When a lattice expansion of 0.1 to 0.5% as crystal-size decreases to ~5nm suggest a negative surface stress in each of the five oxides: CeO2, MgO, Cu2O, Fe3O4, and Co3O4. The finding is different from the positive surface stress observed in nanoparticles of noble metals. Surface stress is calculated from the amount of lattice-contraction as crystallite-size decreases. The present 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, bulk modulus (B) and coefficient of lattice thermal expansion (alpha) were measured. Bulk modulus peaks around 33nm for nano-ceria and 14nm for nano-MgO. In both cases there is a quick decline below the peak. The findings have a number of implications for the bonding, surface-stress and elastic energy stored in thee nanoparticles. Effects of surface adsorbents will be addressed with the help of earlier STM work on Fe3O4 (111) at Columbia University. With binary oxides, the solubility of zirconia in nano-ceria is greater as the crystallite-size decreases and as the partial pressure of oxygen decreases. In reducing atmosphere, total solid solution in cubic fluorite structure is formed for full composition range of cerium and zirconium. Thus, the “bulk” phase diagram cannot predict the phases in nano-oxides. Even pure oxide of zirconium can be prepared in cubic fluorite structure with the right condition. Implication to their catalytic applications will be discussed. Cation valence change, and defect density will be discussed.

Crystallite-size dependent bond length, elastic and thermal properties of nano-oxides

256th National Meeting (2018)

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