Double Doping of NaTaO₃ Photocatalysts with Lanthanum and Manganese for Strongly Enhanced Visible-Light Absorption

Abstract

Perovskite-structured tantalates, NaTaO3 in particular, have been at the forefront of solar energy conversion to generate hydrogen fuel via photocatalytic water splitting. However, their application as a photocatalyst remains impractical due to their inability to absorb long-wavelength light. One promising scheme for extending the material response to long-wavelength light is via the charge compensation of doped cations with lanthanum and a transition metal. In this research, NaTaO3 is doubly doped with lanthanum and manganese, a 3d-block transition metal, for visible-light sensitization. Following the double doping, the light absorption is extended to the near-infrared region. The doubly doped samples not only strongly absorb visible light but also produce electrons under visible-light irradiation (420 < λ < 740 nm). EDX confirms the one-to-one ratio of dopant incorporation, and nanometer-resolution elemental mapping with TEM demonstrating that La and Mn are incorporated at approximately the same location in the particles. EXAFS furthermore reveals that La occupies the Na site, while Mn occupies the Ta site. These findings suggest that La pairs with Mn at the neighboring site to form a LaMnO3–NaTaO3 solid solution.

Dependence of Photoexcited Electron Behavior on Octahedral Distortion in Barium-Doped NaTaO3 Photocatalysts

“…When the B-site cation is substituted with foreign, nonequivalent B′ cation, this symmetry restriction breaks down, producing the 830 cm –1 band. In a number of ABO 3 -perovskites doped with metal cations at the B site, strong Raman bands were detected at ∼820 cm –1 . ,− As also revealed in our previous studies, the substitution of Ta 5+ in NaTaO 3 with first-row transition-metal cations produced additional Raman bands at ∼830 cm –1 . − The intensity of the newly produced Raman bands was linearly correlated with the cation concentration, indicating that these bands were associated with the Ta site occupation by those metal cations.…”

“…As a result, the TaO 6 octahedral sublattice became more distorted to produce additional Raman band at ∼830 cm –1 . The 830 cm –1 band intensity was quantitatively sensitive to the Ta site occupation by metal cations. ,,− , The stronger is the 830 cm –1 band intensity, the higher is the number of metal cations occupying the Ta sites. A decreased band intensity suggests a decreased number of metal cations at the Ta sites.…”

Advances in Z‐scheme semiconductor photocatalysts for the photoelectrochemical applications: A review

“…Furthermore, it is possible to combine properties, such as ferroelectricity or piezoelectricity, with the photocatalytic effect to improve photocatalytic activity. The perovskite photocatalysts can be classified into the following categories: titanates, SrTiO 3 112–118 and BaTiO 3 119–123 ; tantalates, LiTaO 3 , 124 NaTaO 3 125–132 and KTaO 3 133–137 ; niobates, LiNbO 3 , 138,139 NaNbO 3 140–147 and KNbO 3 148–153 ; vanadates, AgVO 3 154–162 ; and ferrites, LaFeO 3 , 164–169 BiFeO 3 170–176 and GaFeO 3 177 . These compounds show great potential for application in visible light‐driven photoreactions.…”

Band Gap Narrowing of Bi-Doped NaTaO3 for Photocatalytic Hydrogen Evolution under Simulated Sunlight: A Pseudocubic Phase Induced by Doping

“…For this reason, several doping and heterojunction strategies have been proposed as an attempt to overcome this limitation by tuning the electronic band structure of the material. 4,20 Some of the most successful approaches for enhancing the photocatalytic activity of NaTaO 3 consist of La 3+ and Sr 2+ doping, 21,22 which improve charge transfer in the material 23,24 and induce surface reconstruction by creating steplike nanostructures that notably increase the surface area for water splitting reactions. 25 Despite these effects, La and Sr doping are only beneficial to the production of hydrogen under UV light, as these elements have negligible influence on the band gap.…”

“…However, its wide band gap only allows for absorption of ultraviolet irradiation, thus hindering its utilization under natural sunlight. For this reason, several doping and heterojunction strategies have been proposed as an attempt to overcome this limitation by tuning the electronic band structure of the material. , …”