PROJECT SUMMARY REPORT

Synthesis and Surface Modification of Carbon Nanotubes

Submitted To

The 2012 Academic Year NSF AY-REU ProgramPart of

NSF Type 1 STEP Grant

Sponsored ByThe National Science FoundationGrant ID No.: DUE-0756921

College of Engineering and Applied Science University of Cincinnati

Cincinnati, Ohio

Prepared By

Ryan Niehauser, Senior, Mechanical EngineeringDerek Moon, Sophomore, Environmental Engineering

Report Reviewed By:

________________________

Dr. Vesselin ShanovAssociate Professor

School of Biomedical, Chemical, and Environmental EngineeringUniversity of Cincinnati

September 9 – December 5, 2013

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NSF Type 1 Step GrantGrant No.: DUE-0756921

Synthesis and Surface Modification of Carbon Nanotubes September 9- December 5, 2013

Ryan Niehauser, Senior, Mechanical EngineeringDerek Moon, Sophomore, Environmental Engineering

Goals and Objectives of Project

The main objective of this project was to functionalize carbon nanotube arrays using a plasma torch. Functionalization consists of adding functional groups, like carboxyl groups, to the outer walls of the carbon nanotubes (CNTs). To accomplish this task we used a dry functionalization process using a plasma torch connected to a robotic arm. It was our goal to use this plasma torch, and its large variety of settings, to functionalize small arrays of CNTs.

Before this project was undertaken, not much was known about what settings the plasma torch should use to achieve optimal functionalization of the CNT array. It was another goal of this project to use a variety of settings of the plasma torch to find an optimal setting. To find the optimal setting, the plasma torch was programed to a set power, oxygen flow rate, and helium flow rate. The robotic arm was also programed to move at a specific speed, and move to a specific position. The array would pass under the plasma torch, and a quick look at the effect of the plasma was done with a Raman Spectroscopy Microscope. The Raman was also used to look at the conditions of the CNT array before plasma functionalization. Later in the project, the objective was to verify that the plasma torch is adding functional groups to the arrays using FTIR.

For each plasma application, the objective was to created CNT sheets and threads using the equipment made specifically for the University of Cincinnati Nanoworld Labs. For each sheet the goal was to find the effects on conductivity using the different intensities of plasma. Since threads were quicker to make, they were used to test for the conductivity and tensile strength. Finding a correlation between plasma settings and conductivity or strength of the CNT material was an important objective for this project.

Research Tasks Undertaken

The first task for this research project was to become familiarized with the equipment and literature on carbon nanotubes. During the first few weeks, we conducted a literature review of different documents on carbon nanotubes, and carbon nanotube functionalization. During this time we also met in the lab to receive training on the equipment. Since neither of us were familiar with the equipment, it was important for us to learn as much as we could and take notes.

After learning about carbon nanotube functionalization and the lab equipment, a plan was made to start making CNT sheets after applying plasma to the arrays. We devised a plan to compare a non-functionalized CNT sheet with different functionalized sheets. We would test the tensile strength and conductivity of each sheet, and see the difference between pristine and functionalized. We also wanted to see if a trend existed for the sheets between the different settings of plasma.

Since making the CNT sheets was time consuming, we made a plan to focus on threading. Before starting the functionalize arrays to make threads, we made a task to program the plasma robotic arm. This would give us more control of the position and speed of the array going under the plasma torch. We also wanted to try to get the plasma torch as close as possible to array. Slowing the speed of the array moving through the array was another task. Not only did we want to experiment these new settings to see the effect on the Raman Microscope spectrum, but we also wanted to see if they were still spinnable. After many trials, we made it our task to compare tensile strength and conductivity of functionalized threads with a pristine threads, given the new robotic arm settings.

Training Received in Use of Specialized Equipment and/or Computer

To graph a Raman Spectrometry curve, a Renishaw inVia Raman Microscope was used. The program WiRE 3.3 was run on a connected computer to display the graph produced. Because the Raman

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microscope was an expensive piece of equipment, many precautions were taken. Before the students were allowed to use it, they had to observe their graduate student advisor operate the microscope first. Also, all operators had to sign in on use-log. After about an hour of training, the students were able to operate the microscope and analyze the Raman plots produced.

The Atomflo Atmospheric Plasma made by Surfx Technologies was the atmospheric system used to produce plasma. Because this equipment was very sensitive, the students had to observe their graduate student advisor operate it before they were allow to operate it themselves. The students were able to operate the plasma system after about an hour of training. The training included how to release gases which were held in canisters and how to purge the system before the gasses were used. Also, since this process produced ozone (O3), the fume hood, which held the plasma output devices, needed to be operated correctly as well.

Training was needed to operate the sheeting system, which was just a platform connected to a roller, which was all connected to a computer. The system was designed by a mechanical engineering graduate student. Training was needed to be able to jog the platform back and forth and also to spin the roller a computer program made by the student. After 30 minutes the students were able to roll arrays into sheets.

The machine used to measure tensile strength was called Intron 5948 and the program used to run the machine from a computer was called BlueHill 3. Training was needed to correctly run the program which allowed the user to measure the tensile strength of the material. After 30 minutes of training, the students were able to correctly set up the thread and sheet for measurement and to run the program to test the tensile strength of sheets and threads. A system made by the Nanoworld Labs at UC was used to measure the resistivity of the threads and sheets. Training was required in order to operate the system and also to correctly place the thread or sheet in order to get a correct 4-probe reading. Training was provided by the graduate student advisor and after 30 minutes, the student were able to correctly measure the resistivity of threads and sheets.

The optical microscope used was called the Fisnar F7300N. Training was required to operate the microscope. The students were able to initialize the microscope and observe threads and sheets up to 1000x optical zoom. The program used to view the image was also named Fisnar. After 30 minutes of training the students were able to fully operate the optical microscope.

Methodologies

Many different systems and processes were used for this experimentation. The CNTs used for experimentation were made from a specific formula used by the University of Cincinnati Nano World labs. The arrays were pre-made by the labs for use in plasma functionalization experimentation.

When the arrays were received a Raman spectroscopy from a Renishaw inVia Raman Microscope was performed on one of the arrays in 3 different areas of the array (See Figure 1). To accomplish this, the microscope was used to focus on a specific part of the array. The focus was adjusted in each spot until it was deemed a clear picture by the viewer. The spectrum was plotted for each location on the array, and an average of the values was taken to create a single Raman plot for that array.

Next, the plasma torch and robot were set up for functionalization. For this the oxygen tank pressure was adjusted and regulated to 40psi. The helium tank was also adjusted and regulated, but set to 50psi. With the gas pressures set, the torch was purged by using the purge action on the torch system. After an arbitrary time, the purge system was turned off manually. The power, helium, and oxygen settings were then adjusted on the plasma control system to the level needed for the experiment. The CNT array, while taped to a glass Petri dish, was then placed on the robotic metal plate. Before activating the torch, it was lowered over the metal plate until the Z value was equal to 30.00. The dish was then moved to so the array was centered with the torch. The robotic metal plate was then moved away before the torch was activated.

In this project, there was great use of an atmospheric plasma system. With the CNT array in place, and settings on the torch control system set, the plasma torch was activated. After the automatic purging and tuning of the system, the CNT array was passed under the plasma torch using the robotic job speed of the system. For the double pass experiments, the array was jogged past the torch to a safe distance where the Petri dish was turned 180° by hand. This was done so the part where the array was taped to the silicon disk would pass under the array first. This was done to keep the array from fluttering up closer

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to the plasma. The array was then passed under the torch in the opposite direction using the robotic jog speed again. The plasma torch was then deactivated from the controls.

After functionalization, a Raman spectroscopy was performed on the newly functionalized array. The same Raman process performed for pre-functionalization was repeated for post-functionalization of the array. Raman data was averaged and a single Raman plot was made for the post-functionalization of the CNT array.

Immediately following a Raman study after the plasma application, the array was set up for the sheeting process. The array was placed on the stationary part of the sheeting system with the taped side facing away from the roller. Teflon was then wrapped around the roller on the location for CNT sheeting. Double sided tape was used to keep the Teflon in place. The tape was strategically placed, so the CNT layers would only be exposed to Teflon. The Teflon layer was then cleaned with a paper tissue and acetone. The CNT array was then stretched and stuck to the Teflon on the roller. The layer of CNT that was stretched was carefully worked so it was uniform across the array. The roller was then activated and set to a speed of 18 seconds per revolution. The CNTs then slowly pull from the array and form layers around the roller. The roller was stopped after 24 minutes, which gives 80 layers. Acetone was dripped on the CNTs on the back of the roller every other revolution (See Figure 2).

Making thread requires a different process than making CNT sheets. For the threading process we used a setup made by the Nanoworld labs at UC. The machine had a motor, which drove a belt that rotated and assembly. The assembly contained another smaller shaft that was rotated by a different motor. The smaller shaft pulled the CNTs from the array and wrapped it around a replaceable metal bobbin. While the bobbin was pulling the CNTs from the array, the assembly was rotated by the larger motor. This caused the CNTs to twist into a single thread as they were pulled from the array. Since it was unnecessary to record the amount of thread being collected, the machine was ran until it was assumed there was enough thread to carry out conductivity and tensile testing.

For testing the sheet was cut from the roller after a day of rest. A 4-prong probe was used to test the conductivity of the sheet (See Figure 3). Measurements were taken at different parts of the sheet. A thickness measurement was also taken in several locations of the sheet. After an average thickness was obtained, the sheets were cut into small strips, about 1-2mm x 35mm. These strips were super-glued to strips of paper at each end. After the glue dried, the width of each strip was measured using a Keyence VHX optical microscope (See Figure 4). The strips were then placed on the Instron 5948 for a tensile test. After the strips were in the correct position, the tensile test was performed and a force measurement was obtained. The measured cross sectional area was then used to calculate the tensile strength of the sheet.

For the threads, testing was a little different. For conductivity, threads were placed on a four prong electrical piece. Liquid silver paint was then used to adhere the CNT thread on the four prong piece. The piece was then plugged into the conductivity setup, made by the Nanoworld lab at UC. A LabView program was used to activate the current and voltage source for the experiment. The LabView program also recorded the data points used to calculate the resistance and conductivity of each thread. Before the conductivity could be calculated, the diameter of the thread was measured by an optical microscope.

As for tensile testing, the procedure was much like that of the sheets. A thread holder layout was created in Microsoft Word, and the gauge length was set to 2.5in. Each tab was cut out from the printed layout, and a window was cut into each tab. A length of thread was placed end to end on the tab, and permanent double sided tape was used to hold the thread in place. The tab was then folded over, pinning the thread between two areas of paper held together by the double sided tape. The threads within the testing paper tabs were taken to the Instron 5948 tensile test machine. The BlueHill 3 program that controlled the testing machine was opened, and a test was set up to run at an extension of 0.4mm/min. Each type of thread was tested 5 times.

Results

There was a definite relationship between the intensity of the plasma used on the spinnable array and the amount of functionalization that occurred. The intensity of the plasma could be adjusted by the atmospheric plasma system controls and the amount of functionalization was determined by observing the G/D curves measured by Raman spectrometry. The G/D ratio of the spinnable array observed by Raman spectrometry curve before functionalization was always greater than the G/D ratio of the spinnable array observed by Raman spectrometry curve after functionalization. This confirmed that the spinnable array was successfully functionalized by the atmospheric plasma system Because of the data,

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it can be confirmed that the atmospheric plasma had the ability to break the carbon-carbon bonds of the CNT and replace them with functional groups from the surrounding system.

There was not an observed definite trend relating the amount of functionalization of the CNT spinnable array and the conductivities of the sheets produced by the spinnable arrays. The trend hypothesized was that the more the array to make the sheet was functionalized, the greater the conductivity of the sheet would be. The increase of conductivity would have resulted from the functional groups that would be attached to the functionalized array. These functional group increase the conductivity of the sheet by acting as a bridge that would allow the electron the pass between the CNTs in the array.

The reason the hypothesis was not observed could have been from a couple different reasons. Although unlikely, the amount of the functional groups in the CNT sheet could not have an impact on the conductivity. One of the errors could have occurred in the sheeting process. During this process the use of acetone was used to compress forming sheet and expel air pockets that form in between the layers of sheet. Although the dispersion of the acetone was supposed to be uniform for all the sheets, the amounts used for each sheet fluctuated for different operators. This could have increased the distance of the layers of the sheet which could have an effect on the resistance values measured. The sheeting process and most of the processes of this project were very physical and comprised of many mechanical processes, which provides many opportunities for error. This was also the first collection of data obtained in this project. The limited experience of the researchers could have been a part of the unfruitful results. Since there were only a few weeks left of the semester after the sheeting of the arrays was completed, and since the sheeting process of the arrays was very time consuming, the decision to move the aim of the project from CNT sheets to CNT threads was made.

A few trends could be made relating the amount of functionalization of the CNT spinnable array and the conductivities and mechanical properties of the threads produced by the spinnable arrays. The trend hypothesized was that the more the array to make the thread was functionalized, the greater the conductivity of the thread would be and the lesser the tensile strength would be.

Because of communication errors between researchers from different projects, poor quality arrays given to the project, and lack of materials needed to conduct the experiments, only three quality threads were produced. More data is required to determine any concrete relationships. The conductivity values were averaged from five different samples of the same thread. There is an incomplete trend that is observed where the pristine thread has a greater conductivity than the thread spun from an array functionalized with 100W, 0.2L/min O2 plasma, but the thread spun from an array functionalized with 100W, 0.3L/min O2 had a greater conductivity than both of them. If more data was present, the expected trend would be that the more the array to make the thread was functionalized, the greater the conductivity of the thread would be.

The results of the mechanical properties show that the tensile strength of the CNT spinnable array thread increased as the amount of functionalization increased. This deviated from the expected hypothesis but it could be a result of the differences in the thread diameter. The smaller the diameter of the thread, the greater the tensile strength should be. However, since the thread comprised of the array functionalized with 100W, 0.2L/min O2 plasma had a lesser diameter than the thread spun from an array functionalized with 100W, 0.3L/min O2, the thread comprised of the array functionalized with 100W, 0.2L/min O2 plasma should have had a greater tensile strength than the thread spun from an array functionalized with 100W, 0.3L/min O2 which was not observed. No observable relationship between the tensile strengths of the thread could be made. However, this is why the specific strength of the threads were calculated. The result of calculating the tenacity of the threads show that there is no relationship between the tenacity of the thread and the amount of functionalization that occurred.

One interesting observation was made where which relates the brittleness of the thread and the amount of functionalization of the array which made up the thread. Since the average tensile strain of the thread decreased as the amount of functionalization of the array which made up the thread increased, it can be determined that the more the array that made up the thread was functionalized, the more brittle the thread becomes. This make sense because it was observed that the structural quality of the array decreased as the amount of functionalization increased.

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Conclusion

Several conclusions were reached after many experiments with the plasma torch. In the first part of the project, it was seen that the G/D ratio does in fact change when the array is exposed to plasma. However, a definite trend could not be found for the change of the G/D ratio with respect to the intensity of the plasma treatment. Looking at the first few plasma applications while making sheets, the difference in the G/D ratio before and after plasma increases as the power and number of swipes increases. This trend was also seen for the arrays while making threads. The higher the oxygen flow rate setting, the greater the difference in the G/D ratio.

Besides checking the G/D ratio through Raman, arrays were also tested for spinnability. In early trials, many of the arrays stayed spinnable after plasma treatment. This was due to the distance from the array to the torch, and the speed the array passed the torch. In later experiments, the torch was moved closer, and the array was moved much slower to a programmed speed of 10mm/s. After several trials, even with the plasma set at the lowest power and oxygen settings, the arrays were unspinnable after plasma treatment. After setting the speed to 25mm/s, there was enough increase in spinnability to make threads. However, the arrays would not spin uniform enough to make sheets. It’s concluded that the array needs less exposure time to the plasma torch to obtain a better spinnability.

Judging from the results of conductivity testing, it’s inconclusive that a trend exists between the intensity of the plasma and the conductivity of the thread or sheet. However, looking at the thread testing, the array exposed to 100W, 0.3L O2 plasma had higher conductivity than the pristine thread. There were also some sheets that possessed higher conductivity than the pristine sheet. Regardless of any trend existing, it can be concluded that functionalization using plasma does change the conductivity.

As for tensile testing, the results show an increase in tensile strength, as the amount of oxygen in the plasma increases. However, the functionalized threads had a shorter elongation until breaking, which means they were more brittle. For another measure of strength, the tenacity was calculated. After this calculation the pristine thread had the greatest strength, followed by the 100W, 0.3L O2 thread. This added strength could be due to the hydrogen bonds created during functionalization.

Overall, this project has showcased the ability of plasma to functionalize CNT arrays. The plasma process offers a clean, fast, and less hazardous way to functionalize nanotubes. Also, plasma functionalization offers a dry process and allows the nanotubes to stay aligned while being pulled into sheets and threads.

The key conclusions:

The application of plasma decreases the G/D ratio.

During plasma application, the array should move at or faster than 25mm/s to functionalize without making the array totally unspinnable.

For lower power settings, increasing the amount of oxygen in the plasma causes the difference in the G/D ratio to increase.

Increasing the amount of oxygen in the plasma increases the tensile strength.

Increasing the amount of oxygen for plasma functionalization makes thread more brittle.

There is no relation between the intensity of functionalization and the conductivity a sheet or thread, but functionalization does change the conductivity.

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Figure 1. Raman Microscope

Figure 2. Sheeting Process

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Figure 3. Sheet Conductivity

Figure 4: Optical Microscope

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