An electrochemical comparison of manganese dioxide microparticles versus α and β manganese dioxide nanorods:: mechanistic and electrocatalytic behaviour

The comparative electrochemical behaviour of both alpha- and beta- nanorods of manganese dioxide (MnO(2)) and microparticles of predominantly beta-phase manganese dioxide is investigated at pHs close to neutral. In order to understand the observed voltammetric behaviour of all three materials the mechanisms of electrodeposition of MnO(2) onto a graphite electrode surface from a solution of Mn(II) at pH 3-7 is also reported. It is proposed that two competing mechanistic pathways operate, both invoking MnOOH as an intermediate species, which are an ECE or a DISP process, respectively. At low pH values (< pH 3) the mechanism likely proceeds via a DISP mechanism whereas at high pHs (pH similar to 7) the mechanism proceeds predominantly as an ECE process with the diffusion of protons out of the MnOOH intermediate being the likely rate determining step. The alpha- and beta-MnO(2) nanorods display a ca. 30 mV difference in the observed reduction potentials corresponding to a ca. 3 kJ mol(-1) difference in the Gibb's energy between the two phases. There is also an observable difference in the reduction potential of the beta-nanorods and beta-microparticles, which probably reflects the differing surface or structural energies of the two materials. Finally the electrocatalytic performance of the three MnO(2) materials with respect to the analytical determination of hydrogen peroxide is investigated. Both phases of the MnO2 nanorods exhibited a lower limit of detection (5.0 +/- 2.5 mu M based on 3 sigma) and greater sensitivity towards H(2)O(2) than the MnO(2) microparticles, likely attributed to an increased surface area.