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.     

Electrochemistry of Prussian Blue in silica sol-gel electrolytes doped with polyamidoamine dendrimers

The inclusion of a generation-4 polyamidoamine (G4-PAMAM) dendrimer in a silica sol-gel yielded a solid electrolyte that was used to encapsulate Prussian Blue (PB), iron(III) hexacyanoferrate(II), and cobalt hexacyanoferrate. The PB was synthesized in the doped silica by sequential immersion of a monolith in 0.1 M K₄Fe(CN)₆, water, and 0.1 M FeCl₃. Inclusion of G4-PAMAM resulted in a nanoporous anion-exchange material with a capacity of 10.1 mmol g^–1, which is about four times greater than the capacity of silica alone. Relative to its G0 counterpart, the G4-PAMAM doped silica increased the rate of formation of PB by a factor of ca. 20. The solid state voltammetry of PB in the doped silica had the usual features for this compound. At 0.1 V vs. a Ag quasi-reference electrode, a reversible reduction was seen; the relationship between current and scan rate was that for a surface-confined redox couple. The quasi-reversible oxidation of PB was observed at 0.85 V. Inclusion of G4-PAMAM increased the lifetime of silica as a solid electrolyte from a few days to at least three months. Raman microprobe mapping analysis demonstrated that PB was homogeneously distributed across the entire width (ca. 1 mm) of the G4-doped monolith with 20-h immersions. Electronic Publication     

Ionic liquids for the convenient synthesis of functional nanoparticles and other inorganic nanostructures

Ionic liquids are a new class of organic solvents with high polarity and a preorganized solvent structure. Very polar reactions can be carried out in these liquid in the absence of or with a controlled amount of water, and crystalline nanoparticles can be synthesized conveniently at ambient temperatures. The pronounced self-organization of the solvent is used in the synthesis of self-assembled, highly organized hybrid nanostructures with unparalleled quality. The extraordinary potential of ionic liquids in materials synthesis is described in this minireview and a physicochemical explanation is given.     

Highly effective sulfated zirconia nanocatalysts grown out of colloidal silica at high temperature

A large surface-to-volume ratio is a prerequisite for highly effective catalysts. Making catalysts in the form of nanoparticles provides a good way to achieve the aim. However, agglomeration of nanoparticles during the preparation and utilization of nanocatalysts remains a formidable problem. Here, we present a novel approach in which nano units of catalysts are formed in the matrix of a colloidal carrier, with assistance of a cross-linking agent, and then grow out of the carrier upon calcination at high temperature. This ensures that the catalysts not only do not agglomerate, but also have a low cost and high catalytic efficiency due to the large surface-to-volume ratio and the absence of carbon deposition. The technique is demonstrated by the successful preparation of a binary nanocatalyst that consists of a silica nanoparticle core and a sulfated zirconia (SZ) nanocrystal shell (JML-1). The synthesis was achieved by converting sulfated zirconia (SZ) and silica solutions into a composite gel by means of sol-gel processing in the presence of triethoxysilane as the cross-linking agent, followed by heating at 50 degrees C and calcining at 550 degrees C. Relative to other catalysts, such as pure SZ, non-nanodispersed SZ over silica (SZ/SiO2), and zeolites Y, Beta, and ZSM-5, JML-1 exhibits superior catalytic activity in many reactions. For example, the activity of JML-1 in the production of gasoline by alkylation of 1-butene with isobutene remained at 95% or higher after 20 h of reaction and was over 90% after being regenerated five times. In sharp contrast, SZ and SZ/SiO2 give a high activity only for 2 h and the initial activity of zeolites Beta and ZSM-5 are about 88 and 60%, respectively. These findings demonstrate that non-agglomerated nanoparticles anchored onto a carrier surface can be prepared and the technique provides a versatile route to new highly effective nanocatalyst systems.     

Sol-gel template synthesis and photoluminescence of n- and p-type semiconductor oxide nanowires.

A sol-gel template technique has been put forward to synthesize single-crystalline semiconductor oxide nanowires, such as n-type SnO2 and p-type NiO. Scanning electron microscopy and transmission electron microscopy observations show that the oxide nanowires are single-crystal with average diameters in the range of 100-300 nm and lengths of over 10 microm. Photoluminescence (PL) spectra show a PL emission peak at 401 nm for n-type semiconductor SnO2, and a PL emission at 407 nm for p-type semiconductor NiO nanowires, respectively. Correspondingly, the observed violet-light emission at room temperature is attributed to near-band-edge emission for SnO2 nanowires and the 3d(7)4s-->3d8 transition of Ni2+ for NiO nanowires.     

Adsorption of cesium using mesoporous silica gel evenly doped by Prussian blue nanoparticles

In this paper, a novel mesoporous silica gel evenly doped by Prussian blue nanoparticles (PBMSG) was successfully synthesized by using N,N-dimethylamide as template with a large Barrett-Emmett-Teller (BET) surface area of 505 m²/g and an average pore size of 2.9 nm. The static adsorption experiments showed that the equilibration time of PBMSG for Cs⁺ was about 30 min. The adsorption isotherm of PBMSG for Cs⁺ accorded with Langmuir model and the theoretical maximum adsorption capacity was 80.0 ± 2.9 mg/g. When the initial concentration of Cs⁺ was 1.00 mg/L, the adsorption partition coefficient K_d could reach 3.5 × 10⁴ mL/g After adsorption, Cs⁺ could be eluted by dilute hydrochloric acid (pH 2) with an efficiency of 89.8%, while no K⁺, Fe³⁺, Fe²⁺ was eluted. PBMSG exhibited good selectivity toward Cs⁺ and Rb⁺. In the presence of high concentration of K⁺, the selective adsorption of PBMSG could change the mass ratio of K⁺, Rb⁺ and Cs⁺ from 96.63:0.83:1.00–1.12:0.73:1.00. The separation of Cs⁺ and Rb⁺ from K⁺ with similar concentration (100 mg/g) was realized by column experiment. This indicated that PBMSG was suitable for rapid recovery of low concentration of rubidium and cesium from complex matrixes, such as wastewater and salt lake brine, etc.     

Coupling sol-gel synthesis and microwave-assisted techniques: a new route from amorphous to crystalline high-surface-area aluminium fluoride

A non-aqueous sol-gel Al-based fluoride has been subjected to the microwave solvothermal process. The final material depends on the temperature heat treatment used. Three types of material have been prepared: 1) for low temperature heat treatment (90 degrees C) X-ray amorphous alkoxy fluoride was obtained; 2) for the highest temperature used (200 degrees C) the metastable form beta-AlF3 was obtained with a very large surface area of 125 m2 g(-1). The mechanism of the amorphous=crystalline transformation has been rationalised by the occurrence of a decomposition reaction of the gel fluoride induced by the microwave irradiation. 3) Finally, at intermediate temperature (180 degrees C) a multi-component material mixture exhibiting a huge surface area of 525 m2 g(-1) has been obtained and further investigated after mild post-treatment fluorination using F2 gas. The resulting aluminium-based fluoride still possesses a high-surface-area of 330 m2 g(-1). HRTEM revealed that the solid is built from large particles (50 nm) identified as alpha-AlF3, and small ones (10 nm), relative to an unidentified phase. This new high-surface-area material exhibits strong Lewis acidity as revealed by pyridine adsorption and catalytic tests. By comparison with other materials, it has been shown that whatever the composition/structure of the Al-based fluoride materials, the number of strong Lewis acid sites is related to the surface area, highlighting the role of surface reconstruction occurring on a nanoscopic scale on the formation of the strongest Lewis acid sites.     

色氨酸在普鲁士蓝修饰玻碳电极上的电化学行为研究

采用电沉积法制备了普鲁士蓝修饰玻碳电极(PB/GCE),使用循环伏安法(CV)研究了色氨酸(Trp)在修饰电极上的电化学特性,并建立了一种电化学检测色氨酸的新方法。实验结果表明,在优化实验条件下,色氨酸在8.0×10-6～5.0×10-4mol·L-1浓度范围内与峰电流呈良好线性关系,线性回归方程为Ip(μA)=5.8954c(μmol·L-1)-32.91,相关系数r=0.9999(n=8),方法检出限(S/N=3)为3.5×10-7mol·L-1。将该修饰电极用于色氨酸样品的测定,结果满意。初步的反应机理探讨表明,色氨酸在普鲁士蓝电极上的反应可能是以两步进行的两电子氧化过程。     

A Cholesterol Biosensor Based on Entrapment of Cholesterol Oxidase in a Silicic Sol‐Gel Matrix at a Prussian Blue Modified Electrode

A cholesterol biosensors fabricated by immobilization of cholesterol oxidase (ChOx) in a layer of silicic sol‐gel matrix on the top of a Prussian Blue‐modified glassy carbon electrode was prepared. It is based on the detection of hydrogen peroxide produced by ChOx at ?0.05 V. The half‐lifetime of the biosensor is about 35 days. Cholesterol can be determined in the concentration range of 1×10^?6?8×10^?5 mol/L with a detection limit of 1.2×10^?7 mol/L. Normal interfering compounds, such as ascorbic acid and uric acid do not affect the determination. The high sensitivity and outstanding selectivity are attributed to the Prussian Blue film modified on the sensor.     

A simple method to fabricate a Prussian Blue nanoparticles/carbon nanotubes/poly(1,2-diaminobenzene) based glucose biosensor

A novel Prussian Blue nanoparticles/carbon nanotubes/poly(1,2-diaminobenzene) based glucose biosensor was fabricated by a simple and fast method. Firstly, the Prussian Blue (PB) nanoparticles were direct electrodeposited onto the surface of a glassy carbon electrode modified with multiwall carbon nanotubes (MWNTs) in a short time. Then an ultrathin conducting poly(1,2-diaminobenzene) film was electrodeposited onto the surface of PB/MWNTs nanocomposite to immobilize the glucose oxidase via glutaraldehyde cross-linking. The experiments results showed that the stability of the PB nanoparticles was greatly improved by the MWNTs and the PB/MWNTs nanocomposite exhibited an excellent synergistic electrocatalytic effect toward reduction of hydrogen peroxide. The biosensor showed excellent performances toward determination of glucose. The linear range of the glucose biosensor is from 10 μM to 2.5 mM, with a detection limit of about 5 μM. The response is within less than 5 sec. Correspondence: Wanzhi Wei, State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Hunan, Changsha 410082, P.R. China     

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.     

Role of acidity on the electrochemistry of Prussian Blue at plain and Nation filmcoated electrodes

Prussian Blue (PB) coated on plain platinum (Pt) shows a redox wave at 0.44V vs SCE in addition to the two usual redox waves at 0.82 and 0.12 V vs SCE when the electrodes were dipped in acidic KC1 solution. PB incorporated into Nation film-coated on Pt electrode exhibited the same behaviour even in the presence of neutral KG solution. In acidic KC1, the additional redox wave observed for PB incorporated into Nation film shifted positively to 0.39V vs SCE and the peak separation was reduced to 30mV. The observed additional redox wave for PB coated on plain Pt electrode and incorporated into Nafion film-coated Pt electrodes was assigned to the partial reduction of PB occurring due to the insertion of protons into the film. The effect of various cations on the electrochemistry of PB incorporated into Nafion film-coated electrode was also studied.     

Ultrahigh density arrays of toroidal ZnO nanostructures by one-step cooperative self-assembly processes: mechanism of structural evolution and hybridization with Au nanoparticles

A quick protocol for the fabrication of ultrahigh density arrays of toroidal ZnO nanostructures with tailored structures on a substrate surface is presented based on the one-step spin coating of a common solution composed of inverse micelles of polystyrene-block-poly(4-vinyl pyridine) copolymers (PS-b-P4VP) and sol-gel precursors without the need of conventional complex lithographic techniques. ZnO toroids decorated with gold nanoparticles are also obtained by subsequent loading and reduction of metallic precursors. It was elucidated that the diethanolamine moiety in the sol-gel precursors, which induces selective swelling and structural reorganization of the P4VP core blocks, plays a key role in the generation of toroidal nanostructures. Toroidal ZnO nanostructures embedded in a PS-b-P4VP matrix films or arrays of pure wurtzite ZnO nanorings are obtained by calcination under inert atmosphere. The structural parameters of the toroidal nanostructures such as the width, height, diameter of the rims as well as the spacing of their 2D arrays are controlled by employing PS-b-P4VP with different molecular weight and varying the mixing protocols.     

Characterization and catalytic-hydrogenation behavior of SiO2-embedded nanoscopic Pd, Au, and Pd-Au alloy colloids

Colloids embedded in a silica sol-gel matrix were prepared by using fully alloyed Pd-Au colloids, and pure Pd and Au colloids stabilized with tetraalkylammonium bromide following a modified sol-gel procedure with tetrahydrofuran (THF) as the solvent. Tetraethoxysilicate (TEOS) was used as the precursor for the silica support. The molar composition of the sol was TEOS/THF/H2O/HCl = 1:3.5:4:0.05 for the bimetallic Pd-Au and TEOS/THF/H2O/HCl = 1:4.5:4:0.02 for Pd and Au monometallic systems. After refluxing, the colloid was added as a 4.5 wt % solution in THF for Pd-Au, 10.2 wt % solution in THF for Pd and 8.4 wt % solution in THF for Au at room temperature. The gelation was carried out with vigorous stirring (4 days) under an Ar atmosphere. Following these procedures, bimetallic Pd-Au-SiO2 catalysts with 0.6 and 1 wt % metal, and monometallic Pd- and Au-SiO2 catalysts with 1 wt % metal were prepared. These materials were further treated following four different routes: 1) by simple drying, 2) in which the dried catalysts were calcined in air at 723 K and then reduced at the same temperature, 3) in which they were directly reduced in hydrogen at 723 K, and 4) in which the surfactant was extracted using an ethanol-heptane azeotropic mixture. The catalysts were characterized by nitrogen adsorption-desorption isotherms at 77 K, H2 chemisorption measurements, solid-state 1H, 13C, 29Si-CP/MAS-NMR spectroscopy, powder X-ray diffraction (XRD), small angle X-ray scattering (SAXS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and 197Au M?ssbauer spectroscopy. The physical characterization by a combination of these techniques has shown that the size and the structural characteristics of the Pd-Au colloid precursor are preserved when embedded in an SiO2 matrix. Catalytic tests were carried out in selective hydrogenation of 3-hexyn-1-ol, cinnamaldehyde, and styrene. These data showed evidence that alloying Pd with Au in bimetallic colloids leads to enhanced activity and most importantly to improved selectivity. Also, the combination of the two metals resulted in catalysts that were very stable against poisoning, as was evidenced for the hydrogenation of styrene in the presence of thiophene.     

Organic templates for the generation of inorganic materials

Mankind's fascination with shapes and patterns, many examples of which come from nature, has greatly influenced areas such as art and architecture. Science too has long since been interested in the origin of shapes and structures found in nature. Whereas organic chemistry in general, and supramolecular chemistry especially, has been very successful in creating large superstructures of often stunning morphology, inorganic chemistry has lagged behind. Over the last decade, however, researchers in various fields of chemistry have been studying novel methods through which the shape of inorganic materials can be controlled at the micro- or even nanoscopic level. A method that has proven very successful is the formation of inorganic structures under the influence of (bio)organic templates, which has resulted in the generation of a large variety of structured inorganic structures that are currently unattainable through any other method.