Hollow copper(I) oxide (Cu2O) nanoparticles (NPs) with a diameter of 7.0 ± 1.3 nm were synthesized by sputter deposition of Cu in a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4), followed by the oxidation of Cu NPs in EMI-BF4 by prompt heating at 373 K in air. The void space was 2.3 ± 0.7 nm in size, and the shell thickness was estimated to be 2.4 nm. The structure was presumably produced by the Kirkendall effect at the nanoscale. Core–shell-structured Cu@Cu2O NPs and solid Cu2O NPs were also synthesized by changing the oxidation conditions. From the action spectra of their cathodic photocurrents, the optical bandgap of the Cu2O phase in these NPs was estimated to be 2.51–2.56 eV, which is larger than that of bulk Cu2O (2.1 eV). The photoelectrochemical activities of the hollow or spherical Cu2O NPs were higher than those of the core–shell Cu@Cu2O NPs.

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