Unlocking the Fluorine-Free Buoy Effect: Surface-Enriched Ruthenium Polypyridine Complexes in Ionic Liquids.

Controlling the local concentration of metal complexes at the surface of ionic liquids (ILs) is a highly sought-after objective due to its pivotal implications in supported ionic liquid phase (SILP) catalysis. Equally important is to avoid per- and polyfluorinated substances due to environmental concerns. Herein, we investigate the surface enrichment of Ru polypyridyl complexes with fluorine-free alkylic side groups of varying lengths and shapes, using the hydrophilic IL [C2C1Im][OAc] as solvent. Additional charged carboxylate groups are included into the polypyridyl ligands to increase the solubility of the complex in the IL. When the ligand system is functionalized with long and hydrophobic alkyl side chains, the complex predominantly localizes at the IL/vacuum interface, as deduced from angle-resolved X-ray photoelectron spectroscopy. Conversely, in the presence of short or more bulky substituents, no surface enrichment is observed. This buoy-like behaviour with fluorine-free side groups is explored for 0.05 %mol to 1 %mol solutions. Intriguingly, surface saturation occurs at approximately 0.5 %mol, which is beneficial to the efficient operation of catalytic systems featuring high surface areas, such as SILP catalysts.

Demetalation of Surface Porphyrins at the Solid-Liquid Interface.

Understanding the factors that control the demetalation of surface porphyrins at the solid-liquid interface is important as the molecular properties of porphyrins are largely determined by their metal centers. In this work, we used X-ray photoelectron spectroscopy (XPS) to follow the demetalation of Zn and Cd tetraphenylporphyrin molecules (ZnTPP and CdTPP) adsorbed as three-monolayer-thin multilayer films on Au(111), by exposing the molecular layers to acidic aqueous solutions. We found that porphyrin molecules at the solid-liquid interface are less prone to lose their metal center than molecules in solution. We propose that this behavior is due to either the incoming protons provided by the solution or the outgoing metal ion having to pass through the hydrophobic porphyrin multilayers where they cannot be solvated. Our results are relevant for the design of molecular devices based on porphyrin molecules adsorbed on solid surfaces.

Metal Nanoparticle Enhancement of Electron Transfer to Tethered Redox Centers through Self-Assembled Molecular Films.

Metal-nanoparticle-mediated electron transfer (ET) across an insulator thin film containing nanoparticles with attached redox centers was studied using electrochemical impedance spectroscopy. Specifically, a gold spherical microelectrode was modified with 16-amino-1-hexa-decanethiol, creating an insulator film. This was followed by the electrostatic adsorption of gold nanoparticles and the covalent attachment of Os2+ redox centers. A variation of the Creager-Wooster method was developed to get quantitative information regarding the ET kinetics of the system. The experimental data obtained from a single measurement was fitted with a model that decouples two or more ET processes with different time constants and considers a Gaussian distribution of tunneling distances. Two parallel ET mechanisms were observed: one in which the electrons flow by tunneling between the surface and the redox couples with a low kET0 = 1.3 s-1 and a second one in which an enhancement of the electron transfer is produced due to the presence of the gold nanoparticles with a kET0 = 7 × 104 s-1. In this study, we demonstrate that the gold nanoparticle electron transfer enhancement is present only in the local environment of the nanoparticle, showing that the nanoscale architecture is crucial to maximize the enhancement effect.

Electronic Structure of a Self-Assembled Monolayer with Two Surface Anchors: 6-Mercaptopurine on Au(111).

The electronic structure of aromatic and aliphatic thiols on Au(111) has been extensively studied in relation to possible applications in molecular electronics. In this work, the effect on the electronic structure of an additional anchor to the S-Au bond using 6-mercaptopurine as a model system has been investigated. Results from X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and density functional theory (DFT) confirm that this molecule adsorbs on Au(111) with S-Au and iminic N-Au bonds. Combined ultraviolet photoelectron spectroscopy and DFT data reveal that formation of the 6MP self-assembled monolayer generates a molecular dipole perpendicular to the surface, with negative charges residing at the metal/monolayer interface and positive charges at the monolayer/vacuum interface, which lowers the substrate work function. Scanning tunneling microscopy shows two surface molecular domains: a well-ordered rectangular lattice where molecules are tilted on average 30° with respect to the substrate and aligned 6MP islands where molecules are standing upright. Finally, we found a new electronic state located at -1.7 eV with respect to the Fermi level that corresponds to a localized π molecular state, while the state corresponding to the N-Au bond is hybridized with Au d electrons and stabilized at much lower energies (-3 eV).

Adsorption of 4,4'-Dithiodipyridine Axially Coordinated to Iron(II) Phthalocyanine on Au(111) as a New Strategy for Oxygen Reduction Electrocatalysis.

The coordination of PySSPy to FePc was monitored by UV/Vis spectroscopy while the adsobed FePc, anchored by PyS-Au(111), was examined by in situ STM in 0.1 M HClO4 and X-ray photoelectron spectroscopy (XPS). Rotating-disc-electrode (RDE) and linear-sweep-voltammetry (LSV) studies on the resulting FePc-modified Au(111) electrodes in an oxygen-saturated 0.1 M NaOH electrolyte exhibit excellent electrocatalytic properties for the oxygen reduction reaction (ORR), with a smaller overpotential than that observed for Au(111) with FePc deposited by direct adsorption from a benzene solution.

Surface Structure of 4-Mercaptopyridine on Au(111): A New Dense Phase.

4-Mercaptopyridine (4MPy) self-assembled on Au(111) has been studied by in situ electrochemical scanning tunneling microscopy (EC-STM) in HClO4, cyclic voltammetry, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). Samples prepared by varying the immersion time at constant concentration named short time (30 s) and long time (3 min) adsorption have been studied. Cyclic voltammetry and XPS showed that the chemistry of the adsorbed molecules does not depend on the adsorption time resulting in a well established chemisorbed thiol self-assembled monolayer on Au(111). EC-STM study of the short time adsorption sample revealed a new self-assembled structure after a cathodic desorption/readsorption sweep, which remains stable only if the potential is kept negative to the Au(111) zero charge potential (EPZC). DFT calculations have shown a correlation between the observed structure and a dense weakly adsorbed phase with a surface coverage of θ = 0.4 and a (5 × âˆš3) lattice configuration. At potentials positive to the EPZC, the weakly adsorbed state becomes unstable, and a different structure is formed due to the chemisorption driven by the electrostatic interaction. Long time adsorption experiments, on the other hand, have shown the typical (5 × âˆš3) structure with θ = 0.2 surface coverage (chemisorbed phase) and are stable over the whole potential range. The difference observed in long time and short time immersion can be explained by the optimization of molecular interactions during the self-assembly process.

Simplified Approach to Work Function Modulation in Polyelectrolyte Multilayers.

The layer-by-layer (LbL) method is based on sequential deposition of polycations and polyanions. Many of the properties of polyelectrolyte thin films deposited via this method depend on the nature of the topmost layer. Thus, these properties show odd-even oscillations during multilayer growth as the topmost layer alternates from polycations to polyanions. The work function of a (semi)conductive substrate modified with an LbL polyelectrolyte multilayer also displays an oscillatory behavior independent of film thickness. The topmost layer modulates the work function of a substrate buried well below the film. In agreement with previous observations, in this work, we show that the work function of a gold substrate changes periodically with the number of adsorbed layers, as different combinations of polycations and polyanions are deposited using the LbL method. For the first time, we rationalize this behavior in terms of formation of a dipole layer between the excess charge at the topmost layer and the charge of the metal substrate, and we put forward a semiquantitative model based on a continuum description of the electrostatics of the system that reproduces the experimental observations.

Molecular and electronic structure of osmium complexes confined to Au(111) surfaces using a self-assembled molecular bridge.

The molecular and electronic structure of Os(II) complexes covalently bonded to self-assembled monolayers (SAMs) on Au(111) surfaces was studied by means of polarization modulation infrared reflection absorption spectroscopy, photoelectron spectroscopies, scanning tunneling microscopy, scanning tunneling spectroscopy, and density functional theory calculations. Attachment of the Os complex to the SAM proceeds via an amide covalent bond with the SAM alkyl chain 40° tilted with respect to the surface normal and a total thickness of 26 Å. The highest occupied molecular orbital of the Os complex is mainly based on the Os(II) center located 2.2 eV below the Fermi edge and the LUMO molecular orbital is mainly based on the bipyridine ligands located 1.5 eV above the Fermi edge.

XPS analysis of enzyme and mediator at the surface of a layer-by-layer self-assembled wired enzyme electrode.

High potential purified Trametes trogii laccase has been deposited in mono- and multilayer thin films on gold surfaces by layer-by-layer electrostatic adsorption self-assembly. The osmium bipyridil redox relay sites on polycation poly(allylamine) backbone efficiently work as a molecular "wire" in oxygen cathodes for biofuel cells. X-ray photoelectron spectroscopy of Cu 2p3/2 and Os 4f signals provided chemical information on the enzyme and redox mediator surface concentrations after different adsorption steps. The electrical charge involved in oxidation-reduction cycles of the osmium sites, the ellipsometric enzyme film thickness, and the mass uptake from quartz crystal microbalance experiments, correlate with the XPS surface concentration, which provides unique evidence on the chemical identity of the composition in the topmost layers. XPS is shown to be an important analytical tool to investigate stratified copper and osmium distribution in LbL thin films relevant to biosensors and biofuel cells.

High-activity mesoporous Pt/Ru catalysts for methanol oxidation.

High activity mesoporous Pt/Ru catalysts with 2D-hexagonal structure were synthesized using a triblock poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) copolymer (Pluronic F127) template. The normalized mass activities for the methanol oxidation reaction (MOR) of the Pt/Ru catalysts with a regular array of pores is higher than those reported for nanoparticulated Pt/Ru catalysts. Different kinetic parameters, as Tafel slope and activation energy, were obtained for the MOR on the mesoporous catalysts. Results indicated that catalysts performance depends on pore size. Mass activities and the CO2 conversion efficiency for large pore size mesoporous catalysts (10 nm) are greater than those reported for smaller pore size mesoporous catalysts with similar composition. The effect of pore size on catalysts performance is related to the greater accessibility of methanol to the active areas inside large pores. Consequently, the overall residence time of methanol increases as compared with mesoporous catalyst with small pores.

Self-assembled monolayers of NH2-terminated thiolates: order, pKa, and specific adsorption.

Self-assembled monolayers (SAMs) of amino-terminated alkanethiols on Au were characterized by a combination of electrochemical (LSV, CV, and EIS) and spectroscopic (XPS and SER) techniques. Clear correlations were obtained between the apparent surface pKa values determined by impedimetric titrations and order parameters such as the content of trans conformers in the SAMs. These results contrast with previous studies that exhibit dispersions of up to 6 pH units in the reported pKa values. In addition, we determined that inorganic and organic phosphate species bind specifically to these SAMs mediating adsorption and heterogeneous electron transfer of positively charged macromolecules such as cytochrome c.

Molecular and electronic structure of electroactive self-assembled monolayers.

Self-assembled monolayers (SAMs) containing electroactive functional groups are excellent model systems for the formation of electronic devices by self-assembly. In particular ferrocene-terminated alkanethiol SAMs have been extensively studied in the past. However, there are still open questions related with their electronic structure including the influence of the ferrocene group in the SAM-induced work function changes of the underlying metal. We have thus carried out a thorough experimental and theoretical investigation in order to determine the molecular and electronic structure of ferrocene-terminated alkanethiol SAMs on Au surfaces. In agreement with previous studies we found that the Fc-containing alkanethiol molecules adsorb forming a thiolate bond with the Au surface with a molecular geometry 30° tilted with respect to the surface normal. Measured surface coverages indicate the formation of a compact monolayer. We found for the first time that the ferrocene group has little influence on the observed work function decrease which is largely determined by the alkanethiol. Furthermore, the ferrocene moiety lies 14 Å above the metal surface covalently bonded to the alkanethiol SAM and its HOMO is located at -1.6 eV below the Fermi level. Our results provide new valuable insight into the molecular and electronic structure of electroactive SAMs which are of fundamental importance in the field of molecular electronics.

Phosphate mediated adsorption and electron transfer of cytochrome c. A time-resolved SERR spectroelectrochemical study.

The study of proteins immobilized on biomimetic or biocompatible electrodes represents an active field of research as it pursues both fundamental and technological interests. In this context, adsorption and redox properties of cytochrome c (Cyt) on different electrode surfaces have been extensively reported, although in some cases with contradictory results. Here we report a SERR spectroelectrochemical study of the adsorption and electron transfer behaviour of the basic protein Cyt on electrodes coated with amino-terminated monolayers. The obtained results show that inorganic phosphate (Pi) and ATP anions are able to mediate high affinity binding of the protein with preservation of the native structure and rendering an average orientation that guarantees efficient pathways for direct electron transfer. These findings aid the design of Cyt-based bioelectronic devices and understanding the modulation by Pi and ATP of physiological functions of Cyt.

O2 induced Cu surface segregation in Au-Cu alloys studied by angle resolved XPS and DFT modelling.

Surface segregation effects on polycrystalline Au-Cu alloys (Au(0.80)Cu(0.20), Au(0.85)Cu(0.15) and Au(0.90)Cu(0.10)) were studied at room temperature by angle resolved XPS (ARXPS) and density functional theory (DFT) before and after exposure to O(2). Au surface enrichment was found as predicted from calculations showing that this process is energetically favourable, with a segregation energy for Au in a Cu matrix of -0.37 eV atom(-1). Surface enrichment with Cu was observed after exposure to O(2) due to its dissociative adsorption, in agreement with DFT calculations that predicted an energy gain of -1.80 eV atom(-1) for the transfer of Cu atoms to a surface containing adsorbed oxygen atoms, thus leading to an inversion in surface population.

Some evidence for the formation of an azo bond during the electroreduction of diazonium salts on Au substrates.

Molecular films obtained by electrochemical reduction of diazoniuim tetrafluoroborate salts [4-carboxybenzene (PhCOOH) and 4-amino-(2,3,5,6-tetrafluoro)-carboxybenzene (PhF(4)COOH)] on Au substrates and post-functionalization with an osmium pyridil-bipyridine complex are studied by a combination of X-ray photoelectron (XPS) and polarization-modulation infrared reflection absorption spectroscopy (PM-IRRAS). The spectroscopic evidence suggests the formation of N=N bonds tethering the complexes to Au. The surface coverage of the azo-bonded osmium complexes strongly depends on the electrode potential. The resulting tethered osmium redox centres were characterized by cyclic voltammetry and impedance spectroscopy. Similar electron transfer-rate constants were measured for both fluorinated and non fluorinated benzene-linked Os complexes.

Self-assembly of thiolated cyanine aggregates on Au(111) and Au nanoparticle surfaces.

Heptamethinecyanine J-aggregates display sharp, intense fluorescence emission making them attractive candidates for developing a variety of chem-bio-sensing applications. They have been immobilized on planar thiol-covered Au surfaces and thiol-capped Au nanoparticles by weak molecular interactions. In this work the self-assembly of novel thiolated cyanine (CNN) on Au(111) and citrate-capped AuNPs from solutions containing monomers and J-aggregates has been studied by using STM, XPS, PM-IRRAS, electrochemical techniques and Raman spectroscopy. Data show that CNN species adsorb on the Au surfaces by forming thiolate-Au bonds. We found that the J-aggregates are preferentially adsorbed on the Au(111) surface directly from the solution while adsorbed CNN monomers cannot organize into aggregates on the substrate surface. These results indicate that the CNN-Au interaction is not able to disorganize the large J-aggregates stabilized by π-π stacking to optimize the S-Au binding site but it is strong enough to hinder the π-π stacking when CNNs are chemisorbed as monomers. The optical properties of the J-aggregates remain active after adsorption. The possibility of covalently bonding CNN J-aggregates to Au planar surfaces and Au nanoparticles controlling the J-aggregate/Au distance opens a new path regarding their improved stability and the wide range of biological applications of both CNN and AuNP biocompatible systems.

Self-assembled monolayers of disulfide Cu porphyrins on Au surfaces: adsorption induced reduction and demetalation.

Metalloporphyrin molecules have a wide range of potential applications in diverse technological areas ranging from electronics to optoelectronics, electrochemistry, photophysics, chemical sensors, and catalysis. In particular, self-assembled monolayers of porphyrin molecules have recently attracted considerable interest. In this work we have studied for the first time the self-assembly of a novel Cu deutero porphyrin functionalized with disulfide moieties using electrochemical techniques, UV-vis absorption spectroscopy, polarization modulation infrared reflection absorption spectroscopy, and photoelectron spectroscopies (XPS and UPS). Experimental results indicate that the molecule adsorbs retaining its molecular integrity without forming molecular aggregates via the formation of Au-S covalent bonds. Furthermore, the monolayer consists of a packed array of molecules adsorbed with the plane of the porphyrin molecule at an angle of around 30° with respect to the surface normal. Interestingly, adsorption induces reduction of the Cu center and its consequent removal from the center of the porphyrin ring resulting in porphyrin demetalation. Our results are important in the design of self-assembled monolayers of metallo porphyrins where not only blocking of the metal center by the functional groups that drive the self-assembly should be considered but also possible adsorption induced demetalation with the consequent loss in the properties imparted by the metal center.

Manipulation of molecular transport into mesoporous silica thin films by the infiltration of polyelectrolytes.

The design of hybrid mesoporous materials incorporating polymeric assemblies as versatile functional units has become a very fertile research area offering major opportunities for controlling molecular transport through interfaces. However, the creation of such functional materials depends critically on our ability to assemble polymeric units in a predictable manner within mesopores with dimensions comparable to the size of the macromolecular blocks themselves. In this work, we describe for the first time the manipulation of the molecular transport properties of mesoporous silica thin films by the direct infiltration of polyelectrolytes into the inner environment of the 3D porous framework. The hybrid architectures were built up through the infiltration-electrostatic assembly of polyallylamine (PAH) on the mesopore silica walls, and the resulting systems were studied by a combination of experimental techniques including ellipso-porosimetry, cyclic voltammetry and X-ray photoelectron spectroscopy, among others. Our results show that the infiltration-assembly of PAH alters the intrinsic cation-permselective properties of mesoporous silica films, rendering them ion-permeable mesochannels and enabling the unrestricted diffusion of cationic and anionic species through the hybrid interfacial architecture. Contrary to what happens during the electrostatic assembly of PAH on planar silica films (quantitative charge reversal), the surface charge of the mesoporous walls is completely neutralized upon assembling the cationic PAH layer (i.e., no charge reversal occurs). We consider this work to have profound implications not only on the molecular design of multifunctional mesoporous thin films but also on understanding the predominant role of nanoconfinement effects in dictating the functional properties of polymer-inorganic hybrid nanomaterials.

Electrochemistry of Os(2,2'-bpy)2ClPyCH2NHCOPh tethered to Au electrodes by S-Au and C-Au junctions.

Osmium pyridine-bipyridine redox centers have been tethered to Au electrodes by chemical modification through Au-S and Au-C bonds respectively. 4-Mercapto benzoic acid and the reduction product of the aryl diazonium salt of 4-amino benzoic acid were reacted on Au surfaces, with further post-functionalization by chemical reaction of the osmium complex amino-pyridine derivative with the surface carboxylates. The resulting modified Au surfaces were characterized by polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), resonant raman spectroscopy and cyclic voltammetry.

Metal-ion responsive redox polyelectrolyte multilayers.

We present polyelectrolyte multilayer modified electrodes exhibiting novel chemically responsive redox behaviour due to the combination of both redox and metal-ion-ligand functionalities on the same sites.