ENERGY RELAXATION MECHANISMS OF ELECTROLUMINESCENCE IN SINTERED TIO2 ELECTRODES

The charge-transfer mechanism of electroluminescence in a sintered Nb-doped TiO2 was investigated by luminescence transient observed when a step bias was applied to the electrode in a Na2S2O8 + NaOH solution. The transient can be characterized by the relaxation time which depends on the concentrations of Na2S2O8 and NaOH. It was found that the existence of NaOH drastically improved the quantum efficiency of the electroluminescence. The analysis of the transient strongly suggested the existence of surface complexes such as Ti-OH-ad and Ti-OH*ad which can mediate charge exchange between the electroactive species in solution such as SO*BAR4 and the surface states which are inherent to the TiO2 surface. Almost all experimental observations of luminescence transients could be explained by the rate equations. The native surface state was assumed to distribute exponentially from the conduction bandedge to the midgap in the forbidden gap. This assumption was successful in explaining the observed spectral distribution which could be divided into a visible region and an infrared region. In the former, the spectral intensity had had a peak at 460 nm and decreased exponentially from 550 to 700 nm while in the latter, the light intensity increased exponentially from 750 to 930 nm which was the detection limit of the photomultiplier used. The intensity of visible light emitted from the sintered TiO2 was about two orders of magnitude larger than that from a single-crystal TiO2 and the intensity of infrared light was more than two orders of magnitude larger than that of visible light emitted from the same sintered sample. These spectral distributions were explained by a model of a generalized energy relaxation mechanism. It is pointed out that the dynamic electron distribution on the surface state determines the spectral distribution of electroluminescence in this system. Another luminescence band, which peaked at 850 nm, was assigned to the electron transfer from the conduction band to the surface complex Ti-OH*ad. Only this mechanism corresponds to the conventionally accepted charge-transfer mechanism for electroluminescence.