Here, a novel method for the functionalization and stable dispersion of carbon nanomaterials in aqueous environments is described. Ozone is injected directly into an aqueous dispersion of carbon nanomaterial that is continuously recirculated through a high-powered ultrasonic cell.
Functionalization of carbon nanomaterials is often a critical step that facilitates their integration into larger material systems and devices. In the as-received form, carbon nanomaterials, such as carbon nanotubes (CNTs) or graphene nanoplatelets (GNPs), may contain large agglomerates. Both agglomerates and impurities will diminish the benefits of the unique electrical and mechanical properties offered when CNTs or GNPs are incorporated into polymers or composite material systems. Whilst a variety of methods exist to functionalize carbon nanomaterials and to create stable dispersions, many the processes use harsh chemicals, organic solvents, or surfactants, which are environmentally unfriendly and may increase the processing burden when isolating the nanomaterials for subsequent use. The current research details the use of an alternative, environmentally friendly technique for functionalizing CNTs and GNPs. It produces stable, aqueous dispersions free of harmful chemicals. Both CNTs and GNPs can be added to water at concentrations up to 5 g/L and can be recirculated through a high-powered ultrasonic cell. The simultaneous injection of ozone into the cell progressively oxidizes the carbon nanomaterials, and the combined ultrasonication breaks down agglomerates and immediately exposes fresh material for functionalization. The prepared dispersions are ideally suited for the deposition of thin films onto solid substrates using electrophoretic deposition (EPD). CNTs and GNPs from the aqueous dispersions can be readily used to coat carbon- and glass-reinforcing fibers using EPD for the preparation of hierarchical composite materials.
The use of carbon nanomaterials to modify polymeric and composite systems has seen intensive research interest over the past 20 years. Recent reviews on both the use of carbon nanotubes1 (CNTs) and graphene nanoplatelets2 (GNPs) provide an indication of the breadth of research. The high specific stiffness and strength of CNTs and GNPs, as well as their high intrinsic electrical conductivity, make the materials ideally suited for incorporation into polymeric systems to enhance both the mechanical and electrical performance of the nanocomposite materials. CNTs and GNPs have also been used for the development of hierarchical composite structures by using the carbon nanomaterials to modify both fiber interfacial adhesion and matrix stiffness3,4.
The homogeneous dispersion of carbon nanomaterials into polymeric systems often requires processing steps, which chemically alter the nanomaterials to improve the chemical compatibility with the polymer matrix, remove impurities, and reduce or remove agglomerates from the as-received materials. A variety of methods to chemically modify carbon nanomaterials are available and may include wet chemical oxidation using strong acids5,6, modification with surfactants7, electrochemical intercalation and exfoliation8, or dry chemical processing using plasma-based processes9.
The use of strong acids in the oxidation step of CNTs introduces oxygen functional groups and removes impurities. However, it has the disadvantage of significantly reducing the CNT length, introducing damage to the CNT outer walls and using dangerous chemicals, which need to be isolated from the treated material for further processing10. The use of surfactants combined with ultrasonication offers a less aggressive method to prepare stable dispersions, but the surfactant is often difficult to remove from the treated material and may not be compatible with the polymer being used to prepare the nanocomposite materials1,11. The strength of the chemical interaction between the surfactant molecule and CNT or GNP may also be insufficient for mechanical applications. Dry plasma treatment processes conducted under atmospheric conditions may be suitable for functionalizing arrays of CNTs, present on fiber or planar surfaces, used to prepare hierarchical composites9. However, the atmospheric plasma is more difficult to apply to dry powders and does not address the problems with agglomerates present in as-manufactured raw carbon nanomaterials.
In the present work, we introduce a detailed description of the ultrasonicated-ozonolysis (USO) method that we have previously applied to carbon nanomaterials12,13,14. The USO process is used to prepare stable, aqueous dispersions that are suitable for electrophoretically depositing (EPD) both CNTs and GNPs onto carbon and glass fibers. Examples of EPD using USO-functionalized CNTs to deposit thin, uniform films onto stainless steel and carbon fabric substrates will be provided. Methods and typical results used to chemically characterize the functionalized CNTs and GNPs will also be provided, using both X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. A brief discussion of the characterization results in comparison with other functionalization techniques will be provided.
Work Health and Safety Notice
The effects of exposure to nanoparticles such as CNTs, on human health are not well understood. It is recommended that special measures be taken to minimize exposure to and avoid environmental contamination with CNT powders. Suggested hazard isolation measures include working within a HEPA filter-equipped fume cupboard and/or glove box. Occupational hygiene measures include wearing protective clothing and two layers of gloves and performing regular cleaning of surfaces using damp paper towels or a vacuum cleaner with a HEPA filter to remove stray CNT powders. Contaminated articles should be bagged for hazardous waste disposal.
Exposure to ozone can irritate the eyes, lungs, and respiratory system, and at higher concentrations may cause lung damage. It is recommended that measures be taken to minimize personal and environmental exposure to generated ozone gas. Isolation measures include working within a fume cupboard. As the return air stream will contain unused ozone, it should be passed through an ozone destruct unit before being releasing into the atmosphere. Dispersions that have had ozone bubbled through them will contain some dissolved ozone. After ozonolysis operations, allow the dispersions to sit for 1 h before undertaking further processing so that the ozone can undergo natural decomposition.
1. Functionalization of CNTs and GNPs by Ultrasonic Ozonolysis
Figure 1: Ultrasonic Ozonolysis System. This schematic diagram illustrates how to connect the various elements of the ultrasonic ozonolysis system. Please click here to view a larger version of this figure.
2. Electrophoresis
Figure 2: Electrophoretic Deposition Cell. This schematic diagram illustrates the configuration of the electrophoretic deposition cell. Please click here to view a larger version of this figure.
3. Chemical Characterization – X-ray Photoelectron Spectroscopy (XPS)15
4. Structural Characterization – Raman Spectroscopy18
5. Film Morphology – Scanning Electron Microscopy (SEM)
Figure 3 shows the XPS wide-scan characterization of CNTs that had undergone USO treatment. CNTs that had not undergone USO show almost no oxygen content. As the USO time increases, the surface oxygen level increases. Figure 4 charts the oxygen-to-carbon ratio increases as a function of USO time. Table 1 shows the deconvoluted carbon species atomic concentrations of GNP treated with USO. The peak fitting used a combination of constrained peaks, represented by Gr1 to Gr6, to represent the inherent peak shape due to graphite and the associated energy loss features17. The oxygen-containing species were then added, and a cross-correlation with the C 1s peak fitting data and the elemental C and O percentages was made to ensure that sensible peak fitting results were achieved. The C-O, C=O, and COO species are at insignificant levels without USO treatment but increased markedly with 12 h of USO.
Raman spectra of CNTs treated by USO are shown in Figure 5. The intensity of the D band at 1,346 cm-1 denotes the presence of defects in the CNTs20 and indicates that defects already existed in the CNTs prior to the USO treatment. With increasing USO treatment time, there is a shift in the G band, from 1,576 cm-1 to 1,582 cm-1, with a second component becoming more pronounced at 1,618 cm-1. This corresponds to an increase in the oxidation level of the CNTs. There is also a noticeable decrease in intensity of the 2D band at 2,698 cm-1 and a small increase in the D+G band at 2,941 cm-1 with USO treatment time, indicating an increased level of oxidation in the CNTs20.
The intensity ratio of the D to G bands, ID/IG, was measured at 1.18 with no USO, increased to 1.37 after 155 min of USO treatment, and continued to increase at a much slower rate with greater USO treatment times. This may indicate that oxidation is occurring primarily at existing defects, and once the existing defect sites are saturated, the oxidizing effect of further USO treatment time occurs at a slower rate. This suggests that there is minimal damage to the as-received CNT structure from USO.
The SEM images in Figure 6 show USO-treated CNTs compared with untreated CNTs. The untreated CNTs are highly clumped, while the USO-treated CNTs are much more evenly dispersed across the surface, showing that the USO has helped to reduce agglomerates present in the as-received material.
The USO-treated CNTs were then electrophoretically deposited using a variety of conditions. Thicker films were produced as the electrophoresis time increased; thicker films were also produced by using higher CNT concentrations, as shown in Figure 7. Similar results are achieved when GNPs are used. Figure 8 shows the rate of electrophoretic deposition at various CNT concentrations, using deposition rates calculated from curve-fitting the data in Figure 7. It appears that the rate of EPD of USO-treated CNTs can be predicted reasonably well if the initial CNT concentration is known. The linear relationship in deposition rate and dispersion concentration is predicted from the Hamaker Law21, but the logarithmic rate in deposition shows that changes in the deposition parameters are occurring with time. Our previous work, in which CNTs were deposited onto carbon12 and glass13 fabrics, also clearly showed that the deposition rate with time is non-linear. Under constant voltage deposition, the resistance of the CNT film increases, leading to a reduction in the deposition rate.
Figure 3: Chemical Characterization – XPS. CNTs that had been subjected to USO for varying times were analyzed using XPS. This image shows the wide scan changes in oxygen (532 eV) and carbon (284.6 eV) intensity, with the oxygen intensity increasing at higher USO times. Please click here to view a larger version of this figure.
Figure 4: Atomic Ratio – XPS. This graph shows the change in the O:C atomic ratio of CNTs with increasing USO times, calculated from the scans shown in Figure 3. The ozonolysis mechanism is very complex, and the polynomial fit is provided, as a guide to the reader, to show the trend in oxidation rate, but not to explain anything about the ozonolysis mechanism. Please click here to view a larger version of this figure.
Figure 5: Structural Characterization – Raman. All spectra showed strong D bands (~1,346 cm-1), G bands (~1,576 cm-1), 2D bands (~2,698 cm-1), and D+G bands (~2,941 cm-1). The D-band increase indicates some increase in the graphitic defects. Please click here to view a larger version of this figure.
Figure 6: Film Morphology – SEM. These images show CNTs that were (a) not chemically treated and (b) USO-treated for 16 h. Please click here to view a larger version of this figure.
Figure 7: Electrophoresis Measurements. This graph shows the density of the EPD films versus the deposition time using three different concentrations of CNTs dispersed in water. Blue diamonds = 2 g/L, red squares = 1.5 g/L, and green triangles = 1 g/L. A 14 V/cm DC electric field was used for these samples. Please click here to view a larger version of this figure.
Figure 8: Electrophoretic Deposition Rate. Plot of the EPD rate versus the CNT concentrations, using deposition rates calculated from the curve fit of the Figure 7 plot of film density versus deposition time. Please click here to view a larger version of this figure.
Name | Posição | Constraint | FWHM | Constraint | Area Constraint | % Atomic Concentration | |
0 h | 12 h | ||||||
Gr 1 | 284.6 | ±0.1 | 0.7 | ±0.2 | Best fit | 29.4 | 17.6 |
Gr 2 | 284.8 | 1.2 | Gr1*1.3 | 36.9 | 22.2 | ||
Gr 3 | 286.1 | 1.3 | Gr1*0.4 | 12.5 | 7.5 | ||
Gr 4 | 287.9 | 1.3 | Gr1*0.2 | 5.9 | 3.5 | ||
Gr 5 | 289.5 | 1.6 | Gr1*0.1 | 3.3 | 1.8 | ||
Gr 6 | 291.3 | 2.4 | Gr1*0.2 | 6.6 | 1.7 | ||
C-C | 285 | 1.2 | Best fit | 2.1 | 13.7 | ||
C-O | 287 | 1.2 | Best fit | 1.5 | 14.3 | ||
C=O | 288.6 | 1.2 | Best fit | 0.9 | 12.4 | ||
COO | 289.4 | 1.2 | Best fit | 0.9 | 5.3 |
Table 1: Chemical Characterization – XPS. This table shows the peak fitting parameters used to deconvolute the C 1s spectrum for the GNPs processed using the USO method for 0 h and 12 h treatment times. It also shows the resulting atomic concentrations of each species14. Gr 1, Gr 2, etc. represent the various graphite peaks used in the fit.
When working with nanoparticles of high hardness, such as CNTs, the potential erosion effect on containers and tubing should not be overlooked. Step 1.14 in the protocol was inserted after the tubing became worn at a bend due to CNTs impinging on the tube side wall, causing a system leak.
Also, note that the CNTs are in suspension, not solution, and that they must be stirred before each use if a homogeneous suspension is desired. For example, this would be necessary to maintain the desired concentrations if smaller volumes are decanted from a large batch.
It is very difficult to obtain a homogeneous dispersion of CNTs without the chemical modification of their surfaces. SEM imaging shows how untreated CNTs in a dispersion deposited on a surface remain highly agglomerated, while USO-treated CNTs are well dispersed across the surface. The EPD of untreated CNTs was also problematic, with untreated CNTs unable to be deposited by electrophoresis due to the absence of surface charge, which led to the zero electrophoretic mobility of the CNTs.
XPS showed that CNTs and GNPs treated with USO had high levels of surface oxygen, compared to insignificant oxygen levels in untreated CNTs or GNPs. The presence of the oxygen functional groups clearly provided the surface charge on the carbon nanomaterials, which facilitated stable dispersion and electrophoretic mobility under the applied field used during EPD. Likewise, Raman spectroscopy demonstrated that oxidation of the CNTs increased with USO treatment time. Raman spectroscopy also suggested that USO caused minimal damage to the graphitic structure of the CNTs.
The USO method was found to be highly effective at improving the compatibility of CNT and GNP in polymeric composite systems. Processes where reinforcing fibers were coated with USO-treated CNTs prior to resin infusion were shown to improve the in-plane shear strength of composite laminates12,13 and the adhesion strength of glass-resin bonds22. CNTs and GNPs that were treated by USO produce stable dispersions that enable EPD on both conducting and non-conducting substrates at a controlled rate.
Compared to other techniques for the chemical modification of carbon nanomaterials, ozonolysis has relatively low workplace health and safety risks. No harsh chemicals are used, and the water in which the carbon nanomaterials are processed can be easily removed, providing a functionalized material that can be readily used in a range of practical applications.
While the USO method has several advantages, such as allowing for the production of functionalized carbon nanomaterials without the use of harsh chemicals, the process does take up to 16 h to produce a stable dispersion suitable for EPD. Clearly, future work will need to examine how the process can be accelerated to enable greater throughput.
The authors have nothing to disclose.
The non-salary component of the work was funded by the Commonwealth of Australia. The author from the University of Delaware gratefully acknowledges the support of the US National Science Foundation (Grant #1254540, Dr. Mary Toney, Program Director). The authors thank Mr. Mark Fitzgerald for his assistance with the electrophoretic deposition measurements.
Ultrasonic bath | Soniclean | 80TD | |
Ultrasonic horn | Misonix | S-4000-010 with CL5 converter | Daintree Scientific |
Flocell stainless steel water jacketed | Misonix | 800BWJ | Daintree Scientific |
Peristaltic pump | Masterflex easy-load | 7518-00 | |
Controller for peristaltic pump | Masterflex modular controller | 7553-78 | |
Ozone generator | Ozone Solutions | TG-20 | |
Ozone destruct unit | Ozone Solutions | ODS-1 | |
Recirculating liquid cooler | Thermoline | TRC2-571-T | |
Multi-mode power supply unit | TTi | EX752M | |
High resolution computing multimeter | TTi | 1906 | |
X-ray photoelectron spectroscopy | Kratos Analytical | Axis Nova | |
XPS analysis software | Casa Software | Casa XPS | www.casaxps.com |
Kratos elemental library for use with Casa XPS | Casa Software | Download Kratos Related Files | http://www.casaxps.com/kratos/ |
Raman dispersive confocal microscope | Thermo | DXR | |
Field emission scanning electron microscope | Leo | 1530 VP | |
Sputter coater with iridium target | Cressington | 208 HR | |
Thickness measurement unit | Cressington | mtm 20 | |
Magnetic stirrer | Stuart | CD162 | |
Analytical balance | Kern | ALS 220-4N | |
Analytical balance | Mettler Toledo | NewClassic MF MS 2045 | |
Laboratory balance | Shimadzu | ELB 3000 | |
Electrodes from 316 stainless steel sheet | RS Components | 559-199 | |
Sanding sheets, P1000 grade | Norton | No-Fil A275 | |
Multi-walled carbon nanotubes | Hanwha | CM-95 | http://hcc.hanwha.co.kr/eng/business/bus_table/nano_02.jsp |
Graphene nanoplatelets | XG Sciences | XGNP Grade C | http://xgsciences.com/products/graphene-nanoplatelets/grade-c/ |