LETTER pubs.acs.org/NanoLett
Directing Substrate Morphology via Self-Assembly: Ligand-Mediated Scission of Gallium Indium Microspheres to the Nanoscale J. Nathan Hohman,†,‡ Moonhee Kim,† Garrett A. Wadsworth,†,§ Heidi R. Bednar,† Jun Jiang,†,|| Mya A. LeThai,† and Paul S. Weiss*,†,‡,§ †
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California NanoSystems Institute and Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90095, United States ‡ Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States § Department of Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California 90095, United States Department of Chemistry, Dickinson College, Carlisle, Pennsylvania 17013, United States
bS Supporting Information ABSTRACT: We have developed a facile method for the construction of liquid-phase eutectic gallium indium (EGaIn) alloy nanoparticles. Particle formation is directed by molecular self-assembly and assisted by sonication. As the bulk liquid alloy is ultrasonically dispersed, fast thiolate self-assembly at the EGaIn interface protects the material against oxidation. The choice of self-assembled monolayer ligand directs the ultimate size reduction in the material; strongly interacting molecules induce surface strain and assist particle cleavage to the nanoscale. Transmission electron microscopy images and diffraction analyses reveal that the nanoscale particles are in an amorphous or liquid phase, with no observed faceting. The particles exhibit strong absorption in the ultraviolet (∼200 nm), consistent with the gallium surface plasmon resonance, but dependent on the nature of the particle ligand shell. KEYWORDS: Gallium, indium, alloy, liquid metal, self-assembled monolayer, nanoparticle, alkanethiol, ultrasound, sonication, hydrogen bonding
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he reactions between sulfhydryl and metal (or semiconductor) surfaces are the basis for many self-assembled monolayers (SAMs).1 3 The order and structure of a SAM on a solid support are ultimately templated by the substrate crystal structure; intermolecular forces are generally weaker than the bonds between atoms in a solid crystal. However, if a liquid substrate is chosen, intermolecular forces within a supramolecular assembly can be used to direct substrate morphology with the aggregate sum of “weak” intermolecular forces.4 Here, we describe the deposition of SAMs on the liquid metallic eutectic alloy of gallium indium (EGaIn, 74.5% Ga and 24.5% In by weight).5 The oxide is separated from the alloy mechanically by application of ultrasound in an ethanolic solution of thiol molecules, resulting in ethanolic nanoscale EGaIn colloid. This method enables the synthesis of liquid-metal alloy nanoparticles with tunable compositions, produced by ligand-mediated scission of larger particles. We then compare the use of chemically bound thiolate SAMs as ligands to the surfactant polyvinylpyrrolidone (PVP)6 for the preparation of EGaIn nanoparticles. There are two basic approaches for the preparation of metallic alloy nanomaterials: bottom-up and top-down. Bottom-up synthetic methods excel at producing nanoparticles with extraordinarily well-controlled shape and size distributions, exploiting the controlled reduction from homogeneous reagent solutions.7 18 It is a significant challenge to control the stoichiometry and limit r 2011 American Chemical Society
phase segregation in complex alloy nanomaterials prepared by bottom-up techniques.17,19 23 Top-down synthetic approaches can be used to circumvent these challenges; alloys can be prepared in bulk and then divided to the nanoscale. Emulsions of molten metals in boiling solvent have been used to prepare a variety of nanoparticles from low-melting metals and alloys, including indium, bismuth, and lead.6,24 Ultrasound, which induces local extremes of temperature and pressure, is proving increasingly effective for producing and dispersing new classes of nanomaterials.25,26 For example, Raabe et al. used ultrasonic cavitation in an emulsion of water and molten Field’s metal (32.5% Bi, 51% In, and 16.5% Sn by weight) to produce micro- and nanoparticles of a complex alloy with a continuous size distribution.27 Aizenberg and co-workers demonstrated that intermolecular interactions between alkanethiolate adsorbates on ultrasonically dispersed mercury produce planar, microscale mercury thiolate crystals.4 To prepare gallium-based alloy nanomaterials, we have taken a top-down, ultrasound-assisted, and SAM-directed approach. The EGaIn alloy is a room-temperature liquid and is a nonNewtonian fluid that flows freely when subjected to shear forces. This characteristic is imbued by the rapid formation of the thin Received: May 6, 2011 Revised: October 14, 2011 Published: October 24, 2011 5104
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Nano Letters (∼1 nm) gallium oxide “skin” on its surface, akin to the protective coating of aluminum oxide on metallic aluminum.28,29 If the alloy is perturbed, the skin takes on a wrinkled appearance, and the alloy flows beneath the oxide crust. In the absence of this oxide skin (for example, in an oxygen-free environment or in the presence of aqueous halide acids), EGaIn behaves similarly to mercury, pulling into a tight sphere because of its high surface energy.29 Scanning electron microscopy (SEM) can be used to reveal the presence or the absence of the oxide; oxidized material will appear wrinkled or rippled, and a droplet or small sphere of the material will deform where it contacts another surface. The gallium oxide is responsible for the mechanical characteristics that make the alloy useful for electrical contacts and other applications.28 30 However, the rapid oxide formation interferes both with EGaIn emulsification and with the assembly of thiols at its interface (oxygen and sulfhydryl compete for surface sites). While SAMs have previously been reported on gallium- and indium-containing materials,31 33 they have not heretofore been reported directly on gallium, indium, or their alloys (although thiol termination has been shown to improve EGaIn wetting in microchannels).30 We used two SAM-forming thiols: 1-dodecanethiol (C12) and 3-mercapto-N-nonylpropionamide (1ATC9), shown schematically in Figure 1B. Figure 1 details the synthetic process with photographs and with SEM and transmission electron microscopy (TEM) images of the reaction products. In a typical experiment, a small quantity of EGaIn (0.18 g) is added to an Eppendorf vial, which is filled to a total volume of 0.5 mL with 1 mM degassed ethanolic thiol (C12 or 1ATC9). For comparison, EGaIn was dispersed in neat ethanol and in an ethanolic solution of PVP surfactant. Air is allowed to fill the vial headspace, and the vessel is placed in an ultrasonic bath for two hours. As the alloy gradually disperses, an opaque, gray slurry is produced (Figure 1A). After sonication, the largest particles precipitate within seconds, and the slurry is removed from the vial. The >100 nm slurry component may be separated by filtration through a 0.1 μm Whatman Luer Lock membrane filter, leaving sub-100-nm particles, or separated by mild centrifugation for subsequent characterization by SEM. Ultrasound dispersal results in a faintly reddish-brown solution for all four experimental cases, a colorimetric indicator for nanosized EGaIn. The particles are purified by centrifugation and removal of the supernatant, followed by redispersal in neat ethanol. After three cycles of centrifugation and redispersion, particles are imaged by TEM. Optical density provides a simple yield metric: high optical densities correlate to higher yields of spherical particles. Yield is primarily ligand-dependent. Filtered suspensions of 1ATC9-capped particles (1ATC9NP) are typically deep reddish brown, the reaction yielding a particle concentration of 300 500 μg/mL. The particles capped with C12 (C12NP) are formed in lower yields, 50 150 μg/mL, giving a faintly colored solution. For comparison, we used PVP as an alternate surfactant for the ultrasonic synthetic method and prepared emulsions in boiling solvent with PVP or thiol ligands. Additional experimental details are included in the Supporting Information. A confluence of mechanical and chemical characteristics enables the preparation of EGaIn nanomaterials by ultrasound. We now describe a series of experiments designed to elucidate the mechanism of stabilized EGaIn particle synthesis in the presence of selfassembling thiols. Molten metals have previously been converted to nanoparticles by emulsification in boiling solvent in the presence of a surfactant (PVP). However, vigorously stirring EGaIn with an adsorbate (PVP or thiol) only divides the alloy to the
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Figure 1. (A) Eutectic gallium indium alloy is ultrasonically dispersed in an ethanolic solution of 1ATC9 or C12 (shown schematically in (B)). Both molecules deposit rapidly at the EGaIn interface as the liquid alloy is dispersed. An opaque slurry is produced, giving a continuous size series of micrometer and submicrometer scale spherical particles. (C) Intermolecular hydrogen-bonding interactions between assembled 1ATC9 chains induce surface strain that assists the scission of larger particles to sub-100nm spherical particles (image acquired by scanning electron microscopy). (D) As sonication time increases, so does cavitation-induced damage to the material, resulting in sub-10-nm nonspherical EGaIn nanoparticles and oxide fragments. (image acquired by transmission electron microscopy).
submillimeter scale; the alloy does not emulsify. Imaging the resultant material by SEM (Figure 2C) reveals the wrinkled surface of the thin oxide skin on the liquid alloy, identical to larger droplets of bulk EGaIn. We conclude that stirring in boiling solvent, however vigorous, is insufficient to separate the oxide from the alloy; without rapid and continuous exposure of the oxide-free alloy, the thiol surfactants cannot bind appreciably, and the alloy remains passivated by its native oxide. Ultrasound efficiently emulsifies EGaIn. The oscillating shear forces within an ultrasonic bath cause the non-Newtonian liquid to flow, fracturing and separating the thin oxide envelope protecting the alloy. The high surface energy of the freshly exposed oxidefree EGaIn drives it to draw into a spherical shape.29 In neat ethanol (without thiols or PVP), ultrasonic treatment results in a mixture of nonuniform particles; the material is divided to the nanoscale, but particles are generally oblong or deformed spheres, each particle surrounded by re-formed oxide (panels A and B of Figure 2 and Figure S1 in the Supporting Information). Without a passivation layer, runaway oxidation degrades the alloy. As fresh metal is exposed under ultrasound, it reoxidizes, then flows and divides again, rapidly reducing the material to the micro- and nanoscale. 5105
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Figure 2. Eutectic gallium indium liquid alloy materials produced without thiol self-assembly, imaged by (A, C, D) scanning electron microscopy and (B) transmission electron microscopy. (A) EGaIn dispersed in neat ethanol results in spheroid, rippled particles; the surface oxidizes in the absence of stabilizing, capping thiol ligands (see Figure S1 in the Supporting Information for additional images). (B) The nanoscale fraction of EGaIn dispersed in ethanol is characterized by disordered, degraded, and discontinuous material; small pockets of metallic gallium indium alloy are observed, but the material is predominantly oxide. (C) Boiling EGaIn under reflux in the presence of polyvinylpyrrolidinone (PVP) is insufficient to emulsify the metal alloy particles, and the product is covered with the characteristic gallium oxide layer. (D) Sonication in the presence of PVP gives spherical micro- and nanoscale particles, but PVP does not inhibit oxidation, the rippled interface again visible.
The use of PVP surfactants partially stabilizes the EGaIn alloy. Exposing EGaIn to ultrasound in the presence of PVP helps droplets remain stable at the microscale, but the surfactant does not inhibit oxidation. The particles are not well-formed. Though the particles are generally spherical, the characteristic gallium oxide is visible, giving the material its characteristic appearance of having a thin skin. The PVP-passivated alloy particles also appear soft, deforming readily when in contact with another particle or surface (Figure 2D). While PVP is effective at assisting the emulsion of EGaIn, the surfactant does not enable the metal to resist oxidation and thus the resulting particles have uncontrolled morphologies. Treating EGaIn with ultrasound in the presence of either 1ATC9 or C12 results in well-formed and predominantly spherical particles (Figure 3A,C). As surface area increases during emulsification, fast alkanethiolate self-assembly protects against the runaway oxidation that otherwise precludes the formation of uniform, stable nanoparticles (Figure 2A,B). The result is a continuous size series of micro- and nanoparticles, passivated by self-assembled monolayers (Figure 3A,C). The absence of gallium oxide gives the particles the appearance of hard spheres, rather than the soft spheres observed in Figure 2D. We conclude that thiol selfassembly inhibits oxidation of the EGaIn interface through chemisorption of the sulfur atoms. Colloidal EGaIn is formed by the mechanical separation of alloy from its thin, passivating oxide. While the oxidation at the interface competes with thiol adsorption and results in a small loss of gallium, the surface oxide is critical to the assembly of the gallium nanoparticles. Without the surface oxide or an equivalent,
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Figure 3. Eutectic gallium indium liquid alloy micro- and nanoparticles imaged by (A, C) scanning microscopy and (B, D) transmission electron microscopy (TEM). EGaIn dispersed in (A and B) 3-mercaptoN-propionamide (1ATC9), and (C and D) 1-dodecanethiol (C12). Microscale spheres of (A) 1ATC9-capped and (C) C12-capped particles are well-formed and show little evidence of oxidation. (B) Large, spherical 1ATC9-capped nanoparticles (1ATC9NP) are produced in high yield and can be easily isolated for analysis by centrifugation. A representative TEM diffraction pattern of a single alloy nanoparticle is consistent with a liquid or amorphous phase (inset). (D) The C12NPs are produced in lower yields and must be concentrated for TEM analysis. Rarely, cuboid particles are observed (red arrow). Additional images can be found in Figure S2 in the Supporting Information.
the alloy is not dispersed by ultrasound. The oxide layer relaxes the alloy surface, such that the oscillating shear forces in the ultrasonic bath can fragment and disperse the alloy. The mechanically separated thin oxide sheets may be recovered by centrifugation and imaged via SEM or TEM, as shown in Figure 4. The PVP surfactant enables the production of large sheets by adhering to and stabilizing the oxide surface (Figure 4A). We have not yet characterized the elemental composition of the oxide sheets. If the sheets contain oxides of indium in addition to gallium, the scission process will not appreciably alter the elemental composition of the alloy nanoparticles. However, if the sheets are composed predominantly of gallium oxide, one might expect a small variation in the elemental composition of the smallest nanoparticles, which have undergone numerous oxidation/scission cycles. Continued ultrasonic treatment decreases particle size to the nanoscale, with SAM deposition protecting the alloy against oxidation. As with the slurry particles, the SAM-protected, spherical EGaIn nanoparticles are formed in a continuous size series down to ∼10 nm. Also observed are flattened, irregular sub-10-nm particles in the raw filtrate (see Figure 1D), attributed to fragments of the native oxide. Both 1ATC9NP and C12NP have similar general morphologies but differ in relative yields. We observe abundant spherical particles in 1ATC9NP samples (∼300 500 μg/mL), while only sparse particles are observed in C12NPs samples (∼50 150 μg/mL). Fractional centrifugation is used to separate and to concentrate the larger spherical nanoparticles for TEM analysis (Figure 3B,D). For comparison, dispersing EGaIn in neat ethanol (without thiols) gives a deep brown solution. There are nanoscale pockets of metallic EGain, 5106
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Figure 4. Native gallium oxide sheets mechanically separated from EGaIn, imaged by (A) scanning electron and (B) transmission electron microscopies. (A) Oxide is visible as large sheets enshrouding multiple particles after ultrasonic emulsification using polyvinylpyrrolidinone (PVP) as a surfactant. Large, free, and continuous sheets are accented in blue to aid visualization. In the case of PVP-stabilized particles, the composition of the free sheets is identical to the composition of the oxidized particle surface. (B) Using thiol surfactants (here, 1-dodecanethiol) stabilizes the alloy particles but does not stabilize the oxide sheets, which are fragmented to smaller sizes than observed when using PVP. Gallium oxide sheets are removed by filtration, and are isolated by centrifugation for TEM analysis.
but analysis reveals that particles are not of uniform shape, and a large quantity of oxidized material is observed (Figure 2B). The EGaIn particle cores appear to remain in the liquid phase after synthesis and thus are expected to have uniform elemental composition. High-resolution images of thiol-protected spherical nanoparticles reveal no crystalline lattice, which would typically have been observed for solid metallic nanocrystalline nanoparticle systems.9 Analysis by TEM electron diffraction reveals no signature of crystalline material, consistent with the metal of the alloy nanoparticles in the liquid or an amorphous glass phase (Figure 3B, inset).34 We note that the particles are held at the cryogenic temperature of the TEM (well below the 16 °C melting point of EGaIn) but that the incident electron beam could melt the particles. The preponderance of SAM-passivated particles remain spherical or spheroid and do not appear distorted by adjacent particles, as observed in the soft, PVP-coated material shown in Figures 2D and 4A. The tendency for all particles to distort on oxidation is further evidence of the liquid phase. Rarely, elongated particles (which appear cuboid in TEM images, an example shown in Figure 3D) are observed with both monolayer systems. Such species are either distortions of spherical particles after ultrasonic treatment, or represent particles in stable, intermediate configurations just prior to particle division. The structure and properties of SAMs on EGaIn have not previously been reported. In the absence of the oxide, molten gallium indium alloys are highly enriched with indium in their outermost atomic layer, in substantial excess of the indium concentration (>90% indium at the oxide-free interface).35 As the chemistries of Ga and In are similar, the thiolate SAMs could bind to either Ga or In; thiolate SAMs on InP and indium tin oxide bind to In,31,32 while thiolate SAMs on GaAs bind both to Ga and As, depending on the specific interface structure.36 We turn to the well-studied thiol on gold SAMs for insight into the structure of these monolayers on gallium indium nanoparticles. The structure of a SAM is defined by the substrate morphology and curvature, 37 the nature of the molecule/substrate interactions,38 and the geometry of (and intermolecular forces between) adsorbates.39 A SAM supported on a substrate with
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high curvature has a different structure, coverage, and reactivity compared to a SAM supported on a flat substrate.40 43 On a spherical surface, the outer interface density of a SAM is necessarily lower than the density close to the substrate surface, with the terminal groups “splayed out” from the particle. This effect becomes more pronounced as curvature increases.37 Molecular dynamics simulations have shown that n-alkanethiolate monolayers on such spherical substrates tend to organize into several large domains, stabilized by van der Waals forces to minimize the energy of the assembly.38 In the case of C12NPs, the alloy surface is passivated against further oxidation by the high thiol coverage and by the large, weakly interacting molecular domains. This passivation enables the preparation of spherical particles with diameters between 100 and 1000 nm but limits synthesis of sub-100-nm particles. In contrast, the secondary amide group in the 1ATC9 monolayers produces intermolecular hydrogen-bonding networks within assembled films, which manifest as organized domains of aligned, hydrogen-bonded molecules.44 49 These interactions dominate the local SAM structure and properties.50 We engineered the 1ATC9 assembly to strain the EGaIn interface and to assist with particle scission by modification of the SAM structure. We postulate that the strong, local, and short-range hydrogenbonding interactions serve to pull molecules on the molten surface toward one another. Rather than stabilizing the alloy interface, the intermolecular interactions direct the continued scission of EGaIn particles to the nanoscale; more (and smaller) 1ATC9 domains leave the particles more susceptible to calving by sonication. As a secondary effect, smaller domains on larger particles lead to higher defect densities and oxidation at defects alters the mechanical properties of the alloy, which induces further scission. Ultrasound is responsible for inducing the majority of particle division. However, as substantially higher yields of spherical (and nonspherical) nanoparticles are observed when formed in the presence of 1ATC9, it follows that the functional amide stratum more effectively assists particle division than C12. Exposure to long-duration ultrasonic cavitation has been shown to cause chemical reactions,25,51 and eventually degrades EGaIn nanomaterials. In the presence of a SAM, particles can retain their spherical shape longer before succumbing to this damage. As the C12 film does not actively assist scission, nanoparticle production is slow, and C12NPs are eventually fragmented. The 1ATC9 monolayer assists scission, improving yield by decreasing the time necessary to obtain nanoparticles. The EGaIn nanoscale component exhibits strong absorption in the ultraviolet (UV), with more moderate extinction throughout the rest of the UV visible spectrum (Figure 5A,B). The characteristic gallium plasmon resonance is centered at 202 nm for C12NP and 207 nm for 1ATC9NP, consistent with the measurements for gallium colloids produced by chemical liquid deposition.52 Elimininating the
