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Abstract
Undoped, single doped and codoped TiO2 thin films were fabricated by spin coating of sol-gel solutions on soda-lime-silica glass substrates followed by annealing at 450°C for 2 h. The mineralogical (GAXRD, Raman spectroscopy), microstructural (SEM, AFM), optical (UV-vis spectroscopy), hydrophilic (wetting angles), crystallographic (TEM) and photocatalytic (dye degradation) properties were determined.
Undoped TiO2 thin films with thicknesses of~50-540 nm were fabricated. The films were discontinuous (1 cycle), continuous (4-11 cycles), and damaged (13-15 cycles), with the highest quality films being the continuous ones. The discontinuous and damaged films showed high wetting angles while the continuous films showed low angles. The free energies were the driving force for oxide bond formation and thus the highest contaminant levels were found for Na+ ions that freely diffused through the microstructure until it achieved saturation solubility.
Co and V single-doped TiO2 thin films were fabricated with dopant levels of 0.05 to 1.00 mol%. All the films were highly transparent and contained anatase as the sole crystalline phase. At the lowest level (0.05 mol%), the dopant ions were distributed irregularly in the film microstructure and this contributed to defect formation and hindrance of nucleation of the TiO2 crystallites. With increasing dopant levels, the dopants distribution became more homogeneous and this favoured anatase recrystallisation and grain growth. Co formed substitutional solid solutions while V formed interstitial solid solutions in the anatase lattice. The changes in grain and microstructural characteristics of the films were owing to variations in dopant solubilites in the lattice. The data showed that Co has low solubility in TiO2 with the saturation limit being ~0.05 mol%. In contrast, for V doping, the solubility limit was higher, approaching 1.00 mol%.
For Co + V codoped TiO2 thin films, substitutional solid solution caused increased anatase recrystallisation at the lowest dopant level. Macroscopic roughening at the highest dopant levels occurred owing to oversaturation, precipitation, and grain boundary pinning. The photocatalytic performance was related to the grain size owing to the associated changes in surface area, number of active sites and partial amorphisation. Intervalence charge transfer effects lowered photoactivity at higher dopant concentrations. Codoping was seen to have a synergistic effect on the photocatalytic performance. Low dopants levels were seen to be ideal for enhancing the photocatalytic performance through modification of the semiconducting properties.