Isotopic Purification of TiO₂ Obtained by Surface-Enhanced Interstitial Injection

Use of isotopically pure semiconductors is important in the fabrication of devices for quantum computing, as spinning nuclei require isolation from environmental perturbation by isotopically pure layers having nuclear spins of zero. Raw materials of sufficiently high isotopic purity are expensive and difficult to obtain, so a post-synthesis method for removing isotopic impurities would be valuable. Through isotopic self-diffusion measurements of oxygen in rutile TiO₂ single crystals immersed in water near room temperature, we demonstrate fractionation of ¹⁸O by a factor of three below natural abundance in a near-surface region up to 10 nm wide.¹ Application of electrochemical biases increases the width of this region by a factor up to five. The submerged surface injects O interstitials that displace lattice ¹⁸O deeper into the solid due to the statistics of interstitialcy-mediated diffusion combined with steep chemical gradients of O interstitials. The steep gradients arise because lowered chemical coordination at clean metal oxide surfaces facilitates the creation of interstitial atoms from adsorbed atoms with energy barriers near or below roughly 1 eV.² The atomic configurations for interstitial injection resemble those for site hopping in the bulk, with barriers only slightly higher. The modest hopping barriers of many interstitial species in oxides, coupled with those for injection, make clean surfaces efficient pathways for populating the nearby bulk with O_i near room temperature. This physical picture is not restricted only to oxygen or TiO₂. Indeed we also show that, when metallic cations such as Mn are present in the aqueous phase, application of electrochemical bias enables the efficient injection of the corresponding metallic interstitials into TiO₂. Such an approach for the host cation (Ti in this case) should lead to isotopic purification of the lattice cation.<br/><br/>References<br/><br/>1. Heonjae Jeong and Edmund G. Seebauer, “Strong Isotopic Fractionation of Oxygen in TiO₂ Obtained by Surface-Enhanced Solid-State Diffusion,” <i>J. Phys. Chem. Lett.</i>, <b>13</b> (2022) 9841-9847.<br/>2. Heonjae Jeong, Elif Ertekin and Edmund G. Seebauer, “Surface-Based Post-synthesis Manipulation of Point Defects in Metal Oxides Using Liquid Water,” <i>ACS Appl. Mater. Interfaces</i>, <b>14</b> (2022) 34059-34068.