Low dimensional carbon enhanced semiconductor nanomaterials for photocatalytic applications

Abstract

The environmental consequences of burning fossil fuels and the global demand for energy has generated interest in renewable and clean energy sources. Solar water splitting to produce hydrogen using semiconductors is an attractive process as no emissions are generated, using water and sunlight only. Low dimensional carbon nanostructures unique structural, optical and electronic properties have led to more research in photocatalysis. TiO2 has a high band gap of 3.2 eV resulting in strong absorbance in the UV region of the electromagnetic spectrum but constitutes only 4% of the solar spectrum. Initially, there was research in synthesising 0D carbon quantum dots to improve visible light activity in TiO2-A NF/CQDs. However, UV-Vis Spectroscopy showed absorbance of CQDs below 435 nm, which illustrated minimal absorbance in the visible light region. Nanocomposites provide more efficient separation of electron-hole pairs as it is difficult for a single semiconductor to obtain a narrow band gap for absorbing photons in visible light and appropriate band positions for efficient charge transfer. The photocurrent response in C-NW/TiO2 NF/Cu2O showed a sharp increase compared to bare TiO2 and Cu2O, which indicated higher interparticle charge transfer between 1D TiO2 NF and Cu2O nanocubes. Carbon nanowires were used as a short electron pathway between both semiconductors, as suggested by the proposed schematic. Cu2O has favourable band energy positions for water splitting but is prone to photocorrosion as reduction and oxidation potentials of Cu2O lie within the band gap. Thus, rGO is used as a versatile catalyst support because it has a high surface area, electronmobility and rGO surfaces have a variety of oxygen functionalities. The synthesised Cu2O/rGO offered higher photocatalytic activity (31 μmol.g-1.h-1) and rate of 4-chlorophenol decomposition (95% after 60 minutes) than previous reports. H2 evolution using BiVO4 has not been consistently reported in literature because the conduction band (CB) position of BiVO4 is near the reduction potential of water. QDBiVO4/rGO exhibited higher photocatalytic activity than BiVO4 and QD-BiVO4 in visible light towards BPA photodecomposition and H2 evolution, which was indicative of the nanocomposite’s high surface area and enhanced light absorption. QD-BiVO4/rGO has excellent photostability after testing in several photodecomposition and transient photocurrent cycles.