Hemin interaction with bare and 4,4′-thio-bis-benzene-thiolate covered n-GaAs (110) electrodes

Abstract

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) investigations on the redox behavior of hemin at bare and 4,4'-thio-bis-benzene-thiolate (TBBT) covered n-GaAs (110) electrodes in dimethylsulfoxide (DMSO) revealed the high irreversibility of the electroreduction process, which appeared to be closely related to the stable adsorbed species strongly interfering with the electronic properties of the semiconducting substrate. The subsequent exploration of the hemin-modified electrodes by second harmonic generation (SHG), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements pointed to significant differences between the iron protoporphyrin species adsorbed on the bare- and TBBT-GaAs (110) electrodes. Only Fe(2+) species having a flat configuration with the porphyrin plane oriented parallel to the surface were detected on GaAs, unlike the TBBT-GaAs, where Fe(2+) and Fe(3+) species having both flat and vertical adsorption positions could be observed. These differences originate from the mutual interactions between the solvent, hemin and dithiolate molecules as well as their competition for the surface sites found to play a key role in the electrochemical process under discussion.

Covalent Grafting of Carbon Nanotubes with a Biomimetic Heme Model Compound To Enhance Oxygen Reduction Reactions

“…The iron content was determined to be 0.84 at % by XPS analysis. The Fe 2p core‐level XP spectrum displayed peaks at 709.9 and 722.1 eV, which were assigned to the BE values of Fe 2p3/2 and Fe 2p1/2 , respectively (see Figure S3 in the Supporting Information) 20a. The N 1s XP spectral region of the (DFTPP)Fe‐Im‐CNTs showed a broad nitrogen signal, which was deconvoluted into three components (Figure 3 c).…”

Immobilisation of Iron Porphyrin from an Equilibrium Solution with Diazonium‐Functionalised Axial Ligand: Dependence of Film Composition on Grafting Potential

“…[4b,6,16] The high-resolution Fe 2p core-level XPS spectrum for the [FeTPP(Im-Ph) 2 ] + -modified sample (Figure 6a) shows major peaks at binding energy (BE) 710.0 eV (Fe-2p 3/2 ) and 723.0 eV (Fe-2p 1/2 ) and a Fe-2p 3/2 satellite at 714.4 eV. This spectrum is typical for Fe in the + 2 oxidation state, [17] indicating that reduction of the Fe centre occurs during XPS measurement. The high-resolution N 1s spectra for [FeTPP(Im-Ph) 2 ] + -modified GC (Figures 6b, c) show very clear differences between the films grafted to À 0.2 and À 0.6 V. The spectra can be fitted with 4 main peaks, listed with their assignments in Table 2.…”

Surface states mediated charge transfer in redox behavior of hemin at GaAs(100) electrodes

“…The lower kinetic energy of the 2p-lines considerably increases their surface sensitivity as compared to the 3d-lines. One may see that hemin adsorbed on p-GaAs(100) completely suppressed both the substrate core-level lines Ga-As (BE ¼ 1117.3 AE 0.1 eV)/As-Ga (BE ¼ 1323.4 AE 0.1 eV) [67][68][69][70] and those of Ga-O (BE ¼ 1118.5 AE 0.1 eV) and As-O (BE ¼ 1326.1 AE 0.2 eV) species, [67][68][69][70] which were replaced by two new species with higher binding energies, BE ¼ 1327.1 AE 0.1 eV and BE ¼ 1119.2 AE 0.1 eV, assigned to the carboxylate species As-COO and Ga-COO, 12 respectively. These data suggest that hemin is bound on p-GaAs(100) by its carboxyl groups, known to be responsible for the self-assembled iron porphyrins on the GaAs surfaces.…”

“…The further examination of the hemin-covered GaAs electrodes by second harmonic generation, X-ray photoelectron spectroscopy and atomic force microscopy pointed to a possible change in the spin conguration of the iron ions preceding the charge transfer step originating in the mutual interactions between solvent, hemin and surface sites. 12 Since surface states and eld effects were found to depend signicantly at GaAs electrodes on both the surface orientation and the charge carrier type, [14][15][16][17] the comparison between the redox behaviour of hemin on Siand Zn-doped GaAs(100) in the same electrolyte should give a good hint about the role played by the charge carrier type in this charge transfer process. It is the aim of this paper to present the results of our electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) investigations in this respect.…”