(22d) In-Situ Gisaxs Study of Pore Orientation Effect on Thermal Transformation Mechanism of Surfactant Templated Mesoporous Titania Thin Films

Titania thin films with orthogonally oriented cylindrical mesopores have numerous potential applications in photovoltaic cells, electrochemical devices and photocatalytic technologies due to their accessible cylindrical straight pores which provide well-defined short diffusion paths for reactants and charge carriers¹.  The Rankin group at the University Of Kentucky has established a technique based on Pluronic P123 surfactant templating to synthesize titania thin films with hexagonally ordered cylindrical mesopores tilted orthogonal to the substrate ² and also showed that these orthogonal structures have better thermal stability than films with mesostructure oriented parallel to the substrate.³ However, the mechanisms of formation and thermal transformation of this ideal mesostructure are in need of further investigation, since the ability to orient pores in these films may allow the films to better withstand stress during heating and crystallization. This study addresses the detailed kinetics of orthogonally oriented mesostructure transformation, and compares them with films oriented parallel to the substrate using in-situ grazing incidence small angle x-ray scattering (GISAXS).  Complementary supporting information is provided by ex situ transmission and electron microscopy.

The pores in the films are template using Pluronic surfactants P123, and are oriented by using a crosslinked P123 modified substrate.  Prior to calcination, the films are aged at 4 °C, and GISAXS indicates that the orthogonal orientation of ordered mesoporous films develops during this time.  In-situ GISAXS measurements dueing thermal treatment show that the orthogonally oriented films have higher activation energy (155 KJ/mole) for mesostructurdeformation loss during crystallization than those with pores oriented parallel to the substrate (126 KJ/mole), and that this makes the orthogonally oriented films more stable during thermal treatment at higher temperatures (> 600 °C).The reasons for this difference are ascribed to greater resistance to anisotropic stress during heating of the orthogonally oriented pores and to nucleation of titania crystallites at the top surface of the films with orthogonally oriented pores rather than throughout the thickness of the film.

References

1. Coakley et al., MRS Bulletin, 2005, 30-37.

2. Koganti et al., Nano Letters, 2006, 6, 2567-2570.

3.  Das et al., The Journal of Physical Chemistry C, 2014, 118, 968-976.