Role of Pr on the Semiconductor Properties of Nanotitania. An Experimental and First-Principles Investigation

Nanostructured praseodymium doped titania (Pr-TiO2) samples were obtained in the 7−10 nm range starting from a classical sol−gel synthesis, and the effects of the dopant on the semiconductor properties have been extensively studied. The materials, synthesized at various nominal Pr/Ti molar ratios (0.2, 0.3, 0.5, and 0.7%), were investigated by Xray powder diffraction, high-resolution transmission electron microscopy, UV−vis spectroscopy, N2 adsorption−desorption isotherm, and EDX analysis. A complete photoelectrochemical characterization was also carried out by means of photocurrent and photovoltage measurements. It was found that Pr doping induces high crystallinity and sometimes slows the recombination of photogenerated electrons and holes in TiO2, modifying the absorption spectra with specific features in the visible region. The effects of the dopant on the band energy level, surface area, pore volume, and crystal size of the Pr-TiO2 samples were systematically investigated as well. The experimental picture was implemented by plane-wave bulk DFT calculations that allowed us to reach a thorough and complete understanding of the energy states originating from the dopant in the bandgap and provided important insights into the interplay among structural and electronic degrees of freedom in the lattice. In particular, strong evidence emerged that the foreign Pr ion should be present as substitutional in the titania lattice and electronic photoexcitation enhancements are generated by the presence of f orbitals just below the conduction band. Therefore, nanostructured Pr-TiO2 can be considered to be a promising photocatalytic material.