Preparation And Application Of Macroscopically Oriented Films By Air Flow Method

Ordered mesoporous films have attracted great interests due to their relatively large surface area,high ordered mesostructure, adjustable mesopores and potential applications in host-guest chemistry,gas sensor, filtration membrane, molecule design and photoelectric device, etc. According to thedefinition of international unit of pure and applied chemistry (IUPAC), mesoporous films are definedas solid porous film with pore size of2~50nm. The mesochannels were randomly oriented in themesoporous films prepared by traditional methods, which limited their applications. Nowadays, how tocontrol the mesochannels macroscopically uniaxially aligned has become a hot topic. Althoughdifferent approaches have been developed for the preparation of parallelly or perpendicularly orientedmesoporous films, most of approached are dependent on specific substrates or complex equipment andthe mesochannels cannot be controlled to align in desired direction. Besides, if the insulatedmesoporous silica film can be substituted by semiconductor films such as mesoporous titania film, theusing value of oriented mesoporous films will be enhanced greatly. However, the main hurdle in thesynthesis of uniaxially oriented mesoporous transition-metal-based films is the high reactivity of metalion precursors toward hydrolysis and condensation. How to prepare the aligned mesoporous titaniafilms and further control the mesochannels in any direction is the main purpose of our work. Thecontent of this dissertation could be summarized as follows:1. Controlling of the orientation of mesochannels in mesostructured thin films is important for thedevelopment of novel molecular devices and, in particular, generating vertically aligned mesochannelswith respect to the substrate plane is extremely challenging for nonsiliceous materials. We describe afacile and highly effective air flow method, which is able to control the unidirectional alignment oftitania mesochannels in a desired direction (e.g., parallel, perpendicular, or oblique) on a large scale,via manipulation of the air flow rate and incident angle. The titania mesochannels were characterized by TEM, SEM, SAXRD, and GISAXS. The unidirectional, vertically aligned mesostructured titaniafilms were found to exhibit excellent ion conductivity.2. Highly crystallized mesoporous titania films with obliquely aligned mesochannels have beensuccessfully synthesized by utilizing a simple air flow method and subsequent layer-by-layerdeposition. The morphology of oriented mesoporous titania films were characterized by cross-sectionalTEM. The obliquely aligned mesoporous titania films outperformed the disordered counterpart of thesame thickness in both short circuit current and efficiency. The4-layer-thick obliquely alignedmesoporous film shoed enhanced solar conversion efficiency by about12%compared to that ofdisordered mesoporous film of the same thickness. The improved cell performance of the alignedmesoporous films is ascribed to the enhanced electron transport and significantly inhibited chargerecombination in the unidirectionally oriented titania mesochannel.3. The air flow method was applied for the orientation control of mesoporous silica films.Macroscopically oriented mesoporous silica films were prepared by horizontal air flow withmesochannels parallelly oriented along the air flow direction. The environment condition and thetemperature and speed of air flow play an important role on the mesochannel orientation. By adjustingthe incident angle of air flow, the obliquely aligned mesoporous silica films can be obtained. Furtherdecreasing the film thickness from650nm to420nm, the alignment of oblique mesochannels was wellenhanced.4. Hybrid film of mesoporous silica film with oriented mesochannels and semiconductor quantumdot has been prepared. Encapsulation of CdS and PbS within the oriented mesochannels leads to aregular arrangement at the macro scale. The hybrid film thus obtained showed remarkable anisotropicphotoelectronic properties due to the confinement effect of the oriented mesochannels. Furthermore,due to the independence of the orientations of the mesochannels on the substrate, bilayer filmscontaining both CdS and PbS could be prepared. This design has allowed an extension of the range oflight absorption by the thin film as well as an amplification of the response to external photoelectroniceffects. Such a hybrid film may prove useful in the design of anisotropic electrodes and electronicnanodevices. 5. The effect of applied air flow on the surface morphology of polystyrene-block-poly(ethylene oxide)copolymers were investigated using atomic force microscopy and ultraviolet ozone etching technique.The adjusted solvent evaporation rate and the applied external shear force conduced to thesandwich-mode films with the orientated cylindrical microdomains perpendicular to both the top andbottom surface and the irregular oriented cylinders located in the middle part. The velocity andtemperature of air flow affected the film surface morphology. This film demonstrated high molecularpermeability and selectivity toward H/V ions.