Carbon doped tin disulphide and methods for synthesizing the same

This non-provisional application claims priority to U.S. provisional patent application Ser. No. 62/479,462 filed on Mar. 31, 2017. This and all other extrinsic materials discussed herein are hereby incorporated by reference in their entirety.

The present invention relates to the carbon doped SnS 2 and related SnS 2 composites as an efficient photocatalyst for conversion of carbon dioxide gas and water vapor to hydrocarbons (acetaldehyde, methanol etc) under visible light.

The increasing demand of reduce fossil-fuel consumption, and renewable solar-energy generation processes are highly challenging global environmental energy issues. In particular, hydrocarbon's formation via photocatalytic carbon dioxide (CO 2 ) reduction under visible-light irradiation offers an attractive solution for solving simultaneously the incoming clean renewable energy and anthropogenic greenhouse emission problems. Since 1979, the pioneer demonstration of photocatalytic CO 2 reduction using various semiconductors the photocatalytic CO 2 reduction to solar fuels has attracted tremendous interest. (Inoue et al. Nature, 1979, 277, 637.) Over the years, many significant efforts have been applied to develop an efficient photocatalyst for the CO 2 reduction to hydrocarbons. Among the entire reported semiconductor photocatalyst, only TiO 2 and TiO 2 based composites are widely used for CO 2 photoreduction because of their comparatively low cost, low toxicity and photocorrosion resistance. (Liu et al. J. Am. Chem. Soc. 2010, 132, 11914, Yi et al. Chem. Rev. 2014, 114, 9987) However, wide band gap TiO 2 or other related wide band gap semiconductor photocatalysts are not enough to use for practical application due to their limited visible light absorption (Navalon et al. ChemSusChem 2013, 6, 562). On the other hand recent years, several carbonaceous hybrid materials GO (Hsu et al. Nanoscale, 2013, 5, 262.), g-C3N4 (Wang et al. Applied Catalysis B: Environmental, 2015, 176,44), TiO 2 /rGO (Tan et al. Nanoscale Research Lett. 2013, 8, 465.), MoS 2 /rGO (Xiang et al. J. Am. Chem. Soc. 2012, 134, 6575) and Cu-GO (Shown et al. Nano Lett. 2014, 14, 6097) have been reported as a promising photocatalyst for CO 2 reduction to solar fuels conversion. However, the CO 2 reduction process is more complicated multi-electron process and forming several hydrocarbon products. Thus, it is highly necessary to introduce a narrow bandgap nanostructure photocatalyst with suitable band structure to drive the CO 2 reduction process under visible light and forming selective hydrocarbons after multi-electron reduction. In addition, several narrow band gap metal sulfides such as CdS, PbS, CdSe, MoS 2 have been introduced as a photocatalyst or a co-catalyst together with another wide-band gap semiconductor for photocatalytic reaction under visible light (Zhang et al. Catal. Sci. Technol., 2013, 3, 1672.). Although CdS and PbS contains sufficient potential characteristics as a photocatalyst, however, they have less explored as a commercial photocatalyst due to the toxicity problem of Cd and Pb. Therefore, it's highly desirable to explore nontoxic narrow bandgap metal sulfide for photocatalytic CO 2 reduction. Among various metal sulfides, SnS 2 is one of the narrow bandgap nontoxic semiconductors associated with dichalcogenide family. Recently, it has been attracted for its potential applications as a light absorber layer for dye-sensitized solar cell (Yang et al. Materials Lett., 2014, 133, 197.), gas sensing (Ou et al. ACS Nano, 2015, 9, 10313), energy storage (Wang et al. RSC Adv., 2015, 5, 89069.) and conversion (Liu et al. ACS Appl. Mater. Interfaces, 2015, 7, 4000.). SnS 2 possesses CdI 2 crystal structure, consisting sandwiched S—Sn—S layers bounded by weak van der Waals's interaction. In the sandwiched structure tin cations are octahedrally coordinated by nearest six sulfur atoms. SnS 2 is an n-type semiconductor with a narrow bandgap around 2.2-2.4 eV at room temperature, and high quantum yield, thus it shows several benefits to be a good photocatalyst under visible light. Recently, Sun et al. have reported the photocatalytic CO 2 reduction to CO over SnS 2 (Sun et al. Materials Letters, 2016, 174, 238). However the overall photocatalytic performance is far low from the practical requirement due to the fast recombination of photogenerated charge carrier. To overcome this problem it is necessary to modify this semiconductor by doping it with metal or nonmetals. In semiconductors, doping with nonmetal carbonaceous materials is very popular in photocatalyst system due to their wide range of light absorption and low photo corrosion as compared with metal. The doped carbon acts as an excellent electron acceptor center and suppresses the charge recombination in the electron transfer process due to electronic interaction between doped carbon and the semiconductor. This doping process effectively improves the photocatalytic performance of the semiconductor.

The present disclosure discloses a simple hydrothermally and microwave synthesized in situ carbon doped SnS 2 nanomaterials as a gas phase photocatalyst for the reduction of CO 2 with water vapor to produce hydrocarbons under visible light. The L-cysteine assisted hydrothermally synthesized carbon doped SnS 2 nanostructure has been selectively forming acetaldehyde with a conversion rate of around 28.6 μmole/hr·g under visible light. The thiourea assisted hydrothermally synthesized carbon doped SnS 2 nanostructure also has been used in the art to obtain a conversion rate of about 1.6 μmole/hr·g under visible light.

In general, the in situ carbon doped SnS 2 photocatalyst provided by this disclosure may be a mixture of SnS/SnS 2 , Sn 2 S 3 /SnS 2 with different ratios and various nanostructures like sheet type, flower type, sphere type or needle shape.

In view of the foregoing objectives, the present disclosure discloses a photocatalyst based on in situ carbon doped tin disulfide nanostructure based on the chemical formula C—SnS x , wherein 1.5≤x≤2, with metal, metal oxide or other metal sulfide co-catalyst deposited on it. The co-catalyst may be selected from the following groups Ag, Cu, Au, Pt, Ni, Zn, TiO 2 , ZnO, WO 3 , Cu 2 O, CuO, SnO 2 , CdS, MoS 2 , ZnS, NiS or mixture thereof.

In another aspect, preparing SnS 2 -GO, SnS 2 -rGO composites as a photocatalyst with mixture of graphene oxide or reduced graphene oxide together with SnS 2 by microwave or hydrothermal process.

In one embodiment, the photocatalyst is a mixture of SnS/SnS 2 or Sn 2 S 3 /SnS 2 .

In one embodiment, the photocatalyst has a nanostructure which is selected from the group consisting of sheet type, flower type, sphere type, needle shape and a mixture thereof.

In one embodiment, the photocatalyst has a nanostructure with a dimension ranging from 3 to 300 nm in length and in diameter.

In one embodiment, the photocatalyst further comprises a co-catalyst which is selected from the group consisting of metal, metal oxide, and metal sulfide and a mixture thereof, and the co-catalyst is deposited on the carbon doped tin sulfide.

In one embodiment, in the photocatalyst, contents of Sn, S and C are 29.49-37.23, 55.32-57.69 and 7.45-12.82 atomic %, respectively.

In one embodiment, the photocatalyst is SnS 1.95 C 0.43 or SnS 1.49 C 0.2 .

In view of the foregoing objectives, the disclosure also provides a carbon doped tin disulphide (C—SnS 2 ) which comprises a sheet-structured SnS 2 of nanometer scale.

In one embodiment, the carbon doped tin disulphide has a size of 3-300 nm.

In one embodiment, the carbon doped tin disulphide has a carbon content ranging from 0.5% to 20%.

In one embodiment, the carbon doped tin disulphide has a signature peak at 312 cm −1 in Raman spectra.

In one embodiment, the carbon doped tin disulphide has a bandgap ranging from 2.0-2.5 eV.

In view of the foregoing objectives, the disclosure further provides a method for synthesizing the aforementioned carbon doped tin disulphide. The method comprises the following steps of: mixing SnCl 4 .5H 2 O and L-cysteine (C 3 H 7 NO 2 S) into distilled water; and allowing SnCl 4 .5H 2 O and L-cysteine (C 3 H 7 NO 2 S) to react under hydrothermal or microwave reactor to form a C—SnS 2 (C) nanostructured photocatalyst.

In one embodiment, the photocatalyst is prepared by a hydrothermal method with L-cysteine.

In one embodiment, the C—SnS 2 (C) is prepared to form nanoflower morphology.

In one embodiment, the photocatalyst contains SnS 2 polycrystalline phases dominated by 001, 100, 101 and 110.

In one embodiment, in the photocatalyst, contents of Sn, S and C of the photocatalyst are 29.49, 57.69 and 12.82 atomic %, respectively.

In one embodiment, the photocatalyst is SnS 1.95 C 0.43 .

In view of the foregoing objectives, the disclosure further provides a method for synthesizing the aforementioned carbon doped tin disulphide, and the method comprises the following steps of: mixing SnCl 4 .5H 2 O and thiourea (CH 4 N 2 S) into PEG-400, and allowing SnCl 4 .5H 2 O and thiourea (CH 4 N 2 S) to react under hydrothermal or microwave reactor to form a C—SnS 2 (T) nanostructured photocatalyst.

In one embodiment, the C—SnS 2 (T) is prepared with nanocage morphology.

In one embodiment, the photocatalyst contains SnS 2 polycrystalline phases dominating 001, 100, 101 and 110.

In one embodiment, the C—SnS 2 (T) nanostructured photocatalyst is composed of several thin SnS 2 nano sheets having 1 to 2 μm.

In one embodiment, the C—SnS 2 (T) nanostructured photocatalyst is composed of Sn, S and C around 37.23, 55.32 and 7.45 atomic %, respectively.

In one embodiment, the C—SnS 2 (T) nanostructured photocatalyst is SnS 1.49 C 0.2 .

In view of the foregoing objectives, the disclosure further provides a method for producing a solar fuel, comprising the following steps: reducing gas phase CO 2 or splitting water with any one of the aforementioned photocatalysts.

In one embodiment, the reduction of CO 2 or splitting of water is carried out under visible light.

In one embodiment, the solar fuel is a hydrocarbon which is selected from the group consisting of acetaldehyde, methanol, ethanol and a mixture thereof.

In one embodiment, the photocatalyst has a maximum solar fuel production rate of 1.6-28.6 μmole/g cat , hr.

In one embodiment, the solar fuel is hydrogen and oxygen.

Accordingly, the photocatalyst, the carbon doped tin disulphide (C—SnS 2 ), the method for synthesizing the aforementioned carbon doped tin disulphide, and method for producing a solar fuel, are provided by the present disclosure. Solar fuels can be generated from the reduction of CO 2 under visible light on the surface of carbon doped SnS 2 photocatalyst. Also, the in situ carbon doped SnS 2 photocatalyst is prepared in an environmentally friendly and energy-efficient manner from nontoxic materials. To the best of our knowledge, this study is the first to describe in situ carbon doped SnS 2 nanostructures and related composites for photocatalytic CO 2 reduction application under visible-light irradiation. The in situ carbon doped SnS 2 absorbs visible light to generate an electron-hole pair, and interspatial doped carbon suppresses the charge recombination process. The photogenerated electrons and holes would migrate to SnS 2 surface and serve as a reducing and oxidizing sites for CO 2 reduction to hydrocarbon and water oxidation to oxygen formation. The present disclosure addresses the simple hydrothermally synthesized in situ carbon doped SnS 2 nanostructure, thin film, nanoparticles as a potential photocatalyst for gas-phase photochemical reduction of CO 2 to hydrocarbon fuels such as acetaldehyde as major product under visible light.

The embodiments will become more fully understood from the detailed description and accompanying drawings, which are given for illustration only, and thus are not limitative of the present invention, and wherein:

FIG. 1 Absorption spectra of the C—SnS 2 (C) and C—SnS 2 (T) obtained by the hydrothermal method of the present invention.

FIG. 2 Tauc plot and band gap calculation of the C—SnS 2 (C) and C—SnS 2 (T) obtained by the hydrothermal method of the present invention.

FIG. 3 Powder XRD pattern of the obtained (a) C—SnS 2 (C) and (b) C—SnS 2 (T) by the hydrothermal method of the present invention.

FIG. 4 SEM images of the (a-b) C—SnS 2 (C) and (c-d) C—SnS 2 (T) obtained by the hydrothermal method of the present invention.

FIG. 5 HRTEM images of the (a-b) C—SnS 2 (C) and (d-e) C—SnS 2 (T) obtained by the hydrothermal method of the present invention. SAED pattern of the (c) C—SnS 2 (C) and (f) C—SnS 2 (T) obtained by the hydrothermal method of the present invention.

FIG. 6 TEM EDX elemental composition and elemental mapping of C—SnS 2 (C).

FIG. 7 TEM EDX elemental composition and elemental mapping of C—SnS 2 (T).

FIG. 8 High resolution XPS spectra of the Sn 3d and the S 2p of the (a-b) C—SnS 2 (C) and (c-d) C—SnS 2 (T) obtained by the method of the present invention.

FIG. 9 Raman spectra of the C—SnS 2 (C) and C—SnS 2 (T).

FIG. 10 The schematic diagram of the photocatalytic reduction of CO 2 with the photocatalysts via hydrothermal method of the present invention.

FIG. 11 CO 2 photoreduction analysis of the photocatalysts obtained by the method of the present invention and representative (a) cumulative acetaldehyde yields for C—SnS 2 (C), C—SnS 2 (T) and (b) acetaldehyde yield comparison for synthesized C—SnS 2 (C) C—SnS 2 (T) and commercial SnS 2 respectively.

FIG. 12 On/off photocurrent response of C—SnS 2 (C) at the external bias of 0.8V under 0.5M Na 2 SO 4 electrolyte.

The embodiments of the invention will be apparent from the following detailed description, which proceeds with reference to the accompanying drawings, wherein the same references relate to the same elements.

Photocatalyst of Carbon Doped Tin Sulfide (C—SnS x )

According to one embodiment of the present disclosure, a photocatalyst comprises a carbon doped tin sulfide. The carbon doped tin sulfide is represented by a formula of C—SnS x , wherein 1.5≤x≤2. The photocatalyst has a nanostructure which is sheet type, flower type, sphere type, needle shape or a mixture thereof, and the dimension of the photocatalyst ranges from 3 to 300 nm in length and in diameter. The tin sulfide part of the photocatalyst can be a mixture of SnS/SnS 2 or Sn 2 S 3 /SnS 2 . Moreover, in the photocatalyst, contents of Sn, S and C may be 29.49-37.23, 55.32-57.69 and 7.45-12.82 atomic %, respectively. For some specific examples, the photocatalyst can be SnS 1.95 C 0.43 or SnS 1.49 C 0.2 .

Alternatively, the photocatalyst may further comprise a co-catalyst which is metal, metal oxide, metal sulfide or a mixture thereof, and the co-catalyst is deposited on the carbon doped tin sulfide. For example, the co-catalyst can be Ag, Cu, Au, Pt, Ni, Zn, TiO 2 , ZnO, WO 3 , Cu 2 O, CuO, SnO 2 , CdS, MoS 2 , ZnS, NiS, or a mixture thereof.

According to another embodiment of the present disclosure, the carbon doped tin sulfide of the photocatalyst is carbon doped tin disulphide (C—SnS 2 ) and the carbon doped tin disulphide comprises a sheet-structured SnS 2 of nanometer scale. Similarly, the carbon doped tin disulphide may have a size ranging from 3-300 nm. In this embodiment, the carbon doped tin disulphide has a carbon content ranging from 0.5% to 20%, and has a signature peak at 312 cm −1 in Raman spectra. Also, the bandgap of the carbon doped tin disulphide may range from 2.0-2.5 eV.

Method for Synthesizing Carbon Doped Tin Disulphides: C—SnS 2 (C) and C—SnS 2 (T)

According to further another embodiment of the present disclosure, a method for synthesizing the aforementioned carbon doped tin disulphide by a hydrothermal method with L-cysteine (such carbon doped tin disulphide is abbreviated as “C—SnS 2 (C)” herein) is provided. The method comprises the following steps of: mixing SnCl 4 .5H 2 O and L-cysteine (C 3 H 7 NO 2 S) into distilled water; and allowing SnCl 4 .5H 2 O and L-cysteine (C 3 H 7 NO 2 S) to react under hydrothermal or microwave reactor to form a C—SnS 2 (C) nanostructured photocatalyst. The C—SnS 2 (C) prepared by such synthetic method may form a nanoflower morphology. The C—SnS 2 (C) nanostructured photocatalyst contains SnS 2 polycrystalline phases dominated by 001, 100, 101 and 110. In the C—SnS 2 (C) nanostructured photocatalyst, contents of Sn, S and C of such photocatalyst are 29.49, 57.69 and 12.82 atomic %, respectively. For a specific example, the C—SnS 2 (C) nanostructured photocatalyst is SnS 1.95 C 0.43 .

Alternatively, the disclosure further provides a method for synthesizing the aforementioned carbon doped tin disulphide with thiourea (such carbon doped tin disulphide is abbreviated as “C—SnS 2 (T)” herein), and the method comprises the following steps of: mixing SnCl 4 .5H 2 O and thiourea (CH 4 N 2 S) into PEG-400, and allowing SnCl 4 .5H 2 O and thiourea (CH 4 N 2 S) to react under hydrothermal or microwave reactor to form a C—SnS 2 (T) nanostructured photocatalyst. The C—SnS 2 (T) prepared by such synthetic method may have a nanocage morphology. The C—SnS 2 (T) nanostructured photocatalyst contains SnS 2 polycrystalline phases dominating 001, 100, 101 and 110. The C—SnS 2 (T) nanostructured photocatalyst can be composed of several thin SnS 2 nano sheets having 1 to 2 μm. Moreover, the C—SnS 2 (T) nanostructured photocatalyst is composed of Sn, S and C around 37.23, 55.32 and 7.45 atomic %, respectively. For a specific example, the C—SnS 2 (T) nanostructured photocatalyst is SnS 1.49 C 0.2 .

Method for Producing Solar Fuel

According to still another embodiment of the present disclosure, the invention further provides a method for producing a solar fuel, comprising the following steps: reducing gas phase CO 2 or splitting water with any one of the aforementioned photocatalysts. In the present embodiment, the reduction of CO 2 or splitting of water can be carried out under visible light. The solar fuel may be hydrogen and oxygen, or a hydrocarbon which is acetaldehyde, methanol, ethanol or a mixture thereof. The maximum solar fuel production rate of the photocatalyst used in the present embodiment ranges from 1.6 to 28.6 μmole/g cat , hr.

To illustrate some characteristics of aforementioned embodiments, such as, but not limited to, the synthesis of carbon doped SnS 2 , and the optical properties, morphology, microstructures, and photochemical CO 2 reduction activities of such carbon doped SnS 2 photocatalyst, there are several examples shown below.

Craig A. Grimes et al. Titania nanotubes arrays, methods of manufacture, and photocatalytic conversion of carbon dioxide using same, US20100213046A1 (2010)

Paul O'connor et al. Artificial photosysnthesis, WO2011020825A1 (2011)

Paul O'connor et al. Direct photoconversion of carbon dioxide to liquid products, WO2012168355A1 (2012)

Sun Yuanyuan et al. Visible light photocatalytic reduction of carbon dioxide over SnS 2 , Materials Letters 174, 238-241 (2016).