Met Mater Int., Vol 20, No (2014), pp 337~342 doi: 10.1007/s12540-014-2013-x Control of morphology and Orientation of Electrochemically Grown ZnO Nanorods 1 1 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 Vietnam National University, Faculty of Materials Science, University of Science, Ho Chi Minh City, Vietnam Vietnam National University, Faculty of Physics and Engineering Physics, University of Science, Ho Chi Minh City, Vietnam Vietnam National University, Laboratory of Advanced Materials, University of Science, Ho Chi Minh City, Vietnam 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 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) 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: pbthang@skku.edu, pbthang@hcmus.edu.vn, sangsub@inha.ac.kr ©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 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 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, the commercial ITO substrate, which had an area of 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 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 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) RESULTS AND DISCUSSION Figure 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 shows top-view SEM images of the ZnO nanorods grown by the one-step process for 40 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 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 XRD pattern and AFM image of ITO substrate Fig Top-view SEM images of ZnO nanorods grown by one-step galvanostatic electrodeposition for 40 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 Top-view SEM images of ZnO nanorods grown by one-step galvanostatic electrodeposition at a fixed current density of 1.2 mA/ cm for various growth times: (a) 30 and (b) 10 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 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 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 XRD patterns of ZnO nanorods grown by one-step galvanostatic electrodeposition for 40 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 was followed by the 0.1 mA/cm process for 40 In the 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 The structure and morphology of the ZnO nanorods were decided by the number of Fig Schematic illustrations of growth behaviors of ZnO nanorods deposited at different current densities: (a) 0.1 mA/cm ; (b) 1.2 mA/ 2 cm , and (c) 1.2–0.1 mA/cm Fig 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 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)) 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 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nanorods deposited at different current densities: