Studies of Electrochemical Synthesis of Ultrathin ZnO Nanorod

Nov 15, 2007 - Electrochemical synthesis and applications of oriented and hierarchically quasi-1D semiconducting nanostructures. Xue-Jun Wu , Feng Zhu...
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CRYSTAL GROWTH & DESIGN

Studies of Electrochemical Synthesis of Ultrathin ZnO Nanorod/ Nanobelt Arrays on Zn Substrates in Alkaline Solutions of Amine-Alcohol Mixtures

2007 VOL. 7, NO. 12 2562–2567

Jinhu Yang, Yongfu Qiu, and Shihe Yang* Department of Chemistry, Institute of Nano Science and Technology, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ReceiVed June 6, 2007; ReVised Manuscript ReceiVed September 15, 2007

ABSTRACT: The electrode reactions and pathways involved in the electrochemical fabrication of highly ordered ultrathin ZnO nanorod/nanobelt arrays in alkaline solutions of amine-alcohol mixtures are systematically studied and revealed. These include electrochemical surface oxidation of the Zn cathode to Zn(OH)42-, simultaneous in situ nucleation of ZnO from the Zn(OH)42- ions on the cathode, and subsequent amine-capped growth into the ZnO nanobelt/nanorod arrays. It is shown that the growth step can also be supported by the Zn(OH)42- source from the anodic oxidation, and the growth from this source actually yields much more uniform and smooth nanobelt/nanorod arrays than those from only the cathodic source. Furthermore, the strategy has been successfully applied to the fabrication of a ZnO nanorod array on an ITO substrate covered with a Zn film, which promises exciting applications in solar cell and sensor devices. Introduction ZnO films have drawn great interest because their unique electric and optical properties make them suitable for various applications, such as light emitting devices and sensors.1 In particular, ZnO films consisting of an ordered array of onedimensional (1D) nanostructures have a high hope of improving optoelectronic properties of the semiconductor.2 A number of methods have been employed to achieve ZnO 1D-nanostructured arrays, including chemical and physical vapor deposition,3–9 hydrothermal process,10,11 metallorganic vapor-phase epitaxial growth,6,12 and templating with films of anodized aluminum oxide (AAO)13 and SBA-16.14 The electrochemical deposition (ECD) technique is becoming an important means for the fabrication of ZnO films due to the low cost, mild conditions, and accurate process control.15 In the process of ECD, ZnO films are usually deposited from weak acidic or nearly neutral aqueous solutions of ZnCl2 or Zn(NO3)2. Recently, researchers reported cathodic electrodeposition of crystalline ZnO films promoted by base generation from electroreduction of NO3-, dissolved O2, or hydrogen peroxide.16–20 To produce porous structures of ZnO films, some additives such as dye molecules and amphiphilic molecules (alkyl sulfates, alkyl sulfonates, and carboxylates) were also employed.21,22 However, in most of the previous studies on ECD of ZnO films, the electrolyte solutions were seldom in alkaline conditions, and this may limit the electrochemical window for the deposition of novel nanoscopic ZnO films. ZnO films consisting of arrays of ZnO nanowires are of particular interest because this novel system has shown capabilities to harvest efficient sound and light energies.2b,23 A current challenge is to prepare well-defined ZnO nanowire arrays with diameters of the ZnO nanowires below 10 nm. Very recently, we reported an electrochemical route for the fabrication of ordered ultrathin ZnO nanorod/nanobelt arrays directly on the Zn electrode substrate in the presence of amine.24 This was the first time the synthesis of both ZnO nanobelts and ZnO nanorods as thin as 8 nm in ordered arrays by using the ECD method was achieved. In this paper, we investigate systemati* To whom correspondence should be addressed. E-mail address: [email protected].

Figure 1. Schematic diagram of the glass cell setup for the electrochemical synthesis of ZnO nanorod/nanobelt arrays.

cally the evolution of ECD-produced ZnO nanostructures by varying electrochemical conditions, identify important factors for the growth of the ZnO nanorod/nanobelt arrays, deduce possible electrode reactions and pathways, and discuss possible growth mechanisms under the alkaline conditions. Building on this study, we have further accomplished ECD deposition of the ZnO nanorod array on a Zn film-covered ITO substrate, which is an important step toward optoelectronic applications of the nanoscale 1D arrays of ZnO. Experimental Section Materials and Setup. The setup for the synthesis of ZnO nanorod and nanobelt arrays is composed of a glass cell (50 mL, Figure 1) with two electrodes and a computerized model 600A electrochemical analyzer. The glass cell was kept closed except when N2 (used for purging) or O2 (used during ECD) gas was fed through the input and output ports. Electrochemical Deposition. A mixed solvent of H2O, isopropanol (General Chemical Corp., New Jersey, U.S.A.), and diethylamine (99%, Sigma-Aldrich, Seelze, Germany) was

10.1021/cg070513i CCC: $37.00  2007 American Chemical Society Published on Web 11/15/2007

Synthesis of Ultrathin ZnO Nanorod/Nanobelt Arrays

prepared at a volume ratio of 1:1:1. Into the glass cell were added 22.5 mL of the mixed solvent, 0.5 mL of a saturated NaCl solution (0.127 M), and 0.5 mL of 30% H2O2 (22 mM) (Merck KGaA Corp., Germany) under slight stirring at 22 °C, resulting in a solution with pH ∼ 12. In the case of synthesis for the ZnO nanorod array, all were similar except that 0.1 mL of 30% H2O2 (4.4 mM) was used instead of 0.5 mL (22 mM). Two pieces of zinc foil (99.99%, 1 cm × 0.5 cm × 0.25 mm) were polished with sandpaper and cleaned by immersing into ethanol (95%) under ultrasonication for 10 min and dried in air. The glass cell containing the electrolyte solution was purged with an N2 flow for 15 min to remove O2 before the two cleaned Zn foils were immersed into the solution in a parallel configuration with an interelectrode separation of 1 cm (Figure 1). To apply a fixed electric potential of 3.0 V between the two Zn electrodes by using the electrochemical analyzer for the ECD, we adopted the linear sweep voltammetry technique to scan from 3.02 to 3.00 V at a rate of 10-6 V/s, essentially keeping the potential at 3.0 V. After 6 h of reaction, a whitish gray film, which appeared obviously different from the bright Zn foil surface before ECD, was generated on the Zn cathode, and simultaneously, a white but rough layer could be seen on the anode. The products on the anode were found to be irregular ZnO particles densely stacked on the rugged substrate (see Figure S1, Supporting Information), which are not the subject of interest in this paper and will not be discussed further. Henceforth, our analysis will focus on the products on the cathode. The Zn cathode with the deposited products was then collected, washed with distilled water, and dried in the air. Characterization. The nanostructured ZnO products were characterized by SEM (scanning electron microscopy), XRD (X-ray diffraction), and TEM (transmission electron microscopy). SEM was conducted using JEOL 6700 at an accelerating voltage of 5 kV. For TEM observations, the ZnO nanobelt/ nanorod products were ultrasonically dispersed in ethanol and then dropped onto carbon-coated copper grids. TEM observations were carried out on JEOL 2010F and JEOL 2010 microscopes both operating at 200 kV. XRD analyses were performed on a Philips PW-1830 X-ray diffractometer with Cu KR irradiation (λ ) 1.5406 Å) at a scanning speed of 0.025°/s over the 2θ range of 20–70°.

Crystal Growth & Design, Vol. 7, No. 12, 2007 2563

Figure 2. SEM images of ZnO nanobelt arrays grown on a Zn substrate at [H2O2] ) 22 mM (A, B) and 44 mM (C, D), respectively. Belt characteristics are labeled with arrows (B, D). E ) 3 V. pH ) 12.

Results and Discussion As we reported already in the previous letter with a Zn-Zn two-electrode system,24 the ordered ZnO nanobelt array was successfully synthesized under relatively high H2O2 concentrations of ca. 22∼44 mM as shown in Figure 2, whereas the ordered ZnO nanorod array was preferentially formed at relatively low H2O2 concentrations of ca. 4.4∼13.2 mM. XRD patterns (see Figure S2, Supporting Information) indicate that the products of nanobelts and nanorods take a Wurtzite crystal structure of ZnO though they were not perfectly crystallized at a low synthetic temperature of 22 °C. TEM images of the ZnO nanobelts and nanorods (Figure 3) show ultrathin nanostructures, and both exhibit a growth direction along [0002].24 In fact, besides the H2O2 concentration, electric potential was also found to display a similar influence on the ZnO nanostructures. In other words, the ZnO nanorods were preferentially formed at lower electric potentials (ca. 0.4 V), whereas the generation of the ZnO nanobelts was preferred at the converse condition (1.5∼3.8 V). The morphological and structural evolution of the ZnO products as a function of applied electric potential is shown in Figure 4. When no electric potential was applied with other conditions being typical, no obvious ZnO nanobelt/nanorod

Figure 3. TEM images of the ultrathin ZnO nanobelts (A, B) and nanorods (C, D) synthesized at H2O2 concentrations of 22 and 4.4 mM, respectively. E ) 3 V. pH ) 12.

product was formed on the electrodes, though a layer of Zn substrate had been rusted by the presence of H2O2 or O2 released from decompositions of H2O2. Clearly, the reactions for the formation of ZnO nanobelts/nanorods can be neglected under the mere influence of H2O2. Vertically arrayed ZnO nanorods with diameter ∼10 nm and length ∼100 nm were produced on the cathode when the applied potential was increased to 0.4 V. Further increase of the potential to 1.5 V resulted in the formation of the slightly longer ZnO nanobelts (200 nm) but unvaried thickness of ∼5 nm and width of ∼15–20 nm, which were assembled roughly into a bundle-like morphology. When the potential was finally elevated to 3.8 V, ZnO nanobelts as long as 1 µm were obtained, and they assembled completely into dense nanobundles due to their structural match and bending flexibility, which is in good accordance with our interpretation given in the previous paper. A further increase in the applied potential up to ca. 6 V did not result in an obvious increase in

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Figure 5. SEM images of a ZnO nanobelt array grown on a Zn cathode in a Pt-Zn two-electrode system. Inset in (B) is a magnified image. E ) 3 V. [H2O2] ) 22 mM. pH ) 12.

Figure 4. SEM images of ZnO products on a Zn cathode at various applied potentials: (A) 0 V, (B) 0.4 V, (C) 1.5 V, and (D) 3.8 V. [H2O2] ) 22 mM. pH ) 12.

the lengths of the ZnO nanobelts probably because the high potential facilitated a formation of a dense ZnO film on the substrate, which insulated the Zn electrode and thus halted the nanobelt growth. We have therefore clearly demonstrated the combined influences of H2O2 concentration and applied electric potential on the ZnO nanostructures, such as the length, morphology, and array structure of the ZnO nanorods/nanobelts. However, the diameters of ZnO nanorods/nanobelts were not effectively tuned upon the changes of reaction conditions. Most likely, the diameter is dictated by the critical size of the nucleus. Once nucleated, the growth will propagate preferentially along the c-axis because the side walls are effectively capped by the amine molecules, leading to the formation of the ultrathin ZnO nanorods/nanobelts. Electrode Reactions. Under the conditions of our experiment, the following reactions are thought to occur at the anode in the Zn-Zn two-electrode electrochemical system:25 Zn f Zn2+ + 2e- φ° ) -0.765 V 2+

Zn

-

+ 4OH f Zn(OH)4 2-

Zn(OH)4

2-

(1) (2)

-

f ZnO + H2O + 2OH

H2O2 f O2 + 2H+ + 2e- φ° ) -0.695 V

(3) (4)

According to reactions 1 and 2, Zn atoms from the anode lose electrons and enter the solution by combining with OHions to form negatively charged Zn(OH)42- ions. Under the DC electric field, the Zn(OH)42- ions can easily reach the anode, leading to the deposition of ZnO particles (reaction 3) on the anode, in accord with our observations (see Figure S1, Supporting Information). One might also suppose that the Zn2+ ions that feed the formation of the ZnO nanorod/nanobelt arrays on the cathode can only come from the Zn anode because the anodic oxidation provides a necessary Zn2+ source in the form of Zn(OH)42- that can not be produced by any well-known cathodic reduction process. To clarify the detailed mechanism for the electrochemical formation of the ZnO nanobelt array, we used a Pt-Zn twoelectrode system instead of the Zn-Zn two-electrode system and conducted the reaction at otherwise the same typical condition. Surprisingly, a ZnO nanobelt array could also grow on the Zn cathode even without the Zn anode, although they were often accompanied by the separation of the whole array to many islands (Figure 5). This result suggests that the Zn2+

Figure 6. SEM images of ZnO nanostructures grown from Zn substrate at various applied potentials in ambient air: (A) 0 V, (B) 1.5 V, (C) 3 V, and (D) 3.8 V. Reaction time: (a) 24 h, and (b-d) 6 h. pH ) 12.

ions necessary for the growth of ZnO nanobelts can also be supplied by the Zn cathode, which must involve oxidation of Zn to Zn2+ at the Zn cathode. One possibility is direct oxidation by O2. To test this hypothesis, we introduced sufficient O2 by flowing air continuously into the same reaction system but without applying potential. Even after reaction for as long as 24 h, however, only a thin layer of ZnO nanoparticles of about 10 nm was found on the Zn substrate (Figure 6A). This implies that the ZnO nanorod/nanobelt arrays were formed most probably by the combined effect of O2 and the applied electric potential. Hence, the reactions that have possibly occurred at the cathode are probably the following (see Supporting Information S2, for the calculation of reduction potential): Zn + O2 + 2H2O + 2e- f Zn(OH)42- φ° ) 0.554 V (5) 2-

Zn(OH)4

-

f ZnO + H2O + 2OH

(6)

Because of its relatively higher reduction potential, the cathode reaction (5) is more favorable than that without involving oxidation of Zn (reaction 7) and much more so than another possible electrode reaction (8) below. O2 + 2H2O + 4e- f 4OH- φ° ) 0.401 V

(7)

2H+ + 2e- f H2 φ° ) 0 V

(8)

By considering all of the results we obtained, we propose a plausible mechanism of in situ nucleation and the subsequent tip growth for the formation of the ZnO nanorod/nanobelt arrays on Zn cathode. It is believed that the nucleation of ZnO on the

Synthesis of Ultrathin ZnO Nanorod/Nanobelt Arrays

Crystal Growth & Design, Vol. 7, No. 12, 2007 2565 Scheme 1. Possible Mechanisms for the Formation of ZnO Nanorod/Nanobelt Arrays

Figure 7. SEM images of ZnO nanorod (A) and nanobelt (B) arrays on a Zn cathode substrate synthesized in typical conditions. Insets of (A) and (B) are magnified images of the ZnO nanorod and nanobelt arrays, respectively.

Zn cathode is accomplished in situ via reaction 5. Conceivably, O2 molecules adsorb on the Zn cathode and get two electrons from Zn and two from the current supply, forming a layer of O2- above a layer of Zn2+ in a very small region. This can be considered as a nucleus for the growth of ZnO nanorod/nanobelt arrays. The layer-by-layer incorporation of O2- and Zn2+ in alternation on the top of the nuclei ensures a growth along the c-direction. Once the layered nuclei are formed, the Zn(OH)42ions in the solution that may come from both anode and cathode substrates shall be more easily deposited on the tips of the embryonic nanorod/nanobelt because the ZnO semiconductor nanocrystal nuclei are expected to be less negatively charged than the Zn cathode. The results above can be briefly summarized here. For the Zn-Zn two-electrode system, aside from reaction 5 on the Zn cathode, reaction 1 on the Zn anode is also a main Zn2+ source for the cathodic generation of the ZnO nanorod/nanobelt since it raises the concentration of Zn(OH)42-. This is supported by a much more smooth growth of the ZnO nanorod/nanobelt arrays on the cathode (Figure 7) than with the Pt-Zn two-electrode system (Figure 5) because the abundant supply of Zn2+ from reaction 1 could suppress the dissolution of Zn at the cathode by partially switching from reaction 5 to 7. For the latter case when the Pt-Zn two-electrode system is used, however, the necessary Zn2+ source can only come from dissolution of the Zn cathode. At the Pt anode, the dominance of reaction 4 and the absence of reaction 1 result in a rapid increase of the O2 concentration at a given potential. This would increase the dissolution rate of Zn at the Zn cathode via reaction 5, which may lead to a less smooth growth of the ZnO nanorod/nanobelt arrays in the form of islands separated by cracks (Figure 5A and B). Growth of Nanobelts vs Nanorods. Notably, ZnO nanobelts are preferentially formed at higher H2O2 concentrations and/or higher electric potentials, whereas the formation of ZnO nanorods is preferred at lower H2O2 concentrations and/or lower electric potentials. It should be pointed out that both a high H2O2 concentration and a high electric potential give rise to a high O2 concentration, which should increase the nucleation and growth rates of ZnO. In general, slow crystallization is required to form products with a thermodynamically stable structure because the crystallizing partners have time to recognize each other and follow the lowest-energy path. However, fast crystallization often leads to kinetically controlled products, such as unstable or metastable crystal structures, and even defects can be formed during a fast nucleation process.26 Hence, we believe that the cylindrical ZnO nanorods are preferably formed from slow nucleation and growth, whereas the noncylindrically symmetric ZnO nanobelts are obtained from fast nucleation and growth processes. Although more studies are needed to work

out the detailed mechanisms for the growth of ZnO nanorods/ nanobelts, based on the data already obtained, the analysis above, and further experimental results to be shown below, a plausible proposition can be given here. Scheme 1 illustrates the proposed pathways for the formation of the ZnO nanorods/ nanobelts arrays under different conditions, which emphasize the critical step of nucleation and subsequent growth assisted by the capping amine molecules. Specifically, at low electric potentials and low O2 concentrations, the rate of reaction 5 and thus the rate of ZnO nucleation are low. Such a slow nucleation process would preferentially form symmetric hexagonal ZnO nuclei (A1) from the thermodynamic point of view. The subsequent growth is expected to be on top of the nuclei. First, this site is more active for alternate deposition of the Zn2+ and O2- layers from Zn(OH)42- than the side walls, which are more inert due to a balanced composition of Zn2+ and O2-. Second, the adsorbed amine molecules cannot inhibit the growth along the c-axis because of high activity of the (001) plane but can stall the transverse growth of the nanosized rods. This can be clearly seen from Figure 8: when diethylamine is insufficient (∼0.6 M), many nanorods are aggregated in a transverse direction. These two main factors account for the formation of vertically aligned ZnO nanorods along the c-axis (A2–3). On the other hand, when sufficient O2 and high electric potentials are employed, a fast nucleation process via reaction 5 should result in a high density of nuclei, which may join together and line up to form rectangular nuclei due to the kinetic favorability. In other words, the rectangular nuclei are actually successors of the original spherical nuclei. Alternatively, the fast nucleation may kinetically prefer the rectangular shape. As a result, the usual ZnO hexagon morphology is distorted with an elongated j pair of parallel facets, e.g., the ((1100) facets (B1). This is plausible because the site with the smaller dimension is expected to be more active than that with the larger dimension, and this would amplify the distortion of the ZnO hexagon morphology to a rectangle one. Once the rectangular ZnO nuclei are formed,

Figure 8. SEM images of sideway-aggregated ZnO nanorod arrays on a Zn cathode in the presence of 1.5 mL of diethylamine. E ) 3 V. [H2O2] ) 22 mM. Vwater ) Visopropanol ) 10.5 mL.

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Figure 9. SEM images of ZnO nanostructures on a Zn cathode in the presence of (A) 7.5 mL of ammonia and (B) 0.01 M NaOH instead of diethylamine. E ) 3 V. [H2O2] ) 22 mM.

the subsequent growth would be the same as that for the nanorods described above; that is, Zn2+ and O2- layers are deposited on top of the nuclei in alternation from Zn(OH)42along the c-axis of Wurtzite ZnO (B2). Finally, the continuous growth gives vertically aligned ZnO nanobelts (B3), which are eventually assembled into a bundle-like array. In addition, adsorption of diethylamine on special side facets j (((1100)) should help to maintain the asymmetry of the ZnO nucleus during the growth process and lead to the final formation of the ZnO nanobelts. This is somewhat similar to the nanorod case described above. There are large amounts of discharged amine at a concentration of about 3 M, and more than 99% of the amine remains in the neutral form in solution (see Supporting Information S2 for calculation), which are presumably adsorbed j on the relatively inert side facets efficiently, ((1100), due to 2+ their strong coordinative ability with Zn . The amine adsorption will not only reduce the growth speed of the side facets but also, more importantly, preserve the asymmetric structure, resulting in the beltlike morphology as we observed (B2–3). The amine-adsorption mechanism can be directly proved by further control experiments. When 7.5 mL of ammonia or 0.01 M NaOH (pH ) 12) is employed instead of diethylamine under otherwise the same conditions, only hierarchical assemblies consisting of shoulder to shoulder ZnO nanoplates or vertically stood ZnO nanoplates with a hexagonal shape are obtained on the Zn substrate (Figure 9). This indicates that the growth along the c-axis has been blocked strongly by adsorption of NH4+ or OH- ions on a charged (0001) plane, whereas sideway aggregation and growth of the nanorods/nanobelts continue because of the lack of neutral amine molecules for adsorption. In addition, our experiments on morphological evolutions during the growth of the ZnO nanorod/nanobelt arrays with different reaction times also reveal that, with increasing reaction time, the lengths of the nanorods/nanobelts increase (see Figures S3 and S4, Supporting Information), in accord with the processes illustrated in Scheme 1. Furthermore, it was observed that the ordered array and assembly could be obtained only when the ZnO nanobelts possess a certain length (∼200 nm) because the nanobelts longer than ∼200 nm may have enough flexibility for the assembly. Taken together, the proposed mechanisms nicely explain the electrochemical formation of the ultrathin nanorod and nanobelt arrays. However, it is cautioned that the real situation may be much more complex, which requires more in-depth studies. Growth of ZnO Nanorod Arrays on an ITO Electrode. This electrochemical method for the synthesis of ZnO nanostructured arrays on a Zn cathode in the presence of alkaline electrolytes can be extended to the Zn film deposited ITO substrate. A Zn film served as both a Zn source and a substrate to support the growth of the 1D ZnO nanostructures. The sputtering technique was employed to deposit the Zn film (250

Figure 10. SEM images (A-C) and UV–vis spectrum (D) of ZnO nanorod arrays grown from a Zn film covered ITO substrate. H2O ) 11.5 mL. Isopropanol ) 7.5 mL. Diethylamine ) 3 mL. [H2O2] ) 22 mM. [NaCl] ) 0.127 M. Potential ) 1.2 V. t ) 6 h.

nm thick) on an ITO substrate. The fabrication setup was nearly the same as the Zn-Zn two-electrode system described above except for the replacement of the Zn foil with the Zn-coated ITO substrate for the cathode. As shown in Figure 10A and B, the ZnO nanorod array was formed uniformly in large scale on the Zn-covered ITO substrate. The enlarged image in Figure 10C shows that the ZnO nanorods are about 30 nm in diameter and 100 nm in length. Close observation in the inset of Figure 10C reveals that some nanorods are composed of three or more single thin nanorods, meaning that the primary nanorods may have diameters of about 10 nm or less. Compared with the ZnO nanobelt/nanorod arrays produced on the Zn substrates, the ZnO nanorod array synthesized on the Zn-coated ITO is somewhat less dense and in fact more tilted, possibly arising from the thinness of the Zn film on ITO. As mentioned above, the nanobelt array should be more ordered and uniform only when the nanorod length exceeds ∼200 nm, and the nanorod array is expected to behave similarly. To achieve better quality of the ZnO nanorod array, it is important to control the reaction rate. For the synthesis of the nanorod array shown in Figure 10, the concentrations of diethylamine and applied potential were both decreased from 3.1 M and 3.0 V, a typical condition for the synthesis of the ZnO nanobelt array, to 1.2 M and 1.2 V, which were found to be appropriate for the synthesis of the ZnO nanorod array on Zn-covered ITO. When those parameters were too high, the Zn film was consumed rapidly from the ITO substrate within several hours, and nearly no ZnO product was found to stick on the ITO substrate in this case. The as-synthesized ZnO nanorod array on Zn-coated ITO was transparent and characterized facilely with a UV–visible spectrophotometer. The UV spectrum of the sample shows an adsorption shoulder located in the UV range, which results from the optical transition to the exciton state (Figure 10D). The wavelength of λ1/2 (the wavelength at about 50% absorbance of the shoulder)27 that is often used to evaluate band gap energy is approximately 350 nm, corresponding to a band gap energy of 3.54 eV. Compared with bulk ZnO (λ1/2 ) 368 nm, 3.37 eV), a blue shift in the exciton absorption is recognized for the as-synthesized ZnO nanorod array, which may be caused, perhaps, by the fact that some of the ZnO nanorods are as thin as a few nanometers. The dependence of the absorption peak position on the ZnO nanoparticle size is well documented, and

Synthesis of Ultrathin ZnO Nanorod/Nanobelt Arrays

a blue-shift would occur if the diameter decreases to several nanometers.28 Conclusion In summary, we have systematically investigated the electrode reactions in the Zn-Zn two-electrode electrochemical system under alkaline electrolyte conditions. A plausible mechanism has been proposed that emphasizes the initial in situ cathode nucleation, the subsequent growth, and the capping amine molecules for the thermodynamically controlled formation of the ZnO nanorod arrays and for the kinetically controlled formation of the ZnO nanobelt arrays on Zn substrates. Moreover, we applied this strategy to fabricate a ZnO nanorod array on a Zn-coated ITO substrate based on the proposed growth mechanism and characterized its optical absorption spectrum with a significantly blue-shifted exciton absorption. The 1D ZnO nanostructures vertically aligned on the transparent conducting electrode may find applications in solar cell and sensor devices. In principle, it should be possible to extend this technique to the fabrication of nanowire arrays of other compound semiconductors. Acknowledgement. We acknowledge support from the Hong Kong University of Science and Technology (HIA05/06.SC02) and RGC (604206) administrated by the UGC of Hong Kong. Supporting Information Available: Additional data including XRD, SEM, and TEM characterizations of the ZnO nanostructures, the effect of reaction time on the ZnO nanorod/nanobelt arrays, and calculations of reaction parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) (a) O’Regan, B.; Schwartz, D. T.; Zakeeruddin, S. M.; Gratzel, M. AdV. Mater. 2000, 12, 1263. (b) Keis, K.; Magnusson, E.; Lindstrom, H.; Lindquist, S.-E.; Hagfeldt, A. Sol. Energy Mater. Sol. Cells 2002, 73, 51. (c) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J. F.; Lieber, C. M. Nature 2001, 409, 66. (2) (a) Wang, Z.; Song, J. Science 2006, 312, 242. (b) Wang, X.; Song, J.; Liu, J.; Wang, Z. Science 2007, 316, 102. (3) Kong, Y. C.; Yu, D. P.; Zhang, B.; Fang, W.; Feng, S. Q. Appl. Phys. Lett. 2001, 78, 407. (4) Han, X.; Wang, G.; Zhou, L.; Hou, J. G. Chem. Commun. 2006, 212.

Crystal Growth & Design, Vol. 7, No. 12, 2007 2567 (5) Park, W. I.; Yi, G. C.; Kim, M.; Pennycook, S. J. AdV. Mater. 2002, 14, 1841. (6) Yang, P.; Yan, H.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R.; Choi, H. AdV. Funct. Mater. 2002, 12, 323. (7) Yao, B. D.; Chan, Y. F.; Wang, N. Appl. Phys. Lett. 2002, 81, 757. (8) Wang, X.; Song, J.; Li, P.; Ryou, J.; Dupuis, R. D.; Summers, C. J.; Wang, Z. J. Am. Chem. Soc. 2005, 127, 7920. (9) Lyu, S. C.; Zhang, Y.; Lee, C. J. Chem. Mater. 2003, 15, 3294. (10) (a) Lu, C. H.; Qi, L. M.; Yang, J. H.; Tang, L.; Zhang, D. Y.; Ma, J. M. Chem. Commun. 2006, 3551. (b) Tak, Y.; Yong, K. J. Phys. Chem. B 2005, 109, 19263. (11) (a) Fang, Y.; Pang, Q.; Wen, X.; Wang, B.; Yang, S. Small 2006, 2, 612. (b) Wen, X.; Fang, Y.; Pang, Q.; Yang, C.; Wang, J.; Ge, W.; Wong, K.; Yang, S. J. Phys. Chem. B 2005, 109, 15303. (12) (a) Park, J. Y.; Lee, D. J.; Kim, S. S. Nanotechnology 2005, 16, 2044. (b) Peterson, R. B.; Fields, C. L.; Gregg, B. A. Langmuir 2004, 20, 5114. (13) Liu, C. H.; Zapien, J. A.; Yao, Y.; Meng, X.; Lee, C. S.; Fan, S.; Lifshitz, Y.; Lee, S. T. AdV. Mater. 2003, 15, 838. (14) Shi, K. Y.; Chi, Y. J.; Yu, H. T.; Xin, B. F.; Fu, H. G. J. Phys. Chem. B 2005, 109, 2546. (15) Pandy, R. K.; Sahu, S. N.; Chandra, S. Hand Book of Semiconductor Electrodeposition; Marcel Dekker, Inc.: New York, 1996. (16) (a) Pauporte, Th.; Lincot, D. J. Electrochem. Soc. 2001, 148, C310. (b) Peulon, S.; Lincot, D. AdV. Mater. (Weinheim, Ger.) 1996, 8, 166. (17) Izaki, M.; Omi, T. Appl. Phys. Lett. 1996, 68, 2439. (18) Oshida, T. Y.; Komatsu, D.; Shimokawa, N.; Minoura, H. Thin Solid Films 2004, 451, 166. (19) Fathy, N.; Ichimura, M. J. Cryst. Growth 2006, 294, 191. (20) Yan, C. L.; Xue, D. F. Electrochem. Commun. 2007, 9, 1247. (21) (a) Yoshida, T.; Miyamoto, K.; Hibi, N.; Sugiura, T.; Minoura, H.; Schlettwein, D.; Oekermann, T.; Schneider, G.; Wohrle, D. Chem. Lett. 1998, 7, 599. (b) Yoshida, T.; Terada, K.; Schlettwein, D.; Oekermann, T.; Sugiura, T.; Minoura, H. AdV. Mater. 2000, 12, 1214. (22) Boeckler, C.; Oekermann, T.; Feldhoff, A.; Wark, M. Langmuir 2006, 22, 9427. (23) Law, M.; Greene, L.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. 2005, 4, 455. (24) Yang, J.; Liu, G.; Lu, J.; Qiu, Y.; Yang, S. Appl. Phys. Lett. 2007, 90, 103109. (25) David, R. L, Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 1990–1991; 71st, 8–16. (26) Li, F.; Ding, Y.; Gao, P. X.; Xin, X. Q.; Wang, Z. L. Angew. Chem., Int. Ed. 2004, 43, 5238. (27) Zhao, Z. W.; Tay, B. K.; Chen, J. S.; Hu, J. F.; Sun, X. W.; Tang, S. T. Appl. Phys. Lett. 2005, 87, 251912. (28) Ramakrishna, G.; Ghosh, H. N. Langmiur 2003, 19, 3006.

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