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 functionality.^7,8 Recent literature, such as the study conducted by Ouyang et al.⁹ or Chen et al.¹⁰, has shown the possibility for the synthesis of these particles, but only limited control in the uniformity of the hybrid species is possible due to a distribution of gold nanoparticle sizes and compounded by the lack of optical characterization coupled with physical characterization at each stage of growth. Zamkov et al. showed similar uniformity in shell formation but only one shell thickness was utilized with different core sizes, with some shells not being fully formed around the nanoparticles. In order to effectively utilize these nanoparticles, the precise optical response must be known and characterized for a variety of shell thicknesses. Higher precision in shell thickness can be accomplished through the use of monodisperse, aqueous gold particles as the template, resulting in higher control over the final hybrid species. Interaction between the core and shell may show limited enhancement in absorption or emission rates due to the small amount of semiconductor material and the proximity to the gold core. Instead of interaction between the semiconductor found in the shell and the gold particle, the shell may be used as a spacer to limit the distance between an external chromophore.¹¹ This will allow for higher control over the spatial separation between the plasmon while, negating the consequences of direct contact with the metal surface.

The extent of the electronic interaction between the surface plasmon resonance and exciton produced in the chromophore, is directly correlated to the distance between the metallic and semiconductor species, the surface environment and strength of the interaction.¹² When the species are separated by distances greater than 25 nm, the two electronic states remain unperturbed and the optical response remains unchanged.¹³ The strong coupling regime is dominant when the particles have more intimate contact and can result in the quenching of any excitation energy via nonradiative rate enhancement or Forester Resonance Energy Transfer (FRET).^14,15 Manipulation of the coupling strength, by tuning the spacing between the chromophore and metal nanoparticle, can result in positive effects as well. The nanoparticle extinction coefficient can be orders of magnitude larger than most chromophores, allowing the nanoparticles to concentrate the incident light much more effectively. Utilizing the increased excitation efficiency of the nanoparticle can result in higher excitation rates in the chromophore.¹² Coupling of the excitation dipole can also increase the emission rate of the chromophore which, can result in increase in quantum yield if nonradiative rates are unaffected.¹² These effects could lead to solar cells or films with increased absorbance, and photovoltaic efficiencies, facilitated by the increased absorption cross-section of the gold and the ease of charge extraction from the semiconductor layer due to the existence of localized surface states.^12,16 This study will also provide useful information on the coupling strength of the plasmon as a function of distance.

Localized surface plasmons have widely been used in sensing¹⁷ and detection¹⁸ applications due to the sensitivity of the plasmon resonance to the local environment. Cronin et al., showed the catalytic efficiency of TiO₂ films can be improved with addition of gold nanoparticles. Simulations showed that this increase in activity is due to coupling of the plasmon electric field with excitons created in the TiO₂, which subsequently increases exciton generation rates.¹⁹ Schmuttenmaer et al., showed that the efficiency of Dye-sensitized (DSSC) solar cells could be improved with the incorporation of the Au/SiO₂/TiO₂ aggregates. The aggregates enhance the absorption through creation of broad localized surface plasmon modes which increase optical absorption over a broader range of frequencies.²⁰ In other literature, Li et al. observed significant reduction in fluorescence lifetime as well as distance dependent enhancement in steady state fluorescence intensity was observed through direct coupling of a single CdSe/ZnS quantum dot and single gold nanoparticle.²¹ In order to take full advantage of this plasmonic enhancement, there is a need for physical coupling with a set distances between the two species.

Synthesis of Hybrid Nanoparticles

Jiatiao et al., described a method to coat semiconductor material onto gold nanoparticles via a cationic exchange in order to produce uniform and tunable shell thicknesses. The shells were uniform in thickness, but the gold templates were not very monodisperse. This will alter the semiconductor to gold ratio from particle to particle and therefore the coupling strength.⁹ An in-depth study on the optical properties of these core shell nanoparticles has been conducted, in order to develop a reproducible synthetic method. Previous methods rely on organic-based nanoparticle synthesis, which can produce samples with broad plasmon resonances due to inhomogeneity in the gold nanoparticle size. A modified aqueous synthesis of gold nanoparticles can provide a reproducible and monodisperse gold nanoparticle template with stability for long periods of time. The aqueous surfactant cetyl trimethyl ammonium chloride forms a double layer on the nanoparticle surface due to interaction between the long carbon chains of nearby cetyl trimethyl ammonium chloride molecules.²² This thick surface layer requires careful washing to remove excess surfactant and allow access to the nanoparticle surface, but can provide higher control over the nanoparticle size and shape.²³ The aqueous addition of a silver shell can be controlled with high precision leading to a more intimate correlation between shell thickness and optical properties.²³ A slower reduction via ascorbic acid is utilized to deposit the silver on the gold surface, requiring the addition of silver salt to be very precise in order to prevent formation of silver nanoparticles in the solution. The third step requires a large excess of sulfur to be added into an organic phase and a phase transfer of the aqueous nanoparticles must occur. With addition of oleylamine as an organic capping agent and oleic acid, which may act as both a capping agent and aid in phase transfer of the nanoparticles, a uniform, amorphous silver sulfide shell can be formed around the nanoparticles.^9,24 The concentration of these molecules must be high enough to prevent aggregation of the nanoparticles in this step, but too much excess can make purification difficult. In the presence of tri butyl phosphine and a metal nitrate (Cd, Zn or Pb), a cationic exchange inside of the amorphous sulfide shell can be conducted. Reaction temperatures must be modified for the different reactivates of the metals⁹ and any excess sulfur must be eliminated to reduce the formation of individual quantum dots. Each step of the synthesis corresponds to a change in the surface environment of the nanoparticle, therefore, a change in plasmon should be observed due to the dependence of the plasmon frequency on surrounding dielectric field. A parallel study of optical absorption as a function of Transmission Electron Microscopy (TEM) characterization was used to characterize the nanoparticles. This synthetic procedure will provide us with well-controlled and uniform samples, providing better correlation from microscopy and spectroscopy data.

Coupling with Fluorophores

Applying a dielectric spacing layer between a plasmonic metal surface and a fluorophore can help to diminish losses due to nonradiative energy transfer of created excitons into the metal. This spacing layer can also aid in the study of distance dependence between the fluorophore and the plasmon resonance on the metal surface. We propose using the semiconductor shell of the hybrid nanoparticles as our dielectric spacing layer. The shell thickness can be tuned with nanometer precision with thicknesses ranging from 2 nm to 20 nm allowing precise distance correlation experiments to be conducted. The shell can also be tuned with Cd, Pb or Zn cations and S, Se and Te anions, allowing for control over not only the distance but also the dielectric constant, electronic band arrangement and even crystal lattice parameters.

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 and place in an oil bath set to 60 °C. Allow to mix for 10 min.
4. Add 1.7 mg (1:1 mole ratio) solid Borane tert-butyl amine (86.97 g/mol) to the gold/CTAC solution and let stir for 30 min.
   Note: Solution should turn deep red. The resulting solution has a gold particle concentration of about 5 μM and can be stored for months at a time or used immediately for the next phase of reaction.

2. Coating with Silver

1. Use precise reagent amounts to coat the nanoparticles with a silver shell. The shell will provide the template for size and shape of the semiconductor shell. Precise reagent amounts will also help to prevent nucleation of silver particles.
2. First calculate the volume of the core, in cm³, and convert to mass per particle using the density of gold. For example, to calculate the core volume, assume a spherical nanoparticle with a diameter of 15 nm to give a volume of 1767.15 nm³ and then convert to cm³ (1.77 x 10^-18 cm³). Multiply the volume by the density of gold (19.3 cm³) to calculate the mass per particle (3.41 x 10¹⁷).
   1. Using 10 ml of a 5.3 μM gold nanoparticle solution, 5.30 x 10^-8 moles of particles are present. Multiply by the molar mass gives to calculate the mass of gold present in the solution (1.04 x 10^-5 g). Divide the mass of gold in the solution by the mass per particle to find the number of gold particles present (3.06 x 10¹¹).
   2. Calculate the volume of the nanoparticles with a 5 nm shell thickness, in cm³ (8.18 x 10¹⁸ cm³) and subtract this from the volume of the core nanoparticle (1.77 x 10^-18 cm³) to determine the shell volume (6.41 x 10^-18). Convert this volume to mass of silver by multiplying by the number of gold particles and the density of silver (2.33 x 10^-4). Shell thicknesses in the range of 1-10 nm will be utilized in this study.  
   3. Convert the mass of silver to moles of silver needed for a 5 nm shell radius (2.33 x 10^-4). From this value, calculate the volume of 4.0 mM silver nitrate 540 μl) solution needed for the amount of gold utilized in the starting solution (10 ml).
3. Prepare a 4.0 mM AgNO₃ (169.87 g/mol) solution in 5 ml water. In a 70 °C oil bath, mix 10 ml of stock gold nanoparticles with ascorbic acid to make a 20 mM solution.
4. Add the silver solution drop-wise to the gold and ascorbic acid solution and allow the reaction to stir for 2 hr.
   Note: The reaction will turn light orange (thinner shell) to dark orange (thicker shell) over the course of the reaction.
5. Centrifuge the nanoparticles at 21,130 x g for 10 min and redisperse into clean water. Decant the supernatant from the pelleted nanoparticles to aid in removal of bare gold nanoparticles or silver nanoparticles which may have been formed.

3. Conversion of the Shell to Silver Sulfide

1. Weigh elemental sulfur in a 200:1 molar ratio to the silver used in the previous stage of the experiment. For 10 ml of Au/Ag core shell particles and a 5 nm shell, dissolve 3 ml of oleylamine and 1.5 ml of oleic acid into 10 ml toluene.
   1. Concentrate the silver colloids, via centrifugation at 21,130 x g for 10 min and disperse in 1 ml water.
      Note: This step helps increase the efficiency of the extraction from the aqueous layer to the organic layer upon formation of the silver shell.
2. Add the colloids, drop-wise to the sulfur solution under stirring for 1 hr.
   Note: The solution will turn dark blue (thinner shells) to purple (thicker shells) as the sulfurization goes to completion.
3. Centrifuge the colloidal solution at 4,000 x g for 10 min after the reaction has stirred 2 hr to remove the water and unreacted sulfur from the solution. Re-disperse the nanoparticles into clean toluene with sonication, if necessary.
   1. Sonicate the nanoparticles in a bath sonicator for 30 sec to 1 min in order to disperse into toluene.
      Note: Excess oleylamine or oleic acid may fall out of solution and can be removed after this step by decanting the solution from the white solid.

4. Cation Exchange

1. Make the metal precursor by dissolving the metal nitrate into 1 ml of methanol, to make a 0.2 M solution of Cd(NO₃) or Zn(NO₃).
   Note: a 0.8 M solution may be used for thicker shells to decrease the amount of methanol in solution.
   1. Mix the metal solution with the silver sulfide-shelled nanoparticles in a 1:1 molar ratio with the silver. Heat to 50 °C for cadmium shell and 65 °C for zinc shells under a nitrogen atmosphere.
2. Add tri-butyl phosphine in a 500:1 molar ratio to the metal precursor. The reaction times are 2 hr for Cadmium and 20 hr for zinc.
3. Purify via centrifugation at 21,130 x g for 10 min in order to remove any isolated CdS or ZnS nanoparticles which may have been formed. Disperse the pelleted nanoparticles into a clean nonpolar solvent such as hexanes, toluene, or chloroform.

5. Ligand Exchange from Oleylamine

1. Mix the nanoparticle solution with 1.5 times volume ratio ethanol to colloidal solution in toluene in a centrifuge tube. Centrifuge at 4,000 x g for 10 min to pellet the nanoparticles.
2. Wash the nanoparticles with ethanol and centrifuge once more to collect the solid particles.
   Note: The particles can be stored at this stage but removal of ethanol is necessary to prevent aggregation.
3. Bind Ligands with a nucleophilic binding group to the surface via unbound cationic sites on the shell. 11-mercaptoundecanoic acid and 3,4-diaminobenzoic acid are appropriate molecules which leave the nanoparticles water-soluble.
   1. Disperse the nanoparticles into the ligand solution in large excess, approximately 10 times higher concentration than the native oleate molecules. Stir the particles at room temperature overnight to allow displacement of any residual oleate molecules.
   2. Centrifuge the solution at 4,000 x g for 10 min. Wash the pelleted particles with methanol and centrifuge at 4,000 x g for 10 min once more to collect the solid nanoparticles.

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

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.