Atomically thin two-dimensional nanosheets of nitrogen-rich C3N4.8, C3N6, and C3N7 are synthesized by sonochemical process. Despite their high nitrogen content, their triazole-based crystal structures remain intact after exfoliation. Among the present materials, the nitrogen-richest C3N7 nanosheets display the highest photocatalytic activity for ammonia production, highlighting the synergetic effect of composition control and exfoliation.