Direct synthesis of carbon nanosheets by the solid-state pyrolysis of betaine

Abstract

Carbon sheets of a few nanometers thick (nanosheets) define a peculiar class of carbon materials with unique surface-to-volume ratio, smooth surface morphologies and thin edges, flexibility and elasticity, high thermal and chemical stability, and lightness [1, 2]. In this respect, carbon nanosheets are promising candidates for hydrogen storage materials, sensors, catalyst supports, fillers, templates, and substrates for further functionalization and single graphene production [3–16]. In early studies, the particular carbon nanomaterials have been synthesized via radio-frequency or microwave plasma-enhanced chemical vapor deposition (CVD), expansion of graphite, chemical reduction of exfoliated graphite oxide, a solvothermal route, or catalytic growth [1–16]. However, these preparative methods suffer (depending on the case) from the following drawbacks: (i) low yield or/and concurrent formation of other carbon morphologies, which limits extensive studies and development; (ii) the thickness of the sheets rarely falls below 10 nm; (iii) from a technical standpoint, they often require a sophisticated apparatus, controlled atmosphere, high temperature, flammable gaseous mixtures, gas flow adjustments, time-consuming steps, catalysts, or highly corrosive and potentially explosive chemicals; and (iv) poor surface functionality, which restricts further derivatization. Unambiguously, the direct formation of customized carbon nanosheets at fairly good yields using simple and safe methods would be highly recommended from the viewpoint of commercial usage and applications. Betaine, (CH3)3N CH2COO , is an important zwitterionic organic compound widely distributed in nature. Although the thermal decomposition of betaine has been studied in detail [17, 18], nonetheless, there is no information available on the structure and morphology of the residual carbon after pyrolysis. To that end, herein we report the pyrolytic formation of ultrathin carbon nanosheets in air using betaine as a molecular precursor. This alternative yet paradox approach towards sheet-like carbons exhibits the following advantages: (i) it produces powder carbon nanosheets at fairly good yields; (ii) the thickness of the sheets is far less than 10 nm; (iii) the method is simple, safe, and inexpensive proceeding under normal conditions; and (iv) it directly introduces oxygen-containing functional groups in the solid, thus providing active sites to the surface for further modification. Overall, the present method offers new possibilities for the cost-efficient production and processing of this kind of materials. Typically, 1 g of anhydrous betaine (Sigma) was calcined in air at 400 C for 2 h at a heating rate of 10 C min to afford a lightweight, black-brown powder at 2% yield [17, 18] (Fig. 1). This sample, thereafter denoted as BET400, contains exclusively carbon nanosheets. The reported yield is sufficient enough for the preparation of bulk quantities of powder nanosheets (Fig. 1). Also note that calcination takes A. B. Bourlinos (&) V. Georgakilas Institute of Materials Science, NCSR ‘‘Demokritos’’, Ag. Paraskevi Attikis, Athens 15310, Greece e-mail: bourlinos@ims.demokritos.gr