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. 2016 Apr 18;120(37):20843–20851. doi: 10.1021/acs.jpcc.6b02103

Structural and Optical Properties of Discrete Dendritic Pt Nanoparticles on Colloidal Au Nanoprisms

Rowan K Leary †,*, Anjli Kumar ‡, Patrick J Straney §, Sean M Collins †, Sadegh Yazdi ⊥, Rafal E Dunin-Borkowski ∥, Paul A Midgley †, Jill E Millstone §,*, Emilie Ringe ‡,⊥,*
PMCID: PMC5036133  PMID: 27688821

Abstract

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Catalytic and optical properties can be coupled by combining different metals into nanoscale architectures in which both the shape and the composition provide fine-tuning of functionality. Here, discrete, small Pt nanoparticles (diameter = 3–6 nm) were grown in linear arrays on Au nanoprisms, and the resulting structures are shown to retain strong localized surface plasmon resonances. Multidimensional electron microscopy and spectroscopy techniques (energy-dispersive X-ray spectroscopy, electron tomography, and electron energy-loss spectroscopy) were used to unravel their local composition, three-dimensional morphology, growth patterns, and optical properties. The composition and tomographic analyses disclose otherwise ambiguous details of the Pt-decorated Au nanoprisms, revealing that both pseudospherical protrusions and dendritic Pt nanoparticles grow on all faces of the nanoprisms (the faceted or occasionally twisted morphologies of which are also revealed), and shed light on the alignment of the Pt nanoparticles. The electron energy-loss spectroscopy investigations show that the Au nanoprisms support multiple localized surface plasmon resonances despite the presence of pendant Pt nanoparticles. The plasmonic fields at the surface of the nanoprisms indeed extend into the Pt nanoparticles, opening possibilities for combined optical and catalytic applications. These insights pave the way toward comprehensive nanoengineering of multifunctional bimetallic nanostructures, with potential applications in plasmon-enhanced catalysis and in situ monitoring of chemical processes via surface-enhanced spectroscopy.

1. Introduction

Metallic nanostructures composed of more than one component have attracted attention because of their vast potential for multifunctionality.1−3 In the best cases, such architectures provide function beyond the sum of their parts, enabling useful or unique combinations such as plasmon-enhanced catalysis,4−7 magnetically addressable drug delivery systems and catalysts,2,8−10 or combined contrast agents and hyperthermal cancer treatment.11−15

The combination of a plasmonic and a catalytic material is particularly interesting because plasmonic materials can interact with light in unique ways. An example is the emergence of localized surface plasmon resonances (LSPRs), which are light-driven, coherent oscillations of the conduction electrons in metallic particles smaller than the wavelength of light. LSPRs create enhanced local electric fields that can increase signal intensity in optical spectroscopies, giving rise to techniques such as localized refractive index sensing16−18 and surface-enhanced Raman,19 fluorescence,20 and infrared absorption spectroscopies.21 These approaches are exceptionally sensitive to subtle changes in surface chemistry, sometimes down to the single-particle level,22,23 and can provide feedback on chemical reactions as they occur.7,19,24−26 The decay of the LSPR can also be useful and may lead to either localized heat27,28 or hot electrons5,6 that can speed up or enable new chemical reactions.4,6,29,30

Combining materials in controllable ways is a prerequisite for the creation of multifunctional optical and catalytic nanostructures. These combinations are particularly interesting in cases where catalytic metals such as Pt and Pd do not sustain strong plasmon resonances (due to significant losses in the visible range31−33) and where more traditional plasmonic metal nanoparticles such as Au and Ag may not be well-suited as catalysts for a target chemical reaction. Several studies have now shown LSPR activity both numerically and experimentally in core–shell bimetallic architectures containing both plasmonic and catalytic materials.34−37 Here, the morphology and optical properties of triangular Au nanoprisms decorated with Pt nanoparticles (NPs) of sizes relevant for catalysis (∼3–6 nm) are reported. The alignment and structure of particles on both the side and large {111} Au facets are disclosed, as are the different LSPRs sustained by these bimetallic architectures.

2. Experimental Details

2.1. Nanoparticle Synthesis

Hexadecyltrimethylammonium bromide (CTAB, 99%), chloroplatinic acid (H2PtCl6, 8 wt % in H2O) hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, 99.999%), l-ascorbic acid (99%), sodium borohydride (NaBH4, 99.99%), sodium hydroxide (99.99%), sodium iodide (NaI, 99.999%), and trisodium citrate (99%) were obtained from Sigma-Aldrich and used as received. NANOpure water (Thermo Scientific, > 18.2 MΩ·cm) was used for all washing, synthesis, and purification protocols, as well as in the preparation of all solutions. All stock solutions were aqueous and prepared fresh before each reaction, unless otherwise noted. All glassware was washed with aqua regia (3:1 ratio of concentrated HCl and HNO3 by volume) and rinsed thoroughly with water. Caution! Aqua regia is highly toxic and corrosive and requires personal protective equipment. Aqua regia should be handled in a fume hood only.

Au nanoprisms were synthesized according to literature protocols.38,39 Two hours after Au nanoparticle seeds were added to the nanoprism growth solution, the reaction mixture was heated in a H2O bath to 37 °C for 1 min to dissolve any CTAB that may have recrystallized during the growth period (this crystallized CTAB can interfere with nanoprism purification by centrifugation). To separate the nanoprisms from pseudospherical nanoparticle reaction byproducts, 90 mL of the reaction mixture was divided into 15 mL conical tubes and centrifuged at 820 relative centrifugal force (rcf) for 15 min (Eppendorf centrifuge 5804 with swing bucket rotor A-4-44). The supernatant and pellet were both extracted, and the nanoprism thin film was resuspended in 1.0 mL of H2O by vortexing for 5 s. To remove additional CTAB and excess reagents, this mixture was transferred to 1.5 mL centrifuge tubes, and the prisms were then centrifuged using a Spectrum mini-centrifuge (SC1006-R) for approximately 5 min. After centrifugation, the supernatant was removed and the prisms were resuspended in 1.0 mL of H2O and subsequently combined in a 15 mL conical tube. The concentration of the purified nanoprisms in the nanoprism stock solution was then adjusted by diluting with H2O to an optical density (O.D.) of 1.0 at the maximum absorption wavelength (λmax, approximately 1300 nm) by ultraviolet–visible–near-infrared (UV–vis–NIR) spectroscopy.

Pt was deposited on the Au nanoprisms using the purified Au nanoprism product (synthesis described above).38 Briefly, 0.5 mL of the prism stock solution (O.D. at λmax = 1.0) was added to a 1.5 mL centrifuge tube and diluted with 0.5 mL of water. To this mixture was added 20 μL of 20 mM ascorbic acid (for a final H2PtCl6:ascorbic acid molar ratio of 1:10), and the solution was briefly mixed by vortexing. Then, 4 μL of 10 mM H2PtCl6 was added (pH 1.8), and the solution was mixed by vortexing again. After allowing 1 h for completion of nanoparticle growth, the reaction mixture was purified from excess reagents by centrifugation (5 min at 2200 rcf using a Spectrum mini-centrifuge (SC1006-R)). After removal of the supernatant, the particles were resuspended in 1.0 mL of H2O by brief sonication (∼10 s). The undecorated and decorated Pt nanoprisms studied were from the same synthesis batch.

2.2. UV–Vis–NIR Spectroscopy

The extinction spectra of the resulting colloid were measured by UV–vis–NIR spectroscopy using a Cary 5000 spectrophotometer (Agilent, Inc.). Spectra were baseline-corrected with respect to the spectrum of water for optical density measurements.

2.3. Electron Microscopy

Particles were drop cast from solution onto Si3N4 membrane windows for electron energy loss spectroscopy (EELS), (TEMWindows Inc.) or thin carbon film supports for tomography and energy-dispersive X-ray spectroscopy (EDS), (Agar Scientific) immediately prior to analysis. Electron energy-loss spectroscopy (EELS) data of decorated Au prisms were obtained on a monochromated, Cs- and Cc-corrected FEI Titan G4 PICO (Forschungszentrum Jülich) equipped with a Gatan Quantum ERS electron spectrometer. EELS data from bare Au nanoprisms, the precursor to the decorated nanoprisms, were obtained on a monochromated, Cs-corrected FEI Titan Themis (Rice University) equipped with a Gatan Quantum ERS electron spectrometer. The pixel dwell time was set to 40–50 μs and 0.02 s on the PICO and Themis, respectively. The beam energy was set to 80 kV in both instruments to optimize the width of the zero-loss peak (ZLP, ∼0.08 eV on the PICO and ∼0.13 eV on the Themis) and to maximize the plasmon interaction cross section. EELS spectrum images were obtained for six decorated Au/Pt nanoprisms and five undecorated Au nanoprisms, from which three representative examples of each are shown in the paper and Supporting Information. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) tilt series images for tomography were acquired on a Cs-corrected FEI Titan G2 ChemiSTEM equipped with a Bruker Super-X quad EDS detector and operated at 80 kV. An EDS spectrum image was acquired with 1024 × 1024 pixels and 4096 energy channels (0.01 keV dispersion) on the same FEI Titan G2 ChemiSTEM instrument. EDS data analysis was performed in HYPERSPY (open source software, available at hyperspy.org) on a 4× spatially binned spectrum image (256 × 256 pixels and 4096 energy channels for improved intensity statistics). X-ray intensities reported are background-subtracted, integrated over an appropriate window centered around the Pt and Au Lα and Lβ1 lines; the M lines for these elements were not used because they present a high degree of overlap.

HAADF-STEM tilt series images for electron tomography were acquired manually, with tilt increments of 2 or 5° and angular ranges of ca. ± 65°, and were aligned using a combination of cross-correlation (FEI Inspect3D) and landmark-based alignment40 (TomoJ). The tilt series images were background-subtracted and normalized such that all images in each tilt series had the same integrated intensity; these steps reduced the impact of carbonaceous contamination and diffraction contrast. Tomographic reconstruction of the background-subtracted tilt series was carried out via a compressed sensing approach,41 using the ASTRA toolbox42,43 (available at https://sourceforge.net/projects/astra-toolbox/) in conjunction with a 3D implementation of TVAL3 (ref (44), available at http://www.caam.rice.edu/~optimization/L1/TVAL3/) for total variation minimization. A thorough description of total variation minimization and its implementation in electron tomographic reconstruction can be found in references (41, 42, 44, and 45). Volume rendering and isosurface visualizations of the 3D reconstructions were generated using ParaView (available at http://www.paraview.org/). The z-direction labeled on the reconstructions is parallel to the optic axis (the “missing wedge” direction; Supporting Information); the y-direction is parallel to the tilt axis, and the x-direction is perpendicular to these.

STEM-EELS data analysis was performed in HYPERSPY. A 200 × 200 pixel × 2048 energy channel (0.01 eV dispersion) spectrum image was acquired for the particle shown in Figures 6 and 7; the spectrum was cropped to 0.25–5.0 eV for analysis. A blind source separation approach based on non-negative matrix factorization (as described previously34,46,47) was used to decompose the spectrum image into a set of n spectral components (or “factors”), which were then used to reconstruct the EEL spectra at each pixel. Because the whole data set is fitted at once, information about both the spectral and spatial distribution of LSPRs is obtained. The same n components are used to fit each spectrum; their relative intensity is varied at each point. The amount by which each component is multiplied for each point is referred to as a loading; mapping this loading provides information about the spatial distribution of the probability of exciting this particular component. Note that components are not constrained to specific lineshapes.

Figure 6.

Figure 6

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Raw (color-coded diamonds, binned 5 × 5 pixels equivalent to 9.3 × 9.3 nm) and fitted (linear combination of NMF-extracted components, black lines) EELS spectra obtained from a Pt-decorated Au nanoprism, at the positions indicated by color-coded squares in the HAADF-STEM image (square sizes exaggerated by a factor of 3 for clarity). Spectra are vertically offset for clarity. The solid and dotted black arrows show the approximate position of the corner and edge LSPRs, respectively. Scale bar, 50 nm.

Figure 7.

Figure 7

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LSPRs and other contributions to EELS spectrum imaging of the Pt-decorated Au nanoprism shown in Figure 6. (a) NMF-extracted spectral components and fit of the raw data, for the EELS response of a 5 × 5 pixel region (9.3 × 9.3 nm) at the position in the inset (purple square size exaggerated by a factor of 3 for clarity). (b) Spatial loadings of each of the components across the full spectrum image; ZLP, IBTs, edge LSPR, and corner LSPR. Scale bars, 50 nm.

3. Results and Discussion

3.1. Structure and Composition

The reduction of H2PtCl6 on large (100–300 nm edge length) colloidal Au nanoprisms yielded an array of small Pt NPs decorating the Au {111} facets (Figure 1). In addition to triangular structures, the reaction mixture contained a small number of various truncated nanoprisms, hexagonal plates, and some elongated, ribbon-like morphologies (Figure S1). All particle shapes are thin plates (typically ca. 7 nm thick), display broad {111} facets, and also support the deposition of Pt NPs.

Figure 1.

Figure 1

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Pt-decorated Au nanoprisms: (a, b) HAADF-STEM images of Au nanoprisms before and after Pt decoration; (c) HAADF-STEM image and (d, e) STEM-EDS maps of Pt and Au. The red and yellow ellipses highlight areas of high and low Pt concentration, respectively. Scale bars, 50 nm.

The composition of the decorated nanoprisms was confirmed to be Au and Pt by EDS. Figure 1d,e shows elemental maps for Pt and Au (obtained from the background-subtracted integrated intensities for both Lα and Lβ1 emission, located at 9.71 and 11.44 keV for Au and 9.44 and 11.07 keV for Pt; a full spectrum and additional maps are available in Figure S2). The EDS map for Au is generally homogeneous, as expected for a prismatic nanoparticle. The EDS map for Pt is not homogeneous. The regions of higher X-ray intensity correlate well with the regions of higher HAADF-STEM intensity, as shown by red ellipses in Figure 1c,d. Thicker regions of material or higher atomic number (Z) material give rise to stronger scattering and thus higher HAADF-STEM intensity. In this case, the values of Z for Au and Pt are nearly identical, and the HAADF contrast is mainly due to thickness effects. The correlation between the Pt EDS map and the HAADF-STEM image indicates that the thicker regions are where Pt NPs have grown. Analogously, the regions where fewer Pt NPs are present, contained in yellow ellipses in Figure 1c,d, generate weaker X-ray emission in the Pt map. Finally, the EDS signal from the NPs located at the edges of the nanoprisms more clearly verifies their chemical identity, containing almost exclusively Pt signals, as shown by the difference map in Figure S2e.

3.2. Electron Tomography

Traditional (electron or other) transmission microscopy techniques provide an image that represents an integral “projection” through an object, meaning that information pertaining to three-dimensional (3D) structure may be hidden. In the case of a very thin nanoprism decorated on both sides, this makes discerning the shape, size, and distribution of the Pt NPs on each respective side difficult, if not impossible. Previous reports used scanning electron microscopy, an essentially surface-sensitive technique, to observe the alignment of the Pt NPs, bypassing the projection obstacle by observing strictly one side of the structure at a time.38 However, that approach does not readily allow analysis of both sides of a given nanoprism, nor of the full 3D structure. Here, tilt series (from large negative to large positive sample tilt angles) of HAADF-STEM images were obtained and used to study the 3D structure of the Pt-decorated Au nanoprisms via tomographic reconstruction (Figure 2 and Supporting Information). This approach is successful because, as noted above, to a sufficient approximation (Supporting Information) the HAADF-STEM signal varies according to the thickness of the specimen (i.e., the thicker the region of the sample, the stronger the high-angle scattering and the higher the image intensity). This enables tomographic reconstruction of a 3D structure involving “back-projection” of the tilt series of images.

Figure 2.

Figure 2

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Volume rendering visualizations of the four Pt-decorated Au nanoprisms studied by electron tomography. (a, b) Planar ∼7 nm thick Au nanoprisms with lower and higher amounts of Pt NP growth, respectively (total thickness ∼15 nm). (c) Smaller (edge length ∼110 nm) but thicker (∼14 nm) Au nanoprism with extensive Pt NP growth (combined thickness ∼25 nm). The inset orthoslice reveals well-defined convex {111} facets of the Au nanoprism. (d) Pt–Au nanoprism with twisted morphology. Further volume rending visualizations are available in Figures S3–S6 and Movies 9–12.

From Figure 2, it can be seen that Pt NPs form in abundance over all of the faces of the nanoprisms (further visualization is provided in the Supporting Information). In addition to enabling more clear analysis of the growth of Pt NPs on each of the large Au {111} surfaces (Figure 2 and vide infra), the 3D reconstructions reveal information about the shape of the underlying Au nanoprism. Interestingly, in addition to the planar morphology expected (Figure 2a–c), one of the nanoprisms is considerably bent (Figure 2d). This is consistent with the occasional observation of strong diffraction contrast in transmission electron microscopy (TEM) and dark-field TEM imaging, a consequence of local bending of the crystal structure. The bent morphology is fully revealed only by electron tomographic reconstruction. However, the vast majority of nanoprisms analyzed exhibit an overall planar morphology dictated by their face-centered cubic crystallography. Figure 2c, for example, shows an orthoslice perpendicular to the large {111} facets, revealing a convex edge structure of the nanoprism, consistent with the presence of slow-growing {111} facets and predicted theoretically from modified Wulff constructions.48,49 These nanoprisms are confirmed here for the first time to be very thin truncated bitetrahedra with eight {111} facets and, as previously known from electron diffraction, containing a central {111} twin plane.50

3.3. Pt NP Morphology

The electron tomography reconstructions also enable detailed analysis of the Pt NP morphologies, as shown in Figures 3 and 4 (further visualization is also provided in the Supporting Information). In Figure 3, orthoslices (2D “slices” through the 3D tomogram) are shown at different heights, intersecting the NPs on the extended faces of the Au nanoprism at different vertical positions, as well as providing cross sections of the NPs on the nanoprism side faces. Projection images (such as conventional 2D HAADF-STEM images) mix the signal from NPs on the top and bottom of the nanoprism, but the 3D tomogram and orthoslices enable separate analysis.

Figure 3.

Figure 3

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Sequential orthoslices (a–g) through the electron tomographic reconstruction of a Pt-decorated Au nanoprism (corresponding to that shown in Figure 2a). Orthoslices are labeled according to the regions of the Pt NPs on the extended faces of the Au nanoprism that they dominantly intersect: roots, trunks, or caps, as labeled on the inset enlarged view of a side-face NP in panel d. Orthoslices for all nanoprisms studied by electron tomography are available in Figures S3–S6 and Movies 5–8.

Figure 4.

Figure 4

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Isosurface renderings of the electron tomographic reconstructions, showing the Pt NP morphologies. (a) Overview of NPs on main and side faces. (b, c) Specific morphologies: (b) distinctive root, trunk and “mushroom-like” caps; (c) irregularly branched (“dendritic-type” cap), and NPs without caps and greater (uncapped trunk) and lesser (stump) height.

The pendant Pt NPs grow epitaxially38 and exhibit a dendrite-like morphology, which is a growth mode observed in several systems involving Pt overgrowth.37,38,51 However, the orthoslices (Figure 3) and 3D isosurface renderings (Figure 4) reveal several distinct motifs. One motif is a “mushroom-like” morphology in which Pt NPs exhibit a narrow “trunk” region that is directly appended to the nanoprism surface and is “capped” by a more broad Pt feature; examples are highlighted in the enlarged region of Figure 3d and in Figure 4b. As highlighted in the enlarged region of Figure 3g and in Figure 4c, there are also NPs that develop an irregularly branched morphology (dendritic-type), as well as particles that are not branched at all. In the case of extensive Pt NP growth (Figure 2c; see also volume renderings and orthoslices in the Supporting Information), the NPs form a dense array in which many of the particle branches are irregularly shaped and in which many Pt NPs are no longer discrete, as their branches are fused to those of neighboring NPs.

3.4. Pt NP Growth Patterns

Pt NPs deposit on both the broad triangular facets and the side facets of the Au nanoprisms and tend to align in parallel to one of the nanoprism edges. This alignment results in the formation of discrete lines of Pt NPs, although these may migrate and/or fuse over time (Figure 2c). The decorated triangular nanoprisms either exhibit Pt NP rows oriented to a single edge of the nanoprism, or there may be 2 or 3 different row directions, each of which is oriented parallel with respect to a different nanoprism edge.

The separable analysis of top and bottom Pt NPs made possible by the 3D tomograms is useful to understand whether there is a relationship between the alignment of the Pt NPs on the top and bottom surfaces. Registration could indicate cooperative alignment across the high curvature nanoprism side facets. To explore this possibility, the alignment of NPs was analyzed using a series of orthoslices parallel to the main {111} facets; each orthoslice enables analysis of the Pt NPs on either the top or the bottom, without any contribution from the other side. Lines were drawn as shown in Figure 5, and the alignment was observed through a stack of orthoslices ranging from the NP roots to their caps. Conservatively, lines not obvious in the entire half-volume were rejected. Although there is some similarity in line directions, this appears related more to growth mediated by the edges of the nanoprisms. The superposition of the top and bottom sets of lines (Figure 5d,h) does not show explicitly consistent coupling of patterns for the top and bottom Pt NPs lines. This result indicates that there is no or minimal cooperative effects between growth on the top and bottom facets.

Figure 5.

Figure 5

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Alignment of the Pt NPs on Au nanoprisms: (a, e) HAADF-STEM images, which comprise a projection through both top and bottom surfaces; orthoslices of the tomographic reconstructions for the (b, c) top and (f, g) bottom surfaces, respectively, taken at heights corresponding approximately to the “trunks” of the Pt NPs; and (d, h) schematic of top and bottom NP growth lines. Scale bars, 50 nm.

Alignment is increased when the nanoprism symmetry is lowered to 2-fold, as seen in parallelograms (Supporting Information), and when the size of the nanoprism is increased. Conversely, NPs tend to align less on small nanoprisms; the particle shown in Figure 2c, with less alignment of Pt NPs, is smaller than those in Figure 2a,b. It is also worth making a brief remark that the Pt NP growth sometimes results in distinct circular patterns (e.g., Figure S4c), as well as localized patches of lower NP density or lower NP growth, as revealed clearly by the orthoslices. Note also that many of these regions of lesser NP growth are not mirrored on the top and bottom surfaces, suggesting NP growth (and/or migration over time) without top–bottom coupling.

3.5. Localized Surface Plasmon Resonances

Pt is an excellent catalyst but a poor plasmonic material.31−33 The use of Pt in the form of small, high surface area discrete NPs on a plasmonic material such as Au offers an opportunity for plasmon-driven catalysis and in situ monitoring of catalytic processes via surface-enhanced spectroscopies.6,7,19,25,26 Using STEM-EELS performed in a monochromated FEI Titan, we confirmed that the presence of discrete Pt NPs did not prevent enhanced fields at the surfaces of the Au nanoprisms.

Electrons passing near the surface of a plasmonic nanostructure can excite LSPRs and lose the amount of energy associated with this resonance;52,53 such electrons are dispersed by an electron spectrometer and imaged on a charge coupled device (CCD) camera, yielding information about the spatial distribution of LSPRs. Spectra at different positions of a ∼190 nm side length Pt-decorated Au nanoprism, binned from areas 5 pixels (9.3 nm) wide, are shown in Figure 6. When the electron beam is near the nanoprism corners, a distinct peak is visible at ∼1 eV, indicating that many electrons lose ∼1 eV of energy. Similarly, when the beam passes near the center of an edge, electrons are likely to lose ∼1.3 eV. These peaks suggest that the nanoprisms may sustain LSPRs with different energy and spatial distribution.

To unravel spatial and spectral information about LSPR excitation on Pt-decorated Au nanoprisms, STEM-EELS spectrum images were analyzed using non-negative matrix factorization (NMF) (see Experimental Details). This approach enables the extraction of both spectral component energy and spatial localization and has been applied to several systems, including bimetallics.34,46,47Figure 7 shows the results of this decomposition applied to the spectrum image of the nanoprism shown in Figure 6. Four components were used because this number was found sufficient to explain the full spectrum image. Systematic analysis (ncomponents = 4–7) for the particle presented in Figures 6 and 7, and other particles, are reported in the Supporting Information.

The overall greatest contribution to the spectrum image of Figure 7 is the tail of the ZLP, i.e., the energy distribution of the incoming electrons. As a result of absorption and scattering, this spectral contribution is weakest inside the particle; it is constant outside the particle because the underlying Si3N4 film thickness is reasonably uniform. The second contribution to the spectrum image is a broad component with an onset at ∼1 eV; this represents the interband transitions (IBTs) in both Au and Pt. Indeed, the fact that Pt contributes to this energy loss can be seen by the considerable intensity of the component in regions exactly corresponding to NPs, i.e., at the edges of the nanoprism. The next component, represented by a blue line in Figure 7a, is the edge LSPR. Its spectrum is centered at ∼1.25 eV, with a full-width half-maximum (fwhm) of ∼0.7 eV. The spatial distribution of the edge LSPR is akin to that observed in undecorated Au nanoprisms.54,55 The other component unraveled by NMF is the corner LSPR, which localizes the electric field at the tips of the nanoprisms.54,55 This component is centered at ∼0.9 eV and is sharper, with a fwhm of ∼0.4 eV. Both LSPRs extend into the Pt NPs, which, combined with studies of far-field optical coupling, may open possibilities for multifunctional nanostructures.

STEM-EELS experiments and NMF data analysis performed on undecorated Au nanoprisms from the same synthesis batch used for producing the decorated nanoprisms (Supporting Information), as well as previous studies on similar bare structures,54,55 reveal nearly identical corner and edge resonances. All undecorated and decorated nanoprisms studied here by STEM-EELS (five and six of each, respectively) showed similar behavior. However, a direct comparison of the plasmon energy before and after Pt decoration is not feasible with STEM-EELS because of the impracticality of locating the same nanoprism(s) before and after decoration, as well as the potentially perturbing effects of a substrate on solution-phase growth. Here, a bulk measurement, UV–vis–NIR spectroscopy, provides additional information about the effects of Pt on the LSPR and complements STEM-EELS by providing statistical, ensemble-averaged behavior. UV–vis–NIR spectra show a slight red-shift and dampening of the plasmon (Supporting Information), consistent with previous studies on core–shell nanorods and nanospheres.37,56 This spectral broadening effect is primarily due to the poor plasmonic activity of Pt caused by interband transitions.31−33 When combined, these analyses strongly corroborate the notion that plasmonic activity is only minimally impacted by the presence of Pt.

4. Conclusions

The reduction of Pt salts can be used to produce discrete dendritic Pt nanoparticles at sizes relevant for catalysis across all surfaces of colloidal Au nanoprisms, with distinct nanoparticle morphologies ranging from pseudospherical to dendritic and spatial distributions that appear to be strongly dictated by the nanoprism geometry. Pt-decorated Au nanoprisms can sustain localized surface plasmon resonances, which extend into the Pt NPs. Given the well-known catalytic properties of Pt and the burgeoning field of plasmon-enhanced catalysis, these insights provide a step toward, and motivation for, precise tailoring of bimetallic nanoparticles that couple catalytic and optical properties to provide versatile multifunctional systems.

Acknowledgments

We thank Francisco de la Peña for several valuable discussions. R.K.L. acknowledges support from a Clare College Junior Research Fellowship. S.M.C. acknowledges support from a Gates Cambridge Scholarship. This work has received funding from the European Union Seventh Framework Programme under Grant Agreement 312483 - ESTEEM2 (Integrated Infrastructure Initiative-I3) and support from the European Research Council, Reference 291522 3DIMAGE. J.E.M. acknowledges support from the Research Corporation for Science Advancement.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02103.

  • Additional STEM images, EDS spectra and maps, EELS spectra and maps for decorated and undecorated nanoprisms, tomography results, and UV−vis−NIR extinction spectra (PDF)

  • Movies of electron tomography tilt series and reconstructions:

  • Tilt series images for the Pt-decorated Au nanoprism tomography reconstructions shown in Figure 2a–d and S3–6: Movie 1 (AVI), Movie 2 (AVI), Movie 3 (AVI), Movie 4 (AVI)

  • Sequential orthoslices in the x–y plane of the Pt-decorated Au nanoprism tomography reconstructions shown in Figure 2a–d and S3–6: Movie 5 (AVI), Movie 6 (AVI), Movie 7 (AVI), Movie 8 (AVI)

  • Rotating volume rendering visualizations of the Pt-decorated Au nanoprism tomography reconstructions shown in Figure 2a–d and S3–6: Movie 9 (AVI), Movie 10 (AVI), Movie 11 (AVI), Movie 12 (AVI)

The authors declare no competing financial interest.

Supplementary Material

jp6b02103_si_001.pdf (3.2MB, pdf)
jp6b02103_si_002.avi (488.2KB, avi)
jp6b02103_si_003.avi (956.6KB, avi)
jp6b02103_si_004.avi (1.2MB, avi)
jp6b02103_si_005.avi (1.9MB, avi)
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jp6b02103_si_012.avi (19.8MB, avi)
jp6b02103_si_013.avi (16.7MB, avi)

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