Broad Solar Spectrum-Responsive and Highly Efficient Photoanode of Nonstoichiometric TiO2 Nanoplates/Reduced Graphene Oxide

Abstract

Although a photoelectrochemical approach offers one of the most promising processes for solar energy conversion, it still suffers from the unavailability of photocatalysts that can absorb the photons across the ultraviolet–visible region and efficiently generate electron–hole pairs with longer lifetimes. Here, we report the fabrication of heterostructure comprising reduced TiO2 (TiOx) and reduced graphene oxide (RGO) through a facile nonhydrolytic route using benzyl alcohol as a solvent, shape-directing, and a strong reducing agent. Reduction of Ti4+ and graphene oxide was observed in the absence of any additional reducing agent. Nearly monoshaped square nanoplates of TiOx with size in the range of 10–20 nm were formed. Nanoplates consisted of Ti3+ and were capable of absorbing a broad solar spectrum (ultraviolet–visible region). Formation of Ti3+ was confirmed by electron paramagnetic resonance spectroscopy, while a Mott–Schottky plot corroborated the band shift in TiOx. Findings suggest that defects were introduced both on the surface and in the bulk. A plausible mechanism for the formation of TiOx endowed with a reduced metal center is proposed. Photoelectrochemical activity was investigated for water oxidation under visible (λ > 420 nm) or UV–visible (λ = 300–600 nm) radiation and compared with those of anatase TiO2 and nitrogen-doped TiO2. Electrochemical impedance spectroscopy was employed to gain further insights into the electrical conductivity and the charge transfer process in anatase TiO2, N-doped TiO2, and benzyl alcohol-derived TiOx and TiOx/RGO under dark, visible, and UV–visible illuminations.