DOI 10.1007/s11182-017-1008-8 Russian Physics Journal, Vol. 59, No. 12, April, 2017 (Russian Original No. 12, December, 2016)

STRUCTURE AND PROPERTIES OF NANOCRYSTALLINE IRON OXIDE POWDER PREPARED BY THE METHOD OF PULSED LASER ABLATION V. A. Svetlichnyi, A. V. Shabalina, and I. N. Lapin

UDC 621.373.826, 621.318.1

Colloidal solution of iron oxide nanoparticles is synthesized by nanosecond pulsed laser ablation (Nd:YAG laser, 1064 nm, 7 ns, and 180 mJ) of a metallic iron target in water, and nanocrystalline powder is prepared from this solution by vacuum drying. A composition and structure of the material obtained are investigated by methods of electron microscopy, x-ray diffraction, and optical spectroscopy. It is established that oxide particles with average size of about 5 nm and Fe3O4 magnetite structure are mainly formed during ablation. Preliminary investigation of magnetic properties of the prepared nanoparticle powders shows that they can be in ferromagnetic and/or superparamagnetic states. Keywords: pulsed laser ablation, magnetic nanoparticles, iron oxide Fe3O4, magnetite. INTRODUCTION Iron oxides are among the most important compounds in the history of science and technology development. Specific properties of these oxides, including magnetic ones, predetermined their application in different fields of industry for synthesis of new materials, development of new technologies, and study of mechanisms of processes taking place in magnetic materials and phenomenon of magnetism in general. The most important polymorphs of iron oxides are hematite (α-Fe2O3), maghemite (γ-Fe2O3), magnetite (Fe3O4), and wustite (FexO and FeO). Nanomaterials, including nanoparticles, exhibit special properties that are not observed in the bulk materials with the same elemental and phase composition. In the case of iron oxides, it is well known that at room temperature maghemite and magnetite with particle sizes decreased down to 30 nm and less can show increased coercive force, decreased saturation magnetization, and superparamagnetism [1, 2]. These properties of nanoparticles can find wide applications, from synthesis of new functional materials in mechanical engineering to construction of complex electronic devices [3]. Biomedical applications for MRI contrasting [4], development of drug delivery systems [5, 6], therapy (for example, magnetic hyperthermia [7]), and so on are also promising for application of iron nano-oxides. In addition, nanodimensional iron oxide particles, regardless of their magnetic properties, exhibit antibacterial activity [8]. By the present time, many different methods of preparation of iron oxide nanoparticles (NP), including maghemite and magnetite nanoparticles, have been developed. Such methods as co-precipitation (for example, with subsequent microwave processing) [9], gas phase condensation [10], and electrochemical synthesis are widespread. Among the well-known methods, pulsed laser ablation (PLA) in a liquid occupies a special place. This method is fast, inexpensive, easy to use, does not require complex and expensive chemical reagents, and allows one to control over the process parameters affecting the composition and structure of the product obtained. As noted in [11], during PLA process the conditions are achieved that cannot be achieved in other methods of synthesis. Explosive evaporation of the target material exposed to pulsed laser radiation leads to spraying of evaporated material into a liquid medium in the

National Research Tomsk State University, Tomsk, Russia, e-mail: [email protected]; [email protected]; [email protected]. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 30–34, December, 2016. Original article submitted May 30, 2016. 2012

1064-8887/17/5912-2012 2017 Springer Science+Business Media New York

form of ions, clusters, and small particles. Unlike the noble materials, iron is not an inert metal; it reacts with the solvent and gases dissolved in it. This leads to the formation of nanoparticles of various iron compounds, mainly oxides. One of the most important advantages of the PLA method compared to many other methods of nanoparticle preparation is an opportunity to obtain nanoparticles in the form of stable dispersions without stabilizers and surfactants (SA). As already mentioned above, the nanoparticle composition that is determined by the content of the target material – metal, oxygen, carbon, and other elements – is controlled by the conditions of the ablation processes. Thus, using different solvents and additives, nanoparticles modified with various compounds, including organic ones, can be prepared. The aim of the present work is investigation of the composition, structure, and magnetic properties of nanocrystalline iron oxide powders prepared from aqueous dispersions produced by laser ablation of a metallic target in water.

EXPERIMENTAL Preparation of colloidal solution of nanoparticles and nanocrystalline powders The colloidal solution of iron oxide NP was prepared by PLA of a metallic target in distilled water (DW). A low-carbon steel (S185) plate with a purity of 99.5% and sizes of 10  40  4 mm was used as the target. Ablation was performed on the setup described in [12]. The target in a special holder was immersed into a 100 ml cylindrical glass reactor with distilled water. Using an X-Y line translator, the target was automatically moved in the XY plane orthogonal to the laser beam. This provides the uniform ablation of the entire target surface without crater formation. Radiation of a Nd:YAG laser (1064 nm, 180 mJ, 7 ns, and 20 Hz) was focused by a short-focus lens with the focal distance F = 50 mm at the target surface through the transparent side wall of the glass reactor. A liquid layer in front of the target did not exceed 5 mm, thereby minimizing effects of the secondary interaction of NP in the dispersion with laser radiation [13]. The initial radiation power density on the target surface was 0.1–1 GW/cm2. Such a high power density for PLA is caused by thermophysical properties of the iron target affecting the efficiency and threshold characteristics of the process [14], namely, high temperature and melting heat. Irradiation time was 3 h. The number of NP produced was estimated from the target mass losses; the average rate of NP production was about 0.2 mg/min. Thus, about 36 mg of particles were accumulated in the dispersion during one PLA cycle. Taking into account the oxidation process, it can be calculated that the amount of NP of iron oxide was about 50 mg per cycle. To obtain powders of iron oxide nanoparticles, nanocolloids synthesized by PLA were placed in a vacuum chamber with a water jet pump creating a residual pressure of 2–3 Torr and then dried at room temperature. Investigation methods The microstructure of the material obtained was studied by the method of transmission electron microscopy using a Philips CM 12 microscope with accelerating voltage of 120 kV. Freshly obtained dispersion of iron oxide NP was deposited on a copper grid for microscopy with a carbon film on the surface and dried at room temperature. The phase composition of the material obtained was investigated by the method of x-ray diffraction of nanoparticle powder obtained by drying the synthesized dispersion in vacuum (see above). X-ray diffraction patterns were obtained using a diffractometer Shimadzu XRD 6000 at the angles from 10 to 85°. The NP Raman spectra were recorded under laser irradiation at a wavelength of 785 nm using a Renishaw InVia Raman microscope. To prevent the transition of nanoparticles from the magnetite to hematite form under laser irradiation [15], the measurements were performed at a minimal radiation power (about 0.5 mW) out of the focus of the objective of the Raman microscope. Surface topology of iron oxide NP obtained after drying of the dispersion in vacuum was investigated using a scanning electron microscope Tescan VEGA 3 SBH without additional sample preparation. The magnetic properties of nanocrystalline powders were measured in the range 0–6000 Oe at room temperature using an automated vibrational magnetometer [16, 17].

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Fig. 1. TEM images (a) and x-ray diffraction patterns (b): curve 1 is for nanoparticles obtained by PLA of the iron target in water, curve 2 is for Fe2O3 (PDF4 Card No. 01-078-6916), curve 3 is for Fe3O4 (PDF4 Card No. 04-013-7114), and curve 4 is for Fe (PDF4 Card No. 01-071-4648).

RESULTS AND DISCUSSION Figure 1а shows a TEM image of nanoparticles prepared from the dispersion produced by PLA of the metallic iron target in water. It can be noted that the nanoparticles have a spherical shape. Most NP have sizes up to 5 nm inclusively, whereas larger particles (up to 50 nm) are also present, but in much smaller amounts. The particles are aggregated in loose agglomerates; smaller particles surround larger spheres. Figure 1b shows x-ray diffraction pattern of the powder prepared after drying of the dispersion in vacuum and diffraction patterns of well-known structures from the Powder Diffraction File (PDF-4) database. From the diffraction data, it can be concluded that the powder most likely contains hematite and/or magnetite and comprises a certain amount of non-oxidized metallic iron. These two oxides are very difficult to distinguish by the x-ray diffraction method, especially in case of broadened reflections of the NP diffraction patterns, since their signals strongly overlap. Figure 2 shows SEM images of the nanocrystalline powder surface after vacuum drying of the dispersions synthesized by PLA. Backscattered electron images (BSE detector) led to the assumption that a small number of large metallic iron particles (bright particles) were present in the sample, which was in agreement with the x-ray diffraction data (Fig. 1b). Since x-ray diffraction and electron microscopy do not allow the structure of iron oxide nanoparticles to be determined unambiguously, other methods were additionally used. It should be noted that Raman spectroscopy is considered to be one of the most effective instruments for the determination of the structure of magnetic iron oxide particles [18]. The Raman spectrum of magnetite is weakly structured; it exhibits low intensity and a characteristic broad band at 670 cm–1 (curve 1 in Fig. 3) [18]. The Raman spectrum of hematite (curve 2 in Fig. 3), obtained after heating of the original powder to 500С in air, shows higher intensity and contains a series of characteristic bands. The bands in the regions of 225 and 495 cm–1 correspond to A1g mode; the weak shoulder at 245 cm–1 and the peaks at 290, 410, and 610 cm–1 correspond to Eg mode [19]. A small amount of hematite was also found to present in the spectrum of the original powder obtained after vacuum drying. This is indicated by the bands at 225 and 290 cm–1 in the spectrum. Results of investigations of the magnetic properties of nanocrystalline iron oxide powder are shown in Fig. 4. This figure shows the field dependence of the residual magnetization M of iron oxide NP. The presence of a relatively thin hysteresis can be noted at room temperature. Presumably, there are two types of particles in the sample: one in the superparamagnetic state (determined by the absence of saturation in the curve), and the second is in a ferromagnetic

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Fig. 2. SEM image of nanocrystalline powders prepared after vacuum drying of the dispersion synthesized by the method of laser ablation: a is for SE and b is for BSE detection.

Fig. 3

Fig. 4

Fig. 3. Raman spectra for nanocrystalline iron oxide powders prepared by vacuum drying of the dispersion synthesized by laser ablation. Fig. 4. Field dependence of the residual magnetization of iron oxide nanoparticles obtained by laser ablation.

state (determined by the presence of the hysteresis loop). To determine precisely the nanoparticle ordering, temperature measurements of the residual magnetization must be performed in a weak magnetic field.

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CONCLUSIONS As a result of the work performed on the ablation of a bulk metallic iron target in water, oxide nanoparticles were synthesized with average size of about 5 nm according to the TEM data. The nanocrystalline powder was prepared by dispersion vacuum drying, and its structure and composition were studied by the methods of scanning electron microscopy, x-ray diffraction, and Raman spectroscopy. The last method allowed us to conclude that the nanoparticles obtained contain predominantly magnetite with small amount of hematite. The magnetic properties of the obtained NP were investigated at room temperature. It was suggested that the material contained the particles of two types with ferriand superparamagnetic properties. These properties will be investigated further. The nanomaterials obtained will find application in biomedical investigations. The authors are grateful to A. E. Sokolov and D. A. Velikfnov, researchers of the Kirensky Institute of Physics of the Siberian Branch of the Russian Academy of Sciences (Krasnoyarsk), for measurements of the magnetic properties of nanoparticles. This work was supported by the Ministry of Education and Science of the Russia Federation (State Assignment No. 2014/223, Project Code 727).

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