Growth of ZnO Nanorods by the Chemical Solution Method withAssisted Electrical Field

Juan Zhao,w Zheng-Guo Jin, Tao Li, Xiao-Xin Liu, and Zhi-Feng Liu

Key Laboratory For Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University,Tianjin 300072, China

Zinc oxide (ZnO) nanorod arrays on the ZnO-coated seedsubstrates were prepared by the solution chemical methodfrom Zn(NO3)2/NaOH under an assisted electrical field. Theinfluence of the electrical field on ZnO nanorod growth wasprimarily explored, and the positive effects of the electrical fieldwere demonstrated by adding polyethylene glycol in growth so-lution. It has been proved that the electrical field enhances ionadsorption to the substrate and lowers the nucleation energybarrier by increasing charge intensity; meanwhile, it producesH

1through oxidation of OH

�and increases properly the de-

gree of solution supersaturation near the substrate surface. XRDresults show that the nanorods grown under the electrical fieldprimarily have a zincite structure. With increasing precursorconcentration, the average diameter and length of ZnO nano-rods increase. The maximum rod growth rate at a given con-centration of Zn21 ion occurs at a specific temperature.

I. Introduction

ZINC oxide (ZnO) nanomaterials have been studied widely fora variety of high-technology applications ranging from sur-

face acoustic wave filters,1 photonic crystals,2 photodetectors,3

photodiodes,4 optical modulator waveguides,5 varistors,6 to gassensors,7 due to their wide bandgap, and excellent chemical andoptoelectronic properties as a II–VI semiconductor with a largeexciting binding energy. In the past decade, interest has beenfocused on ZnO nanorods having a high surface area and aspecific crystalline orientation.8 They have become an alterna-tive to the TiO2 nanostructured electrode in Gratzel-type pho-tocells because of reduction in grain-boundary density leading toimproved photogenerated carrier transport.9,10 ZnO nanorodshad been fabricated by several methods, such as catalytic growthvia vapor–liquid–solid epitaxial (VLSE) mechanisms,11 metal–organic chemical vapor deposition (MOCVD),12 pulsed laserdeposition,13 and templating with anodic alumina membranes.14

More recently, the solution chemical route has been reported asa low-cost, low-temperature method of ZnO column produc-tion, in which thermal deposition techniques utilize the degra-dation of methenamine with zinc nitrate solutions to producecrystal arrays.15 Other solution techniques use sodium hydro-xide16 or complexing agents such as dimethylaminoborane(DMAB), urea, ethylenediamine, and ammonium fluoride tostabilize the presence of zinc ion in solution.17

In this paper, we use the aqueous solution containing NaOHand Zn(NO3)2 and substrate coated with ZnO seeds throughsol–gel processing for ZnO rod growth.18 This method does not

require the use of complexing agents and can produce microm-eter-thick films in comparatively short times. The principle ofthe process is based on heterogeneous nucleation on a specificsurface and subsequent crystal growth that is driven by super-saturation of the aqueous solution. However, in the reactionprocess, the precursors keep on being consumed with time, lead-ing to lowering of both the concentration and the supersatura-tion degree of the precursor solution. Under such a condition,the crystal growth rate gradually decelerates and eventually re-mains constant at a low level. Therefore, we apply an assistedelectrical field on the substrate as an anode to lower appropri-ately the pH value and thus elevate the degree of supersaturationof the solution near the substrate surface in order to promote thecrystal growth. The growth of ZnO nanorods under differentpotentials was explored; the mechanism of how the electricalfield works was discussed and further verified by changing thestability of the aqueous solution with polyethylene glycol (PEG).The effects of other factors, like the precursor concentration anddeposition temperature, on the morphology of the ZnO nano-rods under the applied electric field were also analyzed.

II. Experimental Procedure

(1) Preparation of Seed-Coated Substrates

0.75 mol/L Zn(CH3COO)2 � 2H2O and monoethanolamine werefirst dissolved in a 2-methoxyethanol solution at room temper-ature. The resultant solution was stirred at 601C for 30 min toyield a clear and homogeneous solution, which served as a seed-coating solution. Then, a clean ITO glass substrate was dippedinto the solution, withdrawn at 3.5 cm/min, and dried in air for10 min. Finally, the as-coated substrate was heated in an elec-trical furnace at 3001C for 10 min, drawn out of the furnace, andcooled in air.

(2) Growth and Characterization of ZnO Nanorods

For the experiment without electrical assistance, the ZnO-coatedsubstrates were rinsed with deionized water, and then suspendedin an aqueous solution of 0.001M Zn(NO3)2/0.1M NaOH at701C for 2 h. For the experiment under an electrical field, theelectrical field was applied by a standard three-electrode cellconfiguration, while other conditions were held constant. TheZnO-coated substrate was used as a working electrode (anode),a larger area Pt foil as a counter electrode, and Ag/AgCl (KClsaturated) as the reference electrode. The anodic potential wasadjusted from 0.9 to 1.5 V. Electrochemical measurement wasperformed with a Potentiostat/Galvanostat (TD3691, TianjinZhonghuan Co., Tianjin, China). The resultant samples wererinsed with deionized water and dried in air. The morphologyand dimensions of the nanorods were observed by JEOLJSM6700 FE-scanning electron microscopy and transmissionelectron microscopy (TEM, JEOL 100CX-II, Tokyo, Japan).The rod crystal structure was detected using XRD (Rigaku2500, Tokyo, Japan) with CuKa radiation.

Journal

J. Am. Ceram. Soc., 89 [8] 2654–2659 (2006)

DOI: 10.1111/j.1551-2916.2006.01103.x

r 2006 The American Ceramic Society

2654

M. Paranthaman—contributing editor

This work was supported by Natural Science Foundation of Tianjin (Project No.:033802311).

wAuthor to whom correspondence should be addressed. e-mail: tonyjane666@yahoo.com.cn

Manuscript No. 20903. Received August 22, 2005; approved March 7, 2006.

III. Results and Discussion

(1) Effect of Potential on ZnO Nanorod Growth

Figure 1 shows SEM photographs of the ZnO nanorods grownwithout an electrical field and at the potentials of 0.9–1.5 V froma 10�3M zinc solution at 701C. It was found that nanorodsgrown below 1.1 V were oriented in a perpendicular fashion andwere arranged in highly uniform and densely packed arrays,which was different from those grown without an electrical field.However, the ZnO nanorods grown above 1.1 V began to inclineand the rod tops bound with each other. The length and diam-eter distributions based on the micrographs of the samplesgrown at 1.3 V are depicted in Fig. 2. It can be seen that boththe rod length and diameter have narrow distributions, showinga uniform rod structure. In addition, the average film thickness(measured from cross section of SEM photographs) increasesfrom 650 nm without an electrical field to 700 nm at 1.1 V, andfurther to 750 nm at 1.3 V, which illustrates the positive effect ofan electrical field for rod growth, but it decreases to 600 nm at1.5 V.

Figure 3(A) exhibits the XRD patterns of the ZnO nanorodsgrown without an electrical field and at 1.3 V. All characteristicpeaks of the obtained nanorods can be indexed to the hexagonalphase of ZnO (wurtzite structure), except those of the conduct-ing layer on the substrate. The cell constants were calculated tobe a5 0.325 nm and c5 0.521 nm, which agree with the report-

ed value (JCPDS card No. 36-1451). Moreover, the [002] reflec-tion is greatly enhanced relative to the usual [101] maximumreflection for ZnO due to the preferred orientation along the caxis of ZnO crystals. The structure of the ZnO crystals is furtherconfirmed by TEM and SAED (Fig. 3(B)). The diffraction pat-tern demonstrates the single-crystal nature of the nanorodsgrown along the [001] direction.

The possible chemical reactions for ZnO growth under anelectrical field are described as follows:

Zn2þ þ 4OH� ! ZnðOHÞ2�4 (1)

2OH� � 2e! O2 þ 2Hþ (2)

ZnðOHÞ2�4 þ 2Hþ ! ZnOþ 2H2O (3)

When the potential reaches a certain value, the anodic oxidationof OH� viz. Eq. (2) takes place; the production of H1 increasesthe supersaturation of the solution near the substrate/liquid in-terface, hence promoting the rod growth according to Eq. (3).Figure 4 shows the linear sweep voltammograms for differenttemperatures over the range of 0 and 2.0 V. The potential ofunder which the anodic current or the anodic oxidation beginsdecreases with temperature, from 950 mV for 301C to 0.735 Vfor 701C, and to 0.72 V for 901C.When the potential is 0.9 V for

(b)(a)

(e)(d)

(c)

Fig. 1. SEM photographs of the zinc oxide nanorods grown without an electrical field (a) and at different potentials, (b) 0.9 V, (c) 1.1 V, (d) 1.3 V, and(e) 1.5 V.

15 20 25 300

10

20

30

40

50

60

perc

ent (

%)

diameter (nm)

(a)

400 500 600 700 800 900 1000 11000

5

10

15

20

25

30

35

40

45

perc

ent (

%)

length (nm)

(b)

Fig. 2. Diameter distribution (a) and length distribution (b) of zinc oxide nanorods grown at 1.3 V.

August 2006 Communications of the American Ceramic Society 2655

701C, the current density is only 0.17 mA/cm2 and few H1 areproduced; thus, the electrical field just makes the nanorods standperpendicularly on the substrate, due to repulsion caused by the

same charge. With a further increase in the current density orpotential, H1 ions increase, increasing the supersaturation andpromoting crystal growth. Meanwhile, the growth on the rodtop is especially accelerated, which induces binding of the rodtops with each other to decrease the surface energy. However,when the potential becomes 1.5 V, the electron-exchange rateenhances further, and the continuous production of H1 lowersthe pH of the solution to the area outside the metastable rangeof the ZnO phase diagram,16 leading to homogeneous precipi-tation in the solution and a decrease in rod growth rate.

On the other hand, when an electrical field is applied, both theion adsorption onto the anode and ion diffusion in the solutionare accelerated by an increase in the surface charge density. Ac-cording to the Gibbs adsorption equation19:

dg ¼ �X

i

Gidmi;

where g represents the interfacial tension, G the superfacial ad-sorption density, and m the chemical potential of the adsorbedspecies i; the interfacial tension of a system tends to decrease ifthe adsorption density increases. Meanwhile, a maximum isfound for the first derivative of the free enthalpy of nucleation.Such a maximum depends on the interfacial tension at the cubicpower19:

DG� ¼ 16pn�2g3

3ðRT LnSÞ2

Therefore, reducing the interfacial tension leads to a significantlowering of the nucleation energy barrier, which promotes crys-tal nucleation.

(2) Positive Role of an Electrical Field

Figure 5 shows SEM photographs of the ZnO nanorods grownfrom 10�3M zinc solution at 701C. The symbols (a) and (b)represent the deposition condition without the electrical fieldand at a potential of 1.3 V, respectively. The footnotes 1, 2, and3 signify the PEG adding amount of 1, 2, and 4 g/50 mL in thegrowth solutions, respectively. It can be seen from Fig. 5(a) thatthe addition of PEG weakens the growth of ZnO nanorods, andthere are no rods at all when the amount of PEG reaches 2 g/50mL or above. In contrast, straight nanorods appear on thewhole substrate surfaces with high density under electrical as-sistance, indicating the positive role of electrical field on the rodgrowth. However, when the amount of PEG reaches 4 g/50 mL,the growth of rods seems to be weakened again in spite of ap-plying an electrical field.

It has been known that PEG (H(OCH2–CH2)nOH) is a pol-ymer molecule with hydrophilic–O–and hydrophobic CH2CH2

groups. The atom O on the C–O–C chain of PEG has coordi-nation abilities with metal ions20; thus, it has a strong electro-static attraction to Zn21 and creates excess OH�, whichincreases the stability of the growth solution because of super-saturation lowering. But the electrical field enhances the ion ad-sorption and diffusion and induces the neutralization of H1

with the excessive OH�, both of which lead to rod growth again.However, when the amount of PEG reaches 4 g/50 mL, the co-ordination of Zn21 and then greatly excessive OH� make thesystem too stable to grow nanorods and the compensation of theH1 is invalid, which destroys the rod growth once more.

(3) Effect of Concentrations Under an Electrical Field

Figure 6 shows the SEM photographs of the ZnO nanorods atdifferent precursor concentrations under 1.3 V at 701C. It isfound that there are only very short nanorods in the initialgrowth stage in Fig. 6(a) due to a low concentration and thus ahigh activation energy. And obvious rod growth is seen whenthe Zn21 ion concentration increases, with film thickness vary-ing from 750 nm for 10�3M to 1.32 mm for 10�2M. Moreover,

10 20 30 40 50 60 70 80

Inte

nsity

(a.

u.)

2θ (degree)

(A)

(002)

(100) (101)

(004)

(a)

(b)

(c)

(002)

Fig. 3. Structure investigation of zinc oxide (ZnO) nanorods. (A) XRDpatterns of ITO substrate (a) and the ZnO nanorods grown without anelectrical field (b) and at 1.3 V (c); (B) transmission electron microscopyimage of ZnO nanorods grown under an electrical field and SAED pat-tern of the selected nanorod.

(

0 500 1000 1500 2000

0

1

2

3

4

5

6

7

Cur

rent

den

sity

m

A/c

m2

)

Potential (mV)

(a)

(b)

(c)

Fig. 4. Linear sweep voltammograms of Zn(NO3)2/NaOH solutionsover the range of 0 and 2 V at a rate of 10 mV/S at (a) 301C, (b) 701C,and (c) 901C.

2656 Communications of the American Ceramic Society Vol. 89, No. 8

the rod diameter increases from 25 nm for 10�3M to 50 nm for10�2M.

Figure 7 shows the current density curves for different pre-cursor concentrations at 701C under 1.3 V. All curves are foundto decrease dramatically in the beginning and slowly in the laterstage until they remain constant at a low level. The value ofcurrent density over the whole range increases with increasing

precursor concentration, indicating that the rod deposition rateincreases with the precursor concentration under an electricalfield, a result consistent with that of SEM images. It could alsobe concluded that a certain value of current density is requiredfor the rod growth under an electrical field because the currentdensity at 1 h is about 0.2 mV for 10�3 M, while it is only0.01 mV for 10�4M.

(4) Effect of Temperature Under an Electrical Field

Figure 8 exhibits the SEM photographs of the ZnO nanorodsgrown at the temperature range of 501–901C from a 10�3M zincsolution under 1.3 V. We note from Fig. 8(a) that the rodsgrown at 501C are short and upstanding onto the substrate.With increasing temperature, the growth of nanorods improvesbut the average diameter of the nanorods remains approximate-ly unchanged, except that at 901C. This implies that the growthrate along the [001] direction is more sensitive to temperaturecompared with those along the [101] and [100] directions.

Figure 9 shows the film thickness as a function of tempera-ture. The film thickness is found to first increase from 450 nm at501C to 1 mm at 801C, and then decrease to 550 nm at 901C,indicating that the maximum rod growth rate at the given con-centration of Zn21 occurs at a specific temperature. This can beexplained as follows: the growth factors are the increasing ionicdiffusion rate and the decreasing activation energy with increas-ing temperature. The opposing factor is the increasing solubilityof the ZnO with temperature. The first factor apparently dom-inates at a lower temperature, while the second dominates at ahigher temperature, resulting in a peak in the nanorod growthrate versus temperature.16

(a2) (a3) (a1)

(b3)(b2)(b1)

Fig. 5. SEM photographs of the zinc oxide nanorods grown without (a) and with an electrical field of 1.3 V (b). The added amount of PEG: (a1), (b1)—1g/50 mL, (a2), (b2)—2 g/50 mL, (a3), (b3)—4 g/50 mL.

0 1000 2000 3000 4000 5000 6000 7000−0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Cur

rent

den

sity

(mA

/cm

2)

Time (s)

(a)

(b)

(c)

Fig. 7. Current density transients for potentiostatic deposition from thesolution of Zn21/OH�: (a) 10�4M/0.025M, (b) 10�3M/0.1M, and (c)10�2M/1.6M under 1.3 V at 701C.

(c)(b)(a)

Fig. 6. SEM photographs of the zinc oxide nanorods grown from the solution of Zn21/OH�: (a) 10�4M/0.025M, (b) 10�3M/0.1M, and (c) 10�2M/1.6M under 1.3 V at 701C.

August 2006 Communications of the American Ceramic Society 2657

Figure 10 shows the current density transients for the depositionof these ZnO nanorods. The current density is found to increasewith temperature from 501 to 801C, and decreases at 901C,which is consistent with flourishing of rod growth in Fig. 8.

IV. Conclusion

In the system of Zn(NO3)2/NaOH solution, an assisted electricalfield can change ZnO rod morphology and promote crystalgrowth. Its role in rod growth can be explained on the basis thatthe anodic oxidation of OH� viz. 2OH��2e-O212H1 occurswhen the potential reaches a certain value, and the productionof H1 increases solution supersaturation near the substratesurface. Also, the use of an electrical field enhances ion adsorp-tion and diffusion, which may lower the nucleation barrier. Be-low 1.1 V, the electrical field aids nanorods to standperpendicularly onto the substrates because the rods carry thesame charge induced by anodic potential and repel each other.At 1.1–1.3 V, the production of H1 through the anodic reactionremarkably promotes the rod growth. At 1.5 V, a large amountof H1 lowers the pH to the area outside the deposition region,resulting in precipitation. Under an electrical field, the averagediameter and film thickness of the ZnO nanorods increase withincreasing precursor concentration, from diameter of 25 nm anda film thickness of 750 nm for 10�3M to a diameter of 50 nm anda film thickness of 1.32 mm for 10�2M. Meanwhile, with in-creasing temperature, the nanorods have an optimal growth interms of the competition between growing driving factors andresolvability of the ZnO, and the maximum rod growth rate of10�3M Zn21 occurs at 801C.

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