Alexander J. E. Rettie

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

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.

Synthesis of BiVO4 nanoflake array films for photoelectrochemical water oxidation

Because of the potential for application in photoelectrochemical cells for water splitting, the synthesis of nanostructured BiVO4 is receiving increasing attention. Here we report a simple new drop-casting method for the first time to synthesize un-doped and doped bismuth vanadate (BiVO4) nanoflake array films. Synthesis parameters such as the amount of polyethylene glycol 600 (PEG-600) and the precursor solution drying time are investigated to optimize the films for photoelectrochemical water oxidation. The BiVO4 films consisting of nanoflakes with an average thickness of 20 nm and length of 2 μm were synthesized from a precursor solution containing Bi3+, V3+ and PEG-600 with a Bi:V: PEG-600 volume ratio of 2:2:1, dried at 135 °C for 55 min. Photoelectrochemical measurements show that the BiVO4 nanoflake array films have higher photoelectrochemical activity than the BiVO4 nanoparticle films. Additionally, the nanoflake arrays were tested after incorporating W and Mo to enhance the photoelectrochemical activity. The 2% W, 6% Mo co-doped BiVO4 nanoflake array films demonstrate the best photoelectrochemical activity with photocurrent densities about 2 times higher than the un-doped BiVO4 nanoflake films and greater than the photocurrents of individually Mo doped or W doped BiVO4 films. The origin of enhanced photoelectrochemical activity for the co-doped film may be due to the improved conductivity through the BiVO4 or slightly enhanced water oxidation kinetics.

Unravelling Small-Polaron Transport in Metal Oxide Photoelectrodes

Transition-metal oxides are a promising class of semiconductors for the oxidation of water, a process that underpins both photoelectrochemical water splitting and carbon dioxide reduction. However, these materials are limited by very slow charge transport. This is because, unlike conventional semiconductors, material aspects of metal oxides favor the formation of slow-moving, self-trapped charge carriers: small polarons. In this Perspective, we seek to highlight the salient features of small-polaron transport in metal oxides, offer guidelines for their experimental characterization, and examine recent transport studies of two prototypical oxide photoanodes: tungsten-doped monoclinic bismuth vanadate (W:BiVO4) and titanium-doped hematite (Ti:α-Fe2O3). Analysis shows that conduction in both materials is well-described by the adiabatic small-polaron model, with electron drift mobility (distinct from the Hall mobility) values on the order of 10(-4) and 10(-2) cm(2) V(-1) s(-1), respectively. Future directions to build a full picture of charge transport in this family of materials are discussed.

Facile growth of porous Fe2V4O13 films for photoelectrochemical water oxidation

Porous n-type Fe2V4O13 films on FTO substrates were prepared by a simplified successive ion layer adsorption and reaction method and characterized as photoelectrodes for photoelectrochemical (PEC) water oxidation. Synthesis parameters such as film thickness and annealing temperatures and durations were investigated to optimize the PEC performance. A band gap of ∼2.3 eV and a flat band potential of 0.5 V vs. RHE make Fe2V4O13 a promising photoanode material. Water oxidation was kinetically limited at the surface of Fe2V4O13 film as confirmed by tests in electrolyte with a hole scavenger (Na2SO3). Improved PEC performance was achieved by Mo and W doping because of enhanced carrier densities. The best performance was obtained by 2.5% W-doped Fe2V4O13 films (actual 0.8% W-doped), which efficiently oxidize water to O2via photogenerated holes as confirmed by oxygen evolution measurements. Moreover, the Fe2V4O13 photoanode displayed very stable photocurrent under illumination. Due to the suitable band gap and valence band position, Fe2V4O13 is a promising photoanode for solar water splitting. Co-catalyst loading and doping optimization are identified as routes to improve this materials performance further.

Soft X-ray spectroscopic studies of the electronic structure of M:BiVO4 (M = Mo, W) single crystals

Bismuth vanadate, BiVO4, is a promising material for use as an anode in photoelectrochemical water splitting. However, its conversion efficiency is limited by poor bulk charge transport, which is via small-polarons. We report here the use of a suite of X-ray spectroscopic probes to determine the electronic structure of 0.3–0.6 at% M:BiVO4 (M = Mo or W). The results are interpreted in the context of current theories regarding the influence of doping on the existence of inter-band gap small-polaron states and their effect on the conversion efficiency of BiVO4. Preliminary X-ray absorption and emission measurements reveal that doping widens the band gap from 2.50 to 2.75 eV, whereas the indirect nature of the band gap remains unaffected. X-ray absorption spectroscopy verified that the doping levels did not affect the distorted tetrahedral environment of V5+ in BiVO4. For BiVO4 and W:BiVO4, V L3 resonant inelastic X-ray scattering showed energy loss features related to charge transfer from low lying valence metal/oxygen states to unoccupied V eg conduction band states. A 3.8 eV energy loss feature, coupled with small polaron-like peaks measured in valence band resonant photoelectron spectroscopy of M:BiVO4, point to the population of inter-band gap V 3d states of eg symmetry. The data reveals the existence of a band gap state in the absence of an applied bias in M:BiVO4, linked to small-polaron formation. We tentatively assign it as a deep trap state, which suggests that the improved conversion efficiency of M:BiVO4 relative to the undoped material is largely due to the increased carrier concentration in spite of increased carrier recombination rates.

Synthesis, electronic transport and optical properties of Si:α-Fe2O3 single crystals

We report the synthesis of silicon-doped hematite (Si:α-Fe2O3) single crystals via chemical vapor transport, with Si incorporation on the order of 1019 cm−3. The conductivity, Seebeck and Hall effect were measured in the basal plane between 200 and 400 K. Distinct differences in electron transport were observed above and below the magnetic transition temperature of hematite at ∼265 K (the Morin transition, TM). Above 265 K, transport was found to agree with the adiabatic small-polaron model, the conductivity was characterized by an activation energy of ∼100 meV and the Hall effect was dominated by the weak ferromagnetism of the material. A room temperature electron drift mobility of ∼10−2 cm2 V−1 s−1 was estimated. Below TM, the activation energy increased to ∼160 meV and a conventional Hall coefficient could be determined. In this regime, the Hall coefficient was negative and the corresponding Hall mobility was temperature-independent with a value of ∼10−1 cm2 V−1 s−1. Seebeck coefficient measurements indicated that the silicon donors were fully ionized in the temperature range studied. Finally, we observed a broad infrared absorption upon doping and tentatively assign the feature at ∼0.8 eV to photon-assisted small-polaron hops. These results are discussed in the context of existing hematite transport studies.

One-step waferscale synthesis of 3-D ZnO nanosuperstructures by designed catalysts for substantial improvement of solar water oxidation efficiency

We investigated a rational mechanism for one-step waferscale synthesis of ZnO three dimensional (3-D) nanosuperstructures using designed catalysts via chemical vapor deposition. By precisely engineering the morphology and chemistry of the catalysts, we obtained 3-D nanosuperstructures from one dimensional (1-D) nanowire and two-dimensional (2-D) network catalysts (porous gold) in one-step. Tuning the material chemistry along the lengths of the nanowire catalysts yielded 3-D ZnO nanosuperstructures with distinct morphology on each segment. The growth mechanism and roles of chemistry of catalysts in growth of ZnO nanosuperstructures were studied. Compared with nanowire arrays synthesized from commonly used zero-dimensional (0-D) dot catalysts, 3-D nanosuperstructures significantly enhanced water oxidation efficiency by 150%. This work may inspire a new general paradigm for synthesis of 3-D semiconductor nanostructures as electrode materials for various energy related applications.

High-pressure synthesis, structure, and photoluminescence of a new KSbO3‑type bismuth germanate Bi3Ge3O10.5

A new Bi(3)Ge(3)O(10.5) compound has been synthesized under high pressure, P = 7 GPa, and 700 °C. Instead of the pyrochlore that is normally stabilized under high pressure, the Bi(3)Ge(3)O(10.5) crystallizes in a KSbO(3)-ype crystal structure. The crystal structure has been refined by the Rietveld method from synchrotron X-ray diffraction data. Moreover, we have also characterized the Bi(3)Ge(3)O(10.5) by X-ray photoelectron spectroscopy, photoluminescence, and specific heat.