Photocatalytic CO2 reduction is in high demand for sustainable energy management. Hybrid photocatalysts combining semiconductors with supramolecular photocatalysts represent a powerful strategy for constructing visible-light-driven CO2 reduction systems with strong oxidation power. Here, we demonstrate the novel effects of plasma surface modification of graphitic carbon nitride (C3N4), which is an organic semiconductor, to achieve better affinity and electron transfer at the interface of a hybrid photocatalyst consisting of C3N4 and a Ru­(II)–Ru­(II) binuclear complex (RuRu′). This plasma treatment enabled the “surface-specific” introduction of oxygen functional groups via the formation of a carbon layer, which worked as active sites for adsorbing metal-complex molecules with methyl phosphonic-acid anchoring groups onto the plasma-modified surface of C3N4. Upon photocatalytic CO2 reduction with the hybrid under visible-light irradiation, the plasma-surface-modified C3N4 with RuRu′ enhanced the durability of HCOOH production by three times compared to that achieved when using a nonmodified system. The high selectivity of HCOOH production against byproduct evolution (H2 and CO) was improved, and the turnover number of HCOOH production based on the RuRu′ used reached 50 000, which is the highest among the metal-complex/semiconductor hybrid systems reported thus far. The improved activity is mainly attributed to the promotion of electron transfer from C3N4 to RuRu′ under light irradiation via the accumulation of electrons trapped in deep defect sites on the plasma-modified surface of C3N4.

Graphitic carbon nitride nanosheets (NS-C 3 N 4 ) combined with a binuclear Ru(II)−Re(I) complex (RuRe) consisting of a photosensitizer and catalytic units are capable of selectively reducing CO 2 to CO under visible light (λ > 400 nm) using triethanolamine as an electron donor. In this system, the grafting of the nanoparticulate rutile TiO 2 on the NS-C 3 N 4 surface has previously been shown to enhance photocatalytic performance because of improved charge separation between the NS-C 3 N 4 and the TiO 2 and the reinforced adsorption of the RuRe. Here, a more detailed investigation of various polymorphic TiO 2 species loaded onto the NS-C 3 N 4 and the visible-light CO 2 reduction activity of the resultant photocatalysts was conducted. The experimental results showed that the RuRe/anatase-TiO 2 /NS-C 3 N 4 outperformed analogues with other TiO 2 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 TiO 2 modifiers. The results revealed that the TiO 2 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 TiO 2 . The results also showed that, among the investigated TiO 2 polymorphs, anatase best facilitated the forward electron transfer from the NS-C 3 N 4 to the TiO 2 while suppressing the undesirable back electron transfer reaction.