Academic literature on the topic 'Mesoporous structures'

Hollow-structured mesoporous silica has wide applications in catalysis and drug delivery due to its high surface area, large hollow space, and short diffusion mesochannels. However, the synthesis of hollow structures usually requires sacrificial templates, leading to increased production costs and environmental problems. Here, for the first time, amino-functionalized mesoporous silica hollow spheres were synthesized by using CO2 gaseous bubbles as templates. The assembly of anionic surfactants, co-structure directing agents, and inorganic silica precursors around CO2 bubbles formed the mesoporous silica shells. The hollow silica spheres, 200–400 nm in size with 20–30 nm spherical shell thickness, had abundant amine groups on the surface of the mesopores, indicating excellent applications for CO2 capture, Knoevenagel condensation reaction, and the controlled release of Drugs.

A mesoporous ZSM-5 monolith several millimetres in size has been synthesized employing the template method and using a carbon aerogel with uniform mesopores. Measurement of the pore-size distribution using nitrogen adsorption showed a bimodal pore system of mesopores and micropores whose average pore widths were 8 nm and 0.51 nm, and whose volumes were 0.09 cm3/g and 0.34 cm3/g, respectively.

The high porosity and uniform pore size of mesoporous oxide films offer unique opportunities for microelectromechanical system (MEMS) devices that require low density and low thermal conductivity. This paper provides the first report in which mesoporous films were adapted for MEMS applications. Mesoporous SiO2 and Al2O3 films were prepared by spin coating using block copolymers as the structure-directing agents. The resulting films were over 50% porous with uniform pores of 8-nm average diameter and an extremely smooth surface. The photopatterning and etching characteristics of the mesoporous films were investigated and processing protocols were established which enabled the films to serve as the sacrificial layer or the structure layer in MEMS devices. The unique mesoporous morphology leads to novel behavior including extremely high etching rates and the ability to etch underlying layers. Surface micromachining methods were used to fabricate three basic MEMS structures, microbridges, cantilevers, and membranes, from the mesoporous oxides.

The ability to decorate silicate surface with different organoalkoxysilanes creates powerful new capabilities for catalyst, adsorbents and chemical separation. Mesopororus silica, MCM-41 was modified by grafting of amino and mercaptopropyl functional group. The structures of these materials were characterized by using Fourier Transform Infrared Spectroscopy (FT-IR), and X-Ray diffraction (XRD). The samples were found to exhibit structural properties similar to those reported earlier. Significant functional groups of the modified mesoporous silicates were found in the spectrum of FT-IR. Standard structure of mesoporous silicates were found to be preserved at planar [100] of XRD difractogram of mesoporous silicates. Adsorption of Cu (II) ions were done under different temperatures, initial concentrations and pH. Adsorption process also was determined from kinetic point of view and was found to be better fitted to pseudo second order of kinetic model.

In this paper, we report a simple strategy for fabricating mesoporous magnesium borate (2MgO·B2O3) microspheres. We employed sodium dodecyl sulfate (SDS) in one system, as a template for the controlled growth of mesoporous 2MgO·B2O3microspheres. The products were characterized by XRD, SEM, TEM, EDS, N2sorption and FT-IR. SEM and TEM observations indicate magnesium borate products are composed of a large number of hollow microspheres, with diameters of 1.0~1.5 μm, which are in fact built from fibers with lengths of 100~150 nm. The N2sorption results show that the products have meso-structures. The average pore diameter is 27 nm. The BET surface area is about 53.03 m2/g, and the pore volume is 0.37 m3/g. It was found that SDS plays a key role for the formation of mesoporous structure. A possible mechanism was proposed to interpret the formation of the mesoporous structure. The mesopores will endow the hierarchical microspheres with novel application potentials.

Mesoporous silica materials have attracted great research interest for various applications ranging from (bio)catalysis and sensing to drug delivery. It remains challenging to prepare hollow mesoporous silica nanoparticles (HMSN) with large center-radial mesopores that could provide a more efficient transport channel through the cell for guest molecules. Here, we propose a novel strategy for the preparation of HMSN with large dendritic mesopores to achieve higher enzyme loading capacity and more efficient bioreactors. The materials were prepared by combining barium sulfate nanoparticles (BaSO4 NP) as a hard template and the in situ-formed 3-aminophenol/formaldehyde resin as a porogen for directing the dendritic mesopores’ formation. HMSNs with different particle sizes, shell thicknesses, and pore structures have been prepared by choosing BaSO4 NP of various sizes and adjusting the amount of tetraethyl orthosilicate added in synthesis. The obtained HMSN-1.1 possesses a high pore volume (1.07 cm3 g−1), a large average pore size (10.9 nm), and dendritic mesopores that penetrated through the shell. The advantages of HMSNs are also demonstrated for enzyme (catalase) immobilization and subsequent use of catalase-loaded HMSNs as bioreactors for catalyzing the H2O2 degradation reaction. The hollow and dendritic mesoporous shell features of HMSNs provide abundant tunnels for molecular transport and more accessible surfaces for molecular adsorption, showing great promise in developing efficient nanoreactors and drug delivery vehicles.

Materials with mesoscale structural characteristics have attracted great attention across the fields of chemistry, physics, and materials science. A typical example is mesoporous silica, which are synthesized in water/surfactant/silica systems, and has well-defined mesopores resulting in high surface area. Mesoporous silicas have two defining structural characteristics: (i) disorder at the atomic scale, i.e. only short-range order; and (ii) distinct order at the mesoscale, i.e. long-range order. Atomic-scale structural characterization by common diffraction techniques, such as X-ray single crystal diffraction, is challenging for these partially ordered materials. This is because of the difficulty in obtaining large (> 10 µm) single crystals, and because large-distance periodic features cause diffraction intensities to fall off rapidly with scattering angle, so that only limited small-angle data are available. On the other hand, transmission electron microscopy (TEM) is a powerful tool for structural characterization at the mesoscale level due to the stronger interaction of electrons with matter compared to X-rays, enabling the use of very small crystals. In particular, high-resolution TEM (HRTEM) images give the phase and the amplitude of the crystal structure factors experimentally, leading to a 3D structural model by electron crystallography. Cage-type anionic-surfactant-templated mesoporous silicas display rich structural diversity. Among them, cage-type mesoporous silica with tetrahedrally close-packed (TCP) structures can be described by four types of polyhedra, 5^12, 5^12 6^2, 5^12 6^3, and 5^12 6^4.[1] A variety of structural polymorph have been observed and characterized by TEM. Their structures show a close resemblance to the Frank-Kasper phases, which are well known in intermetallic compounds. We found mesoporous silica with dodecagonal quasicrystalline ordering as one of the TCP structures (Figure).[2] In this presentation, I will discuss structural characterization of aperiodic crystals at the mesoscale, such as mesoporous silicas and binary colloidal crystals, by electron microscopy.

In this study, a series of Co3O4 nanoparticle-functionalized mesoporous SiO2 (Co–SiO2) were successfully synthesized via a spontaneous infiltration route. Co species were firstly infiltrated into the confined spaces between the surfactant and silica walls, with the assistance of grinding CoCl3·6H2O and the as-prepared mesoporous SiO2. Then, Co3O4 nanoparticles (NPs) were formed and grown in the limited space of the mesopores, after calcination. Structures, morphologies, and compositions of the materials were characterized by X-ray diffraction, transmission electron microscopy, energy dispersion spectrum, N2 adsorption, and Fourier transform infrared spectra. Results showed that the high content of Co (rCo:Si = 0.17) can be efficiently dispersed into the mesoporous SiO2 as forms of Co3O4 NPs, and the structural ordering of the mesoporous SiO2 was well-preserved at the same time. The Co3O4 NP functionalized mesoporous SiO2 materials were used as Fenton-like catalysts for removing methylene blue (MB) from aqueous solutions. The catalyst prepared at rCo:Si = 0.17 could completely remove the high-concentration of MB (120 mg·L−1), and also showed an excellent performance with a removal capacity of 138 mg·g−1 to 180 mg·L−1 of MB. Catalytic mechanisms were further revealed, based on the degradation results.

Background: Silver nanosystems have attracted considerable attention for numerous applications in optoelectronics. The localized surface plasmon of silver nanoparticles embedded into mesoporous titania gives rise to an enhancement of local optical field in the vicinity of Ag nanoparticles which act as efficient light-trapping components, resulting in a visible wavelength-dependent photocurrent. Objective: In this paper, we synthetized patterned nanocomposites formed by titania mesoporous thin films loaded with alkanethiol functionalized Ag nanoparticles and we demonstrated that these stable and accessible nanostructures possess a photocurrent response. Method: Mesoporous thin films are created by combining sol-gel synthesis and template selfassembly. Based on a photolithography technique, silver nanoparticles were selectively photodeposited and then stabilized with octanethiols. Current vs. voltage curves with and without light were compared, where selective light wavelength measurements were achieved by using visible bandpass filters. The optofluidic behavior was evaluated by placing a drop of solutions on the mesoporous film. Results: We demonstrate photocurrent in these mesoporous thin film structures decorated with chemistabilized Ag nanoparticle-based conductive arrays, with significantly enhanced photocurrent peak at the plasmon resonant wavelength around 540 nm. Our findings offer a possibility to perform improved fluid detection with silver-mesoporous titania electronic devices. Conclusion: We showed that an optofluidic sensitive nanocomposite circuit consisting of alkanethiol- functionalized metal nanoparticles embedded in a mesoporous oxide thin film matrix can be produced.

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.

Les matériaux mésoporeux sont devenus des nanomatériaux d’une grande importance, et le contrôle des structures des matériaux mésoporeux est essentiel pour une variété d'applications pratiques. Les matériaux «cœur/coquille» structurés sont un type de matériaux hybrides qui non seulement possèdent les propriétés des composants individuels, mais présentent également de effets synergiques entre le «cœur» et la «coquille». La conception de matériaux mésoporeux et «cœur/coquille» structurés pour les appliquer avec succès dans la pratique devrait être une force de progrès importante pour le développement continu. Cette thèse se concentre principalement sur deux aspects: (1) une conception de matériaux mésoporeux «cœur/coquille» structurés en vue de résoudre les problèmes de synthèse, qui entravent leurs nouvelles applications et (2) l'application de matériaux mésoporeux dans la capture du CO2 cyclique pour améliorer la durabilité des sorbants de CO2 en prenant avantage du concept de «cœur/coquille». Visant le cyclage de l’hydroxyde de calcium, une technologie attrayante pour la capture du CO2 à grande échelle, nous avons établi un nouveau mésoporeux «cœur/coquille» structuré à base de CaO qui présentait une grande stabilité et d'excellentes performances de résistance à l’attrition, attribuées aux avantages des matériaux mésoporeux et à la configuration de «cœur/coquille». Notre procédé de fabrication peut être facilement réalisé à grande échelle et répond aux exigences de la circulation entre des réacteurs en lit fluidisé. Les nanoparticules métalliques ont normalement tendance à se coaguler ensemble dans des réactions catalytiques, et sont difficiles à séparer. Par conséquent, nous avons démontré une synthèse de microsphères Fe3O4@C-Pd@mSiO2 à composants multiples et polyvalentes avec une structure «cœur/coquille» bien définie et des nanoparticules catalytiques de Pd confinées, et ayant des canaux mésoporeux ordonnés et facilement accessibles. Récemment, des méthodes diverses ont été proposées pour fabriquer un revêtement de matériaux mésoporeux sur un cœur par un processus de «soft-templating». Cependant, les diamètres des mésopores générés sont généralement très faibles (< 3 nm), ce qui peut limiter leurs nouvelles applications. Ici, nous avons réalisé la synthèse de microsphères «cœur/coquille» structurées superparamagnétiques possédant une coquille externe de silice mésoporeuse ordonnée à larges pores (4,5 nm), en adoptant un copolymère tribloc comme agent tensioactif directeur de structure.
Mesoporous materials, especially ordered ones have become ones of great importance nanomaterials, which possess regular, uniform and interpenetrating mesopores in nanoscale. Morphology and texture controls towards mesoporous materials are critical for a variety of practical applications, the ultimate goal of which are the realization of their functional design. Core/shell composite materials are a type of functional hybrid materials which not only possess the properties of the individual components, but also exhibit some new or synergistic effects between the core and the shell. The design of mesoporous materials with unique core/shell configuration and multifunctions to make them successfully applied in practice, should be an important driving force for the continuous development of current material science. This thesis mainly focuses on two aspects: (1) careful design of core/shell structured mesoporous materials in order to solve the problem and difficulty in synthesis, which hinders their further applications and (2) application of mesoporous materials in cyclic CO2 capture to enhance the durability of CO2 sorbents by taking advantage of the core/shell concept. Aiming at the calcium looping cycle, an attractive technology for large-scale CO2 capture, we have prepared novel mesoporous core/shell structured CaO-based sorbents which exhibit highly stable cyclability and excellent attrition-resistance performances, attributed to advantages of both mesoporous materials and unique core/shell configuration. Our fabrication method could easily be realized in large-scale and meet the requirements of circulating fluidized bed reactors. Owing to their high surface energies, metallic nanoparticles normally tend to aggregate together during catalytic reactions, and their separation from a complex heterogeneous system is another obstacle. In this regards, we have demonstrated a facile and versatile synthesis of multicomponent and multifunctional microspheres Fe3O4@C-Pd@mSiO2 with well-defined core/shell structures, confined catalytic Pd nanoparticles and accessible ordered mesopore channels. Recently, various methods have been proposed for coating mesoporous shells on cores by soft-templating process. However, the generated mesopores are usually very small (< 3 nm), which may limit their further applications. In this work, we have accomplished the synthesis of superparamagnetic core/shell structured microspheres possessing an outer shell of ordered mesoporous silica with large pores (4.5 nm) by adopting triblock-copolymer Pluronic P123 as soft-template.

Doutoramento em Ciência e Engenharia de Materiais
Os materiais multiferróicos possuem simultaneamente pelo menos duas das três propriedades ferróicas: i) ferroelectricidade; ii) ferromagnetismo; e / ou iii) ferroelasticidade. Estes materiais têm despertado considerável interesse na indústria microeletrónica devido ao seu potencial para serem usados em dispositivos de armazenamento de informação com elevada capacidade e eficiência energética. A constante procura pela redução do tamanho e aumento da funcionalidade dos dispositivos, imposta pela Lei de Moore, exige materiais ferróicos, na forma de filmes finos e multifuncionalidade. Contudo, à medida que a espessura dos filmes diminui, as propriedades ferróicas ficam comprometidas em virtude de constrangimentos provocados pelo substrato ou outros efeitos. Neste contexto, esta tese estuda a possibilidade de utilizar a porosidade em filmes funcionais para criar sistemas compósitos multifuncionais. Assim, desenvolveram-se estratégias para a preparação de filmes de ferroeléctricos, ferromagnéticos e multiferroícos com porosidade uniforme e ordenada. O efeito dessa porosidade foi avaliado nas propriedades físicas locais e macroscópicas. Foram estudados óxidos multimetálicos com estrutura de perovisquite ou de espinela por serem promissores para aplicação em sensores; atuadores; condensadores; memórias; etc. Escolheu-se uma metodologia química em que os filmes são depositados por técnica de mergulho em soluções sol-gel contendo um copolímero em bloco que se organiza espontaneamente conjuntamente com os precursores durante o processo de evaporação. PbTiO3 foi a composição inicialmente escolhida para entender o efeito da nanoporosidade nas propriedades eléctricas locais por ser o material piezoeléctrico protótipo que possui o mais alto coeficiente piezoeléctrico conhecido. Assim, foram preparados filmes nanoporosos e densos de PbTiO3 com espessura de cerca de 100 nm e diâmetro de poro na ordem dos 50 nm. A presença da nanoporosidade contribui para a cristalização precoce da fase cristalina por aumento local da temperatura durante a decomposição do copolímero e / ou por funcionarem como núcleos de cristalização. Consequentemente, os fimes porosos exibem melhores coeficientes piezoeléctricos e baixo campo coercivo, sendo mais fácil inverter a direção da polarização por efeito do campo elétrico. Sendo a porosidade um meio para atingir propriedades melhoradas, esta pode funcionar como uma ferramenta para ajustar as propriedades ferroeléctricas à aplicação desejada. Todos os resultados de PFM foram previstos através de modelação teórica usando o modelo de elementos finitos. Foi também investigada a preparação de filmes porosos de titanado de bário enquanto protótipo de um ferroeléctrico sem chumbo. Neste contexto, foi avaliado o efeito de vários parâmetros, tais como: i) o aquecimento da solução de precursores; ii) adição de precursores inorgânicos / solventes orgânicos; e iii) envelhecimento da solução inicial, na estrutura final dos filmes.Verificou-se que o uso de uma solução fresca de precursores sem qualquer ciclo de aquecimento contribuía para uma melhor organização dos filmes porosos de BaTiO3. Verificou-se também que o tamanho dos blocos num copolímero à base de poliestireno e poli(óxido de etileno) era preponderante para a ordem e microestrutura cristalina dos filmes finais. Copolímeros em bloco com cadeias de bloco mais longas são preferíveis para obter uma estrutura ordenada e aparentemente desempenham um papel na cristalização precoce da fase ferroeléctrica tetragonal, contribuindo para uma melhoria da resposta piezoeléctrica. Em analogia com o PbTiO3, os resultados indicam que nos filmes nanoporosos de BaTiO3, a cristalização ocorre a temperaturas mais baixas do que nos filmes densos. Utilizou-se a deposição electroquímica para inserir nanopartículas metálicas de cobalto dentro dos poros dos filmes de BaTiO3. O carácter multiferróico dos filmes foi constatado através da avaliação nanoscópica das propriedades elétricas e pela medida das propriedades magnéticas macroscópicas à temperatura ambiente. Verificaram-se as dificuldades de conseguir um preenchimento uniforme dos poros e de otimizar a interface entre as duas fases ferróicas. Assim com vista a tentar ultrapassar estas dificuldades, prepararam-se filmes mais finos e em que a porosidade estivesse devidamente organizada, com poros perpendiculares à superfície. Conceberam-se filmes nanotexturados ordenados de óxidos multimetálicos com propriedades ferroelétricas, ferromagnéticas e multiferróicas com espessuras e texturas de dimensão inferior a 100 nm. As composições escolhidas foram PbTiO3, CoFe2O4 e BiFeO3. Os filmes finos porosos nanotexturados PbTiO3 apresentaram a fase cristalográfica tetragonal mesmo em espessuras de filme de 22 nm. Os filmes finos de CoFe2O4 apresentaram uma orientação preferencial no plano e elevadas magnetizações de saturação. Deduziu-se que os filmes teriam uma impureza ferromagnética compatível com uma liga metálica rica em platina. A presença desta impureza não só melhora o desempenho magnético dos filmes mas também fornece uma forte evidência para a potencial aplicabilidade dos filmes de CoFe2O4 como catalisadores para a oxidação de hidrocarbonetos através do mecanismo de Mars-Van-Krevelen. Foram também preparados filmes finos porosos nanotexturados de BiFeO3, com 66 nm de espessura e tamanho médio de diâmetro de 100 nm. Verificou-se o caráter multiferróico destes filmes e mais uma vez a melhoria clara das propriedades eléctricas locais induzida pela porosidade. A estrutura porosa também tem um efeito positivo nas propriedades magnéticas no plano, mostrando uma componente ferromagnética 50% maior que a medida em filmes densos. Verificou-se também que porosidade dos filmes de BiFeO3 pode ter interesse para aplicações fotocatalíticas, conjugando reduzido valor do hiato óptico direto (2.58 eV) com relativamente elevada área porosa (ca. 57 %). Para testar a aplicabilidade dos filmes nanotexturados na construção de um filme multiferróico compósito, uma matriz porosas ferroelétricos (BaTiO3) foi funcionalizada por preenchimento dos poros com nanopartículas ferromagnéticas de níquel. A estratégia de funcionalização dos poros foi a deposição por arrastamento com CO2 supercrítico, seguida de redução da espécie metálica a 250 ºC ativada por etanol. Pequenas nanopartículas de níquel com cerca de 21 nm foram depositadas dentro dos poros da matriz porosa, tendo-se verificado as propriedades estruturais e magnéticas do compósito. Esta tese, provou a adequação desta metodologia química de baixo custo na concepção de materiais multifuncionais, criando novas perspectivas para a indústria da microeletrónica na sua abordagem contínua de redução de tamanho e custo, enquanto aumenta a complexidade de funcionamento.
Multiferroic materials, exhibiting simultaneously at least two of the three ferroic properties: i) ferroelectricity; ii) ferromagnetism; and iii) ferroelasticity, have attracted considerable interest from the microelectronics industry. Due to their potential, these materials can be used in information storage applications with significantly high energetic efficiencies and elevated capacities. During the last decades and owing the increasing need for miniaturization of electronic devices, the ferroic materials, mainly in the format of thin films, have been extensively studied both theoretically and experimentally. However, as the film thickness decreases the ferroics properties progressively decreases due to the in-plane strain relaxation constrained by the substrate or others intrinsic and extrinsic effects. Within this context, here we exploit the role of nanoporosity on local and macroscopic properties of ferroelectrics, ferromagnetic and multiferroics thin films. Although, porosity is normally considered as a defect (or secondary phase) having usually a detrimental effect on the electrical macroscopic response; it can also be regarded as an asset, in terms of: i) density (light weight) and ii) capacity to host other functionality/ies. Oxides with perovskite and spinel structures are promising materials because they possess extraordinarily useful properties namely to be used as piezoelectrics sensors, as ferroelectric actuators, capacitors and memories, in high-strength dielectrics, for ferromagnetics or even multiferroics. Among the bottom-up approaches, the sol-gel method and evaporation-induced self-assembly methodology are the most suitable, low-cost and easy preparation method to prepare nanoporous and nanopatterned thin films of different compositions. PbTiO3 is the chosen composition to understand the role of the nanoporosity on the local electric properties. Thus, nanoporous and dense ferroelectric PbTiO3 thin films with 100 nm and ~ 50 nm pore size formed using a block polymer as a structure-directing agent are prepared. The presence of nanoporosity markedly affects the microstructure, crystallization and ferroelectric film properties. The crystallization of tetragonal phase is enhanced in nanoporous films. It seems that the decomposition of the block-copolymer in porous films triggers the crystallization of the perovskite phase at low temperatures via the local increase of temperature. Moreover, pores may work as initiators of the crystallization. Consequently, nanoporous films with improved tetragonality exhibit enhanced piezoelectric coefficients, switchable polarization and low local coercivity. In fact, the porosity induces instability in the dipole-dipole interactions and consequently the reverse polarization can be favoured for low bias values. By providing a means of achieving enhanced properties, nanoporosity may work as a tool to tune electric properties to the desired ferroelectric application. All the PFM results were supported by theoretical modelling using Finite Element Model. To have a more complete picture of the role of the nanoporosity on the crystallization and electric properties, the procedure is applied to prepare a nanoporous lead-free material, BaTiO3. However, this expantion was not trivial whereas the crystallization temperature of the tetragonal phase necessary for the ferroelectric properties is much higher than the decomposition temperature of the block-copolymer used as template. From this, several parameters such as: heating the solution, addition of inorganic precursors / organic solvent and aging time of solution are studied in order to understand the effect of these on the micellization process and consequently in the final porous BaTiO3 films. Based on the results of this study, for this specific multimetallic oxide system it is preferable to use a very fresh solution, without any heating cycles. In addition, block-copolymers based on polystyrene and poly(ethylene oxide) with different block sizes are used to investigate their influence on the order and crystalline microstructure of the final films. Blocks-copolymers with longer block chains are preferable to get an ordered structure and apparently play a role on the earliest crystallization of the tetragonal ferroelectric perovskite phase, contributing to an enhancement of the piezoelectric response. Similarly to PbTiO3, our results indicate that in nanoporous BaTiO3 films the crystallization occurs as well before in dense films. Moreover, besides providing a means of achieving enhanced properties, nanoporosity may work as a tool to tune electric properties to the desired ferroelectric application. BaTiO3 nanoporous films are tested as a kind of “golf course” full of holes to accommodate ferromagnetic particles. In this way, electrochemical deposition is used to insert the cobalt metal nanoparticles into the pores of BaTiO3 films. Films containing cobalt particles within the pores are obtained and piezoelectric and ferromagnetic properties are evaluated. For many applications would be a challenge to prepare ferroelectric thin films with lateral sizes well below 100 nm. Furthermore, the design of nanofeatures, uniformed in size and shape at a reasonable large-range order, i.e. “nanopatterning”, would extend their utility for electronic devices and integrated circuits, which require that each pixel feature can be individually addressable. Additionally, nanopatterned porous ferroelectric thin films may be interesting to develop vertical composite structures with perfect strain coupling at the interface. Thus, and using the chemical self-assembly method, different functional nanopatterned porous thin films: PbTiO3, BiFeO3 and CoFe2O4 are designed. Nanopatterned PbTiO3 thin films display the tetragonal ferroelectric crystallographic phase even when the films are as thin as 22 nm. CoFe2O4 thin films present a preferential in-plane orientation. High saturation magnetizations (close or even higher than in bulk CoFe2O4) are determined in all films, pointing to the presence of a ferromagnetic impurity compatible with a platinum-rich metal alloy. The presence of this impurity not only enhances the magnetic performance but also provides evidence for the catalytic activity of these CoFe2O4 films for hydrocarbon oxidation through a Mars-Van-Krevelen mechanism. For the BiFeO3 composition, crystalline nanopatterned BiFeO3 layers with 66 nm of thickness and average pore diameter of 100 nm at 600 ºC are obtained. The large vertical porosity markedly enhances the local electric and macroscopic magnetic properties when compared with the dense counterparts. The porous structure also has a positive effect on the parallel magnetic characteristics of the system, displaying a 50% larger ferromagnetic component and enhanced remanent magnetization when compared to the dense thin films counterpart. The porosity is also important for the photocatalytic applications conjugating the smallest direct band gap (2.58 eV) and extended porous area (ca. 57 %). The nanopatterned thin films allow the exploitation of a new concept to prepare multiferroic nanocomposite thin films. The multiferroic films based on in two chemical-based bottom-up steps, including: i) the formation of a porous ferroic matrix and ii) the accomodation of nanoparticles from another ferroic phase within the pores. Hexagonal-arranged pores with diameter of ca. 95 nm, running perpendicularly to the substrate are filled with nickel nanoparticles using the supercritical fluid deposition technique from reduction of hydrated nickel nitrate in a supercritical CO2-ethanol mixture at 250 ºC. Small nickel nanoparticles with ca. 20 nm are deposited inside the pores of the porous matrix. Structural and magnetic properties proved the coexistence of both phases. The chemical based methodology offers thus an excellent control of the physical and chemical properties of nanostructured materials such as: stoichiometry, thickness, size, array and porous distribution. Moreover the self-assembly of block-copolymers provides a versatile platform to prepare functional nanostructured materials, namely mesostructured oxide thin films, due to their capability to form large pores and thick walls, apart from being industrially available and hazard-free. Additionally, the chemical-assembly method can allow the direct nanopatterning of large substrate areas with a functional oxide at a cost-effective price, in the absence of expensive equipment or etching processes (which typically affect negatively the ferroic properties).Besides, the functional properties of the porous films by themselves, the porous films are extremely promising to achieve multiferroic composites.

Thesis (Ph.D.)--Kent State University, 2008.
Title from PDF t.p. (viewed Dec. 17, 2009). Advisor: Mietek Jaroniec. Keywords: mesoporous, FDU-1, SBA-16, organosilicas, pendant groups, bridging groups, adsorption, isocyanurate, template removal, cage-like structures. Includes bibliographical references (p. 219-238).

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A pesquisa no desenvolvimento de estruturas do tipo micro-mesoporosas tem por intuito a obtenção de materiais porosos com características superiores, uma vez que busca unir a ácidez elevada da zeólita MCM-22, com o sistema de mesoporos, da peneira molecular MCM-41, consequentemente melhorar a difusão de moléculas volumosas. Nesta pesquisa foram sintetizadas as seguintes estruturas porosas: Inicialmente foi obtido o percussor lamelar MCM22-(P) com razão molar SiO2\Al2O3 = 30 e ativada para obter sua forma zeólítica MCM-22 a 550 0C por 5 horas. Em seguida, foi realizada a síntese do material micro-mesoporoso do tipo MCM-22/MCM-41, tratando 2 g da zeólita MCM-22, com uma solução de 25 mL de brometo de cetiltrimetilamônio (CTABr) a 10 % em massa, onde o material permaneceu em estufa a 1100C por 7 dias. Com o intuito de obter um novo material com melhor organização estrutural, utilizou-se a MCM-22 nas seguintes proporções (5%, 10% e 15%), permanecendo em estufa a 300C por 24 horas, sendo ativado em corrente de ar por 5500C por 5 horas. Os resultados das caracterizações de difratometria de raios-X, evidenciaram a formação do precursor MCM-22 (P) e sua forma zeólítica MCM22, com os picos da topologia MWW. A curvas obtidas por meio da análise termogravimétrica (TG/DrTG), demostraram as perdas de massa da água e demais adsorvatos. As micrografias (MEV), apresentou formato toroidal com depreciamento na região central para a MCM-22. Por meio dos resultados de adsorção física de N2, verifica-se que as zeólitas MCM-22, com isotermas do tipo I e loop de histerese do tipo H4. A partir dos difratogramas de raios - X para os materiais micro-mesoporosos foi possível observar a formação das estruturas porosas, com a identificação dos picos de reflexão pertinentes a fase microporosa da MCM-22 e da peneira molecular MCM-41, coexistindo em uma única fase estrutural. As imagens obtidas por MEV, detectam a formação de aglomerados de partículas da fase mesoporosa sendo constituída em torno da fase microporosa. A análise textural mostraram uma diminuição do volume de microporos e um aumento do volume de mesoporos, com isotermas do tipo IV e histereses 2. Demonstrando assim que as caracterizações foram eficazes na elucidação das estruturas porosas. Foi possível obter os materiais micromesoporosos para ambas as metodologias adotadas, sendo o teor de 5% de zeólita MCM-22 a melhor condição de síntese para obtenção desse novo material.
The research on the development of micro-mesoporous structures has the purpose of obtaining porous materials with superior characteristics, once it seeks to join the high acidity of MCM-22 zeolite with the mesoporous system of MCM41 molecular sieve, consequently improving the diffusion of bulky molecules. In this research, the following porous structures were synthesized: Initially, the MCM-22-(P) lamellar precursor was obtained with molar ratio of SiO2\Al2O3 = 30 and was activated to obtain its MCM-22 zeolite form at 550 °C for 5 hours. Then, MCM-22/MCM-41 micro-mesoporous material was synthesized by treating 2 g of MCM-22 zeolite with a solution of 25 mL of 10% wt cetyltrimethylammonium bromide (CTABr), where the material remained in an incubator at 110 °C for 7 days. In order to obtain a new material with better structural organization, the MCM-22 was used in the following proportions (5%, 10% and 15%), remaining in an incubator at 30 °C for 24 hours, being activated in air stream at 550 °C for 5 hours. The results of the X-ray diffraction characterization demonstrated the MCM-22 (P) precursor formation and its MCM-22 zeolite form, with MWW topology peaks. The curves obtained by means of the thermogravimetric analysis (TG/DrTG), showed the losses of water mass and other adsorbates. The micrographs (SEM) presented toroidal format with depreciation in the central region for MCM-22. By means of the results of physical adsorption of N2, it was verified for MCM-22 zeolites: type I isotherms and hysteresis loops of type-IV. From the X-ray diffractograms for the micro-mesoporous materials, it was possible to observe the formation of the porous structures, with the identification of the reflection peaks pertinent to the microporous phase of MCM-22 and the MCM-41 molecular sieve, coexisting in a single structural phase. The SEM images detected the formation of particle agglomerates of the mesoporous phase being constituted around the microporous phase. The textural analysis showed a decrease in the volume of micropores and an increase in the volume of mesopores, with type IV isotherms and hysteresis loops of type-II. Thus demonstrating that the characterizations were effective in elucidating the porous structures. It was possible to obtain the micro-mesoporous materials for both methodologies, being the 5% content of MCM-22 zeolite the best synthesis condition to obtain this new material.

Mesoporous silica SBA-16 thin films with highly ordered 3D cubic structures were synthesized by evaporation-induced self-assembly method, using an F127 triblock copolymer as the structure-directing agent via dip coating, to investigate proton transport of aqueous solutions confined in mesopores. Using electrochemical measurements of ionic current under DC electric fields, we elucidated proton transport phenomena through mesopores of SBA-16 thin films. At low concentrations, ranging from 10−7 to 10−5 M, the I–V curves of KCl and HCl aqueous solutions were nonlinear. However, at 10−4 and 10−3 M, while I–V curves of KCl aqueous solutions displayed nonlinear behavior, those of HCl aqueous solutions were almost linear. The linear behavior can be attributed to a decrease in the electric potential barrier owing to a reduction in the surface charge density, which is caused by the protonation of silanol groups on the inner surface of mesopores. At high concentrations, ranging from 10−2 to 1 M, the I–V curves of KCl and HCl aqueous solutions were almost linear because the effect of surface charge of mesopores on ion transport was marginal.

Novel synthetic method for the formation of mesoporous silica nanotubes was proposed using glycyldodecylamide (GDA) as an amino acid surfactant, which enabled to control the tube diameter, wall structure and morphology with the diverse structures of amphiphile due to the capability of H-bonds by forming amide bond. Moreover, this sol-gel transcription process could be elucidated at neutral condition that enabled the recyclable use of surfactant and resulted in unique structures depending on the temperatures of self-assembly.

The analysis and control of transport phenomena in fluidic nanopores and nanochannels is important in applications such as biochemical analysis, power generation and environmental protection. A unique aspect of nanofluidics is that the relevant length scale is comparable to the range of various surface and interfacial forces in liquids (such as electrostatic, van der Waals and steric interactions). Thus, to obtain an adequate description of transport phenomena in nanospace, it is necessary to understand the discreteness of molecules, especially when the size decreases to 2 nm. Micelle-templated mesoporous silicas (MPSs) possess highly ordered structures such as 2D hexagonal and 3D cubic structures and pores within the 2–50 nm range. In particular, 2D hexagonal films that generally have pore channels parallel to the surface plane have been widely synthesized by using various types of template molecules. If the pore channels of such materials are aligned in a certain direction, these materials can be employed for various purposes such as the fabrication of oriented nanowires, optoelectronic devices, recording media, selective separations, and nanofluidic systems. 3D cubic structures give large surface areas and become good candidates for highly efficient catalysts and sensors. Advances in the synthesis, measurement and analysis of nanotubes and nanochannels have allowed ion and liquid transport to be routinely examined and controlled in spaces with dimensions that range from 10 to 100 nm. The ability to now explore transport and adsorption phenomena in confined spaces of around 2 nm offers a range of possibilities. We have investigated several unique transport and adsorption phenomena in mesopores measuring a few nanometers in diameter, including nonlinear I–V curves of ionic current passing through MPS thin films filled with aqueous solutions, humidity-dependent adsorption rate of water into MPS, and the reduction of melting and freezing temperature of water in MPS.

In this work, we measure the thermal conductivity of mesoporous silica and aerogel thin-films using a non-destructive optical technique: time domain thermoreflectance (TDTR). Due to the rough surfaces of the optically transparent silica-based films, we evaporate an Al film on a glass cover slide and fabricate the silica structures directly on the Al film, providing a “probe-through-the-glass” configuration for TDTR measurements. This allows the thermal conductivity of mesoporous silica and aerogel thin films to be measured with traditional TDTR analyses. As the thermoreflectance response is highly dependent on the thermal effusivity of the porous structures, we estimate the density of the films by varying the heat capacity in our analysis. This density determination assumes that the solid matrix in the silica structure has the thermal conductivity as bulk SiO2, which is valid if all the lattice vibrations are localized, consistent with the minimum thermal conductivity concept. We independently determine the density of the porous silica films with nitrogen sorption measurements of thin films using a surface acoustic wave (SAW) technique. The difference between the determined from the SAW technique and that estimated by the TDTR effusivity analysis lends insight into the relative contributions of localized and propagating modes to thermal transport.

Mesoporous silica SBA-16 thin films were synthesized on a Si substrate via the dip-coating method. SEM analysis revealed that these films possess highly ordered 3D cubic structure. After these films were filled with either pure water or KCl aqueous solutions, the ionic current passing through the mesopores was measured by applying electric field to find out ion transport phenomena. If the ion transport phenomena in mesoporous silica are completely elucidated, this will enhance its use in applications where it is intended to be employed as a catalyst, filter, and adsorbent. The measured I-V curves were non-linear. This document discusses the relationship between the non-linear I-V curves and the ionic flow inside the 3D cubic pore structure. The effect of the dimensions and surface properties of mesopores on the I-V curves are also discussed.