The present work reports a simple and rapid disassembly/reassembly approach at room temperature to tailor functional metal oxides of 2D and 3D architectures.
Oxygen vacancy concentrations
are critical to the redox/photocatalytic performance of nanoceria,
but their direct analysis is problematic under controlled atmospheres
but essentially impossible under aqueous conditions. The present work
provides three novel approaches to analyze these data from XPS data
for the three main morphologies of nanoceria synthesized under aqueous
conditions and tested using in vacuo analytical conditions. First,
the total oxygen vacancy concentrations are decoupled quantitatively into surface-filled, subsurface-unfilled, and bulk values. Second, the relative surface
areas are calculated for all exposed crystallographic planes. Third,
XPS and redox performance data are deconvoluted according
to the relative surface areas of these planes. Correlations based
on two independent empirical results from volumetric surface XPS, combined with sequential deep XPS
and independent EELS data, confirm that these approaches provide quantitative
determinations of the different oxygen vacancy concentrations. Critically,
the redox/photocatalytic performance depends not on the total oxygen
vacancy concentration but on the concentration of the active sites
on each plane in the form of subsurface-unfilled oxygen vacancies. This is verified by the pH-dependent performance,
which can be increased significantly by exposing these vacancies to
the surroundings. These approaches have significance to the design
and engineering of semiconducting materials exposed to the environment.
Experimental data for Ce-doped TiO2 are interpreted through solubility mechanisms, structural analogies, defect energies, and a new defect equilibria formalism.
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