Laboratory Experiment pubs.acs.org/jchemeduc
Simple Graphene Synthesis via Chemical Vapor Deposition Robert M. Jacobberger,†,∥ Rushad Machhi,‡,∥ Jennifer Wroblewski,‡,§ Ben Taylor,‡ Anne Lynn Gillian-Daniel,*,‡ and Michael S. Arnold*,† †
Department of Materials Science and Engineering, University of WisconsinMadison, Madison, Wisconsin 53706, United States Interdisciplinary Education Group, Materials Research Science and Engineering Center, University of WisconsinMadison, Madison, Wisconsin 53706, United States § Madison West High School, Madison, Wisconsin 53726, United States ‡
S Supporting Information *
ABSTRACT: Graphene’s unique combination of exceptional mechanical, electronic, and thermal properties makes this material a promising candidate to enable next-generation technologies in a wide range of fields, including electronics, energy, and medicine. However, educational activities involving graphene have been limited due to the high expense and complexity associated with fabricating and characterizing graphene. Here, we demonstrate an economical, safe, and simple technique to synthesize multilayer graphene films via chemical vapor deposition in 30−45 min in a classroom setting. Raman spectroscopy indicates that the graphene is of high quality, scanning electron microscopy shows that the films are continuous over large areas, and oxidation studies demonstrate graphene’s high impermeability. The films are also transferred to insulating, optically transparent substrates, which enables measurement of the high electrical conductivity of graphene and direct visualization of several layers of atoms. This graphene synthesis has been successfully implemented in diverse settings with students ranging in education level from 5th grade to undergraduate. In addition to reinforcing fundamental concepts at the core of chemical education, this experiment introduces students to cutting-edge nanotechnology research. KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Chemical Engineering, Interdisciplinary/Multidisciplinary, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, Catalysis, Materials Science, Nanotechnology, Synthesis
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several peeling iterations, small graphene flakes of varying thickness are obtained. While this method allows students to observe graphene under a microscope, the resulting graphene flakes are on the micrometer-scale, making the material difficult to further characterize. In addition, mechanical exfoliation relies on simple physical separation to fabricate graphene rather than chemical processes, restricting its relevance to chemical education. Recently, Blake et al. reported the synthesis of nanoscale chemically modified graphene sheets as part of an undergraduate nanomaterials course.6 In their procedure, graphite is chemically exfoliated in solution to form graphene oxide and is subsequently thermally reduced to form the nanosheet powder product.6 Alternatively, chemical vapor deposition (CVD) is among the most widely used techniques to synthesize graphene, as well as many other materials.7−10 It is also one of the most promising methods to produce graphene on an industrial-scale due to its ability to yield high-quality, continuous films over large areas that are only limited in extent to the size of the
INTRODUCTION Graphene is a unique two-dimensional carbon allotrope that consists of a hexagonal array of sp2-bonded carbon atoms and can be considered the parent material of all other crystalline sp2 forms of carbon. For example, graphene sheets can be wrapped to form zero-dimensional fullerenes, rolled to form onedimensional carbon nanotubes, or stacked to form threedimensional graphite. In addition to being atomically thin, graphene has exceptionally high charge carrier mobility, thermal conductivity, strength, stiffness, impermeability, and current carrying capacity.1,2 Consequently, graphene has the potential to enable major advancements in a wide range of applications, including electronics, photonics, energy harvesting and storage, composites, sensing, catalysis, and biodevices, and give rise to next-generation technologies.3 While graphene has received immense interest in the scientific research community, educational activities involving graphene have been lacking. Graphene can be isolated in a classroom setting via the mechanical exfoliation of graphite.4,5 In this technique, stacked graphene sheets, which are weakly bonded by van der Waals forces to form graphite, are separated using adhesive tape. After © XXXX American Chemical Society and Division of Chemical Education, Inc.
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DOI: 10.1021/acs.jchemed.5b00126 J. Chem. Educ. XXXX, XXX, XXX−XXX
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substrate on which they are grown.11 During CVD, carbon precursors decompose on a catalyst surface to form hydrocarbon intermediate species, which subsequently react to form graphene layers. Unlike deconstructive top-down methods in which bulk graphite crystals are exfoliated to separate graphene flakes or nanosheets from their parent graphite material,4−6 CVD is a bottom-up chemical approach in which relatively simple molecular precursors are reacted and assembled atomby-atom to create more complex and more ordered graphene crystals. However, graphene synthesis via CVD has only been achieved in a research environment due to the requirement for expensive equipment and materials combined with dangerous and complex procedures. Furthermore, optimization of the growth conditions, such as temperature, time, precursor flux and composition, catalyst composition and geometry, and experimental setup, to obtain high-quality continuous films of graphene can be difficult. Here, we report an inexpensive, safe, and simple procedure to synthesize multilayer graphene films via CVD. Raman spectroscopy indicates that the graphene is of high quality, and scanning electron microscopy shows that the films are continuous over the entire catalyst surface. The graphene films are imaged with optical microscopy, and the excellent diffusion barrier properties of graphene are demonstrated. The films are also transferred to insulating, optically transparent substrates to enable measurement of the high electrical conductivity of graphene and to allow direct visualization of a few layers of atoms. In addition to introducing students to concepts and techniques used in cutting-edge materials science research, this graphene synthesis is governed by several basic chemistry principles that are taught at many levels of chemical education.
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oxidize the Ni foil and decrease the quality and coverage of the resulting graphene film. Nitrogen (N2), which serves as the inert carrier gas, is bubbled through IPA in an Erlenmeyer flask. The resulting gaseous N2/IPA is flowed through the sealed reaction tube and is exhausted into a beaker of water. The system is purged for 5 min with the N2/IPA stream at a flow rate of 3 bubbles/s (∼50 cm3/min). The flow rate is controlled by a gas flow regulator and is measured via counting the bubbles flowing through the IPA precursor and the water exhaust. Igniting a Meker burner and positioning the flame beneath the Ni foil starts the graphene reaction. The flame of the Meker burner reaches 575−625 °C and provides sufficient energy for the graphene reaction to occur. After 5−10 min, the flame is extinguished to stop the reaction. The system is cooled for 5 min before the graphene-coated Ni foil, as shown in Figure 2a, is removed. The experiment, including assembly and synthesis, takes 30−45 min. Complete experimental details are provided in the Supporting Information.
GRAPHENE SYNTHESIS AND CHARACTERIZATION
Graphene Synthesis
To synthesize graphene, isopropyl alcohol (IPA), which acts as the carbon precursor, is decomposed on a nickel (Ni) foil catalyst using a Meker burner. The experimental setup is shown in Figure 1. First, Ni foil is submerged in acetic acid to etch the native oxide from its surface. Then, the Ni foil is placed inside a quartz tube and the system is tightly sealed to ensure that oxygen and water vapor from the air do not enter during growth. Minimizing the concentration of oxygen and water vapor throughout growth is critical because these gases can
Figure 2. Characterization of the graphene-coated Ni foil. (a) Optical image. (b) Raman spectrum showing the G, 2D, and D bands. (c) Scanning electron micrograph. Scale is 5 μm. (d) Greyscale optical micrograph. Scale is 20 μm. The bright and dark lines in panels c and d, respectively, correspond to graphene at the Ni grain boundaries. The continuous multilayer graphene samples in a−d are grown on Ni foil at ∼600 °C for 5−10 min using an IPA flow rate of 3 bubbles/ second.
Graphene Characterization
Raman spectroscopy (Thermo Scientific DXR) is used to confirm the presence and quality of graphene on the Ni foil. Figure 2b shows a Raman spectrum from a representative graphene film, which is characterized by the G, 2D, and D peaks at ∼1580, ∼2700, and ∼1350 cm−1, respectively. The G peak corresponds to the optical E2g phonon mode. Intervalley scattering of optical phonons gives rise to (1) the D peak if accompanied by charge scattering on a defect or (2) the 2D peak if accompanied by emission of a second phonon. Therefore, the G peak and 2D peak are always active, whereas the D peak is only active in the presence of defects. The G and 2D Raman bands at 1581 and 2713 cm−1, respectively, in
Figure 1. Experimental CVD apparatus used to synthesize graphene. During growth, the Ni foil catalyst is positioned inside the quartz tube directly above the Meker burner. Figure 3a provides an optical image of the Ni foil catalyst before growth. B
DOI: 10.1021/acs.jchemed.5b00126 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Figure 2b confirm that the films are graphene.12 The intensity ratio of the 2D peak to the G peak is 1 min results in full graphene coverage (Figure 2), whereas using a growth time 106 Ω), the resistance of the graphene films is orders of magnitude lower (1000−5000 Ω).
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HAZARDS Although this laboratory is safer than typical procedures used to grow graphene in research laboratories, there are associated hazards. Gloves and eye protection should be worn. Material safety data sheets for acetic acid, IPA, and FeCl3 should be consulted before using these chemicals. Care must be taken when using sharp needles to avoid puncturing oneself and the needles must be disposed of properly. The N2 flow must be introduced into the system gradually to avoid ejecting the needles from the quartz tube. Caution should be used when operating the Meker burners, which output a hotter flame than typical Bunsen burners by about 200 °C. The flame should not C
DOI: 10.1021/acs.jchemed.5b00126 J. Chem. Educ. XXXX, XXX, XXX−XXX
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be left unattended and a fire extinguisher should be in close proximity. Glass tubes should not be used to replace quartz tubes because glass is less stable and less resistant to cracking at the temperatures used during synthesis.
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CLASSROOM TESTING This graphene synthesis laboratory was tested in diverse settings to ensure its robustness and reproducibility. To simultaneously accommodate several users, multiple experimental setups were implemented by connecting the quartz tubes in series, in which each tube received the same gas stream (see Supporting Information). Classroom testing was predominately conducted as part of a field trip at the Wisconsin Institutes for Discovery at the University of Wisconsin Madison. The field trip was conducted 8 times for 256 students between 5th and 10th grade. Each laboratory station was shared by groups of 3−5 students. Raman spectroscopy confirmed that graphene was grown on selected samples. Evaluation data from students who attended the field trip and had parental consent were used to refine the protocol and to determine that the learning objectives of the laboratory had been achieved. Prior to performing the experiment, 76% of attendees were unaware of graphene. However, after performing the experiment, 75% of the students accurately described three or more properties or applications of graphene. Furthermore, 90% of students reported that they enjoyed performing the experiment. Classroom testing was also conducted in a general chemistry course at Madison West High School with 41 10th grade students and in a nanochemistry course at Beloit College with 10 undergraduate students. Each experimental setup was shared by groups of 2−4 students. The high school students conducted the Ni corrosion experiment and found that the graphene-coated Ni foils oxidized slower than the bare Ni foils. Raman spectroscopy confirmed that their samples contained continuous multilayer graphene. The undergraduate students transferred the graphene films to PET substrates, which enabled them to directly visualize the graphene layers and to measure the high electrical conductivity of the graphene films.
Figure 4. Side-view schematic of graphene growth on Ni foil. The black, light blue, red, and light gray atoms represent carbon (C), Ni, oxygen (O), and hydrogen (H), respectively. The yellow arrows represent segregation of carbon atoms from the Ni bulk to the Ni surface. The green arrows represent diffusion of carbon on the Ni surface. As the growth progresses, more carbon segregates from the Ni bulk, increasing the graphene layer thickness in the direction of the blue arrow.
more difficult and eventually the number of graphene layers saturates. For growth on catalysts with low carbon solubility, such as copper (Cu), processes 3−5 become negligible; thus, the growth mechanism is limited to the catalyst surface.14 These processes are governed by fundamental chemistry concepts,16−19 such as adsorption, solubility, decomposition, precipitation, oxidation, reaction kinetics and thermodynamics, crystal nucleation and growth, phase diagrams, and vapor pressure, and can be manipulated by changing the CVD conditions to affect the graphene growth rate and coverage. For example, by considering reaction thermodynamics and kinetics, students can predict how changing the growth temperature, precursor flow rate, and precursor concentration will affect the reaction rate. We find that with increasing temperature, increasing flow rate, and increasing precursor composition, the reaction proceeds faster, and consequently, after a given reaction time, the graphene coverage on the Ni surface is higher. Additionally, by comparing the melting point and carbon solubility of various transition metals using phase diagrams, the students can predict favorable catalysts for the graphene CVD reaction. Ni foil is selected as the catalyst due to its high carbon solubility, its stability at the temperatures used for growth, and its high reactivity toward cracking hydrocarbons. We found that Cu does not result in high-quality graphene films and aluminum (Al), which has a low melting point, is not stable using our growth conditions. These results are consistent with previous results in literature; continuous graphene films have only been achieved on Cu catalysts at temperatures
