Research Article Special Effect of Urea as a Stabilizer in Thermal …downloads.hindawi.com/journals/jnm/2013/163527.pdf · 2019-07-31 · ZnO nanostructures in vacuum and presence

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2013 Article ID 163527 7 pageshttpdxdoiorg1011552013163527

Research ArticleSpecial Effect of Urea as a Stabilizer in Thermal ImmersionMethod to Synthesis Porous Zinc Oxide Nanostructures

F S Husairi12 Syahirah Mhd Ali12 A Azlinda12 M Rusop13 and S Abdullah12

1 NANO-SciTech Centre (NST) Institute of Science Universiti Teknologi MARA (UiTM) 40450 Shah Alam Selangor Malaysia2 Faculty of Applied Sciences Universiti Teknologi MARA (UiTM) 40450 Shah Alam Selangor Malaysia3 NANO-ElecTronic Centre (NET) Faculty of Electrical Engineering Universiti Teknologi MARA (UiTM)40450 Shah Alam Selangor Malaysia

Correspondence should be addressed to F S Husairi mhusairifadzilahyahoocom

Received 19 March 2013 Revised 28 June 2013 Accepted 1 July 2013

Academic Editor Jinquan Wei

Copyright copy 2013 F S Husairi 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

ZnO nanostructure was prepared by catalytic immersion method (90∘C) with zinc nitrate hexahydrate (Zn(NO3

)2

6H2

O) as aprecursors and urea (CH

4

N2

O) as a stabilizer Different molarity concentration ratio of Zn(NO3

)2

6H2

O to CH4

N2

O 2 1 1 4 1 6and 1 8 is used in this workThe effect of urea concentration used during the synthesis process is discussedTheZnOnanostructureswere characterized by using field emission scanning electron microscope (FESEM) photoluminescene (PL) and I-V probe Porousnanoflakes are successfully synthesized on p-type silicon substrate coated with gold layer with different size and dimension Highintensity photoluminescence (PL) at optimum concentration indicated that urea is good stabilizer to produce ZnO nanostructureswith good crytallinity Rectifying characteristics show dramaticaly change in turn-on voltage when the concentration of ureaincreases in aqueous solution This is related to the theory about p-type doping of ZnO nanostructures by nitrogen from NH

3

1 Introduction

ZnO posses unique characteristics because it is a semicon-ductor material with a direct band gap of 337 eV and largeexcitation binding energy (60meV) which is important fornear-ultraviolet emission and transparent conductivity ZnOalso is a piezoelectric material which suitable for sensor andtranducers application Metal oxide semiconductingmaterialbecomes selecting device comparing solid state sensor dueto small dimension lower cost low power consumptionsimple processing and stableThe conductivity ofmetal oxidesemiconducting material can be improve by introducing adoping material The impurity used to reduce the band gapof TiO

2and Nminus and Cminus dopand successfully narrow it [1]

Nitrogen is a good p-type dopant for IIndashVI semiconductorsNitrogen dopant is used in metal oxide semiconductor toincrease conductivity and produce p-type conductormaterialbecause it more efficient than any other element sinceit contributes excess hole for conduction by forming anNndashZn(O)ndashN complex [2]

Differing from ZnO it is difficult to introduced Nminus inZnO because Zn atoms always preferentially combine withO rather than with N Nitrogen has been known as suitableimpurity for p-type doping in ZnO Based on theoreticalstudy incorporation of N in ZnON can be improved if weincrease the chemical potential in N source From previousstudy researcher used RF sputtering [3] technique CVDmethod [4] spray pyrolysis [5] and so forth to dope ZnOwith nitrogen Most of them report that the conducting ofZnO increases by increasing concentration of N source As-grown ZnO typically has n-type conductivity with back-ground concentrations between 1016 and 1017 cmminus3 [4] SoNminusis introduced in ZnO to increase the conductivity and changeit into p-type conducting material

In this paper we report on the synthesis of ZnON byusing immersion method with varying molar concentrationof urea in order to increase the source of N When ureadissolved in water it will produce CO

2and ammonias [6 7]

This CO2will form a carbonate ion (CO

3

2minus) which is used inZnO formation and ammonia molecules will supply the N

2 Journal of Nanomaterials

atom [8] Due to the much lower NndashH bonding energy inNH3 the decomposition of NH

3H2O will become the main

source for nitrogen in ZnON formationWe found that whenconcentration of urea increases the conductivity of ZnO filmalso increases and N

2response becomes more effective

2 Experimental

In this work zinc oxide nanostructures were grown by thecatalytic immersion method using zinc nitrate hexahydrate(Zn(NO

3)26H2O) as a precursors and urea as a stabilizer p-

type silicon (100) as substrates was cut with the size that is15 cm times 20 cm and clean with acetone methanol and HF(48 HF DI water (1 10)) The silicon substrate was sputtercoated with 6 nm thickness of gold (Au) as a catalyst in argonplasma

21 Synthesis of ZnO Nanostructures For solution prepa-ration the molar ratio of the Zinc Nitrate Hexahydrate[Zn(NO

3)26H2O] to urea [CH

4N2O] in 100 mL was varied

with ratio 2 1 1 4 1 6 and 1 8The aqueous solution stirredand heated at 60∘C for 1 hour and follow by the ageingprocess for 24 hours to produce homogenous solution Afterthe ageing process the silicon substrate inserted into testtube filled with 40mL of the solutions of different molarratio concentration Then the test tubes were immersedin a water bath at temperature of 90∘C After 4 hourssamples were dried in 1 hour at temperature 150∘C and wereannealed at temperature 500∘C for 1 hour The morphologyof the ZnO nanostructures was analyzed by Field EmissionScanning Electron Microscopy (JSM-7600F FESEM) Theoptical properties are characterized by photoluminescence(HORIBA JOBIN YVON PL) spectra at room temperature

22 I-V Characteristics and N2 Response Studies ZnO nanos-tructures were characterized by I-V characteristic to measureelectrical properties and N

2gas response Gold layer (as

a metal) with thickness 6 nm was coated on ZnO nanos-tructures by using sputtering technique like in Figure 1ZnO nanostructures were placed in tight box with inlet andoutlet for N

2gases The current-voltage characteristics of

ZnO nanostructures in vacuum and presence N2gas were

measured by Keithely 2400 multimeter and voltage source

3 Result and Discussion

31 Chemical Decomposition in Thermal Immersion MethodThe particle nucleation involves two steps nucleation andgrowth with the nucleation rate needs to be faster thangrowth rate Growth rate that depends on the amount ofreacting particles available while nucleation will take placeafter supersaturation is achieved and this is affected bythe solubility of the reacting particles [9] Urea is highlysoluble in water and when urea which is one of aminegroups dissolved in water it will slowly displacement bywater molucules to produce ammonia and carbonate anion

2-point terminal+ minus

Metal

SiliconZinc oxide

nanostructures

Figure 1 The schematic diagram of the electrodes plating on thesurface of ZnO nanostructures

(CO3

2minus) The progression of chemical synthesis of ZnO inpresence of urea is suggested as follows [6]

CON2H4+ 3H2O 997888rarr CO

2(g) + 2NH

3H2O (1)

When temperatures increase urea decomposed and pro-duced NH

3 NH

4

+ ions generated from NH3ions will

increase pH aqueous solution and support the ZnO crystalgrowth process

2NH3sdotH2O + CO

2997888rarr 2NH

4

+

+ CO3

2minus

+OHminus (2)

Zinc nitrate hexahydrate will provide Zn2+ ions when dis-solved in water as in

Zn(NO3)2

sdot 6H2O +H

2O rarr Zn2+ + 2NO

3

minus

+ 7H2O (3)

4Zn2+ + CO3

2minus

+ 6OHminus +H2O

997888rarr Zn4CO3(OH)6sdotH2O (s)

(4)

Zn4CO3(OH)6sdotH2O 997888rarr 4ZnO (s) + 4H

2O + CO

2(g) (5)

Zn4CO3(OH)6sdotZnO is formedduring the reaction in aqueous

solution and after heating at 500∘C it decomposed ZnO [10]The degree of supersaturation of Zn(OH)

2in the interrfacial

zone and the adsorption of organicinorganic species onthe surface of ZnO are the factors that can influence thenucleation and growth of ZnO nanostructures [11 12]

32 Morphology of the Zinc Oxide Nanostructures The pres-ence of Au layer on Si substrate is a catalyst because it canprovide an alternating energy pathway with lower activationenergy Heterogeneous nucleation promoted between Auparticle and Zn and O ions is energetically favorable sincethe interfacial energy between the ions and adsorbed sites islower This is comparative to homogenous nucleation of twosolid phases which has a higher activation energy barrier [13]A large lattice match between ZnO nanostructures and Si isanother factor of deposition of Au layer

The morphology of ZnO nanostructures is observed byusing Field Emission Scanning Electron Microscopy (JSM-7600F FESEM) Different molarity of solution had beencontributed to the different size and morphology ZnOnanoconflakes structures Figure 2 shows that the FESEMimages of ZnO nanostructures had grown on the siliconsubstrate coatedwithAuAs the urea concentration increasesthe number of nanoflakes sheet also increases and theirsize reduced At the lowest concentration of urea the ZnOstructures more to agglomerates structures and less sheet

Journal of Nanomaterials 3

(a) (b)

(c) (d)

Figure 2 The FESEM micrographs of samples prepared at different concentration of Zn(NO3

)2

sdot6H2

O to urea in aqueous solution (a)002M 001M (b) 001M 004M (c) 002M 006M and (d) 002M 008M

structures form It may be cause by less of number carbonateions (CO

3

2minus) and OHminus combine with Zn+ to form crystallineZnO nanostructures For the sample ZnO 001M Urea004M the micrograph shows the early structures for ZnObefore nanoflakes structures form We can see that theprimary nanoflakes structures with small size start to growand no porous exists on it From the observation on samplesZnO 001M Urea 006M and ZnO 001M Urea 008M as inFigures 2(c) and 2(d) the nanoflakes show a difference in thesize and number as a concentration increase

When concentration of urea (stabilizer) increases thechance for ZnO nuclei to grow in their orientation is highThe probability of ZnO nuclei to agglomerate decreasesand ZnO will form a large sheet number of nanoflakes Itcan be deduced that smaller amount of Zn2+ ions leads toslower nucleation rate and induces smaller sheets growthToo small a concentration may impede nuclei growth due tolack of starting materialThe porous structures form on sheetflakes will increase the surface area that is good for sensingapplication

33 Photoluminescence (PL) Spectra Optical properties ofZnO nanostructures are being intensively studied for imple-menting photonic devices ZnO-based material [14] Figure 3shows the PL spectrum of ZnO nanoflakes structures at vary-ing stabilizer concentrations Two emitting bands includingstrong UV emission at 385ndash400 nm and weak orange band

Inte

nsity

(cps

)

300 400 500 600 900800700

Wavelengths (nm)(a)

(a)

2 1(b)

(b)

1 4(c)

(c)

1 6(d)

(d)

1 8

Figure 3 PL spectra of ZnO nanostructures at varying Zn2+ Ureaconcentartion ratio (a) 002M 001M (b) 001M 004M (c)001M 006M and (d) 001M 008M

(585ndash620 nm) were observed Peaks centered at ultra violetband-edge attributed to near band gap emission (NBE) andemission on visible range are due to the recombinationof photogenerated holes with singly ionized charge states


Table 1 Energy band gap ZnO nanostructures calculated based onphotoluminescene (PL) spectra at UV emission

(Zn(NO3)2sdot6H2O) (CH4N2O)Wavelength

(nm)Energy bandgap (eV)

(a) 002M 001M 387 321(b) 001M 004M 395 320(c) 001M 006M 399 311(d) 001M 008M 390 319

in intrinsic defects likes oxygen vacancies (119881O) and zincinterstitial (Zn

119894) [15 16]

The excitonic emission in the UV range (from 385to 400 nm) is an intrinsic property of the wurtzite ZnOnanostructures It originates due to excitonic recombinationwhere electrons come back after being excited to this energylevel in band gap and associate with a hole to form a pair ofexciton It can be seen that the emission at 387 nm (321 eV)shifts to 399 nm (311 eV) for ZnO 001M Urea 006M beforeshifted back to short wavelength (Table 1) Due to quantumconfinement effect theory the energy emission shifts tohigher energy when the size of the nanostructures decreases[17] Emission energy of this band edge emission (NBE) obeysan inverse dependence on the size of nanostructures becauseit tale with FESEM result which nanoflake increase untiloptimum ratio (1 to 6) before reduced back Besides theshift of the exciton band to lower energy may be attributedto increase the carrier concentration closed to the valenceband in the band gap [18] and the reducing of band gap[19] When concentration of stabilizer increases (precursorconcentration constant) the intensity of UV emission alsoincreases until lowest energy 311 eV That means that thecrystalline properties of the films improved and the intrinsicdonor defects such as 119881O and Zn

119894 decreased [16] At low

urea concentration (less number of N atom) the formationof ZnON is lower This will produce the structural defectbecause of deficiency of oxygen to form the correct ZnOstructures in the sample The optimum stabilizer ratio wasfound at 001M of precursor to 006M because the intensityUV emission produced is higher compared to others

The emission near the yellow region most probably iscaused by two factors Firstly it can be caused by an excessof oxygen and the presence of hydroxyl (OH) group whichfound in ZnO was synthesized by using thermal immersionmethod [20 21] The formation of interstitial oxygen ions isgiven by aqueous chemical growth because this is an oxygen-rich growth method for ZnO [22] Secondly it may be dueto presence of deep-level defect in ZnO nanostructure layer[23] where it can be reduced substantially by thermal treat-ment like annealing [24] According to the PL spectra ZnOgrew at lowest stabilizer concentration (CH

4N2O) solution

(001M with 002M of Zn(NO3)2sdotH2O) and a weak violet

emission was observed

34 I-V Characteristics of Zinc Oxide Nanostructures TheI-V characteristic of ZnO nanostructures is measured byusing 2-point probe in vacuum condition Figure 4 shows

12

90

60

30

00

minus30

minus60

minus90

minus12

minus40 minus30 minus20 minus10 0 10 20 30 40

1 21 4

1 61 8

Curr

ent (

A)

Voltage (V)

times10minus4

Figure 4 I-V characteristics of ZnOnanostructureswithAu contactin air ambient

the results of the I-V measurements of ZnO deposited onp-Si substrate Rectifying behavior is clearly illustrated forall samples produced but different in turn-on voltage valueRectifying behavior for all samples existence because by thejunction form at interface of ZnOfilm and p-Si substrate [25]ZnO exhibits n-type semiconductors due to their dominantdonor defects such as oxygen vacancies and Zn interstitials[2] The turn-on voltage decreases as the molar ratio of ureato zinc precursor increases The observed values of turn-onvoltage are 327V 279V 158V and 142V for 2 1 1 4 1 6and 1 8 respectivelyThis negative trend is possible when thefilm is getting doped with ldquoNrdquo The decreasing of resistancecan be related to two aspects the first one reduced netcompensation between holes and electrons due to reductionof oxygen vacancy and the second one is doping of N into thelattice [26 27]

Based on the theoretical study of the chemical trends inthe defect energy levels by Kobayashi et al [28] N wouldproduce a shallow acceptor level in ZnO to form ZnONform This prediction made is based on the theoretical studyof the chemical trends in the defect energy levels wherethe substitutional impurities are considered in a numberof wurtzite-structured semiconductors According to thatfinding when the number of nitrogen in ZnO structuresincreases the numbers of free carriers also increase Besidethe introduction of ldquonitrogenrdquo as a p-type dopant is moreefficient than any other element due to formation of anNndashZn(O)ndashN complex and contributes excess ldquoholesrdquo for con-duction [2]

Early the sample may exhibit an n-type As the N atompresent in ZnO film increased it will be activated as electronacceptors of the ZnON thin film and turn to p-type ZnObehavior Minimum turn-on voltage was observed at highconcentration of urea ZnO 001M Urea 008M 142V so


times10minus4

10

80

60

40

20

00

0 10 20 30 40 50

Curr

ent (

A)

Voltage (V)

(a) ZnO 002M Urea 001M 177V

80

60

40

20

00

0 105 15 25 3520 30 40

Curr

ent (

A)

Voltage (V)

times10minus4

(b) ZnO 001M Urea 004M 119 V

times10minus4

10

80

60

40

20

00

In vacuumPresent gas

Curr

ent (

A)

0 105 15 25 3520 30

Voltage (V)

(c) ZnO 001M Urea 006M 10V

0 2 4 6 8 10 12 14 16 18 20

times10minus4

10

80

60

40

20

00

Present gas

Curr

ent (

A)

Voltage (V)

In vacuum

(d) ZnO 001M Urea 008M 02V

Figure 5 I-V characteristics of ZnO nanostructures when exposed to N2

gas ambient

the minimum operating potential for ZnO nanostructures asa based material because of increasing of oxidation and Ndilution into ZnO lattice as a dopand When the nitrogenatom in solution increased the donor defects insufficient tocompensate for the N substitutional acceptor and leads to anacceptor density increases N atom that is present in ZnOfilmis activated as electrons acceptor of ZnON thin film More Natoms is present will lead to the further pronounced p-typeZnO behavior [25]

35 N2 Response of ZnO Nanoflakes Synthesized at DifferentMolar Concentration of Urea Most of research in gas sensorwere done more to gases which contain oxygen moleculeand explained the response based of free electron on oxygen

ions [29] In this project we test the N2gas response

by ZnO nanostructures which had a modification in termof conductivity (doping) Figure 5 shows the change of I-V characteristics of ZnO nanostructures with variety ofmolarity of urea when exposed to N

2gas From that figure

we can see that all the samples give a response to N2gas in

different sensitivity with good response detected at sampleZnO 001M Urea 008M

Table 2 shows the changes of turn-on voltage and sensitiv-ity of the samples when exposed to N

2gas The detecting of

chemical species usually determined by surface defect whichmay act as an active site to absorb the testing gas and the ratioof area to volume [30] The basis of the sensing mechanismfor metal oxide is chemoresistivity which changes in termof conductance or resistance when surface chemical reacts


Table 2 Sensitivity of ZnO nanostructures prepared at different ratio concentration of N2 gas by I-V characteristics of testing

Samples Turn-on volatge 119881O Response 119878Before exposed 119881

119887

After exposed 119881119886

119878 = ((119881119887

minus 119881119886

)119881119887

) times 100ZnO 002M Urea 001M 327V 177V 4587ZnO 001M Urea 004M 279V 119V 5735ZnO 001M Urea 006M 158V 053V 6646ZnO 001M Urea 008M 142V 021 V 8521

with testing gases When ZnO is exposed to N2gas the N

molecules will be adsorbed on the ZnO surface and causethe change in chemical bonding and carrier concentration ofZnON Molecules N arrived antact at the surface so highconcentration of (N

2)O centers will be observed at Zn-rich

conditions [3] Generally nitrogen exists in two forms (N)Owhich acts as an acceptor and (N

2)O which acts as donor

in ZnON [31] (N)O formed when N substitutes at O siteand (N

2)O formed when N

2substitute at O site also Based

on theoretical calculations as-grown ZnON films containhigher electron concentration than undoped ZnO because(N2)O has a smaller formation energy than (N)OThat makes

the resistance of film decreased when ZnON exposed to N2

gas especially for ZnO 001M Urea 008M The responseswere increased by the conductivity of ZnO film The resultof ZnO film responses was detailed in Table 2

4 Conclusion

Porous ZnO nanoflakes were successfully synthesized atvarious molar concentrations of urea by using thermalimmersionmethodThe analysis of FESEM images illustratesnanoflake formation on silicon substrate The PL spectrashow that the ZnO nanoflake has two emission bands oneis relatively strong UV emission centered about 390ndash400 nmand the other is weaker emission observed in the green-yellow range of the visible spectrum with an emission peakabout 600 nm The study of PL and I-V measurements hasdemonstrated that an n-type ZnO doped with N (ZnON)thin film is successfully fabricated by using simple immersionmethod It is shown that by controlling concentration ofurea we can obtain a good conductivity and N

2gas response

of ZnO nanostructures The minimum value of rectifyingcharacteristic is observed at high concentration of urea insolution It can be concluded that when high concentrationof nitrogen atoms is introduced in the film the p-type ZnOnanostructure behavior is observed

Acknowledgments

The authors would like to thank the Universiti TeknologiMARA (UiTM) Malaysian Ministry of Higher Education(MOHE) and the Malaysian Government for their supportand funding

References

[1] R Asahi T Morikawa T Ohwaki K Aoki and Y TagaldquoVisible-light photocatalysis in nitrogen-doped titaniumoxidesrdquo Science vol 293 no 5528 pp 269ndash271 2001

[2] K Minegishi Y Koiwai Y Kikuchi K Yano M Kasuga and AShimizu ldquoGrowth of p-type zinc oxide films by chemical vapordepositionrdquo Japanese Journal of Applied Physics vol 36 no 11pp L1453ndashL1455 1997

[3] Y Yan S B Zhang and S T Pantelides ldquoControl of dopingby impurity chemical potentials predictions for p-type ZnordquoPhysical Review Letters vol 86 no 25 pp 5723ndash5726 2001

[4] A Kaschner UHaboeckM Strassburg et al ldquoNitrogen-relatedlocal vibrational modes in ZnONrdquo Applied Physics Letters vol80 no 11 pp 1909ndash1911 2002

[5] PThilakan DM Radheep K Saravanakumar and G SasikalaldquoDeposition and characterization of ZnO thin films bymodifiedpulsed-spray pyrolysisrdquo Semiconductor Science and Technologyvol 24 no 8 Article ID 085020 2009

[6] G S Wu T Xie X Y Yuan et al ldquoControlled synthesis of ZnOnanowires or nanotubes via sol-gel template processrdquo Solid StateCommunications vol 134 no 7 pp 485ndash489 2005

[7] J Z Romeiro F C Marinho S C S Lemos et al ldquoUrea-basedsynthesis of zinc oxide nanostructures at low temperaturerdquoJournal of Nanomaterials vol 2012 Article ID 427172 7 pages2012

[8] R S Gaikwad R S Mane B N Pawar et al ldquoNitrogen-dopedZnO shells studies on optical transparency and electricalconductivityrdquo Materials Research Bulletin vol 47 no 5 pp1246ndash1250 2012

[9] J D Wright and N A J M Sommerdijk Sol-Gel MaterialsChemistry and Applications Taylor amp Francis Group LondonUK 2001

[10] D Dollimore J A France B W Krupay and R WhiteheadldquoKinetic aspects of the thermal decomposition of zinc carbon-aterdquoThermochimica Acta vol 36 no 3 pp 343ndash349 1980

[11] J P Kar M H Ham S W Lee and J M Myoung ldquoFabricationof ZnO nanostructures of various dimensions using patternedsubstratesrdquo Applied Surface Science vol 255 no 7 pp 4087ndash4092 2009

[12] L Vayssieres ldquoGrowth of arrayed nanorods and nanowires ofZnO from aqueous solutionsrdquo Advanced Materials vol 15 no5 pp 464ndash466 2003

[13] L Vayssieres K Keis S-E Lindquist and A HagfeldtldquoPurpose-built anisotropie metal oxide material 3D highlyoriented microrod array of ZnOrdquo The Journal of PhysicalChemistry B vol 105 no 17 pp 3350ndash3352 2001

[14] Z Fan and J G Lu ldquoZinc oxide nanostructures synthesis andpropertiesrdquo Journal of Nanoscience and Nanotechnology vol 5no 10 pp 1561ndash1573 2005

[15] E Bacaksiz S Yilmaz M Parlak A Varilci and M AltunbasldquoEffects of annealing temperature on the structural and opticalproperties of ZnO hexagonal pyramidsrdquo Journal of Alloys andCompounds vol 478 no 1-2 pp 367ndash370 2009

[16] G Kenanakis M Androulidaki E Koudoumas C Savvakisand N Katsarakis ldquoPhotoluminescence of ZnO nanostructures


grown by the aqueous chemical growth techniquerdquo Superlatticesand Microstructures vol 42 no 1ndash6 pp 473ndash478 2007

[17] M Ghosh and A K Raychaudhuri ldquoShape transition in ZnOnanostructures and its effect on blue-green photolumines-cencerdquoNanotechnology vol 19 no 44 Article ID 445704 2008

[18] D Behera and B S Acharya ldquoNano-star formation in Al-doped ZnO thin film deposited by dip-dry method andits characterization using atomic force microscopy electronprobe microscopy photoluminescence and laser Raman spec-troscopyrdquo Journal of Luminescence vol 128 no 10 pp 1577ndash1586 2008

[19] H-C Hsu C-S Cheng C-C Chang S Yang C-S Chang andW-F Hsieh ldquoOrientation-enhanced growth and optical prop-erties of ZnO nanowires grown on porous silicon substratesrdquoNanotechnology vol 16 no 2 pp 297ndash301 2005

[20] S A Kamaruddin K-Y Chan M Z Sahdan M Rusop andH Saim ldquoZnO microstructures and nanostructures preparedby sol-gel hydrothermal techniquerdquo Journal of Nanoscience andNanotechnology vol 10 no 9 pp 5618ndash5622 2010

[21] Z Khusaimi S Amizam M H Mamat et al ldquoControlledgrowth of zinc oxide nanorods by aqueous-solution methodrdquoSynthesis and Reactivity in Inorganic Metal-Organic and Nano-Metal Chemistry vol 40 no 3 pp 190ndash194 2010

[22] S N Bai H H Tsai and T Y Tseng ldquoStructural and opti-cal properties of Al-doped ZnO nanowires synthesized byhydrothermal methodrdquo Thin Solid Films vol 516 no 2ndash4 pp155ndash158 2007

[23] Z Khusaimi S Amizam H A Rafaie M H Mamat NAbdullah and M Rusop ldquoA surface morphology study on theeffect of annealing temperature to nanostructured ZnO andits reaction mechanism in solution methodrdquo AIP ConferenceProceedings vol 1136 pp 790ndash795 2009

[24] S H Lee H J Lee H Goto M-W Cho and T Yao ldquoFabrica-tion of porous ZnO nanostructures and morphology controlrdquoPhysica Status Solidi (C) vol 4 no 5 pp 1747ndash1750 2007

[25] D Wang Y C Liu R Mu et al ldquoThe photoluminescenceproperties of ZnON films fabricated by thermally oxidizingZn3

N2

films using plasma-assisted metal-organic chemicalvapour depositionrdquo Journal of Physics Condensed Matter vol16 no 25 pp 4635ndash4642 2004

[26] V R Shinde T P Gujar C D Lokhande R S Mane and S-H Han ldquoMn doped and undoped ZnO films a comparativestructural optical and electrical properties studyrdquo MaterialsChemistry and Physics vol 96 no 2-3 pp 326ndash330 2006

[27] C Wang Z Ji J Xi J Du and Z Ye ldquoFabrication andcharacteristics of the low-resistive p-type ZnO thin films by DCreactive magnetron sputteringrdquoMaterials Letters vol 60 no 7pp 912ndash914 2006

[28] A Kobayashi O F Sankey and J D Dow ldquoDeep energy levelsof defects in the wurtzite semiconductors AIN CdS CdSe ZnSand ZnOrdquo Physical Review B vol 28 no 2 pp 946ndash956 1983

[29] F Chaabouni M Abaab and B Rezig ldquoMetrological character-istics of ZNO oxygen sensor at room temperaturerdquo Sensors andActuators B vol 100 no 1-2 pp 200ndash204 2004

[30] Y Cao P Hu W Pan Y Huang and D Jia ldquoMethanal andxylene sensors based on ZnO nanoparticles and nanorodsprepared by room-temperature solid-state chemical reactionrdquoSensors and Actuators B vol 134 no 2 pp 462ndash466 2008

[31] S Limpijumnong X Li S H Wei and S B Zhang ldquoSubsti-tutional diatomic molecules NO NC CO N

2

and O2

theirvibrational frequencies and effects on p doping of ZnOrdquoAppliedPhysics Letters vol 86 no 21 Article ID 211910 3 pages 2005

Submit your manuscripts athttpwwwhindawicom

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MaterialsJournal of


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materials


Journal ofNanomaterials


atom [8] Due to the much lower NndashH bonding energy inNH3 the decomposition of NH

3H2O will become the main

source for nitrogen in ZnON formationWe found that whenconcentration of urea increases the conductivity of ZnO filmalso increases and N

2response becomes more effective

2 Experimental

In this work zinc oxide nanostructures were grown by thecatalytic immersion method using zinc nitrate hexahydrate(Zn(NO

3)26H2O) as a precursors and urea as a stabilizer p-

type silicon (100) as substrates was cut with the size that is15 cm times 20 cm and clean with acetone methanol and HF(48 HF DI water (1 10)) The silicon substrate was sputtercoated with 6 nm thickness of gold (Au) as a catalyst in argonplasma

21 Synthesis of ZnO Nanostructures For solution prepa-ration the molar ratio of the Zinc Nitrate Hexahydrate[Zn(NO

3)26H2O] to urea [CH

4N2O] in 100 mL was varied

with ratio 2 1 1 4 1 6 and 1 8The aqueous solution stirredand heated at 60∘C for 1 hour and follow by the ageingprocess for 24 hours to produce homogenous solution Afterthe ageing process the silicon substrate inserted into testtube filled with 40mL of the solutions of different molarratio concentration Then the test tubes were immersedin a water bath at temperature of 90∘C After 4 hourssamples were dried in 1 hour at temperature 150∘C and wereannealed at temperature 500∘C for 1 hour The morphologyof the ZnO nanostructures was analyzed by Field EmissionScanning Electron Microscopy (JSM-7600F FESEM) Theoptical properties are characterized by photoluminescence(HORIBA JOBIN YVON PL) spectra at room temperature

22 I-V Characteristics and N2 Response Studies ZnO nanos-tructures were characterized by I-V characteristic to measureelectrical properties and N

2gas response Gold layer (as

a metal) with thickness 6 nm was coated on ZnO nanos-tructures by using sputtering technique like in Figure 1ZnO nanostructures were placed in tight box with inlet andoutlet for N

2gases The current-voltage characteristics of

ZnO nanostructures in vacuum and presence N2gas were

measured by Keithely 2400 multimeter and voltage source

3 Result and Discussion

31 Chemical Decomposition in Thermal Immersion MethodThe particle nucleation involves two steps nucleation andgrowth with the nucleation rate needs to be faster thangrowth rate Growth rate that depends on the amount ofreacting particles available while nucleation will take placeafter supersaturation is achieved and this is affected bythe solubility of the reacting particles [9] Urea is highlysoluble in water and when urea which is one of aminegroups dissolved in water it will slowly displacement bywater molucules to produce ammonia and carbonate anion

2-point terminal+ minus

Metal

SiliconZinc oxide

nanostructures

Figure 1 The schematic diagram of the electrodes plating on thesurface of ZnO nanostructures

(CO3

2minus) The progression of chemical synthesis of ZnO inpresence of urea is suggested as follows [6]

CON2H4+ 3H2O 997888rarr CO

2(g) + 2NH

3H2O (1)

When temperatures increase urea decomposed and pro-duced NH

3 NH

4

+ ions generated from NH3ions will

increase pH aqueous solution and support the ZnO crystalgrowth process

2NH3sdotH2O + CO

2997888rarr 2NH

4

+

+ CO3

2minus

+OHminus (2)

Zinc nitrate hexahydrate will provide Zn2+ ions when dis-solved in water as in

Zn(NO3)2

sdot 6H2O +H

2O rarr Zn2+ + 2NO

3

minus

+ 7H2O (3)

4Zn2+ + CO3

2minus

+ 6OHminus +H2O

997888rarr Zn4CO3(OH)6sdotH2O (s)

(4)

Zn4CO3(OH)6sdotH2O 997888rarr 4ZnO (s) + 4H

2O + CO

2(g) (5)

Zn4CO3(OH)6sdotZnO is formedduring the reaction in aqueous

solution and after heating at 500∘C it decomposed ZnO [10]The degree of supersaturation of Zn(OH)

2in the interrfacial

zone and the adsorption of organicinorganic species onthe surface of ZnO are the factors that can influence thenucleation and growth of ZnO nanostructures [11 12]

32 Morphology of the Zinc Oxide Nanostructures The pres-ence of Au layer on Si substrate is a catalyst because it canprovide an alternating energy pathway with lower activationenergy Heterogeneous nucleation promoted between Auparticle and Zn and O ions is energetically favorable sincethe interfacial energy between the ions and adsorbed sites islower This is comparative to homogenous nucleation of twosolid phases which has a higher activation energy barrier [13]A large lattice match between ZnO nanostructures and Si isanother factor of deposition of Au layer

The morphology of ZnO nanostructures is observed byusing Field Emission Scanning Electron Microscopy (JSM-7600F FESEM) Different molarity of solution had beencontributed to the different size and morphology ZnOnanoconflakes structures Figure 2 shows that the FESEMimages of ZnO nanostructures had grown on the siliconsubstrate coatedwithAuAs the urea concentration increasesthe number of nanoflakes sheet also increases and theirsize reduced At the lowest concentration of urea the ZnOstructures more to agglomerates structures and less sheet


(a) (b)

(c) (d)


)2

sdot6H2



3




Inte

nsity

(cps

)

300 400 500 600 900800700

Wavelengths (nm)(a)

(a)

2 1(b)

(b)

1 4(c)

(c)

1 6(d)

(d)

1 8







(a) 002M 001M 387 321(b) 001M 004M 395 320(c) 001M 006M 399 311(d) 001M 008M 390 319


119894) [15 16]





4N2O) solution




12

90

60

30

00

minus30

minus60

minus90

minus12


1 21 4

1 61 8

Curr

ent (

A)

Voltage (V)

times10minus4






times10minus4

10

80

60

40

20

00

0 10 20 30 40 50

Curr

ent (

A)

Voltage (V)


80

60

40

20

00

0 105 15 25 3520 30 40

Curr

ent (

A)

Voltage (V)

times10minus4

(b) ZnO 001M Urea 004M 119 V

times10minus4

10

80

60

40

20

00


Curr

ent (

A)

0 105 15 25 3520 30

Voltage (V)


0 2 4 6 8 10 12 14 16 18 20

times10minus4

10

80

60

40

20

00

Present gas

Curr

ent (

A)

Voltage (V)

In vacuum



gas ambient














119887


119878 = ((119881119887

minus 119881119886

)119881119887








2)O formed when N





4 Conclusion


2gas response


Acknowledgments


References




























N2








2

and O2









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b)

(c) (d)


)2

sdot6H2



3




Inte

nsity

(cps

)

300 400 500 600 900800700

Wavelengths (nm)(a)

(a)

2 1(b)

(b)

1 4(c)

(c)

1 6(d)

(d)

1 8







(a) 002M 001M 387 321(b) 001M 004M 395 320(c) 001M 006M 399 311(d) 001M 008M 390 319


119894) [15 16]





4N2O) solution




12

90

60

30

00

minus30

minus60

minus90

minus12


1 21 4

1 61 8

Curr

ent (

A)

Voltage (V)

times10minus4






times10minus4

10

80

60

40

20

00

0 10 20 30 40 50

Curr

ent (

A)

Voltage (V)


80

60

40

20

00

0 105 15 25 3520 30 40

Curr

ent (

A)

Voltage (V)

times10minus4

(b) ZnO 001M Urea 004M 119 V

times10minus4

10

80

60

40

20

00


Curr

ent (

A)

0 105 15 25 3520 30

Voltage (V)


0 2 4 6 8 10 12 14 16 18 20

times10minus4

10

80

60

40

20

00

Present gas

Curr

ent (

A)

Voltage (V)

In vacuum



gas ambient














119887


119878 = ((119881119887

minus 119881119886

)119881119887








2)O formed when N





4 Conclusion


2gas response


Acknowledgments


References




























N2








2

and O2









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials







(a) 002M 001M 387 321(b) 001M 004M 395 320(c) 001M 006M 399 311(d) 001M 008M 390 319


119894) [15 16]





4N2O) solution




12

90

60

30

00

minus30

minus60

minus90

minus12


1 21 4

1 61 8

Curr

ent (

A)

Voltage (V)

times10minus4






times10minus4

10

80

60

40

20

00

0 10 20 30 40 50

Curr

ent (

A)

Voltage (V)


80

60

40

20

00

0 105 15 25 3520 30 40

Curr

ent (

A)

Voltage (V)

times10minus4

(b) ZnO 001M Urea 004M 119 V

times10minus4

10

80

60

40

20

00


Curr

ent (

A)

0 105 15 25 3520 30

Voltage (V)


0 2 4 6 8 10 12 14 16 18 20

times10minus4

10

80

60

40

20

00

Present gas

Curr

ent (

A)

Voltage (V)

In vacuum



gas ambient














119887


119878 = ((119881119887

minus 119881119886

)119881119887








2)O formed when N





4 Conclusion


2gas response


Acknowledgments


References




























N2








2

and O2









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




times10minus4

10

80

60

40

20

00

0 10 20 30 40 50

Curr

ent (

A)

Voltage (V)


80

60

40

20

00

0 105 15 25 3520 30 40

Curr

ent (

A)

Voltage (V)

times10minus4

(b) ZnO 001M Urea 004M 119 V

times10minus4

10

80

60

40

20

00


Curr

ent (

A)

0 105 15 25 3520 30

Voltage (V)


0 2 4 6 8 10 12 14 16 18 20

times10minus4

10

80

60

40

20

00

Present gas

Curr

ent (

A)

Voltage (V)

In vacuum



gas ambient














119887


119878 = ((119881119887

minus 119881119886

)119881119887








2)O formed when N





4 Conclusion


2gas response


Acknowledgments


References




























N2








2

and O2









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






119887


119878 = ((119881119887

minus 119881119886

)119881119887








2)O formed when N





4 Conclusion


2gas response


Acknowledgments


References




























N2








2

and O2









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials














N2








2

and O2









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



Documents

Research Article Special Effect of Urea as a Stabilizer in Thermal …downloads.hindawi.com/journals/jnm/2013/163527.pdf · 2019-07-31 · ZnO nanostructures in vacuum and presence