Figure 7.

(a) Linear sweep voltammograms (scan rate: 20 mV/s) of BiVO₄ (surface area: 0.27 cm²) in a pH 6.8 phosphate buffer (0.1 M HPO₄^2–/0.1 M H₂PO₄^–) solution containing Ni²⁺ and Co²⁺, with chopped UV–visible light irradiation (xenon lamp, 100 mW/cm²). (b–c) Changes in the photocurrent responses with repeating a potential sweep. (d) Linear sweep voltammograms of BiVO₄ with the highest photocurrent achieved by Ni²⁺ or Co²⁺. In the case of Ni²⁺, the photocurrent was measured in a phosphate buffer containing 2 mM Ni²⁺, and for Co²⁺, BiVO₄ was immersed in the phosphate-buffered solution containing 0.05 mM Co²⁺ with the potential sweeping from 0.2 to 1.0 V (vs Ag/AgCl) (four times) under UV–vis light illumination, and then, the pretreated BiVO₄ was tested in a Co²⁺-free phosphate solution.