Engineering an integrated ZnO/carbon quantum dot heterostructure for the electrochemical detection and photoelectrochemical reduction of hexavalent chromium in water

Hexavalent chromium (Cr(VI)) is a highly toxic water contaminant that requires sensitive monitoring and remediation. Although photoelectrochemical (PEC) technologies have the potential to integrate contaminant detection and remediation, multifunctional photoelectrodes capable of performing both functions are limited. Herein, a zinc oxide/carbon quantum dot (ZnO 700/ CQDs) heterostructure was engineered, immobilised on an indium tin oxide (ITO) substrate, and evaluated for the electrochemical detection and PEC removal of Cr(VI). Structural, optical, and electrochemical characterisation confirmed successful heterostructure formation, enhanced light absorption, improved interfacial charge transfer, an approximately 75.3% reduction in charge-transfer resistance, and a 2.7-fold increase in transient photocurrent upon CQD incorporation. Under optimised square wave voltammetric conditions, ITO/ZnO 700 /CQDs exhibited a linear response over 1-100 μg L −1 with a detection limit of 0.44 μg L −1 , which is below the WHO guideline and outperforms ITO/ZnO 700 . ITO/ZnO 700 /CQDs also achieved 95% Cr(VI) removal within 60 min under PEC conditions, compared with 76% under electrochemical (EC) removal conditions. The 1,5-diphenylcarbazide (DPC) spectrophotometric method independently confirmed the greater depletion of aqueous Cr(VI) under PEC conditions. The enhanced Cr(VI) sensing and remediation performance is attributed to favourable interfacial charge transfer and improved utilisation of photogenerated charge carriers within the ZnO 700 /CQDs heterostructure, demonstrating its potential as an integrated platform for chromium monitoring and water remediation.