Hot electron-induced time-resolved electrogenerated luminescence of Tb(III) ions at stationary oxide-covered aluminium electrodes

Injection of tunnel-emitted hot electrons from a pulse-polarised oxide-covered aluminium electrode into aqueous Tb(III) ion solution induces electrogenerated luminescence which comprises 5D4 → 7FJ transitions of Tb(III). The luminescence lifetime of a hydrated Tb(III) ion is ca. 500 μs while that of the main component of the cathodic intrinsic electrogenerated luminescence of the oxide film is only ca. 6 μs. This allows sensitive detection of Tb(III) ion on a time-resolved basis. Tb(III)-specific electrogenerated luminescence is enhanced by the presence of azide ions or coreactants like hydrogen peroxide and peroxodisulfate ions which produce highly oxidising radicals by one-electron reduction. The Tb(III)-specific electrogenerated luminescence is produced via several parallel mechanisms which usually involve oxidation state changes of Tb(III) that are difficult to produce in an aqueous medium. The cathodically produced primary species are tunnel-emitted hot electrons and F+ centres of the oxide film. These primary species are capable of hard reductions and oxidations not normally encountered in the electrochemistry of aqueous solutions, and can probably produce hydrated electrons and hydroxyl radicals which are able to mediate reductions and oxidations in aqueous solution beyond the tunnelling distance from the oxide/electrolyte interface. The present electrogenerated luminescence is formed as a sum of the luminescence emitted by Tb(III) ions adsorbed at the oxide/electrolyte interface, partially incorporated in the oxide, and located in the solution at the close vicinity of the oxide/electrolyte interface.