Fabrication Of Silicon Nanowire Based Composite Structure For Photocatalytic Water Splitting

Hydrogen energy has the advantages of efficient, clean and pollution-free. It is regarded as the optimal clean energy. Using hydrogen energy to replace fossil fuels has become one of the inexorable trends in the energy development of 21 st century. As one of the important means to obtain hydrogen, hydrogen generation from photocatalytic water splitting has attracted continuous exploration and efforts since it was first realized in 1972. As one of the significant developments in science and technology in this century, the significant progress in Nano preparation technology bring to mankind new hopes to prepare Nano structured photocatalyst for hydrogen generation from water splitting. In recent years, a small band gap, low-cost silicon material has attracted more and more attention in photocatalytic hydrogen production. More importantly, the great potential of nanostructured silicon based composite structure as photocatalyst in water splitting has been realized gradually. This thesis focused the preparation of silicon and its composite structure for hydrogen generation from photocatalytic water splitting. It composes of three parts:In the first part, silicon nanowire prepared via Metal-assisted Chemical Etching(Ma CH) and its photocatalytic performance was studied. Silicon is an important semiconductor material. As the the mainstream material in photovoltaic field, its optical band gap is very small(~1.12 e V). So it can absorp visible light of the solar spectrum effectively. However, the photocatalytic activity of pure silicon is very low because of its special energy band position and photocorrosion in electrolyte solution. Based on this, people adopt applied bias、precious metal loading(Ag, Pt, etc.) and add the sacrificial agent to improve the photocatalytic activity and photochemical stability of silicon. Here, the principle and process of metal-assisted chmecal etching silicon nanowire were introduced thoroughly firstly, models involved in the silicon etching process in literature were summarized comprehensively. Then silicon nanowire with different length was prepared by metal-assisted chemical etching, the relationship between the morphology of Ag nanoparticles and silicon nanowire were studied further. Next, the photocatalytic performance of silicon nanowire was tested in 0.5 M K2SO4 electrolyte solution, the p H value of which is 1.0. Photocurrent will increase with the increase of length of silicon nanowire. Photocatalytic performance of silicon was improved obviously after co-catalyst Mo S3 loaded via annealing treatment after ammonium molybdate was spinned on silicon nanowire arrays.The preparation, optical characteristics, and photocatalytic performance of Si NW/Cu2 O composite structure were introduced in part 2. Cu2 O is an important p-type semiconductor, it has been used extensively in many fields such as solar cell, sensing, and so on. There exist many reports on the application of Cu2 O in photocatalysis research. Because the band gap of Cu2 O is small, that is it can absorb the visible light. In this part, Cu2 O nanocrystallines were deposited on the surface of silicon nanowire arrays to fabricate the Si NW/Cu2 O composite nanoheterojunction array via electroless deposition process. The morphology of Cu2 O crystalline can be transferred from cube to polyhedron by regulating the p H value of growth solution. The obtained nano hetero-junction arrays display good visible light absorption ability, it also demonstrate some new optical properties. For example, the infrared active mode was activated in the nanoheterojunction because of the defects(oxygen vacancy and copper vacancy) in Cu2 O nanocrystallines. In addition, the photoluminescence(PL) peak of silicon nanowire became broader because of the interaction between the core and shell. More interestingly, there appeared a new PL peak at about 615 nm in the PL spectrum of the composite structure. Even though the light absorption ability of Si NW/Cu2 O heterojuncion is a bit poorer than Si NWs, the photocatalytic performance of the composite structure was improved markedly compared with silicon nanowire arrays because of the formation of heterojunction between Cu2 O and silicon nanowire. Heterojunction speeded the separation and transportation of photon-generated carriers and reduced the recombination of electronic-hole pairs, therefore enhanced the photocatalytic activity. The photocatalytic activity of composite structure was improved further after Pt nanoparticles loaded. This work provides a new train of thought for building nano heterojunction through solution growth process.Part 3 focused on fabrication and photocatalytic activity of Cd1-x Znx S with three dimensional micro-nano structure and its composite structure Si NW/ Cd1-x Znx S-r GO. Solid solution semiconductor has been widely studied in photocatalytic water splitting. Because it is a kind of visible light photocatalyst and its band gap can be tuned by altering the ratio of constituents. In this work, one-step solvothermal method was adopted to prepare three dimensional micro-nano structure Cd1-x Znx S solid solution by changing the ratio of water and ethanediamine. This 3D micro-nano structure can greatly enhance the ability of light absorption. Thus improve the photocatalytic activity of catalyst. XRD and Raman demonstrated that the obtained product is solid solution, rather than the mixture of precursors of reactants—Cd S and Zn S. In addition, the obtained solid solution has good crystallization and high degree of order. In addition, the highest photocatalytic activity of Cd1-x Znx S was acquired from experiment, where the x value is 0.75. It is Cd0.24Zn0.75 S with optical band gap of 2.76 e V. As is well known that graphene material possess many specific properties, its compounds have been studied in many fields, and have displayed unique superiority. In this part, the Si NW/Cd0.24Zn0.75S-r GO composite structure was obtained after coating the Cd0.24Zn0.76S-r GO nanocomposite on silicon nanowire. Next, the obtained Si NW/Cd0.24Zn0.76S-r GO composite structure was annealed at 450℃ in vaccum. The obtained composite structure showed good photocatalytic performance. Moreover, Mo Sx is a good electrical catalytic materials, it is expected to a substitution of precious metal Pt as cocatalyst in the field of photocatalysis research, while P25 is a commercialized catalyst with stable catalytic performance. In this part, nano-composites of Mo S2-r GO and P25-r GO were obtained through chemical process. Then, Si NWs /Mo S2-r GO and Si NWs/P25-r GO composite structures were constructed and the corresponding photocatalytic activity was studied. Compared with pure silicon nanowire arrays, the photocatalytic activity of them are all improved apparently. This work expanded the application of silicon nanowire based composite structure in the field of photocatalytic water splitting.