Polymorphic phase study on nitrogen-doped TiO2 nanoparticles: effect on oxygen site occupancy, dye sensitized solar cells efficiency and hydrogen production

Abstract

In this work we show that phase formation and oxygen substitution can be controlled by the source of nitrogen used during the synthesis of TiO₂ nanoparticles. By performing a thorough study on the structure of the nanoparticles, the use of NH₄⁺ or NO₃⁻ was found to influence not only the N-doping level but also the formation of the polymorphic phase. Structural and microstructural refinement obtained by XRD pattern and data processing performed by the Rietveld refinement revealed that TiO₂ obtained with HNO₃ presents ca. 98% of anatase and ca. 2% for rutile. Meanwhile TiO₂ nanoparticles synthesized with NH₄F and NH₄Cl presents a single anatase phase with ca. 7.0% and 4.4% of nitrogen substitutional oxygen sites, respectively. The local structure of N-doped TiO₂ around the Ti atoms was investigated by X-ray absorption spectroscopy. The XANES spectra show that N-doped TiO₂ possesses a characteristic pre-edge of a single anatase structure. The coordination number decreased and the shrinking Ti–O bond distances are due to the N-doping in the TiO₂ structure. The most efficient dye sensitized solar cell and the higher hydrogen production was obtained from the TiO₂/NH₄Cl, which was obtained as a single anatase phase with intermediary concentration of N substitutional oxygen sites.