Mesoporous metal oxide films

The high surface area of the mesoporous metal oxide film is critical to efficient device performance as it allows strong absorption of solar irradiation to be achieved by only a monolayer of adsorbed sensitiser dye. Whereas a dye monolayer absorbed on a flat interface exhibits only negligible light absorption (the optical absorption... 

The family of mesoporous metal oxide films prepared by a particle-based approach is growing rapidly, including Nb Ti02 [66], r -Al203 [193], Li4TisOi2 [194], and many others. 

GENERATION OF MESOPOROUS CRYSTALLINE METAL OXIDE FILMS VIA EVAPORATION-INDUCED SELF-ASSEMBLY... 

Dye-sensitized solar cells (DSSCs) are photoelectrochemical solar devices, currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. DSSCs are based upon the sensitization of mesoporous nanocrystalline metal oxide films to visible light by the adsorption of molecular dyes.5"7 Photoinduced electron injection from the sensitizer dye (D) into the metal oxide conduction band initiates charge separation. Subsequently, the injected electrons are transported through the metal oxide film to a transparent electrode, while a redox-active electrolyte, such as I /I , is employed to reduce the dye cation and transport the resulting positive charge to a counter electrode (Fig. 17.4). 

Figure 8.1 Schematic of a liquid electrolyte dye-sensitised solar cell. Photoexcitation of the sensitiser dye is followed by electron injection into the conduction band of the mesoporous oxide semiconductor, and electron transport through the metal oxide film to the TCO-coated glass working electrode. The dye molecule is regenerated by the redox system, which is itself regenerated at the platinised counter electrode...

Electrodeposited mesoporous metal oxides were investigated as chemochromic hydrogen sensor films. 

Defined-sized mesoporous metal oxide materials are synthesized by templating organic molecular assemblies (Kresge et ah, 1992 Yanagisawa et al., 1990). The templating rodlike micelles form lyotropic LC states, and therefore, the hexagon-ally packed rods are oriented by the structural anisotropy of the surface. The photoorientation of an Az monolayer is transferred to a photooriented spincast film of a polysilane as the first step (Seki et al., 1999), and then deposition of mesoporous silica film is performed (Kawashima et al., 2004, 2002). In this attempt, the photoorientation of the Az monolayer is first transferred to a... 

Xu C, Xu G, Liu Y, Zhao X, Guanghou WG (2002) Preparation and characterization of SnO nanorods by thermal decomposition of SnC O precursor. Seri Mater 46 789-794 Yang HX, Qian JF, Chen ZX, Ai XP, Cao YL (2007) Multilayered nanocrystaUine SnO hollow microspheres S3mthe-sized by chemically induced self-assembly in the hydrothermal environment. J Phys Chem 111 14067-14071 Yuan L, Hyodo T, Shimizu Y, Egashira M (2011) Preparation of mesoporous and/or macroporous SnO -based powders and their gas-sensing properties as thick film sensors. Sensors 11 (2) 1261-1276 Yue W, Zhou W (2008) Crystalline mesoporous metal oxide. Progr Nat Sci 18 1329-1338... 

These cells do not rely on a traditional p-n junction to separate photo-generated charge carriers. Polymer cell materials used include polyester (PET) foil, indium tin oxide (ITO) film, polyethylenedioxythiophene (PEDOT), and aluminum. Nanocrystalline cells use thin film materials and are overlayed on a supporting matrix of conductive polymer or mesoporous metal oxide. 

Invented less than a decade ago,1 EISA has rapidly developed into a universal technique for fabrication of organized porous and patterned nanocomposite materials, ranging from metal oxides and chalcogenides to carbons, polymers, and metals.2,3 In addition to generating ordered mesoporous films, this technique may also be used to incorporate functional molecules and... 

The number of publications concerning utilization of the EISA process for fabrication of different structured materials is counted in the hundreds, which is far beyond the possibilities of this chapter to review in depth. Rather, we intend to provide a brief introduction into EISA and its application to the fabrication of functional thin films for electronic applications (e.g., electro-chromic layers and solar cells), with a special focus on fabrication of crystalline mesoporous films of metal oxides. Attention will also be given to techniques used to evaluate the pore structure of the thin films. For the other aspects of the EISA process, for example its mechanism,4 strategies for preparation of crystalline porous metal oxides,5 mesoporous nanohybrid materials,6 periodic organic silica materials,7,8 or postgrafting functionalization of mesoporous framework,9 we kindly recommend the reader to refer to the referenced comprehensive reviews. 

Brezesinski, T. Smarsly, B. Iimura, K. Grosso, D. Boissiere, C. Amenitsch, H. Antonietti, M. Sanchez, C. 2005. Self-assembly and crystallization behavior of mesoporous, crystalline Hf02 thin films A model system for the generation of mesostructured transition-metal oxides. Small 1 889-898. 

Brezesinski, T. Antonietti, M. Smarsly, B. M. 2007. Self-assembled metal oxide bilayer films with single-crystalline overlayer mesopore structure. Adv. Mater. 19 1074-1078. 

Ad Figure 2.1,2. Zeolite layers can be grown by hydrothermal synthesis onto porous supports (clay, alumina, sintered metal). Especially layers of MFI-type zeolite have been studied [e.g. 5-7]. Such MFI-layers were shown to survive template removal and subsequent thermal cycles up to 350 °C, which is taken as a strong indication for chemical bonding [8] at the support interface. To understand chemical attachment to metals one has to take into consideration that metals - by exposure to air - will be covered with a thin (1-2 nm) oxide film. Sometimes an intermediate mesoporous layer has been applied, e.g. a metakaolin film on clay or on zirconia [5] or metal wool on sintered metal [6]. 

Well ordered mesoporous silicate films were prepared in supercritical carbon dioxide.[218] In the synthesis in aqueous or alcoholic solution, film morphology of preorganized surfactants on substrate cannot be fully prescribed before silica-framework formation, because structure evolution is coincident with precursor condensation. The rapid and efficient preparation of mesostructured metal oxides by the in situ condensation of metal oxides within preformed nonionic surfactants can be done in supercritical CCU- The synthesis procedure is as follows. A copolymer template is prepared by spin-coating from a solution containing a suitable acid catalyst. Upon drying and annealing to induce microphase separation and enhance order, the acid partitions into the hydrophilic domain of the template. The template is then exposed to a solution of metal alkoxide in humidified supercritical C02. The precursor diffuses into the template and condenses selectively within the acidic hydrophilic domain of the copolymer to form the incipient metal oxide network. The templates did not go into the C02 phase because their solubility is very low. The alcohol by-product of alkoxide condensation is extracted rapidly from the film into the C02 phase, which promotes rapid and extensive network condensation. Because the template and the metal oxide network form in discrete steps, it is possible to pattern the template via lithography or to orient the copolymer domains before the formation of the metal oxide network. 

The tailored design of the titanium-coordinated surfactant and its application in an evaporation-induced self-assembly process followed by heat treatment enabled the preparation of periodic mesoporous silica-based films with a high loading and good dispersion of tetrahedral titanium atoms within the silica matrix. This approach is also feasible for a variety of other transition metal oxides. 

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