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Interstitial atoms rather than vacancies mediate the exchange of oxygen between TiO₂and liquid water containing dissolved O₂. 

Injection of interstitial atoms by specially prepared surfaces submerged in liquid water near room temperature offers an attractive approach for post-synthesis defect manipulation and isotopic purification in device structures. However, this approach can be limited by trapping reactions that form small defect clusters. The compositions and dissociation barriers of such clusters remain mostly unknown. This communication seeks to address this gap by measuring the dissociation energies of oxygen interstitial traps in rutile TiO2 and wurtzite ZnO exposed to liquid water. Isotopic self-diffusion measurements using 18O, combined with progressive annealing protocols, suggest the traps are small interstitial clusters with dissociation energies ranging from 1.3 to 1.9 eV. These clusters may comprise a family incorporating various numbers, compositions, and configurations of O and H atoms; however, in TiO2, native interstitial clusters left over from initial synthesis may also play a role. Families of small clusters are probably common in semiconducting oxides and have several consequences for post-synthesis defect manipulation and purification of semiconductors using submerged surfaces. 

Low bond coordination of surface atoms facilitates the injection of oxygen interstitial atoms into the bulk near room temperature from the clean surfaces of semiconducting metal oxides when exposed to liquid water, opening new prospects for postsynthesis defect engineering and isotopic fractionation. The injection rate and penetration depth vary considerably under identical experimental conditions, however, with the adsorption of adventitious carbon suggested as the cause. For water-submerged rutile TiO2(110) and wurtzite ZnO(0001), this work bolsters and refines that hypothesis by combining the isotopic self-diffusion measurements of oxygen with characterization by x-ray photoelectron spectroscopy and atomic force microscopy. Adventitious carbon likely diminishes injection rates by poisoning small concentrations of exceptionally active surface sites that either inject O or dissociate adsorbed OH to injectable O. These effects propagate into the penetration depth via the progressive saturation of Oi traps near the surface, which occurs less extensively as the injected flux decreases. 

Oxygen vacancies (V O ) influence many properties of ZnO in semiconductor devices, yet synthesis methods leave behind variable and unpredictable V O concentrations. Oxygen interstitials (O i ) move far more rapidly, so post-synthesis introduction of O i to control the V O 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(101̄0) and O 2 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.