Ca-assisted nitrogen-rich molecular approach for the synthesis of monodisperse tantalum oxide and (oxy)nitride nanocrystals for visible light–induced water oxidation

The visible-light-induced photocatalytic water oxidation activity without cocatalyst has been demonstrated for the first time on tantalum (oxy)nitride nanocrystals synthesized by a Ca-assisted nitrogen-rich molecular approach. Monodisperse TaON and Ta3N5 nanocrystals were synthesized by using suitable urea/Ta ratios in the presence of Ca2+ ions. The as-synthesized (oxy)nitrides are in the form of spherically-shaped and well-defined nanocrystals with an average diameter of ca. 7 nm. Both nanocrystals showed absorption edges in the visible region. The band gap energies of TaON and Ta3N5 nanocrystals were determined from the Tauc’s plots of the UV–Vis diffuse reflectance spectra to be 2.27 and 2.08 eV for indirect allowed transitions, respectively. The O2 evolution rate was increased and reached the maximum in the following order: 2.01, 3.32, and 4.66 µmol·h−1 for the samples synthesized with Ca2+/urea molar ratio of 0.10, 0.50, and 0.25, respectively, due to the decrease in the optical band gap.