“Surface stress in nanoscale”
To appreciate surface stress in nano-scale, we will start with an overview of the size-dependence lattice-parameter of nano-oxides. Lattice expansion of 0.1 to 0.5% is observed as crystallite-size decreases to ~5nm. Here, a compressive surface stress is experienced in each of the five oxides: MgO, Cu2O, Fe3O4, CeO2 and Co3O4. The finding is different from the tensile surface stress observed in nanoparticles of noble metals notably gold, where surface stress is tensile as lattice-contraction with decreasing crystallite-size. Our investigation is possible because of the mono-dispersed nature of the nano-oxide in each batch We have also studied the pressure and thermal response of the lattice-parameter of nano-ceria and nano-MgO. Hence, size-dependent bulk modulus and coefficient of lattice thermal expansion were measured. Bulk modulus peaks around 33nm for nano-ceria and 14nm for nano-MgO. In both cases there is a quick decline with further size decreases. The coefficients of lattice thermal expansion in both nano-oxides decrease rapidly below 15nm. The findings have a number of implications for bonding, surface-stress, heat capacity and the amount of elastic energy stored in these nanoparticles.