Kateřina Klímová

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.

Layered transition metal oxyhydroxides as tri-functional electrocatalysts

Layered transition metal-based materials have been intensively explored for their electrocatalytic capabilities in energy-related applications such as the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) lately. These reactions are kinetically sluggish, and require catalysts to promote their efficiency. Since their discovery and characterization decades ago, the catalytic properties of metal oxyhydroxides have not been profoundly studied. It was only in recent years when emphasis was placed on them again, mainly as possible OER catalysts. In this work, we wish to delve deeper into several layered first-row transition metal (cobalt, chromium, iron, manganese and nickel) oxyhydroxides, and investigate their inherent electrochemistry and electrocatalytic behaviors for HER and OER, as well as the oxygen reduction reaction (ORR). Characterisation of these materials was performed using scanning electron microscopy, X-ray powder diffraction, high resolution transmission electron microscopy and X-ray photoelectron spectroscopy prior to electrochemical studies. Among the five layered oxyhydroxides examined, cobalt and nickel oxyhydroxides exhibited better electrocatalytic properties than the other three layered oxyhydroxides mainly in HER and OER.

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.

Fluorographenes via thermal exfoliation of graphite oxide in SF6, SF4 and MoF6 atmospheres

Functionalization of graphene with heteroatoms is of paramount interest. Doping of graphene materials with electron withdrawing groups leads to the opening of the band gap which further results in a change of its electronic and electrochemical properties. Fluorine exhibits the largest electronegativity and thus it is expected that fluorographenes will have significantly different properties from graphene. Fluorinated graphene was prepared by a scalable method using thermal treatments (at different temperatures) or microwave plasma exfoliation of graphite oxides (prepared via chlorate or permanganate routes) in atmospheres containing SF6, SF4 or MoF6 fluorination agents. We characterized the resulting fluorographenes by scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, combustible elemental analysis and cyclic voltammetry. It was observed that with increasing fluorine content the heterogeneous electron transfer rates increased. The scalable fluorination of graphene with relatively low-toxic agents which yields high-performance electrochemical materials should find many applications in electrochemical devices, such as sensors or supercapacitors.

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.

Simple Synthesis of Fluorinated Graphene: Thermal Exfoliation of Fluorographite

Fluorinated graphene can be prepared directly by thermal exfoliation of fluorographite. The exfoliation was performed in a dynamic nitrogen atmosphere at various temperatures and the exfoliation products were analysed in detail by GC-MS. The structure and properties of all prepared fluorinated graphenes with various contents of fluorine were characterized by a number of analytical techniques. The results show both the dependence of fluorine concentration on exfoliation temperature and the suitability of this method for the synthesis of graphene with controlled concentration of fluorine. The high-temperature exfoliated fluorographite exhibits a high heterogeneous electron transfer rate and excellent catalytic properties towards the oxygen reduction reaction. These synthetic procedures can open a simple way for the synthesis of fluorinated graphene-based devices with tailored properties.

Highly selective uptake of Ba2+ and Sr2+ ions by graphene oxide from mixtures of IIA elements

We show here that graphene oxide selectively gathers heavy IIA group elements in the order of Ba2+ > Sr2+ > Ca2+ > Mg2+ with enrichment factors >100.