The photocatalytic activity of materials for water splitting is limited by the recombination of photogenerated electron-hole pairs as well as the back-reaction of intermediate species. This review concentrates on the use of electric fields within catalyst particles to mitigate the effects of recombination and back-reaction and to increase photochemical reactivity. Internal electric fields in photocatalysts can arise from ferroelectric phenomena, p-n junctions, polar surface terminations, and polymorph junctions. The manipulation of internal fields through the creation of charged interfaces in hierarchically structured materials is a promising strategy for the design of improved photocatalysts.

Using in situ synchrotron measurements of total reflection x-ray fluorescence, we find evidence of strontium surface segregation in (001)-oriented La0.7Sr0.3MnO3 thin films over a wide range of temperatures (25–900 °C) and oxygen partial pressures (pO2=0.15–150 Torr). The strontium surface concentration is observed to increase with decreasing pO2, suggesting that the surface oxygen vacancy concentration plays a significant role in controlling the degree of segregation. Interestingly, the enthalpy of segregation becomes less exothermic with increasing pO2, varying from −9.5 to −2.0 kJ/mol. In contrast, the La0.7Sr0.3MnO3 film thickness and epitaxial strain state have little impact on segregation behavior.

Titania films, 15-100 nm thick, have been grown on BaTiO 3 substrates and used to photochemically reduce Ag þ to Ag 0 and oxidize Pb 2þ to Pb 4þ under ultraviolet illumination. Atomic force microscopy was used to show that the reactions are spatially selective and that the pattern of products on the film surface reproduces the pattern of products on the bare substrate. The influence of the substrate on the pattern of reactants is diminished as the film thickness increases and is quenched when the film is donordoped with Nb. The results indicate that for thin (15 nm) films, dipolar fields from the ferroelectric domains cause carriers generated in the substrate to travel through the film to react on the surface.

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Using pulsed laser deposition (PLD), metastable perovskite YMnO3 films were grown from a hexagonal YMnO3 target. The stabilization of the metastable phase is a result of the structural similarity between it and the perovskite substrates. X-ray and electron diffraction confirm the films' epitaxial nature but evince that the orientation and residual strains depend on the substrate. The implication of these findings is that PLD is a simple synthetic approach to stabilizing new, more complex, metastable perovskites.

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Photochemical reactions that deposit insoluble products on catalytic surfaces have been used to probe the anisotropy of the reactivity of SrTiO 3 microcrystals. Both reduced and oxidized products are formed preferentially on {100} surfaces. It is proposed that the anisotropic photochemical reactivity can be explained by the electronic band structure. Because direct optical transitions for charge carriers having momentum vectors in the <100> direction overlap well with the spectral distribution of the absorbed photons, more photogenerated carriers are moving toward {100} surfaces than other surfaces and, as a result, {100} surfaces are more active. Knowledge of the electronic band structure and the spectral distribution of the light allows predictions to be made about the anisotropic reactivity of photocatalysts with other crystal structures.

Titania films were grown on BaTiO 3 (BTO) substrates by pulsed laser deposition to create (001) Anatase ||(100) BTO , (100) Rutile ||(111) BTO , and (110) Rutile ||(110) BTO heterostructures. The photochemical reactivity was evaluated by observing the amount of silver reduced by each film under identical conditions. The thickest films (100 nm) had reactivities that varied with the phase of titania and the crystal orientation. Thin films (15 nm), on the other hand, had reactivities that were approximately the same. Furthermore, reaction products were spatially distributed in patterns consistent with the underlying domain structure of the substrate. For the case of (110) Rutile ||(110) BTO , the thin films are more reactive than the thick films, showing that the dipolar fields in the ferroelectric substrate enhance the reactivity of (110) rutile.