Single-Crystal Tungsten Oxide Nanosheets: Photochemical Water Oxidation in the Quantum Confinement Regime

Here we investigate the structure, photophysics, and photocatalytic water splitting properties of single-crystalline WO3 nanosheets (0.75 nm x 90 +/- 38 nm), obtained by exfoliation from Bi2W2O9. Upon delamination, the nanosheets undergo a structural change from tetragonal symmetry in the parent material to monoclinic, as confirmed by powder X-ray diffraction and electron microscopy. Diffuse reflectance optical spectra show band gap energies consistent with quantum confinement in nano-WO3 (E-G = 2.88 eV) and Bi2W2O9 (E-G = 2.81 eV), relative to bulk WO3 (E-G = 2.68 eV). Surface photovoltage measurements on nano-WO3 films on a F:SnO2 substrate demonstrate photochemical carrier formation under band gap excitation and irreversible trapping of holes. Photochemical oxygen formation is observed with 50 mg of the material in aqueous AgNO3 and (NH4)(2)Ce(NO3)(6) solutions under full spectrum (> 250 nm) or visible only (> 400 nm) irradiation. The highest initial O-2 evolution rates (69.7 mu mol h(-1) for bulk and 35.5 mu mol h(-1) for nano-WO3) are observed under > 250 nm illumination in the presence of 8.3 mM AgNO3(aq). Quantum efficiencies (at 375 nm) reach 1.43% and 1.55% for bulk and nano-WO3, respectively. Electrochemical measurements reveal large water oxidation overpotentials (0.96 V) for both nano- and bulk-WO3. On the basis of photo-onset measurements, the conduction band edges in nano-/bulk-WO3 are at +0.11/+0.23 V, respectively. Overall, the data show that the photoelectrochemical water oxidation ability of WO3 is maintained in 0.75 nm nanocrystal WO3 sheets, although more energetic photons are required because of the extended band gap.