Silica thin films

Similarly, monometallic Rh, Pd, and Au and bimetallic Pt-Rh and Pt-Pd nanowires were prepared in FSM-16 or HMM-1 by the photoreduction method [30,33,34]. The bimetallic wires gave lattice fringes in the HRTEM images, and the EDX analysis indicated the homogeneous composition of the two metals. These results show that the wires are alloys of Pt-Rh and Pt-Pd. Mesoporous silica films were also used as a template for the synthesis of uniform metal particles and wires in the channels [35,36]. Recently, highly ordered Pt nanodot arrays were synthesized in a mesoporous silica thin film with cubic symmetry by the photoreduction method [37]. The... 

Northeastern Lau Basin Papatua expedition site (15°17 S, 174"45 W) 2100 Axial region of northeasterly trending active spreading ridge of the northern Lau Basin. Dredged black smoker chimney samples. Wurtzite, pyrite, chalcopyrite, barite, and amorphous silica. Thin film of Mn-oxyhydroxide. 

Ikeue, K., Nozaki, S., Ogawa, M., and Anpo, M. (2002) Characterization of self standing Ti-containing porous silica thin films and their reactivity for the photocatalytic reduction of C02 with H20. Catalysis Today, 74 (3-4), 241—248. 

Dourdain, S. Bardeau, J. F. Colas, M. Smarsly, B. Mehdi, A. Ocko, B. M. Gibaud, A. 2005. Determination by x-ray reflectivity and small angle x-ray scattering of the porous properties of mesoporous silica thin films. Appl. Phys. Lett. 86 113108. 

Etienne, M. Quach, A. Grosso, D. Nicole, L. Sanchez, C. Walcarius, A. 2007. Molecular transport into mesostructured silica thin films Electrochemical monitoring and comparison between p6m, P6(3)/mmc, and Pm3n structures. Chem. Mater. 19 844-856. 

Etienne, M. Cortot, J. Walcarius, A. 2007. Preconcentration electroanalysis at surfactant-templated thiol-functionalized silica thin films. Electroanalysis 19 129-138. 

This chapter deals with the selective preparation, TEM/EXAFS/XPS characterization and catalysis of mono- and bimetallic nanowires and nanoparticles highly ordered in silica FSM-16, organosilica HMM-1 and mesoporous silica thin films. The mechanism of nanowire formation is discussed with the specific surface-mediated reactions of metal precursors in the restraint of nanoscale void space of mesoporous silica templates. The unique catalytic performances of nanowires and particles occluded in mesoporous cavities are also reviewed in terms of their shape and size dependency in catalysis as well as their unique electronic and magnetic properties for the device application. 

Figure 15.30 TEM and electron diffraction images of Pt nanoparticles (diameter 2.5 nm) with mesoporous frameworks of mesoporous silica thin films from different planes [100] (a) and [210] (b) (c) cross-section structure of a single electron soliton device of Pt nanoparticle/mesoporous silica film on a Si substrate connected with two Al electrode.

Further projects dealt with the fabrication of heterogeneous, basic or acidic solid-state catalysts or adsorbents carrying, for instance, amino or sulfonic acid groups. Amino-functionalized silicas were prepared and analyzed for the catalytic activity in Knoevenagel condensation reactions of aldehydes or ketones with ethyl cyanoace-tate ions by Macquarrie et al.154 155 Recently, Zhang et al.156,157 reported on the successful preparation of amino-functionalized silica thin films by means of the EISA approach. 

Another photochemically based gating system is the one by Brinker and coworkers in which the cis-trans isomerization of azobenzene was applied.69 (Triethoxysilylpropylureido)azobenzene was co-condensed with TEOS by evaporation-induced self-assembly to yield a functionalized mesoporous silica thin film. The propylureidoazobenzene groups inside of the mesopores could then be iso-merized from their stable elongated trans structure to the more compact cis geometry upon UV irradiation. The trans geometry could be recovered by thermal treatment. The authors suggested that the results of the transitions from trans to cis could lead to an increase in pore size of approximately 0.7 nm and an increase of the polarity from 0 to 3 D. This could be applied to regulation of ion transport however, they did not confirm such expectations.69... 

Figure 20.5 Simplified drawing depicting the noncovalent imprinting of dopamine in hybrid silica thin film.

One of the first mentions of an imprinted sol-gel-derived silica thin film that was able to discriminate against enantiomers was that of Marx and Liron, who created a chiral cavity for propranolol.66 In a note added to the proof, the authors reported on data that showed a cavity templated with the pure enantiomer, 5-propranolol, that discriminated between it and the R-enantiomer.66 In a later publication, more details were provided on this method and two additional systems (R)- and (5)-2,2,2-trifluoro-1 -(9-anthryl) ethanol and d- and L-3,4-dihydroxyphenylalanine.67... 

W. Yantasee, Y. Lin, X. Li, G. E. Fryxell, T. S. Zemanian, and V. V. Viswanathan, Nanoengineered electrochemical sensor based on mesoporous silica thin-film functionalized with thiol-terminated monolayer, Analyst 128, 899-904 (2003). 

As powders are unfavorable with respect to the potential applications in nonlinear optics and in the optical characterization of the composites, ordered mono and bifunctionalized organo-silica thin films were prepared through evaporation-induced self-assembly (EISA) and characterized. 

Huang, M. H. Soyez, H. M. Dunn, B. S. Zink, J. I., In situ fluorescence probing of molecular mobility and chemical changes during formation of dip-coated Sol-gel silica thin films. Chemistry of Materials 2000, 12(1), 231-235... 

D and 3-D metal nanowire thin films[263] with tunable 3-10 nm wire diameters have been obtained by electrodeposition into mesoporous silica thin-film templates, resulting in nanowire arrays that reflect the pore structure of the template. Removal of silica is achieved via annealing followed by etching to leave mechanically strong freestanding metal nanowire films. 

B.W. Eggiman, M.P Tate, and H.W. Hillhouse, Rhombohedral Structure of Highly Ordered and Oriented Self-assembled Nanoporous Silica Thin Films. Chem. Mater., 2006, 18, 723-730. 

R.E. Williford, R.S. Addleman, X.S. Li, T.S. Zemanian, J.C. Birnbaum, and G.E. Fryxell, Pore Shape Evolution in Mesoporous Silica Thin Films From Circular to Elliptical to Rectangular. J. Non-Cryst. Solids, 2005, 351, 2217-2223. 

D.Y. Zhao, P.D. Yang, D.I. Margolese, B.F. Chmelka, and G.D. Stucky, Synthesis of Continuous Mesoporous Silica Thin Films with Three-dimensional Accessible Pore Structures. Chem. Commun., 1998, 2499-2500. 

There has been extensive research on the synthesis of mesoporous silica thin films since the first reports in 1996. However, most research has focused on two general synthesis techniques evaporation induced self-assembly and spontaneous film growth from solution. 

Fig. 3 Mechanism for the synthesis of a hexagonal silica thin film by dip coating. This is zoomed in at the substrate-bulk solution interface. The dotted line indicates the second critical micelle concentration, above which the micelles form cylindrical micellar rods. These rods then begin to assemble at the air-liquid interface and the liquid-substrate interface. (From Ref. l)...

There have been several other thin film synthesis techniques, such as the use of pulsed laser deposition. " However, the promising processing techniques for the economic mass production of mesoporous silica thin films are all based on EISA. Synthesis times can be as short as a few minutes and the processing equipment is readily available. Thus the remainder of this entry focuses on EISA techniques. 

PROCESSING MESOPOROUS SILICA THIN FILMS BY SPIN AND DIP COATING... 

The chemistry involved in the formation of mesoporous silica thin films is qualitatively well understood. However, specific reaction mechanisms of the individual steps are still debated. In addition, owing to the complexity of the sol-gel reaction pathways and cooperative self-assembly, full kinetic models have not been developed. From the time of mixing, hydrolysis reactions, condensation reactions, protonation and deprotonation, dynamic exchange with solution nucleophiles, complexation with solution ions and surfactants, and self-assembly, all occur in parallel and are discussed here. Although the sol-gel reactions involved may be acid or base catalyzed, mesoporous silica film formation is carried out under acidic conditions, as silica species are metastable and the relative rates of hydrolysis and condensation reactions lead to interconnected structures as opposed to the stable sols produced at higher pH. Silicon alkoxides are the primary silica source (tetramethyl orthosilicate, tetraethyl orthosilicate, tetrapropyl orthosilicate, etc.) and are abbreviated TMOS, TEOS, and TPOS, respectively. Starting from the alkoxide, Si(OR)4, in ROH and H2O solution, some of the general reactions are ... 

Interest in the synthesis and processing of mesoporous silica materials has grown extensively since their discovery in 1992, and the exciting potential that these films hold in low-k dielectrics, sensors, nanowire fabrication, catalysis, membrane separations, and many other applications will continue to fuel academic and industrial interest in these films. While there are many new synthesis routes for processing mesoporous silica thin films, spin coating and dip coating remain the most facile methods available. These methods deliver high quality reproducible films that can be used for any of the variety of applications. 

Cagnol, F. Grosso, D. Soler-Illia, G.J.D.A.A. Crepaldi, E.L. Babonneau, F. Amenitsch, H. Sanchez, C. Humidity-controlled mesostructura-tion in CTAB-templated silica thin film processing. The existence of a modulable steady state. J. Mater. Chem. 2003, 13 (1), 61-66. 

Nishiyama, N. Tanaka, S. Egashira, Y. Oku, Y. Ueyama, K. Enhancement of structural stability of mesoporous silica thin films prepared by spin-coating. Chem. Mater. 2002, 14 (10), 4229-4234. 

Grosso, D. Balkenenda, A.R. Albouy, P.A. Ayral, A. Amenitsch, H. Babonneau, F. Two-dimensional hexagonal mesoporous silica thin films prepared from block copolymers detailed characterization and formation mechanism. Chem. Mater. 2001, B (5), 1848-1856. 

Fig. 4 Nanoporous silica thin films by sol-gel process. (View this art in color at www. dekker.com.)...

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