Deprotonation of phenol linked to a silicon dioxide surface using adaptive feedback laser control with a heterodyne detected sum frequency generation signal.

The development of laser-controlled surface reactions has been limited by the lack of decisive methods for detecting evolving changes in surface chemistry. In this work, we demonstrate successful laser control of a surface reaction by combining the adaptive feedback control (AFC) technique with surface sensitive spectroscopy to determine the optimally shaped laser pulse. Specifically, we demonstrate laser induced deprotonation of the hydroxyl group of phenol bound to a silicon dioxide substrate. The experiment utilized AFC with heterodyne detected vibrational sum frequency generation (HD-VSFG) as the surface sensitive feedback signal. The versatile combination of AFC with HD-VSFG provides a route to potentially control a wide range of surface reactions.

Catalytic Intermediates of CO2 Hydrogenation on Cu(111) Probed by In Operando Near-Ambient Pressure Technique.

The in operando monitoring of catalytic intermediates is crucial for understanding the reaction mechanism and for optimizing the reaction conditions to improve the efficiency of the catalytic protocol; however, until now, this has remained a daunting challenge. Herein, we investigated the interaction of CO2 and H2 with the Cu(111) surface in a CO2 hydrogenation model system by using the in operando technique of near-ambient pressure X-ray photoelectron spectroscopy, which is further assisted by ultraviolet photoemission spectroscopy and low-energy electron diffraction (LEED) measurements. These techniques allowed the direct observation of CO2 dissociation into CO+O on the Cu(111) surface and the adsorption of O on the surface at room temperature. The intermediate HCOO- was unambiguously detected in the CO2 +H2 environment, which corroborated the formate pathway for methanol formation on the Cu(111) surface. We further found that O coverage can prevent the build up of graphitic carbon on the Cu surface. By taking advantage of the competitive interplay between Cu-O and graphitic carbon, we have proposed a feasible strategy for inhibition of the formation of graphitic carbon by tuning the CO2 and H2 partial pressures, which may contribute to sustaining the active Cu catalyst under the reaction conditions.

Two-Dimensional versus Three-Dimensional Self-Assembly of a Series of 5-Alkoxyisophthalic Acids.

Physisorbed self-assembled monolayers (SAMs) have been suggested as potential models for three-dimensional (3D) crystallization. This work studies the effect of altering the chain length of 5-alkoxyisophthalic acid (C nISA) on self-assembled morphology in both two-dimensional (2D) and 3D to explore the extent comparisons can be drawn between dimensions. Previous studies of 5-alkoxyisophthalic acid at solid-liquid interfaces (2D) reported different morphologies for C5ISA and C6ISA-alkoxy chains on the one hand and C10ISA and C18ISA on the other. Independently, also in 3D a dependence of morphology on chain length has been reported, including an unexpected inclusion of a solvent in the 3D morphology of C6ISA, while the previous reports of 2D self-assembly were driven only by molecule-molecule and molecule-substrate interactions. However, a complete set of data for comparison has been missing. Here, we report scanning tunneling microscopy (STM) and molecular dynamics simulations performed for C2ISA self-assembled monolayers (SAMs) and STM imaging of C6ISA-C9ISA SAMs, to further examine self-assembly behavior in 2D. In 3D, X-ray diffraction analysis of C2ISA single crystals was carried out to complete the data set. With a complete set of data, it was observed that regardless of the dimension, short-chain-length C nISAs formed H-bonding-dominated structures, mid-chain-length C nISAs exhibited solvent-dependent morphologies, and long-chain-length C nISAs displayed van der Waals-dominated solvent-independent structures. However, the transition point among morphologies occurred at different chain lengths in 2D and 3D regardless of the dominant interaction. The results of this study inform the design of 2D films and guide the application of knowledge from physisorbed SAMs to 3D systems, including mixed-dimensional (2D/3D) van der Waals heterostructures.

Substituent effects on the kinetics of bifunctional styrene SAM formation on H-terminated Si.

Self-assembled monolayers (SAMs) on metal and semiconductor surfaces are of interest in electronic devices, molecular and biosensors, and nanostructured surface preparation. Bifunctionalized molecules, where one functional group attaches to the surface while the other remains free for further modification, allow for the rational design of multilayer chemisorbed thin films. In this study, substituted styrenes acted as a model system for SAM formation through an alkene moiety. Substituents ranging from activating to strongly deactivating for aromatic reactions were used to probe the effect of the electronic properties of functionalizing molecules on the rate of SAM formation. Substituted styrene SAMs were formed on hydrogen-terminated p-type Si(100) and n-type Si(111) via sonochemical functionalization. Monolayers were characterized via ellipsometry, IR spectroscopy, contact angle goniometry, and X-ray photoelectron spectroscopy (XPS). Initial rates of reaction for molecules that selectively attached through the alkene were further studied. A linear relationship was observed between the initial rates of surface functionalization and the substituent electron donating/withdrawing ability for the substituted styrenes, as described by their respective Hammett constants. This study provides precedent for applying well quantified homogeneous chemical reaction relationships to reactions at the solid-liquid interface.

Anodized indium metal electrodes for enhanced carbon dioxide reduction in aqueous electrolyte.

The interactions of CO2 with indium metal electrodes have been characterized for electrochemical formate production. The electrode oxidation state, morphology, and voltammetric behaviors were systematically probed. It was found that an anodized indium electrode stabilized formate production over time compared to etched indium electrodes and indium electrodes bearing a native oxide in applied potential range of -1.4 to -1.8 V vs SCE. In addition, it was observed that formate is the major product at unprecedentedly low overpotentials at the anodized surface. A surface hydroxide species was observed suggesting a mechanism of formate production that involves insertion of CO2 at the indium interface to form an electroactive surface bicarbonate species.

Potassium spin polarization lifetime for a 30-carbon chain siloxane film.

The siloxane film derived from the 30-carbon chain triacontyltrichlorosilane (TCTS) is studied as an anti-relaxation coating for atomic vapor cells. The longitudinal spin relaxation lifetime of optically pumped potassium atoms in the presence of TCTS is measured and the average number of non-relaxing atom-wall collisions, or bounces, enabled by the coated surface is determined. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) of TCTS were performed to investigate changes in chemical states and surface morphology of TCTS arising from K atom deposition on the film surface. TCTS was found to give approximately 530 bounces. Following lifetime measurements, K2p signals were clearly observed in XPS spectra. AFM images display non-preferential K deposition on the TCTS surface, however additional AFM studies with a TCTS surface exposed to Rb atoms show deposition occurs along surface defects. In agreement, Rb is found to preferentially deposit along the step edges of an 18-carbon chain monolayer film derived from 1-Octadecene. Finally, AFM indicates a much smoother surface for a tetracontane coating relative to TCTS. The importance of siloxane surface morphology versus film thickness with respect to coating performance is discussed.

Can we understand the molecule in molecular electronics?

Using molecules as individual components in electronic devices promises the ultimate in miniaturization coupled with the flexibility of organic synthesis to tune the individual component. Examination of metal/molecule/metal junctions show that organic functionality has little effect on the conductivity and rectification behavior of molecular electronic junctions, thus questioning the possible tunability of molecular electronic devices.

Mild and efficient functionalization of hydrogen-terminated Si(111) via sonochemical activated hydrosilylation.

Efficient chemical functionalization of hydrogen-terminated Si(111) with simple and bifunctional 1-alkenes was achieved via novel sonochemical activated hydrosilylation, utilizing just a simple ultrasonic bath. It is an extremely mild method that allows the specific attachment of unprotected bifunctional alkenes such as undecenol, undecylenic acid, and even a heat/UV-sensitive alkene, bearing an activated leaving group (N-succinimidyl undecylenate), without suffering any degradation.

Direct photochemical functionalization of Si(111) with undecenol.

Direct UV photochemical functionalization of H-terminated Si(111) with bifunctional 10-undecen-1-ol was achieved with selective attachment via its vinyl end, resulting in the formation of a compact monolayer with free terminal alcohol groups. This is due to the faster radical propagation mechanism in hydrosilylation with alkene compared to the nucleophilic attack mechanism of alcohol, which is impeded by intermolecular hydrogen bonding present at room temperature. Evidence from X-ray photoelectron spectroscopy, infrared spectroscopy, and resistance to fluoride etching shows that Si-C is the interfacial bond, and atomic force microscopy shows the presence of a smooth, uniform monolayer conforming to the atomic terraces of the Si(111) surface. The application of such a hydroxyl-terminated monolayer was demonstrated by tethering a bromoinitiator through surface esterification and thereafter subjecting the surface to the surface-initiated atom-transfer radical polymerization of butyl methacrylate. The poly(butyl methacrylate) brushes formed were found to be smooth (R(a) < 0.3 nm) and uniform even for a thin film of 4.0 nm.

Hydrogen-bonding versus van der Waals interactions in self-assembled monolayers of substituted isophthalic acids.

Self-assembled monolayers of a series of isophthalic acids (5-octadecyloxyisophthalic acid, 5-decyloxyisophthalic acid, 5-hexyloxyisophthalic acid, and 5-pentyloxyisophthalic acid) formed on highly ordered pyrolytic graphite (HOPG) at the solid-liquid interface were studied using scanning tunneling microscopy (STM). Although these molecules have the same dicarboxyl headgroup, their hydrocarbon tails are of different lengths. Hydrogen-bonding between headgroups and van der Waals interactions between the hydrocarbon tails control the final morphology of the monolayer. The STM images show that both van der Waals interactions (vdWs) and hydrogen-bonding (H-B) compete to control the structure, but the final structure of the monolayer is determined by balance between the two interactions.

Higher-order complexity through R-group effects in self-assembled tripeptide monolayers.

Self-assembled monolayers of tri-L-leucine and tri-L-valine formed on highly ordered pyrolytic graphite (HOPG) substrates have been examined using scanning tunneling microscopy. These monolayers exhibit markedly different structures, even though the tripeptides differ by only a minor change in the amino acid R-group. This minor change in R-group apparently affects the balance between hydrogen bonding and van der Waals interactions that control the monolayer structures. Implications of this effect for evolution of molecular complexity in prebiotic synthesis on environmental surfaces are discussed.

Probing the Reaction Mechanism in CO2 Hydrogenation on Bimetallic Ni/Cu(100) with Near-Ambient Pressure X-Ray Photoelectron Spectroscopy.

Bimetallic Ni-Cu catalysts feature high activity in CO2 hydrogenation. However, the primary surface intermediates during reaction are still elusive, making the understanding of the reaction mechanism inadequate. Herein, taking advantage of near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), we focused on the mechanistic exploration of CO2 hydrogenation on the Ni/Cu(100) model catalyst under millibar pressures. We show that CO2 dissociates into CO and atomic oxygen on the Ni/Cu(100) surface and gives rise to the formation of chemisorbed O and nickel oxide (NiO). The CO3* species is formed through the reaction of CO2 with surface oxygen during CO2 activation. With the presence of H2, the conversion of adsorbed CO3* into the formate intermediate, HCOO*, is unambiguously demonstrated by the C 1s and O 1s core-level spectra as well as ultraviolet photoelectron spectroscopy. Based on these observations, we conclude that the CO2 hydrogenation route via CO2 dissociation, the formation of CO3*, the conversion of CO3* to formate, and the ensuing hydrogenation of formate to methanol on the Ni-Cu catalyst are feasible.

Interpretation on Nanoporous Network Structure in Rice Husk Silica Layer: A Graph Model.

The rice plant produces an amorphous silica layer in the husk covering the brown rice grain as a part of a protective respiration system. The layer shows high permeation molecular flow while the Brunauer-Emmett-Teller isotherm indicates the existence of nanometer-sized pores. Here, we interpret the inner structure of the layer as a porous network consisting of void spheres with a degree of 2-5 and tunnels with a length of 2-7 nm based on the transmission electron microscopy images. In the network, the gas molecules travel through the tunnels and move in random directions after collisions with the walls of the spheres. A tree network was introduced to understand the permeance of the layer and the reflection of the molecule of the root or parent sphere was estimated for a specific case. The tree becomes a graph with cycles in a finite space such as the silica layer and the reflection of the root sphere in the graph converses to that of the tree. On the basis of the properties of the network, the high permeance of the silica layer in the rice husk can be explained. It is suggested that the specific system restricts the movements of the gas molecules and can be applied to reduce the size of gas phase separation and chemical reactor systems providing a new view to understand nanoscaled porous materials.

Competition and coadsorption of di-acids and carboxylic acid solvents on HOPG.

The self-assemblies of di-acids HOOC-(CH(2))(n)-COOH (n = 20, 18, 16, 14, 12, 10) in three solvents hexanoic acid, octanoic acid, and decanoic acid on highly oriented pyrolytic graphite (HOPG) were studied with scanning tunneling microscopy (STM). In the solvent hexanoic acid, solvent molecules coadsorb with HOOC-(CH(2))(n)-COOH (n = 20, 18, 16) via formation of hydrogen bonds. The solvent octanoic acid coadsorbs with HOOC-(CH(2))(n)-COOH (n = 20, 18). Decanoic acid only coadsorbs with HOOC-(CH(2))(20)-COOH. In each solvent, the trend of coadsorption between solvent molecules and di-acid molecules decreases with decreasing chain-length of di-acid molecules. These systematic investigations suggest that coadsorption of solvent molecules with di-acid solute molecules is mainly dependent on the relative hydrogen-bond densities in the formed monolayer. This is consistent with the maximization of adsorption heat of the self-assembled monolayers of di-acids dissolved in solvents of carboxylic acids.

Self-assembly and odd-even effects of cis-unsaturated carboxylic acids on highly oriented pyrolytic graphite.

The self-assembly of several cis-unsaturated carboxylic acids of the structure cis-CH3(CH2)p-1CH=CH(CH2)m-1COOH on highly oriented pyrolytic graphite (HOPG) was studied. The impact of the interior cis-CH=CH group and the molecular chain length on their self-assembled structures was considered. Due to the cis conformation of the -HC=CH- group in the interior of these molecules, they display self-assembled structures significantly different from saturated acids with all-trans configurations. As an example of the class of molecules cis-CH3(CH2)p-1CH=CH(CH2)2n-1COOH (p not equal 2n) (p=8, n=7), cis-CH3(CH2)7CH=CH(CH2)13COOH self-assembles into two kinds of enantiomer domains with opposite 2-D chirality. Due to the steric restriction of the interior cis-HC=CH group, all chains with acid groups are packed at the same side of a lamella, a head-to-head arrangement which is different from the head-to-tail packing of saturated all-trans acids. However, cis-CH3(CH2)7CH=CH(CH2)8COOH, considered as one example of the group cis-CH3(CH2)p-1CH=CH(CH2)2n-2COOH (p not equal 2n-1) (p=8, n=5), does not form any stable self-assembled domain, consistent with the molecular arrangement model. This difference in self-assembly behavior between cis-CH3(CH2)p-1CH=CH(CH2)2n-1COOH (p not equal 2n) and cis-CH3(CH2)p-1CH=CHC2n-2COOH (p not equal 2n-1) shows an odd-even chain-length effect of cis-CH3(CH2)p-1CH=CH(CH2)m-1COOH (p not equal m, m=2n or 2n-1). For another category of molecules, cis-unsaturated acids with equal numbers of all-trans carbon atoms on both sides of the cis-CH=CH group, cis-CH3(CH2)m-1CH=CH(CH2)m-1COOH (m=2n or 2n-1), display another odd-even effect. cis-CH3(CH2)7CH=CH(CH2)7COOH, one example of cis-CH3(CH2)2n-1-CH=CH(CH2)2n-1COOH (n=4), is predicted to form both an enantiomer and a nonchiral racemic structure, which is in accordance with the experimental observation of its self-assembled monolayer. However, cis-CH3(CH2)2n-2CH=CH(CH2)2n-2COOH does not form a stable self-assembled domain due to the same steric repulsion as that seen in the cis-CH3(CH2)7CH=CH(CH2)8COOH structure. These odd-even effects demonstrate that molecular self-assembly can be significantly tailored by slightly changing the molecular chain length.

Structures formed by the chiral assembly of racemic mixtures of enantiomers: iodination products of elaidic and oleic acids.

The self-assembled monolayer structure of the products of elaidic acid iodination (the racemic mixture of 9,10-(9S,10R)-diiodooctadecanoic acid and 9,10-(9R,10S)-diiodooctadecanoic acid) and the products of oleic acid iodination (the racemic mixture of 9,10-(9R,10R)-diiodooctadecanoic acid and 9,10-(9S,10S)-diiodooctadecanoic acid) are studied by high-resolution scanning tunneling microscopy. For the iodination products of elaidic acid, the separation of enantiomers into distinct chiral domains during the formation of the 2-D crystal on the highly ordered pyrolytic graphite (HOPG) surface is not observed. Instead, within the diiodooctadecanoic acid SAM, each row of molecules is composed of opposite racemates. The two opposite racemates pack alternately inside a row, using different faces to adsorb on the surface. The unit cell is composed of a pair of opposite racemates, forming a heterochiral structure. For the iodination products of oleic acid, the racemic mixture is observed to exhibit quasi-phase separation during the formation of the 2-D crystal on the HOPG surface. Each row is composed of homochiral acid molecules, either the 9,10-(9R,10R)-diiodooctadecanoic acid (R) or the 9,10-(9S,10S)-diiodooctadecanoic acid (S). The R row and the S row pack alternately, with a unit cell composed of four molecules. Two of the molecules in the unit cell are the 9,10-(9R,10R)-diiodooctadecanoic acid (R) molecules; two are the 9,10-(9S,10S)-diiodooctadecanoic acid (S) molecules. In the unit cell, the two molecules that have the same chirality pack antiparallel inside the homochiral row, using different faces to adsorb on the surface. These results suggest that several different types of chiral assembly are possible. Enantiomers with opposite chirality exhibit many chiral assembly patterns, forming heterochiral structures on the surface in addition to separation to form macroscopic chiral domains. By using different conformations, similar enantiomers with opposite chirality will display many chiral assembly patterns to form heterochiral structures on the surface.

Chirality in supramolecular self-assembled monolayers of achiral molecules on graphite: formation of enantiomorphous domains from arachidic anhydride.

The molecular arrangement and chirality of the self-assembled arachidic anhydride monolayer on graphite were investigated using scanning tunneling microscopy (STM). This molecule has two identical alkyl chains, linked by an anhydride group in the middle. In its extended form, one alkyl chain is shifted, with respect to the other, along the molecular backbone. Upon adsorption on graphite, this achiral anhydride spontaneously forms two types of homogeneous domains (denoted as m and m') with mirror symmetry. The angle from the molecular chain to the row-packing direction is 98.0 degrees +/- 0.5 degrees and 82.0 degrees +/- 0.5 degrees for domains m and m', respectively. Domain m is the mirror image of m'. The molecular arrangement of this self-assembled monolayer shows that domains m and m' are two-dimensional enantiomers with opposite chiralities. This new molecular packing motif is confirmed by line-profile analyses along the molecule-chain and the row-packing directions. This finding demonstrates the spontaneous formation of highly ordered homogeneous enantiomorphous domains on graphite resulting only from weak van der Waals forces between the achiral arachidic anhydride molecules.

Direct measurement of surface complex loading and surface dipole and their effect on simple device behavior.

Tin complexes of phenoxide ligands having a range of dipole moments were prepared on the surface of indium-tin oxide (ITO). Surface complex loadings and stoichiometries were measured by quartz crystal microgravimetry. Work functions of ITO substrates treated with these various surface complexes were measured using a Kelvin probe. Surface complex dipole moments were then calculated based on measured surface loadings. Changes in the ITO work function effected by surface phenoxide complex introduction correlate with these surface complex dipole moments and with total surface dipole per unit area, and current densities in simple hole-only diode devices also correlate with these total surface dipoles.

Higher-power supercapacitor electrodes based on mesoporous manganese oxide coating on vertically aligned carbon nanofibers.

A study on the development of high-power supercapacitor materials based on formation of thick mesoporous MnO2 shells on a highly conductive 3D template using vertically aligned carbon nanofibers (VACNFs). Coaxial manganese shells of 100 to 600 nm nominal thicknesses are sputter-coated on VACNFs and then electrochemically oxidized into rose-petal-like mesoporous MnO2 structure. Such a 3D MnO2/VACNF hybrid architecture provides enhanced ion diffusion throughout the whole MnO2 shell and yields excellent current collection capability through the VACNF electrode. These two effects collectively enable faster electrochemical reactions during charge-discharge of MnO2 in 1 M Na2SO4. Thick MnO2 shells (up to 200 nm in radial thickness) can be employed, giving a specific capacitance up to 437 F g(-1). More importantly, supercapacitors employing such a 3D MnO2/VACNF hybrid electrode illustrate more than one order of magnitude higher specific power than the state-of-the-art ones based on other MnO2 structures, reaching ∼240 kW kg(-1), while maintaining a comparable specific energy in the range of 1 to 10 Wh kg(-1). This hybrid approach demonstrates the potential of 3D core-shell architectures for high-power energy storage devices.