Modification of SnO₂ Nanowires with TeO₂ Branches and Their Enhanced Gas Sensing ^†

Abstract

We prepared a highly sensitive and selective NO₂ sensor, based on the TeO₂ branched SnO₂ nanowires (NWs), in terms of vapor-liquid-solid method, with subsequent growing of branches on the stems of SnO₂ NWs. Fabricated sensors showed a high response higher than 10 to 10 ppm of NO₂ gas at 100 °C. We investigated the associated sensing mechanisms, with respect to the enhancement of sensing behaviors by the addition of TeO₂ branches. Based on the results obtained in this work, we believe that the present sensor with an efficient fabrication technique, and high sensitivity and selectivity can be used for detection of NO₂ gas in real applications.

1. Introduction

Nanostructures are attractive and important structure in gas sensor criteria, because they can be manipulated easily and gas sensing properties are different depending on their morphologies [1]. Among that one-dimensional nanowires have been experimented as attractive materials to chemical gas sensors. Especially, the branched nanowires are tested for several reasons [2]. First, they have strength of 1-D nanowires. They have a linear path of charge transportation, which reduces carrier diffusion lengths and improves electron transportation. Second, branched nanowires have large surface area than 1-D nanowires, which enables many reactants to react on the surface of sensing materials. Accordingly, it enhances gas sensing properties such as sensitivity and selectivity.

2. Experimental

The fabrication procedures of TeO₂ branched SnO₂ nanowires are as follows. First, SnO₂ nanowires were fabricated by thermal evaporation of Sn powder. Sn powder (purity: 99.9%, Sigma-Aldrich) was used as the source material. The substrate was set to 900 °C for 1hr to heat 3 nm-Au coated Si substrates. A mixture of O₂ and Ar gases (O₂: 3%; Ar: 97%) was set at a fixed 2 Torr pressure. Then, TeO₂ branches were grown onto the surface of SnO₂ nanowires with the same VLS procedure only with the different temperature of 370 °C. For the sensing experiments, Ti/Au electrodes were sputtered on specimens with a turbo sputter coater (Emitech K575X, Emitech Ltd., Ashford, Kent, UK) and as-fabricated sensors were placed in a horizontal quartz tube furnace for gas sensing test. Through mass flow controllers, concentrations of target gases were manipulated by changing the mixing ratio of the target gas and dry air, with a total flow rate of 500 sccm. The resistances of sensors in the presence of air (R_a) and target gas (R_g) were measured and the sensor response for oxidative gas (NO₂) was calculated as R = R_g/R_a, and for reducing gases was calculated as R = R_a/R_g.

3. Results and Discussion

Figure 1a is SEM images of SnO₂ nanowires and Figure 1b–d is SEM images of TeO₂ branched SnO₂ nanowires with annealing temperature at 320, 370 and 420 °C. Branches were grown randomly onto the surface of SnO₂ nanowires. Figure 2a shows an XRD pattern of pristine SnO₂ nanowires, exhibiting reflection peaks that can be indexed to the tetragonal rutile SnO₂ phase (JCPDS card: No. 41-1445). Figure 2b–d show XRD pattern of SnO₂ nanowires with orthorhombic TeO₂ (JCPDS card: No. 52-1005) branches with different temperature. Figure 3a shows low magnification TEM images of TeO₂ branched SnO₂ nanowires. Figure 3b,c shows SAED pattern and Lattice-resolved TEM image of TeO₂ branched SnO₂ nanowires.

Figure 4 shows the sensing performances of pristine SnO₂ nanowires and TeO₂ branched SnO₂ nanowires. Figure 4a shows the sensor responses, revealing that the responses of branched SnO₂ nanowires are higher than those of pristine SnO₂ nanowires at temperatures in the range of 25–150 °C. Figure 4b shows the response times, indicating that the response time tends to decrease with increasing the temperature and that the response time of TeO₂ branched SnO₂ nanowires is shorter than that of pristine SnO₂ nanowires. Figure 4c shows the recovery times, exhibiting that the response time also tends to decrease with increasing the temperature and that the response time of branched SnO₂ nanowires is shorter than that of pristine SnO₂ nanowires.

We investigated the associated sensing mechanisms, in regard to the enhancement of sensing performances by the incorporation TeO₂ branches. It is revealed that not only the TeO₂ branches themselves but also the heterojunctions of SnO₂/TeO₂ play a crucial role in enhancing the sensing behaviors.

4. Conclusions

In this work, a highly sensitive and selective NO₂ sensor, based on the TeO₂ branched SnO₂ nanowires (NWs) were synthesized by an efficient route. Fabricated sensor showed a high response to 10 ppm of NO₂ gas at optimal temperature of 100 °C, demonstrating an excellent selectivity of sensor towards NO₂ gas. The superior sensing properties of branched NWs sensor relative to the pristine sensor were mainly attributed to the branch-induced high surface area of sensor and formation of homo-and heterojunctions between SnO₂ and TeO₂.