Development of Morphologically engineered Flower-like Hafnium-Doped ZnO with Experimental and DFT Validation for Low-Temperature and Ultrasensitive Detection of NOX Gas

Substitutional doping and different nanostructures of ZnO haverendered it an effective sensor for the detection of volatile organic compoundsin real-time atmosphere. However, the low selectivity of ZnO sensors limitstheir applications. Herein, hafnium (Hf)-doped ZnO (Hf-ZnO) nanostruc-tures are developed by the hydrothermal method for high selectivity ofhazardous NOXgas in the atmosphere, substantially portraying the role ofdoping concentration on the enhancement of structural, optical, and sensingbehavior. ZnO microspheres with 5% Hf doping showed excellent sensing anddetected 22 parts per billion (ppb) NOXgas in the atmosphere, within 24 s,which is much faster than ZnO (90 s), and rendered superior sensing ability (S= 67) at a low temperature (100 degrees C) compared to ZnO (S= 40). The sensorrevealed exceptional stability under humid air (S= 55 at 70% RH), suggestinga potential of 5% Hf-ZnO as a new stable sensing material. Density functionaltheory (DFT) and other characterization analyses revealed that the high sensing activity of 5% Hf-ZnO is attributed to theaccessibility of more adsorption sites arising due to charge distortion, increased oxygen vacancies concentration, Lewis acid base,porous morphology, small particle size (5 nm), and strong bond interaction amidst NO2molecule with ZnO-Hf-Ovacancysites,resulting from the substitution of the host cation (Zn2+) with doping cation (Hf4+).