Nanocrystalline heterojunction materials

#The present invention relates to nanocrystalline materials. More specifically, mesoporous nanocrystalline titanium dioxide materials comprising titanium dioxide and a second metal oxide are disclosed.

#Since the discovery that titanium dioxide can act as a photocatalyst for the splitting of water, the substance has attracted the attention of scientists. The substance, however, exhibits two principal physical limitations. First, titanium dioxide absorbs light at energies greater than 3.2 eV; well outside the most intense region of the ambient solar spectrum (centered at ˜2.6 eV). Second, since titanium dioxide functions as a heterogeneous catalyst, catalytic activity is limited by surface area.

#Titanium dioxide exists in at least three crystalline forms: anatase, rutile, and brookite. Anatase is the form that exhibits the highest catalytic activity and much effort has been directed toward providing anatase powders with increased stability and high surface areas.

#FIG. 1 shows XRPD diffraction data for a series of TiO2—(MoO3)x core-shell compounds.

#FIG. 2 shows a plot of the TiO2—(MoO3)x photoabsorption energy (PE) as a function of TiO2—(MoO3)x core-shell diameter (i) TiO2—(MoO3)0.18: 80 Å, PE=2.88 eV; (ii) TiO2—(MoO3)0.54: 60 Å, PE=2.79 eV; (iii) TiO2—(MoO3)1.1: 50 Å, (iv) TiO2—(MoO3)1.8: 40 Å PE=2.60 eV.

#FIG. 3 shows the Mo K-edge XANES data for α-MoO3 (□), TiO2—(MoO3)0.18 (Δ), TiO2—(MoO3)0.54 (x), TiO2—(MoO3)1.1 (O), and TiO2—(MoO3)1.8(⋄), and MoO2 (solid line).

#New heterojunction materials exhibit red-shifted photoabsorption energies (PE's) that contrast sharply with the blue-shifted PE's typical of zirconium oxide stabilized nanocyrstalline titanium oxide materials and nanocrystalline materials in general. In some embodiments, the new titanium dioxide heterojunction materials show a PE that corresponds closely with the ambient solar maxiumum (˜2.6 eV).

#Disclosed mesoporous nanocrystalline titanium dioxide heterojunction materials comprise a nanocrystalline titanium dioxide phase chemically bonded to a phase of a second metal oxide through a heterojunction formed at an interface between the phases. As used herein, a heterojunction is a boundary between a nanocrystalline titanium dioxide phase and a second metal oxide phase along which their electronic oribitals overlap. Orbital overlap at the interface between the two oxide phases allows electronic transitions to occur between the valence band (HOMO-highest occupied molecular orbital) of one oxide phase and the conduction band (LUMO-lowest unoccupied molecular orbital) of the second oxide phase. Nanocrystalline heterojunction materials therefore possess unique photophysical properties, not seen for materials where contacting metal oxide phases are not chemically bonded (e.g., in zirconia stabilized mesoporous nanocrystalline titantium dioxide). For example, the disclosed nanocrystalline titanium dioxide heterojunction materials are characterized by a red-shift in PE that correlates with the extent of chemical interaction between the nanocrystalline titanium dioxide phase and the second metal oxide phase at their interface. In some instances the red-shift is sufficient to bring the PE of the material below the PE's of both titanium dioxide and the second metal oxide. Heterojunction materials may exhibit photochromism and fluorescencent properties not seen for either metal oxide alone.

#In some embodiments, the disclosed nanocrystalline titanium dioxide heterojunction materials comprise a nanocrystalline titantium dioxide core and a shell of a second metal oxide, wherein the two oxide phases are chemically bonded through a heterojunction at the interface between the core and the shell. The shell may vary from less than a complete monolayer of the second metal oxide on the surface of the core to one or more monolayers of the second metal oxide phase that substantially surround the core. In particular embodiments, a molybdenum oxide shell surrounds a nanocrystalline anatase core. In more particular embodiments the molybdenum oxide shell has a structure substantially similar to α-MoO3. In these materials, the PE systematically shifts to the red as the molybdenum oxide shell varies in size from less than a monolayer to about two full monolayers.