Strategic Modification of BiVO4 for Improving Photoelectrochemical Water Oxidation Performance

Abstract

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/cm(2)). 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 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 (eta(sep)) 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 WO3 layer is deposited under a similarly thick Mo-BiVO4 layer. This binary heterojunction (WO3/Mo-BiVO4) exhibits eta(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-BiVO4 (Mo-BiVO4/Co-Pi) enhances charge injection efficiency (eta(inj)) from similar to 50 to similar to 70% at 1.23 V-RHE. These two binaries are coupled into a ternary heterojunction (WO3/Mo-BiVO4/Co-Pi) in order to improve the charge transfer efficiencies (eta(sep) and eta(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 eta(sep) and eta(inj) of similar to 60 and 90%, respectively.