ARTICLE
Using Scanning-Probe Block Copolymer Lithography and Electron Microscopy To Track Shape Evolution in Multimetallic Nanoclusters Guoliang Liu,† Chuan Zhang,‡ Jinsong Wu, and Chad A. Mirkin* Department of Chemistry and International Institute for Nanotechnology, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States. † Present address: Department of Chemistry, Virginia Tech, 900 West Campus Drive, Blacksburg, Virginia 24061. ‡Present address: School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Rd, Minhang, Shanghai 200240, China.
ABSTRACT Here we describe a general method for synthesizing multimetallic
coreshell nanoclusters on surfaces. By patterning seeds at predesignated locations using scanning-probe block copolymer lithography, we can track shape evolution in nanoclusters and elucidate their growth pathways using electron microscopy. The growth of coreshell nanostructures on surface-bound seeds is a highly anisotropic process and often results in multimetallic anisotropic nanostructures. The shell grows at specific edge and corner sites of the patterned seeds and propagates predominately from the top hemisphere of the seeds. KEYWORDS: scanning-probe block copolymer lithography . coreshell nanoparticle . shape evolution . multimetallic nanocluster
S
canning-probe block copolymer lithography (SPBCL)1,2 is a method for sitespecifically synthesizing nanoparticles on surfaces based upon concepts from scanning-probe molecular printing3,4 and block copolymer lithography.5,6 This method, which utilizes an aliquot of block copolymer loaded with a metal precursor of interest and delivered to a substrate with a scanning-probe microscope, allows one to subsequently synthesize nanoparticles with control over size (150 nm) and composition. The technique has proven useful for a variety of single element metals, metal oxides, and binary alloys.2,7,8 It is also an important tool for tracking and understanding the pathways for nanoparticle growth from atomic precursors, allowing one to distinguish important fundamental processes such as coalescence and Ostwald ripening mechanisms.9 In principle, it also could be a useful tool for understanding shape evolution in nanocrystal syntheses since one can control the number, size, and physical placement of relatively welldefined nucleation sites on a desired substrate. Herein, we explore this possibility LIU ET AL.
in the context of bimetallic and trimetallic nanostructures consisting of spherical Au clusters that can nucleate coreshell architectures including spheres, dendrites, and cubes. Moreover, we study the growth behavior of these materials on substrates and compare them to what is known in solution. We find that the growth of coreshell nanostructures on surfaces is a highly anisotropic process and often results in multimetallic anisotropic nanostructures, in contrast with the isotropic processes that occur in other conventional solution-based seedmediated growth.10,11 Different from the anisotropic nanostructures grown from solution-based seed-mediated synthesis, which is carefully controlled by the reaction kinetics, the anisotropic growth of core shell nanostructures in this article is intrinsically controlled by the surface-immobilized seeds. In particular, growth is initiated at the most active sites (corners and edges) of the isotropic seeds and propagates around them to form anisotropic structures, which leads to binary nanocrystals with the seed located near the surface of each particle as opposed to the center.12,13 VOL. XXX
’
NO. XX
’
* Address correspondence to
[email protected]. Received for review August 19, 2015 and accepted November 3, 2015. Published online 10.1021/acsnano.5b05191 C XXXX American Chemical Society
000–000
’
XXXX
A www.acsnano.org
RESULTS AND DISCUSSION The process for synthesizing coreshell nanoparticles on a surface consists of two steps: (1) patterning and generation of the core via SPBCL and (2) subsequent growth of the shell (Figure 1). In this study, we choose Au nanoparticles as the cores because they can be routinely generated on a surface using SPBCL.2,7 In a typical experiment, HAuCl4 precursor was mixed with an aqueous solution of block copolymer, polyethyleneblock-poly(2-vinylpyridine), to form an ink. The ink was then patterned into an array of nanoscale features on hydrophobic substrates using an array of atomic force microscopy (AFM) tips. The patterned polymer features were subject to two sequential annealing steps at 150 C for 4 h and 500 C for 2 h under an Ar atmosphere, resulting in an array of individual Au nanoparticles on each patterned spot. The diameter of the particles directly corresponds with the volumes of the SPBCL features, as described in the literature.2,7 The substrates with the patterned arrays of Au nanoparticles were floated on growth solutions that contained other metal precursors to effect the growth of half-shell metallic structures. Since the bottom of the Au nanoparticles is sterically blocked by the substrate, the second material grows predominately on top of the Au nanoparticles, thus forming half-shell structures. The shape of such structures can be either hemispherical, dendrite, or cube-like, depending on the growth materials and conditions.14,15 Typically, ascorbic acid (AA) was used as the reducing agent to reduce metal precursors from their oxidized to elemental states. Selectively Growing Au on Patterned Au Seeds on a Surface. As a proof-of-concept, we first tested the feasibility of growing Au on patterned Au nanoparticle seeds. Using standard SPBCL conditions, we generated 12 nm LIU ET AL.
ARTICLE
Herein, we initially explore the feasibility of using patterned Au nanoparticles as seeds to grow larger Au nanoparticles. This is the surface equivalent of the well-known solution-based seed-mediated synthesis14 of nanoparticles. We then explore the generation of two-component particles, where Pd is grown on Au seeds. Importantly, the two components can be differentiated by transmission electron microscopy (TEM), allowing us to use the Au particle as a label to track the evolution of the process.13 Finally, we explore the potential to make other types of binary systems, including AuPt and AuAg bimetallic nanoparticles and one three-component system best described as a AuPdPt coreshelldendrite nanoparticle. In addition to providing insight into the shape evolution of these particles on surfaces, this work begins to lay the foundation for using the SPBCL process to synthesize a wide variety of nanoclusters on surfaces that may become important in a variety of studies and applications pertaining to catalysis, optics, electronics, and chemical and biological sensing.
Figure 1. Schematic procedure for synthesizing coreshell bimetallic nanoparticles on a surface. Patterned arrays of Au nanoparticles (yellow) on a substrate (black) are used as seeds to grow a second metal (orange) and form various bimetallic nanostructures, including Au@Au and Au@Ag spheres (top), Au@Pt dendrites (middle), and Au@Pd cube-like structures (bottom). CTAB, hexadecyltrimethylammonium bromide; AA, ascorbic acid.
diameter Au particles that were used as seeds to grow other structures. The substrates with patterned arrays of Au seeds were floated, particle side down, on a Au(I) growth solution that was prepared by mixing HAuCl4 and the surfactant hexadecyltrimethylammonium bromide (CTAB) with AA.14 Over the course of 2 h, the Au nanoparticles grew to 90 nm diameter spheres (Figure 2). The growth of the Au on patterned Au seeds is a heterogeneous process, which is much faster than homogeneous nucleation of Au nanoparticles in solution.16 Consequently, one can adjust the growth conditions (temperature and the concentrations of CTAB, HAuCl4, and AA)14,1719 to ensure that Au only grows on the patterned seeds, resulting in an array of larger diameter hemispherical Au nanoparticles at the same locations occupied by the seeds on the substrates. In addition, the diameter of the resulting nanoparticles exceeds the range of diameters (up to 60 nm and spherical)2 that can be accessed using SPBCL. The nanoparticles obtained via this seedmediated procedure can be either single crystalline or polycrystalline, depending on the crystallinity of the seeds (single crystalline or polycrystalline). Since the particle structures are characterized by TEM (Figure 2), the substrates used in these experiments are silicon nitride TEM membranes. Other substrates were also tested, including silicon and silicon oxide, and similar arrays of Au nanoparticles can be grown from patterned Au seeds on all of them. Synthesis of Au@Pd Core@Shell Bimetallic Nanoparticles on a Surface. After the proof-of-concept demonstration of growing Au on patterned Au seeds, we then used the method to study the synthesis of bimetallic nanoparticles on surfaces. Bimetallic nanoparticles are interesting and important because multicomponent systems often have synergistic effects in catalysis, and the catalytic activities can be superior to particles made VOL. XXX
’
NO. XX
’
000–000
’
B
XXXX www.acsnano.org
ARTICLE Figure 2. (a) Zoom-out and (b) zoom-in SEM images of an array of Au nanoparticles on a silicon substrate. Low-angle annular dark-field scanning transmission electron microscope images of (c) a Au nanoparticle seed (12 nm in diameter) and (d) a subsequently grown Au nanoparticle (90 nm in diameter).
from either one of the elements alone.10,2024 As a test case, we examined the growth of Pd on patterned Au nanoparticle seeds. Since the core can be differentiated from the shell by a variety of techniques, this study also provides insight into bimetallic particle growth pathways. An array of Au nanoparticle seeds was prepared on a silicon nitride membrane by SPBCL. The membrane with the patterned Au seeds was floated on a solution that contained a Na2PdCl4 precursor for the growth of Pd. Similar to the growth of Au on patterned Au seeds, long growth times (>30 min) can result in homogeneous nucleation of Pd seeds in the solution, which subsequently grow into pure Pd cube-like structures that can be attached to the substrate. By adjusting the growth conditions (time, temperature, and the concentrations of CTAB, Na2PdCl4, and AA, respectively), one can suppress the homogeneous nucleation of Pd in the solution and ensure that Pd only grows on the patterned Au seeds to form Au@Pd core@shell nanoparticles (Figure 3). A magnified view of the nanoparticles shows that each Au@Pd nanoparticle adopts the shape of a cube. Although only an example 2 2 array is shown for clarity, the entire array can span millimeter areas and consist of millions of particles.1,2 X-ray energy-dispersive spectrometry (EDS) mapping of Au (L line) and Pd (L line) on a high-angle annular darkfield (HAADF) scanning transmission electron microscope (STEM) confirms that the bimetallic nanoparticle is composed of a Au core and a cube-shaped Pd shell (Figure 4). Depending on the growth conditions, Pd nanocubes on the substrate exhibit various orientations that differ widely from one another. We observed that the orientation of Pd nanocubes correlates with the size of the patterned Au seeds. For Pd nanoparticles that derive from small Au seeds (Figure 3, seed diameter ∼7 nm), the core does not provide enough constraint to direct the growth of the Pd shell layer, and the Pd tends to grow and form a crystal with the Æ100æ axis normal to the substrate due to the steric effects of the substrate. For Pd nanoparticles that grow from larger LIU ET AL.
Figure 3. (Left) HAADF-STEM images of (top) a patterned array of Au nanoparticle seeds on a silicon nitride membrane and (bottom) Au core/Pd shell nanocubes grown from the patterned Au seeds. (Right) Magnified view of a Au seed and a Au@Pd nanocube. The Au core appears brighter than the Pd shell.
Au seeds (Figure 4, seed diameter ∼35 nm), however, the seed directs the growth of the Pd shell layer and the Pd shell follows a certain crystal orientation, resulting in tilted cube-shaped nanoparticles on the substrate (a flat face is not lying on the substrate). The tilt angle, with respect to the top surface of the substrate, varies from particle to particle. The cube-like structure is independent of the seed size but believed to be determined by the CTAB surfactant in the solution.19 Since Br from CTAB preferentially adsorbs onto Pd{100} surfaces,25 it drives the cube-forming growth process.14 To locate the exact position of the Au cores in the cube-like two-component structures, STEM electron tomography was used to characterize the threedimensional structure of the Au@Pd nanoparticles (Figure 5). For example, the Au@Pd nanocube can be reconstructed from a series of STEM images spanning a rotational angle from 47 to þ59 with an interval of VOL. XXX
’
NO. XX
’
000–000
’
C
XXXX www.acsnano.org
ARTICLE Figure 4. HAADF-STEM-EDS mapping of a Au@Pd nanoparticle. Pd and Au are labeled red and yellow, respectively. EDS spectra and electron diffraction patterns show that both Au and Pd are present in region 1, and only Pd is present at the edges in region 2. In the electron diffraction patterns, two sets of peaks in region 1 correspond to Au and Pd, and one set of peaks in region 2 corresponds to Pd. The diffraction patterns are taken along the Pd Æ100æ zone axis.
Figure 5. HAADF-STEM images of a Au@Pd nanocube at various rotating angles. The Au core appears brighter than the Pd shell. The corresponding rotating angles are shown in the insets in degrees. The scale bar applies to all images.
2 (video S1). From both the static images and the three-dimensional reconstruction, one can clearly see that the Au core is not in the center of the particle or the plane parallel to the substrate. Instead, it is always located off-center and at the bottom of the nanoparticle closest to the patterned surface. The off-center Au core in the Pd shell is similar to what was observed with Pd@Cu coreshell nanostructures reported previously,26 in which the Pd seeds are rarely located in the center of each Pd@Cu coreshell nanostructure. Despite the similarity, the causes of this behavior are different. In the reported Pd@Cu nanostructure,26 the off-center position of the Pd core in the Cu shell is due to the different growth rate of the shell on different facets of the seeds or the localized epitaxial growth of Cu on Pd seeds. In this work, the Au spherical seeds are bound to the substrate, and because of steric effects, the Pd shells can only grow on top of the Au seeds. As a result, the Au seeds are not fully enclosed in the center of the coreshell particle. Tomography reveals that LIU ET AL.
the surfaces of Au@Pd nanocubes are rough and sawtooth-like. Such surfaces have high surface energies and can potentially enhance the activity of nanoparticles to catalyze chemical reactions. Interestingly, despite such rough surfaces, the cube-like Pd shell is single crystalline, as evidenced by electron diffraction patterns taken at the nanoparticle edges (Figure 4). In contrast to the single diffraction pattern at the nanoparticle edges, we observed two in the center of the nanoparticle, corresponding to the Au core along the Æ111æ zone axis and the Pd shell along the Æ100æ zone axis, confirming the presence of both materials. To understand the heterogeneous growth mechanism of Au@Pd nanocubes on surfaces, we monitored the nanoparticle evolution by taking STEM snapshots at various time points during growth. Low-angle annular dark-field (LAADF) STEM images, which show more diffraction contrast than HAADF-STEM images, reveal that the Au seed is penta-twinned (Figure 6). Initially, Pd grows only at the corners of the penta-twinned Au seed VOL. XXX
’
NO. XX
’
000–000
’
D
XXXX www.acsnano.org
ARTICLE
Figure 6. LAADF-STEM images of Au@Pd coreshell nanoparticles at various lengths of growth time. Pd initiates the growth at the most active edges of the Au seed and then crystallizes around it. Note that the Au seed does not necessary locate at the center of the cube.
Figure 7. (A) HAADF-STEM image of Au@Pt coreshell nanoparticles and (BD) bright-field TEM images of stages of Pt growth on a patterned Au seed at various time points. Pt growth is initiated at the most active edges of the Au seed and then crystallizes around it to form dendrite structures.
because these corners have high surface energies compared with the flat facets. Gradually, Pd covers the entire Au seed and forms a primitive nanocube. Eventually, a full cube-shaped particle is grown, which engulfs the Au seed. Continued growth results only in an increase in the edge length and Pd content of the Au@Pd cube. The growth that occurs on surface-bound Au seeds is markedly different from that on solution-immersed Au seeds. In the latter case, which is essentially seedmediated growth of nanoparticles in a bulk solution, the unpatterned Au seeds are fully immersed in the Pd precursor solution, and the reduced Pd grows uniformly and isotropically around the Au seeds.27 In contrast, for the surface-based growth of coreshell nanoparticles, Pd initially grows only locally at certain corners and edges of the crystalline Au seeds (Figure 6, second panel) and then propagates around the top hemisphere of the Au seeds. The growth pathway of the coreshell structure is determined by two factors. First, the steric effects of the surface-immobilized seed confine the diffusion direction of the precursors to the patterned Au seeds; that is, Pd can only diffuse to the Au seeds in a direction from the solution to the top hemisphere and result in anisotropic nanoparticles. To some extent, this observation of anisotropic shell growth on surface-bound seeds differs from the isotropic shell growth reported by Gilroy et al.28 In the previous work by Gilroy et al., they used SEM to characterize the coreshell nanoparticles, and it appeared that the shells grew isotropically on surfaceimmobilized seeds.28 Since SEM is a scanning technique, it can provide information about the top surface layer of the nanoparticles, but it is difficult to probe beneath the surface-bound seeds. A three-dimensional LIU ET AL.
reconstruction of the entire coreshell nanoparticle synthesized here (Figure 5) confirms that shell growth on surface-immobilized seeds is, in fact, anisotropic, with the bottom part of the seeds directly attached to the substrate. This phenomenon is expected based on the fact that the seeds are directly attached to the substrate, and it is impossible to grow the shell between the surface-bound seeds and the substrate. In contrast, when Au nanoparticle seeds are suspended in a Pd precursor solution, Pd can diffuse to the Au seeds from any direction and grow uniformly around the seeds.27 Second, the surface-bound Au seeds have a high degree of crystallinity compared with Au seeds synthesized in solution29 (Figure S2) because they are subject to high-temperature (500 C) annealing during the SPBCL process.2 The Au nanoparticle seeds have developed into well-defined crystalline structures and can be characterized as either single crystalline or twinned polycrystalline structures, as determined by the X-ray diffraction patterns (Figure 4) and STEM images (Figure 4, Figure 6 first panel, and Figure 7 second panel). As a result, certain facets and edges have high surface energies, and the growth of Pd around these high-surface-energy edges is much faster than that in other areas, leading to preferential growth of Pd around these corners. The preferred growth of Pd at high-surface-energy site is in agreement with previous observations by Jungjohann et al.,27 in which Pd tends to incorporate on Au particles at low-coordination sites during the growth of Pd on free-floating Au seeds. Synthesis of Au@Pt and Au@Ag CoreShell Bimetallic Nanoparticles on a Surface. In addition to Pd, metals such as Pt and Ag are also important materials in heterogeneous catalysis.3033 Therefore, we investigated the growth of these materials on surface-immobilized Au seeds. VOL. XXX
’
NO. XX
’
000–000
’
E
XXXX www.acsnano.org
ARTICLE Figure 8. (Left) HAADF-STEM image and the corresponding EDS maps of Ag (green) and Au (yellow) in a Au@Ag coreshell bimetallic nanoparticle. The EDS spectra of the nanoparticle in different regions are shown at the bottom. (Right) Highresolution TEM image of a Au@Ag coreshell bimetallic nanoparticle.
Similar to the Au@Pd system, TEM was used to visualize the growth pathway for the Au@Pt particles (Figure 7). It is documented11 that three conditions must be met to ensure conformal growth of one metal on another following the Frankvan der Merwe mode. (1) The lattice mismatch between the two metals must be less than 5%. In rare cases, conformal growth can still occur when the lattice mismatch is larger than 5%, but the atomic radius of the shell metal is smaller than that of the core to release lattice strain.26 (2) The electronegativity (χ) of the shell is lower than that of the core to avoid galvanic displacement. (3) The bond energy between the shell atoms is smaller than that between the core and shell atoms. For Au@Pt nanoparticles, the first two conditions are met (lattice constants are 4.080 and 3.920 Å for Au and Pt, respectively; a 4.1% mismatch; χPt = 2.28 < χAu = 2.54), but as a consequence of the strong affinity of Pt with itself (PtPt bond dissociation energy, D298 = 306.7 kJ/mol),11,34 conformal growth of Pt on Au is prohibited and dendritic structures form on the patterned Au seeds. Further growth of Pt resulted in raspberry-shaped Au@Pt nanoparticles, similar to the observations in the previous work by Wang et al.10 and Fan et al.11 Despite the similarities, the Au@Pt dendritic structure reported here differs from the previously identified Au@Pt dendritic structure (Figure S2).10,11 Under our experimental conditions, Pt initiates the growth from only one active site (the edges and corners of seeds where surface energies are high) and propagates around the seed to form dendritic structures, while in the growth of Pt dendrites on solution-dispersed seeds,10,11 Pt grows uniformly from multiple sites surrounding each particle to form dendritic structures (Figure S2). In contrast with Pt growth on Au seeds, the lattice match of Ag and Au is quite good (4.080 and 4.090 Å for Au and Ag, respectively); the atomic radii of Ag and Au are similar (1.442 and 1.444 Å for Au and Ag, respectively); the electronegativity of Ag is less than that of Au (χAg = 1.93 < χAu = 2.54), and AgAg bond energy (162.9 kJ/mol) is much lower than AuAg bond energy (203 kJ/mol). All conditions are met for LIU ET AL.
Figure 9. (Left) Schematic procedure for the synthesis of Au@Pd@Pt coreshelldendrite nanoparticles on a surface. (Top right) LAADF-STEM image and (bottom right) bright-field TEM image of Au@Pd@Pt coreshelldendrite nanoparticles. Au appears the brightest in the LAADF-STEM image and the darkest in the bright-field TEM image. Pt dendritic structures can be seen at the crystal facets of Pd.
conformal growth, and therefore, Ag tends to grow on immobilized Au seeds in a conformal manner, resulting in nanoparticles with hemispherical shapes (Figure 8). By TEM, each particle appears as a Au particle encased in a Ag half-shell. Synthesis of Au@Pd@Pt Trimetallic Nanoparticles on a Surface. This method of surface-seed-mediated growth can be further generalized to grow three-component nanoparticles on surfaces, something never explored before (Figure 9). For example, Au@Pd@Pt trimetallic nanoparticles were grown by initially floating the substrate with Au seeds face-down in a solution containing Na2PdCl4 to grow Au@Pd nanocubes. The substrate with Au@Pd nanocubes was removed from the first growth solution, rinsed with deionized water, and then placed face-down in a second growth solution containing H2PtCl6. This resulted in Au@Pd@Pt trimetallic nanoparticles, which were characterized by TEM (Figure 9, right panel). The sequential growth of Pd VOL. XXX
’
NO. XX
’
000–000
’
F
XXXX www.acsnano.org
ARTICLE Figure 10. LAADF-STEM image and the corresponding EDS maps of Au@Pd@Pt coreshelldendrite trimetallic nanoparticles. Au, Pd, and Pt are labeled in yellow, red, and pink, respectively. The EDS spectrum of the entire nanoparticle is shown on the bottom right.
on Au and Pt on Pd allows one to easily distinguish these metals under TEM. EDS mapping was used to further confirm the composition and distribution of the elements in the particles (Figure 10). Au, Pd, and Pt are labeled in yellow, red, and pink, respectively, and it is quite clear that the Au core is embedded inside a Pd cube but not perfectly positioned in the center of the cube. The facets of the Pd cube can be distinguished from the Pt and the Au, and they are coated with speckles of Pt. Due to the threedimensional projection of the cube-like structure, the Pt signal at the edges is stronger than that at the top surface. The spectrum of the entire nanoparticle confirms the presence of all constituent elements including Au, Pd, and Pt, yet Pd exhibits the strongest signal because the majority of the nanoparticle is a Pd cube. The Au signal is relatively weak compared with that of Pd because the amount of Au is small and it is embedded inside the Pd. Since there is only a small amount of Pt around the nanoparticle edges and surfaces, the Pt signal is the weakest among all elements surveyed. CONCLUSION The data presented herein introduce a general method for synthesizing and tracking the growth steps for coreshell multimetallic nanoparticles on surfaces. This surface-based method uses prepatterned Au seeds to grow shells of the same or different elements,
EXPERIMENTAL SECTION Materials. Block copolymer PEO-b-P2VP (Mn = 2.8-b-1.5 kg mol1, polydispersity index = 1.11) was purchased from Polymer Source and used as received. All metal precursors, HAuCl4 3 3H2O, AgNO3, H2PtCl6 3 6H2O, and Na2PdCl4, were purchased from Sigma-Aldrich. CTAB, AA, hexamethyldisilazane, and hexane were purchased from Sigma-Aldrich and used as received. DPN pen arrays (type M, no gold coating) were
LIU ET AL.
resulting in Au@Au, bimetallic Au@Pd, Au@Pt, and Au@Ag, and trimetallic Au@Pd@Pt particles. By imaging particles at various time points during growth, we have elucidated the shape evolution and growth pathways for the different compositions, and the structures obtained are different from what one can make via conventional seed-mediated growth in solution. The reasons for this observation are that (1) the surface acts as a mask and leads to anisotropic rates of diffusion to the seed particle and (2) the lattice match (or mismatch) between the seed and growth material, along with the presence of surfactant, significantly influences the geometric outcome of the particle. The synthesis method developed herein is built upon our previously developed SPBCL technique and significantly expands its capability of patterning nanomaterials on surfaces with control over their composition,2 size,2,7 shape, and elemental distribution. The patterned nanoparticles can potentially provide a platform for fundamental studies in single nanoparticle catalysis,35 combinatorial screening of catalysts,3639 and the development of biological probes.4042 Furthermore, we envision that the on-surface synthesis of multicomponent nanoparticles will not only provide insights into the mechanism of seed-mediated nanoparticle growth but also offer an opportunity to probe the importance of atomic interactions between different elements inside individual nanoparticles.
purchased from Nanoink, Inc. Hydrophobic silicon nitride membranes (membrane thickness = 15 or 50 nm) were purchased from Ted Pella, Inc. Silicon wafers were purchased from Nova Electronic Materials. Synthesis of Au Nanoparticle Seed Arrays. Following our SPBCL protocol2 of synthesizing nanoparticles on a surface, aqueous solutions of HAuCl4 (16.33 mg/mL) and PEO-b-P2VP (5 mg/mL) were mixed together (2VP/Au = 64:1) and stirred overnight.
VOL. XXX
’
NO. XX
’
000–000
’
G
XXXX www.acsnano.org
LIU ET AL.
The three-dimensional reconstruction was performed by using IMOD software.43 High-resolution TEM was performed on a JEOL 2100F at an acceleration voltage of 200 kV. Conflict of Interest: The authors declare no competing financial interest. Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b05191. STEM images and EDS mapping of Au@Pd nanoparticles, and TEM images of Au@Pt nanoparticles from solutionbased seed-mediated growth (PDF) Three-dimensional reconstruction of Au@Pd nanoparticles (AVI)
ARTICLE
To pattern arrays of polymer features that contain the Au nanoparticle precursors, an array of AFM tips was dipped into the solution and then brought into contact with a hydrophobic substrate using an NScriptor (Nanoink, Inc.). The patterned substrates were then subjected to a two-step annealing process in a tube furnace under an Ar atmosphere. Typically, the samples were first annealed at 150 C for 4 h and then at 500 C for 2 h. The annealing time can vary, but the two separate annealing steps are critical, as discussed previously.2 The Ar flow rate is 300 sccm. Synthesis of Au@Au Nanoparticles on a Surface. Ten milliliters of 50 mM CTAB solution was added into a 20 mL vial and then heated to 40 C in a water bath. Afterward, 0.5 mL of 10 mM HAuCl4 and 2 mL of 100 mM ascorbic acid were sequentially added and shaken thoroughly. Subsequently, substrates with patterned arrays of Au nanoparticles were floated on the solution. The entire solution was soaked in a 40 C water bath for 2 h to allow for the growth of a Au shell. After the growth reaction, the substrates were removed from the solution and rinsed with deionized water at least three times. Synthesis of Au@Pd Nanoparticles on a Surface. Ten milliliters of 50 mM CTAB solution was added into a 20 mL vial and then heated to 40 C in a water bath. Afterward, 0.5 mL of 10 mM Na2PdCl4 and 2 mL of 100 mM ascorbic acid were sequentially added and shaken thoroughly. Subsequently, substrates with patterned arrays of Au nanoparticles were floated on the solution. The entire solution was soaked in a 40 C water bath for various lengths of time to allow for the growth of a Pd shell. After the growth reaction, the substrates were removed from the solution and rinsed with deionized water at least three times. Synthesis of Au@Pt Nanoparticles on a Surface. Ten milliliters of 50 mM CTAB solution was added into a 20 mL vial. Afterward, 1.0 mL of 10 mM H2PtCl6 and 2 mL of 100 mM ascorbic acid were sequentially added and shaken thoroughly. Subsequently, substrates with patterned arrays of Au nanoparticles were floated on the solution. The entire solution was soaked in a 40 C water bath for various lengths of time to allow for the growth of Pt. After the growth reaction, the substrates were removed from the solution and rinsed with deionized water at least three times. Synthesis of Au@Ag Nanoparticles on a Surface. Ten milliliters of 50 mM CTAB solution was added into a 20 mL vial. Afterward, 0.5 mL of 10 mM AgNO3 and 2 mL of 100 mM ascorbic acid were sequentially added and shaken thoroughly. Subsequently, substrates with patterned arrays of Au nanoparticles were floated on the solution. The entire solution was soaked in a 40 C water bath for various lengths of time to allow for the growth of Ag. After the growth reaction, the substrates were removed from the solution and rinsed with deionized water at least three times. Synthesis of Au@Pd@Pt Nanoparticles on a Surface. Ten milliliters of 50 mM CTAB solution was added into a 20 mL vial and then heated to 40 C in a water bath. Afterward, 0.5 mL of 10 mM Na2PdCl4 and 2 mL of 100 mM ascorbic acid were sequentially added and shaken thoroughly. Subsequently, substrates with patterned arrays of Au nanoparticles were floated on the solution. The entire solution was soaked in a 40 C water bath for 15 min. After the growth reaction, the substrates were removed from the solution and rinsed with deionized water at least three times. Concurrently, 10 mL of 50 mM CTAB solution was added into a second 20 mL vial. Afterward, a 1.0 mL aliquot of 10 mM H2PtCl6 and a 2 mL aliquot of 100 mM ascorbic acid were sequentially added to the growth solution and shaken thoroughly. Subsequently, the substrates with grown Au@Pd nanoparticles were floated on the solution. The entire solution was soaked in a 40 C water bath for 20 min to allow for the growth of Pt. STEM, STEM Tomography, and STEM-EDS. STEM was performed on a Hitachi STEM HD-2300A by a high-angle annular dark-field detector and a bright-field detector at an acceleration voltage of 200 kV and a current of 78 μA. EDS spectra were obtained with Thermo Scientific NSS 2.3. For electron tomography, the sample was rotated from 47 to þ59 in a 2 increment.
Acknowledgment. This material is based upon work supported by the AFOSR Award FA9550-12-1-0280; the Nonequilibrium Energy Research Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences Award DE-SC0000989; and UOP LLC Research Agreement 7/18/2011.
REFERENCES AND NOTES 1. Chai, J.; Huo, F.; Zheng, Z.; Giam, L. R.; Shim, W.; Mirkin, C. A. Scanning Probe Block Copolymer Lithography. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 20202–20206. 2. Liu, G.; Eichelsdoerfer, D. J.; Rasin, B.; Zhou, Y.; Brown, K. A.; Liao, X.; Mirkin, C. A. Delineating the Pathways for the Site-Directed Synthesis of Individual Nanoparticles On Surfaces. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 887–891. 3. Piner, R. D.; Zhu, J.; Xu, F.; Hong, S. H.; Mirkin, C. A. 00 DipPen00 Nanolithography. Science 1999, 283, 661–663. 4. Salaita, K.; Wang, Y.; Mirkin, C. A. Applications of Dip-Pen Nanolithography. Nat. Nanotechnol. 2007, 2, 145–155. 5. Hamley, I. W. Nanotechnology With Soft Materials. Angew. Chem., Int. Ed. 2003, 42, 1692–1712. 6. Hawker, C. J.; Russell, T. P. Block Copolymer Lithography: Merging 00 Bottom-Up00 With 00 Top-Down00 Processes. MRS Bull. 2005, 30, 952–966. 7. Liu, G.; Zhou, Y.; Banga, R. S.; Boya, R.; Brown, K. A.; Chipre, A. J.; Nguyen, S. T.; Mirkin, C. A. The Role of Viscosity on Polymer Ink Transport in Dip-Pen Nanolithography. Chem. Sci. 2013, 4, 2093–2099. 8. Chen, P. C.; Liu, G. L.; Zhou, Y.; Brown, K. A.; Chernyak, N.; Hedrick, J. L.; He, S.; Xie, Z.; Lin, Q. Y.; Dravid, V. P.; et al. TipDirected Synthesis of Multimetallic Nanoparticles. J. Am. Chem. Soc. 2015, 137, 9167–9173. 9. Chai, J.; Liao, X.; Giam, L. R.; Mirkin, C. A. Nanoreactors for Studying Single Nanoparticle Coarsening. J. Am. Chem. Soc. 2012, 134, 158–161. 10. Wang, S. Y.; Kristian, N.; Jiang, S. P.; Wang, X. Controlled Synthesis of Dendritic Au@Pt Core-Shell Nanomaterials for Use As an Effective Fuel Cell Electrocatalyst. Nanotechnology 2009, 20, 025605. 11. Fan, F.-R.; Liu, D.-Y.; Wu, Y.-F.; Duan, S.; Xie, Z.-X.; Jiang, Z.-Y.; Tian, Z.-Q. Epitaxial Growth of Heterogeneous Metal Nanocrystals: From Gold Nano-Octahedra to Palladium and Silver Nanocubes. J. Am. Chem. Soc. 2008, 130, 6949–6951. 12. Xue, C.; Millstone, J. E.; Li, S. Y.; Mirkin, C. A. Plasmon-Driven Synthesis of Triangular Core-Shell Nanoprisms From Gold Seeds. Angew. Chem., Int. Ed. 2007, 46, 8436–8439. 13. Langille, M. R.; Zhang, J.; Personick, M. L.; Li, S. Y.; Mirkin, C. A. Stepwise Evolution of Spherical Seeds into 20-Fold Twinned Icosahedra. Science 2012, 337, 954–957. 14. Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics? Angew. Chem., Int. Ed. 2009, 48, 60–103. 15. Sun, Y.; Xia, Y. Shape-Controlled Synthesis of Gold and Silver Nanoparticles. Science 2002, 298, 2176–2179. 16. Cao, G.; Wang, Y. Nanostructures and Nanomaterials: Synthesis, Properties & Applications; Imperial College Press: London, 2011. 17. Tao, A. R.; Habas, S.; Yang, P. D. Shape Control of Colloidal Metal Nanocrystals. Small 2008, 4, 310–325.
VOL. XXX
’
NO. XX
’
000–000
’
H
XXXX www.acsnano.org
LIU ET AL.
40. Murphy, C. J.; Gole, A. M.; Stone, J. W.; Sisco, P. N.; Alkilany, A. M.; Goldsmith, E. C.; Baxter, S. C. Gold Nanoparticles in Biology: Beyond Toxicity to Cellular Imaging. Acc. Chem. Res. 2008, 41, 1721–1730. 41. Rosi, N. L.; Mirkin, C. A. Nanostructures in Biodiagnostics. Chem. Rev. 2005, 105, 1547–1562. 42. Katz, E.; Willner, I. Integrated Nanoparticle-Biomolecule Hybrid Systems: Synthesis, Properties, and Applications. Angew. Chem., Int. Ed. 2004, 43, 6042–6108. 43. Kremer, J. R.; Mastronarde, D. N.; McIntosh, J. R. Computer Visualization of Three-Dimensional Image Data Using IMOD. J. Struct. Biol. 1996, 116, 71–76.
VOL. XXX
’
NO. XX
’
000–000
’
ARTICLE
18. Grzelczak, M.; Perez-Juste, J.; Mulvaney, P.; Liz-Marzan, L. M. Shape Control in Gold Nanoparticle Synthesis. Chem. Soc. Rev. 2008, 37, 1783–1791. 19. Wang, Y. W.; He, J. T.; Liu, C. C.; Chong, W. H.; Chen, H. Y. Thermodynamics Versus Kinetics in Nanosynthesis. Angew. Chem., Int. Ed. 2015, 54, 2022–2051. 20. Toshima, N.; Yonezawa, T. Bimetallic Nanoparticles - Novel Materials for Chemical and Physical Applications. New J. Chem. 1998, 22, 1179–1201. 21. Song, H. M.; Anjum, D. H.; Sougrat, R.; Hedhili, M. N.; Khashab, N. M. Hollow Au@Pd and Au@Pt Core-Shell Nanoparticles As Electrocatalysts for Ethanol Oxidation Reactions. J. Mater. Chem. 2012, 22, 25003–25010. 22. Song, H. M.; Moosa, B. A.; Khashab, N. M. Water-Dispersable Hybrid Au-Pd Nanoparticles As Catalysts in Ethanol Oxidation, Aqueous Phase Suzuki-Miyaura and Heck Reactions. J. Mater. Chem. 2012, 22, 15953–15959. 23. Lee, Y. W.; Kim, M.; Kim, Y.; Kang, S. W.; Lee, J. H.; Han, S. W. Synthesis and Electrocatalytic Activity of Au-Pd Alloy Nanodendrites for Ethanol Oxidation. J. Phys. Chem. C 2010, 114, 7689–7693. 24. Fang, P. P.; Duan, S.; Lin, X. D.; Anema, J. R.; Li, J. F.; Buriez, O.; Ding, Y.; Fan, F. R.; Wu, D. Y.; Ren, B.; et al. Tailoring Au-Core Pd-Shell Pt-Cluster Nanoparticles for Enhanced Electrocatalytic Activity. Chem. Sci. 2011, 2, 531–539. 25. Carrasquillo, A.; Jeng, J. J.; Barriga, R. J.; Temesghen, W. F.; Soriaga, M. P. Electrode-Surface Coordination Chemistry: Ligand Substitution and Competitive Coordination of Halides at Well-Defined Pd(100) and Pd(111) Single Crystals. Inorg. Chim. Acta 1997, 255, 249–254. 26. Jin, M. S.; Zhang, H.; Wang, J. G.; Zhong, X. L.; Lu, N.; Li, Z. Y.; Xie, Z. X.; Kim, M. J.; Xia, Y. N. Copper Can Still Be Epitaxially Deposited on Palladium Nanocrystals To Generate CoreShell Nanocubes Despite Their Large Lattice Mismatch. ACS Nano 2012, 6, 2566–2573. 27. Jungjohann, K. L.; Bliznakov, S.; Sutter, P. W.; Stach, E. A.; Sutter, E. A. In Situ Liquid Cell Electron Microscopy of the Solution Growth of Au-Pd Core-Shell Nanostructures. Nano Lett. 2013, 13, 2964–2970. 28. Gilroy, K. D.; Hughes, R. A.; Neretina, S. Kinetically Controlled Nucleation of Silver on Surfactant-Free Gold Seeds. J. Am. Chem. Soc. 2014, 136, 15337–15345. 29. Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Preparation and Characterization of Au Colloid Monolayers. Anal. Chem. 1995, 67, 735–743. 30. Yu, W. T.; Porosoff, M. D.; Chen, J. G. G. Review of Pt-Based Bimetallic Catalysis: From Model Surfaces to Supported Catalysts. Chem. Rev. 2012, 112, 5780–5817. 31. Su, D. S.; Zhang, B. S.; Schlogl, R. Electron Microscopy of Solid Catalysts-Transforming from a Challenge to a Toolbox. Chem. Rev. 2015, 115, 2818–2882. 32. Abou El-Nour, K. M. M.; Eftaiha, A.; Al-Warthan, A.; Ammar, R. A. A. Synthesis and Applications of Silver Nanoparticles. Arabian J. Chem. 2010, 3, 135–140. 33. Chaudhuri, R. G.; Paria, S. Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications. Chem. Rev. 2012, 112, 2373–2433. 34. Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 87th ed., Internet version; Taylor and Francis: Boca Raton, FL, 2007. 35. Sambur, J. B.; Chen, P. Approaches to Single-Nanoparticle Catalysis. Annu. Rev. Phys. Chem. 2014, 65, 395–422. 36. Senkan, S. Combinatorial Heterogeneous Catalysis - A New Path in an Old Field. Angew. Chem., Int. Ed. 2001, 40, 312–329. 37. Cawse, J. N. Experimental Strategies for Combinatorial and High-Throughput Materials Development. Acc. Chem. Res. 2001, 34, 213–221. 38. Jandeleit, B.; Schaefer, D. J.; Powers, T. S.; Turner, H. W.; Weinberg, W. H. Combinatorial Materials Science and Catalysis. Angew. Chem., Int. Ed. 1999, 38, 2494–2532. 39. Mallouk, T. E.; Smotkin, E. S. Combinatorial Catalyst Development Methods. Handbook of Fuel Cells; John Wiley & Sons, Ltd.: New York, 2010.
I
XXXX www.acsnano.org
