Engineering hybrid nanostructure photocatalyst for CO₂ conversion : synthesis, characterization, and mechanism

Abstract

Increased application of fossil fuel accompanied by high carbon dioxide (CO2) emissions generally results in an energy crisis and global warming. The photocatalytic conversion of CO2 to solar fuels has been shown to be an effective technique for resolving these two issues. However, the current method of preparation is complex, time-consuming, and unfeasible for mass production. The issue as to how to enhance the catalytic stability of material is of high significance and thus has caused extensive studies, whereas the underlying mechanism of the reaction still remains unknown. Here, this thesis will provide comprehensive investigations into the synthesis, mechanism, and application of nanomaterials as active catalysts for effective CO2 reduction and energy conversion. The main results in this thesis are divided into three sections, which can be briefly summarized as follows. In the first section, for increasing the light utilization of TiO2 nanomaterials, several techniques which modify their nanoscale properties were proposed to obtain better catalytic performance. A comparison was made between the CO2 conversion yield of samples that were acquired via various methods of engineered TiO2 nanoparticles. The reasoning behind this outcome is further analyzed. In the second section, we investigated the potential of utilizing the synthesized composite nanomaterials in a range of important energy conversion processes and conducted systematic tests to evaluate their catalytic performance, and further uncover the underlying mechanism behind their high catalytic activity. To achieve flexible material property change, we investigated a range of material synthesis methods, including electrochemical synthesis, hydrothermal synthesis, anodization, and chemical synthesis. Furthermore, we synthesized the metal and bi-metal cocatalysts for TiO2 based photoreactive semiconductor nanomaterials, with a focus on their catalytic activity. In the third section, we investigated the possibility of using carbon-based composite nanomaterials to improve charge separation and prolong internal carrier recombination, thereby avoiding the short-lived solar-driven separated electron-hole pairs while also optimizing carbon dioxide reduction performance. This chapter addressed the issue regarding the use of carbon-based compounds for increasing the stability of photocatalysts. The photoreduction technique was used to study the conversion of CO2 to valuable products. The performance of graphene oxide-wrapped materials for CO2 photoreduction was evaluated, with good stability in the initial CO2 conversion rate over hours of illumination. Apart from the measurements above, a combination of DFT calculations and a series of experimental characterizations were performed to provide a fundamental understanding of the catalytic process. It is shown that the charge transfer from graphene oxide to titanium dioxide (TiO2) can delay photogenerated electrons and holes recombination. As a buffer layer, the graphene oxide layer feeds electrons into TiO2 to fill photogenerated holes while extending the time required for photogenerated electron-hole pairs to recombine. Consequently, the stability and performance of the photocatalytic system based on Graphene Oxide wrapped TiO2 nanocomposites are enhanced. These investigations in this thesis may offer valuable insights into reaction mechanisms, thus assisting in the design of photocatalytic materials. (477 characters)