Dual-Functional Surfactant-Templated Strategy for Synthesis of an In Situ N 2 -Intercalated Mesoporous WO 3 Photoanode for Efficient Visible-Light-Driven Water Oxidation.

N2 -Intercalated crystalline mesoporous tungsten trioxide (WO3 ) was synthesized by a thermal decomposition technique with dodecylamine (DDA) as a surfactant template with a dual role as an N-atom source for N2 intercalation, alongside its conventional structure-directing role (by micelle formation) to induce a mesoporous structure. N2 physisorption analysis showed that the specific surface area (57.3 m2  g-1 ) of WO3 templated with DDA (WO3 -DDA) is 2.3 times higher than that of 24.5 m2  g-1 for WO3 prepared without DDA (WO3 -bulk), due to the mesoporous structure of WO3 -DDA. The Raman and X-ray photoelectron spectra of WO3 -DDA indicated intercalation of N2 into the WO3 lattice above 450 °C. The UV/Vis diffuse-reflectance spectra exhibited a significant shift of the absorption edge by 28 nm, from 459 nm (2.70 eV) to 487 nm (2.54 eV), due to N2 intercalation. This could be explained by the bandgap narrowing of WO3 -DDA by formation of a new intermediate N 2p orbital between the conduction and valance bands of WO3 . A WO3 -DDA-coated indium tin oxide (ITO) electrode calcined at 450 °C generated a photoanodic current under visible-light irradiation below 490 nm due to photoelectrochemical water oxidation, as opposed to below 470 nm for ITO/WO3 -bulk. The incident photon-to-current conversion efficiency (IPCE=24.5 %) at 420 nm and 0.5 V versus Ag/AgCl was higher than that of 2.5 % for ITO/WO3 -bulk by one order of magnitude due to N2 intercalation and the mesoporous structure of WO3 -DDA.