Letter pubs.acs.org/NanoLett
Triangular Gold Nanoplate Growth by Oriented Attachment of Au Seeds Generated by Strong Field Laser Reduction Behzad Tangeysh,† Katharine Moore Tibbetts,†,‡ Johanan H. Odhner,†,‡ Bradford B. Wayland,† and Robert J. Levis*,†,‡ †
Department of Chemistry and ‡Center for Advanced Photonics Research, Temple University, Philadelphia, Pennsylvania 19122, United States S Supporting Information *
ABSTRACT: The synthesis of surfactant-free Au nanoplates is desirable for the development of biocompatible therapeutics/diagnostics. Rapid Δ-function energy deposition by irradiation of aqueous KAuCl4 solution with a 5 s burst of intense shaped laser pulses, followed by slow addition of H2O2, results in selective formation of nanoplates with no additional reagents. The primary mechanism of nanoplate formation is found to be oriented attachment of the spherical seeds, which self-recrystallize to form crystalline Au nanoplates. KEYWORDS: Oriented attachment, femtosecond laser processing, triangular Au nanoplate, rapid energy deposition
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of spherical silver nanoparticles through light-induced excitation of their SPR band.17,18 The mechanisms for formation and growth of triangular gold nano- and microplates have been debated since their initial observation.19 Three processes previously invoked to explain the growth of Au plates in chemical routes include aggregation and recrystallization of primary particles,20,21 direction by the inhibition of growth along particular crystal facets with surfaceblocking reagents,22 and the formation of coherent twin boundaries along the {111} plane of gold crystals that kinetically accelerate growth along the edges of the plates.23 Recent work shows the importance of the latter two processes to determining the final crystal morphology.6,7,24 The original model of growth by aggregation and recrystallization has largely been discarded in favor of the Kossel−Stranski mechanism, which describes crystal growth occurring through atom-byatom reduction onto a gold particle surface.25,26 The formation and growth of triangular gold nanoplates using an extract of lemongrass as the reducing and capping agent14 was observed to proceed by aggregation of the spherical particles formed during the initial phase of the reaction, similar to the early experiments.20,21 The authors of that paper ascribed this transformation to sintering of spherical nanoparticle assemblies through a liquid-like state at the gold surface.14 Similar transformations observed in the photo- and thermally induced conversion of Ag nanospheres to triangular Ag nanoplates were ascribed to the light-induced fragmentation and fusion of primary particles, as well as the Ostwald ripening
ailoring the shape-dependent plasmonic properties of nanoparticles is an area of fundamental interest with applications in sensing, biology, catalysis, and quantum electronics.1−4 Anisotropic (nonspherical) gold nanoparticles (AuNPs) are sought for biomedical imaging, diagnosis, and photothermal cancer therapy because their associated surface plasmon resonance (SPR) frequencies can extend into the nearinfrared “biological window” where absorbance in tissue is minimal.1,2 Triangular Au nanoplates present a desirable synthetic target because the sharp tips provide exceptional optical near-field enhancement for sensing applications such as surface-enhanced Raman spectroscopy (SERS) and single molecule fluorescence.1,5 Controlled synthesis of AuNPs with different shapes is a maturing field where judicious choice of the precursors, reducing agents, and surfactants determines the dominant particle shape.6,7 One of the most reliable methods for shapecontrolled synthesis of AuNPs including nanotriangles is the thermal seed-mediated growth approach, which usually uses toxic cationic surfactants such as cetyltrimethylammonium bromide (CTAB) to direct particle growth.8−10 While seedless procedures have been recently developed to eliminate the initial seed formation step,11 a remaining challenge is the elimination of cytotoxic surfactants such as CTAB, which must be exchanged for biocompatible ligands in medical applications.12,13 To avoid the use of CTAB, several robust procedures have been developed to produce comparable yields of Au nanotriangles without CTAB by using biological extracts14,15 and sodium thiosulfate.16 In addition to chemical methods, photochemical routes have also been successfully employed for preparing bulk quantities of silver nanotriangles by conversion © XXXX American Chemical Society
Received: February 20, 2015 Revised: March 26, 2015
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DOI: 10.1021/acs.nanolett.5b00709 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters process.17,18,27 This article reports the first synthesis of Au nanotriangles from near-quantitative transformation of lasergenerated Au clusters to nanoplates in aqueous KAuCl4 solution upon slow reduction by H2O2 without any added surfactant molecule or capping agent. In contrast to commonly used photochemical routes,17,18 the transformation of the particles was observed when the laser is “off”, which rules out the effect of light-induced fragmentation and fusion of the nanoparticles in the synthesis of Au nanoplates. Electron microscopy and electronic spectroscopy measurements performed at different stages of the reaction reveal that oriented attachment28,29 drives the formation and growth of nanoplates, which does not have any precedent in the literature for synthesis of Au nanotriangles. In comparison with the seedmediated growth approach, no toxic reagent was used in the synthesis, and the surfactant-free Au nanoplates produced by this method can readily be used for a variety of applications such as therapeutics, sensing, and photocatalysis. The nanotriangle synthesis relies on the formation of Au clusters through irradiation of aqueous KAuCl4 solution (0.125 mM) by a short burst of intense, spatiotemporally shaped femtosecond laser pulses (3.8 mJ/pulse, 5000 pulses at 1 kHz) (Figure 1a). Simultaneous spatial and temporal focusing
Figure 2. Growth of crystalline Au nanotriangles in aqueous solution over 48 h with slow H2O2 addition. (a) Electronic spectra of KAuCl4 solution and Au nanostructure dispersions during slow H2O2 addition. The inset magnifies the spectral region from 300 to 1100 nm, showing the lack of an SPR feature at ∼520 nm that would indicate spherical Au nanoparticles. (b) Representative TEM image of Au nanoplates formed after adding 75 μL of H2O2. The black region in the upper lefthand corner corresponds to the grid material. (c) HRTEM image of an individual Au nanotriangle showing the corner and the smooth edge of the particle. The inset magnifies the region around the edge of the particle showing continuous lattice fringes with spacing of 0.25 nm, which corresponds to the 1/3{422} facet.36−38 (d) Selected area electron diffraction (SAED) recorded on the surface of the particle shown in panel c. The diffraction spots marked by the circle, square, and triangle correspond to the allowed {220} and {311}, and normally forbidden 1/3{422} Bragg reflections of the fcc Au crystal structure, respectively. (e) Compositional elemental mapping analysis performed on a single Au nanotriangle showing insufficient Cl− on the surface to stabilize the particle. (f) Representative SEM image recorded at 65° angle shows that the thickness of Au nanoplates may be estimated as between 10 and 20 nm.
Figure 1. (a) Schematic representation for femtosecond laser irradiation of KAuCl4 solution using SSTF. (b) Electronic spectra of KAuCl4 solution recorded before irradiation (black) and after irradiation at 10 min (blue), 2 h (green), and 18 h (red). (Inset) Magnification of the region between 400 and 1100 nm. The spectrum at 18 h shows two SPR features corresponding to spherical Au seeds at 520 nm and anisotropic structures at ∼700 nm. (c) Representative TEM image of Au nanostructures formed in aqueous KAuCl4 solution 18 h after laser irradiation. The colored circles indicate spherical Au seeds (blue), irregularly shaped aggregates of seeds (green), and crystalline nanoplates (red). (d) High magnification TEM image recorded 18 h after irradiation shows the diffusion of spherical seed particles into the edge of an irregularly shaped nanoplate (red arrows).
remaining solution, forming homogeneously dispersed nucleation sites. Our previous reports of spherical AuNP synthesis by SSTF irradiation of aqueous KAuCl4 found that in addition to direct reduction of [AuCl4]−, formation of long-lived H2O2 from water during irradiation drives postirradiation AuNP growth until the remaining H2O2 or [AuCl4]− is consumed.32,33 Using Δ-function energy deposition, where the 5 s laser processing time is short compared to the ∼105 s total growth
(SSTF)30 of the laser pulse creates a strong electric field that strips the ligands from every [AuCl4]− molecule in the irradiation volume.31 The ensuing formation of high-energy cavitation bubbles mixes the Au atoms uniformly into the B
DOI: 10.1021/acs.nanolett.5b00709 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
faster than the SPR feature corresponding to the spherical particles (at 520 nm), which is consistent with the formation and growth of Au nanoplates during slow H2O2 reduction (Figure 2a). The UV−vis−NIR spectrum (see Supporting Information Figure S2) of the final product shows a broad band at >1150 nm, which is indicative of the dipole resonance of Au nanoplates.24 The complete disappearance of the SPR feature for spherical AuNPs (at 520 nm) observed in the spectra of the final product suggests that the Au seeds are incorporated into the plates and that the final product is relatively pure (Figure 2a). The formation of Au nanoplates is confirmed with TEM and scanning electron microscopy (SEM), shown in Figures 2b−f and S3−S7 in the Supporting Information. The TEM analysis shows that the majority of the nanoplates are triangular, with average edge length of 346 ± 95 nm (Figures 2b and S3 in the Supporting Information). High-resolution TEM (HRTEM) measurements of the Au nanoplates show well-defined continuous lattice fringes, indicating that the surface of the particles is atomically flat (Figures 2c and S4 in the Supporting Information). Selected area electron diffraction (SAED) measurements, presented in Figure 2d, show the diffraction spots corresponding to the allowed {220} and {311} and normally forbidden 1/3{422} Bragg reflections of the Au facecentered-cubic (fcc) crystal structure. The hexagonal pattern of the diffraction spots indicates that the surfaces of Au nanoplates are terminated by {111} facets.27,35 Observation of relatively strong diffraction from the forbidden 1/3{422} plane is consistent with the occurrence of twinned planes parallel to the {111} surfaces,36−38 which was also confirmed by HRTEM recorded on the side of an individual nanotriangle (see Figure S5 in the Supporting Information). In addition to the twinned planes parallel to the {111} surfaces, some stacking faults and crystal defects were observed perpendicular to the {111} planes (see Figure S5 in the Supporting Information). The SEM measurement shows that the thicknesses of Au nanoplates are in the range of ∼10−20 nm, which is comparable to the diameter of the spherical AuNPs that are present in the aqueous dispersion 18 h after laser processing (Figures 2f and S6 of the Supporting Information). The near-complete disappearance of spherical AuNPs by the end of the synthesis indicates their quantitative incorporation into Au nanoplates when Δ-function laser energy deposition is employed for nucleation. Whereas noble metal nanoplates usually have organic surfactants stabilizing the surface resulting from the synthetic procedure,10,39,40 no additional surfactant is employed in our processing, and thus, we anticipated a Cl− terminated surface.41,42 In order to test this hypothesis, the elemental composition on the surface of Au nanotriangles was evaluated by scanning transmission electron microscopy (STEM) coupled with high-angle annular dark field (HAADF) imaging and energy dispersive X-ray (EDX) spectroscopy, as shown in Figures 2e and S6 in the Supporting Information. These analyses, performed both on the final synthesis products and during an intermediate stage of the reaction where ∼50% of the initial [AuCl4]− had been reduced, show that both chloride (Cl−) and oxygen are present in insufficient quantities to effectively coat the surface of the Au nanoplates (Figures 2e, S7, and S14 (Supporting Information)). This result indicates that these species do not act as stabilizing agents either during particle growth or upon completion of the reaction. However, we cannot exclude the participation of trace amounts of Cl− in
Figure 3. Representative TEM images of the nanostructures formed upon ∼50% [AuCl4]− reduction. (a) Low magnification image showing spherical AuNPs (blue circle), a plate-like structure apparently formed of spherical AuNPs (green arrow), and roughedged nanoplates (red arrows). (b) Higher magnification image of a plate-like structure composed of spherical AuNPs (green arrow) and smaller aggregates of spherical AuNPs (yellow circle). (c) Low magnification image showing several rough-edged plate structures. (d) Magnified image of the region in the yellow box in panel c showing the diffusion of spherical particles into the edge of a nanoplate (blue arrows).
time, only a small fraction (
