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 WO 3 nanosheets (0.75 nm × 90 ± 38 nm), obtained by exfoliation from Bi 2 W 2 O 9 . 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-WO 3 ( E G = 2.88 eV) and Bi 2 W 2 O 9 ( E G = 2.81 eV), relative to bulk WO 3 ( E G = 2.68 eV). Surface photovoltage measurements on nano-WO 3 films on a F:SnO 2 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 AgNO 3 and (NH 4 ) 2 Ce(NO 3 ) 6 solutions under full spectrum (>250 nm) or visible only (>400 nm) irradiation. The highest initial O 2 evolution rates (69.7 μmol h –1 for bulk and 35.5 μmol h –1 for nano-WO 3 ) are observed under >250 nm illumination in the presence of 8.3 mM AgNO 3 (aq). Quantum efficiencies (at 375 nm) reach 1.43% and 1.55% for bulk and nano-WO 3, respectively. Electrochemical measurements reveal large water oxidation overpotentials (0.96 V) for both nano- and bulk-WO 3 . On the basis of photo-onset measurements, the conduction band edges in nano-/bulk-WO 3 are at +0.11/+0.23 V, respectively. Overall, the data show that the photoelectrochemical water oxidation ability of WO 3 is maintained in 0.75 nm nanocrystal WO 3 sheets, although more energetic photons are required because of the extended band gap.