University of Groningen
Nanostructured graphene
Lu, Liqiang
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Publication date: 2018
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Lu, L. (2018). Nanostructured graphene: Forms, synthesis, properties and applications. Rijksuniversiteit Groningen.
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Chapter 8
Summary and outlook
8.1 Summary
Graphene is an extraordinary material due to its rather unique mechanical, thermal and electrical properties. It bears potential applications in various fields such as electronics, energy storage and conversion, semiconductor, environment, composites, sensors, biology and biomedical engineering. The properties and applications of the graphene vary with the dimensionality. This thesis started from 3D foam-like graphene to, graphene film, and to 0D graphene quantum dots, from macroporous to nanoporous. The research is on synthesis, performance and applications in various fields, e.g. electrochemical energy storage, optical and cell imaging.
Nanoporous metals are important templates of the synthesis of 3D foam-like graphene. Chapter 3 illustrates a novel and versatile template-free method for the synthesis of nanoporous metals by hydrogen reduction of metallic salts. It is a one-step approach but involves decomposition, reduction and metal growth. The as-obtained porous architectures of metals own topological disordered structure. The pore size and ligaments size can be adjusted from tens nanometers to micrometers. The forms of porous metals can be powdery, but also in chips and bulks. The porous metals can be Ni, Cu, Fe as well as other metals and alloys. The formation mechanism of the nanoporous metal has been qualitatively explained. In addition to the controllable synthesis, the conductivity and application of nanoporous Ni as binder-free current collectors of lithium-ion batteries are also investigated. In comparison with the commercial macroporous Ni, the nanoporous-based electrodes deliver enhanced reversible capacities and cyclic performances.
To overcome several problems of CVD growth of nanoporous graphene, in
chapter 4 a new approach of solid-state-growth of nanoporous graphene at low
temperatures (600-800 °C) is investigated by the nanoporous metal templates developed as catalysts. The as-synthesized nanoporous graphene is composed of 3D interconnected tubular pores and non-tubular pores. The wall thickness can be controlled from monolayer to multilayer. The pore sizes could be adjustable from tens of nanometers to hundreds of nanometers by varying nanoporous Ni templates. Various forms of powders and up to macroscopic foams are synthesized.
A high specific surface area of 555 m2 g-1 is achieved. Nanoporous graphene is used
novel cathode is proposed by encapsulation of sulfur in the tubular pores. The composites exhibited superb electrochemical performances such as enhanced capacities and cyclic performances. The capacitive and cyclic performances presented great improvements with decreasing the tubular pore size from 1,000 to 50 nm and increasing the pore volume.
Chapter 5 displays a three-dimensional interconnected macroporous graphene foam (3D-MPGF). As our developed template-free synthesis of porous metal occurs at a gaseous atmosphere, the synthesis of porous metal and APCVD growth of graphene are combined together in one processing route, by which 3D-MPGF is synthesized. Compared with multiple-step synthesis of porous metal by dealloying followed with CVD, this method is fast, low-cost, and sustainable because the metallic waste generated from etching can be easily recycled. 3D-MPGF comprises pore size of hundreds of nanometers to several microns, and tunable wall thickness from few to ten atomic layers of graphene. The density is
only 20-37.5 mg cm-3. Because of the good conductivity and low density, the
important application of 3D-MPGF is as lightweight binder-free current collectors of batteries. Hence, 3D-MPGF-S composite electrodes with sulfur loading of 2.5 to 13 mg cm-2 are synthesized. The electrode delivers a high initial capacity of 844
mAh g-1 (based on weight of electrode, the S loading of 2.5 mg cm-2), and maintain
at 400 mAh g-1 after 50 cycles. In addition, the areal capacities of 3D-MPGF/S with
a sulfur loading of 13 mg cm-2 reach 5.9 mAh cm-2 after 50 cycles. The use of 3D
macroporous graphene foam will significantly increase, not only for batteries but also for other energy storage devices their specific capacities and energy densities of overall electrodes.
In chapter 6, synthesis of graphene film by using diffusion-assisted approach at low-temperature is investigated. The processing route is illustrated on a free surface of Ni catalyst film by vacuum thermal processing of amorphous carbon. Key of the approach is that the synthesis is done at below 350 °C, and within a time as short as one minute. The nucleation and growth of graphene on the free surface of
nickel and along the interface between Ni film and SiO2 substrate are investigated.
Raman spectroscopy and HR-TEM microscope demonstrates that the graphene-based carbon films consist of graphitic carbon enriched by defects. The graphene grown on the free Ni surface is a multilayer. The graphene segments are micrometer in size and are interconnected covering the entire surface of Ni catalyst. Growth parameters such as growth time, growth temperature and carbon/Ni ratio are examined in detail for a control of graphene growth kinetics. The results point at several attractive strategies for the facile synthesis of graphene-based carbon films for industrial applications.
Chapter 7 presents an environmentally friendly, fast and industrial
promising method via an ultrasonic-assisted liquid-phase exfoliation of abundant carbon feedstocks for synthesizing GQDs in large scale. The production yield of
different sizes, structures and defect contents were obtained by using different graphitic carbon precursors for exfoliation. Two types of high-defects GQDs and low-defects GQDs are synthesized from acetylene black and nanographite, respectively. By luminescent and absorbance investigations, different light absorption and photoluminescence (PL) properties were identified. The different edge structures, defect contents and sizes of GQDs are responsible for the variation of luminescent properties induced by changing the excitation wavelength and the pH values of the GQDs dispersions. Attributed to the high water dispersion, excellent biocompatibility and controllable fluorescent performances, the as-synthesized HD-GQDs show high potential as fluorescence nanoprobes for bioimaging.
8.2 Outlook
There are several interesting and exciting outcomes of this thesis project. Most of the methodologies, microstructures, and design of nanostructures in the chapters are novel, e.g. the synthesis of nanoporous metals via hydrogen reduction of metallic salts, the application of nanoporous metals for high-capacity density electrodes of batteries, low-temperature solid state growth of nanoporous graphene, applications of the nanoporous and macroporous graphene for energy storages, synthesis of graphene-like carbon film at low temperatures, solid-state synthesis of graphene-like nanoporous films, ultrasonic–assisted synthesis of graphene quantum dots, and graphene-like nanoballs. Many investigations of them are at the initial stages. In future, more thorough examinations are required. Below is a list of interesting points in the future work.
(1) On the synthesis of nanoporous metals, detecting other nanoporous multicomponent metals based on Ti. On the application of nanoporous metal in
batteries, the long-cycling performances and rate performances of NiC2O4·2H2O
with high loading of active materials is still rather poor and require more efforts. Also employing high loading of other active materials with high specific capacities such as Si could significantly increase the gravimetric and volumetric capacity density. In addition, studying the influence of ligament size and pore size of nanoporous metals on the electrochemical performances is also important for
guiding the application of porous metal in electrochemical energy storage devices.1
(2) With respect to the low-temperature solid-state growth of graphene, it also provides a strategy for synthesis of graphene nanofoams. The formation mechanism of low-temperature growth is still obscure. The big challenge for nanoporous graphene is still the developing and engineering nanostructured Ni and other metal catalysts. The application of nanoporous graphene is not only useful for lithium-sulfur batteries, for other devices such as supercapacitors, lithium-ion batteries, lithium-air batteries and catalyst are also interesting. In addition, the graphene foam current collector can significantly increase the
capacity of electrodes of batteries and supercapacitors. More researches on the application of 3D MPGF in other devices are very promising. In addition, exploring the application of graphene foams in other applications such as sensors,
microelectromechanical or nanoelectromechanical systems are also relevant.2
(3) The quality of graphene films prepared in chapter 5 contains a high density of defects. These defects are related to the growth temperatures and depend on the quality of Ni catalyst. Developing appropriate quality (e.g. low density of grain boundaries) of Ni catalyst films is meaningful for diffusion-assisted growth of graphene at low temperature. In addition, strategies are required to reduce the density of domains during low-temperature growth of graphene to increase the domain sizes. Besides, the mechanism of graphene growth across the grain boundaries lacks clear experimental validations.
(4) Regarding the graphene quantum dots, the photoluminescent mechanism of GQDs, the wavelength and pH dependences of GQDs are still obscure. Investigating the band gap of the GQDs with different edge states, sizes, and defects is interesting. Heteroatoms doped GQDs can vary the band gap, enhance the quantum yields, and endow GQDs catalytic performances in chemistry. Exploring GQDs in applications of solar cells, catalysis, and electrochemistry are very relevant.
References
[1] Y. Yue, H. Liang, 3D current collectors for lithium-ion batteries: a topical review, Small Methods 2018, 1800056.
[2] Y.F. Ma, Y. Chen, Three-dimensional graphene networks: synthesis, properties and applications, Nat. Sci. Rev. 2 (2015) 40.
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