J. Phys. Chem. C 2008, 112, 11767–11775
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Multistep Synthesis, Growth Mechanism, Optical, and Microwave Absorption Properties of ZnO Dendritic Nanostructures R. F. Zhuo,† H. T. Feng,† J. T. Chen,† D. Yan,† J. J. Feng,† H. J. Li,† B. S. Geng,† S. Cheng,† X. Y. Xu,† and P. X. Yan*,†,‡ Department of Physics, Lanzhou UniVersity, Lanzhou, 730000, China, and Key Laboratory of Solid Lubrication, Institute of Chemistry and Physics, Chinese Academy of Science, Lanzhou, 730000, China ReceiVed: May 8, 2008; ReVised Manuscript ReceiVed: June 10, 2008
Intriguing ZnO dendritic nanostructures have been synthesized by a two-step chemical vapor deposition process. Regular nanorods grow uniformly to the presynthesized ZnO nanowires on silicon substrate, the secondary nanorods are single-crystal hexagonal ZnO, and each nanorod grows along the [0001] direction. The relationship between the secondary-grown nanorods and the primary ZnO nanowire is not epitaxial due to the high temperature-increasing rate during the rapid grown process. The size and morphology of branches can be controlled by adjusting the temperature and duration of growth. Room temperature photoluminescence (PL) and mircrowave absorption properties of the ZnO dendritic nanostructures have been investigated in detail. The value of minimum reflection loss for the composite with 50 vol % ZnO dendritic nanostructures is -42 dB at 3.6 GHz with a thickness of 5.0 mm. Hierarchical nanostructures of this type are ideal objects for the fabrication of nanoscale functional devices. 1. Introduction Investigation in synthesizing assembled two- and threedimensional hierarchical nanostructures is a brand new area in nanotechnology, which is a decisive step toward the realization of functional nanoscale systems.1,2 Hierarchical nanostructures, in which the primary stems and the secondary branches consisting of either the same or different materials, increase structural complexity and pave the way to assemble 1D nanostructures into functional electronic devices.3,4 Many novel hierarchical nanostructures, such as ZnO,5–8 In2O3,9 MgO,10 SnOx,4,11,12 Si-SiO2,13,14 W-WO3,15 ZnS-CdS,16 MgOZnO,17a SnO2-ZnO,3,17b and SnO-RFe2O318 have been achieved by various gas-phase growth processes. In fact, all these attractive nanostructures could be applied in field emission, optoelectronic devices, and sensors. Although much effort has been devoted to the synthesis of hierarchical nanostructures, it remains a big challenge to understand the growth mechanism thoroughly and develop the new hierarchical structures. Most of the reported hierarchical ZnO nanostructures possess basic 6-, or 4-, or 2-fold (most have 6-fold) structural symmetry because of epitaxial growth. Gao and Wang et al. reported the synthesis of nanowire-nanoribbon junction arrays of ZnO which were grown by a thermal evaporation process. The structure was formed due to the fast growth of ZnO nanowires along [0001] and the subsequent “epitaxial” radial growth of the ZnO nanoribbons along the six 〈011j 0〉 directions around the nanowire.5a,b Later they presented a two-step high-temperature solid-vapor deposition process for the synthesis of polar surface dominated ZnO nanopropeller arrays. The axis of the nanopropellers was a straight nanowire along the c axis and enclosed by {21j1j0} surfaces; the 6-fold symmetric nanoblades were later formed along the crystallographic equivalent a axes (〈21j1j0〉) * To whom correspondence should be addressed. E-mail: pxyan@ lzu.edu.cn. Tel: 86 + 0931 + 8912719. Fax: 86 + 0931 + 8913554. † Lanzhou University. ‡ Chinese Academy of Science.
Figure 1. (a) Schematic diagram of the horizontal furnace. (b) Temperature distribution curve of the furnace.
perpendicular to the nanowire, and the array was formed by epitaxial growth of nanoblades on the nanowire.5c Lao et al. reported that many novel hierarchical ZnO nanostructures were grown (from In2O3 nanowires) by a thermal vapor transport and condensation technique. These nanostructures have basic 6-, 4-, and 2-fold structural symmetries.7 In this paper we report a twostep chemical vapor deposition (CVD) route for rapid growth of ZnO dendritic nanostructures, in which numerous aligned ZnO nanorods have been grown on the surface of the primary single-crystal ZnO nanowire substrates without any catalyst. The secondarily grown ZnO nanorods which utilize the catalystfree vapor-solid (VS) growth mechanism have isotropic crystal symmetries toward the stem because of the high temperatureincreasing rate during the rapid growth process. The relationship between the secondary-grown nanorods and the primary ZnO
10.1021/jp804090q CCC: $40.75 2008 American Chemical Society Published on Web 07/16/2008
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Figure 2. (a) SEM image of the primary ZnO nanowires before secondary growth. (b) SEM image of secondary ZnO nanorods on the primary ZnO nanowire. (c) Typical TEM image of the secondary branch. (d) The stem that all branches were broken off because of sonication during the TEM sample preparation process. (e) The corresponding SAED pattern. (f) HRTEM image of a representative branch/stem interface area of ZnO hierarchical nanostructures.
served in ZnO dendritic nanostructures/paraffin composites. The value of minimum reflection loss for the composite with 50 vol % ZnO dendritic nanostructures is -42 dB at 3.6 GHz with a thickness of 5.0 mm. The dendritic nanostructures which have isotropic crystal symmetry may find applications in a variety of fields. It is important in industrial manufacture. More importantly, it gives us a way to understand the growth mechanism deeply and to develop new hierarchical structures. 2. Experimental Section
Figure 3. (a) XRD patterns and (b) EDS of ZnO hierarchical nanostructures.
nanowire is not epitaxial. The growth conditions have been determined and the morphology evolution process of the ZnO branched hierarchical nanostructures has been characterized. PL properties of the ZnO dendritic nanostructures have been studied. Excellent microwave absorption performances have been ob-
The dendritic nanostructures were synthesized via a simple two-step chemical vapor deposition process. ZnO nanowires were first synthesized to serve as substrates (stems or trunks) on which nanorod branches were grown in the second step. Synthesis of the ZnO nanowire substrates and secondary growth of nanorod branches were both performed in a quartz tube (inner diameter ca. 35 mm, length ca. 800 mm) installed inside a conventional horizontal tube furnace, shown schematically in Figure 1a. A temperature gradient was thus established from the center to the end of the quartz tube, as shown in Figure 1b. The synthesis of single-crystal ZnO nanowires was described in detail in our previous work.19 For the second-step growth, zinc powder (purity of ∼99.99%) and the presynthesized ZnO nanowires which act as the substrate to grow ZnO nanostructures were located in a ceramic boat, in which the substrate was downstream from the zinc source. The ceramic boat was loaded into the inner quartz tube (inner diameter ca. 24 mm, length ca. 500 mm) at the central region of the furnace. After the quartz tube was evacuated by a mechanical rotary pump, the central region of the furnace was heated to a settled temperature of 700 °C with a increasing rate of 100 deg/min. High-purity Ar and O2 gases, whose flow rates were separately controlled by two flowmeters at Ar 100 sccm and O2 20 sccm, were introduced into the inner quartz tube. The temperature of the furnace was
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Figure 4. SEM images of ZnO hierarchical nanostructures synthesized with diverse duration of growth in the 700 °C region. Nanorods grown on nanowire stems (a) for 5 min (the inset shows the enlarged view), (b) for 10 min, (c) for 15 min (the inset is the enlarged view of as-synthesized ZnO hierarchical nanostructures), (d) for 25 min, (e) for 30 min, (f) for 40 min, (g) for 50 min through a fast cooling down process,and (h) for 60 min.
maintained at 700 °C for 5-60 min, and then the furnace was allowed to cool to room temperature. The as-synthesized hierarchical nanostructures were studied by XRD on a X’Pert PRO PHILIPS diffractometer, Cu KR irradiation (λ ) 1.54056 Å), field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800) equipped with energy dispersive X-ray spectroscopy (EDS), TEM and SAED were carried out on a Hitachi H-600 transmission electron microscopy operated at 100 kV, and HRTEM were carried out on a TECNAI F30 high-resolution transmission electron microscopy. A fluorescence spectrophotometer (FLS920T) equipped with a Xenon lamp was used for PL spectroscopy and the PL spectra were obtained at room temperature under an ambient atmosphere. The composite samples used for microwave absorption measurement were prepared by mixing the ZnO dendritic nanostructures with paraffin wax with 50 vol % of the ZnO. The
mixtures were then pressed into toroidal shaped samples (φout: 7.00 mm; φin: 3.04 mm). The complex permittivity and permeability of the mixtures in the 0.1-18 GHz frequency range were measured by using an Agilent E8363B vector network analyzer. 3. Results and Discussion 3.1. Structural Characterization, Analysis, and Growth Mechanism. Typical Field-emission scanning electron microscopy (FE-SEM) images of the as-synthesized products are given in Figure 2. Figure 2a shows the primary ZnO nanowires before secondary growth. The diameters of the primary ZnO nanowire substrates range from tens of nanometers to several hundred nanometers, and the lengths are up to a micron scale. Figure 2b shows a typical FE-SEM image of the secondarily grown ZnO nanorods on the primary ZnO nanowire substrates syn-
11770 J. Phys. Chem. C, Vol. 112, No. 31, 2008 thesized at 700 °C for 1 h. It is clearly seen that a high density of quite uniform well-aligned ZnO nanorods have been synthesized on the surface of the primary ZnO nanowires. From the enlarged view in the inset of Figure 2b, it can be seen that the secondarily grown ZnO nanorods do not have a uniform diameter along their axes, but become thinner toward the tipsthe diameters of the nanorods are 200-300 nm at the root, and have a uniform length about 5-6 µm. It also could be noticed that the hierarchical nanostructures have isotropic crystal symmetry and they have no 6-fold (or 4-fold, or 2-fold) symmetry as general epitaxial growth.5,7 The reason will be discussed later in this paper. TEM was used to investigate the microstructure of the ZnO hierarchical nanorods in more detail. The low-magnification TEM image of an isolated branched ZnO nanostructure in Figure 2c reveals one nanorod grown on the stem. The lattice-resolved HRTEM images of Figure 2c have not been obtained due to the slightly greater thickness of the junction region. The second-level nanorods would be broken off from the stem (Figure 2d) by ultrasonic treatment during TEM sample preparation. The SAED pattern in Figure 2e obtained from the circled area in Figure 2c indicates that the growth direction of secondary ZnO nanrod is along [0002]. Figure 2f shows the high-resolution TEM image of a representative branch/stem interface area of ZnO. It presents the detailed microstructure information between the primary ZnO nanowires and the secondary nanorods. It can be seen that the lattice fringe spacing of the ZnO stem is 0.52 nm and the branch is 0.26 nm, respectively, and both of them correspond to the Zn-terminated + c plane perpendicular to the growth direction. It is clearly seen that orientation relationship between the major ZnO core nanowire and the secondary ZnO nanorod is not epitaxial. There is a thin amorphous region between them, which may be caused by the rapid growth of initial stage of the secondary ZnO nanorod. We will present and discuss the growth model later. The XRD pattern (Figure 3a) reveals the crystal structure and phase purity of the ZnO dendritic nanostructures. All the diffraction peaks of the products match very well with those of wurtzite ZnO (a ) 3.253 Å, c ) 5.209 Å, JCPDS file No. 80-0075). The narrow peaks indicate the nanostructures are highly crystallized. The EDS spectrum in Figure 3b demonstrates the purity of the sample. Only zinc and oxygen are detected with the atomic ratio of Zn to O of 57.53:42.47 (1.35: 1). Figure 4 shows the SEM images of ZnO hierarchical dendritic nanostructures synthesized in the 700 °C region with various growth durations. Figure 4a shows a low magnification SEM image of the product kept for 5 min at 700 °C. It can be seen that a large quantity of nanodots grew on nanowire stems, and from the enlarged view of the inset, it can be concluded that the surface of the stems has been completely coated by the uniformly secondarily grown ZnO nanodots. The size of the nanodots is about 50-100 nm. When grown for 10 min (Figure 4b) and 15 min (Figure 4c), the nanodots became longer to form nanorods. It can be seen from the inset of Figure 4c that the diameter of the nanrods is about 100 nm and the length is about 300-500 nm. The nanorods all present sharp tips (Figure 4d) and their lengths are about 500-800 nm after 25 min of growth, and for 30 min (Figure 4e) they became longer and sharper. Compared with the pattern shown in Figure 4e, these rods in Figure 4f have thicker diameters. Figure 4g shows obtuse cantilevers of the secondary ZnO nanorods grown for 50 min, which come through a fast cooling down process by blowing cold air to the surface of the quartz tube. Figure 4h displays the SEM image of the sample grown for 60 min, which are
Zhuo et al. uniform and well-aligned pine-like hierarchical ZnO nanorods with a length of 5-6 µm and a diameter of 200-300 nm. Figure 4 presents the morphology evolution of secondary branched ZnO hierarchical nanorods synthesized on the primary ZnO nanowire substrates. The size and morphology of branches can be controlled by adjusting the duration of growth. It is clearly seen that they have isotropic crystal symmetry and do not have preferred growth orientations toward the stem, withal, the HRTEM presents that the relationship between the branches and the stem is not epitaxial growth. Other research groups obtained architectures with preferred growth directions which have 6-fold (or 4-fold, or 2-fold) structural symmetry because of homogeneous/heterogeneous epitaxial growth, and most of them reported a mild temperature-increasing rate in the gasphase growth process.5,7 Here we use a very high temperatureincreasing rate of 100 deg/min for the secondary rapid growth process. In our chemical vapor deposition experiment, ZnO crystal growth may undergo the following process. Zn powder is quickly vaporized and delivered into the primary ZnO nanowire substrates by the carrier gas Ar and reaction gas O2 when temperature rises rapidly, and then adsorbs on the surface of the nanowire substrates to form an absorbed layer of solute Zn atoms. The reactions take place between O2 gas and the absorbed Zn atoms on the surface of the primary nanowire substrate to form ZnO crystal particles, and then they diffuse and migrate to a suitable lattice to grow up. But the crystal particles do not migrate to certain lattices and grow up directly. They always undergo a nucleation and growing up process. It is well-known that the droplets can be formed in a supersaturated system as a stable nucleus, which is called spontaneous nucleation, but the presence of a suitable foreign body or “sympathetic” surface can induce nucleation more easily, which is called heterogeneous nucleation at degrees of free energy changing less than those required for spontaneous nucleation. In our experiment, the secondary ZnO nucleus formed on the surface of presynthesized ZnO nanowires is typical heterogeneous nucleation. The rate of nucleation, J, e.g., the number of nuclei formed per unit time per unit volume, can be expressed in the form of the Arrhenius reaction velocity equation commonly used to describe the nucleation rate of a thermally activated process:20a
[
J ) A exp
∆Edes - ∆Ed - ∆Gcrit kT
∆Gcrit ) f(φ) )
16πσ3ν2 f(φ) 3k3T3(InS)2
(2 + cos φ)(1 - cos φ)2 4
]
(1) (2) (3)
where ∆Gcrit is the critical nucleus excess free energy; ∆Edes is the atomic activation energy of desorption, ∆Ed is the surface diffusion activation energy, k is the Boltzmann constant, and T is the temperature. The angle φ is the angle of contact between the crystalline deposit and the foreign solid surface; σ is the interfacial tension, V is the molecular volume, and S is the degree of supersaturation. These equations indicate that the rate of nucleation was governed by four main variables: S, T, σ, and φ. The necessary condition to form epitaxial single-crystal growth is that the rate of nucleation in epitaxial direction is larger than nonepitaxial direction. We use J1 as the rate of nucleation in epitaxial direction φ1, J2 as the rate of nucleation in nonepitaxial direction φ2, from the eq (1), the ratio of the two nucleation rates β is given by20b
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β)
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J1 ) exp{B[f(φ2) - f(φ1)]} J2
(4)
16πσ3V2 3k3T3(InS)2
(5)
B)
Generally speaking, the interfacial specific surface energy in epitaxial direction is smaller than in nonepitaxial direction, thus φ2 > φ1, f(φ2) > f(φ1) and epitaxy takes place. When we have a relative high supersaturation S due to the high temperatureincreasing rate, the corresponding B and β decreased. In this situation, epitaxy has been restrained and the relationship between the secondarily grown nanorods and the primary ZnO nanowire is not epitaxial in the initial nucleation process. When the supersaturation decreases to a stable status, which may be a very quick process since the reactant zinc powder is microdosage and the tube was evacuated by a mechanical rotary pump, it approximately undergoes an equilibrium condition during the growth process, which would result in regular-shaped hexagonal cylinder arms. In Yumoto’s work21 of heating Zn bars to fabricate Zn crystal, no one-dimensional Zn crystals can be fabricated owing to the fast production of Zn vapor (high Zn supersaturation). Although some researchers reported that the relatively high-saturated vapor pressure benefits the growth of 1D nanostructures,22 low supersaturation benefiting the growth of the nanowires also has been proven by many groups.23–25 In the growth of nanowires, the relatively lower supersaturation is probably critical for whisker growth, which should be lower than that required for euhedral crystal growth, otherwise, two- or three-dimensional growth will occur.26,27 In a word, nucleation is very important in the chemical vapor deposition process to synthesize nanoscale crystals. When the supersaturation S is high or a lot of “active centers” exist, the system can have a high rate of nucleation and achieve the fastest growth speed, and then the nucleus could not grow up along certain crystallographic directions and approximately parallel to each other. In this situation, it could not form single-crystal, amorphous particle-shape congeries as is the tendency of the
growth. However, further investigation and modification about the growth model are still needed. To study the growth mechanism of the branches after the initial nonepitaxial growth stage, the morphplogy details of the secondary hierarchical nanorods are given in Figure 5. Figure 5a is the top view of the typical hierarchical nanorods grown for 60 min at 700 °C, which have a fascinating structure and symmetry. Panels b and c of Figure 5 are the enlarged views of Figure 5a. SEM results reveal that there are no nanoparticles observed on any tips of the secondarily synthesized ZnO nanorods which are single-crystal prominent hexagonal structures. Since the only source material used in our synthesis is zinc powders, it is likely that the growth of secondary ZnO nanorods is governed by the vapor-solid (VS) mechanism.28 From the insets the hexagonal growth plane and the growth step can also be clearly identified, providing strong evidence that it grows in a layer-by-layer stacking mode in the direction along [0001]. It is known that the uneven change along the length for the single nanostructure is related to the anisotropy of ZnO materials, and crystal planes with higher surface energy possess faster growth velocity.29 Tong et al.30 reported that the growth velocity of the (0001) plane is higher than that of the {1000} planes due to the different surface energy. As a result, when different crystal planes grow at different velocities, those surfaces with the faster growth velocities will continually decrease their area and the surfaces with slower growth velocities will gradually dominate the morphology of crystal, which leads to the growth of the hexagonal ZnO nanowire with a sharp tip in the [0001] direction. In fact, the length of the nanorods increases while the top area decreases gradually. Hence, the (0001) plane easily disappears, and it is bounded by the lower surface energy facet with higher Miller indices of the {011j0} surfaces. This growth pattern will produce a number of steps on the side surfaces. Therefore, the growth pattern of secondary ZnO nanorods is dominated by a kinetic process, due to the increase in both the surface/interface defects and the
Figure 5. SEM images of ZnO hierarchical nanowires synthesized in the 700 °C region. (a) Nanorods grown for 40 min, (b) grown for 50 min through a fast cooling down process (the inset shows an enlarged view of the stems tip), (c) top view of the hierarchical nanostructure which is grown for 60 min at the 700 °C region, (d) enlarged view of panel c (the inset shows one of the branches), (e) enlarged view of the center region of panel c (the inset shows the tip of the branch), and (f) the schematic illustration of the growth model of the single-crystal branch.
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Figure 6. SEM images of the secondarily grown ZnO nanostructures on the primary ZnO nanowire substrates in (a) the 600 °C region for 40 min, (b) the 650 °C region for 60 min, (c) the 680 °C region for 60 min, and (d) the 700 °C region for 60 min.
system energy in the ZnO nanorods.30 Figure 5d gives the schematic illustration of the growth model of the single-crystal branch. Different shapes and sizes of secondary ZnO nanostructures on the primary ZnO nanowire substrates have been observed from the samples obtained from different temperature regions. Figure 6a shows the SEM image of the product obtained in the temperature zone of 600 °C for 40 min, which is 80 mm away from the quartz tube center with tree-like appearance and lengths of 800 nm to 1 µm. At the higher temperature region of 650 °C grown for 60 min, bigger hierarchical structures were obtained, as shown in Figure 6b, and some nanowires with broken flaglike terminals were also found.. When changed to the 680 °C region grown for 60 min, the nanorods (Figure 6c) have diameters of 200-500 nm and lengths of 5-10 µm, which is longer than that shown in Figure 6b. Figure 6d shows an orderly array and high degree of crystallinity of the hierarchical nanrods, which are grown at 700 °C for 60 min. It is generally believed that the growth temperature and gas-phase supersaturation determine the growth rate of surface planes and the final morphology of the crystals, and higher temperatures favor the epitaxial growth of single-crystal nanostructures.3,31,32 3.2. PL Properties of ZnO Dendritic Nanostructures. The corresponding PL spectra of the primary and ZnO dendritic nanostructures are shown respectively in Figure 7. Figure 7a is the PL spectra of the primary ZnO, and panels a, b, c, and d of Figure 7b are the PL spectra of the secondary products obtained at different temperature regions, respectively. It can be seen that the PL spectra of the primary and secondary ZnO have nearly the same appearances. All spectra are composed of an ultraviolet (UV) emission centered at about 381 nm and a broad green emission centered at about 510 nm, as is clearly shown in Figure 7. The relative peak intensity ratio of the green-light emission to UV emission decreased as the temperature increased (Figure 7b). It is generally accepted that the UV emission of ZnO at 381nm corresponds to the recombination of free excitons between conductive band and valence band, which is called near-bandedge emission. It is also suggested that high crystallinity plays
Figure 7. The room temperature PL spectra of the ZnO (a) primary nanowires and (b) secondary hierarchical nanostructures synthesized at different temperature regions.
a key role in the enhancement of the UV emission.31,33–35 The green-light emission at 510 nm corresponds to the deep level emission. Deep level emission can have many origins such as single ionized oxygen vacancies,36 antisite oxygen,37 donor-acceptor complexes,38,39 and so on. In our work,
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Figure 8. Frequency dependence of (a) real (′) and (b) imaginary (′′) parts of relative complex permittivity for ZnO nanowires composite and dendritic nanostructures composite.
energy-dispersed spectroscopy (EDS) shows the element ratio of Zn:O is 1.35:1, which indicates that the hierarchical ZnO nanostructures are Zn-rich. Therefore, the main point defects in ZnO should be the oxygen vacancy (VO) defects.40 Associated with the EDS results, it is reasonable to conclude that in our experiment single ionized oxygen vacancies should be the most possible origin of the green-light luminescence.41 In addition, the point defect density of the ZnO nanorods grown under high temperature should be lower than those grown under low temperature.3,31 Therefore, when the temperature increased, the intensity of UV emission increased and the intensity of green-light emission decreased, respectively. 3.3. Microwave Absorption Properties of ZnO Dendritic Nanostructures/Paraffin Composite. In recent years, the ZnO nanostructures have shown great attraction for microwave radiation absorbing and shielding material in the high-frequency range due to their many unique chemical and physical properties.42 Some research works focused on ZnO and ZnO have proven them to be promising and vivid microwave absorption material.42,43 However, ZnO is probably the richest family of nanostructures among all materials, both in structures and properties.44 The microwave absorption property of ZnO dendritic nanostructres has not been studied so far. Thus, it is necessary to study the novel properties of these unique nanostructures and the intrinsic reasons for microwave absorption of the ZnO nanocomposites. Figure 8 shows the complex permittivity of the primary ZnO nanowires composite with 50 vol % ZnO nanowires and ZnO dendritic nanostructures composite with 50 vol % ZnO dendritic nanostructures versus frequency. The real permittivity of ZnO nanowires composite is about 5.0, and the imaginary permittivity is about 1.0. However, for the ZnO dendritic nanostructures
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Figure 9. (a) Simulation of reflection loss of 50 vol % ZnO nanowires and 50 vol % ZnO dendritic nanostructures composites with a thickness of 3.0 mm. (b) Simulation of reflection loss of 50 vol % ZnO dendritic nanostructures composite with different thicknesses.
composite, the ′ and ′′ values show a complex variation, the real part of relative permittivity (′) declines with increasing frequency from 30 to 15 in the 0.1-18 GHz range and exhibits a peak in the 13-17 GHz range (Figure 8a). The imaginary part of permittivity (′′) increases from 4 to 7 and the curve exhibits two broad peaks in the 0.1-13 and 13-17 GHz ranges (Figure 8b). It is worthy to notice that the peaks of the ′′ curve appear at 7.5 and 15.5 GHz, suggesting a resonance behavior, which is expected when the composite is highly conductive and skin effect becomes significant.46 Electronic spin and charge polarization due to point effect and polarized centers,47 for example, may have a profound effect on the response of ZnO nanostructures. The imaginary part ′′ of ZnO dendritic nanostructures is relatively higher in contrast to that of primary ZnO nanowires, which implies the distinct dielectric loss properties arising from the morphology variation. It is reasonable that the dielectric loss is attributed to the lags of polarization between the interfaces as the frequency is varied. ZnO dendritic nanostructures possess more complicated interfaces than nanowires. Thus, the complex hierarchical structures may own excellent dielectric loss properties. The reflection loss (RL) curves are calculated according to the following equations:45
Zin ) Z0(µrε)1⁄2 tanh[j(2πfd/c)(µrε)1⁄2]
(6)
RL(dB) ) 20 log |(Zin - Z0)/(Zin + Z0)|
(7)
where, f is the frequency of the EM wave, d the thickness of the absorber, c the velocity of light, Z0 the impedance of free space, and Zin the input impedance of absorber. According to eqs 6 and 7, the simulations of the reflection loss of the two composites with a thickness of 3.0 mm are shown in Figure 9a. The ZnO dendritic nanostructures composite
11774 J. Phys. Chem. C, Vol. 112, No. 31, 2008 possesses astrong microwave absorption property. However, the ZnO primary nanowires composite only gives weak absorption. Figure 9b shows simulations of reflection loss of ZnO dendritic nanostructures composite with different thicknesses. The value of the minimum reflection loss for the ZnO dendritic nanostructures composite is -42 dB at 3.6 GHz with a thickness of 5.0 mm. Our results are much better than those in the literature.42,43 According to the results shown above, ZnO dendritic nanostructures show very strong absorption of microwave compared with nanowires. It can be noticed that the ZnO dendritic nanostructures have special geometrical morphology. Such isotropic crystal symmetry can form isotropic quasiantennas and some incontinuous networks in the composites. It is available for the electromagnetic wave to penetrate the nanocomposites formed by the numerous antenna-like semiconductive ZnO dendritic nanostructures and the energy will be induced into a dissipative current, and then the current will be consumed in the incontinuous networks, which lead to the energy attenuation.43,48 More importantly, the interfacial electric polarization should be considered. The multi-interfaces between branches, paraffin matrix, and air bubbles can be a benefit for the microwave absorption because of the interactions of electromagnetic radiation with charge multipoles at the interfaces. Due to the large aspect ratio and the dielectric confinement effect of the ZnO dendritic nanostructures, the charge concentration at the tips of the branches is distinct when the material is under an electric field. Thus, it is reasonable that concentrated tips of the branches will act as multipoles that will be tuned with the incident microwaves and contribute to strong absorption.43 Compared with ZnO dendritic nanostructures, no complex frame exists in ZnO nanowires composite. The related interfaces are much less than that in ZnO dendritic nanostructures composite, and the other peculiar effects for microwave absorption, such as dissipation results from quasiantenna, multipoles due to charge concentration, and the multi-interfaces in the composites, can be ignored. However, further experimental and theoretical work is needed to make the mechanism clear. 4. Conclusion In summary, we have reported the secondary growth of wellaligned arrays of ZnO nanorods on the surface of the singlecrystal ZnO primary nanowire substrates by a two-step catalystsfree VS procedure. A reasonable mechanism has been presented to understand the rapid growth process. It is found that by tuning the temperature and duration of growth, the size and morphology of branches can be controlled, and the optical property could be correspondingly modulated. Excellent microwave absorption performances have been observed in ZnO dendritic nanostructures/paraffin composites. Our results pave a new way for the synthesis of hierarchical nanostructures with intriguing photoelectronic and microwave absorption properties. The dendritic nanostructures which have isotropic crystal symmetry may find applications in a variety of fields such as field emission, photovoltaics, transparent EMI shielding, supercapacitors, fuel cells, high strength, and multifunctional nanocomposites that require not only high surface area but also structural integrity, such as omnidirectional emitters, microtube cleansing devices, dendritic biosensors, etc. Acknowledgment. This work was supported by a fund from the National Natural Science Foundation of China (Grant No.
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