RESUMO
An (oxy)nitride-based heterostructure for powdered Z-scheme overall water splitting is presented. Compared with the single MgTa2O(6-x)N(y) or TaON photocatalyst, a MgTa2O(6-x)N(y)/TaON heterostructure fabricated by a simple one-pot nitridation route was demonstrated to effectively suppress the recombination of carriers by efficient spatial charge separation and decreased defect density. By employing Pt-loaded MgTa2O(6-x)N(y)/TaON as a H2-evolving photocatalyst, a Z-scheme overall water splitting system with an apparent quantum efficiency (AQE) of 6.8% at 420â nm was constructed (PtO(x)-WO3 and IO3(-)/I(-) pairs were used as an O2-evolving photocatalyst and a redox mediator, respectively), the activity of which is circa 7 or 360â times of that using Pt-TaON or Pt-MgTa2O(6-x)N)y) as a H2-evolving photocatalyst, respectively. To the best of our knowledge, this is the highest AQE among the powdered Z-scheme overall water splitting systems ever reported.
RESUMO
Tantalum nitride (Ta3N5) modified with various O2-evolution cocatalysts was employed as a photocatalyst for water oxidation under visible light (λ>420â nm) in an attempt to construct a redox-mediator-free Z-scheme water-splitting system. Ta3N5 was prepared by nitriding Ta2O5 powder under a flow of NH3 at 1023-1223â K. The activity of Ta3N5 for water oxidation from an aqueous AgNO3 solution as an electron acceptor without cocatalyst was dependent on the generation of a well-crystallized Ta3N5 phase with a low density of anionic defects. Modification of Ta3N5 with nanoparticulate metal oxides as cocatalysts for O2 evolution improved water-oxidation activity. Of the cocatalysts examined, cobalt oxide (CoO(x)) was found to be the most effective, improving the water-oxidation efficiency of Ta3N5 by six to seven times. Further modification of CoO(x)/Ta3N5 with metallic Ir as an electron sink allowed one to achieve Z-scheme water splitting under simulated sunlight through interparticle electron transfer without the need for a shuttle redox mediator in combination with Ru-loaded SrTiO3 doped with Rh as a H2-evolution photocatalyst.
RESUMO
Tantalum nitride (Ta(3)N(5)) is a promising nitride semiconductor photocatalyst for solar water splitting because it has band edge potentials capable of producing hydrogen and oxygen from water under visible light (λ < 590 nm). However, the photocatalytic performance of Ta(3)N(5) has been far below expectations because insufficient crystallization upon thermal nitridation of the oxide precursors enhances undesirable charge recombination limiting the quantum efficiency of the photocatalytic reaction. This problem was successfully rectified in this study by modifying the surface of the starting Ta(2)O(5) with a small amount of alkaline metal (AM) salts. Compared with conventional Ta(3)N(5), Ta(3)N(5) nitrided from AM salt-modified Ta(2)O(5) had better crystallinity and smaller particles with smoother surfaces and, most importantly, demonstrated a 6-fold improvement in photocatalytic activity for O(2) evolution under visible light. AM salt modification was compatible with the loading of an O(2) evolution cocatalyst, such as CoO(x), yielding an apparent quantum efficiency of 5.2% at 500-600 nm. This indicates that the effects of AM modification were attributable to the changes in the crystallinity and the morphology of Ta(3)N(5) rather than to catalytic effects. Detailed characterization of the Na(2)CO(3)-modified Ta(3)N(5) suggested partial dissolution of Ta(2)O(5) and nucleation of NaTaO(3) in the early stages of nitridation, which gave rise to the characteristic particle morphologies and improved the crystallinity of the nitridation products. This study demonstrates that a facile pretreatment of a starting material can improve the physical and photocatalytic properties of photocatalysts drastically, enabling the development of advanced photocatalysts for solar water splitting.
