Thin films hierarchically structured

Both the molecular template and the self-assembly techniques presented above have limited control over the final shape of the solid, since this is generally obtained in the form of a powder, fibers, or thin films. It is possible, however, to control the shape and size of solids by combining the former techniques with techniques that restrict the volume in which the synthesis takes place. The final goal is to have control over the solids at the molecular as well as macroscopic level, in order to have in a single material properties emerging from several levels of scale. Such structures are referred to as hierarchical [2, 6]. 

Maroudas and co-workers have described a hierarchical scheme for atomistic simulations involving the use of electronic structure calculations to develop and test semiempirical potentials that are in turn used for MD simulations. These results can sometimes be used to develop elementary step transition probabilities for use in dynamic Monte Carlo schemes. With Monte Carlo techniques, the well-known length and time scale limitations of MD can be greatly extended. This hierarchical approach appears to have great promise for the development of simulation strategies that will allow studies of a wide range of practical surface and thin-film chemical and physical processes. 

Wang DH, Jakobson HP, Kou R, Tang J, Fineman RZ, Yu DH, Lu YF (2006) Metal and semiconductor nanowire network thin films with hierarchical pore structures. Chem Mater 18 4231 237... 

Aligned multiwall CNT arrays were synthesized as a basis for a microstructured catalyst, which was then tested in the Fischer-Tropsch reaction in a microchannel reactor [269]. Fabrication of such a structured catalyst first involved MOCVD of a thin but dense A1203 film on a FeCrAlY foam to enhance the adhesion between the catalyst and the metal substrate. Then, multiwall CNTs were deposited uniformly on the substrate by controlled catalytic decomposition of ethene. Coating the outer surfaces of the nanotube bundles with an active catalyst layer results in a unique hierarchical structure with small interstitial spaces between the carbon bundles. The microstructured catalyst was characterized by the excellent thermal conductivity inherent to CNTs, and heat could be efficiently removed from the catalytically active sites during the exothermic Fischer-Tropsch synthesis. 

The nanocoating of PPy on natural cellulose fiber was performed without disrupting the hierarchical network structure of individual fibers [245]. Since pyrrole was hardly adsorbed onto a cellulose surface, this approach was based on the adsorption of the growing polymer chain from the solution and on the subsequent immobilization to form thin film. The conformation of the PPy chain was parallel to the surface of the cellulose fiber, because the corresponding oligomer is adsorbed parallel to the surface and further polymerized in the lateral direction. 

Continuous mesoporous carbon thin films were fabricated by direct carbonization of sucrose-silica nanocomposite films and subsequent removal of the silica [236]. The mesoporous carbon film with uniform and interconnected pores had a surface area of 2603 mVg and a pore volume of 1.39 cmVg. Subsequently, nanoporous carbons with bimodal PSD centered at about 2 and 27 nm in diameter were prepared by using both the TEOS-derived silica network and the colloidal silica particles as templates [237]. Figure 2.33 illustrates the preparation pathway. The pore sizes of the carbon are determined by the sizes of the added silica particles and the silica network. As the colloidal silica particles are commercially available with different diameters (e.g., 20 to 500 nm), this dual template synthesis process provides an efficient route to preparing nanoporous carbons with a controllable hierarchical pore structure. 

This review focuses on the most interesting reversible coordination polymers and their application in various hierarchical self-assembled structures, including thin films, microcapsules, micelles, microemulsions, and nanoribbons. These structures have in common that they are hierarchical assemblies containing metal-mediated reversible coordination polymers as a main component. The charges carried by the coordination polymers are utilized to interact with oppositely charged components, including nanoparticles, polyelectrolytes, block copolymers, and surfactants. Specific features of these objects, introduced by the coordination polymers, as well as the influence of additional salt are discussed. 

A different approach to the synthesis of nanosized macromolecules through hierarchical self-assembly is based on Layer-by-Layer (LbL) chemistry. LbL allows the deposition of ultra thin films whose thickness can be controlled by the chemical structure of the molecules and number of deposited layers. The interactions between layers can be ionic, covalent, hydrogen-bonding, and charge-transfer, depending upon the nature of the polymer used in the preparation.The layer-by-layer assembly of an electroactive polymer nanocomposite thin film of cationic linear poly(ethyleneimine) and Prussian Blue nanoparticles, has been exploided... 

Effectiveness factor approaches Macrohomogeneousmodel of ionomer-bound CL Structural (percolation) model of ionomer-bound CL Structural model coupled with water balance in pores Thin-film morphology of ionomer in CL Hierarchical Model, coupling of meso-and macroscale... 

As for the first assumption, the electrolyte phase must be treated as a mixed phase. It consists of a thin-film structure of ionomer at the surface of Pt/C agglomerates and of water in ionomer-free intra-agglomerate pores. The proton density is highest at the ionomer film (pH 1 or smaller), and it is much smaller in water-filled pores (pH > 3). However, the proton density distribution is not incorporated in the statistical utilization Tstat, but in an agglomerate effectiveness factor, defined in the section Hierarchical Model of CCL Operation. ... 

Aksay and coworkers [20] produced mesoscopic patterning of oriented nan-ostructured silica thin films polymerized by a surfactant-templated sol-gel technique [21] in combination with a micromolding technique, which is another microfabrication technique without photolithography proposed by Xia and Whitesides [19]. A network pattern of microcapillaries (submicrometer scale) was transferred to an elastomeric PDMS stamp as a microreplica molding. An aqueous mixture of tetraethoxysilane and a cationic surfactant (CTAC cetyltri-methylammonium chloride) was introduced into the microcapillaries. After hydrolysis of tetraethoxysilane at the cationic interface of the tubular surfactant assemblies, a mesoscopic supramolecular structure hierarchically constituted from hexagonally packed nanoscopic tubules of silica was formed in the microscopic capillary. 

Examples of dense silica, hybrid silica, metal oxides, solid-state metal oxide solutions, or colloidal self-assembly are unlimited. However, the recent developments to accurately control processing conditions (e.g., atmosphere, temperature, and motion) led to films with unique properties (see Figure 9.6) [52,53]. These progresses concern mesoporous coatings with controlled pore size and structure [26], hard template infiltration and/or replication [54-58], nanostructured epitaxial low-quartz thin films [59], ultrathin nanostructured supported networks [60,61], ultrathick porous Ti02 layer prepared from aqueous solutions [51], coatings with hierarchical porosity [62], multilayer porous stacks [63], colloidal MOF layers [64,65], pillar planar nanochannels (PPNs) for nanofluidics [66], and so on. 

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