(Invited) Highly Selective Detection of Propanol and Ethanol Gas Sensing Characteristics of SnO2/NiO and SnO2/NiO/Au Hollowspheres Based Chemiresistive Sensors
Authors: Motaung David Edmond 
Published: May 2020

Introduction
The enormous market demands for gas sensors to detect and monitor volatile organic compounds (VOCs) and toxic gases in environmental protection, medical treatment and safety requirements of industry and daily life have motivated enormous awareness in the fabrication of high performance gas sensing materials. Generally, VOCs are considered as low boiling point carbon-comprising materials that can evaporate simply at room temperature and contribute in atmospheric photochemical reactions, such as photochemistry smog and ozone formation. Besides, controlling and detection of these VOCs, such as methanol, ethanol, propanol and acetone is significant because they are considered as widespread concerns for common pollutants in indoor/outdoor air quality and for testing alcohol levels of drivers [1]. In comparison to other VOCS, propanol is less toxic, nonetheless its usage for numerous daily applications, for example hand sanitizer, cosmetics, may be imagined to be polluting the breathing air. Besides, Iso-propanol was presented to behave as an aesthetic and central nervous system sedative, leading in symptoms that can frighten the mental ability of the individual [2].
Yet, the fabrication of the anticipated gas sensor with superior sensitivity, low detection limit, and exceptional long-term stability is still a significant research issue. In addition, the high operating temperatures and poor selectivity often limits the application and commercialization of semiconductor metal oxide (SMO) gas sensors. Thus, this work is directed to improve the selective, sensitivity, stability and to reduce the operating temperatures of the metal oxides based sensors, by exploiting SnO2 hollowspheres loaded with various NiO nanoparticles contents and SnO2/NiO loaded with various Au nanoparticles contents synthesized using a simple hydrothermal method. Our findings showed improved stability upon loading SnO2 hollowspheres with various NiO contents. Amongst the various NiO contents, the 0.01 wt.% NiO loaded SnO2 demonstrated higher response and sensitivity towards propanol (C3H7OH) in dry air and in the presence of 40 and 60 % RH at an operating temperature of 150 °C. While the as-fabricated SnO2/NiO/Au (2.5 wt.%) based sensor exhibited a response that is more than 2 times higher in comparison to that of SnO2/NiO (0.01wt. %) and lower operating temperature 75 °C towards ethanol (C2H5OH) and C3H7OH. The long-term stability analyses established that both the fabricated SnO2/NiO (0.01 wt.%) and SnO2/NiO/Au (2.5 wt. %) sensors were very stable towards C3H7OH and C2H5OH after a month in the presence of 40% RH. These findings showed that the current sensors can be employed for detecting C3H7OH and C2H5OH in a vastly sensitive and selective way with insignificant interference from ambient humidity.
Method
Porous hollowspheres were synthesized according to the following procedure reported in ref. [3]. While the NiO loaded SnO2 hollowspheres were fabricated using the as-prepared hollowspheres as templates. Gold (Au) loaded NiO/SnO2 hollowspheres were prepared using NiO/SnO2 hollowspheres as template as reported elsewhere.
The gas sensing properties of the Au-loaded SnO2/NiO were measured using the KSGAS6S gas testing system (KENOSISTEC, Italy). Sensing preparations and testing were performed according to the procedure in ref [3, 4].
Results and Conclusions
To investigate the operational temperature influence on the gas sensing characteristics, the sensing layers were analysed at various operating temperatures (75 to 225 °C). The variations in the gas sensing resistance (Ra) of SnO2/NiO loaded with various NiO contents and SnO2/NiO loaded with various Au contents as a function of operating temperature. The Ra of the all sensing materials decreased with an increase in operating temperature, except that that of low Ni loading. A decrease could be when the operational temperature increases more electrons from valence band of the sensing layer will jump to conduction band and accordingly more electrons are available for carrying current, resulting in a decrease of Ra and this is accordance with the previous studies. While the increase in Ra values for SnO2/NiO can be justified by the formation of nanoscale p/n junctions between the NiO and SnO2 phases.
We further explored the stability of the SnO2/NiO (0.01 wt.%)-and SnO2/NiO (0.01 wt.%)/Au best performing based sensors in terms of repeatability and long-term stability measurements in dry air and in the presence of relative humidity (RH). The results exhibited the repeatability behavior of the SnO2/NiO (0.01 wt.%)-based sensor for eight successive cycles at 40 ppm C3H7OH for a month in the presence of 40% RH. The fresh sensor in day one and that tested after 30 days displayed a flawless repeatability of eight successive cycles towards 40 ppm C3H7OH. After 7 to 30 days, only minimal drift on the repeatability performance was witnessed, while the response increased by 8%, validating that our sensor is very stable. Same behavior was observed for the SnO2/NiO (0.01 wt.%)/Au(2.5 wt.%) based sensor. Thus, these findings corroborate that the current sensors are very stable when either exposed to dry air or in real conditions after long exposure to C3H7OH and C2H5OH.