Complete characterization of TiO2 mesoporous thin films by means of XANES, XRR and 2D-SAXS

PAULA C. ANGELOMÉ; M. CECILIA FUERTES; LEANDRO ANDRINI; FÉLIX G. REQUEJO; GALO J. A. A. SOLER-ILLIA

Conferencia; 6th International Conference on Synchrotron Radiation in Materials Science; 2008

Transition-metal Oxide based Mesoporous Thin films (TM-MTF) have being extensively studied in the last ten years because they present very high surface areas (200-1000 m2/g), and an accessible ordered pore array with monodisperse tunable size in the 2-50 nm range. These properties make them interesting for applications such as optoelectronics, sensing, (photo)catalysis, selective nanomembranes, drug delivery, etc. These TM-MTF are obtained by the combination of sol gel reactions and the self assembly of supramolecular templates (surfactants, polymers), by means of the method called Evaporation Induced Self Assembly, which occurs during the dip coating process. The template is then eliminated by calcination or extraction, given rise to empty accessible pores. The final properties of MTF depend on their thickness, pore size and symmetry, nature of the pore surface and crystalline character of the walls. All these factors that affect the final properties should be carefully studied in order to tune the final material properties for a given application. Although some structural studies have been presented, there is a lack of systematic information about the majority of the variables. Synchrotron techniques are ideal to probe the structural features of these materials, due to their high sensitivity, and ability to work with small quantities of matter. In this work, we have studied TiO2 MTF, prepared using a triblock PEO-PPO based copolymer (Pluronics F127®) as a template, using crossed synchrotron techniques. Films were deposited by dip-coating precursor solutions on different substrates (glass, silicon and ITO), stabilized at 130°C and further treated at 200ºC-500 °C in air. Different initial solutions, with variable concentration of copolymer or metal, and several deposition speeds were used. The nature of pore array and the contraction during the thermal treatment was studied by 2D-SAXS, which gives valuable information on the symmetry and spatial orientation of the pore system. Film thickness, porosity and density were determined by X-Ray Reflectometry. Both measurements were made in the LNLS, Campinas, SP, Brazil (SAXS2 and XRD2 beamlines). We have obtained the dependence between the deposition speed (v) and the film thickness (t), and adjust it to a power law t=Avn, where n ≈ 0.5, a behavior typical of viscous precursor solutions. We determined that increasing the metal or template concentration causes an increasing of the film thickness, due to an increase in viscosity. In addition, modification of the template amount results in a change in the pore array symmetry and porosity: from a well ordered cubic Im3m structure with low porosity to a local order structure with high porosity. We have also investigated the effect of the substrate in the crystalline structure of the TiO2 walls through X-ray absorption spectroscopy (XANES) at O K-edges and Ti K and L-edges at the ALS, Berkeley, USA and at the LNLS. The local order of the Ti atoms was determined by Ti K or L-XANES. A clear distinction can be made between Ti in amorphous-like TiO2 with an average coordination number lower than six, and six-fold Ti centers in anatase-TiO2. Oxygen K-edge follows this crystallization by sharpening of the peak feature. We conclude that this crystallization process, which occurs between 300 - 500 °C, strongly depends on the type of substrate. A clear sequence of crystallization temperatures T is found, in the following trend: TSi < TITO ≈ Tglass Finally, we have determined that the accessibility to organic molecules (tested with methylene blue) is strongly dependent on the pore array and porosity. Films with local order and/or low porosity are clearly less accessible than films with high porosity – cubic structures. Our results show that even with a “simple” system, such as TiO2 templated with F127, it is possible to obtain final materials with different physical-chemical behavior by easily tuning the pre or post synthesis conditions. The use of crossed synchrotron techniques permits to adequately characterize these complex materials in several length scales: molecular environment, mesoscopic order, and thickness.