Antonio Campero

On comparing BJH and NLDFT pore-size distributions determined from N2 sorption on SBA-15 substrata

SBA-15 silica materials consisting of a collection of non-intersecting cylindrical pores of varying diameters have been utilized to try to reconcile the pore-size distribution results proceeding from the classical Barrett–Joyner–Halenda (BJH) and modern non-local density functional theory (NLDFT) approaches. To assess such pretended concordance, it is necessary to perform BJH pore-size estimates on the basis of a modified Kelvin equation that makes allowance for the adsorption potential field emanating from the solid walls of the adsorbent towards the adsorbate molecules. Under this context, critical conditions for capillary condensation and evaporation to happen in cylindrical pores have been specifically calculated via a treatment previously developed by Broekhoff and de Boer (BdB). In this way, BJH-BdB pore-size distribution results, obtained from the analyses of both ascending and descending boundary curves of N2 sorption isotherms at 76 K on a series of model SBA-15 substrata that have been synthesized in this work, are compared with homologous curves proceeding from a NLDFT treatment performed on the descending boundary curve and very reasonable agreement has been found.

Structure and texture of self-assembled nanoporous SnO2

Synthesis of nanoporous SnO2 by a surfactant self-assembling sol–gel technique is described in this work. The synthesis was performed at room temperature by employing tin(IV) tetra-tert-amyloxide, Sn (OAmt)4, as the porous SnO2 frame precursor in the presence of the micelles of a cationic surfactant solution of cetyltrimethylammonium bromide (CTAB), which, in turn, acts as the SnO2 particle nanostructure director. The pH of the Sn alkoxide–CTAB mixture was adjusted to a value of 2 with HCl in order to eventually achieve a finely dispersed SnO2 gel structure. The final annealed materials were attained after thermal treatment of the above mentioned SnO2 gel systems between 300 and 500 °C. This annealing procedure burned off different numbers of CTAB micelles and caused some sintering of the substrates depending on the final calcination temperature, thus producing truly mesoporous (cylindrical) SnO2 skeletons of pore sizes 5–9 nm and surface areas between 60 and 100 m2 g−1 as well as some other assorted structural and textural characteristics.

Aggregation Properties of Metallic Tetrasulphophthalocyanines Encapsulated in Sol-Gel Materials

Metallic phthalocyanines exhibit, among many useful properties, the ability to protect sensors against short, intense optical pulses deleterious to efficient sensor operation. It is possible by means of low temperature sol-gel techniques to obtain porous monolithic materials encapsulating metallic ions, and all sorts of complex organic molecules. We report here the study of water-soluble metallic tetrasulphophthalocyanines (MTSPc) encapsulated in silica obtained by the sol-gel technique. These materials are transparent and stable and exhibit interesting optical properties. Absorption in the visible region of the spectrum reveals aggregation of the MTSPc species to form dimers which modify the optical absorption of excited states and the resulting optical properties. We investigate the relative importance of the various physicochemical parameters affecting dimer formation (dye concentration, pH, presence of species such as dimethylformamide, pyridine, etc.) in order to control and inhibit dimer formation. Two species are chosen as examples of the general behavior: CuTSPc, which shows a strong tendency to dimerize in water and in mixtures of solvents, and (OH)AlTSPc, which does not form dimers in aqueous solutions because of its hindering OH axial group. The experiments performed show that addition of dimethylformamide or pyridine does not inhibit aggregation in CuTSPc, simultaneously causing the blue color to disappear from the prepared gels. In the case of (OH)AlTSPc addition of relatively high concentrations of pyridine brings about the formation of non-monomeric species.

Crossed and Linked Histories of Tetrapyrrolic Macrocycles and Their Use for Engineering Pores within Sol-Gel Matrices

The crossed and linked histories of tetrapyrrolic macrocycles, interwoven with new research discoveries, suggest that Nature has found in these structures a way to ensure the continuity of life. For diverse applications porphyrins or phthalocyanines must be trapped inside solid networks, but due to their nature, these compounds cannot be introduced by thermal diffusion; the sol-gel method makes possible this insertion through a soft chemical process. The methodologies for trapping or bonding macrocycles inside pristine or organo-modified silica or inside ZrO2 xerogels were developed by using phthalocyanines and porphyrins as molecular probes. The sizes of the pores formed depend on the structure, the cation nature, and the identities and positions of peripheral substituents of the macrocycle. The interactions of the macrocyclic molecule and surface Si-OH groups inhibit the efficient displaying of the macrocycle properties and to avoid this undesirable event, strategies such as situating the macrocycle far from the pore walls or to exchange the Si-OH species by alkyl or aryl groups have been proposed. Spectroscopic properties are better preserved when long unions are established between the macrocycle and the pore walls, or when oligomeric macrocyclic species are trapped inside each pore. When macrocycles are trapped inside organo-modified silica, their properties result similar to those displayed in solution and their intensities depend on the length of the alkyl chain attached to the matrix. These results support the prospect of tuning up the pore size, surface area, and polarity inside the pore cavities in order to prepare efficient catalytic, optical, sensoring, and medical systems. The most important feature is that research would confirm again that tetrapyrrolic macrocycles can help in the development of the authentic pore engineering in materials science.

Surfactantless synthesis and textural properties of self-assembled mesoporous SnO2

Ordered surfactantless self-assembled, mesoporous SnO(2) adsorbents, consisting of tubular voids of nanometric sizes, are prepared by the sol-gel processing of tin (IV) tetra-tert-amyloxide, Sn(OAm(t))(4), whose molecules have been previously chelated with acetylacetone in the absence of water, to modulate their reactivity and to promote an incipient self-assembling of -O-Sn-O oligomeric species; ultimately, the necessary amount of water to induce the hydrolysis-condensation reactions is added to this aged sol, then producing tubular pore templates within the SnO(2) xerogel network. A collection of mesoporous SnO(2) xerogels of assorted structural properties has been obtained after calcination in air of precursory gels proceeding from an aged mixture of Sn(OAm(t))(4) and acetylacetone at temperatures in the range 200-1000 degrees C. N(2) sorption isotherms measured on these SnO(2) solids evidence mesoporous structures of diverse textural characteristics (i.e. pore widths of 3-50 nm and surface areas of 10-140 m(2) g(-1)) in which voids virtually behave as if they are independent cylindrical pores during capillary condensation and evaporation.

Controlled three-dimensional manipulation of vanadium oxide nanotubes with optical tweezers

Hernandez-Pozos acknowledges the support from the Academia Mexicana de Ciencias and the Royal Society U.K.. K.D. is a Royal Society-Wolfson Merit Award holder. We thank the EPSRC for funding.

Biomimetic sol-gel synthesis of TiO₂ and SiO₂ nanostructures.

We report the heptapeptide-mediated biomineralization of titanium dioxide nanoparticles from titanium alkoxides. We evaluated the influence of pH on the biomineralized products and found that nanostructured TiO2 was formed in the absence of external ions (water only) at pH ~ 6.5. Several variants (mutants) of the peptides with different properties (i.e., different charges, isoelectric points (pIs), and sequences) were designed and tested in biomineralization experiments. Acid-catalyzed experiments were run using the H1 (HKKPSKS) peptide at room temperature, which produced anatase nanoparticles (~5 nm in size) for the first time via a heptapeptide and sol-gel approach. In addition, the peptide H1 was used to synthesize SiO2 nanoparticles. The influence of the pH and the added ions were monitored: at higher pH levels (8-9), SiO2 nanoparticles (20-30 nm in size) were obtained. In addition, whereas borate and Tris ions allowed the formation of colloidal systems, phosphate ions were unable to produce sols. The results presented here demonstrate that biomineralization depends on the sequence and charge of the peptide, and ions in solution can optimize the formation of nanostructures.

On the textural and morphological properties of crystalline and amorphous α-tin phosphate

The synthesis as well as the textural and morphological properties of microporous amorphous and crystalline tin(IV) phosphate, Sn(HPO4)2·H2O (α-SnP), prepared by the sol–gel technique are presented. Tin tetra-tert-amyloxide, Sn(OAmt)4, is first synthesized and a solution of this reactant in tert-amyl alcohol reacts with ortho-phosphoric acid to obtain in due course both the amorphous and the crystalline α-SnP forms. The N2 sorption isotherm of the crystalline powder attains a hysteresis cycle proper of lamellar void structures over a long interval of relative pressures. The differential t-plot of this isotherm reveals the presence of two kinds of supermicropores: (i) a group of narrow slit-like pores with interlamellar void maxima arising at 0.712 nm, 0.774 nm, and 0.828 nm (these values suggest that the primary micropore filling of the interlayer space occurs with at most two layers of N2 indicating likewise the non-rigid character of the plates that are delimiting each slit-like pore), (ii) a second microporous contribution that involves the cooperative micropore filling of wider slit-like pores having widths in the interval 1.01–1.57 nm. The narrow supermicropores are formed by the interlamellar spacing between a pair of elementary α-Sn-P sheets, while the wider micropores are delimited by solid plates that may be built by the parallel stacking of several elementary sheets of α-Sn-P. BET and t surface areas of the α-SnP specimen both correspond to 20.2 m2 g−1, therefore confirming adsorption on flat surfaces and the absence of voids other than supermicropores. Application of Bragg’s equation to the X-ray diffraction pattern of the α-SnP sample renders an elementary interlayer distance of 0.80 nm that compares well with the theoretical 0.78 nm value. NMR spectra depict sharp crystalline peaks at −11.9 and −12.1 ppm for the amorphous and crystalline forms, respectively. SAXS measurements render crystallite sizes between 4 and 32 nm for the amorphous and crystalline forms. Crystalline α-SnP also depicts a wide-ranging fractal structure; this fact is confirmed by the consistent values of the surface fractal dimension obtained from application of the Neimark–Kiselev sorption approach over a long range of relative pressures.

SiO2 Xerogel: A Suitable Inert Support for Microbial Growth

A silica xerogel was prepared at pH 2 by the hydrolysis-condensation reactions of the sol-gel method. Silica xerogel was used as a support for the growth of two filamentous fungi: Aspergillus niger ATCC 9642 and Phanerochaete chrysosporium A594. In both cases, an apparent abundant mycelia growth (5.5 mg biomass/g dry xerogel and 4.7 mg biomass/g dry xerogel respectively) was observed. A phase of rapid consumption of glucose which lasted until 96 h of culture with sugar consumption rate of 0.075 mg sugar/g support h and 0.042 mg sugar/g support h respectively, was also observed. Scanning electron microscopy (SEM) showed a superficial colonisation of both strains even in the occasional imperfections and crevices of the xerogel. This novel application of sol-gel metallic oxide systems suggests the potential use of an inert and versatile support which could be valuable, for example for solid state fermentation fundamental studies.

Sol–gel copper–magnesia–silica mixed oxides

Sol-gel magnesia-silica mixed oxides were prepared with tetraethoxysilane TEOS and magnesium ethoxide as molecular precursors. While still at the fluid phase before gelation, a solution of copper acetylacetonate was added. After gelation, the solids thus obtained were sulfated in two ways: by conventional impregnation and by an in situ impregnation . with H SO . The solids were characterized by TPD NH and CO . The solids impregnated in situ, containing 4.0 and 6.7 24 3 2 mol% of MgO show a high acidity. The mixed oxides showed high specific surface area values when the gelation is carried out at pH 3; however, at pH 9, areas three times smaller were obtained. Their catalytic activity was studied using the decomposition of 2-propanol as a test reaction. q 2000 Elsevier Science B.V. All rights reserved. ¨ 22 sites on mixed oxides; 2-Propanol decomposition