Search

Tuning the crystal phase of bimetallic nanocrystals offers an alternative avenue to improving their electrocatalytic performance. Herein, we present a facile and one-pot synthesis approach that is used to enhance the catalytic activity and stability toward oxygen reduction reaction (ORR) in alkaline media via control of the crystal structure of Pd-Bi nanocrystals. By merely altering the types of Pd precursors under the same conditions, the monoclinic structured Pd5Bi2 and conventional face-centered cubic (fcc) structured Pd3Bi nanocrystals with comparable size and morphology can be precisely synthesized, respectively. Interestingly, the carbon-supported monoclinic Pd5Bi2 nanocrystals exhibit superior ORR activity in alkaline media, delivering a mass activity (MA) as high as 2.05 A/mgPd. After 10,000 cycles of ORR durability test, the monoclinic structured Pd5Bi2/C nanocatalysts still remain a MA of 1.52 A/mgPd, which is 3.6 times, 16.9 times, and 21.7 times as high as those of the fcc Pd3Bi/C counterpart, commercial Pd/C, and Pt/C electrocatalysts, respectively. Moreover, structural characterizations of the monoclinic Pd5Bi2/C nanocrystals after the durability test demonstrate the excellent retention of the original size, morphology, composition, and crystal phase, greatly alleviating the leaching of the Bi component. This work provides new insight for the synthesis of multimetallic catalysts with a metastable phase and demonstrates phase-dependent catalytic performance.

Precise control over the size, shape, composition, structure, and crystal phase of random alloy and intermetallic nanocrystals has been intensively explored in technologically important applications in recent years. Different from the monometallic nanocrystals and other types of structural nanocrystals such as core-shell and heterostructured nanocrystals, well-defined multimetallic random alloy and intermetallic nanocrystals exhibit unique and intriguing physicochemical properties, serving as ideal models for benefiting the structure-to-property studies. As such, random alloy and intermetallic nanocrystals have attracted extensive attention and interest in scientific research and shown huge potential in various fields. In this review, we focus specifically on summarizing the synthetic principles and strategies developed to form random alloy and intermetallic nanocrystals with enhanced performance. Some representative examples are purposely selected for emphasizing basic concepts and mechanistic understanding. We then highlight the fascinating properties and widespread applications of random alloy and intermetallic nanocrystals in electrocatalysis, heterogeneous catalysis, optical and photocatalysis, as well as magnetism and conclude the review by addressing the prospects and current challenges for the controlled synthesis of random alloy and intermetallic nanocrystals.

We report a facile method to fabricate CuNi nano-octahedra and nanocubes using a colloidal synthesis approach. The CuNi nanocrystals terminated with exclusive crystallographic facets were controlled and achieved by a group of synergetic capping ligands in a hot solution system. Specifically, the growth of {111}-bounded CuNi nano-octahedra is derived by a thermodynamic control, whereas the generation of {100}-terminated CuNi nanocubes is steered by a kinetic capping of chloride. Using a reduction of 4-nitrophenol with sodium borohydride as a model reaction, CuNi nano-octahedra and nanocubes demonstrated a strong facet-dependence due to their different surface energies although both exhibited remarkable catalytic activity with the high rate constant over mass (k/m). A kinetic study indicated that this is a pseudo first-order reaction with an excess of sodium borohydride. CuNi nanocubes as the catalysts showed better catalytic performance (k/m = 385 s-1•g-1) than the CuNi nano-octahedra (k/m = 120 s-1•g-1), indicating that 4-nitrophenol and hydrogen were adsorbed on the 100 facets with their molecules parallel to the surface much easier than those on {111} facets.

Ceria (CeO2) possesses a distinctive redox property due to a reversible conversion to its nonstoichiometric oxide and has been considered as a promising catalyst in the oxidative coupling of methane. Since a heterogeneously catalytic process usually takes place only on the surface of catalysts, it is reasonably expected that the performance of a catalyst, such as CeO2, highly relies on its size- and shape-dependent surface structure. We report our recent progress in achieving exclusive crystal facet-terminated CeO2 nanocrystals using a shape-controlled synthesis protocol in a one-pot colloidal system. We modified a two-phase solvothermal approach to fabricate cubic and truncated octahedral CeO2 nanocrystals with a size-control. During the two-phase solvothermal process, we propose that the Ce-precursors transfer from the aqueous layer to the interface of the organic phase, promoted by the capping ligands (as known as phase-transfer catalysts), for the oxidation and nucleation, and subsequently form CeO2 nanocrystals in the organic layer. As different capping ligands favor binding on diverse crystal facets, tuning the composition of the capping ligand with a precise control could generate nanocrystals that are dominated by a single type of facets with a relatively narrow size distribution.

In this work, we demonstrate a size-controlled synthesis of CuNi octahedral nanocrystals (NCs) using a hot colloidal solution approach. Two different sizes of CuNi nano-octahedra are chosen and investigated. It is determined that the reagent concentration remarkably plays a key role in the formation of the size-defined CuNi octahedral NCs. In terms of the reduction of 4-nitrophenol (4-NP), it uncovers that the obtained CuNi octahedral NCs (in both sizes) exhibit higher catalytic activity than those of CuNi spherical NCs reported previously. It further indicates that the catalytic performance is strongly size-dependent due to their devise specific surface areas of the exposed crystallographic planes.

Resolving the structure–property relationships of two-dimensional (2D) organic–inorganic hybrid perovskites is essential for the development of photovoltaic and photoelectronic devices. Here, pressure (0–10 GPa) was applied to 2D hybrid perovskite flakes mechanically exfoliated from butylammonium lead halide single crystals, (C4H9NH3)2PbI4, from which we observed a series of changes of the strong excitonic emissions in the photoluminescence spectra. By correlating with in situ high-pressure X-ray diffraction results, we examine successfully the relationship between structural modifications in the inorganic PbI42– layer and their excitonic properties. During the transition between Pbca (1b) phase and Pbca (1a) phase at around 0.1 GPa, the decrease in ⟨Pb–I–Pb⟩ bond angle and increase in Pb–I bond length lead to an abrupt blue shift of the excitonic bandgap. The presence of the P21/a phase above 1.4 GPa increases the ⟨Pb–I–Pb⟩ bond angle and decreases the Pb–I bond length, leading to a deep red shift of the excitonic bandgap. The total band gap narrowing of ∼350 meV to 2.03 eV at 5.3 GPa before amorphization, facilitates (C4H9NH3)2PbI4 as a much better solar absorber. Moreover, phase transitions inevitably modify the carrier lifetime of (C4H9NH3)2PbI4, where an initial 150 ps at ambient phase is prolongated to 190 ps in the Pbca (1a) phase along with enhanced photoluminescence (PL), originating from pressure-induced strong radiative recombination of trapped excitons.The onset of P21/a phase shortens significantly the carrier lifetime to 53 ps along with a weak PL emission due to pressure-induced severe lattice distortion and amorphization. High-pressure study on (C4H9NH3)2PbI4 nm-thin flakes may provide insights into the mechanisms for synthetically designing novel 2D hybrid perovskite based photoelectronic devices and solar cells. Ministry of Education (MOE) Accepted version T.T.Y., J.X.Y., and Z.X.S, gratefully acknowledge the Ministry of Education (MOE) for the following grants: AcRF Tier 1 (Reference No: RG103/16); AcRF Tier 2 (MOE2015-T2-1- 148); AcRF Tier 3 (MOE2011-T3-1-005). J.X.Y. is supported by the National Natural Science Foundation of China (Grant No. 11704185) and the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20171021). T.C.S. receives funding from the Ministry of Education Academic Research Fund Tier 1 Grant RG173/16, Tier 2 Grants MOE2015-T2-2- 015 and MOE2016-T2-1-034, and from the Singapore (NRF) through the Singapore−Berkeley Research Initiative for Sustainable Energy (SinBeRISE) CREATE Program and the Competitive Research Program NRF-CRP14-2014-03. S.H.W. is supported by the National Key Basic Research Program of China (2016YFB0700700) and National Natural Science Foundation of China (51672023, 11634003, U1530401). S.J. and J.F. thank Dr. Zhongwu Wang and Dr. Ruipeng Li for their assistance and acknowledge the support from Custom Electronics Inc. and Binghamton University. CHESS was supported by the NSF award DMR-1332208.

Chemically controlling crystal structures in nanoscale is challenging, yet provides an effective way to improve catalytic performances. Pt-based nanoframes are a new class of nanomaterials that have great potential as high-performance catalysts. To date, these nanoframes are formed through acid etching in aqueous solutions, which demands long reaction time and often yields ill-defined surface structures. Herein we demonstrate a robust and unprecedented protocol for facile development of high-performance nanoframe catalysts using size and crystallographic facet-controlled PtNi4 tetrahexahedral nanocrystals prepared through a colloidal synthesis approach as precursors. This new protocol employs the Mond process to preferentially dealloy nickel component in the ⟨100⟩ direction through carbon monoxide etching of carbon-supported PtNi4 tetrahexahedral nanocrystals at an elevated temperature. The resultant Pt3Ni alloy tetrahexahedral nanoframes possess an open, stable, and high-indexed microstructure, containin...

The development of visible-light-responsive photocatalysts is a promising challenge in the management of environmental pollution. Silver–silver halide nano-photocatalysts have received intensive attention due to their excellent photocatalytic performance in recent years, where silver nanoparticles/nanoclusters demonstrate plasmonic enhanced light absorption efficiency and have been considered as an important component in various functional photocatalytic nanocomposite materials, serving for harvesting visible light. This review provides an overall survey on the state-of-the-art silver–silver halide-based photocatalysts, fundamental understanding of their plasmonically induced photo-reactions and their major environmental applications. We first discuss the basic concepts of localized surface plasmon resonance, and outline the general mechanism of silver–silver halide-based photocatalysis. We then discuss the latest progress in the design and fabrication of silver halide based photocatalysis using various strategies. Next, we highlight some selected examples to demonstrate the new applications of silver/silver halide nano-photocatalysts. Eventually, we provide an outlook of the present challenges and some perspectives of new directions in this interesting and emerging research area.

The synthesis of noble metal nanocrystals terminated with high-index facets has received increasing attention due to the remarkable improvement in their catalytic performance. Introducing a transition metal to noble metals (bimetallic nanocrystals) could result in a reduced cost and potentially improve properties. Keeping in mind both of these advantages, we have developed a new synthetic approach to fabricate size-controlled Pt3Fe concave nanocubes using a high-temperature organic solution system containing oleylamine and oleic acid. It further demonstrates that the particle size and concavity could be controlled by a number of parameters such as the ratio of oleylamine and oleic acid, the physicochemical properties of the metal carbonyl, the metal valence in the precursor, and the ratio of metal precursors. Catalytic tests show that the high-index-surface-terminated ≈12 nm Pt3Fe concave nanocubes exhibit superior performance in both the hydrogenation of styrene and reduction of 4-nitrophenol in comparison with their counterparts.

Summary In recent years, the syntheses of III–V quantum dots have attracted extensive interests because of their remarkable quantum-confinement-effects, other attractive properties such as large exciton radius and potential applications in a number of areas such as gas sensing and biological probing. Some III–V members are low toxic and suitable to be used in bio-system with their excellent fluorescent characteristics. Although a solution-phase preparation of III–V quantum dots has become possible, the synthesis approach is still in its infancy and the quality of III–V products is inferior to those of II–VI and IV–VI quantum dots due to some difficulties such as lack of suitable precursors, high covalent characteristic, and difficult control of their nucleation and growth. The size, size distribution and crystallinity are strongly dependent on a specific reaction procedure and preparation technique. It is also believed that the selection of suitable precursors and designing corresponding key reactions should hold the keys for a successful production of the desired III–V quantum dots. This article focuses on the solution-phase synthesis of In- and Ga-based III–V quantum dots, and these developments are presented through a classification of their chemical routes including metathesis, thermolysis, dehalosilylation and transmetalation with a discussion on their preparation techniques such as one-pot heating-up and hot-injection. Furthermore, III–V quantum dots acted in gas sensing and bio-imaging applications are outlined as well. Eventually, an outlook of current challenges and promising direction for the development of III–V quantum dot synthesis is provided.

Self-organization of colloidal Pt nanocubes into two types of distinct ordered superlattices, simple-cubic and body-centered-tetragonal structures, has been achieved using a home-built setup. Detailed translational and orientational characteristics of these superstructures were determined using a transmission electron microscopy tomographic technique with 3D reconstruction analysis. The formation of these distinct superlattices is the result of a delicate choice of solvent (i.e., aliphatic hexane or aromatic toluene hydrocarbons), which serves as a dispersion medium to fine-tune the relative strengths of ligand-ligand and ligand-solvent interactions during the self-assembly process. This work provides important insights into the effects of ligand-solvent interactions on superlattice formation from nonspherical nanoparticles.

We report the pressure-induced crystallographic transitions and optical behavior of MAPbI3 (MA=methylammonium) using in situ synchrotron X-ray diffraction and laser-excited photoluminescence spectroscopy, supported by density functional theory (DFT) calculations using the hybrid functional B3PW91 with spin-orbit coupling. The tetragonal polymorph determined at ambient pressure transforms to a ReO3-type cubic phase at 0.3 GPa. Upon continuous compression to 2.7 GPa this cubic polymorph converts into a putative orthorhombic structure. Beyond 4.7 GPa it separates into crystalline and amorphous fractions. During decompression, this phase-mixed material undergoes distinct restoration pathways depending on the peak pressure. In situ pressure photoluminescence investigation suggests a reduction in band gap with increasing pressure up to ≈0.3 GPa and then an increase in band gap up to a pressure of 2.7 GPa, in excellent agreement with our DFT calculation prediction. This work lays the foundation for understanding the pressure-dependent phase transition of MAPbI3 and potentially enriches the toolkit for engineering perovskite polymorphs with exceptional optical properties. NRF (Natl Research Foundation, S’pore) Accepted version

Single-phase Bi0.85La0.15FeO3 ceramics were synthesized under various magnetic fields (Ha=0 T, 3 T, 5 T). Substantially reduced leakage current and hence modified ferroelectric (FE) properties were obtained with magnetic field annealing (MA). The largest magnetization and lowest leakage current with large FE polarization (Pr∼33 μC/cm2) were found in the sample annealed with Ha=3 T. Great changes were also observed in the Raman spectra. All the observed features originate mainly from the different FE domain wall structures induced by MA. These results demonstrate that MA is an effective way to tune the multiferroic and magnetoelectric properties in BiFeO3-based materials.

We report a Kirkwood-Alder transition in a system of nonspherical Pt(3)Cu(2) nanoctahedra coated with oleic acid and oleylamine ligands. Using both transmission electron microscopy tomography with 3D reconstruction analysis and synchrotron-based in-situ grazing-incidence small-angle X-ray scattering (GISAXS) techniques, we specifically determined that these nanoctahedra can assemble into an open structure in which the nanoctahedra are arranged tip-to-tip to form a bcc superlattice with a low packing efficiency. Using in-situ and real-time GISAXS, we further observed a "nanoctahedron crystallization" as a soft Kirkwood-Alder transition, that is, the soft nanoactahedra crystallize at a critical concentration and possess continuous crystalline states during a period of solvent evaporation. Finally, we found a reversible change of the superlattice constant during the solvent annealing and evaporation/drying processes.

Graphite oxide has been synthesized from raw graphite particles and been treated with various mass amounts of oleylamine as intercalants to form intercalation compounds. X-ray diffraction patterns reveal that the inter-sheet distances strongly depend on the graphite oxide to oleylamine mass ratios. The equilibrium-like behavior implies diffusion-dominated oleylamine adsorption on graphite oxide in solution and excluded volume intercalations among oleylamine-adsorbed graphite oxide during restacking. The intercalation compounds are soluble in organic solvents, and their applications in the fabrication of transparent and conductive coatings have been demonstrated.

In(2)O(3)@SiO(2) core-shell nanoparticles were prepared using an organic solution synthesis approach and reverse-microemulsion technique. In order to explore the availability of various silica encapsulations, a partial phase diagram for this ternary system consisting of hexane/cyclohexane (1:29 wt), surfactant (polyoxyethylene(5)nonylphenyl ether, i.e., Igepal CO-520), and aqueous solution containing ammonium hydroxide was also established. It is realized that the shell-thickness can be tuned by several parameters such as the concentration of In(2)O(3) nanocrystal suspension and the dose of the Si-precursor, tetraethyl orthosilicate. It was observed that the deeper energy level emissions of In(2)O(3) were apparently enhanced when In(2)O(3) was confined by the silica-shell in such core-shell nanoparticles. However, this enhancement could be degraded by increasing the shell-thickness.

Using an in situ synchrotron X-ray diffraction technique, a pressure-induced phase transformation of PbTe nanocrystals with sizes of 13 and 5 nm up to ∼20 GPa was studied. Upon an increase of pressure, we observed that the 13 nm PbTe nanocrystals start a phase transformation from rocksalt structure to an intermediate orthorhombic structure and finally CsCl-type structure at 8 GPa, which is 2 GPa higher than that in bulk PbTe. In contrast, the 5 nm PbTe nanocrystals do not display the same type of transition with a further increased transition pressure as expected. Instead of orthorhombic or CsCl-type structure, the 5 nm PbTe nanocrystals turn to amorphous phase under a similar pressure (8 GPa). Upon a release of pressure, the 13 nm PbTe nanocrystals transform from high pressure CsCl-type structure directly to rocksalt structure, whereas the 5 nm PbTe nanocrystals remain their amorphous phase to ambient conditions. The structure stability of rocksalt-type PbTe shows a significant reversal of Hall-Petch effect. On the basis of such an observation with a critical size determination of ∼9 nm, PbTe nanocrystals appear as the first class of material that demonstrates a pressure-induced structural change from order to disorder. By sharing the insight of this reversed Hall-Petch effect with associated transition types, we tuned our experimental protocol and successfully synthesized a sample with "high-pressure metastable structure", amorphous phase at ambient pressure. This integrative study provides a feasible pathway to understand nucleation mechanism as a function of particle size and to explore novel materials with high-pressure metastable structure and unique properties under lab-accessible conditions.

This review focuses on recent developments in the self-assembly of lead chalcogenide nanocrystals into two- and three-dimensional superstructures. Self-assembly is categorized by the shapes of building blocks, including nanospheres, nanocubes, nano-octahedra, and nanostars. In the section on nanospheres, rapid assemblies of lead chalcogenide-based multicomponent nanocrystals with additional components, such as semiconductors, noble metals, and magnetic nanocrystals, are further highlighted. In situ self-assembly of lead chalcogenide nanocrystals into one-dimensional nanostructures at elevated temperatures is also covered. Each section of this paper highlights examples extracted from recent publications. Finally, relatively novel properties and applications arising from lead chalcogenide superlattices as typical examples are also discussed.

Improving the catalytic activity of the anode catalyst is an important task in direct methanol and formic acid fuel cell development. In the present work, catalytic activity of shape-controlled PtCu nanoparticles toward methanol and formic acid oxidation was investigated. The results show that the addition of Cu to Pt increases the catalytic activity of both reactions. In addition, the shape of PtCu nanoparticles plays an important role on improving the reactivity of both reactions. Cubic PtCu nanoparticles are more active for methanol oxidation while spheres are better for formic acid oxidation. The present study demonstrates controlling shape of Pt alloy catalysts is an effective way of improving catalytic activity. Likely mechanisms of the activity enhancement are briefly discussed.

A facile, reliable, and robust synthetic method to prepare high-quality Pt 3 Fe nanocubes is reported. Results of investigation on these Pt 3 Fe nanocubes in shape-/size-distribution, crystallinity, structural characteristics, compositions, electrochemical activity towards small organic molecule oxidation as well as capability of assembling into simple cubic supercrystals are presented, and a possible formation mechanism of the cubic nanocrystals at high temperature in oleylamine/oleic acid solution is also proposed. This study indicates that the catalyst performance may be improved by controlling the shape of particles.

Platinum-based alloys have been extensively shown to be effective catalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Most of these catalysts are nanoparticles without shape control. Recently, extended Pt(3)Ni(111) surfaces prepared in ultrahigh vacuum were demonstrated to possess enhanced ORR catalytic activity as compared to the state-of-the-art carbon supported Pt (Pt/C) nanoparticle catalysts. How and whether this promising surface can be transformed into practical nanoscale electrocatalysts used in PEMFCs remain a challenge. We report a new wet-chemical approach of preparing monodisperse Pt(3)Ni nanoctahedra and nanocubes terminated with {111} and {100} facets, respectively. We further show that the ORR activity on the Pt(3)Ni nanoctahedra is approximately 5-fold higher than that of nanocubes with a similar size. Comparison of ORR activity between carbon-supported Pt(3)Ni nanoctahedra and commercial Pt/C reveals that the Pt(3)Ni nanoctahedra are highly active electrocatalysts. This synthetic strategy may be extended to the preparation of other shape-controlled fuel cell electrocatalysts.

Monodisperse PtNi nanoparticles with various compositions were synthesized by a wet-chemical approach. After electrochemical dissolution of Ni on the particle surface, these particles exhibit a 20–30-fold enhancement in O2 reduction activity as compared to the commercial Pt/C catalysts. Keywords: Platinum–nickel alloys, Nanoparticles, O2 reduction, Electrocatalysts, Fuel cell

Shape-controlled catalysis: High-quality Pt-Cu nanocubes with an average size of about 8 nm (see picture, scale bar = 20 nm) were synthesized from a high-temperature organic solution system in the presence of various capping ligands. These cubic Pt-Cu nanocrystals terminated with {100} facets demonstrated a superior catalytic activity towards methanol oxidation compared to similar sized Pt-Cu and Pt nanospheres.

A facile, self-seeded, solution-liquid-solid growth of soluble InP and GaP nanowires with a very low amount of native point defects with respect to the carrier concentrations have been synthesized (see scheme) and characterized. They are potentially promising building blocks in optoelectronic applications.We demonstrate a facile method for self-seeded, solution-liquid-solid growth of soluble InP and GaP nanowires at a temperature of approximately 300 degrees C. Both types of nanowires are single crystals with very small diameters. The synthesized InP nanowires are almost defect-free, whereas the GaP nanowires have some microtwins. The effect of reaction temperatures and input ligand/III/V (III and V indicate elements of Group 13 and 15 respectively) ratios on wire formation is discussed, and two competitive chemical pathways involved in the nanowire formation are proposed. In addition, electrical properties of these III-V nanowires, generated from the solution-based approach, were investigated for the first time. The current-voltage (I-V) and room temperature resistance investigations indicate that both InP and GaP nanowires possess very low native point defects for carrier concentrations and they could be potentially promising building blocks in optoelectronic applications.

Precursor-reduction method is employed to synthesize nanometer-sized ZnTe. Zinc blende ZnTe nanocrystals growth in three different shapes under various conditions, that is, quasi-spheres, tetrahedrons, and nanorods, have been observed. It is believed that the crystal growth of ZnTe is the rate-controlling step when superhydride was employed at 250 °C, resulting in quasi-spherical ZnTe nanocrystals. Replacement of superhydride with oleylamine alters the precursor-reduction step as the rate-controlling step, giving tetrahedral ZnTe nanocrystals. At low temperature (150 °C) and in the presence of superhydride and oleylamine, kinetic growth and/or surfactant-template dominate the process, causing an anisotropic crystal growth into ZnTe nanorods. ZnTe nanocrystals are typically surface-active with a similar crystal structure of CdSe. The present study provides a clue to understand the zinc blende-type nanocrystal growth mechanism in high-temperature colloidal system, and may lighten certain strategies of futur...

Spherical PbSe@SiO2 nanoparticles have been successfully synthesized within reverse micelles via metal alkoxide hydrolysis and condensation within a microemulsion system. These core-shell nanoparticles were characterized by transmission electron microscopy (TEM), NIR absorption spectroscopy, energy-dispersive X-ray analysis, and TEM electron diffractions. It shows that the obtained core-shell structures have a spherical shape with narrow size distribution (average size approximately 35 nm) and smooth surfaces. The size of the particles and the thickness of the shells can be controlled by manipulating the relative rates of the hydrolysis and condensation reactions of tetraethoxysilane (TEOS) within the microemulsion.

Using photoinduced second-order non-linear optical methods, we have determined that monodisperse In2O3 nanocrystals of size ~11–24 nm incorporated into poly(methyl methacrylate) (PMMA) matrices possess significant second-order optical susceptibilities up to 17 pm V−1 at λ = 1.76 µm; this is almost one order higher than that of traditional nanolayers of In2O3. A substantial increase in the corresponding susceptibilities is dependent on a relatively high monodispersion of the nanocrystals. Calculations such as quantum chemical, molecular dynamics and band structure simulations of interfaces indicate that the principal role in the observed non-linear optical effects is played by interfaces on the borders separating the nanocrystals and the surrounding host polymer matrix. In order to clarify a role of the monodispersion and that of the surfaces, additional experiments were carried out using samples with a larger size-distribution of nanocrystals and highly polarized photopolymers of oligoethercrylates as the host matrix. A drastic decrease in the effective optical nonlinearity in these two cases further supports the crucial role of the nanointerfaces.

CeF3, CeF3:Tb3+, and CeF3:Tb3+/LaF3 (core/shell) nanoparticles were prepared by the polyol method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), UV−vis absorption spectra, photoluminescence (PL) spectra, and lifetimes. The results of XRD indicate that the obtained CeF3, CeF3:Tb3+, and CeF3:Tb3+/LaF3 (core/shell) nanoparticles crystallized well at 200 °C in diethylene glycol (DEG) with a hexagonal structure. The TEM images illustrate that the CeF3 and CeF3:Tb3+ nanoparticles are spherical with a mean diameter of 7 nm. The growth of the LaF3 shell around the CeF3:Tb3+ core nanoparticles resulted in an increase of the average size (11 nm) of the nanopaticles as well as in a broadening of their size distribution. These nanocrystals can be well-dispersed in ethanol to form clear colloidal solutions. The colloidal solutions of CeF3 and CeF3:Tb3+ show the characteristic emission of Ce3+ 5d−4f (320 nm) and Tb3+ 5D4−7FJ (J = 6−3, with 5D4−7F...

Yttrium oxide (Y2O3) microprisms with interesting trilobal cross section were synthesized through a large-scale and facile hydrothermal method followed by a subsequent heat treatment. The size of the Y2O3 trilobal prisms could be modulated from micro- to submicroscale with the increase of pH value. The as-formed products via the hydrothermal process, monoclinic yttrium oxide hydroxide nitrate, Y4O(OH)9(NO3), could transform to cubic Y2O3 with the same morphology after annealing at high temperatures. The formation mechanism for the Y4O(OH)9(NO3) trilobal prisms has been proposed. Under hydrothermal conditions, the synergistic effect of Y-type aggregates of primary Y4O(OH)9(NO3) nanorods and sequent preferential of different crystal planes resulted in the formation of trilobal prisms.

Polymer-coated cobalt nanocrystals were prepared in a polar solvent at high temperature through an organometallic thermal-decomposition route in the presence of poly (N-viny-2- pyrrolidone) as a protective polymer. The as-synthesized PVP-coated cobalt nanocrystals were 20 to 80 nm cubic/hexagonal shapes. HCP was further determined as a main phase in these samples. For comparison purposes, we have also prepared PVP-cobalt nanocrystals using the seed-mediated thermal decomposition method. It revealed that PVP plays a significant role in the synthesis of cubicstructured cobalt nanocrystals. It was also noted that the formation of Co nanocrystals and their magnetic properties were dependent not only on the PVP component but also in the synthetic route. The saturation magnetization (MS) determined from the seed-mediated sample is lower than that of the conventional sample. The coercivity of the former (610 Oe at 5 K), however, is double that of latter (300 Oe at 5 K), indicating that coercivity strongly depends on the crystallinity of the cobalt.

Aluminum–silicon alloys are an important class of commercial casting materials having wide applications in automotive and aerospace industries. Etching the Al–Si eutectic leads to selective dissolution of Al, resulting in novel morphology–macroporous Si spheres with a three-dimensional nano network. Up to 5% Al is dissolved in Si, leading to an expansion of the crystal lattice. The resulting porous Si is electrochemically active with lithium and thus can be used as a high capacity anode for lithium-ion batteries. The etching of Al–Si provides a simple and low-cost method of producing nano-structured Si materials.

A fine strontium ferrite powder with high coercivity was successfully prepared by forming hydroxide precursor particles in the continuous ethanol-based phase of a microemulsion consisting of iso-octane, NP9, and an ethanol solution containing Sr2+ and Fe3+ cations at a molar ratio of 1:12. The microemulsion-derived hydroxide precursor was calcined at various temperatures, ranging from 600° to 1100°C, to develop the hexagonal strontium ferrite phase. X-ray diffractometry and infrared characterizations revealed that the formation mechanisms of strontium ferrite in the microemulsion-derived precursor differed from those of the precursor derived by conventional coprecipitation. The microemulsion resulted in a strontium ferrite of finer particle size and better magnetic properties than those of the conventionally coprecipitated strontium ferrite. The microemulsion-derived strontium ferrite exhibited an intrinsic coercivity of 6195 Oe and a saturation magnetization of 58.28 emu/g when calcined at 900oC. The saturation magnetization increased further, to 69.75 emu/g, when the microemulsion-derived precursor was calcined at 1100oC.

We report a chemical approach for the in situ synthesis of conducting polymer-magnetic inorganic nanomaterials with the integration of a high temperature organometallic method and a new microemulsion system containing organic, surfactant and aqueous phases that were established to enable the formation of our host nanocomposite materials. Polypyrrole-based γ-Fe 2 O 3 nanocomposites were prepared through this colloidal system. These organic–inorganic nanocomposites were characterized using X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, electrical resistivity and magnetization measurements. It was observed that the blocking temperature increases with increasing the γ-Fe 2 O 3 nanocomponent in these samples. The resistivity and the temperature variation of resistivity have also been investigated.

The effect of inter-particle interaction in samples of Co nanoparticles dispersed in different volume fractions in a wax matrix was studied. The evolution of interaction intensity in the Co nanoparticle systems as a function of temperature was determined by measuring the integral generalized Delta-M curves. These experimental curves do not require AC demagnetized states of the sample, which is an important simplification of the experimental procedure. The results are analyzed with a generalized moving Preisach model.

Nanometer-sized maghemite iron oxide ({gamma}-Fe{sub 2}O{sub 3}) particles were produced first by synthesis of a precursor, {gamma}-FeO(OH), in a surfactant-less microemulsion and subsequent heat treatment of the {gamma}-FeO(OH). The precursors and {gamma}-Fe{sub 2}O{sub 3} powder were characterized using thermogravimetric analysis (TGA), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Susceptometer from Quantum Design (SQUID) measurements. TGA and XRD analysis indicated the formation of single cubic phase when the samples were heat-treated at 240 deg. C. TEM reveals that the {gamma}-Fe{sub 2}O{sub 3} particles are needle-shaped with an aspect ratio of {approx}20; typically 5-10 nm wide and over 150 nm long. It was found that these microemulsion-derived {gamma}-Fe{sub 2}O{sub 3} nanoneedles possess an intrinsic coercivity of 28 Oe at 300 K and 950 Oe at 2 K.

Nanosized La0.9Ca0.1MnO3 perovskite-type crystalline complex oxides have been prepared at the low calcination temperature of 300 °C. The preparation procedure was carried out by the two-step process of amorphous formation and calcination. The amorphous phase was obtained by the reaction of metal ions with tetrabutylammonium hydroxide at 245 °C, and then calcined at 300 °C (sample A) or 400 °C (sample B) to prepare the nanocrystalline materials. The magnetic measurement shows that spin-glass behavior exists at 45 K and the blocking temperature increases with an increase of calcination temperature. The result of the spin-glass temperature of 45 K demonstrates that the particle size of the two samples A and B is below 50 nm. The increase of blocking temperature from A to B indicates that the particle size of A is less than that of B. The measurement of the normalized resistivity versus temperature for samples A and B shows that they have the change of the normalized resistive value at 230 K.

Dynamic properties of Co nanoparticle systems were investigated by means of RF transverse susceptibility. To study the effect of interaction between particles, samples of Co nanoparticles dispersed in a wax matrix in different volume fractions were studied. The evolution of interaction intensity in the Co nanoparticle systems as a function of temperature was determined by measuring the Integral Generalized DeltaM curves.

Ultrafine lead titanate (PbTiO 3 ) powders in tetragonal form have been successfully prepared via two processing routes, namely, conventional coprecipitation (CPC) and microemulsion-refined coprecipitation (MCP). The formation process of lead titanate from the resulting precursors was monitored using techniques such as thermal analyses, FTIR spectroscopy, Raman scattering spectroscopy and X-ray diffraction for the phase identification. It was found that the microemulsion-refined processing route led to a lower formation temperature for lead titanate than that observed in the conventional coprecipitation route, and there is no detectable pyrochlore phase during the formation of PbTiO 3 in the former case. The two PbTiO 3 powders have also been comparatively studied in particle morphology and specific surface areas. It indicates that the microemulsion-refined coprecipitation is the technique that results in the formation of the finer powder of lead titanate than the conventional coprecipitation does in the present work.

Bismuth (Bi) is classic semi-metal with a low carrier density, small carrier effective masses, very long mean free path and highly anisotropic Fermi surface. In this work, we present a technique for the synthesis of bismuth nanometer-sized particles using an inverse microemulsion method. To prevent air-oxidation in the characterization stage, poly(vinylpyrrolidone) was used as protecting agent. Our characterizations reveal that single rhombohedral phase of bismuth can be obtained with ultrafine particle with ∼20 nm in size by following this synthetic route. The optical absorption spectrum of the aqueous nanoparticle colloids shows an absorption band at ∼268 nm.

Single-crystalline Bi nanowire arrays have been assembled into the nanochannels of anodic alumina membranes by electrodeposition. X-ray diffraction and transmission electron microscopy investigations revealed that the nanowires with diameter of either 50 nm or 20 nm are essentially single crystalline and highly oriented. The magnetotransport properties of the Bi nanowire arrays were measured, and positive magnetoresistance as high as 45% at 100 K was observed.} \fnm{3}{Author to whom correspondence should be addressed.