World of nanostructures - nanotechnology,surface properties of chosen nanomaterials

Summary The paper presents the basic information on nanotechnology and the recent results of studies of physicochemical properties of chosen nanomaterial surfaces (montmorillonites, carbon nanotubes, smart surfaces) by means of complex measuring methods. Physicochemical properties of nanomaterial surfaces by means of the special thermogravimetry Q-TG, sorptometry, porosimetry, atomic force microscopy (AFM) and scanning electron micrograph (SEM) methods were investigated. A numerical and analytical procedure for the evaluation of total heterogeneous properties (desorption energy distribution and pore-size distribution functions) on the basis of liquid thermodesorption from the sample surfaces under the quasi-equilibrium conditions and sorptometry techniques are presented. The evaluation of the fractal dimensions of nanotubes using the sorptometry, porosimetry, thermogravimetry Q-TG and AFM data are presented. The comparison of fractal coefficients calculated based on them with the results from Q-TG, sorptometry, porosimetry and AFM gave good agreement.     

Studies of heterogeneity properties of selected high-temperature superconductor surfaces

Nitrogen adsorption measured at 77 K was used to characterize the surface heterogeneity of high-temperature superconductor surfaces. Properties relating to adsorption and porosity of the solids (adsorption capacity, specific surface area, radii and volume of the pores, pore-size distribution function) were determined from nitrogen adsorption–desorption isotherms and atomic force microscopy (AFM) for a series of oxide superconductors. It is shown that the adsorption isotherms of all samples are S-shaped and belong to type II according to the IUPAC classification. On the basis of the nitrogen adsorption isotherms and AFM data, fractal dimensions were determined and correlations found with adsorption and porosity parameters.     

Studies of physico-chemical properties and fractal dimensions of MgB2 superconductor surface

The porous structure of MgB₂ has been investigated using atomic force microscopy (AFM) and sorption techniques. The fractal dimension and surface roughness parameters were evaluated from (AFM) and nitrogen adsorption?Cdesorption isotherms measured at ?196?°C for MgB₂ sample. Adsorption capacity, specific surface area, and fractal dimensions were determined from adsorption?Cdesorption isotherms. The sorption isotherms of MgB₂ samples were S-shaped and belong to type II according to the IUPAC classification. The results of fractal dimensions of MgB₂ surface determined on the basis sorptometry and AFM data are compared.     

An experimental study of gas adsorption on fractal surfaces

The validity of the fractal versions of the FHH and BET theories for describing the adsorption of butane and nitrogen on a variety of partially dehydroxylated silica surfaces has been tested. The fractal dimensions obtained from adsorption data have been compared with those obtained completely independently using SAXS. It was found that the fractal dimensions obtained from butane adsorption isotherms, using both the fractal FHH and fractal BET theories, agreed well with the corresponding values obtained from SAXS over overlapping length scales. However, in general, a systematic deviation between the fractal dimension obtained from nitrogen adsorption and that obtained from SAXS was observed. The fractal dimensions obtained from nitrogen adsorption were consistently larger than those obtained from SAXS, which is the opposite of what has often been found in the literature. It has been suggested that the differences in the suitability of the adsorption theories tested to describe butane and nitrogen adsorption is due to the significant difference between the interaction strengths of these two different molecules with silica surfaces. A modified theory that can account for the discrepancy between the fractal dimensions obtained from nitrogen adsorption and SAXS has been proposed. The implications of the new theory for the accuracy of nitrogen adsorption BET surface areas for silicas are discussed.     

Adsorption and porosity properties of carbon-covered alumina surfaces

Uniformly carbon-covered alumina (CCA) was prepared via the carbonisation of sucrose highly dispersed on the alumina surface. Using special thermogravimetry and sorptometry methods physicochemical properties of carbon-covered alumina surfaces were investigated. A numerical and analytical procedure for the evaluation of total heterogeneous properties (desorption energy distribution and pore-size distribution functions) on the basis of liquid thermodesorption from the sample surfaces under the quasi-equilibrium conditions are presented. The desorption energy distribution was derived from the mass loss Q-TG and the differential mass loss Q-DTG curves of thermodesorption of pre-adsorbed polar and apolar liquid films. For the first time, the evaluation of the fractal dimensions of carbon-covered alumina using the sorptometry, thermogravimetry and AFM data is presented.     

Studies of adsorption and total heterogeneity properties of pure and modified carbon nanotube surfaces

Modified carbon multiwall nanotubes were prepared via the oxidation process by means of 65 % nitric acid and/or ferric nitrate dissolved with 65 % nitric acid. Using special thermogravimetry (Q-TG), sorptometry, and AFM methods physicochemical properties of pure and modified nanotube surfaces were investigated. A numerical and analytical procedure for the evaluation of total heterogeneous properties on the basis of liquid thermodesorption from the sample surfaces under the quasi-equilibrium conditions is presented. The calculations of the fractal dimensions of carbon nanotubes using the thermogravimetry Q-TG, sorptometry, and AFM data are presented.     

Collisions of ideal gas molecules with a rough/fractal surface. A computational study

The frequency of collisions of ideal gas molecules (argon) with a rough surface has been studied. The rough/fractal surface was created using random deposition technique. By applying various depositions, the roughness of the surface was controlled and, as a measure of the irregularity, the fractal dimensions of the surfaces were determined. The surfaces were next immersed in argon (under pressures 2 x 10(3) to 2 x 10(5) Pa) and the numbers of collisions with these surfaces were counted. The calculations were carried out using a simplified molecular dynamics simulation technique (only hard core repulsions were assumed). As a result, it was stated that the frequency of collisions is a linear function of pressure for all fractal dimensions studied (D = 2, ..., 2.5). The frequency per unit pressure is quite complex function of the fractal dimension; however, the changes of that frequency with the fractal dimension are not strong. It was found that the frequency of collisions is controlled by the number of weakly folded sites on the surfaces and there is some mapping between the shape of adsorption energy distribution functions and this number of weakly folded sites. The results for the rough/fractal surfaces were compared with the prediction given by the Langmuir-Hertz equation (valid for smooth surface), generally the departure from the Langmuir-Hertz equation is not higher than 48% for the studied systems (i.e. for the surfaces created using the random deposition technique).     

Studies of physico-chemical properties and fractal dimensions of selected high-temperature superconductor surfaces

Properties relating to porosity of solids (fractal dimensions, surface roughness parameters) were evaluated from atomic force microscopy (AFM) and nitrogen adsorption-desorption isotherms measured at 77 K for selected high-temperature [(RE) Ba₂Cu₃O_7−x, RE=Y, Sm] superconductors. Adsorption capacity, specific surface area, fractal dimensions were determined from adsorption-desorption isotherms. The adsorption isotherms of all samples were S-shaped and belong to type II according to the IUPAC classification. A linear relationship was demonstrated between the fractal coefficients calculated by using the two methods and values of adsorption capacity of monolayer.     

Determination of peptide-surface adsorption free energy for material surfaces not conducive to SPR or QCM using AFM

The interactions between peptides and proteins with material surfaces are of primary importance in many areas of biotechnology. While surface plasmon resonance spectroscopy (SPR) and quartz crystal microbalance (QCM) methods have proven to be very useful in measuring fundamental properties characterizing adsorption behavior, such as the free energy of adsorption for peptide-surface interactions, these methods are largely restricted to use for materials that can readily form nanoscale-thick films over the respective sensor surfaces. Many materials including most polymers, ceramics, and inorganic glasses, however, are not readily suitable for use with SPR or QCM methods. To overcome these limitations, we recently showed that desorption forces (F(des)) obtained using a standardized AFM method linearly correlate to standard-state adsorption free energy values (ΔG°(ads)) measured from SPR in phosphate buffered saline (PBS: phosphate buffered 140 mM NaCl, pH 7.4). This approach thus provides a means to determine ΔG°(ads) for peptide adsorption using AFM that can be applied to any flat material surface. In this present study, we investigated the F(des)-ΔG°(ads) correlation between AFM and SPR data in PBS for a much broader range of systems including eight different types of peptides on a set of eight different alkanethiol self-assembled monolayer (SAM) surfaces. The resulting correlation was then used to estimate ΔG°(ads) from F(des) determined by AFM for selected bulk polymer and glass/ceramic materials such as poly(methyl methacrylate) (PMMA), high-density polyethylene (HDPE), fused silica glass, and a quartz (100) surface. The results of these studies support our previous findings regarding the strong correlation between F(des) measured by AFM and ΔG°(ads) determined by SPR, and provides a means to estimate ΔG°(ads) for peptide adsorption on macroscopically thick samples of materials that are not conducive for use with SPR or QCM.     

Studies of modified carbon nanotubes by means of thermogravimetry Q-TG and sorptometry techniques

The paper presents the physicochemical (adsorption and porosity) properties of closed carbon nanotubes, opened through the action of an oxidizing acid and modified with metal ions of nickel, cobalt, and iron/cobalt. The carbon nanotubes were prepared via the oxidation process by means of 65 % nitric acid and/or nickel and cobalt nitrates dissolved with 65 % nitric acid. Using special thermogravimetry Q-TG and sorptometry methods physicochemical properties of pure and modified nanotube surfaces were investigated. A numerical and analytical procedure for the evaluation of heterogeneous properties (adsorption capacity, statistical number of adsorbed liquid layers, desorption energy distribution functions) on the basis of liquid thermodesorption Q-TG from the sample surfaces under the quasi-equilibrium conditions are presented. The calculations of the specific surface areas, pore size and volume, and fractal dimensions of carbon nanotubes were made from sorptometry data.     

Studies of surface properties of Na- and La-montmorillonites

This paper presents possible applications of thermal analysis, sorptometry and porosimetry to study physico-chemical properties of Na- and La-montmorillonite samples, especially for determination of total surface heterogeneity. The quasi-isothermal thermogravimetric (Q-TG) mass loss and its first derivative (Q-DTG) curves with respect to temperature and time obtained during programmed liquid thermodesorption under quasi-isothermal conditions have been used to study adsorbed layers and heterogeneous properties of the Na- and La-montmorillonites. Calculations of the desorption energy distribution functions by analytical procedure using mass loss Q-TG and differential mass loss Q-DTG curves of thermodesorption under quasi-isothermal conditions of polar and non-polar liquid vapours preadsorbed on a material surface are presented. Parameters relating to porosity of samples were determined by sorptometry, mercury porosimetry and atomic force microscopy (AFM). From nitrogen sorption isotherms from sorptometry and porosimetry methods, the fractal dimensions of montmorillonites have been calculated. Moreover, a new approach is proposed to calculate fractal dimensions of materials obtained from Q-TG curve; this is compared with values obtained by the above methods. The total heterogeneous properties (energy distribution function and pore-size distribution functions) of samples studied were estimated. The radius and pore volume of the tested samples calculated on the basis of thermogravimetry, sorptometry and porosimetry techniques were compared and good correlations obtained.     

On the nanostructure of polymer layers formed by adsorption from binary and ternary solutions on a solid SiO2: a combined adsorption and atomic force microscopy (AFM) study

The thickness of nanolayers formed by adsorption from dilute and semi-dilute solutions on a solid SiO₂ surface has been estimated from adsorption isotherms and atomic force microscopy (AFM) measurements for polystyrene, poly(butyl methacrylate), and their mixtures. The thickness of the adsorption layers depends strongly on the adsorption conditions and is controlled by several features of the adsorbing entities. In a low-concentration regime of adsorption, the length of polymer chains and the nature of their interaction with the substrate are the most important factors controlling the adsorption process. Above the critical concentration C*, macromolecular clusters (aggregates of several overlapping chains) are formed in a solution as a result of polymer chains self-assembly. Therefore, the final adsorption layer thickness is determined mainly by the size of the clusters in this concentrated regime of adsorption. We also demonstrate that in the case of polymer mixtures, the adsorption leads to formation of mosaic structures with alternation of the polymeric components in plane of the substrate and a characteristic domain size of approximately 200 nm for each of the components. AFM study reveals that the adsorbed layers are fractal structures whose fractal dimensions depend on the type of the polymer and the adsorption process. We demonstrate therefore that the structure of nanolayers of polymers and their mixtures on the solid surface can be regulated by variation of the adsorption conditions.     

Determination of the textural characteristics of carbon samples using scanning electronic microscopy images: comparison with mercury porosimetry data

In this paper, scanning electron microscopy (SEM) images of carbon samples were analysed, and some textural characteristics were obtained and compared with those determined using mercury porosimetry data. Fractal dimensions were calculated from both mercury porosimetry and SEM images as methods for characterising the porous distribution (heterogeneity) of the samples. Lacunarity is easily determined from SEM images as a measure of the degree of heterogeneity of a porous surface. A relationship between the lacunarity and the fractal dimensions calculated using both methods is shown. Pore-size distributions were also determined from the analysis of SEM images. We show that the analysis of SEM images is a valuable complement to mercury porosimetry measurements and a useful tool for the characterisation of porous surfaces. This method offers the possibility of evaluating the features of porous materials and comparing the results to those obtained using mercury intrusion analysis.     

Atomic force and ultrasonic force microscopy investigation of adsorbed layers formed by two incompatible polymers: polystyrene and poly(butyl methacrylate)

Atomic force microscopy (AFM) and ultrasonic force microscopy (UFM) have been used to study the properties of adsorption layers formed by two incompatible polymers, polystyrene and poly(butyl methacrylate), in the course of simultaneous adsorption on the surface of silica (naturally oxidized surface of a silicon wafer). The adsorption was performed from solutions containing both of the components in a common solvent (carbon tetrachloride) in dilute and semidilute concentration regimes. It was discovered that in both cases the structure of adsorption layers has a complex mosaic structure, the details of which depend on solution composition, on the solution concentration regime, and on the ratio of the components in the adsorption layer. The observed structural inhomogeneity on the length scale of approximately 200-500 nm (distribution of segment density revealed by UFM) appears as result of thermodynamic incompatibility in the system and is conditioned by changes in the conformation states of the adsorbed macromolecules in the route of competitive adsorption of the components. The adsorbed polymer films with thicknesses of approximately 20-500 nm appeared to have fractal properties and could be characterized with fractal dimensions dependent on the ratio of the components at the interface and the adsorption conditions.     

Correlation between desorption force measured by atomic force microscopy and adsorption free energy measured by surface plasmon resonance spectroscopy for peptide-surface interactions

Surface plasmon resonance (SPR) spectroscopy is a useful technique for thermodynamically characterizing peptide-surface interactions; however, its usefulness is limited to the types of surfaces that can readily be formed as thin layers on the nanometer scale on metallic biosensor substrates. Atomic force microscopy (AFM), on the other hand, can be used with any microscopically flat surface, thus making it more versatile for studying peptide-surface interactions. AFM, however, has the drawback of data interpretation due to questions regarding peptide-to-probe-tip density. This problem could be overcome if results from a standardized AFM method could be correlated with SPR results for a similar set of peptide-surface interactions so that AFM studies using the standardized method could be extended to characterize peptide-surface interactions for surfaces that are not amenable for characterization by SPR. In this article, we present the development and application of an AFM method to measure adsorption forces for host-guest peptides sequence on surfaces consisting of alkanethiol self-assembled monolayers (SAMs) with different functionality. The results from these studies show that a linear correlation exists between these data and the adsorption free energy (ΔG(o)(ads)) values associated with a similar set of peptide-surface systems available from SPR measurements. These methods will be extremely useful to characterize thermodynamically the adsorption behavior for peptides on a much broader range of surfaces than can be used with SPR to provide information related to understanding protein adsorption behavior to these surfaces and to provide an experimental database that can be used for the evaluation, modification, and validation of force field parameters that are needed to represent protein adsorption behavior accurately for molecular simulations.     

The dependence of fractal dimension on measuring conditions of scanning probe microscopy

The fractal dimension of sputtered thin film surfaces was determined. Topography was measured using scanning tunneling microscopy (STM) and atomic force microscopy (AFM). It can be shown that measuring conditions have an important influence on the topographic data and the obtained fractal dimension. This influence was investigated systematically. The results of STM- and AFM-measurements have been compared. The results for surfaces imaged with AFM give lower values for the fractal dimension than with STM. Measurements were carried out with a standard cantilever and so the data have been filtered. Dynamic behavior of the measuring system is influenced by scanning speed and loop gain factor. During measurement of topographic signal high scanning speed and low loop gain factor produce a low pass behavior and the fractal dimension will be varied. If there is a disturbance in addition to the topographic signal (e.g. a noise) we found power spectra with more than one linear part. The calculated fractal dimension especially depended on the density of measuring points in the profile. The experimental results were verified by calculated Weierstrass-Mandelbrot functions.     

Formation of fractal porous silica by hydrolysis of TEOS in a bicontinuous microemulsion

Mesoporous silica particles have been prepared by hydrolysis of TEOS (Si(OC₂H₅)₄) in bicontinuous microemulsions containing polyoxyethylene (POE) dodecylether, isooctane and water. TEOS was dissolved in a continuous water phase and hydrolyzed by the dispersed water at around the phase inversion temperature (60°C). Undulating solid materials with layered mesostructures were produced from middle-phase microemulsions in the three phase region (o/w=0.2–0.7). On the other hand, the solids obtained from the lower aqueous phase in the three phase region were found to have a heterogeneous disordered structure. Measurements of the fractal dimensions were performed in the macropore region using a box-counting method for the outline of the SEM texture. We found that the macropore size distribution in the particles prepared from the middle-phase microemulsion follows the fractal rule with a dimension of 1.7. From the results of nitrogen adsorption/desorption curves on the silica, a steep increase in the adsorption amounts was observed at a relative pressure below 0.2, and adsorption/desorption hysteresis was also observed at a relative pressure between 0.3 and 0.5. These studies suggest that the silica synthesized in the bicontinuous microemulsion mesostructure has a very broad size range from micro to macropores with a fractal distribution.     

Studies of physico-chemical properties of mixed adsorbent in the zeolite/SiO2 system

The paper presents physico-chemical properties of mixed adsorbents in the clinoptylolite (mordenite)/SiO₂ system containing 30, 50, 80 mass% zeolite. Adsorption capacity towards polar (water, butanol) and non-polar (n-octane) substances as well as total surface heterogeneity (energetic and geometrical) were determined. Desorption energy distribution functions as well as fractal dimensions were also determined and compared with the low-temperature nitrogen adsorption data. Irregular shapes of the curves q=f(E _d) as well as large values of volumetric fractal dimensions (D _f~2.6) revealed heterogeneous properties of the zeolite/SiO2 system surfaces. Addition of zeolite increases total heterogeneity of the material.     

Determination of total heterogeneity and fractal dimensions of high-temperature superconductors

Using thermo-analytical and sorptometric methods physicochemical properties and especially surface heterogeneity of HgBa₂Ca₂Cu₃O₈₊, (Hg-1223) was investigated. The desorption energy distribution was derived from mass loss Q-TG and differential mass loss Q-DTG curves of thermodesorption in quasi-isothermal conditions of pre-adsorbed n-octane and water vapour. It is shown that the superconducting Hg-1223 phase is highly sensitive to water vapours. The mechanism of water adsorption depends largely on the activation time. By water vapour saturation in a period of 90 min, physisorption takes place. Prolonged periods result in a chemical decomposition. From nitrogen ad- and desorption isotherms the fractal dimension of superconductors were calculated. A new approach is proposed to calculate fractal dimension from Q-TG curves.This revised version was published online in November 2005 with corrections to the Cover Date.     

煤微孔表面的分形维数及其变化规律的研究

本文利用气体吸附数据确定了各种煤阶煤（从褐煤到无烟煤）和煤焦气化反应过程中微孔表面的分形维数及其变化规律。这有助于进一步认识煤的孔结构特征。