Surface-Based Post-synthesis Manipulation of Point Defects in Metal Oxides Using Liquid Water

Abstract

Initial synthesis of semiconducting oxides leaves behind poorly controlled concentrations of unwanted atomic-scale defects that influence numerous electrical, optical, and reactivity properties. We have discovered through self-diffusion measurements and first-principles computations that poison-free oxide surfaces inject interstitial oxygen atoms into the crystalline solid when simply contacted with liquid water near room temperature. These interstitials diffuse quickly to depths of 20 nm–2 μm and are likely to eliminate prominent classes of unwanted defects or neutralize their action. The mild conditions of operation access a regime for oxide fabrication that relaxes important thermodynamic constraints that hamper defect regulation by conventional methods at higher temperatures. The surface-based approach appears well-suited for use with nanoparticles, porous oxides, and thin films for applications in advanced electronics, renewable energy storage, photocatalysis, and photoelectrochemistry.

Strong Isotopic Fractionation of Oxygen in TiO2 Obtained by Surface-Enhanced Solid-State Diffusion

“…A striking feature, visible at both pH levels, is the large "valley" in label concentration near the surface that is up to about 10 nm wide. Counterintuitively, this isotopically depleted valley emerges, despite the enrichment of the liquid water to 10% 18 O. Depletion of the label within the valley region drives the 18 O concentration down to about a factor of 3 below the natural abundance level. 18 O lost from the valley builds up as a peak lying deeper in the bulk.…”

“…Counterintuitively, this isotopically depleted valley emerges, despite the enrichment of the liquid water to 10% 18 O. Depletion of the label within the valley region drives the 18 O concentration down to about a factor of 3 below the natural abundance level. 18 O lost from the valley builds up as a peak lying deeper in the bulk. The amount of 18 O contained within the peak exceeds the amount depleted from the valley, with the difference equaling the total amount of injected 18 O.…”

“…Steep gradients in the concentration of oxygen interstitials (O i ) may be produced by clean surfaces of several binary oxides, which inject sizable fluxes of O i when contacted with liquid water near room temperature. 18 The equilibrium concentration of O i at such temperatures is extremely small, so that sizable injected fluxes produce large concentration gradients. It is therefore plausible that substantial isotopic fractionation should occur near the surface.…”

“…Figure 1 shows example 18 O concentration profiles after 1 h water exposures at pH values of 5 and 9, both at several temperatures. A striking feature, visible at both pH levels, is the large "valley" in label concentration near the surface that is up to about 10 nm wide.…”

“…18 O lost from the valley builds up as a peak lying deeper in the bulk. The amount of 18 O contained within the peak exceeds the amount depleted from the valley, with the difference equaling the total amount of injected 18 O.…”