Nanometer-Thick Nickel Oxide Films Prepared from Alanine-Chelated Coordination Complexes for Electrochromic Smart Windows

The use of coordination complexes as metal precursors in electrodeposition of transition-metal oxides has been shown to be important for film formation at the nanoscale. Here, we present a systematic investigation of nickel oxide (NiOx) films prepared from Ni(II) complexes with bidentate l-alanine ligands or monodentate β-alanine ligands and their potential for use in electrochromic windows. In situ Ni K-edge X-ray absorption fine structure (XAFS) spectra show that the chemical structures of the NiOx films in their colored and bleached states are γ-NiOOH and Ni(OH)2, regardless of the type of Ni(II)-alanine ligand used. O K-edge XAFS spectra indicate that l-alanine is present only near the surface of the film but not in the bulk of the film, whereas β-alanine is absent in both. The Ni (II)-l-alanine complex leads to the deposition of a homogenous film with small cluster and grain sizes compared to the Ni(II)-β-alanine complex; therefore, this film exhibits exceptional electrochromic performance. The coloration efficiency of the film increases by ca. 50% to 74.8 cm2/C. The starting oxidation potential of the film is also lower than that of the other films. Further, there was almost no change in optical modulation for the film with an increase in the ambient temperature to 50 °C, which suggests that the film deposited from the Ni(II)-l-alanine complex is a potential electrochromic material for smart windows. This work opens new avenues for the use of coordination complexes as metal precursors for the preparation of transition-metal oxide nanostructures.