Hydrogen motion in rutile TiO2

Ion diffusion retarded by diverging chemical susceptibility

For first-order phase transitions, the second derivatives of Gibbs free energy (specific heat and compressibility) diverge at the transition point, resulting in an effect known as super-elasticity along the pressure axis, or super-thermicity along the temperature axis. Here we report a chemical analogy of these singularity effects along the atomic doping axis, where the second derivative of Gibbs free energy (chemical susceptibility) diverges at the transition point, leading to an anomalously high energy barrier for dopant diffusion in co-existing phases, an effect we coin as super-susceptibility. The effect is realized in hydrogen diffusion in vanadium dioxide (VO2) with a metal-insulator transition (MIT). We show that hydrogen faces three times higher energy barrier and over one order of magnitude lower diffusivity when it diffuses across a metal-insulator domain wall in VO2. The additional energy barrier is attributed to a volumetric energy penalty that the diffusers need to pay for the reduction of latent heat. The super-susceptibility and resultant retarded atomic diffusion are expected to exist universally in all phase transformations where the transformation temperature is coupled to chemical composition, and inspires new ways to engineer dopant diffusion in phase-coexisting material systems.

Addition of Organic Acids during PEO of Titanium in Alkaline Solution

This research study describes recent advances in understanding the effects of the addition of organic acids, such as acetic, lactic, citric and phytic acids, on the process of plasma electrolytic oxidation (PEO) on Ti using an alkaline bath. As the plasma developed over the workpiece is central to determine the particular morphological and structural features of the growing oxide, the focus is then on the inter-relationships between the electrolyte and the resultant plasma regime established. In situ optical emission spectroscopy (OES) allowed us to verify a marked plasma suppression when adding low-molecular-weight anions such as acetates, resulting in short-lived and well-distributed discharges. Conversely, when more bulky anions, such as lactates, citrates and phytates, were considered, a less efficient shielding of the electrode caused the build-up of long-lasting and destructive sparks responsible for the formation of thicker coatings, even >30 µm, at the expense of a higher roughness and loss of compactness. Corrosion resistance was tested electrochemically, according to electrochemical impedance spectroscopy (EIS), and weight losses evidenced the coatings produced in the solution containing acetates to be more suitable for service in H2SO4.

New monoclinic ruthenium dioxide with highly selective hydrogenation activity

HxRuO2 acts as a standalone catalyst exhibiting selective hydrogenation under mild conditions. Mobile protons embedded in the oxide lattice play an important role in stabilizing the distorted structure, and facile proton dynamics is key to improving catalytic properties.

Dynamic Tuning of a Thin Film Electrocatalyst by Tensile Strain

We report the ability to tune the catalytic activities for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) by applying mechanical stress on a highly n-type doped rutile TiO₂ films. We demonstrate through operando electrochemical experiments that the low HER activity of TiO₂ can reversibly approach those of the state-of-the-art non-precious metal catalysts when the TiO₂ is under tensile strain. At 3% tensile strain, the HER overpotential required to generate a current density of 1 mA/cm² shifts anodically by 260 mV to give an onset potential of 125 mV, representing a drastic reduction in the kinetic overpotential. A similar albeit smaller cathodic shift in the OER overpotential is observed when tensile strain is applied to TiO₂. Results suggest that significant improvements in HER and OER activities with tensile strain are due to an increase in concentration of surface active sites and a decrease in kinetic and thermodynamics barriers along the reaction pathway(s). Our results highlight that strain applied to TiO₂ by precisely controlled and incrementally increasing (i.e. dynamic) tensile stress is an effective tool for dynamically tuning the electrocatalytic properties of HER and OER electrocatalysts relative to their activities under static conditions.

Electronic structure and migration of interstitial hydrogen in the rutile phase of TiO2

The formation and migration energies of interstitial hydrogen in rutile TiO₂ are obtained from first principles calculations. The computational approach was based on density functional theory with a semilocal generalised-gradient approximation functional, supplemented with an on-site Hubbard term to account for correlation among the Ti 3d electrons. Charge-transition levels are calculated and compared to previous theoretical studies. The donor character of hydrogen is examined in depth, focusing in particular on the tendency to form polaron-like configurations with the unpaired electron trapped at nearby titanium ions. Distinct minimum-energy paths of hydrogen migration and associated energy barriers were determined by the nudged elastic-band method. The present findings show clearly the strong anisotropy in the energy barriers for migration within the open c channels as opposed to migration crossing adjacent channels of the rutile lattice. For the rate-limiting step which leads to macroscopic diffusion along the c axis the corresponding rate and diffusion coefficient were also determined from transition-state theory. The results are discussed in connection to existing measurements of hydrogen diffusion and recent findings from electron paramagnetic resonance, electron-nuclear double resonance and muonium spectroscopies that probed the spatial localization of the electron spin.

Reorientation kinetics of hydroxyl groups in anatase TiO2

The reorientation kinetics of hydrogen in a variety of complexes in the anatase polymorph of TiO₂ was investigated by means of stress-induced dichroism. For the hydrogen-defect resulting in an O-H vibrational mode with a frequency of 3389 cm^-1, the energy barrier separating adjacent equivalent in-plane sites of hydrogen was determined to be independent of the isotope and equal to 0.74 ± 0.02 eV, whereas the attempt frequency was found to be (1.10 ± 0.20) × 10¹² and (0.75 ± 0.15) × 10¹² s^-1 for hydrogen and deuterium, respectively. The defect with vibrational modes at 3412 and 3417 cm^-1 previously assigned to isolated hydrogen did not reveal alignment under the stress up to room temperature, which indicates that the barrier of hydrogen motion is above 0.9 eV.