Synthesis, Properties And Application Of Ordered Mesoporous Films

Due to their various framework compositions, fascinating pore architectures, large porosities and diverse morphologies, ordered mesoporous materials have triggered substantial research interests in material science, and possess great potential for the applications in energy, environment and human health fields. On the other hand, the thin film morphology is favorable for device fabrication and opens up enormous opportunities for the practical applications of mesoporous materials. Ordered mesoporous thin film, therefore, is a class of desirable advanced materials nowadays. However, although great progress in mesoporous thin films have been realized during the past decade, there are still challenges as the lack of systematical investigation and structural control of mesoporous carbon films, the difficulty of synthesizing mesoporous titania thin films with combined adjustable thickness, large pore size, high crystallinity and ordering, the limitation of substrates for device fabrications and applications of mesoporous thin films, et al.This thesis focuses on the synthesis, property study and application of mesoporous thin films, including the selection of functional framework compositions as carbon and titania, the modification via forming composite framework, post-deposition or in-situ doping, the synthesis of free-standing films and the usage of novel porous substrate, the device fabrication of electrode, nanofilter, photoanode based on the obtained mesoporous thin films, as well as the application in electrochemical energy storage, solar energy conversion and bio macromolecular separation.In Chapter2, a series of highly ordered mesoporous polymer and carbon thin films on silicon wafers have been synthesized via evaporation induced organic-organic self assembly of resol precursor and Pluronic F127or P123block copolymer templates, followed by carbonization in inert atmosphere. The structural orientation, transformation as well as distortion due to contraction have been studied. Mesoporous polymer/carbon films with various structures of lamellar, centered-rectangular, hexagonal, orthorhombic, worm-like types have been obtained by changing surfactants or adjusting the ratio of resol/surfactant, and the phase diagram for the films synthesized from F127and P123templates has been discussed. In addition, the porosity and pore size distribution of the mesoporous polymer/carbon films are in-situ studied using the sorption isotherm based on refractive index changes.In Chapter3, for the first time, a free-standing mesoporous carbon thin film with highly ordered pore architecture has been fabricated by a simple coating-etching approach, which has an intact morphology with variable sizes as large as several square centimeters and a controllable thickness of90-3000nm. The mesoporous carbon film is first synthesized by coating a resol precursors/F127solution on a pre-oxidized silicon wafer and forming highly ordered polymeric mesostructure based on organic-organic self assembly, followed by carbonation and finally etching of the native oxide layer between the carbon film and the silicon substrate. A polymer-layer assisted transfer method is also developed, by which this free-standing carbon film can be easily transferred onto any substrates even with large curvatures and directly used for novel device applications. We demonstrate that these free-standing carbon thin films can be directly fabricated as a supercapacitor device with a capacitance of136F/g, and a nanofilter device with size-selective permeability of cytochrome c and bovine serum albumin with a difference of-50%.In Chapter4, carbon fiber fabric as a novel porous substrate with high mechanical strength and conductivity has been used for the synthesis of mesoporous carbon thin films. A mesoporous carbon layer is coated on the carbon fiber fabric via organic-organic self assembly of resol precursors and F127template followed by carbonization at high temperature. The mesostructure and pore architecture of mesoporous carbon layer has been adjusted by resol/F127ratio and largely influenced by the unique morphology of carbon fiber fabric substrate. The obtained carbon fiber fabric@mesoporous carbon composite membrane can be directly applied as an electrode of supercapacitor, exhibiting a specific capacitance as high as70F/g in an organic electrolyte system. The electrochemical performance of the composite membrane mainly depends on the pore architectures, connectivity, opening as well as carbonization degree.In Chapter5, in order to enhance the mechanical strength and adjust the electric or optical properties, highly ordered mesoporous carbon/silica composite thin films have been synthesized by an evaporation-induced triconstituent co-assembly method. For the first time, mesoporous carbon/silica films with a3-dimensional orthorhombic structure have been obtained, in which the silica amount can be varied from0to74wt%. The structure, porosity, conductivity and modulus have been studied and it can be concluded that silica composition remarkably decreases the contraction of framework and enlarges the porosity, while a composite film with38wt%silica shows the best mechanical strength (14.1GPa) as well as enough conductivity for electrochemical applicationsIn Chapter6. highly-(?) red, multi-layered, continuous mesoporous TiC2thin films on conductive glass (FTO) substrates have been synthesized by a ligand assited evaporation induced self assembly of Ti(acetylacetone) and diblock copolymer PEO-b-PS. The obtained mesoporous titania films possess an ordered monoclinic mesostructure, uniformly large pores of10-30nm, large surface area of～100m2/g, and high crystalline anatase walls. The film thickness can be well controlled from150nm to several microns to tune the light absorption, with the capability of generating inch-scale free-standing film morphologies. This designed architecture allows for effective post-deposition of small-bandgap semiconductor nanomaterials (such as CdS quantum dots) inside the large, open and interconnecting mesopores, leading to significantly improved solar absorption and photoconversion. A photoanodes made of an8-layered,4.75-μm-thick mesoporous TiO2films with deposited CdS quantum dots exhibit excellent performance in photoelectrochemical (PEC) water splitting, with a photocurrent density of6.03mA/cm" and a photoconversion efficiency of3.9%.In Chapter7, an in-situ incorporation approach, involving the ligand assited evaporation induced co-assembly of Ti(acetylacetone), metal(acetylacetone) and diblock copolymer PEO-b-PS, is employed for the fabrication of transitional metal ion (such as Cr, Ni and Co) doped mesoporous titania thin films. Metal ion doped titania films with the dopants highly dispersing in framework show similar mesostructures and pore architectures as non-doped films with the doping amount lower than5.0wt%, as well as enhanced adsorption of visible light. The metal ion doped mesoporous titania film on glass substrate can be used as a photocatalytic device for the degradation of rhodamine B under simulated sunlight and exhibits high photocatalytic activity.In Chapter8, the whole thesis is summarized.