Open AccessSurface-Induced Changes in the Thermochromic Transformation of an Ionic Liquid Cobalt Thiocyanate Complex
Author(s) -
B. D. May,
Michael Hönle,
Bettina S. J. Heller,
Francesco Greco,
Radha Gobinda Bhuin,
HansPeter Steinrück,
Florian Maier
Publication year - 2017
Publication title -
the journal of physical chemistry letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.563
H-Index - 203
ISSN - 1948-7185
DOI - 10.1021/acs.jpclett.7b00142
Subject(s) - ionic liquid , thermochromism , thiocyanate , chemical physics , cobalt , x ray photoelectron spectroscopy , chemistry , octahedron , solvation , ionic bonding , ion , materials science , crystallography , inorganic chemistry , chemical engineering , catalysis , organic chemistry , engineering
We demonstrate that a thermodynamic complex equilibrium within an ionic liquid film can be significantly influenced by the presence of the liquid-vacuum interface. Using surface-sensitive X-ray photoelectron spectroscopy, we find that the temperature-driven transition from the blue-colored tetrahedral [Co(II) (NCS) 4 ] 2- to the red-colored octahedral [Co(II) (NCS) 6 ] 4- complex already occurs within the outermost nanometers at around +4 °C as compared with -25 °C in the bulk. This thermochromic transformation in the near-surface region goes along with a loss in preferential surface orientation of free [SCN] - anions and with a pronounced decrease in the complex density; both effects are attributed to the formation of a weakly bound solvation shell around the [Co(II) (NCS) 6 ] 4- anion, leading to an effective complex dilution. Our results are not only relevant for high-surface area thin film systems, such as in sensor and catalysis applications, but also shed light on the role of ionic liquid surfaces in particular and liquid surfaces in general.