Unravelling charge carrier dynamics in protonated g-C3N4 interfaced with carbon nanodots as co-catalysts toward enhanced photocatalytic CO2 reduction: A combined experimental and first-principles DFT study

In this work, we demonstrated the successful construction of metal-free zero-dimensional/two-dimensional carbon nanodot (CND)-hybridized protonated g-C3N4 (pCN) (CND/pCN) heterojunction photocatalysts by means of electrostatic attraction. We experimentally found that CNDs with an average diameter of 4.4 nm were uniformly distributed on the surface of pCN using electron microscopy analysis. The CND/pCN-3 sample with a CND content of 3 wt.% showed the highest catalytic activity in the CO2 photoreduction process under visible and simulated solar light. This process results in the evolution of CH4 and CO. The total amounts of CH4 and CO generated by the CND/pCN-3 photocatalyst after 10 h of visible-light activity were found to be 29.23 and 58.82 mu mol . g(catalyst)(-1), respectively. These values were 3.6 and 2.28 times higher, respectively, than the amounts generated when using pCN alone. The corresponding apparent quantum efficiency (AQE) was calculated to be 0.076%. Furthermore, the CND/pCN-3 sample demonstrated high stability and durability after four consecutive photoreaction cycles, with no significant decrease in the catalytic activity. The significant improvement in the photoactivity using CND/pCN-3 was attributed to the synergistic interaction between pCN and CNDs. This synergy allows the effective migration of photoexcited electrons from pCN to CNDs via well-contacted heterojunction interfaces, which retards the charge recombination. This was confirmed by photoelectrochemical measurements, and steady-state and time-resolved photoluminescence analyses. The first-principles density functional theory (DFT) calculations were consistent with our experimental results, and showed that the work function of CNDs (5.56 eV) was larger than that of pCN (4.66 eV). This suggests that the efficient shuttling of electrons from the conduction band of pCN to CNDs hampers the recombination of electron-hole pairs. This significantly increased the probability of free charge carriers reducing CO2 to CH4 and CO. Overall, this study underlines the importance of understanding the charge carrier dynamics of the CND/pCN hybrid nanocomposites, in order to enhance solar energy conversion.