Stable Adlayer Research Articles

Solid State Reaction Epitaxy, A New Approach for Synthesizing Van der Waals heterolayers: The Case of Mn and Cr on Bi2Se3

AbstractVan der Waals (vdW) heterostructures that pair materials with diverse properties enable various quantum phenomena. However, the direct growth of vdW heterostructures is challenging. Modification of the surface layer of quantum materials to introduce new properties is an alternative process akin to solid state reaction. Here, vapor deposited transition metals (TMs), Cr and Mn, are reacted with Bi2Se3 with the goal to transform the surface layer to XBi2Se4 (X = Cr, Mn). Experiments and ab initio MD simulations demonstrate that the TMs have a high selenium affinity driving Se diffusion toward the TM. For monolayer Cr, the surface Bi2Se3 is reduced to Bi2‐layer and a stable (pseudo) 2D Cr1+δSe2 layer is formed. In contrast, monolayer Mn can transform upon mild annealing into MnBi2Se4. This phase only forms for a precise amount of initial Mn deposition. Sub‐monolayer amounts dissolve into the bulk, and multilayers form stable MnSe adlayers. This study highlights the delicate energy balance between adlayers and desired surface modified layers that governs the interface reactions and that the formation of stable adlayers can prevent the reaction with the substrate. The success of obtaining MnBi2Se4 points toward an approach for the engineering of other multicomponent vdW materials by surface reactions.

Tuning N-heterocyclic carbene wingtips to form electrochemically stable adlayers on metals.

Self-assembled monolayers (SAMs) are employed in electrochemical biosensors to passivate and functionalize electrode surfaces. These monolayers prevent the occurrence of undesired electrochemical reactions and act as scaffolds for coupling bioaffinity reagents. Thiols are the most common adlayer used for this application; however, the thiol-gold bond is susceptible to competitive displacement by naturally occurring solvated thiols in biological fluids, as well as to desorption under continuous voltage interrogation. To overcome these issues, N-heterocyclic carbene (NHC) monolayers have been proposed as an alternative for electrochemical biosensor applications due to the strong carbon-gold bond. To maximize the effectiveness of NHCs for SAMs, a thorough understanding of both the steric effects of wingtip substituents and NHC precursor type to the passivation of electrode surfaces is required. In this study, five different NHC wingtips as well as two kinds of NHC precursors were evaluated. The best performing NHC adlayers can be cycled continuously for four days (over 30 000 voltammetric cycles) without appreciably desorbing from the electrode surface. Benchmark thiol monolayers, in contrast, rapidly desorb after only twelve hours. Investigations also show NHC adlayer formation on other biosensor-relevant electrodes such as platinum and palladium.

Adlayer structure and lattice size effects on catalytic rates predicted from KMC simulations: NO oxidation on Pt(111).

Repulsive and/or attractive interactions between surface adsorbates have an important effect on the structure of the adsorbate layer and consequently on the rate of heterogeneous catalytic reactions. Thus, developing reaction models that take into account adsorbate-adsorbate interactions is crucial for making accurate predictions of the catalytic rate and surface coverage during reaction. In the present work, we employ kinetic Monte Carlo simulation to model the catalytic NO oxidation on Pt (111), adopting a cluster expansion (CE) Hamiltonian approach for treating the aforementioned interactions. We investigate CEs of increasing complexity, ranging from pairwise 1st nearest neighbor to long-range and many-body terms. We show that energetic models incorporating solely short-range interactions result in ordered adlayer structures, which are disrupted by anti-phase boundaries and defective regions when the size of the periodic lattice is non-commensurate to the structure of the stable adlayer. We find that O2 dissociates on sites located in these defective regions, which are predominantly responsible for the activity, and the predicted catalytic rate is strongly depended on the lattice size. Such effects are absent when employing non-periodic lattices, whereon the catalytic activity appears more intense on edges/corner sites. Finally, inclusion of long-range interactions in the model Hamiltonian induces relative disorder in the adsorbate layer, which is ascribed to the "softening" of the repulsive interactions between adspecies. Under these circumstances, the distribution of activation energies for O2 dissociation is broader as compared to short-range interaction models and on this basis we explain the disparate catalytic rate predictions when using different CEs.

Exploration of Chirality and Achirality of Self‐Assembled Monolayer Formed by Unsymmetrically Substituted Fluorenone Derivative at the Liquid/Solid Interface

AbstractThe self‐assemblies of 2‐decyloxy‐7‐pentadecyloxy‐9‐fluorenone (DPF) are characterized by scanning tunneling microscopy at the liquid/solid interface. Achiral Dimer and chiral S‐like structures are observed in 1‐octanoic acid. The solvent molecule takes part in the self‐assembly via forming COOH⋯OC and COOH⋯COOH hydrogen bonds. When 1‐phenyloctane and n‐tetradecane are used as the solvents, DPF self‐assembles into chiral Z‐like structure, which are classified into Types I–IV according to the packing styles. For the purpose of exploring the effect of solvent and the competition between these three structures, the self‐assemblies of DPF in mixed solvents and on dry surface are investigated. The results show that S‐like structure is preferred in mixed solvents, while the Z‐like structure has priority when the function of solvent is excluded. DPF molecule shows polarity thus dipole−dipole interaction plays significant roles on forming stable adlayers by forming antiparallel dipole pairs. Furthermore, density functional theory calculation is performed to reveal the driving forces of hydrogen bonds and van der Waals interactions. In light of the popularity of studying chirality at the liquid/solid interface, it is believed that the findings will provide implications for designing chiral systems and will promote the exploration of self‐assembly in the field of surface science.

STM Exploration of the Diverse Polymorphs for Tri‐Substituted Anthraquinone Derivatives via Alkyl Chain Elongation

Fabrication of structural diversity in self‐assembled monolayers has gained considerable attention, not only due to its significance in interface science but also because of its potential application in designing nanomaterials. Here, systematical characterization of the molecular self‐assembly of tri‐substituted anthraquinone derivatives at the 1‐octanoic acid/HOPG interface is provided. Different nanopatterns of Linear I, Butterfly‐like, and Linear II are recorded for 1,2,4‐A‐3OC16 by gradually changing the solution concentration. Diverse polymorphs are also obtained by alkyl chain elongation, meaning that 1,2,4‐A‐3OC15,17,18 adopt Dimer‐Linear, Linear III, and Linear IV structures, respectively. Dipole–dipole, hydrogen bonding, and van der Waals interactions are the dominant forces on forming the stable adlayers. Also, the coadsorption of the solvent molecules is a common phenomenon, ascribing to space matching and gain of enthalpy. Moreover, the 1,2,4‐A‐3OC17,18 self‐assembled into monolayers consists of both ordered and chaotic domains. This loss of regularity is attributed to the large number and long length of the side chains, for entropy reasons. This work provides important insight into the fabrication of complex molecular self‐assembly and the exploration of monolayers at the liquid/solid interface.

Surface-Induced Diastereomeric Complex Formation of a Nucleoside at the Liquid/Solid Interface: Stereoselective Recognition and Preferential Adsorption

With the aim of achieving surface-mediated enantioselective adsorption, the self-assembly of chiral oligo(p-phenylenevinylene) (OPV3T) with nucleosides is investigated at the liquid/solid interface by means of scanning tunneling microscopy and molecular modeling. OPV3T enantiomers form mirror related hexameric rosette patterns. The DNA nucleoside, thymidine, does not self-assemble into stable adlayers but coadsorbs with OPV3T on the surface, leading to a pattern transformation of OPV3T from rosettes to dimers, and a change in chiral expression as well. Diastereoselective recognition between OPV3T and thymidine enantiomers can be used to resolve thymidine enantiomers at an achiral surface with an OPV3T enantiomer as the resolving agent. The impact of molar ratio and concentration on the self-assembly and chiral resolution is systematically investigated. Because there is no interaction between OPV3T and thymidine in solution, the liquid/solid interface acts as the platform for the chiral resolution of thymidine enantiomers.

Sequential Pt(1 1 1) oxide formation in perchloric acid: An electrochemical study of surface species inter-conversion

A significant number of electrocatalytic reactions take place in a potential region in which the surface of platinum is partly covered by oxygenated species. Hence, the mechanism of the electrochemical formation and reduction of surface oxides is fundamental to understand how its properties can determine the reactivity of the catalyst. In this paper, the dynamic of the initial states of the electrochemical oxide growth on Pt(111), in the absence of anion specific adsorption and keeping surface stability, is studied by employing electrochemical techniques. Several species, such as chemisorbed hydroxide and oxygen, initial Pt oxide structures and even sub-surface oxygen, interconvert and interact. Their existences depend more strongly on the total surface coverage than on the electrode potential. Considering structural and thermodynamic data related to the identity of the electrochemical oxide film at platinum, the results suggest that Pt(111) oxide starts by water dissociation to OHads followed by its electrochemical oxidation to Oads at higher potentials. The latter process occurs as a phase transition through a nucleation and growth mechanism, until a stable adlayer is formed. In parallel, Oads transforms to an initial Pt oxide structure. Thus, there are at least two types of processes involved in the growth of the surface oxide layer (i.e. two states of oxide films), and the separation between anodic and cathodic j–E profiles reflects a true hysteresis arising from a change of state of the surface oxide film.

Protein‐Resistant Surfaces through Mild Dopamine Surface Functionalization

The synthesis and evaluation of new dopamine-based catechol anchors coupled to poly(ethylene glycol) (PEG) for surface modification of TiO(2) are reported. Dopamine is modified by dimethylamine-methylene (7) or trimethylammonium-methylene (8) groups, and the preparation of mPEG-Glu didopamine polymer 11 is presented. All these PEG polymers allow stable adlayers on TiO(2) to be generated through mild dip-and-rinse procedures, as evaluated both by variable angle spectroscopic ellipsometry and X-ray photoelectron spectroscopy. The resulting surfaces substantially reduced protein adsorption upon exposure to full human serum.

The Adlayers of 6-Anilino-1,3,5-Triazine-2,4-Dithiol Monosodium on Au(111) Electrode in Acidic Solution Studied by Electrochemical Scanning Tunneling Microscopy

We investigated the electrochemical behavior of the triazinedithiol derivative (TAD), 6-anilino-1,3,5-triazine-2,4-dithiol monosodium (ATDS), on a Au(111) electrode surface in 0.05 M HClO4 using electrochemical measurements and scanning tunneling microscopy (STM). The oxidation of Au was significantly suppressed by modifying Au with ATDS. It was found that each ATDS molecule formed an agglomerate recognized as a set of three bright spots on the Au(111). The agglomerate was also in part linearly ordered along the √3 direction, and no definite phase transition was seen, which was supported by the fact that the CV curve was degenerated by ATDS molecules adsorbed on the electrode in the double-layer region. This result differed from previously reported results of the CV and ECSTM measurements for the 6-octylthio-1,3,5-triazine-2,4-dithiol monosodium (OTDS)-modified Au(111) electrode. The ATDS molecule formed significantly stable adlayer structures on the Au(111). This might contribute to the relatively high corrosion inhibition ability of the ATDS molecule.

Cycloadditions on Diamond (100) 2 × 1: Observation of Lowered Electron Affinity due to Hydrocarbon Adsorption

The adsorption of allyl alcohol, acrylic acid, and allyl chloride, as well as unsaturated organic molecules such as acetylene and 1,3 butadiene, on reconstructed diamond (100) 2 x 1 have been investigated using high-resolution electron energy loss (HREELS) spectroscopy and synchrotron radiation spectroscopy. The cycloadditions of these organic molecules produce chemically adsorbed adlayers with varying degree of coverages on the clean diamond. The organic adsorbed surface has a lowered electron affinity and shows a secondary electron yield that varies between 12 and 40% of the yield obtained from a fully hydrogenated diamond surface. The diamond surface can be functionalized with hydroxyl, carboxylic, and chlorine functionalities by the adsorption of these allyl organics. The [2 + 2] adduct of acetylene on the diamond (100) 2 x 1 surface can be observed. 1,3-butadiene attains a higher coverage as well as forms a thermally more stable adlayer on the diamond surface compared to the other organic molecules, due to its ability to undergo [4 + 2] cycloaddition.

Biomimetic Surface Modifications Based on the Cyanobacterial Iron Chelator Anachelin

Siderophores are natural iron chelators that have been evolutionarily selected to bind to Fe ions with very high binding constants. We utilize these unique properties to bind to metal oxide surfaces using a fragment of the cyanobacterial siderophore anachelin. The resulting poly(ethylene glycol) conjugate forms stable adlayers on TiO2 as has been shown by variable angle spectroscopic ellipsometry and X-ray photoelectron spectroscopy. Moreover, these coated surfaces are highly protein-resistant against the adsorption of full human serum.

Metal-adlayer-stabilized ZnO(0001) surfaces: Toward a new growth mode for oxides

First-principles total energy calculations were performed for clean and In-rich ZnO(0001) surfaces. The calculations indicate that Zn adlayers themselves are not energetically favorable, but that In can form a stable adlayer in Zn-rich conditions. This result opens the possibility that In may serve as a surfactant for the growth of ZnO.

Nanoparticles‐on‐electrode approach for in situ surface‐enhanced Raman spectroscopy studies with platinum‐group metals: examples and prospects

AbstractSurface‐enhanced Raman spectroscopy (SERS) spectra have been obtained for different species (CO, CN−, ethylene and oxalic acid) adsorbed on nanostructured electrodes prepared by deposition of Pt, Pd, Rh and Pd:Pt nanoparticles on either gold or platinum electrodes. These examples not only refer to stable adlayers of typical adsorbates, such as carbon monoxide and cyanide, but also to adsorbates that may be easily either reduced or oxidized at some of these metals, such as ethylene and oxalic acid. On the other hand, the SERS spectrum of CO adsorbed on a transition metal alloy (Pd/Pt = 1) electrode is obtained for the first time. Taking advantage of the high surface enhancement factor found for these nanostructured samples (∼550 for CN− on Pt at − 0.4 V/SCE), ethylene and oxalic SER spectra are obtained for pure platinum electrodes. The importance of the charge‐transfer mechanism for the enhancement in the case of Pt is evidenced according to the ratio of the band intensities for CO or CN− to the oxalic ones. Specific advantages of the nanoparticles‐on‐electrode approach are highlighted. Copyright © 2005 John Wiley & Sons, Ltd.

Surface coatings of PEO–PPO–PEO block copolymers on native and polystyrene-coated silicon wafers

Adsorbed layers of three PEO–PPO–PEO triblock copolymers on a hydrophilic SiO 2 and a hydrophobic polystyrene surface were studied by atomic force microscopy, X-ray reflectometry, ellipsometry, and contact angle measurements. Adsorption from aqueous solutions at the hydrophilic surface is found to depend in a delicate way on N EO and N PO, the total numbers of hydrophilic EO and hydrophobic PO groups of the polymers, as quantified by the asymmetry parameter β = N EO 3/5/ N PO 1/2. Only for the polymer of medial β value, F127 ( β = 2.76), an adsorbed layer is retained after extensive rinsing with pure water. At the hydrophobic surface all polymers, L64 ( β = 1.2), F127 ( β = 2.76), and F68 ( β = 3.47), form stable ad-layers and their thickness scales with the number of EO units of the hydrophilic blocks. From this result and the observed decrease of the contact angle of water droplets caused by the ad-layer, it is conjectured that the polymers adsorb with their PPO block to the hydrophobic substrate and the PEO blocks directed outwards. At the hydrophilic surface the ad-layer of F127 is oriented in the opposite way, i.e., with the PPO blocks oriented outwards. In all cases, the ad-layers form uniform coatings on a micrometer scale, with a thickness in the range 1.0–2.3 nm, again suggesting monolayer coverage of the surface.

Effect of Chemical Structure on the Adsorption of Amino Acids with Aliphatic and Aromatic Substitution Groups: In Situ STM Study

The adsorption of three amino acids, l-valine (Val), l-leucine (Leu), and l-phenylalanine (Phe), bearing different substituted groups was investigated on a Cu(111) electrode by in situ electrochemical scanning tunneling microscopy (ECSTM). All three molecules are found to form well-ordered adlayers on the surface. Two stable adlayers of l-Val with the symmetry of (3 × 3) and (2 × √3) were found at the investigated potential range, while l-Leu and l-Phe form similar (4 × 4) adlayer structures. With high-resolution STM images, the internal structural information of adlayers was obtained and the structural models for the three adlayers were proposed.

Examining a Potential Fuel Cell Poison : A Voltammetry Study of the Influence of Carbon Dioxide on the Hydrogen Oxidation Capability of Carbon-Supported Pt and PtRu Anodes

Polymer electrolyte fuel cells being operated on reformed fossil fuels are exposed to large quantities of carbon dioxide. In order to elucidate the extent of the reverse water gas shift of on hydrogen-covered fuel-cell anodes to CO, an extensive cyclic voltammetry study has been done on Pt and PtRu electrodes in sulfuric acid with varying hydrogen coverage. The electrodes used are representative of those being used in proton exchange membrane fuel cells. The reverse shift reaction is facile on Pt/C, but with the surface not poisoned to the same extent as it does by exposure to CO-saturated solutions. The reaction product is oxidized in a similar fashion to formic acid and methanol on platinum. The results indicate the formation of a stable adlayer, consisting of linear, bridge, and multibonded CO in accordance to what has been proposed for unsupported platinum electrodes. PtRu/C fuel-cell anodes are expected to be considerably more tolerant toward carbon dioxide than Pt/C anodes, with both the reaction suppressed and the adsorbate oxidation occurring at a lower potential. © 2001 The Electrochemical Society. All rights reserved.

Bromine monolayer adsorption on Pt(110) surfaces

Abstract Pt(110)-Br surfaces have been characterized by cyclic voltammetry, charge displacement experiments and scanning tunneling microscopy (STM). The strong specific adsorption of bromide involves charge transfer and gives rise to very stable adlayers of bromine atoms that survive emersion and transfer to a bromide-free electrolyte. These adlayers can be characterized by ex-situ STM. Bromine coverages calculated from charge displacement experiments are in the range 0.6–0.7 Br/Pt, referred to the atomic surface density of the unreconstructed surface. From STM images, the arrangement of bromine adatoms is concluded to be a distorted compact hexagonal lattice, whose coincidence with the substrate gives rise to complex structures, with preferential alignment along the [00 1 ] direction.

Structures and Dynamic Formation Processes of Porphyrin Adlayers on Iodine-Modified Au(111) in Solution: In Situ STM Study

The structures and dynamic formation processes of adlayers of 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)-21H,23H-porphine (TMPyP) on both bare Au(111) and iodine-modified Au(111) in perchloric acid have been investigated in detail by using in situ scanning tunneling microscopy (STM). Highly-ordered TMPyP arrays formed on iodine-modified Au(111), whereas disordered adlayers were consistently found on bare Au(111) surface. High-resolution STM images revealed the characteristic internal shape and orientation of each TMPyP molecule in ordered adlayers. Time-dependent in situ STM allowed direct observation of the dynamics of the self-ordering processes. Before the most stable adlayer was established in a relatively small domain, several structural changes were found to occur, including the formation of a one-dimensional ordered chain at an early stage and several phase transitions in the ordered adlayers. Once the most stable adlayer was formed as a nucleus in a domain, two-dimensional growth of the domain extended over the entire area of the terraces with the final packing arrangement.

An amorphous monolayer: Infrared spectroscopic and theoretical studies of SO2 on NaCl (100)

At temperatures between 100 and 110 K, exposing the (100) face of NaCl to unsaturated SO2 gas yields a stable adlayer. Infrared spectra of adsorbed SO2 contain complex resonances near the origins of the molecular symmetric and asymmetric stretching vibrations. On photometric grounds the absolute coverage of the surface is found to be one molecule per exposed Na+Cl− ion pair. The spectra of this monolayer consist of several sharp lines overlapping one broad feature for each molecular mode. By comparison to vibrational excitons in simpler systems, the coexistence of crystalline and amorphous adlayer structures is strongly indicated. This partial ordering is pressure dependent, and develops spontaneously on a time scale of minutes. The disordered component, in contrast, is never in equilibrium with the gas phase. Computational simulations have detailed the microscopic basis of this behavior. Accurate ab initio models of the SO2 molecule and NaCl(100) surface were used in a Monte Carlo simulation of the experimental conditions. At both half and full coverage, an amorphous two-dimensional condensate developed. This is minimally consistent with the polarized infrared photometry. Seemingly equilibrated Monte Carlo runs retained some memory of the initial molecular configuration, again consistent with hysteresis observed in the spectroscopic experiments. No structural order developed in the simulations, but the energetic state of affairs was clarified: the potential driving adsorption and condensation is deep in comparison to available thermal energy, but relatively insensitive to molecular orientation.

Formation and dissipation of adlayer domains during electrooxidation of carbon monoxide on ordered monocrystalline and disordered platinum and rhodium surfaces as probed by FTIR spectroscopy

Abstract The stability of adlayer domains (“islands”) of carbon monoxide formed during electrooxidation on ordered and disordered monocrystalline platinum and rhodium electrodes in aqueous 0.1M HClO 4 has been characterized by means of real-time surface FTIR spectroscopy. The presence of such domains is discerned in part from the retention of high-frequency CO stretching ( v CO ) bands for both terminal and twofold bridging CO, characteristic of close-packed adlayer structures, during electrooxidative CO removal down to low average coverages, θ. Further evidence for the presence of stable adlayer domains on ordered Pt(111) and Pt(100) was obtained by sequential dosing and partial electrooxidation of 13 CO followed by 12 CO dosing. The dipole-dipole coupling properties of such adlayers indicate partial segregation of the isotopic forms. Subsequent rapid electrooxidation of these mixed isotope layers yields initially preferential formation of 12 CO 2 , pointing to the presence of reactive domain regions on the ordered surfaces. While the adlayer domains are stable for extended periods (ca. 1 h) on most ordered monocrystalline surfaces, they undergo relaxation on Pt(100) as discerned from decreases in the v CO frequency with time at a fixed θ as well as with decreasing coverage. The dissipation of adlayer structures on disordered monocrystalline Pt and Rh was also observed during slow potentiostatic oxidation from θ-induced v CO frequency decreases, which become progressively greater for longer oxidation timescales. Similar findings were also obtained for CO on both mechanically polished and annealed polycrystalline platinum; domain dissipation is especially facile on the former surface. Such electrooxidation-induced CO domain dissipation is attributed to the nucleation of the reaction front at surface defect sites. The stability of adlayer domains during electrooxidation on most ordered surfaces, and for short reaction timescales on disordered surfaces, is ascribed to a lower density of nucleation sites operative under these conditions.