Size-Controllable Growth of Vertical ZnO Nanorod Arrays by a Pd

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Size-Controllable Growth of Vertical ZnO Nanorod Arrays by a Pd-Catalyzed Chemical Solution Process Tsutomu Shinagawa,*,† Seiji Watase,† and Masanobu Izaki‡ † ‡

Electronic Materials Research Division, Osaka Municipal Technical Research Institute, Osaka 536-8553, Japan Department of Mechanical Engineering, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan ABSTRACT: We report on a versatile and low-temperature solution process (e80 °C, containing no annealing and vacuum processes) to grow vertical and dense ZnO nanorod arrays. The ZnO nanorod arrays have been deposited by immersing a Pd-nanoparticle-coated substrate into an aqueous solution containing Zn(NO3)2 and dimethylamineborane (DMAB). Coating substrates with Pd nanoparticles that act not only as a catalyst for DMAB oxidation but also as a ZnO nucleation seed layer allows similar ZnO nanorod arrays to be deposited on different types of substrates such as glass and PET. The effect of deposition conditions on the resulting ZnO morphology is examined, and the size (length and diameter) of ZnO nanorods is demonstrated to be controllable independently by just adjusting the Zn(NO3)2 concentration and deposition time. The degree of c-axis orientation is evaluated in terms of the texture coefficient and its standard deviation, and all samples show a high vertical alignment irrespective of the Zn(NO3)2 concentration and deposition time. As-grown ZnO nanorod arrays exhibit UV and visible light emission at room temperature, and interstitial oxygen is found to be a major lattice defect.

1. INTRODUCTION One-dimensional (1D) semiconductor nanostructures have attracted increasing attention because of their various applications in electronics and photonics. Among them, zinc oxide (ZnO) is one of the most promising materials for fabricating 1D nanostructured functional devices, such as solar cells,1
5 UV-emitting diodes,6
10 and chemical sensors11
13 due to its wide band gap energy of 3.37 eV and large exciton binding energy of 59 meV. ZnO nanorods have been prepared by various methods, such as chemical vapor deposition,14,15 vapor
liquid
solid deposition, 16
18 and solution-based growth methods.19
25 The solution-based methods, including hydrothermal synthesis19
21 and electrochemical deposition,22
25 have some advantages over vapor-phase deposition in terms of low cost, low temperature growth, environmentally friendliness, and ease of morphology control without a template. In both the hydrothermal and electrochemical methods, heteroepitaxial growth on a single-crystalline substrate such as sapphire (Al2O3), Au, Si, and GaN has been developed to give perfectly vertically aligned ZnO nanorod arrays.25
29 On the other hand, growth of ZnO nanorod arrays on common transparent substrates, such as glass and Sn-doped indium oxide (ITO), is of importance from a practical viewpoint. Since using such amorphous or polycrystalline substrates gives rather disordered ZnO growth, seed layers have been used to improve the growth density and vertical alignment of ZnO nanorod arrays.30
32 The seed layer composed of packed ZnO nanocrystals (typically ∼50 nm in thickness) acting as a homoepitaxial nucleation site is generally prepared by a sol
gel method r 2011 American Chemical Society

so as to cover the entire substrate surface. In the sol
gel method, annealing at temperatures above 150 °C is usually required after spin-coating or spray-coating of sol solution containing a zinc precursor. Such a high-temperature heating process, however, detracts from the advantage of the low-temperature solutionbased ZnO growth methods as well as limits usable substrates. In this paper, preparation of vertically aligned dense ZnO nanorod arrays on glass and PET substrates by using a Pdcatalyzed chemical solution process containing no annealing and vacuum processes has been presented. The ZnO deposition mechanism of this chemical process is basically same as that of the electrochemical deposition22,33 except that a reducing agent, dimethylamineborane (DMAB), is employed instead of an external power source.34,35 A local-pH increase in the vicinity of a substrate is caused by reduction reactions of dissolved oxygen and/or nitrate ion, followed by the formation of Zn(OH)2 and dehydration to form ZnO: O2 þ 2H2 O þ 4e
f 4OH


ð1Þ

NO3
þ H2 O þ 2e
f NO2
þ 2OH


ð2Þ

Zn2þ þ 2OH
f ZnðOHÞ2 f ZnO þ H2 O

ð3Þ

While cathodic current from an external power source promotes the reactions of 1 and 2 in the electrochemical deposition, DMAB Received: August 24, 2011 Revised: October 26, 2011 Published: November 14, 2011 5533

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plays the role of providing electrons in the chemical deposition: Pd

ðCH3 Þ2 NHBH3 þ H2 O sf HBO2 þ ðCH3 Þ2 NH2 þ þ 5Hþ þ 6e


ð4Þ

Because this reaction 4 is usually slow in an acidic/neutral pH region and catalyzed by Pd that is coated on a substrate, a series of reactions 1
3 is promoted only on the substrate surface. Hence, the precipitation of ZnO is not generated in the bulk of solution, which remains transparent during the deposition, unlike the hydrothermal synthesis. When aqueous solution containing Zn(NO3)2 and DMAB is employed to deposit ZnO without deaeration, that is, in the presence of ambient dissolved oxygen, two reduction reactions of 1 and 2 may be promoted competitively. In previous work, Murase and Shinagawa et al.36,37 have revealed that the reduction reaction of dissolved oxygen is much faster than that of nitrate ions and adversely affects the ZnO nucleation step, resulting in relatively low crystallographic orientation. In the present study, we employed deionized (DI) water deaerated with Ar gas and prepared ZnO nanorod arrays from aqueous Zn(NO3)2
DMAB solutions with different Zn(NO3)2 concentrations, [Zn(NO3)2]. The ZnO nanorods obtained without any annealing and vacuum processes showed a high growth density and preferred c-axis orientation. The length and diameter of the nanorods were found to be controllable independently by adjusting the [Zn(NO3)2] and deposition time. Furthermore, asgrown ZnO nanorods showed ultraviolet emission at room temperature.

Figure 1. Transmittance spectra of Pd-catalyzed glass and bare glass substrates. Inset is an AFM image of the Pd-catalyzed glass surface.

2. EXPERIMENTAL SECTION General Procedures. Chemical solution deposition of ZnO nanorod arrays was performed in an airtight glass vessel using reagentgrade chemicals and deionized (DI) water purified by a Milli-RX12 Plus system. A Corning glass (no. 1737, alkali-free borosilicate, 20  40  0.7 mm) and a PET sheet (TORAY Lumirror T60, 20  40  0.25 mm) were used as a substrate. Prior to each chemical deposition, the Corning glass was cleaned ultrasonically in acetone and rinsed with DI water, and the PET sheet was dipped into 10% NaOH solution at 60 °C for 5 min and rinsed with DI water. These substrates were then treated with a UV/ ozone cleaner for 10 min and catalyzed using a commercialized threestep Sn/Ag/Pd dipping process (Okuno Chemical Industries, Techno Clear series). This wet-process catalyzation comprises a sequential immersion of the substrate into the three solutions for 1 min each at 25 °C, resulting in the substrate being entirely covered with Pd nanoparticles at a high density. Deposition of ZnO Nanorod Arrays. Zinc nitrate hexahydrate Zn(NO3)2 3 6H2O and dimethylamineborane (DMAB) were dissolved into DI water (0.15 dm3) deaerated with Ar gas for 0.5 h in advance to prepare solutions containing 1.5
50 mM Zn(NO3)2 and 3.3 mM DMAB. The solution in an airtight vessel was heated to 80 °C, and then the Pd-catalyzed substrate was immersed into the solution to deposit ZnO without magnetic stirring. Ar bubbling (∼20 cm3 min
1) was continued during the deposition. The deposits obtained were rinsed with DI water and dried under ambient atmosphere. Characterization of Catalyzed Substrates and ZnO Nanorod Arrays. The Pd-catalyzed substrate was evaluated with a UV
vis spectrophotometer (UV-3150, Shimadzu) and an atomic force microscope (AFM; NanoScope IIIa, Dimension 3000, Digital Instruments). Deposits obtained on the Pd-catalyzed substrates were characterized with a field emission scanning electron microscope (FESEM; JSM-6700F, JEOL), an X-ray diffractometer (XRD; RINT-2500 system, Rigaku) with

Figure 2. FESEM images (a, b) and XRD patterns (c, d) of ZnO nanorod arrays deposited from a 15 mM Zn(NO3)2
3.3 mM DMAB aqueous solution on a Pd-catalyzed glass (a, c) and PET seet (b, d). Scale bars are 1 μm. Cu Kα radiation (40 kV, 100 mA), and a fluorescence spectrometer (F-4500, excitation wavelength 325 nm, Xe lamp, Hitachi).

3. RESULTS AND DISCUSSION Before the growth of ZnO nanorod arrays, a Corning glass and a PET sheet were catalyzed by sequential dipping into three aqueous solutions each containing Sn, Ag, and Pd ions. The catalyzation has little influence on the visible transparency as shown in Figure 1. Transmittance of the Pd-catalyzed glass is above 85% over the visible wavelength region and has a slight absorption of ∼4.5% at around 400 nm in wavelength compared to a bare glass, which is due to the plasmon band of Ag nanoparticles.38 The inset of Figure 1 depicts an AFM image of the Pd-catalyzed glass. Homogeneously dispersed nanoparticles (diameter ∼50 nm, height ∼20 nm) were observed over the catalyzed area (20  40 mm2). By using this catalyzing process, catalytic nanoparticles with such morphology can be provided to different-type substrates without any annealing and vacuum processes. Figure 2a,b shows FESEM images of typical ZnO nanorod arrays deposited on a Pd-coated glass and a Pd-coated PET sheet, 5534

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Figure 3. Tilted and top-view FESEM images of ZnO nanorod arrays deposited on Pd-catalyzed glass substrates from Zn(NO3)2
3.3 mM DMAB aqueous solutions for 3 h at 80 °C. All scale bars are 100 nm. Right-side histogram shows diameter distribution of the ZnO nanorod grains. Zn(NO3)2 concentration: (a
c) 50, (d
f) 30, (g
i) 15, (j
l) 5.0, and (m
o) 1.5 mM.

respectively, from a 15 mM Zn(NO3)2
3.3 mM DMAB solution at 80 °C. In both substrates, we can see dense ZnO nanorods about 80 nm in diameter grown homogeneously and almost vertically. X-ray diffraction (XRD) patterns for these ZnO nanorod arrays are also similar to each other except for the contribution of PET (Figure 2c,d); wurtzite ZnO crystal grains with a c-axis preferred growth orientation are formed directly from the aqueous solution. Such morphological identity, that is, nanorod diameter, growth density, and growth orientation, between the glass and PET substrates is due to not only the same deposition condition but also the catalytic nanoparticles that

cover the two different substrates in the similar manner. Since the catalytic Pd nanoparticles act as a seed layer for ZnO growth (as discussed later) as well as a catalyst for DMAB oxidation, its uniformity affects the resulting ZnO morphology. To obtain the uniform catalytic nanoparticle layer, pretreatment that makes the substrate surface hydrophilic enough is necessary before the catalyzation. Thus, the Pd catalyzation makes it possible that ZnO nanorod arrays with a similar morphology can be deposited on various substrates, including glass, Si wafer, PET, and polycarbonate. In this study, we conducted the following experiments using Corning glass because of its ease of handling. 5535

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Figure 4. Morphological variation in ZnO nanorod arrays deposited on Pd-catalyzed glass substrates from Zn(NO3)2
3.3 mM DMAB aqueous solutions as a function of Zn(NO3)2 concentration. Data of (a, b, d) are from FESEM images comprising Figure 3, and (c, e, f) are calculated from them.

3.1. Variation in Morphology Depending on the Zn(NO3)2 Concentration. ZnO nanorod arrays were deposited on Pd-

catalyzed glass from 3.3 mM DMAB aqueous solutions containing Zn(NO3)2 with different concentrations of 50, 30, 15, 5.0, and 1.5 mM. The deposition temperature and time were fixed at 80 °C and 3 h, respectively. Although all deposits consisted of densely grown ZnO nanorods with a hexagonal columnar structure, the nanorod size was considerably varied depending on the Zn(NO3)2 concentration, [Zn(NO3)2], as shown in Figure 3. With decreasing the [Zn(NO3)2], the nanorod length and diameter increased from 462 to 967 nm and decreased from 125 to 63 nm, respectively; these size values are averaged over 100 nanorods observed with FESEM. The diameter distribution at each concentration is also depicted in Figure 3, indicating near monodispersity. The variations of morphological parameter for nanorod, such as the length, diameter, aspect ratio, and growth density, are plotted against the [Zn(NO 3 )2 ] as shown in Figure 4a
d. Whereas the variation of nanorod length is a curved line, that of nanorod diameter is linear. The calculated aspect ratio, therefore, describes a curve and varies from 15 to 3.7 with increasing [Zn(NO3)2]. The growth density estimated from the top-view FESEM images shows a convex curve with a relatively small variation from 44 to 56 μm
2, which is almost compatible with a density of the catalytic nanoparticles observed clearly in the AFM

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image (Figure 1). This indicates that the catalytic nanoparticles act as a seed layer to yield the initial nuclei of ZnO. We previously reported that the catalytic activity of Pd for DMAB oxidation is relatively high, leading to a rapid increase of pH in the vicinity of the substrate, followed by a rapid nucleation of ZnO; once the catalyst layer is covered over by ZnO, the pH increase rate decreases considerably due to the poor catalytic property of ZnO, resulting in near solid/liquid equilibrium and growth along the caxis.37 Since the initial amount of Zn2+ in the vicinity of the substrate is proportional to the bulk Zn2+ concentration and the number of the nucleation site is regulated by the catalytic nanoparticles, the diameter of ZnO nuclei will increase linearly with increasing [Zn(NO3)2]. In contrast, the decrease in nanorod length with increasing [Zn(NO3)2] suggests that the amount of ZnO deposited does not increase enough to make the lengths increase. A decrease in the growth density at conditions of high and low Zn2+ concentrations (Figure 4d) is probably due to a lateral interference among ZnO grains and a lack of Zn2+ to yield robust grains, respectively. Total surface area and total deposit amount calculated from the data in Figure 4a,b,d are also depicted in the bottom of Figure 4 and interestingly indicate an almost constant and clear linear relationship with respect to the [Zn(NO3)2], respectively. The linear behavior of the total deposit amount implies that a rate determining step under the present ZnO deposition conditions is the diffusion of Zn(NO3)2 from the bulk of solution, where the concentration of Zn(NO3)2 in the vicinity of the substrate is proportional to that of the bulk solution. The other steps, such as (i) the diffusion of DMAB and (ii) a sequence of chemical reactions at the ZnO(001) surface containing the generation and decomposition of intermediates, can be excluded from rate determining step candidates. If the diffusion of DMAB is the rate determining step and Zn(NO3) is supplied sufficiently to the substrate, the total deposit amount will be constant irrespective of the [Zn(NO3)2] because the DMAB concentration is fixed at 3.3 mM for each run. On the other hand, if the chemical reaction at the ZnO(001) surface is the rate determining step, that is, Zn(NO3)2 and DMAB are supplied sufficiently to the substrate and the area density of intermediates on the ZnO(001) surface is constant, the deposit amount per unit of ZnO(001) surface area should also be constant, resulting in the same nanorod length irrespective of [Zn(NO3)2]. 3.2. Variation in Morphology Depending on Deposition Time. In order to clarify the effect of deposition time on the size variation in ZnO nanorod arrays, 5.0 mM Zn(NO3)2
3.3 mM DMAB aqueous solutions were employed. Figure 5 shows FESEM images of ZnO nanorod arrays obtained with different deposition times of 0.5, 1, 2, 4, and 6 h (Figure 3j,k,l corresponds to a 3 h deposition time). Through the deposition up to at least 6 h, the growth density is almost unchanged, and each ZnO nanorod has a constant diameter from the bottom to the top. As shown in Figure 5f, nanorod length and diameter increase linearly from 0.5 to 1.35 μm and from 64 to 85 nm, respectively, in the deposition time range of 1
6 h, revealing that the variation of the length is about 40 times larger than that of the diameter. It is well-known that such growth anisotropy is based on the difference in the surface energy between ZnO crystal planes of (001) and (100). The (001) plane corresponding to the top face of ZnO nanorods has higher surface energy than that of the (100) plane corresponding to the lateral face, acquiring the long and thin columnar grain to minimize its total surface energy. Since the nanorod diameter also depends linearly on the [Zn(NO3)2] as 5536

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Figure 5. Tilted FESEM images of ZnO nanorod arrays deposited on Pd-catalyzed glass substrates from 5.0 mM Zn(NO3)2
3.3 mM DMAB aqueous solutions at 80 °C for (a) 0.5, (b) 1, (c) 2, (d) 4, and (e) 6 h. All scale bars are 100 nm. (f) Length and diameter variation of the ZnO nanorod arrays as a function of deposition time.

mentioned above (Figure 4b), these linearity of crystalline growth nature allows one to control readily the ZnO nanorod length and diameter independently by only adjusting the deposition time and [Zn(NO3)2]. 3.3. XRD Study. Figure 6 shows the X-ray diffraction (XRD) patterns of the ZnO nanorod arrays deposited from various Zn(NO3)2 concentrations (Figure 6a) and deposition times (Figure 6b). The other deposition conditions were held constant ([DMAB] = 3.3 mM, deposition temperature = 80 °C). All the diffraction peaks were identified as the wurtzite ZnO (JCPDS no. 36-1451) and no peaks from the catalyst were observed due to its trace amount. As seen in the FESEM images (Figures 3 and 5), since the ZnO nanorods grow almost vertically from the substrate in the direction of the c-axis, the highest peak is 0002 diffraction for all the samples, and other small peaks recognized are 1011, 1012, and 1013 diffraction. Since one of the factors affecting the diffraction intensity is the crystalline volume, the intensity of 0002 diffraction increases with increasing the [Zn(NO3)2] (see also Figure 4f) and the deposition time (see also Figure 5f). The degree of preferred orientation for the ZnO nanorod arrays was estimated quantitatively by calculating Harris’s texture coefficient, Tc, and its

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Figure 6. XRD patterns of ZnO nanorod arrays deposited on Pdcatalyzed glass substrates from (a) Zn(NO3)2
3.3 mM DMAB aqueous solutions with different Zn(NO3)2 concentrations for 3 h and (c) 5.0 mM Zn(NO3)2
3.3 mM DMAB aqueous solutions with different deposition times. Insets (b) and (d) show the variation of texture coefficient for 0002 diffraction, Tc(0002) and its standard deviation, σ.

standard deviation, σ.39,40 The Tc is defined as Im ðhklÞ=I0 ðhklÞ Tc ðhklÞ ¼ n n Im ðhklÞ=I0 ðhklÞ

∑1

ð5Þ

where Im(hkl) is the measured relative intensity of the peak corresponding to the hkl diffraction, I0(hkl) is the relative intensity from the same diffraction in the standard powder sample (JCPDS no. 36-1451), and n is the total number of diffraction peaks considered in the evaluation. The standard deviation σ of the texture coefficients is also useful to compare the degree of orientation among different samples and is defined as vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi un u u ðTc
1Þ2 t1 ð6Þ σ ¼ n

∑

The Tc and σ values range from 0 to n and from 0 to (n
1)1/2, respectively, and samples having a high and random orientation 5537

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Figure 7. PL spectra of ZnO nanorod arrays deposited on Pd-catalyzed glass substrates from Zn(NO3)2
3.3 mM DMAB aqueous solutions with different Zn(NO3)2 concentrations for 3 h. (a) As-grown and (b) after annealing at 500 °C for 0.5 h under an atmosphere of 3% H2/Ar.

show Tc = ∼n, σ = ∼(n
1)1/2 and each Tc = ∼1, σ = ∼0, respectively. In the present case, we chose four diffractions (n = 4) of 0002, 1011, 1012, and 1013, and the results calculated for Tc(0002) and σ are plotted in Figure 6b,d. Irrespective of the Zn(NO3)2 concentration and deposition time, the Tc(0002) and σ show almost constant values of 3.0
3.4 and 1.2
1.4, and reach 75
85% and 70
80% of their possible maximum values, respectively. These results, therefore, demonstrate that it is possible to control the ZnO nanorod size consistently keeping the highly preferential orientation. 3.4. Photoluminescence Property. Figure 7a shows roomtemperature photoluminescence (PL) spectra for the as-grown ZnO nanorod arrays deposited with different Zn(NO3)2 concentrations of 5
50 mM at a fixed deposition time of 3 h (see also Figure 3 for their morphology). These spectra were recorded at wavelengths from 344 to 800 nm corresponding to photon energies from 3.6 to 1.55 eV with a fluorescent spectrophotometer equipped with a xenon lamp (excitation wavelength: 325 nm). All the ZnO nanorod arrays emitted both ultraviolet (UV) light at a photon energy of ∼3.15 eV (394 nm) and broad visible light with a peak at around 2 eV (620 nm). The energy of the observed UV emission originating from the near-band-edge

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(NBE) is in the range of values reported for as-deposited ZnO nanostructures prepared by electrodeposition (3.12
3.30 eV)25,41
43 and hydrothermal deposition (3.12
3.29 eV) techniques.30,44 The comparatively lower energy band, that is, red shift, of the UV emission may be attributed to some lattice defects such as impurities and distortion. There is also a possibility that DMAB and its decomposition products of dimethylamine and borate ion influence the formation of the lattice defects. On the other hand, the broad visible light emission band which is centered at 2 eV (620 nm) corresponds to yelloworange light. The similar yellow-orange light emission has been reported for ZnO prepared by electrodeposition45 and hydrothermal deposition,30,46 and is considered to be based on a deep level trapping site of interstitial oxygen ions (Oi
).47 The formation of Oi
as a major point defect implies that the ZnOgrowth environment is oxygen rich. In fact, either hydration water of aquo-Zn2+ ion or OH
ion generated from the nitrate reduction (reaction 2) can be an excess oxygen source of Oi
at the growth site even though the aqueous solutions used here was deaerated with Ar gas before and during the deposition. Among the samples, there is almost no difference in both the UV and yellow-orange light positions, whereas the intensity ratio of UV to visible light changed depending on the Zn(NO3)2 concentration; UV/visible intensity ratios are 0.26, 1.0, 3.4, and 0.85 for the concentration of 50, 30, 15, and 5 mM, respectively. This trend probably results from combined factors of ZnO nanorod morphology48 and deposition rate.41 The UV/visible intensity ratio was significantly increased by annealing at 500 °C for 0.5 h under a reductive atmosphere of 3% H2/Ar as shown in Figure 7b. The annealing increases the UV emission intensity by about 2.5 times and causes a blue shift of peak position (3.25 eV, 381.5 nm). Furthermore, broad yelloworange light emission disappeared due to the reaction between Oi
and H2. After the annealing, a narrow shaped band appeared clearly at 1.63 eV (761 nm). The energy of this narrow band is just half of the UV emission and the intensity order is same trend as those in the UV region. Thus, this narrow band corresponds to the second-order diffraction of the NBE UV emission as reported by Mahalingam et al.49

’ CONCLUSIONS In this paper, the Pd-catalyzed chemical deposition of ZnO nanorod arrays from Zn(NO3)2
DMAB aqueous solutions has been reported. Different types of substrates (glass and PET) coated with Pd nanoparticles gave a similar structural morphology of ZnO nanorods by just immersing into the solution (80 °C) without annealing and vacuum processes. With decreasing the Zn(NO3)2 concentration, the nanorod length and diameter increased nonlinearly from 462 to 967 nm and decreased linearly from 125 to 63 nm, respectively. The detailed analysis of the morphology variation revealed that the catalytic Pd particles act as a seed layer and the rate determining step is in the diffusion of Zn(NO3)2. In the case of deposition time dependence, the nanorod length and diameter increased linearly from 0.5 to 1.35 μm and from 64 to 85 nm, respectively in the range of 1
6 h, revealing a highly anisotropic growth manner. These linear relationships of the length and diameter with the deposition time and [Zn(NO3)2] allow one to control the size of nanorods readily. The XRD results indicate that all samples obtained have highly c-axis preferred growth orientation. The texture coefficient and its standard deviation are calculated from 5538

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Crystal Growth & Design the four diffractions to be 75
85% and 70
80% of their possible maximum values, respectively irrespective of the [Zn(NO3)2] and deposition time. The as-grown ZnO nanorods emitted UV light (3.15 eV) derived from the near-band-edge (NBE) and yellow-orange light (2 eV) due to the lattice defects at room temperature, and the UV/visible intensity ratio changed depending on the [Zn(NO3)2]. The reductive annealing (3% H2/Ar gas) led to an increase in intensity and a blue shift of the UV emission as well as the disappearance of the yellow-orange light due to the reaction between a major point defect of Oi
and H2. Since the low-temperature Pd-catalyzed chemical process can be applied to various substrates including insulators and polymers, this unique method will make the development of novel devices using ZnO nanorod arrays possible.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT T.S. thanks Dr. K. Murase (Kyoto Univ.) for helpful discussions and Dr. Y. Kobayashi (OMTRI) for help with XRD experiments. This work was supported in part by the Incorporated Agency New Energy and Industrial Development Organization (NEDO) under the Japanese Ministry of Economy, Trade and Industry (METI). ’ REFERENCES (1) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455. (2) Baxter, J. B.; Aydil, E. S. Appl. Phys. Lett. 2005, 86, 053114l. (3) Chou, T. P.; Zhang, Q.; Fryxell, G. E.; Cao, G. Z. Adv. Mater. 2007, 19, 2588. (4) Leschkies, K. S.; Divakar, R.; Basu, J.; Enache-Pommer, E.; Boercker, J. E.; Carter, C. B.; Kortshagen, U. R.; Norris, D. J.; Aydil, E. S. Nano Lett. 2007, 7, 1793. (5) Musselman, K. P.; Wisnet, A.; Iza, D. C.; Hesse, H. C.; Scheu, C.; MacManus-Driscoll, J. L.; Schmidt-Men, L. Adv. Mater. 2010, 22, E254. (6) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (7) K€onenkamp, R.; Word, R. C.; Schlegel, C. Appl. Phys. Lett. 2004, 85, 6004. (8) Lim, J.-H.; Kang, C.-K.; Kim, K.-K.; Park, I.-K.; Hwang, D.-K.; Park, S.-J. Adv. Mater. 2006, 18, 2720. (9) Nadarajah, A.; Word, R. C.; Meiss, J.; K€onenkamp, R. Nano Lett. 2008, 8, 534. (10) Lupan, O.; Pauporte, T.; Viana, B. Adv. Mater. 2010, 22, 32398. (11) Polarz, S.; Roy, A.; Lehmann, M.; Driess, M.; Kruis, F. E.; Hoffmann, A.; Zimmer, P. Adv. Funct. Mater. 2007, 17, 1385. (12) Ra, H.-W.; Choi, K.-S.; Kim, J.-H.; Hahn, Y.-B.; Im, Y.-H. Small 2008, 4, 1105. (13) Kobrinsky, V.; Rothschild, A.; Lumelsky, V.; Komen, Y.; Lifshitz, Y. Appl. Phys. Lett. 2008, 93, 113502. (14) Wu, J. J.; Liu, S. C. Adv. Mater. 2002, 14, 215. (15) Yuan, H.; Zhang, Y. J. Cryst. Growth 2004, 263, 119. (16) Yang, P.; Yan, H.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R.; Choi, H. J. Adv. Funct. Mater. 2002, 12, 323. (17) Gao, P.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 12653. (18) Wang, S.; Song, J.; Li, P.; Ryou, J. H.; Dupuis, R. D.; Summers, C. J.; Wang, Z. L. J. Am. Chem. Soc. 2005, 127, 7920. (19) Vayssieres, L.; Keis, K.; Hagfeldt, A.; Lindquist, S. Chem. Mater. 2001, 13, 4395. (20) Vayssieres, L. Adv. Mater. 2003, 15, 464.

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dx.doi.org/10.1021/cg2011106 |Cryst. Growth Des. 2011, 11, 5533–5539