(209f) Modeling of Graphene-Titania Interfaces

Authors: 
Deskins, N. A., Worcester Polytechnic Institute

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.