**2. Preparation and characterization of carbon nanocomposites**

#### **2.1. Noncovalent functionalization**

In general, the carbon nanomaterials are chemically inert and highly hydrophobic in nature [10]. Therefore, the dispersion/anchoring of metal nanoparticles on the surface of carbon materials is very challenging task [11]. The noncovalent functionalization is one of the very common methods for the preparation of metal nanoparticles supported carbon nanocomposites. The noncovalent functionalization is mainly referred as a physical absorption, which involves weak interactions (п-interactions) [12]. In general, the noncovalent functionalization method causes no change on the basal plane structure and the electronic properties of carbon materials. However, prior to metal dispersion, in most of the cases, the surface of carbon materials has been modified to improve the hydrophobic nature and better "metal-carbon interactions." There are two main methods for the preparation of metal nanoparticles supported carbon nanocomposites by using the noncovalent functionalization: (1) wet synthesis and (2) dry synthesis.

#### *2.1.1. Wet synthesis*

The wet synthesis method has been widely adopted for the preparation of metal nanoparticles supported carbon nanocomposites. The wet synthesis is quite simple and low timeconsuming processing steps. Moreover, the uniform nucleation and the high possibility of the control of size and morphology of the metal nanoparticles are the key factors, which can be easily achieved through wet synthesis. So far, the researchers have developed numerous highly unique and efficient carbon nanocomposites. Particularly, in recent years, the numbers have been gradually increased due to the high demand of these useful materials in various fields such as catalysis, energy, sensors, biomedical, and textiles.

Noble metals such as silver (Ag), gold (Au), ruthenium (Ru), and palladium (Pd) nanoparticles have been widely employed as promoters and catalysts in various organic transformations. The carbon-based supports such as CNTs and graphene/graphene oxide (G/GO) are often used as support for the immobilization of Ag, Ru, Pd, and Au nanoparticles. It was found that the preparation method has huge influences on the structure and surface morphology of the carbon nanocomposites. Recently, Salam et al. [13] obtained a highly efficient silver-graphene nanocomposite (Ag-G) through a simple wet chemical route. They used silica-coated Ag nanoparticle solution as Ag sources. In a typical wet synthesis, the silica-coated Ag nanoparticle solution was added with aqueous GO solution under stirring for 15 min followed by the addition of hydrazine solution. The solution was heated at 80°C for 15–20 min, and the resultant precipitate (Ag-G) was filtered and dried. The Ag-G has been characterized by XRD, TEM, and Raman spectroscopy. The results confirmed the uniform dispersion of Ag nanoparticles with good attachment with GO. Well known that the NaBH<sup>4</sup> is a strong reducing agent and often used for the preparation of carbon nanocomposites. The Ag/graphene nanocomposites were prepared using NaBH4 as a reducing agent [14]. In a typical procedure, GO was mixed with CH3 COOAg solution and stirred at 100°C, followed by the addition of aqueous NaBH<sup>4</sup> solution and stirred at 100°C. Finally, the Ag/G nanocomposites were obtained by centrifugation, washing, and freeze-drying. The SEM and TEM results confirmed that the Ag nanoparticles (ranging from 5 to 25 nm) were orderly decorated and closely attached on the graphene nanosheets.

straight away into the exploration and modification of graphene and CNTs. To date, the potential applications of graphene and CNTs are diverse, which include catalyst carrier, energy storage, absorbents, biomedical, textiles, and sensors and support in many areas. As a catalyst carrier, the role of graphene and CNTs is just outstanding [5]. Particularly, in heterogeneous catalysts, the carbon materials often employed as a support to disperse the metal nanoparticles [3, 6]. In fact, the metal nanoparticles can easily agglomerate to form big nanoparticles due to their high surface energy, and it can be avoided by using support materials. Generally, the activity of the heterogeneous catalyst is mainly due to the structure of the catalyst, size of the metal nanoparticles, nature of the support, metal-support interaction, and fine dispersion of catalyst in reaction medium [7, 8]. To date, there are several metal nanoparticles supported graphene or CNT catalysts developed and reported for various organic transformations. The catalytic products are highly valuable in various fields including pharmaceutical, biomedical, agricultural, and material sciences [9]. In recent days, the interest on carbon nanocomposites in organic reaction has been increased significantly due to their unexpected positive outcomes. In this chapter, we discuss the main advances in the field over the last few years and explore the novel preparation methods of carbon nanocomposites (metal nanostructures/

carbon materials) and their applications in various catalytic organic transformations.

In general, the carbon nanomaterials are chemically inert and highly hydrophobic in nature [10]. Therefore, the dispersion/anchoring of metal nanoparticles on the surface of carbon materials is very challenging task [11]. The noncovalent functionalization is one of the very common methods for the preparation of metal nanoparticles supported carbon nanocomposites. The noncovalent functionalization is mainly referred as a physical absorption, which involves weak interactions (п-interactions) [12]. In general, the noncovalent functionalization method causes no change on the basal plane structure and the electronic properties of carbon materials. However, prior to metal dispersion, in most of the cases, the surface of carbon materials has been modified to improve the hydrophobic nature and better "metal-carbon interactions." There are two main methods for the preparation of metal nanoparticles supported carbon nanocomposites by using the noncovalent functionalization: (1) wet synthesis and (2) dry synthesis.

The wet synthesis method has been widely adopted for the preparation of metal nanoparticles supported carbon nanocomposites. The wet synthesis is quite simple and low timeconsuming processing steps. Moreover, the uniform nucleation and the high possibility of the control of size and morphology of the metal nanoparticles are the key factors, which can be easily achieved through wet synthesis. So far, the researchers have developed numerous highly unique and efficient carbon nanocomposites. Particularly, in recent years, the numbers have been gradually increased due to the high demand of these useful materials in various

fields such as catalysis, energy, sensors, biomedical, and textiles.

**2. Preparation and characterization of carbon nanocomposites**

**2.1. Noncovalent functionalization**

18 Nanocomposites - Recent Evolutions

*2.1.1. Wet synthesis*

Bozkurt [15] obtained Ag/graphene nanocomposite by the sonochemical method in situ reducing reaction of silver ions and GO with sodium citrate as a green reducing agent. At first, GO was well dispersed in distilled water, and an aqueous solution of AgNO<sup>3</sup> was gradually added to the above suspension under vigorous stirring condition. Finally, sodium citrate was added to the above mixture and sonicated for 1 h. The resultant black solid product (Ag/graphene nanocomposite) was centrifuged and dried in a vacuum. The authors have proposed mechanism for the formation of Ag nanoparticles on GO. Briefly, at first, silver nitrate precursor deposits on the surface of the GO nanosheets. Subsequently, the applied ultrasonic irradiation assists the deposited silver nitrate precursor to homogeneously disperse on the GO surface. The functional groups such as epoxy groups, hydroxyl groups (–OH), carbonyl groups (C = O), and carboxylic acid (–COOH) groups on the surface of GO would also act as the active sites for the metal cations. In general, the oxygen functional groups interact with the metal cations through electrostatic interactions. In the final step, the addition of sodium citrate reduces the GOAg+ to Ag nanoparticles on the GO surface. In comparison with other methods, this ultrasonic irradiation method has advantages such as simplicity and high efficiency. The characterization results confirmed the merit of the ultrasonic irradiation method. TEM results showed the most of Ag nanoparticles deposited on the GO, which are spherical in shape with good attachment over GO surface.

A one-pot strategy was designed for forming the Au-SiO2 -GO composite by Peng and coworkers [16]. To prepare Au-SiO2 -GO composite, tetraethyl orthosilicate (TEOS) and HAuCl<sup>4</sup> were dissolved in TX-100 aqueous solution dispersed with GO, followed by the addition of compressed carbon dioxide (CO2 ). Here the aim of utilizing compressed CO2 is to form carbonic acid by reacting CO2 with water. The carbonic acid can act as a catalyst for TEOS hydrolysis. Certainly, the compressed CO2 can also promote the deposition of nanoparticles on a solid support. The solution mixture was stirred at room temperature for 7 hours. Finally, the CO2 was released, and the product Au-SiO2 -GO composite was obtained. The TEM observation confirmed the uniform dispersion of Au nanoparticles on the GO with a narrow size distribution of 1.4–2.0 nm. The BET surface area and the total pore volume are 429 and 1.01 cm<sup>3</sup> g1 , respectively.

Recently, a facile and green method was developed to synthesize a new type of catalyst by coating Pd nanoparticles on reduced graphene oxide (rGO)-CNT nanocomposite [23]. At first, the three-dimensional microstructure of an rGO-CNT nanocomposite was obtained by hydrothermal treatment. The homogeneous mixture of GO and CNTs was prepared under sonication conditions, and the mixture was subsequently sealed in a 50-ml Teflon-lined autoclave and maintained at 180°C for 12 h. A black gel-like 3D cylinder of rGO-CNT composite was obtained. The resultant rGO-CNT composite was dispersed in aqueous solution, and

bath. Then, the reaction mixture was washed well with pure water to obtain Pd-rGO-CNT

Similarly, CuO nanoparticles were decorated on the surface of GO to obtain CuO/GO catalyst [24]. In a typical procedure, GO was dispersed in methanol and sonicated for 1 h. Then,

slowly evaporated. The resultant slurry was mixed well by a mortar and pestle, and obtained

for 3 h. **Figure 1(a)** shows a schematic illustration for the preparation of CuO/GNS. The CuO/ GNS was completed characterized by TEM, SEM-EDS, XPS, Raman, and XRD (**Figure 1**). The TEM images showed the strong attachment of CuO nanoparticles on the GNS with particle size distribution of 12–35 nm. Raman and XPS results indicated the strong attachment of CuO on GNS through covalent bonding (Cu▬C). The Cu 2p XPS spectrum of CuO/GNS showed shakeup satellite peaks of the Cu 2p3/2 at 942.4 eV and Cu 2p1/2 at 962.6 eV, which confirmed

The dry synthesis is found to be highly efficient and suitable method for the synthesis of carbon nanocomposite. The main advantages of this method are its simplicity, better adhesion, and advantages of least parameters to be controlled [25]. It was found that the dry synthesis is the method, which is highly suitable for the decoration of metal nanoparticles on carbon

**Figure 1.** (a) Schematic illustration of the procedure for the preparation of CuO/GNS, (b–d) TEM images, (e) SEM-EDS,

(f) XPS, (g) Raman, and (h) XRD patterns of CuO/GNS (from Gopiraman et al. [24]).

was added. The mixture was vigorously stirred for 30 min in an ice

Carbon Nanocomposites: Preparation and Its Application in Catalytic Organic Transformations

was calcinated under inert atmosphere at 350°C

http://dx.doi.org/10.5772/intechopen.81109

21

added to the above mixture was refluxed for 5 h (Step 1), and the MeOH was

subsequently, K<sup>2</sup>

nanocomposite.

the Cu(acac)2

*2.1.2. Dry synthesis*

PdCl4

homogeneous mixture of GO and Cu(acac)2

the presence of Cu(II) species (CuO).

Binary Au-Ag catalyst has been widely demonstrated to be one of the highly efficient catalysts for organic reactions. Babu et al. [17] prepared Au-Ag/SLG nanocatalyst from HAuCl4 × H2 O, Ag/ DNA, and single-layer graphene (SLG). Negatively charged Salmon milt DNA was employed as Ag sources. In a typical wet synthesis, mixture of HAuCl4 × H2 O and colloidal Ag/DNA was sonicated for 1 h at room temperature. Then, acid-treated single-layer graphene (*f*-SLG) was added to the above mixture and sonicated. Finally, the mixture was centrifuged to separate the Au-Ag/SLG and calcinated at 700°C for 3 h under inert atmosphere. Similarly, Pt-Ni bimetallic nanoparticles supported on CNTs nanocomposites (xPtNi/CNTs) with different compositions of Pt were synthesized [18]. Chemically modified CNTs were used for the decoration of nanocomposites. The solution phase reduction methods were adopted to prepare the nanocomposites in which ethylene glycol as a reducing agent in the polyol method or using poly (amidoamine) dendrimer as a platform and NaBH4 as a reducing agent were used to deposit the Pt and Ni nanoparticles on the surface of modified CNTs. Recently, Yuan and coworkers [19] found that the bimetallic Pd-Ag nanoparticles supported MWCNTs (Pd-Ag/MWCNTs) are highly active catalyst for the electro-oxidation of ethanol, n-propanol, and *iso*-propanol. The Pd-Ag/MWCNTs was prepared by using the NaBH4 reduction method in a mixed solvent of ethylene glycol and water. In a typical method, MWCNTs were first treated with conc. H<sup>2</sup> SO4 and conc. HNO3 to create oxygen functional groups on the surface of MWCNTs. Subsequently, the acid-treated MWCNTs were added to a mixture of PdCl2 , AgNO3 , and ethylene glycol/water, and then the mixture was stirred for 30 min. Finally, NaBH4 dissolved ethylene glycol was slowly added to the above mixture under vigorous stirring for 4 h. The Pd-Ag/MWCNT nanocomposite was characterized and applied for the electro-oxidation of ethanol, n-propanol, and *iso*-propanol.

Ru was found to be an excellent catalyst for organic reactions due to its wide chemical states (II to +VIII) and tunable properties [20]. Particularly, the Ru catalyst has shown an excellent activity in oxidation reactions because of its redox properties. Interconnected RuO2 nanoparticles anchored GO nanocatalyst (RuO2 /GO) with very good BET surface area (285 m<sup>2</sup> /g) were obtained by Yuan and coworkers [21]. Very simple method was adopted for the preparation of RuO2 /GO. Briefly, Ru(acac)<sup>3</sup> and GO were dispersed in methanol and sonicated for several hours followed by heating at 65°C to evaporate the methanol. The obtained slurry was grinded well with mortar and pestle until the homogeneous mixture was obtained, and then, it was calcinated in the muffle furnace under N<sup>2</sup> atmosphere at 600°C (heating rate of 5°C/min) for 3 h. The RuO2 /GO was completely characterized by various spectroscopic and microscopic techniques.

Wang et al. [22] obtained Pd nanoparticles immobilized GO nanocomposite by a very simple wet chemical method. PdCl2 and hydrazine hydrate were used as Pd sources and reducing agent, respectively. Initially, an aqueous suspension of GO was prepared, and then, PdCl<sup>2</sup> was added under the assistant of mild ultrasound. The hydrazine hydrate was then added to the above mixture and the solution heated at 100°C for 1 h. The black solid of Pd/GO was isolated by filtration and washed copiously with water and methanol. TEM image of the Pd/graphene composite showed that the Pd nanoparticles were supported on the surface of the GO sheets without any agglomeration of the Pd nanoparticles. The Pd nanoparticles are composed of spherical particles. The size of the Pd particles calculated to be 2–6 nm. The metal surface area of Pd/graphene measured to be 161 m2 /g.

Recently, a facile and green method was developed to synthesize a new type of catalyst by coating Pd nanoparticles on reduced graphene oxide (rGO)-CNT nanocomposite [23]. At first, the three-dimensional microstructure of an rGO-CNT nanocomposite was obtained by hydrothermal treatment. The homogeneous mixture of GO and CNTs was prepared under sonication conditions, and the mixture was subsequently sealed in a 50-ml Teflon-lined autoclave and maintained at 180°C for 12 h. A black gel-like 3D cylinder of rGO-CNT composite was obtained. The resultant rGO-CNT composite was dispersed in aqueous solution, and subsequently, K<sup>2</sup> PdCl4 was added. The mixture was vigorously stirred for 30 min in an ice bath. Then, the reaction mixture was washed well with pure water to obtain Pd-rGO-CNT nanocomposite.

Similarly, CuO nanoparticles were decorated on the surface of GO to obtain CuO/GO catalyst [24]. In a typical procedure, GO was dispersed in methanol and sonicated for 1 h. Then, the Cu(acac)2 added to the above mixture was refluxed for 5 h (Step 1), and the MeOH was slowly evaporated. The resultant slurry was mixed well by a mortar and pestle, and obtained homogeneous mixture of GO and Cu(acac)2 was calcinated under inert atmosphere at 350°C for 3 h. **Figure 1(a)** shows a schematic illustration for the preparation of CuO/GNS. The CuO/ GNS was completed characterized by TEM, SEM-EDS, XPS, Raman, and XRD (**Figure 1**). The TEM images showed the strong attachment of CuO nanoparticles on the GNS with particle size distribution of 12–35 nm. Raman and XPS results indicated the strong attachment of CuO on GNS through covalent bonding (Cu▬C). The Cu 2p XPS spectrum of CuO/GNS showed shakeup satellite peaks of the Cu 2p3/2 at 942.4 eV and Cu 2p1/2 at 962.6 eV, which confirmed the presence of Cu(II) species (CuO).

#### *2.1.2. Dry synthesis*

dispersion of Au nanoparticles on the GO with a narrow size distribution of 1.4–2.0 nm. The

Binary Au-Ag catalyst has been widely demonstrated to be one of the highly efficient catalysts

DNA, and single-layer graphene (SLG). Negatively charged Salmon milt DNA was employed

sonicated for 1 h at room temperature. Then, acid-treated single-layer graphene (*f*-SLG) was added to the above mixture and sonicated. Finally, the mixture was centrifuged to separate the Au-Ag/SLG and calcinated at 700°C for 3 h under inert atmosphere. Similarly, Pt-Ni bimetallic nanoparticles supported on CNTs nanocomposites (xPtNi/CNTs) with different compositions of Pt were synthesized [18]. Chemically modified CNTs were used for the decoration of nanocomposites. The solution phase reduction methods were adopted to prepare the nanocomposites in which ethylene glycol as a reducing agent in the polyol method or using poly (amidoamine)

nanoparticles on the surface of modified CNTs. Recently, Yuan and coworkers [19] found that the bimetallic Pd-Ag nanoparticles supported MWCNTs (Pd-Ag/MWCNTs) are highly active catalyst for the electro-oxidation of ethanol, n-propanol, and *iso*-propanol. The Pd-Ag/MWCNTs

create oxygen functional groups on the surface of MWCNTs. Subsequently, the acid-treated

the above mixture under vigorous stirring for 4 h. The Pd-Ag/MWCNT nanocomposite was characterized and applied for the electro-oxidation of ethanol, n-propanol, and *iso*-propanol. Ru was found to be an excellent catalyst for organic reactions due to its wide chemical states (II to +VIII) and tunable properties [20]. Particularly, the Ru catalyst has shown an excellent

obtained by Yuan and coworkers [21]. Very simple method was adopted for the preparation

hours followed by heating at 65°C to evaporate the methanol. The obtained slurry was grinded well with mortar and pestle until the homogeneous mixture was obtained, and then, it was calci-

Wang et al. [22] obtained Pd nanoparticles immobilized GO nanocomposite by a very simple

added under the assistant of mild ultrasound. The hydrazine hydrate was then added to the above mixture and the solution heated at 100°C for 1 h. The black solid of Pd/GO was isolated by filtration and washed copiously with water and methanol. TEM image of the Pd/graphene composite showed that the Pd nanoparticles were supported on the surface of the GO sheets without any agglomeration of the Pd nanoparticles. The Pd nanoparticles are composed of spherical particles. The size of the Pd particles calculated to be 2–6 nm. The metal surface area

agent, respectively. Initially, an aqueous suspension of GO was prepared, and then, PdCl<sup>2</sup>

/g.

/GO was completely characterized by various spectroscopic and microscopic techniques.

activity in oxidation reactions because of its redox properties. Interconnected RuO2

, AgNO3

for organic reactions. Babu et al. [17] prepared Au-Ag/SLG nanocatalyst from HAuCl4

, respectively.

× H2

as a reducing agent were used to deposit the Pt and Ni

reduction method in a mixed solvent of ethylene glycol and

/GO) with very good BET surface area (285 m<sup>2</sup>

and GO were dispersed in methanol and sonicated for several

and hydrazine hydrate were used as Pd sources and reducing

atmosphere at 600°C (heating rate of 5°C/min) for 3 h. The

SO4

dissolved ethylene glycol was slowly added to

, and ethylene glycol/water, and then the

and conc. HNO3

to

nanopar-

/g) were

was

× H2

O and colloidal Ag/DNA was

O, Ag/

BET surface area and the total pore volume are 429 and 1.01 cm<sup>3</sup> g1

water. In a typical method, MWCNTs were first treated with conc. H<sup>2</sup>

as Ag sources. In a typical wet synthesis, mixture of HAuCl4

dendrimer as a platform and NaBH4

20 Nanocomposites - Recent Evolutions

was prepared by using the NaBH4

MWCNTs were added to a mixture of PdCl2

ticles anchored GO nanocatalyst (RuO2

/GO. Briefly, Ru(acac)<sup>3</sup>

nated in the muffle furnace under N<sup>2</sup>

of Pd/graphene measured to be 161 m2

wet chemical method. PdCl2

of RuO2

RuO2

mixture was stirred for 30 min. Finally, NaBH4

The dry synthesis is found to be highly efficient and suitable method for the synthesis of carbon nanocomposite. The main advantages of this method are its simplicity, better adhesion, and advantages of least parameters to be controlled [25]. It was found that the dry synthesis is the method, which is highly suitable for the decoration of metal nanoparticles on carbon

**Figure 1.** (a) Schematic illustration of the procedure for the preparation of CuO/GNS, (b–d) TEM images, (e) SEM-EDS, (f) XPS, (g) Raman, and (h) XRD patterns of CuO/GNS (from Gopiraman et al. [24]).

nanomaterials when compared with wet synthesis method. In fact, several drawbacks of the wet synthesis method have been resolved by the dry synthesis method. Moreover, the carbon materials are highly hydrophobic, and it needs surface modification (with oxygen functional groups (C▬OH, C▬O▬C, C〓O, and COOH or amine groups) prior to the decoration of metal nanoparticles [26]. The oxygen functional groups could play a bridging role between the metal nanoparticles and the carbon materials. However, the creation of the oxygen functional groups is very difficult in case of activated carbon, carbon nanofibers, and carbon black. Interestingly, carbon materials without any surface functional groups could also be utilized successfully for the preparation of carbon nanocomposites. However, the large-scale production of the carbon nanocomposites through dry synthesis is limited.

similar procedures to obtain the corresponding carbon nanocomposites. The mechanochemical process is found to be rapid, versatile, and potentially scalable, making it useful for further exploitation in various applications. **Scheme 1** shows the general procedure for the prepara-

**Scheme 1.** Suzuki reaction of iodobenzene with phenylboronic acid catalyzed by Pd-graphene nanocomposites (from

Carbon Nanocomposites: Preparation and Its Application in Catalytic Organic Transformations

Later, Kim's group [29–33] developed various carbon nanocomposites by using the dry synthesis method also called "mix-and-heat" method. The prepared carbon nanocomposites were utilized as heterogeneous catalysts in various organic reactions. The metallic Ru nanoparticles were decorated on graphene nanosheets (GNSs) by "mix-and-heat" method [29]. Initially, the bi- and few-layered graphene nanosheets (GNSs) were obtained from graphene nanoplatelets (GNPs) by a solution-phase exfoliation method. The obtained GNSs were chemically treated

▬C▬O▬C▬, and ▬OH) on the surface of GNSs. The resultant *f*-GNSs were used for the

mixed well by a mortar and pestle under ambient conditions. Then, the homogeneous mixture

phology of the resultant nanocomposite (GNS-RuNPs) was found to be excellent. Ultrafine Ru nanoparticles were homogeneously dispersed on the surface of GNSs. Interestingly, the size of these attached Ru nanoparticles was found to be below 3.0 nm. Similarly, GNPs-RuO<sup>2</sup>

was prepared by a simple "mix-and-heat" method. **Figure 3** shows the schematic illustra-

NPs, TEM images, RuO<sup>2</sup>

to create oxygen functional groups (▬COOH, ▬C〓O,

was calcinated at 300°C for 3 h under an argon atmosphere. The mor-

was added into *f*-GNS and

http://dx.doi.org/10.5772/intechopen.81109

23

particle distribution, XPS,

NPs, (b and c) TEM images, (d) RuO<sup>2</sup>

NPs (from Gopiraman et al. [32]).

NPs

particle

tion of carbon nanocomposites by mechanochemical process.

and HNO3

decoration of Ru nanoparticles. In a typical preparation, Ru(acac)3

SO4

tion for the preparation of GNPs-RuO2

**Figure 3.** (a) Schematic illustration for the preparation of GNPs-RuO2

distribution, (e and f) XPS, (g) XRD patterns, and (h) Raman spectra of GNPs-RuO<sup>2</sup>

with concentrated H2

Li et al. [39]).

of *f*-GNS and Ru(acac)3

A rapid and solventless dry synthesis method was described for the preparation of carbon nanocomposites by Lin and coworkers [27]. This straightforward two-step process involves the dry mixing of a precursor metal salt with carbon materials (CNTs or GO) followed by heating in an inert atmosphere. They found that the dry synthesis procedure is scalable and applicable to various other carbon substrates (e.g., CNFs, expanded graphite, CNTs, activated carbon, and carbon black) and many metal salts (e.g., Ag, Au, Co, Ni, and Pd acetates). The Ag nanoparticles decorated CNTs have been reported as a model system, and the composites were prepared under various mixing techniques, metal loading levels, thermal treatment temperatures, and nanotube oxidative acid treatments. The TEM and SEM observation confirmed the uniform and strong attachment of Ag nanoparticles on the surface of the CNTs. However, in a wet synthesis, many factors such as solvent, concentration of metal precursor, reducing agent, deposition time, and temperature need to be controlled very carefully. Similarly, Ag nanoparticles of small average diameter (<5 nm) were decorated on the surface of MWCNTs by a simple mechanochemical process [28]. In a typical preparation, the silver acetate and MWCNTs were placed in a zirconia vial. Then, two zirconia balls were placed in a vial, and the set-up was secured in a SPEX CertiPrep 8000D high energy shaker mill and subjected to mechanical shaking for a desired period of time to yield the Ag/MWCNTs nanocomposite. The mechanochemical process requires no solvent, no additional reducing agents, or no applied electrical current. They demonstrated that the mechanochemical process was found to be readily applicable to not only CNTs, but also other carbon materials that are thermally conductive such as graphene, GO, and activated carbon (**Figure 2**). Moreover, different organic metal salts (e.g., Au and Pd acetates and Pt acetylacetonate) were also successfully applied in

**Figure 2.** General procedure for preparation of carbon nanocomposites by mechanochemical synthesis (from Lin et al. [28]).

Carbon Nanocomposites: Preparation and Its Application in Catalytic Organic Transformations http://dx.doi.org/10.5772/intechopen.81109 23

**Scheme 1.** Suzuki reaction of iodobenzene with phenylboronic acid catalyzed by Pd-graphene nanocomposites (from Li et al. [39]).

similar procedures to obtain the corresponding carbon nanocomposites. The mechanochemical process is found to be rapid, versatile, and potentially scalable, making it useful for further exploitation in various applications. **Scheme 1** shows the general procedure for the preparation of carbon nanocomposites by mechanochemical process.

Later, Kim's group [29–33] developed various carbon nanocomposites by using the dry synthesis method also called "mix-and-heat" method. The prepared carbon nanocomposites were utilized as heterogeneous catalysts in various organic reactions. The metallic Ru nanoparticles were decorated on graphene nanosheets (GNSs) by "mix-and-heat" method [29]. Initially, the bi- and few-layered graphene nanosheets (GNSs) were obtained from graphene nanoplatelets (GNPs) by a solution-phase exfoliation method. The obtained GNSs were chemically treated with concentrated H2 SO4 and HNO3 to create oxygen functional groups (▬COOH, ▬C〓O, ▬C▬O▬C▬, and ▬OH) on the surface of GNSs. The resultant *f*-GNSs were used for the decoration of Ru nanoparticles. In a typical preparation, Ru(acac)3 was added into *f*-GNS and mixed well by a mortar and pestle under ambient conditions. Then, the homogeneous mixture of *f*-GNS and Ru(acac)3 was calcinated at 300°C for 3 h under an argon atmosphere. The morphology of the resultant nanocomposite (GNS-RuNPs) was found to be excellent. Ultrafine Ru nanoparticles were homogeneously dispersed on the surface of GNSs. Interestingly, the size of these attached Ru nanoparticles was found to be below 3.0 nm. Similarly, GNPs-RuO<sup>2</sup> NPs was prepared by a simple "mix-and-heat" method. **Figure 3** shows the schematic illustration for the preparation of GNPs-RuO2 NPs, TEM images, RuO<sup>2</sup> particle distribution, XPS,

**Figure 3.** (a) Schematic illustration for the preparation of GNPs-RuO2 NPs, (b and c) TEM images, (d) RuO<sup>2</sup> particle distribution, (e and f) XPS, (g) XRD patterns, and (h) Raman spectra of GNPs-RuO<sup>2</sup> NPs (from Gopiraman et al. [32]).

**Figure 2.** General procedure for preparation of carbon nanocomposites by mechanochemical synthesis (from Lin et al.

nanomaterials when compared with wet synthesis method. In fact, several drawbacks of the wet synthesis method have been resolved by the dry synthesis method. Moreover, the carbon materials are highly hydrophobic, and it needs surface modification (with oxygen functional groups (C▬OH, C▬O▬C, C〓O, and COOH or amine groups) prior to the decoration of metal nanoparticles [26]. The oxygen functional groups could play a bridging role between the metal nanoparticles and the carbon materials. However, the creation of the oxygen functional groups is very difficult in case of activated carbon, carbon nanofibers, and carbon black. Interestingly, carbon materials without any surface functional groups could also be utilized successfully for the preparation of carbon nanocomposites. However, the large-scale produc-

A rapid and solventless dry synthesis method was described for the preparation of carbon nanocomposites by Lin and coworkers [27]. This straightforward two-step process involves the dry mixing of a precursor metal salt with carbon materials (CNTs or GO) followed by heating in an inert atmosphere. They found that the dry synthesis procedure is scalable and applicable to various other carbon substrates (e.g., CNFs, expanded graphite, CNTs, activated carbon, and carbon black) and many metal salts (e.g., Ag, Au, Co, Ni, and Pd acetates). The Ag nanoparticles decorated CNTs have been reported as a model system, and the composites were prepared under various mixing techniques, metal loading levels, thermal treatment temperatures, and nanotube oxidative acid treatments. The TEM and SEM observation confirmed the uniform and strong attachment of Ag nanoparticles on the surface of the CNTs. However, in a wet synthesis, many factors such as solvent, concentration of metal precursor, reducing agent, deposition time, and temperature need to be controlled very carefully. Similarly, Ag nanoparticles of small average diameter (<5 nm) were decorated on the surface of MWCNTs by a simple mechanochemical process [28]. In a typical preparation, the silver acetate and MWCNTs were placed in a zirconia vial. Then, two zirconia balls were placed in a vial, and the set-up was secured in a SPEX CertiPrep 8000D high energy shaker mill and subjected to mechanical shaking for a desired period of time to yield the Ag/MWCNTs nanocomposite. The mechanochemical process requires no solvent, no additional reducing agents, or no applied electrical current. They demonstrated that the mechanochemical process was found to be readily applicable to not only CNTs, but also other carbon materials that are thermally conductive such as graphene, GO, and activated carbon (**Figure 2**). Moreover, different organic metal salts (e.g., Au and Pd acetates and Pt acetylacetonate) were also successfully applied in

tion of the carbon nanocomposites through dry synthesis is limited.

22 Nanocomposites - Recent Evolutions

[28]).

XRD patterns, and Raman of GNPs-RuO<sup>2</sup> NPs. Later, CuO/MWCNTs [30], RuO2 /MWCNTs [31], and GNPs-RuO2 NPs [32] were synthesized by the dry synthesis method. It was demonstrated that the SWCNTs were also utilized to successfully decorate the RuO2 via dry synthesis method [33]. Astonishingly, the mean diameter of the RuO2 nanoparticles attached to SWCNTs was found to be about 0.9 nm. The BET surface area of RuO<sup>2</sup> /SWCNT was found to be 416 m2 g−<sup>1</sup> . Moreover, Raman and XPS results confirmed that the RuO<sup>2</sup> nanoparticles were strongly attached on the surface of SWCNTs.

The complete recovery and excellent reusability are the major advantages of using heterogeneous catalysts [40]. However, in most of the heterogeneous catalytic systems, the isolation of catalysts from the reaction mixture by conventional filtration methods is inefficient and time consuming. Therefore, magnetically recoverable carbon nanocomposites have gained much attention due to it easily and complete recovery of the catalyst from reaction mix-

excellent semi-heterogeneous catalyst for the Suzuki-Miyaura cross-coupling reaction in an environmentally friendly solvent (water/ethanol (1:1)) under ligand-free ambient conditions

also enough to achieve 97% of the product after 30 min of the reaction time. The small size and

10th cycle, which may be due to the easy and efficient magnetic separation of the catalyst and the high dispersion and stability of the catalyst in an aqueous solution. At 10th cycle, the

broad range of highly functionalized substrates in both Suzuki and Heck coupling reactions.

Similarly, various Pd nanoparticles supported graphene nanocomposites were prepared and used as an excellent nanocatalyst for the cross-coupling reaction. Pd nanoparticles supported

and it was used for Suzuki-Miyaura coupling reactions by Sun and coworkers [43]. They found

a complete conversion (100%) of the reactant and a high yield of 97% for biphenyl. Unlike other

Pd atoms, which could favor fine dispersion and stabilization of the ultrafine Pd nanoparticles

MWCNTs) as a support for the decoration of Pd nanoparticles. Both the supported catalysts (Pd/MWCNT)M and (Pd/SWCNT)M) were successfully employed in Suzuki cross-coupling reactions with a wide variety of functionalized substrates. Interestingly, they noticed that the MWCNTs supported Pd nanoparticles catalyst (Pd/MWCNT)M) showed slightly better yield

. Siamak et al. [44] used single- or multi-walled carbon nanotubes (SWCNTs and

high turnover number (TON) of 9250 and turnover frequency (TOF) of 111,000 h−<sup>1</sup>

toxic solvents, and inert atmosphere. Under the optimized conditions, the Pd/g-C3

O4

/s-G catalyst gave 84% of the product. Similarly, magnetically recoverable Pd/Fe3

chemical approach [41]. The prepared carbon nanocomposite Pd/Fe3

(**Scheme 2**). It was found that even a low amount of catalyst Pd/Fe3

O4

N4

**Scheme 2.** Suzuki-Miyaura cross-coupling reaction catalyzed by Pd/Fe3

homogeneous distribution of Pd nanoparticles on the Fe3

for the excellent catalytic activity. The activity of Pd/Fe3

nanoparticles supported graphene nanosheets (Pd/Fe3

Heck coupling reactions (**Figure 4**) [42]. The Pd/Fe3

With 7.6 wt% of Pd, the Pd/Fe3

reused for 10 times (**Figure 4**).

graphitic carbon nitride (Pd/g-C3

N4

N4

that the Pd/g-C3

supports, the g-C3

N4

on g-C3

good magnetic property of the Pd/Fe3

and Pd nanoparticles were decorated on sulfonated graphene (s-G) by a facile

Carbon Nanocomposites: Preparation and Its Application in Catalytic Organic Transformations

O4

O4

O4

was worked well at room temperature without any phase transfer agents,

with plenty of nitrogen-containing anchor sites was a suitable platform for

O4

/s-G catalyst (from Elazab et al. [41]).

/G worked well in Suzuki cross-coupling reaction with a

) was prepared through a one-step photodeposition strategy,

/G, it was easily recovered using a simple magnet and

O4

O4

http://dx.doi.org/10.5772/intechopen.81109

O4

/s-G was used as an

/s-G (0.15 mol% Pd) is

O4

25

. Due to the

N4

achieved

/s-G matrix are the main reason

/s-G did not deteriorate even after

/G) were prepared for Suzuki and

/G system gave excellent yields over a

ture. Fe3

Pd/Fe3 O4

O4
