In photoelectrochemical
(PEC) water splitting, BiVO4 is considered the most promising
photoanode material among metal
oxide semiconductors because of its relatively narrow optical bandgap
and suitable band structure for water oxidation. Nevertheless, until
now, the solar-to-hydrogen conversion efficiency of BiVO4 has shown significant limitations for commercialization because
of its poor charge transport. Various strategies, including the formation
of a heterojunction and doping of electron donors, have been implemented
to enhance the charge transport efficiency; however, fundamental approaches
are required for further enhancement. In this regard, we report the
fundamental approach for BiVO4 thin film photoanodes by
fabricating epitaxial oxide thin films with different crystallographic
orientations for PEC water splitting. The crystalline anisotropy generally
reveals distinct physical phenomena along different crystallographic
orientations. In the same vein, in terms of the anisotropic properties
of BiVO4, the electrical conductivity of BiVO4 is greater along the ab-plane than along the c-axis. Consequently, as the crystallographic orientation
of the BiVO4 thin film changes from (001) to (010), the
charge transport properties in the epitaxial BiVO4 thin
film are significantly enhanced. Thus, at 1.23 VRHE, the
photocurrent density of the epitaxial BiVO4 (010) thin
film (2.29 mA cm–2) is much higher than that of
the epitaxial BiVO4 (001) thin film (0.74 mA cm–2) because of significant enhancement in charge transport properties
even for undoped BiVO4. These results strongly suggest
that the growth of epitaxial BiVO4 thin films with specific
crystallographic orientations has great potential to considerably
improve the charge transport efficiency of photoanodes for solar water
splitting.
Herein, we developed
a fully solution-deposited nanocomposite photoanode
based on silver nanoparticle (NP)-impregnated bismuth vanadate (BiVO4) films. The synthesized Ag NPs exhibit diameters of few nanometers
and uniform matrix dispersion, which were confirmed by high-resolution
transmission electron microscopy. The photoanode composed of the Ag
NP-incorporated nanocomposite BiVO4 showed a remarkable
enhancement in both low potential and the saturated photocatalytic
current densities in comparison with the pristine BiVO4 film. The observed experimental results are attributed to the improved
carrier generation and enhanced charge separation by the localized
surface plasmon resonance-mediated effect as suggested by electrochemical
impedance spectroscopy and a numerical simulation.
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