Controlled Synthesis of Au@AgAu YolkâShell Cuboctahedra with Well

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Controlled synthesis of Au@AgAu yolkshell cuboctahedra with well-defined facets Alejandra Londono-Calderon, Daniel Bahena, and Miguel José-Yacamán Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01888 • Publication Date (Web): 07 Jul 2016 Downloaded from http://pubs.acs.org on July 13, 2016

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Controlled synthesis of Au@AgAu yolk-shell cuboctahedra with well-defined facets Alejandra Londono-Calderon1, Daniel Bahena2 and Miguel J. Yacaman1* 1

University of Texas at San Antonio, Department of Physics and Astronomy, One UTSA Circle, San Antonio, Texas, 78249, USA. 2

Advanced Laboratory of Electron Nanoscopy, Cinvestav, Av. Instituto Politecnico Nacional

2508, Col. San Pedro Zacatenco, Delegacion Gustavo A. Madero, Mexico D.F. C.P. 07360, Mexico. KEYWORDS: Yolk-shell, galvanic replacement, bimetallic nanoparticle, electron tomography, electron diffraction.

ABSTRACT

The synthesis of Au@AgAu yolk-shell cuboctahedra nanoparticles formed by galvanic replacement in a seed mediated method is described. Initially single crystal Au seeds are used for the formation of Au@Ag core-shell nanocubes which serve as template material for the deposition of an external Au layer. The well-controlled synthesis yield to the formation of cuboctahedra

nanoparticles

with

smooth

inner

and

outer

Au/Ag

surfaces.

The

deposition/oxidation process is described in order to understand the formation of cuboctahedra

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and octahedra nanoparticles. The Au core maintains the initial morphology of the seed and remains static on the center of the yolk-shell due to residual Ag. Structural analysis of the shell indicates intrinsic stacking faults near the surface. EDS and XPS compositional analysis shows Au-Ag non-ordered alloy conforming the shell. The three-dimensional structure of the nanoparticles presented open-facets on the [111] as it was observed by electron tomography SIRT reconstruction over a stack of HAADF-STEM images. The geometrical model was validated by analysing the direction of streaks in coherent nanobeam diffraction. The catalytic activity was evaluated using a model reaction based on the reduction of 4-nitrophenol by NaBH4 in the presence of Au@AgAu yolk-shell nanoparticles.

INTRODUCTION Hollow nanostructures attracted important research interests in the past few years due to their high surface area and low material density compared with solid solution particles. Au nanocages show for example, an enhancement on the catalytic activity for redox reaction in comparison with nanoboxes which at the same time are catalytically more active than solid nanoparticles1. Bimetallic2,3 and trimetallic4 nanocages systems are increasingly reaching better plasmonic, catalytic and sensing properties. Hollow-like structures are usually obtained by electrochemical effects due to the difference between reduction potential of two metals (Galvanic Replacement5– 8

), vacancy mediated diffusion (Kirkendall effect9,10), or re-crystallization of larger crystals from

smaller particles (Ostwald Rippening11). An increasing interest has risen regarding a new type of complex structure: Nanorattles and Yolk-Shell, which combine the properties of hollow and core-shell systems. These kinds of structures are denoted as @ containing a hollow space between a core material and an outer shell B12–14. Tuning the shape, size, and composition of


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the inner and outer layers in both the cores and hollows shells can award new exciting properties that can be use in nanoreactors15,16, drug delivery17,18, nanocatalysis

19–22

, lithium-ion

batteries23,24, SERS25 among others. Rattle-like and yolk-shell systems are mostly synthesized as composites in which the shell is made of a hard template such as silica, a polymer or carbon for the formation of M@SiO or M@Carbon where M is usually a noble metal M = Au, Pd, Pt, Ru26–31. Some Metal@Metal yolk-shell32–39 have been recently studied. In 2004 Sun et al.40 reported complex Au/Ag nanorattles by a simple galvanic replacement on Au/Ag alloy cores. Since then more studies over Au-like nanorattle structures have also been reported41–45. However, there is still a general lack of control over the galvanic process which affects the final shape of the material as well as the composition, which is usually a heterogeneous mix of porous island-like surfaces rather than layer-by-layer epitaxial growth over a template. In this paper, a report over highly yielded Au@AgAu yolk-shell cuboctahedra nanoparticles with smooth open surfaces is reported using a combined seed mediated and galvanic replacement method. To determine the mechanism of formation of the synthesized nanostructures, we used a combination of techniques including electron transmission microscopy, Cs-corrected scanning transmission electron microscopy, as well as coherent nanobeam diffraction and electron tomography to characterize the crystalline structure of the yolk-shell nanoparticles. The elemental composition was obtained by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analysis. The catalytic activity was tested by monitoring the UV-Vis absorption spectra of the reduction of 4-nitrophenol by NaBH4 in the presence of Au@AgAu nanoparticles at room temperature. RESULTS AND DISCUSSION


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Single crystalline Au seeds of 19 nm in size were obtained by a two-step growth method described by Fan et al46 with some variations. Figure 1 present low and high magnification images of the high-yielded single crystal Au-CTAC coated seed solution. A low percent of single and multi-twinned nanoparticles were also found (see figure S1). In ideal conditions, the seeds will growth into an octahedral shape; however, to minimize the total surface energy, a decrease in the surface/volume ratio leads to a more favorable formation, the truncated octahedron47. The use of CTAC instead of CTAB, keeping all other parameters the same, shows to promote the formation of more truncated nanoparticles due to the faster reduction rate in the presence of ascorbic acid as it was observed by a faster change in the color during the second growth. Similar observations and a more complete study over the influence of the capping agent can be found elsewhere43.

Figure 1. (a) Low magnification of Au-CTAC coated seed, (b) high magnification of a singlecrystal particle and (c) size distribution histogram obtained from 200 nanoparticles. The capping agent (CTAC) preferentially chemisorb on the {100} facets; therefore, during the growth of a second metal on the surface, the final shape is influenced more by anisotropic interactions on the solid solution with the capping agent, than the one from Wulff construction. The difference between surface energies allows a faster growth of Ag on the


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direction of the Au surfaces to form core-shell Au@Ag nanocubes of 27 nm in size. Figure 2a shows low magnification DF-SEM image at 30kV of Au@Ag nanoparticles; a low percent of tetrahedra and nanorods were also observed due to the presence of of twinned Au seeds. Figure 2b shows a HAADF-STEM image at 200kV and 2c SAED pattern of a single nanocube oriented on the [001] zone axis. We also observed through STEM imaging at 200kV, coalescence on nearby particles after more than 10 minutes of exposure as is presented on figure 2d. Surface migration of adatoms produce a superficial reconstruction of high index facets, such as the {310} shown in figure 2f, for the particle on 2e. Compositional analysis of the core-shell can be found on the supplementary information S2 where the Au core and Ag shell can be differentiated. Figure S3 also presents different stages during shape transformation of a nanocube due to interaction with the electron beam.


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Figure 2. (a-b) Low and high magnification images of Au@Ag core shell truncated cubes. (c) SAED pattern of a single nanocube oriented on the [001] zone axis. (d) Coalescency of nearby particles after interaction with electron beam at 200kV. (e) An initially cubic nanoparticle shows surface transformation, close view of the red square is showed on (f).

For the growth of Au@AgAu yolk-shell cuboctahedra structures, the complete miscibility between Au and Ag allows the formation of highly crystalline hetero-metallic shell. The dynamic of the growth is as follow: A small amount of HAuCl4 is added the Au@Ag solution held at 100°C under vigorous stirring. The galvanic replacement mechanism of Ag in the presence of Au ions is already known48–50 and occurs due to the high difference in reduction potentials between Au ( = +1.5" for Au$% → Au(') ) and Ag (E = +0.8V for Ag % → Ag (') ). The simultaneous reduction/oxidation for Au and Ag occurs as follows: -$% + 3/ → - + 3/0% The Ag shell is quickly titrated and oxidized; while HAuCl2 is slowly reduced from [AuCl4]1to [AuCl2]1- to solid - on the surface of the template. The growing/dissolving process is shown in figsure 3. In the first step, the initial Au@Ag template is heated up; the addition of Au ions triggers the galvanic replacement in the second step. Fast oxidation of the Ag shell starts as pin holes on the corners of the nanocubes on the {111} facets. The produced Au$% is reduced to Au(') and adsorbed on the {111} surfaces of the truncated nanocube. The close packed {111} planes allows to the adatoms to diffuse quickly to the {001} surfaces. The formation of a visible shell is observed in the third step, in which the outer shell resemble the shape of the initial cubic template. After covering completely the cubic surface with the formation of the Au shell (on which some Ag has been segregated forming an Au/Ag alloy), more Au from the aqueous media


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is adsorbed. The lower mobility of the atoms on the {001} planes produce a faster growth of the [001] direction, which allows the transformation from truncated cube to cuboctahedron formed by 8 triangular {111} facets and 6 square {001} surfaces in the fourth step. A key feature for the formation of cuboctahedra yolk-shell nanoparticles is to use truncated nanocubes without sharp corners to incite the titration over the {111} facets51. The residual Ag helps to “hold” the Au core particle in the middle of the hollow structure as is pointed by the red arrow on last step of figure 3.

Figure 3. Galvanic mechanism for the formation of Au@AgAu yolk-shell structures. Starting from Au@Ag templates the fast reduction/oxidation allows the formation of a smooth Au/Ag shell. The final cuboctahedra nanoparticles within inner hollow space are shown in the fourth step. Residual Ag helps to “hold” the Au core in place as indicated by the red arrow. Figure 4 (a-b) shows low magnification TEM and SEM images of the prepared yolk-shell structures. Red arrows on (b) indicate the remaining pin holes from where the titration of Ag ions took place. Nearly 70% of the structures correspond to cuboctahedra; in a small percent of the


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particles the growth on the [001] direction continues until a full transformation from truncated cube to truncated octahedron is achieved. A smaller percent of yolk-shell nanorods was also observed due to multitwinned Au seeds. The three different configurations as well as the geometrical models are shown on part (c-h) on figure 4 (Mathematica Version 7.0, Wolfram Research, Inc., Champaign, IL 2008).

Figure 4. (a-b) Low magnification DF and SE images of the Au@AgAu yolk-shell nanostructures. The red arrows on (b) indicate pin holes where titration of Ag took place. TEM images of the structures obtained with their respective geometrical model (c-d) cuboctahedron, (e-f) low truncated octahedron and (g-h) nanorod particle.

High magnification HAADF-STEM image of a single cuboctahedron is shown in figure 5(a-b). The Au/Ag alloy shell is randomly ordered since no contrast pattern was identified. In HAADF, the resolution is atomic weight sensitive and the difference between Ag and Au (47 and 79 respectively) will be clearly visible on Z-contrast imaging if the elements were present as


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separate aggregates (in agreement with the EDS results on part 5b, which shows no localized Ag precipitates but homogenous distribution on the entire wall of the nanoparticle). A close view on the surface suggests that the formation of smooth Au/Ag planes comes with the formation of intrinsic stacking faults (SF) on which the sequence is …ABCBABC… near the surface 52,53. The Au concentration used in the third step is highly influential on the outer surface formation; high HAuCl2 concentration produced a faster Ag titration, which promotes the formation of open empty shells and single Au cores as illustrated in figure S4.

Figure 5. HAADF-STEM image of a single yolk-shell cuboctahedron (a) high resolution STEM image of three different areas: (1) near the surface an extrinsic SF can be identified; (2) and (3) close view of the inner cavity where smooth Au-Ag alloy can be observed. (b) EDS elemental spectra, line scan and compositional mapping showing the presence of Au on the core and the outer shell and Ag homogenously over the external shell.


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Figure 6a shows a typical TEM image of cuboctahedron Au yolk-shell oriented slightly off the [011] zone axis. Structural information related to the more external facets of the nanoparticle can be obtained using coherent nanobeam diffraction. This technique can be used to analyse large particles on which lattice fringe cannot be observed on TEM and therefore, a two-dimensional projection of the particle can mislead to 3D interpretation of the shape of the particle54. To validate the geometrical model of the particle on 6b; a small probe was used to creates a parallel illumination slightly bigger than the particle (~100 nm) and diffraction pattern was recorded. In contrast with SAED mode, in coherent NBD only the illuminated nanoarea contributes to the diffraction pattern, therefore the shape of the diffracted spots are more sensitive to structural changes and contains information about the 3D nature of the particle. The diffraction streaks on figure 6c correspond to directions normal to the (001) and (1-11) planes, which represent the external facets of the cuboctahedron particle. The long length of the streaks indicates a sharp facet. The tangent lines of the external planes on the particle in (a) creates an 123° angle. From the image, the angle between the planes is 180° 5 123° = 57°. From the diffraction pattern, the angle between streaks from two different reflections are 55.97, 55.57° and 55.97°, which are more close to the theoretical value 54.74° for {001}//{111} planes in an fcc structure; validating the geometrical model of the 3D shape.


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Figure 6. (a-b) TEM image and geometrical model of yolk-shell cuboctahedron, (c) coherent nanobeam diffraction pattern shows streak lines on the spots related to the facets of the nanoparticle, the angles between the direction of the streaks on the {220} reflections correspond to the theoretical angles between external facets {001}//{111}.

Electron tomography was performed on the synthesized Au@AgAu yolk-shell structures in order to study the 3D character of the 2D projections observed on electron microscopy images. The development of advanced methods that recreate the 3D nature of an object in material science and that can reach better resolution has increased in the past years55,56. The most common and simple technique to reconstruct the original object is the Weighted BackProjection, however, this method is very sensitive to the limited capabilities of the tilt angle. Iterative Reconstruction Methods are used more widely in the reconstruction of high resolution nanostructures. More recently electron tomography has been used to monitor the galvanic replacement of metal nanoparticles, not only to corroborate the inner hollow space, but to determine local elemental composition of nanostructures49; showing how powerful this technique is in the understanding of the growing mechanism of complex structures. In this technique, a high tilt holder is used to record images of a single particle at different tilted angles relative to the incident electron beam. The stack of images was then processed by removing residual displacements between the projections and defining a common tilt axis. For a single Au@AgAu cuboctahedron, Simultaneous Iterative Reconstruction algorithm was used within a tilt angle of ±62° for a stack of HAADF images. Figure 7a shows some of the projections used for the reconstruction as well as a visualization of the reconstructed particle. The tomography reconstruction emphasizes the HAADF-STEM observation of a cuboctahedron formation within


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an inner hollow space between the Au core and the alloy shell. The inner voids are ~ 6-7 nm in size measured by a line profile in different directions on a thin slice of the reconstructed tomography (see figure S6). The size of one void does not change when different algorithms and iterations are used, which proves the reliability of the method and accuracy of the reconstruction (see comparison in figure S7). Figure 7b shows a lateral cut of the reconstructed particle as well as the 2D view of the reconstruction; the Au yolk and the remaining material that keep it on place are clearly visible. Figure 7 (c-d) shows another two different orientation of the particle and the comparison with the geometrical model; the cuboctahedron yolk-shell is not fully close and remains some open faces (pin holes) on the {111} triangular facets, which are closer to the center of reference due to lower surface energy according to Wulff construction principle. A video of the reconstructed-cuboctahedra is included on supplementary information showing the rotation around an axis, as well as, a progressive orthoslice of different sections of the yolk-shell structure.

Figure 7. (a) 3D reconstruction of a single Au@AgAu cuboctahedron yolk-shell from a series of tilted projections. (b) Lateral view showing the inner Au yolk and for comparison 2D projection


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of the reconstructed particle. (c-d) Two different orientations of the reconstructed particle are in agreement with the geometrical model of an open cuboctahedra polygon.

Although the cuboctahedra particles are stable under general circumstances, high dose of electron beam radiation promotes the diffusion and merge between the Au yolk and the external Au/Ag shell. Figure 8(a-d) shows the transformation of the same particle under high electron beam dose; between images, the camera was removed and the electron beam was focused to the size of the particle for 5 seconds, providing 511.7 ;/=> . The progression shows a reduction on the size of the Au yolk until it completely merges with the inner walls of the shell. The same result was observed on other particles when the experiment was repeated as it is shown on figure S5. Figure 8 (e-g) shows the geometrical model and HRTEM images of the final Au/Ag hollow shell in which no evidence of the Au yolk can be found. Selected Area Electron Diffraction (SAED) pattern of the same particle oriented near the [011] zone axis is showed on (h).


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Figure 8. (a-d) Progressive diffusion of the Au yolk with the Au/Ag shell after interaction with the electron beam with high electron dose every 5 seconds, the red arrows points an evident reduction of size of the Au core until it fully dissapear in part (d). (e-f) geometrical model and TEM image of the Au/Ag hollow-shell in (d) showing the facets of the cuboctahedron and the thickness of the shell wall. (g) HRTEM image of the area marked with a white square on (f) shows no residue of the Au core and (h) SAED pattern of the particle oriented near the [011] zone axis.

The elemental composition of the Au yolk-shell nanoparticles was analysed by X-ray Photoelectron spectroscopy (XPS) in order to determine the surface composition. Figure 9 (a) shows the complete spectrum where C 1s, O 1s, Ag 3d, Si 2p and Au 4f can be identified. For sample preparation a concentrated aqueous solution containing yolk-shell nanoparticles was dropcasted on a silicon substrate. O 1s and Si 2p peaks are associated to the SiO2 substrate since it can be assume the nanoparticles are not close-packet; the C 1s peak is related to the presence of CTAC coating the nanostructures. Figure 9(b-c) shows high resolution scan over the Au 4f and Ag 3d regions. From (b) two peaks are observed on 83.90 and 87.59eV related to Au 4f 7/2 and 5/2 respectively. The split orbit is around 3.69 eV and the intensity ratio is 3:4. Ag peaks are shown in (c), the ratio between Ag 3d 3/2 and 5/2 is 2:3 and the position of the peaks is 368.02 and 374.00 eV. The spin orbit split is 5.98eV. Both results correspond to the expected for metallic Au and Ag and are in good agreement with other reports57,58. The percentage atomic concentrations of elements on the surface correspond to 38.4% Ag and 61.6% Au (CasaXPS version 2.3.17PR1.1). No significant shift was observed with respect to the monometallic Au and Ag59–61 which indicates the absence of other oxidized states or AgCl species on the surface. These


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results suggest that most likely only zero valent Au and Ag are formed on the surface of the nanoparticles as a random alloy as it was confirmed by EDS analysis.

Figure 9. (a) XPS spectra of an array of Au@AgAu yolk-shell nanoparticles on a silicon substrate. High resolution scans over (b) the Au 4f and (c) Ag 3d region.

The UV-Vis spectra of the particles obtained in each process was compared. Figure 10(a) shows the results for Au seed, Au@Ag core shell, and Au@AgAu yolk-shell. The SPR band for the single crystal Au seed is observed at 521 nm typical for nearly spherical Au nanoparticles of less of 20 nm in size62. Au@Ag nanocubes shows a broad peak in the 350-450 nm region attributable to the Ag cubic shell on the surface43. After the titration of Ag, Au@AgAu yolk-shell structures shows again the initial SPR band of the Au core at 521 nm as well as a weak shoulder around 620-680 nm, which has also been observed in Au-Ag nanorattles and for which the position shift with the size and internal separation gap between the core and the shell41,63. However, the origin of this weak shoulder for yolk-shell on which the core is not movable is still debatably and requires further analysis.A model system to evaluate the catalytic activity of nanoparticles is the reduction of 4-NTP by NaBH4 through extinction spectra. Figure 10 (b-d) shows the evolution of the UV-Vis extinction spectra over time for 19 nm Au seeds, 27 nm


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Au@Ag nanocubes, and 37 nm Au@AgAu yolk-shell respectively.. 4-NTP solution alone exhibits apeak at 400 nm. After the addition of NaBH4 and a dispersion of metallic nanoparticles the peak gradually reduce the intensity. A new peak starts to appear at 317 nm associated with the reduction of 4-NTP to 4-AMP. The complete reduction process was completed in 20, 6, and 3 minutes for Au seeds, Au@Ag nanocubes and Au@AgAu yolk-shell respectively due to the extinction of the 400 nm peak as well as the change to a colorless solution. From the maximum intensity at 400 nm as a function of time, the pseudo first order reaction can be quantify by plotting the logarithm of the normalized extinction peak at 400 nm as a function of time as is shown on figure 10(e). The slope of the plot represent the apparent kinetic constant rate k in sec1

. It is worth noting that all the solutions came from th same batch which makes reasonable to

assume the same particle concentration and therefore is it right to compare the apparent kinetic constant rate k. Figure 10(f) presents the calculated values of the slope; the Au@AgAu yolkshell nanoparticles are 3.5 times faster than the original Au seeds. Another important parameter to mention is the adsorption time (tads) required for the 4-NTP to adsorb on the metallic surface before the degradation reaction start; this time is reduced from 16 minutes to less than 5 seconds from the Au seeds to the Au@AgAu yolk-shell. In the absence of nanoparticles the 4NTP + NaBH4 solution shows no changes in the extinction spectra within 5 days clearly demonstrating the catalytic capability of the yolk-shell.


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Figure 10. (a) UV-Vis extinction spectra of Au seeds, Au@Ag nanocubes and Au@AgAu yolkshell nanoparticles. Reduction of 4-NTP to 4-AMP in the presence of NaBH4 with (b) Au seeds, (c) Au@Ag nanocubes and (d) Au@AgAu yolk-shell nanoparticles. (e) Normalized extinction at 400 nm as a function of time. (f) Table showing the calculated values of the slope (apparent kinetic constant) and adsorption time (tads).

CONCLUSION In summary, a combined seed mediated and galvanic replacement method has been used for the formation of Au@AgAu yolk-shell cuboctahedra and octahedrons nanoparticles. Single crystal Au 5 CTAC seeds were used to form Au@Ag nanocubes which serve as sacrificial template to form the Au@AgAu yolk-shell. The fast titration of Ag shell on the truncated corners of the nanocube and quick migration of adatoms from {111} to {100} facets explain the growth mechanism and the non-resemble of the cubic template shape on the final yolk-shell nanoparticle. The small lattice mismatch and high miscibility between Au and Ag is responsible


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for the formation of randomly ordered Au/Ag alloy on the outer shell. The geometrical model than support the growth mechanism of the yolk-shell was validated by coherent nanobeam observations and electron tomography reconstruction. Further analysis is required in order to understand the fusion mechanism of the Au yolk with the external shell after electron beam interaction. The resulting octahedrons exhibit open facets on the {111} planes, which make them promising materials for electrochemical oxidation and other catalytic applications. The large surface area obtained by both the presence of an accessible core and the inner and outer walls of the shell may result in enhanced properties compared to core-shell nanostructures. The catalytic activity was tested and compared during the degradation of 4-NTP; Au@AgAu yolk-shell nanostructures showed the higher apparent kinetic rate and minimum adsorbtion time in comparison with solid Au seeds and Au@Ag core-shell nanocubes.

EXPERIMENTAL METHODS Chemicals and Materials Silver

Nitrate

(99.999%),

Gold

(III)

Chloride

Trihydrate

(99.9%),

Hexadecyltrimethylammonium Chloride (CTAC > 98.0%), ascorbic acid (99+%), Sodium Borohydride (99%) and 4-nitrophenol solution were all purchased from Sigma-Aldrich. All chemicals were used as received. All the aqueous solutions were prepared with DI water.

BC@BD core-shell template nanocubes: For the synthesis of Au@Ag, a seed mediated method previously reported by Fan et al. was used46. In the first step Au nuclei are formed by the addition of 0.6 ml of NaBH2 to an 10 >G aqueous solution containing HAuCl2 0.25 >H and CTAC 75 >H at room temperature. The


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solution is kept under slow stirring for 2 hours to assure the total decomposition of NaBH2 . 1 >G of the seed solution is diluted to 100 >G to form a hydrosol. In order to obtain Au truncated octahedra-shaped seeds, a secondary growth was prepared, using a 25 >G solution containing 0.1 >G of HAuCl2 10 >H, 2 >G 2 ml CTAC 0.2 H and 1.5 >G ascorbic acid 0.1 H, 0.3 >G of the Au hydrosol solution was added and let undisturbed for 8 hours. For the growth of the Ag cubic shell, 0.5 >G of AgNO$ 10 >H and 2 >G of ascorbic acid 0.1 H were added to the Au seed solution. The mixture was kept in a water bath at 50°C overnight.

BC yolk-shell Cuboctahedra: For the Au yolk-shell structure, 4 >G of Au@Ag solution was heated up to 100°C for 15 minutes under vigorous stirring. 100 IG of HAuCl2 5 >H was added dropwise (approximately 20 IG/J). The solution then changes from light yellow to dark red, indicating the oxidation of the Ag ions of the surface. The solution was left for another 15 min, after that the stirring was stopped and the flask was removed from the heat. The obtained Au@AgAu nanoparticles were centrifuged and washed several times with DI water and ethanol before deposition of 2-3 drops of the colloidal solution on a carbon film coated Cu grid for electron microscopy characterization.

Catalytic Reduction of 4-nitrophenol (4-NTP): For the catalytic test 4>G of H2O were mixed with 0.5 >G 4-NTP 1>H in a glass vial at room temperature. 1 >G NaBH4 0.1H and the respective metallic solution (Au seeds, Au@Ag nanocubes and Au@AgAu yolk-shell) were added while stirring. The reaction was monitored by recording the optical absorption every 20 seconds until the yellow solution became colourless.


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Instrumentation Transmission Electron Microscopy (TEM) low magnification, high resolution images and electron diffraction patterns were acquired on a JEOL 2010-F microscope operating at 200 kV Selected Area Electron Diffraction (SAED) and Coherent Nanobeam Diffraction (NBD) were obtained on a spot size 1 with a camera length of 30 cm. HAADF-STEM images were recorded with a probe size of 0.09 nm on a JEOL ARM 200F fitted with a CEOS probe corrector for spherical aberrations with a 0.08 nm spatial resolution operated at 200 kV. SEM images were obtained on a HITACHI 5500 with DF and BF detectors working at 30 kV. EDS spectroscopy was used to determine the elemental composition with an EDAX Apollo XLT system. Electron tomography was obtained using a high tilt holder Fishione 2030 within - 62° to + 62° for a single particle. The 3D reconstruction of the HAADF stack of images was made using a Simultaneous Iterative Reconstruction Technique algorithm on the software Digital Micrograph. The X-Ray Photoelectron Spectroscopy was carried out using a PHI 5000 VersaProbe II.

Absorption

spectrums were acquired on a Agilent Cary 100-Series UV-Vis Spectrophotometer.

ACKNOWLEDGMENT This project was supported by the Research Centers in Minority Institutions (RCMI) Nanotechnology and Human Health Core (G12MD007591). The authors would like to thank the Welch Foundation (Grant No. AX-1615); the International Center for Nanotechnology and Advanced Materials (ICNAM) and Laboratorio Nacional de Investigaciones en Nanociencias y Nanotecnologia (LINAN) at Instituto Potosino de Investigacion Cientifica y Tecnologica (IPICYT) for the XPS measurements and the Molecular Biophysics Laboratory at The University of Texas at San Antonio Physics & Astronomy Department for the UV-Vis measurements.


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Corresponding Author *

Corresponding author: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally. Alejandra LondonoCalderon, Daniel Bahena and Miguel Jose Yacaman.

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Table of Contents (TOC)

Au@AgAu yolk-shell cuboctahedron nanoparticles were obtained by galvanic replacement in a seed mediated method using Au@Ag nanocubes as sacrificial templates. The fast, wellcontrolled reduction/oxidation process allows the formation of high-yield nanoparticles with well-defined facets, which present an enhanced catalytic activity on the conversion of 4nitrophenol to 4-aminophenol.


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Figure 1. (a) Low magnification of Au-a single crystal CTAC -coated Au seed, (b) high magnification of a single-crystal particle and (c) size distribution histogram obtained fromf 200 nanoparticles. 244x77mm (150 x 150 DPI)


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Figure 2. (a-b) Low and high magnification images of Au@Ag Au@Ag core shell truncated cubes. (c) SAED pattern of a single nanocube oriented on the [001] zone axis. (d) Coalescency of nearby particles after interaction with electron beam at 200kV. (e) An initially cubic nanoparticle shows surface transformation, close view of the red square is showed on (f). 180x109mm (150 x 150 DPI)


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Figure 3. Galvanic mechanism for the formation of Au@AgAu Au@AgAu yolk-shell structures. Starting from Au@Ag Au@Ag templates the fast reduction/oxidation allows the formation of a smooth Au/Ag Au-Ag shell. The final cuboctahedral nanoparticles within inner hollow space are showned ion the fourth step fourth. Residual Ag Ag helps to “hold” the AuAu core ion place as indicated by the red arrow. 192x95mm (150 x 150 DPI)


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Figure 4. (a-b) Low magnification DF and SE images of the Au@AgAuAu@AgAu yolk- shell nanostructures. The red arrows on (b) indicate pin holes where titration of AgAg took place. TEM images of the structures obtained with their respective geometrical model (c-d) cuboctahedronal, (e-f) low truncated octahedronal and (g-h) nanorod particle. 250x124mm (150 x 150 DPI)


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Figure 5. HAADF-STEM image of a single yolk-shell cuboctahedron (a) high resolution STEM image of three different areas: (1) near the surface an extrinsic SF can be identified; (2) and (3) close view of the inner cavity where smooth Au-Ag alloy can be observed. (b) EDS elemental spectra, line scan and compositional mapping showing the presence of Au on the core and the outer shell and Ag homogenously over the external shell. 259x131mm (150 x 150 DPI)


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Figure 6. (a-b) TEM image and geometrical model of yolk-shell cuboctahedron, (c) coherent nanobeam diffraction pattern shows streak lines on the spots related to the facets of the nanoparticle, the angles between the direction of the streaks on the {220} reflections correspond to the theoretical angles between external facets {001}//{111}. 245x71mm (150 x 150 DPI)


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Figure 7. (a) 3D reconstruction of a single Au@AgAu cuboctahedron yolk-shell from a series of tilted projections. (b) Lateral view showing the inner Au yolk and for comparison 2D projection of the reconstructed particle. (c-d) Two different orientations of the reconstructed particle are in agreement with the geometrical model of an open cuboctahedral polygon. 305x140mm (150 x 150 DPI)


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Figure 8. (a-d) Progressive diffusion of the AuAu yolk with the Au/AgAgAu shell after interaction with the electron beam with high electron dose every 5 seconds, the red arrows points an evident reduction of size of the Au core until it fully dissapear in part (d).. (e-f) geometrical model and TEM image of the Au/AgAgAu hollow-shell in (d) showing the facets of the cuboctahedron and the thickness of the shell wall. (g) HRTEM image of the area marked with a white square on (f) shows no residue of the AuAu core and (h) SAED pattern of the particle oriented near the [011] zone axis. 176x86mm (150 x 150 DPI)


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Figure 9. (a) XPS spectraum of an array of Au@AgAuAu@AgAu yolk-shell nanoparticles on a silicon substrate. High resolution scans over (b) the Au 4f and (c) Ag 3d region. 251x77mm (150 x 150 DPI)


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Figure 10. (a) UV-Vis extinction spectraabsorbance spectrum of the Au single crystal seeds, used for the growth of Au@Ag nanocubes and the formation of AAu@AgAu yolk-shell nanoparticles. (b) Absorption spectra of the rReduction of 4-NTP to 4-AMP in the presence of NaBH4 with (b) Au seeds, (c) Au@Ag nanocubes and (d) Au@AgAu yolk-shell nanoparticles. (e) Normalized extinction at 400 nm as a function of time. (f) Table showing the calculated values of the slope (apparent kinetic constant) and adsorption time (tads). 244x123mm (150 x 150 DPI)


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Controlled Synthesis of Au@AgAu YolkâShell Cuboctahedra with Well

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Controlled Synthesis of Au@AgAu YolkâShell Cuboctahedra with Well