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abstracts
The measurement of the X-ray reflectivity curve is a widely used method to obtain structural information about thin films, surfaces and interfaces. With conventional instruments, the reflectivity curve is measured sequentially for a range of incident angles, which takes a long time. A recently developed method using white synchrotron radiation can measure the whole curve at once [1, 2]. In this contribution, the adaption of this method to a laboratory characteristic X-ray source is presented. This will make it possible to do time-resolved or high-throughput measurements using standard laboratory sources. The basic idea of our method is to focus the divergent X-ray beam emitted from a point source with either a doubly-curved Si crystal monochromator or a bent-twisted Si crystal monochromator [1]. Instead of using the whole beam, however, only the fan-shaped beam from a diagonal line on the monochromator is focused onto the sample. This is realized by placing an inclined slit before the monochoromator. The beam reflected from the sample forms a line on a two-dimensional pixel array detector. For each horizontal position on the detector, the incident angle onto the sample, and therefore the momentum transfer, is different. The reflectivity curve for a range of momentum transfers can therefore be measured with a single detector exposure without moving the sample, monochromator or detector. Reflectivity curves from a silicon wafer sample measured by our method are compared with the conventional angle scan method in the figure. The reflectivity down to 10 to the -7th power can be obtained, because the background can be subtracted from the measured intensity. We will show an example of time-resolved (10 s) measurements of specular X-ray reflectivity curves. We will also discuss the momentum transfer range that can be measured simultaneously and factors limiting the resolution of the method.
abstracts
Photocatalysis of titanium dioxides has been extensively studied in the past. Especially, after the discovery of the UV-light induced hydrophobic-hydrophilic transition of the rutile-TiO2(110) surface in the late of 1990's [1], the number of photochemistry-related publications increased dramatically over the last decade to the extent that ~2400 related papers were published in 2008, in which ~80% of the papers involve the TiO2-related materials. The remarkable research activity arises from the potential applications of the photo-induced wettability control such as anti-fog coatings or self-cleaning coatings. However, despite the intensive study, the mechanism of the hydrophilic reaction is not completely clarified yet, mainly due to the lack of the detailed information of the atomic-scale surface structure. We have studied the surface structural change by means of surface X-ray diffraction. By using the recently developed time-resolved x-ray crystal truncation rod (CTR) scattering measurement [2] and the static measurement for the hydrophobic and hydrophilic surfaces, we confirmed that (i) the surface roughness increases during the reaction probably due to the desorption of the surface oxygen atoms and (ii) an ordered water molecular layer formed on the hydrophobic surface disappears in the hydrophilic surface. Considering the previous reports which show the increase of hydrogen bond density in the hydrophilic surface, we suggest that the ill-ordered surface of the hydrophilic phase allows a larger number of water molecules to adsorb by making a hydrogen-bond network.
