Solar water splitting on porous-alumina-assisted TiO2-doped WOx nanorod photoanodes: Paradoxes and challenges
Sprache des Titels:
Arrays of self-organized WO3-based semiconductor nanorods are prepared from a thin W layer, W/Ti bilayer (tungsten-on-titanium), and W-10at.%Ti alloy layer via the porous-anodic-alumina (PAA)-assisted anodization at various conditions to address the radius/length ratio of~13/130 and ~70/700 nm (respectively ?small? and ?big? nanorods). Doping the WO3 nanorods with TiO2 was achieved, for the first time, simply by anodizing the W/Ti and W-10at.%Ti layers through the alumina nanopores. The post-anodizing treatments combined PAA dissolution with annealing in air and vacuum at 500?550 °C to alter the film composition, crystal structure, and
electrical properties. The air-annealed big nanorods comprising monoclinic and triclinic WO3 crystal phases reveal their superior performance in photoelectrochemical (PEC) water splitting, showing a low onset potential (0.5 VRHE) and a competitive value of photocurrent (15.5 mA cm?2) in 0.1 mol dm?3 Na2SO4 solution (pH 5.0)
under chopped illumination at a single wavelength of 405 nm, 1 W cm?2, with no sign of photocorrosion.
Paradoxically, the presence of monoclinic WO2.9 phase in the vacuum-annealed nanorods worsens the PEC behavior and stimulates the peroxo-assisted dissolution. Unexpectedly, electrochemically doping both the WO3and WO2.9 big nanorods with TiO2 causes the photocurrent to decrease dramatically. An advanced approach developed for modeling charge transport processes in the PAA-assisted WOx nanorods predicts a 7-fold further rise in the solar current should the big nanorods grow longer (1.5 ?m) and wider (300 nm) to absorb a bigger
portion of light and support a thicker depletion layer, without, however, getting fully depleted, which is the case of the small nanorods.