Role of oxygen vacancy in tuning of optical, electrical and NO2 sensing properties of ZnO1-x coatings at room temperature

Oxygen vacancy concentration of ZnO1-x coatings were regulated by dipping the coatings into hydrogen peroxide solution for 1, 10, 60 and 120 min (abbreviated as Vo-1, Vo-10, Vo-60 and Vo-120 samples) with an addition of subsequent annealing. XRD, Raman, PL and XPS characterizations were used to determine the evolution of oxygen vacancy concentration in the treated coatings. The oxygen vacancy concentration for Vo-1, Vo-10, Vo-60 and Vo-120 samples was 1.9%, 3.7%, 4.8% and 4.3%, respectively. Optical and electrical measurement showed that the visible light absorption range was extended and electrical resistance decreased when the oxygen vacancy concentration was increased. The Vo-60 samples possessed the widest range of visible light response. First principle calculation and various experimental characterizations were used to study the role of the oxygen vacancy in bandgap of the coatings and the results showed that the bandgap was narrowed when the oxygen vacancy was increased. The sensors exhibited significant sensing responses to 0.9 ppm NO2 at room temperature. The coatings with the highest oxygen vacancy concentration had the best NO2 gas sensing properties. In addition, the Vo-60 coatings showed good selectivity and stability to NO2.