Characterizing Electronic Structure near the Energy Gap of Graphitic Carbon Nitride Based on Rational Interpretation of Chemical Analysis

Graphitic carbon nitride (g-CN) has attracted enormous interest in applications as a visible-light-driven photocatalyst, particularly for hydrogen evolution via water splitting. Despite intensive photocatalytic works to achieve higher hydrogen-evolution rate, the chemical and electronic structures that are essential for the water photolysis reactions have not been comprehensibly understood. To reveal the fundamental properties, we utilized well-oriented g-CN films for reliable analyses with several types of electron spectroscopies. Comparing X-ray photoelectron spectra of the g-CN film with those of a g-CN monomer, melem, provided a definite peak assignment of the spectra, from which we identified g-CN as melon. The analysis with ultraviolet photoelectron and inverse photoemission spectroscopy (UPS and IPES) for the melon film clarified energy distributions of the occupied and unoccupied electronic states near the energy gap of melon, respectively. Band structure calculations of a melon crystal revealed orbital characteristics of the electronic states. The calculations also implied that the energy dispersion of only the lowest unoccupied molecular orbital is present along melon chains. The energy structure of melon, determined by the UPS and IPES spectra, was demonstrated to be preferable for water splitting. The results shown in this study will facilitate designs of superior polymeric photocatalysts.