Engineering Nanostructured Materials for Photocatalytic Conversion of CO2
Creators: name: Shah, Daksh
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340601
subject: Catalysis and mechanisms of reactions

description: Anthropogenic CO2 emissions have resulted in global warming, and subsequently severe weather events. Fossil fuel burning is the largest source of CO2 emissions, and currently the primary source of global energy. The most abundant renewable energy source on Earth is sunlight, and hence offers a means to alleviate the reliance on fossil fuels. Inspired by biological photosynthesis, inorganic semiconductors can be applied to harvest light energy and thereby drive the photocatalytic conversion of CO2 and water to hydrocarbons. 
The majority of inorganic semiconductor photocatalysts for CO2 photoreduction exhibit poor quantum efficiencies of less than 1%, almost 10 times lower than biological photosynthesis. Biological photosynthesis is a highly complex process, incorporating multiple pigments that maximize absorbance of sunlight, intricate enzymes for charge transport and separate oxidation and reduction sites. Similarly, a sophisticated photocatalyst design is required that can facilitate the conversion of thermodynamically stable CO2 into hydrocarbons. Key factors to consider include its microstructure and morphology that govern its light absorbance capability, the availability of active sites on the surface, and electronic properties. 
Presently, the use of 2D materials as a standalone photocatalyst and in nanocomposites has been a popular choice due to their superior electron mobility and rich active sites towards CO2 photoreduction. Therefore, in Chapter 3, promising 2D LDHs were examined for their textural, optical and electronic properties. Cobalt aluminium layered double hydroxides (CoAl LDHs) were synthesized via a modified alkali free coprecipitation and hydrothermal methods to produce visible light photocatalysts. Physicochemical and optical properties of CoAl LDHs were tuned by tailoring the Co:Al molar ratio. A modified alkali-free coprecipitation method produced pristine CoAl LDHs with 6 times greater surface area and CO2 adsorption capacity than hydrothermal CoAl LDHs, reaching 120 m2/g and 61 µmol/g respectively. High Co loadings induced defects and vacancies in the hydroxide layer due to CoOOH formation observed from XPS. Increasing Co:Al ratio from 1.3 to 3.2 lowered the band gap from 2.1 to 1.3 eV, extending light absorbance range from visible to near infrared absorbance. Increase in Co:Al ratio also resulted in linearly proportional decrease in PL intensity, suggesting the formation of defects and vacancies in the LDH sheets that act as charge trap sites. However, due to the narrow band gap and band gap position of the LDHs, coprecipitated LDHs exhibited no photocatalytic activity by itself.
Therefore, to generate charge carriers with sufficient energy, UV light active TiO2 nanostructures were investigated in Chapter 4 and introduced to form a nanocomposite with CoAl LDH in Chapter 5. Nanostructured TiO2 phase, exposed facet, size and shape influence its optical, electronic and photocatalytic properties. Hence, in chapter 4, microstructural engineering of TiO¬2 was conducted. TiO2 nanoparticles, nanotubes, nanorods and nanoprisms were characterized and examined for CO2 photoreduction. It was found that CH4 selectivity of 55% was achieved by nanotubes calcined at 300°C, consisting of 65/35 wt% TiO2(B)/anatase ratio with a quantum efficiency of 0.0511%. It is believed this composition increased the active contact sites that facilitated effective electron transport from TiO2 (B) to anatase valence band for CO2 photoreduction to CH4, in a Direct Z-scheme. Calcined nanotubes displayed the highest selectivity and yield of CH4 amongst the TiO2 nanostructures due to the superior charge transfer kinetics. 
Research efforts have focused on semiconductor-semiconductor composite materials, in which semiconductors with different band gaps are brought in contact that form a heterojunction at the interface. This strategy can improve charge transfer and, in some cases, spatially separates the redox sites, resulting in enhanced quantum efficiencies compared to a single semiconductor catalyst. Therefore, TiO2/CoAl LDH nanocomposites were synthesized to couple UV-visible light semiconductors. Thus harnessing a wider solar spectrum, and further improve charge separation via heterojunction formation [2]. CO2 photoreduction performance of TiO2/CoAl LDHs composites, exceeded that of pristine TiO2 and CoAl LDH. It was found that low loadings of LDH onto TiO2 nanoparticles and nanoprisms boosted their selectivity towards CH4 by as far as twice the pristine counterparts. This finding was ascribed to the successful construction of a nanocomposite material, offering high surface area, basicity, and formation of an effective Z-scheme heterojunctions with complimentary band gaps. TiO2 nanoprism/LDH (referred to as 97 wt% Nprism/LDH) exhibited the highest selectivity switch towards CH4 compared to the pristine nanoprism by 45%. Fine-tuning TiO2 wt% with CoAl LDH, for heterojunction formation is a promising route to an effective CO2 photoreduction system. 
This work presented a bottom-up approach to design advanced, efficient nanocomposites for CO2 photoreduction. Insights are gained into band gap tuning, evaluating the role of crystal phase and nanostructure of the photocatalysts, as well as building effective semiconductor-semiconductor heterojunctions towards CH4 production.