Surface and electronic properties of rutile TiO2 thin films coated with PbO2

Abstract

Binary films of semiconductors have lots of potential applications in material science. Initially, these films were based on In2O3 but, because of its high coast, alternative materials to replace it have been intensely sought after. In this sense, TiO2 and PbO2 are both n-type degenerated semiconductors, which can be good alternatives due to its easy preparation and low cost. Its unique characteristics justify investigating the properties of films composed of both materials. Structural and electronic surface properties of thin films with crystallographic planes (0 0 1), (0 1 0), (1 0 1), and (1 1 0) of TiO2 coated with PbO2 (TiO2/PbO2), both in rutile form, have been investigated in this study. Pure TiO2 and PbO2 bulk and TiO2 thin films have also been studied for comparison purposes. Periodic quantum mechanics DFT calculations have been performed using B3LYP hybrid density functional and taking into account long-range interactions. In both bulks, the energy near the top of the valence band comes mainly from the O 2p orbitals. In the PbO2 the states in the lower energy region of the conducting band are basically composed of Pb 6s and 6p orbitals, while in TiO2 those states come mainly from the Ti 3d orbitals. The relative stability of the TiO2 thin films (surfaces) follows the sequence (1 1 0) > (0 1 0) > (1 0 1) > (0 0 1), but changes substantially in the TiO2/PbO2 binary thin films increasing as (0 1 0) > (1 0 1) > (0 0 1) > (1 1 0). Band gap energies of the four studied pure TiO2 thin films are practically the same as the bulk. However, there is a remarkable decrease in the band gap energies of binary films compared to pure TiO2 films. The decrease in the bottom of the conduction band is due to the contribution of Pb 6s and 6p orbitals at energies below the bottom of the conduction band of the pure TiO2 films.