Surface Segregation Studies in Ternary Noble Metal Alloys: Comparing DFT and Machine Learning with Experimental Data

Surface segregation, whereby the surface composition of an alloy differs systematically from the bulk, has historically been hard to study, because it requires experimental and modeling methods that span alloy composition space. In this work, we study surface segregation in catalytically relevant noble and platinum-group metal alloys with a focus on three ternary systems: AgAuCu, AuCuPd, and CuPdPt. We develop a data set of 2478 slabs with those compositions including all three low-index crystallographic orientations relaxed with Density Functional Theory using the PBEsol functional with D3 dispersion corrections. We fine-tune a machine learning model on this data and use the model in a series of 1800 Monte Carlo simulations spanning ternary composition space for each surface orientation and ternary chemical system. The results of these simulations are validated against prior experimental surface segregation data collected using composition spread alloy films for AgAuCu and AuCuPd. Our findings reveal that simulations conducted using the (110) orientation most closely match experimentally observed surface segregation trends, and while predicted trends qualitatively match observation, biases in the PBEsol functional limit numeric accuracy. This study advances understanding of surface segregation and the utility of computational studies and highlights the need for further improvements in simulation accuracy.

Activation by O2 of AgxPd1-x Alloy Catalysts for Ethylene Hydrogenation

A composition spread alloy film (CSAF) spanning all of AgxPd1-x composition space, xPd = 0 → 1, was used to study catalytic ethylene hydrogenation with and without the presence of O2 in the feed gas. High-throughput measurements of the ethylene hydrogenation activity of AgxPd1-x alloys were performed at 100 Pd compositions spanning xPd = 0 → 1. The extent of ethylene hydrogenation was measured versus xPd at reaction temperatures spanning T = 300 → 405 K and inlet hydrogen partial pressures spanning PH2in = 70 → 690 Torr. The inlet ethylene partial pressure was constant at PC2H4in = 25 Torr, and the O2 inlet partial pressure was either PO2in = 0 or 15 Torr. When PO2in = 0 Torr, only those alloys with xPd ≥ 0.90 displayed observable ethylene hydrogenation activity. As expected, the most active catalyst was pure Pd, which yielded a maximum conversion of ∼0.4 at T = 405 K and PH2in = 690 Torr. Adding a constant O2 partial pressure of PO2in = 15 Torr to the feed stream dramatically increased the catalytic activity across the CSAF at all experimental conditions and catalyst compositions without inducing catalytic ethylene combustion and without measurable O2 consumption. The presence of PO2in = 15 Torr more than doubled the maximum achievable conversion on Pd to ∼0.9 and activated alloys with as little as xPd = 0.6 for ethylene hydrogenation. Measurement of the reaction order with respect to hydrogen, nH2, showed that nH2 ≈ 0 when PO2in = 15 Torr on high xPd alloys but that nH2 increases to values between 0.5 and 1 as xPd decreases or when PO2in = 0 Torr. We attribute this PO2in-induced change in nH2 to a change in the reaction mechanism resulting from different functional catalyst surfaces: one that is O2-activated and Pd-rich and one that is Ag-capped with low activity. Both are extremely sensitive to the bulk alloy composition, xPd, and the reaction temperature, T. These results show that the activity of AgPd catalysts for ethylene hydrogenation depends strongly on the operational conditions. Furthermore, we demonstrate that the exposure of AgPd catalysts to 15 Torr of O2 at moderate temperatures leads to enhanced catalyst performance, presumably by stimulating both Pd segregation to the topmost surface and Pd activation for ethylene hydrogenation.

Alloy corrosion and passivation spanning composition space

In this short review we highlight the importance and the capabilities of composition spread alloy films (CSAFs) for the high-throughput study and comprehensive understanding of corrosion passivation in multicomponent alloys, A_xB_yC_1-x-y, spanning composition space, x ∈ [0, 1] and y ∈ [0, 1 - x]. After first establishing the mechanistic issues associated with corrosion, and the problems arising from the corrosion of metals, we establish the need for further studying and understanding the mechanisms of alloy corrosion and corrosion passivation. In particular, we highlight the development of new combinatorial methods that circumvent the experimental bottleneck associated with preparing, characterizing, and testing many alloy samples having common components at different compositions. We will illustrate the use of CSAFs in studying corrosion across alloy composition space. Because of their structure and inherent composition range, CSAFs enable many novel studies that are otherwise intractable using the traditional methods of preparing and testing one alloy composition at a time.

Molecular Origins of Chiral Amplification on an Achiral Surface: 2D Monolayers of Aspartic Acid on Cu(111)

Recent experiments have demonstrated an intriguing phenomenon in which adsorption of a nonracemic mixture of aspartic acid (Asp) enantiomers onto an achiral Cu(111) metal surface leads to autoamplification of surface enantiomeric excess, ee_s, to values well above those of the impinging gas mixtures, ee_g. This is particularly interesting because it demonstrates that a slightly nonracemic mixture of enantiomers can be further purified simply by adsorption onto an achiral surface. In this work, we seek a deeper understanding of this phenomena and apply scanning tunneling microscopy to image the overlayer structures formed by mixed monolayers of d- and l-Asp on Cu(111) over the full range of surface enantiomeric excess; ee_s = -1 (pure l-Asp) through ee_s = 0 (racemic dl-Asp) to ee_s = 1 (pure d-Asp). Both enantiomers of three chiral monolayer structures are observed. One is a conglomerate (enantiomerically pure), another is a racemate (equimolar mixture of d- and l-Asp); however, the third structure accommodates both enantiomers in a 2:1 ratio. Such solid phases of enantiomer mixtures with nonracemic composition are rare in 3D crystals of enantiomers. We argue that, in 2D, the formation of chiral defects in a lattice of one enantiomer is easier than in 3D, simply because the stress associated with the chiral defect in a 2D monolayer of the opposite enantiomer can be dissipated by strain into the space above the surface.

Breaking the barrier to biomolecule limit-of-detection via 3D printed multi-length-scale graphene-coated electrodes

Sensing of clinically relevant biomolecules such as neurotransmitters at low concentrations can enable an early detection and treatment of a range of diseases. Several nanostructures are being explored by researchers to detect biomolecules at sensitivities beyond the picomolar range. It is recognized, however, that nanostructuring of surfaces alone is not sufficient to enhance sensor sensitivities down to the femtomolar level. In this paper, we break this barrier/limit by introducing a sensing platform that uses a multi-length-scale electrode architecture consisting of 3D printed silver micropillars decorated with graphene nanoflakes and use it to demonstrate the detection of dopamine at a limit-of-detection of 500 attomoles. The graphene provides a high surface area at nanoscale, while micropillar array accelerates the interaction of diffusing analyte molecules with the electrode at low concentrations. The hierarchical electrode architecture introduced in this work opens the possibility of detecting biomolecules at ultralow concentrations.

Deep reinforcement learning for predicting kinetic pathways to surface reconstruction in a ternary alloy

The majority of computational catalyst design focuses on the screening of material components and alloy composition to optimize selectivity and activity for a given reaction. However, predicting the metastability of the alloy catalyst surface at realistic operating conditions requires an extensive sampling of possible surface reconstructions and their associated kinetic pathways. We present CatGym, a deep reinforcement learning (DRL) environment for predicting the thermal surface reconstruction pathways and their associated kinetic barriers in crystalline solids under reaction conditions. The DRL agent iteratively changes the positions of atoms in the near-surface region to generate kinetic pathways to accessible local minima involving changes in the surface compositions. We showcase our agent by predicting the surface reconstruction pathways of a ternary Ni3Pd3Au2(111) alloy catalyst. Our results show that the DRL agent can not only explore more diverse surface compositions than the conventional minima hopping method, but also generate the kinetic surface reconstruction pathways. We further demonstrate that the kinetic pathway to a global minimum energy surface composition and its associated transition state predicted by our agent is in good agreement with the minimum energy path predicted by nudged elastic band calculations.

2D Ising Model for Adsorption-induced Enantiopurification of Racemates

Mechanisms for the spontaneous transformation of achiral chemical systems into states of enantiomeric purity have important ramifications in modern pharmacology and potential relevance to the origins of homochirality in life on Earth. Such mechanisms for enantiopurification are needed for production of chiral pharmaceuticals and other bioactive compounds. Previously proposed chemical mechanisms leading from achiral systems to near homochirality are initiated by a symmetry-breaking step resulting in a minor excess of one enantiomer via statistical fluctuations in enantiomer concentrations. Subsequent irreversible processes then amplify the majority enantiomer concentration while simultaneously suppressing minority enantiomer production. Herein, equilibrium adsorption of amino acid enantiomer mixtures onto chiral and achiral surfaces reveals amplification of surface enantiomeric excess relative to the gas phase; i. e. enantiopurification of chiral adsorbates by adsorption. This adsorption-induced amplification of enantiomeric excess is shown to be well-describe by the 2D Ising model. More importantly, the 2D-Ising model predicts formation of homochiral monolayers from adsorption of racemic mixtures or prochiral molecules on achiral surfaces; i. e. enantiopurification with no apparent chiral driving force.

Chiral metal surfaces for enantioselective processes

Chiral surfaces are critical components of enantioselective heterogeneous processes such as those used to prepare enantiomerically pure pharmaceuticals. While the majority of chiral surfaces in practical use are based on achiral materials whose surfaces have been modified with enantiomerically pure chiral adsorbates, there are many inorganic materials with valuable surface properties that could be rendered enantiospecific, if their surfaces were intrinsically chiral. This Perspective discusses recent developments in the fabrication of intrinsically chiral surfaces exhibiting enantiospecific adsorption, surface chemistry and electron emission. We propose possible paths to the scalable fabrication of high-surface-area, enantiomerically pure surfaces and discuss opportunities for future progress.

Enhancing Thermal Interface Conductance to Graphene Using Ni-Pd Alloy Contacts

To identify superior thermal contacts to graphene, we implement a high-throughput methodology that systematically explores the Ni-Pd alloy composition spectrum and the effect of Cr adhesion layer thickness on thermal interface conductance with monolayer graphene. Frequency domain thermoreflectance measurements of two independently prepared Ni-Pd/Cr/graphene/SiO₂ samples identify a maximum metal/graphene/SiO₂ junction thermal interface conductance of 114 ± (39, 25) MW/m² K and 113 ± (33, 22) MW/m² K at ∼10 at. % Pd in Ni-nearly double the highest reported value for pure metals and 3 times that of pure Ni or Pd. The presence of Cr, at any thickness, suppresses this maximum. Although the origin of the peak is unresolved, we find that it correlates with a region of the Ni-Pd phase diagram that exhibits a miscibility gap. Cross-sectional imaging by high-resolution transmission electron microscopy identifies striations in the alloy at this particular composition, consistent with separation into multiple phases. Through this work, we draw attention to alloys in the search for better contacts to two-dimensional materials for next-generation devices.

Templated Growth of a Homochiral Thin Film Oxide

Chiral surfaces are of growing interest for enantioselective adsorption and reactions. While metal surfaces can be prepared with a wide range of chiral surface orientations, chiral oxide surface preparation is more challenging. We demonstrate the chirality of a metal surface can be used to direct the homochiral growth of a thin film chiral oxide. Specifically, we study the chiral "29" copper oxide, formed by oxidizing a Cu(111) single crystal at 650 K. Surface structure spread single crystals, which expose a continuous distribution of surface orientations as a function of position on the crystal, enable us to systematically investigate the mechanism of chirality transfer between the metal and the surface oxide with high-resolution scanning tunneling microscopy. We discover that the local underlying metal facet directs the orientation and chirality of the oxide overlayer. Importantly, single homochiral domains of the "29" oxide were found in areas where the Cu step edges that templated growth were ≤20 nm apart. We use this information to select a Cu(239 241 246) oriented single crystal and demonstrate that a "29" oxide surface can be grown in homochiral domains by templating from the subtle chirality of the underlying metal crystal. This work demonstrates how a small degree of chirality induced by slight misorientation of a metal surface (∼1 sites/20 nm²) can be amplified by oxidation to yield a homochiral oxide with a regular array of chiral oxide pores (∼75 sites/20 nm²). This offers a general approach for making chiral oxide surfaces via oxidation of an appropriately "miscut" metal surface.

A Most Enantioselective Chiral Surface: Tartaric Acid on All Surfaces Vicinal to Cu(110)

Enantioselective chemistry on intrinsically chiral surfaces is the quintessential form of structure-sensitive surface chemistry, arising purely from the dissymmetry of the surface structure. Identification or design of chiral surface structures that maximize enantioselectivity for a given processes is extremely challenging because of the limited magnitude of the enantiospecific interaction energetics of chiral molecules with chiral surfaces. Using spherical Cu single crystals exposing surfaces with a continuous two-dimensional distribution of crystallographic orientations, we mapped the enantiospecific surface reaction kinetics of tartaric acid decomposition across the surface orientation space. These measurements reveal both the mechanistic origin of enantioselectivity and identify the structural features of the most enantiospecific surface orientation.

Chemical Reactions Impede Thermal Transport Across Metal/β-Ga2O3 Interfaces

The impact of chemical reactions on the thermal boundary conductance (TBC) of Au/metal contact/β-Ga₂O₃ layered samples as a function of contact thickness is investigated using high-throughput thermoreflectance measurements. A maximum in TBC of 530 ± 40 (260 ± 25) MW/m² K is discovered for a Cr (Ti) contact at a thickness of 2.5 (5) nm. There is no local maximum for a Ni contact, for which the TBC saturates at 410 ± 35 MW/m² K for thicknesses greater than 3 nm. Relative to the Au/β-Ga₂O₃ interface, which has a TBC of 45 ± 7 MW/m² K, these nanoscale contacts enhance TBC by factors of 6 to 12. The TBC maximum only exists for metals capable of forming oxides that are enthalpically favorable compared to β-Ga₂O₃. The formation of Cr₂O₃, via oxygen removal from the β-Ga₂O₃ substrate, is confirmed by TEM analysis. The reaction-formed oxide layer reduces the potential TBC and leads to the maximum, which is followed by a plateau at a lower value, as its thickness saturates due to passivation. Many advanced materials are prone to similar chemical reactions, impacting contact engineering and thermal management for a variety of applications.

Kinetics and Mechanism of Aspartic Acid Adsorption and Its Explosive Decomposition on Cu(100)

The mechanism and kinetics of aspartic acid (Asp, HO₂CCH(NH₂)CH₂CO₂H) decomposition on Cu(100) have been studied using X-ray photoemission spectroscopy and temperature-programmed reaction spectroscopy. We investigate the Asp decomposition mechanism in detail using unlabeled d-Asp and isotopically labeled l-Asp-4-¹³C (HO₂CCH(NH₂)CH₂¹³CO₂H), l-Asp- d₇ (DO₂CCD(ND₂)CD₂CO₂D), l-Asp-2,3,3- d₃ (HO₂CCD(NH₂)CD₂CO₂H), and l-Asp-¹⁵N-2,3,3- d₃ (HO₂CCD(¹⁵NH₂)CD₂CO₂H). The monolayer of Asp adsorbed on the Cu(100) surface is in a doubly deprotonated bi-aspartate form (-O₂CCH(NH₂)CH₂CO₂-). During heating, Asp decomposes on Cu(100) with kinetics consistent with a vacancy-mediated explosion mechanism. The mechanistic steps yield CO₂ by sequential cleavage of the C3-C4 and C1-C2 bonds, and N≡CCH₃ and H₂ via decomposition of the remaining CH(NH₂)CH₂ intermediate. Deuterium labeling has been used to demonstrate that scrambling of H(D) occurs during the decomposition to acetonitrile of the CD(NH₂)CD₂ intermediate formed by decarboxylation of l-Asp-2,3,3- d₃ and l-Asp-¹⁵N-2,3,3- d₃.

Enantiomer surface chemistry: conglomerate versus racemate formation on surfaces

Research on surface chirality is motivated by the need to develop functional chiral surfaces for enantiospecific applications. While molecular chirality in 3D has been the subject of study for almost two centuries, many aspects of 2D chiral surface chemistry have yet to be addressed. In 3D, racemic mixtures of chiral molecules tend to aggregate into racemate (molecularly heterochiral) crystals much more frequently than conglomerate (molecularly homochiral) crystals. Whether chiral adsorbates on surfaces preferentially aggregate into heterochiral rather than homochiral domains (2D crystals or clusters) is not known. In this review, we have made the first attempt to answer the following question based on available data: in 2D racemic mixtures adsorbed on surfaces, is there a clear preference for homochiral or heterochiral aggregation? The current hypothesis is that homochiral packing is preferred on surfaces; in contrast to 3D where heterochiral packing is more common. In this review, we present a simple hierarchical scheme to categorize the chirality of adsorbate-surface systems. We then review the body of work using scanning tunneling microscopy predominantly to study aggregation of racemic adsorbates. Our analysis of the existing literature suggests that there is no clear evidence of any preference for either homochiral or heterochiral aggregation at the molecular level by chiral and prochiral adsorbates on surfaces.

Detection of CuAuPd Phase Boundaries Using Core Level Shifts

A high throughput study has been conducted of the Cu 2p_3/2, Au 4f_7/2, and Pd 3d_3/2 X-ray photoemission spectra obtained from a continuous distribution of Cu_xAu_yPd_1-x-y alloy samples prepared as a single composition spread alloy film (CSAF). All three elements exhibit shifts of their core level binding energies with respect to their pure states when diluted into the alloy. The Cu 2p_3/2 core level shift (CLS) exhibits additional shifts over the composition ranges at which the Cu_xAu_yPd_1-x-y alloy transitions between FCC and B2 phases. This discontinuous CLS has been used to map the extent of the B2 phase across the ternary Cu_xAu_yPd_1-x-y alloy composition space. The sensitivity of core level binding energies to the alloy phase offers an opportunity to use XPS to study phases in alloy nanoparticles, ultrathin films, and other morphologies that are not amenable to structure determination by diffraction based methods.

Structure-sensitive enantiospecific adsorption on naturally chiral Cu(hkl) R&S surfaces

The desorption kinetics of a chiral compound, R-3-methylcyclohexanone (R-3MCHO), have been measured on both enantiomers of seven chiral Cu(hkl) ^R&S surfaces and on nine achiral Cu single crystal surfaces with surface structures that collectively span the various regions of the stereographic triangle. The naturally chiral surfaces have terrace-step-kink structures formed by all six possible combinations of the three low Miller index microfacets. The chirality of the kink sites is defined by the rotational orientation of the (1 1 1), (1 0 0) and (1 1 0) microfacets forming the kink. R-3MCHO adsorbs reversibly on these Cu surfaces and temperature programmed desorption has been used to measure its desorption energetics from the chiral kink sites. The desorption energies from the R- and S-kink sites are enantiospecific, [Formula: see text], on the chiral surfaces. The magnitude of the enantiospecificity is [Formula: see text]  ≈  1 kJ mol^-1 on all seven chiral surfaces. However, the values of [Formula: see text] are sensitive to elements of the surface structure other than just their sense of chirality as defined by the rotational orientation of the low Miller index microfacets forming the kinks; [Formula: see text] changes sign within the set of surfaces of a given chirality.

High-Throughput Screening Across Quaternary Alloy Composition Space: Oxidation of (AlxFeyNi1-x-y)∼0.8Cr∼0.2

Composition spread alloy films (CSAFs) are commonly used as libraries for high-throughput screening of composition-property relationships in multicomponent materials science. Because lateral gradients afford two degrees of freedom, an n-component CSAF can, in principle, contain any composition range falling on a continuous two-dimensional surface through an (n - 1)-dimensional composition space. However, depending on the complexity of the CSAF gradients, characterizing and graphically representing this composition range may not be straightforward when n ≥ 4. The standard approach for combinatorial studies performed using quaternary or higher-order CSAFs has been to use fixed stoichiometric ratios of one or more components to force the composition range to fall on some well-defined plane in the composition space. In this work, we explore the synthesis of quaternary Al-Fe-Ni-Cr CSAFs with a rotatable shadow mask CSAF deposition tool, in which none of the component ratios are fixed. On the basis of the unique gradient geometry produced by the tool, we show that the continuous quaternary composition range of the CSAF can be rigorously represented using a set of two-dimensional "pseudoternary" composition diagrams. We then perform a case study of (AlxFeyNi1-x-y)∼0.8Cr∼0.2 oxidation in dry air at 427 °C to demonstrate how such CSAFs can be used to screen an alloy property across a continuous two-dimensional subspace of a quaternary composition space. We identify a continuous boundary through the (AlxFeyNi1-x-y)∼0.8Cr∼0.2 subspace at which the oxygen uptake into the CSAF between 1 and 16 h oxidation time increases abruptly with decreasing Al content. The results are compared to a previous study of the oxidation of AlxFeyNi1-x-y CSAFs in dry air at 427 °C.

Assessment of a High-Throughput Methodology for the Study of Alloy Oxidation using AlxFeyNi1-x-y Composition Gradient Thin Films

The high-temperature oxidation of multicomponent metal alloys exhibits complex dependencies on composition, which are not fully understood for many systems. Combinatorial screening of the oxidation of many different compositions of a given alloy offers an ideal means for gaining fundamental insights into such systems. We have previously developed a high-throughput methodology for studying AlxFeyNi1-x-y alloy oxidation using ∼100 nm thick composition spread alloy films (CSAFs). In this work, we critically assess two aspects of this methodology: the sensitivity of CSAF oxidation behavior to variations in AlxFeyNi1-x-y composition and the differences between the oxidation behavior of ∼100 nm thick CSAFs and that of bulk AlxFeyNi1-x-y alloys. This was done by focusing specifically on AlxFe1-x and AlxNi1-x oxidation in dry air at 427 °C. Transitions between phenomenologically distinguishable types of oxidation behavior are found to occur over CSAF compositional ranges of <2 at. %. The oxidation of AlxFe1-x CSAFs is found to be very similar to that of bulk AlxFe1-x alloys, but some minor differences between CSAF and bulk behavior are observed for AlxNi1-x oxidation. On the basis of our assessment, high-throughput studies of CSAF oxidation appear to be an effective method for gaining fundamental insights into the composition dependence of the oxidation of bulk alloys.

Atomic-Scale Picture of the Composition, Decay, and Oxidation of Two-Dimensional Radioactive Films

Two-dimensional radioactive (125)I monolayers are a recent development that combines the fields of radiochemistry and nanoscience. These Au-supported monolayers show great promise for understanding the local interaction of radiation with 2D molecular layers, offer different directions for surface patterning, and enhance the emission of chemically and biologically relevant low-energy electrons. However, the elemental composition of these monolayers is in constant flux due to the nuclear transmutation of (125)I to (125)Te, and their precise composition and stability under ambient conditions has yet to be elucidated. Unlike I, which is stable and unreactive when bound to Au, the newly formed Te atoms would be expected to be more reactive. We have used electron emission and X-ray photoelectron spectroscopy (XPS) to quantify the emitted electron energies and to track the film composition in vacuum and the effect of exposure to ambient conditions. Our results reveal that the Auger electrons emitted during the ultrafast radioactive decay process have a kinetic energy corresponding to neutral Te. By combining XPS and scanning tunneling microscopy experiments with density functional theory, we are able to identify the reaction of newly formed Te to TeO2 and its subsequent dimerization. The fact that the Te2O4 units stay intact during major lateral rearrangement of the monolayer illustrates their stability. These results provide an atomic-scale picture of the composition and mobility of surface species in a radioactive monolayer as well as an understanding of the stability of the films under ambient conditions, which is a critical aspect in their future applications.

Enantiospecific Adsorption of Amino Acids on Naturally Chiral Cu{3,1,17}R&S Surfaces

Gas-phase equilibrium adsorption of D- and L-serine (Ser) mixtures and D- and L-phenylalanine (Phe) mixtures has been studied on the naturally chiral Cu{3,1,17}(R&S) surfaces. (13)C labeling of the l enantiomers (*L-Ser and *L-Phe) has enabled mass spectrometric enantiodiscrimination of the species desorbing from the surface following equilibrium adsorption. On the Cu{3,1,17}(R&S) surfaces, both equilibrium adsorption and the thermal decomposition kinetics of the D and *L enantiomers exhibit diastereomerism. Following exposure of the surfaces to D/*L mixtures, the relative equilibrium coverages of the two enantiomers are equal to their relative partial pressures in the gas phase, θ(D)/θ(*L) = P(D)/P(*L). This implies that adsorption is not measurably enantiospecific. The decomposition kinetics of Ser are enantiospecific whereas those of Phe are not. Comparison of these results with those for aspartic acid, alanine, and lysine suggests that enantiospecific adsorption on the naturally chiral Cu surfaces occurs for those amino acids that have side chains with functional groups that allow strong interactions with the surface. There is no apparent correlation between amino acids that exhibit enantiospecific adsorption and those that exhibit enantiospecific decomposition kinetics.

Enantiospecific desorption of R- and S-propylene oxide from D- or L-lysine modified Cu(100) surfaces

The enantiospecific desorption kinetics of R- and S-propylene oxide (PO) from a Cu(100) surface modified by enantiomerically pure D- or L-lysine have been studied using temperature programmed desorption. These experiments have used R- or S-PO as the chiral probe for study of enantiospecific adsorption on Cu(100) surfaces modified with D- or L-lysine. This chiral probe/modifier/Cu system manifests a significant diastereomeric effect in the R- and S-PO peak desorption temperatures and, hence, true enantiospecific behavior. The enantiospecificity in the PO desorption kinetics is observed only over a narrow range of lysine modifier coverage with a maximum at a lysine coverage leaving an empty site density of θ(O) ≈ 0.25. The observation of enantiospecific behavior in the PO/lysine/Cu(100) system is in contrast with the failed results of prior attempts to observe enantiospecific desorption from chirally modified Cu surfaces. The potential for hydrogen-bonding interactions between the chiral probe and chiral modifier, which can depend on the coverage and configuration of the adsorbed modifier, may play a crucial role in enantiospecific adsorption on lysine modified Cu surfaces.

Fe2o3 shell growth on Pt nanoparticles

Fe2O3 shells have been synthesized around Pt cores to create Pt@Fe2O3 core-shell nanoparticles. The synthesis conditions allow control of the shell shape and allow the preparation of both hexagonal shells and spherical shells. 2D cross-sectional TEM images show that the cores are not positioned at the centers of the shells. By rotating the nanoparticles and monitoring the apparent motions of the cores in the 2D cross-sectional images, it is possible to determine quantitatively the radial position of the Pt core with respect to the center of the Fe2O3 shell. The distribution of core positions within the core-shell structures is bimodal. These observations suggest that the Fe2O3 shells grow on the Pt cores by a nucleation process, rather than layer-by-layer growth.

Probing enantioselectivity on chirally modified Cu(110), Cu(100), and Cu(111) surfaces

Temperature programmed desorption methods have been used to probe the enantioselectivity of achiral Cu(100), Cu(110), and Cu(111) single crystal surfaces modified by chiral organic molecules including amino acids, alcohols, alkoxides, and amino-alcohols. The following combinations of chiral probes and chiral modifiers on Cu surfaces were included in this study: propylene oxide (PO) on L-alanine modified Cu(110), PO on L-alaninol modified Cu(111), PO on 2-butanol modified Cu(111), PO on 2-butoxide modified Cu(100), PO on 2-butoxide modified Cu(111), R-3-methylcyclohexanone (R-3-MCHO) on 2-butoxide modified Cu(100), and R-3-MCHO on 2-butoxide modified Cu(111). In contrast with the fact that these and other chiral probe/modifier systems have exhibited enantioselectivity on Pd(111) and Pt(111) surfaces, none of these probe/modifier/Cu systems exhibit enantioselectivity at either low or high modifier coverages. The nature of the underlying substrate plays a significant role in the mechanism of hydrogen-bonding interactions and could be critical to observing enantioselectivity. While hydrogen-bonding interactions between modifier and probe molecule are believed to induce enantioselectivity on Pd surfaces (Gao, F.; Wang, Y.; Burkholder, L.; Tysoe, W. T. J. Am. Chem. Soc. 2007, 129, 15240-15249), such critical interactions may be missing on Cu surfaces where hydrogen-bonding interactions are believed to occur between adjacent modifier molecules, enabling them to form clusters or islands.

Chiral surfaces: accomplishments and challenges

Chiral surfaces serve as media for enantioselective chemical processes. Their chirality is dictated by atomic- and molecular-level structure, and their enantioselectivity is determined by their enantiospecific interactions with chiral adsorbates. This Perspective describes three types of chiral metal surfaces: those modified by adsorption of chiral molecules, those templated by chiral lattices of adsorbed species, and those that are naturally chiral. A new paper in this issue of ACS Nano offers insight into the intermolecular interactions that govern chiral templating of surfaces. This Perspective then outlines three major challenges to the field of chiral surface science: development of methods for detection of enantiospecific interactions and enantioselective surface chemistry, preparation of high-area chiral metal surfaces, and the development of a fundamental, predictive-level understanding of the origin of enantioselectivity on chiral surfaces.

Oxidation of fluorinated amorphous carbon (a-CF(x)) films

Amorphous fluorinated carbon (a-CF(x)) films have a variety of potential technological applications. In most such applications these films are exposed to air and undergo partial surface oxidation. X-ray photoemission spectroscopy has been used to study the oxidation of fresh a-CF(x) films deposited by magnetron sputtering. The oxygen sticking coefficient measured by exposure to low pressures (<10(-3) Torr) of oxygen at room temperature is on the order of S approximately 10(-6), indicating that the surfaces of these films are relatively inert to oxidation when compared with most metals. The X-ray photoemission spectra indicate that the initial stages of oxygen exposure (<10(7) langmuirs) result in the preferential oxidation of the carbon atoms with zero or one fluorine atom, perhaps because these carbon atoms are more likely to be found in configurations with unsaturated double bonds and radicals than carbon atoms with two or three fluorine atoms. Exposure of the a-CF(x) film to atmospheric pressures of air (effective exposure of 10(12) langmuirs to O(2)) results in lower levels of oxygen uptake than the low pressure exposures (<10(7) langmuirs). It is suggested that this is the result of oxidative etching of the most reactive carbon atoms, leaving a relatively inert surface. Finally, low pressure exposures to air result in the adsorption of both nitrogen and oxygen onto the surface. Some of the nitrogen adsorbed on the surface at low pressures is in a reversibly adsorbed state in the sense that subsequent exposure to low pressures of O(2) results in the displacement of nitrogen by oxygen. Similarly, when an a-CF(x) film oxidized in pure O(2) is exposed to low pressures of air, some of the adsorbed oxygen is displaced by nitrogen. It is suggested that these forms of nitrogen and oxygen are bound to free radical sites in the film.

Tailoring the shapes of Fe(x)Pt(100-x) nanoparticles

Fe(x)Pt(100-x) nanoparticles of varying composition have been synthesized with various shapes and sizes using a high pressure synthesis method which allows control of synthesis conditions, in particular the reaction temperature. Tailoring the shapes and sizes of Fe(x)Pt(1-x) nanoparticles allows one to control a variety of properties that are relevant to the many potential applications of metallic nanoparticles. Shape and composition can be used to control catalytic activity and to achieve high packing density in self-assembled films. Variation of both nanoparticle size and shape has been achieved by using various different solvents. The solvents used in the nanoparticle synthesis can influence the product because they can play a role as surfactants. Using solvents of various types it has been possible to synthesize Fe(x)Pt(100-x) nanoparticles with a variety of shapes including spherical, rod-like, cubic, hexagonal and high aspect ratio wires. Control of nanoparticle shape opens the door to their being used in various technological applications for which spherical nanoparticles are ineffective.

Dynamics of charging of muscovite mica: measurement and modeling

The advent of a new method for measuring the zeta potential of planar surfaces, the rotating disk, allowed the investigation of the charging process of mica after immersion in water. The zeta potential of freshly cleaved muscovite mica was recorded within seconds of immersion of the sample and in fractions of a second thereafter. The zeta potential of mica in water at pH = 5.6 with no added potassium changed by 40-50 mV over approximately 1 min. A model of adsorption and desorption of potassium ions and protons captured this behavior and provided a framework for determination of surface adsorption rate constants. The charging of mica in alkaline KCl solutions of arbitrary concentration, however, was too fast for observation. The equilibrium zeta potential depended on the logarithm of salt concentration, in agreement with a model based on ion exchange reactions. The average values of the proton adsorption, proton desorption, potassium adsorption, and potassium desorption rate coefficients were 45 L/s +/- 15, 0.0014/s +/- 0.0006, 58 L/s +/- 5, and 0.14/s +/- 0.03, respectively. An equation giving the zeta potential as a function of the rate parameters and the concentrations of potassium ions and protons was derived. It is demonstrated that subtraction of zeta potentials, measured at different solution compositions within the same experiment, eliminates extrinsic factors and brings data from disparate measurements into agreement.

Transition state for alkyl group hydrogenation on Pt(111)

Substituent effects have been used to probe the characteristics of the transition state to hydrogenation of alkyl groups on the Pt(111) surface. Eight different alkyl and fluoroalkyl groups have been formed on the Pt(111) surface by dissociative adsorption of their respective alkyl and fluoroalkyl iodides. Coadsorption of hydrogen and alkyl groups, followed by heating of the surface, results in hydrogenation of the alkyl groups to form alkanes, which then desorb into the gas phase. Temperature-programmed reaction spectroscopy was used to measure the barriers to hydrogenation, DeltaE(H)(double dagger), which are dependent on the size of the alkyl group (polarizability) and the degree of fluorination (field effect). This example is one of only two surface reactions for which the influence of the substituents on DeltaE(H)(double dagger) has been correlated with both the field and the polarizability substituent constants of the alkyl groups in the form of a linear free energy relationship. Increasing both the field and the polarizability constants of the alkyl groups increases the value of DeltaE(H)(double dagger). The substituent effects are quantified by a field reaction constant of rho(F) = 27 +/- 4 kJ/mol and a polarizability reaction constant of rho(alpha) = 19 +/- 3 kJ/mol. These suggest that the transition state for hydrogenation is slightly cationic with respect to the alkyl group on the Pt(111) surface, RC + H <--> {RC(delta+)...H}(double dagger).

Oxidation kinetics of hydrogenated amorphous carbon (a-CH(x)) overcoats for magnetic data storage media

The oxidation kinetics of a-CHx overcoats during exposure to oxygen and water vapor have been measured using X-ray photoemission spectroscopy (XPS) in an apparatus that allows oxidation and analysis of freshly deposited a-CHx overcoats without prior exposure of the overcoats to air. The uptake of oxygen on the surfaces of the a-CHx overcoats has been measured at O2 and H2O pressures in the range 10(-7)-10(-3) Torr at room temperature. The uptake of oxygen during O2 exposures on the order of 10(7) Langmuirs leads to saturation of the a-CHx overcoat surfaces at oxidation levels on the order of 20%. This indicates that the surfaces of a-CHx overcoats are relatively inert to oxidation in the sense that the dissociative sticking coefficient of O2 is approximately 10(-6). Oxygen uptake during exposure to H2O vapor is similar to the uptake during exposure to O2 gas. Although the surfaces of the a-CHx overcoats are quite inhomogeneous, it has been possible to model the uptake of oxygen on their surfaces using a fairly simple Langmuir-Hinshelwood mechanism. Interestingly, the saturation coverage of oxygen during exposure to air at atmospheric pressure is approximately 6%, significantly lower than that obtained during low-pressure exposure to O2 gas or H2O vapor.

Adsorption of fluorinated ethers and alcohols on fresh and oxidized carbon overcoats for magnetic data storage

Temperature programmed desorption has been used to study the desorption kinetics and desorption energies of perfluorodiethylether, (CF3CF2)2O, and 2,2,2-trifluoroethanol, CF3CH2OH, adsorbed on fresh and oxidized hydrogenated amorphous carbon (a-CHx) films. (CF3CF2)2O and CF3CH2OH serve as models for the ether backbone and hydroxyl end-groups of Fomblin Zdol, the lubricant most commonly used to lubricate the surfaces of amorphous carbon overcoats on magnetic data storage hard disks. Our measurements clearly reveal, for the first time, the effects of surface oxidation on the adsorption of fluorocarbon lubricants such as Fomblin Zdol on a-CHx films. Oxidation of the a-CHx surface increases the desorption energy of CF3CH2OH but has no observable impact on the desorption energy of (CF3CF2)2O. These results support the suggestion that the alcohols interact with the surface via hydrogen bonding. From a practical perspective, these results imply that the oxidation of the fresh a-CHx film may serve as a means to control or tailor the a-CHx surface to optimize the properties of the lubricant-overcoat interface in hard disks.