Remote Plasma-Generated Oxygen for Interstitial Atom Injection from a TiO₂ Surface

Using surfaces for rapid, controllable injection of interstitial atoms represents a new and versatile tool for tuning material properties after initial synthesis. Lowered chemical coordination at clean metal oxide surfaces facilitates the creation of interstitial atoms. An example is generation of oxygen interstitials (O_i) from adsorbed O atoms with energy barriers near or below roughly 1 eV.¹ The atomic configurations for interstitial injection resemble those for site hopping in the bulk, with barriers only slightly higher. The modest hopping barriers of O_i in oxides, coupled with those for injection, make clean surfaces effective pathways for populating the nearby bulk with O_i near room temperature. Surfaces of several different oxides have proven capable of generating the requisite adsorbed O when submerged in liquid water,¹ with possible applications in defect engineering and isotopic purification² – both done post-synthesis near room temperature. For oxides, however, generating adsorbed O atoms directly from O₂ gas requires high temperatures. Generation from water requires removal of two H atoms, which can be rather slow even with electrochemical augmentation. However, generation of O atoms from a remote O₂ plasma should overcome these difficulties and avoid lattice damage from ions impacting the surface. Here we confirm this hypothesis using isotopic self-diffusion measurements of O in rutile TiO₂ single crystals exposed to a remote plasma containing ¹⁸O₂. Injected O_i fluxes exceed those from comparable electrochemical methods. Temperatures are not limited to 100°C as with liquid water methods, and injection is less susceptible to surface poisoning. However, to minimize solid-state trapping of O_i that limits diffusional penetration, we show it is important to suppress injection of interstitial H originating from stimulated desorption of H₂O from the chamber walls.<br/><br/><br/><u>References</u><br/>1. 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.<br/>2. 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.