Abstract: Nonaqueous solution routes to metal oxide nanoparticles are a valuable alternative to the well-known aqueous sol-gel processes, offering advantages such as high crystallinity at low temperatures, robust synthesis parameters and avoidance of surfactants in order to control the crystal growth. In the first part of this paper we give an over-view of the various solution routes to metal oxides in organic solvents, with a strong focus on surfactant-free processes developed in our group. In most of these synthesis approaches, the organic solvent plays the role of the reactant that provides the oxygen for the metal oxide, controls the crystal growth, influences particle shape and, in come cases, also determines the assembly behaviour. In general, these routes involve the reaction of metal oxide precursors such as metal halides, alkoxides, or acetylacetonates with benzyl alcohol, benzylamine or carbonyl compounds like ketones and aldehydes. Whereas the reaction between metal halides,and benzyl alcohol enables the direct synthesis of crystalline nanoparticles via simple beaker chemistry, the other reaction systems require a solvothermal treatment at temperatures between 200 degrees C and 250 degrees C. The metal halide-benzyl alcohol system additionally allows for an insitu functionalisation process, where the surface of the nanoparticles can be modified during nanoparticle synthesis in order to tailor the solubility as well as the assembly behaviour. This is an important step towards the use of metal oxides as nanobuilding blocks for the fabrication of structures like nanowires or mesoporous materials. In the second part, various reaction pathways to nanoparticle formation are discussed. Solvothermal processes are not easy to monitor insitu. In order to elucidate possible formation mechanisms, we analyse the reaction solution obtained after synthesis of the nanoparticles as well as after reference experiments with altered reaction conditions. The organic species found in the mixtures allow us to propose possible formation mechanisms. As an important example, we will discuss the formation mechanism of ceria nanoparticles synthesised from cerium(III) isopropoxide and benzyl alcohol, involving a C-C bond formation between the isopropoxy ligand and benzyl alcohol. This reaction pathway was also found to lead to the formation of BaTiO3 nanoparticles. The last part of this paper deals with possible applications of metal oxide nanoparticles, especially with regard to gas sensing devices.
Abstract: Hybrid organic-inorganic nanocomposites are functional materials offering various possibilities in terms of tailoring the chemical and physical properties. In fact, they are determined by both organic and inorganic phases as well by their interface. This Highlight article demonstrates that the '' benzyl alcohol route '' is not only suitable for the formation of highly crystalline nanoparticles but also for hybrid materials. It describes a new one-pot approach leading to ordered hybrid materials composed of metal oxide nanobuilding blocks with at least one dimension in the sub-nanometer regime. Rare-earth hybrid materials, synthesized following this route, show new and interesting optical properties which are strongly influenced by the peculiar organic-inorganic nanostructure.
Abstract: The study focuses on synthesis and characterization of transition-metal doped zinc oxide (Zn1-xMxO), which has been theoretically predicted to be ferromagnetic at room temperature. Although this system has been under experimental study for some time, the vast majority of research conducted on this material has been done on bulk crystals or thin films. There are very few reports on the fabrication of one-dimensional nanostructures of Zn1-xMxO, and all of these syntheses employ high-temperature, vapor-phase methods. While this approach has proven quite effective for the production of a multitude of nanoscale semiconductors, gas-phase synthesis have considerable limits on nanowire yield and reaction scalability. We present the synthesis and characterization of cobalt- or manganese-doped zinc oxide (Zn1-xCoxO and Zn1-xMnxO) nanowires grown from a solution phase synthesis. The article reports the structural and magnetic characterization of the nanowires, the effect of annealing on secondary phase precipitation and on magnetic properties. The dopant concentration was determined by EDX (Energy Dispersive X-ray) spectroscopy, the dopant distribution by EDX and EPR (Electron Paramagnetic Resonance).
Abstract: A general nonaqueous route has been applied for the preparation of rare-earth ordered nanocrystalline hybrid structures. In a simple one-pot reaction process, Y3+ and Gd3+ isopropoxides were reacted in an autoclave with 4-biphenyl methanol at 275 degrees C. This approach leads to crystalline rare-earth oxide thin layers (similar to 0.6 nm) regularly separated from each other by organic layers of biphenolate molecules. The optical properties of these hybrid materials doped with Eu3+ and Nd3+, which are optical active lanthanides ions emitting in the red and near infrared, respectively, are presented. They show significant advantages compared to standard phosphors, such as higher radiance and luminance values, the possibility to tune the emission chromaticity by varying the excitation wavelength, and a much larger excitation range (250-350 nm and 300-500 nm for the Eu3+ and Nd3+ doped nanohybrids, respectively) shifted toward the red.
Abstract: A general non-aqueous route was applied to the preparation of erbium oxide based ordered nanocrystalline hybrid structures. In a simple one-pot reaction process, EP3+ isopropoxide was dissolved in benzyl alcohol and reacted in an autoclave at 300 degrees C. This approach leads to crystalline sub-nanometer thick Er2O3 oxide layers (crystallized in the monoclinic structure) regularly separated from each other by organic layers of intercalated benzoate molecules. They display good thermal stability for temperature up to 450 degrees C and outstanding optical emission properties in the near infrared region interesting for optical fiber technology. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: A non-aqueous sol-gel process was implemented for the preparation of SnO2- and In2O3-nanostructured powders. A detailed transmission electron microscopy (TEM), high resolution TEM (HRTEM) and X-ray diffraction (XRD) characterization carried out on the prepared nanomaterials showed that they were composed of single crystallites only slightly aggregated with an average size of 2.2 nm for SnO2 and 20 nm for In2O3. A mixed oxide of composition Sn0.95In0.05Ox was also prepared showing a crystallite, size of 2.2 nm. The nanoparticles were directly used for thick film sensor deposition by drop coating and tested as gas sensors in the monitoring of trace level of both oxidizing and reducing gases. In particular, the In2O3-based sensor showed enhanced gas sensing properties toward the oxidizing gas NO2, while for the mixed metal oxide Sn0.95In0.05Ox very interesting gas sensing properties were found towards the reducing gases CO and CH4. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: Nonaqueous solution routes to metal oxide nanoparticles are a valuable alternative to the well-known aqueous sol-gel processes, offering advantages such as high crystallinity at low temperatures, robust synthesis parameters and ability to control the crystal growth without the use of surfactants. In the first part of the review, we give an overview of the various nonaqueous routes to metal oxides, their surface functionalization and their assembly into well-defined nanostructures. However, we will strongly focus on surfactant-free processes developed in our group. Within the various reaction systems such as metal halides-benzyl alcohol, metal alkoxides-benzyl alcohol, metal alkoxides-ketones, metal acetylacetonates-benzyl alcohol and metal acetylacetonates-benzylamine we will discuss representative examples in order to show the versatility of this approach. The careful characterization of the organic species in the final reaction mixtures provides information about possible condensation mechanisms. Depending on the system several reaction pathways have been postulated: (i) elimination of organic ethers as result of condensation between two metal alkoxide precursors; (ii) C-C bond formation between the alkoxy ligand of the metal alkoxide precursor and the solvent benzyl alcohol under formation of a metal hydroxyl species, which can undergo further condensation; (iii) ketimine and aldol-like condensation steps, which in the metal acetylacetonate systems are preceded by a solvolysis of the precursor, involving C-C bond cleavage. In the second part of the paper we will focus on the synthesis of indium oxide nanoparticles using different precursors and solvents. Indium oxide represents an instructive example how the oxide precursors and the solvents influence the particle morphology. These findings make it possible to tailor particle size and shape of a particular metal oxide by the appropriate choice of the reaction system.
Abstract: Ultrathin, highly crystalline, mesostructured metal oxide layers, as shown in the figure, have been prepared by sol-gel processing using a modified evaporation-induced self-assembly (EISA) approach. This approach can be exploited for the generation of periodic surface structures featuring a well-defined in-plane mesostructure and a high crystallinity at the same time.
Abstract: Ordered mesoporous and dense films of crystallographically oriented metal oxides (e.g. MoO3, see figure and also cover) are obtained by evaporation-induced self-assembly followed by heating. This surfactant-directed facile methodology works for oxides with anisotropic unit cells, based on the interaction with surfactants during nucleation (soft epitaxy).
Abstract: A general nonaqueous route has been applied for the preparation of lanthanide ordered nanocrystalline hybrid structures. In a simple one-pot reaction process, Ln(III) isopropoxides (Ln = Gd, Sm, Nd) were dissolved in benzyl alcohol and reacted in an autoclave between 250 and 300 degrees C. This approach leads to crystalline lanthanide oxide layers regularly separated from each other by organic layers of intercalated benzoate molecules. They display good thermal stability for temperature up to 400 degrees C. The gadolinium-based nanohybrids showed outstanding optical emission properties when doped with terbium( III) and europium(III).
Abstract: In this article a detailed study of the optical properties of lonthanide doped lamellar nanohybrids synthesized by the "benzyl alcohol route" is presented. The synthetic approach results in the formation of a highly ordered lamellar nanocomposite consisting of yttrium or gadolinium oxide crystalline layers with a confined thickness of about 0.6 nm, separated from each other by organic layers of intercalated benzoate molecules. When the inorganic layers are doped with optically-active lanthanide ions they show outstanding emission properties in the green (Tb3+), red (Eu3+) and near infrared (Nd3+). The local environment of the emitting ions and the energy transfer processes involving the phenyl ring of the benzoate complexes and the lanthanide ions ore presented, as well as radiance and lifetime measurements. The radiance values are comparable and in some cases even larger than those of standard phosphors, proving that these nanohybrids con compete, from an emission efficiency point of view, with commercial phosphors. Furthermore, in these nanohybrids it is possible by simply changing the excitation wavelength, to tune the emission colour chromaticity without loosing the radiance.
Abstract: Mesostructured thin films of CeO2, ZrO(2)and CeO2-ZrO2 mixed oxides with highly crystalline pore walls and ordered arrays of mesopores were obtained by a straightforward fabrication process employing evaporation-induced self-assembly (EISA) and a well-designed temperature treatment, taking advantage of a novel type of amphiphilic block copolymer as template. The mesostructure and crystallinity were studied in detail using small-angle and wide-angle X-ray scattering and electron microscopy. The mesostructured CeO2 films are crack-free, possess a final pore size of ca. 10 nm, and the mesopores are surrounded by an almost completely crystalline matrix of nanoparticles of ca. 5-7 nm in size, as revealed by high-resolution electron microscopy. Additionally, the mesoscopic order (bcc structure) shows high thermal stability. The crystallization of the walls is usually accompanied by stresses and strong uniaxial structural shrinkage, which can, however, be significantly diminished by making mixed CeO2-ZrO2 mesostructured systems. Here, the crystallites represent "solid solutions'' of both binary oxides and exhibit an even higher thermal stability, while the constituting nanocrystals are smaller compared to the pure CeO2.
Abstract: Here, we present a systematic study on the influence of the bioligand deferoxamine mesylate on the crystallization and assembly behavior of tungsten oxide in a soft-chemistry process. Without deferoxamine mesylate, this approach yields pseudo-single crystalline tungstite nanoplatelets consisting of a large number of crystallographically almost perfectly aligned primary crystallites. In the presence of a constant amount of deferoxamine, the particle morphology drastically changes with temperature, ranging from wormlike organic-inorganic hybrid nanostructures to single-crystalline tungsten oxide nanowires, highlighting the role of the bioligand in controlling the crystal growth and assembly behavior. The nanowires have a uniform diameter of about 1.3 nm, an aspect ratio of more than 500, and the structural flexibility of tungsten oxide. The presented process is based on the combination of biomimetic construction principles with nonaqueous sol-gel chemistry, thus combining the advantages of both tools, excellent control over particle morphology and high crystallinity at low temperature.
Abstract: A novel nonaqueous route has been applied for the preparation of nanocrystalline magnetite. In a simple one-pot reaction process, iron(III) acetylacetonate was dissolved in benzyl alcohol and treated in an autoclave between 175 and 200 degrees C. This approach leads to monocrystalline magnetite particles with sizes ranging from 12 to 25 nm, as evidenced by X-ray analysis, HRTEM, and Raman and Mossbauer spectroscopy. The isolated particles can be redispersed either in polar or nonpolar solvents by coating them just after synthesis with undecanoic acid or dopamine. Simple sedimentation after redispersion in hexane can be used to lower the polydispersity of the sample.
Abstract: A nonaqueous synthetic route for the preparation of a regular crystalline yttria mesostructure is presented. The reaction between yttrium alkoxides and benzyl alcohol results in the formation of a highly ordered lamellar nanocomposite consisting of yttria layers with a confined thickness of about 0.6 nm, separated from each other by organic layers of intercalated benzoate molecules. Doping with europium leads to strong red luminescence. The nanostructure formation proceeds via two reactions. A C-C bond formation occurs between benzyl alcohol and the isopropanolate ligand. At the same time, yttrium oxide catalyzes two low-temperature hydride-transfer reactions to form benzoic acid and toluene from benzyl alcohol via benzaldehyde, thus limiting the growth of the thickness of the lamellae.
Abstract: A widely applicable solvothermal route to nanocrystalline iron, indium, gallium, and zinc oxide based on the reaction between the corresponding metal acetylacetonate as metal oxide precursor and benzylamine as solvent and reactant is presented. Detailed XRD, TEM, and Raman studies prove that, with the exception of the iron oxide system, where a mixture of the two phases magnetite and maghemite is formed, only phase pure materials are obtained, gamma-Ga2O3, zincite ZnO, and cubic In2O3. The particle sizes lie in the range of 15-20 nm for the iron, 10-15 nm for the indium, 2.5-3.5 nm for gallium, and around 20 nm for zinc oxide. GC-MS analysis of the final reaction solution after removal of the nanoparticles showed that the composition is rather complex consisting of more than eight different organic compounds. Based on the fact that N-isopropylidenebenzylamine, 4-benzylamino-3-penten-2-one, and N-benzylacetamide were the main species found, we propose a detailed formation mechanism encompassing solvolysis of the acetylacetonate ligand, involving C-C bond cleavage, as well as ketimine and aldol-like condensation steps.
Abstract: We developed novel reaction approaches using non-aqueous and halide-free procedures to synthesize a wide variety of different metal oxide nanoparticles including the binary metal oxides of groups IV and V, SnO2, In2O3, FeOx, ZnO, Ga2O3, perovskites (BaTiO3, SrTiO3, (Ba,Sr)TiO3, BaZrO3) and related compounds (LiNbO3). These routes involve the solvothermal reaction of metal oxide precursors such as metal alkoxides or metal acetylacetonates either with benzyl alcohol, various ketones or benzylamine. The careful characterization of the organic species in the final reaction mixtures provides information about possible condensation mechanisms. In the case of HfO2, a simple ether elimination process between two alkoxide precursors leads to the formation of the Hf-O-Hf bond, whereas BaTiO3 formation occurs via a mechanism involving a C-C bond formation between the isopropoxy ligand and the solvent benzyl alcohol. Binary metal oxide nanoparticles using the reaction of metal acetylacetonates with benzylamine are generated from a mechanism encompassing solvolysis of the acetylacetonate ligand, involving C-C bond cleavage, as well as ketimine and aldol-like condensation steps. The use of non-reducing solvents instead of alcohols allows the preparation of lead-based metal oxides like Pb(Zr,Ti)O-3. SnO2 and In2O3, known to be sensitive to reducing and oxidizing gases, respectively, have been tested as possible gas sensing devices and they showed good sensitivity and selectivity. (C) 2005 Elsevier Ltd. All rights reserved.
Abstract: Crystalline hexagonal-shaped superstructures of calcium carbonate, synthesized in the presence of ammonia, are shown to be assembled by a three-dimensional oriented attachment of vaterite nanoparticles. This unusual crystallographic lock-in mechanism enables the formation of complicated rounded structures with a crystallographic orientation from nanosized building blocks, which has so far only been found for transition metal systems.
Abstract: Semiconducting In2O3 nanocrystals, synthesised via a nonaqueous sol-gel method and doped with 1 wt% of platinum, have shown to possess a unique high sensitivity to oxygen at room temperature (RT). Consequently, a Pt/In2O3-based oxygen sensor for room temperature operation has been developed showing higher performance compared to the state-of-the-art devices.
Abstract: Crystallization Of DL-alanine by cooling of a supersaturated solution in the presence of a chiral double-hydrophilic block copolymer poly(ethylene glycol)-block-poly(ethylene imine)-S-isobutyric acid (PEG(4700)-PEI1200-S-iBAc) yields crystal superstructures with an astonishing morphology. Although analysis by light microscopy reveals these crystals to be apparently well facetted, electron microscopy shows that they consist of three-dimensionally, well-aligned nanocrystals that are scaffolded to a so-called meso-crystal. This mesocrystal is formed by polymer-mediated structuration, and provides evidence for the importance of mesoscopic events in a typical crystallization process.
Abstract: We reveal that the aragonite CaCO3 platelets in nacre of Haliotis laevigata are covered with a continuous layer of disordered amorphous CaCO3 and that there is no protein interaction with this layer. This finding contradicts classical paradigms of biomineralization, e.g., an epitaxial match between the structural organic matrix and the formed mineral. This finding also highlights the role of physicochemical effects in morphogenesis, complementing the previously assumed total control by biomolecules and bioprocesses, with many implications in nanotechnology and materials science.
Abstract: Nanoparticle powders composed of surface-functionalized anatase crystals with diameters of about 3 nm self-organize into different structures upon redispersion in water. The assembly is directed by a small amount of a low-molecular-weight functional ligand (the "assembler") adsorbed on the surface of the nanoparticles. The ligand functionality determines the anisotropy of the resulting structures. Multidentate ligands, such as trizma ((HOCH2)(3)CNH2) and serinol ((HOCH2)(2)CNH2), with a chargeable terminal group preferentially induce the formation of anisotropic nanostructures several hundreds of nanometers in total length, whereas all the other investigated ligands (ethanolamine H2N(CH2)(2)OH, glycine hydroxamate H2NCH2CONHOH, dopamine (OH)(2)C6H3(CH2)(2)NH3Cl, tris (HOCH2)(3)-CCH3) mainly lead to uncontrolled agglomeration. Experimental data suggests that the anisotropic assembly is a consequence of the water-promoted desorption of the organic ligands from the 10011 faces of the crystalline building blocks together with the dissociative adsorption of water on these crystal faces. Both processes induce the preferred attachment of the titania nanoparticles along the [001] direction. The use of polydentate and charged ligands to functionalize the surface of nanoparticles thus provides a versatile tool to control their arrangement on the nanoscale.
Abstract: We studied unsupported VxOy nanoparticles prepared by a novel nonaqueous route in the selective oxidation of n-butane to maleic anhydride. The evolution of the electronic and geometric structure of the material was characterized by X-ray photoemission spectroscopy, electron energy-loss spectroscopy, transmission electron microscopy, and electron diffraction before and after the reaction at different temperatures. A change from a water-mediated C-C bond cracking functionality of the catalyst forming acetic acid to an oxidizing functionality resulting in maleic anhydride was observed. It was found that the particles underwent a radical modification of the geometric and electronic structure that finally resulted in V2O5 crystals. Experimentally derived conclusions are related to some conceptual claims from the literature. (c) 2005 Elsevier Inc. All rights reserved.
Abstract: Highly anisotropic TiO2 nanostructures (see Figure) of several hundreds of nanometers in length were obtained by oriented assembly of preformed TiO2 nanoparticles with typical diameters of 5 nm. The assembly is directed by a particularly small amount of a polydentate ligand with a low molecular weight, which binds selectively to specific crystal faces of the titania nanoparticles.
Abstract: Electronic structure calculations of gamma-V2O5 are presented and compared to the electronic structure of the more commonly known alpha-V2O5. The different VO5 building blocks in gamma-V2O5 are investigated in terms of the angular momentum projected density of states. Since the same structural unit is also found in the alpha-V2O5 phase, it is possible to compare the electronic structure with respect to differences in the geometry of the VO5 pyramids. Electron energy-loss spectroscopy is applied as a sensitive probe of the electronic structure and reveals differences in the fine structure of the oxygen K ionization edge which are confirmed by the simulation.
Abstract: A strategy is shown to fabricate highly organized mesoporous anatase films exhibiting favorable properties for photocatalysis and photovoltaic applications by the hydrolysis/condensation of TiCl4 in the presence of PHB-PEO block copolymer templates. Dipcoating for evaporation-induced-self-assembly followed by a straight thermal treatment was employed. The evaporation/structuration process and the thermal treatment were mechanistically followed by in situ GISAXSIWAXS measurements, and the final product was carefully analyzed by spectroscopic ellipsometry and transmission electron microscopy to reveal the consequences of crystallization onto the micro-, the meso-, and the macroscale.
Abstract: A novel nonaqueous route for the preparation of nanocrystalline BaTiO3, SrTiO3, and (Ba,Sr)TiO3 has been developed. In a simple one-pot reaction process, the elemental alkaline earth metals are directly dissolved in benzyl alcohol at slightly elevated temperatures. After the addition of Ti((OPr)-Pr-i)(4), the reaction mixture is heated to 200 degreesC, resulting in the formation of a white precipitate. XRD measurements prove the exclusive presence of the perovskite phase without any other crystalline byproducts such as BaCO3 or TiO2. TEM investigations reveal that the BaTiO3 nanoparticles are nearly spherical in shape with diameters ranging from 4 to 5 nm. The SrTiO3 particles display less uniform particle shapes, and the size varies between 5 and 10 nm. Lattice fringes observed in HRTEM measurements further prove the high crystallinity of the nanoparticles. Surprisingly, GC-MS analysis of the reaction solution after hydrothermal treatment shows that hardly any ether formation occurs during the BaTiO3 synthesis. Instead, the presence of 4-phenyl-2-butanol in stoichiometric amounts gives evidence that the formation mechanism proceeds mainly via a novel pathway involving C-C bond formation between benzyl alcohol and the isopropanolate ligand.
Abstract: Innovative strategies to produce well-defined nanoparticles and other nanostructures such as nanofibres, quantum wells and mesoporous materials have revitalized materials science(1,2) for the potential benefit to society. Here, we report a controlled process, involving soft-chemistry-based deposition, template-assisted mesostructured growth, and tuned annealing conditions that allows the preparation of ordered mesoporous crystalline networks and mesostructured nano-island single layers, composed of multicationic metal oxides having perovskite, tetragonal or ilmenite structures. This strategy to obtain meso-organized multi-metal-oxide nanocrystalline films (M3NF) bridges the gap between conventional mesoporous materials and the remarkable properties of crystalline ternary or quaternary metallic oxides. Nanocrystalline mesoporous films with controlled wall thickness (10-20 nm) of dielectric SrTiO3, photoactive MgTa2O6 or ferromagnetic semi-conducting CoxTi1-xO2-x were prepared by evaporation-induced self-assembly (EISA) using a specially designed non-ionic block-copolymer template. A tuned thermal treatment of the mesoporous films permits the transfer of the wall structure into nanocrystallites, with all tectonic units being tightly incorporated into mechanically stable ordered tri- or bidimensional nanocrystalline networks. This methodology should allow multifunctionalization, miniaturization and integration during development of devices such as smart sensors and actuators, better-performing photocatalysts, and fast electrochromic devices. On the other hand, organized arrays of dispersed ferromagnetic or ferroelectric nanoparticles are promising materials for spintronics and for cheap, non-volatile 'flash' memories.
Abstract: The reaction between transition metal alkoxides and benzyl alcohol provides a novel soft chemistry route to metal oxide nanoparticles. The method allows the preparation of nanocrystals of two important transition metal oxides, namely V2O3 and Nb2O5. Although the reaction temperatures of 200-220degreesC are comparably low, the obtained particles are highly crystalline. According to TEM investigations, the V2O3 crystals exhibit particle sizes between 20 and 50 nm, and the Nb2O5 crystals display platelet-like particle shapes with sizes of 50-80 nm, Without any indications of amorphous character. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: Ce1-xZrxO2 nanoparticle sols (x = 0-1) are synthesized by hydroxide coprecipitation of a mixed precursor solution of cerium ammonium nitrate and zirconyl chloride followed by redispersion in an aqueous medium by sonication using nitric acid as the peptizing agent. The obtained sols are highly concentrated and stable for weeks. Analytical ultracentrifugation measurements show a particularly narrow particle distribution with an average particle size of about 3.5 nm for pure CeO2 and 2.5 nm for pure ZrO2 nanoparticles. Wide-angle X-ray scattering (XRD) as well as high-resolution transmission electron microscopy give evidence that all of the as-synthesized nanoparticle sols with a ceria content larger than 20 mol % are well crystalline. The formation of a solid solution with an increasing amount of Zr was monitored by XRD and Raman spectroscopy.
Abstract: A soft-chemistry route to (impurity free) Ta2O5 and HfO2 nanopowders (see Figure) based upon a synthesis protocol that involves the reaction of metal alkoxides with benzyl alcohol is presented. The formation of HfO2 proceeds via an unprecedented ether-elimination process and extends the available non-aqueous reaction methodologies in addition to the well-known ester and alkyl halide eliminations.
Abstract: Divanadium pentoxide (gamma-V2O5) nanorods and nanowires have been synthesized by a reverse micelle technique. The length can be tuned easily by keeping the particles in micellar solution after the synthesis from 40 nm to 1 mum. The local structure of these nanocrystals has been studied with high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, and electron energy loss spectroscopy. The obtained results demonstrate that a combination of experimental and theoretical tools permits the accurate characterization of the local structure of nanomaterials.
Abstract: Experimental and calculated reflectivity spectra of silver nanocrystals self-assembled in compact hexagonal networks on various substrates [highly oriented pyrolytic graphite (HOPG), gold, silicon, and Al0.7Ga0.3As] are compared. The calculated spectra are deduced from a model based on a mean-field theory and taking into account the electromagnetic resonances for p (parallel) and s (perpendicular) polarizations. From experimental and calculated spectra, it is concluded that the major change in the responses is due to the refractive index of the substrate. Hence, the optical properties of coated silver nanocrystals organized in a hexagonal network do not depend on the substrate used. That is to say no specific interactions between nanocrystals and substrate take place. Collective optical properties due to the formation of a film made of 5-nm silver nanocrystals are observed. This is pointed out by the appearance of an additional resonance at a higher energy than that of the plasmon resonance of isolated nanocrystals.
Abstract: CdS nanostructures with high crystallinity are widely investigated for their semiconductor properties. This work focuses on the preparation and properties of equilateral-triangular nanocrystals made by colloidal self-assembly. The picture shows the results of high-resolution transmission electron microscopy imaging of a CdS nanostructure prepared by this method.
Abstract: Equilateral CdS nanocrystals with 10-nm sides are produced in a Cd(AOT)(2)/isooctane/water reverse micellar solution. The structures of individual nanocrystals are determined by TENT and HRTEM, and these data were compared with computer simulations and power spectra calculations. This shows that the particles are crystallized in a hexagonal (wurtzite) form. The thickness of these equilateral nanocrystals (around 5 nm) is estimated from optical measurements. These nanocrystals are not highly stable as the coated surfactant desorbs, and they coagulate, forming particles with a cubic structure (zinc blende).
