Tailoring the surface oxygen engineering of a carbon-quantum-dot-sensitized ZnO@H-ZnO_1-x multijunction toward efficient charge dynamics and photoactivity enhancement

A well-steered coordination environment on photoelectrode can offer fruitful active sites and efficaciously alter atomic and electronic configuration to boost the performance in water oxidation. To enhance the performance of the photocatalytic oxygen-evolution reaction, enriched oxygenous-type carbon quantum dots (CQD) were combined with oxygen-deficient ZnO@H-ZnO_1-x homojunctions for the first time. Specifically, hydrogenation created an unstoichiometric H-ZnO_1-x surface through oxygen vacancies, which were responsible for the formation of coordinatively unsaturated Zn centers to drive the photoelectrochemical reaction by precise regulation of the charge density, valence-band edge, atomic geometric structure and electronic structure. The enriched photohole reservoir in carboxyl-conjugated CQD was a great benefit for super-rapid charge tunnels, thereby governing the rate of internal and interfacial charge separation and transport in the multijunction. Accordingly, the CQD/H-ZnO_1-x/ZnO photoanodes showed a photoactivity enhanced five-fold relative to bare ZnO NR and maintained that high performance for 24 h of operation. Comprehensive analyses, involving electrochemical measurements, time-resolved photoluminescence, X-ray absorption spectra in situ and calculations with density-functional theory were undertaken to elucidate the underlying mechanism of enhancement and the route of directional charge transport as well as the interfacial charge dynamics over the CQD/H-ZnO_1-x/ZnO photoelectrode. These findings provide a new track towards photoelectrodes that could yield not only strikingly enhanced photocatalysis but also satisfied stability for realistic application.