Figure 2 shows spectral absorptance A(A) calculated from

A(A) = 1 — T(A) — R(A).

The data in panel (a) represent the bleached state for the nickel-based oxides mentioned before. Prior to the measurements, the films were cycled ten times in a 1 M KOH solution to stabilize the properties. A significant decrease of A(A) was found at short wavelengths for additives being Mg, Al, Si, Zr, Nb, and Ta, whereas the films containing V and Ag did not show any improvement in their optical properties compared to those of pure nickel oxide.

(a) (b)

The strong absorptance at A < 350 nm is due to the semiconductor band-gap, which appears to be widened as a consequence of the addition of Mg, Al, Si, Zr, Nb, or Ta. On the other hand, the addition of V or Ag narrowed the band-gap. Some weak absorption features can be discerned in the spectral data; they are possibly associated with crystal-field effects [16,17]. Alternatively, the additives may affect
optical absorption caused by defects such as vacancies, over-stoichiometry, grain boundaries, etc. Thermodynamically stable nickel oxide is a p-type conductor due to excess oxygen [18,19]. It is then plausible that the p-type conductivity and the residual optical absorption in the bleached state originate from the same electron states, and this may explain why films of pure Ni oxide cannot be made completely colorless. When Al is added, for example, it can act as a donor of electrons and fill the electron (hole) states on Ni, thereby reducing the residual absorption. The addition of V, on the other hand, may provide acceptor states whose effect would be to enhance the residual absorption.

Figure 2(b) reports comparative data for iridium-based films, cycled in 1 M propionic acid. It is found that additives of Mg, Al, Ta, and Zr tend to lower the absorption, as may be understood by the same arguments as those applicable to the nickel-based oxides.