Highly Reversible Ambipolar Photocurrents by Modulating Ta in Ba2Bi2–xTaxO6 Double Perovskites

Ta5+-doped Ba2Bi2–xTaxO6 (0.2 ≤ x ≤ 0.8) double perovskites are synthesized by a soft chemical sol–gel method. X-ray diffraction (XRD) results show the formation of a double perovskite structure. Systematic shifts of Bragg diffraction peaks to larger angles confirm the incorporation of increasing amounts of Ta5+ dopant ions in the Ba2Bi2O6 (BBO) crystal lattice. The optical absorption spectra and Tauc plots reveal a monotonic increase of the optical band gaps for Ba2Bi2–xTaxO6 (0.2 ≤ x ≤ 0.8) from 1.5 to 2.3 eV. Photoelectrochemical (PEC) and electrochemical tests signify the advantages of doping Ta5+ in improving the photoelectrode resistance to electrochemical corrosion. Highly reversible ambipolar photocurrents are observed for photoelectrodes with x ≥ 0.4 that can be further enhanced by anodic or cathodic poling treatment for facilitated interface photocharge injection. Ba2Bi1.6Ta0.4O6 photoelectrodes exhibit both the highest cathodic photocurrent of 65.5 μA cm–2 at 0 VRHE with an onset potential of 1.43 VRHE, and the highest anodic photocurrent of 37 μA cm–2 at 2 VRHE with an onset potential of 1.03 VRHE among other Ta5+ containing counterpart samples. The deteriorating PEC photocurrents of photoelectrodes with x ≥ 0.5 are the result of both inferior light absorption ability due to large band gaps and lower charge carrier concentrations due to defect compensation. The understanding and control of this switchable-photocurrent phenomenon by doping strategies are significant to further tailor the electronic band structure for compatible homojunctions of BBO-based double perovskites.