Project Report on. Synthesis of Graphene: Electrochemical Exfoliation

Transcription

1 UGC-NRC sponsored undergraduate summer research internship At Institute of Chemical Technology, Mumbai Project Report on Under the guidance of: Dr. Parag R Nemade Faculty of Chemical Engineering Department of Chemical Engineering Institute of Chemical Technology Matunga, Mumbai, India By: Rajat Subhra Ghosh 3 rd Year, Chemical Engineering National Institute of Technolgy Trichy Tamil Nadu

2 Index Acknowledgement 2 Introduction 3 Materials and Methods 5 Results and Discussion 8 Conclusions 12 References 13 1

3 ACKNOWLEDGEMENT A summer project at the Institute of Chemical Technology, Mumbai is a golden opportunity for learning and self-development. I consider myself very lucky and honoured to have so many wonderful people lead me through in completion of this project. I wish to express my indebted gratitude and special thanks to Dr.Parag R. Nemade who in spite of being extraordinarily busy with his work, took time out to hear, guide and keep me on the correct path and allowing me to carry out my project work at their esteemed organization and extending during the training. I express my deepest thanks to all the PhD scholars and others, for their guidance and helping us with my work throughout training period. I choose this moment to acknowledge their contribution. 2

4 1. Introduction: Graphene, has hexagonal lattice consisting of sp 2 carbon, possesses exceptional electronic and mechanical characteristics. The sp 2 hybridization present in grapheme plays a crucial role in imparting the excellent electrical properties. The pi cloud of electron above and below helps in making Graphene highly conductive. As well as monolayer, AB stacked bi-layer and tri-layer Graphene have displayed desirable properties for applications in electronics and composites. The first report of monolayer Graphene by Geim, Novoselov and co-workers in 2004, isolated via the scotch tape method, there have been sustained efforts to find a scalable, high yield and high purity production method for Graphene. There are currently two accepted approaches to the bottom up production of Graphene. The first is via growth from a small molecular carbon source, such as chemical vapor deposition (CVD) on a catalytic metal surface, or via epitaxial growth from silicon carbide (SiC). The advantage of the bottom up approaches is that large areas of Graphene are potentially obtainable. Epitaxial growth offers a limited selection of substrates however, and the Graphene film cannot readily be transferred from the growth substrate. CVD provides advantages in this respect, since recent work has shown success in transferring CVD Graphene to various substrates. CVD Graphene however is rarely defect free, and the quality of the film is inferior to mechanically cleaved Graphene, the electronic quality of Graphene is heavily dependent on the presence of grain boundaries within the films. The comparatively high costs associated with CVD growth of high quality Graphene currently make the route unfeasible for large scale industrial synthesis. The alternative route to Graphene production is via exfoliation of a graphite sources. A scalable analogue of this approach focuses on solution phase exfoliation of graphite, which also allows for chemical functionalization if desired. The solution phase methods generally involve the production of Graphene oxide (GO), either by acid exfoliation such as the Hummers method or by electrochemical oxidative exfoliation of graphite to directly get Graphene. Hummer s method has inherent disadvantages such 1) it is time consuming, the reaction period spreads over a week and later washing off and drying takes much longer time. 2) To produce Graphene the GO produced must be reduced using suitable reducing agents which leads to the wastage of the 3

5 products, in addition to that the Graphene produced is not defect free. (CO linkages present limits the electrical and mechanical properties of Graphene hence formed by the stated method). 3) The formation of toxic and corrosive Sulphuric acid also induces severe environmental concerns limiting the large scale production of Graphene using Hummer s Method. Electrochemical Exfoliation on the other hand as will be found later in the report is quite simple and takes less time to synthesize Graphene. The intercalation of the ions of the electrolyte and then oxidation of the ions to gases helps in exfoliation of Graphite to get Exfoliated Graphene. (In Electrochemical Exfoliation the choice of electrolyte plays a major role in synthesizing Graphene, usage of strong oxidizing agents such as Sulphuric acid synthesizes GO instead of Graphene, so a suitable electrolyte must be chosen that does not form any C-O linkages) The suitable choice of electrolytes paves a way for direction synthesis of Graphene from Graphene, which if needed to be functionalized can be done easily. As compared to Hummer s method the Graphene produced as little or no traces of oxidation and hence retains the high mechanical and electrical properties. Sonication in organic solvents is another plausible route to Graphene production, although its disadvantages include low yields and the large amount of energy associated with any scale-up of the process. Consequently, a scalable non-oxidative method of Graphene production is becoming increasingly desirable. The Graphene as from various papers was found out that it can be definitively characterized using X-Ray Diffraction Technique (XRD), where the parameter found out is interplanar distance is found out (d). The Value of d for Graphene is greater than that of Graphite, in addition to that the <002> peak of Graphite at 26 is shifted towards the left of axis (5(e)). To prove the quality of Graphene produced is defect free i.e.) the absence of C-O linkages FTIR studies helps to do the same. The band at 1576 cm -1 stretching indicates the presence of C=C bond structure and any stretching related to C-O bond is absent in case for Graphene (5(a)). This report presents a cheap and easy method to synthesize Graphene by Electrochemical Exfoliation. The Graphite was intercalated with Hydroxide ions with Graphite rods from Battery Cells. 4

6 2. Materials and methods: The Electrodes used were the Graphite rods present in the dry cell. The rods were carefully removed and later washed with water and kept inside air oven at 100 Celsius for about 8hrs to remove any adsorbed substances. The electrolyte was prepared using commercially available KOH pellets and made up to 1M solution using DI water. Similar procedure was followed for other electrolytes. A beaker made of borosilicate with an effective volume of 100ml was used as batch reactor. Deionized water used throughout to prepare the solutions. 2.1 Electrolytic Exfoliation Experiment The setup is shown as given in the diagram. The electrolytes used were KOH, (NH4)2SO4, Oxalic Acid. And each setup was kept for one hour at 10V. The electrodes used as mentioned were graphite rods from dry cell. After the process the solution was cooled. Fig 2.1 Experimental Setup 5

7 Experimental Setup 2.2. Separation of Graphene from solution The batch where KOH was used as electrolyte, very thin graphite flakes where seen floating as shown in Fig 2.2(a). The Floating flakes are FLG (Few Layer Graphene) and were removed using a spatula and transferred to a petri-dish containing water (Fig 2.2(b)). The water present in the petri-dish was removed with help of syringe and later kept under halogen lamp for rapid drying. The dried FLG were scrapped and collected in a vial. After 8 batches of electrolytic process where KOH was used as electrolyte 25mg sample was synthesized which was later sent for XRD. (a) (b) Fig 2.2 (a) Few Layer of Graphene floating over KOH solution (b) FLG floating transferred to a petri-dish containg DI water 6

8 For batches where other electrolytes were used the solution was passed through filter paper, the residue was dried and sample collected was sent for XRD Reduction of KOH solution after electrolysis The KOH solution after electrolysis and filtration was found to have light brownish color. It was suspected that a small amount of GO must have been produced to impart the district light brown color. So the solution was reduced using Hydrazine Hydrate. 3ml of Hydrazine was added to the 50ml KOH electrolyte filtrate. The mixture was heated to 80 Celsius for 12 hours under constant stirring. 7

9 3. Results and Discussion 3.1. Physical Appearance of Electrode In all cases there was degradation of electrode, the least or no change happened in case for Oxalic acid (1M). For (NH4)2SO4 during the process the Graphite rods were exfoliated but it led to the formation of graphite flakes instead of Graphene. In case of KOH as electrolyte FLGs were formed which was later confirmed using XRD (discussed later). CATHODE ANNODE Fig 3.1(a) Graphite Electrodes after electrolysis The Fig 3.1(a) shows that an exfoliation has taken place at the anode. This is due to the intercalation OH - ions from the aqueous solution in between the layers of Graphite of anode. The exfoliation is due to the oxidation of OH - ions to O2 gas as a result the layers expand to give us Graphene or Exfoliated Graphene. (As depicted in Fig 3.1(b)) The Half Reactions taking place at respective electrodes At Cathode: 2H2O + 2e - H (g) + 2OH - At Annode: 4OH - O2 + 2H 2O + 4e - 8

10 Fig 3.1(b) Mechanism of Exfoliation 9

11 3.2. XRD ANALYSIS The Sample collected from KOH electrolyte batch and Ammonium Sulphate batch and Oxalic acid batch were sent for XRD analysis. From the results only in KOH electrolyte batch showed the formation of Graphene as shown in Fig 3.2. <002> <002> Fig 3.2 XRD of the FLG Sample Name Obs. Max 2-Theta d (Obs. Max) Angstrom Graphene Graphite Graphene Table

12 Table 3.2 shows that the interplanar distance of the sample is greater than that of Graphite which confirms the presence of Graphene. The peaks at and have a hexagonal geometry with <hkl> values as <002> similar to that of graphite Reduction Effect of Hydrazine on the KOH solution Earlier it was suspected that Graphene Oxide might be present in the KOH electrolyte solution so it was reduced using Hydrazine. The solution turned black from light brownish color as shown in Fig 3.3 Fig 3.3 Left: GO reduced using Hydrazine Right: GO present in KOH electrolyte 3.4. FTIR Result This result was done to definitely conclude the absence of any C-O linkages and to show that Graphene produced is bereft of any defect. 11

13 4. Conclusions: In conclusion, Graphene was successfully synthesized by electrochemical exfoliation of graphite employing alkaline electrolyte. The as synthesized Graphene has been characterized using XRD. A feasible mechanism of exfoliation based on polarization of graphitic layers by OH- ions has been suggested. The electrostatic interaction of the graphitic layers with OH- ions which move under the influence of applied electric field and its oxidation to O2 gas rather the expansion due to formation of gas leads to the exfoliation. The present study provides a proficient approach to synthesize cost effective and Graphene. 12

14 5. References (a) Pitamber Mahanandia, Frank Simon, Gert Heinrichb and Karuna Kar Nanda An electrochemical method for the synthesis of few layer Graphene sheets for high temperature Applications Chem. Commun., 2014,50, 4613 (b) Adam J. Cooper, Neil R. Wilson, Ian A. Kinloch, Robert A.W. Dryfe Single stage electrochemical exfoliation methodfor the production of few-layer Graphene via intercalation of tetraalkylammonium cations CA R B O N 6 6 ( ) (c) ZHANG Xiong, ZHANG DaCheng, CHEN Yao, SUN XianZhong & MA YanWei Electrochemical reduction of Graphene oxide films: Preparation, characterization and their electrochemical properties Chinese Science Bulletin (d) Jianguo Song,1 Xinzhi Wang,1 and Chang-Tang Chang Preparation and Characterization of Graphene Oxide Journal of Nanomaterials (e) Khaled Parvez,Zhong-Shuai Wu, Rongjin Li, Xianjie Liu, Robert Graf, Xinliang Feng, and Klaus Müllen Exfoliation of Graphite into Graphene in Aqueous Solutions of Inorganic Salts Journal of American Chemical Society 13