Large-Scale Fabrication of Sub-20-nm-Diameter ZnO Nanorod Arrays

May 22, 2009 - We report a very simple and mild wet-chemical synthesis of ZnO nanorods to fabricate sub-20-nm-diameter ZnO nanorod arrays. These large...
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J. Phys. Chem. C 2009, 113, 10452–10458

Large-Scale Fabrication of Sub-20-nm-Diameter ZnO Nanorod Arrays at Room Temperature and Their Photocatalytic Activity Seungho Cho,† Semi Kim,‡ Ji-Wook Jang,† Seung-Ho Jung,† Eugene Oh,† Bo Ram Lee,† and Kun-Hong Lee*,† Department of Chemical Engineering, and Department of Mechanical Engineering, Pohang UniVersity of Science and Technology (POSTECH), San 31, Hyoja-Dong, Nam-Gu, Pohang, Gyungbuk, Korea 790-784 ReceiVed: February 25, 2009; ReVised Manuscript ReceiVed: April 24, 2009

We report a very simple and mild wet-chemical synthesis of ZnO nanorods to fabricate sub-20-nm-diameter ZnO nanorod arrays. These large-scale arrays could be obtained by immersing a Zn sheet in an ammonia aqueous solution containing Al3+ ion at room temperature (25 °C) and normal atmospheric pressure (1 atm). This method uses little energy and requires no complex experimental procedures or equipment. The Zn ions were provided by the Zn sheet; as a consequence, the growth process is self-limiting. The ZnO nanorods produced were ∼250 nm long and ∼18 nm in diameter. On the basis of the results, we propose a mechanism for the growth of the ZnO structures on the Zn sheet with Al3+ ion-containing ammonia aqueous solution. The high surface-to-volume ratio of the sub-20-nm-diameter ZnO nanorod arrays results in enhanced photocatalytic activity. Introduction Zinc oxide (ZnO) is a II-VI semiconductor that has a wide direct band gap of 3.37 eV at room temperature and a high electron-hole binding energy of about 60 meV. In addition, ZnO shows useful characteristics, such as a large piezoelectric constant and easy electrical conductivity modification. Direct synthesis of quasi-one-dimensional (1D) ZnO nanostructures on a substrate is highly desirable in various applications, such as field effect transistors,1 chemical sensors,2 field emitters,3 transparent conductors,4 ultraviolet light emitting devices,5 and photocatalytic processes.6 Various techniques have been used to synthesize 1D ZnO nanostructures, such as hydrothermal synthesis,7 thermal evaporation,8 chemical vapor deposition (CVD),5 microwave-assisted solution phase reaction,9 sol-gel process,10 and metal-organic chemical vapor deposition (MOCVD).11 Among them, the vapor-phase techniques are major physical approaches to fabricate 1D ZnO nanostructures. However, they generally require expensive equipment, complex process control, and stringent reaction conditions such as high temperature (450-900 °C) and low or high pressure. Also, they often produce nanostructures with poor uniformity and can have low yield.12 The solution chemical approaches can allow the growth of ZnO crystals at lower temperatures (60-200 °C), and can be used for large-scale production.7,13-16 However, for the solution synthesis of quasi-1D ZnO nanostructures on a substrate, low-pressure deposition processes such as evaporation and radio frequency magnetron-sputtering techniques are normally used to prepare ZnO seed or lattice mismatch buffer layer coated substrates for subsequent liquid-phase growth of ZnO nanorods.7,15,16 These processes are energy intensive and require specialized equipment. It is highly desirable to develop total chemical syntheses which operate near room temperature under normal atmospheric * To whom correspondence should be addressed. E-mail: ce20047@ postech.ac.kr. † Department of Chemical Engineering. ‡ Department of Mechanical Engineering.

pressure for the purposes of mimicking natural mineralogical or biological processes13,17 and reducing the amount of energy required to fabricate the devices. Methods of preparing small diameter ZnO nanorod arrays with high crystallinity under room temperature and normal atmospheric pressure are also desired, because such conditions allow control of band structures in the quantum confinement regime18 and increase the surface-tovolume (S/V) ratio to increase effective surface area for catalytic applications of ZnO. To the best of our knowledge, there have been no reports on the fabrication of ZnO nanorod arrays under room temperature and normal atmospheric pressure without adding Zn salts, such as zinc acetate, zinc nitrate, zinc chloride, and zinc phosphate, to the reaction solution. In this paper, we use a novel method of fabricating sub-20-nm-diameter ZnO nanorod arrays at room temperature (25 °C) and normal atmospheric pressure (1 atm). To avoid the step of seed layer deposition, a Zn sheet was used as a substrate. Al salt was added to an aqueous ammonia solution and a Zn sheet was immersed in the solution. The surface of the sheet became covered with sub-20-nm-diameter ZnO nanorods. Thus, this method to fabricate ZnO nanorod arrays uses little energy and requires no complex experimental procedures or expensive equipment. Zn ions were provided by the Zn sheet; therefore the growth process is self-limiting. As a result, ZnO nanorods synthesized by this method are ∼250 nm long and ∼18 nm in diameter. On the basis of the results of this fabrication and control experiments, we propose the mechanisms for the growth of the ZnO structures on the Zn sheet with Al3+ ion-containing ammonia aqueous solution. We also measure the photocatalytic effects of the fabricated nanorod arrays. High S/V ratio of the sub-20-nm-diameter ZnO nanorod arrays results in enhanced photocatalytic activity. Experimental Section Fabrication of Sub-20-nm ZnO Nanorod Arrays (Aging in an Aqueous Ammonia Solution with Al Salt, Sample A). All chemicals used in this work were of analytical grade and were used without further purification. A 500 mL transparent

10.1021/jp9017597 CCC: $40.75  2009 American Chemical Society Published on Web 05/22/2009

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TABLE 1: Sample Preparation Conditions experiments

metal salt dissolved in the solna

ammonia-water (mL)

HMT (g)b

deionized water (mL)

reaction temp (°C)

reaction time (h)

initial pH of solution

sample A sample B sample C sample D sample E sample F

ANN (2.34 g) none none CND (1.82 g) ANN (9.36 g) ZNH (2.98 g)

25 25 0.15 25 25 0

0 0 0 0 0 0.35

475 475 500 475 475 500

25 25 25 25 25 90

24 24 24 24 24 2

10.6 10.9 10.6 10.6 10.2 6.8

a

ANN: aluminum nitrate nonahydrate. CND: cobalt nitrate dehydrate. ZNH: zinc nitrate hexahydrate. b HMT: hexamethylenetetramine.

aqueous solution containing aluminum nitrate nonahydrate (Al(NO3)3 · 9H2O, 99.8%, Aldrich, 2.34 g) and ammonia-water (28.0-30.0 wt %, Samchun, 25 mL) was made at room temperature (pH 10.6). A cleaned Zn sheet (Nilaco, 99.5%, feature size ∼3 × 3 cm2, about 0.25 mm in thickness) was transferred into the as-prepared solution and the Zn substratecontaining solution was maintained at 25 °C under constant stirring for 24 h. The substrate was washed by ultrasonication in deionized water for 10 min, after which it was dried in an oven at 60 °C for 12 h. Control Experiment I (Aging in an Aqueous Ammonia Solution without Al Salt, Samples B and C). To prepare a solution with the same ammonia concentration (sample B), a 500 mL transparent aqueous solution containing ammonia-water (25 mL) was made at room temperature (pH 10.9). A Zn sheet was transferred into the solution and the Zn substrate-containing solution was maintained at 25 °C under constant stirring for 24 h. The substrate was washed by ultrasonication in deionized water for 10 min to remove the substances physically and weakly attached to the substrate, after which it was dried in an oven at 60 °C for 12 h. To prepare the same pH solution without Al salt (sample C), a 500 mL transparent ammonia aqueous solution (pH 10.6) was made at room temperature by adjusting the amount of ammonia-water added. A Zn sheet was transferred into the solution and the Zn substrate-containing solution was maintained at 25 °C under constant stirring for 24 h. The substrate was washed by ultrasonication in deionized water for 5 min, after which it was dried in an oven at 60 °C for 12 h. Control Experiment II (Aging in an Aqueous Ammonia Solution with Co Salt, Sample D). A 500 mL transparent aqueous solution containing cobalt nitrate hexahydrate (Co(NO3)2 · 6H2O, 98%, Aldrich, 1.82 g) and ammonia-water (25 mL) was made at room temperature (pH 10.6). A Zn sheet was transferred into the solution and the Zn substrate-containing solution was maintained at 25 °C under constant stirring for 24 h. The substrate was washed by ultrasonication in deionized water for 10 min, after which it was dried in an oven at 60 °C for 12 h. Control Experiment III (Aging in an Aqueous Ammonia Solution with High Concentration of Al Salt, Sample E). A 500 mL transparent aqueous solution containing aluminum nitrate nonahydrate (9.36 g) and ammonia-water (25 mL) was made at room temperature (pH 10.2). A Zn sheet was transferred into the solution and the Zn substrate-containing solution was maintained at 25 °C under constant stirring for 24 h. The substrate was washed by ultrasonication in deionized water for 10 min, after which it was dried in an oven at 60 °C for 12 h. Photocatalytic Activity Measurements. Orange-II (4-(2hydroxy-1-naphthylazo)benzenesulfonic acid, Aldrich) dye was used for the photodecomposition study. For comparing the photocatalytic activities, we prepared ZnO nanorod arrays by a conventional hydrothermal method (sample F). A 500 mL

transparent aqueous solution containing 0.01 M zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O, 98%, Aldrich) and 0.01 M hexamethylenetetramine (HMT, (CH2)6N4, 99%, Junsei) was made at room temperature. To prepare the substrate, a 10-nmthick Ti film as an adhesion layer was deposited on a (100) Si wafer by rf sputtering in a vacuum chamber. A 40-nm Zn film was deposited on top of the Ti layer. The resulting substrate was transferred into the solution and the substrate-containing solution was maintained at 90 °C for 2 h. After the reaction, the substrate was washed by ultrasonication in DI water for 10 min, after which it was dried in an oven at 60 °C for 12 h. ZnO nanorod array substrates prepared by different conditions or methods were transferred into 100 mL of 1.0 × 10-5 M orangeII solution and irradiated by a mercury lamp (1 W cm-2, Model 66905, Newport Co.). The amounts of dye in the solution were determined by measuring the absorption intensity at 486 nm, a main peak position of the orange-II dye, using a UV2501PC (SHIMADZU) spectrometer. The details of control experiments are summarized in Table 1. Characterizations. The morphology and the crystal structure were observed with use of a field-emission scanning electron microscope (FESEM, JEOL JMS-7400F, operating at 10 keV), a high-resolution scanning transmission electron microscope (Cs corrected HR-STEM, JEOL JEM-2200FS with energy-dispersive X-ray spectrometer (EDX), operating at 200 kV), X-ray photoelectron spectroscopy (XPS, VG Scientific, EscaLab 200iXL), and X-ray diffraction (XRD, Mac Science, M18XHF). Results and Discussion Structure and Composition of ZnO Nanorods. Panels a and b of Figure 1 display the X-ray diffraction (XRD) patterns of the Zn sheet before and after the reaction (sample A). Before the reaction, the XRD pattern of the original bare Zn sheet (Figure 1a) shows sharp peaks corresponding to Zn crystalline planes (JCPDS No. 04-0831). After the reaction, the XRD pattern shows peaks in addition to the Zn crystal plane peaks (Figure 1b). These new peaks can be indexed as the hexagonal wurtzite ZnO structure with calculated lattice constants of a ) 0.325 nm and c ) 0.521 nm. These lattice constant values are consistent with previously reported data (JCPDS No. 36-1451). Because no diffraction peaks from other impurities were observed in the XRD patterns, we concluded that pure hexagonalphase ZnO crystals were synthesized by this method. Panels a and b of Figure 2 show the fabricated 3 cm × 3 cm ZnO nanorod arrays and SEM images of the surface of the substrate after the reaction, respectively (sample A). ZnO crystals with rod-like structures (Figure 2b) uniformly distributed on the surface of the Zn sheet. The length, diameter, and shape of the ZnO structures are highly uniform. Dimensions of these rod-like structures are ∼250 nm in length and ∼18 nm in diameter. The more magnified structure of the ZnO nanorods was analyzed by using a high-resolution scanning transmission electron microscope (HR-STEM). Figure 2c shows a TEM

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Figure 1. XRD patterns of the Zn sheet (a) before and (b) after the reaction with the aqueous ammonia solution containing Al ions at 25 °C for 24 h (sample A).

image of ZnO nanorods synthesized and detached from the substrate. An HR-TEM image (Figure 2d) shows that the ZnO nanorod is highly crystalline, with a lattice spacing of about 0.26 nm, corresponding to the distance between (0002) planes in the ZnO crystal lattice. In addition, the selected-area electron diffraction (SAED) pattern of the ZnO nanorod (Figure 2c, inset) confirms that it has a single crystalline structure along the [0001] direction. The composition of the ZnO nanorods was investigated by using energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). EDX elemental maps demonstrate that Zn (Figure 2f) and O (Figure 2g) are distributed homogeneously through the nanorods. The EDX pattern (Figure 2h) indicates that the ZnO structures are composed of only Zn and O because the Cu signal is attributed to the copper mesh used for HR-TEM. Quantitative analyses of the mean atomic ratios (Zn:O) in the ZnO structures were 0.49:0.51 with EDX and 0.497:0.513 with XPS. No evidence of other impurities was found. ZnO nanorods are nearly stoichiometric. These data also confirm the high purity of the ZnO nanorods. Identification of Substances Detached by Sonication. Any substances physically and weakly attached to the substrate after the reaction were removed during the ultrasonication for 10 min. To better understand the mechanism of ZnO nanorod growth, we investigated materials that were detached by the sonication. After 1 min of sonication, the zinc aluminum layered double hydroxides (ZnAl:LDHs) were found on some regions of the substrate (Figure 3a). This substrate was sonicated in DI water again and filtered to collect the attached substances. The XRD pattern of the filtered and gathered powder that was attached weakly to the substrate (Figure 3b) revealed diffraction peaks of ZnAl:LDHs (JCPDS No. 38-0486) in addition to ZnO peaks. The reflections can be indexed as a hexagonal lattice (a ) 0.307 nm; c ) 2.28 nm) with a R3jm rhombohedral symmetry. Parameter a represents the average intermetallic distance

Figure 2. (a) Fabricated 3 cm × 3 cm ZnO nanorod arrays prepared with the aqueous ammonia solution containing Al ions at 25 °C for 24 h (sample A). (b) SEM images of the surface of the ZnO nanorod arrays. (c) TEM image of ZnO nanorods; the inset is the SAED pattern. (d) HR-TEM image of the area marked by the circle in panel c; the inset is the higher magnification image. (e) High-angle annular darkfield scanning transmission electron microscope (HAADF-STEM) image of ZnO nanorods. (f) Elemental mapping of Zn. (g) Elemental mapping of O. (h) EDX pattern of the ZnO nanorods.

calculated from the position of the (110) reflection (2θ ) 60.164°) and parameter c corresponds to 3/2(d003 + 2d006). The 01k (k ) 2, 5, 8) reflections in the mid-2θ regions are characteristics of the 3R1 polytype.19,20 No peaks of any other phases or impurities were detected, indicating that the detached and filtered powder contained mainly ZnAl:LDHs and ZnO. Therefore, we can conclude that ZnAl:LDH formed near the Zn surface. Homogeneous precipitates in the bulk solution were also investigated. Panels c and d of Figure 3 are the SEM image and the XRD pattern of the filtered precipitates from the bulk solution, respectively. The XRD pattern can be indexed as aluminum oxide hydrate (β-Al2O3 · H2O) with calculated lattice constants of a ) 0.504 nm and c ) 0.472 nm. These lattice constant values are consistent with previously reported data (JCPDS No. 08-0096). Al(OH)3 was not observed in the XRD pattern (Figure 3d).

Fabrication of Sub-20-nm-Diameter ZnO Nanorod Arrays

Figure 3. (a) SEM image of the ZnAl:LDH on the Zn substrate (prepared with the aqueous ammonia solution containing Al ions at 25 °C for 24 h) after ultrasonication for 1 min; (b) XRD pattern of powders detached from the Zn substrate (prepared with the aqueous ammonia solution containing Al ions at 25 °C for 24 h and ultrasonicated for 1 min) by ultrasonication and gathered. (c) SEM image and (d) XRD pattern of the filtered precipitates from the bulk solution in the reaction with the aqueous ammonia solution containing Al ions at 25 °C for 24 h.

Effect of Al Salts To Synthesize ZnO Nanorod Arrays. To investigate the role of Al species or NO3- in synthesizing ZnO nanorods on the Zn substrate, we examined a series of control experiments in which Al salt was omitted from the aqueous ammonia solution. When the Zn sheet was developed in an aqueous of ammonia with the same ammonia concentration as with the Al case, but omitting Al ions (Sample B), it was revealed that 1D structures were not synthesized on the Zn surface in the absence of Al ions (Figure 4a). The corresponding XRD pattern (Figure 4b) is almost the same as that of the Zn sheet before the reaction (Figure 1a), which demonstrates that ZnO crystals did not grow on the substrate. The surface image (Figure 4c) and the XRD pattern (Figure 4d) of the Zn substrate after reaction in the pH 10.6 ammonia solution without aluminum salt (sample C) show that ZnO crystals were not synthesized on the substrate. On the basis of the results of these two control treatments, we conclude that either Al species or NO3- were necessary for ZnO nanorods to form on the Zn substrate at room temperature without adding Zn salts, such as zinc acetate, zinc nitrate, and zinc chloride. We also measured the effect of using cobalt nitrate instead of aluminum nitrate (sample D). The pH of the solution (10.6), concentrations of ammonia and metal nitrate, reaction time, and reaction temperature were identical with those of the aluminum nitrate case. 1D structures were not synthesized on the Zn substrate after the reaction (Figure 4e). The corresponding XRD pattern (Figure 4f) shows that ZnO crystals were not grown on the Zn sheet. Therefore, we conclude that the key species for this synthetic method is Al, not NO3-. The Zn surface etching effect was markedly suppressed in comparison to the aluminum nitrate case because Co2+ ions might not react with the Zn substrate surface at room temperature. We also conducted a control treatment (sample E) to test the effect of Al3+ concentration by increasing the amount of

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Figure 4. SEM images and XRD patterns of the substrates: (a and d) after the reaction in the aqueous ammonia solution without Al ions (the same ammonia concentration as when Al was included) at 25 °C for 24 h (sample B); (b) (e) after the reaction in the aqueous ammonia solution without Al ions (the same pH (10.6) as when Al was included) at 25 °C for 24 h (sample C); and (c and f) after the reaction with the aqueous ammonia solution containing Co ions at 25 °C for 24 h (sample D).

aluminum nitrate dissolved in the reaction solution to four times that in the case of sample A: the Zn sheet was much more etched (ionized) and it almost disappeared. This etching effect was dramatically reduced when Al was not used (Figure 4a,c). Proposed Mechanism of ZnO Nanorod Growth with Al Salts. With aluminum salts, the growth mechanism of the sub20-nm-diameter ZnO nanorod arrays can be proposed. The following reactions occurred when aluminum nitrate was added to the aqueous ammonia solutions:

NH3 + H2O f NH4+ + OH-

(1)

Al3+ + 4OH- a Al(OH)4-

(2)

2Al(OH)4- a Al2O3 + 2OH- + 3H2O

(3)

Zn + 4OH- + Al(OH)4- + CO32- + H2O f ZnAl:LDH (4) ZnAl:LDH a Zn2+ + 4OH- + Al(OH)4- + CO32- + H2O (5) We propose that ZnO nanorods grow by the following mechanism (Figure 5). At room temperature, Al3+ ions mainly existed as Al(OH)4- complex or Al2O3 in the alkaline solution as a result of reactions 2 and 3. Al(OH)4-, the dominant Al complex ions in the alkaline solution, reacts with Zn on the Zn sheet surface and OH- to form ZnAl:LDHs by reaction4; this reaction is

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Figure 5. Schematic illustration of the proposed growth mechanism of sub-20-nm-diameter ZnO nanorods on the Zn sheet through addition of Al salts to aqueous ammonia solution.

thermodynamically favored.21-24 ZnAl:LDH is a member of the layered double hydroxides (LDH) family of inorganic layered materials. These materials have structures similar to the brucite structure, Mg(OH)2, in which each Mg2+ ion is surrounded by six OH- ions in an octahedral arrangement. LDHs are obtained when some of the divalent cations are isomorphously replaced by trivalent cations such as Al3+.21,25,26 The higher charge of the trivalent cations generates an overall positive charge in the brucite-type layers, which is balanced by the intercalation of interlayer anions such as CO32-. These synthetic procedures were not CO2 free. CO2 was dissolved in the aqueous solution to form CO32-. CO32- anions readily intercalate into layered double hydroxide. The IR spectrum of the filtered ZnAl:LDH includes a band at 1358 cm-1, which is the vibrational absorption of interlayer CO32- (see the Supporting Information, Figure S1). However, ZnAl:LDH formed through forward reaction 4 was surrounded by a solution with low Zn2+ concentration at this low temperature because sufficient Zn2+ ions could not be supplied. Thus, some ZnAl:LDHs that did not reach critical nuclei size or that did reach critical size were dissolved and ionized by reaction 5 for chemical equilibrium. Therefore, Al species served as a Zn ion carrier. Such release of Zn2+ ions, which was assisted by the Al species, led to the generation of locally sufficient ZnO growth units (Zn(NH3)42+) near the Zn surface. The reactions involved in the formation of ZnO nanorods on the Zn surface in the aqueous solutions involved in are as follows:

Zn2+ + 4NH3 f Zn(NH3)42+

(6)

Zn(NH3)42+ + 2OH- f ZnO + 4NH3 + H2O

(7)

More ZnO growth units existed near the Zn surface when Al3+ were present than when they were absent. This greater density of ZnO units locally increased the degree of supersaturation of ZnO near the substrate. Therefore, due to the increase of driving force, heterogeneous nucleation occurred on the Zn surface by reaction 7. The ZnO crystal structure consists of hexagonally close-packed oxygen and zinc atoms. The crystals

exhibit several main crystal planes: a top tetrahedron cornerexposed polar zinc (0001) face, six symmetric nonpolar {101j0} planes parallel to the [0001] direction, and a basal polar oxygen (0001j) face.27 These different planes have different polarities. The nonpolar {101j0} planes, which have relatively low surface energies, are more stable than the polar (0001) and (0001j) planes having higher surface energies.28-30 Because the system tends to minimize the total surface energy, ZnO crystals grew preferentially along the [0001] direction based on the ZnO seed sites nucleated in the initial growth state. This growth reaction occurred at room temperature, which led to slow growth kinetics. As a result, the possibilities of the crystal growth with lattice mismatch were reduced. Other species are excluded from the ZnO crystals. On account of the slow growth kinetics of this reaction, Al species are excluded form the ZnO matrix. The source of the Zn ions was the Zn substrate itself. Zn ions were not provided when the substrate was covered with grown ZnO structures as the reaction proceeded. Thus, the ZnO growth ceased when the entire surface of the substrate was covered with ZnO nanorods (Figure 5). This self-limiting growth process and the slow growth kinetics due to low reaction temperature allow us to obtain sub-20-nm-diameter ZnO nanorod arrays. Photocatalytic Activity of ZnO Nanorod Arrays. Oxide semiconductors have high photocatalytic activity, and so have been considered for applications of this attribute.31,32 Fine oxide powders such as ZnO and TiO2 have received much attention because high photocatalytic efficiency can be achieved by increasing their effective surface area.33 However, these powders have generally been used in a suspended state in water, which limits their practical use due to separation and recovery problems. These problems become more serious when nanometer-size fine powders are used but can be solved by making a photocatalytic thin film adhered to a rigid substrate. In particular, highly dense 1D nanostructure arrays are a good candidate for photocatalytic applications. The photocatalytic activity of the as-prepared substrates was evaluated by degradation of aqueous Orange-II solutions under ultraviolet light (UV) radiation. We monitored the concentration of the Orange-II in the solutions under exposure to UV (Figures 6a). Under conditions in which the Lambert-Beer law is applicable, the concentration c of the

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A ) εcl

(8)

where l is the length of the light path through the absorbing layer and ε is the molar absorbing coefficient. In the case of the solution without any catalyst, the Orange-II concentration was almost constant during UV irradiation, which confirms the photostability of the dye. When we used the substrate reacted in an aqueous ammonia solution without Al salt at room temperature for 24 h (sample B) as a catalyst shown in Figure 4a, the Orange-II concentration slightly decreased after UV irradiation. This change occurred because there is thin native oxide of the Zn sheet or a small quantity of ZnO particles formed and attached on the Zn substrate in the reaction and some dye molecules adsorbed to the substrate. When we used the sub20-nm-diameter ZnO nanorod arrays fabricated through addition of Al salts to aqueous ammonia solution (sample A, the estimated ZnO mass: 5.136 mg), the photocatalystic activity was much better than that of the ZnO nanorod arrays prepared by conventional hydrothermal method (sample F, the estimated ZnO mass: 6.322 mg). The dimensions of rod-like structures synthesized by the hydrothermal method are ∼350 nm in length and ∼100 nm in diameter (sample F). In the case of the sub20-nm-diameter ZnO nanorod arrays, the main absorption peak at 480 nm, which corresponds to the Orange-II dye molecules, decreases with the extension of the exposure time, and almost disappears after 6 h as shown in panels a and b of Figure 6. Figure 6c shows the Orange-II solutions after UV irradiation for 6 h with different catalysts. It is clear that the orange color of the starting solution disappears with sub-20-nm nanorod catalyst (sample A) after exposure to UV for 6 h. Because photocatalytic activity increases with effective surface area, a rough film surface with a high S/V ratio is beneficial. The selflimiting growth process and slow growth kinetics of this Al salt addition method allow us to obtain sub-20-nm-diameter ZnO nanorod arrays, which have very high S/V ratios. Therefore, these sub-20-nm-diameter ZnO nanorod arrays have high photocatalytic activity. Conclusions

Figure 6. (a) The normalization concentration (from the optical absorbance measurements at 480 nm) of the Orange-II solution (100 mL) with no catalyst; the substrate prepared in the aqueous ammonia solution without Al ions at 25 °C for 24 h (sample B); the ZnO nanorod array prepared by a conventional hydrothermal method (sample F); and the substrate prepared in the aqueous ammonia solution containing Al ions at 25 °C for 24 h (sample A) versus the UV irradiation time. (b) Absorption spectra of Orange-II solution photodegraded by the sub20-nm-diameter ZnO nanorod arrays (sample A), which were prepared with the aqueous ammonia solution containing Al ions at 25 °C for 24 h, for different UV irradiation times. (c) Digital camera image of Orange-II solutions after UV irradiation for 6 h with no catalyst; the substrate prepared in the aqueous ammonia solution without Al ions at 25 °C for 24 h (sample B); the ZnO nanorod array prepared by a conventional hydrothermal method (sample F); and the substrate prepared in the aqueous ammonia solution containing Al ions at 25 °C for 24 h (sample A).

absorbing component is proportional to absorbance A as follows:34

In summary, we report a very simple wet-chemical synthesis of sub-20-nm-diameter ZnO nanorod arrays at room temperature and normal atmospheric pressure. Large-scale arrays could be obtained by immersing a Zn sheet in an aqueous ammonia solution containing Al3+ ions. The key factors in this mild synthetic method for synthesizing large-scale ZnO nanorod arrays are to synthesize ZnO crystals on the Zn substrate at low temperature assisted by Al species and to maintain the sub-20nm-diameter of ZnO nanorods by a self-limiting growth process. This method uses little energy and requires no complex experimental procedures or equipment. Zn ions were provided by the Zn sheet, which caused this process to be self-limiting. ZnO nanorods synthesized by this method with slow kinetics and a self-limiting growth process are ∼250 nm long and ∼18 nm in diameter. On the basis of the results of this fabrication and control experiments, we propose the mechanisms for the growth of the ZnO structures on the Zn sheet with Al3+ ioncontaining ammonia aqueous solution. We also measure the photocatalytic effects of the fabricated nanorod arrays. The high S/V ratio of the sub-20-nm-diameter ZnO nanorod arrays results in enhanced photocatalytic activity. We expect that these high S/V ratio ZnO arrays are also good candidates for various applications. The present findings should be applicable to the synthesis of other oxide materials.

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Acknowledgment. This work was supported by a grant from the second phase BK21 program of the Ministry of Education of Korea and Acceleration Research Program (2009-0079077) of Korea Science and Engineering Foundation (KOSEF). Supporting Information Available: IR absorption spectrum of the filtered precipitates on the Zn substrate. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Arnold, M. S.; Avouris, P.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2003, 107, 659. (2) Rao, G. S. T.; Rao, D. T. Sens. Actuators, B 1999, 55, 166. (3) Lee, C. J.; Lee, T. J.; Lyu, S. C.; Zhang, Y.; Ruh, H.; Lee, H. J. Appl. Phys. Lett. 2003, 81, 3648. (4) Tominaga, K.; Umezu, N.; Mori, I.; Ushiro, T.; Moriga, T.; Nakabayashi, I. Thin Solid Films 1998, 334, 35. (5) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (6) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Mcdermott, M. J.; Rodriguez, M. A.; Konishi, H.; Xu, H. F. Nat. Mater. 2003, 2, 821. (7) Greene, L. E.; Law, M.; Goldberger, J.; Kim, F.; Johnson, J. C.; Zhang, Y.; Saykally, R. J.; Yang, P. Angew. Chem., Int. Ed. 2003, 42, 3031. (8) Lee, J. S.; Islam, M. S.; Kim, S. Nano Lett. 2006, 6, 1487. (9) Cho, S.; Kim, S.; Kim, N.-H.; Lee, U.-J.; Jung, S.-H.; Oh, E.; Lee, K.-H. J. Phys. Chem. C 2008, 112, 17760. (10) Bae, C. H.; Park, S. H.; Ahn, S. E.; Oh, D. J.; Kim, G. K.; Ha, J. S. Appl. Surf. Sci. 2006, 253, 1758. (11) Park, W. I.; Kim, J. S.; Yi, G. C.; Bae, M. H.; Lee, H. J. Appl. Phys. Lett. 2004, 85, 5052. (12) Greene, L. E.; Yuhas, B. D.; Law, M.; Zitoun, D.; Yang, P. Inorg. Chem. 2006, 45, 7535. (13) Liu, B.; Zeng, H. C. Langmuir 2004, 20, 4196. (14) Cho, S.; Jung, S.-H.; Lee, K.-H. J. Phys. Chem. C 2008, 112, 12769.

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