Carbon doped tin disulphide and methods for synthesizing the same

Abstract

Disclosed herein are carbon doped tin disulphide (C—SnS.sub.2) and other SnS.sub.2 composites as visible light photocatalyst for CO.sub.2 reduction to solar fuels. The in situ carbon doped SnS.sub.2 photocatalyst provide higher efficiency than the undoped pure SnS.sub.2. Also disclosed herein are methods for preparing the catalysts.

Claims

1. A photocatalyst, comprising a carbon doped tin sulfide, which is represented by formula I
C—SnS.sub.x  (I), wherein 1.5≤x≤2.

2. The photocatalyst of claim 1, having a nanostructure which is selected from the group consisting of sheet type, flower type, sphere type, needle shape and a mixture thereof.

3. The photocatalyst of claim 2, having a nanostructure with a dimension ranging from 3 to 300 nm in length and in diameter.

4. The photocatalyst of claim 1, further comprising a co-catalyst which is selected from the group consisting of metal, metal oxide, and metal sulfide and a mixture thereof, and wherein the co-catalyst is deposited on the carbon doped tin sulfide.

5. The photocatalyst of claim 4, wherein the co-catalyst is selected from the group consisting of Ag, Cu, Au, Pt, Ni, Zn, TiO.sub.2, ZnO, WO.sub.3, Cu.sub.2O, CuO, SnO.sub.2, CdS, MoS.sub.2, ZnS, NiS and a mixture thereof.

6. The photocatalyst of claim 1, wherein contents of Sn, S and C are 29.49-37.23, 55.32-57.69 and 7.45-12.82 atomic %, respectively.

7. The photocatalyst of claim 6, wherein the photocatalyst is SnS.sub.1.95C.sub.0.43 or SnS.sub.1.49C.sub.0.2.

8. The photocatalyst of claim 1, wherein the carbon doped tin sulfide is carbon doped tin disulphide (C—SnS.sub.2) which is in a size of nanometer scale, and the photocatalyst has a sheet structure.

9. The photocatalyst of claim 8, wherein the carbon doped tin disulphide has a size of 3-300 nm.

10. The photocatalyst of claim 8, wherein the carbon doped tin disulphide has a carbon content ranging from 0.5% to 20%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

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

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

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

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

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

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

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

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

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

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

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

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

DETAILED DESCRIPTION OF THE INVENTION

(14) 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.SUB.x.)

(15) 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.sub.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.sub.2 or Sn.sub.2S.sub.3/SnS.sub.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.sub.1.95C.sub.0.43 or SnS.sub.1.49C.sub.0.2.

(16) 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.sub.2, ZnO, WO.sub.3, Cu.sub.2O, CuO, SnO.sub.2, CdS, MoS.sub.2, ZnS, NiS, or a mixture thereof.

(17) According to another embodiment of the present disclosure, the carbon doped tin sulfide of the photocatalyst is carbon doped tin disulphide (C—SnS.sub.2) and the carbon doped tin disulphide comprises a sheet-structured SnS.sub.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.sup.−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.SUB.2.(C) and C—SnS.SUB.2.(T)

(18) 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.sub.2(C)” herein) is provided. The method comprises the following steps of: mixing SnCl.sub.4.5H.sub.2O and L-cysteine (C.sub.3H.sub.7NO.sub.2S) into distilled water; and allowing SnCl.sub.4.5H.sub.2O and L-cysteine (C.sub.3H.sub.7NO.sub.2S) to react under hydrothermal or microwave reactor to form a C—SnS.sub.2(C) nanostructured photocatalyst. The C—SnS.sub.2(C) prepared by such synthetic method may form a nanoflower morphology. The C—SnS.sub.2(C) nanostructured photocatalyst contains SnS.sub.2 polycrystalline phases dominated by 001, 100, 101 and 110. In the C—SnS.sub.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.sub.2(C) nanostructured photocatalyst is SnS.sub.1.95C.sub.0.43.

(19) 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.sub.2(T)” herein), and the method comprises the following steps of: mixing SnCl.sub.4.5H.sub.2O and thiourea (CH.sub.4N.sub.2S) into PEG-400, and allowing SnCl.sub.4.5H.sub.2O and thiourea (CH.sub.4N.sub.2S) to react under hydrothermal or microwave reactor to form a C—SnS.sub.2(T) nanostructured photocatalyst. The C—SnS.sub.2(T) prepared by such synthetic method may have a nanocage morphology. The C—SnS.sub.2(T) nanostructured photocatalyst contains SnS.sub.2 polycrystalline phases dominating 001, 100, 101 and 110. The C—SnS.sub.2(T) nanostructured photocatalyst can be composed of several thin SnS.sub.2 nano sheets having 1 to 2 μm. Moreover, the C—SnS.sub.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.sub.2(T) nanostructured photocatalyst is SnS.sub.1.49C.sub.0.2.

Method for Producing Solar Fuel

(20) 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.sub.2 or splitting water with any one of the aforementioned photocatalysts. In the present embodiment, the reduction of CO.sub.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.sub.cat, hr.

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

EXAMPLE 1

Synthesis of Carbon Doped SnS.SUB.2 .by Hydrothermal Process Based on Cystine [C—SnS.SUB.2.(C)]

(22) In a typical procedure, 1 mM of tin (IV) chloride pentahydrate (SnCl.sub.4.5H.sub.2O) and 5 mM L-cysteine (C.sub.3H.sub.7NO.sub.2S) were added to a 60 ml of distilled water and gradually dispersed to form a homogeneous solution by vigorous magnetic stirring for 1 hr. at room temperature. Finally, the resulting solution was transferred into a Teflon-lined stainless autoclave. The autoclave was sealed and heated at 180° C. for 24 hrs. After hydrothermal reaction, the sample was cooled to room temperature naturally. The resulting product was collected by centrifugation at 8000 rpm for 10 min and washed several times with distilled water. Finally, the collected yellow C—SnS.sub.2 powder was vacuum-dried at 80° C. overnight.

EXAMPLE 2

Synthesis of Carbon Doped SnS.SUB.2 .by Hydrothermal Process Based on Thiourea [C—SnS.SUB.2.(T)]

(23) In a typical procedure, 1 mM of tin (IV) chloride pentahydrate (SnCl.sub.4.5H.sub.2O) and 5 mM thiourea (CH.sub.4N.sub.2S) were added to a 60 ml of polyethylene glycol-400 (PEG-400) and gradually dispersed to form a homogeneous solution by vigorous magnetic stirring for 3 hr. at room temperature. Finally, the resulting solution was transferred into a Teflon-lined stainless autoclave. The autoclave was sealed and heated at 220° C. for 15 hrs. After hydrothermal reaction, the sample was cooled to room temperature naturally. The resulting product was collected by centrifugation at 8000 rpm for 10 min and washed several times with distilled water. Finally, the collected yellow C—SnS.sub.2 powder was vacuum-dried at 80° C. overnight.

EXAMPLE 3

Characterization

(24) The ultraviolet-visible absorption spectrum of powder samples was measured with a Jasco V-670 spectrophotometer using an integrated sphere. The crystal structures were determined by XRD using CuKα radiation (Bruker, D2 PHASER with XFlash). The surface morphology of all samples was characterized by field emission scanning electron microscopy (FESEM, JEOL, 6700F). The high-resolution transmission electron microscopy (HRTEM, JEOL-2100) studies with selected area electron diffraction (SAED) and EDX were also performed to determine morphology, crystal phase and elemental compositions. X-ray Photoelectron spectroscopy (XPS) analysis was performed on a theta probe ESCA VG Scientific (2002) using a monochromatic AlKa as the exciting source. The peak positions of the XPS were calibrated carefully with respect to the Au 4f peak. Finally, the XPS spectra were deconvoluted by using Voigt fitting function after a Shirley background subtraction procedure.

EXAMPLE 4

Photochemical CO.SUB.2 .Reduction Experiment

(25) The photocatalytic experiment for the reduction of CO.sub.2 was performed at ambient temperature (25±5° C.) in a continuous gas flow reactor. The volume of the cylindrical reactor which was made of stainless steel and covered with Quartz-Glass was 300 ml (11 cm×4 cm). One sample dish containing 0.1 g of the photocatalysts obtained by the method of the present invention was placed in the middle of the reactor. A 300 W commercial halogen lamp was used as the simulated solar-light source. The lamp was vertically placed outside the reactor above the sample dish. Two mini fans were fixed around the lamp to avoid the temperature rise of the flow system. The catalyst powder spread onto the glass disc, with a diameter of around 4 cm. Initially nitrogen gas was purged inside the reactor to remove the air with other gases. After that CO.sub.2 was purged inside the reactor for another 1 hour and control the flow rate at 4 sccm. The CO.sub.2 was flowing through water to control the desire humidity level for entire experiment. The halogen lamp was turned on after one hour while adsorptions desorption of gas and photocatalyst reached the equilibrium. The concentration of methanol was continuously measured by a GC-FID in vapor phase. The detail schematic drawing of the experimental setup is shown in FIG. 9.

EXAMPLE 5

Optical Property of SnS.SUB.2 .Photocatalyst

(26) The UV-vis absorption spectra were performed to determine the optical absorption of the carbon doped SnS.sub.2 photocatalyst prepared in example 1 and 2 respectively. As shown in FIG. 1, C—SnS.sub.2(C) and C—SnS.sub.2(T) present strong intense absorption band from UV to visible region around 300 to 550 nm. From the tauc plot in the FIG. 2, approximate bandgaps of 2.55 and 2.43 eV for C—SnS.sub.2(C) and C—SnS.sub.2(T), respectively, were obtained. It is observed that the narrow bandgap of the carbon doped SnS.sub.2 photocatalyst provides excellent visible light responsive photocatalytic activity.

EXAMPLE 6

Structure of Carbon Doped SnS.SUB.2 .Photocatalyst

(27) The crystal structure and phase composition of the as prepared carbon doped SnS.sub.2 were characterized by XRD. Displayed in FIG. 3 are the X-ray diffraction (XRD) patterns of the as prepared C—SnS.sub.2(C) and C—SnS.sub.2(T) nanostructures photocatalyst in example 1 and 2. All the peaks in FIGS. 3a and b are indexed to the standard diffraction data of hexagonal SnS.sub.2, which well match the literature values (JCPDS no 00-001-1010). The strong reflection and no impurity peak reveal the high purity and crystallinity of the as prepared samples. We observed the peak shifting slightly lower 2θ position which is due to crystal lattice expansion of SnS.sub.2 by the interstitial doping of carbon in layer SnS.sub.2. The (001) facet of the hexagonal SnS.sub.2 shows quite strong intensity compared with standard value, which demonstrates that (001) orientation is preferentially oriented.

EXAMPLE 7

Morphology and Microstructure of SnS.SUB.2 .Photocatalyst

(28) The morphology of the as prepared SnS.sub.2 samples was characterized by FESEM. FIGS. 4(a, b) and 4(c, d) show the typical SEM images of C—SnS.sub.2(C) and C—SnS.sub.2(T) at low magnification and high magnification, respectively. C—SnS.sub.2(C) clearly shows small flower type nanostructure formation with number of nanosheets with a uniform dimension around 300 to 400 nm length and diameter. The high resolution image clearly shows that several nanosheets with rough surface forming the nanoflower morphology. In the C—SnS.sub.2(T) sample we observed nanosheet-interconnected cage type nanostructure, where nanosheets are thin and bigger than the C—SnS.sub.2(C). The higher magnification of C—SnS.sub.2(T) in FIG. 4d clearly shows the nanosheets are around micron size, thinner and with smooth surface morphology. Additionally, to investigate the more details morphology and structure features analysis of C—SnS.sub.2(C) and C—SnS.sub.2(T) samples, HRTEM and SAED were also performed as shown in FIG. 5. FIGS. 5(a, b) shows the microstructure of the C—SnS.sub.2(C), its clearly observed that several thicker nanosheets are forming small nanoflower architecture showing clear lattice space around 0.326 nm which is corresponding to the interspacing of 001 planes of SnS.sub.2. Moreover, C—SnS.sub.2(T) shows that several thinner nanosheets forming a bigger interconnected nanocage microstructure with lattice spacing of the 001 plane at around 0.326 nm. In addition the corresponding SAED was performed for both the samples as shown in FIGS. 5 c and f. The SAED of both samples reveal the polycrystalline and dominating 001, 100, 101 and 110 facets. This result is well consistent with the XRD analysis results.

EXAMPLE 8

Chemical Composition and Raman Spectrum of SnS.SUB.2 .Photocatalyst

(29) The energy dispersive X-ray (EDX) spectra confirm the presence of Sn, S and C elements. The synthesized in situ carbon doped SnS.sub.2 are denoted as C—SnS.sub.2(C) and C—SnS.sub.2(T), with measured C loading of 12.82 and 7.45 atomic % respectively. The observed Sn:S ratio for C—SnS.sub.2(C) and C—SnS.sub.2(T) are 1:1.95 and 1:1.5. FIGS. 6 and 7 show the STEM EDX elemental maps of the C—SnS.sub.2 samples signifying that the Sn, S and C are evenly distributed within the SnS.sub.2 nanostructure. FIG. 8 presents the comparison of high-resolution XPS spectra of Sn 3d and S 2p of the as prepared SnS.sub.2—C and SnS.sub.2-T samples. In FIGS. 8a and 8c, the measured binding energies corresponding to Sn 3d.sub.5/2 and Sn 3d.sub.3/2 are around 486.7 and 495.2 eV, respectively; these binding energies indicate Sn.sup.4+ ions in SnS.sub.2 samples. The difference at around 8.4 eV between the two strong Sn 3d peaks is characteristic of tetravalent Sn 3d states. Furthermore, in FIGS. 8b and 8d, the high resolution S 2p core level analysis at binding energies of around 162.8 and 164.0 eV correspond to S 2p.sub.3/2 and S 2p.sub.1/2, which are typical values for metal sulfide. The observed XPS binding energies of Sn 3d and S 2p spectra confirmed the Sn.sup.4+ and S.sup.2+ of as prepared SnS.sub.2 samples. The Raman spectra of C—SnS.sub.2(C) and C—SnS.sub.2(T) are shown in FIG. 9 with signature peak at 312 cm.sup.−1.

EXAMPLE 9

Photocatalytic CO.SUB.2 .Reduction Activity of SnS.SUB.2 .Photocatalyst

(30) Photoreaction characteristics of the C—SnS.sub.2(C) and C—SnS.sub.2(T) nanostructures photocatalyst prepared in example 1 and 2 were determined through reaction of CO.sub.2 and water in gas phase to produce hydrocarbons as indicated in the schematic in FIG. 10. The products of the CO.sub.2 reduction are shown in FIG. 11. FIG. 11a illustrates the cumulative acetaldehyde production yield after 14 hours for the C—SnS.sub.2(C) and C—SnS.sub.2(T) nanostructures photocatalyst. The observed maximum cumulative acetaldehyde yields after 14 hours are 298.3 μmole/g.sub.cat and 19.9 μmole/g.sub.cat for C—SnS.sub.2(C) and C—SnS.sub.2(T) photocatalyst respectively. It can be seen that the prepared SnS.sub.2 nanostructure photocatalyst exhibited clear photocatalytic CO.sub.2 reduction activity under visible light and produced selective acetaldehyde as a major product under multi-electron reduction. Additionally, FIG. 11b shows a comparison of visible light photocatalytic CO.sub.2 reduction to solar fuel formation rates of commercial SnS.sub.2, C—SnS.sub.2(T) and C—SnS.sub.2(C). The maximum solar fuels formation rates for the commercial SnS.sub.2, C—SnS.sub.2(T) and C—SnS.sub.2(C) photocatalyst are around 0.23±0.05, 1.69±0.08 and 28.6±3.5 μmole/g.sub.cat, hr. In the in situ carbon doped SnS.sub.2 the solar fuel formation rate was increased. The solar fuel formation rate was achieved at the highest value of around 28.6 μmole/g.sub.cat, hr. for C—SnS.sub.2(C). The photocatalytic solar fuel formation rate for C—SnS.sub.2(C) is almost 124 times higher than the commercial SnS.sub.2.

(31) It's well accepted that the photocatalytic CO.sub.2 reduction is a multi-electron reduction. In an initial step, direct photon absorption by the band gap of SnS.sub.2 and generate electron-hole pairs. Therefore, the position of frontier orbital's of CO.sub.2 with respect to the conduction band position SnS.sub.2 would feasible for reduction process. Specifically, the narrow bandgap (around 2.5 eV) and the conduction band position with respect to onset reduction potential energy of CO.sub.2 favoring for ten-electron reduction on the surface of photocatalyst. The ten-electron reduction processes are involved in the production of acetaldehyde in our experiment. On the other hand the excess polysulfide act as a sacrificial agent by hole scavenging to oxidize to elemental sulfur and suppress the corrosion of SnS.sub.2. The overall reactions can be described in the following equations.
SnS.sub.2+hv.fwdarw.SnS.sub.2(e.sup.−+h.sup.+)  (1)
H.sub.2O+2h.sup.+.fwdarw.2H.sup.++1/2O.sub.2  (2)
H.sub.2O+h.sup.+.fwdarw.1/2H.sub.2O.sub.2+H.sup.+  (3)
H.sub.2O+h.sup.+.fwdarw.OH+H.sup.+  (4)
S.sup.2−+2h.sup.+.fwdarw.S  (5)
2CO.sub.2+10H.sup.++10e.sup.−.fwdarw.CH.sub.3CHO+3H.sub.2O  (6)

(32) Although such properties demonstrated for in situ carbon doped SnS.sub.2 nanostructures photocatalyst in these examples, other SnS.sub.2 based hybrid photocatalyst according to the invention can be prepared and tested accordingly to the present disclosure and would exhibit similar catalytic activity.

(33) On the other hand, splitting of water to produce hydrogen and oxygen utilizing C—SnS.sub.2(C) has been verified. As shown in FIG. 12, the on/off photocurrent response of C—SnS.sub.2(C) at the external bias of 0.8V under 0.5M Na.sub.2SO.sub.4 electrolyte shows clear evidence of redox reaction under the light illumination. Despite the fact that additional separation of hydrogen and oxygen is needed to take advantage of this technique, proof of concept utilizing the abovementioned catalysts showed the potential for such application.

(34) It will be appreciated by those skilled in the art of the changes could be made to the embodiments described above without departing from the broad invention concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modification within the spirit and scope of the present invention as defined by the appended claims.

OTHER PUBLICATION

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