Nanosized N-doped TiO2 and gold modified semiconductors — photocatalysts for combined UV–visible light destruction of oxalic acid in aqueous solution Original Research Article

The photooxidation of oxalic acid, catalyzed by nanosized TiO2, Au/TiO2, N-doped TiO2 and Au/N-doped TiO2 has been studied upon irradiation with UV, visible and combined UV–visible light. The average size of the synthesized by sol–gel method TiO2 and N-doped TiO2 nanoparticles is 20 nm. The average size of the Au nanoparticles in Au/TiO2 and Au/N-TiO2 is ≈ 5 nm. The XPS measurements confirm the replacing of oxygen atoms with nitrogen in the crystal lattice of TiO2. The rate constants of oxalic acid visible light photooxidation catalyzed by the studied samples follow the order: Au/N-doped TiO2 > N-doped TiO2 ≫ Au/TiO2 > TiO2. The rate constants of oxalic acid photocatalytic degradation upon irradiation with visible light are much lower compared to the rate constants upon UV irradiation. In the case of combined UV–visible light photooxidation of oxalic acid catalyzed by N-doped TiO2 and Au/N-doped TiO2 the rate constants are equal to these registered upon UV light irradiation. This is due to the fact that the photon energy of the UV irradiation, Ehν ≥ band gap energies of the N-doped TiO2 and Au/N-doped TiO2 semiconductors and they can be excited under UV illumination.