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.

Reaching the goal of economical photoelectrochemical (PEC) water splitting will likely require the combination of efficient solar absorbers with high activity electrocatalysts for the hydrogen and oxygen evolution reactions (HER and OER). Toward this goal, we synthesized an amorphous FeOOH (a-FeOOH) phase that has not previously been studied as an OER catalyst. The a-FeOOH films show activity comparable to that of another OER cocatalyst, Co-borate (Co-Bi), in 1 M Na2CO3, reaching 10 mA/cm(2) at an overpotential of ∼550 mV for 10 nm thick films. Additionally, the a-FeOOH thin films absorb less than 3% of the solar photons (AM1.5G) with energy greater than 1.9 eV, are homogeneous over large areas, and act as a protective layer separating the solution from the solar absorber. The utility of a-FeOOH in a realistic system is tested by depositing on amorphous Si triple junction solar cells with a photovoltaic efficiency of 6.8%. The resulting a-FeOOH/a-Si devices achieve a total water splitting efficiency of 4.3% at 0 V vs RHE in a three-electrode configuration and show no decrease in efficiency over the course of 4 h.

We report that conductive single nanoparticle (NP) collisions can involve a significant component of the mass transport to the electrode of the charged NPs by migration. Previously, collision events of catalytic NPs were described as purely diffusional using random walk theory. However, the charged NP can also be attracted to the electrode by the electric field in solution (i.e., migration) thereby causing an enhancement in the collision frequency. The migration of charged NPs is affected by the supporting electrolyte concentration and the faradaic current flow. A simplified model based on the NP transference number is introduced to explain the migrational flux of the NPs. Experimental collision frequencies and the transference number model also agreed with more rigorous simulation results based on the Poisson and Nernst−Planck equations.

A new dispenser and scanner system is used to create and screen Bi-M-Cu oxide arrays for cathodic photoactivity, where M represents 1 of 22 different transition and post-transition metals. Over 3000 unique Bi : M : Cu atomic ratios are screened. Of the 22 metals tested, 10 show a M-Cu oxide with higher photoactivity than CuO and 10 show a Bi-M-Cu oxide with higher photoactivity than CuBi2O4. Cd, Zn, Sn, and Co produce the most photoactive M-Cu oxides, all showing a 200-300% improvement in photocurrent over CuO. Ag, Cd, and Zn produce the highest photoactivity Bi-M-Cu oxides with a 200-400% improvement over CuBi2O4. Most notable is a Bi-Ag-Cu oxide (Bi : Ag : Cu atomic ratio of 22 : 3 : 11) which shows 4 times higher photocurrent than CuBi2O4. This material is capable of evolving hydrogen under illumination in neutral electrolyte solutions at 0.6 V vs. RHE when Pt is added to the surface as an electrocatalyst.

We report that oxide composite electrodeposition can be used for the facile preparation of metal-doped BiVO 4 photoelectrodes for photoelectrochemical water oxidation. The photoactivity of electrodeposition film was improved by the addition of a small amount of tungstic acid particles during the electrodeposition. These particles are incorporated in the deposit and finally generate tungstendoped bismuth vanadate. The suspended particles in the plating solution were electrostatically attracted to the cathode and accordingly incorporated into the deposit (electrostatic deposition). WO 3 nanoparticles (NPs) can be used instead of tungstic acid, to yield a BiVO 4 with different properties. Enhanced photoelectrochemical (PEC) water oxidation was confirmed via scanning electrochemical microscopy (SECM) by detecting increased oxygen evolution with using optical fiber incorporating a ring electrode.

A rapid screening technique utilizing a modified scanning electrochemical microscope has been used to screen photocatalysts and determine how metal doping affects its photoelectrochemical (PEC) properties. We now extend this rapid screening to the examination of photocatalyst (semiconductor/semiconductor) composites: by examining a variety of ZnWO 4 /WO 3 composites, a 9% Zn/W ratio produced an increased photocurrent over pristine WO 3 with both UV and visible irradiation on a spot array electrode. With bulk films of various thickness formed by a drop-casting technique of mixed precursors and a one-step annealing process, the 9 atomic % ZnWO 4 /WO 3 resulted in a 2.5-fold increase in the photocurrent compared to pristine WO 3 for both sulfite and water oxidation at +0.7 V vs Ag/AgCl. Thickness optimization of the bulkfilm electrodes showed that the optimum oxide thickness was ∼1 μm for both the WO 3 and ZnWO 4 /WO 3 electrodes. X-ray diffraction showed the composite nature of the WO 3 and ZnWO 4 mixtures. The UV/vis absorbance and PEC action spectra demonstrated that WO 3 has a smaller band gap than ZnWO 4 , while Mott−Schottky analysis determined that ZnWO 4 has a more negative flat-band potential than WO 3 . A composite band diagram was created, showing the possibility of greater electron/hole separation in the composite material. Investigations on layered structures showed that the higher photocurrent was only observed when the ZnWO 4 /WO 3 composite was formed in a single annealing step.

Durability of high-energy throughput batteries is a prerequisite for electric vehicles to penetrate the market. Despite remarkable progresses in silicon anodes with high energy densities, rapid capacity fading of full cells with silicon–graphite anodes limits their use. In this work, we unveil degradation mechanisms such as Li+ crosstalk between silicon and graphite, consequent Li+ accumulation in silicon, and capacity depression of graphite due to silicon expansion. The active material properties, i.e. silicon particle size and graphite hardness, are then modified based on these results to reduce Li+ accumulation in silicon and the subsequent degradation of the active materials in the anode. Finally, the cycling performance is tailored by designing electrodes to regulate Li+ crosstalk. The resultant full cell with an areal capacity of 6 mAh cm−2 has a cycle life of >750 cycles the volumetric energy density of 800 Wh L−1 in a commercial cell format.

Bi 2 WO 6 microelectrode arrays on FTO glass substrates were fabricated by a picoliter solution dispensing technique using Bi(NO 3 ) 3 as the Bi source and (NH 4 ) 6 H 2 W 12 -O 40 as the W source in ethylene glycol. The scanning electrochemical microscope modified by using an optical fiber in place of an ultramicroelectrode was employed for rapid screening of the Bi 2 WO 6 arrays and to investigate the effect of 12 different dopants on the photocatalytic oxidation of SO 3 2− . Among the different dopant compositions, addition of 12% Zn showed a photocurrent enhancement of up to 80% compared with that of the pure Bi 2 WO 6 . This result was further confirmed with bulk electrode studies for SO 3 2− and water oxidation. UV− vis absorption, electrochemical impedance spectroscopy, scanning electron microscopy, and X-ray diffraction studies were carried out with the photocatalysts to elucidate the role of Zn in the bulk semiconductors. Absorbed photon-to-current efficiency and incident photon-to-current efficiency determinations further confirm the enhancement of photoelectrochemical behavior upon addition of Zn to Bi 2 WO 6 photocatalysts.