An isolated carbon sheet possesses a geometric surface of about 2600 m 2 /g. However, graphene-based materials are often composed by packed sheets in particular when wet chemical synthesis methods are utilised. The exposed Specific Surface Area (SSA) of the material can give useful information about the degree of packing of the graphene-based material. Gas adsorption methods are among the techniques able to estimate the SSA and the porosity of a material. Being the amount of gas adsorbed on the surface related to the SSA, the analysis of the adsorption isotherms allows evaluating the material morphology qualitatively and quantitatively. In this work, two different graphene samples have been synthesized by thermal reduction of GO following a controlled and an explosive pathway, respectively. Nitrogen adsorption isotherms at 77 K have been measured, permitting to compare the features of the samples. Briefly, GO suspension was obtained by exploiting a modified Hummers method (1) and subsequently subjected to freeze drying (2) process to obtain Lyophilised Graphene Oxide (LGO hereafter). The reduction of the LGO was then promoted by thermal activation exploiting a controlled reduction pathway, obtaining thus a Reduced Graphene Oxide (RGO hereafter), and an explosive reduction pathway, producing an Exploded Graphene Oxide (EGO hereafter). Both the samples were obtained under a vacuum of 10 -2 mmHg. The materials were characterized by means of Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy and Nitrogen adsorption analyses at 77 K. The BET method (3) was applied to calculate the SSA. Nitrogen adsorption isotherms at 77 K are reported in Fig. 1. The sample EGO exhibits a type IV isotherm, typical of mesoporous samples. The isotherm presents a hysteresis loop of type H3 that is commonly attributed to the presence of slit-like pores, typical in the aggregates of sheet-like particles. The sample RGO exhibits a type II isotherm, typical of non-porous or macroporous samples. The BET SSA calculated from the EGO and RGO isotherms are ~580 m 2 /g and ~90 m 2 /g, respectively. By comparing nitrogen adsorption measurements of the RGO and EGO samples it can be evidenced that the RGO can be considered as composed by aggregates of ~30 sheets, the explosively obtained EGO sample can be considered as a 4-5 sheets system. The explosion phenomenon appears to be effective in separating the hexagonal carbon layers. Further analyses are currently in progress to confirm the obtained results by using CO 2 gas as probe molecule. Differences in the reduction process of Graphene Oxide can drastically alter morphological properties of the obtained samples. Nitrogen adsorption measurements show that the exploded sample possesses a significantly higher specific surface area than the reduced one, due to a higher degree of disorder and high spacing between graphene sheets. On the other side, he low area measured for the Reduced Graphene Oxide sample can be due to a slight separation of the graphene sheets.
Morphological characterization of graphene sheets as obtained from thermally reduced graphene oxide.
C Riccucci;GM Ingo;A Capobianchi
2016
Abstract
An isolated carbon sheet possesses a geometric surface of about 2600 m 2 /g. However, graphene-based materials are often composed by packed sheets in particular when wet chemical synthesis methods are utilised. The exposed Specific Surface Area (SSA) of the material can give useful information about the degree of packing of the graphene-based material. Gas adsorption methods are among the techniques able to estimate the SSA and the porosity of a material. Being the amount of gas adsorbed on the surface related to the SSA, the analysis of the adsorption isotherms allows evaluating the material morphology qualitatively and quantitatively. In this work, two different graphene samples have been synthesized by thermal reduction of GO following a controlled and an explosive pathway, respectively. Nitrogen adsorption isotherms at 77 K have been measured, permitting to compare the features of the samples. Briefly, GO suspension was obtained by exploiting a modified Hummers method (1) and subsequently subjected to freeze drying (2) process to obtain Lyophilised Graphene Oxide (LGO hereafter). The reduction of the LGO was then promoted by thermal activation exploiting a controlled reduction pathway, obtaining thus a Reduced Graphene Oxide (RGO hereafter), and an explosive reduction pathway, producing an Exploded Graphene Oxide (EGO hereafter). Both the samples were obtained under a vacuum of 10 -2 mmHg. The materials were characterized by means of Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy and Nitrogen adsorption analyses at 77 K. The BET method (3) was applied to calculate the SSA. Nitrogen adsorption isotherms at 77 K are reported in Fig. 1. The sample EGO exhibits a type IV isotherm, typical of mesoporous samples. The isotherm presents a hysteresis loop of type H3 that is commonly attributed to the presence of slit-like pores, typical in the aggregates of sheet-like particles. The sample RGO exhibits a type II isotherm, typical of non-porous or macroporous samples. The BET SSA calculated from the EGO and RGO isotherms are ~580 m 2 /g and ~90 m 2 /g, respectively. By comparing nitrogen adsorption measurements of the RGO and EGO samples it can be evidenced that the RGO can be considered as composed by aggregates of ~30 sheets, the explosively obtained EGO sample can be considered as a 4-5 sheets system. The explosion phenomenon appears to be effective in separating the hexagonal carbon layers. Further analyses are currently in progress to confirm the obtained results by using CO 2 gas as probe molecule. Differences in the reduction process of Graphene Oxide can drastically alter morphological properties of the obtained samples. Nitrogen adsorption measurements show that the exploded sample possesses a significantly higher specific surface area than the reduced one, due to a higher degree of disorder and high spacing between graphene sheets. On the other side, he low area measured for the Reduced Graphene Oxide sample can be due to a slight separation of the graphene sheets.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
