Silica-metal core-shell nanostructures.

Silica-metal nanostructures consisting of silica cores and metal nanoshells attract a lot of attention because of their unique properties and potential applications ranging from catalysis and biosensing to optical devices and medicine. The important feature of these nanostructures is the possibility of controlling their properties by the variation of their geometry, shell morphology and shell material. This review is devoted to silica-noble metal core-shell nanostructures; specifically, it outlines the main methods used for the preparation and surface modification of silica particles and presents the major strategies for the formation of metal nanoshells on the modified silica particles. A special emphasis is given to the Stöber method, which is relatively simple, effective and well verified for the synthesis of large and highly uniform silica particles (with diameters from 100 nm to a few microns). Next, the surface chemistry of these particles is discussed with a special focus on the attachment of specific organic groups such as aminopropyl or mercaptopropyl groups, which interact strongly with metal species. Finally, the synthesis, characterization and application of various silica-metal core-shell nanostructures are reviewed, especially in relation to the siliceous cores with gold or silver nanoshells. Nowadays, gold is most often used metal for the formation of nanoshells due to its beneficial properties for many applications. However, other metals such as silver, platinum, palladium, nickel and copper were also used for fabrication of core-shell nanostructures. Silica-metal nanostructures can be prepared using various methods, for instance, (i) growth of metal nanoshells on the siliceous cores with deposited metal nanoparticles, (ii) reduction of metal species accompanied by precipitation of metal nanoparticles on the modified silica cores, and (iii) formation of metal nanoshells under ultrasonic conditions. A special emphasis is given to the seed-mediated growth, where metal nanoshells are formed on the modified silica cores with deposited metal nanoparticles. This strategy assures a good control of the nanoshell thickness as well as its surface properties.

Further advancements in predicting adsorption equilibria using excess formalism: calculation of adsorption excesses at the liquid/solid interface.

Prediction of adsorption equilibria for ternary liquid mixtures on solid surfaces by means of adsorption data for the corresponding three binary liquid mixtures can be improved by combining the thermodynamic excess formalism with geometrical models. This new strategy for the prediction of excess adsorption isotherms is examined for four ternary adsorption systems ranging from ideal to highly non-ideal ternary mixtures. The predicted isotherms are discussed and compared with experimental ones as well as with those obtained for a model based on the absolute quantities. The results confirm: (i) superiority of predicting adsorption in terms of excess quantities, and (ii) utility of geometrical models for constructing ternary molar compositions on the basis of binary ones to predict equilibria not only for liquid mixtures alone but also for adsorption of liquid mixtures on solid surfaces.

Features of nitrogen adsorption on nonporous carbon and silica surfaces in the framework of classical density functional theory.

Equilibrium adsorption data of nitrogen on a series of nongraphitized carbon blacks and nonporous silica at 77 K were analyzed by means of classical density functional theory to determine the solid-fluid potential. The behavior of this potential profile at large distance is particularly considered. The analysis of nitrogen adsorption isotherms seems to indicate that the adsorption in the first molecular layer is localized and controlled mainly by short-range forces due to the surface roughness, crystalline defects, and functional groups. At distances larger than approximately 1.3-1.5 molecular diameters, the adsorption is nonlocalized and appears as a thickening of the adsorbed film with increasing bulk pressure in a relatively weak adsorption potential field. It has been found that the asymptotic decay of the potential obeys the power law with the exponent being -3 for carbon blacks and -4 for silica surface, which signifies that in the latter case the adsorption potential is mainly exerted by surface oxygen atoms. In all cases, the absolute value of the solid-fluid potential is much smaller than that predicted by the Lennard-Jones pair potential with commonly used solid-fluid molecular parameters. The effect of surface heterogeneity on the heat of adsorption is also discussed.

Ordered mesoporous silica SBA-15 A new effective adjuvant to induce antibody response

The applicability of SBA-15 mesostructure as an adjuvant and evaluation of its efficiency to induce antibody response, was discussed. It was observed that better encapsulation of biomolecules of variable shape and size can be achieved using a antigen to SBA-15 weight ratio of 1: 2.5. Efficient antibody generation could be achieved because SBA-15 was able to attract antigens effectively due to its high surface area and proper mesopore size. The results show that SBA-15 and related silica mesostructures are promising nanosystems for vaccine delivery.

Equilibrium adsorption in cylindrical mesopores: a modified Broekhoff and de Boer theory versus density functional theory.

This paper presents a thermodynamic analysis of capillary condensation phenomena in cylindrical pores. Here, we modified the Broekhoff and de Boer (BdB) model for cylindrical pores accounting for the effect of the pore radius on the potential exerted by the pore walls. The new approach incorporates the recently published standard nitrogen and argon adsorption isotherm on nonporous silica LiChrospher Si-1000. The developed model is tested against the nonlocal density functional theory (NLDFT), and the criterion for this comparison is the condensation/evaporation pressure versus the pore diameter. The quantitative agreement between the NLDFT and the refined version of the BdB theory is ascertained for pores larger than 2 nm. The modified BdB theory was applied to the experimental adsorption branch of adsorption isotherms of a number of MCM-41 samples to determine their pore size distributions (PSDs). It was found that the PSDs determined with the new BdB approach coincide with those determined with the NLDFT (also using the experimental adsorption branch). As opposed to the NLDFT, the modified BdB theory is very simple in its utilization and therefore can be used as a convenient tool to obtain PSDs of all mesoporous solids from the analysis of the adsorption branch of adsorption isotherms of any subcritical fluids.

Modeling Nitrogen Adsorption in Spherical Pores of Siliceous Materials by Density Functional Theory.

Adsorption of nitrogen in spherical pores of FDU-1 silica at 77 K is considered by means of a nonlocal density functional theory (NLDFT) accounting for a disordered structure of pore walls. Pore size distribution analysis of various FDU-1 samples subject to different temperatures of calcination revealed three distinct groups of pores. The principal group of pores is identified as ordered spherical mesopores connected with each other by smaller interconnecting pores and irregular micropores present in the mesopore walls. To account for the entrances (connecting pores) into spherical mesopores, a concept of solid mass distribution with respect to the apparent density was introduced. It is shown that the introduction of the aforementioned distribution was sufficient to quantitatively describe experimental adsorption isotherms over the entire range of relative pressures spanning six decades.

An improved methodology for adsorption characterization of unmodified and modified silica gels.

An improvement in the adsorption characterization of the surface and structural properties of unmodified and modified mesoporous silica gels is presented. This improvement was achieved by selection of proper macroporous silica as the reference solid for adsorption characterization of porous silica gels. Experimental illustration is provided for unmodified and n-octyl-modified silica gels of different bonding density. The surface and structural properties of these silica gels were characterized by utilizing the standard adsorption data for both unmodified and octyl-modified LiChrospher Si-1000 macroporous silica gels. It was shown that the standard nitrogen adsorption data have an appreciable influence on the analysis of the pore size and surface properties of silica gels. This analysis can be improved by selecting the reference solid of the surface properties close to those of the silica gel studied.

Novel bifunctional periodic mesoporous organosilicas, BPMOs: synthesis, characterization, properties and in-situ selective hydroboration-alcoholysis reactions of functional groups.

A new class of bifunctional periodic mesoporous organosilicas (BPMOs) containing two differently bonded organic moieties in a mesoporous host has been synthesized and characterized. By incorporating bridge-bonded ethylene groups into the walls and terminally bonded vinyl groups protruding into the channel space, both the chemistry and physical properties of the resulting BPMO could be modified. The materials have periodic mesoporous structures in which the bridging ethylene plays a structural and mechanical role and the vinyl groups are readily accessible for chemical transformations. The vinyl groups in the material underwent hydroboration with BH(3).THF and the resulting organoborane in the BPMO was quantitatively transformed into an alcohol using either H(2)O(2)/NaOH or NaBO(3).4H(2)O. The materials retained ordered structures after subsequent in situ reactions with largely unchanged pore volumes, specific surface areas and pore size distributions. Other organic functionalized BPMO materials may be synthesized in a similar manner or by further functionalizing the resulting borylated or alcohol functionalized BPMO materials. The thermal properties of the BPMO materials have also been investigated and are compared to those of the periodic mesoporous organosilica (PMO) materials. Noteworthy thermal events concern intrachannel reactions between residual silanols or atmospheric oxygen and organics in BPMOs. They begin around 300 degrees C and smoothly interconvert bridging ethylene to terminal vinyl groups and terminal vinyl to gaseous ethene and ethane, ultimately producing periodic mesoporous silica at 900 degrees C that exhibits good structural order and a unit-cell size decreased relative to that of the parent BPMO.

Synthesis of ordered and disordered silicas with uniform pores on the border between micropore and mesopore regions using short double-chain surfactants.

Silica molecular sieves with uniform pores on the borderline between micropore (diameter <2 nm) and mesopore (from 2 to 50 nm) ranges were synthesized by a novel method using judiciously chosen mixtures of short double-chain alkylammonium surfactants. These silicas were characterized using X-ray diffraction (XRD), thermogravimetry, and nitrogen and argon adsorption. The calcined materials exhibited either 2-dimensional (2-D) hexagonal or disordered structures with XRD interplanar spacing from 2.51 to 2.93 nm, including the value of as small as 2.69 nm for highly ordered 2-D hexagonal silica. The dependence of the pore size and surfactant content on the surfactant chain length provided strong evidence for supramolecular templating being operative in the formation of small-pore silicas, even for the surfactant chain length of six carbon atoms. Both hexagonally ordered and disordered calcined samples were shown to exhibit narrow pore size distributions with maxima in the range from 1.96 to 2.61 nm (reliably evaluated on the basis of the unit-cell dimension and pore volume for 2-D hexagonal materials, and calculated using a properly calibrated procedure), tailored by the surfactant chain length. The samples exhibited primary pore volumes from 0.28 to 0.54 cm(3) g(-1) and specific surface areas from 730 to 930 m(2) g(-1). Because of their small yet uniform pore size and large specific surface area, the silicas reported herein promise to be useful in applications in adsorption and catalysis. Adsorption studies of these materials provided a unique new insight into the pore-filling mechanism for small-pore materials. Moreover, the approach proposed herein is expected to facilitate the synthesis of not only small-pore silicas but also materials with other framework compositions, thus largely contributing to bridging the gap in attainable pore sizes between micropore and mesopore ranges.

Characterization of Silver-Containing Pitch-Based Activated Carbon Fibers.

Silver-containingpitch-based activated carbon fibers (ACFs) were prepared by activation with steam. Silver particles acted as catalyst by accelerating activation rate and increasing micropore size. Scanning electron microscopy (SEM) was used to investigate the surface morphology and behavior of silver particles in carbon fibers during activation. SEM images of the silver-containing fibers at about 70% burn-off were similar to those of nonactivated carbon fibers. Characterization of the ACFs studied was performed by using nitrogen adsorption isotherms, pore size distributions, silver content data, and adsorption capacities of iodine and methylene blue. Nitrogen adsorption isotherms and specific surface areas for the silver-containing ACFs were similar to those measured on the pure ACFs, i.e., ACFs to which silver had not been added. Also, adsorption capacities of iodine and methylene blue measured from liquid solutions were similar to those of the pure ACFs. However, the average micropore size of ACFs increased with increasing burn-off. Copyright 1999 Academic Press.

Synthesis and Properties of Lanthanide Incorporated Mesoporous Molecular Sieves.

Nanostructured lanthanum and cerium incorporated MCM-41 molecular sieves were synthesized via a hydrothermal method using cethyltrimethylammonium bromide as template and trimethylammonim hydroxide as mineralizer. Characterization of these materials by X-ray diffraction and nitrogen adsorption indicates that the resulting materials possess an MCM-41-like mesoporous structure. Atomic absorption and infrared spectroscopy studies revealed the presence of La and Ce in these materials. Their BET surface areas were ca. 590 and 700 m(2)/g for La- and CeMCM-41, respectively, whereas the average mesopore diameters were in the range between 3.0 and 3.5 nm. Total acidity determined by n-butylamine adsorption showed that the samples possess from weak to strong acid sites, the strengths of which are much higher for CeMCM-41 than for LaMCM-41. Adsorption energy distributions evaluated from low pressure adsorption isotherms indicate that the amount of surface hydroxyls is also higher for the cerium containing materials. Copyright 1999 Academic Press.

Monitoring Changes in Surface and Structural Properties of Porous Carbons Modified by Different Oxidizing Agents.

A series of active carbons and carbon blacks was oxidized with various oxidizing agents such as hydrogen peroxide, perchloric acid, and nitric acid. Their surface and structural properties were evaluated by use of nitrogen and water vapor adsorption isotherms, as well as high-resolution thermogravimetry. A comparative analysis of differential thermogravimetric curves and adsorption isotherms of nitrogen and water vapor for the samples studied showed that the surface properties of oxidized carbons depend on the type of oxidizing agent as well as oxidation conditions. This comparison shows that the modification of carbons with concentrated nitric acid caused the most pronounced surface and structural changes. These changes were much smaller for the samples modified with perchloric acid and hydrogen peroxide. Copyright 1999 Academic Press.

Comparative Studies of Carbon Blacks by Thermogravimetry and Nitrogen Adsorption.

Thermodesorption behavior of water, n-butanol, and n-heptane from different carbon blacks was studied by thermogravimetry. It was shown that differences in interactions of these adsorbates with the carbon blacks studied can be monitored by this technique. The adsorbates' polarity is essential in controlling thermodesorption from low surface area carbon blacks while this factor seems to be less important in the case of high surface area samples. The relative thermodesorption plot was introduced and used to analyze thermogravimetric data. It was shown that this method provides information about surface heterogeneity similar to that obtained from the comparative adsorption plot. In addition, thermogravimetric results are related to nitrogen adsorption data. Copyright 1999 Academic Press.

Adsorption Characterization of Two Clay Minerals Society Standard Kaolinites.

The kaolinite samples KGa-1b and KGa-2 are used as standards by the Clay Minerals Society of America. Recently a comparative study of these samples was carried out using atomic force microscopy (AFM) and X-ray diffraction methods (XRD). The aim of the current work was to characterize the kaolinite standards by high resolution nitrogen adsorption and to investigate how their surface heterogeneity changes upon sample cleaning. Copyright 1998 Academic Press.

Characterization of Structural and Surface Properties of Activated Carbon Fibers.

Nitrogen adsorption isotherms on activated carbon fibers (ACFs) were measured at 77.3 K in the relative pressure range from 10(-7) to 0.995. The low relative pressure data are examined in detail in order to characterize the microporosity and the surface heterogeneity of the samples studied. The pore structure of ACFs was studied by using the alphas plot and DFT methods. Adsorption potential and adsorption energy distributions were also calculated. The results showed that ACFs are highly microporous and in some cases contain small mesopores (2-4 nm). ACFs exhibit a significant structural and energetic heterogeneity. Copyright 1998 Academic Press.

Adsorption Energy Evaluation from Luminescence Spectra of Uranyl Ions (UO22+) Adsorbed on Disperse Silica Surfaces

Luminescent measurements are utilized to evaluate the adsorption energy for hydrated uranyl groups on the disperse silica surface. The position of the UO22+ 0-0 electronic transition and the frequencies of intramolecular vibrations appearing in luminescence spectra were used in that evaluation. The method is based on the dependence of the adsorption bond force constants corresponding to each type of surface sites on the respective perturbation energy of uranyl ions. To calculate the corresponding force constants of adsorption bonds, a vibrational problem is solved. The molecular adsorbate on the silica surface is modeled by means of hypothetical diatomic and triatomic molecules with Cinfinityv and C2v symmetry, in which the UO22+ ion is predominantly fixed on the silica surface through one and two water molecules, respectively. The perturbations of stretching and deformational vibrations of hypothetical molecules by the presence of the substrate are considered. It has been shown that due to a large value of uranium mass the proposed models can be used for the analysis of vibrational spectra of hydrated uranyl groups adsorbed on the silica surface. A comparison of experimental data with theoretical calculations shows one to suggest that hydrated uranyl groups are attached to the surface by means of two water molecules. The adsorption energy for hydrated uranyl groups physically and chemically adsorbed on the silica surface were evaluated. Copyright 1997 Academic Press. Copyright 1997Academic Press

Nitrogen Adsorption Studies of Novel Synthetic Active Carbons

Synthetic active carbons obtained via controlled carbonization of phenolic resins were characterized using low temperature nitrogen adsorption over a wide pressure range. It was shown that, although active carbons under study show appreciable differences in the BET specific surface area and the total pore volume, they exhibit quite similar porous properties. All samples were shown to possess a considerable amount of small micropores (ca. 0.7 nm). However, the active carbons have markedly different amounts of larger micropores (width above ca. 1 nm) and mesopores. It was shown that simple comparative methods, such as the high resolution alphas plot, provide valuable information about structural properties of active carbons. However, reliable adsorption data for suitable reference adsorbents need to be available in order to successfully apply the comparative methods. Therefore, a nitrogen adsorption isotherm was measured on a Cabot BP 280 nongraphitized carbon black over a relative pressure range from 10(-6) to 0.99 and the resulting data (104 points) are reported in a tabular form. In the high pressure region (relative pressure from 0.1 to 0.7), the isotherm for the BP 280 sample exhibits a good agreement with the reference data available in the literature. Moreover, the adsorption isotherm currently reported covers the low pressure range, which is crucial in profound characterization of porous and nonporous solids by means of comparative methods of adsorption analysis.

Surface and Structural Properties of Novel Titanium Phosphates

Two novel fibrous framework titanium phosphates having a similar formula, Ti2O(PO4)2·H2O, and a layered titanium phosphate, Ti2O3(H2PO4)2·2H2O, were synthesized and their properties were characterized with physicochemical methods, such as elemental analysis, thermogravimetry, nitrogen adsorption, X-ray diffraction, and 31P solid-state NMR. Nitrogen adsorption isotherms were measured on synthesized titanium phosphates in order to characterize their surface and structural properties. The specific surface area and total pore volume of the titanium phosphates studied were determined. The low-pressure nitrogen adsorption isotherms were used to evaluate surface heterogeneity of the synthesized materials by employing an advanced numerical procedure based on the regularization method.