Nanorods-Decorated Zinc Oxide

Nov 17, 2010 - Transparent zinc oxide nanowires orderly decorated with silver nanoparticles/nanorods were synthesized by thermal evaporating of metall...
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Transparent Silver-Nanoparticles/Nanorods-Decorated Zinc Oxide Nanowires Guozhen Shen* and Di Chen Wuhan National Laboratory for Optoelectronics and College of Optoelectronic Science and Engineering, Huazhong UniVersity of Science and Technology, Wuhan 430074, P. R. China ReceiVed: August 1, 2010; ReVised Manuscript ReceiVed: October 30, 2010

Transparent zinc oxide nanowires orderly decorated with silver nanoparticles/nanorods were synthesized by thermal evaporating of metallic zinc powders at 550 °C by using Ag nanoparticles-coated silicon as the substrate. As-obtained ZnO nanowires are single crystals with the preferred growth directions along the [0001] plane, with Ag nanoparticles/nanorods orderly attached to the whole length of the nanowires. Single ZnO-nanowirebased devices were fabricated, and it is revealed that as-synthesized Ag-nanoparticle/nanorods-decorated ZnO nanowires are transparent conductors with resistivities down to 6.8 × 10-4 Ωcm and failure-current density up to 4.5 × 107 A/cm2 because of the single-crystalline metallic structure. 1. Introduction As a unique group of materials that offer high optical transparency in the visible range, low electrical resistivity, and high thermal stability, transparent conducting oxides (TCOs) have attracted great attention in recent years. They are widely used as transparent coatings or electrodes in flat-panel displays, solar cells, radiation protection, light emitting diodes, and Liion batteries.1-6 Common TCO materials include donor-doped oxides such as tin oxide (SnO2), indium oxide (In2O3),and zinc oxide (ZnO) and their ternary alloys (ITO, etc.), CuGaO2, CuAlO2, and others.6-10 Recently, it was found that the performance of the TCO devices may be further improved by using one-dimensional (1-D) nanostructures, such as nanowires, nanotubes, and nanobelts.11-15 One efficient way to improve the performance of 1-D TCO devices is to dope 1-D TCO nanostructures with proper dopants, which is of great significance for both technological applications and fundamental understanding.11,13,16,17 For example, Lu et al. synthesized Sn-doped In2O3 (ITO) nanowire arrays, which show a very low resistivity of 6.29 × 10-5 Ωcm and a high failurecurrent density of 3.1 × 107 A/cm2. Transparent metallic Sbdoped SnO2 nanowires, In-doped ZnO nanowires, and so forth were also studied for the purpose of high-performance TCO devices. Zinc oxide (ZnO) is a promising TCO because of its low electrical resistivity, high optical transparency, and high thermal stability.18-22 Formation of 1-D ZnO nanostructures with proper dopants might provide a good way to meet the demand for TCO with improved performance. In this paper, a simple one-step growth process was employed to prepare ZnO nanowires orderly decorated with Ag nanoparticles/nanorods. Our results show that the prepared special nanowires can act as metallic conductors with a resistivity down to 6.8 × 10-4 Ωcm and a failure-current density up to 4.5 × 107 A/cm2 and may provide an inexpensive alternative to ITO as TCO materials. 2. Experimental Section The Ag-nanoparticles/nanorods-decorated ZnO nanowires were synthesized in a horizontal tube furnace by thermal * Author to whom correspondence should be addressed. E-mail: [email protected].

evaporating of metallic zinc powders at 550 °C by using Ag nanoparticles-coated silicon as the substrate. The Ag nanoparticles were prepared via a polyol process according to the literature.23 Briefly, appropriate amounts of AgNO3 and polyvinyl pyrrolidone (PVP) were dissolved in ethylene glocol, which was then fluxed at 197 °C for 30 min. After synthesis, the product (nanoparticles, 50-200 nm in diameters) was centrifuged, dried, and then redispersed in isopropanol (IPA) solution and spin-coated on silicon substrate. In a typical process to the Ag-decorated ZnO nanowires, a quartz tube was mounted inside the tube furnace. An amount of 0.13 g zinc powders (100 mesh, 99.998%) was placed in a ceramic boat, which was placed in the center of the quartz tube. A piece of prepared Agnanoparticle-coated silicon substrate was then put downstream in the tube (about 5 mm away from the center). During growth, the furnace was heated from room temperature to 550 °C under Ar flow at 200 SCCM (SCCM denotes cubic centimeter per minute at STP) with a trace amount of oxygen. The growth time was 1 h. When the furnace was cooled to room temperature, a white layer was obtained on the substrate. 3. Results and Discussion 3.1. Structure and Morphology. The structure of the synthesized products was investigated by using an X-ray powder diffractometer (RINT 2200). The pattern is shown in Figure 1 and can be easily indexed to pure ZnO with wurtzite structure. Besides the peaks from ZnO, no obvious peaks from other crystalline phase were observed, indicating the purity of the product. The morphology of the products was observed by using a Hitachi field-emission scanning electron microscope (SEM, S-4800), and a typical SEM image is shown in Figure 2. Large quantities of nanowires were found to be deposited on the substrate. Typical nanowires have diameters of 50-100 nm and lengths up to several tens of micrometers. Most of the nanowires have smooth surfaces with little size variation along the nanowire axis. The microstructure and chemical composition of the deposited nanowires were further characterized by using a JEOL 300 kV field-emission transmission electron microscope (TEM, JEM3000F) equipped with energy dispersive X-ray spectrometry (EDS). Figure 3a is a TEM image of the synthesized nanowires.

10.1021/jp107213q  2010 American Chemical Society Published on Web 11/17/2010

Transparent ag-Nanoparticles/Nanorods-Decorated ZnO Nanowires

Figure 1. XRD pattern of as-synthesized product.

Figure 2. SEM images of as-synthesized ZnO product.

As shown in this image, each nanowire is actually not a monolithic nanowire but a nanowire with many nanoparticles/ nanorods almost periodically attached to its surface, which can be easily seen from the brightness contrast along the nanowire. Carefully checking hundreds of the synthesized nanowires shows that about 80% of them are nanoparticles-decorated nanowires similar to those shown in Figure 3a. Further analysis of the nanoparticles-decorated nanowires by using EDS, detected by nanobeam with a spot size of 20 nm, was performed on the nanowire itself and the nanoparticles. The EDS results are shown in Figure 3g,h. It was found that the nanowire itself is composed of pure Zn and O, indicating the formation of ZnO nanowires, in agreement with the XRD result. The nanoparticles/nanorods were found to be composed of Ag, indicating the decorated Ag nanoparticles along the ZnO nanowires.24 More TEM data are shown in Figure 3b-e to demonstrate the detailed microstructures of the Ag-decorated ZnO nanowires. Generally, there are two kinds of Ag-decorated nanowires coexisting in the product. One is the ZnO nanowires decorated with Ag nanoparticles (Type-I nanowires), and the other is the ZnO nanowires decorated with short Ag nanorods (Type-II nanowires). Figure 3b,c are TEM images of the synthesized Type-I nanowires. From these images, we can clearly see that as-synthesized ZnO nanowires are ∼50-100 nm in diameters with many Ag nanoparticles attached to them, and the size of the Ag nanoparticles are around 20 nm. Type-II nanowire is shown in Figure 3d,e. Contrary to the Type-I nanowire, the Ag decorated to the nanowires exist as short nanorods instead of nanoparticles shown in Figure 3c. The Ag nanorods are about 5-20 nm in diameter and have lengths in the range of 10-100 nm. In most cases,

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21089 the distance between two Ag nanoparticles/nanorods was found to be around 50-200 nm. Figure 3f is a SAED pattern taken from a single ZnO nanowire, indicating the single-crystal nature of the nanowire. A HRTEM image of the nanowire is shown in Figure 3i. The clearly resolved lattice fringe perpendicular to the growth direction is 0.26 nm, corresponding to the (0002) plane of wurtzite ZnO. The results verified that as-obtained nanostructures are single crystals with the preferred growth directions along the [0001] orientation. We compared the microstructures of the Ag nanoparticles before and after reaction, and the corresponding results are shown in Figure 4. Figure 4a is a TEM image of a typical Ag nanoparticle obtained from the polyol process. An HRTEM image is depicted in Figure 4b, where the calculated lattice distance is found to be around 2.3 6 nm, corresponding to the {111} planes of cubic Ag phase. The TEM image of an Ag nanorod decorated on a ZnO nanowire is shown in Figure 4c; the diameter is ∼30 nm, and the length is ∼200 nm. The clearly resolved lattice shown in Figure 4d is also calculated to be 0.236 nm, in accordance with the {111} planes of cubic Ag phase. Deriving a detailed mechanism for the formation of such an interesting structure is still a challenge. We first tried to give some explanations from the viewpoint of thermodynamics. Reacted at 550 °C, the following reactions may happen:

2Zn + O2 f 2ZnO

(1)

4Ag + O2 f 2Ag2O

(2)

Zn + Ag2O f ZnO + 2Ag

(3)

The Gibbs free energy calculated for these reactions are -535.7, 47.1, and -291.4 kJ/mol, respectively. From these values, it can be seen that Ag will not be oxidized under 550 °C, which is similar with previous reports that Ag2O will decompose at temperature higher than 523 K. It also confirms the HRTEM and EDS results. During the reaction, because no catalyst was used and because there is no particle attached to the tips of the obtained heterostructures, the growth of Ag nanoparticles embedded ZnO nanowires is thought to be governed by vapor-solid (VS) mechanism. At a high reaction temperature, Zn powder evaporates spontaneously, and the vapors are transferred by the carrier gas to a low-temperature region. The transferred Zn vapors react with oxygen to generate ZnO nuclei in the low-temperature region. The growth of ZnO nanowires then occurs via a VS process. Previously, Weller et al. reported the synthesis of size-specific deposition of Ag nanoparticles on ZnO nanorods by utilization of the intrinsic properties of nanocrystals for the assembly, which were extensively investigated by El-sayed, Murphy, et al. in the solution synthetic methods.25-29 Weller et al. found that the photoreduction preferentially occurred at one end of the ZnO nanorods, and a site-selective deposition mechanism was thus proposed to explain the possible growth mechanism. As for our results here, by considering the experimental results and HRTEM analyses in Figure 4d, we propose a similar mechanism to explain the growth of Ag-nanoparticle/nanorods-decorated ZnO nanowires, although our experiments were performed in the vapor phase. In the reaction system, ZnO nanowires were formed via a VS mechanism. At the same time, Ag was evaporated, and the vapors were transferred by the carrier gas and deposited on the surface of ZnO nanowires as site-preferred nuclei. Because of the relatively small lattice mismatch between

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Figure 3. (a-d) Representative TEM images of as-synthesized Ag-decorated ZnO nanowires. (e) High-magnification TEM image showing four Ag nanoparticles orderly attached to a single ZnO nanowire. (f) SAED pattern taken from the ZnO nanowire, indicating its single-crystalline nature. (g,h) EDS spectra collected from the ZnO nanowire and the Ag nanoparticles, respectively. (i) HRTEM image of the Ag-decorated ZnO nanowire.

Figure 4. TEM and HRTEM images of (a,b) Ag nanoparticles before the reaction and (c,d) Ag nanorods decorated on ZnO nanowires.

the Ag {111} planes (d ) 0.236 nm) and the ZnO (0002) plane (d ) 0.26 nm), Ag nanoparticles preferred to deposit on the (0001) plane of ZnO nanowires, which is confirmed from the HRTEM image in Figure 4d. It is well known that bulk Ag has a melting point of around 960 °C; the melting point decreases dramatically when the size of particles lies in the nanoscale range. For example, Ag nanoparticles with a diameter of 5 nm have a melting point of around 100 °C. In our experiment, ZnO nanowires were found to be deposited on substrate with a temperature around 450-500 °C. Under such a temperature, Ag nuclei deposited on ZnO nanowires exist as droplets, and they tended to migrate on the surface of ZnO nanowires and to aggregate into bigger nanoparticles, which made them distributed periodically on ZnO nanowire surface. After reaction, the temperature in the furnace gradually decreased; bigger Ag

nanoparticles tended to crystallize, and smaller Ag nanoparticles still existed as droplets, migrated, and aggregated. Because of the crystallographic relationship between Ag and ZnO analyzed above, Ag nanoparticles tend to grow into nanorods via a process analogous to the Ostwald-ripening process along the {111} planes. The growth stopped in the case of temperatures below the melting point of Ag nanoparticles or when all Ag sources were consumed. Here, it should be mentioned that the growth of Ag-decorated ZnO nanowires in the vapor-phase process is quite complicated. The effect of other factors, such as the competition of Ag source, gas-concentration changes, and flowing gas should also be considered when trying to understand the exact growth process. The Ag-decorated ZnO nanowires deposited on silicon substrate were then physically transferred by a dry transfer process to a glass substrate. The optical transmittance of the glass substrate with transferred nanowires (red line), as well as the bare glass substrate (black line), is shown in Figure 5. The bare glass substrate was found to show an optical transmittance higher than 90%, whereas the glass substrate with transferred nanowires shows ∼80% optical transmittance in the visiblelight range. A colorful logo beneath the substrate due to the penetration of the visible light can easily be seen (Figure 5 inset). 3.2. Optical Properties. The optical properties of the synthesized Ag-decorated ZnO nanowires were also investigated at room temperature by a photoluminescence (PL) system by using a He-Cd laser with an excitation wavelength of 325 nm. The corresponding PL spectrum is shown in Figure 6. For comparison, the PL spectrum of pure ZnO nanowires is also shown in the figure. For both samples, a sharp UV emission at around 385 nm was observed. According to our previous results,30-33 the UV emission corresponds to the near-band-edge emission that is responsible for the recombination of free excitons through an exciton-exciton collision process. Besides the UV emission, broad green emissions centered at around 493

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Figure 5. Optical transmittance curves of (a) bare glass substrate (black curve) and (b) glass substrate with transferred Ag-decorated ZnO nanowires (red curve).

Figure 6. Room-temperature PL spectra of the Ag-decorated ZnO nanowires (black line) and pure ZnO nanowires (red line).

and 543 nm were observed for the Ag-decorated ZnO nanowires and the pure ZnO nanowires, respectively. Contrary to the UV emission, the visible-light emissions for ZnO nanostructures are rather contradictory. There are many explanations for their origination. The general accepted one is that the green emission is a deep-level emission that is related to the singly ionized oxygen vacancies. As for our nanostructures here, it was also thought to partially result from the recombination of a photogenerated hole with a singly ionized charge state of the specific defects related to the special nanostructures. 3.3. Single Nanowire Device and Electric-Transport Properties. To study the electrical properties of the Ag-decorated ZnO nanowires, single nanowire devices were fabricated according to our previously reported technique.34,35 Typically, assynthesized nanowires were sonicated into a suspension with desired concentration in IPA and then deposited onto a degenerately doped silicon wafer covered with 100 nm SiO2. A photolithography process was employed to define electrode patterns on the substrate, followed by metal deposition of Ti/ Au as the source and drain electrodes on both ends of the nanowire. Prior to metal evaporation, the sample was cleaned with O2 plasma to remove the residue photoresist. Figure 7a is the Ids-Vds curve obtained on the single nanowire device. Linear current versus voltage curves were observed on all devices fabricated, indicating good Ohmic contacts between the nanowire and the electrodes. The inset in Figure 7a is a SEM image of the device, where a nanowire with a diameter of ∼150 nm

Figure 7. (a) Room-temperature current-voltage (Ids-Vds) curve of the Ag-decorated ZnO nanowire. Inset is a SEM image of the individual nanowire device. (b) Current-voltage curve recorded from an individual device that breaks down at high current. Inset is a SEM image of the nanowire after current breakdown.

was found, and the channel length between the source and drain electrodes of the device is around 1.5 µm. From the Ids-Vds data and SEM result, we can deduce that the resistivity value of a typical device is around 6.8 × 10-4 Ωcm, which is much lower than that of pure ZnO nanowires reported before. The Ids-Vg curve of the Ag-decorated ZnO nanowire device was also measured. The curve looks rather flat, and no obvious gating effect was found for the Ag-decorated ZnO nanowires. Temperature-dependence resistance was also performed. Along with the result in Figure 7a, they indicate the metallic-like behavior of our special ZnO nanowires. Tens of as-synthesized ZnO nanowires were studied, and it was found that they all show resistivity values below 1.1 × 10-4 Ωcm. During the measurement, we found no obvious correlation between the resistivity values and the nanowire diameter or shape. Figure 7b depicts the current-voltage curve recorded on a single nanowire device at large biases. The nanowire was found to endure a current up to 7.8 mA, which corresponds to a failure-current density of 4.5 × 107 A/cm2. A SEM image of the device after breakdown is shown in the inset of Figure 7b. The failure occurred in the middle of the nanowire, and it can be attributed to the melting of nanowires because of resistive self-heating. The result is in good agreement with the other reports on metallic Sb-doped SnO2 nanowire and other nanowire devices.11,13 For comparison, we synthesized pure ZnO nanowires by using Si as the substrate instead of Ag-nanoparticle-coated Si. Figure 8a shows a general SEM image of as-deposited product, where nanowires are found on a large scale. The XRD result indicates

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Figure 8. SEM images (a,b), TEM image (c), and HRTEM image (d) of the synthesized pure ZnO nanowires synthesized without Ag nanoparticles.

the formation of pure ZnO structure. A high-magnification SEM image of the product is shown in Figure 8b. Typical ZnO nanowires have diameters of ∼30 nm with a smooth surface. A TEM image and lattice-resolved HRTEM image are shown in Figure 8c,d, respectively. The results reveal that assynthesized pure ZnO nanowires are single crystals with the preferred growth directions along the (0001) plane. We also made a single ZnO nanowire device to investigate the electronic-transport behaviors. Figure 9a shows gatedependent current-voltage curves of a single device measured in air. The applied gate voltages to the device range from -20 to 20 V. The as-fabricated devices show very good Ohmic contacts from the linear shape of the curves. Decrease in conductance at Vg < 0 and increase in conductance at Vg > 0 reveal that as-synthesized ZnO nanowires are n-type semiconductors. The Ids-Vg curve of the ZnO nanowire device is shown in Figure 9b. Similarly, the result confirms the n-type semiconducting behavior of as-synthesized ZnO nanowires. From the above results, it can be seen that the Ag-decorated ZnO nanowires show metallic behavior instead of n-type semiconducting behavior for pure ZnO nanowires. It was thought that the present Ag-decorated ZnO nanowires may carry a very high current density because they have a single-crystal nature according to previous reports on other metal oxide nanowires. The low resistivities of the present Ag-decorated ZnO nanowires may be caused by two factors. One is the degenerately decorated Ag nanoparticles on ZnO nanowires. Attaching Ag nanoparticles to ZnO nanowires forms many Ag-ZnO junctions, which greatly change the surface environments of ZnO nanowires, thus decreasing their resistance. The other is the proper doping of Ag into the ZnO nanowires. During our synthesis, it is inevitable to dope ZnO nanowires with Ag at high reaction temperature, although we did not detect any signal from Ag element in Figure 3g. This result is similar to the other work on doped metal oxide nanowires.13,16,17 4. Conclusion In conclusion, we successfully synthesized ZnO nanowires with orderly decorated silver nanoparticles/nanorods via a simple

Figure 9. Electronic-transport behavior of as-synthesized pure ZnO nanowires.

thermal-evaporation method. As-obtained ZnO nanowires are single crystals with the preferred growth directions along the [0001] plane, with Ag nanoparticles/nanorods orderly attached to the whole length of the nanowires. Compared with pure ZnO nanowires, electrical transport characterizations demonstrate that the Ag-decorated ZnO nanowires are metallic conductors with low resistivities down to 6.8 × 10-4 Ωcm and failure-current densities up to 4.5 × 107 A/cm2. As-obtained special Agdecorated ZnO nanowires may find applications in many areas, such as field emitters, transparent conductive oxide electrodes, and so on. Acknowledgment. This work was supported by the National Natural Science Foundation (51002059), the High-level Talent Recruitment Foundation of Huazhong University of Science and Technology, the Basic Scientific Research Funds for Central Colleges (C2009Q045), the Natural Science Foundation of Hubei Province (2009CDB326), and the Research Fund for the Doctoral Program of Higher Education (20090142120059). References and Notes (1) Kawazoe, H.; Yasukawa, M.; Kurita, M.; Yanagi, H.; Hosono, H. Nature 1997, 389, 939. (2) Lewis, J.; Grego, S.; Chalamala, B.; Vick, E.; Temple, D. Appl. Phys. Lett. 2004, 85, 3450. (3) Banerjee, D.; Jo, S. H.; Ren, Z. F. AdV. Mater. 2004, 16, 2028. (4) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455. (5) Ginely, D. S.; Bright, C. MRS Bull. 2000, 25, 15. (6) Kim, D. W.; Hwang, I. S.; Kwon, S. J.; Kang, H. Y.; Park, K. S.; Choi, Y. J.; Choi, K. J.; Park, J. G. Nano Lett. 2007, 7, 3041.

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