Insights into Reinforced Photocatalytic Activity of the CNT–TiO₂ Nanocomposite for CO₂ Reduction and Water Splitting

Using titanium dioxide (TiO2) and its modified forms for the photocatalytic reduction of CO2 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 TiO2 nanoparticles modified with multiwalled carbon nanotubes (CNTs) via a simple combined sonothermal method, followed by a hydrothermal treatment. In comparison to bare TiO2, the synthesized CNT–TiO2 photocatalysts showed improved photocatalytic activities for CO2 reduction under UVA as well as under visible light and water (H2O) splitting under visible light at ambient temperature and pressure. The 2.0CNT–TiO2 has performed the best for methanol, hydrogen, and formic acid production from the reduction of CO2 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–TiO2 under visible light at pH 2. The primary photocatalytic reactions of CNT–TiO2 composites and their intimate structure were studied computationally. It was demonstrated that the binding of CNT to TiO2 nanoparticles is preferable at (101) surfaces than at (001) facets. Interaction of CNT with TiO2 results in common orbitals within the TiO2 band gap that enables visible light excitation of the CNT–TiO2 composites can lead to charge transfer between TiO2 and CNT, whereas UV light excitation can result in charge transfer in any direction from CNT to TiO2 and from TiO2 to CNT. The latter process is operative in the presence of a sacrificial electron donor triethanolamine