Ondřej Jankovský

Synthesis of Strongly Fluorescent Graphene Quantum Dots by Cage-Opening Buckminsterfullerene

Graphene quantum dots is a class of graphene nanomaterials with exceptional luminescence properties. Precise dimension control of graphene quantum dots produced by chemical synthesis methods is currently difficult to achieve and usually provides a range of sizes from 3 to 25 nm. In this work, fullerene C60 is used as starting material, due to its well-defined dimension, to produce very small graphene quantum dots (∼2-3 nm). Treatment of fullerene C60 with a mixture of strong acid and chemical oxidant induced the oxidation, cage-opening, and fragmentation processes of fullerene C60. The synthesized quantum dots were characterized and supported by LDI-TOF MS, TEM, XRD, XPS, AFM, STM, FTIR, DLS, Raman spectroscopy, and luminescence analyses. The quantum dots remained fully dispersed in aqueous suspension and exhibited strong luminescence properties, with the highest intensity at 460 nm under a 340 nm excitation wavelength. Further chemical treatments with hydrazine hydrate and hydroxylamine resulted in red- and blue-shift of the luminescence, respectively.

Uranium- and Thorium-Doped Graphene for Efficient Oxygen and Hydrogen Peroxide Reduction

Oxygen reduction and hydrogen peroxide reduction are technologically important reactions in the fields of energy generation and sensing. Metal-doped graphenes, where metal serves as the catalytic center and graphene as the high area conductor, have been used as electrocatalysts for such applications. In this paper, we investigated the use of uranium-graphene and thorium-graphene hybrids prepared by a simple and scalable method. The hybrids were synthesized by the thermal exfoliation of either uranium- or thorium-doped graphene oxide in various atmospheres. The synthesized graphene hybrids were characterized by high-resolution XPS, SEM, SEM-EDS, combustible elemental analysis, and Raman spectroscopy. The influence of dopant and exfoliation atmosphere on electrocatalytic activity was determined by electrochemical measurements. Both hybrids exhibited excellent electrocatalytic properties toward oxygen and hydrogen peroxide reduction, suggesting that actinide-based graphene hybrids have enormous potential for use in energy conversion and sensing devices.

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.

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.

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.

Carbon fragments are ripped off from graphite oxide sheets during their thermal reduction

Since the discovery of graphene, many different exfoliation processes of graphite oxide have been reported. Thermal reduction is the most often used method for graphene synthesis. It is a general assumption that during the exfoliation process water vapor and carbon-monoxide and -dioxide are produced. In this paper it is shown that more complex products are formed during this process. Graphite oxides, prepared according to Hofmann, Hummers, Staudenmaier and Brodie methods, having different C/O ratios, were exposed to thermal shock. The resulting fragments detected using a time-of-flight spectrometer exhibit that the fragment fingerprints are very similar for all graphite oxides. Our finding challenges the general assumption that only basic gases are formed during thermal exfoliation of graphite oxide. The full understanding of the exfoliation mechanism and products is crucial for reproducible scalable synthesis of reduced graphenes on a large scale.

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.

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.

A New Member of the Graphene Family: Graphene Acid.

A new member of the family of graphene derivatives, namely, graphene acid with a composition close to C1 (COOH)1 , was prepared by oxidation of graphene oxide. The synthetic procedure is based on repeated oxidation of graphite with potassium permanganate in an acidic environment. The oxidation process was studied in detail after each step. The multiple oxidations led to oxidative removal of other oxygen functional groups formed in the first oxidation step. Detailed chemical analysis showed only a minor amount of other oxygen-containing functional groups such as hydroxyl and the dominant presence of carboxyl groups in a concentration of about 30 wt %. Further oxidation led to complete decomposition of graphene acid. The obtained material exhibits unique sorption capacity towards metal ions and carbon dioxide. The highly hydrophilic nature of graphene acid allowed the assembly of ultrathin free-standing membranes with high transparency.

Toward graphene chloride: chlorination of graphene and graphene oxide

Halogenated graphene derivatives are interesting due to their outstanding physical and chemical properties. In this paper, we present various methods for the synthesis of chlorinated graphene derivatives from graphene oxide and thermally reduced graphene. We performed exfoliation of graphene oxide in a chlorine atmosphere, plasma assisted exfoliation of graphite oxide using microwave radiation and finally direct chlorination of thermally reduced graphene by liquid chlorine under deep UV irradiation. The influence of the chlorination method on the resulting chlorinated graphenes was investigated by characterization of the graphenes, which was carried out using various techniques, including SEM, SEM-EDS, high-resolution XPS, FTIR, STA and Raman spectroscopy. Electrochemical properties were investigated by cyclic voltammetry. Although the graphenes were structurally similar, they involved remarkably different chlorine concentrations. The most highly chlorinated graphene exhibits a chlorine concentration of 11.7 at%. Chlorinated graphenes with such properties could be used for reversible chlorine storage or as a starting material for further chemical modifications.