Improved photocatalytic activity of TiO₂ nanoparticles through nitrogen and phosphorus co-doped carbon quantum dots: an experimental and theoretical study

In this work, we develop a photocatalyst wherein nitrogen and phosphorus co-doped carbon quantum dots are scaffolded onto TiO₂ nanoparticles (NPCQD/TiO₂), denoted as NPCT hereafter. The developed NPCT photocatalyst exhibits an enhanced visible light photocatalytic hydrogen production of 533 μmol h⁻¹ g⁻¹ compared to nitrogen doped CQD/TiO₂ (478 μmol h⁻¹ g⁻¹), phosphorus doped CQD/TiO₂ (451 μmol h⁻¹ g⁻¹) and pure CQD/TiO₂ (427 μmol h⁻¹ g⁻¹) photocatalysts. The enhanced photocatalytic activity of the NPCT photocatalyst is attributed to the excellent synergy between NPCQDs and TiO₂ nanoparticles, which results in the creation of virtual energy levels, a decrease in work function and suppressed recombination rates, thereby increasing the lifetime of photogenerated electrons. A detailed mechanism is proposed for the enhancement in visible light hydrogen production by the NPCT photocatalyst from the experimental results, Mott-Schottky plots and ultraviolet photoelectron spectroscopy results. Further, first-principles density functional theory (DFT) simulations are carried out which predict the decrease in the work function and band gap, and the increase in the density of states of NPCT as the factors responsible for the observed enhancement in visible light photocatalytic hydrogen production.