All Density Functional Theory (DFT) calculations were carried out with the periodic Quantum Espresso code , using ultra-soft pseudopotentials and the van der Waalscorrected BEEF-vdW exchange correlation functional . The Brillouin zone was sampled at the Gamma point only, and cut-offs of 500 and 5000 eV were used for the orbitals and charge density, respectively. The electronic structure was self-consistently calculated until energy differences between consecutive steps were smaller than 5E-5 eV, and all nanoparticle structures were relaxed until forces for all atoms were smaller than 0.05 eV/ Å. In order to avoid interactions between NPs in neighboring unit cells, vacuum was added to the supercell ensuring particle-particle distances < 8 Å.