Abstract
Introduction:
Harmful and toxic gases such as nitrogen dioxide (NO2), carbon-monoxide (CO) and VOCs are largely released into environment due to increased industrial revolution. NO2 is considered as one of the most dangerous air pollutants, which plays a vital role in the formation of ozone (O3) to produce acid rains and therefore, it is essential to monitor trace level NO2 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 (WO3) is an important n-type semiconductor material with a bandgap of 2.7 eV, suitable for gas sensor applications [5]. Though, WO3 is widely used gas sensor material, poor selectivity, high operating temperature and reliability hinders their practical application. Similarly, indium oxide (In2O3) has demonstrated as another promising gas sensor material specifically to detect NO2 gas at room-temperature. In this investigation, development of n-n type WO3@ In2O3 heterojunction nanorods has been implemented to form core-shell architecture which exhibited distinguished sensing properties at reduced temperature towards trace level NO2 gas with excellent sensitivity, high selectivity and fast response/recovery characteristics and a plausible mechanism is deduced.
Synthesis of WO3@In2O3 core-shell heterojunction nanorods:
1D WO3 nanorods were synthesized using hydrothermal method as per previous report [6]. In this work, WO3@In2O3 core-shell heterojunction nanorods were subsequently prepared by solvothermal method. Briefly, In(CH3COO)3. xH2O (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 WO3 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 160oC 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 In2O3 impurities. The precipitate was dried in a vacuum oven at 80oC for 24 h. Finally, the resultant product was annealed at 500oC in air for 2 h at a heating rate of 2oC/min. The color of the precipitate was changed to pale yellow, implying the functionalization of In2O3 nanoparticles on the surface of WO3 nanorods.
Results and Discussion:
XRD patterns and the Raman spectral analysis of WO3 nanorods and WO3@In2O3 core-shell heterojunction nanorods confirmed the structural purity and formation of heterojunction materials. The peaks in XRD pattern of the WO3 nanorods could be well-indexed to the hexagonal phase of WO3 (JCPDS 85-2460). No additional peaks were observed, which confirmed the phase purity of synthesized WO3 nanorods. Further, core shell structure of the WO3@In2O3 nanorods was analyzed using HRTEM and SEM. The results indicated the homogenous distribution of In2O3 nanoparticles over WO3 nanorods.
Evaluation of NO2 gas sensor properties:
NO2 gas sensing properties of WO3 nanorods and WO3@In2O3 core-shell heterojunction nanorods were evaluated using in-house gas sensor test station at reduced temperature. The dynamic gas sensing response of WO3@In2O3 core-shell heterojunction nanorods towards trace level detection of NO2 in the range of 500 ppb to 3 ppm exhibited sensitivity upto to S = 280% at 150oC. The enhanced gas sensing response of WO3 nanorods with surface anchored In2O3 nanoparticles is attributed to high adsorption property of NO2 on active sites and also due to directed electron transport mechanism. Upon exposure to NO2, the gas molecules initially physisorbed at the heterojunctions and trap electrons from the heterojunction interfaces of WO3@In2O3 nanorods. Since electron transport between WO3 and In2O3 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 WO3@In2O3 heterojunction nanorods upon exposure to NO2.
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