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. Copyright © 2014 RSC Pub.