Synthesis and physicochemical properties of graphene/ZrO₂ composite aerogels

Abstract

Aerogel materials possess a wide variety of exceptional properties, including a quite low density, high specific surface area, high porosity, etc. Considering that both graphene aerogels and ZrO₂ aerogels have advantages and disadvantages respectively, graphene/ZrO₂ composite aerogels are prepared, by a facile step, to enable them to have low thermal conductivity and to enhance the electronic interaction between the ZrO₂ nanoparticles and graphene sheets. The chemical composition and crystalline structure of the resulting graphene/ZrO₂ composite aerogels, as well as the strong interaction between the graphene sheets and the ZrO₂ nanoparticles, have been disclosed by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and X-ray powder diffraction (XRD). The morphology and hierarchically porous attributes of the resulting graphene/ZrO₂ composite aerogels have been investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen adsorption–desorption tests. The mechanical properties, electrical conductivity, electrochemical properties and thermal conductivity (as well as thermal stability) of the resulting graphene/ZrO₂ composite aerogels have also been revealed in this study.

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1. Introduction

As a new kind of carbon material, graphene showed excellent performance in energy storage, catalysis, polymer composites, electricity, etc., which is mainly attributed to its outstanding properties including a high Young’s modulus, high fracture strength, large thermal conductivity, huge specific surface area, etc.^1–3 Graphene sheets decorated with metal oxide nanoparticles, such as ZnO, Fe₃O₄, TiO₂, MnO₂ and RuO₂, have attracted a great deal of scientific interest not only in basic scientific studies but also for potential applications in fuel cells, solar cells, electronic devices, lithium ion batteries, gas sensors, etc.^4–8 Among the metal oxide nanoparticles, intensive attention should be paid to ZrO₂ with respect to its unique properties including excellent mechanical, electrical, thermal, optical, and stable photochemical properties. However, there have been only a few reports about graphene/ZrO₂ composites with applications in lithium ion batteries, super-capacitors or the detection of organic phosphorus agents so far. Shi et al.⁹ have incorporated ZrO₂ nanoparticles into graphene sheets as a good candidate for use in the anode material of lithium-ion batteries. Das et al.¹⁰ have synthesized graphene/ZrO₂ composites using an in situ hydrothermal method. Du et al.¹¹ have mentioned the preparation, characterization, and electrochemical properties of graphene/ZrO₂ nano-composites and their application in the enrichment and detection of methyl parathion. However none of the mentioned composites have been in the condensed state of an aerogel, and thus have limited applications in many fields due to the quite low specific surface area of these composites.

Aerogel materials possess a wide variety of exceptional properties, including quite low density, high specific surface area, high porosity, etc. Due to the above exceptional properties, aerogels are amongst the best thermal insulation materials and this will likely be a promising insulation media in the future. To develop graphene composites in the form of an aerogel with a low thermal conductivity, combining the graphene with ZrO₂ is a good choice. Tetragonal and cubic ZrO₂ nanoparticles are used in insulation applications such as thermal barrier coatings. Zirconia aerogels are a kind of highly porous three-dimensional (3D) architecture with charming properties including a low ratio of solid volume, and thus possess quite low heat transfer rates. A number of papers on ZrO₂ aerogels, including the design of catalyst supports,^12,13 electrodes in dye-sensitized solar cells¹⁴ and solid oxide fuel cells (SOFCs),¹⁵ have been reported in the past few years. However, quite low electrical conductivities and extremely fragile mechanical properties have limited their applications in many fields. On the other hand, graphene aerogels with huge surface areas, large pore volumes and high conductivity have been successfully synthesized recently. Worsley et al.¹⁶ have presented a unique method for producing ultra-low-density graphene aerogels with high electrical conductivities and large surface areas. An easy method to create graphene aerogels from aqueous gel precursors processed by supercritical CO₂ drying or freeze drying has been reported in our previous work.¹⁷ However the inert nature of the graphene sheets has resulted in graphene aerogels with inactive performances in many application fields. Considering that both graphene aerogels and ZrO₂ aerogels have advantages and disadvantages respectively, the graphene/ZrO₂ composite aerogels are prepared, by a facile step, to enable them to have low thermal conductivity and to enhance the electronic interaction between the ZrO₂ nanoparticles and graphene sheets. By incorporation of ZrO₂ nanoparticles into graphene sheets, the aggregation problem of ZrO₂ nanoparticles could be minimized as well. Therefore, graphene sheets decorated with ZrO₂ nanoparticles combined with the outstanding properties of aerogel materials might result in some particular properties due to the synergetic effect between them.

Differing from all previously reported methods to prepare graphene/ZrO₂ composites, we have developed a facile route to synthesize graphene/ZrO₂ composite aerogels using epichlorohydrin as a proton scavenger to initiate hydrolysis and polycondensation of dichlorooxozirconium (ZrOCl₂·8H₂O) in a graphene oxide/dimethylformamide (GO/DMF) solution to form the gel precursors, and by then employing supercritical drying with CO₂ and carbonization in argon in sequence to obtain the corresponding composite aerogels. The resulting graphene/ZrO₂ composite aerogels show quite low densities (20–70 mg cm⁻³), large specific surface areas (380–490 m² g⁻¹) and rather low thermal conductivities (0.0249–0.0259 W m⁻¹ K⁻¹). The experimental figures also suggest that a strong electronic interaction exists between the ZrO₂ nanoparticles and graphene sheets.

2. Experimental

2.1 Chemical reagents and materials

The graphite powder, K₂S₂O₅, P₂O₅, H₂SO₄, KMnO₄, H₂O₂, DMF, ethanol, etc. were purchased from Beijing Chemical Reagents Company and used without any further purification. The GO used in this work was synthesized according to the procedure reported in our previous study.¹⁷ The high concentration GO/DMF solution was prepared according to a procedure reported elsewhere.¹⁸

2.2 Preparation of the graphene/ZrO₂ composite aerogels

The synthesis of the graphene/ZrO₂ composite aerogels is illustrated in Fig. 1. Briefly, a fixed amount of ZrOCl₂·8H₂O (0.115 g, 0.115 g or 0.043 g) was added respectively to a GO/DMF solution (6 mg ml⁻¹, 12 mg ml⁻¹ and 12 mg ml⁻¹) to form a uniform mixture, and then epichlorohydrin (PPO) was added to the above mixture, with stirring for a short while and then standing still over some time, to form the gel. The resulting graphene/ZrO₂ composite aerogels would be obtained as long as the above composite gels after 3 days of aging were processed by solvent-exchange with ethanol, supercritical fluid drying with CO₂ and then carbonization under 500 °C for 2 h in an Ar atmosphere in sequence. It should be noted that, a lower carbonization temperature could not fully convert the precursors into the final products, and a higher carbonization temperature could reduce the specific surface area of the resulting composite aerogels.

Fig. 1 Schematic illustration for the synthesis of the graphene/ZrO₂ composite aerogels.

2.3 Characterization

The structures and compositions of the as-prepared products were characterized by X-ray powder diffraction (XRD) using a Rigaku Dmax 2200 X-ray diffractometer with Cu K_α radiation (λ = 1.5416 Å). Raman spectra were recorded on a Lab RAM HR800 (Horiba Jobin Yvon) confocal Raman spectrometer with an excitation laser wavelength of 632.8 nm. All samples were deposited on silicon wafers without the use of any solvent. The morphologies of the composites were observed with a scanning electron microscope (SEM, Hitachi S-4800). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) investigations were carried out using a FEI Tecnai 20 microscope. The as-prepared samples were dispersed in deionized water and dropped onto a carbon film supported on a copper grid for the drying process in air. The X-ray photoelectron spectroscopy (XPS) study was performed using a Kratos Axis-ULTRA XPS analyzer. The X-ray source used was a monochromatic Al K_α line (hν = 1486.71 eV) powered with 10 mA and 15 kV. Thermogravimetric analysis (TGA) was performed in air using a Pyris Diamond TG/DTA (PerkinElmer Inc., U.S.A). The samples were heated from room temperature to 900 °C at 10 °C min⁻¹. Specific surface areas were measured at 77 K by Brunauer–Emmett–Teller (BET) nitrogen adsorption–desorption (Shimadzu, Microm eritics ASAP 2010 Instrument), and the pore size distributions were calculated from the desorption branch of the N₂ adsorption isotherm using the Barrett–Joyner–Halenda (BJH) formula. The thermal conductivities of the composite aerogels and graphene aerogel were investigated using a hot wire technique with the Thermal Conductivity Measuring Instrument (TC3010L). The conductivities of the as-prepared composite aerogels and graphene aerogel were characterized by I–V curves (electrochemical workstation). The mechanical properties of the composite aerogels were measured using an Instron 6022 instrument, the stroke was 40% of the sample length, and the velocity was 30% of the sample in length per minute. The ZrO₂/graphene composite aerogels were packed into nickel foam in a sandwich-type manner as the test electrode, and an Ag/AgCl electrode was used as the reference electrode and 1 M KOH was used as the electrolyte. The electrochemical performance was investigated by using the Solartron 1280B electrochemical workstation to carry out cyclic voltammetry (CV) and by using the Arbin cell tester (CT2001A) to carry out galvanostatic charge/discharge tests.

3. Results and discussion

X-ray diffraction (XRD) was employed to determine the crystalline phase of the samples and the XRD patterns of all samples are shown in Fig. 2. After calcination at 500 °C, the graphene/ZrO₂ composite aerogels exhibited a single tetragonal ZrO₂ (t-ZrO₂) structure. Five peaks at 2θ = 30.1, 35.1, 50.2, 59.2 and 74.6° are obvious in the ZrO₂ aerogel and composite aerogels, which are consistent with the (101), (110), (200), (211) and (220) crystalline planes of t-ZrO₂ (JCPDS no. 88-1007). A strong diffraction peak 2θ = 24°, which is ascribed to the (002) diffraction of graphene,^19,20 indicates that the GO was completely reduced to graphene.²¹ There were no obvious peaks corresponding to graphene observed in the graphene/ZrO₂ composite aerogel pattern, which might be attributed to the graphene sheets being evenly dispersed in the composite aerogel owing to the ability of the ZrO₂ nanoparticles to evenly spread among the graphene layers. It also indicates that the growth of the crystalline phase of the ZrO₂ nanoparticles is not influenced by the existence of graphene sheets.

Fig. 2 XRD patterns of ZrO₂ aerogel (a), graphene/ZrO₂ composite aerogels with mass ratios of ZrO₂ to graphene = 3.8 : 1 (b), 1.9 : 1 (c) and 0.7 : 1 (d), and graphene powder (e).

Raman spectroscopy is a powerful tool for characterizing carbonaceous materials, especially the sp² and sp³ hybridized carbon atoms involved in GO and graphene. The graphene powder and graphene/ZrO₂ composite aerogels were investigated by Raman spectroscopy and the results are displayed in Fig. 3a. Both the D and G bands can be observed in the Raman spectra of all the above samples. But the G bands of the graphene/ZrO₂ composite aerogels appear 8 cm⁻¹ blue-shifted in comparison with that of the graphene powder. The results indicate that the graphene/ZrO₂ composite aerogels contain graphene sheets and that the existence of ZrO₂ in the graphene/ZrO₂ composite aerogels can influence the graphene sheets (the existence of an electronic interaction between ZrO₂ and graphene sheets²²). The D band is an indication of the defects and disorder vibrations of the sp³ carbon atoms, and the G band is related to the vibration of sp² carbon atoms in a graphitic 2D hexagonal lattice.²³ Therefore, the intensities of the D and G bands (I_D, I_G) provide the clue to the ordered or disordered crystal structures of graphene sheets.^24–26 The intensity ratios (I_D/I_G) of the composite aerogels after heat treatment at 500 °C increase from 0.96 to 1.17, as the mass ratios of ZrO₂ to graphene decrease from 3.8 : 1 to 0.7 : 1, which are similar to the intensity ratio of the graphene aerogel (I_D/I_G = 1.12) indicating the reduction of graphene oxide to graphene after heat treatment.

Fig. 3 (a) Raman spectra of a graphene aerogel (1) and graphene/ZrO₂ composite aerogels with mass ratios of ZrO₂ to graphene = 3.8 : 1 (2), 1.9 : 1 (3) and 0.7 : 1 (4); (b) TGA curves of the resulting graphene/ZrO₂ composite aerogels with mass ratios of ZrO₂ to graphene = 3.8 : 1 (1), 1.9 : 1 (2) and 0.7 : 1 (3).

The as-prepared graphene/ZrO₂ composite aerogels were analysed by TGA. As is shown in Fig. 3b, the composite aerogels exhibited mainly four stages of weight loss within the temperature range of 25–900 °C. The first stage of weight loss (ca. 2%) up to 100–110 °C can be attributed to the loss of adsorbed water molecules. The second stage from 110 to 300 °C is a ca. 3% weight loss, which is attributed to the removal of chemisorbed water and organics in the mesopores of the composite aerogels, as well as organic functional groups in the graphene sheets located in the composite aerogels. The third weight loss (ca. 3%) occurs at 430–600 °C, and is associated with thermal oxidative decomposition of the graphene sheets and condensation of the ZrO₂ nanoparticle precursors.²⁷ The last stage of weight loss is ca. 7%, which is attributed to further thermal oxidative decomposition of the graphene sheets. From the above results, it is suggested that the graphene/ZrO₂ composite aerogels show good thermal stability.¹⁸

The chemical structures of the GO powder and graphene/ZrO₂ composite aerogels were studied by X-ray photoelectron spectroscopy (XPS) and the plots are displayed in Fig. 4. Fig. 4(a) shows the wide survey XPS spectra of both the graphene/ZrO₂ composite aerogel and GO powder, which reveal the presence of carbon and oxygen elements. In comparison with GO, the Zr peak is only observed for the graphene/ZrO₂ composite aerogel. From Fig. 4(b), the Zr 3d peaks are obviously observed in the graphene/ZrO₂ composite aerogel. The Zr 3d_5/2 and Zr 3d_3/2 peaks have binding energies of 182.5 and 184.9 eV, respectively, which represent the fully oxidized zirconium ions in their Zr⁴⁺ state.^28,29 The C 1s spectrum of the GO powder in Fig. 4(c) shows four functional groups: a non-oxygenated ring C (284.7 eV), C–OH species (286.7 eV), C=O species (288.1 eV) and C=O–OH (289.1 eV).³⁰ Although the C 1s spectrum of the graphene/ZrO₂ composite aerogel shows the same oxygen functionalities as the GO powder, the absorbance peak intensities of the composite aerogel at 286.7 eV (C–OH), 288.1 eV (C=O) and 289.1 eV (C=O–OH) were sharply decreased, and the intensity of the non-oxygenated ring C (284.7 eV) peak increased simultaneously, as shown in Fig. 4(d). The results highlight the fact that most of the oxygenated functional groups are successfully removed after high temperature treatment, suggesting the reduction of graphene oxide to graphene.

Fig. 4 XPS survey profiles of the GO powder and graphene/ZrO₂ composite aerogel (a), Zr 3d XPS pattern of the graphene/ZrO₂ composite aerogel (b), C 1s XPS spectra of the GO powder (c) and graphene/ZrO₂ composite aerogel (d).

The morphology and structural features of the as-prepared graphene/ZrO₂ composite aerogel have been elucidated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 5a presents a representative SEM image of the composite aerogel, revealing a ruffled morphology consisting of a thin wrinkled structure and nanoparticles. It can also be seen that small ZrO₂ nanoparticles are homogeneously distributed on the graphene sheets and between the layers of them. Due to the presence of graphene sheets, a monolithic graphene/ZrO₂ composite aerogel (inset in Fig. 5a) was easily obtained in comparison to a monolithic ZrO₂ aerogel (the latter is too fragile), indicating that the graphene sheets can enhance the mechanical strength of the ZrO₂ aerogel (for more details see ESI: Fig. SI1 and Table SI1†). It can be seen from Fig. 5b that a lot of large open pores are uniformly distributed within the graphene/ZrO₂ composite aerogel. A HRTEM image of the graphene/ZrO₂ composite aerogel is shown in Fig. 5c, showing that the average particle size of the ZrO₂ is less than 10 nm with a lattice spacing of 0.32 nm, corresponding to the d-spacing of the (101) crystal plane of t-ZrO₂. The insets in Fig. 5c and SI2† are the selected area electron diffraction (SAED) patterns of the composite aerogel and pure ZrO₂ aerogel respectively, indicating that the results are consistent with those revealed by XRD. Compared with the composite aerogels, ZrO₂ nanoparticles in the pure ZrO₂ aerogel have aggregated easily, as shown in Fig. S2b–d.† The results suggest that the large surface area of the graphene sheets provides a good support for the ZrO₂ nanoparticles and prevents their aggregation. Fig. 5d is a HRTEM image, also showing that the graphene sheets are uniformly decorated by ZrO₂ nanoparticles. Although the sample was processed with ultrasonic treatment, the ZrO₂ nanoparticles disperse well on the graphene sheets, which indicates the existence of strong interactions between the graphene sheets and ZrO₂ nanoparticles.

Fig. 5 SEM images of the graphene/ZrO₂ composite aerogel with different magnifications (a and b). Inset in (a) is a digital photo of the composite aerogel. TEM images of the graphene/ZrO₂ composite aerogel with different magnifications (c and d). Inset in (c) is the selected area electron diffraction (SAED) pattern of the composite aerogel.

The porous property of the graphene/ZrO₂ composite aerogels is further confirmed by nitrogen sorption investigations. N₂ adsorption–desorption isotherms and pore-size distribution curves of the graphene/ZrO₂ composite aerogels with different mass ratios of ZrO₂ to graphene are presented in Fig. 6. All of the adsorption–desorption curves exhibit type-IV isotherms with a H₃ hysteresis loop, suggesting a characteristic open wedge-shaped mesoporous structure.³¹ The BET surface areas of the graphene/ZrO₂ composite aerogels were in the range 388–490 m² g⁻¹, which was much higher than that of the bare ZrO₂ aerogel.^32,33 A lower mass ratio of ZrO₂ to graphene (e.g., 0.7) would not effectively reduce the aggregation of the graphene sheets, while a higher mass ratio (e.g., 3.8) would not effectively reduce the aggregation of the ZrO₂ nanoparticles, and thus the highest BET surface area would be obtained for the composite aerogel with a mass ratio of ZrO₂ to graphene of 1.9. A high surface area is an essential factor for the low thermal conductivity of the graphene/ZrO₂ composite aerogels. The vast majority of the pores (as shown in Fig. 6b) are in the range of 2–100 nm, which indicates the presence of a relatively well-defined mesoporous structure. The total pore volume is 1.30–1.45 cm³ g⁻¹ for the composite aerogels. The nitrogen sorption data of the graphene/ZrO₂ composite aerogels with quite low apparent densities (20–70 mg cm⁻³) are shown in Table SI2.†

Fig. 6 N₂ adsorption–desorption isotherms (a) and pore-size distribution curves (b) for the graphene/ZrO₂ composite aerogels with different mass ratios of ZrO₂ to graphene.

In order to investigate the heat conduction properties of the graphene/ZrO₂ composite aerogels, thermal conductivity experiments were conducted at room temperature and the results are listed in Table SI3.† ZrO₂ nanoparticles are used in insulation applications such as thermal barrier coatings.³⁴ Fig. 7 illustrates the thermal conductivity of the graphene aerogel and the graphene/ZrO₂ composite aerogels with different mass ratios of ZrO₂ to graphene. The results show that the graphene/ZrO₂ composite aerogels have a lower thermal conductivity than that of the graphene aerogel. At room temperature, the thermal conductivities of the graphene/ZrO₂ composite aerogels with different mass ratios of ZrO₂ to graphene are close to each other. But the thermal conductivity of the graphene aerogel with a higher specific surface area and porosity is about two times larger than those of the graphene/ZrO₂ composite aerogels. It has been pointed out that a number of factors including density, porosity, specific surface area, etc., would effectively have an influence on the thermal conductivity of porous materials. It has been proven that microstructural parameters such as the grain size or porosity of nanoparticles have heavy effects on the thermal conductivity of materials. In particular, a smaller grain size resulting in a higher number of grain boundaries in the heat path can add additional thermal resistance to the polycrystalline solid phase. The presence of small ZrO₂ nanoparticles effectively increases the grain boundaries in the graphene/ZrO₂ composite aerogels in our present work.

Fig. 7 Thermal conductivities of the graphene aerogel and the graphene/ZrO₂ composite aerogels with different mass ratios of ZrO₂ to graphene.

At room temperature, the pure ZrO₂ nanoparticles have an extremely high resistance, namely, ZrO₂ nanoparticles are insulating materials. The presence of reduced graphene oxide in the graphene/ZrO₂ composite aerogels promotes electronic interaction with ZrO₂, which enhances the conductivity of the composite aerogels. The linear cyclic voltammograms of the graphene/ZrO₂ composite aerogels with different mass ratios of ZrO₂ to graphene and the graphene aerogel have been represented in Fig. 8a and SI3.† In comparison with the graphene aerogel, the conductivity of the composite aerogels obviously decreased. Fig. 8b and SI3† show the conductivity of the graphene/ZrO₂ composite aerogels with different mass ratios of ZrO₂ to graphene and the conductivity of the graphene aerogel, respectively. In fact, the experimental observation of the graphene conductivity decreases after ZrO₂ decoration.

Fig. 8 I–V curves (a) and conductivity histograms (b) for the graphene/ZrO₂ composite aerogels with different mass ratios of ZrO₂ to graphene.

To further study the electrochemical energy storage of the composite aerogels, electrochemical investigations have been conducted, as shown in Fig. SI4.† For the composite aerogel with a mass ratio of ZrO₂ to graphene of 0.7, even when the scan rate increases to 100 mV s⁻¹ the cyclic voltammograms (Fig. SI4a†) of the composite aerogel basically remain as rectangle shapes with some deviation at lower potential, implying a quick charge propagation capability of both double layer capacitance and pseudo-capacitance. Meanwhile, the rate performances of the composite aerogels were also evaluated by galvanostatic charge/discharge under an enhanced current density (Fig. SI4b†). The capacitance of the composite aerogel decreases slowly with an increase of the current density, indicating that the composite aerogel shows good rate capability. When the current density is 0.5 A g⁻¹, the specific capacitance of the composite aerogel is 117 F g⁻¹, however when the current density increases to as high as 10 A g⁻¹, the specific capacitance of the composite aerogel still remains at 52 F g⁻¹. Furthermore, the electrochemical impedance spectroscopy (EIS) plots (Fig. SI4c†) of the cell based on the composite aerogel show a typical electric double layer capacitive behaviour. The intercept of the composite aerogel is at 0.471 Ω cm², indicating that the resistance of the cell based on the composite aerogel is not very high. The cycle performance (Fig. SI4d†) of the capacitor based on the graphene/ZrO₂ composite aerogel is relatively stable. However, for the composite aerogels with the higher mass ratios of ZrO₂ to graphene, the electrochemical energy storage performances are not the same as that of the composite aerogel with a mass ratio of ZrO₂ to graphene of 0.7, as shown in Fig. SI5.†

4. Conclusions

A low-cost and simple method was reported for the synthesis of graphene/ZrO₂ composite aerogels, in which the graphene sheets were decorated with ZrO₂ nanoparticles. The resulting composite aerogels possessed a mesoporous structure (pore diameter lies at ca. 13 nm) with low density (20–70 mg cm⁻³) and large specific surface area (380–490 m² g⁻¹). The physicochemical properties of the graphene/ZrO₂ composite aerogels were fully investigated by many experiments, suggesting superior thermal stability, low thermal conductivity at room temperature and an appropriate electrochemical performance.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21373024), the Innovation Program of the Beijing Institute of Technology and the 100 Talents Program of the Chinese Academy of Sciences.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: More SEM and TEM images, nitrogen sorption data, thermal conductivity data, etc. See DOI: 10.1039/c4ra16024j

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