Synthesis, Characterization, and Functionalization of Hybrid Au/CdS and Au/ZnS Core/Shell Nanoparticles

Summary

The synthesis of uniform gold nanoparticles coated with semiconductor shells of CdS or ZnS is performed. The semiconductor coating is conducted by first depositing a silver sulfide shell and exchanging the silver cations for zinc or cadmium cations.

Abstract

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large extinction coefficients which can be tuned across the visible spectrum. Research into the plasmonic enhancement of optical transitions has become popular, due to the possibility of altering and in some cases improving photo-absorption or emission properties of nearby chromophores such as molecular dyes or quantum dots. The electric field of the plasmon can couple with the excitation dipole of a chromophore, perturbing the electronic states involved in the transition and leading to increased absorption and emission rates. These enhancements can also be negated at close distances by energy transfer mechanism, making the spatial arrangement of the two species critical. Ultimately, enhancement of light harvesting efficiency in plasmonic solar cells could lead to thinner and, therefore, lower cost devices. The development of hybrid core/shell particles could offer a solution to this issue. The addition of a dielectric spacer between a gold nanoparticles and a chromophore is the proposed method to control the exciton plasmon coupling strength and thereby balance losses with the plasmonic gains. A detailed procedure for the coating of gold nanoparticles with CdS and ZnS semiconductor shells is presented. The nanoparticles show high uniformity with size control in both the core gold particles and shell species allowing for a more accurate investigation into the plasmonic enhancement of external chromophores.

Introduction

Gold and silver nanoparticles have potential for future technological advances in a variety of applications including photonics,¹ photovoltaics,² catalysis,³ chemical/biological sensing,⁴ biological imaging,⁵ and photodynamic therapy.⁶ Under visible excitation, the surface electrons can oscillate to form a resonance known as a localized surface Plasmon resonance (SPR), which can be utilized to concentrate incident radiation in the visible spectrum. Recently, noble metal nanoparticles have been combined with semiconductor or magnetic nanoparticles to produce hybrid nanoparticles with enhanced and tunable....

Protocol

1. Synthesis of Gold Nanoparticles

1. Weigh the gold salt in the glove box and add to a vial previously cleaned with aqua regia before diluting with water in a volumetric flask. Prepare a 1 mM Gold (III) chloride trihydrate (393.83 g/mol) in 100 ml water for gold stock solution.
2. Weigh out 3.2 g solid CTAC (320 g/mol) and heat, in 25 ml water, to approximately 60 °C for dissolution. Cool to room temperature and dilute the mixture with to 50 ml with water in a volumetric flask to prepare a 0.2 M Cetyl trimethyl ammonium chloride (CTAC).
3. Mix 20 ml of 1 mM gold solution and 20 ml of 0.2 M CTAC solution inside a round bottom boiling flask ....

Results

Normalized absorbance spectra of gold nanoparticles with three different surfactants are shown in Figure 1. The surfactants utilized are oleylamine, tetradecyl trimethyl ammonium chloride (TTAC), and cetyl trimetyl ammonium chloride. CTAC and TTAC surfactants show narrower plasmon resonance absorption band.

The amount of reducing agent not only affects the FWHM but the peak position of the resulting nanoparticle.......

Discussion

Gold nanoparticles

In order to guarantee high quality core shell nanoparticles, a monodisperse sample of gold nanoparticles must first be synthesized as a template.^28,29,30 We modified the gold nanoparticle synthesis to produce long-chain tertiary amines-capped nanoparticles instead of oleylamine-capped nanoparticles. Oleylamine-capped nanoparticles show a rather narrow plasmon resonance, indicative of monodisperse size range, but the particles synthesized via reduction using tert-b.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
MilliQ Water	Millipore	Millipore water purification system	water with 18 MΩ resistivity was utilized in all experiments
Gold(II) chloride trihydrate	Sigma Aldrich	520918	used as gold precursor for nanoparticle synthesis
Cetyl trimethyl ammonium chloride (CTAC)	TCI America	H0082	used as surfactant for gold nanoparticles
Borane tert butyl amine	Sigma Aldrich	180211	used as reducing agent for gold nanoparticles
Silver nitrate	Sigma Aldrich	204390	used as silver source for shell application
Ascorbic acid	Sigma Aldrich	A0278	used as reducing agent for silver shell application
Sulfur powder	Acros	199930500	used as sulfur source for silver sulfide shell conversion
Oleylamine	Sigma Aldrich	O7805	used as surfactant for silver sulfide shell conversion
Oleylamine	Sigma Aldrich	364525	used as surfactant for silver sulfide shell conversion
cadmium nitrate tetrahydrate	Sigma Aldrich	642405	used as cadmium source for cation exchange
zinc nitrate hexahydrate	Fisher Scientific	Z45	used as zinc source for cation exchange
11-Mercaptoundecanoic acid	Sigma Aldrich	450561	used as water soluable ligand during ligand exchange
3,4-diaminobenzoic acid	Sigma Aldrich	D12600	used as water soluable ligand during ligand exchange
UV-Vis absorption spectrophotometer	Cary	50 Bio	used to monitor absorption spectrum of colloidal solutions
JEOL TEM 2100	JEOL	2100	used to analyze size of synthesized nanoparticles. TEM grids were purchased from tedpella
FTIR spectrophotometer	Perkin Elmer	Spec 100	used to monitor chemical compostion of nanoparticle surface after ligand exchange.