Direct and Large-Area Growth of One-Dimensional ZnO

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J. Phys. Chem. C 2007, 111, 5876-5881

Direct and Large-Area Growth of One-Dimensional ZnO Nanostructures from and on a Brass Substrate Kaifu Huo,†,‡,§ Yemin Hu,† Jijiang Fu,‡ Xuebin Wang,† Paul K. Chu,§ Zheng Hu,*,† and Yi Chen† Key Laboratory of Mesoscopic Chemistry of MOE and Jiangsu ProVincial Laboratory for Nanotechnology, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, China, Hubei ProVince Key Laboratory of Refractories and Ceramics, College of Materials and Metallurgy, Wuhan UniVersity of Science and Technology, Wuhan 430081, China, and Department of Physics and Materials Science, City UniVersity of Hong Kong, Tat Chee AVenue, Kowloon, Hong Kong, China ReceiVed: January 8, 2007; In Final Form: February 24, 2007

A convenient method for the direct and large-area growth of one-dimensional (1-D) ZnO nanostructures on a conductive brass substrate has been developed, consisting of thermal oxidation of a Cu0.66Zn0.34 alloy foil in the presence of oxygen. Various 1-D nanostructures such as nanowires, nanobelts, nanocombs, and nanosheets have been in situ grown on the brass substrate under different reaction temperatures and characterized by means of X-ray diffraction, electron microscopy, and X-ray photoelectron spectroscopy. In this preparation, the Cu0.66Zn0.34 alloy foil functions as both Zn source and substrate for the growth of 1-D ZnO nanostructures; thus, the synthesis and assembly of ZnO nanostructures on a metallic substrate is accomplished in one step, and the naturally good adhesion or electrical connection between the ZnO nanostructures and the conductive substrate has been realized. This approach could prepare ZnO nanostructures on a brass substrate without size limitations. Such a configuration of product is a good field emitter as demonstrated in this study. The potential technological importance of the product, the simplicity of the preparation procedure, as well as the cheap commercial precursor of the Cu0.66Zn0.34 alloy foil makes this study both scientifically and technologically interesting.

Introduction With the ongoing miniaturization of devices, the controllable synthesis and assembly of one-dimensional (1-D) nanostructures such as nanowires, nanotubes, and nanobelts into functional devices have attracted overwhelming interest in recent years.1 Zinc oxide, a direct wide band gap (3.37 eV) semiconductor with a large exciton binding energy (60 meV), is considered to be one of the most important semiconductor materials due to its unique properties and wide applications in transparent electronics, ultraviolet light emitters, piezoelectric devices, chemical sensors, spin electronics, and so on.2 In the past few years, the synthesis, characterization, and application of ZnO nanostructures have become the focus of intensive research.3 Various ZnO nanostructures such as nanorods,4 nanowires,5 nanobelts,6 nanotubes,7 nanonails,8 nanocombs,9 nanohelices,10 nanosprings,11 nanorings,12 and hiearachical nanostructures13 have been synthesized, and some exciting properties have been revealed. 3a, 14 Well-aligned 1-D ZnO nanowires/nanorods have been synthesized on some substrates such as insulating sapphire3a and glass15 or semiconducting Si,16 GaN, and Al0.5Ga0.5N17 with the assistance of a gold catalyst or ZnO seed layer. The poor conductivity of these substrates might limit their applications in some electronics and optoelectronics. In addition, the predeposition of the catalyst or ZnO buffer layer may increase the complexity of the experimental procedure, introduce * Corresponding author. Tel: 0086-25-83686015. Fax: 0086-2583686251. E-mail: [email protected]. † Nanjing University. ‡ Wuhan University of Science and Technology. § City University of Hong Kong.

some impurities, and influence the adhesion of ZnO nanostructures to the underlying substrates, thereby degrading the performance of the products. Recently, quasi-aligned ZnO nanobelts were grown from and on the conductive Zn substrate by directly oxidizing Zn foils around 500 to ∼600 °C. Good natural adhesion and conductivity between ZnO nanobelts and conductive Zn substrate was obtained, which shows some promising sensing properties.18 However, since the growth temperature is higher than the melting point of Zn (420 °C), Zn foils are usually roughened and distorted. As a result, wellaligned ZnO nanobelt arrays can only be fabricated in small areas.18 For some practical applications (e.g., in field emission devices), it is important to grow 1-D ZnO nanostructures on conductive substrates in a large area with good adhesion. It is noted that brass (Cu-Zn alloy) with a Zn content less than 35% remains as a solid below 900 °C, which is beneficial for resistance to deformation at high temperatures, yet brass could provide enough Zn species for the formation of ZnO nanostructures. By using Cu0.66Zn0.34 foil as both Zn source and substrate, herein we report a convenient approach for the direct and largearea growth of different 1-D ZnO nanostructures from and on brass substrates simply by annealing the brass foils under different reaction temperatures in the presence of oxygen. These products are natural field emitters due to the good adhesion and conductivity between the ZnO nanostructures and the conductive substrate as demonstrated in this study, which could have wide applications.

10.1021/jp070135s CCC: $37.00 © 2007 American Chemical Society Published on Web 04/03/2007

1-D ZnO Nanostructures from and on Brass Substrate

Figure 1. In situ X-ray diffraction pattern obtained at different temperatures for Cu0.66Zn0.34 foil during the heating process in air with a heating rate of 15 °C/min.

Experimental Procedures Commercial brass foil (Cu0.66Zn0.34, 10 mm × 10 mm) was polished with SiC sandpaper and then ultrasonically cleaned with acetone, ethanol, and deionized water sequentially. After it was dried in air, the brass foil was loaded onto a ceramic substrate located in the center of a corundum tube inside a horizontal tube furnace. The reaction system was flushed with Ar and heated to the desired temperature, and then the Ar/O2 (4%, O2) mixture with a flow rate of 50 sccm was introduced into the tube as the reagent gas. The system was maintained in a pressure range of 100 to ∼150 Pa by a mechanical pump. After 60 min of reaction, the system was cooled down to room temperature in Ar, and a homogeneous white layer was formed on the substrate. Field emission scanning electron microscopy (FE-SEM, LEO1530VP), transmission electron microscopy (TEM, JEOL-JEM-1005), high-resolution TEM (HRTEM, JEM2010), X-ray diffraction (XRD, Philips X’Pert Pro), as well as X-ray photoelectron spectroscopy (XPS, ESCALB MK-II) were employed to characterize the products. Field emission measurements were carried out using a parallel-plate diode configuration in a test chamber maintained at 1.5 × 10-6 Torr. Results and Discussion Figure 1 is the in situ XRD pattern of Cu0.66Zn0.34 foil during heating process in air with a heating rate of 15 °C/min. Below 500 °C, no obvious change in the XRD patterns could be observed, only the slight shift to the low-angle side due to the thermal expansion of the foil with rising temperature. A trace of ZnO peaks appears around 500 °C, and an observable ZnO species is formed around 600 °C. According to the Cu-Zn binary alloy phase diagram,19 Cu0.66Zn0.34 is in a solid state under 900 °C. Hence, the reaction temperature (TR) is chosen to be in between 600 and 900 °C in this study. Figure 2 shows FE-SEM and TEM images of the white products grown on Cu-Zn alloy substrates at 600, 700, 800, and 900 °C for 60 min, respectively. It is shown that the morphologies of the as-synthesized products are highly dependent on TR. For a TR of 600 °C (Figure 2a,b), the product is composed of nanosheets with a thickness of 20-100 nm and a width of about hundreds of nanometers. For a TR of 700 °C (Figure 2c,d), the comb-like nanostructures consisting of nanobelt backbones and parallel nanowire teeth are obtained. The width of the backbones is about 200-600 nm, and the thickness is around 10-40 nm. The teeth of the combs have diameters ranging from 20 to 40 nm and a length of up to 100-400 nm.

J. Phys. Chem. C, Vol. 111, No. 16, 2007 5877 The corresponding electron diffraction (ED) pattern (inset in Figure 2c) indicates that the comb teeth grow along [0001] and that the comb stem elongates along [011h0]. For a higher TR of 800 and 900 °C (Figure 2e,f), nanowires are fabricated, with a diameter of about 20-60 nm and a length of up to tens of micrometers. XRD patterns of these nanostructures are shown in Figure 3, which indicates that all these nanostructures are comprised of hexagonal wurtzite ZnO (JCPDS card no. 361451). It can be clearly seen that, with increasing TR from 600 to 900 °C, the diffraction peaks corresponding to the Cu-Zn substrates shift to higher diffraction angles and approach the standard XRD pattern for face-centered cubic Cu (JCPDS card no. 03-1005) when TR > 800 °C. This suggests that more Zn species in Zn-Cu foil were converted into ZnO nanostructures at higher temperatures, which made the Zn-Cu foil approach the metallic copper substrate above 800 °C. In other words, ZnO nanostructures can be grown on the most commonly used copper electrode when all the Zn species in the Zn-Cu foil are depleted at a high TR in this way. The products are further characterized by HRTEM and XPS, as typically shown in Figure 4. A lattice spacing of 0.52 nm between the neighboring fringes is clearly identified, in agreement with the d-spacing of the (0001) planes of wurtzite ZnO. The XPS spectrum shows there are only Zn and O peaks, suggesting that the as-synthesized nanostructures are pure ZnO. Xia et al. reported the growth of CuO nanowire arrays on Cu foils by a simple thermal oxidation process.20 In our case, although more than 66 atom % is Cu in the brass substrate, the formation of the CuO species was suppressed due to the presence of enough Zn. Even though little Cu was oxidized to CuO by chance, the newly formed CuO would be easily reduced back to pure Cu by the reaction of Zn + CuO ) ZnO + Cu. The previous results show that various ZnO nanostructures including nanosheets, nanocombs, and nanowires have been grown in situ on a brass substrate simply by directly heating the brass foils in Ar/O2 at a TR of 600-900 °C. In this temperature range, the Cu and Cu0.66Zn0.34 alloy are in a solid state, while Zn is in a liquid state whose vapor pressure increases rapidly with temperature.19,21 When the brass foil was heated at an elevated temperature, the Zn species would segregate and diffuse toward the surface to form Zn droplets that were partially vaporized. Once oxygen was introduced, the molten Zn was oxidized to form a thin ZnO grain film on the substrate surface that functioned as the seed for the further growth of ZnO nanostructures. This could be regarded as the nucleation stage. As the reaction proceeded, the subsequently segregated and vaporized Zn species through the grain boundaries or cleavages of the preformed ZnO grain film continuously reacted with oxygen and epitaxially grew on the preformed ZnO seeds and finally resulted in the formation of 1-D ZnO nanostructures on the substrate. This could be regarded as a growth stage, which is similar to the synthesis of 1-D ZnO nanostructures through ZnO seeds or buffer layers in the literature.16,22 When most of the Zn species in the Cu-Zn foil was consumed in this way, the Cu-Zn foil itself would approach the metal copper substrate. As a result, it could be approximately regarded that ZnO nanostructures were grown in situ on a copper substrate that naturally acted as a good conductive electrode. Since the Zn species in the ZnO product comes from the brass substrate, the reaction temperature has an important influence on the segregation and diffusion rates of the Zn atoms, as well as the Zn vapor pressure. The higher the reaction temperature is, the faster the Zn atoms segregate and diffuse, and the higher the Zn vapor pressure is. Since the O2 pressure is kept constant in our reaction,

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Figure 2. Typical SEM images of the products grown on Cu-Zn substrates at different temperatures. (a and b) 600 °C, (c and d) 700 °C, (e) 800 °C, and (f) 900 °C. Inset in panel a is the typical TEM image of the sample. Inset in panel c is the typical TEM image and corresponding ED pattern.

Figure 3. Room-temperature XRD patterns of the nanostructures obtained for different reaction temperatures.

the higher reaction temperature would give rise to a higher Zn/ O2 ratio.21 As learned from the literature,23 the Zn/O2 ratio is the major factor to determine the morphology of ZnO nanostructures. A relatively high Zn/O2 ratio is favorable for the formation of ZnO nanowires,24 and a relatively low Zn/O2 ratio is favorable for the formation of large and branched ZnO

nanostructures.25 Therefore, at a high temperature of 800 or 900 °C, the high Zn/O2 ratio facilitates the growth of nanowires. At a low temperature of 600 °C, a relatively low Zn/O2 ratio is in favor of the growth of ZnO crystals, resulting in the formation of large sheet-like structures. At an intermediate temperature of 700 °C, the sidebranch growth of nanowires on the basal nanobelt may occurr by polarity-induced growth due to a relatively supersaturated Zn vapor,9a, 26 giving comb-like ZnO nanostructures. Systematic studies are still needed to fully illuminate this point, for example, with different Zn-containing Cu-Zn substrates and different oxygen concentrations in the reaction gases. In principle, the morphology of the ZnO nanostructure should depend on the nucleation and growth conditions as demonstrated in Figure 2. Hence, we further tried to modify the morphology of the ZnO nanostructure by regulating the nucleation conditions. In reference to Figure 1, we first annealed the substrate at a low TR of 500 °C in Ar/O2 for 30 min to form a layer of a ZnO grain film on the substrate, as identified in Figure 5a. XRD (inset of Figure 5a) and XPS (see Figure S1a in the Supporting Information) analyses indicate that the grain film is pure ZnO. This sample (Figure 5a) was further annealed at 700 °C for another 60 min. Interestingly, quasi-aligned ZnO nanobelts were obtained on a brass substrate as shown in Figure 5b. A high

1-D ZnO Nanostructures from and on Brass Substrate

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Figure 4. Typical HRTEM image and XPS of the ZnO nanostructures.

Figure 5. (a) SEM image of ZnO grain film on the substrate formed by annealing Cu0.66Zn0.34 foil at a low TR of 500 °C in Ar/O2 for 30 min. Inset is the corresponding XRD pattern, indicating the hexagonal wurtzite ZnO. (b) SEM image of the quasi-aligned ZnO nanobelts obtained on a brass substrate by further annealing the sample (a) at 700 °C for another 60 min.

magnification SEM image (see Figure S1b in the Supporting Information) shows that most nanobelts in the quasi-aligned arrays do not have the aforementioned comb-like structure (Figure 2c,d) obtained by directly annealing the Cu0.66Zn0.34 foil at 700 °C in Ar/O2 for 60 min, although the growth temperatures are both at 700 °C. This resulted from the differences in the preparation procedures. When the Cu0.66Zn0.34 substrate was first annealed in Ar/O2 at 500 °C for 30 min, the partial Zn species segregated from the substrate to form a ZnO grain-like coating on the substrate surface, and the Zn concentration in the remaining substrate is much lower than that in the original substrate (Cu0.66Zn0.34). Hence, further heating at 700 °C would generate a relatively lower Zn vapor pressure in comparison with the case of directly annealing the Cu0.66Zn0.34 foil at 700 °C. As a result, the low Zn vapor pressure was not enough for further sidebranching growth to form a comb-like structure. Most likely, the preformed ZnO grains (Figure 5a) on the brass substrate acted as the seed layer for further epitaxial growth of the ZnO nanostructure, similar to the process by pre-coating a layer of ZnO nanoparticles for growing aligned nanobelts or nanowires.16,22 Hence, the aligned belt-like morphology was obtained by this two-step nucleation and epitaxial growth process. As known, carbon nanotubes (CNTs) have exhibited excellent field emission (FE) properties,27 yet they have some drawbacks such as the easy degradation in oxygen atmosphere,28 which would limit their applications in FE devices under poor vacuum or low-pressure gas-filled environments. The exploration for

some other good FE emitters in 1-D nanostructures is an important topic and of current interest because of their geometrical similarity to carbon nanotubes with a high field enhancement factor.29 Recent results have shown the quite good FE performance of 1-D ZnO nanostructures,8,30 especially the better endurance of oxygen ambient conditions as compared with CNTs.28a As seen, the products synthesized by our method have an ideal configuration, which integrates the 1-D ZnO nanostructures with a good conductive metal electrode. Consequently, improved FE properties and convenient applications could be expected. FE measurements were carried out using a parallel-plate diode configuration in a test chamber maintained at 1.5 × 10-6 Torr. The as-prepared ZnO nanostructures on the brass substrate were used as the FE cathodes. Another plate-shaped stainless steel electrode was used as an anode with a sample-anode distance of 200 µm. Since both sides of the Cu-Zn substrate are coated with a layer of ZnO nanostructures, the bottom side was polished with SiC sandpaper to remove the coating layer when measuring the FE properties. The emission current under the applied voltage with a step of 100 V supplied by a power source (Keithley 248) was measured with a Keithley 6514 electrometer. Figure 6a plots the field emission current density as a function of the applied electric field for the different ZnO nanostructures previously mentioned. A stable emission could be realized by conducting an electrical annealing at a current density of about 1 mA/cm2 for 2 h before the measurement. Here, we define the turn-on field as an electric field to produce a current density of

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Huo et al. important factors for β. In general, a small tip diameter, large aspect ratio, and good alignment of emitters are favorable for a high β value. In addition, the emitters’ density also has an important influence on the β value. The higher density would generate a greater screening effect to decrease the β value.33 Hence, it is expected that the ZnO nanostructures with a smaller emitter size, better alignment, and appropriate density should have a better FE performance. From Figure 2, it is seen that the nanosheets, nanocombs, and nanowires are randomly distributed on the substrate and have a similar density; thus, the sizes and morphologies of these emitters should be the main factors to influence their FE performance. Comparing the three samples, the nanosheets have the largest size, and the nanowires have the smallest tip diameter, while the nanocombs are composed of a mixture of large-sized backbones with smallsized teeth. Hence, the nanowires should possess the lowest turnon field and the nanosheets the highest turn-on field, while the nanocombs possess an intermediate turn-on field, which is consistent with the experimental results shown in Figure 6. As to the quasi-aligned nanobelts in Figure 5b, it is seen that the tip sizes are approximately at the same level as the diameters of the nanowires. The good alignment could further enhance the β value. Hence, the quasi-aligned nanobelts should have even better FE properties than the nanowires. The calculated β is 6720, the highest in the four examined samples. Conclusion

Figure 6. (a) Field emission current density (J) as a function of the applied electric field (E) for the different ZnO nanostructures and (b) the corresponding F-N plots.

10 µA/cm2. It is found that the turn-on field for ZnO nanosheets, nanocombs, nanowires, and quasi-aligned nanobelts is about 3.9, 3.8, 3.0, and 2.3 V/µm, respectively. The applied field for the emission current densities of 1 mA/cm2 is 8.9, 7.6, 6.8, and 5.3 V/µm correspondingly. It is seen that the applied fields for generating a 10 µA/cm2 (turn-on field) or 1 mA/cm2 current density for our ZnO nanostructures are generally lower than those for the ZnO nanostructures in the literature30 and comparable to those for carbon nanotubes.27 The excellent FE performance of the ZnO nanostructures in this study should be attributed to the direct growth of ZnO nanostructures on a metallic substrate with a naturally good adhesion or connection between ZnO and conductive electrode. The field emission current-voltage characteristics are further analyzed by using the Fowler-Nordheim (F-N) equation31

J ) (Aβ2E2/φ) exp(-Bφ3/2/βE) where J is the current density, E is the applied field, φ is the work function of the emitting material, β is the field enhancement factor, and A and B are constants with values of 1.56 × 10-10 (A eV V-2) and 6.83 × 103 (V eV-3/2 µm-1), respectively. The F-N plots of ln(J/E2) versus 1/E are shown in Figure 6b. The linearity of the F-N curves within the measurement range confirms that the electron emission from ZnO nanostructures follows the F-N behavior. Taking the work function of ZnO as 5.3 eV,32 the field enhancement factor β was estimated to be about 1600, 4208, 4611, and 6720 for the nanosheets, nanocombs, nanowires, and quasi-aligned nanobelts, respectively. As known, the aspect ratio and the tip radius are the two

In conclusion, we have developed a convenient method for the direct growth of 1-D ZnO nanostructures on conductive metallic substrates simply by annealing a Cu0.66Zn0.34 alloy foil in Ar/O2. Various 1-D nanostructures such as nanowires, nanobelts, nanocombs, and nanosheets have been grown in situ on the metallic substrate in a temperature range of 500-900 °C. In this growth process, the Cu0.66Zn0.34 foil provides both Zn source and substrate for the growth of 1-D ZnO nanostructures; thus, the synthesis of the ZnO nanostructures and their assembly on the conductive substrate is accomplished in one step. Such a configuration of the product is an ideal field emitter due to the naturally good adhesion or electrical connection between ZnO nanostructures and conductive electrode. As a result, the field emission performances of the products in this study are generally better in comparison with those for the ZnO nanostructures in the literature as expected. The growth mechanism has also been suggested. This technique route could prepare ZnO nanostructures on a brass substrate without a size limitation. The potential technological importance of the product, the simplicity of the preparation procedure, as well as the cheap commercial precursor of the Cu-Zn alloy foil makes this study both scientifically and technologically interesting. Acknowledgment. This work was financially supported by the NSFC (20525312 and 20471028), the Chinese Ministry of Education (NCET-04-0449 and 20040284006), and the City University of Hong Kong Direct Allocation Grant (9360110). K.F. also thanks the Hubei Province Key Laboratory of Refractories and Ceramics Ministry Province Jointly Constructed Cultivation Base for the State Key Laboratory (G0609) for support. Supporting Information Available: XPS pattern of grain film shown in Figure 5a (Figure S1a) and high magnification SEM image of quasi-aligned ZnO nanobelts shown in Figure 5b (Figure S1b). This material is available free of charge via the Internet at http://pubs.acs.org.

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