Mika Matsunaga

Epitaxial growth of Au@Pd core–shell nanocrystals prepared using a PVP-assisted polyol reduction method

Au@Cu core-shell nanocrystals were prepared using a two-step polyol reduction method. First, mixtures of ochedral, triangular and hexagonal platelike, decahedral, and icosahedral Au core seeds were prepared by reducing HAuCl 4 ·4H 2 O in ethylene glycol (EG) using microwave (MW) heating in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant. Then Cu shells were overgrown on Au core seeds by reducing Cu 2 (OAc) 4 in EG with PVP using oil-bath heating. Resultant crystal structures were characterized using TEM, high-resolution (HR)-TEM, TEM-EDS, and selected area electron diffraction (SAED) measurements. A large mismatch exists in lattice constants between Au (0.4079 nm) and Cu (0.3615 nm). No monometallic Cu nanocrystals having well-defined facets were prepared by reducing Cu 2 (OAc) 4 in EG. Therefore, the epitaxial growth ofCu shells over Au cores was expected to be difficult. Nevertheless, flat {111} facets of Cu shells were grown epitaxially on {111} facets of Au cores. The SAED patterns and Moire patterns showed Cu layers parallel to Au layers. The Cu shell growth on sharp Au-core corners was slower than that on flat {111} facets and single twin facets. This report is the first describing epitaxial growth of core—shell nanocrystals despite a large lattice mismatch (11.4%). The Au@Cu nanoparticles were more antioxidative than pure Cu particles prepared under identical conditions.

Rapid Transformation from Spherical Nanoparticles, Nanorods, Cubes, or Bipyramids to Triangular Prisms of Silver with PVP, Citrate, and H2O2

Rapid sphere-to-prism (STP) transformation of silver was studied in aqueous AgNO(3)/NaBH(4)/polyvinylpyrrolidone (PVP)/trisodium citrate (Na(3)CA)/H(2)O(2) solutions by monitoring time-dependent surface plasmon resonance (SPR) bands in the UV-vis region, by examining transmission electron microscopic (TEM) images, and by analyzing emitted gases during fast reaction. Roles of PVP, Na(3)CA, and H(2)O(2) were studied without addition of a reagent, with different timing of each reagents addition, and with addition of H(2)O(2) to mixtures of spheres and prisms. Results show that prisms can be prepared without addition of PVP, although it is useful to synthesize smaller monodispersed prisms. A new important role of citrate found in this study, besides a known role as a protecting agent of {111} facets of plates, is an assistive agent for shape-selective oxidative etching of Ag nanoparticles by H(2)O(2). The covering of Ag nanoparticles with carboxylate groups is necessary to initiate rapid STP transformation by premixing citrate before H(2)O(2) addition. Based on our data, rapid prism formation starts from the consumption of spherical Ag particles because of shape-selective oxidative etching by H(2)O(2). Oxidative etching of spherical particles by H(2)O(2) is faster than that of prisms. Therefore, spherical particles are selectively etched and dissolved, leaving only seeds of prisms to grow into triangular prisms. When pentagonal Ag nanorods and a mixture of cubes and bipyramids were used as sources of prisms, rod-to-prism (RTP), cube-to-prism (CTP), and bipyramid-to-prism (BTP) transformations were observed in Ag nanocrystals/NaBH(4)/PVP/Na(3)CA/H(2)O(2) solutions. Shape-selective oxidative etching of rods was confirmed using flag-type Ag nanostructures consisting of a triangular plate and a side rod. These data provide useful information for the size-controlled synthesis of triangular Ag prisms, from various Ag nanostructures and using a chemical reduction method, having surface plasmon resonance (SPR) bands at a desired wavelength.

Syntheses of Ag/Cu alloy and Ag/Cu alloy core Cu shell nanoparticles using a polyol method

Ag–Cu bimetallic nanoparticles were prepared by reducing mixtures of AgNO3 and Cu(OAc)2·H2O in ethylene glycol (EG) in the presence of poly(vinylpyrrolidone) (PVP) at 175 °C for 5–60 min. At high [Ag]/[Cu] molar ratios of 1 and 2 or at a short reaction time below 5 min, Ag rich Ag/Cu alloy particles were formed. On the other hand, at low [Ag]/[Cu] molar ratios of 0.25 and 0.5 or a long reaction time above ≈15 min, Cu shells were overgrown on Au/Cu cores and new Ag/Cu alloy core Cu shell nanoparticles, denoted as Ag/Cu@Cu, were produced. The formation of Ag/Cu and Ag/Cu@Cu particles was examined using energy dispersed X-ray spectroscopic (EDS) measurements. The growth mechanisms of Ag/Cu and Ag/Cu@Cu particles are discussed on the basis of TEM-EDS and ultraviolet (UV)-visible (Vis)-near infrared (NIR) extinction spectral data. The time dependence of UV-Vis spectra indicated that the Cu component of Ag/Cu@Cu particles has higher antioxidized property than that of Cu and Cu@Ag particles.

Crystal structures and growth mechanisms of octahedral and decahedral Au@Ag core-shell nanocrystals prepared by a two-step reduction method

Octahedral and decahedral Au@Ag core-shell nanocrystals have been prepared in high yields using a two-step reduction method. In the first step, octahedral or decahedral Au core seeds were prepared by reducing HAuCl4·4H2O in tetraethylene glycol (TEG) under microwave (MW) heating or in diethylene glycol (DEG) under oil-bath heating, respectively, in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant. In the second step, Ag shells were overgrown on these Au seeds in N,N-dimethylformamide (DMF) in the presence of PVP under oil-bath heating for three hours or MW heating for ten minutes. Crystal structures of products were characterized using TEM, TEM–EDS, and SEM. Under oil-bath heating, octahedral or decahedral Au@Ag nanocrystals covered by uniform Ag shells were prepared. On the other hand, decahedral Au@Ag core-shell particles are formed through stepwise growth of tetrahedral units after covering thin Ag shells over decahedral Au cores under fast MW heating. Since the growth rate of Ag shells on corners having defects is slower than that on flat {111} facets, non-uniform Ag shells were formed under fast MW heating. Optical properties of each nanocrystal were determined by measuring extinction spectra.

Synthesis of Au@Ag@Cu trimetallic nanocrystals using three-step reduction

Au@Ag@Cu trimetallic nanocrystals were prepared using a three-step reduction method. In the first step, decahedral Au core seeds were prepared by reducing HAuCl4·4H2O in diethylene glycol (DEG) under oil-bath heating in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant. In the second step, Ag shells were overgrown on these Au seeds in N,N-dimethylformamide (DMF) in the presence of PVP under oil-bath heating to prepare decahedral Au@Ag nanocrystals. In the third step, Cu shells were overgrown further on Au@Ag core–shell nanocrystals in ethylene glycol (EG) in the presence of PVP under oil-bath heating. The resultant crystal shapes were characterized using transmission electron microscopic (TEM), TEM-energy dispersed X-ray spectroscopic (EDS), and X-ray diffraction (XRD) measurements. Results show that Cu shells of two kinds are grown over Au@Ag core seeds: a phase-separated major Cu component attached to one or two side edges of decahedral Au@Ag cores, and a minor Cu component that appears as thin Cu shells over decahedral Au@Ag cores. Partial reservation of pentagonal shape and appearance of Moire patterns in Au@Ag@Cu particles suggest that epitaxial growth occurs on some parts of the Au@Ag cores despite a large lattice mismatch between Ag and Cu (11.5%). The growth mechanism of Au@Ag@Cu nanocrystals was discussed in terms of lattice mismatch, decahedral particle defects, and the favorable shape of metallic shells. Optical properties of Au@Ag@Cu nanocrystals were determined by measuring extinction spectra.

Formation of Au@Pd@Cu core–shell nanorods from Au@Pd nanorods through a new stepwise growth mode

Au@Pd@Cu core–shell nanorods (NRs) were prepared using Au@Pd NRs as seeds. The resultant crystal structures were characterized using transmission electron microscopic (TEM), TEM-energy dispersed X-ray spectroscopic (EDS), and X-ray diffraction (XRD) measurements. Au@Pd seeds were prepared by reducing H2PdCl4 with cetyl trimethyl ammonium bromide (CTAB) and ascorbic acid in an aqueous solution. Dumbbell-type Au@Pd particles were formed at low Pd/Au molar ratios of 0.5–2.5, whereas rectangular Au@Pd NRs with {100} facets were produced at high Pd/Au molar ratios of 5 and 10. When Cu shells were further grown over rectangular Au@Pd NRs as seeds, Au@Pd@Cu nanocrystals with {100} facets were grown epitaxially through a new single island-growth mechanism designated as the Tsuji–Ikedo mechanism. In this mechanism, crystal growth of Cu shells over Au@Pd cores starts from the formation of single semispherical nuclei on a wide side facet followed by growth to one rectangular rod shell, further growth of neighboring rectangular rod shells, and eventual full covering by large rectangular Cu shells with {100} facets. In many cases, the growth rates of Cu shells over respective surfaces of Au@Pd NRs differ, so that Au@Pd@Cu particles with different thickness of Cu shells are prepared. Similar Au@Pd@Cu NRs with Au@Pd cores deviated from the center were also grown using dumbbell-type Au@Pd NRs, which indicates that flat Pd surfaces are unnecessary for the formation of rectangular Cu shells over Au@Pd NRs. The optical properties of Au@Pd and Au@Pd@Cu NRs were examined by observing ultraviolet (UV)-visible (Vis)-near infrared (NIR) extinction spectra.

Syntheses of Au–Cu-rich AuAg(AgCl)Cu alloy and Ag–Cu-rich AuAgCu@Cu core–shell and AuAgCu alloy nanoparticles using a polyol method

Core–shell and alloy types of nanoparticles including Au, Ag, and Cu components were prepared by reducing mixtures of HAuCl4·4H2O, AgNO3, and Cu(OAc)2·H2O in ethylene glycol (EG) in the presence of poly(vinylpyrrolidone) (PVP) at 175 °C. At a HAuCl4·4H2O : AgNO3 : Cu(OAc)2·H2O molar ratio of 1 : 2 : 1, mixtures of Au–Cu-rich AuAg(AgCl)Cu alloy nanoparticles and AgCl precipitates were formed after 2.5–35 min heating. On the other hand, at a HAuCl4·4H2O : AgNO3 : Cu(OAc)2·H2O molar ratio of 0.0065 : 2 : 1, at which the formation of AgCl precipitate was suppressed, Ag–Cu-rich AuAgCu alloy particles were prepared via AuAgCu@Cu core–shell particles after 2.5–34 min heating. The growth mechanisms of AuAg(AgCl)Cu, AuAgCu@Cu, and AuAgCu particles were examined using TEM-energy dispersed X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES), and ultraviolet (UV)-visible (Vis)-near infrared (NIR) extinction spectral data. The time dependence of UV-Vis-NIR spectral data indicated that the Cu components of AuAg(AgCl)Cu and AuAgCu alloy particles retained good anti-oxidation properties about 1 month after preparation.

Shape changes in Au–Ag bimetallic systems involving polygonal Au nanocrystals to spherical Au/Ag alloy and excentered Au core Ag/Au alloy shell particles under oil-bath heating

Shape changes in Au–Ag bimetallic nanoparticles involving polygonal Au particles were examined during oil-bath heating in ethylene glycol (EG). Polygonal Au and spherical Ag particles were annealed at 150 °C for 10–60 min with polyvinylpyrrolidone (PVP), yielding spherical Au-rich Au/Ag alloy particles, denoted as Au/Ag, with an average diameter of 207 ± 35 nm, and string-like Au/Ag alloy aggregates. The yield of spherical Au/Ag alloy particles increased with annealing time. Formation of Au/Ag alloy without the presence of etchants indicated that shape and size changes occur through fusion of Au and Ag particles during annealing. This method can be used as a simple technique for preparation of monodispersed Au/Ag alloy particles using mixtures of Au and Ag particles. Addition of polygonal Au seeds to AgNO3/EG/PVP solution preheated at 150 °C caused remarkable shape changes in products. After heating for 30 min, Au and Au@Ag core–shell particles attached to Ag-rich Ag/Au alloy nanowires were produced. They were transformed to excentered Au core Ag-rich Ag/Au alloy shell nanoparticles, denoted as Au@Ag/Au, with an average diameter of 370 ± 73 nm after further heating for 30 min. Results show that a shape change from Ag/Au alloy nanowires to spherical Ag/Au alloy shells occurs from attached parts of Au and Au@Ag particles to nanowires. The effects of HAuCl4 concentration used for the preparation of Au seeds and solvent temperature on the shape, sizes or composition distributions were examined. This report is the first describing new simple technique for preparation of excentered Au@Ag/Au particles during oil-bath heating.

Effects of Au fraction on the morphology and stability of Au–Ag–Cu trimetallic particles prepared using a polyol method

AuAgCu alloy and phase-separated AuAgCu/Cu nanoparticles were prepared by reducing mixtures of Au(OAc)3, AgNO3, and Cu(OAc)2·H2O in ethylene glycol (EG) in the presence of poly(vinylpyrrolidone) (PVP) at 175 °C. At Au(OAc)3:AgNO3:Cu(OAc)2·H2O molar ratios of 1:1:1, short string and peanut types of AuAgCu alloy nanoparticles were formed after 2.5–40 min heating. For Au(OAc)3:AgNO3:Cu(OAc)2·H2O molar ratios of 0.5:1:1 and 0.1:1:1, peanut-type or spherical AuAgCu alloy core Cu shell nanoparticles denoted as AuAgCu@Cu were produced after 5 min heating. They changed to a mixture of AuAgCu alloy and phase-separated AuAgCu/Cu nanoparticles after 17.5–37 min heating. The growth mechanisms of AuAgCu, AuAgCu@Cu, and AuAgCu/Cu particles were examined using TEM energy dispersed X-ray spectroscopic (EDS), XRD, and ultraviolet (UV)-visible (Vis)-near infrared (NIR) extinction spectral data. The time dependence of UV-Vis-NIR spectral data indicated that AuAgCu and AuAgCu/Cu nanoparticles retain high stability including anti-oxidation properties for a long time after preparation.