Synergistic enhancement of ammonia gas-sensing properties at low temperature by compositing carbon nanotubes with tungsten oxide nanobricks
Introduction
Ammonia (NH3) is a hazardous gas produced in the chemical industry, food processing and even human breath [[1], [2], [3]]. Many efforts have been exerted to fulfil the huge demand for ammonia gas sensors [[2], [3], [4], [5], [6], [7], [8], [9], [10]]. Recent research on NH3 chemiresistive sensors mainly focused on using nanostructures [3,4] and nanocomposites [5] to enhance the selectivity, increase the response, reduce the working temperature, lower the power consumption and reduce the resistance of these sensors [[6], [7], [8], [9],11]. Reducing the working temperature not only simplifies the design of sensors and lowers operational power but also avoids the degradation of the gas sensor due to the grain coalescence of nanostructures during long-term operations (at >200 °C) [12]. Lowering the resistance of sensors could increase measurement accuracy and reduce operational power requirement [2,8]. Among numerous potential nanomaterials, carbon nanotubes (CNTs) and tungsten oxide (WO3) are the two most promising [[2], [3], [4], [5],13]. CNT-based NH3 gas sensors have low resistance and low working temperatures, even at room temperature (RT), but poor response [4]. NH3 gas sensors based on 100% WO3 nanostructures show good responses but high resistance and high working temperature (200–400 °C) [14]; they could work at low temperatures but with unstable response [[15], [16], [17]].
Many scholars focused on increasing the response and/or lowering the power consumption and working temperature of sensors [11]. Duy et al. built an n-p junction between n-WO3 nanorods and p-CNTs to reduce the resistance and increase the response of a gas sensor working at high temperatures [8]. Moon et al. designed a self-activated gas sensor working at 139 °C [9]. Muthukumaran et al. fabricated an n-p junction between n-Fe2O3 and p-CNTs to increase the selectivity of sensors to NH3 [18]. Lich et al. modified on-chip CNTs with metal nanoparticles to enhance the responses of a gas sensor working at RT [19]. Shim et al. employed electron transfer through the junction between n-type WO3 and electron-rich metallic Au nanoparticles to increase the response of sensors at high temperatures [20]. In these works, the response, working temperature, or operational power of sensors improved but at the expense of other parameters. Many efforts are still needed to develop a suitable nanocomposite material for NH3 gas sensors.
In this work, we composited CNTs with WO3 nanobricks (NBs) not only to reduce the resistance of gas sensors but also to build a junction between CNTs and WO3 NBs to enhance NH3 gas-sensing properties. Stable monoclinic stoichiometric WO3 NBs were one-step synthesised by a hydrothermal method without any post-annealing process. WO3 NBs were then composited with commercial CNTs at different contents via facile mixing. The NH3 gas-sensing properties of nanocomposite materials were investigated at low temperatures (<200 °C) and compared with those of a pristine CNT-based sensor. In this study, we aim to demonstrate the synergistic effect in CNT/WO3 nanocomposite materials that could be employed in gas sensors with low power consumption and low working temperature. The mechanism underlying the synergistic effect in CNT/WO3 nanocomposites was also discussed in detail.
Section snippets
Synthesis of WO3 nanostructure
All chemicals were of analytical grade and were used without any further purification. Tungsten oxide NBs were synthesised using a hydrothermal method in a highly acidic environment by dissolving 8.25 g of Na2WO4·2H2O into 25 mL of bi-distilled water, adding dropwise 39 mL of HCl (37 wt%) into the above solution to adjust the solution pH to −1.0, creating a highly acidic environment (using Multiparameter pH Meter HI2020-02 Hanna instruments) and then stirring for 4 h at RT. After stirring, the
Morphology and structure of as-grown WO3 nanostructure
Fig. 2 a shows the FESEM image of as-grown WO3 nanostructure. The obtained WO3 nanostructure had a uniform NB morphology with the approximate medium dimensions ~150 × 100 × 100 nm. WO3 NBs had sharp corners and were well separated, which revealed the high crystallinity of the WO3 NBs. The sharp and strong peaks in the XRD pattern of WO3 NBs (Fig. 2b) confirmed the high crystallinity and uniformity of the WO3 NBs. XRD data were analysed using HighScore Plus software, and results showed that WO3
Conclusion
Ammonia gas sensors based on nanocomposite materials of commercial CNTs (0.5, 1.0 and 1.5 wt%) and one-step synthesised monoclinic WO3 NBs were simply fabricated by co-dispersing precursors in DMF. All nanocomposite sensors exhibited p-type response to NH3 at low temperatures and showed the synergistic enhancement of response in comparison with CNT-based sensors, and they were more stable than WO3 NB-based sensors. At 50 °C, the 0.5 wt%-CNT sensor had shorter recovery time, the 1.5 wt%-CNT
Acknowledgments
This research is funded by Hanoi University of Science and Technology (HUST) under project number T2018-PC-126. We gratefully acknowledge fruitful discussions with N. D. Chien. The authors thank the support from the BKEMMA at the AIST-HUST for the FESEM work.
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