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Electrodeposited ZnIn2S4 onto TiO2 thin films for semiconductor-sensitized photocatalytic and photoelectrochemical applications
Accepted Manuscript Title: Electrodeposited ZnIn2 S4 onto TiO2 thin films for semiconductor-sensitized photocatalytic and photoelectrochemical applications Author: Ibtissem Ben Assaker Mounir Gannouni Jamila Ben Naceur Munirah Abdullah Almessiere Amal Lafy Al-Otaibi Taher Ghrib Shouwen Shen Radhouane Chtourou PII: DOI: Reference:
S0169-4332(15)01372-0 http://dx.doi.org/doi:10.1016/j.apsusc.2015.06.038 APSUSC 30557
To appear in:
APSUSC
Received date: Revised date: Accepted date:
7-3-2015 28-5-2015 8-6-2015
Please cite this article as: I.B. Assaker, M. Gannouni, J.B. Naceur, M.A. Almessiere, A.L. Al-Otaibi, T. Ghrib, S. Shen, R. Chtourou, Electrodeposited ZnIn2 S4 onto TiO2 thin films for semiconductor-sensitized photocatalytic and photoelectrochemical applications., Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.06.038 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights
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ZnIn2S4 thin films was grown using electrodeposition route onto TiO2/ITO coated
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glass substrate.
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Study of the heterostructure ZnIn2S4/TiO2 thin films
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Photocatalytic activity of ZnIn2S4/TiO2 heterostructure under visible light irradiation.
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ZnIn2S4/TiO2
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High performance of Photoelectrochemical properties in the presence of the junction
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Electrodeposited ZnIn2S4 onto TiO2 thin films for semiconductor-sensitized photocatalytic and photoelectrochemical applications.
Ibtissem Ben Assaker*,1, Mounir Gannouni1, Jamila Ben Naceur1, Munirah Abdullah
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Almessiere2, Amal Lafy Al-Otaibi2, Taher Ghrib2, Shouwen Shen3, Radhouane Chtourou1 1
Laboratoire Photovoltaïque, Centre de Recherches et des Technologies de l'Energie Technopole borj cedria, Bp 95, hammamm lif 2050, Tunisie 3
Laboratory of Physical Alloys (LPA), Science Faculty of Dammam, University of Dammam, Saudi Arabia.
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2
Advanced Analysis Unit, Technical Service Division Research & Development Center Saudi Aramco.
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Abstract
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Corresponding author: [email protected]
In this study, ZnIn2S4/TiO2 heterostructure was successfully synthesized on ITO-
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coated glass substrates via a facile two-step process from aqueous solution. Firstly, TiO2 thin film was prepared by sol-gel and deposited onto ITO coated glass substrate by spin-coating method. Then the zinc indium sulfide semiconductor was fabricated via electrodeposition
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technique onto TiO2/ITO coated glass electrode. The X-ray diffraction patterns confirm that
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the heterostructure is mixed of both Anatase TiO2 and Rhombohedric ZnIn2S4. The scanning electron microscopy (SEM) images show that the morphology change with the deposition of
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ZnIn2S4 over TiO2 thin film and a total coverage of the electrode surface was obtained. Optical absorption spectroscopy study of ZnIn2S4/TiO2 heterostructure exhibits a remarkable red-shift compared to the TiO2 and ZnIn2S4 achieve the best efficiency of visible light absorption. Therefore, it is expected to apply to visible-light photocatalysis and solar cells. To investigate the effect of the heterojunction on the photocatalytic activity of ZnIn2S4/TiO2 thin films, photodegradation of methylene blue in the presence of ZnIn2S4 was performed. ZnIn2S4/TiO2 heterostructure exhibited strong photocatalytic activity, and the degradation of methylene blue eached 91% after irradiation only for 4h. Also, the study of the photocurrent density produced by ZnIn2S4/TiO2 thin film electrode reached 0.8 mA.cm -2, about four times higher than that measured on TiO2 thin film. These results indicate that the heterojunction have a better photo-electrochemical performance than the pure TiO2 thin films under illumination. As a result, the obtained ZnIn2S4/TiO2 heterostructure would have great potential in photocatalytic and Photoelectrochemical devices.
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Keywords: Heterojunction; ZnIn2S4/TiO2; Electrodeposition of thin film; Photoelectrochemical properties; Photocatalytic applications.
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1. Introduction
Much attention has been paid to water splitting process by light irradiation because of its
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potential to obtain clean and high energy from water [1]. Among variety of materials, TiO2 has drawn much attention because of its promising applications in utilization of solar energy
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such as photocatalysis [2], photovoltaic [3, 4] and photocatalytic water splitting [5, 6]. However, the photocatalytic performance of TiO2 was restricted by its low visible light absorption and its fast recombination of photo-generated electrons and holes. Therefore to
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enlarge the photoresponse spectrum, considerable efforts have been made, such as doping with transition and/or noble metals [7, 8], non-metals [9, 10], deposition in quantum dots [11] and construction of heterojunctions with other semiconductors [12-16].
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Construction of a heterojunction between TiO2 and other semiconductors with a suitable band gap is an effective method to extend the light absorption spectrum and accelerate
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photogenerated electron-hole separation, thus enhancing the solar-to-hydrogen conversion efficiency [17, 18]. Zinc indium sulfide (ZnIn2S4) is a ternary chalcogenide semiconductor
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with a narrow band gap well corresponding to the visible spectrum and considerable
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photostability in aqueous solution under light irradiation [19, 20]. Several methods have been used for the synthesis of various nanostructures of ZnIn2S4 such as nanosheet, nanowire and microsphere for solar water splitting [21-23]. Among these approaches, electrodeposition technique [24] was used for the deposition of ZnIn2S4 thin films onto TiO2. Compared with other deposition methods, electrodeposition provide numerous advantages, including, low temperature processing, arbitrary substrate shapes, controllable film thickness, morphology, and potential low capital cost. It is an isothermal process mainly controlled by electrical parameters [25]. As ZnIn2S4 is non-toxic and pollution-free, and not susceptible to damage from photocorrosion, it is a potential substitute for CdS. To our best knowledge, there are few reports about the synthesis of oxide semiconductor incorporated with ternary semiconductor materials for the application in photoelectrochemical water splitting. In the present work, semiconducting low band gap ternary ZnIn2S4 thin films are used to sensitize highly-oriented TiO2 thin films. In this context, we will prepare ZnIn2S4 coated TiO2 thin films supported on ITO coated glass combining the sol-gel process, spin coating and
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electrodeposition technique. This work reports the synthesis conditions, the main physicchemical characterizations of the resulting hetero-junctions, including X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-Visible absorption spectroscopy; the photocatalytic activity of ZnIn2S4 and heterostructure ZnIn2S4 /TiO2 was measured following the degradation of an organic model substrate, methylene blue under visible light. The
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2.1. Chemical deposition of TiO2 thin films
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2. Experimental details
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photoelectrochemical properties of the samples were also studied.
TiO2 thin films were prepared using sol-gel spin coating onto ITO glass substrates
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[26]. Titanium (IV) isopropoxide (Ti(OCH(CH3)2)4 was used as a precursor, isopropanol and methanol as solvents and acetic acid as a catalyst. Then deionized water was dropped wise to the solution while vigorously stirring. The obtained mixture was continuously stirred for 3 h
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at room temperature. The TiO2 thin film were deposited onto ITO coated glass substrates by using spin coating technique. The withdrawal speed was set at 3000 rpm and the deposition
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time to 30 s. The TiO2 solution was dropped one time onto the substrates and then dried in the oven at 100 °C for 15 min. The drop and dry process were repeated three times onto the same
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substrates. Finally, the film was annealed under oxygen at 450°C for 60 min.
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2.2. Electrodeposition of ZnIn2S4
Room temperature chemical deposition of nanocrystalline ZnIn2S4 thin film onto ITO coated glass substrate from aqueous acidic bath has been reported earlier [24]. The electrodeposition
has
been
carried
out
potentiostatically
using
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Autolab
Potentiostat/Galvanostat PGSTAT 30 (Eco Chemie BV) connected to a three-electrode cell (K0269A Faraday Cage, Par). The working electrode was (ITO)-coated glass substrate (ρ ≤ 5.0 × 10−5 Ω.cm), the reference electrode was an Ag/AgCl (3M NaCl) and a platinum plate was used as counter electrode. Before using, all (ITO)-coated glass substrates were ultrasonically cleaned during 15 min with respectively acetone and iso-propanol and rinsed with deionized water and finally dried in air at room temperature. In order to obtain near stoichiometric ZnIn2S4 compound, the Zn/In ratio was varied from 1:1 to 1:2 with an excess of sulfur. The suitable Zn/In ratio for ZnIn2S4 compound was found to be 1:1.25. Therefore, a mixture of 1mM ZnCl2, for the zinc, 1.25 mM InCl3, for the indium and 10 mM Na2S2O3.
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5H2O as sulfur source were well stirred in the solution baths. All the precursors were dissolved in deionized water (18 m.cm) with 0.1 M KCl as the supporting electrolyte. The pH of the solution has been adjusted to 2~2.5 by adding drops of concentrated 1.0 M HCl in order to decrease the formation of metal complexes such as In(OH)3. The uniform and well
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adherent ZnIn2S4 thin films have been deposited at optimized deposition potential of −1.1 V (vs. Ag/AgCl). Details of the deposition process are reported in a recent past work [24]. The deposition time (td) was kept at 300 seconds with magnetically stirring. After each
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electrodeposition, the obtained ZnIn2S4 thin films were rinsed with deionized water and then
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annealed in air atmosphere at 200°C for 60 min.
2.3. Chemical deposition of TiO2/ZnIn2S4 thin films
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Electrodeposition of ZnIn2S4 film was deposited onto TiO2 thin films, which were previously deposited on ITO coated substrate using same conditions as mentioned in section
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2.2. TiO2/ZnIn2S4 films were annealed at 200°C for 1 h in air and used for characterizations.
2.4. Characterization of ZnIn2S4, TiO2 and TiO2/ZnIn2S4 thin films
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Characterization of all samples was carried out with different techniques. Structural properties were determined by X-ray diffraction technique by the means of an automated
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Bruker D8 advance X-ray diffractometer with CuKa radiations (λ = 1.541 Ǻ) in 2θ ranging from 10° to 70°. Surface morphology of samples examined using a field-emission scanning
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electron microscope (FE-SEM, JSM-5410, JEOL). Accelerating voltage of SEM was set to 20 kV. Optical measurements were deduced from (transmission/reflection) spectra taken from ultraviolet-visible-near-infrared (UV-VIS-NIR) PerkinElmer Lambda 950 spectrophotometer in the wavelength range of 300-1500 nm at room temperature. The photocatalytic activity of the samples was evaluated by the degradation of a
standard organic dye: methylene blue (MB), in aqueous solution under visible light irradiation for different exposition times. For irradiation, an incandescence visible light lamp with a tungsten filament and power 100 W was used at the distance of 10 cm from the solution in a darkness box. Typically, 5 mL of methylene blue aqueous solution (3 mg.L-1) was placed in a box and photocatalyst films (area of 2×2 cm2) were placed into the solution for each test. Prior to each irradiation, the solution was magnetically stirred in the dark for 30 min to promote an adsorption desorption equilibrium. Methylene blue decomposition evaluation was
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carried out using Carry UV-Visible absorption spectroscopy working in a transmission mode, following the methylene blue absorption peak intensity decrease at ca. 660 nm [26, 27]. The photoelectrochemical performance measurements of the samples were carried out in a quartz electrolytic cell. The sample (average area = 1.0 cm2), a Pt plate electrode (average
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area = 1 cm2), and an Ag/AgCl electrode were employed as the working, counter, and reference electrodes, respectively. Aqueous Na2SO4 (0.5 M) solution, prepared using
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deionized water was used as the electrolyte. All measurements were carried out in air environment at room temperature. Current densities, as a function of applied potential (-1 to +
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0.8 V vs. Ag/AgCl electrode) for the samples, were recorded under front-side illumination with a computer-controlled Potentiostat/Galvanostat PGSTAT 30 (Eco Chemie BV) for all PEC experiments. A 300 W Xe short arc lamp (Perkin Elmer Model PE300BF) with white
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light intensity of 200 mW/cm2 was employed to simulate solar light. The intensity of incident
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light from the Xe lamp was measured using a Photometer Model 70310 from Spectra-Physics.
3. Results and discussion
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3.1. Structural analysis
Fig.1 corresponds to the X-ray diffraction (XRD) patterns of pure TiO2, ZnIn2S4 and
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ZnIn2S4/TiO2 composite. For all diagrams, the peaks marked by dashed line are due to the contribution from the ITO substrate. Also, in the range 15-35° a broad hump appears,
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probably due to an amorphous glass contribution [27, 28]. Fig.1a illustrates the XRD patterns of the TiO2 thin films annealed at 450°C obtained by sol-gel method and deposited by spincoating onto ITO glass substrate. From this figure, we can observe two main peaks at 25.6° and 48.7° corresponding respectively to the reflection planes of (101) and (200), respectively. This was verified by JCPDS (joint committee on Power Diffraction Standards) database for Anatase TiO2 structure with card number (JCPDS#01-084-1286). This result is in good agreement with those published earlier by our group [26, 27]. As evident in Fig. 1b, XRD patterns of the ZnIn2S4 thin films electrodeposited on ITO-coated glass substrates shows single phase of ZnIn2S4 Rhombohedric structure according to JCPDS #049-1562. All diffraction peaks corresponding to (009), (018) and (1010) planes were fully indexed. This result is in good agreement with those found previously by our group [24]. After electrodeposition of ZnIn2S4 onto TiO2 thin films, the XRD pattern of ZnIn2S4/TiO2 composite is very similar to the naked ZnIn2S4 and TiO2 Fig.1c. The two intense peaks appear
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at 2 = 33.15° and 25.36° are characteristic of ZnIn2S4 (018) and TiO2 (101), respectively. These two peaks are still present in the XRD pattern of ZnIn2S4/TiO2 composite, indicating the stability of the two compounds during the synthesis process. This result confirms that the ZnIn2S4/TiO2 composite sample was successfully prepared.
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To verify the composite structure, the morphology of three samples was observed by scanning electron microscopy (SEM).
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3.2. Morphological analysis
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Fig.2(a) and (b) shows the scanning electron microscopy (20,000 x magnification) of TiO2 and ZnIn2S4 thin films obtained by sol gel and electrodeposition technique, respectively. Due to homogeneous nucleation process, both films were uniform with dense surface
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morphology covering complete substrate surface area. From figure 2(a), some pinholes in the surface can be appeared. This phenomenon was may be due to the effect of the annealing
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temperature of TiO2 thin film (450°C for 1 h). Covered both TiO2 and ZnIn2S4 films consist of single step of grains (spherical) with little different surface morphology. The average grain size for TiO2 (10 nm) thin films was too much smaller than ZnIn2S4 (100 nm). Difference in
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grain size of the two thin films is attributed to different chemical deposition kinetics (sol gel
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and electrodeposition) and preparative conditions. Fig.2c shows the scanning electron microscopy image of the electrodeposited of ZnIn2S4 over TiO2 thin film. It may be noted
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here that the grains of ZnIn2S4 on TiO2 were spherical in nature but slightly smaller in size compared to the grains of ZnIn2S4 on bare ITO (Fig.2b). The morphology change confirms the deposition of ZnIn2S4 over TiO2 thin films with a total coverage of the electrode surface, which is in agreement with the XRD results. We note also from figure 2c that the number of pinholes has been reduced because the surface of TiO2 was covered by ZnIn2S4 thin film. An approximate value of film thickness was obtained through analysis by Atomic
Force Microscopy as observed in our previous work [24, 26]. In our case, the estimated thickness of ZnIn2S4, TiO2 and hetero-structure ZnIn2S4/TiO2 thin films were 150 nm, 55 nm and 200 nm respectively. We can clearly note that the values of the thickness are not the same for thin films due to the change of surface.
3.3. Optical study
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The optical properties of ZnIn2S4, TiO2 and ZnIn2S4/TiO2 heterostructures were characterized by the UV-vis absorption spectra, as shown in Fig.3. The absorption spectrum of TiO2 thin films (Fig 3.a) shows a strong absorption in ultraviolet region with spectral wavelength between 300 and 400 nm exhibiting typical absorption spectra of TiO2[29, 30]. Fig3.b shows the UV-visible absorption spectra of ZnIn2S4 obtained by electrodeposition
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technique onto ITO substrate. There we can observe that the ZnIn2S4 thin films have a broad absorption in the UV to visible region with a tail extending to 800 nm, which matches the
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criterion of good absorption in the visible light region for photoelectrode application [31, 32]. For fig3.c, it can be seen that ZnIn2S4/TiO2 heterostructure exhibits a remarkable red-shift
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compared to the TiO2 and ZnIn2S4 achieve the best efficiency of visible light absorption. Therefore, it is expected to apply to visible-light photocatalysis and solar cells [33].
and it is given by the following equation Eq (1):
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For semiconductors, the optical absorption coefficient, α, is related with the energy band gap
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h A( h E g ) n
where A is an energy dependent constant, Eg is the band gap of the material, hν is photon energy and n is an index that characterizes the optical absorption process and it is theoretically
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equal to 2, 1/2, 3 or 3/2 for indirect allowed, direct allowed, indirect for-bidden and direct for-
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bidden transitions. The best plot that covers widest range of data in accordance with (Eq. 1) has been obtained for the ( h )2– h dependence. This plot is illustrated in Fig. 4. E-axis
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interception in this figure reflects the direct allowed transitions in our prepared samples. The band gaps for single layer TiO2 and ZnIn2S4 films were estimated to be 3.6 eV and 2.4 eV, respectively. These values are approximate to the values in literatures [24, 26]. We noticed from figure 4 that there are two band gaps for the ZnIn2S4/TiO2 heterojunction film. One is high around 3.4 eV and the other is low at around 2.6. The former seems derived from TiO2 and the latter is approximate of ZnIn2S4. The two estimated gaps could be attributed to the different absorption by ZnIn2S4 and TiO2 since both can absorb UV light but only ZnIn2S4 absorbs visible light [34]. These results considering the possibility of using the ZnIn2S4/TiO2 heterostructure for photocatalytic application in the visible light.
3.4. Photocatalytic activities As explained previously in the experimental section, methylene blue substrate was chosen as a polluent model to evaluate the photocatalytic efficiency of the prepared
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photocatalysts. UV-visible absorption spectra recorded on the methylene blue aqueous solution before and after contact with both ZnIn2S4 and heterojunction ZnIn2S4/TiO2 photocatalysts are given in Fig. 5a. Clearly, the heterojunction ZnIn2S4/TiO2 thin films sample presents higher intrinsic activity for the photodegradation of methylene blue than the only ZnIn2S4. This activity is demonstrated by the considerable decrease in the absorbance, which
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is related to the decrease of concentration of methylene blue in the solution. In the presence of only TiO2 thin films, (curve not presented here), the UV-visible absorption spectra remained
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constant, which demonstrated that TiO2 has no photocatalytic activity under visible light irradiation [33]. As illustrated in Fig.5b the absorbance at 660 nm for methylene blue in the
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presence of ZnIn2S4/TiO2 thin films decreased as increasing the irradiation time, accompanied by the intensity decrease, the maximum absorption wavelength of methylene blue monomer [26]. This confirmed the visible photocatalysis of ZnIn2S4/TiO2 thin films. To investigate the
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effect of the heterojunction on the photocatalytic activity of ZnIn2S4/TiO2 thin films, photodegradation of methylene blue in the presence of ZnIn2S4 was also performed. As shown
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in Fig.5c, only the ZnIn2S4/TiO2 thin films exhibited strong photocatalytic activity and the degradation of methylene blue eached 91% after irradiation for 4h. ZnIn2S4 thin film shows 70%. photocatalytic activities in the degradation of methylene blue. A possible explanation is
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as follows. TiO2 has no photocatalytic activity in the visible region of λ >420 nm due to its
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wide band gap (3.6 eV) [15, 35]. The ZnIn2S4 with a narrower band gap (2.4eV) allows the photodegradation of methylene blue in visible light region [32]. The visible light
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photocatalytic activity of heterojunction ZnIn2S4/TiO2 composite should primarily come from ZnIn2S4. This result is in good agreement with that obtained by W.-H. Yuan et al. [33], which explained that the unique core/shell nanostructure of a composite semiconductor photocatalyst plays a key role resulting in a large increase in the photocatalytic activity for methylene blue degradation. In fact, The intimate contact between TiO2 and ZnIn2S4 favors the formation of junctions between the two components [36], and the higher activity of the ZnIn2S4/TiO2 composite can be attributed to electron transfer between two conduction bands [37, 38] .
In order to better understand the charge transport properties, the possible reason for the increased photocatalytic activity observed in this study can be explained by an approach described in (Fig. 6). By exploiting the different values of the flat band potential esteemed from I-V responses as described in the next section (see Fig. 7), and the band gap obtained from optical measurements, we are able to get the relative position of the conductive band
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edge and valence band edge. This approach was carried out by several authors and recently by our research group to describe the band position of CuIn5S8 [39]. In this study, the flat-band potentials of the ZnIn2S4 and TiO2 are in the range of -0.64 to -0.55 V vs. Ag/AgCl electrode, (-0.44 to -0.35 V vs. normal hydrogen electrode (NHE)), respectively. It is generally known that the difference between the position of the conduction
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band (ECB) and that of the flat band potential (EFB) for many n-type semiconductors is 0.1–0.3 V [40, 41]. Thus, the difference between EFB and ECB is assumed to be -0.3 V for the ZnIn2S4
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and TiO2. From this figure, the conduction band (CB) edges of ZnIn2S4 and TiO2 are -0.74 eV and -0.65 eV vs normal hydrogen electrode (NHE), respectively. Accordingly, the valence
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band (VB) edges are 1.66 eV for ZnIn2S4 and 2.95 eV vs NHE for TiO2. Because of its narrower band gap, the ZnIn2S4 cocatalyst is more easily activated by the wide range of wavelengths emitted by the light source employed. Because of these electronic properties, the
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electrons in ZnIn2S4 are first excited into the conduction band of this material, and the heterojunction structure formed by ZnIn2S4 and TiO2 thin films facilitates the migration of the
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photoexcited electrons to the conductions band of TiO2. So that additional electrons enter the conduction band of TiO2. At the end, the transferred electron-hole pairs from ZnIn2S4 to TiO2 facilitate redox reactions through the formation of absorbed radicals on the ZnIn2S4-TiO2
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surface. Recently, this phenomena was obtained by several authors in the literature described
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the electron transfer in the heterostructure between TiO2 and another semiconductors [16, 4244]. This suggestion will be confirmed in the next part by photoelectrochemical study of
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ZnIn2S4/TiO2 heterostructure .
3.5. Photoelectrochemical properties of ZnIn2S4/TiO2 heterostructures In the recent years, photoelectrochemical (PEC) cells based on semiconductor-
electrolyte junction have been used for quick testing of the quality of solar cell materials [45]. In the present case, linear sweep voltammetry was performed in the dark and under simulated solar light to characterize the ability of the photoelectrochemical cells consisting of ITO/TiO2/ZnIn2S4/Na2SO4/Pt. Through the measurement of I-V curves (Fig. 7), TiO2, ZnIn2S4 and heterostructure ZnIn2S4/TiO2 electrodes led to negligible current under dark conditions. Under illumination, the current densities of the TiO2 and ZnIn2S4 electrodes were evident which increases with the bias potential. The TiO2 electrode exhibited a photocurrent of 0.2 mA/cm2 at 0.5V versus Ag/AgCl. However, ZnIn2S4 electrode exhibited only 0.07 mA/ cm2 at
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the same potential. From fig.7b, we can note a small peak at the negative bias (-0.5 to -0.25V) which originated from oxidation of S2- ion by photogenerated holes [46]. This phenomenon was attributed to the photo-corrosion of metal sulfide semiconductors. Many workers had reported this phenomenon [47, 48]. In the presence of both semiconductors TiO2 and ZnIn2S4 (curve 7c), the current density
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shows an important increasing up to 0.8 mA/cm2 at 0.5V versus Ag/AgCl. The increment in efficiency of photogenerated charge carriers of ZnIn2S4 /TiO2.
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the current value can be explained through the high absorption and the perfect separation This suggests that ZnIn2S4 modified TiO2 could effectively improve the visible-light response
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and the separation of electron–hole pairs compared with pure TiO2 or ZnIn2S4 under the same bias potential. The excellent photoelectrochemical properties could be attributed to the function of ZnIn2S4 /TiO2 heterojunction for reducing the recombination of photo-generated
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electrons and holes. Moreover, it can be seen a small shift in onset potential from -0.5 to 0.1V vs.Ag/AgCl, suggesting a shift in Fermi level as a result of the coupling TiO2 and
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ZnIn2S4. This result gives a slight improvement in photoelectrochemical performance as compared with other reported data available in literature [33, 49]. This suggests that the on the synthesis process.
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performance of the ZnIn2S4/TiO2 thin films in terms of photoelectrochemical cell depending
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Both photoanodes (TiO2 and junction ZnIn2S4/ TiO2) were illuminated intermittently at a given potential (+ 0.5V vs.Ag/AgCl) for several cycles, to appreciate the reproducibility
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of their photoresponses as well as the stability of the device toward oxidation. The corresponding current density response to on–off cycling with total test duration of 300 seconds is presented in Fig.8. From this figure, one can observe that both photoanodes led to an instantaneous change in current upon illumination. The current retracted to the original values almost instantaneously once the illumination is switched off. These results indicated the stability of both thin films. Besides, we can note from fig.8b that the photocurrent density produced by ZnIn2S4/TiO2 thin film electrode reached 0.8 mA.cm-2, about four times higher than that measured on TiO2 thin film. These results prove that the heterojunction have a better photo-electrochemical performance than the pure TiO2 thin films under illumination. In this case, the coupling of ZnIn2S4 and TiO2 thin films could transfer electrons from ZnIn2S4 into TiO2 for their proper conduction of band potentials. This result is in good agreement with that obtained in the photocatalytic activity.
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As a conclusion, it can be clearly inferred that the heterojunction fabrication is an effective
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way to improve the photoelectric performance [50, 51].
4. Conclusion
Heterostructure of ZnIn2S4/TiO2 films was deposited onto ITO coated glass substrate
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by two step low cost methods. Firstly, TiO2 thin film was prepared by sol-gel and deposited onto ITO coated glass substrate by spin-coating method. Then zinc indium sulfide
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semiconductor was fabricated via electrodeposition technique.
Fabrication of heterostructure electrodes in this work show several benefits. First, the
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procedure is very simple and fast. Second, the process is environmentally friendly, and it can be performed using commercially available reagents in water as a green solvent with no need to any additive. Finally, it does not require rigid condition such as high temperature and high
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pressure apparatus so it can perform at room temperature.
Thin films of TiO2, ZnIn2S4 and ZnIn2S4/TiO2 heterostructure are studied and characterized using X-ray diffraction, scanning electron microscopy and optical analysis. The
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difference between single and heterojunction structure has been established. The
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heterostructure materials were applied to the photocatalytic degradation of methylene blue. In comparison with the ZnIn2S4 semiconductor, the heterostructure ZnIn2S4/TiO2 thin films
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results in a significantly increase in the photocatalytic activity. The photoelectrochemical performances of both TiO2 and heterojunction ZnIn2S4/TiO2 have been investigated at room temperature and under 200 W illumination. As a general trend the photoelectrochemical performance of the heterojunction was better than the performance of the TiO2 thin films. These Changes can be correlated with the synthesis methods.
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deposition of ZnIn2S4 nanosheet films on FTO substrates for photoelectric application,
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of 4-nitrophenol, Applied Catalysis B: Environnemental 168 (2015) 266-273. [23] J. Zhou, G. Tian, Y. Chen, X. Meng, Y. Shi, X. Cao, K. Pan, H. Fu, In situ controlled growth of ZnIn2S4 nanosheets on reduced graphene oxide for enhanced photocatalytic hydrogen production performance , Chemical Communications 49 (2013) 2237. [24] I. Ben Assaker, M. Gannouni, A. Lamouchi, R. Chtourou, Physicals and electrochemical properties of ZnIn2S4 thin films grown by electrodeposition route, Superlattices and Microstrutures 75 (2014) 159-170.
[25] M. Gannouni, I. Ben Assaker, R. Chtourou, Influence of electrodeposition potential on the properties of CuIn5S8 spinel thin films. Journal of Electrochemical Society 160 (2013) H446-51. [26] J. Ben Naceur, M. Gaidi, F. Bousbih, R. Mechiakh, R. Chtourou, Annealing effects on microstructural and optical properties of Nanostructured-TiO2 thin films prepared by sol-gel technique, Current Applied Physics 12 (2012) 422-428.
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photocatalyst with enhanced performance for photocatalytic H2 evolution, International Journal of Hydrogen Energy 38 (2013) 1278-1285.
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valence band positions of Ta2O5, TaON, and Ta3N5 by UPS and electrochemical methods.
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complex oxides. Journal of Solid State Chemistry 126 (1996) 227–234.
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[43] S. Cheguetmi, F. Mammeri, S. Nowak, P. Decorse, H. Lecoq, M. Gaceur, J. Ben Naceur,
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S. Achour, R. Chtourou, S. Ammar, Photocatalytic activity of TiO2 nanofibers sensitized with ZnS quantum dots, RSC Adv, 3 (2013) 2572-2580.
[44] T. Du, N. Wang, H. Chen, H. He, H. Lin, K. Liu, TiO2-based solar cells sensitized by
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chemical-bath-deposited few-layer MoS2, Journal of Power Sources 275 (2015) 943-949.
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[45] S.M. Pawar, B. S. Pawar, A. V. Moholkar, D. S. Choi, J. H. Yun, J. H. Moon, S.S. Kolekar, J. H. Kim, Single step electrosynthesis of Cu2ZnSnS4 (CZTS) thin films for solar
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cell application, electrochemical Acta 55 (2010) 4057-4061. [46] M. Dieter, M. Rundiger, K. Bertel. Photoelectrochemistry of cadmium sulfide. 1. Reanalysis of photocorrosion and flat-band potential. The Journal Physical Chemistry 92 (1988) 3476 e 83.
[47] F. DJ, P. EA, P. LM. A kinetic study of CdS photocorrosion by intensity modulated photocurrent and photoelectrochemical impedance spectroscopy. Journal of Electroanalytical Chemistry 473 (1999) 19 2 e203.
[48] J. Jiang, M. Wang, R. Li, L. Ma, L. Guo, Fabricating CdS/BiVO4 and BiVO4/CdS heterostructured film photoelectrodes for photoelectrochemical applications, Inetrnational Journal of Hydrogen Energy 38 (2013) 13069-13076. [49] S. Peng Y. Wu, P. Zhu, V. Thavasi, S. Ramakrishna, S. G. Mhaisalkar, Controlled synthesis and photoelectric application of ZnIn2S4 nanosheet/TiO2 nanoparticle omposite films, Journal of Materials Chemistry, 21 (2011) 15718.
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nanosheets coating, Nano Energy (2014) . [51] Y. Jia, F. Yang, F. Cai, C. Cheng. Y. Zhao, Photoelectrochemical and charge transfer properties of SnS/TiO2 heterostructure nanotube arrays. Electronics Materials Letters 9 (2013)
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an
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287-291.
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(018)
an (200)
(c)
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(1010)
Intensity (a.u)
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Figure 1
¤
¤
(101)
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20
30
(200)
(a)
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*
ITO (01-089-4598)
10
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*
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(b)
*
40
50
60
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Bragg angle 2(degree)
Fig.1. XRD patterns of (a) TiO2, (b) ZnIn2S4 and (c) ZnIn2S4/TiO2 junction.
Page 19 of 27
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an
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cr
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Figure 2
Figure 2: SEM images of (a) TiO2 thin films, (b) ZnIn2S4, and (c) ZnIn2S4/TiO2.
Page 20 of 27
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Figure 3
1.0
(a) (b) (c)
cr us
0.6
0.4
an
Absorbance (a.u)
0.8
0.0 400
500
600
700
800
900 1000 1100 1200 1300 1400 1500
d
300
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0.2
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Wavelength(nm)
Figure 3: UV-Visible absorption spectra of (a) TiO2, (b) ZnIn2S4 thin films and (c) ZnIn2S4/TiO2 heterostructure.
Page 21 of 27
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Figure 4
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20
cr
(a) (b) (c)
h (eV.cm )
-1 2
30
0 1.0
1.5
2.0
2.5
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10
3.0
3.5
4.0
4.5
d
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heV)
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Figure 4: plots of (αhν)2 versus hν of (a) single TiO2, (b) ZnIn2S4 films and (c) heterostructure ZnIn2S4/TiO2 films.
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Figure 5
0.8
0.8
0h
(B)
(a)
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A
0.6
0.6
(c)
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0.2
0.2
0.0
500
550
600
650
700
750
400
800
450
500
550
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450
600
2h
3h 4h
650
700
750
800
Wavelenth (nm)
Wavelenth (nm)
0.8
(C)
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0.7
ZnIn2S4 ZnIn2S4/TiO2
0.6 0.5
d
0.4 0.3
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Absorbance (a.u)
0.0 400
1h
0.4
cr
Absorbance (a.u)
0.4
0.2 0.1
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Absorbance (a.u)
(b)
0.0
0
1
2 3 Irradiation time (min)
4
Figure 5: (A) Absorption spectrum of MB solution obtained after 1 h in the contact of (a) without any sample, (b) with ZnIn2S4 thin films and (c) ZnIn2S4/TiO2 heterojunction; (B) Time-dependent absorption spectrum of MB in the presence of ZnIn2S4/TiO2 heterostructure under the visible light irradiation and (C) visible light photocatalytic activity of MB for ZnIn2S4 and ZnIn2S4/TiO2.
Page 23 of 27
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d
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an
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cr
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Figure 6
Figure 6: Scheme of excitation and charge transfer process between ZnIn2S4 and TiO2 thin films.
Page 24 of 27
Figure 7
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1.4 1.2 -2
I (mA.cm )
(b) ZnIn S dark 2 4 (b) ZnIn S illumination 2 4
1.0 0.8 0.6
(c) Junction dark
0.4
(c) Junction illumination
-1.0
-0.8
-0.6
-0.4
an
0.2
-1.2
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(a) TiO illumination 2
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(a) TiO dark 2
0.0 0.0
-0.2
0.2
0.4
0.6
0.8
1.0
-0.2
E (V / vs.Ag/AgCl)
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-0.4 -0.6
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te
d
-0.8
Figure 7: Photocurrent density-applied voltage plots between range of -1 V to 0.8 V vs. Ag/AgCl for samples (a) TiO2, (b) ZnIn2S4 and (c) ZnIn2S4/TiO2 heterostructure under illuminated and dark conditions in 0.5 M Na2SO4 aqueous solution.
Upon illumination, the photocurrent appeared at about -0.4, -0.5 V vs. Ag/AgCl
(photocurrent onset) and increased gradually on the positive potentials, which was consistent with the flat band potential very well, and also an n-type character of these films. This result is consistent with that observed by M-S plots in our previous work [31]. In fact, under polarization at different potentials, photocurrent can be measured at all potentials more anodic
Page 25 of 27
than Vfb, so that an accumulation of majority carriers occurs when V < Vfb, and a depletion
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layer forms when V > Vfb.
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Figure 8
(b) ZnIn2S4/TiO2
Light on
-2
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0.8 Current density (mA.cm )
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1.0
an
0.6
0.4
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0.2
0.0
Dark
(a) TiO2 0
50
100
150
200
250
300
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d
Times (sec)
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Figure 8 : Current density of (a) TiO2 and (b) junction ZnIn2S4/TiO2 as a function of time with the external bias kept at 0.5V vs.Ag/AgCl electrode in 0.5M Na2SO4 aqueous solution.
Page 27 of 27
S0169-4332(15)01372-0 http://dx.doi.org/doi:10.1016/j.apsusc.2015.06.038 APSUSC 30557
To appear in:
APSUSC
Received date: Revised date: Accepted date:
7-3-2015 28-5-2015 8-6-2015
Please cite this article as: I.B. Assaker, M. Gannouni, J.B. Naceur, M.A. Almessiere, A.L. Al-Otaibi, T. Ghrib, S. Shen, R. Chtourou, Electrodeposited ZnIn2 S4 onto TiO2 thin films for semiconductor-sensitized photocatalytic and photoelectrochemical applications., Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.06.038 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights
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ZnIn2S4 thin films was grown using electrodeposition route onto TiO2/ITO coated
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glass substrate.
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Study of the heterostructure ZnIn2S4/TiO2 thin films
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Photocatalytic activity of ZnIn2S4/TiO2 heterostructure under visible light irradiation.
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ZnIn2S4/TiO2
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High performance of Photoelectrochemical properties in the presence of the junction
Page 2 of 27
Electrodeposited ZnIn2S4 onto TiO2 thin films for semiconductor-sensitized photocatalytic and photoelectrochemical applications.
Ibtissem Ben Assaker*,1, Mounir Gannouni1, Jamila Ben Naceur1, Munirah Abdullah
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Almessiere2, Amal Lafy Al-Otaibi2, Taher Ghrib2, Shouwen Shen3, Radhouane Chtourou1 1
Laboratoire Photovoltaïque, Centre de Recherches et des Technologies de l'Energie Technopole borj cedria, Bp 95, hammamm lif 2050, Tunisie 3
Laboratory of Physical Alloys (LPA), Science Faculty of Dammam, University of Dammam, Saudi Arabia.
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2
Advanced Analysis Unit, Technical Service Division Research & Development Center Saudi Aramco.
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Abstract
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Corresponding author: [email protected]
In this study, ZnIn2S4/TiO2 heterostructure was successfully synthesized on ITO-
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coated glass substrates via a facile two-step process from aqueous solution. Firstly, TiO2 thin film was prepared by sol-gel and deposited onto ITO coated glass substrate by spin-coating method. Then the zinc indium sulfide semiconductor was fabricated via electrodeposition
d
technique onto TiO2/ITO coated glass electrode. The X-ray diffraction patterns confirm that
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the heterostructure is mixed of both Anatase TiO2 and Rhombohedric ZnIn2S4. The scanning electron microscopy (SEM) images show that the morphology change with the deposition of
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ZnIn2S4 over TiO2 thin film and a total coverage of the electrode surface was obtained. Optical absorption spectroscopy study of ZnIn2S4/TiO2 heterostructure exhibits a remarkable red-shift compared to the TiO2 and ZnIn2S4 achieve the best efficiency of visible light absorption. Therefore, it is expected to apply to visible-light photocatalysis and solar cells. To investigate the effect of the heterojunction on the photocatalytic activity of ZnIn2S4/TiO2 thin films, photodegradation of methylene blue in the presence of ZnIn2S4 was performed. ZnIn2S4/TiO2 heterostructure exhibited strong photocatalytic activity, and the degradation of methylene blue eached 91% after irradiation only for 4h. Also, the study of the photocurrent density produced by ZnIn2S4/TiO2 thin film electrode reached 0.8 mA.cm -2, about four times higher than that measured on TiO2 thin film. These results indicate that the heterojunction have a better photo-electrochemical performance than the pure TiO2 thin films under illumination. As a result, the obtained ZnIn2S4/TiO2 heterostructure would have great potential in photocatalytic and Photoelectrochemical devices.
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Keywords: Heterojunction; ZnIn2S4/TiO2; Electrodeposition of thin film; Photoelectrochemical properties; Photocatalytic applications.
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1. Introduction
Much attention has been paid to water splitting process by light irradiation because of its
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potential to obtain clean and high energy from water [1]. Among variety of materials, TiO2 has drawn much attention because of its promising applications in utilization of solar energy
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such as photocatalysis [2], photovoltaic [3, 4] and photocatalytic water splitting [5, 6]. However, the photocatalytic performance of TiO2 was restricted by its low visible light absorption and its fast recombination of photo-generated electrons and holes. Therefore to
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enlarge the photoresponse spectrum, considerable efforts have been made, such as doping with transition and/or noble metals [7, 8], non-metals [9, 10], deposition in quantum dots [11] and construction of heterojunctions with other semiconductors [12-16].
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Construction of a heterojunction between TiO2 and other semiconductors with a suitable band gap is an effective method to extend the light absorption spectrum and accelerate
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photogenerated electron-hole separation, thus enhancing the solar-to-hydrogen conversion efficiency [17, 18]. Zinc indium sulfide (ZnIn2S4) is a ternary chalcogenide semiconductor
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with a narrow band gap well corresponding to the visible spectrum and considerable
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photostability in aqueous solution under light irradiation [19, 20]. Several methods have been used for the synthesis of various nanostructures of ZnIn2S4 such as nanosheet, nanowire and microsphere for solar water splitting [21-23]. Among these approaches, electrodeposition technique [24] was used for the deposition of ZnIn2S4 thin films onto TiO2. Compared with other deposition methods, electrodeposition provide numerous advantages, including, low temperature processing, arbitrary substrate shapes, controllable film thickness, morphology, and potential low capital cost. It is an isothermal process mainly controlled by electrical parameters [25]. As ZnIn2S4 is non-toxic and pollution-free, and not susceptible to damage from photocorrosion, it is a potential substitute for CdS. To our best knowledge, there are few reports about the synthesis of oxide semiconductor incorporated with ternary semiconductor materials for the application in photoelectrochemical water splitting. In the present work, semiconducting low band gap ternary ZnIn2S4 thin films are used to sensitize highly-oriented TiO2 thin films. In this context, we will prepare ZnIn2S4 coated TiO2 thin films supported on ITO coated glass combining the sol-gel process, spin coating and
Page 4 of 27
electrodeposition technique. This work reports the synthesis conditions, the main physicchemical characterizations of the resulting hetero-junctions, including X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-Visible absorption spectroscopy; the photocatalytic activity of ZnIn2S4 and heterostructure ZnIn2S4 /TiO2 was measured following the degradation of an organic model substrate, methylene blue under visible light. The
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2.1. Chemical deposition of TiO2 thin films
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2. Experimental details
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photoelectrochemical properties of the samples were also studied.
TiO2 thin films were prepared using sol-gel spin coating onto ITO glass substrates
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[26]. Titanium (IV) isopropoxide (Ti(OCH(CH3)2)4 was used as a precursor, isopropanol and methanol as solvents and acetic acid as a catalyst. Then deionized water was dropped wise to the solution while vigorously stirring. The obtained mixture was continuously stirred for 3 h
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at room temperature. The TiO2 thin film were deposited onto ITO coated glass substrates by using spin coating technique. The withdrawal speed was set at 3000 rpm and the deposition
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time to 30 s. The TiO2 solution was dropped one time onto the substrates and then dried in the oven at 100 °C for 15 min. The drop and dry process were repeated three times onto the same
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substrates. Finally, the film was annealed under oxygen at 450°C for 60 min.
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2.2. Electrodeposition of ZnIn2S4
Room temperature chemical deposition of nanocrystalline ZnIn2S4 thin film onto ITO coated glass substrate from aqueous acidic bath has been reported earlier [24]. The electrodeposition
has
been
carried
out
potentiostatically
using
An
Autolab
Potentiostat/Galvanostat PGSTAT 30 (Eco Chemie BV) connected to a three-electrode cell (K0269A Faraday Cage, Par). The working electrode was (ITO)-coated glass substrate (ρ ≤ 5.0 × 10−5 Ω.cm), the reference electrode was an Ag/AgCl (3M NaCl) and a platinum plate was used as counter electrode. Before using, all (ITO)-coated glass substrates were ultrasonically cleaned during 15 min with respectively acetone and iso-propanol and rinsed with deionized water and finally dried in air at room temperature. In order to obtain near stoichiometric ZnIn2S4 compound, the Zn/In ratio was varied from 1:1 to 1:2 with an excess of sulfur. The suitable Zn/In ratio for ZnIn2S4 compound was found to be 1:1.25. Therefore, a mixture of 1mM ZnCl2, for the zinc, 1.25 mM InCl3, for the indium and 10 mM Na2S2O3.
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5H2O as sulfur source were well stirred in the solution baths. All the precursors were dissolved in deionized water (18 m.cm) with 0.1 M KCl as the supporting electrolyte. The pH of the solution has been adjusted to 2~2.5 by adding drops of concentrated 1.0 M HCl in order to decrease the formation of metal complexes such as In(OH)3. The uniform and well
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adherent ZnIn2S4 thin films have been deposited at optimized deposition potential of −1.1 V (vs. Ag/AgCl). Details of the deposition process are reported in a recent past work [24]. The deposition time (td) was kept at 300 seconds with magnetically stirring. After each
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electrodeposition, the obtained ZnIn2S4 thin films were rinsed with deionized water and then
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annealed in air atmosphere at 200°C for 60 min.
2.3. Chemical deposition of TiO2/ZnIn2S4 thin films
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Electrodeposition of ZnIn2S4 film was deposited onto TiO2 thin films, which were previously deposited on ITO coated substrate using same conditions as mentioned in section
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2.2. TiO2/ZnIn2S4 films were annealed at 200°C for 1 h in air and used for characterizations.
2.4. Characterization of ZnIn2S4, TiO2 and TiO2/ZnIn2S4 thin films
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Characterization of all samples was carried out with different techniques. Structural properties were determined by X-ray diffraction technique by the means of an automated
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Bruker D8 advance X-ray diffractometer with CuKa radiations (λ = 1.541 Ǻ) in 2θ ranging from 10° to 70°. Surface morphology of samples examined using a field-emission scanning
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electron microscope (FE-SEM, JSM-5410, JEOL). Accelerating voltage of SEM was set to 20 kV. Optical measurements were deduced from (transmission/reflection) spectra taken from ultraviolet-visible-near-infrared (UV-VIS-NIR) PerkinElmer Lambda 950 spectrophotometer in the wavelength range of 300-1500 nm at room temperature. The photocatalytic activity of the samples was evaluated by the degradation of a
standard organic dye: methylene blue (MB), in aqueous solution under visible light irradiation for different exposition times. For irradiation, an incandescence visible light lamp with a tungsten filament and power 100 W was used at the distance of 10 cm from the solution in a darkness box. Typically, 5 mL of methylene blue aqueous solution (3 mg.L-1) was placed in a box and photocatalyst films (area of 2×2 cm2) were placed into the solution for each test. Prior to each irradiation, the solution was magnetically stirred in the dark for 30 min to promote an adsorption desorption equilibrium. Methylene blue decomposition evaluation was
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carried out using Carry UV-Visible absorption spectroscopy working in a transmission mode, following the methylene blue absorption peak intensity decrease at ca. 660 nm [26, 27]. The photoelectrochemical performance measurements of the samples were carried out in a quartz electrolytic cell. The sample (average area = 1.0 cm2), a Pt plate electrode (average
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area = 1 cm2), and an Ag/AgCl electrode were employed as the working, counter, and reference electrodes, respectively. Aqueous Na2SO4 (0.5 M) solution, prepared using
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deionized water was used as the electrolyte. All measurements were carried out in air environment at room temperature. Current densities, as a function of applied potential (-1 to +
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0.8 V vs. Ag/AgCl electrode) for the samples, were recorded under front-side illumination with a computer-controlled Potentiostat/Galvanostat PGSTAT 30 (Eco Chemie BV) for all PEC experiments. A 300 W Xe short arc lamp (Perkin Elmer Model PE300BF) with white
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light intensity of 200 mW/cm2 was employed to simulate solar light. The intensity of incident
M
light from the Xe lamp was measured using a Photometer Model 70310 from Spectra-Physics.
3. Results and discussion
d
3.1. Structural analysis
Fig.1 corresponds to the X-ray diffraction (XRD) patterns of pure TiO2, ZnIn2S4 and
te
ZnIn2S4/TiO2 composite. For all diagrams, the peaks marked by dashed line are due to the contribution from the ITO substrate. Also, in the range 15-35° a broad hump appears,
Ac ce p
probably due to an amorphous glass contribution [27, 28]. Fig.1a illustrates the XRD patterns of the TiO2 thin films annealed at 450°C obtained by sol-gel method and deposited by spincoating onto ITO glass substrate. From this figure, we can observe two main peaks at 25.6° and 48.7° corresponding respectively to the reflection planes of (101) and (200), respectively. This was verified by JCPDS (joint committee on Power Diffraction Standards) database for Anatase TiO2 structure with card number (JCPDS#01-084-1286). This result is in good agreement with those published earlier by our group [26, 27]. As evident in Fig. 1b, XRD patterns of the ZnIn2S4 thin films electrodeposited on ITO-coated glass substrates shows single phase of ZnIn2S4 Rhombohedric structure according to JCPDS #049-1562. All diffraction peaks corresponding to (009), (018) and (1010) planes were fully indexed. This result is in good agreement with those found previously by our group [24]. After electrodeposition of ZnIn2S4 onto TiO2 thin films, the XRD pattern of ZnIn2S4/TiO2 composite is very similar to the naked ZnIn2S4 and TiO2 Fig.1c. The two intense peaks appear
Page 7 of 27
at 2 = 33.15° and 25.36° are characteristic of ZnIn2S4 (018) and TiO2 (101), respectively. These two peaks are still present in the XRD pattern of ZnIn2S4/TiO2 composite, indicating the stability of the two compounds during the synthesis process. This result confirms that the ZnIn2S4/TiO2 composite sample was successfully prepared.
ip t
To verify the composite structure, the morphology of three samples was observed by scanning electron microscopy (SEM).
cr
3.2. Morphological analysis
us
Fig.2(a) and (b) shows the scanning electron microscopy (20,000 x magnification) of TiO2 and ZnIn2S4 thin films obtained by sol gel and electrodeposition technique, respectively. Due to homogeneous nucleation process, both films were uniform with dense surface
an
morphology covering complete substrate surface area. From figure 2(a), some pinholes in the surface can be appeared. This phenomenon was may be due to the effect of the annealing
M
temperature of TiO2 thin film (450°C for 1 h). Covered both TiO2 and ZnIn2S4 films consist of single step of grains (spherical) with little different surface morphology. The average grain size for TiO2 (10 nm) thin films was too much smaller than ZnIn2S4 (100 nm). Difference in
d
grain size of the two thin films is attributed to different chemical deposition kinetics (sol gel
te
and electrodeposition) and preparative conditions. Fig.2c shows the scanning electron microscopy image of the electrodeposited of ZnIn2S4 over TiO2 thin film. It may be noted
Ac ce p
here that the grains of ZnIn2S4 on TiO2 were spherical in nature but slightly smaller in size compared to the grains of ZnIn2S4 on bare ITO (Fig.2b). The morphology change confirms the deposition of ZnIn2S4 over TiO2 thin films with a total coverage of the electrode surface, which is in agreement with the XRD results. We note also from figure 2c that the number of pinholes has been reduced because the surface of TiO2 was covered by ZnIn2S4 thin film. An approximate value of film thickness was obtained through analysis by Atomic
Force Microscopy as observed in our previous work [24, 26]. In our case, the estimated thickness of ZnIn2S4, TiO2 and hetero-structure ZnIn2S4/TiO2 thin films were 150 nm, 55 nm and 200 nm respectively. We can clearly note that the values of the thickness are not the same for thin films due to the change of surface.
3.3. Optical study
Page 8 of 27
The optical properties of ZnIn2S4, TiO2 and ZnIn2S4/TiO2 heterostructures were characterized by the UV-vis absorption spectra, as shown in Fig.3. The absorption spectrum of TiO2 thin films (Fig 3.a) shows a strong absorption in ultraviolet region with spectral wavelength between 300 and 400 nm exhibiting typical absorption spectra of TiO2[29, 30]. Fig3.b shows the UV-visible absorption spectra of ZnIn2S4 obtained by electrodeposition
ip t
technique onto ITO substrate. There we can observe that the ZnIn2S4 thin films have a broad absorption in the UV to visible region with a tail extending to 800 nm, which matches the
cr
criterion of good absorption in the visible light region for photoelectrode application [31, 32]. For fig3.c, it can be seen that ZnIn2S4/TiO2 heterostructure exhibits a remarkable red-shift
us
compared to the TiO2 and ZnIn2S4 achieve the best efficiency of visible light absorption. Therefore, it is expected to apply to visible-light photocatalysis and solar cells [33].
and it is given by the following equation Eq (1):
an
For semiconductors, the optical absorption coefficient, α, is related with the energy band gap
M
h A( h E g ) n
where A is an energy dependent constant, Eg is the band gap of the material, hν is photon energy and n is an index that characterizes the optical absorption process and it is theoretically
d
equal to 2, 1/2, 3 or 3/2 for indirect allowed, direct allowed, indirect for-bidden and direct for-
te
bidden transitions. The best plot that covers widest range of data in accordance with (Eq. 1) has been obtained for the ( h )2– h dependence. This plot is illustrated in Fig. 4. E-axis
Ac ce p
interception in this figure reflects the direct allowed transitions in our prepared samples. The band gaps for single layer TiO2 and ZnIn2S4 films were estimated to be 3.6 eV and 2.4 eV, respectively. These values are approximate to the values in literatures [24, 26]. We noticed from figure 4 that there are two band gaps for the ZnIn2S4/TiO2 heterojunction film. One is high around 3.4 eV and the other is low at around 2.6. The former seems derived from TiO2 and the latter is approximate of ZnIn2S4. The two estimated gaps could be attributed to the different absorption by ZnIn2S4 and TiO2 since both can absorb UV light but only ZnIn2S4 absorbs visible light [34]. These results considering the possibility of using the ZnIn2S4/TiO2 heterostructure for photocatalytic application in the visible light.
3.4. Photocatalytic activities As explained previously in the experimental section, methylene blue substrate was chosen as a polluent model to evaluate the photocatalytic efficiency of the prepared
Page 9 of 27
photocatalysts. UV-visible absorption spectra recorded on the methylene blue aqueous solution before and after contact with both ZnIn2S4 and heterojunction ZnIn2S4/TiO2 photocatalysts are given in Fig. 5a. Clearly, the heterojunction ZnIn2S4/TiO2 thin films sample presents higher intrinsic activity for the photodegradation of methylene blue than the only ZnIn2S4. This activity is demonstrated by the considerable decrease in the absorbance, which
ip t
is related to the decrease of concentration of methylene blue in the solution. In the presence of only TiO2 thin films, (curve not presented here), the UV-visible absorption spectra remained
cr
constant, which demonstrated that TiO2 has no photocatalytic activity under visible light irradiation [33]. As illustrated in Fig.5b the absorbance at 660 nm for methylene blue in the
us
presence of ZnIn2S4/TiO2 thin films decreased as increasing the irradiation time, accompanied by the intensity decrease, the maximum absorption wavelength of methylene blue monomer [26]. This confirmed the visible photocatalysis of ZnIn2S4/TiO2 thin films. To investigate the
an
effect of the heterojunction on the photocatalytic activity of ZnIn2S4/TiO2 thin films, photodegradation of methylene blue in the presence of ZnIn2S4 was also performed. As shown
M
in Fig.5c, only the ZnIn2S4/TiO2 thin films exhibited strong photocatalytic activity and the degradation of methylene blue eached 91% after irradiation for 4h. ZnIn2S4 thin film shows 70%. photocatalytic activities in the degradation of methylene blue. A possible explanation is
d
as follows. TiO2 has no photocatalytic activity in the visible region of λ >420 nm due to its
te
wide band gap (3.6 eV) [15, 35]. The ZnIn2S4 with a narrower band gap (2.4eV) allows the photodegradation of methylene blue in visible light region [32]. The visible light
Ac ce p
photocatalytic activity of heterojunction ZnIn2S4/TiO2 composite should primarily come from ZnIn2S4. This result is in good agreement with that obtained by W.-H. Yuan et al. [33], which explained that the unique core/shell nanostructure of a composite semiconductor photocatalyst plays a key role resulting in a large increase in the photocatalytic activity for methylene blue degradation. In fact, The intimate contact between TiO2 and ZnIn2S4 favors the formation of junctions between the two components [36], and the higher activity of the ZnIn2S4/TiO2 composite can be attributed to electron transfer between two conduction bands [37, 38] .
In order to better understand the charge transport properties, the possible reason for the increased photocatalytic activity observed in this study can be explained by an approach described in (Fig. 6). By exploiting the different values of the flat band potential esteemed from I-V responses as described in the next section (see Fig. 7), and the band gap obtained from optical measurements, we are able to get the relative position of the conductive band
Page 10 of 27
edge and valence band edge. This approach was carried out by several authors and recently by our research group to describe the band position of CuIn5S8 [39]. In this study, the flat-band potentials of the ZnIn2S4 and TiO2 are in the range of -0.64 to -0.55 V vs. Ag/AgCl electrode, (-0.44 to -0.35 V vs. normal hydrogen electrode (NHE)), respectively. It is generally known that the difference between the position of the conduction
ip t
band (ECB) and that of the flat band potential (EFB) for many n-type semiconductors is 0.1–0.3 V [40, 41]. Thus, the difference between EFB and ECB is assumed to be -0.3 V for the ZnIn2S4
cr
and TiO2. From this figure, the conduction band (CB) edges of ZnIn2S4 and TiO2 are -0.74 eV and -0.65 eV vs normal hydrogen electrode (NHE), respectively. Accordingly, the valence
us
band (VB) edges are 1.66 eV for ZnIn2S4 and 2.95 eV vs NHE for TiO2. Because of its narrower band gap, the ZnIn2S4 cocatalyst is more easily activated by the wide range of wavelengths emitted by the light source employed. Because of these electronic properties, the
an
electrons in ZnIn2S4 are first excited into the conduction band of this material, and the heterojunction structure formed by ZnIn2S4 and TiO2 thin films facilitates the migration of the
M
photoexcited electrons to the conductions band of TiO2. So that additional electrons enter the conduction band of TiO2. At the end, the transferred electron-hole pairs from ZnIn2S4 to TiO2 facilitate redox reactions through the formation of absorbed radicals on the ZnIn2S4-TiO2
d
surface. Recently, this phenomena was obtained by several authors in the literature described
te
the electron transfer in the heterostructure between TiO2 and another semiconductors [16, 4244]. This suggestion will be confirmed in the next part by photoelectrochemical study of
Ac ce p
ZnIn2S4/TiO2 heterostructure .
3.5. Photoelectrochemical properties of ZnIn2S4/TiO2 heterostructures In the recent years, photoelectrochemical (PEC) cells based on semiconductor-
electrolyte junction have been used for quick testing of the quality of solar cell materials [45]. In the present case, linear sweep voltammetry was performed in the dark and under simulated solar light to characterize the ability of the photoelectrochemical cells consisting of ITO/TiO2/ZnIn2S4/Na2SO4/Pt. Through the measurement of I-V curves (Fig. 7), TiO2, ZnIn2S4 and heterostructure ZnIn2S4/TiO2 electrodes led to negligible current under dark conditions. Under illumination, the current densities of the TiO2 and ZnIn2S4 electrodes were evident which increases with the bias potential. The TiO2 electrode exhibited a photocurrent of 0.2 mA/cm2 at 0.5V versus Ag/AgCl. However, ZnIn2S4 electrode exhibited only 0.07 mA/ cm2 at
Page 11 of 27
the same potential. From fig.7b, we can note a small peak at the negative bias (-0.5 to -0.25V) which originated from oxidation of S2- ion by photogenerated holes [46]. This phenomenon was attributed to the photo-corrosion of metal sulfide semiconductors. Many workers had reported this phenomenon [47, 48]. In the presence of both semiconductors TiO2 and ZnIn2S4 (curve 7c), the current density
ip t
shows an important increasing up to 0.8 mA/cm2 at 0.5V versus Ag/AgCl. The increment in efficiency of photogenerated charge carriers of ZnIn2S4 /TiO2.
cr
the current value can be explained through the high absorption and the perfect separation This suggests that ZnIn2S4 modified TiO2 could effectively improve the visible-light response
us
and the separation of electron–hole pairs compared with pure TiO2 or ZnIn2S4 under the same bias potential. The excellent photoelectrochemical properties could be attributed to the function of ZnIn2S4 /TiO2 heterojunction for reducing the recombination of photo-generated
an
electrons and holes. Moreover, it can be seen a small shift in onset potential from -0.5 to 0.1V vs.Ag/AgCl, suggesting a shift in Fermi level as a result of the coupling TiO2 and
M
ZnIn2S4. This result gives a slight improvement in photoelectrochemical performance as compared with other reported data available in literature [33, 49]. This suggests that the on the synthesis process.
d
performance of the ZnIn2S4/TiO2 thin films in terms of photoelectrochemical cell depending
te
Both photoanodes (TiO2 and junction ZnIn2S4/ TiO2) were illuminated intermittently at a given potential (+ 0.5V vs.Ag/AgCl) for several cycles, to appreciate the reproducibility
Ac ce p
of their photoresponses as well as the stability of the device toward oxidation. The corresponding current density response to on–off cycling with total test duration of 300 seconds is presented in Fig.8. From this figure, one can observe that both photoanodes led to an instantaneous change in current upon illumination. The current retracted to the original values almost instantaneously once the illumination is switched off. These results indicated the stability of both thin films. Besides, we can note from fig.8b that the photocurrent density produced by ZnIn2S4/TiO2 thin film electrode reached 0.8 mA.cm-2, about four times higher than that measured on TiO2 thin film. These results prove that the heterojunction have a better photo-electrochemical performance than the pure TiO2 thin films under illumination. In this case, the coupling of ZnIn2S4 and TiO2 thin films could transfer electrons from ZnIn2S4 into TiO2 for their proper conduction of band potentials. This result is in good agreement with that obtained in the photocatalytic activity.
Page 12 of 27
As a conclusion, it can be clearly inferred that the heterojunction fabrication is an effective
ip t
way to improve the photoelectric performance [50, 51].
4. Conclusion
Heterostructure of ZnIn2S4/TiO2 films was deposited onto ITO coated glass substrate
cr
by two step low cost methods. Firstly, TiO2 thin film was prepared by sol-gel and deposited onto ITO coated glass substrate by spin-coating method. Then zinc indium sulfide
us
semiconductor was fabricated via electrodeposition technique.
Fabrication of heterostructure electrodes in this work show several benefits. First, the
an
procedure is very simple and fast. Second, the process is environmentally friendly, and it can be performed using commercially available reagents in water as a green solvent with no need to any additive. Finally, it does not require rigid condition such as high temperature and high
M
pressure apparatus so it can perform at room temperature.
Thin films of TiO2, ZnIn2S4 and ZnIn2S4/TiO2 heterostructure are studied and characterized using X-ray diffraction, scanning electron microscopy and optical analysis. The
d
difference between single and heterojunction structure has been established. The
te
heterostructure materials were applied to the photocatalytic degradation of methylene blue. In comparison with the ZnIn2S4 semiconductor, the heterostructure ZnIn2S4/TiO2 thin films
Ac ce p
results in a significantly increase in the photocatalytic activity. The photoelectrochemical performances of both TiO2 and heterojunction ZnIn2S4/TiO2 have been investigated at room temperature and under 200 W illumination. As a general trend the photoelectrochemical performance of the heterojunction was better than the performance of the TiO2 thin films. These Changes can be correlated with the synthesis methods.
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te
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an
us
cr
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287-291.
Page 18 of 27
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(018)
an (200)
(c)
¤
*
(1010)
Intensity (a.u)
¤
(101)
(009)
cr
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Figure 1
¤
¤
(101)
te
20
30
(200)
(a)
d
*
ITO (01-089-4598)
10
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*
¤
(b)
*
40
50
60
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Bragg angle 2(degree)
Fig.1. XRD patterns of (a) TiO2, (b) ZnIn2S4 and (c) ZnIn2S4/TiO2 junction.
Page 19 of 27
Ac ce p
te
d
M
an
us
cr
ip t
Figure 2
Figure 2: SEM images of (a) TiO2 thin films, (b) ZnIn2S4, and (c) ZnIn2S4/TiO2.
Page 20 of 27
ip t
Figure 3
1.0
(a) (b) (c)
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0.6
0.4
an
Absorbance (a.u)
0.8
0.0 400
500
600
700
800
900 1000 1100 1200 1300 1400 1500
d
300
M
0.2
Ac ce p
te
Wavelength(nm)
Figure 3: UV-Visible absorption spectra of (a) TiO2, (b) ZnIn2S4 thin films and (c) ZnIn2S4/TiO2 heterostructure.
Page 21 of 27
ip t
Figure 4
us
20
cr
(a) (b) (c)
h (eV.cm )
-1 2
30
0 1.0
1.5
2.0
2.5
an
10
3.0
3.5
4.0
4.5
d
M
heV)
Ac ce p
te
Figure 4: plots of (αhν)2 versus hν of (a) single TiO2, (b) ZnIn2S4 films and (c) heterostructure ZnIn2S4/TiO2 films.
Page 22 of 27
Figure 5
0.8
0.8
0h
(B)
(a)
ip t
A
0.6
0.6
(c)
us
0.2
0.2
0.0
500
550
600
650
700
750
400
800
450
500
550
an
450
600
2h
3h 4h
650
700
750
800
Wavelenth (nm)
Wavelenth (nm)
0.8
(C)
M
0.7
ZnIn2S4 ZnIn2S4/TiO2
0.6 0.5
d
0.4 0.3
te
Absorbance (a.u)
0.0 400
1h
0.4
cr
Absorbance (a.u)
0.4
0.2 0.1
Ac ce p
Absorbance (a.u)
(b)
0.0
0
1
2 3 Irradiation time (min)
4
Figure 5: (A) Absorption spectrum of MB solution obtained after 1 h in the contact of (a) without any sample, (b) with ZnIn2S4 thin films and (c) ZnIn2S4/TiO2 heterojunction; (B) Time-dependent absorption spectrum of MB in the presence of ZnIn2S4/TiO2 heterostructure under the visible light irradiation and (C) visible light photocatalytic activity of MB for ZnIn2S4 and ZnIn2S4/TiO2.
Page 23 of 27
Ac ce p
te
d
M
an
us
cr
ip t
Figure 6
Figure 6: Scheme of excitation and charge transfer process between ZnIn2S4 and TiO2 thin films.
Page 24 of 27
Figure 7
ip t
1.4 1.2 -2
I (mA.cm )
(b) ZnIn S dark 2 4 (b) ZnIn S illumination 2 4
1.0 0.8 0.6
(c) Junction dark
0.4
(c) Junction illumination
-1.0
-0.8
-0.6
-0.4
an
0.2
-1.2
cr
(a) TiO illumination 2
us
(a) TiO dark 2
0.0 0.0
-0.2
0.2
0.4
0.6
0.8
1.0
-0.2
E (V / vs.Ag/AgCl)
M
-0.4 -0.6
Ac ce p
te
d
-0.8
Figure 7: Photocurrent density-applied voltage plots between range of -1 V to 0.8 V vs. Ag/AgCl for samples (a) TiO2, (b) ZnIn2S4 and (c) ZnIn2S4/TiO2 heterostructure under illuminated and dark conditions in 0.5 M Na2SO4 aqueous solution.
Upon illumination, the photocurrent appeared at about -0.4, -0.5 V vs. Ag/AgCl
(photocurrent onset) and increased gradually on the positive potentials, which was consistent with the flat band potential very well, and also an n-type character of these films. This result is consistent with that observed by M-S plots in our previous work [31]. In fact, under polarization at different potentials, photocurrent can be measured at all potentials more anodic
Page 25 of 27
than Vfb, so that an accumulation of majority carriers occurs when V < Vfb, and a depletion
Ac ce p
te
d
M
an
us
cr
ip t
layer forms when V > Vfb.
Page 26 of 27
ip t
Figure 8
(b) ZnIn2S4/TiO2
Light on
-2
us
0.8 Current density (mA.cm )
cr
1.0
an
0.6
0.4
M
0.2
0.0
Dark
(a) TiO2 0
50
100
150
200
250
300
te
d
Times (sec)
Ac ce p
Figure 8 : Current density of (a) TiO2 and (b) junction ZnIn2S4/TiO2 as a function of time with the external bias kept at 0.5V vs.Ag/AgCl electrode in 0.5M Na2SO4 aqueous solution.
Page 27 of 27