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Abstract

Potassium poly(heptazine imide) (PHI) is a photocatalytically active carbon nitride material that was recently prepared from substituted 1,2,4-triazoles. Here we show that the more acidic precursors, such as commercially available 5-aminotetrazole, upon pyrolysis in LiCl/KCl salt melt yield PHI with the greatly improved structural order and thermodynamic stability. Tetrazole-derived PHIs feature long range crystallinities and unconventionally small layer-stacking distances leading to the altered electronic band structures as shown by Mott-Schottky analyses. Under the optimized synthesis conditions, visible light driven hydrogen evolution rates reach twice the rate provided by the previous golden standard, mesoporous graphitic carbon nitride having much higher surface area. More interestingly, the up to 0.7 V higher valence band potential of crystalline PHI compared to the ordinary carbon nitrides makes it an efficient water oxidation photocatalyst which works even in the absence of any metal-based co-catalysts under visible light. To our knowledge, this is the first case of a metal free oxygen liberation from water as such.