Initial synthesis of semiconducting oxides leaves behind poorly controlled concentrations of unwanted atomic-scale defects that influence numerous electrical, optical, and reactivity properties. We have discovered through self-diffusion measurements and first-principles computations that poison-free oxide surfaces inject interstitial oxygen atoms into the crystalline solid when simply contacted with liquid water near room temperature. These interstitials diffuse quickly to depths of 20 nm–2 μm and are likely to eliminate prominent classes of unwanted defects or neutralize their action. The mild conditions of operation access a regime for oxide fabrication that relaxes important thermodynamic constraints that hamper defect regulation by conventional methods at higher temperatures. The surface-based approach appears well-suited for use with nanoparticles, porous oxides, and thin films for applications in advanced electronics, renewable energy storage, photocatalysis, and photoelectrochemistry.

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Heterogeneous Pt–Ru catalysts are synthesized by atomic layer deposition (ALD) of Ru onto a porous Pt mesh and employed as the anode in direct methanol solid oxide fuel cells (DMSOFCs). The degree of Ru coverage is controlled by the number of deposition cycles employed in catalyst preparation; samples are prepared using 50, 100, 200, 300, and 400 cycles. The dispersed nature of the ALD Ru coating is confirmed by auger electron spectroscopy and high-resolution transmission electron microscopy. DMSOFC performance is measured at several temperatures between 300 and 450 °C. Optimal ALD Ru coverage results in DMSOFC power and electrode impedance values close to ones from SOFCs with Pt electrodes running on hydrogen. Thermal stability is also improved significantly, preventing agglomeration of the Pt mesh. X-ray photoelectron spectroscopy is used to analyze the chemical properties of the surface and confirms that increased ALD Ru coverage results in a dramatic reduction in the amount of surface-bound carbon monoxide (CO) present after cell operation. This suggests that improved anode kinetics resulted from the reduction of the CO-passivated Pt layer.

Atomically clean surfaces of semiconducting oxides efficiently mediate the interconversion of gas-phase O 2 and solidphase oxygen interstitial atoms (O i ). First-principles calculations together with mesoscale microkinetic modeling are employed for TiO 2 (110) to determine reaction pathways, assess appropriate rate expressions, and obtain corresponding activation energies and preexponential 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.

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Properties related to transport such as self-diffusion coefficients are relevant to fuel cells, electrolysis cells, and chemical/gas sensors. Prediction of self-diffusion coefficients from first-principles involves precise determination of both enthalpy and entropy contributions for point defect formation and migration. We use first-principles density functional theory to estimate the self-diffusion coefficient for neutral O0i and doubly ionized Oi2- interstitial oxygen in rutile TiO2 and compare the results to prior isotope diffusion experiments. In addition to formation and migration energy, detailed estimates of formation and migration entropy incorporating both vibrational and ionization components are included. Distinct migration pathways, both based on an interstitialcy mechanism, are identified for O0i and Oi2-. These result in self-diffusion coefficients that differ by several orders of magnitude, sufficient to resolve the charge state of the diffusing species to be Oi2- in experiment. The main sources of error when comparing computed parameters to those obtained from experiment are considered, demonstrating that uncertainties due to computed defect formation and migration entropies are comparable in magnitude to those due to computed defect formation and migration energies. Even so, the composite uncertainty seems to limit the accuracy of first-principles calculations to within a factor of ±103, demonstrating that direct connections between computation and experiment are now increasingly possible.

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...

In the same way that gases interact with oxide semiconductor surfaces from above, point defects interact from below. Previous experiments have described defect–surface reactions for TiO2(110), but an atomistic picture of the mechanism remains unknown. The present work employs computations by density functional theory of the thermodynamic stabilities of metastable states to elucidate possible reaction pathways for oxygen interstitial atoms at TiO2(110). The simulations uncover unexpected metastable states including dumbbell and split configurations in the surface plane that resemble analogous interstitial species in the deep bulk. Comparison of the energy landscapes involving neutral (unionized) and charged intermediates shows that the Fermi energy EF exerts a strong influence on the identity of the most likely pathway. The largest elementary-step thermodynamic barrier for interstitial injection trends mostly downward by 2.1 eV as EF increases between the valence and conduction band edges, while that for annihilation trends upward by 2.1 eV. Several charged intermediates become stabilized for most values of EF upon receiving conduction band electrons from TiO2, and the behavior of these species governs much of the overall energy landscape.