Protons in Room-Temperature, Post-Synthesis Doping and Defect Engineering of Oxides for Energy Applications

Injection of interstitial atoms by specially prepared oxide surfaces under electrochemical bias from aqueous solutions offers a new and attractive approach for post-synthesis cation doping and defect control for energy applications such as photocatalysis and electrocatalysis. In oxides such as TiO₂, the highly mobile injected interstitials can include those of oxygen (O_i), cations such as Mn (Mn_i) and others that facilitate material fabrication in a regime near room temperature wherein kinetic rather than thermodynamic effects dominate defect behavior.¹ Hydrogen atoms within such oxides, present either adventitiously or intentionally, often act as donors and therefore effectively as protons. These protons participate in solid-state acid-base reactions with chemically basic acceptors such as O_i. Here we employ solid-state isotopic diffusion experiments combined with first principles calculations by density functional theory (DFT) to illustrate how such chemistry may be exploited and controlled to improve material properties. For example, diffusion experiments together with depth-profiling by x-ray photoelectron spectroscopy show that O_i reacts with Mn_i to form interstitial clusters that impede the penetration of Mn and subsequent lattice substitutional. However, isotopic self-diffusion measurements using ¹⁸O, combined with progressive annealing protocols² and DFT, suggest that O_i also acts as a polyfunctional base that reacts with protons to form small interstitial clusters having dissociation energies ranging from 1.3 to 1.9 eV. These clusters comprise a family incorporating various numbers, compositions and configurations of O and H atoms. Corresponding experiments with ZnO suggest that families of such clusters are probably common in semiconducting oxides. Thus, a general strategy may be envisioned wherein H is introduced intentionally into oxides to react with injected O_i and thereby suppress undesired clustering with injected dopant interstitials.
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, “Influence of interstitial cluster families on post-synthesis defect manipulation and purification of oxides using submerged surfaces,” J. Chem. Phys., 161 (2024) 121103.