KH2PO4-Assisted Synthesis and Electrochemical Performance of Highly Uniform CuBi2O4 Microspheres Hierarchically Self-Assembled by Nanoparticles

Abstract

The effect of KH2PO4 on the synthesis of CuBi2O4 microstructures was investigated. The samples were characterized by powder x-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) analysis, x-ray photoelectron spectroscopy (XPS) and ultraviolet (UV)-visible diffuse reflectance spectroscopy. It is demonstrated that the use of KH2PO4 leads to the production of highly uniform CuBi2O4 microspheres hierarchically self-assembled by nanoparticles. With increasing the KH2PO4 concentration from 0.5 M to 1.4 M, the average diameter of the resultant microspheres decreases gradually from 3.3 μm to 1.4 μm. However, further increase in the KH2PO4 concentration up to 1.5 M leads to a sudden increase in the average diameter of the resultant microspheres up to 2.3 μm. In addition, a minor amount of bamboo leaf- or pine needle-like structures are visible in the samples prepared at the KH2PO4 concentrations of 1.0–1.5 M. The bandgap energy of the as-prepared samples is measured to be 1.89 eV by UV-visible diffuse reflectance spectroscopy. The electrochemical performance of the samples was investigated by cyclic voltammetry, galvanostatic charge-discharge measurements, and electrochemical impedance spectroscopy in 2 M KOH electrolyte. Among the hierarchical microspheres, those prepared at the KH2PO4 concentration of 1.4 M deliver a relatively higher specific capacitance due to their smaller size (1284 F g−1 at a current density of 2 A g−1).