The nanorods were tilted, hexag-onal, and prismatic at a low current density 0.1 mA/cm2 and vertically aligned and obelisk-shaped at high current densities greater than 0.6 mA/cm2.. By u
Trang 1doi: 10.1007/s12540-014-2013-x
Control of morphology and Orientation of Electrochemically Grown
ZnO Nanorods
Tran Hoang Cao Son 1 , Le Khac Top 1 , Nguyen Thi Dong Tri 1 , Ha Thuc Chi Nhan 1 , Lam Quang Vinh 2 ,
Bach Thang Phan 1,3,* , Sang Sub Kim 4,* , and Le Van Hieu 1
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
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 The
morphology and orientation of the grown ZnO nanorods were investigated as functions of the current
density It is likely that the concentrations of OH- and Zn2+ ions, which could be controlled by varying the
current 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 high
current densities (greater than 0.6 mA/cm2) 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
micros-copy (SEM)
1 INTRODUCTION
Nanostructured ZnO materials have received much
atten-tion from the scientific community owing to their potential
for use in various applications and devices such as gas
sen-sors, 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
[1-4] 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
homo-junction-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-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 chem-ical sensors, sensors have been fabricated using vertchem-ically aligned ZnO nanorod arrays [6-8]
Several methods have been employed for growing verti-cally 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.
*Corresponding author: pbthang@skku.edu, pbthang@hcmus.edu.vn,
sangsub@inha.ac.kr
©KIM and Springer
Trang 2fabricated 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
con-centrations 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
fab-rication 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
deposition device (Series G 300TM Potentiostat/Galvanostat/
ZRA, Gamry Instruments, USA), in which a commercial ITO
substrate 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/□, 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
densities (0.1, 0.6, and 1.2 mA/cm2) 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,
ori-entations, and phases of the synthesized ZnO nanorods The
surface and cross-section morphologies of the nanorods were
observed using scanning electron microscopy (SEM)
(JSM-7401F, JEOL) The surface of the ITO substrate was analyzed
using atomic force microscopy (AFM) (5500, Agilent)
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 cur-rent densities Figure 2(a) shows that, at a low curcur-rent den-sity (0.1 mA/cm2), well-defined, hexagonal, and prismatic ZnO nanorods were formed; however, these were not per-pendicular to the ITO substrate However, for larger current densities (0.6 and 1.2 mA/cm2) obelisk-shaped ZnO nano-rods were formed; the diameter of these nanonano-rods 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/cm 2 , (b) 0.6 mA/cm 2 , and (c) 1.2 mA/cm 2
Trang 3are 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
orienta-tion (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
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 nano-rods grown through the one-step process at a current density
of 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 are
as 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 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 planes composed of tetrahedrally coordinated O2− and Zn2+
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
Zn NO( 3)2→Zn 2++2NO3−
NO3−+H2O e+ −→NO2−+2OH−
Zn 2++2OH−→Zn OH( )2
Zn OH( )2→ZnO H+ 2O
Fig 3 XRD patterns of ZnO nanorods grown by one-step
galvano-static electrodeposition for 40 min at various current densities: (a) 0.1
mA/cm 2 , (b) 0.6 mA/cm 2 , and (c) 1.2 mA/cm 2
Fig 4 Top-view SEM images of ZnO nanorods grown by one-step
galvanostatic electrodeposition at a fixed current density of 1.2 mA/
cm 2 for various growth times: (a) 30 min and (b) 10 min.
Trang 4direction [19] However, in our investigations, at the higher
current densities (0.6 mA/cm2 and 1.2 mA/cm2), which
cor-responded 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
nano-rods 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
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 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
was followed by the 0.1 mA/cm2 process for 40 min In the
first 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 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
dependent not only on the concentration of O2− and Zn2+ 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
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 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 condi-tions 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 ran-domly 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]
Fig 5 Schematic illustrations of growth behaviors of ZnO nanorods
deposited at different current densities: (a) 0.1 mA/cm 2 ; (b) 1.2 mA/
cm 2 , and (c) 1.2–0.1 mA/cm 2
Fig 6 SEM images of ZnO nanorods grown by two-step
galvano-static electrodeposition: (a) 1 st step: 0.1 mA/cm 2 ; (b) 2 nd step: 1.2-0.1 mA/cm 2 Images on left are top-view images while the ones on right are cross-sectional images.
Trang 5The 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
fol-lowing 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
three-dimensional (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
sub-strate 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
num-ber 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
layer-free ITO substrate by means of a galvanostatic
electrodepo-sition 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
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 Uni-versity, 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).