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Preparation of Zn–In–S film electrodes using chemical bath deposition for photoelectrochemical applications
ARTICLE IN PRESS Solar Energy Materials & Solar Cells 94 (2010) 1137–1145
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Preparation of Zn–In–S film electrodes using chemical bath deposition for photoelectrochemical applications Kong-Wei Cheng a,b,n, Chia-Jui Liang a a b
Department of Chemical and Materials Engineering, Chang Gung University, Taoyuan, Taiwan Solar Cell Group, Green Technology Research Center, Chang Gung University, Taoyuan, Taiwan
a r t i c l e in fo
abstract
Article history: Received 14 January 2010 Accepted 26 February 2010 Available online 2 April 2010
Ternary zinc–indium–sulfide film electrodes were fabricated on fluorine doped tin oxide coated glass substrates using chemical bath deposition. New procedures for the growth of Zn–In–S films are presented. The physical and photoelectrochemical properties of the samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–vis spectroscopy, and a potentiostat. The XRD patterns and SEM images of samples reveal that the samples changed from the cubic ZnIn2S4 phase with lamellar microstructures to the cubic In2S3 phase with irregular platelet microstructures when the [Zn]/[In] molar ratio was decreased in precursor solutions. The thicknesses, direct and indirect band gaps of the samples, determined from the surface profile measurement and transmittance and reflectance spectra, are in the ranges of 1135–1714 nm, 2.47–2.74 eV and 1.88–2.11 eV, respectively. All samples were n-type semiconductors. The flat band potentials of the samples in 0.6 M K2SO3 solution (pH ¼ 9.5), determined from Mott–Schottky plots, lie in the range of 0.58 to 0.45 V vs. the normal hydrogen electrode (NHE). The maximum photocurrent density of samples obtained in this study was 0.67 mA/cm2 at an external potential of + 0.5 V vs. an Ag/AgCl electrode in contact with 0.6 M K2SO3 solution under illumination using a 300 W Xe lamp system with the light intensity set at 100 mW/cm2. Crown Copyright & 2010 Published by Elsevier B.V. All rights reserved.
Keywords: Chemical synthesis Photoelectrode Optical property Photocurrent density
1. Introduction The sustainable production of energy with zero CO2 emissions has attracted increasing attention due to increases in the cost of fossil fuels and the global warming effect. One way of achieving clean energy is through hydrogen production with a photoelectrochemical (PEC) system using high-efficiency semiconductors under solar energy irradiation [1,2]. Water splitting using a PEC system was first demonstrated by Fujishima and Honda in 1972 [3]. They utilized a TiO2 photoelectrode and a platinum counter electrode in a PEC system under ultraviolet (UV) light irradiation. However, the efficiency of water splitting using a TiO2 photoelectrode is limited due to its large band gap of 3.0–3.2 eV. To improve the efficiency of water splitting using a PEC system, various photocatalysts have been developed [4–6]. However, most metal oxides only respond to UV light due to their wide band gaps. The band structure of many metal oxides is not suitable for hydrogen production in the PEC process with the solar spectrum [5,7]. In contrast, metal sulfides such as CdS are efficient hydrogen
n Corresponding author at: Department of Chemical and Materials Engineering, Chang Gung University, No. 259 Wen-Hwa 1st Rd., Kwei-Shan, Taoyuan 333, Taiwan. Tel.: + 886 3 2118800 3353; fax: + 886 3 2118668. E-mail address: [email protected] (K.-W. Cheng).
producers in the water splitting process because of their suitable band edge position for reducing/oxidizing water [8–10]. However, photocorrosion [11] and the toxicity of some metal sulfides such as CdS limit their industrial applications. It was reported that the photo-stability of multicomponent metal sulfides is much better than that of binary metal sulfides in aqueous solutions [6,12]. The synthesis and characterization of multicomponent metal sulfides for solar energy applications have been reported [13–16]. Studies have shown that multicomponent semiconductors such as I–III–VI (I¼Cu, Ag; III ¼Al, In, Ga; VI¼S, Se, Te) or II–III–VI (II¼Mg, Hg, Cd, Zn; III ¼Al, In, Ga; VI ¼O, S, Se, Te) materials can be used as photoelectrodes for hydrogen production under solar radiation [6,14–16]. II–III–VI ternary semiconductors have received considerable academic and industrial interests for solar energy application. II–III–VI ternary semiconductors are good photo-absorbers for PEC and solar cell devices because their energy band gaps lie between 1.4 and 2.8 eV [6]. Sirimanne et al. [17] prepared a MgIn2S4/MgIn2O4 photoelectrode with a photocurrent density of 30 mA/cm2 in an aqueous solution containing polysulfide solution using an external bias of + 0.7 V (vs. Ag/AgCl electrode) and a 500 W Xe lamp system with light intensity of 250 mW/cm2. Kokate et al. [18] prepared a CdIn2S4 photoelectrode using the electrodeposition technique. The efficiency of their PEC cell was 2.94% in an aqueous polysulfide solution with a 200 W tungsten
0927-0248/$ - see front matter Crown Copyright & 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.02.049
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lamp as the light source. Sawant et al. [19] prepared CdIn2S4 thin films using the spray pyrolysis technique. The efficiency of their PEC cell was 1.06% in an aqueous polysulfide solution with a 500 W tungsten lamp as the light source. ZnIn2S4 is a visible light active II–III–VI photo-absorber material in solar energy application. The band gap of ZnIn2S4 reported in the literature is in the range of 2.1–2.8 eV, which matches the solar spectrum for energy conversion reasonably well. Shen et al. [20] synthesized the ZnIn2S4 powder photocatalyst using the hydrothermal method, obtaining a hydrogen evolution rate of up to 112.5 mmol/h under visible-light irradiation. Li et al. [6] developed ZnIn2S4 film electrodes using the spray pyrolysis method with a photocurrent density of 0.8 mA/cm2 in an aqueous solution containing 0.1 M Na2SO3 (pH¼9.2) using an external bias of + 1.0 V (vs. a saturated calomel electrode, SCE) and a light intensity of 260 mW/cm2. Asenjo et al. [21] developed 7.9%-efficient CuInS2 solar cells with ZnS–In2S3 as the buffer layer using chemical bath deposition. Vigil et al. [22] prepared n-type ZnIn2S4 single crystal using the vapor-phase chemical transport method. Yu et al. [23] fabricated visible-light-responsive ZnIn2S4 film electrodes on Ti substrates using the electrodeposition method. Their results showed that ZnIn2S4 exhibits a remarkable photocatalytic bactericidal ability under visible-light irradiation. However, thin-film devices that use multicomponent metal sulfides are more convenient for photoelectrolysis of water without the need for further separation procedures. In order to prepare a thin-film device with good efficiency, it is necessary to understand the properties of these semiconductor films. The chemical bath deposition (CBD) technique, which uses diluted baths containing metal ions and sulfide or selenide ions, is a simple method for the low-temperature deposition and crystallization of semiconductor thin films [13–16]. Compared with other film-deposition techniques, the CBD method has the advantages of simplicity, low cost, and good uniformity for large area deposition of semiconductor films, making CBD suitable for the preparation of high-efficiency optoelectronic devices and solar cells. In the present study, Zn–In–S samples were prepared on fluorine doped tin oxide (FTO) coated glass substrates using chemical bath deposition in acidic solutions. The effect of the [Zn]/[In] molar ratio in the precursor solution on the structural, optical, and PEC properties of the samples was studied.
2. Experiment details In this study, the zinc to indium ratio, [Zn]/[In], was varied during the deposition of Zn–In–S samples on substrates. Samples were grown on commercial FTO-coated glass substrates (sheet resistance¼10 O/square, Union Chemical Co., Taiwan) using CBD. Before the samples were deposited on the substrates, the FTO-coated glass substrates were cut into several slides ( 1 cm 5 cm) and cleaned in ultrasonic baths of ethanol, deionized water, and acetone for 30 min in each bath. Substrates were then carefully cleaned using deionized water, blown dry using ultra-pure nitrogen gas, and used immediately to prevent any further contamination of the surface. Aqueous cationic and anionic solutions were separately prepared before deposition. The cationic solution consisted of 5 mL of 0.05–0.3 M zinc nitrate [Zn(NO3)2 6H2O], 5 mL of 0.4 M indium nitrate [In(NO3)3 5H2O], 2.5 mL of 0.4 M ammonium nitrate (NH4NO3, buffer solution), 0.2 mL of 3.7 M triethanolamine [C6H15NO3 (TEA), chelating agent], and 20 mL of water. Concentrated sulfuric acid was used to adjust the pH value of the aqueous cationic solution to around 1 in order to avoid the formation of hydroxide complexes such as In(OH)3. After the cationic solution was stirred for 1 h, 50 mL of
0.32 M thioacetamide (CH3CSNH2, TAA) solution, the source of S2 ion, was added and mixed well. Pre-cleaned substrates were placed vertically into the solution bath. The bath was placed on a hot plate and magnetically stirred to improve the uniformity of samples. The temperature of the reaction solution was kept at 65 1C. After 4 h of deposition, the samples were removed and cleaned ultrasonically in a water bath for 5 min in order to get uniform and compact Zn–In–S films. The thermal treatment of samples was carried out in a vacuum quartz tube at 550 1C for 1 h after CBD. The phase formation and crystallinity of samples were characterized using an X-ray diffractometer (Siemems Model ˚ radiation. The X-ray diffraction D5005) with CuKa (l ¼1.5405 A) (XRD) patterns were recorded in the 2y range of 10–701. The scan rate was set to 1.21/min in order to increase the signal-to-noise ratio. The surface microstructures of the samples were studied using a field-emission scanning electron microscope (FE-SEM, model JEOL JSAM 6700F). The compositions of the samples were also analyzed using a scanning electron microscope (SEM) equipped with energy dispersive analysis of X-ray (EDAX, model Hitachi S-3000N). The accelerating voltage of SEM was set at 5 kV. The surface microstructures and roughnesses of samples were analyzed using an atomic force microscope (AFM, model Park System XE 70) operated in non-contact mode. The optical transmittance and reflectance spectra of the samples were measured using a UV–vis–NIR spectrophotometer with an integrating sphere (JASCO Model V-670) in the wavelength range of 300–1000 nm at room temperature. The thickness of the samples was determined using a surface profile (Surfcorfer ET 3000). The Mott–Schottky plots of samples were measured using a computer-controlled potentiostat (model CHI 600 C) equipped with a frequency response analyzer. A three-electrode setup was employed, where the semiconductor film, a Pt plate electrode with an area of 1.0 cm2, and an Ag/AgCl electrode were the working, counter, and reference electrodes, respectively. A silver wire was attached to the conducting layer of the FTO-coated glass substrate by silver paste. The contacts and edges of the sample electrodes were sealed with epoxy resin and dried at room temperature. The electrolyte, aqueous K2SO3 (0.6 M, pH¼9.5), was freshly prepared using Milli-Q water and degassed by purging with high purity nitrogen, followed by ultrasonication for 30 min before each experiment. All experiments were carried out in a nitrogen environment at a temperature of 25 1C. The applied potentials were in the range of 1.0 to +0.0 V vs. the Ag/AgCl electrode. The PEC performance measurements of the samples were carried out in a quartz electrolytic cell. The sample (average area¼1.0 cm2), a Pt plate electrode (average area¼1.0 cm2), and an Ag/AgCl electrode were employed as the working, counter, and reference electrodes, respectively. A silver wire was attached to the conducting layer of the working electrode with silver paste, and the back contact and edges of the working electrode were sealed with epoxy resin. Aqueous K2SO3 (0.6 M, pH ¼9.5) solution, prepared using double deionized water and degassed by purging with nitrogen gas (99.99% purity) before each experiment, was used as the electrolyte. The electrolyte was put into an ultrasonic bath for 30 min before each experiment in order to decrease the influence of gas solutes. All measurements were carried out in a nitrogen environment at a temperature of 25 1C. Current densities, as a function of applied potential ( 1.0 to + 0.5 V vs. Ag/AgCl electrode) for the samples, were recorded under front-side illumination with a computer-controlled potentiostat (CHI 600 C) for all PEC experiments. A 300 W Xe short arc lamp (Perkin Elmer Model PE300BF) with white light intensity of 100 mW/cm2 was employed to simulate solar light. The intensity of incident
ARTICLE IN PRESS K.-W. Cheng, C.-J. Liang / Solar Energy Materials & Solar Cells 94 (2010) 1137–1145
light from the Xe lamp was measured using a photometer Model 70310 from Spectra-Physics.
3. Results and discussion 3.1. Characterization of samples on FTO-coated glass substrates Zn–In–S samples were grown on FTO-coated glass substrates using chemical bath deposition. The [Zn]/[In] molar ratio in the solution bath was varied to study its effect on the physical properties and PEC performance of the samples. Well-adherent yellow films were produced on the surface of the substrate after 4 h of deposition. The structural, optical, and electrical properties of semiconductor films with various deposition parameters were analyzed. Fig. 1(I) shows the XRD diffraction patterns of samples after heat treatment in the 2y range of 10–701 with various [Zn]/[In] molar ratios in the solution bath. The standard diffraction peaks of FTO [24], b-In2S3 [25], cubic ZnIn2S4 [26] and hexagonal Zn3In2S6 [27] reported in JCPDS cards are also shown in Fig. 1(I). Of note, a change was observed in the crystal phase of the samples with an increase in [Zn]/[In] molar ratio in the precursor solution. All samples showed the polycrystalline cubic spinel Zn–In–S phase. JCPDS Hexagonal Zn3In2S6
Intensity (arb. unit)
JCPDS Cubic ZnIn2S4 a b c d JCPDS Cubic β -In2S3 JCPDS FTO
10
20
30
FTO
40 2θ (degrees)
In2S3(311)
50
60
70
ZnIn2S4(311)
Intensity (arb. unit)
a
b
In2S3 c
d ZnIn2S4
25
26
27 28 2θ (degrees)
29
30
Fig. 1. X-ray diffraction patterns of samples on FTO-coated glass substrates for various [Zn]/[In] molar ratios in the precursor solutions. (a) [Zn]/[In] ¼0.75, (b) [Zn]/[In]¼ 0.5, (c) [Zn]/[In] ¼0.25 and (d) [Zn]/[In]¼ 0.125.
1139
With higher [Zn]/[In] molar ratios in the solution bath (samples (a) and (b)), some hexagonal Zn3In2S6 phase was observed. Fig. 1(II) shows the change in the (3 1 1) reflection in the XRD patterns for all samples in this study. The diffraction peak of the (3 1 1) plane for sample (a) approaches that of the ZnIn2S4 phase. As the molar ratio of [Zn]/[In] in the solution bath decreased (samples (a)-(d)), the peaks of XRD patterns of samples shifted to lower angles, which indicates that the crystal phase of samples approaches that of In2S3. The successive shift of the XRD pattern indicates that In3 + ions were incorporated into the lattice of samples with a decrease in the [Zn]/[In] molar ratio in the solution. This shift is reasonable because the ion radius of In3 + ˚ is larger than that of Zn + (0.74 A) ˚ [28,29]. For high [Zn]/ (0.81 A) [In] molar ratios in the solution (samples (a) and (b)), a phase separation was observed. Some hexagonal Zn3In2S4 phase was observed in samples (a) and (b). The thicknesses of samples obtained from surface profile measurements are shown in Table 1. The thicknesses of the samples are in the range of 1135–1714 nm. Fig. 2 shows the effect of the [Zn]/[In] molar ratios in the solution bath on film thickness and lattice parameter of the samples. With an increase in the [Zn]/[In] molar ratio in the solution bath, the thicknesses of samples increased and the lattice parameters of samples estimated from XRD patterns decreased. The values of lattice parameters for ZnIn2S4 [26] and In2S3 [25] reported in JCPDS cards are 1.062 and 1.072 nm, respectively. The lattice parameters of samples (a), (b), (c), and (d) estimated using XRD diffraction patterns are 1.066, 1.069, 1.071, and 1.072 nm, respectively. The lattice parameter of sample (a) approaches to that of ZnIn2S4 and that of sample (d) approaches to that of In2S3, which agree well with the values reported in the literature. The surface morphology of samples was analyzed using FE-SEM and AFM. The SEM images of samples (a) and (d) are shown in Figs. 3(A) and (C), respectively. The figures show that samples (a) and (d) were dense and uniform. Microspheres with lamellar microstructures were observed on the surface of sample (a) while irregular shaped platelet microstructures were observed for sample (d). Shen et al. [30] reported that microspheres with lamellar microstructures were observed for ZnIn2S4 and Puspitasari et al. [31] reported that irregular platelet microstructures were observed for In2S3. With decreasing [Zn]/ [In] molar ratio in the precursor solution, the microstructures of samples changed from the ZnIn2S4 phase to the In2S3 phase, which agrees well with the results of XRD. The AFM images of samples (a) and (d) are shown in Figs. 3(B) and (D), respectively. The surface roughnesses (dRM) of samples (a) and (d) obtained from AFM were 101.8 and 96.2 nm, respectively. The surface roughnesses of all samples obtained from AFM images are in the range of 95–105 nm, which indicates that there is no significant effect of the [Zn]/[In] molar ratio in the precursor solution on the surface roughness of samples. Quantitative analysis using EDAX was carried out to analyze the atomic ratios of Zn, In, and S in the samples. Changes in the compositions of films were studied by varying the [Zn]/[In] molar ratio in the precursor solution. The weight percentages of zinc, indium, and sulfur obtained from EDAX analysis were converted to atomic ratios. The atomic [Zn]/[Zn+ In] and 2[S]/(2[Zn]+3[In]) ratios of samples are listed in Table 1. Fig. 4 plots the atomic ratios of [Zn]/[Zn+In] and 2[S]/(2[Zn]+ 3[In]) in the samples as a function of the [Zn]/[In] concentration ratio in the precursor solution. The molar ratio of [Zn]/[Zn+ In] in the films decreased from 0.29 to 0.06 when the molar ratio of [Zn]/[In] was decreased from 0.75 to 0.125 in the precursor solution (samples (a)–(d)). The 2[S]/(2[Zn]+ 3[In]) ratio of the samples remained almost constant with decreasing [Zn]/[In] molar ratio in the solutions. The EDAX analysis of samples also shows that there was no sulphur deficit in the samples. The atomic ratios of elements in the film obtained
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Table 1 Physical properties of chemical-bath-synthesized Zn–In–S films on FTO-coated glass substrates. Sample [Zn]/[In] molar Thickness ratios in (nm) solution
(a) (b) (c) (d) a b
0.750 0.500 0.250 0.125
1714 1617 1516 1135
Atomic ratios for sample [Zn]/ [Zn+ In]
2S/ (2Zn+ 3In)
0.29 0.24 0.15 0.06
1.00 1.00 1.04 1.06
Direct Eg (eV)
Indirect Eg (eV)
EFBa (V vs. NHE)
EFBb (V vs. NHE)
ECB (V vs. NHE)
EVB (V vs. NHE)
Conduction type
2.74 2.67 2.61 2.47
2.11 1.96 1.94 1.88
0.58 0.54 0.50 0.45
0.70 0.65 0.59 0.41
0.88 0.84 0.80 0.75
1.86 1.83 1.81 1.72
n n n n
Obtained from Mott–Schottky plots. Determined from photocurrent density–applied voltage plots.
1.080
In an aqueous acid solution bath, thioacetamide undergoes the following reactions [16,33]:
2000 Thickness Lattice parameter
CH3CSNH2 + H2O2CH3CONH2 +H2S
1800
The following equilibrium reactions take place: 1600
1.070 1400
Thickness (nm)
Lattice parameter (nm)
1.075
1.065 1200
H2S + H2O2H3O + +HS
KA1 ¼10 7
(4)
HS +H2O2H3O + + S2
KA2 ¼10 17
(4)
where KA1 and KA2 are the first and second acid-ionization constants of H2S, respectively. TEA is used as a chelating agent for zinc ion to form metal complex: Zn2 + +TEA2[Zn(TEA)]2 +
1.060 0.10
0.20 0.30 0.40 0.50 0.60 0.70 [Zn]/[In] molar ratio in precursor solution
1000 0.80
Fig. 2. Effects of [Zn]/[In] molar ratio in the reaction solution on the film thickness and lattice parameters of samples.
Because the pH value of the aqueous solution is lower than 4, indium in the solution exists as free In3 + ions. Complexes of In3 + may be formed with TEA: In3 + + TEA2[In(TEA)]3 + The equilibrium constant, K, is defined as
from EDAX analysis show that the growth rate of ZnS in samples became slower with decreasing [Zn]/[In] molar ratios in the reaction solution (samples (a)-(d)). According to the phase diagram of ZnS and In2S3 [32], approximately 23 mol% ZnS can be dissolved in In2S3 at 600 1C. It can be estimated that approximately 25 mol% ZnS is dissolved in In2S3 at 550 1C using the phase diagram of the ZnS and In2S3 system. ZnS and In2S3 can form a continuous solid solution series with [Zn]/[Zn+ In] molar ratios less than 0.15. Samples (c) and (d) indicated that ZnS can form a solid solution with In2S3. Some impurities (ZnIn3S6 phase) were observed in samples (a) and (b) due to the [Zn]/[Zn +In] molar ratios being greater than 0.15. The results obtained from EDAX analysis agree well with the results from the XRD analysis.
M+L2ML
K¼[ML]/[M][L]
(4)
where M¼Zn2 + or In3 + ions, and L¼TEA in this study. The value of log K for Zn(TEA)2 + is 2.56 [34]. However, the equilibrium constant of [In(TEA)]3 + has not be reported in the literature [33]. In this study, the simplified method proposed by Chang et al. [16] was used. The concentration of free indium ions is the same as that in the initial reaction solution. The estimated free [Zn2 + ] concentrations of samples (a)–(d) using a equilibrium constant of are 1.0 10 2, 5.8 10 3, 2.45 10 3 and Zn(TEA)2 + 1.1 10 3 M, respectively. The estimate free [In3 + ] concentration of samples (a)–(d) in solution bath is 2.4 10 2 M. Solubility products (Ksp) of zinc sulfide and indium sulfide are given by [35,36]
3.2. Simple nucleation and growth mechanism
Zn2 + +S2 -ZnS
The chemistry of the solution bath affects the physical properties of the film on the substrates. Nucleation and the growth of films change due to the degree of supersaturation, interfacial energy, and the strength of coordination bonds of the chelating reagents. A possible nucleation and growth mechanism of samples is discussed below using the stability of metal complexes and classical nucleation theory.
2In3 + + 3S2 -In2S3
Ksp ¼2.93 10 25
(4)
Ksp ¼5.8 10 74
The sulfur concentrations for saturated zinc sulfide and indium sulfide in these conditions are in the order of 10 22–10 23 and 10 24 M, respectively. This indicates that indium sulfide precipitated more easily under our experimental conditions. With a decrease in zinc ion concentration in the precursor solution
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Fig. 3. SEM and AFM images of samples.
(samples (a)-(d)), the deposition rate of zinc sulfide decreased while the indium sulfide remained almost unchanged in the experiments. Because the deposition rate of zinc sulfide decreased, the thickness of zinc sulfide film decreased with a decrease in the [Zn]/[In] molar ratio in the solution. A thinner sample was obtained with a decrease in the [Zn]/[In] molar ratio in the precursor solution. The atomic ratios of [Zn]/[Zn+In] in samples (a)–(d) decreased with a decrease in zinc ions in the precursor solution due to a decrease in the growth rate of zinc sulfide. After thermal treatment of the samples, the reaction of formation for Zn–In–S can be given as xZnS + yIn2S3-ZnxIn2yS(3y + x)
(4)
3.3. Optical properties The transmittance and reflectance spectra of samples in the wavelength range of 300–1000 nm are shown in Fig. 5. The maximum transmittance spectrum of samples in this study was 80% transparency in the near infrared region. The reflectance
spectra of samples were almost the same in the range of 300– 1000 nm. The energy band gap (Eg) is an important parameter for a PEC device application. The absorption coefficient (a) of samples can be estimated using the Manifacier model:
a ¼ 1=t ln½ð1RÞ=T
ð1Þ
where t is the thickness of the sample and T and R are the transmittance and reflectance data, respectively [37]. The thicknesses of samples in this study obtained from the surface profile measurements are shown in Table 1. The thicknesses of samples are in the range of 1714–1135 nm. They decrease with decreasing [Zn]/[In] molar ratio in the precursor solution, which agrees well with the results obtained from nucleation and growth mechanism. The relationship between the absorption coefficient (a) and the incident photon energy (hv) can be written as ðahvÞ ðhvEg Þn
ð2Þ
Generally, n is 1/2 for a direct band gap and 2 for the indirect band gap [38]. The optical band gaps of samples can be obtained by plotting (ahn)1/n with respect to hv and extrapolating it to
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1.5
1.5
1.0x1010
1.2
1.2
8.0x109
0.9
0.9
0.6
0.6
Zn/(Zn+In) (a) (b) (c) (d)
0.3
0.3
(αhν)2 (eV/cm)2
2S/ (2Zn+3In) molar ratio in samples
Zn/ (Zn+In) molar ratio in samples
2S/(2Zn+3In)
6.0x109
4.0x109
2.0x109
0.0 2.2
0.0 0.1
0.2 0.3 0.4 0.5 0.6 0.7 [Zn]/[In] molar ratio in precursor solutions
2.4
2.6
2.8
3.0
3.2
hν (eV)
0.0 0.8
400
(a) (b) (c) (d)
Fig. 4. The atomic ratios of [Zn]/[Zn+ In] and 2[S]/(2[Zn] +3[In]) for samples as a function of the [Zn]/[In] molar ratio in the reaction solution.
(αhν)1/2 (eV/cm)1/2
300
100 Reflectance / Transmittance (%)
(a) (b) (c) (d)
80
T
60
200
100
40 0 1.6
20 R
1.8
2.0
2.2
2.4
2.6 2.8 hν (eV)
3.0
3.2
3.4
3.6
Fig. 6. Plots of (ahv)1/n vs. hv of samples. (I) n¼1/2 and (II) n¼2.
0 300
400
500
600 700 Wavelength (nm)
800
900
1000
Fig. 5. Transmittance and reflectance spectra of samples prepared in this study.
(ahn)1/n ¼0. Both direct and indirect band to band transitions can be observed in ZnIn2S4 [6,23]. Figs. 6(I) and (II) show the linear dependence of (ahn)2 and (ahn)1/2on hv, respectively. The direct and indirect band gaps of samples obtained from Figs. 6(I) and (II) are shown in Table 1. The direct and indirect band gaps of samples prepared in this study were in the ranges of 2.74–2.47 eV and 1.88–2.11 eV, respectively. The direct and indirect band gaps of samples increased with an increase in zinc ions in the precursor solution. The direct band gaps of In2S3, ZnIn2S4 and Zn3In2S6 are 2.4, 2.8 and 2.81 eV, respectively [39,40]. The direct band gaps for samples (a) and (d) are 2.74 and 2.47 eV, respectively, and those of samples (b) and (c) lie between those of In2S3 and ZnIn2S4, which agree well with the XRD patterns and the values published in the literature. The band gaps of samples increased with an increase in zinc ions in the precursor solution because the phase of samples changed from In2S3 to ZnIn2S4. 3.4. Photoelectrochemical properties The flat band potentials of a semiconductor can be measured in a variety of ways [2,14–15]. One method based on the capacitance
vs. applied potential measurement is the Mott–Schottky relationship for the barrier. The Mott–Schottky equation is 1=C 2 ¼ B ½2=ðee0 eND A2 Þ½EEFB ðkT=eÞ
ð3Þ
where e is the dielectric constant of the semiconductor, A is the surface area of the semiconductor/electrolyte barrier, ND is the carrier density of the semiconductor, EFB is the flat band potential of the semiconductor, e is the electric charge and e0 is the permittivity of the vacuum. For an n-type semiconductor, B is 1, while for a p-type semiconductor, B is 1. The flat band potentials of samples, EFB, can be estimated from the point of intersection of C 2 vs. applied potentials, E, plot. In order to develop a physically reasonable equivalent circuit of the samples in the electrolyte for Mott–Schottky measurements, impendence analysis for samples was performed. Readecka et al. [41] and Loef et al. [42] reported that the equivalent circuit of samples can be simplified into a series of resistance–capacitance (RC) circuits if 0 the real part of impedance (Z ) is independent of frequency. Fig. 7 shows the Mott–Schottky plots for samples in 0.6 M K2SO3 aqueous solution. The inset in the figure shows the relationship 0 between the real parts of impedance spectra (Z ) of samples and frequency with external potentials of 1.0 V vs. an Ag/AgCl electrode. At frequencies higher than 10 KHz, the real part of the impedance is frequency independent and the influence of the constant-phase element on the impedance spectrum becomes
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Z' (Ω)
(a) (b) (c) (d)
a b c d
100
1.2x1011 10 1
10
8.0x1010
100 1000 10000 100000 frequence (Hz)
4.0x1010
0.0 -0.9
-0.8
-0.7
-0.6
-0.5
1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -1.0 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -1.0
Applied potential (V vs. Ag/AgCl electrode)
negligible. In this study, the frequency of impendence was set to 10 kHz for measuring the Mott–Schottky plot for all samples. The flat band potentials of samples obtained from Mott–Schottky plots are shown in Table 1. The flat band potentials in K2SO3 solution at pH¼ 9.5 of these samples lie in the range of 0.80 to 0.67 V vs. Ag/AgCl electrode ( 0.58 to 0.45 V vs. the normal hydrogen electrode, NHE). The Mott–Schottky measurements for samples were also employed in this study using 0.5 M K2SO4 (pH¼7.0) as the electrolyte (which are not shown here). There is no significant effect of the pH value in the electrolyte on the flat band potentials of samples. It is known that sulfur species semiconductors do not react with H + or OH ion in solution. The band edges at the interface do not shift with the pH value of the electrolyte [2,8,14]. Because the B value of Mott–Schottky plots is 1 for all samples in this study, the samples are n-type semiconductors. The position of the conduction band (ECB) for many n-type semiconductors is 0.1–0.3 V more negative than that of EFB [43]. Based on this, the difference between ECB and EFB is assumed to be 0.3 V for determining the band positions of samples by electrochemical measurements. The positions of the valence band and conduction band for samples using the direct energy band gaps from transmittance and reflectance spectra and flat band potentials from Mott–Schottky plots are shown in Table 1. The PEC responses of samples were measured to examine the possibility of their use for photoelectrolysis. With K2SO3 (0.6 M, pH¼ 9.5) solution as the electrolyte, the holes produced in valance 2 band of samples oxidized SO2 3 to form SO4 under irradiation. Water is reduced to hydrogen by the electrons produced in the conduction ion under band of samples accompanied by oxidation of SO2 3 illumination. The following reactions occur in the electrolyte [8,28]: hv
ZnInS samples-e ðCBÞ þh þ ðVBÞ
ð4Þ
2 þ SO2 3 þ 2OH þ2h -SO4 þ H2 O
ð5Þ
2H2 O þ2e -H2 þ2OH
ð6Þ
Fig. 8 shows the photo and dark current densities of the samples with applied potentials in the range of 1.0 V to + 0.5 V vs. an Ag/AgCl electrode. All samples show an anodic photocurrent under illumination. The maximum photocurrent density of samples in this study approached 0.67 mA/cm2 at an external potential of + 0.5 V vs. an Ag/AgCl electrode in contact with 0.6 M K2SO3 solution under illumination with the light intensity set at 100 mW/cm2. The photocurrent is in the positive
0.5
0.5
Fig. 8. Photocurrent density–applied voltage plots in the range of 1.0 V to +0.5 V vs. an Ag/AgCl electrode for samples in 0.6 M K2SO3 aqueous solution.
0.80 Photo-enhancement current density (mA/cm2)
Fig. 7. Mott–Schottky plots of the samples with various deposition parameters in aqueous 0.6 M K2SO3 solution.
1.0 Photocurrent (b) 0.8 Dark current (b) 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.5 0.0 0.5 -1.0 -0.5 0.0 1.0 Photocurrent (d) Photocurrent (c) 0.8 Dark current (d) Dark current (c) 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.5 0.0 0.5 -1.0 -0.5 0.0 Applied voltage (V vs. Ag/AgCl electrode) Photocurrent (a) Dark current (a)
2.8
0.60 2.6
0.40
2.4 0.20
Energy band gap of samples (eV)
1/C2 (cm4/F2)
1.6x1011
1000
Photocurrent density (mA/cm2)
2.0x1011
1143
Photoenhencement effect Energy band gap
0.00 0.05
0.10 0.15 0.20 0.25 [Zn]/[Zn+In] molar ratio in samples
2.2 0.30
Fig. 9. Effects of [Zn]/[Zn+ In] molar ratio in samples on photo-enhancement current density (external bias kept at 0.5 V vs. an Ag/AgCl electrode) and energy band gap of samples.
potential area, which indicates that the samples are n-type semiconductors. This is in agreement with the results of Mott–Schottky plots. The onset of photocurrent density can be treated as a measure of the flat band potentials, EFB [44,45]. Table 1 also shows the flat band potentials of samples obtained from photocurrent density–applied voltage measurements. The values of flat band potentials determined from photocurrent density–applied voltage measurements are in the range of 0.70 to 0.41 V vs. NHE. Fig. 9 shows the relationship of the photoenhancement current density (external bias kept at 0.5 V vs. an Ag/AgCl electrode) and direct energy band gaps of samples and the [Zn]/[Zn+ In] molar ratio in the solution. The definition of photo-enhancement current density for the samples is the difference between photocurrent density and dark current density. With an increase in the molar ratio of zinc ions in the solution, the energy band gaps of samples increased and the photo-enhancement effects of samples decreased. Sample (d) had
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Acknowledgements
Current density (mA/cm2)
0.60
The authors are grateful to Chang Gung University in Taiwan (Grant no. UERPD280271) for supporting this study. The authors would like to thank the Microscopic Center at Chang Gung University and the Nanotechnology R&D Center at Kun Shan University for SEM-EDAX analysis and FE-SEM micrographs, respectively. The authors would also like to thank Dr. Lee Tai-Chou from National Chung Cheng University and Dr. Huang Chao-Ming from Kun Shan University for their invaluable input.
Light on
0.40
Light on Light on
Light on
Light on
0.20
References
Dark
Dark
Dark
Dark
Dark
0.00 0
200
400 600 Time (sec)
800
1000
Fig. 10. Current density of sample (b) as a function of time with the external bias kept at 0.4 V vs. an Ag/AgCl electrode in 0.6 M K2SO3 aqueous solution.
the maximum photo-enhancement current density because it has the narrowest band gap. Sample (a), which had the largest energy band gap, had the lowest photo-enhancement current density. Some impurities, such as Zn2In3S6, were observed in samples (a) and (b) from XRD patterns. The impurity phases act as recombination sites for photogeneated electrons and holes, which decreases the photo-enhancement effect of samples. Similar observations in PEC responses for ZnIn2S4 were reported in the literature [23,30]. The stability of sample (b) was tested with the external bias kept at +0.4 V vs. an Ag/AgCl electrode in 0.6 M K2SO3 aqueous solution. Fig. 10 shows the current density of sample (b) as a function of time for 1000 s using the chopping method. The current density of sample (b) changed from 0.42 to 0.32 mA/cm2 during the course of the experiment.
4. Conclusions In this study, polycrystalline Zn–In–S photoelectrodes were deposited on FTO-coated glass substrates. The influence of the [Zn]/[In] molar ratio in the precursor solution on the structural, electric and optical properties of the samples was investigated. The polycrystalline ZnIn2S4 phase with some impurity (Zn3In2S4 phase) was observed with [Zn]/[In] molar ratios in the solution bath greater than 0.5. As the [Zn]/[In] molar ratio in the solution bath was decreased, the crystal phase of samples approached the In2S3 phase, as revealed by the peaks of XRD patterns. With an increase in the [Zn]/[In] molar ratio in the precursor solution, the [Zn]/[Zn+ In] atomic ratios in samples increased. There was no sulphur deficit in the samples. The thicknesses, direct and indirect energy band gaps of the samples are in the ranges of 1135–1714 nm, 2.47–2.74 and 1.88–2.11 eV, respectively. With an increase in the [Zn]/[In] molar ratio in the precursor solution, the thicknesses and band gaps of samples increased. At an irradiation of 100 mW/cm2 from a Xe lamp, the maximum photocurrent density of samples in this study was 0.67 mA/cm2 at an external potential of + 0.5 V vs. an Ag/AgCl electrode in contact with 0.6 M K2SO3 solution. The results show that ternary Zn–In–S film electrodes have potential in PEC applications.
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Preparation of Zn–In–S film electrodes using chemical bath deposition for photoelectrochemical applications Kong-Wei Cheng a,b,n, Chia-Jui Liang a a b
Department of Chemical and Materials Engineering, Chang Gung University, Taoyuan, Taiwan Solar Cell Group, Green Technology Research Center, Chang Gung University, Taoyuan, Taiwan
a r t i c l e in fo
abstract
Article history: Received 14 January 2010 Accepted 26 February 2010 Available online 2 April 2010
Ternary zinc–indium–sulfide film electrodes were fabricated on fluorine doped tin oxide coated glass substrates using chemical bath deposition. New procedures for the growth of Zn–In–S films are presented. The physical and photoelectrochemical properties of the samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–vis spectroscopy, and a potentiostat. The XRD patterns and SEM images of samples reveal that the samples changed from the cubic ZnIn2S4 phase with lamellar microstructures to the cubic In2S3 phase with irregular platelet microstructures when the [Zn]/[In] molar ratio was decreased in precursor solutions. The thicknesses, direct and indirect band gaps of the samples, determined from the surface profile measurement and transmittance and reflectance spectra, are in the ranges of 1135–1714 nm, 2.47–2.74 eV and 1.88–2.11 eV, respectively. All samples were n-type semiconductors. The flat band potentials of the samples in 0.6 M K2SO3 solution (pH ¼ 9.5), determined from Mott–Schottky plots, lie in the range of 0.58 to 0.45 V vs. the normal hydrogen electrode (NHE). The maximum photocurrent density of samples obtained in this study was 0.67 mA/cm2 at an external potential of + 0.5 V vs. an Ag/AgCl electrode in contact with 0.6 M K2SO3 solution under illumination using a 300 W Xe lamp system with the light intensity set at 100 mW/cm2. Crown Copyright & 2010 Published by Elsevier B.V. All rights reserved.
Keywords: Chemical synthesis Photoelectrode Optical property Photocurrent density
1. Introduction The sustainable production of energy with zero CO2 emissions has attracted increasing attention due to increases in the cost of fossil fuels and the global warming effect. One way of achieving clean energy is through hydrogen production with a photoelectrochemical (PEC) system using high-efficiency semiconductors under solar energy irradiation [1,2]. Water splitting using a PEC system was first demonstrated by Fujishima and Honda in 1972 [3]. They utilized a TiO2 photoelectrode and a platinum counter electrode in a PEC system under ultraviolet (UV) light irradiation. However, the efficiency of water splitting using a TiO2 photoelectrode is limited due to its large band gap of 3.0–3.2 eV. To improve the efficiency of water splitting using a PEC system, various photocatalysts have been developed [4–6]. However, most metal oxides only respond to UV light due to their wide band gaps. The band structure of many metal oxides is not suitable for hydrogen production in the PEC process with the solar spectrum [5,7]. In contrast, metal sulfides such as CdS are efficient hydrogen
n Corresponding author at: Department of Chemical and Materials Engineering, Chang Gung University, No. 259 Wen-Hwa 1st Rd., Kwei-Shan, Taoyuan 333, Taiwan. Tel.: + 886 3 2118800 3353; fax: + 886 3 2118668. E-mail address: [email protected] (K.-W. Cheng).
producers in the water splitting process because of their suitable band edge position for reducing/oxidizing water [8–10]. However, photocorrosion [11] and the toxicity of some metal sulfides such as CdS limit their industrial applications. It was reported that the photo-stability of multicomponent metal sulfides is much better than that of binary metal sulfides in aqueous solutions [6,12]. The synthesis and characterization of multicomponent metal sulfides for solar energy applications have been reported [13–16]. Studies have shown that multicomponent semiconductors such as I–III–VI (I¼Cu, Ag; III ¼Al, In, Ga; VI¼S, Se, Te) or II–III–VI (II¼Mg, Hg, Cd, Zn; III ¼Al, In, Ga; VI ¼O, S, Se, Te) materials can be used as photoelectrodes for hydrogen production under solar radiation [6,14–16]. II–III–VI ternary semiconductors have received considerable academic and industrial interests for solar energy application. II–III–VI ternary semiconductors are good photo-absorbers for PEC and solar cell devices because their energy band gaps lie between 1.4 and 2.8 eV [6]. Sirimanne et al. [17] prepared a MgIn2S4/MgIn2O4 photoelectrode with a photocurrent density of 30 mA/cm2 in an aqueous solution containing polysulfide solution using an external bias of + 0.7 V (vs. Ag/AgCl electrode) and a 500 W Xe lamp system with light intensity of 250 mW/cm2. Kokate et al. [18] prepared a CdIn2S4 photoelectrode using the electrodeposition technique. The efficiency of their PEC cell was 2.94% in an aqueous polysulfide solution with a 200 W tungsten
0927-0248/$ - see front matter Crown Copyright & 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.02.049
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lamp as the light source. Sawant et al. [19] prepared CdIn2S4 thin films using the spray pyrolysis technique. The efficiency of their PEC cell was 1.06% in an aqueous polysulfide solution with a 500 W tungsten lamp as the light source. ZnIn2S4 is a visible light active II–III–VI photo-absorber material in solar energy application. The band gap of ZnIn2S4 reported in the literature is in the range of 2.1–2.8 eV, which matches the solar spectrum for energy conversion reasonably well. Shen et al. [20] synthesized the ZnIn2S4 powder photocatalyst using the hydrothermal method, obtaining a hydrogen evolution rate of up to 112.5 mmol/h under visible-light irradiation. Li et al. [6] developed ZnIn2S4 film electrodes using the spray pyrolysis method with a photocurrent density of 0.8 mA/cm2 in an aqueous solution containing 0.1 M Na2SO3 (pH¼9.2) using an external bias of + 1.0 V (vs. a saturated calomel electrode, SCE) and a light intensity of 260 mW/cm2. Asenjo et al. [21] developed 7.9%-efficient CuInS2 solar cells with ZnS–In2S3 as the buffer layer using chemical bath deposition. Vigil et al. [22] prepared n-type ZnIn2S4 single crystal using the vapor-phase chemical transport method. Yu et al. [23] fabricated visible-light-responsive ZnIn2S4 film electrodes on Ti substrates using the electrodeposition method. Their results showed that ZnIn2S4 exhibits a remarkable photocatalytic bactericidal ability under visible-light irradiation. However, thin-film devices that use multicomponent metal sulfides are more convenient for photoelectrolysis of water without the need for further separation procedures. In order to prepare a thin-film device with good efficiency, it is necessary to understand the properties of these semiconductor films. The chemical bath deposition (CBD) technique, which uses diluted baths containing metal ions and sulfide or selenide ions, is a simple method for the low-temperature deposition and crystallization of semiconductor thin films [13–16]. Compared with other film-deposition techniques, the CBD method has the advantages of simplicity, low cost, and good uniformity for large area deposition of semiconductor films, making CBD suitable for the preparation of high-efficiency optoelectronic devices and solar cells. In the present study, Zn–In–S samples were prepared on fluorine doped tin oxide (FTO) coated glass substrates using chemical bath deposition in acidic solutions. The effect of the [Zn]/[In] molar ratio in the precursor solution on the structural, optical, and PEC properties of the samples was studied.
2. Experiment details In this study, the zinc to indium ratio, [Zn]/[In], was varied during the deposition of Zn–In–S samples on substrates. Samples were grown on commercial FTO-coated glass substrates (sheet resistance¼10 O/square, Union Chemical Co., Taiwan) using CBD. Before the samples were deposited on the substrates, the FTO-coated glass substrates were cut into several slides ( 1 cm 5 cm) and cleaned in ultrasonic baths of ethanol, deionized water, and acetone for 30 min in each bath. Substrates were then carefully cleaned using deionized water, blown dry using ultra-pure nitrogen gas, and used immediately to prevent any further contamination of the surface. Aqueous cationic and anionic solutions were separately prepared before deposition. The cationic solution consisted of 5 mL of 0.05–0.3 M zinc nitrate [Zn(NO3)2 6H2O], 5 mL of 0.4 M indium nitrate [In(NO3)3 5H2O], 2.5 mL of 0.4 M ammonium nitrate (NH4NO3, buffer solution), 0.2 mL of 3.7 M triethanolamine [C6H15NO3 (TEA), chelating agent], and 20 mL of water. Concentrated sulfuric acid was used to adjust the pH value of the aqueous cationic solution to around 1 in order to avoid the formation of hydroxide complexes such as In(OH)3. After the cationic solution was stirred for 1 h, 50 mL of
0.32 M thioacetamide (CH3CSNH2, TAA) solution, the source of S2 ion, was added and mixed well. Pre-cleaned substrates were placed vertically into the solution bath. The bath was placed on a hot plate and magnetically stirred to improve the uniformity of samples. The temperature of the reaction solution was kept at 65 1C. After 4 h of deposition, the samples were removed and cleaned ultrasonically in a water bath for 5 min in order to get uniform and compact Zn–In–S films. The thermal treatment of samples was carried out in a vacuum quartz tube at 550 1C for 1 h after CBD. The phase formation and crystallinity of samples were characterized using an X-ray diffractometer (Siemems Model ˚ radiation. The X-ray diffraction D5005) with CuKa (l ¼1.5405 A) (XRD) patterns were recorded in the 2y range of 10–701. The scan rate was set to 1.21/min in order to increase the signal-to-noise ratio. The surface microstructures of the samples were studied using a field-emission scanning electron microscope (FE-SEM, model JEOL JSAM 6700F). The compositions of the samples were also analyzed using a scanning electron microscope (SEM) equipped with energy dispersive analysis of X-ray (EDAX, model Hitachi S-3000N). The accelerating voltage of SEM was set at 5 kV. The surface microstructures and roughnesses of samples were analyzed using an atomic force microscope (AFM, model Park System XE 70) operated in non-contact mode. The optical transmittance and reflectance spectra of the samples were measured using a UV–vis–NIR spectrophotometer with an integrating sphere (JASCO Model V-670) in the wavelength range of 300–1000 nm at room temperature. The thickness of the samples was determined using a surface profile (Surfcorfer ET 3000). The Mott–Schottky plots of samples were measured using a computer-controlled potentiostat (model CHI 600 C) equipped with a frequency response analyzer. A three-electrode setup was employed, where the semiconductor film, a Pt plate electrode with an area of 1.0 cm2, and an Ag/AgCl electrode were the working, counter, and reference electrodes, respectively. A silver wire was attached to the conducting layer of the FTO-coated glass substrate by silver paste. The contacts and edges of the sample electrodes were sealed with epoxy resin and dried at room temperature. The electrolyte, aqueous K2SO3 (0.6 M, pH¼9.5), was freshly prepared using Milli-Q water and degassed by purging with high purity nitrogen, followed by ultrasonication for 30 min before each experiment. All experiments were carried out in a nitrogen environment at a temperature of 25 1C. The applied potentials were in the range of 1.0 to +0.0 V vs. the Ag/AgCl electrode. The PEC performance measurements of the samples were carried out in a quartz electrolytic cell. The sample (average area¼1.0 cm2), a Pt plate electrode (average area¼1.0 cm2), and an Ag/AgCl electrode were employed as the working, counter, and reference electrodes, respectively. A silver wire was attached to the conducting layer of the working electrode with silver paste, and the back contact and edges of the working electrode were sealed with epoxy resin. Aqueous K2SO3 (0.6 M, pH ¼9.5) solution, prepared using double deionized water and degassed by purging with nitrogen gas (99.99% purity) before each experiment, was used as the electrolyte. The electrolyte was put into an ultrasonic bath for 30 min before each experiment in order to decrease the influence of gas solutes. All measurements were carried out in a nitrogen environment at a temperature of 25 1C. Current densities, as a function of applied potential ( 1.0 to + 0.5 V vs. Ag/AgCl electrode) for the samples, were recorded under front-side illumination with a computer-controlled potentiostat (CHI 600 C) for all PEC experiments. A 300 W Xe short arc lamp (Perkin Elmer Model PE300BF) with white light intensity of 100 mW/cm2 was employed to simulate solar light. The intensity of incident
ARTICLE IN PRESS K.-W. Cheng, C.-J. Liang / Solar Energy Materials & Solar Cells 94 (2010) 1137–1145
light from the Xe lamp was measured using a photometer Model 70310 from Spectra-Physics.
3. Results and discussion 3.1. Characterization of samples on FTO-coated glass substrates Zn–In–S samples were grown on FTO-coated glass substrates using chemical bath deposition. The [Zn]/[In] molar ratio in the solution bath was varied to study its effect on the physical properties and PEC performance of the samples. Well-adherent yellow films were produced on the surface of the substrate after 4 h of deposition. The structural, optical, and electrical properties of semiconductor films with various deposition parameters were analyzed. Fig. 1(I) shows the XRD diffraction patterns of samples after heat treatment in the 2y range of 10–701 with various [Zn]/[In] molar ratios in the solution bath. The standard diffraction peaks of FTO [24], b-In2S3 [25], cubic ZnIn2S4 [26] and hexagonal Zn3In2S6 [27] reported in JCPDS cards are also shown in Fig. 1(I). Of note, a change was observed in the crystal phase of the samples with an increase in [Zn]/[In] molar ratio in the precursor solution. All samples showed the polycrystalline cubic spinel Zn–In–S phase. JCPDS Hexagonal Zn3In2S6
Intensity (arb. unit)
JCPDS Cubic ZnIn2S4 a b c d JCPDS Cubic β -In2S3 JCPDS FTO
10
20
30
FTO
40 2θ (degrees)
In2S3(311)
50
60
70
ZnIn2S4(311)
Intensity (arb. unit)
a
b
In2S3 c
d ZnIn2S4
25
26
27 28 2θ (degrees)
29
30
Fig. 1. X-ray diffraction patterns of samples on FTO-coated glass substrates for various [Zn]/[In] molar ratios in the precursor solutions. (a) [Zn]/[In] ¼0.75, (b) [Zn]/[In]¼ 0.5, (c) [Zn]/[In] ¼0.25 and (d) [Zn]/[In]¼ 0.125.
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With higher [Zn]/[In] molar ratios in the solution bath (samples (a) and (b)), some hexagonal Zn3In2S6 phase was observed. Fig. 1(II) shows the change in the (3 1 1) reflection in the XRD patterns for all samples in this study. The diffraction peak of the (3 1 1) plane for sample (a) approaches that of the ZnIn2S4 phase. As the molar ratio of [Zn]/[In] in the solution bath decreased (samples (a)-(d)), the peaks of XRD patterns of samples shifted to lower angles, which indicates that the crystal phase of samples approaches that of In2S3. The successive shift of the XRD pattern indicates that In3 + ions were incorporated into the lattice of samples with a decrease in the [Zn]/[In] molar ratio in the solution. This shift is reasonable because the ion radius of In3 + ˚ is larger than that of Zn + (0.74 A) ˚ [28,29]. For high [Zn]/ (0.81 A) [In] molar ratios in the solution (samples (a) and (b)), a phase separation was observed. Some hexagonal Zn3In2S4 phase was observed in samples (a) and (b). The thicknesses of samples obtained from surface profile measurements are shown in Table 1. The thicknesses of the samples are in the range of 1135–1714 nm. Fig. 2 shows the effect of the [Zn]/[In] molar ratios in the solution bath on film thickness and lattice parameter of the samples. With an increase in the [Zn]/[In] molar ratio in the solution bath, the thicknesses of samples increased and the lattice parameters of samples estimated from XRD patterns decreased. The values of lattice parameters for ZnIn2S4 [26] and In2S3 [25] reported in JCPDS cards are 1.062 and 1.072 nm, respectively. The lattice parameters of samples (a), (b), (c), and (d) estimated using XRD diffraction patterns are 1.066, 1.069, 1.071, and 1.072 nm, respectively. The lattice parameter of sample (a) approaches to that of ZnIn2S4 and that of sample (d) approaches to that of In2S3, which agree well with the values reported in the literature. The surface morphology of samples was analyzed using FE-SEM and AFM. The SEM images of samples (a) and (d) are shown in Figs. 3(A) and (C), respectively. The figures show that samples (a) and (d) were dense and uniform. Microspheres with lamellar microstructures were observed on the surface of sample (a) while irregular shaped platelet microstructures were observed for sample (d). Shen et al. [30] reported that microspheres with lamellar microstructures were observed for ZnIn2S4 and Puspitasari et al. [31] reported that irregular platelet microstructures were observed for In2S3. With decreasing [Zn]/ [In] molar ratio in the precursor solution, the microstructures of samples changed from the ZnIn2S4 phase to the In2S3 phase, which agrees well with the results of XRD. The AFM images of samples (a) and (d) are shown in Figs. 3(B) and (D), respectively. The surface roughnesses (dRM) of samples (a) and (d) obtained from AFM were 101.8 and 96.2 nm, respectively. The surface roughnesses of all samples obtained from AFM images are in the range of 95–105 nm, which indicates that there is no significant effect of the [Zn]/[In] molar ratio in the precursor solution on the surface roughness of samples. Quantitative analysis using EDAX was carried out to analyze the atomic ratios of Zn, In, and S in the samples. Changes in the compositions of films were studied by varying the [Zn]/[In] molar ratio in the precursor solution. The weight percentages of zinc, indium, and sulfur obtained from EDAX analysis were converted to atomic ratios. The atomic [Zn]/[Zn+ In] and 2[S]/(2[Zn]+3[In]) ratios of samples are listed in Table 1. Fig. 4 plots the atomic ratios of [Zn]/[Zn+In] and 2[S]/(2[Zn]+ 3[In]) in the samples as a function of the [Zn]/[In] concentration ratio in the precursor solution. The molar ratio of [Zn]/[Zn+ In] in the films decreased from 0.29 to 0.06 when the molar ratio of [Zn]/[In] was decreased from 0.75 to 0.125 in the precursor solution (samples (a)–(d)). The 2[S]/(2[Zn]+ 3[In]) ratio of the samples remained almost constant with decreasing [Zn]/[In] molar ratio in the solutions. The EDAX analysis of samples also shows that there was no sulphur deficit in the samples. The atomic ratios of elements in the film obtained
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Table 1 Physical properties of chemical-bath-synthesized Zn–In–S films on FTO-coated glass substrates. Sample [Zn]/[In] molar Thickness ratios in (nm) solution
(a) (b) (c) (d) a b
0.750 0.500 0.250 0.125
1714 1617 1516 1135
Atomic ratios for sample [Zn]/ [Zn+ In]
2S/ (2Zn+ 3In)
0.29 0.24 0.15 0.06
1.00 1.00 1.04 1.06
Direct Eg (eV)
Indirect Eg (eV)
EFBa (V vs. NHE)
EFBb (V vs. NHE)
ECB (V vs. NHE)
EVB (V vs. NHE)
Conduction type
2.74 2.67 2.61 2.47
2.11 1.96 1.94 1.88
0.58 0.54 0.50 0.45
0.70 0.65 0.59 0.41
0.88 0.84 0.80 0.75
1.86 1.83 1.81 1.72
n n n n
Obtained from Mott–Schottky plots. Determined from photocurrent density–applied voltage plots.
1.080
In an aqueous acid solution bath, thioacetamide undergoes the following reactions [16,33]:
2000 Thickness Lattice parameter
CH3CSNH2 + H2O2CH3CONH2 +H2S
1800
The following equilibrium reactions take place: 1600
1.070 1400
Thickness (nm)
Lattice parameter (nm)
1.075
1.065 1200
H2S + H2O2H3O + +HS
KA1 ¼10 7
(4)
HS +H2O2H3O + + S2
KA2 ¼10 17
(4)
where KA1 and KA2 are the first and second acid-ionization constants of H2S, respectively. TEA is used as a chelating agent for zinc ion to form metal complex: Zn2 + +TEA2[Zn(TEA)]2 +
1.060 0.10
0.20 0.30 0.40 0.50 0.60 0.70 [Zn]/[In] molar ratio in precursor solution
1000 0.80
Fig. 2. Effects of [Zn]/[In] molar ratio in the reaction solution on the film thickness and lattice parameters of samples.
Because the pH value of the aqueous solution is lower than 4, indium in the solution exists as free In3 + ions. Complexes of In3 + may be formed with TEA: In3 + + TEA2[In(TEA)]3 + The equilibrium constant, K, is defined as
from EDAX analysis show that the growth rate of ZnS in samples became slower with decreasing [Zn]/[In] molar ratios in the reaction solution (samples (a)-(d)). According to the phase diagram of ZnS and In2S3 [32], approximately 23 mol% ZnS can be dissolved in In2S3 at 600 1C. It can be estimated that approximately 25 mol% ZnS is dissolved in In2S3 at 550 1C using the phase diagram of the ZnS and In2S3 system. ZnS and In2S3 can form a continuous solid solution series with [Zn]/[Zn+ In] molar ratios less than 0.15. Samples (c) and (d) indicated that ZnS can form a solid solution with In2S3. Some impurities (ZnIn3S6 phase) were observed in samples (a) and (b) due to the [Zn]/[Zn +In] molar ratios being greater than 0.15. The results obtained from EDAX analysis agree well with the results from the XRD analysis.
M+L2ML
K¼[ML]/[M][L]
(4)
where M¼Zn2 + or In3 + ions, and L¼TEA in this study. The value of log K for Zn(TEA)2 + is 2.56 [34]. However, the equilibrium constant of [In(TEA)]3 + has not be reported in the literature [33]. In this study, the simplified method proposed by Chang et al. [16] was used. The concentration of free indium ions is the same as that in the initial reaction solution. The estimated free [Zn2 + ] concentrations of samples (a)–(d) using a equilibrium constant of are 1.0 10 2, 5.8 10 3, 2.45 10 3 and Zn(TEA)2 + 1.1 10 3 M, respectively. The estimate free [In3 + ] concentration of samples (a)–(d) in solution bath is 2.4 10 2 M. Solubility products (Ksp) of zinc sulfide and indium sulfide are given by [35,36]
3.2. Simple nucleation and growth mechanism
Zn2 + +S2 -ZnS
The chemistry of the solution bath affects the physical properties of the film on the substrates. Nucleation and the growth of films change due to the degree of supersaturation, interfacial energy, and the strength of coordination bonds of the chelating reagents. A possible nucleation and growth mechanism of samples is discussed below using the stability of metal complexes and classical nucleation theory.
2In3 + + 3S2 -In2S3
Ksp ¼2.93 10 25
(4)
Ksp ¼5.8 10 74
The sulfur concentrations for saturated zinc sulfide and indium sulfide in these conditions are in the order of 10 22–10 23 and 10 24 M, respectively. This indicates that indium sulfide precipitated more easily under our experimental conditions. With a decrease in zinc ion concentration in the precursor solution
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1141
Fig. 3. SEM and AFM images of samples.
(samples (a)-(d)), the deposition rate of zinc sulfide decreased while the indium sulfide remained almost unchanged in the experiments. Because the deposition rate of zinc sulfide decreased, the thickness of zinc sulfide film decreased with a decrease in the [Zn]/[In] molar ratio in the solution. A thinner sample was obtained with a decrease in the [Zn]/[In] molar ratio in the precursor solution. The atomic ratios of [Zn]/[Zn+In] in samples (a)–(d) decreased with a decrease in zinc ions in the precursor solution due to a decrease in the growth rate of zinc sulfide. After thermal treatment of the samples, the reaction of formation for Zn–In–S can be given as xZnS + yIn2S3-ZnxIn2yS(3y + x)
(4)
3.3. Optical properties The transmittance and reflectance spectra of samples in the wavelength range of 300–1000 nm are shown in Fig. 5. The maximum transmittance spectrum of samples in this study was 80% transparency in the near infrared region. The reflectance
spectra of samples were almost the same in the range of 300– 1000 nm. The energy band gap (Eg) is an important parameter for a PEC device application. The absorption coefficient (a) of samples can be estimated using the Manifacier model:
a ¼ 1=t ln½ð1RÞ=T
ð1Þ
where t is the thickness of the sample and T and R are the transmittance and reflectance data, respectively [37]. The thicknesses of samples in this study obtained from the surface profile measurements are shown in Table 1. The thicknesses of samples are in the range of 1714–1135 nm. They decrease with decreasing [Zn]/[In] molar ratio in the precursor solution, which agrees well with the results obtained from nucleation and growth mechanism. The relationship between the absorption coefficient (a) and the incident photon energy (hv) can be written as ðahvÞ ðhvEg Þn
ð2Þ
Generally, n is 1/2 for a direct band gap and 2 for the indirect band gap [38]. The optical band gaps of samples can be obtained by plotting (ahn)1/n with respect to hv and extrapolating it to
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1.5
1.5
1.0x1010
1.2
1.2
8.0x109
0.9
0.9
0.6
0.6
Zn/(Zn+In) (a) (b) (c) (d)
0.3
0.3
(αhν)2 (eV/cm)2
2S/ (2Zn+3In) molar ratio in samples
Zn/ (Zn+In) molar ratio in samples
2S/(2Zn+3In)
6.0x109
4.0x109
2.0x109
0.0 2.2
0.0 0.1
0.2 0.3 0.4 0.5 0.6 0.7 [Zn]/[In] molar ratio in precursor solutions
2.4
2.6
2.8
3.0
3.2
hν (eV)
0.0 0.8
400
(a) (b) (c) (d)
Fig. 4. The atomic ratios of [Zn]/[Zn+ In] and 2[S]/(2[Zn] +3[In]) for samples as a function of the [Zn]/[In] molar ratio in the reaction solution.
(αhν)1/2 (eV/cm)1/2
300
100 Reflectance / Transmittance (%)
(a) (b) (c) (d)
80
T
60
200
100
40 0 1.6
20 R
1.8
2.0
2.2
2.4
2.6 2.8 hν (eV)
3.0
3.2
3.4
3.6
Fig. 6. Plots of (ahv)1/n vs. hv of samples. (I) n¼1/2 and (II) n¼2.
0 300
400
500
600 700 Wavelength (nm)
800
900
1000
Fig. 5. Transmittance and reflectance spectra of samples prepared in this study.
(ahn)1/n ¼0. Both direct and indirect band to band transitions can be observed in ZnIn2S4 [6,23]. Figs. 6(I) and (II) show the linear dependence of (ahn)2 and (ahn)1/2on hv, respectively. The direct and indirect band gaps of samples obtained from Figs. 6(I) and (II) are shown in Table 1. The direct and indirect band gaps of samples prepared in this study were in the ranges of 2.74–2.47 eV and 1.88–2.11 eV, respectively. The direct and indirect band gaps of samples increased with an increase in zinc ions in the precursor solution. The direct band gaps of In2S3, ZnIn2S4 and Zn3In2S6 are 2.4, 2.8 and 2.81 eV, respectively [39,40]. The direct band gaps for samples (a) and (d) are 2.74 and 2.47 eV, respectively, and those of samples (b) and (c) lie between those of In2S3 and ZnIn2S4, which agree well with the XRD patterns and the values published in the literature. The band gaps of samples increased with an increase in zinc ions in the precursor solution because the phase of samples changed from In2S3 to ZnIn2S4. 3.4. Photoelectrochemical properties The flat band potentials of a semiconductor can be measured in a variety of ways [2,14–15]. One method based on the capacitance
vs. applied potential measurement is the Mott–Schottky relationship for the barrier. The Mott–Schottky equation is 1=C 2 ¼ B ½2=ðee0 eND A2 Þ½EEFB ðkT=eÞ
ð3Þ
where e is the dielectric constant of the semiconductor, A is the surface area of the semiconductor/electrolyte barrier, ND is the carrier density of the semiconductor, EFB is the flat band potential of the semiconductor, e is the electric charge and e0 is the permittivity of the vacuum. For an n-type semiconductor, B is 1, while for a p-type semiconductor, B is 1. The flat band potentials of samples, EFB, can be estimated from the point of intersection of C 2 vs. applied potentials, E, plot. In order to develop a physically reasonable equivalent circuit of the samples in the electrolyte for Mott–Schottky measurements, impendence analysis for samples was performed. Readecka et al. [41] and Loef et al. [42] reported that the equivalent circuit of samples can be simplified into a series of resistance–capacitance (RC) circuits if 0 the real part of impedance (Z ) is independent of frequency. Fig. 7 shows the Mott–Schottky plots for samples in 0.6 M K2SO3 aqueous solution. The inset in the figure shows the relationship 0 between the real parts of impedance spectra (Z ) of samples and frequency with external potentials of 1.0 V vs. an Ag/AgCl electrode. At frequencies higher than 10 KHz, the real part of the impedance is frequency independent and the influence of the constant-phase element on the impedance spectrum becomes
ARTICLE IN PRESS K.-W. Cheng, C.-J. Liang / Solar Energy Materials & Solar Cells 94 (2010) 1137–1145
Z' (Ω)
(a) (b) (c) (d)
a b c d
100
1.2x1011 10 1
10
8.0x1010
100 1000 10000 100000 frequence (Hz)
4.0x1010
0.0 -0.9
-0.8
-0.7
-0.6
-0.5
1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -1.0 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -1.0
Applied potential (V vs. Ag/AgCl electrode)
negligible. In this study, the frequency of impendence was set to 10 kHz for measuring the Mott–Schottky plot for all samples. The flat band potentials of samples obtained from Mott–Schottky plots are shown in Table 1. The flat band potentials in K2SO3 solution at pH¼ 9.5 of these samples lie in the range of 0.80 to 0.67 V vs. Ag/AgCl electrode ( 0.58 to 0.45 V vs. the normal hydrogen electrode, NHE). The Mott–Schottky measurements for samples were also employed in this study using 0.5 M K2SO4 (pH¼7.0) as the electrolyte (which are not shown here). There is no significant effect of the pH value in the electrolyte on the flat band potentials of samples. It is known that sulfur species semiconductors do not react with H + or OH ion in solution. The band edges at the interface do not shift with the pH value of the electrolyte [2,8,14]. Because the B value of Mott–Schottky plots is 1 for all samples in this study, the samples are n-type semiconductors. The position of the conduction band (ECB) for many n-type semiconductors is 0.1–0.3 V more negative than that of EFB [43]. Based on this, the difference between ECB and EFB is assumed to be 0.3 V for determining the band positions of samples by electrochemical measurements. The positions of the valence band and conduction band for samples using the direct energy band gaps from transmittance and reflectance spectra and flat band potentials from Mott–Schottky plots are shown in Table 1. The PEC responses of samples were measured to examine the possibility of their use for photoelectrolysis. With K2SO3 (0.6 M, pH¼ 9.5) solution as the electrolyte, the holes produced in valance 2 band of samples oxidized SO2 3 to form SO4 under irradiation. Water is reduced to hydrogen by the electrons produced in the conduction ion under band of samples accompanied by oxidation of SO2 3 illumination. The following reactions occur in the electrolyte [8,28]: hv
ZnInS samples-e ðCBÞ þh þ ðVBÞ
ð4Þ
2 þ SO2 3 þ 2OH þ2h -SO4 þ H2 O
ð5Þ
2H2 O þ2e -H2 þ2OH
ð6Þ
Fig. 8 shows the photo and dark current densities of the samples with applied potentials in the range of 1.0 V to + 0.5 V vs. an Ag/AgCl electrode. All samples show an anodic photocurrent under illumination. The maximum photocurrent density of samples in this study approached 0.67 mA/cm2 at an external potential of + 0.5 V vs. an Ag/AgCl electrode in contact with 0.6 M K2SO3 solution under illumination with the light intensity set at 100 mW/cm2. The photocurrent is in the positive
0.5
0.5
Fig. 8. Photocurrent density–applied voltage plots in the range of 1.0 V to +0.5 V vs. an Ag/AgCl electrode for samples in 0.6 M K2SO3 aqueous solution.
0.80 Photo-enhancement current density (mA/cm2)
Fig. 7. Mott–Schottky plots of the samples with various deposition parameters in aqueous 0.6 M K2SO3 solution.
1.0 Photocurrent (b) 0.8 Dark current (b) 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.5 0.0 0.5 -1.0 -0.5 0.0 1.0 Photocurrent (d) Photocurrent (c) 0.8 Dark current (d) Dark current (c) 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.5 0.0 0.5 -1.0 -0.5 0.0 Applied voltage (V vs. Ag/AgCl electrode) Photocurrent (a) Dark current (a)
2.8
0.60 2.6
0.40
2.4 0.20
Energy band gap of samples (eV)
1/C2 (cm4/F2)
1.6x1011
1000
Photocurrent density (mA/cm2)
2.0x1011
1143
Photoenhencement effect Energy band gap
0.00 0.05
0.10 0.15 0.20 0.25 [Zn]/[Zn+In] molar ratio in samples
2.2 0.30
Fig. 9. Effects of [Zn]/[Zn+ In] molar ratio in samples on photo-enhancement current density (external bias kept at 0.5 V vs. an Ag/AgCl electrode) and energy band gap of samples.
potential area, which indicates that the samples are n-type semiconductors. This is in agreement with the results of Mott–Schottky plots. The onset of photocurrent density can be treated as a measure of the flat band potentials, EFB [44,45]. Table 1 also shows the flat band potentials of samples obtained from photocurrent density–applied voltage measurements. The values of flat band potentials determined from photocurrent density–applied voltage measurements are in the range of 0.70 to 0.41 V vs. NHE. Fig. 9 shows the relationship of the photoenhancement current density (external bias kept at 0.5 V vs. an Ag/AgCl electrode) and direct energy band gaps of samples and the [Zn]/[Zn+ In] molar ratio in the solution. The definition of photo-enhancement current density for the samples is the difference between photocurrent density and dark current density. With an increase in the molar ratio of zinc ions in the solution, the energy band gaps of samples increased and the photo-enhancement effects of samples decreased. Sample (d) had
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Acknowledgements
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The authors are grateful to Chang Gung University in Taiwan (Grant no. UERPD280271) for supporting this study. The authors would like to thank the Microscopic Center at Chang Gung University and the Nanotechnology R&D Center at Kun Shan University for SEM-EDAX analysis and FE-SEM micrographs, respectively. The authors would also like to thank Dr. Lee Tai-Chou from National Chung Cheng University and Dr. Huang Chao-Ming from Kun Shan University for their invaluable input.
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Fig. 10. Current density of sample (b) as a function of time with the external bias kept at 0.4 V vs. an Ag/AgCl electrode in 0.6 M K2SO3 aqueous solution.
the maximum photo-enhancement current density because it has the narrowest band gap. Sample (a), which had the largest energy band gap, had the lowest photo-enhancement current density. Some impurities, such as Zn2In3S6, were observed in samples (a) and (b) from XRD patterns. The impurity phases act as recombination sites for photogeneated electrons and holes, which decreases the photo-enhancement effect of samples. Similar observations in PEC responses for ZnIn2S4 were reported in the literature [23,30]. The stability of sample (b) was tested with the external bias kept at +0.4 V vs. an Ag/AgCl electrode in 0.6 M K2SO3 aqueous solution. Fig. 10 shows the current density of sample (b) as a function of time for 1000 s using the chopping method. The current density of sample (b) changed from 0.42 to 0.32 mA/cm2 during the course of the experiment.
4. Conclusions In this study, polycrystalline Zn–In–S photoelectrodes were deposited on FTO-coated glass substrates. The influence of the [Zn]/[In] molar ratio in the precursor solution on the structural, electric and optical properties of the samples was investigated. The polycrystalline ZnIn2S4 phase with some impurity (Zn3In2S4 phase) was observed with [Zn]/[In] molar ratios in the solution bath greater than 0.5. As the [Zn]/[In] molar ratio in the solution bath was decreased, the crystal phase of samples approached the In2S3 phase, as revealed by the peaks of XRD patterns. With an increase in the [Zn]/[In] molar ratio in the precursor solution, the [Zn]/[Zn+ In] atomic ratios in samples increased. There was no sulphur deficit in the samples. The thicknesses, direct and indirect energy band gaps of the samples are in the ranges of 1135–1714 nm, 2.47–2.74 and 1.88–2.11 eV, respectively. With an increase in the [Zn]/[In] molar ratio in the precursor solution, the thicknesses and band gaps of samples increased. At an irradiation of 100 mW/cm2 from a Xe lamp, the maximum photocurrent density of samples in this study was 0.67 mA/cm2 at an external potential of + 0.5 V vs. an Ag/AgCl electrode in contact with 0.6 M K2SO3 solution. The results show that ternary Zn–In–S film electrodes have potential in PEC applications.
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