Controlling the microstructure of resorcinol–furfural aerogels and derived carbon aerogels via the salt templating approach

The control strategy for the microstructure of resorcinol–furfural (RF) aerogels and derived carbon aerogels is attracting attention in different applications such as adsorbents, electrochemical electrodes, thermal insulation and so on. In this work, RF aerogels with abundant micropores were synthesized successfully by the sol–gel process using resorcinol (R) and furfural (F) as monomers, methanol (M) as the solvent, hexamethylenetetramine (H) as the catalyst, and zinc chloride (Z) as a salt template. The RF aerogels with micro specific surface area up to 228.28 m² g⁻¹ thus obtained have a high specific surface area (547.96 m² g⁻¹), and have a large total pore volume (0.7960 cm³ g⁻¹). The carbon aerogels were synthesized by pyrolyzing RF aerogels under a flowing argon atmosphere. Compared with carbon aerogel synthesized without a salt template, carbon aerogels synthesized with a salt template have higher BET specific surface area and larger total pore volume. Moreover, the mean pore size and particle size of carbon aerogels could be greatly reduced by adding the salt template. The influence of M/R ratio (molar ratio of methanol to resorcinol) and Z/R ratio (molar ratio of zinc chloride to resorcinol) on the microstructure of RF aerogels was systematically investigated. The salt templating is an effective approach for controlling the microstructure of RF aerogels and derived carbon aerogels.

The control strategy for the microstructure of resorcinol–furfural (RF) aerogels and derived carbon aerogels is attracting attention in different applications such as adsorbents, electrochemical electrodes, thermal insulation and so on.     

Preparation of a carbon fibre-reinforced carbon aerogel and its application as a high-temperature thermal insulator

Carbon aerogels (CAs) have attracted attention in thermal insulation. However, the traditional sol–gel method for preparing them involves time-consuming solvent exchange and rigorous supercritical drying processes, and the obtained CAs are brittle and crumble easily. To address these problems, a carbon fibre-reinforced carbon aerogel (CF/CA) was prepared via combining a resorcinol–furfural (RF) gel containing a salt (ZnCl₂) with polyacrylonitrile (PAN) fiber felt. The CF/CA not only has low thermal conductivity (0.6904 W m⁻¹ K⁻¹) even at an ultra-high temperature of 1800 °C in an argon atmosphere but also exhibits relatively high compressive strength (6.10 MPa, 10% ε) and a low density of 0.68 g cm⁻³. The CF/CAs can be used as ultrahigh-temperature thermal insulators (under inert atmospheres or vacuum) in thermal protection systems such as space vehicles or industrial high temperature furnaces. Our novel strategy may lead to lower-cost and large scale industrial processes of CF/CAs.

A carbon fiber reinforced carbon aerogel (CF/CA) was prepared by impregnating polyacrylonitrile (PAN) fibre felts with a resorcinol (R)–furfural (F) sol containing a salt (ZnCl₂), followed by ageing and pyrolysis. The RF sol containing the salt was synthesized by direct polymerisation of R and F in methanol (MeOH) using ZnCl₂ as a salt template. Compared with the traditional sol–gel method for preparing CF/CAs, this procedure eliminates the need for solvent-exchange and supercritical-fluid drying processes. This novel strategy may lead to lower-cost and large scale industrial processes of CF/CAs.     

Effect of concentration of glycidol on the properties of resorcinol-formaldehyde aerogels and carbon aerogels

By using glycidol as a catalyst, high porosity, low-density resorcinol (R) and formaldehyde (F) aerogels and carbon aerogels (CAs) were synthesized via a sol-gel method. The effect of glycidol and water on the color, density, morphology, textual characteristics and adsorption properties of the resultant RF aerogels and CAs were investigated in detail. The results revealed that the properties of RF aerogels and CAs can be controlled by adjusting the amount of glycidol and water. The resultant RF aerogels and CAs were porous materials, the minimum densities of RF aerogels and CAs were 96 and 110 mg cm⁻³ respectively while the maximum specific surface areas of RF aerogels and CAs were 290 and 597 m² g⁻¹. The maximum adsorption capacity of CAs was about 125 mg g⁻¹ on Rhodamine B, which was higher than that of some reported CAs catalyzed by base and acid catalysts. The sol-gel mechanisms of RF aerogels and CAs can be attributed to the opening of the epoxy group of glycidol in the mixture of R and F.

The sol-gel mechanism of glycidol-catalyzed RF aerogels is the opening of the epoxy ring rather than the preservation the of epoxy ring.     

A facile soft template method to synthesize hollow carbon and MnOx composite particles for effective methylene blue degradation

Hollow carbon and MnO_x composite particles (HC-Mn) were fabricated by using polyacrylic acid (PAA) and Mn ion co-assembled colloids as the soft template and resorcinol formaldehyde resin (RF) as the carbon source. The formation process was well studied and a plausible formation mechanism was proposed. The Mn ions played two key roles in the synthesis: first, they promoted the aggregation of the PAA molecules, thus forming the PAA–Mn colloids in solution with high water content, which were suitable for the subsequent RF coating. Secondly, considerable Mn ions were retained after template removal, which were transformed into MnO_x particles simultaneously during carbonization. This approach was facile and effective and the as-prepared HC-Mn showed superior catalytic activity toward methylene blue (MB) degradation.

Hollow carbon and MnO_x composite particles were synthesized for catalytic use and their formation mechanism was proposed.     

Preparation and application of sunlight absorbing ultra-black carbon aerogel/graphene oxide membrane for solar steam generation systems

In this study, sunlight absorbing membranes consisting of ultra-black resorcinol–formaldehyde (RF)-based carbon aerogel (CA) and hydrophilic graphene oxide (GO) suspension were fabricated. To investigate the effect of substrate structure, CA/GO ink was cast onto two different layers including 3D modified copper foam (MCF) and 2D paper sheet. The copper foam (CF) was treated with a new and simple modification method to enhance the hydrophilicity. Finally, the solar steam generation performances of the prepared membranes were evaluated. The optical analyses indicated that 2D and 3D samples respectively reflected ∼4.5% and ∼10%, and transmitted ∼0% of the incident light. The water contact angle measurements revealed a significant change in the wettability of the CF layer representing a contact angle of 139.41° before the modification. Based on the water evaporation rates, the efficiencies of 81.1% and 91.4% (at 1 kW m⁻²) were achieved for 2D and 3D absorbents, respectively. In addition to eliminating the geometrical restrictions of the monolithic absorbents, the results verified that CA/GO ink-based absorbents were promising materials for solar steam generation systems (SSG) due to the high light absorption, superhydrophilicity and porous structure.

The sunlight absorbing membrane consisting of ultra-black resorcinol-formaldehyde (RF)-based carbon aerogel (CA) and graphene oxide (GO) suspension was fabricated. The hydrophilic modified copper foam (MCF) was prepared and used as the substrate.     

Step-freeze-drying method for carbon aerogels: a study of the effects on microstructure and mechanical property

In this paper, a novel step-freeze-drying method was used to prepare carbon aerogels. The effects of step-freeze-drying on the density, linear shrinkage, specific surface area, pore size distribution, microstructure and compressive strength of carbon aerogels were investigated, and compared to traditional freeze-drying methods. It was found that the step-freeze-drying method reduced the density, linear shrinkage and pore size of carbon aerogels compared to traditional freeze-drying. And it also improved the specific surface area, the microstructural homogenization and the compressive strength of carbon aerogels compared to traditional freeze-drying. It is therefore believed that step-freeze-drying is an efficient method to obtain carbon aerogels with fine microstructure and high mechanical property.

In this paper, a novel step-freeze-drying method was used to prepare carbon aerogels.     

Electromagnetic and microwave absorption characteristics of PMMA composites filled with a nanoporous resorcinol formaldehyde based carbon aerogel

Nanostructured carbons have opened up new perspectives in fields of electromagnetic (EM) applications. The present study aims at the processing of microwave absorbing (MA) materials based on carbon aerogels (CAs) in polymethyl methacrylate (PMMA) matrix to be used in X-band frequency. CAs were synthesized by carbonization of a sol–gel derived organic gel from resorcinol and formaldehyde as starting materials. Microwave attenuation properties of the prepared composites were investigated in terms of CAs particle size distribution (PSD) and mass fraction. To do so, the optimal PSD was initially determined by assessing the EM attenuation performance of the CAs/PMMA composites with constant mass loading (10 wt%) and differing particle sizes. Next, the EM properties of the selected CAs with the optimal particle size was measured as a function of mass fraction varying from 1 to 15 wt% in order to obtain a highly efficient CAs based MA. The results indicate that the dielectric loss of CAs composites can be enhanced by optimizing the PSD as well as the mass fraction of CAs. The effective absorption bandwidth of composites containing 10 wt% of CAs exceeded 3.7 GHz at a very thin thickness of 1.9 mm indicating that these materials present advantages as microwave absorbers.

Both PSD and filler content play dominant role in tuning EM absorption performance of CAs composites.     

Mesoporous silica–carbon composites fabricated by a universal strategy of hydrothermal carbonization: controllable synthesis and applications

Mesoporous silica–carbon composite materials, with homogeneous and thickness-controllable carbon coating, were synthesized by using a universal strategy of hydrothermal carbonization, and the carbon layer could be coated on the surface of ordered and disordered mesoporous silica. The electrostatic interaction between amino-modified silica and hydrothermal carbon was regarded as the main driving force for the formation of homogeneous carbon coverage on the silica surface. The obtained composites showed high graphitization degree, and controlled morphology (shape and particle size) and pore size by adjusting the species of carriers and hydrothermal conditions. The application results demonstrated that a thin carbon layer possessed high adsorption capacities for dyes, and the composite could be rapidly recovered by sedimentation (10 min) after adsorption with 30 μm spherical silica gel as the carrier. Besides, baseline chromatographic separation of oligosaccharide isomers could be achieved on the silica–carbon column. These results indicated that the silica–carbon composites should be promising functional materials for the large-molecule-involving processes such as adsorption and chromatographic separation.

A carbon layer with controlled thickness can be coated on the surface of mesoporous silica through the hydrothermal carbonization strategy.     

Zeolitic imidazole framework derived N-doped porous carbon/metal cobalt nanoparticles hybrid for oxygen electrocatalysis and rechargeable Zn–air batteries

Bifunctional electrocatalysts with high catalytic property for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are vital for high-performance zinc–air batteries (ZnABs). In this study, an efficient bifunctional electrocatalyst with hollow structure (C–N/Co (1/2)) has been successfully prepared through carbonization of ZIF-8@ZIF-67 and evaporation of Zn ions at high temperature. With Co nanoparticles encapsulated by an N-doped porous carbon matrix, the catalyst exhibits excellent stability in aqueous alkaline solution over an extended period and good tolerance to the methanol crossover effect. The integration of an N-doped graphitic carbon outer shell and Co nanoparticles enables high ORR and OER activity, as evidenced by ZnAB using the catalyst C–N/Co (1/2) in an air cathode.

An efficient bifunctional electrocatalyst with hollow structure (C–N/Co (1/2)) has been obtained through carbonization of ZIF-8@ZIF-67, which showed high ORR and OER activity, as evidenced by ZnAB using catalyst of C–N/Co (1/2) in air cathode.     

CoSx/C hierarchical hollow nanocages from a metal–organic framework as a positive electrode with enhancing performance for aqueous supercapacitors

Benefiting from abundant redox chemistry and high electrochemical properties, metal sulfides have been broadly employed as electrode materials in supercapacitor systems. However, the predominant limitation in their performance, which arises from indifferent electron and ion dynamics for transportation and a rapid slash in capacitance, is of particular concern. Herein, we portray the cobalt sulfides/carbon (CoS_x/C) hierarchical hollow nanocages using ZIF-67 nanocrystals coated with carbon from resorcinol–formaldehyde (ZIF-67@RF) as a self-sacrificial template. The RF acted as a hard framework to prevent the hollow structure from breaking and was transformed to a carbon layer to enhance the charge transfer process. When used as positive electrodes in supercapacitor systems with aqueous electrolytes, the optimized CoS_x/C hierarchic hollow nanocages exhibited a considerable specific capacitance (618 F g⁻¹ at 2 A g⁻¹), superior rate performance (83.6% capacitance retention of the initial capacity when the current density was amplified from 2 A g⁻¹ to 50 A g⁻¹) and an extraordinary cycle stationarity along with an undiminished specific capacitance after 10 000 cycles. In this study, the meticulously designed hierarchical hollow structure that we conceived not only provides an outstanding electrochemical performance but also provides options for other related materials, such as various MOFs.

Benefiting from abundant redox chemistry and high electrochemical properties, metal sulfides have been broadly employed as electrode materials in supercapacitor systems.     

Boron/nitrogen co-doped carbon synthesized from waterborne polyurethane and graphene oxide composite for supercapacitors

We report B/N co-doped carbon materials synthesized by an efficient and easy one-step carbonization method with ferric catalyst treatment from a precursor with boric acid treatment after the formation of the composite between waterborne polyurethane (WPU) and graphene oxide (GO). The nitrogen content was improved with the introduction of numerous melamine in the synthetic process of WPU. In addition, WPU possessed a repetitive basic unit urethane bond (–NHCOO); thus, nitrogen heteroatom could be efficiently introduced into the WPU/GO composite from WPU as a nitrogen-rich carbon. In addition, the specific surface area was increased by the boric acid treatment and washing process. The ferric catalyst treatment could prevent the formation of inert B–N bonds. Thus, the synthesized B/N co-doped carbon materials exhibited high specific capacitance (330 F g⁻¹ at 0.5 A g⁻¹), superior rate performance, and excellent cycling stability. Furthermore, the assembled symmetric supercapacitor displayed a good energy density (7.9 W h kg⁻¹ at 505 W kg⁻¹) and a good capacitance retention of about 89.9% after 5000 charge–discharge cycles in 6 M KOH electrolyte. Therefore, the as-prepared B/N co-doped carbon materials show a promising future in supercapacitor application.

We report B/N co-doped carbon materials synthesized by a carbonization method with ferric catalyst treatment from a precursor with boric acid treatment after the formation of the composite between waterborne polyurethane and graphene oxide.     

Opal promotes hydrothermal carbonization of hydroxypropyl methyl cellulose and formation of carbon nanospheres

Hydrothermal carbon nanospheres were prepared by introducing opal into the hydrothermal carbonization system of hydroxypropyl methyl cellulose (HPMC). Then the effects of opal on hydrothermal carbonization of HPMC were investigated after different reaction durations (105–240 min). The reaction products were characterized by elemental analysis, gas chromatography-mass spectrometry (GC-MS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR) and N₂ adsorption–desorption. Results of elemental analysis indicated that the H (hydrogen) and O (oxygen) content of HPMC decreased through dehydration, demethylation, decarbonylation and hydrolysis reactions, forming hydrochar with higher carbon content. The addition of opal was confirmed to accelerate the hydrolysis of HPMC. N₂ adsorption–desorption tests and SEM analysis showed that opal with a large specific surface area adsorbed HPMC hydrolysis products, such as furans, and facilitated furan cyclodehydration on its surfaces to form cross-linked carbons, which contributed to the quick formation of hydrochar. Moreover, the adsorption by opal also inhibited hydrochar aggregation, so the final hydrothermal carbon spheres had sizes of 20–100 nm.

Carbon nanospheres were formed under the effect of opal during hydrothermal carbonization of HPMC at 230 °C.     

Facilely controlled synthesis of a core-shell structured MOF composite and its derived N-doped hierarchical porous carbon for CO2 adsorption

A new strategy for controlled synthesis of a MOF composite with a core–shell structure, ZIF-8@resorcinol–urea–formaldehyde resin (ZIF@RUF), is reported for the first time through in situ growth of RUF on the surface of ZIF-8 nanoparticles via an organic–organic self-assembly process by using hexamethylenetetramine as a formaldehyde-releasing source to effectively control the formation rate of RUF, providing the best opportunity for RUF to selectively grow around the nucleation seeds ZIF-8. Compared with the widely reported method for MOF composite synthesis, our strategy not only avoids the difficulty of incorporating MOF crystals into small pore sized materials because of pore limitation, but also effectively guarantees the formation of a MOF composite with a MOF as the core. After carbonization, a morphology-retaining N-doped hierarchical porous carbon characterized by its highly developed microporosity in conjunction with ordered mesoporosity was obtained. Thanks to this unique microporous core–mesoporous shell structure and significantly enhanced porosity, simultaneous improvements of CO₂ adsorption capacity and kinetics were achieved. This strategy not only paves a way to the design of other core–shell structured MOF composites, but also provides a promising method to prepare capacity- and kinetics-increased carbon materials for CO₂ capture.

New strategy for controlled synthesis of core–shell structured ZIF-8 composite and hierarchical N-doped carbon via an effective in situ self-assembly process.     

Fused sphere carbon monoliths with honeycomb-like porosity from cellulose nanofibers for oil and water separation

Carbon monoliths with a unique hierarchical surface structure from carbonized cellulose nanofibers were synthesized in pursuit of developing carbon materials from sustainable natural resources. Through a 2-step hydrothermal – carbonization method, TEMPO-oxidized cellulose nanofibers were turned into carbon-rich hydrochar embedded with polystyrene latex as template for 80 nm-sized pores in a honeycomb pattern, while the triblock copolymer Pluronic F-127 was used for a dual purpose not reported before: (1) an interface between the cellulose nanofibers and polystyrene particles, as well as (2) act as a secondary template as ∼1 μm micelles that form hollow carbon spheres. The use of nanofibers allowed more contact between the carbon spheres to coalesce into a working monolith while optimizing the pore structure. Oil–water separation studies have shown that carbon monoliths have high adsorption capacity due to surface area and hydrophobicity. Testing against commercially available activated carbon pellets show greater performance due to highly-developed macropores.

Carbon monoliths with a unique hierarchical surface structure from carbonized cellulose nanofibers were synthesized in pursuit of developing carbon materials from sustainable natural resources.     

Scalable,hydrophobic and highly-stretchable poly(isocyanurate–urethane) aerogels

Scalable, low-density and flexible aerogels offer a unique combination of excellent mechanical properties and scalable manufacturability. Herein, we report the fabrication of a family of low-density, ambient-dried and hydrophobic poly(isocyanurate–urethane) aerogels derived from a triisocyanate precursor. The bulk densities ranged from 0.28 to 0.37 g cm⁻³ with porosities above 70% v/v. The aerogels exhibit a highly stretchable behavior with a rapid increase in the Young''s modulus with bulk density (slope of log–log plot > 6.0). In addition, the aerogels are very compressible (more than 80% compressive strain) with high shape recovery rate (more than 80% recovery in 30 s). Under tension even at high strains (e.g., more than 100% tensile strain), the aerogels at lower densities do not display a significant lateral contraction and have a Poisson''s ratio of only 0.22. Under dynamic conditions, the properties (e.g., complex moduli and dynamic stress–strain curves) are highly frequency- and rate-dependent, particularly in the Hopkinson pressure bar experiment where in comparison with quasi-static compression results, the properties such as mechanical strength were three orders of magnitude stiffer. The attained outcome of this work supports a basis on the understanding of the fundamental mechanical behavior of a scalable organic aerogel with potential in engineering applications including damping, energy absorption, and substrates for flexible devices.

Scalable, low-density and flexible aerogels offer a unique combination of excellent mechanical properties and scalable manufacturability.     

Robust monolithic polymer(resorcinol-formaldehyde) reinforced alumina aerogel composites with mutually interpenetrating networks

Monolithic polymer(resorcinol-formaldehyde) reinforced alumina (RF/Al₂O₃) aerogel composites were prepared using a sol–gel method and supercritical fluid CO₂ drying. The formation mechanism, chemical compositions, pore structures, morphologies, thermal and mechanical performances of RF/Al₂O₃ aerogel composites with different RF/Al molar ratios were investigated. The results show that the two networks of organic resorcinol-formaldehyde and inorganic alumina are completely independent of one another. The as-synthesized RF/Al₂O₃ aerogels consist of spherical organic carbon particles and fibrous alumina, which possess low bulk density (0.077–0.112 g cm⁻³), low shrinkage (1.55–2.76%), low thermal conductivity (0.024–0.028 W m⁻¹ K⁻¹), and high specific surface area (453.26–722.75 m² g⁻¹). Especially, the sample prepared with molar ratio RF/Al = 1 shows the best network structure with the higher compressive strength (1.83 MPa) and Young''s modulus (122.57 MPa). The resulting robust RF/Al₂O₃ aerogel composites could be potentially used as thermal insulators, catalysts and adsorbents.

Monolithic polymer(resorcinol-formaldehyde) reinforced alumina (RF/Al₂O₃) aerogel composites were prepared using a sol–gel method and supercritical fluid CO₂ drying.     

A self-assembled silicon/phenolic resin-based carbon core–shell nanocomposite as an anode material for lithium-ion batteries

Silicon, with advantages such as high theoretical capacity and relatively low working potential, has been regarded as promising when it is used for lithium-ion battery anodes. However, its practical application is impeded by the intrinsic low electrical conductivity and the dramatic volume change during the lithiation/delithiation process, which leads to a rapid capacity fading of the electrode. In this regard, we design silicon nanoparticles homogeneously coated with a phenolic resin-based carbon layer as a core–shell nanocomposite via a facile self-assembly method followed by carbonization. The surrounding carbon shell, confirmed by transmission electron microscopy and Raman spectroscopy, is not only beneficial to the formation of a stable solid electrolyte interface film, but the electrical conductivity of the electrode is also enhanced. A high and stable specific capacity of nearly 1000 mA h g⁻¹ is achieved at C/3 after 200 cycles with a coulombic efficiency of >99.6%. The entire synthesis process is quite simple and easy to scale up, thus having great potential for commercial applications.

A self-assembled silicon/phenolic resin-based carbon core–shell nanocomposite is reported, which exhibits a high and stable reversible capacity and good rate capability.     

Enhancement of photocatalytic hydrogen evolution with catalysts based on carbonized MOF-5 and g-C3N4

The study presents enhancement of photocatalytic hydrogen generation after metal–organic framework (MOF5) carbonization at 700 °C and its utilization as a co-catalyst of graphitic carbon nitride (gCN). Thermal treatment of MOF5 affected the formation of ZnO nanoparticles which played the role of co-catalyst for H₂ evolution. Moreover, significant band-gap narrowing of MOF5 was observed, which also affected the narrowing of the hybrid band gap. The appropriate conduction band position of the carbonized MOF allowed photogenerated electron transfer from gCN to the carbonized MOF, hence, improving the separation of the charge carriers and reducing the overpotential for H₂ generation. The mechanism of the photocatalytic process was also discussed.

The study presents enhancement of photocatalytic hydrogen generation after metal–organic framework (MOF5) carbonization at 700 °C and its utilization as a co-catalyst of graphitic carbon nitride (gCN).     

Polyimide-derived carbon nanofiber membranes as free-standing anodes for lithium-ion batteries

Free-standing and flexible carbon nanofiber membranes (CNMs) with a three-dimensional network structure were fabricated based on PMDA/ODA polyimide by combining electrospinning, imidization, and carbonization strategies. The influence of carbonization temperature on the physical-chemical characteristics of CNMs was investigated in detail. The electrochemical performances of CNMs as free-standing electrodes without any binder or conducting materials for lithium-ion batteries were also discussed. Furthermore, the surface state and internal carbon structure had an important effect on the nitrogen state, electrical conductivity, and wettability of CNMs, and then further affected the electrochemical performances. The CNMs/Li metal half-cells exhibited a satisfying charge–discharge cycle performance and excellent rate performance. They showed that the reversible specific capacity of CNMs carbonized at 700 °C could reach as high as 430 mA h g⁻¹ at 50 mA g⁻¹, and the value of the specific capacity remained at 206 mA h g⁻¹ after 500 cycles at a high current density of 1 A g⁻¹. Overall, the newly developed carbon nanofiber membranes will be a promising candidate for flexible electrodes used in high-power lithium-ion batteries, supercapacitors and sodium-ion batteries.

Free-standing and flexible carbon nanofiber membranes (CNMs) with a three-dimensional network structure were fabricated based on PMDA/ODA polyimide by combining electrospinning, imidization, and carbonization strategies.     

Understanding the structural transformation of carbon black from solid spheres to hollow polyhedra during high temperature treatment

Understanding the structural transformation of carbon black during high temperature treatment and the underlying mechanism are very important because of the correlation with nanocarbon species such as fullerenes and carbon onions. Herein, we find that carbon black nanoparticles exhibit a solid skin–core structure constructed from small graphene flakes. The skin shows a more ordered structure than the core. During heating treatment in an inert atmosphere, the small graphene flakes coalesce together to form large-area lamellae at 1600 °C. Then, the solid spherical nanoparticles completely transform to hollow polyhedra at 1800 °C with significantly improved crystallinity. What''s more, the inner cores of carbon black can be removed through simple oxidation in air, demonstrating that the cores are more disordered and reactive than the skin. Accordingly, the structural transformation mechanism is ascribed to well-ordered graphitic shells being preferentially formed by coalescing ordered small graphene flakes in the skin region of carbon black nanoparticles. The multilocular hollow structure is subsequently formed by reconstruction of highly disordered and twisted inner cores in a confined space.

Carbon black nanoparticles with a solid skin–core structure gradually transform to hollow nanopolyhedra when treated above 1800 °C.