A composite of titanium(IV) oxide (TiO2) particles (core) and nanoporous silica (shell) was prepared by successive coating of a carbon layer and an octadecyl-functionalized silica layer on TiO2, followed by heat treatment to remove the organic components. Transmission electron microscope (TEM) observation and nitrogen (N-2) sorption analyses showed that the composite has a unique rattle-type structure, i.e., TiO2 particles were encapsulated in the hollow silica shell having well-developed porosity. When the photocatalytic activity for gas phase decomposition of acetone over the composite was compared with that over naked TiO2 without the lateral silica shell, the activity over the composite tended to become higher than that over naked TiO2 as the initial amount of acetone in the system was reduced. The enhancement of decomposition rate under a diluted condition was due to condensation of acetone on the lateral silica shell, which resulted in enhancement of the collision rate between the substrate and the surface of the TiO2 core. (C) 2009 Elsevier B.V. All rights reserved.