Preparation and Characterization of Transparent Nanocrystalline TiO₂ Films Possessing Well-Defined Morphologies

Abstract

We describe the preparation, characterization, and potential-dependent optical absorption spectroscopy of transparent nanocrystalline TiO 2 films possessing well-defined morphologies. Regarding preparation, Langmuir-Blodgett techniques are used to deposit between one and four monolayers of TiO 2 nanocrystallites on conducting glass. As the average spacing between the nanocrystallites constituting a deposited monolayer may be controlled, the degree of nanoporosity of the resulting nanocrystalline film can be determined. Subsequent firing fuses the constituent crystallites of the deposited monolayers and ensures an ohmic contact with the conducting substrate. Characterization by electron microscopy and optical absorption spectroscopy has been undertaken. The former reveals the above films to be nanoporous or close-packed arrays of anatase nanocrystallites. The latter has allowed determination of an absorption coefficient for anatase at 326 nm. Measurement of potential-dependent optical absorption spectra has also proved possible. Results obtained suggest that the degree of charge accumulation possible at a given applied potential is dependent on the morphology of the nanocrystalline film.

Studies on the Diffusion of Molecules through Pores in Monolayer and Multilayer Films

“…In this case, following Chen and Yang's (13) analysis, a molecular flux through the pores is There is increasing interest in transport of molecules derived as follows: a molecule may reside on an adsorption through ultrathin multilayer films because of potential applisite at the external surface of the upstream monolayer, actications in molecular electronics devices (1)(2)(3)(4), electrovated and deactivated, then may jump into a vacant adsorpchemical devices (5,6), separation of gas molecules (7,8), tion site, return back to the original site due to blockage by and in delivering drugs to a patient over a controlled period another adsorbate, move on to the next vacant site or jump of time (9). Some of the applications are based on use of downstream through the pore to the next monolayer.…”

“…The structured multilayer films, such as those produced by the net rate of change of activated molecules coverage, du*/ Langmuir-Blodgett (LB) technique (10,11). Langmuirdt, comprising the above processes, is derived, to include Blodgett films can have pore sizes that are of the order of porosity effects on the diffusion through ultrathin films: the molecular size (6,8). LB films exhibit short diffusion times due to the fact that the film thickness is of the molecudu*/dt Å ku 0 2k m u*u v 0 k m u*u p 0 k b u*u, [1] lar scale.…”

“…LB films exhibit short diffusion times due to the fact that the film thickness is of the molecudu*/dt Å ku 0 2k m u*u v 0 k m u*u p 0 k b u*u, [1] lar scale. Penetration of molecules through monolayer and multilayer films can be characterized by the following mechanism: (a) adsorption of the diffusing molecules and surface where k, k m , and k b , are respectively rate constants of activation, forward migration, and return to the original site due diffusion on the upstream external layer of the multilayer film (12, 13) and subsequent (b) diffusion of the molecules to the occupied site by another adsorbate, u is the fractional surface coverage of the occupied adsorption sites at the first through pores of the multilayer film (6,10,14). Previous theoretical and experimental studies of diffusion through LB (or) external layer, and u v is the fractional vacant sites coverage at the external layer.…”

“…[1] represents the rate of blockage by another adsorbate. The three coverage fractions represent the total J Å Ddu/h, [6] sites of the external surface of the first monolayer. Therefore, where du Å (u 0 u n )/n is a steady-state constant concentra-…”

“…The coverage of activated adsorption sites at steady-state is Equations [5] and [6] yield a concentration-dependent derived from Eqs. [1] and [2]: diffusivity through multilayer films given by…”