Indian Journal of Chemistry Vol. 44A. May 2005, pp. 932-937

Electrodeposition of gold nanorods with a uni-directional crystal growth and lower Au(lll) facets area

Feifei Gao', Mohamed S EI-Deab2 & Takeo Ohsaka*

Department of Electronic Chemistry, Interd isciplinary Graduate School of Science and Engineering. Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan

Email: ohsaka @echem.titech.ac.jp

Received II Novell/ber 2004

Stable Au nanorods with a uni-directional pin-like morpho logy have been prepared on the surface of glassy carbon (GC) e lec trodes via a potent ial-step electrodeposition method from H2S04 solution containing Na[AuCI41 in the presence of cysteine as an additive. The Au nanorods are characterized by a relatively low Au(lll) facets area because of a continuous uni-direc ti onal growth of the crystal along the (I II ) orientation leading to a pin-like morphology of the crystals . In other words, there is a significant enrichment of Au(lOO) and Au(IIO) facets in contrast to the spherical Au anoparticles prepared in the absence of cysteine. The peak potential of the oxygen reduction react ion, measured in Orsaturated 0.5 M H2S0 4, at the thus-prepared Au nanorods e lectrodeposited onto glassy carbon (nano-Au/GC) electrodes shows a s ignificant positive shift compared to that obta ined at the nano Au/GC e lectrode prepared in the absence of cysteine, demonstrating a higher electrocatalytic activity of these Au nanorods.

IPC Code: Int. Cl 7 B82B; C25B; C25D

Introduction Nowadays, synthesis and application of metallic

nanocrystals with control lable morphology and shapes are of great interest. Gold nanoparticles are one of the most stable metal nanoparticles and have been studied for many years due to their quantum size-related electronic, optical and magnetic properties, and wide applications in catalytic, electrochemical, biochemical and materials sciences. For example, rods, cubes, and mixtures of triangular, hexagonal and spherical gold nanoparticles have been successfully prepared 1-3. Additionally, the particle size and crysta llographic morphology of gold particles are important factors in their electrocatalytic behavior, design of nanoelectrode, investigation of catalytic mechanism and the structure sensitivity of some important electrochemical reactions, e.g., oxygen reduction4-6,

etc. Synthesis and optical properties of "branched" gold nanocrystals in high yield (over 90%) via a wet­chemical route have been reported7

. Gold nanoparticles have been dispersed on supports of u­Fe)0 4, C030 4, or NiO oxides by coprecipitation method8

, and utilized not only for the catalytic

'Present address : Department of Chemical System Engineering, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku . Tokyo 11 3-8656. Japan. ~ Permanent address: Department of Chemistry, Faculty o f Science. Cairo University, Cairo, Egypt.

hydrogenation of unsaturated alcohols and aldehydes, but also for oxygen reduction because of their remarkable electrocatalytic activit/ -14. Recently, Daniel and Astruc l5 presented a review on the assembly, supramolecular chemistry, quantum-size­related properties, and applications in biology, catalysis, and nanotechnology. A notable point is that most of the Au crysta ls have a large r area of Au( III ) facets than Au( I 00) facets since the crystal structure of gold is close-packed face-center cubic (fcc) and has cuboctahedral equilibrium shape with (111) facets normally presenting lower surfar;e energy th an ( 100) facets. Facet area is inversely related to the surface energy of its facet; facets of lower surface energy forming larger facets area l6

.

At the same time, most of these gold nanocrystals are prepared in solutions but not fixed on a substrate, since the gathering of such nanocrys tals into large clusters or domair,s would change or cause the disappearance of the advanced properties of nanosi ze. It is well known that several applications of nanocrystals in the fields of electrocatalysis, bio­electrochemistry, functional material s and electric devices are based on their conducting and attaching or binding properties with substrate surfaces. Although specific binding molecules have been used to connect gold nanoparticles and the substrates, such as molecules with -SH or -NH2 groups to bond wi th

GAO el al.: ELECTRODEPOSITION OF UNI-DIRECTIONAL GOLD NANORODS 933

gold particles and silanol or 4-mercaptophenyl groups bonded to the substrates of glass or carbon l7

. 18, gold nanoparticles formed on the conducting substrate without using binder molecules are of great interest l9

,

in view of the fact that the surrounding environment of the nanoparticles strongly influences their catalytic activity and conductivity.

In the present work, pin-like Au nanorods with obviously lower Au( 111) facets area have been obtained directly and stably on the surface of GC electrode. Potential-step electrolysis has been performed to prepare Au nanocrystals in the presence of cysteine as an additive. Such Au nanorods possess a significant enrichment of areas of (100) and (I 10) facets at the expense of (111) facet.~ leading to considerable electrocatalytic activity for the oxygen reduction reaction in 0.5 M H2S04 solution.

Materials and Methods Au nanorods were deposited on GC electrode (<1> =

3 mm) by electrodeposition from Nrsaturated 0 .5 M H2S04 solution containing 1.0 mM Na[AuCi4 ] (Wako Pure Chemical Co., Japan) in the presence of 0.1 mM cysteine (Wako Pure Chemical Co., Japan) by applying potential step electrolysis from 1.1 to 0 Y for different deposition time (td). Self-assembled monolayer (SAM) of cysteine was prepared on the thus-prepared nano-Au/GC electrodes (obtained at td

= 1000 s in the presence and absence of cysteine, respectively) by immersing into 1.0 mM cysteine (aqueous solution) for 20 min.

A computer-controlled electrochemical analyzer (ALS/CHI 630A) and a standard two-compartment three-electrode electrochemical cell were used for the electrochemical measurements , with a platinum wire auxiliary electrode and a potassium chloride-saturated silverlsilver chloride (AgIAgCI, KCi-sat.) reference electrode. Prior to use, GC electrodes were polished with emery paper, and aqueous slurries of successively finer alumina powder (down to 0.06 ).A.m) on a polishing microcloth, wet with Milli-Q water and then carefully rinsed with Milli-Q water in an ultrasonic bath for 10 min. Au electrodes (<1> = I mm) were polished 111 the same way, and then electrochemically pretreated in 0.05 M H2S04

solution (Nrsaturated) by cycling the electrode potential between - 0.2 and 1.5 Y at 100 mY S·I for 10 min , or until the characteristic 1- V curve of a clean Au electrode was obtained. All the electrochemical measurements were performed at room temperature (25 ± I DC), and current densities were calculated on

the basis of the geometric surface areas of the GC and Au electrodes.

The morphology of the different Au particles electrodeposited onto GC electrode was studied by scanning electron microscopy (SEM) measurements, conducted with a JSM-T220 scanning electron microscope (lEOL, Japan) at an acceleration voltage of 15 kY and a working distance of 4-5 mm. The x­ray diffraction (XRD) measurements were performed on a Philips PW1700 Power X-ray diffractometer,

using Cu KUI radiation 0 .. = 1.54056 A) with a Ni filt er working at 40 kY and 30 rnA.

Results and Discussion SEM photographs of the Au nanorods prepared at

different td'S, 1000 and 2000 s, in the presence of cysteine are shown in Fig. 1 (a & b), while those of Au nanoparticles obtained under the same experimental conditions but in the absence of cysteine are presented in Fig. 1 (c & d) for comparison. Au nanoparticles deposited at td = 2000 s were much larger in size than those at td = 1000 s irrespective of the presence or absence of the additive cysteine, which suggests that the size of Au particles increases with increasing {d . A noticeable image of the Au particles electrodeposited in the presence of cysteine [shown in Fig. 1 (a & b)] is that they are pin-like crystals (i.e ., uni-directionally grown nanorods) and do not possess spherical domains such as those prepared in the absence of cysteine (shown in Fig. I (c & d» . This implies that such a preferenti al crystallization of Au particles occurs on a certain crystalline facet , which might be caused by the existence of cysteine that favors an electrodeposition and growth of Au crystals in a particular crystalline orientation.

To study the influence of cysteine on the preferential crystallographic orientation of the electrodeposited Au nanoparticles, SAMs of cysteine on the nano-Au/GC electrodes were prepared. Figure 2 shows the cyclic voltammograms obtained at two successive potential scans for the reducti ve desorption of cysteine SAMs formed on the Au/GC electrode, which were prepared in the presence (curve a) and absence (curve b) of cysteine. It is clearly observed that the peak currents for the nano-Au/GC electrodes prepared in the presence of cysteine are higher than those prepared in its absence, which will lead to a higher total faradaic charge consumed during the reductive desorption process at curve (a) than that at curve (b). That is because the pin-like Au crystals ,

934 INDIAN J CHEM . SEC A. MAY 2005

Fig. I- SEM photographs o f Au particles e lectrodeposited o n GC e lec trode from N2-saturaled 0.5 M H2S04 solution containing 1.0 111M

Na[AuCI41 in the presence (a and b) and absence (c and d) of 0.1 111M cysteine by applying a potent ial step electrolys is fro lll 1. 1 to 0 V for 1000 s (a and c) and 2000 s (b and d). Each di vision of the gauge in the photog raph s is 500 nill .

deposited on the GC electrode with cysteine, contribute more crystalline facets and in turn higher surface area binding with cysteine SAMs, which were desorpted during the two successive potential scans. The reduction peak at ca. -0.78 Y in the first scan (corresponding to the reductive desorption of cysteine from (Ill) facets of the Au crystals20

.21

) disappeared completely in the second scan, which means that the binding between cysteine and (Ill ) facets was easily broken durjolg the potential scans, and therefore no reductive peak appeared at (111 ) facets in the second scan. Meanwhile, the cun·ent response at ca. -1.0 Y and - 1.1 Y (ascri bed to that from the (100) and (110) facets of Au crystals respectivel/o.21

) could be observed clearly in the second scan although considerably decreased . The above results prove that Au(lll) is the most favorable facet of All crystals from which the reductive desorption of cysteine

occurs, and that cysteine adsorbs more strongly on the Au( 100) and AuOIO) facets tbn on the Au(lll ) facets. Thus, the presence of cysteine enriches Au(IOO) and Au(llO) facets by strongly binding with these facets and preventing more Au particles from depositing on these facets during the process of deposition. Consequently, a preferential electrodeposition of Au particles would occur on Au(lll) facet, ultimately leading to a growth of the pin-like Au nanorods along the (111) orientat;on.

The electrodeposition behavior of Au particles was characterized by CY responses on bare GC electrode in N2-saturated 0.5 1\.1 H2S04 solution containing 1.0 mM Na[AuCI4] in the presence and absence of 0.1 mM cysteine, and the results are presented in Fig. 3 (A and B). Gradual increase of the reduction peak (at ca. 1 V vs. AgIAgCI, KCI-sat.) in curve (a) indicates an increased amount of the Au particles deposited on

GAO el al.: ELECTRODEPOSITION OF UNI-DIRECTIONAL GOLD NANORODS 935

1 f.lA. mm-2 I

b

-1.2 -1 -0.8 -0.6 -0.4

E, V vs. AglAgCl

Fig. 2-Cyclic voltammograms for the reducti ve desorpti on of cysteine SAMs on the Au/GC electrodes (<1> = 3 mm) in Nr satu rated 0.5 M KOH solution. Potential scan rate: 100 mY s·'. The Au/GC electrodes were prepared by applying a potential step electrolysis from 1.1 to 0 Y for 1000 s in 0.5 M H~S04 so lution containing 1.0 mM Na[AuCI4J in the presence (a) and absence (b) of 0.1 mM cysteine. The numbers I and 2 refer to the first and the second potenti al cycles. respectively.

GC electrodes. Comparing 3A and 3B, we can see that although both the oxidation peaks in the potential region of l.l to 1.2 V, ascribed to the contribution from Au(l ll ) facet 2

:>, increase to a lmost the same CUITent density after ten successive potential scans, a slightly larger peak cun'ent density could be observed during every scan in the e lectrolyte with cysteine (curve a) in Fig. 3A) as compared to that in the electrolyte without cysteine (curve a in Fig. 38). On the other hand, a the potential of ca. 1.28 V, the oxidation peak current obtained in the presence of cysteine (shown in Fig. 3A) obviously has a much higher intensity than that obtained in the absence of cysteine (Fig. 38). These results suggest that the surface area of the nano-Au/GC e lectrode prepared in the presence of cysteine is larger than that prepared in the absence of cysteine, and that the pin-like Au nanorods electrodeposited in the presence of cysteine possess a larger surface area of crystalline state, compared wi th the round spherical nanoparticles prepared in the absence of cysteine. It also illustrates that such a large surface area is mostly composed of Au(IOO) and Au(110) facets, whi le possess ing a low rati o of the Au( III) facets area.

This result was also proved by the XRD measurements on Au crystals obtained with the pre, ence and absence of the additive cysteine. I n the

6A

4

~ 2

~ b ~ 0 ::1.. . ....,

-2 a

-4

-6 0 0.4 0.8 1.2

E, V vs. AglAgCI

68

4

2

~

§ 0

~ • -2 . ....,

-4

-6 0 0.4 0.8 1.2

E, V vs. AglAgCI

Fig. 3-Cyclic voltammograms of ten successive pOlential scans on bare GC electrodes in N2-satllrated 0.5 M H2S04 solution containing 1.0 mM Na[AlICI41 in the presence (A) and absence (B) of 0.1 mM cysteine. In both cases, the cyclic voltall1mog rams (dolled lines (curve b)) were obtained at bare GC electrode in _ saturated 0.5 M Hl S04 solution. Potential scan rate: 50 mV s·'.

XRD patterns shown in Fig. 4. the strong peaks at co .

26:., 43 and 78° were assigned as ce 11 I), ceO 10) and C( IIO), respectively , due to the substrate of GC on which Au nanoparticles were electrodeposited , and

the peak at ca. 38° was ascribed to the Au( Ill ) facets of the Au nanoparticles. On the basis of the XRD data shown in Fig. 4, ' we can see that the presence o f cysteine as an additive in the electrodeposition of the Au crysta ls decreases the Au( 111 ) domai ns. and that contrary to the expectation from the reducti ve desorption patterns (shown in Fig. 2) , the XRD peak<:

936 INDIAN] CHEM, SEC A, MAY 2005

a

b

100

2 Theta, degree

Fig. 4-X RD palterns for Au/GC e lectrodes , prepared for 2000 s in the same way as in Fig. 2 in the presence (a) and absence (b) of 0.1 mM cystei ne.

corresponding to the Au( I 00) and Au(l10) domains are not observed. This is due to the fact that according to the extinction rule of diffracti on in the XRD measurements of a face-centered lattice crystal , for a given facet of 17kl orientation to have a significantly observed XRD peak, it should have all-odd or all­even II , k and l numbers. Facets with mixed odd and even h, k and l values have a structure factor F hkl of zero23. Thus, the XRD peaks assigned to the Au(ll 0) and Au( I 00) domains could not be observed, and the intensity of the peak corresponding to the Au(lll) facet was taken as the probing parameter to evaluate the effect of cysteine. In our recent work6 we have shown that the presence of cysteine (as an additive) during the electrodeposition of Au nanoparticles onto Au(lll) substrate results in a s ignificant increase in the peak intensity located at 28 of ca. 44° [colTesponding to Au(200»), but we could not observe thi s peak in the present case owing to the high background intensity of the GC substrate used in this study . Figure 4 shows that the XRD peak intensity located at 28 of ca. 38° corresponding to the Au(lll) facets of the samples obtained in the presence of cysteine (curve a) is weaker than that in its absence (curve b), indicating that Au(111) crystalline facets are less favorably formed on the GC surface in the presence of cysteine. This is in agreement with the fac t that the Au electrodeposition in the presence of cys teine would more easily get the larger Au(l 00) and Au( 110) facets area at the expense of Au( 111) facets,

a

b

/SIlAmm-2

-0.8 -0.6 -0.2 o 0.2 E, V vs AglAgCI

Fig. 5-Cyclic voltam mograms for oxygen reduction at (a) bare: GC, (b) bare polycrystall ine Au ((jl = I mm), (c) and (d) nano­Au/GC electrodes in Orsaturated 0.5 M H2S04 so lution . The nano-A u/GC electrodes were prepared in the same way as in Fig . 2 in the absence (c) and presence (d) of 0.1 mM cystei ne. Potenti al scan rate: 50 mY S· I.

in addition to the unique directional gro wth of the Au nanorods.

ElectrocataIytic activity for oxygen reduction

The electrochemical application of such Au nanorods was achieved by their electrocatalytic activity for oxygen reduction. The CY responses for oxygen reduction reaction were measured at bare GC and polycrystalline Au electrodes and nano-Au/GC electrodes prepared in the absence and presence of cysteine in 02-saturated 0.5 M H2S04 solution (Fig. 5). A 2-step 4-electron reduction of oxygen was observed on both nano-Au/GC electrodes (curves c and d). Compared with those at the bare GC and polycrystalline Au electrodes (curves a and b respectively), the oxygen reduction potential at both nano-Au/GC electrodes shifted positively, indicating that the Au nanoparticles possess a more effective catalytic ability for oxygen reduction. In addition, from the comparison of curves (c) and (d), we can see that the nano-Au/GC electrode prepared in the presence of cysteine is superior in its e1ectrocatalytic activity for oxygen reduction to that prepared in the absence of cysteine.

All the electrochemical measurements as well as the SEM and XRD studies were directly carried out on the Au nanocryscals on the GC electrode surface.

GAO el al.: ELECTRODEPOSITION OF UNI-D1RECTIONAL GOLD NANORODS 937

demonstrating that such Au pin-like nanorods were directly and stably electrodeposited on the electrode surface with their conducting property and electrocatal yti cacti vi ty.

Acknowledgement This work was financially supported by Grant-in­

Aids for Scientific Research on Priority Areas (No. 417), Scientific Research (No. 12875164 and No. 14050038) and "Scientific Research (A)" (No. 10305064) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. The authors thank Dr. A. Genseki for his help in the SEM measurements of Au crystals electrodeposited onto GC electrodes. FG and MS El-D thank the Venture Business Laboratory (VBL) of Tokyo lnstitute of Technology for postdoctoral fellowships.

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