Ti/TiO2-CoWO4-Co3(PO4)2 composites: Plasma electrolytic synthesis, optoelectronic properties, and solar light-driven photocatalytic activity

Abstract Co-, W-, and P-containing oxide heterostructures on titanium substrates have been obtained by plasma electrolytic oxidation at current densities of 0.1 and 0.15 A/cm2. The heterostructures have been studied by the X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. Conglomerates of nanoparticles with an elemental composition corresponding to CoWO4, CoO, and Co3(PO4)2 have been found on the surface of the samples. Air-annealing of samples at 700 °C for 1 h leads to the transformation of nanoparticles into nanocrystals with CoWO4 and Co3(PO4)2 phases. The annealed samples and the initial samples obtained at 0.15 A/cm2 are also contain the crystalline phase of TiO2. All samples are characterized by a small bandgap: Eg values for the direct (n = 2) and indirect (n = 1/2) transitions are equal 2.77 and 2.07 eV, respectively, for both initial samples. All samples exhibit photocatalytic activity in indigo carmine (IC) degradation at pH = 2.9 under solar light illumination. The proposed mechanism of solar light-driven photocatalytic activity of Ti/TiO2-CoWO4-Co3(PO4)2 structures includes the formation of p-n heterojunction between TiO2 and CoWO4, capture of holes by Co3(PO4)2, and producing a high-valence cobalt ions Co3+/Co4+, which could further induce oxidation reactions with water molecules and IC.