Structure defects in g-C₃N₄ limit visible light driven hydrogen evolution and photovoltage

Graphitic carbon nitride (g-C₃N₄) is a promising visible-light-responsive photocatalyst for hydrogen generation from water. As we show here, the photocatalytic activity of g-C₃N₄ is limited by structure defects generated during the calcination process. Specifically we find that the photocatalytic hydrogen production rate from aqueous methanol is inversely related to the calcination temperature (520-640°C). The highest activity of 0.301 mmol h^-1 g^-1 is observed for the sample prepared at the lowest processing temperature. Surface photovoltage (SPV) spectroscopy shows that the maximum photovoltage is reduced (from 1.29 V to 0.62 V) as the processing temperature is increased, in accordance with higher defect concentrations and faster electron-hole recombination. The defects also produce additional optical absorption in the visible spectra and cause a red shifted, weakened photoluminescence (PL). Based on the sub-gap signal in the SPV and PL spectra, defect energy levels are +0.97 V and -0.38 V (vs. NHE) in the band gap of the material. According to Fourier transform infrared (FTIR) spectra, the defects are due to amino/imino groups in the g-C₃N₄ lattice.