Influence of Hf doping on structural, morphological, optical and electrochemical properties of NiO thin films: Electrochromic application

Abstract

In this study, the authors investigated the influence of hafnium doping on the properties of NiO thin films using the spray pyrolysis (SP) method. A comprehensive analysis was conducted to evaluate the structural, morphological, optical, and electrochemical characteristics of the deposited films. Techniques such as X-ray diffraction analysis (XRD), Raman spectroscopy, scanning electron microscopy (FESEM), energy dispersive analysis by X-ray (EDS), UV–Vis–NIR spectrophotometry, cyclic voltammetry (CV), and chronoamperometry (CA) were employed. XRD examination revealed that the NiO thin films exhibit a polycrystalline cubic phase, with a preferred orientation along the (111) plane. The successful integration of Hf ions into the NiO matrix was confirmed by the Raman spectrum. SEM images depicted the formation of dense and uniform nanograins on all film surfaces, free from any cracks. EDS analysis and elemental mapping indicated uniform and homogeneous distribution of Ni, O, and Hf elements across the films. The optical analysis suggested a decrease in the optical band gap energy from 3.54 eV to 3.49 eV with an increase in hafnium doping concentration. Electrochemical results revealed a continuous improvement in specific capacitance with Hf doping. The NHf6 sample reached 65 F. g−1 at 5 mV s−1, compared to 46 F. g−1 at 5 mV s−1 for the NHf0 sample. However, the NHf4 film exhibited the highest optical density variation (49%), shortest response times (tb = 3.58 s, tc = 4.67 s), and the largest coloration efficiency (63.39 cm2/C), indicating enhanced electrochromic performance at this Hf doping rate. These findings suggest that Hf-doped NiO holds promise as a candidate for use as a counter-electrode in electrochromic devices.