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 m² 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).