Hot electron injection into aqueous electrolyte solution from thin insulating film-coated electrodes

Abstract

Hot electron injection into aqueous electrolyte solution was studied with electrochemiluminescence and electron paramagnetic resonance (EPR) methods. Both methods provide further indirect support for the previously proposed hot electron emission mechanisms from thin insulating filmcoated electrodes to aqueous electrolyte solutions. The results do not rule out the possibility of hydrated electron being as a cathodic intermediate in the reduction reactions at cathodically pulse-polarized thin insulating film-coated electrodes. However, no direct evidence for electrochemical generation of hydrated electrons could be obtained with EPR, only spin-trapping experiments could give information about the primary cathodic steps. +IntroductionIt has been shown earlier that very different types of luminophores emitting in the visible range can be electrochemically excited during cathodic pulsepolarization of oxide-covered aluminium 1 and n-silicon 2 electrodes. In addition, it was later shown that (9-fluorenyl)methanol (FMOC-OH), emitting strong electrogenerated chemiluminescence (ECL) in the UV range, can be cathodically excited at thin insulating film-coated electrodes such as at oxide-covered aluminium, magnesium and n-silicon electrodes. 3 It was also shown 3 that strong ECL of FMOC-OH could only be produced in the direct field-assisted tunneling regime, 4 while in the Fowler-Nordheim (FN) tunneling regime 4 where the electrons are mainly transferred to the solution from the bottom of the conduction band of the oxide, the ECL intensity decreased exponentially as a function of the oxide film thickness. 3The tunnel-emitted hot electrons and hydroxyl radicals, 2,3,5,6 or in the presence of peroxydisulfate ions, hot electrons and sulfate radicals 2,3 have been proposed to be the primary mediating radicals responsible for the excitation reactions of luminophores in aqueous solution and in electroluminoimmunoassays. 7The aim of this work was to study the other types of luminophores having differing photophysics and redox chemistry would show the analogous behavior when changing from the direct field-assisted tunneling regime to FN tunneling regime. We wanted to study also, whether toluene or phenol could be used instead of FMOC-OH as more simple aromatic model compounds exhibiting high energy ECL in the aqueous medium. Efforts were also made to detect directly the hot or hydrated electrons, and secondary radicals formed from luminophores and coreactants in the cell built in an EPR measurement resonator. Spin-trapping experiments were also conducted.

Cathodic electrochemiluminescence of lucigenin at disposable oxide-coated aluminum electrodes

“…The effect of oxide film thickness on the ECL intensity is presented in Fig.3. The results are analogous to those previously observed with metal chelates and organic luminophores [25,33,43].…”

Hot electron-induced electrochemiluminescence of fluorescein in aqueous solution

“…If the energy of tunnel-emitted hot electrons is above the conduction band edge of water, they may enter the conduction band of water and are likely to become hydrated ðe À aq Þ after thermalization and solvation [19,[21][22][23]. Meanwhile, strongly oxidizing species such as sulfate and hydroxyl radicals can be cathodically generated from purposely added coreactants or from dissolved oxygen [19][20][21][22][23]27].…”

“…Meanwhile, strongly oxidizing species such as sulfate and hydroxyl radicals can be cathodically generated from purposely added coreactants or from dissolved oxygen [19][20][21][22][23]27]. In addition, F + -centers existing in oxide film of C/I/E electrodes may act as oxidants [17,21].…”

“…These thin insulating film-coated electrodes are known to act as pulsed cold-cathodes and tunnel-emit hot electrons ðe À hot Þ into aqueous solutions in an analogous manner to that occurring in solid-state devices [19,21,22,[24][25][26].…”

“…We have earlier shown that some metal chelates such as RuðbpyÞ 2þ 3 [16] and aromatic Tb(III) chelates [17,18], and some organic luminophores as well [19,20] can be excited by cathodic pulse polarization of conductor/insulator/electrolyte (C/I/E) tunnel junction electrodes, e.g., at oxidecoated aluminum, silicon and magnesium electrodes [19,[21][22][23]. These thin insulating film-coated electrodes are known to act as pulsed cold-cathodes and tunnel-emit hot electrons ðe À hot Þ into aqueous solutions in an analogous manner to that occurring in solid-state devices [19,21,22,[24][25][26].…”