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Article 5+
Formation of Sub-Surface W Species in Gasochromic Pt/WO Thin Films Exposed to Hydrogen 3
Pedro Castillero, Victor J. Rico, Carmen Lopez-Santos, Angel Barranco, Virginia Perez-Dieste, Carlos Escudero, Juan Pedro Espinos, and Agustín R. Gonzalez-Elipe J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03385 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on June 26, 2017
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The Journal of Physical Chemistry
Formation of Sub-surface W5+ Species in Gasochromic Pt/WO3 Thin Films Exposed to Hydrogen Pedro Castillero,1 Victor Rico-Gavira,1 Carmen López-Santos,1 Angel Barranco,1 Virginia Pérez-Dieste,2 Carlos Escudero,2 Juan P. Espinós, 1* Agustín R. González-Elipe, 1* 1
Instituto de Ciencia de Materiales de Sevilla, Universidad de Sevilla-CSIC. Avenida Américo
Vespuccio 49, 41092. Sevilla, Spain. 2
ALBA Synchrotron Light Source. Carres de la Llum, 2-26, 08290 Cerdanyola del Vallés,
Barcelona, Spain. KEYWORDS. Gasochromic films, GLAD, OAD, nanoporous structure, magnetron sputtering, tungsten oxide, porphyrins, platinum nanoparticles, optical sensor, hydrogen detection, smart windows.
ABSTRACT. M/WO3 (M: Pt, Pd) systems formed by a porous WO3 thin film decorated by metal nanoparticles are known for its reversible coloring upon exposure to H2 at room temperature. In this work, this gasochromic behavior is investigated “in situ” by means of near ambient photoemission (NAPP). Pt/WO3 systems formed by very small Pt nanoparticles (10 ± 1 nm
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average size) incorporated in the pores of nanocolumnar WO3 thin films prepared by magnetron sputtering at oblique angle, have been exposed to a small pressure of hydrogen at ambient temperature. The recorded UV-vis transmission spectra showed the reversibly appearance of a very intense absorption band responsible for the blue coloration of these gasochromic films. In an equivalent experiment carried out in the NAPP spectrometer, W4f, O1s, Pt4f and valence band photoemission spectra have been recorded at various photon energies to follow the evolution of the reduced tungsten species and hydroxyl groups formed upon film exposure to hydrogen. The obtained results are compared with those of a conventional X-ray photoemission study after hydrogen exposure between 298 and 573 K. As investigated by NAPP, the gasochromic behavior at 298 K is accounted for by a reaction scheme in which hydrogen atoms resulting from the dissociation of H2 onto the Pt nanoparticles are spilt over to the WO3 substrate where they form surface OH- /H2O species and sub-surface W5+ cations preferentially located in buried layers of the oxide network.
1.- Introduction Tungsten oxide is a reference material for reversible electrochromic
1-4
and gasochromic5-8
coloring effects. These latter have been used for the development of photonic sensors of hydrogen7,9,10 or for smart window applications11,12. The synthesis, characterization and electrochemical activation of WO3 thin films used as cathodes in electrochromic devices have deserved an ample attention in literature1-3. Although less widespread, much work also exists on the gasochromic behavior of this oxide, doped or decorated with Pd or Pt nanoparticles, upon exposure to H213-15. The Pt and Pd nanoparticles in these systems promote the room temperature dissociation of the hydrogen molecules into hydrogen atoms that, upon reaction with the tungsten
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oxide support, provoke the coloration (i.e. blue color) of the oxide film.8-10, 13-15 This effect has been associated to the partial reduction of W6+ cations of WO3 according to: WO3 + (3-x) H2 WOx + (3-x) H2O [1]. This process and the electronic structure of WO3 have been investigated by photoemission spectroscopy15-18 and other techniques19,20 and the formation of Wn+ (n
