- Email: [email protected]
Iridium Oxide Films
237
Chapter 14 IRIDIUM OXIDE FILMS The earlier chapters have dealt with electrochromic thin films based on oxides comprising octahedral MeO 6 units with predominant corner-sharing, so that the overall stoichiometry tends towards MeO 3. This chapter, and several following ones, is devoted to rutile-like materials with MeO6 octahedra having both comer-sharing and edge-sharing; the stoichiometry then tends towards MeO 2. Specifically, the present chapter considers Ir oxide. This material has been investigated in detail, so that certain aspects are better known than they are for W oxide. Iridium oxide shows anodic electrochromism, i.e., the optical modulation is opposite to that in oxides of W and Mo. Some comments on bulk-like IrO2-based materials are given first (14.1). Electrochromic thin films of Ir oxide films have been produced by several different techniques, and the ensuing films have been designated by acronyms such as AIROF, SIROF, TOIROF, AEIROF, CEIROF, and PRIROF. Here IROF stands for "iridium oxide film" and A, S, TO, AE, CE, and PR denote "anodic" (339), "sputtered" (3041), "thermally oxidized" (3023), "anodically electrodeposited" (3705), "cathodically electrodeposited" (3705), and electrodeposited by a "periodic reverse" voltage (3749). The references are to the papers in which these acronyms were introduced. These terms are not used below, but the films are referred to simply as "Ir oxide". This chapter basically follows the same format as Ch. 12 dealing with Mo oxide. The various sections cover the preparation and characterization of films made by evaporation and sputterdeposition (14.2) and by electrochemical and chemical techniques (14.3), ion intercalation/ deintercalation reactions and diffusion constants (14.4), ion intercalation/deintercalation studied by electrochemical (14.5) and physical (14.6) techniques, optical properties (14.7), and coloration efficiency (14.8).
14.1
Crystal Structure of Bulk-like Iridium Oxide
The electrochromically active Ir oxide is akin to, but not idential with, IrO 2. The latter material has been studied in detail by Murphy et al. (2441) and Rogers et al. (2926); the structure is of rutile type and can be thought of as constructed from almost octahedral building blocks. More specificlly, the ideal rutile structure may be described as an hexagonal closepacked oxygen lattice with octahedrally coordinated metal ions forming edge-shared infinite chains. The chains are crosslinked so that they form an equal number of identical vacant channels. These channels may allow intercalation/deintercalation of ionic species. The channels contain two tetrahedral and one octahedral site per lr ion, where ion accommodation seems likely. Lil.5IrO2 with (pseudo)tetragonal structure is known. The channel diameter in lrO~ is as small as -0.13 nm, so it is not obvious how
Chapter 14 IRIDIUM OXIDE FILMS The earlier chapters have dealt with electrochromic thin films based on oxides comprising octahedral MeO 6 units with predominant corner-sharing, so that the overall stoichiometry tends towards MeO 3. This chapter, and several following ones, is devoted to rutile-like materials with MeO6 octahedra having both comer-sharing and edge-sharing; the stoichiometry then tends towards MeO 2. Specifically, the present chapter considers Ir oxide. This material has been investigated in detail, so that certain aspects are better known than they are for W oxide. Iridium oxide shows anodic electrochromism, i.e., the optical modulation is opposite to that in oxides of W and Mo. Some comments on bulk-like IrO2-based materials are given first (14.1). Electrochromic thin films of Ir oxide films have been produced by several different techniques, and the ensuing films have been designated by acronyms such as AIROF, SIROF, TOIROF, AEIROF, CEIROF, and PRIROF. Here IROF stands for "iridium oxide film" and A, S, TO, AE, CE, and PR denote "anodic" (339), "sputtered" (3041), "thermally oxidized" (3023), "anodically electrodeposited" (3705), "cathodically electrodeposited" (3705), and electrodeposited by a "periodic reverse" voltage (3749). The references are to the papers in which these acronyms were introduced. These terms are not used below, but the films are referred to simply as "Ir oxide". This chapter basically follows the same format as Ch. 12 dealing with Mo oxide. The various sections cover the preparation and characterization of films made by evaporation and sputterdeposition (14.2) and by electrochemical and chemical techniques (14.3), ion intercalation/ deintercalation reactions and diffusion constants (14.4), ion intercalation/deintercalation studied by electrochemical (14.5) and physical (14.6) techniques, optical properties (14.7), and coloration efficiency (14.8).
14.1
Crystal Structure of Bulk-like Iridium Oxide
The electrochromically active Ir oxide is akin to, but not idential with, IrO 2. The latter material has been studied in detail by Murphy et al. (2441) and Rogers et al. (2926); the structure is of rutile type and can be thought of as constructed from almost octahedral building blocks. More specificlly, the ideal rutile structure may be described as an hexagonal closepacked oxygen lattice with octahedrally coordinated metal ions forming edge-shared infinite chains. The chains are crosslinked so that they form an equal number of identical vacant channels. These channels may allow intercalation/deintercalation of ionic species. The channels contain two tetrahedral and one octahedral site per lr ion, where ion accommodation seems likely. Lil.5IrO2 with (pseudo)tetragonal structure is known. The channel diameter in lrO~ is as small as -0.13 nm, so it is not obvious how