Defect engineering of Heterojunction Photocatalyst for Efficient Solar-Fuel Production

Abstract

The global increase in energy demand and consumption is causing greenhouse gas emissions and environmental pollution, highlighting the importance of alternative energy sources to fossil fuels. Solar fuel, as a sustainable and eco-friendly energy source, is attracting significant attention due to the high potential of solar energy. The method of producing solar fuel using particulate photocatalysts is suitable for large-scale industrial applications because it can utilize only light energy without external bias, has a simple structure, and is cost-effective. However, photocatalysts still have not reached a practical level due to severe charge carrier recombination issues that result in low solar energy conversion efficiency. To overcome this critical limitation, there is a need to develop high-efficiency photocatalysts through precise approaches to charge transfer dynamics. Therefore, we aimed to enhance charge transfer and separation efficiency in graphitic carbon nitride (g-C3N4) by engineering its band structure through heterostructure formation, non-metal elemental doping, and defect introduction, with the goal of developing a high-efficiency solar fuel photocatalyst and elucidating its mechanism by analyzing charge transfer pathways and processes. In Chapter 2, we present the synthesis of W18O49/Au/g-C3N4 photocatalyst via a simple hydrothermal solution reaction, demonstrating efficient photocatalytic hydrogen generation across a broad spectrum beyond visible light. To overcome fundamental issues such as charge recombination and slow reaction rates in photocatalytic water splitting, a Z-scheme heterostructure is proposed. The W18O49 material, with abundant oxygen vacancies, exhibits wide absorption in the near-infrared range but faces challenges in hydrogen production due to not meeting the hydrogen reduction potential standard. By forming heterostructures with g-C3N4 to complement the advantages and disadvantages of W18O49, the effective separation of photogenerated charges and high absorption of visible light to near infrared light enhance hydrogen production. In addition, our W18O49/Au/g-C3N4 demonstrated apparent quantum yield (AQY) of 3.9% at 420 nm, and 9.3% at 1200 nm wavelength. Moreover, continuous runs of hydrogen production using W18O49/Au/g-C3N4 proved its high durability and stability, and analysis of charge transfer efficiency suggested possible mechanisms for photocatalytic hydrogen production under visible to near-infrared light. This study demonstrates the high efficiency of photocatalytic water splitting under visible to infrared irradiation using W18O49/Au/g-C3N4. In Chapter 3, we explored efficient and selective photocatalytic nitrate reduction to ammonia (PcNRA) using band structure engineering of g-C3N4 through non-metal doping and defect introduction. Ammonia (NH3), with high hydrogen density, is noted for its potential as a next-generation energy carrier. However, the traditional Haber-Bosch process emits large amounts of greenhouse gases and operates under high energy conditions due to the strong triple bond in N2. To address these drawbacks, PcNRA offers a sustainable alternative for solar fuel production without CO2 emissions and under ambient conditions, with relatively lower dissociation energy and higher solubility compared to N2 fixation. The key to achieving selective PcNRA is to enhance the adsorption of NO3 on the catalyst surface, which is the rate-determining step in ammonia production, and to increase selectivity by suppressing side reactions with N2 and H2. This study develops a boron and nitrogen vacancy doped g-C3N4 (NVCN) catalyst to accumulate photo-excited electrons, leading to efficient charge separation and surface NO3 reduction. Additionally, by adjusting the doping concentration of boron and nitrogen vacancies through simple temperature variations, NO3 - adsorption and activation are promoted. The co presence of boron atoms and nitrogen vacancies in NVCN synergistically enhances PcNRA activity, achieving remarkable NH3 synthesis activity with an exceptional selectivity of 96.9% and a production rate of 8.83 molh-1 under visible light irradiation, compared to pristine g-C3N4, making it the most efficient g-C3N4-based photocatalyst for NH3 production to date. In Chapter 4, the idea of band structure engineering through non-metal doping and heterojunctions was applied to hydrogen production using g-C3N4-based homojunction catalysts. The distinct band structure changes in boron-doped g-C3N4 (BCN) and oxygen-doped g-C3N4 (OCN) led to improved light absorption and optimized hydrogen reduction potential. Additionally, hydrochloric acid treatment induced surface charge changes that facilitated the formation of an effective BCN/OCN homojunction through electrostatic interactions between 2D nanosheets, enhancing light absorption and charge separation. The BCN/OCN homojunction demonstrated a high hydrogen production rate of 519.34 mol g 1 h 1 and an enhanced AQY of 4.49% at 420 nm and AQY of 3.37% at 450 nm wavelength. Furthermore, the Z-scheme charge transfer mechanism within the BCN/OCN catalyst was systematically analyzed by photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. This study appropriately demonstrates that the combination of a Z-scheme homojunction and band structure engineering through doping and defect introduction can be a promising strategy for designing high-performance photocatalytic platforms for solar fuel production. In summary, this study presents the potential of 2D g-C3N4 nanosheets as photocatalysts for solar fuel production through various strategies, including heterostructure formation, defect introduction, and elemental doping for band structure engineering. Moreover, the performance improvement in photocatalytic hydrogen and ammonia production is demonstrated through detailed charge transfer dynamics analysis. The enhanced catalytic activity is evidenced by interfacial charge transfer, charge trapping, and improved charge transfer efficiency at the homojunction interfaces. This study is expected to provide new directions for the commercialization of graphitic carbon nitride-based photocatalytic solar fuel production and charge transfer dynamics analysis.