液相脫層製備高品質石墨烯之參數探討與其特性分析

Synthesis and Characterization of High-quality Graphene Nanosheets via Three Different Liquid-phased Exfoliation Processes

范晏寧(Yen-Ning Fan)

指導教授 ： 劉偉仁

中原大學/工學院/化學工程研究所/碩士(2016年)

https://doi.org/10.6840/cycu201600288

摘要

本研究以環保且簡單之液相脫層法製備石墨烯，主要分為三個部分，機械研磨法、電化學脫層法和超音波破碎脫層法。
第一部份為機械研磨法，是利用石墨烯微片作為原料，在溶劑中添加不同之界面活性劑，包括PVP K30、SDS、PVP K90、PVA、CMC、PEG，探討界面活性劑對研磨之效果。初步以雷射粒徑分析其石墨烯微片之片徑，其中以添加PVP K90作為界面活性劑有較好的研磨效果，並且石墨烯懸浮液經過1個月沒有沉降的現象。進一步對研磨時間、溶劑pH值和粉球比各別探討。首先探討研磨時間，分別為1小時、3小時和5小時，並使用雷射粒徑分析初步可判斷，3小時為最佳研磨時間。接著探討溶劑pH值，利用在溶液中添加硫酸或氫氧化鈉，控制其pH值在3、7和10，透過此方式探討在酸性、中性、鹼性其不同環境下之研磨效果，透過分析發現以鹼性環境做研磨其效果較佳。最後探討石墨烯微片和二氧化鋯之粉球比對其研磨之影響，以重量比1：20和1：40進行探討，由雷射粒徑可觀察出以1：20效果較佳。經過以上參數探討找出其最佳條件，再將最佳化研磨之石墨烯進行表面分析 (FE-SEM)、元素分析 (EDS)、拉曼光譜 (Raman spectra)和原子力顯微鏡 (AFM)分析。分析結果為其形貌明顯變薄，且研磨後碳氧比為93：7，其氧含量幾乎沒有增加。而透過拉曼分析，ID/IG為0.25，。厚度也從20 ~ 60 nm，下降至12 ~ 14 nm。
第二部份電化學脫層法製備石墨烯，利用高純度的石墨板進行電化學脫層，藉由不同的電流密度和電解液添加不同銨類等參數去探討電化學脫層石墨烯其形貌和產率。首先分別以三種電流密度作探討，電流密度分別0.113、0.198和0.288 A/cm2，可透過肉眼觀察其上層物產量以電流密度為0.113 A/cm2有最多上層物。接著將石墨烯進行FE-SEM分析，發現電流密度0.113 A/cm2之上層物的品質最佳，因此將其進行TEM和拉曼光譜分析。其樣品在TEM下呈現皺摺狀，且顏色近透明，而透過拉曼分析，ID/IG為0.117，以AFM分析可得到其厚度約為1 nm之少層石墨烯。接著固定使用電流密度0.113 A/cm2進行電化學脫層，將其電解液添加不同銨類，如:硫酸銨、甲酸銨或碳酸銨，希望能透過銨根進入陰極石墨層間輔助電化學脫層，增加石墨烯之產率，並將脫層後上、下層之石墨烯初步利用FE-SEM分析。藉由添加不同的銨類進行電化學脫層，發現透過添加碳酸銨可使其上下層產率提升。
第三部分為超音波破碎脫層法，是利用超音波能量讓石墨紙在液相中進行脫層。首先使用去離子水、酒精和NMP作為溶劑，在相同時間、振幅下，初步利用肉眼觀察，使用NMP能將石墨紙完全脫層，獲得均勻分散之懸浮液，因此選擇此溶劑進行後續實驗。接著藉由控制不同振幅來控制超音波之能量，其振幅分別為5、15、25、35，透過FE-SEM、拉曼光譜、AFM等分析石墨烯其表面形貌、缺陷程度與其厚度。由實驗結果得知，以振幅5進行超音波脫層有較好的脫層效果，進而針對震盪不同時間進行探討。震盪時間為10、20、40分鐘，以紫外/可見光譜 (UV-Vis)和拉曼光譜分析其濃度和缺陷，發現以震盪20分鐘有最佳脫層效率。接著透過離心進而將石墨烯懸浮液進行分層，轉速分別為1000、5000、10000 r.p.m.，離心時間為10分鐘，將其不同大小、不同厚度之石墨烯進一步分離，其中以轉速為10000 r.p.m.離心10分鐘其可獲得厚度為2 ~ 5 nm之少層石墨烯。

關鍵字

石墨烯、液相脫層、機械研磨、電化學脫層、超音波

並列摘要

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.

並列關鍵字

graphene, liquid-phase exfoliation, mechanical grinding, electrochemical exfoliation, ultasonication

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