N-type BiVO 4 films coupled with p-type copper oxide nanoparticles were investigated as photoanodes for water oxidation. Compared with the BiVO 4 film, the Cu 2 O/BiVO 4 , CuO/BiVO 4 and Cu x O/BiVO 4 films all showed significantly improved photoelectrochemical water oxidation activity and stability. As expected, the radiative recombination of charge on the three kinds of copper oxide/BiVO 4 films was decreased due to the p-n junction effect between copper oxide and BiVO 4 . However, our investigations indicated that the electrocatalytic water oxidation on the copper oxide could be initiated when the copper oxide/BiVO 4 photoanodes were irradiated by solar light; therefore, the copper oxide-coupling resulted in a more obvious improvement in the water oxidation kinetics of BiVO 4 than the thermodynamic improvement in the charge separation of BiVO 4 . Of the three kinds of copper oxide/BiVO 4 films, the Cu x O/BiVO 4 film comprising BiVO 4 film coupled with a combination of Cu 2 O/CuO nanoparticles showed the best solar water oxidation performance due to the better electrocatalytic activity for water oxidation of Cu x O compared with Cu 2 O or CuO alone. The kinetic investigations and analyses in the present work provide comprehensive insight into the mechanism of the copper oxide/BiVO 4 photoanodes for solar water oxidation.
In conventional photoelectrochemical (PEC) cell for solar water splitting, its solar-to-hydrogen efficiency is limited by the sluggish oxygen evolution on its photoanode. 3-pyridinecarboxylic acid (3-PA) is an important organic in industry, but there are several disadvantages in its conventional synthesis approaches. Here, we reported that the oxidation of 3-methylpyridine (3-MP) into 3-PA as an alternative to the oxygen evolution on WO 3 photoanode for improving the hydrogen-evolution efficiency and the product added-value of PEC cell. In a WO 3 film photoanode-based dual-chamber PEC cell, 3-PA and hydrogen were simultaneously synthetized. Since the oxidation of 3-MP into 3-PA is more thermodynamically and kinetically favorable than water oxidation, higher efficiency of hydrogen evolution was achieved in the PEC cell with function of 3-PA synthesis relative to the conventional PEC cell for water splitting. Our investigations indicated that the oxidation of 3-MP into 3-PA on WO 3 photoanode is co-initiated by the photogenerated holes and the H 2 O 2 intermediate that originated from two-holes pathway of solar water oxidation on WO 3 . Additionally, the Cr 2 O 7 2− /Cr 3+ redox pair could mediate and promote the oxidation of 3-MP into 3-PA on WO 3 photoanode, and thus the PEC efficiency of 3-PA synthesis can be improved by adding Cr 3+ into the H 2 SO 4 -3-MP electrolyte.
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