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

N 2 ‐Intercalated crystalline mesoporous tungsten trioxide (WO 3 ) was synthesized by a thermal decomposition technique with dodecylamine (DDA) as a surfactant template with a dual role as an N‐atom source for N 2 intercalation, alongside its conventional structure‐directing role (by micelle formation) to induce a mesoporous structure. N 2 physisorption analysis showed that the specific surface area (57.3 m 2  g −1 ) of WO 3 templated with DDA (WO 3 ‐DDA) is 2.3 times higher than that of 24.5 m 2  g −1 for WO 3 prepared without DDA (WO 3 ‐bulk), due to the mesoporous structure of WO 3 ‐DDA. The Raman and X‐ray photoelectron spectra of WO 3 ‐DDA indicated intercalation of N 2 into the WO 3 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 N 2 intercalation. This could be explained by the bandgap narrowing of WO 3 ‐DDA by formation of a new intermediate N 2p orbital between the conduction and valance bands of WO 3 . A WO 3 ‐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/WO 3 ‐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/WO 3 ‐bulk by one order of magnitude due to N 2 intercalation and the mesoporous structure of WO 3 ‐DDA.