Article pubs.acs.org/Langmuir
Development of Oxidized Sulfur Polymer Films through a Combination of Plasma Polymerization and Oxidative Plasma Treatment Behnam Akhavan, Karyn Jarvis, and Peter Majewski* School of Engineering, Mawson Institute, University of South Australia, Mawson Lakes, South Australia 5095, Australia ABSTRACT: A novel two-step process consisting of plasma polymerization and oxidative plasma treatment is introduced in this article for the first time for the fabrication of −SOx(H)functionalized surfaces. Plasma-polymerized thiophene (PPT) was initially deposited onto silicon wafers and subsequently SOx(H)-functionalized using air or oxygen plasma. The effectiveness of both air and oxygen plasma treatments in introducing sulfur−oxygen groups into the PPT film was investigated as the plasma input specific energy and treatment time were varied. The surface chemistries of untreated and treated PPT coatings were analyzed by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS), whereas spectroscopic ellipsometry was used to evaluate the film thickness and ablation rate. Surface chemistry analyses revealed that high concentrations of −SOx(H) functionalities were generated on the surface upon either air or oxygen plasma treatment. It was found that, at low plasma input energies, the oxidation process was dominant whereas, at higher energies, ablation of the film became more pronounced. The combination of thiophene plasma polymerization and air/oxygen plasma treatment was found to be a successful approach to the fabrication of −SOx(H)-functionalized surfaces. physical properties.25,26 In this process, a precursor liquid monomer is initially converted to vapor under the low pressure of the system. The monomer vapor is then excited into the plasma state under the influence of an electric field. The plasma state consists of neutral species, electrons, ions, and radicals with an overall neutral charge. As a result of plasma-phase interactions, monomer molecules become fragmented and subsequently recombine and deposit onto any surface in contact with the plasma.27 SOx(H)-containing monomers have low volatilities and therefore do not easily evaporate into the plasma chamber at room temperature. Thus, their direct plasma polymerization is not possible in practice.28 Because of this difficulty, limited research has been undertaken on SOx(H)functionalized plasma polymers in comparison to plasma polymers deposited by a one-step process. These simply fabricated plasma polymers often contain CxHy,29 NH2,30 or COOH31 functionalities. A number of studies have utilized a sulfur-containing plasma, such as SO2, to add SOx(H) functionalities to polymeric materials such as polyethylene, polyester, polyurethanes, and polypropylene.2,7,32−36 Apart from these studies, a few approaches have previously been attempted to deposit oxidized sulfur-containing films through a combination of plasma polymerization/treatment and wetchemistry deposition/derivatization.37,38 However, these approaches are complicated; chemically unpredictable; and still
1. INTRODUCTION Surfaces containing oxidized sulfur functionalities [−SOx(H)] such as sulfonate (SO3), sulfonic acid (SO3H), and sulfate (SO4) are of great interest in a number of critical applications including biomaterials,1,2 fuel cells,3,4 and water purification.5 SOx(H)-functionalized surfaces show remarkably high blood compatibility because of their decreased platelet adhesion6,7 and anti-inflammatory properties.8 These surfaces also exhibit enhanced ionic conductivity, which makes them excellent candidates for proton-exchange membranes applied in fuel cells.3 In aqueous solutions, SOx(H)-functionalized surfaces carry a net negative charge even at very low pH values.9 These surfaces have thus been extensively applied as electrostatic removal agents for the adsorption of positively charged contaminants, such as ammonium,10,11 metallic cations,5,12 and cationic dyes.13,14 Acidic sulfur-containing functionalities can also be applied as catalysts in acetalization reactions15−17 and biodiesel production.18 Oxidized sulfur-containing surfaces are conventionally synthesized by wet-chemistry methods, such as mass polymerization,19,20 formation of self-assembled monolayers,21,22 acid sulfonation,23 and electrochemical polymerization.24 These methods, however, are complicated, substrate-dependent, and are environmentally sustainable because of their extensive rates of waste production. Plasma polymerization is a simple, wastefree, and substrate-independent process that can be a novel alternative for conventional wet-chemistry techniques. Using this process, functionalized polymer films can be deposited onto a variety of substrates regardless of their chemical and © 2014 American Chemical Society
Received: November 28, 2013 Revised: January 5, 2014 Published: January 15, 2014 1444
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reactor. Energy-dependent plasma treatments were conducted using air or oxygen at a constant treatment time of 2 min, with input powers of 2−80 W and flow rates of 1−13 sccm applied to produce W/F ratios ranging from 0.06 to 2.4 kJ·cm−3. Time-variable treatments were carried out at times of 2−60 min, with the W/F ratio kept constant at the optimum value of 0.24 kJ·cm−3 (RF power = 8 W, air/oxygen flow rate = 2 sccm). 2.3. X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectroscopy (XPS) of untreated and treated PPT films was carried out using a SPECS SAGE spectrometer. The X-ray source was a nonmonochromatic Mg Kα source (hν =1253.6 eV) operating at a power of 200 W (10 kV and 20 mA). A hemispherical analyzer (Phoibos 150) with an MCD9 electron detector was used for spectroscopy. The electron takeoff angle was 90° relative to the sample surface. A pass energy of 30 eV and a resolution of 0.5 eV over the energy range of 0−1000 eV were applied for recording the survey spectra of samples. High-energy-resolution (0.1-eV) spectra of C 1s and S 2p signals were acquired at a pass energy of 20 eV to determine the carbon- and sulfur-containing components. To correct for the effects of charging, all of the binding energies were calibrated in reference to the binding energy of the aliphatic C 1s component (285 eV). The analysis area was circular and 3 mm in diameter. Calculations of survey elemental compositions were carried out using CasaXPS software, and a linear background, equal full-width at half-maximum (fwhm), and Gaussian (70%)−Lorentzian (30%) line shape were applied. All XPS analyses were carried out no later than 1 day after the plasma polymerization/treatment. 2.4. Time-of-Flight Secondary Ion Mass Spectroscopy (ToFSIMS). The ToF-SIMS spectra of untreated and air-/oxygen-plasmatreated PPT coatings were acquired using a Physical Electronics Inc. PHI TRIFT V nanoTOF instrument (Physical Electronics Inc., Chanhassen, MN) equipped with a pulsed liquid metal 79+Au primary ion gun (LMIG), operating at 30 kV energy. All secondary ion spectra were collected at base pressures of 5 × 10−6 Pa or lower. Dual charge neutralization was achieved with an electron flood gun and 10 eV Ar+ ions. At least nine areas of ∼400 × 400 μm2 were analyzed per sample. Both positive and negative SIMS counts were collected, but only negative counts, which contained most useful information, are reported. Sample spectra were analyzed using WincadenceN software (Physical Electronics Inc., Chanhassen, MN). 2.5. Spectroscopy Ellipsometry. Film thickness measurements were carried out utilizing a variable-angle spectroscopic ellipsometer (J. A. Woollam Co. Inc.). The high-speed monochromator (HS-190) and control module (VB-400) were operated by WVASE32 software. Measurements were carried out at 65°, 70°, and 75° angles of incidence, after the instrument had been calibrated using a reference silicon wafer. The data were collected in the visible and near-UV regions (wavelength range of 250−1100) at steps of 5 nm. The data were fitted to a Cauchy model. At least four measurements were carried out per sample, and average values are reported. The standard deviations of the mean values were less than 12%.
environmentally questionable, as they use toxic precursors. Few studies have utilized a plasma-assisted technique for the deposition of SOx(H)-functionalized coatings without the use of any wet-chemistry route.28,39,40 In these investigations, sulfur dioxide (SO2) has been applied for copolymerization or postpolymerization treatments. For copolymerization, SO2 has been polymerized with hydrocarbons39 and perfluorobenzene,40 whereas for a postpolymerization treatment, SO2 plasma has been applied to modify 1,7-octadiene and heptylamine plasma polymers.28 These methods are hazardous, however, because of the high toxicity of sulfur dioxide.41 This investigation introduces a novel, greener, and more efficient plasma polymerization/treatment method for the fabrication of SOx(H)-functionalized surfaces. In this approach, thiol groups are initially deposited onto a surface through the plasma polymerization of thiophene. Subsequently, using an oxidative plasma (air or oxygen), thiol groups are oxidized into −SOx(H) moieties, including sulfoxide, sulfonic acid, sulfonate, and sulfate. Optimization studies were initially carried out to determine the plasma input specific energy and deposition time required for the deposition of plasma-polymerized thiophene (PPT) onto silicon wafers. PPT films deposited under optimum conditions were subsequently treated with air or oxygen plasmas under a range of power-to-gas-flow-rate ratios (W/F) and treatment times to obtain the maximum concentration of −SO x(H) functionalities on the surface. The surface chemistries of untreated and air-/oxygen-plasmatreated polymer films were examined by X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) was employed to study the molecular structures of the deposited/treated films. The film thicknesses and ablation rates of the plasma polymers were evaluated by spectroscopic ellipsometry.
2. EXPERIMENTAL SECTION 2.1. Materials. Thiophene monomer (C4H4S) was purchased from Sigma-Aldrich (Castle Hill, Australia) and used as received. Oxygen gas (99.5% purity) was obtained from BOC Australia. Silicon wafers with a surface area of 1 cm2 (MMRC Pty. Ltd.) were ultrasonicated in acetone (Merck Pty. Ltd.), rinsed with Milli-Q water, and dried under a stream of nitrogen prior to plasma polymerization. 2.2. Plasma Polymerization. An inductively coupled radiofrequency (RF) reactor, described previously,27,42 was utilized in stationary mode for both plasma polymerization and plasma treatment processes. The reactor was powered by a 13.56 MHz RF generator and a matching network (Coaxial Power Systems Ltd.). The plasma chamber was pumped down to a pressure of approximately 5 × 10−3 mbar using a rotary vacuum pump, as the silicon wafer substrates were placed in the in-coil region. Samples remained in the same position throughout the study. Liquid thiophene was applied as a precursor monomer after at least three freeze−pump−thaw cycles for the removal of any dissolved gases. For the plasma polymerization of thiophene, initial optimization of the plasma input specific energy was carried out using input powers of 2−80 W and thiophene flow rates of 1−8 sccm (standard cubic centimeters per minute) to produce W/F ratios ranging from 0.03 to 2.4 kJ·cm−3. A constant deposition time of 2 min was applied for energy-dependent coatings. Time-variable samples were coated for a range of polymerization times from 30 s to 20 min with the optimum W/F value of 0.15 kJ·cm−3 (RF power = 5 W, thiophene flow rate = 2 sccm). After plasma polymerization of thiophene, plasma treatment of PPT was conducted using air and oxygen plasmas. For each batch of plasma treatments, thiophene was initially plasma-polymerized at the optimum W/ F ratio (0.15 kJ·cm−3) and a deposition time of 10 min to obtain a PPT film with a thickness of ∼57 nm. Plasma treatment processes were implemented on PPT films immediately after deposition, without removal from the plasma
3. RESULTS AND DISCUSSION 3.1. Deposition of Plasma-Polymerized Thiophene Films. 3.1.1. Influence of Plasma Input Specific Energy. The power-to-monomer-flow-rate ratio (W/F), also known as specific energy, is a crucial parameter in plasma polymerization, as it demonstrates the available energy per unit volume of the monomer.43 Optimization of plasma polymerization parameters including the specific energy and deposition time was carried out to obtain PPT films with the maximum contribution of sulfur-containing groups. To optimize the input specific energy, plasma polymerization of thiophene was conducted at a constant polymerization time of 2 min, as the W/F ratio was varied in a range of 0.03−2.4 kJ·cm−3. XPS survey spectra of uncoated silicon wafers showed silicon, oxygen, and adventitious carbon with concentrations of 62%, 33%, and 5%, respectively. The XPS atomic concentrations and thicknesses of 1445
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the PPT coatings as functions of W/F ratio are shown in Figure 1. As observed, upon an initial increase in specific energy, the
can be explained by the predominance of the ablation process. At these W/F ratios, the available energy per unit volume of the monomer is excessively high, resulting in the formation of super-high-energy ions that can partially ablate the deposited PPT film back into the plasma state.44 We also recently showed that the deposition rate of 1,7-octadiene decreases at high input energies through a similar ablation mechanism.29 According to these data, it can be concluded that a W/F ratio of ∼0.15 corresponds to the maximum deposition of PPT film with the highest contribution of sulfur atoms. 3.1.2. Influence of Plasma Polymerization Time. To determine the influence of polymerization time on PPT surface chemistry and film thickness, the optimum W/F ratio of 0.15 kJ·cm−3 was kept constant as the deposition time was varied from 30 s to 20 min. The elemental chemical compositions and thicknesses of the PPT coatings as functions of deposition time are plotted in Figure 2. With increasing polymerization time,
Figure 1. (a) XPS survey elemental composition and (b) film thickness of PPT coatings as functions of the input specific energy (plasma polymerization time = 2 min).
sulfur atomic concentration increased from 0% for the uncoated silicon wafer to more than 20% for the samples coated in the range of 0.15−0.3 kJ·cm−3. The carbon concentration also increased significantly, whereas the silicon and oxygen signals originating from the underlying substrate decreased. When the input specific energy was increased above 0.3 kJ·cm−3, the atomic concentration of carbon noticeably dropped, whereas that of silicon increases. For these coatings, a slight increase in oxygen concentration and a small decrease in sulfur concentration were also observed. These changes are consistent with an increase in the PPT film thickness as a function of W/F ratio as observed from Figure 1b. Such findings indicate that, as the W/F ratio was increased to approximately 0.15 kJ·cm−3, the deposition of PPT increased because of the availability of more energy for the fragmentation of thiophene monomer. Increasing the W/F ratio beyond 0.3 kJ·cm−3 resulted in the detection of silicon signals. The presence of silicon in the observed spectra implies that the thicknesses of the PPT films deposited at high energies were below the analyzing depth of XPS, which is 8−10 nm for a Mg Kα source and a takeoff angle of 90°.27 These findings are in agreement with ellipsometry results, which showed a thickness of 50 nm), so that the influence of the plasma treatment on the ablation process could also be investigated along with the oxidation process. 3.2. Air/Oxygen Plasma Treatment of Plasma-Polymerized Thiophene Films. 3.2.1. Influence of Plasma Input Specific Energy. Plasma treatment of PPT films with air and oxygen plasmas was carried out to investigate the oxidation and/or ablation of PPT films. Oxidation of polymers with oxidative plasmas is always accompanied by some degree of ablation. The predominance of one of these two processes over the other is predominantly controlled by the processing parameters such as the chemistry of the gas, the input specific energy, and the treatment time.43,49 The kinetic energy of the ions arriving at the surface is highly dependent on the external parameters of the plasma polymerization process, including the 1447
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high W/F ratios, the balance of oxidation and ablation processes tends more toward ablation. Such behavior can once again be explained by a greater number of ions that have sufficient energy to etch the PPT film before it becomes significantly oxidized. These findings are consistent with previously reported results on the modification of conventional polymers with SO2 plasma.6 According to surface chemistry and film thickness data, it can be suggested that, at specific energies in the range of 0.24−0.45 kJ·cm−3, the maximum oxidation of PPT film can be achieved, where the impact of the ablation process is less pronounced. 3.2.2. Influence of Air/Oxygen Plasma Treatment Time. The maximum incorporation of oxygen atoms into the treated PPT films was achieved at an input specific energy of approximately 0.24 kJ·cm−3. Plasma treatment time also influences the surface chemistry and thickness of PPT films through oxidation/ablation processes. The influence of plasma treatment time on the surface chemistry and film thickness of PPT coatings was evaluated by varying the air/oxygen treatment time from 2 to 60 min, while keeping the input specific energy constant at the optimum value of 0.24 kJ·cm−3. The chemical composition of the PPT films as a function of plasma treatment time is plotted in Figure 5. As observed, with the initial increase of the treatment time to 2 min, the oxygen concentration increased from 4% for the untreated PPT film to approximately 16% for both air- and oxygen-treated PPT films. The carbon and sulfur concentrations dropped considerably in the early stages of air/oxygen plasma treatment, whereas a small concentration of nitrogen (∼ 2%) was incorporated into the surface. No silicon was observed for samples treated for less than 45 min. When the treatment time was increased beyond 2 min, the carbon concentration continued to decrease, whereas the sulfur concentration remained almost constant. These results indicate that the oxidized carbon-containing products formed during the plasma treatment process were less stable and more volatile than the sulfur-containing products and could therefore be readily etched and evacuated from the plasma chamber. This behavior was more apparent for PPT films treated with oxygen plasma for longer than 30 min. Less than 5% carbon and approximately 50% silicon were detected for these films, consistent with the film thickness measurements (film thickness < 5 nm). The atomic concentration of sulfur observed for these samples was greater than 12%, however, which initially seems inconsistent with the carbon concentration. Such a high S/C ratio at very late stages of oxygen treatment might suggest a structure composed of sulfur, oxygen, and silicon but almost free of carbon atoms. A proposed structure for these surfaces is depicted in Figure 6. This structure suggests the presence of highly oxidized sulfur compounds such as sulfate and sulfone groups on the surface that are bonded to the silicon substrate. These compounds are highly stable and are not volatile compared to oxidized carbon compounds such as CO2.52 It can also be assumed that the input specific energy of 0.24 kJ·cm−3 was not great enough to produce high-energy ions capable of breaking a significant number of either SiO or SO bonds in 60 min of treatment. Thus, a relatively high concentration of sulfur atoms was observed on the surface, despite the very limited concentration of carbon atoms. The surface chemistry of the PPT coating treated with oxygen plasma for 90 min showed a chemistry similar to that of a silicon wafer, indicating that no film remained on the surface.
contamination as a result of residual air in the chamber or traces of nitrogen in the oxygen gas and (ii) postdeposition grafting of atmospheric nitrogen onto the surface. Long-lived reactive carbon- or sulfur-centered radicals are generated during the plasma treatment process mainly through hydrogen abstraction or polymer chain scission.49 These radical sites are highly susceptible to postdeposition reactions once they are exposed to atmosphere.3,26 Atmospheric oxygen molecules are a better candidate for reacting with surface radicals, however, as their reactivity is considerably higher than that of nitrogen molecules.32 Thus, it can be concluded that plasma-phase contamination played a more significant role in grafting nitrogen atoms onto the treated PPT films. The absence of nitrogen atoms from the surfaces of PPT films treated at W/F ratios lower than 0.24 kJ·cm−3 further supports this conclusion. At the lowest W/F ratio of 0.06 kJ·cm−3, low incorporation of oxygen atoms into the treated film was observed, particularly for the air plasma treatment. It appears that, at low W/F ratios, a minor fraction of electrons gained sufficient energy for the dissociation of oxygen bonds, so fewer oxygen radicals originated in the plasma and arrived at the surface for oxidation reactions. With increasing input specific energy, the oxygen concentration increased and reached approximately 17% for W/F ratios of 0.24−0.45 and 0.24−1.05 kJ·cm−3 for air and oxygen plasmas, respectively. With further increases in the W/F ratio, the oxidation of the PPT films decreased, as indicated by a decrease in the oxygen concentration. Such a decrease in surface oxidation can be attributed to the predominance of the ablation process at very high energies. The concentration of oxygen atoms at high input specific energies was found to be considerably lower for air plasma compared to oxygen plasma. Such a difference can be explained by the higher oxidation kinetics of oxygen plasma, which results in the immediate oxidation of the ablated surface. The film thickness data presented in Figure 4 can help to further elucidate this
Figure 4. Film thicknesses of PPT films treated with air and oxygen plasmas as functions of the plasma specific energy (plasma treatment time = 2 min).
hypothesis. As observed, when the plasma specific energy was increased, the thickness of the PPT film decreased for both air and oxygen plasmas, thus suggesting significant etching of the PPT films. At W/F values greater than 0.45 kJ·cm−3, the etching of the PPT films became more significant. For example, the thickness reduction of the PPT films upon oxygen plasma treatment increased from less than 3% for W/F = 0.06 kJ·cm−3 to more than 40% for W/F = 2.4 kJ·cm−3. It appears that, at 1448
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Figure 5. XPS survey elemental compositions of untreated and air-/oxygen-plasma-treated PPT films as functions of the plasma treatment time (plasma specific energy = 0.24 kJ·cm−3).
structure of the S 2p spectra changed markedly upon plasma treatment, which suggests a significant modification of the surface chemistry. Once the PPT film was treated with air or oxygen plasma, two new peaks with binding energy values centered at 165−167 and 167−169 eV emerged, corresponding to low and high oxidation states, respectively. Individual groups in observed peaks were not defined separately because of the proximity of their binding energies.16,32 The significant formation of oxidized groups as a result of air/oxygen plasma treatment, however, is apparent from these spectra. Calculated atomic concentrations of different sulfur-containing groups are listed in Table 1. As observed in Figure 7a, as the air/oxygen treatment time was increased, there was an increase in the concentration of oxidized sulfur moieties and a decrease in the concentration of sulfur atoms in neutral environments. Upon plasma treatment of the PPT films, the atomic concentration of total oxidized sulfur moieties increased from 4.9% for the untreated PPT film to 54% and 96% for the films treated for 60 min by air and oxygen plasmas, respectively. These results imply that almost complete oxidation of the PPT films was achieved for oxygen plasma in comparison to air plasma. These changes, which are consistent with the increase in the oxygen concentration observed from the survey spectra (Figure 5), indicate the successful production of oxidized sulfur-functionalized plasma polymer films. It is proposed that oxidation of thiol groups in a PPT film is carried out through a combination of activation and oxidation processes. According to this theory, activated free-radical sites initially form on the PPT film as a result of the bombardment of active species from the air/ oxygen plasma. These unstable radicals become oxidized into
Figure 6. Proposed structure for a PPT film treated with oxygen plasma for 60 min.
To quantify the concentrations of contributing sulfurcontaining components in the untreated and plasma-treated PPT films, the S 2p high-resolution peaks were curve-fitted as shown in Figure 7a. The S 2p peak is a doublet due to the spin−orbit splitting of S 2p1/2 and S 2p3/2.53 The photoionization cross section of S 2p1/2 is one-half that of S 2p3/2; thus, the area ratio of S 2p1/2 and S 2p3/2 was set to 1:2 with a splitting binding energy (BE) of 1.2 eV.1,28 According to the data in the literature,28,32,54,55 high-resolution S 2p peaks can include three categories of components with different oxidation states: (i) unoxidized sulfur-containing groups (SH, SC, SS) at BEs of 163−165 eV, (II) oxidized sulfur-containing groups in low oxidation states (SO, SO2) at BEs of 165−167 eV, and (iii) oxidized sulfur-containing groups in high oxidation states (SO3H, SO3, SO4) at BEs of 167−169. For each category, two peaks were fitted corresponding to 1/2 and 3/2 spins, giving a total of six peaks. XPS S 2p high-resolution spectra of PPT films show a peak at a binding energy of 163−165 eV corresponding to thiol or sulfide groups.32 As observed, the 1449
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Figure 7. XPS (a) S 2p and (b) C 1s peak-fitted spectra of PPT films treated with air and oxygen plasmas as functions of the treatment time (plasma specific energy = 0.24 kJ·cm−3).
Table 1. XPS Atomic Concentrations (%) of Curve-Fitted S 2p and C 1s Components for Untreated and Air-/Oxygen-PlasmaTreated PPT Films sulfur-containing moieties oxidation time (min)
S−H, SC, SS
0
95.1
2 6 10 20 30 45 60
82.0 80.0 72.3 56.5 53.3 42.9 32.5
2 6 10 20 30 45 60
79.3 71.4 69.4 56.8 56.9 1.5 3.5
SO, SO2
carbon-containing moieties SO3, SO3H, SO4
Untreated PPT 4.6 0.2 Air-Plasma-Treated PPT 5.2 12.7 6.4 13.6 5.5 22.1 4.0 39.5 3.6 43.0 4.32 52.8 2.5 52.0 Oxygen-Plasma-Treated PPT 6.3 14.4 7.4 21.1 7.0 23.5 5.0 38.1 5.1 37.9 24.7 73.8 23.0 73.4
CC/H
COH/R
CO
COOH
89.3
8.3
1.7
0.6
77.0 74.4 73.0 71.2 69.4 68.0 70.0
13.9 16.0 16.4 15.7 19.0 19.4 19.1
5.3 5.4 5.6 5.7 4.9 6.0 6.7
3.8 4.1 4.7 7.4 6.6 6.5 4.1
75.0 72.0 72.9 67.9 69.1 − −
14.7 17.8 17.6 18.7 18.1 − −
5.0 5.4 5.1 6.2 6.0 − −
5.2 4.6 4.3 7.0 6.8 − −
oxidation process because of their faster recombination. For oxygen-plasma-treated samples, increasing the treatment time beyond 30 min resulted in an abrupt increase in the
more stable groups as they react with oxygen radicals generated in the plasma environment.56 As reported in the literature,56−58 radicals play a more significant role than ions in the activation/ 1450
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concentration of total oxidized moieties from 43% to more than 95%. For these thin coatings (film thickness < 5 nm), almost no thiol and sulfide functionalities were observed, whereas highly oxidized sulfur groups were the major components of S 2p high-resolution peak. Such changes in the S 2p high-resolution peak further support the proposed structure for these surfaces illustrated in Figure 6. To study the formation of different oxygen−carbon moieties during the air/oxygen plasma treatments, C 1s high-resolution peaks were curve-fitted as shown in Figure 7b. Four components were fitted to the C 1s core-level peak at binding energies of 285.0, 286.5, 287.9, and 289.0 eV corresponding to CC/H, COH/R, CO, and COOH, respectively.26 The signal at ∼286.5 eV could also reflect CS and COS groups. These groups were not separately fitted because of the considerable uncertainty arising from their overlap with C OH/R groups.28 According to the peak-fitting results listed in Table 1, through either air or oxygen plasma treatment, the concentration of oxygen-containing moieties increased whereas that of carbon atoms in the neutral environment decreased. The formation of these functionalities can once again be explained by the activation/oxidation mechanism discussed for oxidized sulfur groups. Nevertheless, the increase in the rate of oxygen− carbon moieties was not as significant as that of oxygen−sulfur moieties. As discussed earlier, such a difference can be attributed to the higher stability/lower volatility of oxidized sulfur-containing compounds in comparison to oxidized carbon-containing products. Thus, a lower concentration of carbon-containing groups remained on the surface after the plasma treatment. The absence of carbon signals in PPT films treated with oxygen for 45 and 60 min can also be closely linked to this concept. The surface-incorporated sulfur−oxygen moieties were further identified by ToF-SIMS normalized negative counts as shown in Figure 8. It was observed that air/oxygen plasma treatment of the PPT films resulted in a significant decrease of thiol (SH) counts and an increase in oxidized sulfur fragments. Consistent with the XPS results, sulfur atoms in untreated PPT films were mainly in the form of thiol (SH) and sulfoxide (SO) groups. As expected, sulfur-containing functionalities at high oxidation states were not significantly observed for untreated PPT films. On the contrary, a significant contribution of −SOx(H) species was observed for air-/oxygen-plasma-treated PPT films. The main oxidized sulfur-containing species at high oxidation states included SO2, SO3, SO3H, SO4, and SO4H. These results are of particular significance because of the proximity of these groups in S 2p high-resolution spectra, which make them undistinguishable. Initial treatment of PPT films with oxygen and air plasmas resulted in an abrupt decrease in the counts of thiol groups, whereas that of oxidized sulfur functionalities significantly increased. When the treatment time was increased over 2 min, the counts of sulfur−oxygen species did not markedly change. This behavior might initially appear to be inconsistent with the XPS results, which still showed a significant change in surface functionalities for treatment times longer than 2 min. Such a discrepancy, however, results from the different sampling depths of ToF-SIMS (1−2 nm)59 and XPS (8−10 nm).27 It can therefore be suggested that the oxidative reactions penetrate into the sublayers of PPT films at extended treatment times, whereas the very topmost part of the surface remains saturated. Similar behavior has also been reported for the oxygen plasma treatment of conventional poly(methyl methacrylate).56 The sample treated with oxygen
Figure 8. Average normalized negative SIMS counts of sulfur−oxygen species from ToF-SIMS for PPT films treated with (a) air and (b) oxygen plasmas (95% confidence intervals, N ≥ 9; plasma specific energy = 0.24 kJ·cm−3).
plasma for 60 min showed very limited counts of thiol groups, but high counts of highly oxidated species such as SO3, SO4, and SO4H were still detected. These data are in agreement with the highly oxidated sulfur-reacted structure that was proposed for this surface (Figure 6). In the plasma treatment of a polymer, ablation of the surface is inevitable, and it takes place simultaneously with the oxidation process. From the film thickness data shown in Figure 9, film thickness reduction rates of approximately 0.3
Figure 9. Film thicknesses of PPT films treated with air and oxygen plasmas as functions of the plasma treatment time (plasma specific energy = 0.24 kJ·cm−3). 1451
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and 0.9 nm·min−1 were observed for PPT films treated with air and oxygen plasmas, respectively. Oxygen plasmed shows greater ablation than air. This behavior was expected and can be attributed to the higher reactivity of oxygen atoms compared to nitrogen. It is well-known that the partial pressure of oxygen in the plasma treatment is directly related to the flux of reactive atoms arriving at the surface.60 The lower partial pressure of oxygen in air reduces the formation of volatile species on the surface, thus resulting in a reduction of the ablation rate. Thiophene plasma polymerization followed by air/oxygen plasma treatment was thus found to be an efficient approach to the fabrication of SOx(H)-functionalized plasma polymer films. The novel method introduced here demonstrates a number of advantages over previous approaches.28,37,38,40 This process is substrate-independent and can be accomplished without the incorporation of any solvent. Toxic SO2 gas is not involved in the process, and SOx(H)-functionalized surfaces with higher incorporations of sulfur atoms (sulfur atomic concentration ≈ 20%, S/C > 0.38) can be obtained.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the Government of South Australia, through the Premier Science and Research Fund (PSRF), and the National Centre of Excellence in Desalination Australia (NCEDA). The authors acknowledge the facilities and scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the South Australian Regional Facility (SARF), University of South Australia, a facility that is funded by the University and State and Federal Governments. We are thankful to Dr. John Denman for undertaking ToF-SIMS measurements. The valuable advice of Mr. Thomas Michl is greatly appreciated.
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4. CONCLUSIONS A novel method has been introduced to develop oxidized sulfur-containing surfaces. These surfaces were successfully fabricated through a combination of thiophene plasma polymerization and air/oxygen plasma treatment of deposited films. Plasma treatment of plasma-polymerized thiophene significantly influenced both the surface chemistry and film thickness of the coatings. XPS results, corroborated by ToFSIMS data, suggested that the concentration of oxidized sulfurcontaining functionalities substantially increase upon either air or oxygen plasma treatments. Untreated plasma-polymerized thiophene mainly consists of neutral carbon- and sulfurcontaining moieties, whereas upon plasma treatment, oxidized carbon and sulfur-containing functionalities generate on the surface. Attachment of oxygen-containing groups on plasmapolymerized thiophene film was found to be accompanied by a concurrent etching process. The predominance of these two processes is strongly governed by the applied specific energy. At low energies, the oxidation process is predominant, whereas at higher energies, ablation of the film becomes prevalent. The maximum oxidation of PPT films was achieved at specific energies in the range of 0.24−0.45 kJ·cm−3, where the impact of ablation process was less pronounced. Air plasma showed a moderate ablation potential (thickness reduction rate = 0.3 nm· min−1) compared to oxygen (thickness reduction rate = 0.9 nm·min−1) and is therefore a better candidate for the oxidation of plasma-polymerized thiophene. The novel method developed in this work is green and dry and produces higher concentrations of −SOx(H) functionalities (sulfur atomic concentration ≈ 20%, S/C > 0.38) than previously reported methods. Combined with the substrate independency of the plasma polymerization process, this approach can be employed to deposit −SOx(H)-functionalized coatings onto almost any substrate regardless of its chemical composition. These coatings are of great interest in numerous applications including biomaterials, fuel cells, and water purification.
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