| Summary: | 碩士 === 中原大學 === 化學工程研究所 === 104 === The aim of this research is to develop simple and green methods to prepare graphene nanosheets via liquid-phase exfoliation of graphite which involves three different methods, such as mechanical grinding, electrochemical exfoliation and ultrasonication. The surface morphologies of as-synthesized graphene nanosheets were analyzed by using Field Emission Scanning Electron Microscopy (FE-SEM), Atomic Force Microscopy (AFM) and Transmittance Electron Microscopy (TEM). Meanwhile, Raman Spectroscopy and Energy-dispersive X-ray spectroscopy (EDS) were used for chemical characterization of the samples and Laser Diffraction Particle Size Analyzer was used to evaluate the lateral size of graphene nanosheets.
In the topic of mechanical grinding method, 8 different surfactants, such as PVP K30, SDS, PVP K90, PVA, CMC, and PEG were individually added into the graphene nanoplatelets (GNPs) solution. Initially, the lateral size of GNPs was 13 µm, the laser particle size analyzer revealed that PVP K90 efficiently breaks down GNPs into graphene due to the high interaction of PVP K90 with the functional groups of graphene, resulting in a smaller particle size of 8 µm. Moreover, the influence of the grinding time, pH value of solvent and GNP/ZrO2 ratio were also studied. The grinding time was varied from 1 hour, 3 hours and 5 hours and yielded lateral sizes of are 9 µm, 7 µm and 8 µm, respectively. Sodium hydroxide and sulfuric acid were used to modulate the pH values of the solutions at 3, 7 and 10. It is believed that in aqueous solution, NaOH is hydrolyzed into OH- which leads to the deoxygenation of GNP. Lastly, GNP/ZrO2 ratio was controlled to 1:20 and 1:40 yielding particle sizes of 7 µm and 8 µm, respectively. After obtaining the optimum parameters, the C/O ratio was calculated to be 93:7. Ratio of the intensity of D band and G band (ID/IG) obtained from the Raman spectra was calculated to be 0.25. According to the AFM, the thickness of graphene was decreased from 20 ~ 60 nm to 12 ~ 14 nm.
In second part, electrochemical exfoliation technique was carried out by tuning different parameters, such as applied current density and amine-based intercalants. The applied current densities were varied from 0.113, 0.198 and 0.288 A∙cm-2. Applying current into the solution promotes exfoliation of graphite flakes into few layer and multi-layer graphene. Using 0.113 A∙cm-2, it was observed that more few-layer graphene were obtained and dispersed on the upper part of the solution. This was confirmed using TEM which showed wrinkled morphology with high transparency which indicate successful production of few layer graphene. ID/IG of the obtained samples were calculated to be 0.117 and a thickness of 0.4 nm was attained which is comparable to the thickness of monolayer graphene. Next, the current density was fixed to 0.113 A∙cm-2 while different amine groups were added to the electrolyte such as NH4HCO2, (NH4)2SO4 and (NH4)2CO3. Since the electrolyte is composed of OH- and SO42- groups, it is expected that these ions would intercalate into the positive electrode (Anode) only. In order to intercalate both electrodes, amine groups were introduced into the solution. NH4+ cations were believed to intercalate into the negative electrode (Cathode) and would increase the yield of graphene flakes.
In final part, liquid-phase exfoliation of graphene were prepared using ultrasonication of graphite paper. In this method, solvents, sonication amplitudes and times were varied to obtain the optimum results. Suspending graphite paper in different solvents, DI water, Ethanol and NMP were investigated and it was found out that NMP has the highest dispersion owing to the comparable surface tension of graphene and NMP. Then, the solution was ultrasonicated with varying amplitudes of 5, 15, 25 and 35 Hz. Ultrasonication breaks down the Van der Waals force of the graphite layer subsequently exfoliation the graphite paper into few-layer graphene. The thickness of graphene layer after ultrasonication with 5 and 35 Hz were 2 ~ 5 and 20 ~ 30 nm, respectively. Subsequently, 5 Hz was used to carry out to identify the influence of time (10, 20 and 40 minutes) on the exfoliation of graphite paper. The UV-visible spectroscopy indicate that 40 minutes yield more graphene concentration, however, it possess a high ID/IG ratio of 0.241. On the other hand, when the sample was sonicated for 20 minutes, relatively high graphene concentration were produced with lower ID/IG ratio of 0.086. Therefore, 20 minutes was considered to yield best results to exfoliate graphite paper. Afterwards, the solution was centrifuged for 10 minutes with varying rotational speed of 1000, 5000 and 10000 r.p.m. in order to change or displace the lateral dimensions of graphene. Rotational speed of 1000 r.p.m., 5000 r.p.m. and 10000 r.p.m. can attain few-layer graphene with thickness of 7 ~ 9, 4 ~ 6 and 2 ~ 5 nm, respectively. Thus, using 10,000 r.p.m. few-layer graphene can be obtained.
Based on the obtained results, few-layer graphene were successfully prepared using facile and simple methods which can be utilized in up-scale industry.
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