Combined Charge Carrier Transport and Photoelectrochemical Characterization of BiVO₄ Single Crystals: Intrinsic Behavior of a Complex Metal Oxide

Abstract

Bismuth vanadate (BiVO4) 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 BiVO4 single crystals (undoped, 0.6% Mo, and 0.3% W:BiVO4) 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 c axis, attributed to the layered structure of BiVO4. Measurements of the ac field Hall effect yielded an electron mobility of ~0.2 cm(2) V(-1) s(-1) for Mo and W:BiVO4 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 H2 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.

Structural and electronic properties of oxygen defective and Se-doped p-type BiVO4(001) thin film for the applications of photocatalysis

“…Pristine BiVO 4 (001) is an n-type semiconductor where the electrons play an important role in the photocatalytic reaction. This statement also corroborate the already reported work of A. Rettie et al [31] So, the relative delocalized orbitals at CBM compared to VBM (Fig. 3) confirmed that majority of hole, limit the charge transport in this material.…”

Anisotropic small-polaron hopping in W:BiVO4 single crystals

“…[1][2][3] However, the low electrical conductivity of BVO limits its utility as a photo-electrode. [4][5][6] This low conductivity was preliminarily ascribed to its charge carriers forming small polarons and a low temperature-independent Hall mobility. 4 Small polarons can form when electronic charge carriers move slowly enough to displace surrounding atoms from their equilibrium positions.…”

“…[4][5][6] This low conductivity was preliminarily ascribed to its charge carriers forming small polarons and a low temperature-independent Hall mobility. 4 Small polarons can form when electronic charge carriers move slowly enough to displace surrounding atoms from their equilibrium positions. 7 Thus, small polarons are found in non-crystalline solids [8][9][10] as well as in wellordered transition-metal [11][12][13][14] and organic [15][16][17] compounds, where slow electronic motion is attributed to disorder and to narrow electronic energy bands, respectively.…”

“…[http://dx.doi.org/10.1063/1.4905786] Monoclinic bismuth vanadate (BiVO 4 or BVO) is a promising photo-anode for solar water splitting because (1) its band edges are well positioned for water oxidation, (2) its band gap is modest, $2.5 eV, and (3) it is stable in aqueous environments when coupled with co-catalysts. [1][2][3] However, the low electrical conductivity of BVO limits its utility as a photo-electrode.…”

“…4 Four point collinear conductivity and AC field Hall effect were conducted at Lake Shore Cryotronics on an 8404 AC/DC Hall measurement system. Measurement of the Seebeck coefficient was performed using a laboratory-built apparatus at UT Austin.…”