In this study, TiO₂ heterostructures with anatase and brookite phases, as well as oxygen vacancies, were synthesized using a modified sol-gel method and a heat treatment process. Subsequently, Au nanoparticles (Au NPs) were deposited onto TiO₂ through photo-deposition. Detailed structural and chemical analyses verified the successful creation of anatase-brookite phases, efficient incorporation of Au NPs, and strong interactions between the Au NPs and the oxygen vacancies on the TiO₂ surface. Spectroscopic analysis revealed the presence of localized surface plasmon resonance (LSPR) from the Au NPs, indicating enhanced light absorption properties. The photocatalytic efficiency of the Au-TiO₂ composites was evaluated under solar light irradiation for the conversion of o-phenylenediamine to 2-methylbenzimidazole and hydrogen (H₂) production. Notably, the 2 % Au-TiO₂ catalyst achieved a remarkable 99.7 % conversion rate of o-phenylenediamine, with 90 % selectivity toward 2-methylbenzimidazole and the highest H₂ production rate within 9 h, significantly outperforming 2 % Au/UV100 (commercial TiO₂), 2 % Pd/TiO₂, and pure TiO₂. This enhanced photocatalytic performance is attributed to increased surface acidity (from both Lewis and Brønsted acid sites), efficient charge separation, increased photocurrent, reduced charge transfer resistance and the synergistic interactions between Au NPs and surface oxygen vacancies in TiO₂. These findings highlight the potential of Au-TiO₂ heterostructures for advancing solar-driven catalytic applications, promoting both clean energy generation and efficient organic transformations.