Recent Progress on Synthesis of 3D Graphene, Properties, and Emerging Applications

Abstract

Materials based on 3D graphene, such as aerogels, hydrogels, sponges, and foams, are attracting substantial interest due to their superb electrical conductivity, remarkable mechanical properties, and expedited mass and electron transport. These substances preserve the inherent characteristics of 2D graphene sheets and introduce enhanced features like low density, substantial surface area, high porosity, and steadfast mechanical properties. The applications for 3D graphene-based materials are vast, ranging from flexible electronics, sensors, absorbents, and composites to catalysis, energy storage devices, agricultural uses, water purification, biomedical applications, and solar steam generation devices, among others. In this book chapter, we consolidate the latest advancements in the fabrication of 3D graphene-based materials, discussing their properties and the emerging uses in composites and energy storage apparatuses. The synthesis of 3D graphene-based materials on a larger scale poses substantial challenges, the discussion of which might spur innovation and novel approaches in this domain. We aim to provide a comprehensive overview of the contemporary progress in this field, emphasizing the synthesis, properties, and diverse applications of these advanced materials. Our research is anticipated to establish a groundwork for the widespread preparation, understanding of structure–property relationships, and utilization of 3D graphene-based architectures (3DGAs) across various fields, including but not limited to tissue engineering, electronics, supercapacitors, composites, and energy storage devices.

1. Introduction

Carbon is a crucial building block of our universe and plays a significant role in both natural phenomena and technological advancements. In recent years, our understanding of carbon’s versatility has grown significantly, especially in the field of sp2-hybridized carbon nanomaterials. Graphene, which is an example of a 2D sp2-hybridized carbon lattice that is only one atom thick, has remarkable physicochemical properties. Its derivatives have found application in various domains like low-dimensional physics, energy storage, electronic devices, catalysis, sensors, and medical equipment. However, a major challenge in harnessing graphene’s potential at macroscopic scales is restacking when using 2D graphene sheets, which reduces efficiency and diminishes their unique properties. One solution to this challenge is the conversion of 2D graphene layers into interconnected, 3D frameworks that prevent restacking and retain 2D graphene’s exceptional properties. This requires large-scale production and the conversion of individual graphene sheets into multifunctional 3D architectures. A critical step in manufacturing 3D graphene materials, specifically 3D reduced graphene oxide (3D-rGO), involves the reduction of graphene oxide (GO), which is achieved through different chemical methods designed to eliminate oxygen-containing functional groups on the GO plane. While early research focused primarily on synthesizing high-quality graphene sheets from GO, numerous strategies have since emerged, including thermal reduction in inert atmospheres, chemical reduction using various reducing agents, photocatalytic reduction, hydro/solvothermal techniques, laser/flashlight irradiation, electrochemical reduction, hydrogen-plasma/arc-discharge, microwave treatment, and combinations of these methods. Nonetheless, although significant progress has been made in the development of 3D graphene materials in recent years, there is a need for a comprehensive understanding of 3D architectures and their performance in various applications.

This study aims to bridge this gap by presenting advanced fabrication processes and design considerations for 3D graphene-based architectures (3DGAs). It will explore the relationship between 3D graphene properties, formation mechanisms, and key components, ultimately providing an encompassing overview of the 3D graphene family of materials. This will also highlight the significance of these materials in diverse applications and inspire new directions for their development, particularly focusing on simplifying the preparation and functionalization of 3D graphene materials. In the subsequent sections, we will describe the synthesis methods, properties, and potential applications of 3DGAs [1].

2. Synthesis of 3D graphene structure

Over the last decade, numerous techniques for synthesizing 3D graphene have emerged. These methods include electrospraying, supercritical drying, freeze drying, vacuum drying, chemical vapor deposition (CVD), hydrothermal processes, self-assembly, and various other approaches, which this review will explore and discuss the associated distinct advantages and disadvantages of each.

2.1 Template-assisted method

The templates-assisted method involves reducing GO and subsequently extracting the template from the structure. The template-assisted approach, outlined by Ding and Li [2] as well as by Qiu et al. [3], stands out due to its advantages in controlling the formation of 3D graphene structures, offering meticulously crafted morphologies compared to other fabrication methodologies. Within this technique, the chosen template, predominantly constructed from materials such as polystyrene (PS) or silicon dioxide (SiO₂), holds crucial significance, with graphene sheets gathering around it through electrostatic forces. These forces originate from the interaction between negatively charged graphene sheets and the positively charged templates, culminating in the creation of an organized composite structure. Compared to self-assembly strategies, this technique ensures superior control over the final product’s architecture and morphological attributes.

Several research studies have underscored the efficacy of template-assisted methods in developing 3D graphene-based materials. For instance, positively charged PS spheres were used as templates, coated with GO sheets, which were later reduced to obtain reduced graphene oxide (rGO) using hydrazine. The process was then finalized by calcination to eliminate the PS core, resulting in graphene hollow spheres [4]. This example highlights the extensive potential and adaptability of template-assisted techniques in creating diversified 3D graphene configurations. One remarkable study demonstrated a synthesis pathway for graphene-based hollow spheres, utilizing strong electrostatic interactions between polyethylenimine-functionalized SiO₂ spheres and graphene sheets, forming GO-SiO₂ spherical entities (Figure 1). Graphene-based hollow spheres were acquired after reducing and treating with hydrofluoric acid to eliminate the SiO₂ template, showcasing the method’s versatility [6]. Additionally, Huang and his team have studied developing nanoporous graphene foams (GFs), emphasizing the role of hydrophobic interactions between GO sheets and modified SiO₂ spherical templates, resulting in nanoporous GF structures with customizable pore sizes [7]. This innovation marks a substantial progression in modifying the structural attributes of 3D graphene materials to accommodate specific needs and applications [8].

Figure 1.

Synthesis and water resistance characteristics of nano SiO₂ modified graphene oxide composite [5].

The template-assisted fabrication of 3DGAs unveils a multifaceted and regulated strategy to construct graphene-based configurations with optimal morphologies. Employing templates like PS and SiO₂, coupled with specialized electrostatic interactions and reduction phases, allows scientists to innovate in creating 3D graphene materials with a spectrum of applications that range from catalysis, to sensing, and energy storage. The continuous evolution in this domain suggests that template-assisted techniques are important for fostering groundbreaking progressions in materials science and technology.

2.2 Electrospraying

Electrospinning and electrospraying are simple and versatile techniques that can produce graphene-based fibers, spherical structures, and bead-like materials. The resulting structures can have diameters ranging from a few micrometers to nanometers and have precise control over their shape [9, 10]. The process involves applying a strong electric field between a nozzle containing a graphene-based solution and a grounded metallic collector plate. When the electric field is strong enough, a droplet elongates and forms a continuous jet, ultimately depositing graphene-based fibers or spherical structures on the collector plate. One of the great benefits of electrospinning/electrospraying is its ability to fine-tune the process parameters, allowing for the creation of materials with specific properties. Researchers have made numerous attempts to integrate graphene into fiber structures using both classic and core-shell electrospinning technologies. The core-shell approach has recently gained attention due to its ability to address challenges and enhance control over the resulting structures [11].

Poudeh and colleagues introduced a novel design for creating 3D graphene-based hollow and filled polymeric spheres through a one-step core-shell electrospraying technique (Figure 2) [12]. To achieve the desired spherical morphology, the study determined the optimal polymer concentration using Mark–Houwink–Sakurada equations [12]. Proper polymer concentration and solution viscosity are crucial for obtaining the desired spherical shape. In cases where hollowness is desired, the core material should contain a solvent with a higher vapor pressure than the shell solution. By using this innovative approach, researchers have overcome challenges such as crumbling and agglomeration of 2D graphene sheets and ensured better dispersion of graphene layers through the polymer chains. This technique has expanded the potential applications of fabricated structures across various fields, including drug delivery, energy storage, sensors, and nanocomposites. The final morphology of the produced materials is influenced by a combination of solution properties, such as viscosity and electrical conductivity, and process parameters including applied voltage and flow rate.

Figure 2.

Preparation of 3D graphene-based spheres by tri-axial electrospraying technique [12].

The interactions that occur between polymeric chains and graphene sheets throughout the sphere formation process are essential in determining the characteristics and capabilities of the subsequently formed materials. The core-shell electrospraying method unveils novel prospects for creating sophisticated graphene-centric structures characterized by customized properties, thus presenting a promising path for assorted materials science and technology applications. In essence, the electrospinning and electrospraying techniques grant meticulous control in synthesizing graphene-centric materials, enabling the development of structures with the preferred morphologies and attributes. The cutting-edge progress in core-shell electrospraying unveils pathways for creating versatile materials with multifunctional applications spanning various domains. Incorporating graphene into such structures is poised to fuel advancements in sectors like drug delivery, energy storage, sensing technologies, and nanocomposite materials, showcasing immense potential for breakthroughs in innovation.

2.3 Supercritical drying method

Traditional drying techniques, including air drying and vacuum drying, present substantial challenges when employed on graphene hydrogels due to their minimal solid content. The elevated capillary forces induced by solvent evaporation during these methods can disrupt and collapse the graphene network structure, inflicting irreversible damage. To resolve this problem and uphold the network’s integrity, advanced drying approaches such as supercritical drying and freeze-drying have been introduced. Supercritical drying emerged as a leading method for synthesizing materials like SiO₂ aerogels. This technique transforms the solvent into a supercritical fluid by meticulously modulating temperature and pressure conditions. At the supercritical state, the lack of a distinct liquid–gas interface effectively eradicates capillary pressure within the network, allowing for the gentle removal of the supercritical fluid through depressurization, yielding aerogels. This method has shown tremendous efficacy in retaining the pristine network structure of SiO₂ aerogels with minimal contraction [13]. However, for graphene-based materials such as 3DGAs, certain constraints exist.

In contrast to SiO₂ aerogels, which possess a network with robust Si-O covalent bonds, 3DGAs have graphene sheets bound by π-π interactions. These sheets are prone to slippage during supercritical drying, causing increased volume and density reduction compared to SiO₂ aerogels. Consequently, supercritical drying is not ideal for fabricating large-scale, low-density 3DGAs. Additionally, the technique’s requirement for high pressure and extended durations elevates production costs, making it less advantageous for preparing 3DGAs designated for low-density, large-scale utilization [14]. Therefore, alternative methods, like freeze-drying and air drying, might be more apt for specific graphene-based materials, considering the respective application intents and requisite attributes.

2.4 Freeze drying method

Lyophilization, or freeze drying, is a feasible alternative for mitigating capillary forces during the drying phase of materials like graphene-based hydrogels. This method entails multiple phases to maintain the structural wholeness of the hydrogel, initiated by substituting the solvent in the wet graphene hydrogel with water. The hydrogel is then exposed to freezing at sub-zero temperatures, and the encapsulated water transitions directly to gas via sublimation under vacuum, avoiding the liquid phase completely and thereby preserving the graphene framework. For freeze-drying, specific considerations are essential. When utilized on SiO₂ gels possessing diminutive pore sizes, often around tens of nanometers, the volumetric expansion of water during freezing might compromise the network structure, potentially decreasing the porosity to levels around 80% and halving the material’s specific surface area relative to outcomes from supercritical drying. In some instances, the stress induced by freezing might even induce fractures in the sample [15]. Nonetheless, graphene hydrogels, due to their expansive pore dimensions- ranging from several micrometers to tens of micrometers—are more compatible with freeze-drying compared to SiO₂ hydrogels. 3DGAs fabricated via freeze drying exhibit diminished densities, possibly down to 0.16 mg/cm³, and the method permits structural control by influencing ice crystal formation [16].

However, the process is not devoid of challenges; the water freezing can potentially distort and eject the microstructure of the graphene network, thereby reducing the specific surface area Moreover, freeze drying demands sub-zero temperatures, elevated vacuum levels, and prolonged drying durations, escalating energy usage and preparation expenditures. While lyophilization provides a substantial alternative for contending with capillary forces in the drying of graphene-based hydrogels. It is particularly favorable for graphene hydrogels due to their extensive pore dimensions and potential to produce low-density 3DGAs. However, it involves compromises like potential reductions in specific surface area and heightened energy and cost inputs, which must be weighed against the benefits depending on the intended applications.

2.5 Vacuum drying method

While effective for preserving the structural integrity of 3DGAs, supercritical and freeze drying come with certain drawbacks that hinder their suitability for large-scale production. The main challenges of specific drying methods arise from the requisition of low vacuum, low temperatures, or high-pressure states, which significantly amplify equipment, energy, and time costs. As highlighted earlier, supercritical drying entails modifying a solvent into a supercritical fluid under designated temperature and pressure states, followed by depressurization to yield aerogels. While it proficiently preserves the initial network structure of substances like silica aerogels, it encounters constraints when implemented on 3DGAs. The π–π interactions connecting the graphene sheets tend to displace, causing notable volume reduction in the supercritical drying phase, yielding structures with densities higher than what is optimal for applications necessitating low-density 3DGAs [17]. Furthermore, the time-intensive nature of supercritical drying, coupled with high-pressure conditions, escalates production expenditures. Freeze drying is another alternative, proffering benefits in maintaining the structural soundness of 3DGAs, especially those with expansive pore sizes. Still, it demands elevated vacuum levels, low temperatures, and prolonged durations, elevating energy usage and subsequent costs, especially when juxtaposed with other drying techniques.

In contrast, vacuum drying is a more economically and temporally efficient approach for 3DGAs synthesis, exempting the process from the need for specialized apparatus for temperature, high pressure, or solvent exchange and instead utilizing vacuum conditions for solvent removal. However, the process is challenged by the potential collapse of the delicate network structures of 3D graphene gels, intensifying the density of the final product. While such collapse can be perceived as a constraint, it can also be strategically utilized for the construction of dense 3D graphene network assemblies, marked by their enhanced volumetric/gravimetric energy density, making them ideal for advanced energy storage devices like high-performance batteries. Vacuum drying is a pragmatic and economical technique for developing 3D graphene network assemblies, especially when high volumetric/gravimetric energy density is prioritized. The induced collapse of the 3D network can be optimized to develop materials with properties aligned with the needs of diverse applications, predominantly in advanced energy storage sectors.

2.6 Air drying method

Air drying serves as a simplistic and economical method conducted under atmospheric pressure conditions, presenting a practical solution for drying substances like SiO₂ gels. This method is especially beneficial for the mass production of aerogels compared to intricate techniques such as supercritical drying and freeze-drying. Nonetheless, air drying has challenges; the Young–Laplace equation illustrates that, during the process, the interaction between liquid and air within the pores can create significant capillary pressure due to the minuscule pore size, typically in the tens of nanometers range. This pressure can result in the gradual shrinkage and cracking of the SiO₂ gel skeleton. Various approaches are implemented to counteract these issues, including replacing the solvent in the wet gel with one of lower surface tension to minimize capillary forces and chemically treating the SiO₂ gel skeleton to avoid further condensation of adjacent surface functional groups on the skeleton under capillary stresses. Introducing non-polar groups to the skeleton is vital in maintaining the structural integrity of SiO₂ aerogels throughout the air-drying process [14].

Contrarily, 3DGAs are more adaptable to large-scale production via air drying due to their larger pore sizes, diminishing the capillary forces experienced during the process. Several research endeavors have effectively employed air drying to construct 3DGAs with impressive properties. For instance, durable and flexible 3DGAs have been fabricated by in situ polymerization of polyacrylamide, fortifying the graphene network and allowing it to endure capillary forces during air and vacuum drying while preserving low density. Other investigations have utilized diverse methods, such as GO liquid crystal-stabilized bubbles and ice crystals and air bubbles as templates, to manufacture 3DGAs with the air-drying method, resulting in strong and uniformly porous structures with excellent mechanical properties [18]. Recent research has evidenced the fruitful application of air drying in creating resilient 3DGAs, underscoring the role of aspects like graphene sheet sizes, interconnections between graphene sheets, and the overall graphene network structure in influencing the mechanical characteristics of these materials. Optimization of these elements is paramount for the creation of robust graphene networks with tailored properties suitable for diverse applications.

2.7 Photoinduced reduction

Photoinduced reduction is an innovative process that reduces graphene oxide without the use of chemicals. It delivers high efficiency and rapid processing. This approach harnesses photon energy and has shown remarkable progress as a technique for GO reduction. Researchers began using photon energy for the reduction of GO in solutions as early as 2008 [19]. They employed semiconductor photocatalysts like titanium dioxide (TiO₂) under ultraviolet (UV) light irradiation. This approach paved the way for the development of graphene/semiconductor composites. Later, it was discovered that GO reduction could occur directly under UV light, yielding by-products such as CO, O₂, H₂O, and CO₂ [20]. Additionally, xenon lamp-equipped photographic camera lights and laser lights were found to be capable of thermally deoxygenating GO. In the early days, photoreduction of GO in dilute solutions typically resulted in rGO suspension instead of the desired 3D rGO structures. However, the photoreduction of GO films led to the formation of pore-rich 3D rGO materials characterized by high conductivity and expanded interlayer spacing. Photoinduced reduction of GO occurs through two distinct mechanisms: the photothermal effect and photochemical reduction. Photochemical reduction involves the use of UV light with a wavelength below 390 nm. Wavelengths longer than 390 nm induce the photothermal effect, which involves higher temperatures (approximately 200–230°C) and is highly effective for GO reduction. Laser lights can create intense localized heating by focusing the laser beam on a specific point, leading to rapid reduction [20].

In both mechanisms, strong excitation of the GO surface triggers particle ejection and the formation of plasma plumes. Energy transfer from the plume to the lattice results in the removal of oxygen groups from GO, leading to the formation of CO and CO₂, as well as the evaporation of internal water. This process generates interlayer pressure, causing the rGO layers to expand and form a porous structure. The photoinduced reduction process is also applicable to chemically reduced GO films. Laser lights are widely employed for this purpose. Laser scribing allows for synchronous reduction while patterning GO films. By adjusting laser-processing variables such as intensity, shifting pitch, and scanning pitch, precise control over the degree of reduction in the resulting 3DGAs can be achieved. These 3D graphene patterns, generated through laser-induced reduction, hold significant potential in various biomedical applications, including tissue engineering and cell culture.

The reaction atmosphere plays a crucial role in laser-induced GO reduction, influencing the degree of reduction. The process is notably enhanced in an oxygen-free environment. When the GO precursor is subjected to laser reduction under liquid nitrogen, thermal expansion is suppressed, resulting in 3D rGO films with fewer defects and higher conductivity. Laser intensity can also be directly used to assemble 3D rGO in crystalline GO suspension. By adjusting the focus and intensity of the laser, researchers can create arbitrary homogeneous 3D structures on the inside surface. This approach is particularly favorable for precise 3D localization in electronics and photonics applications. This technique has evolved significantly since its inception, allowing for precise control over the reduction process and opening up a wide range of applications, from composite materials to biomedical scaffolds and electronics.

2.8 Chemical reduction

Chemical reduction is a widely used method to reduce GO to rGO using various chemical routes. Initially, hydrazine was used to reduce dilute GO suspensions, leading to rGO dispersions. Later, researchers focused on synthesizing 3D graphene hydrogels and aerogels using more concentrated GO colloidal solutions and milder reducing agents. The reduction temperature typically ranges from 60 to 100°C, with reagents and absorbed water removed through washing and lyophilization. Several reducing agents, including Na₂S, HI, H₃PO₄/I₂, gelatin, and sodium ascorbate, have been explored to simultaneously reduce and construct 3D GO structures. For instance, sodium ascorbate has been employed for the reduction of GO, resulting in the development of 3D graphene frameworks [21]. The formation of π-π interlinkages among sheets has been identified as a key factor governing the self-assembly of chemically reduced GO. Additionally, the functional groups attached to reducing agents play a crucial role in 3D rGO sheet construction. Covalent bonds form between GO sheets and reducing agents, leading to the substitution of oxygen-containing hydroxyl and epoxy groups and cross-linking of the GO sheets. Carboxylic groups present in reducing agents further promote the development of interlayer hydrogen bonds.

During the chemical reduction of GO, the evolution of CO and CO₂ gases is common, contributing to the restacking of GO and rGO sheets and modulating the pore structure of 3D materials. Water bubbles can also be introduced through controlled heating, and the number of bubbles plays a critical role. Fewer bubbles may fail to suppress restacking, while an excess can affect the self-assembly of rGO sheets. Hydrazine is considered favorable for reducing GO into 3D structures due to its ability to generate an appropriate number of bubbles. Active metals such as Al, Cu, Al, Fe, and Co can also serve as reducing agents, enabling both reduction and the formation of 3DGAs assemblies on their surfaces. This process relies on redox reactions between metals and GO, occurring at room temperature and accelerating at 60°C. Interestingly, when a conductive substance is placed on the active metal substrate, no reduction in the development of 3D rGO assembly is observed. This unique property has been harnessed to create specialized graphene structures, such as microtubes and micropatterns, with applications in rechargeable Li-ion batteries and flexible rGO 3D thin film devices.

Moreover, vapors produced by reducing agents can also effectively reduce both dried GO films and sponges, leading to side-by-side alterations in 3D structures. For instance, when a dense GO film is used as a precursor, vapor-based reduction significantly alters its 3D configuration. Niu et al. performed the reduction of filtrated GO film by positioning it on the top of a hydrazine monohydrate solution at 90°C for 10 hours (Figure 3) [22]. By controlling the concentration of the hydrazine monohydrate solution, the open pore structure can be tuned.

Figure 3.

Schematic of the fermentation process to prepare rGO foam [22].

Despite the progress in the chemical reduction of GO, there is still much to learn about the changes that occur during the reduction process. Minimizing the amount of non-carbon impurities in the final product remains a challenge, necessitating further research and optimization of reduction methods. Chemical reduction offers a versatile approach to obtaining 3D graphene structures, with various reducing agents and strategies available to tailor the properties and morphology of the resulting materials.

2.9 Electrochemical reduction

Electrochemical reduction is a prevalent technique for producing 3D graphene constructs, particularly in crafting electrodes for electrochemical devices directly. It permits the formation of resilient 3D graphene frameworks straight on electrode surfaces, optimizing the efficacy of several electrochemical setups. The procedure of electrochemical reduction requires settling GO on a cathode from a GO mixture. Contrasting typical solid graphite strata, the graphene strata in 3D formations are aligned in a fortifying manner, establishing a stable and integrated 3D graphene grid. This method is frequently termed as the immediate expansion of rGO sheets from the electrode interface. The developed 3D graphene lattice presents numerous perks such as superior porosity and escalated electrochemical efficacy.

A crucial consideration in developing 3D rGO substances electrochemically is the selection of the electrode material. Several substances like stainless steel lattice, platinum (Pt) leaf, nickel (Ni) froth, and gold (Au) fiber are feasible as electrode foundations. Utilizing graphene paper as the foundation for the electrode resulted in a closely bonded structure of highly porous rGO layers onto the substrate. Consequently, this yielded a carbon electrode with outstanding versatility for flexible device applications. Employing Ni foam as the electrode base results in the interior vacuities being occupied by rGO, forging a systematic porous framework with diverse pore dimensions. Moreover, hierarchical 3D structures can be synthesized by employing previously acquired 3D rGO substances as electrode foundations [23]. The electrochemical technique is also viable for reducing pre-shaped GO films on electrodes. This method provides superior regulation over the alignment of GO sheets in the casting phase and assures proficient reduction via the electrochemical mechanism. The electrochemical reduction of GO facilitates the production of high-caliber electrodes with customized 3D configurations, enriching their utilization in energy conservation apparatuses, detectors, and other varied electrochemical setups. Electrochemical reduction is a multifaceted and efficient strategy for developing 3D graphene formations, especially significant for the advancement of electrodes in electrochemical appliances. The 3D graphene frameworks that result possess enhanced electrochemical traits, rendering them instrumental in an extensive array of applications.

2.10 Thermal expansion of GO bulk

Expanding GO bulk materials through thermal reduction or annealing is a well-known and efficient method to eliminate oxygen functional groups from GO, yielding 3DGAs with increased bulk volumes. This technique entails exposing GO precursors to elevated temperatures, usually between 800 and 1000°C, inducing a series of structural and chemical modifications. For 3D graphene, high-temperature annealing can trigger thermal exfoliation, causing the expansion of dried and pre-shaped GO bulk precursors. These precursors may appear in several forms, including granular GO, misaligned GO films, tape-cast layers, and bulk GO materials. During the thermal evolution of bulk GO, oxygen-containing functional groups such as carboxyl, hydroxyl, carbonyl, and ether groups are either partially removed or converted into more stable chemical bonds like anhydrides, quinones, and lactones. This leads to the emission of trace gases including CO₂, CO, and H₂O. The degree of purification of the graphene surface is augmented with increased annealing temperatures. At maximum temperatures of 1000°C, unstable oxygen species are efficiently eradicated, enhancing the electrical conductivity of the resultant rGO sheets significantly [24].

Concurrently, the GO mass morphs into honeycomb-esque 3D nanostructures composed of microscopic sheets and numerous pores. This enlargement process contributes to a notable amplification in the material’s specific surface area. Depending on the specific precursor and the subsequent reaction, the surface area of 3D rGO structures can generally vary from 400 to 800 m²/g. Thermal annealing is also utilized for the reduction of partially reduced 3D GO and 3DGAs achieved through dry processes and intense reduction procedures. In such instances, the main goal might not be the modification of 3D structures, but rather the expulsion of oxygen functional groups and conductivity augmentation. Nonetheless, exposing GO precursors to elevated temperatures swiftly, without a gradual heating process, can induce the creation of additional in-wall pores, increasing the surface areas. For instance, subjecting crumpled graphene spheres to thermal shock at 400°C can produce a surface area of roughly 567 m²/g, whereas maintaining the sample at the same temperature without rapid cooling can yield a lower surface area of 344 m²/g. Thermal expansion through annealing is a flexible and popular approach to fabricating 3DGAs with enhanced bulk volumes and superior electrical conductivity. This technique involves the elimination of oxygen functional groups and the formation of porous 3D nanostructures, positioning it as a valuable technique for diverse applications in material science and nanotechnology.

2.11 Solvothermal and hydrothermal reduction

The solvothermal and hydrothermal reduction techniques are increasingly recognized for their effectiveness in transforming GO to rGO and crafting 3D graphene structures. These procedures revolve around high-temperature reactions in autoclaves, usually ranging between 100 and 250°C, using water or organic solvents as reducers. In hydrothermal treatments, supercritical water driven by thermal activation serves as the reducer, facilitating the transformation of GO sheets into 3D rGO hydrogels. Early in this process, the surface charges are reduced due to the extraction of carboxylic groups from GO sheets. As the reaction progresses, most carboxylic groups vanish, increasing hydrophobicity and enhancing the attractions and interactions among the rGO sheets. This facilitates the formation of a closely-knit 3D structure. By-products commonly include CO₂, with minor amounts of CO and organic acid remnants. Even if some GO sheets remain intact post-treatment, their oxygen-functional groups help link the 3D rGO sheets, forming a monolithic design. Factors like the initial GO suspension’s size, concentration, and surface traits can notably alter the final 3D rGO gel’s makeup.

In solvothermal treatments, organic solvents take the place of water, leading to milder reaction temperatures and decreased GO sheet assembly. Given their inherent high pressure and the solvent’s low surface energy, solvothermal processes are adept at shaping and reducing GO sheets. This results in 3D rGO products with superior conductivity but diminished surface areas and bulkier walls compared to their hydrothermal counterparts. For example, using ethanol in solvothermal reduction can yield 3D rGO aerogels with distinct features, like near-zero super-elasticity.

Post-reduction drying is vital in maintaining the eventual 3D shape. Traditional drying might induce significant volume contraction and cracks, stemming from substantial capillary pressure during solvent evaporation. To mitigate this, alternative techniques like freeze-drying or supercritical CO₂ drying are adopted. They reduce structural degradation since ice or supercritical CO₂ have minimal interactions with graphene. The capillary pressure experienced during solvent evaporation is influenced by aspects like solvent surface tension, contact angle, and pore size. Several methods are explored to achieve ambient drying while preserving the 3D structure and elevating material rigidity. While the degrees of reduction achieved in solvothermal and hydrothermal reactions are somewhat moderate due to their temperature constraints, further treatments like annealing or chemical reduction can be applied to amplify reduction and reinstate π-conjugation. These augmentations also bolster electrical conductivity. Moreover, these processes can be employed to manufacture 3D graphene-infused composites by embedding nanoparticles like Fe₃O₄ or CO₃O₄ into the 3DGAs. Solvothermal treatments especially lead to a more evenly spread nanoparticle distribution within the 3D rGO, creating consistent composites. The solvothermal and hydrothermal reduction techniques are potent tools for devising 3D graphene structures, aerogels, and composites. The selection of solvents and drying methods remains essential in shaping the final material’s attributes and structure.

2.12 Freeze-casting process (pre- or post-reduction)

The freeze-casting process is a well-established solution-phase technique widely employed for the fabrication of 3D structures from GO or partially reduced GO suspensions or gels. This method leverages the freezing point of water and capitalizes on ice crystallization to arrange GO or partially reduced GO sheets into a continuous 3D framework. For this to occur, it is crucial to exceed the percolation threshold by concentrating GO or partially reduced rGO sheets. After freeze-casting, a subsequent reduction step is usually necessary to transform the porous GO monolith into a 3D rGO framework, thereby modifying the surface properties of the sheets. However, it should be noted that the freeze-casting process and subsequent reduction may have a marginal effect on the microscopic morphology of the final product. In the freeze-casting of GO suspensions, the chemical characteristics of GO sheets play a pivotal role. When GO suspensions are frozen directly, they produce monoliths that are brittle and randomly oriented. In contrast, when freeze-drying is applied to GO-filtered gels or specific partially reduced GO dispersions, it results in super-elastic structures with a honeycomb-like cellular orientation. In part, this contrast in monolith structure can be explained by the heightened attraction between partially reduced GO sheets. Furthermore, the capacity of partially reduced GO sheets to adsorb onto ice surfaces, owing to their hydrophobic properties and the presence of abundant oxygen-containing groups, facilitates the development of these super-elastic configurations [25].

The term “super-elasticity,” as applied to freeze-cast partially reduced GO monoliths, characterizes their exceptional ability to endure substantial deformation and rapidly recover, rendering them exceptionally resilient materials. These materials exhibit remarkable load-bearing capabilities, sustaining loads up to 450,000 times their weight and rebounding from 480% compression quite quickly. Moreover, the freezing process’s temperature holds a significant role in shaping the 3D structure during freeze-casting. Various freezing temperatures affect ice crystal growth dynamics, resulting in different wall thicknesses, overall dimensions, and pore architectures. Through precise control of the freezing temperature, researchers gain the ability to tailor the freeze-cast material’s properties to precise specifications. In recent years, bidirectional freezing technologies have also emerged as valuable tools for controlling the freezing process precisely. This innovative approach facilitates the creation of distinctive structures, such as fan-shaped arrangements of GO, which find applications across diverse fields, including water purification. Moreover, freeze-casting can use alternative solvents, such as those characterized by high vapor pressures and melting points slightly higher than room temperature. This variation, known as room-temperature freeze gelation, presents significant energy-saving advantages compared to freeze-drying with water. However, it’s worth noting that the cost associated with these organic solvents may present challenges for large-scale applications. The freeze-casting method stands as a versatile and efficient technique for producing 3D structures from GO or partially reduced GO suspensions or gels. The method allows the fabrication of super-elastic, highly resilient materials that can be tuned by manipulating freezing parameters and applying bidirectional freezing methodologies. This approach holds great promise across a broad spectrum of applications, encompassing domains like water purification and the development of lightweight structural materials.

2.13 3D printing before reduction

The extrusion-based 3D printing method, also known as robot-assisted deposition, robocasting, direct ink writing, or continuous extrusion, stands out for its ability to fabricate complex 3D rGO structures that are challenging to achieve using alternative manufacturing techniques. This technique involves the layer-by-layer deposition of GO or partially reduced GO inks, enabling the creation of intricate 3D structures. The effectiveness of extrusion-based 3D printing depends on the rheological properties of the GO ink. Specifically, the ink should exhibit shear-thinning behavior and viscoelastic characteristics to ensure proper adhesion between layers while maintaining the desired print shape. The rheological properties of GO inks are influenced by various factors, including flake size and GO concentration. Smaller lateral sizes of GO sheets, typically in the range of 150–400 nm, display non-Newtonian fluid behavior at a concentration of 20 mg/mL. Higher concentrations result in increased viscosity and improved printability. To enhance printability further, modified GO suspensions are often used, incorporating additives such as pH-sensitive polymers, hydrophobic fumed silica powder, and basic compounds. These additives help regulate viscosity and shear-yield stress, making it possible to 3D print the inks effectively. In addition to extrusion-based 3D printing, optical 3D printing has emerged as a precise technique for manipulating micrometer-scale structures. This approach combines photo-excited polymerization with layer-by-layer 3D printing to generate complex macro-3D rGO architectures. The photocurable resin used in this process is created by blending diluted GO dispersion with photocurable acrylates and a photoinitiator. This resin can rapidly solidify through light-initiated polymerization. During optical 3D printing, thin layers of the resin are deposited, cross-linked, and solidified using patterned light irradiation. This process is repeated for subsequent layers, resulting in the gradual construction of 3D structures. A light source with a wavelength of 405 nm is typically employed, along with a spatial light modulator, to enable precise control over 3D patterns and resolutions. This technology allows for the fabrication of intricate 3D structures with controlled micron-scale dimensions, maintaining a high resolution of approximately 10 μm (Figure 4) [26].

Figure 4.

Illustration of the optical 3D printing process [26].

2.14 Wet-spinning before reduction

Wet-spinning is a widely employed technique for the fabrication of 3D rGO structures, encompassing a range of forms such as fabrics, films, cylinders, spheres, and fibers. This method offers continuous alignment of rGO materials, resulting in the development of materials with distinctive properties. The wet-spinning process involves extruding a GO suspension through a nozzle into a coagulation bath. The concentration of GO within the suspension plays a crucial role in the successful formation of fibers. High concentrations of GO in the suspension foster strong interactions among GO sheets, promoting alignment and coagulation. Conversely, low concentrations can lead to the formation of brittle fibers and collapsed structures. It’s important to note that the formation of graphene fibers through wet-spinning follows a multi-step mechanism. Initially, a multilayer GO film is generated as negatively charged GO sheets repel each other. As the charge neutralizes within the coagulation bath, the film undergoes bending and folding, resulting in highly aligned fine particles. The versatility of wet-spinning allows for the production of 3D graphene materials with diverse shapes and structures by controlling the rotation process. For instance, the wet-spinning of liquid GO crystals into a rotating coagulation bath can yield super-elastic graphene aerogel millispheres. These millispheres possess continuous shell and core structures, exceptional mechanical strength, and unique jumping properties. Wet-spinning, when followed by a reduction step, emerges as a versatile and efficient approach for manufacturing a wide array of 3D rGO structures with tailored properties. The selection of GO concentration, spinning conditions, and parameters for the reduction process can be finely tuned to achieve specific characteristics in the final material. This method finds applications across various domains that demand high-performance graphene-based materials, showcasing its versatility and adaptability.

3. Properties of 3D graphene

Since their initial creation in 2011, 3DGAs have consistently showcased exceptional properties. The Table below provides an overview of the physical characteristics that define various 3DGAs, encompassing attributes such as specific surface area (SSA), electrical conductivity, pore structure, density, and mechanical properties. It highlights the remarkable qualities inherent to three-dimensional graphene networks, with a specific focus on GFs, graphene sponges (GSs), and graphene aerogels (GAs). These materials stand out for their impressive features, including substantial surface areas and pore volumes, reduced densities, noteworthy electrical conductivities, and robust mechanical performance. In a general context, it is evident that 3DGAs characterized by chemically bonded structures exhibit notably superior properties in comparison to those relying on physically assembled structures. As a result of these chemically bonded variants, they consistently demonstrate advantages such as reduced contact resistance, enhanced electrical conductivity, and superior mechanical properties, such as heightened strength, toughness, and flexibility. As illustrated in theTable 1, chemically bonded GAs have a bulk electrical conductivity of approximately 1 S/cm. This level of conductivity outperforms what has been reported for macroscopic 3DGAs exclusively constructed using physical cross-linkers or partial chemical bonding. For instance, GO hydrogel, with a conductivity of 5 × 10⁻³ S/cm, and 3D RGO, with a conductivity of 2.5 × 10⁻³ S/cm, significantly lag in this regard. In contrast, GFs exhibit significantly higher electrical conductivity due to their continuous and interconnected networks. Notably, GAs consistently demonstrate high electrical conductivities of around 1 S/cm, regardless of whether or not an organic cross-linker is employed.

Synthesis method	Specific surface area (m^2/g)	Pore structure	Density (mg/cm^3)	Electrical conductivity (S/cm)	Mechanical property	Ref.
GF/Ni Foam template-assisted CVD growth	∼ 850 (∼3 Layer graphene)	Porosity ∼ 99.7% Pore size ∼400 μm	∼5 (∼3 Layer graphene)	∼7 (∼3 Layer graphene)	Flexible	[27]
GS/freezing drying of GH	53.4	Pore size 2–100 nm	9	0.54	Good Flexibility, Young’s Moduli 7.56 kPa	[27]
GS/Self-assembly of GO, hydrothermal freeze-drying	423	Long axis ∼570–620 μm short axis ∼150–300 μm	12 ± 5			[28]
GA/supercritical drying or freeze drying of GH	512	Pore volumes 2.48 cm^3/g, pore size 2–100 nm	12–96	∼1	Young’s moduli 1.2–6.2 MPa	[14]
GA/organic cross-linker	584	Pore volume 2.96 cm^3/g, pore size 1–100 nm	10	∼1		[29]
GA/organic cross-linker		Porosity 99.7–99.8%	3–5		Highly compressible	[16]
GA/Chemical reduction			15	0.87		[21]
GH/hydrothermal		Pore size ∼ 0.1–7 μm		5 × 10^−3		[30]
3D RGO/hydrothermal freeze-drying			30	2.5 × 10^−3	Compressive strength 0.042 MPa compress modulus 0.26 MPa	[31]

Table 1.

The physical characteristics of various 3DGAs have been synthesized through diverse methods.

This phenomenon may be attributed to the cross-linking interactions that occur among functional groups present on the surfaces and edges of the graphene oxide during the sol–gel process. Alterations in fabrication conditions lead to variations in structural features, including the orientation of graphene sheets, physical or chemical interconnections between these sheets, pore size, porosity, and the number of graphene sheet layers. Consequently, these structural changes affect the properties of the material. For example, GFs possess an isotropic structure due to the absence of specific graphene sheet orientation, while GSs exhibit anisotropic structures. In the case of GSs, large graphene films align nearly parallel to one another, resulting in an anisotropic arrangement. The adaptable manufacturing process employed for GFs provides precise control over both their macrostructure and microstructure [31]. The pore structure within Ni foam can be carefully manipulated to tailor the pore size and porosity to meet specific requirements. Simultaneously, the adjustment of CH₄ concentration plays a vital role in influencing several key characteristics of GFs, including the average number of graphene layers, surface area, and overall density. The concentration of CH₄ enables more graphene layers to be formed, resulting in a substantial change in SSA, density, and electrical conductivity. Intriguingly, it is noteworthy that the electrical conductivity of GFs exhibits an initial increase followed by a subsequent decrease as the number of graphene layers escalates.

In a similar vein, the microstructure and characteristics of graphene sponges (GSs) can be finely tailored by making adjustments to the synthesis conditions. Particularly, when GSs are fabricated through the freeze-drying process using graphene hydrogels (GH), the freezing temperature emerges as a crucial factor (Figure 5). Changing freezing temperatures cause significant pore size and wall thickness changes, changes in pore morphology from anisotropic lamellar to uniform cellular structures, and changes in Young’s modulus. It’s worth emphasizing that the mean pore size exerts a direct influence on water absorption properties, where larger pores impart water resistance to the sponge, while smaller pores facilitate water absorption. This tunable aspect of GS properties, contingent on freezing temperature, offers remarkable versatility for an array of applications [32].

Figure 5.

(a–d) SEM images of the porous structures of four 3DGAs fabricated at different freezing temperatures of −170, −40, −20, and − 10°C, respectively. (e) Enlargement of the square area in image (a) [32].

4. Applications of 3D graphene

4.1 Drug delivery and cancer treatment

In conventional drug delivery methods, a range of challenges can arise, including low solubility, insufficient selectivity, and toxicity concerns. To address these limitations, researchers have increasingly used carbon nanotubes (CNTs), a subgroup of graphene, as drug delivery platforms. CNTs offer a substantial surface-to-volume ratio, enabling the incorporation of significant drug volumes, encompassing both hydrophilic and lipophilic drugs. Drugs can be loaded either on the exterior or inside of CNTs, making this a viable option for multidrug therapies. Of notable importance, drugs attached to the surface of CNTs possess the capability to recognize specific cell receptors and traverse cellular barriers without inducing toxicity. This innovative approach has demonstrated significant potential in the delivery of medications such as methotrexate, epirubicin, and doxorubicin, particularly in the context of cancer treatment [33].

4.2 Tissue engineering

Tissue engineering constitutes the development of functional tissues for purposes such as transplantation and regenerative medicine. Materials based on graphene have proven valuable in the design of scaffolds in this domain. The scaffolds promote crucial cellular processes, such as adhesion, proliferation, and differentiation. As a noteworthy example, the utilization of graphene oxide-modified 3D acellular cartilage extracellular matrix (ACM) scaffolds has yielded substantial advancements in the realm of cartilage regeneration. These scaffolds were found to promote good cell behavior and biocompatibility, making them promising for repairing cartilage injuries [34]. Additionally, as shown in Figure 6, graphene-based scaffolds have been extensively investigated in various tissue engineering applications, including bone tissue engineering, cardiac tissue engineering, neural tissue engineering, and skin tissue engineering. The versatility and potential of graphene-based scaffolds in tissue engineering are underscored by this multifaceted application.

Figure 6.

Graphene-based scaffolds have been explored for bone tissue engineering [34].

4.3 Sensors for magnetic resonance imaging (MRI)

Graphene’s unique properties have made it suitable for use in medical MRI sensors. Traditional sensors made from metals can introduce distortion and inhomogeneity to the magnetic field, leading to misdiagnosis. Graphene’s high conductivity, strain resilience, and non-toxic nature make it an excellent candidate for replacing traditional metallic conductors in piezoelectric sensors used in MRI. Graphene-based sensors have shown promising results in providing clearer images with minimal distortion during MRI scans [35].

4.4 Stem cell-based transplant

Transplant therapy based on stem cells holds significant promise for the restoration of damaged tissues and organs, particularly within the realm of neural regeneration. Neural stem cells (NSCs) can self-renew and differentiate into a variety of neural cell types, making them invaluable for treating nerve damage, promoting neurogenesis, and stimulating axonal growth. Within the sphere of regenerative medicine, carbon-based materials, including CNTs, carbon nanofibers, and 3DGAs, have attracted considerable attention owing to their distinctive electrical, mechanical, and biological properties [36].

4.5 Dental and oral application

There has been considerable interest in graphene-based materials in the field of dentistry due to their exceptional properties, particularly their potential antibacterial properties [37]. These materials have demonstrated their effectiveness against a broad spectrum of bacterial pathogens, including both gram-positive and gram-negative bacteria [38]. The utilization of 3D GAs in dentistry presents various applications and advantages, as outlined below:

1. Tooth filling: Graphene-based materials have been utilized in tooth-filling procedures. One of their notable advantages is their low cytotoxicity, making them safe for dental applications. These materials can be used to fill cavities and repair damaged teeth [39].

2. Antibacterial properties: Graphene-based materials exhibit robust antibacterial properties, contributing to the prevention of bacterial growth in the oral cavity. This, in turn, reduces the risk of dental infections and enhances overall oral health. The effectiveness of this antibacterial action has been extensively investigated and documented in studies [39].

3. Biocompatibility: Graphene materials are biocompatible with oral tissues. This means they are well-tolerated by the body and do not induce adverse reactions or inflammation when in contact with oral tissues [40].

4. Enhanced imaging: Graphene oxide materials have also been investigated for their potential in enhancing dental imaging techniques such as X-rays. They can improve the contrast and quality of dental radiographs, aiding in more accurate diagnoses [41].

5. Tooth whitening: Graphene-based materials have been explored for their teeth-whitening properties [42]. They can be incorporated into dental products to help whiten teeth effectively.

6. Bioactive coatings: Graphene coatings can be applied to dental implants and orthodontic devices to improve their biocompatibility and reduce the risk of infections or complications [43].

Thus, a variety of dental procedures may benefit from graphene-based materials, including antibacterial properties, biocompatibility, and imaging enhancement. These properties make graphene-based materials promising candidates for improving oral health and dental treatments [42].

4.6 Supercapacitors

Due to their distinctive properties, such as high power density and extended cycle life, supercapacitors have received substantial attention as energy storage devices. These devices are categorized into two primary groups: pseudo-capacitors and electrochemical double-layer capacitors (EDLCs). Pseudo-capacitors, such as transition metal oxides and conducting polymers, store charges via chemical reactions on their surfaces. In contrast, EDLCs, frequently incorporating carbon-based materials such as graphene, store energy through ion adsorption at the electrode-electrolyte interface. Because of its large surface area and varied dimensions, graphene has gained widespread use in EDLC electrodes [44]. Recently, 3D graphene structures have emerged as compelling candidates for supercapacitors, thanks to their porous nature, expansive surface area, and interconnected networks, which enhance electrolyte ion accessibility and bolster electrical conductivity. The following 3D graphene configurations have been investigated for applications in supercapacitors:

1. Graphene spheres: Hollow micro/nanostructured graphene spheres deliver advantages such as a remarkable surface area and abbreviated diffusion length for charge and mass transport. A graphene-wrapped polyaniline hollow sphere demonstrated an exceptional specific capacitance of 614 F/g and maintained over 90% capacitance after 500 charging/discharging cycles.

2. Mesoporous graphene nanoballs: These nanoballs, synthesized via CVD, exhibited a high specific capacitance of 206 F/g at a scan rate of 5 mV/s and retained 96% of their capacitance after 10,000 charging and discharging cycles [44].

3. 3D graphene networks: Structures like GFs, sponges, and hydrogels have garnered attention owing to their porous architectures, which facilitate the movement of electrolyte ions within the graphene frameworks. The high energy density of spongelike graphene nanostructures enables them to retain 98% of their capacitance after 10,000 cycles, for instance [45].

4. Flexible supercapacitors: Flexible supercapacitors have been designed for wearable and portable devices. Graphene hydrogel films have been employed as electrodes in flexible solid-state supercapacitors, exhibiting high gravimetric capacitance and excellent cycling capability [45]. The films are mechanically robust and are well suited to flexible applications.

5. The graphene/MnO₂ composite network has been used to manufacture ultralight, freestanding, flexible supercapacitors. Using CVD on nickel foam and electrochemically depositing MnO₂, these networks achieved a high specific capacitance of 130 F/g and demonstrated low resistance variations upon vending [46].

So, 3DGAs have shown promise in supercapacitor applications, offering enhanced performance, high energy density, and excellent cycling capability. These structures, whether in the form of spheres, networks, or films, hold great potential for energy storage in various electronic devices and portable applications. The utilization of graphene in flexible supercapacitors further expands their range of applications, making them suitable for emerging technologies like electronic textiles and wearable devices [44].

4.7 Lithium-ion batteries

In recent years, 3DGAs have become the focus of extensive research due to their potential application as active electrodes in batteries, particularly in lithium-ion batteries (LIBs). These 3DGAs offer numerous advantages, including enhanced lifetime, higher energy density, and improved electrochemical performance, addressing some of the challenges typically associated with batteries, such as low reversible capacity and limited cyclic life. In the design of batteries, such as LIBs, it is crucial to consider the role of different components, including electrodes and electrolytes, in enhancing overall battery performance. In addition to its exceptional properties, 3D graphene is a highly promising candidate for achieving high-performance LIBs due to its large surface area, porous structure, rapid mass and charge transfer, and interconnected network. Among the key strategies for harnessing 3D graphene’s potential for LIBs is to incorporate metal or metal oxides such as Sn, NiO, Fe₃O₄, and LiFePO₄, along with CNTs into graphene sheets. 3D graphene composites were formed as a result of these efforts, which exhibit remarkable electrochemical properties and significantly improve the performance of LIB electrodes. The incorporation of conductive nanomaterials such as graphene into Fe₃O₄ has attracted considerable attention, primarily due to the appealing characteristics of Fe₃O4 as a prospective electrode material for LIBs. Aside from its high theoretical capacity, Fe₃O₄ has the advantage of being cost-effective and non-toxic.

Stable electrode performance has been hindered by previously encountered challenges, such as Fe₃O₄’s low conductivity and high volume expansion. To tackle these issues, researchers have pioneered the development of 3D GFs-supported Fe₃O₄ LIBs. This innovative approach has yielded LIB electrodes with an impressive capacity of 785 mA h/g at a 1 C charge–discharge rate, demonstrating stable performance over 500 cycles [47]. 3D graphene’s incorporation into LIB electrodes confers several advantages in all the aforementioned studies. Firstly, it establishes a shorter path length for lithium ion transport, expediting charge and discharge processes. Secondly, it enhances electron transport and electrode conductivity. Lastly, it curbs the agglomeration of active materials, ensuring uniform and consistent electrode performance over extended cycling. The 3D graphene structures can significantly enhance LIB performance. Through the integration of diverse metal or metal oxide composites and CNTs, researchers have harnessed the distinctive attributes of 3D graphene, resulting in improved electrode materials characterized by enhanced capacity, stability, and rate capability. These advancements pave the way for the development of high-performance batteries, with applications spanning from portable electronics to electric vehicles [48].

4.8 Sensors

In recent years, graphene-based materials adorned with metals and metal oxides have emerged as highly promising contenders for a diverse array of sensing devices, encompassing electrochemical sensing and biosensing. As a result of graphene’s exceptional optoelectronic attributes and the remarkable catalytic activity conferred by metals and metal oxides, graphene is attracting a growing amount of attention. Several applications involving these materials have yielded impressive results in terms of sensitivity, selectivity, and detection limits. Presented below are noteworthy instances of graphene-based sensing materials and their respective applications [49, 50, 51, 52].

1. Ammonia (NH₃) and nitrogen dioxide (NO₂) detection utilizing 3D graphene networks: Yavari et al. developed a 3D graphene network tailored for the detection of NH₃ and NO₂ under standard room temperature and atmospheric pressure conditions [53]. Due to its high sensitivity, this sensor was able to detect these gases at minute concentrations within the ppm (parts per million) range. The sensor’s exceptional performance can be attributed to the outstanding optoelectronic properties inherent to graphene, complemented by its intricate 3D structural design. This work holds substantial significance for applications such as air quality monitoring and environmental sensing.

2. Detection of hydrogen peroxide (H₂O₂) utilizing platinum-ruthenium bimetallic nanocatalyst-integrated 3D GFs: Kung et al. designed a sensor specifically for the detection of H₂O₂, featuring the integration of a platinum-ruthenium bimetallic nanocatalyst into 3DGFs [54]. Through this innovative approach, the surface area of the electrochemical reaction was increased, and efficient transport was facilitated. The resultant material exhibited remarkable performance in terms of electrochemical oxidation of H₂O₂, showcasing both high sensitivity and an impressively low detection limit. The application of these sensors is wide, ranging from healthcare to environmental monitoring to food safety.

3. Utilizing 3D graphene interconnected with polyaniline (PANI) nanofibers for the determination of guanine and adenine: Yang et al. fabricated 3D graphene structure intricately connected with GO by self-doped PANI nanofibers through a simple adsorption and intercalation route via sonication of the mixed dispersions of both components [55]. The distinctive free-standing, three-dimensional interconnected nanostructure arises from the robust π–π* stacking interaction between PANI backbones and GO basal planes, coupled with electrostatic repulsion between negatively charged PANI and GO sheets. This nanocomposite exhibits a significant specific surface area, maintaining a uniform and stable dispersion with PANI. Consequently, it acquires high conductivity and excellent electrocatalytic activity. The nanocomposite’s negative charge and specific structure facilitate the adsorption of positively charged guanine and adenine through strong π–π* interactions or electrostatic adsorption. Leveraging these properties, the hybrid serves as an outstanding sensing platform for the highly sensitive determination of guanine and adenine. Beyond its applicability to guanine and adenine, this versatile platform lends itself to the determination of a multitude of other analytes, particularly within the realm of bioanalytical and clinical applications. These exemplars underscore the adaptability and potential of graphene-based materials to diverse sensing applications. Due to their unique properties, such as extensive surface areas, excellent electrical conductivity, and amenable functionalization, they are ideal candidates for sensor development. Furthermore, the integration of metals and metal oxides serves to bolster their catalytic prowess, broadening the horizon of possibilities in environmental monitoring, healthcare, and beyond. A graphene-based sensor can detect a wide range of analytes and has several advantages, such as fast response times, greater sensitivity, selectivity, and a wide range of analytes. As continued research in this domain advances, the anticipation of even more intricate and efficient sensing devices, spanning industries from electronics to biomedicine, becomes increasingly foreseeable.

4.9 Fuel cells

In the pursuit of sustainable energy sources and the transition away from finite fossil fuels, scientists and researchers have been exploring various avenues to develop renewable energy technologies. Among these, 3DGAs have garnered significant attention for their potential applications in fuel cells and microbial fuel cells (MFCs), where they serve as catalysts or catalyst carriers. In particular, these graphene structures improve the performance of oxygen reduction reactions (ORRs) in these energy conversion devices. In addition, they have shown promise for improving the power density and efficiency of MFCs, which can be applied to both energy production and environmental bioremediation [56, 57, 58, 59, 60].

1. Fuel cells: enhancing catalyst performance with 3D graphene.

   Fuel cells serve as electrochemical devices, directly converting the chemical energy stored in fuels like hydrogen or methanol into electricity. Fundamentally, they operate by catalyzing reactions: anodes oxidize fuel, and cathodes reduce oxygen. The efficacy and performance of fuel cells heavily depend on the catalyst materials employed. Researchers are actively studying the potential of 3DGAs as catalyst supports or even as catalysts. One illustrative case involves the integration of 3D graphene as an anode electrode within microbial fuel cells (MFCs). MFCs represent devices designed to harness the chemical energy embedded in biodegradable organic compounds through a bio-oxidation process, simultaneously yielding electricity and contributing to environmental bioremediation. Nevertheless, conventional MFCs have issues such as low power density and low bacterial adhesion to electrodes. To tackle these issues, Yong et al. introduced an innovative anode electrode featuring a macroporous and monolithic structure, constructed from a hybrid material merging PANI with 3D graphene. As a result of graphene’s generous surface area, it is more robustly integrated with bacterial films, resulting in enhanced electron transfer along multiple conductive pathways.

2. Microbial fuel cells (MFCs) offer an innovative avenue for converting organic substances, including wastewater, into electricity via microbial-driven processes. There are, however, several challenges associated with MFCs, including low power density and limited bacterial adhesion to electrode surfaces. Recent investigations have delved into the utilization of 3DGAs to tackle these hurdles and enhance MFC performance. To illustrate, researchers developed freestanding anodes for MFCs by adorning 3DGA with platinum (Pt) nanoparticles. This inventive configuration yielded an exceptional power density of 1460 mW/m². The superior performance of these MFCs can be attributed to several factors, including a heightened bacterial loading capacity, streamlined electron transfer between bacteria and the 3D graphene/Pt anode, and expedited ion diffusion facilitated by the porous 3D structure.

3. Applications of 3DGAs in MFCs related to the environment and energy.

   The integration of 3DGAs in microbial fuel cells holds significant promise for both energy generation and environmental bioremediation. These advancements not only enhance the efficiency of MFCs but also contribute to sustainable energy production while mitigating environmental issues through the treatment of organic waste. As a result, the use of 3D graphene in fuel cells and microbial fuel cells represents an important advancement in the field of renewable energy and environmental technology. These graphene structures contribute to improving catalyst performance, power density, and overall efficiency in energy conversion devices. As research continues in this area, we can anticipate further innovations and applications of 3DGAs in addressing global energy and environmental challenges.

5. Conclusion

3DGAs are noble materials for their distinctive 3D porous architectures, expansive specific surface areas, remarkable adsorption capabilities, exceptional electrical conductivity, mechanical robustness, and swift mass and electron transport properties. 3DGAs represent a transformative technology with far-reaching implications across various facets of human life. This review provides a comprehensive overview of recent advancements in 3DGA synthesis and their burgeoning applications in fields such as sensors, fuel cells, lithium-ion batteries, supercapacitors, dental materials, tissue engineering, drug delivery systems, and many other domains. The synthesis of 3DGAs has evolved significantly, embracing methods like electrospraying, 3D printing, and chemical and electrochemical reduction, among others. Efforts aimed at achieving cost-effective large-scale production and utilization of various 3D graphene materials are essential for their widespread adoption in industrial settings. It also holds the promise of mitigating toxicity concerns and ensuring safer applications by prioritizing the development of biocompatible 3DGAs.