Fermi level dependence of gas-solid oxygen defect exchange mechanism on TiO2 (110) by first-principles calculations

Reactions of fluid and lattice oxygen mediated by interstitial atoms at the TiO2(110)-water interface

O2 interacts with TiO2 surfaces in numerous aqueous reactions for clean hydrogen production, wastewater cleanup, reduction of CO2 and N2, and O2 sensing. In many cases, these reactions involve reversible exchange of O with the solid, whose participation is usually thought to require oxygen vacancies (VO). Based on measurements of oxygen isotopic self-diffusion in rutile TiO2 under water, this work proposes a different perspective centered on O interstitial atoms (Oi). Experiments with varying concentrations of O2 and mole fractions of 18O show that the (110) surface facilitates O exchange with both the H2O liquid and its dissolved O2. First-principles calculations indicate that on-top and "surface Oi" configurations of adsorbed O participate sequentially in the exchange process. Adsorbed OH appears to provide a single pathway for H2O and O2 to contribute oxygen, although fitting the diffusion data to simple models indicates that H2O contributes more. Because rutile TiO2 is a prototypical metal oxide, this picture based on Oi probably generalizes in many cases to other oxides - explaining important aspects of their thermal, electrochemical, and photochemical reactions with dissolved O2.

Mechanism of creation and destruction of oxygen interstitial atoms by nonpolar zinc oxide(101[combining macron]0) surfaces

Oxygen vacancies (VO) influence many properties of ZnO in semiconductor devices, yet synthesis methods leave behind variable and unpredictable VO concentrations. Oxygen interstitials (Oi) move far more rapidly, so post-synthesis introduction of Oi to control the VO concentration would be desirable. Free surfaces offer such an introduction mechanism if they are free of poisoning foreign adsorbates. Here, isotopic exchange experiments between nonpolar ZnO(101[combining macron]0) and O2 gas, together with mesoscale modeling and first-principles calculations, point to an activation barrier for injection only 0.1-0.2 eV higher than for bulk site hopping. The modest barrier for hopping in turn enables diffusion lengths of tens to hundreds of nanometers only slightly above room temperature, which should facilitate defect engineering under very modest conditions. In addition, low hopping barriers coupled with statistical considerations lead to important qualitative manifestations in diffusion via an interstitialcy mechanism that does not occur for vacancies.

Kinetic Control of Oxygen Interstitial Interaction with TiO2(110) via the Surface Fermi Energy

Atomically clean surfaces of semiconducting oxides efficiently mediate the interconversion of gas-phase O₂ and solid-phase oxygen interstitial atoms (O_i). First-principles calculations together with mesoscale microkinetic modeling are employed for TiO₂(110) to determine reaction pathways, assess appropriate rate expressions, and obtain corresponding activation energies and pre-exponential factors. The Fermi energy (E_F) at the surface influences the rate-determining step for both injection and annihilation of O_i. The barriers range between 0.72-0.82 eV for injection and 0.60-2.34 eV for annihilation and may be manipulated through intentional control of E_F. At equilibrium, the microkinetic model and first-principles calculations indicate that interconversion of O_i species in the first and second sublayers limits the rate. The effective pre-exponential factors for injection and annihilation are surprisingly low, probably resulting from the use of simple Langmuir-like rate expressions to describe a complicated kinetic sequence.