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Hydrothermal Phase Transformation of Bicontinuous Cubic Mesoporous Material AMS-6
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Nanoteknologi och funktionella material. (Nanotechnology & Functional Materials)
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Nanoteknologi och funktionella material. (Nanotechnology & Functional Materials)
2008 (engelsk)Inngår i: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 20, nr 12, s. 3857-3866Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

The controlled synthesis of ordered anionic surfactant templated mesoporous silica with cubic Ia3 structure (AMS-6) is reported via prolonged periods of hydrothermal treatment (HT). The cubic Ia3 mesophase transforms to hexagonal p6mm after 8 days of HT at 100 °C. Surprisingly, the hexagonal phase is stable only for a limited period after which a reversal to the cubic Ia3 mesostructure is observed. Characterization methods such as powder X-ray diffraction (XRD), electron microscopy (SEM, TEM), N2-isotherms, magic-angle spinning (MAS) 29Si NMR spectroscopy, and thermogravimetric analysis (TGA) have been employed to follow structural and textural changes of the materials prepared. Data show that the resultant mesostructure and its textural properties are highly dependent on the period of HT with less unit-cell shrinkage on calcination after extensive HT. Furthermore, evidence of two different solid−solid phase mechanisms during HT is presented. The initial transition is consistent with a restructuring of the surfactant packing and a depletion of the organic moieties from the organo-silica wall as evident from 29Si NMR spectroscopy. The return to the bicontinuous cubic phase is driven by changes in charge matching at the organic−inorganic interface as a result of increases in the polymerization of the silica wall. The textural properties, and in particular the presence or absence of surface porosity, has been controlled through variations in hydrothermal treatment. These are associated with specific growth directions of cubic AMS-6 crystals. The synthetic method described allows us to easily prepare phase pure and intermediate mesostructured nanoparticles.

sted, utgiver, år, opplag, sider
2008. Vol. 20, nr 12, s. 3857-3866
HSV kategori
Forskningsprogram
Teknisk fysik med inriktning mot nanoteknologi och funktionella material
Identifikatorer
URN: urn:nbn:se:uu:diva-98531DOI: 10.1021/cm702440nISI: 000256854800015OAI: oai:DiVA.org:uu-98531DiVA, id: diva2:174824
Tilgjengelig fra: 2009-02-25 Laget: 2009-02-25 Sist oppdatert: 2017-12-13bibliografisk kontrollert
Inngår i avhandling
1. Novel Syntheses, Structures and Functions of Mesoporous Silica Materials
Åpne denne publikasjonen i ny fane eller vindu >>Novel Syntheses, Structures and Functions of Mesoporous Silica Materials
2010 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

The field of mesoporous silica has been studied for about 20 years but it is still an area attracting a lot of attention. The use of novel templating molecules and several issues related to the synthesis and fine structural details are still poorly understood. These aspects are of special relevance to the theme of this thesis, which includes novel work on three fronts; the synthesis, characterization and applications of mesoporous materials.

The work described in this thesis aims to contribute to the mesoporous field by developing novel methods of mesoporous silica synthesis without relying on surfactant micelles as the templating agent but focusing instead on the stacking arrangement of aromatic molecules such as folic acid. The novel route presented here leads to 2D hexagonal structures with p6mm symmetry possessing high mesoporosity and large surface areas. The versatility of this route at various synthesis temperatures and using hydrothermal treatments has also been investigated.

A novel strategy is also proposed for the synthesis of mesocaged materials with Pm3n symmetry structures. The mechanism relies on the penetration of the neutral propylamino moiety of a co-structure directing agent into the hydrophobic core of the surfactant micelles. Beside these novel pathways, the effect of hydrothermal treatment (HT) at 100 oC on the 3D cubic Ia3d structure (AMS-6) over a long period of time was also examined, and the results show a phase transformation from a 3D cubic Ia3d to a 2D hexagonal p6mm structure and a return to the 3D cubic Ia3d structure at a later stage in the synthesis. This unexpected result is discussed.

In this work, the detailed structural characterization of mesoporous materials using electron microscopy techniques is an important task. In particular, to extend previous knowledge, the fine structural details of mesocaged materials possessing Pm3n symmetry prepared with various amphiphilic surfactants under acidic and alkaline conditions has been investigated using electron crystallography and sorption studies. The results show subtle fine structural differences with materials prepared under alkaline conditions exhibiting the largest mesocage sizes. The cage and window sizes are primarily determined by the charge density of the surfactant and the thickness of the hydration layer surrounding the surfactant micelles.

The relationship between the mesoporous structure and its function has been investigated by evaluating the rate of release of amphiphilic molecules, used as model molecules, from the internal pore structures of mesoporous materials with different pore geometries. In a similar study, the rate of proton diffusion from a liquid surrounding the mesoporous nanoparticles into the pore system of AMS-n was also assessed. The results show that the diffusion coefficients for the proton absorption process are higher than those for the release of the surfactant template molecules, with more complex 3D mesocaged particles showing the highest diffusion coefficients in both cases.

Finally, the quantity of CO2 adsorption was measured by modifying the internal surfaces of mesocaged material with n-propylamino groups. Results show that the cage-connecting window sizes limit the surface coverage of n-propylamino groups by pore blocking and affect the volume of CO2 adsorption. In addition, at the molecular level, CO2 adsorption shows physisorption or chemisorption depending on the localized distribution of n-propylamino groups, as studied by in-situ infrared spectroscopy.

sted, utgiver, år, opplag, sider
Uppsala: Acta Universitatis Upsaliensis, 2010. s. 82
Serie
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 733
HSV kategori
Forskningsprogram
Materialvetenskap
Identifikatorer
urn:nbn:se:uu:diva-122289 (URN)978-91-554-7786-8 (ISBN)
Disputas
2010-05-21, Siegbahnsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:00 (engelsk)
Opponent
Veileder
Tilgjengelig fra: 2010-04-28 Laget: 2010-04-07 Sist oppdatert: 2010-05-18

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