Growth and optical properties of transition metal oxides single crystal solid solutions
Abstract NiO, CoO, MnO single crystals and single crystal solid solutions of Ni c Mg 1− c O, Co c Mg 1− c O and Mn c Mg 1− c O were grown by the method of chemical transport reaction (the “sandwich” technique). The nature of the observed absorption and luminescence bands is explained in terms of the electronic transition of bivalent nickel, cobalt and manganese. The fine structure of the observed bands in NiO and MnO are connected to the pure exciton transition and to the exciton-magnon excitation.
Rock salt type NiCo2O3 supported on ordered mesoporous carbon as a highly efficient electrocatalyst for oxygen evolution reaction
Abstract It is a great challenge to develop cost-effective and stable oxygen evolution reaction (OER) electrocatalysts for energy sustainability. Herein, we report nanocomposites of rock salt type nickel oxide and cobalt oxide supported on ordered mesoporous carbon (OMC) as efficient non-noble metal electrocatalysts for the OER in alkaline solution. The rock salt type NixCoyOx+y@OMC (x/y = 0:1,2:1,1:1,1:2,1:0) nanocomposites are synthesized through a nanocasting and carbon thermal reduction route by using the OMC as a nano-reactor. The OMC is used as the support material, with large specific surface area and pore volume, which can not only effectively disperse metal oxides nanoparticles and prevent the agglomeration of nanoparticles, but also improve electrocatalytic performance of metal oxides by increasing electrical conductivity and electrocatalytic active sites. Interestingly, the rock salt type NiCo2O3@OMC exhibits the lowest overpotential of 281 mV at 10 mA/cm2 in 1 M KOH, which is even lower than those values reported previously for RuO2 and IrO2 precious catalysts, and a small Tafel slope of 96.8 mV/dec for the OER. Notably, the NiCo2O3@OMC has a turnover frequency value of 0.0341 s−1 and an actual oxygen generation rate of ca. 22.5 μmol/h, much higher than its counterparts, confirming its outstanding intrinsic activity as an OER catalyst. Furthermore, the NiCo2O3@OMC has a good durability without obvious attenuation after continuous operation test for 230 h. Evidently, the NiCo2O3@OMC catalyst can remarkably promote the OER, and thus brighten the prospect of non-noble metal for efficient water electrolysis in industrial scale.
High resolution MCD and phosphorescence excitation spectroscopy of Mn2+ in KMgF3 and KZnF3
A combination of high resolution MCD and phosphorescence excitation techniques has been used to determine, for the first time, the optical spectra of single Mn2+ ions in the cubic perovskite hosts KMgF3 and KZnF3. Particular attention has been paid to transition mechanisms, spin-orbit effects and vibronic structure, and the experimental results have been compared with theoretical crystal field calculations based on the complete d 5 configuration. All of the states arising from the 4 G, 4 P and 4 D terms have been observed and a dynamic Jahn-Teller effect has been observed in the 4 T 1g a state. Both magnetic dipole and electric dipole (vibrationally induced) absorption processes occur and the former is most important for transitions to the 4 T 1g a , 4 A 1g , 4 T 1g b states. Calculated and observed magnetic dipole intensities are in good agreement. A complete analysis of the zero phonon (magnetic dipole) lines and vibronic (magnetic and electric dipole) side-bands associated with transitions to 4 A 1g an...