Pd-Doped SnO 2-Based Sensor Detecting Characteristic Fault Hydrocarbon Gases in Transformer Oil

Methane (CH 4 ), ethane (C 2 H 6 ), ethylene (C 2 H 4 ), and acetylene (C 2 C 2 ) 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 SnO 2 nanoparticles were synthesized by hydrothermal method and characterized by X-ray powder diffraction, field-emission scanning electronmicroscopy, and energy dispersive X-ray spectroscopy, respectively.The gas sensors were fabricated by side-heated preparation, and their gas sensing properties against CH 4 , C 2 H 6 , C 2 H 4 , andC 2 H 2 weremeasured. Pd doping increases the electric conductance of the prepared SnO 2 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 C 2 H 4 has the highest occupied molecular orbital energy among CH 4 , C 2 H 6 , C 2 H 4 , and C 2 H 2 , which promotes charge transfer in gas sensing process, and SnO 2 surfaces capture a relatively larger amount of electric charge from adsorbed C 2 H 4 .


Introduction
With the development of ultra-high voltage and extra-high voltage electricity transmission and transformation project [1,2], the quantity and capacity of power transformers with various voltage levels sharply increase.Power transformers are essential electrical apparatus, and their operating conditions directly affect the safety and reliability of the power system [3][4][5].Faults happened in power transformers bring huge economic losses to our national economy [6].Now, most of the large power transformers are in oil-paper insulation structures.When internal insulation faults occur in a transformer, the transformer generates some low molecular hydrocarbon gases [7][8][9], such as methane (CH 4 ), ethane (C 2 H 6 ), ethylene (C 2 H 4 ), and acetylene (C 2 H 2 ), and most of these gaseous components are dissolved in transformer oil.Online monitoring the component contents of these dissolved hydrocarbon gases and their generation rates is one of the most effective and convenient methods for diagnosing transformer incipient faults.This method can distinguish different types of faults which happened in power transformer, such as overheating, partial discharge, spark discharge, and arcing discharge [10][11][12][13].
Gas sensing technology is the core of online monitoring.At present, semiconductor gas sensors [14,15], palladium gate field effect transistors [16], catalytic combustion sensors [17,18], fuel cell sensors [19], and optical sensors [20,21] are mainly methods utilized to detect characteristic fault gases.Given remarkable advantages of simple fabrication process, low maintenance cost, rapid response and recovery time, long service life [22], metal oxide semiconductor materials like SnO 2 [22], ZnO [23], TiO 2 [24], and In 2 O 3 [25] have been receiving scientific and technological importance for many years [26,27] and widely used to detect flammable, explosive, and poisonous gases.However, when used to detect hydrocarbon gases traditional SnO 2 -based gas sensor presents some limitations such as low gas response and high operating temperature [28].Many relevant studies have been conducted to enhance the sensing performances of SnO 2 [29][30][31].Doping noble metal is an effective method to improve the gas sensing properties of conventional thick or thin film gas sensor [32,33].However, due to the same composition and similar structure for the four hydrocarbons, the gas sensing mechanism is still unclear and lack detail understandings.
In this work, SnO 2 samples doped with metallic ions Pd 2+ (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), fieldemission scanning electron microscopy (FESEM), and energy dispersive X-ray spectroscopy (EDS), respectively; as well as their gas sensing properties to CH 4 , C 2 H 6 , C 2 H 4 , and C 2 H 2 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 SnO 2 (110) surface were calculated.O, 30 mL of absolute ethanol, and 30 mL of distilled water were mixed together.Then, the compound metal salt PdCl 2 ⋅2H 2 O 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 AgNO 3 aqueous solutions.Finally, the products were further air-dried for further characterization.

Experimental
The crystalline structures of the products were investigated using X-ray powder diffraction.The surface morphologies of both pure and Pd-doped SnO 2 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.

Fabrication and Measurement of Sensors.
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 Al 2 O 3 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 1 shows 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  = ( 0 − )/ 0 , where  0 and  represent the resistance of the sensor in air and in targeted gas, respectively.Response time and recovery time were defined as the time taken by the sensor to reach 90% of the total resistance change when gas in and 10% when gas out, respectively.All measurements were repeated several times in order to ensure the reproducibility of the gas sensing response.

Theoretical Calculation Method.
The orbital energy was calculated with the DMol 3 module, which is based on the linear combination of the atomic orbits.The exchangecorrelation function between electrons is described by the revised Perdew-Burke-Ernzerh form of the generalized gradient approximation [34][35][36].The orbital cutoff quality was set as 4.3 Å, and the maximum root-mean-square convergent tolerance is 1.0 × 10 −6 eV/atom.The basis set was approximated with a double numerical orbit base group and an orbit polarization function to handle the p orbit polarization of hydrogen atom.

Results and Discussions
3.1.Structural Characterization.X-ray powder diffraction patterns of the synthesized pure and Pd-doped (1, 3, and 5 wt%) SnO 2 samples were shown in Figure 2. It is obvious that the prominent peaks of (110), ( 101), (211), and other smaller peaks are corresponding to the standard data file of rutile SnO 2 (JCPDS 41-1445).As the amount of metallic ions doped in SnO 2 is small, no other metal-oxide diffraction features were observed in the patterns.With the amount of metallic ions doped in SnO 2 increasing, the relative intensity of the diffraction peaks decreases, and the full width at half maximum increases.Based on the Scherrer equation D = 0.89/( cos ), where  presents the X-ray wavelength,  means the half peak width, and  is the Bragg angle, the calculated average particle size of pure SnO 2 samples is about 50 nm, and it is about 35-40 nm for Pd-doped SnO 2 samples.
Field-emission scanning electron microscopy was used for characteristic surface morphologies of the prepared samples.One can clearly see in Figure 3 that the crystal particles of prepared samples are uniform in size and have a nearly spherical shape.As shown in Figure 3(a), the average size of pure SnO 2 nanospheres is about 50 nm, and the image inset in Figure 3(a) presents a high-resolution image of a single nanosphere.As seen in Figures 3(b)-3(d), the diameters of the sphere-like Pd-doped SnO 2 samples are ranging from 25 to 30 nm.From these images, it is clearly seen that doping a moderate amount of noble metallic ions Pd 2+ affects the shapes of SnO 2 slightly but inhibits the crystalline growth of the samples evidently.
In order to further check whether metallic ions Pd 2+ were successfully doped into the SnO 2 particles, energy dispersive X-ray spectroscopy was used to confirm the elemental chemical compositions of the prepared samples.Figure 4 shows the EDS spectra of pure and 3 wt% Pd-doped SnO 2 nanoparticles.As shown in Figure 4(a), the EDS spectra reveal the elemental compositions of Sn, O, C, and Cu, and no other peaks have been found.Nevertheless, the presence of Pd element is confirmed in the EDS date of Figure 4(b), and the atomic percentage of Pd in the samples is calculated to be about 3 wt%.

Gas Sensing Properties.
Figure 5 shows the electric resistance properties of the prepared SnO 2 -based sensors at different temperatures in pure air.As seen in Figure 5, the resistance values of all the sensors decrease when the working temperature increases from 200 to 500 ∘ C, which is the intrinsic characteristic of a semiconductor gas sensor.It is   also observed that the resistance of the SnO 2 sensor decreases somewhat after Pd 2+ ions doping.When worked at a same temperature, the electric resistance value decreases in the order of pure 1, 3, and 5 wt% Pd-doped SnO 2 sensors, which means doping a moderate amount of noble metal can improve the electric conductivity of our prepared sensor, and 5 wt% is the optimum doping content among our samples.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 SnO 2 crystal has four major low-index surfaces (110), ( 101), (100), and (001) [37,38].The (110) surface is the most thermodynamically stable surface and has been widely used to investigate the surface properties of SnO 2 .Thus, the density of states (DOS) of SnO 2 (110) surface before and after Pd 2+ -doping were calculated by replacing partial Sn atoms on the (110) surface (Sn : Pd = 4 : 1).As shown in Figure 6 after Pd 2+ -doping the DOS of SnO 2 (110) surface shifted downward somewhat, the band gap narrowed and new doping levels appeared near the Fermi level.Pd 2+doping promotes electron transfer between conduction band and valence band and improves the electric conductance properties of the sensor.
The gas responses of these sensors against CH 4 , C 2 H 6 , C 2 H 4 , and C 2 H 2 were measured at different operating temperatures to find out their optimum operating temperatures.
The gas responses of the sensors to 100 L/L of CH 4 , C 2 H 6 , C 2 H 4 , and C 2 H 2 at different operating temperatures were indicated in Figure 7, respectively.For each gas, the gas response values of the sensor increase at different degrees firstly and reach its maximum value at the optimum operating temperature and then decrease rapidly with the operating temperature rising.This tendency is commonly observed in pure and Pd 2+ -doped SnO 2 sensors.As shown in Figure 7, the optimum operating temperature of the prepared sensor to 100 L/L of CH 4 , C 2 H 6 , C 2 H 4 and C 2 H 2 are about 400, 400, 350 and 350 ∘ C, respectively, which is attributed to the sensor showing the maximum gas response at the corresponding operating temperature.It is also noted that for each hydrocarbon gas the sensor doped with 5 wt% Pd 2+ exhibits the highest sensitivity among the four prepared sensors.Correspondingly, when worked at optimum operating  temperatures, the gas responses of the 5 wt% Pd 2+ -doped sensor against 100 L/L of CH 4 , C 2 H 6 , C 2 H 4 , and C 2 H 2 are about 4.95, 5.89, 6.78, and 7.22, respectively, which are obviously larger than those of pure 1 and 3 wt% Pd 2+ -doped sensors.
The gas response of the 5 wt% Pd 2+ -doped SnO 2 sensor to different gas concentrations of CH 4 , C 2 H 6 , C 2 H 4 , and C 2 H 2 is shown in Figure 8, where the sensor worked at its own optimum operating temperatures as mentioned above.One can clearly see in Figure 8 that with the gas concentration increasing, the gas response of the sensor has an increasing trend in different degrees.Among the four characteristic hydrocarbons, the sensor exhibits maximum response to C 2 H 4 , followed by C 2 H 2 , C 2 H 6 , and CH 4 with a descending order.The relationship between gas response and concentration is quasilinear, which indicates that our prepared sensor can be used to online monitor and detect characteristic hydrocarbon gases dissolved in transformer oil.
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 [36], which determine the ability of atoms to gain or loss electrons and perform electronic transfer.It is considered that HOMO orbits occupied with the highest energetic electrons are easy to lose electrons, whereas LUMO orbits occupied with the lowest unoccupied electrons are demonstrated to easily capture electrons during the gas sensing process.
Tin oxide is a typical n-type semiconductor material, and characteristic fault hydrocarbons like CH 4 , C 2 H 6 , C 2 H 4 , and C 2 H 2 are reducing gases.During the gas sensing process, reducing gas molecules manifest to lose electrons, and SnO 2based 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% Pd 2+ -doped SnO 2 sensor exhibiting various gas responses to hydrocarbon gases, orbital energies of CH 4 , C 2 H 6 , C 2 H 4 , and C 2 H 2 were calculated through the quantum mechanics program.The models of built hydrocarbon gases are shown in Figure 9.After geometry optimization, orbital energy calculations were performed using the DMol 3 module.
As shown in Figure 10, the HOMO energy values of CH Figure 11 presents the voltage responses of the 5 wt% Pd 2+ -doped SnO 2 gas sensor to 50 L/L of CH 4 , C 2 H 6 , C 2 H 4 , and C 2 H 2 , where the sensor worked at the optimum operating temperature.It is an intrinsic characteristic of the prepared SnO 2 -based sensor that voltage response sharply increased when gas in and dramatically decreased when gas out.As seen in Figure 11, the prepared sensor shows rapid response and recovery to CH 4 , C 2 H 6 , C 2 H 4 , and C 2 H 2 , and for each gas, the recovery time is somewhat longer than the response time.Such a rapid response and recovery property could be attributed to the doping effect of the doped metallic ions, which provide some new activity points and catalyze the gas sensing process [29,33,34].

Conclusions
Pure and Pd-doped nano-SnO 2 particles were successfully prepared and characterized by XRD, FESEM, and EDS, respectively.The sensor doped with 5 wt% Pd 2+ ions shows a higher electric conductance and gas sensing properties to characteristic fault hydrocarbons with rapid response and

Figure 1 :Figure 2 :
Figure 1: The structure of a side-heated gas sensor.

Figure 5 :
Figure 5: The electric resistance properties of the prepared sensors to different temperatures in air (room temperature at 25 ∘ C and relative humidity as 60%).

Figure 7 :
Figure 7: Gas responses of the sensors to 100 L/L of CH 4 , C 2 H 6 , C 2 H 4 , and C 2 H 2 at different operating temperatures (room temperature at 25 ∘ C and relative humidity as 60%).
2.1.Preparation and Characterization of Materials.Pure and Pd-doped SnO 2 powders were prepared by hydrothermal method usingSnCl 4 ⋅5H 2 O, Na 2 SO 4 ⋅5H 2 O, PdCl 2 ⋅2H 2 O, 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 SnCl 4 ⋅5H 2 O, 0.3 g of Na 2 SO 4 ⋅5H 2 4 , C 2 H 4 , C 2 H 2 , and C 2 H 6 are −0.3441,−0.2401, −0.2542, and −0.2995 eV, respectively, and the LUMO energy values for CH 4 , C 2 H 4 , C 2 H 2 , and C 2 H 6 are 0.0834, −0.0272, 0.0121, and 0.0702 eV.Based on the molecular frontier orbital theory, gas molecules easily lose electrons with a higher HOMO.Thus, the ability of losing electron weakens in the following order of C 2 H 4 , C 2 H 2 , C 2 H 6 , and CH 4 .The possibility of electron transfer between adsorbed gas molecules and adsorption surface diminishes with the sequence of C 2 H 4 , C 2 H 2 , C 2 H 6 and CH 4 .Because C 2 H 4 has the highest HOMO among the four hydrocarbon gases, a relatively higher gas response of the prepared sensor to C 2 H 4 is observed.On the contrary, a relatively lower gas response was measured to CH 4 .Theoretical calculation results highly agree with our experimental data as mentioned in Figure 8.This finding provides a qualitative understanding for the 5 wt% Pd 2+ -doped SnO 2 sensor, which indicates different gas responses to CH 4 , C 2 H 6 , C 2 H 4 , and C 2 H 2 with the same gas concentration.
4 , C 2 H 4 , C 2 H 2 , and C 2 H 6 .Figure 11: Response and recovery property of the 5 wt% Pd 2+doped sensor to 50 L/L of CH 4 , C 2 H 6 , C 2 H 4 , and C 2 H 2 (room temperature at 25 ∘ C and relative humidity as 60%).recovery property.The optimum operating temperatures of the 5 wt% Pd-doped sensor are about 400, 400, 350, and 350 ∘ C for 100 L/L of CH 4 , C 2 H 6 , C 2 H 4 , and C 2 H 2 with gas responses of 4.95, 5.89, 6.78, and 7.22, respectively.Based on the molecular frontier orbital theory calculations, the HOMO energy values of CH 4 , C 2 H 6 , C 2 H 4 , and C 2 H 2 are −0.2401,−0.2542, −0.3441, and −0.2995 eV, separately.The ability of lose electrons weakens in the following order: C 2 H 4 , C 2 H 2 , C 2 H 6 , and CH 4 .Therefore, the sensor exhibits a larger gas response to C 2 H 4 among the four characteristic hydrocarbon gases.These findings provide a further insight to understand different gas sensing properties of the prepared sensors to CH 4 , C 2 H 6 , C 2 H 4 , and C 2 H 2 .