Mesoporous Titania Materials - Tuning and Optimizing Nanostructures and Porous Morphologies

This thesis aims at extending the applicability of surfactant-based mesoporous titanium dioxide materials to photovoltaic, photocatalytic a nd energy storage devices, and to exploit their advantages of high surface area, tuneable por size and periodic porosity in these applications. The major limitation of the establish ed procedures for preparing surfactanttemplated mesoporous titania is the low degree of c rystallinity of its walls, and the resulting negative effect on the material’s electronic and se miconductor properties. Furthermore, in thin films the significant density changes accompanying the condensation and crystallization of initially amorphous titania walls lead to mechanica l stress upon heating, deterioration of film quality and cracking or delamination when stacking several layers. One strategy to overcome these drawbacks is based o n intr ducing preformed nanocrystalline precursors into the walls of the mesoporous framewo rk in a novel “brick and mortar” approach for creating highly crystalline transparen t TiO2 coatings. In this way, preformed titania nanocrystalline “bricks” are fused with sur factant-templated sol-gel titania “mortar”, which acts as a structure-directing matrix and as a chemical glue. The interaction of crystalline and amorphous components leads to a str iking synergy, such that a rapid increase in crystallinity occurs upon thermal treatment and highly porous and highly crystalline structures are formed at very mild conditions. Coat ings with a broad variety of periodic mesostructures and thicknesses ranging from few nan ometers to several micrometers are accessible using the same organic template, and the final structures are tunable by varying the fraction of the “bricks”. The beneficial combinatio n of crystallinity and porosity leads to greatly enhanced activity of the films in photocata lytic and photovoltaic processes, and a performance optimum concerning the nanoparticle con tent is found to be around 70 % (Cooperations with the groups of Dr. J. Rathouský, J. Heyrovský Institute, Prague, and Prof. M. Grätzel, EPF Lausanne). The potential of the “brick and mortar” approach to synthesize thick mesoporous titania films of up to 10 μm thickness by multilayer deposition w as specifically investigated. The asproduced films exhibit very high surface area, whic h s ales linearly with the thickness, and roughness factors of up to 1600 cm2/cm2 can be reac hed. This is the first time that surfactantderived mesoporous titanium dioxide films of such a igh thickness and surface area can be prepared without serious cracking, delamination, or deterioration of the porous structure. In dye-sensitized solar cells (DSCs) using a volatile electrolyte, the films feature a remarkably high performance level of over 7 % already at thick nesses below 4 μm due to their high surface area and dye adsorption. Thicker “brick and mortar” films reach up to 8.1 % efficiency, which is until today the highest DSC pe rformance of a surfactant-derived mesoporous material. The advantages of such high surface area mesoporous films can best be exploited in photovoltaic systems which are based on more stable but diffusion-length limited electrolytes, for example room temperature ionic liquids (RTIL, c ooperation with the group of Prof. M. Grätzel, EPF Lausanne). DSCs employing RTILs, a hig h-extinction coefficient organic dye and “brick and mortar” electrodes exhibit very high conversion efficiencies above 6.2 % at low film thicknesses of only 2 μm. This is a very p romising performance for such thin films compared to the record for this system (8.2 % effic i n y, electrode consisting of 20 nm nanoparticles, thickness 7 μm), and considering sig nificant losses due to insufficient light absorption. Thicker films (> 2 μm), however, proved to be not as efficient due to charge transport-limitations, and further thickness and po re size optimization are supposed to increase the maximum performance of “brick and mort ar” electrodes in RTIL-based DSCs. The scope of general strategies employing crystalli ne t tania nanoparticles can be extended, when the dimensions of the nanocrystalline building blocks are further down-sized. A new non-aqueous sol-gel protocol using tert-butanol as reaction medium widens the available siz range of TiO2 nanoparticles at its lower end by producing ultras m ll and highly soluble anatase nanoparticles of around 3 nm employing fast and low-temperature microwave heating. The extremely small size of the nanopartic les and their dispersibility make it possible to use commercial Pluronic surfactants for self-ass embly of the nanoparticulate building blocks to form periodic mesoporous structures with a igh surface area ranging up to 300 m2/g. The advantages of the retention of the mesopo rous order with extremely thin nanocrystalline walls were shown by electrochemical lithium insertion, in which films made using microwave-treated nanoparticles showed fast i nsertion behaviour with high maximum capacity due to quantitative lithiation with a 10-f old increase of charging rates compared to a standard reference electrode made from 20 nm anatas e particles. The use of preformed titania nanocrystals (“bricks” ) as precursors for surfactant-templated mesoporous materials can also be extended to titani a-silica composite materials. In combination with silica sol-gel precursors as “mort ar”, films with a high fraction of crystalline titania particles finely dispersed in p eriodic silica mesostructures were obtained at temperatures as low as 100 °C, and employed as acti ve layers for the photooxidation of NO (cooperation with Dr. J. Rathouský, J. Heyrovský In stitute, Prague). For this material system, the “brick and mortar” approach results in periodic mesoporous nanocomposites with photocatalytically active sites leading to function al coatings which can be processed at low temperatures on a wide range of substrates. In a second strategy towards titania-silica composi tes with control over the crystalline properties of the porous system, colloidal mesoporo us silica (CMS) nanoparticles with a thin titania-enriched outer shell showing a spatially re solved functionality were synthesized by a delayed co-condensation approach (joint project wit h Dr. Valentina Cauda, LMU Munich). The titania shell can serve as a selective nucleati on s te for the growth of nanocrystalline anatase clusters. These fully inorganic pure silica -core titania-enriched shell mesoporous nanoparticles show orthogonal functionality, demons trated through the selective adsorption of a carboxylate-containing ruthenium N3-dye. UV-Vis a nd fluorescence spectroscopy indicate the strong interaction of the N3-dye with the titan a-phase at the outer shell of the CMS nanoparticles. This selectivity is strongly enhance d when anatase nanocrystallites are nucleated at the titania-enriched shell surface. In conclusion, the introduction of crystalline tita ni nanoparticles to the synthesis of mesostructures based on surfactants and sol-gel pre cursors allowed to improve the applicability of the resulting pure titania or sili ca-titania composite materials. This measure provides the possibility to exploit the attractive porous characteristics and versatile chemistry of templated metal oxides, and led to coatings exhi biting very promising performance in photovoltaics and photocatalysis, and bulk material s with orthogonal functional properties.