Potassium Poly(heptazine imides) from Aminotetrazoles: Shifting Band Gaps of Carbon Nitride-like Materials for More Efficient Solar Hydrogen and Oxygen Evolution

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 gold standard, mesoporous graphitic carbon nitride, which has a much higher surface area. More interestingly, the up to 0.7 V higher valence band potential of crystalline PHI compared with 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 metal-free oxygen liberation from water.