Growth Mechanisms and Electronic Properties of Vertically Aligned MoS2.

Thin films of layered semiconductors emerge as highly promising materials for energy harvesting and storage, optoelectronics and catalysis. Their natural propensity to grow as oriented crystals and films is one of their distinct properties under recent focal interest. Specifically, the reaction of transition metal films with chalcogen vapor can result in films of vertically aligned (VA) layers, while metal-oxides react with chalcogens in vapor phase to produce horizontally aligned crystals and films. The growth mechanisms of vertically oriented films are not yet fully understood, as well as their dependence on the initial metal film thickness and growth conditions. Moreover, the resulting electronic properties and the role of defects and disorder had not yet been studied, despite their critical influence on catalytic and device performance. In this work, we study the details of oriented growth of MoS2 with complementary theoretical and experimental approaches. We present a general theoretical model of diffusion-reaction growth that can be applied to a large variety of layered materials synthesized by solid-vapor reaction. Moreover, we inspect the relation of electronic properties to the structure of vertically aligned MoS2 and shed light on the density and character of defects in this material. Our measurements on Si-MoS2 p-n hetero-junction devices point to the existence of polarizable defects that impact applications of vertical transition-metal dichalcogenide materials.

Enhancing the catalytic activity of the alkaline hydrogen evolution reaction by tuning the S/Se ratio in the Mo(SxSe1-x)2 catalyst.

The alkaline hydrogen evolution reaction (HER) plays a key role in photo(electro)catalytic water splitting technologies, particularly in water-alkali electrolyzers. Unfortunately, although transition metal dichalcogenide (TMD) materials, especially MoS2 and MoSe2, are considered efficient, Earth-abundant catalysts for the HER in an acidic electrolyte, they are much less effective under high pH conditions due to a sluggish water dissociation process (Volmer-step) and strong adsorption of the OH- intermediate on their surfaces. Herein we show a novel synergetic effect obtained by tailoring the S/Se ratio in Mo(SxSe1-x)2 alloys. We were able to influence the metal oxidation state and d-band to optimize the catalytic sites for HOH dissociation and OH- desorption while maintaining favourable M-H energetics. The Mo(SxSe1-x)2 (x = 0.58) catalyst exhibited high performance with an onset potential of -43 mV in 0.5 M KOH, i.e., ∼3 and ∼4-fold less than that for MoSe2 and MoS2, respectively. The results obtained in the current study have implications for the rational design of cost-effective electro-catalysts for water reduction based on TMD alloys.

Improved catalytic activity of Mo1-xWxSe2 alloy nanoflowers promotes efficient hydrogen evolution reaction in both acidic and alkaline aqueous solutions.

Layered transition metal dichalcogenides are noble-metal free electrocatalysts for the hydrogen evolution reaction (HER). Instead of using the common hydrothermal synthesis, which requires high pressure and temperature, herein a relatively simple and controlled colloidal synthesis was used to produce an alloy of Mo1-xWxSe2 with nanoflower morphology as a model system for the electrocatalysis of hydrogen evolution in both acidic and alkaline environments. The results show that Mo1-xWxSe2 alloys exhibit better catalytic activity in both acidic and alkaline solutions with low overpotentials compared to pure MoSe2 and WSe2. Moreover, the electrode kinetics was studied using electrochemical impedance spectroscopy (EIS) and the results indicate that the alloys exhibit improved catalytic activity with low Tafel slopes, making them appealing for HER in either environment. Additionally, when MoSe2 nanoflowers (NFs) are prepared by using different metal salts and chalcogenide precursors, changes in the HER catalytic activity were observed, despite the morphology and crystal structure similarities. This finding suggests that different results reported in the literature could originate from different synthetic methods of the TMD, emphasizing that a better understanding of the relationship between the synthetic route and the catalytic performance is still lacking.

Increased calcium absorption from synthetic stable amorphous calcium carbonate: double-blind randomized crossover clinical trial in postmenopausal women.

Calcium supplementation is a widely recognized strategy for achieving adequate calcium intake. We designed this blinded, randomized, crossover interventional trial to compare the bioavailability of a new stable synthetic amorphous calcium carbonate (ACC) with that of crystalline calcium carbonate (CCC) using the dual stable isotope technique. The study was conducted in the Unit of Clinical Nutrition, Tel Aviv Sourasky Medical Center, Israel. The study population included 15 early postmenopausal women aged 54.9 ± 2.8 (mean ± SD) years with no history of major medical illness or metabolic bone disorder, excess calcium intake, or vitamin D deficiency. Standardized breakfast was followed by randomly provided CCC or ACC capsules containing 192 mg elemental calcium labeled with 44Ca at intervals of at least 3 weeks. After swallowing the capsules, intravenous CaCl2 labeled with 42Ca on was administered on each occasion. Fractional calcium absorption (FCA) of ACC and CCC was calculated from the 24-hour urine collection following calcium administration. The results indicated that FCA of ACC was doubled (± 0.96 SD) on average compared to that of CCC (p < 0.02). The higher absorption of the synthetic stable ACC may serve as a more efficacious way of calcium supplementation.

Solubility and bioavailability of stabilized amorphous calcium carbonate.

Since its role in the prevention of osteoporosis in humans was proven some 30 years ago, calcium bioavailability has been the subject of numerous scientific studies. Recent technology allowing the production of a stable amorphous calcium carbonate (ACC) now enables a bioavailability analysis of this unique form of calcium. This study thus compares the solubility and fractional absorption of ACC, ACC with chitosan (ACC-C), and crystalline calcium carbonate (CCC). Solubility was evaluated by dissolving these preparations in dilute phosphoric acid. The results demonstrated that both ACC and ACC-C are more soluble than CCC. Fractional absorption was evaluated by intrinsically labeling calcium carbonate preparations with (45)Ca, orally administrated to rats using gelatin capsules. Fractional absorption was determined by evaluating the percentage of the administrated radioactive dose per milliliter that was measured in the serum, calcium absorption in the femur, and whole-body retention over a 34-hour period. Calcium serum analysis revealed that calcium absorption from ACC and ACC-C preparations was up to 40% higher than from CCC, whereas retention of ACC and ACC-C was up to 26.5% higher than CCC. Absorbed calcium in the femurs of ACC-administrated rats was 30% higher than in CCC-treated animals, whereas 15% more calcium was absorbed following ACC-C treatment than following CCC treatment. This study demonstrates the enhanced solubility and bioavailability of ACC over CCC. The use of stable ACC as a highly bioavailable dietary source for calcium is proposed based on the findings of this study.