h i g h l i g h t s < The initial Au particle size of 2e5 nm increased to 25 nm in one year of storage. < The main mechanisms of Au particle growth are Ostwald ripening and fusion. < The initial spherical morphology changed to regular shapes. < Twin boundaries have an important effect on the evolution of morphology. a b s t r a c t The evolution of size and shape of gold nanoparticles was studied during long-time aging. The initial particle size of 2e5 nm increased to about 25 nm in one year of storage. It was revealed that the main mechanism of particle growth is Ostwald ripening, however, fusion of particles was also observed. Additionally, while the initial particles have spherical morphology, the grown particles show various shapes such as sphere, bipyramid, decahedron, deca-tetrahedron, triangular plate and rod. Twin boundaries with large frequency of w5% were detected inside the particles which have an important effect on the evolution of morphology. This study suggests that aging may be a new way of tuning size and shape of gold nanoparticles. Ó 2013 Elsevier B.V. All rights reserved.