Laser defect engineering of TiO2 and beyond: the effect of defects on surface chemistry, adsorption, and catalysis

Small things can make a big difference. While defects are small by definition, they can profoundly influence material properties. However, studying these minute features is challenging and often requires methods to carefully create defects and techniques to precisely characterize them. In this thesis, we explore the effects of point defects in TiO2 on surface chemistry, adsorption, and catalysis. First, Ti3+/oxygen vacancy defects are systematically introduced in TiO2 through sophisticated control of pulsed laser defect engineering in liquid (PUDEL) using a flat jet setup with single-pulse conditions, while the thermodynamically stable phase of rutile TiO2 is chosen to avoid other structural changes. We then develop a method to quantify surface hydroxyls on metal oxides by substituting them with fluoride ions. This dual approach of laser defect engineering and fluoride substitution reveals that Ti3+/oxygen vacancy defects created by laser only induce the formation of acidic bridging hydroxyls on the surface, without altering the abundance of basic terminal hydroxyls. The effectiveness of fluoride ions as probes for surface hydroxyls is further validated on other metal oxides, including Al2O3, ZrO2, Fe2O3, and Co3O4, where a linear correlation between surface hydroxyl density and surface fluoride density is observed. Furthermore, the impact of local charge deviations on colloidal adsorption is investigated via finite element method simulations, demonstrating that the defect density is more influential than the defect potential. In addition, we examine the effect of Ti3+/oxygen vacancy defects on catalysis using anaerobic methylene blue decolorization and aerobic photocatalytic degradation as case studies. Our findings indicate that these defects tend to reduce photocatalytic activity, likely due to an excess of defects beyond the optimal amount suggested in the literature. Finally, InOx/ZrO2 catalysts prepared via pulsed laser ablation in liquid (PLAL) are tested for CO2 hydrogenation to methanol in a slurry phase, where small, crystalline InOx particles exhibit the highest catalytic activity. Overall, our results demonstrate that defects can be effectively studied through careful control and characterization. Although this thesis is relatively fundamental and mainly focuses on the model material of TiO2, we believe much of the methodology can be extended to real-world catalysts. The role of defects, despite their scarcity, should not be underestimated in understanding material properties, particularly the structure-activity relationship in catalysis.