Coupling effects of NaYF4Yb,Er upconversion nanoshells and au ag metallic nanoshells 4

Chapter 4 Synthesis and characterization of Au-Ag metallic nanoshells 4.1 Introduction Hollow metallic structures have been of interests because of potential applications in drug delive

Chapter 4 Synthesis and characterization of Au-Ag metallic nanoshells

4.1 Introduction

Hollow metallic structures have been of interests because of potential applications in drug delivery,87,88 photothermal therapy,39,40 and fluorescence enhancement.54,89 Optical properties of hollow metallic nanoshells or dielectric cores/metallic shells may be controlled by the size, shape, and shell thickness, demonstrating higher sensitivity in localized surface plasmon resonance (LSPR) as compared with their solid counterparts.57 The LSPR of metallic nanoshells may be tuned between the visible and the NIR wavelengths by changing the dimension of interior cavity and shell thickness.90,91 Au nanoshells are one of the most studied due to its good biocompatibility, thermal and chemical stability.38,92,93 Since the media such

as water, blood, and tissue are relatively transparent in the NIR wavelengths,94

the NIR absorbing Au nanoshells find potential biomedical applications.43 Localized heating for selective destruction of cancer cells was demonstrated in NIR absorbing Au nanoshells.4,54,95

When the incident light is in resonance with the plasmon frequency of the metallic particles, a strong enhancement of local field is produced at the nanoparticle surface.96 This local field enhancement around the metallic particles could increase the fluorescence of nearby fluorophores such as organic dyes, quantum dots, and UC nanoparticles.97,98,99 The metallic nanoshells may exhibit a larger local field enhancement around the particle surface compared with their solid counterparts Therefore, metallic nanoshells

may be a good candidate to enhance the fluorescence of nearby UC nanoparticles

In this thesis, Au-Ag nanoshells were synthesized via galvanic replacement reaction between Ag templates and HAuCl4 in toluene-ODE in the presence of oleylamine.56 The transformation from Ag templates to Au–

Ag nanoshells in the organic medium was studied The microstructure, surface morphology, shell thickness, composition, and optical properties of the sample collected at different stages of the transformation were investigated Further, a size-dependent transformation was demonstrated

For non-truncated decahedral metal nanoparticles, the entire surface was commonly covered by {111} facets,101,102 which is the densest and lowest

surface energy fcc facet The average size (estimated by random

measurements of ~100 particles from the TEM images) was found to be 43 ± 6

nm for the decahedral shape and 53 ± 9 nm in edge length for the triangular prism shape The XRD of the as-synthesized Ag nanoparticles matched well

with the fcc Ag reference [JCPDS file number PDF 4-783], (Fig 4.1c) The

much higher (111) peak intensity indicated the (111) texture Figure 4.1d shows the UV-visible spectrum of Ag nanoparticles stabilized by oleylamine dispersed in toluene Their LSPR extinction peak was ~504-nm wavelength

Fig 4.1 (a) TEM images of as-synthesized Ag nanoparticles consisting of

decahedral (~43 nm in size) and triangular prism (~53 nm in edge length) shapes (b) HRTEM image of a decahedral Ag nanoparticle (c) XRD of as-

synthesized Ag nanoparticles matched well with the fcc Ag reference (JCPDS

file number PDF 4-783) (d) UV-visible extinction spectrum of the oleylamine-stabilized Ag nanoparticles in toluene showed an extinction peak

at ~504 nm wavelength

4.3 Microstructure and surface morphology

Au-Ag nanoshells were synthesized via galvanic replacement reaction between the as-synthesized Ag nanoparticles and HAuCl4 solution To study the changes of microstructure and surface morphology, the samples were collected at different stages of the reaction (Table 2.1) for TEM and SEM analyses Figure 4.2a shows the TEM image of the particles obtained at the initial stage of the reaction, after the reaction between 15 mol of the HAuCl4

and the Ag templates Large voids (light contrasts) in each particle (decahedral and triangular prism shapes) were observed, indicating Ag solids (dark contrasts) at the center of the particles had been removed At this stage, the decahedral particles appeared like equiaxed particles

The solids at the center gradually disappeared (increasing light contrast) with increasing HAuCl4 (20 mol), leaving behind a continuous dark contrast at the particle periphery associated with a nanoshell (Fig 4.2b) The removal of solid Ag was attributed to Ag oxidation via reduction of HAuCl4 The nanoshells were found in both equiaxed and triangular prism shapes Equiaxed nanoshells were likely transformed from decahedral Ag templates, whereas the triangular prismatic nanoshells were from triangular Ag nanoprisms Note that the nanoshells did not collapse or fragment into small solid particles with addition of excess HAuCl4 (25 – 30 mol), as shown by the TEM images in Fig 4.2c, d The contrast at the center of the nanoshells appeared darker and the shells noticeably thicker when HAuCl4 increased to

50 mol and 75 mol (Fig 4.2e, f)

Figure 4.3 shows the HRTEM images of the nanoshells The contrasts

in the HRTEM images indicated the structures could be either a nanoshell

(equiaxed or triangular prism shapes) with an interior cavity or a Ag core/Au shell structure since Ag had a lower scattering contrast This was further investigated using EDX line scanning and elemental mapping, discussed in detail in the following paragraphs

Fig 4.2 TEM images of the particles obtained from the reaction of the Ag

templates with (a) 15 mol, (b) 20 mol, (c) 25 mol, (d) 30 mol, (e) 50

mol, and (f) 75 mol of the HAuCl4 The scale bars for the images (a – f) are

50 nm

Fig 4.3 High-resolution TEM images of the nanoshells obtained from the

reaction between the Ag templates and 30 mol of the HAuCl4, (a) and (b) equiaxed shape, (c) and (d) triangular prism shape

Figure 4.4 shows the compositional line profile across a single particle

by the EDX line scanning analysis The equiaxed- and triangular shaped particles (from left to right in Fig 4.4) were obtained from increasing the amount of HAuCl4 The TEM images (Fig 4.4a, e) showed part of the solids (dark contrasts) at the center of both the equiaxed- and triangular prism-shaped particles were removed at the initial stage of the reaction Their compositional line profile showed the Ag intensity reached a maximum value

prism-at the dark contrast regions (solids) and a minimum value prism-at light contrast regions This confirmed the removal of Ag solids at the center of the sacrificial templates At this stage, the Au intensity was still low across the single particle compared with that of the Ag (Fig 4.4a, e), suggesting a low relative concentration of Au The relative concentration of the Au in the

single particle was 22.5% and 18.7% for the equiaxed- and triangular-shaped particles, respectively

A very thin, most likely incomplete Au shell was deposited on the surface of Ag template when a small amount of HAuCl4 was added at the initial stage of the galvanic replacement reaction.83 The oxidation of Ag likely continued toward the interior of the particles through the sites at Ag template surface that was not covered by Au, leading to subsequent large voids at the center of each particle When the oxidation was allowed to continue with increasing amount of HAuCl4, most of the Ag solids at the center of the particles were removed This was indicated by light contrast at the center of the particles with the minimum corresponding EDX intensity of Ag (Fig 4.4b–d, f–h) At this stage, a continuous dark contrast at the particle periphery with maximum intensities of both Au and Ag was observed This confirmed the formation of Au-Ag shells A previous study reported the deposited Au layer alloyed with the underlying Ag to form the Au-Ag shells.83 The surface energy of Au (1.50 J/m2) is higher than that of Ag (1.25 J/m2) Hence, the deposited Au on the outer surface would therefore be more likely diffuse inward and mix with Ag to decrease the surface energy of Au-Ag system.103

In our study, the Au-Ag shells did not collapse when excess HAuCl4

was added Instead, the shells grew thicker as more Au was formed and deposited on the surface to form Au-Ag shells, as shown by increased relative concentration of Au in the shells (Fig 4.4) The compositional line profile confirmed the Au-Ag nanoshells did not consist of a solid Ag core for both equiaxed and triangular prism shapes, since Ag intensity reached a minimum

at the center of the particle The Au-Ag structure was unlikely a structure of

Ag shell/Au shell since the position of the EDX maximum intensities of both

Ag and Au of the shells overlapped each other

Fig 4.4 TEM images and compositional line profile of Au and Ag across a

single Au-Ag particle by EDX line scanning analysis The particles (equiaxed and triangular prism shapes) from left to right side were obtained with increasing the amount of HAuCl4 The relative concentration of Au in the equiaxed single particles was (a) 22.5%, (b) 44.1%, (c) 52.6%, (d) 65.5% and

in the triangular prism single particles was (e) 18.7%, (f) 26.2%, (g) 61.6%, and (h) 68.1% The scale bars in the TEM images (a – h) are 20 nm

Further, EDX elemental mappings of Au and Ag of a nanoshell structure showed that pure Au or Au-rich shell was not observed at the outer

surface of the nanoshells (Fig 4.5) Instead, it was observed that Au mixed with Ag in the shell to form Au-Ag shell, similar to the previous study.82 No EDX of Cl was detected, suggesting the surface of the single particle was probably free from AgCl contamination

Fig 4.5 (a) TEM image of an equiaxed Au-Ag nanoshell and its elemental

mappings of (b) Au, (c) Ag, and (d) Au and Ag (e) TEM image of a triangular prismatic Au-Ag nanoshell and its elemental mappings of (f) Au, (g) Ag, and (h) Au and Ag The nanoshells were obtained from the reaction between the

Ag templates and excess HAuCl4 The scale bars in the images (a – h) are 20

of the HAuCl4 This indicated the oxidation of Ag templates was initiated at the localized sites at the surface, resulting in pore formation at the surface

Previous studies reported that the deposition of Au would initially occur on the higher energy facets of cuboctahedral Ag templates such as {100} and {110} facets, inhibiting oxidation of Ag initiated from these facets.104 Oxidation would preferentially start from the {111} facets For non-truncated decahedral Ag nanoparticles with entire surfaces enclosed by {111} facets, the highest energy sites would be located at twin-boundaries, where defects and lattice distortion commonly accumulated.105,106 Thus, Au would

be initially deposited on the twin-boundary sites of the decahedral Ag templates, whereas the oxidation of the Ag templates would locally start from the {111} facets In this work, the pores were observed at the {111} facets of the decahedral Ag templates at the initial stage of the reaction (Appendix Fig D.1) This result indicated the oxidation initiated at {111} facets For triangular Ag nanoprisms, the two triangular surfaces were commonly composed of almost {111} crystal facets, whereas their edges were typically {100}, {110}, or {111} facets.107 Our results showed the pores were found at the triangular surface, indicating oxidation of triangular Ag nanoprisms started

at the triangular {111} facets (Fig 4.6a)

Combining the TEM, EDX line scanning, and SEM results, the localized oxidation likely continued toward the interior of the particles through the pores, generating the large voids inside each particle, followed by formation of interior cavity surrounded by a shell structure In the galvanic replacement reaction between Ag templates and HAuCl4, AgCl precipitates would be formed as the interior of the Ag templates was oxidized Oleylamine would form a complex with AgCl, which is soluble in the organic medium.108 The presence of abundant oleylamine in our work would facilitate removal of

AgCl precipitates from the interior cavity to the surrounding medium through the pores

With increasing the amount of HAuCl4 (20 mol), more Au would be deposited on the particles, leading to pore shrinkage and subsequent enclosed shells as shown by the SEM images (Fig 4.6b) The outer surface of the equiaxed Au-Ag nanoshells was rough after formation of the enclosed shells

On further increasing HAuCl4 (25 – 75 mol), the outer surface (Fig 4.6c–f) became noticeable rougher than that of 20 mol of HAuCl4 The surface roughness was not caused by AgCl precipitates on the particle surface as discussed earlier It was reported the presence of surface roughness significantly increased near-field enhancements on the Au nanoshell surface and resulted in the redshift of the LSPR peak as compared with the corresponding smooth nanoshells.109,110 The SEM images showed the decahedral-shaped nanoshells were found (Fig 4.6b) The decahedral-shaped nanoshells gradually became equiaxed as the surface roughened (Fig 4.6c–f) The transformation of the Au-Ag nanoshells from Ag template in the organic medium is shown in Fig 4.7

Fig 4.6 SEM images of the particles derived from the reaction between the

Ag templates and (a) 15 mol, (b) 20 mol, (c) 25 mol, (d) 30 mol, (e) 50

mol, and (f) 75 mol of HAuCl4 The pores were found at the surface of the particle obtained at 15 mol of HAuCl4 as indicated by arrows in image (a), whereas the no pores were found after the addition of 20 – 75 mol of HAuCl4 The scale bars for images (a – f) are 100 nm

The BET surface area analysis of the particles with incomplete shells was performed and compared with the particles with enclosed shells (Appendix Fig D.2) The measured BET surface areas were 28.0 and 14.0

m2/g for the particles with incomplete shells and the particles with enclosed shells, respectively This confirmed the formation of the interior cavity in the Au–Ag particles The average pore diameter of the Au–Ag particles with incomplete shells by the BET measurement was ~17 nm

Fig 4.7 Illustration of the transformation from Ag templates to Au-Ag

nanoshells (a) The transformation from the decahedral Ag templates to the spherical-like or equiaxed Au-Ag nanoshells (b) The transformation from the triangular Ag nanoprisms to the triangular prismatic Au-Ag nanoshells (1) Au was deposited on the higher energy surface of Ag templates (e.g {100}, {110} facets or the twin-boundary sites) as very thin incomplete shells at the initial stage of galvanic replacement reaction Thus, the localized oxidation started

at {111} facets of Ag template surfaces those were not covered by Au, resulting in the formation of pores at the (111) surfaces of the Ag templates and small voids underneath the surface The localized oxidation could start on one to several (111) surfaces of the decahedral Ag templates at this stage The deposited Au formed an alloy with the underlying Ag surface to form Au-Ag alloy shells (2 – 3) The localized oxidation continued toward the interior of the particles through the pores, generating larger voids inside the particles, and (4) followed by the formation of interior cavity surrounded by a shell structure (5) The growth of the shells through the Au deposition led to the pore shrinkage and subsequent enclosed shells

4.4 Shell thickness

Both TEM and SEM results showed the Au-Ag nanoshells (equiaxed and triangular prism shapes) did not collapse or transform into Au-rich solid particles with addition of excess HAuCl4 after the formation of the enclosed shells These results did not follow others’ observations of extensive Ag de-alloying and pinhole formation with subsequent collapse of the shells.108 Instead, the shells grew thicker with addition of excess HAuCl4 The average

increased with increasing amount of HAuCl4 (Fig 4.8) The shell thickness increased from ~5 nm to ~10 nm and ~5 nm to ~8 nm for the equiaxed and triangular prismatic Au-Ag nanoshells, respectively

Fig 4.8 The average shell thickness of the Au-Ag nanoshells collected after

the reaction between the Ag templates and different amounts of HAuCl4 in ODE-toluene medium in the presence of oleylamine at 60 oC

4.5 Composition

In galvanic replacement reaction, AgCl precipitates deposited on the surface of the Ag template would inhibit further reaction between Ag and HAuCl4, preventing the formation of Au-Ag nanoshells Such a problem was not observed in this thesis because of the presence of abundant oleylamine, which facilitated AgCl to form a soluble complex as discussed earlier This soluble complex was removed from the samples by centrifugation and washing using toluene in the presence of oleylamine

Figure 4.9 shows two sets of experimental data of Au composition in the samples collected at the different stages of reaction The Au composition was determined by EDX (Appendix Fig D.3a) and XPS analysis (Appendix Fig D.3b, c), compared with those calculated using three theoretical conditions (Appendix Table D.1): (i) complete galvanic replacement between

Ag nanoparticles and HAuCl4, (ii) no galvanic replacement, and (iii) combination between complete galvanic replacement and no galvanic replacement The theoretical condition (i) (complete galvanic replacement) assumed the Ag templates are stoichiometrically oxidized by HAuCl4 when it

is reduced to Au In galvanic replacement reaction, three moles of Ag are consumed to form one mol of Au The theoretical condition (ii) (no galvanic replacement) assumed the Ag templates were not oxidized This condition assumed that all Ag templates underwent alloying with Au to form Au-Ag alloys The theoretical condition (iii) (the combination) consisted of the complete galvanic replacement in a region of 0 15 mol of the HAuCl4

(region I), followed by no galvanic replacement in a region of 20 75 mol of the HAuCl4 (region II) as shown in Fig 4.9 The absence of Cl in the experimental results indicated the removal of AgCl from the samples of Au-

Ag particles by centrifugation and washing

The composition of the Au measured by the EDX was similar to the results determined by XPS Both the experimental results by the EDX and XPS analysis showing a sharp increase of the Au % in the sample of Au-Ag particles matched with the results calculated using the theoretical condition (i) (the complete galvanic replacement) in the region I (Fig 4.9) The sharp increase of the Au % was attributed to the oxidation of Ag templates by