Engineering hybrid nanostructure photoelectrode for solar energy conversion : synthesis, characterization and performance

Abstract

Solar energy conversion is regarded as an attractive and promising route to meet the growing energy consumption owing to the abundant source of sunlight. Intensive research and great efforts have been focused on the exploration and development of photocatalyst for better utilization of solar light during the past decades. As efficiency of solar energy conversion based on photocatalytic material is the most significant factor for practical applications, various strategies have been investigated to improve the performance of photocatalysis. Theoretically, photocatalyst can be stimulated under irradiation of suitable light, to generate photoinduced electron and hole pairs, that can trigger the redox reactions, initially. Three steps of charge kinetics can heavily impact photocatalytic efficiency: (i) generation rate of photoinduced charge pairs; (ii) transfer rate of photoinduced charge carriers; and (iii) consumption rate of photoinduced charge carriers. Photoelectrocatalysis, i.e. photocatalyst applied with a bias potential to form a construction of photoelectrochemical reaction, is a desirable method to decrease recombination rate of photoinduced charge pairs and accelerate the migration rate of charge carriers. Alternatively, in contrast to single-component material, composited photocatalyst with heterojunction structure may harvest broad range of solar spectrum, and facilitate separation rate of photoinduced charge pairs, resulting in an advanced utilization efficiency of solar energy. The primary purpose of this study is to develop heterojunction nanostructure photoelectrode with low cost, good stability and enhanced photon utilization efficiency for photoelectrocatalytic reactions.  First, rutile titanium dioxide nanorod arrays (TiO2 NRs) has been synthesized by a hydrothermal method, and further modified with palladium (Pd) nanoparticles through a facile electrodeposition process. The loading amount of Pd nanoparticles was controlled to investigate enhancement of Pd-TiO2 NRs on photoelectrocatalytic performance.  To achieve high performance of photoelectrocatalysis, highly-ordered TiO2 nanotube arrays (TiO2 NTs) with mixed crystal phase of anatase/rutile was prepared as photocatalyst-based substrate, which possesses a higher photocurrent than that of the pure TiO2 NRs. Furthermore, three platinum group metals, that is platinum, palladium and ruthenium, were chosen as co-catalyst to decorate TiO2 NTs (denoted as M-TiO2 NTs). Results indicate that M-TiO2 NTs exhibit different enhancement effects on the light responses, as well as different photoelectrochemical performances.  To further maximize photon utilization efficiency, visible-light-respond bismuth vanadate (BiVO4) photoanode has been fabricated and loaded with plasmonic semi-metal bismuth (Bi). The method and condition of loading Bi nanoparticles have been optimized. Photoelectrochemical hydrogen production with addition of phenol was improved. The mechanism of this improvement has been proposed.  BiVO4 photoelectrode coupled with narrow band gap bismuth sulfide (Bi2S3) was synthesized though a two-step conversion, resulting in a staggered type-II heterojunction structure material with a strong capacity of solar light absorption. The formation process of Bi2S3 nanowire has been discussed. Efficiency of photoelectrocatalytic hydrogen production of the hybrid Bi2S3/BiVO4 photoelectrode was enhanced significantly. The research work in this thesis has attempted to study photoelectrode with various hybrid nanostructures for solar-light-conversion and provide strategic direction for possible exploration. Finally, future perspective research works for solar energy conversion were highlighted.