*2.1. Nanodiamonds*

NDs have existed for billions of years in nature within meteorites, crude oil, interstellar dust protoplanetary nebulae, and different sediment layers of the Earth's crust. Nevertheless, the synthetization process of this valuable material only started in the second half of the nineteenth century through either exposing graphite to high pressure and high temperature conditions, or by the explosive detonation of bulk graphite [64–66]. The first is known as the high-pressure and high-temperature (HPHT) approach, whereas the second route is known as the detonation technique. In the literature, it was reported that the first study conducted on the preparation of NDs was performed by Bovenkerk et al. [67], in 1959, after which Danilenko [68] used the detonation technique as part of his synthesis approach. Furthermore, many approaches were developed afterward for fabricating ND, such as the microplasma-assisted formation [69], chemical vapor deposition (CVD) method [70], laser ablation [71], high energy ball milling of microdiamonds produced from high pressure and high temperature conditions [72], high energy ball milling of ultra-fine graphite powder [73,74], ultrasound cavitation [75], chlorination of carbides [76], carbon onions irradiated by electron [77], and irradiation of graphite by ion beam [78]. In addition to the previous synthesizing methods, El-Eskandarany has proposed a novel approach for producing superfine NDs from commercial graphite powders and SWCNTs under ambient temperature and atmospheric pressure conditions, using a high-energy ball mill technique [79]. It is important to note that, according to Ali et al. [66] and Mochalin et al. [80], the most common types of NDs seen today are the detonation NDs (DNDs) and the HPHT-NDs. From the aforementioned production routes, it can be concluded that the synthesized NDs can only be produced as independent solid particles, and therefore cannot be grown within liquids through chemical and/or physical approaches. Regardless of the method used, the production of NDs usually involves three major phases, which are 1—synthesis (methods mentioned earlier), 2—processing, and 3—modification. The processing stage, which follows the synthesis phase, enhances the as-produced NDs purity by removing the metals and metals oxides along with the non-diamond carbons that remain attached to the ND surface. Hence, a high level of sp<sup>3</sup> carbon bonded diamond nanoparticles can be obtained. This can be done by using oxidants such as nitric acid (HNO3), perchloric acid (HCLO4), or hydrochloric acid (HCL) [81]. Furthermore, the modification phase is essential so that the fabricated NDs can meet the requirements of their targeted application. Modification can be performed using either surface functionalization (widely used) or doping of the NDs particles. It is important to note that some researchers have recently started focusing on the doping technique due to the distinct optical properties gained from this NDs modification approach [82,83]. Figure 5 shows the three phases involved in the production of NDs [84].

**Figure 5.** Phases involved in the production process of nanodiamonds, where (**a**) illustrates the synthesis phase, (**b**) demonstrates the processing phase, and (**c**) shows the modification phase. Reproduced with permission from [84]. Elsevier, 2019. **Figure 5.** Phases involved in the production process of nanodiamonds, where (**a**) illustrates the synthesis phase, (**b**) demonstrates the processing phase, and (**c**) shows the modification phase. Reproduced with permission from [84]. Elsevier, 2019.

#### *2.2. Graphene 2.2. Graphene*

Graphene is a type of carbon material that originates from bulk graphite. It has the shape of a 2-dimensional (2D) (i.e., monolayer) sheet of one-atom thickness and lattice of hexagonally arranged sp<sup>2</sup> bonded carbon atoms [85]. The material itself was successfully synthesized for the first time in 2004 by Novoselov et al. [86], through mechanical exfoliating graphite with Scotch tape. Furthermore, the development in the field has resulted in categorizing graphene by the materials architecture structure, which ranges from zerodimensional (0D) graphene quantum dots, one-dimensional (1D) graphene fibers and nanoribbons, and 2D graphene nanomesh, rippled/wrinkled and multisheet [87]. Figure 6 shows an illustration of the different categories of graphene based on their dimensionality and bandgap opening. Regarding 2D graphene sheets, few suitable techniques are commonly employed for producing such material, which are mechanical exfoliation [86], sublimation of silicon carbide (SiC) [88], laser-induced graphene [89,90], covalent [91,92] or Graphene is a type of carbon material that originates from bulk graphite. It has the shape of a 2-dimensional (2D) (i.e., monolayer) sheet of one-atom thickness and lattice of hexagonally arranged sp<sup>2</sup> bonded carbon atoms [85]. The material itself was successfully synthesized for the first time in 2004 by Novoselov et al. [86], through mechanical exfoliating graphite with Scotch tape. Furthermore, the development in the field has resulted in categorizing graphene by the materials architecture structure, which ranges from zero-dimensional (0D) graphene quantum dots, one-dimensional (1D) graphene fibers and nanoribbons, and 2D graphene nanomesh, rippled/wrinkled and multisheet [87]. Figure 6 shows an illustration of the different categories of graphene based on their dimensionality and bandgap opening. Regarding 2D graphene sheets, few suitable techniques are commonly employed for producing such material, which are mechanical exfoliation [86], sublimation of silicon carbide (SiC) [88], laser-induced graphene [89,90], covalent [91,92] or

*Nanomaterials* **2021**, *11*, x FOR PEER REVIEW 9 of 79

non-covalent [93] exfoliation of graphite in liquids, and CVD growth [94]. These fabrication methods produce graphene in a solid form except for the liquid-phase exfoliation, which delivers the material as part of a suspension. non-covalent [93] exfoliation of graphite in liquids, and CVD growth [94]. These fabrication methods produce graphene in a solid form except for the liquid-phase exfoliation, which delivers the material as part of a suspension.

**Figure 6.** Different forms of graphene based on their dimensionality and bandgap opening, where graphene quantum dots, graphene nanoribbon, and both graphene nanomesh and graphene rippled/wrinkled have a structural dimension of 0D, 1D, and 2D, respectively. **Figure 6.** Different forms of graphene based on their dimensionality and bandgap opening, where graphene quantum dots, graphene nanoribbon, and both graphene nanomesh and graphene rippled/wrinkled have a structural dimension of 0D, 1D, and 2D, respectively.

The mechanical exfoliation method was the first approach for obtaining graphene. In this method, the small mesas of highly oriented pyrolytic graphite are repeatedly peeled out with a Scotch tape, and hence the attached thin films on the tape are of monolayer graphene. This production route is highly reliable and allowed the preparation of highquality graphene sheets of up to 100 µm in thickness [86]. Other less common types of mechanical exfoliation are also available, such as ball milling of graphite nanoparticles [95] and hammering graphite [96]. Furthermore, the high temperature sublimation of SiC, which was developed initially for the electronics industry, relies on the thermal decomposition of a SiC substrate via either an electron beam or resistive heating to epitaxial graphene under ultrahigh vacuum condition. This results in the desorption of the silicon (Si) on the wafer surface, and therefore causing the surface atoms to arrange into forming hexagonal lattice. Moreover, fabricating graphene through laser-inducement is performed under ambient atmosphere by subjecting carbon dioxide (CO2) pulsed laser to a substrate containing carbon-based materials. This approach combines 3-dimensional (3D) graphene fabrication and patterning into a single step without having to use wet chemical steps. In addition, exfoliation of graphite in liquids or liquid-phase exfoliation depends on the employment of external peeling force, such as an ultrasonic horn sonicator, to separate the graphene sheets from the immersed bulk graphite in a solvent of suitable surface tension. The solvent used in the process is usually a non-aqueous solution, such as N-methyl-2 pyrrolidone (NMP), but aqueous solutions can also be employed if surfactant was added. It is important to note that the yield of the liquid-phase exfoliation process is relatively low, and thus centrifugation is used to gain a significant fraction of monolayer and fewlayer graphene flakes in the final dispersion [97]. On the other hand, the CVD production The mechanical exfoliation method was the first approach for obtaining graphene. In this method, the small mesas of highly oriented pyrolytic graphite are repeatedly peeled out with a Scotch tape, and hence the attached thin films on the tape are of monolayer graphene. This production route is highly reliable and allowed the preparation of highquality graphene sheets of up to 100 µm in thickness [86]. Other less common types of mechanical exfoliation are also available, such as ball milling of graphite nanoparticles [95] and hammering graphite [96]. Furthermore, the high temperature sublimation of SiC, which was developed initially for the electronics industry, relies on the thermal decomposition of a SiC substrate via either an electron beam or resistive heating to epitaxial graphene under ultrahigh vacuum condition. This results in the desorption of the silicon (Si) on the wafer surface, and therefore causing the surface atoms to arrange into forming hexagonal lattice. Moreover, fabricating graphene through laser-inducement is performed under ambient atmosphere by subjecting carbon dioxide (CO2) pulsed laser to a substrate containing carbon-based materials. This approach combines 3-dimensional (3D) graphene fabrication and patterning into a single step without having to use wet chemical steps. In addition, exfoliation of graphite in liquids or liquid-phase exfoliation depends on the employment of external peeling force, such as an ultrasonic horn sonicator, to separate the graphene sheets from the immersed bulk graphite in a solvent of suitable surface tension. The solvent used in the process is usually a non-aqueous solution, such as N-methyl-2-pyrrolidone (NMP), but aqueous solutions can also be employed if surfactant was added. It is important to note that the yield of the liquid-phase exfoliation process is relatively low, and thus centrifugation is used to gain a significant fraction of monolayer and few-layer graphene flakes in the final dispersion [97]. On the other hand, the CVD production route uses

hydrocarbon gases to grow graphene on a targeted substrate by carbon diffusion and segregation of high carbon solubility metallic substrates, such as nickel (Ni), or by surface adsorption of low carbon solubility metals (e.g., Cu) [98,99]. From all of the previous methods, CVD has shown to be the most successful, promising, and feasible approach in the field for producing monolayer graphene of high quality and large area [94]. For deeper insight into the various graphene synthesis methods, the reader is referred to the published work of Rao et al. [100]. sion and segregation of high carbon solubility metallic substrates, such as nickel (Ni), or by surface adsorption of low carbon solubility metals (e.g., Cu) [98,99]. From all of the previous methods, CVD has shown to be the most successful, promising, and feasible approach in the field for producing monolayer graphene of high quality and large area [94]. For deeper insight into the various graphene synthesis methods, the reader is referred to the published work of Rao et al. [100].

route uses hydrocarbon gases to grow graphene on a targeted substrate by carbon diffu-

*Nanomaterials* **2021**, *11*, x FOR PEER REVIEW 10 of 79

### *2.3. Carbon Nanotubes 2.3. Carbon Nanotubes* Although carbon is known as a ubiquitous material in nature, CNTs are not, where

Although carbon is known as a ubiquitous material in nature, CNTs are not, where this allotrope material is a human-made seamless cylindrical form of carbon. It is believed that the oldest CNTs existed on damascene swords [101]. Still, their first proof of presence was in 1952 through the transmission electron microscopy (TEM) images published by Radushkevich and Lukyanovich [102], after which Boehm [103] and Oberlin et al. [104] obtained similar images along with describing the currently widely accepted CNTs growth model. Conceptually, CNTs are graphene sheets rolled into cylindrical tubes, of less than 1 nm in diameter, with a half fullerenes caped end. Based on the number of consistent tubes (i.e., rolled-up graphene sheets), CNTs can be classified as SWCNTs, DWCNTs, and MWCNTs. As the terms suggest, the SWCNTs consist of only one tube, whereas DWCNTs and MWCNTs comprise two and three (or more) tubes, respectively [105,106]. Figure 7 shows the mechanism in which CNTs are formed and their three different types. It is important to note that some researchers distinguished between the three tubes form of CNTs and those of a higher number of tubes, where they have categorized the first as the triple-walled carbon nanotubes (TWCNTs) and the second as MWCNTs [107,108]. this allotrope material is a human-made seamless cylindrical form of carbon. It is believed that the oldest CNTs existed on damascene swords [101]. Still, their first proof of presence was in 1952 through the transmission electron microscopy (TEM) images published by Radushkevich and Lukyanovich [102], after which Boehm [103] and Oberlin et al. [104] obtained similar images along with describing the currently widely accepted CNTs growth model. Conceptually, CNTs are graphene sheets rolled into cylindrical tubes, of less than 1 nm in diameter, with a half fullerenes caped end. Based on the number of consistent tubes (i.e., rolled-up graphene sheets), CNTs can be classified as SWCNTs, DWCNTs, and MWCNTs. As the terms suggest, the SWCNTs consist of only one tube, whereas DWCNTs and MWCNTs comprise two and three (or more) tubes, respectively [105,106]. Figure 7 shows the mechanism in which CNTs are formed and their three different types. It is important to note that some researchers distinguished between the three tubes form of CNTs and those of a higher number of tubes, where they have categorized the first as the triple-walled carbon nanotubes (TWCNTs) and the second as MWCNTs [107,108].

**Figure 7.** Carbon nanotubes formation and classifications, where (**a**) illustrates the rolling mechanism of graphene sheet into SWCNT and (**b**) demonstrates the three different categories of CNTs, namely SWCNT, DWCNT, and MWCNT. **Figure 7.** Carbon nanotubes formation and classifications, where (**a**) illustrates the rolling mechanism of graphene sheet into SWCNT and (**b**) demonstrates the three different categories of CNTs, namely SWCNT, DWCNT, and MWCNT.

There are three main synthesis methods for producing CNTs, which are the arc discharge, laser ablation, and CVD [109–111]. Other approaches, such as diffusion and

premised flame method, can be used for CNTs fabrication but are less frequently utilized [112]. All three primary production methods depend on the carbon feedstock, either as a solid phased carbon source (arc discharge and laser ablation) or carbonaceous gases (CVD method). An example of the gases employed in the CVD process include carbon monoxide (CO), ethanol, and acetylene. Moreover, the final product is always delivered in a dried form; thus, CNTs cannot be grown within liquids as dispersions. In the arc discharge process, doped graphite rods or two catalysts loaded are vaporized at 4000–5000 K, within a closed chamber, by an electric arc placed between them, after which the resulting deposit is of CNTs. Like the arc discharge method, the laser ablation route relies on the evaporation of a carbon feedstock, usually a graphite rod with a metallic based catalyst, to obtain the CNTs. The difference between this approach and the previous one is that the laser ablation uses high energy laser irradiation to heat the carbon source, and thus causing the phase transformation (i.e., from the solid to gaseous phase). Additionally, the final product gets accumulated in a cold trap located within the chamber. Therefore, this technique is much more efficient than the arc discharge process in terms of the losses in the as-produced CNTs. On the other hand, the CVD, which was mentioned earlier in Section 2.2, decomposes carbonaceous gases on catalytic nanoparticles to produce the CNTs. The catalytic nanoparticles used for this purpose are either grown while conducting the process or are initially fabricated through a separate procedure. Furthermore, the advantage associated with this production technique is the high level of control over the synthesis process parameters such as carbon supply rate, growth temperature, catalyst particles size, and type of substrate used for the CNTs growth.
