Colloidal metal Ag nanowire as an efficient co-catalyst for enhancing the solar water oxidation of fluorinated BiVO4 photoelectrode

Abstract

Bismuth vanadate (BiVO4) is widely thought to be an attractive visible light active (Eg = 2.4 eV) photoanode but its limitations in terms of its fast charge recombination process and retarded charge transport rate have also been well reported. To address these issues, we developed a nanoporous planar BiVO4 film prepared by a facile sol–gel assisted spin-coating method and modified the electrical properties through the doping of F− ions on to the BiVO4 films. The doped film displayed reasonable absorption onset at a wavelength of approximately 530 nm, revealing a modest red shift (~30 nm) compared with that of the undoped BiVO4 film. The doped and undoped photoanode films obtained by post-annealing in air (Air) and argon (Ar) atmospheres are denoted as BiVO4@Air, BiVO4@Ar, F:BiVO4@Air and F:BiVO4@Ar for brevity. Notably, in the F: BiVO4@Ar film, crystalline-amorphous phase boundaries around the surface region were observed, these amorphous phases improved the electronic conductivity through the creation of beneficial electrochemical active sites. In addition, the formation of shallow donor states after the fluorination process lead to improved carrier densities due to increased oxygen vacancies. The photocurrent density (J) of the undoped and F:BiVO4@Air films assessed under 1 sun illumination (100 mW/cm2 with AM 1.5 filter) in 0.5 M Na2SO4 was approximately 0.3 and 0.9 mA/cm2 at 1.23 V vs. reversible hydrogen electrode (RHE) (briefly abbreviated as 1.23 VRHE). Furthermore, the BiVO4@Ar and F:BiVO4@Ar displayed enhanced J values of ~0.5 mA/cm2 and ~1.15 mA/cm2 at 1.23 VRHE, respectively. In addition, the deposition of silver nanowire (Ag NW) with ~28 nm diameter as a co-catalyst onto F:BiVO4@Ar film further promotes PEC performance more than two-fold to ~2.7 mA/cm2 in terms of J value at 1.23 VRHE. The onset potential (Von) is negatively shifted to approximately ~160 mV for this film, this can be ascribed to beneficial charge transfer and transportation rates. These results were confirmed by gas chromatography, which showed the amount of gas evolution rate increases from ~40% for pristine BiVO4 to ~80% for the Ag NW/F:BiVO4@Ar film.