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- Porosity (3) (entfernen)
One of the crucial characteristics of functionalized thin films is their porosity (i.e., the ratio between the pore volume and the volume of the whole film). Due to the very low amount of material per coated area corresponding to thin films, it is a challenge for analytics to measure the film porosity. In this work, we present an Approach to determine the porosity of thin films by means of electron probe microanalysis (EPMA) either by wavelength-dispersive X-ray spectrometry (WDX) or by energy-dispersive X-ray spectrometry (EDX) with a scanning electron microscope (SEM). The procedure is based on the calculation of the film mass deposition from electron-excited X-ray spectra. The
mass deposition is converted into film density by division of measured film thickness.
Finally, the film porosity is calculated from the measured film density and the density of bulk, nonporous film material. The general applicability of the procedure to determine the
porosity is demonstrated on thin templated mesoporous TiO₂
films, dip-coated on silicon
wafer, with controlled porosity in the range of 15 to 50%. The high accuracy of the mass deposition as determined from X-ray spectra was validated with independent methods (ICP-OES and weighing). Furthermore, for the validation of the porosity results, ellipsometry, interference fringes method (IFM), and focused ion beam (FIB) cross sectioning were employed as independent techniques. Hence, the approach proposed
in the present study is proven to be suited as a new analytical tool for accurate and relatively fast determination of the porosity of thin films.
Various applications such as automotive catalysts, photocatalysis and sensors rely on thin coatings with high active surface area. The development of new and improved coatings requires in-depth understanding of the film morphology and texture. Especially the film porosity is a key parameter to identify structure-property relationships in applications. However, an accurate porosity determination of thin porous coatings is a challenging. In this contribution we present and discuss a new approach for determining the porosity of films by x-rays at SEM.
A combined methodical approach is tested for the first time with respect to the characterization of the inner structure of porous TiO2 layers infiltrated with ruthenium molecular sensitizer as typically used in modern dye sensitized solar cells (DSSC). Their performance is directly linked to the surface area ‘offered’ by the pore Network to the dye. The micrometer thick layer employed was manufactured by screen-printing of a starting TiO2 powder constituted of shape-controlled, bipyramidal anatase nanoparticles (NPs) on FTO/glass substrates. The
analytical methods exploited in our study are Focused Ion Beam (FIB) slicing followed by 3D reconstruction as well as the new approach transmission Kikuchi diffraction (TKD) technology in the scanning electron microscope (SEM). While the first method results in the visualization of the 3D pore network within the TiO2 layer, the second one can be successfully applied to analyze the crystal orientation of grains (i.e. NPs in our case) in nanometer resolution. Moreover, size and shape distribution of the TiO2 NPs within the layer can be extracted.
SEM in transmission mode and atomic force microscopy (AFM) have been used to verify the dimensional data obtained by the new combined methodical approach. Its analytical benefits but also the challenges and limitations
are highlighted.