Orientation of diamagnetic layered transition metal oxide particles in 1-tesla magnetic fields

The magnetic field-driven orientation of microcrystals of six diamagnetic layered transition metal oxides (HLaNb(2)O(7), HCa(2)Nb(3)O(10)·0.5H(2)O, KNaCa(2)Nb(4)O(13), KTiTaO(5), KTiNbO(5), and H(2.2)K(1.8)Nb(6)O(17)·nH(2)O) suspended in epoxy resins was studied by X-ray diffraction using permanent magnets producing a 0.8 T field. Although the degree of orientation, quantified as the Hermans order parameter, was strongly affected by the particle size distribution, in all cases microcrystals with ～1-2 μm lateral dimensions were found to orient with the magnetic field vector in the layer plane. Control of the orientation of ionically conducting layered oxides is of interest for practical applications in batteries and fuel cells. The consistent direction of orientation of the lamellar oxides studied can be rationalized in the framework of a quantitative bond anisotropy model developed by Uyeda (Phys. Chem. Miner.1993, 20, 77-80). The asymmetry of metal-oxygen bonding at the faces of the octahedral layers results in long and short M-O bonds perpendicular to the plane of the sheets. This distortion of the M-O octahedra, which is a structural feature of almost all layered materials that contain octahedral bonding frameworks, gives rise to the diamagnetic anisotropy and results in an easy axis or plane of magnetization in the plane of the sheets.     

Modular synthesis of pi-acceptor cyclophanes derived from 1,4,5,8-naphthalenetetracarboxylic diimide and 1,5-dinitronaphthalene

Three neutral cyclophanes were synthesized, and their association with indole, an aromatic pi-donor, was studied. The cyclophanes were designed to contain a rigid, hydrophobic binding cavity with 1,4,5,8-naphthalenetetracarboxylic diimide or 1,5-dinitronaphthalene as the pi-acceptor. Two of the cyclophanes also contain a (S)-(valine-leucine-alanine) tripeptide unit to provide chiral hydrogen bonding interactions with guest molecules. Despite the fact that these cyclophanes contain a hydrophobic binding cavity of appropriate dimensions, their association with indole is very weak. In the case of cyclophanes derived from 1,5-dinitronaphthalene, steric interactions force the nitro groups out of the plane of the naphthalene ring, diminishing their effectiveness as pi-acceptors. A simple UV--visible titrimetric method, using N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) as a pi-donor, was used to rank the pi-acceptor strength of these and other aromatic units. These titrations show that 1,4,5,8-naphthalenetetracarboxylic diimide and 1,5-dinitronaphthalene derivatives are weaker pi-acceptors than viologens, which make good pi-acceptor cyclophanes. Methyl viologen is in turn a weaker pi-acceptor than anthaquinone disulfonate, suggesting that the latter may serve as a useful building block for pi-accepting cyclophane hosts.     

Catalytic nanomotors: autonomous movement of striped nanorods

Rod-shaped particles, 370 nm in diameter and consisting of 1 microm long Pt and Au segments, move autonomously in aqueous hydrogen peroxide solutions by catalyzing the formation of oxygen at the Pt end. In 2-3% hydrogen peroxide solution, these rods move predominantly along their axis in the direction of the Pt end at speeds of up to 10 body lengths per second. The dimensions of the rods and their speeds are similar to those of multiflagellar bacteria. The force along the rod axis, which is on the order of 10(-14) N, is generated by the oxygen concentration gradient, which in turn produces an interfacial tension force that balances the drag force at steady state. By solving the convection-diffusion equation in the frame of the moving rod, it was found that the interfacial tension force scales approximately as SR(2)gamma/muDL, where S is the area-normalized oxygen evolution rate, gamma is the liquid-vapor interfacial tension, R is the rod radius, mu is the viscosity, D is the diffusion coefficient of oxygen, and L is the length of the rod. Experiments in ethanol-water solutions confirmed that the velocity depends linearly with the product Sgamma, and scaling experiments showed a strong dependence of the velocity on R and L. The direction of motion implies that the gold surface is hydrophobic under the conditions of the experiment. Tapping-mode AFM images of rods in air-saturated water show soft features that are not apparent in images acquired in air. These features are postulated to be nanobubbles, which if present in hydrogen peroxide solutions, would account for the observed direction of motion.     

Photoluminescence of perovskite nanosheets prepared by exfoliation of layered oxides, K2Ln2Ti3O10, KLnNb2O7, and RbLnTa2O7 (Ln: lanthanide ion)

Luminescent perovskite nanosheets were prepared by exfoliation of single- or double-layered perovskite oxides, K2Ln2Ti3O10, KLnNb2O7, and RbLnTa2O7 (Ln: lanthanide ion). The thickness of the individual nanosheets corresponded to those of the perovskite block in the parent layered compounds. Intense red and green emissions were observed in aqueous solutions with Gd1.4Eu0.6Ti3O10- and La0.7Tb0.3Ta2O7-nanosheets, respectively, under UV illumination with energies greater than the corresponding host oxide band gap. The coincidence of the excitation spectrum and the band gap absorbance indicates that the visible emission results from energy transfer within the nanosheet. The red emission intensity of the Gd1.4Eu0.6Ti3O10-nanosheets was much stronger than that of the La0.90Eu0.05Nb2O7-nanosheets reported previously. The strong emission intensity is a result of a two-step energy transfer cascade within the nanosheet from the Ti-O network to Gd(3+) and then to Eu(3+). The emission intensities of the Gd1.4Eu0.6Ti3O10- and La0.7Tb0.3Ta2O7-nanosheets can be modulated by applying a magnetic field (1.3-1.4 T), which brings about a change in orientation of the nanosheets in solution. The emission intensities increased when the excitation light and the magnetic field directions were perpendicular to each other, and they decreased when the excitation and magnetic field were collinear and mutually perpendicular to the direction of detection of the emitted light.     

Aqueous Photoelectrochemical CO2 Reduction to CO and Methanol over a Silicon Photocathode Functionalized with a Cobalt Phthalocyanine Molecular Catalyst

We report a precious-metal-free molecular catalyst-based photocathode that is active for aqueous CO₂ reduction to CO and methanol. The photoelectrode is composed of cobalt phthalocyanine molecules anchored on graphene oxide which is integrated via a (3-aminopropyl)triethoxysilane linker to p-type silicon protected by a thin film of titanium dioxide. The photocathode reduces CO₂ to CO with high selectivity at potentials as mild as 0 V versus the reversible hydrogen electrode (vs RHE). Methanol production is observed at an onset potential of −0.36 V vs RHE, and reaches a peak turnover frequency of 0.18 s⁻¹. To date, this is the only molecular catalyst-based photoelectrode that is active for the six-electron reduction of CO₂ to methanol. This work puts forth a strategy for interfacing molecular catalysts to p-type semiconductors and demonstrates state-of-the-art performance for photoelectrochemical CO₂ reduction to CO and methanol.     

Kirkwood–Buff integrals of finite systems: shape effects

The Kirkwood–Buff (KB) theory provides an important connection between microscopic density fluctuations in liquids and macroscopic properties. Recently, Krüger et al. derived equations for KB integrals for finite subvolumes embedded in a reservoir. Using molecular simulation of finite systems, KB integrals can be computed either from density fluctuations inside such subvolumes, or from integrals of radial distribution functions (RDFs). Here, based on the second approach, we establish a framework to compute KB integrals for subvolumes with arbitrary convex shapes. This requires a geometric function w(x) which depends on the shape of the subvolume, and the relative position inside the subvolume. We present a numerical method to compute w(x) based on Umbrella Sampling Monte Carlo (MC). We compute KB integrals of a liquid with a model RDF for subvolumes with different shapes. KB integrals approach the thermodynamic limit in the same way: for sufficiently large volumes, KB integrals are a linear function of area over volume, which is independent of the shape of the subvolume.     

Effect of pH and α-, β- and γ-cyclodextrin on the spectral properties of etoricoxib: spectroscopic and molecular dynamics study

The spectral properties of etoricoxib (ETR) at pH 2.0, 6.0 and 10.0 in the presence of cyclodextrins (CDs) were investigated. The absorption spectrum of ETR in acidic medium exhibited two bands centered at 236 and 273 nm, while in basic medium it exhibited two bands centered at 236 and 285 nm. No change in the spectrum was observed in the presence of CDs. The fluorescence emission spectra of ETR in acidic and basic media exhibited one band at 380 nm and another one at 484 nm. The emission band at 484 nm was enhanced when ETR was complexed with β-CD and γ-CD at pH 2.0, 6.0 and 10.0, while the band at 380 nm was enhanced selectively when ETR was complexed with α-CD at pH 2.0. Molecular dynamics simulations computations revealed that at pH 2.0, the sulfonyl moiety of H₂ETR²⁺ is preferentially included within the α-CD cavity, which is believed to cause the enhancement of the band at 380 nm. Moreover, at pH 6.0 and 10.0, the enhancement of the band at 484 nm was related to the inclusion of the chloropyridinyl and methylpyridinyl groups of the bipyridine moiety of HETR⁺ and ETR within β-CD and γ-CD cavities. Benesi–Hildebrand analysis showed that the ETR/β-CD complex adopts a 1:1 stoichiometry with association constant of K ₁₁?=?64.8 at pH 2.0, K ₁₁?=?105.4 at pH 6.0 and K ₁₁?=?520.5 at pH 10.0.     

A New Synthetic Route to Microporous Silica with Well‐Defined Pores by Replication of a Metal–Organic Framework

Microporous amorphous hydrophobic silica materials with well‐defined pores were synthesized by replication of the metal–organic framework (MOF) [Cu₃(1,3,5‐benzenetricarboxylate)₂] (HKUST‐1). The silica replicas were obtained by using tetramethoxysilane or tetraethoxysilane as silica precursors and have a micro–meso binary pore system. The BET surface area, the micropore volume, and the mesopore volume of the silica replica, obtained by means of hydrothermal treatment at 423 K with tetraethoxysilane, are 620 m²g^?1, 0.18 mL g^?1, and 0.55 mL g^?1, respectively. Interestingly, the silica has micropores with a pore size of 0.55 nm that corresponds to the pore‐wall thickness of the template MOF. The silica replica is hydrophobic, as confirmed by adsorption analyses, although the replica has a certain amount of silanol groups. This hydrophobicity is due to the unique condensation environment of the silica precursors in the template MOF.