Adaptive states of carbon nitride semiconductor materials treated with hydrofluoric acid

The value of the band gap of a semiconductor is a primary factor in determining its suitability for applications, such as in photovoltaics or photocatalysis. Carbon nitride (C₃N₄; E_gap = 2.7 eV) is an attractive representative, as it is a non-toxic material containing abundant elements. Future technologies, e.g., in optical computing, demand semiconductors in which one can reversibly change key characteristics such as E_gap by preferably simple methods (temperature, pressure, presence of ions) and reversibly. The current paper demonstrates that carbon nitride protonated by hydrofluoric acid (HF) fulfills these requirements. The protonation process is studied in detail, and unlike the reaction with hydrochloric acid, the reaction with HF is topotactic. The attachment of the protons to the nitrogen atoms only induces a decrease in the interlayer spacing. However, the intralayer spacing increases due to the incorporation of π-bonded fluoride ions in the voids of the carbon nitride planes. These effects together lead to a pronounced structure-property correlation and a notable blue shift of E_gap to almost 3 eV. Pristine carbon nitride can be restored by simple thermal treatment. Even more interestingly, a third and new state can be reached by applying pressure. Thus, one can switch reversibly between these three states by using HF, temperature, and pressure.