A carbon-quantum-dot-sensitized ZnO:Ga/ZnO multijunction composite photoanode for photoelectrochemical water splitting under visible light irradiation

- The CQDs@ZnO:Ga/ZnO composite combines type I and type II n-n heterojunctions.
- CQD sensitization can eliminate the energy barrier at the type I ZnO:Ga/ZnO junction.
- CQDs@ZnO:Ga/ZnO gives a negatively shifted onset potential.
- The CQDs@ZnO:Ga/ZnO photoanode exhibits remarkably enhanced PEC performance.
- The CQDs@ZnO:Ga/ZnO composite shows better photostability.

A carbon quantum dot (CQD)-sensitized ZnO:Ga/ZnO composite photoanode for photoelectrochemical water splitting has been designed and tested. In the composite, a ZnO array is grown on a piece of indium tin oxide glass and facilitates the essential integrity of the photoanode. An overlayer of Ga-doped ZnO (ZnO:Ga) is then hydrothermally grown on the ZnO nanorods. A type I ZnO:Ga/ZnO junction forms with a ZnO:Ga shell possessing visible light activity and a narrow bandgap. However, a highly positive onset potential exists due to the location of the conduction band (CB) of ZnO:Ga at the high end. The deposition of the CQDs on the surface of the overlayer provides a negatively shifted CB with a type II junction between ZnO:Ga and CQDs. The CQDs@ZnO:Ga/ZnO multijunction composite photoanode exhibits a negative shift of the onset potential and a significantly increased photocurrent density, up to 1.5 mA cm−2 at 1.6 V vs. RHE, with good photostability under AM 1.5 G simulated light. The multijunction composite photoanode also achieves a current density of 0.16 mA cm−2 under visible light at a bias of 1.0 V, which is comparable to that of ZnO under AM 1.5 G simulated light. Electrochemical impedance spectra reveal that the CQD modification acts to decrease the interfacial charge transfer resistance associated with eliminating the energy barrier in the type I ZnO:Ga/ZnO junction that causes charge transfer resistance.

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