Determination by x-ray reflectivity and small angle x-ray scattering of the porous properties of mesoporous silica thin films

Abstract

Two-dimensional hexagonal silica thin films templated by a triblock copolymer were investigated by grazing incident small angle x-ray scattering ͑GISAXS͒ and x-ray reflectivity ͑XR͒ before and after removing the surfactant from the silica matrix. XR curves-analyzed above and below the critical angle of the substrate-are evaluated by the matrix technique to obtain the average electron density of the films, the wall thickness, the electron density of the walls, the radius of the pores, and subsequently the porosity of such mesoporous films. In combination with GISAXS, the surface area of the mesopores is ascertained, thereby providing a complete analysis of the porosity in thin films by x-ray scattering methods.In thin films, the nonintrusive determination of the surface area and of the porosity is a nontrivial issue. 1 Indeed, the BET technique that is used for powders is not applicable due to lack of materials. X-ray reflectivity ͑XR͒ that is sensitive to both the film average electron density and its profile can be used to determine the porosity of disordered films. [2][3][4][5] The aim of this letter is to present the determination of the porous properties of a highly ordered silica thin film by a quantitative analysis of the XR. For this purpose, a silica matrix was initially templated by a polyethylene oxide/ polypropylene oxide ͑PEO/PPO͒ triblock copolymer ͑with the commercial P123 name from BASF͒ that was removed by rinsing the film in ethanol. 6 After rinsing, the film consisted of a regular array of cylindrical pores of diameter t 1 located at the nodes of a two-dimensional ͑2D͒ hexagonal lattice as shown in Fig. 1. By combining grazing incident small angle x-ray scattering ͑GISAXS͒ measurements with the XR analysis, we show how these parameters and the electron density of the silica matrix are obtained and further used to determine the porosity and the specific surface of the mesoporous film.Films were made from initial sols prepared in two steps. First, 3.50 g of tetraethoxysilane ͑TEOS͒, 2.0 g of ethanol, and 2.5 g of H 2 O ͑pH= 1.25͒ were mixed and stirred at room temperature for 1 h. A second solution containing 1.2 g of P123 and 54.4 g of EtOH was then added to this sol. After 2 h of stirring, 4 g of H 2 O ͑pH= 1.25͒ was added. From the resulting sol, thin films were dip coated at a constant withdrawal velocity of 14 cm/ min on clean glass substrates in the final sol of molar composition 1 TEOS: 72 C 2 H 5 OH:21 H 2 O : 0.022 HCl: 0.012 P123. The composition of the sol was adjusted so as to make films about 100 nm thick. For the present study, two identical films were prepared at relative humidity around 60% and the temperature at 25°C. One of the films was thoroughly rinsed in ethanol for 6 h to remove the surfactant so as to produce a mesoporous film and the other was not processed. a͒ Author to whom correspondence should be addressed; electronic mail: gibaud@univ-lemans.fr FIG. 1. Idealized model used to fit the reflectivity data. Films are considered to be made of Layer 1 either composed of s...

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