Graphene oxide aerogels constructed using large or small graphene oxide with different electrical, mechanical and adsorbent properties

Abstract

The performances of graphene oxide aerogels (GOA) are studied by constructing them using different lateral sizes of graphene oxide (GO). Gelatinization of GO hydrosol is realized using a cross-linking agent, poly(vinyl alcohol) (PVA), and then solution-freeze-lyophilization is carried out to construct the cross-linking network of GOA. The contact points in GOA constructed using large sized GO (GOA₃₂₅) were fewer than those in the GOA using small sized GO (GOA_10 000). It is the contact points that decide the micro-structure of GOA, and hence the performances. The size effect of the structural units is studied by investigating the electrical conductivity, mechanical strength, and adsorption capacity. The electrical conductivity of GOA₃₂₅ can reach 0.78 S m⁻¹ after carbonization at 800 °C, which was much higher than that of carbonized GOA_10 000 (0.53 S m⁻¹). The compression modulus of GOA₃₂₅ was 0.51 MPa, while the value for GOA_10 000 was 0.04 MPa. The adsorption capacity of GOA₃₂₅ was also higher than that of GOA_10 000 for adsorbing both hydrophilic and oleophilic media.

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1. Introduction

With the boom of carbon-based nano-materials (such as carbon nanotubes, graphene, etc.), carbon aerogels have become popular in the last five years.^1–4 The excellent properties of low density, a 3D network and a porous structure^5–7 have accelerated the development of carbon aerogels in various applications, including fuel storage, catalyst carriers and pollutant adsorption.^8–11 Lower density is the notable characteristic of carbon aerogel, which may become a development trend in future. CNT–graphene aerogel with an ultralow density of 0.16 mg cm⁻³ is constructed using a simple “sol-cryo” method, and it is light enough to stand on a flower.¹² A continuous 3D network is another notable feature that can work as a high conductivity network. Zhai et al. report that multiwall carbon nanotube constructed aerogel has a high electrical conductivity of 3.2 S m⁻¹ and a recoverable compression property.¹³ Moreover, researchers are also attracted by the high specific surface area of carbon aerogel. The surface area of single-wall carbon nanotube constructed aerogel can be as high as 1291 m² g⁻¹ with a density of 7.3 mg cm⁻³.¹⁴

The constitutional units of the newly nano-carbon aerogel are carbon nanotubes or graphene oxide enhanced with some nanowires, molecules or ions, which can form a 3D cross-linked network and retain the excellent properties of nano-carbon materials simultaneously.^15–19 In particular, with the development of graphene, more outstanding features are obtained.^2,20 Graphene hydrogel assembled using a one-step hydrothermal method is reported with a high mechanical property (storage modulus is about 0.45–0.49 MPa), electrical conductivity (0.5 S m⁻¹), and specific capacitance (175 F g⁻¹).³ Highly conductive and mechanically strong graphene aerogel can be composed with the help of L-ascorbic acid.²¹ Worsley et al. presented graphene aerogel with a high electrical conductivity and large surface area constructed using resorcinol and formaldehyde with sodium carbonate as a catalyst.²² Liu et al. synthesized graphene aerogel with a low density (less than 10 mg cm⁻³), and filled the 3D network with paraffin wax, which showed a strong electromagnetic wave absorption ability.²³ Wang et al. prepared graphene aerogel using the gelation of noble-metal nanocrystals (the compression modulus can reach 0.26 MPa), and used it in fixed-bed catalysis for the Heck reaction with a selectivity as high as 92% and a conversion of 100%.⁹

The construction methods and crosslinking mechanisms of graphene aerogel have been researched in detail, while the lateral size effect of graphene is seldom reported.¹ The gelation process occurs when the balance between the van der Waals attraction from the basal plane and the electrostatic repulsion from the functional groups of the GO sheets is broken. The lateral size of GO is critical to the gelation process when considering the force from the basal plane. As Bai et al. reported, the supramolecular interactions between GO sheets can promote the gelation, and the lateral size of GO affected the gelation process. Large sized GO sheets can be easily constructed into stable hydrogels by acidification.²⁴ However, this only refers to the gelation process and neglects research into the macroscopic properties. Cheng et al. produced graphene sponge with large and small graphene oxide sheets using a hydrothermal method with the assistance of thiourea. The graphene sponges show a high adsorption ability. The research studied the relationship between the structure and adsorption performance of the graphene sponges, but there is less comparison between two types of graphene sponge.²⁵ Afterwards, a hydrothermal-soak method was used to construct graphene/PVA hydrogel by Yao et al. and the results show that the small-sized gel (0.75 μm²) shows better mechanical and electrical properties than the large one (12.5 μm²).²⁶ However, the resulting material is hydrogel and the mechanical properties correspond to hydrogel, which is hard to use in practical applications.

In this paper, in order to easily compare and measure the properties of graphene oxide aerogels, we refer to a very simple and mild method to construct the 3D graphene oxide aerogels with different lateral sizes of graphene oxide and crosslinking agent poly(vinyl alcohol) (PVA). Firstly, we chose different sized graphene oxides from the original material graphite directly and it is easy to distinguish the two types of graphene oxide. Secondly, graphene oxide aerogels were constructed using solution-freeze-lyophilization, which is a very simple and mild method. Thirdly, the graphene oxide aerogels can be in any shape and adjusted to size. Then, the constructed 3D graphene oxide aerogel can maintain the structure of graphene oxide. Using the same amounts of crosslinking agents, we discuss the gelation of different meshes of GO below. Both the resultant microstructure and properties are studied based on the size effect on the GO aerogel.

2. Experimental

2.1 Chemicals and reagents

Graphite powders with an average size of 325 mesh and 10 000 mesh were purchased from Sinopharm Chemical Reagent Co. Ltd. KMnO₄, NaNO₃, H₂SO₄ and PVA with an average polymerization degree of 1750 ± 50 were purchased from Tianjin Tianli Chemical Reagents Ltd.

2.2 Preparation of different sized GO

GO was prepared using the modified Hummers method. In this case, graphite powders with average particle sizes of 325 mesh and 10 000 mesh were oxidized using KMnO₄, NaNO₃ and concentrated H₂SO₄.²⁷ After rinsing, ultrasonic exfoliation, and centrifugation, the upper suspension solution was used in the later experiments and the solutions were denoted as GO₃₂₅ and GO_10 000, separately.

2.3 Preparation of the graphene oxide aerogel

PVA was selected to act as the crosslinking and strengthening agent at the same time. Firstly, 11 mg mL⁻¹ PVA solution was prepared by adding a certain amount of PVA into hot deionized water at 95 °C.²⁸ After 3 h of stirring, 8 mg mL⁻¹ GO solution was dripped into the PVA solution in the mass ratio of 1.3 : 1. Then the mixture was stirred with a magnetic stick for 12 h. 10 mL of the mixture solution was put into a PTFE beaker with ultrasonic treatment for 15 min. After that, the mixture was pre-frozen in a refrigerator for 3 h, and then the freeze-drying processing was performed in a freeze drier for 24 h to obtain GO aerogel. The shape of the aerogel can be controlled by the “freeze-modification” method. The frozen sample was modified by a container with the desired shape. Two types of GO solution (derived from 325 and 10 000 mesh graphite) were treated by the above procedure, and the finally obtained aerogels were named GOA₃₂₅ and GOA_10 000, respectively.

2.4 Structure characterization

Two types of GO solution were measured using AFM (atomic force microscopy, SPA-300HV) to identify the lateral size of the GO sheets. The morphology of GOA₃₂₅ and GOA_10 000 was characterized using SEM (scanning electron microscopy, JSM-7001F). The pore size distribution of the GOAs was characterized using Hg penetration (MicroActive AutoPore V 9600) and the BET specific surface area of the GOAs was measured using N₂ adsorption/desorption (BELSORP-max). The thermal stability of the GO/PVA mixture was decided using TGA (Thermogravimetric Analysis, STA409PC) at a heating rate of 10 °C min⁻¹ from room temperature to 700 °C.

2.5 Property characterization

In order to study the size effect on the electrical conductivity of GOAs, the GO aerogel assembled using large or small sized GO was carbonized. The carbonization temperatures were selected as being 250 °C, 400 °C, 600 °C and 800 °C. The sample was heated at the rate of 5 °C min⁻¹ under a gas flow of argon (20 mL min⁻¹), and then remained at the specific temperature for 30 min. Electrical conductivity was tested using an electrochemical workstation (Autolab 302N), in which two wires were drawn from the two ends of the columned aerogel²⁹ and the electrochemical workstation offered the voltage. In the electrical conductivity experiment, we tested carbonized GOAs. A universal testing machine (CMT6503) was used to characterize the mechanical strength of graphene oxide aerogel. The measurement was carried out with a loading speed of 10 mm min⁻¹ until the samples were collapsed. The two samples were cut into a similar shape and size for an accurate comparison. A fixed-sized cylinder was put between two iron plates. The compression strain was maintained as the same while the compression modulus and maximum force were measured. Different solutions, including alcohol, acetone, dimethyl sulfoxide (DMSO), toluene, hexane and pump oil, were used to test the adsorption ability. The mass (m₁) of the original GOA and the mass (m₂) of the GOA after saturated adsorption were recorded. Then the adsorption ratio (Q) was calculated according to the equation: .

3. Results and discussion

3.1 The lateral size of the GO solution

AFM was employed to confirm the lateral size distribution of GO sheets. It can be seen from Fig. 1a and b that the size control is realized by adjusting the size of the graphite. The size distribution is basically in accordance with Gaussian distribution and the average sizes of GO₃₂₅ and GO_10 000 mainly concentrate in 0.6–0.8 μm and 0.2–0.4 μm, respectively. The differences between large and small lateral sized graphene oxide are obvious. The statistical mean size of GO₃₂₅ is around 1 μm (Fig. 1a), while the size of GO_10 000 in Fig. 1b is about 0.38 μm. The graphite powders with average particle sizes (325 and 10 000 mesh) are shown in Fig. S1(a) and (b).† It is confirmed that graphene oxide prepared using graphite with different meshes has a big difference in lateral size.

Fig. 1 The AFM images of GO solution derived from graphite of (a) 325 mesh and (b) 10 000 mesh; size distribution of (c) GO₃₂₅ and (d) GO_10 000.

3.2 The structural analysis

The detailed process of the GO aerogel formation is schematically shown in Fig. 2. Under the freezing conditions, the shape of the mixture was fixed during ice formation. The PVA is attached on the GO sheet and combined the adjacent GO sheets with each other.³⁰ The macro-morphology of the graphene oxide aerogel is displayed in Fig. 3a. Moreover, graphene oxide aerogel in various shapes also can be obtained using this method, as shown in Fig. 3b. The size and the shape of the bulk can be easily controlled by selecting a corresponding container.

Fig. 2 Procedure of aerogel fabrication.

Fig. 3 (a) The digital photos of GOA₃₂₅ (left) and GOA_10 000 (right), (b) GOA in different shapes and the SEM figures of (c) GOA₃₂₅ and (d) GOA_10 000.

The microstructures of GOA₃₂₅ and GOA_10 000 were characterized using SEM. As shown in Fig. 3c and d, GO sheets are cross-linked with each other to form 3D continuous holes after the sublimation of the ice crystals. However, the microstructures of GOA₃₂₅ and GOA_10 000 were different. The cross-linking networks constructed using large sized GO sheets are obvious (Fig. 3c). Meanwhile the pores constructed using GOA_10 000 are different from those of GOA₃₂₅. Some continuous networks are clear, while more contact points can also be observed.

N₂ adsorption and Hg penetration were carried out to study the pore structure of GOA. As shown in Table 1, GOA₃₂₅ and GOA_10 000 have high porosity (∼95%) with low bulk density. Both S_BET and the pore area of GOA₃₂₅ are higher than those of GOA_10 000. The specific surface area of GOAs measured using BET (Fig. S2a†) is very low, which can act as a reference. The corresponding BJH pore size distribution (Fig. S2b†) exhibits a pore structure including mesopores (2–50 nm) and macropores (>50 nm).

Table 1 Results of Hg penetration and N₂ adsorption

Sample	SBET^a (m^2 g^−1)	Pore area^b (m^2 g^−1)	Bulk density^b (mg cm^−3)	Porosity^b (%)
a Measured by N2 adsorption.b Measured by Hg penetration.
GOA325	19.65	26.70	26.3	95.20
GOA10 000	4.48	8.17	24.3	94.92

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The original materials, GO, PVA and GOA, were tested using TGA in an Ar atmosphere (Fig. 4). The residual mass ratio of GO, PVA and GOA is 60.11%, 17.25% and 50.47%, respectively. The main weight loss is around 200 °C for GO, which is attributed to evaporation of “physical” water (30–150 °C) and intermolecular reactions between functional groups (150–400 °C).³¹ The TG curve of PVA shows that the weight loss at about 100 °C is the evaporation of physically absorbed water. The decomposition of PVA begins at 257 °C. The mass loss in the first section is 62.46%, which is attributed to the partial reduction of hydroxyl.²⁶ Then further chain-fracture takes place at 400 °C. The tendency of the TG curve for GOA is similar to that for PVA. For GOA, before 400 °C, the main residual mass results from GO and PVA, while after 400 °C, with the decomposition of PVA, GO becomes the main residual.

Fig. 4 The TG curves of PVA, GO and GOA.

3.3 Electrical conductivity

In order to distinguish the electrical conductivity of GOAs more clearly, we measured the electrical conductivity of carbonized GOAs. The difference in electrical conductivity between GOA₃₂₅ and GOA_10 000 is not obvious below 400 °C, which can be observed in Fig. 5. The electrical conductivity of GOA reduced below 400 °C is primarily restricted by PVA and GO. As reported before, some oxygen containing functional groups still remain during lower temperature reduction.³¹ It is also clearly seen from the analysis of the TG curves that PVA is partly decomposed before 400 °C. After 400 °C, the oxygen-containing functional groups are removed and the sp² hybridized crystallites are repaired gradually with the increase of annealing temperature. As known from the TG curves, most of the PVA is decomposed with defects left after 400 °C. At this stage, the crosslinking effect by PVA is destroyed and GO is further reduced. Therefore, the resistance of GOA is mainly contributed by the contact resistance between the GO sheets. As discussed above, GOA₃₂₅ owns fewer contact joints than GOA_10 000 for the larger lateral size of GO sheets. Hence, the electrical conductivity of GOA₃₂₅ is supposed to be higher than that of GOA_10 000 which is in accordance with Fig. 5.

Fig. 5 The electrical conductivity of the GOAs.

3.4 Mechanical property

As shown in Table 2, under the conditions of almost the same compressive strain, the compression modulus and maximum force of GOA₃₂₅ are larger than those of GOA_10 000. This demonstrates that the aerogel constructed using large sized GO sheets with this method is stronger than that using small sized GO sheets. In the maintaining the structure, the joints of the GO sheets are the weak points, which prevent the expression of the high mechanical strength of graphene itself. More joints in GOA_10 000 decreased its mechanical strength.

Table 2 Mechanical properties of GOA₃₂₅ and GOA_10 000

	Height/mm	Diameter/mm	Compression modulus/MPa	Maximum force/N	Compressive strain/%
GOA325	27.71	19.22	0.51	5.62	10.82
GOA10 000	27.50	19.05	0.04	0.91	10.91

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3.5 Adsorption ability of GOA

According to the structure analyzed above, the GO aerogel in this paper possesses a porous structure with continuous holes. The character of GO contributes to the adsorption properties of the GO aerogel. A series of solvents are considered to test the adsorption properties of GOA₃₂₅ and GOA_10 000. As shown in Fig. 6, the adsorption capacity of GOA₃₂₅ is higher than that of GOA_10 000 for the adsorption of ethanol, acetone, toluene, chloroform, DMSO, and thermal oil. Among the six media, the adsorption capacity of the GO aerogel is the largest for chloroform within the same volume. The adsorption ratio of chloroform is 140.9 g g^−​1 for GOA₃₂₅, while it is 124.5 g g⁻¹ for GOA_10 000. Previous reports have proved that the adsorption of organic solvent is mainly related to the specific surface area.²⁵ As shown in the detailed analysis of BET and Hg penetration above, GOA₃₂₅ has higher specific surface area. So GOA₃₂₅ shows an obvious advantage in the adsorption of organic solvents compared with GOA_10 000.

Fig. 6 The adsorption properties of GOA₃₂₅ and GOA_10 000.

Generally graphene aerogels were mostly prepared using a hydrothermal method, sol–gel method, template method or any other methods previously. In this paper, we combine the sol–gel method and ice template method to prepare graphene oxide aerogels. Different methods correspond to different properties. We provide some related references just for comparison as follows. Graphene sponge constructed with graphene oxide and thiourea using the hydrothermal method can reach 0.014 MPa with a strain of ∼82%, and the adsorption of chloroform can reach 154 g g⁻¹.²⁵ Graphene/PVA gels are prepared using the hydrothermal treatment with different sized graphene oxide followed by soaking with a PVA solution. The electrical conductivity of the large-sized graphene/PVA gel is 0.11 S m⁻¹ and the compressive fracture strength can reach 0.213 MPa.²⁶ Graphene monoliths prepared using graphene oxide and hexane droplets using the hydrothermal method show the results that the compressive modulus can reach 0.016 MPa and the conductivity is 0.48 S m⁻¹.³²

4. Conclusion

In this paper, GOAs are prepared using the freeze-lyophilization method with controlled lateral dimensions of GO sheets as building blocks and the same amount of PVA as a crosslinking agent. The dimension effects on the structure and properties of GOA are studied. Results show that both the electron transfer and torque delivery are restricted by joints. The joints between the GO sheets are the major factor for the differences between GOA₃₂₅ and GOA_10 000. Large sized GO sheets correspond to fewer joints in the aerogel, resulting in a high electrical conductivity and mechanical strength, and vice versa. Moreover, the adsorption capacity is also increased for the large sized GO sheet assembled aerogel. This work enlightens further research on the 3D construction of graphene and its derivatives with desired performances.

Acknowledgements

The authors are grateful to Prof. Qi-Feng Li for his help with the mechanical testing. We gratefully acknowledge the support of this work by the National Nature Science Foundation of China (No. 51402324, 51402325, 51302281), the Innovative Research Fund of ICC-CAS (2012SCXQT03), the Natural Science Foundation of Shanxi Province (2015021077, 2013011012-7), the Innovative Research Fund of Taiyuan Science and Technology Bureau (2012CXJD0510), and the Shanxi Coal Transportation and Sales Group Co. Ltd (2013WT103).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26922a

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