A Chemiresistor Sensor Based on Zn2SnO4-RGO Hybrids for High Selectivity to ppb-level Oxidizing Gas Under Humidity Conditions
Authors: Wang Ziying , Zhang Tong 
Published: May 2020

Highly selective oxidizing gas sensors are of great importance for environmental pollution monitoring. In this work, Zn2SnO4 nanoparticles (NPs) and reduced graphene oxide (Zn2SnO4-RGO) hybrids were developed as a high-performance gas sensing materials for the detection of ppb-levels of oxidizing gases (NO2 and O3). The structural, morphological and compositional properties of the Zn2SnO4-RGO hybrids were systematically characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), which demonstrated the successful anchoring of Zn2SnO4 NPs on RGO nanosheets. The obtained Zn2SnO4-RGO hybrids exhibited outstanding sensing performance for detecting oxidizing gases (NO2 and O3) with very low cross sensitivities to reducing gases, such as C2H5OH, CH3COCH3 and CO (Fig. 1a). The Zn2SnO4-RGO based sensors exhibited high response values of up to 3.50 for 500 ppb NO2, which is higher than that for detection of 500 ppb O3 (1.78) at 30 °C under 50% relative humidity (RH). Moreover, the NO2 sensing performances of Zn2SnO4-RGO-based sensors were investigated under various RH. In all cases, the sensors based on RGO and Zn2SnO4-RGO hybrids presented p-type behavior. In Fig. 1b and c, the sensors based on Zn2SnO4-RGO hybrids also exhibited high response values of up to 3.62 for 1 ppm NO2 at 50°C in 80% RH, which is much higher than that of pure RGO (1.31). The excellent sensing performances are mainly ascribed to the synergetic effect of Zn2SnO4 NPs and RGO. Furthermore, the surface reaction between Zn2SnO4-RGO hybrids and NO2 can be concluded from Operando diffuse reflectance infrared Fourier transformed spectroscopy (Operando DRIFT). NO2 is a highly electrophilic molecule and can directly capture free electrons from Zn2SnO4-RGO hybrids. The formation of NO3
-, NO and NO2
- can contribute to the decreased resistance of the sensor based on Zn2SnO4-RGO hybrids by loss of electrons as described in the chemical reaction [1,2], as shown in Fig.1d. The DRIFT spectrum of Zn2SnO4-RGO hybrids show absorption peaks at 1367 cm−1, 1535 cm−1 and 1735 cm−1 (Fig. 1e),which could be assigned to the n3 stretching vibration of nitrate species (NO3
-) [3], the NO vibration band of the bridging bidentate nitrate [4] and the formation of nitrite species (NO2
-) [5], respectively. This work is meaningful to design RGO-based sensors for detecting oxidizing gases.

References

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Figure1 (a) The selectivity of the sensor based on Zn2SnO4-RGO hybrids toward different kinds of gases, including 0.15 ppm NO2, 1 ppm O3, 20 ppm C2H5OH, 3.9 ppm CH3COCH3 and 20 ppm CO at 30 °C under 50% RH, the linear fitting curve of the response for the sensors based on (b) RGO and (c) Zn2SnO4-RGO hybrids versus NO2 concentration under different RH (80%, 50%, 20%, 5% and 0%), (d) the schematic illustration of the structure of Zn2SnO4-RGO-based sensors,
(e) Operando DRIFT spectra taken with the sensor based on Zn2SnO4-RGO hybrids heated to 50 °C during exposure to NO2 in the presence (red line) and absence of 10% RH (black line).

Figure 1