Growth of aligned ZnO nanorod arrays from an aqueous solution: effect of additives and substrates

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Copyright copy 2011 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 11 10379ndash10386 2011

Growth of Aligned ZnO Nanorod Arrays from anAqueous Solution Effect of Additives and Substrates

Sriparna Chatterjee1 Smita Gohil1 Avesh K Tyagi2 and Pushan Ayyub1lowast1Department of Condensed Matter Physics and Materials Science

Tata Institute of Fundamental Research 1 Homi Bhabha Road Mumbai 400005 India2Chemistry Division Bhabha Atomic Research Centre Mumbai 400085 India

We report a simple versatile low cost fabrication technique for synthesizing nanorod arrays whosearchitecture is suited for many applications spanning the nanometer to micrometer range Specifi-cally we have covered the range of nanorod diameter from 50 to 1200 nm From a detailed studyof the growth parameters involved in the synthesis of the ZnO nanorod arrays from an aqueoussolution we report in particular the effects of varying the capping agent substrate and substrate-seeding We find that seeding the substrate and selecting the appropriate capping agent play themost crucial roles in the alignment of nanorod arrays Our study on the use of different precursormaterials and varied substrates for the growth of ZnO nanorod arrays should lead to an enhancedunderstanding of the controllable growth of ZnO crystals and nanostructures

Keywords Zinc Oxide Nanorods Solution Synthesis Shape Control

1 INTRODUCTION

The current and future requirements of the semiconduc-tor optoelectronic and chemical industries (among oth-ers) have created a great demand for the cost effectivefabrication of size- and shape-controlled nanostructures ofvarious materials using a range of physical and chemi-cal techniques Nanostructures based on simple as well asmixed oxides of a number of 3d transition metals suchas Zn Ti V Cu and Fe arguably represent the mostdiverse range of size-dependent optical electrical ther-mal mechanical magnetic and catalytic properties andhave already achieved great importance in the fields ofenergy storage1 gas sensing23 field effect transistor4

field emitters56 and so on Among the various possibletypes of nanostructures one-dimensional (1-D) morpholo-gies (nanowires tubes whiskers etc) are fast acquiringa unique status because they have two quantum confineddirections for electrical conduction a high surface to vol-ume ratio as well as a high aspect ratio In particulara roughly parallel array of nanorods exhibits distinctiveproperties7 which may differ significantly from those of asingle nanorod For example a metal nanorod array (whenclustered) shows super-hydrophobicity8 and ultra low elec-trical breakdown voltage in a gas discharge device9 whichare properties not associated with a single metal nanorod

lowastAuthor to whom correspondence should be addressed

On the other hand many important properties such asUV lasing10 photovoltaic effect1 field emission56 etc aresignificantly amplified when the nanorods form an arraythereby making them practically useful as a deviceAmong the various functional semiconducting materi-

als that have been studied zinc oxide (ZnO) is certainlyone of the most promising as it exhibits a direct bandgap (337 eV) at room temperature with a large exci-tonic binding energy of 60 meV11 ZnO possesses a com-bination of excellent semiconducting optoelectronic andpiezoelectric12 properties as well as bio-compatibility13

and high mechanical thermal and chemical stability Itis therefore no surprise that ZnO has many proven aswell as futuristic applications such as in catalysis14

gas sensing15 field effect transistors16 dye sensitizedsolar cells1 and in other optoelectronic and piezoelec-tric devices Further ZnO exhibits a surprisingly diverserange of nanostructures which includes nanowires17

nanobelts1819 nanosprings20 nanoflowers21 nanocages22

nanonails23 nanosquids24 and nanoprisms25 Many of thenovel properties of nanostructured ZnO are controlled bythe crystallite size morphology crystallographic orienta-tion and crystallinity1026ndash29 which are in turn dependenton the synthesis conditions Therefore the control overthe size and shape of ZnO over a rather wide length scale(from micrometer to nanometer) is of great importance andis a challenging area of research It follows that the devel-opment of a suitable synthesis technique for well-aligned

J Nanosci Nanotechnol 2011 Vol 11 No 12 1533-488020111110379008 doi101166jnn20115197 10379



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Growth of Aligned ZnO Nanorod Arrays from an Aqueous Solution Effect of Additives and Substrates Chatterjee et al

arrays of ZnO nanorods on suitable substrates and withcontrolled aspect ratio is extremely important to meet thedemands of potential applicationsZnO nanorod arrays have been synthesized by a variety

of techniques such as template-assisted growth30 vaporndashliquidndashsolid growth31 thermal evaporation32 solndashgeltransition33 electrochemical deposition34 hydrothermalsynthesis35 and aqueous solution36 synthesis Vayssiereset al first reported the successful synthesis of three-dimensional highly oriented hexagonal microrod arraysof ZnO from an aqueous solution37 Such a microrodarray shows high UV photoresponse and excellent electrontransport properties over a wide range of rod lengths38

The same technique was later used for the growth of ZnOmicrotube arrays39 as well as nanorod arrays36 just bylowering the precursor concentration The ZnO nanorodarray appears very promising for the fabrication of costeffective and high performance gas sensors15 Liu et alsynthesized vertically aligned ZnO nanorod arrays directlyon a Zn foil (which acted both as source and substrate) inpure water at low temperature and measured its field emis-sion property40 Yang et al reported the hydrothermal syn-thesis of pencil-like aligned and oriented ZnO nanorodson different substrates41 and found that the nanorod densitywas greater on pre-seeded substrates than on bare Si or CuSong and Lim found that the morphology of ZnO nanorodsdepends strongly on the nature of the seed layer42 whosethickness and crystallite size control the diameter and den-sity of the nanorods while the orientation dictates the crys-tallinity of the nanorods Sugunan et al studied the role ofhexamethylenetetramine (HMT) on the seeded growth ofZnO nanowires by a hydrothermal route43 and concludedthat HMT acts as a shape inducing molecule by selectivelycapping the non polar crystallographic planes of ZnO crys-tals Further the concentration of the precursor solutionhas a significant effect on the growth rate of the nanowiresKim et al found that the alignment of ZnO nanorod arraycritically depends on initial seed layer44 Zhao et al alsostudied the effect of seed layer on the growth of ZnOnanorods45

Here we report a detailed study of the effect of thegrowth conditions on the nanorod diameter and morphol-ogy and the alignment of the nanorod array grown in anaqueous solution In particular we have studied the effectsof different capping agents and different amines in control-ling the growth and morphology of 1-D ZnO nanocrystalsWe have also used different substrates to determine theireffect on the crystallinity alignment aspect ratio and den-sity of nanorods Interestingly we found that by onlychanging the substrate one can switch from microrods tonanorods At the same time the roughness of the substrateplays an important role in controlling the alignment anddensity of the nanorods

2 EXPERIMENTAL DETAILS

One-dimensional ZnO microcrystals and nanocrystals withdifferent morphologies were synthesized using a modi-fied aqueous solution route Each substrate was mountedin an upside down position inside an ordinary screw-cap pyrex glass bottle An aqueous solution of 001 Mzinc nitrate (Zn(NO32 middot 6H2O) was used as the sourceof zinc ions Morphological changes were brought aboutby using a number of different amines and cappingagents each of them at a concentration of 001 MThese were hexamethylenetetramine (HMT (CH26N4n-dodecylamine (C12H27N) hexadecyl trimethyl ammo-nium bromide (CTAB C19H42BrN) urea (CH4N2O) andethylene diamine tetra-acetate (EDTA C10H16N2O8 Assubstrates we used oriented and randomly oriented thinfilms of ZnO on Si wafers as well as commercially avail-able SiO2 LaAlO3 and MgO wafers The reaction vesselswere each kept at 90 C for 12 hours and then cooled downto room temperature After the reaction was over each as-prepared sample was washed with de-ionized water untilthe pH of the wash became asymp65 Finally the sampleswere air driedFor their subsequent use as seed layers nanocrys-

talline ZnO thin films were sputter-deposited on Si wafersmounted on a sample holder of a custom-designed sputter-ing chamber initially evacuated to about 10minus6 torr Usinga 200 mm long axial planar magnetron sputtering sourcesputtering was carried from a 50 mm diameter ZnO targetmade by pressing 999 pure ZnO powder at 20 tons Theaverage grain size in sputter deposited films can be con-trolled with reasonable accuracy by appropriate choice ofprocess parameters46 In the present case sputtering wascarried out in 50ndash100 mtorr of 9999 Ar gas pressure at50 W rf power The sputtering time was 30ndash60 min dur-ing which the substrate was maintained at about 20 C bywater-cooling The other substrates used were of commer-cial originThe crystallographic structure of the 1-D ZnO sam-

ples was investigated using a Panalytical Xrsquopert Pro pow-der X-ray diffractometer (XRD) with Cu K radiationDetailed studies of the microstructure and the elemen-tal composition were obtained using an energy dispersiveX-ray (EDX) analyzer with a Jeol JSM 840 scanning elec-tron microscope (SEM) The topography and the rough-ness of the ZnO seeded substrates were estimated usinga Multimode Scanning Probe Microscope (NanoscopeIV-Veeco) in contact mode with a Si tip at a scan speedof 1 Hz

3 RESULTS

We first study the possibility of controlling the morphol-ogy of the 1-D ZnO structures by means of adding suit-able hydrolyzing agents as well as surfactants For thispart of the study the ZnO nanostructures were grown on

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Chatterjee et al Growth of Aligned ZnO Nanorod Arrays from an Aqueous Solution Effect of Additives and Substrates

sputter-deposited oriented ZnO nanocrystalline thin filmsthat acted as a seed layer The source of the zinc ions was001 M Zn(NO32 middot6H2O

31 Effect of Adding Hexamethylenetetramine (HMT)

Here HMT (001 M) was used both as hydrolyzing as wellas capping agent A plan view SEM image clearly showsan aligned array of ZnO nanorods with tip diameters rang-ing from 170 nm to 250 nm (Fig 1(a)) The cross sec-tional SEM image in Figure 1(b) shows that the height ofthe nanorod array is approximately 5 m The probable

Fig 1 (a) Top view scanning electron micrograph of an oriented ZnOnanorod array synthesized using HMT as capping agent (b) Cross-sectional view of the same nanorod array (c) X-ray diffraction patternof the aligned nanorod array showing preferential growth along [002]

sequence of chemical reactions leading to the synthesis ofZnO is as follows

C6H12N4+4H2Orarr C6H16N4+4OHminus (31)

C6H12N4+6H2Orarr 6HCHO+4NH3 (32)

Zn2++4OHminus rarr ZnOH2minus4 (33)

Zn2++4NH3 rarr ZnNH32+4 (34)

ZnNH32+4 +2OHminus rarr ZnO+4NH3+H2O (35)

ZnOH2minus4 rarr ZnO+H2O+2OHminus (36)

We know that crystalline ZnO exhibits a partially polarcharacter and the (001) plane is the basal polar plane ina typical wurtzite structure Kong et al reported that oneend of the basal polar plane terminates with partially pos-itive Zn lattice points and the other end terminates in par-tially negative oxygen lattice points19 Since HMT is a nonpolar chelating ligand it has a strong tendency to attachto the nonpolar facets of the 1-D ZnO nanostructure As aresult the (002) plane is the only one exposed for epitaxialgrowth resulting in preferential unidirectional growth ofnanorods along the [0002] axis This is supported by theXRD data (Fig 1(c)) of the sample confirming a prefer-ential growth of the nanorods along the hexagonal c-axis

32 Effect of Adding n-Dodecylamine

We next used n-dodecylamine (a primary amine with along non-polar chain) instead of HMT but with the sameconcentration (001 M) In this case we observed the for-mation of randomly oriented ZnO nanostructures with alow aspect ratio (Fig 2(a)) Even though n-dodecylamineis a straight chain amine with a larger number of non-polarcarbon residues in its structure than HMT it clearly doesnot constrain the ZnO growth as efficiently as HMT Infact n-dodecylamine is a potential reagent for hydrolysisof zinc nitrate However due to its poor chelating capacityto the non polar facets of the ZnO crystal epitaxial growthalong the [0002] direction is hindered As a result it leadsto comparatively flattened nanostructures with an aspectratio between 1 and 15 The XRD data of this sampleshowed no preferential growth direction (Fig 2(b))

33 Effect of Adding Cetyl Trimethyl AmmoniumBromide (CTAB)

A further reduction in the aspect ratio of the ZnO nano-structure occurs when a cationic surfactant such as CTABwas used In the presence of 001 M CTAB large plate-shaped microcrystals (Fig 3(a)) were formed It is reportedthat one end of the basal polar plane terminates withpartially positive charged Zn lattice points and the otherend terminates in partially negative charged oxygen latticepoints19 In this case the cationic surfactant probably caps

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Fig 2 (a) Top view scanning electron micrograph of ZnO nano-structure synthesized using n-dodecylamine as capping agent (b) X-raydiffraction pattern of the same

the basal polar plane which terminates with the partiallynegative oxygen lattice point Thus epitaxial growth alongthe hexagonal axis is totally hindered and lateral growthis preferred resulting in the formation of the observedplate like microstructures Further the surface coverage ofsuch plate shaped structures on the substrate was quite lowwhen compared to the two cases discussed above

34 Effect of Adding Ethylenediaminetetraacetic Acid(EDTA)

We also studied the effect of adding EDTAmdasha strong hex-adentate chelating agent with both polar (ndashCOOminus andnon polar (two tertiary amines) co-ordination sitesmdashon themorphology of the resulting ZnO crystals In this case nonano- or micro-structured crystals were detected on thesubstrate EDTA has a very strong tendency to sequesterzinc ions and the chelated compound is soluble in waterThe chemical reaction is as follows

ZnH2O63++H4EDTA ZnEDTAminus+6H2O+4H+

(37)The zinc-EDTA chelate structure is shown in Figure 3(b)In this case it appears that all zinc ions were trapped byEDTA and no ZnO crystals were formed during the aque-ous solution synthesis

Fig 3 (a) Scanning electron micrograph of ZnO microstructure synthe-sized using CTAB (b) representative Zn-EDTA chelate structure (c) ZnOnanostructure synthesized using Urea

35 Effect of Adding Urea

In the next experiment 001 M of urea was used as addi-tive Urea has two primary amine groups (ndashNH2 attachedto a carbonyl (ndashCOndash) residue in its structure Again inaqueous solution synthesis no micro- or nanocrystals ofZnO were observed Some irregular structures did appear(Fig 3(c)) but no definite shape appeared to emerge Inthe aqueous solution urea forms ammonia (NH3 whichprobably starts the hydrolysis of zinc nitrate But due to




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the absence of a side chain residue (polar or non polar)the growth of 1-D crystals of ZnO was not facilitatedFrom the above study we may conclude that an amine

with a suitable non polar side chain is necessary for thegrowth of 1-D nanocrystals of ZnO HMT has four tertiaryamine residues which initiate the hydrolysis of zinc nitrateto ZnO whereas the non polar cyclic side chain residuesof HMT promotes the epitaxial growth of 1-D nanocrystals

Fig 4 (a) Top view scanning electron micrograph of randomly orientedZnO nanorod array synthesized using HMT (b) X-ray diffraction patternof the same nanorod array showing no preferential growth (c) EDXspectrum of the randomly oriented ZnO nanorod array showing presenceof only Zn and O

of ZnO We now investigate the role of the substrate in thegrowth of ZnO nanorods

36 Effect of Using a Randomly Oriented SeedSubstrate

All the samples described previously were grown onsputter-deposited oriented ZnO nanocrystalline thin films(on Si) that acted as a seed layer To determine the impor-tance of having an oriented seed layer we repeated oneof the previous experiments with the same starting mate-rial (001 M of Zn(NO32 middot6H2O) and additive (001 M ofHMT) but using a randomly oriented ZnO thin film as theseed layer Thereby we obtained a layer of ZnO nanorodswith an average tip diameter of 50ndash70 nm (Fig 4(a))

Fig 5 Scanning electron micrograph of ZnO microrods synthesizedusing HMT as capping agent on bare (a) LaAlO3 (b) MgO and (c) SiO2

substrates




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The XRD pattern of the sample (Fig 4(b)) matches withthe hexagonal Wurtzite structure of ZnO (space groupP63mc) EDX analysis was used to study the elementalcomposition of the sample at different locations through-out the film area A representative EDX pattern is shown inFigure 4(c) which also confirms the absence of any impu-rity phase We conclude from this study that the orientationof the ZnO nanorods strongly depends on the orientationof the seed layer in contradiction to earlier assertions thatthe alignment of ZnO nanorods is independent of the sub-strate in aqueous solution synthesis36

37 Effect of Using Bare Substrates

To further investigate the dependence of the nanorod align-ment on the substrate ZnO nanorods were grown on dif-ferent commercially available substrates such as LaAlO3

(010) MgO (200) and SiO2 (polycrystalline) keeping allother growth condition as before In all three cases weobtained well defined 1-D structures of ZnO with strongadherence to the substrate but none of them was alignedin any particular direction (Fig 5)

4 DISCUSSIONS

This study confirms that the nanorod alignment is con-trolled by the seed layer Within the parameters of ourstudy oriented ZnO nanorods were obtained only when theZnO seed layer was oriented whereas a randomly orientedZnO seed layer or even a single crystalline substrate with-out a ZnO seed layer yield randomly oriented 1-D ZnOnanocrystalsWe further note that the substrate plays an important

role in controlling the diameter of the nanorods and hencethe aspect ratio of the 1-D structure Earlier reports sug-gested that the concentration of the precursor solution con-trols the nanorod diameter a precursor concentration of01 M yields micrometer-thick rods while a concentrationof 0001 M produces nanometer-thick wires36 Howeverwe observed that the formation of nanorods or microrodsdoes not depend appreciably on the concentration of pre-cursor solution (varied in the range 01 M to 0001 M) Wehave been able to vary the ZnO rod diameter over a widerange just by changing the microstructure of the seed layerand the substrate Keeping the concentration of both theprecursor solution (Zn(NO32 middot6H2O) and additive (HMT)fixed at 001 M microrods with an average diameter of10ndash12 m were obtained with MgO and SiO2 substrateswhile the average diameter was 450ndash600 nm on LaAlO3

substrates Under identical conditions as above nanorodswith average diameter 50ndash70 nm were obtained on ran-domly oriented ZnO seed substrates while nanorods withan average diameter of 170ndash250 nm were formed on ori-ented ZnO seed substrates

In all the cases investigated the reaction parameterssuch as temperature reaction time and reactant concentra-tions were kept unaltered Clearly therefore the substratessolely play a key role in controlling the overall aspectratio of the 1-D ZnO nanostructures In aqueous solutionsynthesis the movement of the ions is principally guidedby thermal motion In case of MgO SiO2 and LaAlO3

substrates there was no pre-seeding so at the early stageof the reaction the thermally agitated ions do not directlyreach the nucleation centres which results in somewhatrandom deposition on the substrate and lateral growth ofZnO nanorods On the other hand pre-seeding of sub-strates guides the thermally agitated ions to get depositedon the favorable nucleation centres and reduces the chanceof lateral growthInterestingly the areal density of nanorod growth was

also dependent on the substrate The nanorods density was

Fig 6 Atomic force micrograph of (a) oriented ZnO seed substrate and(b) randomly oriented ZnO seed substrate




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much more on the substrates with comparatively roughsurfaces such as LaAlO3 and MgO with rms surfaceroughness of 12 nm and 19 nm respectively On theother hand low density nanostructures were found on SiO2

which had a rms surface roughness of 04 nm The sur-face roughness of the substrate is believed to be advanta-geous for the growth of 1-D nanostructures by reducingits strain47 and increasing the number of nucleation sites48

During aqueous solution growth a ZnO wetting layer isformed on the surface of the substrate The rough surfacemorphology of the substrate plays a key role in controllingthe growth of the wetting layer In order to reduce the sur-face energy the wetting layer forms at the steps and kinksof the rough surface and influences the spontaneous growthof the nanorods in at least two important ways First itinduces an anisotropic strain which confines the diffu-sion of atoms to one dimension and second it generatesnucleation sites such as nanoscale etch pits hillocks andnanocrystallites48 Comparing the growth of nanorods onLaAlO3MgO and SiO2 we observe that denser nanorodsgrow on the rougher surfaces of LaAlO3MgO than onSiO2 The rms surface roughness of the oriented ZnO seedsubstrate was 15 nm (Fig 6(a)) while that of the randomseeded substrate was 28 nm (Fig 6(b)) These values arecomparable with the rms roughness of the bare SiO2 sub-strate (04 nm) However the nanorod density was muchhigher in case of both the seeded substrates The probablereasons for this are (a) the ZnO seed layer on the sub-strate lowers the lattice mismatch between the depositedmaterial and the substrate and (b) it helps to lower downthe free energy barrier of activation

5 CONCLUSIONS

We have presented a detailed study of the effect of thecapping agent the nature of the substrate and substrateseeding on the growth of ZnO nanorods by aqueous solu-tion synthesis By capping the non-polar side facets ofthe growing ZnO crystal HMT (hexamethylenetetramine)promoted the growth of high aspect ratio uniform ZnOnanorods Other capping agents such as dodecylamineresulted in 1-D nanocrystals with low aspect ratio whileplate shaped microcrystals were formed when CTAB wasused We have shown that seeding the substrate with gran-ular ZnO played a very important role in the alignmentof the nanorod array The aspect ratio of ZnO nanorodswas also guided by the substrate used By proper choice ofsubstrate seed layer and capping agent we could obtainboth oriented and random nanorods having a controllablediameter in the range of 50 to 1200 nm Our study ofthe effect of different substrates additives and seed lay-ers on the growth of ZnO nanorod arrays should lead toan improved understanding of the controllable growth ofone dimensional nanostructures that are expected to haveimportant applications in novel nanodevices

Acknowledgment The authors acknowledgeMr N Kulkarni for his assistance with XRDmeasurements

References and Notes

1 M Law L E Greene J C Johnson R Saykally and P YangNature Mater 4 455 (2005)

2 Z Sun H Yuan Z Liu B Han and X Zhang Adv Mater 17 2993(2005)

3 J Chen L Xu W Li and X Gou Adv Mater 17 582 (2005)4 A Yoon W K Hong and T Lee J Nanosci Nanotechnol 7 4101

(2007)5 C J Lee T J Lee S C Lyu Y Zhang H Ruh and H J Lee

Appl Phys Lett 81 648 (2002)6 Y W Zhu T Yu F C Cheong X J Xu C T Lim V B C Tan

J T L Thong and C H Sow Nanotechnology 16 88 (2005)7 P Ayyub J Cluster Science 20 429 (2009)8 P Bhattacharya S Gohil J Mazher S Ghosh and P Ayyub Nano-

technology 19 075709 (2008)9 D Carvalho S Ghosh R Banerjee and P Ayyub Nanotechnology

19 445713 (2008)10 M H Huang S Mao H Feick H Yan Y Wu H Kind E Webber

R Russo and P Yang Science 292 1897 (2001)11 Z L Wang ACS Nano 2 1987 (2008)12 Z L Wang and J Song Science 242 312 (2006)13 J Zhou N Xu and Z LWang Adv Mater 18 2432 (2006)14 J Das and D Khushalani J Phys Chem C 114 2544 (2010)15 J X Wang X W Sun Y Yang H Huang Y C Lee O K Tan

and L Vayssieres Nanotechnology 17 4995 (2006)16 Z Fan D Wang P-C Chang W Y Tseng and J G Lu

Appl Phys Lett 85 5923 (2004)17 M H Huang Y Wu H Feick N Tran E Weber and P Yang

Adv Mater 13 113 (2001)18 Z W Pan Z R Dai and Z L Wang Science 291 1947 (2001)19 X Y Kong and Z L Wang Appl Phys Lett 84 975 (2004)20 X Y Kong and Z L Wang Nano Lett 3 162 (2003)21 A Umar S H Kim J H Kim Y K Park and Y B Hahn

J Nanosci Nanotechnol 7 4421 (2007)22 M Snure and A Tiwari J Nanosci Nanotechnol 7 481 (2007)23 A Umar M M Rahman S H Kim and Y B Hahn J Nanosci

Nanotechnol 8 3216 (2008)24 S L Mensah A Prasad J Wang and Y K Yap J Nanosci Nano-

technol 8 233 (2008)25 D Wang C Song Z Hu W Chen and F Xun Mater Lett 61 205

(2007)26 J Zhang L Sun J Yin H Su C Liao and C Yan Chem Mater

14 4172 (2002)27 B Liu and H C Zeng J Amer Chem Soc 126 16744 (2004)28 Z L Wang Mater Today 7 26 (2004)29 Z R Tian J A Voigt J Liu B Mckenzie M J Mcdermott M A

Rodriguez H Konishi and H F Xu Nature Mater 2 821 (2003)30 Y Li G W Meng L D Zhang and F Phillipp Appl Phys Lett

76 2011 (2000)31 X Duan and C M Lieber J Amer Chem Soc 122 188 (2000)32 A Umar S H Kim J H Kim and Y B Hahn J Nanosci Nano-

technol 7 4522 (2007)33 G S Wu T Xie X Y Yuan Y Li L Yang Y H Xiao and L D

Zhang Solid State Commun 134 485 (2005)34 S Chatterjee S Gohil B A Chalke and P Ayyub J Nanosci

Nanotechnol 9 4792 (2009)35 J Zhang L Sun H Pan C Liao and C Yan New J Chem 26 33

(2002)36 L Vayssieres Adv Mater 15 464 (2003)




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37 L Vayssieres K Keis S-E Lindquist and A Hagfeldt J PhysChem B 105 3350 (2001)

38 K Keis L Vayssieres S-E Lindquist and A Hagfeldt Nanos-truct Mater 12 487 (1999)

39 L Vayssieres K Keis A Hagfeldt and S-E LindquistChem Mater 13 4395 (2001)

40 J P Liu C X Xu G P Zhu X Li Y P Cui Y Yang and X WSun J Phys D Appl Phys 40 1906 (2007)

41 J Yang J Zheng H Zhai X Yang L Yang Y Liu J Lang andM Gao J Alloys Comp 489 5155 (2010)

42 J Song and S Lim J Phys Chem C 111 596 (2007)

43 A Sugunan H C Warad M Boman and J Dutta J SolndashGelSci Techn 39 49 (2006)

44 Y J Kim H Shang and G Cao J SolndashGel Sci Techn 38 79(2006)

45 J Zhao Z-G Jin T Li and X-X Liu J Europ Ceram Soc26 2769 (2006)

46 P Taneja R Chandra R Banerjee and P Ayyub Scripta Mater44 1915 (2001)

47 J S Lee M I Kang S Kim M S Lee and T K Lee J CrystGrowth 249 201 (2003)

48 P Yang and C M Lieber J Mater Res 12 2981 (1997)

Received 12 June 2011 Accepted 8 August 2011




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arrays of ZnO nanorods on suitable substrates and withcontrolled aspect ratio is extremely important to meet thedemands of potential applicationsZnO nanorod arrays have been synthesized by a variety

of techniques such as template-assisted growth30 vaporndashliquidndashsolid growth31 thermal evaporation32 solndashgeltransition33 electrochemical deposition34 hydrothermalsynthesis35 and aqueous solution36 synthesis Vayssiereset al first reported the successful synthesis of three-dimensional highly oriented hexagonal microrod arraysof ZnO from an aqueous solution37 Such a microrodarray shows high UV photoresponse and excellent electrontransport properties over a wide range of rod lengths38

The same technique was later used for the growth of ZnOmicrotube arrays39 as well as nanorod arrays36 just bylowering the precursor concentration The ZnO nanorodarray appears very promising for the fabrication of costeffective and high performance gas sensors15 Liu et alsynthesized vertically aligned ZnO nanorod arrays directlyon a Zn foil (which acted both as source and substrate) inpure water at low temperature and measured its field emis-sion property40 Yang et al reported the hydrothermal syn-thesis of pencil-like aligned and oriented ZnO nanorodson different substrates41 and found that the nanorod densitywas greater on pre-seeded substrates than on bare Si or CuSong and Lim found that the morphology of ZnO nanorodsdepends strongly on the nature of the seed layer42 whosethickness and crystallite size control the diameter and den-sity of the nanorods while the orientation dictates the crys-tallinity of the nanorods Sugunan et al studied the role ofhexamethylenetetramine (HMT) on the seeded growth ofZnO nanowires by a hydrothermal route43 and concludedthat HMT acts as a shape inducing molecule by selectivelycapping the non polar crystallographic planes of ZnO crys-tals Further the concentration of the precursor solutionhas a significant effect on the growth rate of the nanowiresKim et al found that the alignment of ZnO nanorod arraycritically depends on initial seed layer44 Zhao et al alsostudied the effect of seed layer on the growth of ZnOnanorods45

Here we report a detailed study of the effect of thegrowth conditions on the nanorod diameter and morphol-ogy and the alignment of the nanorod array grown in anaqueous solution In particular we have studied the effectsof different capping agents and different amines in control-ling the growth and morphology of 1-D ZnO nanocrystalsWe have also used different substrates to determine theireffect on the crystallinity alignment aspect ratio and den-sity of nanorods Interestingly we found that by onlychanging the substrate one can switch from microrods tonanorods At the same time the roughness of the substrateplays an important role in controlling the alignment anddensity of the nanorods

2 EXPERIMENTAL DETAILS

One-dimensional ZnO microcrystals and nanocrystals withdifferent morphologies were synthesized using a modi-fied aqueous solution route Each substrate was mountedin an upside down position inside an ordinary screw-cap pyrex glass bottle An aqueous solution of 001 Mzinc nitrate (Zn(NO32 middot 6H2O) was used as the sourceof zinc ions Morphological changes were brought aboutby using a number of different amines and cappingagents each of them at a concentration of 001 MThese were hexamethylenetetramine (HMT (CH26N4n-dodecylamine (C12H27N) hexadecyl trimethyl ammo-nium bromide (CTAB C19H42BrN) urea (CH4N2O) andethylene diamine tetra-acetate (EDTA C10H16N2O8 Assubstrates we used oriented and randomly oriented thinfilms of ZnO on Si wafers as well as commercially avail-able SiO2 LaAlO3 and MgO wafers The reaction vesselswere each kept at 90 C for 12 hours and then cooled downto room temperature After the reaction was over each as-prepared sample was washed with de-ionized water untilthe pH of the wash became asymp65 Finally the sampleswere air driedFor their subsequent use as seed layers nanocrys-

talline ZnO thin films were sputter-deposited on Si wafersmounted on a sample holder of a custom-designed sputter-ing chamber initially evacuated to about 10minus6 torr Usinga 200 mm long axial planar magnetron sputtering sourcesputtering was carried from a 50 mm diameter ZnO targetmade by pressing 999 pure ZnO powder at 20 tons Theaverage grain size in sputter deposited films can be con-trolled with reasonable accuracy by appropriate choice ofprocess parameters46 In the present case sputtering wascarried out in 50ndash100 mtorr of 9999 Ar gas pressure at50 W rf power The sputtering time was 30ndash60 min dur-ing which the substrate was maintained at about 20 C bywater-cooling The other substrates used were of commer-cial originThe crystallographic structure of the 1-D ZnO sam-

ples was investigated using a Panalytical Xrsquopert Pro pow-der X-ray diffractometer (XRD) with Cu K radiationDetailed studies of the microstructure and the elemen-tal composition were obtained using an energy dispersiveX-ray (EDX) analyzer with a Jeol JSM 840 scanning elec-tron microscope (SEM) The topography and the rough-ness of the ZnO seeded substrates were estimated usinga Multimode Scanning Probe Microscope (NanoscopeIV-Veeco) in contact mode with a Si tip at a scan speedof 1 Hz

3 RESULTS

We first study the possibility of controlling the morphol-ogy of the 1-D ZnO structures by means of adding suit-able hydrolyzing agents as well as surfactants For thispart of the study the ZnO nanostructures were grown on




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Growth of aligned ZnO nanorod arrays from an aqueous solution: effect of additives and substrates

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Transcript of Growth of aligned ZnO nanorod arrays from an aqueous solution: effect of additives and substrates