sensors Article
Synthesis, Characterization and Enhanced Sensing Properties of a NiO/ZnO p–n Junctions Sensor for the SF6 Decomposition Byproducts SO2, SO2F2, and SOF2 Hongcheng Liu 1 , Qu Zhou 1,2, *, Qingyan Zhang 1 , Changxiang Hong 1 , Lingna Xu 1,2 , Lingfeng Jin 2 and Weigen Chen 2 1 2
*
College of Engineering and Technology, Southwest University, Chongqing 400715, China;
[email protected] (H.L.);
[email protected] (Q.Z.);
[email protected] (C.H.);
[email protected] (L.X.) State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing 400030, China;
[email protected] (L.J.);
[email protected] (W.C.) Correspondence:
[email protected]; Tel.: +86-130-6830-5845
Academic Editor: Vittorio M. N. Passaro Received: 12 March 2017; Accepted: 17 April 2017; Published: 21 April 2017
Abstract: The detection of partial discharge and analysis of the composition and content of sulfur hexafluoride SF6 gas components are important to evaluate the operating state and insulation level of gas-insulated switchgear (GIS) equipment. This paper reported a novel sensing material made of pure ZnO and NiO-decorated ZnO nanoflowers which were synthesized by a facile and environment friendly hydrothermal process for the detection of SF6 decomposition byproducts. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were used to characterize the structural and morphological properties of the prepared gas-sensitive materials. Planar-type chemical gas sensors were fabricated and their gas sensing performances toward the SF6 decomposition byproducts SO2 , SO2 F2 , and SOF2 were systemically investigated. Interestingly, the sensing behaviors of the fabricated ZnO nanoflowers-based sensor to SO2 , SO2 F2 , and SOF2 gases can be obviously enhanced in terms of lower optimal operating temperature, higher gas response and shorter response-recovery time by introducing NiO. Finally, a possible gas sensing mechanism for the formation of the p–n junctions between NiO and ZnO is proposed to explain the enhanced gas response. All results demonstrate a promising approach to fabricate high-performance gas sensors to detect SF6 decomposition byproducts. Keywords: synthesis and characterization; NiO-decorated ZnO; p–n junctions; sensing properties; SF6 decomposition byproducts
1. Introduction Gas-insulated switchgear (GIS) is used extensively In electrical power systems, where it presents unique advantages, including small space occupation, low failure rates, outstanding breaking capacity, and so on [1,2]. In the event of a short circuit in big systems, the high-voltage circuit breaker (CB) in the switchgear is the last line of defense, so the GIS must remain healthy [3]. Sulfur hexafluoride (SF6 ) as a widely used insulating medium in GIS due to its excellent insulating and arc-extinguishing properties [4,5]. Despite of the high reliability of GIS equipment, some internal defects could still lead to different degrees of partial discharge (PD) when the GIS equipment has been running for a long time [6]. When exposed to the energy of partial discharges, the SF6 gas in GIS equipment could decompose and generate SF4 , SF3 , and SF2 [7], as well as various low-fluorine sulfides. The decomposed SF6 reacts
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further with trace moisture, oxygen and solid materials, thus producing multiple chemical products (SOF4 , SOF2 , SO2 F2 , SO2 , H2 S and HF, among others) [8,9]. These products accelerate the aging of the insulation and the corrosion of metal surfaces, which may eventually lead to GIS insulation faults. Some new studies show that the composition and content of these byproducts could be used to evaluate the operating state and insulation level of GIS equipment [10,11]. Consequently, many methods such as infrared absorption spectrometry [12], photoacoustic (PA) spectroscopy technology [13], and test tube method [14] have been used to detect and analyze the partial discharge decomposition products of SF6 . However, these methods are offline laboratory detection methods and it is difficult to achieve on-line monitoring. Recently, metal-oxide semiconductor materials with low operating temperature, good gas responses and stable properties have attracted much research interest for various applications, including gas sensors [15,16], solar cells [17], UV sensors [18]. Particularly zinc oxide (ZnO) [19–22], a typical n-type semiconducting material with a wide band gap, that can easily form p–n junctions with NiO has attracted the attention of researchers [23–25]. Liu et al. reported p-NiO/n-ZnO hetero-structures synthesized by a low-cost and environmentally friendly strategy and investigated their gas sensing performance to acetone [26]. Using a simple photochemical deposition method Dai et al. prepared honeycomb-like NiO/ZnO hetero-structured nanorods exhibiting dramatically enhanced UV detection performance [27]. Tian et al. developed a hydrothermal method followed by calcination to prepare NiO/ZnO p–n heterostructures which exhibited high responses to ethanol vapor [28]. However, to the best of our knowledge, the hydrothermal synthesis of p-NiO/n-ZnO composite nanostructures for SF6 decomposed components gas sensors has rarely been reported. Thus, in this context we have successfully synthesized pure ZnO and NiO-decorated ZnO flower-like nanometer materials and systematically researched their gas sensing performances toward the SF6 decomposition byproducts SOF2 , SO2 F2 , and SO2 . By a comparative study, we conclude that the introduction of NiO plays an important role in improving the sensing performances of pure ZnO microflowers toward the main components of SF6 decomposition, in terms of lower optimal working temperature, higher gas response and shorter response-recovery time. Moreover, a possible sensing mechanism to explain this interesting performance is also proposed, that is, the formation of p–n junctions NiO and ZnO. These results demonstrate a promising approach to fabricate gas sensors to detect SF6 decomposed components in GIS. 2. Experimental Details 2.1. Preparation and Characterization of Materials All the chemical reagents were analytical-grade, purchased from Beijing Chemicals Co. Ltd. (Beijing, China) and used as received without any further purification. Pure ZnO flower-like nanometer materials were synthesized by a facile and environmentally friendly hydrothermal method. In a typical procedure, Zn(NO3 )2 ·6H2 O (10 mmol) was firstly dissolved completely in a mixed solution distilled water and anhydrous ethanol (40 mL of each) to form a precursor solution. After magnetic stirring for about 30 min, the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave, and maintained at 180 ◦ C for 12 h in order to obtain flower-like nanomaterials. Finally, the collected products were washed with distilled water and anhydrous ethanol several times, and dried at 80 ◦ C for 8 h to further use. 3 at-% NiO-decorated ZnO flower-like sensing materials were synthesized by the same process mentioned above, except a certain proportion of Ni(NO3 )2 ·6H2 O was added to the precursor solution. The crystalline parameters of the synthesized sensing materials were characterized by X-ray diffraction (XRD) with Cu Kα radiation (40 kV, 200 mA and λ = 1.5418 Å). Field emission scanning electron microscopy (FESEM, FEI, Hillsboro, OR, USA; operated at 10 kV), transmission electron microscopy (G2 F20 S-TWIN, Tecnai, Hillsboro, OR, USA; operated at 200 kV) and HRTEM combined with select area electron diffraction (SAED) were used to examine the morphology of the products.
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The chemical compositions of the prepared samples were conducted using energy-dispersive X-ray performed on(EDS). an ESCLAB MKII using monochromatic Al was Kα as the X-ray source to spectroscopy X-ray photoelectron spectroscopy (XPS) performed onexciting an ESCLAB MKII investigate the chemical state of elements existing in the products. using monochromatic Al Kα as the X-ray exciting source to investigate the chemical state of elements existing in the products. 2.2. The Fabrication of Sensors 2.2. The Fabrication of Sensors Gas sensors were fabricated by a screen-printing technique with planar ceramic substrates [29]
Figure 1 (the planar sensor) showsby a structural drawingtechnique of the sensor, consists of three Gas sensors were fabricated a screen-printing withwhich planarmainly ceramic substrates [29] parts: substrate, Ag–Pd interdigital electrodes, and of sensing materials. Asmainly prepared samples Figureceramic 1 (the planar sensor) shows a structural drawing the sensor, which consists of were ground into a fine powder and further mixed with distilled water and ethanol to form a three parts: ceramic substrate, Ag–Pd interdigital electrodes, and sensing materials. As prepared homogeneous paste. This screen mixed printedwith onto the planar substrate to samples were ground into awas finesubsequently powder and further distilled waterceramic and ethanol to form generate a uniform sensing dried in air at 50 printed °C for 8onto h. Finally, the fabricated gas sensor a homogeneous paste. Thisfilm, was and subsequently screen the planar ceramic substrate to ◦ was aged at an aging test chamber for 12 days to improve the stability and repeatability before generate a uniform sensing film, and dried in air at 50 C for 8 h. Finally, the fabricated gas sensor was testing. aged at an aging test chamber for 12 days to improve the stability and repeatability before testing. 2.3. Gas Gas Response Response Measurement Measurement 2.3. Gas sensing sensing properties of the fabricated sensors to and SO22 were were measured measured and and Gas to SOF SOF22, SO22F22,, and automatically recorded recorded by by aa CGS-1TP CGS-1TP (Chemical (Chemical Gas Gas Sensor-1 Sensor-1 Temperature Temperature Pressure) intelligent gas automatically sensing analysis analysis system, system, purchased purchased from from Beijing Beijing Elite Elite Tech Tech Co., Ltd. (Beijing, (Beijing, China) China) [30]. Figure Figure 11 sensing shows aa schematic schematic diagram diagram of of the CGS-1TP CGS-1TP system. system. The The planar planar sensor sensor was was laid laid on on the the test test chamber chamber shows and its electrodes fixed by adjusting the two probes on each side to collect electrical signals. It is is and its electrodes collect electrical It convenient totogain sensor resistance, gas response, working temperature, environmental temperature, convenient gain sensor resistance, gas response, working temperature, environmental and relative humidity with this system. temperature, and relative humidity with this system.
Figure Figure 1. 1. Schematic Schematic diagram diagram of of the the CGS-1TP CGS-1TP gas gas sensing sensing analysis analysis system. system.
In this study, the gas response was defined as S = Ra/Rg, where Ra and Rg were the electrical In this study, the gas response was defined as S = Ra/Rg, where Ra and Rg were the electrical resistance of the fabricated sensor in air and in the test gas, respectively. Response and recovery time resistance of the fabricated sensor in air and in the test gas, respectively. Response and recovery time was defined as the time required by the sensor to achieve 90% of the total resistance after injecting was defined as the time required by the sensor to achieve 90% of the total resistance after injecting and and removing the detected gas. All sensing measurements were tested under laboratory condition removing the detected gas. All sensing measurements were tested under laboratory condition with with room temperature◦ (25 °C) and constant humidity (50% relative humidity), and repeated several room temperature (25 C) and constant humidity (50% relative humidity), and repeated several times times to ensure the reliability of the results. to ensure the reliability of the results. 3. 3. Results Results and and Discussion Discussion 3.1. 3.1. Structural Structural and and Morphological Morphological Characterizations Characterizations The crystallineparameters parametersofofthethe as-prepared products investigated by and XRDthe and the The crystalline as-prepared products werewere investigated by XRD results results are presented 2. It isthat obvious all of for theboth peaks for both can be well are presented in Figurein2.Figure It is obvious all of that the peaks products canproducts be well indexed and indexed and exactly match the standard pattern of ZnO (JCPDS. 36-1451). No obvious diffraction peaks of NiO were observed in the pattern, suggesting that the introducing of NiO did not change the original ZnO structure. It may be proposed that the amount of NiO is very small and it is highly
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(101) (101)
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(200) (200) (112) (112) (201) (201)
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Pure-ZnO nanostructures Pure-ZnO nanostructures (102) (102)
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(100) (100)
Intensity Intensity(a.u.) (a.u.)
Sensors2017, 2017,17, 17,913 913 4of of14 14 Sensors exactly match the standard pattern of ZnO (JCPDS. 36-1451). No obvious diffraction peaks of 4NiO were observed in the pattern, suggesting that the introducing of NiO did not change the original ZnO dispersedin inthe theZnO ZnOsamples. samples.Moreover, Moreover,the thehigh high peakintensities intensitiesclearly clearlyindicate indicatethe thehigh highdegree degreeof of dispersed structure. It may be proposed that the amount of NiOpeak is very small and it is highly dispersed in the ZnO crystallization of of both both of of the the samples, samples, corresponding corresponding to to aa lower lower number number of lattice defects defects [31]. No No crystallization samples. Moreover, the high peak intensities clearly indicate the high degreeofoflattice crystallization[31]. of both other doping diffraction peaks were observed from Figure 2, which suggest a high purity of our other doping diffraction peakstowere observed from Figure 2, which a high purity of our of the samples, corresponding a lower number of lattice defects [31].suggest No other doping diffraction as-preparedproducts. products. as-prepared peaks were observed from Figure 2, which suggest a high purity of our as-prepared products.
NiO-ZnO nanostructures NiO-ZnO nanostructures
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Figure2.2.X-ray X-raypowder powderdiffraction diffractionpatterns patternsof ofpure pureand and 3at-% at-%NiO-decorated NiO-decoratedZnO ZnOnanostructures. nanostructures. Figure Figure 2. X-ray powder diffraction patterns of pure and 33 at-% NiO-decorated ZnO nanostructures.
The overall overall surface surface morphologies morphologies of of the the pure pure and and NiO-decorated NiO-decorated ZnO ZnO nanostructures nanostructures were were The The overallbysurface morphologies of theit pure andthat NiO-decorated ZnO nanostructures were firstly studied FESEM. From Figure 3a,b is clear both products consist of a large amount firstly studied by FESEM. From Figure 3a,b it is clear that both products consist of a large amount firstly studiedmicroflowers by FESEM. From Figure 3a,b is clear that both products a large amount of ofbeautiful beautiful withsmooth smooth andit approximately uniform size,consist andno noof other morphologies of microflowers with and approximately uniform size, and other morphologies beautiful microflowers with smooth and approximately uniform size, and no other morphologies are are observed. observed. Comparing Comparing Figure Figure 3a,b, 3a,b, the the micro-morphological micro-morphological structures structures of of the the pure pure and and are observed. Comparing Figure 3a,b, the are micro-morphological structures of the pure and NiO-decorated NiO-decorated ZnO nanostructures almost similar, which demonstrate that the introduction of NiO-decorated ZnO nanostructures are almost similar, which demonstrate that the introduction of ZnO nanostructures are almost similar, which demonstrate that the introduction of NiO only has a NiO only only has has aa slight slight impact impact on on the the morphology morphology of of pure pure ZnO ZnO products. products. Further Further morphology morphology NiO slight impact on the morphology of pure ZnO products. Further morphology characterization was characterization was performed by TEM and shown in Figure 3c. The single microflower was characterization was performed by TEM and shown in Figure 3c. The single microflower was performed by TEM and shown in Figure 3c. The single microflower was composed of 3D nanorods that composed of nanorods that connected to each other through the center to form flower-like composed of nanorods that connected to each other through the center to form 3D flower-like connected eachisother through the center to form 3D flower-like structures. This is nanostructure in agreement with structures.toThis This in agreement agreement with recently reported synthesized flower-like [32]. structures. is in with recently reported synthesized flower-like nanostructure [32]. recently reported synthesized flower-like nanostructure [32]. The HRTEM image is shown in Figure 3dis The HRTEM image is shown in Figure 3d and the interplanar spacing of 0.28 nm as illustrated The HRTEM image is shown in Figure 3d and the interplanar spacing of 0.28 nm as illustrated is and the interplanar of planes 0.28 nmofasZnO illustrated consistent well the (100)distances planes ofofZnO consistent wellwith withspacing the(100) (100) (JCPDS.is36-1451). 36-1451). And thewith interplanar 0.24 consistent well the planes of ZnO (JCPDS. And the interplanar distances of 0.24 (JCPDS. 36-1451). And the interplanar distances of 0.24 nm agree well with the lattice spacing of the nm agree well with the lattice spacing of the (111) planes of the cubic NiO [33], which indicate that nm agree well with the lattice spacing of the (111) planes of the cubic NiO [33], which indicate that (111) planes of theexists cubicin NiO [33], which indicate that NiO has already exists in the composites. NiO has already the composites. NiO has already exists in the composites.
Figure 3. Cont.
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Figure 3. FESEM images ofof(a) pure and (b) NiO-decorated ZnO nanostructures nanostructures (c) TEM and and (d) Figure 3. 3. FESEM (a) pure and NiO-decorated nanostructures (c) TEM Figure FESEM images images of (a) pure and (b)(b) NiO-decorated ZnOZnO (c) TEM and (d) HRTEM image of the NiO-decorated ZnO microflowers. (d)HRTEM HRTEM image of the NiO-decorated ZnO microflowers. image of the NiO-decorated ZnO microflowers.
In order components of the as as synthesized synthesized samples, energy order toinvestigate investigate the element element components the energy In In order totoinvestigate the the element components of the of as synthesized samples,samples, energy dispersive dispersive X-ray spectroscopy measurements were shows theEDS EDSspectra spectra of dispersive X-ray spectroscopy measurements were conducted. conducted. 44shows the 3 3 X-ray spectroscopy measurements were conducted. Figure Figure 4Figure shows the EDS spectra of 3ofat-% at-% NiO-decorated ZnO products. It is clear that the elemental compositions of Ni, O and Zn, and at-% NiO-decorated ZnO products. It is clear that the elemental compositions of Ni, O and Zn, and NiO-decorated ZnO products. It is clear that the elemental compositions of Ni, O and Zn, and the the atomic percentage ofof inin the isis calculated to beat-%. 2.72 at-%. at-%. Meanwhile other EDS the atomic percentage thesamples samples calculated be 2.72 Meanwhile nonoother EDS atomic percentage of Ni inNiNi the samples is calculated to be to 2.72 Meanwhile no other EDS peaks peaks have been found,which whichconfirms confirms ourproducts products high peaks have been found, our are ofpurity. highpurity. purity. have been found, which confirms our products are of high
Figure4.4.EDS EDSspectra spectra of of 33 at-% at-% NiO-decorated NiO-decorated ZnO Figure ZnOnanostructures. nanostructures. Figure 4. EDS spectra of 3 at-% NiO-decorated ZnO nanostructures.
The chemical compositions and chemical states of the samples were studied by X-ray The chemical compositions and chemical states of the samples were studied by X-ray The chemical compositions and and chemical states of theresults samples were studied photoelectron spectroscopy analysis, the corresponding are shown in Figureby5. X-ray As photoelectron spectroscopy analysis, and the corresponding results are shown in Figure 5. As observed photoelectron spectroscopy analysis, and the corresponding results are shown in Figure 5. observed in Figure 5a, the XPS wide survey spectrum of the synthesized hierarchical samplesAs in Figure 5a, the XPS wide survey spectrum of the synthesized hierarchical samples confirms the confirmsinthe existence of Zn, Ni,wide O, and C. It isspectrum proposedofthat includes not hierarchical only C compounds observed Figure 5a, the XPS survey theC synthesized samples existence of Zn,the Ni,samples O, and from C. It is proposed that C includes not only C compounds adsorbed by the adsorbed from the apparatus. Figure shows the high confirms theby existence of Zn, Ni,air, O,but andalso C. oily It is dirt proposed that C includes not 5b only C compounds samples from air, but also oily dirt from the apparatus. Figure 5b shows the high resolution scanning resolution XPSfrom spectra Ni 2p. is observed from Ni 2p3/2 main peak its satellite adsorbed by scanning the samples air,ofbut alsoItoily dirt from thethe apparatus. Figure 5band shows the high XPS spectra of Ni 2p. It855.98 is observed from theeV, Ni respectively, 2p3/2 main peak and itsNisatellite peakpeak are located peak are located at eV and 861.58 while the 2p 1/2 main its at resolution scanning XPS spectra of Ni 2p. It is observed from the Ni 2p3/2 main peak and itsand satellite 855.98 eV and eV, while the Nirespectively. 2p1/2 mainThese peak and its are satellite peak appear satellite peak861.58 appear at respectively, 873.38 eV and 879.68 results in agreement with at peak are located at 855.98 eV and 861.58 eV,eV, respectively, while the Ni 2p 1/2 main peak and its 873.38 eV and 879.68 eV, respectively. These results are in agreement with the previously reported the previously reported valueseV and further confirm the presenceThese of NiOresults [34,35].are From Figure 5c, itwith is satellite peak appear at 873.38 and 879.68 eV, respectively. in agreement values and further confirmpeaks the presence of NiOZn [34,35]. From Figure 5c, itenergies is clearof that the prominent clear that the prominent of Zn 2p 3/2 and 2p 1/2 are at the binding 1021.58 eV and the previously reported values and further confirm the presence of NiO [34,35]. From Figure 5c, it is peaks of Zn and Zn 2pspin-orbit the binding energies of 1021.58 and Zn 1044.68 The observed 1044.68 eV.2pThe of Zn 2p between Zn 2peV 3/2 and 2p1/2 eV. is about 23 eV, 3/2 observed 1/2 are at splitting clear that the prominent peaks of Zn 2p3/2 and Zn 2p1/2 are at the binding energies of 1021.58 eV and spin-orbit ofwith Zn 2p Zn 2p3/2 andofZnpure 2p1/2 is about 23 eV,a which consistent which issplitting consistent thebetween corresponding value ZnO, indicating normalisstate of Zn2+ with in 1044.68 eV. The observed spin-orbit splitting of Zn 2p between Zn 2p 3/2 and Zn 2p1/2 is about 23 eV, 2+ synthesized sample the spectra of Oa 1s as shown peaks are located thethe corresponding value of[36]. pureInZnO, indicating normal stateinofFigure Zn in5d, thethe synthesized sample at [36]. which is consistent with the corresponding value of pure ZnO, indicating a normal2−state of Zn2+ in eV andof531.38 and 532.33 eV respectively, whichare could be attributed O and ions531.38 [37,38].eV, In 530.88 the spectra O 1s eV, as shown in Figure 5d, the peaks located at 530.88toeV the synthesized sample [36]. In the spectra of O 1s as shown in Figure 5d, the peaks are located at 2− ions [37,38]. Therefore, can be deduced thebeZn and Ni to areOpresent as ZnO Therefore, and NiO in thisbesample, and 532.33 eVitrespectively, whichthat could attributed it can deduced 530.88 eV and 531.38 eV, and 532.33 eV respectively, which could be attributed to O2− ions [37,38]. respectively. that the Zn and Ni are present as ZnO and NiO in this sample, respectively.
Therefore, it can be deduced that the Zn and Ni are present as ZnO and NiO in this sample, respectively.
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Therefore, it can be deduced that the Zn and Ni are present as ZnO and NiO in this sample, 6 of 14 respectively.
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Figure survey XPS (b)NiNi2p; 2p;(c)(c)Zn Zn2p; 2p;(d) (d)OO1s. 1s. Figure5.5.XPS XPS surveyspectra spectraofofNiO-decorated NiO-decoratedZnO: ZnO:(a)(a)full fullspectrum; spectrum;(b)
3.2. 3.2.Gas-Sensing Gas-SensingProperties Properties Figure Figure6 6presents resistanceproperties propertiesininpure pureair airofofthe theasasprepared presentsthe theelectric electricresistance preparedpure pureZnO ZnOand and NiO-decorated ZnO sensors at various operating temperatures from 100 °C to 400 °C. As seen inin ◦ ◦ NiO-decorated ZnO sensors at various operating temperatures from 100 °C to 400 °C. As seen NiO-decorated sensors operating temperatures from C C. Figure 6, the resistance values of the two sensors decrease when the operating temperature Figure 6,6,the theresistance resistance values of two the sensors two sensors decrease when the operating temperature values of the decrease when the operating temperature increases, increases, which isisanan intrinsic ofofa asemiconductor with pure increases, which intrinsiccharacteristic characteristic semiconductor gassensor. sensor.Compared Compared with pure which is an intrinsic characteristic of a semiconductor gas sensor.gas Compared with pure ZnO sensor, ZnO sensor, the NiO-decorated ZnO sensor exhibits a higher resistance value at the same working ZnO sensor, the NiO-decorated ZnO sensor exhibits a higher resistance valueworking at the same working the NiO-decorated ZnO sensor exhibits a higher resistance value at the same temperature, temperature, might totothe gas temperature, which mightbe bebeneficial beneficial the following gassensing sensingperformances. performances. which mightwhich be beneficial to the following gasfollowing sensing performances.
Electric resistance(MO ) Electric resistance(MO )
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Figure 6.6. The electric resistance properties ofof the Figure 6. The electric resistance properties of theprepared preparedsensors sensorstoto todifferent differenttemperatures temperatures air. Figure The electric resistance properties the prepared sensors different temperaturesinin inair. air.
InIngeneral, gassensors sensorsisisstrongly bythe general,the thesensing sensingperformance performanceofoffabricated fabricatedgas stronglyinfluenced influencedby the operating temperature. In order to demonstrate that the addition of NiO nanoparticles is an effective operating temperature. In order to demonstrate that the addition of NiO nanoparticles is an effective way gassensor, sensor,the waytotoenhance enhancethe thegas gassensing sensingproperties propertiesofofZnO-based ZnO-basedgas theeffects effectsofofvarious variousdifferent different operating temperatures ranging from 140 °C to 360 °C on fabricated sensors to 100 ppm 2, SOF2, ◦ ◦ operating temperatures temperatures ranging ranging from from 140 140 °C to 360 360 °C sensors to 100 ppm ppmofof ofSO SO C to C on fabricated sensors SO 22, SOF22,, and SO 2F2 were respectively investigated and illustrated in Figure 7. It is found that the response of and SO F were respectively investigated and illustrated in Figure 7. It is found that the response of 2 2
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the pure and NiO-decorated ZnO sensors to SO2 firstly improve with increasing working temperatures and gave the largest values of about 30.35 at 260 °C and 84.26 at 220 °C, respectively, the pure and NiO-decorated ZnO sensors to SO2 firstly improve with increasing working temperatures then it decreased with further rising temperatures. Similar sensing behaviors and gas response and gave the largest values of about 30.35 at 260 ◦ C and 84.26 at 220 ◦ C, respectively, then it decreased curves are observed for SOF2 and SO2F2. The maximum gas response of the pure and NiO-decorated with further rising temperatures. Similar sensing behaviors and gas response curves are observed for ZnO sensors against SOF2 appear at about 300 °C and 260 °C with responses 12.49 and 22.25, SOF2 and SO2 F2 . The maximum gas response of the pure and NiO-decorated ZnO sensors against respectively. The maximum response values to SO2F2 are about 18.46 at 300 °C and 36.67 at 260 °C for SOF2 appear at about 300 ◦ C and 260 ◦ C with responses 12.49 and 22.25, respectively. The maximum the pure and NiO-decorated ZnO sensors. Comparison of these sensing results indicate that the response values to SO2 F2 are about 18.46 at 300 ◦ C and 36.67 at 260 ◦ C for the pure and NiO-decorated introduction of NiO can not only reduce the optimum operating temperature, but also enhance the ZnO sensors. Comparison of these sensing results indicate that the introduction of NiO can not only gas response to SF6 decomposed components, along with obvious changes in the temperature reduce the optimum operating temperature, but also enhance the gas response to SF6 decomposed characteristic curve. components, along with obvious changes in the temperature characteristic curve. Pure ZnO (SO 2) NiO-ZnO (SO 2)
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Figure 7. Gas response of the pure and NiO-decorated ZnO gas sensors to 100 ppm SO2 , SOF2 , Figure 7. Gas response of the pure and NiO-decorated ZnO gas sensors to 100 ppm SO2, SOF2, and and SO2 F2 at different working temperature. SO2F2 at different working temperature.
Figure thethe gasgas responses of theofpure ZnO sensors SO2 at to different Figure8 8depicts depicts responses the and pureNiO-decorated and NiO-decorated ZnO to sensors SO2 at concentrations at their respective optimum operating From Figure 8a, Figure the response different concentrations at their respective optimum temperature. operating temperature. From 8a, the continuously increases increases with gas with concentrations ranging ranging from 5 to 8005ppm. it is clear response continuously gas concentrations from to 800Moreover, ppm. Moreover, it is that the NiO-decorated ZnO sensor demonstrates enhanced sensing properties for SO 2 detection clear that the NiO-decorated ZnO sensor demonstrates enhanced sensing properties for SO2 compared the pure ZnO Figure 8b exhibits linear calibration curve of the sensors with detection with compared with thesensor. pure ZnO sensor. Figurea8b exhibits a linear calibration curve of the SO concentration in the range of 5~100 ppm. In this area, the linear fitting functions of pure and 2 sensors with SO2 concentration in the range of 5~100 ppm. In this area, the linear fitting functions of NiO-decorated ZnO gas sensors are sensors y = 0.288x and y+ = 0.761x +y 13.049, respectively. Their linear pure and NiO-decorated ZnO gas are+y3.397 = 0.288x 3.397 and = 0.761x + 13.049, respectively. 2 are as high as 0.996 and 0.987, which indicates that the response of SO correlation coefficients 2 Their linear correlationR coefficients R2 are as high as 0.996 and 0.987, which indicates that the concentration variation at low concentration is close to linear. response of SO2 concentration variation at low concentration is close to linear. The The gas gas responses responses of of the the pure pure and and NiO-decorated NiO-decorated ZnO ZnO sensors sensors toto SOF SOF2 2 with with different different concentrations at their respective optimum working temperatures were measured and are shown concentrations at their respective optimum working temperatures were measured and are shownin in Figure Figure9.9.The Theresponses responsesof ofthe thefabricated fabricatedsensors sensorsincrease increaserapidly rapidlyininlinearity linearitywith withincreasing increasingSOF SOF2 2 concentration concentration below below 200 200 ppm, ppm, but butincrease increasemuch muchmore moreslowly slowlyabove above200 200ppm. ppm. Moreover, Moreover, we wecan can also find that the response of the NiO-decorated ZnO sensor is nearly two times higher, on average, also find that the response of the NiO-decorated ZnO sensor is nearly two times higher, on average, than than that that of ofthe thepure pureZnO ZnOsensor. sensor. As As Figure Figure 9b 9b exhibits, exhibits, the the linear linear relationship relationship of of the the pure pure and and NiO-decorated ZnO sensors between the sensing response and gas concentration in the range NiO-decorated ZnO sensors between the sensing response and gas concentration in the range of of 5~100 5~100 ppm ppm are are fitted fitted by by the theequation equationyy==0.121x 0.121x++ 0.797 0.797 and and yy== 0.203x 0.203x ++ 1.398, 1.398, with with high high liner liner 2 correlation correlationcoefficients coefficientsRR2of of0.993 0.993and and0.994, 0.994,respectively. respectively.
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Figure 8. 8. (a) Response Response of the the pure and and NiO-decorated ZnO ZnO gas sensors sensors to various various concentrations of of Figure Figure 8. (a) (a) Response of of thepure pure andNiO-decorated NiO-decorated ZnO gas gas sensors to to various concentrations concentrations of 2 at 220 °C; (b) Linear fitting curves of pure and NiO-decorated ZnO sensors to 5~100 ppm of SO 2. SO ◦ SO C; (b) at 220 220 °C; (b) Linear Linear fitting fitting curves curves of of pure pure and and NiO-decorated NiO-decoratedZnO ZnOsensors sensorsto to5~100 5~100 ppm ppmof ofSO SO22.. SO22 at
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Figure 9. (a) Pure and NiO-decorated ZnO gas sensors’ response to different concentrations of SOF2 Figure 9. (a) Pure and NiO-decorated ZnO gas sensors’ response to different concentrations of SOF2 Figure 9. (a) and NiO-decorated ZnO gas sensors’ response to different concentrations SOFand at the 260 °CPure operating temperature; (b) Linear relationship between the sensors’ response of value 2 at at the 260 °C operating temperature; (b) Linear relationship between the sensors’ response value and ◦ the 260 operating temperature; (b) Linear relationship between the sensors’ response value and the SOF2Cconcentration. 2 concentration. the SOF SOF 2 concentration.
Figure 10 shows the responses of the pure and NiO-decorated ZnO sensors as a function of Figure 10 shows the responses of the pure and NiO-decorated ZnO sensors as a function of SO2FFigure 2 with different their respective optimum operating temperatures. shows concentrations the responses ofat pure and NiO-decorated ZnO sensors as a functionAs of Figure SO2 F2 SO2F2 with 10 different concentrations atthe their respective optimum operating temperatures. As Figure 10a shows that the gas responses of the sensors increase rapidly with increasing SO 2 F 2 concentration, with different at their optimum operating temperatures. Figure 10a 10a shows thatconcentrations the gas responses of therespective sensors increase rapidly with increasing SO2F2 As concentration, but increase much more slowlyofwith the further increase of thewith SO2Fincreasing 2 concentration and nearly reach shows that the gas responses the sensors increase rapidly SO F concentration, 2 2 but increase much more slowly with the further increase of the SO2F2 concentration and nearly reach theirincrease saturation at more about 600 ppm. the response of 2the NiO-decorated ZnO sensor is but much withFurthermore, the further increase of the SO F2 concentration and nearly reach their saturation at aboutslowly 600 ppm. Furthermore, the response of the NiO-decorated ZnO sensor is higher than that of the pure one. From Figure 10b the linear fitting functions of pure and NiO-ZnO their saturation at about 600 ppm. Furthermore, the response of the NiO-decorated ZnO sensor is higher than that of the pure one. From Figure 10b the linear fitting functions of pure and NiO-ZnO gas sensors to 5~100 SOFrom 2F2 are y = 0.175x + linear 0.270 and y =functions 0.358x + 1.752, higher than that of theppm pureof one. Figure 10b the pure respectively. and NiO-ZnOAnd gas gas sensors to 5~100 ppm of SO2F2 are y = 0.175x + 0.270 fitting and y = 0.358x +of1.752, respectively. And 2 were calculated to be 0.990 and 0.995, respectively. their linear correlation coefficients R sensors to 5~100 ppm of SO F are y = 0.175x + 0.270 and y = 0.358x + 1.752, respectively. And their 2 2 their linear correlation coefficients R2 were calculated to be 0.990 and 0.995, respectively. 2 were mentioned tocoefficients the sensingRresults above, it can be0.995, concluded that the gas responses of linearAccording correlation calculated to be 0.990 and respectively. According to the sensing results mentioned above, it can be concluded that the gas responses of the NiO-decorated ZnO sensorresults to SO2, SO2F2 andabove, SOF2 is higher that of that the pure one. Moreover, According to the sensing can bethan concluded the gas responses of the NiO-decorated ZnO sensor to SO2mentioned , SO2F2 and SOF2 isithigher than that of the pure one. Moreover, the responses of the prepared sensors to these three test gases at low concentration have a good the ZnO sensor tosensors SO2 , SO SOF2 test is higher thatconcentration of the pure one. Moreover, 2 andthree the NiO-decorated responses of the prepared to2 Fthese gasesthan at low have a good linear relationship. These good linearto relationships indicate that theconcentration fabricated gashave sensors are more the responses of the prepared sensors these three test gases at low a good linear relationship. These good linear relationships indicate that the fabricated gas sensors arelinear more likely to be used in good practice [39,40]. relationship. These linear relationships indicate that the fabricated gas sensors are more likely to likely to be used in practice [39,40]. be used in practice [39,40].
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Figure10. 10.(a) (a)Response Responseofof ofthe thepure pure and NiO-decorated ZnO gas sensors to SO 2F2 with different Figure and NiO-decorated ZnO gas sensors toto SOSO Figure 10. (a) Response the pure and NiO-decorated ZnO gas sensors withdifferent different 2 F22F2with concentration at 260 °C; (b) Linear fitting curves of the prepared the sensors to 5~100 ppm of SOF22FF.22.. ◦ concentration SO concentrationatat260 260 C; °C;(b) (b)Linear Linearfitting fittingcurves curvesofofthe theprepared preparedthe thesensors sensorstoto5~100 5~100ppm ppmofof SO 2 2
Figure 11 11 presents presents the the response response and and recovery recovery curves curves of of the the NiO-decorated NiO-decorated ZnO ZnO sensor to to 100 100 Figure Figure 11 presents the response and recovery curves of the NiO-decorated ZnO sensorsensor to 100 ppm ppm SO 2, SO2F2 and SOF2 working at their respective optimum operating temperature. As seen in ppm SO2, SO2F2 and SOF2 working at their respective optimum operating temperature. As seen in SO 2 , SO2 F2 and SOF2 working at their respective optimum operating temperature. As seen in Figure 10, Figure 10, the the common common feature feature of of these these three three test test gases gases is is that that the the response of of the the sensor sensor increases increases Figure 10, the common feature of these three test gases is that the response of theresponse sensor increases rapidly when rapidly when gas is injected into the gas chamber for sensing and dramatically decreases to its initial initial rapidly wheninto gas is into thefor gas chamber sensing anddecreases dramatically decreases to its gas is injected theinjected gas chamber sensing andfor dramatically to its initial value when value when when the the gas gas is is removed. removed. According According to the the definition definition above, above, the the response response times to to 100 ppm ppm value the gas is removed. According to the definitiontoabove, the response times to 100 ppmtimes SO2 , SO100 2 F2 and SO 2, SO2F2 and SOF2 were calculated to be about 12, 16 and 18 s, while the recovery times were SO2, were SO2Fcalculated 2 and SOF2 were calculated to be about 12, 16 and 18 s, while the recovery times were SOF to be about 12, 16 and 18 s, while the recovery times were calculated to be about 2 calculated to be about about 16, 16, 20 20 and and 22 22 s,s, respectively. respectively. calculated 16, 20 and 22tos,be respectively. 100
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Figure 11. Response-recovery curves of the NiO-decorated ZnO gas sensor to 100 ppm SO2 , SO2 F2 Figure 11. Response-recovery curves of the NiO-decorated ZnO gas sensor to 100 ppm SO2, SO2F2 and and SOF211. . Response-recovery curves of the NiO-decorated ZnO gas sensor to 100 ppm SO2, SO2F2 and Figure SOF2. SOF2.
Finally, Finally,the thelong-time long-timestability stabilityand andrepeatability repeatabilityofofthe theNiO-decorated NiO-decoratedZnO ZnOsensor sensortoto100 100ppm ppm Finally, the long-time stability and repeatability of the NiO-decorated ZnO sensor to 100 ppm SO , SO F and SOF at their respective optimum working temperatures were measured and shown in 2 2, SO 2 2F 2 2 and SOF 2 2 at their respective optimum working temperatures were measured and shown SO SO2, SO 2F 2 and SOF2 atshows, their respective optimum working temperatures were measured and shown Figure 12. As the figure all of the gas responses change slightly and remain at a nearly constant in Figure 12. As the figure shows, all of the gas responses change slightly and remain at a nearly in Figure 12.the As long the figure shows, all of thewhich gas responses change slightly and remain at a nearly value during cycles, the excellent long-term stability and constant value during experimental the long experimental cycles,indicates which indicates the excellent long-term stability constant value during the long experimental cycles, which indicates the excellent long-term stability repeatability of the fabricated sensors for detecting these three kinds of SF decomposition byproducts. and repeatability of the fabricated sensors for detecting these three 6kinds of SF6 decomposition and repeatability of the fabricated sensors for detecting these three kinds of SF6 decomposition byproducts. byproducts.
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3.3. Sensing Mechanisms 3.3. Sensing Mechanisms The sensing mechanism forfor ZnO, a typical n-type semiconductor gas sensing material, has been The sensing mechanism ZnO, a typical n-type semiconductor gas sensing material, has been interpreted in in some former papers [41,42]. It isItbelieved that that the sensing properties of ZnO gas interpreted some former papers [41,42]. is believed the sensing properties of chemical ZnO chemical sensors are mainly determined by the change electric of resistance, which is fundamentally attributed gas sensors are mainly determined by theofchange electric resistance, which is fundamentally toattributed the chemical adsorption and desorption process of target gas molecules the surface ofon gasthe sensing to the chemical adsorption and desorption process of targeton gas molecules surface materials [43]. Inmaterials an air atmosphere, could be absorbed on the surface as asurface trap of gas sensing [43]. In anoxygen air atmosphere, oxygen could beZnO absorbed onacting the ZnO capturing electrons from its conduction band to form a depletion region on the surface. This depletion acting as a trap capturing electrons from its conduction band to form a depletion region on the region would to an region increase of the sensor exposed to a target the test surface. Thislead depletion would lead to anresistance. increase ofWhen the sensor resistance. Whengas, exposed to a gas molecules react with the adsorbed oxygen and the trapped electrons are released back into the target gas, the test gas molecules react with the adsorbed oxygen and the trapped electrons are conduction band, into and thus decreased resistance measured [44]. According to theisdefinition of gas released back the as conduction band, andis thus as decreased resistance measured [44]. response (Ra/Rg) responseofofgas theresponse sensor increases. According to thethe definition (Ra/Rg) the response of the sensor increases. It It was observed in the present study thatthat the sensing properties of ZnO enhanced due was observed in the present study the sensing properties of were ZnO greatly were greatly enhanced todue the introduction of NiO.ofTwo main satisfactorily explain explain this interesting performance. It has to the introduction NiO. Tworeasons main reasons satisfactorily this interesting performance. been acknowledged that the gas of semiconductor nanomaterials with different It has been acknowledged thatsensing the gasproperties sensing properties of semiconductor nanomaterials with morphologies are differentare [45]. In this study, thethis synthesized nanomaterials a larger different morphologies different [45]. In study, theflower-like synthesized flower-like offer nanomaterials surface which will be favorable and desorption ofdesorption target gas molecules, offer aaccessibility, larger surface accessibility, which willtobeadsorption favorable to adsorption and of target gas and this unique helpful to improve theto gas sensingthe response. Moreover, the formation molecules, andmorphology this uniqueismorphology is helpful improve gas sensing response. Moreover, ofthe p–nformation junctions between NiO and between ZnO in the grain results an enhancement of in gasan of p–n junctions NiO andboundaries ZnO in the grainin boundaries results sensing. A hetero has p–n been junction proposedsystem to explain enhanced gastoresponse enhancement of p–n gas junction sensing.system A hetero has the been proposed explain of the Coenhanced O -WO [46], NiO-SnO [47], LaFeO -SnO [48] heterocontacts. 2 [47], LaFeO3-SnO2 [48] heterocontacts. 3 4 3gas response of2 Co3O4-WO3 3[46], NiO-SnO 2 AsAs wewe all know, ZnO shows n-type conductivity by electrons NiO shows conductivity all know, ZnO shows n-type conductivity byand electrons and p-type NiO shows p-type byconductivity holes. By introducing ZnO, theNiO electrons ZnOthe andelectrons holes in NiO transport in opposite by holes.NiO By into introducing into in ZnO, in ZnO and holes in NiO directions the system reaches equalization the Fermi levels, leading to the levels, formation of ato transportuntil in opposite directions until the systemofreaches equalization of the Fermi leading p–n asof shown Figure as 13ashown [49]. in This is the13a thermal equilibrium of theequilibrium formation of the a thejunction, formation a p–n in junction, Figure [49]. This is the thermal p–n junction [50]. When the composite was exposed to air, oxygen molecules can absorb on the formation of a p–n junction [50]. When the composite was exposed to air, oxygen molecules can surface the the material and of form With adsorbed different operating absorbof on surface thechemically material adsorbed and formoxygen. chemically oxygen. temperatures With different chemisorbed oxygen existschemisorbed in various forms [51],exists as shown in the reaction Equations (1)–(4). operating temperatures oxygen in various forms [51], as shown in the reaction Equations (1)–(4). O2 (gas) → O2 (ads) (1) O2 (gas) → O2 (ads) (1) O2 (gas) + 2e− → 2O− (ads) O2 (gas) + 2e− → 2O− (ads) 1 O2 (ads) + 2e− → O2− (ads) 2 ½ O2 (ads) + 2e− → O2− (ads)
(2) (2) (3) (3)
where R is one of SF6 decomposition component gases, and O−(ads) denotes the adsorbed oxygen on the material’s surface. Owing to the electron-hole recombination, the hole concentration of p-type NiO decreases, and as a result the concentration gradient of the p–n junction is reduced. Thus, the effect of carriers transport becomes weak and the depletion layer becomes thin [50,54], as shown in Sensors 2017, 913 resistance of the sensor in the test gas is further decreased [55]. In short, compared 11 of 14 Figure 13b.17,The with a pure ZnO sensor, the formation of p–n junctions greatly increase the resistance of the ZnO sensor in air and further decreases the resistance−in the test gas. Thus, based on the definition of gas O2 (ads) + e → O2 − (ads) (4) response (Ra/Rg), the responses of the gases are greatly enhanced due to the variation of resistance [49].
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Figure Figure 13. 13. Energy Energy band band schematics schematics for for p-type p-type NiO/n-type NiO/n-typeZnO ZnOheterojunction heterojunction(a) (a)in inair air atmosphere; atmosphere; (b) in test gases. Ec: lower level of conduction band; EF: Fermi level; Ev: upper level of valence (b) in test gases. Ec: lower level of conduction band; EF: Fermi level; Ev: upper level of valence band. band.
4. Conclusions Moreover, the resistance of the NiO-decorated ZnO in air (Ra) will be even higher than without the In summary, pure sensing materials were Nevertheless, successfully heterojunctions (pure ZnO)and due3toat-% a newNiO-decorated depletion layer ZnO between NiO and ZnO [33,52]. synthesized via a one-step hydrothermal method and characterized by XRD, FESEM, TEM, HRTEM, when the composite was exposed to test gases (SO2 , SOF2 and SO2 F2 ), the gas molecules react with EDS and XPS, respectively. Their gas sensing properties toward the SF 6 decomposition byproducts the absorbed oxygen species of gas sensing material and release the electrons back to the material. SO SO2FSF 2, and SOF2 were systemically researched with a CGS-1TP gas sensing analysis system. The2, three 6 decomposed components all play the role of an electron donating gas [53]. The reaction Comparative results demonstrate that the as prepared NiO-decorated ZnO sensing material exhibits equation is as follows: − enhanced gas sensing properties, temperature, higher gas R +including O− (ads) «lower = =» ROoptimal (ads) + eworking (5) response and shorter response-recovery time. Such good performance is attributable to the where R is one of SFjunctions component O−work (ads) denotes on 6 decomposition formation of p–n between NiO andgases, ZnO.and This providesthe anadsorbed effectiveoxygen way for the material’s surface. Owing to thesensors electron-hole recombination, the holebyproducts concentration of SO p-type NiO designing high-performance gas to detect SF6 decomposition like 2, SO2F2, decreases, and as a result the concentration gradient of the p–n junction is reduced. Thus, the effect of and SOF2. carriers transport becomes weak and the depletion layer becomes thin [50,54], as shown in Figure 13b. The resistance of the sensor in the test gas is further decreased [55].Natural In short, compared withofaChina pure Acknowledgments: This work has been supported in part by the National Science Foundation ZnO 51507144), sensor, theChina formation of p–n Science junctions greatly increase resistance of the ZnO sensor in air and (No. Postdoctoral Foundation funded the project (Nos. 2015M580771, 2016T90832), the Chongqing Science the and resistance TechnologyinCommission cstc2016jcyjA0400), Postdoctoral Science(Ra/Rg), Funded further decreases the test gas.(CSTC) Thus,(No. based on the definition of gas response Project of Chongqing Scholarship of variation State Key of Laboratory of[49]. Power Transmission the responses of the (No. gasesXm2015016), are greatly Visiting enhanced due to the resistance Equipment & System Security and New Technology (No. 2007DA10512716423) and Fundamental Research Funds for the Central Universities (No. XDJK2015B005). 4. Conclusions
In summary, pure and 3 at-% NiO-decorated ZnO sensing materials were successfully synthesized via a one-step hydrothermal method and characterized by XRD, FESEM, TEM, HRTEM, EDS and XPS, respectively. Their gas sensing properties toward the SF6 decomposition byproducts SO2 , SO2 F2 , and SOF2 were systemically researched with a CGS-1TP gas sensing analysis system. Comparative results demonstrate that the as prepared NiO-decorated ZnO sensing material exhibits enhanced gas sensing properties, including lower optimal working temperature, higher gas response and shorter response-recovery time. Such good performance is attributable to the formation of p–n junctions between NiO and ZnO. This work provides an effective way for designing high-performance gas sensors to detect SF6 decomposition byproducts like SO2 , SO2 F2 , and SOF2 . Acknowledgments: This work has been supported in part by the National Natural Science Foundation of China (No. 51507144), China Postdoctoral Science Foundation funded project (Nos. 2015M580771, 2016T90832), the Chongqing Science and Technology Commission (CSTC) (No. cstc2016jcyjA0400), Postdoctoral Science Funded Project of Chongqing (No. Xm2015016), Visiting Scholarship of State Key Laboratory of Power Transmission Equipment & System Security and New Technology (No. 2007DA10512716423) and Fundamental Research Funds for the Central Universities (No. XDJK2015B005).
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Author Contributions: Hongcheng Liu and Qu Zhou conceived and designed the experiments; Hongcheng Liu and Qingyan Zhang performed the experiments; Changxiang Hong and Lingfeng Jin analyzed the data; Hongcheng Liu and Qu Zhou wrote the paper. Qu Zhou, Lingna Xu and Weigen Chen reviewed and revised the manuscript. All authors read and approved the manuscript. Conflicts of Interest: The authors declare no conflict of interest.
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