CARS diagnostics of molecular media under nanoporous confinement

The CARS spectroscopy is used for the diagnostics of carbon dioxide in a nanoporous glass at temperatures ranging from room temperature (20.5°C) to the subcritical temperature (30.5°C) in the pressure range below the saturated-vapor pressure. The contributions of the gas-phase molecules, the molecular layer adsorbed from the gas phase on the pore surface, the condensed liquid-like phase, and the liquid interface in the vicinity of the pore surface can be selected using the analysis of the nonlinear spectral response. The spectral behavior of the carbon dioxide confined in nanopores at the subcritical temperature indicates a state that is similar to the supercritical fluid. This corresponds to a low-temperature shift of the critical point of the medium confined in nanopores.     

方解石纳米孔隙内二氧化碳毛细凝聚的分子模拟

在二氧化碳地质封存、增产非常规油气以及孔隙材料表征测量方面,纳米孔隙中二氧化碳相态的准确预测具有重要意义.然而,由于纳米尺度下毛细力、分离压等作用力占据主导因素,流体在孔隙中的相行为与体相流体存在根本不同.已有实验和模拟表明,Kelvin毛细凝聚理论无法预测特征尺度10 nm下的,孔隙内流体凝聚压力与体相饱和蒸气压的偏...     

Broadening of vibrational spectra of carbon dioxide upon absorption and condensation in nanopores

Coherent anti-Stokes Raman Spectroscopy (CARS) has been used to study the vibrational Q-branch with the frequency of 1388 cm^?1 of the ν₁ mode of carbon dioxide molecules filling a sample made of nanopore glass at room temperature (20.5°C). The measurements were carried out in a gas cell at pressures approaching saturation P _sat. When pressure was increased above 0.8 P _sat, in addition to the spectral component due to the gaseous phase molecules, the CARS spectra featured a component due to the molecules adsorbed on the pore walls. Simulation of spectra taking the interference of these two contributions into account enabled the estimation of the broadening of the vibrational molecular spectra in the adsorbed layer. The spectral width of the component due to the adsorbed molecules was nearly a factor of two times larger than that of molecules in the bulk liquid phase. At pressures above 0.94 P _sat, the spectral width of the component due to the adsorbed molecules decreased to values close to those measured in the bulk liquid phase, which corresponds to the condensation of molecules in nanopores.     

MD模拟方法研究圆柱形纳米微孔的吸附平衡

本文用MD（分子动力学）模拟方法对圆柱形纳米微孔中的Lennard-Jones流体的物理吸附平衡及界面现象进行了研究。模拟中引入新型势模型,采用了计数法计算平衡压力,预测了在不同孔径下（直径为 3～5nm）的吸附等温线,分析了毛细凝聚理论用于纳米微孔吸附的局限性。并比较了同一纳米尺度的窄缝形微孔和圆柱形微孔吸附的差异。     

Small-angle x-ray scattering in a carbon-sulfur nanocomposite produced from bulk nanoporous carbon

A new nanocomposite material containing approximately 50 vol % S is prepared by filling pores of bulk nanoporous carbon samples with sulfur. The initial nanoporous carbon samples are synthesized from polycrystalline α-SiC through the chemical reaction. A comparative investigation of small-angle x-ray scattering (SAXS) is performed for the prepared nanocomposite and the initial material. The possible changes in the scattering power of the initial material upon filling of its pores with sulfur are considered in the framework of a simple model. The regularities revealed are used to interpret the experimentally observed changes in the scattering power. The size distribution functions of incorporated sulfur nanoclusters in the nanocomposite (or filled nanopores in the initial material) are determined within the Guinier approximation. It is demonstrated that the smallest sized pores (8–16 Å) remain unfilled, whereas the filling factor for larger sized pores can reach several ten percent by volume. The conditions favorable for small-angle x-ray scattering upon filling of the nanopores are analyzed.     

On the role of vacancies in pore formation in the course of anodizing of silicon carbide

Experimental data on the preparation of stoichiometric nanoporous silicon carbide are analyzed. Theoretical calculations are performed under the assumption that nanopores are formed through the vacancy diffusion mechanism. The results obtained confirm the hypothesis that the formation of pores with a steadystate radius of several tens of nanometers in silicon carbide can be associated with the diffusion and clustering of vacancies. The experimental data indicating that the proposed mechanism of formation of nanoporous silicon carbide correlates with the existing model of formation of porous silicon carbide with a fiber structure are discussed. This correlation can be revealed by assuming that nanopores are formed at the first stage with subsequent transformation of the nanoporous structure into a fiber structure due to the dissolution of the material in an electrolyte.     

Freezing of fluids confined in a disordered nanoporous structure

Freezing of a simple fluid in a disordered nanoporous carbon is studied using molecular simulations. Only partial crystallization occurs, and the confined phase is composed of crystalline and amorphous nanodomains. This freezing behavior departs strongly from that for nanopores of simple geometry. We present a method for analyzing the freezing in such disordered materials in terms of a transition in the average size and number of crystalline clusters. The results provide a basis for the interpretation of experiments on freezing in such materials, particularly 1H-NMR and scattering experiments.     

From homogeneous to heterogeneous nucleation of chain molecules under nanoscopic cylindrical confinement

The crystallization of monodisperse linear polyethylene confined in nanoporous alumina is investigated with the calorimetric measurements. We observe a drastic change in crystallization behavior, specifically nucleation, with a decrease in the pore diameter. Crystallization in relatively larger pores with the diameters of 62 and 110 nm occurs at lower temperatures within a very narrow range, whereas crystallization in smaller pores with diameters of 15-48 nm occurs at a higher and broad range of temperatures. Nucleation and crystallization kinetics in nanopores is discussed based on classical nucleation theory as well as the Avrami theory.     

Spectral characteristics of subcritical carbon dioxide in nanopores

Coherent anti-Stokes Raman scattering spectra measured within the Q-branch of the vibrational transition ν₁ are used to gain insights into the state of carbon dioxide molecules in nanopores of Vycor™ glass at room temperature (20.5°C) and a subcritical temperature of 30.5°C and gas pressures up to the saturation point P _sat for each temperature. Along with the main spectral component, belonging to gaseous CO₂ molecules, the spectra recorded at pressures close to P _sat feature a second (low-frequency) component. The second component is associated with the contribution from the CO₂ molecules trapped inside pores. A spectral deconvolution with account for the interference of these two bands makes it possible to estimate the spectral characteristics of the second (low-frequency) component at each temperature. At 20.5°C, the bandwidth of the low-frequency component decreases with CO₂ pressure, a behavior that can be explained by the transition of CO₂ from the adsorbed to the condensed state in the pore. At the subcritical temperature of 30.5°C, the spectral width of the second component is pressure-independent and close to the value measured in the bulk of the supercritical fluid, a result likely associated with a low-temperature shift of the critical point of the substance trapped in nanopores.     

Enhanced two‐photon luminescence from nanoporous gold capped with microcontact‐printed salts

We report that a nanoporous Au film capped with a dielectric surface layer enables the effective absorption of near‐infrared light and intense emission of two‐photon luminescence. In this approach, nanoscale pores in Au are incorporated by chemical dissolution and microscale surface patterns are added on the nanoporous Au by microcontact print lithography. Electromagnetic simulation shows that the strong local electric fields are concentrated in the vicinity of nanopores in the Au film and are further increased by the surface dielectric patterns, which leads to a 40‐fold increase in the intensity of the two‐photon luminescence, as verified by photoemission measurement. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Self-consistent mapping of the ab initio calculations to the multi-orbital <Emphasis Type="Italic">p</Emphasis>–<Emphasis Type="Italic">d</Emphasis> model: Magnetism in α-FeSi<Subscript>2</Subscript> films as the effect of the local environment

The near infrared transmission spectra of nanoporous SiO₂/Al₂O₃ xerogel have been recorded for the first time in the process of filling of nanopores with ammonia and acetone molecules. It has been found that the physical adsorption of these gases results in a reversible increase in the translucence of xerogel samples at the frequencies of vibrational bands of surface OH groups.     

Liquid–gas critical phenomena under confinement: small-angle neutron scattering studies of CO2 in aerogel

Small angle neutron scattering (SANS) is a well-established technique for investigating the behavior of confined binary liquid solutions, as it can probe the correlation length and susceptibility in pores on length scales 1 – 100 nm. We applied SANS to explore the influence of confinement on critical behavior of an individual fluid carbon dioxide (CO₂) in a highly porous aerogel. The results demonstrate that quenched disorder induced by aerogel significantly depresses density fluctuations. Despite the negligible volume occupied by aerogel (< 4%), the macroscopic phase separation of confined CO₂ into coexisting liquid and gaseous phases is suppressed and below the critical temperature of the bulk fluid frozen methastable microdomains are formed. Experimental data show that critical adsorption is as important as the effect of confinement in defining the behavior of confined fluids.     

Optical spectroscopy of the Eu(FOD)<Subscript>3</Subscript> complex impregnated into the free volume of nanoporous glass and polymethylmethacrylate using supercritical carbon dioxide

The absorption and photoluminescence spectra of Eu(FOD)₃ molecules (FOD = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate) impregnated into samples of nanoporous Vycor glass and polymethylmethacrylate using supercritical carbon dioxide were measured. The influence of the matrix on the shape and intensity of the absorption bands of Eu(FOD)₃ molecules and the photoluminescence bands of Eu³⁺ ions was analyzed.     

Synthesis of nanoporous structures in metallic materials under laser action

We defined conditions of the laser-aided formation of nanoporous structures with nanopores ranging in size from 40 to 50 nm using laser pulses of 10.6 μm wavelength at a pulse-repetition rate of up to (4-5)×10³ Hz for a model metallic material (a two-component alloy “brass of 62%”). It has been established that the exposure to a uniform laser light at depths of up to 25-30 μm results in the formation of nanopores with a relatively uniform distribution across the surface. The resulting pattern contains both solitary pores and ramified porous channels. The nanopores are uniformly distributed within a subgrain, being fairly stable in size and shape. The nanopore size and shape feature larger non-uniformity on the subgrain boundary. The resulting metallic structures show promise for use as catalysts and ultrafiltration membranes.     

Effect of surface plasmons at the Al/alumina interface on the formation of nanopores in Al anodization

We investigated the formation mechanism of nanoporous alumina in the electrochemical reaction of aluminium with oxalic acid solution in terms of the pinning effect of Fermi level at the metal–oxide interface. On the Al metal surface, the image potential state pins the standing mode of collectively excited surface electrons and the evanescent wave forms the plasmon mode, which can be reflected in oxide barrier formation. A nanoporous alumina thin film with an amorphous phase oxide layer on the Al surface can enhance light absorption at a shorter wavelength than 382 nm and provide strong Fabry–Perot oscillation in photoreflectance. The cathodoluminescence spectra show the surface plasmon mode as a consequence of the self-ordered oxide nanopores. The Kretschmann configuration of an attenuated total reflection set-up for prism/oxalic solution/Al interface, which also provides a prism-coupled surface plasmon and forms oxide nanopores on the Al surface via a laser-enhanced anodization process.     

IR spectroscopy of the structural states of ethylene in aerogel nanopores

We have investigated for the first time the combination vibrational absorption bands ν₅ + ν₉ and ν₁ + ν₁₁ of ethylene adsorbed by nanopores of a silicon dioxide aerogel in the 5800–6300 cm^–1 region. Our measurements show significant differences in the absorption spectra of gas-phase ethylene and ethylene found in the nanoporous structure of the aerogel, including a change in the shape of the absorption band contours, their frequency shift, and an increase in the absorption intensity.     

Features of magnetic resonance in nanoporous carbon with pores filled with transition-metal atoms

Properties of nanoporous carbon (NPC) free of metal atoms and NPC containing atoms of Ni, Co and Pd in their pores are studied by electron spin resonance (ESR). The asymmetrical ESR line with the so-called Dyson line shape points out that charge carriers are responsible for the resonance spectrum in metal-free NPC. Although the amount of Ni, Co, and Pd introduced into nanopores is small, the NPC properties change significantly. A bulk ferromagnetism is observed in the case of NPC with Co and Pd, but not in NPC:Ni. Co atoms in pores of NPC cause the formation of a new material, namely, a disordered ferromagnetic medium with some features in the Co atom distribution. Magnetic properties are strongly temperature-dependent. The temperature dependences of the conductivity and ESR integral intensity in NPC:Ni reveal an exponential growth with the same activation energy. The magnetic resonance spectrum of NPC:Pd consists of four signals for NPC which is produced from SiC. There are one ferromagnetic and three paramagnetic signals belonging to the carbon dangle sp³- and sp²-bonds kind and to the paramagnetic clusters of Pd atoms.     

A Continuous-Wave Electron Paramagnetic Resonance Study of Carbon Dioxide Adsorption on the Metal–Organic Frame-Work MIL-53

Continuous-wave electron paramagnetic resonance spectroscopy is applied to explore the adsorption of carbon dioxide (CO₂) over the metal organic framework (MOF) MIL-53. Therefore, paramagnetic Cr³⁺ ions, which replace a small amount of the bulk Al³⁺ ions in MIL-53(Al/Cr), are used as magnetically active probes. CO₂ was adsorbed on samples of MIL-53(Al/Cr) at equilibrium pressures between 0 and 2.5 bar. The transformation from the large pore phase to the narrow pore phase of MIL-53 was observed by electron paramagnetic resonance spectroscopy at small CO₂ pressures between 0.2 and 0.4 bar, which is in accordance with adsorption results reported in literature. By analyzing the electron paramagnetic resonance signal intensities of the corresponding Cr³⁺ probes, the ratio between the amount of the narrow pore phase and the large pore phase before and after this phase transformation was quantified. A small fraction of the large pore phase remains even after this phase transition. CO₂ adsorption at 77  K indicates the occurrence of the transformation of this MOF from a narrow pore phase to a large pore phase triggered by the adsorbed CO₂. Similar observations were already made using powder X-ray diffraction or infrared spectroscopy. But in contrast to these methods electron paramagnetic resonance spectroscopy on Cr³⁺ seems to be very sensitive not only to large differences between crystallographic conformations like large pores and narrow pores but also to different amounts and configurations of CO₂ molecules trapped in the same structural phase of MIL-53, taking advantage of the high sensitivity of the fine structure interaction of Cr³⁺.     

Magnetic properties of nickel clusters in nanoporous carbon

The magnetic properties of nanoporous carbon samples whose pores were loaded by nickel are described. It is shown that a sample becomes superparamagnetic for temperatures T<T _C (Ni) only in the case where a noticeable fraction of Ni is contained in the nanopores. The nanopore size estimated from magnetic measurements coincides with the estimates derived earlier from small-angle x-ray scattering studies.