Nitrogen-Doped Carbon Dots/TiO₂ Nanoparticle Composites for Photoelectrochemical Water Oxidation

Carbon dots on photoactive semiconductor nanomaterials have represented an effective strategy for enhancing their photoelectrochemical (PEC) activity. By carefully designing and manipulating a carbon dot/support composite, a high photocurrent could be obtained. Currently, there is not much fundamental understanding of how the interaction between such materials can facilitate the reaction process. This hinders the wide applicability of PEC devices. To address this need of improving the fundamental understanding of the carbon dots/semiconductor nanocomposite, we have taken the TiO2 case as a model semiconductor system with nitrogen-doped carbon dots (NCDs). We present here with in-depth investigation of the structural hybridization and energy transitions in the NCDs/TiO2 photoelectrode via high-resolution scanning transmission microscopy (HR-STEM), electron energy loss spectroscopy (EELS), UV–vis absorption, electrochemical impedance spectroscopy (EIS), Mott–Schottky (M–S), time-correlated single-photon counting (TCSPC), and ultraviolet photoelectron spectroscopy (UPS), which shed some light on the charge-transfer process at the carbon dots and TiO2 interface. We show that N doping in carbon dots can effectively prolong the carrier lifetime, and the hybridization of NCDs and TiO2 is able not only to extend TiO2 light response into the visible range but also to form a heterojunction at the NCDs/TiO2 interface with a properly aligned band structure that allows a spatial separation of the charges. This work is arguably the first to report the direct probing of the band positions of the carbon dot–TiO2 nanoparticle composite in a PEC system for understanding the energy-transfer mechanism, demonstrating the favorable role of NCDs in the photocurrent response of TiO2 for the water oxidation process. This study reveals the importance of combining structural, photophysical, and electrochemical experiments to develop a comprehensive understanding of the nanoscale charge-transfer processes between the carbon dots and their catalyst supports.