TiO₂ nanotubes were synthesized by the solvothermal process at low temperature in a highly alkaline water–methanol mixed solution. Their characteristics were identified by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), specific surface area (BET), Fourier transform infrared spectroscopy (FTIR) and UV–Vis absorption spectroscopy. The as-prepared samples were tested by the photodegradation reaction of methylene blue (MB) dye under visible-light irradiation. The ratios of methanol and water, as well as calcination temperature, affected the morphology, nanostructure and photocatalytic performance. The methanol solvent plays an important role in improving crystallization of the anatase phase, which affects the photocatalytic reaction. Titanate nanotubes were synthesized in methanol–water volume ratios of 10:90, 20:80 and 30:70 which still had high absorbability. Titania nanotubes formed at a calcination temperature of 300 °C using methanol–water volume ratio of 30:70 showed highest photocatalytic performance, much higher than that using water solvent and TiO₂–P25 powder.

TiO_2–𝑥N_𝑦 film has been synthesized successfully through the sol–gel method. It is found that the anatase phase is formed at 400 °C and converted to rutile phase at 600 °C. The response surface methodology (RSM) and Box–Behnken design were employed to optimize the process conditions of sol–gel process. Based on the results in preliminary experiments, we selected molar ratio of surfactant to Ti, molar ratio of acetylacetone to Ti, molar ratio of water to Ti and calcination temperature as the key process factors affecting the roughness of TiO_2–𝑥N_𝑦 film. The adjusted determination coefficient ($R^{2}_{\text{Adj}}$) of the regression model was 0.9651, which indicated that the regression model is significant. By analysing the contour plots of response surface as well as solving the regression model, the optimized conditions were obtained as: 0.19 for molar ratio of surfactant to Ti, 2.01 for molar ratio of acetylacetone to Ti, 1.38 for molar ratio of water to Ti and 500 °C for calcination temperature. The predicted roughness of TiO_2–𝑥N_𝑦 film for the optimized condition was calculated to be 41 nm. Confirmation experiments using the optimized conditions were performed, and a value about 43 nm was obtained. The experimental results are in good agreement with the predicted results.