Modification to the composition of nanocrystalline RuO2 through reactive milling under O2

Crystalline ruthenium dioxide (RuO2) has been ball-milled in an O2 atmosphere and the changes in the physico-chemical properties induced by different milling atmospheres (Ar and O2) or milling sequences have been established. Cyclic voltammetry and BET measurements were used to evaluate the electrochemically active surface charge (qT*, expressed in C g−1) and the specific surface area (expressed in m2 g−1), respectively. The extent of oxygen uptake in the processed samples was determined by X-ray photoelectron spectroscopy (XPS). The qT* value of RuO2 milled under O2 for 30 h (sample S4) is 120.7 C g−1, a factor of two higher than the qT* value of RuO2 milled under Ar for the same period of time (sample S1, 60.3 C g−1). A still higher qT* value is obtained when the O2 atmosphere of the crucible is frequently replenished (sample S5, 138.7 C g−1) or when the milling operation is prolonged up to 81 h (sample S6, 160.5 C g−1). These changes are paralleled by a variation of the BET surface area, which increases from 24 m2 g−1 for sample S1 to 51.5 m2 g−1 for sample S5. The concentration ratio [Obound to Ru]/[Ru] determined by XPS increases steadily from sample S1 to sample S6, indicating that the amount [Obound to Ru] increases with the exposure of RuO2 to oxygen during the milling process. All these changes are explained by the fact that freshly exposed RuO2 surfaces created during the milling process react with O2 molecules, thereby lowering the surface energies and the tendency of the milled material to cluster into larger aggregates.