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Subnanometric metal particles exhibit anomalous chemical activity, suggesting innovative applications as next-generation materials. However, a precise synthesis and detailed characterisation of these subnano-materials remain a major challenge. Here we summarise recent works on the synthesis of size-controlled tin (Sn) oxide subnanoparticles (SNPs) using the dendrimer template method, and on their detailed characterisation. Size-controlled Sn oxide SNPs (Sn12, Sn28 and Sn60) have been synthesised and they showed a size-dependent compositional change containing not only stable Sn(iv) states but also metastable Sn(ii) states so as to form subnano-scaled particle shapes. Detailed vibrational characterisation of SNPs was achieved by employing subnano-sensitive Raman spectroscopy for spectroscopic characterisation. Combined with density functional theory studies, the inherent subnano-structures of Sn oxide SNPs have been elucidated for the first time. Furthermore, the size-dependent activity of Sn oxide SNPs upon CO oxidation was rationally explained from the simulated structure of Sn oxide SNPs. A detailed understanding of the chemical and physical nature of subnano-materials facilitates the rational design of SNPs for practical applications such as catalysis, biosensors, and electronics.

In this paper we combine two operando methods, Raman spectroscopy and X-ray absorption spectroscopy (XAS), in order to probe reduced graphene-oxide supported tinIV oxide nanoparticles ( SnO 2 NPs @ rGO ) as they are being used to catalyse CO2 electroreduction. To achieve high reaction rates it is necessary to apply sufficiently cathodic electrode potentials. Under such conditions, however, not only CO2 is reduced electrochemically, but also the catalyst particles may be transformed from the initial SnIV state to SnII or, in an extreme case, to metallic Sn. While SnII species still favour CO2 electroreduction, yielding formate as a primary product, on metallic Sn CO2 reduction is disfavoured with respect to the competing hydrogen evolution reaction (HER). We show that operando XAS, a robust technique yielding information averaged over a large surface area and a relatively large thickness of the catalyst layer, is a very expedient method able to detect the reduction of SnO 2 NPs @ rGO to metallic Sn. XAS can thus be used to establish an optimum potential for the electroreduction in practical electrolysing cells. It takes, however, a complementary method offered by operando Raman spectroscopy, having greater sensitivity at the catalyst/electrolyte solution interface, to probe reduction intermediates such as the SnII state, which remain undetectable for ex situ methods. As it is shown in the paper, Raman spectroscopy may also find further use when investigating the recovery of catalyst particles following exposure to extreme reducing conditions.

The promoting effect of water on the electrochemical reduction of carbon dioxide (CO2) from non-aqueous solvents has been studied by means of cyclic voltammetry and in-situ surface-enhanced infrared absorption spectroscopy (SEIRAS). CO2 electroreduction on gold is known to be highly selective towards CO formation in aqueous and in non-aqueous media. The use of non-aqueous solvents is advantageous due to the significantly increased solubility of CO2 compared to aqueous systems. However, in the absence of any proton source, extremely high overpotentials are required for the CO2 electroreduction. In this work, we demonstrate for the first time a tremendous accelerating effect of water additives on the electroreduction of CO2 taking place at gold/acetonitrile interfaces. Already moderate amounts of water, in the concentration range of 0.5 to 0.7 M, are sufficient to decrease significantly the overpotential of CO2 reduction while keeping the CO2 concentration as high as in the pure acetonitrile. The effect of water additives on the mechanism of CO2 electroreduction on gold is discussed on the basis of electrochemical and IR spectroscopic data. The results obtained from gold are compared to analogue experiments carried out on platinum.

A major concern of electrocatalysis research is to assess the structural and chemical changes that a catalyst may itself undergo in the course of the catalyzed process. These changes can influence not only the activity of the studied catalyst but also its selectivity toward the formation of a certain product. An illustrative example is the electroreduction of carbon dioxide on tin oxide nanoparticles, where under the operating conditions of the electrolysis (that is, at cathodic potentials), the catalyst undergoes structural changes which, in an extreme case, involve its reduction to metallic tin. This results in a decreased Faradaic efficiency (FE) for the production of formate (HCOO–) that is otherwise the main product of CO2 reduction on SnOx surfaces. In this study, we utilized potential- and time-dependent in operando Raman spectroscopy in order to monitor the oxidation state changes of SnO2 that accompany CO2 reduction. Investigations were carried out at different alkaline pH levels, and a strong co...

Rh modified Au(111-25 nm) electrodes, prepared by electron beam evaporation and galvanostatic deposition, were employed to study adsorption and electro-oxidation of CO on Rh in 0.1 M sulfuric acid solution by in situ attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS). The results of ATR-SEIRAS experiments were compared with those obtained by infrared reflection absorption spectroscopy on three low-index Rh single crystal surfaces. The Rh film deposited on Au(111-25 nm) electrode consists of 3D clusters forming a highly stepped [n(111) × (111)]-like surface with narrow (111) terraces. When CO was dosed at the hydrogen adsorption potential region, CO adsorbed in both atop (COL) and bridge (COB) configurations, as well as coadsorbed water species, were detected on the Rh film electrode. A partial interconversion of spectroscopic bands due to the CO displacement from bridge to atop sites was found during the anodic potential scan, revealing that there is a potential-dependent preference of CO adsorption sites on Rh surfaces. Our data indicate that CO oxidation on Rh electrode surface in acidic media involves coadsorbed water and follows the nucleation and growth model of a Langmuir-Hinshelwood type reaction. The work was supported by the Research Center Jülich, the University of Bern, Swiss National Science Foundation (200020_144471, 200021-124643), the Spanish Ministerio de Economía y Competitividad (project CTQ2013-44083-P) and University of Alicante. QX acknowledges fellowships of the Research Center Jülich; IP acknowledges support by COST Action TD 1002; and AK acknowledges the financial support by CTI Swiss Competence Centers for Energy Research (SCCER Heat and Electricity Storage).

Perchlorate adsorption on Au(1 1 1) was investigated by cyclic voltammetry and surface-enhanced infrared absorption spectroscopy. We found that the electrosorption valency of ClO4− on Au(1 1 1) is ∼ 0.6 and the total coverage of ClO4− on Au(1 1 1) is higher (∼ 0.15) than previously estimated (∼ 0.04). Based on the experimental adsorption isotherms obtained from infrared spectra and the reconstruction-free cyclic voltammograms, we proposed a mechanism for the ClO4− adsorption on Au(1 1 1).

Electrochemical reactivity and structure properties of electrogenic bacteria, Geobacter sulfurreducens (Gs) were studied to explore the heterogeneous electron transfer at the bacteria/electrode interface using electrochemical and in-situ spectroscopic techniques. The redox behavior of Gs adsorbed on a gold electrode, which is modified with a ω-functionalized self-assembled monolayer (SAM) of alkanethiols, depends strongly on the terminal group. The latter interacts directly with outermost cytochromes embedded into the outer membrane of the Gs cells. The redox potential of bacterial cells bound electrostatically to a carboxyl-terminated SAM is close to that observed for bacteria attached to a bare gold electrode, revealing a high electronic coupling at the cell/SAM interface. The redox potentials of bacterial cells adsorbed on amino- and pyridyl-terminated SAMs are significantly different suggesting that the outermost cytochromes changes their conformation upon adsorption on these SAMs. No redox activity of Gs was found with CH3-, N(CH3)3+- and OH-terminated SAMs. Complementary in-situ spectroscopic studies on bacteria/SAMs/Au electrode assemblies were carried out to monitor structure changes of the bacterial cells upon polarization. Spectro-electrochemical techniques revealed the electrochemical turnover of the oxidized and reduced states of outer membrane cytochromes (OMCs) in Gs, providing evidence that the OMCs are responsible for the direct electron transfer to metal electrodes, such as gold or silver, during the electricity production. Furthermore, we observed spectroscopic signatures of the native structure of the OMCs and no conformational change during the oxidation/reduction process of the microorganisms. These findings indicate that the carboxyl-anchoring group provides biocompatible conditions for the outermost cytochromes of the Gs, which facilitate the heterogeneous electron transfer at the microorganism/electrode interface.

On Au(111) electrodes, the investigation of ClO4− adsorption is hampered by a simultaneous surface reconstruction. We demonstrate that these two processes can be decoupled in cyclic voltammograms by a proper choice of the scan rate and of the initial potential. Our approach allowed the establishment of a relation between potentials of zero charge for the reconstructed and unreconstructed Au(111) surfaces. Keywords: Au(111), Perchlorate adsorption, Surface reconstruction, pzc, SEIRAS

Oxygen reduction on Pt single crystal surfaces has been studied in acidic media using a hanging meniscus rotating disk electrode configuration to understand the effect of crystal orientation of the electrode toward electron transfer reactions. The surfaces used belong to the [ 1 1 ¯ 0 ] zone that can be classified in two different series, i.e. surfaces with (1 1 1) terraces separated by monatomic (1 1 0) steps and surfaces with (1 1 0) terraces and (1 1 1) steps. Both in sulphuric and perchloric acid solutions, the Pt(1 1 1) surface showed the lowest catalytic activity among those studied. Conversely, in agreement with the results reported previously on Pt surfaces belonging to the [ 0 1 1 ¯ ] zone [M.D. Macia, J.M. Campina, E. Herrero, J.M. Feliu, J. Electroanal. Chem. 564 (2004) 141], stepped surfaces, irrespectively to their step site symmetry, show higher catalytic activity on oxygen reduction than those on basal low-index surfaces. The effect on the structure sensitivity on the electrocatalytic activity is discussed in relation to the role of adsorbed species, such as hydrogen, oxygen, OH, electrolyte anions and oxide formation.

Copper underpotential deposition (Cu upd) studies have been carried out on flat and stepped Au single crystal surfaces to demonstrate the influence of crystal orientation on Cu upd characteristics. Seven different Au single crystal surfaces were employed, Au(1 1 1), Au(1 1 0), Au(1 0 0), Au(5 5 4), Au(7 7 5), Au(3 3 2) and Au(7 5 5). Voltammetric contributions for Cu upd at both (1 1 0) and (1 0 0) oriented steps were similar to those on the extended (1 1 0) and (1 0 0) surfaces. In contrast, the formation of the ( 3 × 3 ) R30° structure on (1 1 1) terraces is strongly dependent on their width. An increase in step density and the corresponding reduction in (1 1 1) terrace width favour the ( 3 × 3 ) R30° → (1 × 1) phase transition. This could result from the influence of structural or electronic (Smoluchowski) effects on the energetics and/or kinetics of the phase transition.

Electrolyte cations and anions in aqueous solutions are hydrogen bond breaker. We found that most of anions are strong hydrogen bond breaker whereas cations are not. Further, the electro-reduction of Mg(H2O)6(2+) to Mg(OH)2 and the growth of the Mg(OH)2 were observed on Au electrode surface at negative electrode potential. Water molecules in an electric double layer exhibit an ordered and a disordered structure at negative and pzc (point of zero charge) potentials, respectively. Therefore, electrode potential polarization (negative or pzc potential application) from an equilibrium potential operates an electrified interface to cause increased or decreased ordering, orientation and charge transfer of water molecules as well as dissociation of water on the electrode surface.

The bioelectrocatalytic (oxygen reduction reaction, ORR) properties of the multicopper oxidase CueO immobilized on gold electrodes were investigated. Macroscopic electrochemical techniques were combined with in situ scanning tunneling microscopy (STM) and surface-enhanced Raman spectroscopy at the ensemble and at the single-molecule level. Self-assembled monolayer of mercaptopropionic acid, cysteamine, and p-aminothiophenol were chosen as redox mediators. The highest ORR activity was observed for the protein attached to amino-terminated adlayers. In situ STM experiments revealed that the presence of oxygen causes distinct structure and electronic changes in the metallic centers of the enzyme, which determine the rate of intramolecular electron transfer and, consequently, affect the rate of electron tunneling through the protein. Complementary Raman spectroscopy experiments provided access for monitoring structural changes in the redox state of the type 1 copper center of the immobilized enzyme during the CueO-catalyzed oxygen reduction cycle. These results unequivocally demonstrate the existence of a direct electronic communication between the electrode substrate and the type 1 copper center. Financial support from the Spanish Ministerio de Ciencia e Innovacion (CTQ2010−18570) and Generalitat Valenciana (ACOMP/2013/073) is gratefully acknowledged. The work in Bern was supported by the European Union through the FP7 BacWire Project (Contract MNP4-SL-2009-229337), the CTIProject 13696.1 PFFLR-IV, and the University of Bern through a summer fellowship to V.C.

The methanol oxidation on a hydroxylated Pt (Pt(111)-OH) surface has been investigated by means of infrared reflection absorption spectroscopy (IRAS) in ultra-high vacuum (UHV) and in acidic solution. The Pt(111)-OH surface in UHV was prepared by introducing water molecules on a Pt(111)-(2 x 2)-O surface and annealed at temperature higher than 160 K. Methanol was then, introduced to the Pt(111)-OH surface to show the dependence of the reaction intermediate on the annealing temperature. At an annealing temperature below 160 K, IR bands assignable to methanol overlayer were observed and no detectable intermediates, such as CO, formaldehyde and formate, were formed, suggesting that methanol molecules remain stable on Pt(111) surface without dissociation at this temperature region. At an annealing temperature above 160 K, on the other hand, CO and formate were observed. In addition, the oxidation of CO on Pt(111)-OH showed no sign of formate formation, indicating that formate is not derived from CO, but from a direct oxidation of methanol. Methanol oxidation was carried out in 0.1 mol dm(-3) HClO(4) solution on Pt(111) with a flow cell configuration and showed the formation of formate. These results indicate that the formate is the dominant non-CO intermediate both in UHV and in acidic solution, and the preadsorbed oxygen-containing species, in particular OH adsorbates, on Pt(111) surface plays a very important role in the formate formation process in methanol oxidation reaction.

Fullerene is strongly adsorbed on both single crystal and polycrystalline gold surfaces and its specific adsorption resulted in the formation of high coverage large hexagonal rafts with strong interactions between the adsorbed fullerene molecules and the Au substrate. The oxygen reduction reaction (ORR) was investigated on these surfaces to determine their influence on the reduction mechanism. Oxygen reduction did not take place on the fullerene overlayer but proceeded on the sub-nanometer sized exposed pockets of the underlying Au substrate. Reduction at these confined sites produces hydrogen peroxide selectively. This effect is ascribed to the blocking action, or so-called "third body effect", of the adsorbed fullerene molecules, which do not display electrocatalytic properties for oxygen reduction.