Highly Sensing Properties Sensors Based On Ce-Doped ZnO and SnO 2 Nanoparticles to Ethanol Gas

The detection of different types of gases and quantification of their precisely concentration have wide applications in different fields like domestic gas alarms, medical diagnostic equipment, industrial safety, environmental pollution, military field and food industry [Papadopoulos et al (1996), Wang C et al (2009), Wetchakun et al (2011), Brudzewski et al (2012), Mielle et al (2001) and Fleming (2001)]. With numerous industries that utilize or produce ethanol, it is apparent that reliable methods are needed Abstract

The current study aims to compare between the samples which exhibited the best sensing characteristics at the same conditions to define the high sensing properties sensors to ethanol gas among the two oxides sensors.

Synthesis
Zinc oxide nanoparticles were synthesized by chemical precipitation method using zinc acetate dihydrate (99.61%) and diluted ammonium hydroxide as starting materials.
The obtained precipitate was calcined in a muffle furnace at the temperature of 400 oC for 4 hours and then left to cool to room temperature.Preparation of tin oxide nanoparticles was carried out also by chemical precipitation method using a solution of tin tetrachloride with concentration of 0.2 mol/L.The precipitate was then filtered and washed thoroughly until free of chloride by testing the filtrate with silver nitrate solution.The obtained precipitate was dried in air followed by calcination at 400 oC for 4 hours then left to cool to room temperature.Appropriate amounts of CeO2 were added to the prepared ZnO and SnO2 nanoparticles powders with ratio of 0, 2, 4 and 6 wt % for each.The resulting mixtures were ball milled for 2h to get homogenous powder to be used as functional materials to fabricate sensors The prepared samples will be characterized by using X-ray diffraction analysis using Xray diffractometer (model-Bruker AXS D8 advance) with copper radiation, infrared transmission spectra using Nexus 670 FTIR spectrophotometer (Nicolet, USA) and transmission electron microscope using JEOL JEM-1230 operating at 120 KV attached to CCD camera.a. c. electrical conductivity as well as ethanol gas sensing characteristics of the different sensors were measured in the temperature range from 30 up to 410 oC using LCR meter (Hitester, model Hioki 3532, made in Japan) at frequency of 1 KHz and applied 5 V.The sample chamber was a closed glass cell contains the sensor sample surrounded by electrical cylindrical ceramic furnace with controlled temperature device [Hassona et al 2012].The desired ethanol gas concentrations were obtained by injection a known volume of ethanol using a microsyringe through air tight rubber port into the glass chamber.The sensitivity (S) of the sensor toward ethanol gas was measured and calculated as the ratio of the electrical resistance in air atmosphere (Rair) to that in air containing ethanol gas (Rgas) by using the following relation

Characterization
Fig. 1 shows the X-ray diffraction patterns of the prepared zinc oxide and tin oxide. .Also, it can be seen that the electrical conductivity values of ZnO sensors are somewhat higher than that of SnO2 sensors at the same conditions.This may be due to the difference in the particle and change carriers concentration and its mobility.

Ethanol Gas Sensing Properties
Where the pure and cerium doped sensors of both oxides of ZnO and SnO2 sintered at 400 oC exhibited the highest sensitivity among all the prepared sensors, for that reason the study focused on these sensors.Fig. 6 illustrates the variation of the sensitivity with temperature towards 100 ppm ethanol gas of ZnO + x wt% CeO2 and SnO2 + x wt% CeO2 sensors sintered at 400 oC.With respect to the pure and CeO2-doped ZnO sensors, the sensitivity gradually increases with temperature and attains the maximum 310 oC (operating temperature), and then it decreases with increasing the temperature.While, the sensitivity of the pure and CeO2doped SnO2 sensors gradually increase with temperature until 300 oC (operating temperature) and then after it decreased.For comparison, it can be seen that the SnO2 sensors have higher sensitivity values than that of ZnO sensors at the same conditions.Also, the operating temperature of the SnO2 sensors is lower than that in the case of ZnO sensors by 10 oC.
The mechanism of the ethanol detection on the both oxides sensors is depending on the interaction of ethanol gas with the preadsorbed oxygen species on the oxides surface.Where, in atmospheric air the surfaces of ZnO or SnO2 sensors are covered by oxygen species.At relatively low temperature (< 150 oC) the molecular oxygen species (O2−) are adsorbed and with increasing the operating temperature the molecular oxygen dissociate to ionic oxygen species (O2−→2O−→O2−) and its concentration raise gradually until certain temperature by extracting electrons from the metal oxide material which increase the electrical resistance of the materials.The Where, the changes in the electrical resistance of the sensors in the air to that in gas atmosphere (Rair/Rgas) represent the sensitivity of the sensors to the tested gas.
The dependence of the sensitivity on CeO2 content of ZnO and SnO2 sensors sintered at 400 oC is shown in Fig.The variation of the sensitivity with ethanol gas concentration for ZnO + 4 wt % CeO2 and SnO2 + 2 wt % CeO2 sensors sintered at 400 oC is shown in Fig. 8.For ZnO + 4 wt % CeO2 the sensitivity linearly increases with increasing the concentration of ethanol gas up to 400 ppm.While, above 400 ppm, the sensitivity slowly increased.On the other hand, the SnO2 + 2 wt % CeO2 sensor shows a linear increasing in the sensitivity up to 500 ppm then after it slowly increases with increasing the gas concentration until 2000 ppm.
The response time of the sensor is usually defined as the time taken to achieve at least 90 % of the final change in its electrical resistance during exposure to the tested gas.While, the recovery time is generally defined as the time taken by the sensor to get back at least 90 % of its original state after reexposure to air ambient by maintaining the operating temperature constant [Hassona et al 2012].The variation of the sensitivity with time of ZnO + 4 wt % CeO2 and SnO2 + 2 wt % CeO2 sensors sintered at 400 oC after exposure to 100 ppm ethanol gas is shown in Fig. 9.It was found that, the response time of ZnO + 4 wt % CeO2 sensor was 12 second while that of SnO2 + 2 wt % CeO2 sensor was 20 second.Fig. 10 illustrates the variation of the sensitivity with time of ZnO + 4 wt % CeO2 and SnO2 + 2 wt % CeO2 sensors sintered at 400 oC after re-exposure to air atmosphere.It can be seen that, the recovery times of ZnO + 4 wt % CeO2 and SnO2 + 2 wt % CeO2 sensors were 10 and 15 second, respectively.The above mentioned results revealed that all the investigated sensors samples are chemically stable i.e there is no chemical reactions occurred between the sensors and the tested gas led to the change in the chemical composition of the sensors during the exposure to ethanol gas.

Conclusions
From the obtained data it can be concluded that the best sensors among the prepared sensors of the two oxides are ZnO + 4 wt% CeO2 and SnO2 + 2 wt % CeO2 sensors __________________________________________________________________________________________________________________ [El-Sayed et al 2012 and Hassouna et al 2012].S = Rair / Rgas (1) ionic oxygen species O− and O2− are the __________________________________________________________________________________________________________________ ______________ A.M. El-Sayed and S. M. Yakout (2016), Journal of Research in Nanotechnology, DOI: 10.5171/2016.690025 more reactive species in gas sensing process.When the sensors surface exposed to ethanol gas co-adsorption and mutual interaction between the ethanol gas and the adsorb oxygen species with liberating electrons to the sensors materials which decreases the electrical resistance and the final reaction [El-Sayed et al 2012 and Hassouna et al 2012]: ______________ A.M. El-Sayed and S. M. Yakout (2016), Journal of Research in Nanotechnology, DOI: 10.5171/2016.690025 sintered at 400 oC.The SnO2 + 2 wt% CeO2 sensor has high sensitivity towards ethanol gas, rapid response time (20s) and short recovery time (15s) with operating temperature of 300 oC.Also, ZnO + 4 wt% CeO2 sensor has high sensitivity towards ethanol gas, rapid response time (12s) and short recovery time (10s) with operating temperature of 310 oC.

Figure 3 :
Figure 3: Infrared spectra of (a): prepared and sintered ZnO nanoparticles samples and (b): prepared and sintered SnO2 nanoparticles samples.

Table 1 .
It can be seen that at the same CeO2 content the particle size of the ZnO sensors is larger than that of SnO2 sensors.