Insights into Reinforced Photocatalytic Activity of the CNT-TiO 2 Nanocomposite for CO 2 Reduction and Water Splitting

Using titanium dioxide (TiO 2 ) and its modified forms for the photocatalytic reduction of CO 2 reduction and production of hydrogen is a promising route for providing solutions to the world energy demand in the foreseeable future. Here, we report the synthesis of a series of efficient stable TiO 2 nanoparticles modified with multiwalled carbon nanotubes (CNTs) via a simple combined sonothermal method, followed by a hydrothermal treatment. In comparison to bare TiO 2 , the synthesized CNT-TiO 2 photocatalysts showed improved photocatalytic activities for CO 2 reduction under UVA as well as under visible light and water (H 2 O) splitting under visible light at ambient temperature and pressure. The 2.0CNT-TiO 2 has performed the best for methanol, hydrogen, and formic acid production from the reduction of CO 2 with yield rates of 2360.0, 3246.1, and 68.5 μmol g -1 h -1 under UVA, respectively. Its potential was further tested under visible light for methanol production, 1520.0 μmol g -1 h -1 . Also, the highest rate of hydrogen yield from water splitting was 69.41 μmol g -1 h -1 with 2.0CNT-TiO 2 under visible light at pH 2. The primary photocatalytic reactions of CNT-TiO 2 composites and their intimate structure were studied computationally. It was demonstrated that the binding of CNT to TiO 2 nanoparticles is preferable at (101) surfaces than at (001) facets. Interaction of CNT with TiO 2 results in common orbitals within the TiO 2 band gap that enables visible light excitation of the CNT-TiO 2 composites can lead to charge transfer between TiO 2 and CNT, whereas UV light excitation can result in charge transfer in any direction from CNT to TiO 2 and from TiO 2 to CNT. The latter process is operative in the presence of a sacrificial electron donor triethanolamine.