Hydrogen Evolution Activity of Nitrogen-Rich g‑C3–x N4+x Synthesized by Solid–Gas Interface Method

Synthesis of a metal-free carbon nitride (g-C3N4) photocatalyst in the form of nitrogen-rich g-C3–x N4+x derivatives is desirable for efficient solar to hydrogen conversion and remains a challenging task to achieve. Herein we report the development of homogeneous sheets of nitrogen-rich graphitic carbon nitride samples from melamine by a solid–gas interface approach. Using this method, pure g-C3N4 (CN), g-C3–x N4+x under ammonia flow (CN-NH3) and g-C3–x N4+x under nitrogen flow (CN-N2) are prepared. The g-C3–x N4+x (CN-NH3) sample shows better surface conductivity, wide optical absorbance in the visible region, reduced recombination and high electron donor density, and higher performance toward photoelectrochemical hydrogen evolution (HER). The g-C3–x N4+x (CN-NH3) generates a photocurrent of 2.06 μA cm–2, which is 2.5 times higher than that of the pure g-C3N4 (CN) sample (0.85 μA cm–2). It also shows higher photocatalytic water splitting ability compared to the CN and CN-N2 samples, generating 634 μmol g–1 hydrogen without cocatalyst and 1163 μmol g–1 of hydrogen with Pt cocatalyst. Density functional calculations suggest that the progressive band gap reduction with the increase in the N-dopant percentage can be attributed to the gradual increase in the partial π-occupations, which can lead to a significant stabilization of the conduction band minima. The theoretical modeling, however, indicates a saturation in the band gap effect after 75% of N-dopant. The onset potential of g-C3–x N4+x for HER appears at η = 0.43 V in dark and η = 0.34 V vs Ag/AgCl under solar light illumination of 1 sun.