Transition-Metal Decorated Aluminum Nanocrystals - ACS Nano (ACS

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Transition Metal Decorated Aluminum Nanocrystals Dayne Francis 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 ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b04960 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 27, 2017

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Transition Metal Decorated Aluminum Nanocrystals Dayne F. Swearer1, 2, Rowan K. Leary3,*, Ryan Newell2, 4, Sadegh Yazdi2, 4, Hossein Robatjazi2, 6, Yue Zhang2, 5, David Renard1, 2, Peter Nordlander2, 4, 5, 6, Paul A. Midgley3, Naomi J. Halas1, 2 , 4, 5, 6,

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*, Emilie Ringe1, 2,3, 4,*

Department of Chemistry, 2 Laboratory for Nanophotonics, 4 Department of Material Science

and Nanoengineering, 5Department of Physics and Astronomy, 6Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, Houston, Texas 77005, U.S.A. 3

Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, U.K.

Keywords: plasmonics, photocatalysis, aluminum, nanomaterials, antenna-reactor, electron tomography

Corresponding authors R.K.L.: [email protected], N.J.H.: [email protected], E.R.: [email protected]

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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 metals nanoparticle islands, many of which have precedence as heterogeneous catalysts. High resolution and 3D structural analysis using scanning transmission electron microscopy and electron tomography shows that abundant nanoparticle island decoration in the catalytically relevant fewnanometer 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.


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Multicomponent 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 therapies,5 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 photocatalysts composed of Al,11 Cu,12 Ag,13 and Au14 have all proven to be effective photocatalysts, multicomponent 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 hotcarrier driven mechanistic pathways to become accessible, while decreasing reaction energy barriers and increasing selectivity.15,20


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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 plasmonic 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) transition metal NPs into the antenna-reactor design, light-absorption within the catalytic material is enhanced,22 increasing their validity photocatalytic applications. By combining the plasmonic properties of AlNCs with a large palette 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, dimethylethylamine alane, was reduced by Ti(IV) isopropoxide in a 1:4 solution of tetrahydrofuran/1,4-dioxiane to create capping agent free AlNCs as previously described.24 Next, AlNCs were suspended in ethylene glycol, and a 5mM solution of metal salt precursor in ethylene glycol was added directly with


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stirring under the conditions presented in Supplementary Table I. Upon heating, the AlNC suspension gradually darkened in color indicating the reduction of the metal salt precursors to metal as shown in Fig. 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 nanoparticle 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 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


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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 Fig. 1. An example of a 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, low contrast outline on the outside of the AlNC. Each type of Al-TMC antenna-reactor nanostructure displays characteristically small islands (10 nm (Fig 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


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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 Fig. 2; additional data for other Al-TMCs are shown in Figs. 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 Fig. 2a and 2e. Energy dispersive X-ray spectroscopy (EDX) mapping confirmed that a thin oxide shell (Fig. 2c, g) surrounds each Al core (Fig. 2b, f). This oxide layer isolates the Pd and Ru islands in Figs. 2d and 2h, respectively, 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


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transition metal/noble metal contact under photocatalytic conditions. In Al-TMCs, 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 mapping the oxide shell surrounding the Al cores. (d) Pd-Lα X-ray emission line of Pd islands decorating the surface. (h) Ru-Lα X-ray emission line of Ru islands decorating the surface. All scale bars 50 nm.

To verify the optical properties in Al-TMCs, UV-visible extinction spectroscopy was used to characterize the LSPR of pristine AlNCs and decorated varieties of Al-Pt, Al-Ir, Al-Rh, and Al-Ru as shown in Fig. 3. In each case, AlNCs with an average size of ~90 nm from one


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synthetic batch were used for consistency in LSPR after decorated. 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 nanoparticles 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 become higher, the extinction blue shifts with respect to the original LSPR of the undecorated plasmonic nanoparticle. In Fig. 3, slight blue shifts are observable as an effect of the high coverage of metal islands decorating the nanoparticle surface. AlNCs are therefore able to sustain plasmon resonances after decoration with transition metal islands, but allow for small modifications in their LSPR energy based on the degree of transition metal coverage.


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Figure 3. Extinction spectra of Al-TMCs (a) TEM and corresponding extinction spectrum of (a) pristine, undecorated AlNCs (b) Al-Ir (c) Al-Ru (d) Al-Rh (e) Al-Pt. All scale bars 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 2-dimensional (2D) image of a 3-dimensional (3D) object. This means that information about 3D structure may be lost or hidden, and can


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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 provides characterization of the size and spatial distribution of the islands in relation to the AlNC (Figs. 4 & 5 and Supplementary Information). Ru and Pd islands are seen to have formed across all faces of the AlNCs (Figs. 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 (Fig. 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 Fig. 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 (Figs. 4d and 5d), measured here as the distance between the island surfaces. The Al-Ru system has a distribution sharply peaked towards minimal separation, i.e. directly adjacent islands. The Al-Pd system similarly has a distribution sharply peaked towards 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 nearest neighbor distances (insets Figs. 4d and 5d) highlights regions high island density, extensive in the Al-Ru sample in particular. Mapping of the local island density (Fig. S8) shows the prevalence of high island density in a more direct manner and pertains to multifunctional behaviors of the islands that may


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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 (Fig. 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.

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 in (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 supplementary information for corresponding movies.


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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 in (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 supplementary information for corresponding movies.

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 a 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 between the interfacial free energy between the particle and the substrate, surface free energy of the relevant NP face, and surface free energy of the relevant substrate face. To reveal TMC island shape and orientation with respect to the substrate high resolution STEM was


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applied. In all cases, TMC islands were found to nucleate on the alumina layer, yet grow with randomly orientations with respect to the surface (Fig. 6). As predicted by the Winterbottom construction, the metal islands of the same material composition should have consistent shape because of equal contributions between the interfacial free energy between particle and substrate.44 An example of a single Al-Rh nanoparticle with low levels of Rh decoration is shown in Fig 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 Fig. 6b, a second example is given in the form a highly decorated Al-Ir nanoparticle, where the decorating islands take on numerous shapes and orientations 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 AlNCs is a direct consequence of the amorphous alumina interface. Amorphous oxides have some degree of limited short range ordering, leading to subtle changes in the interfacial free energy from point-to-point at the alumina-metal island interface. The small spatial changes between competing interfacial and surface free energies on alumina and the decorating metals result in random orientations and shapes of islands after heterogeneous nucleation on the AlNC surface.


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Figure 6. High Resolution HAADF-STEM imaging of island growth. (a) HAADF-STEM of a single Al-Rh NP decorated with a low coverage of Rh islands. Inset shows a high magnification zoom into a single Rh nanoparticle growing directly from the alumina. (b) HAADF-STEM of a single Al-Ir NP with high coverage of Ir islands. The inset shows a high magnification zoom of one edge with multiple Ir islands visible at atomic-resolution. On-edge observation leads to bright contrast from particle orientation, not Ir distribution, which is uniform across the sample. All scale bars in low-magnification and insets are 50 nm and 3 nm, respectively. Conclusions


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A general synthetic route based on reduction of metal salts in ethylene glycol to heterogeneously nucleate platinum and iron group nanoparticles onto the surface of AlNCs has been presented. The elemental distribution of these nanostructures is such that the plasmonic Al core is distinctly separated from the randomly nucleated metal islands by a 2-4 nm amorphous aluminum oxide layer. The role of this intrinsic, self-limiting oxide is to both stabilize the Al core and provide an inert support for the heterogeneous nucleation of metal islands. Due to the amorphous nature of this oxide, the metal islands decorating the surface tend to be randomly oriented on the surface. This 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 nanoislands shape and high numbers of individual nucleation events under proper synthetic conditions. The small NP island sizes achievable through the polyol synthesis are expected to be excellent for both high catalytic activity and large absorption enhancements. Small NPs (

Transition-Metal Decorated Aluminum Nanocrystals - ACS Nano (ACS

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