Designing the Ultimate Metal Oxide Photoelectrode

Invited presentation at the Fall Meeting of the Materials Research Society (2024)

Transcript

1. Aron Walsh Department of Materials Centre for Processable Electronics Designing the Ultimate Metal Oxide Photoelectrode
2. Sunlight to Electricity or Fuel Electricity Solar Cells Chemical Energy Solar Fuels High efficiency (20–50%) Low efficiency (< 20%) Physics (charge carrier collection) is easier than chemistry (oxidation/reduction reactions) Fusion Reactor 174,000 Terawatts reaches the Earth’s surface
3. Solar Energy Materials Shopping List • Low-cost and non-toxic elements • High optical absorption coefficient • Semiconductor with low carrier concentrations • Tolerant to defects, impurities, microstructure • Chemical and thermal stability • Suitable electrochemical potentials
4. 1. Building Blocks 2. Imperfect Oxides 3. New Avenues Outline
5. Why Metal Oxides? D. W. Davies et al, J. Phys. Chem. Lett. 11, 438 (2020) Most elements heated in air form a metal oxide (+) Relatively stable; (-) Band gaps typically too large Distribution of carrier effective masses for 5548 metal oxides m*h > m*e m*e m*h n-type oxide semiconductors ZnO, In2 O3 , SnO2 , TiO2 … p-type oxide semiconductors Cu2 O, SnO, NiO, BiVO4 …
6. Band Engineering of Metal Oxides Conduction Band – Metal s0 or p0 Valence Band – Oxygen 2p6 Eg ~ 7.8 eV IP EA Example: MgO Exact band structure depends on the nature of chemical bonding and crystal structure
7. Metal d0 Band Engineering of Metal Oxides Conduction Band Valence Band – Oxygen 2p6 IP EA Example: TiO2 Exact band structure depends on the nature of chemical bonding and crystal structure Eg ~ 3.2 eV
8. Metal d0 Band Engineering of Metal Oxides Conduction Band Valence Band Example: BiVO4 Metal s2 IP EA Exact band structure depends on the nature of chemical bonding and crystal structure Eg ~ 2.4 eV
9. Metal d0 Band Engineering of Metal Oxides Conduction Band Valence Band s2 and d0 cations: (Sn, Sb, Pb, Bi)(Ti, W, V, Nb, Ta)Ox Metal s2 Exact band structure depends on the nature of chemical bonding and crystal structure
10. Oxidation State Competition A. Walsh et al, Oxidation states and ionicity, Nature Mater. 17, 958 (2018) Post-transition metals (e.g. Ge, Sn, Pb, Sb Bi) adopt the group oxidation state N or N-2 5s0p0: SnIVO2 Wide bandgap n-type semiconductor 5s2p0: SnIIO Small bandgap p-type semiconductor
11. (Bi 6s + O 2p)* Bi 6s O 2p Case Study: Bismuth Vanadate A. Walsh et al, Chem. Mater. 21, 547 (2009) (Bi 6s + O 2p) Simplified two-level coupling scheme Matches valence band XPS APL 98, 212110 (2011) Electronic density of states (DFT/PBE) Stereochemical and electronic activity of Bi in BiIIIVVO4 involves Bi 6s – O 2p interactions
12. Case Study: Tin Titanate L. A. Burton et al, J. Solid State Chem. 196, 157 (2012) SnII 2 TiIVO4 is more suitable for visible-light solar fuel applications than rutile (SnIV,TiIV)O2 Sn2 TiO4 isostructural to mixed-valence minum (PbII 2 PbIVO4 ) Electron density at the upper valence band (DFT/PBEsol) Eg exp~1.6 eV
13. Case Study: Beyond SnII 2 TiIVO4 E. A. Gabilondo et al, Chem. Mater. 36, 5753 (2024) Exploration of SnII oxides by Paul Maggard and colleagues led to the synthesis of SnIILa4 Ti4 O15
14. Generative AI for Materials Exploration Chemeleon: Hyunsoo Park, A. Onwuli and A. Walsh, Nature Comm. 1, 4379 (2025) Noise Chemeleon can run on a laptop Text-guided denoising diffusion model: “Suggest a crystal structure for Bi2 TiWO8 ” Sample
15. 1. Building Blocks 2. Imperfect Oxides 3. New Avenues Outline
16. From Ideal to Real Materials K. T. Fountaine et al, Nature Comm. 7, 13706 (2016) Values depend on the specific reaction Photoelectrochemical solar to hydrogen Some factors: • Non-radiative processes • Necessary overpotential • Faradaic efficiency
17. Charge Trapping as Polarons Landau (1933)… D. W. Davies et al, J. Phys. Chem. Lett. 11, 438 (2020) (+) Create active sites for catalysis (-) Impact carrier lifetime, photocurrent, overpotential Weak: large polaron with Epol < 0.1 eV; µ > 1 cm2/Vsec Strong: small polaron with Epol > 0.1 eV; µ < 1 cm2/Vsec ψ polaron Electron-phonon coupling regimes
18. Charge Trapping at Defect Sites Review on defects in oxides: E. Pastor et al, Nat. Rev. Mat. 7, 503 (2022) D0 + 𝑒− ⇌ D− + ħ𝜔 D− + ℎ+ ⇌ D0 + ħ𝜔 Hole capture Electron capture github.com/WMD-group/CarrierCapture.jl (+) Create active sites for catalysis (-) Impact carrier lifetime, photocurrent, overpotential
19. “Killer” Transition Metals dn (0<n<10) metals open non-radiative recombination channels detrimental to solar energy conversion dn Example cations High spin (octahedral) SHS Low spin (octahedral) SLS Spin-allowed d-d transitions 0 TiIV, VV t2g 0eg 0 0 ✗ 1 TiIII, VIV t2g 1eg 0 ½ ✓ 2 TiII, VIII t2g 2eg 0 1 ✓ 3 CrIII, MnIV t2g 3eg 0 3/2 ✓ 4 CrII, MnIII t2g 3eg 1 2 t2g 4eg 0 1 ✓ 5 FeIII, MnII t2g 3eg 2 5/2 t2g 5eg 0 ½ ✗/✓ 6 FeII, CoIII t2g 4eg 2 2 t2g 6eg 0 0 ✓ 7 NiIII, CoII t2g 5eg 2 3/2 t2g 6eg 1 ½ ✓ 8 NiII t2g 6eg 2 1 ✓ 9 CuII t2g 6eg 3 ½ ✓ 10 CuI, ZnII t2g 6eg 4 0 ✗ n = 0, 5, 10 are special cases where there are no spin-allowed transitions
20. Transition Metals in Oxides d-d transitions Mq+hν→Mq* Mq+e-+h+→Mq+ℏ⍵ Trap-mediated recombination Conduction Band Valence Band Mq+Mq’+hν→ Mq’+Mq +ℏ⍵ Intervalence charge transfer
21. Transition Metals in Oxides Valence Band Small electron polaron Mq+e-→Mq-1 Mq+h+→Mq+1 Small hole polaron Conduction Band Polaron = charge carrier coupled to a structural distortion Irreversible loss (excess energy lost thermally)
22. Transient Absorption Measurememts Michael Sachs... J. Nelson, A. Walsh, J. R. Durrant, Nature Chemistry, In Press (2025)
23. 1. Building Blocks 2. Imperfect Oxides 3. New Avenues Outline
24. Beyond Oxides: Oxyhalides Many promising families of photo(electro)catalysts based on mixed anions such as Bi2 YO4 Cl and Bi2 YO4 l Kanta Ogawa et al, J. Am. Chem. Soc. 146, 5806 (2024)
25. Intralayer Band Gap Control Excess Bi can replace Y in a high symmetry coordination environment that reduces the band gap Kanta Ogawa et al, J. Am. Chem. Soc. 146, 5806 (2024) IPs from photoelectron yield spectroscopy (PYS) Electron density at the upper valence band BiY
26. Beyond Oxides: Oxyhalides Sillén-Aurivillius (fluorite-perovskite intergrowth) phases such as Bi4 NbO8 Cl and Bi6 NbWO14 Cl Kanta Ogawa et al, Chem. Mater. 35, 5532 (2023) Bi4 NbO8 X (n = 1) Bi O X a Bi/Ba O Ti Bi5 BaTi3 O14 X (n = 3) c Bi Nb/W Bi6 NbWO14 X d Ba2 Bi3 Nb2 O11 X (n = 2) b Bi Bi/Ba Ba O X Nb O Bi4 NbO8 X (n = 1) Bi O X a Bi/Ba O X Ti Bi5 BaTi3 O14 X (n = 3) c X Bi Nb/W Bi6 NbWO14 X d Ba2 Bi3 Nb2 O11 X (n = 2) b Bi Bi/Ba Ba O X Nb O Electronic structure is influenced by chemical interactions within and between the polyatomic building blocks
27. Interlayer Band Gap Control Long-range electrostatics from charged building blocks determines relative band energies Kanta Ogawa and Aron Walsh, J. Am. Chem. Soc. 147, 821 (2025) Upper valence band Lower conduction band Modular approach to engineer band gaps, energies, carrier transport ∆𝑉𝑖𝑛𝑡𝑒𝑟 = 𝑄𝑙𝑎𝑦𝑒𝑟 𝑑 2𝜀𝑆 charge distance area
28. Conclusion Reproducible science: share data and workflows There are established chemical principles for band engineering of metal oxides. Multi-component photoelectrodes offer potential to tailor performance The presentation contained work from: Kanta Ogawa, Lucas Garcia-Verga, Liam Harnett Thanks to my collaboration network, in particular: James Durrant (ICL) and Ernest Pastor (CNRS-Rennes)