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Transition-Metal Decorated Aluminum Nanocrystals Dayne F. Swearer,†,‡ Rowan K. Leary,*,§ Ryan Newell,‡,∥ Sadegh Yazdi,‡,∥ Hossein Robatjazi,‡,# Yue Zhang,‡,⊥ David Renard,†,‡ Peter Nordlander,‡,∥,⊥,# Paul A. Midgley,§ Naomi J. Halas,*,†,‡,∥,⊥,# and Emilie Ringe*,†,‡,§,∥ †
Department of Chemistry, ‡Laboratory for Nanophotonics, ∥Department of Material Science and Nanoengineering, ⊥Department of Physics and Astronomy, and #Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, Houston, Texas 77005, United States § Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, United Kingdom S Supporting Information *
ABSTRACT: Recently, aluminum has been established as an earth-abundant alternative to gold and silver for plasmonic applications. Particularly, aluminum nanocrystals have shown to be promising plasmonic photocatalysts, especially when coupled with catalytic metals or oxides into “antenna-reactor” heterostructures. Here, a simple polyol synthesis is presented as a flexible route to produce aluminum nanocrystals decorated with eight varieties of size-tunable transition-metal nanoparticle islands, many of which have precedence as heterogeneous catalysts. Highresolution and three-dimensional structural analysis using scanning transmission electron microscopy and electron tomography shows that abundant nanoparticle island decoration in the catalytically relevant few-nanometer size range can be achieved, with many islands spaced closely to their neighbors. When coupled with the Al nanocrystal plasmonic antenna, these small decorating islands will experience increased light absorption and strong hot-spot generation. This combination makes transition-metal decorated aluminum nanocrystals a promising material platform to develop plasmonic photocatalysis, surface-enhanced spectroscopies, and quantum plasmonics. KEYWORDS: plasmonics, photocatalysis, aluminum, nanomaterials, antenna-reactor, electron tomography
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plasmonic systems can expand the scope of plasmon-mediated chemistry. The combination of plasmonic materials with catalytic metals15−17 or semiconductors18−20 leads to diversified surface chemistry and reactivity not achievable on single component plasmonic nanostructures. In this geometry the plasmonic nanoantenna is used to harvest light and directly enhance absorption in nearby catalytically active materials, a concept described as an “antenna-reactor” nanostructure.15,17,20 By implementing multicomponent nanostructures, the functions of light-harvesting and surface chemistry can be tuned separately to achieve desired optical properties and specific chemical reactivity. Coupling the surface chemistry of reactive transition metals with plasmonic nanoparticles (NPs) allows new hot-carrier driven mechanistic pathways to become
ulticomponent nanostructures have recently gained traction in applied nanoscience due to increased functionality over their single-component counterparts.1−3 Combinations of metals, metals and semiconductors, and metals on insulating supports have led to enhanced activity in applications ranging from drug delivery4 and cancer therapies5 to chemical transformation6 and sensing.7,8 Multicomponent nanostructures, when appropriately designed, have the potential to integrate otherwise independent functionalities into a single multifunctional material. Plasmon-enhanced catalysis is a nascent field where multicomponent architectures can lead to new, previously inaccessible, chemical activity. Plasmonic photocatalysts utilize the localized surface plasmon resonance phenomenon to produce energetic hot-carriers and photothermal heating to drive chemical reactions.9,10 While single component plasmonic nanostructures composed of Al,11 Cu,12 Ag,13 and Au14 have all proven to be effective photocatalysts, multicomponent © 2017 American Chemical Society
Received: July 13, 2017 Accepted: September 25, 2017 Published: September 25, 2017 10281
DOI: 10.1021/acsnano.7b04960 ACS Nano 2017, 11, 10281−10288
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Cite This: ACS Nano 2017, 11, 10281-10288
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generalized method for AlNC decoration, with a few routes of additional synthetic control investigated. However, given the vast history of polyol reaction development, we have not exhausted all possible reaction conditions in establishing this class of materials. We envision that numerous future variations and optimizations in synthetic conditions could lead to additional levels of morphological control and remains an open area for future research. Utilizing the generalized polyol-based method, all elements from IUPAC groups 8−10, with the exception of osmium, were decorated onto AlNCs, as shown in Figure 1. An example of a
accessible, while decreasing reaction energy barriers and increasing selectivity.15,20 Here, we describe a general synthetic approach for producing a variety of transition-metal decorated aluminum nanocrystals (AlNCs) based on a two-step polyol synthetic process. Aluminum is an earth-abundant, stable, and cheap plasmonic alternative to gold and silver with tunable plasmon resonance across the ultraviolet and visible ranges of the electromagnetic spectrum.21 Transition-metal catalysts (TMCs) have a long history in the field of heterogeneous catalysis, but they often lack strong light-matter interactions and have spectrally broad and weak absorption across the visible spectrum. Consequently, TMCs make poor photocatalysts and are traditionally used in thermal processes. By incorporating small (few-nm) transitionmetal NPs into the antenna-reactor design, light-absorption within the catalytic material is enhanced,22 increasing their validity in photocatalytic applications. By combining the plasmonic properties of AlNCs of TMCs, an expansive library of plasmonic antenna-reactor photocatalysts has been achieved. The wide scope and modularity of plasmonic AlNCs decorated with different catalytically active transition-metal islands should allow for an expanded palette of targeted chemical transformations with plasmon-mediated photocatalysis. Additionally, plasmonic photocatalysis can lead to increased reaction selectivities and diversified chemistry due to hot-carrier driven mechanistic pathways.9,23
RESULTS AND DISCUSSION A two-step process was utilized to create transition-metal decorated aluminum nanocrystals (Al-TMCs). First, an aluminum hydride precursor, N,N-dimethylethylamine alane, was reduced by Ti(IV) isopropoxide in a 1:4 solution of tetrahydrofuran/1,4-dioxane to create capping agent free AlNCs as previously described.24 Next, AlNCs were suspended in ethylene glycol, and a 5 mM solution of metal salt precursor in ethylene glycol was added directly with stirring under the conditions presented in Supplementary Table 1. Upon heating, the AlNC suspension gradually darkened in color indicating the reduction of the metal salt precursors to metal as shown in Figure S1. This general synthesis method, based on reduction of metal salts in high boiling point alcohols containing multiple hydroxyl groups (polyols),25 was developed and used to obtain eight varieties of Al-TMCs. Polyol syntheses have numerous advantages, such as flexibility in choosing the molecular weight and degree of hydroxyl functionality, which allows for additional reaction control. In general, increasing molecular weight and −OH count leads to higher boiling points and viscosity, which can influence metal NP size and shape.26−28 Due to their numerous −OH groups and relatively high polarity, polyols behave as organic analogues to water. Their polarity, while not as high as H2O, allows for common metal salts (halides, acetates, nitrates, etc.) to be used as precursors for metal NP preparation due to good solubility. Additionally, polyols have strong chelating properties that stabilize metal NPs against aggregation throughout nucleation and growth. Here, polyols additionally coordinate with the native aluminum oxide (alumina) layer surrounding the AlNCs, promoting homogeneous suspensions throughout the reaction. Colloidal NP synthesis in polyol solvents is affected by numerous factors including, but not limited to, metal salt concentration, reaction time, temperature, solvent, capping agents, and coordinating anion effects.28 This article presents a
Figure 1. AlNCs decorated with transition-metal islands. Pristine, undecorated Al is shown with a red border in the bottom left; the boundary between the Al core and 2−4 nm aluminum oxide layer is distinctly visible. Group 8 metals (Fe and Ru), group 9 metals (Co, Rh, and Ir), and group 10 metals (Ni, Pd, and Pt) have orange, blue, and green borders, respectfully. All scale bars are 50 nm.
pristine, undecorated AlNC is shown in place of osmium (outlined in red). In this representative image, the 2−4 nm alumina layer is visible as a thin, light outline on the outside of the AlNC. Each type of Al-TMC antenna-reactor nanostructure displays characteristically small islands (10 nm (Figure S4). We believe this effect is due to complex speciation when ligands such as acetate are able to form binuclear intermediates in solution, which slow the reduction kinetics of metal ions in solution.33,34 Even though we have previously synthesized AlNCs decorated with Pd by another protocol,15 the generalizability of the polyol synthesis developed here is difficult to match. For instance, while Al−Pd was readily synthesized via alcoholic reduction in isopropanol, numerous other platinum group metals such as Ru and Pt failed to reduce using this method due to the weak reducing power of isopropanol. Compared to other methods, this generalizable synthetic approach can accommodate multiple materials into a simple, controllable reaction and hence is an attractive method for making decorated AlNCs. The interaction between the metal islands, the interfacial alumina layer, and the subsurface AlNC is expected to play an important role in determining the optical and catalytic properties of Al-TMCs.35 The elemental distributions in an Al−Pd NP and two Al−Ru NPs are shown in Figure 2; additional data for other Al-TMCs are shown in Figures S5−S7. High-angle annular dark-field scanning transmission electron micrographs (HAADF-STEM) show the contrast between the highly scattering transition-metal islands and the Al core in Figure 2a, e. Energy dispersive X-ray spectroscopy (EDX) mapping confirmed that a thin oxide shell (Figure 2c, g) surrounds each Al core (Figure 2b, f). This oxide layer isolates the Pd and Ru islands in Figure 2d, h, from the plasmonic Al. The elemental mapping suggests that the root cause of island formation on the surface of AlNCs is most likely due to heterogeneous nucleation on the surface, rather than a penetration through the oxide and galvanic replacement of Al metal underneath the oxide as previously suggested.36 The separation of the plasmonic Al core and catalytic metal islands by a thin naturally occurring oxide is a defining feature of Al-TMC systems. Other bimetallic plasmonic photocatalysts have frequently depended on Au to be directly in contact with catalytic metals such as Pd or Pt.37−39 However, when transition metals and noble metals are in direct contact, dielectric mixing can occur at the interface altering the optical properties of a pure noble metal.40 Additionally, under illumination, NPs heat up. High temperatures can cause atomic migration; potentially degrading both the optical and catalytic properties of bimetallic systems with direct transition metal/ noble metal contact under photocatalytic conditions. In AlTMCs, the intrinsic 2−4 nm alumina layer acts as a physical barrier that keeps the optical properties of the plasmonic core from mixing with the catalytic properties of the transition metal, preserving the integrity of each component in the system.
Figure 2. Energy dispersive X-ray spectroscopy elemental maps. (a, e) HAADF-STEM image of a single Al−Pd NP and two Al−Ru NPs, respectively. (b, f) Al−Kα X-ray line mapping the plasmonic Al cores. (c, g) O−Kα map showing the oxide shell surrounding the Al cores. (d) (h) Pd-Lα and Ru-Lα maps showing Pd and Ru islands, respectively, decorating the surface. All scale bars are 50 nm.
To verify the optical properties in Al-TMCs, UV−vis extinction spectroscopy was used to characterize the localized surface plasmon resonance (LSPR) of pristine AlNCs, and decorated varieties of Al−Pt, Al−Ir, Al−Rh, and Al−Ru as shown in Figure 3. In each case, AlNCs with an average size of ∼90 nm from one synthetic batch were used for consistency in LSPR after decoration. The normalized extinction spectra for Al-TMCs show a resonance feature nearly identical to the undecorated AlNC. However, slight variation in LSPR energy is observed that we believe can be interpreted in the framework of plasmonic percolation.41 When plasmonic NPs are decorated by increasing levels of other metals, from small islands to a complete shell, a positive-to-negative transition in the real part of the dielectric function is observed in the volume of the decorating metal. This initially results in a red shift in the extinction of the decorated system, but as coverage levels increase, the extinction blue shifts with respect to the LSPR of the original, undecorated plasmonic NP. In Figure 3, slight blue shifts are observable as a result of the high coverage of metal islands decorating the NP surface. AlNCs are therefore able to sustain plasmon resonances after decoration with transitionmetal islands, but allow for small modifications in their LSPR energy based on the degree of transition-metal coverage. 10283
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islands are seen to have formed across all faces of the AlNCs (Figures 4a and 5a and Supplementary Movies 1 and 2), but with differing size, abundance, and spatial distributions in the two systems. Consistent with our results for anion dependence (Figure S4), the Al−Ru synthesized with RuCl3 shows a smaller and narrower size distribution, with a mean and standard deviation of 3.2 ± 0.9 nm (N = 564), compared to Al−Pd synthesized using Pd(OAc)2, where although the size distribution is similar in the few-nm range, several islands, highlighted by the size-color-coded rendering inset in Figure 4c, are able to grow as large as 15 nm. The spatial distribution of the islands can be characterized in terms of a nearest-neighbor distance (Figures 4d and 5d), measured here as the distance between the island surfaces. The Al−Ru system has a distribution sharply peaked toward minimal separation, that is, directly adjacent islands. The Al− Pd system similarly has a distribution sharply peaked toward minimal separation, but also a significant proportion of larger nearest-neighbor distances, consistent with the lower overall abundance of islands. Color-coded rendering of the nearestneighbor distances (insets Figures 4d and 5d) highlights regions of high island density, extensive in the Al−Ru sample in particular. Mapping of the local island density (Figure S8) shows the prevalence of high island density in a more direct manner and pertains to multifunctional behaviors of the islands that may be rendered by many islands with close spacing. Both measures of spatial distribution show the scope for multifunctional behaviors endowed by intimate island spacing, such as hot-spot generation (Figure S9). The contrasting distributions of the Al−Pd and Al−Ru demonstrate the tunability afforded by the flexible synthetic protocols and choice of decorating metal. Independent of metal island composition, the interfacial alumina layer plays an important role in the final shape and orientation of NP islands on AlNCs. Generally, metal islands grown on an oxide support are expected to follow predictions laid out by the Winterbottom construction.43,44 In this model, NP shape on a substrate is determined by thermodynamic energy minimization of the interfacial free energy between the particle and the substrate, the surface free energy of the relevant NP face, and the surface free energy of the relevant substrate face. To reveal TMC island shape and orientation with respect to the substrate, high-resolution STEM was utilized. In all cases, TMC islands were found to nucleate on the alumina layer, yet grow in random orientations with respect to the surface (Figure 6). The Winterbottom construction predicts that metal islands of the same composition should have consistent shapes because of equal contributions between the interfacial free energy between particle and substrate.44 An example of a single Al−Rh NP with low levels of Rh decoration is shown in Figure 6a, with a single 2 nm Rh island magnified in the inset. The truncation of the Rh island along the interface confirms that it has nucleated and grown from the alumina surface, rather than homogeneously nucleated in solution and aggregated when drop cast onto the TEM grid. In Figure 6b, a second example is given in the form of a highly decorated Al−Ir NP, where the decorating islands take on numerous shapes and orientations with respect to the substrate. Because NP shape and thermodynamic energy minimization depends on the interfacial free energy between particle, substrate, and facet-dependent surface free energy of the support material, we put forth that the randomly oriented nature of decorating metal islands on
Figure 3. Extinction spectra of Al-TMCs. TEM and corresponding extinction spectrum of (a) pristine, undecorated AlNCs, (b) Al−Ir, (c) Al−Ru, (d) Al−Rh, and (e) Al−Pt. All scale bars are 100 nm.
The morphology of nanomaterials can have a dramatic influence on their physical properties, particularly in catalytic applications where size and distribution dictate efficiencies.42 Conventional (scanning) transmission electron microscopy ((S)TEM) is a common tool for characterizing nanomaterials, yet only provides a two-dimensional (2D) image of a threedimensional (3D) object. This means that information about 3D structure may be lost or hidden and can obscure true understanding of structure−function relationships, particularly for multicomponent systems with significant 3D morphology, such as Al-TMCs. Therefore, 3D analysis by electron tomography was used to provide a more complete understanding of the AlNCs and island decoration. Focusing on the contrasting Al−Ru and Al−Pd NPs, quantitative 3D analysis and visualization of the electron tomography reconstructions provide characterization of the size and spatial distribution of the islands in relation to the AlNC (Figures 4 and 5 and Supporting Information). Ru and Pd 10284
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Figure 4. 3D morphology of Al−Ru. (a) Surface renderings of the AlNC (blue) and decorating Ru islands (orange). (b) Surface rendering of only the AlNC revealing the faceted morphology. Scale bars for (a) and (b) are 50 nm. (c) Equivalent diameter size analysis of Ru islands. The inset shows size dependent color-coded rendering of the Ru islands according to the color bar. (d) Nearest-neighbor analysis of Ru islands. The inset shows a nearest-neighbor distance dependent color-coded rendering of the Ru islands according to the color bar. The AlNC is rendered semitransparent in (c) and (d) to enable visualization of the size and nearest-neighbor distance distributions throughout the 3D volume. See Supporting Information for corresponding movies.
Figure 5. 3D morphology of Al−Pd. (a) Surface renderings of the AlNC (blue) and decorating Pd islands (green). (b) Surface rendering of only the AlNC revealing the faceted morphology. Scale bars for (a) and (b) are 50 nm. (c) Equivalent diameter size analysis of Pd islands. The inset shows size-dependent color-coded rendering of the Pd islands according to the color bar. (d) Nearest-neighbor analysis of Pd islands. The inset shows a nearest-neighbor distance-dependent color-coded rendering of the Pd islands according to the color bar. The AlNC is rendered semitransparent in (c) and (d) to enable visualization of the size and nearest-neighbor distance distributions throughout the 3D volume. See Supporting Information for corresponding movies.
result can be interpreted in the framework of the Winterbottom construction, where the changing interfacial energy at the alumina-metal interface leads to drastic changes in nanoisland shape and high numbers of individual nucleation events under proper synthetic conditions. The small NP islands achievable through the polyol synthesis are expected to be excellent for both high catalytic activity and large absorption enhancements. Small NPs (
