X-ray Photoelectron Spectroscopy (XPS) Investigation of Alkylation of Glassy Carbon Electrodes Via Reduction of Primary Alkyl Halides

Alkylation of glassy carbon electrodes during the reduction of primary alkyl halides presents a problem for electrochemical experiments involving this class of compounds.  Details of the grafting of primary alkyl moieties onto the surface of a glassy carbon electrode have come to light as a result of recent research conducted by Simonet and co-workers.^1–6  These findings as well as work from our laboratory have revealed that surface alkylation results during an electrolysis of primary alkyl halides.  In a controlled-potential electrolysis, this effect can be especially problematic if the actual reduction potential for a chosen compound shifts to a value that is more negative than the targeted potential.  To understand further the effects and mechanism of this surface alkylation, we have employed X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry to examine and characterize glassy carbon and reticulated vitreous carbon electrodes before and after their surfaces have been alkylated via the reduction of alkyl halides.

Shown in Figure 1 are cyclic voltammograms for the reduction of 1-iodooctane at a glassy carbon electrode in acetonitrile (MeCN) containing 0.050 M tetramethylammonium tetrafluoroborate (TMABF₄).  Between the scans shown in the figure, the electrode was not polished; instead, grafting of octyl moieties onto the electrode surface was allowed to continue with each successive scan until a limiting state was achieved, as evidenced by little or no further negative shift of the cathodic peak potential.  In addition, we have investigated the effects of this surface alkylation on the redox behavior of some common electron-transfer mediator systems involving 1,2,4,5-tetracyanobenzene, 4-nitropyridine N-oxide, nickel(II) salen, and cobalt(II) salen, with significant changes being observed.  As an example, the effects of a surface-octylated glassy carbon electrode on the redox behavior of 1,2,4,5-tetracyanobenzene are depicted in Figure 2.

A previously proposed mechanism for surface alkylation suggests that sp²-hybridized carbon atoms act as carbanions when electrons are forced to the surface of the electrode at negative potentials, thereby promoting an S_N2-type attack on primary alkyl halides to produce grafted alkyl moieties.³  To investigate this process further, the surfaces of glassy carbon and reticulated vitreous carbon electrodes have been examined by means of XPS before and after the electrochemical reduction of perfluorobutyl iodide (C₄F₉I) in DMF containing 0.10 M tetra-n-butylammonium tetrafluoroborate (TBABF₄).  It is noteworthy that cyclic voltammograms for the reduction of C₄F₉I are similar to those shown in Figure 1 for 1-iodooctane, an observation indicating that perfluorobutyl moieties are grafted onto the surface of a glassy carbon cathode at negative potentials.  Using perfluorobutyl iodide as the substrate for electrochemical experiments involving XPS allowed us to distinguish carbon signals for grafted perfluorobutyl groups from the sp² and sp³ carbon signals for the glassy carbon surface itself.  Our XPS results support the proposed mechanism, with the data showing a conversion of sp² to sp³ surface carbon atoms.  A freshly polished glassy carbon electrode from our laboratory was found to have a sp²:sp³ surface-carbon ratio of approximately 4:1.  After cyclic voltammetric experiments involving the reduction of C₄F₉I, the sp² component of the surface carbons was diminished, with the resulting sp²:sp³ ratio being approximately 1:1, a result consistent with the proposed grafting mechanism.  Signals corresponding to the presence of carbon bonded to fluorine (CF₂ and CF₃ moieties) were also observed, with no iodine present, all findings in accord with the grafting of C₄F₉radicals onto the surface of the electrode.

References

1. Simonet, J. Electrochem. Commun. 2011, 13, 107–110.
2. Jouikov, V.; Simonet, J. Appl. Electrochem. 2012, 42, 527–537.
3. Jouikov, V.; Simonet, J. Langmuir 2012, 28, 931–938.
4. Poizot, P.; Durand-Drouhin, O.; Lejeune, M.; Simonet, J. Carbon 2012, 50, 73–83.
5. Jouikov, V.; Simonet, J. J. Electrochem. Soc. 2013, 160, G3008–G3013.
6. Hui, F.; Noël, J.-M.; Poizot, P.; Hapiot, P.; Simonet, J. Langmuir 2011, 27, 5119–5125.