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Oct 11, 2018 - FTO/SnS/SnS2/Pt photo cathode was fabricated to enhance the photo-current densities. The photo-current densities as high as 26.7 µA/cm...
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Cite This: ACS Appl. Energy Mater. 2018, 1, 6497−6504

Facile Synthesis of SnS and SnS2 Nanosheets for FTO/SnS/SnS2/Pt Photocathode Meng Cao,*,†,§ Kefeng Yao,† Chuangsheng Wu,† Jian Huang,†,§ Weiguang Yang,† Lei Zhang,† Fang Lei,† Yan Sun,‡ Linjun Wang,† and Yue Shen*,† †

School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

ACS Appl. Energy Mater. 2018.1:6497-6504. Downloaded from pubs.acs.org by UNIV OF NORTH DAKOTA on 11/26/18. For personal use only.

‡

ABSTRACT: Two-dimensional (2D) materials have attracted great attention recently. SnS/SnS2 with sheet-like morphologies are widely used in photoelectric devices because of their excellent electronic transportation properties. In our study, by changing the mole ratios of SnCl2 and C2H5NS at reaction temperatures as high as 280−300 °C, SnS rectangular nanosheets are converted successfully to SnS2 hexagonal nanosheets with a one-pot method. SEM, XRD, Raman, and XPS analyses were utilized to examine the physical properties of the final products. Diffuse reflection spectra and PL spectra determined the bandgaps of SnS/SnS2 to be about 1.49 and 2.19 eV, respectively. The photoelectric characteristics of SnS/SnS2 nanosheets were also studied. FTO/SnS/SnS2/ Pt photocathode was fabricated to enhance the photocurrent densities. Photocurrent densities as high as 26.7 μA/cm2 were achieved. KEYWORDS: SnS, SnS2, nanosheets, photocathode, photoelectrochemical, water splitting

1. INTRODUCTION As a major pathway for renewable energy utilization, hydrogen production from photoelectrochemical (PEC) water splitting is an efficient way to solve the global energy crisis.1 SnS and SnS2, as important group IV−VI photoelectric materials, have attracted great interest over the past few years.2,3 Both of them comprise nontoxic and earth abundant elements. p-type SnS has a direct bandgap of 1.3 eV.4 Because of its orthorhombic structure, low toxicity, low cost, high conductivity, and high absorption coefficient, SnS can be used in solar cells,5 nearinfrared detectors,6 gas sensing,7 lithium ion batteries,8 photocatalysis,9 and anode materials.10 With bandgap of 2.18−2.44 eV, n-type SnS2 has also exhibited excellent photoelectric properties.11,12 Especially, SnS and SnS2 can form a p−n junction to make solar cells, which is an ideal replacement of toxic CdTe/ CdS thin film solar cells. Besides, SnS/SnS2 p−n junction can also enhance the photoelectrochemical properties of SnS photocathode. As known, PEC splitting of water has been considered as a potential solution for solving global environmental pollution and the energy crisis.13,14 SnS and SnS2 nanomaterials, such as nanosheets,15 nanowires,16 and nanorods,17 can optimize the performance of photoelectric devices due to their excellent migration of electrons and optical properties. SnS and SnS2 2D nanosheets have been synthesized through chemical vapor transport method,18 solvothermal method,19 thermal evaporation20 method, and so on. A one-pot method can also prepare high quality SnS/SnS2 nanosheets with controlled structural and morphological properties, which does not need high cost equipment. The one-pot synthesized ultralarge and thin © 2018 American Chemical Society

Figure 1. XRD patterns of SnS/SnS2 nanosheets synthesized at 280 (a) and 300 °C (b); Raman spectra of SnS/SnS2 nanosheets synthesized at 280 (c) and 300 °C (d).

SnS/SnS2 nanosheets are contributive to enhancement of their PEC properties. However, the used tin or sulfur Received: August 26, 2018 Accepted: October 11, 2018 Published: October 11, 2018 6497

DOI: 10.1021/acsaem.8b01414 ACS Appl. Energy Mater. 2018, 1, 6497−6504

Article

ACS Applied Energy Materials Table 1. Synthesis Parameters of SnS/SnS2 Nanosheets and Their Atomic Compositional Ratios composition determined by EDS (Sn:S) SnCl2·2H2O (mmol)

C2H5NS (mmol)

1.44 0.96 0.48 0.48 0.48

0.48 0.48 0.48 0.96 1.92

Sn:S(280°C) 51.03:48.97 48.91:51.09 44.33:55.67 35.26:64.74 33.20:66.80

(±0.03) (±0.03) (±0.03) (±0.03) (±0.03)

bandgap (eV)

Sn:S(300°C) 52.81:47.19 51.94:48.06 46.79:53.21 41.20:58.80 34.81:65.19

(±0.03) (±0.03) (±0.03) (±0.03) (±0.03)

1.49 1.51 1.52 2.18 2.19

± ± ± ± ±

0.02 0.02 0.02 0.02 0.02

Figure 3. TEM (a) and HRTEM (b) images of SnS nanosheets; TEM (c) and HRTEM (d) images of SnS2 nanosheets.

Figure 2. SEM images of SnS/SnS2 nanosheets synthesized with different mole ratios of SnCl2·2H2O to C2H5NS and reaction temperatures (280 °C, a1, b1, c1, d1, and e1; 300 °C, a2, b2, c2, d2, and e2).

precursors are not easily achieved, such as Sn(Ddtc)2(Phen).21 Selecting commonly used thioacetamide (TAA) as sulfur precursor, the yields of SnS/SnS2 nanosheets are also very high. They can be sprayed or spin coated onto the substrates, which is beneficial to large scale applications. However, not every sulfur precursor can prepare SnS2 with pure phase at temperatures as high as 290−300 °C due to the strong reduction characteristics

Figure 4. XPS spectra of SnS (a1, a2, and a3) and SnS2 (b1, b2, and b3) nanosheets synthesized at 300 °C. 6498

DOI: 10.1021/acsaem.8b01414 ACS Appl. Energy Mater. 2018, 1, 6497−6504

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ACS Applied Energy Materials

Figure 5. Diffuse reflection spectra of SnS/SnS2 synthesized with different mole ratios (a1, b1) and the determined bandgaps (a2, b2); PL spectra of SnS (c1) and SnS2 (c2). were dispersed in a 100 mL three-neck flask. The solution was first heated to 60 °C, degassed by Ar atmosphere for 20 min, and then heated to 280 and 300 °C for 20 min, respectively. After the reaction, SnS/SnS2 nanosheets were centrifuged with toluene and ethanol, respectively. 2.3. Characterizations. X-ray diffraction (XRD, D/MAX2500 V +/PC) and Raman (JY-H800UV) spectra examined the phase purities of SnS/SnS2 nanosheets. The morphological properties of SnS/SnS2 nanosheets were characterized by transmission electron microscopy (TEM, JEOL 2010 F) and scanning electron microscopy (SEM, FEI Sirion 200). X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) analyzed the valence states of Sn and S. UV−vis spectrophotometer (Jasco UV-570) and PL spectra (JY-H800UV) were used to study the optical properties of SnS/SnS2 nanosheets. PEC characterizations, such as electrical impedance spectroscopy (EIS), Mott−Schottky analysis, and assessement of photocurrent performances were performed by using a CHI660B electrochemical workstation. A Xe-based light source provided the light with density of 100 mW/cm2 (AM 1.5G, Newport, Oriel Instruments). A platinum wire was used as the counter electrode and Ag/AgCl rod was used as the reference electrode, respectively. Aqueous solutions of 0.5 M H2SO4 or NaOH were employed.

of the reaction solution. So, no research has been reported on the preparation of SnS/SnS2 nanoparticles at the same reaction condition by just changing the mole ratios of tin and sulfur precursors. However, our studies indicate that SnS nanosheets can be converted to SnS2 nanosheets even at temperatures as high as 300 °C by just varying the mole ratios of TAA and tin precursor, whose reaction mechanism is worthy of attentive study. In this work, one-pot synthesis of SnS/SnS2 nanosheets has been presented. Effects of the mole ratios of sulfur and tin precursors to the physical and photoelectric properties of SnS/SnS2 nanosheets have been studied in detail. SnS/SnS2 p−n junctions have been fabricated to enhance their PEC properties.

2. EXPERIMENTAL SECTION 2.1. Materials. Analytical grade SnCl2·2H2O (≥98.0%), C2H5NS (TAA, ≥ 99.0%), and oleylamine (OLA) were purchased from Sinopharm Chemical Ltd. 2.2. Synthesis. In a typical reaction, 0.48−1.44 mmol of SnCl2·2H2O, 0.48−1.92 mmol of C2H5NS, and 10 mL of OLA 6499

DOI: 10.1021/acsaem.8b01414 ACS Appl. Energy Mater. 2018, 1, 6497−6504

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ACS Applied Energy Materials

3. RESULTS AND DISCUSSION 3.1. Structural Properties. In Figure 1, XRD and Raman spectra present the structures and phase purities of SnS/SnS2 nanosheets synthesized at 280 and 300 °C. When the mole ratio of tin and sulfur precursors is 3:1, diffraction peaks that appeared at 2θ = 22.32°, 26.32°, 27.74°, 30.8°, 31.86°, 32.28°, and 39.34° in Figure 1a,b can be attributed to (110), (120), (021), (101), (111), (040), and (131) planes of SnS (JCPDS No. 39-0354), respectively. In Figure 1c,d, the phase purity of SnS is confirmed by the Raman peaks at 160, 186, and 216 cm−1.22 When the mole ratios of tin and sulfur precursors are adjusted from 2:1 to 1:2, the peak at 2θ = 15.2°, which corresponds to the (001) plane of SnS2, turns stronger gradually. The Raman peak at 315 cm−1 begin to appear, which corresponds to the phase of SnS2. It indicates that both SnS2 and SnS exist in the products. When the mole ratio of tin and sulfur precursors is 1:4, the peaks at 2θ = 15.2°, 28.24°, 30.28°,32.04°, 41.9°, 46.06°, and 50° correspond to (001), (100), (002), (101), (102), (003), and (110) planes of hexagonal SnS2 (JCPDS No. 22-0951). The Raman spectra in Figure 1c,d also confirm the phase purity of SnS2.23 Also it should be mentioned that, at the reaction temperature of 280 °C, the secondary phase of SnS2 appears when the mole ratio of tin and sulfur precursors is 2:1. At the reaction temperature of 300 °C, the secondary phase of SnS2 does not appear until the mole ratio of tin and sulfur precursors is changed to 1:1. This phenomenon indicates that even though SnS2 with pure phase can be obtained at 300 °C, they are really easy to achieve at low reaction temperatures. Due to the strong reductive environment of OLA at high reaction temperatures, SnS2 is reported to be formed at temperatures below 280 °C and SnS is facilely to be formed at temperatures as high as 300 °C.21 However, in our experiments, SnS2 could be synthesized even at the temperature of 300 °C by increasing the amount of TAA. It is worth noting that, at the temperature of 300 °C, SnS2 with pure phase cannot be achieved by using every sulfur precursor. For example, when sulfur powders are selected as the sulfur precursor, the secondary phase of SnS cannot be avoided, even though the mole ratio of SnCl2:sulfur powder is 1:6. It may be due to relative reaction rates between Sn2+ and sulfur precursors. Fast reaction rates between Sn2+ and sulfur precursors contribute to the formation of SnS2, while slow reaction rates between them may lead to a mixed phase of SnS/SnS2. The relative reaction rates of Sn2+ with the sulfur precursors may be TAA > S, which is similar to previous reports.24 But the phase properties of SnS/SnS2 nanosheets influenced by sulfur precursors still need further study. 3.2. Compositional and Morphological Properties. EDS results in Table 1 show that when the mole ratio of tin and sulfur precursors is 3:1 at 280 and 300 °C, tin rich SnS nanosheets are obtained. By increasing the amount of sulfur precursors, the contents of sulfur are increased in the products. The atomic composition ratio of Sn:S is close to 1:2 when the mole ratio of tin and sulfur precursors is 1:4. But at the same mole ratio of tin and sulfur precursors, the contents of sulfur in the products are decreased when the reaction temperature is increased from 280 to 300 °C, which confirms that Sn4+ ions are really tending to be reduced to Sn2+ ions at higher reaction temperatures. Panels a−e of Figure 2 show the morphological properties of SnS/SnS2 nanosheets synthesized with different mole ratios of

Figure 6. Nyquist plots and Mott−Schottky plots of SnS/SnS2 nanosheet thin films synthesized with different mole ratios of SnCl2· 2H2O to C2H5NS.

Table 2. Equivalent Circuit Parameters for EIS Analysis under Light Conditions SnCl2·2H2O (mmol)

C2H5NS (mmol)

Rs (Ω)

1.44 0.96 0.48 0.48 0.48

0.48 0.48 0.48 0.96 1.92

1.5 ± 0.2 2.1 ± 0.2 18 ± 0.2 23 ± 0.2 55 ± 0.2

Rct (Ω)

conductive type

± ± ± ± ±

P P P P P

10 23 30 35 50

0.2 0.2 0.2 0.2 0.2

tin and sulfur precursors at 280 and 300 °C. When the mole ratios of tin and sulfur precursors are 3:1 and 2:1, the products are mainly quadrate and rectangular nanosheets. The sizes are about 0.5 × 0.5 and 1 × 2 μm2 for quadrate and rectangular nanosheets, respectively. When the mole ratio of tin and sulfur precursors is 1:1, small particles with hexagonal shapes appear in the large rectangular shape nanosheets. Nanosheets with hexagonal shapes were obtained when the mole ratios of tin 6500

DOI: 10.1021/acsaem.8b01414 ACS Appl. Energy Mater. 2018, 1, 6497−6504

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Figure 7. Current−voltage response curves of SnS/SnS2 nanosheet thin films synthesized with different mole ratios of SnCl2·2H2O to C2H5NS: (a and b, 0.5 M H2SO4; c, 0.5 M NaOH).

and sulfur precursors are 1:2 and 1:4. The average thicknesses of the nanosheets are decreased when the temperature is increased from 280 to 300 °C. TEM images of SnS/SnS2 nanosheets are shown in Figure 3. When the mole ratio of tin and sulfur precursors is 3:1, the products are rectangular nanosheets (Figure 3a). The lattice spacing of rectangular nanosheets is 2.9 Å in the HRTEM image (Figure 3b), corresponding to the (100) plane of an orthorhombic-phase SnS. The products are mainly hexagonal nanosheets when the mole ratio of tin and sulfur precursors is 1:4 (Figure 3c). Figure 3d demonstrates the HRTEM image of hexagonal nanosheets. The lattice spacing of 3.1 Å corresponds to the (100) plane of SnS2. TEM characterizations correspond well with that of SEM characterizations. Oleylamine has important roles in the reaction process. First, SnCl2 is dissolved in oleylamine and forms a liquid metal [SnOLA] complex. Due to the formation of a N−H···Cl hydrogen bond, these complexes are stable at room temperature.25,26 With increasing reaction temperature, these complexes will release sufficient Sn2+ for nanoparticle growth. Numerous quasi-spherical SnSx nuclei are formed at this early stage of reaction, which has thermodynamically stable shapes with the lowest overall surface energy. Larger nanocrystals will grow at the cost of smaller ones in the following Ostwald ripening process, which will increase both the average sizes and the polydispersities of nanocrystals.27−29 Here, we believe that different crystal facets of SnS have different growth rates. It is because of that OLA is selectively adsorbed on these different crystalline facets. Some facets of SnS nanoparticles are completely passivated by OLA while other facets are partially or not. Then, sheet-like SnS nanosheets are easily formed.30 Decreasing OLA or increasing the reaction temperature will reduce the passivation effect, which will lead to the thickening of SnS nanosheets. During the reaction process, the amount of S2− ions released from TAA has also a great influence on the morphologies of the final products. With increasing of concentrations of S2− ions, the final products are changed from SnS with rectangular shapes to SnS2 with hexagonal shapes.31 3.3. XPS Analysis. The valence properties of the assynthesized SnS/SnS2 nanosheets (mole ratios of tin and sulfur precursors are 3:1 and 1:4; reaction temperature is 300 °C) were also investigated by XPS. Figure 4a1 is a typical XPS survey spectrum of different atoms from SnS. The binding energies obtained in the XPS analysis are standardized for specimen charging using C 1s as the reference at 284.6 eV. There are many XPS peaks from several elements: Sn 4d, Sn 4p,

Figure 8. Current−time response curves of SnS/SnS2 nanosheet thin films synthesized with different mole ratios of SnCl2·2H2O to C2H5NS: (a, 0.5 M H2SO4; b, 0.5 M NaOH).

Sn 4s, Sn 2p, Sn 2s, C 1s, Sn 3d5/2, Sn 3d3/2, Sn 3p3/2, Sn 3p1/2, and Sn 3s. The two peaks at 485.8 and 494.3 eV in Figure 4a2 can be attributed to Sn 3d5/2 and Sn 3d3/2 of SnS, respectively. The 8.4 eV gap between them agrees well with Sn2+.32,33 The peak at 161.6 eV agrees well with the energy of S 2p3/2, which indicates divalent S2− in SnS.34 In the same way, we can find the XPS spectrum of S 2p in Figure 4b3, which belongs to SnS2. The peaks at 486.4 and 162.8 eV correspond to Sn 3d5/2 and Sn 3d3/2 of SnS2, respectively, which indicates Sn4+ and agrees well with previous studies.35,36 3.4. Optical Properties. Panels a1 and b1 of Figure 5 show the diffuse reflection spectra of SnS/SnS2 nanosheet thin films. The used nanosheets are synthesized at the reaction temperature of 300 °C, and their optical bandgaps are determined by Kubelka−Munk equations.37 Panels a2 and b2 of Figure 5 indicate that the direct bandgaps are about 1.49 and 1.51 eV when the mole ratios of tin and sulfur precursors are 3:1 and 2:1, which agrees well with previous studies.38 The determined bandgap is 1.52 eV when the mole ratio is 1:1. The bandgaps are increased relatively due to some secondary phases of SnS2 existing in the products. And the bandgaps are 2.18 and 2.19 eV when the mole ratios of tin and sulfur precursors are 1:2 and 1:4, which match well with the reported 2.21−2.25 eV for hexagonal SnS2.39 At the mole ratios of 3:1, 2:1, and 1:1, their PL peaks are positioned at about 825 nm, whose bandgaps are about 1.5 eV. When the mole ratios are changed to 1:2 and 1:4, the PL peaks are 6501

DOI: 10.1021/acsaem.8b01414 ACS Appl. Energy Mater. 2018, 1, 6497−6504

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Figure 9. Current−voltage response curves (a1, b1) and current−time response curves (a2, b2) of FTO/SnS/Pt and FTO/SnS/SnS2/Pt.

films. But the change tendency is not obvious when the mole ratios of tin and sulfur precursors are 1:2 and 1:4, which agree well with the EIS analysis. As known, the PEC properties of p-type semiconductor are mostly measured in acidic solution and n-type semiconductors are usually measured in alkaline solution.41 Then, PEC properties of SnS2 thin films were measured in 0.5 M NaOH, as shown in Figure 7c. At this condition, the changes of current densities are greatly at positive bias, which confirm that the products are n-type SnS2. Under chopped light (10 s), the time-dependent currents of SnS/SnS2 thin films are measured at −0.4 V, as shown in Figure 8. In the 0.5 M H2SO4 solution, the average current densities of SnS thin films are about 6.1 μA/cm2 when the mole ratio of tin and sulfur precursors is 3:1, as shown in Figure 8a. The average current densities are decreased slightly when the mole ratios of tin and sulfur precursors are 2:1 and 1:1, respectively. Figure 8b shows the current densities of SnS2 thin films in 0.5 M NaOH. We can see the average current densities are about 5.8 and 5.6 μA/cm2 when the mole ratios of tin and sulfur precursors are 1:2 and 1:4, respectively. To enhance the current densities of SnS nanosheets thin films, the structures of FTO/SnS/Pt and FTO/SnS/SnS2/Pt were fabricated. The deposition of Pt can promote the H2 evolution reaction. And the deposition of the n-type SnS2 layer onto the surface of the p-type SnS layer can form a space charge region at the p−n junction, which will accelerate the separation of charge carriers.42 As known, the Fermi levels in p-type SnS are close to its valence band maximum (VBM), while those of n-type SnS2 are close to their conduction band minimum (CBM). Under visible-light irradiation, CBM and VBM of n-type SnS2 are situated at lower energy levels than those of

positioned at about 600 nm, which correspond to the bandgaps of 2.14 eV. The bandgaps determined from the PL spectra correspond well with that measured from the diffuse reflection spectra. 3.5. PEC Characteristics of SnS/SnS2 Nanosheet Thin Films. EIS analysis was carried out to investigate the PEC characteristics of SnS/SnS2 thin films. Impedance parameters have been analyzed by Nyquist diagram. Nyquist plots for SnS/SnS2 thin films are shown in Figure 6a1−d1. Rs is the ohmic series resistance of the electrode system which contributes to the electrical contact of the electrode− electrolyte and resistivity of the electrolyte solution.40 As shown in Table 2, Rs values for SnS/SnS2 thin film are in the range of 1−55 Ω and increase when the products are changing from SnS to SnS2. The charge transfer resistances (Rct) of SnS are also smaller than that of SnS2. The little values of Rct indicate that the charge transfer process of SnS thin films at the electrolyte/photocathode interface is faster than that of SnS2 in 0.5 M H2SO4. The Mott−Schottky plots for SnS/SnS2 thin film are shown in Figure 6a2−d2. From the Mott−Schottky plots, the best fitted straight lines give the slopes of curves. The slopes with negative photocurrents for SnS thin films confirm the p-type electrical conductivity of SnS. The slopes with positive photocurrent for SnS2 thin films confirm the n-type electrical conductivity of SnS2. Panels a and b of Figure 7 show the photocurrent characteristics of as-deposited SnS/SnS2 thin films (reaction temperature of 300 °C) in 0.5 M H2SO4 under AM 1.5 illumination. When the mole ratios of tin and sulfur precursors are 3:1, 2:1, and 1:1, the photocurrents are decreased by changing the potential scan from −0.4 to 0 V, which indicates the p-type conductivity of SnS nanosheet thin 6502

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Photocatalysts: Evidence for a Pseudotetragonal Structural Modification. J. Am. Chem. Soc. 2013, 135, 11634−11644. (5) Ramakrishna Reddy, K. T.; Koteswara Reddy, N.; Miles, R. W. Photovoltaic Properties of SnS Based Solar Cells. Sol. Energy Mater. Sol. Cells 2006, 90, 3041−3046. (6) Ohta, H.; Matsui, T. Mesoscopic SnS Junctions and Their Application to Squids and Millimeter-wave Detectors. Superlattices Microstruct. 1999, 25, 775−784. (7) Afsar, M. F.; Rafiq, M. A.; Tok, A. Two-dimensional SnS Nanoflakes: Synthesis and Application to Acetone and Alcohol Sensors. RSC Adv. 2017, 7, 21556−21566. (8) Vaughn, D. D., II; Hentz, O. D.; Chen, S.; Wang, D. H.; Schaak, R. E. Formation of SnS Nanoflowers for Lithium Ion Batteries. Chem. Commun. 2012, 48, 5608−5610. (9) Wang, L. J.; Zhai, H. J.; Jin, G.; Li, X. Y.; Dong, C. W.; Zhang, H.; Yang, B.; Xie, H. M.; Sun, H. Z. 3D Porous ZnO-SnS p-n Heterojunction for Visible Light Driven Photocatalysis. Phys. Chem. Chem. Phys. 2017, 19, 16576−16585. (10) Li, Y.; Tu, J. P.; Huang, X. H.; Wu, H. M.; Yuan, Y. F. Net-like SnS/carbon Nanocomposite Film Anode Material for Lithium Ion Batteries. Electrochem. Commun. 2007, 9, 49−53. (11) Zhang, Y. J.; Lu, J.; Shen, S. L.; Xu, H. R.; Wang, Q. B. Ultralarge Single Crystal SnS Rectangular Nanosheets. Chem. Commun. 2011, 47, 5226−5228. (12) Gedi, S.; Minnam Reddy, V. R.; Pejjai, B.; Park, C.; Jeon, C.W.; Kotte, T. R. R. Studies on Chemical Bath Deposited SnS2 Films for Cd-free Thin Film Solar Cells. Ceram. Int. 2017, 43, 3713−3719. (13) Cheng, W.; Singh, N.; Elliott, W.; Lee, J.; Rassoolkhani, A.; Jin, X. J.; McFarland, E. W.; Mubeen, S. Earth-abundant Tin Sulfde-based Photocathodes for Solar Hydrogen Production. Adv. Sci. 2018, 5, 1700362. (14) Sun, Y. F.; Cheng, H.; Gao, S.; Sun, Z. H.; Liu, Q. H.; Liu, Q.; Lei, F. C.; Yao, T.; He, J. F.; Wei, S. Q.; Xie, Yi. Freestanding Tin Disulfide Single-layers Realizing Efficient Visible-light Water Splitting. Angew. Chem., Int. Ed. 2012, 51, 8727−8731. (15) Lu, J.; Nan, C. Y.; Li, L. H.; Peng, Q.; Li, Y. D. Flexible SnS nanobelts: Facile Synthesis, Formation Mechanism and Application in Li-ion Batteries. Nano Res. 2013, 6, 55−64. (16) Liu, Y. K.; Hou, D. D.; Wang, G. H. Synthesis and Characterization of SnS Nanowires in Cetyltrimethylammoniumbromide (CTAB) Aqueous Solution. Chem. Phys. Lett. 2003, 379, 67−73. (17) Biswas, S.; Kar, S.; Chaudhuri, S. Thioglycolic Acid (TGA) Assisted Hydrothermal Synthesis of SnS Nanorods and Nanosheets. Appl. Surf. Sci. 2007, 253, 9259−9266. (18) Burton, L. A.; Colombara, D.; Abellon, R. D.; Grozema, F. C.; Peter, L. M.; Savenije, T. J.; Dennler, G.; Walsh, A. Synthesis, Characterization, and Electronic Structure of Single-crystal SnS, Sn2S3, and SnS2. Chem. Mater. 2013, 25, 4908−4916. (19) Hai, B.; Tang, K. B.; Wang, C. R.; An, C. H.; Yang, Q.; Shen, G. Z.; Qian, Y. T. Synthesis of SnS2 Nanocrystals via a Solvothermal Process. J. Cryst. Growth 2001, 225, 92−95. (20) Jamali-Sheini, F.; Cheraghizade, M.; Yousefi, R. SnS nanosheet films deposited via thermal evaporation: The Effects of Buffer Layers on Photovoltaic Performance. Sol. Energy Mater. Sol. Cells 2016, 154, 49−56. (21) Zhang, Y. J.; Lu, J.; Shen, S. L.; Xu, H. R.; Wang, Q. B. Ultralarge Single Crystal SnS Rectangular Nanosheets. Chem. Commun. 2011, 47, 5226−5228. (22) Chao, J. F.; Xie, Z.; Duan, X. B.; Dong, Y.; Wang, Z. R.; Xu, J.; Liang, B.; Shan, B.; Ye, J. H.; Chen, D.; Shen, G. Z. Visible-lightdriven Photocatalytic and Photoelectron -chemical Properties of Porous SnSx (x = 1,2) Architectures. CrystEngComm 2012, 14, 3163− 3168. (23) Gedi, S.; Minnam Reddy, V. R.; Pejjai, B.; Park, C.; Jeon, C.W.; Kotte, T. R. R. Studies on Chemical Bath Ddeposited SnS2 Films for Cd-free Thin Film Solar Cells. Ceram. Int. 2017, 43, 3713−3719. (24) Tiong, V. T.; Zhang, Y.; Bell, J.; Wang, H. X. Phase-selective Hydrothermal Synthesis of Cu2ZnSnS4 Nanocrystals: the Effect of Sulphur Precursor. CrystEngComm 2014, 16, 4306−4313.

p-type SnS. Photogenerated electrons in the condunction band of SnS will transfer to that of SnS2, and photogenerated holes will travel from the valence band of SnS2 to that of SnS.43 This band alignment of SnS/SnS2 p−n junction will accelerate the separation of photogenerated carriers, which is beneficial to enhance its PEC properties. As shown in Figure 9, the average current densities are increased to 11.2 and 26.7 μA/cm2 for FTO/SnS/Pt and FTO/SnS/SnS2/Pt, respectively. The enhanced PEC characteristics are really ascribed to the SnS/ SnS2 p−n junction. However, further studies are still needed to decrease the contact resistances of the FTO/SnS/SnS2/Pt photocathode structure. For example, the contact resistance of FTO/SnS may be decreased by deposition of an Au or Ag layer between FTO and SnS.44 The contact resistances between SnS and SnS2 p−n junction can be controlled by optimizing their interface properties, such as decreasing the interface defects between SnS and SnS2.45 The postannealing process of FTO/SnS/SnS2 is also contributive to decreasing their contact resistances. All of these are worthy of further study.

4. CONCLUSIONS SnS and SnS2 nanosheets have been synthesized at temperatures as high as 280 and 300 °C. The as-synthesized SnS nanosheets have rectangular shapes, and SnS2 nanosheets have hexagonal shapes. The bandgaps of SnS and SnS2 nanosheets were determined to be 1.49 and 2.19 eV, respectively. Under the illumination of AM 1.5, the PEC performances of SnS and SnS2 nanosheet thin films were characterized under acidic and alkaline solution. FTO/SnS/SnS2/Pt photocathode was fabricated to enhance the photocurrent densities. The highest photocurrent density of about 26.7 μA/cm2 was achieved.

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AUTHOR INFORMATION

Corresponding Authors

*(M. Cao) E-mail: [email protected]. *(Y. Shen) E-mail: [email protected]. ORCID

Meng Cao: 0000-0002-8529-7450 Author Contributions §

M.C. and J.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 11775139). REFERENCES

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DOI: 10.1021/acsaem.8b01414 ACS Appl. Energy Mater. 2018, 1, 6497−6504