The cooperative role of nitrogen defects and cyano-group functionalization in carbon nitride towards enhancing its CO2 photoreduction activity

Abstract

Solar-driven CO₂ conversion to produce valuable fuels is an emerging approach that has the potential to shift our reliance away from rapidly depleting fossil fuels in a sustainable and environmentally friendly way. Carbon nitride (CN), a metal-free photocatalyst, is promising due to its high stability and low cost but suffers from low sunlight absorption and fast recombination of photo-generated electron–hole pairs. In this study, we present a facile approach to introduce cyano-functionalization and N vacancies into CN using NaBH₄ reduction treatment to overcome these challenges. Structural analyses using XRD, FTIR and XPS confirmed the incorporation of cyano (–CN) groups and deficiencies of pyridinic nitrogen in the CN framework. The extent of functionalization and defects in CN, and consequently its optical, electron and photocatalytic properties are governed by the temperature and flow conditions for the reduction process. The best resulting material showed a 2.6-fold enhancement of product selectivity towards CH₄ with the generation rates reaching 165.7 μmol g_cat⁻¹ h⁻¹. The optimal NaBH₄ reduction treatment results in a lower bandgap, enhanced surface concentration of electrons, higher thermodynamic driving force of CO₂ reduction and reduced charge recombination, analysed using UV-DRS, VB-XPS, EIS and DFT calculations. The excessive CN and N-defects, however, are shown to greatly affect the electron mobility, charge separation and photoactivity. Our findings provide optimal conditions to introduce CN groups and N-defects to design highly efficient CN for photocatalytic applications.