Development of Functional Mesocrystalline Materials and Ferroelectric Perovskites Wenxiong Zhang March 2019 KAGAWA UNIVERSITY
Development of Functional Mesocrystalline
Materials and Ferroelectric Perovskites
Wenxiong Zhang
March 2019
KAGAWA UNIVERSITY
1
Table of Contents
Chapter I General Introduction ..........................................................................................3
1.1 Overview on mesocrystals ...................................................................................... 4
1.1.1 Structural and formation principles of Mesocrystals ........................................ 4
1.1.2 Characteristic properties of mesocrystals ......................................................... 7
1.1.3 Recent advances and future outlook in mesocrystal materials ....................... 10
1.2 Metal oxides and complex compounds mesocrystals ........................................... 13
1.2.1 TiO2 mesocrystals ........................................................................................... 13
1.2.2 CaCO3 mesocrystals ....................................................................................... 16
1.2.3 SrTiO3 mesocrystals ....................................................................................... 17
1.2.4 Ferroelectric perovskite mesocrystals ............................................................ 20
1.3 Ferroelectric perovskites................................................................................... 20
1.3.1 Inorganic metal oxide ferroelectric perovskites ............................................. 20
1.3.2 Halide perovskites .......................................................................................... 26
1.4 Lattice strain engineering ...................................................................................... 32
1.5 Topochemical synthesis ........................................................................................ 37
1.5.1 Approach of topochemical synthesis .............................................................. 37
1.5.2 Soft chemical process for mesocrystalline nanocomposites ........................... 40
1.6 Purpose of present study ....................................................................................... 41
1.7 Reference .............................................................................................................. 44
Chapter II .........................................................................................................................56
Anomalous Piezoelectric Response of Ferroelectric Mesocrystalline
BaTiO3/Bi0.5Na0.5TiO3 Nanocomposites Designed by Strain Engineering ......................56
2.1 Introduction ........................................................................................................... 56
2.2 Experimental ......................................................................................................... 59
2.2.1 Sample Preparation ......................................................................................... 59
2.2.2 Physical Properties Analysis ........................................................................... 60
2.3 Results and discussion .......................................................................................... 62
2.3.1 Synthesis of mesocrystalline BT/HTO nanocomposite .................................. 62
2.3.2 Synthesis of mesocrystalline BT/BNT nanocomposite .................................. 66
2
2.3.3 Formation reaction mechanism of mesocrystalline BT/BNT nanocomposite 71
2.3.4 Ferroelectric and piezoelectric responses of mesocrystalline BT/BNT
nanocomposite ......................................................................................................... 73
2.3.5 Dielectric responses of mesocrystalline BT/BNT nanocomposite ................. 84
2.4 Conclusion ............................................................................................................ 89
2.5 References ............................................................................................................. 89
Chapter Ⅲ ........................................................................................................................94
Ferroelectric Mesocrystalline BaTiO3/BaBi4Ti4O15 Nanocomposite: Formation
Mechanism, Nanostructure, and Anomalous Ferroelectric Response .............................94
3.1 Introduction ........................................................................................................... 94
3.2 Experimental ......................................................................................................... 97
3.2.1 Sample Preparation ......................................................................................... 97
3.2.2 Physical Properties Analysis ........................................................................... 98
3.3 Results and discussion ........................................................................................ 100
3.3.1 Synthesis of BT/BBT nanocomposites and BBT mesocrystals .................... 100
3.3.2 Nanostructural analysis for BT/BBT nanocomposite ................................... 107
3.3.3 Formation mechanism of BT/BBT nanocomposite ...................................... 108
3.3.4 Ferroelectric, dielectric and piezoelectric responses of BT/BBT
nanocomposite ........................................................................................................ 113
3.4 Conclusions ......................................................................................................... 123
3.5 References ........................................................................................................... 124
Chapter Ⅳ .....................................................................................................................129
Compelling Evidences for Antiferroelectric to Ferroelectric Transition of MAPbI3-xClx
Perovskite in Perovskite Solar Cells..............................................................................129
Chapter V Summary ......................................................................................................131
Publications ...................................................................................................................137
Publications in Journals ......................................................................................... 137
Publications in Conferences .................................................................................. 138
Acknowledgment ...........................................................................................................140
3
Chapter I General Introduction
The study of nanocrystals has become a major interdisciplinary research area and
drawn increasing attentions in the past decades because nanocrystals can be used as
building units to fabricate hierarchically structured solid materials.1 Solids organized by
nanocrystals open up the opportunities of fabricating new materials and devices, which
not only exhibit the properties from individual nanocrystals but also exhibit collective
properties produced via their interactions.2, 3 In these solids, assemblies constructed
from crystallographically oriented nanocrystals have single-crystal-like structures and
much higher porosity than conventional single crystals. Subsequently, the assemblies
have been defined as Mesocrystal, a new class of material, which is a polycrystal
constructed from oriented nanocrystals or microcrystals.3-5 In particular, for the material
chemistry, mesocrystals can offer unique new opportunities for the design of materials,
and be applied to catalysis, sensing, and energy storage and conversion.6-8 Hence, they
have attracted considerable attention of physicists and chemists in recent years, and
become a hot research field. However, the understanding of mesocrystals is still very
limited, such as the preparation approaches, the formation mechanisms, microstructures,
and characteristics, as well as the developments for the various types and
high-performance applications.6
On the other hand, the metal oxide perovskites and perovskite-related halides are
important materials as they possess a number of interesting properties, such as
ferroelectric; piezoelectric; electron-acceptor behavior; a large optical transmission
domain; high resistivity; antiferromagnetic; exceptional magnetic; photoluminescent
properties; anionic conductivity over a wide temperature range.9 Some halide
4
perovskites have the semiconducting property, which has been widely applied to
photovoltaic areas.10 Still, the ferroelectricity is also possible in this kind of halide
perovskites, hence, it has quite interesting and debatable behaviors.11, 12 Therefore,
figuring out the connection between the semiconductor and ferroelectric behavior in the
halide perovskite materials is significant.13
In this chapter, structural and formation principles, characteristic properties, the
applications of conventional mesocrystals, recent advances and future outlook in
mesocrystalline materials are described. Furthermore, the structure, characteristic
properties and the application of the perovskite materials have also been presented. In
addition, the purposes of this dissertation are also clarified.
1.1 Overview on mesocrystals
1.1.1 Structural and formation principles of Mesocrystals
Mesocrystal, as an abbreviation for mesoscopically structured crystalline materials,
represents a structure composed of nanocrystals aligned in a crystallographic pattern but
separated by porosity or a second phase. The first observation of mesocrystal traces
back to 1969 when a porous intermediate structure of BaSO4 was reported by Petres and
co-workers.14 In 2003, Cölfen et al. established some principles and concepts that
illustrate the mesoscale self-assembly of nanocrystals contributing another mechanism
for growing the single crystals.3 In such mesoscale self-assembly (Fig. 1.1, the red
route), the crystal growth by the ordered aggregation results in organized crystals with
iso-oriented directions, which is totally different from the classical crystallization
process (Fig. 1.1, the blue route). For the classical crystallization, including the
5
nucleation and the growth processes, the crystallized primary nanoparticles grow via
ion-by-ion (or molecule-by-molecule) attachment to fulfill the formation of the single
crystal. Until 2005, Cölfen et al. created the word mesocrystal as a replacement for the
iso-oriented crystals and proposed that mesocrystals are kinetically metastable
intermediates, in which the primary units can be still identified.4 Hence, it raises a
challenge to the well-known classification of solids as amorphous, polycrystalline, or
single crystalline. Moreover, mesocrystals show much higher crystallinity than
polycrystalline materials, and in some case even exhibit many characteristic properties
of a conventional single crystal.3, 15, 16 In 2010, Song et al. contributed a more precise
definition that mesocrystals are superstructured crystals consisting of mesoscopically
scaled (1-1000 nm) crystalline subunits that are aligned in the same crystallographic
direction.17
Fig. 1.1 Schematic presentation of classical crystallization (blue route) via ion-by-ion addition
versus single crystal formation (red route) by a mesocrystal intermediate made of nanoparticles.
Although the structure and formation mechanism of nanocrystal superstructures and
6
other nanostructured materials have been investigated for several decades, the
systematic research on mesocrystals only started about 10 years ago. Yet, very little is
known for mesocrystal formation processes, which are fundamental for the
understanding of the fabrication of mesocrystal structure and the control of its synthesis
process.6 Up to now, a large number of mesocrystals have been successfully synthesized,
whereas the growth mechanism of mesocrystal is still a big problem due to the large
range of involved time and length scale during the growth of mesocrystals.5 Recently,
Sturm and Cölfen demonstrated seven scenarios of different mesocrystal formation
pathways, as shown in Fig. 1.2: (a) alignment by organic matrix; (b) alignment by
physical forces; (c) crystalline bridges, epitaxial growth, and secondary nucleation; (d)
alignment by spatial constraints; (e) alignment by oriented attachment; (f) alignment by
face-selective molecules and (g) topotactic (epitaxial) solid phase transformation.6 Each
of these mechanisms involves different driving forces of chemical and physical origin
directing the formation of the mesocrystalline materials, in which (a) to (f) can be
classified to self-assembled materials (red route in Fig. 1.1) and (g) is obtained from
topochemical process (purple route in Fig. 1.1). The topotactic transformation in solids
is well-known and has been quite extensively studied for many classes of solid-state
materials. In other words, if the initial material is single crystalline, the transformation
induces the formation of a new phase in a specific crystallographic orientation, as
shown in Fig. 1.3. It can be obviously seen that the obtained mesocrystals still maintain
the morphologies of the original precursors after one or multi-step transformation,
suggesting that the morphologies of the mesocrystals are inherited from the original
precursors. Usually, this reaction occurs via accompanying the ion exchange,
intercalation, de-intercalation, and topochemical micro/nanocrystal conversion.6, 18
7
Fig. 1.2 Simplified schematic illustration of main formation pathways of mesocrystals. (a)
alignment by organic matrix; (b) alignment by physical forces; (c) crystalline bridges, epitaxial
growth and secondary nucleation; (d) alignment by spatial constraints; (e) alignment by oriented
attachment; (f) alignment by face selective molecules and (g) topotactic (epitaxial) solid phase
transformation. Reproduced with permission.6 Copyright 2017, MDPI.
Fig. 1.3 Various dimensional schematic diagrams for 1 and/or n-step in-situ topotactic
conversion reaction for formations of mesocrystals from original precursors.
1.1.2 Characteristic properties of mesocrystals
It is well-known that the nanocrystal building units with structural multiplicity and
nanoscale size can provide additional opportunities for self-assembly.5 A variety of
8
self-assemblies or topochemical bridge connections for the formations of the
mesocrystals can offer new possibilities for superstructure formations, giving rise to the
mesocrystals presenting some different characteristic properties. The mesocrystals from
functional materials are highly attractive due to the emergent properties of
mesocrystalline materials, such as single-crystal-like behavior, high crystallinity, high
porosity and inner connection bridged by organic components and/or inorganic
nanocrystals.19, 20 Hence, the mesocrystals can exhibit the following characteristic
properties.
Firstly, the mesocrystal is a polycrystal constructed from iso-oriented nanocrystals,
which exhibits a single crystal behavior in X-ray scattering and electron diffraction. For
instance, the octahedron-shaped magnetite (Fe3O4) mesocrystal shows a
single-crystal-like SAED pattern (Fig. 1.4(e)).21 In addition, all the nanocrystals for the
construction of the mesocrystal exhibit the same direction of the interplanar spacing.
These are due to that the mesocrystals are constructed from nanocrystals, and each
nanocrystal is crystal-axis-oriented with each other.
Fig. 1.4 (a, b) TEM images of the 2D monolayer assembly of 21 nm magnetite octahedra in a
9
magnetic field. (c) HRTEM image of one single nanoparticle in the assembly (solid red triangle:
(111) plane at the top; dashed red triangle: (111) plane at the bottom); solid blue triangles:
interparticle spaces. (d) Model of the magnetite octahedra viewed along the [111] zone axis. (e)
Fast Fourier transformation (FFT) pattern and (f) selected area electron diffraction (SAED)
pattern of the 2D mesocrystals shown in (b). Reproduced with permission.21Copyright 2010,
American Chemical Society.
Secondly, the mesocrystals show some special properties of well-aligned and oriented
crystalline assembly, which is unrivalled for the amorphous, polycrystalline, and
single-crystalline materials. Namely, some of desired properties can be satisfied by
using the mesocrystalline superstructure rather than using the same material in
amorphous, polycrystalline aggregate, and single-crystalline. As the case in Fig. 1.4,
magnetic mesocrystals with non-spherical shapes demonstrate more appealing
anisotropic magnetic properties.22
Thirdly, because of the primary crystallites sharing a common crystallographic
orientation, the mechanical properties of the mesocrystals are unusual.23 They can
exhibit higher ductility and toughness than the corresponding single crystalline
materials, and almost all the mesocrystals exhibit the fracture surfaces like amorphous
glasses, but unlike the single crystals.24
Finally, the mesocrystal simultaneously present over two kinds of functional
properties and prevail on the materials having a large improvement in some application
areas. For instance, mechanical toughness and dielectric dissipation, or optical and
magnetic properties can be combined in one system. It is said that these properties
would never be mixed on this nanoscale.5 They combine the high crystallinity with
10
small crystal size, high surface area and high porosity of the mesocrystal as well as
good handling because the size range of mesocrystal is in the nanometer to micrometer.
The existence of the superlattice structure is also the main reason of high attraction due
to the emergent properties of mesocrystals. A self-assembly process for mesocrystals
does not occur by an ion-by-ion manner, however, ionic strength and species of the
solution are still important variables in controlling crystallization to form mesocrystals.
In particular for surfactant phases and microemulsion involved crystallization processes,
the phase equilibrium and physical characteristics of the product can strongly depend on
ionic strength and species, especially if the cationic lyotropic phases are applied.17
1.1.3 Recent advances and future outlook in mesocrystal materials
Recently, the researches on mesocrystals are drawing much attention, not only by
fundamental research but also by its applications, and an increasing number of the
mesocrystals preparations for applications to a wide range of the functional materials
have been reported.21, 25, 26 In addition, the development of the techniques used for the
characterization of the mesocrystals will further allow for the observation of the
mechanisms of mesocrystal formation, and understanding of their structural principles.
The aforementioned techniques mainly include the in-situ techniques like AFM, TEM,
and High-Resolution TEM or other in-situ microscopic and diffraction techniques,
namely SAXS/WAXS, Grazing-Incidence Small-Angle Scattering
(GISAXS)/Grazing-Incidence Wide-Angle Scattering (GIWAXS).
In the research area of the formation mechanism, the carbonates mesocrystals were
studied at the earliest and most. The initial demonstration for the formation of
mesocrystal was originated from the carbonates chemistry, which is a fundamental
11
source of the mesocrystal theory today.27 However, one of the earliest referring
indications of the mesocrystal intermediates was not derived from CaCO3 but the
BaSO4 crystal with the porous structure.28 After exploration of some intermediate metal
oxides mesocrystals, the mesocrystals with even higher definition were reported for
CaCO3 made in silica gels in 1986.29 Therefore, CaCO3 mesocrystals are increasingly
studied and developed via controlling the polymorph and morphology. However, very
little is known that all the carbonate mesocrystals are involved with the
high-performance applications. Subsequently, the study on the metal oxides
mesocrystals not only becomes attracting but also becomes the hottest. Especially in
ZnO and TiO2 mesocrystals, the preparation approaches, the formation mechanisms,
microstructures, and the high-performance applications have been getting increased
attention. At present, the ZnO and TiO2 mesocrystals have been used to catalysis,
sensing, and energy storage and conversion.23, 26 The perovskite metal oxides
mesocrystals have been also developed unsubstantially, they are likely to widely apply
to the high-performance electro-optic field, ferroelectric materials, and other functional
composite materials. Very recently, Hu et al. have developed the platelike
mesocrystalline BaTiO3/SrTiO3 and BaTiO3/CaTiO3 nanocomposite via the
topochemical process with highly elevated dielectric, ferroelectric and piezoelectric
responses.30, 31
Although an increasing number of mesocrystals have been developed, the formation
mechanisms of the mesocrystals are still limited and the mesocrystals are still a new
study field for the solid materials. For example, the metal oxide and carbonate
mesocrystals are predominantly investigated, and they are still understood limitedly,
furthermore, their formation mechanisms are still very difficult to be clear. In addition,
12
the varieties of the mesocrystals are not enough, the application studies of the other
mesocrystals are rarely found out. In the following, the carbonates, metal oxides, and
perovskite mesocrystals are briefly introduced and summarized, respectively. The goal
is to further understand the approach, formation mechanism, and functional application
of the mesocrystals, and is ready for the development and application of new kinds of
functional mesocrystals.
The above contents already reveal the tremendous potential of mesocrystals. Hence,
application-driven research will be increasing, and more applications will be reported
without a doubt. In the near future, the new mesocrystal applications will be
investigated. In the meantime, the development of the aforementioned in-situ techniques
will be achieved for the observation of the formation mechanisms and understanding of
the structuration of the mesocrystals in much more detail than it was possible before. In
addition, given that a large number of optimizations of mesocrystal formation processes
are done by trial and error, so that the introduction of new methodological methods for
characterization and interpretation of the mesocrystal structures is extremely needed. Up
to now, mesocrystals have been applied to Li-ion batteries, catalysts or sensors, and
solar cells like quantum dot based solar cell etc.25, 32, 33 Also, for plasmonic materials,
the two different surface plasmon resonance bands for metal nanorods can lead to
interesting directional couplings in metal nanoparticle mesocrystals, and therefore,
further exploitation of metal nanoparticle mesocrystals can be anticipated in the future.
Furthermore, mesocrystals from magnetic nanoparticles where coupling of the magnetic
fields can also be anticipated. Besides, an untouched area in the mesocrystal research is
mesocrystals of organic nanocrystals, which have a great potential in the pharmaceutical
formulations.6 Therefore, a large number of exciting developments can be foreseen in
13
the field of mesocrystals.
1.2 Metal oxides and complex compounds mesocrystals
1.2.1 TiO2 mesocrystals
As described above, the mesocrystal materials exhibit specific properties in
comparison to its single crystal, which have wide application in sensing, catalysis,
energy storage, and conversion etc. As far as we know, TiO2 (titanium dioxide) is
among the most widely investigated metal oxides materials for its functional properties
and many promising applications in environment, energy, photocatalysts, and sensing
areas.25, 26, 34, 35 Crystal structures for TiO2, including TiO2 (B), TiO2 (Ⅱ), brookite,
anatase, rutile etc. have been widely studied. Among them, the rutile is a
thermodynamic stable phase and others are metastable phases. In addition, anatase is the
most stable phase in the metastable phases.
TiO2 nanocrystals have been increasing investigated and developed into the functional
materials. In addition, the effectiveness of TiO2 in practical applications varied
considerably with its specific surface area and mesoporosity,36 compositions,37
crystallinity38, 39 and, importantly, the morphology and texture of the material.40
Well-defined structural TiO2 materials obtained from the controlled synthesis with, such
as single crystals, ordered mesoporous thin films, nanotubes, and spherical particles, has
attracted much attention in recent decade. For instance, the anatase single crystals
containing high percentage of reactive facets have been fabricated via solvothermal
process with adding the fluorine species. Thermally stable mesoporous TiO2 thin films
with uniformly distributed pores can be fabricated via an evaporation-induced
self-assembly process in the presence of various block copolymers.41 TiO2 nanotubes
14
with different aspect ratios can be synthesized via anodization of titanium foil with
variable potential and electrolyte composition.42 Furthermore, 3D interconnected TiO2
networks with high crystallinity and controllable macropores have been successfully
obtained from preformed templates via a sol-gel nanocasting process.43
Excellent candidates for the applications in sensing, photocatalysis, solar cells and
lithium-ion batteries have been manifested for nanoporous TiO2 materials with large
surface areas. It is noted that anatase TiO2 mesocrystals were first reported by Ye et al.
via the topotactic conversion from NH4TiOF3 mesocrystals, which were fabricated with
the assistance of non-ionic surfactants.44 In such transformation process, the NH4TiOF3
mesocrystal serves as a crystallographically matched template for the subsequent
growth of the TiO2 mesocrystals. Furthermore, the direct mesoscale assembly of TiO2
mesocrystals and their photocatalytic properties have been attracted much attentions. In
the meantime, solid templates and organic additives were gradually introduced during
these mesoscale transformation processes.
Liu et al. have demonstrated that the rutile TiO2 hollow spheres mesocrystals can be
synthesized through the hydrothermal reaction process by using L-serine and
N,N′-dicyclohexylcarbodiimide (DCC) as biologic additives.45 Subsequently, Zhang et
al. have illustrated the formation of the photocatalytically active rutile TiO2
mesocrystals without the help of surfactants or additives, which exhibited the BET
specific surface area of only 16 m2/g. To explore the additive-free approaches for TiO2
mesocrystals with high porosity and high crystallinity, Ye et al. have reported the first
additive-free fabrication of nanoporous anatase TiO2 mesocrystal by using tetrabutyl
titanate as the titanium source and acetic acid as the solvent, as shown in Fig. 1.5.46 In
this formation process, organic titanium firstly reacts with organic acid by a
15
hydrolytic/nonhydrolytic condensation reaction to from amorphous fiber-like titanium
acetate complex precursors with Ti-O-Ti bonds. After two times of continuing
condensation processes, the other crystalline spherical-like precursors come into being
at the expense of the amorphous precursor. Subsequently, the crystalline spherical-like
precursors gradually release soluble titanium including the nucleation and growth of
anatase nanocrystals. Finally, the formed anatase nanocrystals gather along the [001]
direction, accompanying with the entrapment of in situ produced butyl acetate, giving
rise to the formation of the spindle shaped anatase mesocrystals elongated along the
[001] direction. The acetic acid molecules played multiple key roles during the
nonhydrolytic processing of the [001]-oriented anatase mesocrystals. The obtained
anatase mesocrystals with nanoporous exhibits remarkable crystalline stability and
improved performance as anode materials for lithium-ion batteries. In this regard, TiO2
mesocrystals with tunable architectures are promising for a wide range of applications.
16
Fig. 1.5 Schematic illustration of the formation mechanism of nanoporous anatase TiO2
mesocrystals without additives. Reproduced with permission.46 Copyright 2010, American
Chemical Society.
1.2.2 CaCO3 mesocrystals
CaCO3 (calcium carbonate), as one of the most abundant natural existing minerals in
such as the sedimentary rocks47 and the biological skeletons and tissues,3 has been
drawn much attention. There are three kinds of crystalline polymorphs for CaCO3,
including: calcite (dominated phase at lower temperature), aragonite (dominated phase
at higher temperature), and vaterite (at higher supersaturation).48 There are some new
methods for the development of the CaCO3 with the controlling morphologies and
crystallization. Besides, the various inorganic or organic–inorganic composite materials,
superstructure materials, and improved functional materials can be fabricated
accordingly.49, 50 A number of studies on CaCO3 materials have effectively promoted the
development of the mesocrystal chemistry and provided theoretical basis for the
mesocrystal chemistry. A similar gel-sol reaction or a block copolymerization reaction
has been widely used to prepare CaCO3 mesocrystals.
By using the self-assemble in a polyacrylamide gel, a typical calcite mesocrystal with
a pseudo-octahedron morphology formed from rhombohedral nanocrystals has been
fabricated (Fig. 1.6).51 It is obvious that the calcite mesocrystal was obtained by a
precipitation process from an aqueous solution containing Ca2+ and sulfide. It can be
clearly seen that crystallographic block with rough surfaces are constructed from
well-oriented nanocrystalline building units. According to the TEM image (Fig. 1.6b),
the tightly connected nanocrystals and the organic matrix between the individual
17
nanocrystal interspaces can be observed. The inserted SAED pattern reveals a
single-crystal-like crystallographic block, which means the formation of a mesocrystal
structure. In addition, the calcite mesocrystal can be also fabricated by adding the CO2
vapor diffusion into a Ca(OH)2 solution. Since the CO2 vapor diffusion approach has the
advantage of avoiding the interference of the extraneous ions, minimizing ionic strength
and approaching a pH close to biological conditions at the end of the crystallization
reaction, this approach is beneficial to the growth of calcite mesocrystals. As a result,
the vapor diffusion approach gives a better understanding of the driving forces for the
oriented and/or self-assembling of nanocrystals to mesocrystals.
Fig. 1.6 (a) SEM and (b) TEM images of calcite (CaCO3) aggregate with characteristic
pseudo-octahedral morphology obtained from polyacrylamide gel. (Inserted in (b): SAED
pattern of calcite mesocrystal). Reproduced with permission.51 Copyright 2003, American
Mineralogical Society.
1.2.3 SrTiO3 mesocrystals
SrTiO3 (strontium titanate, ST) with a cubic crystal structure has been drawn much
18
attention in the field of solar energy conversion systems and electromagnetic devices.52
In regard to further cutting down human energy usage and controlling environmental
pollution, heterogeneous solid photocatalysts have been anticipated to show promising
solar-to-chemical energy conversion from water since 1972.
For the development of more active ST-based photocatalysts, it is crucial to seek a
versatile route for the structure and property design. It may be expected that the
selectivity and efficiency of the photocatalytic reactions of ST with tailored crystal
facets and morphologies can be achieved for making its mesocrystals. However, there
remain challenges that are relevant to the fabrication of the organized assembly of such
structure-controlled nanoparticles up to the micrometer scale. Only a few of ST
mesocrystals have been reported. Calderone et al. reported the formation of the ST
mesocrystal with cubic morphology by precipitation approach from a suspension of
hydrolyzed TiOCl2 aqueous gel.53 The obtained ST mesocrystal was formed via an
epitaxial self-assembly of nanocrystals with a size of 4-5 nm. Furthermore, the
formation process is a non-additive spontaneous process. In addition, Kalyani et al.
demonstrated that ST mesocrystals with a [010] zone axis along the direction of the
wire-like morphology can be fabricated via an oriented topochemical conversion
approach. In this formation process, the H2Ti3O7 nanowire precursor was firstly
thermally reacted to form the anatase nanowire. The solvothermal and hydrothermal
treatments of the anatase nanowire in Sr(OH)2 solutions were carried out to form the
mesocrystalline ST. The formation of mesocrystals is achieved from a topochemical
reaction.
Very recently, TiO2 mesocrystals, which consist of nanocrystal building blocks, have
showed highly improved performance compared to that of the disorder system due to
19
their efficient charge transfer and separation along the neighboring nanoparticles. The
epitaxial intergrowth of ST from TiO2 has drawn much attention due to the scientific
and technology importance of oxide interface engineering. Zhang et al. reported that the
ST mesocrystals with enhanced photocatalytic activity were produced by topotactic
epitaxial transformation from anatase TiO2 mesocrystals via a facile hydrothermal
process (Fig. 1.7).54 Compared with the conventional disordered system, the material
exhibits the three-fold photocatalytic efficiency (Fig. 1. 7(c)) for the hydro evolution
reaction of water splitting in alkaline aqueous solution because of the ordered
mesocrystalline structure (Fig. 1.7 (d-g))
Fig. 1.7 (a) Schematic presentation of the formation of SrTiO3 mesocrystals via topotactic
epitaxial transformation of TiO2. (b) Structural model of interface between SrTiO3 and TiO2,
suggesting the epitaxial intergrowth of both phases. (c) Simplified scheme of a SrTiO3
mesocrystal showing the photogeneration of electrons and holes and anisotropic electron
transport form the inside to the outside. (d) SEM image of typical SrTiO3 mesocrystals, (e)
zoomed image inside the red frame marked in (d). (f) TEM image and SAED pattern of SrTiO3
mesocrystals. (g) High resolution TEM image of the area marked with red-square in (f).
Reproduced with permission.54 Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA,
20
Weinheim.
1.2.4 Ferroelectric perovskite mesocrystals
Ferroelectric materials have been paid much attention to scientific and technological
fields due to their wide-applications in sensors, actuators, and energy harvesters.55, 56
The ferroelectric mesocrystals with specific morphology can be fabricated via the
bottom-up self-assembly or the topochemical process.57, 58 In our previous research, the
BaTiO3 mesocrystals with platelike morphology have been fabricated by a hydrothermal
soft chemical process from the H1.07Ti1.73O4 (HTO) precursor with a lepidocrocite-like
layered structure.58 Such perovskite mesocrystals with complex chemical compositions
are very difficult to be prepared via the conventional methods. In addition, the platelike
Ba1−xCaxTiO3, Ba0.5(Bi0.5K0.5)0.5TiO3 and Bi0.5Na0.5TiO3 mesocrystals have also been
developed from the same HTO precursor via the topochemical process.59-61 Furthermore,
all of these platelike mesocrystals are promising for making the oriented ceramics with
improved dielectric and ferroelectric properties.62 Very recently, KNbO3 mesocrystals
with different morphologies have been prepared via the self-assembly without polymer
additives by our group.57
1.3 Ferroelectric perovskites
1.3.1 Inorganic metal oxide ferroelectric perovskites
Perovskite is a calcium titanium oxide mineral (calcium titanate, CaTiO3). The
mineral was discovered in the Ural Mountains of Russia by Gustav in 1839 and is
named after Russian mineralogist Lev Perovski. Its name is lent to the class of
21
compounds which have the same type of crystal structure as CaTiO3, known as the
perovskite structure. Generally, perovskites are materials described by the formula
ABX3, where X is an anion and A and B are cations of different sizes (A being larger
than B), and the crystal structure is described in Fig. 1.8(a). Namely, the BX6
octahedrons connect with each other via corner-sharing, forming the 3 dimension space.
The structure was first described by Victor Goldschmidt in 1926, in his work on
tolerance factors.63 Generally, the phase stability of the perovskite materials can be
confirmed through the tolerance factor t, which is defined as below:
t =γA + γX
√2(γ𝐵 + γ𝑋)
Where γA, γB and γX represent for the ionic radii of the A-site cation, B-site cation and
X-site anion in the perovskite ABX3 structure, respectively. A t value between 0.8 and
1.0 is favorable for perovskite structure with rhombohedral or orthorhombic structure,
and larger (>1) one tends to form the tetragonal structure, whereas the smaller (<0.8)
values of tolerance factor usually result in non-perovskite structures.64, 65
22
Fig. 1.8 (a) ABX3 perovskite structure and (b) P-E hysteresis loop of the ferroelectric perovskite
materials. (c) Piezoelectric effect and inverse piezoelectric effect of ferroelectric perovskite
materials.
Ferroelectrics are a section of a much larger class of substances called pyroelectrics.
A pyroelectric has the property that the single crystal with no surface changes is polar;
polarity is masked by the twining or by surface charges and is only revealed by the
heating treatment. A ferroelectric has an additional property that the polarization
direction can be reversed by applying the external electric-field, therefore it exhibits
hysteresis as shown in Fig. 1.8(b).66 Since the discovery of ferroelectricity in
single-crystal materials (Rochelle salt) in 1921 and its subsequent extension into the
realm of polycrystalline ceramics (BaTiO3) during the early to mid-1940s, there has
23
been a continuous succession of new materials and technology developments that have
led to a significant number of industrial and commercial applications.67 All the
ferroelectric materials exhibit an invariably piezoelectric nature, which have
piezoelectric and inverse piezoelectric effects as shown in Fig. 1.8(c). Up to now, the
inorganic metal oxide perovskite ferroelectric materials, such as Pb(Zr1-xTix)O3, BaTiO3,
Bi0.5Na0.5TiO3 etc., have been widely applied to actuators, sensors and energy
harvesters.55, 56, 68
Pb(Zr1-xTix)O3 perovskite ferroelectric. Lead zirconate titanate, namely
Pb(Zr1-xTix)O3 (PZT), was discovered in 1955 by B. Jaffe et al.,69 which has much more
excellent piezoelectric performance than that of BaTiO3 ceramics.55 PZT and related
perovskite compositions have been the mainstream for the high performance actuators
and transducers, owing to their excellent dielectric, piezoelectric, and electromechanical
coupling coefficients.69
PZT is a solid solution constructed from PbZrO3 (PZ) and PbTiO3 (PT) where the x =
0.48 of PT, and its ceramic lies near a morphotropic phase boundary (MPB) separating
the tetragonal and rhombohedral phases. The MPB can usually exhibit anomalously
high dielectric and piezoelectric responses.70-72 The enhancements of the piezoelectric
and dielectric responses are associated with lattice strains derived from the lattice
mismatches between the two different crystalline symmetries with slightly different
lattice constants at their interface.72 Dopants are always used to improve the property of
the PZT materials. With the adding of different kinds of dopants, the PZT material is
categorized into two classes of “soft” and “hard” PZT, respectively, which can be
applied to different areas.55
Nonetheless, lead has gradually been expelled from many commercial applications
24
and materials due to the concerns regarding its poisonousness. The replacement of the
lead-based piezoelectric ceramics represented by PZT has fueled the excitement and
spurned wide-spread scientific activity to find the alternatives in the last decade due to
the publication by Saito et al..70 Since PZTs contain more than 60 weight percent lead,
the work of Saito et al.70 got attention for identifying a mixture of morphotropic and
polymorphic phase transition region in a (K, Na)NbO3-based system (KNN) with
PZT-like values of piezoelectric coefficients in the texture ceramics. Finally, they
achieved a comparable piezoelectric response with PZTs.
The lead-free piezoelectric materials can be categorized into two general groups: one
competes for the same application as PZT and another excels in properties that are
outside the range where PZT can be used. As for the first group (in competition with
PZT), it contains KNN, Bi0.5Na0.5TiO3 (BNT) and (Ba,Ca)(Zr,Ti)O3 (BCZT) based
ceramics. These materials are inferior to PZT in some sense but are superior in another.
The second group includes materials with properties with which PZT cannot compete.
Examples include: BaTiO3, LiNbO3 (single crystal), Bi-based layered compounds with
Aurivillius structure, and other high temperature piezoelectric materials.73
(K, Na)NbO3 (KNN) perovskite ferroelectric. KNN-based ceramics have attracted
considerable attention due to the high Curie temperature, large piezoelectric response
and strong ferroelectricity. These ceramics are supposed to be suitable alternatives for
lead-based PZT.74 Reported in 1955, a morphotropic phase boundary (MPB) separating
two orthorhombic ferroelectric phases was identified at x = 0.5 ((K1-xNax)NbO3).
Inherent to MPBs, a maximum in remnant polarization (Pr) and minimum in coercive
field (Ec) were reported.75 The KNN-based ceramics obtained by solid state reaction
process cannot possess a good piezoelectric and ferroelectric properties as compared to
25
the lead-based PZT systems76 due to that the phase stability of KN is limited to 1040 oC
and KNN is limited to 1140 oC.77 Until now, the high density KNN ceramics can be
only synthesized by hot pressing techniques.78 It is important to note that KNN material
prepared by spark plasma sintering reported significantly higher dielectric and
piezoelectric properties than those synthesized by hot pressing, solid state and molten
salt reaction process, with εr = 700 and d33 = 148 pC/N.79, 80 It is indicated that the
enhancement is the result of extrinsic contributions to the polarizability associated with
submicron grain sizes, similar to that found in fine grain BaTiO3 ceramics.81
Bi0.5Na0.5TiO3 perovskite ferroelectric. Bi0.5Na0.5TiO3 (bismuth sodium titanate, or
BNT) is a well-known lead-free perovskite ferroelectric material. It has a perovskite
structure with a rhombohedral R3c space group (a = 38.91 nm; α= 89.6o) at room
temperature.82 In the perovskite structure with ABO3 formula, where one half of A-site
is filled with Na+ and the other half with Bi3+, and the B-site is filled with Ti4+. The
BNT has also been regarded as one of the most promising candidate materials for the
development of lead-free piezoelectric materials due to its relatively large remnant
polarization (Pr = 38 μC/cm2) and high Curie temperature (Tc = 320 oC).83 However, a
large coercive field and a high conductivity of the un-doped BNT result in difficult
poling, which then directed the exploration of BNT toward BNT-based materials, such
as Bi0.5Na0.5TiO3/BaTiO3 (BNT/BT), Bi0.5Na0.5TiO3/Bi0.5K0.5TiO3 (BNT/BKT),
Bi0.5Na0.5TiO3/BiAlO3 (BNT/BA), Bi0.5Na0.5TiO3/SrTiO3 (BNT/ST) etc., and the
piezoelectric properties along with the electrical resistivity have been greatly
improved.84-86 Processing and properties of the binary BNT-based ceramics have been
extensively reported.87, 88
BaTiO3 perovskite ferroelectric. One of the well-known lead-free ferroelectrics is
26
BaTiO3 (barium titanate, or BT), which exhibits a tetragonal symmetry of perovskite
structure at room temperature having large piezoelectricity and excellent dielectric
properties,89 which make it the most important compound applied in the composition of
ceramic capacitors, especially for the manufacture of multilayer ceramic capacitors
(MLCC).90, 91 Since the performance of the BT ceramics significantly depends on the
microstructure of the calcined body, much attention has been focused on the synthesis
of BT nanoparticles. In recent years, wet-chemistry synthesis techniques, including
sonochemical synthesis,92 sol-gel,93 hydrothermal,58 solvothermal94 and chemical
coprecipitation,95 have been investigated to prepare BT nanoparticles. However, it is
still a challenge to synthesize well dispersive BT nanoparticles with controlled
morphology. In addition, the molten salt method has also been used to prepare many
ceramic materials.96 Above the melting point of the chosen salt, the molten salt forms a
liquid phase to act as a solvent for reactant dissolution, diffusion and precipitation. To
achieve the competition to the lead-based ferroelectric materials, two kinds of effective
approaches, i.e. domain engineering and oriented engineering, have been widely used to
improve the piezoelectric performance for the ferroelectric materials.97-99 The domain
engineering is to reduce the domain size of the ferroelectric materials, while the oriented
engineering is to optimize their orientation direction. The piezoelectric response of
BaTiO3 (BT) can be improved from 200 to 788 pC/N using the combined domain and
oriented engineering, the value is larger than that of 500 pC/N for the commercially
available PZTs.89
1.3.2 Halide perovskites
Researches on the earth-abundant metal halide-based perovskite for high-efficiency
27
photovoltaics have demonstrated this class of materials to be excellent semiconductors
for optoelectronic devices.9, 100, 101 For halide perovskite, perovskite compounds based
on metal halides adopt the ABX3 perovskite structure, which has existed for more than a
century. This structure consists of a network of corner-sharing BX6 (B = Pb(II), Sn(II),
Ge(II); X = Cl, Br, I) octahedra, while the A cation is selected to balance the total charge
and it can even be a Cs+ or a small organic ions (CH3NH3+ or MA, HC (NH2)2
+ or FA).
MAPbX3 perovskites. Examples of insulating, semiconducting, and superconducting
perovskite structured materials are known; they represent a unique system class across
solid-state chemistry and condensed matter physics.9 Particularly, they have phase
transitions with accessible monoclinic, trigonal, orthorhombic, tetragonal and cubic
polymorphs relying on the rotation and tilting of the BX6 octahedra in the lattice.
Stimuli, like temperature, electric field and pressure, can give rise to the reversible
phase transitions. In fact, the Cl-, Br- and I- with different ionic radii have been widely
applied to establish the MAPbX3 (X: Cl, Br, I) perovskite structures, and it has been
found that a smaller ion radius for X- tends to form the cubic phase, such as MAPbCl3
and MAPbBr3, while the MAPbI3 exhibits a tetragonal structure.102 MAPbI3, with a
bandgap of about 1.5-1.6 eV and a light absorption spectrum up to a wavelength of 800
nm (Fig. 1.9), has been extensively used as a light harvester in solar cells.103 In addition,
the ferroelectricity of the MAPbI3, has been confirmed by some researches, in which the
spontaneous polarization of the ferroelectric property gives rise to a more efficient
charge-separation, then improve the power conversion efficiencies (PCEs) of perovskite
solar cells (PSCs).12, 104-110 On the other hand, the MAPbCl3 and MAPbBr3 exhibit
paraelectric property due to their room temperature cubic structure. But, the MAPbBr3
has an advantage of larger band gap of 2.32 eV, which gives rise to a larger voltage
28
potential, whereas the low current density impedes its further improvement of the
PCE.111 In order to fulfill a high open circuit voltage in a solar cell, it is necessary to
combine a suitable energy band structure of the constituent materials with good charge
transfer kinetics. Therefore, the MAPbI-based (MAPbI3-xBrx, or MAPbI3-xClx)
perovskites have been widely studied.112-115 The advantages of utilizing a perovskite
material as the main ingredient are ascribed to its large absorption coefficients, high
carrier mobility, ambipolar transporting properties, and low-cost solution-based
fabrication process.116
Chronologically, Miyasaka et al. demonstrated the formation of the MAPbBr3 and
MAPbI3 in dye-sensitized solar cells (DSSCs) and the efficiency of 2.6 % and 3.8 %,
respectively in 2006 and 2009. Subsequently, Park et al. reported an efficiency of 6.5 %
by creating a cell architecture similar to the extremely thin absorber (ETA) DSSC. In
2012, Park et al. replaced the liquid-based hole transport layer with solid-state
spiro-MeOTAD and immersed perovskite into the TiO2 scaffold because of the
corrosion problems for the liquid electrolytes, in the meantime an efficiency of 9.7 %
was fulfilled. Since then, the researches on the perovskite solar cells show the explosive
growth.10, 117, 118 In 2015, Park reported the MAPbI3 PSCs with a PCE of 19 %.119 Many
studies and investigations on the performances of perovskite solar cells are still ongoing
in the effort to surpass this PCE of 20%, as well as to establish a stable performance for
PSCs, in order to eliminate costly silicon PV cells.120
29
Fig. 1.9 Tuning perovskite bandgap by replacing the A cation. (a), The ABX3 perovskite crystal
structure. (b) The atomic structure of the three A site cations explored. (c) UV-Vis spectra for the
APbI3 perovskites formed, where A is either methylammonium (MA), formamidinium (FA) or
caesium (Cs). Reproduced with permission.121 Copyright 2014, Royal Society of Chemical.
FAPbI3 perovskites. The chemical modification of the X site anions for MAPbI3
(substitution of I for Br or Cl) has been achieved and was shown to increase the
bandgap while modulating the PCEs of PSCs. Similarly, the substitution of the MA+
with a longer chain C2H5NH3+ was reported and gave rise to a larger band gap with a
lower PCE (2.4 %). Therefore, the exploration of the alternative perovskites with
attractive bandgaps is necessary, which can be formed into efficient solar cells.
30
It is known that the optimal bandgap for a single-junction solar cell is between 1.1
and 1.4 eV, currently beyond the range of the MAPbI3 system. Koh et al. have
demonstrated the introduction of a new metal-halide perovskite based on FAPbI3, which
displays a favorable bandgap and exhibits a broader absorption compared to MAPbI3.122
This is because that the FA cation has shown a slightly larger ionic radius than the MA
group, which is expected to result in an increase in perovskite tolerance factor (t).65 An
increase in the tolerance factor t while maintaining the FAPbI3 perovskite structure
generally leads to an increase in symmetry, with an expected reduction in electronic
bandgap (1.47 eV) (Fig. 1.9). This bandgap for FAPbI3 is closer to the optimum value of
∼1.4 eV and presents FAPbI3 as an appealing candidate for photovoltaic applications, as
it displays an extended absorption of light compared to the MAPbI3 analogue.123 Eperon
et al. have reported that the short-circuit currents of FAPbI3 PSCs achieved > 23
mA/cm2, giving rise to PCE of up to 14.2 %.121 Hence, FAPbI3 perovskite is promising
as a new candidate for this class of solar cell.
However, the photovoltaic performance of FAPbI3 has been reported lower than that
of MAPbI3.121 In addition, a black perovskite polymorph (α-phase: stable at temperature
above 160 oC) was discovered to transformed into a yellow FAPbI3 polymorph (δ-phase:
non-perovskite) in an ambient humid atmosphere.122 Given the suitable bandgap that is
lower than that in MAPbI3, the performance of FAPbI3 solar cells can be considerably
improved by stabilizing the perovskite structure of the FAPbI3 phase. Based on this,
Pellet et al. reported an improved PCE by using mixed cation lead iodide perovskites by
gradually substituting MA with FA cations, which increases the absorption range by red
shift, allowing for a higher current density, but the performance was still dominated by
MAPbI3 rather than FAPbI3.124 Therefore, Jeon et al. proposed a strategy for extending
31
the absorption range of solar light by replacing MAPbI3 with FAPbI3 in the combined
composition of FAPbI3 and MAPbBr3.103 Consequently, the (FAPbI3)0.85(MAPbBr3)0.15
perovskite with a trigonal structure at room temperature was fabricated, confirming that
the co-substitution of MA to FA and I to Br can efficiently stabilize the perovskite phase.
Furthermore, the fabricated FAPbI3-based PSCs exhibited a maximum PCE greater than
20%.125
CsPbX3 perovskite. Although all top-performing photovoltaic cells used MA or FA
as the A cation, it is attracting that whether an inorganic A cation would also form light
absorbers with comparable properties to the MA ones. Studies on the all-inorganic
halide perovskites have revealed that these materials have great potential in
optoelectronic applications.126, 127 To find out whether the organic nature or anisotropic
geometry of the A cation is essential for the high performance of the hybrid PSCs, the
CsPbBr3 was chosen because, unlike the CsPbI3 compound, this compound occurs at
standard temperature and pressure in the perovskite structure and exhibits very good
charge transport properties.128 Beal et al. reported that by replacing the volatile MA
cation with cesium, it is possible to synthesize a mixed halide absorber material with
improved optical and thermal stability, a stabilized PCE of 6.5%, and a bandgap of 1.9
eV.129
Although the PCE of PSCs has a tremendous potential to exceed the silicon solar
cells (25 %), conclusive charge separation mechanism is still missing for PSCs because
of the lack of the fundamental understanding of the structure-property-performance
relationship of the perovskite properties, which hampers the optimization and
development of high-performance PSCs. Up to now, a traditional p-n junction charge
separation has been employed to PSCs, however, there are some mysterious behaviors
32
like the strong current-voltage (J-V) hysteresis, yet remains unexplained reasonably.13,
130 And the J-V response of the PSCs could lead to unfaithful estimation of the solar cell
device efficiency, where the reverse scan and forward scan exhibit the overestimated
and underestimated PCEs.131 Therefore, some possible theories have been proposed to
explain the origin of the hysteresis in PSCs, involving ferroelectricity,106
vacancy-assisted ionic migration,132 charge carrier trapping,133 and capacitive effect.13
1.4 Lattice strain engineering
Two-dimension (2D) materials have been paid much attention in the past decade due
to their extraordinary properties and great potential in a wide range of applications.
Strain engineering is regarded as a powerful tool to modulate the properties of 2D
materials because of its direct impact on lattice structure, and thus alters electronic
structure.135 There are five possible methods, like deforming a substrate,136 creating
wrinkles,137 employing a pre-patterned substrates,138 deforming a suspended
membrane139 and lattice mismatch,140 which have been reported in the literatures, as
shown in Fig. 1.10. Among these methods, the lattice mismatch has been widely applied
to ferroelectric 2D thin film materials for enhancing their dielectric and ferroelectric
responses.140-143
33
Fig. 1.10 Schematic presentation of methods to introduce strains to 2D materials. (a) Deforming
a substrate; (b) Creating wrinkles; (c) Employing a pre-patterned substrate; (d) Deforming a
suspended membrane. (e) Lattice mismatch.
Enormous strains can appear in thin film materials when one material is epitaxially
grew on another, originating from differences in the lattice constants of crystals and the
different thermal expansion behaviors between the film and the underlying substrate or
the defects formed during the film deposition. Therefore, some properties of thin film
can be found remarkable different from the corresponding unstrained bulk materials. In
the meantime, the suitable strains are needed for the enhancement of properties of a
chosen material in thin film form, namely lattice strain engineering. It has been widely
applied to 2D thin film materials with superlattice structure, correspondingly causing
the 2D heteroepitaxial interface, as shown in Fig. 1.11(a).
34
Fig. 1.11 Schematic illustrations of local environments for (a) the 2D heteroepitaxial interface in
the superlattice nanocomposite obtained by the MBE approach and (b) the possible 3D
heteroepitaxial interface in the mesocrystalline nanocomposite.
Heteroepitaxial growth has been established as a powerful technique to create
single-crystalline thin films or artificial low-dimensional quantum systems. Furthermore,
it gives the possibility to combine crystalline materials, which can be very different in
symmetry, bonding and lattice constant, respectively. The atomic arrangement at the
interface is the most responsible for the specific epitaxial alignment, and also for the
development of the resulting microstructure. Residual strains as well as extended
defects affect the desired physical properties and, thus, strongly decide the quality of
functionality of heterosystem. Hence, it is possible to synthesize heterosystems with
tailored electrical, optical or mechanical properties.144 For the ferroelectric materials,
the spontaneous polarization direction around the heteroepitaxial interface can be
changed to the other directions due to the lattice distortion originated from different
lattice constants (Fig. 1.12). Therefore, a sloping spontaneous polarization structure can
35
be imported into the interfaces of the
Fig. 1.12 Schematic diagrams of imported heteroepitaxial interfaces from (a)
ferro-tetragonal-ferro-trigonal composite with polarization direction along [001] and [111]
respectively, (b) ferro-tetragonal-ferro-orthorhombic composite with polarization direction
along [001] and [110] respectively, (c) ferro-tetragonal-para-cubic composite.
different substances, which results in producing the polarization reversal sustaining.145
The artificial interfaces such as in artificial superlattices can produce an obviously
significant enhancement of piezoelectric and dielectric constants.140, 141, 143, 146, 147
As mentioned above, the replacement of lead-based Pb(Zr1−xTix)O3 (PZT)
ferroelectric materials has become quite significant for the environmental protection.
However, the reported lead-free materials exhibit quit lower piezoelectric performance
than that PZTs. Although, the piezoelectric response of BaTiO3 (BT) can be improved
from 200 to 788 pC/N using the both domain engineering and oriented engineering, the
value is larger than that of 500 pC/N for the commercially available PZTs.89 However,
the piezoelectric application of BT is restrained in a temperature range of below 130 °C
36
of its Curie temperature (Tc). Therefore, the elevated Tc is also important for the
development of high performance lead-free ferroelectric materials. Haeni et al. first
reported the improvement of Curie temperature (Tc) in SrTiO3 thin film by introducing
the substrate-induced epitaxial strain, in the meantime Choi et al. demonstrated the
same manner in BaTiO3 thin film.140, 141 However, the increase of Tc by means of
substrate control is limited to film materials with only tens of nanometers thick, whereas
many ferroelectric devices need much thicker films. Therefore, a BaTiO3/Sm2O3
composite thick film has been reported with a highly improved Tc of BaTiO3, where the
BaTiO3 lattice bears a tensile strain with +2.35 % lattice mismatch at the BaTiO3/Sm2O3
interface by Harrington et al.143
At present, the most studies on the superlattices of ferroelectric materials are in
regard to 2D BaTiO3/SrTiO3 superlattice structure prepared by the
molecular-beam-epitaxy (MBE) process.141 But the MBE process is inefficient and high
cost, and the industrialization production is difficult to achieve by MBE process.142
Furthermore, 3D superlattice structures (Fig. 1.11(b)) which can achieve a high density
interface are difficult to be fabricated via the MBE process.30, 31, 148 Hence, the
exploration of a new approach to develop the superior superlattice composite materials
is expected. The development of the ferroelectric mesocrystalline nanocomposites with
the 3D superlattice structure by a facile low cost process is a significant subject and will
attract much attention in the ferroelectric materials field.149 In our former research, the
mesocrystalline ferroelectric BaTiO3/SrTiO3 (BT/ST) and BaTiO3/CaTiO3
nanocomposites constructed from the different nanocrystals with same crystal-axis
direction exhibit an both elevated dielectric and ferroelectric responses via the
introduction of the strains originated from the lattice mismatches between BT and ST or
37
BT and CT, which have different lattice constants.30, 31
1.5 Topochemical synthesis
Topochemical synthesis is a quite useful and classical approach for the fabrication of
the targeted particles with the desired morphologies.54, 150 Compared with other
reactions, the topochemical conversion reaction can be regarded as special phase
transformations of the parent crystals into the daughter crystals and is driven by the
crystal structures rather than by the chemical nature of the reactants. Therefore, the
crystallographic directions of parent and daughter crystals have some certain topological
correspondences. Some mesocrystals can be prepared by the topochemical syntheses
method as described above. The conventional synthesis approaches, such as
hydrothermal/solvothermal process, molten salt process, and solid-state reaction process
etc., can be employed for the topochemical synthesis.
1.5.1 Approach of topochemical synthesis
The hydrothermal/solvothermal process is a liquid chemical reaction process under a
higher pressure than 1 atm and a higher temperature than boiling point of the solvent
used. When an aqueous solution is used as the solvent, it is called hydrothermal process.
When other solvents rather than aqueous solvent, such as organic solvents, organic and
aqueous mixed solvents, are used, it is called solvothermal process. These processes are
widely applied to prepare the ceramics powders. The basic mechanism of crystal
nucleation and growth under the hydrothermal and solvothermal conditions is the
dissolution-deposition reactions. The particle size is controlled by the crystal growth
rate, reaction time, and reaction temperature. The particle morphology is dependent on
the crystal growth direction or the non-classical self-assemble direction that is not easy
38
to be controlled in the normal cases. The advantages of the hydrothermal/solvothermal
process are preparations of the products with a controllable morphology, a controllable
crystal facet, a uniform size distribution, a small crystal size at a relatively low
temperature. The hydrothermal/solvothermal process is a potential method for the
topochemical synthesis.
Because of the advancements in modern technology, the study of molten salt
synthesis has achieved considerable progress, and lots of molten salts have been used
for this process. The molten salt process is usually carried out in a low-temperature
molten salt as a reaction medium, and the molten salt can also act as a reagent.151 Salt
melts have a long history as a solvent in research as well as in industry due to their low
toxicity, low cost, low vapor pressure, abundant availability, high heat capacity, large
electrochemical window, and high ionic conductivity. The crystal growth occurs easily
in the salt melt medium, the product particles usually have its original crystal
morphology, uniform and large particle size. The precursor host particles can react
easily with the guest ion or molecule species in the molten-salt via host-guest
mechanism to obtain the desired composition and morphology of the products.
Therefore, the molten salt process is suitable for the topochemical synthesis.
For the normal solid-solid reaction process, the ball-milled precursor powders with
desired compositions should be annealed at high temperatures. This reaction occurs
simply via solid-state diffusion at a high temperature. The obtained products usually
have the characteristics of isometric morphology such as cubic or spherical, large
particle size, and compositional inhomogeneity.152 Nonetheless, the solid-state process
can still be used for the topochemical synthesis.153
39
1.5.2 Layered protonated titanate HTO as precursor for topochemical synthesis
Fig. 1.13 Schematic diagrams of HTO (H1.07Ti1.74O4·H2O) crystal with lepidocrocite structure. (a)
[100] zone axis structure and (b) three-dimensional (3D) structure.
Layered titanates with variety 2D structures have recently been drawn much attention
because of their interesting interlayer chemistry. One of the most studied layered
titanate is lepidocrocite (γ–FeOOH)-type protonated titanate, which has a composition
of H1.07Ti1.73O4·H2O (HTO) and exhibits excellent ion-exchange/intercalation
reactivities, and can be readily exfoliated/delaminated into its structure unit single
sheets with a distinctive 2D morphology and a thickness of about 1 nm.58, 154 In the
HTO crystal structure, the TiO6 octahedrons are combined with each other via angle and
edge-sharing to form a 2D TiO6 octahedral sheet, as illustrated in Fig. 1.13. The host
sheets are stacked with a basal spacing of about 8.82 Å in a body-centered orthorhombic
system (a = 3.7831 Å, b = 17.6413 Å, c = 2.9941 Å), accommodation H2O and H3O+
40
between the octahedral sheets (Fig. 1.13). Approximately 52% of the interlayer sites are
occupied by H3O+ and remaining by H2O. The positive charge of H3O
+ is balanced with
minus one of the host TiO6 octahedral sheets arising from the Ti site vacancies.
In our previous works, we have used the HTO crystal as a precursor to prepare the
various perovskite titanate mesocrystals and the TiO2 platelike mesocrystals,18 and
furthermore as a template to fabricate oriented ceramics by a reactive template grain
growth method.62 Very recently, the phase transition mechanism of the HTO crystal to
anatase TiO2 under supercritical water has been reported.18 These results suggest that
the HTO crystal is an excellent precursor for the preparations of the titanate
mesocrystals and titanium oxides mesocrystals by the topochemical conversion reaction
mechanisms.
1.5.2 Soft chemical process for mesocrystalline nanocomposites
The solvothermal soft chemical process is a useful and unique method for preparation
and design of functional inorganic materials.18, 31, 57, 148, 155 Advantages of the
hydrothermal/solvothermal process are suitable for the soft chemical synthesis,
especially in effectively maintaining the precursor morphologies in the synthesis
process. The solvothermal soft chemical process typically comprises two steps: the first
step is the preparation of a framework precursor with layered structure and insertion of
structural directing-agents (template ions or molecules) into its interlayer space by a soft
chemical reaction; the second step is the structural transformation of the structural
directing-agent- inserted precursor into a desired structure by a solvothermal reaction.
The crystal structure of the product can be controlled by the structural directing-agent
used, and the product particle morphology is dependent on the precursor morphology
41
used. This process has been utilized for the synthesis and design of metal oxides
mesocrystals and organic-inorganic nanocomposites with controlled structure,
morphology, and chemical composition.20, 54, 156 As described above, the platelike
mesocrystalline nanocomposites perovskite mesocrystals, such as BaTiO3/SrTiO3 and
BaTiO3/CaTiO3 nanocomposites, have been successfully fabricated by using the facile
two-step solvothermal soft chemical process, which give rise to highly elevated
dielectric and ferroelectric properties.30, 31
1.6 Purpose of present study
As described above, up to now, studies on the mesocrystals have been reported
mainly on the syntheses and formation mechanisms; however, the understandings on
mesocrystal properties, formation mechanisms, and especially the potential application
possibilities are still not enough. Further development of the functional mesocrystals,
investigations of the mesocrystal performances and the formation mechanisms are
necessary in current nanomaterial research fields. The development of the ferroelectric
perovskite mesocrystals has drawn much attention since the potential improvements on
dielectric, ferroelectric and piezoelectric responses of the mesocrystalline materials. In
addition, the approaches, like oriented engineering, domain engineering and
nanocomposition engineering, can be applied to improve the piezoelectric property of
the ferroelectric materials by using the mesocrystalline materials. However,
simultaneously elevated Curie temperature and piezoelectric responses have not yet
been reported, in which the application temperature ranges of the ferroelectric materials
are restrained by their Curie temperatures.
As described above, the strain engineering can be applied to elevate Curie
42
temperature of the ferroelectric materials by introduction of 2D heteroepitaxial interface.
But for bulk ferroelectric ceramic materials, the strain engineering for simultaneous
improvement of both piezoelectric response and Curie temperature has not been
reported, as far as we know, since the 3D heteroepitaxial interface is very difficult to be
introduced into bulk materials. Therefore, in the present study, the 3D heteroepitaxial
interface is challenged to introduce into the functional mesocrystalline nanocomposites
through a two-step soft chemical process via the in situ topochemical conversion
mechanism. In the meantime, both piezoelectric response and Curie temperature of the
mesocrystalline ferroelectric nanocomposites can be highly improved.
On the other hand, the perovskite compounds possess a number of interesting
properties, such as electron-acceptor behavior; a large optical transmission domain;
antiferromagnetic; exceptional magnetic; piezoelectric; photoluminescent properties;
anionic conductivity over a wide temperature range. The semiconducting properties of
perovskite-related halides have been widely applied to photovoltaic areas. Still, the
ferroelectricity is also possible in perovskite-related halides, which would exhibit quite
interesting and debatable behaviors in the PSCs. Therefore, figuring out the
semiconductor and ferroelectric behaviors in the perovskite-related halides is
significant.
In Chapter II, the ferroelectric mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 (BT/BNT)
nanocomposite synthesized from a layered titanate H1.07Ti1.73O4 (HTO) by an ingenious
two-step topochemical process is introduced. The BT/BNT nanocomposite is
constructed from well-aligned BT and BNT nanocrystals with the same crystal-axis
orientation. The BT/BNT heteroepitaxial interface in the nanocomposite is promising
for the enhanced piezoelectric performance by using the lattice strain engineering,
43
which gives a giant piezoelectric response with a d*33 value of 408 pm/V. The
introduced lattice strain at the BT/BNT heteroepitaxial interface causes transitions of
pseudo-paraelectric BT and BNT nanocrystals to the ferroelectric nanocrystals in the
mesocrystalline nanocomposite, which enlarges ferroelectric, piezoelectric and
dielectric responses. The lattice strain also results in the elevated Curie temperatures
(Tc) of BT and BNT and a new intermediate phase transition.
In Chapter III, the ferroelectric mesocrystalline BaTiO3/BaBi4Ti4O15 (BT/BBT)
nanocomposite synthesized from the layered titanate HTO by a two-step topochemical
process is exhibited. The BT/BBT nanocomposite is constructed from well-aligned BT
and BBT nanocrystals oriented along the [110] and [11-1] crystal-axis directions
respectively. The lattice strain is introduced into the nanocomposite by the formation of
the BT/BBT heteroepitaxial interface, which causes a greatly elevated Curie
temperatures from 400 to 700 °C and an improved piezoelectric response with d*33=130
pm/V. In addition, the BT/BBT nanocomposite is stable up to a high temperature of
1100 oC, therefore the mesocrystalline ceramic can be fabricated as a high-performance
ferroelectric material.
In Chapter IV, the ferroelectric and semiconducting properties of the
CH3NH3PbI3-xClx perovskite are studied by structural analysis, measurements of the
ferroelastic behavior, the ferroelectric hysteresis loops, the piezoelectric response and
conductivity. The results reveal that the CH3NH3PbI3-xClx perovskite exhibits the
anti-ferroelectric and semiconducting natures, and the anti-ferroelectricity can be
switched to ferroelectricity by poling treatment, which gives a solid evidence for the
argument between the non-ferroelectric and ferroelectric nature for this perovskite and
paves the way for the fabrication of high-performance perovskite solar cells.
44
In Chapter V, the main points concluded in each chapter are summarized. In addition,
on the basis of the results of the present study, the future prospects for the applications
of these results are mentioned.
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56
Chapter II
Anomalous Piezoelectric Response of Ferroelectric
Mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 Nanocomposites
Designed by Strain Engineering
2.1 Introduction
Ferroelectric materials have drawn much attention to scientific and technological
fields due to their wide-applications in sensors, actuators, and energy harvesters.1-3
Pb(Zr1-xTix)O3 (PZT) is an excellent and wide-applied ferroelectric material, and a
successful example of application of the morphotropic phase boundary (MPB) around x
= 0.48 with maximized ferroelectric response,4 that occupies the major share of the
piezoelectric ceramic material market. However, the lead-based PZT piezoelectric
ceramics contain more than 60 wt% of poisonous lead component. Recently, the
replacement of the PZTs has fueled the excitement and spread scientific activity to find
the lead-free alternatives, since Saito et al. reported high performance lead-free
piezoelectric materials using MPB and oriented ceramics.5 Nevertheless, the
performances of the lead-free piezoelectric materials are still not comparable to PZTs.
6-8
Some approaches concerning improving the performance of the lead-free
piezoelectric materials have been proposed, such as MPB, oriented ceramics, domain
engineering, and strain engineering.4, 5, 9-21 Among these approaches, the MPB is one of
the effective and common approaches to further elevate the piezoelectric performance
57
of ferroelectric perovskites. The ferroelectric perovskites exhibit elevated piezoelectric
and dielectric responses at MPB due to a lattice strain originating from lattice
mismatching between the two different phases with different symmetrical systems and
little different lattice constants at their interface.4, 5, 14 Then the ferroelectric polarization
rotation of the distorted lattice at the interface becomes sensitive to an applied-bias.22, 23
However, the application of MPB is restrained due to the narrow composition range,
and it is also hard to obtain a temperature-independent MPB.24
The strain engineering is also a promising approach to enhance the piezoelectric and
dielectric responses, and expected to overcome the disadvantages of MPB.9, 10, 12, 15, 16, 18,
19 An artificial superlattice constructed from two kinds of crystals has been employed to
elevate ferroelectric responses, for example large dielectric constant and remanent
polarization have been achieved in BaTiO3/SrTiO3 (BT/ST) artificial superlattice with a
heteroepitaxial interface between the ferroelectric BT and paraelectric ST phases.16, 17,
25-28 The enhanced dielectric and ferroelectric responses are attributed to the horizontal
strain originated from the two-dimensional (2D) BT/ST heteroepitaxial interface, where
little different lattice constants of BT and ST causes lattice strain at the heteroepitaxial
interface.16, 17, 27
Recently, the in-plane biaxial strain regarded as three-dimensions (3D) has drawn
much attention because of its potential for constructing a high density heteroepitaxial
interface.12, 15, 18, 20 Mimura, et al. have fabricated BT/ST heteroepitaxial interface by
self-assembling BT and ST nanocubes to improve piezoelectric response.20 The
enhanced piezoelectric responses have been reported by constructing a 3D BT/KNbO3
(BT/KN) heteroepitaxial interface.12 However, the expected large piezoelectric
responses have not been achieved by employing the 3D heteroepitaxial interfaces. The
58
main reason is attributed to that the perfect 3D heteroepitaxial interfaces are not easy to
be constructed by the bottom-up process by assembling the nanocubes and the surface
coating process using sol-gel or solvothermal method. Therefore, a perfect 3D
heteroepitaxial interface is expected for the further enlarged piezoelectric response.
Mesocrystal is a polycrystal constructed from the nanocrystals with the same
crystal-axis orientation.29, 30 The mesocrystals not only have some potential properties
based on the individual nanocrystals but also exhibit unique collective properties of
nanocrystal ensembles.31 It has become a fascinating research area as a new class of
material for catalysis, sensing, and energy storage and conversion in the past
decade.32-35 Recently, we have reported some ferroelectric perovskite mesocrystals,36-40
and demonstrated that ferroelectric mesocrystalline BT/ST and BT/CaTiO3 (BT/CT)
nanocomposites 15, 18 with the 3D heteroepitaxial interfaces between ferroelectric BT
phase and paraelectric ST phase, and ferroelectric BT phase and ferroelectric CT phase,
respectively, show a highly elevated piezoelectric responses comparing with those of 3D
heteroepitaxial interfaces fabricated by using other methods.19, 20, 41 The results suggest
the potential application of the mesocrystals to the improvement of piezoelectric
performance for the lead-free piezoelectric materials.
In the lead-free piezoelectric families, bismuth sodium titanate (Bi0.5Na0.5TiO3, BNT)
is a promising candidate for the replacement of PZT materials because of its relatively
large remanent polarization (Pr) of 38 µC/cm2 and high Curie temperature (Tc) of 320
○C. The BNT-based Bax(Bi0.5Na0.5)1-xTiO3 (BBNT) materials with a MPB composition
around x = 0.07 have been extensively investigated.14, 42-46 The piezoelectric constant
has been improved from 78 pC/N of BNT to 160 pC/N for non-oriented piezoelectric
BNT-BT bulk ceramic by introducing the MPB.14, 47 Although the BNT-based material
59
is one of the most promising lead-free piezoelectric candidate, but no any strain
engineering approach has been reported on this material.
Herein, we challenge on a mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 (BT/BNT)
nanocomposite for the first time because the large remanent polarization and high Curie
temperature of BNT are expected for the improved ferroelectric behavior of the
mesocrystalline nanocomposite. The BT/BNT nanocomposite constructed from
well-aligned BT and BNT nanocrystals with the same crystal-axis orientation is
synthesized using an ingenious two-step topochemical process. A giant piezoelectric
response of d*33 = 408 pm/V and elevated Tc of 380 °C were achieved by successfully
introducing BT/BNT heteroepitaxial interface. The giant piezoelectric response can be
explained by an optimized combination of Ferro-Tetragonal/Ferro-Rhombohedral
crystal systems with a lattice mismatching of about 2.5 % for the heteroepitaxial
interface. These results will give a trend toward the high performance piezoelectric
materials using the strain engineering.
2.2 Experimental
2.2.1 Sample Preparation
H1.07Ti1.73O4·nH2O (n = 1, abbreviated as HTO) powder sample was prepared from
K0.80Ti1.73Li0.27O4 (KTLO) as reported in our previous study.34 For the synthesis of the
platelike particle sample of mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 (BT/BNT)
nanocomposite, a two-step reaction process, namely the solvothermal reaction and
solid-state reaction, was used. In the first step, the platelike HTO crystals (0.4 g) and
Ba(OH)2·8H2O (mole ratio of Ba/Ti = 0.5) were solvothermally treated in 30 mL
60
distilled water under the stirring conditions at 150 C for 12 h. After the solvothermal
treatment, the obtained sample was washed with distilled water and dried at room
temperature to obtain mesocrystalline BaTiO3/HTO (BT/HTO) nanocomposites. In the
second step, the mesocrystalline BT/HTO sample (0.8 g) was mixed with 20 % mole
excess of Bi2O3 and 40 % mole excess of Na2CO3 in ethanol solvent by ball-milling
with a speed of 50 r/min for 12 h at room temperature, and then the mixture was dried at
60 C for 6 h. Finally, the mixed powders were heated at a desired temperature for 3 h
to obtain mesocrystalline BT/BNT nanocomposites.
A pellet sample of BT/HTO-Bi2O3-Na2CO3 mixture was fabricated by pressing
BT/HTO-Bi2O3-Na2CO3 mixture powder sample using a pellet press mold with a
diameter of 10 mm at 30 M Pa for 3 min. Subsequently, the cold isostatic press (CIP)
was employed with a pressure of 200 M Pa. The obtained pellet samples were heated at
desired temperatures for 3 h to obtain a BT/BNT pellet samples. The obtained pellet
samples were polished with diamond slurry, and cut using a crystal cutter to sizes of
4×4×0.5 mm3. Gold electrodes were printed on the top and bottom surfaces with an area
of 4×4 mm2. A BT mesocrystal sample was prepared by solvothermal treatment of HTO
(0.4 g) in Ba(OH)2 solution (mole ratio of Ba/Ti = 1.2) at 200 oC for 12 h, and then heat
treated at a desired temperature for 3 h. A BNT mesocrystal sample was prepared by
heat-treament of a HTO-TiO2-Bi2O3-Na2CO3 mixture at a desired temperature for 3 h.40
The samples obtained by heat-treatment are designated as BT/BNT-X, BT-X, and
BNT-X, respectively, where X represents for the heating temperature.
2.2.2 Physical Properties Analysis
The structures of powder samples were investigated using a powder X-ray
61
diffractometer (Shimadzu, XRD-6100) with Cu Kα (λ = 0.15418 nm) radiation. The
particle size and morphology of the samples were observed using scanning electron
microscopy (SEM) (JEOL, JSM-5500S) and field emission scanning electron
microscopy (FE-SEM) (Hitachi, S-900). Transmission electron microscopy (TEM)
observation and selected-area electron diffraction (SAED) were performed on a JEOL
Model JEM-3010 system at 300 kV, and the powder sample was supported on a Cu
microgrid.
Piezoelectric responses of the mesocrystal individuals were detected by using a
scanning probe microscopy system (SPM) (SPA-400/Nano Navi Station, SII,
NanoTechnology Inc.) combining atomic force microscopy (AFM) and piezoresponse
force microscopy (PFM). The sample dispersed on an Au-coated silicon substrate (40
nm thickness of Au-film), and an individual platelike particle dispersed on the Si
substrate surface was scanned using the AFM probe tip with a conductive Rh-coated Si
cantilever probe (SI-DF3-R, spring constant: 1.2 N/m) in the contact mode. The
generated strain of the platelike particle was detected using Z/V mode in AFM system
after a DC bias from -10 V to 10 V was employed on the surface of the platelike particle.
And the converse piezoelectric constant d*33 can be calculated by equation (1).
d*33 = D/Va (1)
where D is the displacement (pm), Va is the applied bias (V).48, 49
The polarization-electric field (P-E) loop of the pellet sample was measured using a
ferroelectric testing system (Toyo Corporation, FCE3-4KVSYS) at room temperature.
The dielectric response of the pellet sample was measured using an LCR meter (Agilent
E4980A) in a frequency range of 1 to 2 M Hz. For the measurement of
temperature-dependent dielectric response, the pellet sample was heated in a
62
temperature controller with a heating rate of 3 C/min from room temperature to 460 C
during the measurement.
2.3 Results and discussion
2.3.1 Synthesis of mesocrystalline BT/HTO nanocomposite
In the present study, the platelike mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 (BT/BNT)
nanocomposites are prepared using a two-step reaction process. In the first step, the
platelike H1.07Ti1.73O4·nH2O (HTO) crystals were solvothermally reacted with Ba(OH)2
to obtain a BaTiO3/HTO (BT/HTO) nanocomposite. Fig. 2.1 presents the XRD patterns
of the HTO precursor and BT/HTO nanocomposite obtained by solvothermal treatment
in Ba(OH)2 solution at 150 °C for 12 h with mole ratio of Ba/Ti = 0.5. The HTO
precursor has a lepidocrocite-like (γ–FeOOH) layered structure with a basal spacing of
0.882 nm corresponding to (020) crystal plane.15 After the solvothermal reaction in
Ba(OH)2 solution, except the unreacted HTO phase, a BT phase can be confirmed,
namely, the HTO precursor is partially transformed into the BT phase. The basal
spacing of the HTO crystal decreases slightly from 0.882 to 0.871 nm because of the
H+/Ba2+ ion-exchange in the HTO interlayers. To confirm the nanostructure of BT/HTO
sample and the distribution of the BT in the sample, BT/HTO sample was treated with a
2 M HCl solution to remove BT in the sample. It could be obviously seen that the BT
phase disappeared and a small amount of anatase-type TiO2 phase was found after the
acid treatment (Fig. 2.1(c)). This result reveals that BT phase is dissolved and
transformed to the TiO2 phase after the acid treatment, where the BT in the BT/HTO
nanocomposite formed by the topochemical process have a uniform distribution in the
63
BT/HTO particle, which is a prerequisite for the formation of well-aligned BT/BNT
nanocomposite.
Fig. 2.1 XRD patterns of (a) H1.07Ti1.73O4·nH2O (HTO) and (b) BT/HTO sample obtained by
solvothermal treatment of HTO-Ba(OH)2 mixture with mole ratio of Ba/Ti = 0.5 at 150 ºC for
12 h, and (c) sample obtained by the treatment of BT/HTO with 2 M HCl solution for 12 h.
The nanostructure of BT/HTO sample was investigated by using TEM and FE-SEM.
Fig. 2.2 shows TEM images and SAED spots patterns of HTO, BT/HTO, and
acid-treated BT/HTO samples. HTO has a platelike particle morphology with smooth
surface (Fig. 2.2(a)). The SAED pattern reveals that the platelike HTO particle is a
single crystal located along the [010] zone axis of orthorhombic system, where the
[010] direction (b-axis direction) is perpendicular to the basal plane of the platelike
HTO crystal (Fig. 2.2(b)). BT/HTO retains the platelike particle morphology of HTO
64
precursor, where many nanocrystals with a size of about 40 nm which corresponds to
BT phase are observed in the platelike particle of BT/HTO (Fig. 2.2(c) and Fig. 2.3(c)).
The two sets of SAED spots patterns corresponding to layered HTO phase and BT
perovskite phase, respectively, are simultaneously detected in one platelike crystal,
indicating that the HTO and BT phases coexist in one platelike particle (Fig. 2.2(d)).
This result demonstrates that the BT/HTO nanocomposite can be obtained under the
solvothermal conditions at 150 °C. It is notable that although the platelike particle of
BT/HTO nanocomposite is a polycrystal particle, only one set of SAED pattern is
observed for BT and HTO phases, respectively. The result reveals that all the BT and
HTO nanocrystals in one platelike particle show the same crystal-axis direction,
respectively, namely a mesocrystalline BT/HTO nanocomposite is formed.15 The [200]
and [002] directions of the HTO phase correspond to the [002] and [1-10] directions of
the BT phase, respectively, which reveals the BT phase is formed by a topochemical
structural transformation reaction from HTO phase.
After the acid treatment of the BT/HTO, a porous platelike particle with a pore size of
about 50 nm is generated (Fig. 2.2(e)). The pores are formed by the dissolution of BT
nanocrystals in the BT/HTO nanocomposite. Some nanoparticles with a size of 10 nm
are observed on the surface of the porous platelike particle, which correspond to anatase
TiO2 phase (Fig. 2.3(f)). The acid-treated BT/HTO platelike particle shows a SAED
pattern of single crystal HTO phase (Fig. 2.2(f)), namely, a porous single crystal can be
obtained by the dissolution of BT nanocrystals in the BT/HTO nanocomposite. The
above results indicate that the mesocrystalline BT/HTO nanocomposite is formed by
solvothermal reaction of HTO in Ba(OH)2 solution, and the BT nanocrystals uniformly
distribute in the mesocrystalline BT/HTO nanocomposite particle. An artificial porous
65
single crystal of HTO can be obtained by removing BT nanocrystals with acid treatment.
The formation process of the porous HTO can be confirmed also by FE-SEM
observation results (Fig. 2.3).
Fig. 2.2 (a, c, e) TEM images and (b, d, f) SAED patterns of (a, b) HTO single crystal precursor,
(c, d) BT/HTO sample obtained after solvothermal treatment of HTO in Ba(OH)2 solution with
a mole ratio of Ba/Ti = 0.5 at 150 °C for 12 h, and (e, f) sample obtained by acid treatment of
BT/HTO with 2 M HCl solution.
66
Fig. 2.3 (a, b, c) FE-SEM and (d, e, f) TEM images of (a) HTO precursor, (b) BT/HTO
nanocomposite obtained by solvothermal treatment of HTO-Ba(OH)2 mixtures with mole ratio of
Ba/Ti = 0.5 at 150 °C for 12 h, (c, d, e, f) sample obtained by acid-treatment of BT/HTO
nanocomposite with 2 M HCl solution for 12 h. (f) HRTEM image derived from white pane in TEM
image (d).
2.3.2 Synthesis of mesocrystalline BT/BNT nanocomposite
In the second step of the two-step reaction process for the synthesis of
mesocrystalline BT/BNT nanocomposite, a mixture of BT/HTO, Bi2O3 and Na2CO3 was
calcined. Under the condition of stoichimetric mole ratio of BT/HTO, Bi2O3 and
Na2CO3 for formation of BT/BNT, impurity is formed due to the losses of Bi and Na
contents by evaporation during the calcination at elevated temperature (Fig. 2.4). When
20 and 40 % mole excess of Bi2O3 and Na2CO3, respectively, were introduced into the
67
(BT/HTO)-Bi2O3-Na2CO3 reaction system, high purity BT/BNT can be obtained, and
the XRD patterns of the products prepared at different temperatures are shown in Fig.
2.5. After heat-treatment at 500 °C, no obvious new phase is observed. The basal
spacing of the HTO decreases from 0.871 nm to 0.726 nm because of the dehydration of
its interlayer water. Except BT phase, all other starting material phases disappear when
the temperature was elevated to 600 ºC. The main crystalline phase of the product is the
BT phase, and a small amount of Bi12TiO20 phase that can be well identified by JCPDS
File No. 34-0097 is also found.40 At 700 °C, a mixture of BT and BNT phases are
formed. These results indicate that the HTO firstly reacts with Bi2O3 to form Bi12TiO20,
and then Bi12TiO20 reacts with Na2CO3 to form BNT in the reaction system.
Subsequently, the BT and BNT phases react together gradually to form
Ba0.5Bi0.25Na0.25TiO3 (BBNT) solid solution with the increasing temperature above 800
C, and finally the formation reaction of BBNT almost completes at 1000 C.
Fig. 2.4 XRD patterns of the samples obtained by heating treatments of (BT/HTO)-Bi2O3-Na2CO3
68
mixture with (a) stoichimetric mole ratio, (b) 10 % mole excess of Bi2O3 and Na2CO3, (c) 20 % mole
excess of Bi2O3 and Na2CO3, and (d) 20 % mole excess of Bi2O3 and 40 % mole excess of Na2CO3
for formation of BT/BNT nanocomposite, respectively, at 700 ºC for 3 h.
Fig. 2.5 XRD patterns of (a) (BT/HTO)-Bi2O3-Na2CO3 mixture and samples obtained by
heat-treatments of the mixture at (b) 500, (c) 600, (d) 700, (e) 800, (f) 900, and (g) 1000 ºC for 3 h,
respectively.
In the (BT/HTO)-Bi2O3-Na2CO3 reaction system, the platelike morphology of BT/HTO
retains up to 800 °C, where the mixture of BT and BNT phases is formed, and almost
destroyed at 1000 °C, where the formation reaction of BBNT solid solution is completed
(Fig. 2.6). The formation reaction of BNT in the (BT/HTO)-Bi2O3-Na2CO3 system is
investigated in detail using TEM and SAED, and the results are shown in Fig. 2.7. It can
be clearly seen that the platelike particles constructed from the nanoparticles are formed
at 600 and 700 ºC (Fig. 2.7(a, e)). At 600 ºC, the SAED spots patterns corresponding to
69
the BT phase and the Bi12TiO20 phase respectively can be observed in one platelike
particle (Fig. 2.7 (b)), indicating that BT phase and Bi12TiO20 phase coexist in one
platelike particle. This result is consistent with the XRD result in Fig. 2.5. The
nanocrystals with a size of about 5 nm, which are distributed uniformly on the surface
of the platelike particle, can be confirmed to be Bi12TiO20 phase by HRTEM and FFT
pattern (Fig. 2.7(c, d)). These results suggest that the Bi12TiO20 nanocrystals are formed
on the BT nanocrystal surface by the heteroepitaxial growth mechanism.
70
Fig. 2.6 SEM images of samples obtained by heat-treatments of the (BT-HTO)-Bi2O3-Na2CO3
mixture at (a) 500, (b) 600, (c) 700, (d) 800, (e) 900, (f) 1000, (g) 1100, and (h) 1200 °C for 3 h,
respectively.
Fig. 2.7 (a, e) TEM images and (b, f) SAED patterns of samples obtained by heat-treatments of
(BT/HTO)-Bi2O3-Na2CO3 mixture at (a, b) 600 oC and (e, f) 700 oC for 3 h, respectively. (c)
HRTEM image is an enlarged image derived from white pane in TEM image (a) and (d) the FFT
pattern is obtained from the whole region of the HRTEM (c).
71
At 700 ºC, while two sets of single crystal-like SAED spots patterns corresponding to
the BT phase and the BNT phase are observed simultaneously in one platelike particle
(Fig. 2.7 (f)), revealing that the platelike particle is constructed from the BT and BNT
nanocrystals which have the same crystal-axis orientation direction. Namely, the
mesocrystalline BT/BNT nanocomposite is formed at 700 ºC. The SAED result also
reveals that the BT and BNT nanocrystals in the mesocrystalline BT/BNT
nanocomposite have the same crystal-axis orientation direction in [110]-zone axis, and
lattice constant of BT phase is slightly larger than that of BNT phase, which also agrees
well with the XRD result in Fig. 2.5. This result suggests the formation of a
heteroepitaxial interface between BT and BNT nanoparticles in the mesocrystalline
BT/BNT nanocomposite.
2.3.3 Formation reaction mechanism of mesocrystalline BT/BNT nanocomposite
According to the results described above, a schematic representation for the
formation reaction mechanism of the mesocrystalline BT/BNT nanocomposite from
HTO by the two-step reaction process is given in Fig. 2.8. In the first step, firstly, Ba2+
ions are intercalated into the HTO bulk crystal through its interlayer pathway by a
H+/Ba2+ exchange reaction, subsequently, the Ba2+
ions react with the TiO6 octahedral
layers of HTO to form the BT nanocrystals in the crystal bulk by the topochemical
structural conversion reaction under the solvothermal conditions.36 In the topochemical
solvothermal reaction, about 50% of the HTO phase is transformed to the BT
nanocrystals due to 0.5 of Ba/Ti mole ratio in the reaction system, in which the BT
nanocrystals are uniformly distributed in the HTO platelike particle (Fig. 2.2). The
72
formation of BT phase by a dissolution-deposition reaction is also possible on the
platelike particle surface.39 In the present case, the topochemical conversion reaction is
predominant, owing to the low concentration of Ba(OH)2 and low reaction
temperature.15 There is a definite corresponding relationship between the crystal-axis
directions of HTO precursor and formed BT product in the topochemical reaction
system, in which [200] and [002] directions of HTO phase correspond to [002] and
[1-10] directions of BT phase, respectively, as shown in Fig. 2.2(d). Therefore, all the
formed BT nanocrystals in one platelike crystal of the BT/HTO nanocomposite present
the same crystal-axis orientation in the [110]-zone direction which is consistent with the
[010]-zone direction of the HTO matrix crystal, as shown in Fig. 2.2(d).
Fig. 2.8. Schematic representation for the formation mechanism of the mesocrystalline BT/BNT
nanocomposite from the layered HTO single crystal by a two-step reaction process.
In the second step, firstly the Bi3+ ions immigrate into the HTO bulk crystal of the
BT/HTO via the interlayer pathway of HTO, and react with the TiO6 octahedral layers
73
of HTO framework to form the Bi12TiO20 nanocrystals on the BT nanoparticle surface,
which causes the formation of the BT/Bi12TiO20 nanocomposite, as shown in Fig. 2.7(b).
Secondly, the Bi12TiO20 nanocrystals in the BT/Bi12TiO20 nanocomposite react with
Na2CO3 to form the BNT nanocrystals by a heteroepitaxial growth mechanism, which
results in formation of mesocrystalline BT/BNT nanocomposite, where all BT and BNT
nanocrystals show the same crystal-axis orientation in the [110]-zone direction, as
shown in Fig. 2.7(f). In the mesocrystalline BT/BNT nanocomposite, the heteroepitaxial
interface between BT and BNT nanocrystals is formed, where the BT phase and BNT
phase have little different lattice constants. The BT and BNT nanoparticles in the
mesocrystalline BT/BNT nanocomposite can react together to form BBNT solid
solution at the heteroepitaxial BT/BNT interface over 900 °C, and finally the BT and
BNT nanocrystals are transformed completely to BBNT solid solution over 1000 °C.
2.3.4 Ferroelectric and piezoelectric responses of mesocrystalline BT/BNT
nanocomposite
To figure out the ferroelectric properties of the mesocrystalline BT/BNT
nanocomposite, the pellet samples of the BT/BNT nanocomposite were prepared by
heat-treatment of the (BT/HTO)-Bi2O3-Na2CO3 mixture pellets at different temperatures
for ferroelectric studies. The pellet samples with lower leakage currents can be obtained
by the cold isostatic pressing (CIP) treatment (Fig. 2.9). The samples prepared in a
temperatures range from 600 to 900 oC show the closed ferroelectric-like P-E hysteresis
loops (Fig. 2.10), revealing ferroelectricity of the mesocrystalline nanocomposites. The
dependence of remanent polarization (Pr) on the fabrication heating temperature for the
74
mesocrystalline nanocomposite is exhibited in Fig. 2.11(a). It is interesting that with
elevating the calcination temperature, the Pr value increases, reaches a maximum value
at 700 oC, and then decreases in the studied temperature range. BT/BNT-700 exhibites a
larger ferroelectric response with a remanent polarization Pr value of 2.4 μC/cm2 at an
applied-electric field of 6 kV/cm than the mesocrystalline BT/ST nanocomposite with a
Pr value of 0.6 μC/cm2 at an applied-electric field of 17 kV/cm.15
Fig. 2.9 Plots of leakage current densities against time for the ferroelectric BT/BNT-700 pellet
samples (a) with and (b) without CIP treatment at applied voltage of 6 kV/cm.
75
Fig. 2.10 P-E hysteresis loops of the pellet samples obtained by heat-treatments of
(BT/HTO)-Bi2O3-Na2CO3 mixture at different temperatures measured at 100 Hz.
Fig. 2.11 (a) Variations of remanent polarization and relative permittivity measured at 1 k Hz of
frequency for pellet samples obtained by heat-treatments of (BT/HTO)-Bi2O3-Na2CO3 mixture
at different temperature for 3 h. (b) Nanostructural models of the heteroepitaxial BT/BNT
interface at diferent fabrication heating temperatures.
76
The ferroelectric behavior of the mesocrystalline BT/BNT nanocomposites prepared
at different heating temperatures can be explained by the variation of the nanostructure
at BT/BNT interface in the nanocomposite, as shown in Fig. 2.11(b), based on the TEM
results at the interface (Fig. 2.12). The enhancement of the Pr value from 600 to 700 oC
is attributed to formation of mesocrystalline BT/BNT nanocomposite from
BT/Bi12TiO20 nanocomposite, which generates the heteroepitaxial BT/BNT interface
(Fig. 2.12). Around the heteroepitaxial BT/BNT interface, the BT lattice shrinks and
BNT lattice expands because the lattice constant of BT is slightly larger than that of
BNT with a lattice mismatching of 2.6 % (Table 2.1), which introduces a lattice
distortion around the BT/BNT interface (Fig. 2.12(b)), therefore, the pseudo-cubic
lattices of BT and BNT nanocrystals with paraelectric or weak ferroelectric responses
are distorted to ferroelectric tetragonal and rhombohedral lattices by increasing or
reducing the lattice constants in the direction paralleled to the interface, respectively.
The direction of ferroelectric spontaneous polarization around the interface become
unstable and very sensitive to the applied-bias, which generates an enlarged ferroelectric
response.15, 23, 28
77
Table 2.1. Piezoelectric constants of ferroelectric nanocomposites and single phase of
nanostructured materials, and lattice mismatching in nanocomposites.
Nanocomposite Nanostructured single phase
Material * d*
33
(pm/V)
Lattice
mismatching
(%)
Compound d*
33
(pm/V)
Polarization
direction
BT/ST-C 20
BT/ST-M 15
BT/BNT-M **
BT/KN-S 12
BT/CT-M 18
59
306
408
136
208
2.2
2.2
2.6
0.6
4.3
BaTiO3 (BT) 51
SrTiO3 (ST)
CaTiO3 (CT) 18
Bi0.5Na0.5TiO3 (BNT) 53
KNbO3 (KN) 54
28.0
-
40.9
18.0
19.5
[001]
-
[110]
[111]
[110]
* -C, -M, and -S represent nanocomposites prepared by nanocubes, mesocrystals, and surface coating,
respectively.
** Result of present study.
78
Fig. 2.12 (a, d, g) TEM images and (b, e, h) HRTEM images of BT/BNT-700, BT/BNT-800, and
BT/BNT-1000, respectively. (b, e, h) HRTEM images are an enlarged image derived from white
pane in (a, d, g) TEM images, respectively, and (c, f, i) FFT pattern obtained from the whole
region of HRTEM (b, e, h), respectively.
With further elevating the heating temperature to 800 oC, BBNT solid solution phase
is formed at the BT/BNT interface by diffusing Ba2+ ions of BT phase to BNT phase
and Na+, Bi3+ ions of BNT phase to BT phase, then a BT/BBNT/BNT interface is
formed (Fig. 2.12(e, f)). The lattice distorton effect of the BT/BBNT/BNT interface on
the ferroelectric response is less than that of the BT/BNT interface because the
differences of the lattice constants of BT and BBNT, and BBNT and BNT are less than
that of BT and BNT. The diminution of the lattice distortion causes the dropping of
ferroelectric response. The BT/Bi12TiO20 nanocomposite sample prepared at 600 °C
shows a weak ferroelectric response (Fig. 2.10), which may be ascribed to the formation
of heteroepitaxial interface between Bi12TiO20 and BT nanocrystals, which causes the
lattice distortion in the pseudo-cubic lattice of the BT nanocrystals.
In comparison with the mesocrystalline nanocomposite sample of BT/BNT-700, the
BT single phase mesocrystal sample of BT-700 and the BNT single phase mesocrystal
sample of BNT-700 exhibit a much weaker ferroelectric response (Fig. 2.13(a)) because
BT-700 and BNT-700 mesocrystals are constructed from small BT or BNT nanocrystals
with a size of 60 or 50 nm (Fig. 2.14), where without lattice distortion at nanocrystals
interface. It is well know that the BT and BNT prepared by the low temperature process
have the pseudo-cubic structure due to their small crystal sizes or low crystallinities.15,
50-52 These pseudo-cubic lattices of BT and BNT nanocrystals exhibit the paraelectric or
79
weak ferroelectric responses. The BT and BNT nanocrystals in BT-700 and BNT-700
mesocrystals also have the pseudo-cubic structure and exhibit the weak ferroelectric
response. To further confirm the effect of the heteroepitaxial interface of BT/BNT
nanocomposite on ferroelectric response, the ferroelectric BT-1250 and BNT-1050
samples were also prepared. The BT-1250 and BNT-1050 exhibit ferroelectirc responses
(Fig. 2.13(b)). However, the BT/BNT-700 sample shows an amazing ferroelectirc
response compared to the ferroelectirc BT-1250 and BNT-1050. These results suggest
that the the lattice distortion at BT/BNT interface is highly useful for strengthening
ferroelectric response in the BT/BNT nanocomposite.
Fig. 2.13 P-E hysteresis loops of the pellet samples of (a) BT-700, BNT-700 and BT/BNT-700
as well as (b) BT/BNT-700, BNT-1050 and BT-1250 measured at 100 Hz.
80
Fig. 2.14. (a, c) TEM images and (b, d) SAED spots patterns of (a, b) BT single phase
mesocrystal sample BT-700 and (c, d) BNT single phase mesocrystal sample BNT-700.
The piezoelectric response of the mesocrystalline nanocomposite BT/BNT-700 that
shows the largest ferroelectric response was investigated by using piezoresponse force
microscopy (PFM), and compared with the single phase mesocrystals of BT-700 and
BNT-700 prepared at same temperature. The Displacement-applied voltage (D-V) loops
and d*33-applied voltage (d*
33-V) loops are presented in Fig. 2.15, where converse
piezoelectric constant (d*33) is calculated from D/V value of the D-V loop. It is noted
that the d*33 value of 408 pm/V at 10 V of applied voltage for the BT/BNT-700
nanocomposite is much larger than those of 60 and 50 pm/V for BT-700 and BNT-700
mesocrystals, respectively. The d*33 value of BT/BNT-700 nanocomposite is one order
of magnitude larger than d*33 values of a nanostructured BaTiO3 (28 pm/V)51 and a
81
highly oriented BNT thin film (25 pm/V) fabricated by PLD method, 52 and even higher
than that of a high performance oriented BBNT ceramic (332 pC/N).14 This remarkably
enhanced piezoelectric response can be attributed to the introduced lattice distortion at
the heteroepitaxial BT/BNT interface, which makes the polarization rotation sensitive.22,
23
Fig. 2.15. Displacement-applied voltage loops and d*33-applied voltage loops for (a) BT-700, (b)
BNT-700, and (c) BT/BNT-700 mesocrystalline samples.
82
To impove piezoelectric responses of lead-free piezoelectric materials, some
challenges on the enhanced piezoelectric response using the lattice distortion at
heteroepitaxial interfaces of nanostructured nanocomposites have been reported, and
some d*33 values are summarized in Table 2.1 for the comparison. Most studies have
focused on the BaTiO3/SrTiO3 (BT/ST) heteroepitaxial interface because Harigai et al.
have reported a giant dielectric response of BT/ST heteroepitaxially stacked thin film.27
We think except the nanostructure of the heteroepitaxial interface, the combinations of
the nanocomposite for build-up of heteroepitaxial interface, such as the combinations of
ferroelectric/paraelectric phases and ferroelectric/ferroelectric phases, the combinations
between tetragonal, cubic, rhombohedral, orthorhombic crystal systems, and their lattice
mismatchings at the heteroepitaxial interface, will also affect piezoelectric response.
In the BT/ST combination nanocomposite system, Mimura et al. have reported a
BT/ST nanocomposite (BT/ST-C in Table 2.1) constructed by self-assembling
nanocubes of BT and ST, and it only gives a d*33 value of 59 pm/V.20 The largest d33
value of 306 pm/V in the BT/ST system have been achieved using the mesocrystalline
BT/ST nanocomposite (BT/ST-M in Table 1) by our group.15 The larger piezoelectric
response of mesocrystalline BT/ST nanocomposite can be attributed to the formation of
a perfect and higher density BT/ST heteroepitaxial interface than those in other cases.
Therefore, it can be concluded that the nanostructure of the mesocrystalline
nanocomposite is advantageous for the enhancement of piezoelectric response by the
lattice strain engineering.
In the mesocrystalline nanocomposites, the piezoelectric response increases in a order
of BT/CT-M (208 pm/V) < BT/ST-M (306 pm/V) < BT/BNT-M (408 pm/V) (Table 1).
83
In this case, the difference of the piezoelectric responses can be attributed to the
combinations of the heteroepitaxial interfaces. The BT, CT, ST, and BNT are tetragonal
ferroelectric, orthorhombic ferroelectric, cubic paraelectric, and rhombohedral
ferroelectric phases, respectively, at room temperature. Therefore, the BT/CT, BT/ST,
and BT/BNT heteroepitaxial interfaces can be assigned to Tetra-Ferro/Orth-Ferro
combination with a lattice missmatching of 4.3 %, Tetra-Ferro/Cub-Para combination
with a lattice missmatching of 2.2 %, Tetra-Ferro/Rho-Ferro combination with a lattice
missmatching of 2.6 %, respectively. The BT/ST combination exhibits a larger
piezoelectric response than that of BT/CT, which may be ascribed to that the lattice
mismatching of the BT/CT is too large for the formation of a stable heteroepitaxial
interface in the nanocomposite. We think the much larger piezoelectric response of
BT/BNT combination than that of BT/ST combination can be attributed to the
optimized lattice mismatching of about 2.6 % and the Tetra-Ferro/Rho-Ferro interface
combination. It is well known that PZT exhibits excellent piezoelectric performance at
the morphotropic phase boundary (MPB), in which a Tetra-Ferro/Rho-Ferro interface is
formed also.4 The combination of the polarization directions at the
Tetra-Ferro/Rho-Ferro interface, namely [001]/[111], is important to obtain a large
piezoelectric response. Therefore, we think BT/BNT interface is one of optimized
combination for the enhanced piezoelectric performance.
Wada et al. have reported a BT/KNbO3 (BT/KN-S in Table 1) nanocomposite with a
d33 value of 136 pC/N, although the nanocomposite is fabricated by a simple method,
namely, coating BT grain surface in a porous ceramic with KN layer.12 The BT/KN-S
nanocomposite exhibits a larger d33 value than that BT/ST nanocomposites, except the
mesocrystalline BT/ST nanocomposite, even with its small lattice mismatching of 0.6 %.
84
It hints that the Tetra-Ferro/Orth-Ferro ([001]/[110]) combination may be also a
promising system for the enhanced piezoelectric performance if the lattice mismatching
is appropriate.
2.3.5 Dielectric responses of mesocrystalline BT/BNT nanocomposite
The relative permittivities (εr) of the mesocrystalline BT/BNT nanocomposite pellet
samples prepared at different temperatures were measured using the LCR meter in a
frenquency range of 1 kHz to 2 MHz. The samples prepared above 700 oC with the
ferroelectric-like behavior present large frenquency dependences in the low frenquency
range (Fig. 2.16). The BT/BNT-700 and -800 samples exhibit much larger εr values than
those of mesocrystalline BT-700 and BNT-700 single phases in the frenquency range
measured, because of the enhancing effect of the mesocrystalline nanocomposite on the
dielectric response. The enhancing effect on the dielectric response is strongly
dependent on fabrication heating temperature of the BT/BNT nanocomposite, as shown
in Fig. 2.11(a). With increasing the heating temperature, εr value increases, reaches the
maximum at 700 C, and then gradually decreases above 700 C. This behavior fits well
with the ferroelectric Pr behavior, and can be explained by enhanced ferroelectricity due
to the lattice distortion around the heteroepitaxial BT/BNT interface, where the
polarization directions are tilted gradually with enlarging the lattice distortion, as shown
in Fiure 7(b), which makes the polarization rotations become sensitive.22, 23 With further
increasing the heating temperature above 800 oC, the gradual transformation of the
BT/BNT nanocomposite into the BBNT solid solution at the interface causes a less
lattice distortion, thereby degrading the dielectric response until the complete formation
85
to the BBNT solid solution, similar to the behavior of the ferroelectric response. These
results indicate that the εr value can be enhanced by introducing the heteroepitaxial
BT/BNT interface.
Fig. 2.16. Variations of relative permittivities for the pellet samples prepared by heat-treatment
of (BT/HTO)-Bi2O3-Na2CO3 mixture at different temperatures for 3 h, and BT and BNT pellet
samples prepared by heat-treatments at 700 oC for 3 h, respectively.
86
Fig. 2.17. Temperature dependences of the relativie permittivity (εr) of BT/BNT-700,
BT/BNT-800 and BT-BNT-900 pellet samples at 10 kHz.
The temperature dependences of the dielectric response of the mesocrystalline
BT/BNT nanocomposites prepared at different temperatures were also measured and
compared with those of BT and BNT single phases. The BT-1250 and BNT-1050
samples prepared at 1250 and 1050 °C exhibit a maximum εr value at around 130 and
320 °C (Fig. 2.18(a)), as like their normal bulk crystals, respectively.55-57 These
temperatures correspond to their Curie temperatures (Tc) or phase transitions from
ferroelectric phase to paraelectric phase. However, no obvious phase transitions can be
observed for the both mesocrystalline BT-700 and BNT-700 single phase samples (Fig.
2.18(b)). These samples exhibit the paraelectric or weak ferroelectric responses because
their small crystal sizes of about 60 and 50 nm, respectively (Fig. 2.14), as mentioned
above.
87
Fig. 2.18. Temperature dependences of the relativie permittivity (εr) of (a) BT-1300 and
BNT-1050, and (b) BT-700, BNT-700, and BT/BNT-700 pellet samples at 10 kHz.
Fig. 2.17 exhibits the temperature dependences of the relative permittivities for the
BT/BNT nanocomposites prepared in the temperature range of 700 to 900 oC.
Surprisingly, the BT/BNT-700 nanocomposite exhibits three εr peaks at around 160, 270,
and 380 °C, respectively. The εr peaks at around 160 and 380 °C can be assigned to the
Tc of BT and BNT phases in the nanocomposite, respectively, although they shift to
higher temperatures. Such high-temperature shifts of Tc have been observed by
introducing the lattice strain originated from the heteroepitaxial interface, since the
lattice strain causes the lattice mismatches for BT and BNT, in which restrain the
transformation of the tetragonal to cubic for BT and the rhombohedral to cubic for BNT
with elevated heating temperature, respectively, namely the increasing of the Curie
88
temperature. Furthermore, Haeni et al. have demonstrated hundreds of degrees
increasing in the Tc of ST by introducing the epitaxial lattice strain, resulting in a
room-temperature ferroelectric ST thin film.10 Subsequently, Choi et al. have presented
an elevated Tc for BT thin film with the same method.9 Suzuki et al. have reported a 3D
compressive stress induced mesostructured BT/ST composite film with largely elevated
Tc by using a surfactant-substrate sol-gel method.19
It is notable that the third phase transition at around 270 °C was observed for the first
time in the mesostructured nanocomposite. We think this third phase transition can be
assigned to BT/BNT interface or a distorted BBNT phase between BT and BNT phases
as shown in Fig. 2.11(b) and 2.12(b). Nevertheless, with further increasing fabrication
heating temperature of the BT/BNT nanocomposite, Tc at 270 oC for the third phase
disappears above 900 oC, in fact, as the solid solution process proceeds with the
increasing temperature, the peak of the BBNT solid solution phase peak should appear
but not disappear according to the XRD result in Fig. 2.5, therefore, we speculate the
peak at around 270 °C for BT/BNT-700 sample should be the phase of the BT/BNT
heteroepitaxial interface. Also, the high density of BT/BNT interface in the
mesocrytstalline nanocomposite may be the reason why a distinct third phase is
observed. In addition, the low-temperature shifts and broadings of Tc for the BT and
BNT phases were also observed, this is due to the phase transformation of the BT/BNT
nanocomposite to BBNT solid solution, in which the lattice mismatches between BT
and BBNT or BBNT and BNT become smaller (Fig. 2.14(b)). This behavior
corresponds to the transformation of BT and BNT phases to BBNT solid solution at
high temperature, as shown in Fig. 2.5.
89
2.4 Conclusion
A two-step topochemical process is an effective approach for the synthesis of the
mesocrystalline BT/BNT nanocomposite constructed from well-aligned BT and BNT
nanocrystals with the same crystal-axis orientation. The formation of the
mesocrystalline nanocomposite is attributed to the topochemical transformation
reactions from HTO to BT and HTO to BNT in the two-step process. The BT/BNT
nanocomposite exhibits enlarged ferroelectric, piezoelectric and dielectric responses by
introducing the lattice strain at BT/BNT heteroepitaxial interface. The giant
piezoelectric response suggests the BT/BNT heteroepitaxial interface is one of
optimized combination for the enhanced piezoelectric performance by using the lattice
strain engineering. The introduced lattice strain causes transitions of paraelectric BT and
BNT nanocrystals to the ferroelectric nanocrystals in the mesocrystalline
nanocomposite, and the elevated Tc for BT and BNT, which can expand application
temperature range of this promising lead-free piezoelectric material.
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94
Chapter Ⅲ
Ferroelectric Mesocrystalline BaTiO3/BaBi4Ti4O15
Nanocomposite: Formation Mechanism, Nanostructure, and
Anomalous Ferroelectric Response
3.1 Introduction
Ferroelectric materials with coupled mechanical-electrical and thermal-electrical
responses have drawn much attentions due to the wide applications to sensors, actuators
and energy harvesters.1-3 Although the Pb(Zr1−xTix)O3 (PZT) is an excellent and widely
employed ferroelectric material, it contains more than 60 % toxic lead component,
therefore, the replacement of the PZTs with lead-free alternatives has become quite
significant for the environmental protection.4-6 However, the reported lead-free
materials exhibit quit lower piezoelectric performance than that PZTs.4 Two kinds of
effective approaches, i.e. domain engineering and oriented engineering, have been
widely used to improve the piezoelectric performance for the ferroelectric materials.
The domain engineering is to reduce the domain size of the ferroelectric materials,
while the oriented engineering is to optimize their orientation direction.5, 7-10 The
piezoelectric response of BaTiO3 (BT) can be improved from 200 to 788 pC/N using the
combined domain and oriented engineering, the value is larger than that of 500 pC/N for
the commercially available PZTs.11 However, the piezoelectric application of BT is
restrained in a temperature range of below 130 °C of its Curie temperature (Tc).
95
Therefore, the elevated Tc is also important for the development of high performance
lead-free ferroelectric materials.12
Another approach is the lattice strain engineering by applying an lattice strain at the
heteroepitaxial interface constructed using two kinds of crystals with little different
lattice parameters, which can improve some specific properties of the ferroelectric
materials, including the Tc, piezoelectric and dielectric responses.13-21 The
improvements of the Tc have been achieved by using the lattice strain engineering, but
mainly limited in the thin film or low dimensional quantum systems.14, 22, 23 Choi et al.
have presented a (001)-oriented BT thin film epitaxially grown on a (110) DyScO3
substrate with little different lattice parameters, which results in a large Tc improvement
of BT from 130 to 540 °C.14 It has reported that a mesostructured BT/SrTiO3 (BT/ST)
composite film exhibits an elevated Tc of 230 °C for BT but without obvious
improvement for the piezoelectric response.23 An astonishing dielectric response has
been achieved by construction of an artificial BT/ST superlattice using a molecular
beam epitaxy process.15 Furthermore, the improvement for the piezoelectric response
has been achieved also using BT/KNbO3 (BT/KN) heteroepitaxial interface but without
increase of Tc.24 As far as we know, the improvement has been limited in the individual
effect, such as Tc, piezoelectric response or dielectric response, however the
multi-improvement effect has not been reported yet by using the lattice strain
engineering.
Mesocrystal is a polycrystal constructed from the nanocrystals with the same
crystal-axis orientation.25, 26 The mesocrystals not only have some potential properties
based on the individual nanocrystals but also exhibit unique collective properties of
nanocrystal ensembles.27 It has become a fascinating research area as a new class of
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material for catalysis, sensing, and energy storage and conversion in the past
decade.27-31 Until recently, we have found that the mesocrystalline nanocomposite is a
promising material for the strain engineering to improve the piezoelectric response
because it has high density of the heteroepitaxial interface, and the piezoelectric
responses of the mesocrystalline nanocomposites of BT/ST and BT/CaTiO3 (BT/CT)
can be improved greatly by the introduction of the lattice strain engineering.32, 33
Furthermore, a mesocrystalline BT/Bi0.5Na0.5TiO3 (BT/BNT) nanocomposite exhibits an
anomalous ferroelectric response with a very large piezoelectric response of 408 pm/V
and an elevated Tc effect.34 However, these mesocrystalline nanocomposites are
constructed from two kinds of materials with the same perovskite structure, which are
easily transformed to their solid solution at high temperature, namely disappearances of
the heteroepitaxial interface and the lattice strain effect on the ferroelectric response.32,
34 Therefore, the search for a mesocrystalline nanocomposite that can endure the high
temperature is essential for the practical application of the mesocrystalline
nanocomposites.
Herein, we describe a new challenge on a mesocrystalline BaTiO3/BaBi4Ti4O15
(BT/BBT) nanocomposite constructed from two kinds of nanocrystals with different
crystal structures. BaBi4Ti4O15 (BBT) is a bismuth-layered (Aurivillius) ferroelectric
constructed by stacking the bismuth oxide layers ([Bi2O2]2+) and pseudo-perovskite
layers ([BaBi2Ti4O13]2-), which is different from the normal perovskite structure of BT.35
BBT has been studied as temperature-stable ferro-piezoelectrics with a relatively high
Tc of around 400 °C. 36, 37 The high Curie temperature for BBT and large piezoelectric
response for BT are expected for a simultaneous improvement of the Tc and the
piezoelectric response of the mesocrystalline BT/BBT nanocomposite. Furthermore, the
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different structures of BBT and BT are expected to inhibit formation of their solid
solution at high temperature. The BT/BBT nanocomposite constructed from
well-aligned BT and BBT nanocrystals with the crystal-axis orientation along the [110]
and [11-1] respectively was synthesized using a facile two-step topochemical process. A
highly elevated Tc of BBT from 400 to 700 °C and an enhanced piezoelectric response
of d*33 = 130 pm/V were achieved by successfully introducing BT/BBT heteroepitaxial
interface into the BT/BBT nanocomposite. In addition, the mesocrystalline BT/BBT
nanocomposite can be further made into the bulk ceramic without the formation of the
solid solution at high temperature, which will open the gate to the fabrication of the
mesocrystalline ceramics for the high-performance ferroelectric materials using the
lattice strain engineering.
3.2 Experimental
3.2.1 Sample Preparation
H1.07Ti1.73O4·H2O (HTO) powder sample was prepared from K0.80Ti1.73Li0.27O4
(KTLO) as reported in our previous study.38 For the synthesis of the platelike particle
sample of mesocrystalline BaTiO3/BaBi4Ti4O15 (BT/BBT) nanocomposite, a facile
two-step reaction process was used. In the first step, the platelike HTO crystals (0.4 g)
and Ba(OH)2·8H2O (mole ratios of Ba/Ti = 0.25, 0.5 and 0.75) were hydrothermally
treated in 30 mL distilled water under the stirring conditions at 150 C for 12 h. After
the hydrothermal treatment, the obtained sample was washed with distilled water and
dried at room temperature to obtain mesocrystalline BaTiO3/HTO (BT/HTO)
nanocomposites. The obtained BT/HTO samples are designated as BT/HTO-X, where
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X represents for the Ba/Ti mole ratio in the reaction system. In the second step, the
mesocrystalline BT/HTO-X sample (0.8 g) was mixed with stoichiometric Bi2O3 in
ethanol solvent by ball-milling with a speed of 50 r/min for 12 h at room temperature,
and then the mixture was dried at 60 C for 6 h. Finally, the mixed powders were heated
at a desired temperature for 3 h to obtain mesocrystalline BT/BBT nanocomposites. The
samples obtained by heat-treatment of BT/HTO-X (X = 0.25, 0.5 or 0.75) are
designated as BT/BBT-X-Y, where Y represents for the heating temperature.
Pellet samples of BT/HTO-Bi2O3 mixture was fabricated by pressing a
BT/HTO-Bi2O3 mixture powder sample using a pellet press mold with a diameter of 10
mm at 30 M Pa for 3 min. Subsequently, the cold isostatic press (CIP) was employed
with a pressure of 200 M Pa. The obtained pellet sample was heated at desired
temperatures for 3 h to obtain a BT/BBT pellet sample. A BT mesocrystal powder
sample was prepared by hydrothermal treatment of HTO (0.4 g) in a Ba(OH)2 solution
(mole ratio of Ba/Ti = 1.2) at 200 oC for 12 h. The BT pellet sample was fabricated
using the BT mesocrystal powder sample by analogous manner as BT/BBT pellet
sample at a desired temperature for 3 h. The obtained pellet samples were polished with
diamond slurry, and cut using a crystal cutter to sizes of 4×4×0.5 mm3. The silver paste
was screen-printed on the top and bottom surfaces of the pellet sample with an area of
4×4 mm2 and heated at 500 oC for 30 min.
3.2.2 Physical Properties Analysis
The structures of powder samples were investigated using a powder X-ray
diffractometer (Shimadzu, XRD-6100) with Cu Kα (λ = 0.15418 nm) radiation. The
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particle size and morphology of the samples were observed using field emission
scanning electron microscopy (FE-SEM) (Hitachi, S-900). Transmission electron
microscopy (TEM) observation and selected-area electron diffraction (SAED) were
performed on a JEOL Model JEM-3010 system at 300 kV, and the powder sample was
supported on a Cu microgrid. Raman spectra were detected by a JASCO NRS-3100
Raman spectrometer with a scanning step of 1 cm−1 at an excitation wavelength of 532
nm.
Piezoelectric response of the platelike mesocrystal particle was detected by using a
scanning probe microscopy system (SPM) (SPA-400/Nano Navi Station, SII,
NanoTechnology Inc.) combining atomic force microscopy (AFM) and piezoresponse
force microscopy (PFM). The platelike particles dispersed on an Au-coated silicon
substrate (40 nm thickness of Au-film), and an individual platelike particle dispersed on
the Si substrate surface was scanned using the AFM probe tip with a conductive
Rh-coated Si cantilever probe (SI-DF3-R, spring constant: 1.2 N/m) in the contact mode.
The generated strain of the platelike particle was detected using Z/V mode in AFM
system after a DC bias from -10 V to 10 V was employed on the surface of the platelike
particle. And the converse piezoelectric constant d*33 can be calculated by equation (1).
d*33 = D/Va (1)
where D is the displacement (pm), Va is the applied bias (V).27, 32, 39, 40
The polarization-electric field (P-E) loop of the pellet sample was measured using a
ferroelectric testing system (Toyo Corporation, FCE3-4KVSYS) at room temperature.
The dielectric response of the pellet sample was measured using an LCR meter (Agilent
E4980A) in a frequency range of 1 to 2 M Hz. For the measurement of
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temperature-dependent dielectric response, the pellet sample was heated in a
temperature controlled chamber with a heating rate of 3 C/min from room temperature
to 700 C during the measurement.
3.3 Results and discussion
3.3.1 Synthesis of BT/BBT nanocomposites and BBT mesocrystals
Fig. 3.1 XRD patterns of (a) H1.07Ti1.73O4·H2O (HTO) and BT/HTO samples obtained by
hydrothermal treatment of HTO-Ba(OH)2 mixture with Ba/Ti mole ratios of (b) 0.25, (c) 0.5 and
(d) 0.75 at 150 °C for 12 h, respectively.
The facile two-step process was employed to synthesize the mesocrystalline
BaTiO3/BaBi4Ti4O15 (BT/BBT) nanocomposites. In the first step, the layered titanate
H1.07Ti1.73O4·H2O (HTO) with platelike particle morphology was hydrothermally treated
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with Ba(OH)2 at 150 °C for 12 h. The formation of the BT/HTO nanocomposites can be
achieved when the Ba/Ti mole ratios in the reaction system are 0.5 and 0.75, as shown
in Fig. 3.1, where the formed BT is a cubic perovskite phase (JCPDS File No.74-1964).
However, the sample obtained at mole ratio of Ba/Ti = 0.25 exhibits the layered titanate
structure and without the BT phase is observed, which will be discuss later. Based on
the results of our former research, we conclude that the platelike HTO crystals are
partially transformed into the BT by a topochemical reaction, which results in the
formation of the mesocrystalline BT/HTO nanocomposite at Ba/Ti mole ratios of 0.5
and 0.75,32-34 namely the BT/HTO-0.5 and BT/HTO-0.75 samples are mesocrystalline
BT/HTO nanocomposites. The formation of the platelike mesocrystalline BT/HTO
nanocomposite can be confirmed also from their TEM results, as shown in Fig. 3.2.
Fig. 3.2 (a, c, e, g) TEM images and (b, d, f, h) SAED patterns of (a, b) HTO, (c, d)
BT/HTO-0.25, (e, f) BT/HTO-0.5 and (g, h) BT/HTO-0.75.
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Fig. 3.3 XRD patterns of sample obtained by heat-treatment of (a) (BT/HTO-0.5)-Bi2O3 mixture
at (b) 500, (c) 600, (d) 700, (e) 800, (f) 900, (g) 1000 and (h) 1100 °C for 3 h, respectively.
In the second step, the BT/HTO nanocomposite was mixed with stoichiometric Bi2O3
and calcined at different temperatures for 3 h, where the XRD results of the
BT/BBT-0.5-Y samples prepared from BT/HTO-0.5 are shown in Fig. 3.3. With
increasing the heating temperature, the BT phase and a new phase of Bi12TiO20 were
observed when the heating temperature reached 600 °C, and the blended phases of BT,
Bi12TiO20 and BaBi4Ti4O15 (BBT) (JCPDS File No.73-2184, tetragonal system) were
observed simultaneously at 700 °C. And then the Bi12TiO20 phase gradually disappeared
and was transformed completely into the BBT phase at 800 °C, resulting in the
formation of a mixture of BT and BBT phases. Judging from the FE-SEM results in Fig.
3.4, the BT/BBT-0.5-Y samples exhibit the platelike particle morphology constructed
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from the nanocrystals with uniform crystal size below 900 °C, namely the platelike
particle morphology of BT/HTO is retained. With further elevating the heating
temperature, the nanocrystals constructed the platelike particle continue to grow up and
finally lose the platelike morphology over 1000 °C.
Fig. 3.4 FE-SEM images of the samples obtained by the heat-treatment of the
(BT/HTO-0.5)-Bi2O3 mixture at different temperatures for 3 h.
The mixed BT and BBT phases were observed in the heating temperature range from
800 to 1100 °C (Fig. 3.3). In our previous researches, though the mesocrystalline
BT/Bi0.5Na0.5TiO3, BT/SrTiO3 and BT/CaTiO3 nanocomposites can be successfully
obtained, these perovskite mesocrystalline nanocomposites are transformed to their
solid solution perovskites, respectively, at high temperature because they have the same
perovskite structure, which causes the degradation in ferroelectric and piezoelectric
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responses.32-34 In the present study, BBT owns a layered perovskite structure
constructed by stacking the bismuth oxide layers ([Bi2O2]2+) and pseudo-perovskite
layers ([BaBi2Ti4O13]2-),35 which is different from the perovskite structure of the BT
phase. Therefore, BBT and BT cannot react to forms their solid solution phase, which
reduces the reactivity between BBT and BT nanocrystals in the mesocrystalline
BT/BBT nanocomposite. We think this is the reason why the nanostructure of the
mesocrystalline BT/BBT nanocomposite can be retained even at the high temperature of
1100 °C, which is different from the mesocrystalline BT/Bi0.5Na0.5TiO3, BT/SrTiO3 and
BT/CaTiO3 nanocomposites where solid solution phases are formed respectively at the
high temperature.32-34
Fig. 3.5 XRD patterns of samples obtained by heat-treatments of (a) BT mesocrystal at 800 °C
for 3 h and (BT/HTO-0.75)-Bi2O3 mixture at (b) 600, (c) 700, (d) 800 (e) 900 and (f) 1100 °C
for 3 h.
105
Fig. 3.6 (a, c) TEM images and (b, d) SAED spot patterns of samples obtained by
heat-treatment of (a, b) the BT mesocrystal and (c, d) (BT/HTO-0.75)-Bi2O3 mixture at 800 °C
for 3 h, respectively.
When the (BT/HTO-0.75)-Bi2O3 mixture was heat-treated, the BT/BBT
nanocomposites can be obtained also in the temperature range of 700 to 1100 °C (Fig.
3.5), similar to the case of (BT/HTO-0.5)-Bi2O3 mixture except higher BT content in the
BT/BBT nanocomposite. The sample keeps the platelike particle morphology of
BT/HTO-0.75 after transformation to the BT/BBT nanocomposite by heat-treatment at
800 °C (Fig. 3.6(c)). However, a single BBT phase was obtained when the
(BT/HTO-0.25)-Bi2O3 mixture was heat-treated in a temperature range of 800 to
1100 °C (Fig. 3.7). The formation of the Bi12TiO20 intermediate phase can be detected
also at 600 °C. The BBT sample obtained at 800 °C keeps the platelike particle
morphology of the BT/HTO-0.25 precursor, and loses the platelike particle morphology
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at above 900 °C (Fig. 3.8), similar to the case of (BT/HTO-0.5)-Bi2O3 reaction system
(Fig. 3.4).
Fig. 3.7 XRD patterns of samples obtained by heat-treatment of (a) (BT/HTO-0.25)-Bi2O3 mixture at
(b) 500, (c) 600, (d) 700, (e) 800, (f) 900 and (g) 1100 °C for 3 h, respectively.
Fig. 3.8 FE-SEM images of samples obtained by heat-treatment of (BT/HTO-0.25)-Bi2O3
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mixture at different temperatures for 3 h.
3.3.2 Nanostructural analysis for BT/BBT nanocomposite
To figure out the phase transition reactions in the formation process of the BT/BBT
nanocomposite, a nanostructural study was carried out on the products in the synthesis
process of the BT/BBT nanocomposites by employing TEM observation. Fig. 3.9
presents the TEM images, HRTEM images, and SAED patterns of the
BT/BBT-0.25-800 (BBT-800), BT/BBT-0.5-700, and -800 samples. The BBT and
BT/BBT samples inherited the platelike morphology from the HTO precursor.34 It is
noted that the platelike BT/BBT-0.25-800 particle is a BBT polycrystal, but exhibits a
single-crystal like SAED spot pattern with a [11-1] crystal-axis orientation direction
(Fig. 3.9(b)), namely the formation of the platelike BBT mesocrystal by a topochemical
mechanism from the layered HTO precursor. Such platelike mesocrystals have the
potential applications in the fabrications of oriented ceramics.9, 41-43
Fig. 3.9 (a, c, g) TEM images and (b, d, h) SAED patterns of (a, b) BT/BBT-0.25-800, (c, d)
BT/BBT-0.5-700, and (g, h) BT/BBT-0.5-800 samples. (e, f) HRTEM images obtained from (c)
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and (e) of BT/BBT-0.5-700 sample, respectively.
Three sets of SAED spots corresponding to BT, BBT and Bi12TiO20 phases are
observed simultaneously in one platelike particle of BT/BBT-0.5-700 (Fig. 3.9(d)),
which corresponds to the XRD result in Fig. 3.3. And the BT and BBT phases in the
nanocomposite particle show the crystal-axis orientation along the [110] and [11-1]
directions, respectively. The HRTEM image shows lattice fringes of BT, BBT and the
intermediate phase of Bi12TiO20 (Fig. 3.9(f)), revealing that the platelike particle is
constructed from BT, BBT and Bi12TiO20 nanocrystals. The interface between the BT
and BBT nanocrystals, namely the area between two orange lines marked in Fig. 3.9(f),
can be confirmed clearly in the HRTEM image. The platelike particle of
BT/BBT-0.5-800 exhibits simultaneously two sets of SAED spots corresponding to BT
and BBT phases, that is to say, the platelike particle is a mesocrystalline BT/BBT
nanocomposite. The BT and BBT nanocrystals in the platelike mesocrystalline
nanocomposite show the crystal-axis orientations along the [110] and [11-1] directions,
respectively, which is the same as in the BT/BBT-0.5-700. The result reveals that the
mesocrystalline BT/BBT nanocomposite is formed from the BT/HTO precursor by a
topochemical reaction.34
3.3.3 Formation mechanism of BT/BBT nanocomposite
Based on the results described above, we give a reaction mechanism for the formation
of BT/BBT nanocomposite by the two-step topochemical process, as shown in Fig. 3.10.
In the first step under hydrothermal conditions, the Ba2+ ions in the solution are
intercalated into the HTO bulk crystal by an ion-exchange reaction with H3O+ ions in
the interlayer spaces to form a Ba2+-form HTO that can be expressed with
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BaxH1.07-2xTi1.73O4. Furthermore, the Ba2+ ions in the interlayer spaces can react with the
TiO6 octahedra of the layered titanate to form BaTiO3 nanocrystals in the bulk crystal by
a topotactic structural transformation reaction.34 In the present study, the HTO precursor
is partially transformed into BT phase, and then BT/HTO nanocomposite is obtained at
the mole ratios of Ba/Ti=0.5 and 0.75. However, when the mole ratio of Ba/Ti is less
than 0.25, BT phase cannot be formed because the concentration of Ba2+ is too low in
the hydrothermal reaction system for the formation of the BT phase, where the
Ba2+-form layered titanate BaxH1.07-2xTi1.73O4 is formed.
In the second step, when (BT/HTO)-Bi2O3 mixture is heat-treated, firstly the Bi2O3
phase reacts with the HTO phase to form the intermediate Bi12TiO20 phase on the HTO
surface, and then a BT/HTO/Bi12TiO20 nanocomposite is formed as shown in Fig. 3.9.
Secondly, the Bi12TiO20 phase reacts with the Ba2+-form HTO (BaxH1.07-2xTi1.73O4) to
form BBT nanocrystals in the nanocomposite. Since all the reactions in the
transformation processes of the HTO precursor to the BT/BBT nanocomposite are
topochemical reactions, finally the mesocrystalline BT/BBT nanocomposite is formed.
In the mesocrystalline BT/BBT nanocomposite, the BT and the BBT nanocrystals show
the crystal-axis orientation in the [110]-zone and [11-1]-zone directions, respectively, as
shown in Fig. 3.9(h). Similarly when the mixture of Ba2+-form HTO and Bi2O3 is
heat-treated, the Ba2+-form HTO is transformed to the BBT mesocrystal with the
crystal-axis orientation in the [11-1]-zone direction by the same topochemical
mechanism.
110
Fig. 3.10 Schematic representation of formation mechanism of mesocrystalline BT/BBT
nanocomposite by two-step topochemical reactions from HTO single crystal precursor.
The formation of a heteroepitaxial interface between the BT and BBT nanocrystals in
the mesocrystalline BT/BBT nanocomposite is expected similar to the cases of the
mesocrystalline BT/Bi0.5Na0.5TiO3, BT/SrTiO3 and BT/CaTiO3 nanocomposites in our
previous studies.32-34 The SAED result of the BT/BBT nanocomposite in Fig. 3.9(h)
reveals that the (00-1) facet of the BT nanocrystals faces to the (011) facet of the BBT
nanocrystals in the nanocomposite. The atoms arrangements on the (00-1) facet of BT
and the (011) facet of BBT are presented in Fig. 3.11. The four oxygens constitute a
quadrangular lattice with a side length of 4.001 Å on the (00-1) facet of BT, and a
rectangle lattice with a length of 3.864 Å and a width of 3.732 Å on the (011) facet of
BBT. This result suggests that it is possible to form a heteroepitaxial interface between
the (00-1) facet of BT and the (011) facet of BBT with the lattice mismatches of 3.4 and
6.7 % along the [0-11] and [100]-directions of the BBT lattice cell, respectively, as
111
shown in Fig. 3.11(c). Hayashi et al. have reported a heteroepitaxial interface in the 2D
(Sr,Ba)TiO3/(Ca,Sr)TiO3 superlattice with an effective lattice mismatch of 2.5-5.5 %.44
Trampert has demonstrated a MnAs/GaAs heteroepitaxial system with a lattice
mismatch of 7.5 %.19 Although the lattice mismatch of 6.7 % along [100]-direction of
the BT/BBT interface is relatively large for the formation of the heteroepitaxial
interface, we think the BT/BBT heteroepitaxial interface is possible in the lattice
mismatch range of 3.4 to 6.7 %, especially for the nanocrystals because the lattice strain
is relatively easy to be mitigated at nanocrystals interface than that at large crystals
interface.
Fig. 3.11 Atoms arrangements on (a) (00-1) facet of BT and (b) (011) facet of BBT, and (c)
schematic presentation of lattice mismatches between (00-1) facet of BT and (011) facet of BBT
at BT/BBT heteroepitaxial interface.
Fig. 3.12 shows the Raman spectra of the mesocrystals of BT and BBT, and the
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BT/BBT nanocomposites. The bands at 303, 515 and 714 cm-1 in Fig. 3.12(a) can be
assigned to the characteristic bands of the BT phase, in which the sharp peak at 303
cm-1 illustrates the formation of the tetragonal BT phase for BT-800 mesocrystal, which
has a nanocrystal size of about 75 nm (Table 3.1).32, 45, 46 The characteristic bands of
BBT are observed at 280, 540 and 890 cm-1 in the spectrum of the BBT-800 mesocrystal
(Fig. 3.12(b)), 37, 47 which further confirms the formation of BBT by the heating
treatment of the (BT/HTO-0.25)-Bi2O3 mixture. In the spectrum of the BT/BBT-0.5-800
nanocomposite, the bands corresponding to the cubic BT phase at 515 and 714 cm-1 and
those corresponding to tetragonal BBT phase at 280, 540 and 890 cm-1 are confirmed
(Fig. 3.12(c)). It is noted that the tetragonal BT phase is formed in the BT-800
mesocrystal, whereas the cubic BT phase is formed in the BT/BBT-0.5-800
nanocomposite. The formation of the cubic BT phase in the BT/BBT-0.5-800 can be
ascribed to the smaller size of the BT nanocrystals (40 nm) in the nanocomposite than
that (75 nm) in the BT-800 mesocrystal (Table 3.1).48, 49
In addition, it is quite interesting that some new bands appear at 222, 248, 262, 355,
413, 676 and 802 cm-1, which cannot be assigned to the single BT and BBT phases. We
think these new bands may be ascribed to the introduction of lattice strain at the
BT/BBT heteroepitaxial interface. The spectrum of the BT/BBT-0.75-800
nanocomposite reveals that the tetragonal phases of BT and BBT are formed in the
BT/BBT nanocomposite (Fig. 3.12(d)). The formation of the tetragonal BT phase in the
BT/BBT-0.75-800 nanocomposite can be ascribed to relatively large size of the BT
nanocrystals (70 nm) in the BT/BBT-0.75-800 nanocomposite (Table 3.1), namely the
crystal size of BT increases with increasing its content in the nanocomposite.
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Fig. 3.12 Raman spectra of (a) BT mesocrystal heat-treated at 800 °C, (b) BBT mesocrystal of
BT/BBT-0.25-800 (BBT-800), BT/BBT nanocomposites of (c) BT/BBT-0.5-800 and (d)
BT/BBT-0.75-800.
3.3.4 Ferroelectric, dielectric and piezoelectric responses of BT/BBT
nanocomposite
To study the ferroelectric behavior of the BT/BBT nanocomposites, the P-E
hysteresis measurement was employed as shown in Fig. 3.13. It can be clearly seen that
the BT-1100 and BBT-1100 (BT/BBT-0.25-1100) samples exhibit the largest and the
114
smallest remanent polarizations (Pr), respectively, in these samples. The larger
ferroelectric response of BT than that of BBT is consistent with the results of their
normal ceramic samples reported.14, 37 It is noteworthy that although BT/BBT-0.5-1100
has a lower BT content than BT/BBT-0.75-1100, the remanent polarization of
BT/BBT-0.5-1100 (Pr = 2.8 μC/cm2) is much larger than that of BT/BBT-0.75-1100 (Pr =
1.1 μC/cm2). The larger Pr value of BT/BBT-0.5-1100 than that of BT/BBT-0.75-1100
can be explained by higher density of the BT/BBT heteroepitaxial interface which can
cause a lattice strain around the interface and enhance the ferroelectric response.16, 32, 34
According to the SEM-EDS results, the BT/BBT mole ratios are about 2 in
BT/BBT-0.5-1100 and about 10 in BT/BBT-0.75-1100, respectively (Fig. 3.14). This
result indicates that the chemical composition of BT/BBT-0.5-1100 closes to the
optimum condition for the high density of the BT/BBT heteroepitaxial interface to
enhance the ferroelectric response by using the lattice strain engineering.
Fig. 3.13 P-E hysteresis loops of the BT-1100, BBT-1100, BT/BBT-0.5-1100 and
BT/BBT-0.75-1100 pellet samples.
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Fig. 3.14 SEM-EDS spectra for (a) BBT-1100, (b) BT/BBT-0.5-1100 and (c)
BT/BBT-0.75-1100 samples.
The relative permittivities (εr) measurement was also employed to understand the
influence of the BT/BBT heteroepitaxial interface on the dielectric behavior, as shown
in Fig. 3.15. The εr values of the BT/BBT-0.5 nanocomposite are improved greately
when the heating temeperature increases from 600 to 800 oC due to formation of the
BT/BBT nanocomposite, and then improved slowly from 800 to 1100 oC maybe due to
the enhancemences of dennsity and crystallinity (Fig. 3.16). The εr results for BT/BBT,
BT, BBT samples exhibit the same tendency as their P-E hysteresis results, in which εr
value increases in an order of BBT-1100 < BT/BBT-0.75-1100 < BT/BBT-0.5-1100 <
BT-1100. Generally, the BT-based composites exhibit the enhancing ferroelectric and
116
dielectric responses with increasing BT contents,14, 37, 50 whereas the εr value of
BT/BBT-0.5-1100 is about 1.3 times larger than that of BT/BBT-0.75-1100, namely the
opposite result in the case of the BT/BBT nanocomposite because the higher density of
the BT/BBT heteroepitaxial interface in BT/BBT-0.5-1100 than that in the
BT/BBT-0.75-1100, which corresponds the Raman spectrum results (Fig. 3.12) and the
P-E hysteresis (Fig. 3.13) results.
Fig. 3.15 Variations of relative permittivities (εr) with frequency for BT-1100, BBT-1100,
BT/BBT-0.5-1100 and BT/BBT-0.75-1100 pellet samples.
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Fig. 3.16 Variations of relative permittivity (εr) with frequency for BT/BBT-0.5 samples
obtained at different temperatures.
Fig. 3.17 shows the temperature dependences of the relatively permittivity (εr) for the
BT-1100, BBT-1100, and BT/BBT-0.5-1100 pellet samples. The BT-1100 pellet sample
exhibits a phase transition peak around 130 °C that corresponds to the Curie temperature
(Tc) of the normal ferroelectric BT phase (Fig. 3.17(a)).49, 51 The BBT-1100 pellet
sample shows a phase transition peak around 400 °C that corresponds to the Tc of the
normal ferroelectric BBT phase (Fig. 3.17(b)).52 It is interesting that two phase
transition peaks are observed for the BT/BBT-0.5-1100 nanocomposite at around 40 and
635 °C, which can be assigned to the Tc of BT and BBT nanocrystals in the
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nanocomposite, respectively. We think the anomalous enhancement of the Tc for the
BBT phase can be attributed to the introduced lattice stain at the BT/BBT
heteroepitaxial interface due to their lattice mismatch.13, 14, 23, 34
Fig. 3.17 Temperature dependences of the relative permittivity (εr) for (a) BT-1100, (b)
BT/BBT-0.25-1100 (BBT-1100) and (c) BT/BBT-0.5-1100 nanocomposite at measurement
frequency of 10 kHz.
By considering the larger crystal lattice of BT phase than that of BBT phase (Fig.
3.11), at around the heteroepitaxial BT/BBT interface, BT lattice will bear an in-plane
compressive strain and an out-of-plane tensile strain, while BBT lattice will bear an
in-plane tensile strain and an out-of-plane compressive strain. Haeni et al. have firstly
demonstrated hundreds of degrees increasing in the Tc of SrTiO3 (ST) thin film
deposited on DyScO3 substrate by introducing the in-plane tensile strain with +1 %
119
lattice mismatch.13 A BaTiO3/Sm2O3 composite thick film has been reported with a
highly improved Tc of BT, where the BT lattice bears an in-plane tensile strain with
+2.35 % lattice mismatch at the BT/Sm2O3 interface.12 This result reveals that the
in-plane tensile strain can result in the elevated Tc of the ferroelectric phase. Therefore,
we think the elevated Tc of BBT phase in the BT/BBT nanocomposite can be assigned
to the introduced in-plane tensile strain.
However, it is very interesting that the Tc for BT in the BT/BBT-0.5-1100
nanocomposite was lowered from 130 to 40 °C. We think the decreased Tc of BT in the
BT/BNT nanocomposite could not be ascribed to the introduced in-plane compressive
strain. Choi et al. have presented an elevated Tc for BT thin film epitaxially grown on
the DyScO3 substrate where the BT film undergoes a compressive strain of -1.7% lattice
mismatch because the lattice constant of BT is larger than that of DyScO3 substrate. 14
Suzuki et al. have reported a mesostructured BT/ST composite film with elevated Tc of
BT by introducing an in-plane compressed strain with -4 % lattice mismatch to BT
lattice.23 In our former research, the mesocrystalline ferroelectric BaTiO3/Bi0.5Na0.5TiO3
(BT/BNT) nanocomposite exhibits an elevated Tc for both BT and BNT in the
mesocrystalline nanocomposite, where BNT has a smaller lattice constant than that of
BT, namely, BNT lattice bears an in-plane tensile strain and BT lattice bears an in-plane
compressive strain.34
The above results suggest that both tensile and compressed strains can result in the
elevated Tc of ferroelectric phases, which may be due to that an in-plane tensile strain
accompanies an out-of-plane compressed strain and an in-plane compressed strain
accompanies also an out-of-plane tensile strain. The lattice strain situation in the 3D
system should be quite complicated. To understand lattice strain situation and its effect
120
on ferroelectric phase in the 3D system, a further detail study is necessary, and the phase
field calculation can be an effective approach.53 Another possible reason for the lowered
Tc of BT is due to its small crystal size in the BT/BBT nanocomposite (Table 3.1).
Sánchez-Jiménez and co-workers have reported a lowered Tc of BT to around 80 °C in a
BT-Ni nanocomposite with a crystal size of BT about 45 nm.54
Table 3.1 Sizes of BT nanocrystals in BT/BBT-0.5 nanocomposites obtained at different
temperatures, and in BT/BBT-0.75-800 nanocomposite and BT-800 mesocrystal.
Sample Size of BT nanocrystal*
800 °C 900 °C 1000 °C 1100 °C
BT/BBT-0.5 40 nm 50 nm 55 nm 70 nm
BT/BBT-0.75 70 nm - - -
BT mesocrystal 75 nm - - -
* Scherrer equation (also referred to as the Debye–Scherrer equation) was applied to estimate the
sizes of BT nanocrystals in BT/BNT nanocomposites and BT mesocrystal, as shown followed:
D(2θ) 110 = Kλ/(Bcosθ110)
Where D is the average nanocrystal size, K is a constant (K = 0.89), λ is the wavelength of the X-ray
source (λ = 0.154056 nm), B is the value of the full width at half maximum (FWHW) of the
diffraction peak of plane (110) and θ is the Bragg angle.
The Curie temperatures of the BT and BBT phases in the BT/BBT-0.5 nanocomposite
are dependent on the preparation temperature, as shown in Fig. 3.18. The
BT/BBT-0.5-800 exhibits a BBT phase transition peak at around 700 oC, and an unclear
peak of the BT phase transition at around 40 °C. With increasing the heating temperature,
the Tc of BBT shifts slightly to the lower temperature, while a sharp and clear phase
transition peak of BT is observed at around 40 °C. The low-temperature-shifting Tc of
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BBT may be ascribed to the destruction of the BT/BBT heteroepitaxial interface due to
the crystal growth of the BBT nanocrystals with increasing heating temperature.6 The
appearance of the sharp peak for the BT phase can be ascribed to the transition from the
pseudo-cubic structure to the tetragonal one with increasing heating temperature from
800 to above 900 °C due to the lattice mismatch stress at the BT/BBT heteroepitaxial
interface.34, 49 The Tc of BT is almost constant in the temperature rang of 900 to 1100 °C
because the crystal size of the BT phase is almost constant in this temperature range
(Table 3.1), namely the crystal growth of the BT nanocrystals is limited by the
neighboring BBT nanocrystals.
Fig. 3.18 Temperature dependences of relativie permittivities (εr) for (a) BT/BBT-0.5-800, (b)
BT/BBT-0.5-900, and (c) BT/BBT-0.5-1100 at measurement frequency of 10 kHz.
122
The piezoelectric responses of the mesocrystalline BT/BBT-0.5 nanocomposites
obtained at different temperatures were investigated by using piezoresponse force
microscopy (PFM). The Displacement-applied voltage (D-V) loops and d*33-applied
voltage (d*33-V) loops are presented in Fig. 3.19, where converse piezoelectric constant
(d*33) is calculated from D/V value of the D-V loop. The d*
33 value of 130 pm/V at 10 V
of applied voltage for BT/BBT-0.5-800 is much larger than those of 40 and 60 pm/V for
BT/BBT-0.5-600 and BT/BBT-0.5-700, respectively. The sudden increase of the d*33
value with the increase of heating temeprature from 700 to 800 °C can be ascribed to the
formation of the mesocrystalline BT/BBT nanocomposite above 800 °C (Fig. 3.3),
which introduces the lattice stain at the BT/BBT heteroepitaxial interface. The d*33
value of BT/BBT-0.5-800 nanocomposite is about 5 and 6 times larger than d*33 values
of a nanostructured BT (28 pm/V)55 and BBT ceramic (23 pm/V).37 The result reveals
that the lattice strain engineering is an effective aproach for enhancing the piezoelectric
response.
123
Fig. 3.19 Displacement-applied voltage loops and d*33-applied voltage loops for BT/BBT-0.5
nanocomposites obtained by heat-treatement at (a) 600, (b) 700 and (c) 800 °C, respectively.
3.4 Conclusions
The mesocrystalline BT/BBT nanocomposite can be fabricated by a facile two-step
124
process, including the first step of hydrothermal process and second step of solid state
reaction process. The construction of mesocrystalline BT/BBT nanocomposite using BT
and BBT nanocrystals with different types of crystal structures can improve the stability
of the nanocomposite at high temperature due to no formation of their solid solution,
which provides an opportunity to fabricate a high density of the mesocrystalline
nanocomposite ceramic sample. The introduction of the BT/BBT heteroepitaxial
interface into the mesocrystalline BT/BBT nanocomposite results in the greatly elevated
Curie temperature of the BBT phase, which broadens the application temperature range
as a ferroelectric material. The BT/BBT heteroepitaxial interface also enhances greatly
the ferroelectric, piezoelectric, and dielectric responses of the BT/BBT nanocomposites,
which exposes its potential for the replacement of the lead-based piezoelectric materials.
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Chapter Ⅳ
Compelling Evidences for Antiferroelectric to Ferroelectric
Transition of MAPbI3-xClx Perovskite in Perovskite Solar
Cells
Abstract
Perovskite solar cells based on organic-inorganic perovskites have allured massive
scientific attention due to their excellent photovoltaic performances. The power
conversion efficiencies of CH3NH3PbI3 (MAPbI3) based PSCs have demonstrated
extremely high efficiency of 25.2 %. However, a conclusive charge separation
mechanism is still missing for PSCs because of the lack of the fundamental
understanding of organic-inorganic perovskite properties, which hampers the
optimization and development of high-performance perovskite solar cells. Up to now, a
traditional p-n junction charge separation mechanism has been employed to perovskite
solar cells, however, there are some mysterious behaviors like the strong current-voltage
(J-V) hysteresis, yet remains unexplained reasonably. And the J-V response of the
perovskite solar cells could lead to an unfaithful estimation of the efficiency, where the
reverse scan and forward scan exhibit the overestimated and underestimated power
conversion efficiency. Some possible theories have been proposed to explain the origin
of the hysteresis in PSCs, involving ferroelectricity, vacancy-assisted ionic migration,
charge carrier trapping, and capacitive effect.
130
To figure out the aforementioned problems, in this chapter, we explore the ferroelectric
(FE) behavior of MAPbI3-xClx perovskite by the structural analysis and the
measurements of the piezoelectric, ferroelectric, dielectric and ferroelastic responses.
Antiferroelectric (AFE) nature of MAPbI3-xClx perovskite (I4/mcm space group) was
first experimentally proved by piezoelectric force microscopy (PFM) measurements,
which also undoubtedly demonstrates the transformation of the AFE phase to the FE
phase by applying an electric field at the room temperature, and the FE phase can be
returned to the AFE phase after heat treatment at above its Curie temperature and then
cooling down to room temperature, namely being reversibly tunable between AFE and
FE phases. XRD results reveal that the spontaneous polarization direction of this
perovskite can be switched by applying an electric field and mechanic press. Based on
the AFE behavior, we propose a possible FE semiconductor charge separation
mechanism for perovskite solar cells, which might help us to understand the origin of
J/V hysteresis in the MAPbI3-based perovskite solar cells.
131
Chapter V Summary
In the present study, the application of the lattice strain engineering to a
newly-designed lead-free ferroelectric mesocrystalline nanocomposite has been
demonstrated. Some specific properties of the ferroelectric materials, including the
Curie temperature (Tc), piezoelectric and dielectric responses can be improved by an
enormous strain in the heteroepitaxial interface constructed by two kinds of crystals
with different lattice parameters. Generally, the lattice strain engineering is mainly
applied to the thin film materials because the heteroepitaxial interfaces are relatively
easy to be fabricated by heteroepitaxial crystal growth of the film materials, but it is a
high cost for the fabrication of thin film materials.
The mesocrystals not only have some potential properties based on the individual
nanocrystals, but also exhibit unique collective properties of nanocrystal ensembles. The
mesocrystalline nanocomposite constructed by two kinds of nanocrystals is a promising
material for the lattice strain engineering to improve the ferroelectricity because it has
high density of the heteroepitaxial interface and is low-cost. It is noteworthy that the
mesocrystalline nanocomposites exhibit both improved Tc and piezoelectric response,
which cannot be achieved simultaneously in the thin film materials or bulk materials
without mesocrystalline nanostructure, as far as I know. Therefore, this approach
provides a new concept to design the high-performance lead-free piezoelectric
materials.
In addition, there has been a recent surge of interest in perovskite solar cells (PSCs)
due to soaring power conversion efficiencies (PCEs). However, the fundamental
understanding of organic-inorganic halide perovskites employed as the absorber in the
132
PSCs is still limited. Consequently, systematic studies on further improvements of the
materials and device structures for the commercialization have been severely hampered.
In the present study, the relationship between structure and ferroelectricity of
MAPbI3-xClx perovskite has been investigated by the structural analysis and the
measurements of the piezoelectric, ferroelectric, dielectric and ferroelastic responses.
The transformation from antiferroelectric MAPbI3-xClx phase to its ferroelectric phase
by the poling treatment has been uncovered for the first time.
The swapping behavior between antiferroelectric and ferroelectric phases of the
MAPbI3-based perovskites suggest that the ferroelectricity would affect charge
separation performance, and the ferroelectric phase can possess a higher charge
separation effect than that of the non-ferroelectric phase in the PSCs; hence,
current-voltage (J-V) hysteresis behaviors for the PSCs can be well explained based on
this behavior. The results conclude that the J-V hysteresis is one of the solid evidence
for the exhibition of higher charge separation effect of the ferroelectric perovskites than
that of non-ferroelectric perovskites and the reverse scan J-V curve should be used to
evaluate PSCs of the antiferroelectric or ferroelectric perovskites because it corresponds
to the ferroelectric semiconductor charge separation effect.
The main results and points of the present study are summarized as follow:
In Chapter I, some reviews on the synthesis, the formation mechanisms,
characterizations, and the applications of conventional mesocrystals were described.
The general introduction for the topochemical synthesis, and the layered protonated
titanate as a precursor for the topochemical synthesis of the mesocrystals were
mentioned. In addition, the perovskite and perovskite-related halides were described
also as they possess several interesting properties, such as electron-acceptor behavior, a
133
large optical transmission domain and piezoelectric etc. Furthermore, the purposes of
the present study were clarified.
In Chapter II, the ferroelectric mesocrystalline BT/BNT nanocomposite synthesized
from a layered titanate H1.07Ti1.73O4 (HTO) by a facile two-step topochemical process,
namely first-step solvothermal process and second-step solid-state process, was
introduced. The BT/BNT nanocomposite is constructed from well-aligned BT and BNT
nanocrystals with the same crystal-axis orientation. The BT/BNT heteroepitaxial
interface in the nanocomposite is promising for the enhanced piezoelectric performance
by using the lattice strain engineering, which gives a giant piezoelectric response with a
d*33 value of 408 pm/V. The introduced lattice strain at the BT/BNT heteroepitaxial
interface causes transitions of pseudo-paraelectric BT and BNT nanocrystals to the
ferroelectric nanocrystals in the mesocrystalline nanocomposite, which enlarges
ferroelectric, piezoelectric and dielectric responses. The lattice strain also results in the
elevated Curie temperatures (Tc) of BT and BNT and a new intermediate phase
transition.
In Chapter III, the ferroelectric mesocrystalline BT/BBT nanocomposite synthesized
from the layered titanate HTO by a facile two-step topochemical process, namely
first-step solvothermal process and second-step solid state process, was exhibited. The
BT/BBT nanocomposite is constructed from well-aligned BT and BBT nanocrystals
oriented along the [110] and [11-1] crystal-axis directions respectively. The lattice strain
is introduced into the nanocomposite by the formation of the BT/BBT heteroepitaxial
interface, which causes a greatly elevated Curie temperatures from 400 to 700 °C and an
improved piezoelectric response with d*33=130 pm/V. In addition, the BT/BBT
nanocomposite is stable up to a high temperature of 1100 oC, hence, the mesocrystalline
134
ceramic can be fabricated as a high-performance ferroelectric material.
In Chapter IV, the ferroelectric and semiconducting properties of the
CH3NH3PbI3-xClx perovskites were studied by structural analysis, measurements of the
ferroelastic behavior, the ferroelectric hysteresis loops, the piezoelectric response and
conductivity. The results reveal that the CH3NH3PbI3-xClx perovskite exhibits the
antiferroelectric and semiconducting natures, and the antiferroelectricity can be
switched to ferroelectricity by poling treatment, which gives a solid evidence to put an
end to the heated argument between the non-ferroelectric and ferroelectric nature for the
MAPbI3-based perovskites and paves the way for the fabrication of high-performance
perovskite solar cells by using ferroelectric and antiferroelectric phases.
The results described above conclude that the in situ topochemical mesocrystal
conversion reaction process is an attractive approach. This approach can be employed to
the development of the platelike functional titanate ferroelectric mesocrystalline
nanocomposite. The nanocrystal size, morphology, structure, and composition of the
mesocrystalline nanocomposite can be controlled by adjusting the reaction conditions in
the in situ topochemical mesocrystal conversion reaction process. These mechanisms
will serve also as a guide to develop the topochemical syntheses of other materials in
the solvothermal processes and solid-state processes. Therefore, both the solvothermal
chemical processes and solid-state processes accompanying with the in situ
topochemical conversion reaction are of notable significance for the fundamental
research, and can provide important knowledge for controlling the chemical reaction
process to achieve the materials with advanced functions.
The study of mesocrystalline nanomaterials constructed from well-aligned oriented
nanocrystals has increasingly become an intense and major interdisciplinary research
135
area in the recent decade owing to their potential applications to catalysis, sensing,
ferroelectric, and energy storage and conversion. In addition, strain engineering has
been used to alter the electronic structure of materials, which can greatly change a series
of physical properties of the materials, since its impact is delivered directly on the
lattice. Up to now, the strain engineering has been widely applied to 2D materials with
simple 2D heteroepitaxial interface, while its application to 3D bulk materials have been
rarely reported. In the 3D systems, strain can be effectively introduced to a bulk
material by either pulling or squeezing the lattice. However, the 3D heteroepitaxial
interface is very difficult to be constructed in the 3D bulk materials. Therefore, the
application of the strain engineering to a newly-designed mesocrystalline
nanocomposite with a 3D heteroepitaxial interface is a big breakthrough for making 3D
bulk materials with newly excellent properties. The success in developing these
mesocrystalline nanocomposites not only expand mesocrystalline nanomaterials
chemistry and offers a good opportunity to understand the formation process of this
unique mesocrystalline nanocomposite structure, but also paves a way for the
application of the mesocrystalline materials to improve ferroelectric, piezoelectric, and
dieletric nanomaterials via the strain engineering.
On the other hand, our findings for the MAPbI3-xClx perovskite not only put an end to
the heated argument between its non-ferroelectric and ferroelectric natures, but also
pave a new avenue toward the fabrication of high-performance PSCs using the
antiferroelectric and ferroelectric semiconductor perovskites with optimizing the cell
performances in future developments for the commercialization.
In our next challenges, firstly, figuring out the connection between the atomic
arrangement structure and the anomalous ferroelectric behavior of the mesocrystalline
136
nanocomposite is significant. Of course, this requires the help of some state-of-art
technologies, such as the scanning transmission electron microscopy (STEM), the
energy-dispersive X-ray spectroscopy (EDS) and the electron energy loss spectroscopy
(EELS). Given that ferroelectric mesocrystalline nanocomposites constructed
from the different nanocrystals with the same perovskite structure tend to transform into
the solid solution phase under high heating temperature, therefore, the application of the
mesocrystalline nanocomposite to its ceramic counterpart is scarcely possible. Therefore,
the formation of its film materials or/and polymer-based composite applied to
nanoelectronic devices is promising.
As mentioned above, the lattice strain engineering has been mainly applied to the
ferroelectric super-structured film materials with 2D heteroepitaxial interfaces, which
has been widely studied. Whereas, the ferroelectric mesocrystalline nanocomposite film
materials should exhibit much more complicated 3D heteroepitaxial interface, which
needs much more refined STEM image analysis and other assistant methods to find out
the atomic arrangements near the interfaces and its connection to the anomalous
ferroelectric behavior. Although, some preliminary works have been done, the further
study is still needed.
Besides, the further research on the ferroelectricity of other types of halide
perovskites used for PSCs and discussing its connection to the power conversion
efficiency are meaningful. The ferroelectric semiconductor charge separation
mechanism would not be limited in the halide perovskites, and can be applied also to
other ferroelectric semiconductors, such as metal oxides and metal sulfides. It could
offer a better avenue for the development of the high-performance PSCs with the help
of piezoresponse force microscopy (PFM) technique based on atomic force microscopy
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(AFM) or the strain-electric-field (S-E) texting system.
Publications
Publications in Journals
1. Wenxiong Zhang; Hao Ma; Sen Li; Dengwei Hu; Xinggang Kong; Shinobu
Uemura; Takafumi Kusunose; Qi Feng. Anomalous piezoelectric response of
ferroelectric mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 nanocomposites designed by
strain engineering. Nanoscale 2018, 10, (17), 8196-8206.
2. Wenxiong Zhang; Sen Li; Hao Ma; Dengwei Hu; Xinggang Kong; Shinobu
Uemura; Takafumi Kusunose; Qi Feng. Ferroelectric Mesocrystalline
BaTiO3/BaBi4Ti4O15 Nanocomposite: Formation Mechanism, Nanostructure, and
Anomalous Ferroelectric Response. Nanoscale 2019, DOI: 10.1039/C8NR07587E
3. Dengwei Hu; Wenxiong Zhang; Yasuhiro Tanaka; Naoshi Kusunose,; Yage Peng;
Qi Feng. Mesocrystalline Nanocomposites of TiO2 Polymorphs: Topochemical
Mesocrystal Conversion, Characterization, and Photocatalytic Response. Crystal
Growth & Design 2015, 15, (3), 1214-1225.
4. Dengwei Hu; Xiaomei Niu; Hao Ma; Wenxiong Zhang; Galhenage A. Sewvandi;
DesuoYang; Xiaoling Wang; Hongshei Wang; Xinggang Kong; Qi Feng.
Topological relations and piezoelectric responses of crystal-axis-oriented
BaTiO3/CaTiO3 nanocomposites. RSC Adv. 2017, 7, (49), 30807-30814.
5. Dengwei Hu; Wenxiong Zhang; Fangyi Yao; Fang Kang; Hualei Cheng; Yan Wang;
Xinggang Kong; Puhong Wen; Qi Feng. Structural and morphological evolution of
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an octahedral KNbO3 mesocrystal via self-assembly-topotactic conversion process.
CrystEngComm 2018, 20, (6), 728-737.
6. Ma, H.; Wenxiong Zhang; Xinggang Kong; Shinobu Uemura.; Takafumi
Kusunose.; Qi Feng. BaTi4O9 mesocrystal: Topochemical synthesis, fabrication of
ceramics, and relaxor ferroelectric behavior. Journal of Alloys and Compounds 2019,
777, 335-343.
7. Wenxiong Zhang; Galhenage A. Sewvandi; Sen Li; Xinggang Kong; Dengwei Hu;
Shinobu Uemura.; Takafumi Kusunose.; Qi Feng. Compelling Evidences for
Antiferroelectric to Ferroelectric Transition of MAPbI3-xClx Perovskite in Perovskite
Solar Cells. (In the submission)
Publications in Conferences
1. Wenxiong Zhang, Qi Feng. Topochemical Synthesis of BaTiO3 Platelike
Mesocrystals from Layered Titanate by Flux Method. 第 21 回ヤングセラミスト
ミーティング in 中四国, p103-104, Shimane, 2014/11/15.
2. Wenxiong Zhang, Hao Ma, Qi Feng. Synthesis and Characterization of
Ferroelectric Mesocrystalline BaTiO3-Bi0.5Na0.5TiO3 Nanocomposites. The 54th
Symposium on Basic Science of Ceramics, p82, Saga, 2016/01/07-08.
3. Wenxiong Zhang, Hao Ma, Qi Feng. Fabrication and Characterization of
Ferroelectric Mesocrystalline BaTiO3-Bi0.5Na0.5TiO3 Nanocomposites. 日本化学会
中国四国支部大会, ポスター, 2016/11/05-06.
4. Wenxiong Zhang, Qi Feng. Fabrication of Mesocrystalline BaTiO3-Bi0.5Na0.5TiO3
Nanocomposites and Their Ferroelectric Behavior. The 55th Symposium on Basic
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Science of Ceramics, p109, Okayama, 2017/01/12-13.
5. Wenxiong Zhang, Qi Feng. Fabrication of Mesocrystalline BaTiO3-Bi0.5Na0.5TiO3
Nanocomposites and Their Ferroelectric Behavior. International Symposium on
Advanced Materials: Golden Era in Hydrothermal Research, p36-37, Kochi,
2017/03/27-30.
6. Wenxiong Zhang, Qi Feng. Anomalous Piezoelectric Response of Ferroelectric
Mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 Nanocomposites Designed by Strain
Engineering. The 56th Symposium on Basic Science of Ceramics, p9, Tsukuba,
2018/01/11-12.
7. Wenxiong Zhang, Qi Feng. Anomalous Piezoelectric Response of Ferroelectric
Mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 Nanocomposites Designed by Strain
Engineering. 2018 ISAF-FMA-AMF-AMEC-PFM Joint Conference, p49, Hiroshima,
2018/05/25-06/01.
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Acknowledgment
I could not have completed this dissertation without the help of many people that
have influenced and supported me scientifically, financially and socially over the past 5
years at Kagawa University, Japan.
I would like to firstly thank my supervisor, Prof. Qi Feng, for his kind guidance,
cultivation, and continuous supervision, excellent advice and continuous encouragement
towards the completion of this present research successfully in time. His rigorous and
pragmatic academic attitude is worth studying for my life. Besides the fundamental
knowledge he had taught me, more importantly, he realized early that I have a strong
desire to research, anything, and you gave me the freedom to develop. And I’m also
appreciated for Associate Prof. Lin Yu (Okayama Shoka University), for her much
supports and patience on our research supervisor team, for her precious guidance and
perspectives on my daily life.
I would also like to express my thanks to my vice supervisors Prof. Takafumi
Kusunose and Associate Prof. Shinobu Uemura for their kind advice, valuable
suggestion, necessary support, and enthusiastic assistances to my Ph. D study. In
addition, I would like to express my thanks to Prof. Chengling Pan (Anhui University of
Science and Technology) for recommending me to study in Japan. Grateful
acknowledgements are to Senior Dengwei Hu (Baoji University of Arts and Science),
Changdong Chen (Liaoning Shihua University), Yien Du (Jinzhong University) and
Sewvandi Asha Galhenage (University of Moratuwa) for their valuable suggestion,
enthusiastic assistances and life experience.
I would like to acknowledge the former and current administrative staff at Kagawa
University. Thank you very much for making my life easier in Japan: Especially, I
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would like to thank Sakamoto Ai who helped me out with my application for the
scholarship. In addition, I also want to give my appreciation to Ms. Yuka Kojima for
teaching me Japanese and helping me prepare the interview of the scholarship.
I would like to thank the scientists, who introduced me to the facilities at Kagawa
University. Special thanks to the three scientists, who are the responsible for my favorite
instruments: Associate Prof. Shinobu Uemura (AFM), Mr. Toshitaka Nakagawa
(FE-TEM) and Ms. Ayami Nishioka (TEM). I greatly enjoyed your expertise with the
electron microscopes. Thank you for giving me so many ideas to solve my problems.
I also owe my sincere gratitude to my friends and fellow classmates in our research
group, who gave me many helps and much pleasures on my life and study.
Lastly, I would like to acknowledge my family for their love, understanding and
support throughout my research and life in Japan, in particular, I sincerely appreciate
my lovely girlfriend, Hui Liu, for her endless support and encouragement, as well as
accompanying me through a lot of hard times.
.
Wenxiong Zhang
Feng Lab, Department of Advanced Materials Science,
Faculty of Engineering and Design
Kagawa University, Kagawa, Japan
January, 2018