New Uranium Chalcoantimonates, RbU₂SbS₈ and KU₂SbSe₈, with a Polar Noncentrosymmetric Structure

The new compounds, RbU2SbS8 and KU2SbSe8, were prepared as golden-black, blocklike crystals by the polychalcogenide molten flux method. RbU2SbS8 crystallizes in the monoclinic space group Cm with a = 7.9543(9) Å, b = 11.0987(13) Å, c = 7.2794(10) Å, β = 106.030(2)°, and Z = 2. The compound has a two-dimensional character with layers running perpendicular to the c-axis. The coordination geometry around the U4+ atoms is best described as a bicapped trigonal prism. The trigonal prisms share triangular faces with neighboring prisms, forming one-dimensional columns along the a-axis. The columns are then joined to construct sheets by sharing capping S atoms. Sb3+ ions are sitting at the center of a slightly distorted seesaw coordination environment (CN = 4). Rb+ ions are stabilized in 8-coordinate bicapped trigonal prismatic sites. KU2SbSe8 crystallizes in the monoclinic space group Cm with a = 11.5763(2) Å, b = 8.2033(1) Å, c = 15.2742(1) Å, β = 112.22(2)°, and Z = 4. KU2SbSe8 has essentially the same structure as RbU2SbS8. However, Sb3+ and K+ ions appear disordered in every other layer, resulting in a different unit cell. RbU2SbS8 is a semiconductor with a band gap of 1.38 eV. The band gap of KU2SbSe8 could not be determined precisely due to the presence of overlapping intense f−f transitions in the region (0.5−1.1 eV). The Raman spectra show the disulfide stretching vibration in RbU2SbS8 at 479 cm-1 and the diselenide stretching vibration in KU2SbSe8 at 252 cm-1. Magnetic susceptibility measurements indicate the presence of U4+ centers in the compounds. The compounds do not melt below 1000 °C under vacuum

Effect of a Cobalt-Based Oxygen Evolution Catalyst on the Stability and the Selectivity of Photo-Oxidation Reactions of a WO₃ Photoanode

A bare WO3 electrode and a WO3 electrode coupled with a layer of Co−Pi oxygen evolution catalyst (OEC) were prepared to investigate the effect of Co−Pi OEC on the selectivity of photo-oxidation reactions and photostabilities of WO3 photoanodes. WO3 photoanodes have been reported to produce peroxo species as well as O2 during photo-oxidation reactions, and the accumulation of peroxo species on the surface is known to cause a gradual loss of photoactivity of WO3. The photocurrent to O2 conversion efficiencies of the WO3 and WO3/Co−Pi OEC electrodes were obtained by simultaneously measuring the photocurrent and O2 gas generated during illumination at 0.8 V vs Ag/AgCl. The result shows that the presence of OEC increases the photocurrent to O2 conversion efficiency from approximately 61% to approximately 100%. The complete suppression of peroxo formation provided the WO3/Co−Pi OEC photoelectrode with long-term photostability. The photocurrent−potential characteristics show that the presence of OEC effectively reduces the electron−hole recombination near the flat band potential region and shifts the onset potential of photocurrent by 0.17 V to the negative direction. However, when the applied potential became more positive than approximately 0.35 V vs Ag/AgCl, the WO3/Co−Pi OEC electrode produced less initial photocurrent than the bare WO3 electrode. Mott−Schottky plots reveal the presence of interface states at the WO3/OEC junction that induce more electron−hole recombination when the Fermi level moves below these states. Regardless of the adverse effect on recombination present at 0.8 V vs Ag/AgCl, the WO3/Co−Pi OEC achieved a more efficient and sustainable solar to O2 conversion owing to the ability of Co−Pi OEC to significantly increase the photocurrent to O2 conversion efficiency and prevent the photocurrent decay of the WO3 electrode

Photocurrent Enhancement of n-Type Cu₂O Electrodes Achieved by Controlling Dendritic Branching Growth

Cu2O electrodes composed of dendritic crystals were produced electrochemically using a slightly acidic medium (pH 4.9) containing acetate buffer. The buffer played a key role for stabilizing dendritic branching growth as a pH drop during the synthesis prevents formation of morphologically unstable branches and promotes faceted growth. Dendritic branching growth enabled facile coverage of the substrate with Cu2O while avoiding growth of a thicker Cu2O layer and increasing surface areas. The resulting electrodes showed n-type behavior by generating anodic photocurrent without applying an external bias (zero-bias photocurrent under short-circuit condition) in an Ar-purged 0.02 M K2SO4 solution. The zero-bias photocurrent of crystalline dendritic electrodes was significantly higher than that of the electrodes containing micrometer-size faceted crystals deposited without buffer. In order to enhance photocurrent further a strategy of improving charge-transport properties by increasing dendritic crystal domain size was investigated. Systematic changes in nucleation density and size of the dendritic Cu2O crystals were achieved by altering the deposition potential, Cu2+ concentration, and acetate concentration. Increasing dendritic crystal size consistently resulted in the improvement of photocurrent regardless of the method used to regulate crystal size. The electrode composed of dendritic crystals with the lateral dimension of ca. 12000 μm2 showed more than 20 times higher zero-bias photocurrent than that composed of dendritic crystals with the lateral dimension of ca. 100 μm2. The n-type nature of the Cu2O electrodes prepared by this study were confirmed by linear sweep voltammetry with chopped light and capacitance measurements (i.e., Mott−Schottky plots). The flatband potential in a 0.2 M K2SO4 solution (pH 6) was estimated to be −0.78 vs Ag/AgCl reference electrode. The IPCE measured without applying an external bias was approximately 1% for the visible region. With appropriate doping studies and surface treatment to improve charge transport and interfacial kinetics more efficient n-type Cu2O electrodes will be prepared for use in various photoelectrochemical and photovoltaic devices

Electrochemical Bi/BiPO₄ Cells for a Sustainable Phosphate Cycle

Phosphorus is one of the main components of fertilizer and is also essential for various industrial manufacturing processes. While a continued increase in human population will require more fertilizer production, global phosphate rock reserves are limited. Furthermore, the collection of phosphate rock, its conversion to phosphoric acid, and disposal of phosphate create various environmental concerns. Here, we demonstrate intriguing electrochemical properties of Bi/BiPO4 electrodes which are used to construct phosphate removal and recovery cells. The Bi/BiPO4 cells selectively remove phosphate from a solution via a phosphate-specific electrode reaction and directly recover it as phosphoric acid without needing additional acid or generating any byproduct wastes. This discovery offers an unprecedented opportunity to produce phosphoric acid using phosphate wastes, which can lead to a sustainable phosphate cycle

Electrochemical Valorization of Furfural to Maleic Acid

Maleic acid (MA) is a platform chemical used for various industrial processes. In this study, a new electrochemical oxidation method that can convert furfural, which is one of the most important C5 platform chemicals derived from cellulosic biomass, to MA is reported. This method can produce MA in aqueous media under ambient temperature and pressure without the use of oxidizing agents. The use of acidic media, which promotes opening of the furan ring during oxidation, was critical for the formation of MA. PbO₂, MnO₂, and Pt, which are stable in acidic solutions under strongly oxidizing potentials, were investigated as anodes for furfural oxidation. The results showed that PbO₂ is the only catalyst that can convert furfural to MA as the major product. It was also revealed that the electrochemical conversion of furfural to MA proceeds with 2-furanol as an intermediate

Elucidating the Effect of Additives on the Growth and Stability of Cu₂O Surfaces via Shape Transformation of Pre-Grown Crystals

A new strategy of using pre-grown crystals to study preferential adsorption of various additives is demonstrated for the electrocrystallization of Cu2O. In this method, micron-size Cu2O crystals with well-defined cubic and octahedral shapes were first electrochemically grown, and their crystallization was resumed in a medium containing the additive to be investigated (e.g., Na+, NH4+, SO42-, Cl-, dodecyl sulfate). This method makes it possible to systematically study the interaction of additives with specific planes (e.g., {100} of a cube and {111} of an octahedron) already present. By observing shape transformation over time, the relative stabilities of {100}, {111}, and {110} planes of Cu2O in various growth media could be determined. During this study, a general scheme of forming new crystal shapes containing crystallographic planes that cannot be directly stabilized by preferential adsorption alone was also established (i.e., rhombicuboctahedral shape of Cu2O containing {110} planes). This method can be extended to other crystal systems, which will enable us to classify common features of additives (e.g., charges, type of atoms) and crystallographic planes (e.g., atomic arrangement, surface termination, surface charge) required to allow for strong preferential adsorption

Photodeposition of Co-Based Oxygen Evolution Catalysts on α-Fe₂O₃ Photoanodes

Cobalt-based oxygen evolution catalysts contain-ing phosphates (Co-Pi OEC) were photochemically deposited onto the surface of n-type α-Fe2O3 electrodes to enhance solar O2 production. α-Fe2O3 films used in this study were prepared by electrodepositing Fe films followed by thermal oxidation at 500 °C. The use of a nonaqueous plating solution made it possible to deposit adherent and uniform Fe films, which is difficult to achieve in an aqueous medium. Photodeposition of Co-Pi OEC was carried out by using photogenerated holes in the valence band of α-Fe2O3 to oxidize Co2+ ions to Co3+ ions in a phosphate buffer solution, which resulted in the precipitation of Co-Pi OEC on the α-Fe2O3 surface. Two different deposition conditions, open circuit (OC) and short circuit (SC) conditions, were studied comparatively to understand their effect on the growth and composition of Co-Pi OEC deposits. The results showed that the SC condition where the photoreduction reaction is physically separated from the photo-oxidation reaction significantly increased the yield and nucleation density of Co-Pi OECs, resulting in a better coverage of the α-Fe2O3 surface with Co-Pi OEC nanoparticles. X-ray photoelectron spectroscopy showed that the OC condition resulted in a higher Co2+/Co3+ ratio in the Co-Pi OEC deposits than the SC condition. This difference in composition is due to the simultaneous photoreduction occurring on the α-Fe2O3 surface under OC conditions. Co-Pi OEC improved the photocurrent of α-Fe2O3 electrodes more than Co2+ ions simply adsorbed on the α-Fe2O3 surface and the Co-Pi OEC deposited under SC conditions resulted in the most pronounced photocurrent enhancement. These results demonstrate the advantages of creating a SC condition for photodeposition of Co-Pi OECs. O2 detection measurements show that the presence of photodeposited Co-Pi OEC on the α-Fe2O3 surface not only increases the total amount of photocurrent generated by facilitating electron−hole pair separation but also increases the photocurrent to O2 conversion efficiency by improving O2 evolution kinetics

Sulfosalts with Alkaline Earth Metals. Centrosymmetric vs Acentric Interplay in Ba₃Sb_4.66S₁₀ and Ba_2.62Pb_1.38Sb₄S₁₀ Based on the Ba/Pb/Sb Ratio. Phases Related to Arsenosulfide Minerals of the Rathite Group and the Novel Polysulfide Sr₆Sb₆S₁₇

The new compounds, Sr6Sb6S17, Ba2.62Pb1.38Sb4S10, and Ba3Sb4.66S10 were prepared by the molten polychalcogenide salt method. Sr6Sb6S17 crystallizes in the orthorhombic space group P212121 with a = 8.2871(9) Å, b = 15.352(2) Å, c = 22.873(3) Å, and Z = 4. This compound presents a new structure type composed of [Sb3S7]5- units and trisulfide groups, (S3)2-, held together by Sr2+ ions. The [Sb3S7]5- fragment is formed from three corner-sharing SbS3 trigonal pyramids. The trisulfide groups are separated from the [Sb3S7]5- unit and embedded between the Sr2+ ions. Ba3Sb4.66S10 and Ba2.62Pb1.38Sb4S10 are not isostructural but are closely related to the known mineral sulfosalts of the rathite group. Ba3Sb4.67S10 is monoclinic P21/c with a = 8.955(2) Å, b = 8.225(2) Å, c = 26.756(5) Å, β = 100.29(3)°, and Z = 4. Ba2.62Pb1.38Sb4S10 is monoclinic P21 with a = 8.8402(2) Å, b = 8.2038(2) Å, c = 26.7623(6) Å, β = 99.488(1)°, and Z = 4. The Sb atoms are stabilized in SbS3 trigonal pyramids that share corners to build ribbonlike slabs, which are stitched by Ba/Pb atoms to form layers perpendicular to the c-axis. These materials are semiconductors and show optical band gaps of 2.10, 2.14, and 1.64 eV for Sr6Sb6S17, Ba3Sb4.66S10, and Ba2.62Pb1.38Sb4S10, respectively. Raman spectroscopic characterization is reported. Sr6Sb6S17, Ba3Sb4.66S10, and Ba2.62Pb1.38Sb4S10 melt congruently at 729, 770, and 749 °C, respectively

Si Extraction from Silica in a Basic Polychalcogenide Flux. Stabilization of Ba₄SiSb₂Se₁₁, a Novel Mixed Selenosilicate/Selenoantimonate with a Polar Structure

An unusual compound, Ba4SiSb2Se11, was discovered from a reaction of Ba/Th/Sb/Se. It is assumed that Si was extracted from the silica reaction tube. It forms as silver needlelike crystals in the polar space group Cmc21 with a = 9.3981(3) Å, b = 25.7192(7) Å, c = 8.7748 (3) Å, and Z = 4. A rational synthesis has been devised at 600 °C. The compound is composed of Ba2+ ions stabilized between infinite one-dimensional [SiSb2Se11]8- chains running parallel to the a axis. Each chain is composed of a [SbSe2]-∞ backbone with [SiSe4]4- tetrahedra chelating every other Sb atom from the same side of the backbone. The V-shaped triselenide groups, (Se3)2-, are attached to the rest of the Sb atoms in the chain through one of their terminal Se atoms. The compound has a band gap of 1.43 eV. The Raman spectrum shows a broad shift at 247 cm-1 and a shoulder around 234 cm-1, which are related to the Se−Se vibration of the triselenide groups and/or the Si−Se vibrations of the [SiSe4]4- groups. The compound decomposes at 522 °C

Electrochemical Dehydrogenation Pathways of Amines to Nitriles on NiOOH

Nitriles are highly important synthetic intermediates with applications in a wide variety of organic reactions including production of pharmaceuticals, fine chemicals, and agricultural chemicals. Thus, developing effective green routes to oxidize amines to nitriles is of great interest. One promising method to achieve the oxidation of primary amines to nitriles is through electrochemical oxidation on NiOOH electrodes. This reaction has long been thought to occur through an indirect mechanism consisting of a series of potential independent hydrogen atom transfer steps to catalytic Ni3+ sites in NiOOH, which reduces NiOOH to Ni­(OH)2. The role of the applied potential in this mechanism is simply to regenerate NiOOH by oxidizing Ni­(OH)2. In this work, we demonstrate that a second, potential-dependent pathway recently found to apply to alcohol and aldehyde oxidation on NiOOH and consisting of potential-dependent hydride transfer to Ni4+ sites is the dominant pathway for the oxidation of amines using propylamine and benzylamine as model systems. After qualitatively and quantitatively examining the contributions of indirect and potential-dependent oxidation pathways to amine oxidation on NiOOH, we also examine the effect the amine concentration, solution pH, applied bias, and deuterium substitution have on the two pathways, further clarifying their mechanisms and exploring what factors control their rate. This work provides a comprehensive understanding of the mechanism of primary amine oxidation on NiOOH