Fabrication of Novel 2D NiO Nanosheet Branched on 1D-ZnO Nanorod Arrays for Gas Sensor Application

Research ArticleFabrication of Novel 2D NiO Nanosheet Branched on1D-ZnO Nanorod Arrays for Gas Sensor Application

Le Thuy Hoa Huynh Ngoc Tien and Seung Hyun Hur

School of Chemical Engineering and Bioengineering University of Ulsan Daehak-ro 102 Nam-gu Ulsan 680-749 Republic of Korea

Correspondence should be addressed to Seung Hyun Hur shhurulsanackr

Received 4 August 2014 Revised 27 October 2014 Accepted 28 October 2014 Published 13 November 2014

Academic Editor Santanu K Maiti

Copyright copy 2014 LeThuy Hoa 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

Fabrication of 3D structures composed of 1D n-type ZnO nanorods (NRs) and 2D p-type NiO nanosheets (NSs) by a low-cost low-temperature and large-area scalable hydrothermal process and its use in highly sensitive NO

2gas sensors were studied The p-n

heterojunctions formed byNiO-ZnO interfaces as well as large area two-dimensional NiONSs themselves increased the adsorptionof NO

2 Moreover the charge transfer between NiO and ZnO enhanced the responsivity and sensitivity of NO

2sensing even at

a concentration of 1 ppm The 30-min NiO NS growth on ZnO NRs in the hybrid sensor showed the highest sensitivity due tothe formation of optimum p-n heterojunctions between ZnO NRs and NiO NSs for gas adsorption and carrier transport Lowresponsivity toward reducing gases was also observed

1 Introduction

Among various metal oxide semiconductors zinc oxide(ZnO) has been used extensively due to its beneficial electri-cal properties such as wide band gap (337 eV) and large bind-ing energy (60meV) When hybridized with other materialsZnO showed unique properties that can be used effectivelyfor many types of electronic devices including gas sensors[1 2] solar cells [3 4] and UV sensors [5 6] Moreover itcan be easily grown by hydrothermal methods in the formof nanostructures such as nanorods and nanowires that canexhibit enhanced properties due to increased surface areato volume ratios Due to its excellent resistance to variouschemicals nickel oxide (NiO) has been widely explored forgas sensors [7 8] supercapacitors [9 10] electrochromicdevices [11 12] and lithium ion batteries [13 14] A widerange of NiO nanostructures such as thin films [15 16]nanotubes [17 18] and flake-like structures [19] can beformed by sputtering methods [20 21] sol-gel processes[22 23] and hydrothermal synthesis [24 25] Recent studiesalso demonstrated that the hybrid structures composed ofNiO nanotubes and ZnO shells exhibited highly improvedhydrogen sulfide (H

2S) sensing properties by the formation

of heterointerfaces [26] In our previous work we showed

that optimized hybrid structures of NiO and ZnO can beeffectively used as optical sensors due to improved chargetransfer and decreased recombination of excitons

In this paper the 3D hybrid structures composed of n-type 1D ZnO nanorods (NRs) and p-type 2DNiO nanosheets(NSs) with different NiO growth times were fabricatedusing low-cost and easy hydrothermal methods (Figure 7)These were used to detect toxic gas such as nitrogen oxide(NO2) which has a characteristic sharp biting odor and is

regarded as a prominent air pollutant Due to the chargetransfer between the two nanostructures as well as increasedadsorption sites formed by large-area 2D NiO NSs andelectron-depleted p-n junctions theNiONSZnONRhybridstructures exhibited improved NO

2sensing properties At

optimized conditions the hybrid structures showed a 2800-fold higher sensitivity than pure ZnO NRs and pure NiONSs even at very low NO

2concentrations (1 ppm) They

also exhibited good selectivity toward NO2when other gases

including H2 NH3 and H

2S were tested and compared

2 Experimental

The hybrid structure was fabricated step by step as describedin our previous study [27]

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2014 Article ID 710874 6 pageshttpdxdoiorg1011552014710874

2 Journal of Nanomaterials

Figure 1 The MST-5000 gas sensor system (MST-5000 MS-Tech)and the chamber equipped with a heating plate and probes (inset)gas flow was controlled precisely by mass flow controllers andnitrogen was used as a carrier gas

21 Growth of ZnO NRs The primary step is grow-ing vertically aligned ZnO NRs on the sensing elec-trode via a hydrothermal process In brief diethanolamine(HN(CH

2CH2OH)2 Sigma-Aldrich) was added slowly into

the mixed solution which was composed of zinc acetatedehydrate (Zn(CH

3COO)

2sdot2H2O 98 Sigma-Aldrich) and

2-methoxyethanol (CH3OC2H5OH Sigma-Aldrich) under

continuous magnetic stirring at 70∘C for 2 h Then thesolution was spin-coated onto the sensing electrode pat-terned SiO

2Si wafer and annealed in air at 400∘C for

4 h ZnO NRs were grown by exposing the prepared seedlayer to the mixed aqueous solution of zinc nitrate hexahy-drate 002M (Zn(NO

3)2sdot6H2O Sigma-Aldrich) and hexam-

ethylenetetramine 002M (C6H12N4 Sigma-Aldrich) at 90∘C

for 5 h ZnO NRs were rinsed with deionized water severaltimes and dried in a vacuum oven before the next steps

22 Growth of NiO NSs To grow NiO NSs on the ZnONR surface the Ni seed solution was first prepared bymixing nickel acetate tetrahydrate (Ni(OCOCH

3)2sdot4H2O

Sigma-Aldrich)with 2-methoxyethanol (CH3OCH2CH2OH

Sigma-Aldrich) and diethanolamine (HN(CH2CH2OH)2

Sigma-Aldrich) under continuous magnetic stirring at 70∘Cfor 2 h Then the mixed solution was spin-coated onto theZnO NRs and annealed in air at 400∘C for 4 h to forma Ni seed layer Next the ZnO NRs covered with the Niseed layer were directly exposed to a mixed aqueous solu-tion of nickel nitrate hexahydrate 002M (Ni(NO

3)2sdot6H2O

98 Sigma-Aldrich) and hexamethylenetetramine 002M(C6H12N4 Sigma-Aldrich) at 90∘C to grow NiO NSs Finally

NiO NSsZnO NRs were rinsed and annealed in air at 400∘Cfor 4 h

23 Instrumental Analysis The structures of NiO NSsZnONRs NiO NSs and ZnO NRs were characterized by X-ray diffraction (XRD) and field-emission scanning electronmicroscopy (FE-SEM) after thin film growth on the SiO

2Si

wafer Gas sensing properties were measured using a MST-5000 chamber (MS-Tech Figure 1) at 200∘C Gas flow wasprecisely controlled using a mass flow controller (GMC 1200

ATOVAC) and nitrogen (N2) was used as a carrier gas A

semiconductor parameter analyzer (Hewlett-Packard-4155A)was used to measure the resistance of sensing devices

3 Results and Discussion

The morphology change of NiO NSs on the ZnO NRs atdifferent NiO growth times was investigated and is shownin Figure 2 We can only infer the growth of NiO NSsby the increased roughness of ZnO NRs or some fibril-like structures between ZnO NRs at short growth times(Figures 2(a) and 2(b)) Instead the 3D networks of NiONSs between ZnO NRs are clearly observed after longergrowth times which implies that various morphologies ofNiO NSZnONR hybrid structures that have different NiZnratios can be realized by changing the NiO growth time

In order to verify the crystal structure and phase purityof the samples XRD analysis was used and these results areshown in Figure 3 NiONSs exhibit three characteristic peaksat 3730∘ 4328∘ and 6523∘ which respectively correspondto the (111) (200) and (220) planes of bunsenite structureNiO [28] ZnO NR shows peaks at 3187∘ 3454∘ 5681∘ and6723∘ which respectively correspond to the (100) (002)(110) and (201) planes of hexagonal structure of ZnO [29]The intensity ratio of INi(220)IZn(002) was increased from02538 to 44134 as theNiO growth time increased from 5minto 60min which indicates continuous growth of NiONSs onthe ZnO NRs

The atomic composition and TEM image were shown inFigure 4 Both the Zn and Ni atoms were observed in theenergy dispersive X-ray (EDX) analysis and ZnONR coveredwith NiO NS was also clearly observed in the TEM image

The 119868-119881 characteristics of ZnO NRs NiO NSs and NiONSsZnO NRs were measured to investigate the electricalresistance of each sample The electrodes used in this studywere prepared following our previous study [27] As shownin Figure 5 after decorating the NiO NSs with ZnO NRsthe conductance was decreased due to the formation of localelectron depletion layers at the heterojunctions between p-type NiO and n-type ZnO The electrical resistivity of ZnOnanorods calculated using the thickness of the ZnO seedlayer as shown in the inset of Figure 4 was approximately414 times 10

minus5Ωsdotcm which was higher than that of a previous

report because hydrothermal synthesis was used in this study[28]

NO2responsivity (RS (119877

119892minus 119877

119886)119877119892()) and sensitivity

(RSgas concentration (ppm)) of the NiO NSsZnO NRs andNiONSs alone were measured at low levels of NO

2 where 119877

119886

and119877119892are the resistances of the sensing layermeasured in the

atmosphere of only N2andNO

2N2gasmixture respectively

As shown in Figure 6 the NiO NSZnO NR hybrid structureexhibited around a 6-fold higher responsivity and 9-foldhigher sensitivity than those of NiO NSs which has a largeenough adsorption areaThe sensitivity obtained in this studyof 34026 ppmminus1 from theNiONS (30min growth time)ZnONR sample is three orders (sim2800-fold) higher than that ofpure ZnO NRs and even one order (sim20-fold) higher thanthat of 0-D CuO nanoparticles decorated with 1D ZnO NR

Journal of Nanomaterials 3

(a) (b)

(c) (d)

Figure 2 SEM images of NiO NSs grown on ZnO NRs for (a) 5min (b) 15min (c) 45min and (d) 60min

20 40 60 80

2120579 (deg)

Inte

nsity

(au

)

60min

45min

30min

15min

5min

INi(220)IZn(002) = 44134

INi(220)IZn(002) = 42787

INi(220)IZn(002) = 27742

INi(220)IZn(002) = 23373

INi(220)IZn(002) = 02538

(002)(100)

(111)

(200) (110)

(220)(201)

Figure 3 XRD spectra of NiO NSs grown on ZnO NRs at variousNiO NS growth times from 5min to 60min

that were reported previously [29] The fast response time ofNiO NSZnO NR than that of NiO NS was observed whichcan be due to the high amount of p-n junction sites thatcan effectively adsorb NO

2molecules It is also interesting

to note that the NO2sensitivity increased initially as the

NiO NS growth time increased until 30min then the NO2

sensitivity decreased gradually which indicates the presenceof an optimum ratio ofNiONSs andZnONRs In turn excessgrowth of NiO NSs will cover the whole ZnO NR surfaceand thus synergetic effects between NiO NSs and ZnO NRscannot be expected

The improved NO2response and sensitivity of the NiO

NSZnO NR hybrid structure can be explained as followsFirst increased surface area after NiO NS growth on ZnOprovided a larger NO

2adsorption area In addition the

numerous electron-depletion layers that formed at the p-NiONS and n-ZnO heterojunction region attracted the gasesmore than bare ZnO NRs or NiO NSs alone which resultsin more NO

2adsorption even at low concentration [30]

Another important fact is the charge transfer between p-type NiO NSs and n-type ZnO NRs [28] When NO

2gas

is exposed to the NiO NSs the NO2first reacts with the

adsorbed Ominus ions on NiO as NO2has higher electron affinity

than that of the preadsorbed oxygen [31] this results in thegeneration of holes on NiO surfaces

NO2(gas) 997888rarr NO

2(ads) (1)

NO2(ads) 997888rarr NO

2

minus(ads) + h+ (2)

Then the transfer of holes from p-type NiO to n-typeZnO (loss of electrons of n-type ZnO) leads to a resistanceincrease of the ZnO seed layer that carries charges betweentwo electrodes Without NiO-ZnO heterojunctions (NiO


(keV)

O

Pt

PtPt

Si

Zn

ZnZnNi

Ni

Ni

Spectrum 1

0 2 4 6 8 10 12 14 16 18 20

Full scale 10541 cts cursor 0000 keV

(a)

ZnO NRNiO NS

(b)

Figure 4 (a) EDX spectrum of NiO NSsZnO NRs and (b) TEMimage of ZnO NR covered with NiO NS

20

10

00

minus20

minus10

minus2 minus1 0 21

Voltage (V)

Curr

ent (

A)

ZnO NRNiO NSZnO NR

times10minus3

Figure 5 119868-119881 curves of ZnONRs andNiONSsZnONRsThe insetshows a cross-sectional SEM image of ZnO seed layer

alone) there will be hole accumulation on the NiO surfacewhich will cause low responsivity due to the suppressedfurther adsorption of NO

2 On the other hand in the

presence of p-n heterojunctions hole transfer from NiONSs to ZnO NRs will prevent the accumulation of holesin NiO NSs thus maintaining the adsorption of NO

2 This

will enhance both the responsivity and sensitivity of sensinglayers

The responsivity of fabricated NiO NSZnO NR gassensors toward reducing gases such as H

2S H2 and NH

3

was also investigated As shown in Figure 6 the responsivityfor reducing gases was lower than that for NO

2gas which

can be due to the low catalytic effect during the adsorption ofreducing gases [32] As described in the following reactionwhen reducing gases are adsorbed onto the metal oxide sur-face they are oxidized through the reaction with preadsorbedoxygen ions Due to the low dissociative chemisorptionof hydrogen by the NiO there is less electron generationwhich results in the lower responsivity of NiO NSZnO NRsensors for reducing gases than that for NO

2 The surface

modification by Pt which has excellent H2adsorption and

catalytic dissociation ability can enhance the responsivity ofNiO-based sensors toward reducing gases [33]

H2(gas) +Ominus (ads) 997888rarr H

2O + eminus (3)

The highest responsivity at 30min of NiO NS growthindicates the similar synergetic effect of charge transferbetween NiO NSs and ZnO NRs as the NO

2gas case The

negative responsivity of the hybrid sensor for reducing gasesis due to the electron generated by the oxidation of reducinggases Electrons transferred from the p-type NiO NSs to n-type ZnOmight decrease the resistance of n-type ZnO whichacts as channel materials for carrier transport in the sensingdevices

4 Conclusions

The3D structures composed of 1DZnONRs and 2DNiONSswere fabricated by an easy and cost-effective hydrothermalsynthesis method and these were then used for NO

2sensors

Due to the increased surface areas formation of electrondepletion layers at ZnO-NiO heterojunctions and effectivecarrier transport between two nanostructured semiconduc-tors the NiONSZnONR sensors exhibited highly improvedsensitivity toward NO

2gases over pure NiO NS and ZnO

NR sensors It was also observed that at a 30min NiO NSgrowth on ZnO NRs hybrid sensors exhibited maximumNO2sensitivity due to the formation of optimized 1D-2D p-n

heterojunction hybrid structures Due to the low dissociativechemisorption of H

2on NiO hybrid sensors exhibited low

response toward reducing gases which results in improvedselectivity We think that this type of 3D structures can beeffectively used in many gas-sensing applications due to itssimple fabrication process and high sensing performance


400

300

200

100

0

0 500 1000

Resp

onsiv

ity (

)

ZnO NRNiO NSZnO NRNiO NS

1ppm

5ppm

10ppm

15ppm

Time (s)

(a)

400

300

200

100

0

Resp

onsiv

ity (

)

0 5 10 15 20

5min

15min30min

45min

60min

NiO NS

ZnO NR

k = 34026ppmminus1

k = 22305ppmminus1

k = 1324ppmminus1

k = 03386ppmminus1

k = 03659ppmminus1

k = 00846ppmminus1

k = 00121 ppmminus1

Gas concentration (ppm)

(b)

Figure 6 (a) Responsivity of pure NiO NSs and NiO NSsZnO NRs at 30min NiO growth and (b) the sensitivity of NiO NSsZnO NRs atvarious NiO growth times toward various NO

2gas concentrations

400

350

300

250

200

150

100

minus50

50

0

Resp

onsiv

ity (

)

5min

15min

30min

45min

60min

NO2

H2S NH3H2(4)

Figure 7 Responsivity of NiONSZnONR sensors at different NiOgrowth times under various gases Concentrations of NO

2 H2S and

NH3were 100 ppm and that of H

2was 4 respectively

Conflict of Interests

All authors have no conflict of interests to declare Thisstatement is to certify that all authors have seen and approvedthe paper being submitted The authors warrant that thepaper is the authorsrsquo original work The authors warrantthat the paper has not received prior publication and is notunder consideration for publication elsewhere On behalfof all coauthors the corresponding author shall bear full

responsibility for the submission This research has not beensubmitted for publication nor has it been published in wholeor in part elsewhere

Acknowledgment

This research was supported by the Basic Science ResearchProgram through the National Research Foundation ofKorea (NRF) funded by the Ministry of Education(2013R1A1A2A10004468)

References

[1] 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

[2] 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

[3] S H Ko D Lee H W Kang et al ldquoNanoforest of hydrother-mally grown hierarchical ZnO nanowires for a high efficiencydye-sensitized solar cellrdquo Nano Letters vol 11 no 2 pp 666ndash671 2011

[4] K Matsubara P Fons K Iwata et al ldquoZnO transparentconducting films deposited by pulsed laser deposition for solarcell applicationsrdquo Thin Solid Films vol 431-432 pp 369ndash3722003

[5] H Ohta M Kamiya T Kamiya M Hirano and H HosonoldquoUV-detector based on pn-heterojunction diode composed oftransparent oxide semiconductors p-NiOn-ZnOrdquo Thin SolidFilms vol 445 no 2 pp 317ndash321 2003


[6] C S Lao M-C Park Q Kuang et al ldquoGiant enhance-ment in UV response of ZnO nanobelts by polymer surface-functionalizationrdquo Journal of the American Chemical Societyvol 129 no 40 pp 12096ndash12097 2007

[7] I Hotovy V Rehacek P Siciliano S Capone and L SpiessldquoSensing characteristics of NiO thin films as NO

2gas sensorrdquo

Thin Solid Films vol 418 no 1 pp 9ndash15 2002[8] H Steinebach S Kannan L Rieth and F Solzbacher ldquoH

2

gas sensor performance of NiO at high temperatures in gasmixturesrdquo Sensors and Actuators B Chemical vol 151 no 1 pp162ndash168 2010

[9] JW Lee T Ahn J H Kim JM Ko and J-D Kim ldquoNanosheetsbasedmesoporous NiOmicrospherical structures via facile andtemplate-free method for high performance supercapacitorsrdquoElectrochimica Acta vol 56 no 13 pp 4849ndash4857 2011

[10] J Cheng G-P Cao and Y-S Yang ldquoCharacterization of sol-gel-derived NiOx xerogels as supercapacitorsrdquo Journal of PowerSources vol 159 no 1 pp 734ndash741 2006

[11] X H Xia J P Tu J Zhang X L Wang W K Zhang and HHuang ldquoElectrochromic properties of porous NiO thin filmsprepared by a chemical bath depositionrdquo Solar EnergyMaterialsamp Solar Cells vol 92 no 6 pp 628ndash633 2008

[12] Z Jiao M Wu Z Qin and H Xu ldquoThe electrochromic char-acteristics of sol-gel-prepared NiO thin filmrdquo Nanotechnologyvol 14 no 4 pp 458ndash461 2003

[13] B Varghese M V Reddy Z Yanwu et al ldquoFabrication of NiOnanowall electrodes for high performance lithium ion batteryrdquoChemistry of Materials vol 20 no 10 pp 3360ndash3367 2008

[14] S A Needham G X Wang and H K Liu ldquoSynthesis ofNiO nanotubes for use as negative electrodes in lithium ionbatteriesrdquo Journal of Power Sources vol 159 no 1 pp 254ndash2572006

[15] K-W Nam W-S Yoon and K-B Kim ldquoX-ray absorptionspectroscopy studies of nickel oxide thin film electrodes forsupercapacitorsrdquo Electrochimica Acta vol 47 no 19 pp 3201ndash3209 2002

[16] P S Patil and L D Kadam ldquoPreparation and characterization ofspray pyrolyzed nickel oxide (NiO) thin filmsrdquo Applied SurfaceScience vol 199 no 1ndash4 pp 211ndash221 2002

[17] G Malandrino L M S Perdicaro I L Fragala R L Nigro MLosurdo and G Bruno ldquoMOCVD template approach to thefabrication of free-standing nickel(II) oxide nanotube arraysstructural morphological and optical properties characteriza-tionrdquoTheJournal of Physical ChemistryC vol 111 no 8 pp 3211ndash3215 2007

[18] H Pang Q Lu Y Li and F Gao ldquoFacile synthesis of nickeloxide nanotubes and their antibacterial electrochemical andmagnetic propertiesrdquo Chemical Communications no 48 pp7542ndash7544 2009

[19] J-W Lang L-B KongW-JWu Y-C Luo and L Kang ldquoFacileapproach to prepare loose-packed NiO nano-flakes materialsfor supercapacitorsrdquo Chemical Communications no 35 pp4213ndash4215 2008

[20] H-L Chen Y-M Lu and W-S Hwang ldquoCharacterization ofsputtered NiO thin filmsrdquo Surface and Coatings Technology vol198 no 1ndash3 pp 138ndash142 2005

[21] Y M Lu W S Hwang and J S Yang ldquoEffects of substratetemperature on the resistivity of non-stoichiometric sputteredNiOx filmsrdquo Surface and Coatings Technology vol 155 no 2-3pp 231ndash235 2002

[22] J L Garcia-Miquel Q Zhang S J Allen et al ldquoNickel oxide sol-gel films from nickel diacetate for electrochromic applicationsrdquoThin Solid Films vol 424 no 2 pp 165ndash170 2003

[23] E O Zayim I Turhan F Z Tepehan and N Ozer ldquoSol-geldeposited nickel oxide films for electrochromic applicationsrdquoSolar Energy Materials amp Solar Cells vol 92 no 2 pp 164ndash1692008

[24] D-B Kuang B-X Lei Y-P Pan X-Y Yu and C-Y Su ldquoFabri-cation of novel hierarchical 120573-Ni(OH)

2and NiO microspheres

via an easy hydrothermal processrdquo The Journal of PhysicalChemistry C vol 113 no 14 pp 5508ndash5513 2009

[25] LWang YHao Y Zhao Q Lai andX Xu ldquoHydrothermal syn-thesis and electrochemical performance of NiO microsphereswith different nanoscale building blocksrdquo Journal of Solid StateChemistry vol 183 no 11 pp 2576ndash2581 2010

[26] L Xu R Zheng S Liu et al ldquoNiOZnO heterostructurednanotubes coelectrospinning fabrication characterization andhighly enhanced gas sensing propertiesrdquo Inorganic Chemistryvol 51 no 14 pp 7733ndash7740 2012

[27] L T Hoa H N Tien and S H Hur ldquoA highly sensitive UVsensor composed of 2D NiO nanosheets and 1D ZnO nanorodsfabricated by a hydrothermal processrdquo Sensors and Actuators APhysical vol 207 pp 20ndash24 2014

[28] L T Hoa H N Tien V H Luan J S Chung and S HHur ldquoFabrication of a novel 2D-graphene2D-NiO nanosheet-based hybrid nanostructure and its use in highly sensitive NO

2

sensorsrdquo Sensors and Actuators B Chemical vol 185 pp 701ndash705 2013

[29] L T Hoa and S H Hur ldquoHighly sensitive NO2sensors based on

local pminusn heterojunctions composed of 0D CuO nanoparticlesand 1D ZnO nanorodsrdquo Physica Status Solidi (A) vol 210 no 6pp 1213ndash1216 2013

[30] C W Na H-S Woo I-D Kim and J-H Lee ldquoSelectivedetection of NO

2and C

2H5OH using a Co

3O4-decorated ZnO

nanowire network sensorrdquo Chemical Communications vol 47no 18 pp 5148ndash5150 2011

[31] N D Hoa and S A El-Safty ldquoSynthesis of mesoporous NiOnanosheets for the detection of toxic NO

2gasrdquo Chemistry vol

17 no 46 pp 12896ndash12901 2011[32] A Borgschulte R J Westerwaal J H Rector B Dam and R

Griessen ldquoHydrogen sorption mechanism of oxidized nickelclustersrdquo Applied Physics Letters vol 85 no 21 pp 4884ndash48862004

[33] I Hotovy J Huran P Siciliano S Capone L Spiess and VRehacek ldquoEnhancement of H

2sensing properties of NiO-based

thin films with a Pt surfacemodificationrdquo Sensors and ActuatorsB Chemical vol 103 no 1-2 pp 300ndash311 2004

Submit your manuscripts athttpwwwhindawicom

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materials


Journal ofNanomaterials


Figure 1 The MST-5000 gas sensor system (MST-5000 MS-Tech)and the chamber equipped with a heating plate and probes (inset)gas flow was controlled precisely by mass flow controllers andnitrogen was used as a carrier gas

21 Growth of ZnO NRs The primary step is grow-ing vertically aligned ZnO NRs on the sensing elec-trode via a hydrothermal process In brief diethanolamine(HN(CH

2CH2OH)2 Sigma-Aldrich) was added slowly into

the mixed solution which was composed of zinc acetatedehydrate (Zn(CH

3COO)

2sdot2H2O 98 Sigma-Aldrich) and

2-methoxyethanol (CH3OC2H5OH Sigma-Aldrich) under

continuous magnetic stirring at 70∘C for 2 h Then thesolution was spin-coated onto the sensing electrode pat-terned SiO

2Si wafer and annealed in air at 400∘C for

4 h ZnO NRs were grown by exposing the prepared seedlayer to the mixed aqueous solution of zinc nitrate hexahy-drate 002M (Zn(NO

3)2sdot6H2O Sigma-Aldrich) and hexam-

ethylenetetramine 002M (C6H12N4 Sigma-Aldrich) at 90∘C

for 5 h ZnO NRs were rinsed with deionized water severaltimes and dried in a vacuum oven before the next steps

22 Growth of NiO NSs To grow NiO NSs on the ZnONR surface the Ni seed solution was first prepared bymixing nickel acetate tetrahydrate (Ni(OCOCH

3)2sdot4H2O

Sigma-Aldrich)with 2-methoxyethanol (CH3OCH2CH2OH

Sigma-Aldrich) and diethanolamine (HN(CH2CH2OH)2

Sigma-Aldrich) under continuous magnetic stirring at 70∘Cfor 2 h Then the mixed solution was spin-coated onto theZnO NRs and annealed in air at 400∘C for 4 h to forma Ni seed layer Next the ZnO NRs covered with the Niseed layer were directly exposed to a mixed aqueous solu-tion of nickel nitrate hexahydrate 002M (Ni(NO

3)2sdot6H2O

98 Sigma-Aldrich) and hexamethylenetetramine 002M(C6H12N4 Sigma-Aldrich) at 90∘C to grow NiO NSs Finally

NiO NSsZnO NRs were rinsed and annealed in air at 400∘Cfor 4 h

23 Instrumental Analysis The structures of NiO NSsZnONRs NiO NSs and ZnO NRs were characterized by X-ray diffraction (XRD) and field-emission scanning electronmicroscopy (FE-SEM) after thin film growth on the SiO

2Si

wafer Gas sensing properties were measured using a MST-5000 chamber (MS-Tech Figure 1) at 200∘C Gas flow wasprecisely controlled using a mass flow controller (GMC 1200

ATOVAC) and nitrogen (N2) was used as a carrier gas A

semiconductor parameter analyzer (Hewlett-Packard-4155A)was used to measure the resistance of sensing devices

3 Results and Discussion

The morphology change of NiO NSs on the ZnO NRs atdifferent NiO growth times was investigated and is shownin Figure 2 We can only infer the growth of NiO NSsby the increased roughness of ZnO NRs or some fibril-like structures between ZnO NRs at short growth times(Figures 2(a) and 2(b)) Instead the 3D networks of NiONSs between ZnO NRs are clearly observed after longergrowth times which implies that various morphologies ofNiO NSZnONR hybrid structures that have different NiZnratios can be realized by changing the NiO growth time

In order to verify the crystal structure and phase purityof the samples XRD analysis was used and these results areshown in Figure 3 NiONSs exhibit three characteristic peaksat 3730∘ 4328∘ and 6523∘ which respectively correspondto the (111) (200) and (220) planes of bunsenite structureNiO [28] ZnO NR shows peaks at 3187∘ 3454∘ 5681∘ and6723∘ which respectively correspond to the (100) (002)(110) and (201) planes of hexagonal structure of ZnO [29]The intensity ratio of INi(220)IZn(002) was increased from02538 to 44134 as theNiO growth time increased from 5minto 60min which indicates continuous growth of NiONSs onthe ZnO NRs

The atomic composition and TEM image were shown inFigure 4 Both the Zn and Ni atoms were observed in theenergy dispersive X-ray (EDX) analysis and ZnONR coveredwith NiO NS was also clearly observed in the TEM image

The 119868-119881 characteristics of ZnO NRs NiO NSs and NiONSsZnO NRs were measured to investigate the electricalresistance of each sample The electrodes used in this studywere prepared following our previous study [27] As shownin Figure 5 after decorating the NiO NSs with ZnO NRsthe conductance was decreased due to the formation of localelectron depletion layers at the heterojunctions between p-type NiO and n-type ZnO The electrical resistivity of ZnOnanorods calculated using the thickness of the ZnO seedlayer as shown in the inset of Figure 4 was approximately414 times 10

minus5Ωsdotcm which was higher than that of a previous

report because hydrothermal synthesis was used in this study[28]

NO2responsivity (RS (119877

119892minus 119877

119886)119877119892()) and sensitivity

(RSgas concentration (ppm)) of the NiO NSsZnO NRs andNiONSs alone were measured at low levels of NO

2 where 119877

119886

and119877119892are the resistances of the sensing layermeasured in the

atmosphere of only N2andNO

2N2gasmixture respectively

As shown in Figure 6 the NiO NSZnO NR hybrid structureexhibited around a 6-fold higher responsivity and 9-foldhigher sensitivity than those of NiO NSs which has a largeenough adsorption areaThe sensitivity obtained in this studyof 34026 ppmminus1 from theNiONS (30min growth time)ZnONR sample is three orders (sim2800-fold) higher than that ofpure ZnO NRs and even one order (sim20-fold) higher thanthat of 0-D CuO nanoparticles decorated with 1D ZnO NR


(a) (b)

(c) (d)


20 40 60 80

2120579 (deg)

Inte

nsity

(au

)

60min

45min

30min

15min

5min

INi(220)IZn(002) = 44134

INi(220)IZn(002) = 42787

INi(220)IZn(002) = 27742

INi(220)IZn(002) = 23373

INi(220)IZn(002) = 02538

(002)(100)

(111)

(200) (110)

(220)(201)













2gas





2(ads) (1)


2

minus(ads) + h+ (2)



(keV)

O

Pt

PtPt

Si

Zn

ZnZnNi

Ni

Ni

Spectrum 1

0 2 4 6 8 10 12 14 16 18 20


(a)

ZnO NRNiO NS

(b)


20

10

00

minus20

minus10

minus2 minus1 0 21

Voltage (V)

Curr

ent (

A)

ZnO NRNiO NSZnO NR

times10minus3





2 This



2S H2 and NH

3


2gas which


2 The surface




2O + eminus (3)


2gas case The


4 Conclusions


2sensors








400

300

200

100

0

0 500 1000

Resp

onsiv

ity (

)


1ppm

5ppm

10ppm

15ppm

Time (s)

(a)

400

300

200

100

0

Resp

onsiv

ity (

)

0 5 10 15 20

5min

15min30min

45min

60min

NiO NS

ZnO NR

k = 34026ppmminus1

k = 22305ppmminus1

k = 1324ppmminus1

k = 03386ppmminus1

k = 03659ppmminus1

k = 00846ppmminus1

k = 00121 ppmminus1


(b)


2gas concentrations

400

350

300

250

200

150

100

minus50

50

0

Resp

onsiv

ity (

)

5min

15min

30min

45min

60min

NO2

H2S NH3H2(4)


2 H2S and


2was 4 respectively




Acknowledgment


References









2gas sensorrdquo


2
























2





2and C

2H5OH using a Co

3O4-decorated ZnO
















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b)

(c) (d)


20 40 60 80

2120579 (deg)

Inte

nsity

(au

)

60min

45min

30min

15min

5min

INi(220)IZn(002) = 44134

INi(220)IZn(002) = 42787

INi(220)IZn(002) = 27742

INi(220)IZn(002) = 23373

INi(220)IZn(002) = 02538

(002)(100)

(111)

(200) (110)

(220)(201)













2gas





2(ads) (1)


2

minus(ads) + h+ (2)



(keV)

O

Pt

PtPt

Si

Zn

ZnZnNi

Ni

Ni

Spectrum 1

0 2 4 6 8 10 12 14 16 18 20


(a)

ZnO NRNiO NS

(b)


20

10

00

minus20

minus10

minus2 minus1 0 21

Voltage (V)

Curr

ent (

A)

ZnO NRNiO NSZnO NR

times10minus3





2 This



2S H2 and NH

3


2gas which


2 The surface




2O + eminus (3)


2gas case The


4 Conclusions


2sensors








400

300

200

100

0

0 500 1000

Resp

onsiv

ity (

)


1ppm

5ppm

10ppm

15ppm

Time (s)

(a)

400

300

200

100

0

Resp

onsiv

ity (

)

0 5 10 15 20

5min

15min30min

45min

60min

NiO NS

ZnO NR

k = 34026ppmminus1

k = 22305ppmminus1

k = 1324ppmminus1

k = 03386ppmminus1

k = 03659ppmminus1

k = 00846ppmminus1

k = 00121 ppmminus1


(b)


2gas concentrations

400

350

300

250

200

150

100

minus50

50

0

Resp

onsiv

ity (

)

5min

15min

30min

45min

60min

NO2

H2S NH3H2(4)


2 H2S and


2was 4 respectively




Acknowledgment


References









2gas sensorrdquo


2
























2





2and C

2H5OH using a Co

3O4-decorated ZnO
















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(keV)

O

Pt

PtPt

Si

Zn

ZnZnNi

Ni

Ni

Spectrum 1

0 2 4 6 8 10 12 14 16 18 20


(a)

ZnO NRNiO NS

(b)


20

10

00

minus20

minus10

minus2 minus1 0 21

Voltage (V)

Curr

ent (

A)

ZnO NRNiO NSZnO NR

times10minus3





2 This



2S H2 and NH

3


2gas which


2 The surface




2O + eminus (3)


2gas case The


4 Conclusions


2sensors








400

300

200

100

0

0 500 1000

Resp

onsiv

ity (

)


1ppm

5ppm

10ppm

15ppm

Time (s)

(a)

400

300

200

100

0

Resp

onsiv

ity (

)

0 5 10 15 20

5min

15min30min

45min

60min

NiO NS

ZnO NR

k = 34026ppmminus1

k = 22305ppmminus1

k = 1324ppmminus1

k = 03386ppmminus1

k = 03659ppmminus1

k = 00846ppmminus1

k = 00121 ppmminus1


(b)


2gas concentrations

400

350

300

250

200

150

100

minus50

50

0

Resp

onsiv

ity (

)

5min

15min

30min

45min

60min

NO2

H2S NH3H2(4)


2 H2S and


2was 4 respectively




Acknowledgment


References









2gas sensorrdquo


2
























2





2and C

2H5OH using a Co

3O4-decorated ZnO
















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




400

300

200

100

0

0 500 1000

Resp

onsiv

ity (

)


1ppm

5ppm

10ppm

15ppm

Time (s)

(a)

400

300

200

100

0

Resp

onsiv

ity (

)

0 5 10 15 20

5min

15min30min

45min

60min

NiO NS

ZnO NR

k = 34026ppmminus1

k = 22305ppmminus1

k = 1324ppmminus1

k = 03386ppmminus1

k = 03659ppmminus1

k = 00846ppmminus1

k = 00121 ppmminus1


(b)


2gas concentrations

400

350

300

250

200

150

100

minus50

50

0

Resp

onsiv

ity (

)

5min

15min

30min

45min

60min

NO2

H2S NH3H2(4)


2 H2S and


2was 4 respectively




Acknowledgment


References









2gas sensorrdquo


2
























2





2and C

2H5OH using a Co

3O4-decorated ZnO
















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






2gas sensorrdquo


2
























2





2and C

2H5OH using a Co

3O4-decorated ZnO
















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



Fabrication of Novel 2D NiO Nanosheet Branched on 1D-ZnO Nanorod Arrays for Gas Sensor Application

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