Hindawi Publishing CorporationJournal of NanomaterialsVolume 2013 Article ID 135147 7 pageshttpdxdoiorg1011552013135147
Research ArticleApplication of Flower-Like ZnO Nanorods Gas Sensor DetectingSF6 Decomposition Products
Shudi Peng Gaolin Wu Wei Song and Qian Wang
Chongqing Electric Power Research Institute Chongqing 401123 China
Correspondence should be addressed to Shudi Peng psdzqyahoocn
Received 21 November 2012 Accepted 3 January 2013
Academic Editor Wen Zeng
Copyright copy 2013 Shudi Peng et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited
Gas insulated switchgear (GIS) is an important electric power equipment in a substation and its running state has a significantrelationship with stability security and reliability of the whole electric power system Detecting and analyzing the decompositionbyproducts of sulfur hexafluoride gas (SF
6) is an effective method for GIS state assessment and fault diagnosisThis paper proposes
a novel gas sensor based on flower-like ZnO nanorods to detect typical SF6decompositions Flower-like ZnO nanoparticles were
synthesized via a simple hydrothermal method and characterized by X-ray powder diffraction and field-emission scanning electronmicroscopy respectively The gas sensor was fabricated with a planar-type structure and applied to detect SF
6decomposition
products It shows excellent sensing properties to SO2 SOF
2 and SO
2F2with rapid response and recovery time and long-term
stability and repeatabilityMoreover the sensor shows a remarkable discrimination among SO2 SOF2 and SO
2F2with high linearity
which makes the prepared sensor a good candidate and a wide application prospect detecting SF6decomposition products in the
future
1 Introduction
Gas insulated switchgear (GIS) filled with pressurized sulfurhexafluoride gas (SF
6) is widely used in electric power system
in recent decades with the advantages of small floor spacehigh stability and reliability high-strength insulation nonesmeary oil lower maintenance cost and so on [1ndash6] Sulfurhexafluoride gas has excellent insulating performance andarc extinction function and it can dramatically improve theinsulation intensity when used as an insulating mediumSo it is widely applied to GIS and other gas insulationequipments [1 3] However there exist some unavoidableinsulating defects in the process of GIS design manufactureinstallation and operation [4]
As an inert gas pure SF6is colorless tasteless nontoxic
and noninflammable and its decomposition temperature isas high as 500∘C [7] Although SF
6is of great chemical
inertness and the reliability of GIS is very high inevitableinsulating faults based on arc discharge spark discharge orpartial discharge may occur due to the internal insulatingdefects Researches both at home and aboard demonstratethat such internal insulation faults would cause SF
6gas to
decompose and generate several kinds of low-fluorine sul-fides such as SF
4 SF3 and SF
2[2 4 5 8 9] If the SF
6in GIS
is pure the decomposed low-fluorine sulfides will reduce toSF6fast with the decrease of operating temperature Actually
it always contains a certain amount of impurities such as airand water Some low-fluorine sulfides are very active to reactwith trace moisture and oxygen and generate the compoundsof SOF
4 SOF
2 SO2F2 SO2 HF and so on As the GIS
insulating defects vary the decomposed gas mixtures will bedifferent And the composition contents and decompositionrates are also various Therefore detecting and analyzing thedecomposed chemical byproducts accurately can efficientlyidentity and diagnose fault type occurred in GIS
At present many methods [10ndash13] are used to detectthe SF
6decomposition components in GIS for instance
gas chromatography gas detection tube infrared absorptionspectrometry and semiconductor gas sensor Gas chro-matography [10] is mainly used for offline testing and it takesa quite long time Gas detection tube [11] has no responseto some decomposition components and its stability dependson environment condition Infrared absorption spectrometry[12 13] has cross-response on SF
6and cannot quantitatively
2 Journal of Nanomaterials
05 mm
3 mm
6 mm
Sensing materialsAg-Pd
interdigitated electrodes Ceramic substrate
Figure 1 Schematic representation of planar ZnO gas sensor struc-ture
detect the decomposition components In recent years metaloxide semiconductor gas sensor based on ZnO [14] SnO
2
[15] TiO2[16] Fe
2O3[17] WO
3[18] or In
2O3[19] has
been widely used for detecting and online monitoring targetgas owing to advantages of simple fabrication process rapidresponse and recovery time low maintenance cost longservice life long-term stability and repeatability and so onWith the development of nanotechnology various gas sensorshave been fabricatedwith small particle size and high surface-to-volume ratio [20] However most of these gas sensorsmainly focus on toxic gas [21 22] organic gas [23 24] carbondioxide [25] hydrogen [26] and rare studies concerning theSF6decompositions Meanwhile the cross-sensitivity among
the decomposition components is tough so investigatingsensing properties especially selectivity is the most crucialissue for online monitoring SF
6decompositions
In this work we proposed a simple and effectivehydrothermal synthesis route to prepare flower-like ZnOnanorods X-ray powder diffraction (XRD) and field-emission scanning electron microscopy (FESEM) were usedto characterize the microstructures and morphologies of theprepared samples Then a gas sensor based on the flower-likeZnO nanorods was fabricated and its gas sensing propertiesagainst SF
6decompositions were investigated Particularly
the study mainly focused on the sensing behaviors of theprepared sensor against SOF
2 SO2F2 and SO
2 and its cross-
sensitivitywas also demonstratedTheprepared sensor exhib-ited excellent gas response to different SF
6decompositions
at different working temperature with high linearity rapidresponse-recovery and long-time stability and repeatability
2 Experimental
21 Preparation and Characterization of ZnO NanorodsFlower-like zinc oxide nanorods samples were successfullysynthesized through a hydrothermal method using ammo-nium hydroxide (NH
4OH 28wt NH
3in H2O) as the base
source and zinc nitrate hexahydrate (Zn(NO3)2sdot6H2O) as
the source of Zn2+ ions All chemicals were of analyticalreagent grade and purchased from Beijing Chemicals CoLtd In a typical synthesis process an adequate amount of
20 30 40 50 60 70 80
0
50
100
150
200
250
300
350
201
112
200
10311
0
102
101
002
100
Inte
nsity
(au
)
2120579(∘)
Figure 2 XRD patterns of the ZnO nanorods
Zn(NO3)2sdot6H2Owas dissolved in deionized water (DI water)
with a large beaker and NH4OH was added slowly to the
solution under intense magnetic stirringThemixed solutionwas stirred for 30min and then transferred into a sealedTeflon autoclave with 100mL of inner volume and 80 offill ratio After 24 h reaction at 180∘C the reactor was cooledto room temperature naturally Subsequently the preparedwhite products were centrifuged washed two or three timeswith DI water and ethanol alternately and dried at 80∘C in airfor further use
XRD analysis was conducted on a Rigaku Dmax-2500X-ray diffractometer with the 2120579 range of 20ndash80∘C at roomtemperature and Cu 119870
1205721as the source of X-ray at 40 kV
40mA and 120582 = 15418 A FESEM images were performedon a JEOL JEM-6700F microscope operating at 3 and 5 kVrespectively
22 Fabrication and Measurement of ZnO Sensor ZnOnanorods gas sensor was fabricated based on a planar con-structionwith a simple and convenient fabrication procedureThe schemeof the planarZnOgas sensor structurewas shownin Figure 1 where prepared planar ZnO nanorods gas sensoris constituted of planar ceramic substrate Ag-Pd interdigi-tated electrodes and sensing materialThe length width andheight of the planar ceramic substrate are suggested to beabout 6 3 and 05mm respectively There are five pairs ofAg-Pd interdigitated electrodes on planar ceramic substratewith both width and distance about 015mm As-preparedsamples were further ground into fine powder and mixedwith diethanolamine and ethanol to form a paste with aweight ratio of 100 10 10 It was subsequently screen printedonto the planar ceramic substrate to form a sensing film andthe thickness was about 10 um and then dried in air at 60∘Cfor 5 h Finally the sensor was further aged at an aging testchamber for 240 h
Gas sensing properties of the prepared planar ZnO gassensor to SF
6decomposition byproducts were investigated
using an intelligent gas detecting system Targeted gases were
Journal of Nanomaterials 3
2 120583m
(a)
200 nm
(b)
Figure 3 (a) Low-resolution FESEM image and (b) high-resolution FESEM image of the ZnO nanorods
120 180 240 300 360 420
0
Gas
resp
onse
SO2SO2F2SOF2
Temperature (∘C)
minus10
minus20
minus30
minus40
Figure 4Gas response versus temperature curves to 50120583LL of SO2
SOF2 and SO
2F2
mixed with N2by a dynamic gas distributing system which
worked with high accuracy mass flow controllers and theninjected into the gas sensing chamber The concentrationof detecting gas was controlled and detected by gas massflow meter The operating temperature of the gas sensor wascontrolled by varying current flow of the heater And thesurface temperature of the planar sensor was measured bya thermocouple in real time When the testing sensor waspreheated at 300∘C for some time in air and the baselineof resistance was smooth and stable we could start our gassensing properties test
Gas response was defined as the relative variation of theelectrical resistance of the gas sensor 119878 = (119877 minus 119877
0)119877
0times
100 119877 is the resistance of flower-like ZnO nanorods gassensor in target gas environment and119877
0being in pure airThe
01005040302010
Gas
resp
onse
SO2
SO2F2
SOF2
minus10
minus20
minus30
minus40
minus50
Gas concentration (120583LL)
Figure 5 Gas response versus concentrations curves to SO2 SOF
2
and SO2F2
response time was defined as the time taken by the sensor toachieve 90 of the total resistance change in the case of gasin or the recovery time in the case of gas out All experimentswere repeated several times to ensure the reproducibility andstability of the sensor
3 Results and Discussion
31 Structure and Morphology Figure 2 shows the XRDpatterns of the as-prepared ZnO nanorods All the diffractionpeaks are consistent with the values in the standard card(JCPDS 36-1451) and can be indexed as typical wurtzitehexagonal ZnO crystal structure with lattice constants 119886 =3249 A and 119888 = 5206 A No other diffraction peaks from anyimpurities are detected
4 Journal of Nanomaterials
0 20 40 60 80 1000
Gas
resp
onse
SO2
SO2F2
SOF2
minus10
minus20
minus30
minus40
minus50 119910 = 0363119909 minus 1295minus
1198772 = 0982
1198772 = 0979
119910 = 0205119909 minus 6376minus
1198772 = 0963
119910 = 0159119909 minus 2947minus
Gas concentration (120583LL)
Figure 6 The linear calibration curves of SO2 SOF
2 and SO
2F2
0 100 200 300 400 500
0
05
1
15
2
25
3
Gas out
Time (s)
Volta
ge (V
)
Gas in
SO2
SO2F2
SOF2
minus05
Figure 7 The response and recovery behaviors of the sensor to10120583LL of SO
2 SOF
2 and SO
2F2
Figures 3(a) and 3(b) are typical low-resolution andhigh-resolution FESEM images of the prepared flower-likeZnO nanorods samples synthesized with the hydrothermalmethod The nanoparticles have a high uniform flower-likebundle structure and self-assemble into flowers The averagelength of ZnO nanorods is about 400 nmwith an aspect ratioof 4 1
32 Gas Sensing Properties and Sensing Mechanism The gassensing performances of metal oxide semiconductor gassensor are dominantly influenced by working condition Gas
sensing experiments are performed with an intelligent gasdetecting system at different operating temperatures to findout the optimum working temperature Figure 4 shows thegas responses of the prepared flower-like ZnO nanorods gassensor against 50 120583LL of SF
6compositions as a function of
operating temperature which ranges from 120∘C to 420∘CAs seen in Figure 4 the measured gas response curves havea common change trend in which gas response increasesfirstly with rising operating temperature and reaches themaximum and then decreases with an continuous increaseof the operating temperature
This behavior can be understood by a dynamic equi-librium mechanism between gas adsorption and desorptionprocess of gasmolecule on the surface of ZnOor other similarsemiconducting metal oxides In the beginning the rate ofgas adsorption is much higher than that of desorption andthe amount of net adsorbed gas increases as the operatingtemperature rises It would reach a saturated adsorption stateand maintain a dynamic balance at the constant operatingtemperature With a sequential increase of the operatingtemperature the balancewill be broken and it changes to a netdesorption process which ultimately results in a decreasinggas response As shown in Figure 4 the optimal operatingtemperatures of the sensor to 50 120583LL of SO
2 SOF
2 and
SO2F2are 250 300 and 300∘C with gas response of minus3344
minus1247 and minus1806 respectively which are applied in all thefollowing investigations in this paper
At their optimal operating temperatures we performedthe gas responses of the prepared plane flower-like ZnO gassensor against different concentrations of SO
2 SOF
2 and
SO2F2 Figure 5 shows the relationship between gas responses
and 10 20 30 40 50 and 100 120583LL of SO2 SOF2 and SO
2F2
respectively The gas response measured is manifested topersistently increase with a rising gas concentration At thesame level of gas concentration the gas response values ofthe sensor to the three targeted gases decrease in the orderof SO
2 SO2F2 and SOF
2
If the gas response curve is linear or quasilinear thesensor can be applied to engineering application in practiceTherefore based on the linear fitting tool in Origin softwarelinear characteristics of the prepared sensor to SO
2 SO2F2
and SOF2were discussed Figure 6 shows the linear cali-
bration curves of the sensor to SO2 SO2F2 and SOF
2with
gas concentrations in the range of 10ndash100 120583LL As seen inFigure 6 all the three gas response curves meet highly linearwith gas concentration and the linear correlation coefficient119877
2 for SO2 SO2F2 and SOF
2is suggested to be about
0982 0979 and 0963 respectively Such a higher lineardependence indicates that our prepared flower-like ZnO gassensor can be used as promising materials for detecting SF
6
decompositions such as SO2 SO2F2 and SOF
2
Response time and recovery time are other two key indi-cators to evaluate gas sensor performances Figure 7 showsthe response and recovery characteristic of the preparedsensor to 10 120583LL of SO
2 SO2F2 and SOF
2with the sensor
working at its optimum operating temperature As shownin Figure 7 the response times for 10 120583LL of SO
2 SO2F2
and SOF2are about 21 13 and 10 s and correspondingly
the recovery times are about 45 32 and 17 s respectively
Journal of Nanomaterials 5
0 500 1000 1500 2000 2500 3000
0
1
2
3
4
5
6
Time (s)
Volta
ge (V
)
minus1
SO2
10 120583LL20 120583LL
30 120583LL40 120583LL 50 120583LL
100 120583LL
Figure 8The response and recovery behaviors of the sensor to SO2
0 5 10 15 20 25 300
Gas
resp
onse
Time (days)SO2
SO2F2
SOF2
minus10
minus20
minus30
minus40
minus50
Figure 9 The stability and repeatability of the sensor against50 120583LL of SO
2 SO2F2 and SOF
2
Such rapid response and recovery characteristic could beascribed to the structure of the prepared flower-like sensorwhich has a much bigger specific surface area than otherconventional sensing structures provides a larger adsorptionarea and increases the amount of gas molecules adsorbedon the surface Those advantages increase the rate of chargecarriers and facilitate the movement of carriers through thebarriers consequently fast response and response propertyare observed
The response and recovery behaviors versus SO2with
concentration at 10 20 30 40 50 and 100120583LL are shown inFigure 8 With the concentration of detected gas increasing
the gas response amplitude increases apparently neverthelessthe response and recovery property changes slightly whichindicates a very good and satisfying reproducibility of pre-pared sensor against the decompositions Figure 9 shows thelong-term stability and repeatability of the sensor against50120583LL of SO
2 SO2F2 and SOF
2 One can clearly see in
Figure 9 that the gas response changes slightly and keepsat a nearly constant value during the long experimentalcycles which confirms the excellent longtime stability andrepeatability of the prepared flower-like ZnO nanorods gassensor for detecting SO
2 SO2F2 and SOF
2
For most metal oxide semiconductor gas sensors such aszinc oxide tin oxide titanium oxide ferric oxide and indiumoxide the sensing properties are dominantly controlled by thechange of electrical resistance [27] which is fundamentallyattributed to the chemical adsorption and desorption processof gas molecules on sensing surface of the sensor
It is well known to all that zinc oxide is a typical n-type semiconducting material and there exist many oxygenvacancies in the crystal lattices [28ndash30] where various kindsof oxygen could be adsorbedThe species of adsorbed oxygenare closely related to the ambient temperature [31] At roomtemperature oxygen is likely to be adsorbed on ZnO surfaceor grain boundaries with a typical physical adsorption modeAnd it would turn into chemical adsorption by thermalexcitation or electric excitation with certain energy
As shown in Figure 10(a) oxygenwould capture electronsand form a depletion region on the surface area which resultsin a decrease in the concentration of charge carrier and elec-tron mobility thus gas sensor shows a higher electrical resis-tance Figure 10(b) illustrates the gas sensing process of SO
2
as an example exploring the gas sensing mechanism of theprepared sensor detecting SF
6decompositionsWhen flower-
like ZnO nanorods are reducing gas ambient at moderatetemperature (such as in certain concentration of SO
2 SO2F2
and SOF2) the reducing gas reacts with chemical adsorbed
oxygen and then trapped electrons would be released backinto ZnO surface Electrons released from chemical adsorbedoxygen would reduce the height of barriers in the depletionregion and increase the number of charge carriers [32 33]which promotes the movements of charge carriers betweenconduction band and valence band and eventually increasesthe electrical conductivity of the sensor [34 35]
With temperature rising chemical adsorbed oxygenexists in various forms namely O
2adsminus Oads
minus and Oads2minus as
shown in the following reaction equations
O2gas 997888rarr O
2ads O2ads + e
minus997888rarr O
2adsminus
O2adsminus+ eminus 997888rarr 2Oads
minus Oadsminus+ eminus 997888rarr Oads
2minus
(1)
As mentioned above the state of adsorbed oxygen ismainly determined by the ambient temperature At lowerexperimental temperatures oxygen dominantly exists inthe form of a ldquomolecular ionrdquo O
2adsminus and transfers into
ldquoatomic ionrdquo Oadsminus and Oads
2minus with a further rising operatingtemperature Experimental results indicate that the transitiontemperature for oxygen from ldquomolecular ionrdquo to ldquoatomic ionrdquois about 450sim500K As performed in Figure 4 the optimum
6 Journal of Nanomaterials
ZnO surface
O atomElectron
O2ads + 119890minus rarr O2adsminus
O ads minus2adsminus + 119890minus rarr 2OO ads 2minusads minus + 119890minus rarr O
(a) Oxygen adsorbed on ZnO surface
O atomElectron
ZnO surface
2SO2 + O2adsminus rarr 2SO2minusO + 119890minus
2 ads 2SO + O minus rarr SO minusO + 119890minus
SO2 + Oads 2minus rarr SO2minusO + 2119890minus
(b) SO2 gas sensing on ZnO surface ZnO
Figure 10 Schematic plot illustrating the sensing mechanism of prepared sensor to SO2
working temperatures for SO2 SO2F2 and SOF
2are about
250 300 and 300∘C respectivelyThus we draw a conclusionthat the sensing behavior of the prepared sensor to SO
2gas
may belong to the ldquomolecular ionrdquo reaction pattern while itis an ldquoatomic ionrdquo gas response mode for SO
2F2and SOF
2
4 Conclusions
In summary Flower-like ZnO nanorods have been success-fully synthesized and characterized by XRD and FESEMTheoptimum operating temperatures of the prepared sensor toSO2 SO2F2 and SOF
2are about 250 300 and 300∘C The
response (recovery) time of the sensor to 10 120583LL of SO2
SO2F2 and SOF
2is 21 (45) 13 (32) and 10 (17) s respectively
Especially the flower-like ZnO nanorods gas sensor showshigh linearity to SO
2 SO2F2 and SOF
2at the range of 10ndash
100 120583LL with excellent linear correlation coefficient 1198772 at0982 0979 and 0963 separately These findings demon-strate that our prepared flower-like ZnO nanorods have someexcellent potential advantages for using as gas sensors todetect and online monitor the SF
6decompositions such as
SO2 SOF
2 and SO
2F2in practice although further studies
are still needed
References
[1] J Tang F Liu X X Zhang Q H Meng and J B Zhou ldquoPartialdischarge recognition through an analysis of SF
6decomposition
products Part 1 decomposition characteristics of SF6under four
different partial dischargesrdquo IEEE Transactions on Dielectricsand Electrical Insulation vol 19 no 1 pp 29ndash36 2012
[2] M Shih W J Lee and C Y Chen ldquoDecomposition of SF6
and H2S mixture in radio frequency plasma environmentrdquo
Industrial and Engineering Chemistry Research vol 42 no 13pp 2906ndash2912 2003
[3] J Tang F Liu X X Zhang Q H Meng and J G Tao ldquoPartialdischarge recognition through an analysis of SF
6decomposition
products part 2 feature extraction and decision tree-based
pattern recognitionrdquo IEEE Transactions on Dielectrics andElectrical Insulation vol 19 no 1 pp 37ndash44 2012
[4] R J Van Brunt and J T Herron ldquoFundamental processes of SF6
decomposition and oxidation in glow and corona dischargesrdquoIEEE Transactions on Electrical Insulation vol 25 no 1 pp 75ndash94 1990
[5] M Shih W J Lee C H Tsai P J Tsai and C Y ChenldquoDecomposition of SF
6in an RF plasma environmentrdquo Journal
of the Air andWaste Management Association vol 52 no 11 pp1274ndash1280 2002
[6] I Sauers H W Ellis and L G Christophorou ldquoNeutraldecomposition products in spark breakdown of SF
6rdquo IEEE
Transactions on Electrical Insulation vol EI-21 no 2 pp 111ndash120 1986
[7] W T Tsai ldquoThe decomposition products of sulfur hexafluoride(SF6) reviews of environmental and health risk analysisrdquo
Journal of Fluorine Chemistry vol 128 no 11 pp 1345ndash13522007
[8] L Vial AM Casanovas I Coll and J Casanovas ldquoDecomposi-tion products from negative and 50Hz ac corona discharges incompressed SF
6and SF
6N2(10 90) mixtures Effect of water
vapour added to the gasrdquo Journal of Physics D vol 32 no 14pp 1681ndash1692 1999
[9] C T Dervos and P Vassiliou ldquoSulfur hexafluoride (SF6) Global
environmental effects and toxic byproduct formationrdquo Journalof the Air and Waste Management Association vol 50 no 1 pp137ndash141 2000
[10] E Duffour ldquoMolecular dynamic simulations of the collisionbetween copper ions SF
6molecules and a polyethylene surface
a study of decomposition products and an evaluation of the self-diffusion coefficientsrdquoMacromolecularTheory and Simulationsvol 19 no 2-3 pp 88ndash99 2010
[11] J I Baumbach P Pilzecker and E Trindade ldquoMonitoring ofcircuit breakers using ion mobility spectrometry to detect SF
6-
decompositionrdquo International Journal for Ion Mobility Spec-trometry vol 2 no 1 pp 35ndash39 1999
[12] R Kurte C Beyer HMHeise andD Klockow ldquoApplication ofinfrared spectroscopy to monitoring gas insulated high-voltageequipment electrode material-dependent SF
6decompositionrdquo
Journal of Nanomaterials 7
Analytical and Bioanalytical Chemistry vol 373 no 7 pp 639ndash646 2002
[13] W Ding R Hayashi K Ochi et al ldquoAnalysis of PD-generatedSF6decomposition gases adsorbed on carbon nanotubesrdquo IEEE
Transactions on Dielectrics and Electrical Insulation vol 13 no6 pp 1200ndash1207 2006
[14] J SinghAMukherjee S K Sengupta J ImGW Peterson andJ E Whitten ldquoSulfur dioxide and nitrogen dioxide adsorptionon zinc oxide and zirconium hydroxide nanoparticles and theeffect on photoluminescencerdquo Applied Surface Science vol 258no 15 pp 5778ndash5785 2012
[15] BWang L F Zhu Y H Yang N S Xu and GW Yang ldquoFabri-cation of a SnO
2nanowire gas sensor and sensor performance
for hydrogenrdquo Journal of Physical Chemistry C vol 112 no 17pp 6643ndash6647 2008
[16] J Gong Y Li Z Hu Z Zhou and Y Deng ldquoUltrasensitive NH3
gas sensor from polyaniline nanograin enchased TiO2fibersrdquo
Journal of Physical Chemistry C vol 114 no 21 pp 9970ndash99742010
[17] X Liu J Zhang X Guo S Wu and S Wang ldquoPorous 120572-Fe2O3decorated byAunanoparticles and their enhanced sensor
performancerdquo Nanotechnology vol 21 no 9 Article ID 0955012010
[18] B Cao J Chen X Tang and W Zhou ldquoGrowth of monoclinicWO3nanowire array for highly sensitive NO
2detectionrdquo
Journal of Materials Chemistry vol 19 no 16 pp 2323ndash23272009
[19] S E Moon H Y Lee J Park et al ldquoLow power consumptionand high sensitivity carbon monoxide gas sensor using indiumoxide nanowirerdquo Journal of Nanoscience and Nanotechnologyvol 10 no 5 pp 3189ndash3192 2010
[20] WZeng T Liu ZWang S TsukimotoM Saito andY IkuharaldquoSelective detection of formaldehyde gas using a Cd-DopedTiO2-SnO2sensorrdquo Sensors vol 9 no 11 pp 9029ndash9038 2009
[21] M Chen Z Wang D Han F Gu and G Guo ldquoPorous ZnOpolygonal nanoflakes synthesis use in high-sensitivity NO
2gas
sensor and proposed mechanism of gas sensingrdquo Journal ofPhysical Chemistry C vol 115 no 26 pp 12763ndash12773 2011
[22] E Oh H Y Choi S H Jung et al ldquoHigh-performance NO2gas
sensor based on ZnO nanorod grown by ultrasonic irradiationrdquoSensors and Actuators B vol 141 no 1 pp 239ndash243 2009
[23] K Zheng L Gu D Sun X Mo and G Chen ldquoThe propertiesof ethanol gas sensor based on Ti doped ZnO nanotetrapodsrdquoMaterials Science and Engineering B vol 166 no 1 pp 104ndash1072010
[24] A Wei L-H Pan X-C Dong and W Huang ldquoRoom-temperature NH
3gas sensor based on hydrothermally grown
ZnO nanorodsrdquo Chinese Physics Letters vol 28 no 8 pp 702ndash706 2011
[25] CWen Y JuW Li et al ldquoCarbon dioxide gas sensor using SAWdevice based on ZnO filmrdquo Applied Mechanics and Materialsvol 135-136 pp 347ndash352 2012
[26] O Lupan G Chai and L Chow ldquoNovel hydrogen gas sensorbased on single ZnOnanorodrdquoMicroelectronic Engineering vol85 no 11 pp 2220ndash2225 2008
[27] W Zeng T Liu and ZWang ldquoEnhanced gas sensing propertiesby SnO
2nanosphere functionalized TiO
2nanobeltsrdquo Journal of
Materials Chemistry vol 22 no 8 pp 3544ndash3548 2012[28] J Kim andK Yong ldquoMechanism study of ZnOnanorod-bundle
sensors for H2S gas sensingrdquo Journal of Physical Chemistry C
vol 115 no 15 pp 7218ndash7224 2011
[29] D Velasco-Arias D Dıaz P Santiago-Jacinto G Rodrıguez-Gattorno A Vazquez-Olmos and S E Castillo-Blum ldquoDirectinteraction of colloidal nanostructured ZnO and SnO
2withNO
and SO2rdquo Journal of Nanoscience andNanotechnology vol 8 no
12 pp 6389ndash6397 2008[30] Q Qi T Zhang Q Yu et al ldquoProperties of humidity sensing
ZnO nanorods-base sensor fabricated by screen-printingrdquo Sen-sors and Actuators B vol 133 no 2 pp 638ndash643 2008
[31] M-W Ahn K-S Park J-H Heo et al ldquoGas sensing propertiesof defect-controlled ZnO-nanowire gas sensorrdquo Applied PhysicsLetters vol 93 no 26 Article ID 263103 2008
[32] M W Ahn K S Park J H Heo D W Kim K J Choi and JG Park ldquoOn-chip fabrication of ZnO-nanowire gas sensor withhigh gas sensitivityrdquo Sensors and Actuators B vol 138 no 1 pp168ndash173 2009
[33] J Zhang S Wang M Xu et al ldquoHierarchically porous ZnOarchitectures for gas sensor applicationrdquo Crystal Growth andDesign vol 9 no 8 pp 3532ndash3537 2009
[34] Z Yuan X Jiaqiang X Qun L Hui P Qingyi and XPengcheng ldquoBrush-like hierarchical zno nanostructures syn-thesis photoluminescence and gas sensor propertiesrdquo Journalof Physical Chemistry C vol 113 no 9 pp 3430ndash3435 2009
[35] J Zhang X Liu SWu BCao and S Zheng ldquoOne-pot synthesisof Au-supported ZnO nanoplates with enhanced gas sensorperformancerdquo Sensors and Actuators B vol 169 pp 61ndash66 2012
Figure 1 Schematic representation of planar ZnO gas sensor struc-ture
detect the decomposition components In recent years metaloxide semiconductor gas sensor based on ZnO [14] SnO
2
[15] TiO2[16] Fe
2O3[17] WO
3[18] or In
2O3[19] has
been widely used for detecting and online monitoring targetgas owing to advantages of simple fabrication process rapidresponse and recovery time low maintenance cost longservice life long-term stability and repeatability and so onWith the development of nanotechnology various gas sensorshave been fabricatedwith small particle size and high surface-to-volume ratio [20] However most of these gas sensorsmainly focus on toxic gas [21 22] organic gas [23 24] carbondioxide [25] hydrogen [26] and rare studies concerning theSF6decompositions Meanwhile the cross-sensitivity among
the decomposition components is tough so investigatingsensing properties especially selectivity is the most crucialissue for online monitoring SF
6decompositions
In this work we proposed a simple and effectivehydrothermal synthesis route to prepare flower-like ZnOnanorods X-ray powder diffraction (XRD) and field-emission scanning electron microscopy (FESEM) were usedto characterize the microstructures and morphologies of theprepared samples Then a gas sensor based on the flower-likeZnO nanorods was fabricated and its gas sensing propertiesagainst SF
6decompositions were investigated Particularly
the study mainly focused on the sensing behaviors of theprepared sensor against SOF
2 SO2F2 and SO
2 and its cross-
sensitivitywas also demonstratedTheprepared sensor exhib-ited excellent gas response to different SF
6decompositions
at different working temperature with high linearity rapidresponse-recovery and long-time stability and repeatability
2 Experimental
21 Preparation and Characterization of ZnO NanorodsFlower-like zinc oxide nanorods samples were successfullysynthesized through a hydrothermal method using ammo-nium hydroxide (NH
4OH 28wt NH
3in H2O) as the base
source and zinc nitrate hexahydrate (Zn(NO3)2sdot6H2O) as
the source of Zn2+ ions All chemicals were of analyticalreagent grade and purchased from Beijing Chemicals CoLtd In a typical synthesis process an adequate amount of
20 30 40 50 60 70 80
0
50
100
150
200
250
300
350
201
112
200
10311
0
102
101
002
100
Inte
nsity
(au
)
2120579(∘)
Figure 2 XRD patterns of the ZnO nanorods
Zn(NO3)2sdot6H2Owas dissolved in deionized water (DI water)
with a large beaker and NH4OH was added slowly to the
solution under intense magnetic stirringThemixed solutionwas stirred for 30min and then transferred into a sealedTeflon autoclave with 100mL of inner volume and 80 offill ratio After 24 h reaction at 180∘C the reactor was cooledto room temperature naturally Subsequently the preparedwhite products were centrifuged washed two or three timeswith DI water and ethanol alternately and dried at 80∘C in airfor further use
XRD analysis was conducted on a Rigaku Dmax-2500X-ray diffractometer with the 2120579 range of 20ndash80∘C at roomtemperature and Cu 119870
1205721as the source of X-ray at 40 kV
40mA and 120582 = 15418 A FESEM images were performedon a JEOL JEM-6700F microscope operating at 3 and 5 kVrespectively
22 Fabrication and Measurement of ZnO Sensor ZnOnanorods gas sensor was fabricated based on a planar con-structionwith a simple and convenient fabrication procedureThe schemeof the planarZnOgas sensor structurewas shownin Figure 1 where prepared planar ZnO nanorods gas sensoris constituted of planar ceramic substrate Ag-Pd interdigi-tated electrodes and sensing materialThe length width andheight of the planar ceramic substrate are suggested to beabout 6 3 and 05mm respectively There are five pairs ofAg-Pd interdigitated electrodes on planar ceramic substratewith both width and distance about 015mm As-preparedsamples were further ground into fine powder and mixedwith diethanolamine and ethanol to form a paste with aweight ratio of 100 10 10 It was subsequently screen printedonto the planar ceramic substrate to form a sensing film andthe thickness was about 10 um and then dried in air at 60∘Cfor 5 h Finally the sensor was further aged at an aging testchamber for 240 h
Gas sensing properties of the prepared planar ZnO gassensor to SF
6decomposition byproducts were investigated
using an intelligent gas detecting system Targeted gases were
Journal of Nanomaterials 3
2 120583m
(a)
200 nm
(b)
Figure 3 (a) Low-resolution FESEM image and (b) high-resolution FESEM image of the ZnO nanorods
120 180 240 300 360 420
0
Gas
resp
onse
SO2SO2F2SOF2
Temperature (∘C)
minus10
minus20
minus30
minus40
Figure 4Gas response versus temperature curves to 50120583LL of SO2
SOF2 and SO
2F2
mixed with N2by a dynamic gas distributing system which
worked with high accuracy mass flow controllers and theninjected into the gas sensing chamber The concentrationof detecting gas was controlled and detected by gas massflow meter The operating temperature of the gas sensor wascontrolled by varying current flow of the heater And thesurface temperature of the planar sensor was measured bya thermocouple in real time When the testing sensor waspreheated at 300∘C for some time in air and the baselineof resistance was smooth and stable we could start our gassensing properties test
Gas response was defined as the relative variation of theelectrical resistance of the gas sensor 119878 = (119877 minus 119877
0)119877
0times
100 119877 is the resistance of flower-like ZnO nanorods gassensor in target gas environment and119877
0being in pure airThe
01005040302010
Gas
resp
onse
SO2
SO2F2
SOF2
minus10
minus20
minus30
minus40
minus50
Gas concentration (120583LL)
Figure 5 Gas response versus concentrations curves to SO2 SOF
2
and SO2F2
response time was defined as the time taken by the sensor toachieve 90 of the total resistance change in the case of gasin or the recovery time in the case of gas out All experimentswere repeated several times to ensure the reproducibility andstability of the sensor
3 Results and Discussion
31 Structure and Morphology Figure 2 shows the XRDpatterns of the as-prepared ZnO nanorods All the diffractionpeaks are consistent with the values in the standard card(JCPDS 36-1451) and can be indexed as typical wurtzitehexagonal ZnO crystal structure with lattice constants 119886 =3249 A and 119888 = 5206 A No other diffraction peaks from anyimpurities are detected
4 Journal of Nanomaterials
0 20 40 60 80 1000
Gas
resp
onse
SO2
SO2F2
SOF2
minus10
minus20
minus30
minus40
minus50 119910 = 0363119909 minus 1295minus
1198772 = 0982
1198772 = 0979
119910 = 0205119909 minus 6376minus
1198772 = 0963
119910 = 0159119909 minus 2947minus
Gas concentration (120583LL)
Figure 6 The linear calibration curves of SO2 SOF
2 and SO
2F2
0 100 200 300 400 500
0
05
1
15
2
25
3
Gas out
Time (s)
Volta
ge (V
)
Gas in
SO2
SO2F2
SOF2
minus05
Figure 7 The response and recovery behaviors of the sensor to10120583LL of SO
2 SOF
2 and SO
2F2
Figures 3(a) and 3(b) are typical low-resolution andhigh-resolution FESEM images of the prepared flower-likeZnO nanorods samples synthesized with the hydrothermalmethod The nanoparticles have a high uniform flower-likebundle structure and self-assemble into flowers The averagelength of ZnO nanorods is about 400 nmwith an aspect ratioof 4 1
32 Gas Sensing Properties and Sensing Mechanism The gassensing performances of metal oxide semiconductor gassensor are dominantly influenced by working condition Gas
sensing experiments are performed with an intelligent gasdetecting system at different operating temperatures to findout the optimum working temperature Figure 4 shows thegas responses of the prepared flower-like ZnO nanorods gassensor against 50 120583LL of SF
6compositions as a function of
operating temperature which ranges from 120∘C to 420∘CAs seen in Figure 4 the measured gas response curves havea common change trend in which gas response increasesfirstly with rising operating temperature and reaches themaximum and then decreases with an continuous increaseof the operating temperature
This behavior can be understood by a dynamic equi-librium mechanism between gas adsorption and desorptionprocess of gasmolecule on the surface of ZnOor other similarsemiconducting metal oxides In the beginning the rate ofgas adsorption is much higher than that of desorption andthe amount of net adsorbed gas increases as the operatingtemperature rises It would reach a saturated adsorption stateand maintain a dynamic balance at the constant operatingtemperature With a sequential increase of the operatingtemperature the balancewill be broken and it changes to a netdesorption process which ultimately results in a decreasinggas response As shown in Figure 4 the optimal operatingtemperatures of the sensor to 50 120583LL of SO
2 SOF
2 and
SO2F2are 250 300 and 300∘C with gas response of minus3344
minus1247 and minus1806 respectively which are applied in all thefollowing investigations in this paper
At their optimal operating temperatures we performedthe gas responses of the prepared plane flower-like ZnO gassensor against different concentrations of SO
2 SOF
2 and
SO2F2 Figure 5 shows the relationship between gas responses
and 10 20 30 40 50 and 100 120583LL of SO2 SOF2 and SO
2F2
respectively The gas response measured is manifested topersistently increase with a rising gas concentration At thesame level of gas concentration the gas response values ofthe sensor to the three targeted gases decrease in the orderof SO
2 SO2F2 and SOF
2
If the gas response curve is linear or quasilinear thesensor can be applied to engineering application in practiceTherefore based on the linear fitting tool in Origin softwarelinear characteristics of the prepared sensor to SO
2 SO2F2
and SOF2were discussed Figure 6 shows the linear cali-
bration curves of the sensor to SO2 SO2F2 and SOF
2with
gas concentrations in the range of 10ndash100 120583LL As seen inFigure 6 all the three gas response curves meet highly linearwith gas concentration and the linear correlation coefficient119877
2 for SO2 SO2F2 and SOF
2is suggested to be about
0982 0979 and 0963 respectively Such a higher lineardependence indicates that our prepared flower-like ZnO gassensor can be used as promising materials for detecting SF
6
decompositions such as SO2 SO2F2 and SOF
2
Response time and recovery time are other two key indi-cators to evaluate gas sensor performances Figure 7 showsthe response and recovery characteristic of the preparedsensor to 10 120583LL of SO
2 SO2F2 and SOF
2with the sensor
working at its optimum operating temperature As shownin Figure 7 the response times for 10 120583LL of SO
2 SO2F2
and SOF2are about 21 13 and 10 s and correspondingly
the recovery times are about 45 32 and 17 s respectively
Journal of Nanomaterials 5
0 500 1000 1500 2000 2500 3000
0
1
2
3
4
5
6
Time (s)
Volta
ge (V
)
minus1
SO2
10 120583LL20 120583LL
30 120583LL40 120583LL 50 120583LL
100 120583LL
Figure 8The response and recovery behaviors of the sensor to SO2
0 5 10 15 20 25 300
Gas
resp
onse
Time (days)SO2
SO2F2
SOF2
minus10
minus20
minus30
minus40
minus50
Figure 9 The stability and repeatability of the sensor against50 120583LL of SO
2 SO2F2 and SOF
2
Such rapid response and recovery characteristic could beascribed to the structure of the prepared flower-like sensorwhich has a much bigger specific surface area than otherconventional sensing structures provides a larger adsorptionarea and increases the amount of gas molecules adsorbedon the surface Those advantages increase the rate of chargecarriers and facilitate the movement of carriers through thebarriers consequently fast response and response propertyare observed
The response and recovery behaviors versus SO2with
concentration at 10 20 30 40 50 and 100120583LL are shown inFigure 8 With the concentration of detected gas increasing
the gas response amplitude increases apparently neverthelessthe response and recovery property changes slightly whichindicates a very good and satisfying reproducibility of pre-pared sensor against the decompositions Figure 9 shows thelong-term stability and repeatability of the sensor against50120583LL of SO
2 SO2F2 and SOF
2 One can clearly see in
Figure 9 that the gas response changes slightly and keepsat a nearly constant value during the long experimentalcycles which confirms the excellent longtime stability andrepeatability of the prepared flower-like ZnO nanorods gassensor for detecting SO
2 SO2F2 and SOF
2
For most metal oxide semiconductor gas sensors such aszinc oxide tin oxide titanium oxide ferric oxide and indiumoxide the sensing properties are dominantly controlled by thechange of electrical resistance [27] which is fundamentallyattributed to the chemical adsorption and desorption processof gas molecules on sensing surface of the sensor
It is well known to all that zinc oxide is a typical n-type semiconducting material and there exist many oxygenvacancies in the crystal lattices [28ndash30] where various kindsof oxygen could be adsorbedThe species of adsorbed oxygenare closely related to the ambient temperature [31] At roomtemperature oxygen is likely to be adsorbed on ZnO surfaceor grain boundaries with a typical physical adsorption modeAnd it would turn into chemical adsorption by thermalexcitation or electric excitation with certain energy
As shown in Figure 10(a) oxygenwould capture electronsand form a depletion region on the surface area which resultsin a decrease in the concentration of charge carrier and elec-tron mobility thus gas sensor shows a higher electrical resis-tance Figure 10(b) illustrates the gas sensing process of SO
2
as an example exploring the gas sensing mechanism of theprepared sensor detecting SF
6decompositionsWhen flower-
like ZnO nanorods are reducing gas ambient at moderatetemperature (such as in certain concentration of SO
2 SO2F2
and SOF2) the reducing gas reacts with chemical adsorbed
oxygen and then trapped electrons would be released backinto ZnO surface Electrons released from chemical adsorbedoxygen would reduce the height of barriers in the depletionregion and increase the number of charge carriers [32 33]which promotes the movements of charge carriers betweenconduction band and valence band and eventually increasesthe electrical conductivity of the sensor [34 35]
With temperature rising chemical adsorbed oxygenexists in various forms namely O
2adsminus Oads
minus and Oads2minus as
shown in the following reaction equations
O2gas 997888rarr O
2ads O2ads + e
minus997888rarr O
2adsminus
O2adsminus+ eminus 997888rarr 2Oads
minus Oadsminus+ eminus 997888rarr Oads
2minus
(1)
As mentioned above the state of adsorbed oxygen ismainly determined by the ambient temperature At lowerexperimental temperatures oxygen dominantly exists inthe form of a ldquomolecular ionrdquo O
2adsminus and transfers into
ldquoatomic ionrdquo Oadsminus and Oads
2minus with a further rising operatingtemperature Experimental results indicate that the transitiontemperature for oxygen from ldquomolecular ionrdquo to ldquoatomic ionrdquois about 450sim500K As performed in Figure 4 the optimum
6 Journal of Nanomaterials
ZnO surface
O atomElectron
O2ads + 119890minus rarr O2adsminus
O ads minus2adsminus + 119890minus rarr 2OO ads 2minusads minus + 119890minus rarr O
(a) Oxygen adsorbed on ZnO surface
O atomElectron
ZnO surface
2SO2 + O2adsminus rarr 2SO2minusO + 119890minus
2 ads 2SO + O minus rarr SO minusO + 119890minus
SO2 + Oads 2minus rarr SO2minusO + 2119890minus
(b) SO2 gas sensing on ZnO surface ZnO
Figure 10 Schematic plot illustrating the sensing mechanism of prepared sensor to SO2
working temperatures for SO2 SO2F2 and SOF
2are about
250 300 and 300∘C respectivelyThus we draw a conclusionthat the sensing behavior of the prepared sensor to SO
2gas
may belong to the ldquomolecular ionrdquo reaction pattern while itis an ldquoatomic ionrdquo gas response mode for SO
2F2and SOF
2
4 Conclusions
In summary Flower-like ZnO nanorods have been success-fully synthesized and characterized by XRD and FESEMTheoptimum operating temperatures of the prepared sensor toSO2 SO2F2 and SOF
2are about 250 300 and 300∘C The
response (recovery) time of the sensor to 10 120583LL of SO2
SO2F2 and SOF
2is 21 (45) 13 (32) and 10 (17) s respectively
Especially the flower-like ZnO nanorods gas sensor showshigh linearity to SO
2 SO2F2 and SOF
2at the range of 10ndash
100 120583LL with excellent linear correlation coefficient 1198772 at0982 0979 and 0963 separately These findings demon-strate that our prepared flower-like ZnO nanorods have someexcellent potential advantages for using as gas sensors todetect and online monitor the SF
6decompositions such as
SO2 SOF
2 and SO
2F2in practice although further studies
are still needed
References
[1] J Tang F Liu X X Zhang Q H Meng and J B Zhou ldquoPartialdischarge recognition through an analysis of SF
6decomposition
products Part 1 decomposition characteristics of SF6under four
different partial dischargesrdquo IEEE Transactions on Dielectricsand Electrical Insulation vol 19 no 1 pp 29ndash36 2012
[2] M Shih W J Lee and C Y Chen ldquoDecomposition of SF6
and H2S mixture in radio frequency plasma environmentrdquo
Industrial and Engineering Chemistry Research vol 42 no 13pp 2906ndash2912 2003
[3] J Tang F Liu X X Zhang Q H Meng and J G Tao ldquoPartialdischarge recognition through an analysis of SF
6decomposition
products part 2 feature extraction and decision tree-based
pattern recognitionrdquo IEEE Transactions on Dielectrics andElectrical Insulation vol 19 no 1 pp 37ndash44 2012
[4] R J Van Brunt and J T Herron ldquoFundamental processes of SF6
decomposition and oxidation in glow and corona dischargesrdquoIEEE Transactions on Electrical Insulation vol 25 no 1 pp 75ndash94 1990
[5] M Shih W J Lee C H Tsai P J Tsai and C Y ChenldquoDecomposition of SF
6in an RF plasma environmentrdquo Journal
of the Air andWaste Management Association vol 52 no 11 pp1274ndash1280 2002
[6] I Sauers H W Ellis and L G Christophorou ldquoNeutraldecomposition products in spark breakdown of SF
6rdquo IEEE
Transactions on Electrical Insulation vol EI-21 no 2 pp 111ndash120 1986
[7] W T Tsai ldquoThe decomposition products of sulfur hexafluoride(SF6) reviews of environmental and health risk analysisrdquo
Journal of Fluorine Chemistry vol 128 no 11 pp 1345ndash13522007
[8] L Vial AM Casanovas I Coll and J Casanovas ldquoDecomposi-tion products from negative and 50Hz ac corona discharges incompressed SF
6and SF
6N2(10 90) mixtures Effect of water
vapour added to the gasrdquo Journal of Physics D vol 32 no 14pp 1681ndash1692 1999
[9] C T Dervos and P Vassiliou ldquoSulfur hexafluoride (SF6) Global
environmental effects and toxic byproduct formationrdquo Journalof the Air and Waste Management Association vol 50 no 1 pp137ndash141 2000
[10] E Duffour ldquoMolecular dynamic simulations of the collisionbetween copper ions SF
6molecules and a polyethylene surface
a study of decomposition products and an evaluation of the self-diffusion coefficientsrdquoMacromolecularTheory and Simulationsvol 19 no 2-3 pp 88ndash99 2010
[11] J I Baumbach P Pilzecker and E Trindade ldquoMonitoring ofcircuit breakers using ion mobility spectrometry to detect SF
6-
decompositionrdquo International Journal for Ion Mobility Spec-trometry vol 2 no 1 pp 35ndash39 1999
[12] R Kurte C Beyer HMHeise andD Klockow ldquoApplication ofinfrared spectroscopy to monitoring gas insulated high-voltageequipment electrode material-dependent SF
6decompositionrdquo
Journal of Nanomaterials 7
Analytical and Bioanalytical Chemistry vol 373 no 7 pp 639ndash646 2002
[13] W Ding R Hayashi K Ochi et al ldquoAnalysis of PD-generatedSF6decomposition gases adsorbed on carbon nanotubesrdquo IEEE
Transactions on Dielectrics and Electrical Insulation vol 13 no6 pp 1200ndash1207 2006
[14] J SinghAMukherjee S K Sengupta J ImGW Peterson andJ E Whitten ldquoSulfur dioxide and nitrogen dioxide adsorptionon zinc oxide and zirconium hydroxide nanoparticles and theeffect on photoluminescencerdquo Applied Surface Science vol 258no 15 pp 5778ndash5785 2012
[15] BWang L F Zhu Y H Yang N S Xu and GW Yang ldquoFabri-cation of a SnO
2nanowire gas sensor and sensor performance
for hydrogenrdquo Journal of Physical Chemistry C vol 112 no 17pp 6643ndash6647 2008
[16] J Gong Y Li Z Hu Z Zhou and Y Deng ldquoUltrasensitive NH3
gas sensor from polyaniline nanograin enchased TiO2fibersrdquo
Journal of Physical Chemistry C vol 114 no 21 pp 9970ndash99742010
[17] X Liu J Zhang X Guo S Wu and S Wang ldquoPorous 120572-Fe2O3decorated byAunanoparticles and their enhanced sensor
performancerdquo Nanotechnology vol 21 no 9 Article ID 0955012010
[18] B Cao J Chen X Tang and W Zhou ldquoGrowth of monoclinicWO3nanowire array for highly sensitive NO
2detectionrdquo
Journal of Materials Chemistry vol 19 no 16 pp 2323ndash23272009
[19] S E Moon H Y Lee J Park et al ldquoLow power consumptionand high sensitivity carbon monoxide gas sensor using indiumoxide nanowirerdquo Journal of Nanoscience and Nanotechnologyvol 10 no 5 pp 3189ndash3192 2010
[20] WZeng T Liu ZWang S TsukimotoM Saito andY IkuharaldquoSelective detection of formaldehyde gas using a Cd-DopedTiO2-SnO2sensorrdquo Sensors vol 9 no 11 pp 9029ndash9038 2009
[21] M Chen Z Wang D Han F Gu and G Guo ldquoPorous ZnOpolygonal nanoflakes synthesis use in high-sensitivity NO
2gas
sensor and proposed mechanism of gas sensingrdquo Journal ofPhysical Chemistry C vol 115 no 26 pp 12763ndash12773 2011
[22] E Oh H Y Choi S H Jung et al ldquoHigh-performance NO2gas
sensor based on ZnO nanorod grown by ultrasonic irradiationrdquoSensors and Actuators B vol 141 no 1 pp 239ndash243 2009
[23] K Zheng L Gu D Sun X Mo and G Chen ldquoThe propertiesof ethanol gas sensor based on Ti doped ZnO nanotetrapodsrdquoMaterials Science and Engineering B vol 166 no 1 pp 104ndash1072010
[24] A Wei L-H Pan X-C Dong and W Huang ldquoRoom-temperature NH
3gas sensor based on hydrothermally grown
ZnO nanorodsrdquo Chinese Physics Letters vol 28 no 8 pp 702ndash706 2011
[25] CWen Y JuW Li et al ldquoCarbon dioxide gas sensor using SAWdevice based on ZnO filmrdquo Applied Mechanics and Materialsvol 135-136 pp 347ndash352 2012
[26] O Lupan G Chai and L Chow ldquoNovel hydrogen gas sensorbased on single ZnOnanorodrdquoMicroelectronic Engineering vol85 no 11 pp 2220ndash2225 2008
[27] W Zeng T Liu and ZWang ldquoEnhanced gas sensing propertiesby SnO
2nanosphere functionalized TiO
2nanobeltsrdquo Journal of
Materials Chemistry vol 22 no 8 pp 3544ndash3548 2012[28] J Kim andK Yong ldquoMechanism study of ZnOnanorod-bundle
sensors for H2S gas sensingrdquo Journal of Physical Chemistry C
vol 115 no 15 pp 7218ndash7224 2011
[29] D Velasco-Arias D Dıaz P Santiago-Jacinto G Rodrıguez-Gattorno A Vazquez-Olmos and S E Castillo-Blum ldquoDirectinteraction of colloidal nanostructured ZnO and SnO
2withNO
and SO2rdquo Journal of Nanoscience andNanotechnology vol 8 no
12 pp 6389ndash6397 2008[30] Q Qi T Zhang Q Yu et al ldquoProperties of humidity sensing
ZnO nanorods-base sensor fabricated by screen-printingrdquo Sen-sors and Actuators B vol 133 no 2 pp 638ndash643 2008
[31] M-W Ahn K-S Park J-H Heo et al ldquoGas sensing propertiesof defect-controlled ZnO-nanowire gas sensorrdquo Applied PhysicsLetters vol 93 no 26 Article ID 263103 2008
[32] M W Ahn K S Park J H Heo D W Kim K J Choi and JG Park ldquoOn-chip fabrication of ZnO-nanowire gas sensor withhigh gas sensitivityrdquo Sensors and Actuators B vol 138 no 1 pp168ndash173 2009
[33] J Zhang S Wang M Xu et al ldquoHierarchically porous ZnOarchitectures for gas sensor applicationrdquo Crystal Growth andDesign vol 9 no 8 pp 3532ndash3537 2009
[34] Z Yuan X Jiaqiang X Qun L Hui P Qingyi and XPengcheng ldquoBrush-like hierarchical zno nanostructures syn-thesis photoluminescence and gas sensor propertiesrdquo Journalof Physical Chemistry C vol 113 no 9 pp 3430ndash3435 2009
[35] J Zhang X Liu SWu BCao and S Zheng ldquoOne-pot synthesisof Au-supported ZnO nanoplates with enhanced gas sensorperformancerdquo Sensors and Actuators B vol 169 pp 61ndash66 2012
Figure 3 (a) Low-resolution FESEM image and (b) high-resolution FESEM image of the ZnO nanorods
120 180 240 300 360 420
0
Gas
resp
onse
SO2SO2F2SOF2
Temperature (∘C)
minus10
minus20
minus30
minus40
Figure 4Gas response versus temperature curves to 50120583LL of SO2
SOF2 and SO
2F2
mixed with N2by a dynamic gas distributing system which
worked with high accuracy mass flow controllers and theninjected into the gas sensing chamber The concentrationof detecting gas was controlled and detected by gas massflow meter The operating temperature of the gas sensor wascontrolled by varying current flow of the heater And thesurface temperature of the planar sensor was measured bya thermocouple in real time When the testing sensor waspreheated at 300∘C for some time in air and the baselineof resistance was smooth and stable we could start our gassensing properties test
Gas response was defined as the relative variation of theelectrical resistance of the gas sensor 119878 = (119877 minus 119877
0)119877
0times
100 119877 is the resistance of flower-like ZnO nanorods gassensor in target gas environment and119877
0being in pure airThe
01005040302010
Gas
resp
onse
SO2
SO2F2
SOF2
minus10
minus20
minus30
minus40
minus50
Gas concentration (120583LL)
Figure 5 Gas response versus concentrations curves to SO2 SOF
2
and SO2F2
response time was defined as the time taken by the sensor toachieve 90 of the total resistance change in the case of gasin or the recovery time in the case of gas out All experimentswere repeated several times to ensure the reproducibility andstability of the sensor
3 Results and Discussion
31 Structure and Morphology Figure 2 shows the XRDpatterns of the as-prepared ZnO nanorods All the diffractionpeaks are consistent with the values in the standard card(JCPDS 36-1451) and can be indexed as typical wurtzitehexagonal ZnO crystal structure with lattice constants 119886 =3249 A and 119888 = 5206 A No other diffraction peaks from anyimpurities are detected
4 Journal of Nanomaterials
0 20 40 60 80 1000
Gas
resp
onse
SO2
SO2F2
SOF2
minus10
minus20
minus30
minus40
minus50 119910 = 0363119909 minus 1295minus
1198772 = 0982
1198772 = 0979
119910 = 0205119909 minus 6376minus
1198772 = 0963
119910 = 0159119909 minus 2947minus
Gas concentration (120583LL)
Figure 6 The linear calibration curves of SO2 SOF
2 and SO
2F2
0 100 200 300 400 500
0
05
1
15
2
25
3
Gas out
Time (s)
Volta
ge (V
)
Gas in
SO2
SO2F2
SOF2
minus05
Figure 7 The response and recovery behaviors of the sensor to10120583LL of SO
2 SOF
2 and SO
2F2
Figures 3(a) and 3(b) are typical low-resolution andhigh-resolution FESEM images of the prepared flower-likeZnO nanorods samples synthesized with the hydrothermalmethod The nanoparticles have a high uniform flower-likebundle structure and self-assemble into flowers The averagelength of ZnO nanorods is about 400 nmwith an aspect ratioof 4 1
32 Gas Sensing Properties and Sensing Mechanism The gassensing performances of metal oxide semiconductor gassensor are dominantly influenced by working condition Gas
sensing experiments are performed with an intelligent gasdetecting system at different operating temperatures to findout the optimum working temperature Figure 4 shows thegas responses of the prepared flower-like ZnO nanorods gassensor against 50 120583LL of SF
6compositions as a function of
operating temperature which ranges from 120∘C to 420∘CAs seen in Figure 4 the measured gas response curves havea common change trend in which gas response increasesfirstly with rising operating temperature and reaches themaximum and then decreases with an continuous increaseof the operating temperature
This behavior can be understood by a dynamic equi-librium mechanism between gas adsorption and desorptionprocess of gasmolecule on the surface of ZnOor other similarsemiconducting metal oxides In the beginning the rate ofgas adsorption is much higher than that of desorption andthe amount of net adsorbed gas increases as the operatingtemperature rises It would reach a saturated adsorption stateand maintain a dynamic balance at the constant operatingtemperature With a sequential increase of the operatingtemperature the balancewill be broken and it changes to a netdesorption process which ultimately results in a decreasinggas response As shown in Figure 4 the optimal operatingtemperatures of the sensor to 50 120583LL of SO
2 SOF
2 and
SO2F2are 250 300 and 300∘C with gas response of minus3344
minus1247 and minus1806 respectively which are applied in all thefollowing investigations in this paper
At their optimal operating temperatures we performedthe gas responses of the prepared plane flower-like ZnO gassensor against different concentrations of SO
2 SOF
2 and
SO2F2 Figure 5 shows the relationship between gas responses
and 10 20 30 40 50 and 100 120583LL of SO2 SOF2 and SO
2F2
respectively The gas response measured is manifested topersistently increase with a rising gas concentration At thesame level of gas concentration the gas response values ofthe sensor to the three targeted gases decrease in the orderof SO
2 SO2F2 and SOF
2
If the gas response curve is linear or quasilinear thesensor can be applied to engineering application in practiceTherefore based on the linear fitting tool in Origin softwarelinear characteristics of the prepared sensor to SO
2 SO2F2
and SOF2were discussed Figure 6 shows the linear cali-
bration curves of the sensor to SO2 SO2F2 and SOF
2with
gas concentrations in the range of 10ndash100 120583LL As seen inFigure 6 all the three gas response curves meet highly linearwith gas concentration and the linear correlation coefficient119877
2 for SO2 SO2F2 and SOF
2is suggested to be about
0982 0979 and 0963 respectively Such a higher lineardependence indicates that our prepared flower-like ZnO gassensor can be used as promising materials for detecting SF
6
decompositions such as SO2 SO2F2 and SOF
2
Response time and recovery time are other two key indi-cators to evaluate gas sensor performances Figure 7 showsthe response and recovery characteristic of the preparedsensor to 10 120583LL of SO
2 SO2F2 and SOF
2with the sensor
working at its optimum operating temperature As shownin Figure 7 the response times for 10 120583LL of SO
2 SO2F2
and SOF2are about 21 13 and 10 s and correspondingly
the recovery times are about 45 32 and 17 s respectively
Journal of Nanomaterials 5
0 500 1000 1500 2000 2500 3000
0
1
2
3
4
5
6
Time (s)
Volta
ge (V
)
minus1
SO2
10 120583LL20 120583LL
30 120583LL40 120583LL 50 120583LL
100 120583LL
Figure 8The response and recovery behaviors of the sensor to SO2
0 5 10 15 20 25 300
Gas
resp
onse
Time (days)SO2
SO2F2
SOF2
minus10
minus20
minus30
minus40
minus50
Figure 9 The stability and repeatability of the sensor against50 120583LL of SO
2 SO2F2 and SOF
2
Such rapid response and recovery characteristic could beascribed to the structure of the prepared flower-like sensorwhich has a much bigger specific surface area than otherconventional sensing structures provides a larger adsorptionarea and increases the amount of gas molecules adsorbedon the surface Those advantages increase the rate of chargecarriers and facilitate the movement of carriers through thebarriers consequently fast response and response propertyare observed
The response and recovery behaviors versus SO2with
concentration at 10 20 30 40 50 and 100120583LL are shown inFigure 8 With the concentration of detected gas increasing
the gas response amplitude increases apparently neverthelessthe response and recovery property changes slightly whichindicates a very good and satisfying reproducibility of pre-pared sensor against the decompositions Figure 9 shows thelong-term stability and repeatability of the sensor against50120583LL of SO
2 SO2F2 and SOF
2 One can clearly see in
Figure 9 that the gas response changes slightly and keepsat a nearly constant value during the long experimentalcycles which confirms the excellent longtime stability andrepeatability of the prepared flower-like ZnO nanorods gassensor for detecting SO
2 SO2F2 and SOF
2
For most metal oxide semiconductor gas sensors such aszinc oxide tin oxide titanium oxide ferric oxide and indiumoxide the sensing properties are dominantly controlled by thechange of electrical resistance [27] which is fundamentallyattributed to the chemical adsorption and desorption processof gas molecules on sensing surface of the sensor
It is well known to all that zinc oxide is a typical n-type semiconducting material and there exist many oxygenvacancies in the crystal lattices [28ndash30] where various kindsof oxygen could be adsorbedThe species of adsorbed oxygenare closely related to the ambient temperature [31] At roomtemperature oxygen is likely to be adsorbed on ZnO surfaceor grain boundaries with a typical physical adsorption modeAnd it would turn into chemical adsorption by thermalexcitation or electric excitation with certain energy
As shown in Figure 10(a) oxygenwould capture electronsand form a depletion region on the surface area which resultsin a decrease in the concentration of charge carrier and elec-tron mobility thus gas sensor shows a higher electrical resis-tance Figure 10(b) illustrates the gas sensing process of SO
2
as an example exploring the gas sensing mechanism of theprepared sensor detecting SF
6decompositionsWhen flower-
like ZnO nanorods are reducing gas ambient at moderatetemperature (such as in certain concentration of SO
2 SO2F2
and SOF2) the reducing gas reacts with chemical adsorbed
oxygen and then trapped electrons would be released backinto ZnO surface Electrons released from chemical adsorbedoxygen would reduce the height of barriers in the depletionregion and increase the number of charge carriers [32 33]which promotes the movements of charge carriers betweenconduction band and valence band and eventually increasesthe electrical conductivity of the sensor [34 35]
With temperature rising chemical adsorbed oxygenexists in various forms namely O
2adsminus Oads
minus and Oads2minus as
shown in the following reaction equations
O2gas 997888rarr O
2ads O2ads + e
minus997888rarr O
2adsminus
O2adsminus+ eminus 997888rarr 2Oads
minus Oadsminus+ eminus 997888rarr Oads
2minus
(1)
As mentioned above the state of adsorbed oxygen ismainly determined by the ambient temperature At lowerexperimental temperatures oxygen dominantly exists inthe form of a ldquomolecular ionrdquo O
2adsminus and transfers into
ldquoatomic ionrdquo Oadsminus and Oads
2minus with a further rising operatingtemperature Experimental results indicate that the transitiontemperature for oxygen from ldquomolecular ionrdquo to ldquoatomic ionrdquois about 450sim500K As performed in Figure 4 the optimum
6 Journal of Nanomaterials
ZnO surface
O atomElectron
O2ads + 119890minus rarr O2adsminus
O ads minus2adsminus + 119890minus rarr 2OO ads 2minusads minus + 119890minus rarr O
(a) Oxygen adsorbed on ZnO surface
O atomElectron
ZnO surface
2SO2 + O2adsminus rarr 2SO2minusO + 119890minus
2 ads 2SO + O minus rarr SO minusO + 119890minus
SO2 + Oads 2minus rarr SO2minusO + 2119890minus
(b) SO2 gas sensing on ZnO surface ZnO
Figure 10 Schematic plot illustrating the sensing mechanism of prepared sensor to SO2
working temperatures for SO2 SO2F2 and SOF
2are about
250 300 and 300∘C respectivelyThus we draw a conclusionthat the sensing behavior of the prepared sensor to SO
2gas
may belong to the ldquomolecular ionrdquo reaction pattern while itis an ldquoatomic ionrdquo gas response mode for SO
2F2and SOF
2
4 Conclusions
In summary Flower-like ZnO nanorods have been success-fully synthesized and characterized by XRD and FESEMTheoptimum operating temperatures of the prepared sensor toSO2 SO2F2 and SOF
2are about 250 300 and 300∘C The
response (recovery) time of the sensor to 10 120583LL of SO2
SO2F2 and SOF
2is 21 (45) 13 (32) and 10 (17) s respectively
Especially the flower-like ZnO nanorods gas sensor showshigh linearity to SO
2 SO2F2 and SOF
2at the range of 10ndash
100 120583LL with excellent linear correlation coefficient 1198772 at0982 0979 and 0963 separately These findings demon-strate that our prepared flower-like ZnO nanorods have someexcellent potential advantages for using as gas sensors todetect and online monitor the SF
6decompositions such as
SO2 SOF
2 and SO
2F2in practice although further studies
are still needed
References
[1] J Tang F Liu X X Zhang Q H Meng and J B Zhou ldquoPartialdischarge recognition through an analysis of SF
6decomposition
products Part 1 decomposition characteristics of SF6under four
different partial dischargesrdquo IEEE Transactions on Dielectricsand Electrical Insulation vol 19 no 1 pp 29ndash36 2012
[2] M Shih W J Lee and C Y Chen ldquoDecomposition of SF6
and H2S mixture in radio frequency plasma environmentrdquo
Industrial and Engineering Chemistry Research vol 42 no 13pp 2906ndash2912 2003
[3] J Tang F Liu X X Zhang Q H Meng and J G Tao ldquoPartialdischarge recognition through an analysis of SF
6decomposition
products part 2 feature extraction and decision tree-based
pattern recognitionrdquo IEEE Transactions on Dielectrics andElectrical Insulation vol 19 no 1 pp 37ndash44 2012
[4] R J Van Brunt and J T Herron ldquoFundamental processes of SF6
decomposition and oxidation in glow and corona dischargesrdquoIEEE Transactions on Electrical Insulation vol 25 no 1 pp 75ndash94 1990
[5] M Shih W J Lee C H Tsai P J Tsai and C Y ChenldquoDecomposition of SF
6in an RF plasma environmentrdquo Journal
of the Air andWaste Management Association vol 52 no 11 pp1274ndash1280 2002
[6] I Sauers H W Ellis and L G Christophorou ldquoNeutraldecomposition products in spark breakdown of SF
6rdquo IEEE
Transactions on Electrical Insulation vol EI-21 no 2 pp 111ndash120 1986
[7] W T Tsai ldquoThe decomposition products of sulfur hexafluoride(SF6) reviews of environmental and health risk analysisrdquo
Journal of Fluorine Chemistry vol 128 no 11 pp 1345ndash13522007
[8] L Vial AM Casanovas I Coll and J Casanovas ldquoDecomposi-tion products from negative and 50Hz ac corona discharges incompressed SF
6and SF
6N2(10 90) mixtures Effect of water
vapour added to the gasrdquo Journal of Physics D vol 32 no 14pp 1681ndash1692 1999
[9] C T Dervos and P Vassiliou ldquoSulfur hexafluoride (SF6) Global
environmental effects and toxic byproduct formationrdquo Journalof the Air and Waste Management Association vol 50 no 1 pp137ndash141 2000
[10] E Duffour ldquoMolecular dynamic simulations of the collisionbetween copper ions SF
6molecules and a polyethylene surface
a study of decomposition products and an evaluation of the self-diffusion coefficientsrdquoMacromolecularTheory and Simulationsvol 19 no 2-3 pp 88ndash99 2010
[11] J I Baumbach P Pilzecker and E Trindade ldquoMonitoring ofcircuit breakers using ion mobility spectrometry to detect SF
6-
decompositionrdquo International Journal for Ion Mobility Spec-trometry vol 2 no 1 pp 35ndash39 1999
[12] R Kurte C Beyer HMHeise andD Klockow ldquoApplication ofinfrared spectroscopy to monitoring gas insulated high-voltageequipment electrode material-dependent SF
6decompositionrdquo
Journal of Nanomaterials 7
Analytical and Bioanalytical Chemistry vol 373 no 7 pp 639ndash646 2002
[13] W Ding R Hayashi K Ochi et al ldquoAnalysis of PD-generatedSF6decomposition gases adsorbed on carbon nanotubesrdquo IEEE
Transactions on Dielectrics and Electrical Insulation vol 13 no6 pp 1200ndash1207 2006
[14] J SinghAMukherjee S K Sengupta J ImGW Peterson andJ E Whitten ldquoSulfur dioxide and nitrogen dioxide adsorptionon zinc oxide and zirconium hydroxide nanoparticles and theeffect on photoluminescencerdquo Applied Surface Science vol 258no 15 pp 5778ndash5785 2012
[15] BWang L F Zhu Y H Yang N S Xu and GW Yang ldquoFabri-cation of a SnO
2nanowire gas sensor and sensor performance
for hydrogenrdquo Journal of Physical Chemistry C vol 112 no 17pp 6643ndash6647 2008
[16] J Gong Y Li Z Hu Z Zhou and Y Deng ldquoUltrasensitive NH3
gas sensor from polyaniline nanograin enchased TiO2fibersrdquo
Journal of Physical Chemistry C vol 114 no 21 pp 9970ndash99742010
[17] X Liu J Zhang X Guo S Wu and S Wang ldquoPorous 120572-Fe2O3decorated byAunanoparticles and their enhanced sensor
performancerdquo Nanotechnology vol 21 no 9 Article ID 0955012010
[18] B Cao J Chen X Tang and W Zhou ldquoGrowth of monoclinicWO3nanowire array for highly sensitive NO
2detectionrdquo
Journal of Materials Chemistry vol 19 no 16 pp 2323ndash23272009
[19] S E Moon H Y Lee J Park et al ldquoLow power consumptionand high sensitivity carbon monoxide gas sensor using indiumoxide nanowirerdquo Journal of Nanoscience and Nanotechnologyvol 10 no 5 pp 3189ndash3192 2010
[20] WZeng T Liu ZWang S TsukimotoM Saito andY IkuharaldquoSelective detection of formaldehyde gas using a Cd-DopedTiO2-SnO2sensorrdquo Sensors vol 9 no 11 pp 9029ndash9038 2009
[21] M Chen Z Wang D Han F Gu and G Guo ldquoPorous ZnOpolygonal nanoflakes synthesis use in high-sensitivity NO
2gas
sensor and proposed mechanism of gas sensingrdquo Journal ofPhysical Chemistry C vol 115 no 26 pp 12763ndash12773 2011
[22] E Oh H Y Choi S H Jung et al ldquoHigh-performance NO2gas
sensor based on ZnO nanorod grown by ultrasonic irradiationrdquoSensors and Actuators B vol 141 no 1 pp 239ndash243 2009
[23] K Zheng L Gu D Sun X Mo and G Chen ldquoThe propertiesof ethanol gas sensor based on Ti doped ZnO nanotetrapodsrdquoMaterials Science and Engineering B vol 166 no 1 pp 104ndash1072010
[24] A Wei L-H Pan X-C Dong and W Huang ldquoRoom-temperature NH
3gas sensor based on hydrothermally grown
ZnO nanorodsrdquo Chinese Physics Letters vol 28 no 8 pp 702ndash706 2011
[25] CWen Y JuW Li et al ldquoCarbon dioxide gas sensor using SAWdevice based on ZnO filmrdquo Applied Mechanics and Materialsvol 135-136 pp 347ndash352 2012
[26] O Lupan G Chai and L Chow ldquoNovel hydrogen gas sensorbased on single ZnOnanorodrdquoMicroelectronic Engineering vol85 no 11 pp 2220ndash2225 2008
[27] W Zeng T Liu and ZWang ldquoEnhanced gas sensing propertiesby SnO
2nanosphere functionalized TiO
2nanobeltsrdquo Journal of
Materials Chemistry vol 22 no 8 pp 3544ndash3548 2012[28] J Kim andK Yong ldquoMechanism study of ZnOnanorod-bundle
sensors for H2S gas sensingrdquo Journal of Physical Chemistry C
vol 115 no 15 pp 7218ndash7224 2011
[29] D Velasco-Arias D Dıaz P Santiago-Jacinto G Rodrıguez-Gattorno A Vazquez-Olmos and S E Castillo-Blum ldquoDirectinteraction of colloidal nanostructured ZnO and SnO
2withNO
and SO2rdquo Journal of Nanoscience andNanotechnology vol 8 no
12 pp 6389ndash6397 2008[30] Q Qi T Zhang Q Yu et al ldquoProperties of humidity sensing
ZnO nanorods-base sensor fabricated by screen-printingrdquo Sen-sors and Actuators B vol 133 no 2 pp 638ndash643 2008
[31] M-W Ahn K-S Park J-H Heo et al ldquoGas sensing propertiesof defect-controlled ZnO-nanowire gas sensorrdquo Applied PhysicsLetters vol 93 no 26 Article ID 263103 2008
[32] M W Ahn K S Park J H Heo D W Kim K J Choi and JG Park ldquoOn-chip fabrication of ZnO-nanowire gas sensor withhigh gas sensitivityrdquo Sensors and Actuators B vol 138 no 1 pp168ndash173 2009
[33] J Zhang S Wang M Xu et al ldquoHierarchically porous ZnOarchitectures for gas sensor applicationrdquo Crystal Growth andDesign vol 9 no 8 pp 3532ndash3537 2009
[34] Z Yuan X Jiaqiang X Qun L Hui P Qingyi and XPengcheng ldquoBrush-like hierarchical zno nanostructures syn-thesis photoluminescence and gas sensor propertiesrdquo Journalof Physical Chemistry C vol 113 no 9 pp 3430ndash3435 2009
[35] J Zhang X Liu SWu BCao and S Zheng ldquoOne-pot synthesisof Au-supported ZnO nanoplates with enhanced gas sensorperformancerdquo Sensors and Actuators B vol 169 pp 61ndash66 2012
Figure 7 The response and recovery behaviors of the sensor to10120583LL of SO
2 SOF
2 and SO
2F2
Figures 3(a) and 3(b) are typical low-resolution andhigh-resolution FESEM images of the prepared flower-likeZnO nanorods samples synthesized with the hydrothermalmethod The nanoparticles have a high uniform flower-likebundle structure and self-assemble into flowers The averagelength of ZnO nanorods is about 400 nmwith an aspect ratioof 4 1
32 Gas Sensing Properties and Sensing Mechanism The gassensing performances of metal oxide semiconductor gassensor are dominantly influenced by working condition Gas
sensing experiments are performed with an intelligent gasdetecting system at different operating temperatures to findout the optimum working temperature Figure 4 shows thegas responses of the prepared flower-like ZnO nanorods gassensor against 50 120583LL of SF
6compositions as a function of
operating temperature which ranges from 120∘C to 420∘CAs seen in Figure 4 the measured gas response curves havea common change trend in which gas response increasesfirstly with rising operating temperature and reaches themaximum and then decreases with an continuous increaseof the operating temperature
This behavior can be understood by a dynamic equi-librium mechanism between gas adsorption and desorptionprocess of gasmolecule on the surface of ZnOor other similarsemiconducting metal oxides In the beginning the rate ofgas adsorption is much higher than that of desorption andthe amount of net adsorbed gas increases as the operatingtemperature rises It would reach a saturated adsorption stateand maintain a dynamic balance at the constant operatingtemperature With a sequential increase of the operatingtemperature the balancewill be broken and it changes to a netdesorption process which ultimately results in a decreasinggas response As shown in Figure 4 the optimal operatingtemperatures of the sensor to 50 120583LL of SO
2 SOF
2 and
SO2F2are 250 300 and 300∘C with gas response of minus3344
minus1247 and minus1806 respectively which are applied in all thefollowing investigations in this paper
At their optimal operating temperatures we performedthe gas responses of the prepared plane flower-like ZnO gassensor against different concentrations of SO
2 SOF
2 and
SO2F2 Figure 5 shows the relationship between gas responses
and 10 20 30 40 50 and 100 120583LL of SO2 SOF2 and SO
2F2
respectively The gas response measured is manifested topersistently increase with a rising gas concentration At thesame level of gas concentration the gas response values ofthe sensor to the three targeted gases decrease in the orderof SO
2 SO2F2 and SOF
2
If the gas response curve is linear or quasilinear thesensor can be applied to engineering application in practiceTherefore based on the linear fitting tool in Origin softwarelinear characteristics of the prepared sensor to SO
2 SO2F2
and SOF2were discussed Figure 6 shows the linear cali-
bration curves of the sensor to SO2 SO2F2 and SOF
2with
gas concentrations in the range of 10ndash100 120583LL As seen inFigure 6 all the three gas response curves meet highly linearwith gas concentration and the linear correlation coefficient119877
2 for SO2 SO2F2 and SOF
2is suggested to be about
0982 0979 and 0963 respectively Such a higher lineardependence indicates that our prepared flower-like ZnO gassensor can be used as promising materials for detecting SF
6
decompositions such as SO2 SO2F2 and SOF
2
Response time and recovery time are other two key indi-cators to evaluate gas sensor performances Figure 7 showsthe response and recovery characteristic of the preparedsensor to 10 120583LL of SO
2 SO2F2 and SOF
2with the sensor
working at its optimum operating temperature As shownin Figure 7 the response times for 10 120583LL of SO
2 SO2F2
and SOF2are about 21 13 and 10 s and correspondingly
the recovery times are about 45 32 and 17 s respectively
Journal of Nanomaterials 5
0 500 1000 1500 2000 2500 3000
0
1
2
3
4
5
6
Time (s)
Volta
ge (V
)
minus1
SO2
10 120583LL20 120583LL
30 120583LL40 120583LL 50 120583LL
100 120583LL
Figure 8The response and recovery behaviors of the sensor to SO2
0 5 10 15 20 25 300
Gas
resp
onse
Time (days)SO2
SO2F2
SOF2
minus10
minus20
minus30
minus40
minus50
Figure 9 The stability and repeatability of the sensor against50 120583LL of SO
2 SO2F2 and SOF
2
Such rapid response and recovery characteristic could beascribed to the structure of the prepared flower-like sensorwhich has a much bigger specific surface area than otherconventional sensing structures provides a larger adsorptionarea and increases the amount of gas molecules adsorbedon the surface Those advantages increase the rate of chargecarriers and facilitate the movement of carriers through thebarriers consequently fast response and response propertyare observed
The response and recovery behaviors versus SO2with
concentration at 10 20 30 40 50 and 100120583LL are shown inFigure 8 With the concentration of detected gas increasing
the gas response amplitude increases apparently neverthelessthe response and recovery property changes slightly whichindicates a very good and satisfying reproducibility of pre-pared sensor against the decompositions Figure 9 shows thelong-term stability and repeatability of the sensor against50120583LL of SO
2 SO2F2 and SOF
2 One can clearly see in
Figure 9 that the gas response changes slightly and keepsat a nearly constant value during the long experimentalcycles which confirms the excellent longtime stability andrepeatability of the prepared flower-like ZnO nanorods gassensor for detecting SO
2 SO2F2 and SOF
2
For most metal oxide semiconductor gas sensors such aszinc oxide tin oxide titanium oxide ferric oxide and indiumoxide the sensing properties are dominantly controlled by thechange of electrical resistance [27] which is fundamentallyattributed to the chemical adsorption and desorption processof gas molecules on sensing surface of the sensor
It is well known to all that zinc oxide is a typical n-type semiconducting material and there exist many oxygenvacancies in the crystal lattices [28ndash30] where various kindsof oxygen could be adsorbedThe species of adsorbed oxygenare closely related to the ambient temperature [31] At roomtemperature oxygen is likely to be adsorbed on ZnO surfaceor grain boundaries with a typical physical adsorption modeAnd it would turn into chemical adsorption by thermalexcitation or electric excitation with certain energy
As shown in Figure 10(a) oxygenwould capture electronsand form a depletion region on the surface area which resultsin a decrease in the concentration of charge carrier and elec-tron mobility thus gas sensor shows a higher electrical resis-tance Figure 10(b) illustrates the gas sensing process of SO
2
as an example exploring the gas sensing mechanism of theprepared sensor detecting SF
6decompositionsWhen flower-
like ZnO nanorods are reducing gas ambient at moderatetemperature (such as in certain concentration of SO
2 SO2F2
and SOF2) the reducing gas reacts with chemical adsorbed
oxygen and then trapped electrons would be released backinto ZnO surface Electrons released from chemical adsorbedoxygen would reduce the height of barriers in the depletionregion and increase the number of charge carriers [32 33]which promotes the movements of charge carriers betweenconduction band and valence band and eventually increasesthe electrical conductivity of the sensor [34 35]
With temperature rising chemical adsorbed oxygenexists in various forms namely O
2adsminus Oads
minus and Oads2minus as
shown in the following reaction equations
O2gas 997888rarr O
2ads O2ads + e
minus997888rarr O
2adsminus
O2adsminus+ eminus 997888rarr 2Oads
minus Oadsminus+ eminus 997888rarr Oads
2minus
(1)
As mentioned above the state of adsorbed oxygen ismainly determined by the ambient temperature At lowerexperimental temperatures oxygen dominantly exists inthe form of a ldquomolecular ionrdquo O
2adsminus and transfers into
ldquoatomic ionrdquo Oadsminus and Oads
2minus with a further rising operatingtemperature Experimental results indicate that the transitiontemperature for oxygen from ldquomolecular ionrdquo to ldquoatomic ionrdquois about 450sim500K As performed in Figure 4 the optimum
6 Journal of Nanomaterials
ZnO surface
O atomElectron
O2ads + 119890minus rarr O2adsminus
O ads minus2adsminus + 119890minus rarr 2OO ads 2minusads minus + 119890minus rarr O
(a) Oxygen adsorbed on ZnO surface
O atomElectron
ZnO surface
2SO2 + O2adsminus rarr 2SO2minusO + 119890minus
2 ads 2SO + O minus rarr SO minusO + 119890minus
SO2 + Oads 2minus rarr SO2minusO + 2119890minus
(b) SO2 gas sensing on ZnO surface ZnO
Figure 10 Schematic plot illustrating the sensing mechanism of prepared sensor to SO2
working temperatures for SO2 SO2F2 and SOF
2are about
250 300 and 300∘C respectivelyThus we draw a conclusionthat the sensing behavior of the prepared sensor to SO
2gas
may belong to the ldquomolecular ionrdquo reaction pattern while itis an ldquoatomic ionrdquo gas response mode for SO
2F2and SOF
2
4 Conclusions
In summary Flower-like ZnO nanorods have been success-fully synthesized and characterized by XRD and FESEMTheoptimum operating temperatures of the prepared sensor toSO2 SO2F2 and SOF
2are about 250 300 and 300∘C The
response (recovery) time of the sensor to 10 120583LL of SO2
SO2F2 and SOF
2is 21 (45) 13 (32) and 10 (17) s respectively
Especially the flower-like ZnO nanorods gas sensor showshigh linearity to SO
2 SO2F2 and SOF
2at the range of 10ndash
100 120583LL with excellent linear correlation coefficient 1198772 at0982 0979 and 0963 separately These findings demon-strate that our prepared flower-like ZnO nanorods have someexcellent potential advantages for using as gas sensors todetect and online monitor the SF
6decompositions such as
SO2 SOF
2 and SO
2F2in practice although further studies
are still needed
References
[1] J Tang F Liu X X Zhang Q H Meng and J B Zhou ldquoPartialdischarge recognition through an analysis of SF
6decomposition
products Part 1 decomposition characteristics of SF6under four
different partial dischargesrdquo IEEE Transactions on Dielectricsand Electrical Insulation vol 19 no 1 pp 29ndash36 2012
[2] M Shih W J Lee and C Y Chen ldquoDecomposition of SF6
and H2S mixture in radio frequency plasma environmentrdquo
Industrial and Engineering Chemistry Research vol 42 no 13pp 2906ndash2912 2003
[3] J Tang F Liu X X Zhang Q H Meng and J G Tao ldquoPartialdischarge recognition through an analysis of SF
6decomposition
products part 2 feature extraction and decision tree-based
pattern recognitionrdquo IEEE Transactions on Dielectrics andElectrical Insulation vol 19 no 1 pp 37ndash44 2012
[4] R J Van Brunt and J T Herron ldquoFundamental processes of SF6
decomposition and oxidation in glow and corona dischargesrdquoIEEE Transactions on Electrical Insulation vol 25 no 1 pp 75ndash94 1990
[5] M Shih W J Lee C H Tsai P J Tsai and C Y ChenldquoDecomposition of SF
6in an RF plasma environmentrdquo Journal
of the Air andWaste Management Association vol 52 no 11 pp1274ndash1280 2002
[6] I Sauers H W Ellis and L G Christophorou ldquoNeutraldecomposition products in spark breakdown of SF
6rdquo IEEE
Transactions on Electrical Insulation vol EI-21 no 2 pp 111ndash120 1986
[7] W T Tsai ldquoThe decomposition products of sulfur hexafluoride(SF6) reviews of environmental and health risk analysisrdquo
Journal of Fluorine Chemistry vol 128 no 11 pp 1345ndash13522007
[8] L Vial AM Casanovas I Coll and J Casanovas ldquoDecomposi-tion products from negative and 50Hz ac corona discharges incompressed SF
6and SF
6N2(10 90) mixtures Effect of water
vapour added to the gasrdquo Journal of Physics D vol 32 no 14pp 1681ndash1692 1999
[9] C T Dervos and P Vassiliou ldquoSulfur hexafluoride (SF6) Global
environmental effects and toxic byproduct formationrdquo Journalof the Air and Waste Management Association vol 50 no 1 pp137ndash141 2000
[10] E Duffour ldquoMolecular dynamic simulations of the collisionbetween copper ions SF
6molecules and a polyethylene surface
a study of decomposition products and an evaluation of the self-diffusion coefficientsrdquoMacromolecularTheory and Simulationsvol 19 no 2-3 pp 88ndash99 2010
[11] J I Baumbach P Pilzecker and E Trindade ldquoMonitoring ofcircuit breakers using ion mobility spectrometry to detect SF
6-
decompositionrdquo International Journal for Ion Mobility Spec-trometry vol 2 no 1 pp 35ndash39 1999
[12] R Kurte C Beyer HMHeise andD Klockow ldquoApplication ofinfrared spectroscopy to monitoring gas insulated high-voltageequipment electrode material-dependent SF
6decompositionrdquo
Journal of Nanomaterials 7
Analytical and Bioanalytical Chemistry vol 373 no 7 pp 639ndash646 2002
[13] W Ding R Hayashi K Ochi et al ldquoAnalysis of PD-generatedSF6decomposition gases adsorbed on carbon nanotubesrdquo IEEE
Transactions on Dielectrics and Electrical Insulation vol 13 no6 pp 1200ndash1207 2006
[14] J SinghAMukherjee S K Sengupta J ImGW Peterson andJ E Whitten ldquoSulfur dioxide and nitrogen dioxide adsorptionon zinc oxide and zirconium hydroxide nanoparticles and theeffect on photoluminescencerdquo Applied Surface Science vol 258no 15 pp 5778ndash5785 2012
[15] BWang L F Zhu Y H Yang N S Xu and GW Yang ldquoFabri-cation of a SnO
2nanowire gas sensor and sensor performance
for hydrogenrdquo Journal of Physical Chemistry C vol 112 no 17pp 6643ndash6647 2008
[16] J Gong Y Li Z Hu Z Zhou and Y Deng ldquoUltrasensitive NH3
gas sensor from polyaniline nanograin enchased TiO2fibersrdquo
Journal of Physical Chemistry C vol 114 no 21 pp 9970ndash99742010
[17] X Liu J Zhang X Guo S Wu and S Wang ldquoPorous 120572-Fe2O3decorated byAunanoparticles and their enhanced sensor
performancerdquo Nanotechnology vol 21 no 9 Article ID 0955012010
[18] B Cao J Chen X Tang and W Zhou ldquoGrowth of monoclinicWO3nanowire array for highly sensitive NO
2detectionrdquo
Journal of Materials Chemistry vol 19 no 16 pp 2323ndash23272009
[19] S E Moon H Y Lee J Park et al ldquoLow power consumptionand high sensitivity carbon monoxide gas sensor using indiumoxide nanowirerdquo Journal of Nanoscience and Nanotechnologyvol 10 no 5 pp 3189ndash3192 2010
[20] WZeng T Liu ZWang S TsukimotoM Saito andY IkuharaldquoSelective detection of formaldehyde gas using a Cd-DopedTiO2-SnO2sensorrdquo Sensors vol 9 no 11 pp 9029ndash9038 2009
[21] M Chen Z Wang D Han F Gu and G Guo ldquoPorous ZnOpolygonal nanoflakes synthesis use in high-sensitivity NO
2gas
sensor and proposed mechanism of gas sensingrdquo Journal ofPhysical Chemistry C vol 115 no 26 pp 12763ndash12773 2011
[22] E Oh H Y Choi S H Jung et al ldquoHigh-performance NO2gas
sensor based on ZnO nanorod grown by ultrasonic irradiationrdquoSensors and Actuators B vol 141 no 1 pp 239ndash243 2009
[23] K Zheng L Gu D Sun X Mo and G Chen ldquoThe propertiesof ethanol gas sensor based on Ti doped ZnO nanotetrapodsrdquoMaterials Science and Engineering B vol 166 no 1 pp 104ndash1072010
[24] A Wei L-H Pan X-C Dong and W Huang ldquoRoom-temperature NH
3gas sensor based on hydrothermally grown
ZnO nanorodsrdquo Chinese Physics Letters vol 28 no 8 pp 702ndash706 2011
[25] CWen Y JuW Li et al ldquoCarbon dioxide gas sensor using SAWdevice based on ZnO filmrdquo Applied Mechanics and Materialsvol 135-136 pp 347ndash352 2012
[26] O Lupan G Chai and L Chow ldquoNovel hydrogen gas sensorbased on single ZnOnanorodrdquoMicroelectronic Engineering vol85 no 11 pp 2220ndash2225 2008
[27] W Zeng T Liu and ZWang ldquoEnhanced gas sensing propertiesby SnO
2nanosphere functionalized TiO
2nanobeltsrdquo Journal of
Materials Chemistry vol 22 no 8 pp 3544ndash3548 2012[28] J Kim andK Yong ldquoMechanism study of ZnOnanorod-bundle
sensors for H2S gas sensingrdquo Journal of Physical Chemistry C
vol 115 no 15 pp 7218ndash7224 2011
[29] D Velasco-Arias D Dıaz P Santiago-Jacinto G Rodrıguez-Gattorno A Vazquez-Olmos and S E Castillo-Blum ldquoDirectinteraction of colloidal nanostructured ZnO and SnO
2withNO
and SO2rdquo Journal of Nanoscience andNanotechnology vol 8 no
12 pp 6389ndash6397 2008[30] Q Qi T Zhang Q Yu et al ldquoProperties of humidity sensing
ZnO nanorods-base sensor fabricated by screen-printingrdquo Sen-sors and Actuators B vol 133 no 2 pp 638ndash643 2008
[31] M-W Ahn K-S Park J-H Heo et al ldquoGas sensing propertiesof defect-controlled ZnO-nanowire gas sensorrdquo Applied PhysicsLetters vol 93 no 26 Article ID 263103 2008
[32] M W Ahn K S Park J H Heo D W Kim K J Choi and JG Park ldquoOn-chip fabrication of ZnO-nanowire gas sensor withhigh gas sensitivityrdquo Sensors and Actuators B vol 138 no 1 pp168ndash173 2009
[33] J Zhang S Wang M Xu et al ldquoHierarchically porous ZnOarchitectures for gas sensor applicationrdquo Crystal Growth andDesign vol 9 no 8 pp 3532ndash3537 2009
[34] Z Yuan X Jiaqiang X Qun L Hui P Qingyi and XPengcheng ldquoBrush-like hierarchical zno nanostructures syn-thesis photoluminescence and gas sensor propertiesrdquo Journalof Physical Chemistry C vol 113 no 9 pp 3430ndash3435 2009
[35] J Zhang X Liu SWu BCao and S Zheng ldquoOne-pot synthesisof Au-supported ZnO nanoplates with enhanced gas sensorperformancerdquo Sensors and Actuators B vol 169 pp 61ndash66 2012
Figure 8The response and recovery behaviors of the sensor to SO2
0 5 10 15 20 25 300
Gas
resp
onse
Time (days)SO2
SO2F2
SOF2
minus10
minus20
minus30
minus40
minus50
Figure 9 The stability and repeatability of the sensor against50 120583LL of SO
2 SO2F2 and SOF
2
Such rapid response and recovery characteristic could beascribed to the structure of the prepared flower-like sensorwhich has a much bigger specific surface area than otherconventional sensing structures provides a larger adsorptionarea and increases the amount of gas molecules adsorbedon the surface Those advantages increase the rate of chargecarriers and facilitate the movement of carriers through thebarriers consequently fast response and response propertyare observed
The response and recovery behaviors versus SO2with
concentration at 10 20 30 40 50 and 100120583LL are shown inFigure 8 With the concentration of detected gas increasing
the gas response amplitude increases apparently neverthelessthe response and recovery property changes slightly whichindicates a very good and satisfying reproducibility of pre-pared sensor against the decompositions Figure 9 shows thelong-term stability and repeatability of the sensor against50120583LL of SO
2 SO2F2 and SOF
2 One can clearly see in
Figure 9 that the gas response changes slightly and keepsat a nearly constant value during the long experimentalcycles which confirms the excellent longtime stability andrepeatability of the prepared flower-like ZnO nanorods gassensor for detecting SO
2 SO2F2 and SOF
2
For most metal oxide semiconductor gas sensors such aszinc oxide tin oxide titanium oxide ferric oxide and indiumoxide the sensing properties are dominantly controlled by thechange of electrical resistance [27] which is fundamentallyattributed to the chemical adsorption and desorption processof gas molecules on sensing surface of the sensor
It is well known to all that zinc oxide is a typical n-type semiconducting material and there exist many oxygenvacancies in the crystal lattices [28ndash30] where various kindsof oxygen could be adsorbedThe species of adsorbed oxygenare closely related to the ambient temperature [31] At roomtemperature oxygen is likely to be adsorbed on ZnO surfaceor grain boundaries with a typical physical adsorption modeAnd it would turn into chemical adsorption by thermalexcitation or electric excitation with certain energy
As shown in Figure 10(a) oxygenwould capture electronsand form a depletion region on the surface area which resultsin a decrease in the concentration of charge carrier and elec-tron mobility thus gas sensor shows a higher electrical resis-tance Figure 10(b) illustrates the gas sensing process of SO
2
as an example exploring the gas sensing mechanism of theprepared sensor detecting SF
6decompositionsWhen flower-
like ZnO nanorods are reducing gas ambient at moderatetemperature (such as in certain concentration of SO
2 SO2F2
and SOF2) the reducing gas reacts with chemical adsorbed
oxygen and then trapped electrons would be released backinto ZnO surface Electrons released from chemical adsorbedoxygen would reduce the height of barriers in the depletionregion and increase the number of charge carriers [32 33]which promotes the movements of charge carriers betweenconduction band and valence band and eventually increasesthe electrical conductivity of the sensor [34 35]
With temperature rising chemical adsorbed oxygenexists in various forms namely O
2adsminus Oads
minus and Oads2minus as
shown in the following reaction equations
O2gas 997888rarr O
2ads O2ads + e
minus997888rarr O
2adsminus
O2adsminus+ eminus 997888rarr 2Oads
minus Oadsminus+ eminus 997888rarr Oads
2minus
(1)
As mentioned above the state of adsorbed oxygen ismainly determined by the ambient temperature At lowerexperimental temperatures oxygen dominantly exists inthe form of a ldquomolecular ionrdquo O
2adsminus and transfers into
ldquoatomic ionrdquo Oadsminus and Oads
2minus with a further rising operatingtemperature Experimental results indicate that the transitiontemperature for oxygen from ldquomolecular ionrdquo to ldquoatomic ionrdquois about 450sim500K As performed in Figure 4 the optimum
6 Journal of Nanomaterials
ZnO surface
O atomElectron
O2ads + 119890minus rarr O2adsminus
O ads minus2adsminus + 119890minus rarr 2OO ads 2minusads minus + 119890minus rarr O
(a) Oxygen adsorbed on ZnO surface
O atomElectron
ZnO surface
2SO2 + O2adsminus rarr 2SO2minusO + 119890minus
2 ads 2SO + O minus rarr SO minusO + 119890minus
SO2 + Oads 2minus rarr SO2minusO + 2119890minus
(b) SO2 gas sensing on ZnO surface ZnO
Figure 10 Schematic plot illustrating the sensing mechanism of prepared sensor to SO2
working temperatures for SO2 SO2F2 and SOF
2are about
250 300 and 300∘C respectivelyThus we draw a conclusionthat the sensing behavior of the prepared sensor to SO
2gas
may belong to the ldquomolecular ionrdquo reaction pattern while itis an ldquoatomic ionrdquo gas response mode for SO
2F2and SOF
2
4 Conclusions
In summary Flower-like ZnO nanorods have been success-fully synthesized and characterized by XRD and FESEMTheoptimum operating temperatures of the prepared sensor toSO2 SO2F2 and SOF
2are about 250 300 and 300∘C The
response (recovery) time of the sensor to 10 120583LL of SO2
SO2F2 and SOF
2is 21 (45) 13 (32) and 10 (17) s respectively
Especially the flower-like ZnO nanorods gas sensor showshigh linearity to SO
2 SO2F2 and SOF
2at the range of 10ndash
100 120583LL with excellent linear correlation coefficient 1198772 at0982 0979 and 0963 separately These findings demon-strate that our prepared flower-like ZnO nanorods have someexcellent potential advantages for using as gas sensors todetect and online monitor the SF
6decompositions such as
SO2 SOF
2 and SO
2F2in practice although further studies
are still needed
References
[1] J Tang F Liu X X Zhang Q H Meng and J B Zhou ldquoPartialdischarge recognition through an analysis of SF
6decomposition
products Part 1 decomposition characteristics of SF6under four
different partial dischargesrdquo IEEE Transactions on Dielectricsand Electrical Insulation vol 19 no 1 pp 29ndash36 2012
[2] M Shih W J Lee and C Y Chen ldquoDecomposition of SF6
and H2S mixture in radio frequency plasma environmentrdquo
Industrial and Engineering Chemistry Research vol 42 no 13pp 2906ndash2912 2003
[3] J Tang F Liu X X Zhang Q H Meng and J G Tao ldquoPartialdischarge recognition through an analysis of SF
6decomposition
products part 2 feature extraction and decision tree-based
pattern recognitionrdquo IEEE Transactions on Dielectrics andElectrical Insulation vol 19 no 1 pp 37ndash44 2012
[4] R J Van Brunt and J T Herron ldquoFundamental processes of SF6
decomposition and oxidation in glow and corona dischargesrdquoIEEE Transactions on Electrical Insulation vol 25 no 1 pp 75ndash94 1990
[5] M Shih W J Lee C H Tsai P J Tsai and C Y ChenldquoDecomposition of SF
6in an RF plasma environmentrdquo Journal
of the Air andWaste Management Association vol 52 no 11 pp1274ndash1280 2002
[6] I Sauers H W Ellis and L G Christophorou ldquoNeutraldecomposition products in spark breakdown of SF
6rdquo IEEE
Transactions on Electrical Insulation vol EI-21 no 2 pp 111ndash120 1986
[7] W T Tsai ldquoThe decomposition products of sulfur hexafluoride(SF6) reviews of environmental and health risk analysisrdquo
Journal of Fluorine Chemistry vol 128 no 11 pp 1345ndash13522007
[8] L Vial AM Casanovas I Coll and J Casanovas ldquoDecomposi-tion products from negative and 50Hz ac corona discharges incompressed SF
6and SF
6N2(10 90) mixtures Effect of water
vapour added to the gasrdquo Journal of Physics D vol 32 no 14pp 1681ndash1692 1999
[9] C T Dervos and P Vassiliou ldquoSulfur hexafluoride (SF6) Global
environmental effects and toxic byproduct formationrdquo Journalof the Air and Waste Management Association vol 50 no 1 pp137ndash141 2000
[10] E Duffour ldquoMolecular dynamic simulations of the collisionbetween copper ions SF
6molecules and a polyethylene surface
a study of decomposition products and an evaluation of the self-diffusion coefficientsrdquoMacromolecularTheory and Simulationsvol 19 no 2-3 pp 88ndash99 2010
[11] J I Baumbach P Pilzecker and E Trindade ldquoMonitoring ofcircuit breakers using ion mobility spectrometry to detect SF
6-
decompositionrdquo International Journal for Ion Mobility Spec-trometry vol 2 no 1 pp 35ndash39 1999
[12] R Kurte C Beyer HMHeise andD Klockow ldquoApplication ofinfrared spectroscopy to monitoring gas insulated high-voltageequipment electrode material-dependent SF
6decompositionrdquo
Journal of Nanomaterials 7
Analytical and Bioanalytical Chemistry vol 373 no 7 pp 639ndash646 2002
[13] W Ding R Hayashi K Ochi et al ldquoAnalysis of PD-generatedSF6decomposition gases adsorbed on carbon nanotubesrdquo IEEE
Transactions on Dielectrics and Electrical Insulation vol 13 no6 pp 1200ndash1207 2006
[14] J SinghAMukherjee S K Sengupta J ImGW Peterson andJ E Whitten ldquoSulfur dioxide and nitrogen dioxide adsorptionon zinc oxide and zirconium hydroxide nanoparticles and theeffect on photoluminescencerdquo Applied Surface Science vol 258no 15 pp 5778ndash5785 2012
[15] BWang L F Zhu Y H Yang N S Xu and GW Yang ldquoFabri-cation of a SnO
2nanowire gas sensor and sensor performance
for hydrogenrdquo Journal of Physical Chemistry C vol 112 no 17pp 6643ndash6647 2008
[16] J Gong Y Li Z Hu Z Zhou and Y Deng ldquoUltrasensitive NH3
gas sensor from polyaniline nanograin enchased TiO2fibersrdquo
Journal of Physical Chemistry C vol 114 no 21 pp 9970ndash99742010
[17] X Liu J Zhang X Guo S Wu and S Wang ldquoPorous 120572-Fe2O3decorated byAunanoparticles and their enhanced sensor
performancerdquo Nanotechnology vol 21 no 9 Article ID 0955012010
[18] B Cao J Chen X Tang and W Zhou ldquoGrowth of monoclinicWO3nanowire array for highly sensitive NO
2detectionrdquo
Journal of Materials Chemistry vol 19 no 16 pp 2323ndash23272009
[19] S E Moon H Y Lee J Park et al ldquoLow power consumptionand high sensitivity carbon monoxide gas sensor using indiumoxide nanowirerdquo Journal of Nanoscience and Nanotechnologyvol 10 no 5 pp 3189ndash3192 2010
[20] WZeng T Liu ZWang S TsukimotoM Saito andY IkuharaldquoSelective detection of formaldehyde gas using a Cd-DopedTiO2-SnO2sensorrdquo Sensors vol 9 no 11 pp 9029ndash9038 2009
[21] M Chen Z Wang D Han F Gu and G Guo ldquoPorous ZnOpolygonal nanoflakes synthesis use in high-sensitivity NO
2gas
sensor and proposed mechanism of gas sensingrdquo Journal ofPhysical Chemistry C vol 115 no 26 pp 12763ndash12773 2011
[22] E Oh H Y Choi S H Jung et al ldquoHigh-performance NO2gas
sensor based on ZnO nanorod grown by ultrasonic irradiationrdquoSensors and Actuators B vol 141 no 1 pp 239ndash243 2009
[23] K Zheng L Gu D Sun X Mo and G Chen ldquoThe propertiesof ethanol gas sensor based on Ti doped ZnO nanotetrapodsrdquoMaterials Science and Engineering B vol 166 no 1 pp 104ndash1072010
[24] A Wei L-H Pan X-C Dong and W Huang ldquoRoom-temperature NH
3gas sensor based on hydrothermally grown
ZnO nanorodsrdquo Chinese Physics Letters vol 28 no 8 pp 702ndash706 2011
[25] CWen Y JuW Li et al ldquoCarbon dioxide gas sensor using SAWdevice based on ZnO filmrdquo Applied Mechanics and Materialsvol 135-136 pp 347ndash352 2012
[26] O Lupan G Chai and L Chow ldquoNovel hydrogen gas sensorbased on single ZnOnanorodrdquoMicroelectronic Engineering vol85 no 11 pp 2220ndash2225 2008
[27] W Zeng T Liu and ZWang ldquoEnhanced gas sensing propertiesby SnO
2nanosphere functionalized TiO
2nanobeltsrdquo Journal of
Materials Chemistry vol 22 no 8 pp 3544ndash3548 2012[28] J Kim andK Yong ldquoMechanism study of ZnOnanorod-bundle
sensors for H2S gas sensingrdquo Journal of Physical Chemistry C
vol 115 no 15 pp 7218ndash7224 2011
[29] D Velasco-Arias D Dıaz P Santiago-Jacinto G Rodrıguez-Gattorno A Vazquez-Olmos and S E Castillo-Blum ldquoDirectinteraction of colloidal nanostructured ZnO and SnO
2withNO
and SO2rdquo Journal of Nanoscience andNanotechnology vol 8 no
12 pp 6389ndash6397 2008[30] Q Qi T Zhang Q Yu et al ldquoProperties of humidity sensing
ZnO nanorods-base sensor fabricated by screen-printingrdquo Sen-sors and Actuators B vol 133 no 2 pp 638ndash643 2008
[31] M-W Ahn K-S Park J-H Heo et al ldquoGas sensing propertiesof defect-controlled ZnO-nanowire gas sensorrdquo Applied PhysicsLetters vol 93 no 26 Article ID 263103 2008
[32] M W Ahn K S Park J H Heo D W Kim K J Choi and JG Park ldquoOn-chip fabrication of ZnO-nanowire gas sensor withhigh gas sensitivityrdquo Sensors and Actuators B vol 138 no 1 pp168ndash173 2009
[33] J Zhang S Wang M Xu et al ldquoHierarchically porous ZnOarchitectures for gas sensor applicationrdquo Crystal Growth andDesign vol 9 no 8 pp 3532ndash3537 2009
[34] Z Yuan X Jiaqiang X Qun L Hui P Qingyi and XPengcheng ldquoBrush-like hierarchical zno nanostructures syn-thesis photoluminescence and gas sensor propertiesrdquo Journalof Physical Chemistry C vol 113 no 9 pp 3430ndash3435 2009
[35] J Zhang X Liu SWu BCao and S Zheng ldquoOne-pot synthesisof Au-supported ZnO nanoplates with enhanced gas sensorperformancerdquo Sensors and Actuators B vol 169 pp 61ndash66 2012
O ads minus2adsminus + 119890minus rarr 2OO ads 2minusads minus + 119890minus rarr O
(a) Oxygen adsorbed on ZnO surface
O atomElectron
ZnO surface
2SO2 + O2adsminus rarr 2SO2minusO + 119890minus
2 ads 2SO + O minus rarr SO minusO + 119890minus
SO2 + Oads 2minus rarr SO2minusO + 2119890minus
(b) SO2 gas sensing on ZnO surface ZnO
Figure 10 Schematic plot illustrating the sensing mechanism of prepared sensor to SO2
working temperatures for SO2 SO2F2 and SOF
2are about
250 300 and 300∘C respectivelyThus we draw a conclusionthat the sensing behavior of the prepared sensor to SO
2gas
may belong to the ldquomolecular ionrdquo reaction pattern while itis an ldquoatomic ionrdquo gas response mode for SO
2F2and SOF
2
4 Conclusions
In summary Flower-like ZnO nanorods have been success-fully synthesized and characterized by XRD and FESEMTheoptimum operating temperatures of the prepared sensor toSO2 SO2F2 and SOF
2are about 250 300 and 300∘C The
response (recovery) time of the sensor to 10 120583LL of SO2
SO2F2 and SOF
2is 21 (45) 13 (32) and 10 (17) s respectively
Especially the flower-like ZnO nanorods gas sensor showshigh linearity to SO
2 SO2F2 and SOF
2at the range of 10ndash
100 120583LL with excellent linear correlation coefficient 1198772 at0982 0979 and 0963 separately These findings demon-strate that our prepared flower-like ZnO nanorods have someexcellent potential advantages for using as gas sensors todetect and online monitor the SF
6decompositions such as
SO2 SOF
2 and SO
2F2in practice although further studies
are still needed
References
[1] J Tang F Liu X X Zhang Q H Meng and J B Zhou ldquoPartialdischarge recognition through an analysis of SF
6decomposition
products Part 1 decomposition characteristics of SF6under four
different partial dischargesrdquo IEEE Transactions on Dielectricsand Electrical Insulation vol 19 no 1 pp 29ndash36 2012
[2] M Shih W J Lee and C Y Chen ldquoDecomposition of SF6
and H2S mixture in radio frequency plasma environmentrdquo
Industrial and Engineering Chemistry Research vol 42 no 13pp 2906ndash2912 2003
[3] J Tang F Liu X X Zhang Q H Meng and J G Tao ldquoPartialdischarge recognition through an analysis of SF
6decomposition
products part 2 feature extraction and decision tree-based
pattern recognitionrdquo IEEE Transactions on Dielectrics andElectrical Insulation vol 19 no 1 pp 37ndash44 2012
[4] R J Van Brunt and J T Herron ldquoFundamental processes of SF6
decomposition and oxidation in glow and corona dischargesrdquoIEEE Transactions on Electrical Insulation vol 25 no 1 pp 75ndash94 1990
[5] M Shih W J Lee C H Tsai P J Tsai and C Y ChenldquoDecomposition of SF
6in an RF plasma environmentrdquo Journal
of the Air andWaste Management Association vol 52 no 11 pp1274ndash1280 2002
[6] I Sauers H W Ellis and L G Christophorou ldquoNeutraldecomposition products in spark breakdown of SF
6rdquo IEEE
Transactions on Electrical Insulation vol EI-21 no 2 pp 111ndash120 1986
[7] W T Tsai ldquoThe decomposition products of sulfur hexafluoride(SF6) reviews of environmental and health risk analysisrdquo
Journal of Fluorine Chemistry vol 128 no 11 pp 1345ndash13522007
[8] L Vial AM Casanovas I Coll and J Casanovas ldquoDecomposi-tion products from negative and 50Hz ac corona discharges incompressed SF
6and SF
6N2(10 90) mixtures Effect of water
vapour added to the gasrdquo Journal of Physics D vol 32 no 14pp 1681ndash1692 1999
[9] C T Dervos and P Vassiliou ldquoSulfur hexafluoride (SF6) Global
environmental effects and toxic byproduct formationrdquo Journalof the Air and Waste Management Association vol 50 no 1 pp137ndash141 2000
[10] E Duffour ldquoMolecular dynamic simulations of the collisionbetween copper ions SF
6molecules and a polyethylene surface
a study of decomposition products and an evaluation of the self-diffusion coefficientsrdquoMacromolecularTheory and Simulationsvol 19 no 2-3 pp 88ndash99 2010
[11] J I Baumbach P Pilzecker and E Trindade ldquoMonitoring ofcircuit breakers using ion mobility spectrometry to detect SF
6-
decompositionrdquo International Journal for Ion Mobility Spec-trometry vol 2 no 1 pp 35ndash39 1999
[12] R Kurte C Beyer HMHeise andD Klockow ldquoApplication ofinfrared spectroscopy to monitoring gas insulated high-voltageequipment electrode material-dependent SF
6decompositionrdquo
Journal of Nanomaterials 7
Analytical and Bioanalytical Chemistry vol 373 no 7 pp 639ndash646 2002
[13] W Ding R Hayashi K Ochi et al ldquoAnalysis of PD-generatedSF6decomposition gases adsorbed on carbon nanotubesrdquo IEEE
Transactions on Dielectrics and Electrical Insulation vol 13 no6 pp 1200ndash1207 2006
[14] J SinghAMukherjee S K Sengupta J ImGW Peterson andJ E Whitten ldquoSulfur dioxide and nitrogen dioxide adsorptionon zinc oxide and zirconium hydroxide nanoparticles and theeffect on photoluminescencerdquo Applied Surface Science vol 258no 15 pp 5778ndash5785 2012
[15] BWang L F Zhu Y H Yang N S Xu and GW Yang ldquoFabri-cation of a SnO
2nanowire gas sensor and sensor performance
for hydrogenrdquo Journal of Physical Chemistry C vol 112 no 17pp 6643ndash6647 2008
[16] J Gong Y Li Z Hu Z Zhou and Y Deng ldquoUltrasensitive NH3
gas sensor from polyaniline nanograin enchased TiO2fibersrdquo
Journal of Physical Chemistry C vol 114 no 21 pp 9970ndash99742010
[17] X Liu J Zhang X Guo S Wu and S Wang ldquoPorous 120572-Fe2O3decorated byAunanoparticles and their enhanced sensor
performancerdquo Nanotechnology vol 21 no 9 Article ID 0955012010
[18] B Cao J Chen X Tang and W Zhou ldquoGrowth of monoclinicWO3nanowire array for highly sensitive NO
2detectionrdquo
Journal of Materials Chemistry vol 19 no 16 pp 2323ndash23272009
[19] S E Moon H Y Lee J Park et al ldquoLow power consumptionand high sensitivity carbon monoxide gas sensor using indiumoxide nanowirerdquo Journal of Nanoscience and Nanotechnologyvol 10 no 5 pp 3189ndash3192 2010
[20] WZeng T Liu ZWang S TsukimotoM Saito andY IkuharaldquoSelective detection of formaldehyde gas using a Cd-DopedTiO2-SnO2sensorrdquo Sensors vol 9 no 11 pp 9029ndash9038 2009
[21] M Chen Z Wang D Han F Gu and G Guo ldquoPorous ZnOpolygonal nanoflakes synthesis use in high-sensitivity NO
2gas
sensor and proposed mechanism of gas sensingrdquo Journal ofPhysical Chemistry C vol 115 no 26 pp 12763ndash12773 2011
[22] E Oh H Y Choi S H Jung et al ldquoHigh-performance NO2gas
sensor based on ZnO nanorod grown by ultrasonic irradiationrdquoSensors and Actuators B vol 141 no 1 pp 239ndash243 2009
[23] K Zheng L Gu D Sun X Mo and G Chen ldquoThe propertiesof ethanol gas sensor based on Ti doped ZnO nanotetrapodsrdquoMaterials Science and Engineering B vol 166 no 1 pp 104ndash1072010
[24] A Wei L-H Pan X-C Dong and W Huang ldquoRoom-temperature NH
3gas sensor based on hydrothermally grown
ZnO nanorodsrdquo Chinese Physics Letters vol 28 no 8 pp 702ndash706 2011
[25] CWen Y JuW Li et al ldquoCarbon dioxide gas sensor using SAWdevice based on ZnO filmrdquo Applied Mechanics and Materialsvol 135-136 pp 347ndash352 2012
[26] O Lupan G Chai and L Chow ldquoNovel hydrogen gas sensorbased on single ZnOnanorodrdquoMicroelectronic Engineering vol85 no 11 pp 2220ndash2225 2008
[27] W Zeng T Liu and ZWang ldquoEnhanced gas sensing propertiesby SnO
2nanosphere functionalized TiO
2nanobeltsrdquo Journal of
Materials Chemistry vol 22 no 8 pp 3544ndash3548 2012[28] J Kim andK Yong ldquoMechanism study of ZnOnanorod-bundle
sensors for H2S gas sensingrdquo Journal of Physical Chemistry C
vol 115 no 15 pp 7218ndash7224 2011
[29] D Velasco-Arias D Dıaz P Santiago-Jacinto G Rodrıguez-Gattorno A Vazquez-Olmos and S E Castillo-Blum ldquoDirectinteraction of colloidal nanostructured ZnO and SnO
2withNO
and SO2rdquo Journal of Nanoscience andNanotechnology vol 8 no
12 pp 6389ndash6397 2008[30] Q Qi T Zhang Q Yu et al ldquoProperties of humidity sensing
ZnO nanorods-base sensor fabricated by screen-printingrdquo Sen-sors and Actuators B vol 133 no 2 pp 638ndash643 2008
[31] M-W Ahn K-S Park J-H Heo et al ldquoGas sensing propertiesof defect-controlled ZnO-nanowire gas sensorrdquo Applied PhysicsLetters vol 93 no 26 Article ID 263103 2008
[32] M W Ahn K S Park J H Heo D W Kim K J Choi and JG Park ldquoOn-chip fabrication of ZnO-nanowire gas sensor withhigh gas sensitivityrdquo Sensors and Actuators B vol 138 no 1 pp168ndash173 2009
[33] J Zhang S Wang M Xu et al ldquoHierarchically porous ZnOarchitectures for gas sensor applicationrdquo Crystal Growth andDesign vol 9 no 8 pp 3532ndash3537 2009
[34] Z Yuan X Jiaqiang X Qun L Hui P Qingyi and XPengcheng ldquoBrush-like hierarchical zno nanostructures syn-thesis photoluminescence and gas sensor propertiesrdquo Journalof Physical Chemistry C vol 113 no 9 pp 3430ndash3435 2009
[35] J Zhang X Liu SWu BCao and S Zheng ldquoOne-pot synthesisof Au-supported ZnO nanoplates with enhanced gas sensorperformancerdquo Sensors and Actuators B vol 169 pp 61ndash66 2012
Analytical and Bioanalytical Chemistry vol 373 no 7 pp 639ndash646 2002
[13] W Ding R Hayashi K Ochi et al ldquoAnalysis of PD-generatedSF6decomposition gases adsorbed on carbon nanotubesrdquo IEEE
Transactions on Dielectrics and Electrical Insulation vol 13 no6 pp 1200ndash1207 2006
[14] J SinghAMukherjee S K Sengupta J ImGW Peterson andJ E Whitten ldquoSulfur dioxide and nitrogen dioxide adsorptionon zinc oxide and zirconium hydroxide nanoparticles and theeffect on photoluminescencerdquo Applied Surface Science vol 258no 15 pp 5778ndash5785 2012
[15] BWang L F Zhu Y H Yang N S Xu and GW Yang ldquoFabri-cation of a SnO
2nanowire gas sensor and sensor performance
for hydrogenrdquo Journal of Physical Chemistry C vol 112 no 17pp 6643ndash6647 2008
[16] J Gong Y Li Z Hu Z Zhou and Y Deng ldquoUltrasensitive NH3
gas sensor from polyaniline nanograin enchased TiO2fibersrdquo
Journal of Physical Chemistry C vol 114 no 21 pp 9970ndash99742010
[17] X Liu J Zhang X Guo S Wu and S Wang ldquoPorous 120572-Fe2O3decorated byAunanoparticles and their enhanced sensor
performancerdquo Nanotechnology vol 21 no 9 Article ID 0955012010
[18] B Cao J Chen X Tang and W Zhou ldquoGrowth of monoclinicWO3nanowire array for highly sensitive NO
2detectionrdquo
Journal of Materials Chemistry vol 19 no 16 pp 2323ndash23272009
[19] S E Moon H Y Lee J Park et al ldquoLow power consumptionand high sensitivity carbon monoxide gas sensor using indiumoxide nanowirerdquo Journal of Nanoscience and Nanotechnologyvol 10 no 5 pp 3189ndash3192 2010
[20] WZeng T Liu ZWang S TsukimotoM Saito andY IkuharaldquoSelective detection of formaldehyde gas using a Cd-DopedTiO2-SnO2sensorrdquo Sensors vol 9 no 11 pp 9029ndash9038 2009
[21] M Chen Z Wang D Han F Gu and G Guo ldquoPorous ZnOpolygonal nanoflakes synthesis use in high-sensitivity NO
2gas
sensor and proposed mechanism of gas sensingrdquo Journal ofPhysical Chemistry C vol 115 no 26 pp 12763ndash12773 2011
[22] E Oh H Y Choi S H Jung et al ldquoHigh-performance NO2gas
sensor based on ZnO nanorod grown by ultrasonic irradiationrdquoSensors and Actuators B vol 141 no 1 pp 239ndash243 2009
[23] K Zheng L Gu D Sun X Mo and G Chen ldquoThe propertiesof ethanol gas sensor based on Ti doped ZnO nanotetrapodsrdquoMaterials Science and Engineering B vol 166 no 1 pp 104ndash1072010
[24] A Wei L-H Pan X-C Dong and W Huang ldquoRoom-temperature NH
3gas sensor based on hydrothermally grown
ZnO nanorodsrdquo Chinese Physics Letters vol 28 no 8 pp 702ndash706 2011
[25] CWen Y JuW Li et al ldquoCarbon dioxide gas sensor using SAWdevice based on ZnO filmrdquo Applied Mechanics and Materialsvol 135-136 pp 347ndash352 2012
[26] O Lupan G Chai and L Chow ldquoNovel hydrogen gas sensorbased on single ZnOnanorodrdquoMicroelectronic Engineering vol85 no 11 pp 2220ndash2225 2008
[27] W Zeng T Liu and ZWang ldquoEnhanced gas sensing propertiesby SnO
2nanosphere functionalized TiO
2nanobeltsrdquo Journal of
Materials Chemistry vol 22 no 8 pp 3544ndash3548 2012[28] J Kim andK Yong ldquoMechanism study of ZnOnanorod-bundle
sensors for H2S gas sensingrdquo Journal of Physical Chemistry C
vol 115 no 15 pp 7218ndash7224 2011
[29] D Velasco-Arias D Dıaz P Santiago-Jacinto G Rodrıguez-Gattorno A Vazquez-Olmos and S E Castillo-Blum ldquoDirectinteraction of colloidal nanostructured ZnO and SnO
2withNO
and SO2rdquo Journal of Nanoscience andNanotechnology vol 8 no
12 pp 6389ndash6397 2008[30] Q Qi T Zhang Q Yu et al ldquoProperties of humidity sensing
ZnO nanorods-base sensor fabricated by screen-printingrdquo Sen-sors and Actuators B vol 133 no 2 pp 638ndash643 2008
[31] M-W Ahn K-S Park J-H Heo et al ldquoGas sensing propertiesof defect-controlled ZnO-nanowire gas sensorrdquo Applied PhysicsLetters vol 93 no 26 Article ID 263103 2008
[32] M W Ahn K S Park J H Heo D W Kim K J Choi and JG Park ldquoOn-chip fabrication of ZnO-nanowire gas sensor withhigh gas sensitivityrdquo Sensors and Actuators B vol 138 no 1 pp168ndash173 2009
[33] J Zhang S Wang M Xu et al ldquoHierarchically porous ZnOarchitectures for gas sensor applicationrdquo Crystal Growth andDesign vol 9 no 8 pp 3532ndash3537 2009
[34] Z Yuan X Jiaqiang X Qun L Hui P Qingyi and XPengcheng ldquoBrush-like hierarchical zno nanostructures syn-thesis photoluminescence and gas sensor propertiesrdquo Journalof Physical Chemistry C vol 113 no 9 pp 3430ndash3435 2009
[35] J Zhang X Liu SWu BCao and S Zheng ldquoOne-pot synthesisof Au-supported ZnO nanoplates with enhanced gas sensorperformancerdquo Sensors and Actuators B vol 169 pp 61ndash66 2012
![Review Article Photocatalysis and Bandgap Engineering ... · ZnO-CdS Core-shell nanorods August [ ] CdS@ZnO Nanourchins December [ ] Enhanced eciency due to speci c morphology which](https://static.documents.pub/doc/80x56/60d77bce92ec5444c6044d7f/review-article-photocatalysis-and-bandgap-engineering-zno-cds-core-shell-nanorods.jpg)
