Role of oxygen vacancies on Li-doped Ni:ZnO thin films for enhanced NO2 gas sensing applications

In this study, pure ZnO, 5 wt.% Ni doped-ZnO (Ni:ZnO), 1 wt.% Li doped-ZnO (Li:ZnO), and (1 wt.% Li-5 wt.% Ni) co-doped ZnO (Ni/Li:ZnO) thin films were deposited on a glass substrate via sol-gel spin coating. A series of tests were conducted on all samples to determine their structural, electrical, morphological, elemental, optical, and NO2 gas sensing properties. According to the XRD study, all fabricated samples displayed a hexagonal wurtzite structure. FESEM analysis reveals that the Ni/Li:ZnO sample exhibits smaller grain sizes and numerous voids among the fabricated samples. According to Four-Probe Hall measurements, Ni/Li:ZnO exhibits the highest carrier concentration among all thin film samples explored. Photoluminescence and XPS studies confirmed that Ni/Li:ZnO exhibits higher levels of surface oxygen vacancies. The NO2 gas sensing test results suggested that the Ni/Li:ZnO showed enhanced gas sensitivity (246) with high selectivity, stability, repeatability, and fast response (22 s) and recovery (157 s) times at 210 degrees C. The interaction between the defect states created by the lithium and nickel and the inherent defect states of ZnO contributes to better charge transportation, thereby yielding good gas sensing capabilities.