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Морилова Виктория Михайловна
старший преподаватель кафедра общей физики Снежинский физико-технический институт Национального исследовательского ядерного университета «МИФИ» Бржезинская Мария Михайловна доктор физико-математических наук, старший научный сотрудник Институт нанометровой оптики и технологии Гельмгольц-Центра
Берлин, Германия Байтингер Евгений Михайлович
доктор физико-математических наук, старший научный сотрудник Федеральный институт по материаловедению Берлин, Германия Вилков Олег Юрьевич научный сотрудник Санкт-Петербургский государственный университет Санкт-Петербург Институт физики твёрдого тела, Технический университет Дрездена
Дрезден, Германия Нелюбов Александр Васильевич научный сотрудник Санкт-Петербургский государственный университет Санкт-Петербург Институт химии и биохимии, Свободный университет Берлина
Берлин, Германия Евсюков Сергей Евгеньевич кандидат химических наук Заведующий лабораторией Evonik Industries AG Доссенхайм, Германия Песин Леонид Абрамович доктор физико-математических наук, профессор,
кафедра физики и методики обучения физике Челябинский государственный педагогический университет
г. Челябинск
Morilova Viktoria Mikhailovna
Senior lecturer Department of General Physics Snezhinsk Physics and Technology Institute of the National Research Nuclear University «MEPhI» Brzhezinskaya Mariya Mikhailovna Doctor of Science (Physics and Mathematics),
Senior Scientist
Institute for Nanometre Optics and Technology, Helmholtz-Zentrum Berlin für
Materialien und Energie Berlin, Germany Baitinger Eugeniy Mikhailovich Doctor of Science (Physics and Mathematics),
Senior Scientist Bundesanstalt für Materialforschung und -prüfung Berlin, Germany Vilkov Oleg Yurievich Research associate St. Petersburg State University St. Petersburg
Institut für Festkörperphysik, Technische Universität Dresden,
Dresden, Germany Nelyubov Alexander Vasilievich Research associate St. Petersburg State University St. Petersburg
Institut für Chemie und Biochemie, Freie Universität Berlin
Berlin, Germany Evsyukov Sergey Evgenievich Candidate of Science (Chemistry)
Head of Laboratory Evonik Industries AG Dossenheim, Germany Pesin Leonid Abramovich Doctor of Science (Physics and Mathematics)
Professor,
Department of physics and methodology of teaching physics Chelyabinsk state pedagogical University Chelyabinsk vika_m@list.ru
Исследование модификации поливинилиденфторида под действием синхротронного излучения методами NEXAFS и РФЭС A NEXAFS and XPS study of poly (vinylidene fluoride) modification under synchrotron radiation
Проведены исследования радиационной карбонизации частично кристаллической плёнки поливинилиденфторида методами рентгеновской абсорбционной спектроскопии (NEXAFS) и рентгеновской фотоэлектронной спектроскопии (РФЭС). С помощью проводящего клея удалось уменьшить электростатическую зарядку образцов, что позволило получить NEXAFS спектры с высоким разрешением и хорошим статистическим усреднением. Форма спектров и эволюция их интенсивности свидетельствует о разрушении CF2 и CH2 групп и формировании п-связей, характерных для sp и sp2 гибридных состояний атомов углерода. Результаты РФЭС спектроскопии показывают появление CF групп и множества атомов углерода, не связанных непосредственно со фтором, но имеющих в ближайшем соседстве атомы углерода, ковалентно связанные со фтором.
NEXAFS and XPS of partially crystalline PVDF film have been studied in the course of carbonization under synchrotron radiation. High quality NEXAFS spectra of partially degraded PVDF have been achieved using conducting glue which allows significant suppression of electrostatic charging of a polymer sample. Spectra shape and intensity evolution suggests destruction of CF2 and CH2 groups and formation of п bonds relevant to sp and sp hybrid states of carbon atoms. XPS spectra show creation of CF groups and a variety of carbon atoms not attached to fluorine directly but having the nearest carbon neighbors covalently bonded with fluorine.
Ключевые слова: поливинилиденфторид (ПВДФ), NEXAFS, РФЭС, радиационная карбонизация
Key words: Poly (vinylidene fluoride) (PVDF), NEXAFS, XPS, radiative carbonization Introduction
Poly(vinylidene fluoride) (PVDF) film is a promising raw material to produce a variety of carbonized derivatives on its surface using chemical treatment, ion and electron bombardment, and X-ray irradiation [1-5]. Dehydrofluorination occurs in all cases without scissions of the carbon backbone, thus opening up an opportunity to
create ’naked’ carbon chains. However, one could expect the formation of not only
1 2 nanoscaled one-dimensional sp -carbon, but also graphite- (sp ) and diamond-like
(sp ) clusters due to probable cross-linking of the chains. The proportion of each of these phases can affect specific properties of the modified polymer surface, depending on the duration of chemical treatment, peculiarities of bombardment, and radiation dose. One-dimensional carbon structures are expected to become promising materials for future applications in microelectronics, microwave and electrical technologies, medicine, etc. [1, 6] Using a conventional XPS, our team has revealed that dur-
ing profound carbonization of PVDF under X-ray radiation in ultrahigh vacuum the cross-linking effects are negligible [7]. We believe that the reason of such an unusual behavior is stabilization of carbynoid chains by the residual short fragments of original polymer interspersed between the carbonized moieties. A special important problem is the detection of one-dimensionally structured carbon [3, 7]. Features of unoccupied states can be revealed via NEXAFS technique and are expected to be even better suitable for identification of the carbon atomic arrangement than Auger spectroscopy due to a wider spatial spreading of the empty orbitals forming the unoccupied states.
Several studies have been published, where NEXAFS experiments with PVDF and its co-polymers are reported. Some studies are dealing with ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) co-polymer [8-10], which is a prospective material for non-volatile memory devices. Relatively few data concerned with pristine PVDF without copolymers are available. Ohta et al. [11] have studied an elongated thin (~0.3 ^m) film with 90% of polar p phase. Its C K-edge absorption spectra have been obtained in the transmission mode with radiation electric vector E parallel and perpendicular to the chain direction. The only NEXAFS study on the PVDF degradation under synchrotron radiation has been reported by Okudaira et al. [12-14]. Although spectra in that experiment showed rather high level of ‘noise’, it has nevertheless revealed a very important feature of the degradation process, namely, the creation of double carbon-carbon bonds in the backbone chain of PVDF due to the efficient H+ desorption induced by the irradiation with photons corresponding to the transition between C1s and o(C-H)* states. On irradiating with photons near the C1s region, a new peak appears in the C1s NEXAFS spectra at photon energy of 285 eV, which is about 3 eV lower than that of the lowest peak in the NEXAFS spectrum of the pristine PVDF film. The appearance of this new NEXAFS feature in irradiated PVDF film is assigned to the transition to the n* electron state. An analogous feature was reported to appear and to rise during exposure of (PVDF-TrFE) co-polymer to synchrotron radiation [8]. The ions eliminated from PVDF due
to the irradiation with photons near C1s region are H+ and F+ [13], while H+ was reported to predominate [14]. On the other hand, Morikawa et al. [3] reported on predominant elimination of H2 as well as simultaneous ejection of hydrogen and fluorine as molecular HF from the PVDF surface under the synchrotron broad-band (~0.5-2.0 KeV) X-ray radiation centered at around 1.2 KeV. This fact may testify to different mechanisms of radiative carbonization of PVDF, depending on photon energy, and opens up an exciting field for further in-depth studies. To our best knowledge, there are no publications dealing with the modification of the NEXAFS line-shape during degradation in connection with the residual fluorine content. The last parameter could be necessary to monitor a degree of the surface carbonization before and after each NEXAFS scan. The aim of the present study was to obtain well-resolved C K-edge NEXAFS spectra of partially carbonized PVDF and to reveal modifications of spectra due to the polymer degradation caused by monochromatic synchrotron radiation with simultaneous monitoring of residual fluorine content using XPS.
Experimental
The PVDF samples studied were partially (~50%) crystalline Kynar films (Atofina, France), 50 pm thick and 10*10 mm in size. They were fixed on copper sample holders by means of conductive glue. The same glue has been used also to form a kind of a conductive grid on the working side of the sample with empty spaces sufficient to register spectra from polymer. This provided significant stabilization and decrease of an electrostatic charging effect, thus making possible registration of C K-edge NEXAFS and XPS (^©=1030 eV) spectra. Photons with the same energy were also used to induce radiative carbonization of the samples. A piece of highly oriented pyrolytic graphite (HOPG) was used as a test object to evaluate the effect of PVDF charging upon the XPS parameters (energy position and line width). As a rule, the XPS lines and Auger bands were strongly but uniformly shifted towards higher binding energies by up to 50-75 eV. Nevertheless the broadening effect proved to be negligible. As the degradation of PVDF proceeds, the XPS lines slowly but steadily move towards higher kinetic energies, thus demonstrating increase in electrical con-
ductivity of the irradiated film surface. The XPS spectra necessary to determine the residual fluorine content were scanned at passing energy of 50 eV before and after each NEXAFS measurement. Relative fluorine atomic content F/C was measured from a ratio of integral F1s and C1s XPS intensities, taking into account the ionization cross-sections, the transmission function of analyzer, and different mean escape depths of these two groups of photoelectrons. The C K-edge NEXAFS spectra were registered in a total electron yield (TEY) mode in the photon energy range of 280-317 eV with energy resolution 70 meV.
In attempt to reveal the C1s fine structure and its radiation-induced modification, an additional XPS experiment was carried out at passing energy of 20 eV. A total of 19 C1s spectra were recorded in succession. Survey spectra (50 eV passing energy) were scanned twice: before and after registration of the first and the last C1s spectra. As the energy position of C1s lines gradually moved in the course of carbonization, only one-scan measurements were possible and, therefore, statistical averaging of the spectra was rather poor. Nevertheless, it proved to be sufficient to notice variations in the C1s line-shape. In order to match positions of the CF2 feature in all spectra, the spectra 2 through 19 were shifted towards lower kinetic energy.
All measurements were carried out at the Russian-German beamline of the electron storage ring BESSY II (Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin, Germany) [15].
Results and discussion
The C K-edge NEXAFS spectra for several doses of irradiation are shown in Fig. 1. The assignment of the spectral features is made according to [8, 11]. The lowest solid and dashed curves 1 are adapted from Ref. 11 for comparison. The modification of spectral line-shape with increasing dose (decreasing F/C ratio) is clearly evident. Two growing features A and B at 285 and 287 eV are associated in Ref. 8 with C1s^ n* excitations. The dominant peak D (~293 eV) corresponds to C1s^o*(C-F) resonance. Its intensity decrease is consistent with a well-known effect of fluorine elimination in irradiated PVDF [2, 4]. Peak E (295 eV) corresponds to C1s^o*(C-C)
transition and its shape and intensity seem to be stable in accordance with previous observations [8]. Our starting NEXAFS spectrum (curve 2) contains a sharp feature at 288.5 eV (peak C), which at first becomes less intensive and then is replaced by a constantly growing shoulder at 288-291 eV. These changes of spectra shape are consistent with those observed for (PVDF-TrFE) co-polymer under synchrotron radiation with photon energy ranged from 280.0 to 320.0 eV [8] and can be associated with a decrease in C-H bonds content under synchrotron radiation.
Fig. 1. C K-edge NEXAFS TEY line-shape modification caused by partial carbonization of PVDF film. Numbers to the right of curves indicate a relative atomic F/C ratio. The lowest solid and dashed spectra 1 have been obtained by Ohta et al. for vector E parallel and perpendicular to the pristine PVDF chains direction respectively and are adapted from Ref. 11 for comparison.
In pristine PVDF one could expect mean relative content of fluorine F/C in the surface layer available for XPS to be close to 1 and no C1s^ n* transitions. On the contrary, the F/C parameter declines from 0.77 to 0.42 during the entire time of the experiment. Besides, small peaks at 285 and 287 eV are observed just in the first NEXAFS spectrum. We believe that these discrepancies could be explained by extremely rapid degradation of a polymer sample in the very beginning of its exposition to synchrotron radiation during scanning of the first survey XPS spectrum. Nevertheless the NEXAFS data for partially carbonized PVDF are qualitatively consistent with somewhat “noisier” spectra reported by Okudaira et al. [12-14].
The NEXAFS line-shape of the sample after its maximum exposition differs from those of HOPG, polyethylene, and diamond known from various sources [see, for example, [16-20]. One can find the most close resemblance of this spectrum to those of amorphous diamond-like and graphite-like carbons, but splitting of C1s^ n* feature (285 and 287 eV) obviously shows a pronounced distinction, which probably reflects an existence of two n electrons per atom in the chain-like carbon.
The XPS C1s spectra obtained at passing energy of 20 eV by scans 1 (upper) and 19 are displayed in Fig. 2. Both the ring current and the radiation intensity were gradually decreasing during the registration of spectra. Besides, as we have already mentioned previously, the elevation of surface conductivity causes shifting of spectra towards higher kinetic energies. Therefore, both spectra are normalized to equal area, and the latter one is shifted to equalize energy positions of CF2 features for better visualization and further data subtraction. The most prominent peaks are formed by the photoelectrons emitted from the carbon core states of CF2-(left peak) and CH2-groups in residual fragments of intact PVDF. Nevertheless, even the line-shape of spectrum 1 is evidently distinct from that of pristine PVDF [21] due to fast polymer degradation during preliminary registration of the first survey spectrum. The right dominant peak is significantly wider than the CF2 feature due to appearance of carbon atoms experiencing a weaker secondary chemical shift in comparison with atoms surround-
ed by CF2 groups. Besides, between the dominant spectral peaks there are noticeable additional features caused by primary chemical shift in CF-groups.
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Fig. 2. Modification of C1s XPS line-shape in the course of partial carbonization of PVDF film. The upper (circles) and lower (squares) spectra correspond to scans 1 and
19, respectively. The spectra are normalized to an equal area. The latter spectrum is shifted to the left so that positions of CF2 features match in both spectra.
These distinctions become stronger with increasing irradiation exposure and are easily seen in the difference spectra. One of them, obtained by subtraction of spectrum 1 from spectrum 19 and shown in Fig. 3, demonstrates maximum modification of spectral line-shape. Two negative features of nearly equal widths and intensities
correspond to decrease in contents of difluoromethylene (CF2) groups and CH2-groups located in the polymer chain between two CF2-groups.
Kinetic Energy [eV]
Fig. 3. C1s XPS difference spectra obtained via subtraction of spectrum 1 from spectrum 19.
A weak positive feature between 679 and 681 eV is attributable to CF-groups. A wide and more intensive positive feature in the range of 684-690 eV reflects the existence of a variety of carbon atoms, which are not bonded to fluorine directly, but have in their next surrounding other carbon atoms covalently bonded with different numbers of fluorine atoms. One can assume four cases corresponding to these secondary chemical shifts, i.e. 3, 2, 1, and 0 atoms of fluorine attached to the neighboring car-
bon atoms. This assumption can explain why the feature being considered is enormously wide.
Conclusions
NEXAFS and XPS study of partially crystalline PVDF film has shown a significant modification of its chemical content and electronic structure in the course of carbonization under synchrotron radiation. An electrostatic charging reduction demonstrates an elevation of electrical conductivity of the irradiated film surface. The spectra shape and intensity evolution is consistent with the destruction of CF2 groups and formation of n electrons inherent in sp and sp hybrid states of carbon atoms. In the difference XPS spectra two dominant negative features of nearly equal widths and intensities correspond to decrease of CF2 groups content as well as to reduction of the secondary chemical shift of C1s (CH2) line because CH2 groups initially are surrounded from both sides with CF2 ones in a pristine polymer chain. A weak feature between CF2 and CH2 C1s lines can be attributed to the formation of CF groups in the first rapid stage of dehydrofluorination. A wide and more intensive feature in the 684-690 eV kinetic energy range reflects an existence of a variety of carbon atoms in which core levels are subjected to different secondary chemical shifts caused by different number of residual fluorine atoms bonded with their next carbon neighbors in the partially carbonized chain.
Acknowledgements
This work was supported by the bilateral program “Russian-German Laboratory at BESSY”. The authors are cordially grateful to HZB stuff (a project 110064) and to Chelyabinsk State Pedagogical University administration (grants № UG-20/11/A, № UG-05/12/A) for financial support.
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