Strategic Modification of BiVO4for Improving Photoelectrochemical Water Oxidation Performance

To improve the photoelectrochemical (PEC) performance of BiVO4, 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/cm2). When a hexavalent element (Cr6+, W6+, or Mo6+) is doped at various levels, the Mo6+-doping effect is most significant at 10 atomic % with about two times higher photocurrent generation at the oxygen evolution potential (1.23 VRHE). Such enhancement is attributed to a decrease in charge transfer resistance (Rct) by donor doping, resulting in an approximate 2-fold increase in charge separation efficiency (ηsep) to about 25%. W6+is less effective than Mo6+, whereas Cr6+has a detrimental effect. To further improve the charge separation efficiency of Mo6+-doped BiVO4(Mo-BiVO4), a approximate 600 nm thick WO3layer is deposited under a similarly thick Mo-BiVO4layer. This binary heterojunction (WO3/Mo-BiVO4) exhibits ηsepof 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-BiVO4(Mo-BiVO4/Co-Pi) enhances charge injection efficiency (ηinj) from ∼50 to ∼70% at 1.23 VRHE. These two binaries are coupled into a ternary heterojunction (WO3/Mo-BiVO4/Co-Pi) in order to improve the charge transfer efficiencies (ηsepand ηinj). The PEC performance of this ternary is significantly high with photocurrent density of about 2.4 mA/cm2at 1.23 VRHE(corresponding to the solar-to-hydrogen efficiency of ca. 3%) due to ηsepand ηinjof ∼60 and 90%, respectively.