Controllable Fabrication of Regular Hexagon-Shaped SnS2

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Controllable Fabrication of Regular Hexagon-Shaped SnS2 Nanoplates and Their Enhanced Visible-Light-Driven H2 Production Activity Wenli Fu, Jinming Wang, Shengyin Zhou, Renjie Li, and Tianyou Peng ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00563 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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ACS Applied Nano Materials

Controllable Fabrication of Regular Hexagon-Shaped SnS2 Nanoplates and Their Enhanced Visible-Light-Driven H2 Production Activity Wenli Fu,† Jinming Wang,† Shengyin Zhou,‡,* Renjie Li,† and Tianyou Peng†,* †

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, PR China

‡

Hubei Provincial Supervision and Inspection Research Institute for Products Quality, Wuhan 430061, PR China

KEYWORDS: SnS2 nanoplate; visible-light driven H2 production, morphology controlling, hydrothermal process, growth mechanism

1 Environment ACS Paragon Plus

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ABSTRACT: SnS2 nanoplate-like products were fabricated via a facile hydrothermal process of a mixed solution containing SnCl4 and thiourea (SC(NH2)2) without organic capping agent, and their composition, crystallinity and morphology can be adjusted by varying the SC(NH2)2/SnCl4 molar ratio. Especially, regular hexagon-shaped SnS2 nanoplates with an average size of ∼275 nm and thickness of ∼56 nm were attained when the SC(NH2)2/(SnCl4) molar ratio is 6:1. The obtained SnS2 nanoplates exhibit layered structures with exposed {001} facets and singlecrystalline feature, and its growth mechanism was proposed according to the hydrothermal time-dependent experimental results. The regular hexagon-shaped SnS2 nanoplates achieve high photocatalytic H2 production activity of 356 µmol h-1 under visible light (λ ≥ 420 nm) irradiation, much better than that of the irregular nonaoplate-like products. The higher crystallinity and fewer defects of the regular hexagon-shaped SnS2 nanoplates compared to the irregular ones can more efficiently retard the photogenerated charge recombination, while the S atoms with higher density in the exposed {001} facets might be beneficial for the formation of H-bonds with H2O molecules, which then causing good dispersity and photocatalytic activity for H2 production of the SnS2 nanoplates. These results demonstrate the potential application of SnS2 nanoplates in the photocatalytic H2 production field, and might provide guidance to the controllable syntheses of the family of MS2 photocatalysts with high-efficient H2 production property.


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INTRODUCTION Photocatalytic water splitting for H2 production over semiconductors has attracted extensive attention since it was regarded as an economically feasible approach to converting the solar energy into clean hydrogen fuel.1-4 Although TiO2 is the most extensively adopted photocatalysts for its specific properties such as low cost, nontoxicity, good stability and easy availability, its broad bandgap energy (∼3.2 eV) makes it impossible to utilize the visible light of solar radiation.4 In this context, visible-light-responsive CdS has been extensively investigated because of its bandgap energy (∼2.3 eV) corresponding well with the solar spectrum and its conduction band (CB) edge more negative than the H2O/H2 redox potential.5-8 Nevertheless, CdS is high toxicity and easy photocorrosion during the photoreaction, which greatly retard its large-scale application in the photocatalytic H2 production field.5,6 Therefore, there are still pressing needs for exploring novel metal sulfide semiconductors with suitable bandgap energy and good photostability.4,9 Due to the inexpensive, stable, excellent electronic and optical properties, some layered metal disulfides (MS2, M = Mo, W, or Sn) with a structure stacked by three atomic layers (S-M-S), which are held together by van der Waals forces, are considered to be potential materials for photocatalytic H2 production.10-12 For instance, MoS2 and WS2 are promising low-cost alternatives to the Pt cocatalyst for the H2 evolution reaction since the unsaturated S atoms on its exposed edges can strongly bind with H+ ions in the aqueous solution, which are easily reduced to H2 by the photogenerated electrons of semiconductor.13-17 Also, layered SnS2 stacked by S-Sn-S layered structural units has excellent optical and electronic properties, and has already been applied in Li- or Na-ion batteries,18-21 photodetectors,22 and supercapacitor.23 As a n-type semiconductor, SnS2 has narrower bandgap energy (1.91-2.35 eV) and better performance (such as low toxicity, good chemical and thermal stability) than CdS, which make it charming in visible-light-responsive photocatalysts.24-33 Nevertheless, the photocatalytic applications of SnS2 are mainly focused on the photodegradation of organic pollutants and photoreduction of Cr(VI),27-31 and only few literatures reported the photocatalytic H2 production activity of SnS2-related catalysts to the best of our knowledge.32,33 Recently, layered SnS2/SnS superstructures with a lattice mismatch between the two alternating layers were synthesized through a solvothermal method, whereby the formed S vacancies led to bandgap narrowing of SnS2 superstructures and an improved H2 production activity with an apparent quantum yield (AQY) of ca. 4-7% at 410



nm light irradiation.32 Also, large-size SnS2 nanosheets with diameters of 0.8-1 µm and thicknesses of ∼22 nm were prepared via a solvothermal procedure by using polyvinylpyrrolidone (PVP) as a capping and steric stabilizer agent, which exhibit higher visible-light-responsive H2 production activity of 1.06 mmol h-1 g-1 (20 mg catalyst), much higher than that of irregular SnS2 nanosheets.33 Herein, a series of SnS2 nanoplate-like products were synthesized through hydrothermally treating a mixed solution containing SnCl4 and thiourea (SC(NH2)2) without organic capping agent. It was found that the SC(NH2)2/SnCl4 molar ratio in the hydrothermal solution has a critical effect on the composition, crystallinity and morphology of the products, and regular hexagon-shaped SnS2 nanoplates with an average size of ∼275 nm and thickness of ∼56 nm were attained when the SC(NH2)2/SnCl4 molar ratio is 6:1. The obtained SnS2 nanoplates have layered structures with exposed {001} facets and single-crystalline feature, and the corresponding growth mechanism was investigated according to the hydrothermal time-dependent experimental results by maintaining the SC(NH2)2/SnCl4 molar ratio at 6:1. The resultant regular hexagon-shaped SnS2 nanoplates display better visiblelight-responsive H2 production activity than the irregular ones, and the photogenerated charge transfer/separation procedures of the SnS2 nanoplate-like products under visible light (λ ≥ 420 nm) irradiation were investigated by using photoelectrochemical behaviors and photoluminescence spectra for exploring the reasons for the differences in H2 production activity.

EXPERIMENTAL SECTION Material Preparation. SnS2 product was synthesized through a facile hydrothermal process as follows: SnCl4·5H2O (1.0 mmol) and suitable amount of SC(NH2)2 were dissolved into distilled water (40 mL) under magnetically stirring, the mixed solution was then transferred into a Teflon-lined stainless steel autoclave (volume of 50 mL) for hydrothermally treating at 190 °C for 12 h. After cooling naturally, the precipitate was collected by centrifugation, washed with deionized water and ethanol several times, and then dried at 60 °C in a vacuum oven overnight. By varying the SC(NH2)2/SnCl4 molar ratio while maintaining the SnCl4 addition amount at 1.0 mmol, a series of SnS2 products were synthesized through the same hydrothermal process. The corresponding products were denoted as SnS2-x, where x represents the SC(NH2)2/SnCl4 molar ratio in the reaction solution. For instance, SnS2-6 was derived from the reaction solution containing SC(NH2)2/SnCl4 molar ratio of 6:1.


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Pt cocatalyst was loaded on the photocatalyst as follows: SnS2-x (100 mg) and H2PtCl6 solution (0.133 mL, 0.039 M) were dispersed in methanol aqueous solution (50 mL, 20vol% methanol) with an ultrasonic bath for 20 min and then irradiated by a 500 W high-pressure Hg-lamp for 3 h under magnetically stirring. 2.0wt% Pt-loaded photocatalyst was obtained by centrifugation, washed with water and then dried at 60 °C overnight. Material Characterization. The crystal phases of the products were analyzed using a Miniflex 600 X-ray diffractometer with Cu Kα radiation at 40 kV and 15 mA and a scan rate of 4° min-1 in the range of 10° ≤ 2θ ≤ 70°. The morphology was observed using a Zeiss-Sigma field emission scanning electron microscope (FESEM). Highresolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were performed at a 200 kV field-emission electron microscope (JEM-2100(HR) with an ultrahigh-resolution pole piece. UV−vis diffuse reflectance absorption spectra (DRS) were recorded on a Shimadzu UV-3600 using BaSO4 as a reference. X-ray photoelectron spectroscopy (XPS) was measured using a Thermo Fisher ESCALAB 250Xi X-ray photoelectron spectroscope equipped with a standard and monochromatic source (Al Kα) operated at 300 W. Liquid N2 adsorption-desorption isotherms were measured on a Micrometrics ASAP 2010 system after sample was degassed at 120 °C. Fourier transform infrared (FTIR) spectra were recorded on a FTIR5700 spectrometer using KBr. Raman spectra was recorded on a RM1000 microlaser Raman spectrometer. Steady-state photoluminescence (PL) spectra were measured on a Hitachi Model F-4500 fluorescence spectrophotometer. Transient photocurrent curves were obtained using a conventional three-electrode system, whereby Pt plate and Ag/AgCl electrode were used as counter electrode and reference electrode, respectively. The work electrode was prepared by depositing catalyst suspension (1.0 mL, 1.5 g L-1) on the fluorine tin oxide (FTO)coated glass (1 × 1 cm2), which was heated at 60 °C for 1 h to volatilize the solvent and steady the catalyst. The three electrodes were immersed into Na2SO4 solution (100 mL, 0.5 M) as electron media, which was continuously purged by N2 flow for 30 min before irradiation. A 300 W Xe-lamp with 420 nm cutoff filter was employed as incident light source to investigate the photoelectrochemical behavior of the catalyst at a bias potential of 0.5 V (vs. SCE) using a CHI Model 618C electrochemical analyzer. The flat-band potential (Efb) of the product was measured according to the method reported in our previous literature.34 For example, the SnS2-6 paste was prepared by adding the corresponding product (10 mg) into a mixed solvent (0.5 mL) containing alcohol, α-terpineol and hydroxyethyl cellulose under grind and sonication for 60 min, and then the resultant paste was spread onto a clean fluorine-doped tin oxide (FTO) glass. After air-drying and



calcination at 380 °C for 1 h in Ar atmosphere, the obtained SnS2-6 film electrode was used as working electrode for the electrochemical testing in Na2SO4 solution (0.5 M) with various frequencies by using Ag/AgCl (saturated KCl solution, Eθ = 0.198 V vs. NHE) as reference electrode, and the corresponding Efb value was evaluated by the Mott−Schottky relation (C-2 = (2/eεε0Nd)[Va – Efb – kT/e]),34 in which C is the space charge layer capacitance, e is the electron charge, Nd is the electron donor density, ε is the dielectric constant, ε0 is the permittivity of the vacuum, k is the Boltzmann constant (1.38 × 10−23 J K−1), T is the temperature, Va and Efb is the applied potential and flat band potential, respectively. H2 Production Property Test. The H2 production reaction was performed in a sealed reactor with top irradiation of a 300 W Xe-lamp (PLS-SXE300, Beijing Trusttech Co. Ltd., China).8,9 In a typical run, the photoreaction was performed in aqueous suspension (50 mL) containing catalyst (3.0 mg) and ascorbic acid (AA) solution (50 mM) as sacrificial reagent. The suspension was dispersed in an ultrasonic bath for 30 min, and then irradiated from the top after thoroughly removing air. Visible light (λ ≥ 420 nm) irradiation was obtained by a 300 W Xe-lamp combined with a UV-cutoff filter, and the H2 production amount was analyzed using an online gas chromatograph (GC, SP6890, TCD detector, 5Å molecular sieve columns, and Ar as carrier gas). Apparent quantum yield (AQY) was measured under the same reaction condition but using monochromatic light illumination acquired by inserting an appropriate band-pass filter (λ = 420, 435, 450, 475, 500, 520, 550 and 570 nm, where λ ± 10 nm at 10% of peak height) ahead of the 300 W Xe-lamp, and the light intensity was calibrated using a monocrystalline silicon cell (SRC-1000-TC-QZ-N, Oriel, USA). The AQY value was estimated as follows:9,35 AQY (%) =

number of reacted electrons × 100 number of incident photons

RESULTS AND DISCUSSION Microstructures Analyses of the SnS2-x Products. By varying the SC(NH2)2/SnCl4 molar ratio in the SnCl4 (1.0 mmol) solution, a series of products (SnS2-x) were hydrothermally synthesized at 190 °C for 12 h, and their Xray diffraction (XRD) patterns are depicted in Figure 1. SnS2-0 derived from the hydrothermal solution without SC(NH2)2 shows an amorphous-like XRD pattern with very low and broad diffraction peaks at 2θ = 26.6o, 33.9o and 51.8o, which correspond to the reflections of (110), (101) and (211) planes of rutile SnO2 (JCPDS No. 41-1445),31 respectively. After adding SC(NH2)2 into the SnCl4 solution, SnS2 can be obtained during the hydrothermal process


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as shown in Figure 1. Although SnS2-1 and SnS2-2 derived from the hydrothermal solutions with SC(NH2)2/SnCl4 molar ratio of 1:1 and 2:1 still exhibit the broad diffraction peaks of rutile SnO2, some new and intensive peaks ascribable to the hexagonal SnS2 can be observed. Especially, SnS2-4 derived from the hydrothermal solution with SC(NH2)2/SnCl4 molar ratio of 4:1 exhibits more obvious and intensive peaks at 2θ = 15.0°, 28.2°, 32.1°, 41.9°, 46.1°, 50.0°, 52.4° and 55.0°, corresponding well with the reflections of (001), (100), (011), (102), (003), (110), (111) and (103) planes of hexagonal SnS2 (JCPDS No. 23-0677),33 respectively. Further enhancing the SC(NH2)2/SnCl4 molar ratio leads to SnS2-6 and SnS2-8 having higher crystallinity and crystal phase purity.

Figure 1. XRD patterns of the SnS2-x products hydrothermally synthesized from SnCl4 solutions with different SC(NH2)2/SnCl4 molar ratios at 190 °C for 12 h.

FESEM images indicate that SnS2-0 derived from the hydrothermal solution without SC(NH2)2 shows shapeless aggregates of nanoparticles (Figure 2a), which can be ascribed to the low-crystallized SnO2 nanoparticles as shown in Figure 1. SnS2-1 still contains SnO2 nanoparticles’ aggregates co-existed with some irregular large sized plate-like particles (Figure 2b), while SnS2-2 is mainly composed of large aggregates with the surfaces covered by few amorphous particles (Figure 2c). Since SnS2-2 is mainly composed of hexagonal SnS2 according to the above XRD analysis results, those large aggregates and amorphous particles might be attributed to hexagonal SnS2 and rutile SnO2, respectively. Once the SC(NH2)2/SnCl4 molar ratio is enhanced to 4:1, the corresponding product (SnS2-4) is almost entirely transformed into nanoplate-like morphology (Figure 2d). Compared to SnS2-4, SnS2-6 possesses hexagon-shaped nanoplates with more regular boundaries and slightly smaller sizes (Figure 2e). Further enhancing the molar ratio to 8:1 causes those SnS2 nanoplates in SnS2-8 having decreased ordered structure and more fragments on surfaces (Figure 2f) compared to SnS2-6. These results indicate that the SC(NH2)2 addition amount in the hydrothermal solution has a critical effect on the composition, crystal phase, crystallinity and morphology of the



SnS2-x products, and SnS2-6 derived from the hydrothermal solution with SC(NH2)2/SnCl4 molar ratio of 6:1 has more regular hexagon-shaped morphology.

Figure 2. FESEM images of the SnS2-x products hydrothermally synthesized from SnCl4 solutions with different SC(NH2)2/SnCl4 molar ratios at 190 °C for 12 h. SnS2-0 (a), SnS2-1 (b), SnS2-2 (c), SnS2-4 (d), SnS2-6 (e), SnS2-8 (f). The scale bars are 200 nm.

The low-magnification FESEM image (Figure 3a) indicates that those SnS2 nanoplates in SnS2-6 have very uniform hexagon shapes and size distribution with an average size of ∼275 nm as shown in the inset of Figure 3a. The average thickness of the SnS2 nanoplates is estimated to be ∼56 nm from those marked nanoplates (Figure 3b), and each nanoplate possesses clear boundaries and smooth surfaces, indicating that the SnS2 nanoplates of SnS2-6 have high crystallinity. Moreover, SnS2-6 exhibits a significantly stronger (001) peak than the JCPDS card of hexagonal SnS2 as shown in Figure 1, implying those SnS2 nanoplates have preferred growth direction. This conjecture can be confirmed by the TEM observation in Figure 3c. The SnS2 nanoplate is made of multi-layered structure, and the lattice-resolved HRTEM image (lower right corner) shows the three lattice spacings of 0.315 nm whose crystal facet angles between each other are ∼60°, consistent with the included angles of (010), (100) and (110) planes of hexagonal SnS2,32 respectively. The corresponding fast Fourier transform (FFT) pattern (lower left corner) resembles the diffraction pattern of a hexagonal phase along the [001] zone axis, and those bright and clear spots in the corresponding selected-area electron diffraction (SAED) pattern (top right corner) indicates the singlecrystalline feature of the SnS2 nanoplates. Therefore, it can be concluded that the exposed top and down surfaces of


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SnS2 nanoplates are {001} facets according to the hexagonal structure. Moreover, the perfectly matched FFT and SAED patterns of SnS2 nanoplates indicate a characteristic figure of hexagonal crystal system,33 and thus reveal the high crystallinity and single-crystalline feature of those SnS2 nanoplates in SnS2-6. From those XRD patterns (Figure 1) and FESEM images (Figure 2), it can be found that both SnS2-6 and SnS2-8 contain hexagon-shaped nanoplates with very similar crystallinity and crystal phase purity even though SnS2-8 has slightly decreased ordered structure and more fragments (Figure 2f) compared to SnS2-6, and thus it can be conjectured that those nanoplates in SnS2-8 also have single-crystalline feature with exposed {001} facets.

Figure 3. FESEM (a, b) and TEM (c) images of the SnS2-6 product. The lower left corner of c is the FFT pattern, the top right corner of c is the SAED pattern, and the lower right corner of c is the HRTEM image of the red square region.

The obvious changes in the composition, crystallinity and morphology of the SnS2-x products once the SC(NH2)2/SnCl4 molar ratio is enhanced to larger than 2:1 also cause some differences in specific surface area and optical absorption property. The liquid N2 adsorption-desorption isotherms (Figure S1a) indicate that SnS2-2 exhibits a type II-like isotherm with an insignificant H2-like hysteresis loop at a low partial pressure region (P/P0 = 0.42) and a H3 hysteresis loop at a high partial pressure (P/P0 > 0.90). Since the large amorphous aggregates coexist with nanoparticles in SnS2-2 (Figure 2c), those hysteresis loops of SnS2-2 can be attributed to total contribution of the stacked nanoparticles and the large aggregates. Differently, those nanoplate-like products (SnS2-4, -6 and -8) display similar type III-like isotherms with H2 hysteresis loops at high partial pressure region (P/P0 > 0.80), which can be attributed to the N2 capillary condensation in the slit pores formed by the accumulation of those nanoplates (Figure 2d-f). Moreover, SnS2-2 displays a Brunauer-Emmett-Teller (BET) specific surface area (SBET) of 40.2 m2 g-1, much higher than that of the nanoplate-like products, which have the similar SBET values (SnS2-4 (10.5 m2 g-1), SnS2-6 (10.3 m2 g-1), SnS2-8 (11.8 m2 g-1)). The Barret-Joyner-Halenda (BJH) pore size distribution plots (Figure S1b) determined from the desorption branches reveal that SnS2-2 has much smaller pore sizes than the nanoplates, which



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can be ascribed to the co-existing smaller nanoparticles covered on those large aggregates as shown in Figure 2c. Moreover, SnS2-4, -6 and -8 exhibit similar BJH pore size distribution plots and SBET values, which can be due to their similar nanoplate-like morphologies. Energy Band Structures Analyses of the SnS2-x products. The UV-vis diffuse reflectance absorption spectra (DRS, Figure 4a) of the SnS2-x products indicate that SnS2-0 has an absorption band edge (λbe) at ~342.8 nm, corresponding to an optical bandgap energy (Eg) of ~3.62 eV according to the formula (Eg = 1240/λbe), very close to the previously reported Eg (3.66 eV) of rutile SnO2.31 It is consistent with the above XRD and FESEM analysis results that SnS2-0 is low-crystallized SnO2 nanoparticles’ aggregates. However, the other SnS2-x products exhibit very similar DRS spectra since these products have almost transformed into SnS2. Taking SnS2-6 as an example, the Eg value is estimated to be ~2.21 eV according to the formula (Eg = 1240/λbe), similar to the previously reported Eg (2.25 eV) of SnS2.31 Also, very similar Eg values of ∼3.68 eV (SnS2-0) and ∼2.21 eV (SnS2-6) can be obtained from the DRS spectra of SnS2-0 and SnS2-6 (Figure S2a) according to the Tauc equation [(αhν)1/2 = A(hν − Eg), where A is constant, hν is light energy, and α is the measured absorption coefficient].33

Figure 4. (a) UV-vis diffuse reflectance absorption spectra (DRS) of the SnS2-x products; (b) The possible energy band structures of SnS2 and SnO2.

To investigate the energy band structures of the obtained SnS2 nanoplates in SnS2-6, its flat-band potential (Efb) was determined from the Mott−Schottky plots (Figure S2b). As can be seen, the Mott−Schottky plots of SnS2-6 film show positive slope, implying SnS2-6 is an n-type semiconductor.34 All plots have the same intercept with the X-axis, indicating that the Efb value is −0.81 V vs. Ag/AgCl (saturated KCl solution, Eθ = 0.198 V vs. NHE).36 Namely, the Efb value of SnS2-6 is −0.81 + 0.198 = −0.61 V vs. NHE. Generally, the bottom of conduction band (CB) is more


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negative by ca. −0.1 V than the Efb for many n-type semiconductors,37 and thus the CB level (ECB) of SnS2-6 can be estimated to be ca. -0.71 V.34,38 Accordingly, the valence band (VB) level (EVB) of SnS2-6 can be calculated to be 1.50 V from the formula (Eg = EVB − ECB).34,36 It was reported that the CB bottom of hexagonal SnS2 is mainly composed of Sn5s and S3p orbitals, and the VB top is dominated by S3p orbitals,24 while the CB bottom of rutile SnO2 is mainly composed of Sn5s and O2p orbitals, and the VB top is dominated by O2p orbitals.39 Based on the above data and the previous literatures,24,31,39 the potential energy diagrams for SnS2 and SnO2 can be drawn as Figure 4b. The lower Eg (~2.21 eV) of SnS2 with ECB level more negative than the reduction potential of H+/H2 demonstrates the thermodynamic feasibility for the photogenerated electron transfer and photocatalytic H2 production reaction under visible light irradiation, while the much larger Eg (~3.62 eV) of SnO2 implies that SnS2-0 would have no visible-light-responsive ability for H2 production. Photoactivity Analyses of the SnS2-x Products. The primary experiments indicate that Na2S/Na2SO3 mixed solution, which was usually used as sacrificial reagent in CdS-based photocatalytic systems for H2 production, is not suitable for the present SnS2 products since it will occur the reaction of SnS2 + Na2S = Na2SnS3. Therefore, ascorbic acid (AA) was chosen as a sacrificial reagent for the visible-light-driven photoactivity test by considering that SnS2 possesses favorable stability in neutral and even acid solutions. Using SnS2-6 as a reference, the photoreaction condition (Pt-loading amount and photocatalyst dosage) for H2 production were optimized as shown in Figure S3. Under the optimized photoreaction condition, the SnS2-x products show distinctly different H2 production activities under visible light (λ ≥ 420 nm) irradiation as shown in Figure 5. With enhancing the SC(NH2)2/SnCl4 molar ratio to 6:1, the corresponding SnS2-x products show an increasing trend in the H2 production activity, which then slightly decreases once the molar ratio is enhanced to 8:1. Namely, SnS2-6 exhibits the best H2 production activity among those SnS2-x products. The above changing trend of photoactivity for the SnS2-x products along with the SC(NH2)2/SnCl4 molar ratio can be ascribed to the changes in composition, crystallinity and morphology, which could influence the surface feature, optical absorption and charge carrier generation/separation processes. SnS2-0 has no activity for H2 production since it is aggregates of low-crystallized SnO2 nanoparticles which cannot be excited under λ ≥ 420 nm light irradiation due to its larger bandgap energy (∼3.62 eV) as mentioned above. SnS2-1 only achieves very limited activity for H2 production, indicating the formed SnS2 with relatively poor crystallinity has visible-light-responsive activity for H2 production, and the enhanced H2 production activity (49 µmol h-1) of SnS2-2 implies that more SnS2



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formation would be beneficial for the H2 production. Although the SBET (10.5 m2 g-1) of SnS2-4 is much lower than that (40.2 m2 g-1) of SnS2-2, the H2 production activity (110 µmol h-1) of SnS2-4 is much higher than that (49 µmol h1

) of SnS2-2. As mentioned above, the corresponding product has almost entirely transformed into SnS2 with

nanoplate-like morphology and improved phase purity and crystallinity once the SC(NH2)2/SnCl4 molar ratio is enhanced to 4:1, which might contribute the enhanced activity for H2 production. Compared to SnS2-4, SnS2-6 possesses significantly improved activity (356 µmol h-1), while SnS2-8 gives a slightly decreased one (319 µmol h-1). Namely, SnS2-6 with more regular hexagon shapes exhibits the best activity among those nanoplate-like products even though they have very similar SBET values and optical absorption properties.

Figure 5. Photocatalytic H2 production activity of the SnS2-x products. Conditions: 2wt% Pt-loaded photocatalyst in 50 mL AA solution (50 mM), visible light (λ ≥ 420 nm) irradiation.

From the steady-state photoluminescence (PL) spectra under excitation wavelength of 420 nm (Figure 6a), it can be found that SnS2-6 has the lowest PL peak intensity among those nanoplate-like products (SnS2-4, -6 and -8), demonstrating the fastest photogenerated charge transfer/separation dynamics. The higher PL peak of SnS2-8 compared to SnS2-6 implies a faster photogenerated charge recombination due to its slightly decreased crystallinity (Figure 1) and ordered structure (Figure 2e,f). Namely, the smoother exposed {001} surfaces and fewer defects of the regular hexagon-shaped SnS2 nanoplates in SnS2-6 can more efficiently retard the charge recombination, which then lead to the better activity for H2 production as shown in Figure 5. This consequence can be confirmed by the experimental result shown in Figure 6b. SnS2-6 possesses the largest transient photocurrent response among those nanoplate-like products (SnS2-4, -6 and -8) under λ ≥ 420 nm light irradiation, demonstrating that SnS2-6 has more efficient charge transfer/separation for the H2 production reaction. Although SnS2-2 has a poor H2 production activity (Figure 5), it still shows an extremely higher transient photocurrent response (Figure 6b) compared to SnS2-6,


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implying much faster charge separation process of SnS2-2. This can be due to the co-existence of SnS2 and lowcrystallized SnO2 in SnS2-2. According to the diagram of energy band structures shown in Figure 4b, the SnS2 component in SnS2-2 can be excited under λ ≥ 420 nm light irradiation, and the photo-induced electrons can transfer to the SnO2 component even though it cannot be excited at this situation, which might result in the faster charge separation and the higher photocurrent response. Nevertheless, the low crystallinity of both components (SnO2 and SnS2) in SnS2-2 would lead to severe charge recombination (it can be validated by the strongest PL peak intensity as shown in Figure 6a). Overall, the more regular morphology structure of SnS2-6 compared to the other SnS2-x products is responsible for the above phenomena. Namely, the smoother exposed {001} surfaces and fewer defects of the regular hexagon-shaped SnS2 nanoplates in SnS2-6 can more efficiently retard the photogenerated charge recombination, and thus causing more efficient charge separation (Figure 6b) and better photoactivity (Figure 5).

Figure 6. (a) Steady-state photoluminescence (PL) spectra of the SnS2-x products with excitation wavelength of 420 nm; (b) Photocurrent-time curves of the SnS2-x products under λ ≥ 420 nm light irradiation.

Photostability Analyses of the Nanoplates in SnS2-6. The apparent quantum yield (AQY) values for H2 production over 2wt % Pt-loaded SnS2-6 irradiated with various monochromatic lights are shown in Figure 7a. As can be seen, the wavelength-dependent AQY values match well with the absorption band edge of DRS spectrum of SnS2-6, and the AQY values is up to 9.43% and 3.81% at 420 and 450 nm monochromatic light irradiation, respectively. It should be noted that this AQY value (9.43%) at 420 nm is higher than that (ca. 4-7%) of the reported SnS2 superstructures at 410 nm light irradiation.32 Furthermore, SnS2-6 also exhibits favorable stability for the visible-light-responsive H2 production during the 4 runs of total 20 h irradiation (Figure 7b) even though the activity after 1 h light irradiation for each run shows a decreasing trend. SnS2-6 achieves an average H2 production activity of 238.8 µmol h-1 in the first run of 5 h photoreaction, which then slightly decreases to 232.2, 225.4 and 214.0 µmol h-1



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in the second, third and fourth run, respectively. The percentages of the average activity of the second, third and fourth run as compared with the first run are estimated to be 97.2%, 94.4% and 89.6%, respectively. It indicates that SnS2-6 has relatively good stability in the present photoreaction system.

Figure 7. (a) The apparent quantum yield (AQY) values and DRS spectrum of 2wt% Pt-loaded SnS2-6; (b) Time course of the photocatalytic H2 production over the 2wt% Pt-loaded SnS2-6. Conditions: 2wt% Pt-loaded photocatalyst in 50 mL AA solution (50 mM), visible light (λ ≥ 420 nm) irradiation.

The XPS spectra (Figure S4) of the recovered SnS2-6 after 20 h irradiation are very similar to that of 2wt% Ptloaded SnS2-6 before irradiation, indicating the element valence states of SnS2-6 are unchanged during the long-term photoreaction processes. The XPS spectra (Figure S4a) of SnS2-6 before/after irradiation only contain Sn and S elements with no evident peak of other element (for example O 1s at 531.8 eV31,33). It indicates that all Sn ions can be transformed into SnS2 during the present hydrothermal reaction condition. The high-resolution XPS spectra of Sn 3d (Figure S4b) consists of two continuous peak with binding energies at 495.2 (Sn 3d3/2) and 486.8 (Sn 3d5/2) eV, indicating the valence state of Sn is +4.31,33 Besides, the S 2p XPS spectra (Figure S4c) displays two overlapping peaks with binding energies at 162.9 (S 2p1/2) and 161.8 (S 2p3/2) eV, indicating that the S in SnS2-6 is present as −2 oxidation state.31,33 Moreover, the XRD pattern (Figure S5a) of the recovered 2wt% Pt-loaded SnS2-6 after 20 h irradiation matches well with the standard card of hexagonal SnS2 (JCPDS No. 23-0677) and shows no difference when compared with that of 2wt% Pt-loaded SnS2-6 before irradiation. It indicates that the phase composition of SnS2-6 did not change during the long-term photoreaction processes. Also, the low-magnification FESEM image (Figure S5b) shows that the recovered SnS2-6 after 20 h irradiation still maintains the hexagon-shaped morphology. The above results confirm that the SnS2 nanoplates of SnS2-6 have both high phase purity and fairly good stability for H2 production during the long-term photoreaction process.


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Overview on the Growth Mechanism of the SnS2 Nanoplates. To explore the growth mechanism of the hexagon-shaped SnS2 nanoplates, a series of SnS2-6 products were synthesized by varying the hydrothermal treatment times at 190 oC. As can be seen from the XRD patterns (Figure 8a), SnS2-6 derived from 1 h reaction time shows very low and broad diffraction peaks, which is similar to that of SnS2-0 as shown in Figure 1, and can be ascribed to rutile SnO2 (JCPDS No. 41-1445).31 It indicates that the S2− amount released from the thermal decomposition of SC(NH2)2 is not enough to cause the growth of SnS2 crystalline in a short-term reaction. The SnS26 product derived from 2 h reaction time exhibits obvious diffraction peaks of hexagonal SnS2 (JCPDS No. 23-0677) even though the broad diffraction peaks of rutile SnO2 still can be observed. Those XRD peaks of hexagonal SnS2 show an obviously increasing trend upon prolonging the reaction time from 3 to 12 h (Figure 8a), and then a slightly decreasing one along with further prolonging the reaction time from 15 to 24 h (Figure 8b). Moreover, no peak shift or other peak except for the hexagonal SnS2 can be observed from those SnS2-6 products derived from hydrothermal treatment for longer than 3 h, indicating their high crystal phase purity.

Figure 8. XRD patterns of the SnS2-6 products hydrothermally synthesized at 190 °C for 1-12 h (a) and 15-24 h (b), respectively.

Also, the hydrothermal reaction time at 190 °C has obvious effect on the microstructures of those SnS2-6 products as shown in the FESEM images (Figure 9). The SnS2-6 product derived from 1 h reaction time is mainly composed by amorphous-like structure (Figure 9a), which corresponds to the low-crystallized SnO2 nanoparticles as mentioned in the above XRD analyses (Figure 8a). The SnS2-6 product derived from 2 h reaction time changes into large and irregular plate-like particles (Figure 9b), which can be ascribed to the low-crystallized SnS2, while those co-existed amorphous structures can be due to the low-crystallized SnO2 nanoparticles based on the above XRD analyses



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(Figure 8a). Along with prolonging the reaction time from 3 to 12 h, more and more regular nanoplate-like morphologies with fewer co-existed nanoparticles are formed (Figure 9c-f). Especially, the SnS2-6 product derived from 12 h reaction time displays perfect hexagon-shaped nanoplates with clear and regular boundaries and no obvious nanoparticle coexisted on the surfaces (Figure 9f). Along with further prolonging the reaction time longer than 12 h, the boundaries of hexagon-shaped nanoplates in those SnS2-6 products tend to collapse and form more and more nanosheet-like debris on the surfaces (Figure 9g-i). Namely, a reaction time longer than 12 h would lead to the microstructures change and collapse, which would decrease the degree of crystallinity of the corresponding product, and then causing the slightly decreased diffraction peak intensities as shown in Figure 8b.

Figure 9. FESEM images of the SnS2-6 products hydrothermally synthesized at 190 °C for different reaction times. 1 h (a), 2 h (b), 3 h (c), 6 h (d), 9 h (e), 12 h (f), 15 h (g), 18 h (h), and 24 h (i). The scale bars are 100 nm.

The effects of hydrothermal reaction temperature on the crystal phase and microstructure of the SnS2-6 products were also investigated. The XRD patterns (Figure S6) indicate that the SnS2-6 product hydrothermally synthesized at 130 °C for 12 h exhibits broad diffraction peaks of rutile SnO2 and narrow ones of hexagonal SnS2, and the


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corresponding FESEM image (Figure S7a) shows that this product mainly consists of amorphous nanoparticles. With enhancing the hydrothermal reaction temperature to 160 °C, the SnS2-6 product was transformed into irregular hexagonal nanoplates co-existed with some nanoparticles (Figure S7b), which can be ascribed to hexagonal SnS2 nanoplates co-existed with some rutile SnO2 nanoparticles according to the XRD pattern shown in Figure S6. Moreover, those SnS2-6 products hydrothermally synthesized at 190 °C and 220 °C for 12 h exhibit significantly enhanced diffraction peaks of hexagonal SnS2 without no rutile SnO2 (Figure S6). The FESEM images (Figure S7c and d) indicate that those hexagon-shaped nanoplates in the SnS2-6 product hydrothermally synthesized at 220 °C have larger size and more debris than that in the SnS2-6 product hydrothermally synthesized at 190 °C. The above results demonstrate that enhancing the hydrothermal reaction temperature from 130 °C to 190 °C can promote the formation of SnS2 nanoplates with more regular shape, higher crystallinity and phase purity, while the reaction temperature higher than 190 °C would cause the microstructure change and collapse. These composition and microstructure changing trends along with enhancing the hydrothermal temperature lead to the different photocatalytic activities as shown in Figure S8, and the SnS2-6 product hydrothermally synthesized at 190 °C achieves the best H2 production activity among those products derived from reaction temperatures. For making clear the initial reaction processes of the hydrothermal solution with the SC(NH2)2/SnCl4 molar ratio of 6:1, four precursors were prepared as following: 1) SnCl4·5H2O (1.0 mmol) dissolved into 40 mL of water; 2) SC(NH2)2 (6.0 mmol) dissolving into 40 mL of water; 3) SnCl4·5H2O (1.0 mmol) and SC(NH2)2 (6.0 mmol) dissolved into 40 mL of water; 4) the mixed solution of SnCl4·5H2O (1.0 mmol) and SC(NH2)2 (6.0 mmol) was hydrothermally treated at 190 °C for 1 h. These solutions were rotary evaporated at 60 °C to remove water, and the corresponding products are labeled as P-1, P-2 and P-3, respectively. As can be seen from the FTIR spectra (Figure S9a), P-1 (SnCl4) shows the ν(Sn-Cl) deformation modes at 1624 cm-1,40 while P-2 (SC(NH2)2) exhibits relatively more complex IR absorption with four main bands, in which the IR peaks at 3382/3281/3175 cm-1 can be attributed to the ν(N-H) stretching modes, and that at 1620/1476 cm-1 to the δ(N1-H)/δ(N2-H) normal modes of the intermolecular H-bonds.41,42 Moreover, the IR peaks at 735/630 cm−1 can be assigned to the ν(C=S) stretching mode, and that at 1405/1080 cm−1 to the δ(N-H)/ν(C-N) vibration.41,42 P-3 derived from the rotary evaporation of the mixed solution (SnCl4 and SC(NH2)2) shows a FTIR spectrum very similar to that of P-2, which should mainly result from the intensive IR signal of SC(NH2)2 with higher amount. The IR peak intensities of ν(N-H), δ(N1-H) and δ(N2-H) of P-4 seriously decrease as compared to P-2 and -3, implying the partial decomposition of SC(NH2)2 in the



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hydrothermal heating for 1 h. The microscopic confocal laser Raman spectra (Figure S9b) indicate that P-2 (SC(NH2)2) has Raman peaks at 485, 1096 and 735/1388 cm−1, which can be assigned to the N-C-N asymmetric bending, C-N symmetric stretching and C=S symmetric/asymmetric stretching vibrations of SC(NH2)2,43,44 respectively. Compared to P-2, the Raman spectrum of P-3 has an extra peak at 281 cm−1, which roughly disappeared in the Raman spectrum of P-4 derived from the hydrothermal treatment at 190 °C for 1 h. Since Sn-S bond has very strong peak at ∼316 cm−1 as shown in the Raman spectrum of SnS2,45 it can be concluded that there is no Sn-S bond formation in P-4, and the new peak at 281 cm−1 of P-3 might correspond to Sn-N bond in P-3 derived from the rotary evaporation of the mixed solution since N and S atoms in SC(NH2)2 can coordinate with metal ions. The significantly decreased ν(C-N) peaks at 1080 cm−1 of P-3 and -4 compared to P-2 as shown in the FTIR spectra (Figure S9a) imply that the C-N bonds in SC(NH2)2 might combine with Sn4+. Therefore, it can be conjectured that SC(NH2)2 is initially coordinated with Sn4+ ions through N atoms in the solution, and the Sn-N bonds will split companied by SC(NH2)2 decomposition during the hydrothermal process. On the bases of the above results and discussion, the structural evolution of SnS2 nanoplates can be briefly summarized as shown in Figure 10. In the SC(NH2)2/SnCl4 solution with molar ratio of 6:1, each SC(NH2)2 molecule can coordinate with two Sn4+ ions through its N atoms to from Sn-SC(NH2)2 complex, and the interlaced H-bonds formed between H atoms and N atoms of two nearby SC(NH2)2 molecules further lead to the formation of Sandwich layered structures containing SC(NH2)2 molecules and Sn atoms layers. The orientation of the S atoms in each SC(NH2)2 molecule layer is the same, and the S atoms in the both sides of the Sn atom layer face to opposite directions (Figure 10a). During the hydrothermal process, the SC(NH2)2 molecules in the Sandwich layered SnSC(NH2)2 complexes decompose and the released S2- ions combine with Sn4+ ions to form SnS2 nucleation, which then grow into layered structures with each layer containing two S-atom layers and one Sn-atom layer (Figure 10b). After a long time of reaction, SnS2 nanoplates tend to form and grow larger undergoing the dissolutionrecrystallization-growth processes (Figure 10c), and finally SnS2 nanoplates with regular hexagon shape, high crystallinity and phase purity were fabricated through minimizing the surface energy of SnS2 and Ostwald ripening processes (Figure 10d). This conjecture can be validated from the FESEM images (Figure 9). For instance, at the initial 3 h reaction time, the layered structures of S-Sn-S embodies as the early irregular SnS2 flakes (Figure 9c), which further grow into nanoplates (Figure 9d) with prolonging the reaction time to 6 h. Finally, SnS2 nanoplates


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gradually grown into perfect hexagonal nanoplates (Figure 9f) with time prolonged to 12 h. After the reaction time longer than 12, SnS2 nanoplates tend to grow larger and collapse (Figure 9g-i).

Figure 10. Possible growth mechanism of the SnS2 nanoplates. (a) Sandwich layered structures containing SC(NH2)2 molecules and Sn atoms layers; (b) layered structures with each layer containing two S-atom layers and one Sn-atom layer; (c) SnS2 nanoplates formation/growth undergoing dissolution-recrystallization-growth processes; (d) regular hexagon-shaped SnS2 nanoplates formation undergoing Ostwald ripening process.

The hydrothermal treatment of SnCl4 solution with SC(NH2)2/SnCl4 molar ratio of 6:1 at 190 °C for 12 h is beneficial for the growth of regular hexagon-shaped SnS2 nanoplates with high crystallinity and phase purity, which also has the best H2 production activity as shown in Figure 11a. The SnS2-6 product derived from 1 h reaction time shows no activity under λ ≥ 420 nm light irradiation, implying the corresponding SnO2 product have no obvious visible-light responsive property. Whereas those SnS2-6 products derived from 2, 3, 6, 9 and 12 h reaction time achieve an increasing activity of 10, 67, 180, 273 and 356 µmol h-1, respectively. Once the reaction time prolonged to 15, 18 and 24 h, the corresponding SnS2-6 products give a gradually decreasing activity of 304, 203 and 151 µmol h-1, respectively. As mentioned above, the regular hexagon-shaped SnS2 nanoplates would collapse in some degree upon prolonging the reaction time longer than 12 h, which results in the decreased crystallinity, and then the decrease in the photocatalytic activity for H2 production.



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Figure 11. (a) Effect of the hydrothermal reaction time on the H2 production activity of the SnS2-6 products hydrothermally prepared from SC(NH2)2/SnCl4 solution with molar ratio of 6:1 at 190 °C. Conditions: 2wt% Ptloaded photocatalyst in 50 mL AA solution (50 mM), visible light (λ ≥ 420 nm) irradiation; (b) The probable mechanism for H2 evolution over the SnS2-6 product.

The above results demonstrate that the regular hexagon-shaped SnS2 nanoplates in SnS2-6 have the best activity among those nanoplate-like products. As shown in Figure 3c, the surfaces of SnS2 nanoplates are correspond to {001} facets. The high-density S atoms with two lone-pair electrons on the exposed {001} facets might be beneficial for the formation of H-bonds with H2O molecules in the acidic photoreaction solution (pH2.8) containing AA as sacrificial reagent, which then induce good dispersity and better photocatalytic activity for H2 production of the SnS2 nanoplates. This conjecture can be validated by the pH value-dependent photoactivity experimental results of SnS2-6 shown in Figure S10, whereby the pH value of the photoreaction AA solution (initial pH2.8) was adjusted to 4.0, 5.0, or 7.0 by adding NaOH solution (1.0 M). As can be seen, the H2 production activity of SnS2-6 exhibits obviously decreasing trend along with the photoreaction solution’s pH enhancing from 2.8 to 7.0. It means that the H-bonds between higher density of S atom and H2O molecules would be gradually destroyed when the acidity of the photoreaction solution continues to decrease, which then leads to the decreased photoactivity. That is to say, the S atoms on the exposed {001} facets of SnS2-6 can not only form H-bonds with H2O molecules, but also can promote the photogenerated electrons transferring to the Pt co-catalyst since the CB bottom of SnS2 is mainly composed of Sn5s and S3p orbitals, and then to the surface-adsorbed water for the H2 production reaction (Figure 11b). Moreover, the higher crystallinity and fewer defects of the regular hexagon-shaped SnS2 nanoplates in SnS2-6 compared to the irregular ones can more efficiently retard the photogenerated charge recombination, which then causing the efficient H2 production under visible light irradiation.


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CONCLUSIONS A series of SnS2 nanoplates were synthesized through hydrothermally treating a mixed solution of SnCl4 and SC(NH2)2 without organic capping agent. It was found that the SC(NH2)2/SnCl4 molar ratio and hydrothermal reaction condition have critical effects on the composition, crystallinity and morphology of the SnS2 products, and regular hexagon-shaped SnS2 nanoplates with an average size of ~275 nm and thickness of ∼56 nm were attained when the SC(NH2)2/SnCl4 molar ratio is 6:1. The obtained regular hexagon-shaped SnS2 nanoplates with exposed {001} facets and single-crystalline feature display better visible-light-responsive H2 production activity than the irregular SnS2 nanoplates. The higher crystallinity and fewer defects of the regular hexagon-shaped SnS2 nanoplates compared to the irregular ones can more efficiently retard the photogenerated charge recombination, while the S atoms with higher density in the exposed {001} facets might be beneficial for the formation of H-bonds with H2O molecules, which then causing good dispersity and photocatalytic activity for H2 production. The present results provide new insights into the significance of crystallinity and exposed facets in the visible-light-responsive activity of SnS2, and might provide guidance to the controllable syntheses of the family of MS2 photocatalysts with highefficient H2 production property.

ASSOCIATED CONTENT Supporting Information. Liquid N2 adsorption-desorption isotherms and BJH pore size distribution curves of the SnS2-x products. Tauc and Mott-Schottky plots of the SnS2-6 product. Effects of Pt-loading amount, catalyst dosage, and photoreaction solution’s pH value on the H2 production activity of SnS2-6 under λ ≥ 420 nm light irradiation. Effects of hydrothermal temperature on the composition, crystallinity, morphology and H2 production activity of the SnS2-6 products. XRD pattern and low-magnification FESEM image f 2wt% Pt-loaded SnS2-6 after 20 h irradiation. XPS spectra of 2wt% Pt-loaded SnS2-6 before and after 20 h irradiation. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION



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Corresponding Author * E-mail: [email protected] (T. Y. Peng); [email protected] (S. Y. Zhou). Author Contributions The manuscript was written through contributions of all authors. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (21573166 and 21271146), the Funds for Creative Research Groups of Hubei Province (2014CFA007), and the Natural Science Foundation of Jiangsu Province (BK20151247), China.

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Controllable Fabrication of Regular Hexagon-Shaped SnS2

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