BiVO4/Cs2PtI6 Vacancy-Ordered Halide Perovskite Heterojunction for Panchromatic Light Harvesting and Enhanced Charge Separation in Photoelectrochemical Water Oxidation.

Photoelectrochemical water oxidation is a challenging reaction in solar water splitting due to the parasitic recombination process, sluggish catalytic activity, and electrode stability. Oxide semiconductors are stable in an aqueous medium but show huge charge carrier recombination. Creation of a heterojunction is found to be effective for extracting the photogenerated electrons/holes before they recombine to the ground state. In this work, we created a heterojunction of BiVO4 with vacancy-ordered halide perovskite Cs2PtI6 and used it as a photoanode in PEC water oxidation. Cs2PtI6 is the only halide perovskite that is found to be extremely stable even in strong acids and bases. We utilized the stability of this material and its panchromatic visible light absorption property and made the first unprotected heterojunction dual-absorber photoanode for PEC water oxidation. At 1.23 V (vs RHE), bare BiVO4 gave 0.6 mA cm-2 photocurrent density, whereas the BiVO4/Cs2PtI6 heterojunction shows 0.92 mA cm-2. With the addition of IrOx cocatalyst, at 1.23 V (vs RHE), the heterojunction gave ∼2 mA cm-2. To obtain 2 mA cm-2 photocurrent, pure BiVO4 requires 560 mV overpotential, whereas the heterojunction requires 250 mV. The increase in the photocurrent arises from the increase in the efficiency of charge separation from BiVO4 to Cs2PtI6 and the complementary absorption offered by the latter.