Influence of oxygen vacancies on the lithium-doped Mn:ZnO thin films for improved NO2 gas-sensing applications

Abstract

Herein, the ${(\mathrm{Zn}}_{0.97-x}{\mathrm{Li}}_x{\mathrm{Mn}}_{0.03})\mathrm{O}$ ($x=0,0.01,0.03,\mathrm{and}0.05$) thin films were prepared on a glass substrate via the sol–gel spin coating technique to study the influence of lithium on Mn-doped ZnO thin films for structural, optical, electrical, morphological, chemical, and NO₂ gas-sensing applications. According to the XRD analysis, all samples display a hexagonal wurtzite crystal structure. A FESEM analysis revealed that the incorporation of lithium into Mn-doped ZnO results in a smaller grain size with more voids than Mn-doped ZnO. Four-probe Hall measurements revealed the n-type conductivity on ${(\mathrm{Zn}}_{0.97-x}{\mathrm{Li}}_{\mathrm{x}}{\mathrm{Mn}}_{0.03})\mathrm{O}$ ($x=0\mathrm{and}0.01),$ whereas samples with ($x=0.03\mathrm{and}0.05$) exhibited p-type conductivity, which was well explained. XPS and PL spectra confirmed the abundance of surface oxygen vacancies on the prepared sample. It is revealed that interaction between the defect states of lithium and manganese with inherent defect states of ZnO play a crucial role in carrier transfer for the gas-sensing process. In contrast to Mn-doped ZnO, ${(\mathrm{Zn}}_{0.96}{\mathrm{Li}}_{0.01}{\mathrm{Mn}}_{0.03})\mathrm{O}$ exhibits smaller grains and a ninefold gas sensitivity (62.01) toward 75 ppm of NO₂ gas at 210 °C toward 75 ppm of NO₂ gas with a rapid response (30 s) and recovery (125 s) time.