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

Abstract

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.