Sheath Nanowires Prepared by a Simple Redox

NANO LETTERS

Cu2S/Au Core/Sheath Nanowires Prepared by a Simple Redox Deposition Method

2002 Vol. 2, No. 5 451-454

Xiaogang Wen and Shihe Yang* Department of Chemistry and Institute of Nano Science and Technology, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Received February 4, 2002

ABSTRACT We have deposited continuous Au films on the Cu2S nanowire surfaces using a simple redox reaction, forming a novel type of Cu2S/Au core/sheath nanowires. The core/sheath nanostructures were characterized by various spectroscopic and microscopic techniques. The Cu2S core that was used as the template was typically ∼60 nm thick, and the Au coating was from a few nm up to 100 nm thick depending on the coating time period and the concentration of Au3+ in the coating solution. The Au coating layer was found to be polycrystalline. For relatively thick Au coatings, we have successfully fabricated Au nanotubes by etching away the Cu2S core.

Introduction Metal nanowires are expected to be crucial components in future nanoscale devices for chemical, mechanical, electrical, magnetic, optical, and many other applications. During the past few years, considerable efforts have been spent to develop viable methods for the fabrication of metal nanowires. These methods usually involve conventional lithography,1 surfactant-induced anisotropic growth,2-5 electrolysis,6 extrusion,7 and techniques based on templates of “TrackEtch” polycarbonate membranes,8 alumina membranes,9-13 carbon nanotubes,14-16 zeolites,17,18 mica,19 DNA,20 and calix[4]hydroquinone nanotubes.21 We have recently found a simple method for the synthesis of straight and isolated Cu2S nanowires arrayed on copper surfaces at room temperature.22-24 Although the semiconductor nanowires themselves are potentially active elements of functional devices, they may be also excellent templates for the growth of nanorods, nanowires, or nanotubes of other materials due to their straightness, controllable surface adsorption characteristics, and rough alignment on a conductive substrate. Here, we demonstrate the possibility of coating gold on the Cu2S nanowire surfaces using a simple redox reaction in an aqueous solution. This represents an attractive approach for fabricating gold nanowire arrays because of the simplicity of the method and the possibility for large-scale production. Au nanowires were synthesized mainly by depositing Au in one-dimensional (1D) pores, which we refer to as negative templates. For example, Martin’s group studied the preparation of gold nanotubules and their applications in “TrackEtch” polycarbonate membranes.25-28 Most recently, striped * To whom correspondence should be addressed. E-mail: [email protected]. 10.1021/nl0202915 CCC: $22.00 Published on Web 04/02/2002

© 2002 American Chemical Society

multi-metal nanorods were successfully fabricated in the alumina membranes using this method.8 Electrostatic assembly of positively charged gold nanoparticles on the negatively charged phosphate backbone of DNA molecules has been demonstrated.18 Interestingly, carbon nanotubes have been employed as a positive template for coating metals on their outer surfaces.12,14 In this work, we use the Cu2S nanowires as a positive template for the fabrication of Au nanowires. Unlike the negative templates, the diameters of the Au nanowires are not restricted by the template size and can be controlled by adjusting the reaction conditions such as the concentration of the coating solution and reaction time. It is also possible to tailor new properties through the judicious combination of different cores and sheaths at the nanometer size regime. Furthermore, if we remove the core materials after sheath deposition, we will be able to fabricate Au nanotubes, which might be useful in areas such as microreactors and sensors, among others. Experimental Section The synthesis of Cu2S nanowire arrays has been described in detail elsewhere.22-24 For TEM characterization, fresh Cu2S nanowires were grown directly on copper grids. The Cu2S nanowire samples for other measurements were prepared on copper foils. Hydrogen tetrachloroaurate (III) trihydrate (HAuCl4‚3H2O) (Aldrich Chem. Co.) was used as precursor for the gold coating. A 0.1 M HAuCl4 aqueous solution was prepared by dissolving 0.395 g HAuCl4‚3H2O in 10 mL deionized water. This solution was then diluted to different concentrations for the gold coating. A 1 M NaOH solution was used

Scheme 1. Schematic Diagram for the Fabrication of Cu2S/Au Core/Sheath Nanowires

to adjust the pH value of the gold-coating solution. Before coating, the nanowire templates were immersed into a 1 M HCl solution for 20 min to remove the surface oxide layer and other absorbed species, followed by washing 3 times with deionized water. The pretreated Cu2S nanowire templates were fixed on a glass capillary and immersed into a coating solution. After coating for 1 h, the samples were collected and washed 3 times with deionized water and then with absolute alcohol. Finally, the samples were dried in air. Results and Discussion Our strategy for the growth of Cu2S/Au core/sheath nanowires is illustrated in Scheme 1. Because the Cu2S nanowires were grown directly on a copper substrate, it was possible to coat a gold layer on the nanowires by a redox deposition method. During the coating process, the yellow color of the coating solution became lighter and eventually disappeared. At the same time, the original black Cu2S nanowires on a copper surface turned gold yellow after coating, indicating the formation of elemental gold coating on the nanowires. The success of this coating strategy hinges on the fact that the Cu2S nanowires are fairly good electric conductors. Basically, gold-coating on the Cu2S nanowires was accomplished via the redox reactions of galvanic cells. During the redox processes, the Cu2S nanowires act as cathodes where Au-deposition occurs, and the copper substrate serves as an anode where copper dissolution takes place. The electrolyte was the coating solution that contains HAuCl4. At least during the initial phase, the Cu2S nanowires short the circuits, ensuring the continuation of the gold-coating reactions. Later on, the gold sheath itself should constitute a much better conducting path. The actual electrochemical reactions can be given as follows. At the Cu2S nanowire surfaces AuCl4- + 3 e ) Au + 4Cl-

(1)

Figure 1. Scanning electron microscope (SEM) images of Cu2S/ Au core/sheath nanowires.before (A) and after (B) being coated with Au. (C) EDX spectrum taken near the tip of a single Cu2S/ Au core/sheath nanowire (inside the white circle in (B)).

E° ) 1.002 V At the copper substrate surface Cu2+ + 2 e ) Cu E° ) 0.3419 V

(2)

The overall cell reaction 3Cu + 2HAuCl4 ) 3CuCl2 + 2Au + 2HCl 452

(3)

Note that the E° values are referenced to that of the standard hydrogen electrode.29 Because the E° value of Cu+ + e ) Cu is 0.521 V, it is expected to be more difficult to form Cu+ than to form Cu2+. Because the reduction potential of AuCl4-/Au is much larger than that of Cu2+/Cu, the deposition of Au on the Cu2S nanowires and the simultaneous dissolution of Cu from the copper substrate were spontaneous. The scheme for gold nanowire growth described above has been borne out by our experiments using various spectroscopic and microscopic techniques. Shown in Figure 1 Nano Lett., Vol. 2, No. 5, 2002

Figure 2. Au 4f XPS spectrum of the Cu2S/Au core/sheath nanowires.

are scanning electron microscopic (SEM) images of the Cu2S nanowires on a copper substrate before (A) and after (B) coating with gold. A close examination of the SEM images shows that after Au coating, the Cu2S nanowires are thicker and the surfaces become rough compared to the uncoated nanowires. This indicates that the Au nanoparticles have been successfully deposited on nearly all the Cu2S nanowire surfaces along their entire lengths. This was confirmed by other measurements as will be described below. Noticeable from the SEM images is also the thickening of the nanowires at their tips after gold-coating. This is presumably due to the concentration gradient between the nanowire roots on the copper substrate and the nanowire tips toward the solution; the nanowire tips, being in touch with the bulk solution as opposed to the nanowire roots, are surrounded by more abundant Au3+ ions and thus grow faster. As shown in Figure 1C, energy-dispersive X-ray analysis (EDX) near the tip of a single Cu2S/Au nanowire shown in Figure 1B confirmed the presence of copper, sulfur and gold in the nanowires. X-ray photoelectron spectroscopy (XPS) not only confirmed the presence of gold in the samples but also clarified the oxidation state of the gold deposit. XPS spectra were recorded on a PHI 5600 X-ray photoelectron spectrometer using a monochromatic Al KR X-ray as the excitation source. Figure 2 gives the Au 4f XPS spectrum of the core/sheath Cu2S/Au nanowire sample collected from the copper surface. The two peaks at 84.3 and 88.1 eV correspond to the core level excitations of 4f7/2,5/2 of Au(0), respectively. In addition, no peak of Cl was detected in the XPS spectrum. This proves the formation of metallic gold from AuCl4-. Although Au may have also been deposited directly on the copper substrate, we believe that the XPS signal of gold we observed is mainly from the gold species on the Cu2S nanowire surfaces. Transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM) were used to examine the structures of single nanowires. From the TEM image in Figure 3A, the Cu2S nanowires before gold-coating but after treatment with HCl are relatively straight, thin (∼60 nm in diameter), and smooth on the surfaces. The TEM image of the Cu2S nanowires coated with Au is shown in Figure 3B. Nano Lett., Vol. 2, No. 5, 2002

Figure 3. Transmission electron microscopic (TEM) images of Cu2S nanowires before and after being coated with Au. (A) TEM image of Cu2S nanowires; (B) TEM image of a Cu2S/Au core/ sheath nanowire; (C) HRTEM image of a Cu2S/Au core/sheath nanowire; and (D) Electron diffraction (ED) pattern of a single Cu2S/Au core/sheath nanowire.

Here, the concentration of HAuCl4 is 1 × 10-5 M and the reaction time is 1 h. Clearly, the nanowire becomes thicker (∼100 nm in diameter). Although the nanowire surface becomes more rough after coating, it appears to be completely covered by the gold deposit along the entire length of the nanowire. The roughness of the nanowire surface indicates that the gold coating is polycrystalline. We believe that Au is deposited in the form of Au nanocrystals, which are aggregated on the Cu2S nanowire surfaces to form a continuous coating layer. The Cu2S/Au core/sheath nanostructure of the nanowire can be better appreciated from the HREM image shown in Figure 3C. It can be seen that the core/sheath nanowire is ∼100 nm in diameter, with a ∼60 nm thick core of Cu2S and a ∼20 nm thick outer sheath of Au. The aggregates of the Au nanocrystals in the outer sheath are clearly perceptible. The electron diffraction (ED) pattern of this core/sheath nanowire is shown in Figure 3D. This ED pattern confirms the polycrystalline nature of the outer sheath of Au (ring pattern) as well as the single-crystal nature of the inner core of Cu2S (discrete diffraction spots). Therefore, the monoclinic Cu2S nanowires are coated with face-center-cubic polycrystallites of Au. 453

strated the formation of pure gold nanotubes by removing the core materials, which may be used as nanoreactors or drug delivery carriers. This represents an alternative route for the fabrication of core/sheath nanowires and nanotubes. Future work will focus on better control over the diameters of core/sheath nanowires and robustness of the coating. Extension of the coating to other materials is also going to be pursued. Acknowledgment. This work was supported by a RGC grant administered by the UGC of Hong Kong. We thank Dr. Y. S. Zheng and MCPF of HKUST for the sample characterizations. References

Figure 4. Transmission electron microscopic (TEM) image of a gold nanotube after the removal of the Cu2S core. Inset: ED pattern of the Au nanotube.

Finally, we demonstrate the formation of Au nanotubes with the Cu2S nanowire templates. After the formation of Cu2S/Au core/sheath nanowires, we removed the Cu2S core by etching with 6 M HCl. Specifically, the Cu2S/Au core/ sheath nanowire arrays on a copper foil were immersed in an aqueous solution of 6 M HCl, and laid aside for 7 d. The precipitate was then washed with deionized water and ethanol successively. Finally, a drop of the precipitate suspension in ethanol was deposited on a copper grid. After ethanol was evaporated in air, the sample on the copper grid was analyzed using TEM. Figure 4 shows the TEM picture of a typical gold nanotube. It has diameter of ∼300 nm and a wall thickness of 50 nm. Clearly, one end of the gold tube is closed and the other is open. The ED pattern showed the polycrystalline fcc structure of the gold tube and the absence of the Cu2S crystals originally in the core. For thinner tubes with thinner walls, it was found that the nano-structure is easy to collapse after removing the core owing to the relatively weak aggregation forces between the gold nanoparticles.

Conclusion We have demonstrated a simple redox deposition method for the preparation of Cu2S/Au core/sheath nanowires. The Cu2S nanowires are shown to be a good template for the fabrication of core/sheath nanostructures. The unique structure of the Cu2S nanowire arrays on copper surfaces allows gold to be deposited on the nanowire surfaces by taking advantage of the straightforward galvanic process. In this way, continuous Au nanotubes compactly filled with Cu2S could be synthesized easily. In addition, we have demon-

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NL0202915

Nano Lett., Vol. 2, No. 5, 2002