Mechano-Electrochemistry and Fuel-Forming Mechano-Electrocatalysis on Spring Electrodes

Each material, in principle, possesses a continuum of electrochemical and electrocatalytic properties that can be reversibly tuned by mechanical stress over its elastic range. As an initial test of this hypothesis we investigate stainless steel extension springs as electrodes. Stretching the springs reversibly doubles the heterogeneous rate constant for electron transfer to a redox species in solution, Ru­(NH₃)₆Cl₃, while the charge transfer rate through a surface film of Ni­(II/III) oxy-hydroxide increases ∼4-fold. Straining the springs near their elastic limit in 1 M NaOH increases the electrcatalytic hydrogen evolution current by ∼50% and the oxygen evolution current by ∼300%. Thus, even the small elastic strain (∼0.1% lattice deformation) that can be applied by stretching a spring leads to significant and reversible increases in the rates of: 1) electron transfer to a redox couple in solution, 2) charge transport through a surface film, and 3) electrocatalysis

Semiconductor-to-Metal Transition in Rutile TiO₂ Induced by Tensile Strain

We report the first observation of a reversible, degenerate doping of titanium dioxide with strain, which is referred to as a semiconductor-to-metal transition. Application of tensile strain to a ∼50 nm film of rutile TiO₂ thermally grown on a superelastic nitinol (NiTi intermetallic) substrate causes reversible degenerate doping as evidenced by electrochemistry, X-ray photoelectron spectroscopy (XPS), and conducting atomic force microscopy (CAFM). Cyclic voltammetry and impedance measurements show behavior characteristic of a highly doped <i>n</i>-type semiconductor for unstrained TiO₂ transitioning to metallic behavior under tensile strain. The transition reverses when strain is removed. Valence band XPS spectra show that samples strained to 5% exhibit metallic-like intensity near the Fermi level. Strain also induces a distinct transition in CAFM current–voltage curves from rectifying (typical of an <i>n</i>-type semiconductor) to ohmic (metal-like) behavior. We propose that strain raises the energy distribution of oxygen vacancies (<i>n</i>-type dopants) near the conduction band and causes an increase in carrier concentration. As the carrier concentration is increased, the width of the depletion region is reduced, which then permits electron tunneling through the space charge barrier resulting in the observed metallic behavior