Magnetocaloric effect in reduced graphene oxide

Highlights

• Two samples of reduced graphene oxide with different degrees of defectiveness.

• Magnetocaloric effect of graphene has experimentally obtained in the room temperatures.

• Degree of graphene defectiveness act on the heat capacity and magnetocaloric effect.

Abstract

The magnetocaloric effect (MCE) in multiplayer graphene flakes was for the first time detected at deferent temperatures. When the magnetic field induction varied from 0 to 1.0 T, the MCE was positive. The MCE field dependence has a linear character. At 298 K, the MCE is equal to 0.01 K. Graphene samples prepared in different manner were characterized by different degree of structural imperfection. It was established that the MCE increases as the content of surface defects in graphene increases. Graphene samples obtained by different methods and having different degree of structural imperfection have different heat capacities.

Introduction

Theoretical and experimental studies reported in the last decade demonstrated that specific features of the electronic structure of carbon-containing samples can lead to the onset of magnetic (including ferromagnetic) properties persisting up to room temperature [1], [2], [3], [4], [5], [6], [7]. For example, ferromagnetic ordering in fullerene-based samples was reported [8], [9], [10], [11], [12], [13], [14].

A special place among carbon nanostructures is occupied by graphene, a one-atom-thick crystal of sp²-bonded C atoms arranged in a two-dimensional (2D) hexagonal lattice as in a single graphite layer. Graphene is interesting both theoretically and experimentally due to a combination of unique properties caused by the behavior of its π-electron system [15] that is responsible for high electrophysical characteristics [16] and mechanical strength of the material [17].

Although magnetic ordering in graphene samples of different origin was observed repeatedly, the mechanism of the onset of ferromagnetism in such carbon nanostructures is still unclear; however, there is an evident connection between magnetism and the presence of defects of different nature in the graphene samples under study [18](a), [18](b).

Defects in graphene can be divided into several groups [19], viz., structural defects caused by the presence of “pentagons” or “heptagons”; replacement of C atoms by other (e.g., N and P) atoms in the hexagonal crystal lattice; defects caused by factors other than Csp² bonds (vacancies, breaks in the boundary connections, adsorbed atoms, interstitial atoms, deformation of graphene sheets, etc.).

There are specific edge states of carbon atoms with dangling bonds along the perimeter of graphene flakes, namely, “armchair”, “zigzag” and “bearded” ones [18](a), [18](b). Unsaturated valence bonds at the edges of graphene flakes are filled with stabilizing elements. Usually, it is assumed that a “zigzag” edge is stabilized by one hydrogen atom. A “bearded” edge is bonded to two hydrogen atoms [20]. There is a great difference between the electronic states depending on the edge shape. The differences are directly related to the onset of magnetic ordering. For example, Fujita [21], [22] used the Hubbard model and assumed that π-electrons at the “zigzag” edge can create a ferromagnetic spin system. However, “armchair”-type graphene structures have no localized states. The structure of the perimeter of arbitrary-shaped graphene flakes is usually described as a combination of “zigzag” and “armchair” edges. Edge states with poorly developed “zigzag” structure consisting of three or four teeth cause significant changes in the electronic structure [23].

If both edges of a graphene ribbon are “zigzag”-shaped or “bearded”, the total spin momentum of the graphene ribbon is equal to zero since the π-electron system produces a two-sublattice structure with the same number of positions in the sublattices, i.e., the local magnetic moments interact antiferromagnetically.

The edge effects are strongly dependent on the medium where the sample is. For example, hydrogen trapping by dangling bonds along the perimeter of the graphene ribbon can induce a finite magnetization or suppress it. A theoretical study of a graphene ribbon in which each carbon is linked to two hydrogen atoms on one edge (“bearded” edge) and to one hydrogen atom on the other edge (“zigzag” edge) showed that the structure has a finite total magnetic moment, namely, a double lattice with different number of sites in each sublattice is formed [24].

Unusual magnetism in graphene was predicted theoretically [25], [26] and observed experimentally [27], [28] in nanocrystals of a carbon material representing a stack of graphene layers. There is a significant difference in the magnetic behavior of nanographite systems depending on the order of alternation of graphene planes. A layer displacement by half the cell leads to finite magnetization of the sample.

Thus, we considered certain variants of the onset of magnetic ordering in graphene samples of different origin and showed that all of them are related to structural defects in the material. In this work, magnetic ordering is inferred from experimental observation of the magnetocaloric effect (MCE) in the graphene samples under study (reduced graphene oxide, RGO).

Normally, the MCE manifests itself in changes in the magnetic state of a magnetic material under the action of external magnetic field. Depending on the magnetic field application conditions, the MCE is measured based on the adiabatic change in temperature ΔT_MCE or on the isothermal change in entropy ΔS_(M). Under adiabatic conditions, the relationship between the MCE and magnetism is given by ΔT = - (T/C_H)(∂M/∂T)_HΔH provided that the change in the magnetic field is finite [29]. The heat capacity, as a function of magnetic field and temperature, is the third most important parameter characterizing the ability of a material to accumulate thermal energy.