PH-Dependent Growth of Zinc Oxide Nanorods

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Journal of Crystal Growth 311 (2009) 2549–2554

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

�Corr

E-m

journal homepage: www.elsevier.com/locate/jcrysgro

pH-dependent growth of zinc oxide nanorods

Sunandan Baruah, Joydeep Dutta �

Centre of Excellence in Nanotechnology, School of Engineering and Technology, Asian Institute of Technology, P.O. Box-4, Klong Luang, Pathumthani 12120, Thailand

a r t i c l e i n f o

Article history:

Received 4 November 2008

Received in revised form

26 January 2009

Accepted 28 January 2009

Communicated by S. Udachange gradually from 6.4 to 7.3 in 5 h during the growth process. The growth of the ZnO nanorods was

Available online 4 February 2009

PACS:

61.46.HK

61.46.Km

81.07.�b

81.16.Dn

Keywords:

A1. PH

A2. Hydrothermal

B1. Hexamine

B1. Nanoparticle

B1. Nanorod

B2. Zinc oxide

48/$ - see front matter & 2009 Elsevier B.V. A

016/j.jcrysgro.2009.01.135

esponding author. Tel.: +66 2 524 5680; fax: +

ail address: [email protected] (J. Dutta).

a b s t r a c t

Here we study the effect of pH variation on the dimension and morphology of zinc oxide (ZnO) nanorods

grown through hydrothermal process at temperatures less than 100 1C. ZnO nanorods were grown on

pre-seeded glass substrates using zinc nitrate hexahydrate as the source of Zn ions and

hexamethylenetetramine as the source of hydroxyl ions. The pH of the reaction bath was found to

observed to be faster, both laterally and longitudinally, when the growth solution was in basic

conditions. However, flower petal like ZnO nanostructures were obtained when the growth process was

initiated in basic condition (pH 8–12), indicating that initial acidic conditions were required to obtain

nanorods with well-defined hexagonal facets. ZnO is known to erode in acidic condition and the final

dimension of the nanorods is determined by a competition between crystal growth and etching. ZnO

nanorods of different dimensions, both laterally (diameters ranging from 220 nm to 1mm) and

longitudinally (lengths ranging from 1 to 5.6mm) were successfully synthesized using the same

concentration of zinc nitrate and hexamine in the reaction bath and the same growth duration of 5 h

simply through appropriate control of the pH of the reactant solution between 6 and 7.3.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

Numerous reports on the growth and characterization of one-dimensional nanowires of elemental and compound semiconduc-tors such as silicon (Si) [1], germanium (Ge) [2], indiumphosphide (InP) [3], gallium arsenide (GaAs) [4] and zinc oxide(ZnO) [5–7] are available in the literature. Nanostructures of ZnOsuch as nanowires and nanorods [8], nanocombs [9], nanorings[10], nanoloops and nanohelices [11], nanobows [12], nanobelts[13] and nanocages [14] have been reported. These structureshave been synthesized under controlled growth conditions[15,16]. Zinc Oxide nanostructures can be synthesized eitherthrough gas-phase synthesis or through solution-phase synthesis.Gas-phase synthesis is carried out in a gaseous environment inclosed chambers at high temperatures (500–1500 1C). Somecommonly used gas-phase methods are vapor-phase transport,which includes vapor solid (VS) [17] and vapor liquid solid (VLS)[18] growth, physical vapor deposition (PVD) [19], chemical vapordeposition (CVD) [20], metalorganic chemical vapor deposition(MOCVD) [21] and thermal oxidation of pure Zn [22] followed bycondensation. In the solution-phase synthesis, the growth process

ll rights reserved.

66 2 524 5617.

is carried out in a liquid. Normally aqueous solutions are used andthe process is then referred to as hydrothermal growth process.Some of the solution-phase synthesis processes reported are thezinc acetate hydrate (ZAH) derived nano-colloidal sol–gel route[23], ZAH in alcoholic solutions with sodium hydroxide (NaOH)[24], tetra methyl ammonium hydroxide (TMAH) [25] or lithiumhydroxide (LiOH) [26], template-assisted growth [27] or spraypyrolysis [28,29] and electrophoresis [30].

One of the most energy-efficient strategies for synthesizingZnO nanorods is the hydrothermal process that does not requirehigh temperature and complex vacuum environment. The hydro-thermal process induces an epitaxial, anisotropic crystal growth ina solution [26,31] (normally aqueous solution). The hydrothermalprocess is usually substrate independent [32] and the morphologyof the nanorods can be easily controlled through slight changes inthe reaction conditions [33]. There are reports on the successfulgrowth of ZnO nanowires on flat substrates like Si [15], glass[33,34], TCO [35], polyethylene fibers [8], carbon cloth [36] and Alfoil [37].

In one of the reported processes [31], an equimolar solution ofzinc nitrate hexahydrate [Zn(NO3)2 �6H2O] and hexamethylenetetramine [C6H12N4], popularly known as hexamine is utilized toepitaxially grow ZnO rods on substrates by fixing pre-synthesizedZnO nanoparticles on the substrate as seeds. The ZnO crystal ishexagonal wurtzite and exhibits partial polar characteristics [16]

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0

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Chemicals replenished

Single pot synthesis

5 10 15 20

Fig. 2. ZnO nanorod growth rate increased when the chemicals were replenished

after every 5 h.

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time (hrs)

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S. Baruah, J. Dutta / Journal of Crystal Growth 311 (2009) 2549–25542550

with lattice parameters a=0.3296 and c=0.52065 nm. The aniso-tropy in the crystal structure of ZnO assists the growth of thenanorods. The most common polar surface is the basal plane(0 0 1). One end of the basal polar plane terminates in partiallypositive Zn lattice points and the other end terminates in partiallynegative oxygen lattice points. The anisotropic growth of thenanorods takes place along the c-axis in the [0 0 0 2] direction.

It was reported by Sugunan et al. [33] that hexamine, a non-ionic tertiary amine derivative and a non-polar chelating agent,would preferentially attach to the non-polar facets of the ZnOcrystal as it builds up, thereby exposing only the (0 0 1) plane forepitaxial growth [33]. Thus a preferential growth along the[0 0 0 2] direction is made possible. Seeding of the substrate withZnO nanoparticles was found to lower the thermodynamic barrierby providing nucleation sites, further improving the aspect ratioof the synthesized nanorods. Seeding of the substrate is thus animportant parameter for the uniform growth of ZnO nanorodsthrough hydrothermal process. Seeding can be done by dipcoating [8] and spin coating [32] using a colloidal solution ofZnO nanoparticles or sputtering a thin layer of ZnO on thesubstrate [38].

A lot of factors come into play during the growth of the ZnOnanorods like concentration of the chemical bath, temperature,duration of growth, pH, etc., which directly affect the finalmorphology of the rods grown. There are reports available inthe literature about the synthesis of ZnO nanorods and othermorphologies through a variation in pH of the reaction bath[39–41]. However, this study is aimed at optimizing the pHconditions to obtain ZnO nanorods of different dimensionsstarting with the same concentration of the reactant mixture. Itwas possible to grow ZnO nanorods of different dimensions (bothlateral and longitudinal), with the same concentration of Zn(NO3)2

and hexamine in the chemical bath and the same growthduration, simply by varying the pH of the growth solutionbetween 6 and 7.3.

Fig. 3. Change in pH of the reaction bath over a period of 5 h during the growth of

ZnO nanorods.

The ZnO nanorods were synthesized modifying a methodinitially suggested by Vayssieres et al. [31] and proposed bySugunan et al. [33]. The method consisted of seeding substrateswith ZnO nanocrystallites followed by a chemical bath growth ofthe nanorods as described in Section 2.3.

Fig. 1. HRTEM image showing the ZnO nanoparticles synthesized in isopropanol.

Measurements of lattice spacings done on images of different particles indicated

the presence of the (10 0), (0 0 2), (10 1) and (10 2) planes of the wurtzite

structure.

2.1. Materials used

All chemicals used in this study were analytical grade and wasused without further purification. Zinc acetate dihydrate[(CH3COO)2Zn �2H2O] procured from Merck was used as the zincion source, sodium hydroxide [NaOH] from Merck as the reducingagent and ethanol [C2H5OH] from J.T.Baker as the solvent for thesynthesis of ZnO nanoparticles. Zinc nitrate hexahydrate[Zn(NO3)2 �6H2O, Aldrich, 99% purity] and hexamethylene tetra-mine [C6H12N4, Carlo Erba, 99.5%] were used as the reactants inthe chemical bath for the ZnO nanorod growth.

2.2. Synthesis of ZnO nanoparticles

The synthesis of ZnO nanoparticles that were used as seedswas carried out following a procedure reported by Bahnemann etal. [42] 1 mM zinc acetate solution was prepared in 2-propanolunder rigorous stirring at 50 1C. The solution was then furtherdiluted and cooled, after which aliquots of 20 mM sodiumhydroxide in 2-propanol was added under continuous stirring.The mixture was then kept in a water bath at 60 1C for 2 h [43].

2.3. Hydrothermal growth of ZnO nanorods

ZnO nanorods were grown on glass slides, which were firstthiolated by dipping in a 1% solution of dodecane thiol in ethanol

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[8] as the surface functionalization of silica using thiol allows theirreversible binding of metal oxide particles from a colloidalsolution [44]. The substrates were seeded by dipping the thiolated

Fig. 4. SEM images of ZnO nanowires grown hydrothermally for 15 h with different co

1000

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00 2 4 6 8 10 12 14 16

time (hrs)

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eter

(nm

)

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dic

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ic

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ic

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ic

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5 mM

1 mM

Fig. 5. Dimensions of ZnO nanorods grown over a period of 15 h using 1, 5 an

Fig. 6. FESEM images showing the nanostructures obtained when growth was carried ou

pH of 8 (C) pH of 10 and (D) pH of 12.

substrates into a concentrated colloidal solution of ZnO nanopar-ticles in isopropanol for 15 min. Three dippings were made andthe substrates were heated at 150 1C for 15 min after each dipping

ncentration of the reactants (A) 1 mM (B) 5 mM and (C) 10 mM. Scale bars=1mm.

0 2 4 6 8 10 12 14 16time (hrs)

leng

th (n

m)

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dic

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d 10 mM solution of zinc nitrate and hexamine (A) diameters (B) lengths.

t at basic pH using 10 mM reaction bath and growth duration of 5 h (A) pH of 7.3 (B)

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to ensure that the seeds were securely attached. The wires weregrown in a sealed chemical bath containing an equimolar solutionof zinc nitrate hexahydrate and hexamethylene tetramine at 90 1C.Different concentrations of the precursor solution were used tostudy the growth of the ZnO nanowires. As the growth rate wasobserved to decrease after about 5 h [8], the precursor solutionwas changed every 5 h and growth was continued for up to 15 h.pH of the reaction bath was controlled by titrating the reactantsolution with HCl and NaOH. The samples were then heated at250 1C for 30 min to vaporize any organic deposits. The character-ization of the nanowires was performed using scanning electronmicroscopy (SEM, IE350FSG FESEM) images through image-processing software. High-resolution transmission electron mi-croscopy (HRTEM) was carried out using a JEOL JEM 2010operating at 100 kV. pH measurements were carried out with apH meter from Denver Instrument.

3. Results and discussions

Organometallic synthesis method was selected for the growthof the ZnO seed nanoparticles as their nucleation and growth aremuch faster in alcohol (isopropanol in the present case) than inaqueous solution, water being a highly polar solvent. The ZnOseed nanoparticles were almost spherical and about 5–6 nm indiameter, as can be observed from the HRTEM image shown inFig. 1. The nanoparticles synthesized by this method exhibit thewurtzite structure of ZnO [8] and lattice spacing measurements

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Fig. 7. Length and width of the ZnO nanorods after a growth of 5 h as a function of the t

final pH of 7.3: (A) initial pH 6, (B) initial pH 6.5, (C) initial pH 6.8 and (D) initial pH 7

done using Scion image-processing software show almostcomparable dominance of the (10 0), (0 0 2), (10 1) and (10 2)crystallographic planes [33]. The ZnO nanorods were initiallysynthesized using a reaction bath containing 20 mM aqueoussolution of zinc nitrate hexahydrate and hexamethylenetetraminefor 20 h, recording the length after every 5 h. It was observed thatthe growth was very fast in the initial 5 h and the growth rateshowed a gradual decrease after that as was reported earlier [8].This may be accounted to the decrease in concentration of Zn2+

ions in the chemical bath. To maintain a high growth rate, thechemicals in the reaction bath was replenished after every 5 h.

Fig. 2 shows the comparison between the lengths of thenanorods with and without replenishment of the reaction bath. Itwas observed that the growth of the ZnO nanorods was notconsistent throughout and the growth rate at any particularconcentration is not a linear function of time. Hexamine is knownto degrade upon prolonged thermal treatment thereby releasinghydroxyl ions, which gradually changes the pH of the reactionbath [45,46]. To understand the role of pH in the growth of ZnOnanorods, pH variations during the growth process was carefullyfollowed for 3 different concentrations (1, 5 and 10 mM) of thechemical bath and a plot of pH versus time recorded over a periodof 5 h is shown in Fig. 3.

It can be concluded from Fig. 3 that the pH of the reaction bathchanges from acidic to basic in about an hour of growth time ofthe ZnO nanorods. After 5 h the pH stands at about 7.3 for allconcentrations. To correlate the lateral and longitudinal growth ofthe ZnO nanorods with the pH, measurements were done on SEM

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ime for which the initial growth was carried out in different pH conditions and the

.

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images of the nanorods grown over a total duration of 15 h. Someof the SEM images are shown in Fig. 4. The chemical reaction bathwas replenished after every 5 h with similar concentration ofreactants. Fig. 5 shows the length and diameter of the nanorodsplotted as a function of time over a period of 15 h. The growth rate,both lateral and longitudinal, was observed to be higher in basicconditions than in acidic conditions, which is understandable asZnO nanorods are known to get eroded in acidic conditions andthe final growth depends upon the competition between growthand etching [47]. The initial phase of nanorod growth is slow(Fig. 5) as the nucleation sites are limited and these smaller rodswould merge during growth to form bigger rods. The enhancedrate of lateral and longitudinal growth during the period between8 and 10 h can be attributed to the higher availability of surfaceatoms on the ZnO nanorods for the crystal growth. Once thenanorods reach a thermodynamically stable size, the growth rateshows a decline (after about 10 h of growth) that arises due to thelarger number of surface adatoms to which the available Zn2+orO2� ions can bond as compared to the thinner rods.

In continuation with the observation that the growth rate wasfaster in basic pH conditions, a few controlled experiments wereconducted to monitor the growth and morphology of the grownnanorods. Hydrothermal crystallization of ZnO carried out atvarious basic pH values (7.3, 8, 10, 12) showed the transition ofmorphology from rod like to flower like with a shift towards morebasic pH. Fig. 6 shows the SEM images of the nanostructuresobtained at basic pH. Comparing the rods obtained with growth

Fig. 8. ZnO nanorods grown under different initial pH with same concentration (10 mM)

10 min and 7.3 for 4 h 50 min, (C) pH 6 for 1 h and 7.3 for 4 h, and (D) pH 6.5 for 40 mi

initiation in mild acidic condition (Fig. 3) and the ones grown atbasic condition (Fig. 6(a) and (b)), it can be concluded that in theformer case the growth was much better with sharp hexagonalfacets clearly visible. Further, for the nanorods shown in Fig. 3,initial growth was in mild acidic condition, which graduallychanged to basic condition after about 1 h (for 5 and 10 mMsamples) and 1.5 h for the 1 mM sample (Fig. 3). In order toobserve the growth variation with changes in pH, a set ofexperiments were conducted using 10 mM growth solution andgrowth duration of 5 h for all the samples. Starting in a slightlyacidic media (pH �6, 6.5 and 6.8) and also at a neutral pH of 7 fordifferent durations of time (10, 30, 40 and 60 min), further growthwas continued after transferring the substrates to a reactantmixture at a pH of 7.3 (pH at which fastest growth was observed).Observations were done on FESEM images and measurementswere carried out using Scion image-processing software. Figs. 7show the length and width of the nanorods as functions of thetime when the sample was kept in slightly acidic condition (pH�6, 6.5, 6.8 and 7).

From Fig. 7, we can observe that the maximum growth, both inthe lateral and longitudinal directions, was obtained when thegrowth was started with a pH of 6.8 for 10 min followed by growthat pH 7.3 (Fig. 7(A)). The smallest rods, both in diameter andlength, was obtained with a starting pH of 6.5 for 10 min and therest of the growth at a pH of 7.3 (Fig. 7(B)). Fig. 7(D) shows thelength and diameter of the nanorods grown with an initial neutralpH and continued at a pH of 7.3. Negligible variations in the

and growth duration (5 h): (A) pH 6 for 40 min and 7.3 for 4 h 20 min, (B) pH 6.8 for

n and 7.3 for 4 h 20 min. Scale bars: 1mm.

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dimensions were observed, which can be accounted to the lack ofetching of the ZnO crystal facets at neutral pH. The growth rate ofthe ZnO crystal at pH 7 and 7.3 are observed to be comparable. Therods obtained after keeping the reaction bath at pH 7 for differentdurations and continuing in pH 7.3 were therefore of similardimensions (Fig. 7(D)). The ZnO nanorods obtained a taperedmorphology when the initial growth was carried out at a pH of 6.8for 30–40 min and continued for 5 h at pH 7.3 (Fig. 7(C) and 8(D)).This tapering of the rods may result from the growth of multiplelayers of the ZnO crystal with decreasing width along thec-axis.

The growth rate of ZnO is known to be the fastest along the c-axis [48]. The disappearance rate of the (0 0 0 1) face of ZnOnanorods is also reported to be the fastest compared to the otherfaces when eroded in acidic medium [47]. This corroborates withthe observation that the ZnO nanorods grown initially with a pHof 6 for 60 min followed by another 4 h growth at pH 7.3 gives theshortest length (3.5mm) and the largest width (1.8mm). Thelength was probably checked by the erosion of the (0 0 0 1) faceleading to a higher lateral growth.

An interesting point to note is that, in just 5 h, it was possibleto grow ZnO nanorods with length (5.6mm in 5 h using 10 mMconcentration) up to twice that obtained using the conventionalmethod (2.8mm in 5 h using 10 mM concentration) simplythrough controlling the pH of the reactant bath. However, themaximum width that could be obtained in 5 h is 1.8mm ascompared to 242 nm using the conventional method (an increaseof over 7 times) that needs closer look for future studies.

4. Conclusion

ZnO nanorods with diameters varying between 220 nm and1.8mm and lengths between 1 and 5.6mm were successfullysynthesized through a simple hydrothermal method, using thesame concentration and the same growth duration, simply bycontrolling the pH of the reaction bath during the growth process.It was clearly demonstrated that the pH plays a major part in themorphology and dimensions of the nanorods grown throughhydrothermal process. The growth of the nanorods was observedto be faster in basic medium than in acidic medium. However,slightly acidic conditions during the initial growth from thenucleation sites resulted in appreciable variations in the lateraland longitudinal dimensions, opening up possibilities of tailoringthe width and length of the nanorods to suit different applica-tions. This may be an attractive option for manufacturing ZnOnanorod-based devices at a commercial level as the dimension ofthe nanorods can be controlled simply through the variation in asingle parameter i.e. the pH of the reaction bath, keeping all otherparameters like concentration of the bath, duration of growth andtemperature constant.

Acknowledgements

The authors would like to acknowledge partial financialsupport from the Centre of Excellence in Nanotechnology at theAsian Institute of Technology and the National NanotechnologyCenter (NANOTEC) belonging to the National Science & Technol-ogy Development Agency (NSTDA), Thailand.

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