doi: 10.1007/s12540-014-2013-xMet. Mater. Int., Vol. 20, No. 2 (2014), pp. 337~342

Control of morphology and Orientation of Electrochemically GrownZnO Nanorods

Tran Hoang Cao Son1, Le Khac Top1, Nguyen Thi Dong Tri1, Ha Thuc Chi Nhan1, Lam Quang Vinh2,Bach Thang Phan1,3,*, Sang Sub Kim4,*, and Le Van Hieu1

1Vietnam National University, Faculty of Materials Science, University of Science,Ho Chi Minh City, Vietnam

2Vietnam National University, Faculty of Physics and Engineering Physics, University of Science,Ho Chi Minh City, Vietnam

3Vietnam National University, Laboratory of Advanced Materials, University of Science, Ho Chi Minh City, Vietnam

4Inha University, Department of Materials Science and Engineering, Korea

(received date: 11 June 2013 / accepted date: 16 August 2013)

We report the direct electrochemical deposition of ZnO nanorods on an indium tin oxide substrate. Themorphology and orientation of the grown ZnO nanorods were investigated as functions of the currentdensity. It is likely that the concentrations of OH- and Zn2+ ions, which could be controlled by varying thecurrent density, determine the shape and alignment of the ZnO nanorods. The nanorods were tilted, hexag-onal, and prismatic at a low current density (0.1 mA/cm2) and vertically aligned and obelisk-shaped at highcurrent densities (greater than 0.6 mA/cm2). By using the low and high current densities sequentially in atwo-step growth process, vertically aligned, hexagonal, and prismatic ZnO nanorods could be grownsuccessfully. The underlying mechanism responsible for the growth of the ZnO nanorods is also discussed.

Key words: ZnO nanorod, electrochemical deposition, orientation, growth mechanism, scanning electron micros-copy (SEM)

1. INTRODUCTION

Nanostructured ZnO materials have received much atten-tion from the scientific community owing to their potentialfor use in various applications and devices such as gas sen-sors, photodetectors, light-emitting diodes (LEDs), and solarcells, to name a few. The output power of GaN LEDs can beenhanced by up to 50% by the use of ZnO nanotip arrays [1-4]. A heterojunction LED could be fabricated by the growthof vertically aligned ZnO nanowires on a p-GaN substrate,which was combined with a indium tin oxide (ITO)/glass layerand packaged [2,3]. Most of the currently available ZnOLEDs are based on heterojunctions. However, a p-n homo-junction-based LED with a layer of ion-implanted P-dopedp-type ZnO nanorods has also been reported [4]. BecauseZnO nanorods have surface-to-volume ratios much larger thanthose of their thin-film and bulk counterparts, they should behighly suited for use in miniaturized, highly sensitive chemi-

cal sensors. Oh et al. fabricated CO sensors based on alignedZnO nanorods grown on a substrate; these sensors exhibitedhigh sensitivity to CO gas and had a detection limit as low as1 ppm at 350 °C [5]. Despite the significant progress made inthe fabrication of chemical sensors based on individual ZnOnanorods, the application of such nanorods in practical devicesstill remains a challenge. Recently, in order to overcome theshortcomings associated with single ZnO nanorod-based chem-ical sensors, sensors have been fabricated using verticallyaligned ZnO nanorod arrays [6-8].

Several methods have been employed for growing verti-cally arrayed ZnO nanorods. These include solution-basedtechniques [9-12], metal organic chemical vapor deposition(MOCVD) [13,7,8], and pulsed laser deposition (PLD) [14,15].Some of these techniques such as MOCVD and PLD involvehigh temperatures. This poses limitations with respect to thegrowth of ZnO nanorods on plastic substrates. In addition, inorder to grow ZnO nanorods vertically, a seed layer is oftenused. However, this layer and the subsequently grown ZnOnanorods have to be deposited using different techniques,resulting in the overall growth process being complex. Effortsare underway to counter this problem. For instance, Gao et al.

*Corresponding author: pbthang@skku.edu, pbthang@hcmus.edu.vn,sangsub@inha.ac.kr©KIM and Springer

338 Tran Hoang Cao Son et al.

have reported that, using a simple inorganic aqueous solu-tion, well-spaced, vertically grown wurtzite ZnO nanorodscould be deposited on a seed layer-free glass substrate after20 deposition cycles [11].

One of the low-temperature techniques available for growingZnO nanomaterials is the electrochemistry-based method.Cao et al. reported [1] that a ZnO nanorod array could befabricated on an ITO substrate by a two-step process. Thetwo steps were the seeding or formation of ZnO islands andthe subsequent growth of the nanorod array on these seedsthrough electric field-assisted nucleation and subsequentthermal annealing [12]. In this deposition technique, the con-centrations of the OH− and Zn2+ ions are strongly influencedby the deposition current density, which, in turn, affects thenucleation and growth of the ZnO nanorods.

In this study, a simple two-step growth process for the fab-rication of vertically aligned ZnO nanorods on a seed layer-freeITO substrate at low temperatures was investigated. During theprocess, the deposition current density was varied in order tocontrol the morphology and orientation of the grown nanorods.

2. EXPERIMENTAL PROCEDURES

The ZnO nanorods were grown using an electrochemicaldeposition device (Series G 300TM Potentiostat/Galvanostat/ZRA, Gamry Instruments, USA), in which a commercial ITOsubstrate was set as the cathode. Prior to the growth process,the commercial ITO substrate, which had an area of 2 cm2

and a sheet resistance of approximately 10 Ohm/□, wassequentially cleaned by ultrasonication in acetone, ethanol,and deionized water. The precursor electrodeposition bath wasformed by mixing 0.005 M Zn(NO3)2·6H2O and 0.005 MC6H12N4. The temperature of the bath was maintained at90 °C. The ITO substrate was immersed into the bath andgalvanostatically subjected to deposition currents of differentdensities (0.1, 0.6, and 1.2 mA/cm2) for different periods(10–40 min). After the completion of the growth process, theITO substrate, which was now covered with ZnO nanorods,was taken out from the solution and rinsed immediately withdeionized water to remove any residual impurities remainingon its surface. It was then dried in air 150 °C for 60 min. Twotypes of electrochemical deposition processes are availablefor growing ZnO nanorods. The first one is a one-step process,in which a fixed current density is used, and the second oneis a two-step process, in which two different current densitiesare employed in sequence while all other parameters are keptconstant. X-ray diffraction (XRD) analyses (D8 ADVANCE,Bruker Corp.) were performed to identify the structures, ori-entations, and phases of the synthesized ZnO nanorods. Thesurface and cross-section morphologies of the nanorods wereobserved using scanning electron microscopy (SEM) (JSM-7401F, JEOL). The surface of the ITO substrate was analyzedusing atomic force microscopy (AFM) (5500, Agilent).

3. RESULTS AND DISCUSSION

Figure 1 shows the XRD pattern of the ITO glass substrate.The peaks in the XRD pattern correspond to a polycrystal-line ITO film, with peaks attributable to the (211), (222), and(400) orientations being present. The inset AFM image ofthe ITO substrate shows that its surface was rough and had aroot mean square (RMS) roughness of 5.1 nm. Figure 2shows top-view SEM images of the ZnO nanorods grown bythe one-step process for 40 min for various deposition cur-rent densities. Figure 2(a) shows that, at a low current den-sity (0.1 mA/cm2), well-defined, hexagonal, and prismaticZnO nanorods were formed; however, these were not per-pendicular to the ITO substrate. However, for larger currentdensities (0.6 and 1.2 mA/cm2) obelisk-shaped ZnO nano-rods were formed; the diameter of these nanorods decreasedas their length increased (Figs. 2(b) and 2(c)). In contrast tothe abovementioned hexagonal, prismatic ZnO nanorods,the obelisk-shaped ZnO nanorods grew more perpendicularto the ITO substrate. In addition, their diameter increasedwith the increase in current density (top diameters are below20 nm and above 20 nm for the ZnO nanorods grown for 10and 40 mins, respectively).

The XRD patterns of the three above-mentioned samples

Fig. 1. XRD pattern and AFM image of ITO substrate.

Fig. 2. Top-view SEM images of ZnO nanorods grown by one-stepgalvanostatic electrodeposition for 40 min at various current densities:(a) 0.1 mA/cm2, (b) 0.6 mA/cm2, and (c) 1.2 mA/cm2.

Morphology and orientation of ZnO nanorods 339

are shown in Fig. 3. Representative diffraction peaks of the(100), (002), and (101) planes of wurtzite ZnO can be clearlyidentified. There is a change in the crystallographic orienta-tion (i.e., a change in the ratio of the intensities of the peakscorresponding to the (002) and (100) planes, I002/I100) withthe current density. In Fig. 3(a), the (100) and (002) peaks have

comparable intensities. A higher current density leads to thesurfaces of the (002) planes being exposed preferentially(Figs. 3(b) and 3(c)). The XRD pattern for the 0.1 mA/cm2

sample exhibited the smallest I002/I100 ratio, while that of the1.2 mA/cm2 sample had the largest ratio. This suggests that theZnO nanorods were preferentially oriented along the (002)plane and that they grew vertically with their c-axis beingperpendicular to the ITO substrate. On the other hand, therelatively high intensity of the (100) peak was indicative ofthe generation of misaligned and tilted ZnO nanorods on theITO substrate.

Figure 4 shows top-view SEM images of the ZnO nano-rods grown through the one-step process at a current densityof 1.2 mA/cm2 for different growth durations. The ZnO nan-orods maintained their obelisk-like shape. The electrodepo-sition processes that control the growth of the nanorods areas follows [16,17]:

(1)

(2)

(3)

(4)

Zn2+ and OH− ions are generated as shown in Eqs. (1) and(2). They are likely to react with each other and eventuallyproduce Zn(OH)2 (Eq. (3)), which forms the basic growthunits of the ZnO nanorods (Eq. (4)). The structure of ZnOcan be described as consisting of a number of alternatingplanes composed of tetrahedrally coordinated O2− and Zn2+

ions that are alternately stacked along the c-axis. The growthrate (n) follows the sequence ν(001) > ν(010) > ν(001) [17,18].Therefore, preferential growth along c-axis is to be expected.

The abovementioned results reveal the effect that thegrowth parameters have on the morphology and size of the

Zn NO3( )2 Zn2+→ 2NO3−+

NO3− H2O e−+ + NO2

− 2OH−+→

Zn2+ 2OH−+ Zn OH( )2→

Zn OH( )2 ZnO H2O+→

Fig. 3. XRD patterns of ZnO nanorods grown by one-step galvano-static electrodeposition for 40 min at various current densities: (a) 0.1mA/cm2, (b) 0.6 mA/cm2, and (c) 1.2 mA/cm2.

Fig. 4. Top-view SEM images of ZnO nanorods grown by one-stepgalvanostatic electrodeposition at a fixed current density of 1.2 mA/cm2 for various growth times: (a) 30 min and (b) 10 min.

340 Tran Hoang Cao Son et al.

ZnO nanorods. In particular, the deposition current densityhas a significant effect on the morphology of the ZnO nano-rods. During the growth process, the concentration of theOH− ions can be electrochemically controlled by varying thecurrent density (Eq. 2). An increase in the OH− concentrationhinders the growth of the ZnO nanorods along the [001]direction owing to the shielding effect of the plane along thisdirection [19]. However, in our investigations, at the highercurrent densities (0.6 mA/cm2 and 1.2 mA/cm2), which cor-responded to larger OH- concentrations, the high growth ratein the [100] direction limited the area of the (001) plane;thus, other high-index, low-energy surfaces (such as the (010)planes) grew preferentially, resulting in the obelisk-shapedZnO nanorods. It is likely that the shape of the ZnO nano-rods is affected not only by the OH− ion concentration butalso by the rate of diffusion of the Zn2+ ions from the bulksolution to the substrate. In the low-current-density process(i.e., low Zn2+ ion concentration at the ITO substrate), thegrowth rate of the side surfaces was reduced, and consequently,hexagonal, prismatic ZnO nanorods were formed (Fig. 1(a)).In the case of the high-current-density process, the formationof the obelisk-shaped ZnO nanorods that takes place is likelyowing to the rapid transport of Zn2+ ions to the ITO substrate.The higher Zn2+ ion concentration leads to an increase in thegrowth rate of the side surfaces of the ZnO nanorods (Fig.1(b) and Fig. 1(c)).

In order to obtain well-defined, highly oriented, hexagonal,and prismatic ZnO nanorods, we combined the advantagesof the low-current-density process (which results in well-defined, hexagonal, and prismatic nanorods) and the high-current-density process (which results in highly oriented ones).ZnO nanorods were grown by dividing the growth processinto two steps, that is, by using both the low-current-densityand the high-current-density processes. ZnO nanorods werefirst grown using the 1.2 mA/cm2 process for 10 min; thiswas followed by the 0.1 mA/cm2 process for 40 min. In thefirst step, i.e., during the high-current-density step (1.2 mA/cm2,10 min), the ZnO nanorods grew preferentially in the longi-tudinal direction. The second step, that is, the low-current-density (0.1 mA/cm2, 40 min) step, resulted in reduced growthin the longitudinal direction and an increase in lateral growth.Eventually, the shape of the ZnO nanorods switched frombeing obelisk-like to being column-like. It was found that theresulting ZnO nanorods grew almost vertically on the ITOsubstrate. In summary, the shape of the ZnO nanorods wasdependent not only on the concentration of O2− and Zn2+ ionsin the bulk solution and at the ITO substrate but also on therate of diffusion of the ions, which changed with the currentdensity.

The growth mechanism of the ZnO rods is modeled in Fig.5; that the model is accurate was confirmed by the experi-mental data, which is shown in Fig. 6. The structure and mor-phology of the ZnO nanorods were decided by the number of

nuclei formed in the initial stage of growth; these continuedto grow and form the nanorods. The number of nuclei formedis likely determined by the lattice structure, the number ofdefects on the substrate surface, and the experimental condi-tions. It has been reported that during the initial stage of thedeposition of ZnO from an aqueous solution by electrochemicaldeposition, islands of ZnO form on the substrate [20]. It hasalso been reported that a polycrystalline ITO film with a ran-domly oriented surface is not atomically [21] as the roughsurface would favor the formation of small clusters or islandsduring the initial deposition stage [12,22-24].

Fig. 5. Schematic illustrations of growth behaviors of ZnO nanorodsdeposited at different current densities: (a) 0.1 mA/cm2; (b) 1.2 mA/cm2, and (c) 1.2–0.1 mA/cm2.

Fig. 6. SEM images of ZnO nanorods grown by two-step galvano-static electrodeposition: (a) 1st step: 0.1 mA/cm2; (b) 2nd step: 1.2-0.1mA/cm2. Images on left are top-view images while the ones on rightare cross-sectional images.

Morphology and orientation of ZnO nanorods 341

The orientation exhibited by the nanorods in this study canbe explained on the basis of the roughness of the ITO surfaceas well as a structural mismatch between the polycrystallineITO glass substrate and the hexagonal ZnO nanorods. Asshown in Fig. 1, the XRD pattern of the ITO glass substratecorresponded to that of a polycrystalline film with the fol-lowing orientations: (211), (222), and (400). The AFM imageof the ITO substrate shows that it has a rough surface. Thesefactors can induce the formation of ZnO clusters or islandsduring the initial stage of growth. In addition, the crystallinestructures of ZnO (wurtzite; a = b = 3.249 Å and c = 5.206 Å)and ITO (bixbyite, a = 10.117 Å) [24] are different. The three-dimensional (3D) growth of a crystalline material on a substrateusually occurs when the interfacial energy is high owing to alarge lattice mismatch between the material being grown andthe substrate. On the polycrystalline ITO substrate, first a layerof ZnO grows following the formation of the 3D ZnO islands;this type of growth is called Volmer-Weber (VW) growth [25].It is well known that during VW growth, adatom-adatominteractions are stronger than those between the adatoms andthe substrate surface, leading to the formation of 3D adatomclusters or islands. The low current density (0.1 mA/cm2) yieldssmaller ZnO nuclei, which might not cover the entire sub-strate surface, resulting in the formation of rough 3D ZnOislands on the smooth ITO surface (Fig. 5(a) and Fig. 6(a)).Further growth on the 3D islands leads to a less-dense arrayof tilted, hexagonal ZnO nanorods. On the other hand, thehigher current density (1.2 mA/cm2) increases the size and num-ber of the coalesced 3D ZnO islands, which now can coverthe entire substrate surface and form a continuous layer. Thislayer promotes the growth of vertical, obelisk-shaped ZnOnanorods and their alignment along a direction that is moreperpendicular to the substrate (Fig. 5(b) and Fig. 6(b)). Thesequence growth under the low current density (0.1 mA/cm2)switched the vertical obelisk-shaped ZnO nanorods into thevertical and hexagonal, prismatic ZnO nanorods (Fig. 5(b)and Fig. 6(c)).

4. CONCLUSIONS

In conclusion, ZnO nanorods were grown on a seed layer-free ITO substrate by means of a galvanostatic electrodepo-sition technique. The morphology and orientation of the ZnOnanorods were strongly influenced by the current densityused during the process. Growth using a single, low currentdensity resulted in hexagonal, prismatic ZnO nanorods thatwere tilted, while growth using a high current density generatedvertically aligned, obelisk-shaped nanorods. By using differentcurrent densities (i.e., both low and high current densities) insequence, vertically aligned, hexagonal, and prismatic ZnOnanorods could be grown. The mechanism of growth of theZnO nanorods and their shape transition as functions of thedeposition current density were discussed on the basis of the

role of the OH− and Zn2+ ions. The method developed in thisstudy has potential for use in the mass production of alignedZnO nanorods.

ACKNOWLEDGMENTS

This work was supported by the Vietnam National Uni-versity, Ho Chi Minh City (VNU-HCM), through Grant No.B2011-18-3TD.

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