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Structural, optical and photoelectrochemical properties of phase pure SnS and SnS2 thin films prepared by vacuum evaporation method
Journal of Alloys and Compounds 822 (2020) 153653
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Structural, optical and photoelectrochemical properties of phase pure SnS and SnS2 thin films prepared by vacuum evaporation method Dipika Sharma, Navpreet Kamboj, Khushboo Agarwal, B.R. Mehta* Thin Film Laboratory, Department of Physics, Indian Institute of Technology, New Delhi, 110016, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 September 2019 Received in revised form 2 January 2020 Accepted 2 January 2020 Available online xxx
Two-dimensional (2D) layered semiconducting materials, such as MoS2, SnS2 and SnS, have received remarkable consideration due to their thickness dependent properties and potential applications. In this study, SnS and SnS2 thin films were grown using a simple method of thermal evaporation and their possible application in photoelectrochemical splitting of water reaction have been investigated. X-ray diffraction, Scanning electron microscopy, Raman spectroscopy, and UVeVis spectroscopy techniques results show the presence of SnS and SnS2 pure phases with orthorhombic and hexagonal crystal structure, having band gap values of 1.8 eV and 2.75 eV, respectively. The photoelectrochemical response of as grown SnS and SnS2 thin films was investigated and the results showed the photoanodic behaviour for SnS2 films with photocurrent density of 1 mA cm2 and photocathodic nature for SnS with photocurrent density of 0.4 mA cm2 at 0.95 V vs. Ag/AgCl. The above obtained properties of SnS and SnS2 opens a pathway for using these films for SnSeSnS2 heterojunctions for further improving the photoelectrochemical response. © 2020 Elsevier B.V. All rights reserved.
Keywords: Tin disulfide Tin monosulfide Thermal evaporation Photoelectrochemical response
1. Introduction In the quest of a suitable route for solar hydrogen generation to meet the future energy needs, solar light induced photoelectrochemical splitting of water is a clean, environmental friendly, low cost and renewable promising method [1,2]. For photoelectrochemical water splitting optimization using solar light a number of methods like bulk nanostructured semiconductors through doping, swift heavy ion irradiation and by creating oxideoxide interfaces etc. have been used for the enhancement of photoresponse [3e6]. Two-dimensional (2D) materials have received remarkable attention since the isolation of graphene and possibility of growing 2D materials by low cost methods [7,8]. Recently, two dimensional layered metal sulfides have been emerged as promising photocatalysts for hydrogen evolution reaction on account of their narrow band gap and high carrier mobility [9,10]. People are giving efforts on synthesis of these 2D inorganic metal sulfides such as MoS2 [11], SnS [12e14] and SnS2 [15,16] for an active photocatalyst. However, most of the compounds do not show maximum theoretical efficiencies which may be due to pinning of fermi level
* Corresponding author. E-mail address: [email protected] (B.R. Mehta). https://doi.org/10.1016/j.jallcom.2020.153653 0925-8388/© 2020 Elsevier B.V. All rights reserved.
at semiconductor/electrolyte interface, increased charge carrier recombination in small size grain and interfacial defects [17]. Among the 2D inorganic metal sulfide materials, SnS and SnS2, have received enormous attention due to their structural multiformity, low cost, high electrical conductivity, and unique layered structure [18]. SnS2 is n-type semiconductor having layered cadmium iodide (CdI2) type structure with band gap of 2.2e2.5 eV. Due to its chemical stability in acidic or neutral solutions it is potentially suitable visible light photocatalyst. In contrast, SnS thin film is ptype semiconductor with orthorhombic structure exhibiting direct and indirect band gap energies in range 1.3e1.5eV and 1.0e1.2eV respectively [19,20]. Both SnS2 and SnS with suitable band gap are being explored for possible applications in solar cells and photoelectrochemical water splitting due to non-toxic earth abundant constituent elements, and high chemical and environmental stability [21,22]. Yi Xie et al. used exfoliated SnS and SnS2 mono layers for enhanced photoelectrochemical response, which are may be due to reduced recombination rate of charge carrier between the interlayers due to the exfoliation process [12,16]. There are reports on improved photoresponse of doped SnS and SnS2 with antimony, indium and copper elements [23,24], due to donor/acceptor carrier concentration. As exfoliation method is complex and expensive method for SnS and SnS2 thin film preparation. In the present study, we report the preparation of SnS and SnS2 thin films using simple
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Fig. 1. XRD patterns for SnS2 (at different sulphurization temperatures) and SnS (at different substrate temperatures) thin films deposited on borosilicate glass substrate.
thermal evaporation method followed by sulphurization in rapid thermal processing (RTP) system. As deposited SnS and SnS2 thin films were characterized for their structural, electrical and optical properties using X-Ray Diffraction (XRD), Raman spectroscopy, UVeVisible spectroscopy, and Scanning electron microscope (SEM).
were optimized on to borosilicate glass substrates then deposited on ITO (Sn:In2O3) conducting glass substrate for investigating their photoelectrochemical activity [25,26]. For SnS and SnS2 thin film deposition ~85 and 50 mg amount of SnS and SnO2 powder was used, which results in the ~75 nm and ~50 nm thickness of SnS and SnS2 thin films respectively. SnS and SnS2 thin film thickness was controlled by the thickness controller.
2. Experimental 2.1. SnS and SnS2 thin film preparation
3. Characterization techniques
SnS thin films were deposited on ultrasonically cleaned borosilicate glass substrate using thermal evaporation method. For this highly pure (99.99%) SnS powder was kept in the tungsten boat and substrate at the height of 15 cm under high vacuum condition (5 105 Pa). SnS thin films were deposited (time of deposition: 1e2 min) by thermal evaporation of SnS powder with same deposition rate (100 nm/min) at three different substrate temperatures (250 C, 350 C and 450 C. In addition, synthesis of SnS2 thin films were carried out in two steps: a) SnO2 thin films were deposited by thermally evaporating SnO2 powder at room temperature under high vacuum condition (5 105 Pa). b) Sulphurization of SnO2 thin films in a closed graphite box using RTP furnace at different sulphurization temperatures (350 C, 450 C and 550 C). For sulphurization reaction rate was 5 per minute, and time of reaction (mean hold time or stay at 350 C, 450 C and 550 C) was 10 min, then cooling done at same rate (5 per minute). The gas flow rate of Ar was maintained at 2 sccm. Initially films
X-ray diffraction patterns of SnS and SnS2 thin films were measured in the 2q range of 20 e60 in Goniomode (Configuration ¼ IR Stage (PhiPsi X YZ)) using Philip’s X’Pert PROPW vertical system operating in reflection mode employing CuKa radiation (l ¼ 1.5418 A ). Raman analysis of samples were carried out on Invia Raman microscope with excitation using 514 nm Argon ion laser pulse. SEM images were obtained using field emission scanning electron microscope (FE-SEM), (INCA Penta FET X3, TESCAN) at an operating voltage of 20 kV. To understand the optical behaviour, absorption spectra were measured using UVevisible spectrophotometer (PerkinElmer Lambda 35). X-ray photoelectron spectroscopy (XPS) measurements were performed using the HAXPES (Hard X-ray photoelectron spectroscopy). Al Ka line (1486.6 eV) was used as the monochromatic X-ray excitation source. The analyzer collects excited photoelectrons from the small area on the sample (~5.0 mm2). The obtained spectra were calibrated using amorphous carbon (C 1s peak at 284.6 eV) present in
Fig. 2. Raman spectra for SnS2 (at 450 C sulphurization temperature) and SnS (at 450 C substrate temperature) thin films deposited on borosilicate glass substrate.
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Fig. 3. Shows the absorbance and Tauc plots for SnS2 (at 450 C sulphurization temperature) and SnS (at 450 C substrate temperature) thin films deposited on borosilicate glass substrate.
the sample, and data analysis was done with XPS peak fit software by Shirley background subtraction. The high-resolution core spectra were obtained for the individual XPS features of interest. To remove the surface abnormalities we have performed the depth profiling up to 2 layers using Ar þ ions of energy ~1.5 kV. Crosssectional SEM was used for thickness measurement of samples. For Cross-sectional SEM analysis, we have cut the sample into half to make an edge. After that the sample was placed at 90 on the half edge of the sample holder. Photoelectrochemical response of SnS and SnS2 thin films deposited on ITO substrate was also investigated using photoelectrochemical cell. The photoelectrochemical cell is made up of quartz glass consisting of three electrodes was used for current voltage measurements. Ag/AgCl and platinum wire was used as reference electrode and counter electrode respectively, and SnS and SnS2 thin film samples with 1 cm2 exposure area were used as working electrode, dipped in aqueous 0.5 M Na2SO4. 150 W xenon lamp having output illumination intensity of 100 mW/cm2 was utilized as the UVevisible light source. Current-voltage curves were recorded using scanning potentiostat (Autolab, Netherlands), under dark and illumination in the potential range 1.0 to þ1.0 V vs. Ag/AgCl with a scan rate of 20 mV/s. 4. Results and discussion 4.1. XRD and Raman analysis The crystal structure of as grown SnS and SnS2 thin films was investigated using XRD and Raman measurements as shown in Figs. 1 and 2. For SnS films deposited at 250 C and 350 C substrate
temperature, observed peaks were due to elemental S and Sn along with Sn2S3 phase [27], whereas, at substrate temperature of 450 C, a high intensity peak at 2q ¼ 31.7 corresponding to orthorhombic phase of SnS was observed with other peaks at 2q angles of 27.5⁰, 30.4⁰, and 39.1⁰ correspond to (021), (101), and (131) planes of orthorhombic crystal phase of SnS (JCPDS data 65e3766) [28]. It indicates the exclusively growth of orientated crystallites along (111) direction. Devika et al., observed a similar (111) preferred orientation for SnS thin films using thermal evaporation technique [27]. The XRD patterns of SnS2 thin films prepared at temperature 350 C and 450 C displayed high intensity peak at 2q ¼ 15.2 , and low intensity peak at 29.8 , corresponding to diffraction from (001) and (100) planes and indicate the formation of hexagonal phase of SnS2 (JCPDS data 83e1706). At 550 C, intensity of the observed peaks were very low [29]. No corresponding impurities could be detected in the XRD patterns, indicating the phase purity of SnS and SnS2 thin films. In addition to the XRD, Raman analysis was also performed to confirm phase purity of SnS and SnS2 thin films as shown in Fig. 2. The typical Raman active bands for SnS observed at 97 cm1 (Ag), 159 cm1 (B2g), 187 cm1 (Ag) and 225 cm1 (Ag) revealed the predominant SnS phase at 450 C [28]. On the other hand for SnS2 Raman active modes observed at 204 cm1 and 315 cm1 corresponds to Eg and A1g modes, confirmed pure SnS2 phase at 450 C [29]. 4.2. Absorption spectra The optical absorption spectra of SnS and SnS2 films were taken
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Fig. 4. SEM and Cross sectional SEM images of SnS2 (at 450 C sulphurization temperature) and SnS (at 450 C substrate temperature) thin films.
with respect to a reference substrate. Both SnS and SnS2 thin film exhibited a predominant absorbance edge in the visible region as shown in Fig. 3, which is in good agreement with earlier reported values [29,30]. Optical absorbance data were also examined for band gap value calculation using Tauc plots which shows the relationship between absorption coefficient (a) and incident photon energy (hv) of a crystalline semiconductor as equation given below [31]:
n 2 hv Eg hv =
a¼
Where n is an integer and depends on whether the band gap is direct (n ¼ 1) or indirect (n ¼ 4), a, h, n, Eg and A are the absorption coefficients, Planck constant, light frequency, band gap and a constant, respectively. Plotting of (ahv) n vs hv where n ¼ ½ and n ¼ 2 corresponds to indirect band gap and direct band gap, respectively. The optical band gap of SnS and SnS2 thin films were estimated from the intercept of the extrapolated linear fit for the plotted experimental data of (ahn)2 versus incident photon energy (hn) near the absorption edge. Fig. 3 represents Tauc plots for SnS and SnS2 thin films, at 450 C which indicate direct band gap energy of 1.8 eV [32], for SnS and 2.75eV for SnS2 respectively with an error of ±0.05 eV. 4.3. Morphological studies Scanning electron microscope (SEM) and cross sectional SEM analysis was used to study the morphology and thickness of SnS and SnS2 thin films deposited on ITO glass substrate at 450 C as
shown in Fig. 4. The morphology of both the thin film shows vertically aligned 2D flakes-like growth, having uniform surface and good adhesion to the substrate, similar to films obtained by spray pyrolysis by TH Sajeesh et al., [[33]]. SnS2 thin films exhibited thin and dense growth of flakes as compared SnS films. SnS2 and SnS flakes size is around 250 nm. Cross-sectional SEM analysis of the samples shows the thickness of ~75 and ~50 nm for SnS and SnS2 films respectively. Chemical composition of SnS (sample a) and SnS2 (sample b) in found to be Sn:S ¼ 0.97 and 0.48 which is quite close to stoichiometry values. Elemental distribution analysis was done by EDS spectrum. 4.4. XPS analysis High-resolution XPS spectra of as deposited SnS and SnS2 thin films were recorded and shown in Fig. 5. XPS results show the dominant presence of Sn and S elements, with no other elemental impurities. Sn 3d5/2 and Sn 3d3/2 peaks are observed at 486.2 and 494.6 eV for SnS2 sample and at 485.3 and 494.2 eV for SnS sample, which are in good agreement with reported values for Sn4þ and Sn2þ, respectively [34]. XPS peaks corresponding to S 2p3/2 and S2p1/2 are observed at 161.1 and 162.3 eV for SnS2 and at 161.5 and 162.8 eV for SnS and agree very well with the values for S2respectively. The position of Sn 3d5/2 peak was observed, at 0.9 eV higher binding energy for SnS2 than for SnS, which confirm the single oxidation states of Sn and unadulterated phase for both the SnS and SnS2 as compared to the earlier studies which show the presence of three phases of Sn 3d5/2 peak due to SneO, Sn2þ, Sn4þ [35,36]. Irrespective to this, the oxidation states of tin in different phases of tin sulfide, can be easily determined using energy-
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Fig. 5. High-resolution XPS spectrum of (a) Sn 3d region and (b) S 2p region for SnS (at 450 C substrate temperature (top)) and SnS2 (at 450 C sulphurization temperature (bottom)) deposited on borosilicate glass substrate.
referenced high resolution XPS on clean samples [37,38].
4.5. Photoelectrochemical response The SnS and SnS2 thin films deposited on ITO substrate with variation in thickness were used as working electrode or photoelectrode in photoelectrochemical cell and current-voltage characteristics were recorded under dark and illumination. The externally applied bias was varied from - 1.0 V/SCE (cathodic bias) to þ1.0 V/SCE (anodic bias). The photocurrent for both SnS and SnS2
was calculated by subtracting dark current from current under illumination. Fig. 6 shows the photocurrent density values of SnS and SnS2 (two different film thicknesses) with applied potential vs. Ag/AgCl in 0.5 M Na2SO4 electrolytic solution (pH 7.0). The photoelectrochemical measurements indicate that SnS2 thin film showed an oxidative photocurrent, which was principally ascribed to the photoelectrochemical water oxidation and confirmed the n-type behaviour of SnS2. While SnS thin film displayed cathodic photocurrent attributed to water reduction and exhibited the p-type conductivity [39,40]. For SnS (75 nm) and SnS2 (50 nm) maximum
Fig. 6. Photocurrent density under UVevisible light illumination in 0.5 M Na2SO4 vs. Applied bias for SnS2 (at 450 C sulphurization temperature) and SnS (at 450 C substrate temperature) thin films deposited on ITO substrate.
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Fig. 7. (aeb) Schematic illustration of energy band bending diagram for SnS/electrolyte and SnS2/electrolyte interfaces for photoelectrochemical splitting of water reaction.
photocurrent density of 0.4 mA cm2 and 1.0 mA cm2 at 0.95 V vs. Ag/AgCl was obatined respectively, which may be attributed to their favorable band edges position w.r.t. redox potential of water, absorption in visible region and surface area for the reaction. The obtained values for photocurrent density for both SnS and SnS2 films is much higher than reported values by spin coating mixedphase synthesis via a hot injection technique on Au/Cr substrate [41]. On the basis of valence band position and band gap value, an energy band bending diagram was constructed for SnS and SnS2 at to explain the probable mechanism for photogenerated charge carriers transfer, as illustrated in Fig. 7. The band edge positions of SnS and SnS2 were calculated using the equation EVB ¼ X -Eeþ0.5Eg and ECB ¼ EVBeEg Refs. [42,43], where Eg is the experimental band gap of material, Ee is free electron energy on the hydrogen scale, and X is the absolute electronegativity (geometric mean of electronegativity of constituents atoms). As shown in Fig. 7, the Fermi level at the surface is close to the conduction band for n-type SnS2 and close to the valence band for p-type SnS. Due to the energy difference between surface and bulk, electrons will move from the bulk to the surface, resulting in an upward band bending (depletion layer formation) for SnS2 and, electrons flow from the surface to the bulk lead to downward bending (accumulation layer formation) for SnS [44,45]. Due to upward bending for SnS2, electrons move from bulk to surface and then towards Pt electrode through external
circuit where combine with Hþ ion and form Hydrogen on the other hand holes remain on the SnS2 surface oxidizes the OH ion into O2. In case of p-type SnS downward bend bending leads to electron accumulation at the SnS surface, where electrons combine with Hþ ion and form Hydrogen and holes moves towards Pt cathode and oxidizes the OH ion into O2. A similar energy band diagram for the Bi2O3/BiVO4 heterojunction has also been reported earlier [46]. The above photoelectrochemical results were also supported by the Mott-Schottky analysis. 4.5.1. Photostability Photostability investigation of SnS and SnS2 photoelectrode samples was performed under applied potential of 0.65 V/Ag/AgCl for 1200 s illumination under UVevisible light source in 0.5 M Na2SO4 aqueous electrolyte. No decrement in photocurrent was observed for SnS and SnS2 samples. Observations made under chopped illumination clearly show role of light in photocurrent generation (Fig. 8) and confirm the stability of the thin films [47]. 4.6. Mott Schottly and EIS analysis One of the important property determining photoelectrochemical performance of a semiconductor is flat band potential. It can be calculated using the Mott-Schottky equation (given below), which relates semiconductor-electrolyte interfacial
Fig. 8. Photocurrent density under illumination and on-off illumination cycles (inset) for SnS2 (at 450 C sulphurization temperature) and SnS (at 450 C substrate temperature) thin films deposited on ITO substrate.
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Fig. 9. (a) Mott- Schottky plots (b) Nyquist plots under illumination of SnS2 (at 450 C sulphurization temperature) and SnS (at 450 C substrate temperature) thin films deposited on ITO substrate.
capacitance (C) to applied voltage (Vapp).
1
2 ¼ 2 2 qε ε C 0 s A ND
! Vapp Vfb
kB T q
where q is the electronic charge (1.6021019 C), εs is the relative permittivity of semiconductor, ε0 is the vacuum permittivity (8.8541012 F m1), Vapp (V vs. SCE) is the applied potential, C is the space-charge capacitance, A (cm2) is the active geometric area, ND (cm3) is the donor density, Vfb (V vs. SCE) is the flat-band potential, kB is the Boltzmann constant (1.381 1023 J K1) and T is the temperature (298 K). Capacitance (C) at the semiconductor/ electrolyte junction was measured using an LCR meter (Agilent technology, Model 4263B) at varying electrode potentials of the AC signal frequency of 1000 Hz [47]. The obatined flat band potential values of 0.47 and 0.32 V vs. Ag/AgCl for SnS and SnS2 respectively were calculated from the slopes of Mott-Schottky plots as shown in Fig. 9 (a). Negative and positive slopes with obtained values of Vfb are consistant with p-type conductivity for SnS and n-type conductivity for SnS2 respectively. Charge transfer process occurring at the semiconductor/electrolyte interface was also investigated using electrochemical impedance spectroscopy (EIS) (from Nyquist plots) measurements under illumination. Fig. 9(b) shows Nyquist plots for SnS and SnS2 thin films deposited on ITO glass substrate. It can be seen from the Nyquist plots that semicircle arc is smaller for SnS2 sample as compared to SnS confirming the effective enhancement of charge carriers transfer at semiconductor/electrolyte interface and also validate the high value of photocurrent density [48].
5. Conclusions In summary, phase pure SnS and SnS2 thin films were prepared using simple method of thermal evaporation in order to study their photoelectrochemical performance. SnS films were prepared by direct thermal evaporation of SnS powder with substrate kept at 450 C. SnS2 films were obtained by deposition of SnO2 film and then sulphurization process at 450 C. XRD and Raman spectroscopy confirm the crystal structure and phase of the SnS and SnS2 films. XPS results confirm the presence of single phase SnS and SnS2. Optical spectra show the band gap values of 2.75 1st 1.8 eV for SnS2 and SnS films respectively. Photoelectrochemical results show that SnS2 and SnS thin films displayed n-type and p-type conductivity with photocurrent density of 1mA/cm2 and 0.4 mA cm2 at 0.95 V vs. Ag/AgCl respectively. Two major factors attributing to the high photocurrent density may be the appropriate band edge alignment w. r.t redox potential of water and suitable band gap for visible light absorption. Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements "Author (Dipika Sharma) gratefully acknowledge financial support received from the Department of Science & Technology, New
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Delhi, India through National Post Doc fellowship (reg:PDF/2016/ 001988/CS) and IIT Delhi for Institute Post Doc fellowship. Authors are also thankful to IUAC for extending FESEM facility funded by Ministry of Earth Sciences (MoES) under Geochronology" project [MoES/P.O (Seismic) 8(09)-Geochron/2012]. References [1] A. Fujishima, A.T. RaO, D. Tryk, Titanium dioxide photocatalysis, J. Photochem. Photobiol. C Photochem. Rev. 1 (2000) 1e21. [2] T. Bak, J. Nowotny, M. Rekas, C.C. Sorrell, Photo-electrochemical hydrogen generation from water using solar energy: materials-related aspects, Int. J. Hydrogen Energy 27 (2002) 91e102. [3] D. Sharma, V.R. Satsangi, R. Shrivastav, U.V. Waghmare, S. Dass, Nanostructured BaTiO3/Cu2O heterojunction with improved photoelectrochemical activity for H2 evolution: experimental and first-principles analysis, Appl. Catal. B Environ. 189 (2016) 75e85. [4] K. H Reddy, K.M. 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Minemoto, Impact of growth temperature on the properties of SnS film prepared by thermal evaporation and its photovoltaic performance, Curr. Appl. Phys. 15 (2015) 897e901. [29] E.W. Huang, X.L. Yao, W.C. Wang, G.J. Wu, N.J. Guan, L. D Li, SnS2 nanoplates with specific facets exposed for enhanced visible-light-driven photocatalysis, ChemPhotoChem 1 (2016) 60e69. [30] V. Reddy, M. Reddy, S. Gedi, C. Park, K.T. Reddy, Development of sulphurized SnS thin film solar cells, Curr. Appl. Phys. 15 (2015) 588e598. [31] D. Sharma, A. Verma, V.R. Satsangi, R. Shrivastav, S. Dass, Nanostructured SrTiO3 thin films sensitized by Cu2O for photoelectrochemical hydrogen generation, Int. J. Hydrogen Energy 39 (2014) 4289e4297. [32] M. Rajalakshmi Sohila, C. Muthamizhchelvan, S. Kalavathi Chanchal Ghosh, R. Divakar, C.N. Venkiteswaran, N.G. Muralidharan, A.K. Arora, E. Mohandas, Synthesis and characterization of SnS nanosheets through simple chemical route, Mater. Lett. 65 (2011) 1148e1150. [33] T. H Sajeesh, A. R Warrier, C. S Kartha, K. P Vijayakumar, Optimization of parameters of chemical spray pyrolysis technique to get n and p-type layers of SnS, Thin Solid Films 518 (2010) 4370e4374. [34] C.C. Huang, Y.J. Lin, C.Y. Chuang, C.J. Liu, Y.W. Yang, Conduction-type control of SnSx films prepared by the Solgel method for different sulfur contents, J. Alloy. Comp. 553 (2013) 208e211. [35] J. Thomas, J. Whittles, Lee A. Burton, M. Jonathan, A. Walsh, D. Tim, R. Dhanak, Band Alignments, valence bands, and core levels in the tin sulfides SnS, SnS2, and Sn2S3: experiment and theory, Chem. Mater. 28 (2016) 3718e3726. [36] M.V. Reddy, P. Babu, K.T. Ramakrishna Reddy, R.W. Miles, X-ray photoelectron spectroscopy and X-ray diffraction studies on tin sulfide films grown by sulfurization process, J. Renew. Sustain. Energy 5 (2013), 031613. [37] L. Kover, G. Moretti, Z. Kovacs, R. Sanjines, I. Cserny, G. Margaritondo, J. Palinkas, H. Adachi, High resolution photoemission and auger parameter studies of electronic structure of tin oxides, J. Vac. Sci. Technol. 13 (1995) 1382e1388. [38] B.J. Lindberg, K. Hamrin, G. Johansson, U. Gelius, A. Fahlman, C. Nordling, K. Siegbahn, Molecular spectroscopy by means of ESCA II. Sulfur compounds. Correlation of electron binding energy with structure, Phys. Scr. 1 (1970) 286e298. [39] D. Sharma, B.R. Mehta, Nanostructured TiO2 thin films sensitized by CeO2 as an inexpensive photoanode for enhanced photoactivity of water oxidation, J. Alloy. Comp. 749 (2018) 329e335. [40] D. Sharma, S. Upadhyay A, A. Verma, V.R. Satsangi, R. Srivastav, U.V. Waghmare, S. Dass, Nanostructured Ti-Fe2O3/Cu2O heterojunction photoelectrode for efficient hydrogen production, Thin Solid Films 574 (2015) 125e131. [41] X. Lin, J. Xing, W. Wang, Z. Shan, F. Xu, F. Huang, Photocatalytic activities of heterojunction semiconductors Bi2O3/BaTiO3: a strategy for the design of efficient combined photocatalysts, J. Phys. Chem. C 111 (2007) 18288e18293. [42] W. Wang, J. Wang, Z. Wang, X. Wei, L. Liu, Q. Ren, W. Gao, Y. Lianga, H. Shi, pen junction CuO/BiVO4 heterogeneous nanostructures: synthesis and highly efficient visible-light photocatalytic performance, Dalton Trans. 63 (2014) 6735e6744. [43] Y. Wu, T. Wei, X. An, L.M. Liu, Colloidal synthesis of SnS nanocrystals with dimension dependent photoelectrochemical properties, New J. Chem. 43 (2019) 7457. [44] P. Huang, Y.M. Shen, S. Brahma, M.O. Shaikh, J.L. Huang, S.C. Wang, SnSx (x ¼ 1, 2) nanocrystals as effective catalysts for photoelectrochemical water splitting, Catalysts 7 (2017) 252e261. [45] L. Chen, Q. Zhang, R. Huang, S.F. Yin, S.L. Luo, C.T. Au, Porous peanut-like Bi2O3/ BiVO4 composites with heterojunctions: one-step synthesis and their photocatalytic properties, Dalton Trans. 41 (2012) 9513e9518. [46] W.B. Ingler Jr., S.U.M. Khan, Photoresponse of spray pyrolytically synthesized magnesium- doped iron (III) oxide (Fe2O3) thin films under solar simulated light illumination, Thin Solid Films 461 (2004) 301e308. [47] D. Sharma, R.K. Upadhyay, B. Satpati, V.R. Satsangi, R. Srivastav, U.V. Waghmare, S. Dass, Electronic band-offsets across Cu2O/BaZrO3 heterojunction and its stable photo-electro-chemical response: first-principles theoretical analysis and experimental optimization, Renew. Energy 113 (2017) 503e511. [48] R. Upadhyay, D. Sharma, Fe doped BaTiO3 sensitized by Fe3O4 nanoparticles for improved photoelectrochemical response, Mater. Res. Express 5 (2018), 015913.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Structural, optical and photoelectrochemical properties of phase pure SnS and SnS2 thin films prepared by vacuum evaporation method Dipika Sharma, Navpreet Kamboj, Khushboo Agarwal, B.R. Mehta* Thin Film Laboratory, Department of Physics, Indian Institute of Technology, New Delhi, 110016, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 September 2019 Received in revised form 2 January 2020 Accepted 2 January 2020 Available online xxx
Two-dimensional (2D) layered semiconducting materials, such as MoS2, SnS2 and SnS, have received remarkable consideration due to their thickness dependent properties and potential applications. In this study, SnS and SnS2 thin films were grown using a simple method of thermal evaporation and their possible application in photoelectrochemical splitting of water reaction have been investigated. X-ray diffraction, Scanning electron microscopy, Raman spectroscopy, and UVeVis spectroscopy techniques results show the presence of SnS and SnS2 pure phases with orthorhombic and hexagonal crystal structure, having band gap values of 1.8 eV and 2.75 eV, respectively. The photoelectrochemical response of as grown SnS and SnS2 thin films was investigated and the results showed the photoanodic behaviour for SnS2 films with photocurrent density of 1 mA cm2 and photocathodic nature for SnS with photocurrent density of 0.4 mA cm2 at 0.95 V vs. Ag/AgCl. The above obtained properties of SnS and SnS2 opens a pathway for using these films for SnSeSnS2 heterojunctions for further improving the photoelectrochemical response. © 2020 Elsevier B.V. All rights reserved.
Keywords: Tin disulfide Tin monosulfide Thermal evaporation Photoelectrochemical response
1. Introduction In the quest of a suitable route for solar hydrogen generation to meet the future energy needs, solar light induced photoelectrochemical splitting of water is a clean, environmental friendly, low cost and renewable promising method [1,2]. For photoelectrochemical water splitting optimization using solar light a number of methods like bulk nanostructured semiconductors through doping, swift heavy ion irradiation and by creating oxideoxide interfaces etc. have been used for the enhancement of photoresponse [3e6]. Two-dimensional (2D) materials have received remarkable attention since the isolation of graphene and possibility of growing 2D materials by low cost methods [7,8]. Recently, two dimensional layered metal sulfides have been emerged as promising photocatalysts for hydrogen evolution reaction on account of their narrow band gap and high carrier mobility [9,10]. People are giving efforts on synthesis of these 2D inorganic metal sulfides such as MoS2 [11], SnS [12e14] and SnS2 [15,16] for an active photocatalyst. However, most of the compounds do not show maximum theoretical efficiencies which may be due to pinning of fermi level
* Corresponding author. E-mail address: [email protected] (B.R. Mehta). https://doi.org/10.1016/j.jallcom.2020.153653 0925-8388/© 2020 Elsevier B.V. All rights reserved.
at semiconductor/electrolyte interface, increased charge carrier recombination in small size grain and interfacial defects [17]. Among the 2D inorganic metal sulfide materials, SnS and SnS2, have received enormous attention due to their structural multiformity, low cost, high electrical conductivity, and unique layered structure [18]. SnS2 is n-type semiconductor having layered cadmium iodide (CdI2) type structure with band gap of 2.2e2.5 eV. Due to its chemical stability in acidic or neutral solutions it is potentially suitable visible light photocatalyst. In contrast, SnS thin film is ptype semiconductor with orthorhombic structure exhibiting direct and indirect band gap energies in range 1.3e1.5eV and 1.0e1.2eV respectively [19,20]. Both SnS2 and SnS with suitable band gap are being explored for possible applications in solar cells and photoelectrochemical water splitting due to non-toxic earth abundant constituent elements, and high chemical and environmental stability [21,22]. Yi Xie et al. used exfoliated SnS and SnS2 mono layers for enhanced photoelectrochemical response, which are may be due to reduced recombination rate of charge carrier between the interlayers due to the exfoliation process [12,16]. There are reports on improved photoresponse of doped SnS and SnS2 with antimony, indium and copper elements [23,24], due to donor/acceptor carrier concentration. As exfoliation method is complex and expensive method for SnS and SnS2 thin film preparation. In the present study, we report the preparation of SnS and SnS2 thin films using simple
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Fig. 1. XRD patterns for SnS2 (at different sulphurization temperatures) and SnS (at different substrate temperatures) thin films deposited on borosilicate glass substrate.
thermal evaporation method followed by sulphurization in rapid thermal processing (RTP) system. As deposited SnS and SnS2 thin films were characterized for their structural, electrical and optical properties using X-Ray Diffraction (XRD), Raman spectroscopy, UVeVisible spectroscopy, and Scanning electron microscope (SEM).
were optimized on to borosilicate glass substrates then deposited on ITO (Sn:In2O3) conducting glass substrate for investigating their photoelectrochemical activity [25,26]. For SnS and SnS2 thin film deposition ~85 and 50 mg amount of SnS and SnO2 powder was used, which results in the ~75 nm and ~50 nm thickness of SnS and SnS2 thin films respectively. SnS and SnS2 thin film thickness was controlled by the thickness controller.
2. Experimental 2.1. SnS and SnS2 thin film preparation
3. Characterization techniques
SnS thin films were deposited on ultrasonically cleaned borosilicate glass substrate using thermal evaporation method. For this highly pure (99.99%) SnS powder was kept in the tungsten boat and substrate at the height of 15 cm under high vacuum condition (5 105 Pa). SnS thin films were deposited (time of deposition: 1e2 min) by thermal evaporation of SnS powder with same deposition rate (100 nm/min) at three different substrate temperatures (250 C, 350 C and 450 C. In addition, synthesis of SnS2 thin films were carried out in two steps: a) SnO2 thin films were deposited by thermally evaporating SnO2 powder at room temperature under high vacuum condition (5 105 Pa). b) Sulphurization of SnO2 thin films in a closed graphite box using RTP furnace at different sulphurization temperatures (350 C, 450 C and 550 C). For sulphurization reaction rate was 5 per minute, and time of reaction (mean hold time or stay at 350 C, 450 C and 550 C) was 10 min, then cooling done at same rate (5 per minute). The gas flow rate of Ar was maintained at 2 sccm. Initially films
X-ray diffraction patterns of SnS and SnS2 thin films were measured in the 2q range of 20 e60 in Goniomode (Configuration ¼ IR Stage (PhiPsi X YZ)) using Philip’s X’Pert PROPW vertical system operating in reflection mode employing CuKa radiation (l ¼ 1.5418 A ). Raman analysis of samples were carried out on Invia Raman microscope with excitation using 514 nm Argon ion laser pulse. SEM images were obtained using field emission scanning electron microscope (FE-SEM), (INCA Penta FET X3, TESCAN) at an operating voltage of 20 kV. To understand the optical behaviour, absorption spectra were measured using UVevisible spectrophotometer (PerkinElmer Lambda 35). X-ray photoelectron spectroscopy (XPS) measurements were performed using the HAXPES (Hard X-ray photoelectron spectroscopy). Al Ka line (1486.6 eV) was used as the monochromatic X-ray excitation source. The analyzer collects excited photoelectrons from the small area on the sample (~5.0 mm2). The obtained spectra were calibrated using amorphous carbon (C 1s peak at 284.6 eV) present in
Fig. 2. Raman spectra for SnS2 (at 450 C sulphurization temperature) and SnS (at 450 C substrate temperature) thin films deposited on borosilicate glass substrate.
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Fig. 3. Shows the absorbance and Tauc plots for SnS2 (at 450 C sulphurization temperature) and SnS (at 450 C substrate temperature) thin films deposited on borosilicate glass substrate.
the sample, and data analysis was done with XPS peak fit software by Shirley background subtraction. The high-resolution core spectra were obtained for the individual XPS features of interest. To remove the surface abnormalities we have performed the depth profiling up to 2 layers using Ar þ ions of energy ~1.5 kV. Crosssectional SEM was used for thickness measurement of samples. For Cross-sectional SEM analysis, we have cut the sample into half to make an edge. After that the sample was placed at 90 on the half edge of the sample holder. Photoelectrochemical response of SnS and SnS2 thin films deposited on ITO substrate was also investigated using photoelectrochemical cell. The photoelectrochemical cell is made up of quartz glass consisting of three electrodes was used for current voltage measurements. Ag/AgCl and platinum wire was used as reference electrode and counter electrode respectively, and SnS and SnS2 thin film samples with 1 cm2 exposure area were used as working electrode, dipped in aqueous 0.5 M Na2SO4. 150 W xenon lamp having output illumination intensity of 100 mW/cm2 was utilized as the UVevisible light source. Current-voltage curves were recorded using scanning potentiostat (Autolab, Netherlands), under dark and illumination in the potential range 1.0 to þ1.0 V vs. Ag/AgCl with a scan rate of 20 mV/s. 4. Results and discussion 4.1. XRD and Raman analysis The crystal structure of as grown SnS and SnS2 thin films was investigated using XRD and Raman measurements as shown in Figs. 1 and 2. For SnS films deposited at 250 C and 350 C substrate
temperature, observed peaks were due to elemental S and Sn along with Sn2S3 phase [27], whereas, at substrate temperature of 450 C, a high intensity peak at 2q ¼ 31.7 corresponding to orthorhombic phase of SnS was observed with other peaks at 2q angles of 27.5⁰, 30.4⁰, and 39.1⁰ correspond to (021), (101), and (131) planes of orthorhombic crystal phase of SnS (JCPDS data 65e3766) [28]. It indicates the exclusively growth of orientated crystallites along (111) direction. Devika et al., observed a similar (111) preferred orientation for SnS thin films using thermal evaporation technique [27]. The XRD patterns of SnS2 thin films prepared at temperature 350 C and 450 C displayed high intensity peak at 2q ¼ 15.2 , and low intensity peak at 29.8 , corresponding to diffraction from (001) and (100) planes and indicate the formation of hexagonal phase of SnS2 (JCPDS data 83e1706). At 550 C, intensity of the observed peaks were very low [29]. No corresponding impurities could be detected in the XRD patterns, indicating the phase purity of SnS and SnS2 thin films. In addition to the XRD, Raman analysis was also performed to confirm phase purity of SnS and SnS2 thin films as shown in Fig. 2. The typical Raman active bands for SnS observed at 97 cm1 (Ag), 159 cm1 (B2g), 187 cm1 (Ag) and 225 cm1 (Ag) revealed the predominant SnS phase at 450 C [28]. On the other hand for SnS2 Raman active modes observed at 204 cm1 and 315 cm1 corresponds to Eg and A1g modes, confirmed pure SnS2 phase at 450 C [29]. 4.2. Absorption spectra The optical absorption spectra of SnS and SnS2 films were taken
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Fig. 4. SEM and Cross sectional SEM images of SnS2 (at 450 C sulphurization temperature) and SnS (at 450 C substrate temperature) thin films.
with respect to a reference substrate. Both SnS and SnS2 thin film exhibited a predominant absorbance edge in the visible region as shown in Fig. 3, which is in good agreement with earlier reported values [29,30]. Optical absorbance data were also examined for band gap value calculation using Tauc plots which shows the relationship between absorption coefficient (a) and incident photon energy (hv) of a crystalline semiconductor as equation given below [31]:
n 2 hv Eg hv =
a¼
Where n is an integer and depends on whether the band gap is direct (n ¼ 1) or indirect (n ¼ 4), a, h, n, Eg and A are the absorption coefficients, Planck constant, light frequency, band gap and a constant, respectively. Plotting of (ahv) n vs hv where n ¼ ½ and n ¼ 2 corresponds to indirect band gap and direct band gap, respectively. The optical band gap of SnS and SnS2 thin films were estimated from the intercept of the extrapolated linear fit for the plotted experimental data of (ahn)2 versus incident photon energy (hn) near the absorption edge. Fig. 3 represents Tauc plots for SnS and SnS2 thin films, at 450 C which indicate direct band gap energy of 1.8 eV [32], for SnS and 2.75eV for SnS2 respectively with an error of ±0.05 eV. 4.3. Morphological studies Scanning electron microscope (SEM) and cross sectional SEM analysis was used to study the morphology and thickness of SnS and SnS2 thin films deposited on ITO glass substrate at 450 C as
shown in Fig. 4. The morphology of both the thin film shows vertically aligned 2D flakes-like growth, having uniform surface and good adhesion to the substrate, similar to films obtained by spray pyrolysis by TH Sajeesh et al., [[33]]. SnS2 thin films exhibited thin and dense growth of flakes as compared SnS films. SnS2 and SnS flakes size is around 250 nm. Cross-sectional SEM analysis of the samples shows the thickness of ~75 and ~50 nm for SnS and SnS2 films respectively. Chemical composition of SnS (sample a) and SnS2 (sample b) in found to be Sn:S ¼ 0.97 and 0.48 which is quite close to stoichiometry values. Elemental distribution analysis was done by EDS spectrum. 4.4. XPS analysis High-resolution XPS spectra of as deposited SnS and SnS2 thin films were recorded and shown in Fig. 5. XPS results show the dominant presence of Sn and S elements, with no other elemental impurities. Sn 3d5/2 and Sn 3d3/2 peaks are observed at 486.2 and 494.6 eV for SnS2 sample and at 485.3 and 494.2 eV for SnS sample, which are in good agreement with reported values for Sn4þ and Sn2þ, respectively [34]. XPS peaks corresponding to S 2p3/2 and S2p1/2 are observed at 161.1 and 162.3 eV for SnS2 and at 161.5 and 162.8 eV for SnS and agree very well with the values for S2respectively. The position of Sn 3d5/2 peak was observed, at 0.9 eV higher binding energy for SnS2 than for SnS, which confirm the single oxidation states of Sn and unadulterated phase for both the SnS and SnS2 as compared to the earlier studies which show the presence of three phases of Sn 3d5/2 peak due to SneO, Sn2þ, Sn4þ [35,36]. Irrespective to this, the oxidation states of tin in different phases of tin sulfide, can be easily determined using energy-
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Fig. 5. High-resolution XPS spectrum of (a) Sn 3d region and (b) S 2p region for SnS (at 450 C substrate temperature (top)) and SnS2 (at 450 C sulphurization temperature (bottom)) deposited on borosilicate glass substrate.
referenced high resolution XPS on clean samples [37,38].
4.5. Photoelectrochemical response The SnS and SnS2 thin films deposited on ITO substrate with variation in thickness were used as working electrode or photoelectrode in photoelectrochemical cell and current-voltage characteristics were recorded under dark and illumination. The externally applied bias was varied from - 1.0 V/SCE (cathodic bias) to þ1.0 V/SCE (anodic bias). The photocurrent for both SnS and SnS2
was calculated by subtracting dark current from current under illumination. Fig. 6 shows the photocurrent density values of SnS and SnS2 (two different film thicknesses) with applied potential vs. Ag/AgCl in 0.5 M Na2SO4 electrolytic solution (pH 7.0). The photoelectrochemical measurements indicate that SnS2 thin film showed an oxidative photocurrent, which was principally ascribed to the photoelectrochemical water oxidation and confirmed the n-type behaviour of SnS2. While SnS thin film displayed cathodic photocurrent attributed to water reduction and exhibited the p-type conductivity [39,40]. For SnS (75 nm) and SnS2 (50 nm) maximum
Fig. 6. Photocurrent density under UVevisible light illumination in 0.5 M Na2SO4 vs. Applied bias for SnS2 (at 450 C sulphurization temperature) and SnS (at 450 C substrate temperature) thin films deposited on ITO substrate.
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Fig. 7. (aeb) Schematic illustration of energy band bending diagram for SnS/electrolyte and SnS2/electrolyte interfaces for photoelectrochemical splitting of water reaction.
photocurrent density of 0.4 mA cm2 and 1.0 mA cm2 at 0.95 V vs. Ag/AgCl was obatined respectively, which may be attributed to their favorable band edges position w.r.t. redox potential of water, absorption in visible region and surface area for the reaction. The obtained values for photocurrent density for both SnS and SnS2 films is much higher than reported values by spin coating mixedphase synthesis via a hot injection technique on Au/Cr substrate [41]. On the basis of valence band position and band gap value, an energy band bending diagram was constructed for SnS and SnS2 at to explain the probable mechanism for photogenerated charge carriers transfer, as illustrated in Fig. 7. The band edge positions of SnS and SnS2 were calculated using the equation EVB ¼ X -Eeþ0.5Eg and ECB ¼ EVBeEg Refs. [42,43], where Eg is the experimental band gap of material, Ee is free electron energy on the hydrogen scale, and X is the absolute electronegativity (geometric mean of electronegativity of constituents atoms). As shown in Fig. 7, the Fermi level at the surface is close to the conduction band for n-type SnS2 and close to the valence band for p-type SnS. Due to the energy difference between surface and bulk, electrons will move from the bulk to the surface, resulting in an upward band bending (depletion layer formation) for SnS2 and, electrons flow from the surface to the bulk lead to downward bending (accumulation layer formation) for SnS [44,45]. Due to upward bending for SnS2, electrons move from bulk to surface and then towards Pt electrode through external
circuit where combine with Hþ ion and form Hydrogen on the other hand holes remain on the SnS2 surface oxidizes the OH ion into O2. In case of p-type SnS downward bend bending leads to electron accumulation at the SnS surface, where electrons combine with Hþ ion and form Hydrogen and holes moves towards Pt cathode and oxidizes the OH ion into O2. A similar energy band diagram for the Bi2O3/BiVO4 heterojunction has also been reported earlier [46]. The above photoelectrochemical results were also supported by the Mott-Schottky analysis. 4.5.1. Photostability Photostability investigation of SnS and SnS2 photoelectrode samples was performed under applied potential of 0.65 V/Ag/AgCl for 1200 s illumination under UVevisible light source in 0.5 M Na2SO4 aqueous electrolyte. No decrement in photocurrent was observed for SnS and SnS2 samples. Observations made under chopped illumination clearly show role of light in photocurrent generation (Fig. 8) and confirm the stability of the thin films [47]. 4.6. Mott Schottly and EIS analysis One of the important property determining photoelectrochemical performance of a semiconductor is flat band potential. It can be calculated using the Mott-Schottky equation (given below), which relates semiconductor-electrolyte interfacial
Fig. 8. Photocurrent density under illumination and on-off illumination cycles (inset) for SnS2 (at 450 C sulphurization temperature) and SnS (at 450 C substrate temperature) thin films deposited on ITO substrate.
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Fig. 9. (a) Mott- Schottky plots (b) Nyquist plots under illumination of SnS2 (at 450 C sulphurization temperature) and SnS (at 450 C substrate temperature) thin films deposited on ITO substrate.
capacitance (C) to applied voltage (Vapp).
1
2 ¼ 2 2 qε ε C 0 s A ND
! Vapp Vfb
kB T q
where q is the electronic charge (1.6021019 C), εs is the relative permittivity of semiconductor, ε0 is the vacuum permittivity (8.8541012 F m1), Vapp (V vs. SCE) is the applied potential, C is the space-charge capacitance, A (cm2) is the active geometric area, ND (cm3) is the donor density, Vfb (V vs. SCE) is the flat-band potential, kB is the Boltzmann constant (1.381 1023 J K1) and T is the temperature (298 K). Capacitance (C) at the semiconductor/ electrolyte junction was measured using an LCR meter (Agilent technology, Model 4263B) at varying electrode potentials of the AC signal frequency of 1000 Hz [47]. The obatined flat band potential values of 0.47 and 0.32 V vs. Ag/AgCl for SnS and SnS2 respectively were calculated from the slopes of Mott-Schottky plots as shown in Fig. 9 (a). Negative and positive slopes with obtained values of Vfb are consistant with p-type conductivity for SnS and n-type conductivity for SnS2 respectively. Charge transfer process occurring at the semiconductor/electrolyte interface was also investigated using electrochemical impedance spectroscopy (EIS) (from Nyquist plots) measurements under illumination. Fig. 9(b) shows Nyquist plots for SnS and SnS2 thin films deposited on ITO glass substrate. It can be seen from the Nyquist plots that semicircle arc is smaller for SnS2 sample as compared to SnS confirming the effective enhancement of charge carriers transfer at semiconductor/electrolyte interface and also validate the high value of photocurrent density [48].
5. Conclusions In summary, phase pure SnS and SnS2 thin films were prepared using simple method of thermal evaporation in order to study their photoelectrochemical performance. SnS films were prepared by direct thermal evaporation of SnS powder with substrate kept at 450 C. SnS2 films were obtained by deposition of SnO2 film and then sulphurization process at 450 C. XRD and Raman spectroscopy confirm the crystal structure and phase of the SnS and SnS2 films. XPS results confirm the presence of single phase SnS and SnS2. Optical spectra show the band gap values of 2.75 1st 1.8 eV for SnS2 and SnS films respectively. Photoelectrochemical results show that SnS2 and SnS thin films displayed n-type and p-type conductivity with photocurrent density of 1mA/cm2 and 0.4 mA cm2 at 0.95 V vs. Ag/AgCl respectively. Two major factors attributing to the high photocurrent density may be the appropriate band edge alignment w. r.t redox potential of water and suitable band gap for visible light absorption. Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements "Author (Dipika Sharma) gratefully acknowledge financial support received from the Department of Science & Technology, New
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D. Sharma et al. / Journal of Alloys and Compounds 822 (2020) 153653
Delhi, India through National Post Doc fellowship (reg:PDF/2016/ 001988/CS) and IIT Delhi for Institute Post Doc fellowship. Authors are also thankful to IUAC for extending FESEM facility funded by Ministry of Earth Sciences (MoES) under Geochronology" project [MoES/P.O (Seismic) 8(09)-Geochron/2012]. References [1] A. Fujishima, A.T. RaO, D. Tryk, Titanium dioxide photocatalysis, J. Photochem. Photobiol. C Photochem. Rev. 1 (2000) 1e21. [2] T. Bak, J. Nowotny, M. Rekas, C.C. Sorrell, Photo-electrochemical hydrogen generation from water using solar energy: materials-related aspects, Int. J. Hydrogen Energy 27 (2002) 91e102. [3] D. Sharma, V.R. Satsangi, R. Shrivastav, U.V. Waghmare, S. Dass, Nanostructured BaTiO3/Cu2O heterojunction with improved photoelectrochemical activity for H2 evolution: experimental and first-principles analysis, Appl. Catal. B Environ. 189 (2016) 75e85. [4] K. H Reddy, K.M. 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