Sandwich-nanostructured n-Cu₂O/AuAg/p-Cu₂O Photocathode with Highly Positive Onset Potential for Improved Water Reduction

A 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 thermodynamic shift of the flat-band potentials. The obtained extremely positive onset potential, 0.8 V_RHE 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. Acting as a plasmonic photosensitizer, the alloy increased the efficiency of conversion of solar energy to electric current. The mechanism of plasmonic energy transfer involved a direct transfer of the plasmonic hot carriers; the interfacial Schottky barrier height modulated the plasmonic hot electron transfer. This facile sandwich structure combines both electrical and optical functions of alloy nanoparticles into a single structure, which has implications for the design of efficient devices to harvest solar energy.