This work was supported by the National Nature Science Foundation of China (Grant&nbsp;No. 21776095), the Guangzhou Science and Technology Key Program (Grant&nbsp;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.

1. Exfoliation of graphite in a liquid phase
<ol>
	<li>Preparation of reagents
	<ol>
		<li>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.</li>
		<li>Gently swirl the flask until the PVA fully dissolves. 
		CAUTION: PVA is harmful to humans; thus, protective gloves and surgical&#160;masks should be used.</li>
		<li>Add 50 ...

<ol>
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J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Controllable+size-selective+method+to+prepare+graphene+quantum+dots+from+graphene+oxide.">Controllable size-selective method to prepare graphene quantum dots from graphene oxide.</a> Nanoscale Research Letters. 10, 1-8 (2015).</li><li>Oikonomou, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Scalable+bottom-up+assembly+of+suspended+carbon+nanotube+and+graphene+devices+by+dielectrophoresis.">Scalable bottom-up assembly of suspended carbon nanotube and graphene devices by dielectrophoresis.</a> Physica Status Solidi-Rapid Research Letters. 9 (9), 539-543 (2015).</li><li>Liu, Y., Zhang, D., Pang, S. W., Liu, Y. Y., Shang, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Size+separation+of+graphene+oxide+using+preparative+free-flow+electrophoresis.">Size separation of graphene oxide using preparative free-flow electrophoresis.</a> Journal of Separation Science. 38 (1), 157-163 (2015).</li><li>Cui, C. N., Huang, J. T., Huang, J. H., Chen, G. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Size+separation+of+mechanically+exfoliated+graphene+sheets+by+electrophoresis.">Size separation of mechanically exfoliated graphene sheets by electrophoresis.</a> Electrochimica Acta. 258, 793-799 (2017).</li><li>Sun, Z. Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-yield+exfoliation+of+graphite+in+acrylate+polymers:+A+stable+few-layer+graphene+nanofluid+with+enhanced+thermal+conductivity.">High-yield exfoliation of graphite in acrylate polymers: A stable few-layer graphene nanofluid with enhanced thermal conductivity.</a> Carbon. 64, 288-294 (2013).</li><li>Sun, Z. Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Amine-based+solvents+for+exfoliating+graphite+to+graphene+outperform+the+dispersing+capacity+of+N-methyl-pyrrolidone+and+surfactants.">Amine-based solvents for exfoliating graphite to graphene outperform the dispersing capacity of N-methyl-pyrrolidone and surfactants.</a> Chemical Communications. 50 (72), 10382-10385 (2014).</li><li>Du, B. L., Jian, Q. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Size+controllable+synthesis+of+graphene+water+nanofluid+with+enhanced+stability.">Size controllable synthesis of graphene water nanofluid with enhanced stability.</a> Fullerenes Nanotubes and Carbon Nanostructures. 27 (1), 87-96 (2019).</li><li>Tao, H. C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Scalable+exfoliation+and+dispersion+of+two-dimensional+materials+-+an+update.">Scalable exfoliation and dispersion of two-dimensional materials - an update.</a> Physical Chemistry Chemical Physics. 19 (2), 921-960 (2017).</li><li>Phiri, J., Gane, P., Maloney, T. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-concentration+shear-exfoliated+colloidal+dispersion+of+surfactant-polymer-stabilized+few-layer+graphene+sheets.">High-concentration shear-exfoliated colloidal dispersion of surfactant-polymer-stabilized few-layer graphene sheets.</a> Journal of Materials Science. 52 (13), 8321-8337 (2017).</li></ol>

The authors have nothing to disclose.

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 p...

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.

<table>
	<tbody>
		<tr>
			<td>Beaker</td>
			<td>China Jiangsu Mingtai Education Equipments Co., Ltd.</td>
			<td>500 mL</td>
			<td></td>
		</tr>
		<tr>
			<td>Beaker</td>
			<td>China Jiangsu Mingtai Education Equipments Co., Ltd.</td>
			<td>5000 mL</td>
			<td></td>
		</tr>
		<tr>
			<td>Deionized water</td>
			<td>Guangzhou Yafei Water Treatment Equipment Co., Ltd.</td>
			<td></td>
			<td>analytical grade</td>
		</tr>
		<tr>
			<td>Electronic balance</td>
			<td>Shanghai Puchun Co., Ltd.</td>
			<td>JEa10001</td>
			<td></td>
		</tr>
		<tr>
			<td>Filter membrane</td>
			<td>China Tianjin Jinteng Experiment Equipments Co., Ltd.</td>
			<td>0.2 micron</td>
			<td></td>
		</tr>
		<tr>
			<td>Graphite powder</td>
			<td>Tianjin Dengke chemical reagent Co., Ltd.</td>
			<td></td>
			<td>analytical grade</td>
		</tr>
		<tr>
			<td>Hand gloves</td>
			<td>China Jiangsu Mingtai Education Equipments Co., Ltd.</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Laboratory shear mixer</td>
			<td>Shanghai Specimen and Model Factory</td>
			<td>jrj-300</td>
			<td></td>
		</tr>
		<tr>
			<td>Long neck flat bottom flask</td>
			<td>China Jiangsu Mingtai Education Equipments Co., Ltd.</td>
			<td>1000 ml</td>
			<td></td>
		</tr>
		<tr>
			<td>Nanoparticle analyzer</td>
			<td>HORIBA, Ltd.</td>
			<td>SZ-100Z</td>
			<td></td>
		</tr>
		<tr>
			<td>PVA</td>
			<td>Shanghai Yingjia Industrial Development Co., Ltd.</td>
			<td>1788</td>
			<td>analytical grade</td>
		</tr>
		<tr>
			<td>Raman spectrophotometer</td>
			<td>HORIBA, Ltd.</td>
			<td>Horiba LabRam 2</td>
			<td></td>
		</tr>
		<tr>
			<td>Scanning electron microscope</td>
			<td>Zeiss Co., Ltd.</td>
			<td>LEO1530VP</td>
			<td>SEM</td>
		</tr>
		<tr>
			<td>Surgical mask</td>
			<td>China Jiangsu Mingtai Education Equipments Co., Ltd.</td>
			<td>for one-time use</td>
			<td></td>
		</tr>
		<tr>
			<td>Thermal Gravimetric Analyzer</td>
			<td>German NETZSCH Co., Ltd.</td>
			<td>NETZSCH TG 209 F1 Libra</td>
			<td>TGA analysis</td>
		</tr>
		<tr>
			<td>Transmission electron microscope</td>
			<td>Japan Electron Optics Laboratory Co., Ltd.</td>
			<td>JEM-1400plus</td>
			<td>TEM</td>
		</tr>
		<tr>
			<td>UV-Vis spectrophotometer</td>
			<td>Agilent Technologies, Inc.+BB2:B18</td>
			<td>Varian Cary 60</td>
			<td></td>
		</tr>
	</tbody>
</table>
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synthesis of graphene nanofluids with controllable flake size distributions

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.

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...

Watch this Scientific Journal Video about Synthesis of Graphene Nanofluids with Controllable Flake Size Distributions at JoVE.com

Synthesis of Graphene Nanofluids with Controllable Flake Size Distributions

A method for synthesizing graphene nanofluids with controllable flake size distributions is presented.

A method for synthesizing graphene nanofluids with controllable flake size distributions is presented. Graphene nanoflakes can be obtained by the ...

The proposed method employs exfoliation process and centrifugation process to control the lower limits and the upper limits of size distributions of resulting graphinate suspension separately. The main advantage of the proposed method is that the final size distribution is controllable by adjusting the process parameters of the exfoliation step and the centrifugation step. In a dry, clean, flat bottom flask, add 20 grams of PVA, and then add 1000 milliliters of distilled water.Gently swirl the flask until the PVA fully dissolves. Then add 50 grams of graphite powder to the flat bottom flask and gently swirl the flask until the graphite powder fully disperses in the suspension. Transfer 500 milliliters of the resulting suspension to a 500-milliliter 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. Lower the mixing head to its lowest position, 30 millimeters from the base plane. Next, prepare a water bath by filling a 5000-milliliter beaker with room temperature water, and position the 500-milliliter beaker in the bath.Start the mixer, and increase the speed gradually to 4, 500 rpm. Mix at this speed for 120 minutes. Change the water every 30 minutes.Perform this exfoliation step five more times with different duration. The mixing time determines the lower lateral size limit of the graphene nanoflakes. Collect the 500-milliliter generated suspensions after each exfoliation step.Label each suspension with the exfolation time, and centrifuge the collected suspension at 140 times g for 45 minutes. To remove the unexfoliated graphite, use a pipette to collect the top 80%of the supernatant from each centrifuge tube for further centrifugation. Centrifuge the supernatant suspension from the last centrifugation step at 8, 951 times g for 45 minutes.Collect the upper 50%of the supernatant in the centrifuge tube, and label the sample with a number. Then, to recycle the sediment on the bottom of the centrifuge tube, add 50 milliliters of the previously prepared PVA water reagent to the sediment, and shake the tube vigorously by hand until the sediment is well-dispersed in the suspension. Centrifuge the suspension at 8, 951 times g for 45 minutes.Collect the upper 80%for further measurements. Repeat the centrifugation step for the pellet four more times with four different centrifugation speeds. The centrifugation speed determines the upper lateral size limit of the graphene nanoflakes.Now, prepare the ultraviolet visible spectroscopy. With the previously prepared PVA water solution in a dry, clean sample cell, calibrate the ultraviolet visible spectrometer, setting the PVA water concentrations to 0%Then, add the PVA water resuspension after centrifugation to a dry, clean sample cell, with a path length of 10 millimeters, and obtain a readout using the manufacturer's software. Click the Obtain button to get the measurement results graph, and save the results.Next, to determine the graphene weight, vacuum filter the sample suspension using a nylon membrane with a pore size of 0.2 microns. Obtain the membrane film, and wash with approximately 200 milliliters of water into a beaker. Repeat the wash three times, until all the solids are washed away from the membrane.Determine the washed water mass with a high-precision microbalance to obtain the weight of the solids. Again, vacuum filter the water suspensions using a nylon membrane with a pore size of 0.2 microns. Obtain the membrane, and dry it at room temperature for over 12 hours.Subsequently, rinse the film with 200 milliliters of deionized water into a beaker. If the desired concentration is less than the production rate at one milligram per milliliter, add the prepared PVA water solution to obtain the desired concentration. If the desired concentration is higher than 1%dry the deionized water under vacuum in the dryer for 24 hours to obtain the graphene nanosheets.In this protocol, the ultraviolet visible measurement of the various flake size distributions shows the spectra absorbance peak obtained at a wavelength of 270 nanometers, indicating the evidence of the graphene flakes. Suspension with different concentration has different 660-nanometer absorption. The D band and 2D band of the Raman spectroscopy determines the flake thickness of the graphene nanoflakes.The D band of the Raman spectrum, which is related to graphene sp3 carbon atoms, distinguishes between the initial graphite and the graphene nanoflakes. Low D band intensity indicates the defect-free graphene nanosheets. Distinctive size distribution was observed for the resulting suspension, prepared using different centrifugation speeds.Both the transmission electron microscopy and the scanning electron microscopy show that graphene was produced, and the exfoliation was successful. However, the centrifugation step only worked on nanoparticles with mean diameters larger than 1000 nanometers. The relationship between the upper limits of size distributions with centrifugation speed should be pre-determined.Since the size does not influence with the proposed method, the heat transfer efficiency is possible to be manipulated under conditions like some connectivity, convection, and transfer applications at the center. The PVA polymer is harmful to human. Face mask and gloves should be used to protect the operator.

synthesis-graphene-nanofluids-with-controllable-flake-size

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JoVE Journal

Chemistry

6.4K visualizações.  South China University of Technology. O método proposto emprega processo de esfoliação e processo de centrifugação para controlar os limites inferiores e os limites superiores das distribuições de tamanho da suspensão de grafinato resultante separadamente. A principal vantagem do método proposto é que a distribuição do tamanho final é controlável ajustando os parâmetros de processo da etapa de esfoliação e da etapa de centrifugação. Em um frasco de fundo seco, limpo e plano, adicione 20 gramas de PVA e adicione...