Abstract:

Layered-type metal phosphates of BaNb_2-xTa_xP₂O₁₁ (x = 0, 0.5, 1.0, 1.5, and 2.0) were synthesized using a solid-state reaction method. The photophysical, optical, and photocatalytic hydrogen production properties of the resulting powders were investigated for the first time. Phase-pure and homogeneous powders with irregular morphologies were obtained at a calcination temperature of 1200 °C. As the Ta content increased, the interlayer distance along the c-axis increased by up to 0.14%. Additionally, the optical bandgap values increased from 3.32 to 3.59 eV. The energy band positions were estimated from the Mott-Schottky measurements. BaNb₂P₂O₁₁ (x = 0) exhibited the lowest conduction band edge position (-0.14 V vs. the normal hydrogen electrode, NHE), which is located above the water reduction potential (0.0 V vs. NHE). In comparison, BaTa₂P₂O₁₁ (x = 2.0) exhibited the highest conduction band edge position (-0.29 V vs. NHE), comparable to that of TiO₂. The photocatalytic activity for hydrogen produced from splitting water was measured under ultraviolet light irradiation. Notably, BaTa₂P₂O₁₁ exhibited the highest activity (7.3 μmol/h), which was 15 and 10 times larger than BaNb₂P₂O₁₁ (0.5 μmol/h) and nano-TiO₂ (0.7 μmol/h), respectively. The activity of BaTa₂P₂O₁₁ increased to 24.4 μmol/h after deposition of the NiO_x co-catalyst (1 wt.%), which remained stable during continuous operation (~35 h).

Key words: BaNb₂P₂O₁₁, BaTa₂P₂O₁₁, Layered crystal structure, Bandgap, Band edge positions, Photocatalytic hydrogen production