Characterization and Mechanism of Efficient Visible-Light-Driven Water Oxidation on an in Situ N₂-Intercalated WO₃ Nanorod Photoanode

Nanorod architecture of a N₂-intercalated WO₃ photoanode has been developed by emphasizing the dual role of N₂H₄, which functioned simultaneously as a structure-directing agent and as a nitrogen source for in situ N₂ intercalation. The N content for WO₃–N₂H₄ increased with increase of calcination temperature from 350 to 420 °C and thereafter decreased up to 550 °C. This is different from the monotonously decreased N content with the calcination temperature increase for WO₃–NH₃. X-ray diffraction data indicated that the phase-pure monoclinic WO₃ crystals are formed over 350 °C calcination for WO₃–N₂H₄, in contrast to a mixed phase of hexagonal and monoclinic WO₃ crystals below 500 °C for WO₃–NH₃.The crystallinity of the monoclinic phase for WO₃–N₂H₄ is higher than those of WO₃-neat and WO₃–NH₃ at each calcination temperature of 350–550 °C. The β values of lattice parameters of the monoclinic phase changed significantly with the calcination temperature; this is consistent with the calcination temperature dependence of the N content, clarifying the intercalation of N₂ in the WO₃ lattice. The UV–visible diffuse reflectance spectra of WO₃–N₂H₄ exhibited a shoulder at 470–630 nm, which became more intense as the calcination temperature increased from 350 °C and then decreased up to 550 °C through the maximum at 420 °C. This temperature dependence is consistent with the cases of the N content and the lattice parameter of β. This indicates that N₂ intercalation into the WO₃ lattice is responsible for the considerable red shift in the absorption edge, with a new shoulder appearing at 470–630 nm due to transition from a new intermediate N 2p orbital formed in the conduction band of WO₃. Based on this transition, the WO₃–N₂H₄ photoanode can utilize the visible light in longer wavelength below 520 nm for photoelectrochemical water oxidation compared to 470 nm for WO₃-neat. The high incident photon-to-current conversion efficiency of the WO₃–N₂H₄ photoanode is due to efficient electron transport through the WO₃ nanorod film.