Graphene hydroxide is a reduced form of graphene oxide. It exhibits properties such as a positive temperature coefficient of resistance and a large electron-acoustic phonon coupling constant. While graphene hydroxide remains relatively unexplored, this article provides an overview of its properties and applications reported thus far.
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Graphene Hydroxide – Overview
Following the discovery of graphene, its two-dimensional (2D) structure and its electronic and mechanical properties have gained much attention from the research community. Graphene is extensively used in electronic devices owing to its high electron mobility.
Because graphene is a zero-bandgap material, much effort is being made to create a bandgap in graphene for device applications. There are two ways to create a band gap: one is by cutting graphene into nanoribbons, thereby breaking its transverse symmetry, and the other is through chemical functionalization.
An article published in The Journal of Physical Chemistry Letters reported the functionalization of graphene with hydroxyl groups. Here, hydroxyl radicals were adsorbed on graphene in pairs, forming the most stable graphene hydroxide with a bandgap of ∼1 eV.
Thus, the formed graphene hydroxide was highly stable at room temperature and was proposed for use in electronic devices. Crystallographic studies of graphene hydroxide showed that its surface was half-covered with hydroxyl radical pairs with alternative sp2 and sp3 hybridization between carbon atoms.
Preparation and Properties of Graphene Hydroxide
Among the graphene derivatives, graphene hydroxide is a promising candidate for preparing graphene-based devices. The dispersion of graphene oxide in a sodium hydroxide (NaOH) solution exfoliates the layers of graphene oxide, and lengthy immersion followed by drying leads to the formation of graphene hydroxide, which exhibits a large electron–acoustic phonon coupling constant and a positive temperature coefficient of resistance.
Owing to the high ratio of sp3:sp2 carbon atoms in the structure of graphene hydroxide, the chemical groups on its surface provide a large surface area, which enhances its chemical reactivity. Moreover, subjecting graphene hydroxide to high-temperature and high-vacuum conditions during the reduction process opens up many large pores between its layers, thereby increasing its chemical reactivity.
The large pores in the structure of graphene hydroxide enhance the surface area, allowing for its application as an electrode in sensors and supercapacitors. The supercapacitor electrodes made out of graphene hydroxide demonstrated high gravimetric and volumetric capacitance.
Graphene Hydroxide Vs Graphene Oxide – Structural Difference
An article published in scientific reports investigated the structure of graphene hydroxide and compared it with that of graphene oxide. This study reports the three-dimensional (3D) structure of graphene hydroxide, which offers unique properties to the material.
The study reported that when the brown powder of graphene oxide was immersed in NaOH, it turned black, signifying the decomposition of epoxy groups on graphene oxide and the formation of hydroxide groups. The material started to become flexible after one week, and after two weeks, the graphene hydroxide film was obtained after washing with deionized water.
Extended immersion in NaOH for approximately two weeks resulted in multilayered graphene hydroxide with many wrinkles, as observed in scanning electron microscopy (SEM) images. Thus, the multilayer and wrinkles confirmed the large surface area of graphene hydroxide.
Reduction of Graphene Oxide into Graphene Hydroxide using NaOH – Mechanism
An article published in Physical Chemistry Chemical Physics proposed a reduction mechanism for graphene oxide when immersed in a NaOH solution, as demonstrated by density functional theory (DFT) simulations. This was an attempt to avoid toxic reducing agents during the reduction process.
This study proposes a reduction process in the presence of all main components, such as sodium cations, hydroxyl anions, and neutral water molecules. The polar oxide groups on the surface of graphene oxide reduced the reduction barrier of graphene oxide. Here, a metal atom was used as the electron donor, which aided the opening of the epoxy ring.
Second, the reduction of graphene oxide in NaOH alkaline solution is aided by the electron-donating metal in the presence of water molecules. The study results revealed that electron transfer is important for the oxidation–reduction reaction.
Applications of Graphene Hydroxide
Graphene hydroxide is a less explored material; its applications align with that of graphene oxide:
Catalysis
The wrinkled surface of graphene hydroxide and the large pores between the layers provide a large surface area for the material, thus showing promise as a catalyst in chemical reactions.
Material Synthesis
The enhanced surface area of graphene hydroxide is also useful for preparing electrodes for supercapacitors with high volumetric and gravimetric capacitance.
Energy Storage
Graphene hydroxide is also utilized in energy storage systems due to its high volumetric capacitance, which is required for the development of electric double-layer capacitor (EDLC) materials in supercapacitors.
Nanomaterial Synthesis
Owing to the presence of large pores in graphene hydroxide, it can be used to prepare nanomaterials and in applications such as drug delivery systems.
Conclusion
Overall, graphene hydroxide, a graphene derivative, has unique characteristics, such as a strong electron–acoustic phonon coupling constant and a positive temperature coefficient of resistance.
Graphene hydroxide is a material that shows great promise in a variety of fields. The incorporation of hydroxyl groups into graphene allows for the synthesis of graphene hydroxide. In addition to improving the bandgap, this functionalization makes it suitable for energy storage and electronic devices.
The high sp3:sp2 ratio of the carbon atoms in graphene hydroxide is a distinctive feature of its structure. It is extremely reactive and appropriate for various applications because of its wrinkled surface and large surface area caused by the chemical creation of pores.
In particular, their use as electrodes in supercapacitors and sensors has shown remarkable capacitance performance. Its superiority over graphene oxide in material synthesis applications, catalytic potential, and energy storage capabilities indicates a broad range of prospective applications.
Future investigations and studies on the structural alterations, synthesis techniques, and a wide range of uses of graphene hydroxide may fully realize its promise and open the door to ground-breaking breakthroughs in materials science, electronics, energy storage, and catalysis.
See More: A Guide to Graphene
References and Further Reading
Zhu, Q., et al. (2011). Stability and properties of two-dimensional graphene hydroxide. The Journal of Physical Chemistry Letters, 2(11), pp.1310-1314. https://sci-hub.se/10.1021/jz200398d
Lee, D. & Seo, J. (2014). Three-dimensionally networked graphene hydroxide with giant pores and its application in supercapacitors. Scientific Reports, 4, p.7419. doi.org/10.1038/srep07419
Chen, C., et al. (2014). Theoretical simulation of reduction mechanism of graphene oxide in sodium hydroxide solution. Physical Chemistry Chemical Physics, 16(25), pp.12858-12864. https://pubs.rsc.org/en/content/articlelanding/2014/cp/c4cp01031k/unauth
