High Performance N-N Type WO3@in2O3 core-Shell Heterojunction Nanowires Based NO2 Gas Sensor for Environmental Monitoring

Introduction: Harmful and toxic gases such as nitrogen dioxide (NO 2 ), carbon-monoxide (CO) and VOCs are largely released into environment due to increased industrial revolution. NO 2 is considered as one of the most dangerous air pollutants, which plays a vital role in the formation of ozone (O 3 ) to produce acid rains and therefore, it is essential to monitor trace level NO 2 gas in environment for human safety. One-dimensional (1D) nanostructures have become an attractive candidate for sensors owing to their superior spatial resolution and rapid response due to the high surface-to-volume ratio compared to thin film gas sensors [1–4]. Metal oxide based tungsten oxide (WO 3 ) is an important n -type semiconductor material with a bandgap of 2.7 eV, suitable for gas sensor applications [5]. Though, WO 3 is widely used gas sensor material, poor selectivity, high operating temperature and reliability hinders their practical application. Similarly, indium oxide (In 2 O 3 ) has demonstrated as another promising gas sensor material specifically to detect NO 2 gas at room-temperature. In this investigation, development of n-n type WO 3 @ In 2 O 3 heterojunction nanorods has been implemented to form core-shell architecture which exhibited distinguished sensing properties at reduced temperature towards trace level NO 2 gas with excellent sensitivity, high selectivity and fast response/recovery characteristics and a plausible mechanism is deduced. Synthesis of WO 3 @In 2 O 3 core-shell heterojunction nanorods: 1D WO 3 nanorods were synthesized using hydrothermal method as per previous report [6]. In this work, WO 3 @In 2 O 3 core-shell heterojunction nanorods were subsequently prepared by solvothermal method. Briefly, In(CH 3 COO) 3 . x H 2 O (0.5 mmol) was dissolved in a binary solvent mixture (1:2) with ethylene glycol (17 mL) and ethanol (34 mL), followed by vigorous stirring for 40 min. Meanwhile, the surface treatment of 200 mg of WO 3 nanorods were carried out under UV-irradiation (254 nm) for 2 min. The above mixture was subjected to ultrasonication for 30 min. The solution was further transferred into a Teflon-lined stainless-steel autoclave (100 mL capacity) and maintained at 160 o C for 5 h. The product was separated, washed and centrifuged several times with ultrapure water followed by ethanol in order to eliminate the organic and redundant In 2 O 3 impurities. The precipitate was dried in a vacuum oven at 80 o C for 24 h. Finally, the resultant product was annealed at 500 o C in air for 2 h at a heating rate of 2 o C/min. The color of the precipitate was changed to pale yellow, implying the functionalization of In 2 O 3 nanoparticles on the surface of WO 3 nanorods. Results and Discussion: XRD patterns and the Raman spectral analysis of WO 3 nanorods and WO 3 @In 2 O 3 core-shell heterojunction nanorods confirmed the structural purity and formation of heterojunction materials. The peaks in XRD pattern of the WO 3 nanorods could be well-indexed to the hexagonal phase of WO 3 (JCPDS 85-2460). No additional peaks were observed, which confirmed the phase purity of synthesized WO 3 nanorods. Further, core shell structure of the WO 3 @In 2 O 3 nanorods was analyzed using HRTEM and SEM. The results indicated the homogenous distribution of In 2 O 3 nanoparticles over WO 3 nanorods. Evaluation of NO2 gas sensor properties: NO 2 gas sensing properties of WO 3 nanorods and WO 3 @In 2 O 3 core-shell heterojunction nanorods were evaluated using in-house gas sensor test station at reduced temperature. The dynamic gas sensing response of WO 3 @In 2 O 3 core-shell heterojunction nanorods towards trace level detection of NO 2 in the range of 500 ppb to 3 ppm exhibited sensitivity upto to S = 280% at 150 o C. The enhanced gas sensing response of WO 3 nanorods with surface anchored In 2 O 3 nanoparticles is attributed to high adsorption property of NO 2 on active sites and also due to directed electron transport mechanism. Upon exposure to NO 2 , the gas molecules initially physisorbed at the heterojunctions and trap electrons from the heterojunction interfaces of WO 3 @In 2 O 3 nanorods. Since electron transport between WO 3 and In 2 O 3 is based on work function difference, the electrons can accumulate at the heterojunction interfaces leading to increased space-charge depletion region. Hence, the resistance increases due to the depletion of electrons which is evident from the enhanced gas sensing behaviour of WO 3 @In 2 O 3 heterojunction nanorods upon exposure to NO 2 . 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