Preparation And Water-Splitting Activities Of Non-metal Doped WO₃ Powders And Nanoporous WO₃ Films

F-doped WO3-x (F-WO3) and S-doped WO3-x (S-WO3) powder photocatalysts were prepared by solid-state annealing method, and nanoporous WO3 films with different structures and morphologies were prepared by anodization in neutral F--containing strong electrolytes. Structures and photocatalytic performance of samples were characterized.Photo absorption efficiency and electron transport propertise were influenced by doping, which changed grain size and the amount of W5+ and oxygen vacancies,and enhanced samples' photocatalytic activity. In photocatalysts containing suspensions, using Fe3+ as sacrificial agent, the photocatalytic activities for water splitting were enhanced by nonmetal-doping. Under ultraviolet (UV) irradiation of 12 h, the highest average oxygen evolution rates obtained for F-WO3 (1.0%F precursor concentration) and S-WO3 (2.0%S precursor concentration) were 102.1 and 99.9μmol·L-1·g-1·h-1 compared with 80.2μmol·L-1·g-1·h-1 for WO3-x, respectively increased by 27% and 25%.Under 12 h of visible light (VIS) irradiation, the water splitting activities were inhibited by F-doping. However, the highest average oxygen evolution rate obtained for S-WO3 (2.0% S precursor concentration) was 102.1μmol·L-1·g-1·h-1 under VIS irradiation, which is 57% higher than 48.9μmol·L-1·g-1·h-1 for WO3-x.Before and after annealing, both compact and nanoporous films consist essentially of WO3.The as-anodized nanoporous films were amorphous and converted to a monoclinic phase with preferential orientation in the (002) planes after annealing. Annealing removed lots of lattice imperfection, which could serve as recombination centers of photoelectrons and holes.In this way, conductivities and water splitting activities of annealed films were enhanced significantly. Under visible light irradiation, the photocurrent density (at 1.6 V vs.Ag/AgCl) and maximum photoconversion efficiency generated by the annealed nanoporous film (0.5% NaF,50 V,25℃,30 min) were 5.75 mA/cm2 and 1.68%,respectively. These values were 4.75 and 5.09 times of that generated by annealed compact WO3 film. Due to the larger surface area, nanoporous WO3 films can not only absorb more light energy, but also contact fully with the electrolyte, which is conducive to charge separation and transfer. Compared with compact films, nanoporous WO3 has smaller interface charge transfer resistance, larger carrier concentration, smaller transient photocurrent kinetics constant and larger transient time constant. Moreover, morphologies of the samples could be controlled by adjusting fluoride concentration, voltage, reaction temperature and time, and thus improve the photocatalytic water splitting activity. Nanoporous films prepared under optimal conditons (0.5% NaF,50 V,15℃,60 min) can generate photocurrent of 6.70 mA/cm2(1.6 V vs.Ag/AgCl),5.54 times the value of annealed compact WO3 films.