Triangular Core-Shell Structure Ag@AgAu

Mohammad M. Shahjamali , Lim Ming Pin, Mustafa Hussain Kathawala, Satkunarajah Pratheepan, Norsuzana Bte Saini, Maimanah Bte Mansor, Can Xue

School of Materials Science and Engineering, Nanyang Technological University, Block N4.1, 50 Nanyang Avenue, Singapore 639798, Singapore

Abstract- Novel Triangular Ag@AgAu nanoparticles were

synthesized in aqueous solution using a seed-mediated approach in the absence of any surfactants. The formation of the gold layer on the silver nanoprism seeds leads to structures with better stability and tunability. The triangular Ag@AgAu has been synthesized by the reduction of hydrogen tetrachloraurate onto Ag nanoprisms. This unusual structure exhibited interesting optical properties and the extinction spectra can be engineered in a broad range based upon the thickness of the outer AuAg shell.

I. INTRODUCTION

Obviously size, shape, and specially composition of nanomaterials can dramatically affect their physical and chemical properties, and that technologies based on nanoscale materials have the potential to revolutionize various fields. Anisotropic particles are particularly interesting because the decreased symmetry of such particles often leads to new and unusual chemical and physical behavior [1] with unusual optical, catalytic, electronic and magnetic properties [2-7]. Complex nanostructures made of noble metals, silver and gold in particular, have received renewed interests in recent years due to their interesting optical, catalytic and electronic properties [8].

Nanosized bimetallic colloidal particles exhibit unique optical properties. It has been confirmed that co-operative and synergistic interactions between the metallic components could lead to an overall more useful functionality [2]. Core-shell bimetallic triangular Ag@Au is an ongoing area of research, due to their unique optical properties involving surface plasmon resonance (SPR), which are tunable by their geometric dimensions, composition and Size. In this study, the triangular Ag@Au has been synthesized by the reduction of hydrogen tetrachloraurate onto Ag nanoprisms. However, such a synthesis pathway is not easy as the galvanic replacement reactions occurring between (AuCl4

-) and Ag nanoprisms result in changing the triangular plate morphology.

Experimental

In a typical experiment, Triangular Ag@AgAu particles were synthesized by reduction of Au ions onto Ag triangular nanoprism seeds (prepared according to literature procedures).[6] The prism mixture was then grown by continuous addition of 5.76 mM hydroxylamine

hydrochloride and 0.2745mM , HAuCl4 solution by using syringe pump. Total volume of aqueous Ag nanoprism solution before additions was 12.5 ml, where concentration of Ag nanoprism seed solution was determined by optical density of 0.5 O.D. after dilution with 30 ml of DI water. The process takes 165 min to be complete and the rate of infusion of both of the reagent is 0-120 min and 120-160 min is 1 and 2.66 mm/hour, respectively.

Instrumentation

Samples for TEM images were examined using a JEOL 2010 TEM operated at 200 kV. They were prepared by directly placing 4 �l of the as-prepared samples on a PEI modified carbon coated copper grid for better dispersion and dried at room temperature. Optical spectra were acquired on Shimadzu UV-1800 UV–vis spectrophotometer using 10 mm light path quartz cuvette.

II. RESULTS AND DISCUSSION

UV-vis-NIR spectroscopy allows one to easily follow the

growth of the triangular Ag@AgAu. Since the dipole plasmon resonance is very sensitive to the amount of gold deposited on the triangular silver prism, it can provide lots of information. In Fig. 1 and 2, we show typical extinction spectra for the resulting triangular particles during the synthesis and Ag nanoprism seeds.

0

0.1

0.2

0.3

0.4

0.5

350 450 550 650 750 850 950 1050

Wavelength�(nm)

Extinction

�

0��min.

30�min.

60�min.

Fig. 1. UV-vis spectra of the Ag@AgAu particles synthesized during 0-60 min.

Fig. 2. UV-vis spectra of the Ag@AgAu particles synthesized during 90-165 Min.

Initial silver nanoprism had only one intense SPR peak at

720 nm , the 30 min sample had a red-shifted as much as 36 nm due to formation of the gold frame on the rims of the structure as it is depicted in Fig 1 . In 60 min sample still red shift is noticed and the peak reaches 766 nm .The increase in thickness of the nanoframe around the particles’ rims should

be responsible for this continuous red shift. Starting from 90 min. we had blue shift in the spectrum

which continues till end of the process. In this sample, 5 and 15 nm blue shift is noticed from 60 to 90 minutes and 90 to 120 minutes.

As you can see in the Fig. 2 some pinholes has been

produced in the particles but in next step it is refilled by reduced silver and gold ions. In 120 min the number and area of pinholes decreased and from this point the gold coating layer on the {111} facets of the silver nanoprism becomes thicker due to deposition of gold atoms on the rough etched-surface of silver prism.

Even though the chemical identity of the gold coating on

the {111} facets of the silver nanoprism is not instantaneously apparent from the contrast in the image of the {111} lattice planes, elemental analysis of these faces by X-ray energy dispersive spectroscopy (EDS) shows the presence of both Ag and Au on the surface of the particles which is another proof for having a Bimetallic core-shell structure.

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0.4

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350 450 550 650 750 850 950 1050

Wavelength��(nm)

Extinction�

90�min.

120�min.

165�min.

A B C

D E F

Fig. 3. TEM images of (A) Silver nanoprism, Ag@AgAu at (B) 30 min., (C) 60 min. (D) 90 min. (E) 120 min. and (F) 165 min. The pinholes created at 90 min vanish in the next steps of synthesis during 120 and 165 min.

Fig. 4. Elemental mapping analysis of Ag@AgAu nanoparticles

probed by X-ray energy dispersive spectroscopy (EDS) (A) silver counts (B) gold counts of a single structure.

In conclusion, we have shown that the triangular

Ag@AgAu can be synthesized using a seed-mediated approach. Triangular Ag@AgAu is more stable than silver prism and can be used as SERS substrate and also in biosensing applications. The optical properties and the physical attributes of this core-shell structure can be engineered by using various stoichiometric ratios of gold and silver.

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C