Strategic Modification of BiVO4 for Improving Photoelectrochemical Water Oxidation Performance

Abstract

To improve the photoelectrochemical (PEC) performance of BiVO 4, three different modifications (doping, heterojunction, and catalyst deposition) using earth-abundant elements are performed and their effects are compared in a 0.1 M phosphate electrolyte at pH 7 under AM1.5 light (100 mW/cm 2 ). When a hexavalent element (Cr 6+, W 6+, or Mo 6+ ) is doped at various levels, the Mo 6+ -doping effect is most significant at 10 atomic % with about two times higher photocurrent generation at the oxygen evolution potential (1.23 V RHE ). Such enhancement is attributed to a decrease in charge transfer resistance ( R ct ) by donor doping, resulting in an approximate 2-fold increase in charge separation efficiency (η sep ) to about 25%. W 6+ is less effective than Mo 6+, whereas Cr 6+ has a detrimental effect. To further improve the charge separation efficiency of Mo 6+ -doped BiVO 4 (Mo-BiVO 4 ), a approximate 600 nm thick WO 3 layer is deposited under a similarly thick Mo-BiVO 4 layer. This binary heterojunction (WO 3 /Mo-BiVO 4 ) exhibits η sep of about 50% along with more than 3 times higher photocurrent generation. On the other hand, an oxygen evolving cobalt-phosphate (Co-Pi) catalyst electrodeposited to Mo-BiVO 4 (Mo-BiVO 4 /Co-Pi) enhances charge injection efficiency (η inj ) from ∼50 to ∼70% at 1.23 V RHE . These two binaries are coupled into a ternary heterojunction (WO 3 /Mo-BiVO 4 /Co-Pi) in order to improve the charge transfer efficiencies (η sep and η inj ). The PEC performance of this ternary is significantly high with photocurrent density of about 2.4 mA/cm 2 at 1.23 V RHE (corresponding to the solar-to-hydrogen efficiency of ca. 3%) due to η sep and η inj of ∼60 and 90%, respectively.