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This study presents a novel method for synthesizing hierarchical porous zinc oxide (ZnO) nanostructures by thermally decomposing green-synthesized hydrozincite. The hydrozincite was prepared at room temperature using zinc acetate dihydrate and calcium carbonate (CaCO 3) in cellulose printing papers as the precipitating agent. The resulting ZnO particles exhibit a distinctive flower-like morphology, consisting of porous nanosheets composed of interconnected nanoparticles. The average crystallite size of the hierarchically structured zinc oxide nanoparticles, as determined via X-ray diffraction Rietveld refinement method, was found to be 18.8 nm when calcined at 500 °C. However, with increasing calcination temperature to 600, 700, and 800 °C, the average crystallite size also increased to 24.3, 30.4, and 47.2 nm, respectively. The ZnO synthesized at temperatures of 500 °C, 600 °C, and 700 °C exhibits higher electrocatalytic activity toward the hydrogen evolution reaction (HER) compared to both the ZnO synthesized at 800 °C and commercial ZnO. This enhanced performance can be primarily attributed to the nanosized ZnO particles, which provide a substantial surface area, high accessibility of active sites, and fast charge transfer kinetics. In terms of photocatalytic activity, all the samples show HER improvement under the UV light irradiation, attributed to the generation of photoexcited charge carriers. The ZnO synthesized at 800 °C and the commercial ZnO exhibit greater enhancements in photocatalytic activity compared to the other samples. This implies that larger solid particles with fewer nanopores result in a greater exposed surface area for UV-light absorption. The hierarchically structured ZnO nanoparticles demonstrate promising electrocatalytic and photocatalytic activities, as well as stability, opening up potential applications in various fields, including energy conversion and environmental remediation. [ABSTRACT FROM AUTHOR]