Design, Synthesis And Photocatalytic Property Of Metal-Semiconductor Hybrid Nanomaterials

With the developing demands and requirements for the human’s living and manufacture, energy consumption is dramatically increasing while environmental pollution becomes more serious. For this reason, it has been a grand challenge to exploit the clean and sustainable energy. Semiconductors which can convert the solar energy to chemical energy through photocatalysis, have been paid extensive attention. In the past decade, numerous types of photocatalysts have been synthesized and used for a wide range of applications such as water splitting and CO2 reduction for clean energy, as well as wastewater treatment and air purification for environmental rescue. However, with the development of photocatalysts and photocatalytic reactions, the research community has recognized that deeper understanding on the principles and processes of photocatalysis should be gained in efforts to develop more efficient photocatalysts.In recent years, the rapid development of composite materials provides new opportunities for fabricating novel catalysts with higher activity and selectivity. To date, the composite materials have been widely used in photocatalysis and electrocatalysis to solve the environmental and energy problems. As compared with bare semiconductors, semiconductor-metal hybrid materials exhibit the improved photocatalytic performance due to their unique structures. Recent studies have demonstrated that catalytic performance can be further improved by tailoring the exposed facets of both semiconductor and metal as well as their interface. However, further development of photocatalysts is largely limited by the synthetic techniques and fundamental understanding on catalytic mechanisms.In general, there are two major challenges for the design of semiconductor-metal hybrid materials:1) selection rules for the appropriate facets to utilize the interfacial effects by combining semiconductor with metal nanoparticles; 2) intrinsic spatial charge distribution of photogenerated electrons and holes in semiconductor. In this dissertation, taking advantage of maneuvering the facets of semiconductors, we built the Schottky barrier to improve charge migration and separation. Meanwhile, the spatial charge separation is synergistically utilized to further improve the charge migration and separation, and eventually the photocatalytic activities are efficiently enhanced.The main research results are summarized as follows:1. Metal nanoparticles have been widely used in various reactions as efficient catalysts. Shape control of a metal nanostructure is a central research theme in nano techno logy, which allows tuning the physical and chemical properties. Notably, high-index facets can be exposed on surface when a certain morphology is formed. Benefiting from the unsaturated coordinative sites, the exposed high-index facets provide more active sites compared with low-index ones, and boost the chemical and catalytic activity. Among various shapes, five-fold twinned structures are a class of important members in the family of metallic nanocrystals with a face-centered cubic (fcc) structure. In this section, taking advantage of anisotropic growth of five-fold twinned structures, we successfully synthesized the palladium nanotapers enclosed by high-index facets by maneuvering kinetic growth. These nanocrystals exhibited superior chemical activity to their low-index counterparts, as proven in the galvanic replacement.2. The low migration rate of photogenerated holes largely limits the photocatalytic efficiency. Taking advantage of the Schottky barrier between p-type semiconductor and metal, the rate of hole migration can be promoted to improve photocatalytic water splitting. In this section, we demonstrated that work function serves as an important selection rule for surface facets to build the desired Schottky junction between semiconductor and metal. Meanwhile, the intrinsic spatial charge distribution has to be taken into account when selecting the facets, as it results in accumulation of photoexcited electrons and holes on certain semiconductor facets. Based on this finding, we synergized the Schottky barrier and spatial charge distribution to successfully design a Cu2O-Pd model system p-type semiconductor-metal hybrid material, which exhibited the improved photocatalytic water splitting.3. As the polarization plane of hexagonal zinc oxide (ZnO) is well exposed on surface, photogenerated charges can be selectively accumulated on the different exposed facets. Tailoring the length-to-diameter ratios of ZnO nanorods can optimize the efficiency of electron-hole separation Moreover, introducing the co-catalysts can reduce the overpotential of photocatalytic hydrogen and oxygen evolution, which further enhances the separation of photogenerated charges. In this section, we firstly optimized the length-to-diameter ratios of ZnO nanorods, and then deposited Au and MnOx on different facets to serve as co-catalysts of hydrogen and oxygen evolution, respectively. Benefiting from the synergistic effect of intrinsic spatial charge distribution and co-catalysts, the performance of photocatalytic hydrogen evolution has been dramatically improved.4. Based on the local plasmonic effect of noble metal nanoparticles, Au nanoparticles can absorb the visible light. In this section, we combined TiO2 with Au nanoparticles to broaden the range of solar harvesting by utilizing the hot electrons and enhanced local electromagnetic field induced by plasmonic effect. Furthermore, Pd nanoparticles were introduced as a co-catalyst to further enhance photocatalytic efficiency. As a result, the synergistic effect of plasmonic effect and Schottky barrier efficiently improved the full-spectrum photocatalytic performance.