research article high performance indium-doped zno gas...

Research ArticleHigh Performance Indium-Doped ZnO Gas Sensor

Junjie Qi1 Hong Zhang1 Shengnan Lu1 Xin Li1 Minxuan Xu1 and Yue Zhang12

1School of Materials Science and Engineering University of Science and Technology Beijing Beijing 100083 China2Key Laboratory of New Energy Materials and Technologies University of Science and Technology Beijing Beijing 100083 China

Correspondence should be addressed to Junjie Qi junjieqiustbeducn and Yue Zhang yuezhangustbeducn

Received 3 November 2014 Accepted 23 December 2014

Academic Editor Antonios Kelarakis

Copyright copy 2015 Junjie Qi et alThis is an open access article distributed under the Creative Commons Attribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Gas sensors for ethanol and acetone based on ZnO nanobelts with doping element indium were fabricated Excellent sensitivityaccompanied with short response time (10 s) and recovery time (23 s) to 150 ppm ethanol is obtained For In-doped sensors aminimum concentration of 375 ppm at 275∘C in acetone was observed with an average sensitivity of 7144 which is 7 times largerthan that of the pure sensors and much larger than that reported response (16) of Co-doped ZnO nanofibers to acetone Theseresults indicate that doping elements can improve gas sensitivity which is associated with oxygen space and valence ions In-dopedZnO nanobelts exhibit higher sensitivity to acetone than that to ethanol These results indicate that doped ZnO nanobelts cansuccessfully distinguish acetone and ethanol which can be put into various practical applications

1 Introduction

As we all know volatile organic compounds (VOCs) arewidely used and vapor of some VOCs is hazardous tohuman health For example acetone in environment cancause permanent eye damage and its long-time exposurecan cause kidney liver and nerve damage [1] Controllingand monitoring ethanol are important in some fields as well[2] such as testing alcohol levels of drivers and monitoringchemical synthesisThus the detection of such hazardous andcommonly used chemical is of crucial importance

Semiconductor sensors due to their particular advan-tages such as high response low cost and portability arewidely used for the detection of toxic or dangerous gases andmonitoring of air pollution in the environment [3] As ann-type semiconductor ZnO has been extensively used as agas sensing material due to its high mobility of conductionelectrons and good chemical and thermal stability under theoperating conditions of sensors [4 5] Compared with bulkand thin-film materials the nanostructure materials exhibita large response due to their large length-to-diameter aspectratio and high surface-to-volume ratio which is remarkablybeneficial to the adsorption and desorption of testing gases[6]

Appropriate doping can provide electronic defects thatincrease the influence of oxygen partial pressure on the

conductivity Recently the search for improving the perfor-mance of ZnO-based gas sensors has expanded in two direc-tions one is doping withmetal elements [7ndash9] or heavymetalmodified [10] and the other is fabricating sensors by usingone or several nanostructures as gas sensing elements [6 11]Up to now the applications of doped ZnO nanostructures ingas sensors have been intensively studied [12] However as tothe majority of gas sensors based on ZnO films now existingthe detection concentration is still high [13 14] (usually isseveral hundreds of ppm) and the response is relatively verylow So it is still a great challenge to reduce the limitation ofdetection concentration and improve the response propertyof the gas sensor Moreover there are few reports on thegas sensing of In-doped ZnO nanobelts In this paper ZnOnanobelts doped with indium element were synthesized andcharacterized By assembling the In-doped ZnO nanobeltswith comb-shaped gold electrodes the high performancegas sensor was achieved The mechanism and the influenceof doping elements on the sensing characteristics were alsodiscussed

2 Experimental Methods

Pure ZnO nanobelts were synthesized by normal pressurethermal evaporation using zinc powders (99wt) in a hor-izontal quartz furnace system Before the process substrates

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2015 Article ID 954747 6 pageshttpdxdoiorg1011552015954747

2 Journal of Nanomaterials

were coated with seed solution (the solution is composed ofZn(NO

3)2sdot6H2O and CH

3OCH2CH2OH the concentration

is 02molL) In-doped ZnO nanobelts were synthesized bythermal evaporation with Au catalyst The mixture of ZnIn2O3 andCpowderswith themole ratio of 5 1 2was placed

in a quartz bar (with a hole in a head) inside a quartz tube asthe evaporation source A quartz substrate coated with 5 nmthickness of Au was then positioned on the top of the sourceboat Ar (99 sccm)was used as the carrier gas andO

2(1 sccm)

was the reaction gas After the reaction at a temperature of970∘C for 30min yellow-green color samples were observed

Themorphology and structure of the synthesized productwere characterized by using scanning electron microscopy(SEM JEOL 6490) equipped with an energy dispersive X-rayspectroscopy (EDS) and X-ray diffraction (XRD DMAX-RB) Photoluminescence (PL) spectra of the synthesizedproducts were taken at the room temperature using the325 nm line of a He-Cd laser as the excitation source Anelectrochemical workstation (IM6e) was used to record theresistivity of sensors the exchange electronic potential ofdisturbance is 10mV and scan frequency ranges from 01sim105Hz

For the device assembly ten pairs of Au interdigitalelectrodes (30 nm thickness with the gap of two toes being04mm) were printed on an alumina ceramic substrate bya screen-printing technology using ion evaporation As-synthesized ZnO nanobelts were assembled on the electrodesfollowed by connecting two Cu wires using Ag paste to theAu electrodes The ZnO nanobelts sensor was connectedto a computer-controlled ceramic plate heater as shown inFigure 1(a) I-V curve of the element was measured as shownin Figure 1(b) which reveals that the contact between ZnOand Ag paste is ohmic contact Compared to the resistanceof the ZnO film the contact resistance is so small that it canhardly influence themeasurement of the gas sensing propertyTherefore the contact resistance can be omitted

A gas distribution method was used for the test of gassensing properties The measurement was operated in a gassensor test system WS-30A which consists of a gas chamber(with heating equipment) temperature control circuit anddisturbed flow fan The sample signals were led out andconnected to IM6e electrochemical workstationThe effectivevolume of the chamber is 30 cmlowast30 cmlowast20 cmThe volumeof the gas chamber and the gas exchange speed have a greateffect on the response time and recovery property The gasdiffusion was driven by the disturbed flow fan after the targetgas was introduced And then the chamber cover was movedaway when the sample stabilizedThe vent of the gas relies onthe air diffusion driven by the disturbed flow fanTheworkingtemperature of a sensor was adjusted through varying theheating voltage The resistance of the chamber is preset theheating voltage can be tuned from 1 to 10 v The samplewas assembled on the resistance gage and the resistance valeis constant so the maximum measurement temperature is300∘C in our experiment Detecting gases were injected intothe test chamber and mixed with air The heating area ofsensor is smaller than the volume of test chamber so the testchamber works at room temperature Thus the environmentof the test chamber is recognized as standard condition

3 Results and Discussion

The morphology of the pure and doped ZnO nanobelts isshown in Figures 2(a) and 2(b) respectively These nanobeltshave a width of about 50ndash100 nm and a length up to dozensof micrometers Only O and Zn elements were observedas demonstrated in EDS spectra of pure ZnO which areindicated in Figure 2(c) In addition to O and Zn indiumelement was observed as demonstrated in Figure 2(d) andthe indium content was about 296

XRD spectra of the as-prepared doped and undopedZnO nanomaterial samples were shown in Figure 3(a) whichindicate that all the samples have the wurtzite structure andno secondary phase As compared to the standard ZnOpowders (JCPDS No 36-1415) a shift of the peaks to lowerangle was observed in In-doped ZnO nanobelts which canbe attributed to lattice change caused by introduced ions [15]

Figure 3(b) shows the room-temperature PL spectrumrecorded from the as-deposited pure ZnO and doping ZnOnanobelts and part spectrum of In-doped samples rangingfrom350 to 400 nm is shown in insetTheultraviolet emissionof pure ZnO is located at 390 nm corresponding to thenear band edge (NBE) peak which is responsible for therecombination of free excitons of ZnO [16]With regard to In-doped samples a strong defects peak at 540 nm leads to theinvisible intrinsic peak It is likely that In3+ can combinemoreoxygen atoms than bivalent cations because of the largercoordinate linkage number [15] Therefore there are moreoxygen related defects in In-dopedZnOwhich cause a strongdeep level emission

For gas sensor the response value (S) is defined asthe ratio of resistance in the air (Ra) and resistance in thetested gas (Rg) [17] A gas distribution method was used forthe test of gas sensing properties The calculating method ofconcentration is as follows (taking ethanol as an example)

119862 =

119881ethanolgas

119881

119904

(1)

where 119881ethanolgas is the volume of gaseous state ethanol119862 (ppm) is the concentration of the tested gas and 119881

119904is the

volume of the systemrsquos test chamber Consider

119881ethanolgas =119881injection times 120588 times 119881119898 times 120596

119872

(2)

where 119881injection is the volume of injection ethanol 120588 (gmL)is the density of the liquid 119881

119898is the molar volume under

normal conditions119872 (gmol) is the molar mass of ethanoland 120596 is the mass fraction of the liquid Combined with thesetwo formulas we can get the general formula for converting119881injection into concentration

119862 =

119881injection times 120588 times 119881119898 times 120596

(119872 times 119881

119904)

(3)

Put the constant value that each letter stands for into formula(3)

119862 = 001934184119881injection molmL (4)

Journal of Nanomaterials 3

ZnO

Cu wires

Substrate

Au electrodes

(a)

0

0

1

20

40

60

80

2minus2minus80

minus60

minus40

minus20

minus1

Curr

ent (

nA)

Voltage (V)

(b)

Figure 1 (a) Schematic of the gas sensor (b) typical I-V curve of the gas sensor

(a)

(c) (d)

(b)

O Zn

Zn

2

2

4

8

6

10

12

14

16

0

4 6 8 10 12

(keV)

(cps

eV

)

(cps

eV

)

Zn

Zn

InInInO

(keV)2 4 6 8 10 1412

2

4

8

6

10

12

0

Figure 2 SEM images of ZnO nanobelts (a) pure ZnO (b) In-doped ZnO The corresponding EDS spectrum (c) pure ZnO (d) In-dopedZnO

In the same way we calculated the formula which is appro-priate for acetone

119862 = 001511961119881injection molmL (5)

Through the two equations above how much liquidshould be injected to the test chamber to get needed con-centration can be calculated Figure 4 shows the responsesof pure and doped ZnO samples to different concentrationof ethanol and acetone at a fixed temperature (275∘C) In


30 35 40 45 50 55 60 65

Au (2

00)

ZnO

(103

)

ZnO

(110

)

ZnO

(102

)

Au (1

11)

ZnO

(101

)

ZnO

(002

)Zn

O (1

00)

Inte

nsity

(au

)

Pure ZnOIn-doped ZnO

2120579 (∘)

(a)


300 400 500 600 700 800

350 375 400

Inte

nsity

(au

)

Wavelength (nm)

In-doped

(b)

Figure 3 (a) X-ray powder diffraction patterns of pure and In-doped ZnO (b) PL spectrum at room temperature of pure and In-doped ZnOWavelength from 350 to 400 nm of In-doped samples is shown in inset

0 100 200 300 400 500 600 700 800100

150

200

250

Resp

onse

Concentration (ppm)


(a)

0 100 200 300 400 500 600 700 800

100

200

300

400

500

600

700

800

Resp

onse

Concentration (ppm)


(b)

Figure 4 Responses of gas sensors to (a) ethanol and (b) acetone range from 375 ppm to 750 ppm at 275∘C

the range of 375ndash750 ppm the sensitivity of all samples isfound to enhance with the increase of concentration Theaverage sensitivity of pure ZnO gas sensor at 275∘C in ethanolambiance is 1584 and the average sensitivity of In-dopedZnO sensor at the same condition is 2204 which achieved40 increase compared with pure ZnO gas sensor Thedetection limit of In-doped ZnO sensor has gone down to375 ppm compared with pure ZnO ones (150 ppm) as shownin Figure 4(a) Figure 4(b) displays the responses of gassensors to acetone The average response of In-doped sensorto acetone (CH

3COCH

3) is 7144 which is 7 times larger than

that of the pure ZnO sensor (around 100) showing higher

sensitivity than the result in previous similar experiments inliteratures [18 19] where they reported that the response of05 wtCo-dopedZnOnanofibers to 100 ppmacetone is onlyabout 16These results indicate that In-doped ZnO nanobeltsgas sensor can successfully distinguish acetone and ethanolwhich could be put into various practical applications

The rapid response and recovery of doped ZnO nanobeltsgas sensor to 150 ppm of ethanol at 275∘C are shown inFigure 5 as compared to pure ZnO gas sensor The responseor recovery time is defined as the time for reaching 90of the full response change of sensor after testing gas isintroduced It was found that the response time of the doped


0 50 100 15040

50

60

70

80

90

100

Time (s)


Resis

tanc

e (kΩ

)

Figure 5 Response and recovery curve of the sensors for 150 ppmethanol at 275∘C

ZnO sensors was less than 10 s whereas the recovery timewasabout 23 s both of which are much less than the pure onesRapid response and recovery of the sensors reveal potentialvalue in the practical application

Figure 6 shows the responses of pure and doped ZnOnanobelts gas sensor to 450 ppm testing gas at differentoperating temperatures The responses of all samples arefound to increase with increasing the operating temperatureAt the lower temperature scope ranging from 175sim250∘C thesensitivity grew slowly while there is a drastic increase from250∘C which is possibly due to the physical adsorption ofoxygen at low temperature and the chemical adsorption ofoxygen at higher temperature on the surface of the sensor[20] The sensitivity does not show saturation when the testreached to the maximum experiment temperature of 300∘C

To measure the long-term stability of those sensors werepeated some of the sensors many times within 2 monthsDuring the test no appreciable variations were detectedThus the obtained results showed that both sensitivity andelectrical conductance were reproducible enough

A conventional model is introduced to elaborate the gassensing mechanism As n-type semiconductor ZnO adsorbsoxygenmolecules when it is exposed to air Absorbed oxygencan form O119899minus (O

2

minus O2

2minus and O2minus) ions by capturingelectrons from the conductance band [21] Since gas sensorsare usually operated at elevated temperature (around 573K)the O

2

2minus type is more important than other states of oxygen[22] Since ZnO is a basic oxide the target gas (ethanol)may undergo two-step decomposition reaction which comesdown to a dehydrogenation process [3]

O119899minus + 119899C2H5OH 997888rarr 119899CH

3CHOminus +H

2O (g) (6)

119899CH3CHOminus 997888rarr 119899CH

3CHO (gas) + neminus (7)

CH3CHO (ad) + 5O

2

2minus

(ad) 997888rarr 4CO2+ 4H2O + 10eminus (8)

180 200 220 240 260 280 30075

150

225

300

375

450

Resp

onse

(S

)


Temperature (∘C)

Figure 6 Response of the sensors exposed to ethanol gas withconcentration of 450 ppm at operating temperatures from 175∘C to300∘C

The general reaction formula in acetone is given asfollows

H3COCH

3(ads) + 8Ominus (ads) 997888rarr 3CO

2(gas)

+ 3H2O (gas) + 8eminus

(9)

Finally electrons return to the semiconductor conduct-ing the increase of electrons concentration in the conductionband As a result the surface resistance decreased showinghigher gas sensitivity

The high response and short responserecovery timesof In-doped ZnO gas sensor were attributed to many pos-sible influencing factors (a) The nanostructure of ZnOnanobelts possesses large surface-to-volume ratio which isan important factor for high sensing performance [23] (b)The photoluminescence (PL) spectra revealed the existenceof large amount of oxygen vacancies in the doped ZnOnanobelts (especially in In-doped ZnO) which boosts theadsorption and response of oxygen resulting in the improvedperformances in the gas sensors (c) According to the highvalence ionic conduction mechanism high metal ion M3+which entered into ZnO semiconductor will form donorcentre the reaction is expressed as follows [24]

M2O3= 2MZn + 2O119909 +

1

2

O2(g) + 2e1015840 (10)

where MZn represents the positive charge center which isformed by M3+ occupying the position of Zn2+ with electronlosing O

119909means that oxygen atom dissociates from M

2O3

and e1015840 is the losing electron which ionized after M3+ occupythe position of Zn2+ We could conclude that the doping ofhigh valence ion into the semiconductor surface can producemore electrons so the materials could absorb more gasmolecules finally leading to high performance of gas sensor


4 Conclusions

In summary pure and In-doped ZnO nanobelts are syn-thesized by chemical vapor deposition method Gas sensinginvestigation reveals that In-doping can enhance the sensingproperties of ZnO nanobelts gas sensor efficiently in bothethanol and acetone For the In-doped sensors a minimumconcentration of 375 ppm at 275∘C in acetone was observedwith an average sensitivity of 7144 which is much largerthan that reported response of Co-doped ZnO nanofibers Inparticular In-doped ZnO nanobelts gas sensor can success-fully distinguish acetone and ethanol The high responsivityand quick responserecovery of the gas sensors are explainedby high valence ions mechanism and oxygen space effectThe results demonstrate the potential application of In-dopednanobelts for fabricating high performance gas sensors

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This work was supported by the National Major ResearchProgram of China (2013CB932602) the Major Project ofInternational Cooperation and Exchanges (2012DFA50990)the Program of Introducing Talents of Discipline to Uni-versities NSFC (51172022 51232001) and the Program forChangjiang Scholars and Innovative Research Team in Uni-versity (FRF-SD-12-032 and FRF-AS-13-001)

References

[1] T Colborn F S Vom Saal and A M Soto ldquoDevelopmen-tal effects of endocrine-disrupting chemicals in wildlife andhumansrdquo Environmental Health Perspectives vol 101 no 5 pp378ndash384 1993

[2] L Zhang H Qin P Song J Hu and M Jiang ldquoElectricproperties and acetone-sensing characteristics of La

1minusxPbxFeO3perovskite systemrdquoMaterials Chemistry and Physics vol 98 no2-3 pp 358ndash362 2006

[3] J Xu J Han Y Zhang Y Sun and B Xie ldquoStudies on alcoholsensing mechanism of ZnO based gas sensorsrdquo Sensors andActuators B Chemical vol 132 no 1 pp 334ndash339 2008

[4] M H Huang S Mao H Feick et al ldquoRoom-temperatureultraviolet nanowire nanolasersrdquo Science vol 292 no 5523 pp1897ndash1899 2001

[5] Q Wan Q H Li Y J Chen et al ldquoFabrication and ethanolsensing characteristics of ZnO nanowire gas sensorsrdquo AppliedPhysics Letters vol 84 no 18 pp 3654ndash3656 2004

[6] L Li H Yang H Zhao et al ldquoHydrothermal synthesis andgas sensing properties of single-crystalline ultralong ZnOnanowiresrdquo Applied Physics A Materials Science and Processingvol 98 no 3 pp 635ndash641 2010

[7] A Yu J Qian H Pan et al ldquoMicro-lotus constructed byFe-doped ZnO hierarchically porous nanosheets preparationcharacterization and gas sensing propertyrdquo Sensors and Actua-tors B Chemical vol 158 no 1 pp 9ndash16 2011

[8] K Zheng L Gu D Sun X Mo and G Chen ldquoThe propertiesof ethanol gas sensor based on Ti doped ZnO nanotetrapodsrdquo

Materials Science and Engineering B Solid-State Materials forAdvanced Technology vol 166 no 1 pp 104ndash107 2010

[9] N Han X Wu D Zhang G Shen H Liu and Y Chen ldquoCdOactivated Sn-doped ZnO for highly sensitive selective andstable formaldehyde sensorrdquo Sensors and Actuators B Chemicalvol 152 no 2 pp 324ndash329 2011

[10] Q Xiang GMeng Y Zhang et al ldquoAg nanoparticle embedded-ZnO nanorods synthesized via a photochemical method and itsgas-sensing propertiesrdquo Sensors and Actuators B Chemical vol143 no 2 pp 635ndash640 2010

[11] O Lupan V V Ursaki G Chai et al ldquoSelective hydrogen gasnanosensor using individual ZnO nanowire with fast responseat room temperaturerdquo Sensors and Actuators B Chemical vol144 no 1 pp 56ndash66 2010

[12] A R Raju and C N R Rao ldquoGas-sensing characteristics ofZnO and copper-impregnated ZnOrdquo Sensors and Actuators BChemical vol 3 no 4 pp 305ndash310 1991

[13] W Xudong D Yong L Zhou S Jinhui and L W ZhongldquoSingle-crystal mesoporous ZnO thin films composed ofnanowallsrdquo The Journal of Physical Chemistry C vol 113 no 5pp 1791ndash1794 2009

[14] C Gu J Huang YWuM Zhai Y Sun and J Liu ldquoPreparationof porous flower-like ZnOnanostructures and their gas-sensingpropertyrdquo Journal of Alloys and Compounds vol 509 no 13 pp4499ndash4504 2011

[15] J Zhao X Yan Y Yang Y Huang and Y Zhang ldquoRamanspectra and photoluminescence properties of In-doped ZnOnanostructuresrdquo Materials Letters vol 64 no 5 pp 569ndash5722010

[16] T Matsumoto H Kato K Miyamoto M Sano E A Zhukovand T Yao ldquoCorrelation between grain size and optical proper-ties in zinc oxide thin filmsrdquo Applied Physics Letters vol 81 no7 pp 1231ndash1233 2002

[17] N Hongsith E Wongrat T Kerdcharoen and S ChoopunldquoSensor response formula for sensor based on ZnO nanostruc-turesrdquo Sensors and Actuators B Chemical vol 144 no 1 pp67ndash72 2010

[18] S-J Chang T-J Hsueh I-C Chen et al ldquoHighly sensitiveZnO nanowire acetone vapor sensor with Au adsorptionrdquo IEEETransactions on Nanotechnology vol 7 no 6 pp 754ndash759 2008

[19] L Liu S Li J Zhuang et al ldquoImproved selective acetone sensingproperties of Co-doped ZnO nanofibers by electrospinningrdquoSensors and Actuators B Chemical vol 155 no 2 pp 782ndash7882011

[20] Z Yang Y Huang G Chen Z Guo S Cheng and S HuangldquoEthanol gas sensor based on Al-doped ZnO nanomaterialwith many gas diffusing channelsrdquo Sensors and Actuators BChemical vol 140 no 2 pp 549ndash556 2009

[21] N Yamazoe J Fuchigami M Kishikawa and T SeiyamaldquoInteractions of tin oxide surface with O

2

H2

O andH2

rdquo SurfaceScience vol 86 pp 335ndash344 1979

[22] G S V Coles G Williams and B Smith ldquoSelectivity studieson tin oxide-based semiconductor gas sensorsrdquo Sensors andActuators B Chemical vol 3 no 1 pp 7ndash14 1991

[23] A Kolmakov and M Moskovits ldquoChemical sensing and catal-ysis by one-dimensional metal-oxide nanostructuresrdquo AnnualReview of Materials Research vol 34 pp 151ndash180 2004

[24] M T Mohammad A A Hashim and M H Al-MaamoryldquoHighly conductive and transparent ZnO thin films preparedby spray pyrolysis techniquerdquoMaterials Chemistry and Physicsvol 99 no 2-3 pp 382ndash387 2006

Submit your manuscripts athttpwwwhindawicom

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NanoparticlesJournal of



International Journal of

Biomaterials


NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials


were coated with seed solution (the solution is composed ofZn(NO

3)2sdot6H2O and CH

3OCH2CH2OH the concentration

is 02molL) In-doped ZnO nanobelts were synthesized bythermal evaporation with Au catalyst The mixture of ZnIn2O3 andCpowderswith themole ratio of 5 1 2was placed

in a quartz bar (with a hole in a head) inside a quartz tube asthe evaporation source A quartz substrate coated with 5 nmthickness of Au was then positioned on the top of the sourceboat Ar (99 sccm)was used as the carrier gas andO

2(1 sccm)

was the reaction gas After the reaction at a temperature of970∘C for 30min yellow-green color samples were observed

Themorphology and structure of the synthesized productwere characterized by using scanning electron microscopy(SEM JEOL 6490) equipped with an energy dispersive X-rayspectroscopy (EDS) and X-ray diffraction (XRD DMAX-RB) Photoluminescence (PL) spectra of the synthesizedproducts were taken at the room temperature using the325 nm line of a He-Cd laser as the excitation source Anelectrochemical workstation (IM6e) was used to record theresistivity of sensors the exchange electronic potential ofdisturbance is 10mV and scan frequency ranges from 01sim105Hz

For the device assembly ten pairs of Au interdigitalelectrodes (30 nm thickness with the gap of two toes being04mm) were printed on an alumina ceramic substrate bya screen-printing technology using ion evaporation As-synthesized ZnO nanobelts were assembled on the electrodesfollowed by connecting two Cu wires using Ag paste to theAu electrodes The ZnO nanobelts sensor was connectedto a computer-controlled ceramic plate heater as shown inFigure 1(a) I-V curve of the element was measured as shownin Figure 1(b) which reveals that the contact between ZnOand Ag paste is ohmic contact Compared to the resistanceof the ZnO film the contact resistance is so small that it canhardly influence themeasurement of the gas sensing propertyTherefore the contact resistance can be omitted

A gas distribution method was used for the test of gassensing properties The measurement was operated in a gassensor test system WS-30A which consists of a gas chamber(with heating equipment) temperature control circuit anddisturbed flow fan The sample signals were led out andconnected to IM6e electrochemical workstationThe effectivevolume of the chamber is 30 cmlowast30 cmlowast20 cmThe volumeof the gas chamber and the gas exchange speed have a greateffect on the response time and recovery property The gasdiffusion was driven by the disturbed flow fan after the targetgas was introduced And then the chamber cover was movedaway when the sample stabilizedThe vent of the gas relies onthe air diffusion driven by the disturbed flow fanTheworkingtemperature of a sensor was adjusted through varying theheating voltage The resistance of the chamber is preset theheating voltage can be tuned from 1 to 10 v The samplewas assembled on the resistance gage and the resistance valeis constant so the maximum measurement temperature is300∘C in our experiment Detecting gases were injected intothe test chamber and mixed with air The heating area ofsensor is smaller than the volume of test chamber so the testchamber works at room temperature Thus the environmentof the test chamber is recognized as standard condition

3 Results and Discussion

The morphology of the pure and doped ZnO nanobelts isshown in Figures 2(a) and 2(b) respectively These nanobeltshave a width of about 50ndash100 nm and a length up to dozensof micrometers Only O and Zn elements were observedas demonstrated in EDS spectra of pure ZnO which areindicated in Figure 2(c) In addition to O and Zn indiumelement was observed as demonstrated in Figure 2(d) andthe indium content was about 296

XRD spectra of the as-prepared doped and undopedZnO nanomaterial samples were shown in Figure 3(a) whichindicate that all the samples have the wurtzite structure andno secondary phase As compared to the standard ZnOpowders (JCPDS No 36-1415) a shift of the peaks to lowerangle was observed in In-doped ZnO nanobelts which canbe attributed to lattice change caused by introduced ions [15]

Figure 3(b) shows the room-temperature PL spectrumrecorded from the as-deposited pure ZnO and doping ZnOnanobelts and part spectrum of In-doped samples rangingfrom350 to 400 nm is shown in insetTheultraviolet emissionof pure ZnO is located at 390 nm corresponding to thenear band edge (NBE) peak which is responsible for therecombination of free excitons of ZnO [16]With regard to In-doped samples a strong defects peak at 540 nm leads to theinvisible intrinsic peak It is likely that In3+ can combinemoreoxygen atoms than bivalent cations because of the largercoordinate linkage number [15] Therefore there are moreoxygen related defects in In-dopedZnOwhich cause a strongdeep level emission

For gas sensor the response value (S) is defined asthe ratio of resistance in the air (Ra) and resistance in thetested gas (Rg) [17] A gas distribution method was used forthe test of gas sensing properties The calculating method ofconcentration is as follows (taking ethanol as an example)

119862 =

119881ethanolgas

119881

119904

(1)

where 119881ethanolgas is the volume of gaseous state ethanol119862 (ppm) is the concentration of the tested gas and 119881

119904is the

volume of the systemrsquos test chamber Consider

119881ethanolgas =119881injection times 120588 times 119881119898 times 120596

119872

(2)

where 119881injection is the volume of injection ethanol 120588 (gmL)is the density of the liquid 119881

119898is the molar volume under

normal conditions119872 (gmol) is the molar mass of ethanoland 120596 is the mass fraction of the liquid Combined with thesetwo formulas we can get the general formula for converting119881injection into concentration

119862 =

119881injection times 120588 times 119881119898 times 120596

(119872 times 119881

119904)

(3)

Put the constant value that each letter stands for into formula(3)

119862 = 001934184119881injection molmL (4)


ZnO

Cu wires

Substrate

Au electrodes

(a)

0

0

1

20

40

60

80

2minus2minus80

minus60

minus40

minus20

minus1

Curr

ent (

nA)

Voltage (V)

(b)


(a)

(c) (d)

(b)

O Zn

Zn

2

2

4

8

6

10

12

14

16

0

4 6 8 10 12

(keV)

(cps

eV

)

(cps

eV

)

Zn

Zn

InInInO

(keV)2 4 6 8 10 1412

2

4

8

6

10

12

0



119862 = 001511961119881injection molmL (5)



30 35 40 45 50 55 60 65

Au (2

00)

ZnO

(103

)

ZnO

(110

)

ZnO

(102

)

Au (1

11)

ZnO

(101

)

ZnO

(002

)Zn

O (1

00)

Inte

nsity

(au

)


2120579 (∘)

(a)


300 400 500 600 700 800

350 375 400

Inte

nsity

(au

)

Wavelength (nm)

In-doped

(b)


0 100 200 300 400 500 600 700 800100

150

200

250

Resp

onse

Concentration (ppm)


(a)

0 100 200 300 400 500 600 700 800

100

200

300

400

500

600

700

800

Resp

onse

Concentration (ppm)


(b)



3COCH






0 50 100 15040

50

60

70

80

90

100

Time (s)


Resis

tanc

e (kΩ

)






2

minus O2


2



3CHOminus +H

2O (g) (6)



CH3CHO (ad) + 5O

2

2minus


180 200 220 240 260 280 30075

150

225

300

375

450

Resp

onse

(S

)


Temperature (∘C)



H3COCH


2(gas)


(9)



M2O3= 2MZn + 2O119909 +

1

2

O2(g) + 2e1015840 (10)



2O3



4 Conclusions




Acknowledgments


References
























2

H2

O andH2












CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




ZnO

Cu wires

Substrate

Au electrodes

(a)

0

0

1

20

40

60

80

2minus2minus80

minus60

minus40

minus20

minus1

Curr

ent (

nA)

Voltage (V)

(b)


(a)

(c) (d)

(b)

O Zn

Zn

2

2

4

8

6

10

12

14

16

0

4 6 8 10 12

(keV)

(cps

eV

)

(cps

eV

)

Zn

Zn

InInInO

(keV)2 4 6 8 10 1412

2

4

8

6

10

12

0



119862 = 001511961119881injection molmL (5)



30 35 40 45 50 55 60 65

Au (2

00)

ZnO

(103

)

ZnO

(110

)

ZnO

(102

)

Au (1

11)

ZnO

(101

)

ZnO

(002

)Zn

O (1

00)

Inte

nsity

(au

)


2120579 (∘)

(a)


300 400 500 600 700 800

350 375 400

Inte

nsity

(au

)

Wavelength (nm)

In-doped

(b)


0 100 200 300 400 500 600 700 800100

150

200

250

Resp

onse

Concentration (ppm)


(a)

0 100 200 300 400 500 600 700 800

100

200

300

400

500

600

700

800

Resp

onse

Concentration (ppm)


(b)



3COCH






0 50 100 15040

50

60

70

80

90

100

Time (s)


Resis

tanc

e (kΩ

)






2

minus O2


2



3CHOminus +H

2O (g) (6)



CH3CHO (ad) + 5O

2

2minus


180 200 220 240 260 280 30075

150

225

300

375

450

Resp

onse

(S

)


Temperature (∘C)



H3COCH


2(gas)


(9)



M2O3= 2MZn + 2O119909 +

1

2

O2(g) + 2e1015840 (10)



2O3



4 Conclusions




Acknowledgments


References
























2

H2

O andH2












CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




30 35 40 45 50 55 60 65

Au (2

00)

ZnO

(103

)

ZnO

(110

)

ZnO

(102

)

Au (1

11)

ZnO

(101

)

ZnO

(002

)Zn

O (1

00)

Inte

nsity

(au

)


2120579 (∘)

(a)


300 400 500 600 700 800

350 375 400

Inte

nsity

(au

)

Wavelength (nm)

In-doped

(b)


0 100 200 300 400 500 600 700 800100

150

200

250

Resp

onse

Concentration (ppm)


(a)

0 100 200 300 400 500 600 700 800

100

200

300

400

500

600

700

800

Resp

onse

Concentration (ppm)


(b)



3COCH






0 50 100 15040

50

60

70

80

90

100

Time (s)


Resis

tanc

e (kΩ

)






2

minus O2


2



3CHOminus +H

2O (g) (6)



CH3CHO (ad) + 5O

2

2minus


180 200 220 240 260 280 30075

150

225

300

375

450

Resp

onse

(S

)


Temperature (∘C)



H3COCH


2(gas)


(9)



M2O3= 2MZn + 2O119909 +

1

2

O2(g) + 2e1015840 (10)



2O3



4 Conclusions




Acknowledgments


References
























2

H2

O andH2












CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




0 50 100 15040

50

60

70

80

90

100

Time (s)


Resis

tanc

e (kΩ

)






2

minus O2


2



3CHOminus +H

2O (g) (6)



CH3CHO (ad) + 5O

2

2minus


180 200 220 240 260 280 30075

150

225

300

375

450

Resp

onse

(S

)


Temperature (∘C)



H3COCH


2(gas)


(9)



M2O3= 2MZn + 2O119909 +

1

2

O2(g) + 2e1015840 (10)



2O3



4 Conclusions




Acknowledgments


References
























2

H2

O andH2












CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




4 Conclusions




Acknowledgments


References
























2

H2

O andH2












CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



research article high performance indium-doped zno gas...

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