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Growth Characteristics and Optical Properties of Core/Alloy Nanoparticles Fabricated via the Layer-by-Layer Hydrothermal Route Peter N Njoki, Wenjie Wu, Patrick Lutz, and Mathew M. Maye Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm401286w • Publication Date (Web): 04 Jul 2013 Downloaded from http://pubs.acs.org on July 5, 2013

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Chemistry of Materials

Growth Characteristics and Optical Properties of Core/Alloy Nanoparticles Fabricated via the Layer-byLayer Hydrothermal Route Peter N. Njoki, Wenjie Wu, Patrick Lutz, Mathew M. Maye* Department of Chemistry, Syracuse University, Syracuse New York 13244 U.S.A. *[email protected]

Abstract The layer-by-layer formation of core/alloy nanoparticles is described. Using pre-synthesized gold nanoparticle cores, AuxAg1-x alloy shells were deposited and annealed with sub-nanometer precision using a microwave irradiation (MWI) mediated hydrothermal processing method. The alloy composition, thickness, and nanoparticle morphology governed the surface plasmon resonance characteristics of the particles, as well as growth characteristics. The mechanism for alloy deposition, annealing, and interdiffusion was explored using two gold precursors, [AuBr4]- and [AuCl4]-, and two hydrothermal temperatures (120, 160 oC). Findings indicate that use of [AuCl4]- results in significant galvanic displacement, leading to non-uniform alloy formation and phase segregation at low annealing temperatures, which leads to loss of morphology control at intermediate compositions (x ≈ 0.25 - 0.75). In contrast, use of [AuBr4]- results in alloy shells with low galvanic interactions, leading to optimum alloy distribution and high fidelity control of alloy-shell thickness, that in combination with higher hydrothermal processing temperatures, leads to uniform and monodisperse core/alloy microstructure across all compositions. The alloy deposition and core/alloy nanoparticle growth was followed in-situ by monitoring the change in surface plasmon resonance (SPR) signatures by UV-vis, which were unique to alloy shell thickness, as well as composition, and morphology. The interfacial alloy composition was 1 ACS Paragon Plus Environment


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probed by modeling the SPR with Discrete Dipole Approximation (DDA), the results of which suggest the final alloy shells are Au-rich compared to the feed ratios, owing in large part to both galvanic displacements sas well as core-to-shell alloy interdiffusion.

Keywords: alloys, plasmon resonance, nanoparticle, processing, alloy, core/alloy,

TOC Graphic

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Introduction The post-synthetic processing of nanomaterials may allow researchers to reach specific properties, morphologies, or phase regimes that are not accessible by simple synthesis alone.1-6 This approach takes advantage of the diffusion of dopants, defects, or atoms at nano-interfaces to manipulate composition, structure, or morphology of a nanomaterial in ways analogous to bulk materials processing. The resulting changes to composition, microstructure, or lattice type are driven by redox potential, as well as the rapid atom and defect diffusion at both a nanoparticle (NP) interface and its interior.2-4 This type of processing typically takes place after the initial synthesis and to date has resulted in the tailoring of catalytic, optical, or morphological properties of a nanomaterial. For example, the use of galvanic reactions at the NP interface has proven to be especially interesting.5-6 Using sacrificial palladium or silver nanocubes, researchers have shown that hollow gold shells or cubic gold cages can be fabricated. The thickness of the cages and the morphology can be tuned by tailoring the diffusion of ions, and the process can be followed in-situ by monitoring the rich plasmonic behavior.4-6 In these processes ion diffusion and accessibility of atoms or ions in the nano superlattice is key, as has been shown recently for Cu2S or Ag2Se qdots, which can undergo galvanic displacement with Cd2+ or Pb2+ cations, thus altering the optical and catalytic properties of the quantum dots.3 This approach has also been employed for transforming metal nanoparticles to semiconductive ones. For instance, researchers recently processed Co NPs into both Co2P and CoP ones using chemical transformation and Kirkendall diffusion.7 The final NPs have hollow centers, due to defect formation arising from the oxidation state change of the Co. Other hollow NP systems, such as Ag2S transformed from Ag NPs, have also been created using a similar processing approach.8

This approach is particularly interesting for research related to alloy nano-interfaces.1,3 Research on alloy NPs has grown recently,9 with primary focus being on noble metal alloys for heterogeneous 3 ACS Paragon Plus Environment


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catalysis,10-12 including fuel cell electrocatalysis.10a,11 These alloys are typically homogeneous, with diameters of less than 10 nanometers, and alloy composition and particle size that are closely related to the co-reduction of the precursors.2,14 On the other hand, alloy interfaces of NPs with larger diameters, in the 10-50 mid-nano ranges are under-explored by comparison. Such sizes are unique in that the NPs will posses a rich optical behavior, as well as potentially complex phase behavior. Alloys in this size range are challenging to fabricate from a pure wet-chemical approach due to differences in redox potentials of precursors affecting nucleation and growth, and the diffusion and segregation of atoms after nucleation, which oftentimes results in core/shell morphology.

Because of these synthetic

challenges, a number of theoretical descriptions have recently studied the extent that nanoscale interfacial energies may have on alloy formation.2,13,14 For example, alloy interdiffusion is expected to be thermodynamically and kinetically favorable,2a with the resulting nano-alloys being highly stable due in large part to relieved interfacial energies.2b Computational approaches have also provided insight into formation of bimetallic nanoparticles.15 Monte Carlo simulations studies have shown that surface aggregation and atomic-scale structures of Au/Ag NPs are affected by not only particle size, but also temperature, and alloy composition ratio.15a Atomistic modeling of Au/Ag NPs alloy showed that Ag segregates at the outer shell depending on the reaction temperature because Ag has less surface energy than Au.15b

In an attempt to probe these interesting alloy properties for mid-nano NP sizes, we recently developed a processing protocol to deposit alloy interfaces at a pre-synthesized core, thus fabricating core/alloy NPs.16-17

A key feature of this approach is the use of a layer-by-layer hydrothermal

deposition route that employs microwave irradiation (MWI) as the heat source that activates layer growth, core-shell interdiffusion, and annealing.

The use of a synthetic MWI reactor allows for

automated control of hydrothermal heating and cooling rates, and may induce the non-equilibrium heating of the nano-interface.16 The benefits of using an automated microwave reactor was first shown 4 ACS Paragon Plus Environment

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by Strouse and co-workers for quantum dots,18 and since the approach has been used by a number of researchers for a variety of nanomaterials, including metal nanoparticle catalysts.19,20

In this report, we describe the full details of the alloy deposition and growth mechanism of AuxAg1-x alloy shells at pre-synthesized Au NP cores. The resulting Au/AuxAg1-x core/alloy NPs were fabricated using two gold precursors, and two hydrothermal processing temperatures. Results indicate that galvanic displacements at the alloy interface during growth leads to larger size distributions that are more pronounced at higher temperatures. In contrast, use of alloy precursors with matched reduction potentials resulted in improved tunability of alloy growth, and composition control.



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Experimental details

Chemicals:

Hydrogen

tetrachloroaurate

(III)

hydrate

(HAuCl4.xH20,

99.999%),

sodium

tetrabromoaurate (III) hydrate (NaAuBr4. xH20), silver nitrate (AgNO3, 99%), sodium citrate tribasic dihydrate (Na2C6H5O7 · 2H2O, >99%), bis(p-sulfonatophenyl) phenylphosphine dihydrate dipotassium salt (C18H13K2O6PS2 · 2H2O, 97%) were purchased from Sigma. Ultrapure water (18.2 MΩ) was provided from a Sartorius Stedim Arium 61316 reverse osmosis unit combined with an Arium 611 DI polishing unit. All chemicals were used as received.

Synthesis of Gold Nanoparticle Cores: Gold nanoparticles (Au, 15.4 ± 0.7 nm) were synthesized by a slightly modified citrate (Cit) reduction procedure.21 Briefly, an aged 1 mM HAuCl4 solution was heated to ~95 °C for 30 minutes. To this solution a warm 38 mM trisodium citrate solution (10 ml) was added in one aliquot. Upon initial color change to red, the solution was then immediately cooled to ~80 °C and annealed for 1 hr. The sample was then let to cool naturally to room temperature and allowed to stir overnight. The solution was then stored protected from light. The Au concentrations were calculated via a measured extinction coefficient of 2.2 x 108 L mole-1 cm-1.

Layer-by-Layer Alloy Shell Growth: We begin with the pre-synthesized Au cores described above and deposit shells of AuxAg1-x (x = 0.00, 0.15, 0.50, 0.85, 1.00) in controllable sub nanometer layers (n). Here, tetrachloroaurate (III) ([AuCl4]-) or tetrabromoaurate (III) ([AuBr4]-) and silver nitrate (AgNO3) reduction is achieved using a minimum amount of reducing agent, sodium citrate (Cit), which reduce Ag+ and Au3+ ions at the hydrothermal temperatures employed. Furthermore, Ag+ and Au3+ ions are added and reduced in a step-by-step (e.g. layer-by-layer) fashion at a ratio (r = [(Ag+ + [AuX4]-)]/[Au]; X = Cl- or Br-) required to deposit a 0.25~0.50 nm thick shell (tS), based on model calculations that consider volume change due to shell growth at a constant Au-core diameter and NP concentration. In a 6 ACS Paragon Plus Environment

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typical experiment, a 2.2 mL ultrapure water (18.2 MΩ) solution of Au ([Au] = 4.6 nM), trisodium citrate ([Cit] = 1.36 mM), and Ag+ plus Au3+ ions ([AuX4-] + [AgNO3] = 0.045 mM) are hermetically sealed in 10 mL glass microwave reaction vessels. The alloy feed ratio was manipulated by tuning the [AuX4-] : [Ag+] ratio. Next, the sample is rapidly heated to hydrothermal temperatures (TH) and pressures (PH) using computer controlled microwave irradiation (MWI). A typical reaction time is 5 minutes (including heating and cooling times). After each layer (n) deposition (heating cycle), a 100 µL aliquot was collected for UV-vis and TEM analysis, and a fresh 100 µL aliquot of [AuX4-] : [Ag+] at different ratios are added. Thus, the total [NP] is decreasing over the course of the reaction because of sampling. The process is then repeated an n number of times, resulting in the growth of the Au/AuxAg1-x core/shell nanostructure. It is important to note that the expected growth is highly susceptible to initial NP core size and concentration, as well as the volumes sampled during the course of shell addition. The final Au/AuxAg1-x products were stored in the reaction mother liquor, and protected from light. Under these storage conditions, the NPs were stable indefinitely.

Instrumentation Synthetic Microwave Reactor: A Discovery-S (CEM Inc.) synthetic microwave reactor was employed. The instrument is computer controlled, and operates at power values between 0 - 300W; temperatures ranging from 30-300 oC, and pressures from 0-200 PSI. Temperature is monitored in-situ during synthesis via the use of an integrated IR-sensor, or via an immersed fiber optic temperature probe. The instrument is equipped with an active pressure monitoring system, which provides both pressure monitoring and added safety during synthesis. Taken together, this combination allows the MWI power to be dynamically attenuated by temperature feedback measured via the integrated infrared detector or fiber optic probe, that results in fine control of annealing temperature, the ability to rapidly achieve hydrothermal conditions, as well as control of heating and cooling rates. Pressure rated glass reaction vials with volumes of 10 or 35 mL were employed during synthesis. Active cooling was 7 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

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provided by the influx of the MW cavity with compressed N2, which rapidly cools the sample at a controlled rate. UV-visible Spectrophotometry (UV-vis): The UV-vis measurements were collected on a Varian Cary100 Bio UV-vis spectrophotometer. The instrument is equipped with an 8-cell automated holder with high precision Peltier heating controller. Transmission Electron Microscopy (TEM): TEM measurements were performed on a FEI T12 Twin TEM operated at 120 kV with a LaB6 filament and Gatan Orius dual-scan CCD camera at the Cornell Center for Materials Research (CCMR). Samples were drop cast onto carbon coated copper grids. Particle size was analyzed manually by modeling each particle as a sphere, with statistical analysis performed using ImageJ software on populations of at least 100 counts.

X-ray Photoelectron Spectroscopy (XPS): XPS analysis was performed at the CCMR on Surface Science Instruments (SSI) model SSX-100 that utilizes monochromated aluminum Kα X-rays (1486.6 eV). The nanoparticles were drop-cast onto freshly cleaved HOPG and studied at an angle of 55°, which corresponds to an analysis depth of ≈5 nm. The data were processed using CasaXPS software. Before analysis, each sample was purified free of any excess metallic ions by centrifugation. Discrete Dipole Approximation (DDA) Modeling: The NP and core/shell NP surface plasmon resonance (SPR) extinction spectra was modeled using the discrete dipole approximation (DDA) method developed by Draine and Flatau.22 The open source software package DDSCAT 7.07 22 was employed on a Linux workstation equipped with an Intel i7 processor and 12 GB SDRAM running Ubuntu. Isotropic cores were calculated via the ELLIPSOID DDSCAT routine, whereas core/shell morphologies were calculated via the CONELLIPS routine with defined core diameter, and shell thicknesses. Typical calculation times ranged from minutes for simple structures, to 12-24 hr for large diameters or complex core/shell geometries. Each simulation consisted of ~7000 dipoles being used. In DDA (eq. 1) a numerical SPR solution is defined by dividing a NP into 8 ACS Paragon Plus Environment

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elemental cubic volumes that are characterized by their coordinates within the NP, and their subsequent polarizability.22,23,24 Thus, each unit can be treated as a dipole, the collection of which have shown great accuracy in describing not just SP λmax, but also the entire shape of the SP band (i.e. accurate NP mapping):

σ ext =

4πk N (E*loc, J • PJ ) 2 ∑ E 0 j= i

(1)

Here, the SPR extinction (σext,, Qext) is related to the sum of N discrete dipole vectors (fields) E* and PJ, corresponding to electrical field and polarization, and k is a constant (k=m0(2π/λ); m0 = related to material index of refraction (eqn. 1). Wavelength dependent dielectric tables for both Au and Ag were generated using well-established optical constants.25 For the AuxAg1-x solid solution alloys, we used calculated dielectric constants that take into account interband contributions at the nanoscale.26



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Results and Discussion In this section we present the optical, morphological, and chemical analysis results for the fabrication of Au/AuxAg1-x core/alloy nanoparticles (NPs). We first describe the layer-by-layer synthesis at hydrothermal temperatures (TH) of TH = 120 and 160 °C using [AuCl4]- as the gold precursor, and then describe the use of the [AuBr4]-. We then propose a growth mechanism for the nanosystems. It is important to note that the Au-Ag nanosystem has been studied in detail using a number of different synthetic strategies, and a number of different morphologies. The advantage of the Au-Ag nanosystem over other metal nanostructures is due in large part to the ease of both Au and Ag NP synthesis, the noble alloys resistance to oxidation, a highly miscible phase diagram, and an optically rich surface plasmon resonance (SPR) property.19,27-33 Some previously described fabrication strategies include galvanic replacement,21, 34-38 photochemical methods,39 radiolytic techniques,4,19,40 laser irradiation,41 coprecipitation,42 two-phase method43 and thermal evolution.44 The morphology of these Au-Ag NPs can also be tuned. Morphologies include alloys,16a,17,31,37,43,45 core/shell nanoparticles,16a,28,36,44b,46 core/shell nano-rods,39 nano-shells,33a,47 and Au-Ag dimers48 amongst others. Our approach is different in that we desire to form alloy interfaces at pre-synthesized NP cores, which will provide a better platform on which to study the size and shape dependence of alloys, as well as provide a novel route to fine tune surface plasmon resonance (SPR).

Scheme 1a shows an idealized illustration of the core/alloy processing approach. The pre-synthesized AuNP cores (d = 15.4 ± 0.7 nm) are first combined with known feed ratios of alloying precursors ([AuCl4]-, [AuBr4]-, AgNO3). This feed ratio (r = ([Ag+] + [AuX4]-])/[AuNP], X = Cl-, Br-) is limited to that required to grow a shell with thickness (tS) of only 0.25 – 0.5 nm. Once added, the reaction is heated to the desired hydrothermal temperature (TH) via the automated MWI.16-17 A representative set of hydrothermal temperature (TH) and pressure (PH) profiles are shown in Scheme 1b-c. Due to the 10 ACS Paragon Plus Environment

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programmed nature of the heating, which includes an active feedback from temperature and applied MWI power, each deposition layer has a consistent temperature and pressure profile, leading to very systematic results. After this heat treatment, and aliquot of sample is collected for both UV-vis and TEM analysis. This process is then repeated for n-layers depending on the desired shell thickness. In this study, we fabricated shells up to n =10.

SCHEME 1 Scheme 1: (a) A schematic illustration for the layer-by-layer formation of Au/AuxAg1-x core/alloy NPs. A presynthesized Au NP core of known diameter (dC) is hydrothermally heated to TH = 120 or 160 °C in the presence of [AuX4]- (X = Cl- , Br-), and Ag+ the molar ratio of which defines the alloy feed ratio. The process is repeated n-times, and the optical and morphological properties are dependent upon shell thickness (tS), core+shell diameter (dC+S), and alloy ratio (x). The hydrothermal processing temperature is tailored using a synthetic microwave reactor, which allows for a reproducible thermal history, and in-situ measurement of both temperature (b) and pressure (c).

Figure 1 shows the UV-vis and TEM results for the Au/AuxAg1-x formation using [AuCl4]- at TH = 120 oC, at n = 1-10, and alloy feed ratios of x = 0.00 (a), 0.15 (b), 0.50 (c), and 0.85 (d). Focusing first on the UV-vis results, two key observations can be made. First, the SPR band shape, and extinction progression is highly sensitive to both the shell layer (n) and alloy composition (x). For example at x = 0.0 (a) and 0.15 (b), the SPR initially undergoes a blue-shift from λSPR = 522 to 500 nm, followed by the rise in extinction of the Ag-rich SPR centered at 390 and 400 nm. An additional characteristic is the profile of the SPR itself, which are broad. The Au-rich alloys at x = 0.5 (c) and 0.85 (d) show less blueshift, and the maintaining of a single-SPR characteristic at λSPR = 490 and 510 nm, as well as an overall decreased extinction compared to the x < 0.5. The alloy shell growth was confirmed via TEM analysis. Figure 1 shows a set of TEM results at n = 3 (ii) and 7 (iii) at x = 0.00 (a), 0.15 (b), 0.50 (c) and 0.85 (d). In general, the core+shell diameter (dC+S) of the NPs increases with n, and the core/alloy NPs maintained the spherical morphology of the core. However, the size distribution of the NPs did increase, with some examples of anisotropic growth. These results indicate that the SPR signature of the core/alloy NP is related to alloy composition and shell thickness. It is well understood that the SPR of a 11 ACS Paragon Plus Environment


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NP, as well as a core/shell NP is derived from the size, shape, structure, composition, and surrounding medium of the nanostructure. 23,49-52 It has been shown previously that the λSPR for a binary AuxAg1-x NP is linearly correlated to composition,23,28b,37,41b,42,49-53 with higher Ag concentrations exhibiting blue shifted SPR. Importantly, such solid-solution alloys are known to maintain single SPR and near linear SPR for particles of similar core-sizes.23,49-50,54 In addition, the spontaneous alloying of AuAg NPs with d < 5nm showed composition dependent SPR.28b,37,53 In comparison, the SPR of a core/alloy NP is less well established (see below).16-17


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FIGURE 1

Figure 1: Optical and morphological comparisons for Au/AuxAg1-x prepared at TH=120 °C with x = 0.0 (a), 0.15 (b), 0.50 (c), and 0.85 (d) with [AuCl4]- as Au precursor, comparing (i) UV-vis spectra showing experimental data for n = 1-10. TEM micrographs for shell layers n = 3 (ii) and n = 7 (iii) for the four compositions. Statistical analysis (insets). UV-vis offset for clarity.

The SPR response, as well as NP growth was highly susceptible to processing TH. Figure 2 shows the UV-vis (i) and TEM (ii, iii) of Au/AuxAg1-x prepared using [AuCl4]- at TH = 160 °C. Compared to those processed at TH = 120 °C (Fig. 1), a different SPR signature is noted. A major characteristic of which is a lower extinction, and broad SPR. For example, at x = 0.15 (b) the main SPR is located at λSPR ≈495 nm, which is associated with a broad SPR at higher energy (λSPR ≈430 nm). At x = 0.5, the λSPR ≈ 495 nm, and the resonance is broad, which increases with n. Meanwhile, the SPR at x = 0.85 is consistently uniform, with a single SPR at λSPR ≈522 nm. A representative set of TEM micrographs sampled at n = 3 (ii) and 7 (iii) showed two trends. First, by comparison to TH = 120 oC, the shell growth is less controlled as the NPs become polydisperse, particularly at x = 0.15 - 0.50. Second, small NPs can also be observed at n = 7. This can be observed at x = 0.15 (b) and 0.50 (c), where the NPs show considerable morphological changes that have both increased polydispersity, as well as shape. These polydispersity are likely the reason for the broadened SPR observed. In contrast, the alloy shells prepared at x = 0.00 (i) and 0.85 (ii) showed uniform morphological growth, suggesting that intermediate ratios are more prone to uncontrolled growth. FIGURE 2

Figure 2: Optical and morphological comparisons for Au/AuxAg1-x prepared at TH=160 °C with x = 0.0 (a), 0.15 (b), 0.50 (c), and 0.85 (d) with [AuCl4]- as Au precursor, comparing (i) UV-vis spectra showing experimental data for n = 1-10. TEM micrographs for shell layers n = 3 (ii), n = 7 (iii) and Statistical analysis (insets). UV-vis offset for clarity.

Figure 3 quantifies the UV-vis and TEM findings as a function of alloy shell feed ratio (x) and shell layer (n). The NPs prepared at both TH show shell dependent growth (dC+S), with those at TH = 160 13 ACS Paragon Plus Environment


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o

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C showing poly dispersed particles for x = 0.15 – 0.50, especially at n = 7 (Fig. 3b). The tunability of

λSPR with both dC+S and n is illustrated in Fig. 3 at TH = 120 (c) and 160 °C (d). Here, the λSPR value is qualitatively determined as the SPR with highest extinction. At x > 0.50, the λSPR can be shifted to higher energy by up to 30-40 nm, despite an increase in overall NP diameter (i.e., dC+S). At x < 0.50, the shift extends to a Ag dominant SPR at 400nm (Fig. 3c). At TH = 160 °C, this λSPR is far less tunable (Fig. 3d), due in large part to loss of control of NP growth.

FIGURE 3 Figure 3. A summary of the influence of using [AuCl4]- precursor on the Au/AuxAg1-x NP core+size diameter (dC+S) dependence on Ag content at TH = 120 °C (a) and 160 °C (b) for n = 3 and 7. The corresponding relationship between dC+S and λSPR at TH = 120 (c) and 160 °C (d).

These results suggest that the galvanic reaction between the initial metallic Au NP core, Ag+ and [AuCl4]- precursors is significant in these systems, and that this effect is increased at the elevated TH, as observed by the uncontrolled growth. In this aqueous system, the standard reduction potential for the alloy precursors (E0(Ag+|Ag) = 0.80 V, E0([AuCl4]-|Au) = 1.00 V)55a dictates that [AuCl4]- reduction is favorable and that it will oxidize any metallic Ag0 at the interface. To probe this, we compare these results with those we recently communicated in reference 17 that used the [AuBr4]- gold precursor analogue. The E0([AuBr4]-|Au) = 0.86 V is similar to that of E0(Ag+|Ag), which allows for little galvanic displacement after deposition is expected. Figure 4 shows a summary of the results using [AuBr4]-. In general, the SPR trends from [AuBr4]- are consistent with those shown above (Fig. 3), with each showing SPR blue-shift that is related to both x and n. The data however is more consistent in the layer dC+S dependence, particularly at x = 0.15 and 0.50.17 This suggests a more uniform shell growth and composition. At TH = 160 oC even more control in dC+S and λSPR was observed (Fig. 4b,d). In general, both TH show much narrow size distributions across the composition range, with the NPs prepared at TH = 160 °C showing particularly uniform growth. FIGURE 4 14 ACS Paragon Plus Environment

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Figure 4: A summary of the influence of using [AuBr4]- precursor on the Au/AuxAg1-x NP core+size diameter (dC+S) dependence on Ag content at TH = 120 °C (a) and 160 °C (b) for n = 3 and 7. The corresponding relationship between dC+S and λSPR at TH = 120 (c) and 160 °C (d). Reprinted with permission from ref 17. Copyright 2011 American Chemical Society.

X-ray Photoelectron Spectroscopy (XPS) analyzed the composition of the Au/AuxAg1-x NPs. Figure 5a and Table S1 shows a comparison of the synthetic feed ratio (x) and XPS determined ratio (x) for the Au/AuxAg1-x NPs prepared to n = 7 at TH = 120 oC using the [AuCl4]- (i) and [AuBr4]- (ii) precursors. Both precursors showed a linear trend between the feed ratio composition and the XPS determined composition of the NPs. In general, the sample prepared using the [AuBr4]- precursors show slightly gold rich compositions by comparison. A similar set of data are shown in Figure 5b for the NPs prepared to n = 7 at TH = 160 oC, which shows a higher linearity, with Au-rich compositions at low feed ratios (x < 0.5), and Ag-rich compositions at high ratios (x > 0.5). In addition, halide reside was also observed in each NP despite being copiously purified, with samples prepared using [AuCl4]- showing Cl- compositions of up to 1-3%, and those prepared with [AuBr4]- with Br- at ~1%, suggesting strong anion absorption to the NP interface. It is important to note that under the XPS conditions, a penetration depth of ~5 nm is expected. Since the TEM results show the NP increases in size to dC+S ≈ 19-21 nm (Fig. 3-4) from a Au core size of dC ≈ 15.5, these results indicate that a considerable amount of the pure Au-core may contribute to the XPS determined composition. FIGURE 5 Figure 5: XPS determined composition (x) of Au/AuxAg1-x prepared to n = 7 using [AuCl4]- precursor (i) and [AuBr4]- precursor (ii) for Au/AuxAg1-x NP preparation at TH = 120 ºC (a) and 160 ºC (b). Inset: Trend line (dotted) showing a linear feed ratio to XPS ratio trend for comparison.

Taken together these results show that interfacial composition can be tuned by alloy shell thickness and feed ratio composition. Collectively, this trend is further indicated by the SPR signatures of the core/alloy NPs. To better correlate these SPR with morphology (dC+S), alloy composition (x), and alloy shell thickness (tS), discrete dipole approximation (DDA) simulations were performed using a 15 ACS Paragon Plus Environment


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model Au NP core with diameter of 15.5 nm, alloy shell thickness of 2.75 nm, and compositions ranging from x = 0.0 - 1.0. These dimensions were chosen to best match the final NP sizes prepared at n = 7.

Figure 6a shows the representative DDA simulations, which reveal the expected shift in

wavelength (denoted as λDDA) with composition, but also reveal overall band shapes that are similar to those observed by UV-vis. For example, at x = 0.50, a broad SPR is observed, with λDDA centered at ≈ 465 nm. Figure 6b shows the corresponding linear relationship between λDDA with alloy composition. Others have showed similar linear relationships for homogeneous alloy NPs.49,

56

It is an interesting

finding that the core/alloy NPs have a similar relationship, in addition to the new finding of the broadness. We next related these model λDDA values with those experimentally observed, as shown in the overlay in Fig. 6b. One general trend is that each NP showed a red-shift in the λDDA compared to λSPR, compared to the DDA simulations, with samples prepared at TH = 160 oC showing considerably longer λDDA (open circles) compared to TH = 120 oC (closed circles). This is most pronounced at shells with high Ag-content.

We have previously shown that interdiffusion can occur at the elevated

hydrothermal temperatures of TH = 160 oC using MWI.16a Thus, there must be a considerable core-toshell (and shell-to-core) diffusion occurring, which may be further influenced by the galvanic displacement resulting in Au enrichment towards the interface (see below). This is most likely the situation for the x = 0.15 NPs, where the NPs prepared with [AuCl4]- have λSPR ≈500 nm, whereas the NPs from the [AuBr4]- precursors are λSPR ≈460 nm.

FIGURE 6 Figure 6: (a) DDA simulation of the Au/AuxAg1-x core/alloy NPs using Au NP core (dC = 15.5nm), and alloy shell (tS = 2.75 nm) at x = 0.0, 0.2, 0.4, 0.5, 0.8, 1.0; in a medium of water. This particle size was chosen to best match the samples prepared at n = 10. (b) A comparison of the λSPR dependence with alloy shell composition between the DDA simulations composition and the NPs prepared in this study. Here, λSPR is defined as the wavelength of the maximum extinction.

SCHEME 7


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Figure 7: Proposed Core/Alloy growth mechanisms using [AuCl4]- (i) and [AuBr4]- (ii) gold precursors to a presynthesized Au NP core. The Ag+ and Au3+ precursors are added to a pre-synthesized Au NP core in the presence of a weak reducing agent (a). In the moments before heating, reduction of the ions is possible, which will result in galvanic interactions occurring (b). Due to the standard reduction values (E0) of the precursors, such redox will be much more favorable using the [AuCl4]- precursors (∆E0 = +0.20 V) than the [AuBr4]- (∆E0 = +0.04 V). This results in a more segregated Au/Ag alloy shell using [AuCl4]- (i), which leads to higher interactions with Cl- ions in solution, and uncontrolled growth during heating. In contrast, when [AuBr4]- is used, a more homogeneous AuAg alloy is deposited, which undergoes further annealing during heating. This improved alloy distribution and less interaction with Br- ions, leads to much improved alloy shell growth.

Discussion: Taken together, these results demonstrate that this core/alloy NP processing strategy is effective at fine-tuning alloy-shell thickness, composition, as well as the SPR characteristics of the NP. However, the results do indicate that a large difference exists in the morphology observed using either the [AuCl4](Figs. 1-2), or [AuBr4]-

17

gold precursors. This is due to the alloy formation mechanism, the co-

reduction of the metal precursors, and the resulting galvanic interactions that occur before complete reduction and MWI mediated hydrothermal heating.

Figure 7 shows a proposed alloy growth

mechanism for these systems based on the observed UV-vis and TEM results. When the [AuCl4]precursors are employed a significant redox potential exists (∆E0 = 0.20V), that will drive the oxidation of any deposited Ag0 at the surface until either an alloy is formed, or the [AuCl4]- is consumed (Fig. 7, top panel). This reaction will take place during both the addition of the precursors (Fig. 7. Step b), as well as during the heating cycle (step c). Upon this galvanic displacement, the Ag+ ions formed will be reduced by the excess reducing agent present in the system (Cit), leading to the outermost layer to be Ag-rich. Thus, when the precursors are added, within each layer (n), there will exist a core/shell composition gradient that will be Ag-rich towards the interface. This effect will be highest for the Aurich shells (high x), as is indicated by the higher values of Ag% measured by XPS (Fig. 5). Recently, a number of reports have described similar phenomena,15 and recently theoretical work describes the effect in detail.2,13-14 In addition, the excess Cl- ions present will be able to coordinate to the interface, or complex Ag+. It is important to note that no AgCl precipitation was observed in our system during the reaction, and the concentrations were below the solubility of AgCl (KSP(AgCl) = 1.8×10-10).21b,55b 17 ACS Paragon Plus Environment


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However this does not rule out Ag-Cl interactions at the interface, which may account for precipitation or agglomeration during shell growth, since Cl- was detected via XPS. The effect of Cl- during Ag NP growth has been described previously.44c,57 This room temperature mixing and galvanic displacement occurs within the ~1 min it takes to add precursors, seal the reaction vessel, and initiate MWI heating. Upon heating, the alloy layer will first complete galvanic replacement, and then undergo reorganization and annealing, forming a more homogenous alloy.

From these galvanic interactions during shell

growth, the experimental TEM and SPR results suggest that the NPs prepared at the TH = 120 oC resulted (Fig. 1) in an increased size distribution with a degree of asymmetry (i.e. TEM), and interfaces that are more Ag-rich (Fig. 6b).

The alloy growth change characteristics at the high processing

temperature of TH = 160 (Fig. 2), resulting in larger size distributions, aggregate like shapes, and regions that suggest phase segregation. On the other hand, using the [AuBr4]- precursor significantly lessens the galvanic interactions during the initial alloy deposition (∆E0 = 0.04V), and provides weaker halide interaction between any unreacted Ag+ with Br- (KSP(AgBr) = 5.35×10-13).55 This results in a more homogeneous/simultaneous deposition of Ag and Au atoms at the surface, which upon heating, further anneal forming a homogeneous solid solution. This result is indicated by the improved morphological control and homogeneous alloy deposition.

Conclusion: The deposition of AuxAg1-x alloy shells at pre-synthesized Au NPs has been described. The resulting core/alloy NPs have optical SPR characteristics that are tunable based on alloy composition and shell thickness. Results indicate that the use [AuCl4]- as a gold precursor results in a heterogeneous growth at intermediate compositions, especially at high MWI-mediated hydrothermal temperatures of > 120 oC. If the galvanic interactions between Au3+ and Ag+ are lessened using the [AuBr4]- complex, much improved properties are shown, with higher temperatures resulting in the most uniform NPs, and homogeneous alloying observed throughout the composition range.

Insights into alloying were 18

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provided by monitoring the change in SPR characteristics and comparing them to a DDA model. These results will enhance the understanding of nanostructured core/alloy interface in future studies for other metals and phase behaviors, leading to the engineering the alloy interface for specific catalytic or plasmonic applications.

Acknowledgement This work was supported by a grant from the ACS Petroleum Research Fund (51303-DNI10). TEM work was carried out at the Cornell Center for Materials Research (CCMR, NSF/DMR 0520404).

Supporting Information Table S1 and S2 are available free of charge via the Internet at http://pubs.acs.org.



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55. (a) “Electrochemical Series” in CRC Handbook of Chemistry and Physics, 92nd Edition (Internet Version 2012), W. M. Haynes, ed., CRC Press/Taylor and Francis, Boca Raton, FL. (b) “Solubility Products Constants” in CRC Handbook of Chemistry and Physics, 92nd Edition (Internet Version 2012), W. M. Haynes, ed., CRC Press/Taylor and Francis, Boca Raton, FL. 56. Russier-Antoine, I.; Bachelier, G.; Sabloniere, V.; Duboisset, J.; Benichou, E.; Jonin, C.; Bertorelle, F.; Brevet, P-F. Phys. Rev. B 2008, 035436. 57. Tsuji, M.; Nishio, M.; Jiang, P.; Miyamae, N.; Lim, S.; Matsumoto, K.; Ueyama, D.; Tang, X.-L. Colloid Surf. A: Physicochem. Eng. Aspects 2008, 317, 247–255.


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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


93x71mm (300 x 300 DPI)



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

150x113mm (300 x 300 DPI)


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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


150x113mm (300 x 300 DPI)



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

142x77mm (300 x 300 DPI)


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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


124x68mm (300 x 300 DPI)



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

149x63mm (300 x 300 DPI)


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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


159x61mm (300 x 300 DPI)



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

102x70mm (300 x 300 DPI)


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Alloy

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