An n-Cu2O layer formed a high-quality buried junction with p-Cu2O to increase the photovoltage and thus to shift the turn-on voltage positively. Mott–Schottky measurements confirmed that the improvement benefited from a positive shift in flat-band potential. The obtained extremely positive onset potential, 0.8 VRHE in n-Cu2O/AuAg/p-Cu2O, is comparable with measurements from water reduction catalysts. The AuAg alloy sandwiched between the homojunction of n-Cu2O and p-Cu2O improved the photocatalytic performance. This alloy both served as an electron relay and promoted electron-hole pair generation in nearby semiconductors; the charge transfer between n-Cu2O and p-Cu2O in the sandwich structure was measured with X-ray absorption spectra. The proposed sandwich structure can be considered as a new direction for the design of efficient solar-related devices.
