Kleme [l] (hence forward abbreviated KA) commented on the observation of hydrogen evolution at activated silver electrodes by hot electrons as published by D. Diesing, S. RiiRe, A. Otto and M.M. Lohrengel in this journal [2]. KA proposed" that the effect of tunneling currents has mainly been based on electrochemical generation of hydrated electrons by tunnel emission of hot electrons from metal/insulator junctions acting as cold cathodes followed by ballistic or near ballistic transport of hot electrons through thin Ag films into the conduction band of water in cases where the Fermi level of electronemitting A l or Mg film was driven above -1.3 eV on vacuum scale".KA's reasoning is based on the following point: "Tunnel emission occurs at an electric field strength where the triangular potential barrier is sufficiently thinned by electric field for quantum mechanical tunnelling to occur." [In other words, tunneling occurs when the tunneling voltage UT is big enough to change the trapezoidal tunneling barrier (see Fig. 2) into a triangular barrier]. Tunneling in this case is known as Fowler-Nordheim tunneling. However it should be noted, that tunneling currents in the MIMs produced by Diesing et al. sets in directly when I (IT I > 0 V as demonstrated by the nonlinear tunneling characteristic IT(UT) (see Fig. 1). 0 -2 -3 -4 N , 1 c .--10 0.0 -1 2 -2.0 -1.5 -1 .o -0.5 Fig. 1 tunnel voltage U , [V] Absolute tunneling current I, as function of the tunneling voltage U, for an aluminium-aluminium oxide-silver tunneling contact for forward and reverse sweeps with dU,/dt = 50 mV/s at room temperature The onset of Fowler-Nordheim tunneling has been observed for negative UT at UT = -2.71 V and for positive UT at UT = 2.05 V [3]. These values deliver the energetic distances between the Fermi levels of A1 and Ag and the bottom of the conduction band of aluminium oxide in Fig. 2. Fig. 2 Energy scheme of the tunnel junctions (in ev) and of the lower edge of the electron conduction band PE ( = D state in [4]) and of the onset potential of the hydrogen evolution at a platinum electrode H,O/OH, both measured in the acetate electrolyte [2] and the position of the lower edge W of the socalled wet electron state and the center H of the hydrated state of the electron in an aqueous electrolyte [4], all levels at the electrochemical potential of the top silver electrode of 0 VSCE. (In this case the Fermi levels of the silver electrode and of the metallic mercury in the SCE (EF(SCE)) are at the same level.) 1 eV on the energetic scale corresponds to 1 V,,, on the electrochemical potential scale KA's diagram of the electronic levels is based on values of the workfunctions of the metals and of the electron affinity of the oxide. The neglect of dipole layers between the oxide and the metals by KA explains the differences between our experimentally determined scheme and that of KA. The analogous neglect in the case of Schottky barriers leads to a wrong prediction of the activation energies. The position of the bottom of the electronic cond...
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