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Synthesis-Dependent Oxidation State of Platinum on TiO2 and Their Influences on the Solar Simulated Photocatalytic Hydrogen Production from Water

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ic . K ers fo Ni b c fr yd g is d sy of tar c eri en om CS PHYSICAL REVIEW B 68, 094302 ~2003! wavelengths. Further increase of the hydrogen content leads to nucleation of the g-YH3 phase. At a hydrogen-to-yttrium ratio of about 2.8 the resistivity increases sharply and the film becomes transparent to visible light. From the position of the absorption edge in substoichiometric YH32d with d ;0.1, an optical gap close to 2.7 eV was deduced4,5 for YH3. The large increase in resistivity when the trihydride phase is formed, as well as its negative temperature coeffi- cient and inverse proportionality to d in YH32d suggest that YH3 is a true semiconductor. However, so far the size of the fundamental gap has not been determined experimentally. If the H2 pressure is reduced sufficiently, hydrogen desorbs un- til the dihydride phase is recovered. Since the process of switching between the reflecting dihydride and the transpar- ent trihydride phase is reversible, these ‘‘switchable mirrors’’ may become suitable for a number of different applications. At the time of their discovery a good understanding of the mechanism underlying the metal-insulator transition in these materials was lacking. The properties of YH2 had been stud- ied by Weaver et al.6,7 and successfully interpreted on the basis of self-consistent electronic structure calculations by Peterman et al.8 However, straightforward application of band theory to YH3 failed to produce a gap. Using band structure calculations based upon the local density approxi- ments and even more recently by Remhof et al.14 for epitaxi- ally grown films of the type used in the switching experiments.15 For the HoD3 structure, both theoretical stud- ies found a band structure characteristic of a semimetal with a large band overlap of about 1.3 eV ~Ref. 10! at the center of the Brillouin zone. The failure of DFT-LDA calculations to explain the large band gap in YH3 and LaH3 prompted theoretical work into the nature of the band gap in t

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