ORIGINAL PAPER Photoelectrochemical water splitting over mesoporous CuPbI 3 films prepared by electrophoretic technique Rabia Naeem 1 • Rosiyah Yahya 1 • Muhammad Adil Mansoor 1 • Mohd Asri Mat Teridi 2 • Mehran Sookhakian 1 • Asad Mumtaz 3 • Muhammad Mazhar 1 Received: 20 May 2016 / Accepted: 21 November 2016 Ó Springer-Verlag Wien 2017 Abstract Copper lead iodide, CuPbI 3 , nano powder has been synthesized by co-precipitation of PbI 2 on aqueous suspension of CuI followed by solid-state chemical reac- tion; its film was fabricated on FTO substrates by electrophoretic deposition technique and tested for photo- electrochemical water splitting using solar light. The synthesized CuPbI 3 nano powder has been characterized by thermogravimetric, differential thermal analysis, differen- tial scanning calorimetry, and X-ray diffraction. The surface morphology and the elemental composition of the electrophoretically deposited film have been characterized by XRD, Raman spectroscopy, X-ray photoelectron spec- troscopy, field emission scanning electron microscope, and energy dispersive X-ray mapping. The direct band gap energy of CuPbI 3 film of average thickness 96 lm has been estimated at 1.82 eV and the film shows a current density of 216 lA/cm 2 at 0.62 V measured in 0.1 M Na 2 SO 4 solution vs. Ag/AgCl/3 M KCl. This study explores the viability of synthesis of a variety of inorganic halide pho- tocatalysts for enhanced stability and improved solar capturing. Graphical abstract Keywords Cuprous lead iodide Solid solution film Electrophoresis Water splitting Introduction Metal halide perovskites have been demonstrated to be a relatively new class of photoactive materials for optoelec- tronic applications [1–4], high-efficiency photovoltaic cells [2], light emitting diodes [3], dye-sensitized solar cells [5–8], lasers [4], and photodetectors [9]. The oxide perovskite and organic/inorganic hybrid perovskite materials have revealed their potential in photoelectrochemical and photovoltaic applications [10–16]. When using optimized synthesis and fabrication procedures, carefully chosen perovskite design exhibits broad absorption spectra in the UV region making them excellent harvesters that demonstrate properties as either n- or p-type semiconductors with close to optimal band gaps for solar energy conversion. These favourable charac- teristics coupled with the feasibility of their fabrication by solution processing and low production costs make them an excellent candidate as optoelectronic materials [17–19]. Recently, developed organic/inorganic metallic halide Electronic supplementary material The online version of this article (doi:10.1007/s00706-016-1880-x) contains supplementary material, which is available to authorized users. & Muhammad Mazhar [email protected]1 Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia 2 Solar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia 3 Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak Darul Ridzuan, Malaysia 123 Monatsh Chem DOI 10.1007/s00706-016-1880-x
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ORIGINAL PAPER
Photoelectrochemical water splitting over mesoporous CuPbI3films prepared by electrophoretic technique
Rabia Naeem1• Rosiyah Yahya1 • Muhammad Adil Mansoor1 • Mohd Asri Mat Teridi2 •
Mehran Sookhakian1 • Asad Mumtaz3 • Muhammad Mazhar1
Received: 20 May 2016 / Accepted: 21 November 2016
� Springer-Verlag Wien 2017
Abstract Copper lead iodide, CuPbI3, nano powder has
been synthesized by co-precipitation of PbI2 on aqueous
suspension of CuI followed by solid-state chemical reac-
tion; its film was fabricated on FTO substrates by
electrophoretic deposition technique and tested for photo-
electrochemical water splitting using solar light. The
synthesized CuPbI3 nano powder has been characterized by
exhibits broad absorption spectra in the UV region making
them excellent harvesters that demonstrate properties as
either n- or p-type semiconductorswith close to optimal band
gaps for solar energy conversion. These favourable charac-
teristics coupled with the feasibility of their fabrication by
solution processing and low production costs make them an
excellent candidate as optoelectronic materials [17–19].
Recently, developed organic/inorganic metallic halide
Electronic supplementary material The online version of thisarticle (doi:10.1007/s00706-016-1880-x) contains supplementarymaterial, which is available to authorized users.
purity, and optical band gap, respectively. Further potential
of these thin-film electrodes for harvesting sunlight for the
generation of photoelectric current was investigated in the
presence of 0.1 M Na2SO4 and is reported here.
Results and discussion
The solid-state synthesis of CuPbI3 was carried out using a
precipitated mixture of CuI and PbI2 from an aqueous
solution. Since PbI2 is soluble in DMF, while CuI is
insoluble, we precipitated PbI2 in slight excess over CuI to
ensure complete utilization of the latter in solid-state
reaction, while excess of the former was removed from the
product by washing with DMF. Henceforth, a dry mixture
of co-precipitated CuI-PbI2 (1.65 g) was charged in a
5 cm3 Pyrex glass ampule, evacuated, and then sealed
under vacuum. The charged ampule was placed in a hori-
zontal tube furnace and heated at 620 �C for 72 h before it
was cooled to room temperature. The mustard-coloured
final product was mechanically removed from the ampule,
ground to a fine powder in an agate pestle and mortar and
washed with several 5 cm3 portions of DMF to ensure
complete removal of unreacted PbI2 to yield 99.9% pure
CuPbI3 (m.p.: 307 �C). The synthesized CuPbI3 is stable in
air and is insoluble in common polar and nonpolar solvents.
Pyrolytic and structural aspects
The thermal decomposition pattern of CuPbI3 was exam-
ined by thermogravimetric/derivative thermogravimetric
(TG/DTG) analyses, performed in the temperature range of
50–900 �C, under a flow of dinitrogen (20 cm3 min-1) at a
heating rate of 20 �C min-1. The TG/DTG plot (Fig. 1)
shows no appreciable loss in weight until the temperature
reaches to 432 �C where its pyrolysis begins. This pyrol-
ysis step is sharp and is completed at 693 �C with a mass
loss of 53.31% of the original weight of the sample. TG/
DTG curves also indicate that there is no mentionable loss
Fig. 1 Simultaneous TG and DTG plots of CuPbI3 recorded under an
inert atmosphere of nitrogen gas with constant flow of 20 cm3 min-1
and heating rate of 20 �C min-1
R. Naeem et al.
123
in weight at the start in the temperature range of
50–400 �C, which indicates that CuPbI3 is thermally
stable from room temperature to above its melting point of
307 �C. Further heating to 900 �C yields a stable residual
mass of 41.53% of the original weight of the sample
indicating the formation of Cu-Pb (1:1) alloy with the
liberation of iodine as indicated in Scheme 1.
Scheme 1CuPbI3 Cu-Pb (alloy)
+ 1.5 I2
The DSC trace of CuPbI3 recorded under an inert
atmosphere of nitrogen gas with constant flow of
20 cm3 min-1 and heating rate of 20 �C min-1 is shown in
Fig. 2. The DSC curve displays two endothermic peaks at
240 and 307 �C. The well-defined sharp endotherm at
307 �C indicates melting point of CuPbI3. The onset
melting begins at 294.51 �C and the melting process is
completed at 315 �C with maximum heat flow at 307 �C.The sharp and well-defined endotherm at 307 �C indicates
that the material is highly crystalline in nature needing an
enthalpy of -19.00 J g-1 for melting. A weak endotherm
with small enthalpy value of -2.79 J g-1 in the DSC plot
further suggests that CuPbI3 passes through a phase change
process at 240 �C before melting.
X-ray diffraction patterns of the powder and elec-
trophoretically deposited film of CuPbI3 are displayed in
Fig. 3 and Fig. S1, respectively. The XRD pattern of
powdered CuPbI3 agrees well with the pattern obtained for
the films of CuPbI3. This observation suggests that FTO
does not chemically interfere during the electrophoretic
fabrication of CuPbI3 film. The diffraction peaks at 2hvalues of 12.9�, 25.7�, 39.7�, and 52.5� that correspond to
the (002), (004), (302), and (312) planes, respectively, of
the hexagonal CuPbI3 lattice are in good agreement with
literature data [37, 41]. The crystallite size of the CuPbI3thin film is calculated by Debye Scherer equation L = k k/b cosh, where k represents the Scherer constant, k is the
wavelength, and h is the Braggs angle. The estimated
crystallite size of CuPbI3 is L = 62.5 nm which agrees
well with the literature value [41].
Raman spectroscopy with polarized laser beams
(k = 514 nm) at dissimilar geometries with respect to the
crystal orientation should be used to distinguish the
Raman active modes of different symmetries. The Raman
scattering of as-synthesized CuPbI3 film was recorded in
the range of 100–1000 cm-1 and is shown in Fig. 4. It is
reported in the literature that pure CuI shows a Raman
scattering mode at 140 cm-1 [47], while an intense peak
was observed at 120 cm-1 which is attributed to the Pb–I
bond vibration [10], while further PbI2 modes are
observed at 112 and 164 cm-1 [48, 49]. It is found that
Raman active modes related to CuI in CuPbI3 fall at lower
wave number with a difference of 4 cm-1 and detected at
136 cm-1 [27]. The broad band at 165 cm-1 character-
istic of PbI2 has been found at its place as reported [49]. A
Fig. 2 DSC trace of CuPbI3 recorded under an inert atmosphere of
nitrogen gas with constant flow of 20 cm3 min-1 and heating rate of
20 �C min-1
Fig. 3 Comparison of powder XRD diffraction pattern of a as-
synthesized CuPbI3 powder with b reported in the literature [40]
Fig. 4 Raman spectrum of CuPbI3 film
Photoelectrochemical water splitting over mesoporous CuPbI3 films prepared by…
123
new broad and intense Raman scattering mode that
appeared at 213 cm-1 is considered as characteristic
mode for CuPbI3 [10].
Surface and optical studies
The surface morphology of the EPD deposited films has
been investigated by FE-SEM and is shown in Fig. 5a. The
CuPbI3 films consist of uniform grain structure with com-
pact morphology. The films prepared by electrophoresis
tend to grow with small mesoporous grains grouped toge-
ther in different sizes to show smooth uniform juxtaposed
layers. The growth of small grains on the boundary layer of
the FTO substrate can be seen and the average film
thickness was measured to be 96 lm as depicted in cross-
sectional view of Fig. 5b. The results of FE-SEM exami-
nation combined with EDX mapping for the elements Cu,
Pb, and I are shown in Fig. 5c, i, ii, and iii. The bright
regions with different colours correspond to the presence of
the elements Cu, Pb, and I, respectively, and indicate that
all these elements are distributed uniformly maintaining the
stoichiometric proportion of 1:1:3 throughout the whole
area as indicated in Fig. S3. The atomic concentrations of
Cu, Pb, and I elements were also determined by XPS and
compared with EDX results. A comparison of the results is
displayed in SI Table 1. It can be seen that EDX results
show slightly higher atomic % for Cu, Pb, and I as com-
pared to XPS.
The surface and sub-surface chemical states were
investigated by high-resolution narrow scan XPS spectra in
the Cu2p, Pb4f, and I3d region of CuPbI3 film, and are
recorded as Fig. 6a–c. Figure 6a shows the binding energy
of Cu 2p3/2 as 931.5 eV which is closer to the 931.9 eV
that was found in CuI [50]. The binding energies of Pb 4f7/2and Pb 4f5/2 (Fig. 6b) are 137.3 and 142.1 eV, respectively,
indicating a spin orbital splitting of 4.8 eV. These values
are in good agreement for the reported energy values for Pb
in Cs0.2FA0.8PbI3 and PbI2 [26, 51, 52]. The binding
energies of the peaks I3d5/2 and I3d3/2 (Fig. 6c) are 618.0
and 629.3 eV, respectively, which are in close agreement
Fig. 5 a Illustration of FE-SEM images of CuPbI3 film deposited by electrophoresis on FTO substrate; b cross-sectional view of the film;
c(i) red, c(ii) green, and c(iii) violet colours represent EDX mapping of Cu, Pb, and I, respectively (color figure online)
R. Naeem et al.
123
to the I3d values reported for I in CuI, Cs0.2FA0.8PbI3, and
PbI2 [26, 50, 52].
For the precise measurement of the valence band max-
imum (VBM) energy position, 200 measurement scan
cycles were carried out. A plot of normalized intensity and
binding energy ranging from -6 to 8 eV is shown in
Fig. 6d. The VBM was determined by linear extrapolation
method and was found to be at 1.20 V vs. fermi level [53].
The position of conduction band minimum (CBM) was
estimated on the basis of band gap energy determined by
UV–Vis spectrophotometry and the energy determined by
XPS. The value obtained for CBM of CuPbI3 was found to
be -0.62 eV vs. Fermi level, as shown in Fig. 7.
The optical band gap of CuPbI3 film was studied by
measuring the UV–Vis absorption spectra recorded in the
wavelength range of 300–900 nm using a similar FTO
substrate as reference to minimize the contribution from
the substrate. It can be seen that the UV–Vis spectrum
(Fig. 8a) of the CuPbI3 thin film shows wide range
absorption which gradually increases towards lower
wavelengths with maximum absorption in the range of
330–790 nm. The Tauc’s plot of energy versus (ahv)2
(Fig. 8b) shows a direct optical band gap energy of
1.82 eV, while the reported band gap value of CuPbI3 thin
film deposited by high vacuum method is 1.64 eV [54].
This optical band gap value also agrees with the band gap
value calculated for CH3NH3PbI3 by UV–Vis spec-
troscopy [29]. It is also reported that the presence of
methylammonium cation or Cs? does not affect the band
gap region and it is the inorganic component of metal
halide perovskites that play a dominant role in ascer-
taining the band gap energy [55, 56]. In case of CuPbI3,
the band gap energy value remains unchanged inferring
that replacement of CH3NH3? or Cs? by a univalent
transition d10 metal ion, e.g., Cu? did not affect the band
gap. After successful deposition of high purity film of
CuPbI3 and evaluation of its thermal properties, structure,
stoichiometric composition, surface morphology, and
optical band gap, we investigated its photoelectrochemi-
cal (PEC) behavior.
Photoelectrochemical studies
The photoelectrochemical behavior of the CuPbI3 film was
investigated to evaluate its ability to perform water oxi-
dation using linear sweep voltammetry (LSV) technique
Fig. 6 XPS spectra of a Cu 2p3/2, b Pb 4f7/2, 4f5/2, c I 3d5/2,
I3d3/2, and d plot of normalized
intensity and binding energy
ranging from -6 to 8 eV of
CuPbI3 film
Fig. 7 Energy of conduction band minimum calculated from XPS
determined valence band maximum and UV–Vis band gap of CuPbI3film
Photoelectrochemical water splitting over mesoporous CuPbI3 films prepared by…
123
under simulated solar AM 1.5G irradiation in the presence
of 0.1 M Na2SO4 at a scan rate of 50 mV/s. Under applied
bias, the film undergoes photo-induced charge separation,
thereby promoting the valence band electrons to the con-
duction band resulting in the formation of holes at the
valence band. The holes produced at the valence band can
be readily scavenged through water oxidation that produces
O2 and H? ions. The electron present in the conduction
band can be readily collected as a photocurrent generated
by the film. The H? ions are transported through the
electrolyte towards the counter electrode where they react
with photo-generated electrons to produce hydrogen. Fur-
ther enhancement in photocurrent can be brought about by
improving film thickness, morphology, surface roughness,
structure, and geometry. The photoelectrode shows active
response under illumination and represents an anodic
photocurrent pattern. The onset of photocurrent begins at
-0.1 V under illumination and increases with increasing
applied potential, as depicted in Fig. 9a. In the dark,
however, no current can be observed until a bias of 0.1 V
and after that small currents appear before it increases
sharply beyond 0.8 V (vs. Ag/AgCl). Moreover, the mini-
mum necessary device voltage to start water oxidation or
hydrogen reduction potential is ?0.62 V vs. Ag/AgCl at
pH 7, which is equivalent to 1.23 V vs. RHE as referred in
the equation given below [57]:
ERHE ¼ EAg=AgCl þ 0:059V:pHþ 0:199V :
Under illumination, a photocurrent density of 216 lA/cm2 can be observed at the thermodynamic potential of
?0.62 V vs. Ag/AgCl, pH 7. However, the dark current for
the CuPbI3 film has slightly increased probably due to the
leakage of electrons from the electrolyte to the FTO due to
the pinhole effect. Nevertheless, many of the pin holes
already have been covered by the compact thin film as seen
in high-resolution SEM picture shown in Fig. 5a. We
assume that the appropriate band gap and compact
morphology facilitate better electronic flow under
illumination that enhances photocurrent density.
The role of Cu? with d10 configuration is just to balance
the charge and does not have any important contribution
for the conduction and valence band states except for
donating one electron to the Pb-I framework as is reported
for CH3NH3? and Cs? ion in CH3NH3PbI3 and CsPbI3,
respectively [29, 58]. The molecular orbital diagram of
[PbI3]-1 ion shown in Fig. S4 depicts that I (EN: 2.5) and
atomic orbitals (AO) are at low energy and Pb (EN: 2.0)
Fig. 8 a UV–Vis spectrum of wavelength vs. absorbance; b Tauc’s
plot of energy vs. (aht)2 of thin film of CuPbI3 deposited by
electrophoretic technique
Fig. 9 a Photocurrent density—applied voltage (J-V) plots obtained
for the electrophoretic produced CuPbI3 films dipped in 0.1 M
Na2SO4 at a scan rate of 50 mV/s in light and dark; b chronoamper-
ometry (I-t) profiles (on–off cycles) of CuPbI3 films at an applied
potential of ?0.62 V vs. Ag/AgCl under 100 mW cm-2 illumination
(AM 1.5) in 0.1 M Na2SO4 aqueous solution
R. Naeem et al.
123
abd AO are at high energy. After linear combination, AO
orbitals split into two sets of VB and CB orbitals. The VB
orbitals are further divided into two, lowest and middle
order energy state orbitals, while CB orbitals are the
highest energy orbitals. Hence, r Pb 6p-I 5 s and r Pb 6p-I
5pz are in the lowest energy state, while nonbonding Pb
6 s, I 5px, and 5py orbitals are of intermediate energy,
which remain as nonbonding orbitals. Pb 6 s nonbonding
orbital is at a slightly higher energy state than I 5px and
5py. After crossing the Fermi level, there begins a region
where the empty CB orbitals r* Pb 6p-I 5pz and r* Pb 6p-
I 5 s are located. The energy difference between the top of
VB and the bottom of CB orbitals is estimated to be
1.82 eV. The process of photoelectrochemical water split-
ting begins with the photo activation of VBM Pb 6 s
nonbonding electrons that, on solar activation, cross the
fermi level (Ef) and fall into r* Pb 6p-I 5pz CB orbitals
from where they are removed by the applied bias. The hole
generated by the photo activation reacts with water to split
water into oxygen and H? ions. It is believed that mostly
Pb electrons are being used in photocatalytic process
[55–59].
The stability of the material has been tested under illu-
mination on–off cycles and reported as Fig. 9b, which
shows that the material is stable after 18 on–off cycles over
a period of 60 min [60–63].
The CuPbI3 has distinct advantages in terms of its
environmental stability, wide light absorption range due to
its appropriate band gap. The combination of these unique
properties enables this class of halide materials to adopt a
solar cell structure in which high efficiencies could possi-
bly be achieved.
Conclusions
A practicable synthetic route has been established for the
preparation of CuPbI3 by the thermal fusion of an evacu-
ated co-precipitated homogeneous mixture of CuI and PbI2at 620 �C for 72 h and successfully used it to deposit
reproducible and robust thin films by electrophoretic
technique. The UV–Vis spectrophotometry, XRD, XPS,
and FE-SEM/EDX indicate that CuPbI3 film has a band gap
of 1.82 eV and exists in a hexagonal phase containing
Cu:Pb:I in the ratio of 1:1:3. Photoelectrochemical studies
show that the film is photoactive and is capable of pro-
ducing photocurrent density of 216 lA/cm2 at 0.62 V as
measured in 0.1 M aqueous Na2SO4 vs Ag/AgCl/3 M KCl.
Overall, these results suggest that deposited photosensitive
CuPbI3 has significant potential for application in solar
energy harvesting devices, such as photoelectrochemical
cells.
Experimental
Materials and methods
Copper(I) iodide (m.p.: 606 �C), lead(II) nitrate, potassiumiodide, and dimethylformamide (DMF) were purchased
from Sigma-Aldrich. Fluorine doped tin oxide (FTO)
conducting glass slides with surface resistivity of 8 X/sqwere also supplied by Sigma-Aldrich.
Differential scanning calorimetry (DSC) curve of
CuPbI3 powder was recorded on Mettler Toledo DSSC
822e instrument in the temperature range of 50–400 �C.The controlled thermal analyses were investigated using a
Mettler Toledo TGA/SDTA 851e Thermogravimetric
Analyzer with a computer interface. The thermal mea-
surements were carried out in an alumina crucible under an
atmosphere of flowing dinitrogen gas (20 cm3 min-1) with
a heating rate of 20 �C min-1. The CuPbI3 powder and film
were characterized using X-ray diffraction (XRD) on a D8
Advance X-ray Diffractometer-Bruker AXS using CuKaradiation (k = 1.542 A), at a voltage of 40 kV and current
of 40 mA at ambient temperature. Surface analysis of the
films was carried using XPS-type axis ultra from Kratos
Analytical with monoenergetic soft X-ray MgKa(1253.6 eV) as an irradiation source. The Raman spectra of
the films were acquired using a Renishaw inVia Raman
spectrophotometer with Ar laser excitation (k = 514 nm).
The optical absorption spectra of the films were recorded
on a Lambda 35 PerkinElmer UV–Vis spectrophotometer
in the wavelength range of 300–900 nm. The surface
morphology of CuPbI3 was studied using a field emission
scanning electron microscope (FE-SEM, FEI Quanta400)
coupled with energy dispersive X-ray spectrometer [INCA
Energy 200 (Oxford Inst.)], at an accelerating voltage of
20,000 magnification and a working distance of 9.6 mm.
The photoelectrochemical properties of the elec-
trophoretic produced CuPbI3 films were studied using