Different morphologies SnS2 supported on 2D g-C3N4 for excellent

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Different morphologies SnS2 supported on 2D g-C3N4 for excellent and stable visible light photocatalytic hydrogen generation Liquan Jing, Yuanguo Xu, Zhigang Chen, Minqiang He, Meng Xie, Jie Liu, Hui Xu, Shuquan Huang, and Huaming Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04792 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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ACS Sustainable Chemistry & Engineering

Different morphologies SnS2 supported on 2D g-C3N4 for excellent and stable visible light photocatalytic hydrogen generation Liquan Jing,† Yuanguo Xu,†* Zhigang Chen,† Minqiang He,† Meng Xie,‡ Jie Liu,† Hui Xu,§ Shuquan Huang,† and Huaming Li, †,§* † School of Chemistry and Chemical Engineering, School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, PR China. ‡ School of Pharmacy, Jiangsu University, Zhenjiang 212013, PR China.

§ Institute for Energy Research, Jiangsu University, Zhenjiang 212013, PR China. *E-mail: [email protected]; [email protected]

Abstract Highly efficient different morphologies SnS2 (nanoparticles, nanosheets and 3D flower-like)/g-C3N4 composites were respectively prepared via an elementary hydrothermal method that was integrated with the calcination means. The result of XRD showed that the relative intensities of several diffraction peaks, especially (001), (100), (101) and (102), indicating that the as-prepared samples (SnS2 nanoparticle, SnS2 nanosheet and 3D flower-like SnS2) should be dominated by different specific preferred growth of facets. In comparison with SnS2 and pure g-C3N4, the SnS2/g-C3N4 composites exhibited much higher H2 development performance under visible light irradiation in presence of Na2SO3 and Na2S as the sacrificial agent. The SnS2 nanoparticles/g-C3N4 composites exhibit the highest visible-light-driven H2-generation rate of 6305.18 µmol h-1 g-1 without any noble-metal as co-catalyst, which is approximately 16.98 times higher than that of SnS2 nanoparticles. In all, SnS2 nanoparticle (SnS2 nanoparticle/g-C3N4 composite) dominated by (001) and (100) preferred growth of facets exhibit significant photocatalytic activity resulting from their suitable band edges to realize the photocatalytic redox reaction. The analysis of photocurrent response, linear sweep voltammograms and photoluminescence demonstrated that the low recombination rate and the efficacious charge transfer of photogenerated carriers could be assigned to the interactive impact of g-C3N4 and SnS2.

Keywords: SnS2; different morphologies; g-C3N4; photocatalytic; Hydrogen evolution Introduction Nowadays, rapid industrial growth and overuse of fossil fuels have led to the energy crisis and environmental pollution. Thus, hydrogen (H2) is considered as an environmentally-friendly combustion product to idealize substitution for fossil fuels.1,2 Recently, the utilization of semiconductor photocatalysts for H2 production has been recognized as one of the most promising technologies.3,4 Moreover, the design and construction of high-efficiency noble-metal-free photocatalysts are quite appealing with the aim of providing sustainable and cost-competitive H2. In this sense, graphite-like carbon nitride (g-C3N4), as an engrossing visible-light-driven photocatalyst has entertained the dramatically attention that increases interest in visible-light-driven H2-generation by reason of appropriate conduction band level for water reduction/oxidation, the unique two-dimensional (2D) layered structure and high chemical stability.5,6 Even through, the photocatalytic performance of g-C3N4 is greatly limited by its depressed visible light utilization, miserable electric conductivity, stupid charge flexibility and

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rapid recombination rate of photogenerated carriers.7-9 To conquer the above-mentioned issues, a variety of methods had been developed to improve catalytic performance, including textural property design, metal or nonmetal deposition and constructing semiconductor heterojunctions, etc.10-13 Among various strategies, g-C3N4-based heterojunctions are critical for hydrogen production due to its high effectiveness in improving the rate of electron-hole pair separation. So far, the manipulation of composite heterojunctions coupled with g-C3N4 has already been designed for water splitting catalysts, such as g-C3N4/CoO,14 Co0.5Cd0.5S/g-C3N4,15 Nb2O5 microspheres/g-C3N4,16 CdS/g-C3N4/CuS,17 g-C3N4/SiC,18 CuS/g-C3N4,19 CdIn2S4/g-C3N4,20 CdLa2S4/mesoporous g-C3N4,21 CdZnS quantum dots/2D g-C3N4,6 MoS2/pyridine-modified g-C3N4,22 MoS2/g-C3N4,23 CoTiO3/g-C3N4,24 Mn0.8Cd0.2S/g-C3N4,25 FeOx/g-C3N4,26 and so on. Therefore, composite photocatalyst is a very effective means to improve its photocatalytic properties Among above various semiconductors, metal sulfide, because of the appropriate band gap and catalytic function, has been received significant attention in photocatalytic H2 production.27-31 Tin sulfide (SnS2), as an n-type IV−VI semiconductor with a layered cadmium iodide (CdI2) constitution and a comparatively narrow-minded band gap (2.0–2.5 eV),32, 33 sandwiched tin atom between two layers of hexagonally disposed close-packed sulfur atoms possesses the interesting applications in photocatalysis, solar cell, and anode materials and so on.34-41 As a promising visible-light-driven photocatalyst, SnS2 has shown great ability in the removal of organic pollutants and aqueous Cr6+ and photocatalytic water splitting.31, 42-45 Recently, Yu et al. 45 have prepared monodispersed SnS2 nanosheets for high efficient photocatalytic hydrogen generation, and the photocatalyst showed high photocatalytic activity with the H2 evolution rate of 1.06 mmol h−1 g−1 under visible light irradiation. Huang et al. 31 have reported SnS2 nanoplates that was reigned by using {101} exposed facets display substantial photocatalytic H2 evolution that results from their appropriate band edges to actualize the photocatalytic redox reaction. According to our knowledge, there have been reports of SnS2/g-C3N4 composites for photocatalytic degradation of organic pollutants.46-51 However, no previous study regarding the application of different morphologies (different specific preferred growth of facets) SnS2/g-C3N4 hybrid photocatalyst for H2 production has been reported to date. In this work, different morphologies SnS2 (nanoparticles, nanosheets and 3D flower-like SnS2)/g-C3N4 composites were respectively made via an elementary hydrothermal process along with the calcination means. As-prepared samples (SnS2 nanoparticle, SnS2 nanosheet and 3D flower-like SnS2) should be dominated by different specific preferred growth of facets, which result in different photocatalytic behaviours in photocatalytic hydrogen evolution. The contrast of interactive impacts between g-C3N4 and different morphologies SnS2 was studied by XPS analysis. Their photocatalytic activities were examined for water splitting under sunshine irradiation. Additionally, conceivable mechanism of SnS2/g-C3N4 systems in the photocatalytic action were dealt with and put forward in particular.

Experimental section Synthesis of photocatalysts All chemicals (analytical grade purity) used in this research were purchased from Aladdin (Shanghai) Chemistry Co., Ltd. The bulk g-C3N4 powders were prepared via pyrolysis of urea in a tube furnace. In a typical steps, 10 g of urea was placed into a porcelain boat with cover and heated to 550 °C for 2 h with 20 °C/min. After cooling naturally, the final g-C3N4 yellow powder


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was obtained. Synthesis of SnS2 nanoparticle: Briefly, 2 mmol of SnCl4·5H2O and 4 mmol of thioacetamide (TAA) were dispersed in a 50 mL Teflon-lined stainless steel autoclave filled with 40 mL of deionized water with stirring for 30 min, the autoclave was sustained at 180 ◦C for 12 h and left to air-cooled to opportunity temperature. The yellow precipitates were collected, washed with absolute ethanol and deionized water for several times, and finally dried at 80 ◦C for 12 h to get the SnS2 nanoparticles. Synthesis of SnS2 nanosheet: 1 mmol of SnCl4·5H2O and 5 mmol of citric acid (CA) were dispersed in 40 mL of deionized water with stirring for 30 min, followed by adding 0.6 g thiourea with stirring for an additional 30 minutes. After transferring the mixture into a 50 mL Tenflon-lined stainless steel autoclave sustained at 180 ◦C for 12 h, then air-cooled to opportunity temperature. The yellow suspension was collected, rinsed with deionized water and ethanol for several times, and finally dried at 80 ◦C for 12 h. Synthesis of 3D flower-like SnS2 microspheres: 2 mmol of SnCl4·5H2O and 4 mmol of thioacetamide (TAA) in 40 mL polyethylene glycol (PEG-600). After stirring to form a transparent solution, the above mixture was moved into a Teflon-lined stainless steel autoclave and then sustained at 180 oC for 12 h. After solvent thermal reaction, the brown sample was collected, washed with ethanol and deionized water for several times and eventually dried at 80 oC for 12 h. The fabrication of SnS2/g-C3N4 composites was carried out via a simple heat treatment method. Typically, a certain amount of g-C3N4 and SnS2 were dispersed completely into 5 mL of ethanol with ultrasonic, and then stirred at room temperature for 10 h. The residual ethanol was dried in a vacuum oven at 80 ◦C. At last, the obtained yellow powder was heated to 400 ◦C with 2 ◦C min−1 and kept for 2 h in argon, resulting in the SnS2/g-C3N4 composites. Characterization The crystal structure and phase purity of the obtained samples was performed using Powder X-ray diffraction (XRD, D/MAX-2550, = 0.15418 nm) with monochromatized Cu K radiation. Infrared spectra were recorded on KBr pellets using a FT-IR spectrophotometer. The morphologies structures of the obtained samples were measured by Scanning electron microscopy (SEM; XL-30 ESEM FEG,) and transmission electron microscopy (TEM, JEOL JEM-2010). The surface analysis of the samples were tested by X-ray photoelectron spectroscopy (XPS PHI ESCA-5000C). UV−vis diﬀuse reflectance spectra (DRS) over a range of 200–800 nm were measured by a UV–vis spectrophotometer (Shimadzu UV-2401). The photocurrent measurements were executed on a CHI660E electrochemical apparatus in a standard three-electrode system with Pt as counter electrode and Ag/AgCl as the citation electrode. The working electrode was made and Na2S (0.5 M) aqueous solution was used as the electrolyte. The photoluminescence (PL) spectra of the samples were acquired utilizing a Varian Cary Eclipse spectrometer with an excitation wavelength of 325 nm at opportunity temperature. Photocatalytic activity measurement The photocatalytic hydrogen evolution experimentations were executed in a 100 mL quartz reactor that was been connected with retreat system and a shut gas circulation employing a 300 W Xenon lamp (Beijing, PLS-SXE300C, Perfect light) coupled with a cut-off filter (λ > 400 nm). In a characteristic procedure, 50 mg of samples was distributed in 100 mL of aqueous solution that contained 0.35 M Na2SO3 and 0.25 M Na2S. The reaction container was emptied for 30 min to empty melted oxygen in front of the experiments by N2. The reaction cell was maintained at



opportunity temperature utilizing cooling water. The generated H2 was observed on an on-line gas chromatography (GC7900, Shimadzu, Japan, TCD, with N2 carrier and 5 Å molecular sieve column).

Results and discussions Characterization Fig. 1 shows the XRD patterns of g-C3N4, different morphologies SnS2 and SnS2/g-C3N4 composites. The C3N4 sample exhibits a strong diffraction peak at 27.3o and a feeble peak at 13.3o, which corresponds to the (002) and (100) crystal faces of g-C3N4, respectively (JCPDS 87-1526).47 For the different morphologies SnS2 sample, diffraction pattern of SnS2 nanoparticle sample can be indexed as (001), (100), (101), (102), (110), (111), (200) and (201) planes of the 2T-type hexagonal berndtite SnS2 structure (JCPDS 23-0677).52 And diffraction patterns of SnS2 nanosheet and 3D flower-like SnS2 samples can be matched with the (001), (100), (002), (101), (102), (003), (110), (111), (103), (200), (201), (004), (202) and (113) planes of the 2T-type hexagonal berndtite SnS2 structure. Moreover, contrast the diffraction patterns of SnS2 nanoparticle, SnS2 nanosheet and 3D flower-like SnS2, and there is no diffraction peak of other phases. However, it can be seen that the intensity of the preferred growth of crystal surface of the three topographies is not the same. The relative intensities of several diffraction peaks, especially (001), (100), (101) and (102), change distinctly as a result of the preferential orientation of crystals, indicating that the as-prepared samples (SnS2 nanoparticle, SnS2 nanosheet and 3D flower-like SnS2) should be dominated by different specific preferred growth of facets. Huang et al. 31 have reported that SnS2 samples dominated by (101) and (100) preferred growth of facets possess different band-energy diagrams, which result in different photocatalytic behaviours in photocatalytic hydrogen evolution. For the composites, it can be seen that the diffraction patterns of g-C3N4 and SnS2 are present in all the SnS2/g-C3N4 composites, which indicates that different morphologies SnS2 and g-C3N4 have existed in the composites by using the ethanol that stirs calcination and dispersion.

SnS2(flower) SnS2(nanosheet)

Intensity (a.u.)

SnS2(nanoparticale) SnS2(flower)/g-C3N4 SnS2(nanosheet)/g-C3N4

10

20

30

(102) (003) (110) (111) (103)

g-C3N4 (101)

(100) (002)

SnS2(nanoparticale)/g-C3N4 (001)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40

50

JCPDS 23-0677

60

70

80

2 Theta (degree) Fig. 1. XRD patterns for g-C3N4, different morphologies SnS2 and SnS2/g-C3N4 composites.


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Fig. 2 shows the FT-IR spectra of g-C3N4 and different morphologies SnS2/g-C3N4 composites. For pure g-C3N4, the peaks at 1243, 1324, 1422, and 1575 cm-1 are attributed to aromatic C-N stretching vibration modes and the peak at 1644 cm-1 is attributed to C=N stretching vibration modes. Besides, the peak at 813 cm-1 is associated with s-triazine units' distinctive breathing style.47 For different morphologies SnS2/g-C3N4 composites, the typical peaks of g-C3N4 were detected, suggesting that the structure of g-C3N4 is not destroyed by the introduction of SnS2 and still exist in the composites. For pure SnS2, its FT-IR spectra was not strong, and its FT-IR absorption peak could not be seen in the complexs. Moreover, the presence of g-C3N4 and SnS2 will be further tested by means of SEM, TEM and XPS.

SnS2(flower)/g-C3N4 SnS2(nanosheet)/g-C3N4 SnS2(nanoparticale)/g-C3N4 g-C3N4

T (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4000

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-1

Wavenumber (cm ) Fig. 2. FT-IR spectra of g-C3N4, different morphologies SnS2/g-C3N4 composites. The microstructure and morphology of all the samples were investigated through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. 3A and Fig. S1, the pure g-C3N4 shows a typically layered structure, which suggests that it possesses stacked graphitic-like structure of one or more layers.53 The SEM image of SnS2 nanoparticle (Fig. 3B) exhibits highly uniform, homogenous and irregular particle with diameters of about 50 nm. For SnS2 nanoparticles/g-C3N4 composites (Fig. 3C), it is clearly observed that SnS2 nanoparticles were immobilized on the surface of g-C3N4. The morphology of the SnS2 nanosheet shows (Fig. 3D) 2D hexagonal plates with a lateral size of ca. 1µm. As shown in Fig. 3E, it can be observed that amounts of SnS2 nanosheets are dispersed on the surface of g-C3N4. For pure SnS2 flower, Fig. 3F displays a random arrangement of three-dimensional flower-like structure. However, when a certain amount of 3D flower-like SnS2 was introduced to combine with g-C3N4, the obtained 3D flower-like SnS2/g-C3N4 composites exhibited a perfect 3D flower-like nanostructure embedded in the layered structure of g-C3N4 and supported on the surfaces of that. Such tight contacts can be further proved by TEM observation. Fig. 4A shows that the pure g-C3N4 is a typical two-dimensional layered structure, which is in agreement with the results of previous SEM image (Fig. 3A). The TEM of single SnS2 nanoparticle (Fig. 4B) shows a flaky formation with a lateral size of 20-50 nm, meanwhile, the thickness of the SnS2 nanoparticles measured by AFM (Fig. S2) is only about 15-35



nm. Fig. 4C displays that the SnS2 nanoparticles are uniformly immobilized on the g-C3N4 surface with excellent dispersion. The TEM image (Fig. 4D) of SnS2 nanosheet reveals a lamellar morphology, which is obviously consistent with SEM results (Fig. 3D). For SnS2 nanosheets/g-C3N4 composites, SnS2 nanosheets are homogeneously dispersed on the g-C3N4 layer (Fig. 3E). As depicted in Fig. 4F, it can be seen that the morphology of SnS2 shows a perfect 3D flower-like nanostructure composed of many nanosheets, which is accordance with the result of SEM analysis (Fig. 3F). Fig. 4G indicates that the 3D flower-like SnS2 are affiliated to the surface of g-C3N4. In summary, contrast the different morphologies SnS2/g-C3N4 composites, the above results reveal that SnS2 nanoparticle disperse on the g-C3N4 uniformly, which would efficiently strengthen the interaction between SnS2 nanoparticles and g-C3N4 and be better combined together.


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Fig. 3 SEM images of the as-synthesized products: (A) g-C3N4, (B) SnS2 nanoparticles, (C) SnS2 nanoparticles/g-C3N4 composites, (D) SnS2 nanosheets, (E) SnS2 nanosheets/g-C3N4 composites, (F) 3D flower-like SnS2 and (G) 3D flower-like SnS2/g-C3N4 composites



Fig. 4. TEM images of the as-synthesized products: (A) g-C3N4, (B) SnS2 nanoparticles, (C) SnS2 nanoparticles/g-C3N4 composites, (D) SnS2 nanosheets, (E) SnS2 nanosheets/g-C3N4 composites, (F) 3D flower-like SnS2 and (G) 3D flower-like SnS2/g-C3N4 composites X-ray photoelectron spectroscopy (XPS) was utilized to analyze the chemical state and surface chemical substance composition of the as-fabricated products. Fig. 5 was the XPS survey spectra for different samples and corresponding C 1s, N 1s, Sn 3d, S 2p high-resolution spectra. Fig. 5A shows the XPS survey spectra of g-C3N4, SnS2 nanoparticles/g-C3N4 composites, SnS2 nanosheets/g-C3N4 composites and 3D flower-like SnS2/g-C3N4 composites. Compared with g-C3N4 and different morphologies SnS2/C3N4 composites, C and N are the common elements of the above samples and the survey spectra of the composites show two obvious peaks of Sn 3d and S 2p. Three obvious peaks at 284.7, 286.5 and 288.3 eV could be attributed to the adventitious hydrocarbon, C=N or C≡N coordination and C-N bonds from sp2−hybridized carbon atoms present in the C1s high-resolution spectra of g-C3N4 (Fig. 5B), respectively.54-57 The N 1s spectrum of g-C3N4 (Fig. 5C) can be partitioned into three peaks centered at 398.8, 400.1, 401.1 and 404.4 eV, corresponding to the nitrogen atoms in the aromatic rings (C=N-C), tertiary nitrogen (N–(C)3), C-N-H and π-excitation, respectively.58-60 Comparing to g-C3N4, the N 1s and C 1s peaks of different morphologies SnS2/g-C3N4 composites exhibit a different degree of shift, suggesting there is interaction between g-C3N4 and SnS2 to form the heterojunction. The different morphologies SnS2 samples show Sn 3d5/2 and Sn 3d3/2 level (Fig. 5D-F) with binding energies at 486.6-487.1 eV, 495.0-495.6 eV, respectively. The binding energies are close to that reported for SnS2.61 The S 2p3/2 and S 2p1/2 level of all SnS2 samples (Fig. 5G-I) shows a symmetric peak close to 162.0 eV and


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163.0 eV, respectively, which is characteristic of S2− species.62 Compared with different morphologies SnS2 samples, the Sn 3d and S 2p spectra of different morphologies SnS2/g-C3N4 composites have a certain degree of deviation. The above results demonstrated that the presence of chemical interaction force between g-C3N4 and SnS2, rather than physical mixture. Combined with the result of XRD and IR, the composite photocatalyst has been successfully synthesized.

A

g-C3N4 SnS2(nanoparticale)/g-C3N4

N 1s

Intensity (a.u.)

Intensity (a.u.)

C 1s SnS2(flower)/g-C3N4

SnS2(flower)/g-C3N4 Sn 3d

S 2p

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C 1s


SnS2(particale)/g-C3N4

g-C3N4

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Binding Energy (eV)

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SnS2(nanoparticale)/g-C3N4

Intensitt (a.u.)

SnS2(flower)/g-C3N4

SnS2(nanosheet)/g-C3N4 SnS2(particale)/g-C3N4

Intensity (a.u.)

SnS2(nanoparticale)

g-C3N4 392

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Binding Energy (eV)

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Binding Energy (eV)

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SnS2(flower)/g-C3N4


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Intensity (a.u.)

SnS2(nanosheet)

Intensity (a.u.)

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Binding Energy (eV)


500


H

S 2p

SnS2(nanoparticle) /g-C3N4

S 2p

SnS2(nanosheet)

Intensity (a.u.)

G Intensity (a.u.)

/g-C3N4

SnS2(nanoparticle)

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SnS2(nanosheet) 156

158

Binding Energy (eV)

I

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Binding Energy (eV)

S 2p

Intensity (a.u.)

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SnS2(flower)/g-C3N4

SnS2(flower) 156

158

160

162

164

166

168

Binding Energy (eV)

Fig. 5. XPS spectra of g-C3N4 and different morphologies SnS2/g-C3N4 composites: (A) survey; (B) C 1s; (C) N 1s; (D), (E) and (F) Sn 3d; (G) (H) and (I) S 2p. The optical performance of g-C3N4, different morphologies SnS2 and SnS2/g-C3N4 composites were analyzed by DRS technique and the results are demonstrated in Fig. 6. As illustrated in Fig. 6A, the absorption edge of pure g-C3N4 is approximately 450 nm and its absorption range extends from the UV to the visible. For SnS2 nanoparticle, SnS2 nanosheet and 3D flower-like SnS2, these absorption edge is about 610, 575 and 600nm, respectively. Comparing the absorbance of the different morphologies SnS2, SnS2 nanoparticle shows the strongest and widest absorption range from the UV to the visible. The SnS2/g-C3N4 composites display increased visible light absorption ability than pure g-C3N4, which are imputed to the interaction between g-C3N4 and different morphologies SnS2 in the composites. The interaction in all probability plays a significant position in ameliorating the photogenerated carriers' separation, and finally enhancing the photocatalytic activity. Single semiconductor's optical band gaps can be ascertained through DRS.63 As displayed in the plots of (αhv)2 versus (hv) in Fig. 6B, the band gaps of sample at g-C3N4, SnS2 nanoparticle, SnS2 nanosheet and 3D flower-like SnS2 are calculated to be 2.87, 2.25, 2.32 and 2.21 eV, respectively.


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A

1.2

B

SnS2(flower) SnS2(particle)

1.0

SnS2(nanosheet)

10

8

0.8


6

(ahv)

2

SnS2(particale)/g-C3N4 g-C3N4

0.6

SnS2(nanosheet) SnS2(flower)

4 2.25 eV

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2.21 eV

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0 1.5

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g-C3N4 SnS2(nanoparticale)

SnS2(flower)/g-C3N4

Absorbance

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2.0

Wavelength (nm)

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hv (eV)

Fig. 6. UV–vis diffuse-reflectance spectra of g-C3N4, different morphologies SnS2 and SnS2/g-C3N4 composites. The photocatalytic H2-production over the different morphologies SnS2 and SnS2/g-C3N4 composites was tested under visible light irradiation (λ > 400 nm) with Na2S and Na2SO3 as sacrificial agents to devour photoinduced holes. Fig. 7A and 7B displays the H2-generation rates with the samples of pure g-C3N4, different morphologies SnS2 and SnS2/C3N4 composites as photocatalysts. The photocatalytic H2-production rate of pure g-C3N4 is 106.82 µmol h-1 g-1 with Pt (3 wt%) as co-catalyst. For SnS2 nanoparticle, SnS2 nanosheet and 3D flower-like SnS2, their photocatalytic rates are 371.27, 318.97 and 103.16 µmol h-1 g-1 without Pt, respectively. This indicates that changes in the morphology and structure of SnS2 make an important effect on the photocatalytic performance towards hydrogen production. However, after hybridization of g-C3N4 with different morphologies SnS2, the different morphologies SnS2/g-C3N4 composites exhibited a high performance of H2-generation without noble-metal as cocatalyst. The H2-generation rates of SnS2 nanoparticles/g-C3N4 composites, SnS2 nanosheets/g-C3N4 composites and 3D flower-like SnS2/g-C3N4 composites are 6305.18, 5112.12 and 2336.65 µmol h-1 g-1, respectively. It can be seen that SnS2 nanoparticles/g-C3N4 composite exhibits the highest H2-generation rate, which is approximately 16.98 times higher than that of SnS2 nanoparticles. The reusability and stability of the SnS2 nanoparticles/g-C3N4 composite photocatalysts were tested through the cycling hydrogen evolution experimentation and the consequences are demonstrated in Fig. 7C, which indicates that the H2 production of SnS2 nanoparticles/g-C3N4 does not display obvious degradation after three consecutive cycles, which shows that the SnS2 nanoparticles/g-C3N4 composite sample has sufficient stability during photocatalytic reactions. In addition, the analysis of the TEM images of the composites with the three morphologies after recycling photocatalytic H2 evolution test (Fig. S3) did not show any significant structure change, indicating that the composite structure is very stable.



B

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Fig. 7 (A) H2 production under visible-light irradiation and the rate of H2 production of different morphologies SnS2 and SnS2/g-C3N4 composites (with Na2S and Na2SO3 were used as the hole scavengers without noble-mental cocatalysts), g-C3N4 (with triethanolamine used as the hole scavengers and Pt (3 wt%) as cocatalyst); (B) Rate of H2 evolution over g-C3N4, different morphologies SnS2 and SnS2/g-C3N4 composites; (C) recycling photocatalytic H2 evolution test of SnS2 nanoparticles/g-C3N4 composites Fig. 8A displays the photocurrent response obtained from both the pure g-C3N4 and different morphologies SnS2/g-C3N4 composites with several on-off cycles. It can be seen that the transient photocurrent intensity for the above four samples follows the order SnS2 nanoparticle/g-C3N4 composite>SnS2 nanosheet/g-C3N4 composite>3D flower-like SnS2/g-C3N4 composite> g-C3N4. Pure g-C3N4 exhibits very low photocurrent density, which may because of the fast recombination of photogenerated carriers. The SnS2 nanoparticle/g-C3N4 composite illustrates the most excellent photocurrent intensity among the four samples, which suggests that the constituted SnS2 nanoparticle/g-C3N4 composite can realize more photogenerated carrier to separate than pure g-C3N4 and other morphologies SnS2/g-C3N4 composites. Above all, it can be terminated that the efficient photo-induced electrons transfer between SnS2 nanoparticles and g-C3N4 facilitates the separation of electron-hole separation, which lead to the enhanced photocatalytic performance. Comparison of the different morphologies SnS2/g-C3N4 composites, SnS2 nanoparticle evenly dispersed on the g-C3N4 made a crucial contribution to increasing photocatalytic activities. Fig. 8B illustrates linear sweep voltammagrams (LSV) of the stripped g-C3N4 and different morphologies SnS2/g-C3N4 composites. The linear sweep voltammograms were registered in 0.5 M Na2SO4 buffer solution at pH 7 and at 10 mV/s in a potential range from -0.4 to 1.0 V versus Ag/AgCl


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under illumination with a 300 W Xenon lamp. The linear sweep voltammagrams apparently illustrate that the SnS2 nanoparticle/g-C3N4 composite exhibits the fastest light response, which on account of transportation and efficient charge separation by heterojunctions that is built between the interface of SnS2 nanoparticle and g-C3N4.64-66 More importantly, comparing to the photocurrent density produced on the pure g-C3N4 (~2.0 µA cm−2), the one on the SnS2 nanoparticle/g-C3N4 heterostructure is ~13 µA cm−2 at 1.0 V versus Ag/AgCl, which is 6.5 times higher than that for pure g-C3N4 acquired at the equal potential. In addition, the SnS2 nanosheet/g-C3N4 composite and 3D flower-like SnS2/g-C3N4 is ~10 and 7.5 µA cm−2 at 1.0 V versus Ag/AgCl, respectively, which is also higher than that for pure g-C3N4.

B

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Fig. 8 (A) The transient photocurrent responses and (B) Linear sweep voltammograms of g-C3N4, different morphologies SnS2/g-C3N4 composites under visible-light irradiation. The transfer process of photoinduced interfacial charge of both SnS2 and g-C3N4 within the composites were further understood by photoluminescence (PL) spectra with an excitation wavelength of 325 nm, just as shown in Fig.9. It can be examined that the pure g-C3N4 illustrates a substantial emission peak at approximately 469 nm, which is equivalent to the recombination of electron-hole pairs of g-C3N4. Comparing to g-C3N4, the PL intensities of different morphologies SnS2/g-C3N4 composites are much more dispirited, which means that the composites possess a lower recombination rate of carriers. Furthermore, the order of PL intensity is g-C3N4>3D flower-like SnS2/g-C3N4 composite > SnS2 nanosheet/g-C3N4 composite >SnS2 nanoparticle/g-C3N4 composite, which agrees well with the photocatalytic activity results.



g-C3N4 SnS2(flower)/g-C3N4 SnS2(nanosheet)/g-C3N4

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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360

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Wavelength (nm) Fig. 9 PL spectra of g-C3N4, different morphologies SnS2/g-C3N4 composites As shown in Fig. 10, VB-XPS spectra indicate the valence band top of g-C3N4, SnS2 nanoparticles, SnS2 nanosheets and 3D flower-like SnS2 is 1.73, 1.70, 1.39 and 1.90 eV (vs. NHE), respectively. According to the UV–vis and VB-XPS results, the conduction band (CB) positions of g-C3N4, SnS2 nanoparticles, SnS2 nanosheets and 3D flower-like SnS2 are estimated to be -1.14, -0.55, -0.96 and -0.31 eV (vs. NHE), respectively. Among the different morphologies SnS2 (nanoparticles, nanosheets and 3D flower-like SnS2)/g-C3N4 composites, it can be seen by TEM that the SnS2 nanoparticle/g-C3N4 composite has the best contact surface and the best interaction force with the other two materials, and also has the best advantage of favoring the hydrogen production of the band structure due to the preferred growth of the crystal plane. According to the experimental results above, a conceivable photocatalytic reaction mechanism was put forward and concisely depicted in Scheme 1. Under the visible-light irradiation, the CB and VB positions of g-C3N4 straddle those of SnS2 nanoparticles, forming the “type I” band alignment.67 When this composite is excited by visible light, the photo-induced carriers of g-C3N4 would migrate to the CB and VB of SnS2 nanoparticles, respectively. Then the photo-induced charge carriers recombination of g-C3N4 could be suppressed effectively by the synergistic effect of SnS2 nanoparticles/g-C3N4 heterostructure. Accordingly, the amount of photo-induced charge carriers on SnS2 nanoparticles is obvious increased based on the photo-induced interfacial charge transfer. During the photocatalytic H2 production process, the photo-induced electrons assembled on the CB of SnS2 nanoparticles could facilitate the catalytic proton reduction to H2. Accordingly, the photo-induced holes move from the VB of g-C3N4 to the VB of SnS2 nanoparticles were destroyed through the sacrificial reagents (S2−, SO32−). In this approach, the effective charge transfer at the interface between the SnS2 nanoparticles and g-C3N4 leads in the enhanced ability on H2 production.


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A

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Fig. 10 Valence band XPS spectra of (A) g-C3N4 and (B) different morphologies SnS2, (C) Schematic illustration of band structures (CB: conduction band, VB: valence band) of the g-C3N4, SnS2 nanoparticles (SnS2-p), SnS2 nanosheets (SnS2-s) and 3D flower-like SnS2 (SnS2-f) samples

Scheme 1. Possible photocatalytic reaction mechanism about the charge separation and transformation in the SnS2 nanoparticles (SnS2-p)/g-C3N4 during photocatalytic H2 production under sunlight irradiation.

Conclusions In conclusion, we have synthesized different morphologies SnS2 (nanoparticles, nanosheets and



3D flower-like SnS2)/g-C3N4 composites successfully via an elementary hydrothermal treatment technique that was merged with the calcination technique. It was found that the SnS2/g-C3N4 systems exhibited noticeably higher performance on account of photogenerated carriers' ameliorated separation capability. More importantly, comparing to SnS2 nanosheets/g-C3N4 and 3D flower-like SnS2/g-C3N4 composites, the SnS2 nanoparticles presented better uniform distribution on g-C3N4, contributing to more straightforward contact of SnS2 nanoparticles and g-C3N4, which enabled the electrons transfer significantly and augmented electron-hole pairs' separation. At the same time, it is revealed that SnS2 samples dominated by (001), (100), (101) and (102) preferred growth of facets possess different band-energy diagrams, which result in different photocatalytic hydrogen evolution. Therefore, an unusually increased photocatalytic activity for hydrogen output of SnS2 nanoparticles/g-C3N4 has been illustrated, which demonstrates excellent stability as well. The results presented herein could improve our current understanding of the facet-dependent semiconductors to search and construct new types with other semiconductors to further improve photocatalytic performance. Hence, this composite is likewise making a promise for solar cells, photonic and optoelectronic instruments and sensors. Further in-depth researches of the material with distinct expected applications are being under process.

Supporting Information Fig. S1-S3 showing data on the SEM image of g-C3N4, AFM image of obtained SnS2 nanoparticales and TEM images of the different morphologies SnS2/g-C3N4 composites (after recycling photocatalytic H2 evolution test).

Acknowledgements This work is financially supported by the National Natural Science Foundation of China (No. 21777063, 21407065), Natural Science Foundation of Jiangsu Province (BK20140533, BK20161363), China Postdoctoral Science Foundation (2015T80514). A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.


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The different morphological of the SnS2 with different specific preferred growth facets have strong influence in their photoactivity hydrogen production


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Different morphologies SnS2 supported on 2D g-C3N4 for excellent

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