Article pubs.acs.org/cm
Reactive AgAuS and Ag3AuS2 Synthons Enable the Sequential Transformation of Spherical Nanocrystals into Asymmetric Multicomponent Hybrid Nanoparticles Xuefei Li and Raymond E. Schaak* Department of Chemistry and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: Nanoscale heterostructures that interface with multiple distinct materials provide opportunities to engineer functional complexity into single-particle constructs. However, existing synthetic pathways to such hybrid nanoparticles emphasize surface-seeded growth, which limits the scope of accessible systems. Here, we introduce an alternative approach that transforms isotropic nanocrystals into asymmetric, multicomponent Janus particles through sequential deposition, reactive phase segregation, and cation exchange processes that are mediated by an unusual class of reactive synthons. After Ag−Au seed particles had formed and had reacted with sulfur, a series of segregated Au1−xAgx−AgAuS and Au1−xAgx−Ag3AuS2 hybrid nanoparticles form. The AgAuS and Ag3AuS2 domains provide a synthetic entryway into solution-mediated cation exchange reactions, with the compositions of the Ag−Au−S synthons defining the components, morphologies, and interfaces of the hybrid nanoparticle products. Upon cation exchange with Pb2+, Au1−xAgx−AgAuS forms Ag1−xAux−PbS heterodimers while Au1−xAgx−Ag3AuS2 forms Ag1−xAux−Ag2S−PbS heterotrimers. The process by which isotropic metal nanoparticles transform into asymmetric hybrid nanoparticles through reactive Ag−Au−S synthons provides important insights that will be applicable to the retrosynthetic design of complex nanoscale heterostructures having expanded multifunctionality and synergistic properties.
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INTRODUCTION Nanoscale heterostructures that contain multiple distinct material domains coupled together through direct solid−solid interfaces are important synthetic targets for achieving multifunctionality and synergistic properties.1 While the optical, magnetic, electronic, and photocatalytic properties of nanoparticles can often be tuned by modifying their sizes, compositions, or morphologies,2−4 integrating them into a single hybrid construct enables additional tunability through control over their interfaces, spatial arrangements, and configurations.5 For example, synergistic optical properties emerge in dumbbell-shaped Au−CdSe−Au heterostructures because of strong interparticle coupling, which is absent in related CdS−Au matchstick-shaped hybrids that have different particle sizes and interfacial characteristics.6 Also, precise placement of multiple individual nanoparticle components in rationally designed Ru−CdSe@CdS−Pt heterostructures permits control over charge carrier separation and directional transfer, which cannot be achieved by simply mixing together the individual components.7 To access functional target heterostructures that meet these stringent function-driven design criteria, synthetic pathways must facilitate the rational incorporation of diverse materials in precise locations. Typical strategies for synthesizing multifunctional hybrid nanoparticles rely on heterogeneous seeded nucleation and growth, whereby a new domain is added to the © 2017 American Chemical Society
surface of a preexisting particle. For example, to access twocomponent heterodimers, one type of nanoparticle serves as a seed onto which a second type of nanoparticle is grown.3 A diverse library of nanoparticle heterodimers has been synthesized using this approach, including the functional examples mentioned above. However, when constructing higher-order hybrid nanoparticles that contain three or more components, successive seeded growth steps are needed, and surface chemistry considerations define the preferred sites at which additional nanoparticles grow. This, in turn, places constraints on the types of materials that can be incorporated into hybrid nanoparticles, as well as the morphologies, interfaces, and arrangements of the constituent nanoparticles. Methods that move beyond surface-mediated nucleation and growth open the door to the design and synthesis of more complex nanoscale heterostructures with features that traditional seeded growth strategies cannot achieve. For example, supersaturation−precipitation strategies produce hybrid nanoparticles by extracting an insoluble component from a seed nanoparticle, as demonstrated in the formation of Au−Ge heterodimers by precipitation of Ge from Au seeds8 and Fe3O4−Ge−Au heterotrimers by insertion of Ge between the Au and Fe3O4 domains of Au−Fe3O4 heterodimers.9 SolutionReceived: April 8, 2017 Published: April 17, 2017 4153
DOI: 10.1021/acs.chemmater.7b01449 Chem. Mater. 2017, 29, 4153−4160
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Chemistry of Materials
structures, such as CdSe−AgInSe2, were synthesized and shown to exhibit enhanced photocatalytic activities.25 Existing multistep reaction sequences to high-order hybrid nanoparticles primarily apply the same synthetic pathway multiple times, as described above for successive seeded growth and ion exchange processes. The sequential application of multiple distinct synthetic pathways (for example, by combining seeded growth, reactive phase segregation, cation exchange, and other nanoparticle transformation reactions) offers a powerful framework for further expanding the total synthesis toolbox for accessing high-order hybrid nanoparticles with increasingly complex features and material components. For example, sequentially applied reactions involving deposition of Ag onto Au, reaction with sulfur to form Au@Ag2S core−shell particles, and cation exchange yielded a variety of multicomponent core−shell particles, including Au@(CdS−PbS).26 Accordingly, here we show that multiple distinct types of synthetic strategies can be combined in succession to produce multicomponent, asymmetric hybrid nanoparticles with complex morphologies, interfaces, compositions, and constituent materials from simple, readily available, isotropic nanoparticle seeds. Specifically, we demonstrate that isotropic Au nanoparticles serve as seeds for Ag deposition to form either Au-rich or Ag-rich Ag−Au nanoparticles, which in turn can serve as reactive precursors for the selective, stepwise transformation into a variety of asymmetric hybrid nanoparticles. For example, asymmetric Au1−xAgx−AgAuS and Au1−xAgx−Ag3AuS2 hybrid nanoparticles form through a phase segregation process triggered by reaction with dissolved sulfur, and the formation of AgAuS versus Ag3AuS2 occurs selectively on the basis of the Ag:Au ratio defined by the isotropic Ag−Au seed particles. Ag1−xAux−PbS and Ag1−xAux− Ag2S−PbS hybrid particles then form through cation exchange of Au1−xAgx−AgAuS and Au1−xAgx−Ag3AuS2, respectively. The formation of AgAuS and Ag3AuS2 domains upon reaction of colloidal Ag−Au nanoparticles with dissolved sulfur is unexpected and particularly noteworthy, as these rare mixed metal sulfides serve as reactive synthons that open the door to cation exchange in a system for which such a synthetic pathway would not otherwise be applicable.27−32 The design guidelines that emerge from sequentially applying multiple synthetic pathways for strategically chosen model systems significantly expand our capabilities in complex hybrid nanoparticle retrosynthesis and nanoparticle chemical transformation reactions, which is directly relevant to functional targets in application areas that include photocatalysis, electronics, optoelectronics, and photovoltaic devices.
mediated cation exchange reactions, in which cations in a precursor nanoparticle are replaced with cations from a solution, offer a postsynthetic modification strategy for achieving changes in the composition of nanoparticles while retaining morphology,10,11 as well as crystal structure in some cases.12 Additionally, partial cation exchange reactions can produce segmented multicomponent hybrid particles, where interfacial formation energies define the spatial arrangements of the components.13 For example, striped CdS−Ag2S superlattices form during Ag+ exchange of CdS nanorods because of the large lattice strain between the resulting CdS and Ag2S domains.14 Partial exchange of CdS nanorods with Cu+ produces asymmetric CdS−Cu2S hybrid nanorods, driven by facet-selective cation exchange on one end of the CdS nanorods.15 Chemically triggered segregation of multielement nanoparticles with preprogrammed compositions offers yet another powerful alternative route for synthesizing anisotropic, multidomain hybrid nanoparticles from an isotropic single-domain nanoparticle seed.16 Such reactive phase segregation processes typically use oxygen or chalcogen reagents to trigger the extraction of reactive elements from the multielement seed particle, concomitant with the formation of oxide or chalcogenide domains that remain attached to the particle containing the remaining components of the seed. For example, dual-plasmonic Au−In2O3 heterodimers were obtained by oxidative extraction of the In component of AuIn alloy nanoparticles.17 Similarly, Au−Cu2S18,19 and Pt−CuS20 heterodimers were synthesized by reacting AuCu and PtCu nanoparticle seeds, respectively, with dissolved sulfur or sulfur reagents, which preferentially react with the Cu to form copper sulfides while leaving behind the remaining Au and Pt. Such reactive segregation methods provide an alternative pathway for forming nanoscale heterostructures from single-domain seed particles that does not require surface-seeded nucleation and growth.21 The strategies mentioned above, along with others that offer complementary capabilities, are part of a growing toolbox of reactions that can be used to produce hybrid nanoparticles with compositional, structural, morphological, and interfacial features that map onto their targeted applications. Applied rationally and sequentially, these reactions offer the possibility of retrosynthetically designing functional hybrid nanoparticles using a multistep approach that is conceptually analogous to the total synthesis framework that organic chemists use to access complex natural products.22,23 Most examples of multistep pathways to hybrid nanoparticles have involved combined sequences of seeded growth steps, which utilize surfacemediated nucleation and growth and are therefore limited in the diversity of materials, morphologies, and interfaces that can be accessed. For example, Ag−Pt−Fe3O4 heterotrimers21,23 and PbS−Au−Pt−Fe3O421 heterotetramers were synthesized by successive seeded growth reactions; other products and configurations cannot be achieved in high yields using traditional seeded growth pathways. Nanoparticle ion exchange reactions have also been applied sequentially to facilitate the formation of complex nanoscale heterostructures. For example, ZnS@CdS core−shell tetrapods were accessed by complete anion exchange of ZnO tetrapods to form ZnS, followed by partial exchange of the Zn2+ cations with Cd2+ to form a CdS shell.24 Similarly, using Ag2Se as seeds, multiple types of similarly shaped II−VI and I−III−VI semiconductor hetero-
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EXPERIMENTAL SECTION
Chemicals and Materials. Oleylamine (technical grade, 70%), oleic acid (technical grade, 90%), 1-octadecene (technical grade, 90%), trioctylphosphine [TOP, 97%, (C8H17)3P], hydrogen tetrachloroaurate trihydrate (99.99%, HAuCl4·3H2O), and tetrabutylamine borane complex (TBAB, 97%) were purchased from Sigma-Aldrich. Silver acetate [Ag(C2H3O2), anhydrous, 99%], lead(II) acetate trihydrate [Pb(C2H3O2)2·3H2O, 99%], and sulfur powder (325 mesh, 99.5%) were purchased from Alfa Aesar. Solvents, including hexanes, toluene, and ethanol, were of analytical grade. All chemicals were used as received. Synthesis of Au Particles.33 In a 100 mL round-bottom flask, 100 mg of HAuCl4, 10 mL of toluene, and 10 mL of oleylamine were mixed while being stirred at room temperature in air. After the mixture had been stirred for 10 min, a solution previously prepared by sonicating 1 mL of oleylamine, 1 mL of toluene, and 46 mg of TBAB 4154
DOI: 10.1021/acs.chemmater.7b01449 Chem. Mater. 2017, 29, 4153−4160
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Chemistry of Materials for 5 min was quickly injected. After the injection, the orange, transparent HAuCl4 solution immediately changed dark brown/red and was left to stir for ∼60 min. After the reaction was stopped, the suspension was transferred to centrifuge tubes and isolated by the addition of ethanol followed by centrifugation at 13500 rpm for 3 min. The particles were redispersed in hexanes and washed again by centrifugation. The Au seeds were then redispersed in hexanes and stored. Synthesis of Au-Rich and Ag-Rich Ag−Au Seed Particles. Aurich and Ag-rich Ag−Au seed particles were synthesized by introducing a silver acetate solution into a suspension of the as-synthesized Au nanoparticles. For the Au-rich Ag−Au seed particles, in a 50 mL threeneck flask, 30 mg of Ag(C2H3O2) was dissolved in 5 mL of 1octadecene and 5 mL of oleylamine by sonication at room temperature. Next, 40 mg of Au seed particles dispersed in 2−4 mL of hexanes was added to the flask, and the hexanes were evaporated using a Schlenk line. After the hexanes were evaporated, the system was exposed to air and heated to 150 °C at a heating rate of 5 °C/min using a heating mantle. Once the temperature reached 150 °C, the suspension was cooled by taking off the heating mantle and the product was isolated by the addition of ethanol followed by centrifugation at 13500 rpm for 3 min. The particles were redispersed in hexanes, washed again with ethanol, and then centrifuged under identical conditions. The Au-rich Ag−Au seed particles were then redispersed in hexanes and stored for further use. For Ag-rich Ag−Au seed particles, the synthesis procedure was identical, except that 40 mg of Au nanoparticles in 2 mL of hexanes and 144 mg of Ag(C2H3O2) were used. Synthesis of Au1−xAg x−AgAuS and Au1−xAgx−Ag3AuS2 Hybrid Nanoparticles. The synthesis of Au1−xAgx−AgAuS and Au1−xAgx−Ag3AuS2 hybrid nanoparticles was performed by introducing a sulfur−oleyamine complex to the as-synthesized Au-rich and Agrich Ag−Au seed particles, respectively. First, 20 mg of Au-rich or Agrich Ag−Au seed particles dispersed in 2−3 mL of hexanes was mixed with 5 mL of oleylamine and 5 mL of 1-octadecene in a 50 mL threeneck flask, and then the hexanes were evaporated using a Schlenk line. Next, 40 mg of sulfur powder was dissolved in 2.5 mL of oleylamine by sonication for ∼10 min to form a clear, dark-red solution. The sulfur− oleylamine solution was quickly injected into the seed particle suspension at room temperature in air, and then the reaction flask was heated at a rate of 5 °C/min using a heating mantle. A 1 mL aliquot was removed at 60−70 °C to capture the intermediate core− shell Au1−xAgx@AgAuS or Au 1−xAgx@Ag3AuS2 particles. The remainder of the suspension was heated to 150 °C before being cooled by removing the heating mantle. The particles were then isolated by the addition of ethanol followed by centrifugation at 13500 rpm for 3 min. The particles were redispersed in hexanes, washed again by centrifugation, and then dispersed in hexanes for storage. Pb2+ Cation Exchange of Au1−xAgx−AgAuS and Au1−xAgx− Ag3AuS2 Hybrid Nanoparticles. First, 10 mg of the as-synthesized Au1−xAgx−AgAuS and Au1−xAgx−Ag3AuS2 hybrid nanoparticles dispersed in 1 mL of hexanes was transferred into 0.5 mL of TOP and 2 mL of oleylamine, and then hexanes were evaporated by vacuum using a Schlenk line. The suspensions of hybrid nanoparticles were quickly injected into a 50 mL three-neck flask containing 5 mL of oleylamine, 5 mL of 1-octadecene, and 0.5 mL of oleic acid preheated at 70 °C under vacuum. The reaction mixture was kept at 67−70 °C for 10 min before being cooled by removing the heating mantle. The particles were washed three times; each cycle included redispersion of the particles in toluene and precipitation with ethanol, followed by centrifugation at 13500 rpm for 3 min. Characterization. Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, selected area electron diffraction (SAED) patterns, high-angle annular dark-field scanning transmission electron microscopy (STEM-HAADF) images, and energy dispersive X-ray spectroscopy (EDS) spectra and element maps were obtained using a FEI Talos field emission transmission electron microscope operating at 200 kV. Cliff−Lorimer quantification was performed on the deconvoluted EDS data using Bruker Esprit software. Quantitative maps (Qmaps) were obtained using Bruker
Esprit software to deconvolute the overlapping gold, lead, and sulfur signals in the range of 2.1−2.4 eV (2.12 eV for Au M, 2.31 eV for S Kα, and 2.34 eV for Pb M). Using the Qmap process, the Au Lα peak at 9.71 eV, the Pb Lα peak at 10.55 eV, and the deconvoluted S Kα peak were used to reconstruct the Qmap signals. ES Vision software (Emispec) was used for SAED and FFT data processing. Powder X-ray diffraction (XRD) patterns were collected on a Bruker Advance D8 Xray diffractometer with Cu Kα radiation at room temperature. XRD samples were drop-cast in thick layers on a low-background Si substrate. Simulated powder XRD patterns were made using the CrystalMaker and CrystalDiffract software suite.
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RESULTS AND DISCUSSION Formation of Au 1−x Ag x −AgAuS and Au 1−x Ag x − Ag3AuS2 Hybrid Nanoparticles. Spherical Au and Ag−Au nanoparticles are well known34 and therefore were chosen as a starting point for studying the transformation of isotropic colloidal nanocrystals into complex, asymmetric, multicomponent hybrid nanoparticles through sequential application of multiple reactive modifications. Au nanoparticles were synthesized first, and then Ag was deposited to form Au-rich Ag−Au particles with an average Ag:Au ratio of 45:55, as determined by EDS (Figure S1). Panels a and b of Figure 1 show HRTEM
Figure 1. Representative microscopy data showing the evolution of the particles from Au seeds to Au-rich Ag−Au particles and Au1−xAgx@ AgAuS intermediate particles. HRTEM images of (a) Au and (b) Aurich Ag−Au particles. (c) HAADF-STEM image and corresponding STEM-EDS element maps of (d) Au and (e) Ag, showing the Ag-rich Ag−Au shell surrounding the Au-rich Ag−Au core. (f) HRTEM image of a Au1−xAgx@AgAuS particle formed during the reaction of Au-rich Ag−Au seed particles with the sulfur−oleylamine complex at 70 °C. (g) HAADF-STEM image and corresponding STEM-EDS element maps of (h) Au, (i) Ag, and (j) S for Au1−xAgx@AgAuS particles.
images of the Au and Au-rich Ag−Au particles, and the corresponding TEM images are shown in Figure S1. Figure 1c shows a HAADF-STEM image of the Au-rich Ag−Au particles, and the corresponding Au and Ag STEM-EDS element maps are shown in panels d and e of Figure 1, respectively. The STEM-EDS element maps indicate that some Ag alloys with the Au seed particles, but the majority coats the surface to form largely core−shell particles. Such a morphology is not 4155
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Chemistry of Materials unexpected, as Ag is known to deposit on Au seeds and the elevated temperature is also sufficient to facilitate diffusion of Ag into Au.35 The resulting particles are best described as primarily Ag with some Au as the shell and primarily Au with some Ag as the core, e.g., Ag1−xAux@Au1−xAgx. For the sake of simplicity, and given the 45:55 Ag:Au composition, these particles are hereafter termed Au-rich Ag−Au seed particles. The diffraction rings observed in the SAED pattern for ensembles of both the Au and Au-rich Ag−Au seed particles in Figure S1 indicate that the particles are highly crystalline, which is consistent with the corresponding HRTEM images in panels a and b of Figure 1 and also with the powder XRD data shown in Figure S1. The isotropic, spherical Au-rich Ag−Au seed particles were reacted with sulfur dissolved in oleylamine to begin transforming them into more complex, asymmetric, hybrid particles. The phase segregation reaction was initiated by injecting a sulfur−oleylamine solution into the suspension of Ag−Au seed particles under ambient conditions. Immediately upon injection of the sulfur−oleylamine solution, the solution containing the Au-rich Ag−Au seed particles turned from orange to black, indicating loss of the local surface plasmon resonance (LSPR) associated with the Ag−Au particles. The reaction mixture was then heated at a rate of 5 °C/min, and an aliquot was taken when the reaction temperature reached 70 °C. The HRTEM image in Figure 1f reveals a core−shell structure more pronounced than that of the Au-rich Ag−Au seeds. The powder XRD pattern in Figure S2 shows weak, broad reflections consistent with small domains of Ag−Au, as well as a broad and complex pattern having peak maxima that match with a ternary AgAuS compound rather than a silver sulfide phase. The SAED pattern in Figure S2, which corresponds to a collection of particles, shows spots and rings that are consistent with the XRD data. The HAADF-STEM image and corresponding STEM-EDS element maps in Figure 1g−j indicate that the core remains an Au-rich Ag−Au alloy while the shell contains colocalized Au, Ag, and S. Taken together, the data in Figures 1, S1, and S2 are consistent with the formation of a core−shell nanoparticle intermediate of approximate Au1−xAgx@AgAuS composition upon initial reaction of the Au-rich Ag−Au seeds with sulfur. With an increase in the reaction temperature to 150 °C, the particles shown in the TEM images in panels a and b of Figure 2 are formed. The particles now appear as asymmetric Janus nanostructures that are hybrids of two distinct materials. Corresponding STEM-EDS element maps in panels d−f of Figure 2 indicate that Au, Ag, and S are all present in the hybrid nanoparticles. The bright regions of the HAADF-STEM image in Figure 2c, which correspond to the heaviest elements, match with the regions in the STEM-EDS maps where Au is the predominant element; a small amount of Ag is also present in these regions. The Ag is primarily located in the larger domains and is colocalized with sulfur. The STEM-EDS maps are therefore consistent with the formation of hybrid nanoparticles that contain an Au1−xAgx domain fused to a domain containing Au, Ag, and S. The XRD pattern in Figure 3a does not show a significant contribution from the Au1−xAgx domain, which is consistent with the HRTEM image in Figure 2b that shows it is small and poorly crystalline. The majority crystalline phase determined by XRD matches best with AgAuS, which is the mineral petrovskaite.27 To the best of our knowledge, pertrovskaite AgAuS has not been observed previously in colloidal nano-
Figure 2. Representative microscopy data showing the Au1−xAgxAgAuS hybrid nanoparticles. (a) TEM and (b) HRTEM images of Au1−xAgx−AgAuS particles. (c) HAADF-STEM image and corresponding STEM-EDS element maps of (d) Au, (e) Ag, and (f) S for a single Au1−xAgx−AgAuS particle, showing the colocalization of Au and Ag in the brighter-contrast domain in the HAADF image, and of Au, Ag, and S in the darker-contrast sulfide domain.
Figure 3. (a) Powder XRD data of the Au1−xAgx−AgAuS hybrid nanoparticles. The experimental and simulated patterns are colored blue and black, respectively, and characteristic peaks are labeled. The crystal structure of AgAuS viewed in the [100] direction is shown in the inset. (b) SAED pattern, highlighting two characteristic peaks. (c) HRTEM image showing a single Au1−xAgx−AgAuS particle.
particle syntheses. The crystal structure of AgAuS, shown in the inset of Figure 3a, is unusual, as it contains a distorted bodycentered cubic anion sublattice with interpenetrating networks incorporating silver atoms in a distorted tetrahedron of sulfur,27 as well as gold and silver linearly coordinated with sulfur. Figure 3b shows the SAED pattern corresponding to the ensemble of 4156
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Chemistry of Materials Au1−xAgx−AgAuS particles in Figure 2a, and it matches well with that expected for AgAuS and also with the XRD pattern of the bulk sample. The HRTEM image in Figure 3c highlights the AgAuS region, which matches well with the [100] projection of the AgAuS crystal structure. Unusual Au1−xAgx− AgAuS colloidal hybrid nanoparticles therefore form through the stepwise deposition of Ag onto premade Au nanoparticle seeds, followed by reaction with sulfur and subsequent heating. The observation that AgAuS forms rather than Ag2S is interesting and surprising, given the known reactivity of silver toward sulfidation34 and the corresponding lack of reactivity of sulfur with gold, as gold sulfides are rarely observed as products in colloidal nanoparticle syntheses. In addition to the Au-rich Ag−Au seed particles shown in Figure 1b, similar Ag-rich Ag−Au particles can be produced by depositing a larger amount of silver on the premade Au seeds. Figure S3 shows TEM, HRTEM, and SAED data characterizing the evolution from spherical Ag-rich Ag−Au particles to Au1−xAgx@Ag3AuS2 core−shell intermediates upon reaction with the sulfur−oleylamine complex and heating to 60 °C. Figure 4 shows the final Au1−xAgx−Ag3AuS2 hybrid nano-
Ag−Au system that unexpectedly produced crystalline pertrovskaite (AgAuS), this reaction involving the Ag-rich Ag−Au system produces hybrid nanoparticles containing crystalline Ag3AuS2, which is a different mineral, uytenbogaardtite.28 The powder XRD data in Figure 5a, along with the
Figure 5. (a) Powder XRD data of the Au1−xAgx−Ag3AuS2 hybrid nanoparticles. The experimental and simulated patterns are colored blue and black, respectively, and characteristic peaks are labeled. The crystal structure of Ag3AuS2 viewed in the [100] direction is shown in the inset. (b) SAED pattern, highlighting two characteristic peaks. (c) HRTEM image showing a single Au1−xAgx−Ag3AuS2 particle.
SAED pattern in Figure 5b that corresponds to the TEM image in Figure 4a, are consistent with those expected for crystalline Ag3AuS2. The HRTEM image of the Ag3AuS2 region, shown in Figure 5c, matches well with the crystal structure of Ag3AuS2 viewed from the [100] direction. Ag3AuS2,28 along with Ag3AuSe2,31 has been reported a few times as colloidal nanoparticles. The crystal structure of Ag3AuS2, which is shown in the inset of Figure 5a, has structural features similar to those of AgAuS, including a related distorted body-centered cubic anion sublattice, distorted silver−sulfur tetrahedra, and linear metal−sulfur chains.27,28 The ratio of the tetrahedrally coordinated cations and the chain motifs for Ag3AuS2 is different from that for AgAuS. Pb2+ Cation Exchange of Au1−xAgx−AgAuS and Au1−xAgx−Ag3AuS2 Hybrid Nanoparticles. While the formation of crystalline AgAuS and Ag3AuS2 domains through the reaction of Ag−Au nanoparticle seeds with sulfur is inherently interesting, the reactivity of the AgAuS and Ag3AuS2 domains makes them powerful new synthons for subsequent chemical transformation reactions. We find that the Ag and Au, which are oxidized, are amenable to cation exchange, allowing AgAuS and Ag3AuS2 to open the door to subsequent chemical modifications for the construction of increasingly sophisticated hybrid nanoparticles. Cation exchange with Pb2+ is well-
Figure 4. Representative microscopy data showing the Au1−xAgx− Ag3AuS2 hybrid nanoparticles. (a) TEM and (b) HRTEM images of the Au1−xAgx−Ag3AuS2 particles. (c) HAADF-STEM image and STEM-EDS element maps of (d) Au, (e) Ag, and (f) S for a single Au1−xAgx−Ag3AuS2 particle, showing the colocalization of Au and Ag in the brighter-contrast domain in the HAADF image, and of Au, Ag, and S in the darker-contrast sulfide domain.
particles after heating to 150 °C. The STEM-EDS element maps in Figure 4 confirm the colocalization of Ag, Au, and S in the large domains, as well as the majority presence of Ag in the smaller domains. The reaction pathway that produces the Au1−xAgx−Au3AuS2 hybrid nanoparticles is analogous to that shown in Figures 1 and 2, which produces Au1−xAgx−AgAuS. As for the Au-rich 4157
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Chemistry of Materials established in colloidal nanocrystal systems36 and therefore was used to study the reactivity of the AgAuS and Ag3AuS2 domains of the Au1−xAgx−AgAuS and Au1−xAgx−Ag3AuS2 hybrid nanoparticles when they were heated at 70 °C for 10 min with Pb2+ in a mixture of trioctylphosphine, oleylamine, oleic acid, and octadecene. As shown in the TEM images in Figure 6a, Au1−xAgx−AgAuS reacts with Pb2+ to form hybrid
Interestingly, there is a concomitant decrease in the amount of Au in the Ag−Au domain during the transformation, as the Aurich Au1−xAgx domain becomes Ag-rich. To rationalize this, we hypothesized that trioctylphosphine sulfide (TOP-S), which could form in situ from residual sulfur and TOP, may facilitate some leaching of Au. TOP itself is not known as an etchant for Au, but some sulfur-containing species are known to etch gold nanoparticles under certain conditions.37 Indeed, control experiments indicate that Au-rich AgAu alloy nanoparticles (by themselves, not part of a hybrid construct) transform to Ag-rich AgAu alloy nanoparticles upon reaction with a TOP−S complex (prepared ex situ) in oleylamine upon being heated to 70 °C, which are conditions analogous to those used during the cation exchange reaction. A different type of hybrid nanoparticle product forms when Au1−xAgx−Ag3AuS2 reacts with Pb2+ under identical conditions. TEM and HRTEM images of the cation-exchanged product, shown in panels a and b of Figure 7, reveal hybrid nanoparticles with three distinct regions. The corresponding HAADF-STEM image and STEM-EDS maps in panels c−h of Figure 7 show Pb in part of the region that corresponds to the original Ag3AuS2 domain, while Ag is present in that domain where Pb is not. The sulfur signal is co-located with both Pb and Ag. The corresponding powder XRD pattern (Figure 7i) shows prominent peaks for PbS, as well as weaker reflections that match well with Ag2S, while the SAED pattern in the inset of Figure 7i shows rings that match with Ag1−xAux, PbS, and Ag2S. The data are therefore consistent with the formation of threecomponent hybrid nanoparticles containing distinct Ag1−xAux, Ag2S, and PbS segments. Unlike cation exchange of AgAuS with Pb2+ that goes to completion, in Ag3AuS2 partial cation exchange occurs, leading to a segmented product that can be described as the three-component hybrid nanoparticle Ag1−xAux−Ag2S−PbS. Effective cation exchange of both AgAuS and Ag3AuS2 with Pb2+ can be rationalized using hard soft acid base theory that predicts preferential coordination of Ag+ and Au+, which are softer cations than Pb2+, with TOP, which is a soft ligand. Such soft−soft cation−anion interactions in solution are well-known to help drive colloidal nanocrystal cation exchange reactions.38,39 For AgAuS, both Ag+ and Au+ can be exchanged with a single Pb2+ cation, resulting in complete cation exchange of the AgAuS domain to form PbS. For Ag3AuS2, which contains more Ag+ than Au+, we speculate that the same initial exchange of one Ag+ and one Au+ for a single Pb2+ occurs. Such a pathway would leave behind two Ag+ cations and a single S2− anion per Ag3AuS2 unit, resulting in the transformation of the Ag3AuS2 domain into Ag2S and PbS, which are both stable compounds. It is anticipated that other soft cations may be amenable to similar cation exchange processes. Figure 8 summarizes the sequences of reactions that transform isotropic spherical Au nanoparticles into several distinct types of asymmetric Janus particles. Depositing different amounts of Ag onto the Au nanoparticles to form either Au-rich Ag−Au seeds or Ag-rich Ag−Au seeds provides a tunable entryway into the selective formation of Au1−xAgx− AgAuS or Au1−xAgx−Ag3AuS2 hybrid nanoparticles, respectively, upon reaction with dissolved sulfur; the formation of these hybrid constructs proceeds through an intermediate type of nanoparticle containing Ag−Au−S shells. The AgAuS and Ag3AuS2 nanocrystal domains, which contain oxidized Ag and Au, are amenable to cation exchange to form Ag1−xAux−PbS and Ag1−xAux−Ag2S−PbS, respectively. The formation of
Figure 6. Characterization of the hybrid nanoparticles obtained after Pb2+ cation exchange of the AgAuS domains in Au1−xAgx−AgAuS. (a) TEM and HRTEM (inset) images of the resulting Ag1−xAux−PbS particles. (b) Powder XRD data for the Ag1−xAux−PbS hybrid nanoparticles. The experimental and simulated patterns are colored blue and black, respectively, and characteristic peaks are labeled. The SAED pattern shown in the inset confirms the presence of both Ag1−xAux and PbS in the hybrid nanoparticles. (c) HAADF-STEM image and corresponding STEM-EDS element maps of (d) Ag, (e) Au, (f) Pb, (g) S, and (h) superimposed Ag, Pb, and S for a single Ag1−xAux−PbS particle.
nanoparticles that remain similar in size and morphology. The HAADF-STEM image in Figure 6c and the STEM-EDS element maps in Figure 6d−h reveal that Ag, along with a small amount of Au, is present in the small domains while Pb and S are present in the large domains; no Ag or Au signal above baseline is observed in the PbS domains. The powder XRD pattern and SAED pattern in Figure 6b both confirm the formation of PbS and the absence of AgAuS, and the HRTEM image in the inset of Figure 6a is also consistent with lattice fringes matching the spacings of the PbS (200) planes. PbS is known to form faceted nanocrystals, which accounts for a subpopulation of particles that are oriented on the TEM grid such that the Au−Ag domain is facing up rather than on the side. Reaction of Pb2+ with Au1−xAgx−AgAuS therefore results in the formation of Ag1−xAux−PbS through transformation of the AgAuS domain into PbS. AgAuS contains Ag+ and Au+, which together can be replaced with Pb2+ considering charge balance. 4158
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Chemistry of Materials
Ag1−xAux−Ag2S−PbS is particularly noteworthy, as it represents an asymmetric trimeric hybrid nanoparticle having a complex Janus-type nanostructure and multijunction buried interface that is constructed rationally through a sequence of reactions that do not involve, and would not form through, traditional heterogeneous seeded growth processes.
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CONCLUSIONS In summary, we have demonstrated that asymmetric Janus nanoparticles can be synthesized through the stepwise application of several distinct types of chemical transformation reactions, producing hybrid constructs with features that would not otherwise be accessible through existing heterogeneous seeded growth methods. For example, the Ag1−xAux−Ag2S− PbS heterotrimers having segmented Ag2S−PbS domains attached to Ag1−xAux alloy particles contain a large, buried, triple-junction interface that is distinct from the smaller particle−particle interfaces typically accessible in hybrid nanoparticles synthesized through seeded growth methods. The introduction of AgAuS and Ag3AuS2 nanocrystals as reactive synthons for cation exchange provides a new entryway into nanoparticle chemical transformation reactions, thereby expanding the scope of such capabilities.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01449. Additional experimental details and supplemental TEM, HRTEM, SAED, and XRD data (PDF)
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AUTHOR INFORMATION
Corresponding Author
Figure 7. Characterization of high-order Ag1−xAux−Ag2S−PbS hybrid nanoparticles obtained after partial Pb2+ cation exchange of the Ag3AuS2 domains in Au1−xAgx−Ag3AuS2. (a) TEM and (b) HRTEM images of the resulting Ag1−xAux−Ag2S−PbS particles. (c) HAADFSTEM image and corresponding STEM-EDS element maps of (d) Ag, (e) Au, (f) Pb, (g) S, and (h) superimposed Ag, Pb, and S for a single Ag1−xAux−Ag2S−PbS particle. (i) Powder XRD data for the Ag1−xAux−Ag2S−PbS particles. The experimental and simulated patterns are colored blue and black, respectively, and characteristic peaks are labeled. The SAED pattern shown in the inset confirms the presence of Ag1−xAux, Ag2S, and PbS in the hybrid nanoparticles.
*E-mail:
[email protected]. ORCID
Raymond E. Schaak: 0000-0002-7468-8181 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Penn State Materials Research Science and Engineering Center (MRSEC, National Science Foundation Grant DMR-1420620). TEM imaging was performed at the Materials Characterization Laboratory of the
Figure 8. Schematic representation of the evolution of the nanoparticles from Au particles to Au-rich and Ag-rich Ag−Au particles, followed by conversion of the isotropic Ag−Au particles to asymmetric Au1−xAgx−AgAuS and Au1−xAgx−Ag3AuS2 hybrid particles with reactive AgAuS and Ag3AuS2 synthons that are further transformed to Ag1−xAux−PbS and Ag1−xAux−Ag2S−PbS hybrid nanoparticles. 4159
DOI: 10.1021/acs.chemmater.7b01449 Chem. Mater. 2017, 29, 4153−4160
Article
Chemistry of Materials
(20) Ding, X.; Zou, Y.; Ye, F.; Yang, J.; Jiang, J. Pt−CuS heterodimers by sulfidation of CuPt alloy nanoparticles and their selective catalytic activity toward methanol oxidation. J. Mater. Chem. A 2013, 1, 11880. (21) Buck, M. R.; Bondi, J. F.; Schaak, R. E. A total-synthesis framework for the construction of high-order colloidal hybrid nanoparticles. Nat. Chem. 2012, 4, 37. (22) Buck, M. R.; Schaak, R. E. Emerging strategies for the total synthesis of inorganic nanostructures. Angew. Chem., Int. Ed. 2013, 52, 6154. (23) Hodges, J. M.; Morse, J. R.; Williams, M. E.; Schaak, R. E. Microscopic Investigation of Chemoselectivity in Ag−Pt−Fe3O4 Heterotrimer Formation: Mechanistic Insights and Implications for Controlling High-Order Hybrid Nanoparticle Morphology. J. Am. Chem. Soc. 2015, 137, 15493. (24) Hodges, J. M.; Kletetschka, K.; Fenton, J. L.; Read, C. G.; Schaak, R. E. Sequential Anion and Cation Exchange Reactions for Complete Material Transformations of Nanoparticles with Morphological Retention. Angew. Chem., Int. Ed. 2015, 127, 8793. (25) Prusty, G.; Guria, A. K.; Mondal, I.; Dutta, A.; Pal, U.; Pradhan, N. Modulated Binary−Ternary Dual Semiconductor Heterostructures. Angew. Chem. 2016, 128, 2755. (26) Zhang, J.; Tang, Y.; Lee, K.; Ouyang, M. Nonepitaxial growth of hybrid core-shell nanostructures with large lattice mismatches. Science 2010, 327, 1634. (27) Seryotkin, Y. V.; Bakakin, V. V.; Pal’Yanova, G. A.; Kokh, K. A. Synthesis and crystal structure of silver−gold sulfide AgAuS. Four-fold interpenetrated three-dimensional [(Au, Ag)10S8]-networks. CrystEngComm 2014, 16, 1675. (28) Seryotkin, Y. V.; Bakakin, V. V.; Pal’yanova, G. A.; Kokh, K. A. Synthesis and Crystal Structure of the Trigonal Silver (I) Dithioaurate (I), Ag3AuS2. Cryst. Growth Des. 2011, 11, 1062. (29) Siva, C.; Iswarya, C. N.; Baraneedharan, P.; Sivakumar, M. Lcysteine assisted formation of mesh like Ag 2 S and Ag 3 AuS 2 nanocrystals through hydrogen bonds. Mater. Lett. 2014, 134, 56. (30) Liu, M.; Zeng, H. C. General Synthetic Approach to Heterostructured Nanocrystals Based on Noble Metals and I−VI, II−VI, and I−III−VI Metal Chalcogenides. Langmuir 2014, 30, 9838. (31) Dalmases, M.; Ibáñez, M.; Torruella, P.; Fernández-Altable, V.; López-Conesa, L.; Cadavid, D.; Piveteau, L.; Nachtegaal, M.; Llorca, J.; Ruiz-González, M. L.; Estrade, S.; Peiró, F.; Kovalenko, M. V.; Cabot, A.; Figuerola, A. Synthesis and Thermoelectric Properties of Noble Metal Ternary Chalcogenide Systems of Ag−Au−Se in the Forms of Alloyed Nanoparticles and Colloidal Nanoheterostructures. Chem. Mater. 2016, 28, 7017. (32) Mikhlin, Y. L.; Nasluzov, V. A.; Romanchenko, A. S.; Shor, A. M.; Pal’yanova, G. A. XPS and DFT studies of the electronic structures of AgAuS and Ag3AuS2. J. Alloys Compd. 2014, 617, 314. (33) Peng, S.; Lee, Y.; Wang, C.; Yin, H.; Dai, S.; Sun, S. A Facile Synthesis of Monodisperse Au Nanoparticles and Their Catalysis of CO Oxidation. Nano Res. 2008, 1, 229. (34) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Shape-Controlled synthesis of metal nanocrystals: simple chemistry meets complex physics? Angew. Chem., Int. Ed. 2009, 48, 60. (35) Shore, M. S.; Wang, J.; Johnston-Peck, A. C.; Oldenburg, A. L.; Tracy, J. B. Synthesis of Au(Core)/Ag(Shell) Nanoparticles and their Conversion to AuAg Alloy Nanoparticles. Small 2011, 7, 230−234. (36) Luther, J. M.; Zheng, H.; Sadtler, B.; Alivisatos, A. P. Synthesis of PbS nanorods and other ionic nanocrystals of complex morphology by sequential cation exchange reactions. J. Am. Chem. Soc. 2009, 131, 16851. (37) Dreier, T. A.; Ackerson, C. J. Radicals Are Required for Thiol Etching of Gold Particles. Angew. Chem., Int. Ed. 2015, 54, 9249−9252. (38) Pearson, R. G. Absolute electronegativity and hardness: application to inorganic chemistry. Inorg. Chem. 1988, 27, 734. (39) Son, D. H.; Hughes, S. M.; Yin, Y.; Alivisatos, A. P. Cation exchange reactions in ionic nanocrystals. Science 2004, 306, 1009.
Penn State Materials Research Institute. The authors thank Jennifer Gray and Ke Wang for assistance in collecting and processing some of the TEM images.
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REFERENCES
(1) Cozzoli, P. D.; Pellegrino, T.; Manna, L. Synthesis, properties and perspectives of hybrid nanocrystal structures. Chem. Soc. Rev. 2006, 35, 1195. (2) Yin, Y.; Talapin, D. The chemistry of functional nanomaterials. Chem. Soc. Rev. 2013, 42, 2484. (3) Cortie, M. B.; McDonagh, A. M. Synthesis and optical properties of hybrid and alloy plasmonic nanoparticles. Chem. Rev. 2011, 111, 3713. (4) Selinsky, R. S.; Ding, Q.; Faber, M. S.; Wright, J. C.; Jin, S. Quantum dot nanoscale heterostructures for solar energy conversion. Chem. Soc. Rev. 2013, 42, 2963. (5) Banin, U.; Ben-Shahar, Y.; Vinokurov, K. Hybrid semiconductor− metal nanoparticles: from architecture to function. Chem. Mater. 2014, 26, 97. (6) Shaviv, E.; Schubert, O.; Alves-Santos, M.; Goldoni, G.; Di Felice, R.; Vallée, F.; Del Fatti, N.; Banin, U.; Sönnichsen, C. Absorption Properties of Metal−Semiconductor Hybrid Nanoparticles. ACS Nano 2011, 5, 4712. (7) Amirav, L.; Oba, F.; Aloni, S.; Alivisatos, A. P. Modular Synthesis of a Dual Metal−Dual Semiconductor Nano-Heterostructure. Angew. Chem., Int. Ed. 2015, 54, 7007. (8) Read, C. G.; Biacchi, A. J.; Schaak, R. E. Au−Ge and Ag−Ge heterodimers with tunable domain sizes: a supersaturation-precipitation route to colloidal hybrid nanoparticles. Chem. Mater. 2013, 25, 4304. (9) Read, C. G.; Gordon, T. R.; Hodges, J. M.; Schaak, R. E. Colloidal hybrid nanoparticle insertion reaction for transforming heterodimers into heterotrimers. J. Am. Chem. Soc. 2015, 137, 12514. (10) Hodges, J. M.; Morse, J. R.; Williams, M. E.; Schaak, R. E. Microscopic Investigation of Chemoselectivity in Ag−Pt−Fe3O4 Heterotrimer Formation: Mechanistic Insights and Implications for Controlling High-Order Hybrid Nanoparticle Morphology. J. Am. Chem. Soc. 2015, 137, 15493. (11) Beberwyck, B. J.; Surendranath, Y.; Alivisatos, A. P. Cation exchange: a versatile tool for nanomaterials synthesis. J. Phys. Chem. C 2013, 117, 19759. (12) De Trizio, L.; Manna, L. Forging colloidal nanostructures via cation exchange reactions. Chem. Rev. 2016, 116, 10852. (13) De Trizio, L.; Li, H.; Casu, A.; Genovese, A.; Sathya, A.; Messina, G. C.; Manna, L. Sn Cation Valency Dependence in Cation Exchange Reactions Involving Cu2‑xSe Nanocrystals. J. Am. Chem. Soc. 2014, 136, 16277. (14) Robinson, R. D.; Sadtler, B.; Demchenko, D. O.; Erdonmez, C. K.; Wang, L. W.; Alivisatos, A. P. Spontaneous Superlattice Formation in Nanorods through Partial Cation Exchange. Science 2007, 317, 355. (15) Sadtler, B.; Demchenko, D. O.; Zheng, H.; Hughes, S. M.; Merkle, M. G.; Dahmen, U.; Wang, L. W.; Alivisatos, A. P. Selective Facet Reactivity During Cation Exchange in Cadmium Sulfide Nanorods. J. Am. Chem. Soc. 2009, 131, 5285. (16) Gu, H.; Zheng, R.; Zhang, X.; Xu, B. Facile one-pot synthesis of bifunctional heterodimers of nanoparticles: a conjugate of quantum dot and magnetic nanoparticles. J. Am. Chem. Soc. 2004, 126, 5664. (17) Gordon, T. R.; Schaak, R. E. Synthesis of Hybrid Au-In2O3 Nanoparticles Exhibiting Dual Plasmonic Resonance. Chem. Mater. 2014, 26, 5900. (18) Motl, N. E.; Bondi, J. F.; Schaak, R. E. Synthesis of Colloidal Au−Cu2S Heterodimers via Chemically Triggered Phase Segregation of AuCu Nanoparticles. Chem. Mater. 2012, 24, 1552. (19) Ding, X.; Zou, Y.; Jiang, J. Au−Cu2S heterodimer formation via oxidization of AuCu alloy nanoparticles and in situ formed copper thiolate. J. Mater. Chem. 2012, 22, 23169. 4160
DOI: 10.1021/acs.chemmater.7b01449 Chem. Mater. 2017, 29, 4153−4160
