We have successfully developed an effective supercritical CO2 (sc-CO2) emulsion-assisted electrochemical approach for cathodic deposition of ZnO mesocrystals. The sc-CO2 was introduced along with a nonionic surfactant in the process to form emulsified electrolyte, which significantly increased the supersaturation degree and promoted the molecular diffusion to affect the crystal growth of ZnO. The deposition involved the generation of primary nanocrystals with substantially high surface energy, followed by the preferred attachment of the primary nanocrystals along an energetically favorable orientation to form ZnO mesocrystals. The as-deposited ZnO mesocrystals exhibited remarkable optical properties at room temperature in terms of prominent near band-edge emission and substantially long exciton lifetime, attributable to the highly oriented crystallinity of mesocrystals that effectively suppressed the nonradiative charge recombination to extend the exciton decay dynamics. As compared to the structures prepared without the addition of surfactant or sc-CO2, ZnO mesocrystals from sc-CO 2 emulsion displayed greatly improved photoactivity toward photoelectrochemical water oxidation, revealing their promising potential as photoanodes in relevant photoelectrochemical processes. The current study delivers the first demonstration of directly depositing ZnO mesocrystals on conductive substrates, which paves the way for utilization of mesocrystals as practical electrodes in technologically important energy conversion fields, such as electrochemical cells, photovoltaic devices, as well as photoelectrochemical water splitting, where the advantageous structural characteristics of mesocrystals, i.e., high crystallinity and abundant porosity, can be fully exploited.