Electron
transfer kinetics between donor and acceptor molecules
in electrolytes has been described by Marcus theory using reorganization
energy (λ), electronic coupling (H), and free
energy difference (ΔG°). In solution,
the molecules can collide freely, while collision occurs only at the
exposed area of the molecules when the donors or the acceptors are
anchored onto an electrode, altering the values of λ and H. To date, these structural effects of electrode-bound
molecules have not been considered in detail. To study geometrical
effects, we fabricate TiO2 electrodes with nine different
donor-(π-bridge)-acceptor type molecules and measure the kinetics
of electron transfer from five different Co complexes in electrolytes.
For densely adsorbed electrodes, the molecules with larger donor moieties
have faster reduction kinetics and the kinetics are independent of
the length of the π-bridge. When the amount of the adsorbed
molecules is reduced, the kinetics become faster and the kinetics
depend on the π-bridge length. These phenomena can be partially
correlated to the increased exposed area of the molecules to the electrolyte.
By fitting the data, we obtain lower λ values for lower dye-loading
conditions, which is not expected if only the effect of solvent molecules
is considered. Obtained H values with various geometries
suggest that it is important not only to increase the exposed area
but also to expose the point giving high H values
to increase the kinetics. One example found is designing molecules
with small molecular orbitals to increase H values,
though this would also give large λ values.
Ti-B-N cast alloys were prepared by a reactive arc-melting technique using elemental titanium and boron nitride powders. The alloys obtained were composites of titanium matrix containing nitrogen of 3 mol% or less and a small amount of TiB reinforcement. The microstructure of the matrix was a cellular structure consisting of elongated dislocation cells with 1-2 µm in width and 5-10 µm in length, for Ti-1B-1N alloy. The TiB particles were rod-like in shape with size of less than 0.5 µm in diameter and agglomerated to form clusters along the grain boundary. The mechanical properties were mainly dependent on the content of nitrogen and the influence of TiB on the mechanical properties was relatively small. The Ti-B-N cast alloys containing suitable concentration of nitrogen, Ti-0.5B-0.5N alloys, had a high strength of 920 MPa with adequate ductility of more than 5%.
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