Incorporation of P5+ and P3− from phosphate precursor in TiO2:P coatings produced by PEO: XPS and DFT study

Abstract

The study of the incorporation of P ions in TiO2 coatings was carried out in this work using Plasma Electrolytic Oxidation (PEO) technique, which was analyzed using XPS and compared with simulations using DFT. The synthesis was carried out using the PEO technique making variations of the useful duty cycle between 10 and 50%. The solution used is composed of Na3PO4 and NaOH, with a process duration of 7 min. The TiO2/P coatings were characterized by XPS with high resolution spectra and analyzed by deconvolution of the spectra. The XPS results showed the incorporation into the crystal lattice of P ions in the form of phosphate ions [(PO4)3−] and phosphide ions (P3−). Density functional theory (DFT) calculations were carried out to demonstrate the formation of various structural defects in titanium dioxide systems. The results show the coexistence of TiO2, TiO2/PO4 and TiO2/P. In this case, the reduction of phosphorus from P5+ to P3− favors the generation of titanium phosphide (TiP) that occurs as a consequence of the high temperature generated in discharge channels (ton). Single and multiple P substitutions formation energies are presented showing that all the substitutions studied are energetically favorable. The single replacement of the P atom in TiO2 sites were calculated showing that the replacement of P at the Ti site is energetically preferred over the replacement of P at the O site with replacement energies of 0.47 eV and 1.42 eV respectively. Along the same lines, a narrowing of the band gap and a shift to the left of the states were observed in all the calculations presented due to the presence of P in the general structure. Finally, the calculation of various substitutions of P in TiO2 with the amount of P replaced in the experimental observations showed an accumulation of electron density in the energy gap attributed mainly to the orbitals of the substituted P.