Hydrothermal-Electrochemical Synthesis of ZnO Nanorods Seong Kyong Park,† Jae Hyoung Park,† Ki Young Ko,† Sungho Yoon,† Kyo Seon Chu,‡ Woong Kim,‡ and Young Rag Do*,† Department of Chemistry, Kookmin UniVersity, Seoul 136-702, Korea, and Department of Materials Science and Engineering, Korea UniVersity, Seoul 136-713, Korea
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 8 3615–3620
ReceiVed April 1, 2009; ReVised Manuscript ReceiVed May 26, 2009
ABSTRACT: Vertically aligned ZnO nanorods having high optical quality were prepared by a hydrothermal-electrochemical method. The nanorods were synthesized in a Zn(NO3)2 aqueous solution on Si substrates which were coated with a platinum conducting layer and a ZnO seed layer. They possessed a single-crystal wu¨rtzite structure and grew along the c-axis, perpendicular to the substrates. The height and diameter of the ZnO nanorods were up to ∼4.3 µm and 90-150 nm, respectively. The morphological, structural, and photoluminescence properties of the ZnO nanorods were examined with respect to the growth temperature (120-180 °C) and the presence of NaOH additive. The nanorods synthesized at high temperature (180 °C) exhibited a strong UV emission and a weak defect-related visible emission leading to a UV-visible ratio of ∼230. This high optical quality was attributed to the increased growth rate of ZnO nanorods (∼4.3 µm/h) which was caused by the high growth temperature (180 °C). This was based on the fact that the ZnO phase is thermodynamically more favorable than the defect-related Zn(OH)2 phase at higher temperature. Since the growth temperature was compatible with polymer materials, our synthetic method may provide a promising way for fabricating high performance optoelectronic devices on flexible polymer substrates.
1. Introduction The development of synthetic methods to produce high quality one-dimensional (1D) nanomaterials, such as ZnO nanorods and nanowires, has attracted considerable attention. They are considered as the key building block materials for constructing nanoscale devices such as lasers,1 luminescent devices,2 light emitting diodes,3 sensors,4 and general electronic devices.5-7 Various growth methods including vapor-8-10 and solutionbased techniques11-18 have been developed. Among them, the electrochemical approach allows the facile control of both the thickness and morphology of the deposited film, as well as the crystallization at relatively low temperatures. Moreover, the equipments are inexpensive and the production efficiency is high. Izaki et al.15,16 and Peulon et al.17,18 first performed their pioneering research on the electrodeposition of ZnO using ZnCl2 or Zn(NO3)2 as Zn precursors. However, several groups, including our lab, have observed that there are strong defectrelated visible emissions in the photoluminescence (PL) spectra in the electrochemically grown nanorods. The UV emission in the PL spectrum of ZnO results from the radiative recombination between the electrons in the conduction band and the holes in the valence band, where the enhanced UV emission reflects a decrease in the defect density that may trap the photogenerated holes and/or electrons. On the other hand, the visible emission is attributed to the localized defect levels in the band gap. It is generally assumed that the crystal quality of ZnO nanorods is strongly related to the stability of the UV emission and the ratio of UV to visible emission.19 However, it was reported that the ratio of the intensities of UV and defect-related visible emission is dependent on the excitation density and the excitation area.20,21 Even though the ratio of these two emissions cannot be used as absolute factors for determining the crystalline quality of ZnO nanorods, they are still useful in comparing the quality of different samples when * To whom correspondence should be addressed. Tel: +82-2-910-4893; fax: +82-2-910-4415; e-mail:
[email protected]. † Kookmin University. ‡ Korea University.
identical excitation conditions are used during the PL measurements.23 The development of an effective wet-chemical synthetic method for increasing the UV emission and decreasing the intensity of the visible defect emission is of practical importance for optoelectronic device applications. Regarding the electrochemical deposition of ZnO nanorods, it was reported that the defect density of deposited nanorods decreased with increasing bath temperature.24-27 The results imply that further improvement in optical properties of ZnO nanorods could be achieved if the growth occurs at the elevated temperature. Since electrochemical reactions in aqueous solution are usually carried out at temperatures below 100 °C, we employed a hydrothermal-electrochemical method and grew ZnO nanorods in an autoclave at higher temperatures (120-180 °C). In the fields of biomaterials and dielectric materials, the hydrothermal-electrochemical method has been used to synthesize a variety of a high-quality or new structured compounds, such as apatites,28 mixed titanium oxides,29,30 tantalates,31,32 and vanadates.33 As previously reported, the capability of coating the substrates with a variety of materials at relatively low working temperatures may be the major benefits of this method compared to the gas phase synthetic methods. Although there are many advantages in hydrothermal-electrochemical synthesis for growing oxide thin films, there are no reports on the preparation of high-quality 1D ZnO nanorods or nanowires by this synthetic method so far. This paper demonstrates that deposition of high-quality ZnO nanorods is possible via the hydrothermal-electrochemical approach. Owing to the high growth rate facilitated by high temperatures (120-180 °C), the nanorods showed high ratio of UV to visible emission. The study of temperature dependence of photoluminescence confirms that the higher the growth temperature, the higher the optical qualities of the nanorods are produced. We also demonstrate that high-optical-quality nanorods can be synthesized on polyethylene terephthalate (PET) plastic substrates. Previously, ZnO nanorods grown during the solution phase have shown strong defect related visible emission. Although it could be suppressed by high temperature annealing (∼500 °C), this process is certainly not compatible with plastic
10.1021/cg9003593 CCC: $40.75 2009 American Chemical Society Published on Web 06/12/2009
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Park et al. spectroscopy using a He-Cd laser in association with a spectrophotometer (Darsa, PSI Co. Ltd.). The temperature dependence of PL measurements was done at the surface normal direction with the temperature ranging from 10 to 300 K. A closed-cycle He cryostat (CitiCryogenics, Helix Technology Co.) equipped with a digital thermometer controller was also used.
3. Results and Discussion
Figure 1. (a) Schematic diagram of the apparatus used for the hydrothermal-electrochemical deposition of ZnO nanorods (RE: reference electrode, CE: counter electrode, WE: working electrode). Top and cross-sectional FE-SEM images of the nanorod arrays prepared with (a) electrochemical deposition without additives at 90 °C for 1 h and hydrothermal-electrochemical deposition without additives at a bath temperature of (b) 120, (c) 150, and (d) 180 °C. The right-side columns show FE-SEM images of the junction between ZnO nanorods and the substrates.
substrates. Our demonstration of growing high quality ZnO nanorods on polymer film can be further developed to fabricate optoelectronic devices with high performance on flexible substrates.
2. Experimental Section Working electrodes were prepared by having both 50-nm-thick Cr as adhesion layer and 50-nm-thick Pt as electrode layer subsequently deposited on a single-crystal Si (100) wafer or PET by DC sputter. The Pt/Cr coated substrates showed resistivity smaller than 5 Ω/cm2. Then, ZnO thin film of 35 nm thickness was deposited on the Pt-coated Si substrates by atomic layer deposition (ALD) at 150 °C. The ZnO film was used as a seed layer for the growth of ZnO nanorods. Electrochemical deposition of ZnO nanorods were carried out in an aqueous solution of zinc nitrate (∼1 × 10-4 M) at 90 °C.23 A Pt sheet (99.99%) was used as a counter electrode. A Ag/AgCl electrode in a saturated KCl solution was used as the reference electrode. The ZnO/ Pt/Cr/Si or ZnO/Pt/Cr/PET was used as a working electrode. The deposition was performed at one potential of -1.0 V (WPG-100, Wonatech) with respect to the Ag/AgCl reference electrode under stirred and bubble-free conditions. The hydrothermal-electrochemical experiments were carried out in a homemade stainless steel autoclave with a Teflon liner.29,32 Threeelectrode cell configuration was used as shown in Figure 1a. All the electrodes were immersed in a 420 mL of a 1 × 10-4 M Zn(NO3)2 solution in the autoclave. The growth conditions were the same as those of the electrochemical experiments, except that the growth temperature was 120, 150, and 180 °C, and a small amount of concentrated NaOH stark solution was added to prevent ZnO seed layer from being etched (final concentration ∼3.5 × 10-5 M). The temperature was increased at the rate of 1.25 °C/min. Once the target temperature was reached, -1.0 V of potential was applied to the ZnO/Pt/Cr/Si working electrode for a certain period of time. The pressures inside the autoclave was 1.6-10.5 atm. After the deposition, the autoclave was naturally cooled down to room temperature. After electrochemical and hydrothermal-electrochemical deposition, all nanorod films were rinsed with distilled water and dried at room temperature for 24 h. The surface morphology of the as-deposited ZnO nanorods was analyzed by field emission scanning electron microscopy (FE-SEM) (JSM7401F, Jeol). The crystal structure of the nanorods was investigated using X-ray diffraction (XRD) (X’pert system, Philips) with Cu KR radiation. Transmission electron micrographs and complementary selected area electron diffraction (SAED) patterns of the ZnO nanorods were obtained by transmission electron microscopy (TEM; JEM2100F, Jeol). The optical properties were measured by PL
The growth conditions greatly affected the surface of the ZnO seed layers, resulting in different morphologies of ZnO nanorods.34 Figure 1b-e shows FE-SEM images of the top and side view of the ZnO nanorods synthesized under different conditions. All ZnO nanorod arrays were produced from a 1 × 10-4 M Zn(NO3)2 solution without any additives, with an applied voltage of -1.0 V vs Ag/AgCl, and with an hour of reaction time. Nanorods grown electrochemically at 90 °C showed a narrow diameter distribution with an average diameter of ∼110 nm, while the nanorods grown hydrothermally-electrochemically at higher temperature showed a broader diameter distribution. For example, a high amount of very thick nanorods (d > ∼300 nm) were observed when the growth temperature was 180 °C as shown in Figure 1e. To find out the cause of this difference, scanning electron microscopy (SEM) investigation on the surface of ZnO seed layer was carried out. Since the diameter of the nanorods could be determined at the very early stages of the growth, the growth reaction was stopped as soon as the target temperature was reached. No electrochemical reaction was done on the substrates. The etching was clearly observed when the temperature was higher than 120 °C but not at 90 °C. The reduction of thickness of the ZnO due to the etching can be clearly observed in the SEM side view as shown in the rightcolumn of Figure 1b-e. Our experimental results imply that nanorods with welldefined morphologies can be produced if the ZnO layers remain intact. As ZnO materials can be etched in a solution with a high H+ or OH- concentration, adjusting the pH of the growth solution may prevent the etching problem associated with the ZnO seed layers.35 It was discovered that this could be achieved by adding a small amount of NaOH to the growth solution, which increased the pH from 4.68 to 5.18. Reduction in the thickness of the ZnO layer was not observed at any temperature owing to the NaOH additive as shown on the right-side columns in Figure 2. Since there was no noticeable etching on the ZnO seed layer, nanorods showed similar morphologies regardless of the growth conditions. Figure 2 shows the top and side views of vertically oriented ZnO nanorods with the diameter of ∼120 nm and the length ∼4.3 µm prepared under different conditions; Figure 2a shows nanorods produced electrochemically without any additive at 90 °C. Figure 2b-d shows nanorods prepared hydrothermally-electrochemically with NaOH solution as an additive (3.5 × 10-5 M in reaction vessel) at bath temperatures of 120, 150, and 180 °C, respectively. To produce the samelength nanorods, the growth time was reduced accordingly as the growth temperature was increased. This indicates that the growth rate was higher at a higher temperature. Our results indicate that the similar morphologies of ZnO nanorods can be obtained regardless of the synthetic conditions, as long as the ZnO seed layers remained intact (as shown on the right-side columns in Figure 2). This indicates nanowires can grow via a similar nucleation process (Figure S1, Supporting Information). It should be noted that the ZnO morphology can also be influenced by the growth conditions, including cathodic potentials, precursor electrolytes, and seed layers on the cathodic substrate. In this experiment, all the other growth conditions were fixed, except for the bath temperature and NaOH addition.
Hydrothermal-Electrochemical Synthesis of ZnO Nanorods
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As soon as the growth temperature (120, 150, and 180 °C) was reached, the hydrothermal-electrochemical reaction was immediately stopped without applying any potential to the working electrode, in order to examine the nucleation stage of the growth. It was discovered that a small amount of NaOH played an important role at this initial stage. When NaOH was not used (solution pH ) 4.68), no ZnO nanoparticles or nanorods were observed on the ZnO seed layers. This indicates that nucleation can occur only when external voltage is applied to the ZnO/Pt/Cr/Si electrodes. In other words, the nanorods grow electrochemically, not hydrothermally, in this slightly acidic condition. On the other hand, when NaOH additive was used (solution pH ) 5.14), low-density ZnO nanoparticles or nanorods were observed on the substrates (Figure 3a). This means ZnO nucleation can start hydrothermally without any applied voltage when NaOH was added. We speculate that this hydrothermal growth can happen because the OH- anions in NaOH solution react with Zn2+ cations to form Zn(OH)2 in the vicinity of the ZnO seed layer. Zn(OH)2 therefore becomes ZnO nanoparticles. Once the voltage was applied, ZnO nanowires grew similarly to electrochemical deposition.15-18 Figure 2. (a) Top and cross-sectional FE-SEM images of ZnO nanorods formed by electrochemical deposition at 90 °C for 4 h. Hydrothermalelectrochemical deposition with 3.5 × 10-5 M of NaOH additive was carried out at various conditions: (b) 120 °C, 3 h 20 min, (c) 150 °C, 2 h, (d) 180 °C, 1 h. Applied voltage was -1.0 V with respect to Ag/ AgCl reference. It should be noted that the higher the reaction temperature, the higher the growth rate of the ZnO nanorods. The rightside columns show FE-SEM images of the junction between ZnO nanorods and the substrates.
Because growth temperature can be higher than 100 °C in an autoclave, hydrothermal-electrochemical synthesis may have advantages over electrochemical or hydrothermal synthesis in terms of producing high quality ZnO nanorods. Lincot et al. reported that the preferred c-axis orientation of the ZnO sample prepared electrochemically from zinc chloride solution became stronger with increasing preparation temperature.25 Recently, Otani et al. also reported that the crystallinity of the ZnO film improved with increasing bath temperature during electrochemical deposition, and that the strongly preferred c-axis orientation was detected for the sample prepared at 70 °C.24 In addition, our previous observations in electrochemical depositions26,27 indicated that the growth rate, defined as the height of nanorod per unit time, increased as the bath temperature increased. The results of current hydrothermal-electrochemical deposition were also consistent. On the other hand, hydrothermally grown ZnO nanorods at a lower temperature (
