Photoelectrochemical degradation of naphthol blue black diazo dye on WO 3 film electrode

Abstract

Photoelectrochemical degradation of a textile diazo dye, naphthol blue black (NBB), has been investigated using different semiconductor electrodes. A higher photoelectrocatalytic activity has been observed for WO 3 film electrodes, prepared by electrodeposition, than for TiO 2 nanoparticulate film electrodes. By annealing of the WO 3 film deposited on a Pt substrate, we were able to increase the photodegradation rate of NBB, owing to the reduced losses due to the electron–hole recombination. This treatment also increased the long-term photostability of the electrode. The effect of the supporting electrolyte was examined using NaCl, NaClO 4, KNO 3, and Na 2SO 4 solutions. The highest NBB degradation rate was found in the acidic Cl − medium. With increasing solution pH, the degradation rate decreased to a lower level indicating a change in the NBB photodecomposition mechanism. The reaction is of the first order with respect to NBB in the concentration range up to ca. 6×10 −5 M NBB. The apparent rate constants for the photoelectrocatalytic degradation of NBB in 0.5 M NaCl+8.8×10 −5 M NBB solution, on a WO 3 film electrode at E=1.08 V versus SCE, are 1.302×10 −4 and 3.41×10 −5 cm −2 s −1, under illumination with ultraviolet and visible light, respectively. It has been found that the mechanism of the sub-band excitation with the dye self-sensitization is a minor degradation path for NBB, while the main path involves the electron–hole generation and heterogeneous oxidation with participation of valence-band electron holes. The latter reactions involve oxidation of solution components with generation of powerful oxidants, such as the OH , Cl 2 −, and Cl radicals, being directly responsible for the fast dye decomposition.