This paper describes a facile one-step synthesis of large-scale floating TiO_2 nanorod arrays via a hydrothermal reaction without using any surfactants, which opens a new way to prepare floating photocatalysts for photodecomposition of floating organics and free-standing nanorod arrays for flexible dye-sensitized solar cells. A general model is developed to analysis the driving force for the floating TiO_2, which may be also useful for other two-dimensional (2D) materials to determine the obtainable size of a arbitrary shaped film floated by surface tension and buoyancy.