Electrochromism in Nickel-based Oxides Coloration Mechanisms and Optimization of Sputter-deposited Thin Films

Abstract: Electrochromic properties of sputter-deposited nickel-based oxide films have been studied with a two-fold goal. From a practical point of view, the optical switching performance has been improved by optimizing the deposition conditions and film stoichiometry with respect to oxygen and hydrogen, and further by adding Mg, Al, Si, Zr, Nb or Ta to the films. From a theoretical point of view, details of the coloration mechanism have been studied by means of electrochemical intercalation (CV, GITT), optical measurements (UV, VIS, NIR and MIR), RBS, XRD, XPS and EXAFS.Optimization of deposition conditions has been illustrated by the example of films made by sputtering of a non-magnetic Ni(93)V(7) % wt. target in an atmosphere of Ar/O2/H2. The optimized films exhibit transmittance modulation between 20% and 75 % at 18 mC/cm2 charge intercalation. The remaining problem with nickel oxide and nickel vanadium oxide films is their residual yellow-brown color tint in the bleached state, which disappears as the short-wavelength transmittance increases upon addition of Mg, Al, Zr or Ta. Optimization of deposition conditions by co-sputtering from two targets and the film composition for mixed oxide films has been illustrated by the example of nickel aluminium oxide.The mechanisms of coloration upon electrochemical charge insertion and ozone exposure have been investigated. In the beginning of the electrochemical cycling, first, a reconstruction and crystallization is observed with the outer most part of the grain surface being transformed from oxygen rich nickel oxide into nickel oxy-hydroxide and hydroxide by transfer of H+ and OH- groups. After the charge capacity has been stabilized, only a transfer of H+ occurs with two reversible reactions involved: the first one from Ni(OH)2 to NiOOH and the second one from NiO and Ni(OH)2 to Ni2O3.Ozone coloration is described by a similar reaction scheme. The ozone molecule is split on the surface and dehydrogenates Ni(OH)2 into NiOOH. Further dehydrogenation produces Ni2O3 as in the electrochemical coloration.