Search

ACS catalysis 12, 3256 - 3268 (2022). doi:10.1021/acscatal.1c05169, 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 Co$_3$O$_4$ 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 Co$_3$O$_4$ 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., Published by ACS, Washington, DC

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).

International audience; The photo isomerization characteristic parameters (photo-stationary states (PSS), cross section quantum yield) of indolyl fulgimide compounds dispersed in solution are quantitatively compared to those of the same compounds covalently immobilized as a monolayer on a functionalized Si(111) surfaces (see Advanced Materials 25, 416 (2013)). The photo isomerization kinetics of fulgimide compounds in solution is monitored using UV-Vis spectroscopy and that of monolayers by in situ calibrated FTIR spectroscopy. While the isomeric composition at photo stationary states are very similar in solution and at surfaces, a strong influence of the incident light polarization is found on photo isomerization cross section  (). Accounting for the local excitation electromagnetic field indicates that the fulgimide groups adopt a preferential tilt angle with respect to the surface normal and that their transition dipole is almost parallel to the surface plane for either C or E/Z isomers. DFT indicates that this orientation corresponds to photochromic groups that are lying in contact with the underlying monolayer. This interaction plausibly explains why the quantum yield of photoisomerization is found nearly twice lower at the surface than in the solution.

In the search for precious-metal-free electrode materials for electrochemical water splitting, transition-metal oxides have been receiving much recent interest as active and stable electrocatalysts...

HAL Id: hal-03021641 https://hal.archives-ouvertes.fr/hal-03021641, 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 Fe3O4 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., We gratefully acknowledge financial support by AGENCIA PICT 20141415 and 2016069, EC‐MEC (ANR‐15‐CE30‐0024‐01 and DFG‐Ma1618/2020), European Research Council—European Union's Horizon 2020 (864628), Austrian Science Fund FWF (Project Z‐250 Wittgenstein Prize), and RyC program RYC‐2017‐21931 and Basque Government Project (IT‐1255‐19). D.G. acknowledges Federico López for assistance with the TOC. We thank the ESRF for the PhD fellowship for T.W. and the ID03 beamline staff—in particular H. Isern.

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

In this work, compact titanium oxide layers are formed by anodization in 1 M H3PO4 aqueous electrolyte containing LiClO4. Elastic Recoil Detection Analysis (ERDA) measurements demonstrate that incorporation of lithium ions occurs during anodization, and cell-voltage transient measurements evidence the different electronic properties of Li-doped TiO2 layers. The mechanism of direct incorporation of Li during the fabrication of TiO2 layers is discussed.

We present experimental data on the electric field effect on the magnetic domain wall dynamics in Co ultrathin films in direct contact with an aqueous electrolyte and in the absence of any oxide layer. We use a three electrode electrochemical setup to apply a large and uniform electric field and to precisely separate chemical effects induced by hydrogen from pure electric field effects. We show that in the case of the pure electric field effect, the domain wall velocity varies exponentially with the electric field and that these variations are larger than those observed previously on similar systems due to a larger magnetoelectric coefficient in our case.We present experimental data on the electric field effect on the magnetic domain wall dynamics in Co ultrathin films in direct contact with an aqueous electrolyte and in the absence of any oxide layer. We use a three electrode electrochemical setup to apply a large and uniform electric field and to precisely separate chemical effects induced by hydrogen from pure electric field effects. We show that in the case of the pure electric field effect, the domain wall velocity varies exponentially with the electric field and that these variations are larger than those observed previously on similar systems due to a larger magnetoelectric coefficient in our case.

The oxidation behavior of ultrathin electrodeposited Co films on Au(111) in alkaline electrolyte was studied using in situ surface X-ray scattering techniques employing synchrotron radiation and complementary optical reflectivity and electrochemical measurements. The films are formed at pH 4 and consist of (001)-oriented hcp Co crystallites that are several nm high, a few ten nm in diameter, and remain largely unchanged after electrolyte exchange to pH 12 solution. In the pre-oxidation peak only minor changes were observed in the diffraction studies, excluding the formation of Co(OH)2 layers. In the potential regime of Co hydroxide formation a rapid reduction of the amount of Co is observed, while the characteristic height of the islands decreases only slightly. On longer times scales, growth of 3D crystals of Co(OH)2 occurs as well as irreversible Co dissolution into the electrolyte is found. On the basis of the structural observations oxidation of the Co film is proposed to proceed via fast formation of an ultrathin passivating layer, followed by nucleation and growth of 3D hydroxide crystals at the grain boundaries in the Co deposit.

We study the initial stage of the electrodeposition of Ag, Pd and Au on Ni submonolayers deposited on unreconstructed Au(111) by in–situ scanning tunneling microscopy. At submonolayer coverage all the deposited Ag forms islands on the Au substrate. Ag partially wets the Ni island edges but does not nucleate on top of the Ni islands. On the other hand, we found a completely different behavior for Pd and Au which nucleate on top of the Ni islands, undergo place exchange with the Ni atoms and also form rims wetting the entire Ni island edges. By selectively dissolving the Ni, we evidence an unexpected behavior: the initial stage of Pd place exchange occurs at the minima of the Ni moire whereas it occurs at the maxima in the case of Au. The origin of the difference between Pd and Au preferential place exchange site is attributed to the presence of a hydrogen overlayer.

The growth and atomic structure of thin Co films electrodeposited onto Au(111) have been studied using in situ surface X-ray diffraction (SXRD) and scanning tunneling microscopy (STM). SXRD is used to monitor the growth in real-time and, following Co deposition, the strain of the Co film is characterized as a function of the Co thickness. In-plane SXRD measurements show that Co deposition leads to the formation of a reconstructed Au layer at the Au–Co interface that is composed of an unfaulted, single Au monolayer uniformly compressed with respect to the underlying bulk Au lattice. The surface normal structure of the Au–Co interface and Co films are examined by measurement of the extended X-ray reflectivity (or specular crystal truncation rod (CTR)) for a nominally two monolayer thick (2 ML) Co deposit. In situ STM has also been used to obtain the morphology of the Co films and the Au(111) reconstruction pattern both before and after Co dissolution. The STM data give complementary information to the SXRD ...

We investigate by in situ scanning tunnelling microscopy (STM) the potential dependence of the electrochemical dealloying of NiPd monoatomic layers electrodeposited on Au(111). The dealloying process is achieved by Ni selective dissolution and was studied as a function of NiPd composition: for an alloy with a Ni content ≥70%, quasi-complete Ni dissolution is achieved at a potential of −0.9 VMSE whereas for a Ni content

Iron-group oxides are among the best earth abundant catalysts for the oxygen evolution reaction (OER) in alkaline and neutral electrolytes [1],[2]. Such oxides have been prepared with an amorphous or a crystalline structure, using thermal salt decomposition [3], autoclave synthesis [4], photochemical reactions [5], sol-gel synthesis [6], metal electrodeposition and subsequent oxidation, [7] and direct oxide electrodeposition [1], [8]. This work focuses on well-defined epitaxial CoOx [9] electrocatalysts grown by direct electrodeposition on Au(111) according to an approach similar to that introduced by Switzer [8]. The growth modes of catalysts and the mechanisms of deposition will be discussed based on electrochemical characterizations, XRD characterizations and microscopic observations. In particular to explain that CoOOH may be obtained at a potential of +1V/RHE by oxidation of a Co(II) complex diluted in an alkaline solution while thermodynamics predicts that Co3O4 should be formed. The electrochemical properties in pre-OER and within OER will be discussed as a function of the surface structure and morphology of layers. In particular to show that the large pseudocapacitance measured at the electrode is not related to its ECSA but to electrical charging within the bulk of the electrode. [1] M. W. Kanan and D. G. Nocera, Science 321, 1072 (2008). [2] C. C. L. McCrory, S. Jung, I. M. Ferrer et al., Journal of the American Chemical Society 137, 4347 (2015). [3] C. Pirovano and S. Trasatti, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 180, 171 (1984). [4] D. Yuming, H. Kun, Y. Lin et al., Nanotechnology 18, 435602 (2007). [5] S. R. Alvarado, Y. Guo, T. P. A. Ruberu et al., The Journal of Physical Chemistry C 116, 10382 (2012). [6] A. Bergmann, E. Martinez-Moreno, D. Teschner et al., Nature Communications 6, 8625 (2015). [7] M. W. Louie and A. T. Bell, Journal of the American Chemical Society 135, 12329 (2013). [8] J. A. Koza, Z. He, A. S. Miller et al., Chem. Mat. 24, 3567 (2012). [9] F. Reikowski, F. Maroun, I. Pacheco et al., ACS Catalysis 9, 3811 (2019).

Iron-group oxides are among the best precious-metal-free catalyst for the oxygen evolution reaction (OER) in alkaline media [1] and thus of great interest for large-scale electrochemical water splitting into O2 and H2. A wide variety of different catalysts, based on oxides of Co, Fe, and Ni, have been synthesized and studied, exhibiting rather diverse nanoscale morphologies and usually an unknown surface structure. This makes it difficult to compare their reactivity and correlate it with ab initio theoretical studies, which hinders the development of clear structure-reactivity relationships and unambiguous determination of the OER reaction mechanism. In this work we report in situ and operando surface x-ray diffraction (SXRD) studies of well-defined Co3O4 and b-CoOOH electrocatalysts [2]. Ultrathin epitaxial films (10-20 nm thickness) of these materials were electrodeposited on Au single crystals from a dilute Co(II) tartrate solution of pH 14, using a modified version of the procedure described in Ref. [3]. Characterization by SXRD and AFM revealed that epitaxial Co3O4(111) and β-CoOOH(001) deposits were obtained in 2 M and 5 M NaOH, respectively. The film orientation is independent of the substrate and in the surface plane aligned with the Au(111) and Au(001) substrate lattice. The β-CoOOH(001) film deposits were found to be smooth with clearly visible atomic steps. The Co3O4 films exhibits a more island-like morphology, which can be controlled by the deposition parameters. The potential-dependent structure of these model oxide electrocatalysts during cyclic voltammograms was studied at pH values between 7 and 13 over a wide potential range up to OER current densities of 150 mA/cm2. For this, a custom designed electrochemical cell was used that allows high-resolution SXRD characterization together with electrochemical and optical reflectivity measurements while gas evolution proceeds. The two oxide materials show very different behavior. The CoOOH(001) samples were perfectly stable over a wide potential range. In the case of Co3O4(111), fast and fully reversible structural changes are observed, in good agreement with a previous operando study of polycrystalline Co3O4 catalysts [4]. Specifically, the near surface region of Co3O4(111) is converted to a new phase, which is assigned to highly disordered CoOOH. This restructuring starts at potentials 300 mV negative of the onset of the OER, indicating that the process is related to the thermodynamically predicted Co3O4 / β-CoOOH(001) transition rather than to the catalytic reaction. The formed skin layer is of defined thickness, which changes linearly with applied potential. In addition, continuous changes of the Co3O4 unit cell volume are found over a wide potential range, which are assigned to changes in the oxide bulk, related to the pseudo-capacitive charging current in the pre-OER potential range. The kinetics of these structural changes is fast and highly reversible. Furthermore, systematic studies of Co3O4(111) films with different morphology show a smaller skin layer thickness and lower magnitude of the bulk oxide changes for smoother deposits, suggesting an important role of more open oxide facets for the structural conversion. Surprisingly, the catalytic activity of the skin layer covered Co3O4(111) and that of the smooth β-CoOOH(001) are almost identical, if the true microscopic surface area is taken into account. This indicates that the number of OER active sites on the two oxides is similar, despite the very different defect density, and is at variance with previous suggestions that di-µ-oxo bridged Co cations are exclusively responsible for the OER activity of Co oxides. For the smooth β-CoOOH(001) a turnover frequency of 4.2 s-1 per surface atom is determined at an overpotential of 400 mV, which exceeds that estimated in most studies of polycrystalline Co oxide electrocatalysts. [1] C. C. L. McCrory et al., Journal of the American Chemical Society 2015, 137, 4347. [2] F. Reikowski, et al., ACS Catalysis, 2019, 9, 3811 [3] J. A. Koza et al., Chem. Mat. 2012, 24, 3567. [4] A. Bergmann, et al., Nature Communications, 2015, 6, 8625. Figure 1

International audience; 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 as grafted surfaces, AFM (topography and phase imaging) and calibrated FTIR analysis demonstrate that a mixed monolayer is formed, free of residue. When the acid-molecule fraction (Γ SOL) is > 0.1 mixed monolayers are homogeneous on the scale of observations and they are only slightly enriched in acid chains with respect to the solution. Conversely, when Γ SOL < 0.1 the acid chain fraction within the ML becomes quasi-independent of the solution composition and may become much larger than Γ SOL. In addition, dark domains are observed in AFM phase images. Correlations between the characteristic parameters of CO IR band 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.

In this work we investigate the magnetic properties and structure of ultrathin Pd/Co/Au(111) stacks prepared by sequential electrodeposition. A combination of in situ real-time magneto optical Kerr effect (MOKE), in situ surface X-ray diffraction (SXRD), and complementary in situ scanning tunneling microscopy, electrochemical studies, and optical reflectivity measurements is employed to characterize the formation of the Pd/Co deposit and its potential-dependent structure. For the preparation of the Pd/Co film a specific procedure is used that prevents Co dissolution. The SXRD results reveal a 2D growth of a (111)-oriented Pd layer, which is in epitaxy with the underlying Co(001)/Au(111). A potential dependent study demonstrates that the Co magnetic properties are reversibly modified upon hydrogen absorption in and release out of the Pd capping film, respectively. Analysis of the SXRD data indicates a Pd lattice expansion upon H-absorption, which is strongly inhibited within the surface plane due to clamping by the substrate. The transmission of hydrogen-induced strain to the Co layer is measured and the resulting impact on the magnetic properties is estimated and discussed.

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.

A recent spectroscopic study showed that alkoxy monolayers grafted on a (111) silicon surface thermally decompose in the 200–400 °C range by fragmentation of the alkyl chains [Surf. Sci. 601 (2007) 3961]. Here this behavior is reproduced by a numerical simulation, assuming that the elementary fragmentation steps have different probabilities, depending on the length of the fragments formed. The variation of the CH 2 /CH 3 ratio, as determined experimentally by infrared spectroscopy, is reproduced with a set of fragmentation probabilities consistent with the enthalpy variation associated with each corresponding step.

In this work, we study the electrochemical deposition of NiAu monolayers on Au(111) surfaces using in situ STM. This system which is characterized by a miscibility gap at room temperature in the case of a bulk alloy is important for connectors in electronic circuits. The microstructure of the alloy is investigated as a function of the composition and the deposition rate. At intermediate Au contents and using a relatively slow alloy deposition rate, a complete phase separation on a 10 nm length scale is observed. At larger deposition rates, the alloy structure presents different phases with several characteristic length scales. For a Au content smaller than 10%, Au and Ni phase separation yields Au clusters of 1–2 nm size embedded in a Ni layer at a specific position with respect to the Ni moire pattern. This phenomenon is interpreted in light of preliminary atomic-scale simulations of the equilibrium structure of the pure Ni monolayer pattern and site-dependent total energy calculations of exchanging Ni atoms by Au.

Creating photoactive monolayers of photochromes is of considerable technological interest. This paper describes the construct of fulgimide monolayers on silicon surfaces and presents a quantitative IR analysis studies of their composition and switching properties. The scheme on top shows the structure of the starting C-form terminated Si(111) surface and the graph below sketches the surface composition at the photostationnary states under visisble and UV irradiation, as derived from in situ IR spectroscopy after several UV/vis irradiation cycles.

The silicon surface may be functionalized with organic groups through direct Si-C bonding, which provides the obtained surfaces with improved chemical stability and allows for the immobilization of probe molecules for the elaboration of sensors. We here review recent work done in our group, using carboxyl-terminated alkyl monolayers on model Si(111) surfaces as a benchmark to study in details further surface functionalization, then extending the processes to amorphous hydrogenated silicon and amorphous hydrogenated silicon/carbon alloys, which can be deposited in thin-film form on any substrate. By designing appropriate structures, fluorescence biochips were then obtained, with strongly improved performance in terms of sensitivity, stability and reliability.

In this work we study the electrochemical growth and dissolution of a Ni on Pd–Au(1 1 1) bimetallic surfaces using in situ scanning tunnelling microscopy. We also compare Ni deposition on monometallic electrodes, i.e. Au(1 1 1) and Pd(1ML)/Au(1 1 1), using electrochemical characterizations. Results evidence that the first Ni monolayer grows preferentially on Au(1 1 1) in a wide potential range, and that a full Ni monolayer covering the entire Pd–Au surface can be selectively dissolved from Pd islands. No such selectivity is observed upon growth of subsequent Ni atomic planes. We demonstrate that the Ni–substrate interactions play a key role in the above mentioned selectivity. The binding energy of Ni to Pd is found to be 80 meV smaller than of Ni to Au. The sign and the amplitude of this difference are discussed in light of the d band filling of the Pd–Au(1 1 1) bimetallic surface and the presence of adsorbed H on Pd before deposition.

Magnetism is highly sensitive to the local atomic environment, a property that is at the origin of the unique magnetic behavior induced by the large ratio of surface or interface atoms to bulk atoms in nanostructures and ultrathin films. One key property of strong relevance in technology is interface-induced perpendicular magnetization anisotropy, which often is observed in ultrathin magnetic layers of only a few atomic planes. Establishing the existence of this phenomenon required an improved control of the growth modes, and advances in its understanding required detailed structural studies coupled with sensitive magnetic characterization. While magnetic nanostructures have been mainly realized using molecular beam epitaxy, the preparation of magnetic nanostructures in the electrolytic environment has become possible, with a degree of control that is comparable to that achieved in ultrahigh vacuum. This article reviews recent studies about epitaxial ultrathin magnetic layers electrodeposited on Au(111) single crystal electrodes with emphasis on in situ structural and magnetic characterization. This article also demonstrates that surface magnetism is very sensitive to the electrochemical interface.

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.

The incorporation of foreign atoms in trace amounts modifies electronic properties and spectral response by the so-called band gap engineering 1. Among different dopants, the incorporation of Li+ cations into TiO2 is important for expanding the oxide’s technological applications. Li+ and H+ doping of TiO2 was carried out using electrochemical processes or salt impregnation, which improved solar to electricity efficiency of Dye Sensitized Solar cells and Perovskite solar cells (where a TiO2 layer is used as electronic transporting layer), as well as photocatalytic performance 2-3. Thermochemical treatments, N and V doping by standard ion implantation or Ag doping by metal plasma ion implantation was also reported 4-6. We recently reported 7 that Li+ is incorporated in the bulk of compact TiO2 layers during the anodization of Ti in lithium perchlorate (LiClO4), moving in the opposite direction expected for a cation under the applied electric field. Using Elastic Recoil Detection Analysis (ERDA) we observed that Li+ is accumulated in the form of more or less flat concentration profile along the thickness of compact layer (Fig. 1). Moreover, cell voltage profiles are strongly affected when Li+ is added to the electrolyte independent of anion of the salt. When the Li+ concentration is increased from 0.1 M to 0.5 M by addition of perchlorate or nitrate, the cell voltage transient changes from a curve typical for a compact anodizing with sparking to that of porous anodizing (Fig. 2). This transient change and the fact that the potential plateau during anodizing decreases as [Li+] is increased indicates that Li+ incorporation into the oxide greatly increases ionic conductivity and Ti oxide dissolution. Preliminary results show that a thick irregular tubular oxide structure is formed. The direct incorporation of Li+ into tubular TiO2 during its formation opens new possibilities for techniques of tubular oxide preparation. A mechanism for Li incorporation against the electric field is presented, as well as for the enhanced oxide dissolution. 1. Y.-C. Nah, I. Paramasivam and P. Schmuki, ChemPhysChem, 2010, 11, 2698–2713. 2. B. H. Meekins and P. V. Kamat, ACS Nano, 2009, 3, 3437–3446. 3. L. Tsui, M. Saito, T. Homma and G. Zangari, J Mater Chem A, 2015, 3, 360–367. 4. F. Giordano, A. Abate, J. P. Correa Baena, M. Saliba, T. Matsui, S. H. Im, S. M. Zakeeruddin, M. K. Nazeeruddin, A. Hagfeldt and M. Graetzel, Nat. Commun., 2016, 7, 10379. 5. N. A. Kyeremateng, N. Plylahan, A. C. S. dos Santos, L. V. Taveira, L. F. P. Dick and T. Djenizian, Chem Commun, 2013, 49, 4205–4207. 6. Y.-Y. Chang, Y.-N. Shieh and H.-Y. Kao, Thin Solid Films, 2011, 519, 6935–6939. 7. J. A. Penafiel-Castro, B. Hahn, R. L. Maltez, G. Knornschild, P. Allongue, and L. F. P. Dick, Chemical Communications , 2018, 54, 3251-3254. Figure 1

Photoluminescence, surface photovoltage, interface capacitance and vibrational infrared spectroscopy measurements have been used to monitor the electronic properties of silicon surfaces protected by covalently grafted organic monolayers. The grafted surfaces present electronic properties equivalent to those of freshly prepared H-terminated surfaces. Moreover, the organic layers are found to improve the lifetime of the surface passivation in ambient atmosphere by orders of magnitude, as compared to a hydrogenated surface. In liquid water, the interfaces are stable up to pH ∼ 9. Above that value, and though silicon oxidation is barely detectable, one observes a marked decrease in the photoluminescence intensity, a sharp change in the value of the surface photovoltage, and a strong increase in the capacitance peak associated with surface states. These changes are not fully reversible when pH is brought back to acidic values. Complementary experiments on the wetting properties of paraffin films as a function of pH suggest that the abrupt degradation of electronic properties above pH ∼ 9 is triggered by interactions between molecular water and the aliphatic chains, which leads to permeation of water at structural defects of the organic monolayer and to localized oxidation of silicon at the interface. This sets an upper limit to the pH range where sensors based on silicon grafted with an organic layer can be used.

In this work, we report on a multi-step route fully compatible with the mild conditions required for anchoring amino acids onto the porous silicon (PSi) surface. A dense monolayer of acid chains is first grafted via thermal hydrosilylation of undecylenic acid at the surface of hydrogenated PSi. The reaction takes place at the terminal C=C double bond and yields an organic monolayer covalently attached to the surface through Si-C bonds. Next, the acid terminal groups are transformed to succinimidyl ester terminations. This reaction, called activation, is achieved using N-hydroxysuccinimide (NHS) in the presence of the watersoluble N-ethyl-N'-(3-dimethylaminopropyl)carbodiimide (EDC). The surface, now bearing an 'activated ester' termination, is subsequently made to react with the amino end of the amino acid, allowing for its covalent attachment through the formation of an amide bond. The reaction efficiency at each stage of functionalization was confirmed using Fourier transform infrared (FT-IR) measurements. SIMS depth profiling showed a significant level of carbon incorporation throughout the PSi layer. Electrochemical behavior of the amino-acid-modified PSi in the presence of copper ions was studied by means of cyclic voltammetry measurements.

The magnetism in ultrathin films and multilayers is an active field of research in strong relationship with applications. Advances in the understanding of such systems came from an improved control of growth modes and detailed structural studies coupled with magnetic measurements, especially in the UHV environment. Thanks to the development of in situ methods, similar studies can now be conducted in the electrolytic environment and investigating the magnetism of nanostructures on the nanometre scale has become a reality. After a brief review of our work about the epitaxial growth and magnetism of ultrathin magnetic layers of Ni, Co and Fe on Au(1 1 1), with an emphasis on the early stages of their electrochemical growth, this article introduces ongoing works which draw attention about the specificity and the opportunities offered by the electrochemical interface.

The electronic properties of hydrogenated and alkyl-grafted silicon surfaces have been investigated by photoluminescence, surface photovoltage and electrochemical capacitance measurements, which are sensitive methods for monitoring the presence of electronic defects. On p-type silicon, the electronic quality of the interface was found to depend critically on the surface preparation and was studied with special care. The thermal grafting of one monolayer of linear organic chains on atomically flat silicon allows the formation of stable alkyl-grafted p-Si surfaces, exhibiting a very low density of recombination centers comparable to those measured on hydrogenated silicon surfaces. Photoluminescence measurements show that the high electronic quality of the grafted surfaces is preserved on a month scale in air, indicating a much more efficient long term passivation by comparison with hydrogenated Si surfaces. The stability of the silicon/organic layer interface in aqueous buffer solution has been assessed by in situ photoluminescence and surface photovoltage measurements. Some degradation of the electronic properties was evidenced during exposure in basic solution of pH > 9.

This paper reviews our recent work about gold electrodeposition on H-terminated Si(111). It is shown that Au(111)/Si(111) epitaxial layers are grown and that the film morphology can be varied according to the deposition conditions (potential and solution pH). At pH = 14 (cyanide solution) selective gold nucleation at the substrate monatomic step edges is observed. A homogeneous nucleation is obtained at pH 4 (chloride solution). In the former case 3D growth occurs and in the latter case ultra smooth buffer layers are obtained. Application of these substrates to formation of magnetic nanostructures with tuneable properties is demonstrated.

In order to grow magnetic layers on silicon substrates, a non-magnetic buffer layer is often needed to avoid silicide formation and to reproduce the perpendicular magnetic anisotropy obtained on metal single crystals, as in the case of Co on Au(1 1 1) and Pt(1 1 1). In this context, we have studied the electrochemical growth of Au buffer layers, and show that it is possible to obtain different film morphologies on hydrogen-terminated vicinal Si(1 1 1) surfaces by varying the electrochemical deposition parameters and solution composition. Two different morphologies have been obtained as observed by atomic force microscopy: continuous 2D Au films (chloride solution at pH 4), and films consisting in flat top 3D Au islands decorating the Si(1 1 1) step edges (cyanide solution at pH 14). X-ray diffraction measurements reveal that the gold layer and islands have Au(1 1 1) orientation and are in epitaxy with the Si(1 1 1) surface. In the case of islands, the lateral facets have also Au(1 1 1) orientation. Results are discussed within a model in which the breaking of the Si–H surface bonds plays a major role in the Au nucleation and growth mechanisms.

Au/Co(4–8 ML)/Au single magnetic layers and Au(8 ML)/Co(4 ML)/Au(8 ML)/Co(8 ML)/Au bilayer were sequentially grown by electrodeposition on an Au(1 1 1) buffer layer electrodeposited on Si(1 1 1). The technique used in this work provides full control on the structure and the chemical composition of the different layers (no alloying) as well as on the chemistry at interfaces. scanning tunneling microscopy (STM) and atomic force microscopy (AFM) imaging and X-ray diffraction measurements show that atomically flat continuous Co(0 0 0 1) layers (4–8 ML) can be grown in epitaxy with the Au(1 1 1) substrate and that the 2 nm-thick spacer is also a continuous Au(1 1 1) layer. The Co ultrathin layers (4 and 8 ML) exhibit perpendicular magnetic anisotropy. The lateral magnetic homogeneity and magnetization reversal process have been investigated by scanning magneto-optical Kerr effect (MOKE) magnetometry and global Kerr microscopy. The correlation between magnetization switching behaviour in each layer of the Co-bilayer stack has been evidenced from in-depth sensitive MOKE measurements and microscopy. The strong coupling observed between the two Co layers is attributed to magnetostatic interaction at domain wall boundaries.

Fourier Transform Infrared (FTIR) spectroscopy was used to study in-situ the anodic oxidation of n-type Si(111) in KOH solution. Changes in surface chemistry were followed during oxide growth. The results are considered on the basis of a model developed from electrochemical measurements.

Gold (111) is a widely used substrate in vacuum and in electrolytes for studying the initial stages of metal growth. Its surface reconstruction is unavoidable using common preparation procedures in vacuum and this affects the growth mechanism of metal overlayers. In the electrochemical environment, the same surface reconstruction is obtained at potentials negative of the Au(111) potential of zero charge, whereas above this potential the surface structure is (1 × 1). In this paper we present a preparation procedure which allows blocking the Au step edges by Pd islands, inhibiting the formation of the Au(111) reconstruction and yielding the unreconstructed Au(111) surface (1 × 1) at any electrochemical potential. Using in-situ scanning tunnelling microscopy, we compare the Ni/Au(111) electrochemical epitaxy on reconstructed and on unreconstructed Au(111). Observations show that performing Ni growth on the (1 × 1) surface drastically improves the long-range order of the (8 × 8) moire pattern created by the lattice mismatch between the Au substrate and Ni adlayer. Detailed characterizations of the moire structure and of its electrochemical dependence provide new insights into the relative energy landscape of the multiple adsorption sites of the Ni atoms within the (8 × 8) unit cell. The possibility of stabilizing a Au(111)-1 × 1 surface opens up new perspectives for studying metal growth and molecular organisation.

This paper is a short review about the principle, mechanisms and applications of epitaxial electrochemical growth of ultrathin metal films on single crystal metal electrode surfaces. Starting from an atomic scale view of electrodeposition the parallel is made with the parent technique molecular beam epitaxy (MBE) in ultra high vacuum (UHV). A first section is devoted to homoepitaxy studies. Subsequent sections deal with selected studies of heteroepitaxy of magnetic materials and electrocatalysts.

This paper is a brief review of self-ordered electrochemical growth on single-crystal electrode surfaces on which arrays of nanostructures are created by replication of patterns made by atomic steps or surface reconstruction. Whenever possible the parallel is made between electrochemical growth and molecular beam epitaxy in ultrahigh vacuum. An atomistic view of electrodeposition is given first to help with identifying the similarities of and specific differences between the two techniques of growth. Recent examples where self-organized nanostructures are prepared on metals and silicon substrates are discussed.

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.

The magnetic state of electrodeposited iron on Au(111) substrates is investigated during the electrodeposition process by means of in situ alternating gradient force magnetometer (AGFM) and in situ perpendicular magneto-optic Kerr effect measurements (PMOKE). A perpendicular magnetic anisotropy (PMA) is observed for a thickness below 2 monolayers (ML). In situ STM observations of the initial stages of iron growth and ex situ EXAFS are used to correlate the structure of films to their magnetic properties. r 2004 Elsevier B.V. All rights reserved.

As a first step toward the development of photocathodes for use in water-purification reactors, we have investigated the behaviour of p-type Si(1 0 0)-oriented single crystals and glow-discharge a-Si:H layers as photocathodes for the reduction of dissolved dioxygen. The electrode surfaces were prepared by HF rinsing and were characterised by cyclic voltammetry in the dark and under tungsten-halogen lamp illumination. This study has been performed in dilute-buffered fluoride medium, in dilute sulphuric acid, and lastly in three buffered phosphate media at pH 2, 7 and 12. A study using a rotating-disk electrode (RDE) arrangement permits to distinguish the contribution from dioxygen reduction (mass-transport dependent) and that from hydrogen evolution (kinetically limited). Crystalline Si and a-Si:H appear to exhibit rather similar behaviours. The Si surface acquires catalytic properties for dioxygen reduction upon negative pre-polarisation. However, these catalytic properties do not appear usable in practice for efficient operation, which calls for chemical modifications of the silicon surface.

Electrochemical micromachining (ECM) of p-type Si substrates is accomplished in HF-based solutions by applying nanosecond potential pulses between the substrate and a tungsten tool electrode. With sufficiently high potential pulses, the silicon potential locally reaches the electropolishing regime and microstructures may be machined. ECM precision is investigated as a function of pulse height, pulse duration, solution composition, and silicon doping level. Results show that micrometer precision may be obtained with highly doped substrates and that experimental data can be explained within a simple model, taking the charging time of the interface capacitance into account. In highly doped p-Si, well-defined microstructures can be realized without application of a mask on the surface. In addition, the isotropy of the process allows fabrication of structures not constrained by the crystal direction. In the case of low-doped material, ECM is only possible for very short pulses (

The oxidation and reduction processes of both polycrystalline tin and Sn(100) single crystals are investigated in borate buffer solution of pH 7.5 using cyclic voltammetry and in-situ characterizat...

International audience; The growth mechanisms of Co and Ni/Au(1 1 1) ultrathin layers electrodeposited from dilute sulfate solutions are investigated using in situ STM and electrochemical measurements. Bulk cobalt deposition occurs for potentials 0.25 V negative of the Nernst potential ECo0 of the reaction Co 2+ + 2e - → Co, while bulk nickel may be deposited very close to ENi0. Closer to the Nernst potential slow nucleation is driven by place exchange for both metals. Subsequent growth leads to ML-thick cobalt nanostructures confined onto hcp regions of reconstructed Au(1 1 1) surface, which only cover a small faction of the gold surface, and to a progressive needle growth of nickel until full surface coverage. At greater overpotentials both Co and Ni films grow epitaxially on Au(1 1 1) in a layer-by-layer fashion under appropriate conditions of polarization. The main difference concerns the first Co layer which is strained (4.4%) and biatomic whereas nickel growth starts with a relaxed monolayer. The mechanisms of deposition are discussed from a detailed analysis of STM images, atomic models of the interface are presented and a strain relief mechanism is discussed in light of dissolution experiments.

The possibility of preparing structurally well-defined and periodically arranged metal islands on a semiconductor is very attractive for optical and magnetic applications. In this work we used electrochemical deposition to grow nm-sized gold islands that decorate steps on defect-free vicinal H terminated Si(1 1 1) surfaces prepared by chemical etching. The gold deposits were studied by atomic force microscopy and X-ray diffraction. Results show that gold nucleates exclusively along the steps and that the density of nuclei is controlled by the electrode potential. Nearly prefect replication of the periodic array of straight monatomic steps is achieved at sufficiently negative potential. XRD indicates that the structure of gold films evolves from powder-like, close to the onset potential of nucleation (−1.6 V), to strongly epitaxial with the (1 1 1) orientation at more negative potentials. A reaction model and a growth mechanism are proposed to account for the origin of the selective nucleation and the excellent epitaxy obtained. In particular they discuss whether or not the H-monolayer remains intact under the deposit.