Unravelling the Mechanisms of GoldâSilver CoreâShell Nanostructure

Subscriber access provided by Kaohsiung Medical University

Communication

Unravelling the mechanisms of gold-silver core-shell nanostructure formation by in situ TEM using an advanced liquid cell design Andreas Hutzler, Tilo Schmutzler, Michael P.M. Jank, Robert Branscheid, Tobias Unruh, Erdmann Spiecker, and Lothar Frey Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03388 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 22, 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 27 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

Nano Letters

Unravelling the mechanisms of gold-silver coreshell nanostructure formation by in situ TEM using an advanced liquid cell design Andreas Hutzler‡,*, Tilo Schmutzler§, Michael P. M. Jank⊥ , Robert Branscheid†, Tobias Unruh§, Erdmann Spiecker†, and Lothar Frey‡,⊥ ‡

Chair of Electron Devices (LEB), Department of Electrical, Electronic and Communication

Engineering, Friedrich-Alexander University Erlangen-Nürnberg (FAU), Cauerstraße 6, 91058 Erlangen, Germany §

Institute for Crystallography and Structural Physics, Department of Physics, Friedrich-

Alexander University Erlangen-Nürnberg (FAU), Staudtstraße 3, 91058 Erlangen, Germany ⊥

Fraunhofer Institute for Integrated Systems and Device Technology IISB, Schottkystraße 10, 91058 Erlangen, Germany

†

Institute of Micro- and Nanostructure Research (IMN) & Center for Nanoanalysis and Electron

Microscopy (CENEM), Department of Materials Science and Engineering, Friedrich-Alexander University Erlangen-Nürnberg (FAU), Cauerstraße 6, 91058 Erlangen, Germany KEYWORDS Liquid cell TEM, Ag@Au, core-shell, electron beam induced growth, plasmonic structures, graphene

ACS Paragon Plus Environment

1

Nano Letters 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 27

ABSTRACT

The growth of silver shells on gold nanorods is investigated by in situ liquid cell transmission electron microscopy using an advanced liquid cell architecture. The design is based on microwells in which the liquid is confined between a thin Si3N4 membrane on one side and a few-layer graphene cap on the other side. A well-defined specimen thickness and an ultraflat cell top allow for the application of high-resolution TEM and the application of analytical TEM techniques on the same sample. The combination of high-resolution data with chemical information is validated by radically new insights into the growth of silver shells on CTAB stabilized AuNRs. It is shown that AgBr particles already formed in the stock solution play the important role in the exchange of silver ions. The Ag shell growth can be directly correlated with the layer-by-layer dissolution of AgBr nanocrystals which can be controlled by the electron flux density via distinctly generated chemical species in the solvent. The derived model framework is confirmed by in situ UV-Vis absorption spectroscopy evaluating the blue shift in the longitudinal surface plasmon resonance of anisotropic NRs in a complementary batch experiment.


2

Page 3 of 27 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

Nano Letters

TEXT An extensive understanding of the growth processes involved in the formation of noble metal core-shell nanostructures is essential for controlling their plasmonic characteristics which are strongly size and shape dependent.1,2 As an example, the plasmon resonance frequency of silvercoated gold nanorods (AuNR) can be tuned over a wide range of the visible spectrum by adjustment of the shell thickness.3 For a microscopic understanding of the growth dynamics, advanced characterization techniques are needed. In situ liquid cell transmission electron microscopy (LCTEM) is a highly attractive method for direct study of processes in liquid solutions with high spatial and temporal resolution.4 Recent applications range from cell biology5–9 and energy storage10–12 until the realization of advanced nanomaterials like complex or hybrid gold13–16 and silver-based17,18 nanoparticles. A very early concept of a liquid cell for electron microscopy was already presented in 1944 by Abrams and McBain.19 However, the field gained significant momentum only after the implementation of a vacuum compatible liquid cell by Williamson et al.20 in 2003 using thin-film and bulk micromachining fabrication processes. In their approach, a thin film of the specimen solution is enclosed between two electron-transparent membranes to avoid vaporization within the vacuum inside the electron microscope. The thickness of the liquid film is defined by distinct spacer features that are customized to the particular application. This concept was subsequently adapted to conventional TEM specimen holders for application in materials science by Zheng et al.21 and specialized fluid-flow TEM holder systems for biological applications by de Jonge et al.6. Since then, different liquid cell designs have been developed for specific experimental issues22–26. As membrane material amorphous silicon nitride (Si3N4) is utilized in most cases because very thin films (down to 10 nm in thickness27) can be processed using conventional semiconductor technologies like low


3

Nano Letters 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 4 of 27

pressure chemical vapor deposition (LPCVD). In general, liquid cells suffer from the fact that the lateral dimensions of the electron transparent windows have to be rather small and the membranes have to be thick enough to reduce window bulging caused by the pressure difference between the inside of the liquid cell and the ultra-high vacuum surroundings.28,29 Moreover, since the membranes are held by thick silicon frames, shadowing effects arising from {111} edges limit the applicability of energy dispersive X-ray spectroscopy (EDXS) and electron tomography for chemical analysis or 3D studies, respectively. In order to overcome these limitations, Yuk et al.30 implemented the graphene-based liquid cell (GLC) which utilizes graphene for liquid encapsulation. The high mechanical strength and elasticity of graphene enables its application as ultra-thin (down to one atomic layer) window material. Additionally, the low nuclear charge of carbon reduces the scattering background compared to silicon nitride enhancing the achievable resolution.28 Furthermore, the electrical conductivity of graphene decreases charging effects generated by electron-beam irradiation. The drawback of Yuk’s approach, however, is the bad controllability of the final thickness of the liquid film between the graphene membranes.31 Due to the absence of a distinct spacer, the liquid enclosure has a curved shape with the maximum thickness of the droplet being defined by the wettability of the base layer and the volume of the specimen.32 Furthermore, the success rate has been reported to be as low as of 25%-50%.31 A recent development of Kelly et al. addresses the problem of controlling the thickness of the liquid by introducing a defined spacer layer of hBN between the graphene membranes for liquid cell TEM.33,34 Furthermore, microchannel arrays have been demonstrated to be suitable for enclosing liquids in the SEM by Yualev et al.35,34 Furthermore, Dahmke et al. presented a liquid cell architecture where biological specimen are encapsulated on a silicon nitride window by a graphene cover layer.36,34


4

Page 5 of 27 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

Nano Letters

In this letter we introduce the novel concept of a graphene-supported microwell liquid cell (Fig. 1), which combines the advantages of both, the silicon-based and the graphene-based architectures.37,38 It features thin structured silicon nitride wells with their bottom acting as a membrane. The sheet-style well array is supported by a robust silicon frame which strongly increases the success rate of preparation to values above 75%. The liquid is enclosed in the individual microwells by an additional graphene membrane covering the entire array structure. This architecture provides excellent properties for correlative electron microscopy methods as it can be utilized with various TEM holders. The graphene membrane supports an operation of transmission electron microscopy (TEM) in high resolution (HRTEM) if shallow wells are utilized. A combination of scanning transmission electron microscopy (STEM) with analytical techniques like energy dispersive X-ray spectroscopy (EDXS) is enabled by the flat topography of the upper side of the liquid cell. In contrast to conventional static cells, shadowing effects39,40 originating from silicon edges or the specimen holder have not to be taken into account. This enables the exposure of small angle detectors or large angular windows for in situ tomography. Furthermore, as a consequence of the liquid being enclosed in microwells, the viewing area is extremely enlarged to several square millimeters without accompanying bulging effects. The outlined advantages are exploited for LCTEM in order to gain insights into the dynamics of the growth of silver (Ag) shells from aqueous silver nitrate (AgNO3) solution onto cetrimonium bromide (CTAB) stabilized gold nanorods (AuNR). In parallel, the chemical composition of the grown nanostructures is studied by EDXS. The observed microscopic behavior of the Au-Ag core-shell nanostructures is correlated with the macroscopic kinetics deduced in native solution by complementary in situ ultraviolet-visible (UV-vis) absorption spectroscopy, leading to a comprehensive, multi-scale description of the observed mechanisms.


5

Nano Letters 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 6 of 27

Figure 1: Schematical structure of the graphene-supported microwell liquid cell. LCTEM investigations were conducted using a Philips CM 30 (S)TEM and a double corrected FEI Titan³ Themis 80-300 (S)TEM equipped with a Super-X EDXS detector, both operated at 300 kV. AuNRs have been prepared using the seed-mediated growth procedure41 and is described in detail in the supporting information. An aqueous dispersion of AuNRs stabilized with 1 mmol/L CTAB is mixed with silver nitrate (AgNO3) to yield final salt concentrations from 1 mmol/L to 10 mmol/L. This specimen solution is then filled into liquid cells which are mounted in an EDXS optimized Fishione cryo transfer holder and inserted into the electron microscope within a time span of about five minutes. Experimental details on filling of the liquid cell can be taken from supporting information as well (Fig. S1). For the study of growth processes, the reduction of the silver ions during LCTEM is controlled in situ using solvated electrons (and also hydrogen radicals) which are generated intrinsically by radiolysis of the aqueous solution during electron beam irradiation42. For complementary in situ UV-Vis spectroscopy, a TIDAS S 500K spectrometer is utilized and the reduction of silver ions is conducted by adding ascorbic acid to the specimen solution.


6

Page 7 of 27

The analysis of the stock solution and the dynamics of shell growth under constant electron irradiation at approx. 105 e-nm-2s-1 are depicted in Figs. 2 and 3. Besides the gold nanorods, LCTEM of the initial specimen (i.e. at a very initial stage of the observation) directly reveals precipitates of silver bromide (AgBr) (cf. Fig. 2a, Fig. 3) which will later be shown to play an important role concerning the occurring reaction mechanisms. Apparently the bromide ions must originate from CTAB which is used as stabilizer of the AuNRs. Kinetic simulations of the chemical processes induced by radiolysis were performed using the model of Schneider et al. (Fig. S7 SI).42 These simulations indicate an oxidizing atmosphere as large amounts of oxidizing species like hydroxide radicals and hydrogen peroxide are present. However, dissolved silver ions get reduced into elementary silver above dose rates of about 10-4 e-nm-2s-1 while high concentrations of bromide ions are present at each dose rate. This already indicates a deficit of dissolved silver ions in the specimen solution which will be discussed in the following. Figure 2a shows an image series of heteroepitaxial growth of a silver shell on a single AuNR in the vicinity of a silver bromide particle. a)

l

AgBr

Ag

r

EDXS

Au t0 + 7.3 min c) 20

30

Au Ag Br

20 10

t0 + 25.7 min

10 nm

d) 10

increase

15

t0 + 35.3 min

dAg,t / dAg,l

b) 50 40

t0 + 15.6 min

reff,Ag-shell (nm)

t0 atom percent (%)

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

Nano Letters

10

5

saturation

1

0 0 10 20 30 40 position in scan direction (nm)

0.1

1 10 time (min)

0.1

1 10 time (min)


7

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

Page 8 of 27

Figure 2: a) LCTEM bright field image series of the growth of a Ag-shell on a AuNR, b) post mortem EDXS line scan across the core-shell structure and c) evolution of the equivalent radii describing the overall volume of Ag plotted over time of eight different Au-Ag core-shell nanostructures (see Fig. S2 SI for BF TEM micrographs) and d) aspect ratio (thickness over length) of the Ag-shell over time. AgNO3 concentration was 1 mmol/L in the case of a) and 10 mmol/L in the case of c) and d) and the electron dose rate during imaging was adjusted between 2·104 e-nm-2s-1 and 3·105 e-nm-2s-1.

The nucleation of the Ag shell on the AuNR starts at the position marked by a red circle in Figure 2a (see in situ movie 1 of the growth process in SI). The fact that the growth starts at the opposite side of the AuNR with respect to the AgBr particle indicates that the growth process takes place by reduction of free Ag ions out of the solution and not directly by reduction of AgBr. The whole shell growth takes about 35 minutes. After 25 minutes a layer-by-layer dissolution of the adjacent AgBr particle is observed (indicated by a white, dashed line in Fig. 2a, image 4) and can be directly correlated with preferential growth of the Ag shell in the direction of the AgBr particle. Owing to our liquid cell design, the layer-by-layer dissolution can be monitored in situ at atomic scale using HRTEM, as discussed in more detail below (Fig. 3). The dissolution of the AgBr particle and the associated growth of the Ag shell get accelerated once the AgBr particle shrinks below a critical size at which the particle becomes unstable providing more dissolved silver ions per time, in accordance with classical nucleation theory.43 As soon as the AgBr particle is completely dissolved, the growth of the Ag shell stops as well indicating depletion of free silver ions in the proximity of the nanorod. After the growth process and subsequent drying, an EDXS line scan across the grown nanostructure was collected (Fig 2a,


8

Page 9 of 27 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

Nano Letters

red line in the last image). Figure 2b shows the corresponding atomic compositions, calculated from EDX spectra of the core-shell particle. The shell around the AuNR consists of pure Ag, the Ag signal in the Au core region can be explained by a projection effect. The small Br signal can be attributed to the presence of bromide adsorbed on the particle surface. The slight increase of the Br signal in the region of the core is likely due to a fluorescence effect. In order to reveal the kinetics of the growth of the Ag-shells, the growth exponent was determined, which can be extracted from the evolution of the particle radius over time. 44–46 For this purpose both, the longitudinal as well as the transversal thickness of the Ag-shell, were measured separately. For illustration of the growth kinetics, effective radii were determined by calculating the radius of an Ag-nanoparticle with a volume equivalent to the volume of the silver shell derived from subtraction of the AuNR volume from the overall nanoparticle volume. This normalization is necessary in order to eliminate size and shape differences and to ensure the comparability of the anisotropic growth processes with results obtained from complementary UV-Vis absorption spectroscopy measurements. Furthermore, this analytical approach is chosen as a facile approximation in order to compare the obtained results to results from other growth experiments reported in literature.21,45,47 However, for a more precise evaluation the knowledge of the exact, time dependent geometry as well as the surface energy of each crystal facet would be required. Figure 2c shows the evolution of the effective radii for eight different core-shell nanostructures studied by in situ LCTEM (Fig. S2, see also Fig. S3 SI for further micrographs of core shell nanostructures). Wagner showed in 1961 that reaction and diffusion-limited growth of particles both can be approximated with a power law dependence r(t)  tβ of the particle radius r as function of growth time t with the growth exponent β being characteristic for the growth mode44. A β of 1/3 points towards diffusion-limited growth whereas ½ characterizes reaction-


9

Nano Letters 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 10 of 27

limited growth. Previous studies on the growth of silver45 and zinc oxide46 nanoparticles from precursor solutions by in situ (T)EM have shown that this value corresponds to a reaction that can be controlled by variation of the beam currents. Diffusion-limited growth can preferentially be observed at high densities of the beam current, whereas low currents effect reaction-limited growth behavior.45,46 From the slope of the double-logarithmic plot, β was determined to 0.11±0.05. The measured growth exponent is exceptionally small compared to the expected value of 1/3 for purely diffusion limited growth processes.44–46,48,49 However, it is in excellent agreement with the observations for electron beam induced silver nanocrystal growth from 1 mmol/L AgNO3solution by Woehl et al. who suggested a suppressed diffusion limited growth process with growth exponents in the range of 1/8 for high beam current conditions. The authors concluded that the deviation from the ideal behavior could be attributed to the vicinity to the membrane affecting the reaction kinetics at the particle surface. Accordingly, the conditions differ from the condition of a free solution.45 In our case the slow growth can be attributed to the weak availability of silver ions which are needed for the growth of the shell. Although the concentration of AgNO3 added into the specimen solution is relatively high, the overall amount of silver ions during observation is low because of the precipitation into AgBr which already occurs during the preparation of the specimen solution before imaging. Due to the solubility product of AgBr being as low as 5.32·10-13 mol2/L2, the resulting concentration of silver ions in solution is only 0.73 µmol/L. I.e. the vast majority of silver ions is bound in the AgBr nanoparticles. In order to promote shell growth these silver ions have to dissolve first. Another limiting factor is the low liquid thickness which is assumed to be on the order of below 100 nanometers. It is known from various liquid cell experiments13,28,50,51 that the viscosity of the


10

Page 11 of 27 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

Nano Letters

specimen solution increases with decreasing thickness of the liquid. Hence, for thin liquid films as in our case, the viscosity can get as large as 106 Pa·s which is a billion times larger than expected for bulk water at room temperature. This decreases the diffusivity of precursor ions further slowing down the growth rate. Finally, competing oxidation reactions can slow down the overall reduction rate.52 Figure 2d shows the evolution of the ratio of transversal (radial) to longitudinal thickness of the Ag-shell with time. It is obvious that the Ag-shell grows faster in radial direction. This is a direct consequence of the interaction with the AgBr particle. Whereas dissolved Ag⁺ ions are the source for growth of the shell, Br⁻ ions support the thermodynamic stability of distinct crystallographic facets making them more stable, i.e. less prone to reacting with silver ions.53,54 This leads to an anisotropic underpotential deposition (UPD) of silver on gold nanorods.53–55 Furthermore, the silver shell partially exhibits well-defined crystal facets (Figs. S2 and S3 SI) which were also observed in studies of seed-mediated Ag growth by Gómez-Graña et al53. They revealed a favored growth in direction and attributed this to silver-halide adsorbates which preferentially reduce the surface energy of distinct crystallographic orientations leading to structures mainly showing {100} and {110} facets. In the present work this hypothesis is supported by the shape of a common Ag shell growing around two adjacent gold nanoparticles (Fig. S3a and in situ movie 4). The surface area of the overall nanostructure is minimized by preferential growth at topological discontinuities. The pendentive at the contact between the tip of the AuNR and the surface of the AuNP is filled first to yield a maximum reduction of the surface area. Figures S3d-f show high-resolution TEM micrographs and the corresponding diffractograms of two individual AuNRs and an AuNR with a Ag shell approving a heteroepitaxial growth along the direction. Due to the anisotropic growth process, the


11

Nano Letters 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 27

overall deposition rate of silver is reduced. In some cases, the growth of the Ag-shell ran into saturation. This has already been described in detail for the particle in Figure 2a and is observed for other particles as well by the slope of the curves in Figure 2c approaching zero (e.g. red, green and blue data points). The presence of AgBr particles affects the individual growth rate by providing Ag ions which can be reduced. Furthermore, these silver atoms either nucleate to single silver nanoparticles (Fig. S2 SI, arrows) or feed the growth of the Ag shell at the surface of the AuNR (in situ movie 3 in the SI). a)

t0

t0 + 0.25 s

t0 + 0.5 s

t0 + 1 s

t0 + 3.5 s

t0 + 3.75 s

t0 + 4 s

AgBr 5 nm t0 + 3.25 s

b)

t0 + 61 s

t0 + 81 s

t0 + 191 s

t0 + 331 s

t0 + 371 s

AgBr

Ag

Ag

5 nm


12

Page 13 of 27

sum volume AgBr platelet left AgNP right AgNP

2.0 1.5

e) 50

d) AgBr Ag

1.0 0.5

Ag

atom percent (%)

volume (10³ nm³)

c) 2.5

Au

Ag

Br

40 30 20 10 0

0.0 0

2

4 6 time (min)

8

10 50 nm

0 20 40 60 80 100 position in scan direction (nm)

800

5.0 2.5

700

0.0

600

-2.5

500

-5.0

400

irradiated area (nm²) at constant current

f) 7.5

etching / growth (nm²)

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

Nano Letters

-7.5 0

1

2 3 4 time (min)

5

6

Figure 3: a) layer-by-layer dissolution of AgBr, b) beam induced non-equilibrium reactions between AgBr and dissolved ions, c) volume change of the particles over time, d) STEMHAADF micrograph of an AgBr particle with Ag particles at its rim, e) post mortem EDXS line scan of the shown particle and f) layer-by-layer dissolution of AgBr related to the process shown in a). By application of in situ HRTEM with the graphene-supported microwell liquid cell, the layerby-layer dissolution of AgBr particles can be studied in more detail. The process itself was observed mainly at high dose rates between 104 e-nm-2s-1 and 106 e-nm-2s-1. The blue arrows in Figure 3a indicate the individual atomic planes that are dissolved at the edge of the AgBr particle (see in situ movie 2 of the growth process in SI). It was not possible to resolve whether this dissolution proceeds atom-by-atom or molecule-by-molecule. Nevertheless, for different observations of this particular process it could be shown that dissolution is strongly dependent on changes in the electron dose rate and can even be reversed by variation of the current density.


13

Nano Letters 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 14 of 27

Although the obtained resolution of approx. 2 Å indicates that the liquid film around the AgBr particle must be rather thin, the reversibility of the process provides clear evidence for the presence of a liquid film facilitating ion exchange reactions. In the experiment shown in Figure 3f the beam current is held constant while the beam diameter and thus the dose rate per area is varied (green data). The actual growth rates were evaluated by comparison of the projected area of the AgBr particle in two subsequent images. The growth of single atomic planes could be observed under an edge-on condition, flickering in both directions until the dose rate exceeds a certain point, where only dissolution of the AgBr particle takes place. If the particle size is above a certain threshold giving the particle stability, the dissolution can be stopped and even be reversed by lowering the current density. In that case the surface steps of the AgBr particle act as nucleation sites and single atomic rows grow again (peaks in growth regime of blue data). We explain the phenomenon of AgBr dissolution at higher dose rates, i.e. above 104 e-nm-²s-1 as follows: electron-hole pairs are generated in AgBr by inelastic scattering of primary electrons. Electrons in the conduction band of AgBr which are known to cause direct reduction of silver ions to silver atoms, are compensated by large amounts of oxidative species. Thus, the slow process of dissolution caused by a disruption of the solution equilibrium of AgBr and its solvated ions is favored. This is not the case at dose rates below 104 e-nm-²s-1, where the compensation of reducing electrons in the conduction band of AgBr is less efficient as less oxidative species are present. The consequence is that free silver ions originating from dissolving AgBr are reduced and thus neutralized to silver atoms by hydrated electrons generated by electron beam irradiation of the aqueous solution56.. When the reduction rate decreases by spreading the beam, the solution immediately supersaturates with Ag ions and AgBr precipitation occurs. A random sequence of such forward and backward reactions can be


14

Page 15 of 27 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

Nano Letters

observed in situ near the equilibrium state, even when keeping the dose rate at a constant level. Figure 3b shows the interplay of a AgBr particle and attached silver patches over a time frame of 6 min. The depletion of silver patches is guided by the oxidation of silver atoms yielding the formation of additive AgBr. The subsequent re-growth of patches is driven by silver ions originating from AgBr that are reduced to pure silver again. While the relative volumes of the Ag and AgBr particles may change, the cumulative volume of all particles remains relatively constant (Fig. 3c). A post mortem EDXS line scan was performed over the nanostructure shown in the STEM micrograph in Figure 3d (red line). The atomic compositions calculated from the EDX spectra are shown in Figure 3e. This gives evidence that the main particle consists of AgBr with the Ag and Br signal running conformal. The precipitates at the rim of the AgBr particle consist of pure Ag. The observed rivaling redox reactions are possible only because of the reversible reaction between AgBr and its dissolved ions and consequently reduce the effective growth rate of the silver shell on AuNRs. Furthermore, the narrow channel confinement of the liquid solution is supposed to increase the viscosity of the surrounding fluid and can reduce diffusivities, i.e. ion mobilities.48,57,50 Additional factors influencing ion mobility could be electric fields evolving from accumulated secondary and Auger electrons. However, this effect is assumed to be suppressed at least under TEM illumination conditions because of the utilization of an electrically conducting graphene membrane which is assumed to dissipate free electrons.58 Besides the aforementioned chemical reactions, another possible mechanism could be the formation of elementary silver and bromine molecules from silver bromide that is also known to happen under vacuum or dried conditions e.g. in photographic dispersions59. The latter reaction can be observed when examining the solids content of the solution after drying on a conventional TEM grid (Fig. S4 SI). In this case, the desiccation-induced formation of micrometer sized AgBr


15

Nano Letters

platelets as well as AgBr wires (Fig. S4a,b SI) can be observed which react into polydisperse, partially anisotropic pure silver particles (Fig. S4c SI) under electron beam irradiation. However, this process is very fast (< 1 min), irreversible and is thus not a viable explanation for our observations. In fact, primary electrons penetrating AgBr generate electron-hole pairs due to inelastic scattering which would cause reduction of silver ions. However, this process is believed to be strongly suppressed in solution when large amounts of scavengers are present. Most likely, oxidative species, like hydroxide radicals, capture electrons from the conduction band of AgBr57 at high dose rates that would be needed for the direct reduction of silver ions via irradiation. This in turn compensates oxidative species and thus a reduction of dissolved silver ions is favored. Furthermore, small AgBr particles are an indicator for precipitation from a supersaturated aqueous solution. This gives further evidence for the ion-exchange reactions being enabled by a

a) 100

-1

60 s 300 s 523.5 s 868.5 s 946.5 s 1800 s

AuNR (nm)

10

LSPR

c)

b) 750

ex situ

10-2 300 400 500 600 700 800 wavelength (nm)

sample 1 2 3 4

700 650 600 550 0

5

10 15 20 time (min)

25

30

Ag-shell thickness (nm)

surrounding liquid phase.

absorption

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 16 of 27

10 8 6

averaged data power law fit

4  = 0.45 2 1 10-1

 = 0.05 100 101 time (min)

102

Figure 4: UV-Vis absorption spectra of the time-dependent shift of the LSPR of silver-coated AuNRs to smaller wavelength with increasing silver-shell thickness, b) temporal evolution of the LSPR absorption wavelength of four samples and c) the calculated and averaged evolution of the silver shell thickness calculated from b). Complementary UV-Vis absorption spectroscopy was applied to appraise the nanoscale growth kinetics of the silver-shell formation on individual AuNRs observed by LCTEM in the light of the macroscale kinetics averaged over a large particle ensemble. The reduction of Ag ions in the


16

Page 17 of 27 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

Nano Letters

batch is induced by ascorbic acid in complementary experiments. Although the reactions examined by UV-Vis absorption spectroscopy are different from the reactions observed by LCTEM, some parallels can be drawn in terms of the radiation sensitivity of AgBr which forms in both cases. Pristine AuNRs show two absorption bands in the UV-Vis-NIR range that are related to the transversal (TSPR, at around 520 nm) and longitudinal surface plasmon absorption resonance (LSPR, above 700 nm).60,41 The LSPR absorption band is the more intense one and is sensitive to the aspect ratio of the AuNRs.61 Coating the AuNRs with a Ag shell leads to a blueshift of the LSPR depending on the thickness of the silver shell.3 The exact position of the LSPR is used to analyze the evolution of the thickness of the silver shell in situ (c.f. Fig. S6 SI) which is exemplarily depicted in Figure 4a-c. Ex situ UV-Vis absorption spectroscopy and TEM were used to calibrate the LSPR maxima against the thickness of the silver-shell (Fig. S6 SI). An additional strong absorption band at around 400 nm is related to pure Ag particles or AuNRs with silver shell thicknesses larger than 11 nm (Fig. S6 SI). Obviously, the shift of the AuNRLSPR towards smaller wavelengths is accompanied by a decreasing absorption at 400 nm. The growth of the silver shells can thus be directly correlated to a dissolution of AgNPs. The effect of sample illumination is discussed in the light of the electron-beam interaction with AgBr particles described above (Figure 3b) for in situ TEM. In a first reaction step AgBr precipitates directly transform into silver particles via reduction of Ag ions by light-generated electrons in the AgBr conduction band. This leads to the formation of the absorption band at 400 nm in the UV-Vis absorption spectrum. Subsequently, Ag atoms from the particles are oxidized and dissolved in the liquid environment providing silver ions as source for the shell formation. It is noteworthy that we recognized this mechanism being also influenced by illumination with light (Fig. S6 SI). A thinner silver shell was observed on AuNRs that were


17

Nano Letters 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 27

exposed to the light source of the UV-Vis absorption spectrometer when compared to samples that were stored in the dark during the reaction. Regarding the results shown in Figure 3 it is assumed that this behavior is related to the illumination dependent transformation of silver bromide into silver and vice versa. Depending on the time we observed distinct phases with a change of the growth rate of the shell (Fig. 4c) in between. The growth exponent of 0.05 within the first phase is limited by the dissolution of AgBr particles after consumption of dissolved silver ions leading to a growth exponent of 0.45. After consumption of the free silver particles the sudden dissolution of AgBr particles with a size smaller than the critical size releases new silver ions and leads to an increase of the thickness of the silver shell which is consistent with LCTEM observations. Also, the macroscopic growth rates obtained from UV-Vis spectroscopy are comparable to the growth rates of silver-shells analyzed by LCTEM (Fig. 2). Nevertheless, a direct comparison is not possible since UV-Vis spectroscopy cannot be used to distinguish between longitudinal and transversal growth of the silver shell separately. In this letter, we introduce a hybrid liquid cell design, with two different electron transparent membranes, being capable for correlative and thus highly comprehensive studies into complex growth mechanisms of bimetallic nanostructures. With this advanced liquid cell architecture an extensive study into the growth mechanisms of Ag shells from AgNO3 precursor solution on CTAB stabilized AuNRs is conducted. It is shown that AgBr nanoparticles precipitate due to the presence of free Ag+ ions originating from AgNO3 and free Br- ions originating from CTAB. The reduction of solvated Ag+ ions was conducted utilizing solvated electrons which are generated in situ via radiolysis of the aqueous solution during LCTEM electron interaction. This caused nucleation of silver atoms at the surface of AuNRs and lead to underpotential deposition of Ag shells. Chemical analysis of the grown nanostructures was conducted by high yield EDXS which


18

Page 19 of 27 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

Nano Letters

is enabled by the unique architecture of the graphene-supported microwell liquid cell. The observed growth model of silver-shells is confirmed in a complementary batch-type experiment by a blue shift of the LSPR measured by complementary in situ UV-Vis absorption spectroscopy. LCTEM further shows that heteroepitaxial growth of Ag shells on AuNRs is faster along the transversal axis of the AuNRs compared to the longitudinal axis. This is due to bromide ions reducing the surface energy of distinct crystal orientations which favors a growth along direction. A fundamental difference between an initiation of the reaction by illumination and by electron beam irradiation is found to be the direct reduction of AgBr by light. In case of electron beam irradiation in liquid this reduction proceeds from solution only causing a subsequent dissolution of AgBr into its solved ion pair. Under light illumination the AgBr particles are transferred into pure Ag particles first with a subsequent oxidation of the silver atoms by dissolved bromide ions. The overall reaction route is graphically depicted in Figure 5. Various in situ TEM techniques were shown to be applicable on the same sample utilizing the novel liquid cell architecture. This makes it highly suitable for addressing further advanced questions in material science. With applications in e.g. biology being conceivable as well this versatile cell design can go far beyond.


19

Nano Letters 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 20 of 27

Figure 5: Reaction route of the formation of Ag shells on AuNRs in dependence of external stimuli (visible light and electron irradiation). ASSOCIATED CONTENT Supporting Information In situ movies showing growth of Ag shell on AuNRs (movie 1,3 and 4) and of layer-by-layer dissolution of AgBr (movie 2), description of processes used for liquid cell fabrication and preparation for LCTEM, supplementary TEM micrographs (color code corresponding to Figure 2c and 2d), supplementary overview TEM micrographs and HRTEM micrographs of Ag@Au core shell particles, AuNRs, additional data for UV-Vis calibration and measurement method, results of kinetic simulations of radiolysis chemistry AUTHOR INFORMATION Corresponding Author *E-mail: (A.H.) [email protected]


20

Page 21 of 27 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

Nano Letters

Present Addresses Author Contributions Funding Sources Notes ACKNOWLEDGMENT Financial support by the DFG via the Research Training Group GRK1896 "In situ microscopy with electrons, X-rays and scanning probes" and through the Cluster of Excellence “Engineering of Advanced Materials” (EAM) is gratefully acknowledged. ABBREVIATIONS REFERENCES 1. Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107 (3), 668– 677, DOI:10.1021/jp026731y. 2. Jorge Pérez-Juste; Isabel Pastoriza-Santos; Luis M. Liz-Marzán; Paul Mulvaney. Coord. Chem. Rev. 2005, 249 (17-18), 1870–1901, DOI:10.1016/j.ccr.2005.01.030. 3. Becker, J.; Zins, I.; Jakab, A.; Khalavka, Y.; Schubert, O.; Sönnichsen, C. Nano Lett. 2008, 8 (6), 1719–1723, DOI:10.1021/nl080720k. 4.

Zheng,

H.;

Meng,

Y.

S.;

Zhu,

Y.

MRS

Bull.

2015,

40

(01),

12–18,

DOI:10.1557/mrs.2014.305. 5. de Jonge, N.; Pfaff, M.; Peckys, D. B. Adv. Imag. Elect. Phys. 2014, 186, 1–37, DOI:10.1016/b978-0-12-800264-3.00001-0.


21

Nano Letters 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 22 of 27

6. de Jonge, N.; Peckys, D. B.; Kremers, G. J.; Piston, D. W. Proc. Natl. Acad. Sci. U.S.A. 2009, 106 (7), 2159–2164, DOI:10.1073/pnas.0809567106. 7. Peckys, D. B.; de Jonge, N. Nano Lett. 2011, 11 (4), 1733–1738, DOI:10.1021/nl200285r. 8. Ring, E. A.; Peckys, D. B.; Dukes, M. J.; Baudoin, J. P.; de Jonge, N. J. Microsc. 2011, 243 (3), 273–283, DOI:10.1111/j.1365-2818.2011.03501.x. 9. Peckys, D. B.; de Jonge, N. Microsc. Microanal. 2014, 20 (2), 346–365, DOI:10.1017/s1431927614000099. 10. White, E. R.; Singer, S. B.; Augustyn, V.; Hubbard, W. A.; Mecklenburg, M.; Dunn, B.; Regan, B. C. ACS Nano 2012, 6 (7), 6308–6317, DOI:10.1021/nn3017469. 11. Sun, M.; Liao, H.-G.; Niu, K.; Zheng, H. Sci. Rep. 2013, 3 (3227), 1–6, DOI:10.1038/srep03227. 12. Holtz, M. E.; Yu, Y.; Gunceler, D.; Gao, J.; Sundararaman, R.; Schwarz, K. A.; Arias, T. A.; Abruña, H. D.; Muller, D. A. Nano Lett. 2014, 14 (3), 1453–1459, DOI:10.1021/nl404577c. 13. Zheng, H.; Claridge, S. A.; Minor, A. M.; Alivisatos, A. P.; Dahmen, U. Nano Lett. 2009, 9 (6), 2460–2465, DOI:10.1021/nl9012369. 14. Kraus, T.; de Jonge, N. Langmuir 2013, 29 (26), 8427–8432, DOI:10.1021/la401584z. 15. Alloyeau, D.; Dachraoui, W.; Javed, Y.; Belkahla, H.; Wang, G.; Lecoq, H.; Ammar, S.; Ersen, O.; Wisnet, A.; Gazeau, F.; Ricolleau, C. Nano Lett. 2015, 15 (4), 2574–2581, DOI:10.1021/acs.nanolett.5b00140.


22

Page 23 of 27 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

Nano Letters

16. Park, J. H.; Schneider, N. M.; Grogan, J. M.; Reuter, M. C.; Bau, H. H.; Kodambaka, S.; Ross, F. M. Nano Lett. 2015, 15 (8), 5314–5320, DOI:10.1021/acs.nanolett.5b01677. 17. Noh, K. W.; Liu, Y.; Sun, L.; Dillon, S. J. Ultramicroscopy 2012, 116, 34–38, DOI:10.1016/j.ultramic.2012.03.012. 18. Ahn, T.-Y.; Hong, S.-P.; Kim, S.-I.; Kim, Y.-W. RSC Adv. 2015, 5 (100), 82342–82345, DOI:10.1039/c5ra14879k. 19.

Abrams,

I.

M.;

McBain,

J.

W.

Science

1944,

100

(2595),

273–274,

DOI:10.1126/science.100.2595.273. 20. Williamson, M. J.; Tromp, R. M.; Vereecken, P. M.; Hull, R.; Ross, F. M. Nat. Mater. 2003, 2 (8), 532–536, DOI:10.1038/nmat944. 21. Zheng, H.; Smith, R. K.; Jun, Y.-w.; Kisielowski, C.; Dahmen, U.; Alivisatos, A. P. Science 2009, 324 (5932), 1309–1312, DOI:10.1126/science.1172104. 22. Leenheer, A. J.; Sullivan, J. P.; Shaw, M. J.; Harris, C. T. J. Microelectromech. Syst. 2015, 24 (4), 1061–1068, DOI:10.1109/jmems.2014.2380771. 23. Dukes, M. J.; Dukes, A. D.; Klein, K. L.; Thomas, R.; Kelly, D. F.; Damiano, J. Microsc. Microanal. 2014, 20 (S3), 1090–1091, DOI:10.1017/s143192761400717x. 24. Liu, K.-L.; Wu, C.-C.; Huang, Y.-J.; Peng, H.-L.; Chang, H.-Y.; Chang, P.; Hsu, L.; Yew, T.-R. Lab. Chip 2008, 8 (11), 1915–1921, DOI:10.1039/b804986f. 25. Wu, F.; Yao, N. Nano Energy 2015, 11, 196–210, DOI:10.1016/j.nanoen.2014.11.004.


23

Nano Letters 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 24 of 27

26. Huang, T.-W.; Liu, S.-Y.; Chuang, Y.-J.; Hsieh, H.-Y.; Tsai, C.-Y.; Huang, Y.-T.; Mirsaidov, U.; Matsudaira, P.; Tseng, F.-G.; Chang, C.-S.; Chen, F.-R. Lab. Chip 2011, 12 (2), 340–347, DOI:10.1039/c1lc20647h. 27. Liao, H.-G.; Zherebetskyy, D.; Xin, H.; Czarnik, C.; Ercius, P.; Elmlund, H.; Pan, M.; Wang, L.-W.; Zheng, H. Science 2014, 345 (6199), 916–919, DOI:10.1126/science.1253149. 28. Ring, E. A.; de Jonge, N. Microsc. Microanal. 2010, 16 (5), 622–629, DOI:10.1017/s1431927610093669. 29. Woehl, T. J.; Jungjohann, K. L.; Evans, J. E.; Arslan, I.; Ristenpart, W. D.; Browning, N. D. Ultramicroscopy 2013, 127, 53–63, DOI:10.1016/j.ultramic.2012.07.018. 30. Yuk, J. M.; Park, J.; Ercius, P.; Kim, K.; Hellebusch, D. J.; Crommie, M. F.; Lee, J. Y.; Zettl, A.; Alivisatos, A. P. Science 2012, 336 (6077), 61–64, DOI:10.1126/science.1217654. 31. Hauwiller, M. R.; Ondry, J. C.; Alivisatos, A. P. J. Vis. Exp. 2018 (135), DOI:10.3791/57665. 32. Park, J.; Park, H.; Ercius, P.; Pegoraro, A. F.; Xu, C.; Kim, J. W.; Han, S.-H.; Weitz, D. A. Nano Lett. 2015, 15 (7), 4737–4744, DOI:10.1021/acs.nanolett.5b01636. 33. Kelly, D. J.; Zhou, M.; Clark, N.; Hamer, M. J.; Lewis, E. A.; Rakowski, A. M.; Haigh, S. J.; Gorbachev, R. V. Nano Lett. 2018, 18 (2), 1168–1174, DOI:10.1021/acs.nanolett.7b04713. 34.

Textor,

M.;

de

Jonge,

N.

Nano

Lett.

2018,

18

(6),

3313–3321,

DOI:10.1021/acs.nanolett.8b01366.


24

Page 25 of 27 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

Nano Letters

35. Yulaev, A.; Guo, H.; Strelcov, E.; Chen, L.; Vlassiouk, I.; Kolmakov, A. ACS Appl. Mater. Interfaces 2017, 9 (31), 26492–26502, DOI:10.1021/acsami.7b02824. 36. Dahmke, I. N.; Verch, A.; Hermannsdörfer, J.; Peckys, D. B.; Weatherup, R. S.; Hofmann, S.; de Jonge, N. ACS Nano 2017, 11 (11), 11108–11117, DOI:10.1021/acsnano.7b05258. 37. Hutzler, A.; Branscheid, R.; Jank, M. P. M.; Frey, L.; Spiecker, E. Graphene-supported microwell liquid cell for in situ studies in TEM and SEM; pp 209–210. 38. Hutzler, A.; Branscheid, R.; Jank, M. P.M.; Frey, L.; Spiecker, E. Microsc. Microanal. 2016, 22 (S5), 78–79, DOI:10.1017/s1431927616012423. 39. Zaluzec, N. J.; Burke, M. G.; Haigh, S. J.; Kulzick, M. A. Microsc. Microanal. 2014, 20 (2), 323–329, DOI:10.1017/s1431927614000154. 40. Lewis, E. A.; Haigh, S. J.; Slater, T. J. A.; He, Z.; Kulzick, M. A.; Burke, M. G.; Zaluzec, N. J. Chem. Commun. 2014, 50 (70), 10019–10022, DOI:10.1039/c4cc02743d. 41. Huang, X.; Neretina, S.; El-Sayed, M. A. Adv. Mater. 2009, 21, 4880–4910, DOI:10.1002/adma.200802789. 42. Schneider, N. M.; Norton, M. M.; Mendel, B. J.; Grogan, J. M.; Ross, F. M.; Bau, H. H. J. Phys. Chem. C 2014, 118 (38), 22373–22382, DOI:10.1021/jp507400n. 43. Kleber, W.; Bautsch, H. -J.; Bohm, J. Einführung in die Kristallographie, 17th edition; Verlag Technik GmbH Berlin, 1990. 44. Wagner, C. Z. Elektrochem. 1961, 65 (7/8), 581–591, DOI:10.1002/bbpc.19610650704.


25

Nano Letters 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 26 of 27

45. Woehl, T. J.; Evans, J. E.; Arslan, I.; Ristenpart, W. D.; Browning, N. D. ACS Nano 2012, 6 (10), 8599–8610, DOI:10.1021/nn303371y. 46. Liu, Y.; Tai, K.; Dillon, S. J. Chem. Mater. 2013, 25 (15), 2927–2933, DOI:10.1021/cm303522z. 47. De Clercq, A.; Dachraoui, W.; Margeat, O.; Pelzer, K.; Henry, C. R.; Giorgio, S. The Journal of Physical Chemistry Letters 2014, 5 (12), 2126–2130, DOI:10.1021/jz500690a. 48. Jeong, M.; Yuk, J. M.; Lee, J. Y. Chem. Mater. 2015, 27 (9), 3200–3202, DOI:10.1021/acs.chemmater.5b00334. 49. Abellan, P.; Parent, L. R.; Al Hasan, N.; Park, C.; Arslan, I.; Karim, A. M.; Evans, J. E.; Browning, N. D. Langmuir 2016, 32 (6), 1468–1477, DOI:10.1021/acs.langmuir.5b04200. 50. Verch, A.; Pfaff, M.; de Jonge, N. Langmuir 2015, 31 (25), 6956–6964, DOI:10.1021/acs.langmuir.5b00150. 51. Liao, H.-G.; Cui, L.; Whitelam, S.; Zheng, H. Science (New York, N.Y.) 2012, 336 (6084), 1011–1014, DOI:10.1126/science.1219185. 52. Woehl, T. J.; Abellan, P. J. Microsc. 2017, 265 (2), 135–147, DOI:10.1111/jmi.12508. 53. Gómez-Graña, S.; Goris, B.; Altantzis, T.; Fernández-López, C.; Carbó-Argibay, E.; Guerrero-Martínez, A.; Almora-Barrios, N.; López, N.; Pastoriza-Santos, I.; Pérez-Juste, J.; Bals, S.; van Tendeloo, G.; M. Liz-Marzán, L. J. Phys. Chem. Lett. 2013, 4 (13), 2209–2216, DOI:10.1021/jz401269w.


26

Page 27 of 27 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

Nano Letters

54. Dong, X.; Zhou, J.; Liu, X.; Lin, D.; Zha, L. J. Raman Spectrosc. 2014, 45 (6), 431–437, DOI:10.1002/jrs.4495. 55. Xiang, Y.; Wu, X.; Liu, D.; Li, Z.; Chu, W.; Feng, L.; Zhang, K.; Zhou, W.; Xie, S. Langmuir 2008, 24 (7), 3465–3470, DOI:10.1021/la702999c. 56. Hill, M. A.; Smith, F. A. Radiat. Phys. Chem. 1994, 43 (3), 265–280, DOI:10.1016/0969806x(94)90190-2. 57. Belloni, J. Catalysis Today 2006, 113 (3-4), 141–156, DOI:10.1016/j.cattod.2005.11.082. 58. Jiang, N. Ultramicroscopy 2017, 179, 81–83, DOI:10.1016/j.ultramic.2017.04.012. 59. Gurney, R. W.; Mott, N. F. Proc. Royal Soc. A 1938, 164 (917), 151–167, DOI:10.1098/rspa.1938.0011. 60. Orendorff, C. J.; Sau, T. K.; Murphy, C. J. Small 2006, 2 (5), 636–639, DOI:10.1002/smll.200500299. 61. Brioude, A.; Jiang, X. C.; Pileni, M. P. J. Phys. Chem. B 2005, 109, 13138–13142, DOI:10.1021/jp0507288.

BRIEFS SYNOPSIS


27

Unravelling the Mechanisms of GoldâSilver CoreâShell Nanostructure

Recommend Documents

Unravelling the Mechanisms of GoldâSilver CoreâShell Nanostructure