Ligand-free Au nanoparticles with controlled sizes are synthesized via a volume-confined method. In this synthesis, mesoporous hollow silica shells (mHSS) are used as nano-containers for the impregnation of HAuC14 solution before they are separated from the bulk solution. With a simple heating process, the Au precursor confined within the cavity of the isolated hollow shells is converted into ligand-free Au nanoparticles. The size of the Au nanoparticles can be tuned precisely by loading HAuC14 solution of different concentrations, or by using mHSS with different cavity volumes. The ligand-free Au nanoparticles demonstrate superior catalytic activity than sodium citrate-capped Au nanoparticles. (C) 2014 Elsevier B.V. All rights reserved.