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Pt photocathode
Accepted Manuscript Chemical bath deposition of SnS nanosheet thin films for FTO/SnS/CdS/Pt photocathode Jiahe Jing, Meng Cao, Chuangsheng Wu, Jian huang, Jianming Lai, Yan Sun, Linjun Wang, Yue Shen PII:
S0925-8388(17)32688-9
DOI:
10.1016/j.jallcom.2017.07.303
Reference:
JALCOM 42719
To appear in:
Journal of Alloys and Compounds
Received Date: 10 May 2017 Revised Date:
15 July 2017
Accepted Date: 28 July 2017
Please cite this article as: J. Jing, M. Cao, C. Wu, J. huang, J. Lai, Y. Sun, L. Wang, Y. Shen, Chemical bath deposition of SnS nanosheet thin films for FTO/SnS/CdS/Pt photocathode, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.07.303. 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.
ACCEPTED MANUSCRIPT Chemical bath deposition of SnS nanosheet thin films for FTO/SnS/ CdS/Pt photocathode Jiahe Jinga, Meng Caoa*, Chuangsheng Wua, Jian huanga, Jianming Laia,
a
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Yan Sunb, Linjun Wanga, Yue Shena* School of Materials Science and Engineering, Shanghai University, Shanghai, 200072,
China
National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese
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b
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Academy of Sciences, Shanghai 200083, China
Abstract:
SnS nanosheet thin films were prepared by chemical bath deposition in acidic solution, which used tin (II) chloride dihydrate and thioacetamide as precursors of Sn
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and S, respectively. The influences of pH levels, precursors’ mole ratios, deposition times on the physical properties of SnS nanosheet thin films were investigated. The sheet-like morphologies and compositions of as-deposited SnS thin film were
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characterized by Scanning electron microscope (SEM). X-ray diffraction (XRD),
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Raman and X-ray photoelectron spectra (XPS) were used to confirm the crystal structures and phase purities of SnS nanosheet thin films. When employed as a photocathode for photo-electrochemical (PEC) solar hydrogen production, the as-deposited SnS nanosheet thin films yielded photocurrent densities of 31.94 µA·cm-2 at -0.4 V under illumination of AM 1.5G. After deposition of CdS and Pt layers, the cathodic photocurrent densities of FTO/SnS/CdS/Pt and FTO/annealed SnS/CdS/Pt were improved to 0.572 mA·cm-2 and 0.702 mA·cm-2, respectively, 1
ACCEPTED MANUSCRIPT which demonstrated the great potential of using earth-abundant SnS for efficient hydrogen production. Key
Words:
SnS;
nanosheet;
chemical
bath
deposition;
photocurrent;
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photoelectrochemical water splitting * To whom correspondence should be addressed: Corresponding author: Meng Cao, Yue Shen
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*E-mail address: [email protected], [email protected]
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ACCEPTED MANUSCRIPT 1. Introduction Efficiently splitting water into usable hydrogen has become a promising pathway for solar energy conversion [1]. Water splitting cells, which are composed of
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economical and stable semiconductors, have been designed to split water directly at the semiconductor’s surface. These direct semiconductor/liquid contacts can avoid significant manufacturing costs [2]. To split water into H2 and O2, the semiconductor
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must absorb radiant light with photon energies of >1.23 eV [3]. Tin monosulphide
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(SnS) is a low cost and non-toxic IV-VI group binary compound semiconductor [4]. It has a direct bandgap of 1.3-1.5 eV [5] and large absorption coefficient (>104 cm−1) [6] at room temperature, which is close to the optimum level for maximum absorption of solar radiation. So, it has attracted more attention in photoelectronic devices,
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especially for photoelectrochemical (PEC) water splitting [7]. As known, the morphologies of thin films have a great influence to their photoelectronic properties [8, 9]. And sheet-like SnS thin films have several unique advantages, such as
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excellent migration of electrons, high collection efficiency of majority carriers and
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large surface reaction area [10, 11]. Hence, sheet-like SnS thin films will be of great potential in PEC solar hydrogen applications. SnS thin films have been prepared by using various methods such as vacuum
evaporation [12], spray pyrolysis [13], successive ionic layer adsorption and reaction (SILAR) method [14], solvothermal method [15], chemical bath deposition (CBD) [16], electro-deposition [17]. In the above methods, the CBD is a simple and inexpensive technique, which is suitable to prepare high quality layers over large 3
ACCEPTED MANUSCRIPT areas. However, there are many factors that influence the preparation and qualities of SnS thin films, such as precursors’ mole ratios, reaction temperatures, pH levels, deposition times, substrate nature and so on [18, 19].
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In this study, SnS nanosheet thin films were deposited by CBD method. The structural, morphological and optical properties of SnS nanosheet thin films deposited with different reaction conditions were studied. The PEC response properties of SnS
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nanosheet thin films were investigated under simulated AM 1.5 G radiation. The
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structure of SnO2:F (FTO)/SnS/CdS/Pt was also fabricated to optimize the PEC response properties of SnS nanosheet thin films. 2. Experiments 2.1 Materials
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Tin (II) chloride dihydrate (SnCl2·2H2O, ≥98.0%) and thioacetamide (TAA, CH3CSNH2, ≥99%) were used as the precursors of Sn and S. In addition, sulfuric acid (95%~98%) was used to adjust the pH levels of reaction solution. All reagents were
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analytical grade and purchased from Sinopharm chemical LTD (Shanghai). All
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solutions were prepared by using deionized water. 2.2 Preparation of SnS nanosheet thin films Before depositing the SnS nanosheet thin films, the FTO substrates would be
ultrasonically cleaned by acetone, absolute ethanol and deionized water for 15 mins, respectively. Then 4 mmol SnCl2·2H2O and 5-16 mmol TAA were firstly dissolved in 200 ml deionized water under magnetic stirring. The pH levels of the solution (pH = 0.2-1.5) were adjusted by adding sulfuric acid. Then FTO substrates were inserted 4
ACCEPTED MANUSCRIPT into the solution and the temperature of the solution was maintained at 80 °C for 12-180 mins. During the deposition, the solution was magnetic stirred at 600 rpm. After the deposition, the as-deposited thin films were rinsed by deionized water and
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then collected. 2.3 Annealing treatment of SnS nanosheet thin films and preparation of FTO/SnS/CdS/Pt
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SnS nanosheet thin films were annealed in high pure N2 (99.999%) atmosphere at
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300 °C for 40 mins in a tube furnace. About 15 nm-thick CdS thin films were deposited on the surfaces of SnS nanosheet thin films by CBD method. A solution containing 34 ml deionized water, 5 ml CdSO4 (0.025 mM), 6.52 ml NH4OH (0.17 M) and 5 ml CS(NH2)2 (3.24 mM) were used in the CBD process, which was carried out
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at 60 ºC for 10 mins. 5 nm-thick Pt thin films were sputtered on the surface of SnS/CdS with the deposition rate of 0.3 nm/s by using the machine of Baltec 500. 2.4 Characterization of SnS thin films and FTO/SnS/CdS/Pt
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X-ray diffraction (XRD) analysis was carried out using 18 KW D/MAX2500V+/PC
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with Cu-Kα radiation (λ = 0.154 nm). Raman spectra were measured with JY-H800UV (λ = 514 nm). Scanning electron microscopy (SEM, FEI Sirion 200) was used to measure the morphologies of as-deposited SnS thin films. The compositions of SnS thin films were determined by Energy Dispersive Spectrometry (EDS) attached to SEM. The valance states of Sn and S were examined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). The diffuse reflectance spectra were measured by using a UV-vis spectrophotometer (Jasco UV-570). Photocurrent 5
ACCEPTED MANUSCRIPT responses of SnS thin films and FTO/SnS/CdS/Pt (1cm2) were recorded with a CHI660B electrochemical workstation under illumination of AM 1.5G with a light density of 100 mW/cm2 (Newport, Oriel Instruments). SnS nanosheet thin films were
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used as working electrode while an Ag/AgCl rod and a platinum plate were used as the reference electrode and the counter-electrode, respectively. 0.5 M Na2SO4 (pH =
3. Results and discussions
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3.1 Influences of the deposition parameters
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0.5) solution was used as the electrolyte in the measurements.
In order to optimize the deposition process of SnS nanosheet thin films, a number of experiments were carried out based on different conditions, which included various pH levels, mole ratios of SnCl2·2H2O to TAA and deposition times.
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3.1.1 Influences of pH levels
SnS thin films can be obtained from an aqueous bath containing SnCl2‧2H2O and TAA. The deposition process is based on the slow release of Sn2+ and S2- in the
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solution, which is shown in the following equations [20-22]:
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CH3CSNH2+H2O+H3O+⇆CH3COOH+NH4++H2S
(1)
H2S ⇆ HS- + H+
(2)
HS- ⇆ S2- + H+
(3)
Sn2++ S2- ⇆ SnS
(4)
TAA will be dissociated in acidic solution at a temperature higher than 60 °C and release H2S. Then, most deposition temperatures are choosen to be 80 °C ~ 90 °C. Beside these, the release rates of S2- are also influenced greatly by pH levels. So, ：
ACCEPTED MANUSCRIPT keeping the mole ratio of SnCl2·2H2O to TAA = 1:1.5, the reaction solutions of different pH levels are used to deposit SnS nanosheet at 80 °C for 120 mins. The surface and cross-section SEM images of SnS nanosheet thin films deposited with
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different pH levels are shown in Fig. 1a1-1e1 and Fig. 1a2-1e2, respectively. At the pH level range of 0.2-0.7, the deposited SnS thin films are compact and have sheet-like morphologies. When pH levels are 1.0 and 1.5, sheet-like particles are becoming less
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and the average sizes are also decreased. The deposited thin films are loose relatively
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and there are many big sticks at the surface of the thin films. The thicknesses of SnS thin films increase from 608 nm to 4.37 µm with decreasing the pH levels. It is because of that the releasing rates of S2- from TAA are enhanced with decreasing the pH levels, which can lead to fast deposition rates. At high pH levels, more Sn(OH)2
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will precipitate, which will decrease the thickness of SnS nanosheet thin films [20]. The XRD patterns of SnS nanosheet thin films deposited with different pH levels are shown in Fig. 1f. At pH = 0.2, 0.4, 0.7 and 1.0, the strong diffraction peaks at 2θ =
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30.7°, 31.74°, 44.56°, 45.60°, 64.32°, 64.92° correspond to (101), (111), (141), (002),
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(251), (008) planes of orthorhombic SnS (JCPDS No. 39-0354). The peak at 2θ = 37.68° corresponds to SnO2 (JCPDS no.46-1088), which comes from FTO substrate. At pH = 1.5, the diffraction peaks of SnS are not clear due to the strong XRD peaks of SnO2. When the pH value is above 0.7, a small diffraction peak at 2θ = 26.56° also appears, which belongs to sulfur powder (JCPDS no.34-0941). At high pH levels, the deposition of Sn(OH)2 will decrease the concentration of Sn2+ in the reaction solution and too much of S2- have no chance to form SnS. Then, sulfur powders are easy to be 7
ACCEPTED MANUSCRIPT deposited during the hydrolysis of TAA. Raman spectra of deposited SnS nanosheet thin films are shown in Fig. 1g. Three peaks are observed at 162 cm-1, 189 cm-1 and 220 cm-1, which agree well with SnS [23]. At pH = 1.5, the Raman peaks of SnO2
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from FTO substrate are not detected. As known, the laser from the Raman spectrum instrument can only penetrate tens nanometers under the sample’s surface, while X-ray can penetrate tens micrometers under the sample’s surface. The thickness of
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SnS thin films deposited at pH = 1.5 is about 608 nm. So, XRD patterns are
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influenced, but Raman spectra are not influenced by FTO substrate. Beside these, the average spot sized (D) of the laser from the Raman instrument is only about 721 nm (D = 1.22 λ / NA, λ = 514 nm, NA = 0.9). So, Raman spectra of sulfur powders are also not easy to be found because of that the little sulfur powders are dispersed in only
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some very small regions.
The atomic percents of the SnS films were estimated from the EDS results, as shown in Table 1. The atomic percents of Sn to S in the SnS thin films are close to the
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stoichiometric ratio of SnS. With the increasing of pH levels, the atomic percents of
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Sn are decreased, which agree well with the SEM and XRD characterizations. From the above discussion, it is found that SnS nanosheet thin films deposited with pH = 0.2-0.4 have better morphological and structural properties. But it will cost too much sulfuric acid to decrease the solution’s pH values from 0.4 to 0.2. So, pH = 0.4 is the best choice. The valence states and phase purities of the as-deposited SnS nanosheet thin films (pH = 0.4, mole ratio of SnCl2·2H2O to TAA = 1:1.5, 80 °C ,120 mins) were also 8
ACCEPTED MANUSCRIPT investigated by XPS. In Fig. 2a, Sn, S, C and O are found in the films. In Fig. 2b and 2c, the peaks at 486.88 and 495.35 eV correspond to the binding energy of Sn 3d5/2 and Sn 3d3/2, respectively, and the gap between Sn 3d5/2 and Sn 3d3/2 level is 8.4 eV,
which is indicative of sulfur as divalent S2− in SnS [14, 24]. 3.1.2 Influences of mole ratios of SnCl2·2H2O to TAA
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which indicates Sn2+. The peak at 161.6 eV corresponds to the energies of S 2p3/2,
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At a certain pH value, the amounts of S2- released from TAA will also have a great
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influence to the physical properties of the final products, such as thicknesses of the films and the phase purities. So, SnS nanosheet thin films were deposited with different precursors’ mole ratios at 80 °C for 120 mins. The pH value of the solution is 0.4. The SEM images of SnS nanosheet thin films are shown in Fig. 3a1-3c1 and Fig.
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3a2-3c2. As shown, all the deposited SnS thin films are very compact and consist of many nanosheets with the thicknesses of about 200 to 400 nm. With the decreasing of mole ratios of SnCl2·2H2O to TAA, the average sizes and the thicknesses of SnS
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nanosheet thin films are decreased. When mole ratio of SnCl2·2H2O to TAA is 1:4,
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there are some small and irregular clusters at the sample’s surface. The XRD patterns of SnS nanosheet thin films with different precursors’ mole
ratios are shown in Fig. 3d. All the XRD patterns correspond well with the orthorhombic SnS (JCPDS No. 39-0354). And the peak at 2θ = 37.68° corresponds to SnO2 (JCPDS no.46-1088), which comes from the FTO substrate. When the mole ratio of SnCl2·2H2O to TAA is 1:4, a small diffraction peak at 2θ = 26.56° appears, which belongs to sulfur powder (JCPDS no.34-0941) and accords well with the 9
ACCEPTED MANUSCRIPT irregular particles in the SEM images. The Raman spectra of SnS nanosheet thin films deposited with different mole ratios are shown in Fig. 3e. Three peaks in the as-deposited SnS thin films match well with Raman spectra of SnS.
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Table 1 shows the atomic percents of SnS thin films deposited with different precursors’ mole ratios, which are close to 1:1. The atomic percents of Sn are slightly smaller than that of S. And the atomic percents of S are increased slightly with the
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increasing of the amounts of sulfur precursor. In fact, too much TAA will cost H+ in
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the reaction solution greatly. And more Sn(OH)2 will be formed and deposited onto the bottom of the solution. Sulfur powder is also easy to appear due to lack of Sn2+ in the reaction solution. So, the thicknesses of SnS thin films decrease with the decreasing of mole ratios of SnCl2·2H2O to TAA and the atomic percents of S in SnS
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thin films do not increase greatly. Then, the mole ratio of SnCl2·2H2O to TAA = 1:1.5 is the most economic choice.
3.1.3 Influences of the deposition times
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It is important to control the thicknesses of SnS thin films, which influences the
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photoelectronic properties of SnS thin films greatly. By studying time dependent deposition process, it is also helpful to reveal the morphological evolution of the sheet-like SnS thin films. Figure 4 show the SEM images of SnS thin films deposited with different deposition times. The precursors’ mole ratio of SnCl2·2H2O to TAA = 1:1.5. The deposition temperature is 80 °C and pH = 0.4 during reaction process. When the deposition times are shorter than 20 mins, there are only some clusters, which consist of sheet-like particles. In fact, nucleations of SnS appear firstly and 10
ACCEPTED MANUSCRIPT grow perpendicular to the surface of the substrate and form sheet-like particles due to the concentration gradients, which are generated around a growing crystal. Then sheet-like particles will asemble together to form a cluster. With the increasing of
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deposition times, more and more clusters consisting of sheet-like particles will be deposited. At about the deposition times of 20 mins, compact nanosheet thin films will be formed. This reaction process is similar to the nucleation and crystal growth of
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layered metal hydroxides [25]. In Fig. 4c2-4g2, the thicknesses of the SnS nanosheet
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thin films increase from 664 nm to 5.15 µm with the deposition times increasing from 20 mins to 180 mins. The XRD patterns and Raman spectra of SnS nanosheet thin films are shown in Fig. 4h and 4i. The crystal qualities of SnS nanosheets are gradually enhanced with the increasing of the deposition times, which are indicated
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by the peak intensities of XRD patterns and Raman spectra. When the deposition times are 20 mins, the predominant diffraction peaks at 2θ = 26.57°, 37.67°, 51.42° and 65.53° correspond to SnO2 and the atomic percent of Sn to S = 53.83:46.17. It is
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because of the existence of Sn in the FTO substrates. In fact, the atomic percents of
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Sn are also influenced greatly by the FTO substrates when the deposition times are shorter than 20 mins. When the reaction times are beyond 20 mins, the XRD patterns match well with orthorhombic SnS (JCPDS no.39-0354) and the atomic percents of Sn to S are close to 1:1 in the SnS nanosheet thin films, as shown in Table 2. The contents of S increase and that of Sn decrease gradually with the increasing of deposition times. 3.2 Optical properties 11
ACCEPTED MANUSCRIPT Figure 5a1-5c1 show the UV-Vis-NIR diffuse reflection spectra of SnS nanosheet thin films deposited with different pH levels, precursors’ mole ratios and deposition times. And the optical band gaps of SnS nanosheets can be calculated by diffuse
αhν = B (hν − E g )2 1
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reflection spectra according to the Kubelka–Munk equation [26]: (5)
F (R ) = α = 1 − R )2 /(2R )
(6)
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Here α is the optical absorption coefficient, hv is the photon energy, Eg is the band
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gap energy, R is the diffuse reflectance and B is a constant relative to the material. Figure 5a2-5c2 show the direct band gaps of SnS nanosheet thin films deposited with different conditions. These values match well with the reported 1.2-1.5 eV for the orthorhombic SnS, as shown in Table 1 and Table 2. And the band-gaps decrease
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with the increasing of the thicknesses of SnS thin films. It is because of that the grain sizes of SnS nanosheet thin films increase with the increasing of the thicknesses of SnS thin films, which can be calculated by Scherrer equation [27]. And the band gaps
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decrease with the increasing of the grain size as determined by the experimental
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formula[28]: Eg = Eg(bulk) + ((Eb (2παB)2)/ (Ø 2), where αB is the exciton Bohr radius and Ø is the grain size.
3.3 Photoelectrochemical properties of SnS thin films and FTO/SnS/CdS/Pt PEC responses of the SnS thin films and FTO/SnS/CdS/Pt were measured with 0.5
M Na2SO4 electrolyte solution (pH = 0.5). The photo-induced current-time curves were measured at -0.4 V and the current-potential curves were measured from -0.4 to 0.4 V under chopped light illumination condition. 12
ACCEPTED MANUSCRIPT Figure 6a1 shows the current-potential curves of SnS thin films deposited with different deposition times. Under illumination, the photocurrents are decreased in the cathodic direction when the cathodic potential scan range is from -0.4 to 0 V, which
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indicates that SnS nanosheet thin films are p-type conductivity. At the anodic bias range of 0 to 0.4 V, there are no obvious changes in the photo-response, which also confirms the conductivity type. From the current-time curves in the Fig. 6a2, the
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photocurrent densities of SnS thin films deposited at 30, 60, 90, 120 and 180 mins are
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about 15.85, 23.85, 26.57, 31.94 and 18.07 µA·cm-2, respectively. The photocurrent densities increase with enhancing the thickness of SnS thin films until the thickness is beyond 4 µm. The photocurrent densities of SnS nanosheet thin film deposited with 120 mins (4.03 µm) are higher and more stable than that of the other samples. It is
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because of that, the photocurrents are measured in acidic solution, which is corrosive to SnS thin films. So, a relative big thickness is beneficial to the PEC properties of SnS thin films. And these chemical bath deposited SnS thin films have better
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photoelectronic properties than that prepared by using SnS nanoparticles, which are
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synthesized at about 280 °C [29, 30]. It is mainly due to the compact and sheet-like morpholgoy.
It is known that SnS nanoseet thin films without any additional layers cannot
generate high photocurrents because of the loss of electrons via recombination. In order to increase the photocurrents, the n-type CdS thin films were deposited on the as-deposited and annealed (300 °C) SnS nanosheet thin films, whose deposition times are 120 mins. Such a p-n junction (SnS/CdS) is a most efficient way to separate the 13
ACCEPTED MANUSCRIPT photogenerated charge carriers [31, 32]. And 5 nm Pt thin films were sputtered on the SnS/CdS and annealed SnS/CdS films, which were used as a hydrogen generation catalyst to enhance the surface reaction rate in water splitting and can also protect
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SnS/CdS thin film. Figure 7 shows the current-potential and current-time curves of FTO/SnS/CdS/Pt and FTO/annealed SnS/CdS/Pt thin films under chopped light. According to Fig. 7a2 and 7b2, after depositing CdS and Pt thin films, the photocurrent
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density of FTO/SnS/CdS/Pt was increased to 0.572 mA·cm-2 at -0.4 V bias potential.
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The FTO/annealed SnS/CdS/Pt films produced a photocurrent density value of 0.702 mA·cm-2. Beside the annealing process, there are two possible steps to improve the PEC properties of FTO/SnS/CdS/Pt photocathode. The first is to enhance the electron transport characteristics of SnS nanosheet thin films by doping metal ions, such as Cu,
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In and so on. The second is to enhance the currents of FTO/SnS/CdS/Pt photocathode by selecting In2S3/CdS double layer instead of CdS single layer, which has been applied successfully in FTO/Cu2ZnSnS4/SnS/CdS/In2S3/ Pt photocathode [33].
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4. Conclusions
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SnS nanosheet thin films with orthorhombic structure were prepared by CBD method. The influences of reaction conditions on the properties of the SnS nanosheet thin films were studied. According to the results, compact SnS nanosheet thin films are easy to be deposited at 80 °C with pH = 0.4. At this condition, the thicknesses of SnS nanosheet thin films can be controlled by changing the deposition times. Depositing the CdS and Pt layers can obviously increase the photocurrent densities of as-deposited and annealed SnS thin films. A highest photocurrent density of 0.702 14
ACCEPTED MANUSCRIPT mA·cm-2 has been achieved for FTO/annealed SnS/CdS/Pt. The enhanced performance of FTO/SnS/CdS/Pt photocathode is beneficial to develop the PEC water
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splitting of SnS thin films.
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ACCEPTED MANUSCRIPT Acknowledge: This work was supported by the National Key Basic Research Program of China (973 program, Grant No. 2012CB934300), Shanghai city committee of Science and
China
(61006089),
Innovation
program
of
Shanghai
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Technology (15520500200,16010500500), National Nature Science Foundation of City
(CXSJ-13-077,
CXSJ-14-098). We acknowledge Instrumental Analysis & Research Center of
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Shanghai University for the help of characterization works.
1：
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ACCEPTED MANUSCRIPT [16] M. Safonova, P.K. Nair, E. Mellikov, A.R. Garcia, K. Kerm, N. Revathi, T. Romann, V. Mikli, O. Volobujeva, Chemical bath deposition of SnS thin films on ZnS and CdS substrates, Journal of Materials Science: Materials in Electronics 25 (2014)
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3160-3165. [17] B. Ghosh, R. Roy, S. Chowdhury, P. Banerjee, S. Das, Synthesis of SnS thin films via galvanostatic electrodeposition and fabrication of CdS/SnS heterostructure
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for photovoltaic applications, Applied Surface Science 256 (2010) 4328-4333.
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[18] U. Chalapathi, B. Poornaprakash, S.-H. Park, Chemically deposited cubic SnS thin films for solar cell applications, Solar Energy 139 (2016) 238-248. [19] Y. Jayasree, U. Chalapathi, P. Uday Bhaskar, S.R. V, Effect of precursor concentration and bath temperature on the growth of chemical bath deposited tin
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sulphide thin films, Applied Surface Science 258 (2012) 2732-2740. [20] M. Mnari, N. Kamoun, J. Bonnet, M. Dachraoui, Chemical Bath Deposition of tin sulphide thin films in acid solution, Comptes Rendus Chimie 12 (2009) 824-827.
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[21] M. Gharabaghi, M. Irannajad, A.R. Azadmehr, Selective Sulphide Precipitation
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of Heavy Metals from Acidic Polymetallic Aqueous Solution by Thioacetamide, Industrial & Engineering Chemistry Research 51 (2012) 954-963. [22] E.H. Swift, E.A. Butler, Precipitation of Sulfides form Homogeneous Solutions by Thioacetamide, Analytical Chemistry 28 (1956) 146-153. [23] N. Revathi, S. Bereznev, J. Iljina, M. Safonova, E. Mellikov, O. Volobujeva, PVD grown SnS thin films onto different substrate surfaces, Journal of Materials Science: Materials in Electronics 24 (2013) 4739-4744. 19
ACCEPTED MANUSCRIPT [24] D. Avellaneda, B. Krishnan, A.C. Rodriguez, T.K. Das Roy, S. Shaji, Heat treatments in chemically deposited SnS thin films and their influence in CdS/SnS photovoltaic structures, Journal of Materials Science: Materials in Electronics 26
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(2014) 5585-5592. [25] B.E. Hosono, S. Fujihara, I. Honma, H. Zhou, The Fabrication of an Upright-Standing Zinc Oxide Nanosheet for Use in Dye-Sensitized Solar Cells,
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Advance materials 17 (2005) 2091-2094.
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[26] T. Baikie, Y. Fang, J.M. Kadro, M. Schreyer, F. Wei, S.G. Mhaisalkar, M. Graetzel, T.J. White, Synthesis and crystal chemistry of the hybrid perovskite (CH3NH3)PbI3 for solid-state sensitised solar cell applications, Journal of Materials Chemistry A 1 (2013) 5628.
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[27] E. Guneri, C. Ulutas, F. Kirmizigul, G. Altindemir, F. Gode, C. Gumus, Effect of deposition time on structural, electrical, and optical properties of SnS thin films
1189-1195.
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deposited by chemical bath deposition, Applied Surface Science 257 (2010)
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[28] A. Cortes, Grain size dependence of the bandgap in chemical bath deposited CdS thin films, Solar Energy Materials and Solar Cells 82 (2004) 21-34. [29] W. Gao, M. Cao, J. Yang, J. Shen, J. Huang, Y. Zhao, Y. Sun, L. Wang, Y. Shen, Controllable synthesis of SnS2/SnS nanosheets and their photoelectric properties, Materials Letters 180 (2016) 284-287. [30] W. Gao, C. Wu, M. Cao, J. Huang, L. Wang, Y. Shen, Thickness tunable SnS nanosheets for photoelectrochemical water splitting, Journal of Alloys and 20
ACCEPTED MANUSCRIPT Compounds 688 (2016) 668-674. [31] L. Sinatra, A.P. LaGrow, W. Peng, A.R. Kirmani, A. Amassian, H. Idriss, O.M. Bakr, A Au/Cu2O–TiO2 system for photo-catalytic hydrogen production. A
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pn-junction effect or a simple case of in situ reduction, Journal of Catalysis 322 (2015) 109-117.
[32] J. Cao, J. Xing, Y. Zhang, H. Tong, Y. Bi, T. Kako, M. Takeguchi, J. Ye,
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photoelectrodes, Langmuir 29 (2013) 3116-24.
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Photoelectrochemical properties of nanomultiple CaFe2O4/ZnFe2O4 pn junction
[33] F. Jiang, Gunawan, T. Harada, Y. Kuang, T. Minegishi, K. Domen, S. Ikeda, Pt/In2S3/CdS/Cu2ZnSnS4 Thin Film as an Efficient and Stable Photocathode for Water Reduction under Sunlight Radiation, Journal of the American Chemical Society 137
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(2015) 13691-7.
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ACCEPTED MANUSCRIPT Figure Captions: Table 1: The atomic percents of Sn to S, the grain sizes and the band gaps of SnS thin films deposited with different pH levels and different precursors’ mole ratios.
films deposited with different deposition times.
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Table 2: The atomic percents of Sn to S, the grain sizes and the band gaps of SnS thin
Figure 1: Surface and cross-section SEM images of SnS nanosheet thin films
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deposited with different pH levels: (a1, a2) pH = 0.2, (b1, b2) pH = 0.4, (c1, c2) pH =
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0.7, (d1, d2) pH = 1.0, (e1, e2) pH = 1.5. The XRD patterns (f) and Raman spectra (g) of SnS nanosheet thin films deposited with different pH levels. Figure 2: XPS analysis of SnS nanosheet thin films (pH = 0.4, mole ratio of SnCl2·2H2O to TAA = 1:1.5, 80 °C, 120 mins): (a) survey scan, (b) S region, (c) Sn
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region.
Figure 3: Surface and cross-section SEM images of SnS nanosheet thin films deposited with different mole ratios of SnCl2·2H2O to TAA: (a1, a2) 1:1.25, (b1, b2)
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1:2, (c1, c2) 1:4. The (d) XRD patterns and Raman spectra (e) of SnS nanosheet thin
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films deposited with different mole ratios of SnCl2·2H2O to TAA. Figure 4: SEM images of SnS nanosheet thin films deposited with different deposition times: (a1, a2) 12 mins, (b1, b2) 16 mins, (c1, c2) 20 mins, (d1, d2) 30 mins, (e1, e2) 60 mins, (f1, f2) 90 mins, (g1, g2) 180 mins. The XRD patterns (h) and Raman spectra (i) of SnS nanosheet thin films deposited with different deposition times. Figure 5: The UV-Vis-NIR diffuse reflection spectra and the curves of (αhv)2 vs. hv of the SnS nanosheet thin films deposited with different reaction conditions: (a1, a2) 22
ACCEPTED MANUSCRIPT different pH levels, (b1, b2) different precursors’ mole ratios of SnCl2·2H2O to TAA, (c1, c2) different deposition times. Figure 6: The PEC response curves of SnS nanosheet thin films with different
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deposition times at 30 mins, 60 mins, 90 mins, 120 mins, 180 mins: (a1) current-potential, (a2) current-time. Figure 7:
The current-potential and current-time response curves of SnS nanosheet
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thin films (120 mins): (a1, a2) FTO/SnS/CdS/Pt, (b1, b2) FTO/annealed SnS/CdS/Pt.
23
Table1
Atomic percents
pH levels
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Grain size (nm)
Band gaps (eV)
CH3CSNH2
(mmol)
(mmol)
4
6
49.82 50.18
4
6
48.46 51.54
0.4
39.438
1.35
4
6
48.11 51. 89
0.7
38.505
1.36
4
6
47.47 52. 53
1.0
32.263
1.44
4
6
46.66 53. 34
1.5
22.143
1.52
4
5
48.63 51. 27
0.4
35.799
1.37
4
8
47.06 52. 94
0.4
33.721
1.40
4
16
0.4
26.584
1.48
of Sn:S
42.009
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0.2
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SnCl2 2H2O
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46.90 53.10
24
1.28
Table 2
12
57.91 42.09
-
16
54.58 45.42
-
20
53.83 46.17
30
50.52 49.48
60
49.25 50.75
90
48.71 51.29
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Grain size (nm)
25
Band gaps (eV)
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Atomic percents of Sn:S
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Time (mins)
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-
-
25.916
1.43
33.446
1.41
33.746
1.38
38.321
1.36
38.975
1.36
Figure 1
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Figure 4
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Figure 5
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Hightlight · SnS nanosheet thin films were deposited with simple CBD method. of the morphological evolution of SnS nanosheets was
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· Mechanism studied.
· FTO/SnS/CdS/Pt was fabricated to enhance the properties of PEC water
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splitting.
S0925-8388(17)32688-9
DOI:
10.1016/j.jallcom.2017.07.303
Reference:
JALCOM 42719
To appear in:
Journal of Alloys and Compounds
Received Date: 10 May 2017 Revised Date:
15 July 2017
Accepted Date: 28 July 2017
Please cite this article as: J. Jing, M. Cao, C. Wu, J. huang, J. Lai, Y. Sun, L. Wang, Y. Shen, Chemical bath deposition of SnS nanosheet thin films for FTO/SnS/CdS/Pt photocathode, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.07.303. 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.
ACCEPTED MANUSCRIPT Chemical bath deposition of SnS nanosheet thin films for FTO/SnS/ CdS/Pt photocathode Jiahe Jinga, Meng Caoa*, Chuangsheng Wua, Jian huanga, Jianming Laia,
a
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Yan Sunb, Linjun Wanga, Yue Shena* School of Materials Science and Engineering, Shanghai University, Shanghai, 200072,
China
National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese
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b
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Academy of Sciences, Shanghai 200083, China
Abstract:
SnS nanosheet thin films were prepared by chemical bath deposition in acidic solution, which used tin (II) chloride dihydrate and thioacetamide as precursors of Sn
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and S, respectively. The influences of pH levels, precursors’ mole ratios, deposition times on the physical properties of SnS nanosheet thin films were investigated. The sheet-like morphologies and compositions of as-deposited SnS thin film were
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characterized by Scanning electron microscope (SEM). X-ray diffraction (XRD),
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Raman and X-ray photoelectron spectra (XPS) were used to confirm the crystal structures and phase purities of SnS nanosheet thin films. When employed as a photocathode for photo-electrochemical (PEC) solar hydrogen production, the as-deposited SnS nanosheet thin films yielded photocurrent densities of 31.94 µA·cm-2 at -0.4 V under illumination of AM 1.5G. After deposition of CdS and Pt layers, the cathodic photocurrent densities of FTO/SnS/CdS/Pt and FTO/annealed SnS/CdS/Pt were improved to 0.572 mA·cm-2 and 0.702 mA·cm-2, respectively, 1
ACCEPTED MANUSCRIPT which demonstrated the great potential of using earth-abundant SnS for efficient hydrogen production. Key
Words:
SnS;
nanosheet;
chemical
bath
deposition;
photocurrent;
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photoelectrochemical water splitting * To whom correspondence should be addressed: Corresponding author: Meng Cao, Yue Shen
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*E-mail address: [email protected], [email protected]
2
ACCEPTED MANUSCRIPT 1. Introduction Efficiently splitting water into usable hydrogen has become a promising pathway for solar energy conversion [1]. Water splitting cells, which are composed of
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economical and stable semiconductors, have been designed to split water directly at the semiconductor’s surface. These direct semiconductor/liquid contacts can avoid significant manufacturing costs [2]. To split water into H2 and O2, the semiconductor
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must absorb radiant light with photon energies of >1.23 eV [3]. Tin monosulphide
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(SnS) is a low cost and non-toxic IV-VI group binary compound semiconductor [4]. It has a direct bandgap of 1.3-1.5 eV [5] and large absorption coefficient (>104 cm−1) [6] at room temperature, which is close to the optimum level for maximum absorption of solar radiation. So, it has attracted more attention in photoelectronic devices,
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especially for photoelectrochemical (PEC) water splitting [7]. As known, the morphologies of thin films have a great influence to their photoelectronic properties [8, 9]. And sheet-like SnS thin films have several unique advantages, such as
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excellent migration of electrons, high collection efficiency of majority carriers and
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large surface reaction area [10, 11]. Hence, sheet-like SnS thin films will be of great potential in PEC solar hydrogen applications. SnS thin films have been prepared by using various methods such as vacuum
evaporation [12], spray pyrolysis [13], successive ionic layer adsorption and reaction (SILAR) method [14], solvothermal method [15], chemical bath deposition (CBD) [16], electro-deposition [17]. In the above methods, the CBD is a simple and inexpensive technique, which is suitable to prepare high quality layers over large 3
ACCEPTED MANUSCRIPT areas. However, there are many factors that influence the preparation and qualities of SnS thin films, such as precursors’ mole ratios, reaction temperatures, pH levels, deposition times, substrate nature and so on [18, 19].
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In this study, SnS nanosheet thin films were deposited by CBD method. The structural, morphological and optical properties of SnS nanosheet thin films deposited with different reaction conditions were studied. The PEC response properties of SnS
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nanosheet thin films were investigated under simulated AM 1.5 G radiation. The
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structure of SnO2:F (FTO)/SnS/CdS/Pt was also fabricated to optimize the PEC response properties of SnS nanosheet thin films. 2. Experiments 2.1 Materials
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Tin (II) chloride dihydrate (SnCl2·2H2O, ≥98.0%) and thioacetamide (TAA, CH3CSNH2, ≥99%) were used as the precursors of Sn and S. In addition, sulfuric acid (95%~98%) was used to adjust the pH levels of reaction solution. All reagents were
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analytical grade and purchased from Sinopharm chemical LTD (Shanghai). All
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solutions were prepared by using deionized water. 2.2 Preparation of SnS nanosheet thin films Before depositing the SnS nanosheet thin films, the FTO substrates would be
ultrasonically cleaned by acetone, absolute ethanol and deionized water for 15 mins, respectively. Then 4 mmol SnCl2·2H2O and 5-16 mmol TAA were firstly dissolved in 200 ml deionized water under magnetic stirring. The pH levels of the solution (pH = 0.2-1.5) were adjusted by adding sulfuric acid. Then FTO substrates were inserted 4
ACCEPTED MANUSCRIPT into the solution and the temperature of the solution was maintained at 80 °C for 12-180 mins. During the deposition, the solution was magnetic stirred at 600 rpm. After the deposition, the as-deposited thin films were rinsed by deionized water and
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then collected. 2.3 Annealing treatment of SnS nanosheet thin films and preparation of FTO/SnS/CdS/Pt
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SnS nanosheet thin films were annealed in high pure N2 (99.999%) atmosphere at
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300 °C for 40 mins in a tube furnace. About 15 nm-thick CdS thin films were deposited on the surfaces of SnS nanosheet thin films by CBD method. A solution containing 34 ml deionized water, 5 ml CdSO4 (0.025 mM), 6.52 ml NH4OH (0.17 M) and 5 ml CS(NH2)2 (3.24 mM) were used in the CBD process, which was carried out
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at 60 ºC for 10 mins. 5 nm-thick Pt thin films were sputtered on the surface of SnS/CdS with the deposition rate of 0.3 nm/s by using the machine of Baltec 500. 2.4 Characterization of SnS thin films and FTO/SnS/CdS/Pt
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X-ray diffraction (XRD) analysis was carried out using 18 KW D/MAX2500V+/PC
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with Cu-Kα radiation (λ = 0.154 nm). Raman spectra were measured with JY-H800UV (λ = 514 nm). Scanning electron microscopy (SEM, FEI Sirion 200) was used to measure the morphologies of as-deposited SnS thin films. The compositions of SnS thin films were determined by Energy Dispersive Spectrometry (EDS) attached to SEM. The valance states of Sn and S were examined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). The diffuse reflectance spectra were measured by using a UV-vis spectrophotometer (Jasco UV-570). Photocurrent 5
ACCEPTED MANUSCRIPT responses of SnS thin films and FTO/SnS/CdS/Pt (1cm2) were recorded with a CHI660B electrochemical workstation under illumination of AM 1.5G with a light density of 100 mW/cm2 (Newport, Oriel Instruments). SnS nanosheet thin films were
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used as working electrode while an Ag/AgCl rod and a platinum plate were used as the reference electrode and the counter-electrode, respectively. 0.5 M Na2SO4 (pH =
3. Results and discussions
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3.1 Influences of the deposition parameters
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0.5) solution was used as the electrolyte in the measurements.
In order to optimize the deposition process of SnS nanosheet thin films, a number of experiments were carried out based on different conditions, which included various pH levels, mole ratios of SnCl2·2H2O to TAA and deposition times.
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3.1.1 Influences of pH levels
SnS thin films can be obtained from an aqueous bath containing SnCl2‧2H2O and TAA. The deposition process is based on the slow release of Sn2+ and S2- in the
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solution, which is shown in the following equations [20-22]:
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CH3CSNH2+H2O+H3O+⇆CH3COOH+NH4++H2S
(1)
H2S ⇆ HS- + H+
(2)
HS- ⇆ S2- + H+
(3)
Sn2++ S2- ⇆ SnS
(4)
TAA will be dissociated in acidic solution at a temperature higher than 60 °C and release H2S. Then, most deposition temperatures are choosen to be 80 °C ~ 90 °C. Beside these, the release rates of S2- are also influenced greatly by pH levels. So, ：
ACCEPTED MANUSCRIPT keeping the mole ratio of SnCl2·2H2O to TAA = 1:1.5, the reaction solutions of different pH levels are used to deposit SnS nanosheet at 80 °C for 120 mins. The surface and cross-section SEM images of SnS nanosheet thin films deposited with
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different pH levels are shown in Fig. 1a1-1e1 and Fig. 1a2-1e2, respectively. At the pH level range of 0.2-0.7, the deposited SnS thin films are compact and have sheet-like morphologies. When pH levels are 1.0 and 1.5, sheet-like particles are becoming less
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and the average sizes are also decreased. The deposited thin films are loose relatively
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and there are many big sticks at the surface of the thin films. The thicknesses of SnS thin films increase from 608 nm to 4.37 µm with decreasing the pH levels. It is because of that the releasing rates of S2- from TAA are enhanced with decreasing the pH levels, which can lead to fast deposition rates. At high pH levels, more Sn(OH)2
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will precipitate, which will decrease the thickness of SnS nanosheet thin films [20]. The XRD patterns of SnS nanosheet thin films deposited with different pH levels are shown in Fig. 1f. At pH = 0.2, 0.4, 0.7 and 1.0, the strong diffraction peaks at 2θ =
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30.7°, 31.74°, 44.56°, 45.60°, 64.32°, 64.92° correspond to (101), (111), (141), (002),
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(251), (008) planes of orthorhombic SnS (JCPDS No. 39-0354). The peak at 2θ = 37.68° corresponds to SnO2 (JCPDS no.46-1088), which comes from FTO substrate. At pH = 1.5, the diffraction peaks of SnS are not clear due to the strong XRD peaks of SnO2. When the pH value is above 0.7, a small diffraction peak at 2θ = 26.56° also appears, which belongs to sulfur powder (JCPDS no.34-0941). At high pH levels, the deposition of Sn(OH)2 will decrease the concentration of Sn2+ in the reaction solution and too much of S2- have no chance to form SnS. Then, sulfur powders are easy to be 7
ACCEPTED MANUSCRIPT deposited during the hydrolysis of TAA. Raman spectra of deposited SnS nanosheet thin films are shown in Fig. 1g. Three peaks are observed at 162 cm-1, 189 cm-1 and 220 cm-1, which agree well with SnS [23]. At pH = 1.5, the Raman peaks of SnO2
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from FTO substrate are not detected. As known, the laser from the Raman spectrum instrument can only penetrate tens nanometers under the sample’s surface, while X-ray can penetrate tens micrometers under the sample’s surface. The thickness of
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SnS thin films deposited at pH = 1.5 is about 608 nm. So, XRD patterns are
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influenced, but Raman spectra are not influenced by FTO substrate. Beside these, the average spot sized (D) of the laser from the Raman instrument is only about 721 nm (D = 1.22 λ / NA, λ = 514 nm, NA = 0.9). So, Raman spectra of sulfur powders are also not easy to be found because of that the little sulfur powders are dispersed in only
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some very small regions.
The atomic percents of the SnS films were estimated from the EDS results, as shown in Table 1. The atomic percents of Sn to S in the SnS thin films are close to the
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stoichiometric ratio of SnS. With the increasing of pH levels, the atomic percents of
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Sn are decreased, which agree well with the SEM and XRD characterizations. From the above discussion, it is found that SnS nanosheet thin films deposited with pH = 0.2-0.4 have better morphological and structural properties. But it will cost too much sulfuric acid to decrease the solution’s pH values from 0.4 to 0.2. So, pH = 0.4 is the best choice. The valence states and phase purities of the as-deposited SnS nanosheet thin films (pH = 0.4, mole ratio of SnCl2·2H2O to TAA = 1:1.5, 80 °C ,120 mins) were also 8
ACCEPTED MANUSCRIPT investigated by XPS. In Fig. 2a, Sn, S, C and O are found in the films. In Fig. 2b and 2c, the peaks at 486.88 and 495.35 eV correspond to the binding energy of Sn 3d5/2 and Sn 3d3/2, respectively, and the gap between Sn 3d5/2 and Sn 3d3/2 level is 8.4 eV,
which is indicative of sulfur as divalent S2− in SnS [14, 24]. 3.1.2 Influences of mole ratios of SnCl2·2H2O to TAA
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which indicates Sn2+. The peak at 161.6 eV corresponds to the energies of S 2p3/2,
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At a certain pH value, the amounts of S2- released from TAA will also have a great
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influence to the physical properties of the final products, such as thicknesses of the films and the phase purities. So, SnS nanosheet thin films were deposited with different precursors’ mole ratios at 80 °C for 120 mins. The pH value of the solution is 0.4. The SEM images of SnS nanosheet thin films are shown in Fig. 3a1-3c1 and Fig.
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3a2-3c2. As shown, all the deposited SnS thin films are very compact and consist of many nanosheets with the thicknesses of about 200 to 400 nm. With the decreasing of mole ratios of SnCl2·2H2O to TAA, the average sizes and the thicknesses of SnS
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nanosheet thin films are decreased. When mole ratio of SnCl2·2H2O to TAA is 1:4,
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there are some small and irregular clusters at the sample’s surface. The XRD patterns of SnS nanosheet thin films with different precursors’ mole
ratios are shown in Fig. 3d. All the XRD patterns correspond well with the orthorhombic SnS (JCPDS No. 39-0354). And the peak at 2θ = 37.68° corresponds to SnO2 (JCPDS no.46-1088), which comes from the FTO substrate. When the mole ratio of SnCl2·2H2O to TAA is 1:4, a small diffraction peak at 2θ = 26.56° appears, which belongs to sulfur powder (JCPDS no.34-0941) and accords well with the 9
ACCEPTED MANUSCRIPT irregular particles in the SEM images. The Raman spectra of SnS nanosheet thin films deposited with different mole ratios are shown in Fig. 3e. Three peaks in the as-deposited SnS thin films match well with Raman spectra of SnS.
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Table 1 shows the atomic percents of SnS thin films deposited with different precursors’ mole ratios, which are close to 1:1. The atomic percents of Sn are slightly smaller than that of S. And the atomic percents of S are increased slightly with the
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increasing of the amounts of sulfur precursor. In fact, too much TAA will cost H+ in
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the reaction solution greatly. And more Sn(OH)2 will be formed and deposited onto the bottom of the solution. Sulfur powder is also easy to appear due to lack of Sn2+ in the reaction solution. So, the thicknesses of SnS thin films decrease with the decreasing of mole ratios of SnCl2·2H2O to TAA and the atomic percents of S in SnS
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thin films do not increase greatly. Then, the mole ratio of SnCl2·2H2O to TAA = 1:1.5 is the most economic choice.
3.1.3 Influences of the deposition times
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It is important to control the thicknesses of SnS thin films, which influences the
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photoelectronic properties of SnS thin films greatly. By studying time dependent deposition process, it is also helpful to reveal the morphological evolution of the sheet-like SnS thin films. Figure 4 show the SEM images of SnS thin films deposited with different deposition times. The precursors’ mole ratio of SnCl2·2H2O to TAA = 1:1.5. The deposition temperature is 80 °C and pH = 0.4 during reaction process. When the deposition times are shorter than 20 mins, there are only some clusters, which consist of sheet-like particles. In fact, nucleations of SnS appear firstly and 10
ACCEPTED MANUSCRIPT grow perpendicular to the surface of the substrate and form sheet-like particles due to the concentration gradients, which are generated around a growing crystal. Then sheet-like particles will asemble together to form a cluster. With the increasing of
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deposition times, more and more clusters consisting of sheet-like particles will be deposited. At about the deposition times of 20 mins, compact nanosheet thin films will be formed. This reaction process is similar to the nucleation and crystal growth of
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layered metal hydroxides [25]. In Fig. 4c2-4g2, the thicknesses of the SnS nanosheet
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thin films increase from 664 nm to 5.15 µm with the deposition times increasing from 20 mins to 180 mins. The XRD patterns and Raman spectra of SnS nanosheet thin films are shown in Fig. 4h and 4i. The crystal qualities of SnS nanosheets are gradually enhanced with the increasing of the deposition times, which are indicated
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by the peak intensities of XRD patterns and Raman spectra. When the deposition times are 20 mins, the predominant diffraction peaks at 2θ = 26.57°, 37.67°, 51.42° and 65.53° correspond to SnO2 and the atomic percent of Sn to S = 53.83:46.17. It is
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because of the existence of Sn in the FTO substrates. In fact, the atomic percents of
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Sn are also influenced greatly by the FTO substrates when the deposition times are shorter than 20 mins. When the reaction times are beyond 20 mins, the XRD patterns match well with orthorhombic SnS (JCPDS no.39-0354) and the atomic percents of Sn to S are close to 1:1 in the SnS nanosheet thin films, as shown in Table 2. The contents of S increase and that of Sn decrease gradually with the increasing of deposition times. 3.2 Optical properties 11
ACCEPTED MANUSCRIPT Figure 5a1-5c1 show the UV-Vis-NIR diffuse reflection spectra of SnS nanosheet thin films deposited with different pH levels, precursors’ mole ratios and deposition times. And the optical band gaps of SnS nanosheets can be calculated by diffuse
αhν = B (hν − E g )2 1
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reflection spectra according to the Kubelka–Munk equation [26]: (5)
F (R ) = α = 1 − R )2 /(2R )
(6)
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Here α is the optical absorption coefficient, hv is the photon energy, Eg is the band
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gap energy, R is the diffuse reflectance and B is a constant relative to the material. Figure 5a2-5c2 show the direct band gaps of SnS nanosheet thin films deposited with different conditions. These values match well with the reported 1.2-1.5 eV for the orthorhombic SnS, as shown in Table 1 and Table 2. And the band-gaps decrease
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with the increasing of the thicknesses of SnS thin films. It is because of that the grain sizes of SnS nanosheet thin films increase with the increasing of the thicknesses of SnS thin films, which can be calculated by Scherrer equation [27]. And the band gaps
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decrease with the increasing of the grain size as determined by the experimental
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formula[28]: Eg = Eg(bulk) + ((Eb (2παB)2)/ (Ø 2), where αB is the exciton Bohr radius and Ø is the grain size.
3.3 Photoelectrochemical properties of SnS thin films and FTO/SnS/CdS/Pt PEC responses of the SnS thin films and FTO/SnS/CdS/Pt were measured with 0.5
M Na2SO4 electrolyte solution (pH = 0.5). The photo-induced current-time curves were measured at -0.4 V and the current-potential curves were measured from -0.4 to 0.4 V under chopped light illumination condition. 12
ACCEPTED MANUSCRIPT Figure 6a1 shows the current-potential curves of SnS thin films deposited with different deposition times. Under illumination, the photocurrents are decreased in the cathodic direction when the cathodic potential scan range is from -0.4 to 0 V, which
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indicates that SnS nanosheet thin films are p-type conductivity. At the anodic bias range of 0 to 0.4 V, there are no obvious changes in the photo-response, which also confirms the conductivity type. From the current-time curves in the Fig. 6a2, the
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photocurrent densities of SnS thin films deposited at 30, 60, 90, 120 and 180 mins are
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about 15.85, 23.85, 26.57, 31.94 and 18.07 µA·cm-2, respectively. The photocurrent densities increase with enhancing the thickness of SnS thin films until the thickness is beyond 4 µm. The photocurrent densities of SnS nanosheet thin film deposited with 120 mins (4.03 µm) are higher and more stable than that of the other samples. It is
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because of that, the photocurrents are measured in acidic solution, which is corrosive to SnS thin films. So, a relative big thickness is beneficial to the PEC properties of SnS thin films. And these chemical bath deposited SnS thin films have better
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photoelectronic properties than that prepared by using SnS nanoparticles, which are
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synthesized at about 280 °C [29, 30]. It is mainly due to the compact and sheet-like morpholgoy.
It is known that SnS nanoseet thin films without any additional layers cannot
generate high photocurrents because of the loss of electrons via recombination. In order to increase the photocurrents, the n-type CdS thin films were deposited on the as-deposited and annealed (300 °C) SnS nanosheet thin films, whose deposition times are 120 mins. Such a p-n junction (SnS/CdS) is a most efficient way to separate the 13
ACCEPTED MANUSCRIPT photogenerated charge carriers [31, 32]. And 5 nm Pt thin films were sputtered on the SnS/CdS and annealed SnS/CdS films, which were used as a hydrogen generation catalyst to enhance the surface reaction rate in water splitting and can also protect
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SnS/CdS thin film. Figure 7 shows the current-potential and current-time curves of FTO/SnS/CdS/Pt and FTO/annealed SnS/CdS/Pt thin films under chopped light. According to Fig. 7a2 and 7b2, after depositing CdS and Pt thin films, the photocurrent
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density of FTO/SnS/CdS/Pt was increased to 0.572 mA·cm-2 at -0.4 V bias potential.
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The FTO/annealed SnS/CdS/Pt films produced a photocurrent density value of 0.702 mA·cm-2. Beside the annealing process, there are two possible steps to improve the PEC properties of FTO/SnS/CdS/Pt photocathode. The first is to enhance the electron transport characteristics of SnS nanosheet thin films by doping metal ions, such as Cu,
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In and so on. The second is to enhance the currents of FTO/SnS/CdS/Pt photocathode by selecting In2S3/CdS double layer instead of CdS single layer, which has been applied successfully in FTO/Cu2ZnSnS4/SnS/CdS/In2S3/ Pt photocathode [33].
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4. Conclusions
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SnS nanosheet thin films with orthorhombic structure were prepared by CBD method. The influences of reaction conditions on the properties of the SnS nanosheet thin films were studied. According to the results, compact SnS nanosheet thin films are easy to be deposited at 80 °C with pH = 0.4. At this condition, the thicknesses of SnS nanosheet thin films can be controlled by changing the deposition times. Depositing the CdS and Pt layers can obviously increase the photocurrent densities of as-deposited and annealed SnS thin films. A highest photocurrent density of 0.702 14
ACCEPTED MANUSCRIPT mA·cm-2 has been achieved for FTO/annealed SnS/CdS/Pt. The enhanced performance of FTO/SnS/CdS/Pt photocathode is beneficial to develop the PEC water
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splitting of SnS thin films.
15
ACCEPTED MANUSCRIPT Acknowledge: This work was supported by the National Key Basic Research Program of China (973 program, Grant No. 2012CB934300), Shanghai city committee of Science and
China
(61006089),
Innovation
program
of
Shanghai
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Technology (15520500200,16010500500), National Nature Science Foundation of City
(CXSJ-13-077,
CXSJ-14-098). We acknowledge Instrumental Analysis & Research Center of
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Shanghai University for the help of characterization works.
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ACCEPTED MANUSCRIPT Figure Captions: Table 1: The atomic percents of Sn to S, the grain sizes and the band gaps of SnS thin films deposited with different pH levels and different precursors’ mole ratios.
films deposited with different deposition times.
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Table 2: The atomic percents of Sn to S, the grain sizes and the band gaps of SnS thin
Figure 1: Surface and cross-section SEM images of SnS nanosheet thin films
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deposited with different pH levels: (a1, a2) pH = 0.2, (b1, b2) pH = 0.4, (c1, c2) pH =
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0.7, (d1, d2) pH = 1.0, (e1, e2) pH = 1.5. The XRD patterns (f) and Raman spectra (g) of SnS nanosheet thin films deposited with different pH levels. Figure 2: XPS analysis of SnS nanosheet thin films (pH = 0.4, mole ratio of SnCl2·2H2O to TAA = 1:1.5, 80 °C, 120 mins): (a) survey scan, (b) S region, (c) Sn
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Figure 3: Surface and cross-section SEM images of SnS nanosheet thin films deposited with different mole ratios of SnCl2·2H2O to TAA: (a1, a2) 1:1.25, (b1, b2)
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1:2, (c1, c2) 1:4. The (d) XRD patterns and Raman spectra (e) of SnS nanosheet thin
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films deposited with different mole ratios of SnCl2·2H2O to TAA. Figure 4: SEM images of SnS nanosheet thin films deposited with different deposition times: (a1, a2) 12 mins, (b1, b2) 16 mins, (c1, c2) 20 mins, (d1, d2) 30 mins, (e1, e2) 60 mins, (f1, f2) 90 mins, (g1, g2) 180 mins. The XRD patterns (h) and Raman spectra (i) of SnS nanosheet thin films deposited with different deposition times. Figure 5: The UV-Vis-NIR diffuse reflection spectra and the curves of (αhv)2 vs. hv of the SnS nanosheet thin films deposited with different reaction conditions: (a1, a2) 22
ACCEPTED MANUSCRIPT different pH levels, (b1, b2) different precursors’ mole ratios of SnCl2·2H2O to TAA, (c1, c2) different deposition times. Figure 6: The PEC response curves of SnS nanosheet thin films with different
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deposition times at 30 mins, 60 mins, 90 mins, 120 mins, 180 mins: (a1) current-potential, (a2) current-time. Figure 7:
The current-potential and current-time response curves of SnS nanosheet
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thin films (120 mins): (a1, a2) FTO/SnS/CdS/Pt, (b1, b2) FTO/annealed SnS/CdS/Pt.
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Table1
Atomic percents
pH levels
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Grain size (nm)
Band gaps (eV)
CH3CSNH2
(mmol)
(mmol)
4
6
49.82 50.18
4
6
48.46 51.54
0.4
39.438
1.35
4
6
48.11 51. 89
0.7
38.505
1.36
4
6
47.47 52. 53
1.0
32.263
1.44
4
6
46.66 53. 34
1.5
22.143
1.52
4
5
48.63 51. 27
0.4
35.799
1.37
4
8
47.06 52. 94
0.4
33.721
1.40
4
16
0.4
26.584
1.48
of Sn:S
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SnCl2 2H2O
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46.90 53.10
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1.28
Table 2
12
57.91 42.09
-
16
54.58 45.42
-
20
53.83 46.17
30
50.52 49.48
60
49.25 50.75
90
48.71 51.29
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Grain size (nm)
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Band gaps (eV)
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-
-
25.916
1.43
33.446
1.41
33.746
1.38
38.321
1.36
38.975
1.36
Figure 1
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Figure 5
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Hightlight · SnS nanosheet thin films were deposited with simple CBD method. of the morphological evolution of SnS nanosheets was
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· Mechanism studied.
· FTO/SnS/CdS/Pt was fabricated to enhance the properties of PEC water
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splitting.