2, University of Southampton, Southampton, , United Kingdom
3, University of Warwick, Coventry, , United Kingdom
Mesoporous thin films have attracted growing interest for developing advanced functional materials considering their high surface area, ordered porosity, and versatile functionalisation of the pore surface.
The evaporation induced self-assembly (EISA) process is the most versatile method to produce mesoporous silica films but often leads to 2D mesophases oriented parallel to the surface. Cylindrical mesochannels oriented perpendicularly to the substrate are required for technological devices such as separation membranes, sensors or optoelectronic devices.
The preparation of such aligned mesoporous silica films can be achieved by different strategies; using anisotropic external fields (electric or magnetic) or chemically modified substrates. Another way is to use confined space to guide the mesochannels in a direction. In this way, the structure of the deposited mesoporous materials is greatly influenced by the synthesis conditions, but the surface chemistry, dimensions and structure of the confined regions need to be considered as well.
Under the Advanced Devices by Electroplating project (EPSRC EP/ N035437/1) we are focused on creating hierarchically ordered porous structures by combining top-down lithographical technologies and bottom-up self-assembly chemistry. The aim of this research is to make a reliable template for the electrodeposition of metal and semiconductor nanowires.
Mesoporous silica pores with a vertical alignment and hexagonal packing were deposited into the pores of anodic alumina membranes and patterned silica substrates using the EISA process. This approach employs homogeneous coating solution combining volatile solvent, surfactant concentrations below the critical micelle concentration (CMC) and silica precursor obtained by acid-catalysed hydrolysis of TEOS. The solvent evaporation drives the self-assembly process towards the CMC and induce the silica condensation around the surfactant micelle template leading to formation of 2D mesophases.
The effect of deposition method, composition of the sol coating solution, aging conditions after deposition and pore surface modification on the filling, structure and morphology of the silica mesopores was studied. Two different surfactants were used in this study (CTAB: ionic surfactant and P123: neutral surfactant) to attempt the formation of columnar hexagonal mesochannels of different size. Interestingly, the design of lithographic patterned substrates has made possible to study the influence of pore shape and pore size ratio on the structuration of silica pores as well.
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