Atomic and Electronic Structure of the BaTiO\u3csub\u3e3\u3c/sub\u3e(001) (√5×√5)\u3cem\u3eR\u3c/em\u3e26.6° Surface Reconstruction

This contribution presents a study of the atomic and electronic structure of the (√5×√5)R26.6° surface reconstruction on BaTiO3 (001) formed by annealing in ultrahigh vacuum at 1300 K. Through density functional theory calculations in concert with thermodynamic analysis, we assess the stability of several BaTiO3 surface reconstructions and construct a phase diagram as a function of the chemical potential of the constituent elements. Using both experimental scanning tunneling microscopy (STM) and scanning tunneling spectroscopy measurements, we were able to further narrow down the candidate structures, and conclude that the surface is either TiO2-Ti3/5, TiO2-Ti4/5, or some combination, where Ti adatoms occupy hollow sites of the TiO2 surface. Density functional theory indicates that the defect states close to the valence band are from Ti adatom 3d orbitals (≈1.4  eV below the conduction band edge) in agreement with scanning tunneling spectroscopy measurements showing defect states 1.56±0.11  eV below the conduction band minimum (1.03±0.09  eV below the Fermi level). STM measurements show electronic contrast between empty and filled states’ images. The calculated local density of states at the surface shows that Ti 3d states below and above the Fermi level explain the difference in electronic contrast in the experimental STM images by the presence of electronically distinctive arrangements of Ti adatoms. This work provides an interesting contrast with the related oxide SrTiO3, for which the (001) surface (√5×√5)R26.6° reconstruction is reported to be the TiO2 surface with Sr adatoms

Effects of the Aqueous Environment on the Stability and Chemistry of β‑NiOOH Surfaces

Nickel oxyhydroxide (NiOOH)-based anodes are among the most promising materials for the electrocatalytic production of oxygen from water under alkaline conditions. We explore the stability of the low-index facets of the catalytically active β-NiOOH phase, namely the (0001), {101̅<i>N</i>} surfaces, and the as yet unexplored {112̅<i>N</i>} surfaces, via density functional theory with a Hubbard-<i>U</i> like correction on Ni. We find that their relative stabilities depend strongly on the coordination number of the exposed Ni (cn_Ni) and O (cn_Ni). In the vacuum, where passivation of the surface dangling bonds is limited, the stability order is as follows: (0001) > {101̅<i>N</i>} ≫ {112̅<i>N</i>}, noting that the coordination numbers for each phase are, respectively, cn_Ni = 6, 5, and 4, and cn_O = 3–4, 2–3, and 2–3. In aqueous media, the order of stability is (0001) > {101̅<i>N</i>} ≈ {112̅<i>N</i>}, as the cn_Ni and cn_O of the latter two surface types increase due to water coordination and dissociation. Water adsorption is found to be most favorable on the {112̅<i>N</i>} surfaces, giving rise to fivefold-coordinated Ni (Ni^5c) or Ni^6c from Ni^4c. Our work suggests that a plethora of facets are likely to coexist on β-NiOOH crystallites with water serving to equalize the stabilities of the different surfaces

Excited-State N₂ Dissociation Pathway on Fe-Functionalized Au

Localized surface plasmon resonances (LSPRs) offer the possibility of light-activated chemical catalysis on surfaces of strongly plasmonic metal nanoparticles. This technology relies on lower-barrier bond formation and/or dissociation routes made available through energy transfer following the eventual decay of LSPRs. The coupling between these decay processes and a chemical trajectory (nuclear motion, charge-transfer, intersystem crossing, etc.) dictates the availability of these alternative (possibly lower barrier) excited-state channels. The Haber–Bosch method of NH₃ synthesis from N₂ and H₂ is notoriously energy intensive. This is due to the difficulty of N₂ dissociation despite the overall reaction being thermodynamically favorable at ambient temperatures and pressures. LSPRs may provide means to improve the kinetics of N₂ dissociation via induced resonance electronic excitation. In this work, we calculate, via embedded <i>n</i>-electron valence second-order perturbation theory within the density functional embedding theory, the excited-state potential energy surfaces for dissociation of N₂ on an Fe-doped Au(111) surface. This metal alloy may take advantage simultaneously of the strong LSPR of Au and the catalytic activity of Fe toward N₂ dissociation. We find the ground-state dissociation activation energy to be 4.74 eV/N₂, with Fe as the active site on the surface. Consecutive resonance energy transfers (RETs) may be accessed due to the availability of many electronically excited states with intermediate energies arising from the metal surface that may couple to states induced by the Fe-dopant and the adsorbate molecule, and crossing between excited states may effectively lower the dissociation barrier to 1.33 eV. Our work illustrates that large energetic barriers, prohibitive toward chemical reaction, may be overcome through multiple RETs facilitating an otherwise difficult chemical process

Effects of the Aqueous Environment on the Stability and Chemistry of β‑NiOOH Surfaces

Nickel oxyhydroxide (NiOOH)-based anodes are among the most promising materials for the electrocatalytic production of oxygen from water under alkaline conditions. We explore the stability of the low-index facets of the catalytically active β-NiOOH phase, namely the (0001), {101̅<i>N</i>} surfaces, and the as yet unexplored {112̅<i>N</i>} surfaces, via density functional theory with a Hubbard-<i>U</i> like correction on Ni. We find that their relative stabilities depend strongly on the coordination number of the exposed Ni (cn_Ni) and O (cn_Ni). In the vacuum, where passivation of the surface dangling bonds is limited, the stability order is as follows: (0001) > {101̅<i>N</i>} ≫ {112̅<i>N</i>}, noting that the coordination numbers for each phase are, respectively, cn_Ni = 6, 5, and 4, and cn_O = 3–4, 2–3, and 2–3. In aqueous media, the order of stability is (0001) > {101̅<i>N</i>} ≈ {112̅<i>N</i>}, as the cn_Ni and cn_O of the latter two surface types increase due to water coordination and dissociation. Water adsorption is found to be most favorable on the {112̅<i>N</i>} surfaces, giving rise to fivefold-coordinated Ni (Ni^5c) or Ni^6c from Ni^4c. Our work suggests that a plethora of facets are likely to coexist on β-NiOOH crystallites with water serving to equalize the stabilities of the different surfaces

Effects of the Aqueous Environment on the Stability and Chemistry of β‑NiOOH Surfaces

Nickel oxyhydroxide (NiOOH)-based anodes are among the most promising materials for the electrocatalytic production of oxygen from water under alkaline conditions. We explore the stability of the low-index facets of the catalytically active β-NiOOH phase, namely the (0001), {101̅<i>N</i>} surfaces, and the as yet unexplored {112̅<i>N</i>} surfaces, via density functional theory with a Hubbard-<i>U</i> like correction on Ni. We find that their relative stabilities depend strongly on the coordination number of the exposed Ni (cn_Ni) and O (cn_Ni). In the vacuum, where passivation of the surface dangling bonds is limited, the stability order is as follows: (0001) > {101̅<i>N</i>} ≫ {112̅<i>N</i>}, noting that the coordination numbers for each phase are, respectively, cn_Ni = 6, 5, and 4, and cn_O = 3–4, 2–3, and 2–3. In aqueous media, the order of stability is (0001) > {101̅<i>N</i>} ≈ {112̅<i>N</i>}, as the cn_Ni and cn_O of the latter two surface types increase due to water coordination and dissociation. Water adsorption is found to be most favorable on the {112̅<i>N</i>} surfaces, giving rise to fivefold-coordinated Ni (Ni^5c) or Ni^6c from Ni^4c. Our work suggests that a plethora of facets are likely to coexist on β-NiOOH crystallites with water serving to equalize the stabilities of the different surfaces

Thermodynamic Constraints in Using AuM (M = Fe, Co, Ni, and Mo) Alloys as N₂ Dissociation Catalysts: Functionalizing a Plasmon-Active Metal

The Haber-Bosch process for NH₃ synthesis is arguably one of the greatest inventions of the 20th century, with a massive footprint in agriculture and, historically, warfare. Current catalysts for this reaction use Fe for N₂ activation, conducted at high temperatures and pressures to improve conversion rate and efficiency. A recent finding shows that plasmonic metal nanoparticles can either generate highly reactive electrons and holes or induce resonant surface excitations through plasmonic decay, which catalyze dissociation and redox reactions under mild conditions. It is therefore appealing to consider AuM (M = Fe, Co, Ni, and Mo) alloys to combine the strongly plasmonic nature of Au and the catalytic nature of M metals toward N₂ dissociation, which together might facilitate ammonia production. To this end, through density functional theory, we (i) explore the feasibility of forming these surface alloys, (ii) find a pathway that may stabilize/deactivate surface M substituents during fabrication, and (iii) define a complementary route to reactivate them under operational conditions. Finally, we evaluate their reactivity toward N₂, as well as their ability to support a pathway for N₂ dissociation with a low thermodynamic barrier. We find that AuFe possesses similar appealing qualities, including relative stability with respect to phase separation, reversibility of Fe oxidation and reduction, and reactivity toward N₂. While AuMo achieves the best affinity toward N₂, its strong propensity toward oxidation could greatly limit its use

Chemical Pressure-Driven Enhancement of the Hydrogen Evolving Activity of Ni₂P from Nonmetal Surface Doping Interpreted via Machine Learning

The activity of Ni₂P catalysts for the hydrogen evolution reaction (HER) is currently limited by strong H adsorption at the Ni₃-hollow site. We investigate the effect of surface nonmetal doping on the HER activity of the Ni₃P₂ termination of Ni₂P­(0001), which is stable at modest electrochemical conditions. Using density functional theory (DFT) calculations, we find that both 2<i>p</i> nonmetals and heavier chalcogens provide nearly thermoneutral H adsorption at moderate surface doping concentrations. We also find, however, that only chalcogen substitution for surface P is exergonic. For intermediate surface concentrations of S, the free energy of H adsorption at the Ni₃-hollow site is −0.11 eV, which is significantly more thermoneutral than the undoped surface (−0.45 eV). We use the regularized random forest machine learning algorithm to discover the relative importance of structure and charge descriptors, extracted from the DFT calculations, in determining the HER activity of Ni₂P­(0001) under different doping concentrations. We discover that the Ni–Ni bond length is the most important descriptor of HER activity, which suggests that the nonmetal dopants induce a chemical pressure-like effect on the Ni₃-hollow site, changing its reactivity through compression and expansion

Strong Reciprocal Interaction between Polarization and Surface Stoichiometry in Oxide Ferroelectrics

We present a systematic evaluation of the effects of polarization switchability on surface structure and stoichiometry in BaTiO₃ and PbTiO₃ ferroelectric oxides. We show that charge passivation, mostly by ionic surface reconstructions, is the driving force for the stability of the surfaces, which suggests that varying the substrate polarization offers a new mechanism for controlling surface reconstructions in polar systems and inducing highly nonstoichiometric structures. Conversely, for thin-films the chemical environment can drive polarization switching via induced compositional changes on the surface. We find that the value of the oxygen partial pressure for the positive-to-negative polar transition is in good agreement with the recent experimental value for thin-film PbTiO₃. For BaTiO₃, we show that it is harder for oxygen control to drive polar transition because it is more difficult to reduce. This study opens up the possibility of real-time control of structure and composition of oxide surfaces

Stable Phosphorus-Enriched (0001) Surfaces of Nickel Phosphides

In heterogeneous catalysis, catalyst synthesis precedes operation and, in most cases, is conducted in an altogether different chemical environment. Thus, determination of the structure and composition of the catalyst surface(s) due to fabrication is essential in accurately evaluating their eventual structure(s) during operation, which provides the origin of their catalytic activities and are therefore key to catalyst optimization. We explore the reconstructions of both Ni₂P­(0001) and Ni₅P₄(0001)/(0001̅) surfaces with first-principles density functional theory (DFT). Most of the stable terminations under realistic synthesis conditions are determined to be P-rich on both materials. A P-covered reconstruction of the Ni₃P₂ termination of Ni₂P­(0001) is found to be most stable, consistent with the current literature. By contrast, the most energetically favorable surfaces of Ni₅P₄ are found to be the Ni₃P₃ and Ni₄P₃ bulk-derived terminations with P-adatoms. The preferred excess P binding sites and their energies are identified on each surface. We find that the P₃ site, which is present on Ni₅P₄, and the Ni₃ site, which is present on both Ni₂P and Ni₅P₄, strongly bind excess P. Additionally, we predict the presence of stable P_<i>n</i> (<i>n</i> = 2, 4) agglomerates on Ni₅P₄ at the P₃-hollow and Ni–Ni bridge sites. This study highlights the importance of considering the aggregation behavior of nonmetal components in predicting the surface reconstruction of transition metal compounds