Impact of the AuAg NPs Composition on Their ... - ACS Publications

Subscriber access provided by Uppsala universitetsbibliotek

Article

Impact of the AuAg NPs Composition on Their Structure and Properties: A Theoretical and Experimental Investigation Janaina Fernandes Gomes, Amanda Cristina Garcia, Cleiton Marconi Pires, Eduardo de Barros Ferreira, Rodrigo Queiroz de Albuquerque, Germano Tremiliosi-Filho, and Luiz Henrique S. Gasparotto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp509437a • Publication Date (Web): 14 Nov 2014 Downloaded from http://pubs.acs.org on November 18, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Impact of the AuAg NPs Composition on Their Structure and Properties: a Theoretical and Experimental Investigation Janaina F. Gomes,1,2* Amanda C. Garcia, Cleiton Pires, Eduardo B. Ferreira, Rodrigo Q. Albuquerque,1* Germano Tremiliosi-Filho, Luiz H. S. Gasparotto3 São Carlos Institute of Chemistry, University of São Paulo, P. O. Box 780, 13560-970 São Carlos, SP, Brazil. ABSTRACT. Bimetallic AuAg NPs (NPs) were synthesized via chemical reduction of AuCl3 and AgNO3 and fully characterized by several experimental techniques and theoretical calculations. The plasmon absorptions of these NPs were correlated with the most stable particle structure through different simulations, revealing the most stable structure to be consisted of a gold core and a silver shell. This structural motif was then confirmed by HR-TEM coupled with line-scan EDS and molecular dynamics. Finally, the impact of the nanoparticle composition on their catalytic performance for glycerol electro-oxidation was evaluated. The better catalytic performance in terms of the onset potential found for the Au50Ag50 and Au75Ag25 catalysts suggests the existence of a synergistic effect between Au and Ag. KEYWORDS: Nanoparticles, AuAg, catalysis, glycerol electro-oxidation, molecular dynamics

1

Corresponding authors: [email protected], [email protected]

(Tel: +55 16 3373-8779, Fax: +55 16 3373-9903) 2

Current address: Departamento de Engenharia Química, Universidade Federal de

São Carlos, Rodovia Washington Luiz, km 235, 13565-905, São Carlos, SP, Brazil. 3

Current address: Instituto de Química, Universidade Federal do Rio Grande do

Norte, Lagoa Nova, 59072-970, Natal, RN, Brazil. 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. INTRODUCTION Metallic NPs (NPs) exhibit optical properties that differ significantly from those of the respective bulk material. As examples bulk Ag and Au are metallic-colored, while their respective solutions of spherical NPs are usually yellow and deep red. The solution color is due to the surface plasmon absorption originating from the coherent motion of the outermost electrons of the nanoparticle when they interact with an external electromagnetic field.1 In addition to particular optical properties, NPs also exhibit better catalytic performances than usual catalysts, such as transition-metal complexes or salts.2 Mixing different metals to produce bimetallic NPs gives rise to a second generation of nanocatalysts, for which the catalytic performance is even better than those of the pure metallic NPs separately.3-4 The interesting optical and catalytic properties of metallic NPs and nanoalloys is responsible for the appearance of many new important applications in the fields of optical data storage,5 tumor imaging,6 hydrogen storage7 or other applications.8 AuAg NPs is probably one of the most studied bimetallic systems.9-12 This is partially because the surface plasmon absorptions of Ag and Au NPs are distinctive and generally restricted to around 400 nm and 520 nm, respectively, while that resulting from mixing Ag and Au into a bimetallic nanoparticle can be tuned between 400 nm and 520 nm, depending on the particle composition, size and morphology.13-14 In the bulk phase, Ag and Au form alloys for all compositions with very little surface segregation due to their similar lattice constants, namely 4.09 Å for Ag and 4.08 Å for Au.15 Differently, in the nanometer size regime, either alloyed16-17 or core–shell18-20 structure can be synthesized through various methods.21-25 As a consequence, the optical properties of these bimetallic NPs can be tuned not only by varying their 2


Page 2 of 24

Page 3 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


composition, size and external morphology, but also by changing the internal structure from alloy to core-shell.13 Optical absorption spectroscopy has been the most widely used technique for inferring the internal structure of the AuAg NPs. However, it provides average information that may not reflect the heterogeneity of bimetallic NPs.26 Therefore, it is extremely important to develop methodologies for accurate internal structure studies that allow us to correlate nanostructure with optical properties. In this scenery, NPs imaging may provide detailed inputs needed for precise modeling to understand the underlying physical mechanisms that favor either mixing or segregation in bimetallic NPs formation.27 Among the theoretical methods currently available, molecular dynamics (MD) is particularly interesting to access general properties of transition-metal NPs. Information about relative stabilities of nanoalloys,3 size effect on properties28 or even trends of catalytic activity29 can be gained using MD simulations. In this work, we prepared bimetallic AuAg NPs with different compositions through a very simple chemical route, recently reported by our group,30 and established a methodology based on the use of experimental techniques and molecular dynamics simulations to fully characterize the structures of the synthesized NPs, which are correlated with their optical properties. This represents an advance with respect to earlier works on the characterization of AuAg NPs. The rather small difference observed for the lattice parameters of both metals (408.53 pm for Ag and 407.82 pm for Au) makes it very difficult to predict segregation patterns experimentally for AuAg NPs31 and for this reason the use of MD simulations to discuss the NP properties at different compositions and atomic arrangements is very important. 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

From a previous study30 we know that adding silver to gold in a 50:50 ratio causes a reduction of the gold 5d band occupancy. A practical consequence of this was a decrease in the overpotential for glycerol electro-oxidation. Although in that work we presented evidences that Ag was segregated at the particle surface, the morphology of the particles was still unknown. In the present paper we elucidate the NPs structure and show how the composition of AuAg NPs impacts their catalytic properties. 2. EXPERIMENTAL 2.1 Reactants and instrumentation All chemicals (Aldrich) used in this work were of analytical grade and were used without further purification. A Shimadzu UV-2600 spectrophotometer was used to acquire UV-Vis spectra of the nanoparticle suspensions. For the transmission electron microscopy (TEM) experiments, copper grids coated with carbon film were immersed into the nanoparticle colloidal suspensions and allowed to dry overnight in a desiccator. The grids were then analyzed with a TEM FEI Tecnai (200 kV accelerating voltage). Electrochemical experiments were carried out with either a 1285 Solartron potentiostat/galvanostat controlled by the CorrWare software or an Autolab 30 potentiostat/galvanostat controlled by the GPES software. They were conducted in a conventional three-electrode cell. The counter electrode was a Pt foil. The reference electrode was Hg/HgO and all potentials in this work are referred to it. The working electrodes consisted of carbon-supported Ag, Au25Ag75, Au50Ag50, Au75Ag25 and Au on a glassy carbon disc. 2.2 Synthesis and Characterization of Colloidal AuAg NPs For the production of bimetallic colloidal NPs, the following stock solutions were produced: 40 g L-1 polyvinylpyrrolidone (PVP) (MW = 10,000), 10 mmol L-1 AuCl3 4


Page 4 of 24

Page 5 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(30% wt in HCl), 10 mmol L-1 AgNO3 and 1.0 mol L-1 glycerol + 0.10 mol L-1 NaOH. For instance, to produce colloidal NPs with 50% of each metal (Au50Ag50), 50 µL of the AuCl3 solution, 50 µL of the AgNO3 solution and 2.5 mL of the PVP solutions were mixed in a beaker with water to yield a 5.0-mL solution. In a separate vessel, 1.0 mL of the glycerol-NaOH solution was dissolved in ultrapure water to yield a 5.0-mL solution. The glycerol-NaOH solution was then added to the AuCl3-AgNO3-PVP solution to generate a 10-mL solution with the following final concentrations: 0.050 mmol L-1 Au3+, 0.050 mmol L-1 Ag+, 0.10 mol L-1 glycerol, 0.010 mol L-1 NaOH and 10 g L-1 PVP. The other nanoparticle compositions (Au25Ag75 and Au75Ag25) were obtained by simply varying the volumes of AuCl3 and AgNO3 to achieve the desired molar fraction. During the synthesis of the bimetallic colloidal NPs, the formation of some AgCl cannot be excluded due to its low Ksp (1.8 x 10-10). If formed, however, AgCl does not hinder the formation of the AuAg NPs since glycerol in alkaline medium was found to be capable of generating Ag NPs when added to pure AgCl solution.30 Gold and silver monometallic NPs were also synthesized to compare the results with those of AuAg NPs. For the preparation of monometallic NPs, the procedure was similar however the concentrations of Au3+ and Ag+ were 0.10 mmol L-1 for both, whereas the concentrations of glycerol, NaOH and PVP remained the same. Kinetic experiments at room temperature were carried out during the synthesis, through which the absorbances of the suspension containing the just-mixed reactants were monitored as a function of time. This procedure allows one to evaluate how the NP composition kinetically influences its growth, as well gives some clues about the final atomic arrangement of the synthesized NP. The measurements were performed 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

in triplicate. The synthesized NPs were then analyzed by transmission electron microscopy (TEM) and line-scan X-ray energy-dispersive spectroscopy (EDS). 2.3 Electro-catalytic experiments For the electro-catalytic experiments, all NPs were synthesized using the same procedure applied for the synthesis of colloidal NPs. However, the NPs were produced directly onto Vulcan carbon (NPs/C) without stabilization by PVP, thus avoiding any influence of PVP on the reactions under study. The synthesis of the bimetallic catalyst consisted in sonicating 40 mg of XC-72 Vulcan carbon in 50 mL of ultrapure water and then adding a fixed amount of AuCl3 and AgNO3 under stirring to promote homogenization. Afterwards, another aqueous solution containing glycerol and NaOH was added to give the following concentrations: 1.0 mol L-1 glycerol, 0.010 mol L-1 NaOH and 0.10 mmol L-1 Au3+ and 0.30 mmol L-1 Ag+ for Au25Ag75, 0.20 mmol L-1 Au3+ and 0.20 mmol L-1 Ag+ for Au50Ag50 and 0.30 mmol L-1 Au3+ and 0.10 mmol L-1 Ag+ for Au75Ag25. The black suspensions were kept during 24 h under stirring at room temperature and then washed, filtered and dried at 80 °C for 12 h. To produce Au/C and Ag/C monometallic catalysts the method employed was also similar, although for these materials the concentrations were 0.40 mmol L-1 Au3+ and 0.40 mmol L-1 Ag+. For the preparation of the catalytic layer 2.0 mg of Ag/C, Au25Ag75/C, Au50Ag50/C, Au75Ag25/C or Au/C powder was suspended in a mixture containing 1 mL of isopropyl alcohol and 20 μL of a Nafion solution (5 wt % in low aliphatic alcohol, from DuPont). After ultrasonic homogenization, 20 μL of this ink was deposited onto a glassy carbon disc and the solvent was then evaporated at room 6


Page 6 of 24

Page 7 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


temperature. The catalysts were applied for glycerol electro-oxidation in O2-free 0.10 mol L-1 glycerol and 0.1 mol L-1 NaOH at 0.05 V s-1. These electrochemical measurements were performed in quadruplicate. Given the arbitrary nature of some of the normalization methods commonly adopted by the literature, as for example mass of catalyst or electroactive area of the catalyst, we have chosen not to use any normalization method for the faradaic currents. However all measures have been made with the same electrode geometric area and metal load on the support, assuring their comparison. In the cyclic voltammograms of glycerol electro-oxidation, the onset potential, defined as the potential at which the oxidation reaction starts to occur, was determined by tracing a baseline on the double layer current and taking the potential corresponding to the initial rise of the faradaic current (in the positive-going potential scan) with respect to the baseline. 2.4 Theoretical calculations The NPs were investigated with Molecular Dynamics (MD) simulations using the LAMMPS program.32 The MD was done in vacuum with timesteps of 1 fs using the canonical ensemble NVT and the Nosé-Hoover thermostat. The equations of motion were integrated using the Verlet algorithm. The embedded-atom model (EAM)33 was used to describe the many-body interactions within the NP. According to the EAM, the total energy of a given atom depends on a pair-wise potential involving this atom and each neighbor, as well as on the electron density around it. The following cutoff distances (in Å) shown inside the potential tables containing the EAM parameters34 were used here: 5.6 (Au) and 5.6 (Ag). The EAM was already successfully used to simulate transition-metal NPs.3, 35 Simulations at 298 K were carried out for NPs of 1289 atoms of truncated octahedron geometry with different compositions and atomic 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 24

arrangements. Simulated annealing runs were also done for the same NPs, where the structures were first equilibrated at 1400 K for 1 ns and then slowly cooled down to 10 K with a cooling rate of 7x10-5 K.step-1. The absorption spectrum of the simulated NPs was predicted using the Mie theory simulation software,36 which solves Maxwell's equations to predict the percentage of light scattered and absorbed by particles.37 For the AuxAgy nanoalloys, where the atoms are randomly distributed, the absorption intensity at each wavelength λ was calculated using the mixed refraction index, n, given by: 𝑛 𝜆! =

![!!" !! ]!![!!" !! ] !!!

(1)

where nAu and nAg are the refraction indices of the pure metals, which are wavelength dependent. The mixing rule shown in Eq. (1) works better for larger wavelengths. In these calculations, the NPs were assumed to be spherical and their size and composition were obtained from TEM images and line-scan EDS analysis.

3. RESULTS AND DISCUSSION Fig. 1 shows the UV-Vis spectra and the respective TEM images of AuAg NPs with different Au contents. The spectrum of colloidal Au NPs had an absorbance maximum at around 520 nm, a wavelength corresponding to quasi-spherical gold NPs,38-39 which is corroborated by the respective TEM image. The colloidal Ag NPs presented a maximum absorbance at around 410 nm, a value attributed to the surface plasmon resonance from silver NPs.40 Fig. 1 also presents UV-Vis spectra of bimetallic AuAg NPs with molar ratios of 1:3, 1:1, and 3:1 (namely Au25Ag75, Au50Ag50, and Au75Ag25, respectively). The formation of bimetallic NPs is evidenced by the appearance of a single band whose wavelength of maximum absorbance (λmax) 8


Page 9 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


is located between those of pure Au and Ag NPs. In the photograph of the colloidal solutions displayed in Fig. 1 one can see that the color gradually moves from pale yellow to deep red passing through orange upon increasing the Au content.

Fig. 1. Absorption spectra of Ag (I), Au25Ag75 (II), Au50Ag50 (III), Au75Ag25 (IV) and Au (V) NPs suspended in water at room temperature, together with the respective photograph, transmission electron microscopy (TEM) images and size distribution histograms.

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The existence of a single peak in the absorption spectra II-IV (Fig. 1) is consistent with the formation of bimetallic AucoreAgshell NPs, in which the Au core is completely covered by the Ag shell.41 On the other side, the smooth red shift of those peaks upon increasing the Au content has been reported to be an evidence for the formation of alloyed NPs.42 Therefore, the assignment of the configuration based solely on UV-Vis spectra is not trivial and must be complemented with other experimental and theoretical techniques, as discussed further on. According to the histograms shown above, the average size of the NPs tends to grow with the increasing percentage of silver. For the pure Ag NPs the bimodal features observed in their histogram are probably due to ripening of big particles, thus generating extra small ones. As mentioned in the Experimental section AgCl may be formed during the synthesis of the AuAg particles. This might explain why the mean atomic percentage of Ag in the Au25Ag75, Au50Ag50 and Au75Ag25 NPs are 59.1, 51.1 and 34.7 %, respectively, i.e., less than 25 % different from the nominal values. However, contamination of the AuAg particles with AgCl can be excluded since Xray energy-dispersive spectroscopy (EDS) analysis of those particles shows only Au and Ag, without any presence of Cl. In order to get insight on the relative stability of the NPs, molecular dynamics simulations at 298 K were carried out to predict average total energies of AucoreAgshell, AushellAgcore and alloyed AuAg (≡ AuAgalloy) NPs having 1289 atoms and different compositions (see Fig. 2). In contrast to the narrower experimental compositions found for the NPs, the Au contents used in the MD simulations were 25, 50 and 75 %, which were chosen in order to provide a clear trend of the nanoparticle stability over a broader range of compositions.

10


Page 10 of 24

Page 11 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 2. Average total energies of MD simulations at 298 K of NPs of different atomic arrangements and compositions (bars), whose respective geometries are shown close to each bar, indicated by dashed lines. Standard deviations were too small (~1.70 eV) to be plotted. The thermal treatment of these structures using simulated annealing (SA, 298K → 1400 K → 10 K) is indicated by an arrow with the label "SA", where the final annealed structures are shown in each case. The number over each final structure is the corresponding percentage of the NP surface covered by Ag atoms.

The vertical bars of Fig. 2 reveal that the AushellAgcore configuration is the least stable one, while the AucoreAgshell and AuAgalloy NPs have similar stabilities. The comparison of the core-shell and alloy configurations shows that the former becomes gradually more stable than the latter upon increasing the Au content. This nicely agrees with another recent experimental work on hollow AuAg NPs, which shows that the AucoreAgshell configuration is more stable at higher Au contents.31

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ag has a lower surface energy43 and slightly larger lattice parameter than Au and for this reason Ag would tend to be on the surface. This may help to understand why the AushellAgcore configuration (Fig. 2, black bars) has exhibited the highest energy or smallest stability. The other two possibilities, AucoreAgshell and AuAgalloy, have very similar energies and their relative stabilities strongly depend on composition. For the AuAgalloy configuration, the energetic penalty of having few Au atoms on the surface (at lower Au contents) seems to be compensated by the formation of thermodynamically-preferred Au-Ag bonds. On the other side, at higher Au content, the large number of Au atoms on the surface for the AuAgalloy configuration (see Fig. 2, white bar at 75 % Au) destabilizes more than the small number of Au-Ag bonds formed and therefore the AushellAucore may become more stable. The rather small difference observed in the lattice parameters of both metals (408.53 pm for Ag and 407.82 pm for Au) makes it very difficult to predict segregation patterns experimentally for such NPs31 and for this reason use of MD simulations to discuss the NP properties at different compositions and atomic arrangements is fully justified. The simulated annealing (SA, 298 K → 1400 K → 10 K) of all nine structures bounded to the vertical bars by dashed lines in Fig. 2 gave rise to the annealed structures shown above each initial structure, where the number over each structure is the percentage of the annealed nanoparticle surface covered by Ag atoms. Note that all NPs were melted and slowly cooled down to give rise to thermodynamically more stable structures. One can see that the number of Ag atoms found at the nanoparticle surface of the annealed structures decreases with increasing Au content, indicating that the bimetallic particles with 25% Au content have the Ag-richest shell, while the

12


Page 12 of 24

Page 13 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


shell of the 50% and 75% Au content NPs might be alloyed AuAg, as previously discussed from the results shown in Fig. 1. Fig. 3 shows how λmax (expressed in energy units) obtained from the simulated absorption spectrum (Fig. S1) of each nanoparticle compares with the corresponding experimental value. The theoretical calculations of the absorption spectra took into account the exact experimental compositions obtained from the EDS measurements, since a direct comparison between theory and experiment is done here. For the pure Au and Ag NPs, the predicted maxima of absorption are in excellent agreement with the experimental values. For 41 % Au, it is evident that the configuration AucoreAgshell fits better the experimental result than the configurations AgcoreAushell and AuAgalloy.

Fig. 3. Comparison between the experimental and theoretical transition energies evaluated at the λmax values obtained from the corresponding absorption spectra of Ag, Au25Ag75, Au50Ag50, Au75Ag25 and Au NPs. The theoretical spectra are shown in Fig. S1.

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

At 65 % Au content, the differences among the three configurations are very small, although the predominance of Au in the core still agrees better with the experimental value. For 49 % Au, the configuration AuAgalloy presents the best agreement with the experiment. It is interesting to realize that the configuration AucoreAgshell is the only theoretical one for which the λmax values decrease monotonically upon increasing the Au content, similarly to the experimental trend. The predicted absorption spectrum of bimetallic NPs using the Mie theory is based on the arithmetic mean of refraction indices shown in Eq. (1) and assuming spherical particles. At extreme compositions (e.g. very high or very low Au contents), any error induced by this mixing rule can be neglected, while at intermediate compositions, such errors become more pronounced, indicating that a different mixing rule would be more appropriate. However, the theoretical results shown in Fig. 3 can be still qualitatively related to experimental data, and are useful to discuss the trends found for different atomic arrangements and compositions. The AushellAgcore arrangement (Fig. 3, red circles) clearly represents the least stable arrangement for Au contents in the range 40-50%, which nicely agrees with a completely different theoretical analysis, based on MD simulations, as shown in Fig. 2 (black bars). Figs. 4a and 4b present the kinetics of formation of Au25Ag75 NPs and the TEM images of Au, Au25Ag75 and Ag NPs, respectively. The UV-Vis experiments were carried out by fixing the wavelength at 412 nm, 421 nm and 550 nm for Ag, Au25Ag75 and Au, respectively, and starting measuring the absorbance immediately after mixing the reactants directly in the cuvette. One notices that the three curves evolve similarly at the beginning of the experiment, with those for Ag and Au25Ag75 depicting a

14


Page 14 of 24

Page 15 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


somewhat longer induction time than that of pure gold, as revealed by the inset of Fig. 4a.

Fig. 4. a) Normalized absorbances taken at 412 nm, 421 nm and 550 nm, corresponding to Ag, Au25Ag75 and Au, respectively, as function of time for the synthesis of Au25Ag75 NPs. The inset shows the beginning of the experiment. b) Highresolution transmission electron microscopy (HR-TEM) images of a single Ag, Au25Ag75 and Au NP. c) High-angle annular dark field (HAADF) intensities of the line-scan energy-dispersive X-ray spectroscopy (EDS) measurements of a single Au25Ag75 NP.

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

This means that during the formation of bimetallic NPs gold-rich nuclei are firstly formed with subsequent incorporation of silver, thus spawning NPs with a silverricher shell. Line-scan EDS conducted on a single Au25Ag75 NP (Fig. 4c) confirmed the latter result by revealing the particle to have an edge richer in silver. Note that at the position 0.010 µm the signal of silver is roughly twice as much as that of gold. In the high-resolution TEM image of the Au25Ag75 nanoparticle (Fig 4b) one can actually see a circumference in lighter shade, primarily constituted of silver, which was not observed in the Ag and Au images. This result agrees with the theoretical ones shown in Figs. 2 and 3 in which the AucoreAgshell configuration is slightly more stable than the alloyed one. Also, AucoreAgshell NPs have been reported to exhibit one single peak in the absorption spectrum, just as we report in Fig. 1, when the gold core is completely covered by the silver shell.41 All these theoretical and experimental results suggest that AucoreAgshell NPs are preferentially formed. On the other side, the fact that the peaks in the absorption spectra shown in Fig. 1 continuously red shift upon increasing the Au content of the NPs indicates the formation of alloyed AuAg NPs.42 In view of both evidences, the most plausible description of the synthesized NPs may be of a gold core covered by a gold-silver alloyed shell. This shell becomes more alloyed for higher Au contents, as previously discussed. The NPs produced in this work were then applied for glycerol electro-oxidation. In the cyclic voltammograms (Fig. S2) the onset potential for glycerol electrooxidation was found to be one of the parameters mostly impacted by the amount of silver. Fig. 5 shows the onset potential for glycerol electro-oxidation as a function of the catalyst Au content.

16


Page 16 of 24

Page 17 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 5. Onset of glycerol electro-oxidation over Au and AuAg NPs with different compositions. The cartoon shows the simplified glycerol electro-oxidation steps, where carbon (C) supported Au NPs were taken as an example of catalyst.

NPs of pure silver failed in delivering currents other than the background ones (Fig. S2). In other words, pure silver NPs oxidize glycerol only marginally, hence there is no onset potential to be included in Fig. 5. While pure gold NPs delivered an onset potential of about -0.23 V vs. Hg/HgO, the enrichment in silver shifted the potentials to about -0.33 V for the Au75Ag25 and Au50Ag50 catalysts. The lower the onset potential the better is the catalyst in terms of the ability to electro-oxidize the adsorbed intermediates of the glycerol electro-oxidation. Therefore, in this respect, Au75Ag25 and Au50Ag50 catalysts are more efficient than the Au catalyst. This 100 mV variation in the onset potential is due to the ligand effect.44 According to previous XANES results,30 the presence of silver in the catalyst surface promoted a slight but significant reduction in the gold 5d band occupancy, which in turn made glycerol to adsorb less strongly on gold culminating in a lower onset potential. At high silver contents (over 75 %) there is a positive potential shift as a consequence of drastic gold 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

depletion on the surface (according to MD simulations shown in Fig. 2). The availability of gold on the surface of NPs is pivotal for the electro-oxidation of glycerol. Fig. 5 also reveals the existence of a clear synergistic effect between Au and Ag, since both pure NPs separately are less promising than the combination of both. This synergy is the basis of the volcano shape usually observed for trends of the catalytic performance with composition for bimetallic NPs.3-4, 29 In addition to the impact of the catalyst composition on the onset potential, kinetic of the glycerol electro-oxidation was also influenced by the Ag addition to the AuAg catalyst (Fig. S2). The maximum current of the most pronounced oxidation peak for the Au25Ag75, Au50Ag50 and Au75Ag25 catalysts is 0.11 mA, 0.56 mA and 0.5 mA, respectively, much smaller than that observed for the Au catalyst, which is 4.96 mA. The kinetics of a given reaction on a catalyst reflects the catalyst activity. Therefore, although AgAu catalysts present a gain in terms of the onset potential (energy to start the oxidation reaction) with respect to the Au catalyst, they are less active than pure Au at high potentials. A possible explanation for the difference between the reaction rate on the Au and bimetallic catalysts may be possibly related to the fact that in the bimettalic catalysts silver is segregated on the nanoparticle surface, as revealed by MD simulations shown in Fig. 2. Since silver oxidizes only marginally glycerol (Figure S2), the presence of silver on the surface of the bimetallic materials might decrease the rate of glycerol electro-oxidation with respect to the pure Au catalyst. To summarize, on one hand the onset potential delivered by the Au75Ag25 and Au50Ag50 is 100 mV lower than that provided by the Au catalyst, which means the reaction needs less energy to start to occur. On the other hand the Au catalyst oxidizes glycerol with a higher reaction rate than the AuAg catalysts. The reaction on the Au 18


Page 18 of 24

Page 19 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


catalyst occurs however at a higher overpotential, i.e., the reaction needs more energy to initiate. Therefore, the catalytic activity gain is uncompensated by the energy costs necessary to start the oxidation of the reaction intermediates. The optimization of the bimetallic particle structure and size might lead to decrease even more the overpotential and compensate for its low reaction rate and to improve the catalytic activity, in addition to the gain already obtained in the onset potential for the glycerol electro-oxidation. Furthermore, another advantage of using AuAg catalysts in place of the Au catalyst is the reduction of costs during the particles synthesis, since the precursor salt of Ag ions is considerably less expensive than that of Au ions. Therefore, for example, the costs involved in the synthesis of a catalyst with 50% of Ag and 50% of Au are lower than those related to the synthesis of a catalyst with 100% Au. 4. CONCLUSIONS In the present work, we established a methodology based on the use of experimental techniques and molecular dynamics simulations to fully characterize the structure of AuAg NPs, which are correlated with their optical properties. In particular, nanoparticle structure was elucidated by transmission electron microscopy (TEM) images combined with line-scan X-ray energy-dispersive spectroscopy (EDS) analysis

and

theoretical

predictions

considering

three

different

structures

(AucoreAgshell, AgcoreAushell and AuAgalloy). Surface plasmon absorptions of the investigated NPs were examined and correlated with the most stable particle structure, which was found to be consistent with a gold core and a silver shell. The molecular dynamics simulations and predictions of absorption spectra were revealed to be very important theoretical tools to characterize the NPs investigated in this work. Both

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

theoretical approaches indicate that the AucoreAgshell is the most stable configuration found for NPs of higher Au contents. In addition to correlating the particle structure with optical properties for AuAgbased colloidal materials with different chemical compositions, we also explored here the catalytic properties of the carbon-supported AuAg NPs for glycerol electrooxidation in alkaline medium. Our results indicate that the oxidation of glycerol needs less energy to start to occur, i.e., it has a lower overpotential, when catalyzed by bimetallic NPs containing 25% and 50% of silver, as compared with the same reaction catalyzed by pure Au NPs. This evidences that synergistic effects between Au and Ag are taking place during the catalysis. Although the rate of the reaction catalyzed by pure Au NPs was higher than that of the bimetallic NPs, the lower energy required for the latter shows that it is still possible to optimize the material to decrease even more the overpotential and compensate for its low reaction rate, as well as to increase the reaction rate, in addition to the onset potential gain already obtained for the glycerol electro-oxidation. Supporting Information Theoretical absorption spectra of AuAg NPs and cyclic voltammograms of the electro-oxidation of glycerol over carbon-supported AuAg NPs. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGEMENTS Financial support from the Brazilian agencies FAPESP (grants 2009/08511-9, 2012/02955-5, 2012/10856-7, 2014/02071-5), CAPES (grant A061_2013) and CNPq (grants 305082/2013-2 and 471794/2012-0) are gratefully acknowledged. 20


Page 20 of 24

Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) Mie, G. Beiträge zur Optik trüber Medien, speziell kolloidaler Metallösungen. Ann. Phys 1908, 25, 377-445. (2) Bönnermann, H.; Braun, G.; Brijoux, W.; Brinkmann, R.; Tilling, A. S.; Seevogel, K.; Siepen, K. Nanoscale Colloidal Metals and Alloys Stabilized by Solvents and Surfactants Preparation and Use as Catalyst Precursors. J. Organomet. Chem. 1996, 520, 143-162. (3) Hermannsdörfer, J.; Friedrich, M.; Miyajima, N.; Albuquerque, R. Q.; Kümmel, S.; Kempe, R. Ni/Pd@Mil-101: Synergistic Catalysis with Cavity-Conform Ni/Pd Nanoparticles. Angew. Chem. Int. Ed. 2012, 51, 11473-11477. (4) Zhang, T.; Li, X.; Kang, S.-Z.; Qin, L.; Li, G.; Mu, J. Facile Assembly of Silica Gel/Reduced Graphene Oxide/Ag Nanoparticle Composite with a Core-Shell Structure and Its Excellent Catalytic Properties. J. Mater. Chem. A 2014, 2, 29522959. (5) Ditlbacher, H.; Lamprecht, B.; Leitner, A.; Aussenegg, F. R. Spectrally Coded Optical Data Storage by Metal Nanoparticles. Opt. Lett. 2000, 25, 563-565. (6) Lima, K. M. G.; Junior, R. F. A.; Araujo, A. A.; Oliveira, A. L. C. S. L.; Gasparotto, L. H. S. Environmentally Compatible Bioconjugated Gold Nanoparticles as Efficient Contrast Agents for Colorectal Cancer Cell Imaging. Sensor. Actuat. BChem. 2014, 196, 306-313. (7) Vinayan, B. P.; Nagar, R.; Ramaprabhu, S. Solar Light Assisted Green Synthesis of Palladium Nanoparticle Decorated Nitrogen Doped Graphene for Hydrogen Storage Application. J. Mater. Chem. A 2013, 1, 11192-11199. (8) He, X.; Yue, C.; Zang, Y.; Yin, J.; Sun, S.; Li, J.; Kang, J. Multi-Hot Spot Configuration on Urchin-Like Ag Nanoparticle/Zno Hollow Nanosphere Arrays for Highly Sensitive Sers. J. Mater. Chem. A 2013, 1, 15010-15015. (9) Deng, L.; Hu, W.; Deng, H.; Xiao, S.; Tang, J. Au-Ag Bimetallic Nanoparticles: Surface Segregation and Atomic-Scale Structure. J. Phys. Chem. C 2011, 115, 1135511363. (10) Barron, H.; Fernandez-Seivane, L.; Weissker, H. C.; Lopez-Lozano, X. Trends and Properties of 13-Atom Ag-Au Nanoalloys I: Structure and Electronic Properties. J. Phys. Chem. C 2013, 117, 21450-21459. (11) Contreras-Caceres, R.; Dawson, C.; Formanek, P.; Fischer, D.; Simon, F.; Janke, A.; Uhlmann, P.; Stamm, M. Polymers as Templates for Au and Au@Ag Bimetallic Nanorods: Uv-Vis and Surface Enhanced Raman Spectroscopy. Chem. Mater. 2013, 25, 158-169. (12) Tsao, Y.-C.; Rej, S.; Chiu, C.-Y.; Huang, M. H. Aqueous Phase Synthesis of Au-Ag Core-Shell Nanocrystals with Tunable Shapes and Their Optical and Catalytic Properties. J. Am. Chem. Soc. 2014, 136, 396-404. (13) Liz-Marzán, L. M. Tailoring Surface Plasmons through the Morphology and Assembly of Metal Nanoparticles. Langmuir 2005, 22, 32-41. (14) Mulvaney, P. Surface Plasmon Spectroscopy of Nanosized Metal Particles. Langmuir 1996, 12, 788-800. (15) Li, Z. Y.; Wilcoxon, J. P.; Yin, F.; Chen, Y.; Palmer, R. E.; Johnston, R. L. Structures and Optical Properties of 4-5 Nm Bimetallic Agau Nanoparticles. Faraday Discuss. 2008, 138, 363-373. 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(16) Mallin, M. P.; Murphy, C. J. Solution-Phase Synthesis of Sub-10 Nm Au-Ag Alloy Nanoparticles. Nano Lett. 2002, 2, 1235-1237. (17) Zhang, H.; Haba, M.; Okumura, M.; Akita, T.; Hashimoto, S.; Toshima, N. Novel Formation of Ag/Au Bimetallic Nanoparticles by Physical Mixture of Monometallic Nanoparticles in Dispersions and Their Application to Catalysts for Aerobic Glucose Oxidation. Langmuir 2013, 29, 10330-10339. (18) Xu, S. P.; Zhao, B.; Xu, W. Q.; Fan, Y. G. Preparation of Au-Ag Coreshell Nanoparticles and Application of Bimetallic Sandwich in Surface-Enhanced Raman Scattering (Sers). Colloid Surface A 2005, 257-58, 313-317. (19) Chen, H. M.; Liu, R. S.; Jang, L. Y.; Lee, J. F.; Hu, S. F. Characterization of Core-Shell Type and Alloy Ag/Au Bimetallic Clusters by Using Extended X-Ray Absorption Fine Structure Spectroscopy. Chem. Phys. Lett. 2006, 421, 118-123. (20) Tsuji, M.; Miyamae, N.; Lim, S.; Kimura, K.; Zhang, X.; Hikino, S.; Nishio, M. Crystal Structures and Growth Mechanisms of Au@Ag Core-Shell Nanoparticles Prepared by the Microwave-Polyol Method. Cryst. Growth Des. 2006, 6, 1801-1807. (21) Shankar, S. S.; Rai, A.; Ahmad, A.; Sastry, M. Rapid Synthesis of Au, Ag, and Bimetallic Au Core-Ag Shell Nanoparticles Using Neem (Azadirachta Indica) Leaf Broth. J. Colloid. Interf. Sci. 2004, 275, 496-502. (22) Hodak, J. H.; Henglein, A.; Giersig, M.; Hartland, G. V. Laser-Induced InterDiffusion in AuAg Core-Shell Nanoparticles. J. Phys. Chem. B 2000, 104, 1170811718. (23) Wilson, O. M.; Scott, R. W. J.; Garcia-Martinez, J. C.; Crooks, R. M. Synthesis, Characterization, and Structure-Selective Extraction of 1-3-Nm Diameter AuAg Dendrimer-Encapsulated Bimetallic Nanoparticles. J. Am. Chem. Soc. 2005, 127, 1015-1024. (24) Shibata, T.; Bunker, B. A.; Zhang, Z. Y.; Meisel, D.; Vardeman, C. F.; Gezelter, J. D. Size-Dependent Spontaneous Alloying of Au-Ag Nanoparticles. J. Am. Chem. Soc. 2002, 124, 11989-11996. (25) Zhang, H.; Okuni, J.; Toshima, N. One-Pot Synthesis of Ag-Au Bimetallic Nanoparticles with Au Shell and Their High Catalytic Activity for Aerobic Glucose Oxidation. J. Colloid. Interf. Sci. 2011, 354, 131-138. (26) Lozano, X. L.; Mottet, C.; Weissker, H. C. Effect of Alloying on the Optical Properties of Ag-Au Nanoparticles. J. Phys. Chem. C 2013, 117, 3062-3068. (27) Ghosh, T.; Satpati, B. Direct Experimental Evidence of Nucleation and Kinetics Driven Two-Dimensional Growth of Core-Shell Structures. J. Phys. Chem. C 2013, 117, 10825-10833. (28) Leppert, L.; Albuquerque, R. Q.; Kümmel, S. Gold-Platinum Alloys and Vegard's Law on the Nanoscale. Phys. Rev. B 2012, 86, 241403. (29) Leppert, L.; Albuquerque, R. Q.; Foster, A. S.; Kümmel, S. Interplay of Electronic Structure and Atomic Mobility in Nanoalloys of Au and Pt. J. Phys. Chem. C 2013, 117, 17268-17273. (30) Garcia, A. C.; Lopes, P. P.; Gomes, J. F.; Pires, C. M.; Ferreira, E.; Gasparotto, L. H. d. S.; Tremiliosi-Filho, G.; Lucena, R. Eco-Friendly Synthesis of Bimetallic AuAg Nanoparticles. New J. Chem. 2014, 38, 2865-2873. (31) Slater, T. J. A.; Macedo, A.; Schroeder, S. L. M.; Burke, M. G.; O'Brien, P.; Camargo, P. H. C.; Haigh, S. J. Correlating Catalytic Activity of Ag-Au Nanoparticles with 3d Compositional Variations. Nano Lett. 2014, 14, 1921-1926. (32) Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117, 1-19. 22


Page 22 of 24

Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(33) Daw, M. S.; Baskes, M. I. Embedded-Atom Method: Derivation and Application to Impurities, Surfaces, and Other Defects in Metals. Phys. Rev. B 1984, 29, 6443-6453. (34) Calle-Vallejo, F.; Koper, M. T. M. Theoretical Considerations on the Electroreduction of Co to C2 Species on Cu(100) Electrodes. Angew. Chem. Int. Ed. 2013, 52, 7282-7285. (35) Molayem, M.; Grigoryan, V. G.; Springborg, M. Theoretical Determination of the Most Stable Structures of NimAgn Bimetallic Nanoalloys. J. Phys. Chem. C 2011, 115, 7179-7192. (36) Charamisinau, I. Mie Theory Calculator. http://engr.smu.edu/ee/smuphotonics/Software/Software_main.htm. (37) Van de Hulst, H. C. Light Scattering by Small Particles, Wiley, 1964. (38) Moskovits, M.; Srnová-Sloufová, I.; Vlcková, B. Bimetallic Ag-Au Nanoparticles: Extracting Meaningful Optical Constants from the Surface-Plasmon Extinction Spectrum. J. Chem. Phys. 2002, 116, 10435-10446. (39) Daniel, M.-C.; Astruc, D. Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum-Size-Related Properties, and Applications toward Biology, Catalysis, and Nanotechnology. Chem. Rev. 2003, 104, 293-346. (40) Garcia, A.; Gasparotto, L. S.; Gomes, J.; Tremiliosi-Filho, G. Straightforward Synthesis of Carbon-Supported Ag Nanoparticles and Their Application for the Oxygen Reduction Reaction. Electrocatal. 2012, 3, 147-152. (41) Mallik, K.; Mandal, M.; Pradhan, N.; Pal, T. Seed Mediated Formation of Bimetallic Nanoparticles by UV Irradiation: A Photochemical Approach for the Preparation of “Core−Shell” Type Structures. Nano Letters 2001, 1, 319-322. (42) Link, S.; Wang, Z. L.; El-Sayed, M. A. Alloy Formation of Gold-Silver Nanoparticles and the Dependence of the Plasmon Absorption on Their Composition. J. Phys. Chem. B 1999, 103, 3529-3533. (43) Gong, H. R. Electronic Structures and Related Properties of Ag-Au Bulks and Surfaces. Mater. Chem. Phys. 2010, 123, 326-330. (44) Liu, P.; Logadottir, A.; Nørskov, J. K. Modeling the Electro-Oxidation of Co and H2/Co on Pt, Ru, PtRu and Pt3Sn. Electrochim. Acta 2003, 48, 3731-3742.

23


Au75Ag 25

Page 24 of 24

40 20 0 1

3

5

7

9

11 13

10 5 0 6

10

14

18

22 26

100 nm 100#nm#

20 10 0

9 11 13 15 19 21 23 25

Diameter / nm

Norm. Absorbance / a.u.

Diameter / nm

20 10 0 1

5

9

13

17

21 25

29

Diameter / nm

1.0

100 nm

Au

0.8

50 nm 50#nm#

0.6 0.4 Frequency / %

2

Frequency / %

Diameter / nm

Ag

Frequency / %

100#nm# 100 nm

200 nm 200#nm#

Frequency / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Au50Ag 50


Frequency / %

Au25Ag 75

0.2 0.0

30 20 10 0

4

5

6

7

8

Diameter / nm

ACS350 Paragon Environment 280 420Plus490 560 630 700

Wavelength / nm

9

Impact of the AuAg NPs Composition on Their ... - ACS Publications

Recommend Documents