25 research outputs found
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), {101Ì…<i>N</i>} surfaces, and the as yet unexplored
{112Ì…<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<sub>Ni</sub>) and O (cn<sub>Ni</sub>). 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<sub>Ni</sub> = 6, 5,
and 4, and cn<sub>O</sub> = 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<sub>Ni</sub> and cn<sub>O</sub> of the latter two surface types increase
due to water coordination and dissociation. Water adsorption is found
to be most favorable on the {112Ì…<i>N</i>} surfaces,
giving rise to fivefold-coordinated Ni (Ni<sup>5c</sup>) or Ni<sup>6c</sup> from Ni<sup>4c</sup>. 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 N<sub>2</sub> 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 NH<sub>3</sub> synthesis from N<sub>2</sub> and H<sub>2</sub> is notoriously energy intensive. This is due to the difficulty of
N<sub>2</sub> dissociation despite the overall reaction being thermodynamically
favorable at ambient temperatures and pressures. LSPRs may provide
means to improve the kinetics of N<sub>2</sub> 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 N<sub>2</sub> 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 N<sub>2</sub> dissociation. We find the ground-state dissociation activation energy
to be 4.74 eV/N<sub>2</sub>, 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), {101Ì…<i>N</i>} surfaces, and the as yet unexplored
{112Ì…<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<sub>Ni</sub>) and O (cn<sub>Ni</sub>). 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<sub>Ni</sub> = 6, 5,
and 4, and cn<sub>O</sub> = 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<sub>Ni</sub> and cn<sub>O</sub> of the latter two surface types increase
due to water coordination and dissociation. Water adsorption is found
to be most favorable on the {112Ì…<i>N</i>} surfaces,
giving rise to fivefold-coordinated Ni (Ni<sup>5c</sup>) or Ni<sup>6c</sup> from Ni<sup>4c</sup>. 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), {101Ì…<i>N</i>} surfaces, and the as yet unexplored
{112Ì…<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<sub>Ni</sub>) and O (cn<sub>Ni</sub>). 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<sub>Ni</sub> = 6, 5,
and 4, and cn<sub>O</sub> = 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<sub>Ni</sub> and cn<sub>O</sub> of the latter two surface types increase
due to water coordination and dissociation. Water adsorption is found
to be most favorable on the {112Ì…<i>N</i>} surfaces,
giving rise to fivefold-coordinated Ni (Ni<sup>5c</sup>) or Ni<sup>6c</sup> from Ni<sup>4c</sup>. 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 N<sub>2</sub> Dissociation Catalysts: Functionalizing a Plasmon-Active Metal
The
Haber-Bosch process for NH<sub>3</sub> 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 N<sub>2</sub> 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 N<sub>2</sub> 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 N<sub>2</sub>, as well as their ability
to support a pathway for N<sub>2</sub> 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 N<sub>2</sub>. While AuMo achieves the best affinity toward N<sub>2</sub>, its
strong propensity toward oxidation could greatly limit its use
Chemical Pressure-Driven Enhancement of the Hydrogen Evolving Activity of Ni<sub>2</sub>P from Nonmetal Surface Doping Interpreted via Machine Learning
The
activity of Ni<sub>2</sub>P catalysts for the hydrogen evolution
reaction (HER) is currently limited by strong H adsorption at the
Ni<sub>3</sub>-hollow site. We investigate the effect of surface nonmetal
doping on the HER activity of the Ni<sub>3</sub>P<sub>2</sub> termination
of Ni<sub>2</sub>PÂ(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 Ni<sub>3</sub>-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 Ni<sub>2</sub>PÂ(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 Ni<sub>3</sub>-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 BaTiO<sub>3</sub> and PbTiO<sub>3</sub> 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 PbTiO<sub>3</sub>. For BaTiO<sub>3</sub>, 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 Ni<sub>2</sub>PÂ(0001) and Ni<sub>5</sub>P<sub>4</sub>(0001)/(0001Ì…)
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 Ni<sub>3</sub>P<sub>2</sub> termination of Ni<sub>2</sub>PÂ(0001)
is found to be most stable, consistent with the current literature.
By contrast, the most energetically favorable surfaces of Ni<sub>5</sub>P<sub>4</sub> are found to be the Ni<sub>3</sub>P<sub>3</sub> and
Ni<sub>4</sub>P<sub>3</sub> bulk-derived terminations with P-adatoms.
The preferred excess P binding sites and their energies are identified
on each surface. We find that the P<sub>3</sub> site, which is present
on Ni<sub>5</sub>P<sub>4</sub>, and the Ni<sub>3</sub> site, which
is present on both Ni<sub>2</sub>P and Ni<sub>5</sub>P<sub>4</sub>, strongly bind excess P. Additionally, we predict the presence of
stable P<sub><i>n</i></sub> (<i>n</i> = 2, 4)
agglomerates on Ni<sub>5</sub>P<sub>4</sub> at the P<sub>3</sub>-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