In this study, we prepared Si clathrate films (Na 8 Si 46 and Na x Si 136) using a single-crystalline Si substrate. Highly oriented film growth of Zintl-phase sodium silicide, which is a precursor of Si clathrate, was achieved by exposing Na vapor to Si substrates under an Ar atmosphere. Subsequent heat treatment of the NaSi film at 400 °C (3 h) under vacuum (<10 −2 Pa) resulted in a film of Si clathrates having a thickness of several micrometers. Furthermore, this technique enabled the selective growth of Na 8 Si 46 and Na x Si 136 using the appropriate crystalline orientation of Si substrates.

A thin film of a type II Ge clathrate, Na x Ge 136 , was epitaxially grown on a (111) substrate of Ge with a diamond structure (α-Ge). A Zintl phase NaGe film was synthesized in advance by a reaction of the substrate surface with Na vapor under an Ar atmosphere, and was highly oriented such that the NaGe(100) planes were parallel to the Ge(111) surface. The NaGe film was transformed to the Na x Ge 136 film by heat treatment under dynamic vacuum. XRD measurements demonstrated that the prepared film consisting of twin crystals with a (111) twin plane was epitaxially grown with the <111> direction normal to the substrate surface. It was also suggested that the lattice mismatch between Na x Ge 136 and the Ge substrate is relaxed by a buffer layer of α-Ge having a triple-period superlattice. The electrical resistivity of the Na x Ge 136 film was estimated from the I-V measurements to be in the order of 10 1 -10 2 Ω m. ExperimentalFrequently, type II clathrates are synthesized from Zintl alloys of alkali elements and Si or Ge. 1,3,12,16,20,21 Fig. 1(b) shows 5630

Water splitting to produce H 2 and O 2 is a fundamental reaction for artificial photosynthesis on semiconductor photocatalysts. The mechanism of the multistepped reaction, especially four-electron oxidation to O 2 , has not yet been understood. Although some intermediate states have been detected in transient spectroscopy, O 2 evolution kinetics remain unknown at the end of consecutive reaction steps. We apply operando O 2 detection with a microelectrode to determine the absolute evolution rate on a highly efficient SrTiO 3 photocatalyst film casted on a glass plate. The evolution rate was determined with a time resolution of 0.1 s, which was improved by 1000 times compared with that in widely used gas-chromatographic detection. The observed rate did not respond instantaneously to excitation light irradiation. When light was turned on, the photocatalyst film was inactive for evolution and light-activated in seconds. It was proposed that the first absorbed photons were consumed to fill trap states on SrTiO 3 surface and then the latter photons drove steady O 2 evolution. When excitation light stopped, the O 2 evolution rate exponentially decayed in seconds. The microelectrode method demonstrated herein will be useful for understanding many other reaction kinetics at liquid−solid interfaces.

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