Post-Synthesis Metal Cation Doping of TiO₂ at Room Temperature by Electrochemically Augmented Interstitial Atom Injection from Surfaces

Metal doping of semiconducting metal oxides has been long investigated for photocatalysis applications including water splitting for solar hydrogen production. However, incorporation of metal cation dopants during synthesis is often not readily controllable with respect to concentration, spatial distribution, or formation of secondary chemical states or phases. Existing post-synthesis doping methods also have problems – ion implantation causes irreversible lattice damage, and inward diffusion suffers from poor controllability and requires high-temperature annealing that degrades nanostructures. Here we demonstrate an electrochemically augmented method for metal dopant introduction (specifically, Mn) into single-crystal rutile TiO₂ from aqueous solution by injection of dopant interstitial atoms from surfaces. The concept generalizes an idea already demonstrated for injecting adsorbed O into oxides as interstitials,¹ whereby lowered chemical coordination at clean surfaces facilitates the conversion of adsorbates into interstitials with energy barriers near or even below 1 eV. The atomic configurations for interstitial injection resemble those for site hopping in the bulk, with barriers only slightly higher. The modest hopping barriers of metal interstitials in oxides suggest a similar concept may work for metal doping, especially with electrochemical facilitation of any change in oxidation state during injection. We describe diffusion measurements near room temperature using a conventional 3-electrode electrochemical cell with MnCl₂ dissolved in water labeled with ¹⁸O. Post-diffusion depth profiling of the TiO₂ with secondary ion mass spectrometry (SIMS) demonstrates Mn penetration up to depths of 200 nm after 1 h in an exponentially shaped profile, with near-surface concentrations &gt;1 atom%. Depth-profiling x-ray photoelectron spectroscopy reveals Mn mainly in oxidation states of +4 and +2 when an N₂ atmosphere is used during diffusion. SIMS shows that ¹⁸O simultaneously injects into the solid. Evidence is presented for clustering of some Mn and O interstitials to form small complexes.<br/> <br/>References<br/><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.