*2.2.2. Chemical reduction approaches*

They employed a two-step procedure to synthesize MnCo2O4–graphene oxide nanohybrids. In the first nucleation step, Co(OAc)2 and Mn(OAc)2 were mixed at 80°C with mildly oxidized graphene oxide in an ethanol/water NH4OH solution. In the second step, hydrothermal treatment was done at 150°C to achieve the nitrogen-doped graphene. And the final material showed an excellent electrocatalytic activity for oxygen reduction reaction (ORR). Dai et al. [19] also used cobalt acetate and GO as a precursor for the hydrothermal synthesis of Co3O4/ graphene hybrid bifunctional catalyst for ORR and water oxidation or oxygen evolution

382 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**Figure 1.** Schematic illustration of the hydrothermal synthesis of 3D graphene/MnO2 (a), and schematic illustration of electrons transfer on the 3D hybrid material (b). (Reproduced with permission from ref. 44 Copyright 2014 American

Wang et al. [20] reported a hydrothermal approach for the synthesis of CoO/rGO nanocom‐ posite using GO, Co (Ac)2.4H2O, Co(NH2)2 as a precursor (190°C, 2 h). Mullen et al. [21] successfully synthesized 3D nitrogen-doped graphene aerogel-supported Fe3O4 nanoparticles by hydrothermal approach for the efficient electrocatalysis of ORR. Graphene oxide, iron acetate, and polypyrrole were hydrothermally assembled at 180°C for 12 h to form a 3D graphene-based hydrogel. The hydrogel was further dehydrated and annealed at 600°C for 3

Hydrothermal methods for the synthesis of graphene-based hybrid materials can be carried out at different temperature ranges, up to 190°C [22]. Figure 1 shows an example for the preparation of 3D graphene–MnO2 composites. There are several advantages of using hydrothermal process. *Firstly*, the strong electrolyte water possesses a high diffusion coeffi‐ cient and dielectric constant under hydrothermal reaction conditions, which helps to remove the oxygen-containing groups via dehydration and also accelerates heterolytic bond cleavage.

reaction (OER).

Chemical Society)

h under nitrogen atmosphere.

Chemical reduction is another method among the most common approaches for the prepara‐ tion of different graphene-based nanohybrid electrocatalysts. Different research groups have used different types of reducing agents for the specific synthesis purposes. For example, Qiao et al. [23] reported a simple chemical reduction method for the preparation of CuO/N-rGO nanohybrid using NaOH in the mixture of CuCl2 and N-doped reduced graphene oxide. Guo et al. [24] used diethylene glycol (DEG) as a reducing agent for the preparation of Cu2O/RGO composite material. Yang et al. [25] prepared PDDA-G/Fe3O4 nanohybrid material using ammonia solution as a reducing agent. Khezrian et al. [26] also used ammonia solution as a reducing agent for the synthesis of Fe3O4 magnetic nanoparticles/RGO hybrid nanosheets. Wang et al. [27] used citric acid for the preparation of MnO2/GO. Ruoff et al. [28] used hydrazine as a reducing agent for the synthesis of RGO/tin oxide (TiO2) nanocomposite.

The major advantage of this approach is that one can tune the degree of reduction and other properties by using specific reducing agents. And also for most of cases the reactions are very energy-efficient due to low temperatures and slow time. An obvious drawback is the need of purifying the final product from different reducing agents, which is in some cases quite challenging.

## *2.2.3. MW-assisted synthetic approaches*

Microwave (MW)-assisted synthesis is a simple and popular technique for the fast production of nanomaterials with small particle dimensions, uniform particle size distribution, and high purity. It is a uniform heating procedure compared to the other conventional heating systems. Moreover, microwave can facilitate the nucleation of nanoparticles and shorten the synthesis time. There are some excellent examples of using this approach. For instance, Peng et al. [29] synthesized CuO/SG hybrid materials by MW-assisted method using graphene oxide and cupric acetate as a precursor (Fig. 2). Ruoff et al. [30] also reported MW-assisted method for the synthesis of RGO/Fe2O3. Ferric chloride and graphene oxide were used as precursors for this method. And the as-synthesized nanocomposite was used as a high-performance anode material for lithium ion batteries. Wang et al. [31] synthesized highly dispersed titanium dioxide (TiO2) nanoclusters on RGO in a toluene–water system by MW-irradiation-assisted method. The main advantages for the MW-assisted synthetic approaches include rapid reaction time, possibility for scale-up production and impurity-free final nanohybrid product. The relatively high cost needed for experimental setups could be a major drawback.

**Figure 2.** Schematic illustration of the MW-assisted synthesis of CuO nanoparticle supported on S-doped graphene/SG and CuO/SG on glassy carbon electrode for glucose sensing. (Reproduced with permission from ref. 29 Copyright Elsevier 2015)

## *2.2.4. Electrochemical synthetic approaches*

The electrochemical synthetic method is an efficient technique for transforming electronic states by regulating the external power source to change the Fermi energy level of the electrode material surface [32]. Kong et al. [33] successfully prepared CuO nano needle/graphene/carbon nanofiber modified electrodes by electrochemical synthetic approaches for nonenzymatic glucose sensing in saliva. Duan et al. [34] reported a novel electrochemical approach to deposit MnO2 nanowires on graphene for the sensor applications. The MnO2 nanowires were anodi‐ cally electrodeposited onto a graphene paper electrode using a CV technique in the potential range from 1.4 V to 1.5 V with a scan rate of 250 mV s−1 (Fig. 3) by cyclic voltametry.

The electrochemical synthetic approach is a fast, controllable, and green technique. By this procedure, one can achieve impurity-free nanohybrid material using little power consump‐ tion. However, this approach normally yields solid nanocomposite products, which is difficult for further processing. And also it would be difficult for large-scale production.

## *2.2.5. Other synthetic approaches*

There are several other techniques that have also been explored for the synthesis of different nanocomposites, such as electrospinning, template-based synthesis, light- or radiationinduced methods, etc. However, although among them each technique has its specific advantages for specific systems; to date, few have been widely used.

**Figure 3.** Schematic illustration of electrodeposition-assisted synthesis of MnO2 nanoparticle supported on reduced graphene oxide paper electrode for biosensor applications. (Reproduced with permission from ref. 34 Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

**Figure 2.** Schematic illustration of the MW-assisted synthesis of CuO nanoparticle supported on S-doped graphene/SG and CuO/SG on glassy carbon electrode for glucose sensing. (Reproduced with permission from ref. 29 Copyright

The electrochemical synthetic method is an efficient technique for transforming electronic states by regulating the external power source to change the Fermi energy level of the electrode material surface [32]. Kong et al. [33] successfully prepared CuO nano needle/graphene/carbon nanofiber modified electrodes by electrochemical synthetic approaches for nonenzymatic glucose sensing in saliva. Duan et al. [34] reported a novel electrochemical approach to deposit MnO2 nanowires on graphene for the sensor applications. The MnO2 nanowires were anodi‐ cally electrodeposited onto a graphene paper electrode using a CV technique in the potential

The electrochemical synthetic approach is a fast, controllable, and green technique. By this procedure, one can achieve impurity-free nanohybrid material using little power consump‐ tion. However, this approach normally yields solid nanocomposite products, which is difficult

There are several other techniques that have also been explored for the synthesis of different nanocomposites, such as electrospinning, template-based synthesis, light- or radiationinduced methods, etc. However, although among them each technique has its specific

range from 1.4 V to 1.5 V with a scan rate of 250 mV s−1 (Fig. 3) by cyclic voltametry.

for further processing. And also it would be difficult for large-scale production.

advantages for specific systems; to date, few have been widely used.

Elsevier 2015)

*2.2.4. Electrochemical synthetic approaches*

384 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

*2.2.5. Other synthetic approaches*

In summary, a number of methods have been developed for the synthesis of metal oxide/ graphene composites, among which hydrothermal procedures have been most extensively used in particular. However, each method has its advantages as well as disadvantages. Table 1 summarizes the preparation methods and their major applications of most studied metal oxide/graphene nanocomposites.

 Summary of the main synthetic methods for preparation of metal oxide–graphene nanocomposites and their major applications.