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| Author(s): |
Murilo Feitosa Cabral
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Carlos. |
| Institution: | Universidade de São Paulo (USP). Instituto de Química de São Carlos (IQSC/BT) |
| Defense date: | 2008-08-29 |
| Examining board members: |
Sergio Antonio Spinola Machado;
Hamilton Brandão Varela de Albuquerque;
Nerilso Bocchi;
Pedro de Lima Neto;
Paulo Olivi
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| Advisor: | Sergio Antonio Spinola Machado |
| Abstract | |
Cd and Pb underpotential deposition (UPD) on gold electrode and selenium thin film, from acid media, were studied using cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM). Different form of deposition for cadmium and lead were observed on gold electrode and selenium thin film. The presence of one peak in the positive sweep with a total charge density value between 42 60 µC cm-2 was observed from the results obtained for Cd upd on Au. This result is attributed to a covering of 0.15 0.20 Cd monolayer on gold electrode with the occupation of one active site by each Cd ad-atom. EQCM results suggest a large increase in mass upon Cd upd, which appears to result from co-adsorbed perchlorate and bisulfate anions. The upd of cadmium on the selenium films were similar to the observed on the gold electrode; however with a higher associated charge of 196 µC cm-2. Moreover, the cadmium upd process on selenium is accompanied by the diffusion of cadmium into the bulk of Selenium films. Also, the co-adsorption of anions and water during cadmium deposition on Selenium films were negligenceable. The UPD process of Pb ad-atoms on gold electrodes occurs with the occupation of two gold atoms for ad-atom ones. Two electrochemical processes are observed and the charge values were conferred a low covering the 0.3 monolayer, with a wrapping up of the type loosely packed. The co-adsorption of perchlorate anions and water were observed during UPD process. The UPD process of Pb ad-atoms on the selenium films is complex, and the formation of H2Se modifies the type of wrapping up of lead ad-atoms on the electrode surface. In this case, that the gotten covering were 0.7 monolayer of lead on selenium, were observed the co-adsorption of anions and water in this UPD process. During the studies carried through for cronoamperometry in high times of polarization (UPD potentials) on the Se films were observed that the cadmium spreads out for the phase of the selenium film. This phenomenon occurs due to the high coefficient of diffusion in the solid state of the Cd, allowing the formation of the CdSe. On the other hand, this process of diffusion was not so evident for the lead, that presents a low coefficient of diffusion in the solid state, and thus, do not support the formation of PbSe. The UPD studies by cyclic voltammetry combined with electrochemical quartz crystal microbalance measurements allowed to observe the changes in the monolayers formation mechanisms of cadmium and lead, as well as the inhibition of H2Se formation. This effect was found in the lead UPD, where was modified the form of packing up of the adatom in the surface. (AU) | |