الفهرس 
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Abstract
Many industrial and commercial activities required the detection, quantification and controlling of ethanol gas concentration. Ethanol gas sensors have wide applications in different field such as breath analyzer of drivers to reduce the number of roads accidents, monitoring of fermentation, foodstuffs conserving, medical processes and other processes in chemical industries. This thesis aimed to prepare and study the structural, electrical conductivity and ethanol gas sensing properties of pure ZnO, CeO2-doped ZnO, SnO2 and CeO2-doped SnO2 nanoparticles sensors. The effects of the sintering temperature and CeO2 additions on the sensing properties were investigated and discussed. The prepared sensors were investigated and characterized by using X-ray diffraction (XRD), infrared spectra (IR), scanning electron microscope (SEM) and transmission electron microscope (TEM). The electrical conductivity as well as ethanol gas sensing properties were measured and discussed. A comparison between the obtained results of the two oxides materials was carried out to choose the suitable sensors samples that can be used for ethanol gas sensing applications. The procedure of preparation, investigation and the obtained results can be summarized as follow:
1- ZnO nanoparticles sensors
Zinc oxide nanoparticles were synthesized by using zinc acetate dihydrate solution with concentration of 0.2 mol/L. Diluted NH4OH solution was added DROP wise to this solution with continuous stirring to yield precipitates of zinc hydroxide at pH of 8. The obtained precipitate was calcined at 400 oC for 4 hours in a muffle furnace. Proper amounts of CeO2 were added to the prepared ZnO with ratio of 0, 2, 4 and 6 wt %. The resulting mixtures were used to fabricate sensors pellets of 12 mm in diameter and 2 mm thick, and then sintered at 400, 600 and 800 oC for 2 hours.
X-ray diffraction pattern of the prepared ZnO showed that all the diffraction peaks can be indexed to the hexagonal wurtzite structure of ZnO with average crystallite size of 34.5 nm. The X-ray diffraction patterns of pure and CeO2-doped ZnO sensors sintered at 400, 600 and 800 oC were indexed to the hexagonal wurtzite structure of ZnO and to the cubic structure of CeO2. No change in the position of the diffraction peaks for ZnO and CeO2 in the doped samples and there is no evidence for interaction between the two components during the sintering processes. The average crystallite size of the sensors samples increased with increasing the sintering temperature. While, the average crystallite size of the pure ZnO decreased with CeO2 additions.
The IR spectrum of the prepared ZnO showed five absorption bands. The strong broad absorption band centered at ~ 3437.49 cm-1 was attributed to the O-H stretching mode of the absorbed water. The weak absorption band appeared at ~ 2924 cm-1 was assigned to C-H mode of the acetate group which indicated that a few organic groups were absorbed on the surface of ZnO. The two bands observed at ~1516 and 1389 cm-1 related to asymmetric and symmetric stretching modes of the carbonate group. The characteristic absorption band of the prepared ZnO appeared at ~ 461cm−1 was attributed to stretching vibration of Zn-O bond. The IR spectrum of CeO2 showed strong absorption band centered at 433 cm-1, which was assigned to Ce-O bond vibration.
The Infrared spectra of pure and CeO2-doped ZnO sensors showed only one broad absorption band. Where, the characteristic absorption band of ZnO overlapped with the CeO2 absorption. The characteristic absorption band of pure ZnO sample is slightly shifted to lower wavenumber with increasing the sintering temperature. This may be due to the increasing in the particle size or aggregation of the particles. In addition, the overlapped bands in the doped samples became broader and extended over a wider wavelength with increasing the sintering temperature and CeO2 content.
The scanning electron microscope photographs of pure and CeO2-doped ZnO sensors samples sintered at 400, 600 and 800 oC depicted that the grain size of the samples increased with increasing the sintering temperature. On the other hand, the grain size of ZnO decreased with CeO2 addition. The sensors samples sintered at 400 oC showed fine structure and composed of nearly homogeneous spherical grains. The sensors samples doped with 6 wt % CeO2 sintered at 600 and 800 oC showed the presence of ZnO grains having hexagonal shape beside the CeO2 grains having cubic structure.
The transmission electron microscope images of pure and CeO2-doped ZnO sensors samples sintered at 400, 600 and 800 oC demonstrated that the particle size of the samples increased with increasing the sintering temperature. The image of the pure ZnO sintered at 400 oC illustrated some agglomeration and the particles nearly have a hexagonal shape. For the doped samples, the particle size of ZnO decreased with CeO2 doping. Also, the hexagonal shape of the ZnO particles was clearly enhanced by the addition of CeO2.
The variation of the electrical conductivity with temperature for pure and CeO2-doped ZnO sensors samples sintered at 400, 600 and 800 oC was studied. Generally, the features of the curves are nearly the same. Each curve consists of three different temperature regions. At the low temperature region (30-150 oC), the electrical conductivity is slightly increased with increasing the temperature which may be attributed to the thermal excitation of electrons into the conduction band. On the other hand, the decreasing in the electrical conductivity with increasing the temperature from 150-320 oC was related to the adsorption of oxygen species (O−, O2−) on the surface of the sensors samples. With further increase in the temperature (320-410 oC), the electrical conductivity increased, probably due to the thermal excitation of electrons and desorption of oxygen species. The electrical conductivity of the pure and CeO2-doped ZnO sensors was found to increase with the increase of the sintering temperature and CeO2 additions.
The relation between the sensitivity and temperature for pure and CeO2-doped ZnO sensors samples sintered at 400, 600 and 800 oC towards 100 ppm ethanol gas was investigated. It was found that, the sensitivity gradually increased with increasing the temperature and reached its maximum value at nearly 310 oC. While, above 310 oC, the sensitivity of the sensors samples decreased. The sensitivity of the sensors samples decreased with increasing the sintering temperature from 400 up to 800 oC. Where, with increasing the sintering temperature, the average particle size of the sensors samples is increased and consequently the surface area is decreased which provides less active sites for adsorption and interaction between the adsorbed oxygen species and ethanol gas.
The sensitivity of the ZnO sensors towards ethanol gas is increased with CeO2 addition up to 4 wt% and then decreased with further CeO2 addition. Where, the additions of CeO2 up to 4 wt% can promote the ethanol dehydrogenation reaction in the form of catalysts and also improve the surface basicity of ZnO which enhances the sensitivity. While, the increasing in CeO2 doping concentration to 6 wt % may cause a high covering of ZnO surface by CeO2 which reduce the available adsorption sites on ZnO surface and this led to the observed decrease in the sensitivity.
The variation of the sensitivity with ethanol gas concentration for pure and CeO2-doped ZnO sensors samples sintered at 400, 600 and 800 oC was investigated. With respect to the pure ZnO sensors, the sensitivity to ethanol gas is linearly increased with increasing the concentration up to 200 ppm and then it is slowly increased up to 2000 ppm. On the other hand, the doped sensors samples showed a linear increase in the sensitivity with concentrations up to 400 ppm and then it is slowly increased until 2000 ppm. The response times of pure and 4 wt% CeO2-doped ZnO sensors sintered at 400 oC after exposures to 100 ppm ethanol were 15 and 12 seconds respectively. While, the recovery times of pure and 4 wt% CeO2-doped ZnO sensors sintered at 400 oC after re-exposure to air were 12 and 10 seconds respectively.
2- SnO2 nanoparticles sensors
Synthesis of tin oxide nanoparticles were carried out using an aqueous solution of tin tetrachloride 0.2 mol/L. Diluted NH4OH solution was added dropwise to this solution with continuous stirring until the pH reached 9, whereupon white gelable precipitate of Sn(OH)4 was obtained. The obtained precipitate was calcined at 400 oC for 4 hours in a muffle furnace. Proper amounts of CeO2 were added to the prepared SnO2 with ratio of 0, 2, 4 and 6 wt %. The resulting mixtures were used to fabricate sensors pellets of 12 mm in diameter and 2 mm thick, and then were sintered at 400, 600 and 800 oC for 2 hours.
XRD pattern of the prepared SnO2 showed that all the diffraction peaks can be indexed to SnO2 tetragonal rutile structure with average crystallite size of 7.2 nm. The XRD patterns of pure and CeO2-doped SnO2 sensors samples sintered at 400, 600 and 800 oC showed that all the diffraction peaks can be indexed to the tetragonal rutile structure of SnO2 and to the cubic structure of CeO2. The average crystallite size of the sensors samples was increased by increasing the sintering temperature. Whereas, the average crystallite size of the pure SnO2 was decreased with CeO2 doping.
The infrared spectrum of the prepared tin oxide showed four absorption bands. The broad absorption band appeared at 3423 cm-1 was assigned to the stretching vibration of –OH group of the adsorbed water. The band centered at 1632 cm-1 was attributed to the bending vibration of adsorbed molecular water. The characteristic absorption bands of tin oxide were appeared at 620 cm-1 and 540 cm-1 which were assigned to Sn-O-Sn stretching vibration and terminal oxygen vibration of Sn–OH respectively. The infrared spectra of pure and CeO2-doped SnO2 sensors sintered at 400, 600 and 800 oC showed two main absorption bands at 615-619 cm-1 and 540 cm-1 which were attributed to Sn-O-Sn stretching vibration and terminal oxygen vibration of Sn–OH respectively. The samples doped with 4 and 6 wt % CeO2 showed small band at 433 cm-1, which was assigned to Ce-O bond vibration. With increasing the sintering temperature the intensity of the Sn-O-Sn band was increased while that of the Sn–OH was decreased.
The scanning electron microscope photographs of pure and CeO2-doped SnO2 samples sintered at 400, 600 and 800 oC showed that the grain size of the samples was increased with increasing sintering temperature. While, the grain size of SnO2 was decreased with CeO2 doping. The photographs of the samples sintered at 400 oC revealed the presence of particles of fine structure with spherical shape.
The transmission electron microscope images of pure and CeO2-doped SnO2 sensors sintered at 400, 600 and 800 oC depicted that the particle size of the samples was increased with increasing the sintering temperature. While, the particle size of the pure SnO2 was decreased with CeO2 addition. The images of the samples sintered at 400 oC showed that the particles were nearly having spherical shape with average particle size ranging from 11.5 to 22.2 nm.
The variation of the electrical conductivity with temperature of pure and CeO2-doped SnO2 sensors sintered at 400, 600 and 800 oC was investigated. Generally, the features of the curves are nearly similar. Each curve consists of three different temperature regions. At the low temperature region (30-150 oC), the electrical conductivity was slightly increased with temperature which may be attributed to the thermal activation of electrons into the conduction band. The decrease in the electrical conductivity at mediate temperature (150-320 oC) were likely to be in relation with the adsorbed of oxygen species (O−, O2−) on the surface of the sensors samples. With increasing the temperature (320-410 oC) the electrical conductivity was increased, which was probably due to the thermal excitation of electrons and desorption of oxygen species. With increasing the sintering temperature the electrical conductivity of the sensors samples was increased. Also, the electrical conductivity of the pure SnO2 increased with CeO2 additions. The activation energy values of the pure and doped samples increased with were increasing the sintering temperature.
The variation of the sensitivity with temperature of pure and CeO2-doped SnO2 sensors sintered at 400, 600 and 800 oC was investigated. The sensitivity of the sensors was gradually increased with increasing the temperature and attained the maximum values at 300 oC. While, above 300 oC, the sensitivity of the sensors samples decreased. The sensitivity of the sensors was decreased with increasing the sintering temperature from 400 up to 800. Where, with increasing the sintering temperature the particle size of the sensors was increased and consequently the surface area was decreased which reflected in the decrease in the sensitivity.
The variation of the sensitivity with CeO2 concentration for SnO2 + x wt % CeO2 sensors sintered at 400, 600 and 800 oC was studied. The highest sensitivity values were obtained with the addition of 2 wt % CeO2. The additions of 2 wt % CeO2 seem to be benefit for ethanol dehydrogenation. While, the additions of 4 and 6 wt %, CeO2 may reduce the available adsorption sites on the SnO2 surface which worsen the gas sensing properties.
The variation of the sensitivity with ethanol gas concentration for pure and CeO2-doped SnO2 sensors sintered at 400, 600 and 800 oC was studied. For the pure SnO2 sensors, the sensitivity was linearly increased with increasing the concentration up to 300 ppm and then it was slowly increased with concentration up to 2000 ppm. While, the doped sensors showed linear increased sensitivity up to 500 ppm and then it was slightly increased with increasing the gas concentration until 2000 ppm. The response times of pure and 2 wt% CeO2-doped SnO2 sensors sintered at 400 oC after exposure to 100 ppm ethanol gas was 25 and 20 seconds respectively. While, the recovery times of pure and 2 wt% CeO2-doped SnO2 sensors sintered at 400 oC after re-exposure to air were 20 and 15 seconds respectively.
3- Comparison between the obtained results of ZnO and SnO2 sensors
The comparison between the obtained results of ZnO and SnO2 sensors was focused on the samples sintered at 400 oC owing to these sensors exhibited the best sensing characteristics.
The averages of the crystallite size of ZnO sensors were higher than that of SnO2 sensors. For both oxides the average crystallite size was decreased with CeO2 addition and was increased with sintering temperature.
In general, the electrical conductivity behavior of both oxides sensors is similar. Also, it was found that the electrical conductivity values of ZnO sensors are higher than those of SnO2 sensors at the same conditions.
The sensitivity values of SnO2 sensors are higher than those of ZnO sensors under the same conditions. Also, the maximum sensitivity values of SnO2 sensors were observed at 300 oC while those of ZnO sensors were observed at 310 oC. It was found that the addition of 4 wt % CeO2 is the more suitable concentration to enhance the sensitivity of ZnO sensors towards ethanol gas, while the addition of 2 wt % CeO2 is the more appropriate concentration to enhance the sensitivity of SnO2 sensors towards ethanol gas. The response time of ZnO + 4 wt % CeO2 sensor was 12 seconds while that of SnO2 + 2 wt % CeO2 sensor was 20 seconds. The recovery times of ZnO + 4 wt % CeO2 and SnO2 + 2 wt % CeO2 sensors sintered at 400 oC after re-exposure to air were 10 and 15 seconds.
from the above mentioned data it can be concluded that the best sensors among the two oxides sensors which can be used as ethanol gas sensor with high sensitivity, rapid response time and short recovery time are:
(a): ZnO + 4 wt% CeO2 sensor sintered at 400 oC
(b): SnO2 + 2 wt % CeO2 sensor sintered at 400 oC.
where SnO2 + 2 wt% CeO2 sensor has a high sensitivity towards ethanol gas with operating temperature of 300 oC and response time of 20 second while ZnO + 4 wt% CeO2 sensor has a slightly rapid response time and shorter recovery time than that of SnO2 + 2 wt% CeO2 sensor. Among the investigated sensors in this work it was found that the SnO2 sensor containing 2 wt% CeO2 is the best obtained sensor for ethanol gas detection.