8144
2008, 112, 8144–8146 Published on Web 05/08/2008
ZnO Nanotube Arrays and Nanotube-Based Paint-Brush Structures: A Simple Methodology of Fabricating Hierarchical Nanostructures with Self-Assembled Junctions and Branches Soumitra Kar*,† and Swadeshmukul Santra*,†,‡,§ NanoScience Technology Center, Department of Chemistry, and Biomolecular Science Center, UniVersity of Central Florida, Orlando, Florida 32826 ReceiVed: April 03, 2008; ReVised Manuscript ReceiVed: April 25, 2008
ZnO nanotube arrays and self-assembled hierarchical nanostructures are reported here. The ZnO nanostructures were synthesized on Zn foils by a simple solvothermal approach. The hierarchical nanostructures resemble the paint-brush morphology, which consisted of a bunch of self-assembled thin nanowires on a comparatively thick ZnO nanotube wall. Successful creation of surface roughness through controlled etching of the ZnO nanotube walls provided the secondary nucleation centers for the nanowires. Introduction One-dimensional (1-D) nanostructures are considered to be the building block as well as interconnect for fabricating nanoscale devices.1–9 Integration of nanostructures as various components in devices is extremely challenging due to their nanodimensions. In this regard, the hierarchical nanostructures with self-assembled junctions and branches could be utilized as potential interconnects in nanoscale devices.10–13 Thus, controlled fabrication of self-assembled structures with nanoscale components of technologically important materials such as ZnO is important from the fundamental as well as technological point of view.10,11,14–20 Since the first report of ultraviolet lasing from ZnO nanorods,1 substantial effort has been devoted to the development of new synthetic methodologies2,18 for various 1-D and 1-D-based hierarchical ZnO nanostructures due to their promising applications in electronic and optoelectronic devices. The hierarchical ZnO nanostructures reported so far are self-assembled solid 1-D nanostructures.2,10,11,14–19,21,22 On the other hand tubular nanostructures offer higher porosity and surface area compared to their solid counterparts and thus offer enhanced efficiency and activity. Thus, the fabrication of nanotube based hierarchical ZnO nanostructures is desirable. In this letter, we report a novel ZnO “paint-brush” like hierarchical nanostructure having two components: ZnO nanotubes and ZnO nanowires. By precisely manipulating reaction parameters, ZnO nanowires bristles were grown on one end of the nanotube “paint-brush” handle by a simple synthetic methodology. Experimental Procedure The ZnO “paint-brush” nanostructures were synthesized in a closed cylindrical Teflon lined stainless steel chamber with 110 mL capacity. All of the reagents were of analytical grade and * Corresponding author. Telephone: 1-407-882-2848. Fax: 1-407-8822819. E-mail:
[email protected] (S.K.);
[email protected] (S.S.). † NanoScience Technology Center. ‡ Department of Chemistry. § Biomolecular Science Center.
10.1021/jp802893t CCC: $40.75
were used without further purification. Zn foil with dimensions of 1 cm2 and 0.5 mm thickness were used as the substrate that also served as the Zn ion source. In a typical procedure, Zn foils were taken in the Teflon chamber and filled with about 60 mL of ethanol. An appropriate amount of NaOH was then added to adjust the pH to ∼13.6. The chamber was then sealed and kept in a furnace at 170 and 200 °C for 12 h. After the desired time period, the system was allowed to cool naturally. The foils collected from the reaction vessel were washed with water several times and dried in air. A thin white layer was found deposited on the Zn foils, which were characterized by X-ray diffractometer (XRD, Seifert 3000P) with Cu KR radiation, and the compositional analysis was done by energy dispersive analysis of X-ray (EDAX, Kevex, Delta Class I). The surface morphology of the white layer on the Zn foils was studied by scanning electron microscopy (SEM, Hitachi S-2300). Results and Discussion While working on the solvothermal synthesis of ZnO nanorod arrays on Zn foils by a solvothermal process at 200 °C,18 we have observed an improvement in the alignment of the nanorods with the increase in the alkalinity of the solvent. At the same time, the diameter of the nanorods increased systematically with the increase in pH level. It was also observed that an increase in the pH level beyond a limit (>10.6) caused an abrupt increase in the diameter of the nanorods at the expense of the alignment of the nanorods due to corrosion of the underlying Zn foils. It was reported that in solution growth under basic conditions the larger diameter of the 1-D nanoforms could trigger the formation of a hollow structure.23 Thus by controlling the pH we should be able to fabricate ZnO nanotubes by the solvothermal approach. It is also necessary to regulate the temperature in order to minimize the corrosion of the underlying Zn substrate. Based on this hypothesis, in this study, we performed a series of experiments where the solvent alkalinity and pH were systematically varied. We were not only able to validate the concept of growing aligned nanotubes directly from the Zn foils but also were able to fabricate novel self-assembled hierarchical 2008 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 112, No. 22, 2008 8145
Figure 1. XRD patterns of the samples synthesized at different synthesis temperatures.
Figure 3. SEM images of the ZnO paint-brush structures produced at 200 °C at the same pH in which ZnO nanotubes were formed at 170 °C. Part a shows a general view, and part b shows the closer view of the top part of a paint-brush revealing the hollow interior of the nanotube handle. The image in the inset of part b exhibits the mesoporous nature of a nanotube handle towards their brush attached end.
Figure 2. SEM image of the ZnO nanotube array produced at 170 °C.
“paint-brush” like ZnO nanostructures. We have produced ZnO nanotube arrays at 170 °C and a ZnO pant-brush at 200 °C under pH 13.6. The Zn foils with white deposits were characterized by XRD, and the XRD plots are shown in Figure 1. The diffraction peaks positioned at 2θ values of 31.76, 34.42, 36.25, 47.53, 56.6, and 62.8 can be indexed to the hexagonal wurtzite phase of zinc oxide (JCPDS card no. 36-1451). The diffraction peaks from the zinc substrate show because of X-ray beam penetration through the hollow ZnO nanostructures on the surface of the zinc foil. This penetration appears to be a bit better through the more open nanotube array versus the more crowded nanopaintbrush one (see Figures 2 and 3). The presence of the (002) peak as the most intense peak for the sample synthesized at 170 °C indicated a preferred growth of the nanostructures at this temperature, whereas the XRD pattern of the sample synthesized at 200 °C show no such preferred growth; that is, the XRD peak intensities represent a random ZnO pattern. The SEM studies (see Figures 2 and 3) indicated that the XRD peak intensities in Figure 1a are influenced by the basal orientation (note enhanced 002 relative to 101 and 100 reflections) of the nanotubes. The morphology of the nanostructures on the Zn foil was investigated through the SEM. Figure 2 shows the SEM image of the sample synthesized at 170 °C that confirmed the formation of approximately 500 nm long nanotube arrays. The diameter varied between 80 and 120 nm. Figure 3a shows the general SEM image of the sample synthesized at 200 °C that exhibited a “paint-brush” like nanostructure. The SEM image of the
“paint-brush” revealed the formation of a bunch of thin (diameter 12-15 nm) nanowires from one end of a relatively thick (∼500 nm in diameter) handle like a mesoporous tubular (Figure 3b) structure. The origin of the ZnO “paint-brush” like nanostructures is the secondary growth of thin nanowires onto the nanotube wall. The growth mechanism could be explained on the basis of the following reactions and the crystal habits of wurtzite ZnO as presented in the schematic diagram (Figure 4). In a typical solution based synthetic approach, [Zn(OH)4]2- serve as a basic growth unit for the fabrication of ZnO nanoforms.18,24 Therefore in the present solvothermal condition at elevated temperature and pressure, Zn+ ions are produced from the Zn foils which then react with the OH-1 ions to produce Zn(OH)42-. These [Zn(OH)4]2- ions then decompose to produce ZnO molecular species, forming a ZnO seed.
Zn f Zn+ + OH-1 f Zn(OH)42- f ZnO
(1)
The growth of the ZnO nanostructures can be explained on the basis of the schematic view presented in Figure 4. These ZnO seeds grew to form a hexagonal planar nucleus. The hexagonal wurtzite ZnO with polar structure can be described as a hexagonal close packing (HCP) of oxygen and zinc atoms in point group 3m and space group P63mc with zinc atoms in tetrahedral sites. Thus, the crystal habits of wurtzite ZnO exhibit well defined crystallographic faces [i.e., basal (001) and non polar low symmetry (100) faces (and C6V symmetric ones)]. In the case of wurtzite ZnO crystals, the zinc terminated (001) planes are active that promote one-dimensional growth. Thus, one face of the hexagonal sheet is Zn rich that forms the (001) planes, whereas the opposite face is the (00-1) plane. Thus, the ZnO crystals are polar in nature. Since the Zn rich positive (001) surface is more reactive than the oxygen rich negative
8146 J. Phys. Chem. C, Vol. 112, No. 22, 2008
Letters modate the newly formed ZnO species. The secondary growth process competes with the original one-dimensional tubular growth, forming the “paint-brush” like hierarchical nanostructures. The small pits formed at the nanotubes walls were mostly covered by the secondary growth of the nanowires. However, the comparatively large holes formed in some of the nanotube handles could not be covered by the thin secondary nanowires resulting in to the formation of a few paint-brush structures with mesoporous handles. Conclusions In summary, we have synthesized ZnO nanotube arrays on Zn foils by a simple solvothermal approach. In addition we have also fabricated “paint-brush” like self-assembled nanostructures of ZnO. These unique self-assembled structures could be appropriate as multiport contact probe. Their large surface areas are also suitable for dye-sensitized solar cell applications. In addition they could be utilized as waveguide splitters and recombiners in nanoscale.
Figure 4. Schematic view of the growth mechanism of the ZnO nanotubes and nanotube based paint brush structures.
Acknowledgment. Authors dedicate this paper to the memory of Prof. Subhadra Chaudhuri, Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata, INDIA. References and Notes
(00-1) surface, it can attract new ZnO species or the opposite ionic species to its surface to promote the anisotropic growth along the (001) direction. The six side facets are generally bounded by the (100) family of planes as shown in the scheme (Figure 4). From the crystal habits of wurtzite ZnO, it is wellknown that the growth rate of the different family of planes follows the sequence (001) > (101) > (100). Thus, normally ZnO columnar structures bounded by six (100) facets are grown along the (001) direction. Thus, the polar (001) surface is metastable in nature and is subjected to etching at high pH conditions.25 In this reaction condition, it is possible that the Zn atoms of the (001) metastable polar surface react with the OH-1 ions, forming the hydroxide species. At elevated temperature and pH, the equilibrium shifts toward the right thus facilitating the metastable Zn-terminated surface etching process.
Zn+ + OH-1 T Zn(OH)42-
(2)
It is also well-known that the well defined side facets are more stable compared to the central portion of the hexagonal sheet like nucleus and are also much more defect prone.25 These factors enhance the etching in the central zone of the disks, and hence, hollow ring like structures appear. At the same time, the one-dimensional growth continues to give rise to the tubular structures. On the other hand, an increase in temperature (200 °C) increases the etching effect that even changes the surface textures of the nanotubes and creates small “pot-hole” like spots on the surface of the nanotubes toward the nanotube tip. The growth ends of the one-dimensional nanostructures are always at a higher energy state compared to the back portion and this is why the front portions are more vulnerable to etching. The continuous adsorption of the elementary species at the growth front and a time delay before they could be perfectly arranged to provide the faceted nanowire surface leads to the higher energy state at the growth front. These spots serve as energetically favored sites (i.e., secondary nucleation sites) to accom-
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