TACC 2016 Talk - Reduced Amorphous Titania

A talk at the Theory and Applications of Computational Chemistry conference in Seattle, WA 2016 on reduced amorphous titania.

Transcript

1. Amorphous Titania for Photoelectrochemical Applications N. Aaron Deskins1, Jincheng Du2, Dunwei Wang3, Pratap Rao1 1 Worcester Polytechnic Institute 2 University of North Texas 3 Boston College
2. Worcester Polytechnic Institute Photocatalyst Material Development 2 §  Water Splitting §  CO2 Reduction (Solar Fuels) §  Organic Oxidation Cheap Stable Efficient
3. Worcester Polytechnic Institute Computational Studies of TiO2 3 1000’s of Studies! Amorphous TiO2 Who Cares?!
4. Worcester Polytechnic Institute Amorphous Titania as a Protective Coating 4 Narrow Band Semiconductor Amorphous Titania holes electrons reduction reactions oxidation reactions CB VB Band Gap Low Band Gap = Good
5. Worcester Polytechnic Institute Amorphous Coatings of p-type semiconductors •  J. Gu, Y. Yan, J.L. Young, K.X. Steirer, N.R. Neale, J.A. Turner, Nat. Mater. 15 (2015) 456. •  Y. Lin, R. Kapadia, J. Yang, M. Zheng, K. Chen, M. Hettick, et al., J. Phys. Chem. C. 119 (2015) 2308–2313. •  J. Qiu, G. Zeng, M.-A. Ha, M. Ge, Y. Lin, M. Hettick, et al., Nano Lett. (2015) 150814112530005. •  J. Qiu, G. Zeng, M. Ge, S. Arab, M. Mecklenburg, B. Hou, et al., J. Catal. 337 (2016) 133–137. •  C. Das, M. Tallarida, D. Schmeisser, Nanoscale. (2015) 7726–7733. •  Etc., Etc. 5 Narrow band p-type semiconductors: Si, InP, GaAs, GaInP2 , etc. p-type Narrow Band Semiconductor Amorphous Titania
6. Worcester Polytechnic Institute Leaky hole conductivity: protection of n-type semiconductors 6 n-type Narrow Band Semiconductor Amorphous Titania electrons holes reduction reactions oxidation reactions Narrow band n-type semiconductors: Si, GaAs, GaP, CdTe, etc. •  S. Hu, M.R. Shaner, J. a Beardslee, M. Lichterman, B.S. Brunschwig, N.S. Lewis, Science. 344 (2014) 1005–9. •  M.T. McDowell, M.F. Lichterman, A.I. Carim, R. Liu, S. Hu, B.S. Brunschwig, et al., ACS Appl. Mater. Interfaces. 7 (2015) 15189–15199. O vacancies? Dopants? Other Defects?
7. Worcester Polytechnic Institute Disordered Black TiO2 7 Crystalline Core Disordered Shell Variety of synthesis and treatment approaches to form black titania. X. Chen, L. Liu, F. Huang, Chem. Soc. Rev. 44 (2015) 1861–1885. T. Lin, C. Yang, Z. Wang, H. Yin, X. Lü, F. Huang, et al., Energy Environ. Sci. 7 (2014) 967.
8. Worcester Polytechnic Institute Methodology 8 Melt and Quench for Initial Structure Generation (Molecular Dynamics) •  DL_POLY •  ~216 atoms •  Shell Model (Polarizable O atoms and Ti4+/Ti3+) S. Kerisit, N.A. Deskins, K.M. Rosso, M. Dupuis, J. Phys. Chem. C. 112 (2008) 7678–7688. •  Temperature Quench Density Functional Theory for Structure Refinement (Geometry Optimization) •  VASP Planewave Code •  PAW Pseudopotentials •  DFT+U (U = 4.0 eV) •  Spin-polarized •  Gamma k-point sampling 0 500 1000 1500 2000 2500 3000 3500 4000 4500 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Temperature Step Stoichiometries Considered (TiOx ) 2.0 1.99 1.97 1.96 1.94 1.90 1.85 1.81 1.75 a-TiO1.77 Synthesized T. Leichtweiss, R. a. Henning, J. Koettgen, R.M. Schmidt, B. Holländer, M. Martin, et al., J. Mater. Chem. A. 2 (2014) 6631.
9. Worcester Polytechnic Institute Stability of Reduced a-TiOx 9 OLattice à VO + ½ O2 (g) * H.H. Pham, L.-W. Wang, Phys. Chem. Chem. Phys. 17 (2015) 541–550. a-TiO1.99 1 VO a-TiO1.97 2 VO Rutile Anatase Brookite 1 VO 2VO 5.59* 4.42* 5.20* 1.50 (1.35*) 6.56 ΔEO2-formation (all in eV)
10. Worcester Polytechnic Institute Geometry Analysis: Coordination of Atoms 10
11. Worcester Polytechnic Institute Effect of Stoichiometry on Structure 11 Basic Structure (Ti-O6 ) Intact!
12. Worcester Polytechnic Institute Gap States Arise From VO 12 Zero eV = Conduction Band Edge O2- à VO + ½ O2 (g) + 2e-
13. Worcester Polytechnic Institute Do under-coordinated sites have special electronic properties? 13 Under-coordinated sites at disordered interface between rutile/anatase preferred sites for electron localization Ti4+ + e- à Ti3+ J.C. Garcia, M. Nolan, N.A. Deskins, J. Chem. Phys. 142 (2015) 024708.
14. Worcester Polytechnic Institute Does coordination link to electronic states? 14 Ti4+ + e- à Ti3+
15. Worcester Polytechnic Institute What about hybrid functionals? 15
16. Worcester Polytechnic Institute Electron Transport 16 Hopping or Polaron Model Ti4+ Ti4+ Ti4+ Ti4+ Ti4+ e- e- e- e- e- •  N.A. Deskins, M. Dupuis, J. Phys. Chem. C. 113 (2009) 346–358. •  N.A. Deskins, M. Dupuis, Phys. Rev. B. 75 (2007) 195212.
17. Worcester Polytechnic Institute Hole Transport via VO : Ti3+ states 17 Ti3+ Ti3+ Ti3+ Ti3+ Ti3+ O2- à VO + ½ O2 (g) + 2e- Oxygen vacancies induce Ti3+ states
18. Worcester Polytechnic Institute Hole Transport via VO 18 Proposed Model of Hopping Ti3+ Ti3+ Ti3+ Ti3+ Ti3+ h+ h+ h+ h+ h+ H.H. Pham, L.-W. Wang, Phys. Chem. Chem. Phys. 17 (2015) 541–550.
19. Worcester Polytechnic Institute Real Systems Mix of Ti3+/Ti4+ •  Ti4+-Ti4+ distances key for photoexcited electron transport •  Ti3+-Ti3+ distances key for photoexcited hole transport 19 Ti3+ Ti4+ Ti4+ Ti4+ Ti3+
20. Worcester Polytechnic Institute Charge Transport •  Ti4+-Ti4+ distances key for photoexcited electron transport •  Ti3+-Ti3+ distances key for photoexcited hole transport 20 Ti3+ Ti4+ Ti4+ Ti4+ Ti3+
21. Worcester Polytechnic Institute Experimental Efforts: WS2 Photoanodes/Corrosion 21 0 5 10 15 20 25 0 0.4 0.8 1.2 Current Density J (mA/cm2) Voltage (V vs. RHE) on off Bare WS2 ! single-crystal! in H2 SO4! 10 µm! Before! After! After coating with ! amorphous-TiO2! Voltage (V vs. RHE) 0.4 0.8 1.2 1.6 0 2 4 6 8 10 Current Density J (mA/cm2) before 0.5h 1.5h 10 µm! Before! After! 1 2
22. Worcester Polytechnic Institute WS2 /a-TiO2 interfaces 22 TiO2 WS2 Staggered Band Gaps Type II Junction
23. Worcester Polytechnic Institute Conclusions •  Structural Properties •  Electronic Properties •  Charge Transport Properties •  Interfaces 23