Chemical nature of catalysts of oxide nanoparticles in environment prevailing during growth of carbon nanostructures by CCVD

摘要

Carbon nanostructures (CNS) are often grown using oxide nanoparticles as catalyst in chemical vapour deposition and these oxides are not expected to survive as such during growth. In the present study, the catalysts of cobalt- and nickel oxide-based nanoparticles of sizes varying over a range have been reduced at 575(A degrees)C under environment resulting from the introduction of C2H2 + NH3 during growth of CNS as well as under introduction of C2H2 and NH3 separately. The structure of the reduced nanoparticles is often different from the equilibrium structure of the bulk. Nanoparticles of cobalt oxide with and without doping by copper oxide are reduced to cobalt alloy or cobalt nanoparticles having fcc structure, but the rate of reduction is relatively less in NH3 environment. However, reduced nickel oxide nanoparticles with and without doping shows a mix of fcc and hcp phases. The presence of hydrogen and nitrogen in the environment appears to discourage the formation of hcp nickel as inferred from the results in NH3 environment. Cobalt carbide forms when the 10 wt.% or less doped cobalt oxide is reduced in C2H2 + NH3 environment. At higher level of doping of 20 wt.%, separate metallic phase of copper appears and carbide formation gets suppressed.