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Ex Situ and in Situ Surface Plasmon Monitoring of TemperatureDependent Structural Evolution in Galvanic Replacement Reactions at a Single-Particle Level Youngchan Park,†,‡ Chanhyoung Lee,†,‡ Seol Ryu,§ and Hyunjoon Song*,†,‡ †

Department of Chemistry, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea ‡ Center for Nanomaterials and Chemical Reactions, Institute for Basic Science, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea § Department of Chemistry, Chosun University, Gwangju 501-759, Korea S Supporting Information *

ABSTRACT: The galvanic replacement reaction has recently been established as a standard protocol to create complex hollow structures with various compositions and morphologies. In the present study, the structural evolution of Ag nanocubes with Au precursors is monitored at the single-particle level by means of ex situ and in situ characterization tools. We explore two important features distinct from previous observations. First, the peak maximum of localized surface plasmon resonance (LSPR) spectra abruptly shifts at the initial stage and reaches a steady wavelength of ∼600 nm; however, the structure continuously evolves to yield a nanobox even during the late stages of the reaction. This steady wavelength results from a balance of the LSPR between the red-shift by the growth of the inner cavity and the blue-shift by the deposition of Au on the interior, as confirmed by theoretical simulations. Second, the change in morphology at different temperatures is first analyzed by both ex situ and in situ monitoring methods. The reaction at 25 °C forms granules on the surface, whereas the reaction at 60 °C provides flat and even surfaces of the hollow structures due to the large diffusion rate of Ag atoms in Au at a higher temperature. These plasmon-based monitoring techniques have great potentials to investigate various heterogeneous reaction mechanisms at the single-particle level.

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INTRODUCTION For the past several decades, metal nanostructures have been extensively studied due to their unique physical and chemical properties and versatile applications in many areas. To control the properties of metal nanostructures so as to make them suitable for specific applications, the creation of complex nanostructures with multiple components and high-dimensional morphologies has been attempted, such as core−shell,1,2 hollow,3,4 and branched nanoparticles.5,6 For the fabrication of these nanostructures, solid-state chemical reactions on the nanoscale are commonly applied, including galvanic replacement,7,8 void formation via the Kirkendall effect,9 and cationic and anionic exchanges10 as well as the induction of kinetic growth during the synthesis step. In particular, galvanic replacement has been established as a standard method to make complex hollow nanostructures with a variety of compositions and morphologies. Galvanic replacement is basically a redox process between two metals with distinct reduction potentials. Oxidation occurs on the metal with a low reduction potential, and the reduction and deposition concomitantly occur on the other metal with a high reduction potential. The replacement reaction is known as © XXXX American Chemical Society

a major cause of the corrosion of metal surfaces in the bulk form, and Xia et al. revitalized it as a simple and versatile route for the generation of metal hollow nanostructures.11 The formation mechanism of this reaction was elucidated through a change in the morphology as observed in TEM images along the reaction progress. These outcomes included the generation of a specific spot, a continuous hollow formation, morphology changes to nanoboxes, and dealloying and fragmentation.12 This process involves multiple steps with a large particle-toparticle variation, and thus some researchers have shifted toward high-resolution measurements with in situ techniques. Sun et al. reported the in situ monitoring of galvanic replacement on Ag nanowires using transmission X-ray microscopy.13 Jain et al. analyzed the replacement reaction on Ag spheres using dark-field scattering at the single-particle level and identified a critical step in the nanosized void formation process followed by an exchange reaction with nonlinear reaction kinetics.14 Received: June 10, 2015 Revised: July 17, 2015

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DOI: 10.1021/acs.jpcc.5b05541 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Supporting Information). Some Ag nanocubes were attached to the inner surface of the flow cell through nonspecific binding. The location of each particle was identified in the DFM image, and one of the particles was chosen to focus and measure its plasmon spectrum. Either the HAuCl4 (10 μM) aqueous solution at 25 °C was directly used or the HAuCl4 (25 μM) aqueous solution was heated to 60 °C using a heating mantle and a heating band. The above HAuCl4 solution was injected through the flow cell using a syringe pump with a continuous flow rate of 0.025−0.050 mL min−1. The DFM image and LSPR spectrum of an individual Ag nanocube were measured for 2 h. Single-Particle Dark-Field Spectroscopy. Single-particle spectra were collected with an inverted optical microscope equipped with a dark-field condenser (Carl Zeiss, Axiovert 40). Dark-field microscopy was performed with a 35 W halogen lamp as a light source, which was focused through a dark-field condenser (NA = 1.2−1.4) with an immersion oil. The scattered light from the samples was collected through a 40× microscope objective lens (NA = 0.98). The dark-field images were captured by a 640 × 480 pixel color video camera (SONY, SSC-DC80), and the plasmon spectra were obtained with a CCD camera (ANDOR, NEWTON DU971N) coupled with a monochromator (Dongwoo Optron Co., Ltd., 500i). The extinction spectrum was integrated for 30 s and was corrected by a background subtraction and divided by the lamp spectrum. FDTD Simulations. An FDTD solution (Lumerical Solutions, Inc.) was used to simulate the interaction between light and electrons on the nanoparticle surface. A wavelength range from 300 to 900 nm was radiated into a box containing a model of a Au−Ag nanostructure (dielectric functions of Au and Ag were taken from the experimental data conducted by Johnson and Christy21) on a substrate (refractive index is 1.5). The surrounding medium inside the box was divided into the meshes of 0.5 nm, and its refractive index was set to be 1.33 for water. The edges in the model of a Ag nanocube were rounded in a radius of 12 nm based on the TEM image.

Although reasonable reaction sequences of galvanic replacement have been proposed based on data from multiple characterization techniques, the actual mechanism still needs to be explored in detail because the replacement reaction is highly sensitive to the reaction environment. The clear formation of Au−Ag nanoboxes only occurs at 100 °C, and facet-specific replacement reactions are frequently observed under specific conditions.15−17 The precipitation of AgCl on the surface was the most dominant factor with regard to the final morphology of the hollow structures.18 Simultaneous coupling with the Kirkendall effect generated complex morphologies with multiple walls.19 In the present study, we attempt to draw a detailed picture of the formation of metal hollow structures by means of both ex situ and in situ techniques at a single-particle resolution. For the ex situ reaction monitoring, Ag nanocubes were deposited on a substrate and were then immersed in a reaction mixture. The samples were taken out at various stages of the reaction process and were directly characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and dark-field microscopy (DFM). The scattering signals of an individual particle by localized surface plasmon resonance (LSPR) were analyzed by the finite-difference time-domain (FDTD) simulation method. For in situ reaction monitoring, a flow cell setup was combined with DFM to maintain a constant reaction environment, and the changes of the LSPR signals at an individual particle were continuously monitored as the reaction proceeded. Importantly, the reaction behaviors at distinct temperatures, 25 and 60 °C, were successfully analyzed at the single-particle level. This integrative monitoring technique enabled us to observe the temperature dependency as well as the time-dependent features of the galvanic replacement reaction and to determine the critical factors dominating these reaction behaviors.

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EXPERIMENTAL METHODS Ex Situ Monitoring of Galvanic Replacement Reactions. A slide glass was cleaned with an Alconox (Alconox, Inc.) powder and sonicated with a 1:1 mixture of acetone and ethanol for 30 min and then rinsed with deionized water, followed by drying under a N2 gas flow. Ag nanocubes were prepared according to the literature.20 The original Ag nanocube dispersion was diluted by 100 times, and the resulting dispersion (10 μL) was dropped onto the clean slide glass. The sample was washed with water and dried under a N2 gas flow. Then, the sample was immersed in a HAuCl4 (40 mL, 1.0 μM) aqueous solution, followed by stirring in a shaker incubator for a specific reaction time (1 min, 1 h, 3 h, 6 h, and 24 h) at 25 and 60 °C. After the reaction, the slide glass was taken out from the reaction mixture, washed with ethanol and water, and dried under a N2 gas flow. To measure the scattering signals under a surrounding aqueous medium at a singleparticle level, a frame-shaped adhesive tape was put on the slide glass, and a drop of water was filled inside the frame, followed by covering with a thin cover glass. For TEM measurements, the nanoparticles were separated by the addition of acetone and redeposited on a carbon-supported 300 mesh Cu grid (Ted Pellar, Inc.). For SEM measurements, an indium tin oxide (ITO) glass was used as a substrate. In Situ Monitoring of Galvanic Replacement Reactions. The original Ag nanocube dispersion was diluted with deionized water by 10 000 times and then injected into a flow cell using a syringe pump (KD Scientific) (Figure S1,

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RESULTS AND DISCUSSION For the galvanic replacement reaction, the formation of Au hollow cubes by the addition of a HAuCl4 solution to Ag cubes has been demonstrated as a typical example.16 The standard reduction potential of the AuCl4−/Au pair (0.99 V vs standard hydrogen electrode (SHE)) is higher than that of the Ag+/Ag pair (0.80 V vs SHE); thus, the Au ions are reduced to Au atoms, while the Ag atoms are oxidized to Ag ions and are dissolved in the aqueous solution, as follows: 3Ag(s) + AuCl4 −(aq) → Au(s) + 3Ag +(aq) + 4Cl−(aq) (1)

The reaction mechanism has been proposed using information from bulk-level experiments.12 During the reaction, the Ag+ ions dissolved from the Ag surface formed AgCl in the presence of Cl− ions in water, which was precipitated and disrupted the deposition of Au atoms over the Ag cubes. Accordingly, galvanic replacement reactions were commonly conducted at a temperature as high as 100 °C to increase the solubility of AgCl in an aqueous solution and to prevent the formation of a rough surface and an uneven size distribution. However, when the reaction is carried out on a large scale using a high concentration of precursors, the synthesis tends not to be reproducible though the reaction temperature is high enough, and the resulting products are not uniform in terms of their B


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Figure 1. SEM and TEM (insets) images of Au−Ag alloy nanoboxes by galvanic replacement reactions under the following conditions: (a) using a Ag cube dispersion in a large scale at 100 °C and using Ag cubes sparsely deposited on ITO substrates at (b) 25 °C and (c) 60 °C. The bars represent (a−c) 200 nm and (insets) 20 nm.

troscopy with DFM is therefore helpful when used to analyze the reaction progress in detail, particularly for individual intermediate structures. Figure 2 displays DFM and SEM images, and the insets show TEM images of Ag nanocubes collected at different reaction periods at 25 and 60 °C. The original Ag nanocubes were measured before the reaction, and at each stage, the substrate containing the Ag nanocubes was immersed in a HAuCl4 aqueous solution (1.0 μM) for 1 min, 1 h, 3 h, 6 h, and 24 h, and the images were captured after washing the substrate with solvents. For the reaction at 25 °C, the original Ag nanocubes appear as bright shining blue dots in the DFM image (Figure 2a) with the sharp edges of regular cubes in the SEM and TEM images (Figure 2b). The reaction period of 1 min abruptly diminishes the scattering intensity of each dot (Figure 2c) despite the fact that the particle morphology observed in the microscopy images remains unchanged (Figure 2d). Within a reaction time of 1 h, the bright dots changes into dimly scattered dots (Figure 2e). The cubes have rough surfaces (Figure 2f). After 3 h of the reaction, minute shiny dots are shown in the DFM image, and tiny granules are formed on the cube surface (Figure 2g,h). The weak scattering signals are not greatly changed up to 24 h; however, the morphology of the cubes continuously changes to a hollow structure composed of granular walls (Figure 2i,j). Eventually, multiple holes are generated on the walls at 24 h, indicating the initiation of a dealloying process (Figure 2k,l). In some particles, metal lumps are attached to the outer surface. These observations indicate that the scattering intensity decreases sharply at the initial reaction period within 1 h, reaching a steady level in the DFM measurement. On the contrary, the evolution in the morphology observed in the SEM and TEM images occurs continuously up to 24 h. For the reaction at 60 °C, the DFM images with the reaction progress represent a sudden decay of the scattering intensity within 1 h, similar to the reaction at 25 °C (Figure 2m−w); however, the change in the morphology observed in the SEM and TEM images follows a different trend. At the reaction time of 1 h, the cube surface has a bumpy appearance (Figure 2r). Then, the center of each face in the individual cubes is regularly etched to form an octapod-like morphology (Figure 2t). At 6 h, the faces of the original cube are reconstructed to generate flat walls, and the structure has a large cavity as identified by a contrast difference (Figure 2v).12,22 The structure completely

morphology. Figure 1a demonstrates a common feature of the hollow cubes. The individual hollow cubes exhibited different degrees of void formation with two distinguished surface structures of a flat face (∼80%) or with a granular surface (∼20%) via the synthesis at a bulk level (Figure S2, Supporting Information). This large particle-to-particle variation blurs the spectral and functional properties through an ensemble average effect. To reduce the large variation in the particle morphology, Ag nanocubes were deposited onto a glass substrate with a low surface density, and the resulting substrate was immersed in a Au precursor solution to induce the galvanic replacement reaction under a high speed stirring. Unlike the previous synthesis, the reactions yielded uniform hollow structures, only with granular surfaces at 25 °C (Figure 1b) and with flat surfaces at 60 °C (Figure 1c). This outcome is attributed to the sparse dispersion of the Ag nanocubes with a relatively high stirring speed, which effectively provided even reaction conditions over the entire particle surface (Figure S3, Supporting Information). The energy dispersive X-ray spectroscopy (EDX) analysis of each nanobox exhibited the signals of Ag and Au without a trace of the Cl peak (Figure S4, Supporting Information), indicating that there was no deposition of AgCl during the reaction at room temperature. In this experimental condition, the amount of Ag+ generated for the reaction to form the precipitation with Cl− is much lower than the solubility product constant (Ksp) of AgCl in water at room temperature. Consequently, the effect of AgCl deposition on the surface can be excluded with regard to the formation of the Au−Ag hollow structures. The EDX line profiles of individual hollow nanostructures prepared at both temperatures indicate that the Au atoms are evenly deposited onto the external walls of the hollow structures (Figure S5, Supporting Information). Ex Situ Monitoring of the Structural Evolution by Single-Particle Scattering Measurements. Using the synthetic technique on the substrate, it is easy to monitor the reaction progress by varying the reaction time. In addition to the microscopic tools, the Ag and Au nanostructures exhibit intense scattering signals in the visible region according to LSPR, which is highly dependent upon the composition and morphology of the nanomaterials. DFM is an excellent tool to measure LSPR signals at the single-particle level. Heterogeneity in the particle morphology blurs the scattering features and makes them appear broad. A single-particle scattering specC


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Figure 2. DFM and SEM images of samples after the galvanic replacement reaction of Ag nanocubes on substrates at 25 °C for (a, b) 0 h, (c, d) 1 min, (e, f) 1 h, (g, h) 3 h, (i, j) 6 h, and (k, l) 24 h and at 60 °C for (m, n) 0 h, (o, p) 1 min, (q, r) 1 h, (s, t) 3 h, (u, v) 6 h, and (w, x) 24 h. The insets are TEM images of individual nanocubes. The dark-field image contrasts of all panels are identical for the direct comparison of intensities. The bars represent 200 nm for SEM and 20 nm for TEM images.

during the galvanic replacement reaction at 25 °C. LSPR spectra were collected and then fitted by the Lorentz curve to determine the position of the peak maximum. The peak maximum for the original Ag nanocube is 520 nm, which is ∼30 nm red-shifted from that of the Ag nanocubes dispersed in ethanol, due to the existence of the glass substrate that the nanocubes lie on.23 With a reaction time of 1 min, the peak maximum largely shifts to a longer wavelength of 550 nm despite the fact that the particle morphology observed in the TEM image remains unchanged. It is not surprising that this sudden shift is also observed in bulk experiments and in a single-particle measurement of a Ag sphere.14,24 As the reaction proceeds, the peak shifts to 590 nm for the reaction period up to 3 h. Then, the peak shows little change at ∼600 nm up to 24

changes into a nanobox with continuous flat walls, including large holes on the faces at 24 h (Figure 2x). To investigate the reaction mechanism and change in morphology more deeply, the LSPR scattering spectra were acquired for more than ten individual nanocubes at each reaction step (Figures S6 and S7, Supporting Information). In the ex situ experiments, each scattering spectrum was measured from the particles collected at a different reaction period; therefore, the analysis based on the peak shift is more meaningful than that of the intensity change. It is notable that the peak shift of the scattering spectrum is closely correlated to the change of intensity in the in situ experiments (vide inf ra). Figure 3 demonstrates the scattering spectra of representative nanocubes as a function of the reaction time D


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Figure 3. (a) Single-particle scattering spectra of the representative Ag nanocubes with the galvanic replacement reaction for different reaction periods at 25 °C in the ex situ measurements and (b) scattering peak maximum as a function of the reaction period. The error bars represent the standard deviations estimated from 11 individual nanoparticles (Figure S6, Supporting Information).

Figure 4. (a) Single-particle scattering spectra of representative Ag nanocubes with the galvanic replacement reaction for different reaction periods at 60 °C in the ex situ measurements and (b) scattering peak maximum as a function of the reaction period. The Lorentzian fit curves were depicted while the scattering peak was clearly deconvoluted into multiple components. The error bars represent the standard deviations estimated from 11 individual nanoparticles (Figure S7, Supporting Information).

h; however, the morphology in the microscopy images continuously changes to the hollow structure with a large cavity. Figure 3b represents a trend of the peak shifts for individual nanocubes, indicating the initial rapid red-shift and eventual steady state of the peak maximum. The scattering peak of the individual nanoparticles broadened along the reaction progress. It is known that alloy nanoparticles usually show broad extinction peaks as a result of scattering of the conduction electrons at the compositional interfaces.25−27 For the galvanic replacement reaction at 60 °C (Figure 4a), the LSPR scattering spectra also exhibit a rapid peak shift from 520 nm before the reaction to 605 nm at 1 h. Then, the spectral features exhibit significant particle-to-particle variations at the reaction periods up to 24 h. In many of the individual

nanocubes, the major peaks can be deconvoluted into multiple distinguishable peaks, indicative of sophisticated morphologies in the intermediate structures (Figure S7, Supporting Information). For instance, the reaction for 3 h generated the octapod-like nanostructure, which causes a complicated peak pattern (vide inf ra). The average peak maximum of the major peaks observed on each stage of the galvanic replacement reaction at 60 °C is plotted in Figure 4b with respect to the reaction time. The peak position shows the initial rapid peak shift up to 1 h and the eventual steady peak at the late stage of the reaction. The large error bars over the reaction periods E


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Figure 5. Structural models and FDTD calculation results of the galvanic replacement reaction. (a) Au deposition on the outer surface of the Ag nanocube, (b) the formation and growth of the holes inside the cube, and (c) Au deposition on the inner surface of the hollow nanocubes. (d) Overall simulation of the galvanic replacement reaction and (e) the spectral shifts as calculated by the FDTD method.

with the edges rounded with a radius of 12 nm, which is the average shape of the Ag cubes as measured in the SEM images (Figure 2b,j). The structural models at each stage are designed based on the morphology observed in the microscopy images (Figure 2). For the first stage, Au deposition on the external surface of the Ag cube can be modeled from 1 to 3 by increasing the Au shell thickness. The FDTD calculation data shows a large red-shift of the peak (Figure 5a). Then, a hole is formed and continuously grown inside the Ag cube, with the concomitant Au deposition on the external surface, from 4 to 6 (Figure 5b). The FDTD simulation estimates that the peak maximum at this stage also shifts to a longer wavelength. The position of the inner hole is not necessarily at the center of the nanocubes, as the peak maximum is only sensitive to the diameter but not to the location.15,30 When the hole diameter increases further until all of the Ag core disappears to form a Au nanobox, the LSPR peak continuously shifts to a much long wavelength up to the nearinfrared region. However, the present and other experimental data unanimously represent that the peak shift of Au−Ag alloy nanocubes during the galvanic replacement reaction reaches a certain steady state,12 which resembles a saturation kinetics curve (Figures 3b and 4b). Additional FDTD calculation confirms that the existence of a hole on each facet of the nanobox does not significantly change its spectral features. The peak shifts approaching a certain wavelength as the reaction progress can be resolved by the third stage, i.e., the deposition of Au atoms on both interior and external surfaces of the hollow cube. With identical thicknesses of the Au layers outside

indicate the complex and diverse structural evolution during the reaction. Correlation between Structural Evolution and Spectral Change by Theoretical Simulation. To analyze the relationship between the intermediate structure and the LSPR spectrum, FDTD calculations of the corresponding structural models were conducted. The simulated model is fundamentally limited for the expression of Au−Ag alloys during the reaction. Although the alloy formation between Au and Ag has been intensively studied, the dielectric constants of the Au−Ag alloys were estimated only using a semiempirical model.28 Moreover, the composition ratio of the Au−Ag alloys cannot be fixed during the galvanic replacement process because each intermediate structure is not in an equilibrium state. Recently, Liz-Marzán et al. reported a three-dimensional EDX mapping of the hollow cubes by galvanic replacement, in which thin Au layers were formed on the surface of Ag nanocubes.29 Based on this observation, the hollow structure generated by the galvanic replacement is reasonably modeled with a Ag hollow cube covered with a thin Au layer on its surface. The galvanic replacement process can be divided into the following three stages to determine key factors leading to the major spectral shifts: (1) Au deposition on the external surface of the Ag nanocube alongside the dissolution of Ag+, (2) the formation and growth of holes inside the cube with the continuous deposition of Au, and (3) Au deposition on the inner surface of the Ag nanoboxes as well as on the external surface (Figure 5 and Figure S8, Supporting Information). The model of the original Ag nanocube has an edge length of 72 nm F


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Figure 6. (a) An octapod-like model for the reaction period of 3 h at 60 °C (k is the wave vector, and E is the electric-field polarization of light) and (b) the spectrum as calculated by the FDTD method. Electric field distributions on the model (c) at 475 nm and (d) at 560 nm. The dotted lines indicate the positions of the glass substrates.

by the selective indentation of {100} facets from a regular cube. The LSPR spectrum is analyzed via the construction of a corresponding model lying on a glass substrate, where a Au layer covers the surface of a Ag nanocube with rounded vertices. The center of each face is then indented to obtain a round concave surface (Figure 6a). The spectrum calculated by the FDTD method from the model exhibits a strong peak at 560 nm with a distinct shoulder at 475 nm. To identify the nature of the LSPR modes, the electric field distributions at both peaks are calculated. Based on the three-dimensional distributions of the contour maps, the largest peak at 560 nm is assigned as a corner dipole mode, in which the electric fields are distributed on all edges of the cubes. The LSPR is known to be hybridized with the substrate, which breaks the symmetry of the resonances and splits them into their proximal and distal components.33−35 Consequently, the shoulder peak at 475 nm is assigned to a proximal component of the dipole mode. A distal component of the dipole mode is located at ∼350 nm (Figure S9, Supporting Information), in which other peaks such as proximal and distal edge modes also appear. The minor peaks that appear in the other intermediates also results from their structural complexity with the hybridization of the substrate. Temperature Dependency of the Galvanic Replacement Reaction. Based on observations of the ex situ singleparticle spectra and the TEM images, the reaction mechanisms are proposed at both temperatures, i.e., 25 and 60 °C. As depicted in Figure 7, two very distinct features are observed. At

the Ag cubes with an increase in the thickness of the Au deposition layer on the inner faces, the simulated peak maximum shifts to a shorter wavelength (Figure 5c). It has been reported how the ratio of outer edge length to wall thickness gives impact on the LSPR properties of the Au−Ag nanostructures.31,32 These models are reasonable because as the galvanic replacement process continues, the holes and pores on the Ag surface increase such that the Au precursor can reach the inner surface of the Ag cube, resulting in the direct deposition of Au atoms on both faces, as observed in the threedimensional EDX mapping.24 Considering these three stages together, the overall galvanic replacement process is proposed in Figure 5d. At the initial stage, the surface Ag atoms are dissolved with the simultaneous deposition of the Au atoms (A to B). Then, the hole begins to form inside the cube (C), the Au atoms are continuously deposited on both the inner and outer surfaces as the hole grows (D), and the nanobox or the hollow nanocube made of thin walls is finally yielded with a large Au fraction (E). Through the FDTD calculations, the peak maximum rapidly shifts from 492 nm (A) to 582 nm (B) and 618 nm (C), after which it reaches a steady wavelength of 625 nm (D and E) due to a balance of the LSPR between the red-shift by the growth of the inner cavity and the blue-shift by the deposition of Au on the interior (Figure 5e). In the galvanic replacement reaction at 60 °C, multiple peaks are detected in the intermediate structures, particularly at a reaction period of 3 h (Figure 4a). The TEM image demonstrates an octapod-like structure, which can be obtained G


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Figure 7. Schematic illustration of the evolution of the morphology depending on the reaction temperature by the galvanic replacement reaction.

Figure 8. (a, c) DFM images and (b, d) LSPR spectra of individual Ag nanocubes by galvanic replacement reactions at (a, b) 25 °C and (c, d) 60 °C using the in situ scattering measurements.

25 °C, the Au atoms deposited on the surface generate a granular morphology. Even with the deposition of a large amount of Au, Au granules still form on the surface, thus becoming connected to neighboring ones to construct a hollow cube. At a late stage of the reaction, the granules grow and agglomerate to form a metal lump outside the hollow cube (Figure 7a). In contrast, the reaction at 60 °C always yields smooth surfaces (Figure 7b). This difference in the surface morphology is attributed to the interdiffusion rate between Au and Ag. The diffusion coefficients of Ag atoms in Au are estimated to be ∼10−24 m2 s−1 at 25 °C and 10−22 m2 s−1 at 60 °C.36,37 Such a large increase of 2 orders of magnitude changes the surface morphology such that it becomes smooth at a higher temperature. The second distinctive feature of the reaction is that the selective indentation only occurs at 60 °C (Figure 7b). In a perfect Ag nanocube, it is known that the surfactant, poly(vinylpyrrolidone), is selectively bound to the {100} facets; thus, the edges with {111} and high-index facets are exposed to the reaction media. Therefore, the Au atoms are preferentially deposited on the edges of the cube during the initial stage of the reaction. The Ag atoms on the {100} facets

then are oxidized and dissolved into the solvent. As a result, the selective deposition of Au on the edges and the dissolution of Ag on the facets provide an octapod-like morphology. When the Au concentration increases to a certain threshold on the surface, the Au (or Au−Ag) atoms generate a flat and continuous surface, caused by the rapid diffusion rate of Ag in Au at a high temperature. This induces the coalescence of voids into a single cavity and thus resultantly produces a hollow structure. Further deposition of Au and the growth of the cavity result in a Au−Ag alloy nanobox. In contrast, the deposition of the Au atoms at 25 °C provides the formation of a granule on the Ag surface (Figure 7a). The diffusion rate of Ag in Au is significantly lower than the deposition rate of Au, resulting in a large number of granules covering the surface. This implies that the surface Au layers have many defects, in which multiple voids are generated. These voids coalesce into multiple cavities inside the cube, finally yielding a Au−Ag alloy nanobox with many granules on its walls. Interestingly, the reaction progress of each reaction occurs at a similar rate to each other, despite the fact that the reaction temperature is significantly different. Because the galvanic replacement reaction is an electrochemical phenomenon, the H


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Figure 9. Trajectories of (a, b) the peak maximum and (c, d) scattering intensity in individual Ag nanocubes during the galvanic replacement reactions at (a, c) 25 °C and (b, d) 60 °C. The scattering intensity for each nanocube is integrated in the range of 300−900 nm. LSPR spectra were collected and then fitted by the Lorentz curve to determine the position of the peak maximum.

intensity decreases and the peak maximum shifts to a longer wavelength in the initial stage, reaching a constant value at the late stage, as observed in the ex situ measurements. Figure 9a,b shows trajectories of the peak maxima for three representative nanocubes during the reaction at each temperature. Interestingly, the spectral shifts of all samples tend to be similar to each other. At 25 °C, each spectral shift has three distinguished stages. The initial step can be referred to as the waiting time, when no chemical shift is observed. This waiting time is different for each nanocube from 20 to 45 min, resulting from the stochastic nature of the initial step and difference in the local environment of the surface, such as the presence of a surface defect or a different density of the surfactant molecules bound to the surface. This value is significantly longer than 200−600 s for the Ag nanospheres,14 presumably due to the {100} facets of the nanocube surface strongly stabilized by the surfactant, poly(vinylpyrrolidone). The peak then shifts rapidly to a longer wavelength within 10−20 min. Finally, the peak maximum reaches a steady wavelength of ∼600 nm, but this does not mean no change in the morphology. As analyzed in the ex situ experiments with a theoretical simulation, the balance of the LSPR between the red-shift by the growth of the cavity and the blue-shift by continuous deposition of Au atoms over the interrior surface leads to a steady state of the peak maximum. At 60 °C, the trajectories also follow a similar pattern: an initial waiting time, a rapid red-shift, and a steady state. However, the waiting time and the rapid red-shift period are not distinguished as clearly as those at 25 °C, and the rapid red-shift period (10−40 min) varies significantly according to

reaction occurs instantaneously; thus, the rate of morphological change is determined by the diffusion of the Au precursor onto the Ag surface. As a consequence of the low concentration of the Au precursor in the aqueous solution, the self-diffusion coefficient of water can be considered, which is 2.299 × 10−9 m2 s−1 at 25 °C and 4.748 × 10−9 m2 s−1 at 60 °C.38−40 In comparison to the diffusion rate of Ag atoms in Au, this increment of a factor of 2 with the temperature has a minor effect on the reaction kinetics. In Situ Monitoring by Single-Particle Scattering Measurements. Compared to the ex situ monitoring, the in situ spectroscopic method, that is, the direct observation of objects without quenching the reaction progress, has many advantages. In situ monitoring can measure the change actually occurring on the particle of interest. It is particularly effective in a system with rapid reaction kinetics. LSPR is a characteristic suitable for in situ measurements, and the combination with DFM has provided critical information about the reaction kinetics at the single-particle resolution.41,42 For continuous reaction monitoring, the reaction environment should remain constant. The introduction of a flow cell into the sample stage of DFM and the proper alignment enable in situ DFM measurements with a constant supply of reactants at variable temperatures higher than room temperature.43,44 Figure 8 shows scattering images and the corresponding spectral changes of individual particles during the reaction at 25 and 60 °C. At both temperatures, the individual Ag nanocubes abruptly change their light scattering intensities during the reaction period of 30 min. In the LSPR spectra, the light I



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the individual nanocube. The peaks of the steady states fluctuate during the late stages of the reaction. These findings indicate that the intermediate structures are sophisticated in their morphology due to the selective galvanic replacement reaction over the distinct facets, which results in a considerable particle-to-particle variation. We have checked the scattering intensity changes of the present nanocubes as well, of which the trajectories also show three distinct stages (Figure 9c,d). Jain et al. analyzed the LSPR intensity change of Ag nanospheres during the galvanic replacement reaction at the single-particle level, reporting that the sudden drop of the peak intensity is due to the bulk conversion from an Ag nanoparticle to a Au− Ag cage.14 In comparison with the ex situ measurements, this in situ plasmon monitoring exhibits similar spectral changes with a significantly long initial waiting time. In the in situ monitoring, the reaction is conducted under the continuous slow flow rate of the reaction mixture, and thus the stochastic feature of the initial step can be fully detected. On the contrary, no initial waiting time is observed in the ex situ experiments owing to a rapid stirring of the reaction mixture during the reaction, which enhances the exposure of naked surfaces to the active reaction media.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.S.). Author Contributions

Y.P. and C.L. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by IBS-R004-D1 and the National Research Foundation of Korea (NRF) funded by the Korea Government (MSIP) (2012-005624, R11-2007-050-00000-0). S.R. acknowledges the support by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (NRF-2013-R1A1A4A01-010384).

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CONCLUSIONS The galvanic replacement reaction of Ag nanocubes by a Au precursor in an aqueous solution is monitored at the singleparticle level by means of ex situ and in situ characterization tools and by FDTD calculations. The reaction mechanism follows a typical change in the morphology: initial Au deposition on the surface, the formation and growth of cavities inside the structure with the continuous deposition of the Au atoms, and the generation of an Au−Ag alloy nanobox. Distinct from previous observations, the present study explores two important features: (1) at a late stage of the reaction, the LSPR peak shift reaches a steady state, but the evolution of the morphology continuously occurs, leading to the generation of the nanobox structure, and (2) the room-temperature reaction forms granules on the surface, whereas the high-temperature reaction provides flat and even surfaces with complex intermediate structures, mainly due to the high diffusion rate of the Ag atoms in Au at a high temperature. In situ monitoring displays rapid peak shifts at the early stages of the reaction at both temperatures. This integration of multiple ex situ and in situ characterization techniques has the potential to provide detailed mechanistic pictures of chemical reactions at the single-particle level. In addition to the changes in the morphology of the metal nanostructures themselves, the reactions conducted on the metal surface can be monitored at a high resolution, which would have an important impact on our understanding of the essential mechanisms of various heterogeneous catalytic reaction systems.

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Article

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05541. Installation of DFM with a flow cell, preparation of a thermostable flow cell, EDX data, TEM and HADDFSTEM images, EDX line-scan profile, scattering spectra, and FDTD simulations of Au−Ag nanostructures (PDF) J


Article

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