Structural anisotropy in three dimensional macroporous graphene: A polarized XANES investigation
Introduction
Graphene is the first thermodynamically stable two-dimensional material discovered in nature. Its properties are remarkable: from the very high electric mobility based on linear dispersion electrons (Dirac electrons), strong interaction with the electromagnetic field, high thermal conductivity, to the remarkable mechanical hardness [1]. In recent years research has focused on providing a third dimension to graphene. Recently three-dimensional (3D) graphene-like materials have been discovered with micro-porous structures, or made by mesoscopic filaments with a distribution on macroscopic scale [2]. While low dimensional and porous materials represent extraordinary properties due to their size and surface states [[3], [4], [5]], these structures are also expected to allow preservation of the extraordinary electrical and thermodynamic properties of 2D graphene in three dimensions. Moreover, the porous (or filamentous) nature with a high surface to volume ratio in these 3D architectures can open interesting opportunities for fundamental physics researches as well as novel applications. In particular they can be employed in batteries and supercapacitors as electrodes as well as in combination with the doping materials to improve the electrode performance via the unique surface properties in these structures [[6], [7], [8], [9], [10]]. Furthermore, it has recently been shown that they are promising candidates in IR and THz photonics and plasmonics [11] and finally, in novel highly-efficient devices capable of transducing light into sound [12,13]. However, in order to perceive and enhance distinct properties in these structures, a profound understanding of the basic structural characteristics and the 2D content proportion within macroporous structures is fundamental.
In oriented materials such as highly oriented pyrolytic graphite (HOPG), the electronic transitions from an initial K-shell to unoccupied σ⁎ or π⁎ state is progressively enhanced when the electric field vector of the linearly polarized incident light is parallel to either the direction of σ⁎ or π⁎ orbitals. Hence, such orientation dependence of transition probabilities plays a significant role in making the X-ray absorption spectroscopy (XAS), a sensitive method to probe the structural anisotropy of ordered systems and/or the local alignment and orientations within the molecular layers and thin films [[14], [15], [16], [17], [18]]. Actually (polarized) XAS spectroscopy is usually applied to characterize the vast variety of 2D and 3D materials and in particular the carbon based ones, providing deep insight into structural dynamics, surface and interface physico-chemical evolution and the electronic structures of occupied and unoccupied density of states (DOS) [[19], [20], [21], [22], [23]]. In this work, we have performed an angular resolved XAS investigation of three dimensional macroporous graphene sponges with different nominal densities and with linearly polarized incident beam. Despite the random structure of the sponge, XAS spectra show the presence of a local order in agreement with the unidirectional oriented hexagonal carbon planes. The evolution of the order parameter and the dichroic ratio within these macroporous structures with different structures and densities is discussed.
Section snippets
Sample fabrication and morphology
For the present study samples were fabricated using a macroscopic (2 × 2 × 0.5 cm) and highly porous (~94%) template consisting of interconnected ceramic ZnO tetrapods [24,25] was infiltrated with an aqueous dispersion (2 mg/ml) of electrochemically exfoliated graphene flakes [26]. After evaporation of the solvent and self-assembly of the graphene flakes (polydisperse system with a size distribution peaked around 2–3 μm) on the template surface, the ZnO template was removed by chemical etching
Results and discussion
X-ray absorption spectroscopy measurements were performed at the C K-edge by TEY in P polarization. The evolution of the absorption versus the electric field direction was obtained by rotating the sample with respect to the incident beam normal to the plane of incidence (the experimental geometry is shown in Fig. 2). In the absorption process, electrons are also excited from the initial 1s core level (K-shell) into σ⁎ or π⁎ states, depending on the orientation of the incident photon
Conclusions
Three dimensional macroporous sponges containing graphene with different networks and densities were characterized by angle resolved X-ray absorption spectroscopy at the C K-edge. We showed that the intense π⁎ excitation in these samples exhibits a cosine-squared dependence vs. the incident angle of the radiation. The behavior can be explained with a consistent alignment degree among 2D-graphene like flakes, embedded in these low-density matrices. The quantitative analysis of the dichroic ratio
CRediT authorship contribution statement
Conceptualization: S.J.R, A.M., S.N, R.A. and S.L.; Methodology: S.J.R., A.D., S.M., F.S., F.R., B.L., Z.Z., L.Q., X.F., A.R.V. Data Analysis: S.J.R., A.D., A.M.; Data Curation: S.J.R., A.D., S.M. Funding acquisition: S.L., A.M, Z.Z. Original Draft: S.J.R. and A.M. The draft is reviewed and edited by all co-authors.
Declaration of competing interest
It is declared that there is no conflict of interest.
Acknowledgments
S. Lupi and A. Marcelli would like to acknowledge the financial support of the Bilateral Cooperation Agreement between Italy and Japan of the Italian Ministry of Foreign Affairs and of the International Cooperation (MAECI) in the framework of the project of major relevance N. PGR0072. L. Qu and Z. Zhang acknowledge the financial support of the project NSFC-MAECI (51861135202). We also thank Dr. Ali Shaygan Nia for his contribution to the synthesis of the samples. S.J.R would like to thank K.
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