Structural and vibrational investigations of Nb-doped TiO2 thin films

Acid-catalyzed sol–gel and spin-coating methods were used to prepare Nb-doped TiO2 thin film. In this work, we studied the effect of niobium doping on the structure, surface, and absorption properties of TiO2 by energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray reflectometry (XRR), X-ray photoelectron spectroscopy (XPS), Raman, and UV–vis absorption spectroscopy at various annealing temperatures. EDX spectra show that the Nb:Ti atomic ratios of the niobium-doped titania films are in good agreement with the nominal values (5 and 10%). XPS results suggest that charge compensation is achieved by the formation of Ti vacancies. Specific niobium phases are not observed, thus confirming that niobium is well incorporated into the titania crystal lattice. Thin films are amorphous at room temperature and the formation of anatase phase appeared at an annealing temperature close to 400 °C. The rutile phase was not observed even at 900 °C (XRD and Raman spectroscopy). Grain sizes and electron densities increased when the temperature was raised. Nb-doped films have higher electron densities and lower grain sizes due to niobium doping. Grain size inhibition can be explained by lattice stress induced by the incorporation of larger Nb5+ ions into the lattice. The band gap energy of indirect transition of the TiO2 thin films was calculated to be about 3.03 eV. After niobium doping, it decreased to 2.40 eV.