One-pot synthesis and first-principles elasticity analysis of polymorphic MnO2 nanorods for tribological assessment as friction modifiers

Abstract

One-pot synthesis of single-crystalline α and β-MnO₂ nanorods was carried out by selectively varying the acidic concentrations. Ultrafine one dimensional nanorods with diameters of about 10–40 nm are achieved. The respective phases of the nanorods were then altered through simple optimization in the molar concentration of H₂SO₄. Morphological transition from microstructure to nanostructure is also examined by changing the acid concentration from high to low. Elastic and tribological properties of these nanomaterials were subsequently explored, with a view to their possible applications as nanoadditives in green lubricants. While β-MnO₂ nanorods showed a reduction in the coefficient of friction by about 15%, α-MnO₂ nanorods turned out to be even better nanoadditives yielding a reduction of as high as 30%. Moreover, both the polymorphs of MnO₂ nanostructures led to lower roughness when used as nanoadditives in the base oil. Our analysis suggests that such enhancement of antiwear properties originates primarily from the mutual interplay between the rolling action and the protective layer formation by respective polymorphs of quasi-1D MnO₂. From the first-principles analysis, we envisage that α-MnO₂ nanorods may potentially serve as efficient nanoadditives in comparison with β-MnO₂ nanorods due to superior elastic properties of the former.