Graphene encapsulated hollow TiO2 nanospheres: efficient synthesis and enhanced photocatalytic activity

Graphene encapsulated mesoporous hollow TiO2 nanospheres (GT NSs), consisting of small TiO2 grains, have been successfully prepared via a novel graphene protected calcination process. This method involves preparation of SiO2 nanospheres and sequential coating of the TiO2 layer, co-assembly of amine-modified TiO2-SiO2 nanospheres and graphene oxide (GO), calcination mediated transformation of the TiO2 layer into anatase TiO2, thermal reduction of graphene oxide to graphene, and final etching of the inner SiO2 template to produce GT NSs. Graphene plays a substantial role in inhibiting the aggregation of TiO2 grains during the high-temperature treatment. A high specific surface area up to 133 m(2) g(-1) for GT NSs can be obtained when the weight ratio of modified TiO2-SiO2 spheres to GO is 100 : 2 (GT NSs-2). Benefiting from the electron transfer from inner hollow TiO2 to outer graphene shells, graphene can substantially suppress the recombination of photo-excited electrons and holes. As a result, GT NSs-2 exhibits excellent photocatalytic activity for the decomposition of rhodamine B (RhB) with an efficiency up to 91% in 90 min, much higher than 65% for TiO2 NSs without involving graphene protection, and 71% for the blended catalyst of TiO2 NSs and reduced GO (TiO2-G).