Combined Charge Carrier Transport and Photoelectrochemical
Characterization of BiVO<sub>4</sub> Single Crystals: Intrinsic Behavior
of a Complex Metal Oxide
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Abstract
Bismuth
vanadate (BiVO<sub>4</sub>) is a promising photoelectrode
material for the oxidation of water, but fundamental studies of this
material are lacking. To address this, we report electrical and photoelectrochemical
(PEC) properties of BiVO<sub>4</sub> single crystals (undoped, 0.6%
Mo, and 0.3% W:BiVO<sub>4</sub>) grown using the floating zone technique.
We demonstrate that a small polaron hopping conduction mechanism dominates
from 250 to 400 K, undergoing a transition to a variable-range hopping
mechanism at lower temperatures. An anisotropy ratio of ∼3
was observed along the <i>c</i> axis, attributed to the
layered structure of BiVO<sub>4</sub>. Measurements of the ac field
Hall effect yielded an electron mobility of ∼0.2 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> for Mo and W:BiVO<sub>4</sub> at 300 K. By application of the Gärtner model, a hole
diffusion length of ∼100 nm was estimated. As a result of low
carrier mobility, attempts to measure the dc Hall effect were unsuccessful.
Analyses of the Raman spectra showed that Mo and W substituted for
V and acted as donor impurities. Mott–Schottky analysis of
electrodes with the (001) face exposed yielded a flat band potential
of 0.03–0.08 V versus the reversible H<sub>2</sub> electrode,
while incident photon conversion efficiency tests showed that the
dark coloration of the doped single crystals did not result in additional
photocurrent. Comparison of these intrinsic properties to those of
other metal oxides for PEC applications gives valuable insight into
this material as a photoanode