Controlled Synthesis And Photocatalytic Properties Of SnS₂ And SnS₂/Graphene Nanocomposites

As an ideal green technology for solar-to-chemical energy conversion and environmental purification, semiconductor photocatalysis has been paid increasing attentions in recent years. The surface structure of a photocatalytic material plays a crucial role in its photocatalytic activity because the photocatalytic reaction takes place only when the electrons and holes are available on the surface. As an important layered structure material, SnS2 possesses interesting optical, electrical, gas sensing and catalytic properties, and has attracted much attention for its applications in solar cell, Li-ion battery anode material, gas sensing and catalytic material. So far, however, the influence of surface structure of SnS2 on the photocatalytic performance has been studied rarely. In this thesis, different morphologies of SnS2 with high visible light photocatalytic activity were synthesized via solvothermal method. Photocatalytic property and photoelectrochemical performance of the as-prepared samples were evaluated. First principle theoretical calculation and molecular dynamic simulation were employed to study the electronic structure and the surface property of SnS2, respectively. Moreover, to suppress recombination between photoexcited electrons and holes, we developed a one-step hydrothermal method to synthesize SnS2/graphene composites, and the photocatalytic degradation of methyl orange was conducted. The main results are as follows:1. A solvothermal method was used to synthesize sheetlike and flowerlike SnS2 by using SnCl₄·5H₂O and TAA as the precursors, respectively. Different kinds of solvents were added for obtaining different morphologies of SnS2. XRD analysis, SEM and TEM observation indicated that the exposed crystal planes of sheetlike SnS2 and flowerlike SnS2 were {001} and {010}, respectively. While the valence band XPS spectra show that the VB maximum of sheetlike SnS2 is identical to that of flowerlike SnS2, UV-vis absorbtion spectra demonstrate that the sheetlike SnS2 possesses a bigger band gap than flowerlike SnS2. As a result, the sheetlike SnS2 possesses a higher conduction band minimum, leading to a stronger reductive ability, and thus better photocatalytic activity. Degradation of methyl orange under visible light showed that, although the flowerlike SnS2 possessed a larger specific surface area than that of sheetlike Sn S2, its activity for MO degradation was substantially lower than that of the sheetlike one. Photocatalytic water splitting results showed that the H2 evolution rate over sheetlike SnS2 was about 11 times greater than that of flowerlike one. A significantly enhanced photocurrent was obtained on sheetlike SnS2, nearly 3 times larger than that on flowerlike one. In order to further clarify the mechanism of the superior photocatalytic activity of sheetlike SnS2,we further studied the surface energies and electronic structures through density functional theory?DFT?. The results show that the surface energy of {001} facets is lower than that of {010}. Nonetheless, the {001} facets with superior electronic band structure exhibit better photocatalytic activity.2. A facile hydrothermal method was developed to prepare SnS2/graphene composites by in situ growth of SnS2 in the interlayer of GO. The GO was obtained from natural graphite using a modified Hummers method. XRD, SEM, TEM and Raman measurements were carried out to characterize the crystal structure and morphology of the as-prepared SnS2/graphene composite. Photocatalytic activity of samples was evaluated by performing methyl orange degradation under visible light irradiation. The results show that, due to the enhanced separation of photoexcited electrons and holes, the photocatalytic activity of as-prepared SnS2/graphene composites is superior to both SnS2 nanosheet and physically mixed SnS2/graphene composites.