Michal Nováček

Concurrent phosphorus doping and reduction of graphene oxide.

Doped graphene materials are of huge importance because doping with electron-donating or electron-withdrawing groups can significantly change the electronic structure and impact the electronic and electrochemical properties of these materials. It is highly important to be able to produce these materials in large quantities for practical applications. The only method capable of large-scale production is the oxidative treatment of graphite to graphene oxide, followed by its consequent reduction. We describe a scalable method for a one-step doping of graphene with phosphorus, with a simultaneous reduction of graphene oxide. Such a method is able to introduce significant amount of dopant (3.65 at. %). Phosphorus-doped graphene is characterized in detail and shows important electronic and electrochemical properties. The electrical conductivity of phosphorus-doped graphene is much higher than that of undoped graphene, owing to a large concentration of free carriers. Such a graphene material is expected to find useful applications in electronic, energy storage, and sensing devices.

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.

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.

Use of deuterium labelling—evidence of graphene hydrogenation by reduction of graphite oxide using aluminium in sodium hydroxide

Highly hydrogenated graphene is one of the main focuses in graphene research. Hydrogenated graphene may have many unique properties such as fluorescence, ferromagnetism, and a tuneable band gap. The most widely used techniques for the fabrication of highly hydrogenated graphene are based on technically challenging methods concerning the usage of liquid ammonia reduction pathways with alkali metals or plasma hydrogenation. However, the reduction of graphene oxide by nascent hydrogen is a simple and effective method leading to the formation of highly hydrogenated graphene at room temperature. Using deuterium labelling, we studied the hydrogenation of graphene oxides prepared by chlorate and permanganate methods. The nascent hydrogen/deuterium was formed by the reaction of aluminum powder with a solution of sodium deuteroxide in deuterated water. The synthesis of hydrogenated graphene was confirmed and it was characterized in detail.

Reducing emission of carcinogenic by-products in the production of thermally reduced graphene oxide

In the last decade, researchers have been trying to find out a simple method for large scale fabrication of high-quality graphene. A typical method for the fabrication of gram quantities of graphene materials is the oxidation of graphite to graphite oxide and consequent thermal reduction and exfoliation to reduced graphene oxide at T = ∼1000 °C. Here we show that highly reduced graphene oxide can be prepared at lower temperatures than 1000 °C while keeping the properties of graphene suitable for various electrochemical applications. The high temperature exfoliation of graphite oxide typically leads to the formation of a large amount of highly toxic volatile organic hydrocarbons such as benzene and its derivatives. The amount of volatile aromatic hydrocarbons can be reduced using low temperature exfoliation procedures that we present here. The application of a lower exfoliation temperature is highly beneficial as it also significantly reduces the etching of the graphene skeleton and the formation of toxic aromatic hydrocarbons. The effect of thermal exfoliation was investigated in detail for the temperature range of 400 °C up to 1000 °C under hydrogen as well as nitrogen atmospheres. Our findings show the route for the preparation of thermally reduced graphene oxide suitable for various electrochemical applications without the formation of toxic hydrocarbons as reaction byproducts. These findings are of high importance for the industrial scale production of thermally reduced graphene oxide.

Concentration of Nitric Acid Strongly Influences Chemical Composition of Graphite Oxide

Graphite oxide is the most widely used precursor for the synthesis of graphene through top-down methods. We demonstrate a significant influence of nitric acid concentration on the structure and composition of the graphite oxide prepared by graphite oxidation. In general, two main chlorate-based oxidation methods are currently used for graphite oxide synthesis: the Staudenmaier method using 98 wt % nitric acid, and the Hofmann method with 68 wt % nitric acid. However, a gradual change in nitric acid concentration allows a continuous change in the graphite oxide composition. The prepared samples are thoroughly characterised by microscopic techniques as well as various spectroscopic and analytical methods. Lowering the nitric acid concentration leads to an increase in oxidation degree, and in particular, to the concentration of epoxy and hydroxyl groups. This knowledge is not only useful for the large-scale synthesis of graphite oxide with tuneable size and chemical composition, but the use of nitric acid in lower concentrations can also reduce the overall cost of the synthesis significantly.