Magnetism in graphene oxide nanoplatelets: The role of hydroxyl and epoxy bridges
Graphical abstract
Introduction
In recent years, magnetism induced in graphene-based systems as reduced graphene oxide (rGO) and graphene oxide (GO) has stirred growing interest due to its unconventional origin and future spintronic applications [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40]. Graphene-induced magnetism has been found to originate from different mechanisms [1], as extended defects (voids, cracks, etc.,) [1], [4], crystal lattice defects (heptagons, pentagons, etc.,) [1], [5], defects of different nature and length-scale point defects (vacancies, ad-atoms, etc.,) [1], [2], [3], topological defects (corrugations, wrinkles, curvatures, etc.,) [1], and boundary defects (edges, grain boundaries, etc.,) [1], [6], among others.
Considerable effort is made to provide a basic theoretical explanation of graphene magnetism induced by defects. First-principles calculations were used to study the magnetic properties of ad-atoms [1], [6], vacancies (including single vacancy, H, Si, or N partially saturated vacancy, and vacancy clusters) [6], [9], [10], [11], coordinated atoms or edges [12], hydrogen adsorption [7], [8], surface curvature [1], topological defects [3], as well as intrinsic magnetism in pure graphene systems [13]. Dutta et al., [14] employing first-principles calculations reported magnetic moments in a range of 0.2 to 2.7 µB in pristine graphene attributed to localized states at the grain boundaries, as defects separating domains with different crystallographic directions.
Presence of FM has been further studied in curved graphene sheets. Particularly, topological defects leading to graphene curvature, such as Stone-Wales defects caused by the rotation of carbon atoms, can lead to generating localized spin moments [1]. Sharma et al., [15] also proposed that an FM ground state might be expected in large deformed graphene. Evidence of room temperature FM in metal-free carbon-based structures has been reported elsewhere [9], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. Irradiation studies in carbon structures also provided convincing proof of the magnetic order induced by defects. Studies of irradiation with light ions (H+, He) can induce defect formation, such as vacancies, vacancy-interstitial pairs (Frenkel pairs), interstitial carbon atoms, and clusters. In these cases, unbounded electrons can strongly modify the structure of the crystal, providing a non-zero magnetic moment. A review of defects formed by irradiation and their correlation with magnetic properties in graphitic systems has been reported [21], [22].
In addition, oxygen contained as carbonyl (CO), carboxyl (COOH), epoxy (COC), and/or hydroxyl (OH) groups has been suggested as responsible for induced magnetic moments in GO and rGO. However, experimental evidence seems to demonstrate that magnetism in these systems originates from defect/vacancy states [9], [10]. As reported in previous work [38], the expected theoretical magnetic moments were listed for several possible mechanisms responsible for FM order in GO samples. In this list (Table 1 of Ref. [38]), it is possible to observe that the highest contribution to FM in GO is attributed to the presence of grain boundary defects, with a magnetic moment of 2.76 μB/Å and hydrogen/OH chemisorption defects with the magnetic moment around 1.00 μB/Å [14], [38].
According to [6], [14], [24], extended defects in GO and rGO systems play an important role in inducing magnetization; theoretical and experimental evidence of this effect for these systems has been recently reported [38]. Also, previous reports [30], [31], [32], [33], [34], [35], [36], [37], [38], have demonstrated that GO nanoplatelets synthetized from bamboo pyroligneous acid, as source material, exhibit semiconductor-like behaviour, like a narrow band-gap material in energies ranging from 0.11 to 0.30 eV. In addition, the presence of hydroxyl and epoxy bridges in graphene can produce electron orbit stretching, which modifies the localized magnetic moment and spin-spin interaction [38].
Furthermore, when the external magnetic field is applied to the initial disordered state, combined with spin–spin interactions, the magnetic moments can be aligned with the external magnetic field [38]. By decreasing hydroxyl bridges, defect density increases and it is possible to enhance the FM order in graphene oxide, as proposed and discussed in this work.
There is no experimental evidence of the role of hydroxyl and epoxy bridges in the FM observed in GOs. Here, first-principle calculations were performed in the atomic structures, which were previously proposed and optimized in geometry and energy from density functional theory (DFT) studies [37], [39], and demonstrate that adsorption of OH and O atoms as bridges produce an uncompensated spin, giving rise to a magnetic signal that scales with the amount of hydroxyl and epoxy functional groups.
Section snippets
Sample preparation
The GO nanoplatelets were prepared by double-thermal decomposition (DTD)-method in a pyrolysis system from bamboo (Guadua angustifolia Kunth) as source material. Controlled nitrogen atmosphere was used during the carbonization process while keeping the temperature fixed. In a first pyrolysis step (thermal decomposition), the pyroligneous acid was obtained at 973 K and collected in a decanting funnel glass. In a second pyrolysis step, the pyroligneous acid was used as source material to obtain
Hydroxyl and epoxy bridges on GO individual nanoplatelets
Fig. 1a presents the SEM image of a superposition of GO nanoplatelets studied here for an OC of 5.3%. The GO thickness of individual nanoplatelets varied from 65.51 to 92.65 nm and agrees with previous studies [30], [43]. By considering these GO nanoplatelets as multilayered material, with thicknesses <100 nm, as mentioned, and the XRD-interlayer d-spacing from 0.3367 to 0.3496 nm, as reported [37], [38], the number of layers varied from 298 to 287 layers for each nanoplatelet synthesized at
Conclusions
In summary, we have provided experimental and theoretical evidence of the role of hydroxyl and epoxy bridges on room-temperature FM in graphene oxide nanoplatelets. On one side, bulk magnetization measurements have shown that the defect density correlates with oxide coverage, giving rise to an enhanced FM signal at low oxide coverage, as expected. On the other side, first-principle calculations on equivalent GO atomic structures revealed FM order induced by uncompensated spin-charge density
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was funded in part by the Interdisciplinary Institute of Sciences at Universidad del Quindío and the Center of Excellence on Novel Materials (CENM) at Universidad del Valle. JR acknowledges support from Facultad de Ciencias and Vicerrectoría de Investigaciones at Universidad de los Andes. RG gratefully acknowledges the computing time granted on the supercomputer Granado hpc at Universidad del Norte. Special acknowledgement to Dr. Guillermo Antorrena for XPS spectroscopy measurements
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