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Photoelectrochemical study of ZnIn2Se4 electrodes fabricated using selenization of RF magnetron sputtered Zn–In metal precursors
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 3 7 6 3 e1 3 7 6 9
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Photoelectrochemical study of ZnIn2Se4 electrodes fabricated using selenization of RF magnetron sputtered ZneIn metal precursors Kong-Wei Cheng*, Ya-Hsin Cheng, Miao-Syuan Fan Department of Chemical and Materials Engineering, Chang Gung University, 259 Wen-Hwa 1st Rd., Kwei-Shan, Taoyuan 333, Taiwan
article info
abstract
Article history:
Polycrystalline ZnIn2Se4 samples are grown on glass substrates and fluorine-doped tin
Received 4 December 2011
oxide coated glass substrates using the selenization of radio-frequency magnetron sput-
Received in revised form
tered ZneIn metal alloys. The effect of the [Zn]/[Zn þ In] molar ratio in the metal alloys on
16 March 2012
the physical and photoelectrochemical properties of the samples is investigated. X-ray
Accepted 16 March 2012
diffraction patterns of samples reveal that the samples are polycrystalline tetragonal
Available online 14 April 2012
ZnIn2Se4. The thicknesses and direct band gaps of the samples are in the ranges of 1.15 e1.44 mm and 1.68e1.81 eV, as obtained from surface profile measurements and trans-
Keywords:
mittance/reflectance spectra, respectively. The flat-band potentials of the samples in 0.6 M
Hydrogen production
K2SO3 electrolyte are in the range of 0.41 to 0.95 V vs. an Ag/AgCl reference electrode.
Photoelectrode
The highest photoelectrochemical response of samples was 1.84 mA/cm2 at an external
ZnIn2Se4
potential of þ1.0 V vs. an Ag/AgCl electrode in 0.6 M K2SO3 solution under illumination
Magnetron sputtering
from a 300 W Xe lamp system with the light intensity set at 100 mW/cm2. Crown Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Solar energy can be converted into hydrogen or electric power to reduce society’s dependence on fossil fuel. Interest in solar energy has led to increased research into photoelectrochemical (PEC) technology, whose major applications include solar cells and photocatalytic reactions. Hydrogen production from water using a PEC process has been extensively studied since the Fujishima-Honda effect, which involves a n-type TiO2 photoelectrode, was reported [1]. Ternary IIeIIIeVI (II ¼ Mg, Hg, Cd, Zn; III ¼ Al, In, Ga; VI ¼ S, Se, Te) semiconductors have received considerable interest for photochemical applications because they exhibit interesting optical and electrical properties in photo-absorbers of PEC and
solar cell devices [2,3]. Kokate et al. [4] prepared a CdIn2S4 photoelectrode with a conversion efficiency of 2.94% in an aqueous polysulfide solution obtained using a 200 W tungsten lamp as the light source. Sirimanne et al. [5] prepared a MgIn2S4/MgIn2O4 photoelectrode in an aqueous solution containing polysulfide solution that produced a photocurrent density of 30 mA/cm2 using an external bias of þ0.7 V (vs. an Ag/AgCl electrode) and a light intensity of 250 mW/cm2. Sawant et al. [6] prepared CdIn2S4 thin films with an efficiency of 1.06% in an aqueous polysulfide solution obtained using a 500 W tungsten lamp as the light source. Yadav et al. [3] prepared n-ZnIn2Se4 films on fluorine-doped tin oxide (FTO) coated glass substrates by spraying an equimolar aqueous solution of zinc sulphate, indium tri-chloride, and selenourea.
* Corresponding author. Tel.: þ886 3 2118800 3353; fax: þ886 3 2118668. E-mail address: [email protected] (K.-W. Cheng). 0360-3199/$ e see front matter Crown Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.03.087
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The efficiency of their liquid junction n-ZnIn2Se4/ 1M(NaOH þ Na2S þ S)/C solar cell was 1.47%. Yu et al. [7] prepared ZnIn2S4 film on Ti substrates using the electrodeposition; the film exhibited a remarkable photocatalytic bactericidal ability under visible-light irradiation. For industrial applications, thin-film devices are convenient for collecting hydrogen in PEC cells because no further separation procedures are required. In order to prepare an efficient thinfilm reactor, it is necessary to understand the properties of newly developed semiconductor films. Recently, an interesting reactive selenization approach for the deposition of CuInSe2 film was reported [8]. The selenization of metal precursors can be done in a gas mixture containing Ar and H2Se or in the presence of solid selenium. The reactive selenization process of a metal alloy is a simple method for obtaining large-area and high-quality photo-absorbing thin films. However there are few reports on the synthesis and characterization of ZnIn2Se4 ternary semiconductor films prepared using the reactive selenization process of a metal alloy. In the present study, ZnIn2Se4 samples were prepared on glass substrates and FTO-coated glass substrates using the reactive selenization process of sputtered ZneIn metal alloy. The effect of the [Zn]/[Zn þ In] molar ratio in the metal alloys on the physical and PEC properties of the samples was studied.
2.
Experimental details
Samples on substrates were prepared using the reactive selenization process with the sputtering of ZneIn metal alloys. The ratio of zinc to the total amount of precursors ([Zn]/[Zn þ In]) was verified during the deposition of the ZneIn metal alloys. Metal alloys with a ZneIn bi-layer structure were deposited on glass substrates and FTO (sheet resistance ¼ 10 U/square)-coated glass substrates using radio-frequency (RF) magnetron sputtering at room temperature. The metal precursors were deposited on substrates using alternate RF sputtering with 2-inch Zn (4N) and In (4N) elemental targets. After the deposition chamber was evacuated to a base pressure of 5 106 Torr, highpurity (99.995%) argon gas was used to provide the plasma at a base pressure of 2.5 103 Torr. The RF sputtering power was kept at 20 W. Due to the poor adhesion of the zinc layer to the substrate, an indium layer was first deposited on the substrate, followed by the deposition of the zinc layer. The sputtering rates of Zn and In on the substrates obtained from a digital quartz controller (Filtech SQM-180) were 16.5 nm/min and 17.5 nm/min, respectively. The [Zn]/[Zn þ In] molar ratio in the metal alloys was set in the range of 0.35e0.60. The selenization process of metal alloys was carried out in a closed container made of aluminum oxide, which was similar to that used for the sulfurization of CuInS2 [9]. The metal alloys and a sufficient amount of elemental selenium were placed in the container and loaded into an evacuated quartz tube with a vacuum of around 103 Torr. A two-stage temperature profile was used for the selenization of the metal alloys. In the first stage, the metal precursors and Se powders were annealed at 250 C for 2 h in order to
incorporate the latter into the former. The formation of ZnIn2Se4 on the substrate was carried out at 500 C for 2 h. The phase formation and crystallographic study of ZneIn metal alloys and samples on glass substrates was conducted using an X-ray diffractometer (Siemems D5005) with CuKa ˚ ) radiation. The X-ray diffraction (XRD) patterns (l ¼ 1.5405 A were recorded in the 2q range of 10e80 . The surface microstructures of the samples were studied using a field-emission scanning electron microscope (FE-SEM, JEOL JSM 6700F). The compositions of the samples were analyzed using a scanning electron microscope (SEM, Hitachi S-3000N) equipped with an energy-dispersive analysis of X-ray (EDAX). The optical transmittance and reflectance of the samples were measured in the wavelength range of 350e600 nm at room temperature using an ultravioletevisibleenear-infrared (UVeViseNIR) spectrophotometer with an integrating sphere (JASCO V-650). The thickness of the samples was determined using surface profile measurements (Sloan Dektak 3030). The MotteSchottky (MeS) plots of the films on FTO-coated glass substrates were measured using a computer-controlled potentiostat (CHI 600 C) equipped with a frequency response analyzer. The measurements were carried out in an electrolyte solution of 0.6 M K2SO3. For frequencies above 10 kHz, the equivalent circuit of samples in the electrolyte can be simplified into a resistanceecapacitance (RC) circuit. The frequency of impendence was set to 10 kHz to measure the MeS plots for various samples. The applied potentials were in the range of 1.2 V to þ 0.2 V vs. an Ag/AgCl reference electrode for MeS measurements. The PEC performance measurements of the samples on FTO-coated glass substrates were carried out in a quartz electrolytic cell with the sample (average area ¼ 1.0 cm2), a Pt plate electrode (average area ¼ 1.0 cm2), and an Ag/AgCl electrode serving as the working, counter, and reference electrodes, respectively. Aqueous 0.6 M K2SO3 solution was used as the electrolyte. All measurements were carried out in a nitrogen environment at a temperature of 25 C. Current density, as a function of applied potential (1.0 w þ1.0 V vs. an Ag/AgCl electrode) for samples, was recorded with a computer-controlled potentiostat (CHI 600 C) in the dark and under illumination. A 300-W Xe short arc lamp (Perkin Elmer PE300BF) with a white-light intensity of 100 mW/cm2 was employed to simulate solar light. The intensity of incident light from the Xe lamp was measured using a photometer (Newport 818P-15e19).
3.
Results and discussion
ZnIn2Se4 thin films were deposited on glass and FTO-coated glass substrates, respectively, using the reactive selenization process with the sputtering of ZneIn metal alloys. The [Zn]/ [Zn þ In] molar ratio in the RF-sputtered ZneIn metal alloys was varied to study its effect on the physical and PEC properties of the samples. Adherent films on glass substrates were obtained after the two-stage reactive selenization process with the metal precursors. The structural and optical properties of semiconductor films with various deposition parameters were analyzed. Table 1 shows the [Zn]/[Zn þ In] molar ratios in the as-deposited metal precursors on substrates obtained using EDAX analysis. The [Zn]/[Zn þ In] molar ratio in
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Table 1 e Physical properties of ZnIn2Se4 samples on substrates used in this study. Sample
(a) (b) (c) (d) (e)
[Zn]/[Zn þ In] ratio in metal precursors (from EDAX analysis) 0.35 0.40 0.47 0.50 0.59
Molar ratio of ZnIn2Se4 sample (from EDAX analysis)
Thickness
Direct Eg
EFBa
EFBb
[Zn]/[Zn þ In]
[2Se]/[2Zn þ 3In]
(mm)
(eV)
(vs. Ag/AgCl)
(vs. Ag/AgCl)
0.24 0.27 0.32 0.37 0.39
1.1 1.09 1.0 1.03 1.06
1.15 1.44 1.37 1.31 1.34
1.68 1.69 1.77 1.78 1.81
0.41 0.95 0.65 0.44 0.62
0.74 0.75 0.82 0.80 0.42
a Obtained from MotteSchottky plots. b Determined from current density-applied voltage plots.
the as-deposited metal precursors were in the range of 0.35e0.59. Fig. 1(I) shows the XRD patterns of as-deposited ZneIn metal alloys with various [Zn]/[Zn þ In] molar ratios in the metal alloys on glass substrates. Only tetragonal In (JCPDS card no. 85-1409) and hexagonal Zn (JCPDS card no. 4-831) phases were detected in the ZneIn metal precursors on glass substrates. With an increase in the [Zn]/[ZnþIn] molar ratio in the metal alloys, the intensities of peaks at 2q of 36.4。 and 39.2。for Zn phase and that at 2q of 33.2。for In phase deceased. The possible reason is some structure defects formed in the metal precursors, which effectively decrease the crystallinity of the ZneIn metal alloys. Fig. 1(II) shows the XRD patterns of samples after the selenization process with the ZneIn metal alloys. The peaks at 2q of 27 , 44.9 and 53.2 correspond to the crystal planes of (112), (204) and (312) for the tetragonal ZnIn2Se4 phase. With an increase in the [Zn]/[Zn þ In] molar ratio in the metal alloys (samples (a)e(e)), the XRD patterns of the samples become almost the same as that of the tetragonal ZnIn2Se4 phase, except that the diffraction peaks were slightly shifted to higher angles. The successive shift of the XRD pattern indicates that Zn2þ was incorporated into the lattice of ZnIn2Se4 samples with an increase in the [Zn]/[Zn þ In] molar ˚ ) is ratio in metal alloys because the ionic radius of Zn2þ (0.74 A ˚ ) [10]. With an increase in the smaller than that of In3þ (0.76 A [Zn]/[Zn þ In] molar ratio in the metal alloys, the number of Zn2þ ions occupying the In3þ sites in the samples increased, which shifted the peaks of XRD patterns to higher angles. The thicknesses of samples obtained from surface profile measurements are shown in Table 1. The thicknesses of the samples are in the range of 1.15e1.44 mm. Quantitative analysis using EDAX was carried out to analyze the atomic ratios of Zn, In, and Se in the samples. The weight percentages of zinc, indium and selenium from EDAX analysis were converted to atomic ratios. The [Zn]/[Zn þ In] molar ratio in the metal alloys for samples (a)e(e) are 0.35, 0.40, 0.47, 0.50 and 0.59, respectively. The [Zn]/[Zn þ In] molar ratio in the samples (a)e(e) obtained from EDAX analysis are 0.24, 0.27, 0.32, 0.37 and 0.39, respectively. The results listed in Table 1. When the [Zn]/[Zn þ In] molar ratio in the metal alloys was increased from 0.35 to 0.59 (samples (a)e(e)), that in the samples increased from 0.24 to 0.39. The 2[Se]/[2Zn þ 3In] molar ratio of the samples was in the range of 1.0e1.1. Fig. 2 shows the [Zn/(Zn þ In)] and 2[Se]/[2Zn þ 3In] molar ratios in the samples as function of the [Zn/(Zn þ In)] molar ratio in the metal alloys. The [Zn/(Zn þ In)] molar ratio in the samples
Fig. 1 e X-ray diffraction patterns of samples with various [Zn]/[Zn D In] molar ratio in (I) as-deposition metal alloys and (II) after selenization process. (a) [Zn]/[Zn D In] [ 0.35, (b) [Zn]/[Zn D In] [ 0.40, (c) [Zn]/[Zn D In] [ 0.47,(d) [Zn]/ [Zn D In] [ 0.50 and (e) [Zn]/[Zn D In] [ 0.59 in metal precursors.
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1.20
1.20
1.00
1.00
0.80
0.80
0.60
0.60
0.40
0.40
[Zn]/[Zn+In]
0.20
[2Se]/[2Zn+3In] ratio in sample
[Zn]/[Zn+In] ratio in sample
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0.20
[2Se]/[2Zn+3In] 0.00 0.35
0.40
0.45
0.50
0.55
0.60
0.00 0.65
[Zn]/[Zn+In] molar ratio in metal precursors
Fig. 2 e Molar ratios of [Zn]/[Zn D In] and 2[Se]/(2[Zn]D3[In]) for samples as functions of the [Zn]/[Zn D In] molar ratio in the metal precursors.
increased with increasing in the [Zn/(Zn þ In)] molar ratio in the metal alloys. The EDAX analysis of samples shows that there is no apparent selenium deficit in the samples. The EDAX analysis also shows that the number of Zn2þ ions occupying the In3þ sites in the samples increases with increasing in [Zn/(Zn þ In)] molar ratio in the metal alloys. The results obtained from EDAX analysis agree with those obtained from XRD patterns of samples. FE-SEM images were used to observe the microstructures of the sample surfaces. Fig. 3 shows that samples (a), (c), and (e) are dense and uniform and have no observable voids or pinholes. Plate-like aggregation microstructures were observed on the surface of sample (a). Furthermore, the grain size of samples tended to decrease with increasing in [Zn]/ [[Zn þ In] molar ratio in the metal alloys. With a [Zn]/[Zn þ In] molar ratio of 0.59 in the ZneIn metal alloys, flower-like microstructures were observed on the surface of sample (e). The transmittance and reflectance spectra of samples for wavelengths of 350 w 600 nm are analyzed using UVeViseNIR spectrophotometer. The energy band gaps (Eg) of samples are important properties for photoluminescence and PEC device applications. The absorption coefficients (a) of samples can be estimated using the Manifacier model [11]: a ¼ 1=t ln½ð1 RÞ=T
(1)
where t is the thickness of the sample and R and T are the reflectance and transmittance data, respectively. The thicknesses of samples determined from surface profile measurements are listed in Table 1. The relationship between the absorption coefficient (a) and the incident photon energy (hv) can be written as: n ahv ¼ A hv Eg
(2)
Generally, n is 2 for an indirect band gap and 1/2 for a direct band gap [12]. The optical energy band gaps of samples can be
Fig. 3 e SEM images of (I) sample (a), (II) sample (c), and (III) sample (e).
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1=C2 ¼ B 2=
2 3 3 0 eND A
½E EFB ðkT=eÞ
(3)
where 3 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 sample, e is the electric charge, and 3 0 is the permittivity of the vacuum. For a n-type semiconductor, B is 1, whereas for a p-type semiconductor, B is 1. The flat-band potentials of samples, EFB, can be estimated from the point of intersection in the C2 vs. applied potentials E, plot. Readecka et al. [19] and Loef et al. [20] reported that the equivalent circuit of samples can be simplified into a series of resistanceecapacitance (RC) circuits if the real part of the imped0 ance (Z ) is independent of frequency. All the samples during the measurement of the MeS plot from AC impendence were controlled at 10 kHz because at frequencies higher than 10 kHz, the real part of the impedance reveals no frequency dependence and the effect of the constant-phase element on the impedance spectrum becomes negligible. Fig. 5 shows the MeS plots for samples in K2SO3 (0.6 M) aqueous solution. The
9
(a) (b) (c) (d) (e)
1.0x10
8
8
6.0x10
2
(αhυ) (eV/cm)
2
8.0x10
8
4.0x10
8
2.0x10
0.0 1.5
1.6
1.7
1.8
1.9
hν (eV)
Fig. 4 e Plots of (ahv)2 versus hv of samples.
2.0
11
2.0x10
11
1.5x10
(a) (b) (c) (d) (e)
2
4
2
1/C (cm/F )
obtained by plotting (ahn)1/n with respect to hv and extrapolating the curve to (ahn)1/n ¼ 0. Fig. 4 shows the linear dependence of (ahn)2 on hv. The direct energy band gaps of samples obtained from Fig. 4 are shown in Table 1. The direct energy band gaps of samples prepared in this study varied from 1.68e1.81 eV. The energy band gap of ZnIn2Se4 is in the range of 1.41e1.74 eV [13,14]. With an increase in the [Zn]/[Zn þ In] molar ratio in the ZneIn metal alloys, the energy band gap of the samples increased. The energy bad gap of sample (e) (Znrich sample) is 1.81 eV. The energy band gap of ZnSe is in the range of 2.42e2.72 eV [15]. Because the [Zn]/[Zn þ In] molar ratio in sample (e) is 0.39, the energy band gap of sample (e) is larger than those of samples (a)e(d). An important property for the PEC response is the flatband potential. The flat-band potential of a sample is a useful quantity in photoelectrochemistry as it facilitates the determination of the energetic position of the valence and conduction band edges of a given semiconductor material [16,17]. Various methods can be used to measure the flat-band potential [17,18]. In this study, the capacitance vs. applied potential based on the MeS relationship is used [17,18]:
11
1.0x10
10
5.0x10
0.0 -1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
Applied potential (V vs. Ag/AgCl)
Fig. 5 e MotteSchottky plots of samples on FTO-coated glass substrates in aqueous 0.6 M K2SO3 solution.
flat-band potentials of samples (a)e(e) obtained from MeS plots are shown in Table 1. The flat-band potentials in 0.6 M K2SO3 aqueous solution of samples (a)e(e) are in the range of 0.41 to 0.95 V vs. an Ag/AgCl electrode. It can be observed that the slope of the MeS plot is positive, which indicates that the samples are n-type semiconductors. The samples were installed in a three-electrode PEC cell to feasibility the possibility of their application in solar energy conversion. The applied potential between the working and reference electrodes was varied from 1.0 to þ1.0 V vs. an Ag/ AgCl electrode. 0.6 M K2SO3 aqueous solution was used as the electrolyte. For the 0.6 M K2SO3 solution, the following reactions may occur [10]: hv
samples / e þ hþ
(4)
2H2 O þ 2e /H2 þ 2OH
(5)
2 þ þ SO2 3 þ H2 O þ 2h /SO4 þ 2H
(6)
Current densities of sample (a) with an applied potential in the range of 1.0 V to þ1.0 V vs. an Ag/AgCl electrode obtained using the chopping method in 0.6 M K2SO3 aqueous solution is shown in Fig. 6(I). Sample (a) has the anodic photoenhancement effect. Fig. 6(II) shows the relationship of the photo-enhancement current densities (J ) of samples and the [Zn/(Zn þ In)] molar ratio in the samples with applied voltage kept at 0.5 V, 0.0 V, þ 0.5 V and þ1.0 V (vs. an Ag/AgCl) in 0.6 M K2SO3 solution. The photo-enhancement current density for the samples is defined as the difference between photocurrent density and dark-current density at the same applied voltage. With an increase in applied voltage (vs. an Ag/ AgCl) in the samples, the photo-enhancement current density of all samples increased, which indicates that the samples were n-type semiconductors. The results obtained from the current density-applied voltage plots for samples agree with those obtained from the MeS measurements. The onset of the photocurrent density can be treated as a measure of the flat-band potential, EFB [21]. The flat-band potentials of samples obtained from photocurrent density-applied voltage
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band gap. Sample (e), which had the largest energy band gap, had the lowest photo-enhancement current density. Similar observations in PEC responses for ZnIn2S4 were reported in the literature [22]. The ZnIn2Se4 samples obtained using the reactive selenization process with the sputtering of ZneIn metal alloys are suitable for PEC application.
2.5
sample (a) 2.0 2
Current density (mA/cm )
light
↓
1.5
1.0
4.
0.5
↓
0.0
-0.5 -1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
dark
0.4
0.6
0.8
1.0
Applied voltage (V.vs Ag/AgCl)
(I) 2.50
External bias kept at -0.5 V vs. Ag/AgCl External bias kept at 0.0 V vs. Ag/AgCl External bias kept at +0.5 V vs. Ag/AgCl 2.00
2 J (mA/cm )
External bias kept at +1.0 V vs. Ag/AgCl
1.50
1.00
0.50
0.00 0.24
0.28
0.32
0.36
Conclusion
The photoelectrochemical properties of ZnIn2Se4 thin films prepared using the reactive selenization process with the sputtering of ZneIn metal precursors on glass and FTO-coated glass substrates, respectively, were studied. The influence of the [Zn]/[Zn þ In] molar ratio in the metal alloys on the structural, electric, and optical properties of the samples was investigated. XRD patterns of as-deposited ZneIn metal alloys show that the metal precursors are the tetragonal In and hexagonal Zn mixing phase. Only the tetragonal ZnIn2Se4 phase was detected in the samples on glass substrates after the selenization process with the Zn-In metal precursors. The thicknesses and the energy band gaps of the samples were in the ranges of 1.15e1.44 mm and 1.68e1.81 eV, respectively. The flat-band potentials in 0.6 M K2SO3 aqueous solution of samples (a)e(e) were in the range of 0.41e0.95 V vs. an Ag/ AgCl electrode. All samples show n-type conductivity, as determined using the MeS plot and photocurrent densityapplied voltage measurements. The highest PEC response (1.84 mA/cm2) was obtained for sample (a) in solution with an applied bias of þ1.0 V vs. an Ag/AgCl electrode under illumination with a 300-W xenon-arc lamp at a light intensity of 100 mW/cm2. The results show that ZnIn2Se4 thin films are a promising semiconductor material for photoelectrochemical applications.
0.40
[Zn]/[Zn+In] molar ratio in sample
(II)
Acknowledgements
Fig. 6 e (I) Current density-applied voltage plots for sample (a) in aqueous 0.6 M K2SO3 solution and (II) relationship between the photo-enhancement current densities of samples and the [Zn/(Znl D In)] ratio of samples in 0.6 M K2SO3 aqueous solution.
This study was supported by the National Science Council, Taiwan, R.O.C., under grants NSC 97-2221-E-182-041-MY3 and NSC 100-2628-E-182-005-MY3.
measurements in 0.6 M K2SO3 aqueous solution are shown in Table 1. When [Zn]/[Zn þ In] molar ratio in the samples was increased from 0.24 to 0.39 (samples (a)e(e)), the photoenhancement current density of samples decreased from 1.84 mA/cm2 to 0.42 mA/cm2 at the applied voltage kept at þ1.0 V vs. an Ag/AgCl. The highest photo-enhancement current density with the applied voltage kept at þ1.0 V vs. an Ag/AgCl electrode was observed when the [Zn]/[Zn þ In] molar ratio was 0.24 (sample (a)). With an increase in the [Zn]/ [ZnþIn] molar ratio in the metal alloys, the energy band gaps of samples increased and the photo-enhancement effects of samples decreased. Sample (a) had the highest photoenhancement current density due to it having the narrowest
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[14] Choe SH. Optical energy band gaps of undoped and Co-doped ZnIn2Se4 single crystal. Curr Appl Phys 2009;9:1e3. [15] Popa ME, Rusu GI. Structural characteristics and optical properties of zinc selenide thin films, optoelectronics and advanced materials. Rapid Commun 2011;5:830e5. [16] Tell B, Shay JL, Kasper HM. Room-temperature electrical properties of ten I-III-VI2 semiconductors. J Appl Phys 1972; 43:2469e70. [17] Nozik AJ, Memming R. Physical chemistry of semiconductorliquid interfaces. J Phys Chem 1996;100:13061e78. [18] Chun WJ, Ishikawa A, Fujisawa H, Takata T, Kondo JN, Hara M, et al. Conduction and valence band positions of Ta2O5, TaON, and Ta3N5 by UPS and electrochemical methods. J Phys Chem B 2003;107:1798e803. [19] Radecka M, Rekas M, Trenczek-Zajac A, Zakrzewska K. Importance of the band gap energy and flat band potential for application of modified TiO2 photoanodes in water photolysis. J Power Source 2008;181:46e55. [20] Loef R, Houtepen AJ, Talgorn E, Schoonman J, Goossens A. Study of electronic defects in CdSe quantum dots and their involvement in quantum dot solar cells. Nano Lett 2009;9: 856e9. [21] Newman J, Karen E, Alyea T. Electrochemical systems. 3rd ed. John wiley & sons, inc publication; 2004. [22] Cheng KW, Liang CJ. Preparation of Zn-In-S film electrodes using chemical bath deposition for photoelectrochemical applications. Solar Energ Solar Cells 2010;94:1137e45.
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Photoelectrochemical study of ZnIn2Se4 electrodes fabricated using selenization of RF magnetron sputtered ZneIn metal precursors Kong-Wei Cheng*, Ya-Hsin Cheng, Miao-Syuan Fan Department of Chemical and Materials Engineering, Chang Gung University, 259 Wen-Hwa 1st Rd., Kwei-Shan, Taoyuan 333, Taiwan
article info
abstract
Article history:
Polycrystalline ZnIn2Se4 samples are grown on glass substrates and fluorine-doped tin
Received 4 December 2011
oxide coated glass substrates using the selenization of radio-frequency magnetron sput-
Received in revised form
tered ZneIn metal alloys. The effect of the [Zn]/[Zn þ In] molar ratio in the metal alloys on
16 March 2012
the physical and photoelectrochemical properties of the samples is investigated. X-ray
Accepted 16 March 2012
diffraction patterns of samples reveal that the samples are polycrystalline tetragonal
Available online 14 April 2012
ZnIn2Se4. The thicknesses and direct band gaps of the samples are in the ranges of 1.15 e1.44 mm and 1.68e1.81 eV, as obtained from surface profile measurements and trans-
Keywords:
mittance/reflectance spectra, respectively. The flat-band potentials of the samples in 0.6 M
Hydrogen production
K2SO3 electrolyte are in the range of 0.41 to 0.95 V vs. an Ag/AgCl reference electrode.
Photoelectrode
The highest photoelectrochemical response of samples was 1.84 mA/cm2 at an external
ZnIn2Se4
potential of þ1.0 V vs. an Ag/AgCl electrode in 0.6 M K2SO3 solution under illumination
Magnetron sputtering
from a 300 W Xe lamp system with the light intensity set at 100 mW/cm2. Crown Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Solar energy can be converted into hydrogen or electric power to reduce society’s dependence on fossil fuel. Interest in solar energy has led to increased research into photoelectrochemical (PEC) technology, whose major applications include solar cells and photocatalytic reactions. Hydrogen production from water using a PEC process has been extensively studied since the Fujishima-Honda effect, which involves a n-type TiO2 photoelectrode, was reported [1]. Ternary IIeIIIeVI (II ¼ Mg, Hg, Cd, Zn; III ¼ Al, In, Ga; VI ¼ S, Se, Te) semiconductors have received considerable interest for photochemical applications because they exhibit interesting optical and electrical properties in photo-absorbers of PEC and
solar cell devices [2,3]. Kokate et al. [4] prepared a CdIn2S4 photoelectrode with a conversion efficiency of 2.94% in an aqueous polysulfide solution obtained using a 200 W tungsten lamp as the light source. Sirimanne et al. [5] prepared a MgIn2S4/MgIn2O4 photoelectrode in an aqueous solution containing polysulfide solution that produced a photocurrent density of 30 mA/cm2 using an external bias of þ0.7 V (vs. an Ag/AgCl electrode) and a light intensity of 250 mW/cm2. Sawant et al. [6] prepared CdIn2S4 thin films with an efficiency of 1.06% in an aqueous polysulfide solution obtained using a 500 W tungsten lamp as the light source. Yadav et al. [3] prepared n-ZnIn2Se4 films on fluorine-doped tin oxide (FTO) coated glass substrates by spraying an equimolar aqueous solution of zinc sulphate, indium tri-chloride, and selenourea.
* Corresponding author. Tel.: þ886 3 2118800 3353; fax: þ886 3 2118668. E-mail address: [email protected] (K.-W. Cheng). 0360-3199/$ e see front matter Crown Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.03.087
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The efficiency of their liquid junction n-ZnIn2Se4/ 1M(NaOH þ Na2S þ S)/C solar cell was 1.47%. Yu et al. [7] prepared ZnIn2S4 film on Ti substrates using the electrodeposition; the film exhibited a remarkable photocatalytic bactericidal ability under visible-light irradiation. For industrial applications, thin-film devices are convenient for collecting hydrogen in PEC cells because no further separation procedures are required. In order to prepare an efficient thinfilm reactor, it is necessary to understand the properties of newly developed semiconductor films. Recently, an interesting reactive selenization approach for the deposition of CuInSe2 film was reported [8]. The selenization of metal precursors can be done in a gas mixture containing Ar and H2Se or in the presence of solid selenium. The reactive selenization process of a metal alloy is a simple method for obtaining large-area and high-quality photo-absorbing thin films. However there are few reports on the synthesis and characterization of ZnIn2Se4 ternary semiconductor films prepared using the reactive selenization process of a metal alloy. In the present study, ZnIn2Se4 samples were prepared on glass substrates and FTO-coated glass substrates using the reactive selenization process of sputtered ZneIn metal alloy. The effect of the [Zn]/[Zn þ In] molar ratio in the metal alloys on the physical and PEC properties of the samples was studied.
2.
Experimental details
Samples on substrates were prepared using the reactive selenization process with the sputtering of ZneIn metal alloys. The ratio of zinc to the total amount of precursors ([Zn]/[Zn þ In]) was verified during the deposition of the ZneIn metal alloys. Metal alloys with a ZneIn bi-layer structure were deposited on glass substrates and FTO (sheet resistance ¼ 10 U/square)-coated glass substrates using radio-frequency (RF) magnetron sputtering at room temperature. The metal precursors were deposited on substrates using alternate RF sputtering with 2-inch Zn (4N) and In (4N) elemental targets. After the deposition chamber was evacuated to a base pressure of 5 106 Torr, highpurity (99.995%) argon gas was used to provide the plasma at a base pressure of 2.5 103 Torr. The RF sputtering power was kept at 20 W. Due to the poor adhesion of the zinc layer to the substrate, an indium layer was first deposited on the substrate, followed by the deposition of the zinc layer. The sputtering rates of Zn and In on the substrates obtained from a digital quartz controller (Filtech SQM-180) were 16.5 nm/min and 17.5 nm/min, respectively. The [Zn]/[Zn þ In] molar ratio in the metal alloys was set in the range of 0.35e0.60. The selenization process of metal alloys was carried out in a closed container made of aluminum oxide, which was similar to that used for the sulfurization of CuInS2 [9]. The metal alloys and a sufficient amount of elemental selenium were placed in the container and loaded into an evacuated quartz tube with a vacuum of around 103 Torr. A two-stage temperature profile was used for the selenization of the metal alloys. In the first stage, the metal precursors and Se powders were annealed at 250 C for 2 h in order to
incorporate the latter into the former. The formation of ZnIn2Se4 on the substrate was carried out at 500 C for 2 h. The phase formation and crystallographic study of ZneIn metal alloys and samples on glass substrates was conducted using an X-ray diffractometer (Siemems D5005) with CuKa ˚ ) radiation. The X-ray diffraction (XRD) patterns (l ¼ 1.5405 A were recorded in the 2q range of 10e80 . The surface microstructures of the samples were studied using a field-emission scanning electron microscope (FE-SEM, JEOL JSM 6700F). The compositions of the samples were analyzed using a scanning electron microscope (SEM, Hitachi S-3000N) equipped with an energy-dispersive analysis of X-ray (EDAX). The optical transmittance and reflectance of the samples were measured in the wavelength range of 350e600 nm at room temperature using an ultravioletevisibleenear-infrared (UVeViseNIR) spectrophotometer with an integrating sphere (JASCO V-650). The thickness of the samples was determined using surface profile measurements (Sloan Dektak 3030). The MotteSchottky (MeS) plots of the films on FTO-coated glass substrates were measured using a computer-controlled potentiostat (CHI 600 C) equipped with a frequency response analyzer. The measurements were carried out in an electrolyte solution of 0.6 M K2SO3. For frequencies above 10 kHz, the equivalent circuit of samples in the electrolyte can be simplified into a resistanceecapacitance (RC) circuit. The frequency of impendence was set to 10 kHz to measure the MeS plots for various samples. The applied potentials were in the range of 1.2 V to þ 0.2 V vs. an Ag/AgCl reference electrode for MeS measurements. The PEC performance measurements of the samples on FTO-coated glass substrates were carried out in a quartz electrolytic cell with the sample (average area ¼ 1.0 cm2), a Pt plate electrode (average area ¼ 1.0 cm2), and an Ag/AgCl electrode serving as the working, counter, and reference electrodes, respectively. Aqueous 0.6 M K2SO3 solution was used as the electrolyte. All measurements were carried out in a nitrogen environment at a temperature of 25 C. Current density, as a function of applied potential (1.0 w þ1.0 V vs. an Ag/AgCl electrode) for samples, was recorded with a computer-controlled potentiostat (CHI 600 C) in the dark and under illumination. A 300-W Xe short arc lamp (Perkin Elmer PE300BF) with a white-light intensity of 100 mW/cm2 was employed to simulate solar light. The intensity of incident light from the Xe lamp was measured using a photometer (Newport 818P-15e19).
3.
Results and discussion
ZnIn2Se4 thin films were deposited on glass and FTO-coated glass substrates, respectively, using the reactive selenization process with the sputtering of ZneIn metal alloys. The [Zn]/ [Zn þ In] molar ratio in the RF-sputtered ZneIn metal alloys was varied to study its effect on the physical and PEC properties of the samples. Adherent films on glass substrates were obtained after the two-stage reactive selenization process with the metal precursors. The structural and optical properties of semiconductor films with various deposition parameters were analyzed. Table 1 shows the [Zn]/[Zn þ In] molar ratios in the as-deposited metal precursors on substrates obtained using EDAX analysis. The [Zn]/[Zn þ In] molar ratio in
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Table 1 e Physical properties of ZnIn2Se4 samples on substrates used in this study. Sample
(a) (b) (c) (d) (e)
[Zn]/[Zn þ In] ratio in metal precursors (from EDAX analysis) 0.35 0.40 0.47 0.50 0.59
Molar ratio of ZnIn2Se4 sample (from EDAX analysis)
Thickness
Direct Eg
EFBa
EFBb
[Zn]/[Zn þ In]
[2Se]/[2Zn þ 3In]
(mm)
(eV)
(vs. Ag/AgCl)
(vs. Ag/AgCl)
0.24 0.27 0.32 0.37 0.39
1.1 1.09 1.0 1.03 1.06
1.15 1.44 1.37 1.31 1.34
1.68 1.69 1.77 1.78 1.81
0.41 0.95 0.65 0.44 0.62
0.74 0.75 0.82 0.80 0.42
a Obtained from MotteSchottky plots. b Determined from current density-applied voltage plots.
the as-deposited metal precursors were in the range of 0.35e0.59. Fig. 1(I) shows the XRD patterns of as-deposited ZneIn metal alloys with various [Zn]/[Zn þ In] molar ratios in the metal alloys on glass substrates. Only tetragonal In (JCPDS card no. 85-1409) and hexagonal Zn (JCPDS card no. 4-831) phases were detected in the ZneIn metal precursors on glass substrates. With an increase in the [Zn]/[ZnþIn] molar ratio in the metal alloys, the intensities of peaks at 2q of 36.4。 and 39.2。for Zn phase and that at 2q of 33.2。for In phase deceased. The possible reason is some structure defects formed in the metal precursors, which effectively decrease the crystallinity of the ZneIn metal alloys. Fig. 1(II) shows the XRD patterns of samples after the selenization process with the ZneIn metal alloys. The peaks at 2q of 27 , 44.9 and 53.2 correspond to the crystal planes of (112), (204) and (312) for the tetragonal ZnIn2Se4 phase. With an increase in the [Zn]/[Zn þ In] molar ratio in the metal alloys (samples (a)e(e)), the XRD patterns of the samples become almost the same as that of the tetragonal ZnIn2Se4 phase, except that the diffraction peaks were slightly shifted to higher angles. The successive shift of the XRD pattern indicates that Zn2þ was incorporated into the lattice of ZnIn2Se4 samples with an increase in the [Zn]/[Zn þ In] molar ˚ ) is ratio in metal alloys because the ionic radius of Zn2þ (0.74 A ˚ ) [10]. With an increase in the smaller than that of In3þ (0.76 A [Zn]/[Zn þ In] molar ratio in the metal alloys, the number of Zn2þ ions occupying the In3þ sites in the samples increased, which shifted the peaks of XRD patterns to higher angles. The thicknesses of samples obtained from surface profile measurements are shown in Table 1. The thicknesses of the samples are in the range of 1.15e1.44 mm. Quantitative analysis using EDAX was carried out to analyze the atomic ratios of Zn, In, and Se in the samples. The weight percentages of zinc, indium and selenium from EDAX analysis were converted to atomic ratios. The [Zn]/[Zn þ In] molar ratio in the metal alloys for samples (a)e(e) are 0.35, 0.40, 0.47, 0.50 and 0.59, respectively. The [Zn]/[Zn þ In] molar ratio in the samples (a)e(e) obtained from EDAX analysis are 0.24, 0.27, 0.32, 0.37 and 0.39, respectively. The results listed in Table 1. When the [Zn]/[Zn þ In] molar ratio in the metal alloys was increased from 0.35 to 0.59 (samples (a)e(e)), that in the samples increased from 0.24 to 0.39. The 2[Se]/[2Zn þ 3In] molar ratio of the samples was in the range of 1.0e1.1. Fig. 2 shows the [Zn/(Zn þ In)] and 2[Se]/[2Zn þ 3In] molar ratios in the samples as function of the [Zn/(Zn þ In)] molar ratio in the metal alloys. The [Zn/(Zn þ In)] molar ratio in the samples
Fig. 1 e X-ray diffraction patterns of samples with various [Zn]/[Zn D In] molar ratio in (I) as-deposition metal alloys and (II) after selenization process. (a) [Zn]/[Zn D In] [ 0.35, (b) [Zn]/[Zn D In] [ 0.40, (c) [Zn]/[Zn D In] [ 0.47,(d) [Zn]/ [Zn D In] [ 0.50 and (e) [Zn]/[Zn D In] [ 0.59 in metal precursors.
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1.20
1.20
1.00
1.00
0.80
0.80
0.60
0.60
0.40
0.40
[Zn]/[Zn+In]
0.20
[2Se]/[2Zn+3In] ratio in sample
[Zn]/[Zn+In] ratio in sample
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0.20
[2Se]/[2Zn+3In] 0.00 0.35
0.40
0.45
0.50
0.55
0.60
0.00 0.65
[Zn]/[Zn+In] molar ratio in metal precursors
Fig. 2 e Molar ratios of [Zn]/[Zn D In] and 2[Se]/(2[Zn]D3[In]) for samples as functions of the [Zn]/[Zn D In] molar ratio in the metal precursors.
increased with increasing in the [Zn/(Zn þ In)] molar ratio in the metal alloys. The EDAX analysis of samples shows that there is no apparent selenium deficit in the samples. The EDAX analysis also shows that the number of Zn2þ ions occupying the In3þ sites in the samples increases with increasing in [Zn/(Zn þ In)] molar ratio in the metal alloys. The results obtained from EDAX analysis agree with those obtained from XRD patterns of samples. FE-SEM images were used to observe the microstructures of the sample surfaces. Fig. 3 shows that samples (a), (c), and (e) are dense and uniform and have no observable voids or pinholes. Plate-like aggregation microstructures were observed on the surface of sample (a). Furthermore, the grain size of samples tended to decrease with increasing in [Zn]/ [[Zn þ In] molar ratio in the metal alloys. With a [Zn]/[Zn þ In] molar ratio of 0.59 in the ZneIn metal alloys, flower-like microstructures were observed on the surface of sample (e). The transmittance and reflectance spectra of samples for wavelengths of 350 w 600 nm are analyzed using UVeViseNIR spectrophotometer. The energy band gaps (Eg) of samples are important properties for photoluminescence and PEC device applications. The absorption coefficients (a) of samples can be estimated using the Manifacier model [11]: a ¼ 1=t ln½ð1 RÞ=T
(1)
where t is the thickness of the sample and R and T are the reflectance and transmittance data, respectively. The thicknesses of samples determined from surface profile measurements are listed in Table 1. The relationship between the absorption coefficient (a) and the incident photon energy (hv) can be written as: n ahv ¼ A hv Eg
(2)
Generally, n is 2 for an indirect band gap and 1/2 for a direct band gap [12]. The optical energy band gaps of samples can be
Fig. 3 e SEM images of (I) sample (a), (II) sample (c), and (III) sample (e).
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1=C2 ¼ B 2=
2 3 3 0 eND A
½E EFB ðkT=eÞ
(3)
where 3 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 sample, e is the electric charge, and 3 0 is the permittivity of the vacuum. For a n-type semiconductor, B is 1, whereas for a p-type semiconductor, B is 1. The flat-band potentials of samples, EFB, can be estimated from the point of intersection in the C2 vs. applied potentials E, plot. Readecka et al. [19] and Loef et al. [20] reported that the equivalent circuit of samples can be simplified into a series of resistanceecapacitance (RC) circuits if the real part of the imped0 ance (Z ) is independent of frequency. All the samples during the measurement of the MeS plot from AC impendence were controlled at 10 kHz because at frequencies higher than 10 kHz, the real part of the impedance reveals no frequency dependence and the effect of the constant-phase element on the impedance spectrum becomes negligible. Fig. 5 shows the MeS plots for samples in K2SO3 (0.6 M) aqueous solution. The
9
(a) (b) (c) (d) (e)
1.0x10
8
8
6.0x10
2
(αhυ) (eV/cm)
2
8.0x10
8
4.0x10
8
2.0x10
0.0 1.5
1.6
1.7
1.8
1.9
hν (eV)
Fig. 4 e Plots of (ahv)2 versus hv of samples.
2.0
11
2.0x10
11
1.5x10
(a) (b) (c) (d) (e)
2
4
2
1/C (cm/F )
obtained by plotting (ahn)1/n with respect to hv and extrapolating the curve to (ahn)1/n ¼ 0. Fig. 4 shows the linear dependence of (ahn)2 on hv. The direct energy band gaps of samples obtained from Fig. 4 are shown in Table 1. The direct energy band gaps of samples prepared in this study varied from 1.68e1.81 eV. The energy band gap of ZnIn2Se4 is in the range of 1.41e1.74 eV [13,14]. With an increase in the [Zn]/[Zn þ In] molar ratio in the ZneIn metal alloys, the energy band gap of the samples increased. The energy bad gap of sample (e) (Znrich sample) is 1.81 eV. The energy band gap of ZnSe is in the range of 2.42e2.72 eV [15]. Because the [Zn]/[Zn þ In] molar ratio in sample (e) is 0.39, the energy band gap of sample (e) is larger than those of samples (a)e(d). An important property for the PEC response is the flatband potential. The flat-band potential of a sample is a useful quantity in photoelectrochemistry as it facilitates the determination of the energetic position of the valence and conduction band edges of a given semiconductor material [16,17]. Various methods can be used to measure the flat-band potential [17,18]. In this study, the capacitance vs. applied potential based on the MeS relationship is used [17,18]:
11
1.0x10
10
5.0x10
0.0 -1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
Applied potential (V vs. Ag/AgCl)
Fig. 5 e MotteSchottky plots of samples on FTO-coated glass substrates in aqueous 0.6 M K2SO3 solution.
flat-band potentials of samples (a)e(e) obtained from MeS plots are shown in Table 1. The flat-band potentials in 0.6 M K2SO3 aqueous solution of samples (a)e(e) are in the range of 0.41 to 0.95 V vs. an Ag/AgCl electrode. It can be observed that the slope of the MeS plot is positive, which indicates that the samples are n-type semiconductors. The samples were installed in a three-electrode PEC cell to feasibility the possibility of their application in solar energy conversion. The applied potential between the working and reference electrodes was varied from 1.0 to þ1.0 V vs. an Ag/ AgCl electrode. 0.6 M K2SO3 aqueous solution was used as the electrolyte. For the 0.6 M K2SO3 solution, the following reactions may occur [10]: hv
samples / e þ hþ
(4)
2H2 O þ 2e /H2 þ 2OH
(5)
2 þ þ SO2 3 þ H2 O þ 2h /SO4 þ 2H
(6)
Current densities of sample (a) with an applied potential in the range of 1.0 V to þ1.0 V vs. an Ag/AgCl electrode obtained using the chopping method in 0.6 M K2SO3 aqueous solution is shown in Fig. 6(I). Sample (a) has the anodic photoenhancement effect. Fig. 6(II) shows the relationship of the photo-enhancement current densities (J ) of samples and the [Zn/(Zn þ In)] molar ratio in the samples with applied voltage kept at 0.5 V, 0.0 V, þ 0.5 V and þ1.0 V (vs. an Ag/AgCl) in 0.6 M K2SO3 solution. The photo-enhancement current density for the samples is defined as the difference between photocurrent density and dark-current density at the same applied voltage. With an increase in applied voltage (vs. an Ag/ AgCl) in the samples, the photo-enhancement current density of all samples increased, which indicates that the samples were n-type semiconductors. The results obtained from the current density-applied voltage plots for samples agree with those obtained from the MeS measurements. The onset of the photocurrent density can be treated as a measure of the flat-band potential, EFB [21]. The flat-band potentials of samples obtained from photocurrent density-applied voltage
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band gap. Sample (e), which had the largest energy band gap, had the lowest photo-enhancement current density. Similar observations in PEC responses for ZnIn2S4 were reported in the literature [22]. The ZnIn2Se4 samples obtained using the reactive selenization process with the sputtering of ZneIn metal alloys are suitable for PEC application.
2.5
sample (a) 2.0 2
Current density (mA/cm )
light
↓
1.5
1.0
4.
0.5
↓
0.0
-0.5 -1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
dark
0.4
0.6
0.8
1.0
Applied voltage (V.vs Ag/AgCl)
(I) 2.50
External bias kept at -0.5 V vs. Ag/AgCl External bias kept at 0.0 V vs. Ag/AgCl External bias kept at +0.5 V vs. Ag/AgCl 2.00
2 J (mA/cm )
External bias kept at +1.0 V vs. Ag/AgCl
1.50
1.00
0.50
0.00 0.24
0.28
0.32
0.36
Conclusion
The photoelectrochemical properties of ZnIn2Se4 thin films prepared using the reactive selenization process with the sputtering of ZneIn metal precursors on glass and FTO-coated glass substrates, respectively, were studied. The influence of the [Zn]/[Zn þ In] molar ratio in the metal alloys on the structural, electric, and optical properties of the samples was investigated. XRD patterns of as-deposited ZneIn metal alloys show that the metal precursors are the tetragonal In and hexagonal Zn mixing phase. Only the tetragonal ZnIn2Se4 phase was detected in the samples on glass substrates after the selenization process with the Zn-In metal precursors. The thicknesses and the energy band gaps of the samples were in the ranges of 1.15e1.44 mm and 1.68e1.81 eV, respectively. The flat-band potentials in 0.6 M K2SO3 aqueous solution of samples (a)e(e) were in the range of 0.41e0.95 V vs. an Ag/ AgCl electrode. All samples show n-type conductivity, as determined using the MeS plot and photocurrent densityapplied voltage measurements. The highest PEC response (1.84 mA/cm2) was obtained for sample (a) in solution with an applied bias of þ1.0 V vs. an Ag/AgCl electrode under illumination with a 300-W xenon-arc lamp at a light intensity of 100 mW/cm2. The results show that ZnIn2Se4 thin films are a promising semiconductor material for photoelectrochemical applications.
0.40
[Zn]/[Zn+In] molar ratio in sample
(II)
Acknowledgements
Fig. 6 e (I) Current density-applied voltage plots for sample (a) in aqueous 0.6 M K2SO3 solution and (II) relationship between the photo-enhancement current densities of samples and the [Zn/(Znl D In)] ratio of samples in 0.6 M K2SO3 aqueous solution.
This study was supported by the National Science Council, Taiwan, R.O.C., under grants NSC 97-2221-E-182-041-MY3 and NSC 100-2628-E-182-005-MY3.
measurements in 0.6 M K2SO3 aqueous solution are shown in Table 1. When [Zn]/[Zn þ In] molar ratio in the samples was increased from 0.24 to 0.39 (samples (a)e(e)), the photoenhancement current density of samples decreased from 1.84 mA/cm2 to 0.42 mA/cm2 at the applied voltage kept at þ1.0 V vs. an Ag/AgCl. The highest photo-enhancement current density with the applied voltage kept at þ1.0 V vs. an Ag/AgCl electrode was observed when the [Zn]/[Zn þ In] molar ratio was 0.24 (sample (a)). With an increase in the [Zn]/ [ZnþIn] molar ratio in the metal alloys, the energy band gaps of samples increased and the photo-enhancement effects of samples decreased. Sample (a) had the highest photoenhancement current density due to it having the narrowest
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