-
RAA
ReceiveAcceptA
Keywords:
Hydrogen productionSolar cells
erties of modied TiO2 nanotube arrays make them excellent
candidates for
splitting, solar cells and CO2 conversion.
. . . . . .lutantsotocatrays asube arr2 nanoy hete
. . . . . . . . . . . . . 29. . . . . . . . .. . . . . . . . ..
. . . . . . . .
7. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . 34
References . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 34
Corresponding author. Tel./fax: +86 10 89732300.E-mail address:
[email protected] (Q. Zhou).
Microporous and Mesoporous Materials 202 (2015) 2235
Contents lists available at ScienceDirect
Microporous and Mesoporous MaterialsAcknowledgements . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 343.3. Sample
pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . .4. Applications in sensitized
solar cells (SSCs) . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . .5. Hydrogen production . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . .6. Photocatalytic conversion of CO2 . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.http://dx.doi.org/10.1016/j.micromeso.2014.09.0401387-1811/ 2014
Elsevier Inc. All rights reserved.. . . . 30
. . . . 32
. . . . 333. Environmental analytical chemistry . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . 283.1. Sensors . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . 28
3.1.1. Gas monitoring . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . 283.1.2. Detection of heavy metal ions . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
283.1.3. Detection of organic pollutants . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29
3.2. Measurement of COD . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 29Contents
1. Introduction . . . . . . . . . . . . . . . . .2.
Photocatalytic degradation of pol
2.1. TiO2 nanotube arrays as ph2.2. Modified TiO2 nanotube
ar
2.2.1. Doping TiO2 nanot2.2.2. Loading on the TiO2.2.3. TiO2
nanotube arra 2014 Elsevier Inc. All rights reserved.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 23
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 23alysts. . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 25ays. . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 25tube arrays
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 25rojunctions . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 28PhotocatalysisEnvironmental
analytical chemistry
use in solar cells and sensitive sensors for trace compounds,
etc. This review focuses on the recentapplications of TiO2 nanotube
arrays in removal of pollutants, environmental analytical
chemistry, waterTiO2 nanotube arraystinct electrochemical
propvailable online 29 September 2014catalytic abilities in several
cases: in the degradation of environmental inorganic and organic
pollutantsto less toxic compounds, water splitting, and in the
reduction of atmospheric CO2 levels by incorporationof CO2 into
hydrocarbons, among others. Moreover, the wide absorption spectrum
characteristics and dis-d 31 December 2013d in revised form 1
September 2014ed 15 September 2014
have been demonstrated to serve as multifunctional materials
which show great promise in addressingmany challenges in both
environmental and energy technology elds. They have exhibited
extraordinarya r t i c l e i n f o
Article history:Receive
a b s t r a c t
TiO2 nanotube arrays, novel TiO2-based nanomaterials with unique
chemical and physical properties,Qingxiang Zhou , Zhi Fang, Jing
Li, Mengyun WangBeijing Key Laboratory of Oil and Gas Pollution
Control, College of Geosciences, China University of Petroleum
Beijing, Beijing 102249, Chinavieweview
pplications of TiO2 nanotube arrays in environmental and energy
elds:rejournal homepage: www.elsevier .com/locate /micromeso
-
Q. Zhou et al. /Microporous and Mesoporous Materials 202 (2015)
2235 231. Introduction
Due to the rapid acceleration of technological advances
acrossthe globe, each year more and more products are produced
tomake our lives more comfortable and convenient. Most of
theseproducts originate from natural sources such as petroleum,
coal,natural gas and mineral sources; meanwhile, many
environmen-tal problems occur during the various steps of the
extraction,transportation, and transformation of raw materials into
nalproducts. These problems include CO2 emission, which
contrib-utes greatly to global warming, ozone depletion, and air
andwater pollution, which increase health risks to living
things.Although these problems may only appear to impact local
areas,they actually pose cumulative hazards on a global level.
Forexample, it is reported that pesticides and heavy metals
havebeen detected in remote Antarctic areas, far from their
points-of-use. As a result, each country around the world is facing
chal-lenges to look for better strategies to solve such energy and
envi-ronmental problems.
Nanomaterials are novel material forms that emerged in the1980s
and have been studied intensely in recent years. Thenano
designation stems from the fact that the unit size ofthese
materials is about 1100 nm along any single dimensionalscale. A new
form of TiO2 nanomaterial, the TiO2 nanotube array,has attracted
much attention recently. Currently, many syntheticmethods for the
preparation of TiO2 nanotube arrays exist,including the most
commonly-used synthetic methods: the tem-plate and anode oxidation
methods. In the template method,common templates include porous
alumina, zinc oxide and var-ious organic polymers [1,2]. The
morphology of each resultingnanotube array depends on the size and
shape of the template,and the main disadvantage is the subsequent
destruction ofnanotube arrays during the downstream separation
process.The anode oxidation method is an electrochemical method
inwhich the Ti substrate is anodized in an electrolyte containingF
ions, either with or without organic solvents. Gong et al.
rstintroduced this facile method to fabricate highly-ordered
verticalnanotube arrays [3], and this has been the preferred method
forTiO2 nanotube array production in recent years. Many
research-ers have investigated the parameters that would affect the
mor-phology of prepared samples such as potential,
electrolytecomposition, oxidation time and annealing temperature.
TiO2nanotube arrays possess unique chemical and physical
propertiessuch as chemical inertness, gas sensitivity, large
surface area,biocompatibility, high photocatalytic and
electrochemical activi-ties. These characteristics have led
researchers to adapt themto improve upon old technologies,
including solar cells, removalof pollutants, water splitting [4],
sensors [5], sample pretreat-ment [6], drug delivery [7], and CO2
conversion [8]. Currently,research into new nanostructures derived
from TiO2-based mate-rials holds great promise to help address many
urgent globalchallenges.
Several reviews have been published which have focused onthe
synthesis and applications of TiO2 nanotube arrays [9],including a
short summary (only available in Chinese) byl ourgroup on the
applications of TiO2 nanotube arrays to addressenvironmental
challenges in 2012 [10]. Thus, the purpose of thisreview is to
provide a thorough survey of recent TiO2 nanotubearray research to
a global audience by focusing on severalaspects of environmental
and energy eld topics including: (1)photocatalytic degradation of
pollutants, (2) applications in envi-ronmental analytical
chemistry, (3) hydrogen generation, (4) sen-sitized solar cells
(SSCs), and (5) CO2 conversion to
hydrocarbons.2. Photocatalytic degradation of pollutants
2.1. TiO2 nanotube arrays as photocatalysts
Environmental pollution has become a growing problem,resulting
in more and more research interest in this area. To thisend,
photocatalytic degradation has been successfully developedand is
rapidly becoming the best way to deal with environmentalpollutants.
In this method, when semiconductive materials areilluminated with
light with energy equal to or higher than theband gap energy of the
semiconductors, the electrons in thevalence band (VB) are excited
to enter the conduction band(CB), and then are transferred to the
surface of particles withavailable holes in the VB. These
photo-generated electronholepairs possess high redox activity.
Since the band gap of anataseTiO2 is about 3.2 eV, electrons in
this material can be excited byillumination using light with
wavelengths less than 387 nm,allowing the electrons in the VB of
TiO2 to be excited to the CBand give rise to electronhole pairs.
Once these pairs reach theTiO2 surface, some pairs can recombine,
releasing energy as lightor heat, while others can react with O2,
H2O and OH adsorbed tothe TiO2 surface to form radicals which can
oxidize macromolec-ular pollutants to form CO2 and water, etc.
During the photocat-alytic process, it is of great importance to
choose a suitablecatalyst. Metal oxide semiconductor materials have
been success-fully developed so that these photocatalytic reactions
may occurin mild conditions [11].
TiO2 powders have been used for many years in the
photocata-lytic eld and their advantages have fueled ongoing
interest. How-ever, this material possesses several disadvantages:
it is difcult todisperse, agglomerates easily, is difcult to reuse,
possesses a lowlight response, and causes environmental pollution
when usedinappropriately. The introduction of TiO2 nanotube arrays,
a novelform of TiO2, has solved these problems. Wender et al. [12]
pre-pared TiO2 nanotube arrays employing an anode oxidation
methodusing ethylene glycol (EG) electrolytes containing
1-n-butyl-3-methyl imidazolium tetrauoroborate (BMIBF4) and
water(Fig. 1). The photocatalytic activity was investigated using
methylorange (MO) as the model pollutant and 13% of the MO was
miner-alized under UV irradiation in 150 min, indicating that the
TiO2nanotube arrays possessed high photocatalytic activity. For
salicylicacid (SA) and salicylaldehyde (SH), the TiO2 nanotube
arrays exhib-itedmuch higher photocatalytic activity upon
irradiation using UVvisible light [13]. It found that 83% of the SA
was oxidized in 2 h,while SH was almost completely eliminated under
the same condi-tions. These results indicated that this method can
be used as apotential treatment to process SA and SH
pollutants.
It is obvious that the structural parameters of TiO2
nanotubearrays, such as specic surface area, wall thickness, tube
lengthand crystalline phase, have important effects on the
photocatalyticactivity of TiO2 nanotube arrays. Liang et al. [14]
investigated theseeffects on the photocatalytic activity of TiO2
nanotube arrays using2,3-dichlorophenol (2,3-DCP) as the target
pollutant. They foundthat a larger specic nanotube surface area can
lead to greaterabsorption of aqueous reactants, while a higher pore
volume canaccelerate the diffusion of various aqueous species
during the pho-tocatalytic reaction, both enhancing the reaction
rate. Using opti-mally-calcined nanotube arrays under UV
illumination, 93% ofadded 2,3-DCP was degraded 2.6 times more
rapidly, as comparedto the result obtained with TiO2 lms under the
same conditions.Zhang et al. fabricated TiO2 nanotube arrays on
uorine-dopedtin oxide (FTO) glass [15], which contained a top
porous nanopar-ticle layer and nanotube array bottom layer. This
new material
resulted in the enhancement of the photocatalytic activity
in
-
liquid and used in H2 production and pollutant degradation
[12].
sopFig. 1. Schematic of TiO2 nanotube arrays synthesized in
ionic
24 Q. Zhou et al. /Microporous and Meglucose photooxidation by
60% over that of TiO2 nanotube arraysbased on simple Ti foil. This
effect was attributed to the fact thatthese nanotube arrays
provided more stable electron pathwaysresulting in high
photocatalytic activity. Wang et al. discoveredthat the increase of
the surface area of the photocathode couldgreatly accelerate the
photoelectrocatalytic reduction rates ofCr(VI) [16]; short TiO2
nanotube arrays (S-TNTs) had higher elec-tron transfer efciency
than that of long TiO2 nanotube arrays (L-TNTs), and nearly
complete reduction of Cr(VI) using S-TNTs wasachieved with UV
irradiation in 60 min (Fig. 2). Their ndings dem-onstrated that
this simple and efcient method could removeCr(VI) from aqueous
samples and was a good start for developinghighly-efcient methods
for heavy metal pollution removal fromwater samples.
Using a different strategy, Zhang et al. designed a
photocatalyticreactor incorporating a rotating disk composed of a
TiO2-nanotube(TNT)/Ti photocatalyst (Fig. 3) [17]. They
investigated the effect ofparameters such as the rotation velocity
and irradiation time onthe photocatalytic activity of the TiO2
nanotube arrays and foundthat the rotating velocity had an
important impact on the photo-catalytic activity. When the rotating
velocity increased up to30 rpm, the removal rate of rhodamine B
approached 90% in 3 h,an improvement of 2540% over results using a
TiO2 nanoparticle
Fig. 2. Schematic illustration of the PEC reduction of Cr(VI)
with S-TNTs as theorous Materials 202 (2015) 2235disk. The results
indicated that a higher rotational velocityincreased delivery of
polluted thick water lm to the reaction,opening a potentially
valuable new direction in the developmentof methodologies for
environmental pollutant removal.
photoanode and a Ti mesh as the photocathode under UV
irradiation [16].
Fig. 3. Schematic diagram (a) and TBPC combined mechanism (b) of
the rotatingdisk photocatalytic reactor [17].
-
ns.
esop2.2. Modied TiO2 nanotube arrays as photocatalysts
In general, anatase TiO2 responds only to the UV fraction of
sun-light; however, the fraction of UV light in sunlight is very
small rel-ative to the fraction of visible light. Based on this
fact, much efforthas been put forth to determine ways to utilize
visible light in thisphotocatalytic system. The simplest successful
approaches havebeen to modify the TiO2 material itself to enhance
its response tovisible light via doping, depositing, loading of
TiO2 nanotubearrays, and other methods [18,20,33,4652] (Table
1).
2.2.1. Doping TiO2 nanotube arraysDoping is an often-used method
to change material properties
in which one or more elements or compounds are doped into
thesubstrate to generate specic electrical and/or optical
properties.Many studies have shown that doping TiO2 with elements
suchas nitrogen, carbon, uorine, iodine and/or iron can lead to the
nec-essary narrow band gap to allow a greater response to visible
light,which can enhance the overall photocatalytic activity. Li et
al. pre-pared N and F co-doped TiO2 nanotube arrays by anodizing a
Tisubstrate in NH4F and NH4Cl solution [18]. They found that
anneal-ing of the doped TiO2 nanotube arrays in an N2 atmosphere
couldeffectively reduce the phenomenon of F atom replacement by
Oatoms, resulting in a higher photocatalytic activity towards
meth-ylene blue. In fact, annealing TiO2 nanotube arrays in a
specicatmosphere has also been developed as a new doping method.
Inthe process of preparing N and S co-doped TiO2 nanotube
arraysthrough the annealing of TiO2 nanotube arrays in thiourea
at500 C [19], NTiO and NOTi bonds were formed between Natoms and
the nanotube arrays, and some of the O atom positionswere replaced
by S atoms, which improved their degree of crystal-lization. Such
doping markedly increased the photocatalytic activ-ity towards
methylene blue, and photocatalytic activity was 1.29times higher
than that of an un-doped array. Boron-doped TiO2nanotube arrays
have also exhibited a phenol degradation rate
Table 1Advances of modied TiO2 nanotube arrays as a catalyst in
photocatalytic degradatio
Modication Methods Pollutants
N and F Anodization MBB Electrodeposition PhenolZnTe
Electrodeposition 9-AnCOOHTi Hydrothermal method Rhodamine BC
Cyclic voltammetry 9-AnCOOHFe Liquid phase deposition MBAg and N
Electrodeposition method AO-IICu2O Electrodeposition AO-I IC
Hydrothermal treatment MBWO3 Immersion Cr(VI)
Q. Zhou et al. /Microporous and Mabout 10% higher than that of
undoped arrays [20]. Owing to itslow cost and easy preparation, Fe
is considered one of the mostsuitable elements for industrial
applications. Doping of TiO2 withFe3+ is an effective approach to
reduce electronhole recombina-tion rates and increase the
photocatalytic efciency due to itssemi-full electronic conguration
and an ion radius close to thatof Ti4+. Sun et al. prepared
Fe-doped TiO2 nanotube arrays by anod-izing Ti in an electrolyte
containing Fe(NO3)3 [21]. By controllingthe concentration of
Fe(NO3)3 in the electrolyte, various concentra-tions of Fe-doped
TiO2 nanotube arrays were obtained. The resultsshowed that a red
shift occurred in the absorption spectrum andthe photocatalytic
performance was enhanced, as expected. Wuet al. prepared Fe-doped
TiO2 nanotube arrays using ultrasound-assisted impregnating and
calcination [22]. The rst step of thistwo-step approach was to
fabricate the TiO2 nanotube arraysdirectly on Ti foils via
electrochemical anodic oxidation and thento immerse the resulting
TiO2 nanotube arrays in an 0.01 MFe(NO3)39H2O aqueous solution
under ultrasound assistance. Thesecond was to anneal the modied Ti
foils at different tempera-tures under ambient conditions for 2 h.
SEM analysis indicated thatFe2O3 nanoparticles with a size of 1020
nm were deposited ontothe TiO2 nanotubes and some Fe3+ ions were
doped into the TiO2lattice. These structural modications were
thought to induce thered-shift of the absorption spectral edge of
the TiO2 nanotubearrays into the visible light range. These FeTiO2
nanotube arraysexhibited a much higher visible-light photocatalytic
activity forthe degradation of methyl blue (MB) than undoped TiO2
nanotubearrays (Fig. 4). Electrochemical impedance spectroscopy
(EIS)showed that Fe incorporation could efciently promote the
separa-tion and transfer of photogenerated charge carriers, a key
factor ineffecting improved photocatalytic performance. Xu et al.
dopedTiO2 nanotube arrays by anodizing TiNb alloys, followed by
heattreatment in a ow of ammonia gas to obtain Nb/N co-dopedTiO2
nanotube arrays [23]. They found that the Nb dopant inTiO2 nanotube
arrays can enhance both the adsorption of NH3 mol-ecules and the
subsequent nitrogen doping of the TiO2 nanotubearrays. These arrays
demonstrated a signicantly-enhanced visiblelight response with a
markedly higher visible light-induced photo-catalytic degradation
of methylene blue when compared toundoped TiO2 nanotube arrays.
2.2.2. Loading on the TiO2 nanotube arraysThe noble metals have
lower fermi levels than that of TiO2.
Thus, they can absorb the electrons excited from TiO2,
effectivelyreducing the recombination rate of photogenerated
electronholepairs, ultimately increasing photocatalytic activity.
Xie et al. pre-pared highly-dispersed Ag nanoparticles on TiO2
nanotube arraysusing a pulse current deposition technique [24]. The
TiO2 nanotubearrays with Ag particles under electrodeposited charge
densities of1800 mC cm2 resulted in the highest absorption peak. In
the deg-radation experiment using methyl orange (MO) under visible
lightirradiation, the photocatalytic kinetic rate constant of the
Ag/TiO2
Light sources Degradation rate (%) Refs.
Visible light 90 [18]Visible light 66 [20]Simulated solar light
100 [33]UV light 80 [46]Simulate solar light 100 [47]Visible light
50 [48]Visible light 37 [49]Visible light 90 [50]Visible light 80
[51]UV light 100 [52]
orous Materials 202 (2015) 2235 25nanotube array was 5.16 times
that of the undoped TiO2 nanotubearray, indicating that the TiO2
nanotube array modied with Agnanoparticles could efciently inhibit
electronhole recombina-tion. This method could also be used to
modify other metal nano-particles on the TiO2 nanotube arrays. Liu
et al. dispersed Agnanoparticles onto the surface of TiO2 nanotube
arrays using anelectrodeposition method [25]. The uniform Ag
nanoparticlesincreased the separation efciency of electrons and
holes. In addi-tion, the modied TiO2 nanotube arrays led to the
highest photo-catalytic activity towards MO when the
electrodeposition timewas 60 min. It was postulated that Ag
nanoparticles acted as elec-tron reservoirs to suppress the
electronhole recombination, mak-ing more holes available for the
oxidation reactions. Tan et al.fabricated Pd-functionalized TiO2
nanotube arrays [26], whichwere utilized to photodegrade methylene
blue (MB) and stearicacid (SA). The photodegradation efciency of
76% (MB) wasachieved under UV irradiation in 4 h, while only 62%
was
-
und
sopdecomposed when TiO2 nanotube arrays without Pd coating
wereused. These results were attributed to the higher surface area
ofmodied arrays and to the catalytic activity of Pd
nanoparticlesand to the effective separation of the electronhole
pairs. Thisgroup also investigated the photocatalytic activity for
solid con-taminants such as SA lm. The results indicated that the
TiO2 nano-tube arrays demonstrated much better photocatalytic
activity thanthat of TiO2 lm under the same conditions. Liu et al.
deposited CdSnanoparticles onto a TiO2 nanotube array surface using
a chemicalbath deposition method [27]. This modied photocatalyst
showed
Fig. 4. Schematic of illustrating the separation and transport
of charge carriers
26 Q. Zhou et al. /Microporous and Meexcellent photocatalytic
performance and cycling stability undervisible light. The
photodegradation rate of MO was up to 96.7%higher under visible
light irradiation using CdS/TiO2 nanotubearrays after 180 min. Xiao
et al. found that gold nanoparticle-func-tionalized TiO2 nanotube
arrays could also effectively enhance thephotocatalytic performance
of TiO2 nanotube arrays [28,29]. Theydiscovered that Au
nanoparticles acted as electron traps, thusprolonging the
separation lifetime of photoexcited electronholecharge carriers.
Iron oxide-modied TiO2 nanotube arrays weresynthesized and the
photocatalytic activity was investigated with2-naphthol as the
model pollutant [30]. The results indicated thatthe modied TiO2
nanotube arrays obtained by annealing at 873 Kafter anodization
showed high 2-naphthol degradation efciency,and the degradation
rate was 4.26 times higher than that ofunmodied TiO2 nanotube
arrays. This was due to iron oxide medi-ating the transmission of
electrons in the VB and CB of TiO2. Theelectrons then effectively
reduced the adsorbed O2 to form radicals,which had high
oxidizability and rapidly degraded the 2-naphtholinto small
molecular compounds. Chen et al. developed Au nano-particles and
used reduced graphene oxide (RGO) co-modiedTiO2 nanotube arrays to
serve to photocatalyze the degradationof methyl orange [31]. The
results exhibited that, in addition toAu, RGO could also capture
the photoinduced electrons of TiO2nanotube arrays to suppress the
recombination of the electronhole pairs. Owing to the simultaneous
electron transfer of TiO2nanotube arrays to Au and RGO, the minimal
recombination ofphotogenerated charges in Au/RGO-TiO2 nanotube
arrays resultedin more effective charge separation, which made it a
good photo-catalyst for the degradation of organic pollutants. The
results alsodemonstrated that the prepared photocatalyst displayed
high cat-alytic activity, excellent stability, and easy
recyclability.
Different semiconductors possess distinct band gaps in
whichphotocarriers can transfer light energy between
semiconductors,and a coupling effect would then be expected to
occur under illu-mination [32]. ZnTe is a semiconductor with a
narrow band gap of2.232.28 eV, which means that this material will
be excited undervisible light. So if this type of semiconductor is
introduced intoTiO2 nanotube arrays, the visible light response
will be increasedand lead to signicant enhancement of
photocatalytic ability. Liu
er visible light irradiation for TiO2 nanotube arrays, Fe3+/Fe4+
and a-Fe2O3 [22].
orous Materials 202 (2015) 2235et al. fabricated ZnTe-modied
TiO2 nanotube arrays using an elec-trodeposition method, and
investigated the resulting photocata-lytic activity under visible
light radiation. ZnTe-modied TiO2nanotube arrays displayed much
higher photocatalytic activitytowards 9-AnCOOH [33]. The mechanism
for the enhanced photo-catalytic activity was demonstrated in Fig.
5. Under the illumina-tion of both visible and UV light, the
photogenerated electrons inthe conduction band (CB) of ZnTe were
transferred to the conduc-tion band (CB) of TiO2 and then reacted
with O2 to produce O2 rad-icals, which combined with H+ from
hydrogen peroxide (H2O2). Asthe holes of the valence band of TiO2
moved to the valence band ofZnTe, they then reacted with H2O2 to
form hydroxyl radicals (OH).The OH radicals then oxidized 9-AnCOOH
into the end-products.Zhu et al. prepared BiFeO3-modied TiO2
nanotube arrays as thephotoelectrocatalytic catalyst [34]. Their
results showed that thecomposites had stronger absorption in the
visible region and muchhigher photocatalytic efciency than the
undoped TiO2 nanotubearrays with rhodamine B as the model
pollutant. The modicationgave rise to a synergistic effect between
the lowered electronholerecombination rate and the wider spectral
response. ZnFe2O4/TiO2nanotube arrays were prepared by an
electrodeposition method[35], in which Zn2+ and Fe3+ were reduced
to Zn and Fe, and thentreated with oxidation to form the proposed
catalyst. From the sur-face photovoltaic spectra (SPV), it was
found that the adsorptionarea of modied TiO2 nanotube arrays
extended from the UV tothe visible light region, when compared to
the unmodied one.Under UV illumination, the degradation efciency of
4-chlorophe-nol (4-CP) by loaded TiO2 nanotube arrays was 1.08
times greaterthan that of unmodied TiO2 nanotube arrays, and at a
bias
-
Zhang et al. modied TiO2 nanotube arrays with reduced
grapheneoxide (RGO) and PbS nanoparticles (NP) in one step [40].
Theresults showed that PbS was successfully dispersed inside and
out-
O2 n
Fig. 6. UVvis diffuse reectance absorption spectra of the (a)
plain TiO2 nanotubearrays, (b) CdS/TiO2 nanotube arrays, and (c)
sonication-CdS/TiO2 nanotube arrays[38].
esoppotential of 0.8 V, all contaminants could be degraded by
loadedTiO2 nanotube arrays. TiO2 nanotube arrays loaded with
theBi2O3 nanoparticles also achieve better
photoelectrochemicalproperties [36], and the experimental results
showed that the pho-tocurrent could be easily observed in the
Bi2O3/TiO2 nanotubearrays, due to Bi2O3 enhancement of the
photocurrent. Theremoval rate of 4-CP with Bi2O3/TiO2 nanotube
arrays was 2 timesthan that of unmodied arrays under the same
conditions. Theband gap of Bi2O3 is 2.85 eV and can be excited by
the light withwavelength less than 435 nm, but the photocatalytic
activity ofBi2O3 is very low due to the high recombination rate of
electronand hole pairs in Bi2O3. However, the electrons of the VB
of TiO2can be transferred to the VB of Bi2O3 under visible light,
and holesgenerated in TiO2 can initiate the photocatalytic
reactions, whichenhance the photocatalytic activity of the
catalyst.
Zhu et al. fabricated CdS/TiO2 nanotube arrays by an
electro-chemical atomic layer deposition method [37] in which CdS
wasloaded on the inner and outer of the walls of TiO2 nanotube
arrays.This method could also reduce the CdS deposited at the
entrance ofthe nanotube to avoid pore-clogging. In addition, this
coaxial het-erogeneous structure signicantly enlarged the contact
areabetween CdS/TiO2 and the CdS/electrolyte, which decreased
thetravel distance that electrons and holes must move to react
withpollutants, thus increasing the adsorption of protons and the
pho-tocurrent of modied TiO2 nanotube arrays. The results
showedthat the introduction of CdS increased the photocatalytic
activityof TiO2 nanotube arrays by 5-fold over that of unmodied
TiO2
Fig. 5. Illustrative diagrams of the electron and hole transfer
in ZnTe/Ti
Q. Zhou et al. /Microporous and Mnanotube arrays. Xie et al.
[38] developed a new sonication-assisted chemical batch
electrodeposition approach to prepareCdS quantum dots (QDs)
sensitized TiO2 nanotube arrays. Theresults demonstrated that the
absorption spectrum of the modiedarrays signicantly moved into the
visible region approaching550 nm (Fig. 6). When compared with a
conventional sequentialchemical bath deposition method, this method
can modify theCdS QDs both on the nanotube and on the tube walls,
which caneffectively reduce clogging. The photocatalytic
degradation ratetowards methyl orange was 1.158 times higher than
that ofunmodied TiO2 nanotube arrays. The band gap of the
sonica-tion-CdS/TiO2 nanotube arrays was calculated to be 2.20 eV.
Itwas much lower than the band gap of TiO2 nanotube arrays,
whichmeant that it could absorb more visible light. Wang et al.
loadeddifferent amounts of Cu2O nanoparticles onto TiO2 nanotube
arraysusing ultrasonication-assisted sequential chemical bath
deposition[39]. Cu2O nanoparticles with narrow band gaps acted as
the sen-sitizer to promote the charge transfer to TiO2, which led
to efcientphotogenerated charge carrier separation. The modied TiO2
nano-tube array composite showed enhanced absorption of visible
lightand improved separation of photogenerated electrons and
holes.anotube arrays and the mechanism of photocatalysis
degradation [33].
orous Materials 202 (2015) 2235 27side the walls of TiO2
nanotube arrays and a reduced grapheneoxide lm was formed on the
top of the surface of TiO2 nanotubearrays. Almost 100% of added
pentachlorophenol was removed in120 min vs. only 61% using bare
TiO2 nanotube arrays. The reducedgraphene oxide surface lm could
successfully suppress the photo-corrosion of PbS, which led to high
photoactivity of these modiedTiO2 nanotube arrays. They also
replaced PbS nanoparticles withAg nanoparticles and investigated
their photocatalytic activitytowards 2,4-dichlorophenoxyacetic
acid. The obtained catalyst alsoachieved good photocatalytic
activity and 93% of the target pollu-tant was degraded [41].
TiO2 nanoparticles have also exhibited good
photocatalyticactivity, although their structure is different from
TiO2 nanotubearrays. The combination of these two distinct types of
TiO2 materi-als has been shown to exhibit excellent photocatalytic
perfor-mance. Zhang et al. prepared novel high-activity
TiO2nanoparticle-lled TiO2 nanotube arrays using
vacuum-assistedlling methods [42]. These novel arrays demonstrated
theexpected properties and 4-fold higher degradation rate for
MOthan that of unmodied TiO2 nanotube arrays, presumably due toan
increase in reaction sites contributed by highly active TiO2
-
and 1000 ppm of H2. Meanwhile, Pd-modied TiO2 nanotube
chain. In order to evaluate environmental safety, it is
important
sopnanoparticles on the surface of TiO2 nanotube. Deng et al.
reportedBi2WO6-modied TiO2 nanotube arrays synthesized by
hydrother-mal deposition [43]. The addition of Bi2WO6 led to the
shifting ofthe absorption band edges to higher wavelengths and
exhibitedstronger light absorption in both UV and visible light
regions.Due to the enhanced separation of photogenerated
electronholepairs, the photocurrent of the modied TiO2 nanotube
arrays was4 times higher than that of the unmodied annealed TiO2
nanotubearrays. The photocatalytic activity of Bi2WO6/TiO2 nanotube
arrayswas 2 times higher than that of the unmodied TiO2
nanotubearrays under visible light irradiation, and was about 1.5
times asmuch as that with Bi2WO6.
2.2.3. TiO2 nanotube array heterojunctionsA Z-scheme system, by
mimicking natural photosynthesis, was
proposed to enhance photocatalytic efciency, which combinestwo
photoexcitation systems and an electron-transfer mediator[44]. Xie
et al. reported a Z-scheme type CdSAgTiO2 nanotubearray [45]. They
found that the recombination rate of photogener-ated carriers was
reduced on CdS or Ag nanoparticle-modied TiO2nanotube arrays, and
also for the CdSAgTiO2 nanotube array sys-tem. The photocurrent
density of CdSAgTiO2 nanotube arraysexceeded that of the AgTiO2,
CdSTiO2, and TiO2 nanotube arraysystems under UV light irradiation
at 2 h. For the degradation ofMB, the removal rate was about 63%
under UV irradiation by 2 h,and only 23% could be degraded when
using pure TiO2 nanotubearrays. The enhanced performance could be
explained by a two-step excitation of CdS and TiO2, and Ag as a
mediator which ledto efcient charge separation of the
photogenerated electronholepairs.
Wang et al. synthesized graphite-like carbon-modied TiO2nanotube
arrays by impregnating TiO2 nanotube array lms in asucrose solution
[53]. They found that the photoresponse initiallyincreased with
increasing sucrose concentration and decreasedwhen the sucrose
concentration exceeded 0.01 g mL1, achievingtwice the photoresponse
of TiO2 nanotube arrays under the sameconditions. The EIS spectra
and HR-TEM image of these modiedTiO2 nanotube arrays demonstrated
that the graphite-like carbonlayer that formed on the sample
surface and nanotube walls playeddifferent roles in
photoelectrocatalytic responses. The two graph-ite-like carbon
layers on the nanotube walls could decrease thedepleting and
Helmholtz layer resistance and then enhance theseparation of
charges. On the contrary, the graphite occulesmainly acted as a
light blocking layer, decreasing photoabsorption.The optimum
photoresponse of TiO2 nanotube arrays was observedunder UV
illumination when the sucrose concentration was0.1 mg L1, the
response was twice as high as that of pure TiO2nanotube arrays and
three times higher than that of pure TiO2nanotube arrays under the
illumination of visible light. The syner-getic effect between
carbon and TiO2 nanotube arrays resulted inthe high efciency of the
charge separation process, thus enhanc-ing photoelectrocatalytic
activity.
Although many improvements on TiO2 nanotube arrays
asphotocatalysts have been achieved, there are still some
deciencieswhich need to be resolved. Until now, the approach to
enhance theutilization of visible light has gained some progress
and opens thedoor to greater prospects, but there is still much
work ahead tosolve practical difculties in wastewater treatment
methodologies.Meanwhile the most widely-researched model pollutants
are sim-ple dyes such as MO and MB, which differ from real
industrial pol-lutants that are released into the environment.
Moreover, manycomplex pollutants and persistent organic pollutants
haveattracted much more attention and seriously threaten the
environ-
28 Q. Zhou et al. /Microporous and Mement and human health.
Thus, nanotube arrays and other methodsshould be applied to develop
methods to achieve degradation ofthese compounds.to develop rapid
and sensitive methods for monitoring them atlow levels. Many
methods have been developed based on ameatomic absorption
spectrometry (FAAS), graphite furnace atomicabsorption spectrometry
(GFAAS), inductively coupled plasmamass spectrometry (ICP-MS),
atomic uorescence spectrometry(AFS), and inductively coupled plasma
atomic emission spectrom-etry, etc. In addition to these methods,
sensors provide a noveltechnique for monitoring heavy metal
pollutants.
A DNA-modied TiO2 nanotube array sensor was reported forthe
determination of Pb2+ in water samples [54]. The determina-tion
procedure involved two steps. First, Pb2+ was applied to thesensor
by immersing it into the sample solution. Second, the elec-trode
was rinsed and then transferred to an electrolytic cell with-out
Pb2+, and a differential pulse anodic stripping voltammetry(DPASV)
method was utilized to determine Pb2+concentration.The results
showed that the experimental values detected by thissensor were in
agreement with that of AAS for real samples (Fig. 7).arrays have
the potential to signicantly increase the sensitivityof this sensor
by up to 107-fold with a shorter response time[59]. Due to its good
insulating performance and arc extinction,SF6 has been widely used
in gas-insulated switchgears (GIS). Wheninsulation faults occur in
GIS, discharging electrical energy causesthe SF6 gas to undergo a
decomposition reaction and then generategases such as SOF4, SOF2,
SO2F2, and SO2. Thus, there is an increas-ing interest to develop
such gas sensors. Zhang et al. [56] used TiO2nanotube arrays as a
sensor to detect SO2. They found that the sen-sitivity of the
sensor increased with increase in temperature, and inaddition, the
response time decreased, because higher tempera-tures accelerated
the movement and diffusion of the molecules.At the optimal working
temperature of 200 C, they also found thatthe higher the
concentration of SO2 gas, the higher the response(sensitivity).
There was a good linear relationship between theresponse signal and
the concentration over the range of 1050 ppm with a relation
coefcient of 0.992, which indicated thatthis sensor could be used
to determine SO2 gas at low levels.
3.1.2. Detection of heavy metal ionsToxic heavy metal ions in
water and soils pose a serious threat
to the environment and human beings when they enter the food3.
Environmental analytical chemistry
3.1. Sensors
Environmental pollutants have attracted much attention due
totheir toxic effects on human health and environment. In order
toevaluate their environmental safety and prevent human exposureto
naturally and industrially generated inorganic and organic
con-taminants, it is of great value to develop rapid, sensitive,
and low-cost monitoring methods and devices. To date, many useful
androbust methods have been developed. Sensors are important
andvaluable tools for the detection of hazardous pollutants. TiO2
nano-tube arrays have great potential to play a role in development
ofnew sensors. Recently, electrochemical sensors based on
TiO2nanotube arrays have been employed for a variety of
applications,including monitoring of heavy metals [54], amines
[55], SO2 [56],glucose [57] and hydrogen [58].
3.1.1. Gas monitoringIn 2003, Grimes et al. found that TiO2
nanotube arrays could be
used as a good sensor for H2 and showed a sensitivity between
104
orous Materials 202 (2015) 2235Arsenic is an important inorganic
pollutant, requiring rapid andsensitive detection methods.
Currently there are many useful ana-lytical methods based on
different principles. Yang et al. fabricated
-
reproducibility [66]. The new TiO2 nanotube array method also
has
ube
esopAu shrubs-modied TiO2 nanotube arrays as a novel sensor
todetect the concentration of arsenic [60]. This composite
possesseda high surface area compared with other modied TiO2
nanotubearrays. The results showed a high sensitivity between
currentchanges and concentration of arsenic with a value of 25.7 lA
cm2
at 5 lg L1 As3+. However, only 10.6 lA cm2 was obtained whenAu
lm-modied TiO2 nanotube arrays were used, which indicatedthat more
surface area and the unique 3D structures accounted forthe high
performance.
3.1.3. Detection of organic pollutantsCai et al. reported a
sensor composed of molecularly imprinted
polymer-modied TiO2 nanotube arrays for peruorooctane sulfo-nate
in water samples. The direct detection of peruorooctane sul-fonate
by electrocatalytic reduction reaction was fullled usingmodied TiO2
nanotube arrays with a detection limit of86 ng mL1. The selectivity
of this sensor was also very good[61]. Cai et al. developed an
octachlorostyrene (OCS) photoelectro-chemical (PEC) immunosensor by
cross-linking anti-OCS antibodyonto CdTe/CdS-sensitized TiO2
nanotube arrays [62]. The PECimmunosensor exhibited high specicity
and high sensitivity witha limit of detection of 2.58 pM, and a
linear range from5 pM 50 nM. Due to the excellent photoelectronic
performanceof the CdTe/CdSTiO2 nanotube arrays, the label-free PEC
immuno-sensor showed a highly sensitive and selective response to
OCS.The testing time was 4 min. Compared with conventional
opticalmethods, the PEC immunoassay was simpler in
instrumentationand more easily miniaturized.
3.2. Measurement of COD
Chemical oxygen demand (COD) is an important parameter forthe
evaluation of water pollution and is also the most commonitem in
water monitoring. The most commonly-used method todetermine COD is
the potassium bichromate method, which hasadvantages such as
reliable results and good reproducibility, butthe defects are also
obvious. For example, the procedure takes arelatively long time (24
h), and consumes a large quantity ofexpensive and poisonous agents,
such as Ag2SO4 and HgSO4, etc.In recent years, many novel
technologies have been developed todetect COD [63,64] based on
electrochemical methods, photocata-lytic oxidation, and
photoelectrochemical oxidation. Recently, aTiO2 nanotube array was
used to develop a new determinationmethod for COD. Using
photoelectrochemical oxidation, Zhanget al. developed a new
determination method for COD using TiO2
Fig. 7. Schematic illustration for DNA/C-TiO2 nanot
Q. Zhou et al. /Microporous and Mnanotube arrays as the work
electrode [65]. They found that TiO2nanotube arrays, which were
prepared in the solution with 1%HF electrolyte (pH = 2) at the
anodic potential of 20 V and annealedat 450 C, showed the highest
photocurrent density. The principleof COD determination is based on
Faradays law, and the COD valuecould be calculated using the
following equation:
CODmg=L of O2 nC4 3200 Q4FV
3200 KQ
During the COD determination experiments, they found
thatinterferences such as pH variation and coexisting ions such
asNH4+ and Cl had no effect on the determination of COD.
Thisadvantages such as short experiment time and is free from
sometoxic reagents, which are often used in the conventional
method.The key to obtaining the real COD value is to oxidize the
organiccomponents completely, and the excellent photocatalytic
oxidabil-ity of TiO2 nanotube arrays makes them ideal electrode
materialsfor determination of COD. Chen et al. assessed performance
of thismethod using water samples containing some refractory and
lowconcentration organic compounds [67]. A comparison was
madebetween the theoretical chemical oxygen demand (ThCOD)
andresponse COD, and the related equation was used, as follows:
COD a ThCOD:They chose recalcitrant organic compounds including
sugars,
benzene derivatives, organic acids, alcohols, amino acids and
pyri-dines, etc. The COD values achieved using the
photoelectrochemi-cal sensor were larger than those obtained using
the CODCrmethod, especially in determination of the COD values of
pyridineand amino acid solutions, where the CODCr method showed
alower a value of 4.5% and 0.5%, while the
photoelectrochemicalmethod obtained a higher a value of 95.1% and
97.6%, which exhib-ited the super-oxidation capability of TiO2
nanotube arrays. Thisnew COD determination method based on TiO2
nanotube arrayspossessed many advantages such as effective
catalysis, fast masstransport, large effective surface area, and
good control over theelectrode microenvironment; these properties
make it applicableto water monitoring. A Cu2O-loaded TiO2 nanotube
arrays elec-trode was fabricated by an electrodeposition process
and used asa sensor to detect COD value [68]. By modication with
Cu2O,TiO2 nanotube arrays showed high absorption intensity in the
vis-ible light region and a much higher sensitivity to visible
light. Add-ing a positive bias potential of 0.3 V in visible light
achieved a lowdetection of limit of 15 mg L1 and a good linear
range of 20300 mg L1. However, environmental samples often have
unknownor changing matrix characteristics, which will result in
difculty inachieving precise concentrations of focused pollutants.
This chal-lenge will impact the development of measurement devices
[69].
3.3. Sample pretreatment
With the technological advances, trace pollutants havemethod
worked within a linear range of 0850 mg L1. Comparedwith the
conventional K2Cr2O7 method, this method, based onhighly ordered
structure of TiO2 nanotube arrays, resulted in good
arrays construction and its Pb ion monitoring [54].
orous Materials 202 (2015) 2235 29attracted more attention.
However, it is also difcult to achieveaccurate detection due to the
varieties of pollutants in water, soiland air, low concentrations
of the pollutants and strong matrixeffects. Thus, a sample
pretreatment step is necessary in the ana-lytical procedure. In
general, it is estimated that more than 60%of analysis time is
spent on the sample pretreatment, especiallyon trace and ultra
analysis. Enrichment is an effective method thatconcentrates trace
targets to the concentration that matches thedetection sensitivity
of instruments. The sample pretreatment stepnot only concentrates
the targets, but also cleans the samples,which decreases matrix
effects and increases the accuracy of ana-lytical methods.
-
Conventional sample treatment methods have been used formany
years such as liquidliquid extraction (LLE) and Soxhletextraction,
etc. However, the typical disadvantage of these meth-ods is the
usage of a large quantity of organic reagents, which willgenerate
secondary pollution. In recent years, some novel pretreat-ment
methods have been developed to address this issue. Solid-phase
micro-extraction (SPME) is a novel pretreatment methodbased on
partition equilibrium, which allows analytes to reachequilibrium
between stationary phase and liquid phase. Jianget al. fabricated
TiO2 nanotube arrays on a Ti wire, which was usedas the extraction
ber in SPME [70]. The highly ordered TiO2 nano-tube arrays showed
high selective adsorption ability to differentanalytes. The results
indicated that PAHs and alkanes could beeffectively adsorbed on a
TiO2 nanotube array with enrichmentfactors in the range of
82.696.9, while lower adsorption abilities
to the TiO2 nanotube. Large holes would decrease the specic
sur-face area, reducing the absorption ability, and a potential of
20 Vwas nally chosen. The other parameters that affected the
l-SPEEprocedure included the eluting solvent, pH value, salt
effect, equi-librium time and desorption time. After optimization
of conditions,the LODs were in the range of 0.0180.073 lg L1 (S/N =
3) and thelinear ranges were in the range of 0.180 lg L1 for
pyrethroidsbifenthrin, fenpropathrin and fyhalothrin, and 0.2160 lg
L1 and0.3210 lg L1 for fenvalreate and deltamethrin,
respectively.When this method was used for the determination of ve
fungi-cides, lower limits of detection were obtained in contrast to
thecommonly-used SPE method [80].
The surfactants could be adhered onto TiO2 nanotube
arraysthrough electrostatic interactions between the charges of
metallicoxide and opposing charges of the surfactants to form
micelles
(PAHs) [82]. The results showed that when the concentration
ofCTAB was 90 mg L1, the maximum adsorption capacity was
30 Q. Zhou et al. /Microporous and Mesoporous Materials 202
(2015) 2235were found for anilines and phenols. Limits of detection
were inthe range of 0.0010.1 lg L1 for the targeted PAHs. When
realwater samples were analyzed, the recoveries were in the rangeof
78.57119.28%. These results demonstrated that TiO2 nanotubearray
bers had many advantages over commercial SPME berssuch as high
rigidity, long lifetime and good resistance to pH vari-ation and
high temperature conditions.
Due to the excellent properties of TiO2 nanotube arrays,
ourgroup has focused on them and developed a new type of
pretreat-ment method called micro-solid phase equilibrium
extraction (l-SPEE) technique (Fig. 8) [6,7174]. Previously, we
investigatedthe applicability of TiO2 nanotube powders as the
adsorbent inSPE and developed many enrichment and determination
methodsfor monitoring copper [66], nickel [75] and cadmium [76],
ben-zoylurea insecticides [77], paraquat and diquat [78] and
DDTsand their main metabolites [79] in water samples. The
resultsshowed that TiO2 nanotubes had better enrichment
capabilitiesand the analytical methods exhibited low detection
limits for tar-geted analytes. The l-SPEE method developed recently
was alsobased on the equilibrium principle, and the TiO2 nanotube
arrayswere used as the adsorbent. The procedures of adsorption
anddesorption of pollutants on sorbent occurred at the same
time.When the rates of adsorption and desorption was equal, the
TiO2nanotube array was taken out and then a little solvent was
usedto elute analytes adsorbed on the TiO2 nanotube array. The
nalsolution was dried and redissolved with an appropriate
solventand analyzed with selected analytical instruments such as
GC,HPLC and AES, etc. We then used this developed method to
deter-mine trace levels of pyrethroids in environmental water
samples[71]. We rst investigated the effect of anodic potential on
theadsorption performance of a TiO2 nanotube array. The
nanotubearray prepared under low potential possessed a
small-diameterhole, which made pollutants difcult to transfer from
liquid phaseFig. 8. The principle of lobtained. With this modied
TiO2 nanotube array as the l-SPEEsorbent, a rapid and sensitive
determination method was devel-oped and the detection limits of 16
PAHs were obtained in therange of 0.0260.82 lg L1. Pan et al.
reported a new method forthe determination of PAHs with TiO2
nanotube arrays fabricatedon Ti wire and modied with Au
nanoparticles and n-octadeca-nethiol, and the detection limits of
selected PAHs were in the rangeof 0.13.0 ng L1 [83]. TiO2 nanotube
and modied TiO2 nanotubearrays will have many applications, and our
efforts will focus ondevelopment of highly selective, rapid,
reliable and sensitiveenrichment and determination methods for
trace pollutants suchas new persistent organic pollutants and
typical pollutants in theenvironment.
4. Applications in sensitized solar cells (SSCs)
Solar energy is a great renewable source and would serve as
theideal future clean energy supply for the world, due to its
accessibil-ity when compared with wind, nuclear and biomass energy.
Ef-cient solar cells must absorb enough light over a broad
spectralrange from visible to near-infrared (near-IR) wavelengths
(350950 nm) and convert the incident light effectively into
electricitywhen the concentration of the surfactant reached
critical micelleconcentration (CMC). As a result, the metallic
oxide surface coatedwith surfactants became hydrophobic, useful for
the enrichment ofpollutants. Niu et al. used a cetyltrimethyl
ammonium bromide(CTAB)-coated titanate nanotube as the adsorbent
for solid phaseextraction to enrich phthalate esters in water
samples, obtaininggood results [81]. We prepared CTAB-coated TiO2
nanotube arraysand investigated the adsorption of polyaromatic
hydrocarbons-SPEE method [82].
-
esop[84]. Among many solar cells devices, the dye-sensitized
solar cell(DSSC) is a new solar cell with special properties.
ORagan et al.rst developed this sensitized solar cell using a TiO2
particle lmas the photoanode [85]. Many efforts were dedicated to
enhancethe light conversion of these cells. Owing to the advantages
suchas their low cost, easy manufacture and high efciency,
DSSC-based TiO2 materials have been shown to be a potential
alternativeto conventional solid-state solar cells [8690]. As one
of the impor-tant components of solar cells, the photoanode has a
great effect onlight conversion efciency. The TiO2 nanotube array,
a new mate-rial with unique properties, has been used in
photovoltaic studieswhere many experiments have also veried
desirable properties[9194] such as highly ordered nanotube
structure which leadsto accelerated electron transport and an
ordered surface whichincreases sensitizer absorption [95]. Park et
al. transplanted TiO2nanotube arrays onto FTO (uorine-doped tin
oxide) glass [96],which can improve the performance of the
photoresponse whencompared with TiO2 nanoparticles. The efciency
was enhancedto 5.36% by a post-treatment with a TiCl4 solution. The
studyshowed treatment with TiCl4 could inhibit the recombination
ofcharges and accelerated the move of electrons, which increasedthe
charge density in the photoanode. They also fabricated a
nano-porous layer-covered TiO2 nanotube array in TiCl4 solution
[97].Both the electron transport rate and electron lifetime
wereimproved, and more surface defects were found on the surface
ofTiO2 nanotube arrays over conventional arrays, which
alsoincreased the performance of the DSSCs. Wang et al. rst
depositedTi lm on the FTO and then prepared TiO2 nanotube arrays on
it foruse as the photoanode in the DSSCs [98]. Their study showed
thatthe adhesive force between Ti lm and FTO depended on the
tem-perature of the sputtering process. At lower temperatures,
TiO2nanotube arrays were easily peeled off, while the stability
wasimproved in higher temperatures. This transparent
photoanodeexhibited a high conversion of light to electricity. A
bamboo-likestructured TiO2 nanotube array was fabricated by
altering theanodization voltage [99]. They found that the bamboo
rings couldprovide much larger surface area for dye loading which
led to ahigh conversion efciency. Wang et al. fabricated a
bamboo-typeTiO2 nanotube array by using a square-wave voltage
[100]. TheEIS measurements showed reduced interfacial resistance
andincreased the interfacial capacitance in the bamboo-type
TiO2nanotube arrays compared with the smooth type arrays.
Anincrease in surface area of the bamboo-type TiO2 nanotube
arraysresulted in dye loading in both the inner and outer walls,
whichincreased the conversion efciency by 7.36% when compared
withsmooth TiO2 nanotube arrays.
Luo et al. immersed the TiO2 nanotube arrays in deionizedwater
to remove Ti foil and then formed free-standing membranenanotube
arrays, and then coupled them with TiO2 nanoparticles,FTO and
electrolyte to form solar cells [101]. The improved lightscattering
performance and improved I3 diffusion were observedin this DSSC.
Wang et al. modied the TiO2 nanotube arrays withRu(dcbpy)2(NCS)2 as
a dye-sensitized solar cell [102]. They foundthat the TiO2 nanotube
arrays fabricated for 50 h showed the high-est conversion efciency,
however the morphology of the materialprepared for 60 h was
destroyed, which would provide morerecombination centers for
electrons, and cause the reduction inefciency. Wang et al. [92]
reported results after TiO2 nanoparti-cles were deposited onto TiO2
nanotube arrays. They found thatdeposition of TiO2 nanoparticles
can remarkably improve theabsorption of N3 dye with an enhancement
of 47.2% due to theincreasing surface area of nanostructure. A
conversion efciencyof 6.28% was achieved for DSSC with TiO2
nanoparticles as the ex-
Q. Zhou et al. /Microporous and Mible photoanodes. This
electrodeposition method of nanoparticleshas great potential uses,
such as in photovoltaic, photodegradationand sensor
applications.Liu investigated the effect of the length of nanotube
on the per-formance of solar cells [103]. The results indicated
that a longernanotube has a positive effect on photocurrent density
and conver-sion efciency. Meanwhile, the open-circuit photovoltage
wasdecreased, which offset the short-circuit photocurrent density,
sothe overall performance was improved.
Many studies have shown that the use of a photonic crystal
(PC)layer on top of a mesoporous TiO2 layer can enhance light
harvest-ing [104,105]. Huang et al. developed a single-step method
to cou-ple a PC layer to TiO2 nanotube arrays [106]. The TiO2
nanotubearrays layer was obtained by normal electrochemical
anodizationand the TiO2 PC layer was fabricated by a periodic
current pulseanodization. This bi-layer structure DSSC showed a
signicantlyenhanced power conversion efciency (PCE) of 50% over
that ofsingle layer DSSC. They proposed a novel photonic
crystal-basedphotoanode composed of a TiO2 nanoparticle (TiO2 NP)
absorptionlayer and a thin TiO2 nanotube array photonic crystal
(TiO2 NT PC).The TiO2 NP worked as the adsorbing layer and TiO2 NT
PC con-ferred the PC effect and acted as the scattering layer.
Comparedwith conventional TiO2 NP-based DSSCs, the PCE increased
by39.5% due to the combined effects [107].
Wang et al. reported a spiral structure of TiO2 nanotube
arrayson Ti wire and found that it could efciently trap scattered
light[108]. Alivov et al. transformed TiO2 nanotube arrays to
formTiO2 nanoparticle lms using similar annealing conditions as
usedto create the photoanodes in DSSC applications [109]. With a
nano-particle size of 65 nm, a maximum nominal efciency of 9.05%
wasobserved. The lowest efciency of 1.48% was observed for
DSSCswhen nanoparticle size was 350 nm, which indicated that
thenanoparticle size had a signicant inuence on the performanceof
the solar cells. Bandara et al. [110] fabricated solid-state
dye-sensitized solar cells with different thicknesses of
transparentTiO2 nanotube array electrodes coupled with a
Ru-(II)-donorantenna dye. A power conversion efciency of 1.94% was
obtained.They also found that a linear increase in the cell current
wasobserved with the increase in length of the TiO2 nanotube
arrays.Mirabolghasemi et al. fabricated single-walled TiO2
nanotubearrays and demonstrated that a signicant gain in electrical
andphotoelectrochemical properties could be reached with
theseunique tubes [111]. They also found that the short circuit
currentdensity and efciency of the solar cell were higher than
those fordouble-walled nanotube ones.
However, the cost of preparing dye-sensitized solar cells is
rel-atively high, which interferes with the commercialization of
DSSCs.This, it is necessary to look for substitutes that replace
dye in theconversion of light energy to electrical energy. New
sensitizedsolar cells such as quantum dot-sensitized solar cells
and hetero-junction solar cells have been introduced.
Semiconductors arewidely used in the modied TiO2 nanotube arrays to
improve thephotoelectrochemical properties by broadening the light
regionthat is absorbed and by reducing the recombination rate of
thephotogenerated electronhole pairs. In recent years, the
semicon-ductor material has received more attention and has been
usedas the sensitizer, replacing the dye sensitizer [112114].
Hossainet al. synthesized CdSe nanoclusters on highly-ordered TiO2
nano-tube arrays using chemical bath deposition [112]. That
studyshowed that parameters such as photocurrent, photovoltage,
llingfactor, and conversion efciency were enhanced signicantly
byincreasing the deposition time due to increased deposition of
CdSeonto TiO2 nanotube arrays under that condition. Because CdSe is
anarrow band gap material, it could be used to absorb the
longwavelength light in the visible spectrum. The results exhibited
thathigher conversion efciency was achieved when compared with
orous Materials 202 (2015) 2235 31other forms of CdSe QDs [115].
CdS nanoparticles adsorbed ontoTiO2 nanotube arrays could be also
used in solar cells [116]. Theresults indicated that the method
effectively decreased the
-
tube arrays from the TiO2 substrate and then fabricated a
TiO2/ZnO nanotube array heterojunction [117]. The results
indicated
age
sopthat the DSSCs based on the heterojunction exhibited a better
shortcircuit current density of 8.67 mA cm2 and a higher PCE of
3.98%under AM 1.5 illumination. Fan et al. reported results after
afxingAgAg2S hybrid nanoparticles onto TiO2 nanotube arrays,
whichexhibited a photocurrent density of 2.76 mA cm2 and
approxi-mately 92 times greater photoelectric efciency than that of
bareTiO2 nanotube arrays. The excellent performance of the
compositewas attributed to the surface plasmonic resonance effect,
whichwas further enhanced by an Ag2S outer-layer [118]. Wu et
al.reported that CdS-sensitized ZnO nanorod arrays on the TiO2
nano-tube arrays could be used to generate hydrogen from water
photo-electrolysis [119]. The existence of the one-dimensional
structureof TiO2 nanotube arrays, acting as an electron collector
and trans-porter, could provide a direct and quick electron pathway
for pho-toinjected electrons along the photoanode and reduce
electronhole recombination. The applications of TiO2 nanotube
arrays usedas the photoanode are shown in Table 2.
5. Hydrogen production
Hydrogen is regarded as an ideal energy resource and is
mainlyaggregation during modication and the photoelectric
conversionefciency increased by 65.8% when compared with the
sequentialchemical bath-deposition method. Ren et al. detached TiO2
nano-
Table 2TiO2 nanotube arrays as a photoanode in solar cells.
Photoanode Jsca Voca
TiO2NA/N719 6.48 0.76TiO2NA/N719/ 7.85 0.77TiO2NA/N719 12.39
0.637TiO2 NA/N719 7.63 0.68TiO2NA/Ti wire/N719 10.9
0.522TiO2NA/CdSe 7.19 0.438TiO2NA/N719 7.21 0.65P3HT@CdS@TiO2NA
3.00 0.7TiO2NA/Nanocrystalline CdS 5.17 0.77TiO2 NA/N719 15.00
0.59TiO2NA based on mash/N719 12.4 0.68TiO2NA/FTO/N719 15.46
0.814TiO2NA/CdS/CdSe/ZnS 13.52 0.48TiO2NA/N719 13.5 0.7
a Jsc = short circuit photocurrent density (mA cm2); Voc = open
circuit photovolt
32 Q. Zhou et al. /Microporous and Meproduced from natural gas
via steam methane reforming [128].However, hydrogen production from
the splitting of water hasbeen considered as another effective way
to solve the currentenergy shortage. Highly-ordered TiO2 nanotube
arrays have beenexplored as a catalyst for hydrogen production
[129]. The experi-mental results exhibited that the Pt
nanoparticles loaded on theTiO2 nanotube arrays by a microwave
irradiation methodenhanced hydrogen generation rate up to 0.613 ml
h1 cm2 com-pared with 0.313 ml h1 cm2 of the unmodied one. The
reasonwas that the Pt nanoparticles loaded onto the TiO2 nanotube
arrayswould trap the photoinduced charge carriers and accelerate
theinterfacial charge-transfer process, which increased the
photocata-lytic reaction rate. Kei reported a catalyst membrane
that consistedof TiO2 nanotube arrays and a Pd layer. The Pd layer
could be usedto separate H2 from other by-products [130]. The
effect of theannealing temperature of TiO2 nanotube arrays [131] on
hydrogenproduction was also investigated. The annealing temperature
had asignicant effect on the crystal phase, morphology and
photoelect-rochemical properties of TiO2 nanotube arrays. Low
annealing tem-perature had no effect on the crystal phase and
morphology of pre-existing material. With further increase in
temperature, the crystalphase transferred from anatase to the
rutile phase, resulting indestruction of the tubular structure for
vectorial charge transfer,resulting in a sharp decrease in
photocurrent. TiO2 nanotube arraysannealed at 450 C showed the
highest photoconversion efciencyof 4.49% and the highest hydrogen
production reported, a rate of122 lmol h1 cm2. Sang et al. reported
that TiO2 nanotube arrayscreated via sonoelectrochemical anodic
oxidation and annealed indifferent gases showed distinct
photoelectrochemical properties inhydrogen production [132], and
they fabricated different element-doped TiO2 nanotube arrays in
their studies [133]. C-doped TiO2nanotube arrays demonstrated more
surface-active sites and anegative at band potential, which
improved the photoelectro-chemical properties for hydrogen
production. Smith et al. reporteda facile method to synthetize
hierarchical TiO2 nanotube arrays byrst etching them in a solution
of HF and HNO3, followed by anod-ization [134]. The unique
morphology provided an increased sur-face area for light
utilization and accelerated the separation ofelectron and charge
pairs. Water splitting efciencies were 0.34%and 0.15% at 1.23 (RHE)
using hierarchical TiO2 nanotube arraysgrown on wires and foils,
respectively. This hydrogen generationrate was increased by over
40% for hierarchical TiO2 nanotubesarrays vs. plain TiO2 nanotube
arrays, and over 25% increased forwire substrate vs. foil.
Based on the same principles of photocatalytic oxidization
men-tioned in the pollutant- degradation section above, it is of
greatimportance to decrease the recombination rate of
electronholepairs within the TiO2 nanotube arrays during hydrogen
production.
FFa Conversion efciency (%) Refs.
0.58 3.18 [33]0.61 3.7 [94]0.5549 4.38 [98]0.708 3.68 [101]0.48
2.78 [103]0.495 1.56 [112]0.45 2.13 [120]0.55 1.16 [121]0.47 1.87
[122]0.49 4.29 [123]0.6 5 [124]0.641 8.070 [125]0.53 3.44 [126]0.7
6 [127]
(V); FF = ll factor; TiO2NA = TiO2 nanotube arrays.
orous Materials 202 (2015) 2235Many researchers have made great
strides in achieving this goal byloading semiconductors or metals
onto TiO2 nanotube arrays suchas Cu2O/Cu [135], and CdS (Fig. 9)
[136]. When CdS-modied TiO2nanotube arrays were used as electrodes
for hydrogen production[129], the presence of CdS nanoparticles led
to improved photo-electrochemical reactivity, which efciently
facilitated the separa-tion and transfer of photogenerated
electronhole pairs. Lai et al.used an impregnation method to load
WO3 onto the TiO2 nanotubearrays [137]. By controlling the soaking
time, different amounts ofWO3 loaded TiO2 nanotube arrays were
obtained. Their studyshowed that a small amount of WO3 could
signicantly improvethe PEC water splitting performance, which was
approximately1.5 times higher than that of pure TiO2 nanotube
arrays. An exces-sive amount of WO3 would decrease the
photocatalytic activity dueto formation of agglomerates. Palladium
quantum dots (Pd QDs)-sensitized TiO2 nanotube arrays were prepared
by hydrothermalmethod. When this material was used as the
photoanode, a hydro-gen production rate of 592 lmol h1 cm2 was
obtained, andwhich was 1.6 times than that of unmodied one. This
enhance-ment was ascribed to the synergetic effects between TiO2
and PdQDs, and Pd QDs acted as electron sinks and catalytic
centers,
-
s ele
esoporous Materials 202 (2015) 2235 33which reduced the
recombination rate of photogenerated electronsand holes and
increased the rate of water decomposition [138].Smith et al.
fabricated TiO2 nanotube arrays onto Ti wire, whichacted as the
catalyst to split water into hydrogen. An enhancementof 40% was
observed when compared with TiO2 nanotube arrays onTi foil [135].
Zhang et al. loaded carbon quantum dots onto the TiO2nanotube
arrays using a deposition method and prepared a carbonquantum
dots-sensitized catalyst for hydrogen production [139]. Itwas found
that enhanced optical absorption was seen in both thevisible and
NIR (near infrared) regions. The photocurrent densityof the
composite was 4 times higher than that of unmodiedTiO2 nanotube
arrays. The hydrogen production rate was about14.1 mmol h1 for a
carbon quantum dots-sensitized TiO2 nano-tube array under simulated
sunlight illumination (AM 1.5G,100 mW cm2). Zhang et al. [140] used
Cu(OH)2 modied TiO2nanotube arrays as the catalyst for hydrogen
production and foundthat the H2-production yield was 20.3 times
higher than that withunmodied TiO2 nanotube arrays due to the
synergistic effectbetween Cu(OH)2 and TiO2 nanotube arrays. Wang et
al. usedCdSe/CdS/TiO2 nanotube arrays for hydrogen production
[141].The results showed a 7-fold enhancement in photoconversion
ef-ciency and its band gap generated an obvious red shift to
broadenthe visible light response.
Hydrogen also can be generated from the decomposition of
Fig. 9. Conguration model consists of CdS/TNAs glasQ. Zhou et
al. /Microporous and Malcohols like ethanol. A gas phrase
photocatalytic decompositionof alcohols in high vacuum conditions
was reported [142]. TheH2 production of 2.8 108 Torr was obtained
by using Pt-modi-ed TiO2 nanotube arrays. The experimental results
showed thata longer TiO2 nanotube was favorable for hydrogen
generationdue to its ability to provide more reaction sites.
6. Photocatalytic conversion of CO2
The main energy sources currently are fossil fuels such as
petro-leum, natural gas and coal because of their usability,
stability andhigh caloric values. The large amounts of CO2
emissions are pro-duced during the burning or transforming of these
fossil fuels, withresulting greenhouse effects. How to effectively
reduce CO2 is animportant issue facing the countries around the
world. In addition,the shortage of fossil fuels and the need to nd
alternative renew-able and sustainable fuels are triggering
increasing interest in thephotocatalytic reduction of CO2
[143145].
TiO2 has been successfully incorporated in methodologies
forphotocatalytic conversion of CO2. Different crystalline phases
ofTiO2 have been evaluated for their performance in
photocatalyticreactions. It has been known that increasing the
annealing temper-ature changes the crystalline phase of TiO2,
causing an increase inthe composition of the rutile phase and
decrease in anatase phase.Because the highest photocatalytic
reactivity was observed in crys-tals composed of a mixture of
anatase and rutile phases, controlover the ratio of the phases
using temperature annealing, and thusthe overall reaction rate, was
observed [146]. For CO2 reduction,methane production rate in
visible light reached the highest levelswith annealing temperature
at 480 C and 680 C (Fig. 10) [147]. At480 C, crystals were mostly
(91%) anatase phase, while at 680 C,the crystals were mostly rutile
phase (91%). Thus, methane produc-tion was highest when both phases
were present but one predom-inated. It was postulated that crystals
containing anataserutileinterfaces possess more numerous active
sites for photoreductionthan in crystals with only one phase.
Varghese et al. also studied reduction of CO2 to methane,
butused actual sunlight and modied TiO2 nanotube arrays as a
cata-lyst [148]. They found that co-catalysis using Pt
nanoparticles andCu nanoparticles that were loaded onto the surface
of nanotubearrays had an important effect on the conversion rate,
and couldalso effectively drive the total conversion. The product
species var-ied owing to the amounts of loaded co-catalysts on the
TiO2 nano-tube arrays. Ultrane Pt nanoparticles distributed on
TiO2nanotube arrays acting as co-catalysts could provide many
reac-tion sites for CO2 conversion reaction, enhancing the
transforma-
ctrode and Pt cathode for hydrogen generation [136].tion rate
from CO2 and H2O to methane [149]. In general theconversion rate
was lower than 5% when using pure TiO2 nanotubearrays as the
catalyst, while Pt nanoparticle-modied TiO2 nano-tube arrays
exhibited a higher conversion rate of 25%. Meanwhile
Fig. 10. Methane production under 365 nm irradiation of TiO2
nanotube arraysannealed in different temperature [147].
-
Acknowledgements
sopThis work was nancially supported by the National
NaturalScience Foundation of China (21377167), Program for New
CenturyExcellent Talents in University (NCET-10-0813) and
ScienceFoundation of China University of Petroleum, Beijing
(KYJJ2012-01-15).
References
[1] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 28912959.[2] G.K.
Mor, O.K. Varghese, M. Paulose, K. Shankar, C.A. Grimes, Sol.
Energy
Mater. Sol. Cells 90 (2006) 20112075.[3] D. Gong, C. Grimes,
O.K. Varghese, W. Hu, R. Singh, Z. Chen, E.C. Dickey, J.
Mater. Res. 16 (2001) 33313334.[4] S.K. Mohapatra, K.S. Raja,
V.K. Mahajan, M. Misra, J. Phys. Chem. C 112 (2008)
1100711012.[5] P. Benvenuto, A.K.M. Ka, A. Chen, J. Electroanal.
Chem. 627 (2009) 7681.[6] Q. Zhou, Y. Huang, G. Xie, J. Chromatogr.
A 1237 (2012) 2429.[7] Y.-Y. Song, F. Schmidt-Stein, S. Bauer, P.
Schmuki, J. Am. Chem. Soc. 131 (2009)
42304232.[8] T.J. LaTempa, S. Rani, N. Bao, C.A. Grimes,
Nanoscale 4 (2012) 22452250.[9] P. Roy, S. Berger, P. Schmuki,
Angew. Chem. Int. Ed. 50 (2011) 29042939.[10] Z. Fang, Q.X. Zhou,
Acta Chim. Sinica 70 (2012) 17671774.[11] F. Al Momani, N. Jarrah,
Environ. Technol. 30 (2009) 10851093.[12] H. Wender, A.F. Feil,
L.B. Diaz, C.S. Ribeiro, G.J. Machado, P. Migowski, D.E.
Weibel, J. Dupont, S.R. Teixeira, ACS Appl. Mater. Interfaces 3
(2011) 1359a microwave-assisted modication method has been
developedand has been very useful for fabricating other
nanoparticle- mod-ied TiO2 nanotube arrays. Ping et al. used modied
TiO2 nanotubearrays as the photocatalyst for CO2 conversion [150].
The band gapof TiO2 nanotube arrays decreased by approximately 0.2
eV whencompared to TiO2 nanoparticles, which induced the nanotube
redshifted absorption edge and enhanced photocatalytic activities
ofTiO2 nanotube arrays. The products were predominately methanoland
ethanol, and the production rates of methanol and ethanolwere
calculated to be about 10 and 9 nmol cm2 h1.
The advances of CO2 transformation technologies will have
afar-reaching effect on environmental protection and
sustainabledevelopment. It makes sense that CO2 conversion to
hydrocarbonfuels would not only reduce the quantity of CO2 in the
atmosphere,but would also trap it in organic matter and speed up
the overallrecycling of carbon. Although much progress has been
achievedin recent years, the low utilization of solar energy, easy
recombina-tion of electrons and holes, and poor absorption of CO2
by TiO2nanotube arrays are problems that must be addressed before
wewill see a high conversion rate of CO2 which will have a
greatimpact on the world.
7. Conclusions
The new architecture of vertically-aligned TiO2 nanotube
arrayshas gained much interest during recent decades due to their
novelphysical attributes as well as their numerous potential
applicationsin various elds. The rapidly-developing techniques
based on TiO2nanotube arrays have provided new solutions to address
chal-lenges in the removal of environmental pollutants, greater
utiliza-tion of solar energy, decreases in greenhouse gases,
creation of newenergy sources and others. However, there is still
much work to do,including increasing the conversion efciency of
solar energy tocreate electrical and chemical energy, signicantly
improvingcatalysis in response to visible light, and developing new
catalystswith higher photocatalytic activities. TiO2 nanotube
arrays holdgreat promise in playing a key role in the development
of newtechnologies to address growing challenges that must be
overcometo assure a bright future for our world.
34 Q. Zhou et al. /Microporous and Me1365.[13] M. Tian, B.
Adams, J. Wen, R.M. Asmussen, A. Chen, Electrochim. Acta 54
(2009) 37993805.[14] H.-C. Liang, X.-Z. Li, J. Hazard. Mater.
162 (2009) 14151422.[15] S. Zhang, J. Qiu, J. Han, H. Zhang, P.
Liu, S. Zhang, F. Peng, H. Zhao, Electrochem.
Commun. 13 (2011) 11511154.[16] Q. Wang, J. Shang, T. Zhu, F.
Zhao, J. Mol. Catal. A 335 (2011) 242247.[17] A. Zhang, M. Zhou, L.
Han, Q. Zhou, J. Hazard. Mater. 186 (2011) 13741383.[18] Q. Li,
J.K. Shang, Environ. Sci. Technol. 43 (2009) 89238929.[19] G. Yan,
M. Zhang, J. Hou, J. Yang, Mater. Chem. Phys. 129 (2011)
553557.[20] J. Li, N. Lu, X. Quan, S. Chen, H. Zhao, Ind. Eng.
Chem. Res. 47 (2008) 3804
3808.[21] L. Sun, J. Li, C.L. Wang, S.F. Li, H.B. Chen, C.J.
Lin, Sol. Energy Mater. Sol. Cells 93
(2009) 18751880.[22] Q. Wu, J. Ouyang, K. Xiea, L. Sun, M. Wang,
C. Lin, J. Hazard. Mater. 199 (2012)
410417.[23] Z. Xu, W. Yang, Q. Li, S. Gao, J.K. Shang, Appl.
Catal. B 144 (2014) 343352.[24] K. Xie, L. Sun, C. Wang, Y. Lai, M.
Wang, H. Chen, C. Lin, Electrochim. Acta 55
(2010) 72117218.[25] X. Liu, Z. Liu, J. Lu, X. Wu, B. Xu, W.
Chu, Appl. Surf. Sci. 288 (2014) 513517.[26] L.K. Tan, M.K. Kumar,
W.W. An, H. Gao, ACS Appl. Mater. Interfaces 2 (2010)
498503.[27] L. Liu, J. Lv, G. Xu, Y. Wang, K. Xie, Z. Chen, Y.
Wu, J. Solid State Chem. 208
(2013) 2734.[28] F. Xiao, J. Mater. Chem. 22 (2012)
78197830.[29] F.-X. Xiao, RSC Adv. 2 (2012) 1269912701.[30] Y.
Muramatsu, Q. Jin, M. Fujishima, H. Tada, Appl. Catal. B 119 (2012)
7480.[31] Y. Chen, Y. Tang, S. Luo, C. Liu, Y. Li, J. Alloys Compd.
578 (2013) 242248.[32] M. Yang, J. Xu, J. Wei, J.-L. Sun, W. Liu,
J.-L. Zhu, Appl. Phys. Lett. 100 (2012)
253113.[33] Y. Liu, X. Zhang, R. Liu, R. Yang, C. Liu, Q. Cai,
J. Solid State Chem. 184 (2011)
684689.[34] A. Zhu, Q. Zhao, X. Li, Y. Shi, ACS Appl. Mater.
Interfaces 6 (2014) 671679.[35] Y. Hou, X.-Y. Li, Q.-D. Zhao, X.
Quan, G.-H. Chen, Adv. Funct. Mater. 20 (2010)
21652174.[36] X. Zhao, H. Liu, J. Qu, Appl. Surf. Sci. 257
(2011) 46214624.[37] W. Zhu, X. Liu, H. Liu, D. Tong, J. Yang, J.
Peng, J. Am. Chem. Soc. 132 (2010)
1261912626.[38] Y. Xie, G. Ali, S.H. Yoo, S.O. Cho, ACS Appl.
Mater. Interfaces 2 (2010) 2910
2914.[39] M. Wang, L. Sun, Z. Lin, J. Cai, K. Xie, C. Lin,
Energy Environ. Sci. 6 (2013)
12111220.[40] X. Zhang, Y. Tang, Y. Li, Y. Wang, X. Liu, C. Liu,
S. Luo, Appl. Catal. A 457 (2013)
7884.[41] Y. Wang, Y. Tang, Y. Chen, Y. Li, X. Liu, S. Luo, C.
Liu, J. Mater. Sci. 48 (2013)
62036211.[42] Z. Zhang, D. Pan, J. Feng, L. Guo, L. Peng, C. Xi,
J. Li, Z. Li, M. Wu, Z. Ren, Mater.
Lett. 66 (2012) 5456.[43] F. Deng, Y. Liu, X. Luo, D. Chen, S.
Wu, S. Luo, Sep. Purif. Technol. 120 (2013)
156161.[44] D. Walker, New Phytol. 121 (1992) 325345.[45] K.
Xie, Q. Wu, Y. Wang, W. Guo, M. Wang, L. Sun, C. Lin,
Electrochem.
Commun. 13 (2011) 14691472.[46] K. Huo, H. Wang, X. Zhang, Y.
Cao, P.K. Chu, ChemPlusChem 77 (2012) 323
329.[47] C. Liu, Y. Teng, R. Liu, S. Luo, Y. Tang, L. Chen, Q.
Cai, Carbon 49 (2011) 5312
5320.[48] Y.-F. Tu, S.-Y. Huang, J.-P. Sang, X.-W. Zou, Mater.
Res. Bull. 45 (2010) 224
229.[49] S. Zhang, F. Peng, H. Wang, H. Yu, S. Zhang, J. Yang,
H. Zhao, Catal. Commun.
12 (2011) 689693.[50] S. Zhang, S. Zhang, F. Peng, H. Zhang, H.
Liu, H. Zhao, Electrochem. Commun.
13 (2011) 861864.[51] H. Yang, C. Pan, J. Alloys Compd. 501
(2010) L8L11.[52] L. Yang, Y. Xiao, S. Liu, Y. Li, Q. Cai, S. Luo,
G. Zeng, Appl. Catal. A 94 (2010)
142149.[53] Y. Wang, J. Lin, R. Zong, J. He, Y. Zhu, J. Mol.
Catal. A 349 (2011) 1319.[54] M. Liu, G. Zhao, Y. Tang, Z. Yu, Y.
Lei, M. Li, Y. Zhang, D. Li, Environ. Sci.
Technol. 44 (2010) 42414246.[55] Z. Xu, J. Yu, Nanotechnology 21
(2010) 245501245506.[56] X. Zhang, J. Zhang, Y. Jia, P. Xiao, J.
Tang, Sensors 12 (2012) 33023313.[57] Z. Zhang, Y. Xie, Z. Liu, F.
Rong, Y. Wang, D. Fu, J. Electroanal. Chem. 650
(2011) 241247.[58] J. Lee, D.H. Kim, S.-H. Hong, J.Y. Jho, Sens.
Actuators B 160 (2011) 14941498.[59] G.K. Mor, K. Shankar, O.K.
Varghese, C.A. Grimes, J. Mater. Res. 19 (2004)
29892996.[60] L. Yang, S. Luo, F. Su, Y. Xiao, Y. Chen, Q. Cai,
J. Phys. Chem. C 114 (2010) 7694
7699.[61] T.T. Tran, J. Li, H. Feng, J. Cai, L. Yuan, N. Wang,
Q. Cai, Sens. Actuators B 190
(2014) 745751.[62] J. Cai, P. Sheng, L. Zhou, L. Shi, N. Wang,
Q. Cai, Biosens. Bioelectron. 50 (2013)
6671.[63] J.X. Qiu, S.Q. Zhang, H.J. Zhao, J. Hazard. Mater. 211
(2012) 381388.[64] C.A. Almeida, P. Gonzalez, M. Mallea, L.D.
Martinez, R.A. Gil, Talanta 97 (2012)
273278.[65] J. Zhang, B. Zhou, Q. Zheng, J. Li, J. Bai, Y. Liu,
W. Cai, Water Res. 43 (2009)
orous Materials 202 (2015) 223519861992.[66] Q. Zheng, B. Zhou,
J. Bai, L. Li, Z. Jin, J. Zhang, J. Li, Y. Liu, W. Cai, X. Zhu,
Adv.
Mater. 20 (2008) 10441049.
-
[67] H. Chen, J. Zhang, Q. Chen, J. Li, D. Li, C. Dong, Y. Liu,
B. Zhou, S. Shang, W. Cai,Anal. Methods 4 (2012) 17901796.
[68] C.Wang, J. Wu, P.Wang, Y. Ao, J. Hou, J. Qian, Sens.
Actuators B 181 (2013) 18.[69] C.M.A. Brett, Pure Appl. Chem. 73
(2001) 19691978.[70] H. Liu, D. Wang, L. Ji, J. Li, S. Liu, X. Liu,
S. Jiang, J. Chromatogr. A 1217 (2010)
18981903.[71] Y. Huang, Q. Zhou, J. Xiao, Analyst 136 (2011)
27412746.[72] Q. Zhou, Y. Huang, J. Xiao, G. Xie, Anal. Bioanal.
Chem. 400 (2011) 205212.[73] Q. Zhou, Z. Fang, RSC Adv. 4 (2014)
74717475.[74] Q. Zhou, W. Wu, G. Xie, Y. Huang, Anal. Methods 6
(2014) 295301.[75] X.N. Zhao, Q.Z. Shi, G.H. Xie, Q.X. Zhou, Chin.
Chem. Lett. 19 (2008) 865867.[76] Q.-X. Zhou, X.-N. Zhao, J.-P.
Xiao, Talanta 77 (2009) 17741777.[77] Y. Huang, Q. Zhou, G. Xie, H.
Liu, H. Lin, Microchim. Acta 172 (2011) 109115.[78] Q. Zhou, J.
Mao, J. Xiao, G. Xie, Anal. Methods 2 (2010) 10631068.[79] Q. Zhou,
Y. Ding, J. Xiao, G. Liu, X. Guo, J. Chromatogr. A 1147 (2007)
1016.[80] Y. Huang, Q. Zhou, G. Xie, Chemosphere 90 (2013)
338343.
[113] S. Huang, Q. Zhang, X. Huang, X. Guo, M. Deng, D. Li, Y.
Luo, Q. Shen, T.Toyoda, Q. Meng, Nanotechnology 21 (2010)
375201375207.
[114] Y. Xie, S.H. Yoo, C. Chen, S.O. Cho, Mater. Sci. Eng. B
177 (2012) 106111.[115] L. Wonjoo, K. Soon Hyung, K. Jae-Yup, B.K.
Govind, S. Yung-Eun, H. Sung-
Hwan, Nanotechnology 20 (2009) 335706.[116] X. Ma, Y. Shen, G.
Wu, Q. Wu, B. Pei, M. Cao, F. Gu, J. Alloys Compd. 538 (2012)
6165.[117] J. Ren, W. Que, X. Yin, Y. He, H.M.A. Javed, RSC Adv.
4 (2014) 74547460.[118] W. Fan, S. Jewell, Y. She, M.K.H. Leung,
Phys. Chem. Chem. Phys. 16 (2014)
676680.[119] L. Wu, J. Li, S. Zhang, L. Long, X. Li, C. Cen, J.
Phys. Chem. C 117 (2013) 22591
22597.[120] H.Y. Hwang, A.A. Prabu, D.Y. Kim, K.J. Kim, Sol.
Energy 85 (2011) 15511559.[121] Y. Hao, Y. Cao, B. Sun, Y. Li, Y.
Zhang, D. Xu, Sol. Energy Mater. Sol. Cells 101
(2012) 107113.[122] J.-Y. Hwang, S.-A. Lee, Y.H. Lee, S.-I.
Seok, ACS Appl. Mat. Interfaces 2 (2010)
13431348.
Q. Zhou et al. /Microporous and Mesoporous Materials 202 (2015)
2235 35[81] H. Niu, Y. Cai, Y. Shi, F. Wei, S. Mou, G. Jiang, J.
Chromatogr. A 1172 (2007)113120.
[82] Y. Huang, Q. Zhou, G. Xie, J. Hazard. Mater. 193 (2011)
8289.[83] D. Pan, C. Chen, F. Yang, Y. Long, Q. Cai, S. Yao,
Analyst 136 (2011) 47744779.[84] M.M. Lee, J. Teuscher, T.
Miyasaka, T.N. Murakami, H.J. Snaith, Science 338
(2012) 643647.[85] B. ORegan, M. Gratzel, Nature 353 (1991)
737740.[86] S. Ko, H. Choi, M.-S. Kang, H. Hwang, H. Ji, J. Kim, J.
Ko, Y. Kang, J. Mater. Chem.
20 (2010) 23912399.[87] J.Y. Kim, S. Lee, J.H. Noh, H.S. Jung,
K.S. Hong, J. Electroceram. 23 (2009) 422
425.[88] I. Kartini, D. Menzies, D. Blake, J.C.D. da Costa, P.
Meredith, J.D. Riches, G.Q. Lu,
J. Mater. Chem. 14 (2004) 29172921.[89] J. Dewalque, R. Cloots,
F. Mathis, O. Dubreuil, N. Krins, C. Henrist, J. Mater.
Chem. 21 (2011) 73567363.[90] B.W. Jing, M.H. Zhang, T. Shen,
Chin. Sci. Bull. 42 (1997) 19371948.[91] C. Xu, P.H. Shin, L. Cao,
J. Wu, D. Gao, Chem. Mater. 22 (2010) 143148.[92] S. Wang, J.
Zhang, S. Chen, H. Yang, Y. Lin, X. Xiao, X. Zhou, X. Li,
Electrochim.
Acta 56 (2011) 61846188.[93] S. Li, Y. Liu, G. Zhang, X. Zhao,
J. Yin, Thin Solid Films 520 (2011) 689693.[94] C. Rho, J.-H. Min,
J.S. Suh, J. Phys. Chem. C 116 (2012) 72137218.[95] J. Lee, K.S.
Hong, K. Shin, J.Y. Jho, J. Ind. Eng. Chem. 18 (2012) 1923.[96] H.
Park, W.-R. Kim, H.-T. Jeong, J.-J. Lee, H.-G. Kim, W.-Y. Choi,
Sol. Energy
Mater. Sol. Cells 95 (2011) 184189.[97] J.-H. Park, J.-Y. Kim,
J.-H. Kim, C.-J. Choi, H. Kim, Y.-E. Sung, K.-S. Ahn, J. Power
Sources 196 (2011) 89048908.[98] H. Wang, H. Li, J. Wang, J. Wu,
Mater. Lett. 80 (2012) 99102.[99] D. Kim, A. Ghicov, S.P. Albu, P.
Schmuki, J. Am. Chem. Soc. 130 (2008) 16454
16455.[100] S. Wang, X. Zhou, X. Xiao, Y. Fang, Y. Lin,
Electrochim. Acta 116 (2014) 2630.[101] J. Luo, L. Gao, J. Sun, Y.
Liu, RSC Adv. 2 (2012) 18841889.[102] S. Wang, W. Tan, J. Zhang, Y.
Lin, Chin. Sci. Bull. 57 (2012) 864868.[103] Z. Liu, M. Misra, ACS
Nano 4 (2010) 21962200.[104] S. Nishimura, N. Abrams, B.A. Lewis,
L.I. Halaoui, T.E. Mallouk, K.D. Benkstein,
J. van de Lagemaat, A.J. Frank, J. Am. Chem. Soc. 125 (2003)
63066310.[105] J.I.L. Chen, G. von Freymann, V. Kitaev, G.A. Ozin,
J. Am. Chem. Soc. 129 (2007)
11961202.[106] C.T. Yip, H. Huang, L. Zhou, K. Xie, Y. Wang, T.
Feng, J. Li, W.Y. Tam, Adv. Mater.
23 (2011) 56245628.[107] M. Guo, K. Xie, J. Lin, Z. Yong, C.T.
Yip, L. Zhou, Y. Wang, H. Huang, Energy
Environ. Sci. 5 (2012) 98819888.[108] Y. Wang, Y. Liu, H. Yang,
H. Wang, H. Shen, M. Li, J. Yan, Curr. Appl. Phys. 10
(2010) 119123.[109] Y. Alivov, Z.Y. Fan, J. Mater. Sci. 45
(2010) 29022906.[110] J. Bandara, K. Shankar, J. Basham, H.
Wietasch, M. Paulose, O.K. Varghese, C.A.
Grimes, M. Thelakkat, Eur. Phys. J. Appl. Phys. 53 (2011)
2026120266.[111] H. Mirabolghasemi, N. Liu, K. Lee, P. Schmuki,
Chem. Commun. 49 (2013)
20672069.[112] M.F. Hossain, S. Biswas, Z.H. Zhang, T.
Takahashi, J. Photochem. Photobiol. A
217 (2011) 6875.[123] P. Zhong, W. Que, J. Chen, X. Hu, J. Power
Sources 210 (2012) 3841.[124] C.S. Rustomji, C.J. Frandsen, S. Jin,
M.J. Tauber, J. Phys. Chem. B 114 (2010)
1453714543.[125] B.-X. Lei, J.-Y. Liao, R. Zhang, J. Wang, C.-Y.
Su, D.-B. Kuang, J. Phys. Chem. C
114 (2010) 1522815233.[126] H.J. Lee, J. Bang, J. Park, S. Kim,
S.-M. Park, Chem. Mater. 22 (2010) 5636
5643.[127] P.-T. Hsiao, Y.-J. Liou, H. Teng, J. Phys. Chem. C
115 (2011) 1501815024.[128] J.A. Turner, Science 305 (2004)
972974.[129] K.-C. Sun, Y.-C. Chen, M.-Y. Kuo, H.-W. Wang, Y.-F.
Lu, J.-C. Chung, Y.-C. Liu, Y.-
Z. Zeng, Mater. Chem. Phys. 129 (2011) 3539.[130] M. Hattori, K.
Noda, K. Matsushige, Appl. Phys. Lett. 99 (2011).[131] Y. Sun, K.
Yan, G. Wang, W. Guo, T. Ma, J. Phys. Chem. C 115 (2011) 12844
12849.[132] L.X. Sang, Z.Y. Zhang, C.F. Ma, Int. J. Hydrogen
Energy 36 (2011) 47324738.[133] L.-X. Sang, Z.-Y. Zhang, G.-M. Bai,
C.-X. Du, C.-F. Ma, Int. J. Hydrogen Energy 37
(2012) 854859.[134] Y.R. Smith, B. Sarma, S.K. Mohanty, M.
Misra, Int. J. Hydrogen Energy 38
(2013) 20622069.[135] Z. Li, J. Liu, D. Wang, Y. Gao, J. Shen,
Int. J. Hydrogen Energy 37 (2012) 6431
6437.[136] J. Bai, J. Li, Y. Liu, B. Zhou, W. Cai, Appl. Catal.
A 95 (2010) 408413.[137] C.W. Lai, S. Sreekantan, Int. J. Hydrogen
Energy 38 (2013) 21562166.[138] M. Ye, J. Gong, Y. Lai, C. Lin, Z.
Lin, J. Am. Chem. Soc. 134 (2012) 1572015723.[139] X. Zhang, F.
Wang, H. Huang, H.T. Li, X. Han, Y. Liu, Z.H. Kang, Nanoscale 5
(2013) 22742278.[140] S. Zhang, H. Wang, M. Yeung, Y. Fang, H.
Yu, F. Peng, Int. J. Hydrogen Energy
38 (2013) 72417245.[141] H. Wang, W. Zhu, B. Chong, K. Qin, Int.
J. Hydrogen Energy 39 (2014) 9099.[142] M. Hattori, K. Noda, K.
Kobayashi, K. Matsushige, Gas phase photocatalytic
decomposition of alcohols with titanium dioxide nanotube arrays
in highvacuum, in: S. Fujita (Ed.), Physica Status Solidi C:
Current Topics in SolidState Physics, vol. 8, WILEY-VCH Verlag
GmbH, Pappelallee 3, W-69469Weinheim, Germany, 2011, pp.
549551.
[143] G. Liu, K. Wang, N. Hoivik, H. Jakobsen, Sol. Energy
Mater. Sol. Cells 98 (2012)2438.
[144] S.A.A. Yahia, L. Hamadou, A. Kadri, N. Benbrahim, E.M.M.
Sutter, Catal. Today185 (2012) 263269.
[145] N. Ahmed, M. Morikawa, Y. Izumi, Catal. Today 185 (2012)
263269.[146] J. Yu, B. Wang, Appl. Catal. B 94 (2010) 295302.[147]
K.L. Schulte, P.A. DeSario, K.A. Gray, Appl. Catal. B 97 (2010)
354360.[148] O.K. Varghese, M. Paulose, T.J. LaTempa, C.A. Grimes,
Nano Lett. 9 (2009) 731
737.[149] X. Feng, J.D. Sloppy, T.J. LaTemp, M. Paulose, S.
Komarneni, N. Bao, C.