Ternary Ag–In–S polycrystalline films deposited using chemical bath deposition for photoelectrochemical applications

Materials Chemistry and Physics 120 (2010) 307–312

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Ternary Ag–In–S polycrystalline films deposited using chemical bath deposition for photoelectrochemical applications Wen-Sheng Chang a , Ching-Chen Wu b , Ming-Shan Jeng a , Kong-Wei Cheng c , Chao-Ming Huang d , Tai-Chou Lee b,∗ a

Energy and Environmental Laboratories, Industrial Technology Research Institute, 195 Sec. 4, Chung-Hsing Road, Hsin-Chu 310, Taiwan Department of Chemical Engineering, National Chung Cheng University, 168 University Road, Min-Hsiung, Chia-Yi 621, Taiwan Department of Chemical and Materials Engineering, Chang Gung University, 259 Wen-Hwa 1st Road, Kwei-Shan, Tao-Yuan 333, Taiwan d Department of Environmental Engineering, Kun Shan University, 949 Da Wan Road, Yung-Kang City, Tainan Hsien 710, Taiwan b c

a r t i c l e

i n f o

Article history: Received 26 November 2008 Received in revised form 4 November 2009 Accepted 5 November 2009 Keywords: Thin films Chemical synthesis Electrochemical techniques Optical properties

a b s t r a c t This paper describes the preparation and characterization of ternary Ag–In–S thin films deposited on indium tin oxide (ITO)-coated glass substrates using chemical bath deposition (CBD). The composition of the thin films was varied by changing the concentration ratio of [Ag]/[In] in the precursor solutions. The crystal structure, optical properties, and surface morphology of the thin films were analyzed by grazing incidence X-ray diffraction (GIXRD), UV–vis spectroscopy, and field-emission scanning electron microscopy (FE-SEM). GIXRD results indicate that the samples consisted of AgInS2 and/or AgIn5 S8 crystal phases, depending on the composition of the precursor solutions. The film thicknesses, electrical resistivity, flat band potentials, and band gaps of the samples were between 1.12 and 1.37 ␮m, 3.73 × 10−3 and 4.98 × 104  cm, −0.67 and −0.90 V vs. NHE, and 1.83 and 1.92 eV, respectively. The highest photocurrent density was observed in the sample with [Ag]/[In] = 4. A photocurrent density of 9.7 mA cm−2 was obtained with an applied potential of 0.25 V vs. SCE in the three-electrode system. The photoresponse experiments were conducted in 0.25 M K2 SO3 and 0.35 M Na2 S aqueous electrolyte solutions under irradiation by a 300 W Xe light (100 mW cm−2 ). The results show that ternary Ag–In–S thin film electrodes have potential in water splitting applications. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Due to the high oil prices in recent years, researchers have become more and more interested in advanced semiconductor material systems that can convert solar energy into hydrogen energy. Photoelectrochemical water splitting is environmentally friendly because hydrogen is generated using natural resources such as water and solar energy [1–3]. In 1972, Honda and Fujishima reported the discovery of water photolysis on TiO2 semiconductor electrodes [4]. The photocatalytic splitting of water at the liquid–solid interface in a semiconductor-electrode system has attracted a lot of attention for the possible conversion and storage of solar energy into hydrogen and oxygen. In the solar spectrum, ultraviolet light ( < 400 nm) accounts for only 4% of the total energy, whereas the visible light region (400 nm <  < 800 nm) accounts for about 43%. Therefore, it is necessary to develop a visible lightresponsive photocatalyst to produce hydrogen from water using solar energy [1–10].

∗ Corresponding author. Tel.: +886 5 2720411x33409; fax: +886 5 2721206. E-mail address: [email protected] (T.-C. Lee). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.11.003

Ternary I–III–VI compounds are direct energy-gap semiconductors which have been utilized in light emitting diodes, nonlinear optics, and photovoltaic solar cells [11–21]. I–III–VI based chalcopyrite semiconductors, especially AgInS2 , are expected to make good solar cell absorber layers and tandem solar cells (for example, CdS/AgInS2 /CuInSe2 ) [13,14]. Bulk crystals of AgInS2 have two crystal forms (chalcopyrite and orthorhombic), and a band gap between 1.87 and 2.03 eV [11–21]. AgIn5 S8 has one crystal form (cubic) and a band gap between 1.7 and 1.8 eV [21,22–25]. There are a lot of preparation methods for Ag–In–S thin films, including thermal evaporation [11], spray pyrolysis [12–15], vertical gradient freezing (VGF) [16], hot-wall epitaxy [17,18], hot-press (HP) method [19], chemical bath deposition (CBD) [20,22,24,25], direct fusion of Ag, In, and S elements [21], and the Bridgman–Stockbarger technique [23]. Comparing these preparation methods, CBD has the following advantages: (i) the reaction occurs under atmospheric pressure and relatively low temperature; (ii) equipment is inexpensive; (iii) the process is simple and easy to control; and (iv) fabrication of uniform large area thin films is possible [22,24–28]. Various preparation parameters of CBD, such as concentration of metal ions and sulfur, bath temperature, pH of the resultant solution, and deposition time, are directly related to the thin film performance [22,26,28].

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Table 1 The deposition parameters of chemical bath deposition for the growth of Ag–In–S polycrystalline thin films. Sample

[Ag]/[In]

Solution A 0.4 M AgNO3 (mL)

A B C D E

1 2 3 4 5

5 5 5 5 5

Solution B In(NO3 )3 ·5H2 O (M)

(mL)

0.4 0.2 0.13 0.1 0.08

5 5 5 5 5

0.4 M NH4 NO3 (mL)

7.4 M TEA (mL)

18 M H2 SO4 (mL)

pH value

0.4 M TAA (mL)

2.5 2.5 2.5 2.5 2.5

2.5 2.5 2.5 2.5 2.5

0.8 1.2 1.5 2.0 2.5

0.5 0.5 0.5 0.5 0.5

44.1 44.1 44.1 44.1 44.1

In this study, a deposition technology for preparing ternary Ag–In–S thin films based on the CBD method is developed. The silver to indium molar ratio ([Ag]/[In]) in the precursor solution was varied systematically in order to determine a better formula for depositing Ag–In–S thin films on an ITO glass substrate. The crystallinity, microstructure, and physicochemical properties of the samples were investigated by grazing incidence X-ray diffraction (GIXRD), field-emission scanning electron microscopy (FE-SEM), and UV–vis spectroscopy. The photoresponses of the thin-film electrodes were recorded by a scanning potentiostat in a standard three-electrode electrochemical setup under illumination. 2. Experimental details 2.1. Preparation of Ag–In–S ternary thin films The Ag–In–S semiconductor thin films were deposited on indium tin oxide (ITO)-coated glass substrates. The substrates were cut into slices (∼1 cm × 5 cm, resistivity 17.5 /) and cleaned ultrasonically in methanol, DI-water, and acetone respectively for 30 min. The slides were then rinsed thoroughly in DI-water and subsequently dried with a stream of pure nitrogen. For the Ag–In–S thin films, the silver to indium molar ratio ([Ag]/[In]) was varied from 1 to 5. The reaction solutions consisted of a mixture of 5 mL of 0.4 M silver nitrate (AgNO3 , J.T. Baker; 99.9%), 5 mL of indium nitrate (In(NO3 )3 ·xH2 O, Alfa Aesar; 99.99%), 2.5 mL of 0.4 M ammonium nitrate (NH4 NO3 , buffer solution, Riedel-de Haën; 98%), and 2.5 mL of 7.4 M triethanolamine (N(CH2 CH2 OH)3 , complex agent, Merck; ≥99%). Sulfuric acid (Merck; 95–97%) was used to adjust the pH value of the aqueous solution to around 0.5. Finally, 44.1 mL of 0.4 M thioacetamide (CH3 CSNH2 , Merck; ≥99%) was added to the mixture as the source of sulfur ions. The concentration of indium nitrate was varied from 0.4 to 0.08 M, as shown in Table 1. The substrates were placed vertically in a beaker containing a freshly prepared aqueous mixture, maintained at 80 ◦ C in a thermostat bath, and dipped for 30 min. After the reaction, the as-deposited thin films were washed with DI-water in an ultrasonic bath for 5 min and then heat treated at 120 ◦ C in an oven for 60 min. The films were then annealed in a vacuum at 400 ◦ C for 1 h.

which was prepared by using MILLIPORE water (resistivity 18.2 M cm). The electrolyte was degassed by purging with high purity nitrogen, and then ultrasonicated for 30 min before each experiment. All the photoelectrochemical experiments were carried out in a nitrogen environment. The samples were placed 5 cm from the quartz window in the electrochemical cell. The photocurrent was recorded every 2 s as a function of applied potential (−1.5 to +0.25 V vs. SCE) under front-side illumination with a computer-controlled potentiostat (AUTOLAB Model PGSTAT 30) using the chopping method [29–31]. The monochromatic photocurrent-wavelength measurements were carried out by placing a monochromator (Sciencetech Model 9030), assisted by an automatic filter wheel, between the photoelectrochemical cell and the light source.

3. Results and discussion The growth mechanism of ternary Ag–In–S thin films on indium tin oxide (ITO)-coated glass substrates is briefly described below. Silver nitrate, indium nitrate, ammonia nitrate, triethanolamine (TEA), and thioacetamide (TAA) in the solution served as Ag+ cations, In3+ cations, buffer solution, complex agent, and sulfur source, respectively. The decomposition of thioacetamide (TAA) produces S2− in the acidic solution, as shown below [20,22,24,25]: CH3 CSNH2 + H+ → H2 S + CH3 CNH+ −

A field-emission scanning electron microscope, Hitachi FE-SEM S4800, was used to investigate the morphology of the samples. EDS attached to FE-SEM was employed to analyze the composition of the thin films. The sample structures were determined by a grazing incidence X-ray diffractometer (MAC SIENCE MXP-18). The GIXRD patterns were recorded in the 2 range from 20◦ to 70◦ and at a scan rate of 2◦ min−1 . The transmittance spectra of the films were measured using a UV–vis spectrophotometer (Shimadzu UV-2450) in the wavelength range of 300–800 nm at room temperature. An identical ITO-coated glass substrate was used as the reference. The thicknesses of the films were determined by ␣-Step (Sloan Dektak 3030). The electrical resistivity of samples was determined using Hall measurements at room temperature (Accent Optical Technologies ACC-HL5500PC). 2.3. Photoelectrochemical (PEC) measurements A silver wire was attached to the Ag–In–S thin films with silver paste. The back and sidewalls of the samples were covered with epoxy resin to prevent current leakage [10,20,30]. All samples had an area of 1.0 cm2 . Photoelectrochemical measurements were carried out using a three-electrode system, with the semiconductor thin films, a Pt plate electrode, and a saturated calomel electrode (SCE) acting as the working, counter, and reference electrodes, respectively. A Xe lamp (PerkinElmer Model PE300UV) and AM = 0 and 1.5 filters were used to simulate the solar spectrum. Illumination intensity during the experiments was kept at 100 mW cm−2 . The intensity was monitored by an optical power meter (Oriel Model 70310). An aqueous solution of 0.25 M potassium sulfite (K2 SO3 , Aldrich; 90%) and 0.35 M sodium sulfide (Na2 S·xH2 O, Riedel-de Haën; 60%) (pH = 13.3) was used as the electrolyte,

(1)

H2 S + H2 O → HS + H3 O

(2)

HS− + H2 O → S2− + H3 O+

(3)

Triethanolamine was used as the complex agent for the film deposition. With the addition of triethanolamine, Ag+ and In3+ cations in the solution can become complexes [20,22,25]: Ag+ + nTEA → Ag(TEA)n + 3+

In 2.2. Characterization of films

+

+ mTEA → In(TEA)m

3+

(4) (5)

The sulfur ions and complex ions migrate to the substrate surface, where a heterogeneous process takes place to form Ag–In–S films. The overall reaction is given by [20,22,24,25]: Ag(TEA)n + + In(TEA)m 3+ + S2− → AgInS2 /AgIn5 S8

(6)

3.1. Characterization of ternary Ag–In–S semiconductors Fig. 1 shows the crystal phases that were measured by the GIXRD with Cu K␣1 radiation (␭ = 1.54056 Å). Curves A, B, C, D, and E represent the X-ray patterns of the corresponding samples listed in Table 1. The standard diffraction peaks of orthorhombic-AgInS2 (JCPDS CARD Number 25-1328) and cubic-AgIn5 S8 (JCPDS CARD Number 25-1329) are also plotted in Fig. 1. Sample A ([Ag]/[In] = 1) shows a dominant cubic-AgIn5 S8 phase, sample E ([Ag]/[In] = 5) shows a dominant orthorhombic-AgInS2 phase, and the other samples show AgInS2 /AgIn5 S8 solid mixtures. The peak intensities of (1 2 0), (1 2 1), and (3 2 0) crystal planes at 2 of 24.992◦ , 28.382◦ , and 44.53◦ , respectively, for orthorhombic-AgInS2 increased with increasing [Ag]/[In] molar ratio. It was found that the structure transformed from cubic-AgIn5 S8 (sample A) to orthorhombicAgInS2 (sample E) when the indium ion concentration in the

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Fig. 1. Grazing incidence X-ray diffraction patterns for samples with [Ag/In] molar ratios of (A) 1, (B) 2, (C) 3, (D) 4, and (E) 5 in the precursor solution. All samples were annealed in a vacuum at 400 ◦ C for 1 h.

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solution decreased. This significant change indicates that the formations of polycrystalline AgInS2 /AgIn5 S8 thin films are greatly affected by the indium concentration in the precursor solution. Fig. 2 shows the SEM images of samples with various [Ag]/[In] molar ratios. The SEM images indicate that the microstructure of the ternary Ag–In–S thin film is related to the indium concentration. As the [Ag]/[In] molar ratio increased, the morphology changed from fiber-like (A) to spinel (D) and to a mixture of fiberlike and spinel structures (E). A reasonable interpretation of this significant change in morphology is that different ion-by-ion and cluster-by-cluster depositions are related to different indium concentrations. In general, the CBD process is based on three chemical reaction steps: (i) the formation or the dissociation of solvated ionic metal–liquid complexes; (ii) hydrolysis of the complexes; and (iii) the formation of solid phases [22,28,32,33]. Yamaguchi et al. prepared In2 S3 thin films using chemical bath deposition in an acidic aqueous solution, with a bath temperature of around 70 ◦ C for 1 h. Their SEM morphology results showed a fiber-like structure [27]. However, Nair et al. reported a bulk structure for Ag2 S thin films [34]. In this study, during the initial 5 min of reaction, the solution color turned black, which is close to the color of silver sulfide (Ag2 S), and remained dark for 15 min. The final solutions of samples A, B, and C became orange, which is similar to the color of indium sulfide,

Fig. 2. SEM images showing the surface morphologies of samples with [Ag/In] molar ratios of (A) 1, (B) 2, (C) 3, (D) 4, and (E) 5 in the precursor solution.

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Table 2 The properties of Ag–In–S polycrystalline thin films on an ITO substrate. Sample A B C D E a b

Thickness (␮m) 1.37 1.12 1.35 1.27 1.21

Atomic molar ratio of Ag:In:Sa 1:7.0:8.7 1:4.9:6.2 1:3.3:4.8 1:2.1:3.7 1:1.2:2.2

Resistivityb ( cm) −3

3.73 × 10 4.13 × 10−1 2.91 × 102 1.17 × 103 4.98 × 104

Band gap (eV)

NHE EFb (V)

NHE ECB (V)

NHE EVB (V)

Vacuum ECB (V)

EVacuum (V) VB

1.83 1.86 1.88 1.89 1.92

−0.67 −0.86 −0.88 −0.90 −0.77

−0.97 −1.16 −1.18 −1.20 −1.07

0.86 0.70 0.70 0.69 0.85

−3.53 −3.34 −3.32 −3.33 −3.43

−5.36 −5.20 −5.20 −5.19 −5.35

The atomic molar ratio of Ag:In:S was determined by EDS. The electrical resistivities were obtained using Hall measurements.

and those of samples D and E remained dark until the end of the reaction. It was reported that AgIn5 S8 growth is a two-step reaction mechanism because of the distinct solubility products (Ksp ) of Ag2 S (Ksp = 1.6 × 10−49 ) and In2 S3 (Ksp = 5.8 × 10−74 ) [20,22]. Additionally, during the annealing process at 400 ◦ C for 1 h, interdiffusion of Ag2 S and In2 S3 occurs at the interface and ternary compounds are formed. As the indium concentration in the solution decreased, the In2 S3 content on top of the as-deposited thin film decreased. This allows Ag2 S to diffuse more easily to the surface of the thin films [35]. Table 2 lists the EDS analysis results for samples with various [Ag]/[In] molar ratios. The indium atomic ratio in the thin films decreases with indium concentration in the solution, which was confirmed by the X-ray diffraction results mentioned earlier. Sample A shows a composition ratio of [Ag]:[In]:[S] = 1:7:8.7, indicating that an indium-rich AgIn5 S8 was deposited on the ITO substrate. On the other hand, sample E shows a composition ratio of [Ag]:[In]:[S] = 1:1.2:2.2, indicating that an indium-rich AgInS2 was deposited on the ITO substrate. These results are in good agreement with XRD analyses. Fig. 3 shows the transmission spectra recorded for all the samples by a UV–vis spectrometer. The highest transmittance was between 50% and 70%. There were distinct absorption edges in the visible light region. The band gap energy can be determined from the intersection of the extrapolation from the straight region of the transmittance and the abscissa. The band gaps of the ternary Ag–In–S thin films were found to be from 1.83 to 1.92 eV, as shown in Table 2, which closely agrees with values previously reported [11–25]. The low transmission for sample D might be due to the scattering caused by the grains in the sample. According to the literature, the band gap energy of AgIn5 S8 is close to that of AgInS2 . Therefore, all the samples are expected to have similar optical properties. The electrical resistivity of thin films, obtained using Hall

Fig. 3. Transmittance spectra. Sample A: 䊉, sample B: , sample C: , sample D: , and sample E: .

measurements at room temperature, was between 3.73 × 10−3 and 4.98 × 104  cm. This results agree with that reported in the literature: the resistivity of orthorhombic-AgInS2 is between 103 and 106  cm and cubic-AgIn5 S8 is 10−2 –10−3  cm. [14,20,23,25] The samples exhibit n-type conductivity because In-rich thin films were obtained [36,37]. 3.2. Photoelectrochemical properties The samples were cut to have an active area of 1.0 cm2 . They were then put into a photoreactor cell filled with an electrolyte containing sacrificial reagents 0.25 M K2 SO3 and 0.35 M Na2 S. The following reactions take place under visible light irradiation [8,24,38]: hv

photocatalyst−→e− (CB) + h+ (VB)

(7)

2H2 O + 2e− (CB) → H2 + 2OH−

(8)

SO3 2− + H2 O + 2 h+ (VB) → SO4 2− + 2H+

(9)

2−

2S S2

2−

+

+ 2 h (VB) → S2 + SO3

2−

→ S2 O3

2−

2−

+S

(10) 2−

SO3 2− + S2− + 2 h+ (VB) → S2 O3 2−

(11) (12)

Fig. 4 shows the photocurrent densities vs. applied potential in a three-electrode system with sacrificial regents. The scanning range was from −1.5 to +0.25 V vs. SCE reference electrode under a scanning rate of 2.5 mV s−1 . The inset in Fig. 4 shows the typical current density measured using the chopping method (sample

Fig. 4. Photocurrent density jp (mA cm−2 ) as a function of applied potential Eapp under 100 mW cm−2 illumination in K2 SO3 (0.25 M) and Na2 S (0.35 M) aqueous electrolyte solution. Sample A: 䊉, sample B: , sample C: , sample D: , and sample E: .

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Fig. 6. Flat band potential from [jp ()]2 vs. applied potential. Sample A: 䊉, sample B: , sample C: , sample D: , and sample E: . Fig. 5. Thin film element analysis of the ternary compounds and the [Ag]/[In] atomic ratios after annealing at 400 ◦ C in a vacuum for 1 h as a function of the [Ag]/[In] molar ratios in the precursor solution. The thin film element analysis was conducted by EDS.

D). The photocurrent and dark-current were obtained simultaneously. The photocurrent jp (mA cm−2 ) was obtained by subtracting dark-current density from current density under irradiation. The photocurrent densities of samples with relatively higher AgInS2 content (D and E) were larger than those of AgIn5 S8 -based samples (A), which is in good agreement with the results obtained by Ueno et al. [39]. The exceptional decrease in photocurrent for sample E can be explained by the poor crystallinity of the sample. The photoresponse of a deposited electrode is greatly affected by the indium concentration in the precursor solution. The photocurrent densities of samples B, C, and E were 5.3, 7.1, and 4.4 mA cm−2 , respectively, as shown in Fig. 4. The highest photocurrent density of 9.7 mA cm−2 was observed in sample D under +0.25 V bias vs. the SCE reference electrode. According to the results of Barbé et al. [40] and Park et al. [41], TiO2 with coexisting of rutile and anatase (e.g., P 25) shows a better photochemical property than that of pure anatase or rutile TiO2 . A good photocurrent performance of AgInS2 /AgIn5 S8 mixed film obtained from this work could be possibly attributed to the similar origin. More investigations will be done. Photocurrent densities (three-electrode setup) and [Ag]/[In] atomic ratios determined by EDS as a function of [Ag]/[In] in the precursor solutions are plotted in Fig. 5. The figure shows the effects of compositional variation on the photoelectrochemical behavior of silver indium sulfide thin film electrodes. The applied potential of a photoelectrode can also be reported vs. the normal hydrogen electrode (NHE), as follows: O ESCE = 0.244 V at 298 K

Evaccum = −ENHE − 4.5

O with ENHE = −ESCE + ESCE

(13) (14)

The flat band potentials (Efb ) can be determined using the monochromatic photocurrent as a function of the applied potential (E). The photocurrent density jp () and the applied potential E are related by the following equation, derived based on small band bending assumption [42]: E − Efb = Bjp2 ()

(15)

where B is a constant that is a function of the optical absorption coefficient, the width of the depletion layer for band bending, the electronic charge, and the flux of the incident photons. Fig. 6 shows [jp ()]2 vs. applied potential referenced to the SCE electrode for samples A, B, C, D, and E. Linear regions are evident in the plots.

Fig. 7. Band positions of samples in K2 SO3 (0.25 M) and Na2 S (0.35 M) aqueous electrolyte. CB: conduction band; VB: valence band; Efb : Fermi energy.

The extrapolation of these straight lines to the x-axis gives the flat band potentials. The flat band potentials determined using this photocurrent method are between −0.91 and −1.14 V vs. SCE reference electrode, corresponding to −0.67 and −0.90 V vs. normal hydrogen electrode (NHE). These samples are n-type semiconductors according to the observed anodic current. Therefore, the positions of the conduction band and the valance band can be estimated from the flat band potentials and the band gap energy. Fig. 7 depicts the band positions of the samples in a pH 13.3 electrolyte. The conduction bands of all the samples are more negative than the hydrogen reduction potential (vs. NHE), and making them suitable for water splitting applications. The stability of sample D was initially tested. Sample D was irradiated by a 300 W Xe lamp in the sacrificial reagents with a bias at −0.5 V vs. SCE for 3 h. The photocurrent density changed from 2.2 to 1.9 mA cm−2 during the course of the experiment. 4. Conclusion In this study, ternary Ag–In–S thin films were prepared in an aqueous solution using CBD method. GIXRD results indicate that the samples were AgInS2 /AgIn5 S8 crystals. The film thicknesses, electrical resistivity, flat band potentials, and band gaps of the samples were between 1.12 and 1.37 ␮m, 3.73 × 10−3 –4.98 × 104  cm, −0.67 and −0.90 V vs. NHE, and 1.83 and 1.92 eV, respectively. A highest photocurrent density of 9.7 mA cm−2 (0.25 V vs. SCE)

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under 100 mW cm−2 irradiation was observed by the sample with [Ag]/[In] = 4. These results suggest that Ag:In:S ratio of this sample is a better composition for the formation of Ag–In–S electrodes using the CBD method. The ternary compounds of AgInS2 /AgIn5 S8 are suitable for photocatalytic water splitting applications. Acknowledgments The authors are grateful to the National Science Council of Taiwan (grant no. NSC 97-2221-E-194-026) and the Bureau of Energy, Ministry of Economic Affairs (grant no.7455VH7200), for supporting this study. We would like to thank Instrument Center at NCCU for SEM and National Tsing-Hua University for GIXRD measurements. References [1] Y. Matsumoto, U. Unal, N. Tanaka, A. Kudo, H. Kato, J. Solid State Chem. 177 (2004) 4205. [2] M. Ashokkumar, Int. J. Hydrogen Energy 23 (1998) 427. [3] M. Radecka, M. Rekas, A.T. Zajac, K. Zakrzewska, J. Power Sources 181 (2008) 46. [4] A. Fujishima, K. Honda, Nature 238 (1972) 37. [5] M. Radecka, K. Zakrzewska, M. Wierzbicka, A. Gorzkowska, S. Komornicki, Solid State Ionics 157 (2003) 379. [6] Z.G. Zou, J.H. Ye, K. Sayama, H. Arakawa, Nature 414 (2001) 625. [7] S.U.M. Khan, M. Al-Shahry, W.B. Ingler, Science 297 (2002) 2243. [8] A. Kudo, I. Tsuji, H. Kato, Chem. Commun. (2002) 1958. [9] A.J. Nozik, R. Memming, J. Phys. Chem. 100 (1996) 13061. [10] W.J. Chun, A. Ishikawa, H. Fujisawa, T. Takata, J.N. Kondo, M. Hara, M. Kawai, Y. Matsumoto, K. Domem, J. Phys. Chem. B 107 (2003) 1798. [11] Y. Akaki, S. Kurihara, M. Shirahama, K. Tsurugida, S. Seto, T. Kakeno, K. Yoshino, J. Phys. Chem. Solids 66 (2005) 1858. [12] Z. Aissa, M. Amlouk, T.B. Nasrallah, J.C. Bernede, S. Belgacem, Sol. Energy Mater. Sol. Cells 91 (2007) 489. [13] M.L.A. Aguilera, J.R.A. Hernandez, M.A.G. Trujillo, M.O. Lopez, G.C. Puente, Thin Solid Films 515 (2007) 6272. [14] M.L.A. Aguilera, M.O. López, V.M.S. Resendiz, J.A. Hernández, M.A.G. Trujillo, Mater. Sci. Eng. B 102 (2003) 380.

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