Pyrolysis, Crystallization, and Sintering of Mesostructured Titania Thin Films Assessed by in Situ Thermal Ellipsometry

Abstract

In-situ thermal ellipsometric analysis is used to elucidate new and fine-scale details on the thermally driven densification, pyrolysis, crystallization, and sintering of dense and ordered mesoporous titania thin films prepared by evaporation-induced self-assembly. The role of the heating schedule, initial film thickness, nature of the substrate and templating agent, solution aging, and presence of water and other additives in the calcination environment is examined. Each of these parameters is shown to have unique and often substantial effects on the final film structure, while the technique itself provides detailed insight into the chemical origin and evolution of these effects. In-situ monitoring and control over the governing chemical processes, such as high-temperature adsorption phenomena that impact nanocrystal growth, is also demonstrated. The evolution of both the porosity and chemical processes occurring inside these materials are evaluated, including extraction of kinetic parameters for the pyrolysis of the template and crystallization of the matrix walls. The latter is shown to be strongly dependent on the presence of mesoscale ordering with ordered cubic films indicating a 1D diffusion-limited crystallization process and dense films following a 3D diffusion-limited process. Less well-ordered mesoporous films, despite similarities in pore volume and pore size distributions, are kinetically more reminiscent of dense films in terms of crystallization. In-situ thermal ellipsometry, by detailing the evolution of the thermally driven chemistry and ceramization that dictate the final film properties, provides immensely important insight into the synthesis and optimization of advanced functional materials based on titania and other metal oxide thin films.

Lithium transition metal (Ti, Nb, V) oxide mesoporous thin films: Contrasting results when attempting direct synthesis by evaporation-induced self assembly

“…The absorption peaks of the P123 structuring agent (at ~2900 cm -1 and ~1100 cm -1 ) can be detected up to 300°C (5 min) and are no longer visible at 350°C (5 min). These results are in good agreement with the in-situ thermal ellipsometric analysis reported by Bass et al [22], where pyrolysis of a similar Pluronic structuring agent (F127, EO 109 -PO 70 -EO 109 ) was found to occur between 250°C and 380°C with a maximum rate of pyrolysis at 310°C.…”

Block copolymer-templated synthesis of highly organized mesoporous TiO2-based films and their photoelectrochemical applications

“…Subsequently, the films are subjected to a humidity aging process to facilitate the cooperative assembly between Ti species and block copolymer molecules, constructing a robust and highly organized mesostructure. After two-step thermal treatment, gelation at a baking temperature and subsequent calcination at a temperature above 300 • C, block copolymer can be completely removed from the gradually solidified film, and mesoporous TiO 2 films are thus obtained [7,17,27,28].…”

“…Sanchez's group [7,27] applied in-situ thermal ellipsometric analysis to investigate the pyrolysis, crystallization, and sintering of mesoporous TiO 2 thin films. They found that slow heating rate resulted in is a lower crystallization temperature and less shrinkage of mesostructure with lower refraction index.…”

“…Grosso and co-workers [27] explored the change in crystallization temperature of mesoporous TiO 2 films deposited on various substrates, and ascribed the difference to be a consequence of adhesive constraints imposed on the films by the substrates. The crystallization temperature is lowest on the Si substrate, while higher on the soda lime glass whose alkaline ions diffusion into the mesoporous films retards the crystal transformation and growth of TiO 2 .…”

Colloidal Route for Preparing Optical Thin Films of Nanoporous Metal–Organic Frameworks

“…In addition, studies on the soft sintering of MOF thin layers via the recently developed thermoellipsometry method for periodically organized mesoporous thin films of transition metal oxides are currently being performed. [20] Experimental Thin films of flexible porous iron muconate MOFs were prepared from a colloidal solution of iron(III) acetate and muconic acid in ethanol. Iron(III) acetate was first obtained as previously reported by Dziobkowski et al [21].…”