Surface And Interfacial Design Of Semiconductor Nanoarray Materials For Solar Energy Conversion

Since energy and environment issues are getting increasingly severe, it becomes highly important to seek renewable energy sources and eco-friendly energy conversion paths. As a clean, sustainable and worldwide abundant energy,solar energy has been regarded as an ideal alternative energy source and received tremendous attention in recent decades. The solar energy can be efficiently utilized through the solar-to-electricity and solar-to-chemical energy conversion. Given their potential values in fundamental study and practical applications, nanomaterials can play a significant role at solar energy conversion. In particular, nanoarray materials have demonstrated their outstanding performance in electron transport and light absorption, which can provide new opportunities for the efficient utilization and conversion of solar energy.Meanwhile, the design of surface and interface has shown its increasing importance to optimizing the performance of catalysis and charge transport. However, the working mechanisms still remain elusive while the related applications have not fully explored,which hinders the development of rational surface and interface engineering.In this dissertation, we have fabricated the inorganic nanoarray materials based on"top-down" and "bottom-up" methods, in which surface dangling bonds, co-catalysts,plasmonic nanostructures can be well controlled at the surface and interface. As such,we can investigate the effect of surface and interface conditions on solar-driven catalysis processes and explore the function of surface plasmon on photovoltaic devices.Moreover, working mechanisms have been further studied by establishing the relationship between surface/interface structures and solar energy conversion.Specifically, the surface dangling bonds, surface electron state and reaction environment have been tailored to elucidate their effects on solar-driven catalysis; the hot-electron injection by plasmonic Ag nanoplates has been implemented into silicon nanowires-based photovoltaic devices. The main results can be summarized as follows:1. We combined micro- and nanofabrication techniques with wet-chemical method so that the types and amounts of dangling bonds could be selectively controlled on the surface of silicon nanowire arrays. We further studied the influence of different silicon dangling bonds on photocatalytic water splitting and elucidated the related mechanism in pure water system. The H-terminated silicon nanowire arrays can produce hydrogen and oxygen with a ratio higher than 2 in pphotocatalytic water solitting.Through photoelectrochemical and IR measurements, we established the coorelation of surface dangling bonds with photogenerated charge lifetime. We further investigated the effect of surface Si-H and Si-OH bonds on photocatalysis together with simulation,and concluded that the reaction would occur via Si(h+) + Si-H(e-) + H2O ? Si-OH + Si+ H2.2. Silicon nanowires with Pd and Pt nanoparticles as co-catalysts were used in the solar-driven catalytic organic reactions. For the first time, the surface Si-H dangling bonds on silicon were used for in-situ solar-driven hydrogenation reactions. By intergrating Pd and Pt nanoparticles on the surface, the surface electron state of silicon nanowires can be changed to impact on solar-driven hydrogenation and oxidation reactions. In addition, the activation of oxygen molecules by porous silicon nanowires for solar-driven oxidation reactions was also studied. This work establishes the foundation for the surface and interface engineering of silicon nanostructures towards solar-driven organic catalysis.3. The approach to synergetic utilization of plasmonic effect of Ag nanoplates and OER catalyst Co(OH)2 for photoelectrochemical oxidation was developed by employing BiVO4 nanoporous arrays. For the first time, we used BiVO4 for photoelectrochemical selective oxidation of glyceroll, and investigated the impact of reaction solution pH values and applied bias on the productivity and selectivity of products. Our result showed that, the production rates, selectivity and efficiency of dihydroxypropanone (DHA) and other valuable chemicals were the highest in acidic condition (pH=2), which achieved a selectivity at 50% and a productive rate at 55.92 mmol/g/h of DHA (1.2V vs RHE, H-type cell). The concentration of H+ had a direct impact on the catalysis path and the selectivity of products. The applied bias promoted charge separation and enhance current density, but cannot change catalysis path.Moreover, Ag nanoplates and Co(OH)2 layers were introduced in this reaction system to study their effect on catalytic performance in a neutral solution. This work provides a broad research perspective for the utilization of solar light to produce high-value chemicals in a photoelectrochemical system.4. N-type silicon nanowires-PEDOT:PSS inorganic-organic heterojunction photovoltaic devices and N-type silicon nanowires-reduced graphene oxide Schottky photovoltaic devices were used as model systems to demonstrate the function of plasmonic hot electrons. The working mechanism for the enhancement of photovoltaic conversion efficiency in IR region by plasmonic Ag nanoplates has been systematically studied. Ag nanoplates offered the plasmonic effect in IR region to generate hot electrons. When the Ag nanoplates were in direct contact with silicon nanowires, the hot electrons could be injected into the conduction band of silicon, so that the photovoltaic efficiency would be increased in the IR region. For example, the efficiency could be improved by 59% at 800 nm. The working mechanism was elucidated by comparing the performance of N-type and P-type silicon-based devices. Through micro- and nanofabrication methods, we further fabricated the flexible photovoltaic devices based on this working mechanism. This work provides the fabrication foundation for the interface design of photovoltaic devices toward broaden light absorption.