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Designing the electronic structure and reactivity of metal oxides
through strain, d-band filling and oxidation state
John Kitchin, Paul Salvador Department of Chemical Engineering, Carnegie Mellon
University, Pittsburgh, PA 15213 Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213
Funding sources: DOE Early Career Award DOE SECA program DOE NETL-RUA
NSF funding: Proposal is pending
Why do we need to control metal oxide reactivity?
• Applications of oxides – Catalysis – Energy – Electronics – Thermal barrier coatings – Corrosion resistant coatings – Paints – Personal products
• In some applications we want reactivity, in others we do not
• How do we design materials that do what we want?
Understanding the reactivity of d-metals
d-band
Fermi Level
Adsorbate
resonance
Ni, Pt, transition metals
Strong bonding
En
erg
y
B
d-band
Fermi Level
Adsorbate
resonance
Au - Noble metals
Very weak bonding
B
• d-band properties dominate the trends in adsorption behavior • Changing d-band properties changes adsorption behavior
Alloying leads to combinations of strain and ligand effects
Straining metal films leads to changes in d-
band width
“Ligand” effects lead to changes in d-band
width
tensile
compressive
Kitchin, J.R., J.K. Nørskov, M.A. Barteau, and J.G. Chen, Physical Review Letters, 2004. 93(15): 156801. Kitchin, J.R., J.K. Nørskov, M.A. Barteau, and J.G. Chen, The Journal of Chemical Physics, 2004. 120(21): 10240.
Electronic structure to chemistry
Determined by active site composition and geometry
Estimated reactivity Correlation due to constant d-band filling
We are at a stage where we know a lot about metals and their reactivity
İnoğlu, N. and J.R. Kitchin. Physical Review B, 2010. 82(4): 045414. I ̇noğlu, N. and J.R. Kitchin. ACS Catalysis, 2011. 1(4): 399-407.
So far, nothing like this exists for metal oxides
Why metal oxides are harder than metals
• Many more different crystal structures and stoichiometries
• Many different defect and oxidation states that can be determined by the environment
• More complex bonding – Covalent and ionic
• Different adsorption mechanisms
Are there simple concepts in oxide reactivity that are analogous to metals?
– Can strain be used to control reactivity?
– What electronic structure features dominate trends in reactivity?
Part I – Reactivity of supported thin oxide films
• The conductivity of thin oxide films varies in reactive environments (gas sensors)
• Conductivity is related to concentration of oxygen vacancies • Sensitive probe to surface reactivity • Focus here on La0.7Sr0.3MnO3 (LSM) films
– Perovskite used in solid oxide fuel cells, catalysis
Thin films synthesized by pulsed laser deposition
Havelia, S., K.R. Balasubramaniam, S. Spurgeon, F. Cormack, and P.A. Salvador. Journal of Crystal Growth, 2008. 310(7-9): p. 1985-1990.
Heteroepitaxial growth of oxides to introduce strain
• Film – La0.7Sr0.3MnO3 (LSM) a=3.876 Å
• Single crystal substrates – SrTiO3 (100) a=3.905 Å
(heteroepitaxial growth leads to tensile strain)
– NdGaO3 (110) a=3.864 Å (heteroepitaxial growth leads to compressive strain)
• Grow films with varying thickness – Films are smooth and polycrystalline
Characterizing the strain in thin films
Thick, relaxed film
Epitaxial, thin film under tensile strain
Epitaxial, thin film under compressive strain • XRD rocking curves
characterize the lattice parameters of the films
• FWHM of rocking peaks related to defect density
• Thin films (50 nm) are epitaxial and fully strained at the substrate lattice constant
• Thick films relax by defect formation
Strain has a big impact on reactivity
• Tensile strained films have larger kchem (higher O2 exchange rate) than compressively strained films
• Order of magnitude difference!
The question is: how do we understand why? 1. Are vacancies easier to form under strain? 2. Is adsorption more favorable under strain? 3. Is this unique to LSM? How would changing the composition affect reactivity?
Tensile
Compressive
Part II – Computational investigations of oxide reactivity
• Trends in properties of cubic 3d-perovskites – Composition
• B = Sc through Cu • LaBO3 (B+3) • SrBO3 (B+4)
– Strain • At lattice constants of ±5%, ±2.5% and DFT equilibrium volumes
– Electronic structure • Properties of the atom-projected d-band density of states
– Reactivity • Dissociative adsorption energy • Surface oxygen vacancy energy
• DACAPO (DFT/US-PP/planewaves) – No hybrid functionals or DFT+U, no spin-polarization
Oxide reactivity with oxygen
• Adsorption and vacancy formation energies are reactivity prototypes
• We anticipate that trends in these prototype reactions will be representative of other types of reactivity
Clean slab
Reactive surface
Adsorbed oxygen atom
Missing oxygen
2
2
*
2
2
slab + 1/2 O slab+O
slab slab+vac + 1/
( ) 1/ 2
2
1/ 2
O
ads slab O slab O
vac slab vac O slab
E eV E E E
E E E E
12
Perovskite electronic structure evolution with strain
• Strong hybridization between B atom d-orbitals and O s,p orbitals
• Convergence of bands occurs as the B-atom d-band filling increases
• Tensile strain causes band narrowing
13
LaTiO3
LaCuO3
d-band widths of B atom in strained perovskites
– d-s/p orbital
coupling
• The strain dependence of the bulk d-band width depends on the size of the B atom
3
2
7
2
ddw
r
d
0
0.5
1
1.5
2
2.5
0.075 0.095 0.115 0.135
d-b
and
wid
th (
Wd
)
d-7/2
LaScO3LaTiO3LaVO3LaCrO3LaMnO3LaFeO3LaCoO3LaNiO3LaCuO3
0
0.5
1
1.5
2
2.5
0.075 0.095 0.115 0.135
d-b
and
wid
th (
Wd
)
d-7/2
SrTiO3SrVO3SrCrO3SrMnO3SrFeO3SrCoO3SrNiO3SrCuO3
LaBO3
SrBO3
14
Pseudo-rectangular band model for perovskite d-band
• The ABO3 series shows a wd/d correlation consistent with a rectangular band model
• Nearly constant d-band filling observed
• This is the same behavior as observed in metals
-3
-2
-1
0
1
2
3
4
5
6
7
8
0.8 1.3 1.8 2.3
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
d-band width (eV)
d-b
and
cen
ter(
eV
)
-1
0
1
2
3
4
5
0.8 1.3 1.8 2.3
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
d-band width (eV)
d-b
and
cen
ter(
eV)
LaBO3
SrBO3
15 Akhade, S.A. and J.R. Kitchin,. J. Chemical Physics, 2011. 135(10): p. 104702.
Electronic structure-reactivity correlations of adsorption energy
• For most perovskites the adsorption energy is related to the d-band center of the B-atom
• Higher oxidation states tend to have weaker adsorption energies
• LaScO3 and SrTiO3 are different – Probably an error in
the pseudopotentials which do not have semicore states
16
LaCuO3
LaTiO3
LaVO3
LaCrO3
LaMnO3
LaFeO3
LaScO3
LaCoO3
LaNiO3
LaScO3
LaTiO3 LaVO3
LaCrO3
LaMnO3
LaFeO3
LaCuO3
LaCoO3
LaNiO3
LaCuO3
LaTiO3
LaVO3 LaCrO3
LaMnO3
LaFeO3
LaScO3
LaCoO3 LaNiO3
-6.00
-5.00
-4.00
-3.00
-2.00
-1.00
0.00
1.00
2.00
3.00
4.00
-2.00 -1.00 0.00 1.00 2.00 3.00 4.00 5.00
En
ger
y (
eV)
d-band center (ed)
Eads (O2) strain = -5%
Eads (O2) equilibrium
Eads (O2) strain =5%
SrCuO3
SrTiO3
SrVO3
SrCrO3
SrMnO3
SrFeO3
SrNiO3
SrCoO3
SrNiO3
SrTiO3
SrVO3
SrCrO3
SrMnO3
SrFeO3
SrCuO3
SrCoO3
SrCuO3
SrTiO3
SrVO3 SrCrO3
SrMnO3
SrFeO3
SrNiO3
SrCoO3
-6.00
-5.00
-4.00
-3.00
-2.00
-1.00
0.00
1.00
2.00
3.00
4.00
-1 0 1 2 3 4
En
ger
y (
eV)
d-band center (ed)
Eads (O2) strain = -5%
Eads (O2) equilibrium
Eads (O2) strain =5%
Chemical correlations in reactivity
• Adsorption is strongest on materials where oxygen vacancies are hard to make
• Correlated properties make it difficult to identify key factor(s) in reactivity
• We know other correlations also exist
17
0
2
4
6
8
-5 0 5
5%
0%
-5%
Dissociative Adsorption energy (eV)
Vac
ancy
fo
rmat
ion
en
ergy
(eV
)
-2
-1
0
1
2
3
4
5
6
-5 0 5
5%
0%
-5%
Dissociative adsorption energy (eV) Vac
ancy
fo
rmat
ion
en
ergy
(eV
)
LaBO3
SrBO3
Correlations can limit performance Example: oxygen evolution on oxides
• Correlations in reaction energies lead to limits in reactivity
2H2O → HO* + H2O + H+ + e-
HO* + H2O + H+ + e- → O* + H2O + 2H+ + 2e-
O* + H2O + 2H+ + 2e- → HOO* + 3H+ + 3e-
HOO* + 3H+ + 3e- → O2 + 4H+ + 4e-
2 H2O → 4(H+ + e-) + O2
Man, I. C., Su, H.-Y., Calle-Vallejo, F., Hansen, H. A., Martínez, J. I., Inoglu, N. G., Kitchin, J., Jaramillo, T. F., Nørskov, J. K. and Rossmeisl, J. (2011), Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces. ChemCatChem, 3: 1159.
Thermodynamic limit
Actual material limit
Conclusions • The bulk electronic structure of perovskite
B-atoms is reasonably modeled as a rectangular band
• d-band filling is the dominant factor in determining reactivity, followed by oxidation state
• The oxygen adsorption energy and vacancy formation energies are correlated in this class of perovskites
• Correlations limit degrees of freedom for materials design
• The outlook for systematically understanding oxide reactivity is very bright!
19
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