Plasma electrolytic preparation of film CoWO4/WO3 p-n heterostructures and their photocatalytic and electrochemical properties

Abstract

Photoactive CoWO₄/WO₃ coatings were fabricated by single-stage plasma electrolytic oxidation (PEO) of titanium in homogeneous electrolytes containing tungstate ions and stable EDTA-chelated Co(II) in different Co:W molar ratio. Changing the Co:W ratio in the electrolyte in the order of 1:3, 1:2, 1:1 allows obtaining coatings containing o-WO₃ and m-CoWO₄ in ratios approximately equal to 2:1, 1:1 and 2:3. The optical band gap values of the samples for direct allowed transitions are 2.5–2.6 eV. The formed CoWO₄/WO₃ coatings exhibit photocatalytic activity in the methyl orange degradation (10 mg/L MO, 10 mmol/L H₂O₂, pH 6.8) when irradiated with visible and UV light. Under UV irradiation, the highest activity (82 % in 180 min) is exhibited by PEO-coated samples with a predominance of WO₃, and in the visible region (33 % in 180 min) – by samples with CoWO₄:WO₃ = 1:1. The photoactivity of the samples is due to the formation of p-CoWO₄/n-WO₃ junction, which was detected in the Mott-Schottky plots.

The MO solution after photocatalytic tests was studied by COD, FTIR and GC–MS. It was found that MO degradation products are low-molecular compounds, derivatives of benzene and long-chain alkanes. Based on the data of electrochemical and optical measurements, a possible mechanism of MO degradation is proposed.