**3. Structural characterization and physicochemical properties of graphene– metal oxide nanocomposites**

As the synthesized graphene–metal oxide nanohybrid materials have tremendous effects on electrocatalytic applications, structural characterization is of paramount importance for understanding the correlation between their nanostructures and catalytic activity. The fundamental knowledge about the structural features of a nanocomposite is also essential for the building up of catalysts with optimal electrocatalytic activity. These structural features also offer helpful clues for further modification of the catalysts. In this section, we briefly summa‐ rize characterization of the structural features of different graphene–metal oxide nanocompo‐ sites by various instrumental techniques. These widely used techniques include scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray photo‐ electron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), and thermogravi‐ metric analysis (TGA).

### **3.1. Microscopic imaging of chemically exfoliated graphene and composites**

The atomic structural features of a material affect its electronic, chemical, and catalytic properties. Various microscopic imaging techniques such as SEM, TEM, and AFM are proven as very essential tools to characterize nanomaterials. Their high-resolution capability enables to provide detailed information regarding shape, size, chemical composition, and phase of nanomaterials. As a typical example, it has been well-shown that particle size, morphology, and exposed active facets have significant impacts on the catalytic efficiency of metal or metal oxide nanomaterials toward ORR electrocatalysis.

**Metal oxidesNanohybrid**

Vanadium

**electrocatalyst Preparation method Precursors Applications Ref**

isobutoxide

V2O5, GO

**Table 1.** Summary of the main synthetic methods for preparation of metal oxide–graphene nanocomposites and their

**3. Structural characterization and physicochemical properties of graphene–**

As the synthesized graphene–metal oxide nanohybrid materials have tremendous effects on electrocatalytic applications, structural characterization is of paramount importance for understanding the correlation between their nanostructures and catalytic activity. The fundamental knowledge about the structural features of a nanocomposite is also essential for the building up of catalysts with optimal electrocatalytic activity. These structural features also offer helpful clues for further modification of the catalysts. In this section, we briefly summa‐ rize characterization of the structural features of different graphene–metal oxide nanocompo‐ sites by various instrumental techniques. These widely used techniques include scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray photo‐ electron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), and thermogravi‐

**3.1. Microscopic imaging of chemically exfoliated graphene and composites**

The atomic structural features of a material affect its electronic, chemical, and catalytic properties. Various microscopic imaging techniques such as SEM, TEM, and AFM are proven as very essential tools to characterize nanomaterials. Their high-resolution capability enables to provide detailed information regarding shape, size, chemical composition, and phase of

freeze drying GO, HVO3 Li-ion batteries [59]

High-performance cathodes in Li-Ion

[58]

[60]

[61]

batteries

High-capacity supercapacitor electrodes

Supercapacitors and electrochemical

Intracellular H2O2 sensors [62]

sensors

oxide Graphene/ V2O5 Chemical reduction GO, vanadyl tri

treatment

Zinc Oxide Graphene/ZnO Hydrothermal method ZnCl2, Graphene

dots/ZnO Electrospinning GO, ZnO

V2O5/GO Chemical reduction,

388 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

3D VO2 NR/ rGO Hydrothermal/heat

rGO quantum

**metal oxide nanocomposites**

metric analysis (TGA).

major applications.

**Figure 4.** (a) Optical micrograph of graphene oxide sheets on the Quantifoil (QF) TEM grid. (b) Electron diffraction pattern of a bilayer area, displaying the stacking structure of the sheets. (c) Diffraction pattern from a single layer. (d) TEM image of RGO sheets on the QF grid. (e) TEM image with the sample tilted to 60°. The region between the hori‐ zontal dashed lines is a single layer (region above is a double layer, below is vacuum). Arrows indicate horizontal dark lines where the RGO sheet appears parallel to the beam, indicating local deformations up to 30°. Scale bars are 10 μm (a), 200 nm (d), and 10 nm (e). (f) Atomic resolution, aberration-corrected TEM image of a single layer reduced gra‐ phene oxide membrane. (g) Highlight with color in different areas. The light-gray color indicates the defect-free crys‐ talline graphene area. Dark-gray shaded regions show the contaminated area. Blue regions indicate the disordered single-layer carbon networks, or extended topological defects due to the remnants of the oxidation–reduction process. Red areas indicate individual ad-atoms or substitutions. Green areas highlight isolated topological defects, that is, sin‐ gle bond rotations or dislocation cores. Holes and their edge reconstructions are marked in yellow color. Scale bar in (f) and (g) is 1 nm. (Reproduced with permission from ref. 65. Copyright ACS 2010)

SEM is mainly used to get the overall morphology of nanocomposites at large scales from several microns to 500 nm. However, to get more detailed and descriptive view of metal oxide nanocomposites and the crystal lattices or defects on graphene sheets, TEM and high-resolu‐ tion TEM (HRTEM) are the most appropriate tools [63, 64]. The working principles of HRTEM are based on the interference of different transmitted and diffracted electron beams for building an image that can show the variation in phase. This is quite different from other traditional microscopic techniques, where sample image is derived from the variation of beam amplitudes due to the absorption of specimens. Compared to mechanically exfoliated gra‐ phene, the chemically synthesized graphene contains notable structural defects, but they could hardly be detected by normal spectroscopic and microscopic characterizations. TEM is one of the leading methods for atomic scale imaging of graphene-based materials. For example, Fig. 4a shows an optical micrograph of RGO-coated Quantifoil (QF) TEM grid with the coverage visible as grayish spots [65]. TEM imaging analysis of the sample (Fig. 4d) showed that only ∼1% of the holes of the whole grid were covered with sheets. Diffraction analysis was done to find holes covered by single-layered graphene sheets, which exhibited only one hexagonal pattern (Fig. 4c). These hexagonal patterns indicate the presence of a long-range hexagonal order orientation in the graphene sheets. Figure 4b indicates the parallel-beam diffraction pattern from a bilayer area, where two hexagonal patterns can be clearly detected. Then, the sample imaging was done under a high tilt (60°, Fig. 4e), which showed a high level of roughness, much more than that in mechanically exfoliated graphene sheets. The horizontal dark lines in Fig. 4e (arrows) was explained as representing the graphene layer in parallel to the electron beam. This image interprets that the wrinkles might be due to solution processing and drying, to stress, or to form defects in RGO. The high-resolution imaging of single-layered graphene sheets showed the actual atomic structure of the RGO layers, as represented in Figs. 4f and 4g. Various regions of the image are highlighted by colors in Fig. 4g. It is clearly observed that the largest part of the layer is formed of clean well-crystallized graphene areas where the hexagonal lattice is clearly seen (light-gray color in Fig. 4f). The visible well-crystallized areas are around 3–6 nm in the domain size and they cover ∼60% of the whole plane. However, it is impossible to determine the exact structure of graphene with the adsorbed contamination, which covers ∼30% of the total area. As most of the contaminants prefer to stick on the defects, the part of the defective areas is most likely underestimated. In spite of the presence of such a significant number of topological defects, the long-range oriented order of RGO is maintained. But such defects were normally not seen in any mechanically exfoliated graphene samples. Because the RGO and mechanically cleaved graphene samples were prepared from the same graphite source, it can be ruled out that the defects were from the starting material. These observations suggest that the high density of topological defects in these samples were introduced during graphene exfoliation by strong chemical oxidation and reduction. Although HRTEM can magnify the micro view into 1 nm, there is still limitation at around 0.2 nm. And, as electron beam heating can destroy small nanoparticles, the possibility of particle melting should be considered.

Song et al. [66] reported an alternative important strategy using scanning transmission electron microscopy (STEM), which combines the advantages of SEM and TEM and has extensively used in characterization of morphology and crystal structures of nanomaterials. Scanning probe microscopy techniques such as AFM and scanning tunneling microscopy (STM) also play an important role in the structural characterization of material surfaces with atomic resolution. Although the theoretical thickness of monolayer graphene is approximately 0.34 nm, the detected thickness by AFM analysis of a single layer of chemically synthesized GO is around 0.6–1.2 nm [67]. Although AFM can partially provide information regarding the number of layers in graphene, it is always better to associate it with Raman and XPS meas‐ urements for a complete chemical information. In addition, the STEM technique is also very helpful to obtain the lattice structure, surface morphology, particle size, and distribution of graphene-based materials surface at atomic resolution [68],[69],[70]. Figure 5a shows an overview of blue-shaded and uniformly folded graphene nanosheets taken by STEM at 200 kV with a high-angle annular dark field (HAADF) detector. Figures 5b and 5c show the 3 and 28 layers of graphene edges corresponding to the two arrows pointed area in Fig. 5a [66]. Figure 5d displays AFM images of chemically synthesized RGO nanosheets with ~0.6 nm thickness and with lateral dimensions in a few hundred nanometers to one micrometer [67]. And Figs. 5e and 5f show the AFM images of self-assembled RGO nanosheets with a 0.9 nm thickness [71].

**Figure 5.** (a) HAADF-STEM image showing an overview of graphene flakes supported by a holey Formvar film cov‐ ered with Cu grids. The arrows indicate areas where graphene is freely suspended on the holey film. (b) High-resolu‐ tion TEM images of a three-layered graphene edge. (c) High-resolution TEM images of a 28-layered graphene sheet edge. (d) An AFM image of RGO nanosheet. (e,f) AFM images as an example of the self-assembled patterns of RGO nanosheets forming a bilayer nanofilm on mica surfaces. Scanned areas of AFM images are 80 μm × 80 μm (c) and 20 μm × 20 μm. (Combiningly reproduced with permission from ref. 66 Copyright Elsevier 2010, ref. 67 copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, and ref. 71 copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Figure 6 shows the morphology and structure of the 3D graphene/Co3O4 nanowire nanohybrid materials by SEM and TEM imaging. Figure 6a shows the SEM images of 3D porous structured graphene. Figure 6b-6d shows the uniform coverage of Co3O4 nano‐ wire on 3D graphene skeleton. The high-resolution SEM image shows that the diameter of Co3O4 nanowire on 3D graphene is around 200–300 nm and the length is around several micrometers (Fig. 6d). And the TEM image (Fig. 6e) shows that the Co3O4 nanowires are composed of numerous nanoparticles.

## **3.2. Spectroscopic characterization**

the leading methods for atomic scale imaging of graphene-based materials. For example, Fig. 4a shows an optical micrograph of RGO-coated Quantifoil (QF) TEM grid with the coverage visible as grayish spots [65]. TEM imaging analysis of the sample (Fig. 4d) showed that only ∼1% of the holes of the whole grid were covered with sheets. Diffraction analysis was done to find holes covered by single-layered graphene sheets, which exhibited only one hexagonal pattern (Fig. 4c). These hexagonal patterns indicate the presence of a long-range hexagonal order orientation in the graphene sheets. Figure 4b indicates the parallel-beam diffraction pattern from a bilayer area, where two hexagonal patterns can be clearly detected. Then, the sample imaging was done under a high tilt (60°, Fig. 4e), which showed a high level of roughness, much more than that in mechanically exfoliated graphene sheets. The horizontal dark lines in Fig. 4e (arrows) was explained as representing the graphene layer in parallel to the electron beam. This image interprets that the wrinkles might be due to solution processing and drying, to stress, or to form defects in RGO. The high-resolution imaging of single-layered graphene sheets showed the actual atomic structure of the RGO layers, as represented in Figs. 4f and 4g. Various regions of the image are highlighted by colors in Fig. 4g. It is clearly observed that the largest part of the layer is formed of clean well-crystallized graphene areas where the hexagonal lattice is clearly seen (light-gray color in Fig. 4f). The visible well-crystallized areas are around 3–6 nm in the domain size and they cover ∼60% of the whole plane. However, it is impossible to determine the exact structure of graphene with the adsorbed contamination, which covers ∼30% of the total area. As most of the contaminants prefer to stick on the defects, the part of the defective areas is most likely underestimated. In spite of the presence of such a significant number of topological defects, the long-range oriented order of RGO is maintained. But such defects were normally not seen in any mechanically exfoliated graphene samples. Because the RGO and mechanically cleaved graphene samples were prepared from the same graphite source, it can be ruled out that the defects were from the starting material. These observations suggest that the high density of topological defects in these samples were introduced during graphene exfoliation by strong chemical oxidation and reduction. Although HRTEM can magnify the micro view into 1 nm, there is still limitation at around 0.2 nm. And, as electron beam heating can destroy small nanoparticles, the possibility of particle melting

390 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

Song et al. [66] reported an alternative important strategy using scanning transmission electron microscopy (STEM), which combines the advantages of SEM and TEM and has extensively used in characterization of morphology and crystal structures of nanomaterials. Scanning probe microscopy techniques such as AFM and scanning tunneling microscopy (STM) also play an important role in the structural characterization of material surfaces with atomic resolution. Although the theoretical thickness of monolayer graphene is approximately 0.34 nm, the detected thickness by AFM analysis of a single layer of chemically synthesized GO is around 0.6–1.2 nm [67]. Although AFM can partially provide information regarding the number of layers in graphene, it is always better to associate it with Raman and XPS meas‐ urements for a complete chemical information. In addition, the STEM technique is also very helpful to obtain the lattice structure, surface morphology, particle size, and distribution of graphene-based materials surface at atomic resolution [68],[69],[70]. Figure 5a shows an overview of blue-shaded and uniformly folded graphene nanosheets taken by STEM at 200

should be considered.

FTIR spectroscopy has been widely used to characterize different functional groups with specific chemical bonds (such as hydroxyl, carbonyl, carboxylic, and epoxy), which absorb

**Figure 6.** SEM images of (a) 3D graphene structure, (b) 3D graphene/Co3O4 nanowire nanohybrid (c, d) Low- and highmagnification SEM images of 3D graphene/Co3O4 nanowire nanohybrid material. Inset panel d shows an enlarged im‐ age. (e, f) Low- and high-resolution TEM images of Co3O4 nanowire grown on the 3D graphene surface. (Reproduced with permission from ref. 17 Copyright © 2012 American Chemical Society)

light energy (4000 cm−1 to 400 cm−1) and exhibit a frequency corresponding to the fundamental vibrations [72]. Raman spectroscopy deals with the frequencies of Raman-scattered mono‐ chromatic light. As a supplementary to FTIR, Raman spectroscopy can provide sufficient information for characterization of graphene-based materials. Vibrations of different groups in polar/nonpolar molecules can be efficiently detected by these two methods. From the Raman spectra of graphene, three typical peaks of the G band at around 1580 cm−1, the D band at around 1350 cm−1, and the 2D band at 2700 cm−1 are often observed. The G band is an indicator of the stacking structures; the D is generally associated with the order/disorder of the material; the 2D-band is sensitive to the layer number of graphene sheets [2]. The ratio of the intensity of the two bands (D/G) is used for determining the number of layers in a particular graphene sample and its overall stacking behavior [2]. From Fig. 7a [16], two prominent peaks for the D and G bands are observed in the range of 1000–2000 cm-1. And the intensity ratio of the D and G band indicates the density of structural defects on the graphene surface. In Fig. 7a, the ID/IG ratio of CuO/GO composite is 1.03 (S3), 0.96 (S2), and 0.87 (S1), which is much higher than the calculated value of GO (0.77). These ratios clearly indicate that the CuO modification intro‐ duced additional defects into the GO structure [16].

Figure 7b displays the FT-IR spectra of GO and different metal oxide nanocomposites. GO shows some typical peaks at 3420 and 1712 cm−1, respectively, for the stretching vibrations of O–H and C=O. And also the bending vibration peak at 1408 cm−1 for O–H, at 1223 cm−1 for C–OH, the stretching peak at 1052 cm−1 for C–O, and the vibration peak at 1633 cm−1 for C=C [73]. For the spectra of Mn3O4–GNS, the vibration peaks at 610 cm−1 and 491 cm−1 are an indi‐

**Figure 7.** (a) Raman spectra of GO and CuO/GO composites and (b) FTIR spectra of GO, and Mn3O4–GNS, Fe3O4–GNS, and Co3O4–GNS nanocomposites. (Reproduced with permission from ref. 16. Copyright ACS 2013; from ref. 74 Copy‐ right RSC 2012)

cation of the stretching modes of the Mn–O. In the spectrum of Fe3O4–GNS, the peak at 570 cm−1 is assigned to the vibration of the Fe–O bonds. In addition, for the spectrum of Co3O4– GNS, the peaks at 611 cm−1 and 575 cm−1 correspond to Co–O bonds [74].

light energy (4000 cm−1 to 400 cm−1) and exhibit a frequency corresponding to the fundamental vibrations [72]. Raman spectroscopy deals with the frequencies of Raman-scattered mono‐ chromatic light. As a supplementary to FTIR, Raman spectroscopy can provide sufficient information for characterization of graphene-based materials. Vibrations of different groups in polar/nonpolar molecules can be efficiently detected by these two methods. From the Raman spectra of graphene, three typical peaks of the G band at around 1580 cm−1, the D band at around 1350 cm−1, and the 2D band at 2700 cm−1 are often observed. The G band is an indicator of the stacking structures; the D is generally associated with the order/disorder of the material; the 2D-band is sensitive to the layer number of graphene sheets [2]. The ratio of the intensity of the two bands (D/G) is used for determining the number of layers in a particular graphene sample and its overall stacking behavior [2]. From Fig. 7a [16], two prominent peaks for the D and G bands are observed in the range of 1000–2000 cm-1. And the intensity ratio of the D and G band indicates the density of structural defects on the graphene surface. In Fig. 7a, the ID/IG ratio of CuO/GO composite is 1.03 (S3), 0.96 (S2), and 0.87 (S1), which is much higher than the calculated value of GO (0.77). These ratios clearly indicate that the CuO modification intro‐

**Figure 6.** SEM images of (a) 3D graphene structure, (b) 3D graphene/Co3O4 nanowire nanohybrid (c, d) Low- and highmagnification SEM images of 3D graphene/Co3O4 nanowire nanohybrid material. Inset panel d shows an enlarged im‐ age. (e, f) Low- and high-resolution TEM images of Co3O4 nanowire grown on the 3D graphene surface. (Reproduced

Figure 7b displays the FT-IR spectra of GO and different metal oxide nanocomposites. GO shows some typical peaks at 3420 and 1712 cm−1, respectively, for the stretching vibrations of O–H and C=O. And also the bending vibration peak at 1408 cm−1 for O–H, at 1223 cm−1 for C–OH, the stretching peak at 1052 cm−1 for C–O, and the vibration peak at 1633 cm−1 for C=C [73]. For the spectra of Mn3O4–GNS, the vibration peaks at 610 cm−1 and 491 cm−1 are an indi‐

duced additional defects into the GO structure [16].

with permission from ref. 17 Copyright © 2012 American Chemical Society)

392 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**Figure 8.** C 1s XPS spectra of (a) GO, (b) Mn3O4–GNS composite, (c) Fe3O4–GNS composite, and (d) Co3O4–GNS com‐ posite. The blue, pink, and green curves denote C–C, C–O, and C=O spectra, respectively. (Reproduced with permis‐ sion from ref. 74 Copyright RSC 2012)

XPS is a powerful tool for the investigation of chemical composition, elemental states, and the nature of heteroatom functionalized or doped-graphene-based nanohybrid materials. For example, the presence of different metal or different heteroatom on functionalized graphene can be reliably identified by XPS. Zhang et al. [74] synthesized different inorganic–organic hybrid nanocomposite materials based on reduced graphene oxide and three different metal oxides. Figure 8a shows the C 1s spectrum of GO with two strong peaks of C–C, C–O, and a relatively weak peak of C=O species. After the formation of the composites, the peaks of C–O and C=O are significantly weakened due to the removal of oxygen-containing functional groups on GO [75]. Also, by comparing Fig. 8(b)–(d), Fe3O4–GNS displays the lowest C–O and C=O intensity, indicating that Fe2+ is the most efficient ion for the reduction of GO. In addition, C 1s, O 1s, and M 2p peaks existing in the wide scan spectra of different metal nanocomposites with graphene clearly indicate the combination of graphene nanosheets and different metal oxide NPs.

Other spectroscopic techniques are also available for the characterization of nanocompo‐ sites. UV-vis spectroscopic analysis often shows some typical absorption peaks for graphene and graphene oxide at around 268, 230, and 300 nm, which is relevant to the π–π\* transi‐ tions of aromatic C=C bonds, and the n-π\* transitions of C=O bonds, respectively [76]. TGA technique has been extensively used in characterizing the thermal stability and loading amount of different metal nanoparticles in graphene-based nanocomposites [77, 79]. Energy dispersive X-ray (EDX) spectroscopy is very helpful for qualitative and quantitative analysis of the element distribution on graphene-based nanocomposite materials. EDX is more effec‐ tive for determination of the locations of different metals in nanocomposites when com‐ bined with TEM or SEM [80, 81].

## **3.3. Main physicochemical properties**

The physicochemical properties of different graphene–metal oxide nanohybrid material can be characterized by a series of electrochemical methods and instruments, such as cyclic voltammetry (CV) measurements, linear sweep voltammetry analysis (LSV), and electrochem‐ ical impedance spectroscopy (EIS). In some cases, rotating-disk electrodes (RDE) and rotating ring-disk electrodes (RRDE) are needed. Each method has its specific advantages for studying electrocatalytic performance of the hybrid materials. For the different characterizations, one method is normally not sufficient but several methods involved. In this section, we focus on discussing some electrochemical characterization techniques and their specialties and limita‐ tions.

## *3.3.1. Cyclic Voltammetry (CV)*

CV is arguably the most common and straightforward method to determine the electrocatalyt‐ ic activity of a nanocomposite material. CV is normally measured in a typical electrolyte solution at room temperature. An electrochemical cell consists of three electrodes, i.e., a working electrode loaded with the catalyst, a reference electrode, and a Pt wire as a counter electrode [82].

As shown in Fig. 9, the electrochemical kinetics of CuO/GO/GCEs compared with CuO/GCEs was studied by CV systematically. The CV was performed in 0.1 M NaOH electrolyte solution with a scan rate of 100 mV/s. Figure 9a shows a peak at +0.67 V versus Ag/AgCl from all the three CuO/GCEs electrodes, which corresponds to the Cu(II)/Cu(III) redox couple. The three

example, the presence of different metal or different heteroatom on functionalized graphene can be reliably identified by XPS. Zhang et al. [74] synthesized different inorganic–organic hybrid nanocomposite materials based on reduced graphene oxide and three different metal oxides. Figure 8a shows the C 1s spectrum of GO with two strong peaks of C–C, C–O, and a relatively weak peak of C=O species. After the formation of the composites, the peaks of C–O and C=O are significantly weakened due to the removal of oxygen-containing functional groups on GO [75]. Also, by comparing Fig. 8(b)–(d), Fe3O4–GNS displays the lowest C–O and C=O intensity, indicating that Fe2+ is the most efficient ion for the reduction of GO. In addition, C 1s, O 1s, and M 2p peaks existing in the wide scan spectra of different metal nanocomposites with graphene clearly indicate the combination of graphene nanosheets and different metal

Other spectroscopic techniques are also available for the characterization of nanocompo‐ sites. UV-vis spectroscopic analysis often shows some typical absorption peaks for graphene and graphene oxide at around 268, 230, and 300 nm, which is relevant to the π–π\* transi‐ tions of aromatic C=C bonds, and the n-π\* transitions of C=O bonds, respectively [76]. TGA technique has been extensively used in characterizing the thermal stability and loading amount of different metal nanoparticles in graphene-based nanocomposites [77, 79]. Energy dispersive X-ray (EDX) spectroscopy is very helpful for qualitative and quantitative analysis of the element distribution on graphene-based nanocomposite materials. EDX is more effec‐ tive for determination of the locations of different metals in nanocomposites when com‐

The physicochemical properties of different graphene–metal oxide nanohybrid material can be characterized by a series of electrochemical methods and instruments, such as cyclic voltammetry (CV) measurements, linear sweep voltammetry analysis (LSV), and electrochem‐ ical impedance spectroscopy (EIS). In some cases, rotating-disk electrodes (RDE) and rotating ring-disk electrodes (RRDE) are needed. Each method has its specific advantages for studying electrocatalytic performance of the hybrid materials. For the different characterizations, one method is normally not sufficient but several methods involved. In this section, we focus on discussing some electrochemical characterization techniques and their specialties and limita‐

CV is arguably the most common and straightforward method to determine the electrocatalyt‐ ic activity of a nanocomposite material. CV is normally measured in a typical electrolyte solution at room temperature. An electrochemical cell consists of three electrodes, i.e., a working electrode loaded with the catalyst, a reference electrode, and a Pt wire as a counter electrode [82].

As shown in Fig. 9, the electrochemical kinetics of CuO/GO/GCEs compared with CuO/GCEs was studied by CV systematically. The CV was performed in 0.1 M NaOH electrolyte solution with a scan rate of 100 mV/s. Figure 9a shows a peak at +0.67 V versus Ag/AgCl from all the three CuO/GCEs electrodes, which corresponds to the Cu(II)/Cu(III) redox couple. The three

oxide NPs.

tions.

bined with TEM or SEM [80, 81].

*3.3.1. Cyclic Voltammetry (CV)*

**3.3. Main physicochemical properties**

394 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**Figure 9.** CV curves of (a) CuO NPs synthesized at 120, 150, and 180°C. The inset is the CuO NPs synthesized at 120°C in 0.1 M NaOH before (black trace) and after (red trace) the injection of 5 mM glucose. (b) CuO/GO composite S3 and GO sheet (inset) in 0.1 M NaOH before (black trace) and after (red trace) the injection of 5 mM glucose. (Reproduced with permission from ref. 16. Copyright ACS 2013)

different types of CuO NP generated different peak currents. CuO NPs obtained at 120°C displayed the most efficient electron transfer. The inset in Fig. 9a represents the CVs of before and after the addition of 5 mM glucose to the electrolye solutions for CuO/GCE based on 120°C CuO NPs. The Cu (II)/Cu(III) redox couple is the important factor for nonenzymatic glucose detection. Figure 9b shows the CV response of the CuO/GO/GCE before and after the addition of 5 mM glucose to the electrolye solutions, and the inset shows the CV curve of the GO/GCE electrode as a reference. As GO is electroinactive, there is no electrocatalytic oxidation of glucose. In contrast, CuO/GO/GCE showed high electrocatalytic activity, and the peak current significantly increases. Thus, the cyclic voltammetry offers a convenient way to study the electrocatalytic oxidation process of glucose.

**Figure 10.** (a) CVs of ORR on different electrodes in N2- and O2-saturated 0.1 M KOH with a scan rate of 10 mV s−1. (b) CV curves of MnCo2O4/N-rmGO, MnCo2O4 + N-rmGO mixture, Co3O4/N-rmGO, and N-rmGO on GCEs in O2-saturat‐ ed (solid line) or N2-saturated (dash line) 1 M KOH. The peak position of Pt/C is displayed as a dashed line for com‐ parison. (Reproduced with permission from ref. 19 Copyright Nature 2011; from ref. 18 Copyright ACS 2012)

As shown in Fig. 10a, the commercial Pt/C displays typical CV responses of ORR in N2 and O2-saturated 0.1 M KOH. The black dash line shows CV response in the N2-saturated 0.1 M KOH within a potential window from 0.36 to 1.1 V. It is clearly seen that there is no sign of the typical ORR peak of Pt/C at +0.9 V [19]. However, a distinct performance of ORR (black solid line) is clearly seen in the case of O2-saturated 0.1 M KOH. The clear peak at ~0.9 V of Pt/C indicates that the Pt/C exhibits excellent ORR activity with its standard onset potential (~1.0 V) and peak potential (~0.9 V). Figure 10b compares the electrocatalytic ORR performance of MnCo2O4/N-rmGO, N-rmGO mixture, Co3O4/N-rmGO, and N-rmGO in an N2- and O2 saturated 0.1 M KOH solution, respectively. However, these four electrodes showed different ORR activity in O2 saturated electrolyte solutions. The ORR activity of MnCo2O4/N-rmGO (red solid line) has a more positive peak potential and higher peak current density (0.88 V, 0.5 mA cm−2) than those of the N-rmGO mixture (~0.84 V, 0.38 mA cm−2), Co3O4/N-rmGO (~0.86 V, 0.44 mA cm−2), and N-rmGO (~0.82 V, 0.29 mA cm−2). Therefore, MnCo2O4/N-rmGO material is more promising for ORR application, with a similar performance to that obtained at commer‐ cially available Pt/C [18].
