Methane (CH4), ethane (C2H6), ethylene (C2H4), and acetylene (C2C2) are important fault characteristic hydrocarbon gases dissolved in power transformer oil. Online monitoring these gaseous components and their generation rates can present the operational state of power transformer timely and effectively. Gas sensing technology is the most sticky and tricky point in online monitoring system. In this paper, pure and Pd-doped SnO2 nanoparticles were synthesized by hydrothermal method and characterized by X-ray powder diffraction, field-emission scanning electron microscopy, and energy dispersive X-ray spectroscopy, respectively. The gas sensors were fabricated by side-heated preparation, and their gas sensing properties against CH4, C2H6, C2H4, and C2H2 were measured. Pd doping increases the electric conductance of the prepared SnO2 sensors and improves their gas sensing performances to hydrocarbon gases. In addition based on the frontier molecular orbital theory, the highest occupied molecular orbital energy and the lowest unoccupied molecular orbital energy were calculated. Calculation results demonstrate that C2H4 has the highest occupied molecular orbital energy among CH4, C2H6, C2H4, and C2H2, which promotes charge transfer in gas sensing process, and SnO2 surfaces capture a relatively larger amount of electric charge from adsorbed C2H4.
With the development of ultra-high voltage and extra-high voltage electricity transmission and transformation project [
Gas sensing technology is the core of online monitoring. At present, semiconductor gas sensors [
In this work, SnO2 samples doped with metallic ions Pd2+ (1, 3, and 5 wt%) were prepared using a simple hydrothermal synthesis route. The crystalline structures, chemical compositions, and surface morphologies of the prepared samples were performed by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), and energy dispersive X-ray spectroscopy (EDS), respectively; as well as their gas sensing properties to CH4, C2H6, C2H4, and C2H2 were measured. Furthermore, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LOMO) of hydrocarbon gases and the density of states (DOS) of SnO2 (110) surface were calculated.
Pure and Pd-doped SnO2 powders were prepared by hydrothermal method using SnCl4·5H2O, Na2SO4·5H2O, PdCl2·2H2O, NaOH, absolute ethanol, and distilled water as precursors. All chemical reagents were of analytical grade and purchased from Beijing Chemicals Co. Ltd. In this study, 0.903 g of NaOH, 1.262 g of SnCl4·5H2O, 0.3 g of Na2SO4·5H2O, 30 mL of absolute ethanol, and 30 mL of distilled water were mixed together. Then, the compound metal salt PdCl2·2H2O was added drop by drop to the mixed solution with intense magnetic stirring. The mass ratio of the total metallic ions added was estimated in a molar ratio of 1, 3, and 5 wt%, respectively. The reaction mixtures were magnetically stirred for about 30 min and then transferred into a 100 mL Teflon autoclave. The vessel was sealed and heated at 180°C for 24 h in an electric furnace. The prepared products were centrifuged and then washed several times with distilled water and absolute ethanol until Cl− could not be detected by 0.1 mol/L AgNO3 aqueous solutions. Finally, the products were further air-dried for further characterization.
The crystalline structures of the products were investigated using X-ray powder diffraction. The surface morphologies of both pure and Pd-doped SnO2 samples were characterized by field-emission scanning electron microscopy. An energy dispersive X-ray spectroscopy analysis was utilized to confirm the chemical compositions of the prepared samples.
To fabricate the sensors, the synthesized samples were mixed with absolute ethanol and distilled water at a weight ratio of 80 : 10 : 10 to form a paste. The paste was then screen-printed on an Al2O3 ceramic tube, in which a pair of gold electrodes was previously printed. And an Ni-Cr heating wire was inserted into the tube to form a side-heated gas sensor. Figure
The structure of a side-heated gas sensor.
Its gas sensing properties were measured by a chemical gas sensor-8 (CGS-8) intelligent gas sensing analysis system (Beijing Elite Tech Co., Ltd., China). The sensors were preheated at different operating temperatures for about 30 min. When electric resistances of all the sensors were stable, a targeted gas was injected into the test chamber (20 L in volume) by a microinjector through a rubber plug. The targeted gas was mixed with air by using two fans in the analysis system. After the sensor resistance values reached a new constant value, the test chamber was opened to recover the sensors. All measurements were performed in a laboratory fume hood. Sensor electric resistance and gas response values were automatically acquired by the analysis system. The whole experiment process was performed in a clean room with constant humidity and temperature, which were monitored by the analysis system.
The relative variation of the gas response was defined as
The orbital energy was calculated with the DMol3 module, which is based on the linear combination of the atomic orbits. The exchange-correlation function between electrons is described by the revised Perdew-Burke-Ernzerh form of the generalized gradient approximation [
X-ray powder diffraction patterns of the synthesized pure and Pd-doped (1, 3, and 5 wt%) SnO2 samples were shown in Figure
XRD patterns of (a) pure, (b) 1 wt%, (c) 3 wt%, and (d) 5 wt% Pd-doped SnO2 nanoparticles.
Field-emission scanning electron microscopy was used for characteristic surface morphologies of the prepared samples. One can clearly see in Figure
FESEM images of (a) pure, (b) 1 wt%, (c) 3 wt%, and (d) 5 wt% Pd-doped SnO2 nanoparticles.
In order to further check whether metallic ions Pd2+ were successfully doped into the SnO2 particles, energy dispersive X-ray spectroscopy was used to confirm the elemental chemical compositions of the prepared samples. Figure
EDS spectra of (a) pure and (b) 3 wt% Pd-doped SnO2 nanoparticles.
Figure
The electric resistance properties of the prepared sensors to different temperatures in air (room temperature at 25°C and relative humidity as 60%).
It is well known that gas sensing performances of metal oxide semiconductor gas sensor are attributed to the changes of electric conductance and particularly dominantly controlled by band structure, conduction band, and valence band near the Fermi level.
Rutile SnO2 crystal has four major low-index surfaces (110), (101), (100), and (001) [
DOS of SnO2 (110) surface before and after Pd2+-doping.
The gas responses of these sensors against CH4, C2H6, C2H4, and C2H2 were measured at different operating temperatures to find out their optimum operating temperatures. The gas responses of the sensors to 100
Gas responses of the sensors to 100
The gas response of the 5 wt% Pd2+-doped SnO2 sensor to different gas concentrations of CH4, C2H6, C2H4, and C2H2 is shown in Figure
Gas response of the 5 wt% Pd2+-doped sensor to different concentrations of CH4, C2H6, C2H4, and C2H2 (room temperature at 25°C and relative humidity as 60%).
Based on the molecular frontier orbital theory, a large number of gas molecular properties are decided by the orbits, particularly HOMO orbits and LOMO orbits [
Tin oxide is a typical n-type semiconductor material, and characteristic fault hydrocarbons like CH4, C2H6, C2H4, and C2H2 are reducing gases. During the gas sensing process, reducing gas molecules manifest to lose electrons, and SnO2-based gas sensor would capture the same number of electrons lost by adsorbed gas molecules. The electrons received by sensing materials increase the number of carriers and decrease the height of the barrier in the depletion region. In order to further understand the difference of the 5 wt% Pd2+-doped SnO2 sensor exhibiting various gas responses to hydrocarbon gases, orbital energies of CH4, C2H6, C2H4, and C2H2 were calculated through the quantum mechanics program. The models of built hydrocarbon gases are shown in Figure
The gas molecule models of CH4, C2H6, C2H4, and C2H2.
As shown in Figure
The HOMO and LUMO values of CH4, C2H4, C2H2, and C2H6.
Figure
Response and recovery property of the 5 wt% Pd2+-doped sensor to 50
Pure and Pd-doped nano-SnO2 particles were successfully prepared and characterized by XRD, FESEM, and EDS, respectively. The sensor doped with 5 wt% Pd2+ ions shows a higher electric conductance and gas sensing properties to characteristic fault hydrocarbons with rapid response and recovery property. The optimum operating temperatures of the 5 wt% Pd-doped sensor are about 400, 400, 350, and 350°C for 100
This work was supported in part by the National Natural Science Foundation of China (no. 51277185), China Postdoctoral Science Foundation (no. 2012M511904), and National Basic Research Program of China (973 Program: 2012CB215205).