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(2)-intercalated tungsten trioxide (WO(3)) nanorod, has been controllably synthesized by using the dual role of hydrazine (N(2)H(4)), which functioned simultaneously as a structure directing agent and as a nitrogen source for N(2) intercalation. The SEM results indicated that the controllable formation of WO(3) nanorod by changing the amount of N(2)H(4). The β values of lattice parameters of the monoclinic phase and the lattice volume changed significantly with the n(W): n(N2H4) ratio. This is consistent with the addition of N(2)H(4) dependence of the N content, clarifying the intercalation of N(2) in the WO(3) lattice. The UV-visible diffuse reflectance spectra (DRS) of N(2)-intercalated exhibited a significant redshift in the absorption edge with new shoulders appearing at 470–600 nm, which became more intense as the n(W):n(N2H4) ratio increased from 1:1.2 and then decreased up to 1:5 through the maximum at 1:2.5. This addition of N(2)H(4) dependence is consistent with the case of the N contents. This suggests that N(2) intercalating into the WO(3) 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(3). The N(2) intercalated WO(3) photoanode generated a photoanodic current under visible light irradiation below 530 nm due to the photoelectrochemical (PEC) water oxidation, compared with pure WO(3) doing so below 470 nm. The high incident photon-to-current conversion efficiency (IPCE) of the WO(3)-2.5 photoanode is due to efficient electron transport through the WO(3) nanorod film.