Well-defined Nanocrystals Research Articles

Controlled Synthesis of Thorium and Uranium Oxide Nanocrystals

Very little is known about the size and shape effects on the properties of actinide compounds. As a consequence, the controlled synthesis of well-defined actinide-based nanocrystals constitutes a fundamental step before studying their corresponding properties. In this paper, we report on the non-aqueous surfactant-assisted synthesis of thorium and uranium oxide nanocrystals. The final characteristics of thorium and uranium oxide nanocrystals can be easily tuned by controlling a few experimental parameters such as the nature of the actinide precursor and the composition of the organic system (e.g., the chemical nature of the surfactants and their relative concentrations). Additionally, the influence of these parameters on the outcome of the synthesis is highly dependent on the nature of the actinide element (thorium versus uranium). By using optimised experimental conditions, monodisperse isotropic uranium oxide nanocrystals with different sizes (4.5 and 10.7 nm) as well as branched nanocrystals (overall size ca. 5 nm), nanodots (ca. 4 nm) and nanorods (with ultra-small diameters of 1 nm) of thorium oxide were synthesised.

CO and O overlayers on Pd nanocrystals supported on TiO2(110)

We have prepared a model catalytic system by depositing Pd onto a TiO2(110) surface held at approximately 720 K. Scanning tunneling microscopy (STM) reveals well-defined Pd nanocrystals consisting of (111) top facets with {111} and {100} side facets. The Pd nanocrystals go down to about 10 nm in width and 1.3 nm in height. Top facets can be imaged with atomic resolution, indicating the absence of TiOx encapsulation. The model catalyst was probed by exposure to CO and O2. By varying the CO exposure, different CO overlayers were formed on the (111) top facets, with coverages ranging from 0.33 to 0.75 of a monolayer. Near edge X-ray absorption fine structure (NEXAFS) measurements at 300 K reveal that at around 0.5 ML coverage, CO is oriented with the molecular axis more or less normal to TiO2(110). Dosing small amounts of 02 separately on a Pd/ TiO2(110) surface led to an overlayer of p(2 x 2)-O formed on the (111) top facet of the Pd nanocrystals at 190 K.

Color-tunable and enhanced luminescence of well-defined sodium scandium fluoride nanocrystals

In this paper, well-defined and regular-shaped Na3ScF6 nanocrystals (NCs) have been synthesized in high boiling organic solvents 1-octadecene (ODE) and oleic acid (OA), via the thermal decomposition of rare-earth oleate precursors. It is found that highly uniform monoclinic Na3ScF6 NCs with narrow size distribution have been obtained, which can easily be dispersed in cyclohexane solvent to form transparent colloid solutions. Upon 980 laser diode (LD) excitation, the relative up-conversion (UC) emission intensities of different colors in Yb(3+)/Er(3+), Yb(3+)/Tm(3+) and Yb(3+)/Ho(3+) doped Na3ScF6 can be tuned by altering the Yb(3+) doping concentration, resulting in the tunable multicolor in a wide range. On the basis of the emission spectra and the plot of luminescence intensity to pump power, the UC mechanisms of the co-doped Na3ScF6 NCs were investigated in detail. Moreover, the UC emission intensities can be significantly improved by coating a layer of Na3ScF6:Yb(3+)/Ln(3+) shell on Na3ScF6:Yb(3+)/Ln(3+) cores with respect to that of pure Na3ScF6:Yb(3+)/Ln(3+) core NCs. Furthermore, transparent and UC luminescent NCs/polydimethylsiloxane (PDMS) composites with regular dimensions were also fabricated by an in situ polymerization route. Uniform NCs with a wide variation of luminescence colors will show potential applications in diverse fields.

ChemInform Abstract: Scalable Strategies for the Synthesis of Well‐Defined Copper Metal and Oxide Nanocrystals

AbstractReview: 76 refs.

Scalable strategies for the synthesis of well-defined copper metal and oxidenanocrystals

This tutorial review highlights the most promising methods for the preparation of well-defined copper metal and oxide nanocrystals. These methodologies could be applied to other metals. We present the main synthetic strategies and associated mechanisms to control monodispersity, size, morphology and structure of metal and oxide nanomaterials which can adopt spherical, polyhedral, cubic, rod, wire, plate shapes and possibly hollow structures. We also consider the scale-up of the production of these nanocrystals, which is crucial for a wide range of potential applications such as catalysis, photovoltaics, electronics, optics and electrocatalysis.

Hydrogen Incorporation and Storage in Well-Defined Nanocrystals of Anatase Titanium Dioxide

Hydrogen incorporation into well-defined nanocrystals of anatase titanium dioxide (TiO2) has been investigated by a combination of experimental studies and density functional theory (DFT) calculations. The hydrogenation of TiO2 nanocrystals was determined at 450 °C with an initial hydrogen pressure of 7.0 MP, and storage capacities of 1.0 wt % and 1.4 wt % were achieved for nanocrystals with predominant (001) and (101) surface terminations, respectively. X-ray diffraction and Raman spectroscopy measurements indicate that the TiO2 crystal structure is very well preserved during the hydrogenation. DFT calculations show that hydrogen occupies the interstitial sites between titanium–oxygen octahedra and the energy barrier for hydrogen incorporation through the anatase (101) surface is lower than that through (001).

Shape-Controlled Synthesis of Well-Defined Matlockite LnOCl (Ln: La, Ce, Gd, Dy) Nanocrystals by a Novel Non-Hydrolytic Approach

We report here a novel synthetic route for the preparation of well-defined and faceted nanocrystals of ternary rare earth oxychlorides based on the ligand exchange and condensation of rare earth halides and alkoxides in the presence of coordinating solvents. Nanocubes, faceted 2D nanosheets, and nanodisk morphologies are obtained as a result of preferential growth along specific crystallographic directions dictated by the choice of the rare earth ion and the capping ligand. The synthetic approach reported here represents a unique low-temperature route for the preparation of LnOCl in the PbFCl matlockite phase. The synthetic strategy can further be adapted to incorporate dopant ions. The potential applicability of these nanostructures as phosphors is illustrated by demonstrating the upconversion of near-infrared illumination to green and red emission by Er(3+):GdOCl nanocrystals.

Facile synthesis and properties of spherical assemblies of NaYF4 nanocrystals with consistent crystalline orientation

Synthesis of NaYF4 nanocrystals with well-defined shapes and/or architectures has been of great interest in recent years, owing to their unique optical properties and wide applications in biological fields. NaYF4 nanocrystals with conventional morphologies have been developed through a variety of chemical or physical methods; however, it still remains challenging to build complex architectures by assembly well-defined NaYF4 nanocrystals in a simple system. Here, we present a facile solution-based method for synthesizing assembled NaYF4 nanocrystals at large scale. These assemblies exhibit a spherical appearance within which NaYF4 nanocrystals are tightly attached and arranged along the same crystalline orientation. This interesting hierarchical structure not only offers a high surface area and easy modification surface, but also provides a new efficient host for doping various lanthanide ions to give strong down- or up-conversion emissions. It is anticipated that these unique assembled NaYF4 nanostructures will serve as biolabels in various biomedical applications.

Nanosensors: towards morphological control of gas sensing activity. SnO2, In2O3, ZnO and WO3case studies

Anisotropy is a basic property of single crystals. Dissimilar facets/surfaces have different geometric and electronic structure that results in dissimilar functional properties. Several case studies unambiguously demonstrated that the gas sensing activity of metal oxides is determined by the nature of surfaces exposed to ambient gas. Accordingly, a control over crystal morphology, i.e. over the angular relationships, size and shape of faces in a crystal, is required for the development of better sensors with increased selectivity and sensitivity in the chemical determination of gases. The first step toward this nanomorphological control of the gas sensing properties is the design and synthesis of well-defined nanocrystals which are uniform in size, shape and surface structure. These materials possess the planes of the symmetrical set {hkl} and must therefore behave identically in chemical reactions and adsorption processes. Because of these characteristics, the form-controlled nanocrystals are ideal candidates for fundamental studies of mechanisms of gas sensing which should involve (i) gas sensing measurements on specific surfaces, (ii) their atomistic/quantum chemical modelling and (ii) spectroscopic information obtained on same surfaces under operation conditions of sensors.

Responsive Plasmonic Assemblies of Amphiphilic Nanocrystals at Oil−Water Interfaces

We report a new class of amphiphilic nanocrystals with mixed polymer brush coatings, which can spontaneously assemble into two-dimensional arrays at oil-water interfaces. The plasmon coupling of gold nanoparticles and nanorods in the assembly can be reversibly modulated by conformational changes of the stimuli-responsive polymer brushes. Amphiphilic gold nanocrystals (nanoparticles and nanorods) with mixed polymer brushes were synthesized via sequential "grafting to" (ligand exchange) and "grafting from" (surface initiated atom transfer radical polymerization) reactions. The integration of "grafting to" and "grafting from" reactions offers great flexibility for the surface modification of the nanocrystal scaffolds, allowing the combination of polymers with distinctly different properties on well-defined nanocrystals. For nanocrystals with poly(ethylene glycol) and poly(methyl methacrylate) coatings, the collective plasmonic property of the assembly can be tailored by changing solvent quality. In the case of the amphiphilic nanocrystal with poly(ethylene glycol) and poly(2-(diethylamino)ethyl methacrylate), the pH-sensitivity of poly(2-(diethylamino)ethyl methacrylate) provides an additional means to reversibly tune the assembly by varying the pH. All of the components, including nanocrystals, materials for self-assembled monolayers (polymers and biomacromolecules) on nanocrystal surfaces, and monomers suitable for surface-initiated living radical polymerization, in this construct have a wealth of possibilities available, indicating the potential of our strategy for developing hybrid materials with integrated and collective functionalities.

Control of Gas Sensing Activity in Tailored Nanomaterials for Gas Sensors

For the fundamental studies of gas reception there is a need in materials that (i) have well-defined morphology with only type of surfaces exposed to ambient gas and (ii) preserve same morphology over different length scales 1. The size- and shape- controlled nanocrystals of sensing materials with controlled surface properties will not only allow for separation of so-called “smoothly scalable” size-dependent properties from qualitatively new effects appeared as particle size decreases, but also will provide control over gas sensing activity in nanosensors. The latter can be achieved either through management over reception as that in well-controlled {012} rh-In2O3 nanocrystals 2 or through the synergy of inorganic and organic functionalities as that in hybrid self-assembled organotin thin films 3. The first step toward such nanomorphological control is the design and synthesis of well-defined nanocrystals which are uniform in size, shape and structure; such materials are ideal candidates for fundamental studies of mechanisms of gas sensing.

Epitaxial overgrowth of platinum on palladium nanocrystals

This paper describes a systematic study on the epitaxial overgrowth of Pt on well-defined Pd nanocrystals with different shapes (and exposed facets), including regular octahedrons, truncated octahedrons, and cubes. Two different reducing agents, i.e., citric acid and L-ascorbic acid, were evaluated and compared for the reduction of K₂PtCl₄ in an aqueous solution in the presence of Pd nanocrystal seeds. When citric acid was used as a reducing agent, conformal overgrowth of octahedral Pt shells on regular and truncated octahedrons of Pd led to the formation of Pd-Pt core-shell octahedrons, while non-conformal overgrowth of Pt on cubic Pd seeds resulted in the formation of an incomplete octahedral Pt shell. On the contrary, localized overgrowth of Pt branches was observed when L-ascorbic acid was used as a reducing agent regardless of the facets expressed on the surface of Pd nanocrystal seeds. This work shows that both the binding affinity of a reducing agent to the Pt surface and the reduction kinetics for a Pt precursor play important roles in determining the mode of Pt overgrowth on Pd nanocrystal surface.

Volumetric flame synthesis of well-defined molybdenum oxide nanocrystals

Well-defined faceted inorganic Mo oxide nanocrystals are synthesized in the gasphase using a solid-fed-precursor flame synthesis method. The solid crystals haverectangular cross-section with characteristic size of 10–20 nm and with lengths rangingfrom 50 nm to a few hundred nanometres. A 1 mm diameter high purity Mo probeintroduced in the oxygen-rich part of the flame serves as the material source. Acombination of the strong temperature gradient and varying chemical speciesconcentrations within the flame volume provides the ideal conditions for the rapidand direct formation of these unique nanocrystals. Oxidation and evaporation ofMoO3 in the oxygen-rich zone are followed by reduction toMoO2 in the lower temperature, more fuel-rich zone. TheMoO3 vapours formed are pushed in the direction of the gas flow and transformed into maturewell-defined convex polyhedron nanocrystals bounded with six faces resembling rectangularparallelepipeds.

Sub-10 nm Platinum Nanocrystals with Size and Shape Control: Catalytic Study for Ethylene and Pyrrole Hydrogenation

Platinum nanocubes and nanopolyhedra with tunable size from 5 to 9 nm were synthesized by controlling the reducing rate of metal precursor ions in a one-pot polyol synthesis. A two-stage process is proposed for the simultaneous control of size and shape. In the first stage, the oxidation state of the metal ion precursors determined the nucleation rate and consequently the number of nuclei. The reaction temperature controlled the shape in the second stage by regulation of the growth kinetics. These well-defined nanocrystals were loaded into MCF-17 mesoporous silica for examination of catalytic properties. Pt loadings and dispersions of the supported catalysts were determined by elemental analysis (ICP-MS) and H(2) chemisorption isotherms, respectively. Ethylene hydrogenation rates over the Pt nanocrystals were independent of both size and shape and comparable to Pt single crystals. For pyrrole hydrogenation, the nanocubes enhanced ring-opening ability and thus showed a higher selectivity to n-butylamine as compared to nanopolyhedra.

Synthesis of Nickel Disilicide Nanocrystal Monolayers for Nonvolatile Memory Applications

AbstractNickel silicide nanocrystals (NCs) were formed by thermally annealing SiOxNy films either implanted with Ni or coated with an evaporated Ni film. It is observed that the NCs grow into well-defined single crystalline structures embedded in a SiOxNy matrix, and that their size can be directly controlled by adjusting the concentrations of either silicon or nickel in the SiOxNy layer. The formation of well-defined NC monolayers was also demonstrated by depositing an ultra-thin Ni layer between two SiOxNy layers. These structures are shown to exhibit characteristic capacitance-voltage hysteresis suitable for nonvolatile memory applications.

Pump-probe experiments at 1.54μm on silicon-rich silicon oxide waveguides

Optical pump-probe measurements were performed on slab waveguides containing excess silicon in the form of nanoclusters or nanocrystals and erbium. The measurements were performed by prism coupling a 1.54μm probe beam into a waveguide formed by silicon-rich oxide and monitoring its intensity and temporal response as the waveguide was optically pumped from above with a chopped 477nm excitation source. Induced absorption (losses) of the 1.54μm probe beam in erbium-doped and undoped silicon-rich silicon oxide waveguides was observed in all cases. For the samples containing only well-defined nanocrystals, a fast (∼60μs) induced absorption component associated with free carriers within the silicon nanocrystals is reported, while for samples containing defective nanocrystals or nanoclusters, a much slower (>10min) component is observed. The free carrier absorption is shown to be reduced by delaying the probe beam relative to the pump beam in cases where it dominates.

Nanomedical imaging: In vivo imaging with smart nanohybrid

Abstract The unique optical, magnetic, and electronic properties of nanocrystals enable them to be key probes and vectors in the next generation of biomedical applications. Semiconductor quantum dots have been demonstrated as excellent fluorescent probes. However, they suffer from the intrinsic limitations of optical imaging systems such as low penetration depth and large background fluorescence. Magnetic nanocrystals are now emerging in biomedical applications with new possibilities. We have demonstrated the production of high quality water-soluble iron oxide (WSIO) nanocrystals by thermal decomposition method. One of the current challenges now is how to develop well-defined magnetic nanocrystals with optimal nanoscale magnetism, high bio-stability to withstand harsh biological conditions, Conjugation of biologically functional molecules to nanocrystals produce nanohybrids that have multi-functionality such as detection, diagnosis, and therapeutics. However, further development of in vivo imaging applications of these molecules has been very limited with very few successful in vivo demonstrations. In addition to therapeutic antibody, the ability to incorporate genes or drugs into detectable site-targeted nanosystems may represent a whole new paradigm in therapeutics. The application for molecular and cellar imaging in nanoscience have not been fully developed. However, it has enormous potential for development and nanocrystals will bring improvement in bio-medical sciences.

Hierarchically Structured Monolithic Silicalite-1 Consisting of Crystallized Nanoparticles and Its Performance in the Beckmann Rearrangement of Cyclohexanone Oxime

In this study, we present a synthetic pathway for the fabrication of self-supporting zeolite monoliths consisting of crystallized nanoparticles. A resorcinol-formaldehyde-based organic aerogel is used as a template, and silicalite-1 is used as the zeolite example. The silicalite-1 monoliths obtained consist of individual well-defined zeolite nanocrystals with sizes of 30-40 nm. The monoliths exhibit a high mechanical stability and have hierarchical porosity, with micropores within the zeolite particles, a mesopore system formed by the packing of the nanoparticles, and a macropore system on the monolith level. Such monolithic zeolites show high selectivity typically above 80% to epsilon-caprolactam combined with a high rate of reaction of 0.46 g(caprolactame)/(g(catalyst).h) in the Beckmann rearrangement of cyclohexanone oxime.

Kinetic aspects associated with the lean reduction of no with C3H6 over well-structured pt nanocrystals

Well-defined platinum nanocrystals (» 52% cubic), with an average diameter of 12 nm, were prepared by colloid method and then supported on alumina. The effect of the exposed Pt surface on lean de-NOx reaction was investigated. The catalytic data free of transport effects were identified and then used to calculate the kinetic parameters of lean de-NOx reaction.

Controllable Growth of Well-Defined Regular Multiporphyrin Array Nanocrystals at the Water−Chloroform Interface

On the basis of the coordination geometry of metal ions, regular cubic, clubbed, and wirelike nanocrystals of Cd(2+)-/PtCl(6)(2-)-mediated, and Hg(2+)-/Ag(+)-/PtCl(4)(2-)-mediated multiporphyrin arrays have been grown at the water-chloroform interface. The nanocrystal growth process was monitored by the transmission electron microscopy (TEM), which revealed (1) an intrinsic rule for coordination polymers, that is, the geometries of metal ions (as connects for the coordination polymers) dominate the frameworks of the related polymeric nanocrystals, and (2) one kind of intuitive nanocrystal growth processes at the interfaces. Both electron diffraction and X-ray diffraction patterns indicated the formation of well-defined nanocrystals. It was found that single-/microcrystals were formed at first, and then they grew into polycrystals. The nanocrystal layer was transferred onto Si and quartz substrate surfaces by the Langmuir-Blodgett method, with its composition analyzed by X-ray photoelectron spectroscopy as well as the arrangement of porphyrin macrocycles in the nanocrystals by UV-vis absorption spectroscopy.