3.2 Hydrothermal method

angle of 120° and a length of a = 0.14 nm, resulting in a hexagonal honeycomb lattice, which can be simplified to a trigonal unit cell with two atoms per unit, as

Graphene can have other allotropic forms at the same time, since it is the basic building structure of the other forms such as graphite (3D), nanotubes (1D), and fullerenes (0D). In all its structures, it has very good properties of thermal, electrical, and other properties such as mechanical, optical, and magnetic properties. The movement of electrons through a sheet of graphene is governed mainly by the relativistic law of Dirac. For a perfect sheet of graphene, it is estimated that its

affected by defects that generate dispersion centers generated mainly by substrates,

Since the nineteenth century, oxidized graphite has been produced by different methods: the Brodie method in 1859 [51], the Hummers method in 1958 [52], and the Hummers method modified in July 2004; and in October of the same year, Andre Geim and Kostya Novoselov, professors at the University of Manchester, London, managed to isolate a sheet of graphite a few atoms thick, by mechanical exfoliation, from which they discovered the exceptional properties of this material that has attracted enormous attention since then [53]. However, the synthesis of graphene remains a great challenge, since, to be considered high purity, it is necessary to take into account certain characteristics such as the quality, size, quantity, complexity, and control of the synthesis method. For this reason, it is very important to know the different methods, degrees of purity, and qualities expected for graphene [54]. In the work presented by Hirata et al. [55], they obtained sheets of several nanometers thick and 20 μm wide on average. In the same work, they propose several categories of graphene: oxidized graphene, reduced graphene,

The most used synthesis processes for ZnO are chemical precipitation at normal conditions, synthesis by hydrothermal, solvothermal techniques, and the sol-gel method. The advantage and novelty of these techniques is the obtaining of a great diversity of geometries, with relatively simple processes and with very accessible

The technique is used specifically for ZnO, part of a Zn precursor, normally a salt or inorganic compound, soluble in water commonly and as a reducing agent either an acid or a base, which, when reacting in solution with the salt, will form a precipitate or solid of insoluble that will commonly require a calcination process for crystallization. This type of reactions could be expressed in a general equation in the

This type of reactions usually occurs between ionic compounds where one of the products is insoluble, and because each component exchanges pairs, these types of

AC þ BD ! ADð Þ insoluble þ BC (7)

/Vs; however, this intrinsic property is greatly

shown in Figure 5.

conductivity reaches 200,000 cm<sup>2</sup>

Zinc Oxide Based Nano Materials and Devices

functionalized graphene, and graphene in its pure state.

3.1 Chemical precipitation at normal conditions

reactions are usually called double-displacement reactions.

3. Methods of synthesis of ZnO

precursors.

following way:

120

dopants, and quiralities.

The method or technique of hydrothermal synthesis is one of the most used for the synthesis of ZnO, and with the same principle of the technique of chemical precipitation, this technique gets its name because the conditions of synthesis, which is carried out in water and temperatures above 25 °C up to 430 °C, reaching autogenous pressures of over 221 bar at 375 °C, which is the critical point where the liquid phase and gas are not distinguished from water [56]. This type of synthesis is also governed by the precipitation equation but adding temperature and pressure variables.

In various works such as that of Alver et al. [40], they synthesized ZnO doped with boron by the hydrothermal method and subsequently formed a nanocomposite with graphene using the same technique. Yoo et al. [41] with the same method obtained hemispherical nanoparticles of 25 nm. Wang et al. [57] synthesized ZnO in flower form, doped with Mn, with the hydrothermal method. Bøjesen et al. [36] conducted an in situ study of the growth of ZnO nanoparticles by hydrothermal synthesis. In this work, they conclude that temperature is one of the biggest factors that influences the size and shape characteristics of glass, this, without forgetting other factors such as time.

### 3.3 Solvothermal method

The solvothermal synthesis method is a variant of the hydrothermal one, since, in this technique, solvents different to water are used, but with the same principles of hydrothermal synthesis and governed by the precipitation equation. The said solvents can be alcohols, acids, bases, or mixtures, since, with this, a greater dissolution or changes in the pressures generated in the synthesis are sought [56]. With the solvothermal synthesis technique, authors such as Wi et al. [19] and Wang et al. [42] have obtained nanoparticle agglomerates, of spherical shape and porous sheet type, respectively. In the case of Wang et al. [42], the synthesis process was carried out at room temperature reaching thicknesses of up to 10 nm in the sheets.

### 3.4 Sol-gel

On the other hand, the sol-gel synthesis technique consists of a chemical synthesis in which, from a colloidal solution or "sol", small precipitates of a solid phase gradually form inside a continuous network called "gel". The peculiarity of the technique is the nanometric size of the particles that can be obtained by this technique as shown in the work of Spanhel et al. [32], who obtained ZnO nanoparticles of colloidal sizes between 3 and 6 nm. Hjiri et al. [33] obtained sizes between 20 and 40 nm for ZnO and up to 3 nm for ZnO doped with Al. Li et al. [34] used the technique to obtain in situ a nanocomposite of nanoparticles deposited in sheets of graphene, whose reported particle size was an average of 9 nm.

### 4. Graphene synthesis methods

To date, various synthesis methods have been developed for graphene, but basically, there are only two ways: one of them is to obtain the sheets of an existing crystal of graphite which is known as exfoliation methods, and on the other hand, the sheets of graphene can be grown directly on a substrate.

quantities and the possibility of being used industrially in different carbon prod-

The modified Hummers method is currently one of the most used to obtain graphene, due to the sufficient quantity and the relative ease of the process, that once the GO is reduced, its properties are very favored, so that different groups of work have pointed their investigations using this technique to obtain graphene with

The Hummers method modified for the synthesis of graphene consists in exposing the graphite particles to prolonged times of combined oxidation to washing and purification processes. The process of Hirata et al. [55] is described as follows: 10 g of natural graphite (99.97 % purity and 24 μm particle diameter) and 7.5 g of NaNO3 (99 % purity) were added to a flask. Then 621 g of H2SO4 (96 %) were added and kept stirring while it was cooled in an ice water bath. Next 45 g of KMnO4 (99 %, purity) were added and gradually added over 1 h. The cooling was reached after 2 h, and the mixture remained for 5 days under constant agitation at 20 °C to obtain a

The cleaning and purification process was carried out in the following way; to the obtained liquid, 1 L of solution with 5 % of H2SO4 was added during 1 h, while it was maintained in agitation, and the obtained mixture in the end was agitated during 2 h more. Then 30 g of H2O2 (30 % by weight solution) were added and kept under stirring for 2 h and finally separated by centrifugation. This process was repeated 15 times, for the removal of the ions originated in the oxidation process. The synthesis process used by Vargas et al. [64] has modifications such as the change in the use of NaNO3 by HNO3 and some modifications in synthesis and cleaning methodology. The process they used was the following: 2 g of graphite powder was added to an aqueous solution with 120 mL of H2SO4 and 80 mL of HNO3 in an ice water bath. Then, 10 g of KMnO4 was added slowly and remained in reaction for 2 h, during which the temperature remained around 35 °C. The dark brown suspension obtained was diluted with 400 mL of deionized water and turned dark yellow, to then add 8.6 mL of H2O2 (35 %). A dark brown gel was obtained, after washing with 100 mL of a 10 % solution of HCl and neutral pH obtained after several cycles of washing and centrifugation. The GO was finally obtained by

Wan et al. [65] synthesized graphene using the modified Hummers method by applying the following process: 1 g of natural graphite was placed in a mixture with a concentrated solution of H2SO4 (98 %, 92 mL) and concentrated HNO3 (65 %, 24 mL) while stirring in an ice water bath, as a safety measure and maintaining the temperature below 10 °C. Then 6 g of KMnO4 was added to the solution gradually so that the temperature of the solution did not exceed 20 °C; then the solution was left for 2 h in the ice water bath. Then a temperature controller was used for water flow, to maintain the temperature at 35 °C for 30 min. Subsequently, the temperature was maintained at 85 °C for 30 min. About 100 mL of deionized water was slowly added to the solution, and the temperature was again maintained at 85 °C for 30 min, until a bright yellow product was obtained. After cooling to room temperature and diluting with 10 mL of 30 % H2O2. The solution was centrifuged, washed with 1 L of deionized water and a 1:10 HCl solution (1 L) to remove the remaining

Of the three processes described above, the modified Hummers method differs in several aspects of the process such as the change of NaNO3 by HNO3 and the increase in temperature to reduce the oxidation time, as well as the optimization of the process of cleaning and obtaining the GO. According to the results obtained in what was reported by Wan et al. [65], they obtained GO of five sheets of thickness,

with very promising results in the electrochemical tests they carried out.

metal ions, and finally dried at 50 °C and vacuum for 12 h.

ucts, one of which being Li-ion batteries [58, 59].

DOI: http://dx.doi.org/10.5772/intechopen.86169

Anodic ZnO-Graphene Composite Materials in Lithium Batteries

applications for the anode of Li-ion batteries [61–63].

highly viscous liquid.

vacuum drying at 80 °C.

123

### 4.1 Exfoliation

The exfoliation methods can be classified in two: the micromechanical exfoliation method and the chemical methods, the latter can be by dispersion or oxidation. According to what was reported by Novoselov et al. [53], they used the technique of micromechanical exfoliation, obtaining sheets of graphene up to a single layer; however, this technique is relatively complex, and the yield or number of sheets is very low.

The dispersion method allows obtaining a greater amount of graphene and higher quality than the micromechanical exfoliation. The synthesis of graphene by oxidation is similar to that of dispersion since it is also in the liquid phase, but the graphene that is obtained is of a slightly lower quality, and this is because the GO has a large amount of defects and oxygen at the edges compared to the pristine [54, 57]. To improve the quality of the GO, it can undergo reduction processes and improve the quality of the graphene sheets, obtaining what was called reduced graphene sheets (rG) [54].

The method of Hummers and Offeman [52], reported since 1958, is a method to obtain large quantities of what they called oxidized graphite; in 2004, Hirata et al. [55] modified this method obtaining thinner sheets of better quality. To be reduced, either by heat treatments or chemical means, you can decrease the amounts of oxygen or eliminate it completely under certain conditions. By eliminating oxygen, the graphene sheets increase considerably their electrical conductivity, since the sp<sup>3</sup> bonds decrease while the sp<sup>2</sup> bonds increase, which attribute the excellent properties to graphene. The GO has defects associated with oxygen bonds, at the edges as well as within the plane, forming different functional groups as carbonyls (C = O), hydroxyls (—OH), and epoxy groups (—O—); in Figure 6 a representative scheme of the structure of a GO sheet is shown; this model was proposed by Santos et al. [59] In addition, this method allowed the production of graphene at higher

Figure 6. Insertion or formation of functional groups in the sheets of graphene.

### Anodic ZnO-Graphene Composite Materials in Lithium Batteries DOI: http://dx.doi.org/10.5772/intechopen.86169

4. Graphene synthesis methods

Zinc Oxide Based Nano Materials and Devices

4.1 Exfoliation

very low.

Figure 6.

122

Insertion or formation of functional groups in the sheets of graphene.

graphene sheets (rG) [54].

To date, various synthesis methods have been developed for graphene, but basically, there are only two ways: one of them is to obtain the sheets of an existing crystal of graphite which is known as exfoliation methods, and on the other hand,

The exfoliation methods can be classified in two: the micromechanical exfoliation method and the chemical methods, the latter can be by dispersion or oxidation. According to what was reported by Novoselov et al. [53], they used the technique of micromechanical exfoliation, obtaining sheets of graphene up to a single layer; however, this technique is relatively complex, and the yield or number of sheets is

The dispersion method allows obtaining a greater amount of graphene and higher quality than the micromechanical exfoliation. The synthesis of graphene by oxidation is similar to that of dispersion since it is also in the liquid phase, but the graphene that is obtained is of a slightly lower quality, and this is because the GO has a large amount of defects and oxygen at the edges compared to the pristine [54, 57]. To improve the quality of the GO, it can undergo reduction processes and improve the quality of the graphene sheets, obtaining what was called reduced

The method of Hummers and Offeman [52], reported since 1958, is a method to obtain large quantities of what they called oxidized graphite; in 2004, Hirata et al. [55] modified this method obtaining thinner sheets of better quality. To be reduced, either by heat treatments or chemical means, you can decrease the amounts of oxygen or eliminate it completely under certain conditions. By eliminating oxygen, the graphene sheets increase considerably their electrical conductivity, since the sp<sup>3</sup> bonds decrease while the sp<sup>2</sup> bonds increase, which attribute the excellent properties to graphene. The GO has defects associated with oxygen bonds, at the edges as well as within the plane, forming different functional groups as carbonyls (C = O), hydroxyls (—OH), and epoxy groups (—O—); in Figure 6 a representative

scheme of the structure of a GO sheet is shown; this model was proposed by Santos et al. [59] In addition, this method allowed the production of graphene at higher

the sheets of graphene can be grown directly on a substrate.

quantities and the possibility of being used industrially in different carbon products, one of which being Li-ion batteries [58, 59].

The modified Hummers method is currently one of the most used to obtain graphene, due to the sufficient quantity and the relative ease of the process, that once the GO is reduced, its properties are very favored, so that different groups of work have pointed their investigations using this technique to obtain graphene with applications for the anode of Li-ion batteries [61–63].

The Hummers method modified for the synthesis of graphene consists in exposing the graphite particles to prolonged times of combined oxidation to washing and purification processes. The process of Hirata et al. [55] is described as follows: 10 g of natural graphite (99.97 % purity and 24 μm particle diameter) and 7.5 g of NaNO3 (99 % purity) were added to a flask. Then 621 g of H2SO4 (96 %) were added and kept stirring while it was cooled in an ice water bath. Next 45 g of KMnO4 (99 %, purity) were added and gradually added over 1 h. The cooling was reached after 2 h, and the mixture remained for 5 days under constant agitation at 20 °C to obtain a highly viscous liquid.

The cleaning and purification process was carried out in the following way; to the obtained liquid, 1 L of solution with 5 % of H2SO4 was added during 1 h, while it was maintained in agitation, and the obtained mixture in the end was agitated during 2 h more. Then 30 g of H2O2 (30 % by weight solution) were added and kept under stirring for 2 h and finally separated by centrifugation. This process was repeated 15 times, for the removal of the ions originated in the oxidation process. The synthesis process used by Vargas et al. [64] has modifications such as the change in the use of NaNO3 by HNO3 and some modifications in synthesis and cleaning methodology. The process they used was the following: 2 g of graphite powder was added to an aqueous solution with 120 mL of H2SO4 and 80 mL of HNO3 in an ice water bath. Then, 10 g of KMnO4 was added slowly and remained in reaction for 2 h, during which the temperature remained around 35 °C. The dark brown suspension obtained was diluted with 400 mL of deionized water and turned dark yellow, to then add 8.6 mL of H2O2 (35 %). A dark brown gel was obtained, after washing with 100 mL of a 10 % solution of HCl and neutral pH obtained after several cycles of washing and centrifugation. The GO was finally obtained by vacuum drying at 80 °C.

Wan et al. [65] synthesized graphene using the modified Hummers method by applying the following process: 1 g of natural graphite was placed in a mixture with a concentrated solution of H2SO4 (98 %, 92 mL) and concentrated HNO3 (65 %, 24 mL) while stirring in an ice water bath, as a safety measure and maintaining the temperature below 10 °C. Then 6 g of KMnO4 was added to the solution gradually so that the temperature of the solution did not exceed 20 °C; then the solution was left for 2 h in the ice water bath. Then a temperature controller was used for water flow, to maintain the temperature at 35 °C for 30 min. Subsequently, the temperature was maintained at 85 °C for 30 min. About 100 mL of deionized water was slowly added to the solution, and the temperature was again maintained at 85 °C for 30 min, until a bright yellow product was obtained. After cooling to room temperature and diluting with 10 mL of 30 % H2O2. The solution was centrifuged, washed with 1 L of deionized water and a 1:10 HCl solution (1 L) to remove the remaining metal ions, and finally dried at 50 °C and vacuum for 12 h.

Of the three processes described above, the modified Hummers method differs in several aspects of the process such as the change of NaNO3 by HNO3 and the increase in temperature to reduce the oxidation time, as well as the optimization of the process of cleaning and obtaining the GO. According to the results obtained in what was reported by Wan et al. [65], they obtained GO of five sheets of thickness, with very promising results in the electrochemical tests they carried out.

### 4.2 Substrate growth

This is a totally different way to the previous methods since the graphene sheets can be grown directly on a surface. And the size of the sheets does not depend directly on the size of a graphite crystal, as in the exfoliation methods. Growth can occur in two ways: whether the carbon exists on the surface of the substrate or it is added by chemical vapor deposition (CVD). Graphene can be obtained simply by heating and cooling a silicon carbide crystal, under suitable conditions, obtaining sheets up to a single layer [54].

4.3 Composite ZnO-graphene materials

DOI: http://dx.doi.org/10.5772/intechopen.86169

Anodic ZnO-Graphene Composite Materials in Lithium Batteries

shapes, and crystallinity [13, 66].

An CM of ZnO-graphene (ZnO-G) could have a large number of assembly structures, however, can be considered six principally, the most used in this type of CM for Li-ion batteries: (a) anchored, (b) wrapped, (c) encapsulated, (d) type sandwich, (e) laminar, and (f) mixed. In all cases, graphene, being a twodimensional material, is the one that functions as a support for dispersed nanoparticles whose three-dimensional morphology can vary in different sizes,

Currently, the CM based on ZnO and carbon are very diverse and with variations in the morphology of both phases of the components. Figure 7 shows the obtaining of an CM of possible interest with the specific case of obtaining the dispersed phase of the phases of the ZnO with a Wurtzite type crystal structure, whose structure is more stable to standard conditions; this phase is the most attractive for, it is mainly used in CM for Li-ion batteries, also presented with a wide variety of morphologies; while for graphene, it usually presents different charac-

In recent years, a great amount of research has been carried out regarding the morphology of ZnO, since most of these works seek to increase the area, modifying the morphology and thus increasing the electrochemical properties, for its application in Li-ion batteries [16]. The investigations that have been carried out regarding the control of the crystallinity in the particles of the ZnO, to increase the capacity, are very few. Recently Mei et al. [17] published a paper in which they analyzed the degree of crystallinity and structural patterns of ZnO as an anode in Li-ion batteries. In this work, they used the hydrothermal synthesis in which they modified concentrations, to obtain different morphologies and then carried out treatments at different temperatures, this to control the degree of crystallinity. Finally, Mei et al. concluded that morphology was an important part in the capacity, since certain morphologies present a greater quantity of internal spaces, which help to compensate the volumetric changes; however, the samples whose crystallinity was controlled presented a specific capacity of 1328 mAhg<sup>1</sup> in the first cycle and 663 mAhg<sup>1</sup> at 50 cycles. In this work, it is worth mentioning that the particles obtained are of the micrometric order, contrary to what is generally reported with nanometric materials, such as Li et al. [18] that dispersed ZnO nanoparticles in

Microparticles distributed radially 320 100 [6] Nanobars 310 40 [20] Microbars 663 50 [17] Nanoplates 400 100 [25] Nanoplates 421 100 [25] Nanoparticles 318 100 [42] Flower type 662 50 [17] Spheres 109 100 [70] Cluster 800 50 [71] Microbars 811 80 [72] Nanoparticles 318 100 [73]

) Number of cycles Ref.

teristics and properties according to its synthesis method [25, 69].

Morphology Reversible capacity (mAhg<sup>1</sup>

Electrochemical characteristics of ZnO applied as an anode material in Li-ion batteries.

Table 2.

125

The chemical vapor deposition method is perhaps one of the most promising and relatively low-cost techniques to obtain good quality graphene. Broadly speaking, the technique consists in the deposition of a solid film on a substrate, where the chemical species of the material deposited come from species in vapor phase and are deposited through chemical reactions. In an ideal CVD process, the transport kinetics of gases is often complicated and complex, since convection and diffusion phenomena occur in different regions of a reactor [66].

The process for obtaining graphene by CVD can be divided into two stages: the first is the pyrolysis of the carbon precursor and the second the formation of the graphene structure. In an ideal synthesis to obtain graphene, temperatures of up to 2500 °C would be needed to overcome the energy barrier that allows the reaction on the surface of the substrate; for this reason, catalysts are used, which are mostly elemental metals, which contribute to the pyrolization of carbon precursors. One of the most used substrates is Ni (111) since it has a structure very similar to that of graphene, with a mismatch in network parameters of 1.3 % [54, 64].

The CVD process, as previously described, is a relatively complex process because of the equipment necessary to carry out the synthesis, but it allows obtaining graphene of higher quality at low cost, with larger sizes and more complex forms than the exfoliation processes of graphite [54, 57, 65, 66]. The team of Kim et al. [67] was among the first to report obtaining graphene by the CVD method on a Ni substrate, proving that the monolayers obtained were of much better quality than those of exfoliation. Since then, several authors have continued research to improve the technique, either by lowering the synthesis temperature as reported by Jang et al. [68], who obtained graphene at temperatures between 100 and 300 °C deposited on copper sheets, using benzene. Other authors such as Sagar et al. [58] have synthesized highly porous structures based on interconnected sheets of graphene and have proposed an anode in Li-ion batteries.

Figure 7.

Scheme of obtaining the MC type ZnO-rG. Left: material of the precursor phase Zn(OH)2-GO. Right: the MC ZnO-rG obtained after a thermal treatment at 600 °C.
