1. Introduction and Purpose
Conclusions
Acknowledgements
GRAPHENE SYNTHESIS VIA CHEMICAL VAPOR DEPOSITION FOR USE IN SUPERCAPACITOR ElECTRODES
Michael Curtin, Jian Liu,William E. Bernier, and Wayne E. Jones, Jr
Department of Chemistry, Binghamton University - State University of New York, NY, 13902
Research Experiences For Undergraduates Poster Session, JULY 31, 2014
References
1 Wonbong Choi, et al., Critical Reviews in Solid State and Materials Sciences, 35:1 (2010) 52-71 2 Chenguang Liu, et al., Nano Letters, 10 (2010) 4863–4868 3 Xuesong Li, et al., Science Express, 324 (2009) 1312 4 A.M. Awad, et al., Applied Surface Science 256 (2010) 4370–4375 5 Ajay Kumar and Chee Huei Lee (2013). Synthesis and Biomedical Applications of Graphene: Present and Future Trends, Advances in Graphene Science, Dr. M. Aliofkhazraei (Ed.), ISBN: 978-953-51-1182-5, InTech, DOI: 10.5772/55728.
Results
Graphene, a mono-layer sheet of carbon atoms oriented in a honeycomb crystal lattice has captured considerable attention due to its exceptional electronic and optoelectronic properties. With its high electrical conductivity, optical transparency, and superb thermal properties, graphene has generated extensive research for use in supercapacitor applications.[1] Supercapacitors are electrical energy storage devices that can store and release energy, quite similar to electrochemical batteries. Unlike batteries, supercapacitors are able to charge and discharge rapidly, generating an extremely large amount of power upon discharge. Batteries however remain to have a higher energy density, allowing for extended storage and discharge of energy.[2] Graphene has the capability of enhancing the energy density of a supercapacitor primarily due to its large surface-to-volume ratio. When paired with the electrodes of a supercapacitor cell, graphene’s surface area and high electron mobility create a tremendous amount of space for efficient electrical charge storage.
According to the Raman spectroscopy analysis, the (2D/G) ratio increases with an increasing flow rate of H2 gas, which indicates mono-layer graphene results from increased H2. Also when shortening the growth time, the (2D/G) ratio increased as well. When running an experiment with a much lower concentration of hydrogen under the same conditions, there was no detection of carbon deposition, concluding the pivotal role of hydrogen involved in the chemical vapor deposition of graphene films.
This work was supported by the Research Experiences for Undergraduates (REU) program of the National Science Foundation under Award Number 1263004. This work could not have been completed without the help of the professional and graduate personnel at the Binghamton University Department of Chemistry. Also special thanks to the CASP Lab at Binghamton University for providing the resources necessary for this work to be completed.
The graphene films were prepared using the chemical vapor deposition (CVD) method. Chemical vapor deposition is an efficient deposition process where chemical precursors are transported in the vapor phase to decompose on a heated substrate to form a film [3]. This growth technique was used to produce high-purity, high-performance solid materials. The target substrate for graphene growth included a thin piece of copper foil. The precursor gases used involved both hydrogen (H2) and methane (CH4). The methane gas provides the carbon source for deposition and the hydrogen supply plays the role of a carrier gas for deposition of carbon. Both gases are carefully controlled using digital monitors and gauges that regulate their flow rate and pressure. The process begins with loading the copper sample into a tube furnace and evacuating any air inside to create a vacuum. The furnace is then heated to approximately 1000 °C while the hydrogen gas enters the chamber. Once the temperature reaches1000 °C the copper is annealed for 15 minutes, after which methane is introduced for a desired period of time.[3] This is the effective growth time for the graphene layer. The furnace is rapidly cooled to room temperature, while the remaining waste gas is evacuated through an external pump.
Diagram 1: Double Layer Supercapacitor with Graphene-Coated Aluminum Electrodes
Experiment A
Results and Discussion
Diagram 2: Chemical Vapor Deposition Process
H2
CH4
Flow Rate Controller
Tube Furnace
Copper
Pump/ Evacuation
Pressure Gauge
Carbon Deposition
Heat Flow
Pressure Gauge
Flow Rate (sccm)
Gas Ratio (H2/CH4)
Growth Time (min)
Hydrogen Supply
Sample #
H2
CH4
1
2
35
0.0571
30
Pure Hydrogen
2
10
35
0.286
30
Pure Hydrogen
3
20
35
0.571
30
Pure Hydrogen
4
2
35
0.0571
15
Pure Hydrogen
9
20
35
0.571
30
5% Hydrogen
Figure 1: Raman spectrum for graphene varying H2 concentration
Figure 2: Raman spectrum for graphene varying growth time
Figure 3: Peak intensity ratios for increasing H2 concentration
Table 1: Growth parameters for CVD trials
Running parallel to the cvd experiment included an exploration into the surface morphology of the target substrate Cu, and potential methods for developing a cleaner and smoother surface. The electrochemical technique of electropolishing was further investigated. This process involves removing metal ion by ion from the surface material, leaving behind a microscopically featureless and smooth surface.
Experiment B
The electropolishing process involves making the workpiece anodic by negatively biasing the two electrodes. When an electric potential is applied, a current will travel through a conductive electrolyte and remove ions from the surface of the metal workpiece. To run a successful electropolish, parameters including current density, current duration, concentration of acidic electrolyte and electrode positioning must be carefully chosen and monitored.[4]
Diagram 3: Electropolishing Mechanism
Figure 6: AFM image of Copper sample surface profile before electropolish
Figure 7: AFM image of Copper sample surface profile after electropolish
Experiment C
Although less studied than the graphene growth process, the transfer of graphene from the Copper substrate to another target substrate for use in supercapacitor electrodes has generated interest and investigation. This tricky and delicate process has the potential for developing convenient and much cheaper supercapacitors on an industrial scale
Diagram 4: Graphene transfer process [5]
Figure 5: Raman spectrum for graphene with a varying H2 supply
Figure 4: Peak intensity ratios for varying growth time
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
H2 =2 CH4 =35
H2 =10 CH4 =35
H2 =20 CH4 =35
Intensity
(2D/G) Ratio
(D/G) Ratio
0
0.2
0.4
0.6
0.8
1
1.2
Growth =30 min
Growth = 15 min
Intensity
(2D/G) Ratio
(D/G) Ratio
-1.00E+02
0.00E+00
1.00E+02
2.00E+02
3.00E+02
4.00E+02
5.00E+02
6.00E+02
1.00E+03
1.50E+03
2.00E+03
2.50E+03
3.00E+03
Raman Intensity
Wavenumber (cm-1)
5% Hydrogen Supply
Pure Hydrogen Supply
According to the AFM images, the the copper sample appears to have an increased surface roughness than before the polish. This could be the result of a very aggressive acid concentration.
-1.00E+02
-5.00E+01
0.00E+00
5.00E+01
1.00E+02
1.50E+02
2.00E+02
1.20E+03
1.70E+03
2.20E+03
2.70E+03
Raman Intensity
Wavenumber (cm-1 )
H2=2 CH4=35
H2=10 CH4=35
H2=20 CH4=35
2D
G
D
-1.00E+02
-5.00E+01
0.00E+00
5.00E+01
1.00E+02
1.50E+02
2.00E+02
1.00E+03
1.50E+03
2.00E+03
2.50E+03
3.00E+03
Raman Intensity
Wavenumber (cm-1 )
Growth Time =30 min
Growth Time = 15 min