Mg-Ti-H thin films are found to have very attractive optical properties: they absorb 87% of the solar radiation in the hydrogenated state and only 32% in the metallic state. Furthermore, in the absorbing state Mg-Ti-H has a low emissivity; at 400 K only 10% of blackbody radiation is emitted. The transition between both optical states is fast, robust, and reversible. The sum of these properties highlights the applicability of such materials as switchable smart coatings in solar collectors. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2212287͔ Many metal hydrides behave as switchable mirrors ͑i.e., their optical properties switch from reflective in the metallic state to transparent in the hydrogenated state͒. [1][2][3][4] In addition, magnesium-rare-earth ͑Mg-RE͒ and magnesium-transitionmetal ͑Mg-TM͒ switchable mirrors also exhibit an intermediate highly absorbing optical state on hydrogenation. The possibility to switch a film from a reflective state to an absorbing state suggests that such materials might be interesting for smart solar collectors, which absorb light in normal operation condition and switch to a reflective state to avoid overheating. Limiting the stagnation temperature of the solar collector makes it possible to use cheap materials such as plastics ͑generally not designed for high temperatures͒.In this letter we show that Mg y Ti 1−y thin films prepared by dc magnetron co-sputtering of Mg and Ti at room temperature ͑on quartz and CaF 2 ͒ satisfy the following requirements for a smart solar coating: ͑i͒ high absorption in the solar regime ͑0.5Ͻប Ͻ 4 eV͒, ͑ii͒ low emissivity in the thermal regime ͑ប Ͻ 0.5 eV͒, and ͑iii͒ reversibility. Three compositions are studied in detail: y = 0.70, y = 0.80, and y = 0.90. Typical deposition rates are 2 Å / s for Mg ͑150 W rf power͒, 0.1-1 Å / s for Ti ͑25-160 W dc power͒, and 1.3 Å / s for Pd ͑50 W dc power͒. All the films are covered with a Pd layer ͑10-50 nm͒ to promote dissociation of H 2 and to prevent oxidation of the underlying film.Reflection ͑R͒ and transmission ͑T͒ spectra are measured simultaneously during hydrogenation ͑pressures up to 1 bar H 2 ͒ in a Perkin Elmer Lambda 900 diffraction grating spectrometer ͑0.495Ͻប Ͻ 6.51 eV͒ and a Bruker IFS 66 Fourier transform infrared spectrometer ͑0.2Ͻប Ͻ 1.1 eV͒. The R-T spectra are measured through the transparent substrate at near normal incidence of the incoming beam. Figures 1͑a͒ and 1͑b͒ show the reflection and transmission spectra measured for 200 nm Mg y Ti 1−y / 10 nm Pd films ͑y = 0.90, 0.80, and 0.70͒ in the as-prepared and hydrogenated states ͑in 1 bar H 2 at room temperature͒. In the metallic state ͓Fig. 1͑a͔͒ all the films have a relatively high and featureless reflection that decreases with increasing Ti content.After hydrogen absorption, the reflection is low for all compositions, whereas significant transmission is observed only for the y = 0.90 sample. The combination of low reflection and low transmission in the hydrogenated state ͑y = 0.80 and 0.70 samples͒ gives rise to a highly a...

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Magnetite nanoparticles, commonly found in subsurface environments, are extensively used in various applications such as environmental remediation, catalysis, electronics and medicine. However, the oxidative transformation of magnetite (mixed-valent Fe-oxide) into...