Gold Speciation and Co-Reduction Control the Morphology of AgAu

Subscriber access provided by READING UNIV

C: Physical Processes in Nanomaterials and Nanostructures

Gold Speciation and Co-Reduction Control the Morphology of AgAu Nanoshells in Formaldehyde-Assisted Galvanic Replacement Josée R. Daniel, Lauren A. McCarthy, Sadegh Yazdi, Matthew Chagnot, Emilie Ringe, and Denis Boudreau J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05771 • Publication Date (Web): 23 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

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 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 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.

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 32 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

Gold Speciation and Co-Reduction Control the Morphology of AgAu Nanoshells in FormaldehydeAssisted Galvanic Replacement Josée R. Daniel,a Lauren A. McCarthy,b Sadegh Yazdi,c Matthew Chagnot,c Emilie Ringe,b,c,*,† and Denis Boudreaua,* a

Département de chimie et Centre d'optique, photonique et laser (COPL), Université Laval,

Québec (QC), G1V 0A6, Canada b

Department of Chemistry and cDepartment of Materials Science and NanoEngineering, Rice

University, Houston, Texas, 77005, USA

ABSTRACT. Hollow AgAu nanostructures have a myriad of potential applications related to their strong and tunable localized surface plasmon resonances (LSPRs). Here we describe how the hydrolysis of the Au precursor, AuCl4-, produces AuCl4-x(OH)x- where x is both time and pHdependent, and how this can be used to control the morphology of hollow nanoshells in the coreduction-assisted galvanic replacement of Ag by Au. Controlling the degree of hydrolysis is key to obtain smooth shells: too small values of x (low hydrolysis) yield inhomogeneously replaced rough shells while too large values of x lead to the dominance of Au nucleation over galvanic replacement. Kinetics studies reveal two time constants for the galvanic replacement varying with temperature and composition; a short (< 10 min) half-life component associated with the

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 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 2 of 32

initial void creation and a long (> 100 min) half-life component associated with the continuous reduction and replacement of Ag. By optimizing the reaction’s pH and Au speciation, we obtained smooth alloy shells with fine control of composition, size, and shape over a broad range, thereby tuning the optical properties. This framework for understanding and controlling reaction kinetics and nanoshell morphology is applicable to other metallic systems and precursors, providing new ways to rationally design nanostructure syntheses.

INTRODUCTION Metallic nanoparticles (NPs) can sustain intense light-driven resonant oscillations of their conduction electrons called localized surface plasmon resonances (LSPR). For Mg, Al, Cu, Ag, and Au, these resonances are located in some or most of the UV-Vis-NIR region of the electromagnetic spectrum.1–3 In addition to wavelength-dependent absorption and scattering, bestowing NPs vibrant colors, this strong interaction leads to the enhancement of the local electromagnetic field responsible for plasmon-enhanced and surface-enhanced spectroscopies4,5 and to plasmon-enhanced hot electron ejection and photocatalysis.6 The properties of this enhancement can be manipulated by varying the NP shape, size, environment and composition.1,7–9 An increasingly popular addition to shape control is the creation of hollow structures, where the shell thickness adds an additional degree of freedom in designing LSPR properties, and the internal void offers opportunities for zeptoliter-scale electrochemistry,10 drug delivery,11–13 photothermal cancer therapy,14–17 and atom-efficient, high surface area catalysis.18– 24

Hollow nanostructures can be synthesized by either coating followed by dissolution of a template, i.e., the nanoscale Kirkendall effect,25 or by the replacement of a sacrificial NP, for instance by galvanic replacement,25,26 a reaction in which nanoscale effects play an important


2

Page 3 of 32 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


role.27 Much of the interest in the latter stems from the availability of shape-control protocols for Ag NPs, which impart a controlled shape to the resulting hollow NPs. This is further enhanced by Ag’s ability to be replaced by more noble metals (M), following nAg(s) + Mn+ nAg+ + M(s)

(eq. 1)

where M represents a catalytic metal such as Pd and Pt as well as Au.28–32 The latter is particularly interesting owing to its versatile chemistry, stability, and optical properties. Further, AgAu alloying and partial replacement – controllable with stoichiometry, addition of a coreducing agent during galvanic replacement, and/or post-synthesis annealing – provide expanded means to tune the LSPR energy beyond what is achievable for single-metal NPs,9 as well as manipulate the composition of the exposed NP surface. The driving force for galvanic replacement is the difference between the reduction potential of the two metals, which is dictated by the metal oxidation state. Here, we show that metal speciation in solution can be used to alter this potential difference and therefore control the outcome of reducing agent-assisted galvanic replacement. We demonstrate this concept using Ag NPs and Au in the form of AuCl4-x(OH)x-, where x is pH-controlled. We further explore the effects of concentration and temperature on the reaction kinetics, morphologies, and optical properties of the AgAu alloy NPs obtained. These results describe means to attain finer control on NP composition (hence plasmon resonance), geometry, and surface quality, for applications in a plethora of important and emerging fields impacted by nanotechnology.

EXPERIMENTAL SECTION Materials. Tannic acid (C76H52O46, ACS Reagent), sodium citrate tribasic (Na3C6H5O7, 99.0% ACS Reagent), polyvinylpyrrolidone (PVP10, MW ˜10 000), gold(III) chloride trihydrate


3


Page 4 of 32

(HAuCl4·3H2O, ≥ 99.9%, trace metal grade) and ICP calibration standards (TraceCERT®, 1000 mg/L Au in hydrochloric acid; 1000 mg/L Ag in nitric acid; 1000 mg/L in nitric acid) were purchased from Sigma Aldrich. Silver nitrate (AgNO3, 99.9995%) was obtained from Strem Chemicals. Aqueous formaldehyde solution, (CH2O, 37% ACS) was obtained from Fisher Scientific. Aqueous hydrochloric acid (HCl, 36.5−38.0%, ACS reagent) and aqueous nitric acid (HNO3, 68.0−70.0%, ACS reagent) were purchased from VWR International. Potassium carbonate anhydrous (K2CO3, ACS ISO reagent) was purchased from EMD Millipore. Ultrapure water (18 MΩ) was used for all experimental processes. All chemicals and reagents were used without further purification. All glassware used for synthesis was previously washed with concentrated acid (nitric acid or aqua regia) and then rinsed thoroughly with ultrapure water. Synthesis Procedures. Ag Nanospheres. Ag nanosphere templates were synthesized using a previously published procedure.33 The synthesis procedure described here is for 55-60 nm diameter NPs, related protocols are reported in Table S9. Briefly, 50 mg of tannic acid (0.29 mM) and 72 mg of sodium citrate (2.45 mM) were dissolved in 100 mL of water in a 250-mL round bottom flask and refluxed under vigorous stirring. Then, 18 mg of AgNO3 dissolved in 1 mL of water was quickly added to the flask. The reaction mixture was stirred for 60 min, then left to cool under light agitation. The solution was centrifuged twice (7500 RCF, 12 min) and redispersed in 100 mL of water. To completely remove tannic acid from the media, a ligand exchange with PVP10 was performed, where 100 mL of NP solution was slowly agitated with 200 mg of PVP10 for a minimum of 12 h at 40°C followed by centrifugation (7500 RCF, 12 min, twice). The resulting PVP-coated NPs were dispersed in 100 mL of water and silver was quantified by ICP-AES.


4

Page 5 of 32 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


Formaldehyde-Assisted Galvanic Replacement. In a typical reaction, the Ag NP solution was diluted to 0.5 mM of Ag then put under agitation at room temperature. 50 µL of 1.5 mM of PVP10 and 10 µL of methanol-stabilized 37% formaldehyde aqueous solution were added per mL of Ag NP solution (i.e., 250 µL of PVP10 and 50 µL of formaldehyde for synthesis using 5 mL of Ag NPs). The K2CO3/HAuCl4 solution was prepared by dissolving K2CO3 in 49 mL of water and adding 1 mL of a 25 mM HAuCl4·3H2O aqueous solution, for a final Au concentration of 0.5 mM. The solution was then aged prior to use. Final concentrations of K2CO3 are given in Table S2, here a concentration of 2.20 mM aged 15 to 30 min resulting in smooth hollow NPs. To obtain the final product, the desired volume of K2CO3/HAuCl4 solution was injected into the reaction mixture and left to stir at room temperature for at least 24 h or for 30 min at elevated temperature without the use of a condenser. The solution was then centrifuged twice and the NPs were dispersed in water. Since the Au:Ag concentration ratio in the Ag NP and K2CO3/HAuCl4 solutions is constant at 1:1, the volume ratio dictates the atomic stoichiometry (i.e., 0.25Au:Ag, 0.5Au:Ag, 0.75Au:Ag and Au:Ag are prepared with 1.25, 2.5, 3.75 and 5 mL of K2CO3/HAuCl4 for syntheses using 5 mL of Ag NP solution). Through this manuscript, Ag and Au concentrations are described as atomic ratio between Ag and Au; given that the stoichiometry of galvanic replacement is 3Ag:1Au (Eq. 1), a reaction Au:Ag, or 1 Au equivalent, contains three times more Au than is required to replace all the Ag present, in the absence of a reducing agent. Characterization Techniques. ICP-AES and ICP-MS. Elemental analysis of the Ag and AgAu NPs were performed with a Perkin-Elmer Optima 3000 ICP-AES spectrometer, except for the supernatant after 48 hours of reaction time, for which the analysis was performed on a Agilent ICP-MS 8800 Triple Quadrupole for additional sensitivity. To digest the Ag samples,


5


Page 6 of 32

100 µL of suspended NPs was mixed with 400 µL of concentrated nitric acid in a 10-mL volumetric flask. After two hours, the flask was filled to 10 mL with water. For AgAu samples, 100 µL of NPs suspension was mixed with 100 µL of concentrated nitric acid and 300 µL of concentrated hydrochloric acid in a 10-mL volumetric flask and left to digest for two hours, then completed to volume with water. Details of the calibration are reported in the supporting information. To determine the elemental concentration in supernatants at different reaction times, a 500-µL aliquot was extracted and immediately centrifuged in a 1.5-mL tube at 12 000 RCF for 4 min. This process takes ~5 min, which was considered in the calculation of the reaction time. The particle-free supernatant was then transferred in a new tube and immediately centrifuged again at 12 000 RCF for 4 min. 250 µL of this solution was used for elemental analysis. Concentrations are given in relative % with respect to Au and Ag present in the media without any purification or centrifugation. Electron microscopy. TEM images were obtained on a FEI Tecnai Spirit G2 Biotwin equipped with an AMT (Orca HR, 11 Megapixels) bottom-mounted camera and operated at 120 kV. NP size and concentration determination: the average sizes and size distributions of nanoparticles was determined from TEM images with the ImageJ software. X-ray photoelectron spectroscopy. The X-ray photoelectron spectroscopy and Auger measurements were conducted using an AXIS Ultra from Kratos Analytical. The samples were centrifuge, then supernatant is removed and concentrate NPs were drop cast on silicon wafers. All XPS measurements were performed using Al Kα radiation. Spectra were measured at 0° detection angles defined with respect to the surface plane of the substrate. The survey spectra used for semi-quantitative analysis were collected with an analyzer pass energy and energy step


6

Page 7 of 32 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


of 160 eV of 1.000 eV respectively. For all high-resolution core-level spectra, a bias of 1.0 V was applied, an analyzer pass energy and energy step of 10 eV of 0. 050 eV was used. Optical spectroscopy. All bulk extinction spectroscopy measurements were performed on a Cary 50 UV-Vis spectrophotometer, except for the study of Au solutions, which was performed on a Cary 5000 UV-Vis-NIR spectrophotometer. In-situ monitoring of the LSPR peak position during the reaction was performed using the same synthesis procedure detailed in the experimental section and a bifurcated transmission dip probe connected to a compact spectrometer and a deuterium/halogen light source (Models TP300-UV-VIS, USB-2000+ and DH-2000, Ocean Optics).

RESULTS AND DISCUSSION Speciation of HAuCl4. The influence of oxidation state was investigated by Au et al.34 and Gonzalez et al.35, who observed marked differences in galvanic replacement reaction outcome when using either Au3+ or Au+. Speciation was shown to affect the nucleation and growth of Au NPs in the aqueous reduction of HAuCl4 by ascorbic acid.36 We hypothesize that galvanic replacement can also be rationally controlled by such speciation. AuCl4- complexes are indeed not stable in aqueous solution and undergo spontaneous stepwise hydrolysis, following AuCl4- + xH2O → AuCl4-x(OH)x- + xH+, where x is pCl and pH dependent.37 Due to the stabilization provided by the complexing ligands, the standard reduction potentials of AuCl4-x(OH)xcomplexes will range from 1.0 V38 for AuCl4- (x = 0) to 0.60 V39 for Au(OH)4- (x = 4). For large x, Au complexes have a lower reduction potential than that of Ag+ (0.8 V).38 Consequently, at low pH, a faster replacement is expected (fewer OH-), while a pH too high should stop the reaction by eliminating its driving force.


7


Page 8 of 32

Since pH affects the availability of hydroxide ions, it can be used to systematically control the Au speciation in solution,37 thereby manipulating the reaction thermodynamics. Here, the pH of AuCl4- solutions is adjusted with the addition of K2CO3; the resulting solution is sometimes referred to as “K-Gold”.40–42 The pH-dependent speciation of HAuCl4 in this solution (Figure S1 and Table S1) involves a slow shift from chlorinated to hydroxylated species across pH 3 to 11; i.e. x in AuCl4-x(OH)x- increases from 0 below pH 3 to 4 at pH 11 and above. This speciation can be readily analyzed by UV-Vis extinction spectroscopy as the planar AuCl4- complex displays strong absorption bands at ~ 230 and ~ 310 nm.43,44 The intensity and energy of these ligandmetal charge transfer bands (LMCT) change as a function of pH due to hydrolysis, i.e., AuCl4x(OH)x complex

formation. At AuCl4- concentrations representative of the NP synthesis, only the

~ 310 nm band could be accurately tracked. Figure 1A shows the pH-dependent speciation at equilibrium, i.e., 24 hours after the preparation of the solution45 (results at the 48-hour point are similar). Before reaching this equilibrium, the hydrolysis of AuCl4- proceeds over time as shown in Figure 1B for a solution initially at pH 9.7. Note that pH evolves during the speciation process (Table S2); here we report pH values measured at the time of utilization of a solution. Au speciation thus depends on both pH and time (before equilibration), providing two ways to control the precursors available during a galvanic replacement reaction.


8

Page 9 of 32

2

pH 2.9 pH 3.1 pH 3.4 pH 3.8 pH 4.4 pH 5.0 pH 5.5 pH 6.0 pH 6.5 pH 9.5

Extinction (a.u.)

A

0 250

300

350 400 450

500

550

Wavelength (nm)

B

2

HAuCl₄ 0 min 1 min 2 min 3 min 4 min 5 min 10 min 15 min 20 min 30 min 40 min 50 min 1h 6h 12 h 24 h

Extinction (a.u.)

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


0 250

300

350

400

450

500

550

Wavelength (nm)

Figure 1. (A) UV-Vis extinction spectroscopy of equilibrated AuCl4- solutions with pH controlled by the addition of K2CO3. (B) K2CO3-free HAuCl4 solution spectrum (dashed line) and time-dependent speciation of AuCl4- in K2CO3 solution with initial pH at 9.7 (solid lines). Speciation Control for Nanoparticle Synthesis. Control of the NP composition and morphology beyond that of traditional galvanic replacement approaches can be achieved by coupling galvanic replacement with co-reduction. This can be realized by introducing a reducing agent to the reaction medium, which allows for tunable surface roughness and alloy composition.46,47 This additional process increases the complexity of the reaction mechanism through competitive reduction of Au(III) by both the sacrificial template and the reducing agent in solution and the reduction of Ag(I) ions by the reducing agent. Therefore, control of these


9


Page 10 of 32

reactions and the resulting NP morphology involve the manipulation of the relative co-reduction and galvanic replacement reactions rates, a subject little explored thus far. Formaldehyde is ideally suited for co-reduction as it is a soft reducing agent active at room temperature that can reduce Ag and Au ions efficiently in basic conditions (Table S3). The efficiency of the reducing agent ascorbic acid at higher pH to control Au reduction over galvanic replacement has been previously demonstrated by Yang et al.48,49 Formaldehyde adds a requirement for basic reaction conditions that initially appears orthogonal to the previously discussed pH-controlled speciation of AuCl4-, whereas species become inactive after some time at high pH. This challenge can be overcome by either 1) adding an equilibrated, Au/K2CO3 solution to an adjusted, basic NP solution or 2) introducing a briefly hydrolyzed, basic Au/K2CO3 solution to the native NP solution. Reaction using either approaches show evidence of galvanic replacement, indicating incomplete hydrolysis of the precursors. The TEM images shown in Figure 2 illustrate the reaction’s acute dependence on pH (see also Figure S2-S3 for data at other pH values). The speciation of an equilibrated pH 5 K-Au solution is equivalent to that of a briefly hydrolyzed pH 9 solution (Figure S4); both approaches can therefore yield shells with varying roughness, and either one can be chosen according to the specifics of the reaction design. The top panel shows NPs obtained using the first approach (equilibrated acidic (pH 5) K-Au added to Ag NP solutions at various pH). With an acidic final pH (Figure 2A), reduction by formaldehyde is less efficient, leading to cage-like structures and fragments created primarily by galvanic replacement. With a basic (pH 9) Ag NP solution, the final pH is slightly basic (pH 8), ideal for both galvanic replacement and reduction by formaldehyde; such conditions produce smooth alloy shells (Figure 2B and S5). At higher pH (Figure 2C), particles show significant surface roughness due


10

Page 11 of 32 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


to the increase in reduction efficiency. The bottom panel of Figure 2 shows NPs obtained following the second approach (Au briefly hydrolyzed, pH 9). When using Ag NP solutions as-is (i.e., pH 4 in the presence of PVP and formaldehyde), the resulting pH after Au addition is slightly basic such that replacement and reduction both occur and smooth shells are obtained (Figure 2D). However, if the Ag NP solution pH is also basic (pH 9.5), the final pH in the reaction mixture is too high, leading to fast reduction kinetics and rough structures (Figure 2E). At still higher pH (pH 11), almost no replacement occurs due to the further hydrolysis of Au, and homogeneous Au nucleation dominates over replacement, leading to the rough, filled Ag@Au core-shell like structures shown in Figure 2F. The assignment of this core-shell structure is based on TEM image contrast and the blue shift from 570 nm to 535 nm observed in the extinction spectra (Figure 2G). Binding energies of the Au (4f) and Ag (3d) electrons were investigated by X-ray photoelectron spectroscopy (XPS) as a means to differentiate rough Ag@Au nanostructures from alloyed AgAu nanoshells. XPS is known to be a surface-sensitive technique with a signal collection depth given by 3ߣcosθ, where ߣ is the attenuation length and θ is the electron emission collection angle (0° in the present case).50 Using an aluminum source, the depth of analysis was thus estimated at ∼5 nm for Ag (3d) and Au (4f) electrons (see details for calculation of ߣ in Table S5).51 For smooth nanoshells (Figure 2D), the Au (4f) and Ag (3d) peaks show no significant shift compared to the energies measured for monometallic gold and silver nanoparticles. Furthermore, the results obtained are comparable those reported for alloys of similar atomic ratios (i.e., 0.5Au:Ag).52–55 By contrast, for rough Ag@Au (Figure 2F), a negative shift of 0.2 eV was observed for the binding energies of both Au (4f) and Ag (3d) electrons. For Ag, it is also possible to determine the Auger parameter (α’), which is reported to be more


11


reliable and independent of surface charging of the specimen, where α’ is the sum of energy binding of the photoelectron emission peak and the kinetic energy of the Auger transition. The calculated α’ for AgAu nanoshells and rough Ag@Au are 726.1 eV and 725.9 eV, respectively. These results suggest a difference in composition and morphology between the two samples.

Stabilized K-Au pH5

Results for all pH values are shown in Table S4.

G

1

pH 4 pH 8 pH 9.5 pH 11

Nomalized Extinction

Briefly hydrolyzed K-Au pH9

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 12 of 32

0 300

400

500

600

700

800

900

Wavelength (nm)

Figure 2. pH-dependence of the galvanic replacement of Ag by Au in the presence of formaldehyde, obtained from (A-C) a K2CO3- AuCl4- (K-Au) solution at pH 5 and Ag NPs solution at (A) pH 4 (B) pH 9 and (C) pH 10; D-F: K-Au solution at pH 9, aged 15-30 min, and Ag NPs solution at (D) pH 4 (E) pH 9.5 (F) pH 11. Note that the pH of the Ag NPs solution was adjusted with 0.1 M K2CO3 solution after the addition of the stabilizing (PVP) and reducing (formaldehyde) agents to avoid pH variation from those components. The reaction conditions B and D led to the formation of smooth shells. Scale bars, 100 nm. (G) UV-Vis extinction spectra of NPs obtained from K-Au solution at pH 9, aged 15-30 min, and Ag NPs solution at pH 4 (red), pH 8 (orange), pH 9.5 (green) and pH 11 (blue).


12

Page 13 of 32

The Au surface enrichment as function of pH was investigated by XPS. Measurements were performed on nanoparticles prepared using a briefly aged K-Au solution at pH 9 where the pH of the nanoparticle solution was adjusted (see bottom panel of Figure 2 and Figure S3). Here, a 0.5 equivalent of Au with respect to Ag atoms was added to the nanoparticle suspension, giving a theoretical 33% of Au and 67% of Ag in the final particle composition. The elemental analysis was carried out with scanning electron microscopy-energy dispersive X-ray spectroscopy (SEMEDX), in which the collection depth largely exceeds the dimensions of the nanoparticles, and gave a constant Au percentage of ∼34% regardless of the final pH of the reaction media, which corresponds to the theoretical value. However, the elemental analysis carried out by XPS, which probes to a depth of about 5 nm, shows the relative concentration in Au as a function of the pH going from 31% at pH 7 to 55 % at pH 11 (Figure 3), suggesting an enrichment of gold at the surface of the particles. 60 55 50

Au relative %

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


45 40 35 30 25 20 6

7

8

9

10

11

12

pH

Figure 3. Relative surface Au concentration as a function of pH as determined by XPS measurements. Nanoparticles were prepared using a briefly aged K-Au solution at pH 9 where the pH of the nanoparticle solution was adjusted (see bottom panel of Figure 2 and Figure S3). Error bars were obtained from three measurements on different regions of the sample.


13


Page 14 of 32

The aging of the K2CO3/AuCl4- solution controls the speciation of Au, as shown in Figure 1B. We hypothesize that this speciation is key to controlling the kinetics of the galvanic replacement reaction, as supported by the pH dependence shown in Figure 2. To explore this concept, we compared reaction products from a HAuCl4 solution containing no K2CO3 with reactions at idealized pH (Au solution at pH 9, NPs solution at pH 4) carried out by systematically increasing the age of the K2CO3/AuCl4- solution (Figure 4). The solution without K2CO3 led to Ag NP fragments and Au NPs (Figure 4A), the signature of a fast galvanic replacement process and of the inability of formaldehyde to reduce Ag in the reaction’s acidic pH. Without any ageing of the Au/K2CO3 solution, galvanic replacement occurs at a faster timescale than reduction due to the predominance of AuCl4- ions, leading to rough structures (Figure 4B). With ageing times of 15 and 30 minutes, smooth alloy shells are obtained (Figure 4C-D), indicating that at least some galvanic replacement and reduction occur concurrently. As Au is further hydrolyzed prior to reaction with Ag NPs, it loses activity; longer ageing leads to fewer and fewer NPs being replaced (Figure 4E-F). This study shows that the time-dependent speciation, via hydrolysis, of K2CO3/AuCl4- prior to reaction with Ag NPs plays an important role in the control of the morphology of alloy nanoshells.


14

Page 15 of 32 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


(no K2CO3)

(0 min)

(15 min)

(30 min)

(6 h)

(24 h)

Figure 4. Effect of Au speciation on NP shape, where the Au speciation is modulated by timedependent AuCl4- hydrolysis. TEM images of final NPs obtained with (A) HAuCl4 solution (no K2CO3). TEM images of NPs obtained with pH 9, K2CO3/HAuCl4 solutions aged (B) 0 min, (C) 15 min, (D) 30 min, (E) 6 h, and (F) 24 h. Scale bars, 100 nm. The use for this reaction design by speciation control is further supported by results using AuBr4-. Stability of gold halogenide complexes depends on the ligand electronegativity, which leads to a stability order of [AuI4]- > [AuBr4]- > [AuCl4]- > [AuF4]-, i.e., AuBr4- is expected to be more stable in water, as observed by UV-Vis extinction spectroscopy (Figure S6).56 The reduction potential of the AuBr4- ion is 0.85 V, slightly higher than that of Ag (0.80 V), but below that of freshly prepared AuCl4- (1.00 V). Galvanic replacement from AuBr4- is thus


15


Page 16 of 32

expected to be slower than AuCl4- at equivalent speciation. We performed formaldehyde-assisted galvanic replacement of Ag NPs with the same K2CO3 concentration and a briefly (15 min) aged AuBr4- solution, with all other parameters identical to those used for the AuCl4- reaction. The results show cage-like structures and a second nucleation consistent with the fast galvanic replacement expected from the AuBr4-x(OH)x- speciation (i.e. lower x than AuCl4-x(OH)x- for same initial pH condition) (Figure S7). Finally, it is worth noting that while speciation plays a dominant role in the control of reducing agent-assisted galvanic replacement, other factors such as the PVP molecular weight, formaldehyde concentration, and NP concentration can also affect the outcome of the reaction, as discussed in the supporting information. Reaction Kinetics. The kinetics of the galvanic replacement reaction using experimental conditions optimized to obtain smooth hollow shells (i.e. Au solution at pH 9 hydrolyzed 15-30 min) was investigated. In this scenario, Ag ions are an intermediate product, being first displaced by Au, and then reduced back by formaldehyde onto the NP. To test this hypothesis, the Au consumption and Ag release kinetics for various initial Au concentrations (all in constant Ag NP concentration) were measured by tracking Au and Ag concentration with inductively coupled plasma atomic emission or mass spectrometry (ICP-AES/MS), performed on the supernatants of centrifuged reaction mixtures. Note that we describe Ag and Au concentrations as the atomic ratio between Ag and Au. Figure S8 show the consumption of Au as a function of time for 0.24, 0.49, 0.81, and 1.01 Au equivalents (i.e., Au:Ag) with the signal fitted by a bi-exponential model revealing a fast (t1) and a slow time (t2) constant that are both concentration-dependent (Figure 5A and Table S6). The fast time constant reflects the rapid formation of an initial cavity via galvanic replacement, for which larger Au concentrations lead to faster reactions, except for the lowest Au concentration where cavities formed are mostly partial, leading to an even faster initial


16

Page 17 of 32 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


step. Rapid initial steps have indeed been directly observed at the single particle level in optical57,58 and electron microscopy59–61 experiments. The slow time constant is likely related to the co-reduction of Au and is more strongly affected by the concentration of Au than the fast time constant. This dependence occurs since dealloying slows as the NP surface composition becomes predominantly Au. A small contribution from the evolution of the speciation of Au in solution may exist: as the pH becomes more acidic, formaldehyde activity decreases, also slowing down the reaction. No significant increase in the concentration of Ag ions in solution was observed during the course of the reaction (Figure S9), indicating that the reduction by formaldehyde is fast and not rate-limiting. The morphology of the reaction products at various stages of the reaction further supports the proposed mechanism for this formaldehyde-assisted galvanic replacement reaction. Figure 5B-E shows NPs synthesized with 0.49Au:Ag after 5, 10, 15 minutes and 24 hours (additional time data in Figure S10). At t = 5 min, the majority of the NPs are semi-hollow with visible pinholes, consistent with a rapid initial galvanic replacement and previous reports.57–61 At t = 10 min, the NPs are fully hollow with rough surfaces, while at t = 15 min the shells become smoother due to the continuous Au co-reduction and Ag re-deposition, a process that continues over a long period to yield even smoother particles after 24 hours. No further morphological changes are observed beyond 24 hours. After 48 hours, only trace amounts of Ag and Au are present in the supernatant solution, indicating a full reduction of both metals, further supported by the match between the Au:Ag ratio in the centrifuged NP (without supernatant) and the Au:Ag ratio in the initial reaction mixture (Table S7). Reactions performed with even larger excesses of Au lead to loss of the structural integrity of the shells as well as homogeneous nucleation forming small Au NPs (Figure S11).


17


Half-life time t₁ (min)

A

10

800 t₁

8

600

6 400 4 200

2 0 0.00

0.25

0.50

0.75

1.00

Half-life time t₂ (min)

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 18 of 32

0 1.25

Au:Ag Ratio

B

C

D

E

Figure 5. Concentration-dependent reaction kinetics and time-dependent NP morphology. (A) Half-life times (values presented in Table S6). TEM images of reaction products for the 0.49Au:Ag reaction after (B) 5, (C) 10, (D) 15 min and (E) 24 h. Pinholes are indicated by arrows (see text for details). Scale bars, 100 nm. Given the acute dependence of the NPs’ optical response on their size and shape, the LSPR characteristics can be used to investigate reaction kinetics with a better time resolution than that provided by ex-situ elemental analysis. Here, changes in the plasmon resonance frequency, width, and intensity for the 0.5 equivalent Au reaction was tracked in real-time and in-situ by UV-Vis extinction spectroscopy (Figure 6). The pronounced LSPR energy change in the first 15 minutes (Figure 6A and S12A) agrees well with the early change in the slope of Au consumption


18

Page 19 of 32

traces for 0.49 equivalents Au in Figure S8. After 15 minutes, the resonance energy stabilizes along with the NP overall shape and composition, also in agreement with results shown in Figure 5. The full width at half maximum (FWHM) initially increases significantly, due to changes in composition as the particles incorporate Au, a metal with broader resonances. It then decreases as the shell gradually smoothens and thickens during the second stage of the reaction (Figure 6A and S12D). Note that the change in shape from sphere to shell is also expected to contribute to the initial increase in FWHM. The maximum extinction intensity follows almost exactly the opposite trend as the FWHM. The intensity initially decreases due to changes in roughness and shell formation caused by the addition of Au, then it slowly increases and reaches a plateau as the NP becomes smoother with a thicker shell and nearly all the Au is deposited.

LSPR Wavelength (nm)

A

900 800 700 600 500 400

100

101

10 2

103

Time (min)

B LSPR Wavelength (nm)

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


600

550

500

RT

450

50˚C 75˚C

400 0

5

10

15

Time (min)


19


Page 20 of 32

Figure 6. In-situ tracking of the reaction kinetics and temperature dependence of the formaldehyde-assisted, pH-controlled galvanic replacement of Ag by Au with 0.5Au:Ag. (A) LSPR spectrum tracked over the course of the reaction at room temperature, with a log time axis. (B) LSPR maximum peak position tracked during the reaction at room temperature, 50°C and 75°C. In-situ measurements also enabled the monitoring of the reaction’s kinetics at elevated temperature, i.e., 50°C and 75°C (Figure 6B and S12). The reaction rate dramatically increases with temperature (i.e., < 2 min), as is expected for co-reduction. Indeed, replacement, alloying, and dealloying are all thermodynamically driven processes running faster at elevated temperature. Further, the efficiency of the reducing agent, e.g., formaldehyde, also depends on temperature, impacting the reduction rate and consequently shifting the relative rates of reduction vs. galvanic replacement. Other factors such as changes in standard reduction potential (Tables S3 and S8) and solution activity with temperature are also expected to have minor effects on the overall reaction rates. This change in rate modifies the composition profile of the final product: scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) results (Figure S13-S14) show that NPs from reactions at 75˚C have comparatively more Au on their surface than those from room temperature protocols. This is likely attributable to the fast reduction, by formaldehyde, of species with reduction potential below that of Ag (AuCl4-x(OH)x-, with large x) at the surface of the particle, which limits the alloying-dealloying step. This approach provides a means to fine-tune the exposed surfaces for surface-sensitive applications such as catalysis and biological sensors.


20

Page 21 of 32 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


SUMMARY AND CONCLUSIONS A novel framework to describe and control the galvanic replacement of Ag NPs by Au for the synthesis of alloy nanoshells was detailed and supported by experimental evidence. In this framework, Au speciation, which plays a major role in the reaction kinetics, can be controlled by pH. We demonstrated that highly chlorinated Au species of the form AuCl4-x(OH)x- aggressively replace Ag NPs and lead to broken structures. Heavily hydrolyzed species, however, are inactive towards galvanic replacement due to their lower reduction potential. This understanding helps us obtain smooth shells by using partially hydrolyzed metal ions. This speciation control was supported by results on HAuBr4 and is expected to be a broadly applicable reaction design tool for galvanic replacement. The atomic composition and the optical properties of alloy NPs were characterized as a mean to track the reaction progress. Finally, concentration and temperature effects were studied to expand the potential control of the kinetics and final product composition profile. We show a two-step replacement mechanism in the presence of a reducing agent, and that temperature has a pronounced effect on the reaction kinetics beyond what is expected from the temperaturedependence of the reduction potential. This multifaceted study provides a framework for future synthetic design of hollow and semihollow nanostructures with wide applications in plasmonics, sensors, drug delivery, and catalysis.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge.


21


Page 22 of 32

Additional tables and figures for speciation control for nanoparticles synthesis, reaction kinetics and optical properties; Additional discussion on factors affecting the synthesis and additional methods (PDF).

AUTHOR INFORMATION Corresponding Authors *E-mail: D.B. [email protected]; E.R.: [email protected]. Present Address † Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK. Author Contributions The manuscript was prepared through contributions of all the authors. J.R.D., D.B., and E.R. designed the experiment. L.A.M and M. G. synthesized and characterized the rods. J.R.D. performed all the other syntheses, characterization, kinetic studies and TEM. S.Y., J.R.D., and E.R. performed high-resolution electron microscopy and spectroscopy. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This research was supported by the American Chemical Society Petroleum Research Fund under grant number 56256 DNI5 (E.R., S.Y.). L.A.M. wishes to acknowledge financial support from a National Science Foundation Graduate Research Fellowship #1450681. D.B. and J.R.D. acknowledge funding from the Natural Sciences and Engineering Research Council of Canada,


22

Page 23 of 32 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 Canadian Foundation for Innovation and the Fonds de la Recherche du Québec – Nature et Technologies, Dominic Larivière as well as technical support from Julie-Christine Levesque, Stéphan Gagnon, Sonia Blais and Alain Adnot.

REFERENCES (1)

Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B 2003, 107, 668–677.

(2)

Chan, G. H.; Zhao, J.; Schatz, G. C.; Duyne, R. P. Van. Localized Surface Plasmon Resonance Spectroscopy of Triangular Aluminum Nanoparticles. J. Phys. Chem. C 2008, 112, 13958–13963.

(3)

Biggins, J. S.; Yazdi, S.; Ringe, E. Magnesium Nanoparticle Plasmonics. Nano Lett. 2018, acs.nanolett.8b00955.

(4)

Stiles, P. L.; Dieringer, J. A.; Shah, N. C.; Van Duyne, R. P. Surface-Enhanced Raman Spectroscopy. Annu. Rev. Anal. Chem. 2008, 1, 601–626.

(5)

Lakowicz, J. R. Radiative Decay Engineering 5: Metal-Enhanced Fluorescence and Plasmon Emission. Anal. Biochem. 2005, 337, 171–194.

(6)

Linic, S.; Aslam, U.; Boerigter, C.; Morabito, M. Photochemical Transformations on Plasmonic Metal Nanoparticles. Nature Materials. 2015, 567–576.

(7)

Petri, M. V.; Ando, R. A.; Camargo, P. H. C. Tailoring the Structure, Composition, Optical Properties and Catalytic Activity of Ag-Au Nanoparticles by the Galvanic


23


Page 24 of 32

Replacement Reaction. Chem. Phys. Lett. 2012, 531, 188–192. (8)

Ringe, E.; Langille, M. R.; Sohn, K.; Zhang, J.; Huang, J.; Mirkin, C. A.; Van Duyne, R. P.; Marks, L. D. Plasmon Length: A Universal Parameter to Describe Size Effects in Gold Nanoparticles. J. Phys. Chem. Lett. 2012, 3, 1479–1483.

(9)

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.

(10)

Yazdi, S.; Daniel, J. R.; Large, N.; Schatz, G. C.; Boudreau, D.; Ringe, E. Reversible Shape and Plasmon Tuning in Hollow AgAu Nanorods. Nano Lett. 2016, 16, 6939–6945.

(11)

You, J.; Zhang, G.; Li, C. Exceptionally High Payload of Doxorubicin in Hollow Gold Nanospheres for Near-Infrared Light-Triggered Drug Release. ACS Nano 2010, 4, 1033– 1041.

(12)

Huang, C.-C.; Lin, P.-H.; Lee, C.-W. OFF/ON Galvanic Replacement Reaction for Preparing Divergent AuAg Nano-Hollows as a SERS-Visualized Drug Delivery System in Targeted Photodynamic Therapy. RSC Adv. 2016, 6, 64494–64498.

(13)

Jang, H.; Kim, D. E.; Min, D. H. Self-Assembled Monolayer Mediated Surface Environment Modification of Poly(Vinylpyrrolidone)-Coated Hollow Au-Ag Nanoshells for Enhanced Loading of Hydrophobic Drug and Efficient Multimodal Therapy. ACS Appl. Mater. Interfaces 2015, 7, 12789–12796.

(14)

Gobin, A. M.; Lee, M. H.; Halas, N. J.; James, W. D.; Drezek, R. A.; West, J. L. NearInfrared Resonant Nanoshells for Combined Optical Imaging and Photothermal Cancer


24

Page 25 of 32 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


Therapy. Nano Lett. 2007, 7, 1929–1934. (15)

Loo, C; Lin, A; Hirsch, L; Lee, Mh; Barton, J; Halas, N; West, J and Drezek, R. “Nanoshell- Enabled Photonics-Based Imaging and Therapy of Cancer.” Technol. Cancer Res. Treat. 2004, 3, 33–40.

(16)

Jiang, T.; Song, J.; Zhang, W.; Wang, H.; Li, X.; Xia, R.; Zhu, L.; Xu, X. Au-Ag@Au Hollow Nanostructure with Enhanced Chemical Stability and Improved Photothermal Transduction Efficiency for Cancer Treatment. ACS Appl. Mater. Interfaces 2015, 7, 21985–21994.

(17)

Jang, H.; Kim, Y. K.; Huh, H.; Min, D. H. Facile Synthesis and Intraparticle Self-Catalytic Oxidation of Dextran-Coated Hollow Au-Ag Nanoshell and Its Application for ChemoThermotherapy. ACS Nano 2014, 8, 467–475.

(18)

Chen, S.; Thota, S.; Wang, X.; Zhao, J. From Solid to Core@shell to Hollow Pt–Ag Nanocrystals: Thermally Controlled Surface Segregation to Enhance Catalytic Activity and Durability. J. Mater. Chem. A 2016, 4, 9038–9043.

(19)

Cobley, C. M.; Campbell, D. J.; Xia, Y. Tailoring the Optical and Catalytic Properties of Gold-Silver Nanoboxes and Nanocages by Introducing Palladium. Adv. Mater. 2008, 20, 748–752.

(20)

Fu, T.; Fang, J.; Wang, C.; Zhao, J. Hollow Porous Nanoparticles with Pt Skin on a Ag – Pt Alloy Structure as a Highly Active Electrocatalyst for the Oxygen Reduction Reaction †. J. Mater. Chem. A Mater. Energy Sustain. 2016, 4, 8803–8811.

(21)

Jiang, Y.; Fan, X.; Xiao, X.; Qin, T.; Zhang, L.; Jiang, F.; Li, M.; Li, S.; Ge, H.; Chen, L.


25


Page 26 of 32

Novel AgPd Hollow Spheres Anchored on Graphene as an Efficient Catalyst for Dehydrogenation of Formic Acid at Room Temperature. J. Mater. Chem. A 2016, 4, 657– 666. (22)

Liu, R.; Guo, J.; Ma, G.; Jiang, P.; Zhang, D.; Li, D.; Chen, L.; Guo, Y.; Ge, G. Alloyed Crystalline Au-Ag Hollow Nanostructures with High Chemical Stability and Catalytic Performance. ACS Appl. Mater. Interfaces 2016, 8, 16833–16844.

(23)

Sun, D.; Li, P.; Yang, B.; Xu, Y.; Huang, J.; Li, Q. RSC Advances Monodisperse AgPd Alloy Nanoparticles as a Highly Active Catalyst towards the Methanolysis of Ammonia Borane for Hydrogen Generation. RSC Adv. 2016, 6, 105940–105947.

(24)

Xu, L.; Luo, Z.; Fan, Z.; Zhang, X.; Tan, C.; Li, H.; Zhang, H.; Xue, C. Triangular Ag – Pd Alloy Nanoprisms : Rational Synthesis with High-e Ffi Ciency for Electrocatalytic Oxygen Reduction †. Nanoscale 2014, 6, 11738–11743.

(25)

Lewis, E. A.; Slater, T. J. A.; Prestat, E.; Macedo, A.; O’Brien, P.; Camargo, P. H. C.; Haigh, S. J. Real-Time Imaging and Elemental Mapping of AgAu Nanoparticle Transformations. Nanoscale 2014, 6, 13598–13605.

(26)

Wang, X.; Feng, J.; Bai, Y.; Zhang, Q.; Yin, Y. Synthesis, Properties, and Applications of Hollow Micro-/Nanostructures. Chem. Rev. 2016, 116, 10983–11060.

(27)

Moreau, L. M.; Schurman, C. A.; Kewalramani, S.; Shahjamali, M. M.; Mirkin, C. A.; Bedzyk, M. J. How Ag Nanospheres Are Transformed into AgAu Nanocages. J. Am. Chem. Soc. 2017, 139, 12291–12298.

(28)

Xia, X.; Wang, Y.; Ruditskiy, A.; Xia, Y. 25th Anniversary Article: Galvanic


26

Page 27 of 32 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


Replacement: A Simple and Versatile Route to Hollow Nanostructures with Tunable and Well-Controlled Properties. Adv. Mater. 2013, 25, 6313–6332. (29)

Choi, Y.; Hong, S.; Liu, L.; Kim, S. K.; Park, S. Galvanically Replaced Hollow Au-Ag Nanospheres: Study of Their Surface Plasmon Resonance. Langmuir 2012, 28, 6670– 6676.

(30)

Métraux, G. S.; Cao, Y. C.; Jin, R.; Mirkin, C. A. Triangular Nanoframes Made of Gold and Silver. Nano Lett. 2003, 3, 519–522.

(31)

Polavarapu, L.; Zanaga, D.; Altantzis, T.; Rodal-Cedeira, S.; Pastoriza-Santos, I.; PérezJuste, J.; Bals, S.; Liz-Marzán, L. M. Galvanic Replacement Coupled to Seeded Growth as a Route for Shape-Controlled Synthesis of Plasmonic Nanorattles. J. Am. Chem. Soc. 2016, 138, 11453–11456.

(32)

Chen, J.; Wiley, B.; McLellan, J.; Xiong, Y.; Li, Z.-Y.; Xia, Y. Optical Properties of Pd−Ag and Pt−Ag Nanoboxes Synthesized via Galvanic Replacement Reactions. Nano Lett. 2005, 5, 2058–2062.

(33)

Rainville, L.; Dorais, M.; Boudreau, D. Controlled Synthesis of Low Polydispersity Ag@SiO2 Core–Shell Nanoparticles for Use in Plasmonic Applications. RSC Adv. 2013, 3, 13953.

(34)

Au, L.; Lu, X.; Xia, Y. A Comparative Study of Galvanic Replacement Reactions Involving Ag Nanocubes and AuCl 2 − or AuCl 4 −. Adv. Mater. 2008, 20, 2517–2522.

(35)

Gonzalez, E.; Arbiol, J.; Puntes, V. F. Carving at the Nanoscale: Sequential Galvanic Exchange and Kirkendall Growth at Room Temperature. Science. 2011, 334, 1377–1380.


27


(36)

Page 28 of 32

Wang, S.; Qian, K.; Bi, X.; Huang, W. Influence of Speciation of Aqueous HAuCl4on the Synthesis, Structure, and Property of Au Colloids. J. Phys. Chem. C 2009, 113, 6505– 6510.

(37)

Brown, P. L.; Ekberg, C. Hydrolysis of Metal Ions; Brown, P. L., Ekberg, C., Eds.; WileyVCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2016.

(38)

Lide, D. R. CRC Handbook of Chemistry and Physics, 89th Edition (Internet Version), CRC Press/Taylor and Francis, Boca Raton, FL; 2009.

(39)

Bratsch, S. G. Standard Electrode Potentials and Temperature Coefficients in Water at 298.15 K. J. Phys. Chem. Ref. Data 1989, 18, 1–21.

(40)

Li, C.; Jamison, A. C.; Rittikulsittichai, S.; Lee, T.-C.; Lee, T. R. In Situ Growth of Hollow Gold–Silver Nanoshells within Porous Silica Offers Tunable Plasmonic Extinctions and Enhanced Colloidal Stability. ACS Appl. Mater. Interfaces 2014, 6, 19943–19950.

(41)

Vongsavat, V.; Vittur, B. M.; Bryan, W. W.; Kim, J.; Lee, T. R. Ultrasmall Hollow Gold– Silver Nanoshells with Extinctions Strongly Red-Shifted to the Near-Infrared. ACS Appl. Mater. Interfaces 2011, 3, 3616–3624.

(42)

Kim, J.; Bryan, W. W.; Lee, T. R. Preparation, Characterization, and Optical Properties of Gold, Silver, and Gold-Silver Alloy Nanoshells Having Silica Cores. Langmuir 2008, 24, 11147–11152.

(43)

Wojnicki, M.; Rudnik, E.; Luty-Błocho, M.; Pacławski, K.; Fitzner, K. Kinetic Studies of Gold(III) Chloride Complex Reduction and Solid Phase Precipitation in Acidic Aqueous


28

Page 29 of 32 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


System Using Dimethylamine Borane as Reducing Agent. Hydrometallurgy 2012, 127– 128, 43–53. (44)

Paclawski, K.; Fitzner, K. Kinetics of Gold ( III ) Chloride Complex Reduction Using Sulfur ( IV ). Metall. Trans. B 2004, 35, 1071–1085.

(45)

Bjerrum, N. La Stabilith Dee Chlorures d’or. Bull. Sco. Chim. Belg 1948, 57, 432–445.

(46)

Jing, H.; Wang, H. Structural Evolution of Ag-Pd Bimetallic Nanoparticles through Controlled Galvanic Replacement: Effects of Mild Reducing Agents. Chem. Mater. 2015, 27, 2172–2180.

(47)

Park, J.-E.; Kim, S.; Son, J.; Lee, Y.; Nam, J.-M. Highly Controlled Synthesis and SuperRadiant Photoluminescence of Plasmonic Cube-in-Cube Nanoparticles. Nano Lett. 2016, 16, 7962–7967.

(48)

Yang, Y.; Liu, J.; Fu, Z.; Qin, D. Galvanic Replacement-Free Deposition of Au on Ag for Core − Shell Nanocubes with Enhanced Chemical Stability and SERS Activity. J. Am. Chem. Soc. 2014, 136, 8153–8156.

(49)

Sun, X.; Yang, Y.; Zhang, Z.; Qin, D. Mechanistic Roles of Hydroxide in Controlling the Deposition of Gold on Colloidal Silver Nanocrystals. Chem. Mater. 2017, 29, 4014–4021.

(50)

Watts, J. F.; Wolstenholme, J. Front Matter. In An Introduction to Surface Analysis by XPS and AES; 2005.

(51)

Tanuma, S.; Powell, C. J.; Penn, D. R. Calculations of Electron Inelastic Mean Free Paths. Surf. Interface Anal. 1991, 17, 911–926.


29


(52)

Page 30 of 32

Tyson, C. C.; Bzowski, A.; Kristof, P.; Kuhn, M.; Sammynaiken, R.; Sham, T. K. Charge Redistribution in Au-Ag Alloys from a Local Perspective. Phys. Rev. B 1992, 45, 8924.

(53)

Watson, R. E.; Hudis, J.; Perlman, M. L. Charge Flow and d Compensation in Gold Alloys. Phys. Rev. B 1971, 4, 4139–4144.

(54)

Srnová-Šloufová, I.; Vlčková, B.; Bastl, Z.; Hasslett, T. L. Bimetallic (Ag)Au Nanoparticles Prepared by the Seed Growth Method: Two-Dimensional Assembling, Characterization by Energy Dispersive X-Ray Analysis, X-Ray Photoelectron Spectroscopy, and Surface Enhanced Raman Spectroscopy, and Proposed Mechanism of Grow. Langmuir 2004, 20, 3407–3415.

(55)

Zhang, G.; Du, M.; Li, Q.; Li, X.; Huang, J.; Jiang, X. Green Synthesis of Au – Ag Alloy Nanoparticles Using Cacumen Platycladi Extract. RSC Adv. 2013, 3, 1878–1884.

(56)

Gilroy, K. D.; Ruditskiy, A.; Peng, H. C.; Qin, D.; Xia, Y. Bimetallic Nanocrystals: Syntheses, Properties, and Applications. Chem. Rev. 2016, 116, 10414–10472.

(57)

Smith, J. G.; Yang, Q.; Jain, P. K. Identification of a Critical Intermediate in Galvanic Exchange Reactions by Single-Nanoparticle-Resolved Kinetics. Angew. Chemie Int. Ed. 2014, 53, 2867–2872.

(58)

Park, Y.; Lee, C.; Ryu, S.; Song, H. Ex Situ and in Situ Surface Plasmon Monitoring of Temperature-Dependent Structural Evolution in Galvanic Replacement Reactions at a Single-Particle Level. J. Phys. Chem. C 2015, 119, 20125–20135.

(59)

Sutter, E.; Jungjohann, K.; Bliznakov, S.; Courty, A.; Maisonhaute, E.; Tenney, S.; Sutter, P. In Situ Liquid-Cell Electron Microscopy of Silver–Palladium Galvanic Replacement


30

Page 31 of 32 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


Reactions on Silver Nanoparticles. Nat. Commun. 2014, 5, 4946. (60)

Sutter, E. A.; Sutter, P. W. In Situ Liquid Cell Electron Microscopy of Ag–Au Galvanic Replacement Reactions. Nanoscale 2017, 9, 1271–1278.

(61)

Tan, S. F.; Lin, G.; Bosman, M.; Mirsaidov, U.; Nijhuis, C. A. Real-Time Dynamics of Galvanic Replacement Reactions of Silver Nanocubes and Au Studied by Liquid-Cell Transmission Electron Microscopy. ACS Nano 2016, 10, 7689–7695.


31


Page 32 of 32

TOC Graphic


32

Gold Speciation and Co-Reduction Control the Morphology of AgAu

Recommend Documents