Plasmonic Solar Fuels Based on Nanostructured Oxide Photocatalysts

Solar energy can be directly converted to electrical energy via photovoltaic devices. Photocatalysis provides an alternative route to the conversion of solar energy that can be stored in chemical fuels through photoelectrochemical cells or particulate photocatalytic systems. While a number of strategies were developed to overcome the deficiencies of existing semiconductors, incorporation of plasmonic nanostructures with semiconductors has gained increasing attention as a promising paradigm to improve the solar energy conversion efficiency. In this presentation, we introduce diverse approaches for the fabrication of nanostructured semiconductors integrated with plasmonic structures that can be exploited to exhibit enhanced photo-conversion efficiencies.

First, a surface plasmon (SP) induced visible light active photocatalyst system composed of silica-titania core-shell (SiO₂@TiO₂) nanostructures decorated with Au nanoparticles (Au NPs) was developed. The influence of size and distribution of Au NPs on photocatalysis, its fabrication methods and exploration of the mechanism of visible light activity were investigated. A favorable architecture of SiO₂ beads with a thin layer of TiO₂ was decorated with Au NP arrays having different size and areal density. Surface modification of SiO₂@TiO₂ leads to a viable and homogenous loading of Au NPs on the surface of TiO₂, which renders visible light-induced photocatalytic activity on the whole TiO₂ surface. An optimized system employing Au NP arrays with 15 nm size and 700/µm² density showed best catalytic efficiency, due to a synergistic effect of the firm contact between Au NPs and TiO₂ and efficiently coupled SPR excitation. A brief mechanism relating the electron transfer from surface plasmon stimulated Au NPs to the conduction band of TiO₂ is proposed. The same system was further tailor-modified to exhibit far-field scattering effect, to lead to maximized performance toward solar water splitting.

Next, we focus on our design principle for the plasmon-enhanced oxide-based photocatalytic systems based on inverse opal structures and demonstrate the SP coupling effect on the efficiency of water splitting. Inverse opals of representative semiconductor oxide photocatalysts were fabricated by well-known colloid templating, and the surface was decorated with Au NPs. The effect of size and areal density of Au NP arrays were explored in terms of oxidation reactions and plasmon-enhancement effect was supported by FDTD analysis. An advance strategy to improve the efficiency of solar fuel generation includes the combined block-copolymer-based self-assembly, leading to hierarchically organized inverse opals with tailored surface area and nano-textured surface morphology. Further, we show that carbon-doped analogue inverse opal structures could be readily obtained by direct carbonization of block copolymer templates, based on which simultaneous carbon-doping and plasmon-enhancement effect are synergistically realized.