Thin films of mesoporous silicas

Petrov, O.V., Vargas-Florencia, D., and Fur6,1.2007. Surface melting of octamethylcyclotetrasiloxane confined in controlled pore glasses Curvature effects observed by H NMR. J. Phys. Chem. B 111 1574-1581. Pevzner, S., Regev, O., andYerushalmi-Rozen, R. 2000. Thin films of mesoporous silica Preparation and characterization. Curr. Opin. Colloid Interface Sci. 4 420-427. 

Wang D, Zhou WL, McCaughy BF, Hampsey JE, Ji Z, Jiang Y-B, HuifangXu TJ, Schmehl RH, O Connor C, Brinker CJ, Lu Y (2003) Electrodeposition of metaUic nanowire thin films using mesoporous silica templates. Adv Mater 15 130-133... 

Canevari, T.C., Raymundo-Pereira, P.A., Landers, R., Benvenutti, E.V., and Machado, S.A. (2013) Sol-gel thin-film based mesoporous silica and carbon nanotubes for the determination of dopamine, uric acid and paracetamol in urine. Talanta, 116, 726-735. 

A new generation of mesoporous silica (SG) materials obtained by sol-gel technique where polymers and ionic or non-ionic surfactant act as stmcture - directed templates is widely developed during last year s. Final materials can be synthesized as thin films and used as sensitive elements of optical and electrochemical sensors. 

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. 

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.

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. 

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. 

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 ... 

Typically, SEM is used to determine the continuity and the thickness of the film. Large-scale cracks and defects of the film can be determined by top view SEM images, and film thickness is easily obtained from cross-sections. It has been used extensively with powdered samples of mesoporous silica to determine morphology and thus assist in phase determination. However, thin films produced by dip and spin coating typically have a scarcity of features that may be resolved in SEM. This can be seen in Figs. 6A and 6B, SEM images of a mesoporous films templated with P123. 

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. 

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. 

Mesostructured thin silica films can be prepared as freestanding films or supported by a variety of different substrates. In the following, a short description of the different synthesis methods is given the synthesis and the formation mechanism of mesoporous silica films has been reviewed in detail elsewhere. " ... 

Fukumoto H, Nagano S, Kawatsuki N, Seki T. 2005. Photo orientation of mesoporous silica thin films on photo crosslinkable polymer film. Adv Mater 17 1035 1039. 

Figure 13.6. Representative TEM images of mesoporous silica thin films. Insets are the corresponding electron diffraction patterns.

A variety of supports have been explored for supporting a thin film of IL [44, 45], such as flame-dried silica [43], mesoporous silica [46], diatomic earth [24], and other oxides. The biopolymer chitosan consisting mainly of 2-amino-2-deoxy-(l,4)-P-D-glucopyranose units is well suited because of its high polarity [47]. Also, carbon nanotubes are feasible supports when their surface is suitably modified for anchoring the thin film of IL, for example, by covalently binding one layer of the IL [48]. Polymers resins are modified by covalently bound ILs [39, 49-51] frequently without covering the material with additional IL. A summary on suitable support materials is presented in Chapter 3. 

Gunji, T. Shimano, R. Arimitsu, K. Abe, Y., Preparation and Characterization of Mesoporous Silica Thin Films from Polyethoxysiloxanes. J. Polym. Sci., Part B Polym. Phys. 2006,44, 2542-2550. 

Dourdain, S., Mehdi, A., Bardeau, J.F., and Gibaud, A. 2006. Determination of porosity of mesoporous silica thin films by quantitative X-ray reflectivity analysis and GISAXS. Thin Solid Films 495 205-209. 

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