Stability of the missing-row reconstruction on fcc (110) transition-metal surfaces

Computational Design of Catalysts with Experimental Validation: Recent Successes, Effective Strategies, and Pitfalls

Computation has long proven useful in understanding heterogeneous catalysts and rationalizing experimental findings. However, computational design with experimental validation requires somewhat different approaches and has proven more difficult. In recent years, there have been increasing successes in such computational design with experimental validation. In this Perspective, we discuss some of these recent successes and the methodologies used. We also discuss various design strategies more broadly, as well as approximations to consider and pitfalls to try to avoid when designing for experiment. Overall, computation can be a powerful and efficient tool in guiding catalyst design but must be combined with a strong fundamental understanding of catalysis science to be most effective in terms of both choosing the design methodology and choosing which materials to pursue experimentally.

Low-index mesoscopic surface reconstructions of Au surfaces using Bayesian force fields

Metal surfaces have long been known to reconstruct, significantly influencing their structural and catalytic properties. Many key mechanistic aspects of these subtle transformations remain poorly understood due to limitations of previous simulation approaches. Using active learning of Bayesian machine-learned force fields trained from ab initio calculations, we enable large-scale molecular dynamics simulations to describe the thermodynamics and time evolution of the low-index mesoscopic surface reconstructions of Au (e.g., the Au(111)-'Herringbone,' Au(110)-(1 × 2)-'Missing-Row,' and Au(100)-'Quasi-Hexagonal' reconstructions). This capability yields direct atomistic understanding of the dynamic emergence of these surface states from their initial facets, providing previously inaccessible information such as nucleation kinetics and a complete mechanistic interpretation of reconstruction under the effects of strain and local deviations from the original stoichiometry. We successfully reproduce previous experimental observations of reconstructions on pristine surfaces and provide quantitative predictions of the emergence of spinodal decomposition and localized reconstruction in response to strain at non-ideal stoichiometries. A unified mechanistic explanation is presented of the kinetic and thermodynamic factors driving surface reconstruction. Furthermore, we study surface reconstructions on Au nanoparticles, where characteristic (111) and (100) reconstructions spontaneously appear on a variety of high-symmetry particle morphologies.

Competing adsorption mechanisms of pyridine on Cu, Ag, Au, and Pt(110) surfaces

We explore the adsorption of pyridine on Cu, Ag, Au, and Pt(110) surfaces using density functional theory. To account for the van der Waals interaction, we use the optB86b-vdW, optB88-vdW, optPBE-vdW, revPBE-vdW, and rPW86-vdW2 functionals. For comparison, we also run calculations using the generalized gradient approximation-PBE (Perdew-Burke-Ernzerhof) functional. We find the most stable adsorption site to depend on both metal and functional, with two energetically favorable adsorption sites, namely, a vertically oriented site and a flat pyridine site. We calculate that every functional predicts pyridine to lie in the vertical configuration on the coinage metals at a low coverage. On Pt(110), by contrast, we calculate all the functionals-except rPW86-vdW2-to predict pyridine to lie flat at a low coverage. By analyzing these differences for these adsorption configurations, along with various geometric and electronic properties of the adsorbate/substrate system, we access in detail the performance of the 6 functionals we use. We also characterize the nature of the bonding of pyridine on the coinage metals from weak to strong physisorption, depending on the functional used. On Pt(110), we characterize the nature of the bonding of pyridine as ranging from strong physisorption to strong chemisorption depending again on the functional used, illustrating both the importance of the van der Waals interaction to this system and that this system can make a stringent test for computational methods.

Atomically Visualizing Elemental Segregation-Induced Surface Alloying and Restructuring

Using in situ transmission electron microscopy that spatially and temporally resolves the evolution of the atomic structure in the surface and subsurface regions, we find that the surface segregation of Au atoms in a Cu(Au) solid solution results in the nucleation and growth of a (2 × 1) missing-row reconstructed, half-unit-cell thick L1₂ Cu₃Au(110) surface alloy. Our in situ electron microscopy observations and atomistic simulations demonstrate that the (2 × 1) reconstruction of the Cu₃Au(110) surface alloy remains as a stable surface structure as a result of the favored Cu-Au diatom configuration.

In situ studies of NO reduction by H2 over Pt using surface X-ray diffraction and transmission electron microscopy

Exposure to H₂ induces faceting of the Pt nanoparticle, while exposure to NO induces rounding of the nanoparticle.

Substrate-Induced Nanoscale Undulations of Borophene on Silver

Two-dimensional (2D) materials tend to be mechanically flexible yet planar, especially when adhered on metal substrates. Here, we show by first-principles calculations that periodic nanoscale one-dimensional undulations can be preferred in borophenes on concertedly reconstructed Ag(111). This "wavy" configuration is more stable than its planar form on flat Ag(111) due to anisotropic high bending flexibility of borophene that is also well described by a continuum model. Atomic-scale ultrahigh vacuum scanning tunneling microscopy characterization of borophene grown on Ag(111) reveals such undulations, which agree with theory in terms of topography, wavelength, Moiré pattern, and prevalence of vacancy defects. Although the lattice is coherent within a borophene island, the undulations nucleated from different sides of the island form a distinctive domain boundary when they are laterally misaligned. This structural model suggests that the transfer of undulated borophene onto an elastomeric substrate would allow for high levels of stretchability and compressibility with potential applications to emerging stretchable and foldable devices.

Finding needles in haystacks: scanning tunneling microscopy reveals the complex reactivity of Si(100) surfaces

Many chemical reactions-etching, growth, and catalytic-produce highly faceted surfaces. Examples range from the atomically flat silicon surfaces produced by anisotropic etchants to the wide variety of faceted nanoparticles, including cubes, wires, plates, tetrapods, and more. This faceting is a macroscopic manifestation of highly site-specific surface reactions. In this Account, we show that these site-specific reactions literally write a record of their chemical reactivity in the morphology of the surface-a record that can be quantified with scanning tunneling microscopy. Paradoxically, the sites targeted by these highly site-specific reactions are extremely rare. This paradox can be understood from a simple kinetic argument. An etchant that produces atomically flat surfaces must rapidly etch every surface site except the terrace atoms on the perfectly flat surface. As a result, the etch morphology is dominated by the least reactive species (here, the terrace sites), not the most reactive species. In contrast, the most interesting chemical species-the site where the reaction occurs most rapidly and most selectively-is the hardest one to find. This highly reactive site, the key to the reaction, is the needle in the haystack, often occurring in densities far below 1% of a monolayer and thus invisible to surface spectroscopies. This kinetic argument is quite general and applies to a wide variety of reactions, not just etching reactions. Understanding these highly site-specific reactions requires a combination of experimental and computational techniques with both exquisite defect sensitivity and high chemical sensitivity. In this Account, we present examples of highly site-specific chemistry on the technologically important face of silicon, Si(100). In one example, we show that the high reactivity of one particular surface site, a silicon dihydride bound to a silicon monohydride, or an "α-dihydride", provides a fundamental explanation for anisotropic silicon etching, a technology widely used in micromachining to selectively produce flat Si{111} surfaces. Fast-etching surfaces, such as Si(100) and Si(110), have geometries that support autocatalytic etching of α-dihydrides. In contrast, α-dihydrides exist only at kink sites on Si(111) surfaces. As a result, the etch rate of surfaces vicinal to Si(111) scales with the step density, approaching zero on the atomically flat surface. In a second example, we explain the chemistry that underlies pyramidal texturing of silicon wafers, a technique that is sometimes used to decrease the reflectivity of silicon solar cells. We show that a subtle change in chemical reactivity transforms a near-perfect Si(100) etchant into one that spontaneously produces nanoscale pyramids. The pyramids are not static features; they are self-propagating structures that evolve in size and location as the etching proceeds. The key to this texturing is the production of a very rare defect at the apex of each pyramid, a site that also etches autocatalytically. These experiments show that simple chemical reactions can enable an exquisite degree of atomic-scale control if only we can learn to harness them.

Quick low temperature coalescence of Pt nanocrystals on silica exposed to NO – the case of reconstruction driven growth?

We report an operando XRD/MS experiment on nanocrystalline Pt supported on silica, monitoring quick, low temperature coalescence of Pt in an NO atmosphere accompanied by surface reconstruction deduced from an apparent lattice parameter (ALP) evolution.

Iridium silicide nanowires on Si(001) surfaces

An iridium (Ir) modified silicon (001) (Si(001)) surface is studied using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The surface exhibits p(2 × 2) domains in LEED intensity images. The STM images show that the basis of the crystal lattice consists of an Ir atom and a Si dimer and, like Si(001) dimer rows, they are aligned parallel to the orthogonal [110] directions.

Surface X-ray scattering of stepped surfaces of platinum in an electrochemical environment: Pt(331) = 3(111)-(111) and Pt(511) = 3(100)-(111)

Real surface structures of the high-index planes of Pt with three atomic rows of terraces (Pt(331) = 3(111)-(111) and Pt(511) = 3(100)-(111)) have been determined in 0.1 M HClO(4) at 0.1 and 0.5 V(RHE) with the use of surface X-ray scattering (SXS). The surfaces with two atomic rows of terraces, Pt(110) = 2(111)-(111) and Pt(311) = 2(100)-(111) = 2(111)-(100), are reconstructed to a (1 × 2) structure according to previous studies. However, the surfaces with three atomic rows of terraces have pseudo-(1 × 1) structures. The interlayer spacing between the first and the second layers, d(12), is expanded 13% on Pt(331) compared to that of the bulk, whereas it is contracted 37% on Pt(511). The surface structures do not depend on the applied potential on either surface.

Spectral signatures of the surface reconstructions of Au(110)/electrolyte interfaces

It is demonstrated that the (1 × 1) structure and the (1 × 2) and (1 × 3) surface reconstructions that occur at Au(110)/electrolyte interfaces have unique optical fingerprints. The optical fingerprints are potential, pH and anion dependent and have potential for use in monitoring dynamic changes at this interface. We also observe a specific reflection anisotropy spectroscopy signature that may arise from anions adsorbed on the (1 × 1) structure of Au(110).

Domain boundary formation in helical multishell gold nanowires

Helical multishell gold nanowires are studied theoretically for the formation mechanism of the helical domain boundary. Nanowires with a wire length of more than 10 nm are relaxed by quantum mechanical molecular dynamics simulation with a tight-binding form Hamiltonian. In the results, non-helical nanowires are transformed into helical ones with the formation of atom pair defects at the domain boundary, where the defective atom pair is moved from an inner shell. Analysis of local electronic structure shows a competitive feature of the energy gain of reconstruction on the wire surface and the energy loss of the defect formation. A simple energy scaling theory gives a general explanation of domain boundary formation.

First-Principles Study of CO Adsorption and Vibration on Au Surfaces

A CO stretching frequency analysis is presented for the adsorption of CO on various Au(110) surfaces from density functional theory calculations. The structure sensitivity of the adsorption has been studied by considering the unreconstructed (1 x 1) surface, the missing-row reconstructed (1 x 2) surface, the vicinal stepped (20) surface, and the adsorption on adatoms deposited on the (110)-(1 x 2) surface. The calculated CO stretching frequencies are compared with infrared reflection-absorption spectroscopy (IRAS) measurements carried out at room temperature and pressure below 1 atm. The overall stability of the systems is discussed within the calculations of surface free energies at various coverages. At room temperature, the adsorption of CO on the ridge of the missing-row reconstructed surface competes in the high pressure regime with more complex adsorption structures where the molecule coadsorbs on the ridge and on adatoms located along the empty troughs of the reconstruction. This result is supported by the CO stretching frequency analysis.

Chevron defect at the intersection of grain boundaries with free surfaces in Au

We have identified a new defect at the intersection between grain boundaries and surfaces in Au using atomic resolution transmission electron microscopy. At the junction line of 90 degrees <110> tilt grain boundaries of (110)-(001) orientation with the free surface, a small segment of the grain boundary, about 1 nm in length, dissociates into a triangular region with a chevronlike stacking disorder and a distorted hcp structure. The structure and stability of these defects are confirmed by atomistic simulations, and we point out the relationship with the one-dimensional incommensurate structure of the grain boundary.

Monitoring the transitions of the charge-induced reconstruction of Aau(110) by reflectance anisotropy spectroscopy

Missing-row reconstructions on Au(110) immersed in electrolytes have been studied by in situ reflectance anisotropy spectroscopy. Transitions between the 1 x 3, 1 x 2, and 1 x 1 surface structures were monitored as a function of the applied potential. A kinetic model allowed us to reproduce the data satisfactorily. These results confirm the theoretical predictions showing that the surface charge determines the surface reconstruction. The transition potentials and the activation barriers were determined.

Adsorption-induced step formation

Through an interplay between density functional calculations, Monte Carlo simulations and scanning tunneling microscopy experiments, we show that an intermediate coverage of CO on the Pt(110) surface gives rise to a new rough equilibrium structure with more than 50% step atoms. CO is shown to bind so strongly to low-coordinated Pt atoms that it can break Pt-Pt bonds and spontaneously form steps on the surface. It is argued that adsorption-induced step formation may be a general effect, in particular at high gas pressures and temperatures.