Stanislava Matějková

Searching for Magnetism in Hydrogenated Graphene: Using Highly Hydrogenated Graphene Prepared via Birch Reduction of Graphite Oxides

Fully hydrogenated graphene (graphane) and partially hydrogenated graphene materials are expected to possess various fundamentally different properties from graphene. We have prepared highly hydrogenated graphene containing 5% wt of hydrogen via Birch reduction of graphite oxide using elemental sodium in liquid NH3 as electron donor and methanol as proton donor in the reduction. We also investigate the influence of preparation method of graphite oxide, such as the Staudenmaier, Hofmann or Hummers methods on the hydrogenation rate. A control experiment involving NaNH2 instead of elemental Na was also performed. The materials were characterized in detail by electron microscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy both at room and low temperatures, X-ray fluorescence spectroscopy, inductively coupled plasma optical emission spectroscopy, combustible elemental analysis and electrical resistivity measurements. Magnetic measurements are provided of bulk quantities of highly hydrogenated graphene. In the whole temperature range up to room temperature, the hydrogenated graphene exhibits a weak ferromagnetism in addition to a contribution proportional to field that is caused not only by diamagnetism but also likely by an antiferromagnetic influence. The origin of the magnetism is also determined to arise from the hydrogenated graphene itself, and not as a result of any metallic impurities.

Synthetic routes contaminate graphene materials with a whole spectrum of unanticipated metallic elements

Significance Graphene is well-poised to revolutionize many industries because of its multitude of exceptional properties. Current bulk synthesis of graphene materials typically starts with the oxidation of graphite to graphite oxide followed by a reduction step. Many different methods exist for both the oxidation and reduction steps, leading to highly variable types and amounts of metallic contaminations that originate from the reagents themselves. These impurities are able to alter the graphene materials’ properties significantly, which impacts the range of potential applications for which these graphene materials are suitable. Thus, proper characterization of metallic contamination is highly important to ensure the suitability of a chosen set of synthetic procedures to the final application of the graphene material. The synthesis of graphene materials is typically carried out by oxidizing graphite to graphite oxide followed by a reduction process. Numerous methods exist for both the oxidation and reduction steps, which causes unpredictable contamination from metallic impurities into the final material. These impurities are known to have considerable impact on the properties of graphene materials. We synthesized several reduced graphene oxides from extremely pure graphite using several popular oxidation and reduction methods and tracked the concentrations of metallic impurities at each stage of synthesis. We show that different combinations of oxidation and reduction introduce varying types as well as amounts of metallic elements into the graphene materials, and their origin can be traced to impurities within the chemical reagents used during synthesis. These metallic impurities are able to alter the graphene materials’ electrochemical properties significantly and have wide-reaching implications on the potential applications of graphene materials.

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.

Tuning of graphene oxide composition by multiple oxidations for carbon dioxide storage and capture of toxic metals

Graphene oxide (GO) is a material used as a precursor for the synthesis of graphene and its derivatives. Chemical properties of graphene are strongly influenced by the chemical composition of the original GO. In this paper we would like to show that the amount as well as the type of functional groups can be significantly increased and controlled by multiple oxidations of GO. For this purpose we performed multiple oxidations using two chlorate methods (Staudenmaier and Hofmann) and a permanganate method (Hummers). The results show a possibility of tuning the composition of GO functionalities by multiple oxidations. The obtained results also show that the second and third subsequent reoxidation reactions significantly increase the amount of oxygen containing groups in GO, mainly carboxylic groups. The multiple oxidation of graphene oxide led to a significant increase of carbon storage capacity. The high concentration of oxygen functionalities led to an increase of sorption capacity by more than one order of magnitude.

Partially Hydrogenated Graphene Materials Exhibit High Electrocatalytic Activities Related to Unintentional Doping with Metallic Impurities.

Partially hydrogenated graphene materials, synthesized by the chemical reduction/hydrogenation of two different graphene oxides using zinc powder in acidic environment or aluminum powder in alkaline environment, exhibit high electrocatalytic activities, as well as electrochemical sensing properties. The starting graphene oxides and the resultant hydrogenated graphenes were characterized in detail. Their electrocatalytic activity was examined in the oxygen reduction reaction, whereas sensing properties towards explosives were tested by using picric acid as a redox probe. Findings indicate that the high electrocatalytic performance originates not only from the hydrogenation of graphene, but also from unintentional contamination of graphene with manganese and other metals during synthesis. A careful evaluation of the obtained data and a detailed chemical analysis are necessary to identify the origin of this anomalous electrocatalytic activity.

Measuring the sugar consumption of larvae in bumblebee micro-colonies: a promising new method for tracking food economics in bees

The consumption of sugar is an important part of the energy intake of social insect. Its monitoring provides information regarding the costs and efficiency of energy flow in a colony. This study aims at tracking the sugar flow from a sugar source to artificial bumblebee micro-colonies and at quantifying the amount of sugar consumed by the larvae. We developed a new method of sugar tracking that utilises an inert lanthanide complex (GdDOTA) dissolved in an aqueous sugar solution. The delayed defecation of bee larvae enabled the collection of all faeces from a cocoon. The amount of digested sugar corresponded to the amount of the lanthanide in the faeces, which was quantified using inductively coupled plasma spectrometric techniques. We highlight the possibility of the novel developed method to be extended for tracking the energy flow within a colony using up to 15 different metal markers without the necessity of killing individuals.

Synthesis and properties of phosphorus and sulfur co-doped graphene

The derivatisation of graphene significantly extends its application potential beyond just a highly conductive material. The introduction of sulfur based functionalities is relatively difficult and remains quite unexplored. Here, we demonstrated effective functionalization of graphene oxide with sulfur and phosphorus based functionalities by a simple one step reaction using phosphorus(V) sulfide known in organic synthesis as the Berzelius reagent. The chemistry of the starting graphene oxide significantly influenced the degree of functionalization. A detailed chemical analysis of the modified graphene showed the presence of various sulfur based functional groups as well as thiophosphate derivatisation. The resulting graphene highly functionalized with thiol based groups proved to be an effective immobilizer for gold nanoparticles.

One-Step Synthesis of B/N Co-doped Graphene as Highly Efficient Electrocatalyst for the Oxygen Reduction Reaction: Synergistic Effect of Impurities

In the last decade, numerous studies of graphene doping by various metal and nonmetal elements have been done in order to obtain tailored properties, such as non-zero band gap, electrocatalytic activity, or controlled optical properties. From nonmetal elements, boron and nitrogen were the most studied dopants. Recently, it has been shown that in some cases the enhanced electrocatalytic activity of graphene and its derivatives can be attributed to metal impurities rather than to nonmetal elements. In this paper, we investigated the electrocatalytical properties of B/N co-doped graphene with respect to the content of metallic impurities introduced by the synthesis procedures. For this purpose, a permanganate (Hummers) and a chlorate (Hofmann) route were used for the preparation of the starting graphene oxides (GO). The GO used for the synthesis of B/N co-doped graphene had significantly difference compositions of oxygen functionalities as well as metallic impurities introduced by the different synthetic procedures. We performed a detailed structural and chemical analysis of the doped graphene samples to correlate their electrocatalytic activity with the concentration of incorporated boron and nitrogen as well as metallic impurities.