Photoexcited Electrons Driven by Doping Concentration Gradient: Flux-Prepared NaTaO₃ Photocatalysts Doped with Strontium Cations

Abstract

Electron–hole recombination always competes with desired reactions on semiconductor photocatalysts. Reducing recombination probability is essential for increasing the quantum efficiency of the reactions. Previous studies demonstrated that doping with lanthanoid or alkaline-earth metal cations reduced recombination probability in NaTaO3 photocatalysts for artificial photosynthesis. The motivation behind this study was to reveal how the guest metal cations reduced recombination probability. NaTaO3 photocatalysts were doped with Sr cations through crystallization in molten NaCl flux to produce 50–100 nm sized particles of NaTaO3–Sr­(Sr1/3Ta2/3)­O3 solid solution. Intraparticle distribution of Sr cations was sensitive to immersion time in the hot flux with a fixed Sr concentration of 2 mol % relative to Ta. Extended immersion for 60 h resulted in a homogeneous Sr distribution. Curtailed immersion for 1 h yielded particles capped with a 3 nm thick Sr-accumulated layer. The population of electrons bandgap-excited under Hg–Xe lamp irradiation was enhanced in the 1-h immersed photocatalyst by 160 times relative to that in a Sr-free NaTaO3 photocatalyst. In the 60-h immersed photocatalyst, population enhancement was not more than 9 times. We interpreted the large population enhancement in the 1-h immersed photocatalyst with a concentration gradient of Sr cations from the surface to bulk. The concentration gradient induced an energy gradient of conduction-band minimum. Photoexcited electrons were driven on the energy gradient to be separated from holes. The overall water splitting reaction rate was evaluated on the photocatalysts to show a 4-times enhancement on the 1-h immersed photocatalyst relative to the rate on the Sr-free photocatalyst. The reaction-rate enhancement less than the electron population enhancement was ascribed to a limited fraction of electrons overriding the energy gradient and returning back to the surface.

Sodium Tantalate Photocatalysts Doped with Metal Cations: Why Are They Active for Water Splitting?

“…The water splitting rate was examined over the four Srcontaining photocatalysts (Sr concentration=2 mol %) prepared in the NaCl flux . H 2 and O 2 production rates are plotted in Figure as a function of integrated absorbance change evaluated with the spectra shown in Figure (i.e., the photoexcited electron population).…”

“…A more recent study further evidenced the essential role of the graded composition . NaTaO 3 photocatalysts were doped with Sr cations through crystallization in molten NaCl flux maintained at 1423 K. Chemical composition and simultaneous doping to form the NaTaO 3 –Sr(Sr 1/3 Ta 2/3 )O 3 solid solution were checked and confirmed.…”

Charge Carrier Dynamics in Sr-Doped NaTaO3 Photocatalysts Revealed by Deep Ultraviolet Single-Particle Microspectroscopy

“…This concentration dependence is similar to the one that electron population in Sr-NTO-SS increased with increasing Sr concentration and then decreased at Sr concentrations above 1.8 mol%. 15,38 It should be noted that the blue shift of approximately 0.1 eV in the lower concentration region is comparable to the increase of the band gap energy (~0.1 eV) by Sr-doping. 15,38 Meanwhile, the higher doping amount (> 3 mol%) results in the red shift of the PL peak, possibly due to delocalization of the excited states and/or involvement of deeper trap states caused by crystal deformations and surface reconstructions.…”

“…15,38 It should be noted that the blue shift of approximately 0.1 eV in the lower concentration region is comparable to the increase of the band gap energy (~0.1 eV) by Sr-doping. 15,38 Meanwhile, the higher doping amount (> 3 mol%) results in the red shift of the PL peak, possibly due to delocalization of the excited states and/or involvement of deeper trap states caused by crystal deformations and surface reconstructions. 28,39 It was suggested that excess K-doping (> 5 mol%) creates defect states in K-doped NaTaO3, thus lowering the photocatalytic activity.…”

“…Recently, Onishi and co-workers proposed an alternative mechanism for the enhanced photocatalytic activity of Sr-doped NaTaO3. 15,38 The conduction band (CB) of pristine NaTaO3 mainly comprises Ta 5d orbitals. [44][45] By replacing the Ta sites with Sr cations, SrO6 octahedra are formed in the corner-shared network of the TaO6 octahedra.…”