Mesoporous Ordered Silica Films

This book introduces the fascinating world of self-assembly in mesoporous ordered silica films. Beginning from a single droplet, it guides the reader, in a step-by-step learning process, how to obtain and control ordered porous mesophases in thin films by varying only the precursor chemistry and the process. It explains, in great detail, how order control is achieved through chemical design and post-deposition processing, the latter of which is a unique property in materials science. The book places a special focus on silica, whose particularly complex chemistry enables order control over a range of different length scales. This book is suitable for students and researchers in the fields of sol-gel or colloidal chemistry and interested in the topics of self-assembly and mesoporous phases.

The fabrication of advanced electronic devices that operate on quantum effects requires the patterning of semiconductors on the scale of 50 A, which cannot be achieved by any of the currently available patterning technologies. This project pursued a novel approach: the fabrication of a self-assembling template which would allow the deposition of ordered arrays of germanium dots on silicon substrates, on length scales permitting the operation of quantum devices at room temperature. The template is the mesoporous silicate MCM-41, discovered by researchers at Mobil Chemical Corp. This material consists of highly ordered, two-dimensional, hexagonal arrays of very uniform pores in silicon dioxide, with diameters tunable from 20 A to over 100 A. If pore arrays of this material can be grown as thin films on silicon substrates, with the pores oriented normal to the substrate surface, the resulting structure will provide a template for the deposition of germanium dots. Germanium can then be deposited through the pores in the film and onto the silicon substrates by chemical or physical vapor deposition. The template film can then be etched away, leaving a hexagonally ordered array of germanium dots on the silicon substrate. Mesoporous silica films were grown on oxidized silicon substrates by acidic synthesis. The substrates were first patterned by optical lithography to produce vertical features with dimensions of the order of microns. The substrates were then coated with hydrophobic polymer monolayers to alter their surface energy. This monolayer was selectively removed from the horizontal surfaces of some of the substrates, leaving it only on the vertical surfaces of the patterned features. It was thought that the difference in surface energy between horizontal and vertical surfaces would induce the pores to align along the vertical surfaces.

Mesoporous materials are a class of molecules with a large and uniform pore size, highly regular nanopores, and a large surface area. This book is devoted to all aspects and types of these materials and describes, in an in-depth and systematic manner, the step-by-step synthesis and its mechanism, as well as the characterization, morphology control, hybridization, and applications, of mesoporous molecular sieves. In so doing, it covers silicates, metal-doped silicates, nonsilicates, and organic-inorganic hybrids. Although the emphasis is on synthesis, the expert authors also discuss characterization and applications, ranging from catalysis and biochemistry to optics and the use of these materials as templates for nanomaterial synthesis. Both the fundamentals and the latest research results are covered, ensuring that this monograph serves as a reference for researchers in and newcomers to the field.

Nanostructured materials with tailored properties are regarded as a fundamental element in the development of future science and technology. Research is still ongoing into the nanosized construction elements required to create functional solids. The recently developed technique, nanocasting, has great advantage over others in terms of the synthesis of special nanostructured materials by the careful choice of suitable elements and nanoengineering steps. This new book summarizes the recent developments in nanocasting, including the principles of nanocasting, syntheses of novel nanostructured materials, characterization methods, detailed synthetic recipes and further possible development in this area. The book focuses on the synthesis of porous solids from the viewpoint of methodology and introduces the science of nanocasting from fundamental principles to their use in synthesis of various materials. It starts by outlining the principles of nanocasting, requirements to the templates and precursors and the tools needed to probe matter at the nanoscale level. It describes how to synthesize nano structured porous solids with defined characteristics and finally discusses the functionalization and application of porous solids. Special attention is given to new developments in this field and future perspectives. A useful appendix covering the detailed synthetic recipes of various templates including porous silica, porous carbon and colloidal spheres is included which will be invaluable to researchers wanting to follow and reproduce nanocast materials. Topics covered in the book include: * inorganic chemistry * organic chemistry * solution chemistry * sol-gel and interface science * acid-base equilibria * electrochemistry * biochemistry * confined synthesis The book gives readers not only an overview of nanocasting technology, but also sufficient information and knowledge for those wanting to prepare various nanostructured materials without needing to search the available literature.

SBA-15 mesoporous silica thin films encapsulating Degussa P-25 TiO2 particles were synthesized using a block copolymer templating method. The thin films were characterized using small angle X-ray diffraction (XRD), variable angle spectroscopic ellipsometry (VASE), polarized light microscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The SBA-15/TiO2 thin films were examined for applications in advanced oxidation processes (AOP) and photovoltaics. A study of the photodegradation of 2,4-dichlorophenol (2,4-DCP) was used to assess the photoactivity of the thin films using UV-visible spectroscopy and high performance liquid chromatography (HPLC). It was shown that the thin films were 88.8% as photoactive as dispersions of P-25 TiO2 on a per photon basis. The SBA-15/TiO2 thin films were also dip-coated onto indium tin oxide (ITO) conducting glass substrates. These electrodes were placed in photoelectrochemical cells in order to determine the photovoltaic efficiency of the thin films. The thin films were shown to have photovoltaic efficiencies between 8.0 x 10 -4 to 0.26% depending on the amount of TiO2 present in the sample. It was also shown that there was an increase in photovoltaic efficiency of approximately 44% for SBA-15/TiO2 thin film electrodes when compared with nonporous silica electrodes containing P-25 TiO2 particles. The development of ordered crystalline mesoporous TiO2 thin films was also examined. Different methods of mesostructure stabilization were explored such as the addition of phosphates and the electrodeposition of nickel within the mesopores. The thin films were calcined to various temperatures, and the thermal effects were examined. The mesoporous TiO2 samples were characterized using small angle XRD, TEM, and SEM.

This book provides a comprehensive overview of the fundamental properties, preparation routes and applications of a novel class of organic–inorganic nanocomposites known as periodic mesoporous organosilicas (PMOs). Mesoporous silicas are amorphous inorganic materials which have silicon and oxygen atoms in their framework with pore size ranging from 2 to 50 nm. They can be synthesized from surfactants as templates for the polycondensation of various silicon sources such as tetraalkoxysilane. In general, mesoporous silica materials possess high surface areas, tunable pore diameters, high pore volumes and well uniformly organized porosity. The stable chemical property and the variable ability for chemical modification makes them ideal for many applications such as drug carrier, sensor, separation, catalyst, and adsorbent. Among such mesoporous silicas, in 1999, three groups in Canada, Germany, and Japan independently developed a novel class of organic–inorganic nanocomposites known as periodic mesoporous organosilicas (PMOs). The organic functional groups in the frameworks of these solids allow tuning of their surface properties and modification of the bulk properties of the material. The book discusses the properties of PMOs, their preparation, different functionalities and morphology, before going on to applications in fields such as catalysis, drug delivery, sensing, optics, electronic devices, environmental applications (gas sensing and gas adsorption), biomolecule adsorption and chromatography. The book provides fundamental understanding of PMOs and their advanced applications for general materials chemists and is an excellent guide to these promising novel materials for graduate students majoring in chemical engineering, chemistry, polymer science and materials science and engineering.

Ordered mesoporous films and membranes refer to materials in a thin layer geometry possessing pore diameters in the 2-50 nm range. Their extremely high surface areas (up to ~ 1000 m2/g) and precise tuning of pore sizes are among the many desirable properties that have made such thin films and membranes a focus of great attention for emerging applications in the synthesis of nanomaterials and separation membranes. Ordered mesoporous silica films are highly promising hosts to grow semiconductor nanoparticles with fine-tuned size if the film thickness, pore size and orientation can be controlled. This important potential use of ordered mesoporous silica materials is one major focus area of this dissertation. Ordered mesoporous silica thin films can also be grown as membrane layers on poroussupports possessing three-dimensionally interconnected pore structures. The investigation of ordered mesoporous silica membranes modified by amine groups for CO2 separation is another major focus area of this work. Potential application of these membranes is to capture CO2 from flue gas which is a critical step preceding a variety of CO2 sequestration approaches to curb its atmospheric emissions. We systematically investigated the effect of pore size and pore symmetry on the growth of CdS nanoparticles inside the functionalized ordered mesoporous silica hosts. The 2D hexagonal mesoporous silica, SBA-15 possessing 4-22 nm pores and 3D cubic SBA-16 with 5 nm pores was employed to investigate the effect of pore size and pore symmetry on the microstructure of resulting CdS nanoparticles. As the pore size of SBA-15 increased, an increase in the average size of the CdS nanocrystals was observed. The pore size distributions of the CdS/SBA-15 and CdS/SBA-16 composites suggested that CdS nanoparticles nucleated primarily in the micropores of SBA-15 and SBA-16 hosts. Furthermore, SBA-16 possessing smaller pores as compared to SBA-15 resulted in larger crystal sizes of CdS nanoparticles. These results provided new important insights into the mechanism of initial growth of CdS nanoparticles inside the pores of mesoporous silica hosts possessing different pore symmetries. Ordered mesoporous MCM-48 membranes possessing a cubic pore structure were prepared by solution growth method on -alumina supports. The surfactant was removed by calcination and Soxhlet extraction. The best quality membranes were obtained when the surfactant was removed by extraction. The membranes prepared on asymmetric - alumina supports displayed higher gas permeance (N2 permeance> 2x 10-7 mol/m2.s. Pa) than those fabricated on symmetric supports (N2 permeance

The original properties of mesoporous molecular sieves are so unique that the design of most existing catalysts could be reconsidered. It might indeed be of interest to introduce MMS either as a support or as the active phase, merely on the basis of their high surface areas, narrow pore size distribution and flexibility in composition. The recent literature provides examples of MMS based catalysts of many types such as acid-base solids, supported metals and supported oxides, mixed oxides, anchored complexes and clusters, grafted organic functional groups and others. Examples of all these developments are documented in the present proceedings including some spectacular new proposals. The new metallic (Pt) mesophases are specially worth mentioning because they represent a new approach to producing non-supported highly dispersed metals. In these proceedings the reader will find feature articles and regular papers from many worldwide groups, covering all aspects of synthesis, physical characterization and catalytic reactivity of MMS and their chemically modified forms. It is actually remarkable that this recent development brought together an even broader spectrum of scientists from traditionally unrelated fields such as those of liquid crystals, surfactants, sol-gels, amorphous oxides and mixed oxides, solid state, adsorbents and heterogeneous catalysts. Obviously, this is a fast-growing research area which triggers the imagination and creativity at the cross-road between material design, molecular surface tailoring and catalytic applications.