Properties, assembly and characterization of carbon nanotubes: their application in water purification, environmental pollution control and biomedicines—a comprehensive review

Abstract

Owing to their simplicity and ease of synthesis, carbon nanotubes (CNTs) have captivated attention of researchers. Many engineering applications have investigated the new features of nanostructured carbon nanotubes, such as large surface area, stiffness and durability. CNTs have opened up new opportunities for environmental improvement, pollution management and application in a variety of fields. Multiple types of pollution are produced as a result of population growth, urbanization and industrialization. CNTs are used to solve a variety of challenges, including environmental difficulties, water pollution, biomedical applications, and so on. It becomes an unavoidable present and future material. Different applications of CNTs have been presented in this review paper. CNTs are potential material having number of uses, including water purification, drug delivery, preservatives, catalysis, genetic engineering and artificial implants which are reviewed in this review article. This paper is presenting an explicit and systematic progress of CNTs for water treatment, medicinal uses drug delivery, artificial implants and so on, and a multitude of CNT applications in broad disciplines and their purification methods have been covered. The issues related to synthesis technologies, purification technology, bio-medicinal application and catalytic property of CNTs within the framework of different engineering applications and environmental impact are discussed in this study.

Graphical abstract

The modified CNTs synthesis methods, structure, catalytic property, as preservatives, water purification technology and genetic engineering applications are all summarized in this paper. Due to their outstanding water purification property, high stability, high surface area and electroluminescent properties, these tubes are highly applicable in multiple fields; this review also discusses these tubes as a useful material in water purification, as a catalyst, and as a drug delivery agent.

1 Introduction

CNTs are undoubtedly attracting a lot of attention in today's world due to their unique characteristics and range of uses. CNTs are basically “one dimensional tube like carbon nanostructures formed as a result of rounded graphene sheets” [1], the structure of which is shown in Fig. 1. The discovery of CNTs by Iijima in 1991 [2] (as previously published in the year 1952 by Radushkevich et al. [3]) has driven global scientific research and technological attention with the hope to revolutionize various frontiers in the field of nanotechnology [2]. They were discovered in Damascus steel in the seventeenth century, and they most likely contributed to the swords' legendary strength [4, 5].

Fig. 1

Basic hexagonal structure of graphene

CNTs are made of graphite and are allotropes of carbon [6], manufactured with L/D ratio of ≫ 132,000,000:1 and are cylindrical nanostructured tubes with several millimeters in length [7], represented in Fig. 2. The original CNTs have been derived from arc discharge experiments [8], yet catalytic chemical vapor deposition (CCVD) method has been favored for single-walled carbon nanotube (SWCNT) synthesis and this method is utilized on commercial scales [8, 9]. Such SWCNT film has an advantage of replacement to other secondary substrate as it has weak adhesion to the substrate [10].

Fig. 2

a Diamond; b graphite; c spherical fullerene C_60; and d tubular fullerene SWCNT [11]

In 1985, the discovery of carbon buck balls or fullerenes was considered as an initial step in the evolution of nanotechnology [12]. The young's modulus and tensile strength of CNT are higher than those of the metals such as iron and steel. The synthesis of CNT involves various methods such as plasma rotating method (PRM), chemical vapor deposition (CVD), electric-arc discharge (EAD) and hydrothermal methods (HM). PRM is used for producing CNTs on large scale [13]. The capacity of CNT production has increased recently more than thousands of tons annually, bringing down the price of CNTs [14]. CNTs serve as transporters for molecules from various sources, including enzymes, DNA or RNA, proteins and medicines. It was shown that CNTs successfully penetrate cells and bypass the blood–tissue barrier in addition to their well-established transport capabilities. These discoveries sparked research toward the designing of CNT-based drug delivery platforms [15].

Using CNTs as super capacitors allows for less environmental waste to be dumped because of their great mobility and high power density, small size, exceptional energy and long suitability [16]. Recent research has demonstrated that CNTs have positive impacts on plants both in vivo and in vitro. These investigations also show that SWCNTs and multi-walled carbon nanotubes (MWCNTs) can travel to particular cellular organelles via crossing the cell walls of plant cells [17]. Furthermore, doping of CNTs effectively provides a way to regulate their physiochemical properties toward specific applications. Heteroatoms, like boron and nitrogen, can be used as dopants to adjust the properties of CNTs and making them applicable in the area of catalysis, electrocatalysis, hydrogen storage and sensors in depth [18] as represented in Fig. 3.

Fig. 3

a Adding heteroatoms to the carbon framework of CNTs to dope them; and b elements reportedly appropriate for doping CNTs with heteroatoms, particularly B, N, P, O and S [14]

2 Structure

CNTs are classified on the basis of number of layers, shape, size, dimension and rolling angle. Details about different classes of CNTs have been described in the below section.

2.1 On the basis of number of layers

Based on the number of concentric cylindrical layers in the nanostructure, CNTs are classified into three types [19] as shown in Fig. 4.

Fig. 4

Rolling of graphite layer into SWCNT [20] and MWCNT [21]

2.1.1 Single-walled carbon nanotubes (SWCNTs)

The 1993 [22], discovery of SWCNTs consists of a single graphite sheet that has been twisted into a seamless cylindrical tube [5, 23] with diameter ranging from 1 to 2 nm [24] and a tube length in orders of centimeters. Graphene, which is the identical polyaromatic and monoaromatic layer constructed of a hexagonal display of sp² hybridized carbon atoms, can be assumed to be the perfect graphene sheet that is wrapped in a one-atom thick layer of graphene as a cylinder to form the structure of SWCNT [22]. SWCNT possesses ultimate tensile strengths in 100–200 GPa range, highest among the materials already in use [25].

Optimized geometry of SWCNT is important to high-performance transparent conductive films [26] and have been of great use in a number of areas such as touch panels, photovoltaics, liquid crystal displays and organic light-emitting diodes [27]. SWCNTs are broadly utilized in electronic applications like telecommunications, fuel batteries, biosensors, photonics, fuel cells, high frequency transistors, molecular contacts and cancer research. They have been anticipated as innovative structural materials because of their exceptional theoretical strength per weight [28].

2.1.2 Double-walled carbon nanotubes (DWCNTs)

DWCNTs are a type of MWCNT made up of two concentrically rolled up graphene sheets [5, 21]. They have properties and morphology similar to SWCNTs except that they possess high resistance to chemicals. When functionality is required to add new attributes to the material, this property is crucial. DWCNTs, which are a synthetic blend of both SWCNT and MWCNT, have higher thermal and electrical stability and flexibility than SWCNTs and MWCNTs. DWCNT undergo modification only on the outer wall, thus maintaining its internal properties. DWCNTs find extensive use in field-emission displays, dielectrics, gas sensors and technically challenging applications like nanocomposite materials [22].

2.1.3 Multi-walled carbon nanotubes (MWCNT)

MWCNTs are made up of nested tubes with increasing widths ranging from 3 to 30 nm [24] enfolded around one another in a tube-shaped structure and with an interlayer dispersion of 3.4 Å [24] due to van der Waals forces between neighboring layers [11]. The basic structure of SWCNT, DWCNT and MWCNT is shown in Fig. 5.

Fig. 5

Basic structure of a SWCNT; b DWCNT; and c MWCNT [5]

There are two models for MWCNT as shown in Fig. 6.

• (a) The Russian Doll Model Concentric cylinders are formed from graphene sheets.

• (b) The Parchment Model A single graphene sheet is folded over on itself like a newspaper [5, 23].

Fig. 6

a Russian doll model; b parchment model [29]

2.2 On the basis of rolling angle

On the basis of rolling angle of graphene sheets, CNTs are classified into three classes as represented in Fig. 7.

• (a) Armchair

• (b) Zigzag and

• (c) Chiral [30]

Fig. 7

Chiral-, zigzag- and armchair-shaped CNT structures [31]

They differ in chiral angle and diameter: armchair CNTs share electrical properties similar to metals. The chiral and zigzag CNTs have electrical characteristics same as that of semiconductors [31].

2.3 On the basis of structure of CNTs

2.3.1 Polymerized SWCNT

These are the fullerenes and associated chemicals and minerals in their solid-state manifestation. A large number of SWCNTs entangle to create polymerized SWCNTs, which are as hard as diamond. Nowadays, SWCNT-Si solar cells are gaining attention because of their high efficiency and low cost [32].

2.3.2 Nanotorus

A CNT is bent into a torus (donut shape) to form a nanotorus, as shown in Fig. 8. They possess a variety of unusual qualities, such as magnetic moment that is 1000 times higher than was previously anticipated for a given set of radii [33].

Fig. 8

A complete nanotorus structure [33]

2.3.3 Nanobuds

Carbon nanobuds shown in Fig. 9 are a newly found substance that combine two previously identified carbon allotropes: CNTs and fullerenes. Nanobuds have “buds” that resemble fullerenes which are covalently attached to the outer sidewalls of the underlying CNT [34].

Fig. 9

Nanobuds [33]

3 Properties of CNTs

CNTs peculiar structure gives them special physical, chemical [20, 21], optical [21, 35], vibrational [36], electronic [16], thermal, mechanical and adsorption properties [21, 37] including ultralight weight [38], large specific surface area [20], which makes them economical in various applications [16].

The perfect hexagonal structure of CNTs is responsible for the exceptional properties including their high length to diameter or aspect ratio which is specified for most of the nanostructured materials. These specific attributes make them interact with their surrounding environment more effectively, particularly in adsorption properties and in gaseous environment [12]. Thus, CNTs have been extensively used in a number of applications such as sensors, electromagnetic shielding, energy, environment, catalysis [20], gene therapy, drug delivery, neuro engineering, biological and chemical sensor technology, biomedical and tissue engineering. CNT is one of the most extensively investigated nanostructured materials as a result of all these applications.

The sp² covalent bonds found between the carbon atoms of CNTs contribute to raise the material's elastic modulus and tensile strength, they also make the material harder and stronger. Covalent and non-covalent bonding can be used to change the surface of CNTs in order to increase their sorption capacity and increase the materials' thermal and chemical stability. [30]. It is possible to achieve rapid detection of biological species at low concentrations due to the high surface to volume ratio of CNT. Thus, CNT-based biosensors are acknowledged as the next-generation component for ultra-sensitive bio-sensing systems [39].

SWCNT-based thin films provide a unique combination of high surface area, mechanical flexibility and optical transparency [40], high mobility, high current carrying density, ballistic transport, and they offer great potential for applications in large-scale integration, filed effect transistors, energy conversion device [41] flexible displays, electronic textiles, energy storage [40] and for photovoltaic applications [42]. To alter the surface of an electrode made of glassy carbon, MWCNT applications in the field of electrochemistry may be helpful [30]. The extraordinary adsorption property of CNTs is of great importance in the removal of heavy metals from waste water. CNTs may provide a route for further technological progress, upgrading our living standards [43].

CNT-based nanofluids have attracted the attention of numerous researchers due to their better heat transfer characteristics [44, 45]. The CNTs being used as materials for removal of organic pollutants have gradually become an exponentially emerging field for researchers. The removal of organic pollutants can be facilitated by CNTs with large specific surface area, specific nanosized structure and electrically conductive properties by adsorption process. Due to their abundance on earth, environmental friendliness and other benefits, carbon-based materials have greatly accelerated societal growth [20]. CNTs are most favorable candidates in the area of nanoelectronics, especially for interconnect applications [5]. Additionally, CNTs have been shown to be excellent at removing pollutants from the environment, including heavy metals, nitric oxide, pharmaceutical chemicals, dyes and hydrogen sulfide with degradation efficiency of up to 99% [46].

Among all engineered nanoparticle (ENPs), the carbon-based ENPs are drawing attention for potential biomedical applications such as drug design, drug delivery, tissue engineering, biosensors design, tumor therapy due to their optical, mechanical and electronic properties [47]. Presently, effects of different surface modification methods (surfactant, aid, base, sulfate and amide) on MWCNT are studied [48]. Furthermore, CNTs are thought as effective building blocks of future nanoelectronics technology [49]. Despite the advancement in the applications of CNTs in pollution treatment and environmental clean-up, major issues of CNTs include high manufacturing cost, the accumulation in the post-treated solutions and the pollution caused by chemicals during surface modification, should be addressed in future for large-scale applications [50] (Table 1).

Table 1 Comparative analysis of SWCNT and MWCNT [6, 33, 38]

4 Preparation techniques

To make MWCNTs or SWCNTs, a variety of procedures are used. CNTs can be synthesized by applying number of processes. This sections explains these strategies in detail.

4.1 Electric-arc discharge (EAD)

CNTs are mostly used by hitting an arc between graphite electrodes in a non-reacting environment such as Ar and He, a process which also yield carbon soot exhibiting molecules of fullerenes [51]. The carbon arc is a convenient and reliable method to produce high temperatures required to evaporate carbon atoms into plasma (> 3000 °C) [52,53,54]. The product of CNTs determined by plasma stability produced between electrodes, current density, inert gas pressure and cooling chamber and electrode [52, 54]. He gas gave the excellent outcomes among inert gases, due to its high ionization potential [55]. In arc growth procedure, cooled electrodes and arc chambers facilitate to increase the product of nanotubes. The circumstances for preparing MWCNTs have been optimized in order to produce minimum amount of soot during arc evaporation and evaporated carbon from a pure graphite anode that is 75% has been synthesized to deposit onto the facing graphite cathode surface. The arc deposited has been prepared by hard grey pyrolytic graphite outer shell and soft black powder with almost 2/3 CNTs and 1/3 graphitic nanoparticles inside. The optimized preparing conditions were 20–25 V, 50–100 A direct current (DC), and 500 torr of He gas pressure. Arc discharge is an easy method that produce structurally good, high quality CNTs. On the other hand, traditional arc discharge is one of the discontinuous and less stable method to prepare large quantities of CNTs. Because of the synthesis of CNTs on the surface of cathode and changes in spacing of electrodes, flow of current not remain uniform and electric fields are also not uniform. As a result, carbon's vapor density and temperature distribution is different, and carbon nanoparticles and contaminants co-exist with nanotubes at all times. Number of efforts have been attempted to generate a continuous and high efficient discharge, and number of studies have been conducted for the understanding of growth mechanism of nanotubes [56, 57].

To prepare high quantities of CNTs, Lee et al. [58] applied plasma rotating arc discharge method. To prepare CNTs, graphite anode is made to rotate with high velocity in the method. The anode rotates, distributing the micro-discharges equally and producing steady plasma. The turbulence is generated by rotation through centrifugal force, which accelerates carbon vapor perpendicular to anode. It is collected on graphite collector, that is placed closer to the plasma's edge, instead of condensed at cathode surface. By increasing rotation speed of anode yield of nanotubes increases and collector move near to the plasma, because of improvement in two conditions. For nucleation, one must possess high carbon vapor density, while other must possess enough temperature of collectors to prepare nanotubes. The plasma rotating electrodes method is a continuous stable discharge method which is analyzed to produce high quality nanotubes in good amounts. CNTs were prepared in large amounts by using plasma arc jets [59, 60] by improving quenching method in an arc between anode made of graphite and cooled Cu electrode.

For the continuous synthesis of CNTs, Ishigami et al. [61] proposed the simplified arc approach. To prepare CNTs, pumps, seals, water-cooled vacuum chambers, and purge-gas controlling setup were not used, rather than this, only DC power supply, electrodes of graphite and liquid N₂ container were used. In general, nanotubes produced were in a good quality, having 4–8 layers, straight and long parallel walls, and with minor surface pollution. Even then, the tubes prepared by liquid N₂ on average seems to have clean surfaces than tubes grown with conventional methods. No sign was found for nitrogen incorporation in the tubes, which were completely prepared by carbon. By comparing traditional nanotubes production method, this carbon arc nanotube preparation method removes almost all the difficult and uneconomical instrument. The arc discharge approach does not require a catalyst for the synthesis of MWCNTs, although catalyst species are required for the development of SWCNTs.

Iijima et al. [62] published the first report on the manufacture of SWCNTs. They prepared SWCNTs by arcing a Fe-graphite electrode in CH₄-Ar atmosphere. In the graphite anode, a hole was drilled, that was later on filled with metal and graphite powder composite, while the cathode was purely made of graphite. Few transition metals such as Fe, Co, Ni and rare earth metals such as Y, Gd have been used to prepare isolated SWCNTs [62,63,64,65,66], while composite catalysts like Fe–Ni and Co–Ni have been applied to prepare piles of SWCNTs [67]. The tubes used had 1.2 nm average diameter investigated by Saito et al. [68] who developed SWCNTs made of various catalysts and discovered that a Co or Fe–Ni bimetallic catalyst synthesized tubes that synthesize a highway-junction like pattern. Ni catalysts prepared lengthy as well as thin tubes which produced readily from the metal particles. A good product of SWCNTs was prepared by using a DC arc discharge with a less quantity of a combination of Ni, Fe and powdered graphite at a low He gas pressure such as 100 torr with small quantity of He gas [69]. To enhance product, sulfur promotor was added, that produced the best yield. Various oxides such as Y₂O₃, La₂O₃ and CeO₂ were also used as catalysts to make SWCNTs [70].

Journet et al. [71] investigated that high products, approximately 80% of SWCNTs with average diameter of nearly 1.4 nm, can be synthesized for high-yield SWCNTs prepared around the world. Shi et al. [72] applied graphite rod having hole filled with powder of a combination of Y–Ni alloy and graphite or CaC₂–Ni and Ni as anode and gave large scale preparation of SWCNTs with arc conditions of 40–50 A DC supply and 500 or 700 torr He pressure. Liu et al. [73] applied a semi-continuous hydrogen arc procedure to increase SWCNT formation, gaining 2 g/h SWCNT bundles, having a 30% increment in SWCNT synthesis in bundles, with 1.72 nm diameter of SWCNTs. Ando et al. [74] formulated a DC arc plasma jet procedure, having maximum production of 1.24 g/min and nearly 50% purity.

Takizzawa et al. [75] found the environmental temperature effect in the arc vaporization procedure for preparing SWCNTs. Highly pure SWCNTs have been prepared by applying an arc discharge at regulated temperature with Fe–Ni–Mg multi-metal catalysts in He atmosphere [76]. The yield of SWCNTs is greatly influenced by temperature, and it rises as the temperature rises. SWCNTs bundles were produced with 45.3 g/h production rate and 7–20 nm width at 600 °C. Because the arc approach uses highly pure graphite electrodes, powdered metal and highly pure He and Ar gases, the cost of synthesizing SWCNTs and MWCNTs is considerable. No doubt, materials crystallinity is good, there is no control on the size of tubes like length and diameter. Polyhedral graphite particles that is by product, encapsulated the metal particles in the formation of SWCNTs, and unfortunately amorphous carbon is produced [77, 78].

4.2 Laser vaporization (LV)

The concepts and mechanics of this approach, which was originally proven by Smalley's group132, was much similar to arc discharge, except that the energy was provided by the laser striking the graphite pellet having catalysts like Ni or Co. Catalysts were vaporized by the laser, that may operate in pulsed or continuous mode. Neutral gas was flowed to direct the vaporized species to a water-cooled Co collector, where it condensed as presented in Fig. 10 [79].

Fig. 10

Schematics of LV reactor

This is an example of a horizontal laser-furnace setup. The sample was evaporated in Ar background gas passing slowly that is 5 mm/s in the tube made of quartz with 25 mm diameter which was inserted in an outer 50-mm tube at a pressure of 6.6 × 10⁴ Pa. The graphite target is made up of raw natural graphite that has been adorned having particles of Ni and Co at 1% concentration for each one. Ni and Co chlorides were reduced with NaBH₄ to form metal particles. The upper tube of quartz was placed in 42-cm split tube furnace that flows at 1000 °C.

The targeted was placed in the furnace center. Laser pulsed material was used to evaporate and was further carried out by the Ar stream and collected on a water-cooled collector made of brass situated at the exit point of the furnace. Graphite irradiation was done via double-pulse Nd:YAD Ekspla 303 D laser. The technique consists of two waves of the uniform wavelength possessing adjustable inter-pulse delay time. The laser was working at a wavelength of 355 or 532, and 1064 nm with 8 nm wave time period with repetition rate of 20 Hz and a fluence range of 1–6 J cm². At the end, the carbon soot with nanotubes was removed off from the collector surface and was analyzed. Both pulses had a similar fluence, with the second pulse being 30 ns later. As a result, fluence signifies the fluence of each of two pulses in the following section of this work; total influence is about two times larger. On the contrary, analysis of two pulses was done by adding fluence of both pulses. It was not rational as both pulses were placed in a distance of 30 ns so cannot be considered as a one long wave [80, 81].

Gated intensified charge-coupled device pictures of the plume emission in ambient Ar at a 7.3 × 10⁴ Pa pressure reveals that a well prepared 250-mm-thick plume forms after 30 ns of delay following graphite vaporization [80, 81] shown in Fig. 11. This plasma plume absorbs a lot of laser light and alters the energy exchange by laser pulse to the target. As a result, the plume is heated more than the target by the second pulse. For the generation of large stain on the target wave was defocused, producing a wave intensity distribution resembling the top-hat distribution. Well-known practical configurations were applied: one just having single upper tube, while other having a double inner and outer tube [82, 83].

Fig. 11

Reactor to synthesize CNT by LV

The benefit of this method includes a better yield of CNT and a reduction in the amount of metallic impurities in them because metallic catalysts with an ability to evaporate from the tubes end point when it is closed. About 70–90 wt% of CNT is produced by LV in the growth product. The reaction temperature can better regulate the CNT's diameter prepared by LV compared to arc discharge. The quantity of amorphous or graphite carbon that impure the SWCNT is decreased by LV [84,85,86]. However, because of requirement for highly pure graphite rods and high power beams, economically LV is not useful since it is becoming uneconomical than CVD technique. Additionally, CNT made by LV exhibits branching rather of being uniformly straight. LV does not produce as much CNT as the arc discharge method does [84]. Ultra-fast beam from a free electron laser (FEL) and the continuous laser powder technique are two innovative processes used for the mass manufacture of SWCNT. Although the procedure can be scaled up, the approach is relatively expensive because a laser and a lot of power are needed [84, 87,88,89,90].

4.3 Chemical vapor deposition (CVD)

Due to clarity, inexpensive, less temperature and low pressure, CVD is the most used method for creating CNTs [2]. Using combinations of CO and H₂ over Fe, carbon’s catalytic vapor phase deposition was originally explained in 1959 [91]. This method was used in 1993 to create CNT through heating acetylenes on Fe particles at temperature of 700 °C [92, 93]. In 1996, CVD was identified as a possible technique for synthesizing CNT on a wide scale [94, 95]. Laser grown and arc CNTs are very crystalline in nature than CVD-grown ones. Nevertheless, CVD and arc methods are excellent in terms of product and purity. In order to decompose a hydrocarbon, CVD demands flow of hydrocarbon vapors via a tube reactor containing a transition metal catalyst like Fe, Co, or Ni, at a high temperature ranging from 600–1200 °C mentioned in Fig. 12. System is cooled down to room temperature, and CNT develops on the catalyst and can be collected [85, 88, 95,96,97,98]. Based on where the catalyst is located, CVD method may be classified further such as supported [99] and floating catalyst-chemical vapor deposition (FC-CVD) [100] techniques. Supported CVD is a laborious process since SWCNT must first be produced after the catalyst has been manufactured and reduced. The synthesis procedure is also hindered through combination between support and catalyst particles [101]. Contrarily, the catalyst and SWCNT synthesis are completed in a gaseous atmosphere using the single-step scalable FC-CVD approach [102].

Fig. 12

Schematic diagram of CVD apparatus

Additionally, contamination-free high-purity SWCNT is produced using FC-CVD. The morphology and chirality of SWCNT have been found to be significantly influenced by the selection of catalyst particles. To grow narrow chirality of SWCNT, bimetallic catalysts have demonstrated more promising outcomes [103]. If a liquid hydrocarbon is being used, like benzene, and alcohol. It is warmed in a flask and purged via inert gas, which passes the vapors of hydrocarbon into the reaction zone. When solid hydrocarbon is stored in region of low temperature in reaction tube, it is used as the CNT precursor. Camphor, naphthalene, ferrocene and other compounds that are volatile directly convert from solid to vapor and undergo CVD as they passed over the catalyst placed in the high temperature region. The catalyst reactants can be applied in any form that includes solid, liquid or gas and that can be rightly placed inside or filled from reactor's outside, exactly similar to the CNT precursors. In CVD, metal catalyst is applied to attain thermal breakage of a vapors of hydrocarbon. As a result, this is named as heat CVD or catalytic CVD. The floating catalyst method includes pyrolyzing the vapors of catalyst at a required temperature that generates metal nanoparticles on the surface. As an alternative, substrates that have been coated with a catalyst can be put in the hot zone of the furnace to stimulate CNT development [104]. By using the CVD process, a high yield of nanotubes can frequently be obtained; nevertheless, these nanotubes have more structural flaws than those made using the arc discharge and LV methods [98, 105].

Regarding purity, yield, crystallinity, structure control and other factors, CVD has many benefits over arc discharge and LV methods. In the first place, CVD is adaptable in which it fabricates usage of a lot of hydrocarbons in any way like powder, films, entangled, upright or helical nanotubes, or a required form of nanotubes on preselected locations of designed substrate [97, 98, 106]. The metallic catalyst would be placed on a substrate, that enhance the growth of arranged nanotubes in the proper direction with rest to the substrate [2, 87, 98, 104]. Next, it allows good control of growth parameters as growth take place at a lower temperature that ranges from 550 to 1000 °C. The best approach for growing aligned CNT on required substrates for particular purposes is CVD [98]. Li et al. [107] used a sol–gel technique to build dense MWCNT arrays on mesoporous silicon that had been iron-impregnated. CNTs have been created on Co-coated quartz substrates by Terrones et al. [108].

According to Pan et al. [109], acetylene-derived aligned CNT with a length of more than 2 mm grew over mesoporous substrates. Extremely aligned nanotubes were created using several catalysts [110] or on various substrates [111, 112] depending on the desired use. By adjusting the CO₂ concentration in an FC-CVD system, Liao et al. [39] successfully controlled SWCNT shape and further explored the functionality of SWCNT-based transparent conducting films (TCFs). The concentration of CO₂ fed into the reactor is a crucial factor in producing various SWCNT geometries. It has been found that as the CO₂ concentration rises, the tube's length and diameter both increase, from 2.7–7.5 nm and 1.1–2.1 nm, respectively. Additionally, a suitable concentration of CO₂ such as 0.31% can increase output by more than double. In a N₂-H₂ atmosphere that is safe for the environment, preparation of high performance SWCNT-TCFs was explained by Hussain et al. [40] by applying low flow of ethylene. The distribution of chirality in the nanotubes is random, with neither zigzag nor armchair configurations being preferred.

Based on a study of the SWCNT using electron diffraction, 35–38% of them were metallic. Optimizing the feeding location resulted in the production of lengthy tubes with a 13-µm mean diameter. Furthermore, the bundling of SWCNT was successfully reduced, and up to 28% of the SWCNTs were individual. By maximizing the supply of ethanol gas, Okada et al. [8] showed that SWCNTs may be produced from Co catalysts below 300 °C, making world record of preparation of SWCNTs in CVD at low temperature. The product of SWCNTs through Co catalyst was in comparison with Rh catalysts at 270 °C. Furthermore, this method became economical as a result of low temperature requirement and less cost of Co catalyst. Acetonitrile reversibly affects SWCNT diameter during CVD development, as demonstrated by Eveleens et al. [7]. The radicals produced from acetonitrile actively removed hydrogen surface of hydrocarbon compounds. On the other hand, the SWCNTs nucleates. During nucleation, a primary byproduct is the formation of hydrogen isocyanide, and the overall surface carbon density falls. In a different publication, Eveleens et al. [113] demonstrated that chemical potential of carbon and hydrogen on the surface of catalyst and addition of chemical etchants, like NH₃ to the CVD feedstock, can both be used to modify SWCNT chirality and diameter.

Romanenko et al. [114] prepared SWCNT by adopting a vertical style of CVD reactor and disproportionation in the presence of catalyst of CO with ferrocene as catalyst precursor [114] created SWCNT transparent films on polyester substrates. The main drawback of the CVD process is the susceptibility for cracking and shrinkage in broad growth areas (several millimeters). The creation of nanotube structures of inferior quality is another drawback.

4.4 Plasma-enhanced chemical vapor deposition (PECVD)

In the current years, CNTs have been grown in large quantities using CVD. But most recently, the capacity of PECVD to create nanotubes with vertical alignment has been studied. It is an effective way for creating hybrid materials of CNTs by varying their surface characteristics [87]. Because of the reason that few methods cannot withstand the high temperatures of PECVD that initially appeared in microelectronics, for many procedures, PECVD offers a substitute at very low temperatures that is at room temperature and possesses since emerged as a crucial step in the production of integrated circuits [115]. Figure 13 depicts the PECVD's setup. In this method, at both ends of the reaction chamber high-frequency voltage was applied to create a glow discharge (reaction furnace). Substrate was used to cover grounded electrode. In order to synthesize homogenous film, reaction gas was provided from the plate across from it.

Fig. 13

Schematic diagram of PECVD

Using thermal CVD, catalyst of transition metals like Fe, Ni and Co is coated on the substrate like Si, SiO₂ and glass. Following the formation of nanoscale-sized fine metal particles, carbon with reaction gas, like acetylene, methane, ethane, ethene, CO, is provided while discharge [116], and by high-frequency power generated by glow discharge, CNTs are generated on the surface of metal particles of the substrate [98]. The catalyst has an impact over the formation rate, thickness of wall and shape of nanotubes. The excellent catalyst to form arranged MWCNT appears to be Ni [117]. The MWCNT has a 15-nm diameter, approximately. The largest yield of CNTs that is about 50% was produced by Chen et al. [116] at comparatively low temperatures that is below 330 °C [116].

The PECVD approach enables CNT growth at low temperatures, making it suitable for substrates that are sensitive to temperature. PECVD can also be employed in a number of various ways, including microwave-chemical vapor deposition (MW-PECVD), DC, diffusion and radio frequency-chemical vapor deposition (RF-PECVD). Ren et al. [118] proposed hot filament PECVD that effectively produced CNTs by using heat energy to create plasma [118]. The greater concentration of reactive radicals in RF-PECVD was the primary distinction between RF- and DC-PECVD. But in case of DC-PECVD, the synthesis of CNT tubes was aligned vertically [87, 98]. Teo et al. [119] produced CNTs by applying DC-PECVD on n+-doped Si (100) substrates with conductive TiN or SiO₂ diffusion barriers as thin as 8 nm. With the exception of the substrate that it is biased to initiate a DC glow discharge plasma at 600 V, the approach was substantially the same as a conventional technique. The CNT grew vertically because of the electric field in the plasma sheath. RF-PECVD was used by Boskovic et al. [120] to create carbon nanofibers (CNFs) at room temperature. Similar to this, Minea et al. [121] used PECVD to show the formation of CNFs at ambient temperature. Hofmann et al. [122, 123] showed the procedure of arranged CNFs at low temperature of 120 °C onto plastic substrate using DC-PECVD. The effective preparation of vertically arranged nanocomposites of CNT using Ni catalyst on TiN–SiO₂–Si substrate via RF-PECVD was demonstrated by Hussain et al. [124].

4.5 Catalytic pyrolysis of hydrocarbon (CPH)

It is believable that SWCNTs may be synthesized via catalytic pyrolysis of hydrocarbons as 3d transition metals seems very important to prepare SWCNTs along with the growth of vapor grown carbon fibers. According to Fonseca et al. [125], the breakdown of hydrocarbons over supported catalysts resulted in the formation of MWCNTs [125] and aligned bundles of nanotubes with 1 nm diameter [126]. However, it was discovered that a difficult to precisely control factor in shaping the generated nanotubes was the quantity and size of the spread catalytic particles on the support.

Fonseca et al. [125] have presented the better floating catalyst procedure to synthesize SWCNTSs, where at 1100–1200 °C benzene was pyrolyzed catalytically. This process produces SWCNTs with a high degree of purity, in significant quantities, and at comparatively moderate costs compared with the EAD and LV methods. It also allows for simpler control of the growth conditions and lower growth temperatures. In this method, a horizontal reactor was used to feed vapor phase catalyst in order to attain semi-continuous development of SWCNTs. This was done using an enhanced floating catalyst approach. Figure 14 displays the flowchart. Benzene was used as a source of carbon, H₂ as a carrier gas and ferrocene as a catalyst precursor. Thiophene, a sulfur-containing compound, was used to promote the growth of SWCNTs. It is well known that ferrocene decomposes over 400 °C and starts to vaporize at about 185 °C. Ferrocene was evaporated during preparation and introduced in the reaction tube along with mixture of benzene, thiophene and H₂ gas.

Fig. 14

Schematic diagram for the synthesis of CNTs by CPH

The H₂ first reduced the vaporized ferrocene to create atomic Fe, which then aggregated into Fe clusters suitable for the development of SWCNTs. Despite the fact that the preparation conditions were not optimal, even then researchers were able to produce SWCNTs in significant quantities by passing benzene directly through the oven at temperatures between 18 and 25 °C and through a solution of benzene containing 0.5 and 5 wt% thiophene, along vaporization speed of 1.5–4.5 × 10^–7 mol/s for ferrocene, and hydrogen gas flow rate of 70–90 and 150–225 ml/min, respectively. Synthesis periods ranged from 1 to 30 min, but they can be considerably time taking The reaction temperature was kept between 1100 and 1200 °C. The growing nanotubes were carried by the flowing gases out of the reaction area, where they were then gathered onto the graphitic plate, a tube wall or tube for protecting thermocouple. The advantages of the floating catalyst approach are fully utilized since the ferrocene flows constantly across the reaction zone, allowing for continuous SWCNTs growth and fairly significant SWCNT production [32].

4.6 Alcohol catalytic chemical vapor deposition (ACCVD)

Large-scale low-cost SWCNT synthesis via ACCVD is shown in Fig. 15. Over zeolite-supported Fe and Co catalytic metal particle catalysts, evaporated methanol and ethanol are used. At a minimal temperature of roughly 550 °C, CNT can be produced. High-purity SWCNT production appears to be hampered by the removal of carbon atoms with dangling bonds by hydroxyl radicals, which are produced when alcohol reacts with catalytic metal particles. The manufactured SWCNTs have a diameter of roughly 1 nm [127].

Fig. 15

Schematic diagram of ACCVD

4.7 Hydrothermal methods (HM)

Another effective way for creating various carbonaceous nanoarchitectures, including nanoonions, nanorods, nanowires, nanobelts, and MWCNTs, is the sonochemical/hydrothermal process proposed by Gogotsi et al. [128]. The starting components are simple to produce and stable at room temperature; the process is low temperature that is between 150 and 180 °C; and throughout the process no carrier gas was required, process, which gives it several advantages over other processes. A combination of Ni catalyst along water and polyethylene was warmed at 700–800 °C under 60–100 MPa pressure to prepare MWCNTs [128]. The production of MWCNTs with closed and open ends and wall thicknesses ranging from a few to more than 100 carbon layers. Hydrothermal nanotubes' thin walls and big inner core diameter, which ranges from 20 to 800 nm, are key characteristics. Gogotsi et al. [128] used solution of ethylene glycol in the presence of Ni catalyst at a temperature of 730–800 °C under pressure of 60–100 MPa to create graphitic CNTs [129, 130]. These CNTs include Ni inclusions in the tips and lengthy, wide internal channels, according to transmission electron microscopy (TEM). Hydrothermal nanotubes typically have a 7–25 nm thick wall and 50–150 nm outer diameter. There have also been constructed thin-walled carbon tubes with internal diameter of 10–1000 nm. The preparation fluid, a supercritical combination of CO, CO₂, H₂O H₂, and CH₄, enters in a tube as it grows.

A significant amount of CNT was produced by Manafi et al. [131] utilizing a sonochemical/hydrothermal technique. Starting ingredients included dichloromethane, and metallic Li in an aqueous solution of 5 mol/l NaOH. For 24 h, at 150–160 °C hydrothermal preparation was carried out. The prepared nanotubes had 60 nm diameter with 2–5 µm length. As a result, precursor's ultrasonic pre-treatment, evenly distributed catalyst nanoparticles were investigated by scanning electron microscopy (SEM). At below 800 °C and without using metal catalysts, multi-walled carbon nanocells and MWCNTs have been synthesized from amorphous carbon [130]. At 600 °C, graphitic carbon multi-walls connected to produce carbon nanocells. In macroscopy, the mass of interconnected hollow spherical cells looks like disorganized carbon. The internal chambers of the nanocells have sizes ranging from 10 to 80 nm, whereas the exterior diameters are less than 100 nm and range from 15 to 100 nm. The sample contains nanotubes with length in hundreds of nanometers and diameter in tens range [131].

4.8 CNTs’ growth mechanism

CNTs' development method has been debated since they were discovered. Several organizations have put out a number of often-contradictory options, mostly depend on reaction conditions and post-deposition yield's evaluations. As a result, no single CNT growth mechanism has been thoroughly studied to date. The significance of the observation made by Journet et al. [132] that nanotube samples prepared using various techniques shared similarities demonstrated that the growth of nanotubes is less dependent of specifics of the procedure reaction conditions and high on the thermodynamic conditions required by the procedure. The following steps make up the growth's progression, and one or more of them may be rate-controlling, depending on the circumstance and requiring careful experimental analysis:

• (a) Precursor(s) diffusing to the substrate through a thin boundary layer.

• (b) Species adhering to the surface.

• (c) Surface reactions that cause the growth of films.

• (d) Product species desorption.

• (e) The spread of species into the bulk stream through the boundary layer.

According to in situ TEM measurements, one of the postulated processes has two phases. On the catalyst surface, CNT precursor synthesis occurs first. The carbide particles of metastable state readily transform into a rod-shaped carbon. The following phase involves graphitizing its wall [88, 97, 104, 133]. The most well-accepted models of growth mechanisms are tip growth [134] and base growth [135] presented in Fig. 16.

Fig. 16

CNT's growth mechanism a tip growth [134]; and b root growth [135]

A tubule tip is left opened in the first model, known as tip growth, so carbon atoms may be added in its perimeter. In addition, metal catalyst helps in growth of reaction along it prevents the tip of tubule from closing. According to one study, molecules of fullerenes used as a growth of nuclei, also the CNTs size would be determined by the tubule's diameter. The metal and phase diagram of a carbon made the foundation for the latter model, known as base growth. SWCNT expand with the precipitation of carbon after molten metal that was dissolving the carbon has cooled and hardened. To comprehend the growth mechanism better, stronger experimental evidence is required. On the basis of particle catalyst size, either SWCNT or MWCNT was generated. Tip growth denotes a moderate substrate–catalyst contact, while root growth denotes a robust substrate–catalyst interaction [88, 97, 98, 136, 137].

4.9 Comparison among CNTs synthesis methods

Even though other techniques have been devised to create MWCNTs and SWCNTs, only the first three techniques EAD, LV and CVD have gained widespread acceptance. Between carbon electrodes an arc discharge with or without catalyst, produces a vapor in an arc discharge. A powerful laser beam is applied on the carbon volume having feedstock gas in the LV method (CH₄ or CO). The amount of pure nanotubes produced by LV is quite little, whereas the amount of impure material produced by an arc discharge is typically quite large. Because of its relatively easiest processing, control mechanism, energy efficiency, utilization of raw material, tendency to increase as large unit operation, high product, and purity, the CNT's formation by using CVD is one of the most reliable technology for potential industrial improvement. Any hydrocarbons can be used in this method, which produces vast quantities of highly pure CNTs at high temperatures that is 600–1000 °C in catalyst's presence such as Fe, Co, or Ni. Table 2 compares various methods based on the synthesis temperature, yield, CNT's graphitization, and relative CNTs quantity. The CVD technique may be scaled to create SWCNTs or MWCNTs alignment, diameter, and length by the kilogram and is relatively straightforward, reproducible, highly selective. Based on these characteristics, the CVD is the most promising synthesis technique for making high-quality CNTs at a reasonable price.

Table 2 Comparison among characteristics of different CNTs' synthesis techniques

5 Purification of CNTs

Amorphous carbon, metal fragments and multi-shell contaminants are frequently found in nanotubes. Purification of nanotubes involves a number of procedures [138]. Most commonly used methods are described below:

5.1 Sonication, filtration and annealing by surfactants

After acid refluxing, CNTs were purified but the tubes got entangled, holding the majority of the contaminants, including particles of catalyst and carbon that were challenging to filter out. So, sonication with the aid of surfactant was done. As settling of CNTs take large time, pointing out the achievement of suspension condition, sodium dodecyl benzene sulfonate helped sonication by ethanol or methanol because organic solvent was favored. In the next step, resulted product was filtered off by using an ultra-filtration equipment, and this was annealed at 1273 K in N₂ atmosphere for up to 4 h. The CNT structures can be optimized using annealing. The effectiveness of the surfactant-assisted sonication in disengaging CNTs and liberating the entrapped particulate contaminants was demonstrated. Additionally, a multi-step purification process can be used to clean nanotubes [139,140,141,142].

5.2 Air oxidation

The quality of carbon nanotubes is decreasing; the average purity is between 5 and 10%. Therefore, purification is required before attaching medications to CNTs. SWCNT purification was based on the dynamic oxidation of carbonaceous impurities by heating at gradually increasing temperature. Reduction in the number of metal catalyst particles like Ni, Y and amorphous carbon via air oxidation is beneficial. It is discovered that 673 K for 40 min is the ideal oxidation condition. Dynamic oxidation enables the elimination of carbonaceous contaminants effectively while minimizing nanotube loss [143, 144].

5.3 Acid refluxing

Acid refluxing is a useful technique for lowering the concentration of metal particles and amorphous carbon in a sample. Sulfuric acid (H₂SO₄), hydrochloric acid (HCl) and nitric acid (HNO₃) were placed among other acids used, but HCl was proved to the best refluxing acid [144, 145].

6 Characterization of CNTs

Characterization is used to identify and quantify the chemicals and materials and or to characterize their physical properties. The characterization of CNTs has been described in detail in this section.

6.1 Raman spectroscopy (RS)

RS [34] is the most powerful technique used for characterization of CNTs [146]. It is fast and non-destructive process. For RS, there is no need of sample preparation. For SWCNTs, this technique gives quantitative and qualitative data about diameter, crystallinity, purity and electronic structure, which allow to differentiate between metallic and semiconducting CNTs [147]. In addition, it supports studying and bundle CNTs [148, 149]. In Raman spectroscopy, the characteristic bands of nanotubes include:

• (a) Band related to the diameter of the tube is A_1g or “breathing mode”.

• (b) Residual ill-organized graphite is assigned by D-line.

• (c) Highly ordered CNT sidewalls are related to G-band.

The relationship between both D-bands and G-bands (I_D:I_G) gives quantitative data about damage sidewalls, and changes occur by functionalization [150]. Unfortunately, D-band contributes many CNTs which contain amorphous carbon that is absorbed on the sidewalls. However, in absence of amorphous carbon, the ratio between both bands I_D:I_G can be very helpful for determination of structural changes that is produced by functionalization such as it involves the attachment of organic moieties. For SWCNTs, radial breathing mode (RBM) is also called as vibrational mode that is used for the determination of SWCNTs diameter based on the position of peaks [151].

Now authors have taken an example from the scientific literature to represent how RS can be effectively used to analyze different modification strategies on covalent surface where chemical moieties attached to the CNTs sidewalls. Raman spectroscopy studied by Marcoux et al. [152], fluorination of SWCNTs done with F₂ and deflourination done with hydrazine by using this spectroscopy technique [152]. They used Raman D-band to monitor the intensities. Figure 17a shows that the intensity of D-band increases with the increase in disorder that occurred as a result of increasing fluorination, and Fig. 17b shows that the intensity of D-band decreases after deflourination observed by hydrazine as the sidewall was repaired.

Fig. 17

SWCNTs Raman D-bands as a function of fluorine concentration a (C (triangles) > B (crosses) > A (squares)); and b before fluorinated SWCNTs (C (triangles) and after (D (circles)) exposure to hydrazine. In both a and b, Raman D-band of pristine SWCNTs is displayed for comparison [150]

Raman spectroscopy can also be used for the interpretation of MWCNTs. The radial contraction–expansion of carbon atoms in the radial direction is comparable to the RBM, which is between 120 and 350 cm⁻¹. The presence of defects at structural locations causes the D-band value, which is around 1350 cm⁻¹ and represents materials that resemble graphite, to exist. The value of G-band is around 1600 cm⁻¹ demonstrate the tangential vibrations of carbon atoms. The long range order in the structure is represented by the overtone value of both bands, such as the D- and G′-band, which is found at 2600 cm⁻¹ [151,152,153,154]. Figure 18 displays the Raman spectra of MWCNTs and micro-emulsified multi-walled carbon nanotubes (μEMWCNTs) [155].

Fig. 18

The spectra of Raman sample μEMWCNTs and MWCNTs [155]

The well-separated D (1278 cm⁻¹), G (1602 cm⁻¹) and G′-band (2567 cm⁻¹) can be seen in the Raman spectra, which shows the characteristic of tubular CNTs. The vibration at 121 cm⁻¹ can be allocated to the RBM. After modification, the position of peaks remains unchanged.

By using the intensity ratios of D/G band, we can investigate the CNTs purity and structural quality. The intensity of G band is sensitive to carbon impurities. The intensity ratios of G'/D band should be more accurately used for the assessment of quality and purity of CNTs [156]. For MWCNTs, the calculated values of intensity ratios I_D/I_G and I_G/I_D are 1.32 and 1.31, respectively, while the corresponding values for μEMWCNTs are 1.01 and 1.28. As a result of micro-emulsion modification the value of decreasing intensity ratio indicates a slightly increased impurity or disorder in the structure of MWCNTs.

6.2 X-ray photoelectron spectroscopy (XPS)

The XPS [34] analysis was performed to detect the chemical structure of CNT (except for H₂), and structural modification occurred as a result of chemical functionalization [157]^. The XPS technique irradiates the CNTs with the X-rays and that is also used to calculate the binding energy of ejected photoelectrons.

Figure 19a shows the XPS analysis that was performed to detect the surface composition and functionalization of MWCNTs [158]. The elemental changes that occur through acidification, acyl chloride and amidation of MWCNTs are also shown in Fig. 19a. After functionalization, the value of peaks appears at 532.9, 399.8 and 200.1 eV shows an increase in oxygen element of MWCNTs–COOH, amination of MWCNTs shows the presence of N element, and the absence of Cl element [159, 160]. During the acidification process, the content of carboxylate was increased and carboxylic acid was introduced to the defects of MWCNTs shown in Fig. 19(b). During amidation process, the 1,12-diaminododecane multi-walled carbon nanotubes not only contained a secondary amide but it also has a secondary amine and then it was transferred to a polyurethane foam doped with MWCNTs and polydopamine given in Fig. 19a. AS shown in Fig. 19, the value of electron binding energies shown at 200.9 and 202.7 eV was electron generations of acyl chloride group with energy levels j = 3/2 and j = 1/2, respectively [158]. The presence of Cl elements on MWCNTs is in the form of acyl chloride groups.

Fig. 19

XPS spectra of functionalized MWCNTs a N-narrow scan spectra of amidated MWCNTs; b C-narrow scan after MWCNTs acidification; c O-narrow scans after MWCNTs acidification; and d Cl-narrow scan of MWCNTs-COCl [158].

6.3 Fourier-transform infrared spectroscopy (FT-IR)

Among many others, FT-IR is the most used vibrational spectroscopic technique which is helpful to determine number of chemical bonds and number of surface and functional groups in a material. Along with the position and the type of shape of the absorption peak possesses in the spectrum [161].

As an example of FT-IR, Fig. 20 shows the FT-IR ranges of β-cyclodextrin (β-CD), MWCNT–COOH, Fe₃O₄/MWCNT, β-CD@Fe₃O₄/MWCNT [162]. A peak with usual properties was observed at 3427 cm⁻¹ within the sample; this peak was because of the stretching vibration of the hydroxyl group [163]. The carbonyl group was in MWCNT-COOH, Fe₃O₄/MWCNT, β-CD@Fe₃O₄/MWCNT; its usual peak was observed at 1740 cm⁻¹ due to tensile vibration. The range of O–H bond in β-CD appears at 1630 cm⁻¹ and 1400 cm⁻¹; this is because of bending vibration, and such ranges can also be visualized in β-CD@Fe₃O₄/MWCNT. The ranges of C–O–C and C–O bonds appear because of stretching vibration near 1160 cm⁻¹ and 1040 cm⁻¹ [164]. Along with this the stretches of Fe₃O₄/MWCNT, β-CD@Fe₃O₄/MWCNT have the range of 584 cm⁻¹due to the vibration of ferro-oxygen bond [165]. The overall data show the attachment of β-CD at the top of magnetic CNTs through covalent bond.

Fig. 20

FT-IR spectra of β-CD, MWCNT-COOH, Fe₃O₄/MWCNT and β-CD@Fe₃O₄/MWCNT [162].

6.4 Thermogravimetric analysis (TGA)

The proportion and thermal stability of the vapor form, CNTs can be seen through TGA [166]. This investigation was made by changing the weight with respect to the temperature and warming up the analyte directly in air of inert gases like He or Ar [167,168,169]. Sometimes the analysis of N₂ or He is done with very little amount of atmospheric oxygen that is 1–5% to slow the process of oxidation [170]. During the TGA analysis of CNT sample in the atmosphere. The loss in weight of analyte is shown in Fig. 21 mainly by the oxidation of C to CO₂; the solid oxides are accountable for superposition of analyte after the oxidation of metallic catalyst [171,172,173]. TGA technique is used to find out the % yields of carbon deposits. Generally, the weight of sample that is oxidized occurs in temperature ranging from 200 to 680 °C [173]. The amount of carbon can be calculated through percentage of (m ₁− m₂)/m₁, where m₁ is weight of analyte before and m₂ is weight of analyte after oxidation [174].

Fig. 21

TGA analysis of the CNT sample [166, 173]

6.5 Scanning and transmission electron microscopy (SEM and TEM)

The morphology of MWCNTs was analyzed by using JEOL2100 high-resolution TEM and spreading of pore size and consistency of MWCNTs surface was analyzed by using quanta FE250 SEM.

The morphology of MWCNTs can be seen by utilizing TEM and SEM in Fig. 22a and c. The crystalline tabular shapes of nanotubes were seen. The inner diameter of MWCNTs is 5–10 nm, and outer diameter is 10–50 nm, and length 5–30 nm with 90% purity. The images of TEM and SEM in Fig. 22b and d show MWCNTs after it has adsorbed the dye. The images show the group of adsorbed dyes on MWCNTs surface [175].

Fig. 22

a and c TEM and SEM images of MWNTs [175]; b and d TEM and SEM images of MWNTs after adsorption of dyes [175]

7 Applications of CNTs

7.1 Drug delivery

In the past, efforts for targeted drug delivery have quickened to achieve this goal, inorganic and organic nanomaterials like gold nanoparticles (Au-NPs) [176, 177], dendrimers [178, 179], polymeric nanoparticle [180, 181], SiO₂ [182, 183], magnetic nanoparticles [184, 185], liposome [186, 187] and carbon-based nanocarriers [188,189,190,191] have played a significant role [192]. CNTs plays a vital role for drug delivery purpose because of excellent physical and chemical properties like ultra-light weight, high tensile strength, high chemical and thermal stability and special electronic structures. Functionalized nanotubes have many advantages over un-functionalized, un-functionalized having some disadvantages like high toxicity and low solubility. Highly water soluble and functionalized pristine nanotubes are lower in cytotoxicity compare to non-functionalized pristine nanotubes [193, 194]. To deal with that various compound having N, O, H atom introduce on CNTs to minimize the problem. Functional groups having previously given atom form hydrogen bonding with water to enhance the activity of CNT as drug delivery.

7.1.1 CNTs in brain targeted drug delivery

Alzheimer’s disease, a brain disease, is caused by deficiency of acetylcholine. Its nature is hydrophilic; this causes toughness for acetylcholine to cross the hydrophobic cross blood–brain barrier (BBB). Non-covalently acetylcholine loaded into SWCNTs. This strategy was new to improve pierce of drug into BBB [195, 196]. In vivo and in vitro result shows that SWCNTs could be used as an excellent and safe drug carrier for the remedy of any disorder of central nervous system (CNS) like Alzheimer disease [197]. The drug dosage can be controlled to make sure lysosomal except mitochondrial attack. Carboxylated SWCNT can be very effective against Parkinson’s disease; results also reveal that the viability of cells did not compromise by newly synthesized nanohybrid. Another approach to reach tumor cell is to target receptor sites. Less dense lipoprotein receptors (LPR) showed to be overexpressed in brain tumor BBB. Rahamathulla et al. [195] have worked via polyethylene glycolated (PEGylated) oxidized MWCNTs on dual targeting of doxorubicin against brain glioma. Angiopep-2 decorated on MWCNTs carrying doxorubicin was hypothetical analysis. In vivo and in vitro study shows that new dual targeted MWCNTs found to be better antiglioma producing than doxorubicin (DOX) in glioma-bearing mice. CD68 immunohistochemistry and hematology analysis shows that it is safe formulation and has no toxicity. For brain glioma PEGylated oxidized MWCNTs targeted on DOX functionalize with angiopep-2. By comparing DOX with anti-glioma, mice bearing C6 cytotoxicity and median survival period of glioma showed more anti-glioma effect doxorubicin-oxidative multi-walled carbon nanotubes-polyethylene glycol-angiopep-2. Histopathological analysis reveals that oxidative multi-walled carbon nanotubes-polyethylene glycol-angiopep-2 has lower cardiac toxicity and excellent biocompatibility than DOX [193]. Figure 23 shows the functioning of functionalized CNTs against tumor.

Fig. 23

CNT in brain targeted drug delivery [198]

MWCNTs is tri-stimuli-responsive wrapped with mesoporous SiO₂-grafted poly (N-isopropyl acrylamide-block-poly(2-(4-formylbenzoyloxy) ethyl methacrylate) loaded with DOX through disulfide linkage. This express the finest release practices in cancer conditions as compare to normal cell upon synchronized triggering by reductant stimuli, pH, temperature and was able to work as a well-organized gatekeeper to limit mesopore on–off and thus to regulate drug release, Furthermore, drug loaded on nanocomposite showed low toxicity than free DOX [199]. DOX put together with TiO₂–AuNPs-decorated MWCNT showed excellent performance against A549 and MCF-7 cancer cell lines as compare to free drug, so the drug delivering capacity of 90.66% for 10 h was observed [200].

7.1.2 Targeted drug delivery for other cancerous cells

The drug releasing system functions in three crucial steps: very first step is the administration of device, second step is release of active components from product and the last step is taking active components to the site of action carried out through biological membrane [201]. This study emphasis on anticancer drug also has functional groups, 5-minolevulinic acid (5-amino-4-oxopentanoic acid), that has functional groups –COOH, –NH₂ and –CO functionalization shown in Fig. 24 having ability to treat skin [202], breast [203], gastric [204], oral [205] cancers.

Fig. 24

Graph of CN/ALA1-4 [206]

Additional theoretical research, Hashemzadeh et al. [207] estimated the effect of many functional groups (such as (–COOH), (–CH₂NH₂) and PEG groups) on releasing efficacy paclitaxel (PTX) based SWCNT. They foresee that the PEG-SWCNT has best working in PTX surface assimilation than the other functional groups because of their greater interplay with drug this will achieve through π–π pile up and polar interactions. Different results appeared while loading DOX on functionalized CNT as shown in Fig. 25. Furthermore, molecular dynamics results show that due to polar interaction PEG-SWCNT system shows greater aqueous solubility.

Fig. 25

Functionalized CNT loaded with DOX [208]

7.2 CNTs for water treatment

7.2.1 Organic dye removal

Waste water containing different pollutants are harmful for the health of living organisms. Near world health organization somewhere in the world among new born one death has been reported due to bad quality of water. For those, different methods have been used; one of them is the use of CNTs adsorbents.

SWCNT, DWCNT and MWCNTs are very effective against dye removal if modified by the specific functional groups like amine, carbonyl, carboxyl or other functional groups and due to distinctive electrical, mechanical and thermal properties are widely used for removal of pollutants from water.

Role of chemically modified CNTs and pristine CNT has checked separately. Anyhow CNTs are chemically inert for good functionalization and reactions must be carried out in critical or inert environment resulting structural destruction to CNTs scaffolding [208, 209]. Chemical modification on CNTs increases the removal efficiency of dye. The influence of distinctive chemical modifications, like cycloaddition, fluorination, reaction of diazonium salts and oxidation process on CNTs, is discussed in Fig. 26.

Fig. 26

Adsorption on water contaminants using CNTs [210]

Other functional groups also introduce on CNTs such as pristine CNTs, pristine choline chloride CNTs and functionalized CNT sonicated with N,N-diethylethanolammonium chloride and ethylene glycol; CNTs screening model shows that choline chloride-functionalized CNT is very effective against methyl orange dye removal. These above-mentioned functionalized CNTs give different dye removal efficiency as shown in Fig. 27.

Fig. 27

Screening model of different adsorbents [211]

Another research describes role of poly aniline multi-walled carbon nanotubes (PANI-MWCNTs) and results compared with unmodified MWCNT and PANI adsorbents. PANI-MWCNTs shows effective results against an anionic azo dye methyl orange (MO) as given in Fig. 28.

Fig. 28

Time vs percentage removal a use of different adsorbent; b changing quantity of PANI-MWCNTs composites; c different temperatures; d by changing initial quantity of dye; and e % age removal of MO after 60 min on different pH [212]

It was found that PANI-MWCNTs showed excellent adsorption results than other functionalized CNTs.

7.2.2 Removal of heavy metals from water

Several heavy metals are rigorous pollutants, highly toxic and carcinogenic. Such as Pb that is well-known lethal poison; in the body, it can pile up from waste water and spoiled food. Although a small amount of Pb can create consequential destruction to the CNS, liver, kidney, basic cellular process reproductive system and human brain, mainly infant and children [213].

It includes removal of less amount of Cd(II) and Pd(II) through surface assimilation process using variety of MWCNT (oxMWCNT 3 h and oxMWCNT 6 h). Through Langmuir model, we get high adsorption capacities, i.e., 10.5 and 13.5 mg g⁻¹ for Cd(II) and 23.4 and 27.9 mg g⁻¹ for Pb(II) (so: oxMWCNT 6 h > oXMWCNT 3 h and Pb(II) and Cd(II)). Positively influence of increase in pH will cause strongly increase in adsorption of Pb(II) and Cd(II) and negative effect of pH will cause increase in the ionic strength of solution [214].

For both the adsorbents, adsorption reaction of Cd(II) and Pd(II) was extremely fast; to achieve state of equilibrium, it just takes 20 min (within 5 min 95.5% of the removal takes place). When compared to some other adsorbents, still preferred in terms of adsorption rate, but at first glance the adsorption capabilities for Cd(II) and Pb(II) of oxMWCNT 3 h and oxMWCNT 6 h may seem inferior [215,216,217].

Another lignin-grafted carbon nanotubes (L-CNTs) an eco-friendly new type of nanocomposite adsorbent for water remediation is reported. This adsorbent is not only ecofriendly and has excellent water dispersibility, but also exhibits good adsorption capacity because of larger surface area for both oil droplet and Pb ions. Through this method, we can remove diesel from water. L-CNTs have lot of oxygen atom on the surface by increasing pH lot of active sites form on the surface for maximum adsorption of Pb(ll).

Agents such as 1-isatin-3-thiosemicarbazone [218], poly-2-aminothiophenol [219], ethylenediamine [220], and dithizone [221] functionalized on MWCNTs for Pb pre-concentration have also been reported.

7.2.3 Miscellaneous applications

7.2.3.1 Genetic engineering

CNTs and carbon nanohorn (CNHs) are used in genetic engineering to control atoms and genes for bio-imaging establishment in genetic engineering, tissue engineering and proteomics. The uncoiled single-stranded DNA coils around SWCNT through linking its particular nucleotides and gives rise to swap its electrostatic property. That generates its powerful applications in therapeutics and in (polymerase chain reaction) diagnostics. Covering of CNTs with s-DNA was shown pattern-dependent and now could be used in DNA analysis [222].

7.2.3.2 Artificial implants

For artificial implants, human body might exhibit a “rejection” reaction and post-administration pain. Yo deal with it, tiny nanohorns and amino acid with attached nanotubes and with other proteins are used to protect against implant rejection. They can also be used as insert in the kind of artificial joints without host rejection reaction. Furthermore, due to excellent tensile strength, Ca filled in CNTs and ordered in the bone structure to use as bone alternate [223].

7.2.3.3 Preservative

Nanohorn and CNTs are considered as antioxidant. So, they can be used for the preservation of drugs that have possibility to oxidize. To protect skin components from oxidation, these antioxidants can be exercised in anti-aging and with ZnO used as sunscreen [224].

7.2.3.4 Diagnostic tool

Nanotubes filled with encapsulated protein or protein/enzyme, due to bioluminescent property, can be used as insert-able biosensor. Even, magnetic material, radioisotope enzymes filled in nanocapsule can be used as biosensor. Nanotubes with nanosized robots and motors can be used in studying cells and biological system [225].

7.2.3.5 As catalyst

Nanohorns have large surface area, and consequently, molecular catalyst could be fused with nanotube in excess amount and in the same time released at specific time in required rate. Hence, by the use of CNTs and CNHs amount of catalyst addition and abatement in the frequency can be achieved [225].

8 Limitations of CNTs

• (a) Less soluble in most solvents well suitable with biological in most solvents compatible with the biological environment (aqueous based).

• (b) The preparation of structurally and chemically reform-able batches of CNTs with similar characteristics.

• (c) Difficulty in maintaining excellent quality and minimal impurities.

9 Future perspective

The latest applications of CNTs in many disciplines, including water purification, water disinfection, catalysis, medication delivery, preservatives, genetic engineering, and artificial implant applications, are discussed in this review article. CNTs are thought to have been used for decades in a variety of applications. Nanomaterials, particularly CNTs, can be used as a replacement material for hereafter defendable energy sources. CNTs are used in a diversity of applications as a potent adsorbent, energy storage material, and in genetic engineering. It is also employed in a variety of biomedicine application, such as biosensors, drug administration, tumor treatment, and so on. CNTs are nowadays profit-making an approachable, and there is no way to avoid using them. These technologies, such as CNTs, will potentially pave the way for further successful and economical toxicant technologies in the near future. Moreover, the success of these CNTs is dependent on a greater knowledge of their potential health and environmental effects. In future, CNTs have potential applications in agricultural sector, electrode materials, air pollutants, hybrid materials, food science, etc.

10 Conclusion

As a member of the fullerenes structural family, CNTs are one of the most remarkable discoveries in the field of nanotechnology. They differ from amorphous carbon according to their kinds, chemical, physical characteristics, and electrochemical qualities. The material utilized in the future must have exceptional mechanical, thermal and chemical qualities that can withstand harsh environmental conditions while still being cost-effective to employ. CNTs can help with environmental cleanup through a variety of methods, including preservatives, water purification, disinfection, hybrid catalysts, medication delivery, and genetic engineering. CNTs must be oxidized or synthesized into various forms in order to perform their function in the treatment of various types of contaminants. Despite this, the price of production, toxicity, ecological dangers and public acceptance have stymied its efforts to safeguard the environment and reduce pollution. CNTs are expected to have safety guidelines and risk assessments to assess the safety of using CNTs in the near future, which will help to promote and enhance the use of CNTs. Such type of materials is in great demand, and CNTs are promising candidate for future applications [226].