Conventional analytical techniques for determining chromium levels often require skilled operators, extensive sample preparation, and sophisticated centralised laboratory instrumentation, leading to high maintenance and operational costs. Similarly, traditional remediation methods like membrane filtration, adsorption, and ion-exchange can involve high operational costs and may generate secondary waste or provide incomplete chromium removal. These limitations have generated growing interest in technologies capable of integrating chromium monitoring and remediation within a single platform. Photoelectrochemical technologies have emerged as promising alternatives because semiconductor photoexcitation can be coupled with an applied electrochemical bias to improve the utilisation of photogenerated charge carriers.

Researchers from the University of South Africa have engineered a zinc oxide and carbon quantum dot heterostructure immobilised on an indium tin oxide substrate to address these challenges. This single electrode architecture is designed for two sequential functions: electrochemical detection of hexavalent chromium under dark conditions and photoelectrochemical chromium removal under ultraviolet irradiation and cathodic bias. The study systematically investigated the influence of carbon quantum dots on the structural, optical, electrochemical, and photoelectrochemical properties to establish structure-property-performance relationships across both functions, with particular emphasis on the relationship between carbon quantum dot-mediated interfacial properties and the resulting sensing and remediation performance.

The zinc oxide nanoparticles were synthesised using a modified precipitation method and calcined at 700 degrees Celsius to improve crystallinity and lattice ordering, thereby enhancing electron mobility and efficient charge transport. The carbon quantum dots were incorporated into the zinc oxide matrix using a solution-based assembly process involving sonication and prolonged continuous stirring to maximise interfacial contact. X-ray diffraction analysis confirmed that the incorporation of carbon quantum dots preserved the crystalline wurtzite structure of zinc oxide without introducing detectable secondary crystalline phases. Fourier transform infrared spectroscopy revealed the presence of oxygen-containing functional groups on the carbon quantum dots, which interacted with the hydroxyl groups in the zinc oxide nanoparticle framework to facilitate hybridisation.

Ultraviolet-visible spectroscopy demonstrated that the heterostructure exhibited significantly enhanced absorption throughout the visible region compared with pristine zinc oxide, indicating an improved light-harvesting capability. Field emission scanning electron microscopy showed that the carbon quantum dots promoted the assembly and aggregation of individual zinc oxide nanoparticles into more compact hierarchical structures, with intimate interfacial contact expected to facilitate efficient electron transport. These structural, optical, and morphological characteristics suggest improved charge generation, separation, and transfer within the nanocomposite, making it a promising photoactive material for chromium detection and reduction.

Electrochemical characterisation revealed that the incorporation of carbon quantum dots substantially modified the interfacial properties of the electrode. Cyclic voltammetry studies using a ferri/ferrocyanide redox probe showed that the heterostructure exhibited slightly enhanced anodic and cathodic peak current responses compared to pristine zinc oxide. Electrochemical impedance spectroscopy provided direct evidence of enhanced interfacial electron-transfer kinetics, with the charge-transfer resistance decreasing by approximately 75 percent upon the incorporation of carbon quantum dots. Mott-Schottky analysis further indicated that the heterostructure formation increased the apparent donor density and shifted the flat-band potential, supporting enhanced charge availability and interfacial charge transport within the material.

Under optimised conditions using sodium sulfate as the supporting electrolyte at pH 4, the developed electrode achieved sensitive electrochemical detection of hexavalent chromium over a linear concentration range of 1 to 100 micrograms per liter. The limit of detection was determined to be 0.44 micrograms per liter, representing a significant improvement over the pristine zinc oxide electrode. Recovery studies in spiked tap water samples yielded values ranging from 84 to 98 percent, indicating satisfactory analytical accuracy for the detection of chromium in real water samples. The sensor also demonstrated excellent selectivity, reproducibility, and long-term stability, retaining approximately 87 percent of its initial current response after 20 days of storage.

The multifunctionality of the electrode was further explored by evaluating its ability to reduce and remove hexavalent chromium from aqueous solution through photoelectrochemical processes. Under photoelectrochemical conditions, ultraviolet irradiation generated additional electron-hole pairs while the applied cathodic bias promoted efficient separation, resulting in the highest removal efficiencies. At an initial concentration of 5 milligrams per liter, the chromium removal efficiency increased from approximately 76 percent under electrochemical conditions to approximately 95 percent under photoelectrochemical conditions. The reduction kinetics followed a pseudo-first-order model, with the rate constant increasing by approximately 2.1-fold following ultraviolet illumination, and the half-life decreasing from 29.2 minutes to 14.0 minutes.

Active-species trapping experiments identified photogenerated electrons as the dominant reducing species, while superoxide radicals provided an additional contribution to the removal process. Transient photocurrent measurements confirmed that the incorporation of carbon quantum dots facilitated more efficient generation, separation, and transport of photogenerated charge carriers, resulting in a 2.7-fold enhancement in photocurrent density compared to pristine zinc oxide. Independent validation using the 1,5-diphenylcarbazide spectrophotometric method confirmed the faster depletion of aqueous chromium under photoelectrochemical conditions, with kinetic parameters in good agreement with the electrochemical analysis. The integration of sensitive electrochemical sensing with efficient photoelectrochemical remediation demonstrates the potential of this heterostructure as a multifunctional platform for simultaneous water quality monitoring and chromium remediation.

Subject of Research: Photoelectrochemical detection and reduction of hexavalent chromium using a ZnO/CQD heterostructure

Article Title: Engineering an Integrated ZnO/Carbon Quantum Dot Heterostructure for the Electrochemical Detection and Photoelectrochemical Reduction of Hexavalent Chromium in Water

Article References: Ndebele, N., & Fuku, X. (2026). Engineering an Integrated ZnO/Carbon Quantum Dot Heterostructure for the Electrochemical Detection and Photoelectrochemical Reduction of Hexavalent Chromium in Water. Environmental Advances, Article 100771. https://doi.org/10.1016/j.envadv.2026.100771

Image Credits: AI Generated

DOI: 10.1016/j.envadv.2026.100771

Keywords: hexavalent chromium, zinc oxide, carbon quantum dots, photoelectrochemistry, water remediation, electrochemical sensing, nanomaterials, environmental monitoring, semiconductor heterostructure, contaminant removal, Engineering, Integrated