ISSN 1063-7834, Physics of the Solid State, 2017, Vol. 59, No. 4, pp. 784–790. © Pleiades Publishing, Ltd., 2017. Original Russian Text © A.E. Shumskaya, E.Yu. Kaniukov, A.L. Kozlovskiy, M.V. Zdorovets, V.S. Rusakov, K.K. Kadyrzhanov, 2017, published in Fizika Tverdogo Tela, 2017, Vol. 59, No. 4, pp. 766–772.
LOW-DIMENSIONAL SYSTEMS
Structure and Physical Properties of Iron Nanotubes Obtained by Template Synthesis A. E. Shumskayaa, *, E. Yu. Kaniukova, A. L. Kozlovskiyb, c, M. V. Zdorovetsb, d, e, V. S. Rusakov f, and K. K. Kadyrzhanovc a Scientific
and Practical Material Research Centre, National Academy of Sciences of Belarus, Minsk, 220072 Belarus b Institute of Nuclear Physics of the Republic of Kazakhstan, Almaty, 050032 Kazakhstan c Gumilyov Eurasian National University, Astana, 010008 Kazakhstan d Ural Federal University, Yekaterinburg, 620002 Russia e National Research Nuclear University “MEPhI,” Moscow, 115409 Russia f Moscow State University, Moscow, 119991 Russia *e-mail:
[email protected] Received September 14, 2016
Abstract—Iron nanotubes with an aspect ratio of approximately 100 are synthesized by electrochemical deposition using polyethylene terephthalate templates. The structural and morphological features of the nanotubes are studied in detail by scanning electron microscopy, energy dispersive analysis, transmission electron microscopy, electron diffraction, X-ray diffraction analysis, and gas permeability. The main magnetic parameters and their dependence on temperature are determined by vibration magnetometry and Mössbauer spectroscopy. DOI: 10.1134/S1063783417040266
1. INTRODUCTION Magnetic nanostructures synthesized from iron triad metals (Fe, Co, and Ni) are of great interest both in terms of the establishment of the fundamental bases of magnetic interactions and from the perspective of potential applications. They are promising for use in catalysis [1], the delivery of drug [2] and genes [3], studying microrheological processes [4], and as contrast fluids [5]. Despite the fact that the properties of the iron triad metals in massive bodies are well studied, their structural and magnetic characteristics in nanosized objects such as nanoparticles, nanowires, and nanotubes require further investigation. Template synthesis is a simple method for the preparation of nanomaterials with a predetermined shape and size [6–8]. It is the most technologically advanced, as it enables the control of the morphology, structure, composition, and, respectively, magnetic properties of nanostructures by changing a number of external parameters: temperature, composition and acidity of the electrolyte, electrode voltage, and deposition time [6, 9–11]. At present, the mechanisms for obtaining magnetic nanowires using substrate of silicon [12], alumina [13], or silica [14] are fairly well developed. However, the production of nanotubes on these substrates is limited by a feature of the synthesis procedure [15, 16]. By virtue of the specific structure
and unique properties, hollow nanostructures are more attractive for practical application, which, because of their lower specific density and larger surface area, are promising for using as elements of nanoelectronic devices [17], catalysts [1], carriers in targeted drug delivery [18], and chemical reactors [19]. In the present work, we propose a simple procedure for design iron nanotubes by template synthesis using ion-track polymer membranes and electrochemical deposition; a detailed study of their morphological and magnetic properties is also performed. 2. METHODS Track membranes based on polyethylene terephthalate (PET) served templates in the synthesis of nanotubes. For templates design, we prepared a PET film 12 μm in thickness, irradiated with 132Xe22+ swift heavy ions at 1.75 MeV/nucleon and a fluence of 1 × 109 cm–2, using a DTs-60 accelerator (Astana, Kazakhstan). The irradiated films were sensitized using a UV lamp (at a wavelength of 253.7 nm) on each side for 30 min. High-defective regions (latent tracks) formed by the irradiation were transformed into pores by chemical etching with a 2.2 M NaOH solution at a temperature of 85 ± 1°C for 4.5 min; the resulting templates were treated in a neutralizing solution (a 1% aqueous acetic acid solution) and washed with deion-
784
STRUCTURE AND PHYSICAL PROPERTIES OF IRON NANOTUBES
ized water. The pores have a cylindrical shape with an average size of 110 ± 3 nm. The pores were filled with metal by electrochemical deposition. For this purpose, individual pieces of rectangular shape (10 × 15 mm) were cut off from the resulting track membranes. A layer of gold, 10 nm in thickness, was deposited on these pieces by magnetron sputtering under vacuum, which served as a working electrode (cathode) during deposition. Templates with gold sputtered films were tightly pressed against the holder so that the electrolyte could reach the cathode through the pores only. Deposition was carried out in a potentiostatic mode at voltage U = 1.25 V in the electrolyte of the following composition: FeSO4 ⋅ 7H2O (234.5 g/L), FeCl2 ⋅ 6H2O (1.16 g/L), H3BO3 (45 g/L), and C6H8O6 (1 g/L). The degree of filling of pores with metal was controlled chronoamperometrically, recording the current intensity by an Agilent 34410A multimeter. The composition, morphological, and structural features of the electrodeposited iron nanostructures were studied using a Hitachi TM3030 scanning electron microscope (SEM) equipped with a Bruker XFlash MIN SVE energy-dispersive analysis (EDA) system at an accelerating voltage of 15 kV. The inner diameters of the nanotubes were measured by the manometric method of determining gas permeability [20] using a Sartocheck® 3 Plus 16290 device. The range of measured pressure was in the range of 8 to 20 kPa. The precise control of the external and internal diameters of the nanotubes was carried out by transmission electron microscopy (TEM) using a JEM-100 instrument at an accelerating voltage of 100 kV. The crystal structure of the metallic phase was investigated by electron diffraction in a selected region using a JEM-100 transmission electron microscope; the X-ray diffraction (XRD) analysis was also performed using a D8 ADVANCE diffractometer with CuKα-irradiation and a graphite monochromator. Diffraction patterns were recorded in the angular range of 2θ = 10°–90° with a 0.02° increment for 5 s at each point. The values of the main magnetic parameters were determined using the vibration magnetometer data of a universal “Liquid Helium Free High Field” measurement system (Cryogenic). Measurements were performed by the induction method, while the induced electromotive force of induction in the signal coils was varied by a magnetized sample, oscillating at a certain frequency, in the range of magnetic fields B ± 3 T at temperatures T = 100–300 K. Fine magnetic parameters were determined using an MS1104Em Mössbauer spectrometer, running in a mode of constant acceleration with a triangular shape of changes in the Doppler velocity of the source relative to the absorber. Nuclei 57Co in a rhodium matrix served as a source. The Mössbauer spectrometer was calibrated at room temperature using a conventional PHYSICS OF THE SOLID STATE
Vol. 59
No. 4
2017
785
α-Fe absorber. For processing and analysis of the results, the methods for assessing the distributions of hyperfine parameters of the Mössbauer spectrum and for model decoding of the spectrum were used, based on a priori information about the object of research, implemented in the SpectrRelax program. 3. RESULTS AND DISCUSSION It is known that the electrodeposition of iron into the pores of templates usually occurs in four stages [21–23], which are clearly visible on the time dependence of the process (Fig. 1a). The first stage (Ia) corresponds to the nucleation process at the cathode gold layer and on an inner surface of pores. Because at this stage, the reduction of metal has a three-dimensional character, the current value I has high values (more than 32 mA). Stage Ia is completed after approximately t = 20 s, when the base of a nanotubes is formed, which is a ring of iron sediment on the cathode layer (Ib). The value of current lowers to approximately 27 mA due to a decrease in the concentration of iron ions in the pore volume and the transition to a two-dimensional (layer by layer) growth of the nanotube. The second stage (II), starting at t = 60 s, is the main phase of the process, during which the length of nanotubes increases. The direction of the growth of nanostructures corresponds to the priority crystallographic directions and depends on the conditions of the deposition process. The current strength during this phase of iron deposition remains practically the same (I = 27 mA) because the concentration of ions inside the pore varies only slightly. The beginning of the third stage (III, t = 150 s)—the deposition of metal on the template surface to form “caps” over the tubes—is characterized by the increase in current due to a higher concentration of iron ions over the surface of the template than in the pores and the three-dimensional growth of the metal deposit. In the fourth stage (not marked on the dependence of the process I(t)), sprawling “caps” form a continuous layer on the surface of the template, and further deposition leads only to an increase in the metal layer thickness. It should be noted that the behavior of I(t) in chronoamperograms depends of the deposition parameters [22, 24] and the shape of pores [25]. To study the structural and magnetic characteristics of the nanotubes, we limited their growth to the third deposition stage; the deposition time was t = 140 s. SEM images of nanotubes obtained after the removal (etching) of the polymer template are presented in Fig. 1b. It is seen that the length of the nanotubes corresponds to the initial thickness of the template (12 μm), and their external diameter is approximately the average size of the pores (110 nm), wherein the aspect ratio is about 100. The SEM image of a cross-section of the nanotube (inset in Fig. 1b) ensures that the obtained nanostructures are of a hollow shape, but this study is not to determine their
786
SHUMSKAYA et al.
(a)
(b)
50 nm
100 nm
Fig. 1. (a) Chronoamperogram of the electrochemical deposition of iron nanotubes in the pores of a PET template and (b) SEM image of an array of nanotubes after removing the template. Inset: nanotube cross section at the bottom.
(b) (a)
20 nm
Fig. 2. (a) X-ray diffraction pattern and (b) TEM image of iron nanotubes. Inset: electron diffraction pattern in the selected region.
internal diameter and the thickness of walls. To find these parameters, the method of gas permeability was used [26]; it was found that the average internal diameters of the nanotubes were 69 ± 1 nm and the wall thickness was 20 nm. The determination of the chemical composition of the nanotubes, based on the EDA spectra, showed that 100% of nanotubes consist of iron without oxide or salt impurities. The analysis of the phase composition of the nanotubes by XRD (Fig. 2a) revealed that the studied samples are single-phase, have a bodycentered cubic (BCC) structure of α-iron (space group Im3m) with the selected growth planes (110) and (211). Halo at 2θ = 15°–35° and an average maximum at 2θ = 54° match the template material (PET) [27]. In the diffraction pattern of the test sample, there are broadened peaks, characteristic of the diffraction at nanoscale objects. The average crystallite size τ, calcu-
lated according to the Scherrer equation, was 21.9 nm. The crystal lattice parameter (a = 2.8627 Å) differs from the reference value (a = 2.866 Å). Analysis of the TEM images (Fig. 2b) confirms that the values of the outer diameter of iron nanotubes are 110 nm over the entire length with slight deviations. The surface of nanostructures is rough, which is a consequence of the use of porous PET films as templates for the growth of the nanostructures [28]. The study of the electron diffraction patterns in the selected region (inset in Fig. 2b) confirms that the nanotubes have a BCC structure. The broadening of the halo in the diffraction pattern indicates that the walls have a finedispersed morphology. The calculation of the size of individual iron crystallites from the radial broadening of the diffraction ring showed that they are of the order of 20 nm, which agrees well with the XRD data. The polycrystalline structure of the nanotubes with an
PHYSICS OF THE SOLID STATE
Vol. 59
No. 4
2017
STRUCTURE AND PHYSICAL PROPERTIES OF IRON NANOTUBES
787
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 3. Magnetic properties of iron nanotubes in the parallel and perpendicular directions of the magnetic field relative to the orientation of the nanotubes. Dependence of the magnetization of the magnetic field at (a) 100 and (b) 300 K. Insets: enlarged fragments of hysteresis loops. Temperature dependences of (c) coercivity, (d) residual magnetization, (e) saturation magnetization, and (f) squareness ratio of hysteresis loops. Solid squares in parts (c–f) correspond to the parallel orientation of the field relative to the axis of the nanotubes, and clear squares are for the perpendicular orientation.
average size of the individual crystallite, corresponding to the wall thickness, along with the lattice parameter different from the reference value, indicate their deformation due to defects formed in the growth of the nanotubes. The dependence of the magnetization on the magnetic field M(H) for its parallel and perpendicular orientation relative to the axis of the nanotubes was deterPHYSICS OF THE SOLID STATE
Vol. 59
No. 4
2017
mined at temperatures of 100–300 K. The value of the magnetic field density was varied within ±3 T. The typical shape of the dependences of M(H) is shown in Figs. 3a and 3b. Studies of the behavior of the magnetization of iron nanotube arrays in the magnetic field demonstrate that the shape of the hysteresis loops corresponds to a group of hard magnetic materials; however, the value
788
SHUMSKAYA et al.
(a)
(b)
v Fig. 4. (a) Processed Mössbauer spectrum of nuclei 57Fe and (b) restored distribution of hyperfine magnetic field Hn. The bottom line in part (a) reflects a discrepancy between the experimental data on the measurement of the Mössbauer spectrum with the results of processing.
of coercivity at the same time is smaller than that for massive iron bodies. It should also be noted that the magnetic properties of iron nanotubes are close to the properties of similar nanowires [29–31]. Based on the hysteresis loops, we determined the main magnetic properties of the nanotubes (Hc is coercivity, Mr is the residual magnetization, Ms is the saturation magnetization, and Mr/Ms is the squareness ratio of the hysteresis loop) and a their temperature dependences (Figs. 3c–3f). The coercivity of the nanotubes (Fig. 3c) is substantially constant throughout the entire temperature range. The dependence of Hc(T) is weak because the monitored range is far from the Curie temperature of iron (1043 K). Note that in the test samples, a magnetic anisotropy is observed: over the entire temperature range, the values of Hc for the parallel orientation of the field relative to the axis of the nanotubes are three times higher than the values for the perpendicular direction of the field. Since no crystalline anisotropy was found in the implementation of structural studies, the factor determining the appearance of the magnetic anisotropy is the shape anisotropy [32, 33]. Its effect is explained by the fact that in the introduction of an external magnetic field, the energy of demagnetization along the axis of nanotubes having a high aspect ratio (~100) has larger values than in the perpendicular direction of the field. It is seen from the curves of the residual magnetization (Fig. 3d) and the saturation magnetization of the nanotubes (Fig. 3e) that these quantities for different orientations of the tubes relative to the magnetic field over the entire temperature range vary greatly, and with increasing temperature, this difference decreases.
This difference, as in the case of coercivity, is a consequence of the shape anisotropy, and its decrease is due to the destruction of the magnetic order by an increasing effect of thermal fluctuations when the sample is heated. Figure 3f shows the difference in the squareness ratio at different temperatures. This dependence can be related to either exchange or magnetostatic dipole interaction. Given the fact that a PET template cannot be a link in such processes even at a distance of more than a few interatomic distances, the mutual effect between the nanotubes occurs only through magnetostatic dipole interactions. Fine magnetic parameters of iron nanotubes were determined by Mössbauer spectroscopy; the results are presented in Fig. 4. The Mössbauer spectrum for iron nanotubes (Fig. 4a), obtained at room temperature, is a Zeeman sextet and two quadrupole doublets. The spectrum is processed by restoring the distribution of the hyperfine magnetic field (Fig. 4b) and using the method of model decoding. It is seen that the maximum of the recovered distribution of the hyperfine magnetic field is achieved at Hn ≅ 330 kOe. The average values δ of the Mössbauer line and the quadrupole offset ε for the Zeeman sextet are close to zero. Consequently, the Zeeman sextet corresponds to α-Fe. The offset values of the quadrupole doublets, occurring in the range of 1.27–1.35 and 0.20– 0.42 mm/s, enable the identification of them as partial spectra of cations Fe2+ and Fe3+ in the paramagnetic state. Regarding these data, the effect of the contribution of partial spectra of cations Fe2+ and Fe3+ is determined, which for Fe2+ is 24%, and for the Fe3+ is 15%. Because EDA, XRD, and electron diffraction showed
PHYSICS OF THE SOLID STATE
Vol. 59
No. 4
2017
STRUCTURE AND PHYSICAL PROPERTIES OF IRON NANOTUBES
no iron salts or oxides in the nanotubes, cations Fe2+ and Fe3+ were detected by means of a specific distribution of the electron cloud in the crystal lattice. By the model decoding of the Mössbauer spectra of iron nanotubes, we obtained the values of quadrupole offset ε = 0.002 ± 0.004 mm/s and shift of the Mössbauer line δ = 0.002 ± 0.004 mm/s. The average values of quadrupole offset ε and isomeric shift δ are approximately zero, while the average value of the hyperfine magnetic field Hn = 330 ± 0.6 kOe. These values of the hyperfine parameters correspond to the values for nuclei 57Fe in massive reference samples of α-Fe. The ratio of the intensities of the second and fifth resonance lines to the intensities of the first and sixth resonance lines of the found Zeeman sextet (Fig. 4a) depends on the angle between the direction of flight of the gamma ray and the magnetic field in the sample and point to a lack of magnetic texture in them. At the same time, a random distribution of the directions of the magnetic moments of the iron atoms is observed. Taking into account the weak temperature dependence of Hc and Ms (Fig. 3), we can conclude that there is no magnetocrystalline anisotropy in the sample. 4. CONCLUSIONS Using the ion-track technology, 12 µm in thickness PET templates containing cylindrical pores with a size of 110 nm are obtained. Iron nanostructures are synthesized electrochemically, shaped as hollow nanotubes with external geometric dimensions corresponding to the parameters of the PET template and an inner diameter of 70 nm. The walls of the nanotubes have a polycrystalline structure; the size of an individual crystallite is approximately 20 nm, which corresponds to the wall thickness. This fact indicates that the wall is formed of individual crystallites that grow over each other. Analysis of the composition of the nanotubes has shown that they are 100% composed of iron, without oxide or salt impurities. Nanostructures are single-phase and have a BCC structure of α-Fe with the lattice parameter of a = 2.8627 Å. The analysis of the magnetic properties of samples led to the conclusion that the characteristics of the nanotubes are comparable with the magnetic properties of iron nanowires. The presence of anisotropy of magnetic properties is demonstrated, which is associated with the shape anisotropy of nanotubes having a large aspect ratio (~100). A slight decrease in the main magnetic parameters of the test samples is found upon heating by an increasing contribution of thermal fluctuations that destroy the magnetic order. It is determined that the nanotubes do not have a magnetic texture due to the random distribution of the directions of the magnetic moments of the iron atoms. PHYSICS OF THE SOLID STATE
Vol. 59
No. 4
2017
789
REFERENCES 1. D. T. Mitchell, S. B. Lee, L. Trofin, N. Li, T. K. Nevanen, H. Söderlund, and C. R. Martin, J. Am. Chem. Soc. 124, 11864 (2002). 2. S. K. Yen, P. Padmanabhan, and S. T. Selvan, Theranostics 3, 986 (2013). 3. B. Kalska-Szostko, E. Orzechowska, and U. Wykowska, Colloids Surf., B 111, 509 (2013). 4. M. Safi, M. Yan, M. A. Guedeau-Boudeville, H. Conjeaud, V. Garnier-Thibaud, N. Boggetto, A. BaezaSquiban, F. Niedergang, D. Averbeck, and J. F. Berret, ACS Nano 5, 5354 (2011). 5. S. R. Dave and X. Gao, Wiley Interdiscip. Rev.: Nanomed. Nanobiotechnol. 1, 583 (2009). 6. C. R. Martin, Science (Washington) 266, 1961 (1994). 7. L. Dauginet-De Pra, E. Ferain, R. Legras, and S. Demoustier-Champagne, Nucl. Instrum. Methods Phys. Res., Sect. B 196, 81 (2002). 8. E. Y. Kaniukov, J. Ustarroz, D. V. Yakimchuk, M. Petrova, H. Terryn, V. Sivakov, and A. V. Petrov, Nanotechnology 27, 115305 (2016) 9. L. G. Vivas, Y. P. Ivanov, D. G. Trabada, M. P. Proenca, and O. Chubykalo-Fesenko, Nanotechnology 24, 105703 (2013). 10. Y. A. Ivanova, D. K. Ivanov, A. K. Fedotov, E. A. Streltsov, S. E. Demyanov, A. V. Petrov, E. Y. Kaniukov, and D. Fink, J. Mater. Sci. 42, 9163 (2007). 11. V. Haehnel, S. Fähler, P. Schaaf, M. Miglierini, C. Mickel, L. Schultz, and H. Schlörb, Acta Mater. 58, 2330 (2010). 12. L. Boarino, S. Borini, and G. Amato, J. Electrochem. Soc. 156, K223 (2009). 13. J. Qin, J. Nogués, M. Mikhaylova, A. Roig, J. S. Muñoz, and M. Muhammed, Chem. Mater. 17, 1829 (2005). 14. S. E. Demyanov, E. Y. Kaniukov, A. V. Petrov, E. K. Belonogov, E. A. Streltsov, D. K. Ivanov, Y. A. Ivanova, C. Trautmann, H. Terryn, M. Petrova, J. Ustarroz, and V. Sivakov, J. Surf. Invest. 8, 805 (2014). 15. Z. Hua, S. Yang, H. Huang, L. Lv, M. Lu, B. Gu, and Y. Du, Nanotechnology 17, 5106 (2006). 16. D. Zhou, T. Wang, M. G. Zhu, Z. H. Guo, W. Li, and F. S. Li, J. Magnetics 16, 413 (2011). 17. X. F. Han, Z. C. Wen, and H. X. Wei, IEEE Trans. Magn. 47, 2957 (2011). 18. H. Hillebrenner, F. Buyukserin, J. D. Stewart, and C. R. Martin, Nanomedicine (London) 1, 39 (2006). 19. S.-H. Liao, K.-L. Chen, C.-M. Wang, J.-J. Chieh, H.-E. Horng, L.-M. Wang, C. Wu, and H.-C. Yang, Sensors 14, 21409 (2014). 20. B. Tylkowski and I. Tsibranska, J. Chem. Technol. Metall. 50, 3 (2015). 21. B. Yoo, F. Xiao, K. N. Bozhilov, J. Herman, M. Ryan, and N. V. Myung, Adv. Mater. 19, 296 (2007). 22. M. Motoyama, Y. Fukunaka, T. Sakka, and Y. H. Ogata, Electrochim. Acta 53, 205 (2007). 23. T. N. Narayanan, M. M. Shaijumon, L. Ci, P. M. Ajayan, and M. R. Anantharaman, Nano Res. 1, 465 (2008).
790
SHUMSKAYA et al.
24. C. Schönenberger, B. M. I. van der Zande, L. G. J. Fokkink, M. Henny, C. Schmid, M. Krüger, A. Bachtold, R. Huber, H. Birk, and U. Staufer, J. Phys. Chem. 101, 5497 (1997). 25. M. Motoyama, Y. Fukunaka, T. Sakka, Y. H. Ogata, and S. Kikuchi, J. Electroanal. Chem. 584, 84 (2005). 26. E. Kaniukov, A. Kozlovsky, D. Shlimas, D. Yakimchuk, M. Zdorovets, and K. Kadyrzhanov, Inst. Phys. Conf. Ser.: Mater. Sci. Eng. 110, 012013 (2016). 27. C. Guillén and J. Herrero, Thin Solid Films 480, 129 (2005). 28. M.E. Toimil-Molares, Beilstein J. Nanotechnol. 3, 860 (2012). 29. M. Krajewski, W. S. Lin, H. M. Lin, K. Brzozka, S. Lewinska, N. Nedelko, A. Slawska-Waniewska,
J. Borysiuk, and D. Wasik, Beilstein J. Nanotechnol. 6, 1652 (2015). 30. S. Yang, H. Zhu, D. Yu, Z. Jin, S. Tang, and Y. Du, J. Magn. Magn. Mater. 222, 97 (2000). 31. H. Zeng, R. Skomski, L. Menon, Y. Liu, S. Bandyopadhyay, and D. J. Sellmyer, Phys. Rev. B: Condens. Matter 65, 134426 (2002). 32. M. Almasi Kashi, A. Ramazani, and A. S. Esmaeily, IEEE Trans. Magn. 49, 1167 (2013). 33. D. J. Sellmyer, M. Zheng, and R. Skomski, J. Phys.: Condens. Matter 13, R433 (2001).
PHYSICS OF THE SOLID STATE
Translated by O. Zhukova
Vol. 59
No. 4
2017
