Tunable Mesoporous Structure of Crystalline WO3 Photoanode toward Efficient Visible-Light-Driven Water Oxidation

Tunable mesoporous crystalline tungsten trioxide (WO3) was synthesized by in situ surfactant-thermal-carbonization method, using the series of 2(alkylaminomethyl)pyridine (PAL2-n, n = 8, 12, and 16) surfactant templates with varied alkyl chain length. The d-spacing, corresponding to the pore center-to-center distance, for mesoporous WO3 annealed at 550 degrees C decreased from 3.8 nm to 3.1 nm with a shortened chain length of PAL2-n from n = 16 to n = 12, in response to changes in the smaller size of micellar self-assemblies of PAL2-n templates. The mesoporous structure was partially collapsed for the PAL2-8 template during the crystallization of WO3 framework, when annealed at 550 degrees C. The longer alkyl chain of PAL2-n yielded the thicker pore walls in WO3/PAL2-n mesocomposites, affording better thermal stabilization of the organized mesoporous structure of WO3, along with the formation of higher content of carbonaceous species in mesopores through surfactant carbonization, which act as a protective support during growth of WO3 nanocrystals by annealing. The degree of organization in mesoporous structures revealed direct impact on the visible-light-driven water oxidation performance. The incident photon-to-current conversion efficiency (IPCE) of mesoporous WO3 based on water oxidation increased from 24% to 36% at 420 nm and 0.5 V vs Ag/AgCI with the increase of the alkyl chain length of PAL2-n from n = 8 to n = 16, and the IPCE values of mesoporous samples are ca. 5-7 times higher than that (5%) of untemplated WO3.