Elsevier

Talanta

Volume 88, 15 January 2012, Pages 152-159
Talanta

Selective detection of carbon dioxide using LaOCl-functionalized SnO2 nanowires for air-quality monitoring

https://doi.org/10.1016/j.talanta.2011.10.024Get rights and content

Abstract

In spite of the technical important of monitoring CO2 gas by using a semiconductor-type gas sensor, a good sensitive and selective semiconductor CO2 sensor has been not realized due to the rather unreactive toward CO2 of conventional semiconductor metal oxides. In this work, a novel semiconductor CO2 sensor was developed by functionalizing SnO2 nanowires (NWs) with LaOCl, which was obtained by heat-treating the SnO2 NWs coating with LaCl3 aqueous solution at a temperature range of 500–700 °C. The bare SnO2 NWs and LaOCl–SnO2 NWs sensors were characterized with CO2 (250–4000 ppm) and interference gases (100 ppm CO, 100 ppm H2, 250 ppm LPG, 10 ppm NO2 and 20 ppm NH3) at different operating temperatures for comparison. The SnO2 NWs sensors functionalized with different concentrations of LaCl3 solution were also examined to find optimized values. Comparative gas sensing results reveal that LaOCl–SnO2 NWs sensors exhibit much higher response, shorter response–recovery and better selectivity in detecting CO2 gas at 400 °C operating temperature than the bare SnO2 NWs sensors. This finding indicates that the functionalizing with LaOCl greatly improves the CO2 response of SnO2 NWs-based sensor, which is attributed to (i) p–n junction formation of LaOCl (p-type) and SnO2 nanowires (n-type) that led to the extension of electron depletion and (ii) the favorable catalytic effect of LaOCl to CO2 gas.

Highlights

► We have demonstrated a facile semiconductor gas sensor platform for selective detection of CO2 gas. ► This technique can solve the problem of semiconductor nanowires gas sensor that is poor sensitive to CO2 gas. ► The results are important because it can be applied to develop multi-sensors based on SnO2 nanowires for environmental monitoring.

Introduction

The recent increase in the emission of carbon dioxide (CO2) due to human activities has led to the considerable attention to CO2 gas as a noxious substance and a global warming factor. Thus, the demand in detection of CO2 gas has been increasing, particularly in controlling environmental condition, such as air-quality monitoring, fire detection, engine exhausts, agricultural development, bio-related and chemical processes [1], [2]. Feasible and low-cost monitoring of CO2 gas from low to high concentrations allow early detection of environmental hazards and provides time for an effective countermeasure [3]. Various types of CO2 sensors such as infrared [4], solid electrolyte [5], capacitive [6], surface acoustic wave s [8], fluorescent chemo-sensors [9], and semiconductive metal oxide (SMO)-based sensors [10], [11], [12], [13], [14], [15], [16], [17], have been demonstrated. Among of these sensing platforms, SMO sensors have important advantages, such as low cost, good reliability, small size, easy mass production, and potential development of array-integrated gas sensors [1]. For the past decades, much effort has been exerted to develop CO2 gas sensors based on conventional SMO materials [10], [11], [12], [13], [14], [15], [16], [17]. According to these reports, the sensitivity, selectivity, and response–recovery time of SMO-based sensors to CO2 gas still need to be improved for particular applications.

SnO2 nanowires (NWs) are among the most promising material systems for semiconductor gas sensors, with advantages, such as large surface-to-volume ratio, higher crystallinity, and better stoichiometric control [18]. The NWs sensors function by converting surface chemical processes, which are often catalytic processes into observable conductance variations in the NWs. Significant progress, including NWs surface functionalization [19], [21], NWs doping [22], NWs core–shell [23], hierarchical NWs [24], on-chip grown NWs [25], [26], wire-diameter control [27], and self-heating sensing [28], has been made to enhance the performance of SnO2 NWs-based sensors. Among these methods, functionalizing NWs with catalytic nanoparticles is one of the most cost-effective and feasible methods enhancing sensitivity and selectivity to particular gases for SnO2 NWs-based sensors [29]. In addition, this method would be very powerful to develop SnO2 NWs multi-sensors for the environmental monitoring by using an alternative catalyst coating. The multi-sensors is a technical important of environmental monitoring, because it can be used to detect a series of toxic gases, such as CO, CO2, SO2, NO2, NO, N2O, H2S, hydrocarbons, VOCs (volatile organic compounds) [30]. SnO2 NWs coated with Pd [19], [31], La2O3 [21], Au [32], and CuO [33] for the corresponsive detection of H2, C2H5OH, CO and H2S gases with very good sensitivity and selectivity has been reported. However, the development of high performance CO2 gas sensors remains a challenge for SnO2 NWs. LaOCl is one of the most promising materials for sensitive and selective detection of CO2 gas because of the favorable absorption of CO2 on the LaOCl surface through the formation of a carbonate based on the lanthanum site [16]. In addition, LaOCl is a p-type semiconductor, used forming the p–n junction with n-type SnO2 NWs to enhance the performance of the gas sensor [34]. Therefore, the present study proposes a simple and effective process to develop a CO2 gas sensor with good sensitivity, selectivity and relatively short response–recovery time using the LaOCl-functionalized SnO2 NWs (LaOCl–SnO2 NWs). The mechanism which the CO2 sensing properties of LaOCl–SnO2 NWs are enhanced is also discussed.

Section snippets

Experimental

SnO2 WNs were synthesized according to previous works [24], [26]. The SnO2 NWs were synthesized on Au-coated Si substrates through a simple thermal evaporation of Sn metal powders (99.9%). The source material was loaded in an alumina boat, placed at the center of a quartz tube in a horizontal-type furnace. The furnace was heated to 800 °C and kept for 30 min during the synthesis of the NWs. The pressure in the quartz tube was controlled at 5–10 Torr using O2 gas with a flow rate of 0.4–0.5 sccm.

Results and discussion

As-grown SnO2 NWs and functionalized SnO2 NWs samples were used for structure and morphology investigations. The functionalized SnO2 NWs sample was prepared by drop-coating the LaCl3 aqueous solution, followed by heat treatment at 600 °C for 5 h. The morphologies of the as-grown SnO2 NWs and functionalized SnO2 NWs recorded by FE-SEM are shown in Fig. 2a and b, respectively. The as-grown SnO2 NWs have a diameter of 50–150 nm and a length of several micrometers, with relative smooth surface and

Conclusion

In the present study, a novel LaOCl–SnO2 NWs sensor with good sensitivity, selectivity, and favorable response–recovery in detecting CO2 gas was developed. This sensing material can solve the problem of conventional SMO sensing materials that could hardly detect CO2 gas because of the chemical stability of the gas. The functionalization with LaOCl can increase the response of SnO2 NWs sensor to 4000 ppm CO2 gas by a factor about 6. Therefore, the selectivity to CO2 is enhanced compared with

Acknowledgments

This work was financially supported by the Vietnam's National Foundation for Science and Technology Development (Nafosted, Code: 104.05.13.09) and by the application-oriented basic research program (2009–2012, Code: 05/09/HÐ-DTÐL).

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