DSpace at VNU: Control of morphology and orientation of electrochemically grown ZnO nanorods

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

Control of morphology and Orientation of Electrochemically Grown
ZnO Nanorods
1

1

1

1

2

Tran Hoang Cao Son , Le Khac Top , Nguyen Thi Dong Tri , Ha Thuc Chi Nhan , Lam Quang Vinh ,
Bach Thang Phan1,3,*, Sang Sub Kim4,*, and Le Van Hieu1
1

Vietnam National University, Faculty of Materials Science, University of Science,
Ho Chi Minh City, Vietnam
2
Vietnam National University, Faculty of Physics and Engineering Physics, University of Science,
Ho Chi Minh City, Vietnam
3
Vietnam National University, Laboratory of Advanced Materials, University of Science,
Ho Chi Minh City, Vietnam
4
Inha 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. The

morphology and orientation of the grown ZnO nanorods were investigated as functions of the current
2+
density. It is likely that the concentrations of OH and Zn ions, which could be controlled by varying the
current density, determine the shape and alignment of the ZnO nanorods. The nanorods were tilted, hexag2
onal, and prismatic at a low current density (0.1 mA/cm ) and vertically aligned and obelisk-shaped at high
2
current densities (greater than 0.6 mA/cm ). By using the low and high current densities sequentially in a
two-step growth process, vertically aligned, hexagonal, and prismatic ZnO nanorods could be grown
successfully. 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 microscopy (SEM)

1. INTRODUCTION
Nanostructured ZnO materials have received much attention from the scientific community owing to their potential
for use in various applications and devices such as gas sensors, photodetectors, light-emitting diodes (LEDs), and solar
cells, to name a few. The output power of GaN LEDs can be
enhanced by up to 50% by the use of ZnO nanotip arrays [14]. A heterojunction LED could be fabricated by the growth
of vertically aligned ZnO nanowires on a p-GaN substrate,
which was combined with a indium tin oxide (ITO)/glass layer
and packaged [2,3]. Most of the currently available ZnO
LEDs are based on heterojunctions. However, a p-n homojunction-based LED with a layer of ion-implanted P-doped
p-type ZnO nanorods has also been reported [4]. Because
ZnO nanorods have surface-to-volume ratios much larger than
those of their thin-film and bulk counterparts, they should be
highly suited for use in miniaturized, highly sensitive chemi*Corresponding author: , ,

©KIM and Springer

cal sensors. Oh et al. fabricated CO sensors based on aligned
ZnO nanorods grown on a substrate; these sensors exhibited
high sensitivity to CO gas and had a detection limit as low as

1 ppm at 350 °C [5]. Despite the significant progress made in
the fabrication of chemical sensors based on individual ZnO
nanorods, the application of such nanorods in practical devices
still remains a challenge. Recently, in order to overcome the
shortcomings associated with single ZnO nanorod-based chemical sensors, sensors have been fabricated using vertically
aligned ZnO nanorod arrays [6-8].
Several methods have been employed for growing vertically arrayed ZnO nanorods. These include solution-based
techniques [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 involve
high temperatures. This poses limitations with respect to the
growth of ZnO nanorods on plastic substrates. In addition, in
order to grow ZnO nanorods vertically, a seed layer is often
used. However, this layer and the subsequently grown ZnO
nanorods have to be deposited using different techniques,
resulting in the overall growth process being complex. Efforts
are underway to counter this problem. For instance, Gao et al.


338

Tran Hoang Cao Son et al.

have reported that, using a simple inorganic aqueous solution, well-spaced, vertically grown wurtzite ZnO nanorods
could be deposited on a seed layer-free glass substrate after
20 deposition cycles [11].
One of the low-temperature techniques available for growing
ZnO nanomaterials is the electrochemistry-based method.
Cao et al. reported [1] that a ZnO nanorod array could be
fabricated on an ITO substrate by a two-step process. The

two steps were the seeding or formation of ZnO islands and
the subsequent growth of the nanorod array on these seeds
through electric field-assisted nucleation and subsequent
thermal annealing [12]. In this deposition technique, the concentrations of the OH− and Zn2+ ions are strongly influenced
by the deposition current density, which, in turn, affects the
nucleation and growth of the ZnO nanorods.
In this study, a simple two-step growth process for the fabrication of vertically aligned ZnO nanorods on a seed layer-free
ITO substrate at low temperatures was investigated. During the
process, the deposition current density was varied in order to
control the morphology and orientation of the grown nanorods.

2. EXPERIMENTAL PROCEDURES
The ZnO nanorods were grown using an electrochemical
TM
deposition device (Series G 300 Potentiostat/Galvanostat/
ZRA, Gamry Instruments, USA), in which a commercial ITO
substrate was set as the cathode. Prior to the growth process,
2
the commercial ITO substrate, which had an area of 2 cm
and a sheet resistance of approximately 10 Ohm/ □ , was
sequentially cleaned by ultrasonication in acetone, ethanol,
and deionized water. The precursor electrodeposition bath was
formed by mixing 0.005 M Zn(NO3)2·6H2O and 0.005 M
C6H12N4. The temperature of the bath was maintained at
90 °C. The ITO substrate was immersed into the bath and
galvanostatically subjected to deposition currents of different
2
densities (0.1, 0.6, and 1.2 mA/cm ) for different periods
(10–40 min). After the completion of the growth process, the
ITO substrate, which was now covered with ZnO nanorods,

was taken out from the solution and rinsed immediately with
deionized water to remove any residual impurities remaining
on its surface. It was then dried in air 150 °C for 60 min. Two
types of electrochemical deposition processes are available
for growing ZnO nanorods. The first one is a one-step process,
in which a fixed current density is used, and the second one
is a two-step process, in which two different current densities
are employed in sequence while all other parameters are kept
constant. X-ray diffraction (XRD) analyses (D8 ADVANCE,
Bruker Corp.) were performed to identify the structures, orientations, and phases of the synthesized ZnO nanorods. The
surface and cross-section morphologies of the nanorods were
observed using scanning electron microscopy (SEM) (JSM7401F, JEOL). The surface of the ITO substrate was analyzed
using 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 polycrystalline ITO film, with peaks attributable to the (211), (222), and
(400) orientations being present. The inset AFM image of
the ITO substrate shows that its surface was rough and had a
root mean square (RMS) roughness of 5.1 nm. Figure 2
shows top-view SEM images of the ZnO nanorods grown by
the one-step process for 40 min for various deposition current densities. Figure 2(a) shows that, at a low current density (0.1 mA/cm2), well-defined, hexagonal, and prismatic
ZnO nanorods were formed; however, these were not perpendicular to the ITO substrate. However, for larger current
2
densities (0.6 and 1.2 mA/cm ) obelisk-shaped ZnO nanorods were formed; the diameter of these nanorods decreased
as their length increased (Figs. 2(b) and 2(c)). In contrast to
the abovementioned hexagonal, prismatic ZnO nanorods,
the obelisk-shaped ZnO nanorods grew more perpendicular
to the ITO substrate. In addition, their diameter increased
with the increase in current density (top diameters are below

20 nm and above 20 nm for the ZnO nanorods grown for 10
and 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-step
galvanostatic electrodeposition for 40 min at various current densities:
(a) 0.1 mA/cm2, (b) 0.6 mA/cm2, and (c) 1.2 mA/cm2.


339

Morphology and orientation of ZnO nanorods

Fig. 4. Top-view SEM images of ZnO nanorods grown by one-step
galvanostatic electrodeposition at a fixed current density of 1.2 mA/
2
cm for various growth times: (a) 30 min and (b) 10 min.

comparable intensities. A higher current density leads to the
surfaces 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 the
1.2 mA/cm2 sample had the largest ratio. This suggests that the
ZnO nanorods were preferentially oriented along the (002)
plane and that they grew vertically with their c-axis being
perpendicular to the ITO substrate. On the other hand, the
relatively high intensity of the (100) peak was indicative of
the generation of misaligned and tilted ZnO nanorods on the

ITO substrate.
Figure 4 shows top-view SEM images of the ZnO nanorods grown through the one-step process at a current density
of 1.2 mA/cm2 for different growth durations. The ZnO nanorods maintained their obelisk-like shape. The electrodeposition processes that control the growth of the nanorods are
as follows [16,17]:
−

2+

Zn ( NO3 )2 → Zn + 2NO3
−

−

−

NO3 + H2 O + e → NO2 + 2OH
−

2+

are shown in Fig. 3. Representative diffraction peaks of the
(100), (002), and (101) planes of wurtzite ZnO can be clearly
identified. There is a change in the crystallographic orientation (i.e., a change in the ratio of the intensities of the peaks
corresponding to the (002) and (100) planes, I002/I100) with
the current density. In Fig. 3(a), the (100) and (002) peaks have

−

(2)

Zn + 2OH → Zn ( OH )2

(3)

Zn ( OH )2 → ZnO + H2 O

(4)

2+

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

(1)

−

Zn and OH ions are generated as shown in Eqs. (1) and
(2). They are likely to react with each other and eventually
produce Zn(OH)2 (Eq. (3)), which forms the basic growth
units of the ZnO nanorods (Eq. (4)). The structure of ZnO
can be described as consisting of a number of alternating
2−
2+
planes composed of tetrahedrally coordinated O and Zn
ions that are alternately stacked along the c-axis. The growth
rate (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 the
growth parameters have on the morphology and size of the

340

Tran Hoang Cao Son et al.

ZnO nanorods. In particular, the deposition current density
has a significant effect on the morphology of the ZnO nanorods. During the growth process, the concentration of the
OH− ions can be electrochemically controlled by varying the
current density (Eq. 2). An increase in the OH− concentration
hinders the growth of the ZnO nanorods along the [001]
direction owing to the shielding effect of the plane along this
direction [19]. However, in our investigations, at the higher
current densities (0.6 mA/cm2 and 1.2 mA/cm2), which corresponded to larger OH- concentrations, the high growth rate
in 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-shaped
ZnO nanorods. It is likely that the shape of the ZnO nanorods is affected not only by the OH− ion concentration but
also by the rate of diffusion of the Zn2+ ions from the bulk
solution to the substrate. In the low-current-density process
(i.e., low Zn2+ ion concentration at the ITO substrate), the
growth 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 formation
of the obelisk-shaped ZnO nanorods that takes place is likely
owing to the rapid transport of Zn2+ ions to the ITO substrate.
The higher Zn2+ ion concentration leads to an increase in the
growth 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 advantages
of the low-current-density process (which results in welldefined, hexagonal, and prismatic nanorods) and the highcurrent-density process (which results in highly oriented ones).
ZnO nanorods were grown by dividing the growth process
into two steps, that is, by using both the low-current-density
and the high-current-density processes. ZnO nanorods were
first grown using the 1.2 mA/cm2 process for 10 min; this
2
was followed by the 0.1 mA/cm process for 40 min. In the
2
first step, i.e., during the high-current-density step (1.2 mA/cm ,
10 min), the ZnO nanorods grew preferentially in the longitudinal direction. The second step, that is, the low-current2
density (0.1 mA/cm , 40 min) step, resulted in reduced growth
in the longitudinal direction and an increase in lateral growth.
Eventually, the shape of the ZnO nanorods switched from
being obelisk-like to being column-like. It was found that the
resulting ZnO nanorods grew almost vertically on the ITO
substrate. In summary, the shape of the ZnO nanorods was
2−
2+
dependent not only on the concentration of O and Zn ions
in the bulk solution and at the ITO substrate but also on the
rate of diffusion of the ions, which changed with the current
density.
The growth mechanism of the ZnO rods is modeled in Fig.
5; that the model is accurate was confirmed by the experimental data, which is shown in Fig. 6. The structure and morphology of the ZnO nanorods were decided by the number of

Fig. 5. Schematic illustrations of growth behaviors of ZnO nanorods
2
deposited at different current densities: (a) 0.1 mA/cm ; (b) 1.2 mA/
2

2
cm , and (c) 1.2–0.1 mA/cm .

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

nuclei formed in the initial stage of growth; these continued
to grow and form the nanorods. The number of nuclei formed
is likely determined by the lattice structure, the number of
defects on the substrate surface, and the experimental conditions. It has been reported that during the initial stage of the
deposition of ZnO from an aqueous solution by electrochemical
deposition, islands of ZnO form on the substrate [20]. It has
also been reported that a polycrystalline ITO film with a randomly oriented surface is not atomically [21] as the rough
surface would favor the formation of small clusters or islands
during the initial deposition stage [12,22-24].


Morphology and orientation of ZnO nanorods

The orientation exhibited by the nanorods in this study can
be explained on the basis of the roughness of the ITO surface
as well as a structural mismatch between the polycrystalline
ITO glass substrate and the hexagonal ZnO nanorods. As
shown in Fig. 1, the XRD pattern of the ITO glass substrate
corresponded to that of a polycrystalline film with the following orientations: (211), (222), and (400). The AFM image
of the ITO substrate shows that it has a rough surface. These
factors can induce the formation of ZnO clusters or islands
during the initial stage of growth. In addition, the crystalline

structures of ZnO (wurtzite; a = b = 3.249 Å and c = 5.206 Å)
and ITO (bixbyite, a = 10.117 Å) [24] are different. The threedimensional (3D) growth of a crystalline material on a substrate
usually occurs when the interfacial energy is high owing to a
large lattice mismatch between the material being grown and
the substrate. On the polycrystalline ITO substrate, first a layer
of 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-adatom
interactions are stronger than those between the adatoms and
the substrate surface, leading to the formation of 3D adatom
clusters or islands. The low current density (0.1 mA/cm2) yields
smaller ZnO nuclei, which might not cover the entire substrate surface, resulting in the formation of rough 3D ZnO
islands on the smooth ITO surface (Fig. 5(a) and Fig. 6(a)).
Further growth on the 3D islands leads to a less-dense array
of tilted, hexagonal ZnO nanorods. On the other hand, the
higher current density (1.2 mA/cm2) increases the size and number of the coalesced 3D ZnO islands, which now can cover
the entire substrate surface and form a continuous layer. This
layer promotes the growth of vertical, obelisk-shaped ZnO
nanorods and their alignment along a direction that is more
perpendicular to the substrate (Fig. 5(b) and Fig. 6(b)). The
sequence growth under the low current density (0.1 mA/cm2)
switched the vertical obelisk-shaped ZnO nanorods into the
vertical and hexagonal, prismatic ZnO nanorods (Fig. 5(b)
and Fig. 6(c)).

4. CONCLUSIONS
In conclusion, ZnO nanorods were grown on a seed layerfree ITO substrate by means of a galvanostatic electrodeposition technique. The morphology and orientation of the ZnO
nanorods were strongly influenced by the current density
used during the process. Growth using a single, low current
density resulted in hexagonal, prismatic ZnO nanorods that

were tilted, while growth using a high current density generated
vertically aligned, obelisk-shaped nanorods. By using different
current densities (i.e., both low and high current densities) in
sequence, vertically aligned, hexagonal, and prismatic ZnO
nanorods could be grown. The mechanism of growth of the
ZnO nanorods and their shape transition as functions of the
deposition current density were discussed on the basis of the

341

role of the OH− and Zn2+ ions. The method developed in this
study has potential for use in the mass production of aligned
ZnO nanorods.

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

REFERENCES
1. J. Zhong, H. Chen, G. Saraf, Y. Lu, C. K. Choi, J. J. Song,
D. M. Mackie, and H. Shen, Appl. Phys. Lett. 90, 203515
(2007).
2. C. H. Chen, S. J. Chang, S. P. Chang, M. J. Li, I. C. Chen,
T. J. Hsueh, and C. L. Hsu, Appl. Phys. Lett. 95, 223101
(2009).
3. X. M. Zhang, M. Y. Lu, Y. Zhang, L. J. Chen, and Z. L.
Wang, Adv. Mater. 21, 2767 (2009).
4. X. W. Sun, B. Ling, J. L. Zhao, S. T. Tan, Y. Yang, Y. Q. Shen,
Z. L. Dong, and X. C. Li, Appl. Phys. Lett. 95, 133124
(2009).

5. E. Oh and S.H. Jeong, J. Korean Phys. Soc. 59, 8 (2011).
6. Z. L. Zhang, Annu. Rev. Phys. Chem. 55, 159 (2004).
7. J. Y. Park, D. E. Song, and S. S. Kim, Nanotechnology 19,
105503 (2008).
8. J. Y. Park, S. W. Choi, and S. S. Kim, Nanoscale. Res. Lett.
5, 353 (2010).
9. J. Elias, R. T. Zaera, F. Y. Wang, and C. L. Clement, Chem.
Mater. 20, 6633 (2008).
10. J. Y. Park, S. W. Choi, K. Asokan, and S. S. Kim, J. Am.
Ceram. Soc. 93, 3190 (2010).
11. X. D. Gao, X. M. Li, W. D. Y, L. Li, and J. J. Qiu, Appl.
Sur. Sci. 253, 4060 (2007).
12. Y. J. Kim, H. M. Shang, and G. Z. Cao, J. Sol-gel Sci.
Techn. 38, 79 (2006).
13. K. N. Chung, C. H. Lee, and G. C. Yi, Science 29, 655
(2010).
14. Z. W. Liu and C. K. Ong, Mater. Lett. 61, 3329 (2007).
15. R. Nishimura, T. Sakano, T. Okata, T. Saiki, and M. Obara,
Jpn. J. Appl. Phys. 47, 4799 (2008).
16. S. Peulon and D. Lincot, J. Electrochem. Soc. 145, 864
(1998).
17. S. Baruah and J. Dutta, Sci. Technol. Adv. Mater. 10, 013001
(2009).
18. F. Xu, Y. Lu, Y. Xie, and Y. Liu, J. Solid State Electr. 14, 63
(2010).
19. H. Sun, M. Luo, W. Weng, K. C. Heng, P. Du, G. Shen, and
G. Han, Nanotechnology 19, 125603 (2008).
20. T. Pauporte, R. Cortes, M. Froment, B. Beaumont, and D.
Lincot, Chem. Mater. 14, 4702 (2002).
21. Y. Han, D. Kim, J. Cho, and S. Koh, J. Vac. Sci. Technol. B,

21, 288 (2003).


342

Tran Hoang Cao Son et al.

22. J. Nayak, M. K. Son, J. K. Kim, S. K. Kim, J. H. Lee, and
H. J. Kim, J. Electr. Eng. Technol. 7, 965 (2012).
23. J. H. Yang, J. H. Lang, C. S. Li, L. L. Yang, Q. Han, Y. J.
Zhang, D. D. Wang, M. Gao, and X. Y. Liu, Appl. Surf. Sci.
255, 2500 (2008).

24. Y. Ishikawa, H. Nagayama, H. Hoshino, M. Ohgai, N. Shibata, T. Yamamoto, and Y. Ikuhara, Mater. Trans. 50, 959
(2009).
25. K. Ouva, V. G. Lifshits, A. A. Saranin, A. V. Zotov, and M.
Katayama, Sur. Sci: An Introduction, Springer, Berlin (2003).