Electric Properties of Dirac Fermions Captured into 3D Nanoporous Graphene Networks

All-Solid-State Mg-Air Battery Enhanced with Free-Standing N-Doped 3D Nanoporous Graphene

Nitrogen (N) doping of graphene with a three-dimensional (3D) porous structure, high flexibility, and low cost exhibits potential for developing metal-air batteries to power electric/electronic devices. The optimization of N-doping into graphene and the design of interconnected and monolithic graphene-based 3D porous structures are crucial for mass/ion diffusion and the final oxygen reduction reaction (ORR)/battery performance. Aqueous-type and all-solid-state primary Mg-air batteries using N-doped nanoporous graphene as air cathodes are assembled. N-doped nanoporous graphene with 50-150 nm pores and ≈99% porosity is found to exhibit a Pt-comparable ORR performance, along with satisfactory durability in both neutral and alkaline media. Remarkably, the all-solid-state battery exhibits a peak power density of 72.1 mW cm-2 ; this value is higher than that of a battery using Pt/carbon cathodes (54.3 mW cm-2 ) owing to the enhanced catalytic activity induced by N-doping and rapid air breathing in the 3D porous structure. Additionally, the all-solid-state battery demonstrates better performances than the aqueous-type battery owing to slow corrosion of the Mg anode by solid electrolytes. This study sheds light on the design of free-standing and catalytically active 3D nanoporous graphene that enhances the performance of both Mg-air batteries and various carbon-neutral-technologies using neutral electrolytes.

Coexistence of Urbach-Tail-Like Localized States and Metallic Conduction Channels in Nitrogen-Doped 3D Curved Graphene

Nitrogen (N) doping is one of the most effective approaches to tailor the chemical and physical properties of graphene. By the interplay between N dopants and 3D curvature of graphene lattices, N-doped 3D graphene displays superior performance in electrocatalysis and solar-energy harvesting for energy and environmental applications. However, the electrical transport properties and the electronic states, which are the key factors to understand the origins of the N-doping effect in 3D graphene, are still missing. The electronic properties of N-doped 3D graphene are systematically investigated by an electric-double-layer transistor method. It is demonstrated that Urbach-tail-like localized states are located around the neutral point of N-doped 3D graphene with the background metallic transport channels. The dual nature of electronic states, generated by the synergistic effect of N dopants and 3D curvature of graphene, can be the electronic origin of the high electrocatalysis, enhanced molecular adsorption, and light absorption of N-doped 3D graphene.

Homogeneous Spatial Distribution of Deuterium Chemisorbed on Free-Standing Graphene

Atomic deuterium (D) adsorption on free-standing nanoporous graphene obtained by ultra-high vacuum D2 molecular cracking reveals a homogeneous distribution all over the nanoporous graphene sample, as deduced by ultra-high vacuum Raman spectroscopy combined with core-level photoemission spectroscopy. Raman microscopy unveils the presence of bonding distortion, from the signal associated to the planar sp2 configuration of graphene toward the sp3 tetrahedral structure of graphane. The establishment of D–C sp3 hybrid bonds is also clearly determined by high-resolution X-ray photoelectron spectroscopy and spatially correlated to the Auger spectroscopy signal. This work shows that the low-energy molecular cracking of D2 in an ultra-high vacuum is an efficient strategy for obtaining high-quality semiconducting graphane with homogeneous uptake of deuterium atoms, as confirmed by this combined optical and electronic spectro-microscopy study wholly carried out in ultra-high vacuum conditions.

The cross-interface energy-filtering effect at organic/inorganic interfaces balances the trade-off between thermopower and conductivity

The energy-filtering effect has been widely employed to elucidate the enhanced thermoelectric properties of organic/inorganic hybrids. However, the traditional Mott criterion cannot identify the energy-filtering effect of organic/inorganic hybrids due to the limitations of the Hall effect measurement in determining their carrier concentration. In this work, a carrier concentration-independent strategy under the theoretical framework of the Kang-Snyder model is proposed and demonstrated using PANI/MWCNT composites. The result indicates that the energy-filtering effect is triggered on increasing the temperature to 220 K. The energy-filtering effect gives a symmetry-breaking characteristic to the density of states of the charge carriers and leads to a higher thermopower of PANI/MWCNT than that of each constituent. From a morphological perspective, a paracrystalline PANI layer with a thickness of 3 nm is spontaneously assembled on the MWCNT network and serves as a metallic percolation pathway for carriers, resulting in a 5.56-fold increase in conductivity. The cooperative 3D carrier transport mode, including the 1D metallic transport along the paracrystalline PANI and the 2D cross-interface energy-filtering transport, co-determines a 4-fold increase in the power factors of PANI/MWCNT at 300 K. This work provides a physical insight into the improvement of the thermoelectric performance of organic/inorganic hybrids via the energy-filtering effect.

Alkaline Phosphatase Electrochemical Micro-Sensor Based on 3D Graphene Networks for the Monitoring of Osteoblast Activity

Alkaline phosphatase (ALP) is a significant biomarker that indicates osteoblast activity and skeletal growth. Efficient ALP detection methods are essential in drug development and clinical diagnosis. In this work, we developed an in-situ synthesized three-dimensional graphene networks (3DGNs)-based electrochemical sensor to determine ALP activity. The sensor employs an ALP enzymatic conversion of non-electroactive substrate to electroactive product and presents the ALP activity as an electrochemical signal. With 3DGNs as the catalyst and signal amplifier, a sample consumption of 5 μL and an incubation time of 2 min are enough for the sensor to detect a wide ALP activity range from 10 to 10,000 U/L, with a limit of detection of 5.70 U/L. This facile fabricated sensor provides a quick response, cost-effective and non-destructive approach for monitoring living adherent osteoblast cell activity and holds promise for ALP quantification in other biological systems and clinical samples.

Selective synthesis of Kagome nanoporous graphene on Ag(111) via an organometallic template

Kagome nanoporous graphenes (NPGs) are fascinating due to their exotic electronic and magnetic properties. The emerging on-surface synthesis (mostly on metal surfaces) provides a new opportunity to fabricate Kagome NPGs with atomic resolution. Previously the Kagome NPGs synthesized on surfaces were largely heteroatom-doped and suffer from morphological defects (evidently on metal surfaces). The on-surface synthesis of pristine Kagome NPG with improved structural quality is extremely desirable. In this paper, using a halogenated precursor, we report a bottom-up fabrication of pristine NPG with Kagome topology on Ag(111) via classic Ullmann coupling. The templating effect of organometallic (OM) intermediates for subsequent covalent coupling is determined by comparing the OM phase and resultant covalent product. The reaction parameters are found to have a significant impact on the topology and quality of OM intermediates. Specifically, a higher surface temperature and lower evaporation rate favor the growth of better-quality and higher-yield OM Kagome NPGs. The covalent Kagome NPGs obtained by further annealing of these OM networks are affected likewise due to the template effect of OM intermediates. Our work further confirms the generality of the OM template effect. It also offers a novel method to achieve the selective synthesis of Kagome lattice networks.

Gap Opening in Double-Sided Highly Hydrogenated Free-Standing Graphene

Conversion of free-standing graphene into pure graphane─where each C atom is sp³ bound to a hydrogen atom─has not been achieved so far, in spite of numerous experimental attempts. Here, we obtain an unprecedented level of hydrogenation (≈90% of sp³ bonds) by exposing fully free-standing nanoporous samples─constituted by a single to a few veils of smoothly rippled graphene─to atomic hydrogen in ultrahigh vacuum. Such a controlled hydrogenation of high-quality and high-specific-area samples converts the original conductive graphene into a wide gap semiconductor, with the valence band maximum (VBM) ∼ 3.5 eV below the Fermi level, as monitored by photoemission spectromicroscopy and confirmed by theoretical predictions. In fact, the calculated band structure unequivocally identifies the achievement of a stable, double-sided fully hydrogenated configuration, with gap opening and no trace of π states, in excellent agreement with the experimental results.

3D Continuously Porous Graphene for Energy Applications

Constructing bulk graphene materials with well-reserved 2D properties is essential for device and engineering applications of atomically thick graphene. In this article, the recent progress in the fabrications and applications of sterically continuous porous graphene with designable microstructures, chemistries, and properties for energy storage and conversion are reviewed. Both template-based and template-free methods have been developed to synthesize the 3D continuously porous graphene, which typically has the microstructure reminiscent of pseudo-periodic minimal surfaces. The 3D graphene can well preserve the properties of 2D graphene of being highly conductive, surface abundant, and mechanically robust, together with unique 2D electronic behaviors. Additionally, the bicontinuous porosity and large curvature offer new functionalities, such as rapid mass transport, ample open space, mechanical flexibility, and tunable electric/thermal conductivity. Particularly, the 3D curvature provides a new degree of freedom for tailoring the catalysis and transport properties of graphene. The 3D graphene with those extraordinary properties has shown great promises for a wide range of applications, especially for energy conversion and storage. This article overviews the recent advances made in addressing the challenges of developing 3D continuously porous graphene, the benefits and opportunities of the new materials for energy-related applications, and the remaining challenges that warrant future study.

Bandgap evolution in nanographene assemblies

Recently cycloarene has been experimentally obtained in a self-assembled structure, forming graphene-like monoatomic layered systems. Here, we established bandgap engineering/prediction in cycloarene assemblies within a combination of density functional theory and tight-binding Hamiltonians. Our results show that the inter-molecule bond density rules the bandgap. The increase in such bond density increases the valence/conduction bandwidth decreasing the energy gap linearly. We derived an effective model that allows the interpretation of the arising energy gap for general particle-hole symmetric molecular arrangements based on inter-molecular bond strength.

Deuterium Adsorption on Free-Standing Graphene

A suitable way to modify the electronic properties of graphene—while maintaining the exceptional properties associated with its two-dimensional (2D) nature—is its functionalisation. In particular, the incorporation of hydrogen isotopes in graphene is expected to modify its electronic properties leading to an energy gap opening, thereby rendering graphene promising for a widespread of applications. Hence, deuterium (D) adsorption on free-standing graphene was obtained by high-energy electron ionisation of D2 and ion irradiation of a nanoporous graphene (NPG) sample. This method allows one to reach nearly 50 at.% D upload in graphene, higher than that obtained by other deposition methods so far, towards low-defect and free-standing D-graphane. That evidence was deduced by X-ray photoelectron spectroscopy of the C 1s core level, showing clear evidence of the D-C sp3 bond, and Raman spectroscopy, pointing to remarkably clean and low-defect production of graphane. Moreover, ultraviolet photoelectron spectroscopy showed the opening of an energy gap in the valence band. Therefore, high-energy electron ionisation and ion irradiation is an outstanding method for obtaining low defect D-NPG with a high D upload, which is very promising for the fabrication of semiconducting graphane on large scale.

Dirac Fermion Kinetics in 3D Curved Graphene

3D integration of graphene has attracted attention for realizing carbon-based electronic devices. While the 3D integration can amplify various excellent properties of graphene, the influence of 3D curved surfaces on the fundamental physical properties of graphene has not been clarified. The electronic properties of 3D nanoporous graphene with a curvature radius down to 25-50 nm are systematically investigated and the ambipolar electronic states of Dirac fermions are essentially preserved in the 3D graphene nanoarchitectures, while the 3D curvature can effectively suppress the slope of the linear density of states of Dirac fermion near the Fermi level are demonstrated. Importantly, the 3D curvature can be utilized to tune the back-scattering-suppressed electrical transport of Dirac fermions and enhance both electron localization and electron-electron interaction. As a result, nanoscale curvature provides a new degree of freedom to manipulate 3D graphene electrical properties, which may pave a new way to design new 3D graphene devices with preserved 2D electronic properties and novel functionalities.

Bismuth/Porous Graphene Heterostructures for Ultrasensitive Detection of Cd (II)

Heavy metals pollution is one of the key problems of environment protection. Electrochemical methods, particularly anodic stripping voltammetry, have been proven a powerful tool for rapid detection of heavy metal ions. In the present work, a bismuth modified porous graphene (Bi@PG) electrode as an electrochemical sensor was adopted for the detection of heavy metal Cd2+ in an aqueous solution. Combining excellent electronic properties in sensitivity, peak resolution, and high hydrogen over-potential of bi-continuous porous Bi with the large surface-area and high conductivity on PG, the Bi@PG electrode exhibited excellent sensing ability. The square wave anodic stripping voltammetry response showed a perfect liner range of 10-9-10-8 M with a correlation coefficient of 0.9969. The limit of detection (LOD) and the limit of quantitation (LOQ) are calculated to be 0.1 and 0.34 nM with a sensitivity of 19.05 μA·nM-1, which is relatively excellent compared to other carbon-based electrodes. Meanwhile, the Bi@PG electrode showed tremendous potential in composite detection of multifold heavy metals (such as Pb2+ and Cd2+) and wider linear range.

3D Graphene Materials: From Understanding to Design and Synthesis Control

Carbon materials, with their diverse allotropes, have played significant roles in our daily life and the development of material science. Following 0D C₆₀ and 1D carbon nanotube, 2D graphene materials, with their distinctively fascinating properties, have been receiving tremendous attention since 2004. To fulfill the efficient utilization of 2D graphene sheets in applications such as energy storage and conversion, electrochemical catalysis, and environmental remediation, 3D structures constructed by graphene sheets have been attempted over the past decade, giving birth to a new generation of graphene materials called 3D graphene materials. This review starts with the definition, classifications, brief history, and basic synthesis chemistries of 3D graphene materials. Then a critical discussion on the design considerations of 3D graphene materials for diverse applications is provided. Subsequently, after emphasizing the importance of normalized property characterization for the 3D structures, approaches for 3D graphene material synthesis from three major types of carbon sources (GO, hydrocarbons and inorganic carbon compounds) based on GO chemistry, hydrocarbon chemistry, and new alkali-metal chemistry, respectively, are comprehensively reviewed with a focus on their synthesis mechanisms, controllable aspects, and scalability. At last, current challenges and future perspectives for the development of 3D graphene materials are addressed.

Ionic Liquid Gating Enhanced Photothermoelectric Conversion in Three-Dimensional Microporous Graphene

The photothermoelectric (PTE) effect can effectively convert light into electricity through photothermal and thermoelectric processes and has great potential applications in light energy harvesting and bandgap-independent photodetection. It is particularly applicable for the terahertz (THz) range featuring low photon energy but has not been well established due to lack of high-performance PTE materials in this range. Three-dimensional microporous graphene (3DMG) foam possesses ultrahigh THz absorptivity and outstanding photothermal conversion and can serve as a promising candidate. Here, enhancement of the THz PTE response of 3DMG foam by fine-tuning its thermoelectric properties using the ionic liquid electric double layer (EDL) technique was demonstrated. Continuous and reversible control of the Seebeck coefficient of 3DMG highlights the effectiveness of EDL gating in manipulating the electronic structures of such bulk and porous material. An approximate 1 order of magnitude enhancement in the Seebeck coefficient as well as the PTE responsivity was observed. In addition, a double-cell 3DMG EDL device with a p-n junction like channel configuration enabled further improvement of the photoresponse. This work opens a new avenue to optimize the PTE performance of 2D nanosheet-assembled 3D porous materials for highly efficient energy harvesting and detection of THz radiation.

Understanding the Detection Mechanisms and Ability of Molecular Hydrogen on Three-Dimensional Bicontinuous Nanoporous Reduced Graphene Oxide

Environmental safety has become increasingly important with respect to hydrogen use in society. Monitoring techniques for explosive gaseous hydrogen are essential to ensure safety in sustainable hydrogen utilization. Here, we reveal molecular hydrogen detection mechanisms with monolithic three-dimensional nanoporous reduced graphene oxide under gaseous hydrogen flow and at room temperature. Nanoporous reduced graphene oxide significantly increased molecular hydrogen physisorption without the need to employ catalytic metals or heating. This can be explained by the significantly increased surface area in comparison to two-dimensional graphene sheets and conventional reduced graphene oxide flakes. Using this large surface area, molecular hydrogen adsorption behaviors were accurately observed. In particular, we found that the electrical resistance firstly decreased and then gradually increased with higher gaseous hydrogen concentrations. The resistance decrease was due to charge transfer from the molecular hydrogen to the reduced graphene oxide at adsorbed molecular hydrogen concentrations lower than 2.8 ppm; conversely, the resistance increase was a result of Coulomb scattering effects at adsorbed molecular hydrogen concentrations exceeding 5.0 ppm, as supported by density functional theory. These findings not only provide the detailed adsorption mechanisms of molecular hydrogen, but also advance the development of catalyst-free non-heated physisorption-type molecular detection devices.

Interfacial properties of 3D metallic carbon nanostructures (T6 and T14)-reinforced polymer nanocomposites: A molecular dynamics study

Herein, the interfacial properties of new three-dimensional (3D) configurations of metallic carbon, namely T6 and T14, incorporated to different polymer matrices (T6 and T14@polymers) are studied using molecular dynamics (MD) simulations. The effects of two types of shape models for T6 and T14, i.e. beam- and plate-like models, various square cross-sectional areas for the reinforcements, pull-out velocity and polymer structure on the interaction energy and pull-out force of final system are investigated. The results reveal that the interfacial resistance of the system is improved by imposing a high pull-out velocity to the nanofillers. For each pull-out velocity, the effect of beam-like T6 and T14@polycarbonate (beam-like T6 and T14@PC) on increasing average pull-out force is more remarkable than that of similar models surrounded by polypropylene (PP). The beam- and plate-like structures@polymers possess the lowest and highest interfacial resistance, respectively. As the aspect ratio (length-to-width ratio) of nanofillers changes from the lowest value to the highest one, the average pull-out force decreases. The average pull-out force of plate-like T6@polymers is higher than their plate-like T14 counterparts. Besides, higher absolute values of interaction energy in plate-like T6 and T14@polymers in comparison with others imply that the load-carrying capacity from the surrounding matrix to the plate-like nanofillers is significantly increased.

Extraordinary tensile strength and ductility of scalable nanoporous graphene

While the compressive strength-density scaling relationship of ultralight cellular graphene materials has been extensively investigated, high tensile strength and ductility have not been realized in the theoretically strongest carbon materials because of high flaw sensitivity under tension and weak van der Waals interplanar bonding between graphene sheets. In this study, we report that large-scale ultralight nanoporous graphene with three-dimensional bicontinuous nanoarchitecture shows orders of magnitude higher strength and elastic modulus than all reported ultralight carbon materials under both compression and tension. The high-strength nanoporous graphene also exhibits excellent tensile ductility and work hardening, which are comparable to well-designed metamaterials but until now had not been realized in ultralight cellular materials. The excellent mechanical properties of the nanoporous graphene benefit from seamless graphene sheets in the bicontinuous nanoporosity that effectively preserves the intrinsic strength of atomically thick graphene in the three-dimensional cellular nanoarchitecture.

Three-dimensional porous graphene networks expand graphene-based electronic device applications

In recent years, there has been increasing demand for 3D porous graphene structures with excellent 2D characteristics and great potential. As one avenue, several approaches for fabricating 3D porous graphene network structures have been proposed to realize multi-functional graphene materials with 2D graphene structures. Herein, we overview characteristics of 3D porous graphene for applications in future electronic devices along with physical insights into "2D to 3D graphene", in which the characters of 2D graphene such as massless Dirac fermions are well preserved. The present review thus summarizes recent 3D porous graphene studies with a perspective for providing new and board applications of graphene in electronic devices.

Two-Dimensional Hallmark of Highly Interconnected Three-Dimensional Nanoporous Graphene

Scaling graphene from a two-dimensional (2D) ideal structure to a three-dimensional (3D) millimeter-sized architecture without compromising its remarkable electrical, optical, and thermal properties is currently a great challenge to overcome the limitations of integrating single graphene flakes into 3D devices. Herewith, highly connected and continuous nanoporous graphene (NPG) samples, with electronic and vibrational properties very similar to those of suspended graphene layers, are presented. We pinpoint the hallmarks of 2D ideal graphene scaled in these 3D porous architectures by combining the state-of-the-art spectromicroscopy and imaging techniques. The connected and bicontinuous topology, without frayed borders and edges and with low density of crystalline defects, has been unveiled via helium ion, Raman, and transmission electron microscopies down to the atomic scale. Most importantly, nanoscanning photoemission unravels a 3D NPG structure with preserved 2D electronic density of states (Dirac cone like) throughout the porous sample. Furthermore, the high spatial resolution brings to light the interrelationship between the topology and the morphology in the wrinkled and highly bent regions, where distorted sp² C bonds, associated with sp³-like hybridization state, induce small energy gaps. This highly connected graphene structure with a 3D skeleton overcomes the limitations of small-sized individual graphene sheets and opens a new route for a plethora of applications of the 2D graphene properties in 3D devices.

Terahertz and mid-infrared plasmons in three-dimensional nanoporous graphene

Two-dimensional (2D) graphene emerged as an outstanding material for plasmonic and photonic applications due to its charge-density tunability, high electron mobility, optical transparency and mechanical flexibility. Recently, novel fabrication processes have realised a three-dimensional (3D) nanoporous configuration of high-quality monolayer graphene which provides a third dimension to this material. In this work, we investigate the optical behaviour of nanoporous graphene by means of terahertz and infrared spectroscopy. We reveal the presence of intrinsic 2D Dirac plasmons in 3D nanoporous graphene disclosing strong plasmonic absorptions tunable from terahertz to mid-infrared via controllable doping level and porosity. In the far-field the spectral width of these absorptions is large enough to cover most of the mid-Infrared fingerprint region with a single plasmon excitation. The enhanced surface area of nanoporous structures combined with their broad band plasmon absorption could pave the way for novel and competitive nanoporous-graphene based plasmonic-sensors.

Recently, fabrication processes have realised three-dimensional nanoporous graphene. Here, the authors reveal two-dimensional Dirac plasmons in three-dimensional nanoporous graphene disclosing strong plasmonic absorptions tunable from terahertz to mid-infrared via controllable doping level and porosity.

Hierarchical nanoporous metals as a path toward the ultimate three-dimensional functionality

Nanoporous metals prepared via dealloying or selective leaching of solid solution alloys and compounds represent an emerging class of materials. They possess a three-dimensional (3D) structure of randomly interpenetrating ligaments/nanopores with sizes between 5 nm and several tens of micrometers, which can be tuned by varying their preparation conditions (such as dealloying time and temperature) or additional thermal coarsening. As compared to other nanostructured materials, nanoporous metals have many advantages, including their bicontinuous structure, tunable pore sizes, bulk form, good electrical conductivity, and high structural stability. Therefore, nanoporous metals represent ideal 3D materials with versatile functionality, which can be utilized in various fields. In this review, we describe the recent applications of nanoporous metals in molecular detection, catalysis, 3D graphene synthesis, hierarchical pore formation, and additive manufacturing (3D printing) together with our own achievements in these areas. Finally, we discuss possible ways of realizing the ultimate 3D functionality beyond the scope of nanoporous metals.

Linear magnetoresistance in gold foams

Classical linear magnetoresistance is observed in ultralow density gold foams with strong spatial inhomogeneity.