Understanding the Roles of the Intercalator, Oxidant, and Graphite Particle Size in the Hummers' Method for Preparing High-Quality Graphene Oxide

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High-quality preparation of graphene oxide via the Hummers' method:

Understanding the roles of the intercalator, oxidant, and graphite particle size

Yonggang Hou

^a,b

, Shenghua Lv

^a,c,^∗

, Leipeng Liu

^a

, Xiang Liu

^a

aCollege of Bioresources Chemical and Materials Engineering, Shaanxi University of Science and Technology, Xi'an, 710021, China

bSchool of Materials Science and Chemical Engineering, Xi'an Technological University, Xi'an, 710021, China

cNational Demonstration Center for Experimental Light Chemistry Engineering Education, Shaanxi University of Science and Technology, Xi'an, 710021, China

A R T I C L E I N F O

Keywords:

Graphite Graphene oxide Hummers method Intercalator Oxidant

A B S T R A C T

The Hummers' method, in which concentrated sulfuric acid (H2SO4) acts as the intercalator and potassium permanganate serves as the oxidant, is a commonly used method to prepare graphene oxide (GO). The amounts of intercalator and oxidant along with the particle size of graphite are important factors that affect the structure and properties of GO. In this study, Fourier-transform infrared spectroscopy, X-ray diffraction, Raman spectroscopy, ultraviolet–visible spectroscopy, scanning electron microscopy, dynamic light scattering, X-ray photoelectron spectroscopy, thermogravimetric analysis, and atomic force microscopy were used to characterize the effects of these factors on the structure and properties of GO. The results show that the amount of intercalator and oxidant clearly affect the types of oxygen-containing functional groups in the GO structure along with the oxidation degree of GO. Increasing the dosages of intercalator and oxidant can improve the oxidation degree of GO, facilitating the preparation of typical GO. Sodium nitrate (NaNO₃) has a synergistic effect with H₂SO₄during the process of graphite oxidation, which is helpful for the intercalation and oxidation of graphite. Increasing the oxidation degree of graphite can increase the interlayer spacing, which is conducive to the exfoliation of GO.

However, the use of excessive NaNO₃is not conducive to improving GO oxidation. The eﬀect of graphite particle size on GO interlayer spacing is greater than that of NaNO₃. The obtained results provide a reference for the preparation of GO with controllable structure.

1. Introduction

The Hummers' method is the most commonly used method to syn- thesize graphene oxide (GO) from graphite [1–3]. Dimiev [4] reported that the formation of GO from bulk graphite involves three steps. In the ﬁrst step, graphite is converted into a stage-1 graphite intercalation compound (GIC), with sulfuric acid (H2SO4) commonly used as the intercalation agent [5]. The stoichiometry of the stage-1 H2SO4-GIC can be represented by C(21-28)+∙HSO4−∙2.5H2SO4. The stage-1 GIC is a critical intermediate in the synthesis of GO because it allows the diﬀusion of oxidizing agents into the interlayer spaces of graphite. In the second step, the stage-1 GIC is converted into pristine oxidized graphite (PGO).

This rate-determining step makes the entire process diﬀusion con- trolled. In the third step, PGO is converted into GO upon exposure to water. Thus, the intercalation of graphite is required for subsequent exfoliation when preparing GO. To overcome the shortcomings of the traditional Hummers' method, researchers have proposed improved

Hummers methods that use potassium permanganate (KMnO4) as the main oxidant of graphite [6] and some auxiliary agents including sodium nitrate (NaNO3) to improve the yield of GO. However, the synthetic process of GO affects itsfinal properties [7–10], making it difficult to accurately control the structure of GO.

In recent decades, improvements to the Hummers' method have mainly been based on avoiding the use of NaNO3[6,11,12] and increase the amount of KMnO4 to replace NaNO3 [13]. These studies suggest that the use of NaNO3does not aﬀect the preparation of GO;

however, the mechanism of action of NaNO3during GO preparation has not been systematically studied. On the other hand, many recent studies used NaNO3in the preparation of GO, and the amount of NaNO3used for the same weight of graphite varied [14,15].

Regarding the mechanism of action of NaNO3during GO production, Sherif [16] reported that NaNO3serves as an oxidant. However, other researchers, including Tour [17], excluded the possibility of NaNO3as an oxidant because of its negligible role in graphite oxidation.

https://doi.org/10.1016/j.ceramint.2019.09.231

Received 8 July 2019; Received in revised form 20 September 2019; Accepted 23 September 2019

∗Corresponding author. College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science and Technology, Xi'an, 710021, China E-mail address:[email protected](S. Lv).

Please cite this article as: Yonggang Hou, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.231

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In consideration of the low oxidation potential of NaNO3, Chen [18]

suggested that the main role of NaNO3is to promote H2SO4intercala- tion, which is conducive to the oxidation of graphite by the oxidant. Wu [19] reported that NaNO3acts as an intercalation agent for graphite prior to GO formation. Mombeshora [20] studied the eﬀect of the graphite/NaNO3 ratio on the oxygen content and physicochemical properties of GO.

Flake graphite is the most common source of graphite used to prepare GO. The structure offlake graphite contains some defects that can serve as reaction sites for the oxidation. However, the complex structure offlake graphite and its inherent defects make it difficult to de- termine the formation mechanism of GO [21]. Most research has fo- cused on converting C]C bonds to C–O bonds [22]. However, most existing studies did not consider the structural changes of graphite sheets or the effect of oxidant diffusion. In fact, the original graphite source consists of many graphene layers; thus, the oxidant must pene- trate into the interlayers before the oxidation reaction can occur [23].

As a result, the particle size of raw graphite also aﬀects graphite oxidation. Trung [24] conﬁrmed that the oxidation degree of GO increases with decreasing graphite particle size. Seyyedeh [25] found that the oxidation time depends strongly on the particle size of the initial graphite. The oxidation time can be decreased by reducing the particle size, indicating that small graphite particles are suitable for large-scale GO preparation. Under identical conditions, graphite particles with relatively small sizes should save considerable time compared to larger graphite particles because of their lower resistance [5].

In summary, the intercalator and oxidant used in the preparation of GO along with the particle size of raw graphite are critical factors controlling the chemical structure of GO. However, few systematic studies on this topic are available in the literature. In this study, a series of GOs were prepared from graphite with diﬀerent particle sizes along with diﬀerent dosages of H2SO4, KMnO4, and NaNO3. The factors af- fecting GO prepared using the Hummers' method were systematically studied via Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Raman spectroscopy, ultraviolet–visible (UV–Vis) spectroscopy, scanning electron microscopy (SEM), dynamic light scattering (DLS), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and atomic force microscopy (AFM). The results provide some theoretical support for the preparation of high- quality GO.

2. Experimental 2.1. Materials

All chemicals were analytical grade and used as received without further puriﬁcation. The chemicals used were as follows: naturalﬂake graphite (99%, Qingdao Huatai Lubricant Sealing S&T Co. Ltd.); concentrated sulfuric acid (H2SO4; 98%, Sinopharm Chemical Reagent Co.

Ltd.); potassium permanganate (KMnO4; A.R., Shanghai Shanpu Chemical Co. Ltd.); sodium nitrate (NaNO3; A.R., Tianjin Kemiou Chemical Reagent Co. Ltd.); and hydrogen peroxide solution (H2O2; 30%, Tianjin Kemiou Chemical Reagent Co. Ltd.).

2.2. Synthesis of GO

GO was prepared by the oxidation of graphite according to the Hummers' method [26]. Typically, H2SO4and NaNO3were placed into a 250-mLﬂask under mechanical stirring in an ice bath followed by the slow addition of 1.0 g of graphite powder and KMnO4. The temperature of the suspension was kept lower than 5 °C. After the system had reacted for 2 h, the temperature of the system was increased to 35 °C for 4 h.

Subsequently, 46 mL of deionized water was added to the reaction system, and the reaction temperature was increased to 95 °C for another 30 min. After decreasing the temperature to 40 °C, 60 mL of deionized water was added slowly followed by 5 mL of 30% H2O2, causing the

color of the solution to change from dark brown to yellow. The solution was centrifuged and washed with 5% HCl aqueous solution to remove metal ions and was washed with distilled water to remove the acid.

Finally, it was freeze dryed to obtained solid GO. The reaction conditions and GO samples are listed inTable 1.

2.3. Characterization

FTIR spectra were recorded on a Vertex V70 Fourier-transform infrared spectrometer (Bruker, Germany) in the range of 400–4000 cm⁻¹ with a resolution of 2 cm⁻¹. The samples were pressed with KBr into pellets before FTIR measurement. XRD was carried out using a D8 Advance X-ray diﬀractometer (Bruker, Germany) with Cu Kαradiation (λ= 0.15418 nm) at a scan rate of 5°/min⁻¹. Raman spectra were recorded using an inVia Raman microscope (Renishaw, UK). A 532-nm- wavelength laser was used for excitation. UV–Vis spectra were collected in the range of 200–500 nm using a Lambda 25 spectrophotometer (PerkinElmer, USA). SEM was carried out using a S-4800ﬁeld-emission scanning electron microscope (Hitachi, Japan). SEM specimens were prepared from ultrasonically dispersed 0.02-mg/mL GO suspensions.

The suspensions were dropped onto pieces of aluminum foil and allowed to dry naturally. The pieces of aluminum foil were then stuck to the aluminum sample stage using conductive paste and sprayed with gold. DLS data was acquired with Zetasizer Nano-ZS90 (Malvern, UK) to measure the lateral size of GO. XPS was carried out using an AXIS Supra XPS spectrometer (Shimadzu, Japan) with an Al Kα(1487 eV) X- ray source. The thermal stability of GO was evaluated via a STA 449F3- 1053-M TGA (Netzsch, Germany) under nitrogen atmosphere at a heating rate of 10 °C/min over a temperature range of 30–800 °C. AFM images of GO sheets were obtained using a SPI3800 N/SPN400 scanning probe microscope (Seiko, Japan). The samples used for AFM characterization were deposited on monocrystalline silicon wafers.

3. Results and discussion 3.1. FTIR analysis

Fig. 1shows the FTIR spectra of all of the prepared GO samples. The deformation vibration peaks of GO prepared using the lower doses of intercalator and oxidant near 1384 cm⁻¹were very weak (Fig. 1(A)). In addition, the–C]O stretching vibration peaks near 1730 cm⁻¹and the –C–O stretching vibration peak near 1408 cm⁻¹were absent. The peaks at 1081, 3445, and 1632 cm⁻¹belong to the absorption of C–O–C and the stretching and bending vibrations of–OH, respectively. These data indicate that at the low doses of intercalator and oxidant, the main oxygen-containing functional groups in GO were hydroxyl and epoxy groups, and only a small amount of carboxyl groups was present, regardless of the particle size of raw graphite and the amount of NaNO3. The strong absorption peak at 3435 cm⁻¹ belongs to the stretching vibration of–OH, the peak at 1734 cm⁻¹belongs to the stretching vibration of C]O, the peak at 1630 cm⁻¹belongs to C]C in the aromatic Table 1

Samples and reaction conditions.

Sample Graphite particle size (mesh)

H2SO4

dosage (mL) NaNO3

dosage (g)

KMnO4

dosage (g)

H0.5(50) 50 23 0.5 3.0

H0.5(325) 325 23 0.5 3.0

H0.5(1200) 1200 23 0.5 3.0

H0(325) 325 23 0 3.0

H2.0(325) 325 23 2.0 3.0

MH0.5(50) 50 46 0.5 6.0

MH0.5(325) 325 46 0.5 6.0

MH0.5(1200) 1200 46 0.5 6.0

MH0(325) 325 46 0 6.0

MH2.0(325) 325 46 2.0 6.0

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ring, the peak at 1408 cm⁻¹belongs to the stretching vibration peak of –COOH, and the peak at 1084 cm⁻¹ belongs to C–O–C. Among the prepared GO samples, the sample prepared from 1200-mesh graphite exhibited the weakest absorption at 1632 cm⁻¹but the strongest absorption at 1084 and 1047 cm⁻¹, indicating that it had the highest oxidation degree. In comparison, GO prepared from 50-mesh graphite using 2.0 g NaNO3 exhibited weak absorption at 1734, 1408, and 1084 cm⁻¹, indicating a low degree of oxidation and few oxygen-containing functional groups.

3.2. XRD analysis

Fig. 2shows the XRD spectra of raw graphite with diﬀerent sizes.

Strong absorption was observed at 2θ= 26.308°, 26.308°, and 26.349°

in the spectra of raw graphite with the three sizes. According to the Bragg equation, the spacing of graphite layers in 50-, 325-, and 1200- mesh graphite were calculated to be 0.3388, 0.3388, and 0.3382 nm, respectively. The absorption peaks of the 50-mesh graphite were much stronger than those of the 325- and 1200-mesh graphite, and the peak was sharper. Compared to 1200-mesh graphite, the absorption peaks of 325-mesh graphite were stronger. These results indicate that the crys- tallinity of graphite decreased with decreasing graphite particle size.

Fig. 3shows the XRD spectra of GO prepared with low doses of intercalator and oxidant.

As shown inFig. 3(A), GO prepared from 50-, 325-, and 1200-mesh graphite exhibited strong absorption at 2θ= 11.099°, 10.608°, and 10.506°, respectively. According to the Bragg equation, the interlayer spacings of the corresponding GO samples were calculated to be 0.7972, 0.8339, and 0.842 nm, respectively. The 325- and 1200-mesh graphite had more edges compared to the 50-mesh graphite. These

edges allowed the intercalator and oxidant to diffuse more easily, resulting in larger interlayer spacing. The interlayer spacing of GO prepared from 325-mesh graphite was much larger than that of GO prepared from 50-mesh graphite, while the interlayer spacing of GO prepared from 1200-mesh graphite is not significantly larger than that of GO prepared from 325 mesh graphite. Therefore, in terms of particle size, 325-mesh graphite was the best choice for improving the degree of intercalation and oxidation. As shown inFig. 3(B), GO prepared from 325-mesh graphite with 0 and 0.5 g NaNO3showed strong absorption at 2θ= 10.629° and 10.608°, corresponding to interlayer spacing of 0.8323 and 0.8339 nm, respectively. This indicates that using no NaNO3or a small amount of NaNO3had little effect on the interlayer spacing of GO. When the dosage of NaNO3was 2.0 g, the resulting GO showed strong absorption at 2θ= 10.178°, corresponding to an interlayer spacing of 0.8691 nm and a half-peak width of 0.712°. These results indicate that graphite only exhibited obvious intercalation ex- pansion when a sufficient amount of NaNO3was used. Owing to the synergistic action of NaNO3and H2SO4, the oxidant easily diffused to the graphite layer for oxidation, leading to a greater degree of oxidation.

Fig. 4shows the XRD spectra of GO prepared with high dosages of intercalator and oxidant. As shown inFig. 4(A), GO prepared from 50-, 325-, and 1200-mesh graphite exhibited strong absorption at 2θ= 10.6°, 11.6°, and 10.1°, respectively, corresponding to interlayer spacing of 0.8300, 0.7601, and 0.8753 nm, respectively. As shown in Fig. 4(B), the absorption peaks of GO prepared with 0, 0.5, and 2.0 g NaNO3were located at 2θ= 11.7°, 11.6°, and 11.7°, respectively, corresponding to interlayer spacing of 0.7562, 0.7601, and 0.7548 nm, respectively. Similar to the GO prepared with lower doses of intercalator and oxidant, the GO prepared from 1200-mesh graphite had a Fig. 1.FTIR spectra of GO samples prepared using (A) low (23 mL H2SO4and 3.0 g KMnO4) and (B) high (46 mL H2SO4and 6.0 g KMnO4) dosages of intercalator/

oxidant.

Fig. 2.XRD spectra of raw graphite: (A) 50-, 325-, and 1200-mesh graphite; and (B) 325- and 1200-mesh graphite in the region of the primary peak.

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large interlayer spacing because the 1200-mesh graphite had many edges, facilitating the diﬀusion of intercalator and oxidant. The GO prepared from 50-mesh graphite was more complicated. On the one hand, 50-mesh graphite has fewer edges than graphite with smaller particle sizes, which is not conducive to the diﬀusion of intercalator and oxidant. On the other hand, when more intercalator and oxidant were used, more intercalator and oxidant were available to oxidize graphite, resulting in a higher interlayer spacing compared to the GO prepared from 325-mesh graphite.

When large doses of intercalator and oxidant were used, the amount of NaNO3 had little effect on the interlayer spacing of GO. This is consistent with the synergistic effect of NaNO3and H2SO4on graphite intercalation and oxidation when lower amounts of intercalator and oxidant were used and suggests that the effect of NaNO3is not obvious when a sufficient amount of H2SO4is used.

In conclusion, when the dosages of intercalator and oxidant were large, the graphite particle size had a greater eﬀect on GO interlayer spacing than NaNO3.

3.3. Raman analysis

Fig. 5shows the Raman spectra of the prepared GO samples. Re- gardless of the dosages of intercalator and oxidant, the Raman spectra of the prepared GO showed typical D and G peaks near 1355 and 1587–1600 cm⁻¹, respectively.

The average grain size in the sp²region of GO was calculated using Formula (1)[27,28]:

= ^×

⎡

⎣

⎤

⎦

L nma( ) [2.4 10− ( ) ]λi ,

ID IG

10 4

(1) whereLais the average grain size,IDandIGare the intensities of the D and G peaks, respectively, and λi is the wavelength of the laser (532 nm). The calculation results are shown inTable 2.

TheID/IGratio of GO can be used to judge the proportion of irre- gular sp³region in graphite sp²region [29]. When GO is prepared with less intercalator and oxidant, it can be seen fromTable 2that theID/IG

of GO prepared from graphite of 50 mesh, 325 mesh and 1200 mesh were 0.8259, 0.8769 and 0.8793 respectively, and the corresponding average grain sizes were 23.28 nm, 21.92 nm and 21.86 nm, respectively. This indicates that small graphite sheets exhibit larger ID/IG

because they have more edges and are easily oxidized to form more oxygen-containing functional groups. TheID/IGof GO prepared with 0 g, 0.5 g and 2.0 g of NaNO3 were 0.8076, 0.8769 and 0.9066, respectively. The corresponding average grain sizes were 23.28 nm, 21.92 nm and 21.20 nm, respectively. It shows that NaNO3plays a synergistic role with H2SO4in graphite oxidation process, which is helpful for graphite intercalation and oxidation.

When larger dosages of intercalator and oxidant were used, theID/IG

values of GO prepared from 50-, 325-, and 1200-mesh graphite were 0.8435, 0.7411, and 0.6644, respectively, and the corresponding average grain sizes were 22.79, 25.94, and 28.94 nm, respectively.

Under these conditions, the larger dosages of intercalator and oxidant resulted in more oxidative damage to graphite; as a result, the structure of the graphite sheets was dominated by disordered regions [30]. The strength of the D-band decreased with decreasing graphite particle size and increasing defect density. TheID/IGvalues of GO prepared using 0, Fig. 3.XRD spectra of GO (23 mL H₂SO₄and 3.0 g KMnO₄) synthesized (A) with 0.5 g NaNO₃from graphite with diﬀerent particle sizes (50, 325, and 1200 mesh) and (B) from 325-mesh graphite with diﬀerent NaNO3dosages (0, 0.5, and 2.0 g).

Fig. 4.XRD spectra of GO (46 mL H₂SO₄and 6.0 g KMnO₄) synthesized (A) with 0.5 g NaNO₃from graphite with diﬀerent particle sizes (50, 325, and 1200 mesh) and (B) from 325-mesh graphite with diﬀerent NaNO3dosages (0, 0.5, and 2.0 g).

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0.5, and 2.0 g of NaNO3were 0.8162, 0.7411, and 0.7493, respectively, and the corresponding average grain sizes were 23.55, 25.94, and 25.66 nm, respectively. The addition of NaNO3was found to increase the oxidation of graphite owing to its synergistic eﬀect with H2SO4. This resulted in more defects and reduced ID/IG. In addition, the diffusion of oxidant decreases with increasing NaNO3concentration in the system. Thus, the oxidation degree of GO decreased with the use of NaNO3. In this respect, when using large dosages of intercalator and oxidant to prepare GO, if a higher degree of oxidation is required, one may choose not to use NaNO3.

In conclusion, the synergistic eﬀect of NaNO3and H2SO4can improve the oxidation degree of graphite when using low dosages of intercalator and oxidant, which is conducive to the exfoliation of graphite oxide by increasing the interlayer spacing. When more intercalator and oxidant are used, NaNO3is not conducive to the diﬀusion of intercalator and oxidant.

3.4. UV–vis analysis

Fig. 6shows the UV–Vis spectra of the prepared GO samples.

The spectra of all GO samples showed strong absorption peaks at 232 nm (corresponding to theπ–π* transition of C]C) and 300 nm (corresponding to the n–π* transition of C]O).

3.5. SEM analysis

Fig. 7shows the SEM images of GO samples prepared from graphite with diﬀerent particle sizes and dosages of intercalator/oxidant. When 0.5 g NaNO3was used, the size of GO decreased with decreasing graphite particle size regardless of the amounts of intercalator and oxidant was used. The GO prepared using low dosages of intercalator/oxidant was less damaged and larger in size than the GO prepared using high dosages of intercalator/oxidant.

Fig. 8shows the SEM images of GO prepared with diﬀerent dosages

of NaNO3. When the dosages of intercalator and oxidant were small, signiﬁcantly fewer GO tablets were obtained without NaNO3than with NaNO3. The GO tablets obtained using diﬀerent amounts of NaNO3

were similar. When the amounts of intercalator and oxidant were large and NaNO3was not used, graphite was oxidized into ﬁne fragments owing to the strong oxidation of H2SO4. Because of the agglomeration of GO with high speciﬁc surface area, GO sheets were not observed in the SEM images. When more NaNO3was used, the oxidation damage of H2SO4was reduced because of the synergistic intercalation of NaNO3

with H2SO4, and a large number of lamellar structures appeared.

The lateral size of GO was related to the chemical characteristics of GO. Table 3 shows the lateral size and C]O/C–O ratio of GO. The results indicated that the lateral size of GO are consistent with the size of graphite, the dosage of intercalator/oxidant and the C]O/C–O ratio of GO. The all size of GO is smaller than the size of the corresponding graphite due the oxidization and dispersion. Meanwhile the size of GO decrease gradually with the increase of intercalator/oxidant and C]O/

C–O ratio of GO. The reason is that the dosages of intercalator/oxidant increase resulting in increasing the destroying of GO structure and the edge selective oxidation to produce more carboxyl groups, which make the GO have uniform dispersion in the aqueous with fewer layers and smaller size.Table 3shows that the variation of particle size was also consistent with the results of SEM analysis (Fig. 7). This testing particle diameter method assumes that the tested particles are spherical, but for the layered structure of GO, the test results ofTable 3 by DLS can characterize its lateral size at certain degree [22].

3.6. XPS analysis

Fig. 9shows the full-scan XPS spectra of GO prepared from graphite with diﬀerent particle sizes and low dosages of intercalator and oxidant. The C/O ratio of GO prepared from 325-mesh graphite was smaller than that of GO prepared from 50-mesh graphite. In contrast, the C/O ratio of GO prepared from 1200-mesh graphite was basically similar to that of GO prepared from 325-mesh graphite. Theseﬁndings are consistent with the XRD and Raman results.

Fig. 10shows the full-scan XPS spectra of GO prepared with different dosages of NaNO3and low dosages of intercalator and oxidant.

Generally, if GO is prepared successfully, the C/O ratio is between 2.1 and 2.9. As shown inFig. 10, the C/O ratio of GO prepared without NaNO3fell within this range, while the C/O ratios of GO prepared with 0.5 and 2.0 g NaNO3exceeded this range. This indicates that when low dosages of H2SO4and KMnO4were used, it was diﬃcult to suﬃciently intercalate and oxidize the graphite, and typical GO was not successfully prepared. When NaNO3was not used, the GO oxidation degree was higher because no additional H2SO4was consumed in the reaction, resulting in stronger oxidation.

Combined with the FTIR, XRD, and Raman analyses, the XPS results Fig. 5.Raman spectra of GO samples prepared using (A) low (23 mL H₂SO₄and 3.0 g KMnO₄) and (B) high (46 mL H₂SO₄and 6.0 g KMnO₄) dosages of intercalator/

oxidant.

Table 2

Raman data of GO.

Sample ID/IG La(nm)

H0.5(50) 0.8259 23.28

H0.5(325) 0.8769 21.92

H0.5(1200) 0.8793 21.86

H0(325) 0.8076 23.80

H2.0(325) 0.9066 21.20

MH0.5(50) 0.8435 22.79

MH0.5(325) 0.7411 25.94

MH0.5(1200) 0.6644 28.94

MH0(325) 0.8162 23.55

MH2.0(325) 0.7493 25.66

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indicate that the amounts of H2SO4and KMnO4should be increased to successfully prepare GO.

Fig. 11shows the full-scan XPS spectra of GO prepared by graphite with diﬀerent particle sizes under high dosages of intercalator and

oxidant.

ComparingFigs. 11 and 9indicates that the C/O ratio of GO prepared using more intercalator and oxidant decreases than using lower intercalator and oxidant. The C/O ratio of GO prepared from 325-mesh and 1200-mesh graphite were 2.886 and 2.841, respectively, which is lie within the typical range of GO. This indicates that GO with a high degree of oxidation can be prepared by graphite reacting with more active sites in the system when enough intercalator and oxidant was used.

Fig. 12shows the full-scan XPS spectra of GO prepared with different dosages of NaNO3 under high dosages of intercalator and oxidant.

When the dosages of intercalator and oxidant were high, the C/O ratios of GO prepared without NaNO3and 0.5 g NaNO3were 2.724 and 2.886, respectively, which lie within the typical range for GO. The C/O ratio of GO prepared with 2.0 g NaNO3was 3.002, which exceeds the typical range but is lower than that obtained when using lower dosages of intercalator and oxidant. This indicates that the oxidation of the system can be improved by increasing the dosages of H2SO4 and KMnO4, facilitating the preparation of typical GO. The C/O ratios of GO prepared without NaNO3and with 0.5 g NaNO3were relatively small, indicating a relatively high degree of oxidation. In contrast, the C/O ratios of GO prepared using 2.0 g NaNO3and 50-mesh graphite were larger. Using larger amounts of intercalator and oxidant can increase the oxidation degree of GO.

The above results can be explained as follows. (1) The use of NaNO3

consumed part of the H2SO4and reduced its oxidation, making it difficult to improve the oxidation degree of GO. (2) Since 50-mesh graphite has a large particle size and few edges, it was difficult to suffi- ciently intercalate and oxidize the interlayer, even when the dosages of intercalator and oxidant were increased; thus, the degree of oxidation was difficult to improve. (3) Since 325 and 1200-mesh graphite has small particle size and many edges than the 50-mesh graphite, it has more edge active sites, which is favorable for oxidation.

To discuss the chemical nature of GO, relative atomic percent composition and C]O/C–O ratio of GO was given.Table 4shows the analysis result of the high-resolution C1s spectra XPS of GO. The results indicated that the amount of oxygen-containing groups and C]O ratio of GO have a close relationship with the intercalator and oxidant as well as the size of graphite. GO prepared with larger dosages of intercalator and oxidant as well as small sized graphite has more oxygen-containing groups than that with fewer dosages of intercalator and oxidant and large sized graphite. The reason is that greater dosage of intercalator and oxidant can react with more graphite and produce GO with more oxygen-containing groups. Small sized graphite has more granular and more edge as well as surface to react with the oxidant, so it has more oxygen-containing and C]O ratio than that greater sized graphite [31].

Therefore, the result hint that the choice of using large doses Fig. 6.UV–Vis spectra of GO samples prepared using (A) low (23 mL H₂SO₄and 3.0 g KMnO₄) and (B) high (46 mL H₂SO₄and 6.0 g KMnO₄) dosages of intercalator/

oxidant.

Fig. 7.SEM images of GO prepared by graphite with diﬀerent particle sizes and dosages of intercalator/oxidant: (A) H0.5(50), (B) H0.5(325), (C) H0.5(1200), (D) MH0.5(50), (E) MH0.5(325), and (F) MH0.5(1200).

Fig. 8.SEM images of GO prepared from 325-mesh graphite with diﬀerent dosages of NaNO3: (A) H0(325), (B) H0.5(325), (C) H2.0(325), (D) MH0(325), (E) MH0.5(325), and (F) MH2.0(325).

Table 3

Lateral size and C]O/C–O ratio of GO.

Sample lateral size of GO (nm) C=O/C–O (%)

H0.5(50) 1615 31.88

H0.5(325) 1473 38.01

H0.5(1200) 1183 45.07

MH0.5(50) 1221 33.46

MH0.5(325) 1088 42.50

MH0.5(1200) 980 46.43

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intecalator/oxidant and small sized graphite are beneﬁcial to preparation of GO with high oxidation degree.

In conclusion, when the dosages of intercalator and oxidant were large, typical GO with a high oxidation degree was prepared from small size graphite without NaNO3. Increasing the amounts of intercalator and oxidant improved the oxidation degree of GO. The use of excessive NaNO3was not conducive to GO oxidation. GO with a high oxidation degree could be prepared without using NaNO3, even with low dosages of intercalation agent and oxidant.

3.7. TGA analysis

Fig. 13shows the TGA curves of GO prepared with low dosages of intercalator and oxidant. When low dosages of intercalator and oxidant were used, the GO showed rapid weight loss near 200 °C, regardless of the graphite particle size or whether NaNO3was used. This can be at- tributed to the decomposition of unstable oxygen-containing functional groups in the GO structure. The percentages of thermal weight loss of GO prepared from 50-, 325-, and 1200-mesh graphite at 800 °C were 54.64%, 52.35%, and 54.25%, respectively. The percentages of thermal weight loss of GO prepared using 0, 0.5, and 2.0 g NaNO3at 800 °C were 60.08%, 52.35%, and 55.27%, respectively, consistent with the C/

O ratios of the corresponding samples; thus, the cumulative thermal weight loss of GO corresponded to the content of oxygen-containing functional groups in the structure.

Fig. 14shows the TGA curves of GO prepared with high dosages of intercalator and oxidant. The TGA curves of the GO samples prepared from high and low dosages of intercalator and oxidant were similar;

rapid thermal weight loss was observed around 200 °C. The percentages of thermal weight loss of GO prepared from 50-, 325-, and 1200-mesh graphite at 800 °C were 50.62%, 59.15%, and 72.59%, respectively, while those of GO prepared using 0, 0.5, and 2.0 g NaNO3at 800 °C were 61.55%, 59.15%, and 59.15%, respectively.

According to the TGA data, the GO prepared with high doses of intercalator and oxidant had a higher degree of oxidation than GO prepared with low dosages of intercalator/oxidant, resulting in greater thermal weight loss. Among graphite particle sizes, GO prepared from

1200-mesh graphite had the highest rate of thermal weight loss, which related to its high oxygen-containing functional groups. The weight loss rate of GO prepared without NaNO3was higher than that of GO prepared with NaNO3, which is consistent with the XPS results.

3.8. AFM analysis

Fig. 15shows the AFM images of GO prepared from graphite with diﬀerent particle sizes under low dosages of intercalator and oxidant.

When the dosages of intercalator and oxidant were small, GO sheets with thicknesses of 1–3 nm were prepared regardless of graphite particle size, and the number of layers was approximately 1–3. Although the particle size of the graphite raw material used to prepare GO dif- fered, the sizes of the GO fragments after breaking were similar owing to the high ultrasonic power (1800 W) used to prepare samples for AFM.

Fig. 16shows the AFM images of GO prepared with different dosages of NaNO3 under low dosages of intercalator and oxidant. GO sheets with thicknesses of 2–4 nm were prepared with or without NaNO3, and the number of layers was approximately 2–4. The lateral size of GO prepared using 2.0 g NaNO3 was larger than those of GO prepared without NaNO3and with 0.5 g NaNO3. This result confirms that the synergistic effect of NaNO3 and H2SO4facilitated the intercalation and peeling of graphite. In addition, the GO prepared without NaNO3exhibited an uneven lamellar size. In addition to large lamellae, many smaller lamellae were observed. This may be because the concentration of H2SO4was higher when NaNO3was not used compared to when NaNO3was used. Thus, the excessive oxidation of edge graphite led to the breakage of the graphite sheets.

Fig. 17shows the AFM images of GO prepared under high dosages of intercalator and oxidant. When high dosages of intercalator and oxidant were used, GO sheets with thicknesses of 1.75 nm were prepared from 50-mesh graphite, and the number of layers was approximately 2. However, the lateral size of the GO sheet was very small owing to the high dosages of intercalator and oxidant, strong oxidation, and high-power ultrasound treatment. GO tablets with thicknesses of 1.75 nm and two layers were prepared when 2.0 g NaNO3was used. It Fig. 9.XPS spectra of GO prepared from graphite with diﬀerent particle sizes under low dosages of intercalator/oxidant: (A) H0.5(50), (B) H0.5(325), and (C) H0.5(1200).

Fig. 10.XPS spectra of GO prepared from graphite with diﬀerent amounts of NaNO₃and low dosages of intercalator/oxidant: (A) H0(325), (B) H0.5(325), and (C) H2.0(325).

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was diﬃcult to observe layer structures in the AMF images of GO prepared from 325- and 1200-mesh graphite either without NaNO3or with 0.5 g NaNO3. This result may be attributable to the excessive use of intercalator and oxidant and resulting in the strong oxidation of

graphite to form GO with small lateral size. In addition, the high power of the ultrasound (1800 W) used to prepare the AFM samples caused the GO sheets to be very small, resulting in the formation of spherical particles.

4. Conclusion

In this paper, a series of GO samples were prepared via Hummers' method from graphite with different particle sizes and different dosages of H2SO4, KMnO4, and NaNO3. The factors influencing the prepared GO were systematically studied using FTIR, XRD, Raman, UV–Vis, SEM, DLS, XPS, TGA, and AFM analyses. The results can be summarized as follows:

(1) Under low dosages of intercalator and oxidant, GO with a thickness of 1–4 nm was prepared. The main oxygen-containing functional groups in the GO structure were hydroxyl and epoxy groups, and a small amount of carboxyl groups was also detected. When NaNO3

Fig. 11.XPS spectra of GO prepared from graphite with diﬀerent particle sizes under high dosages of intercalator/oxidant: (A) MH0.5(50), (B) MH0.5(325), and (C) MH0.5(1200).

Fig. 12.XPS spectra of GO prepared from graphite with diﬀerent amounts of NaNO₃under high dosages of intercalator and oxidant: (A) MH0(325), (B) MH0.5(325), and (C) MH2.0(325).

Table 4

Relative atomic percent composition and C]O/C–O ratio of GO.

Sample Atomic ratio (%) C=O/C–O (%)

C–C &

C=C

C–O &

C–O–C

C=O &

O=C–O/ >

H0.5(50) 67.9 24.34 7.76 31.88

H0.5(325) 57.81 30.57 11.62 38.01

H0.5(1200) 58.06 28.91 13.03 45.07

MH0.5(50) 53.53 34.82 11.65 33.46

MH0.5(325) 50.61 34.66 14.73 42.50

MH0.5(1200) 50.11 34.07 15.82 46.43

Fig. 13.TGA curves of GO (23 mL H₂SO₄and 3.0 g KMnO₄): (A) 0.5 g NaNO₃and diﬀerent graphite particle sizes; and (B) 325-mesh graphite and diﬀerent amounts of NaNO3.

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was not used, GO with a high degree of oxidation was prepared, even with low dosages of intercalator and oxidant. NaNO3had a synergistic eﬀect with H2SO4in the graphite oxidation process. The lateral size of GO prepared with 2.0 g NaNO3was larger than those of GO prepared without NaNO3and with 0.5 g NaNO3.

(2) When high dosages of intercalator and oxidant were used, the oxidation degree of GO was higher than when low dosages of intercalator and oxidant were applied. Furthermore, the oxidation degree of GO prepared without NaNO3was higher than that of GO prepared with NaNO3. The use of NaNO3was not conducive to the diﬀusion of intercalator and oxidant. The eﬀect of graphite particle size on GO interlayer spacing was greater than that of NaNO3. (3) Increasing the amounts of intercalator and oxidant improved the

degree of graphite oxidation and thus facilitated the preparation of

typical GO. The use of excessive NaNO3was not conducive to improving GO oxidation.

Declaration of interests

The authors declare that they have no known competingﬁnancial interests or personal relationships that could have appeared to inﬂu- ence the work reported in this paper.

Acknowledgements

This work was supported by National Natural Science Foundation of China((Grant No. 21276152), Innovational Industrialization Foundation of Shaanxi Province of China(Grant No. 2016KTCL01-14).

Fig. 14.TGA curves of GO (46 mL H2SO4and 6.0 g KMnO4): (A) 0.5 g NaNO3and diﬀerent graphite particle sizes; and (B) 325-mesh graphite and diﬀerent amounts of NaNO3.

Fig. 15.AFM images of GO (23 mL H₂SO₄and 3.0 g KMnO₄) prepared from graphite with diﬀerent particle sizes and 0.5 g of NaNO₃: (A) H0.5(50), (B) H0.5(325), and (C) H0.5(1200).

Fig. 16.AFM images of GO (23 mL H2SO4and 3.0 g KMnO4) prepared from 325-mesh graphite with diﬀerent amounts of NaNO3: (A) H0(325), (B) H0.5(325), and (C) H2.0(325).

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