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As an alternative to the conventional Haber–Bosch process for NH3synthesis that operates under harsh conditions, an electrochemical process has recently been pursued. Here, using a joint experiment–density functional calculation approach, we determine the activity trend of four transition metals (TMs) (Fe, Ru, Rh, and Pd) for N2reduction to NH3: Fe > Ru > Pd > Rh, where the protonation step of *N2to form *N2H (* indicates surface sites) is a potential determining step (PDS). The activity trend of the electrocatalysts is determined by the ability of the adsorbate (*N2) over the catalyst surfaces to easily obtain electrons at the PDS with an assumption of a scaling relationship between the activation energy barrier and the free energy difference. In electronic structures, the ability can be estimated by the energy difference between the lowest unoccupied molecular orbital (LUMO) of the adsorbed N2on the TM surfaces and the fermi energy (EF). For early TMs (e.g., Sc and Ti) where the PDS is *NH protonation reaction to form *NH2, the activity of the TMs can be similarly explained with an electronic structural feature that is the energy difference between the LUMO of the *NH and the EF. Based on the origin, we additionally consider 10 TMs (Ni, Cr, Mn, Co, Cu, Mo, Ag, W, Pt, and Au) and then determine the activity trend of the total 16 diverse TMs for NH3synthesis. We expect that this work could pave the way to novel alloy catalysts with a high activity for electrochemical NH3synthesis.

Dye-sensitized solar cells (DSSCs) are highly efficient and reliable photovoltaic devices that are based on nanostructured semiconductor photoelectrodes. From their inception in 1991, colloidal TiO2nanoparticles (NPs) with the large surface area have manifested the highest performances and the particle size of around 20 nm is generally regarded as the optimized condition. However, though there have been reports on the influences of particle sizes in conventional DSSCs employing iodide redox electrolyte, the size effects in DSSCs with the state-of-the-art cobalt electrolyte have not been investigated. In this research, systematic analyses on DSSCs with cobalt electrolytes are carried out by using various sizes of NPs (20–30 nm), and the highest performance is obtained in the case of 30 nm sized TiO2NPs, indicating that there is a reversed power conversion efficiency trend when compared with those with the iodide counterpart. Detailed investigations on various factors—light harvesting, charge injection, dye regeneration, and charge collection—reveal that TiO2particles with a size range of 20–30 nm do not have a notable difference in charge injection, dye regeneration, and even in light-harvesting efficiency. It is experimentally verified that the superior charge collection property is the sole origin of the higher performance, suggesting that charge collection should be prioritized for designing nanostructured TiO2photoelectrodes for DSSCs employing cobalt redox electrolytes.

Photoactivities of lead chromates with various combinations of Pb and Cr are rapidly screened using scanning electrochemical microscopy (SECM). In the rapid screening investigation, the metal oxide spot electrode with a Pb/Cr ratio of 2:1 exhibits the highest photoactivity among the semiconductor prepared with different compositions. The photoactivity and electrochemical properties of thin-film electrodes of PbCrO4, Pb2CrO5, and Pb5CrO8are further studied following the combinatorial screening. In the bulk electrode measurements, the Pb2CrO5bulk electrode displays the highest photocurrent of 0.23 mA/cm2for SO32–oxidation at 0.4 V vs Ag/AgCl under 100 mW/cm2UV–vis light irradiation. Pb2CrO5presents visible light activity with an absorption wavelength up to 550 nm and an incident photon to current conversion efficiency (IPCE) of 10% at the wavelength of 340 nm. The onset wavelength observed in the UV–vis absorption spectrum increases with increasing Pb contents in lead chromates. Optically obtained direct band gaps decreased from 2.38 to 2.25 to 2.07 eV for PbCrO4, Pb2CrO5, and Pb5CrO8, respectively. However, the onset wavelength that appeared in IPCE is 2.26 ± 0.02 eV for all three lead chromates where the photocurrent under longer wavelength light irradiation is insignificant. The results imply that more Pb 6s orbitals form interband states, increasing optical transitions in lead chromates. The band structures of PbCrO4, Pb2CrO5, and Pb5CrO8are also determined by electrochemical analyses and ultraviolet photoelectron spectroscopy (UPS).

We report that oxide composite electrodeposition can be used for the facile preparation of metal-doped BiVO4photoelectrodes 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 tungsten-doped bismuth vanadate. The suspended particles in the plating solution were electrostatically attracted to the cathode and accordingly incorporated into the deposit (electrostatic deposition). WO3nanoparticles (NPs) can be used instead of tungstic acid, to yield a BiVO4with 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 ZnWO4/WO3composites, a 9% Zn/W ratio produced an increased photocurrent over pristine WO3with 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 % ZnWO4/WO3resulted in a 2.5-fold increase in the photocurrent compared to pristine WO3for both sulfite and water oxidation at +0.7 V vs Ag/AgCl. Thickness optimization of the bulk-film electrodes showed that the optimum oxide thickness was ∼1 μm for both the WO3and ZnWO4/WO3electrodes. X-ray diffraction showed the composite nature of the WO3and ZnWO4mixtures. The UV/vis absorbance and PEC action spectra demonstrated that WO3has a smaller band gap than ZnWO4, while Mott–Schottky analysis determined that ZnWO4has a more negative flat-band potential than WO3. 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 ZnWO4/WO3composite was formed in a single annealing step.

Reaction kinetics and surface coverage of water oxidation intermediates at a W/Mo-BiVO4photoanode were studied using surface interrogation scanning electrochemical microscopy (SI-SECM). Adsorbed hydroxyl radicals (OH•) were produced during water oxidation at the semiconductor surface under UV–visible irradiation and were subsequently electrochemically titrated by tip-generated reductant without irradiation. The IrCl62–/3–redox couple was used to determine the surface concentration of OH• in acidic solution. On W/Mo-BiVO4, ∼6% of the absorbed photons generate surface OH• with a coverage of 5.8 mC cm–2. Less than 1% of the irradiated photons were eventually used for water oxidation under high intensity irradiation (∼1 W cm–2) at the photoanode. Assuming that the primary decay mechanism of the adsorbed OH• on W/Mo-BiVO4is dimerization to produce hydrogen peroxide (H2O2), the rate constant was determined to be 4 × 103mol–1m2s–1. A faster decay rate of OH• was observed in the presence of excess methanol (a radical scavenger) in aqueous solution. In addition, quantitative analysis of the water oxidation processes at W/Mo-BiVO4along with the quantum efficiency for the oxygen evolution reaction was determined using SECM.

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.

Nanostructured Ta3N5photoanodes (band gap of ∼2.0 eV) were synthesized via a two-step process: first, nanocolumnar Ta2O5films were deposited by evaporation of tantalum metal in a vacuum chamber in a low pressure oxygen ambient followed by heating in an ammonia gas flow to convert Ta2O5into orthorhombic Ta3N5. Under Xe lamp irradiation (∼73 mW/cm2), a 100 nm nanoporous Ta3N5electrode achieved an anodic photocurrent of ∼1.4 mA/cm2at +0.5 V versus Ag/AgCl in 1 M KOH solution. By comparison, a dense film achieved ∼0.4 mA/cm2clearly illustrating the importance of nanostructuring for improving the performance of Ta3N5photoanodes. However, Ta3N5films suffered from inherent self-oxidation under light illumination, and application of a cobalt cocatalyst layer was found to improve the stability as well as photocatalytic activity of the Ta3N5films.

Tantalum nitride (Ta3N5) is a promising material for photoelectrochemical (PEC) water oxidation with a narrow band gap (2.1 eV) that can effectively utilize visible light in the solar spectrum. Ta3N5nanotube (NT) arrays were synthesized on a Ta foil by electrochemical anodization followed by an ammonia treatment at 800 °C. The photocurrent of nanostructured Ta3N5was over 3 times higher than that of a dense regular Ta3N5film in 0.1 M Na2SO4aqueous solution at pH 11. Several electrocatalysts (IrO2nanoparticles (NPs), Co3O4NPs, cobalt phosphate, and Pt NPs) were used to modify Ta3N5NTs for PEC water oxidation. The photocurrent of Ta3N5NTs modified with IrO2and Co3O4was ca. four times higher than that of unmodified NTs. Cobalt phosphate also showed a positive improvement for PEC water oxidation on Ta3N5NTs, whereas Pt was ineffective. Scanning electrochemical microscopy was used to measure the faradaic efficiency of the Ta3N5photoanodes for water oxidation, which can reach as high as 88% for a Co3O4–Ta3N5NTs photoanode, but is less than 15% at best, for Ta3N5without the electrocatalyst. The results indicate that cobalt oxide and cobalt phosphate are promising candidates as electrocatalysts on Ta3N5for water oxidation because Co is an earth-abundant material.

Metal doping of the metal oxide photoelectrocatalyst, BiVO4, dramatically increases its activity for water oxidation. Scanning electrochemical microscopy (SECM) was used to screen various dopants for their photoelectrochemical performance and to optimize the used dopant material concentrations with this photocatalyst. For example, adding Mo to W-doped BiVO4enhanced the performance. The photocatalytic activity was examined on larger electrodes by means of photoelectrochemical and electrochemical measurements. The developed photoelectrocatalyst, W- and Mo-doped BiVO4, shows a photocurrent for water oxidation that is more than 10 times higher than undoped BiVO4. Factors that affect performance are discussed, and enhanced separation of excited electron–hole pairs by doping onto the semiconductor is suggested by first-principles density-functional theory (DFT) calculations. Distortion of the crystal structure of monoclinic scheelite-like BiVO4by addition of W and Mo doping predicted by DFT is also revealed by X-ray diffraction and Rietveld refinement analysis. The results indicate that the consecutive doping of W and Mo into the metal oxide photocatalyst introduces improved electron–hole separation without a significant change of the band gap or the material's optical properties.

Oxygen evolution reaction (OER) electrocatalyst arrays for photoelectrochemical (PEC) water oxidation were fabricated on a metal oxide semiconductor photoelectrode, W-doped BiVO4(BiVW-O). The electrocatalysts (IrOx, Pt, Co3O4) were prepared on a drop cast film of BiVW-O on fluorine-doped tin oxide (FTO) with a picoliter solution dispenser or photodeposition with light irradiation through an optical fiber. The prepared arrays were tested for PEC water oxidation in 0.2 M sodium phosphate buffer (pH 6.8) using scanning electrochemical microscopy modified with an optical fiber. Pt and Co oxide electrocatalysts showed an enhanced photocurrent for PEC water oxidation, while the other metal oxide catalysts including IrOx, which is known as an excellent water oxidation electrocatalyst on a metal substrate, were not effective. These results were confirmed with bulk film studies. A cobalt phosphate (Co-Pi) electrocatalyst was also tested as a bulk film on BiVW-O and showed improvement for PEC water oxidation. Preliminary characterization (X-ray diffraction and X-ray photoelectron spectroscopy) was also performed for these catalysts. The results indicate that considerations of the semiconductor photocatalyst/electrocatalyst interface are important in determining the effectiveness of materials for the photodriven OER.

Photoelectrochemical (PEC) studies of CuInSe2(CIS) and Cu(In1−xGax)Se2(CIGS) thin films prepared by electrodeposition and spray coating of nanocrystals were carried out. PEC reductions of methyl viologen (MV) in aqueous solution and ethyl viologen (EV) in acetonitrile were used to evaluate the thin-film performance by measuring the photocurrent. Deposition of a CdS layer on CIS and CIGS films strongly enhanced the photocurrent; for example, a nanocrystal-CIS film showed about 100 times higher photocurrent with a thin CdS layer. Capacitance measurements and Mott−Schottky plots were obtained to find the flat band potential. Incident photon to current conversion efficiencies (IPCE) and absorbed photon to current conversion efficiencies (APCE) obtained from PEC measurements were about 20% and 40−70%, respectively.