Stability,structure,and electronic properties of chemisorbed oxygen and thin surface oxides
on Ir(111)
Hong Zhang,1,2Aloysius Soon,1,*Bernard Delley,3and Catherine Stampfl1
1School of Physics,The University of Sydney,Sydney NSW2006,Australia
2School of Physical Science and Technology,Sichuan University,Chengdu610065,People’s Republic of China
3Paul Scherrer Institut,WHGA/123,CH-5232,Villigen PSI,Switzerland
͑Received18February2008;revised manuscript received10June2008;published31July2008͒We present ab initio calculations for atomic oxygen adsorption on Ir͑111͒for a wide range of oxygen
coverages,⌰,namely from0.11to2.0monolayers͑ML͒,including subsurface adsorption and thin surface-
oxide-like structures.For on-surface adsorption,oxygen prefers the fcc-hollow site for all coverages consid-
ered.Similarly to oxygen adsorption on other transition metal surfaces,as⌰increases from0.25ML to1.0
ML,the binding energy decreases,indicating a repulsive interaction between the adsorbates.For the coverage
range of0.11to0.25ML,there is an attractive interaction,suggesting the possible formation of a local
͑2ϫ2͒periodicity with a local coverage of⌰=0.25ML.Pure subsurface oxygen adsorption is found to be
metastable and endothermic with respect to the free O2molecule.For structures with coverage beyond one full
ML,wefind the incorporation of oxygen under thefirst Ir layer to be exothermic.As the subsurface O coverage
increases in these structures from0.5to1.0ML,the energy becomes slightly more favorable,indicating an
attractive interaction between the O atoms.The structure with the strongest average O binding energy is
however a reconstructed trilayer-like structure that can be described as a͑ͱ3ϫͱ3͒R30°oxide-like layer in
p͑2ϫ2͒surface unit cell,with coverage1.5ML.Through calculation of the surface Gibbs free energy of
adsorption,taking into account the pressure and temperature dependence through the oxygen atom chemical
potential,the calculations predict only three thermodynamically stable regions,namely,the clean surface,the
p͑2ϫ2͒-O phase,and bulk IrO2.Thin trilayer surface oxide structures are predicted only to form when kinetic
hindering occurs,in agreement with recent experimental work.
DOI:10.1103/PhysRevB.78.045436PACS number͑s͒:81.65.Mq,68.43.Ϫh,68.47.Gh
I.INTRODUCTION
Obtaining a detailed knowledge of the surface structure and stoichiometry is crucial for understanding t
he physical and chemical properties of advanced materials such as those used in heterogeneous catalysis,corrosion resistance,elec-tronic devices,sensors,and fuel cells.1–3This knowledge is also central for enhancing the performance of existing cata-lysts as well as developing new ones.4Many current indus-trial processes involve catalytic oxidation reactions,5where the catalysts are typically transition metal particles dispersed on oxide supports.6The importance of transition metals ͑TMs͒for such reactions has motivated large numbers of studies on oxygen-metal interactions at low index surfaces of
TMs with the aim of obtaining a better understanding of the
underlying mechanisms.7–9For example,oxygen adsorption
on Ru͑0001͒,10–14Rh͑111͒,15,16Pd͑111͒,17,18Ag͑111͒,19–24
Ni͑111͒,25Cu͑111͒,5Pt͑111͒26,27,and Au͑111͒28surfaces has
been studied in detail theoretically.Recently,a trend study
addressing the incorporation of oxygen into the basal plane
of the late4d TMs from Ru,Rh,Pd to Ag was carried out.29
It was found that occupation of subsurface sites is connected
with a significant distortion of the host lattice,rendering it
initially less favorable than on-surface chemisorption.Oxy-
gen penetration below the surface of the substrate only starts
after a critical coverage,and is a key signature for oxide
formation at transition metal surfaces.The initial coverage
was found to be lower for the TMs to the right in the Peri-
odic Table,which bind O more weakly.
On the experimental side,many techniques such as AES ͑Auger electron spectroscopy͒,EELS͑electron-energy-loss spectroscopy͒,HREELS͑high-resolution electron-energy-loss spectroscopy͒,LEED͑low-energy electron diffraction͒, NEXAFS͑near-edge x-ray-absorptionfine structure͒,STM ͑scanning tunneling microscopy͒,TDS͑thermal desorption
spectroscopy͒,TPD͑temperature programmed desorption͒, and XPS͑x-ray photoelectron spectroscopy͒have been ap-plied to help determine the structure of surfaces.30,31One or several of these techniques have been used to study oxygen adsorption on Ru͑0001͒,32Rh͑111͒,33Pd͑111͒,34,35 Ni͑111͒,36,37Cu͑111͒,38,39Pt͑111͒40,and Au͑111͒.41 As a late5d transition metal,iridium shows potential in a great variety of applications,particularly as a heterogeneous catalyst in various industrial chemical reactions:42Ir and Ir-alloy catalysts are widely used in reactions that require the activation of strong C-H bonds.It has been shown that oxygen-precovered Ir͑111͒catalyzes the oxidization of pro-pylene and isobutylene to produce acetone.43These olefins are cleaved at the C=C double bond on the iridium surface to form ketones and carboxylic acids,44producing no side products which are often seen when other catalysts are used. An example of such organic reaction catalysis is the use of Ir-based catalysts to improve the production of acetic acid by a methanol carbonylation process.45With the increased de-mand for clean alternative energy,iridium is also now seen as a potential catalyst for CO x-free production of hydrogen from ammonia46and gasoline47to be used as fuel in auto-mobile fuel cells.In addition,it is also considered as an improvement to the automobile catalytic converter because of its unique ability to decompose NO as well as reduce NO x in the presence of hydrocarbons.48Clearly,a more detailed
PHYSICAL REVIEW B78,045436͑2008͒
atomic-level understanding of the interactions of these gas
phase species with Ir surfaces would be very valuable,which
could lead to improved Ir-based catalysts with greater selec-
tivity and activity.
Since the interaction of the iridium catalyst with an oxi-
dizing environment is common to several important hetero-
geneous reactions mentioned above,we address this in the
present paper usingfirst-principles calculations.We focus on
the͑111͒surface and present results for oxygen adsorption
and initial oxidation,and determine the pressure-temperature
phase diagram for conditions extending from ultrahigh
vacuum to those typical of technical catalysis,comparing the
results to other O/TM systems.The interaction of atomic
oxygen with single crystal Ir͑111͒surfaces has been the sub-
ject of several experimental studies.LEED͑Ref.49͒and
ultraviolet photoelectron spectroscopy͑UPS͒studies showed
that exposure of a clean Ir͑111͒surface to oxygen produces a ͑2ϫ2͒LEED pattern.50Such a pattern could either be caused by a p͑2ϫ2͒surface structure or by three domains of
a͑1ϫ2͒surface structure rotated by120°with respect to
one another.The͑1ϫ2͒surface structure corresponds to a
coverage of1/2ML.XPS and HREELS͑Refs.51and52͒
studies found that1/2ML was the maximum coverage for
atomic oxygen.A single chemisorbed state for atomic oxy-
gen on Ir͑111͒was perceived from the observation of a
single loss peak in EELS spectra at550cm−1at the satura-
tion coverage.53
With regard to theoretical investigations,chemisorption of
atomic O on Ir͑111͒was studied by usingfirst-principles
density functional theory͑DFT͒calculations.54,55The pre-
ferred binding site,atomic structure and vibrational frequen-
cies at0.25ML coverage,calculated in a p͑2ϫ2͒surface
unit cell,were reported.It was found that atomic oxygen
adsorbs preferentially in the threefold fcc-hollow site.Ab
initio investigations of oxygen adsorption on Ir͑111͒have therefore been limited to a very narrow range of
oxygen coverage,and to zero pressure and zero temperature.Often the results obtained in such studies cannot be extrapolated directly to the technologically relevant situation offinite temperature and high pressure.2In particular,possible oxida-tion of the surface in a reactive oxygen-rich environment has been thought to lead to the formation of an inactive surface oxide outer layer,poisoning the catalytic reaction.However, conversely it could well play the role of the active centers,as seen in other O/TM systems.56Upon exposure to an oxygen atmosphere,the structures formed on the surfaces may vary from simple adlayers of chemisorbed oxygen,to oxygen dif-fusion into the subsurface region and the formation of ox-ides,depending on the partial pressure,temperature,and ori-entation of the metal substrate.Oxidation catalysts can be rather complex,often involving multiple phases and various active sites.Hence a careful study of the role of each phase and its specific interaction under working conditions is re-quired to suggest efficient ways of catalyst improvement.
II.CALCULATION METHOD
All calculations are performed using DFT as implemented in the all-electron DMol3code,57,58where we employ the generalized gradient approximation͑GGA͒of Perdew, Burke,Ernzerhof͑PBE͒for the exchange-correlation functional.59The Ir͑111͒surface is modeled using a supercell approach,where we use seven-layer Ir͑111͒slabs with a vacuum region of25Å.Oxygen atoms are adsorbed on both sides of t
he slab,preserving inversion symmetry.The oxygen atoms and the outmost two Ir layers are allowed to fully relax.To obtain highly converged surface properties,it is necessary that bulk and surface calculations are performed with the same high accuracy.60The wave functions are ex-panded in terms of a double-numerical quality localized ba-sis set with a real-space cutoff of10bohr for both the bulk and the surface.Polarization functions and scalar-relativistic corrections are also incorporated explicitly.We consider oxy-gen coverages from0.11ML to  2.00ML using͑3ϫ3͒,͑2ϫ2͒,and͑1ϫ1͒surface unit cells in which adsorption in various on-surface and subsurface sites,as well as surface-oxide-like structures,were investigated as explained below. The total energy,force on the atoms,and displacements are converged to within1ϫ10−6Ha͑2.7ϫ10−5eV͒,3
ϫ10−4Ha/Bohr͑1.5ϫ10−2eV/Å͒,and3ϫ10−4Bohr͑1.6ϫ10−2Å͒,respectively,in the DFT self-consistent cycles. The Brillouin-zone integrations are performed using a͑12ϫ12ϫ1͒Monkhorst-Pack͑MP͒grid for the͑1ϫ1͒surface unit cell,yielding19special k-points in the irreducible part of the surface Brillouin zone.Wefind that the change in cohesive energy of bulk Ir is less than10meV per Ir atom when increasing the real-space cutoff radius from8to12 bohr.To test the variation of the change in cohesive energy of bulk Ir as a function of the k-point mesh density,we vary the MP integration grids,denoted by͑MϫMϫM͒,with M taking͑even͒values of6to16.The variation is found to be less than3meV per Ir atom wh
en changing the k-point mesh from M=10to16.Thus,for bulk Ir calculations,we adopt a cut-off radius of10bohr and a MP k-point mesh of ͑12ϫ12ϫ12͒.Using the same cut-off radius for the slab calculations,we alsofind that increasing the k-mesh for the surface unit cell from͑6ϫ6ϫ1͒to͑16ϫ16ϫ1͒changes the surface energy of Ir͑111͒by3meV/Å2.For the surface calculations,we use a MP k-point mesh of͑12ϫ12ϫ1͒for the surface unit cell,and this k-point mesh is folded accord-ingly for larger surface cells.
We address the stability of O/Ir͑111͒structures with re-spect to adsorption of O by calculating the average binding energy per O adatom.The average binding energy per oxy-gen atom,E b O/Ir,is defined as
E b O/Ir=−1/N O͓E O/Ir−͑E Ir+N O E O͔͒,͑1͒where N O,E O/Ir,E Ir,and E O,are the number of oxygen atoms in the surface unit cell,the total energies of the adsorbate-substrate system,the clean surface,and the free oxygen atom,respectively.The binding energy is the energy that a free oxygen atom gains upon adsorption on the Ir surface. For the formation of a surface oxide,the average adsorption energy is defined as
E ad surf.-oxide=−1/N O͓E O/Ir−͑E Ir+N O E O+⌬N Ir E Ir bulk͔͒,
͑2͒where⌬N Ir is defined to be the difference in the number of Ir atoms of the surface structure compared to the ideal Ir͑111͒
ZHANG et al.PHYSICAL REVIEW B78,045436͑2008͒
substrate layer,and E Ir bulk
is the total energy of an iridium atom in bulk.This term appears since the missing Ir atoms are assumed to be rebound at kink sites at steps,which con-tribute an energy equal to that of a bulk Ir atom.To analyze the nature of bonding,we consider the difference electron density,
n ⌬͑r ͒=n ͑r ͒−n 0͑r ͒−n O ͑r ͒,
͑3͒
where n ͑r ͒is the total electron density of the adsorbate-substrate system,and n 0͑r ͒and n O ͑r ͒are the electron densi-ties of the clean substrate and the free oxygen atom,respec-tively,where the atomic geometry of the substrate is that of the relaxed adsorbate system ͑but without the O atoms ͒.This quantity then shows from which regions the electron density has been depleted and increased due to O adsorption on the surface.
Using the Helmholtz equation,the surface dipole moment ͑in Debye ͒is calculated according to the formula
␮=
A ⌬⌽
12␲⌰
,͑4͒
where A is the surface area in Å2per ͑1ϫ1͒surface unit cell,and ⌬⌽is the work-function change in eV .In order to in-vestigate the effect of pressure and temperature on the sta-bility of the various structures,we calculate the surface free energy of adsorption,
␥͑T ,p ͒=͑⌬G −⌬N Ir ␮Ir −N O ␮O ͒/A ,͑5͒
where ⌬G =G O /Ir ͑111͒slab −G Ir ͑111͒slab
,and the first and second terms on the right-hand side are the free energies of the O/Ir sur-face under c
onsideration and the clean Ir ͑111͒slab,respec-tively.␮O and ␮Ir are the O and Ir atom chemical potentials,which for ␮Ir is the free energy of an Ir atom in bulk fcc iridium.The temperature ͑T ͒and pressure ͑p ͒dependences enter mainly through the oxygen chemical potential,␮O ,61
␮O ͑T ,p ͒=1/2ͫE O 2
total
+␮˜O 2͑T ,p 0͒+k B T ln ͩp O 2p 0
ͪͬ
,͑6͒
where p 0corresponds to atmospheric pressure and ␮˜O 2
͑T ,p 0͒includes the contribution from rotations and vibrations of the molecule,as well as the ideal-gas entropy at 1atmosphere.61E O 2
total is the total energy of the oxygen molecule.For ␮˜O 2
͑T ,p 0͒we use the experimental values from thermody-namic tables.62
When calculating the difference ⌬G =G O /Ir ͑111͒slab −G Ir ͑111͒slab
,one needs to calculate the Gibbs free energies of both the adsorption and reference systems.Recent studies ͑e.g.,Ref.63͒have shown that for O/TM systems a good approxima-tion is to use the total energies from the DFT calculations which is what we have done in the present work.The rela-tionship between the total energies of DFT calculations and the Gibbs free energies of the systems has been discussed in detail in the literature.61Briefly,the contributions due to the vibrational free energy,configurational entropy and the pressure-volume ͑pV ͒term are present in the Gibbs free en-ergies.The pV term is of the order of tenths of meV /Å2,from a dimensional analysis for the ͑p ,T ͒ranges we are interested in,and hence can be safely neglected.The vibra-
tional contribution is usually small for such systems,typi-cally less than 10meV /Å2͑see Appendix for details ͒.The
contribution from configurational entropy is known to be non-negligible at phase transition boundaries,64but it is omitted for this study since we only focus on the relative stability of the various structures.
III.RESULTS
A.Clean Ir(111),bulk Ir and the oxygen molecule
We first consider the properties of bulk Ir and the Ir ͑111͒surface.The calculated properties are listed in Table I ͑the free Ir atom is calculated including spin polarization ͒.The calculated bulk lattice constant is a 0=3.85Åneglecting zero-point vibrations.The cohesive energy,E coh ,is calcu-lated to be 7.45eV and the bulk modulus,B 0=3.57Mbar.The corresponding experimental values are 3.84Å,6.94eV ,and 3.55Mbar.67Our values are also in line with other re-ported DFT-GGA results of a 0=3.89Åand
E coh =7.46eV.67
The obtained interlayer relaxations d i ,j be-tween layers i and j with respect to the bulk spacing ͑d =2.224Å͒are ⌬12=−1.57%and ⌬23=−0.49%for the top-most layers.70To the best of our knowledge,there are no recent experimental results for the surface relaxation of the Ir ͑111͒surface,except the early report of a contraction of 2.5Ϯ5%for the first interlayer spacing.71We can compare our results with that of Rh,the upper neighbor of Ir in the Periodic Table.The contractions of the topmost two inter-layer spacings of Ir ͑111͒are slightly smaller than those of the Rh ͑111͒surface obtained from DFT-GGA calculations as implemented in the all-electron full-potential-linearized aug-m
ented plane-wave method ͑FP-LAPW ͒,71which are ⌬12=−1.8%and ⌬23=−0.9%,though the trend is the same.For Rh,the experimental results determined by recent LEED analyses are ⌬12=−1.4Ϯ0.9%and ⌬23=−1.4Ϯ1.8%.72For Pt,the right neighbor of Ir,the change in the uppermost two
TABLE I.Properties of bulk Ir and the Ir ͑111͒surface and com-parison with other ab initio calculations and with experiment.a 0is the lattice constant ͑in Å͒,B 0is the bulk modulus ͑in Mbar ͒,E coh is the cohesive energy ͑in eV ͒,and ⌽is the work function ͑in eV ͒.
Present work
Other ab initio calculations Experimental results
a 0  3.85  3.86a ͑PW91͒  3.84
b 3.89
c ͑GGA ͒
B 0  3.57  3.55b E coh 7.457.46c    6.94b ⌽
5.88
6.63d
5.76e
Reference 65,calculated using DFT-GGA and the plane-wave pseudopotential approach.b
Reference 66.
c Reference 67,calculate
d using DFT-GGA and th
e plane-wave pseudopotential approach.
d Referenc
e 68,calculated using the tight-binding linear-muffin-tin-orbital Green’s function technique.e Reference 69.
STABILITY ,STRUCTURE,AND ELECTRONIC …
PHYSICAL REVIEW B 78,045436͑2008͒
interlayer spacings are⌬12=1.20%and⌬23=−0.50%,as re-ported from DFT-GGA calculations using the FP-LAPW method,60while the experimental result for⌬12is 1.0Ϯ0.1%.73The calculated work function for the clean sur-face of Ir͑111͒is5.88eV and is in line with the reported experimental value5.76eV.74
For the oxygen atom and molecule,spin-unrestricted cal-culations using nonspherical densities are performed where the real-space cutoff for the calculation of both the oxygen atom and oxygen molecule is increased to20bohr,the larg-est basis set available in the DMol3code.The binding energy of O2is calculated to be3.04eV/O atom,while the bond length and vibrational frequency are1.22Åand1527cm−1, respectively,in excellent agreement with other theoretical results.58,75,76The corresponding experimental values77are 2.56eV/atom,1.21Åand1580cm−1.The typical overesti-mation of DFT-GGA is observed in the binding energy.The values presented here are indicative of well-converged DFT-GGA calculations,and since our interest lies mainly in the relative stability of various structures,this overbinding will not affect the qualitative conclusions in this paper.
B.On-surface,subsurface,and thin surface-oxide-like
structures of oxygen on Ir(111)
For on-surface oxygen adsorption,we calculate the bind-ing energies for a range of coverages⌰:͑3ϫ3͒-O͑⌰=0.11ML͒,͑2ϫ2͒-O͑⌰=0.25ML͒,͑2ϫ2͒-2O͑⌰=0.50ML͒,͑2ϫ2͒-3O͑⌰=0.75ML͒,and͑2ϫ2͒-4O͑⌰=1.00ML͒.We consider adsorption in the fcc-and hcp-hollow sites,and top sites.For subsurface sites,we calculate adsorption in͑i͒the octahedral site,denoted hereafter as “octa,”and͑ii͒the tetrahedral sites.There are two types of tetrahedral sites;one is where there are three Ir atoms above it and one below,denoted as tetra-I,and the alternative one, tetra-II,is just the opposite with one surface Ir atom directly above and three below it in the second Ir layer.For0.25ML coverage,we also consider the bridge site.For structures involving both on-surface and subsurface O atoms,we start from the͑2ϫ2͒-4O on-surface configuration and add sub-surface oxygen atoms below the surface Ir layer.We inves-
tigated three possible site configurations:fcc/tetra-I,fcc/ tetra-II,and hcp/octa for various coverages.We performed calculations for oxygen in these different sites up to a total coverage2.0ML͑see Fig.1͒.
Previous studies for O/Rh15and O/Ru10identified a re-constructed surface-oxide-like structure that is energetically more favorable than the homogeneous chemisorbed phases discussed above.In particular,for⌰=1.50ML,the atomic configuration of this surface oxide is similar to that of the2.0 ML“mixed”on-/subsurface structure,except that the O-M-O trilayer͑where M=Rh or Ru͒is laterally expanded and ro-tated30°relative to the underlying substrate such that it consists of three metal atoms
and six oxygen atoms in the p͑2ϫ2͒cell͓instead of four metal and eight oxygen atoms as for in the2.0ML“mixed”on-surface+subsurface struc-ture͑Fig.1͑d͔͒͒.The metal atoms are located in high-symmetry sites,namely,fcc,hcp,and on-top sites.The stoichiometry of this surface oxid-like layer is1Ir:2O,the same as that of bulk iridium dioxide,IrO2.This surface structure can be described as a͑ͱ3ϫͱ3͒R30°oxide layer on a p͑2ϫ2͒/Ir͑111͒surface unit cell͑see Fig.2,labeled as “p2:IrO2”and referred to“͑3/ͱ2ϫͱ2͒”hereafter͒.The av-erage oxygen binding energy of this structure is3.94eV,and it is energetically more favorable than the on-surface O/Ir͑111͒structure at1.0ML oxygen coverage͑which has an average binding energy of3.83eV͒.From Fig.2,it can be seen that the coupling of this O-Ir-O trilayer to the underly-ing metal is via the lower O.Thefirst Ir͑111͒interlayer dis-tance,d12,is notably expanded to3.01Åwhich is about 35%larger compared to the Ir bulk value.
We also consider a similar structure,where the O-Ir-O trilayer is laterally shifted such that the lower lying oxygen atoms occupy the above-mentioned high-symmetry sites͑in-stead of the Ir atoms͒.It is labeled as“p2:IrO2-SR.”The average binding energy of oxygen in this structure is calcu-lated to be3.78eV.
FIG.1.͑Color online͒Atomic geometry of oxygen structures with a full monolayer of oxygen on the surface in the fcc site,for increasing subsurface oxygen concentrations,as calculated using a ͑2ϫ2͒surface unit cell.͑a͒Full monolayer͑four oxygen atoms per cell͒plus one subsurface oxygen atom in the tetra-I site,͑b͒as for ͑a͒but with two oxygen atoms in the tetra-I site,͑c͒and͑d͒as for ͑b͒but with three and four oxygen atoms in the tetra-I sites,respec-tively.The average adsorption energy with respect to the clean Ir͑111͒substrate and free oxygen atoms,as well as the correspond-ing coverage,are given at the bottom of eachfigure.The relative variation of the atomic interlayer spacings,with respect to the bulk value,is also given to the right of thefigures.The large͑gray͒and small͑red͒spheres represent iridium and oxygen atoms, respectively.
ZHANG et al.PHYSICAL REVIEW B78,045436͑2008͒
C.Energetics
The binding energies,E b,of on-surface oxygen on the
Ir͑111͒surface in the fcc,hcp,on-top,bridge,octa,tetra-I and tetra-II sites,at coverage0.25ML are listed in Table II given with respect to the free oxygen atom.In Fig.3,the average binding energies for the various oxygen structures are plotted as a function of coverage.
It can be seen from Table II that the fcc-hollow site is energetically most favorable.This is in agreement with the experiment LEED study for the͑2ϫ2͒superstructure.49The fcc preference for adsorbed oxygen has been observed on the ͑111͒faces of several other fcc transition metals.52Of the
subsurface sites considered,the tetra-I site is most favorable. It is,however,significantly less stable than on-surface chemisorption,presumably because of the additional energy cost of distorting the substrate lattice and breaking metal-metal bonds.
From Fig.3,it can be seen that the binding energy for O on Ir͑111͒increases modestly in the coverage range from ⌰=0.11to0.25ML,and then decreases with oxygen cover-age for both the fcc and hcp sites up to⌰=1.00ML.For oxygen adsorbed in the on-top site,the binding energy varies little with the coverage,with an average value of3.57eV. For oxygen in the subsurface octa,tetra-I and tetra-II sites, the binding energy increases rapidly with the oxygen cover-age,indicating an effective attractive interaction between O atoms.For the“mixed”on-surface+subsurface structures,it can be seen that they are less favorable than the on-surface configurations,but with increasing coverage they exhibit a slight increase in the binding energy,indicating a weak ef-fective attractive interaction.This increase in average bind-ing energy for increasing coverage͑from0.50to1.0ML͒of subsurface oxygen is similar to what has been found for other transition metals͑e.g.,Rh and Ru͒.The most energeti-cally favorable of all
are the surface-oxide-like structures
FIG.2.͑Color online͒͑a͒The lowest energy surface-oxide-like
structure considered can be described as a reconstructed ͑ͱ3ϫͱ3͒R30°surface-oxide-like trilayer in a p͑2ϫ2͒surface unit cell.͑b͒The next most favorable structure,which is the same as͑a͒
except that the upper O-Ir-O trilayer is laterally shifted compared to ͑a͒.Oxygen atoms are shown as small dark͑red͒spheres,while the small gray͑yellow͒spheres͑labeled Ir TL͒are the uppermost Iri-dium atoms͑in the trilayer͒.The large gray spheres are the second and third layer͑unreconstructed͒iridium atoms.O U and O L denote the upper and lower O atoms,respectively.
TABLE II.The binding energy of oxygen,relative to a free O atom,on Ir͑111͒for various adsorption sites for0.25ML coverage, and comparison with other ab initio calculations.The unit of energy is eV.
Site Present work Other ab initio a ͑PW91,RPBE͒
fcc  4.62  4.57,4.00
hcp  4.42  4.32,3.75 Bridge  3.97  4.02,3.51
Top  3.54  3.46,3.04
Octa−0.38
Tetra-I0.78
Tetra-II0.52
Reference54,DFT-GGA calculations using the pseudopotential plane-wave method.
FIG.3.͑Color online͒Average binding energy of oxygen on
Ir͑111͒in the on-surface and subsurface sites for various coverages,
with respect to the energy of a free oxygen atom.The horizontal
upper and lower lines are half the experimental and theoretical
binding energies of O2,respectively.The inset shows the top view
of the atomic structure of the͑2ϫ2͒-4O fcc/O tetra-I structure con-taining four oxygen atoms in fcc sites and one oxygen atom in the
subsurface tetra-I site͑with total coverage1.25ML͒.The large ͑yellow͒and small͑red͒spheres represent iridium and oxygen at-oms,respectively.
STABILITY,STRUCTURE,AND ELECTRONIC…PHYSICAL REVIEW B78,045436͑2008͒
consisting of an O-Ir-O trilayer with a͑ͱ3ϫͱ3͒R30°peri-odicity in a͑2ϫ2͒surface unit cell͑shown in Fig.2͒.For the most favorable structure,it has an average binding en-ergy slightly more favorable than the full monolayer on-surface structure.As indicated by the dashed line in Fig.3, for increasing coverages of oxygen,the results indicate that there will be a phase transition from on-surface adsorption to the reconstructed surface-oxide-like structure with local cov-
erage of1.5ML.
It is interesting to compare these results with those for O
adsorption on Rh͑111͒,71,78Ir’s upper neighbor in the Peri-
odic Table,and neighbors to the right of it in the Periodic
Table,Pt͑111͒and Au͑111͒.In Fig.4,we have plotted the
binding energies of these systems,where oxygen occupies
the fcc sites.It can be seen that the binding energy of oxygen
on Rh͑111͒is stronger than that of Ir͑111͒,e.g.,at coverage
=0.25ML it is about0.60eV larger.The less exothermic
binding energy of O on Ir͑111͒compared with Rh can be
expected from the comparison of the experimental enthalpy
of formation of bulk IrO2per oxygen atom͑−1.42eV͒and
that of Rh2O3͑−1.78eV͒.79According to the“Tanaka-Tamaru rule,”the initial enthalpies of chemisorption of oxy-
gen and other molecules are linearly related to the enthalpies
of formation of the most stable oxides.80
The binding energies shown in Fig.4decrease progres-
sively for the elements to the right in the Periodic ,
for Pt and Au,which is due to the continuedfilling of the d
band in the late TMs,leading to an increased occupation of
antibonding oxygen-metal states.81At0.25ML,the binding
energy is about0.8eV less for O/Pt͑111͒compared to
O/Ir͑111͒.The more exothermic binding energy of O on
Ir͑111͒when compared to O/Pt͑111͒and O/Au͑111͒is also
consistent with the enthalpy of formation of the bulk oxide:
For bulk IrO2,per oxygen atom,it is−1.42eV͑Ref.79͒͑theoretical value−1.45eV͒while the experimental en-thalpy of formation of PtO2is−0.69eV,79and for Au2O3it is−0.135eV.82,83
D.Atomic structure
The calculated atomic geometries of the O/Ir͑111͒struc-tures͑for⌰=0.11to1ML͒are listed in Table III,where the binding energy is also included.The relaxed interlayer dis-tances d12and d23for the clean Ir͑111͒surface are2.19Åand2.21Årespectively,and the interlayer distance for bulk Ir is2.22Å.On adsorption of oxygen at the low coverage of 0.11ML,the interlayer distances d12and d23are both2.21Åshowing an expansion of1.57%for d12relative to the͑re-laxed͒clean surface,and no change for the second interlayer distance.Increasing the oxygen coverage to0.25ML,the interlayer distances d12and d23are2.23Åand2.21Å,re-
FIG.4.͑Color online͒Binding energy͑a͒of oxygen on the
Ir͑111͒,Pt͑111͒͑Ref.27͒,Au͑111͒͑Ref.28͒,and Rh͑111͒͑Ref.15͒
surfaces in the fcc-hollow site for various oxygen coverages.The
reactive materials studies
horizontal upper and lower lines are half the experimental and the-
oretical binding energies of O2,respectively.͑b͒The corresponding
work-function change and͑c͒surface dipole moment for oxygen on
Ir͑111͒,Au͑111͒͑Ref.28͒,and Rh͑111͒͑Ref.15͒.
TABLE III.Calculated structural parameters͑inÅ͒for various coverages of O in the fcc-hollow site on Ir͑111͒.d Ir/O is the bond length between oxygen and thefirst-nearest-neighbor iridium atom,d01is the
vertical height of oxygen above the topmost iridium layer,and d12and d23are thefirst and second metal
interlayer spacings,respectively,where the center of mass of the layer is used.The calculated interlayer
distance for bulk iridium is2.22Å.E b O/Ir is the binding energy in eV with respect to atomic oxygen.
Coverage0.110.250.500.75  1.00
d O/Ir  2.07  2.06  2.04a  2.03  2.04  2.02
d01  1.33  1.30  1.22a  1.29  1.28  1.27
d12  2.21  2.23  2.25  2.26  2.25
d23  2.21  2.21  2.22  2.22  2.23
E b O/Ir  4.37  4.62  4.57a  4.37  4.14  3.83
Reference54,DFT-GGA͑PW91͒calculations using the pseudopotential plane-wave approach.
ZHANG et al.PHYSICAL REVIEW B78,045436͑2008͒

版权声明：本站内容均来自互联网，仅供演示用，请勿用于商业和其他非法用途。如果侵犯了您的权益请与我们联系QQ:729038198，我们将在24小时内删除。