Self-Polycondensation Flux Synthesis of Ultrastable Olefin-Linked Covalent Organic Frameworks for Electrocatalysis.

Creating new functional materials that efficiently support noble metal catalysts is important and in high demand. Herein, we develop a self-polycondensation flux synthesis strategy that can produce olefin-linked covalent organic framework (COF) platforms with high crystallinity and porosity as the supports of Pd nanoparticles for electrocatalytic nitrogen reduction reaction (ENRR). A series of "two in one" monomers integrating aldehyde and methyl reactive groups are rationally designed to afford COFs with square-shaped pores and ultrahigh chemical stability (e.g., strong acid or alkali environments for >1 month). Functionalizing Fluorine significantly boosts the hydrophobicity of fluoro-functionalized COFs, which can inhibit the competing hydrogen evolution reaction (HER) and enhance ENRR performances. The COFs loading Pd nanoparticles show high NH3 production yields up to 90.0 ± 2.6 µg·h-1·mgcat.-1 and the faradaic efficiency of 44% at -0.2 V versus reversible hydrogen electrode, the best comprehensive performance among all reported COFs. Meanwhile, the catalysts are easy to recover and recycle, as demonstrated by their use for 15 cycles and 17 hours, with good performance retention. This work not only provides a new synthesis strategy for olefin-linked COFs, but also paves a new avenue for the design of highly efficient ENRR catalysts.

Two-Step Flux Synthesis of Ultrapure Transition-Metal Dichalcogenides.

Two-dimensional transition-metal dichalcogenides (TMDs) have attracted tremendous interest due to the unusual electronic and optoelectronic properties of isolated monolayers and the ability to assemble diverse monolayers into complex heterostructures. To understand the intrinsic properties of TMDs and fully realize their potential in applications and fundamental studies, high-purity materials are required. Here, we describe the synthesis of TMD crystals using a two-step flux growth method that eliminates a major potential source of contamination. Detailed characterization of TMDs grown by this two-step method reveals charged and isovalent defects with densities an order of magnitude lower than those in TMDs grown by a single-step flux technique. For WSe2, we show that increasing the Se/W ratio during growth reduces point defect density, with crystals grown at 100:1 ratio achieving charged and isovalent defect densities below 1010 and 1011 cm-2, respectively. Initial temperature-dependent electrical transport measurements of monolayer WSe2 yield room-temperature hole mobility above 840 cm2/(V s) and low-temperature disorder-limited mobility above 44,000 cm2/(V s). Electrical transport measurements of graphene-WSe2 heterostructures fabricated from the two-step flux grown WSe2 also show superior performance: higher graphene mobility, lower charged impurity density, and well-resolved integer quantum Hall states. Finally, we demonstrate that the two-step flux technique can be used to synthesize other TMDs with similar defect densities, including semiconducting 2H-MoSe2 and 2H-MoTe2 and semimetallic Td-WTe2 and 1T'-MoTe2.

Impact of Ball Milling on the Hydrogen Evolution Performance of Cu2Sn0.38Ge0.62S3 Photocatalytic Particles Synthesized via a Flux Method.

Ball milling has been shown empirically to produce fine photocatalytic particles from large bulky particles but to drastically reduce the photocatalytic activity of such material during water splitting due to mechanical damage to the photocatalyst surfaces. If the damaged photocatalyst surfaces could be removed or reconstructed, the reduced particle sizes resulting from milling would be expected to provide enhanced photocatalytic activity. In the present study, fine particles of crystalline Cu2Sn0.38Ge0.62S3 (CTGS), which is responsive to long wavelength light up to the near-infrared region, were synthesized by a flux method and subsequent ball milling. A photocathode made of such particles showed significantly enhanced photoelectrochemical (PEC) performance under simulated sunlight while the photocatalytic hydrogen evolution activity of a powder suspension system made from the same material exhibited a typical decrease. The CTGS crystalline particles synthesized using the flux method were found to be highly crystalline but to have relatively large micrometer-scale sizes. Ball milling reduced the particle size but produced an amorphous coating of oxidized species that lowered the photocatalytic activity of the powder suspension system. Typical surface modifications of a photocathode made from this material, consisting of wet chemical processes, also served as an etching treatment to successfully remove the minimally crystalline surface layer and provide greater PEC activity. These data suggest the benefits of combining flux crystal growth with ball milling and the appropriate chemical etching process to obtain high-crystallinity fine photocatalytic particles responsive to long wavelength light with improved PEC hydrogen evolution activity.

Sr(Ag1-x Lix )2 Se2 and [Sr3 Se2 ][(Ag1-x Lix )2 Se2 ] Tunable Direct Band Gap Semiconductors.

Synthesizing solids in molten fluxes enables the rapid diffusion of soluble species at temperatures lower than in solid-state reactions, leading to crystal formation of kinetically stable compounds. In this study, we demonstrate the effectiveness of mixed hydroxide and halide fluxes in synthesizing complex Sr/Ag/Se in mixed LiOH/LiCl. We have accessed a series of two-dimensional Sr(Ag1-x Lix )2 Se2 layered phases. With increased LiOH/LiCl ratio or reaction temperature, Li partially substituted Ag to form solid solutions of Sr(Ag1-x Lix )2 Se2 with x up to 0.45. In addition, a new type of intergrowth compound [Sr3 Se2 ][(Ag1-x Lix )2 Se2 ] was synthesized upon further reaction of Sr(Ag1-x Lix )2 Se2 with SrSe. Both Sr(Ag1-x Lix )2 Se2 and [Sr3 Se2 ][(Ag1-x Lix )2 Se2 ] exhibit a direct band gap, which increases with increasing Li substitution (x). Therefore, the band gap of Sr(Ag1-x Lix )2 Se2 can be precisely tuned via fine-tuning x that is controlled by only the flux ratio and temperature.

Bi4TaO8Cl as a New Class of Layered Perovskite Oxyhalide Materials for Piezopotential Driven Efficient Seawater Splitting.

Piezocatalytic water splitting is an emerging approach to generate "green hydrogen" that can address several drawbacks of photocatalytic and electrocatalytic approaches. However, existing piezocatalysts are few and with minimal structural flexibility for engineering properties. Moreover, the scope of utilizing unprocessed water is yet unknown and may widely differ from competing techniques due to the constantly varying nature of surface potential. Herein, we present Bi4TaO8Cl as a representative of a class of layered perovskite oxyhalide piezocatalysts with high hydrogen production efficiency and exciting tailorable features including the layer number, multiple cation-anion combination options, etc. In the absence of any cocatalyst and scavenger, an ultrahigh production rate is achievable (1.5 mmol g-1 h-1), along with simultaneous generation of value-added H2O2. The production rate using seawater is somewhat less yet appreciably superior to photocatalytic H2 production by most oxides as well as piezocatalysts and has been illustrated using a double-layer model for further development.

Visible-Light-Responsive Oxyhalide PbBiO2Cl Photoelectrode: On-Site Flux Synthesis on a Fluorine-Doped Tin Oxide Electrode.

The performance of photoelectrodes is hugely affected by the preparation method. Although a flux synthesis is useful to endow semiconductor particles with the desired properties such as high crystallinity, there are only a few reports on its application to photoelectrode fabrication, probably because relatively high temperatures are necessary. In the present study, we introduce a new concept for on-site flux synthesis of semiconductor crystals on a commonly used fluorine-doped tin oxide (FTO) substrate; a seed layer is predeposited and then treated with an appropriate flux containing other required elements at a right temperature lower than the limit temperature of FTO but sufficiently high to transform the seed layer to the target material with the aid of flux. Here, an oxyhalide PbBiO2Cl, one of the promising semiconductors for achieving visible-light water splitting, is selected as a target material. Combination of a BiOCl seed layer and the NaCl-PbCl2 flux containing other precursors enables the seed layer to transform into PbBiO2Cl crystals even at 450 °C. The thickness of the PbBiO2Cl layer can be controlled by changing the thickness of the BiOCl seed layer for efficient photon-to-current conversion. Owing to a good contact at the semiconductor-substrate interfaces as well as the high quality of PbBiO2Cl crystals, the flux-synthesized PbBiO2Cl photoelectrode shows a significantly improved PEC performance compared with those prepared from the particulate PbBiO2Cl samples via the conventional squeegee method. In addition, the present PbBiO2Cl photoelectrodes exhibit both anodic and cathodic photoresponses with substantially high current values depending on the applied potentials; the unusual phenomenon is affected by the conditions in flux-assisted synthesis. The present study provides a new and effective way for fabricating efficient photoelectrodes of various semiconductors on various substrates and a possible option to control their morphologies and p/n types for further improvement in performance.

Hydro-flux synthesis and crystal structure of Tl3IO.

Single-crystals of thallium(I) iodide oxide Tl3IO were obtained as by-product in a hydro-flux synthesis at 473 K for 10 h. A potassium hydroxide hydro-flux with a water-base molar ratio of 1.6 and the starting materials TlNO3, RhI3 and Ba(NO3)2 was used, resulting in a few black needle-shaped crystals. X-ray diffraction on a single-crystal revealed the hexa-gonal space group P63/mmc (No. 194) with lattice parameters a = 7.1512 (3) Šand c = 6.3639 (3) Å. Tl3IO crystallizes as hexa-gonal anti-perovskite (anti-BaNiO3 type) and is thus structurally related to the alkali-metal halide/auride oxides M 3 XO (M = K, Rb, Cs; X = Cl, Br, I, Au). The oxygen atoms center thallium octa-hedra. The [OTl6] octa-hedra share trans faces, forming a linear chain along [001]. Twelve thallium atoms surround each iodine atom in an [ITl12] anti-cubocta-hedron. Thallium and iodine atoms together form a hexa-gonal close-sphere packing, in which every fourth octa-hedral void is occupied by oxygen.

Flux Synthesis of Layered Oxyhalide Bi4NbO8Cl Photocatalyst for Efficient Z-Scheme Water Splitting Under Visible Light.

An oxyhalide photocatalyst Bi4NbO8Cl has recently been proven to stably oxidize water under visible light, enabling the Z-scheme water splitting when coupled with another photocatalyst for water reduction. We herein report the synthesis of Bi4NbO8Cl particles via a flux method, testing various molten salts to improve its crystallinity and hence photocatalytic activity. The eutectic mixture of CsCl/NaCl with a low melting point allowed the formation of single-phase Bi4NbO8Cl at as low as 650 °C. Thus, synthesized Bi4NbO8Cl particles exhibited a well-grown and plate-like shape while maintaining surface area considerably higher than those grown with others fluxes. They showed three times higher O2 evolution rate under visible light than the samples prepared via a solid-state reaction. Time-resolved microwave conductivity measurements revealed greater signals (approximately 4.8 times) owing to the free electrons in the conduction band, indicating much improved efficiency of carrier generation and/or its mobility. The loading of RuO2 or Pt cocatalyst on Bi4NbO8Cl further enhanced the activity for O2 evolution because of efficient capturing of free electrons, facilitating the surface chemical reactions. In combination with a H2-evolving photocatalyst Ru/SrTiO3:Rh along with an Fe3+/Fe2+ redox mediator, the RuO2/Bi4NbO8Cl is an excellent O2-evolving photocatalyst, exhibiting highly effective water splitting into H2 and O2 via the Z-scheme.

Synthesis of KCa2Nb3O10 Crystals with Varying Grain Sizes and Their Nanosheet Monolayer Films As Seed Layers for PiezoMEMS Applications.

The layered perovskite-type niobate KCa2Nb3O10 and its derivatives show advantages in several fields, such as templated film growth and (photo)catalysis. Conventional synthesis routes generally yield crystal size smaller than 2 µm. We report a flux synthesis method to obtain KCa2Nb3O10 crystals with significantly larger sizes. By using different flux materials (K2SO4 and K2MoO4), crystals with average sizes of 8 and 20 µm, respectively, were obtained. The KCa2Nb3O10 crystals from K2SO4 and K2MoO4 assisted synthesis were protonated and exfoliated into monolayer nanosheets, and the optimal exfoliation conditions were determined. Using pulsed laser deposition, highly (001)-oriented piezoelectric stacks (SrRuO3/PbZr0.52Ti0.48O3/SrRuO3, SRO/PZT/SRO) were deposited onto Langmuir-Blodgett films of Ca2Nb3O10(-) (CNO) nanosheets with varying lateral nanosheet sizes on Si substrates. The resulting PZT thin films showed high crystallinity irrespective of nanosheet size. The small sized nanosheets yielded a high longitudinal piezoelectric coefficient d33 of 100 pm/V, while the larger sized sheets had a d33 of 72 pm/V. An enhanced transverse piezoelectric coefficient d31 of -107 pm/V, an important input parameter for the actuation of active structures in microelectromechanical systems (MEMS) devices, was obtained for PZT films grown on CNO nanosheets with large lateral size, while the corresponding value on small sized sheets was -96 pm/V.

Flux-Aided Synthesis of Lu2O3 and Lu2O3:Eu-Single Crystal Structure, Morphology Control and Radioluminescence Efficiency.

Li2SO4 or (Li2SO4 + SiO2)-mixture fluxes were used to prepare a Lu2O3:Eu powder phosphor as well as an undoped Lu2O3 utilizing commercial lutetia and europia as starting reagents. SEM images showed that the fabricated powders were non-agglomerated and the particles sizes varied from single microns to tens of micrometers depending largely on the flux composition rather than the oxide(s)-to-flux ratio. In the presence of SiO2 in the flux, certain grains grew up to 300-400 µm. The lack of agglomeration and the large sizes of crystallites allowed making single crystal structural measurements and analysis on an undoped Lu2O3 obtained by means of the flux technique. The cubic structure with a = 10.393(2) Å, and Ia space group at 298 K was determined. The most efficient radioluminescence of Lu2O3:Eu powders reached 95%-105% of the commercial Gd2O2S:Eu.