Abstract
The trapping of electrons at surfaces of nanocrystalline titanium dioxide can be decisive in controlling performance for diverse applications in photocatalysis, energy storage, and solar energy generation. Here, we employ first-principles calculations to elucidate the factors which influence electron trapping for all low index surfaces of rutile TiO2. We show that different surface orientations exhibit markedly different electron affinities: some preferring to trap electrons with others repelling electrons. We demonstrate that local variations in trapping energy are linked to variations in electrostatic potential and ion coordination providing atomistic insight into this effect. The equilibrium nanocrystal morphology exposes both electron-trapping and electron-repelling facets and therefore is predicted to possess highly anisotropic electron-trapping properties. We discuss how knowledge of surface-specific trapping properties can be utilized to design a number of nanocrystal morphologies which may offer improved performance for applications.
Highlights
Electron trapping in nanocrystalline titanium dioxide (TiO2) is an issue of fundamental and technological significance underpinning applications in areas such as solar energy generation, photocatalysis, and portable energy storage.[1−6] Both firstprinciples calculations[7−10] and electron paramagnetic resonance (EPR) studies[11−14] provide clear evidence that electrons form small polarons in the bulk rutile TiO2 crystal
We note that this issue is not specific to the density functional theory (DFT)+U approach but applies in general to other SI-corrected methods such as hybrid functionals
We note that a scheme has recently been developed that can correct calculated total energies for charged defects at surfaces to remove the effect of such spurious electrostatic interactions.[59]
Summary
Electron trapping in nanocrystalline titanium dioxide (TiO2) is an issue of fundamental and technological significance underpinning applications in areas such as solar energy generation, photocatalysis, and portable energy storage.[1−6] Both firstprinciples calculations[7−10] and electron paramagnetic resonance (EPR) studies[11−14] provide clear evidence that electrons form small polarons in the bulk rutile TiO2 crystal. The trapping of photogenerated charge at nanocrystal surfaces can facilitate oxidation or reduction reactions. This is the principle of operation of TiO2 photocatalysts which find applications in water splitting,[3] water purification,[19] and self-cleaning glass.[5] optimal nanocrystals for photocatalysis should expose surfaces which can sustain a high concentration of active Ti3+ surface sites. A different example is the trapping of photoinjected electrons in the photoanode of TiO2 dye-sensitized solar cells (DSSCs).[20]
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