Surface Interrogation Scanning Electrochemical Microscopy (SI-SECM) of Photoelectrochemistry at a W/Mo-BiVO₄ Semiconductor Electrode: Quantification of Hydroxyl Radicals during Water Oxidation

Reaction kinetics and surface coverage of water oxidation intermediates at a W/Mo-BiVO₄ photoanode 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 IrCl₆^2–/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-BiVO₄ is dimerization to produce hydrogen peroxide (H₂O₂), the rate constant was determined to be 4 × 10³ mol^–1 m² s^–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-BiVO₄ along with the quantum efficiency for the oxygen evolution reaction was determined using SECM

Single Collision Events of Conductive Nanoparticles Driven by Migration

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

Visible Light Photoelectrochemical Properties of PbCrO₄, Pb₂CrO₅, and Pb₅CrO₈

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 Pb₅CrO₈ are further studied following the combinatorial screening. In the bulk electrode measurements, the Pb₂CrO₅ bulk electrode displays the highest photocurrent of 0.23 mA/cm² for SO₃^2– oxidation at 0.4 V vs Ag/AgCl under 100 mW/cm² UV–vis light irradiation. Pb₂CrO₅ presents 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 Pb₅CrO₈ are also determined by electrochemical analyses and ultraviolet photoelectron spectroscopy (UPS)

Reliable Multistate Data Storage with Low Power Consumption by Selective Oxidation of Pyramid-Structured Resistive Memory

Multilevel data storage using resistive random access memory (RRAM) has attracted significant attention for addressing the challenges associated with the rapid advances in information technologies. However, it is still difficult to secure reliable multilevel resistive switching of RRAM due to the stochastic and multiple formation of conductive filaments (CFs). Herein, we demonstrate that a single CF, derived from selective oxidation by a structured Cu active electrode, can solve the reliability issue. High-quality pyramidal Cu electrodes with a sharp tip are prepared via the template-stripping method. Morphology-dependent surface energy facilitates the oxidation of Cu atoms at the tip rather than in other regions, and the tip-enhanced electric fields can accelerate the transport of the generated Cu ions. As a result, CF growth occurs mainly at the tip of the pyramidal electrode, which is confirmed by high-resolution electron microscopy and elemental analysis. The RRAM exhibits highly uniform and low forming voltages (the average forming voltage and its standard deviation for 20 pyramid-based RRAMs are 0.645 and 0.072 V, respectively). Moreover, all multilevel resistance states for the RRAMs are clearly distinguished and show narrow distributions within 1 order of magnitude, leading to reliable cell-to-cell performance for MLC operation

ZnWO₄/WO₃ Composite for Improving Photoelectrochemical Water Oxidation

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/WO₃ composites, a 9% Zn/W ratio produced an increased photocurrent over pristine WO₃ 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 % ZnWO₄/WO₃ resulted in a 2.5-fold increase in the photocurrent compared to pristine WO₃ for 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 WO₃ and ZnWO₄/WO₃ electrodes. X-ray diffraction showed the composite nature of the WO₃ and ZnWO₄ mixtures. The UV/vis absorbance and PEC action spectra demonstrated that WO₃ has a smaller band gap than ZnWO₄, while Mott–Schottky analysis determined that ZnWO₄ has 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/WO₃ composite was formed in a single annealing step

Tantalum Cobalt Nitride Photocatalysts for Water Oxidation under Visible Light

Tantalum cobalt nitride photocatalysts were prepared using a simple drop coating method on a Ta foil substrate followed by thermal ammonia treatment, and their photoelectrochemical (PEC) properties for water oxidation under visible light were studied. The resulting Ta_0.9Co_0.1N_<i>x</i> films showed a photocurrent of ca. 1.5 mA/cm² (12 times higher than that of Ta₃N₅) under 100 mW/cm² visible light irradiation at 0.7 V vs Ag/AgCl in a 0.1 M Na₂SO₄ aqueous solution (pH 11). The good PEC performance was attributed to the introduction of cobalt and the formation of cobalt nitride, which efficiently facilitates electron transfer and suppresses the recombination of photogenerated electron–hole pairs. Some cobalt nitride could further be oxidized to generate cobalt oxide, which serves as an efficient electrocatalyst for water oxidation. The enhanced visible light activity and film stability under light irradiation make tantalum cobalt nitride a promising semiconductor for PEC water oxidation

Unraveling the Role of Liquid Metal Catalysts in Electrochemical Growth of Solar Si from SiO₂ in CaCl₂‑Based Molten Salt: Enhancement of Crystallization, Purity, and Photoresponse

The electrochemical growth of Si has been developed as an alternative method of producing highly crystalline and pure semiconducting solar Si. The use of liquid catalysts for the electrochemical growth of semiconductors has been suggested to improve their quality. In this study, the role of liquid catalysts during the direct electrochemical growth of Si from SiO2 in CaCl2-based molten salts was investigated by comparing the crystallinity, purity, and photoresponse of Si deposited with and without liquid catalysts. A liquid catalyst was introduced into Si via the codeposition of Au or Ag, thereby forming a eutectic liquid mixture with Si during electrochemical growth. The use of liquid catalysts did not affect the growth rate of Si. Conversely, the liquid catalysts cause significant morphological changes associated with an approximately 2-fold increase in the grain size of Si, as estimated from X-ray diffraction and Raman analyses, by offering a low interface energy for Si nucleation. In addition, secondary ion and inductively coupled plasma mass spectroscopy analyses revealed that Al from the SiO2 feedstock was the primary impurity within the Si deposit and that its amount decreased when a liquid catalyst was used to filter the impurities during Si growth. Subsequently, the enhanced crystallization and purification of the liquid catalysts increased the photoresponse of the Si deposit, as confirmed by photoelectrochemical measurements. These results suggest that the electrochemical growth of Si using liquid catalysts can enhance the quality of Si, reflecting its potential to replace the conventional process of Si production

Synthesis of Ta₃N₅ Nanotube Arrays Modified with Electrocatalysts for Photoelectrochemical Water Oxidation

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. Ta₃N₅ nanotube (NT) arrays were synthesized on a Ta foil by electrochemical anodization followed by an ammonia treatment at 800 °C. The photocurrent of nanostructured Ta₃N₅ was over 3 times higher than that of a dense regular Ta₃N₅ film in 0.1 M Na₂SO₄ aqueous solution at pH 11. Several electrocatalysts (IrO₂ nanoparticles (NPs), Co₃O₄ NPs, cobalt phosphate, and Pt NPs) were used to modify Ta₃N₅ NTs for PEC water oxidation. The photocurrent of Ta₃N₅ NTs modified with IrO₂ and Co₃O₄ was ca. four times higher than that of unmodified NTs. Cobalt phosphate also showed a positive improvement for PEC water oxidation on Ta₃N₅ NTs, whereas Pt was ineffective. Scanning electrochemical microscopy was used to measure the faradaic efficiency of the Ta₃N₅ photoanodes for water oxidation, which can reach as high as 88% for a Co₃O₄–Ta₃N₅ NTs photoanode, but is less than 15% at best, for Ta₃N₅ without the electrocatalyst. The results indicate that cobalt oxide and cobalt phosphate are promising candidates as electrocatalysts on Ta₃N₅ for water oxidation because Co is an earth-abundant material

Electrochemical Synthesis of NH₃ at Low Temperature and Atmospheric Pressure Using a γ‑Fe₂O₃ Catalyst

The electrochemical synthesis of NH₃ by the nitrogen reduction reaction (NRR) at low temperature (<65 °C) and atmospheric pressure using nanosized γ-Fe₂O₃ electrocatalysts were demonstrated. The activity and selectivity of the catalyst was investigated both in a 0.1 M KOH electrolyte and when incorporated into an anion-exchange membrane electrode assembly (MEA). In a half-reaction experiment conducted in a KOH electrolyte, the γ-Fe₂O₃ electrode presented a faradaic efficiency of 1.9% and a weight-normalized activity of 12.5 nmol h^–1 mg^–1 at 0.0 V_RHE. However, the selectivity toward N₂ reduction decreased at more negative potentials owing to the competing proton reduction reaction. When the γ-Fe₂O₃ nanoparticles were coated onto porous carbon paper to form an electrode for a MEA, their weight-normalized activity for N₂ reduction was found to increase dramatically to 55.9 nmol h^–1 mg^–1. However, the weight- and area-normalized N₂ reduction activities of γ-Fe₂O₃ decreased progressively from 35.9 to 14.8 nmol h^–1 mg^–1 and from 0.105 to 0.043 nmol h^–1 cm^–2_act, respectively, during a 25 h MEA durability test. In summary, a study of the fundamental behavior and catalytic activity of γ-Fe₂O₃ nanoparticles in the electrochemical synthesis of NH₃ under low temperature and pressure is presented

Influence of TiO₂ Particle Size on Dye-Sensitized Solar Cells Employing an Organic Sensitizer and a Cobalt(III/II) Redox Electrolyte

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 TiO₂ nanoparticles (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 TiO₂ NPs, 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 TiO₂ particles 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 TiO₂ photoelectrodes for DSSCs employing cobalt redox electrolytes