Development of TiO2-based Catalyst for Solar Fuel Production and Charge-transfer Mechanism Study

Abstract

As energy demand is increasing significantly worldwide due to explosive population growth and industrial evolution, the importance of alternatives to fossil fuels has been emphasized. Among environmentally-friendly and thus sustainable options, solar fuel has been attracting enormous attention due to the high potential of solar energy. Particulate photocatalyst, one of the three representative methods of producing solar fuel, has the great advantage of utilizing only light energy without external bias, and is also beneficial for scaling-up in terms of simplicity and low cost. However, photocatalysis suffers from severe charge carrier recombination which is the most radical reason for low solar energy conversion efficiency. Despite many efforts, photocatalysts still fall short of the practical standards, and in order to overcome this critical limitation, a more delicate approach to charge transfer dynamics is required. Moreover, through this approach, the development a highly active photocatalyst is demanded. Accordingly, we modified the benchmark photocatalyst, titanium dioxide, with various strategies, such as dye sensitization, heterostructure creation with cocatalyst deposition, and single metal atom incorporation, enhancing charge transfer efficiency and systematically analyzed the charge transfer process within the catalysts using high-end in situ instruments such as Kelvin probe force microscopy and transient absorption spectroscopy, to elucidate possible mechanisms. In Chapter 2, we present the synthesis CuPc/TiO2 photocatalyst composites by a facile solution reaction and demonstrate their efficient photocatalytic hydrogen generation under visible-light irradiation. The amorphous CuPc coating layer plays key roles in solar water splitting in terms of efficient separation of the photogenerated charges and high absorption in visible-light region. More surprisingly, our CuPc/TiO2 does not demand any noble-metal cocatalyst for exhibiting high photocatalytic activity with 6.68 % of apparent quantum yield (AQY) at 420 nm. Moreover, several continuous runs of hydrogen generation using CuPc/TiO2 composite proved the high durability of our products. The detailed charge transfer dynamics of CuPc/TiO2 composites by TRPL demonstrated that our noble metal-free CuPc/TiO2 shows a comparably high charge transfer efficiency as that of Pt cocatalyst modified samples. On the basis of the analysis, a possible mechanism for visible-light driven photocatalytic hydrogen generation of CuPc/TiO2 composites is postulated. Our study suggests that CuPc/TiO2 is a highly efficient photocatalyst for solar water splitting without noble-metal cocatalysts. In Chapter 3, a Z-scheme heterojunction with spatially separated cocatalysts is proposed for overcoming fundamental issues in photocatalytic water splitting, such as inefficient light absorption, charge recombination, and sluggish reaction kinetics. For efficient light absorption and interfacial charge separation, Z-scheme organic/inorganic heterojunction photocatalysts are synthesized by firmly immobilizing ultrathin g-C3N4 on the surface of TiO2 hollow spheres via electrostatic interactions. Additionally, two cocatalysts, Pt and IrOx, are spatially separated along the Z-scheme charge transfer pathway to enhance surface charge separation and reaction kinetics. The as-prepared Pt/g-C3N4/TiO2/IrOx (PCTI) hollow sphere photocatalyst exhibits an exceptional H2 evolution rate of 8.15 mmol h−1 g−1 and a remarkable AQY of 24.3% at 330 nm in the presence of 0.5 wt% Pt and 1.2 wt% IrOx cocatalysts on g-C3N4 and TiO2, respectively. Photo-assisted Kelvin probe force microscopy is used to systematically analyze the Z-scheme charge transfer mechanism within PCTI. Furthermore, the benefits of spatially separating cocatalysts in the PCTI system are methodically investigated in comparison to randomly depositing them. This work adequately demonstrates that the combination of a Z-scheme heterojunction and spatially separated cocatalysts can be a promising strategy for designing high-performance photocatalytic platforms for solar fuel production. In Chapter 4, a single-atom Cu-doped TiO2 nanosheet (Cu-TNS) photocatalyst is presented for efficient and selective photocatalytic nitrate reduction to ammonia (PcNRA). Recently, ammonia (NH3) has been gaining enormous attention as a next-generation energy carrier owing to its high hydrogen density. However, the primary manufacturing routes for NH3, such as the Haber-Bosch process, have been linked to high levels of CO2 emissions and energy consumption. PcNRA is a sustainable alternative that is considered advantageous for the relatively low dissociation energy and high aqueous solubility compared to N2 fixation. Although PcNRA has recently been shown to achieve excellent selectivity, its sluggish kinetics greatly restrict its NH3 production efficiency. Within Cu-TNS photocatalyst, single Cu atoms displacing Ti sites accumulate photogenerated electrons, thereby ensuring efficient charge separation and surface NO3‒ reduction. Moreover, incorporating Cu atoms into the TiO2 matrix induces spontaneous defect formation, resulting in oxygen vacancies and lattice strain that promote NO3‒ adsorption and activation. The simultaneous presence of single Cu atoms and structural defects in Cu-TNS synergistically stimulates PcNRA, leading to a 62-fold enhancement over pristine TiO2 in NH3 production with 97.6% selectivity and an AQY of 11.7% at 330 nm under optimized conditions. In summary, we presented various strategies to improve the photocatalytic performance for fuel production of TiO2 materials in different morphologies of nanoparticles, hollow spheres, and nanosheets. Moreover, the charge transfer dynamics within the catalysts were systematically investigated to elucidate the origin of the enhanced catalytic activity for photocatalytic hydrogen and ammonia production. The enhanced catalytic activity was strongly reflected by the improved charge transfer efficiency through interfacial charge migration, cocatalytic charge trapping, and charge accumulation at the isolated atomic sites. This study adequately demonstrated the opening up future landscape of solar fuel production via photocatalytic approaches.