CRYSTAL GROWTH & DESIGN
Synthesis, Characterization, and Photocatalytic Application of Different ZnO Nanostructures in Array Configurations
2009 VOL. 9, NO. 7 3222–3227
Yang Liu,†,‡ Z. H. Kang,†,‡ Z. H. Chen,† I. Shafiq,† J. A. Zapien,† I. Bello,*,† W. J. Zhang,† and S. T. Lee† Center Of Super-Diamond and AdVanced Films (COSDAF) & Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong SAR, China, and Functional Nano & Soft Materials Laboratory (FUNSOM), Suzhou UniVersity, Suzhou, Jiangsu, China ReceiVed NoVember 26, 2008; ReVised Manuscript ReceiVed March 25, 2009
ABSTRACT: Arrays of well-aligned one-dimensional ZnO nanostructures (nanowires, nanorods, nanoribbons, nanobuds, and flocky nanorods) with high aspect ratios have been grown on zinc substrates by a solution-phase method using a mixture of ethylenediamine, ethanol, and water. The morphology of the ZnO nanostructures has been modulated by controlling the concentration of ethylenediamine and ethanol and regulating the reaction temperature. Chemical and structural analyses and emission spectra show that the arrays of ZnO nanorods favor nearly stoichiometric composition and good crystallization quality, whereas the arrays of ZnO nanowires, nanoribbons, nanobuds, and flocky nanorods confine a considerable amount of oxygen vacancies. The photocatalytic effect investigated at decomposition of methyl red correlates with the defect-related emission properties of these nanoarrays. Particularly ZnO nanobuds and flocky nanorods arrays have been found to be effective photocatalysts. Introduction The interest in the fabrication of one-dimensional (1D) ZnO nanostructures, such as nanowires, nanoribbons, nanorods, and nanotubes, is driven by their exceptionally unique optical, electronic, and mechanical properties that give a wide range of applications. For instance, well-aligned and patterned arrays of 1D ZnO nanostructures have potential use in electronic, optoelectronic, and sensing devices. It was demonstrated that 1D ZnO nanoarrays on proper substrates can emit coherent ultraviolet light at room temperature.1 They can also be used for construction of dye-sensitized solar cells,2 reversible switching of superhydrophobicity to superhydrophilicity,3 piezoelectric nanogenerators,4,5 photocatalysts,6,7 and field-emission (FE),8 and nano-optoelectronics and nanosensing devices.9 Thus, the design and controlled synthesis of 1D ZnO nanoarrays with different morphological configurations on a large scale represent very attractive scientific and technological problems to be solved. Since the first report on the ultraviolet lasing of ZnO nanorods,1 substantial effort has been devoted to the development of novel synthetic methodologies for 1D ZnO nanostructures and their arrays.10-32 The principal techniques used for growing 1D ZnO nanoarrays mainly include template-assisted growth,6,10,11 noble metal catalytic growth,1,7,12-16 and chemical and physical vapor deposition.17-22 Although these methods can produce high-quality ZnO nanostructures, they often confront problems of templates/catalyst removal, tedious operation procedures, high energy consumption, poor adhesion of nanostructures to the underlying substrates, etc. Recently, chemical solution-based processes were found to be more attractive because of the moderate temperatures, simple manipulations, and great potential for scaling up the technology. Notably, directly growing arrays of oriented metal oxide nanostructures on metal substrates was demonstrated by the surface oxidation of metal foil in solution.23-32 This method facilitates the * To whom correspondence should be addressed. E-mail: apibello@ cityu.edu.hk. † City University of Hong Kong. ‡ Suzhou University.
simplicity, efficiency, and low cost of preparing oxide nanomaterials and provides the possibility of extrapolating it to device fabrication. In principle, patterned nanowire arrays created using this method can be electrically addressed by the supporting substrate electrodes. As a result, we have synthesized arrays of ZnO nanorods/tubes and CuO nanowires on zinc and copper foils, respectively, employing solution-based processes.27,31,32 Inspired by these works, in this study, we have grown well-aligned 1D ZnO nanostructures arrays over large areas by direct oxidation of Zn substrates in an aqueous solution of ethylenediamine. The shape of the ZnO nanostructures has been modulated from wires to ribbons, rods, and buds. In addition to chemical and structural properties, the photocatalytic activities of these arrays have also been examined. Experimental Section In a typical procedure, a piece of clean zinc foil was immersed into a solution of ethylenediamine (12 mL), ethanol (10 mL) and water (13 mL). The solution placed in a Teflon-lined stainless steel autoclave (50 mL) was heated to and kept at a constant temperature of 170 °C for 30 h. After the hydrothermal treatment, the resulting zinc foil was taken out and thoroughly rinsed with ethanol and dried in air for further characterization. Unless specifically indicated, the proportional volume of water has been controlled to provide required volume concentration of ethylenediamine and ethanol keeping the total volume of solutions (35 mL) constant. The photocatalytic activity of the prepared ZnO nanoarrays for decomposing or removing methyl red (MR) in aqueous solution were investigated. The evaluation of the photocatalytic activity was carried out in a photochemical reactor made of quartz at room temperature. The ZnO nanoarrays, on substrates of equivalent sizes, were immersed into 100 mL MR solutions (2.5 × 10-4 mol/L) that were subsequently irradiated using a UV lamp (35 W; 4.0 mW/cm2 at a distance of 6 in.). At different time intervals, 5 mL samples were withdrawn for analysis using a UV-vis spectrophotometer. The degradation efficiency η ) (1 - ([MR])/([MR0])) × 100% of MR was calculated, where [MR0] and [MR] are the equilibrium concentration of MR before and after UV irradiation. Here, the content of MR is in linear proportion to the absorption (A), then ([MR])/([MR0]) ) (A)/(A0). The morphology and structure of the ZnO samples were analyzed with a Philips XL30 FEG scanning electron microscope (SEM) and a FEI/Philips Techal 12 BioTWIN transmission electron microscope
10.1021/cg801294x CCC: $40.75 2009 American Chemical Society Published on Web 05/14/2009
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Figure 1. SEM images and structure of synthesized ZnO nanowires array grown on zinc foil: (a) large view field of ZnO nanowires; (b) magnified image of ZnO nanowires; (c) cross-sectional view indicating growth direction of ZnO nanowires; (d) XRD pattern corresponding to the ZnO structures in SEM images. (TEM), whereas high-resolution transmission electron microscopic (HRTEM) analysis was performed with a CM200 FEG TEM at 200 kV. X-ray diffraction (XRD) patterns were recorded by a Siemens D500 diffractometer. The photoluminescence (PL) spectra were obtained at room temperature employing a Perkin-Elmer Luminescence spectrometer LS50B. The UV-vis absorption spectra were acquired by an Aglient 8453 UV-vis Diode Array spectrophotometer.
Results and Discussion Morphological analyses of formed white overlayers reveal furlike arrays of highly uniform nanowires grown on the Zn substrate as illustrated by the large-area SEM image in Figure 1a. The image with higher magnification in Figure 1b shows high density of nanowires with a diameter of 120 nm on average, whereas the cross-sectional view, in Figure 1c, indicates that these ZnO nanowires tend to be perpendicular to the substrate surface at their roots. The considerable length of the nanowires (3 µm) causes slanting in their top regions. The XRD pattern of the product layer (Figure 1d) corresponds to a typical diffraction pattern of pure Wurtzite ZnO phase denoting P63mc space group as cross-referenced to a JCPDS 36-1451 card. The enhanced (002) diffraction peak at 2θ ) 34.48° indicates the preferential orientation of the crystals along the c-axis of the ZnO nanostructures, perpendicular to the substrate surface. The chemical compositional analysis of the ZnO nanowires array reveals only Zn and oxygen constituents in a nearly stochiometric ratio (atomic ratio Zn/O ≈ 1:1). A slight oxygen deficiency of ZnO structures therefore may cause oxygen vacancies in these ZnO nanostructures. Figure 2 shows a typical TEM image of a single free-standing ZnO nanowire abstracted from the array of nanowires by sonication. The nanowire has a uniform diameter along its entire
Figure 2. TEM image of a single ZnO nanowire; corresponding SAED pattern in the upper inset; and HRTEM image of a single ZnO nanowire in the lower inset.
length, which indicates that the growth anisotropy along the +c axis is strictly maintained over the whole growth process. The absence of branching implies that the ZnO nanowires grow with high crystal perfection23 in a growth process following spontaneous nucleation. The selected area electron diffraction (SAED) pattern and the corresponding high-resolution TEM image of the nanowire reveal its single-crystalline nature. These analyses show the growth orientation along the c-axis and lattice spacing of 0.52 nm corresponding to the interplanar spacing of the (001) crystallographic planes which indicate the growth of the ZnO nanowires along the [001] direction. Preparation at a lower reaction temperature (140 °C) results in the growth of ZnO tapered nanoribbons arrays as seen in
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Figure 3. SEM images of an ZnO tapered nanoribbons array grown on a zinc foil at a lower reaction temperature (140 °C): (a) large view field of ZnO tapered nanoribbons array; (b) ZnO array of tapered nanoribbons taken at higher magnification; and (c) array of ZnO tapered nanoribbons taken at a cross-sectional view.
Figure 4. SEM images of synthesized ZnO nanorods array grown on a zinc foil grown with 5 mL of ethanol at 170 °C, showing (a) uniform nanostructures over a large view field of the sample; (b) faceted hexagonal nanorods suggesting a single-crystalline nature of the nanorods.
Figure 3a. The width of the tapered nanoribbons at their bases is larger than that at the top and their lengths are nonequivalent (Figure 3b). Some portion of the tapered nanoribons tends to be oriented vertically to the substrate surface, but the majority of them deviate from the surface normal in some angle degrees as shown in Figure 3c. It is well-known that the Ostwald ripening becomes more operative during the synthesis of nanostructures using solution methods.27 High temperature not only enhances the crystallinity and the growth rate along the [001] direction of ZnO nanostructures, but can also enhance the uniformity of Ostwald ripening around the c axis of 1D ZnO nanostructures. In our case, ZnO nanowires with long length and uniform diameter can be obtained at the higher reaction temperatures (170 °C). In contrast, at a lower reaction temperature (140 °C), the reaction proceeds more slowly and therefore the ZnO nuclei can grow in the lateral directions thus resulting in the formation of nanoribbons. The volume concentrations of ethanol and ethylenediamine have been found to be critical parameters controlling the morphology of the product layer. The growth process with a small amount of ethanol (5 mL) yields large-area arrays of nanorods as illustrated in Figure 4a. The magnified image of a selected area of these arrays, in Figure 4b, shows a faceted hexagonal-prism morphology of the rod sections, suggesting single-crystalline structure of the rods. It has been demonstrated that the alcoholic environment is crucial in ensuring the formation of [ZnO2]2- ions and a controlled release of these species from the alcohol/water mixed phase to the growing 1D ZnO nanostructures.26,33 On the other hand, short-line alcohols have the ability to reduce the surface energy of the solution. So when the volume of ethanol decreases from 10 to 5 mL, the surface energy of the nanosized ZnO nuclei will increase to
Liu et al.
Figure 5. SEM images of a synthesized ZnO nanobuds array, grown on a zinc foil with 8 mL ethylenediamine at 170 °C, indicating (a) high density, evenly distributed nanostructures over a large view field of the sample; (b) nanorods terminated with budlike structures.
Figure 6. SEM images of a synthesized ZnO flocky nanorods array grown on a zinc foil, grown with 8 mL ethylenediamine at 140 °C, showing (a) uniform distribution of nanorods over a large view field of the sample; (b) flocky nanostructures on parental nanorods; and (c) a single parental nanorod uniformly covered by secondary small flocky nanorods with radial orientation.
some extent, thus restricting the growth rate of 1D ZnO nanostructures along the preferred c axis direction. Consequently, the morphology of ZnO nanostructures turns from nanowires to nanorods. Nanostructures were also grown with a small amount of ethylenediamine (8 mL). The SEM images of the corresponding samples are shown in Figure 5. Interestingly, an array of budlike structures with six flocky petals was developed on the top of the nanorods as shown in Figure 5b. Decreasing the reaction temperature to 140 °C and keeping the amount of ethylenediamine at 8 mL led to turning the deposit to nanorods with flocky structures as shown in Figure 6. The magnified SEM image, in Figure 6c, clearly shows that the flocky structures are secondary ZnO nanorods with uniform density and small sizes, grown on the main parental nanorods in radial directions. The diameter and length of the secondary nanorods are 30 and 180 nm, respectively. To understand the growth of ZnO structures in an ethylenediamine solution with a Zn foil and the role of the aqueous solution composition, we propose the following reactions as the primary mechanisms governing the growth of ZnO nanostructures
Zn + O2 + 2H2O f Zn2+ + 4OH-
(1)
Zn2+ + 2 en f [Zn(en)2]2+
(2)
[Zn(en)2]2+ + 4OH- f [ZnO2]2- + 2en + 2H2O (3) [ZnO2]2- + H2O f ZnO + 2OH-
(4)
Here, bidentate ethylenediamine [(en)2] does not only offer a basic medium, but the mixture of ethylenediamine/ethanol/water
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Figure 7. Photoluminescence spectra in UV (left) and visible (right) wavelength bands acquired from the synthesized arrays of (a) ZnO nanorods; (b) ZnO nanowires; (c) ZnO nanoribbons; (d) ZnO nanobuds; and (e) ZnO flocky nanorods grown on zinc foils.
also plays a major role in enhancing the growth rate along the c axis of 1D ZnO nanostructures. As mentioned above, the alcoholic environment is crucial in controlling the release of [ZnO2]2-species from the alcohol/water mixed phase to the growing 1D ZnO nanostructures.26,33 Moreover, in the case of adding a neutral ethylenediamine molecule, the electrostatic force between the positive polar plane of (0001) and the negative [ZnO2]2- growth unit is much stronger than the adsorption affinity between the (0001) plane and the neutral ethylenediamine molecule. Therefore, adsorption of [ZnO2]2- and the ethylenediamine molecule should occur on the (0001) face and the lateral {101j0} surfaces group, respectively. Thus, the radial enlargement of the rods may largely be inhibited because of the strong chelating ability of ethylenediamine toward the divalent zinc species. As a result, the crystal growth rate along the c-axis is enhanced.27,34 At the same time, the solution pH may change with the amount of ethylenediamine changing, and the start time to precipitate ZnO will be influenced under different pH.35,36 In our case, the solution pH will decrease with decreasing the volume of ethylenediamine from 12 to 8 mL, which accelerates the speed of reaction 4 toward the right, and more ZnO nuclei will then precipitate simultaneously. As a result, ZnO nanobud array with a larger diameter is obtained. Room-temperature photoluminescence (PL) spectra induced by 244 nm excitation wavelength (second harmonic of the 488 nm laser line of a CW argon laser) were collected in the UV range from 360 to 480 nm in Figure 7 (left) for the different ZnO nanoarray morphologies presented above. The spectra collected from the arrays of rods, wires, ribbons, buds, and flocky rods (curves a-e, respectively) are characteristic of ZnO nanostructures with different contributions of near band gap emission at ∼380 nm and visible emission (>450 nm). Clearly, the spectrum of ZnO nanorods arrays (curve a) is dominated by band gap emission peak at ∼380 nm and very weak green emission. For ZnO nanowires, nanoribbons, nanobuds, and flocky nanorods arrays, the intensity of the UV emission reduces, respectively, while at the same time the visible emission, in Figure 7 (right) increases (curves b-e). The UV emission is attributed to the near band-edge emission of ZnO, which is namely associated with the recombination of free excitons through an exciton-exciton collision process.37 On the other hand, the visible spectral emission in Figure 7 (right) is related to singly ionized oxygen vacancies in ZnO. This emission originates in recombination of photogenerated holes with single charge states of these defects.38 Its intensity is determined by the concentration of oxygen vacancies in the ZnO crystal.39 The wavy spectral structures are induced by spectral collection using notch filters. However, the important feature is that whenever
Figure 8. Degradation rate of methyl red, in linear (top) and semilogarithmic (bottom) scales, assisted with arrays of (a) ZnO nanorods; (b) ZnO nanowires; (c) ZnO nanoribbons; (d, d*) ZnO nanobuds; (e, e*) ZnO flocky nanorods.
the band gap emission in the UV band is weaker, the emission in visible band is more intense (Figure 7). The higher relative intensities of the visible peak to UV peak point at higher defect density in the nanostructure. Deviation from the illustrated trend is observed in the relative intensities of the spectra collected from ZnO nanobuds and ZnO flocky nanorods (lines d and e). This discrepancy might be caused by higher crystallinity of nanobuds at their roots giving raise to more intense UV peak (curve d) than that (curve e) of flocky nanorods, which are entirely covered by tiny radial nanowires. The intense UV and weak visible emission induced in ZnO nanorods reasonably indicates their good crystallization quality and near stoichiometric nature. However, ZnO nanowires, nanoribbons, nanobuds, and flocky nanorods emissions are characteristic of higher defect densities. The dramatic differences in the PL emission spectra of these nanoarrays synthesized under different conditions show that the optical properties of ZnO crystals are very sensitive to the morphology, defect densities, and thus the preparation conditions. The photocatalytic activity of the prepared ZnO arrays on the MR degradation was also investigated. Figure 8 shows the degradation rates of MR using the prepared ZnO nanoarrays. After UV irradiation for 5 h, the degradation efficiency of MR was found to be about 14.6% (curve a), 38.1% (curve b), and 49.6% (curve c) when assisted with the arrays of ZnO nanorods, nanowires, and nanoribbons, respectively. On the other hand, using arrays of ZnO nanobuds and flocky nanorods on the substrates of equivalent size led to the 100% degradation of the identical MR solutions after UV irradiation for 5 h and 3.5 h, respectively (curves d and e). Obviously, the prepared ZnO arrays of nanobuds and flocky nanorods can work as effective photocatalysts. Although the surface photochemical reaction is the actual conversion process in MR degradation, it occurs in sequence
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of dispersion, diffusion, adsorption surface reaction, and final desorption. Each of these elementary processes can affect the surface reaction rate. The dispersion is the step process determining the heterogeneous photocatalytic reaction to a large extent. After the elimination of the diffusion effect by controlled stirring, the overall rate of the photocatalytic reaction is determined by the surface reaction rate assuming that adsorption and desorption rates are relatively high. In such a case, the reaction rate is expressed by the folllowing equation
r ) kθMRθZnO
(5)
where k is the rate constant of surface reaction, θMR is the MR coverage of ZnO nanoarrays surface, and θZnO is the coverage of surface active centers of the considered ZnO nanoarrays. The coverage θZnO can be constant for equivalent ZnO nanoarrays. When the MR adsorption is weak, the MR coverage θMR can be determined from Langmuir equation, which is θMR ) (KMR[MR])/(1 + KMR[MR]), where [MR] is the concentration of methyl red and KMR is the adsorption equilibrium constant of MR on the ZnO nanoarray surface. If we denote k′ ) kθZnO, eq 1 can be written in the form
r)
kKMR[MR] k′KMR[MR] θZnO ) 1 + KMR[MR] 1 + KMR[MR]
(6)
Hence at a low [MR] concentration when KMR[MR] , 1, the reaction rate is r ) k′KMR[MR]. Substitution for the reaction rate r ) -(d[MR])/(dt) into eq 2 and separation of variables and integration of the resulting equation gives
ln [MR] ) -k′KMRt + C1
(7)
which shows that the logarithm of MR concentration, ln [MR], is a linear function of the reaction time t, which is the first order reaction. Considering the initial condition at time t ) 0 when MR concentration is [MR0], the integration constant C1 can be determined by substitution of the initial conditions into the calculated integral value (3), which gives C1 ) ln[MR0]. Thus introduction of the determined constant C1 into eq 3 and a couple algebraic operations yields
ln [MR]/[MR0] ) -k′KMRt
(8)
The plot of this function is a line with a slope of - k′KMR, which represents one limiting case, i.e., the first-order reaction (line d*, after 3.5 h; line e*, after 2 h). Thus the product of the rate constant of surface reaction k and adsorption equilibrium constant KMR determines the slope of the lines d* and e* indicated in the right plot of Figure 8, and is - k′KMR ) -1.5650/h (for line d*) and -1.3176/h (for line e*). The different k′KMR values result from the surface structure of dissimilar ZnO nanoarrays, leading to the different adsorption equilibrium of MR on the ZnO nanoarrays surfaces. On the other hand, when concentration [MR] is high, then KMR[MR] . 1, adsorption of MR on catalyst surface reaches the saturated state, i.e., coverage θMR ≈ 1, and thus r ) k′ ) -d[MR]/dt. Hence the methyl concentration, [MR] ) -k′t +C2, is a linear function of the reaction time t denoting the second limiting case, which is the zeroth-order reaction (line a, 0-5 h; line b, 0-3.5 h; line c, 0-3 h; line d, 0-2 h; line e, 0-1 h). Again, the initial condition of the concentration [MR]0 of methyl red at reaction time t ) 0 enables us to determine the integration constant, which is C2 ) [MR0], and substitutions to the solution of the integral gives
[MR] - [MR0] ) -k′t
(9)
Because the plot of this function is a line with the slope of -k′, the slope determines the reaction constant. Obviously at optimal value of the MR concentration [MR], the reaction characteristic is between zeroth- and first-order reactions. Following the reaction kinetics discussed above, the MR degradation rates are plotted in Figure 8 using linear (top) and semilogarithmic (bottom) scales. In descending order, the highest degradation rate, in Figure 8, exhibits the array of flocky nanorods, then nanobuds, nanoribbons, nanowires, and the lowest degradation rate shows the array of faceted nanorods. Because the degradation rate (curve a) assisted by ZnO faceted nanorods is a straight line in a linear scale plot over entire time zone of 5 h, the MR degradation follows zeroth-order reaction. For example, curve e in Figure 8 (top) shows that the photocatalytic reaction rate is initially zeroth order (apparent kinetics), but changes to first-order (apparent kinetics) at a longer reaction time as MR concentration reduces. The tangent to e graph in Figure 8 (top) denotes zeroth-order reaction from 0 up to 1 h, whereas the tangent to the curve e* in a semilogarithmic scale, in Figure 8 (bottom), indicates the first-order reaction between the second and third hour. The transition from the zeroth to the first-order reaction is between the first and second hour. All other reactions are similar but with different time zone of the zero-first-order transition. The illustrated sequential degradation rates (capacity) of nanostructures correlates with defect related emission properties (Figure 7) of the prepared nanostructures. In general, the photocatalytic activity of a catalyst for the degradation of pollutants is related to its band gap energy because of their nanosized structures and high surface area.7,40,41 The catalytic capacity of different ZnO nanoarrays is associated with the actual surface areas because of the sized effect. On the basis of the morphological features, it can easily be recognized that the arrays with ZnO nanorod flocky structures represent the largest effective surface area of all the investigated nanostructures herein. It is therefore not surprising that this structure enhances the photocatalytic capacity to some degree, when compared to other investigated structures. Recent work of Bohle and Spina42 reports that the oxygen defect-related vacancy in ZnO nanostructures promotes photocatalytic ability. Accordingly, the higher density of oxygen defect-related vacancies in ZnO flocky nanorods array (Figure 7e) should be the second reason for greater photocatalytic capacity of these structures. The oxygen vacancy in ZnO nanostructures can serve as the electron capturing center, which will restrain the combination of photogenerated electrons (eCB-) and cavities (hVB+). In addition, the oxygen vacancies assist in the generation of active species on the surfaces of ZnO nanostructures and are therefore beneficial to the photodegradation of organic dye. Photooxidation may occur by an indirect pathway involving hydroxyl radicals as the oxidizing intermediate43 as follows + ZnO + hν f ZnO(eCB + hVB )
(10)
+ f H+ + •OH H2O + hVB
(11)
dye + •OH f oxidation products
(12)
whereas on the surface of ZnO nanostructures, oxygen is also reduced via following possible reactions: eCB- + O2 f O2-;
ZnO Nanostructures in Array Configurations
O2- + H+ f HO2•; HO2• + O2- + H+ f H2O2 + O2; H2O2 + eCB- f OH- + •OH; and dye + •OH f oxidation products. Conclusions In this paper, we report the growth of well-aligned onedimensional ZnO nanostructure (nanowires, nanorods, nanoribbons, nanobuds, and flocky nanorods arrays) arrays using oxidation reactions of zinc substrates in an ethylenediamine/ ethanol/water mixture under hydrothermal conditions. By altering some reaction parameters, such as the amounts of ethylenediamine and ethanol and the reaction temperature, we have manipulated the morphology of the 1D ZnO nanostructures. The photoluminescence spectra of the nanoarrays indicate that the ZnO nanorods arrays grow with nearly stoichiometric composition and good crystallization quality. However, a higher density of oxygen vacancies are present in ZnO nanowires, nanoribbons, nanobuds, and flocky nanorods arrays as deduced from PL analysis. The photocatalytic activity at decomposition of methyl red indicates the photocatalytic capacity of these ZnO nanoarrays is consistent with the significant defect-related emission property of these nanoarrays. The flocky structures on the surface of the nanorods array increase the effective surface area, which directly results in the good photocatalytic activity of the ZnO flocky nanorods array. It is reasonable to expect that these 1D ZnO nanoarrays may possess other interesting properties. Finally, the presented method is fairly simple and it can easily be scaled up to fabricate various ZnO and other metal oxides nano/microstructures. Acknowledgment. The work was fully supported by the Research Grant Council of Hong Kong under project number CityU 110208, and we also acknowledge use of the facilities funded by instrument CAV grant CityU 1/03C.
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