Enhancement of electrochemical hot electron injection into electrolyte solutions at oxide-covered tantalum electrodes by thin platinum films

The previously reported experimental evidence for hot solution-phase electrons generated at Ta2O5-covered Ta electrodes in both acetonitrile and aqueous solutions using electrogenerated chemiluminescence (ECL) and electrochemical measurements was extended by observing the effect of thin Pt films. Hot electron injection was monitored by noting the photoemission following reduction of the thianthrene radical cation (TH.+), signaling the direct formation of the excited state (TH.+ + e(h)(s) --> TH*), a process that does not occur at a bulk Pt electrode. We report the enhancement of hot electron injection efficiency by a factor of similar to 5 by deposition of a thin (<40 nm) Pt film on the Ta/Ta2O5 electrode. This enhancement of ECL efficiency at the metal/oxide/Pt/liquid interface is ascribed to the suppression of the Ta2O5 surface states by the Pt film. The effect of Pt film thickness was investigated and showed a decrease in emission with increased film thickness, in accord with the expected mean free path of hot electrons in the Pt (e(h)(Pt)). In examining the Ta/Ta2Os/Pt/solution system, one can contrast the behavior observed when an electrical connection is made directly to the Pt with that observed when a connection to the Pt is made via the Ta, where e(h)(Pt) species are generated.