Article pubs.acs.org/JPCB
Low-Temperature Solution Growth of ZnO Nanocone/Highly Oriented Nanorod Arrays on Copper Yongmei Xia, Youfa Zhang,* Xinquan Yu,* and Feng Chen Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, People’s Republic of China S Supporting Information *
ABSTRACT: Solution-phase approaches to one-dimensional (1D) ZnO nanostructure arrays are appealing because of their good potential for scale-up. Allowing for a wide variety of substrate material compatibility and saving energy, it is very essential to further research the low-temperature growth process of 1D ZnO nanostructure arrays and its detailed growth mechanism. In this study, large-scale misaligned hexagonal ZnO nancone arrays were synthesized on bare copper foil, while large-scale well-aligned, and highly oriented ZnO nanorod arrays were grown on seeded copper foil through a facile solution processing method at normal atmospheric pressure at 35 °C. X-ray diffraction analysis verified the crystalline nature of the ZnO nanocone/nanorods, and transmission electron microscopy further confirmed the single-crystal nature and the preferential growth direction of the ZnO nanocone/nanorods. The room-temperature photoluminescence measurement qualitatively identified the intrinsic point defects in the ZnO nanocones/nanorods. Besides, the detailed growth behavior of ZnO was discussed with and without a ZnO seed layer, which provides useful information to propose the growth mechanism of the nanocone/nanorods in the lowtemperature solution. The method developed here can be easily scaled up to fabricate ZnO nanostructures for many important applications in field emission display, gas sensors, and superhydrophobic surfaces. growth.13,19 Accordingly, there are two principal problems concerning the present SP techniques (general reaction temperature ≥ 60 °C), which greatly limits the application range of ZnO. The first one is to lower the growth temperature for saving energy and for a wide variety of substrate material compatibility, and the second one is to probe its detailed growth mechanism for better control of the experiment process and design for high-performance devices in the future. On the other hand, as an important engineering material, copper is widely used in many industrial applications, such as electric industry, electronics, heat-transfer systems, and machinery manufacturing, due to its excellent thermal and electrical conductivity, good anticorrosion, and high ductility. Considering the properties of 1D ZnO nanostructure arrays and metal copper, once appropriately growing 1D ZnO nanostructure arrays on copper substrate, the potential application fields will very likely be realized in industrial applications, such as field emission displays, gas sensors, and superhydrophobic surfaces, in the near future. However, it is worth noting that copper is easily oxidized and etched in alkali solution at higher temperature (>45 °C), especially in the
1. INTRODUCTION Zinc oxide (ZnO) has attracted a great deal of attention and has been one of the most important semiconductors because of its unique properties (a direct wide band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature) in recent years. A broad range of potential high-performance applications, which is in the fields of ultraviolet lasers,1 photodetectors,2 field -effect transistors,3 solar cells,4 chemical sensors,5,6 superhydrophobic surfaces,7 and nanogenerators,8,9 is based on one-dimensional (1D) ZnO nanomaterials, especially nanocone/nanorod arrays. Present various approaches to build 1D ZnO nanostructure arrays are mainly based on the vapor-phase transport technique (VPT),10 pulse laser deposition (PLD),11 metal−organic chemical vapor deposition (MOCVD), 12 electrochemical deposition (ECD), 13 and solution-phase (SP) approaches. 4,5,14−23 Among these, the SP route is the most appealing because of its low cost, simple technology, and good potential for scale-up. However, the reaction temperature of current SP approaches is usually more than 60 °C. Even though Wu et al. have realized how to fabricate highly oriented ZnO nanoneedles/nanorods arrays on zinc foil substrates using a SP oxidation reaction process and electrochemical anodization deposition method at room temperature, Zn foil substrates were required to participate in reaction and be consumed during ZnO © XXXX American Chemical Society
Received: March 23, 2014 Revised: August 25, 2014
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with the molar ratio of zinc nitrate hexahydrate (Zn(NO3)2· 6H2O, A.R., Xilong Chemical Co., Ltd., China) to potassium hydroxide (KOH, A.R., Sinopharm Chemical Reagent Co., Ltd., China) of 1:8. Second, the bare and seeded copper substrates were floated in the growth solution, the object surface had to be put downward, and then the system was sealed in a beaker. Third, the growth system was put into an electrothermostatic water cabinet, and the growth temperature and time were set at 35 °C and 12 h, respectively. Finally, the samples were taken out and rinsed with deionized water and ethanol in sequence and dried in a vacuum oven at 25 °C for 4 h before characterization. 2.3. Instruments and Characterization. Scanning electron microscopy (SEM) images were taken by a Sirion field-emission scanning electron microscope (FEI) at 20 kV. Xray diffraction (XRD) patterns was performed with an X-ray diffractometer model D8-Discover (Bruker) with Cu Kα radiation (λ = 1.5418 Å). Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) were carried out with a Tecnai G2 (200 kV) (FEI) transmission electron microscope. Photoluminescence (PL) measurement was performed at room temperature with an F-7000 FL spectrophotometer at 240 nm excitation wavelength.
solution including amine or ammonium. So far, very few studies have been reported for fabricating well-aligned 1D ZnO nanostructures on copper substrates by the existing solution route. As far as we are aware, it is still a challenge to fabricate highly oriented 1D ZnO nanostructure arrays on a copper substrate by a solution route. Here, we report for the first time large-scale growth of highly oriented ZnO nanorod arrays and misaligned hexagonal ZnO nancone arrays on copper foils through a facile solution processing method at normal atmospheric pressure at 35 °C. The growth patterns and the growth kinetics are systematically discussed. The possible growth mechanism is proposed. It is worth mentioning that the current process is substrateindependent, which can produce similar high-quality nanostructure arrays on FTO glass and glass slides (Figure S1, Supporting Information), and the copper substrate remains nearly intact during crystal growth (Figure S2, Supporting Information). In addition, the particular architecture of the ZnO/copper hybrid allows in-depth study of the field emission performance, the responsive variation of the surface chemisorbed gases, and wettability of its surface modified by FAS. The field emission properties, the change of electrical properties induced by the adsorption of gas molecules, and the superhydrophobic performance of such a novel ZnO/ copper hybrid structure are currently under investigation in our department.
3. RESULTS AND DISCUSSION The morphology of the ZnO nanocone arrays prepared on bare copper foil at 35 °C for 12 h was characterized by SEM. As shown in Figure 1a and b, the hexagonal cone-like ZnO arrays
2. EXPERIMENTAL SECTION Highly oriented ZnO nanorod arrays were grown on copper foil using a two-step process, combining seeding fabrication by sol−gel spin coating and the rapid thermal treatment and nanorod array growth by a SP route. The misaligned nanocone arrays were directly synthesized on bare copper foil through the SP route. The growth methods and more details are as follows. 2.1. Fabrication of ZnO Seed Layers on Copper Substrates. Seed layers were fabricated on copper substrates (size: 2.846 cm × 2.663 cm × 0.024 cm) by modified sol−gel spin coating and the rapid thermal treatment.24 Prior to the fabrication, the substrates were thoroughly cleaned. The precursor concentration of zinc acetate dihydrate (Zn(CH3COO)2·2H2O, A.R., Xilong Chemical Co., Ltd., China) in the ethylene glycol monomethyl ether (CH3OCH2CH2OH, A.R., Sinopharm Chemical Reagent Co., Ltd., China) solution was 0.5 M. Ethanolamine (NH2CH2CH2OH, A.R., Shanghai Lingfeng Chemical Reagent Co., Ltd., China) was used as the stabilization reagent, and an appropriate amount of water was added to adjust the hydrolysis of zinc acetate. After forming a yellowish transparent sol and aging for 24 h, an appropriate amount of polyethylene glycol 4000 (HO(CH2CH2O)nH, A.R., Guangzhou Guanghua Chemical Factory Co., Ltd. China), which acts as a surfactant, was added to the sol and stirred with a magnetic stirrer at 60 °C for 30 min; the final sol was achieved. The sol was spun onto the substrates at a speed of 600 rpm for 9 s and then 3000 rpm for 20 s at room temperature; then, the sol film was dried by a cold air blower to remove the residual solvent. The procedures from coating to drying were repeated three times to ensure a complete and uniform coverage of ZnO seeds. At last, the wet films were carefully put into an argon atmosphere furnace (ZRX-12-11, Shanghai Chenhua Instrument Co., Ltd. China) horizontally and keep at 350 °C for 10 min. 2.2. SP Growth of ZnO Nanocone/Nanorod Arrays. First, the growth solution, 0.25 M [Zn(OH)4]2−, was prepared
Figure 1. SEM (a,b), XRD (c), and TEM (d) of as-grown ZnO nanocones on a bare copper surface at 35 °C for 12 h.
were produced in this condition; the cones with anisotropic morphology and orientation compactly covered the substrate on a large scale. They are typically 600−1000 nm in length, 20−100 nm in diameter at their top parts, and 80−240 nm in diameter at their bottom parts, as the images of Figure 1b and d demonstrate. According to the XRD pattern (Figure 1c), wurtzite structure ZnO is the only detectable crystallographic phase in the misaligned nanocones, except for those peaks marked with diamonds arising from the copper substrate. Compared with the powder diffraction pattern, the XRD spectrum of ZnO nanocone arrays shows a dominant diffraction peak corresponding to (002) planes, which indicates that the ZnO nanocones are grown with a rough orientation of B
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their c-axis (perpendicular to the substrate). Further structural characterization of the ZnO hexagonal nanocones was performed by TEM. Figure 1d displays a TEM image of several individual ZnO nanocones. The dimensions of the cones under TEM observation are in good agreement with the SEM results. The SAED pattern taken from one of the cones is shown in the bottom-left inset of Figure 1d, which confirms the single-crystal nature of the ZnO cones grown along the c-axis. The HRTEM image shown in the top-right inset of Figure 1d gives a lattice fringe of about 0.26 nm, corresponding to the (002) plane of wurtzite ZnO, which further indicates that the [001] direction is the preferential growth direction of ZnO nanocone. The morphology of the ZnO nanostructures was sensitive to the presence or absence of a ZnO seed layer.25 Figure 2
Figure 3. Room-temperature PL spectra of the arrays of as-prepared ZnO nanocones (black) and nanorods (red) grown on the bare and seeded copper surfaces at 35 °C for 12 h, respectively.
(UV) emission induced by the near-band-edge (NBE) exciton recombination of ZnO, where very weak visible emission related to defects can be observed, confirming the rare point defects in the ZnO nanocones. As for the ZnO nanorods, the PL spectrum is similar to that of the ZnO nanocones with the strong UV emission, but the observable visible emissions peaked at different wavelengths appear. The deep level visible emissions (DLEs) in ZnO usually occur near the blue−green (480−550 nm), yellow (550−610 nm), and orange−red (610− 750 nm) regions. Although their defect origins are still controversial, it is gradually indicated that the blue−green emission is related to the zinc vacancy (VZn), and the green emission originates from the singly charged oxygen vacancy (VO+), while the yellow one is assigned commonly to the doubly charged oxygen vacancy (VO2+), and the orange−red emission is associated with the interstitial oxygen (Oi−) on the ZnO surface, which results mainly from the annealing or the surface modification.18 Thus, the obvious green emission and broad observable orange−red emission in the PL spectrum of the ZnO nanorods suggest that the nanorod arrays may possess a small number of defects of VO+ and Oi−, and the Oi− defects may mainly originate from the seed layer annealed to introduce these defects. In order to fully understand the effect of the seed layer and explore the growth mechanism of the ZnO nanostructure arrays in the solution consisting of Zn(NO3)2·6H2O and KOH, we investigated the growth kinetics of ZnO nanostructures grown with time on the bare and seeded copper surfaces, respectively. Figure 4a, c, and e shows the morphologies of the ZnO nanostructures grown on the bare copper surfaces at 35 °C for 10 min, 30 min, and 1 h, respectively. Figure 4a displays the surface images of the bare copper substrates before and after the growth in solution for 10 min. From the inset of Figure 4a, the nanoscale roughness of the bare copper surface can clearly be seen. In accordance with higher-magnification SEM observation, the substrate possesses a mean roughness of less than 10 nm. In contrast, the surface of the copper substrate after the solution growth for 10 min is covered with numerous nanoparticles that are typically 30−90 nm in size (Figure 4a). In combination with the whole experimental results of growth kinetics, the formation of these particles can be considered as the nucleation process. When the growth time was increased to 30 min, the growth result displayed underdeveloped morphology of nanocone arrays (Figure 4c). We call the growth stage the initial growth stage of arrays. From Figure 4a and c, the
Figure 2. SEM (a,b), XRD (c), and TEM (d) of as-grown ZnO nanorods on a seeded copper surface at 35 °C for 12 h.
illustrates the morphology and structural characterization results of the ZnO nanorod arrays prepared on seeded copper foil at 35 °C for 12 h. As shown in Figure 2a, highly uniform and aligned ZnO rods were grown on the seeded copper surface and oriented perpendicularly to the substrate. Figure 2b is a 30° tilted view of the rod arrays. It is clear that the rods are typically 700−900 nm in length, 20−50 nm in diameter at their top parts, and 40−70 nm in diameter at their bottom parts. From the statistical result, the areal density of the ZnO nanorod arrays is on the order of 108 rods mm−2, which is comparable to the previously reported results from Yang’s group15,26 and Wang’s group27 by the hydrothermal method. Additionally, it is estimated that the ZnO nanorods were mechanically broken when they were removed from the substrate. Thus, the length of ZnO nanorods was not uniform in Figure 2b and d. As expected, the (002) diffraction peak in Figure 2c exhibits an overwhelming intensity, further manifesting that nanorods are much better perpendicular to the substrate. Furthermore, the SAED and HRTEM results shown in Figure 2d also confirm that the nanorods are single crystals and that they were grown preferentially in their [001] direction. To qualitatively identify the intrinsic point defects in the ZnO nanocones and nanorods, we carried out their roomtemperature PL measurements. Figure 3 shows the roomtemperature emission PL spectra of the nanocones (black) and nanorods (red) at a 240 nm excitation wavelength. The PL spectrum of the ZnO nanocones exhibits a strong ultraviolet C
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time on the growth of ZnO nanorod arrays, according to the statistical results, we roughly drew out a graph showing the trends and patterns of the dependence of ZnO nanorod arrays on growth time, as shown in Figure 5. These results indicated
Figure 5. Dependence of the average areal density, buffer layer thickness, rod length, and top size of the nanorods grown on the seeded copper foils on growth time under an identical growth solution and growth temperature.
Figure 4. SEM images of the evolutionary morphologies of the asgrown ZnO nanostructures with the change of growth time on the bare and seeded copper foils, respectively: (a) 10 min, on the bare copper foil; (b) 10 min, on the seeded copper foil; (c) 30 min, on the bare copper foil; (d) 30 min, on the seeded copper foil; (e) 1 h, on the bare copper foil; (f) 1 h, on the seeded copper foil. The top-right insets of images a and b are the surface images of the bare and seeded copper foil, respectively; the bottom-left insets of images b, d, and f are their corresponding 30° tilted view images. The precursor concentration and growth temperature were 0.25 M and 35 °C, respectively.
that, with the growth time prolonged, the longitudinal growth of ZnO nanorods was continuously and steadily proceeding, while the areal density and average diameter were nearly unaltered, which further manifests that the length of nanorods can be experimentally tailored by adjusting the growth time. Also, the buffer layer between the substrate and nanorods becomes gradually thick with the growth time prolonged. Additionally, from the morphologies of the nanorod arrays, we can see that the top of the each rod has a hemispherical nanoparticle whose size is nearly unchanged with the growth time prolonged, which is very similar to the catalytic particle of the vapor−liquid−solid (VLS) process.28,29 The growth of ZnO nanorods on seeded layers prepared by sol−gel and their growth mechanism is already well reported,30 but the existing mechanism could not explain our experimental phenomena and results well. Hence, we believe that the original ZnO seed layer provides a wetting surface for the nucleation of the ZnO crystal and ensures the c-oriented growth of the ZnO buffer layer and the subsequent nanorods, and a self-catalyst growth mechanism played an important role in this growth process. We speculate that the possible growth process is as follows. First, the ZnO crystal in the growth solution is adsorbed on the seed layer and aggregates and forms into a ZnO hemispherical nanoparticle. These hemispherical nanoparticles act as self-formed catalyst particles, which will assist in growing the c-oriented ZnO buffer layer and the following well-aligned ZnO nanorod arrays, at the very beginning of the growth experiment. Second, with the continuous precipitation of ZnO crystal from the solution, a new ZnO crystal will epitaxially grow at the interface between the ZnO catalyst particle and the ZnO seed substrate. Nanorods with almost the same size as the ZnO catalyst particle will start to grow and gradually form the c-oriented ZnO buffer layer. Third, as the reaction proceeds, the concentration of ZnO precursors gradually decreases and less and less new ZnO crystal will continuously grow at the interface of the ZnO catalyst particle and the formed ZnO nanorod to gradually form the highly oriented nanorod arrays. In addition, compared with the almost unchanged size of the ZnO catalyst particle, the size of the nanorod gradual shrinks
crystal growth has the characteristics of an Ostwald ripening mode with the Volmer−Weber growth mechanism. Upon further increasing the growth time to 1 h, as can be seen from Figure 4e, the ZnO nanostructures obtained at this growth stage have a similar morphology as those formed after 12 h of growth (Figure 1a). We call the growth stage the full growth stage of arrays. With the growth time extending, the evolutionary growth of ZnO nanocone arrays follows the pattern of Ostwald ripening. At the same time, these cones grown from the randomly oriented ZnO nuclei begin to impinge on other neighboring crystals, giving rise to the roughly preferred orientation of the arrays. Figure 4b, d, and f shows the morphologies of the ZnO nanostructures grown on the seeded copper substrates at 35 °C for 10 min, 30 min, and 1h, respectively. The roughness of the seeded copper surface, which is also nanoscale size, is larger than that of the bare copper surface, as can clearly be seen in the top-right insets of panels a and b. In combination with all experimental results with the seed layer, the ubiquitous surface roughness logically serves as nucleation sites for nanorod growth. The typical diameter of the ZnO nanorods grown for 10 min was 20−50 nm, and the length was a few tens of nanometers, which indicates that the nucleation process has completed in far less than 10 min. Also, these nanorods were very short, which makes us believe that they had just begun to grow. In contrast with the nanostructure grown on the bare copper surface, the nanorods grown on the seeded copper surface more uniformly and densely covered the entire substrate surface. With the prolonging of growth time, no remarkable change in the top view and average diameter was observed, but the longitudinal growth was very obvious from the tilted view of the rods. To further probe the effect of growth D
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from its root to top, which was confirmed by the mushroomlike rods revealed by the SEM image (Figure S3, Supporting Information). As shown in Figure 4, the nucleation at the initial growth stage is crucial for the growth of a highly oriented perpendicular nanostructure array. On the ZnO-covered substrate, the nucleation density and uniformity were remarkably higher than that on the bare copper surface, which is consistent with the previously reported results.25,31 Furthermore, on bare copper surfaces, an uneven distribution of island nuclei leads to the uneven diameter and orientation of the nanocones. As for the superior alignment and uniformity of ZnO nanorods grown on the ZnO seed layer seeded surface, it is due to the matching lattice structure, and the nucleation on the seeded surface has a lower activation energy barrier than that on the bare copper surface. In addition, ZnO nanocones and nanorods here are found to grow along the [001] direction, which may be as a result of the fact that hexagonal structure of ZnO is a polar crystal with a dipole moment along the [001] direction, and this dipole moment will try to align itself to minimize energy. The growth of ZnO nanostructures in the Zn(OH)42− solution consisting of Zn(NO3)2·6H2O and KOH can be simply represented by the following reaction: Zn(OH)42− ↔ ZnO ↓ + H2O + 2OH−. During the ZnO crystal growth, the gradual consumption of ZnO precursors caused a concentration gradient of zincate ion from the root to the top regions of the ZnO structure in the solution. As a result, the growing rates along the ZnO [001] direction gradually decreased, and finally, nanocones and mushroom-like nanorods were formed. However, the effect of the concentration gradient might be weakened by the seed layer, which could lower the energy barrier of nucleation to promote epitaxial nucleation and growth and to retard the homogeneous nucleation, which can rapidly consume ZnO precursors and result in depleting the supply of Zn2+ earlier in the crystal development process, in the bulk solution. Therefore, ZnO nanorods were produced on the seeded copper surface, while nanocones were generated on the bare copper surface. On the basis of the above results, the possible growth processes of the ZnO arrays grown on bare and seeded copper surfaces are schematically illustrated in Figure 6. The processes are driven to minimize the interfacial energy between the new crystals and the primay crystal/substrate, which lowers the overall energy of the system. Furthermore, it can be noteworthy that the uniform ZnO seed layers were indispensable for the well-aligned and highly oriented growth of ZnO nanorods on the lattice-mismatched substrate under the growth condition. Because the lattice mismatch between the copper substrate and ZnO crystal is beyond 5% ((0001)[112̅0]ZnO//(110)[001]Cu lattice mismatch 11.2%; perpendicular direction: (0001)[1̅100]ZnO//(110)[11̅0]Cu lattice mismatch 9.2%), the copper surface lacks dangling bonds and is almost chemically inert in the growth solution, and only a weak van der Waals interaction exists at the epilayer−substrate interface, it is deemed that the heteroepitaxy growth of ZnO on the copper substrate belongs to van der Waals epitaxy.32 Therefore, we propose that on the bare copper substrate, ZnO heteroepitaxy nucleation classified as van der Waals epitaxy and subsequent ZnO homoepitaxy growth complies with the Volmer−Weber growth accompanied by the Ostwald ripening mode; on the seeded copper substrate, a self-catalyst growth mechanism played an important role in the growth
Figure 6. Schematic illustration of the growth mechanism of the misaligned ZnO nanocone arrays on bare copper surface (A) and the well-aligned ZnO nanorod arrays on seeded copper surface (B).
process of the highly oriented nanorod arrays in the [Zn(OH)4]2− solution system.
4. CONCLUSIONS In conclusion, large-scale misaligned hexagonal ZnO nancone arrays were synthesized on bare copper foil, and large-scale, well-aligned, and highly oriented ZnO nanorod arrays were successfully grown on seeded copper foil through a facile solution processing method at 35 °C. The room-temperature PL measurement qualitatively identified the intrinsic point defects in the ZnO nanocones and nanorods. Furthermore, the detailed growth behavior of ZnO in solution was discussed with and without a ZnO seed layer, and the possible growth mechanisms, van der Waals epitaxy, and Volmer−Weber growth accompanied by Ostwald ripening governed by the nucleation and growth of ZnO nanocone arrays on a bare copper substrate, while a self-catalyst growth mechanism played an important role in the growth process of the highly oriented nanorod arrays on the seeded copper substrate in the [Zn(OH)4]2− solution system, were proposed. These findings will cast a new light on the general understanding of the growth mechanism of 1D ZnO arrays in the SP. The method developed here can be easily scaled up to fabricate 1D ZnO nanostructures for many important applications in field emission display, gas sensors, and superhydrophobic surfaces.
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ASSOCIATED CONTENT
S Supporting Information *
SEM images of as-grown ZnO nanostructure arrays, photographs of a bare copper foil, a copper foil covered with ZnO nanocone arrays without a seed layer, and a copper foil covered with ZnO nanorod arrays with a seed layer, and a highmagnification SEM image of the as-grown ZnO nanorod arrays. This material is available free of charge via the Internet at http://pubs.acs.org. E
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Tel: +86 25 5209 0674. Fax: +86 25 5209 0658 (Y.Z.). *E-mail:
[email protected]. Tel: +86 25 5209 0674. Fax: +86 25 5209 0658 (X.Y.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51101035) and the Natural Science Foundation of Jiangsu Province (BK2011255). We also thank the support from the Specialized Research Fund for the Doctoral Program of Higher Education of China (20110092120066) and the Specialized Research Fund for Nanotechnology of Suzhou City (ZXG2012020).
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