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

Chapter 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 55    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. 4.2 Ag nanoparticles Silver nanoparticles were used as a sacrificial template in the galvanic replacement reaction with HAuCl4 to form Au-Ag nanoshells. A TEM image of as-synthesized Ag nanoparticles is shown in Fig. 4.1a. The Ag nanoparticles consisted of ~85% in decahedral and ~15% in triangular prism shapes, from counting the particles in 10 TEM images. The HRTEM image showed pentagonal crossed lines on the particle, further confirming the decahedral shape with pentagonal cyclic twinning (Fig. 4.1b).100,101 The decahedral Ag particles consisted of five rounded edges rather than five straight edges. Therefore, the decahedral Ag particles lying down on the TEM substrate appeared like equiaxed shape in the low magnification TEM image (Fig. 4.1a) since the pentagonal crossed lines on the particle were not clearly observed. For non-truncated decahedral metal nanoparticles, the entire surface was commonly covered by {111} facets,101,102 which is the densest and lowest 56    surface energy fcc facet. The average size (estimated by random measurements of ~100 particles from the TEM images) was found to be 43 ± nm for the decahedral shape and 53 ± 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 assynthesized 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. 57    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 58    (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. 59    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 prismshaped 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 prismshaped particles were removed at the initial stage of the reaction. Their compositional line profile showed the Ag intensity reached a maximum value at the dark contrast regions (solids) and a minimum value 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 60    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 61    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 62    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 nm.     The surface morphology and surface area were studied using SEM and BET surface area analysis, respectively. Figure 4.6 shows the SEM images of the particles obtained at different stages of the reaction. The arrows in Fig. 4.6a shows the pores at the surface of both equiaxed- and triangular-shaped particles obtained at the initial stage of reaction, after the addition of 15 mol 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. 63    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 nontruncated 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 64    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 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 AuAg 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 69    HAuCl4 and Au formation. When HAuCl4 increased from 20 mol to 75 mol (the region II), the experimental results of the Au % in the metallic particles closely matched with the results calculated using the theoretical condition (ii) (no galvanic replacement). Fig. 4.9 Two experimental data of the percentage composition of Au in the bimetallic particles synthesized using different amounts of HAuCl4 was determined by ( ) EDX and ( ) XPS analysis and compared with those calculated using three theoretical conditions: (x) condition (i) (complete galvanic replacement), (o) condition (ii) (no galvanic replacement), and (+) condition (iii) (combination). The condition (iii) combined the complete galvanic replacement in the region I (0 15 mol of the HAuCl4), followed by no galvanic replacement in the region II (20 75 mol of the HAuCl4). Overall, the experimental results in both the region I and II matched with the theoretical condition (iii) combining the complete galvanic replacements in the region I, followed by no galvanic replacement in the region II. These results confirmed the oxidation of Ag templates via the 70    galvanic replacement took place at the initial stages of the reaction (region I), whereas no extensive oxidation or Ag de-alloying in the Au-Ag shells occurred in the region II. Thus, the increase of the Au % in the region II was not as sharp as that in the region I. It may be expected that the Au % in the sample in the region II would increase as sharply as that in the region I toward a value of 100% if the extensive Ag de-alloying process in the Au-Ag shells had continued in the region II. The experimental results in Fig. 4.9 were consistent with the TEM and SEM results, showing the extensive Ag dealloying did not occur to the Au-Ag nanoshells, despite adding excess HAuCl4. The increase of Au % in the region II may be due to Au deposition from reduction of HAuCl4. A previous study showed oleylamine served as a reducing agent in the synthesis of the Au nanoparticles from HAuCl4. In this thesis, oleylamine could help reduce HAuCl4 in the region II to form Au.111 A previous study reported the alloying of the deposited Au layer with the underlying Ag surface occurred during the galvanic replacement reaction, leading to the formation of Au–Ag alloy shells.83 These homogeneous Au–Ag alloys are thermodynamically more stable than either pure Ag or Au.112,113 A larger driving force would therefore be required to oxidize Ag in the Au–Ag alloy shells than that for pure Ag template.114 4.6 Optical properties In this thesis, the optical properties of the samples collected at different stages of the reaction of Ag templates and HAuCl4 solution were investigated using UV-visible-NIR extinction spectra. At initial stages of the reaction, the extinction peak of the oleylamine-stabilized particles in toluene red shifted 71    from the visible (~670 nm) to NIR (~840 nm) after addition of 10  25 mol of the HAuCl4 (Fig. 4.10a, b). The formation of Au-Ag nanoshells was observed at these stages of reaction as shown by TEM and SEM results. When the HAuCl4 increased from 25 mol to 75 mol, the extinction peak blue shifted from ~840 nm to ~675 nm. At these stages, the shell thickness of Au-Ag nanoshells increased with increasing HAuCl4 (Fig. 4.8). The progressive change of color of the samples collected at different stages of the reaction is shown in Fig. 4.10c. The change in the extinction peaks and color could be associated to the changes of microstructure, surface morphology, composition, and shell thickness of the particles collected at different stages of the reaction, which discussed earlier. The formation of nanoshell structures from their solid templates, the increase of Au %, and surface roughness contributed to the red shift of the LSPR extinction peak,82,109,115,116 whereas the increase of shell thickness would lead to the blue shift.90 The LSPR extinction of metallic nanoshells may be described as follows. It was reported LSPR extinction of metallic nanoshells results from the coupling of the inner shell surface (cavity) and the outer shell surface (sphere) plasmons over a separation distance essentially given by their shell thickness.58 The sphere and cavity plasmons coupled with each other, leading to a splitting into two new plasmons, the lower energy symmetric or “bonding” plasmon and the higher energy antisymmetric or “antibonding” plasmon (Appendix B). The lower energy plasmon strongly interacted with the incident optical field, whereas the higher energy mode showed weak interactions. The strength of the coupling between the sphere and cavity plasmons increased with decreasing the shell thickness (representing the 72    separation distance between the sphere and cavity plasmons), leading to a larger fractional plasmon shift. This explains the optically active plasmon resonance shifted to a longer wavelength with decreasing shell thickness.     Fig. 4.10 (a) UV-visible-NIR extinction spectra, (b) extinction peaks, and (c) photographs of the oleylamine-stabilized particles in toluene were taken after the reaction between the Ag templates and different amounts of HAuCl4 in ODE-toluene medium in the presence of oleylamine at 60 oC.       4.7 Size-dependent transformation To investigate the size effects on the transformation of Ag templates to Au-Ag nanoshells in the organic medium, Ag nanoparticles with smaller size were synthesized. Figure 4.11a, b show TEM images of as-synthesized decahedral Ag nanoparticles collected at different magnifications. The HRTEM image (the inset of Fig. 4.11b) confirmed the decahedral structure of as-synthesized Ag nanoparticles. The average size of the decahedral Ag nanoparticles, estimated by random measurement of 200 particles from the TEM images, was found to be 20 ± nm. The XRD spectra confirmed the fcc 73    structure of Ag nanoparticles (Fig. 4.11c). UV-visible spectrum of the oleylamine-stabilized decahedral Ag nanoparticles in toluene showed an extinction peak at ~407 nm wavelength (Fig. 4.11d). As-synthesized decahedral Ag nanoparticles (~20 nm) were used as sacrificial templates in galvanic replacement reaction with HAuCl4. The nanostructural transformation of such small Ag decahedrons via galvanic replacement reaction with HAuCl4 in the organic medium was compared with that of ~43nm Ag decahedrons which discussed earlier. Fig. 4.11 (a) and (b) TEM images of as-synthesized decahedral Ag nanoparticles (~20 nm in size) collected at magnifications of 30000x and 50000x, respectively. The inset of (b) shows HRTEM image of a decahedral Ag nanoparticle. (c) XRD of as-synthesized decahedral 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 ~407-nm wavelength. 74    Figure 4.12 shows the TEM images of samples obtained from galvanic replacement reaction between the ~20-nm Ag decahedrons and different amounts of HAuCl4 in ODE-toluene medium in the presence of oleylamine at 60 oC. The TEM images collected at lower magnifications are shown in Appendix Fig. D.4. The results showed that voids (light contrast) in each particle were found, indicating part of the Ag solids (dark contrasts) in each ~20-nm decahedrons had been oxidized and removed after addition of 10 mol of HAuCl4 (Fig. 4.12a). At this stage of reaction, the surface openings of the particles were found (arrows in Fig. 4.12a), indicating the formation of the pores at the particle surface, which was similar to that of the ~43-nm Ag decahedrons. The Ag solids at the center of each particle were gradually oxidized and removed with increasing amount of HAuCl4 (Fig. 4.12b), followed by the disappearance of the surface openings, subsequently forming an equiaxed shell structure with interior cavity (Fig. 4.12c, d). The average interior cavity size and shell thickness of these equiaxed nanoshells (estimated by random measurement of 200 particles from the TEM images) were 17 ± nm and ± nm, respectively. Atomic diffusion in smaller particles is faster than the larger ones.117 The TEM results showed the removal of Ag solids at the center of the templates was faster for the small Ag decahedrons (~20 nm) compared with the larger Ag decahedrons (~43 nm). Addition of excess HAuCl4, the nanoshells gradually shrank (Fig. 4.12e–h) and subsequently transformed into solid nanoparticles (Fig. 4.12i). The relative Au and Ag concentrations in the solid nanoparticles measured by 75    EDX analysis were ~90% and ~10 %, respectively. These results could indicate extensive Ag de-alloying in small Au-Ag nanoshells (~17-nm interior cavity/~3-nm shell), transforming Au-Ag nanoshells into Au-rich solid nanoparticles. Once de-alloying of Ag occurred, vacancies would be generated from the removal of Ag atoms in the Au-Ag shells, facilitating further diffusion and de-alloying.118 Incomplete shells were observed during the shrinkage of the nanoshells as indicated by arrows in Fig. 4.12e– h. It has been reported that pinholes in the wall of Au-Ag nanoboxes can be attributed to the coalescence of the lattice vacancies due to the de-alloying of Ag, leading to the formation of porous nanoboxes.83 76    Fig. 4.12 TEM images of the metallic particles obtained from the reaction of ~20-nm decahedral Ag templates with (a) 10 mol, (b) 15 mol, (c) 20 mol, (d) 25 mol, (e) 30 mol, (f) 40 mol, (g) 50 mol, (h) 60 mol, and (i) 75 mol of HAuCl4. The scale bars for the images (a – i) are 20 nm. Figure 4.13 shows the size of metallic nanostructures measured during the transformations of the ~20-nm and ~43-nm decahedral Ag templates via galvanic replacement reaction with different amounts of HAuCl4 in the organic medium. At the initial stages (addition of – 25 mol of HAuCl4), the size of both the metallic nanostructures increased as more Au were formed and deposited on the particles with increasing HAuCl4. At these stages, the ~20-nm and ~43-nm Ag templates transformed into ~17-nm interior 77    cavity/~3-nm shell and ~39-nm interior cavity/~6-nm shell Au-Ag particles, respectively. When the reaction continued with further increasing amount of HAuCl4 (25 – 75 mol), the size (outer diameter) of the small equiaxed AuAg nanoshells (~17-nm interior cavity/~3-nm shell) obtained from the ~20-nm decahedral Ag templates decreased. A sharp decrease in the size was observed after addition of 60 – 75 mol of HAuCl4 as the small nanoshells were transforming into Au-rich solid nanoparticles as indicated by the TEM images (Fig. 12h, i). However, the size (outer diameter) of larger equiaxed Au-Ag nanoshells obtained from the transformation of the ~43-nm decahedral Ag templates increased with further increasing amount of HAuCl4. These AuAg nanoshells (~39-nm interior cavity/~6-nm shell) did not collapse or transform into Au-rich solid nanoparticles when the reaction was continued with addition of excess amount of HAuCl4. Instead, the size of the nanoshells increased as the shells grew thicker outward due to the deposition of Au with increasing HAuCl4 as discussed earlier. These results indicated size- dependent transformation from decahedral Ag templates to equiaxed Au-Ag nanoshells in the organic medium. The size-dependent transformation for the triangular Ag nanoprisms warrants further work. The interior cavity size of Au-Ag nanoshells obtained from the transformations of the ~20-nm and ~43-nm decahedral Ag templates were ~3 nm and ~4 nm smaller, respectively, than the size of their Ag templates. This could be attributed to alloying between the deposited Au layer and the underlying Ag in the templates, consistent with a previous report.82 78    Fig. 4.13 Size of the metallic nanoparticles measured during the transformations of ~20-nm and ~43-nm decahedral Ag templates via galvanic replacement reaction with different amounts of HAuCl4. The size at zero mol of HAuCl4 was the Ag template size. The theoretical and experimental studies showed that the standard redox potential of the metal nanoparticles decreases with decreasing particle size.119 Further, the redox potential of small metal nanoparticles negatively shifted in proportional to (1/particle radius) compared with the bulk metal estimated from the difference in surface free energy between the metal nanoparticles and their bulk counterparts.120 In this work, the potential of the metal nanoparticles (Enano) compared with that of the bulk counterparts (Ebulk) were expressed as Enano  Ebulk  G nF ( 4.1) where ΔG is the surface free energy, n is the number of electrons involved in the reaction (number of mole of electrons per mole of product), and F is 79    Faraday’s constant. The change of surface free energy with surface area (A) is given by dG  dA ( .2 ) where γ is the surface tension. From Eq. 4.2 and assuming constant γ, the surface free energy for spherical solid metal nanoparticles can be derived to G  2Vm r ( 4.3) where Vm is the molar volume of the metal and r is the particle radius. Combining Eqs. 4.1 and 4.3, the relation between the redox potential of the metal nanoparticles (Enano) and the particle radius (r) is given by 120 Enano  Ebulk  2Vm ( 4.4) nF r Equation 4.4 shows the redox potential of metal particles decreases with decreasing the particle size. The potential was calculated to be 0.206 V for the 43-nm Ag templates and 0.188 V for the 20-nm Ag templates, assuming a spherical shape. In a galvanic replacement reaction, the decrease in the standard redox potential of metal templates would increase the potential difference in the oxidation of the metal templates (lower potential) by the metal salt precursor of the higher potential metal being reduced. These results indicated the oxidation of small solid metal templates would be more spontaneous than that of the bigger ones. Assuming a spherical hollow structure, the surface-to-volume ratio of the small nanoshells with ~17-nm interior cavity/~3-nm shell is ~2 times higher than that of the larger nanoshells with ~39-nm interior cavity/~6-nm shell. Equations 4.1 and 4.2 indicate the potential of metallic particles would decrease as the surface area (for same amount of the metal) or surface-to80    volume ratio increases. This suggests the oxidation (de-alloying) of the small nanoshells (~17-nm interior cavity/~3-nm shell) obtained from the transformation of the ~20-nm Ag decahedrons would be more spontaneous than that of the larger nanoshells (~39-nm interior cavity/~6-nm shell) obtained from the ~43-nm Ag decahedrons since the former nanoshells have a larger surface-to-volume ratio than the latter nanoshells. Further, the surface energy of Ag is less than that of Au, which Ag more likely diffuses to the surface of Au-Ag particles rather than Au as more Ag atoms at the surface can decrease the surface energy of the system. The de-alloying process of Ag in Au-Ag nanoshells would start by dissolution of Ag atoms at the surface of the nanoshells as the surface atoms have fewer nearest neighbors, thus they are more weakly bound and less thermally constrained in motion compared with the interior atoms. As the small Au-Ag nanoshells with ~17-nm interior cavity/~3-nm shell have larger number of surface atoms, they would be more active to the de-alloying process of Ag than the larger ones (~39-nm interior cavity/~6-nm shell) with fewer surface atoms. Our results showed the small Au-Ag nanoshells obtained from the transformation of ~20-nm Ag decahedrons shrank and subsequently transformed into Au-rich solid particles due to extensive Ag de-alloying in the shell. However, the larger Au-Ag nanoshells (~39-nm interior cavity/~6-nm Au-Ag shell) obtained from the ~43-nm Ag decahedrons did not collapse or transform into solid particles, indicating these nanoshells were more stable than the small ones (~17-interior cavity/~3-nm Au-Ag shell). Their shells grew thicker as more Au would be formed and deposited on the nanoshells as discussed earlier. The stability of these nanoshells would increase with the 81    shell thickness. A recent study of molecular dynamics simulations reported the thermal stability of hollow Au nanoparticles was dependent on the shell thickness and the ratio of outer radius-to-shell thickness.121 It showed the stability of hollow Au nanoparticles progressively increased with increasing the shell thickness for the same ratio of outer radius-to-shell thickness. The progressive changes of UV-visible-NIR spectra and associated color of the metallic nanoparticles during the transformation from 20-nm decahedral Ag templates to Au-rich solid particles are shown in Fig. 4.14a, b, respectively. The LSPR extinction peak of the oleylamine-stabilized particles in toluene red shifted from ~585 nm to ~725 nm and the color changed from purple to greenish blue after addition of 10 – 25 mol HAuCl4. At these stages, the decahedral Ag templates transformed into Au-Ag nanoshells as indicated by TEM images (Fig. 4.12a–d). The extinction peak slightly blue shifted from ~725 nm to ~698 nm when HAuCl4 increased from 25 mol to 40 mol. No noticeable change in the solution color was observed at these stages. The extinction peak further blue shifted and the spectra broadened with increasing HAuCl4 to 50 mol and the solution color started change to violet. With further increasing HAuCl4 (60 mol), a new extinction peak appeared at ~545 nm and the color changed to violet. At this stage, part of the Au-Ag nanoshells had transformed into Au-rich solid nanoparticles as indicated by the TEM image in Fig. 4.12h. When HAuCl4 increased to 75 mol, the extinction peak blue shifted to ~534 nm, the spectra became narrower, and the color changed to red as most of the nanoshells had transformed into Au-rich solid nanoparticles. 82    Fig. 4.14 (a) UV-visible-NIR extinction spectra and (b) photographs of the oleylamine-stabilized particles in toluene were taken after the reaction between ~20-nm Ag templates and different amounts of HAuCl4 in ODEtoluene medium in the presence of oleylamine at 60 oC. 4.8 Summary Gold-silver nanoshells were successfully synthesized via a galvanic replacement reaction of Ag templates with HAuCl4 in an organic medium in the presence of oleylamine. The transformation from Ag templates to Au-Ag nanoshells was studied. The decahedral (~43 nm in size) and triangular prism (~53 nm in edge length) Ag templates transformed into the equiaxed and triangular prismatic Au-Ag nanoshells, respectively. Two distinct steps were observed in the transformation from the Ag templates to the Au-Ag nanoshells. The first step involved structural and morphological changes from 83    the Ag templates to the Au-Ag nanoshells via galvanic replacement reaction between the Ag templates and HAuCl4. In the second step, the growth of the shells continued through the deposition of Au as indicated by the increase of the shell thickness and the relative Au concentration. The shell thickness increased from ~5 nm to ~10 nm for the equiaxed Au-Ag nanoshells and ~5 nm to ~8 nm for the triangular prismatic Au-Ag nanoshells. The compositional line profile analysis confirmed both the Au-Ag nanoshells did not consist of a Ag core. The LSPR peak was tunable from ~670 nm to ~840 nm. For the Au-Ag nanoshells derived from the 20-nm Ag decahedrons, further reaction in excess HAuCl4 collapsed the nanoshells into Au-rich solid fragments due to extensive Ag de-alloying in the shell. However, the nanoshells derived from the 43-nm Ag decahedrons, the nanoshell structure not only persisted in excess HAuCl4, but its shell thickness also increased. 84    [...]... nm) to NIR (~ 840 nm) after addition of 10  25 mol of the HAuCl4 (Fig 4. 10a, b) The formation of Au- Ag nanoshells was observed at these stages of reaction as shown by TEM and SEM results When the HAuCl4 increased from 25 mol to 75 mol, the extinction peak blue shifted from ~ 840 nm to ~675 nm At these stages, the shell thickness of Au- Ag nanoshells increased with increasing HAuCl4 (Fig 4. 8) The progressive... the equiaxed Au- Ag nanoshells and ~5 nm to ~8 nm for the triangular prismatic Au- Ag nanoshells The compositional line profile analysis confirmed both the Au- Ag nanoshells did not consist of a Ag core The LSPR peak was tunable from ~670 nm to ~ 840 nm For the Au- Ag nanoshells derived from the 20-nm Ag decahedrons, further reaction in excess HAuCl4 collapsed the nanoshells into Au- rich solid fragments due... 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... (f) 40 mol, (g) 50 mol, (h) 60 mol, and (i) 75 mol of HAuCl4 The scale bars for the images (a – i) are 20 nm Figure 4. 13 shows the size of metallic nanostructures measured during the transformations of the ~20-nm and ~43 -nm decahedral Ag templates via galvanic replacement reaction with different amounts of HAuCl4 in the organic medium At the initial stages (addition of 0 – 25 mol of HAuCl4), the... obtained from the ~43 -nm Ag decahedrons since the former nanoshells have a larger surface-to-volume ratio than the latter nanoshells Further, the surface energy of Ag is less than that of Au, which Ag more likely diffuses to the surface of Au- Ag particles rather than Au as more Ag atoms at the surface can decrease the surface energy of the system The de-alloying process of Ag in Au- Ag nanoshells would... transformation from the Ag templates to the Au- Ag nanoshells The first step involved structural and morphological changes from 83    the Ag templates to the Au- Ag nanoshells via galvanic replacement reaction between the Ag templates and HAuCl4 In the second step, the growth of the shells continued through the deposition of Au as indicated by the increase of the shell thickness and the relative Au concentration... transformation of the ~43 -nm decahedral Ag templates increased with further increasing amount of HAuCl4 These AuAg nanoshells (~39-nm interior cavity/~6-nm shell) did not collapse or transform into Au- rich solid nanoparticles when the reaction was continued with addition of excess amount of HAuCl4 Instead, the size of the nanoshells increased as the shells grew thicker outward due to the deposition of Au with... size of their Ag templates This could be attributed to alloying between the deposited Au layer and the underlying Ag in the templates, consistent with a previous report.82 78    Fig 4. 13 Size of the metallic nanoparticles measured during the transformations of ~20-nm and ~43 -nm decahedral Ag templates via galvanic replacement reaction with different amounts of HAuCl4 The size at zero mol of HAuCl4 was... indicating part of the Ag solids (dark contrasts) in each ~20-nm decahedrons had been oxidized and removed after addition of 10 mol of HAuCl4 (Fig 4. 12a) At this stage of reaction, the surface openings of the particles were found (arrows in Fig 4. 12a), indicating the formation of the pores at the particle surface, which was similar to that of the ~43 -nm Ag decahedrons The Ag solids at the center of each particle... further blue shifted and the spectra broadened with increasing HAuCl4 to 50 mol and the solution color started change to violet With further increasing HAuCl4 (60 mol), a new extinction peak appeared at ~ 545 nm and the color changed to violet At this stage, part of the Au- Ag nanoshells had transformed into Au- rich solid nanoparticles as indicated by the TEM image in Fig 4. 12h When HAuCl4 increased to 75 . of 62  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. 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. . 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