Valentina Lazarescu

pH effects on the potentiodynamic behavior of the Pt(111) electrode in acidified NaClO4 solutions

The pH increase was found to bring important changes in the potentiodynamic behavior of the Pt(111) electrode in perchlorate media: an additional couple of anodic-cathodic electrode processes is developed in the double layer region limited by the usual and unusual adsorption states, while all the peaks are shifted toward more negative potentials. The shift of the anomalous anodic wave (60 mV/pH unit) is present, however, only at pH < 4. The pH values around 4 proved to be critical for the other electrode processes too. The hydrogen evolution reaction ceased to occur around this pH value, being replaced by the water cathodic reduction. The surface H + discharge step disappears also at a pH of 4.5 as a consequence of the site blocking with species formed in the high potential region. The results plead for the dissociative adsorption of the water dependent on the local changes of the pH within the solution close to the electrode induced by the surface reactions themselves.

Potential-Induced Conformational Changes in an α-CN-terthiophene Thiolate Film on GaAs(110)

Second harmonic generation (SHG) investigations on alpha-CN-terthiophene-thiolate-covered GaAs(110) electrodes in 1 N H2SO4 solution revealed significant changes in the rotational anisotropy of the SH response. The enhancement of the 1- and the 3-fold contributions around -250 mV suggests changes in the symmetry properties of the delocalized electron system due to an alteration of the adsorption geometry induced by the applied potentials. The analysis of the EIS data showed that in the potential region where the SH signal exhibits the more important changes the Mott-Schottky plot undergoes a pronounced shift to more negative potentials as a result of the charging of the surface states grouped about 1.06 eV below the conduction band edge. Semiempirical MO calculations suggest that the most energetically favorable interaction implies electron transfer from the semiconductor conduction band to the lowest unoccupied molecular orbital of the organic molecule with epsilon(LUMO)=-1.707 eV. Such a chemisorption bond bringing the organic molecule to a quasi-planar position is well supported by the major changes in the XPS spectra of the electrochemically biased samples with respect to the as-prepared ones. Two distinct N 1s species instead of one and a shift of 1.6 eV to higher BE of the terthiophene S 2s core level are strong evidence for a potential-induced change in the adsorption geometry. Taking into account that the acceptor-like surface state group located close to the semiconductor valence edge (EC,S=-1.06 eV) may correspond to the LUMO level (shifted downward by the adsorption process), we assume that the organic molecule, initially adsorbed by the thiolate end, undergoes a conformational change from a tilted to an almost flat position when the applied potential brings the semiconductor Fermi level into its neighborhood. This assumption is in a very good agreement with the potential-induced variation in the thiol thickness estimated from the thiol capacitance.

Fermi Level Pinning at n-GaAs(110) Electrodes

Abstract Mott-Schottky plots and potential-dependent second harmonic generation measurements performed at n-GaAs(110) electrodes in 1 N H2SO4 revealed the Fermi level pinnig near midgap. The possible nature of the bandgap states is discussed on the basis of the information provided by electrochemical impedance spectroscopy and in situ spectroscopic ellipsometry studies