Water-assisted production of honeycomb-like g-C₃N₄ with ultralong carrier lifetime and outstanding photocatalytic activity

Graphitic carbon nitride (g-C3N4) is a visible light photocatalyst, limited by low activity mainly caused by rapid recombination of charge carriers. In the present work, honeycomb-like g-C3N4 was synthesized via thermal condensation of urea with addition of water at 450 °C for 1 h. Prolonging the condensation time caused the morphology of g-C3N4 to change from a porous honeycomb structure to a velvet-like nanoarchitecture. Unlike in previous studies, the photocatalytic activity of g-C3N4 decreased with increasing surface area. The honeycomb-like g-C3N4 with a relatively low surface area showed highly enhanced photocatalytic activity with an NO removal ratio of 48%. The evolution of NO2 intermediate was dramatically inhibited over the honeycomb-like g-C3N4. The short and long lifetimes of the charge carriers for honeycomb-like g-C3N4 were unprecedentedly prolonged to 22.3 and 165.4 ns, respectively. As a result, the honeycomb-like g-C3N4 was highly efficient and stable in activity and could be used repeatedly. Addition of water had the following multiple positive effects on g-C3N4: (1) formation of the honeycomb structure, (2) promotion of charge separation and migration, (3) enlargement of the band gap, (4) increase in production yield, and (5) decrease in energy cost. These advantages make the present preparation method for highly efficient g-C3N4 extremely appealing for large-scale applications. The active species produced from g-C3N4 under illumination were confirmed using DMPO-ESR spin-trapping, the reaction intermediate was monitored, and the reaction mechanism of photocatalytic NO oxidation by g-C3N4 was revealed. This work could provide an attractive alternative method for mass-production of highly active g-C3N4-based photocatalysts for environmental and energetic applications.