Infrared reflectance spectroscopy of electrochromic NiO x H y films made by reactive dc sputtering
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A. Gorenstein and F. Decker DFA/IFGW, UNICAMP, CP 6165 Campinas, SP, Brazil (Received 2 January 1991; accepted 4 April 1991)
Films of NiO^Hy were made by reactive dc magnetron sputtering of Ni followed by electrochemical treatment in KOH. Infrared reflectance spectroscopy with obliquely incident p-polarized light documented a unique topochemical reaction with the bleached state being /3-Ni(OH) 2 and the colored state being /3-NiOOH. The same technique yielded clear, though not yet fully understood, data on the evolution of electrochromism during potentiodynamic cycling.
I. INTRODUCTION AND MOTIVATION This paper reports infrared reflectance spectra for NiO^Hy films, made by reactive dc sputtering of Ni and subsequent electrochemical treatment in KOH, and interprets the electrochromism in terms of a well-defined topochemical reaction. Electrochromic materials exhibit reversible and persistent changes of their optical properties under the action of an applied electric field.1'2 Thin films of these materials enable a dynamic control of the throughput of radiant energy, which makes them suitable for energyefficient fenestration1-3 ("smart windows"), antidazzling devices for vehicles,1 high-contrast nonemissive information displays,1'4'5 etc. In a practical transparent device, the electrochromic film is integrated in a multilayer configuration, which also comprises a counter electrode, an ion conductor, and transparent electronic conductors. Optical modulation is associated with electrochemical insertion/extraction of ions in the electrochromic film. Numerous inorganic and organic materials exhibit electrochromism.12'6 NiO^Hy is of particular interest7"9 and can show a large span in luminous and solar transmittance between fully bleached and fully colored states, good durability, and low materials cost. Further, it can be deposited9 by reactive dc magnetron sputtering, which is an attractive technique for large-area high-rate thin film manufacturing. Most investigations on electrochemically induced changes in NiO^Hy elucidate features relevant to the uses of this material, in bulk-like form, in rechargeable alkaline batteries.10'11 From such battery-related studies it has been shown that the pertinent reversible charging/discharging reaction can be written, in a way a)
Current and permanent address: Universidad Nacional de Ingenieria, Facultad de Ciencias, Apartado 1301, Lima, Peru. J. Mater. Res., Vol. 6, No. 8, Aug 1991
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that illustrates the topochemical mechanism, as /?-Ni(OH) 2 +± (3-WiOOK2-z
1012
ze
(1)
Here /3-Ni(OH) 2 has a brucite C6-type structure (hexagonal D 3 d 3 site symmetry) with stacked layers of nickeloxygen octahedra; the hydroxyl groups are parallel to the c-axis and lie alternately above and below the (0001) plane.11'13 The hydroxyl groups are "free"; i.e., hydrogen bonding is absent, and therefore the vibrational modes of the OH~ do not couple to the lattice modes. This leads to a characteristic infrared absorption signature10'11'13 with
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