Electrodeposition of tubular-rod structure gold nanowires using nanoporous anodic alumina oxide as template

Electrodeposition of tubular-rod structure gold nanowires using nanoporous anodic alumina oxide as template

Electrochemistry Communications 11 (2009) 2019–2022 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www...

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Electrochemistry Communications 11 (2009) 2019–2022

Contents lists available at ScienceDirect

Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom

Electrodeposition of tubular-rod structure gold nanowires using nanoporous anodic alumina oxide as template Hong-Jun Wang a,b, Chang-Wei Zou a,b, Bing Yang a,b, Hong-Bing Lu a,b, Can-Xin Tian a,b, Hui-Juan Yang a,b, Ming Li a,b, Chuan-Sheng Liu a,b, De-Jun Fu a,b,*, Jia-Rui Liu c a b c

Accelerator Laboratory, Department of Physics, Wuhan University, Wuhan 4372, China Key Laboratory of Beam Technology and Material Modification of Ministry of Education, Beijing Normal University, China Department of Physics and Texas Center for Superconductivity, University of Houston, Houston, TX 77023, USA

a r t i c l e

i n f o

Article history: Received 11 August 2009 Received in revised form 21 August 2009 Accepted 21 August 2009 Available online 28 August 2009 Keywords: Gold Tubular-rod structure Nanowire Electrodeposition Anodic alumina oxide

a b s t r a c t One-dimensional tubular-rod structure gold nanowires have been prepared using electrodeposition method at constant current mode with confined nanochannels of porous anodic aluminum oxide template. The reduction mechanism of gold ions and formation process of tubular-rod structure gold nanowires are studied. Electron microscopy results show that the tubular-rod structure gold nanowires transform to solid nanorods when the electrodeposition time is long enough. The tubular-rod structure gold nanowires have an average diameter of 180 nm, which coincide with the diameter of the template used. X-ray diffraction results confirm that the tubular-rod structure gold nanowires are crystalline structure. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction One-dimensional (1D) metallic nanostructures have attracted considerable attention in recent years because of their huge potential applications, such as magnetic storage, optoelectronic, cooling system, etc. [1–4]. There are several methods that can be used for preparing nanostructures with controllable demands. Among those methods, the template assisted method has proven to be a simple approach with many advantages, such as easy controllable profiles of nanomaterials and low cost. Due to the self-assembled regular hexagonal arrays, the porous anodic aluminum oxide (AAO) membranes become the most widely used template to fabricate nanostructures including conductive polymers, metals, semiconductors and other materials [5–9]. Recent efforts have been directed to one-dimensional gold nanostructure because of its unique properties, for instances, its field ionization properties for generating ion beams and neutron generation [10]. However, most of the works concerned nanorods and nanotubes [10,11]. There are few reports about the reduction mechanism of gold ions and relation between gold nanotubes and nanorods because the situation becomes complex by the presence of template. In this paper, we present the tubular-rod

* Corresponding author. Address: Accelerator Laboratory, Department of Physics, Wuhan University, Wuhan 4372, China. Tel./fax: +86 27 6875 3587. E-mail address: [email protected] (D.-J. Fu). 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.08.042

structure gold nanowires which are formed when gold nanotubes transform to nanorods during electrodeposition process. 2. Experimental The nanoporous AAO membranes which are 60 lm thick and 180 nm in diameter bought from Whatman Co. were used as templates in the experiments. In order to fabricate one-dimensional nanostructure, a layer of metal Pt was deposited on the bottom side of the AAO membranes using a vacuum sputtering apparatus. The sputtering current was maintained as 45 mA with duration of 150 s. The membranes served as the working electrode of a standard three-electrode electrochemical system. The gold nanowires were electrodeposited from the following solution: 5 g/L HAuCl43H2O, 2 g/L EDTA, 65 g/L Na2SO3, 12.5 g/L K2HPO4. All the reagents were analytically pure and used without purification during the procedure. Ultrapure distilled water was used in all synthesis procedures. Electrodeposition was taken in three-electrode standard configuration with platinum gauze as counter electrode and Ag/AgCl as reference electrode. Gold nanowires were grown in a constant current density mode at 4.5 mA/ cm2. After deposition the membranes were attached with conductive glue from the bottom side to silicon wafer. The membranes were then dissolved in 1 M NaOH solution for 10 min and rinsed in distilled water for two times, exposing the top of gold nanowires.

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3. Results and discussion Fig. 1a shows the SEM image of highly ordered gold nanowires prepared by electrodeposition for 150 s at optimized solution and current density. It can be seen that the average height of gold the nanowires is 60 lm, which is consistent with the AAO length reported by the manufacturer of porous membranes. Some gold nanowires are bent, indicating that they have a ductile rather than a rigid and brittle nature. Fig. 1b shows the high magnification view of the top segment of the gold nanowires and the corresponding energy EDX spectra. The average diameter of the nanowires is 180 nm, which is also consistent with the diameter of template porous. We can clearly see that the top segments of the nanowires are hollow. This is because the electrodeposition time is limited and as a result, the top parts of pores are not completely filled with gold. The detailed formation process of the gold nanowires is illustrated in Fig. 4. The EDX spectra were recorded to prove the chemical composition of the gold nanowires. The resulting measurements unambiguously revealed the presence of oxygen, aluminum, silicon and gold. The gold peak shows the most prominent intensity indicating that the wires were made up of metallic gold. The silicon comes from the silicon substrate on which the membranes were bonded. The weak peaks of aluminum as well as oxygen are originated from AAO templates. Fig. 2a shows the XRD results of the gold nanowires prepared with electrodeposition duration of 150 and 270 s, respectively. Both of the gold nanowires include four obvious diffraction peaks of Au (1 1 1), (2 0 0), (2 2 0) and (3 1 1) associated with face-centered cubic (fcc) structure, indicating that the face-centered cubic structure of gold is preserved in these nanowires. The diffraction peak intensity corresponding to the (1 1 1) plane is larger than the other peaks, indicating that both of the nanowires have well preferred orientation along [1 1 1] direction. This is because the

(a) (111)

deposition time: 150 s

(200)

(220) (311)

Intensity (a.u)

The morphology of the samples was measured by field emission scanning electron microscopy (FE-SEM) at 25 kV working voltage. Energy dispersive X-ray spectroscopy (EDX) was used to measure the chemical compositions of the nanowires. A 2010 JEOL JEM transmission electron microscopy (TEM) was used to characterize the crystalline structure at accelerating voltage of 200 kV. The Xray diffraction (XRD) analysis was carried out with a scanning rate 0.02 s 1 with Cu Ka radiation. The chemical bonding states were investigated by X-ray photo electron spectroscopy (XPS, Kratos Ltd XSAM800) with Mg Ka excitation.

270 s

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(b)

4f 7/2 4f 5/2

Intensity (a.u)

2020

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Binding energy (eV) Fig. 2. (a) XRD patterns of gold nanowires electrodeposited for 150 and 270 s, respectively. (b) XPS spectra of gold nanowires electrodeposited for 150 s.

{1 1 1} surface is the most densely packed and hence the energetically most favorable for the surface growth for the fcc gold, as reported in the literature [12,13]. In addition, the diffraction peak of

Fig. 1. (a) SEM image of highly ordered gold naonowires prepared by electrodeposition for 150 s. (b) SEM image and EDX graph of high magnification view of top segment of the corresponding gold nanowires.

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(1 1 1) plane exhibits a very sharp profile, which indicates a high crystallinity of the gold nanowires. To confirm the chemical state of the gold in the nanowires, an XPS experiment was carried out on the surface of gold nanowires with electrodeposition duration of 150 s as shown in Fig. 2b. The high-resolution Au 4f peaks shows a spin–orbit doublet with two attributions at binding energy of 84.0 eV and 87.4 eV, which can be attributed to Au0 state. Fitting curve with Gaussian function matches well with the Au 4f peaks, indicating that the gold nanowires are composited of pure Au0 state. Fig. 3a shows a TEM image of top segment of a randomly selected Au nanowire electrodeposited for 150 s. The wire is straight with a diameter of about 180 nm, which is consistent with the average diameter of the template pores. It can be seen that the top part of the wire is more transparent than the bottom. The top part of the nanowire is tubular structure and in contrast, the bottom part is rod structure. The tubular-rod structure gold nanowires were formed because the gold atoms first aggregated on the porous walls and the deposition time 150 s is not enough to fill the pores fully. A selected area electron diffraction (SAED) of the bottom part of nanowire shows that it has a polycrystalline structure. The four rings can be indexed to the {1 1 1}, {2 0 0}, {2 2 0} and {3 1 1} planes of facecentered cubic gold. It can be seen that the {111} plane has the most prominent intensity, indicating that the {1 1 1} plane is the growth orientation, the consistent with the results of XRD patterns (Fig. 2a). Fig. 3b shows the TEM image of a randomly segment of a selected gold nanowires that was prepared for 270 s. The wire is straight with a diameter close to the 150 s electrodeposited nanowires. Compared with tubular-rod structure, we can see that the sample with duration of 270 s is nanorod, because the electrodeposition time is long enough to completely fill the pores. EDTA is a polyamino carboxylic acid and is used as the chelating agent because of its ability to sequester metal ions in the solution. During the gold solution deposition process, the metal-EDTA complexes with amines groups would contact the wall surface of alumina [14]. It is well known that the wall of the AAO consist of hydroxyl-terminated amorphous alumina and HAuCl4 is a strong oxidant. The AuCl4Cl would convert to gold at room temperature when in contact with porous amorphous anodic aluminum which has a high specific area and a large number of reducing hydroxyl groups. This is because the reducing groups on the pore surface result in high surface energy and in turn of high chemical energy [15]. As a result, the wall of the pores would aggregate with a film of gold soon after the start of electrodeposition process.

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Fig. 4. Schematic diagram of the gold nanowire formation process.

Fig. 4 shows a schematic diagram of the gold nanowire formation process. The electrodeposition takes place bilaterally, both through the pores and at the exterior of the AAO. As the Pt layer sputter-deposited before electrodeposition cannot fill the pores completely, the metal exposed to the solution would be a circlelike shape as shown in Fig. 4a. Soon after the beginning of electrodeposition process, a film of gold is aggregated on the wall, as shown in Fig. 4b, indicating the formation of gold nanotubes. As time goes on, the bottom part of the gold nanowires become solid while the top part is still hollow, resulting in the tubular-rod structure gold nanowires as shown in Fig. 4c. The nanotubes are eventually completely filled from bottom to top when the electrodeposition time is long enough. The final result is that the gold nanorods are formed as shown in Fig. 4d. In sum, the formed gold nanotubes would be transformed to tubular-rod structure nanowires and solid nanorods eventually form as the electrodeposition process goes on. 4. Conclusion By using the electrodeposition method at constant current mode, tubular-rod structure gold nanowires were successfully prepared in the pores of AAO templates. The reduction mechanism of the gold ions and formation process of tubular-rod structure gold

Fig. 3. TEM image of (a) top segment of a randomly selected gold nanowire prepared by electrodeposition for 150 s, the inset shows the SAED of the corresponding nanowire. (b) The randomly segment of a selected gold nanowire electrodeposited for 150 s.

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nanowires were discussed. We found that the tubular-rod structure gold nanowires transformed to solid nanorods when the electrodeposition time is long enough. X-ray diffraction results showed the tubular-rod structure gold nanowires have preferred orientation along [1 1 1] direction. The XPS spectra showed that the reduced gold was in Au0 state and the SAED result showed the gold nanowires had polycrystalline structure.

[2] [3] [4] [5] [6] [7] [8] [9]

Acknowledgement This work was supported by NSFC under contracts 10675095, 10875090 and 10875091. References [1] K. Uosaki, K. Okazaki, H. Kita, H. Takahashi, Anal. Chem. 62 (1990) 652.

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