Abstract
Nanocrystalline anatase titanium dioxide is an efficient electron transport material for solar cells and photocatalysts. However, low-coordinated Ti cations at surfaces introduce low-lying Ti 3d states that can trap electrons, reducing charge mobility. Here, a number of dopants (V, Sb, Sn, Zr, and Hf) are examined to replace these low-coordinated Ti cations and reduce electron trapping in anatase crystals. V, Sb, and Sn dopants act as electron traps, while Zr and Hf dopants are found to prevent electron trapping. We also show that alkali metal dopants can be used to fill surface traps by donating electrons into the 3d states of low-coordinated Ti ions. These results provide practical guidance on the optimization of charge mobility in nanocrystalline TiO2 by doping.
Highlights
We examined two approaches to reduce the contribution of low-coordinated surface Ti cations toward electron trapping in nanocrystals of anatase TiO2: (1) substitutional doping to remove the Ti 3d states associated with electron traps and (2) introduction of electron-donating interstitials to fill the associated Ti 3d electron trap states
Increasing the concentration of these dopants could eliminate more of the Tisurf electron traps leading to improved mobility in a-TiO2. Using nonreducible dopants such as Zr is beneficial to reduce electron trapping in anatase TiO2 since this species removes low-lying electron trapping states at the CBM
Using electron-donating species is beneficial to reduce electron trapping in anatase TiO2
Summary
The self-trapping of electron and hole charge carriers in anatase TiO2 affects electron transport, charge recombination rates, and overall device efficiency. Shallow traps lie close to the conduction or valence band edges and mediate transport in TiO2,32 while deeper traps in the band gap promote carrier recombination.[33] Electrochemical and photoluminescence studies provide valuable insights into the nature of traps in metal oxide samples.[34−37] There is no direct evidence to suggest electron that electron trapping does trapping occur at occurs surfaces in of abnualtkasea-TTiiOO22.,37b−u39t
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