| Photocatalysis is considered as one of most promising technologies in the field of pollution control. However, low efficiency of light utilization has limited its practical application. Therefore, the enhancement of efficiency for light utilization is the key scientific problem. Silicon is one of the most abundant elements, whose abundance is up to29.4%on the earth. Moreover, the photoconversion efficiency of silicon materials can be up to33%in theory. If silicon materials can be applied into photocatalysis for pollution control, the light utilization efficiency would be enhanced significantly. However, silicon materials can be easily passivated in aqueous solution, which limited its application in aqueous solution. The aim of this research is to make use of novel materials as protective layer and quantum confinement effect to slow down the passivation of silicon materials effectively, by which can make silicon materials display high photoconversion efficiency. This work helps to improve light utilization efficiency of photocatalytic technology and to promote its practical application in pollution control. The following several parts of work have been done in this dissertation:In order to retard the passivation of silicon in aqueous solution, macroporous silicon/graphene (MPSi/Gr) heterostructure was fabricated by electrophoresis depositing Gr sheets on the surface of MPSi. In the photoelectrochemical measurements, the photocurrent of MPSi/Gr heterostructure at-1.5V was110%higher than that of MPSi,80%higher than that of MPSi/carbon film(C). After5circles of cyclic voltammetry, the photocurrent of MPSi/Gr kept stably. The kinetic constant of MPSi/Gr (1.13h-1) heterostructure in photoelectrocatalytic process was26times as much as that of MPSi and5times as much as that of MPSi/C heterostructure (0.22h-1). The majority (87.1%) of debromination was induced by visible light which was confirmed by comparing reaction efficiency under xenon lamp irradiation to that under the same lamp irradiation via a422nm filter. According to the results of GC-MS and ESR, debromination mechanism of BDE-47was proposed as a reductive debromination by photogenerated electrons.Silicon nanowires arrays (SiNW) possess more excellent antireflection ability and larger absorption area than those of MPSi. In order to improve photoconversion efficiency further, SiNW/platinum(Pt) heterostructure was prepared by electroless deposition. This study systematically investigated photocatalytic ability of SiNW/Pt photocathode for removal of2,4-DCP under various conditions, which indicated that the contribution of photocatalysis for2,4-DCP removal at-0.6V took the most during photoelectrocatalysis process. In this condition, the removal efficiency of2,4-DCP was up to95%in20minutes, and the the kinetic constant of SiNW/Pt was0.13min-1, which is2times,3times,6times as much as those of Si/Pt, SiNW and MPSi/Gr, respectively. Furthermore, the reserves of noble metal Pt is limited an"d its cost is expensive, so low-cost materials for protective layer should be developed. Therefore, novel monocrystal titanium nitride (TiN) nanoparticles with cubic structures, prepared by arc diacharge method, were introduced to substitute Pt. The TiN electrode acting as cathode exhibited stable electrochemical performance during the process of cyclic voltammetry measurement for20cycles in1～10pH value region. The overpotential of TiN cathode for electrochemical reduction of water (the main side reaction during electrocatalytic reduction of pollutants in aqueous solution) was figured out as0.54V, which was much higher than those of Pt wafer (0.12V) and Pt film (0.07V). Then, SiNW/TiN heterostructure was prepared by electrophoresis, depositing discontinuous TiN nanoparticles on the surface of SiNW. The removal efficiency of2,4-DCP on SiNW/TiN at-1.0V was up to85%in40min, which is90%as much as that on SiNW/Pt under the same condition.Although protective layer can retard passivation of silicon, it blocks light absorption inevitably. Furthermore, the protective layer is easily fall off during using processes. On the other hand, the valence band of silicon is too negative to oxidize pollutants without bias. Therefore, in order to inhibit passivation of silicon in aqueous solution without protective layer and to enhance the photocatalytic oxidation ability, hierarchically porous silicon was fabricated through electro-assisted chemical etching using a silicon wafer as a substrate. Pores with an average diameter of ca.1200nm (macropores) were observed and a large number of nanopores with a diameter of less than5nm were uniformly distributed over the surface of the macropore, forming the hierarchically porous silicon with nanopores in macropores structure (NP-MPSi):According to UV-Vis diffuse reflection measurements, the bandgap of NP-MPSi was figured out as2.12eV, which is1.0eV higher than that of the original silicon wafer due to the quantum confinement effect caused by the nanopores. Mott-Schottky experiments further demonstrated that the increasement of bandgap of NP-MPSi arises from the positive shift of the valence band potential, meaning that its capability for photocatalytic oxidation was enhanced. Using phenol as an example, photocatalytic experiments under irradiation with a Xe lamp demonstrated that the kinetic constant of phenol degradation and total organic carbon removal using NP-MPSi were0.49h-1and45%, respectively, which were nearly3.5and8.0times larger, respectively, than those using MPSi. Furthermore, the degradation efficiency of phenol was nearly unchanged using the same NP-MPSi5times, exhibiting its stable photocatalytic ability in aqueous solution. In conclusion, in this study, graphene, platinum and titanium nitride were used as protective layer materials and quantum confinement effect was brought in to retard passivation of silicon materials in aqueous solution. The prepared silicon nanomaterials achieved improved photoconversion efficiencies and photocatalytic degradation efficiencies for pollutants in aqueous solution. These results are expected to promote the practical processes of photocatalytic technology in pollution control. |
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