van der Waals (vdW) metal chalcogenides have been extensively investigated as electrocatalysts for the hydrogen evolution reaction (HER); however, for the majority of these materials only the edges are active, thereby wasting most of the materials' surfaces. Recent research has focused on finding new materials with active basal planes. Herein, for the first time, we demonstrate that the hexagonal vdW Fe 3 GeTe 2 (FGT), also a spintronic candidate material, shows both basal plane and edge site HER activities. Partial exfoliation of bulk FGT through sonication increases both basal plane and edge sites leading to significantly improved overpotential. A subsequent compacting of the sample (using plasma sintering at room temperature) to produce a densified electrode leads to an impressive overpotential to drive a current density of 10 mA/cm 2 of −0.105 V. DFT free energy calculations not only showed that the high activity is due to the abundant active sites present on the hexagonal Te layer in FGT but also presented an even more HER active layer (106) containing both iron and tellurium active sites. Furthermore, the presence of a thin oxide layer on top of the active FGT layers, as found by XPS, suggests that the real active surface is likely a hybrid FGT/oxide layer. These results demonstrate the high basal plane and edge sites HER activity of FGT, thus opening a new avenue for the design and screening of related iron-based vdW materials, their composites, and their surface functionalization as high-performing electrocatalysts.
Glial cells perform numerous functions to support neuron development and function, including axon wrapping, formation of the blood brain barrier, and enhancement of synaptic transmission. We have identified a novel gene, raw, which functions in glia of the central and peripheral nervous systems in Drosophila. Reducing Raw levels in glia results in morphological defects in the brain and ventral nerve cord, as well as defects in neuron function, as revealed by decreased locomotion in crawling assays. Examination of the number of glia along peripheral nerves reveals a reduction in glial number upon raw knockdown. The reduced number of glia along peripheral nerves occurs as a result of decreased glial proliferation. As Raw has been shown to negatively regulate Jun N-terminal kinase (JNK) signaling in other developmental contexts, we examined the expression of a JNK reporter and the downstream JNK target, matrix metalloproteinase 1 (mmp1), and found that raw knockdown results in increased reporter activity and Mmp1 levels. These results are consistent with previous studies showing increased Mmp levels lead to nerve cord defects similar to those observed upon raw knockdown. In addition, knockdown of puckered, a negative feedback regulator of JNK signaling, also causes a decrease in glial number. Thus, our studies have resulted in the identification of a new regulator of gliogenesis, and demonstrate that increased JNK signaling negatively impacts glial development.
Semiconductors have been studied as the most promising thermoelectric materials for the past decades due to their high Seebeck coefficient. Semimetals usually have a small Seebeck coefficient due to a small difference in the density of states between electrons and holes, compensating each other in the longitudinal direction. However, recent studies suggested that semimetals with large asymmetry between electrons and holes could be good thermoelectric candidates. Here, we report the crystal structure and thermoelectric properties of layered van der Waals semimetal ZrTiSe4. Micrometer-sized single crystals of ZrTiSe4 were obtained by the solid-state reaction. The single-crystal X-ray diffraction study suggests that this material exhibits a trigonal structure with space group P3̅m1 (no. 164), instead of a P2/m (no. 10) monoclinic phase reported in the previous work. ZrTiSe4 has the same crystal structure as ZrSe2 and TiSe2 with Zr and Ti atoms occupying the same site with equal occupancy. Polycrystalline ZrTiSe4 samples consolidated by cold pressing show a semi-metallic type of resistivity in the temperature range of 2–400 K and an n-type conducting behavior. The samples exhibit an unusually large Seebeck coefficient of −202 ± 11 μV K–1 at 300 K, notably higher than the parent phases of ZrSe2 and TiSe2 as well as other semimetals. Based on the first-principles calculations of the electronic band structure, the large Seebeck coefficient of ZrTiSe4 could arise from the conduction band convergence at the M point, resulting in a large density of states (DOS) effective mass for electrons and a highly asymmetric DOS about the chemical potential. Furthermore, owing to a low sound velocity and strong phonon scattering by boundaries and defects, the thermal conductivity exhibits weak temperature dependence and a low value of 2.2 ± 0.4 W m–1 K–1 at 300 K. This work provides useful insights into the crystal structure and thermoelectric properties of semimetal ZrTiSe4.
Recent research on van der Waals (vdW) metal chalcogenides electrocatalysts for the hydrogen evolution reaction (HER) has been devoted to finding new catalysts with active basal planes. Here, we report on experimental and theoretical investigations of the HER activity of a recently discovered iron‐rich vdW spintronic material, Fe5Ge2Te2 (FG2T) in alkaline media (1 M KOH). We show that a densified electrode of FG2T requires an overpotential of only −90.5 mV to drive a current density of 10 mA/cm2. Free energy calculations of hydrogen adsorption using density functional theory (DFT) proved that the numerous sites present on the hexagonal Te layer are more active than those found in the recently proposed Fe3GeTe2 (FGT) catalyst, supporting higher activity for the new Fe‐richer catalyst. Like in FGT, XPS analysis has found that a thin oxide layer covers the active FG2T layers, suggesting the real active surface to be a hybrid FG2T/oxide layer. These results strengthen the idea of continued screening of iron‐based vdW materials to replace the non‐abundant platinum group electrocatalysts toward HER and other electrocatalytic processes.
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