Effects of a Nanoparticulate TiO₂ Modifier on the Visible-Light CO₂ Reduction Performance of a Metal-Complex/Semiconductor Hybrid Photocatalyst

Graphitic carbon nitride nanosheets (NS-C3N4) combined with a binuclear Ru(II)–Re(I) complex (RuRe) consisting of a photosensitizer and catalytic units are capable of selectively reducing CO2 to CO under visible light (λ > 400 nm) using triethanolamine as an electron donor. In this system, the grafting of the nanoparticulate rutile TiO2 on the NS-C3N4 surface has previously been shown to enhance photocatalytic performance because of improved charge separation between the NS-C3N4 and the TiO2 and the reinforced adsorption of the RuRe. Here, a more detailed investigation of various polymorphic TiO2 species loaded onto the NS-C3N4 and the visible-light CO2 reduction activity of the resultant photocatalysts was conducted. The experimental results showed that the RuRe/anatase-TiO2/NS-C3N4 outperformed analogues with other TiO2 polymorphs in terms of the CO generation rate, with a maximum catalytic turnover number of ∼100. Transient absorption and emission spectroscopy measurements were carried out to clarify the origin of the different CO evolution activities provided by different TiO2 modifiers. The results revealed that the TiO2 modifiers not only affected the charge separation ability but also controlled the efficiency of back electron transfer from the Ru-photosensitizer unit in the RuRe to the TiO2. The results also showed that, among the investigated TiO2 polymorphs, anatase best facilitated the forward electron transfer from the NS-C3N4 to the TiO2 while suppressing the undesirable back electron transfer reaction.