Synthesis and electrical property of metal/ZnO coaxial nanocables
© Li et al.; licensee Springer. 2012
Received: 1 March 2012
Accepted: 19 June 2012
Published: 19 June 2012
Ag/ZnO and Cu/ZnO coaxial nanocables were fabricated using AgNO3 or copper foil as source materials by the vapor-liquid-solid process. The coaxial nanocables consist of a crystalline metallic Ag or Cu core and a semiconductor ZnO shell. The evolution of the Ag/ZnO products having different morphologies was investigated by stopping the heating at different temperatures. The diameters of the Ag/ZnO nanocables and the Ag cores could be modulated by changing Ag ratio in the source. The electrical characteristics of the Ag/ZnO contact and the influence of annealing reveal a Schottky diode behavior for a single Ag/ZnO nanocable device. The nanocables with uniform shape and controlled size are expected to provide a new choice in various applications of biological detection, nanothermometer, and photocatalysis.
Metal-semiconductor heterogeneous nanostructures have attracted particular attention due to their unique optical, electrical, and catalytic properties [1–3]. ZnO, with a direct wide bandgap energy of 3.37 eV at room temperature, is an important short-wavelength optoelectronic material and has drawn much attention. An enormous variety of ZnO nanostructures such as nanowires, nanobelts, and nanocables has been synthesized by a variety of techniques [4–6]. In particular, metal-ZnO nano-heterostructures have become an active frontier because of their wide application in dye-sensitized solar cells, photocatalysis, and biological detection [7–14]. For example, metal-ZnO Schottky diode is a fundamental component of a device for realizing one-dimensional (1D) nanoelectronics, which is useful for hydrogen sensor, strain sensor, and electrical switch [15–17]. Recently, some metal-ZnO nanostructures have been prepared by wet chemical routes, such as Au/ZnO hybrid nanoparticles, Pb/ZnO nanocables, and ZnO loaded with metal tips or dots [18–24]. However, nanostructures synthesized by wet chemical route usually have poor crystalline quality. The vapor-liquid-solid (VLS) method has been considered as the most promising method for fabricating 1D nanostructures, owing to the fact that the size of the nanostructures can be precisely controlled by the metal catalyst. A variety of functional 1D nanostructures, including silicon nanowires, carbon nanotubes, GaP nanowires, and ZnO nanobelts, has been demonstrated [25–28]. In addition, as far as we know, only Pb/ZnO and Zn/ZnO nanocables were studied in the case of metal/ZnO nanocables [20, 29] Moreover, few properties of the metal-ZnO heterostructures were investigated, especially their electrical characteristics. Therefore, it is important to explore reliable approaches to synthesize metal/ZnO coaxial nanocables with different metal components and to study their contact characteristics.
Here, we report a one-step thermal evaporation route to fabricate the Ag/ZnO and Cu/ZnO coaxial nanocables via the VLS growth mechanism. The coaxial nanocables consist of a crystalline metallic Ag or Cu core and a semiconductor ZnO shell. The diameters of Ag/ZnO nanocables and the Ag cores could be modulated by changing Ag ratio in the source. The electrical characteristics of the Ag/ZnO contact and the influence of annealing on the contact were investigated. This approach can be extended to synthesize other metal/semiconductor coaxial nanocables for applications of the nanodevices.
Ag/ZnO nanocables were synthesized in a conventional tube furnace using AgNO3 and ZnO as the source. An amount of 0.5 g of AgNO3 powder was placed at the bottom of an alumina boat, and 1 g of ZnO powder was added on the AgNO3 layer. The boat was put into the center of a ceramic tube that was mounted on the tube furnace. As a substrate, a Si (100) wafer was put in the low-temperature zone. The tube was evacuated to 10−2 Torr before it was heated, and the pressure was kept through the synthesizing process. No carrier gas was used in the whole process. It took about 35 min before the tube reached the desired temperature of about 1,300°C when the substrate temperature was about 950°C. Then, the heating was turned off, and the system was naturally cooled to room temperature. Cu/ZnO nanocables were obtained under the same condition except that 0.12 g copper foil, instead of AgNO3 powder, was used as the source material and that the substrate temperature was about 1,050°C.
The as-grown products were characterized by X-ray diffraction (XRD) with Cu Kα radiation (wavelength, 1.5045 Å) used as an incident X-ray source. Scanning electron microscopy (SEM) images were obtained on a field-emission SEM (JEOL JSM-6700 F, JEOL Ltd., Akishima, Tokyo, Japan). The products were also examined using a high-resolution transmission electron microscope (HRTEM, JEOL 2010, JEOL Ltd., Akishima, Tokyo, Japan) operating at 200 kV and X-ray photoelectron spectroscopy (XPS; VG ESCALAB X-ray photoelectron spectra spectrometer, VG Scientia Inc., Pleasanton, CA, USA). Their components were determined via energy-dispersive X-ray spectroscopy (EDS) attached in the HRTEM system. Photoluminescence (PL) spectra were measured at room temperature using a He-Cd laser (325 nm) as excitation source.
The electrical measurements of a single Ag/ZnO nanocable were performed by a one-axis manual linear translation stage. A nanocable was affixed to the tip of a tungsten probe attached on the stage by conductive silver epoxy. The nanocable was driven by the stage to approach an Au foil gradually until the Ag particle at the tip of the nanocable was in contact with the Au foil. The current-voltage (I-V) characteristics were measured by a picoammeter/voltage source (Keithley 6487 model, Keithley Instruments Inc., Cleveland, OH, USA) when voltage was applied between the ZnO shell and the Ag core.
Results and discussion
Schottky barrier properties of Ag-ZnO contact before and after 400°C annealing
As-grown Ag/ZnO nanocables
1.004 × 10−10
8.051 × 104
Ag/ZnO nanocable after 400°C annealing in air
1.811 × 10−10
1.164 × 105
In summary, Ag/ZnO and Cu/ZnO coaxial nanocables were fabricated by thermally evaporating the source material of ZnO and AgNO3 (or copper foil) using the vapor-liquid-solid mechanism. SEM, TEM, and EDS results reveal that the coaxial nanocables consist of a crystalline metallic Ag or Cu core and a semiconductor ZnO shell. The diameters of Ag/ZnO nanocables and the Ag cores could be modulated by changing Ag ratio in the source. PL measurements show that Ag/ZnO and Cu/ZnO coaxial nanocables have the band edge emissions and the deep trap emissions due to defect states. The electrical characteristics of the Ag/ZnO contact and the influence of annealing reveal a Schottky diode behavior for a single Ag/ZnO nanocable device.
This work was supported by the National Basic Research Program of China (2009CB939901, 2011CB921400) and the Natural Science Foundation of China (Grant Nos. 50772110, 50721091).
- Enache DI, Edwards JK, Landon P, Solsona-Espriu B, Carley AF, Herzing AA, Watanabe M, Kiely CJ, Knight DW, Hutchings GJ: Solvent-free oxidation of primary alcohols to aldehydes using Au-Pd/TiO2 catalysts. Science 2006, 311: 362. 10.1126/science.1120560View ArticleGoogle Scholar
- Lee JS, Shevchenko EV, Talapin DV: Au-PbS core-shell nanocrystals: plasmonic absorption enhancement and electrical doping via intra-particle charge transfer. J Am Chem Soc 2008, 130: 9673. 10.1021/ja802890fView ArticleGoogle Scholar
- Zhang WQ, Lu Y, Zhang TK, Xu W, Zhang M, Yu SH: Controlled synthesis and biocompatibility of water-soluble ZnO nanorods/Au nanocomposites with tunable UV and visible emission intensity. J Phys Chem C 2008, 112: 19872. 10.1021/jp804547eView ArticleGoogle Scholar
- Kong XY, Ding Y, Yang RS, Wang ZL: Single-crystal nanorings formed by epitaxial self-coiling of polar-nanobelts. Science 2004, 303: 1348. 10.1126/science.1092356View ArticleGoogle Scholar
- Fouad OA, Glaspell G, El-Shall MS: Structural, optical and gas sensing properties of ZnO, SnO2 and ZTO nanostructures. NANO: Brief Report and Reviews 2010, 5: 185.View ArticleGoogle Scholar
- Fouad OA, Khder AS, Dai Q, El-Shall MS: Structural and catalytic properties of ZnO and Al2O3 nanostructures loaded with metal nanoparticles. J Nanoparticle Res 2011, 13: 7075. 10.1007/s11051-011-0620-8View ArticleGoogle Scholar
- Chen ZH, Tang YB, Liu CP, Leung YH, Yuan GD, Chen LM, Wang YQ, Bello I, Zapien JA, Zhang WJ, Lee CS, Lee ST: Vertically aligned ZnO nanorod arrays sentisized with gold nanoparticles for Schottky barrier photovoltaic cells. J Phys Chem C 2009, 113: 13433. 10.1021/jp903153wView ArticleGoogle Scholar
- Ren CL, Yang BF, Wu M, Xu J, Fu ZP, Lv Y, Guo T, Zhao YX, Zhu CQ: Synthesis of Ag/ZnO nanorods array with enhanced photocatalytic performance. J Hazard Mater 2010, 182: 123. 10.1016/j.jhazmat.2010.05.141View ArticleGoogle Scholar
- Zeng HB, Liu PH, Cai WP, Yang SK, Xu XX: Controllable Pt/ZnO porous nanocages with improved photocatalytic activity. J Phys Chem C 2008, 112: 19620. 10.1021/jp807309sView ArticleGoogle Scholar
- Lin DD, Wu H, Zhang R, Pan W: Enhanced photocatalysis of electrospun Ag-ZnO heterostructured nanofibers. Chem Mater 2009, 21: 3479. 10.1021/cm900225pView ArticleGoogle Scholar
- Georgekutty R, Seery MK, Pillai SC: A highly efficient Ag-ZnO photocatalyst: synthesis, properties, and mechanism. J Phys Chem C 2008, 112: 13563. 10.1021/jp802729aView ArticleGoogle Scholar
- Zheng YH, Zheng LR, Zhan YY, Lin XY, Zheng Q, Wei KM: Ag/ZnO heterostructure nanocrystals: synthesis, characterization, and photocatalysis. Inorg Chem 2007, 46: 6980. 10.1021/ic700688fView ArticleGoogle Scholar
- Gu CD, Cheng C, Huang HY, Wong TL, Wang N, Zhang TY: Growth and photocatalytic activity of dendrite-like ZnO@Ag heterostructure nanocrystals. Cryst. Growth Des 2009, 9: 3278. 10.1021/cg900043kView ArticleGoogle Scholar
- Subramanian V, Wolf EE, Kamat PV: Green emission to probe photoinduced charging events in ZnO-Au nanoparticles. Charge distribution and Fermi-level equilibration. J Phys Chem B 2003, 107: 7479. 10.1021/jp0275037View ArticleGoogle Scholar
- Das SN, Kar JP, Choi J-H, Lee TI, Moon K-J, Myoung J-M: Fabrication and characterization of ZnO single nanowire-based hydrogen sensor. J Phys Chem C 2010, 114: 1689. 10.1021/jp910515bView ArticleGoogle Scholar
- Zhou J, Gu YD, Fei P, Mai WJ, Gao YF, Yang RS, Bao G, Wang ZL: Flexible piezotronic strain sensor. Nano Lett 2008, 8: 3035. 10.1021/nl802367tView ArticleGoogle Scholar
- Zhou J, Fei P, Gu YD, Mai WJ, Gao YF, Yang RS, Bao G, Wang ZL: Piezoelectric-potential-controlled polarity-reversible Schottky diodes and switches of ZnO wires. Nano Lett 2008, 8: 3973. 10.1021/nl802497eView ArticleGoogle Scholar
- Li P, Wei Z, Wu T, Peng Q, Li YD: Au-ZnO hybrid nanopyramids and their photocatalytic properties. J Am Chem Soc 2011, 133: 5660. 10.1021/ja111102uView ArticleGoogle Scholar
- Herring NP, AbouZeid K, Mohamed MB, Pinsk J, El-Shall MS: Formation mechanisms of gold-zinc oxide hexagonal nanopyramids by heterogeneous nucleation using microwave synthesis. Langmuir 2011, 27: 15146. 10.1021/la201698kView ArticleGoogle Scholar
- Wang C-Y, Gong N-W, Chen L-J: High-sensitivity solid-state Pb(core)/ZnO(shell) nanothermometers fabricated by a facile galvanic displacement method. Adv Mater 2008, 20: 4789. 10.1002/adma.200703233View ArticleGoogle Scholar
- Im J, Singh J, Soares JW, Steeves DM, Whitten JE: Synthesis and optical properties of dithiol-linked ZnO/gold nanoparticle composites. J Phys Chem C 2011, 115: 10518. 10.1021/jp202461fView ArticleGoogle Scholar
- Lin DD, Wu H, Zhang W, Li H, Pan W: Enhanced UV photoresponse from heterostructured Ag–ZnO nanowires. Appl Phys Lett 2009, 94: 172103. 10.1063/1.3126045View ArticleGoogle Scholar
- Sun ZH, Wang CX, Yang JX, Zhao B, Lombardi JR: Nanoparticle metal–semiconductor charge transfer in ZnO/PATP/Ag assemblies by surface-enhanced Raman spectroscopy. J Phys Chem C 2008, 112: 6093. 10.1021/jp711240aView ArticleGoogle Scholar
- Lee J, Shim HS, Lee M, Song JK, Lee D: Size-controlled electron transfer and photocatalytic activity of ZnO-Au nanoparticle composites. J. Phys. Chem. Lett. 2011, 2: 2840. 10.1021/jz2013352View ArticleGoogle Scholar
- Morales AM, Lieber CM: A laser ablation method for the synthesis of crystalline semiconductor nanowires. Science 1998, 279: 208. 10.1126/science.279.5348.208View ArticleGoogle Scholar
- Hu JT, Odom TW, Lieber CM: Chemistry and physics in one dimension: synthesis and properties of nanowires and nanotubes. Acc Chem Res 1999, 32: 435. 10.1021/ar9700365View ArticleGoogle Scholar
- Gudiksen MS, Lieber CM: Diameter-selective synthesis of semiconductor nanowires. J Am Chem Soc 2000, 122: 8801. 10.1021/ja002008eView ArticleGoogle Scholar
- Zhao MH, Wang ZL, Mao SX: Piezoelectric characterization of individual zinc oxide nanobelt probed by piezoresponse force microscope. Nano Lett 2004, 4: 587. 10.1021/nl035198aView ArticleGoogle Scholar
- Hu JQ, Li Q, Meng XM, Lee CS, Lee ST: Thermal reduction route to the fabrication of coaxial Zn/ZnO nanocables and ZnO nanotubes. Chem Mater 2002, 15: 305.View ArticleGoogle Scholar
- Kong XH, Sun XM, Li XL, Li YD: Catalytic growth of ZnO nanotubes. Mater Chem Phys 2003, 82: 997. 10.1016/j.matchemphys.2003.09.004View ArticleGoogle Scholar
- Li YB, Bando Y, Golberg D: Single-crystalline In2O3 nanotubes filled with In. Adv Mater 2003, 15: 581. 10.1002/adma.200304539View ArticleGoogle Scholar
- Wei DP, Chen Q: Evolution of catalyst droplets during VLS growth and cooling process: a case of Ge/ZnO nanomatchsticks. Cryst. Growth Des. 2009, 10: 122.View ArticleGoogle Scholar
- Yang SH, Chen PC, Hong SY: Characterizations of Ag-catalyzed ZnO nanostructures prepared by vapor-solid mechanism. Curr. Appl. Phys. 2009, 9: e180. 10.1016/j.cap.2008.12.056View ArticleGoogle Scholar
- Glaspell G, Hassan HMA, Elzatahry A, Fuoco L, Radwan NRE, El-Shall MS: Nanocatalysis on tailored shape supports: Au and Pd nanoparticles supported on MgO nanocubes and ZnO nanobelts. J Phys Chem B 2006, 110: 21387. 10.1021/jp0651034View ArticleGoogle Scholar
- Li CR, Lu NP, Mei J, Dong WJ, Zheng YY, Gao L, Tsukamoto K, Cao ZX: Polyhedral to nearly spherical morphology transformation of silver microcrystals grown from vapor phase. J. Cryst. Growth 2011, 314: 324. 10.1016/j.jcrysgro.2010.10.221View ArticleGoogle Scholar
- Khanna R, Ip K, Heo YW, Norton DP, Pearton SJ, Ren F: Thermal degradation of electrical properties and morphology of bulk single-crystal ZnO surfaces. Appl Phys Lett 2004, 85: 3468. 10.1063/1.1801674View ArticleGoogle Scholar
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