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 fcc 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.

2D Ising Model for Enantiomer Adsorption on Achiral Surfaces: L- and D-Aspartic Acid on Cu(111).

The 2D Ising model is well-formulated to address problems in adsorption thermodynamics. It is particularly well-suited to describing the adsorption isotherms predicting the surface enantiomeric excess, ees, observed during competitive co-adsorption of enantiomers onto achiral surfaces. Herein, we make the direct one-to-one correspondence between the 2D Ising model Hamiltonian and the Hamiltonian used to describe competitive enantiomer adsorption on achiral surfaces. We then demonstrate that adsorption from racemic mixtures of enantiomers and adsorption of prochiral molecules are directly analogous to the Ising model with no applied magnetic field, i.e., the enantiomeric excess on chiral surfaces can be predicted using Onsager's solution to the 2D Ising model. The implication is that enantiomeric purity on the surface can be achieved during equilibrium exposure of prochiral compounds or racemic mixtures of enantiomers to achiral surfaces.

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.

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.

Enantiospecific equilibrium adsorption and chemistry of d-/l-proline mixtures on chiral and achiral Cu surfaces.

A fundamental understanding of the enantiospecific interactions between chiral adsorbates and understanding of their interactions with chiral surfaces is key to unlocking the origins of enantiospecific surface chemistry. Herein, the adsorption and decomposition of the amino acid proline (Pro) have been studied on the achiral Cu(110) and Cu(111) surfaces and on the chiral Cu(643)R&S surfaces. Isotopically labelled 1-13 C-l-Pro has been used to probe the Pro decomposition mechanism and to allow mass spectrometric discrimination of d-Pro and 1-13 C-l-Pro when adsorbed as mixtures. On the Cu(111) surface, X-ray photoelectron spectroscopy reveals that Pro adsorbs as an anionic species in the monolayer. On the chiral Cu(643)R&S surface, adsorbed Pro enantiomers decompose with non-enantiospecific kinetics. However, the decomposition kinetics were found to be different on the terraces versus the kinked steps. Exposure of the chiral Cu(643)R&S surfaces to a racemic gas phase mixture of d-Pro and 1-13 C-l-Pro resulted in the adsorption of a racemic mixture; i.e., adsorption is not enantiospecific. However, exposure to non-racemic mixtures of d-Pro and 1-13 C-l-Pro resulted in amplification of enantiomeric excess on the surface, indicative of homochiral aggregation of adsorbed Pro. During co-adsorption, this amplification is observed even at very low coverages, quite distinct from the behavior of other amino acids, which begin to exhibit homochiral aggregation only after reaching monolayer coverages. The equilibrium adsorption of d-Pro and 1-13 C-l-Pro mixtures on achiral Cu(110) did not display any aggregation, consistent with prior scanning tunneling microscopy (STM) observations of dl-Pro/Cu(110). This demonstrates convergence between findings from equilibrium adsorption methods and STM experiments and corroborates formation of a 2D random solid solution.

Chemical Reactions Impede Thermal Transport Across Metal/ß-Ga2O3 Interfaces.

The impact of chemical reactions on the thermal boundary conductance (TBC) of Au/metal contact/ß-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/ß-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 ß-Ga2O3. The formation of Cr2O3, via oxygen removal from the ß-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.

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.

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.

Enantiospecific Adsorption and Decomposition of D- and L-Asp Mixtures on Cu(643)R&S.

The study of molecular chirality is essential to understanding the fundamentals of enantiospecific chemical interactions that are ubiquitous in the biochemistry of life on Earth. At a molecular level, there is insufficient understanding of chiral recognition and enantiomer-enantiomer interaction (aggregation) of chiral molecules adsorbed on surfaces. Here, using enantiospecific isotopic labelling and surface sensitive techniques, we show that when the two enantiomers of chiral aspartic acid (Asp) are adsorbed on the naturally chiral Cu(643)R&S surfaces, they decompose enantiospecifically depending on the chirality of the surface. The non-linear kinetics of the surface decomposition mechanism amplifies the difference between the decomposition rate constants of the two adsorbed enantiomers resulting in highly enantiospecific decomposition rates. Further, we also demonstrate that Asp enantiomers aggregate homochirally on several chiral and achiral surfaces, amplifying the enantiomeric excess on the surface with respect to that in the gas phase, |ees |>|eeg. Our results show that it is possible to discern the enantiospecific behavior of a complex adsorbate such as Asp and shed light on molecular level enantiospecific interactions on surfaces. The enantiospecific isotope labelling methods discussed in this paper allow probing of both the qualitative features of the Asp decomposition mechanism on Cu(643)R&S and quantitative aspects of the adsorption equilibria of enantiomer mixtures.

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.

Atomic-scale origin of the enantiospecific decomposition of tartaric acid on chiral copper surfaces.

The origin of the enantiospecific decomposition of L- and D-tartaric acid on chiral Cu surfaces is elucidated on a structure-spread domed Cu(110) crystal by spatially resolved XPS and atomic-scale STM imaging. Extensive enantiospecific surface restructuring leads to the formation of surfaces vicinal to Cu(14,17,2) which are responsible for the enantiospecificity.

Superenantioselective chiral surface explosions.

Chiral inorganic materials predated life on Earth, and their enantiospecific surface chemistry may have played a role in the origins of biomolecular homochirality. However, enantiospecific differences in the interaction energies of chiral molecules with chiral surfaces are small and typically lead to modest enantioselectivities in adsorption, catalysis, and chemistry on chiral surfaces. To yield high enantioselectivities, small energy differences must be amplified by reaction mechanisms such as autocatalytic surface explosions which have nonlinear kinetics. Herein, we report the first observations of superenantiospecificity resulting from an autocatalytic surface explosion reaction of a chiral molecule on a naturally chiral surface. R,R- and S,S-tartaric acid decompose via a vacancy-mediated surface explosion mechanism on Cu single crystal surfaces. When coupled with surface chirality, this leads to decomposition rates that exhibit extraordinarily high enantiospecificity. On the enantiomorphs of naturally chiral Cu(643)(R&S), Cu(17,5,1)(R&S), Cu(531)(R&S) and Cu(651)(R&S) single crystal surfaces, R,R- and S,S-tartaric acid exhibit enantiospecific decomposition rates that differ by as much as 2 orders of magnitude, despite the fact that the effective rates constants for decomposition differ by less than a factor of 2.

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, AxByC1-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, ees, to values well above those of the impinging gas mixtures, eeg. 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; ees = -1 (pure l-Asp) through ees = 0 (racemic dl-Asp) to ees = 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.

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.

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.

Enantioselective separation on chiral Au nanoparticles.

The surfaces of chemically synthesized Au nanoparticles have been modified with d- or l-cysteine to render them chiral and enantioselective for adsorption of chiral molecules. Their enantioselective interaction with chiral compounds has been probed by optical rotation measurements during exposure to enantiomerically pure and racemic propylene oxide. The ability of optical rotation to detect enantiospecific adsorption arises from the fact that the specific rotation of polarized light by (R)- and (S)-propylene oxide is enhanced by interaction with Au nanoparticles. This effect is related to previous observations of enhanced circular dichroism by Au nanoparticles modified by chiral adsorbates. More importantly, chiral Au nanoparticles modified with either d- or l-cysteine selectively adsorb one enantiomer of propylene oxide from a solution of racemic propylene oxide, thus leaving an enantiomeric excess in the solution phase. Au nanoparticles modified with l-cysteine (d-cysteine) selectively adsorb the (R)-propylene oxide ((S)-propylene oxide). A simple model has been developed that allows extraction of the enantiospecific equilibrium constants for (R)- and (S)-propylene oxide adsorption on the chiral Au nanoparticles.

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.