Preparation and characterization of VOx/TiO2 catalytic coatings on ...
water on TiO2
Transcript of water on TiO2
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Mechanism and activity of water oxidation on TiO2-based
materials
Jia Chen, Jun Hee Lee, Ye Fei Li, AS Department of Chemistry, Princeton University
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Scheme for Photoelectrochemical Water Splitting
TiO2 abundant and very stable
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OER Overpotential on Transition-Metal-Doped TiO2 Nanowires
Bin Liu; Hao Ming Chen; Chong Liu; Sean C. Andrews; Chris Hahn; Peidong Yang; J. Am. Chem. Soc. 2013, 135, 9995-9998.
Thermodynamic redox potential for water oxidation at pH = 13.6
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Outline
• TiO2 is widely used b/c cheap and very stable; yet not very efficient
• TiO2 modifications ⇒ TiO2/Ferroelectric Heterostructures
• Try to understand detailed kinetics first!
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Oxygen Evolution Reaction – (a)
*H2O + h+ → *OH + H+(aq)
*OH + h+ → *O + H+(aq) H2O(l) + *O + h+ → *OOH + H+(aq)
*OOH + h+ → O2(g) + H+(aq)
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2 H2O (l) + 4×1.23 eV ↔ O2(g) + 4H+ + 4 e−
4 PCET steps
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Oxygen Evolution Reaction – (b)
Practical scheme used in periodic calculations
Compute ΔG1- ΔG4 SHE: H+ + e−↔ ½ H2 at U=0 & pH =0
*H2O → *OH + H+ + e−
*OH → *O + H+ + e− H2O(l) + *O → *OOH + H+ + e−
*OOH → O2(g) + H+ + e−
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Y.-F. Li, Z.-P Liu, W. Gao and L. Liu, J. AM. CHEM. SOC., 132, 13008.
Energetics pathway of the OER on anatase (101) Rate determining step
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First proton coupled electron transfer of water oxidation at the interface of water
with photoexcited TiO2 anatase (101)
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TiO2 anatase surface
minority (001)
O2c Ti5c
majority (101)
Lazzeri et al., Phys Rev B, 2001
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Rate determining step: first proton release
*H2O + h+ *HO• + H+(aq)
What about kinetics?
General interest
Specific issues
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R. Nakamura and Y. Nakato, J. AM. CHEM. SOC, 126, 1290.
P. Salvador, J. Elec. Chem. Soc., 128, 1895.
Influence of pH for OER on TiO2
OER is faster at high pH
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*H2O + h+ *HO• + H+(aq)
∆G/e (pH) = ∆Go/e – 0.059eV pH
EO2/H2O = EoO2/H2O – 0.059eV pH
Theoretical analysis of energetics predicts the onset overpotential to be independent of pH η(pH)=∆G(pH)/e – EO2/H2O = ∆Go/e – Eo
O2/H2O =constant
Overpotential’s dependence on pH
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Rate determining step: first proton release
*H2O + h+ *HO• + H+(aq)
How to deal with the kinetics?
Difficulties:
(i) Localized hole
(ii) Solvation effect
(iii) Reaction coordinate for proton
coupled electron transfer
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Watanabe & Hayashi, J. Lumin. (2005)
Luminescence band from self trapped (triplet ) excitons in anatase TiO2
Experimental evidence of self-trapped excitons/ holes in TiO2
Self-trapped polarons well described by hybrid functionals*
* C. Di Valentin, AS, JPC Lett. 2, 2223 (2011)
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Order-N PBE0 hybrid functional based on Maximally
Localized Wannier Functions implemented in Car-Parrinello
Molecular Dynamics in Quantum-Espresso.
Xifan Wu, Annabella Selloni, and Roberto Car, Physical Review B 79, 085102
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Rate determining step: first proton release
*H2O + h+ *HO• + H+(aq)
How to deal with kinetics?
Difficulties:
(i) Localized hole
(ii) Solvation effect
(iii) Reaction coordination for proton
coupled electron transfer
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System size: 24 TiO2 units +48 H2O
Solvation effect
(i) 10ps CPMD, 330K, PBE;
(ii) Select three snapshots; (iii)
Add a hole then relax with
PBE0.
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Water-hole state Surface-hole state
Spin density plot
Relaxed system
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Rate determining step: first proton release
*H2O + h+ *HO• + H+(aq)
How to deal with the kinetics?
Difficulties:
(i) Localized hole
(ii) Solvation effect
(iii) Reaction coordinate for proton
coupled electron transfer
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Reaction Coordinate for Proton Coupled Electron Transfer
Oa H
Ob
Difference between the distances of the proton to two water oxygens ∆d = d(Oa-H)-d(Ob-H)
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Reaction Coordinate (∆d)
Water-hole state
Surface-hole state
PT
PT
∆d ~ -0.5 ∆d ~ +0.5
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λ=0 ET
∆d (A)
λ
Proton transfer
∆E
Potential Energy Surfaces
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PCET likely sequential: *H2O *OH- + H+(aq)
*OH- + h+ *OH•
Additional snapshots
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Why PT first?
OH radical OH− & surface hole
Top views
OH− accepts H-bond donates H-bond
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Kinetics of *OH- + h+ *OH•
Surface hole
Shared hole
Hydroxyl radical
Ow-Osurf distance is used as reaction coordinate
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Additional snapshots
ET barrier ~ 0.1 eV or less, and << PT barrier
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Partial summary
pH < pzc PT occurs first: *H2O *OH- + H+ (aq)
ET is next: *OH- + h+ *OH• PT barrier in the range 0.2- 0.5 eV
pH > pzc *OH- + h+ *OH•
ET barrier ~ 0.1 eV ⇒ OER faster at high pH
Polaronic effects essential to make transfer possible
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TiO2 modification via
nano-structuring TiO2/SrTiO3 heterostructures for
water oxidation Jun Hee Lee & AS
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Anatase TiO2(001) can be grown epitaxially on LAO and STO
HRTEM image of TiO2/LAO interfacial region taken in the [010] zone axis of the film.
~40 nm
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semiconductor/ferroelectric
heterostructures
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Interface dipole can shift electron band energy positions & affect the surface chemistry (see e.g. work by Rohrer & co.)
FE
+
+
+
+P
FE
-
-
-
-P
Ec
Ev
TiO2
QUALITATIVELY
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More relevant for chemistry applications: Core-shell particles
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Computational model: AnataseTiO2(001)/STO
-Pz (dn) = -0.8 C/m2 (negative)
Pt SrTiO3 (3.5u)
TiO2 (1u4L)
Dipole correction
+Pz (up) = +0.8 C/m2 (Positive) Fixed to strained bulk
SrTiO3
H2O (0.5ML)
2a
a = 3.786 Å (lattice const of TiO2, 3% compressive for
SrTiO3)
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+Pz (up)
W(up)
-Pz (dn)
W(dn) W(Pt)
Pt SrTiO3 TiO2
εF
Hartree potential shift
Substrate polarization shifts the work-function
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Layer-resolved
DOS – Structure 1
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Oxygen Evolution Reaction – (b) Practical scheme used in periodic calculations
Compute ΔG1- ΔG4 SHE: H+ + e−↔ ½ H2 at U=0 & pH =0
*H2O → *OH + H+ + e−
*OH → *O + H+ + e− H2O(l) + *O → *OOH + H+ + e−
*OOH → O2(g) + H+ + e−
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Layer-resolved
DOS – Structure 1’
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1’’ 2
-H -O2
TiO2
-H
1’
2’
2’’
0
+H2O
1
-H
-H
Rate-limiting
OER on TiO2
+H2O
1
1
1’
1’’
2
2’
2’’
0
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-H -O2
TiO2
Down -H
+H2O
-H
-H
rate-limiting
No effect from
negative dipole !
+H2Oa
1’’ 2
1’
2’
2’’
0
1
1
1
1’
1’’
2
2’
2’’
0
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-H(*) -O2
TiO2
Down
Up -H
+H2O
-H
-H
Up
no barrier
small barrier
+H2O
1’’ 2
1’
2’
2’’
0
1
1
1
1’
1’’
2
2’
2’’
0
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Pure
Down
Up
+H2O
-O2 +H2O +H2O
Bare surface
Dynamically Induced polarization of TiO2 layers
P of bottom layer of SrTiO3
No dipole
Local dipole
No dipole -H
-H
-H
-H
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-1.0%
0%
+2.4%
+3.8% (STO lattice)
TiO2 Strain effect with positive dipole
+H2O
-H
-H
-H
-H
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L=2.5
L=4.5
L=3.5
L
Catalysts on high-K Overcome the rate-limiting step
even without built-in P.
1’’
2
2’
1
2’’
0 1 1
1’
1’’
1’
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Li et al., J. Am. Ceram. Soc. (2012)
Recent experimental work
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Summary & Conclusions
Many open questions!
• more realistic models: larger surface area,
water environment, defects(?)
• beyond GGA
• Add real holes/electrons
• …
• Expts: Single crystal monodomain
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