Brandon Bukowski, N. Aaron Deskins Worcester Polytechnic Institute Worcester, MA 

Brief Intro to Photocatalyis _ Valence Band Conduction Band e- hν h+ Œ photoexcitation  charge diffusion, trapping, and recombination Ž molecular adsorption and reaction + TiO2 Nanoparticle H+ Photocatalytic Process Overview R R’ C O O O C R R’ O2 O2 R· O O C R’ •  Water/air treatment - Photocatalytic organic oxidation •  Water splitting 

TiO2 /Graphene Heterostructures Graphene TiO2 

State of the Field Various Density Functional Theory Results Available Graphene à Rutile Electron Transfer •  Du, Y.H. Ng, N.J. Bell, Z. Zhu, R. Amal, S.C. Smith J. Phys. Chem. Lett. 2 (2011) 894–899. •  R. Long, N.J. English, O. V Prezhdo, J. Am. Chem. Soc. 134 (2012) 14238– 48. Anatase à Graphene Electron Transfer •  H. Gao, X. Li, J. Lv, G. Liu, J. Phys. Chem. C. 117 (2013) 16022–16027. Graphene à Anatase Electron Transfer •  X. Li, H. Gao, G. Liu, Comput. Theor. Chem. 1025 (2013) 30–34. •  Y. Masuda, G. Giorgi, K. Yamashita, Phys. Status Solidi. 251 (2014) 1471– 1479. TiO2 Gap States Improve Photoexcitation •  N. Yang, Y. Liu, H. Wen, Z. Tang, H. Zhao, Y. Li, et al, ACS Nano. 7 (2013) 1504–12. Graphene + small TiO2 Pristine Graphene + TiO2 

Non-Uniform Graphene/Relevance of Defects reduced graphene oxide Graphite oxidation à separation à reduction (thermal, chemical) Reality: Defect + oxygen containing groups S. Pei, H.-M. Cheng, Carbon 50 (2012) 3210–3228. 

Graphene/TiO2 Models (TiO2 )n Clusters •  N= 1 to 8 from Qu and Kroes, The Journal of Physical Chemistry B, 2006, 110, 8998-9007. •  N= 15 from Hamad et al. The Journal of Physical Chemistry B, 2005, 109, 15741-15748. (6x6) surface a – Pristine graphene b – Graphene with C vacancy c – Graphene with epoxide d – Graphene with hydroxyl 

TiO2 over Pristine Graphene Surface/Cluster Distances 2.4 to 2.9 Å Previous DFT Results for n = 1 -0.8 eV1 -1.22 eV2 -1.27 eV3 -1.55 eV4 1 M. Favaro, S. Agnoli, C. Di Valentin, C. Mattevi, M. Cattelan, L. Artiglia, E. Magnano, F. Bondino, S. Nappini and G. Granozzi, Carbon, 2014, 68, 319-329. 2 S. Ayissi, P. A. Charpentier, N. Farhangi, J. A. Wood, K. Palotás and W. A. Hofer, The Journal of Physical Chemistry C, 2013, 117, 25424-25432. 3 E. H. Song and Y. F. Zhu, Nanoscience and Nanotechnology Letters, 2013, 5, 198-203. 4 M. I. Rojas and E. P. M. Leiva, Physical Review B, 2007, 76, 155415. 1 eV = 96.5 kJ/mol 

What binds TiO2 to graphene? TiO2 binding – no site preference! van der Waals forces play key role in binding TiO2 1 eV = 96.5 kJ/mol 

What about defects/functional groups? Adsorption Energies in eV C Vacancy Epoxide Hydroxyl 1 eV = 96.5 kJ/mol 

Charge Density Difference Plots Pure Graphene Graphene With Vacancy Graphene With Epoxide Graphene With Hydroxyl 

Electronic Interactions between Graphene and TiO2 – Band Gaps 

Electronic Transitions between Graphene and TiO2 Conduction Band Valence Band TiO2 Pure Graphene light 1 2 3 1 2 3 

Defects Affect Transitions Pure Graphene: graphene à TiO2 C Vacancy: graphene ßà TiO2 Epoxide: graphene ß TiO2 Hydroxyl: graphene ß TiO2 

Summary of the work •  Realistic graphene surfaces and high-level molecular model •  Pure Graphene-TiO2 interactions dominated by van der Waals forces •  Graphene defects can serve as binding/anchor sites for TiO2 •  Defects can affect electronic transitions between graphene-TiO2 Graphene TiO2 Defect Phys. Chem. Chem. Phys., 2015, Advance Article, DOI: 10.1039/C5CP04073F 

Acknowledgments/Co-Authors Brandon Bukowski We wish to thank: Sia Najafi, WPI, for Computer Support Environmental Molecular Science Laboratory (EMSL) at Pacific Northwest National Laboratory in Richland, WA for providing computational resources. EMSL is a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. 

Computational Method-Density Functional Theory ¨ CP2K for Geometry Optimization ¨ Gamma-point k- point sampling ¨ Dual Basis Set - Gaussian and Plane wave (GPW) Method ¨ GGA (PBE) Exchange Correlation Functional ¨ van der Waals corrections – method of Grimme ¨ VASP for single- point electronic structure ¨ 4x4x1 kpt mesh ¨ Plane Wave Basis ¨ DFT+U to correct self-interaction errors ¨ Density of states, electronic charge 

Calculated Bader Charges Surface Type Cluster Size (n) Pristine graphene Graphene with C vacancy Graphene with epoxide Graphene with hydroxyl 1 -0.03 -0.11 -0.90 -0.43 3 -0.14 -0.02 -1.17 -0.83 5 -0.08 -0.23 -1.32 -0.63 8 -0.07 -0.15 -1.29 -0.86 15 -0.27 -0.56 -1.29 -0.80 ! C Vacancy Epoxide Hydroxyl 1 eV = 96.5 kJ/mol