Elsevier

Materialia

Volume 23, June 2022, 101442
Materialia

In-plane and through-plane dielectric properties of graphene oxide membrane: Effect of Al3+ modification

https://doi.org/10.1016/j.mtla.2022.101442Get rights and content

Abstract

A graphene oxide (GO) membrane can be used in structural dielectric capacitors and planar micro-supercapacitors as a dielectric separator material due to its excellent through-plane and in-plane dielectric properties. The ability to tailor the dielectric properties of the GO membrane is crucial for dielectric applications. We believe this to be the first demonstration describing how Al3+ ions impact the dielectric properties of the GO membrane through cation to oxygenated functional groups interactions. In this study, the Al3+-modification to the GO membrane was realized through a facile pre-coordination method followed by layer-by-layer assembling. The influence of Al3+ on the GO membranes’ through-plane and in-plane dielectric properties (dielectric constant and dielectric loss) was investigated in the frequency range from 0.01 Hz to 105 Hz. Plane-to-plane and edge-to-edge Al3+coordination with the functional group on the basal plane or edge of the GO sheets was proposed to understand the metal ion's impact on the dielectric properties. The results demonstrated that the Al3+ modification was an effective strategy to tailoring the GO membranes’ dielectric properties in a low frequency regime and further suggested the possibility of designing dielectric GO membranes with controllable dielectric performance. At a low frequency range (0.01–1 Hz), the addition of Al3+ increased the through-plane dielectric constant and decreased the in-plane dielectric constant. In terms of dielectric loss, unmodified GO membranes exhibited a higher in-plane dielectric loss than the through-plane dielectric loss. Al3+-modified GO membranes exhibited increased through-plane dielectric loss and decreased in-plane dielectric loss compared to the unmodified GO membranes.

Introduction

Graphene oxide (GO), which is monolayer graphene sheets decorated with abundant oxygenated functional groups (OFGs) on the basal plane and edges, can be economically produced from graphite through the hummus method, evenly dispersed in solvents, and easily formed in thin membranes [1], [2], [3], [4]. The OFGs on GO sheets not only facilitate further chemical modification, but also serve as structural defects that keep graphene sheets apart, cut off the charge transfer route between the GO sheet, and yield a high dielectric constant with low dielectric loss [5,6]. The high dielectric constant (order of 106) and low dielectric loss of GO make it fit for dielectric and energy related applications such as capacitors, supercapacitors, and dielectric gates [7], [8], [9], [10]. The dielectric property of GO is frequency dependent and sensitive to structural change. For example, the dielectric losses of GO at different frequency ranges can be attributed to structural defects, space charge, and dipole polarization [5], [6], [7]. If the functionalization brings in structural defects, increases intersheet space, or forms new polar covalent bonds, the intensity and peak of related dielectric loss will change or shift accordingly.

Metal cation (e.g., Na+, Ca2+, Mg2+, Fe3+, or Al3+) coordination was reported to be one effective strategy to improve the aqueous stability, ion selectivity, and mechanical strength of GO membranes [11], [12], [13], [14], [15], [16], [17], [18]. High valent metal cations such as trivalent Al3+ show superior stability and strength improvements compared to bivalent and monovalent ones [18]. Al3+ can interact with GO sheets through either stronger dative bonding with OFGs, electrostatic attraction to negatively charged functional groups, or cation−π interactions with the sp2-conjugated structure of the graphene part of GO sheets [4,17]. A GO membrane, which is GO sheets laminated in the form of “brick-and-mortar” stacking configuration, is an orthotropic material. This means that GO will have different dielectric properties parallel to the GO sheets (in-plane) and perpendicular to the GO sheets (through-plane). The Al3+ has the potential to interact with OFGs of GO sheets on the edge (carboxyl groups), which mainly impacts in-plane properties, or on the basal plane (hydroxyl, carbonyl, and carboxyl groups), which mainly impacts through-plane properties. The uneven interaction of Al3+with the functional group on the edge and basal planes of GO sheets could lead to different modification effects of the in-plane and through-plane dielectric properties of the GO membrane. The impact of Al3+ modification on the through-plane and in-plane dielectric properties of the GO membrane is important for potential applications, however, a systemic study of this impact is lacking.

The method of incorporating metal cations in the GO membrane can be grouped into three categories. The first is “post-coordination,” in which the as-prepared pure GO membrane is either soaked in or filtered with a cation rich solution [13], [14], [15]. “Post-coordination” has the least impact on the stacking pattern of GO sheets from metal cation coordination because GO sheets are laminated before the addition of metal cations. The second method is called “in-situ coordination,” in which metal cations are added by filtering the GO solution through a filter membrane containing the cation. For example, anodized aluminum oxide (AAO) filters will release the Al3+ during the filtration process and impact the stacking of GO sheets [4,11,14]. The “in-situ-coordination” allows the metal cation to be released during the stacking of GO sheets, but it is difficult to control the amount of metal cation release and coordination through this method. The third method is “pre-coordination,” in which metal cations are added to a GO solution in the form of either a salt or metal foil so that coordination between the metal cations and oxygenated groups form before membrane formation [16], [17], [18]. For “pre-coordination,” metal cations are coordinated with GO sheets in the solution, which can influence the stacking of GO sheets during the drying process. All the above-mentioned methods can effectively coordinate the GO sheets with metal cations and strengthen the stability of the resultant GO membranes [14]. All three coordination categories usually involve a vacuum filtration assisted drying process, which can accelerate the stacking but limits coordination-oriented laminating of GO sheets. In contrast, the layer-by-layer assembly process is slow, but can improve the intralayer bonding and increase the stability of the as-fabricated GO membrane [3].

In this work, we investigate the influence of Al3+ coordination with GO sheets on the through-plane and in-plane dielectric properties of the GO membrane. The Al3+-modified GO (AGO) membrane was prepared by directly dissolving aluminum foil into the GO solution, followed by a layer-by-layer self-assembling process. The structure, morphology, and dielectric properties of the AGO membrane were compared to pure GO membranes and correlated to the interaction between Al3+and the OFGs on the edge and basal planes of the GO sheets.

Section snippets

Materials and methods

Graphene oxide water dispersion (4 mg/ml) was purchased from MSE Supplies. Aluminum (Al) foil (0.024 cm thick, Puratronic, 99.997% purity) with an area density of 0.65 mg/mm2 was purchased from Alfa Aesar. All the reagents were used as received without further purification. A mixed solution of GO and Al with Al foil weight concentration varying from 0 to 0.5 wt% in increments of 0.1 wt% was prepared by simply dissolving aluminum foil in the GO solution followed by 7∼14 days of stirring for

Results and discussion

The unmodified GO membrane is formed by crosslinking the GO sheet through face-to-face interaction and edge-to-edge interaction between the functional groups [19,20]. The interaction between the functional group on the basal plane and edge of the GO sheets consists of cation-π interaction, electrostatic interaction, and covalent hydrogen bond [3,21,22]. Compared with the vacuum filtration process, the layer-by-layer assembly is able to let the GO sheets laminate in a more stabilized stacking

Conclusion

Unmodified GO and Al3+-modified GO membranes were successfully prepared through a simple layer-by-layer self-assembling method. The GO sheets were modified with Al3+ using a pre-coordination method with Al metal foil. The addition of Al3+, coordinated with oxygenated functional groups on the edge and basal planes of the GO sheets, impacted the dielectric properties of the GO membranes. The addition of 0.3 wt% Al increased the through-plane dielectric constant from 764 to 2743 (a 300% increase),

Data availability

The authors declare that the data supporting the findings of this study are available within the paper and supplement. Also, the data that support the plots within this paper and other finding of this study are available from the corresponding author upon reasonable request.

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.

Acknowledgments

This work is supported by faculty research funding from Union College. Material characterization was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. The authors thank Luke Kilby and Hayden Qualls for assistance with the experimental work. YS is grateful for funding from Union College through the Summer Research Fellowship program.

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