Boosting charge transfer and photoelectrochemical activity in ZnO photoelectrodes with quantum dot carbon decorations

Zinc oxide (ZnO) stands as a promising semiconductor with vast potential across various domains, including photocatalysis, electronics, and optoelectronics. In the realm of photoelectrochemical (PEC) devices, ZnO photoelectrodes have emerged as pivotal components for harnessing solar energy to drive hydrogen and oxygen production. However, the inherent wide band gap of ZnO poses challenges, retarding light absorption rates and fostering rapid electron-hole recombination. To tackle this, ZnO has been ingeniously augmented with carbon quantum dots (CQDs) and nitrogen-doped carbon quantum dots (N-CQDs), synthesized via the hydrothermal method. Notably, ZnO photoelectrodes fabricated in 1 M NaOH alkaline solution exhibit a nanocubic structure, while those produced in 1 M urea solution adopt a nanorod configuration. Among these, ZnO/Urea, characterized by enhanced photocurrent, has been selected for further enhancement with CQDs and N-CQDs, both at a concentration of 8.5 mg/ml. UV–Vis spectra analysis reveals that ZnO/Urea/CQDs and ZnO/Urea/NCQDs exhibit superior light absorption compared to pristine ZnO/Urea, concurrently reducing the quantum band gap to 2.89 eV and 3.01 eV, respectively. Notably, ZnO/Urea/CQDs achieve a noteworthy photocurrent density of 0.56 mA/cm² at 1.23 V vs. RHE. This study underscores the pivotal role of quantum dot decoration in enhancing charge transfer rates and augmenting photocatalytic activity within ZnO photoelectrodes.