Enhancement of photoelectrochemical organics degradation and power generation by electrodeposited coatings of g-C3N4 and graphene on TiO2 nanotube arrays

Abstract

New g-C₃N₄ coatings obtained via electropolymerization (EP) of melamine followed by a heat treatment and graphene oxide (GO) coatings based on combining GO sheets via EP of GO phenolic groups are used to improve the performance of photoanodes composed of TiO₂ nanotube arrays towards the photoelectrochemical (PEC) oxidation of methanol. This process, as examined in Na₂CO₃ solution (pH 11.4) for the two types of coatings and serving as a model for the degradation of an organic pollutant, demonstrates enhanced PEC performance as compared to that obtained using electrochemically reduced GO coatings. PEC oxidation currents obtained with 1 M methanol reach saturation at potentials as low as ∼−0.4 V vs. Ag/AgCl, with the highest saturation current density of ∼2.6 mA cm⁻² and photon-to-current efficiency of 52% as observed for the new TiO₂NTs/g-C₃N₄ photoanodes. Electrochemical impedance spectroscopy measurements for these photoanodes show a charge transfer resistance one order of magnitude lower than that obtained by the other types of coatings. This indicates an enhanced charge separation ability for the photogenerated electron–hole pairs and faster interfacial charge transfer between the electron donor (methanol) and acceptor (holes). It is also demonstrated that the process of organics degradation can be achieved not only via an applied potential but also in a galvanic photofuelcell with methanol and oxygen serving as the fuel and oxidant, respectively. The power densities achieved with the electrochemically prepared g-C₃N₄ photoanodes (∼0.5 mW cm⁻²) are at least one order of magnitude higher than those reported for other TiO₂-based systems.