Effect of reaction time andPVPcontents on morphologies of hierarchical3Dflower-likeZnCo2O4microstructures for energy storage devices

Hierarchical 3D flower-like ZnCo2O4(ZCO) microstructures assembled from petal-like nanosheets/flakes of non-uniform sizes were engineered by a polyvinylpyrrolidone (PVP)-assisted hydrothermal method. Four different samples/morphologies of ZCO were obtained (PVP-L@6, PVP-H@6, PVP-L@12, and PVP-H@12) by altering the reaction parameters such as surfactant concentration (PVP) and reaction time, which can play a significant role in the formation of flower-/petal-/flake-like architectures. The alteration of the reaction parameters not only resulted in morphological changes but also affected the surface area, pore size/volume, crystalline nature, non-stoichiometry of Zn, Co, and O in ZCO, and their electrochemical performance. The metal (Zn/Co)/O deficiencies of ZCO samples were investigated via X-ray photoelectron spectroscopy and supported by the Rietveld refinement method. Furthermore, a plausible growth mechanism for these flower-like ZCO microstructures was projected based on the experimental results. The four dissimilar samples/morphologies of ZCO, which exhibit different electrochemical performances, were investigated. Our results show that PVP-H@12 exhibits higher specific capacitance (761/680 F g(-1)at 0.35/1 A g(-1)) and good cycling constancy (90% capacitive retention after 2000 cycles at 5 Ag-1) among all the four samples.