Moderate Electron Doping Assists in Dissociating Water on a Transition Metal Oxide Surface (n-SrTiO₃)

Abstract

Water dissociation on transition metal oxide (TMO) surfaces regulates their catalytic activity in aqueous media. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) has differentiated TMO surfaces by the population of their first hydration layer on a scale between water molecularly absorbed and water fully dissociated into hydroxyl groups. Here, we show that electron-doping a single TMO (SrTiO 3 : STO) can also span this range, with the data on lightly (0.1 wt % Nb) and moderately (0.7 wt % Nb) doped STO suggestive of partial and full water dissociation, respectively. The hydroxyl coverage is a factor of 9 greater in 0.7% Nb STO than in 0.1% Nb STO at low relative humidity (∼10 −3 % RH) and a factor of 2 greater at 1% RH, for which multilayers have already formed. Given the lack of a clear differentiation in surface morphology and termination, stoichiometry, or chemical environment of the two doped surfaces, the suggestion is made that the factor of ∼10 increase in electron density is the most likely origin of the marked increase in water dissociation across RH. Nonetheless, since the electron density delocalizes across many titanium oxygen bonds while surface characterizations are site-based, defining similar enough surfaces for which such a collective effect dominates remains a topic of future work. This work provides a benchmark for the effect of delocalized electron density, which can now be further tested by theoretical calculations and a broader material scope of low-to-moderately doped TMOs.

Ultrafast electronic and vibrational spectroscopy of electrochemical transformations on a metal-oxide surface during oxygen evolution from water

“…14 Part II C summarizes original work cited by ref. [15][16] A. The Schottky diode at the n-STO/electrolyte interface:…”

“…In the following, the methodology chosen-a surface scan during data taking-is described first 15,26 . Then, the surface characterization prior to 16 , and post catalysis 15 , is explained.…”

“…While these experiments are conducted at neutral conditions and therefore are limited, they are specific enough for adsorbed water (-OH2) and hydroxylated (-OH) sites to lend a molecular picture. In Figure 7a the data at 1% RH are shown for both 0.1% Nb and 0.7% Nb STO 16 ; by 1 % RH, at least a full hydration layer has accrued. They are fit with cross-sections representing lattice oxygen, hydroxylated sites, adsorbed water, water vapor, and carbon contaminants.…”

Ultrafast Electronic and Vibrational Spectroscopy of Electrochemical Transformations on a Metal-Oxide Surface during Oxygen Evolution Catalysis

“…In the following, the methodology chosena surface scan during data takingis described first . Then, the surface characterization prior to and post catalysis is explained.…”

“…AP-XPS provides a unique opportunity to investigate the surface under increasing ambient relative humidity (RH), where the first hydration layer accrues on the surface from vacuum conditions. , While these experiments are limited to neutral conditions, they are specific enough for adsorbed water (−OH 2 ) and hydroxylated (−OH) sites to lend a molecular picture. In Figure a, the data at 1% RH are shown for both 0.1% Nb and 0.7% Nb STO; by 1% RH, at least a full hydration layer has accrued. They are fit with cross-sections representing lattice oxygen, hydroxylated sites, adsorbed water, water vapor, and carbon contaminants.…”