1 Introduction

Different methods have been adopted by researchers for synthesis of carbon nanostructures; particularly, these synthesis methods are divided into two categories: One is bottom-up and second one is top-down approaches [1,2,3,4,5,6]. In the bottom-up approach, special emphasis will be given to the use of ionic liquids owing to their low volatility and hence allowing high-yield pyrolysis and covalent organic frameworks as a new low-temperature method for high structural control [7]. With a top-down approach, more focus will be on the metal organic frameworks as sacrificial templates and carbides for nanostructured carbon [8].

Bottom-up

  • Chemical vapor deposition

  • Sol–gel nanofabrication

  • Laser pyrolysis synthesis

  • Ionic liquid

  • Covalent organic framework.

Top-down

Lithography.

  • Chemical/template etching

  • MOF-derived carbon

  • Carbide-derived carbon and carbon onion.

During the earth civilization, carbon has occupied a significant place mainly in the energy sector and has undergone a drastic transformation from coal as an energy source to activated carbon for electrical energy (batteries, solar cells, supercapacitor, etc.) [9]. The incredible properties of carbon to form linear (sp1), planar (sp2) and tetragonal configuration (sp3), high abundance, and low specific weight have evoked great interest in the scientific community [10]. Over a few years, significant efforts have been made in the development of efficient carbon materials for different applications (energy storage, sensing, energy conversion, and harvesting, etc.); one of the advancements in this direction is the synthesis of carbon nanostructures. Low-dimensional materials with dimensions of structural elements (clusters, crystallites, or molecules) in the range of 1–100 nm are chiefly defined as nanostructured carbons. There is an entire range of dimensionality in the carbon nanostructures from zero-dimensional (fullerenes), one-dimensional (carbon nanotubes), two-dimensional (graphene sheets), to three-dimensional (fullerites, CNT ropes) with one another form “inverted carbons” that are basically mesoporous materials [11]. Nanodimension carbon structures comprise distinctiveness in the form of superior electrical conductivity, high surface area volume ratio, high porosity with outstanding mechanical, chemical, thermal, and optical properties [12]. In energy storage and conversion application, the role of nanostructuring is vast, and some of them are pointed here [13,14,15,16]:

  • Nanostructuring provides more active sites for charge storage.

  • High surface area owes to nanostructuring ensures a large contact between electrode and electrolyte.

  • Stress generated during electrochemical reactions also gets reduced due to porous structure.

  • Hollow and porous surfaces also provide spaces for filling other guest materials and hence can be used in multifunctional applications.

  • Porous network also reduces the ionic diffusion length.

Hence, owing to these advantages, enormous research has been carried out to investigate the mechanism involved in controlling the dimension and surface chemistry of nanostructures. There are several ways in which nanostructuring can be done such as chemical vapor deposition, pyrolysis, covalent organic framework based, template oriented, and hydrothermal treatment, etc. [3]. On the basis of the process involved during synthesis, these methods are divided into two approaches: bottom-up and top-down. The bottom-up approach involves the building of structures from small atoms and molecules through covalent or other force interactions, whereas the top-down approach involves the etching of bulk structures to generate the needed low-dimensional structures. Basically, the bottom approach is mainly focused on controlling the dimension, and a top-down approach is used to generate hollow or porous regions in the structures. However, with a top-down approach, the chances of defects generation and quantum effect increase due to the synthesis of small dimensions. The origination of defects with the nanostructuring would have an impact on the properties of carbon nanostructures. Therefore, in most cases, the combination of these approaches is carried out which is named a hybrid approach [17]. In this chapter, some of the particular bottom-up approaches and top-down approaches have been discussed that are majorly used by the researchers in pursuit of controlling the nanostructures.

Nanostructures are synthesized either by a bottom-up or top-down approach. The bottom-up approach relies on the attractive forces between the building blocks, whereas in the top-down approach, large materials are deconstructed to give nanostructures [5, 6]. Though these approaches comprise various synthesis methods, the chapter will be confined to some recent non-traditional synthesis methods.

2 Bottom-Up Approach

2.1 Chemical Vapor Deposition

In CVD, decomposition of gas-phase molecules to reactive species results in thin films with controlled stoichiometry and morphology. CVD allows for controlled deposition rates to give high-quality products having desired conformality [18]. Recently, the synthesis of carbon nanospheres via a non-catalytic chemical vapor deposition using low-rate acetylene gas with nitrogen gas was reported [19]. This strategy resulted in fluffy carbon nanospheres with spherical shape and regular size.

A new dimension to this synthesis method was added by Fischer and coworkers, where they combined CVD and template synthesis [20]. With this combined approach, graphitic carbon nanofibers were obtained. These are open-ended uniform hollow tubes and can be converted to carbon nanofibers with increasing deposition time. Although the crystallinity of CVD grown nanotubes is low, it is superior to other methods in terms of yield and purity.

2.2 Laser Pyrolysis

This is a recent method for the synthesis of nanoscale particles especially for carbides (Fig. 1). In laser pyrolysis, a dilute mixture of vaporized precursors is decomposed by a laser to initiate nucleation. These nuclei aggregate and transported by inert gas and depending on the amount of air different nanoparticles are formed. Owing to the fast nucleation and very short time in the reaction chamber, this method yields smaller nanoparticles than other vapor-phase methods.

Fig. 1
A layout exhibits a two-phase configuration reactor for the laser pyrolysis method with labeled parts: laser power meter, ethylene insertion, and vectorization gas.

Schematic representation of two-stage configuration reactor for laser pyrolysis. Reprinted with permission from Sourice et al. [21]. Copyright 2015 American Chemical Society

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One-step synthesis of silicon-carbon nanoparticles using laser pyrolysis was reported by Reynaud and coworkers [21]. The authors utilized a CO2 laser to vaporize the reactants and beautifully utilized the working principle of laser pyrolysis. In the typical synthesis, firstly in the bottom stage, silicon cores are synthesized and transferred to the upper stage with a carrier gas where a carbon shell is deposited. This configuration prevents aerial oxidation of silicon surface. Silicon-carbon nanoparticles formed by this method were found to be very efficient for application in lithium-ion batteries with improved cycling stability.

2.3 Ionic Liquids

Although ionic liquids were discovered decades ago, their application to derive carbon materials is a relatively new emerging field. There are a variety of benefits that ILs offer,

  1. (i)

    most general is their negligible vapor pressure allowing much easier shaping, processing, and high yield under ambient thermal treatment,

  2. (ii)

    availability of a wide range of different cation/anion pairing enables a wide range of carbon nanostructures with varying heteroatoms,

  3. (iii)

    as ILs are good solvents so carbon composites with various other substances can be easily prepared by simply mixing.

Functional groups can be selectively introduced in ILs depending on the type of carbon required. Davis coined the term task-specific ionic liquids (TSILs) for ILs having specific functional groups attached for the desired property [22] as shown in Fig. 2. They selected ILs containing a nitrile group so that it can undergo cross-linking. They studied the effect of cations using 1-cyanomethyl-3-methylimidazolium [MCNIm]+ and 1,3-bis(cyanomethyl)imidazolium [BCNIm]+ and interestingly found that rather than cation, the surface properties of carbonaceous materials are affected to a great extent by anion. BET surface area of bis(trifluoromethylsulfonylimide) ([Tf2N]−) and Cl− was found to be 700 m2/g and 10 m2/g, respectively [23]. Similarly, for [C(CN)3]− anion species, BET surface area was found to be 90 m2/g, whereas boron and nitrogen-doped carbon from [B(CN)4]− anion exhibits BET surface area of 500 m2/g [24].

Fig. 2
A schematic representation of the composition route for task-specific ionic liquids - obtained N - doped porous carbon, contains a nitrile group.

Synthesis route for TSILs-derived N-doped porous carbon. Reprinted with permission from Lee et al. [23]. Copyright 2009 American Chemical Society

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Nowadays, ILs containing more than one type of cation/anion forming complex supramolecular structure, referred to as deep eutectic solvents (DES), were discovered. When quaternary ammonium salts in ILs complexes with hydrogen bond donors, then hydrogen bonding facilitates charge delocalization resulting in a eutectic mixture with a reduced melting point is formed.

Carbon and carbon–carbon nanotube composites can be obtained by using a choice of DES [25]. They utilized DES composed of ethylene glycol and CCl to dissolve the reactants (resorcinol and formaldehyde). On heating monolith, resins were formed which were carbonized using thermal treatment. Results indicated high surface area for DES-based carbon monoliths than monoliths derived from aqueous media due to the structure effect of DES. Furthermore, it was found that MWCNTs form a homogeneous dispersion in DES by simple addition. This fact was observed in the SEM micrographs where MWCNTs were uniformly distributed in DES as compared to aqueous solvents.

Although ILs are gaining much importance for designing functional carbon materials, in-depth investigation of the correlation between precursor and carbon nanostructure is required to design novel need tailored materials.

2.4 Covalent Organic Frameworks (COFs)

Covalent organic frameworks are porous crystalline polymers composed of ordered periodic building block skeletons extended in 2/3-dimensions. They provide a low-temperature synthesis method for crystalline carbonaceous materials with a controlled structure. Based on the building blocks, different geometries and dimensions can be obtained. COFs are usually classified based on the linkage topology involved. These include (i) B–O (borates), (ii) C=N (imines), (iii) C=Naromatic (azines), (iv) C=C (alkenes), (v) C–N (β-ketoenamines), and (vi) B=N (borazines) [26]. Among these, C–N linkage possess high structural stability even in strong acids.

Depending on the requirement, robust materials can be designed by Schiff’s base reaction of amines with aldehydes resulting in imines. Based on this strategy, Feng group reported the synthesis of the wafer-sized multifunctional conjugated polymer [27]. They carried out this synthesis of the 2D multifunctional polymer at the air–water interface by Schiff’s base polycondensation. Owing to the high stability of C=N, this polymer possesses a Young’s modulus comparable to graphene and conjugation lowers the band gap to 1.4 eV making it suitable for semiconductors.

3 Top-Down Approach

3.1 Lithography

The term lithography originated from the Greek word lithos meaning stone and graphein meaning to write. Traditional lithography can generate various patterns by masking the required material and etching the exposed surface. Etching can be done by either chemicals or mechanically using high-energy electron beams. This gives rise to different types of lithography like optical lithography, soft lithography, block copolymer lithography, scanning probe, nanoimprint lithography, etc. Here, we will restrict our discussion to the recently emerged advanced technique, nanoimprint lithography (NIL) only. This is the most important lithographic technique for designing advanced nanostructures due to its ability to generate high-resolution nanopatterns with high throughput and low cost [28]. In conventional methods, the resolution is limited due to diffraction or beam scattering but in NIL, there is no such restriction. In NIL, a nanostructured mold is pressed on substrate precoated with a polymeric material. NIL is further classified as—thermoplastic, photo, and electrochemical NIL. In thermoplastic NIL, the substrate is modified with a thin layer of a thermoplastic polymer. This is pressed with a patterned mold at the desired pressure. On heating, the polymer above glass transition temperature results in the required pattern on polymer [29]. In photo NIL, a substrate is modified with a photo-sensitive resist, and the mold is made transparent. After pressing in UV light, the resist becomes solid, and the pattern is transferred. Usually, polydimethylsiloxane (PDMS) is used for the transparent stamp as it offers a high-resolution pattern, and its low surface energy makes it easy to separate [30]. Fang group reported the use of stamps made of silver sulfide, a superionic conductor using electrochemical nanoimprint lithography (ECNIL) [31]. Henceforth pressing, when voltage is applied, it results in electrochemical etching to give the required pattern. Using ECNIL, three-dimensional micro-nanostructures (3D-MNS) were synthesized where ECNIL allowed for the synthesis of 3D-MNS with multilevel and continuously curved profiles on a crystalline GaAs wafer [32]. This suggests the high potential of ECNIL in semiconductor materials.

3.2 MOF-Derived Carbons

Metal organic frameworks (MOFs) consist of a regular array of metal ions and organic linkers to give crystalline porous materials [33, 34]. Uniform pore structure with tunable porosity makes them attractive candidates for various applications. To derive high surface area carbon, they can be directed carbonized or can be used as a template [35,36,37,38]. In 2008, the first synthesis of porous carbon from the MOF template was reported [35]. They carbonized furfuryl alcohol (additional carbon source) filled MOF-5 at 1000 ℃. Such a high temperature helped in removing vaporized Zn metal with inert gas flow and attained metal-free carbon. The resulting carbon exhibited a high specific surface area of 2872 m2 g−1, useful for charge storing applications. This process was further refined by selecting particular MOFs to reduce the need of additional carbon sources to reduce the number of steps involved. It was found that using ZIF-8 as precursor, high surface area (3148 m2 g−1) carbon can be obtained without additional carbon [39].

Later on, it was realized that for some applications, bare carbon is not sufficient, and some heteroatoms may be useful for chemical functionalities and desired properties; heteroatom-doped carbon materials were searched. Here, zeolitic imidazole frameworks (ZIFs) seemed to be a suitable precursor for such materials. The same can be achieved by encapsulating the desired heteroatom rich molecule in MOF pores and then carbonizing the MOF. Using this strategy, synthesis of N, P, and S ternary-doped metal-free carbon from MOF was reported previously [40]. They selected MOF-5 as a template with dicyandiamide, with two heteroatom precursors (triarylphosphine and dimethylsuphoxide) to give high-performance carbon owing to the synergistic effect of heteroatoms (Fig. 3).

Fig. 3
A series of photographs represent 3 D structures. 'A' to c, three typical 3 D structures of M O F - 5, 177 and U M C M - 1. D to i, optical micrographs of M O F - 5, 177 and U M C M - 1 were observed before and after carbonization at 900 degrees Celsius.

Copyright 2014 Springer Nature

a–c Representative 3D structures, d–i optical micrographs before and after carbonization for MOF-5, MOF-177, and UMCM-1. Reprinted with permission from Li et al. [40].

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Although MOFs offer several advantages over traditional methods like tunable morphology, porosity, and heteroatom doping, there are some challenges to be considered. There is a strong need for low-cost synthesis methods for MOFs, and also, high-temperature carbonization makes it difficult to have full control over morphology. So, low temperature and cost-effective methods are required to expand the use of MOFs.

3.3 Carbide-Derived Carbon (CDC)

CDC is a collective term used for carbon derived from binary/ternary carbides (MAX phase) [41]. Depending on the method of metal removal and various experimental conditions, a variety of carbon nanostructures can be obtained. In this method, selective removal of metal atoms does not disturb the parent structure. The most common methods used in CDC are halogenation, hydrothermal treatment, and vacuum decomposition [42].

  1. (i)

    Halogenation

Selective removal of metal atoms from carbide was first reported in 1918 [43]. In this method, carbon was formed as a byproduct of treating dry chlorine gas with hot silicon carbide.

$${\text{SiC}}\left( {\text{s}} \right) + 2{\text{Cl}}_2 \left( {\text{g}} \right){\text{SiCl}}_4 \left( {\text{g}} \right) + {\text{C}}\left( {\text{s}} \right)$$
(1)

Annealing of resulting porous carbon will remove residual chlorine. Halogenation of carbides up to 1000 ℃ produces disordered carbon, whereas above 1000 ℃ graphitization predominates. Although fluorine can also be used for some metal carbides but direct fluorination is aggressive and will result in either fluorocarbon or disintegrated SiC films. Using XeF2 for etching was found to produce non-fluorinated SiC that also at low temperature (120 ℃) than the conventional chlorination method [44]. These equations and the choice of halogen become more complex in the case of ternary carbides due to the possibility of the formation of more than one solid reaction product.

CDC formation from carbides through halogenation is also known as conformal transformation as it maintains the shape and volume of the parent carbide precursor [45, 46]. This fact was recently confirmed that even for a carbide precursor-like β-SiC having complex morphology, it maintains it on chlorination [47] (Fig. 4). A similar conformal transformation was observed for polymer-derived carbides (PDC) also [48, 49].

Fig. 4
A set of 3 T E M micrographs expose beta-silicon carbide captured under 200 nanometers at different chlorination stages, and an S E M micrograph exhibits a conformal modification.

Copyright 2006 John Wiley and Sons

TEM micrographs of β-SiC at various chlorination stages, a as received, b chlorinated at 700 ℃, and c at 1200 ℃. d SEM micrograph showing the conformal transformation. Reprinted with permission from Cambaz et al. [47].

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The structure of CDC mainly depends on carbide precursor and temperature. For TiC, the effect of halogenation temperature was studied and the results indicated that with increase in temperature, there is a constant increase in ordered structure (Fig. 5). Whereas at low temperatures, amorphous carbon was found. These results were further supported by quenched molecular dynamics (QMD) studies [50, 51].

Fig. 5
A set of 3 simulated structures of titanium carbide - C D C, depicts the increased ordered structures at various temperatures, including fast, medium, and slow quenching rates.

Copyright 2010 Elsevier

Simulated structures of TiC-CDC quenched at different rates a 600 ℃/fast quenching, b 800 ℃/medium quenching, and c 1200 ℃/slow quenching. Reprinted from Palmer et al. [51].

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  1. (ii)

    Hydrothermal treatment

The hydrothermal method was first described for the synthesis of Tyranno (Si–Ti–C–O) fibers in the 1990s. In this method, the metal carbides area is treated with supercritical water at high pressure in the temperature range of 200–1000 ℃ [52, 53]. Jacobson performed Gibbs energy minimization for a large number of metal carbides at various temperatures and pressures under hydrothermal conditions [54] and found some common reaction products including carbon, MOx, CH4, CO2, CO, and H2 according to the following equations:

$$n/2{\text{MC}} + n{\text{H}}_2 {\text{O}} \to M_{n/2} O_n + n/2{\text{CH}}_4$$
(2)
$${\text{MC}} + \left( {n + 1} \right){\text{H}}_2 {\text{O}} \to {\text{MO}}_n + {\text{CO}} + \left( {n + 1} \right){\text{H}}_2$$
(3)
$${\text{MC}} + \left( {n + 2} \right){\text{H}}_2 {\text{O}} \to {\text{MO}}_n + {\text{CO}}_2 + \left( {n + 2} \right){\text{H}}_2$$
(4)
$${\text{MC}} + n{\text{H}}_2 {\text{O}} \to {\text{MO}}_n + {\text{C}} + n{\text{H}}_2$$
(5)
$${\text{MO}}_n + y{\text{H}}_2 {\text{O}} \to {\text{MO}}_n \cdot y{\text{H}}_2 {\text{O}}$$
(6)

Experimental studies indicated that water/carbide (SiC) ratio is an important factor for deciding the yield. When this ratio is small, both carbon and silica are deposited and for intermediate ratio both are formed but due to excess of water, silica gets dissolved. At a high ratio, neither carbon nor silica was formed.

The hydrothermal method produces amorphous carbon as indicated by the appearance of D and G bands in the Raman spectra of the product. But in the case of ternary carbides, the metal oxide phase may be present. As in Ti3Si2C, on hydrothermal treatment rutile/anatase was reported along with carbon [55]. The use of organic compounds like polyethylene was found to produce carbon nanotubes [56].

  1. (iii)

    Thermal decomposition

Metal carbides on thermal decomposition in the vacuum will also generate CDC. It is based on the principle of incongruent decomposition. As the melting of carbon is higher than that of carbide, it remains while carbide evaporates. This method is also a conformal process like halogenation, and due to high temperatures, carbon nanostructures thus produced are more ordered. Using this method, Kusunoki group reported epitaxial carpets of self-organized carbon nanotubes (CNTs) from SiC [57]. Interestingly, this was the first time when no metal catalyst was used to obtain CNTs of the same chirality. They also presented a model to explain the mechanism of CNT formation based on residual oxygen. According to this, residual oxygen is the driving force; at low temperatures (< 1000 ℃), only graphite sheets are formed no CNT were formed. At temperatures around 1300 ℃, there is SiO gas generated which forms nanotubes (Fig. 6).

Fig. 6
Two S E M micrographs of C N Ts growth captured under 200 (a) and 100 (b) nanometers. C, two representations and a S E M micrograph exhibit the reaction of surface modification on C N T growth.

Copyright 2008 Elsevier

SEM micrographs of vacuum decomposited 6H-SiC carbon nanostructures for a C-face oriented in high vacuum and b Si-face oriented in low vacuum. c Effect of surface modification on CNT growth. Reprinted with permission from Cambaz et al. [50].

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This method can also be used to produce graphene from SiC either by vacuum decomposition or heating in an inert environment. Its formation mechanism is as that for CNT assuming that prior to CNT formation, there is the formation of a thin graphene layer [58,59,60,61].

Comparisons of various properties are given in Table 1.

Table 1 Comparison of various properties of carbon materials discussed
Full size table

Using the above methods, a variety of carbon nanostructures can be synthesized. Aggregation of atomic sheets of 2D materials like graphene hinders its applications and requires advanced methods to prevent it. Some important methods are:

  1. (1)

    Shape/orientation modified growth of graphene

    Synthesis of graphene directly on the substrate leads to restacking but one way to avoid this is to synthesize vertically oriented graphene. Using the CVD method, vertically oriented graphene was synthesized on Ni substrate where mostly edges are exposed and fully accessible [62]. On some substrates when this is not possible then instead of flat graphene sheets, modification of the shape of graphene to curved or crumbled sheets will also prevent aggregation. This modification provides additional surface area and pore volume essential for several applications [63].

  2. (2)

    Intercalated graphene

    After exfoliation, insertion of typical guest molecules in graphene interlayers is an effective strategy to prevent aggregation with a maintained flat sheets structure. To date, various spacers have been reported including metal nanoparticles, carbon nanotubes, conducting polymers, etc. Based on the type of application, the interlayer gallery height can be fine-tuned by selecting definite spacers. Recently, polyoxometalates (an emerging class of materials) have been reported as a spacer [64]. Polyoxometalates not only prevented aggregation but also provided additional redox-active sites for charge storage.

  3. (3)

    3D interconnected graphene

    This strategy is utilized in certain cases when no foreign element other than graphene is required. Interconnection of a large number of 2D nanosheets results in a complex 3D network. In this complex network, each graphene sheet is supported by other sheets, resulting in a porous structure having a very low tendency to aggregate. Although there are a variety of materials reported with the 3D networks, graphene hydrogels are the important ones. In the simplest way, hydrothermal reduction of graphene oxide will induce cross-linking at multiple points leading to the hydrogel. This generates a porous structure with vast pore size distribution making it useful for a large number of applications [65].

4 Conclusion

The peculiar properties of carbon nanostructures require the attention of researchers to develop advanced synthesis approaches. There are many ways to synthesize carbon materials, where the structures and functions are governed by the processing conditions and precursors used. Depending on the requirement and initial conditions available, this chapter summarizes various methods for the top-down and bottom-up synthesis of carbon nanostructures. Among different approaches, some are outdated and traditional methods that are inefficient to fulfill the present need of advanced nanostructures. This chapter discusses some of the recent methods comprising high yield and ecofriendly nature including laser pyrolysis, ionic liquids precursor, etc. Independently, both these approaches can generate different nanostructures with unique properties; however, they suffer from their limitations which demand consideration toward the generation of some new techniques comprising both top-down and bottom-up approaches for developing novel materials in various research fields.