Nanoscience and Nanotechnology for Chemical and Biological Defense

Downloaded by STANFORD UNIV GREEN LIBR on June 27, 2012 | http://pubs.acs.org Publication Date: December 24, 2009 | doi: 10.1021/bk-2009-1016.ch024

Chapter 24

Scanning Atmospheric Plasma Processes for Surface Decontamination and Superhydrophobic Deposition Seong H. Kim,1* Jeong-Hoon Kim,2 and Bang-Kwon Kang3 1. Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802. 2. APPLASMA, 111 Gibbons Street, State College, PA 16801. 3. APPLASMA Co, Ltd. 211 Sanhakwon, Ajou University, Suwon, Korea.

A scanning atmospheric radio-frequency (rf) plasma source was developed for surface decontamination and protective coating deposition. This plasma source was a dielectric barrier discharge system that utilizes a cylindrical electrode and a dielectric barrier surrounding the electrode. The gaseous species present in the plasma were analyzed with in-situ spectroscopic technique. Since the plasma generation region was open to ambient air, nitrogen, oxygen and water molecules in the air were readily excited. This system was used for decontamination of organophosphorus nerve agents and deposition of hydrophobic and superhydrophobic coatings on various substrates including ceramics, metals, fabrics, and nanofibers.

© 2009 American Chemical Society

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In Nanoscience and Nanotechnology for Chemical and Biological Defense; Nagarajan, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

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Introduction There is a great need for plasma-based surface cleaning processes for pesticide decontamination, chemical and biological warfare agent decontamination, biomaterial sterilization, polymer film stripping, etc (1-3). The use of a plasma for surface cleaning and modification provides many advantages over conventional solution-based processes which may raise material compatibility issues or generate toxic liquid wastes (4). In the case of polymeric materials, the penetration of solvent into the bulk can alter the bulk properties of the polymer. The plasma-based surface modification approach can circumvent these problems since the reactive gaseous species modify only the surface and do not alter the bulk chemistry. In addition, the use of a high energy plasma process allows the generation of reactive species (such as excited atoms and radicals as well as electrons and ions) that are much more potent than chemical reagents in solution (5-8). Among various plasma types, an atmospheric nonthermal plasma is of specific interest since it can be used to scan over large surface area samples without enclosing the samples in a vacuum chamber. So, it can be operated in an in-line mode, not in a batch mode, and scaled up for applications to large substrate surfaces or continuous processing. We have developed a new scanning atmospheric rf plasma generation source that can operate at low voltage and low power and generated the glow plasma without arcing over a large area (9). This paper reviews the characterization of the scanning rf plasma with optical emission spectroscopy and transmission Fouriertransform infrared spectroscopy and its uses for decontamination of organophosphorus (OP) nerve agents on model sample surfaces and deposition of hydrophobic and superhydrophobic coatings as a protective layer.

Experimental The schematic of the scanning atmospheric rf plasma system is shown in Figure 1. A stainless steel cylindrical electrode (1.5 cm dia., 12 cm long) is shielded inside a quartz tube (inner dia. = 1.5 cm, wall thickness = ~2 mm) which separates the electrode from the grounded cover. The plasma was generated with Ar or He gas (flow rate = 4 liter per minute) by applying a 13.56 MHz rf electric field across the center electrode and the grounded cover. Typical operating rf power was 150 ~ 400 W. Various process gases can be fed into the plasma depending on the application purpose. The plasma generated in the open space between the dielectric barrier and the ground electrode landed on the substrate surface when the substrate was brought ~3 mm from the plasma head. The effective plasma area was ~0.9 cm wide and 10 cm long. Since the plasma was operated in a glow discharge mode, it could be directly applied to metallic substrates as well as non-conducting substrates without arc or streamer damages. The plasma exposure to the sample was made by automated scanning of the sample under the fixed plasma source. The typical scan speed was varied within the 3~10 mm/sec range.


325 Ar or He & process gas

Dielectric barrier Electrode (rf power)

Plasma Sample


Translating sample stage

Figure 1. Schematic of the scanning atmospheric rf plasma system. The optical emission from the excited gaseous species in the Ar and Ar/O2 plasma was analyzed using a Varian Cary Eclipse spectrophotometer with a 1 nm wavelength resolution. A clean blank substrate was placed ~5 mm below the plasma head and the optical emission was analyzed from the side of the plasma generated between the head and the substrate. The center of the plasma was positioned at the focal point of the spectrometer. The gas phase species produced in the plasma used for the hydrophobic and superhydrophobic coating deposition were analyzed with transmission FTIR spectroscopy (Thermo Nicholet Nexus 670) with a 1 cm-1 spectral resolution. The IR beam was passed through the plasma zone between the plasma head and the clean blank substrate. The vibrational spectrum of the process gas was collected in atmospheric flow conditions before turning on the plasma and used as a background. In the case of OP decontamination, parathion and paraoxon were spincoated on glass or gold film surfaces from 0.023 mM solution in ethanol. The loadings of parathion and paraoxon were determined with a quartz crystal microbalance (QCM). The deposited amount of parathion and paraoxon film was ~8.5 μg/cm2 and ~5.0 μg/cm2, respectively. These OP films were then treated with an O2/Ar plasma. The treated samples were analyzed with polarization-modulcation reflection absorption infrared spectroscopy (PMRAIRS; Thermo Nicholet Nexus 670 with a Hindz polarization modulation unit and a GWC demodulator) and tested for residual toxicity. For deposition of hydrophobic coatings, a mixture of CH4/He gases was used in the plasma. Superhydrophobic coatings were created with a CF4/H2/He plasma. The deposited coatings were analyzed with PM-RAIRS, x-ray photoelectron spectroscopy, scanning electron microscopy (SEM, JEOL 6700 F - FESEM), atomicforce microscopy (AFM, Molecular Imaging SPM microscope with RHK controllers), ellipsometry, and water contact angle measurements.


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Results and Discussion


Organophosphorus Compound Decontamination Using O2/Ar Plasma The plasma-generated gaseous species are typically in electronically excited states. Figure 2a shows a typical optical emission spectrum of the Ar plasma produced in air (10). The characteristic emission peaks from the excited Ar molecules are clearly seen in the region from 696 nm to 812 nm. The peaks at 337 nm and 674 nm originate from the excited N2 molecule (11). The peak at 309 nm is due to the OH radical produced by dissociation of water vapor in the plasma (11). The peaks at 777 nm and 845 nm are due to the excited atomic oxygen.

(a)

(b)

Figure 2. Optical emission spectra of (a) Ar and (b) O2/Ar plasma generated in ambient air (rf power = 200 W).(10) When oxygen is added into the plasma (Figure 2b), the ratio of the atomic oxygen peaks to the excited Ar peaks increases significantly. This is accompanied by the increase of the peaks corresponding to the electronic transitions involving highly excited OH and N2 species (11). Although the optical emission at ~322 nm from the excited ozone molecules is not detected, it is very likely that the atmospheric rf plasma also produces ozone. It is well known that the atomic oxygen and other electronically-excited OH, N2, and Ar species undergo a three body reaction with molecular oxygen to produce ozone (12,13). These highly excited OH radicals, atomic oxygen, and ozone species are very efficient for oxidative modification of surface chemistry.


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(a)

(b)

(c)

Figure 3. PM-RAIRS spectra of parathion films after being treated with (a) O2/Ar plasma (rf power = 200 W), (b) UV, and (c) UV/ozone.(10) The decomposition products of parathion and paraoxon after the atmospheric rf plasma treatment were analyzed with PM-RAIRS. In order to find whether photolysis or oxidation is responsible for the decomposition reaction, the plasma-induced decomposition products were compared with the surface species remaining on the surface after treatments with UV (200W Hg lamp) and ozone (a commercial UV/ozone cleaner). These results are shown in Figure 3. Upon a single pass of the Ar/O2 plasma (rf power = 200 W, scan rate = 3 mm/sec) over the OP film, the nitrobenzyl and alkyl peaks disappear almost completely and broad peaks corresponding to amorphous phosphate glass (1055 and 1245 cm-1) (14-16), water adsorbed in the glassy film (1650 cm-1), and carbonyl species (1750 cm-1) appear. The carbonyl peak eventually decreases as the plasma exposure time is increased. The changes in the paraoxon spectrum upon O2/Ar plasma exposure are basically the same as the parathion case. The spectral changes of parathion and paraoxon films upon the oxygen plasma exposure are not due to the irradiation of high-flux UV photons from the plasma source. The PM-RAIRS data for the OP films irradiated with UV (Figure 3b) show the monotonic and slow decrease of the nitrobenzyl peaks; but the phosphorus ether peaks are not changed much. The plasma-induced reaction products appear to be similar to the ozonolysis product (Figures 3c). The decrease of the nitrobenzyl characteristic peaks is accompanied by the appearance of the broad peaks at 1055, 1245, and 1750 cm-1. From the degree of peak intensity decrease as a function of process time, it is obvious that the atmospheric plasma treatments are several orders of magnitude faster than the UV and UV/ozone treatments. The efficacy of the OP decomposition and the nontoxicity of the decomposition reaction product were tested by culturing Drosophila in a confined space containing the substrates which were initially coated with OP films and treated with the O2/Ar plasma, UV, and UV/ozone.(10) The UV and


328 UV/ozone treated parathion and paraoxon samples (treatment time = 5 min) were still toxic enough to kill all Drosophila in less than 12 hours in the culture bottle. In contrast, the Drosophila in the bottle containing the plasma-treated sample (treatment time = 10 sec) lived their full life cycle and laid eggs. Even the newly hatched larvae were able to develop into adult flies. These culture test results qualitatively demonstrate the efficacy of the atmospheric rf plasma process for the complete decomposition and detoxification of OP nerve agents.


Hydrophobic Coating Deposition Using CH4/He Plasma CH4 is used as a process gas and He as a plasma generation gas to deposit hydrophobic coatings on various surfaces (17). The transmission FTIR analysis of the gas phase region of the atmospheric CH4/He plasma showed the consumption of the CH4 process gas (negative peaks at 3017.6 cm-1 and 1306 cm-1) and water (small negative peaks near 1600 cm-1 and 3700 cm-1) in the plasma and the positive absorbance is due to production of new species in the plasma ( 18 ). The changes in rotational fine structures and width of these vibration peaks indicated that CH4 molecules are highly excited in the plasma. Although hydrocarbon radicals and ions are expected to be the primary dissociation products of CH4 in the plasma, they are not directly observed in the FTIR spectrum of the scanning atmospheric rf plasma. Instead, the products of their reactions with other gas-phase species are observed in FTIR. These include C2H2, C2H4, and C2H6 (formed by reactions between hydrocarbon radicals) as well as CO (oxidation by atomic oxygen and/or OH radical) (18). These are the consequence of the short mean free path of molecules (10-9 ~ 10-8 m) at the atmospheric pressure. Since typical molecular speed at room temperature is about 500 m/s, the collision frequency in the gas phase is about ~1012 per second. The activation and dissociation of CH4 near the substrate surface lead to deposition of hydrophobic coatings on the surface. The hydrophobic coating layer thickness can be easily controlled with the number of passes under the scanning atmospheric CH4/He plasma. The deposition rate is ~14 nm for the first pass and then slightly decreases in subsequent passes. (Figure 4). The deposited film surface is very smooth. The coating thickness is very uniform over the entire treated area.



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Figure 4. Thickness of hydrophobic coatings deposited with scanning atmospheric CH4/He plasma (rf power = 250 W). The insets show the AFM image (1.5μm×1.5μm) and line profiles of the 11-pass deposited film.(17) The chemical composition of the deposited hydrophobic coatings is determined using XPS. Figure 5 shows the XPS analysis results for the coating deposited on a clean gold film substrate. The C1s peak is the only main peak detected in XPS. The elemental analysis shows that the oxygen concentration is less than 2% regardless of substrates. The incorporation of this small amount of oxygenated species is inevitable because the plasma is generated in atmospheric air. The exact nature of the carbonaceous species is determined with infrared vibration spectroscopy. The C-H vibration peak positions in the PM-RAIRS spectra (Figure 5) are very close to those of polypropylene IR spectrum (19). This indicates that the deposited film is composed mostly of CH2 polymeric backbones with CH3 side groups. The peaks at 1708 cm-1 and 1660 cm-1 are due to carbonyl containing groups which are inevitable because the plasma is generated in atmospheric air.


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(9)Æ (7)Æ (5)Æ


(3)Æ (1)Æ

Figure 5. PM-RAIRS and XPS (inset) spectra of the hydrophobic coating deposted with scanning atmospheric CH4/He plasma on a gold film. The number of plasma scanning is indicated in the figure.(17)

Figure 6. Water contact angles on hydrophobic coating produced with scanning atmospheric CH4/He plasma (rf power = 250 W).(17) All coatings deposited by the atmospheric CH4-He plasma process show good hydrophobicity regardless of the substrate materials. Water contact angles on these coatings are plotted in Figure 6 as a function of the plasma deposition cycles. On flat surfaces such as Cu foil, Si wafer, and glass slide, the contact angle increases to ~90o after a single treatment with the CH4 plasma. This value is very close to the water contact angle of polypropylene (93o). The contact angle remains unchanged upon further deposition. The paper and cotton substrates require at least 3 deposition cycles to be hydrophobic. Until then, the water droplet is completely absorbed into the substrate. This probably arises from lower deposition yields on side walls present in rough surfaces, so repeated


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treatments are required for conformal coating. After multiple treatments with the scanning atmospheric CH/He plasma, the contact angles on paper and cotton substrates show 112.5±1o and 150±1o, respectively. The high contact angle on these substrates is due to the surface roughness effect that amplifies the hydrophobicity (20,21). In general, the water contact angle becomes larger on rough surfaces than on flat surfaces. On the treated cotton sample, the water droplet is easily rolled off with slight tilting of the substrate, showing a selfcleaning capability. It should be noted that the untreated side of the cotton sample still absorbs water readily. These properties will make the plasma-treated cotton fabric a good candidate for self-cleaning garments. Superhydrophobic Coating Deposition Using CF4/H2/He Plasma A mixture of CF4 and H2 is used in the He plasma to deposit superhydrophobic coatings on various surfaces (9). The transmission FTIR analysis of the gas-phase region of the atmospheric CF4/H2/He plasma allowed to monitor the consumption of the CF4 and the gas-phase reaction products. The consumption of CF4 in the plasma was the highest when the CF4 and H2 ratio was 2:1, which is the optimum gas composition ratio for the superhydrophobic coating depositon (18). The CF4 consumption in the atmospheric rf plasma was accompanied by the production of HF (sharp peaks between 3593 cm-1 and 3920 cm-1), C2F6 (peaks at 1250 cm-1 and 1114 cm-1), CF3H (a weak peak at 1153 cm-1), and COF2 (a sharp peak at 774 cm-1) (18). The CF4/H2/He plasma deposited film shows excellent superhydrophobic properties with a water contact angle of >170o on all surfaces tested (silicon wafer, smooth gold film, paper, and cotton). Figure 7 shows selected timesequence images taken with a high speed camera. Upon landing on the superhydrophobic coating surface (Figures 7a and 7c), water droplet advances (panels 3 and 4) and then recedes (panels 5 and 6) with contact angle greater close to 170o. At this high contact angle, the three-phase (solid-liquid-air) contact line circumference is so small that the momentum loss due to liquid surface tension at the contact line is negligible and the droplet bounces back off the surface almost elastically (panels 7 and 8). On hydrophilic surfaces (Figure 7b), the water droplet spreads with a very low contact angle and never recedes back. On untreated cotton surface (Figure 7d), the water droplet advances with a contact angle greater than the hydrophobic flat surfaces, but does not recede. The water is eventually absorbed into the cotton.



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Figure 7. Time-sequence images of water droplet falling on (a) plasmatreated superhydrophobic gold film, (b) clean Si wafer, (c) plasmatreated superhydrophobic cotton and (d) untreated cotton. All panels are at 2 msec interval, except the ones separated with dots.(9)

(16)

(11) (9) (7) (5) (3) (1)

Figure 8. PM-RAIRS and XPS (inset) of superhydrophobic coatings deposited with scanning CF4/H2/He plasma (rf power = 300 W). The number of plasma treatments is indicated in the figure.(9) The chemical functional groups in the superhydrophobic coatings deposited via the scanning atmospheric CF4/H2/He plasma process are identified with PMRAIRS (Figure 8). The observed vibrational peaks are the C-F stretching vibrations at 1245 cm-1, 1285 cm-1, and 1348 cm-1 and the C=O stretching vibration at 1730 cm-1. The fact that C-F and C=O vibrations of the coating are broad and not well resolved indicates that the coated material is amorphous and highly cross-linked.



333 The chemical composition of the superhydrophobic coating is obtained from XPS analysis. Figure 8 shows a survey XPS spectrum of a 3-pass coating on a gold film. The main components in the coating are carbon and fluorine. There are trace amounts of nitrogen and oxygen detected. These species are incorporated because the plasma is generated in air. The high-resolution C1s region shows 4 different carbon species (22). [C-C species at 285 eV, C-F and C=O species at 287.2 eV, CCF2 at 289.8 eV, and CF2−CF2 species species at 292.4 eV]. The atomic composition of the superhydrophobic coating is represented as CFx(x

Nanoscience and Nanotechnology for Chemical and Biological Defense

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