Highly active and stable multi-walled carbon nanotubes-graphene-TiO<inf>2</inf> nanohybrid: An efficient non-noble metal photocatalyst for water splitting

Novel multiwalled carbon nanotubes-graphene-TiO2 (CNT-GR-TiO2) composite materials without noble metal co-catalysts are designed for photocatalytic decomposition of water using solar light. The CNT-GR-TiO2 nanocomposite shows the highest H2 production rate of 29 mmol h−1g−1 under the full spectrum of solar light irradiation. The rate of H2 production is 8-fold higher than the commercial TiO2 (Degussa P25) and the estimated solar energy conversion efficiency is 14.6%. Spectroscopic and photocatalytic studies reveal that graphene acts as an electron reservoir through which interfacial charge transfer occurs for water splitting. The UV-Vis-DRS study shows that the absorption peak maximum for anatase TiO2 occurs at ∼315 nm, which is shifted to ∼355 nm and 380 nm for GR-TiO2 and CNT-GR-TiO2 composites, respectively. The EPR spectra of GR-TiO2 and CNT-GR-TiO2 composites indicate that graphene and multiwalled carbon nanotubes in the composites promote the generation of Ti3+ and oxygen vacancies and in turn reduce the band gap of anatase TiO2 from 3.32 eV to 2.79 eV. This is corroborated by XPS and photoluminescence analyses of the samples. The role of CNTs is to prevent the restacking of graphene nanosheets and provide additional electron transport channels thereby suppressing the recombination rate of electron-hole pairs in the CNT-GR-TiO2 composite. The combination of all these factors results in increasing the hydrogen production rate from 19 mmol h−1 g−1 (anatase TiO2) to 22 mmol h−1 g−1 (GR-TiO2) to 29 mmol h−1 g−1 (CNT-GR-TiO2).