Jerzy Choma

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.

New opportunities in Stöber synthesis: preparation of microporous and mesoporous carbon spheres

The recent extension of the Stober recipe to the synthesis of carbon particles creates tremendous opportunities in the design of novel carbon spheres having micropores, mesopores, or both, as well as composite carbon spheres with incorporated inorganic nanoparticles such as silica and silver.

Comparison of adsorption methods for characterizing the microporosity of activated carbons

Adsorption methods for characterizing the microporosity of activated carbons are discussed critically. Three methods—the αs-method and those based on the Dubinin-Radushkevich and Jaroniec-Choma isotherm equations—are compared with respect to the parameters that characterize the microporous structure of a solid. It is shown that the isotherm equations that account for the structural heterogeneity of activated carbons give values of the micropore volume similar to that obtained by the αs-method.

Critical discussion of simple adsorption methods used to evaluate the micropore size distribution

The well-known simple adsorption methods used to evaluate the micropore size distribution from low pressure adsorption isotherms were examined by employing model isotherms for slit-like graphite micropores obtained from nonlocal density functional theory. It was shown that in the range of pore sizes from about 0.4 to 0.9 nm, the Horvath Kawazoe (HK) method satisfactorily reproduces the shape of the micropore size distribution, but the pore sizes are underestimated. In the case of micropores wider than 0.9 nm, the method fails as the formation of the monolayer on the pore walls produces a peak corresponding to 0.6 nm micropores on the HK pore size distribution. Therefore, the HK method indicates the presence of microporosity even for nonporous samples. The Dubinin-Astakhov adsorption isotherms were also examined and it was shown that their application to represent local adsorption isotherms for homogeneous pores is questionable. However, the adsorption potential distributions seem to be promising for micropore analysis.

Improved Pore-Size Analysis of Carbonaceous Adsorbents:

An improvement was proposed for the pore-size analysis of active carbons based on low-temperature (77 K) nitrogen adsorption isotherms measured over a wide range of relative pressures (5 × 10−7–0.995). It was shown that the applicability of the Barrett, Joyner and Halenda (BJH) computational method based on the Kelvin equation could be extended significantly towards small mesopores and large micropores when a proper t-curve was used to represent the film thickness of nitrogen adsorbed on the carbon surface. It was proposed that the aforementioned t-curve be obtained from the nitrogen adsorption isotherm at 77 K on a macroporous carbon black by fitting its multilayer part to the calibrated t-curve for nitrogen adsorbed at 77 K on a macroporous silica. To date, the Harkins–Jura or Halsey t-curves have been used to describe the pressure-dependence of the film thickness. This appears to be inaccurate, especially in the range of low relative pressures. It was shown that this inaccuracy makes the pore-size analysis questionable. However, the t-curve proposed in this work gave the pore-size distribution functions for the carbons studied thereby reproducing the total pore volume and showing realistic behaviour in the range at the borderline between micropores and mesopores.

Distribution functions characterizing structural heterogeneity of activated carbons

Abstract Two main types of distribution functions are recommended to characterize the structural heterogeneity of activated carbons. The first of them characterizes the distribution of micropores with respect to their dimensions. The second function is the distribution of adsorption potential. A method for evaluating these distributions from benzene adsorption isotherm is discussed and examined by utilizing the experimental isotherms for different types of activated carbons.

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.

Characterization of activated carbons by distribution functions of adsorption potential and micropore dimension

Abstract A method is proposed for evaluating the adsorption potential distributions, which characterize structural and surface heterogeneities of activated carbons. These distributions are calculated from the adsorption isotherms. The benzene adsorption isotherms measured for two different activated carbons are used to examine the above method. Structural heterogeneity of activated carbons is additionally characterized by the micropore distribution.

Assessment of reliability of the Horvath–Kawazoe pore size analysis method using argon adsorption isotherms on ordered mesoporous silicas

Abstract A set of argon adsorption isotherms measured on MCM-41 ordered mesoporous silicas of different pore sizes was used to assess the reliability of the Horvath–Kawazoe (HK) method for mesopore size analysis. It was shown that for small mesopores the HK-type relation between the pore size and argon condensation pressure for cylindrical oxide-type pores underestimates the pore width of MCM-41 materials but this underestimation is much smaller than that obtained from nitrogen adsorption data. In contrast, the relation established on the basis of well-defined argon/MCM-41 adsorption systems allows for a correct evaluation of the pore width of MCM-41 but does not improve the shape of the pore size distribution (PSD). Both aforementioned pore width-condensation pressure relations underestimated the PSD height because of the appearance of an artificial tail in the range of fine pores. This artificial tail erroneously suggests the presence of micropores for all MCM-41 samples studied. Therefore, the recognition of the artificial nature of this tail is crucial for avoiding gross misinterpretations of the HK PSDs.

Chapter 14. Characterization of geometrical and energetic heterogeneities of active carbons by using sorption measurements

A thermodynamic approach to the characterization of active carbons is presented including a brief review of selected methods and extensive experimental illustrations. This approach represents a comprehensive way to characterize the energetic and geometrical heterogeneities of active carbons, and is based on the differential adsorption potential distribution, which provides a quantitative measure of all changes in the Gibbs free energy of a given gas/solid sorption system. The distribution in question is associated with the differential enthalpy, differential entropy and immersion enthalpy via simple relationships. Also, it can be easily converted to the mesopore and micropore volume distributions.