David Sedmidubský

Pristine Basal- and Edge-Plane-Oriented Molybdenite MoS2 Exhibiting Highly Anisotropic Properties

The layered structure of molybdenum disulfide (MoS2 ) is structurally similar to that of graphite, with individual sheets strongly covalently bonded within but held together through weak van der Waals interactions. This results in two distinct surfaces of MoS2 : basal and edge planes. The edge plane was theoretically predicted to be more electroactive than the basal plane, but evidence from direct experimental comparison is elusive. Herein, the first study comparing the two surfaces of MoS2 by using macroscopic crystals is presented. A careful investigation of the electrochemical properties of macroscopic MoS2 pristine crystals with precise control over the exposure of one plane surface, that is, basal plane or edge plane, was performed. These crystals were characterized thoroughly by AFM, Raman spectroscopy, X-ray photoelectron spectroscopy, voltammetry, digital simulation, and DFT calculations. In the Raman spectra, the basal and edge planes show anisotropy in the preferred excitation of E2g and A1g phonon modes, respectively. The edge plane exhibits a much larger heterogeneous electron transfer rate constant k(0) of 4.96×10(-5) and 1.1×10(-3)  cm s(-1) for [Fe(CN)6 ](3-/4-) and [Ru(NH3 )6 ](3+/2+) redox probes, respectively, compared to the basal plane, which yielded k(0) tending towards zero for [Fe(CN)6 ](3-/4-) and about 9.3×10(-4)  cm s(-1) for [Ru(NH3 )6 ](3+/2+) . The industrially important hydrogen evolution reaction follows the trend observed for [Fe(CN)6 ](3-/4-) in that the basal plane is basically inactive. The experimental comparison of the edge and basal planes of MoS2 crystals is supported by DFT calculations.

Water-soluble highly fluorinated graphite oxide

Water-soluble highly fluorinated graphite oxide is a promising candidate for applications in biosensing and for fluorescent probes due to its variable fluorescence properties. We report on a simple process for the preparation of a fluorinated graphite oxide (FGO). This process is based on fluorination of graphite oxide (GO) in a fluorine atmosphere at an elevated temperature and pressure. We used two different GO precursors, which were prepared by Staudenmaier and Hummers methods. The method of GO synthesis has a strong influence on the concentration of fluorine in the obtained product. The mechanism of GO fluorination is associated with the presence of reactive groups, mostly epoxides, and is accompanied by etching of graphite oxide. Our analyses highlighted that the FGO prepared by Hummers method contains a significantly higher amount of bounded fluorine and can be used as a starting material for the synthesis of chemically reduced fluorine doped graphene. Water soluble fluorinated graphene can be easily processed in aqueous solutions to create hydrophilic particles and films with tunable fluorescence properties.

High temperature enthalpy and heat capacity of GaN

The heat capacity and the heat content of gallium nitride were measured by calvet calorimetry (320–570 K) and by drop calorimetry (670–1270 K), respectively. The temperature dependence of the heat capacity in the form Cpm = 49.552+5.440× 10 −3 T − 2.190× 10 6 T −2 + 2.460× 10 8 T −3 was derived by the least squares method. Furthermore, thermodynamic functions calculated on the basis of our experimental results and literature data on the molar entropy and the heat of formation of GaN are given.

Precise tuning of the charge transfer kinetics and catalytic properties of MoS2 materials via electrochemical methods

MoS2 has become particularly popular for its catalytic properties towards the hydrogen evolution reaction (HER). It has been shown that the metallic 1T phase of MoS2 , obtained by chemical exfoliation after lithium intercalation, possesses enhanced catalytic activity over the semiconducting 2H phase due to the improved conductivity properties which facilitate charge-transfer kinetics. Here we demonstrate a simple electrochemical method to precisely tune the electron-transfer kinetics as well as the catalytic properties of both exfoliated and bulk MoS2 -based films. A controlled reductive or oxidative electrochemical treatment can alter the surface properties of the film with consequently improved or hampered electrochemical and catalytic properties compared to the untreated film. Density functional theory calculations were used to explain the electrochemical activation of MoS2 . The electrochemical tuning of electrocatalytic properties of MoS2 opens the doors to scalable and facile tailoring of MoS2 -based electrochemical devices.

Layered Black Phosphorus: Strongly Anisotropic Magnetic, Electronic, and Electron‐Transfer Properties

Layered elemental materials, such as black phosphorus, exhibit unique properties originating from their highly anisotropic layered structure. The results presented herein demonstrate an anomalous anisotropy for the electrical, magnetic, and electrochemical properties of black phosphorus. It is shown that heterogeneous electron transfer from black phosphorus to outer- and inner-sphere molecular probes is highly anisotropic. The electron-transfer rates differ at the basal and edge planes. These unusual properties were interpreted by means of calculations, manifesting the metallic character of the edge planes as compared to the semiconducting properties of the basal plane. This indicates that black phosphorus belongs to a group of materials known as topological insulators. Consequently, these effects render the magnetic properties highly anisotropic, as both diamagnetic and paramagnetic behavior can be observed depending on the orientation in the magnetic field.

Towards highly electrically conductive and thermally insulating graphene nanocomposites: Al2O3–graphene

Highly electrically conductive materials with low heat transfer rates are of very high importance for high temperature fuel cell technologies and the refractory material industry. We aim to develop such materials with high electrical conductivities/high thermal resistivities by creating composite materials of graphene and Al2O3. Here we describe a novel and facile method for the synthesis of Al2O3–graphene composites. Graphite oxide, which was prepared by the Hofmann method, was reduced by active hydrogen generated by the reaction of aluminum with a solution of sodium hydroxide. This reaction led to the formation of a nanocrystalline composite of graphene and aluminum hydroxide. The Al(OH)3–graphene composite was then calcined and pressed into pellets. Sintering of the pellets yielded a nanostructured Al2O3–graphene composite. We characterized the properties of the Al(OH)3–graphene and Al2O3–graphene composite materials in all steps to get an understanding of the process of the nanocomposite formation. The materials were analyzed by XRD, high resolution XPS, Raman spectroscopy, SEM, SEM-EDS, STEM, STA and AFM. The resistivity and thermal conductivity of the final Al2O3–graphene composite were measured. The Al2O3–graphene nanocomposite is a promising conductive material for high-temperature applications.

On the magnetic properties of Gd implanted GaN

The wurzite type gallium nitride doped by gadolinium, Ga1−xGdxN (x∼0.01–0.07), was prepared by Gd ion implantation of the parent GaN thin films deposited on sapphire substrates. The material obtained exhibits a weak ferromagnetism (FM) persisting up to 700K. At higher Gd concentrations, the minute FM component coexists with much more pronounced Curie-type paramagnetism. In a dilute limit (x⩽0.01), the latter part is substantially reduced and the saturated FM moment reaches the value M∼2μB∕Gd atom.

Insight into the Mechanism of the Thermal Reduction of Graphite Oxide: Deuterium-Labeled Graphite Oxide Is the Key

For the past decade, researchers have been trying to understand the mechanism of the thermal reduction of graphite oxide. Because deuterium is widely used as a marker in various organic reactions, we wondered if deuterium-labeled graphite oxide could be the key to fully understand this mechanism. Graphite oxides were prepared by the Hofmann, Hummers, Staudenmaier, and Brodie methods, and a deuterium-labeled analogue was synthesized by the Hofmann method. All graphite oxides were analyzed not only using the traditional techniques but also by gas chromatography-mass spectrometry (GC-MS) during exfoliation in hydrogen and nitrogen atmospheres. GC-MS enabled us to compare differences between the chemical compositions of the organic exfoliation products formed during the thermal reduction of these graphite oxides. Nuclear analytical methods (Rutherford backscattering spectroscopy, elastic recoil detection analysis) were used to calculate the concentrations of light elements, including the ratio of hydrogen to deuterium. Combining all of these results we were able to determine graphite oxides thermal reduction mechanism. Carbon dioxide, carbon monoxide, and water are formed from the thermal reduction of graphite oxide. This process is also accompanied by various radical reactions that lead to the formation of a large amount of carcinogenic volatile organic compounds, and this will have major safety implications for the mass production of graphene.

Single-phase region of the 2212-BiSrCaCuO superconductor

Abstract The phase relationships and solid-solution region of the Bi 2 Sr 2 CaCu 2 O 8+δ (2212) superconductor were investigated at 850°C under air atmosphere in a cross-section of the system BiSrCaCu(O) defined by the formula Bi 2+ y Sr 3− y − x Ca x Cu 2 O 8+δ . The system was studied by means of XRD and EMA and an approximate position of the 2212 single-phase region is delimited by the values of y =0.1 and 0.8≤ x ≤1.3. The single-phase region determined may be divided into two subregions, Sr-rich in the range 0.8≤ x ≤1.0 and Ca-rich in the range 1.1≤ x ≤1.3, these differing in the number and character of intergrowths. The highest superconducting transition temperature ( T c ) was found for x =0.8.

Synthesis of HgBa2CuO4+δ by sol–gel method under controlled oxygen pressure; electron and thermal transport properties

Abstract Samples of mercury superconductor HgBa2CuO4+δ were prepared employing highly homogeneous and reactive precursor Ba2CuO3+x obtained by the sol–gel method. Mixture of Mn3O4/Mn2O3 was used to adjust p(O2) during the synthesis. This approach allows to achieve the appropriate p(O2) at the reaction temperature 720°C and multizone furnace is not required. The enhancement of the thermal conductivity below Tc, measured for the first time in Hg-type superconductor, indicates unusually strong phonon–electron coupling. The enhancement is accompanied by a sharp resistivity and thermopower decrease.