Electrochemical Enhancement of Interstitial-Based ¹⁸O Isotopic Fractionation in TiO₂

Isotopically pure materials are critical for quantum computing and cooling of electronic devices, but obtaining the requisite purities is technologically challenging. Submersion of specially prepared wide-bandgap oxide surfaces into aqueous solutions near room temperature injects oxygen interstitial atoms (O_i) into the solid.¹ The highly mobile O_i facilitates post-synthesis processing in a regime wherein kinetic rather than thermodynamic effects dominate defect behavior. In particular, a counterintuitive “uphill diffusion” phenomenon becomes possible that offers a solid-state mechanism for reducing the concentration of isotopic impurities through the statistics of interstitialcy-mediated diffusion combined with steep interstitial gradients.² Isotopic fractionation emerges as a near-surface minimum in the ¹⁸O concentration that is below the natural abundance of ¹⁸O. Here, isotopic self-diffusion measurements of ¹⁸O in single-crystal rutile TiO₂ show that an applied potential bias increases the width of the ¹⁸O-depleted region from approximately 2.8 nm to as high as 90 nm. Isotopic depletion increases from approximately 1.4x below natural abundance to as much as 3x. In addition, injection fluxes under potential bias increase by up to 25x, resulting in penetration of O-interstitials up to 10 mm below the surface. Important side benefits thereby arise for defect engineering.¹ Surprisingly, the augmentations appear independent of applied potential. Possible mechanisms for these electrochemical effects include removal of surface contamination that otherwise inhibits surface injection of O_i, electrostatic effects from TiO₂ surface charging, and the application of an electric field. Microkinetic modeling simulations of ¹⁸O injection and diffusion demonstrate how the uphill diffusion mechanism induces fractionation to evolve over time. This physical picture applies not only to oxygen and TiO₂, but also to metal cations and other wide-bandgap semiconductors.
References

1. Heonjae Jeong, Elif Ertekin and Edmund G. Seebauer, “Surface-Based Post-synthesis Manipulation of Point Defects in Metal Oxides Using Liquid Water,” ACS Appl. Mater. Interfaces, 14 (2022) 34059-34068.
2. Heonjae Jeong and Edmund G. Seebauer, “Strong Isotopic Fractionation of Oxygen in TiO₂ Obtained by Surface-Enhanced Solid-State Diffusion,” J. Phys. Chem. Lett., 13 (2022) 9841-9847.