Template-free synthesis of hierarchical graphitic carbon nitride (H-gC₃N₄) embedded with NiO for water splitting and CO₂ reduction with the role of hole scavenger: A comparative investigation

Hierarchical graphitic carbon nitride (H-gC₃N₄) embedded with nickel oxide (NiO) were synthesized using a template-free hydrothermal approach. The performance of NiO/H-gC₃N₄ composite was investigated for photocatalytic water splitting in a slurry system to produce H₂. Similarly, photocatalytic CO₂ reduction to generate CO under visible light was conducted in a gas-phase photoreactor system. Optimized 2% NiO/H-gC₃N₄ composite enables H₂ production of 1010 μmol g⁻¹ h⁻¹, which was 13.93 folds higher than using pure H-gC₃N₄. Similarly, the highest CO evolution rate of 96.7 μmol g⁻¹ h⁻¹ was obtained, 3.85 folds higher than using H-gC₃N₄. The enhanced photoactivity resulted mainly from the distinctive interlayer structure, improved light penetration, reduced charge carrier recombination and increased surface-reactive active sites. The methanol sacrificial reagent, owing to its capacity to generate extra protons, proved pathways in augmenting the yields of both H₂ and CO. While the efficiency of water splitting for H₂ production surpassed that of CO₂ reduction, it exhibited lower stability. The recently developed composite demonstrated consistent and sustained photocatalytic efficiency for CO production throughout five consecutive cycles.