The supramolecular photocatalysts in which a Ru­(II) complex as a molecular redox photosensitizer unit and a Re­(I) complex as a molecular catalyst unit are connected with a various alkyl or ether chain have attracted attention because they can efficiently photocatalyze CO2 reduction with high durability and high selectivity of CO formation, especially on various solid materials such as semiconductor electrodes and mesoporous organosilica. The intramolecular electron transfer from the one-electron reduced photosensitizer unit to the catalyst unit, which follows excitation of the photosensitizer unit and subsequent reductive quenching of the excited photosensitizer unit by a reductant, is one of the most important processes in the photocatalytic reduction of CO2. We succeeded in determining the rate constants of this intramolecular electron transfer process by using subnanosecond time-resolved IR spectroscopy. The logarithm of rate constants shows a linear relationship with the lengths of the bridging chain in the supramolecular photocatalysts with one bridging alkyl or ether chain. In conformity with the exponential decay of the wave function and the coupling element in the long-distance electron transfer, the apparent decay coefficient factor (β) in the supramolecular photocatalysts with one bridging chain was determined to be 0.74 Å–1. In the supramolecular photocatalyst with two ethylene chains connecting between the photosensitizer and catalyst units, on the other hand, the intramolecular electron transfer rate is much faster than that with only one ethylene chain. These results strongly indicate that the intramolecular electron transfer from the one-electron reduced species of the Ru photosensitizer unit to the Re catalyst unit proceeds by the through-bond mechanism.

We have designed and synthesized a new tris-chelating polypyridine ligand (bpy3Ph) suitable to be used as a bridging ligand (BL) for constructing various supramolecular photocatalysts.

The silicon etching rate by chlorine trifluoride gas is systematically studied using a reactor having a very small cross section above the silicon substrate and achieving a very high efficiency of etchant gas consumption and very large etching rate, larger than 20 m min Ϫ1 . The silicon etching rate is shown to be proportional to the flow rate of the chlorine trifluoride gas. However, this rate is, for the first time, found to be independent of the initial silicon substrate temperature. This study shows that the silicon substrate is automatically heated to the temperature determined by the balance of the reaction heat and the heat transport in the reactor. Since this temperature increment processes an extremely large-surface chemical reaction rate, the etching rate is governed by the transport rate of the chlorine trifluoride gas. This study concludes that a high efficiency silicon etching by chlorine trifluoride gas is possible without any supplemental heating.Chlorine trifluoride (ClF 3 ) gas has a very high reactivity for various materials. 1-5 This gas is especially suitable for plasmaless etching 2,6-10 near room temperature at atmospheric and reduced pressures. In silicon crystal technology, chlorine trifluoride gas has been known to be used for the in situ cleaning 2,11,12 of a chemical vapor deposition ͑CVD͒ reactor in order to remove any polysilicon film deposited on the susceptor and on the inner wall of the chamber.For the further development of new industrial etching and cleaning processes using chlorine trifluoride gas, its chemical reaction should be systematically studied. For this purpose, our previous study 13 reported the chemical reaction between a silicon surface and chlorine trifluoride gas in ambient nitrogen at atmospheric pressure. It also reported that chlorine trifluoride gas has been shown to work as a source of active fluorine atoms to form inorganic fluorides, for example, silicon tetrafluoride for silicon etching. However, this previous study was performed only to evaluate the overall chemical reaction and the produced gas species. The other fundamental properties of the chemical reaction by chlorine trifluoride gas, such as the etching rate and the rate-determining parameters, have, unfortunately, not been studied.Therefore, in this study using the chlorine trifluoride gas in the CVD reactor designed for achieving the industrially applicable highperformance process, the silicon etching rate and its ratedetermining parameters are experimentally evaluated. ExperimentalIn order to etch silicon by chlorine trifluoride gas, the horizontal cold-wall CVD reactor shown in Fig. 1 was used. This reactor consists of a gas supply system, a quartz chamber, and infrared lamps. A 30 ϫ 50 mm silicon substrate is horizontally held on the bottom wall of the quartz chamber. The silicon substrate is cut from the n-type ͑100͒ 200 mm diam semiconductor silicon wafer, which was grown using the Chzochralski method.The silicon substrate is heated by infrared rays from the infrared lamps through the transpar...

Dye-sensitized photocatalysts that consist of a light-absorbing dye and a wide-gap oxide semiconductor have been studied extensively as components of solar energy conversion systems. Although surface modification by a metal and/or metal oxide has a significant impact on the photocatalytic efficiency, the mechanism by which these modifications increase the activity has not been fully understood. Here, a dye-sensitized H2 evolution system was constructed by using Pt-intercalated HCa2Nb3O10 nanosheets, Ru­(II) complex photosensitizers ([Ru­(4,4′-(CH3)2-bpy)2(4,4′-(PO3H2)2-bpy)]2+ and [Ru­(4,4′-(CH3)2-bpy)2(4,4′-(CH2PO3H2)2-bpy)]2+, abbreviated as RuP 2+ and RuCP 2+ ; bpy = 2,2′-bipyridine), and amorphous Al2O3 as building blocks. In the presence of iodide as the electron donor, the H2 evolution rate from Pt/HCa2Nb3O10 nanosheets sensitized by RuP 2+ was increased by modification of the nanosheets with Al2O3. On the other hand, Al2O3 had a negative impact on the H2 evolution rate when RuCP 2+ was employed. These hybrid materials were studied by transient diffuse reflectance spectroscopy and steady-state emission spectroscopy. A detailed analysis of the transient absorption profiles of the adsorbed Ru­(II) complexes revealed that there are at least three states of the complexes on the nanosheet surface. The transient bleaching of the ground-state absorbance had different lifetime components ranging from a few μs to several hundred μs, which mainly reflect back electron-transfer rates from HCa2Nb3O10 to the oxidized Ru­(II) complexes. The Al2O3 modifier could inhibit not only the back electron-transfer events but also electron injection from the excited-state photosensitizer. Interestingly, the negative effect of Al2O3 on the electron injection rate was negligible in the case of RuP 2+, which also had a higher H2 evolution rate. This work highlights that suppressing fast back electron transfer from Pt/HCa2Nb3O10 to the oxidized Ru­(II) complex, which occurs on a time scale of a few μs, and maximizing the electron injection efficiency are both necessary for improving dye-sensitized H2 evolution.

The development of CO2-reduction photocatalysts is one of the main targets in the field of artificial photosynthesis. Recently, numerous hybrid systems in which supramolecular photocatalysts comprised of a photosensitizer and catalytic-metal-complex units are immobilized on inorganic solid materials, such as semiconductors or mesoporous organosilica, have been reported as CO2-reduction photocatalysts for various functions, including water oxidation and light harvesting. In the present study, we investigated the photocatalytic properties of supramolecular photocatalysts comprised of a Ru­(II)-complex photosensitizer and a Re­(I)-complex catalyst fixed on the surface of insulating Al2O3 particles: the distance among the supramolecular photocatalyst molecules should be fixed. Visible-light irradiation of the photocatalyst in the presence of an electron donor under a CO2 atmosphere produced CO selectively. Although CO formation was also observed for a 1:1 mixture of mononuclear Ru­(II) and Re­(I) complexes attached to an Al2O3 surface, the photocatalytic activity was much lower. The activity of the Al2O3-supported photocatalyst was strongly dependent on the adsorption density of the supramolecular moiety, where the initial rate of photocatalytic CO formation was faster at lower density and higher photocatalyst durability was achieved at higher density. One of the main reasons for the former phenomenon is the decreased quenching fraction of the excited state of the photosensitizer unit by the reductant dissolved in the solution phase in the case of higher density. This is due to the self-quenching of the excited photosensitizer unit and steric hindrance between the condensed supramolecular photocatalyst molecules attached to the surface. The higher durability of the more condensed system is caused by intermolecular electron transfer between reduced supramolecular photocatalyst molecules, which accelerates the formation of CO in the photocatalytic CO2 reduction. Coadsorption of a Ru­(II) mononuclear complex as a redox photosensitizer could drastically reinforce the photocatalysis of the supramolecular photocatalyst on the surface of the Al2O3 particles: more than 10 times higher turnover number and about 3.4 times higher turnover frequency of CO formation. These investigations provide new architectures for the construction of efficient and durable hybrid photocatalytic systems for CO2 reduction, which are composed of metal-complex photocatalysts and solid materials.