Photocatalytic Activity of Heterostructured Powders: Nanostructured Titanium Dioxide Shell Surrounding Microcrystalline Cores.

摘要

The efficiency of photocatalytic water splitting with particulate catalysts is low because of the recombination of photogenerated charge carriers and the back reaction of intermediate chemical species. To efficiently separate the intermediate species and charge carriers, a heterostructured ceramic powder composed of a micron sized light absorbing ferroelectric core surrounded by a nanostructured coating has been synthesized. It is hypothesized that the internal fields from the core can enhance the photocatalytic activity of heterostructured powders both under UV and visible light. The present work compares the photocatalytic activity of heterostructured powders with that of their components and other related formulations. The heterostructured powders investigated include micron-sized (mc-) crystallites of ATiO3 (A=Ba, Sr, Pb, Fe) and AFeO3 (A=Bi, La, Y) coated with nanostructured (ns- ) TiO2.;Three parameters that affect the photocatalytic activity of heterostructured powders, namely annealing temperature, cocatalyst loading, and coating thickness, were optimized. The annealing temperature was shown to change the photocatalytic activity of the shell as well as the interface properties. Photocatalysts loaded with 1 wt % Pt showed the greatest photocatalytic hydrogen production rate. An increase of coating thickness, within a certain range, improved the photocatalytic activity of the heterostructured powders.;Ferroelectric PbTiO3, having a narrower band gap than BaTiO 3, was coated with ns-TiO2 and the heterostructure showed enhanced visible light photocatalytic activity for methylene blue degradation (when compared with the components). This confirms that micro-nano heterostructures are promosing for the design of efficient visible-light photocatalysts.;Visible light absorbing mc-FeTiO3 and mc-AFeO3 (A=Bi, La, Y) were also coated with ns-TiO2 for photocatalytic methylene blue degradation under visible light irradiation. Combined with the results from (Ba, Sr, Pb)TiO 3, it is observed that mc-ATiO3/ ns-TiO2 (A=Ba, Sr, Pb, Fe) heterostructures show greatly enhanced photocatalytic activities, when compared with their components, while mc-AFeO3/ns-TiO2 (A=Bi, La, Y) do not. It is speculated that the similarities of the electronic properties (band gap energy and band positions) of the core materials to the TiO 2 coating and the bonding between the core and shell also influence the photocatalytic activity of the micro-nano heterostructures.;The results show that mc-(Ba, Sr)TiO3/ ns-TiO2 heterostructures annealed at 600 °C are more efficient for photocatalytic hydrogen production than their components alone. That a sonomechanical mixture of BaTiO3 and commercial TiO 2 (Degussa P25) did not show enhanced photocatalytic hydrogen production, indicates that the interface between the core and the shell is important for enhanced photocatalytic activity. Comparison of the absorbance coefficient of each component implies that the micron sized core is the primary component that absorbs photons. This is also confirmed by the fact that a non-absorbing Al2O3 core coated with TiO2 showed a similar photocatalytic hydrogen production rate to TiO2 alone. It is argued that electrons from the core can transfer to the surface of the coating to participate in redox reactions. Micron sized BaTiO3 coated with nanostructrued TiO2 is more photocatalytically active than a nano sized BaTiO3/TiO2 heterostructure, which further indicates the advantage of the micro-nano core/shell heterostructure.