Abstract: Sustainable development is key to addressing energy and environmental challenges. Photocatalysis presents a viable alternative for converting and storing solar energy into fuels such as hydrogen (H₂), while also contributing to the reduction of carbon dioxide (CO₂) emissions and the degradation of pollutants in air and water. The design of efficient, cost-effective, and stable photocatalysts is essential for advancing solar energy conversion technologies. Titanium dioxide (TiO₂) is the most commonly used photocatalyst, known for its low toxicity, low cost, good photocatalytic activity, and chemical and biological stability. However, its use is limited by the rapid and high rate recombination of the electron-hole pair (exciton) and the fact that it can only be excited under UV irradiation due to its large band gap (3.0–3.2 eV). This doctoral thesis focuses on the development of surface-modified photocatalysts with metallic nanoparticles (NPs), which exhibit a synergy between their optical and electronic properties, enhancing the photocatalytic process. Phenomena such as localized surface plasmon resonance (LSPR) in certain transition metals (Au, Pd, Ag) and the formation of the Schottky barrier allow for greater absorption of radiation in the visible spectrum and reduce exciton recombination, thereby increasing photocatalytic activity. In this project, the surface of TiO₂ has been modified with plasmonic mono- and bi-metallic NPs, as well as metal oxide NPs (Au, Pd, AuPd, NiFe, NiO), synthesized through chemical and radiolysis methods. These NPs are used as cocatalysts for green hydrogen generation via photocatalysis. We also demonstrated that NPs shape is crucial when using anisotropic gold nanostar-shaped NPs (AuNSs) in contaminated water treatment, effectively reducing the toxic compound 4-nitrothiophenol (4-NTP) to 4-aminothiophenol (4-ATP) under visible light irradiation. This doctoral research contributes to the understanding of photocatalytic and plasmonic properties, highlighting the potential of nanostructured materials to improve catalytic performance and stability for sustainable energy and environmental applications.