Comparative analysis of Ni and Li doped ZnO with enhanced structural, optical, and electrochemical performance in energy storage applications

Doping is a reliable approach for tailoring the optical, electrical, and structural properties of materials in energy storage applications. In this study, nickel (Ni) and lithium (Li), well-known for their significant influence on semiconductor behavior, were employed as dopants to modify the properties of ZnO thin films. The films of undoped ZnO, Ni-doped ZnO, and Li-doped ZnO were synthesized using the cost-effective sol–gel spin coating technique. Using scanning electron microscopy (SEM) and X-ray diffraction (XRD), the structural and morphological features were examined. The average crystallite diameters for pure, Ni-, and Li-doped ZnO films were found to be 23.79 nm, 20.82 nm, and 22.5 nm, respectively. All films exhibited a wrinkled surface morphology with a nanograined microstructure. UV–Vis spectroscopy revealed that the band gap values for ZnO, Ni-ZnO, and Li-ZnO were 3.33 eV, 3.30 eV, and 3.315 eV, respectively. Other optical properties such as refractive index, extinction coefficient, optical conductivity, and packing density were also investigated. Better optical transparency in the visible region was observed for Li-doped film as compared to Ni-doped film, with the refractive index values less than 2. Likewise, less optical loss in visible region was observed for Li-doped film as compared to Ni-doped film, with the lower values of extinction coefficient. Water contact angle (WCA) experiments revealed the hydrophobic characteristics of the Ni-doped films and the hydrophilic behavior of the Li-doped films. UV irradiation caused a quick decrease in contact angle for both doped samples, with Ni-doped ZnO having the fastest transition rate (0.07). Electrical studies revealed that Li-doped ZnO films exhibited higher conductivity and lower resistance (27.1 MΩ/sq and 0.159 S/m), highlighting their potential for electronic applications. Furthermore, cyclic voltammetry (CV) confirmed electrical double-layer capacitor (EDLC) behavior in both undoped and doped ZnO films with the specific capacitance values 33.54 and 50.16 F/g for Ni-ZnO and Li-ZnO films respectively at 0.2 mA/g current density via galvanostatic charge–discharge (GCD) measurements, underscoring their suitability for energy storage applications, particularly thin-film supercapacitors.

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