A Templateless, Surfactantless, Simple Electrochemical Route to a

Jan 25, 2010 - Meltzer, Sheffer; Mandler, Daniel. Journal of the Electrochemical Society (1995), 142 (6), L82-L84CODEN: JESOAN ; ISSN:0013-4651...
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A Templateless, Surfactantless, Simple Electrochemical Route to a Dendritic Gold Nanostructure and Its Application to Oxygen Reduction Xiaolong Xu,†,‡ Jianbo Jia,† Xiurong Yang,*,†,‡ and Shaojun Dong*,† †

State Key Laboratory of Electroanalysis Chemistry, Changchun Institute of Applied Chemistry, Changchun, China, 130022, and ‡Chemistry Department, University of Science and Technology of China, Hefei, China, 230026 Received November 9, 2009. Revised Manuscript Received December 21, 2009

A templateless, surfactantless, simple electrochemical route to prepare dendritic gold nanostructure is reported. The morphology, composition, and structure of as-prepared dendritic gold nanostructure were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. The whole nanostructure was constructed by pristine metallic gold. The formation mechanism related to experimental conditions was discussed. The synthesis promises indium tin oxide (ITO) electrode can be easily modified with the dendritic pristine gold nanostructure. The as-prepared modified ITO electrode has excellent catalytic activity to oxygen reduction in neutral KCl solution.

Introduction At the present time, micro/nanoscaled materials, due to their unique optical, electrical, magnetic, and catalysis properties, are being synthesized and investigated for their potential application in various fields, such as chemical or biosensing,1-3 optoelectronics,4 energy generation/storage,5 and catalysis.6,7 Recently, considerable efforts have been focused on the design, synthesis, and application of high ordered inorganic crystals with specific sizes, shapes, and hierarchies. Different kinds of materials, such as metal,5 semiconductors, oxides,7 and carbon nanotubes, and different structures, such as flower-like,8,9 urchin-like,10 mushroomlike,11 and dendrite-like12-17 have been reported based on different *Corresponding author. Tel: þ86 431 85682419. Fax: þ86 431 85689711. E-mail: [email protected] (X.Y.). Tel: þ86 431 85262101. Fax: þ86 431 85689711. E-mail: [email protected] (S.D.). (1) Barreca, D.; Comini, E.; Gasparotto, A.; Maccato, C.; Sada, C.; Sberveglieri, G.; Tondello, E. Sens. Actuators B: Chem 2009, 141(1), 270–275. (2) Chen, Y.; Reyes, P. I.; Duan, Z. Q.; Saraf, G.; Wittstruck, R.; Lu, Y. C.; Taratula, O.; Galoppini, E. J. Electron. Mater. 2009, 38(8), 1605–1611. (3) Khanderi, J.; Hoffmann, R. C.; Gurlo, A.; Schneider, J. J. J. Mater. Chem. 2009, 19(28), 5039–5046. (4) Gupta, M. K.; Sinha, N.; Singh, B. K.; Singh, N.; Kumar, K.; Kumar, B. Mater. Lett. 2009, 63(22), 1910–1913. (5) Zhang, Y.; Sun, H.; Chen, C. F. Phys. Lett. A 2009, 373(31), 2778–2781. (6) Andrade, A. L.; Souza, D. M.; Pereira, M. C.; Fabris, J. D. J. Nanosci. Nanotechnol. 2009, 9(6), 3695–3699. (7) Polshettiwar, V.; Baruwati, B.; Varma, R. S. ACS Nano 2009, 3(3), 728–736. (8) Ma, L.; Xu, L. M.; Xu, X. Y.; Luo, Y. L.; Chen, W. X. Mater. Lett. 2009, 63 (23), 2022–2024. (9) Zhao, B.; Chen, F.; Huang, Q. W.; Zhang, J. L. Chem. Commun. 2009, 34, 5115–5117. (10) Ding, H. J.; Wang, G.; Yang, M.; Luan, Y.; Wang, Y. N.; Yao, X. X. J. Mol. Catal. A: Chem. 2009, 308(1-2), 25–31. (11) Shiigi, H.; Morita, R.; Yamamoto, Y.; Tokonami, S.; Nakao, H.; Nagaoka, T. Chem. Commun. 2009, 24, 3615–3617. (12) Lu, G. W.; Li, C.; Shi, G. Q. Chem. Mater. 2007, 19(14), 3433–3440. (13) Hu, Y.; Pan, N.; Zhang, K.; Wang, Z.; Hu, H.; Wang, X. Phys. Status Solidi A 2007, 204(10), 3398–3404. (14) Qin, Y.; Song, Y.; Sun, N. J.; Zhao, N.; Li, M. X.; Qi, L. M. Chem. Mater. 2008, 20(12), 3965–3972. (15) Tang, X. L.; Jiang, P.; Ge, G. L.; Tsuji, M.; Xie, S. S.; Guo, Y. J. Langmuir 2008, 24(5), 1763–1768. (16) Zhang, J. C.; Meng, L. J.; Zhao, D. B.; Fei, Z. F.; Lu, Q. H.; Dyson, P. J. Langmuir 2008, 24(6), 2699–2704. (17) Saltykova, N. A.; Semerikova, O. L.; Molchanova, N. G. Russ. Electrochem. 2007, 43(8), 863–869.

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advanced strategies. Among these works, dendritic gold nanostructures have simulated great research interest because of their promising applications in fuel cell14 (FC), surface-enhanced Raman scattering12,15 (SERS), and photoluminescence13 (PL). Since the intrinsic properties and relevant applications of dendritic gold nanostructure are depended on their size and shape, great efforts have been devoted to the morphology-controlled synthesis in recent years. Saltykova17 et al. obtained dendritic structure of electrolytic gold deposited from molten chlorides. Lu and co-workers16 fabricated dendritic gold nanoparticles by using ionic polymer as template. This method involved in situ generation of ionic polymer, and it took a few days. Qi and co-workers14 reported a single-crystalline dendritic gold nanostructure synthesized by the reaction between a zinc plate and a solution of tetrachloroaurate in ionic liquid. Tang and co-workers15 synthesized dendritic gold nanoparticles by one-step hydrothermal reduction of tetrachloroaurate using ammonium formate in the presence of poly(N-vinyl-2-pyrrolidone). Shi and co-workers12 obtained dendritic gold nanostructure in the aqueous/organic interface. These works evolved either time-consuming, complex stepping procedures or surfactant/assistant reagent. Mandler and co-workers patterned gold nanoparitcles using scanning electrochemical microscope either using electrochemical or electroless method.18-20 Heinze and co-workers conducted galvanic deposition of metals and electroless deposition of conducting polymers to realize surface micromodification.21-23 Wang and co-workers13 fabricated dendritic gold nanostructure by a simple electrochemical method, but only a few pieces were obtained, which constrains its further application. Herein, we report a templateless, surfactantless, simple electrochemical route to dendritic gold nanostructure, which is also a fast and feasible way to modify an electrode with pristine gold (18) (19) (20) (21) (22) 393. (23)

Sheffer, M.; Mandler, D. Electrochim. Acta 2009, 54, 2951. Malel, E.; Mandler, D. J. Electrochem. Soc. 2008, 155, D459. Meltzer, S.; Mandler, D. J. Electrochem. Soc. 1995, 142, L82. Radtke, V.; Hess, C.; Heinze, J. Z. Phys. Chem. 2007, 221, 1221. Radtke, V.; Hess, C.; Souto, R. M.; Heinze, J. Z. Phys. Chem. 2006, 220, Radtke, V.; Heinze, J. Z. Phys. Chem. 2004, 218, 103.

Published on Web 01/25/2010

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Scheme 1. Schematics of the Experiment Setup and the Formation Processes of the Dendritic Gold Nanostructure

in a Faraday cage at room temperature (∼ 25 °C) for all the measurements.

Results and Discussion

nanostructure. The morphology, structure, and formation mechanism are discussed. The as-prepared Au-modified indium tin oxide (ITO) electrode had excellent catalytic activity toward oxygen reduction, which can attribute to the pristine and high surface-to-volume ratio of the dendritic gold nanostructure.

Experimental Section The configuration (Scheme 1) of the synthesis consists of four electrodes: a gold tip electrode (1 mm diameter), an ITO substrate electrode, an Ag/AgCl reference electrode (saturated KCl), and a Pt counter electrode. The substrate electrode served as the bottom of the electrochemical cell, and the reference and counter electrode were attached to the inner perimeter of the electrochemical cell. The tip electrode was hanging at a distance (i.e., 75 μm) from the ITO substrate. The area of the ITO electrode was controlled by totally covering the bare ITO with insulating tape, which had a 1 mm diameter hole. We used a PC camera monitoring from the bottom of the ITO to confirm that the tip electrode was positioned right up to the “hole”. In a typical experiment, the Au atoms on the surface of the tip electrode were etched to a Au(III) ion under a positive voltage (i.e., 1.5 V) and released to electrolyte (i.e., 0.5 M HCl þ 0.5 M KCl). On the other side, reduction, nucleation, and growth took place on the substrate electrode under a negative voltage (i.e., -0.2 V). Investigations under different experiment conditions, such as the distance between the tip and substrate electrode (d), the voltage applied to the tip and substrate electrode (Et, Es), and different kinds of anions in the electrolyte and their concentrations, were conducted. The distance between the tip and the substrate electrode was controlled by the position-manipulator part of CHI 900 (CH Instrument, Inc., China). The tip electrode was brought in mechanical contact with the substrate, and then retracted a preset distance. All electrochemical experiments were performed by a CHI 832B (CH Instrument, Inc.) electrochemical work-station. All reagents were of analytic grade and used as received. Aqueous solutions were prepared using deionized water. The morphology of the dendritic gold nanomaterial was characterized with a XL30 ESEM FEG scanning electron microscope (SEM) at an accelerating voltage of 20 kV and a Tacnai G2 transmission electron microscope (TEM) at an accelerating voltage of 200 kV. The chemical composition of the dendritic gold nanomaterial was determined by energy-dispersive X-ray spectroscopy (EDX). We transferred the sample from ITO to a copper grid. First, a droplet of water covered the sample on ITO; then we scratched the sample gently with a blade within the droplet; after that, the droplet was transferred to a copper grid. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and selected area electron diffraction (SAED) provided structure information. The electrochemical reduction of oxygen was performed using a conventional three-electrode system, a homemade Ag/AgCl (sat. KCl) reference electrode, a Pt counter electrode, and the as-prepared gold modified ITO working electrode. Three aliquots of 0.1 M KCl were saturated with nitrogen and oxygen, respectively, and used immediately. The electrochemical cell was located 7628 DOI: 10.1021/la904245q

The morphology, component, and crystal structure of a typical dendritic gold nanosturcture are given by Figure 1. The lower magnification image (Figure 1a) indicates that the product consists of a large amount of dendritic nanostructures. The width of these dendritic nanostructures is several micrometers and the length is up to about 10 μm. There are also some small pieces and rod-like nanostructures (circled in Figure 1a). High-magnification images (Figure 1b) reveal the detail of this dendritic gold nanostructure. The nanostructure is symmetrical to the bone-axis, and composed of small nanoparticles, which is similar to other reports.13,14 The composition of this nanostructure is determined by EDX (Figure 1c). EDX spectroscopy with significant peaks associated with Au (other peaks originated from substrate) is observed, indicating only element Au in this nanostructure. XRD analysis (Figure 2) was used to characterize the chemical composition and crystal structure of the nanostructure. Comparing with the substrate, a typical powder XRD pattern is shown in Figure 2b. Four peaks associated with the [111], [200], [220] and [311] diffractions of the face-centered cubic (fcc) gold structure (Joint Committee on Powder Diffraction Standards (JCPDS) files: 04-0784) indicate that the nanostructure is made up of pristine crystalline gold. To further confirm the existence of pure gold in the nanostructure, an XPS experiment was employed for the surface analysis. The XPS (Figure 3) pattern of the nanostructure shows the significant Au4f signals corresponding to the binding energy of metallic Au. Thus, we can affirm that the nanostructure is composed of metallic Au. A TEM image (Figure 1d) and an electron diffraction (ED) pattern (Figure 1d, inset) were also provided to determine the crystal orientation of the dendritic gold nanostructures. Figure 1d shows a typical image of the tiny tip of dendritic nanostructure. The ED pattern of the tip shows clear spots indexed to the [112] zone axis of cubic gold, indicating that the Au dendrite is a single crystal along the [111] direction. To understand the formation mechanism of dendrite nanostructure, we inspected the relationship between the morphology and reaction time. Figure 4 shows the morphology of the dendritic nanostructure with different reaction times. From Figure 4a-d, one can figure out that there is a tendency that the nanostructure grows more and more complex when reaction time increases. A 5 s reaction time results in only irregular nanoparticles. When the reaction time is prolonged, the as-prepared nanostructure becomes rod-like (Figure 4b) and dendritic (Figure 4c,d and Figure 1a; note that the reaction conditions of Figure 1 are the same as that of Figure 4 except for the reaction time.). The formation of the nanostructure experienced a growth of the nanoparticle to a 3D dendritic nanostructure. On the basis of these observations, a possible formation mechanism of the dendritic nanostructure is briefly present as follows. When a positive voltage (high enough) is applied to the tip electrode, the surface gold atoms lose electrons, form AuCl4- with Cl-, and then escape into the electrolyte. After that, AuCl4- ions diffuse to the substrate electrode, are reduced to Au there, and release Clback to electrolyte. The deposited gold atoms first form nuclei and distribute uniformly on the substrate electrode (ITO) surface (Figure 4a). The subsequent growth of gold crystals would preferentially occur on the preformed gold nuclei rather than on a bare ITO surface through a surface reaction, which is possibly due to the relatively high activation energy for the surface Langmuir 2010, 26(10), 7627–7631

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Figure 1. SEM images of the dendritic gold nanostructure of different scale bars: 5 and 0.5 μm for panels a and b, respectively. (c) EDS spectrum of a selected region of panel a. (d) TEM image of the tip of dendritic gold nanostructure; inset in d is the SAED pattern. Et = 1.5 V, Es = -0.2 V, d = 45 μm, t = 30 s.

Figure 2. XRD patterns of ITO substrate (a) and dendritic Au on ITO substrate (b).

Figure 4. SEM images of the dendritic gold nanostructure following different reaction times: (a) 5, (b) 15, (c) 45, and (d) 90 s. Et = 1.5 V, Es = -0.2 V, d = 45 μm. Scale bar, 2 μm.

Figure 3. XPS pattern of dendritic Au on ITO substrate.

reaction.14 In other words, the preformed gold nuclei act as a local cathode in the following electrochemical reduction process. The continuous cathode reaction leads to the gradual electrochemical deposition of gold onto the existing gold nuclei, which evolved into the final dendritic nanostructure. Langmuir 2010, 26(10), 7627–7631

In the system reported herein, beside reaction time, some other variable might also affect the formation of the dendritic nanostructure, such as the distance between tip and substrate electrodes, the potential applied to the electrodes, the electrolyte and its concentration. We have conducted experiments using 1 M KNO3/ HNO3 or 1 M K2SO4/H2SO4 as the electrolyte, and no dendritic structure was obtained, which indicates that the anion in the electrolyte plays a key role in the reaction. Nitrate and sulfate do not coordinate with Au (III), thus they could not intermediate this reaction. We also changed the ratio and/or concentration of DOI: 10.1021/la904245q

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Figure 5. SEM images of the dendritic gold nanostructure of different distance between tip and substrate: (a) 30, (b) 75, and (c) 150 μm. (d) Magnified SEM image of the 75 μm sample. Et = 1.5 V, Es = -0.2 V, t = 30 s. Scale bars for a, b, c and d are 2, 5, 10, and 2 μm, respectively.

KCl/HCl, and there were no distant changes of the morphology of the dendritic nanostructure. The distance between the tip and the substrate electrodes plays an important role in the formation of the dendritic nanostructure. SEM images provide direct view of the morphology of dendritic nanostructure with different tip/substrate distances (Figure 5). From Figure 5a to Figure 5c, the distance extends. Dendritic nanostructures were obtained for 45 (Figure 1a), 75 (Figure 5b,d) and 150 (Figure 5c) μm distance, but not for 30 μm distance (Figure 5a). On one hand, a shorter distance means a shorter length for intermediate ion (AuCl4-) diffusion to the substrate electrode. In other words, when experiments are conducted with shorter distance, more intermediate ions will diffuse to the substrate, be reduced, and grow there (others will be in the electrolyte). On the other hand, a shorter distance means a higher intensity of the electric field (given the potential of the tip and substrate electrodes keep constant). Obviously, the electric field exerts electrostatic force on positive ions (AuCl4- and Cl-) toward the tip electrode side, which is against the direction of diffusion. So there is a competition between diffusion and migration. For estimation, the local region could be treated as a parallel plate capacitor, and the intensity of the electric field increases from about 11 000 to 57 000 V/m when the distance decreases from 150 to 30 μm. The absence of dendritic nanostructure under 30 μm distance condition might be due to the electrostatic force playing an overwhelming part of the competition at this distance, so few AuCl4- ions reach the substrate electrode and are reduced there. Therefore, there must be a compromise between the diffusion and the electric field force to obtain dendritic nanostructure. The potential applied to the tip and the substrate electrodes are also key factors to the dendritic nanostructure formation. Each half reaction is driven by the potential applied to the corresponding electrode. A faster Au etching rate would be expected when a higher potential is applied to the tip electrode, and a faster Au deposit rate would be expected when a lower potential is applied to the substrate electrode. We have conducted experiments to investigate these two situations. Figure 6 shows the SEM images corresponding to these two experiments. In Figure 6a,b, the potential applied to tip electrode is up to 1.7 V, while the substrate potential does not change (compare to Figure 1). The etching rate 7630 DOI: 10.1021/la904245q

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Figure 6. SEM images of dendritic gold nanostructures of different applied potentials. (a,b) Et = 1.7 V, Es = -0.2 V; (c,d) Et = 1.5 V, Es = -0.4 V; d = 75 μm, t = 30 s. Scale bar for a and c is 20 μm, and that for b and d is 5 μm.

is accelerated while the reduction rate is not. Under such conditions, the formation of the dendritic nanostructure seems to not be affected. In another experiment, we decreased the substrate potential (to -0.4 V), so that the reduction process is accelerated. In this case, much more rod-like structures and few dendritic nanostructures were obtained (Figure 6 c,d). On the basis of the results above, we can conclude that the reduction rate has a profound effect on the formation of the dendritic structure. As mentioned above, either raising the tip potential or decreasing the substrate electrode will result in a higher intensity of the electric field. For example, rising the tip potential from 1.5 to 1.7 V (or decreasing the substrate potential from -0.2 to -0.4 V, 75 μm distance) increases the intensity of the electric field from about 22 700 to 25 300 V/m. The change is so small that it could not remarkably affect the intensity of the electric field like changing the tip/substrate distance does. We have also conducted experiments of increasing the tip potential to 2.0 V or decreasing the substrate potential to -0.6 V; bubbles were visible between the tip and substrate electrodes. The bubbles might be H2 and/or Cl2 due to electrolyzation of the HCl/KCl solution. It is known that gold nanomaterials have catalytic activity on electrochemical reduction of oxygen. The as-prepared gold dendritic nanostructure has a large surface-to-volume ratio and could provide a good electron-conducting tunnel and facilitate the electron transfer. Figure 7 shows the cycle voltammograms of bulk Au electrode and bare and dendritic gold-modified ITO electrode in O2-saturated KCl solutions. Bare ITO electrode shows no catalytic activity toward oxygen reduction in the potential range investigated. Comparing with bulk Au electrode, dendritic gold-modified ITO shows clear catalytic activity toward oxygen reduction, as the peak position shifted about 0.2 V positively (dash line vs solid line), and peak current increased significantly at the same time. A well-defined voltammogram for the reduction of oxygen was observed on the dendritic gold-modified ITO. There are two reduction peaks at around -0.05 and -0.16 V corresponding to the two-step two-electron reduction of oxygen. The peak at -0.05 V corresponds to the reduction of oxygen to H2O2, whereas the peak at -0.16 V is the result of a further reduction of electrogenerated H2O2 to H2O. This could be confirmed by adding H2O2 to the solution. The results are similar to others’ work,24 in (24) Jena, B. K.; Raj, C. R. Langmuir 2007, 23(7), 4064–4070.

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that the as-prepared nanostructure enhances the oxygen reduction. Eliminating some technical obstacles, the dendritic nanostructure could be readily constructed on other kinds of electrodes, such as a glassy carbon electrode and a metal electrode. It will provide a rapid route to prepare a modified electrode with pristine gold dendritic nanostructure on an electrode surface.

Conclusions

Figure 7. Cyclic voltammograms of a bare Au electrode (dash-dotted line, right axis), bare (dotted line, left axis), and dendritic gold-modified (dashed line, left axis) ITO in 0.1 M KCl solutions saturated with oxygen, and cyclic voltammograms of dendritic goldmodified ITO in oxygen-saturated 0.1 M KCl solutions with addition of about 10 μM H2O2 (solid line, left axis). Scan rate, 50 mV/s.

which a flower-like gold nanostructure was used for the electrochemical reduction of oxygen. The peak potentials in our work are more positive than the flower-like catalyst, which may indicate

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In conclusion, a dendritic nanostructure constructed with pristine metallic gold was obtained by a simple and rapid electrochemical route. The synthesis involved neither template nor surfactant. Because only HCl and KCl were used all through the experiments, the synthesis strategy is “green”. Experimental conditions affecting the formation mechanism were also discussed. The synthesis is also a feasible and fast way to modify an ITO electrode. The as-prepared modified ITO electrode has excellent catalytic activity for oxygen reduction in neutral KCl solution. Acknowledgment. We gratefully acknowledge financial support from the National Natural Science Foundation of China (Nos. 20805044, 20935003, 20820102037); 863 Project (No. 2007AA061501), and 973 Project (No. 2010CB933603).

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