Unexpected enhancement of pH-stability in Au3+/Ag+ loaded H-bonded layer-by-layer thin films.

In this work, a polymeric film was synthesized through a layer-by-layer (LBL) self-assembly technique using polyacrylic acid (PAA) and polyethylene oxide (PEO), resulting in the formation of a hydrogen-bonded LBL film. The formation of these films was evaluated by PMIRRAS and QCM-D. The synergy of these techniques allowed the understanding of the mechanism of formation of the film by showing the H-bonding formation and film growth. Au and Ag metal ions were successfully incorporated into the films, as corroborated by the combination of the information obtained by XRR and PMIRRAS. The films were exposed to increasing pH, showing a pronounced improvement in stability in films loaded with Au ions, extending the stability from pH 4 to 10. This behavior allows the use of this system in a wider range of applications, including the possibility of working in biological conditions. On the other hand, films loaded with Ag disintegrated at pH above 4. At acidic pH (below 3), these films released the Ag ions, which may be useful for the preparation of antibacterial stimuli-responsive nanomaterials. In both cases, the films were adequate to produce metal nanoparticles by metal loading and in situ reduction.

Electrochemical stability of glyme-based electrolytes for Li-O2 batteries studied by in situ infrared spectroscopy.

In situ subtractively normalized Fourier transform infrared spectroscopy (SNIFTIRS) experiments were performed simultaneously with electrochemical experiments relevant to Li-air battery operation on gold electrodes in two glyme-based electrolytes: diglyme (DG) and tetraglyme (TEGDME), tested under different operational conditions. The results show that TEGDME is intrinsically unstable and decomposes at potentials between 3.6 and 3.9 V vs. Li+/Li even in the absence of oxygen and lithium ions, while DG shows a better stability, and only decomposes at 4.0 V vs. Li+/Li in the presence of oxygen. The addition of water to the DG based electrolyte exacerbates its decomposition, probably due to the promotion of singlet oxygen formation.

Interpolymer complexes of poly(sulfonic acid)s and poly(ethylene oxide): an unexpected association.

The formation of novel interpolymer complexes (IPCs) between poly(sulfonic acid)s and poly(ethylene oxide) (PEO) is reported. The complexes were precipitated from polymer mixtures in aqueous solution and deposited on surfaces as layer-by-layer films. Based on evidence from infrared spectroscopy, the interpolymer association in poly(sulfonic acid)/PEO IPCs is ascribed to hydrogen bonding between the sulfonic acid and the ether in PEO. This interaction is not anticipated because sulfonic acids are fully dissociated in aqueous solutions due to their strong acidity. Theoretical calculations suggest that the unexpected association of PEO and poly(sulfonic acid)s results from the formation of very strong sulfonic-acid/ether hydrogen bonds, which increase the apparent pKas of the poly(sulfonic acid)s and, therefore, decrease the net charge of these polymers. It is shown that while poly(styrene sulfonic acid) (PSSA) and Nafion form IPCs with PEO, poly(vinyl sulfonic acid) (PVSA) does not. This result is explained in terms of the hydrodrophobic nature of PSSA and Nafion, which stabilizes their IPCs, and the fact that hydrogen bonds in PSSA/PEO IPCs are predicted to be stronger than in PVSA/PEO IPCs.

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.

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.

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.

New insight into the electrochemical desorption of alkanethiol SAMs on gold.

A combination of Polarization Modulation Infrared Reflection Absorption Spectroscopy (PMIRRAS) under electrochemical control, Electrochemical Scanning Tunneling Microscopy (ECSTM) and Molecular Dynamics (MD) simulations has been used to shed light on the reductive desorption process of dodecanethiol (C12) and octadecanethiol (C18) SAMs on gold in aqueous electrolytes. Experimental PMIRRAS, ECSTM and MD simulations data for C12 desorption are consistent with formation of randomly distributed micellar aggregates stabilized by Na(+) ions, coexisting with a lying-down phase of molecules. The analysis of pit and Au island coverage before and after desorption is consistent with the thiolate-Au adatoms models. On the other hand, PMIRRAS and MD data for C18 indicate that the desorbed alkanethiolates adopt a Na(+) ion-stabilized bilayer of interdigitated alkanethiolates, with no evidence of lying down molecules. MD simulations also show that both the degree of order and tilt angle of the desorbed alkanethiolates change with the surface charge on the metal, going from bilayers to micelles. These results demonstrate the complexity of the alkanethiol desorption in the presence of water and the fact that chain length and counterions play a key role in a complex structure.

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.

Methylene blue incorporation into alkanethiol SAMs on Au(111): effect of hydrocarbon chain ordering.

A detailed polarization modulation infrared reflection absorption spectroscopy, scanning tunneling microscopy, and electrochemical study on methylene blue (MB) incorporation into alkanethiolate self-assembled monolayers (SAMs) on Au(111) is reported. Results show that the amount of MB incorporated in the SAMs reaches a maximum for intermediate hydrocarbon chain lengths (C10-C12). Well-ordered SAMs of long alkanethiols (C > C12) hinder the incorporation of the MB molecules into the SAM. On the other hand, less ordered SAMs of short alkanethiols (C < or = C6) are not efficient to retain the MB incorporated through the defects. For C12 the amount of incorporated MB increases as the SAM disorder is increased. This information is essential to the design of efficient thiol-based Au vectors for transport and delivery of molecules as well as thiol-based Au devices for molecular sensing.

Nitro group as a means of attaching organic molecules to silicon: nitrobenzene on Si(100)-2 x 1.

This paper will present the computational and experimental infrared studies of the reactions of nitrobenzene on a Si(100) surface, a prototypical model reaction for understanding the behavior of bifunctional molecules on semiconductor surfaces. The initial reaction of nitrobenzene with the Si(100)-2 x 1 occurs via 1,3-dipolar cycloaddition of the nitro group to the silicon surface dimer. Computational exploration of the initial adsorption configurations suggests that two stable structures can be formed: one with the phenyl ring essentially perpendicular to the surface; the other one with the tilt angle of approximately 113 degrees with respect to the surface normal. The barrier for converting the latter into the former, more stable by approximately 13 kJ/mol, is 19.1 kJ/mol. Further thermal reactions are analyzed, and the reaction pathways are compared for the computational models with fixed vs relaxed subsurface silicon atoms. While all the surface species resulting from nitrobenzene transformations on the Si(100)-2 x 1 surface studied here are thermodynamically stable, most of the reaction pathways can be ruled out on the basis of the analysis of the transition states leading to these species and on the comparison of predicted and measured vibrational spectra. As a result, the exact adsorption configurations can be pinpointed.

Chemistry of 1,1,1,5,5,5-hexafluoro-2,4-pentanedione on Si(100)-2x1.

The surface chemistry of 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (hfacH), a hydrogenated form of the most common ligand in metal and metal oxide deposition, on Si(100)-2x1 has been investigated using multiple internal reflection Fourier transform infrared spectroscopy (MIR-FTIR), Auger electron spectroscopy (AES), thermal desorption mass spectrometry, and computational analysis. The main goal of these studies was to understand if hfacH is a source of fluorine, carbon, and oxygen contamination for a variety of deposition processes where the hfac ligand is involved. In its molecular form, hfacH may potentially have up to 10 isomers including two ketonic and eight enolic forms. One of the enolic forms is shown to be the most stable upon adsorption on a clean Si(100)-2x1 surface at submonolayer coverages at cryogenic temperatures. Even though only the enolic form is present at cryogenic temperatures, at room temperature any of these isomers can exist and all the possibilities of their interaction with the Si(100)-2x1 surface, including several [2 + 2] and [2 + 4] addition pathways as well as O-H dissociation, should be considered. Despite such an array of possibilities, the room-temperature adsorption is governed by the thermodynamic stability of the final addition products between the hfacH and silicon surface. These adducts are stable at room temperature and decompose upon surface annealing.

Conductance study of association in aqueous CaCl2, Ca(CH3COO)2, and Ca(CH3COO)2.nCH3COOH from 348 to 523 K at 10 MPa.

The conductances of aqueous CaCl2, Ca(CH3COO)2, and Ca(CH3COO)2.nCH3COOH were measured from 348 to 523 K at pressures near 10 MPa using a flow conductometric technique. The first association constant for calcium chloride and the first and second association constants for calcium acetate in water, were derived from a least-squares fit of the measurements to a conductance model in which activity coefficients were calculated using the mean spherical approximation (MSA) and the conductance was modeled by the Turq, Blum, Bernard, and Kunz (TBBK) equation, plus a mixture model.

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.

Azide reactions for controlling clean silicon surface chemistry: benzylazide on Si(100)-2 x 1.

A combination of experimental and computational studies presents direct proof of a novel reaction pathway that delivers aromatic compounds onto a Si(100)-2 x 1 substrate. Benzylazide chemisorbs on a Si(100)-2 x 1 surface, and this chemisorption is followed by nitrogen elimination, leading to a stable surface adduct based on a Si-Si-N cyclic entity. This reaction occurs via a stable surface intermediate with the surface-bound nitrogen molecule stabilized by the presence of a neighboring aromatic group, which eventually releases nitrogen into the gas phase and forms the final product.