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

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 CuPbI3 film of average thickness 96 lm has been

estimated at 1.82 eV and the film shows a current density

of 216 lA/cm2 at 0.62 V measured in 0.1 M Na2SO4

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 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.

& Muhammad Mazhar

mazhar42pk@yahoo.com

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

perovskites of the general formula ABX3 (where A is an

organic cation, B a Pb?2 ions, and X a halide or a mixture of

halides) are abundantly available, cost efficient, and easy to

manufacture using a simple solution process with more than

21% of solar efficiency [20–27]. To reduce the environ-

mental effect of Pb?2 [25], lead-free hybrid perovskites

[28, 29] and derivatives of other divalent metal ions, such as

Sn?2 and Ge?2, have been reported [30, 31], but their main

problem is the inherent instability in ?2 oxidation state

under reaction conditions. There are very few reports

[8, 32, 33] dealing with all-inorganic halide perovskites of

typeMM’X3, whereM is a univalentmetal ion,M’ a divalent

metal ion, and X a halide or a mixture of halide ions [34, 35].

The interest in these ternary halides centers around their

applications as a solid electrolyte in solid-state batteries

[36, 37], ion selective electrodes, or sensitive layers in gas

sensors. Thin films of these materials are particularly desir-

able because of their conductivity, photoluminescence

[38, 39], possible wide compression range with thin films,

large area coverage, and relative chemical stability [40]. It is

reported in the literature that purely inorganic perovskite

materials, such as CuPbBr3, CuPbI3, CuCdCl3, and CuSnI3,produced from the binary combination of the halides of these

respective metals exhibit appreciable conductivity

[33, 41–43]. CsPbI3 and CsPbBr3 are drawing attention for

use as luminescent materials and in high-energy radiation

detectors. Perovskite solar cells utilizing CsPbBr3 have

exhibited excellent thermal stability with high photo volt-

ages [44]. The main problem associated with these materials

is their synthesis through solid-state reaction, solubility, and

transformation of powder into films [45]. Several techniques,

such as vapor-phase co-evaporation [8], spin coating [26],

and solution deposition [33], have been used for the depo-

sition of thin films of inorganic halide perovskite (ABX3)

using a mixture of AX and BX2. Each fabrication method

produces a different film morphology (grain size/shape, film

roughness, surface coverage, etc), and, in turn, these differ-

ent morphologies influence the optical and photocatalytic

properties of the thin film [46].

The current work details the synthesis, structural, and

thermal investigations of CuPbI3, and its utilization for the

fabrication of robust and environmentally stable thin films

on FTO conducting glass substrate using electrophoretic

deposition (EPD) technique. The deposited films were

characterized by field emission scanning electron micro-

scope (FE-SEM)/EDX, XRD, Raman scattering, XPS

analysis, and UV–Vis spectrophotometry to determine their

morphology, elemental composition, particle size, phase

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

Metrohm autolab (NOVA 1.10) electrochemical worksta-

tion with a conventional three-electrode system. The

CuPbI3 films were used as a working electrode, platinum as

a counter, and Ag/AgCl as a reference electrode. For

photocurrent measurement, the CuPbI3 films were dipped

into the supporting electrolyte (0.1 M Na2SO4) and irra-

diated with a 150 W xenon arc lamp (Newport, Model

69907) containing a simulated AM 1.5G filter.

Synthesis of CuPbI3

Copper(I) iodide (0.50 g, 2.62 mmol) was suspended in

20 cm3 distilled water in a 100 cm3 beaker. Lead(II) nitrate

(0.87 g, 2.64 mmol) dissolved in 10 cm3 of distilled water

was added drop by drop to the suspension of copper(I) iodide

Photoelectrochemical water splitting over mesoporous CuPbI3 films prepared by…

123

with constant stirring for 1 h. Potassium iodide (0.87 g,

5.28 mmol) dissolved in 10 cm3 of distilled water was added

very slowly to the vigorously stirred mixture for 2 h. The

obtained yellow–brown precipitates were filtered and

washed several times with distilled water until free from

lead, nitrate, and iodide ions. Finally, the precipitates were

washed with ethyl alcohol and dried in air.

The obtained yellow–brown powder was charged in a

5 cm3 Pyrex glass ampule and evacuated for several hours.

The ampule was sealed under vacuum and heated at 620 �Cfor 72 h in a tube furnace [37]. The furnace was allowed to

cool to room temperature before the ampule was taken out.

The ampule was opened carefully and the mustard-

coloured solid was carefully scratched from the ampule and

ground to fine powder in an agate pestle mortar. The finely

ground powder of copper lead iodide was washed with

several 5 cm3 portions of DMF to remove excess of

unreacted lead(II) iodide to give crystals of CuPbI3 (m.p.:

307 �C) of 99.9% purity and in 100% yield.

Electrophoretic deposition (EPD) of CuPbI3-

modified FTO electrodes

CuPbI3-modified FTO electrodes were prepared by adopt-

ing EPD technique as reported in the literature [64]. In a

typical experiment, two milligrams of the as-synthesized

CuPbI3 powder were dispersed in 40 cm3 of

0.025 M Mg(NO3)2 in isopropanol. The mixture was son-

icated for 30 min to obtain a homogeneous suspension

containing 0.05 mg dm-3 of CuPbI3. The pH of the sus-

pension was adjusted at 3 by utilizing 1 M HCl solution

before carrying out EPD experiment. The FTO glass sub-

strates with an area of 10 mm 9 20 mm were immersed in

a 5% HF solution for a few minutes to remove the native

oxide layer followed by washing in acetone and distilled

water prior to being vertically immersed into the suspen-

sion. The linear distance between the electrodes was

maintained at 10 mm and the dc potential and deposition

time were adjusted to 80 V and 5 min, respectively. The

coated film was dried at 50 �C in a vacuum oven to remove

the excess solvent from the EPD process.

Acknowledgements All the authors acknowledge High-Impact

Research schemes Grant Nos. UM.C/625/1/HIR/242) and UMRG,

UM.TNC2/RC/261/1/1/RP007-13AET, IPPP Grant No. PG111-

2013A, MOSTI, and UKM for providing financial assistance with

Grant No 03-01-02-SF1231.

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