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.
Despite the importance of gallium nitride (GaN) nanostructures for photocatalytic activity, relatively little attention has been paid to their geometrical optimization on the basis of wave optics. In this study, we present GaN truncated nanocones to provide a strategy for improving solar water splitting efficiencies, compared to the efficiency provided by the conventional geometries (i.e., flat surface, cylindrical, and cone shapes). Computational results with a finite difference time domain (FDTD) method and a rigorous coupled-wave analysis (RCWA) reveal important aspects of truncated nanocones, which effectively concentrate light in the center of the nanostructures. The introduction of nanostructures is highly recommended to address the strong light reflection of photocatalytic materials and carrier lifetime issues. To fabricate the truncated nanocones at low cost and with large-area, a dry etching method was employed with thermally dewetted metal nanoparticles, which enables controllability of desired features on a wafer scale. Experimental results exhibit that the photocurrent density of truncated nanocones is improved about three times higher compared to that of planar GaN.
We have fabricated high quality bismuth vanadate (BiVO) polycrystalline thin films as photoanodes by pulsed laser deposition (PLD) without a postannealing process. The structure of the grown films is the photocatalytically active phase of scheelite-monoclinic BiVO which was obtained by X-ray diffraction (XRD) analysis. The change of surface morphology for the BIVO thin films depending on growth temperature during synthesis has been observed by scanning electron microscopy (SEM), and its influence on water splitting performance was investigated. The current density of the BiVO film grown on a glass substrate covered with fluorine-doped tin oxide (FTO) at 230 °C was as high as 3.0 mA/cm at 1.23 V versus the potential of the reversible hydrogen electrode (V) under AM 1.5G illumination, which is the highest value so far in previously reported BiVO films grown by physical vapor deposition (PVD) methods. We expect that doping of transition metal or decoration of oxygen evolution catalyst (OEC) in our BiVO film might further enhance the performance.
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