Demonstrating the impact of Band Gap Modilation on Semiconductor Metal Oxide Gas-sensing Performance
Authors: Wang Tianshuang , Lu Geyu 
Published: July 2021

Introduction
Gas sensors as the front-end device is the core component of indoor air quality monitoring system and plays an important role in detecting toxic gases. Due to various characteristics of simple structure, all solid state, well stability, and low cost, the semiconducting metal oxides (SMOs) based gas sensors have showed potential [1,2]. Recently, some researches have found that the alteration of electronic band structure will impact the gas sensing characteristics via controlling the charge carrier concentration and enhancing the surface adsorbed oxygen species [2-5].
Herein, we reported the synthesis of honeycomb-like 3D inverse opal (IO) CdO-CdGa2O4 microspheres (MS) by using the combination of the one-step USP method with self-assembled S-PS spheres template. Such a novel structure provides both inside and outside reaction sites with high gas accessibility. Since the CdGa2O4 is an n-type semiconductor metal oxide with suitable band gap, and have been found to be a promising material as a transparent electronic conductor [6], we use it as base material of sensitive element. Importantly, the incorporation of CdO with narrow band gap (2.15-2.7 eV) and high electrical conductivity [7] is implied to deeply investigate the relationship between sensing materials’ electronic band structure and gas sensing characteristics. Accordingly, we successfully prepare 3D IO CdO-CdGa2O4 MS based chemiresistor sensor with alterable selectivity and high sensitivity via tuning the ratio of Cd/Ga.
Results and Conclusions
In order to demonstrate the sensing capability of the 3D IO CdGa2O4, CdO-CdGa2O4 MS based gas sensor, we measure their gas sensing response to 100 ppm benzene, toluene, acetone, methanol, formaldehyde, and ethanol, as well as 1 ppm NH3, CH3, H2S, CO, SO2, and NO2 at the range of 175-275 °C, respectively. As shown in Figure 1(A)-(C), the CdGa2O4 based gas sensor exhibits the highest response to 100 ppm formaldehyde at 225 °C, however, after adding of CdO element, the CdO-CdGa2O4 based gas sensor shows lower sensitivity to formaldehyde than primary CdGa2O4 at 225 °C, and possesses higher sensitivity to low concentration of NO2 at 175 °C.
Fig.1 (A-C) Gas responses of the 3D IO CdGa2O4, CdO-CdGa2O4 MS based gas sensor to various gases at 175-275 °C, respectively.
The UV-vis absorption spectra of the CdGa2O4, CdO-CdGa2O4, and Cd-abundant CdO-CdGa2O4 samples show continuous red shifts of absorption edges with increasing CdO element content (Figure 2(A)), suggesting an decrease in band gap. In addition, the band gaps of all above samples can be calculated from the transformation of UV-vis diffuse reflectance spectra by utilizing the Tauc plot equation (Figure 2(B)). The calculated results further show that the band gaps tend to be narrower values compared with incorporation of CdO (from 4.16 to 2.03 eV). Thus, the baseline resistances in air of Cd-abundant CdO-CdGa2O4 sensor is the lowest (Figure 2(C)). Accordingly, this study indicates that the gas-sensing mechanism of semiconductor metal oxide chemiresistor should consider electron energy level structure. Meantime, we will further study this relationship in subsequent experiments based on above analysis.
Fig. 2 (A) UV/visible diffuse reflection spectra and (B) corresponding band gaps of CdGa2O4, CdO-CdGa2O4, and Cd-abundant CdO-CdGa2O4 samples; (C) Resistance in air of gas sensors based on CdGa2O4, CdO-CdGa2O4, and Cd-abundant CdO-CdGa2O4 at 200 °C.
References
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