Research on Acetylene Sensing Properties and Mechanism of SnO 2 Based Chemical Gas Sensor Decorated with Sm 2 O 3

AcetyleneC 2 H 2 gas is one of themost important fault characteristic hydrocarbon gases dissolved in oil immersed power transformer oil. This paper reports the successful preparation and characterization of samarium oxide Sm 2 O 3 decorated tin oxide SnO 2 based sensors with hierarchical rod structure for C 2 H 2 gas detection. Pure and Sm 2 O 3 decorated SnO 2 sensing structures were synthesized by a facile hydrothermal method and characterized by XRD, FESEM, TEM, EDS, and XPS measurements, respectively. Planar chemical gas sensors with the synthesis samples were fabricated, and their sensing performances to C 2 H 2 gas were systematically performed and automatically recorded by a CGS-1 TP intelligent gas sensing analysis system.The optimum operating temperature of the Sm 2 O 3 decorated SnO 2 based sensor towards 50μL/L of C 2 H 2 is 260C, and its corresponding response value is 38.12, which is 6 times larger than the pure one. Its response time is about 8–10 s and 10–13 s for recovery time. Meanwhile good stability and reproducibility of the decorated sensor to C 2 H 2 gas are also obtained. Furthermore, the proposed sensor exhibits excellent C 2 H 2 selectivity among some potential interface gases, like H 2 and CO gas. All sensing results indicate the sensor fabricated with oxide Sm 2 O 3 decorated SnO 2 nanorods might be a promising candidate for C 2 H 2 detection in practice.


Introduction
Large-scale power transformers are expensive and significant electric apparatus in electric grid system [1,2].At present, a large number of power transformers are still in oil-paper insulation structure, and some insulating defects unavoidably exist during transformer design, manufacturer, installation and operation [3].Once potential insulating faults happened on power transformers, some fault characteristic gases, like hydrogen, carbon monoxide, carbon dioxide, methane, ethane, ethylene, acetylene, and so forth, would appear and then dissolve into transformer oil [4,5].Among them, acetylene gas C 2 H 2 is the most effective one to identify thermal and electrical faults.Thus, how to rapidly and accurately detect C 2 H 2 gas is currently the subject of intensive research and great attention has been focused on this issue for the past few years [1][2][3][4][5][6].
In recent years, various types of gas sensing technologies have been proposed to detect transformer fault characteristic gases, such as metal oxide semiconductors [7,8], gas chromatograph, carbon nanotubes [9], and photoacoustic spectroscopy and Raman spectroscopy [10,11].Gas chromatography is mainly used as offline experiment, and spectroscopy is only in the stage of laboratory study and has a long way for practical application.With the advantages of simple manufacture technique, low cost, long life, rapid response, and recovery time, semiconductor SnO 2 may be the most promising sensing technology for detecting and recognizing dissolved fault characteristic gases in transformer oil [12][13][14][15].However, some limitations, like high operating temperature, unsatisfactory selectivity, and poor long-term stability, are still needed to be further improved [16][17][18][19][20][21].Doping modification with noble, rare-earth, and transition metals has been proved to be an effective method to improve the sensing properties of metal oxide semiconductors [22][23][24][25][26][27][28][29].
Hence, in this work we proposed the research of semiconductor SnO 2 based chemical gas sensor decorated with rare-earth oxide Sm 2 O 3 for C 2 H 2 detection.Firstly Sm 2 O 3 decorated SnO 2 nanorods were successfully synthesized with a facile and environment friendly hydrothermal method and characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) measurements, respectively.And then planar chemical gas sensors with the synthesis samples were fabricated with screen-printing technology, and their sensing performances towards C 2 H 2 gas were systematically performed and automatically recorded by a CGS-1 TP intelligent gas sensing analysis system.Finally a possible sensing mechanism was discussed and proposed.In a typical procedure, 0.81 g of Na 2 SnO 3 ⋅3H 2 O, 0.68 g of NaOH, 30 mL of absolute ethanol, and 30 mL of distilled water were firstly mixed together.Then, 0.05 g of compound metal salt Sm(NO 3 ) 3 ⋅6H 2 O and 0.24 g of C 6 H 8 O 7 ⋅H 2 O were added to the mixed solution with intense magnetic stirring.It was magnetically stirred for about 45 min and then transferred into a 100 mL Teflon-lined stainless steel autoclave, sealed and maintained at 180 ∘ C for 24 h.After the reaction was completed, the autoclave was cooled to room temperature naturally.The white precipitates were collected and washed with distilled water and absolute ethanol several times to eliminate the unwanted residues.Finally, the products were further air-dried and heated for further use.Pure SnO 2 nanostructures were also prepared for comparison with a similar synthesis route mentioned above.

Experimental
The crystalline structures of the prepared pure and Sm 2 O 3 decorated SnO 2 nanostructures were investigated by X-ray powder diffraction (XRD, Rigaku D/Max-1200X) with Cu K radiation (40 kV, 200 mA and  = 1.5418Å).The microstructures and morphologies of prepared samples were characterized by means of field emission scanning electron microscope (FESEM, Hillsboro equipped with energy dispersive X-ray (EDS) spectroscopy) and transmission electron micrographs (TEM, Hitachi S-570).Analysis of the X-ray photoelectron spectra (XPS) was performed on an ESCLAB MKII using Al as the exciting source.
Planar chemical gas sensors were fabricated with screenprinting technique, and the ceramic substrates were purchased from Beijing Elite Tech Co., Ltd.,China [18].Figure 1 shows the structure chart of the planar chemical gas sensor.As demonstrated in Figure 1 the planar chemical sensor is composed of three significant components, ceramic substrate, Ag-Pd interdigital electrodes, and sensing materials.The length, width, and height of the planar ceramic substrate are about 13.4, 7, and 1 mm, respectively.The synthesized sensing nanostructures were dispersed with distilled water and absolute ethanol in a weight ratio of 100 : 20 : 10 to  form a homogeneous paste.Then the paste was subsequently screen-printed onto the planar ceramic substrate to generate a uniform gas-sensing film.Finally, the fabricated sensor was dried in air at 80 ∘ C to volatilize the organic solvent and further aged at 300 ∘ C for 2 days to improve its stability before testing.
Gas-sensing properties were measured using the Chemical Gas Sensor-1 Temperature Pressure (CGS-1TP) intelligent gas sensing analysis system [18], which was purchased from Beijing Elite Tech Co., Ltd., China.Figure 2 illustrates the schematic diagram of the CGS-1TP gas sensing analysis system, which could offer an external temperature control ranging from room temperature to 500 ∘ C with an adjustment precision of 1 ∘ C. The first step in testing process was to put the fabricated gas sensor into the test chamber and fix its electrodes by adjusting the two probes on each side to collect electrical signals.When the sensor resistance reached a constant value, certain amount of C 2 H 2 was injected into the chamber by a microinjector through a rubber plug.Open the upper cover of the test chamber to recover the sensor until the resistance attained a new stable value.The sensor resistance and sensitivity were automatically collected and analyzed by the system.And the environmental temperature, relative humidity, and working temperature were also recorded by the analysis system.
The sensor sensitivity () in this paper was defined as  =   /  , where   represented the resistance of the sensor in air and   in certain concentration of C 2 H 2 mixed with air,  respectively [30].The time taken by the sensor to achieve 90% of the total resistance change was designated as the response time in the case of gas adsorption and the recovery time in the case of gas desorption [31].All gas sensing measurements were repeated several times to ensure the repeatability of the sensor against C 2 H 2 gas [31,32].

Results and Discussion
XRD measurement was firstly performed to determine the crystalline structures of the as-synthesized samples.Figure 3 presents the typical XRD patterns of the prepared pure and Sm 2 O 3 decorated SnO 2 nanostructures.One can clearly see in Figure 3 that these nanostructures are polycrystalline in nature.The prominent peaks of (110), (101), and (211) and other smaller peaks are well in accordance with the standard spectrum of the tetragonal rutile SnO 2 given in the standard data file (JCPDS file no.41-1445).No peaks from samarium atom and its oxide are observed, which might be attributed to the high dispersion and the low amount of Sm in the synthesized SnO 2 samples.
To check whether Sm element has been successfully doped into the synthesized SnO 2 nanostructures, energy dispersive X-ray spectroscopy measurement was performed.Figure 4 is the EDS spectrum of the synthesized Sm 2 O 3 decorated SnO 2 nanostructures.Peaks from Sn, Sm, and O are observed and the atomic ratio of Sm to Sn is calculated to be about 3.13 at %, which confirms the availability of Sm dopant in the synthesized SnO 2 nanostructures.
To further verify the existence of Sm atom and its valence in the synthesized SnO 2 samples, XPS analyses (Figure 5) were performed and XPS data was collected.Adventitious hydrocarbon C 1s binding energy at 285 eV was used as a reference to correct the energy shift of O 1s. Figure 6 shows the wide survey spectrum of the samples, confirming the existence of Sn, O, and Sm.The binding energies from Sn 3d, 3p, and 3s correspond to Sn 4+ .And the peak at 1084.3 eV is identified as Sm 2d 5/2 , which could be attributed to Sm 3+ ions.The overall surface morphologies and structural features of the synthesized pure and Sm 2 O 3 decorated SnO 2 samples were performed by FESEM, TEM, and SAED measurements and represented in Figure 6.As shown in Figures 6(a)-6(c) numerous rod-like nanostructures with uniform shape and size can be clearly seen, and no other morphologies were observed, revealing a high yield of the products.The TEM image in Figure 6(c) illustrates that both the shape and size of the Sm 2 O 3 decorated SnO 2 are exactly consistent with the demonstrated FESEM images.The corresponding SAED pattern as shown in Figure 6(d) verifies the polycrystalline structures of the synthesized Sm 2 O 3 decorated SnO 2 nanorods, which coincides well with the XRD results shown in Figure 3.
It is known to all that operating temperature is an important and fundamental characteristic for a semiconductor gas sensor, which has a significant influence on its sensing response.Figure 7 demonstrates the response curves of the prepared pure and Sm 2 O 3 decorated SnO 2 nanorods sensors to 50 L/L of C 2 H 2 as a function of working temperature ranging from 140 ∘ C to 410 ∘ C with an interval of 30 ∘ C. Apparently, for each sensor the gas response increases quickly and obtains its maximum and then decreases rapidly with further increase of working temperature.Compared with pure SnO 2 sensor, Sm 2 O 3 decorated SnO 2 nanorods sensor exhibits a higher resistance value at the same working temperature.The optimum operating temperature of the decorated one to 50 L/L of C 2 H 2 is 260 ∘ C with the corresponding maximum response value 38.12.And it is 7.53 for the pure one at 320 ∘ C, where the sensor exhibits the maximum gas response.
Figure 8 demonstrates the gas responses of the prepared sensors as a function of C 2 H 2 concentration with sensor working at its optimum operating temperature measured above.As represented, the sensing responses of the sensors versus C 2 H 2 increase greatly with increasing gas concentration in the range of 1-100 L/L, change much more slowly from 100 L/L to 400 L/L, and obtain saturation when exposed to more than 400 L/L.The saturated sensing responses were measured to be about 68.87 and 18.61 for the Sm 2 O 3 decorated SnO 2 nanorods sensor and the pure one.
To investigate the response-recovery characteristic, stability, and repeatability of the Sm 2 O 3 decorated SnO 2 nanorods sensor, it was sequentially exposed to various concentrations of C 2 H 2 gas as shown in Figure 9 (5, 10, 20, 50, and 100 L/L) and equal concentration as shown in Figure 10 (20 L/L).As shown in Figures 9 and 10, the sensor response increases rapidly when exposed to certain concentration of C 2 H 2 and decreases dramatically when exposed to air for recovering.The time spent for gas sensing is measured about 8-10 s and 10-13 s for sensor recovering.Meanwhile, the gas response of the sensor always returns to its initial value during the continuous test period, implying a very satisfying reproducibility of the prepared Sm 2 O 3 decorated SnO 2 nanorods sensor.
Figure 11 depicts the histogram of the gas responses of the pure and Sm 2 O 3 decorated SnO 2 nanorods sensors to 20 L/L of various gases at 260 ∘ C, including C 2 H 2 , CO, and H 2 .It can be clearly seen in Figure 11 that the decorated sensor shows excellent C 2 H 2 sensing response among these two potential interface gases.
SnO 2 is a typical n-type oxide semiconducting sensing material, and its gas sensing properties are predominantly controlled by its surface resistance [32][33][34][35].As it is known, when the sensor is exposed to air, oxygen would be absorbed on SnO  the carrier concentration and an increased sensor resistance.As C 2 H 2 gas is introduced, the relevant chemical reactions take place between C 2 H 2 gas and the ionized oxygen.The trapped electrons are released back to the conduction band of SnO 2 surface, increasing the conductivity of the sensor; thus a decreased resistance is measured.
Based on the sensing experiments demonstrated above, oxide Sm 2 O 3 has a significant impact on improving the sensing properties of SnO 2 based gas sensor to C 2 H 2 .A possible sensing mechanism of the Sm 2 O 3 decorated SnO 2 based sensor to C 2 H 2 may be explained as follows [37,38].Firstly, oxide Sm 2 O 3 might work as an effective catalyst, which benefits the effect of active center for C 2 H 2 adsorption.Secondly, the incorporation of Sm 3+ ions might change the electronic movement and the overlap of electron cloud of SnO 2 sensing material, which further strengthens the electronegativity of carbon-hydrogen triple bond of C 2 H 2 and makes it easier for hydrogen dissociation to combine with O 2− .Therefore, much more electrons have been released in this process so that SnO 2 resistance declines dramatically.Thirdly, the addition of Sm 2 O 3 could restrict the crystallite growth of SnO 2 .That is to say, after decorating both the diameter and the length of the synthesized SnO 2 nanorods diminish, which would provide more active sites for oxygen adsorption and channels for gas sensing.Therefore, the Sm 2 O 3 decorated SnO 2 based sensor exhibits a higher gas response at a relatively lower operating temperature to C 2 H 2 gas with rapid responserecovery, excellent stability, reproducibility, and selectivity.

Conclusions
In this work, pure and oxide Sm 2 O 3 decorated SnO 2 nanorods were successfully synthesized with a facile and simple hydrothermal method and carefully characterized by XRD, FESEM, TEM, EDS, and XPS measurements, respectively.Planar chemical gas sensors with the synthesis samples were fabricated, and their sensing performances to C 2 H 2 gas were systematically performed and automatically recorded by a CGS-1 TP intelligent gas sensing analysis system.Compared with the pure one, the Sm 2 O 3 decorated SnO 2 nanorods based sensor exhibits lower optimum operating temperature, higher sensing response, quick response and response time, good stability and reproducibility, and excellent selectivity among potential interface gases.All results indicate the sensor fabricated with oxide Sm 2 O 3 decorated SnO 2 nanorods might be a promising candidate for C 2 H 2 detection in practice.

Figure 1 :
Figure 1: Structure chart of the fabricated planar sensor.

Figure 7 :Figure 8 :Sm 2
Figure 7: Gas responses of the sensors to 50 L/L of C 2 H 2 at various working temperature.
All the raw chemical reagents were of analytical grade purchased form Chongqing Chuandong Chemical Reagent Co., Ltd.(Chongqing, China), and used as received without any further purification.Sm 2 O 3 decorated SnO 2 nanostructures were prepared by a facile and environment friendly hydrothermal method using Na 2 SnO 3 ⋅3H 2 O, Sm(NO 3 ) 3 ⋅6H 2 O, C 6 H 8 O 7 ⋅H 2 O, NaOH, absolute ethanol, and distilled water as precursors.