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The photocatalytic properties of titanium dioxide have been widely studied since the discovery in 1972 of water photolysis by TiO2 electrodes. Rutile and anatase are the most studied of the different TiO2 polymorphs due to their chemo-physical properties. When irradiated with ultraviolet light, TiO2 is able to absorb photons, creating on the surface an electron and a positive “hole”. This electron-hole pair then reacts with water and oxygen, generating chemical radicals. These are very unstable and reactive species which can neutralise pollutants. In the introduction pollutants and their influence in the air quality are described as well as the state of art of TiO2, photochemistry; semiconductor doping, chemo-physical principles, and TiO2 coatings. Various protocols to test photoactivity of both powders and coatings are discussed studying both gas phase and liquid phase reactions looking at UV and visible light irradiation. The sol-gel synthesis of pure and tungsten- and cobalt-doped TiO2, as well as graphene/TiO2 hybrids, was explored modifying the conditions and parameters to optimise the photocatalytic activation of TiO2 in the visible light range. A comprehensive study of manufactured and commercially available TiO2 powders and coatings was performed to identify the differences between photocatalytic properties, using electron microscopy, Raman and UV-vis diffusive spectroscopy and X-ray diffraction. An important question that was answered in this thesis is whether the physical properties of nanoparticles or their electronic properties are critical for their photocatalytic behaviour. Results for doped powders of different particle size and surface area showed how the positioning of their electronic band gap with the wavelength of the visible light source was fundamental for an effective photocatalytic process for the application wantedIn the application of TiO2 for the built environment, lime and MDF were considered as substrates for coatings. Lime-based coatings were prepared mixing commercial K7000 TiO2 with the plaster and photoactivity of the coatings was evaluated looking at formaldehyde degradation. MDF based coating were produced using both produced sol-gel pure, doped TiO2 and tested by observing degradation of Ink Intelligent inks under both UV and visible light confirming the photoactivity. Throughout this work sol-gel was employed for the production of pure and doped TiO2 nanoparticles with the anatase crystalline phase. Photocatalytic tests on the synthesised particles under UV light shows comparable performances with commercial particles. Synthesised particles and coatings shows promising performances, higher than commercially available particles when irradiated with visible light confirming a possible application in indoor environment.