Štěpán Huber

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.

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.

Layered Metal Thiophosphite Materials: Magnetic, Electrochemical, and Electronic Properties

Beyond graphene, transitional metal dichalcogenides, and black phosphorus, there are other layered materials called metal thiophosphites (MPSx), which are recently attracting the attention of scientists. Here we present the synthesis, structural and morphological characterization, magnetic properties, electrochemical performance, and the calculated density of states of different layered metal thiophosphite materials with a general formula MPSx, and as a result of varying the metal component, we obtain CrPS4, MnPS3, FePS3, CoPS3, NiPS3, ZnPS3, CdPS3, GaPS4, SnPS3, and BiPS4. SnPS3, ZnPS3, CdPS3, GaPS4, and BiPS4 exhibit only diamagnetic behavior due to core electrons. By contrast, trisulfides with M = Mn, Fe, Co, and Ni, as well as CrPS4, are paramagnetic at high temperatures and undergo a transition to antiferromagnetic state on cooling. Within the trisulfides series the Néel temperature characterizing the transition from paramagnetic to antiferromagnetic phase increases with the increasing atomic number and the orbital component enhancing the total effective magnetic moment. Interestingly, in terms of catalysis NiPS3, CoPS3, and BiPS4 show the highest efficiency for hydrogen evolution reaction (HER), while for the oxygen evolution reaction (OER) the highest performance is observed for CoPS3. Finally, MnPS3 presents the highest oxygen reduction reaction (ORR) activity compared to the other MPSx studied here. This great catalytic performance reported for these MPSx demonstrates their promising capabilities in energy applications.

Nanosized graphane (C1H1.14)n by hydrogenation of carbon nanofibers by Birch reduction method

Graphane, fully hydrogenated graphene with the composition (C1H1)n, has been theoretically predicted but never experimentally realized. Graphane stands out of the variety of heteroatom modified graphene for its well defined structure. Here we show that by employing Birch reduction on graphite nanofibers, one can reach hydrogenation levels close to 100%. We name this material graphane or graphane-like since its composition is relatively close to ideal theoretical stoichiometry C1H1. We systematically study the effect of the size and structure of the starting material and conditions of the synthesis. The morphology and properties of the synthesized graphane-like material are strongly dependent on the structure of the starting material. The extremely highly hydrogenated nanographanes should find applications ranging from nanoelectronics to electrochemistry such as in supercapacitors or electrocatalysts.

Hydrogenated Graphenes by Birch Reduction: Influence of Electron and Proton Sources on Hydrogenation Efficiency, Magnetism, and Electrochemistry

Interest in chemical functionalisation of graphenes today is largely driven by associated changes to its physical and material properties. Functionalisation with hydrogen was employed to obtain hydrogenated graphenes (also termed graphane if fully hydrogenated), which exhibited properties including fluorescence, magnetism and a tuneable band gap. Although the classical Birch reduction has been employed for hydrogenation of graphite oxide, variation exists between the choice of alkali metals and alcohols/water as quenching agents. A systematic study of electron (Li, Na, K, Cs) and proton sources (tBuOH, iPrOH, MeOH, H2O) has been performed to identify optimal conditions. The proton source exerted a great influence on the resulting hydrogenation with water and out-performed alcohols, and the lowest carbon-to-hydrogen ratio was observed with sodium and water with composition of C1.4H1O0.3. Although ferromagnetism at room temperature correlates well with increasing hydrogen concentrations, small contributions from trace iron impurities cannot be completely eliminated. In contrast, hydrogenated graphenes exhibit a significant paramagnetic moment at low temperatures that has no correlation with impurities, and therefore, originates from the carbon system. This is in comparison to graphene, which is strongly diamagnetic, and concentrations of paramagnetic centres in hydrogenated graphenes are one order of magnitude larger than that in graphite. Nonetheless, hydrogenation over a particular level might also excessively disrupt intrinsic sp(2) conjugation, resulting in unintended reduction of electrochemical properties. This was observed with heterogeneous electron-transfer rates and it was postulated that hydrogenated graphenes should generally have high defect densities, but only moderately high hydrogenation, should they be employed as electrode materials.

Toxicity of layered semiconductor chalcogenides: beware of interferences

The absence of bandgap in graphene has opened exploration in a new class of 2D nanomaterials: layered semiconductor chalcogenides. Research has found that they have promising properties which are advantageous for applications in a wide range of fields such as solar energy conversion, field effect transistors, optoelectronic devices, energy storage, and is expanding into biomedical applications. However, little is known about their toxicity effects. In view of the possibility of employing these materials into consumer products, we investigated the cytotoxicity of two common layered semiconductor chalcogenides, namely GaSe and GeS, based on cell viability assessments using water-soluble tetrazolium salt (WST-8) and methyl-thiazolyldiphenyl-tetrazolium bromide (MTT) assays after a 24 h exposure to varying concentrations of the nanomaterials on human lung carcinoma epithelial cells (A549). The cytotoxicity results indicated that GaSe is relatively more toxic than another group of 2D layered chalcogenide: transition metal dichalcogenides (MoS2, WS2, WSe2). On the other hand, GeS appeared to be non-toxic, with the concentration of GeS introduced having a positive correlation with the cell viability. Control experiments in cell-free conditions revealed that both GaSe and GeS interfered with the absorbance data gathered in the two assays, but the interference effect induced by GaSe could be minimized by additional washing steps to remove the nanomaterials prior to the cell viability assessments. In the case of GeS, however, the interference effect between GeS and both assay dyes were still significant despite the washing steps adopted, thereby giving rise to the false cytotoxicity results observed for GeS. Therein, we wish to highlight that control experiments should always be carried out to check for any possible interferences between the test specimen and cell viability markers when conducting cell viability assessments for cytotoxicity studies.

Mn doped GaN thin films and nanoparticles

Magnetically doped GaN in the form of thin films and nanoparticles has been investigated. The Mn doped GaN layers were grown on sapphire substrates by MOVPE. The influence of deposition condition on surface morphology, magnetic and structural properties was investigated. GaN:Mn epitaxial layers exhibit magnetic moment persisting up to room temperature. The magnetically doped layers were also prepared by ion implantation of GaN layers by Mn. The influence of free carrier concentration and other parameters on magnetic properties were investigated. The pure and transition metal (Cr, Mn and Fe) doped GaN nanoparticles were synthesised by decomposition of fluoride–based complex compound in ammonia atmosphere. Mn doped nanoparticles exhibit pure paramagnetic behaviour.

Universal Method for Large-Scale Synthesis of Layered Transition Metal Dichalcogenides

The layered transition metal dichalcogenides are currently amongst the most intensively investigated materials. These compounds constitute a broad family of materials, with characteristic layered structures, covering both semiconductors and metallic materials. The great attention arises from the possibility to exfoliate these materials down to single layers with many unique properties, such as thickness dependent band-gap energy, and the possibility of tuning transport properties by phase transitions. The research in the field of transition metal dichalcogenides is also motivated by their high electrocatalytic activity towards several industrially important reactions, such as the hydrogen evolution reaction, as well as many other applications in nano- and optoelectronics. Although these materials are studied intensively, their availability is extremely limited and only disulfides of molybdenum and tungsten are broadly commercially available. Here an optimized procedure for simple direct synthesis of transition metal dichalcogenides using powder metals and elemental chalcogens is reported. The optimized thermal treatment allowed the synthesis scaling of the sulfides, selenides and tellurides of 4th, 5th, 6th, and 7th group of layered-structure dichalcogenides. The synthesized transition metal dichalcogenides were single phase. The phase purity, structure, and morphology were investigated in detail by electron microscopy and EDS, X-ray diffraction, and Raman spectroscopy.

Synthesis and Properties of Nanosized Stoichiometric Cobalt Ferrite Spinel

Nanoparticles with controllable sizes of ferrite spinel CoFe2O4 were formed by thermal treatment of cobalt-iron glycerolate. Thermal behavior during the heating was studied by differential thermal analysis combined with thermogravimetry. The precursor, as well as the prepared nanoparticles, were analyzed by a broad spectrum of analytic techniques (X-Ray photoelectron spectroscopy (XPS), X-Ray diffraction (XRD), Energy dispersive spectroscopy (EDS), Atomic absorption spectroscopy (AAS), Scanning electron microscopy (SEM), and Raman spectroscopy). The particle size of nanoparticles was obtained from Transmission electron microscopy and also calculated using Scherrer formula. A vibrating sample magnetometer (VSM) in a Physical Property Measurement System was used to analyze the magnetic properties of nanoparticles.