Synthesis of Graphene Nanofluids with Controllable Flake Size Distributions
Summary
A method for synthesizing graphene nanofluids with controllable flake size distributions is presented.
Abstract
A method for synthesizing graphene nanofluids with controllable flake size distributions is presented. Graphene nanoflakes can be obtained by the exfoliation of graphite in the liquid phase, and the exfoliation time is used to control the lower limits of the graphene nanoflake size distributions. Centrifugation is successfully used to control the upper limits of the nanoparticle size distributions. The objective of this work is to combine exfoliation and centrifugation to control the graphene nanoflake size distributions in the resulting suspensions.
Introduction
Traditional methods used to synthesize graphene nanofluids often use sonication to disperse graphene powder1 in fluids, and sonication has been proven to change the size distribution of graphene nanoparticles2. Since the thermal conductivity of graphene depends on the flake length3,4, the synthesis of graphene nanofluids with controllable flake size distributions is vital to heat-transfer applications. Controlled centrifugation has been successfully applied to liquid exfoliated graphene dispersions to separate suspensions into fractions with different mean flake sizes5,6. Different terminal velocities used in centrifugation lead to different critical settling particle sizes7. The terminal velocity could be used to eliminate large graphene nanoparticles8.
Recently, size-controllable methods used to synthesize graphene via liquid-phase exfoliation have been introduced to overcome the fundamental problems encountered by conventional methods9,10,11,12,13. Liquid phase exfoliation of graphite has been proven to be an effective way to produce graphene suspensions14,15,16, and the underlying mechanism shows that the process parameters are related to the lower limits of the graphene nanoparticles size distributions. The graphene nanofluids were synthesized by the liquid exfoliation of the graphite with the help of surfactants17.While the lower limits of the graphene nanoparticle size distribution could be controlled by adjusting the parameters during the exfoliation, less attention is paid to the upper limits of the graphene nanoparticle size distribution.
The goal of this work is to develop a protocol that can be used to synthesize graphene nanofluids with controllable flake size distributions. Because exfoliation is responsible only for the lower size limit of the resulting graphene nanoflakes, additional centrifugation is introduced to control the upper size limit of the resulting graphene nanoflakes. However, the proposed method is not specific to graphene and could be appropriate for any other layered compounds that cannot be synthesized using traditional methods.
Protocol
1. Exfoliation of graphite in a liquid phase
- Preparation of reagents
- In a dry clean flat-bottom flask, add 20 g of polyvinyl alcohol (PVA), and then add 1,000 mL of distilled water.
NOTE: If the suspension was not processed to satisfaction, the step could be repeated to obtain an additional suspension. - Gently swirl the flask until the PVA fully dissolves.
CAUTION: PVA is harmful to humans; thus, protective gloves and surgical masks should be used. - Add 50 g of graphite powder to the flat-bottom flask, and gently swirl the flask until the graphite powder fully disperses in the suspension.
- Transfer 500 mL of the resulting suspension to a 500 mL beaker.
- Place the beaker under a shear mixer, positioning the beaker near the center of the mixing vessel to prevent the formation of a vortex.
NOTE: All chemical reagents used are of analytical grade.
- In a dry clean flat-bottom flask, add 20 g of polyvinyl alcohol (PVA), and then add 1,000 mL of distilled water.
- Equipment setup
- Lower the mixing head to its lowest position (30 mm from the base plane).
- Make a water bath by filling a 5,000 mL beaker with room temperature (25 °C) water and position the 500 mL beaker in the bath. Change the water every 30 min.
- Exfoliation
- Start the mixer and increase the speed gradually to 4,500 rpm; mix at this speed for 120 min.
- Perform the exfoliation step five times for five predetermined times: 40 min, 60 min, 80 min, 100 min, and 120 min. The mixing time determines the lower lateral size limit of the graphene nanoflakes.
- Collect the suspensions after each exfoliation step. Each exfoliation step will generate a 500 mL suspension. Label each suspension with the exfoliation time for further treatment.
- Centrifuge the collected suspension at 140 x g for 45 min to remove the unexfoliated graphite.
- Collect the top 80% of the supernatant from each centrifuge tube for an additional centrifugation step.
2. Centrifugation
- Centrifuge the resulting suspension at 8,951 x g for 45 min.
- Collect the upper 50% of the supernatant in the centrifuge tube, and label the sample with a number.
- Recycle the sediment on the bottom of the centrifuge tube from step 2.2. Add the PVA/water reagent prepared in step 1.1.1 to the sediments and shake the tube vigorously by hand until the sediment is well dispersed in the suspension.
- Centrifuge the suspension at 8,951 x g for 45 min; collect the upper 80% for further measurements.
- Repeat the abovementioned centrifugation step four times with four different centrifugation speeds: 5,035 x g, 2,238 x g, 560 x g, and 140 x g. The centrifugation speed determines the upper lateral size limit of the graphene nanoflakes.
NOTE: The protocol can be paused here.
3. Concentration measurements of the resulting nanofluids
- Obtain absorption spectra at a wavelength of 660 nm using ultraviolet-visible (UV-Vis) spectroscopy.
- Use the PVA/water solution prepared in step 1.1.1 to calibrate a UV-Vis spectrometer; set the PVA/water concentrations to 0%.
- Add the PVA/water suspension to a dry clean sample cell with a path length of 10 mm and obtain a readout using the manufacturer’s software. Click the obtain button to get the measurement results graph and save the results.
- Repeat step 3.1.2 for each of the different samples prepared in step 2.5.
NOTE: The sample cell must be cleaned carefully with distilled water and dried before use each time.
- Determine the graphene weight in the resulting suspension.
- Vacuum filter the 100 mL sample suspension using a nylon membrane with a pore size of 0.2 µm.
- Wash the membrane film with approximately 1,000 mL of water; repeat this step three times until all the solids are washed from the membrane.
- Determine the washed water mass with a high-precision microbalance to obtain the weight of the solids in the 100 mL suspension.
NOTE: The weights include both the weight of the graphene nanoflakes and the PVA polymers. - Analyze the water with thermogravimetric analysis (TGA)18 to determine the PVA concentration.
- Calculate the mean extinction coefficient values of the PVA-stabilized system:
where A is the absorbance measured at 660 nm using UV-Vis spectroscopy, and I is the path length travelled by the UV light during the measurement; the relationship between the absorbance A and the graphene concentration CG is linear. The extinction coefficient ε is the slope of the curve plotted for the absorbance A as a function of the graphene concentration CG. When the extinction coefficient ε is determined, CG can be determined by the absorbance A.
4. Adjusting the concentration of resulting nanofluids
- Vacuum-filter the suspensions using a nylon membrane with a pore size of 0.2 µm.
- Dry the membrane at room temperature for over 12 h.
- Subsequently, rinse the film with hot deionized water.
- Dry the deionized water under a vacuum for 24 h to obtain the graphene nanosheets.
NOTE: The production rate of graphene is approximately 1 mg/mL. If the desired concentration is lower than this, then it is easy to obtain it only by adding PVA/water. If the desired concentration is higher than 1%, then the drying process is necessary. Here, we demonstrate a condition with a desired concentration of 2%. - Add the PVA/water solution or graphene nanosheets to adjust the concentration.
- If the desired concentration is less than the production rate, add the PVA/water solution prepared in step 1.1.1 to obtain the desired concentration.
5. Measuring the size distributions with dynamic light scattering
- Turn on the nanoparticle analyzer and adjust the detector to C label. Place the sample suspension on the test panel.
- Open the correlator control window software.
- Click Non-Negative Constrained least square: Multiple Pass in the menu.
- Set the elapsed time to 2 min.
- Select water as the solvent type.
- Change the diameter of the detector to 100 nm.
- Click the test button to obtain the readout and save the results.
- Repeat steps 5.1-5.7 for each of the samples prepared after step 4.
Representative Results
The existence of graphene nanosheets can be validated by various characteristic techniques. Figure 1 shows the results of the UV-Vis measurement for the various flake size distributions produced by the abovementioned protocol. The spectra absorbance peak obtained at a wavelength of 270 nm is evidence of the graphene flakes. Different absorbances correspond to different concentrations. The lowest absorbance observed corresponds to the highest centrifugation speed. The spectra strongly confirm that graphene exists.
The D band and 2D band of the Raman spectroscopy could be used to determine the flake thickness of the graphene nanoflakes. Figure 2 shows the Raman analysis for the resulting nanoflakes. The D-band of the Raman spectrum is related to graphene sp3 carbon atoms that can help to distinguish between the initial graphite and the graphene nanoflakes. Using Raman spectroscopy, it was discovered that the intensities of the D-band peaks increase with increasing centrifugation speed. At the same time, the D-band intensity is low because the graphene nanosheets that are produced could be defect-free.
Dynamic light scattering is often used to investigate the nanoparticle size distributions of the dispersion. During the experiments, more than 3,000 nanoparticles of each sample were scanned to study the size distribution. The D50 saucer diameter was used to represent the mean diameter of the resulting dispersion. Figure 3 shows the size distribution of the resulting suspension prepared using different centrifugation speeds.
A TEM image is one of the most instinctive ways to distinguish the graphene nanosheets and graphite nanostructures. The layer number could be easily determined from the TEM image. Figure 4 shows the transmission electron microscopy (TEM) results for the resulting nanoflakes, clearly showing that graphene is produced. Figure 5 shows the scanning electron microscopy (SEM) results, showing that the exfoliation is successful.
As the resulting graphene dispersion has two clear size distributions, the mean diameter of each size distribution was presented in Figure 6 to show the effect of the centrifugation step. The figure shows that the centrifugation step only worked on nanoparticles with mean diameters larger than 1,000 nm. Figure 6 shows the mean flake sizes of the two peaks present in the size distribution, validating the assumption that centrifugation only affects large flakes.
Figure 1. UV-Vis extinction spectra after centrifugation at different centrifugation speeds.
Please click here to view a larger version of this figure.
Figure 2. Raman spectra of the initial graphite powders and the centrifuged graphene nanoflakes obtained using different centrifugation speeds.
Please click here to view a larger version of this figure.
Figure 3. Size distributions of the resulting suspensions obtained using different centrifugation speeds.
Please click here to view a larger version of this figure.
Figure 4. TEM results for the resulting nanoflakes.
The samples were prepared with 4500 rpm rotor speeds, and the centrifugation speed was 8,951 x g. Please click here to view a larger version of this figure.
Figure 5. SEM results for the exfoliated nanoflakes.
The sample was prepared using an exfoliation time of 60 min and a rotor speed of 4500 rpm. Please click here to view a larger version of this figure.
Figure 6. Mean flake sizes of two peaks in the size distribution.
The size distributions of the resulting suspension show two peaks. The graph shows that centrifugation only works on nanoparticles with mean diameters larger than 1,000 nm. Please click here to view a larger version of this figure.
Discussion
We have proposed a methodology for synthesizing graphene nanofluids with controllable flake size distributions. The method combines two procedures: exfoliation and centrifugation. Exfoliation controls the lower size limit of the nanoparticles, and centrifugation controls the upper size limit of the nanoparticles.
Although we employed liquid-phase exfoliation of graphite to produce graphene nanoparticles, the following modifications to the protocol should be considered. Additional exfoliation parameters (e.g., rotor speed, graphite concentration, and the use of other surfactants) should be considered to obtain the lower size limit of the graphene nanosheets. During centrifugation, the terminal velocity is vital to determine the critical settling particle size, which could be used to control the upper limit of the nanoparticle size distributions. The terminal velocity, which is determined by the centrifugation speed, should be varied with different types of centrifuges. The use of a supercritical liquid, as well as other assistance methods, could be used to boost the efficiency of the proposed method.
The method presented in this work relies on several techniques (e.g., UV-Vis spectroscopy) to measure the concentration, and the flake size was not well controlled. Additionally, the method described in this work will increase the cost of production. Although this method may be sufficient to produce graphene suspensions, the graphene layer could not be controlled to obtain more efficient heat transfer.
The significance of the proposed method is that the flake lengths have a narrow size distribution. Traditional methods, such as sonication, change the size distributions of the graphene nanoflakes. This leads to unknown effects on the use of graphene nanoflakes in heat transfer applications.
As the production technology of graphene via liquid-phase exfoliation rapidly grows, supercritical liquid-phase CO2 and ultrasound could be applied to a shear mixer to help fabricate smaller graphene nanosheets. In addition, this method could also be applied to produce other layered compounds.
Divulgaciones
The authors have nothing to disclose.
Acknowledgements
This work was supported by the National Nature Science Foundation of China (Grant No. 21776095), the Guangzhou Science and Technology Key Program (Grant No. 201804020048), and Guangdong Key Laboratory of Clean Energy Technology (Grant No. 2008A060301002). We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.
Materials
| Beaker | China Jiangsu Mingtai Education Equipments Co., Ltd. | 500 mL | |
| Beaker | China Jiangsu Mingtai Education Equipments Co., Ltd. | 5000 mL | |
| Deionized water | Guangzhou Yafei Water Treatment Equipment Co., Ltd. | analytical grade | |
| Electronic balance | Shanghai Puchun Co., Ltd. | JEa10001 | |
| Filter membrane | China Tianjin Jinteng Experiment Equipments Co., Ltd. | 0.2 micron | |
| Graphite powder | Tianjin Dengke chemical reagent Co., Ltd. | analytical grade | |
| Hand gloves | China Jiangsu Mingtai Education Equipments Co., Ltd. | ||
| Laboratory shear mixer | Shanghai Specimen and Model Factory | jrj-300 | |
| Long neck flat bottom flask | China Jiangsu Mingtai Education Equipments Co., Ltd. | 1000 ml | |
| Nanoparticle analyzer | HORIBA, Ltd. | SZ-100Z | |
| PVA | Shanghai Yingjia Industrial Development Co., Ltd. | 1788 | analytical grade |
| Raman spectrophotometer | HORIBA, Ltd. | Horiba LabRam 2 | |
| Scanning electron microscope | Zeiss Co., Ltd. | LEO1530VP | SEM |
| Surgical mask | China Jiangsu Mingtai Education Equipments Co., Ltd. | for one-time use | |
| Thermal Gravimetric Analyzer | German NETZSCH Co., Ltd. | NETZSCH TG 209 F1 Libra | TGA analysis |
| Transmission electron microscope | Japan Electron Optics Laboratory Co., Ltd. | JEM-1400plus | TEM |
| UV-Vis spectrophotometer | Agilent Technologies, Inc.+BB2:B18 | Varian Cary 60 |
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