Controllable Synthesis of N 2 -Intercalated WO 3 Nanorod Photoanode Harvesting a Wide Range of Visible Light for Photoelectrochemical Water Oxidation.

A highly efficient visible-light-driven photoanode, N-intercalated tungsten trioxide (WO) nanorod, has been controllably synthesized by using the dual role of hydrazine (NH), which functioned simultaneously as a structure directing agent and as a nitrogen source for N intercalation. The SEM results indicated that the controllable formation of WO nanorod by changing the amount of NH. The values of lattice parameters of the monoclinic phase and the lattice volume changed significantly with the n: n ratio. This is consistent with the addition of NH dependence of the N content, clarifying the intercalation of N in the WO lattice. The UV-visible diffuse reflectance spectra (DRS) of N-intercalated exhibited a significant redshift in the absorption edge with new shoulders appearing at 470-600 nm, which became more intense as the n:n ratio increased from 1:1.2 and then decreased up to 1:5 through the maximum at 1:2.5. This addition of NH dependence is consistent with the case of the N contents. This suggests that N intercalating into the WO lattice is responsible for the considerable red shift in the absorption edge, with a new shoulder appearing at 470-600 nm owing to formation of an intra-bandgap above the VB edges and a dopant energy level below the CB of WO. The N intercalated WO photoanode generated a photoanodic current under visible light irradiation below 530 nm due to the photoelectrochemical (PEC) water oxidation, compared with pure WO doing so below 470 nm. The high incident photon-to-current conversion efficiency (IPCE) of the WO-2.5 photoanode is due to efficient electron transport through the WO nanorod film.