Visible light responsive titanium-oxides for enhanced photoelectrochemical water splitting application

Abstract

Semiconductor-based photoelectrochemical (PEC) water splitting under sunlight has the potential to produce sustainable hydrogen at a low cost and on a large scale. Titanium dioxide (TiO2) is extensively studied as one of the most promising photoelectrodes owing to its Earth-abundance, stability, and nature-friendly. However, many challenges remain in improving the system performance, such as utilizing longer wavelengths of visible light energy, boosting photoelectrical conversion efficiency at a given wavelength, and extending the lifetime for long-term stable production of hydrogen. In the thesis, visible light responsive titanium-oxides composites, composed of metallic Ti and its oxides, have been proposed and studied for PEC application. The content is divided into chapters, including introduction, numerical modeling/calculation, material systems of the titanium-oxides composites, experimental results and discussion, and conclusions. The used methods for calculations and experiments are combined with the results in each of the chapters. Some results have been presented at conferences and discussed in more detail in published papers enclosed at the end. The latest strategies to engineer TiO2 systems for visible light response are briefly reviewed. With the ambition to develop cost-effective titanium-oxides composites with specific visible responsivity towards PEC application, light-matter interactions are numerically calculated on modeled nanocomposites. Augmented intensity of electric near-field and confined visible light capture is reviewed by the numerical investigation. Titanium-oxides composites are further prepared through controlled thermal oxidation process. Uniquely resonant visible absorption bands are observed and tunable by adjusting the process parameters. Enhanced PEC activity is obtained on the optimal composites. To promote the light to chemical energy conversion, synergistic effect of visible light responsivity and hybrid-junction has been studied for the heterostructure of titanium-oxides composites and molybdenum disulfide (MoS2), i.e., MoS2/TiO2-x/Ti photocathode. Improved hydrogen evolution activities are obtained with reduced overpotential closing to zero-bias under illumination, as well as an increased lifetime by more than ten times. The novel hybrid-junction mechanism is disclosed as a facile and effective engineering strategy to accelerate charge separation and transfer, improving PEC performance. Finally, to address and understand the underlying principles of resonant visible light absorption band for enhanced solar-driven water splitting reactions, titanium-oxides nanocomposites have been designed by combined processes of chemical etching and thermal diffusion. Strongly enhanced light-matter interactions are presented in the nanocomposites exhibiting a highly strong resonant light absorption band in the visible region. Efficient transfer of photonic energy to charge carriers and accelerated charge separation and migration lead to achieving significant PEC conversion efficiency and extended promising long-term stability. Our results strongly suggest that a proper level of oxygen vacancies in metal-oxides composites like titanium-oxides may generate an abundance of free charge states near the Fermi level and support surface plasmon resonance under specific visible irradiation. The titanium-oxides nanocomposites with gradient distribution of oxygen atoms represent a conceptually different approach to enable the specific light capture and promote the conversion of solar to chemical energy for water splitting application and sustainable hydrogen production. Our findings show that titanium-oxides composites could exhibit resonant visible light absorption band and promote the separation and transportation of photoexcited charge carriers. The work opens new paths to engineer titanium-oxides composites for specific visible light response and demonstrates the ability of harvesting visible light energy for enhanced PEC water splitting and potential application capability in other fields.