Operando Identification of the Reversible Skin Layer on Co3O4 as a Three-Dimensional Reaction Zone for Oxygen Evolution.

Co oxides and oxyhydroxides have been studied extensively in the past as promising electrocatalysts for the oxygen evolution reaction (OER) in neutral to alkaline media. Earlier studies showed the formation of an ultrathin CoO x (OH) y skin layer on Co3O4 at potentials above 1.15 V vs reversible hydrogen electrode (RHE), but the precise influence of this skin layer on the OER reactivity is still under debate. We present here a systematic study of epitaxial spinel-type Co3O4 films with defined (111) orientation, prepared on different substrates by electrodeposition or physical vapor deposition. The OER overpotential of these samples may vary up to 120 mV, corresponding to two orders of magnitude differences in current density, which cannot be accounted for by differences in the electrochemically active surface area. We demonstrate by a careful analysis of operando surface X-ray diffraction measurements that these differences are clearly correlated with the average thickness of the skin layer. The OER reactivity increases with the amount of formed skin layer, indicating that the entire three-dimensional skin layer is an OER-active interphase. Furthermore, a scaling relationship between the reaction centers in the skin layer and the OER activity is established. It suggests that two lattice sites are involved in the OER mechanism.

Insights into the Ochratoxin A/Aptamer Interactions on a Functionalized Silicon Surface by Fourier Transform Infrared and UV-Vis Studies.

The association of a mycotoxin-ochratoxin A (OTA)-with a high-affinity DNA aptamer (anti-OTA) immobilized on a functionalized surface has been investigated at the molecular level. Anti-OTA aptamers are coupled by aminolysis in several steps on an acid-terminated alkyl monolayer grafted on a silicon substrate, and Fourier transform infrared spectroscopy in attenuated total reflection geometry is used to assess the immobilization of anti-OTA (in its unfolded single-strand form) and determine its areal density (ca. 1.4/nm2). IR spectra further demonstrate that the OTA/anti-OTA association is efficient and selective and that several association/dissociation cycles may be conducted on the same surface. The areal density of OTA measured after association on the surface (IR spectroscopy) and after dissociation from the surface (UV-vis spectroscopy) falls in the range 0.16-0.3/nm2 which is close to the areal density of a closed-packed monolayer of anti-OTA aptamers folded to form their G-quadruplex structure. The interactions between OTA and its aptamer at the surface are discussed with the help of density functional theory calculations-to identify the complex IR vibrational modes of OTA in solution-and UV-vis spectroscopy-to determine the protonation state of the adsorbing species (i.e., OTA dissolved in the buffer solution).

Electrochemical Stability of the Reconstructed Fe3 O4 (001) Surface.

Establishing the atomic-scale structure of metal-oxide surfaces during electrochemical reactions is a key step to modeling this important class of electrocatalysts. Here, we demonstrate that the characteristic (√2×√2)R45° surface reconstruction formed on (001)-oriented magnetite single crystals is maintained after immersion in 0.1 M NaOH at 0.20 V vs. Ag/AgCl and we investigate its dependence on the electrode potential. We follow the evolution of the surface using in situ and operando surface X-ray diffraction from the onset of hydrogen evolution, to potentials deep in the oxygen evolution reaction (OER) regime. The reconstruction remains stable for hours between -0.20 and 0.60 V and, surprisingly, is still present at anodic current densities of up to 10 mA cm-2 and strongly affects the OER kinetics. We attribute this to a stabilization of the Fe3 O4 bulk by the reconstructed surface. At more negative potentials, a gradual and largely irreversible lifting of the reconstruction is observed due to the onset of oxide reduction.

Structure of Mixed Acid/Decyl Monolayers Grafted on Oxide-Free Si(111) Surfaces.

The structure of mixed acid/decyl monolayers (MLs) grafted on oxide-free Si(111) surfaces by photochemical hydrosilylation in a mixture of neat undecylenic acid and 1-decene is studied in detail. After appropriate surface cleaning of the as-grafted surfaces, atomic force microscopy (AFM) (topography and phase imaging) and calibrated FTIR analysis demonstrate that a mixed monolayer is formed, free of residue. When the acid-molecule fraction (ΓSOL) is >0.1, mixed MLs are homogeneous on the scale of observations and they are only slightly enriched in acid chains with respect to the solution. Conversely, when ΓSOL < 0.1, the acid chain fraction within the ML becomes quasi-independent of the solution composition and may become much larger than ΓSOL. In addition, dark domains are observed in AFM phase images. Correlations between the characteristic parameters of νCO IR bands and AFM phase images suggest a strong phase separation of acid and alkyl chains at various length scales. A model involving a structuration of the grafting solution is proposed to explain observations.

Transmission Surface Diffraction for Operando Studies of Heterogeneous Interfaces.

Processes at material interfaces to liquids or to high-pressure gases often involve structural changes that are heterogeneous on the micrometer scale. We present a novel in situ X-ray scattering technique that uses high-energy photons and a transmission geometry for atomic-scale studies under these conditions. Transmission surface diffraction gives access to a large fraction of reciprocal space in a single acquisition, allowing direct imaging of the in-plane atomic arrangement at the interface. Experiments with focused X-ray beams enable mapping of these structural properties with micrometer spatial resolution. The potential of this new technique is illustrated by in situ studies of electrochemical surface phase transitions and deposition processes.

On the origin of the efficient nanoparticle mediated electron transfer across a self-assembled monolayer.

Recent advances in bioelectrochemistry came from the elaboration of conducting electrodes modified by an organic layer onto which nanoparticles are adsorbed. Self-assembled monolayers on noble-metal electrodes are known to hinder the electrochemical kinetics of fast-transfer redox systems. Surprisingly, fast kinetics are recovered when metal nanoparticles are deposited on top of the monolayer. We show that this surprising behavior can be fully accounted for when realizing that electron transfer from metal to metal is intrinsically easier than transfer between metal and redox system by many orders of magnitude.

Epitaxial growth of gold on H-Si(111): the determining role of hydrogen evolution.

The potential dependence of gold electrodeposition on H-terminated Si(111) is studied in acidic electrolyte by means of atomic force microscopy and X-ray diffraction. The Au films (≤66 monolayers (ML)≈16 nm) are found to be (111)-oriented and in strong epitaxy with the Si(111) surface lattice, with two in-plane orientations separated by 180°. The deposit morphology is controlled by the deposition potential and can be islandlike or atomically flat. The flat morphology is accompanied by a preferential growth of 180°-rotated Au planes with respect to the Si bulk lattice which takes place at potentials where the hydrogen evolution reaction occurs. Obtaining ultraflat Au layers on Si(111) contrasts with the commonly observed islandlike morphology of electrodeposited films on semiconductors. This behavior is discussed in terms of a nucleation coupled with hydrogen evolution reaction (HER) and an enhanced Au adatom mobility induced by this reaction.

Well-defined carboxyl-terminated alkyl monolayers grafted onto H-Si(111): packing density from a combined AFM and quantitative IR study.

This work demonstrates that well-defined mixed carboxyl-terminated/methyl-terminated alkyl monolayers can be prepared in one step on H-terminated Si(111) via direct photochemical hydrosilylation of undecylenic acid and 1-decene mixtures. As evidenced by AFM imaging and IR spectroscopy, a final rinse in hot acetic acid leaves the functionalized surface atomically smooth and perfectly free of physisorbed contaminants while unwanted material remains atop the monolayer with most other common solvents. The compositional surface chemistry was determined from a truly quantitative IR (ATR geometry) study in the range of 900-4000 cm(-)(1). Results prove that neither surface oxidation nor grafting through the carboxyl end groups occurs. Monolayers are fairly dense for such bulky end groups, with a total molecular surface density of approximately 2.7 10(14) cm(-)(2) corresponding to a surface coverage of 0.35 (maximum theoretical value approximately 0.5). Careful analysis of the CH- and COOH-related IR bands reveals that the composition of the grafted layers is richer in acid chains than the starting grafting mixture. A simple model is presented that shows that the grafting kinetics is about twice as fast for undecylenic acid as for 1-decene. Complementary electrochemical impedance measurements indicate the excellent electronic properties of the interface with a very low density of gap states. They also show that the acid terminal groups promote the penetration of water in the outer part of the organic film.

Truly quantitative XPS characterization of organic monolayers on silicon: study of alkyl and alkoxy monolayers on H-Si(111).

The quantitative characterization of the chemical composition (bonding at grafted and ungrafted sites, surface coverage) is a key issue for the application of silicon-organic monolayer hybrid interfaces. The primary purpose of this article is to demonstrate that X-ray photoelectron spectroscopy (XPS) requires to be truly quantitative to deal with two main questions. The first one is accounting for X-ray photodiffraction (XPD), a well-known phenomenon that is responsible for azimuthal variations of the XPS signal intensity. A simple procedure is proposed to account for XPD in angle-resolved measurements. The second critical point concerns the choice of photoelectron attenuation lengths (AL). This article demonstrates that n-alkanethiol self-assembled monolayers on Au(111) can be used as a reference system to derive the effective monolayer thickness on silicon substrates and that one may use the empirical relationship established by Laibinis and co-workers to calculate the relevant ALs (Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991, 95, 7017). A self-consistent approach is presented to justify the above assertions and to give a complete compositional description of alkyl and alkoxy monolayers directly grafted on atomically flat H-Si(111) surfaces. Direct evidences are provided that a Si-C and a Si-O-C linkage is formed, respectively, after reaction with decene and decanol and that the ungrafted sites remain saturated with H atoms. Moreover, the quantitative spectra analysis of satellite peaks at fixed polar angle and three independent angle-resolved Si2p and C1s spectra all give the same surface coverage very close to its theoretical limit.