Organic reaction pathways in the nonaqueous synthesis of metal oxide nanoparticles

Nonaqueous-solution routes to metal oxide nanoparticles are a valuable alternative to the 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 a detailed overview of the various solution routes to metal oxides in organic solvents, with a strong focus on surfactant-free processes. 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 some cases, also determines the assembly behavior. We have a closer look at the following reaction systems in this order: 1) metal halides in alcohols, 2) metal alkoxides, acetates, and acetylacetonates in alcohols, 3) metal alkoxides in ketones, and 4) metal acetylacetonates in benzylamine. All these systems offer some peculiarities with respect to each other, providing many possibilities to control and tailor the particle size and shape, as well as the surface and assembly properties. In the second part we present general mechanistic principles for aqueous and nonaqueous sol-gel processes, followed by the discussion of reaction pathways relevant for nanoparticle formation in organic solvents. Depending on the system several mechanisms have been postulated: 1) alkyl halide elimination, 2) elimination of organic ethers, 3) ester elimination, 4) C--C bond formation between benzylic alcohols and alkoxides, 5) ketimine and aldol-like condensation reactions, 6) oxidation of metal nanoparticles, and 7) thermal decomposition methods.     

Nonaqueous synthesis of metal oxide nanoparticles: Short review and doped titanium dioxide as case study for the preparation of transition metal-doped oxide nanoparticles

The liquid-phase synthesis of metal oxide nanoparticles in organic solvents under exclusion of water is nowadays a well-established alternative to aqueous sol–gel chemistry. In this article, we highlight some of the advantages of these routes based on selected examples. The first part reviews some recent developments in the synthesis of ternary metal oxide nanoparticles by surfactant-free nonaqueous sol–gel routes, followed by the discussion of the morphology-controlled synthesis of lanthanum hydroxide nanoparticles, and the presentation of structural peculiarities of manganese oxide nanoparticles with an ordered Mn vacancy superstructure. These examples show that nonaqueous systems, on the one hand, allow the preparation of compositionally complex oxides, and, on the other hand, make use of the organic components (initially present or formed in situ) in the reaction mixture to tailor the morphology. Furthermore, obviously even the crystal structure can differ from the corresponding bulk material like in the case of MnO nanoparticles. In the second part of the paper we present original results regarding the synthesis of dilute magnetic semiconductor TiO₂ nanoparticles doped with cobalt and iron. The structural characterization as well as the magnetic properties with special attention to the doping efficiency is discussed.     

Simultaneous formation of ferrite nanocrystals and deposition of thin films via a microwave-assisted nonaqueous sol–gel process

Combination of the surfactant-free nonaqueous sol–gel approach with the microwave technique makes it possible to synthesize Fe₃O₄, CoFe₂O₄, MnFe₂O₄, and NiFe₂O₄ nanoparticles of about 5–6 nm and with high crystallinity and good morphological uniformity. The synthesis involves the reaction of metal acetates or acetylacetonates as precursors with benzyl alcohol at 170 °C under microwave irradiation of 12 min. Immersion of glass substrates in the reaction solution results in the deposition of homogeneous metal ferrite films whose thickness can be adjusted through the precursor concentration. If preformed nickel nanoparticles are used as a type of curved substrate, the ferrite nanoparticles coat the seeds and form core–shell structures. These results extend the microwave-assisted nonaqueous sol–gel approach beyond the simple synthesis of nanoparticles to the preparation of thin films on flat or curved substrates.     

Methods of production,structure, and physicochemical characteristics of phosphorylated carbon adsorbents

The paper presents a review of methods for the production of phosphorylated carbon adsorbents, their structure, and their physicochemical characteristics. It was shown that the phosphorylated carbons contain phosphoric acid residues (condensed phosphates) attached to the carbon matrix mainly by a C—O—P bond and, to a lesser degree, by a C—P bond. The presence of the condensed phosphates in the structure of the carbon determines the hydrophilicity and the acidic characteristics and also the ability to absorb cations. The acidic characteristics of the surface of the phosphorylated carbons gives rise to their high catalytic activity in acid catalysis. The presence of the phosphates/polyphosphates improves the electrochemical characteristics of carbon materials for use as electrodes in supercapacitors. Potential fields for the practical application of phosphorylated carbons are the purification of water from heavy metal ions, the acceleration of reactions of the acid–base type, and the accumulation and storage of energy in double-layer capacitors.     

Surfactant-free nonaqueous synthesis of metal oxide nanostructures

Surfactant-free nonaqueous (and/or nonhydrolytic) sol-gel routes constitute one of the most versatile and powerful synthesis methodologies for nanocrystalline metal oxides with high compositional homogeneity and purity. Although the synthesis protocols are particularly simple, involving only metal oxide precursors and common organic solvents, the obtained uniform nanocrystals exhibit an immense variety of sizes and shapes. The small number of reactants in these routes enables the study of the chemical mechanisms involved in metal oxide formation. Nonhydrolytic routes to inorganic nanomaterials that used surfactants as size- and shape-controlling agents have been discussed recently. This Minireview supplements this topic by discussing surfactant-free processes, which have become a valuable alternative to surfactant-assisted as well as to traditional aqueous sol-gel chemistry routes.     

Synthesis of anisotropic PbS nanoparticles using heterocyclic dithiocarbamate complexes

We report the synthesis of lead piperidine and lead tetrahydroquinoline dithiocarbamate (DTC) complexes and their use as single source precursors for the preparation of anisotropic PbS nanoparticles. The complexes were thermolysed in coordinating solvents such hexadecylamime (HDA), tri-n-octylphosphine oxide (TOPO), oleylamine (OA) and decylamine (DA) at various reaction temperatures. The variation of the reaction conditions and precursors produced PbS particles with shapes ranging from spheres to cubes and rods. The size of the particles is generally larger than those synthesized by conventional precursor routes. The electron microscopy and X-ray diffraction data confirm the particles to be very crystalline with the dominant cubic rock salt phase present in all samples.     

One-minute synthesis of crystalline binary and ternary metal oxide nanoparticles

Highly crystalline metal oxide nanoparticles such as CoO, ZnO, Fe(3)O(4), MnO, Mn(3)O(4), and BaTiO(3) were synthesized in just a few minutes by reacting metal alkoxides, acetates or acetylacetonates with benzyl alcohol under microwave heating.     

Controlling the duality of the mechanism in liquid-phase oxidation of benzyl alcohol catalysed by supported Au-Pd nanoparticles

In the solvent-free oxidation of benzyl alcohol to benzaldehyde using supported gold-palladium nanoparticles as catalysts, two pathways have been identified as the sources of the principal product, benzaldehyde. One is the direct catalytic oxidation of benzyl alcohol to benzaldehyde by O(2), whereas the second is the disproportionation of two molecules of benzyl alcohol to give equal amounts of benzaldehyde and toluene. Herein we report that by changing the metal oxide used to support the metal-nanoparticles catalyst from titania or niobium oxide to magnesium oxide or zinc oxide, it is possible to switch off the disproportionation reaction and thereby completely stop the toluene formation. It has been observed that the presence of O(2) increases the turnover number of this disproportionation reaction as compared to reactions in a helium atmosphere, implying that there are two catalytic pathways leading to toluene.     

A general nonaqueous route to binary metal oxide nanocrystals involving a C-C bond cleavage

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-Ga(2)O(3), zincite ZnO, and cubic In(2)O(3). 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.     

Morphology and photoluminescence study of titania nanoparticles

Titania nanoparticles are prepared by sol–gel chemistry with a poly(ethylene oxide) methyl ether methacrylate-block-poly(dimethylsiloxane)-block-poly(ethylene oxide) methyl ether methacrylate triblock copolymer acting as the templating agent. The sol–gel components—hydrochloric acid, titanium tetraisopropoxide, and triblock copolymer—are varied to investigate their effect on the resulting titania morphology. An increased titania precursor or polymer content yields smaller primary titania structures. Microbeam grazing incidence small-angle X-ray scattering measurements, which are analyzed with a unified fit model, reveal information about the titania structure sizes. These small structures could not be observed via the used microscopy techniques. The interplay among the sol–gel components via our triblock copolymer results in different sized titania nanoparticles with higher packing densities. Smaller sized titania particles, (∼13–20 nm in diameter) in the range of exciton diffusion length, are formed by 2% by weight polymer and show good crystallinity with less surface defects and high oxygen vacancies.     

石墨烯负载的金属氧化物纳米颗粒的可控制备及催化性能

采用直接浸渍法、过氧化氢均相氧化沉积法和氨水催化水解法制备了石墨烯负载的铁、钴、镍金属氧化物纳米颗粒.研究了三种沉积方法对颗粒尺寸分布的影响;采用透射电子显微镜、傅里叶变换红外光谱、X射线衍射和X射线光电子能谱表征了催化剂的形貌与结构.用过氧化氢均相氧化沉淀法可制得粒径分布最均匀的纳米颗粒.过氧化氢的氧化作用可使石墨烯表面的氧化基团含量最大化,为纳米颗粒提供了足够的吸附与成核点.氨水加速了金属离子的水解与成核,导致纳米颗粒的粒径增大与不均.以苯甲醇氧化为探针反应考察了催化剂的性能.催化剂的活性按以下顺序逐渐下降:过氧化氢辅助沉积法>直接浸渍法>氨水催化水解法,与纳米颗粒尺寸增长趋势一致.纳米催化剂颗粒尺寸与其活性的良好关联性显示,发展石墨烯负载尺寸可控的纳米催化剂的方法具有重要意义.     

Precursor and Solvent Effects in the Nonhydrolytic Synthesis of Complex Oxide Nanoparticles for Bioimaging Applications by the Ether Elimination (Bradley) Reaction

Investigation of the solvent and alkoxide precursor effect on the nonhydrolytic sol–gel synthesis of oxide nanoparticles by means of an ether elimination (Bradley) reaction indicates that the best crystallinity of the resulting oxide particles is achieved on application of aprotic ketone solvents, such as acetophenone, and of smallest possible alkoxide groups. The size of the produced primary particles is always about 5 nm caused by intrinsic mechanisms of their formation. The produced particles, possessing the composition of natural highly insoluble minerals, are biocompatible. Optical characteristics of the perovskite complex oxide nanoparticles can easily be controlled through doping with rare earth cations; for example, by Eu³⁺. They can be targeted through surface modification by anchoring the directing biomolecules through a phosphate or phosphonate moiety. Testing of the distribution of Eu‐doped BaTiO₃ particles, modified with ethylphosphonic acid, demonstrates their facile uptake by the plants with active fluid transport, resulting finally in their enhanced concentration within the cell membranes.     

Benzyl alcohol and transition metal chlorides as a versatile reaction system for the nonaqueous and low-temperature synthesis of crystalline nano-objects with controlled dimensionality

The reaction of transition metal chlorides with benzyl alcohol leads at low temperatures to oxidic nanostructures with low-dimensional particle shapes such as nearly spherical titania anatase nanoparticles, vanadium oxide nanorods, and tungsten oxide nanoplatelets. The process is simple, allows a scale-up in gram quantities, and leads to highly crystalline materials. Ethanol solutions containing tungsten oxide nanoparticles exhibit blue luminescence upon UV irradiation at room temperature.     

Influence of addition of cobalt oxide on microstructure and electrochemical capacitive performance of nickel oxide

Ni–Co oxide nanocomposite was prepared by thermal decomposition of the precursor obtained via a new method—coordination homogeneous coprecipitation method. The synthesized sample was characterized physically by X-ray diffraction, scanning electron microcopy, energy dispersive spectrum, transmission electron microscope, and Brunauer–Emmett–Teller surface area measurement, respectively. Electrochemical characterization of Ni–Co oxide electrode was examined by cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance measurements in 6-mol L⁻¹ KOH aqueous solution electrolyte. The results indicated that the addition of cobalt oxide not only changed the morphology of NiO but also enhance its electrochemical capacitance value. A specific capacitance value of 306 F g⁻¹ of Ni–Co oxide nanocomposite with n _Co = 0.5 (n _Co is the mole fraction of Co with respect to the sum of Co and Ni) was measured at the current density of 0.2 A g⁻¹, nearly 1.5 times greater than that of pure NiO electrode. Lower resistance and better rate capability can also be observed.     

Swollen poly(dimethylsiloxane) (PDMS) as a template for inorganic morphologies

We report a series of silica, titania, and zirconia microstructures synthesized within swollen poly(dimethylsiloxane) (PDMS). Voids created by solvent-swelling the polymer are used to template the product. The inorganic morphologies range from spheres to networks, depending upon the nature of the polymer, its degree of swelling, and the synthetic conditions. Organic solvents as well as pure metal alkoxide liquids have been used to swell the polymer. Once the alkoxide precursor is inside the swollen polymer, water is introduced to bring about hydrolysis and condensation polymerization. The product is a textured metal oxide within a PDMS matrix. Scanning electron microscopy (SEM), optical microscopy, nuclear magnetic resonance (NMR), and powder X-ray diffraction (PXRD) were used to characterize the products. Microstructures formed in this manner have potential use as an inexpensive route to catalysts, fillers, capsules, or membranes for separations.     

Density functional study of the l-proline-catalyzed α-aminoxylation of aldehydes reaction: The reaction mechanism and selectivity

The reaction mechanism of the l-proline-catalyzed α-aminoxylation reaction between aldehyde and nitrosobenzene has been investigated using density functional theory (DFT) calculation. Our calculation results reveal following conclusions [1]. The first step that corresponds to the formation of C–O bond, is the stereocontrolling and rate-determining step [2]. Among four reaction channels, the syn-attack reaction channel is more favorable than that of the anti one, and the TS-ss channel dominates among the four channels for this reaction in the step of C–O bond formation [3]. The intermolecular hydrogen bond between the acidic hydrogen of l-proline and the N atom of the nitrosobenzene in an early stage of the process catalyzes very effectively the C–O bond formation by a large stabilization of the negative charge that is developing at the O atom along the electrophilic attack [4]. The effect of solvent decreases the activation energy, and also, the calculated energy barriers are decrease with the enhancement of dielectric constants for C–O bond formation step. These results are in good agreement with experiment, and allow us to explain the origin of the catalysis and stereoselectivity for l-proline-catalyzed α-aminoxylation of aldehyde reaction. The addition of H₂O to substituted imine proline, intermolecular proton-transfer steps, and the l-proline elimination process were also studied in this paper.     

Promoted aerobic oxidation of benzyl alcohol on CNT supported platinum by iron oxide

The catalytic activity of Pt/CNT for benzyl alcohol aerobic oxidation was remarkably improved by decorating iron oxide on Pt nanoparticles, and electrochemical measurements evidenced the enhanced activation of oxygen and benzyl alcohol at the FeO(x)/Pt interface.     

Corrosion and passivity of metals in methanol solutions of electrolytes

Electrochemical and corrosion behaviour of metals in alcohols are the subject of numerous investigations because of the application of mentioned solvents in chemical engineering, production of oxide nanoparticles (sol-gel techniques) and application of alcohols as fuels. Despite relatively rich bibliography related to electro-catalytic oxidation of alcohols on metal surface in mixed aqueous–alcohol solutions, the knowledge of the mechanism of reactions on metal/anhydrous alcohol interface is still not sufficient. Anodic oxidation of metal surface in alcohol leads to several electro-catalytic reactions with formation of surface compounds being the product of metal and alcohol oxidation. Identification of these products is very difficult. Therefore, our knowledge of the composition and structure of passive films or corrosion products on metal surface in anhydrous alcohol solvents is poor. Our paper presents the investigations of anodic behaviour of metals (Cu, Zn, Fe, Ni, Al and Ti) and semiconductors (p-Si) in methanol solutions of electrolytes, performed in our laboratory within the last 10 years. On the base of electrochemical measurements (linear sweep voltammetry, electrochemical impedance spectroscopy), spectroscopic investigations (X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and low-energy electron diffraction) and scanning electron microscopy techniques, the role of metal–alcohol intermediates in the formation of surface and soluble compounds is discussed. The practical application of electrochemical etching of metals as a method of production of micro- and nanoparticles of metals and oxides is also shown.     

Oxidation of Benzyl Alcohol Using Cobalt Oxide Supported Inside and Outside Hollow Carbon Spheres

Cobalt oxide nanoparticles (6 nm) supported both inside and outside of hollow carbon spheres (HCSs) were synthesized by using two different polymer templates. The oxidation of benzyl alcohol was used as a model reaction to evaluate the catalysts. PXRD studies indicated that the Co oxidation state varied for the different catalysts due to reduction of the Co by the carbon, and a metal oxidation step prior to the benzyl alcohol oxidation enhanced the catalytic activity. The metal loading influenced the catalytic efficiency, and the activity decreased with increasing metal loading, possibly due to pore filling effects. The catalysts showed similar activity and selectivity (to benzaldehyde) whether placed inside or outside the HCS (63 % selectivity at 50 % conversion). No poisoning was observed due to product build up in the HCS.     

A novel efficient deoxygenation process for N-heteroarene N-oxides

A novel deoxygenation process for N-heteroarene N-oxides is described. The deoxygenation process has been carried out by utilizing some short C-chain alcohols, benzyl alcohol, or 1-phenylethanol as the solvent in the presence of a base, such as sodium alkoxide or sodium hydroxide. A series of N-heteroarene N-oxides was submitted to the developed conditions to provide the corresponding N-heteroarenes with high yield and excellent selectivity. When the deoxygenation is carried out with benzyl alcohol or 1-phenylethanol as the reaction medium, the process can be performed under very mild conditions, at only 30 degrees C. The deoxygenation process is in contrast to several other methods performed without the presence of any transition metal as a catalyst or stoichiometric reagent. DFT calculations suggest that the alkoxide performs a nucleophilic attack on the N-heteroarene in the ortho or para position. This bond is cleaved homolytically with the overall result being that a single electron-transfer step has occurred. The products of this process are an N-heteroarene N-oxide radical anion and an alkoxyl or benzyloxy radical, depending on the solvent that has been used. Successive steps of the mechanism result in an oxygen transfer from the N-oxide to give the deoxygenated N-heteroarene and 1 equiv of the aldehyde, which is the oxidation product of the solvent alcohol.