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

Abstract

Graphitic carbon nitride (g-C 3 N 4 ) is a promising visible-light-responsive photocatalyst for hydrogen generation from water. As we show here, the photocatalytic activity of g-C 3 N 4 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 À1 g À1 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 À0.38 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-C 3 N 4 lattice.

Impact of the annealing temperature on Pt/g-C 3 N 4 structure, activity and selectivity between photodegradation and water splitting

“…This is in line with previous findings on amorphous carbon doped with nitrogen suggesting that as the size of the sp 2 C=N clusters increases, the state density of π bonding in the sp 2 cluster increases leading to a decrease of the optical band gap [13][14][15]. It has to be noted that the material synthetized at 600 °C presents two different band gaps, which has not been reported in the literature so far [6,8,9,16].The lower one being in the continuity with the band gaps obtained for the materials synthesized at lower temperature while the higher one is closer to the data obtained at 650 °C. The synthesis temperature of 600 °C appears to be a turning point in the absorbance properties of g-C3N4.…”

“…The trend observed for hydrogen evolution was similar to that of CO2. Similar finding was made by Wu et al; when using methanol as a sacrificial agent, they saw a clear decrease of the hydrogen evolution from 301µmol.h -1 .g -1 at 520 °C to 59 µmol.h -1 .g -1 at 640 °C [9]. Regarding our bare sample (see An interesting finding was made when comparing the ratio between the rates of CO2 and H2.…”

“…The two samples synthetized at the highest temperatures are displaying an absorbance shoulder around 500nm, matching with their darker color [9]. The band gap of the different photo-catalysts were calculated using the Tauc plot for a direct allowed transition (n=1/2) [13].…”

“…Studies on the effect of the annealing temperature of g-C3N4 have previously been conducted by other workers [6][7][8][9] (among others), however few of those included EPR. Synthesis temperature affects the extent of the 2D ordering within the polymer and hence the number of defects present in the structure, which may be paramagnetic [10].…”

Nanoscale Effects in Water Splitting Photocatalysis

“…We recently employed graphitic carbon nitride g-C 3 N 4 to observe the effect of structure defects on the ability of the material to reduce protons photocatalytically and to generate a photovoltage [149]. Specifically, we observe an inverse relation between the photocatalytic hydrogen production rate from aqueous methanol and the calcination temperature (Fig.…”