Scalable oxygen vacancy by aliovalent charge-compensation doping in tetragonal zirconium oxide nanoparticles for energy storage applications

Abstract

Scalable oxygen vacancies achieved from aliovalent doping of Fe³⁺/Co²⁺/Ni²⁺ ions in Zr⁴⁺ position of zirconium dioxide nanoparticles by co-precipitation method. The work aims to identify the modifications in the valence band states due to oxygen deficiency by charge-compensation and its effect on optical, dielectric and electrochemical charge storage capacity of zirconium oxide samples. All samples are crystallized into tetragonal structure, x-ray photoelectron core level spectrum identifies the presence of multiple charge states of elements and quantify their oxygen vacancies. Deconvoluted valence band spectrum identifies its contributed elemental states, help to measure Fermi energy and modification in the valence band edge values of three samples. The band gap of pristine samples were reported about 5 eV but measured direct band gap energy ranges from 2.0 to 3.7 eV and indirect band gap value ranges from 1.95 to 3.34 eV were calculated and exact transitions occurred were analysed using Urbach energy calculations. Dielectric and electrochemical results confirmed that all samples possess their unique values which is relatable to the valence band states and band edge value. Comparing three samples, cobalt doped zirconium oxide (CZO) sample showed better dielectric property, electrochemical energy storage with good retention of about 94 % at room temperature, 75 % at 40 °C and 100 % at 60 °C up to 1000 cycles. The superior performance of CZO compared to other two samples was justified using changes in the band gap edge, valence band states and multiple oxidation states of elements. The favourable re-arrangement of electronic states are responsible for high electronic conduction that leads better electrochemical performance in CZO.