Synthesis, physicochemical properties and visible light photocatalytic studies of molybdenum, iron and vanadium doped titanium dioxide

In this work, we use an inorganic titanium precursor (titanium oxysulfate, TiOSO4) for the preparation of doped TiO2 by the sol-gel method. Molybdenum (Mo), iron (Fe) and vanadium (V) in the concentration range of 0.0312-2.0 mol% are added as dopants. The prepared samples were characterized by XRD, FTIR, N-2 isotherms, SEM-EDX, TEM, UV-Vis DRS and PL techniques. X-ray diffraction revealed that an increase in the concentration of Mo causes the transformation of brookite into anatase phase. In contrast, Fe and V favor the formation of brookite phase. A decrease in crystallite size was observed on the addition of dopant ions compared to that of pure TiO2 (7 nm), for 0.0312 mol% concentration of dopant in TiO2, a crystallite size of 6.3 nm (Mo), 6.2 nm (Fe) and 6.4 nm (V) was calculated. A noticeable change in anatase lattice parameter along the c direction was observed with increasing dopant concentration, this is evidence that the dopant ions have entered the TiO2 crystal lattice by substitutional doping, which induces local distortion of the crystal structure and variation in interatomic distances. An increase in BET surface area and a decrease in pore size were observed with increasing dopant concentration, this may be due to their substitutional doping in crystal lattice and or their presence in the grain boundaries hindering the crystal growth. Diffuse reflectance studies indicate that doped TiO2 showed visible light absorption compared to that of pure TiO2 (3.1 eV). For 0.25 mol% concentration of dopant in TiO2, energy values of 2.6 eV (Mo), 2.95 eV (Fe) and 2.92 eV (V) were obtained. PL analysis clearly shows that the dopant ions have the capability to alter the oxygen vacancies and to produce Ti+3 surface species which trap the photogenerated electron and transfer the electron to surface absorbed oxygen and results in the decrease of electron hole pair recombination rate. Visible light photoactivity tests for the discoloration of methylene blue demonstrated that the doped samples showed enhanced photoactivity compared to undoped TiO2 and commercially available Degussa P25, because of composite nature, surface hydroxyl groups, narrowed band gap and decreased recombination rate of photogenerated electron-hole pairs. The highest photocatalytic activity was observed with 0.25 mol% dopant concentration, the calculated reaction rate constant (k(obs)) was 0.97 min(-1) (Mo), 1.18 min(-1) (Fe) and 1.17 min(-1) (V) compared to 0.18 min(-1) (undoped TiO2) and 0.17 min(-1) (Degussa P25).