Silvia Guerra

Biobased Janus molecule for the facile preparation of water solutions of few layer graphene sheets

A biobased Janus molecule was used to prepare water solutions of nano-stacks made by few layer graphene. The Janus molecule was 2-(2,5-dimethyl-1H-pyrrol-1-yl)-1,3-propanediol (serinol pyrrole, SP), a serinol derivative obtained through the neat reaction of 2-amino-1,3-propandiol with 2,5-hexanedione, with atom efficiency of about 85%. SP contains the pyrrole ring, suitable for the interaction with carbon allotropes and hydroxy groups, that can easily interact with polar surroundings. Adducts, with about 14% by mass of SP, were prepared by reacting SP with a high surface area nano-sized graphite (HSAG), with about 35 graphene layers stacked in crystalline domains. Green methods were adopted, such as ball milling (HSAG–SP-M) and heating. Infrared spectra revealed peaks due to both HSAG and SP and additional peaks that could be attributed to the adduct. Both thermal and mechanical reactions left substantially unaltered the order in the graphitic layers and the interlayer distance, as shown by X-ray diffraction patterns. The relative intensity of the G and D band in the Raman spectrum was not modified by the thermal reaction, whereas enhancement of the D peak was observed after ball milling. Stable water solutions of HSAG–SP-M were prepared in a concentration range from 0.1 to 1 mg mL−1. Centrifugation allowed isolation of adducts with few stacked graphene layers, as revealed by high resolution transmission electron microscopy. An image of the adduct showed an organic layer tightly adhered to the carbon surface. SP appears a suitable molecule for the easy functionalization of carbon allotropes, such as nano-stacks of graphene layers, without substantially affecting the bulk crystalline organization and promoting the separation of the aggregates into stacks containing a low/very low number of graphene layers.

Polyhydroxylated few layer graphene for the preparation of flexible conductive carbon paper

Electrically conductive flexible carbon papers were prepared, based on hydroxyl functionalized few layer graphene (G-OH). G-OH was obtained from the reaction of KOH with nanosized graphite with a very high surface area (HSAG), greater than 300 m2 g−1, and with high shape anisotropy, with the help of mechanical and thermal energy. Wide angle X-ray and Raman analyses showed that the core of G-OH had the structure of infinite and ideal graphene layers and that the interlayer distance between the few stacked graphene layers was the same as in pristine HSAG. Hydroxyl groups were thus essentially located in peripheral positions. High resolution transmission electron microscopy revealed the presence of graphene aggregates made by a number of layers as low as 6. Stable water suspensions were obtained, with concentrations up to 4 mg mL−1. Mild centrifugation of such suspensions had interesting efficiency as a method for producing few layer graphene: about 35% by mass of pristine HSAG was isolated as few layer graphene from supernatant suspensions. Flexible and electrically conductive carbon papers were prepared by coating a paper support with a G-OH water suspension. This work demonstrates that carbon papers can be prepared without adopting the traditional oxidation–reduction procedure, avoiding harsh reaction conditions, dangerous and toxic reagents, solvents and catalysts.

Carbon Papers and Aerogels Based on Graphene Layers and Chitosan: Direct Preparation from High Surface Area Graphite

In this work, carbon papers and aerogels based on graphene layers and chitosan were prepared. They were obtained by mixing chitosan (CS) and a high surface area nanosized graphite (HSAG) in water in the presence of acetic acid. HSAG/CS water dispersions were stable for months. High resolution transmission electron microscopy revealed the presence of few graphene layers in water suspensions. Casting or lyophilization of such suspensions led to the preparation of carbon paper and aerogel, respectively. In X-ray spectra of both aerogels and carbon paper, peaks due to regular stacks of graphene layers were not detected: graphene with unaltered sp2 structure was obtained directly from graphite without the use of any chemical reaction. The composites were demonstrated to be electrically conductive thanks to the graphene. Chitosan thus makes it possible to obtain monolithic carbon aerogels and flexible and free-standing graphene papers directly from a nanosized graphite by avoiding oxidation to graphite oxide and successive reduction. Strong interaction between polycationic chitosan and the aromatic substrate appears to be at the origin of the stability of HSAG/CS adducts. Cation-π interaction is hypothesized, also on the basis of X-ray photoelectron spectroscopy findings. This work paves the way for the easy large-scale preparation of carbon papers through a method that has a low environmental impact and is based on a biosourced polymer, graphene, and water.