Abstract
We study the dependence of the photochromic effect on environment and triggering light. We demonstrate that the first darkening/bleaching cycle of freshly grown films is accompanied by a release of weakly bound hydrogen, most likely present at the grain boundaries. For consecutive photochromic cycles, we do not find further exchange of material with the environment. Moreover, we report bleaching kinetics dependent on the gas environment after darkening with light of energies below the optical bandgap of the film. For darkening with photon energies above the bandgap of the film, we report a linear relation between the degree of darkening and bleaching relaxation time irrespective of gas environment.
Highlights
Metal hydrides are often used as catalysts or reducing agents1 and battery materials,2,3 but they have a strong tendency to oxidize and are typically covered with a protective layer
Photochromism is observed in a composition range δ between 0.5 and 1.5, with the sample composition following the overall non-stoichiometric equation: MeH2ÀδOδ
The diffracted intensity may just be redistributed among these two phases, e.g., hydrogen and/or oxygen may migrate between the subphases resulting in the photochromic effect as it was reported in a pressurized photochromic yttrium foil
Summary
Metal hydrides are often used as catalysts or reducing agents and battery materials, but they have a strong tendency to oxidize and are typically covered with a protective layer. Based on charge neutrality considerations assuming single-phase metal oxyhydrides, Cornelius et al. proposed a different formula (MeH3À2δOδ) Both these studies agree on the composition range in which photochromism is found. The diffracted intensity may just be redistributed among these two phases, e.g., hydrogen and/or oxygen may migrate between the subphases resulting in the photochromic effect as it was reported in a pressurized photochromic yttrium foil.. The diffracted intensity may just be redistributed among these two phases, e.g., hydrogen and/or oxygen may migrate between the subphases resulting in the photochromic effect as it was reported in a pressurized photochromic yttrium foil.18 This theory is further supported by Remhof et al. who showed that tiny changes in hydrogen concentration, in the range from YH1:9 (higher transmission) to YH2:1 (lower transmission), in the fcc, β-phase go in hand with strong relative changes in optical transmission. We demonstrate that the dependency of bleaching on gas environment is only found if the films are darkened with photons of energy smaller than the bandgap but not for photons of energy larger than the bandgap
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