Surface-Specific Modification of Graphitic Carbon Nitride by Plasma for Enhanced Durability and Selectivity of Photocatalytic CO₂ Reduction with a Supramolecular Photocatalyst

Abstract

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.

Probing Charge Carrier Behavior in Engineered Photocatalysts with Time-Resolved Visible to Mid-IR Absorption Spectroscopy

“…The decay curves are typically fitted with exponential functions. 168 Suzuki et al have constructed a simple but effective formula to transform the electron decay kinetics to obtain a hole decay kinetics which agrees well with the experimental data observed by TRVIR. 80 Upon a detailed theoretical analysis, relevant photophysical parameters, which include rate constants for recombination and trapping, energy levels of trap states, and densities of trap states can be determined.…”

“…Only few TRVIR studies provide quantitative data, such as lifetimes of charge carriers. The decay curves are typically fitted with exponential functions . Suzuki et al have constructed a simple but effective formula to transform the electron decay kinetics to obtain a hole decay kinetics which agrees well with the experimental data observed by TRVIR .…”

“…An important finding on the surface modification of a particulate photocatalyst was reported by Sakakibara and co-workers. 168 Here, plasma treatment was applied on carbon nitride (C 3 N 4 ) to improve its surface affinity and electron transfer to the photosensitizer. A world-record efficient HCOOH production from CO 2 reaching a turnover number of 50000 after coupling with a Ru(II)−Ru(II) binuclear complex (RuRu′) was achieved.…”

“…Indeed, in many instances, the photocatalyst surface needs to be specifically modified to facilitate the desired redox reactions. An important finding on the surface modification of a particulate photocatalyst was reported by Sakakibara and co-workers . Here, plasma treatment was applied on carbon nitride (C 3 N 4 ) to improve its surface affinity and electron transfer to the photosensitizer.…”

“…TEM images and the proposed electron transfer mechanism are also shown. Adapted with permission from ref . Copyright 2023 American Chemical Society.…”