3D honeycomb-like structured graphene and its high efficiency as a counter-electrode catalyst for dye-sensitized solar cells.

Graphene, a two-dimensional carbon sheet, has attracted great interest due to its unique properties. To explore its practical applications, large-scale synthesis with controllable integration of individual graphene sheets is essential. To date, numerous approaches have been developed for graphene synthesis, including mechanical cleavage, epitaxial growth, and chemical vapor deposition. All of those techniques are used to prepare flat graphene sheets on a substrate. Chemical exfoliation of graphite has been applied to prepare graphene oxide solutions and graphene-based composite materials. Recently, tuning graphene shapes is attracting much attention. Cheng and co-workers synthesized graphene foam using porous Ni foam as a template for the CVD growth of graphene, followed by etching away the Ni skeleton. The graphene foam consists of an interconnected flexible network of graphene as the fast transport channel of charge carriers for high electrical conductivity. Ruoff et al. prepared porous graphene paper from microwave exfoliated graphene oxide by KOH activation. The porous graphene, which has an ultra-high surface area and a high electrical conductivity, was exploited for supercapacitor cells, leading to high values of gravimetric capacitance and energy density. Feng, M llen, and co-workers synthesized hierarchical macroand mesoporous graphene frameworks (GFs). The GFs exhibited excellent performance for electrochemical capacitive energy storage. Yu et al. and Qu et al. fabricated graphene tubes that could be selectively functionalized for desirable applications. Choi et al. synthesized macroporous graphene using polystyrene colloidal particles as sacrificial templates in graphene oxide suspension, and the pore sizes can be tuned by controlling template particle size. These important results represent a significant topic—tuning the properties of graphene sheets by controlling their shapes. However, it is still a challenge to synthesize three-dimensional graphene (3D) with a desirable shape. Herein, we develop a novel strategy for the synthesis of a new type of graphene sheet with a 3D honeycomb-like structure by a simple reaction between Li2O and CO. Furthermore, these graphene sheets exhibited excellent catalytic performance as a counter electrode for dye-sensitized solar cells (DSSCs) with an energy conversion efficiency as high as 7.8%, which is comparable to that of an expensive platinum electrode. Li2O is widely exploited as a promoter in catalysts to inhibit carbon formation. However, this general principle is challenged by this work, in which Li2O is used to react with CO to form graphene-structured carbon [Eq. (1)]