Quantum critical points separating weak ferromagnetic and paramagnetic phases trigger many novel phenomena. Dynamical spin fluctuations not only suppress the long‐range order, but can also lead to unusual transport and even superconductivity. Combining quantum criticality with topological electronic properties presents a rare and unique opportunity. Here, by means of ab initio calculations and magnetic, thermal, and transport measurements, it is shown that the orthorhombic CoTe2 is close to ferromagnetism, which appears suppressed by spin fluctuations. Calculations and transport measurements reveal nodal Dirac lines, making it a rare combination of proximity to quantum criticality and Dirac topology.
Cu2TSiS4 (T = Mn and Fe) polycrystalline and single-crystal materials were prepared with high-temperature solid-state and chemical vapor transport methods, respectively. The polar crystal structure (space group Pmn21) consists of chains of corner-sharing and distorted CuS4, Mn/FeS4, and SiS4 tetrahedra, which is confirmed by Rietveld refinement using neutron powder diffraction data, X-ray single-crystal refinement, electron diffraction, energy-dispersive X-ray spectroscopy, and second harmonic generation (SHG) techniques. Magnetic measurements indicate that both compounds order antiferromagnetically at 8 and 14 K, respectively, which is supported by the temperature-dependent (100–2 K) neutron powder diffraction data. Additional magnetic reflections observed at 2 K can be modeled by magnetic propagation vectors k = (1/2,0,1/2) and k = (1/2,1/2,1/2) for Cu2MnSiS4 and Cu2FeSiS4, respectively. The refined antiferromagnetic structure reveals that the Mn/Fe spins are canted away from the ac plane by about 14°, with the total magnetic moments of Mn and Fe being 4.1(1) and 2.9(1) μB, respectively. Both compounds exhibit an SHG response with relatively modest second-order nonlinear susceptibilities. Density functional theory calculations are used to describe the electronic band structures.
A metastable polycrystalline Ag2GeS3 compound was prepared at 1000 °C with binary Ag2S and GeS2 as starting materials. At room temperature, Ag2GeS3 was determined to adopt a polar orthorhombic crystal structure (space group Cmc21) based on the Rietveld refinement of synchrotron X-ray diffraction data. The crystal structure consists of layers of distorted AgS4 and GeS4 tetrahedra stacked along the crystallographic c axis. UV–vis diffuse reflectance spectra identify Ag2GeS3 as a semiconductor with an optical indirect band gap of 2.04 eV. Thermal analysis inside a sealed tube indicates that Ag2GeS3 undergoes several phase transitions but reforms upon cooling after heating to 1000 °C. Thermoelectric conductivity measurements show that Ag2GeS3 exhibits glass-like ultralow lattice thermal conductivity of 0.26 W/m·K at 300 K and 0.22 W/m·K at 380 K. The theoretical calculation of lattice thermal conductivity based on density functional theory using the modified Debye–Callaway model shows a good qualitative agreement with experimental results between 50 and 400 K. A combination of the zig-zag atomic arrangement that decorates the crystal structure, electronic structure features dominated by the hybridization of Ag–S bonds near the Fermi energy, and nearly flat or dispersion-less low-energy optical phonon branches where Ag acts as the rattler atom provides plausible reasons for the observed ultralow lattice thermal conductivity.
In this study, the metal salt, Ni(II) Chloride, was reacted to the organic ligands of disodium terephthalate (C8H4Na2O4) to synthesize the nickel terephthalate complex as metal organic framework (Ni-MOF) in aqueous media. The Ni-MOF was synthesized at room temperature (RT), with crystal formation occurring instantly and continuing to increase for up to a week. The catalytic efficiency of the Ni-MOF catalyst was also tested in aqueous solution with sodium borohydride. Reduction of sodium borohydride via the Ni-MOF showed an overall increase in hydrogen production over the metal-salt catalyst with increased volumes of 43.56 mL to 70.03 mL for the MOF at 303 K. The rates of reaction were also increased for the Ni-MOF over the nickel metal salt from 0.006 min -1 to 0.02 min -1 as a second order reaction.
As the worlds supply of fossil fuels dwindles, more and more research is being conducted to find a reliable alternative energy source. One promising source of energy is hydrogen, which is not only the most abundant element in the universe, but when burned as a fuel source produces only water as a biproduct. The main issue preventing hydrogen from being utilized as a fuel source is the storage. Hydrogen primarily exists as a gas and must be stored in compressed tanks. These tanks run the risk of explosions, so work is being done to produce hydrogen gas over time with a hydrogen feedstock material (HFM). Sodium borohydride (NaBH4) is an HFM that contains 10.8% hydrogen by weight and readily reacts with water to produce hydrogen gas.1 This reaction occurs slowly and needs a catalyst to make its hydrogen production viable. In the past, work has been done with precious metal catalysts and metal catalyst support on carbon materials.2,3 The goal of this study was to explore a novel method of synthesizing nickel borides supported on graphene (NiB@G). NiB@G was formed by the reduction of a metal organic framework precursor (NiMOF-5@GO) at room temperature (RT 295K). Both NiMOF-5@GO and NiB@G were characterized using Powder X-Ray Diffraction (PXRD), Scanning Electron Mircroscopy (SEM, Figure 1), Energy Dispersive X-Ray Spectroscopy (EDS), Transmission Electron Microscopy (TEM), and Fourier Transform Infrared Spectroscopy (FTIR). NiB@G was additionally characterized with Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and tested for its catalytic ability at temperatures of 283K-303K. This product showed an excellent activation energy of 49 kJ mol-1 compared to previous work with nickel, cobalt, and raney-nickel catalysts.4 This product also showed good stability, performing well after multiple uses of the same catalyst. A novel method of producing an efficient catalyst for the hydrolysis of sodium borohydride will help scientists move forward in the mass production of hydrogen fuel. Works Cited: Schlesingehr H.I.; Brown, H.C.; Finholt A.E.; Gilbreath J.R.; Hoekstra H.R.; Hyde E.L.; Sodium Borohydride, Its Hydolysis and its Use as a Reducing Agent in the Generation of Hydrogen; Chem. Soc., (1953), 75 (1), pp 215–219 Clay Huff, Julia M. Long, Austin Heyman, and Tarek M. Abdel-Fattah Palladium Nanoparticle Multiwalled Carbon Nanotube Composite as Catalyst for Hydrogen Production by the Hydrolysis of Sodium BorohydrideACS Applied Energy Materials 2018 1 (9), 4635-4640 DOI: 10.1021/acsaem.8b00748 Huff, C., Long, J. M., Aboulatta, A., Heyman, A., & Abdel-Fattah, T. M. Silver Nanoparticle/Multi-Walled Carbon Nanotube Composite as Catalyst for Hydrogen Production. ECS Journal of Solid State Science and Technology (2017) 6(10), M115–M118. doi:10.1149/2.0051710jss Kaufman, C. M., & Sen, B. (). Hydrogen generation by hydrolysis of sodium tetrahydroborate: effects of acids and transition metals and their salts. Journal of the Chemical Society, Dalton Transactions, 1985 (2), 307. doi:10.1039/dt9850000307 Figure 1: SEM images at 50 microns of the graphene support for the nickel borides. The clear presence of graphene like sheets is seen in the image with conglomerates of sheets have dimensions of much larger than 50 microns Figure 1
The current world energy crisis is concerned with the depletion of natural resources used by power industries. Hydrogen energy produced from feedstock materials such as sodium borohydride (NaBH4) arises as a potential solution for energy demand, but it meets a challenge in finding a durable, efficient and economical catalyst.1,2 A catalyst is required to produce a usable amount of hydrogen through the hydrolysis reaction of NaBH4, due to the limitation that the procedure does not occur quickly enough on its own.2 Metal borides have been researched and applied in various industries as an effective catalyst, but the method of production appears to be costly and eco-unfriendly.3,4 In this study, manganese metal organic frame works (Mn-MOFs) and multi-walled carbon nanotubes (MWCNTs) were applied to synthesize supported manganese borides at room temperature in aqueous conditions. The MOFs have been applied in solving environmental pollution issues and enhance the catalytic effect.5 When combined with MWCNT, it further improves the stability, surface area, and reusability of the manganese borides. The reduction of Mn-MOFs leads to the formation of manganese borides on MWCNTs in the presence of NaBH4, which acts as both a reduction agent and a hydrogen feedstock material in this experiment. The physicochemical structures of the obtained composite, MnB@MWCNT was characterized via x-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and inductively coupled plasma (ICP). The catalytic capability of MnB@MWCNT was tested in an aqueous solution of NaBH4 under various conditions. The catalytic ability and reusability of MnB@MWCNT was measured by a water displacement method while the activation energy was found using a plot of the Arrhenius equation which utilized various temperature trials of 283 K, 295 K and 303 K.6 Five consecutive reusability trials were performed separately in which the production of hydrogen fell from 62.12 mL to 18.44 mL by the final trial and the activation energy was ultimately calculated to be 119.86 kJ mol-1. The results of this synthesis support the idea that supported manganese borides supported over multi-walled carbon nanotubes can act as an efficient catalyst in hydrogen evolution reactions, in the presence of hydrogen feedstock materials. References: 1Schlapbach, L., & Züttel, A. (2010). Hydrogen-storage materials for mobile applications. Materials for Sustainable Energy, 265–270. doi: 10.1142/9789814317665_0038 2Kojima, Y.; Suzuki, K.-I.; Fukumoto, K.; Sasaki, M.; Yamamoto, T.; Kawai, Y.; Hayashi, H. (2002). Hydrogen Generation Using Sodium Borohydride Solution and Metal Catalyst Coated on Metal Oxide. International Journal of Hydrogen Energy, 27 (10), 1029–1034. doi: 10.1016/S0360-3199(02)00014-9 3Nagy, J., Nos B., Bodart-Ravet, I., Derouane, E., (1989). Preparation, Characterization and Catalytic Activity of Monodisperse Colloidal Metal Borides. Faraday Discussions of the Chemical Society, vol. 87, 189-198. doi:10.1039/dc9898700189. 4Bondarchuk S.S. et al. (2018) Synthesis and Properties of Energetics Metal Borides for Hybrid Solid-Propellant Rocket Engines. In: Anisimov K. et al. (eds) Proceedings of the Scientific-Practical Conference "Research and Development - 2016", 511-519. 5Messegee, Z.; Osborne, J.; Abdel-Fattah, T. M. (2017) Green Synthesis of Nickel Terephthalate MOF at Room Temperature for the Catalysis of Hydrogen Generation. ECS Transactions, 80 (10), 1489–1493. 6Huff, C., Long, J. M., Aboulatta, A., Heyman, A., & Abdel-Fattah, T. M. (2017). Silver Nanoparticle/Multi-Walled Carbon Nanotube Composite as Catalyst for Hydrogen Production. ECS Journal of Solid State Science and Technology, 6(10), 115–118. doi: 10.1149/2.0051710jss Figure 1: SEM Image of the MnB@MWCNT Catalyst at a 50µm scale. Figure 1
In the last decade, the push towards reliable, economical, and environmentally clean power systems for energy production has seen an increase in demand. Development of the hydrogen fuel cell has supplied the desired green energy source demanded but the cells lack in safety and efficiency. To increase efficiency and reduce the need for unsafe pressurized hydrogen, efficient catalysts need to be developed for the hydrogen fuel cell process. MOFs (metal organic frameworks) are becoming popular catalysts of interest due to their distinct properties as efficient energy converters and high capacities to store energy. The metal salt, NiCl2•6H2O, was coordinated to the organic ligands of disodium terephthalate (C8H4Na2O4) to synthesize the nickel terephthalate (Ni[C6H4(COO)2]•4H2O) MOF. The MOF was synthesized at 22 °C with crystal formation occurring between one and a half to two weeks. The efficiency of the MOF catalyst was also tested in aqueous solution with sodium borohydride (NaBH4) at room temperature. The hydrogen produced was collected and weighed using a previously described water displacement system [1,2]. With one micromole of MOF material, the catalyst produced 55 mL of hydrogen within a 120 minute period. The reaction rate constant was consistently 63.54 L mol-1 hr-1. References T. Dushatinski, C. Huff, and T. Abdel-Fattah, Applied Surface Science, 385, 282 (2016). 2. C. Huff, T. Dushatinski, A. Barzanji, N. Abdel-Fattah, K. Barzanji, and T. Abdel-Fattah, ECS J Solid State, 6, M69-M71 (2017).
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