Modeling of Graphene-Titania Interfaces

TiO₂-graphene composite photocatalyst materials show promise for a wide variety of environmental applications¹, including pollutant degradation, water treatment, and energy/fuel production. When combined with graphene, TiO₂ shows increased photocatalytic activity, which has been attributed to a variety of reasons¹. Narrowing of the band gap of TiO₂ into the visible region may occur due to formation of TiO₂-C moieties. The large surface area of graphene may facilitate surface reactions between adsorbates and catalyst. Finally, increased charge separation (which lowers charge recombination rates, the opposite of photoexcitation) may occur as electrons are shuttled away by graphene. Still many questions exist for these materials. For instance, experimental evidence suggests that graphene with low number of defects makes for an ideal TiO₂-graphene photocatalyst². However, many popular production methods involve reduction of graphene oxide, which may produce graphene with many defects. It is crucial to identify the role of such defects in these photocatalysts. Other work also suggests that intimate contact between TiO₂ and graphene will increase catalytic activity³. 

We present density functional theory simulations of TiO₂-graphene systems in order to elucidate the nature of the interface between the two materials. We have specifically focused on TiO₂ clusters up to 1 nm in size and rutile (110) surfaces. We considered graphene as defect-free or with common defects such as hydroxyls, vacancies, or epoxides. Our results indicate that significant binding occurs at defect sites, much more so than over pristine graphene. Most notably defects on the graphene surface lower the band gap of TiO₂, indicating that new composite electronic states may arise and increase photoexcitation yield in such materials. We also analyzed how surface-surface distance (indicative of degree of contact between graphene and TiO₂) affects the binding and TiO₂ band gaps. Our results highlight the potential role of the graphene surface state in TiO₂-graphene photoactive materials, and suggest design strategies for graphene-based composite materials based on the TiO₂-graphene interface structure.

(1)      An, X. Q.; Yu, J. C. RSC Advances 2011, 1, 1426.

(2)      Liang, Y. T.; Vijayan, B. K.; Gray, K. A.; Hersam, M. C. Nano Letters 2011, 11, 2865.

(3)      Zhang, Y.; Zhang, N.; Tang, Z.-R.; Xu, Y.-J. Physical Chemistry Chemical Physics 2012, 14, 9167.