Ultrasensitive NO_X Detection in Simulated Exhaled Air: Enhanced Sensing via Alumina Modification of In‐Situ Grown WO₃ Nanoblocks

Abstract

Seedlessg rowth of vertically aligned nanostructures, which can induce smoother transport and minimize Ohmic contact between substrate ands emiconductor,c an be fabricated by in situ growth utilizing modified hydrothermal methods.S uch devices can be useful in designing noninvasive ultrasensitive hand-held sensors for diagnostic identification of volatile organic compounds (VOCs) in exhaled air,o ffering pain-free and easier detection of long-term diseases such as asthma. In the present work, WO 3 nanoblocks, with ah igh surfacea rea and porosity,h ave been grown directly over transparent conducting oxide to minimize Ohmic resistance,f acilitating smoother electron transfer and enhanced current response. Further modification with porous alumina (g-Al 2 O 3 ), by electrodeposition, resulted in the selective and ultrasensitive detection of NO X in simulated exhaled air.C rystal phase purity of as-fabricatedp ristine as well modified samples is validated with X-ray diffraction analysis. Morphologicala nd microstructural analyses reveal the suc-cessfuld eposition of porous alumina over the surface of WO 3 .I mproved surface area and porosity is presented by porousa luminai nt he modified WO 3 device, suggesting more active sites for the gas molecules to get adsorbed and diffuse through the pores. Oxygen vacancies, which are detrimental in the transport phenomenoni nt he presented sensors, have been studied using X-ray photoelectrons pectroscopic (XPS) analysis. Gas sensing studies have been performed by fabricating chemiresistor devices based on bare WO 3 and Al 2 O 3 -modified WO 3 .T he highers ensitivity for NO X gas in case of g-Al 2 O 3 -modified WO 3 based devices, as compared to bare WO 3 -based devices, is attributed to the better surfacea rea and charget ransportk inetics. The presented devices trategy offers crucial understanding in the design and development of non-invasive, hand-held devices for NO gas present in the human breath,w ith potential application in medicaldiagnostics.