New chromogenic single and multi-layered transition metal oxide (TMO) thin films of ReO3-WO3 [1], MeO-IrO2 [2], WO3/Cu/WO3 and rare-earth metal oxy-hydrides (REHO) YHO [3, 4] were developed and fabricated by advanced Reactive High Power Impulse Magnetron Sputtering (R-HiPIMS) technique and industrially scalable roll-to-roll (R2R) technology for applications in Smart Windows (SW). Thin films of rare-earth metal oxy-hydrides (REHO) are a new class of inorganic mixed-anion materials with prominent photochromic effect and a light-induced resistivity change.

Here we examine chromogenic metal oxides of tungsten, molybdenum, iridium, nickel and mixed oxides and compare them to related crystalline species in order to set a foundation for the structure-property relationships in these oxides. X-ray absorption spectroscopy (XAS) is an ideal non-destructive technique for characterising chromogenic materials, devices as well as for in-operando study chromogenic processes. Synchrotron radiation XAS, comprised of both the high-resolution near-edge structure (XANES) and extended fine structure (EXAFS), is element sensitive to the local physical and electronic structure about both the metal (W, Ir, Mo, Ni, etc.) and oxygen target atoms in the chromogenic oxides. Starting from our pioneering works in the 90s, we have applied XAS to the study of cathodic electrochromic oxides such systems as WO3, WO3-NiO, MoO3, ReO3-WO3 [5] and to anodic oxides such as NiOx, IrOx, and recently ZnO-IrOx, NiO-VOx, NiO-IrOx. In the study of thermochromic materials, XAS has been applied to V(W)O2, CuWO4-ZnWO4, and recently, to the photochromic YH3 and Y-O-H systems [3,4].

In the amorphous electrochromic oxides (a-WO3, a-MoO3, a-ReO3, a-IrOx, a-Ta2O5, a-V2O5), the EXAFS analysis indicates the first coordination shell consists of oxygen atoms in a distorted octahedral geometry, with a second shell consisting of metals. No higher shells are observed beyond 3.5 Å for all samples, indicating the metal oxides are "truly" amorphous, consistent with X-ray diffraction results. While in the nanocrystalline anodic films (n-NiO, n-NiO-IrOx), the EXAFS analysis indicates the structural peaks well visible up to 6 Å. The effect of size reduction is observed as a progressive decrease in peak amplitude upon increasing distance. The oxidation state and local geometry of the metal ions in amorphous or nanocrystalline thin films of WO3 (W6+/W5+ oxidation state), IrOx (Ir4+/Ir3+), Y-O-H (Y3+/Y2+) are determined using XANES spectroscopy.

Financial support was provided by the Project “Smart Windows for Zero Energy Buildings” (SWEB) GA nr. 101087367.

References

[1] B. Polyakov, E. Butanovs, A. Ogurcovs, A. Sarakovskis, M. Zubkins, L. Bikse, J. Gabrusenoks, A. Kuzmin, J. Purans ACS Omega, 2022, 7, 1827.

[2] M. Zubkins, J. Timoshenko, J. Gabrusenoks, K. Pudzs, A. Azens, Q. Wang, J. Purans Phys. Status Solidi B 2022, 259, 2100374.

[3] J. Purans, A. P. Menushenkov, S. P. Besedin, A. A. Ivanov, V. S. Minkov, I. Pudza, A. Kuzmin, K. V. Klementiev, S. Pascarelli, O. Mathon, A. D. Rosa, T. Irifune, M. I. Eremets Nature Commun, 2021, 12, 1765.

[4] M.Zubkins, I.Aulika, E.Strods, V.Vibornijs, L.Bikse, A.Sarakovskis, G.Chikvaidze, J.Gabrusenoks, H.Arslan, J.Purans Vacuum, 2022, 203, 111218.

[5] J. Kleperis, G. Bajars, L. Grnberga, G. Vaivars, J. Purans, Ģ. Vitiņs, G. Kucinskis, J Solid State Electrochem, 2023, 27, 1641.