Thin Organic Layers Grown on the Surface of Iron Particles under High

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Thin Organic Layers Grown on the Surface of Iron Particles under High-Energy Ball Milling in the Presence of Polystyrene and Various Surfactants: X-ray Absorption and Photoelectron Spectroscopy Studies Alena N. Maratkanova,*,† Alexander V. Syugaev,† Anatoly A. Shakov,† Oleg Yu. Vilkov,‡,§ and Svetlana F. Lomayeva† †

Physical-Technical Institute UB RAS, 132 Kirov Street, 426000 Izhevsk, Russia V.A. Fock Institute of Physics, Saint-Petersburg State University, 198504 St. Petersburg, Russia § Institute of Solid State Physics, Dresden University of Technology, D-01062 Dresden, Germany ‡

ABSTRACT: High-energy mechanical milling is conventionally and successfully used for fabrication of magnetic filler particles for metal−polymer composites applied in different microwave absorption devices. Chemical modification of the metal surface by wet mechanochemical synthesis allows one to improve chemical compatibility between a metal particle and a host polymer matrix in the composites. In this paper, we have studied the structure of an as-modified thin organic layer depending on different surfactants used under mechanical milling of iron powder in the polystyrene solution. The study was performed with high-resolution X-ray spectroscopic techniques, using synchrotron radiation from the BESSY II storage ring. It has been shown that stearic acid, added as a surfactant into the milling environment, forms a close-packed thin layer, but chemical inertness of its alkyl groups does not provide strong anchoring of the polystyrene fragments to the iron surface. The perfluorononanoic acid molecules form a thin layer with preferably normal orientation of their backbones to the metal surface. The molecules are partially defluorinated under mechanochemical synthesis facilitating appropriate linkage between polystyrene fragments and iron. Mechanical milling with the use of perfluorinated carboxylic acids has the potential to modify the metal surface by polystyrene fragments and then to improve the chemical compatibility and adhesion between the constituents in the metal−dielectric composites.

1. INTRODUCTION Nowadays the investigation of interfaces between thin organic films and metals attracts increasing attention and efforts of the researchers working in the fields of applied physics, chemistry, and materials science because of great importance of surface and interface phenomena affecting many distinguished properties of materials. However, the influence of the surfaces and interfaces on the materials characteristics and fundamental aspects of the interface formation and behavior are still far from being comprehensively understood. Granular ferromagnetic metal/dielectric matrix composites are of vivid interest because of their unique properties favorable for microwave applications, e.g., as microwave absorbing materials, frequency filters, or substrates for microstrip antennas.1−3 Such composites belong to the class of advanced composites, so-called magnetodielectrics, and may combine appropriate magnetic parameters with high electrical resistivity due to the dielectric layer between magnetic inclusions. The structure of particles is composed of core and shell phases with completely different physical−chemical characteristics. Fabricating the magnetodielectric composites, it is important to provide the chemical compatibility between the inclusions and dielectric matrix, i.e., to form the interfacial region which © 2012 American Chemical Society

provides their good adhesion and prevents degradation of the composite properties. This region plays a key role in nanomaterials due to the increasing interaction surface. As the particles size decreases, the surface and interface effects become more important, particularly in the materials applied at microwave frequencies because of an extremely short skin depth considered here. To achieve better interconnection between the dielectric matrix and metal inclusion, we proposed an approach based on high-energy mechanical milling of the metal filler in an organic medium that is chemically close to the dielectric matrix material used for subsequent fabrication of magnetodielectrics.4 It is well-known that mechanical milling is a very effective, simple, and promising way to produce nanostructured materials.5,6 The surfactants added into the milling mixture affect significantly the milling process, including controlling not only the particle’s size but also their shape due to the effects of lowering the surface energy, fracturing the powder particles along some preferred crystalline orientations and the impediment of cold Received: March 23, 2012 Revised: May 28, 2012 Published: June 12, 2012 14005

dx.doi.org/10.1021/jp302788s | J. Phys. Chem. C 2012, 116, 14005−14013

The Journal of Physical Chemistry C

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welding and agglomeration of flakelike particles.7 The use of organic liquids as a milling environment was shown to allow one to form an intermediate shell layer on the metal particle’s surface which stabilizes and protects it,8,9 providing its chemical affinity to the host matrix and then their better adhesion. To tailor the desirable physical properties of the metal/ dielectric matrix composites, thorough analytical characterization of both metal inclusions and shell layer of the mechanically synthesized filler particles is extremely important. To get a deeper insight into the bonding between the metal inclusions and organic molecules of the shell layer, the use of surface sensitive analyzing techniques is necessary. X-ray photoelectron spectroscopy (XPS) is often and successfully used to analyze the chemical structure of ultrathin films and organic molecules adsorbed on solid surfaces10,11 owing to its high surface sensitivity. The analysis of the surface chemical bonds and understanding of the mechanism of the interaction between polymer and metal in the interface layer are crucial to control materials properties. A great number of works dealing with polymers adsorbed on metal surfaces, e.g. in adhesion science12−14 or corrosion protection,15,16 are currently performed with this technique. The chemical shifts and number of peaks in the XPS core-level spectrum of the element analyzed are a valuable source of information on the chemical functionality of a surface. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy has become a powerful tool to probe electronic and structural properties of new materials,17−20 adsorbates,21 thin films, and liquids.17,22−24 The necessary requirement for measuring the NEXAFS spectra as a function of photon energy is the use of synchrotron radiation which provides a monochromatic light source with tunable photon energy and high brilliance. Since its development in the 1980s, NEXAFS spectroscopy has been applied mostly to the materials consisting of low-Z molecules and atoms such as carbon, nitrogen, oxygen, and fluorine.17 NEXAFS spectroscopy measures the photoabsorption cross section for the excitation of the tightly bound core electrons into unoccupied molecular orbitals or vacuum continuum as a function of photon energy. This technique is element specific because each chemical element has a characteristic core-level binding energy. In the C K-edge NEXAFS spectroscopy, an electron is excited from the ground 1s state into unoccupied π* or σ* molecular orbitals. The peak positions and shape in the spectrum reflect the nature of these unoccupied electronic states. Because of its sensitivity to molecular orbitals instead of atoms, the NEXAFS technique is capable of distinguishing between C−C and CC bonds. This feature makes NEXAFS spectroscopy a valuable technique that is complementary to XPS conventionally used for characterization of organic materials. In some cases, in particular, when applied to polymer science, NEXAFS turns out to be much more suitable compared to Xray photoelectron spectroscopy. NEXAFS spectroscopy has been successfully applied to studying carbonaceous materials such as synthetic and natural polymers which exhibit a rich carbon, nitrogen, and oxygen K-edge structure.25−29 NEXAFS spectroscopy has proved to be a powerful technique for investigation of orientation of molecules and polymers on surfaces.30−35 When studying well-oriented long-chain organic films, a variety of factors are taken into account, including the chain length, the substrate type, and the deposition parameters (substrate temperature, deposition rate, etc.).36 However, the mechanisms of growth and structure of the thin films formed

under severe plastic deformation in organic liquids have not been studied so far. In this work for the first time we have studied the molecular orientation of the thin organic films grown on the metal surface under mechanical milling in organic liquids. The objective of the present study is to carry out the detailed experimental study of mechano-synthesized particles surface layer by means of high-resolution NEXAFS and XPS spectroscopies to extract information on the elemental composition, chemical bonds, electronic and atomic structure of the layer, and its molecular orientation. These data are necessary to trace the transformations on the metal surface under mechanical milling of iron in organic liquids. The results obtained will be helpful for the development of an experimental procedure (including the starting mixture composition, organic medium composition, type and quantity of the surfactant, etc.) for the fabrication of metal fillers with the surface layer providing its good affinity to the matrix and the desirable frequency response.

2. EXPERIMENTAL SECTION Samples and Preparation. The samples were produced by mechanical milling of the Fe powder (99.98%) in the polystyrene (PS, Sigma-Aldrich) solution in p-xylene. The starting Fe powder particles had a stonelike shape, with the size of less than 5 μm. Mechanical milling was carried out in a planetary ball mill Fritsch P7. The vials and balls (16 pcs, D = 12 mm) were made of hardened steel containing 1.0 wt % C and 1.5 wt % Cr. The sum weight of Fe and PS in the starting mixture was 10 g, with the Fe:PS ratio being 80:20 vol %. The reaction mixture was prepared immediately in the vial. The total amount of p-xylene was 20 g. Polystyrene was preliminarily dissolved in the heated distilled p-xylene (10 g) and then transferred quantitatively into the vial with an additional 10 g of solvent. After that, the iron powder (9.673 g) was loaded into the vial with an appropriate surfactant (0.6 or 0.09 g) if that was the case. We used C17H35COOH (stearic acid, SA), C8F17COOH (perfluorononanoic acid, PFNA), and C18H37NH2 (octadecylamine, ODA) as surfactants in the ratios listed in Table 1. In the samples synthesized with mixed surfactants the ratio between SA and PFNA was calculated proceeding from the ratio of CFx/COOH groups equal to 1 and 3 for the MIX-F1 and MIX-F2 samples, respectively. The designations of the samples are given in Table 1; the Table 1. Sample Designation Based on the Composition of Surfactant in the Starting Mixture for Mechanochemical Synthesisa surfactant composition, wt % sample S-0 SA ODA PFNA-1 PFNA-2 MIX-N MIX-F1 MIX-F2

SA

PFNA

ODA

3 3 0.5 3 2.5 2.5 1.9

0.5 0.5 1.1

a

Percentage of each surfactant is given with respect to the weight of the solvent (p-xylene). The sum weight of Fe and PS in the starting mixture was 10 g with the Fe:PS ratio of 80:20 vol. %.

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Figure 1. SEM images of (a) S-0 sample powder synthesized without surfactants and (b) MIX-F1 sample synthesized with the mixed surfactant (stearic acid plus perfluorononanoic acid).

spectra that do not depend on the irradiation time. The measured spectra were analyzed using the procedure described in detail elsewhere.38 The spectra were normalized to the incident photon flux. The C K-edge absorption spectral features were calibrated with respect to the energy position of the C 1s → π* resonance of the measured absorption spectrum of the highly oriented pyrolytic graphite (HOPG) as being at 285.35 eV.39 In order to study the molecular orientation of the surface layer, we have measured the polarization-dependent C K-edge NEXAFS spectra for some samples. NEXAFS spectroscopy is known to be sensitive to the molecular orientation, since the resonance intensities vary as a function of the direction of the polarization vector E of the incident X-ray beam relative to the axes of the molecular symmetry.17 The peak intensity is enhanced when the polarization vector of the X-rays is parallel to the direction of the final state orbital. To obtain the polarization-dependent NEXAFS spectra, the vertically mounted samples were rotated around the vertical axis of the manipulator lying in the surface plane of the samples. When the X-rays are incident close to the normal direction to the sample surface, the E vector is parallel to the sample surface. For the cases of linear long-chain molecules, if the molecule backbones are oriented normally to the surface, the E vector is perpendicular to the backbone axis and the peaks due to the transitions into the side group orbitals (e.g., C−H or C−F) will be dominant. In opposite, if the chains are laterally oriented, i.e. parallel to the substrate surface, the angle between the E vector and the molecule chain will change from 90° to 0°, giving rise to the intensities of resonances due to the transitions into the backbone orbitals (e.g., C−C). As a result, we can estimate the molecular orientation of the molecule backbones on the metal surface and find out whether the molecules have normal or lateral orientation or disorder. All the measured NEXAFS spectra were postedge normalized, using the procedure described elsewhere40 to compare the spectra measured at different angles and the spectra of different samples. The X-ray photoelectron spectra were measured at the photon energies varied from 385 to 1000 eV with the monochromator energy resolution of 125−500 meV within this range. All the XPS spectra were acquired using the normal photoemission geometry with the SPECS PHOIBOS 150 hemispherical electron energy analyzer. The energy scales of the spectra were calibrated to eliminate the charging effect by referencing the C 1s peak of hydrocarbons to 285.0 eV.41 The high-resolution spectra were analyzed in order to determine the various chemical species present. Each spectrum was

concentration of the surfactant is indicated with respect to the solvent weight (20 g). The duration of mechanochemical synthesis in a planetary ball mill was 24 h. The temperature of the vial walls did not exceed 60 °C during the milling procedure due to the forced air-cooling used. After milling, the vials were cooled to the room temperature and the powder was separated by decantation. The precipitate was boiled in 10 mL of n-hexane for 1 min and then ultrasonicated for 15 min. This procedure was repeated 5 times with decantation of the solvent and its substitution for a new portion, preventing the precipitate from its contact with air. For a long-term protection from oxidation by atmospheric oxygen, the precipitate was stirred under a layer of hexane with 1 g of melted paraffin, and then the solvent was evaporated at 90 °C. The same rinsing procedure was used to remove carefully the paraffin layer that served as a conserving shell for the powder surface. For the analysis by means of NEXAFS and XPS spectroscopies the powder suspension in hexane was dropped onto a rough copper substrate immediately before loading into the spectrometer vacuum chamber. So, after all the preparation stages the chemisorbed organic shell layers on the metal particles surface were under study. Techniques and Instrumentation. The NEXAFS and XPS spectra were measured at the Russian-German (RG-PGM) dipole beamline at BESSY II, Helmholtz-Zentrum Berlin. The beamline uses the plane-grating monochromator with the 400 and 1200 lines/mm gratings to select the photon energies in the range from 40 to 1500 eV, with the resolving power of up to E/ΔE =100 000. A detailed description of the beamline can be found elsewhere.37 All the X-ray absorption and photoemission spectra were taken under ultrahigh vacuum conditions (∼5 × 10−10 Torr). The NEXAFS data were acquired in a total electron yield (TEY) mode by measuring the sample drain current with varying photon energy. The photon beam incidence angle was varied between 25° and 75° relative to the sample surface. The energy resolution was equal to ∼80 and ∼150 meV for the range of C K-edge (∼285 eV) and F K-edge (∼695 eV), respectively. The sample damage under synchrotron radiation exposure was not significant owing to (1) use of the low-alpha multibunch hybrid operation mode that is characterized by 10 times reduced length of the photon pulses and correspondingly decreased number of photons per second and (2) use of radiation from a dipole beamline which is focused to a rather large spot (0.5 × 0.5 mm2) on the sample surface. This is confirmed by the line shapes of both the NEXAFS and XPS 14007



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Figure 2. Carbon K-edge TEY NEXAFS spectra for all the samples synthesized with use of (a) no fluorine-containing surfactants and (b) fluorinecontaining surfactants. The sample designation based on the composition of the surfactant in the starting mixture used for mechanochemical synthesis is given in Table 1.

∼286.8 eV, which can be attributed to the aromatic rings with an electron-acceptor group (−OH, −COH, ⟩CO, −COOH). Evidently, this absorption mechanism, which is not faultless and rather sporadic, could not allow polymer molecules to bind strongly to the metal surface. Another adsorption mechanism can be attributed to the dehydrocondensation of the polystyrene fragments to the graphitic structures formed on the surface under mechanical milling. The clarification of the C 1s → π*(CC) peak assignment for this case is under further investigation. Besides the peaks mentioned above for the pure polystyrene, the spectrum is characterized by a sharp peak at 288.5 eV from the C 1s → π*(CO) resonance and the C 1s → σ*(C−H) resonance as a shoulder at ∼287.3 eV. The polarization-dependent C K-edge absorption spectra for the S-0 sample do not reveal any substantial differences with changing the sample orientation relative to the X-ray beam. This fact may be interpreted by the stonelike shape of the particles and their agglomeration, giving rise to a very rough surface (Figure 1). The spectrum for the ODA sample is similar to that of the synthesized one without any surfactant (Figure 2a). Thus, the presence of this surfactant in the milling liquid does not prevent adsorption of the polystyrene fragments or the graphitic layer formation on the metal surface. Apart from the resonances already mentioned, the spectrum exhibits a weak structure below 285 eV, which can be attributed to the amorphous-like or carbidic carbon and may be caused by a marginal radiation damage during the measurements, as it was observed elsewhere46,47 as well as for some other samples in this study. In the case of the SA sample, the resonance from the phenyl group is very weak and broad. Stearic acid is known to be a very strong surfactant which is chemisorbed on the metal surface with the formation of a thin,48 mono- and bimolecular, closepacked layer. The chemisorption of SA occurs due to the formation of stable carboxylate complexes revealed by the C 1s → π*(CO) resonance at 288.5 eV. The inertness of the alkyl group prevents strong coupling of the polystyrene molecules to the outermost layer of the metal particle surface.

deconvoluted using the generalized regular algorithm for solving the ill-posed problems.42 The morphology of the particles was characterized by scanning electron microscopy (SEM) with the use of LEO 982 FE-SEM microscope. The samples for microscopic study were prepared using the procedure described above in the Samples and Preparation section. After dissolving the matrix, the powders were deposited on a glass substrate and dried.

3. RESULTS AND DISCUSSION The secondary electron microscopy images are shown in Figure 1 for the S-0 sample (a) obtained without any surfactant and the MIX-F1 sample (b) fabricated with the addition of the mixed surfactant (SA + PFNA) to demonstrate the effect of surfactants on the particle shape and size. It can be seen that in the first case the particles are stonelike shaped (a) while for the case of the surfactant-assisted mechanical synthesis the particles are thin plates with an average size of 5−10 μm (b). NEXAFS Spectroscopy Results. The C K-edge NEXAFS spectra are shown in Figure 2 for the samples mechanosynthesized with use of (a) no fluorine-containing surfactants and (b) fluorine-containing surfactants. The very prominent C 1s → π*(CC) resonance from the phenyl group and broad peaks from the C 1s → σ*(C−C) transitions are characteristic for the polystyrene C K-edge X-ray absorption spectrum.27 The assignment of the peaks in the measured spectra was made, using the most probable transitions according to the published data available for polymers17,27−35,43 and polymer−metal systems.44,45 The spectrum of the S-0 sample, fabricated without any surfactants, contains high-intensity C 1s → π*(CC) resonance of aromatic rings at 285.35 eV from either the polystyrene fragments or the graphitic layer formed due to the catalytic decomposition of the organic environment under mechanical milling. The polystyrene fragments can be adsorbed on the surface by the oxygen-containing groups that appear due to the partial oxidation of polystyrene, and it is confirmed indirectly by the appearance of a weak shoulder located at 14008



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weaken facilitating the formation of biradicals after the π-bond activation. The as-formed radicals react with the polystyrene fragments containing hydrogen atoms in α-position to the phenyl group, which are poorly bound and can easily withdraw. Such interaction results in free-radical reactions and binding the polystyrene fragments to the terminal defluorinated groups of the PFNA molecules as it is schematically illustrated in Figure 4. Beside free radical reactions, the linkage of the F-substituted unsaturated bonds to the phenyl groups having nucleophilic activity may be possible.

When adding perfluorononanoic acid that is most probably comparable with stearic acid as a surfactant, a similar result of chemisorption reaction might be expected. Notwithstanding, an intense C 1s → π*(CC) resonance from the aromatic rings is observed at 285.35 eV, which may be explained by the chemical bond of the fluorine-substituted tail of the PFNA molecules to the polystyrene fragments. The bump at ∼286.7 eV is attributed to the aromatic rings substituted for an electron-acceptor group C 1s(C−R) → π*(CC) which is more intense in the case of the PFNA surfactant additives as compared to all other samples. This peak appears due to the sensitivity of NEXAFS spectroscopy to electronic delocalization which is evidenced by the energy shift of the C 1s → π*(C C) transitions.49 Above 290 eV much broader peaks are observed which are assigned to the transitions corresponding to the resonances C 1s → σ*(C−F) at ∼292.5 eV, C 1s → σ*(C− C) at ∼295.5 eV, and C 1s → σ*(C−F) at ∼298.7 eV, whose intensities rise with increasing the amount of the F-containing surfactant in the milling environment. The same correlation is observed for the intensities of the F 1s → σ*(C−F) resonances at ∼684 and ∼688 eV in the F K-edge absorption spectra (Figure 3). A weak signal from the iron fluorides at ∼678.5 eV

Figure 4. Chemical reactions occurring under mechanical milling of iron powder in the polystyrene solution with the perfluorononanoic acid as a surfactant.

For the sample synthesized with a small amount of PFNA, the polarization-dependent C K NEXAFS spectra were measured (Figure 5). The intensity ratio of the σ*(C−F)/ σ*(C−C) resonances changes when going to the more grazing angle α = 75°. These changes can be interpreted on the basis of the NEXAFS studies of the surface structure and orientation of

Figure 3. Fluorine K-edge TEY NEXAFS spectra for the samples synthesized with the F-containing surfactants along with the spectrum of FeF2.

is seen in the spectra. The intensity of this signal (inset in Figure 3) correlates with the quantity of the formed iron fluorides and governed not only by the quantity of fluorine in the surfactant but also by its qualitative composition. It can be seen that even a small amount of PFNA added into the milling liquid leads to the strong anchoring of the polystyrene fragments on the metal surface due to a sequence of possible reactions at the interface: (1) The interaction of the carboxyl group with iron oxides on the surface leads to the formation of the carboxylate complexes of F-substituted acids, which are highly chemisorbed and retained even when washed with the hydrocarbon solvent. (2) The next possible mechanism is the reaction of the F-containing groups of the adsorbed PFNA with the iron atoms followed by the formation of FeF2 and unsaturated derivatives. (3) Because of the strong negative inductive effect of fluorine atoms and F-substituted groups, the π-bonds in the as-formed unsaturated derivatives

Figure 5. Polarization-dependent carbon K-edge TEY NEXAFS spectra for the sample synthesized with a small amount of perfluorononanoic acid (PFNA-1). α is the angle between the incident X-ray beam direction and the normal vector n to the sample surface. 14009



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the well-ordered fluorocarbons such as poly(tetrafluoroethylene)35,50 and semifluorinated alkane selfassembled monolayers.34,35,51 A strong polarization-dependent intensity variation of the NEXAFS features upon alteration of the angle between the projection of the E polarization vector onto the surface and the molecular axis was found in these studies. It is well-known that the carbon atoms in the PFNA molecule are sp3-hybridized with a nearly tetrahedral geometry of the F−C−F and F−C−C bonds.52 So, the antibonding orbitals of the C−C and C−F moieties are oriented roughly parallel and perpendicular, respectively, to the main helical axis. In our C K NEXAFS spectra, the intensity ratio of the σ*(C− F)/σ*(C−C) resonances has its maximum at α = 25° (close to the normal incidence) and reduces at α = 75° (grazing angle). When α is equal to 25°, the E vector is almost perpendicular to the PFNA molecular backbone and then parallel to the C−F bond direction. On the basis of such angular dependence and the published data for the well-ordered fluorocarbons,34,35,50,51 we can conclude that the PFNA molecule helix is preferentially oriented perpendicular to the surface of the metal particles, as shown schematically in Figure 6. This effect, however, is less

chemically inert SA molecules block sterically the shorterchain molecules of PFNA, preventing their binding to the polystyrene molecules as illustrated schematically in Figure 7.

Figure 7. Schematic drawing for the proposed molecular structure of the chemisorbed layer formed on the iron surface under mechanochemical synthesis in the polystyrene solution with additives of the mixed surfactant (stearic acid plus perfluorononanoic acid). The longer-chain chemically inert stearic acid molecules prevent the shorter-chain molecules of perfluorononanoic acid from their linkage with the polystyrene fragments.

The polarization-dependent C K-edge NEXAFS spectra for the MIX-F1 sample are shown in Figure 8. A decrease in the intensity of the contribution from the C 1s → π*(CO) transition when changing to the grazing angle proves that the carboxylic headgroups of both perfluorononanoic and stearic acids are in the deep-lying layer, which is cross-linked to the iron particle through the formation of the carboxylate complexes. A gradual shift of the C 1s → σ*(C−F) resonance to higher energies is observed on going to the grazing angle incidence (α = 75°). The σ* resonance energy position is established to be sensitive to the intramolecular distance between the pair of atoms giving rise to the resonance.53 This correlation may be used to evaluate the changes in the intramolecular bond lengths with the σ-shape resonances. According to this rule, the interatomic distance in the C−F molecular orbital increases when changing α from nearly normal incidence to the grazing angle. Indeed, the deeper lying layers may contain the −CFCF− double bonds which are characterized by a shorter C−F distance compared with the interatomic distances in the CF2 and CF3 groups located preferentially in the outermost surface layers and detected with the grazing angle. As a result, the position of the C 1s → σ*(C−F) resonance shifts to higher energy. The accumulation of the −CFCF− double bonds within the molecule backbone results in the change of the valence angles in the chain and declination of the molecule axis from the normal orientation relative to the metal surface. As a result, the C−F

Figure 6. Schematic drawing for the proposed molecular structure of the chemisorbed layer formed on the iron surface under mechanochemical synthesis in the polystyrene solution with an additive of the perfluorononanoic acid as a surfactant. The perfluorononanoic acid molecule helix is preferably oriented normally to the metal particle surface.

pronounced than in the well-ordered, grown in equilibrium conditions, thin fluorocarbon films. Invariance of the CO resonance intensity shows its weak polarization dependence that can be explained by the contributions of the carbonyl groups from both deep-lying (carboxylates) and outermost (oxidized polystyrene fragments) layers. As for the samples synthesized with the mixed fluorinecontaining surfactants (MIX-F1 and MIX-F2), simultaneous adsorption of PFNA and SA takes place. The intensities of the C 1s → σ*(C−F) resonances in the C K NEXAFS spectrum (Figure 2) and the corresponding F 1s → σ*(C−F) resonances (at ∼684 and ∼687.5 eV) in the F K NEXAFS spectrum (Figure 3) are consistent with the PFNA content in the starting milling mixture. For all the samples with the mixed surfactant the C 1s → π*(CC) peak from the aromatic groups is very weak, showing a little amount of the polystyrene fragments in the surface layer measured. Under simultaneous interaction of PFNA and SA with the metal surface followed by the formation of the close-packed chemisorbed layers, the longer-chain 14010



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Figure 8. Polarization-dependent carbon K-edge TEY NEXAFS spectra for the sample synthesized with the mixed surfactant (MIXF1), containing a small amount of the perfluorononanoic acid (0.5 wt %). α is the angle between the incident X-ray beam direction and the normal vector n to the sample surface.

Figure 10. Deconvolution of the C 1s X-ray photoelectron spectra for the SA and MIX-F2 samples.

bonds may have a variety of directions and the σ*(C−F)/ σ*(C−C) intensity ratio reveals no pronounced angular dependence on going from nearly normal to the grazing angle. Moreover, the analysis of the σ*(C−F)/σ*(C−C) intensity ratio is complicated in this case since the surfactant is comprised of not only fluorine-containing acid but also stearic acid, whose extended σ*(C−C) resonances are also located within the energy range above 290 eV. XPS Results. The C 1s XPS spectra for the measured samples are shown in Figure 9. The spectra for the SA and MIX-F1 samples are presented in Figure 10 as examples of the applied regularization technique used for the spectra deconvolution. The results of deconvolution for all the spectra

along with the possible assignments41,54 for the spectral features are shown in Table 2. It should be noted that in the case of stearic acid and fluorine-substituted carboxylic acid (SA and PFNA-2 samples) the feature at 288.7 is well pronounced from the iron carboxylates responsible for chemisorption. For the samples obtained with the use of fluorine-substituted carboxylic acid the intense features appear in the spectra at the binding energy above 289.0 eV characteristic for the fluorinated functional groups, the content of which increases in proportion to the content of this acid in the milling mixture. This result is consistent with the data obtained by NEXAFS spectroscopy. In addition to the information extracted from the NEXAFS spectra, the XPS revealed the following. The ratio of the CF3/ CF2 groups is in general equal to 1/3, whereas it is 1/7 in the pristine perfluorononanoic acid. It can be assumed that the defluorination of the terminate CF3 groups proceeds slower than that of CF2. This effect can be explained by the higher steric accessibility of the chain CF2 groups compared with the terminal CF3 ones under interaction with the metal surface. It might be assumed that on average the chemisorbed acid molecule loses about four fluorine atoms and forms a derivative with two F-substituted double bonds. The second observation deals with a decrease of the binding energy of the peaks from the fluorine-containing groups compared with the features characteristic of pristine perfluorononanoic acid (292.2 and 294.3 eV, respectively). The XPS shows the appearance of −CH2−CF2− and −CHF− groups, which are the products of interaction of the chemisorbed Fcontaining acid with the polystyrene fragments and their linkage. The intensity of corresponding peaks is much higher for the samples in which PFNA was only used as a surfactant. For the cases of mixed surfactants these peaks are less pronounced because the stearic acid isolates the fluorinesubstituted acid from its interaction with the components of the milling liquid preventing, in particular, anchoring of the

Figure 9. C 1s X-ray photoelectron spectra for the samples synthesized. The sample designation based on the composition of surfactant in the starting mixture is given in Table 1. 14011



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Table 2. Energy Positions, Intensities, and Assignments of the Features in the C 1s XPS Spectra for the Measured Samples intensity of the feature for different samples [%] energy position [eV]

S-0

SA

ODA

PFNA-2

MIX-N

MIX-F1

MIX-F2

assignment

285.0 285.8 286.8 288.0 288.7 289.0 290.8 291.6 292.2 294.3

90 5 ∼2 ∼1 ∼1 ∼1

86 5 ∼2 3 3 ∼1

80 8 4 7 ∼1 0

28 8 5 3 8 4 6 22 7 9

82 11 ∼2 ∼2 ∼1 ∼2

86 5 ∼2 ∼2 ∼1 ∼2 0 ∼1 0 ∼1

66 7 ∼2 ∼1 ∼1 3 ∼1 3 11 5

−CH2−, −CH3 α-CH2 −C−OR (R = H, alkyl), −CH2−CF2− ⟩CO −C(O)O− −CO(O)H, −CHF−CF2− −CH2−CF2− −CHF−CF2− −CF2−CF2−CF2− −CF2−CF3

the iron fluorides reference samples. Financial support by the Russian Foundation for Basic Research (project no. 09-0800158), bilateral “Russian-German Laboratory at BESSY II” Program, and Program for Basic Scientific Research of UB RAS Presidium (project no. 12-C-2-1019) is gratefully acknowledged.

polystyrene fragments on the metal surface according to the mechanism described above in the NEXAFS Results section.

4. SUMMARY AND CONCLUSIONS The mechanical activation of iron in the presence of surfactants (stearic and perfluorononanoic acids) has been shown to result in the formation of a chemisorbed layer on the oxidized metal surface due to the formation of carboxylate complexes. These chemisorbed layers formed under such an extremely nonequilibrium process as mechanochemical synthesis comprises long-chain molecules of carboxylic acids which are oriented preferably normally to the iron surface as it was observed earlier for the films grown under equilibrium conditions. Using the perfluorononanoic acid as a surfactant facilitates effective anchoring of the polystyrene fragments on the metal particle surface. The mechanism of anchoring proceeds through defluorination of the perfluorononanoic acid molecule tails when reacting with iron, followed by the formation of fluorinesubstituted double bonds, which are then coupled with the polystyrene fragments predominantly by the free-radical mechanism. The chemisorbed layer of the stearic acid does not provide good anchoring of the polystyrene fragments to the metal surface because of the chemical inertness of the stearic acid alkyl groups. When using simultaneously both stearic and perfluorononanoic acids, the linkage between iron particle and polystyrene fragments is also impeded, as the longer inert tails of the stearic acid block sterically the shorter tails of the perfluorononanoic acid. The surfactant-assisted mechanical milling with the use of fluorinated carboxylic acids is a promising way to modify the surface of iron particles by polymers of different nature. Asformed organic shell layers allow one to improve the chemical compatibility of metal filler particles with different organic matrices and hence their better adhesion.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the staff of the Russian-German Beamline at BESSY II for excellent technical support. We are grateful to Prof. E. Rühl for helping us with chemicals during our experiment at BESSY II and Dr. A. B. Preobrazhenski for providing us with 14012



Article

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