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Synthesis and characterization of maghemite nanosheets
Materials Letters 65 (2011) 439–441
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis and characterization of maghemite nanosheets Miha Drofenik a,b, Gregor Ferk a, Matjaž Kristl a, Darko Makovec b,⁎ a b
Faculty of Chemistry and Chemical Engineering, University of Maribor, 2000 Maribor, Slovenia Department for Materials Synthesis, Jožef Stefan Institute, 1000 Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 27 August 2010 Accepted 1 November 2010 Available online 6 November 2010 Keywords: Nanoparticles Magnetic materials Maghemite Nanosheets Synthesis
a b s t r a c t Magnetic maghemite nanoparticles in the form of nanosheets were prepared by a topotactic transformation during the dehydroxylation of a γ-FeO(OH) precursor. The precursor was synthesized from tetrapyridin Fe(II) chloride (Fe(py)4Cl2). The nanosheets are several hundreds of nanometers wide, and less than 5 nm thick; they frequently bend and curl at the edges; they are nanocrystalline; and they are composed of smaller maghemite nanostructured domains, from a few nanometers up to several tens of nanometers wide. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nanoparticles of magnetic iron oxides have attracted a great deal of attention due to their interesting and applicable magnetic properties. In the form of monodomain nanoparticles with sizes below a certain critical value, they exhibit superparamagnetic properties at room temperature. These superparamagnetic ironoxide nanoparticles have been intensively studied and used in biomedicine and biotechnology, for example, as contrast agents in magnetic resonance imaging, as drug carriers for magnetically assisted drug delivery [1], or in the therapy of cancer using magnetic hyperthermia [2]. Among the various iron oxides, maghemite is the most frequently applied material in medicine. The properties of fine magnetic particles are governed by a finite size effect and various interaction effects [3]. The competition between the surface and the core magnetic properties determines the spin structure of the particle. Due to the increase in the surface-to-volume ratio, the spin structure can differ considerably from that in the bulk material, especially as the particle size decreases [4]. Thus, the dimensions of the magnetic particles might have a remarkable effect on their magnetic properties. Apart from the particle size, their shape is also of large importance. For the preparation of 2D (plate-like) maghemite particles, γ-FeO(OH) can be utilized because of its favourable crystalline structure and its tendency to form particles with a 2D morphology. At elevated temperatures, γ-FeO (OH) particles can transform by thermal dehydroxilation to the spinel maghemite without changing their morphology. During the topotactic
⁎ Corresponding author. Tel.: +386 14773579; fax: +386 1 2519 385. E-mail address: [email protected] (D. Makovec). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.11.009
transformation to maghemite, γ-FeO(OH) particles preserve their shape [5]. Among the most common methods that yield the γ-FeO(OH) precursor are the following: the oxidative precipitation of an aqueous ferrous salt solution near neutral conditions [6], the hydrolysis of Fe3+ cations [7], and the oxidation of tetrapyridino-ferrous chloride [8]. We used the last method to study the synthesis of 2D maghemite nanoparticles. The method excludes the formation of magnetite as an intermediate phase, and for this reason it avoids a sensitive heat treatment that might lead to the formation of hematite. 2. Experimental In a typical procedure, 2 mmol of FeCl2∙4H2O was dissolved in 100 mL of mQ oxygen-free water, which was previously deoxygenated under an Ar flow. The solution was stirred under a flow of Ar (300 mL/min) for a period of 30 min, after which 2 mL of pyridine was added. The colour of the solution changed from yellowgreen to green-blue. The solution was allowed to stand for 1 h in an argon flow. After that the oxygen gas was vigorously bubbled through the reaction mixture with a flow of 300 mL/min for 15 min, followed by 3 h with an oxygen flow of 50 mL/min. The solution immediately turned orange as the pyridine complex oxidized to γ-FeO(OH). The solid product was centrifuged and washed several times with distilled water. The dehydroxilation of the γ-FeO(OH) was performed in a furnace at 265 °C for 1 h in air. The samples were characterized by x-ray diffractometry (XRD) and high-resolution transmission electron microscopy (HRTEM) using a field-emission electron-source TEM (JEOL 2010 F) operated at 200 kV. For the TEM investigations the materials were deposited on a copper-grid-supported perforated transparent carbon foil. The
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M. Drofenik et al. / Materials Letters 65 (2011) 439–441
magnetizations of the samples were measured using a vibratingsample magnetometer (Lake Shore 7307 VSM). 3. Results and discussion For the synthesis of 2D maghemite nanoparticles the synthesis route based on tetrapyridino-ferrous chloride (Fe(py)4Cl2) was selected [7]. The synthesized γ-FeO(OH) precursor was then used for the synthesis of maghemite nanoparticles with a 2D morphology. The x-ray diffractometry of the material obtained by the oxidation of Fe(py)4Cl2 showed the single phase lepidocrocite, γ-FeO(OH). The particle size was estimated to be approximately 5 nm from the XRD-peak broadening. The TEM (Fig. 1a) revealed that the synthesized γ-FeO(OH) precursor is in the form of very thin, sheet-like nanoparticles, i.e., nanosheets. The edges of the nanosheets, which are several hundreds of nm wide, are usually curled and bent. Thus, the edges of the nanosheets are frequently oriented edge-on, parallel with the electron beam, showing a small thickness, below 10 nm. These nanosheets frequently bend and they usually exfoliate at the edges. The electron diffraction pattern (Fig. 1a) corresponds to nanocrystalline lepidocrocite, γ-FeO(OH). The size of the nanosheets observed with the TEM is much larger than the crystallite size estimated from the XRD, most probably because of the poor crystallinity, also observed by HREM imaging. At least partially, the poor crystallinity observed by the HREM might be a consequence of the instability of the material in the high vacuum (below 2 × 10−5 Pa) and under the electron beam in the TEM. Intense irradiation of the material with the electron beam resulted in the gradual transformation of the lepidocrocite into the spinel, as can be seen from following the change in the electron diffraction pattern. When the HREM images were taken using a low beam intensity, the lattice fringes corresponding to the
Fig. 1. TEM image (a) and HREM image (b) of as-synthesized γ-FeO(OH). The corresponding electronic diffraction pattern (inset of Fig. 2a) was indexed according to the lepidocrocite structure.
(200) planes of the lepidocrocite orthorhombic structure were visible in the part of the nanosheets lying parallel to the electron beam (Fig. 1(b)). The γ-FeO(OH) precursor was dehydroxylatized by heating at 265 °C. The process is associated with a topotactic transformation in the maghemite, where the morphology of the γ-FeO(OH) crystallites is completely preserved. The XRD spectrum of the sample after the dehydroxilation of γ-FeO(OH) by heating was very diffuse due to the poor crystallinity of the product; however, the electron diffraction (Fig. 2a) confirmed that the dehydroxylation of the γ-FeO(OH) resulted in the formation of a phase with the spinel structure, i.e., maghemite. The TEM image in Fig. 2a shows the morphology of the maghemite, obtained with the dehydroxylation of γ-FeO(OH). The morphology of the maghemite product is similar to that of the γ-FeO(OH) precursor. Also, after the dehydroxylation the particles retained the form of thin nanosheets. The HREM analysis of these maghemite nanosheets (Fig. 2b) revealed that they are nanocrystalline and composed of smaller nanostructured domains, from a few nm up to several tens of nm wide. γ-FeO(OH) is orthorhombic, containing four formula units per unit cell, with a = 1.252 nm, b = 0. 387 nm and c = 0.307 nm [9]. The structure consist of ccp anions (O2−/OH−) stacked along the [150] direction with Fe3+ ions occupying the octahedral interstices [10]. The [150] direction of the orthorhombic unit cell corresponds to the [111]
Fig. 2. (a) TEM image of the sample obtained with dehydroxylation of the γ-FeO(OH) during heating for 1 h at 265 °C. The corresponding electron diffraction pattern (inset) was indexed according to the spinel structure of maghemite. (b) HREM image shows that the nanosheets of material are nanocrystalline, composed of smaller nanostructured domains (marked with broken lines).
M. Drofenik et al. / Materials Letters 65 (2011) 439–441
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nanosheets display only approximately 25% of the bulk magnetization. The main reason for this low magnetisation is the small size of the nanostructured domains. The largest domains observed with HREM imaging are a few tens of nm wide, but very thin, with a thickness below 6 nm. However, they have a broad size distribution and a significant proportion of the nanostructured domains are much smaller. Due to the small size, the proportion of the nonmagnetic surface layer, i.e., the magnetically “dead” layer, is large. In addition, the nanostructured domains are separated by incoherent grain boundaries, which provide an additional source of magnetic disorder and frustration through internal stresses. 4. Conclusions
Fig. 3. Room-temperature hysteresis of the maghemite nanosheets, synthesized with the dehydroxylation of γ-FeO(OH) during heating for 1 h at 265 °C (a) are compared with the hysteresis loop for typical isotropic maghemite nanoparticles with an average size of 14.5 nm (b) (data from ref. 11).
direction of a distorted cubic cell, and this relationship facilitates the dehydroxylation to the spinel phase. γ-FeO(OH) consists of double chains of Fe(O,OH)6 octahedra running parallel to the c-axes. These double chains share edges with adjacent double chains and each chain is displaced by half, with respect to the neighbour, thus forming corrugated sheets of octahedra. These sheets are stacked perpendicular to the [100] direction and are separated by double rows of empty octahedral sites. The sheets are held together by hydrogen bonds [5]. The dehydroxylation of the Fe(O,OH)6 sheets in the γ-FeO(OH) structure, which are bonded together with hydrogen bonds, results in the formation of the maghemite structure. The magnetic hysteresis loop of the maghemite in the form of nanosheets is shown in Fig. 3. It is clear that the hysteresis curve does not show any coercivity, indicating superparamagnetic behaviour at room temperature. The magnetisation of the sample at room temperature and a magnetic field of 1 T reaches 20 emu/g. This value is much lower than that of the bulk, and much lower than that of isotropic maghemite nanoparticles of comparable size. Isotropic particles with sizes around 10 nm typically show magnetizations of approximately 80% of the bulk value [11]. In contrast, the maghemite
γ-FeO(OH) in the form of thin, 2D nanoparticles, i.e., nanosheets, was synthesized by precipitation from an aqueous solution of tetrapyridino-ferrous chloride (Fe(py)4Cl2) with pyridine, followedby oxidation using oxygen bubbling. The dehydroxylation of the γ-FeO (OH) precursor with heating for 1 h at 265 °C in air resulted in a topotactic transformation into maghemite, without losing the nanosheet morphology of the nanoparticles. Acknowledgements The support by the Ministry of Higher Education, Science and Technology of the Republic of Slovenia within the National Research Program is gratefully acknowledged. References [1] Sun C, Lee JSH, Zhang M. Adv Drug Deliv Rev 2008;60:1252–65. [2] Ennas G, Marongiu G, Mosinu A, Falqui A, Ballirano P, Caminiti R. Mater Res 1999;14:1570–5. [3] Dorman JL, Fiorani D, Tronc E. Adv Chem Phys 1997;98:273–6. [4] Kodama RH, Berkowitz AE. Phys Rev 1999;59:6321–36. [5] Cornell RM, Schwertmann U. The iron Oxides. Weinheim: Wiley-VCH; 2003. [6] Hibst H, Schwab E. Magnetic recording media. In: Cahn RW, Hassen P, Kramer EJ, editors. Materials Science and Technology, 3B. Weinheim: VCH; 1994. p. 212–6. [7] Frini A, El. Maaoui MJ. Colloid Interface Sci 1997;190:269–77. [8] Budisch O, Hartung WH. Inorg Synth 1939;1:184–9. [9] Christensen H, Norlund-Christensen A. Acta Chim Scand 1978;32:87–8. [10] Fasiska EJ. Corros Sci 1967;7:833–9. [11] Makovec D, Čampelj S, Bele M, Maver U, Zorko M, Drofenik M, et al. Colloids Surf A 2009;334:74–9.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis and characterization of maghemite nanosheets Miha Drofenik a,b, Gregor Ferk a, Matjaž Kristl a, Darko Makovec b,⁎ a b
Faculty of Chemistry and Chemical Engineering, University of Maribor, 2000 Maribor, Slovenia Department for Materials Synthesis, Jožef Stefan Institute, 1000 Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 27 August 2010 Accepted 1 November 2010 Available online 6 November 2010 Keywords: Nanoparticles Magnetic materials Maghemite Nanosheets Synthesis
a b s t r a c t Magnetic maghemite nanoparticles in the form of nanosheets were prepared by a topotactic transformation during the dehydroxylation of a γ-FeO(OH) precursor. The precursor was synthesized from tetrapyridin Fe(II) chloride (Fe(py)4Cl2). The nanosheets are several hundreds of nanometers wide, and less than 5 nm thick; they frequently bend and curl at the edges; they are nanocrystalline; and they are composed of smaller maghemite nanostructured domains, from a few nanometers up to several tens of nanometers wide. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Nanoparticles of magnetic iron oxides have attracted a great deal of attention due to their interesting and applicable magnetic properties. In the form of monodomain nanoparticles with sizes below a certain critical value, they exhibit superparamagnetic properties at room temperature. These superparamagnetic ironoxide nanoparticles have been intensively studied and used in biomedicine and biotechnology, for example, as contrast agents in magnetic resonance imaging, as drug carriers for magnetically assisted drug delivery [1], or in the therapy of cancer using magnetic hyperthermia [2]. Among the various iron oxides, maghemite is the most frequently applied material in medicine. The properties of fine magnetic particles are governed by a finite size effect and various interaction effects [3]. The competition between the surface and the core magnetic properties determines the spin structure of the particle. Due to the increase in the surface-to-volume ratio, the spin structure can differ considerably from that in the bulk material, especially as the particle size decreases [4]. Thus, the dimensions of the magnetic particles might have a remarkable effect on their magnetic properties. Apart from the particle size, their shape is also of large importance. For the preparation of 2D (plate-like) maghemite particles, γ-FeO(OH) can be utilized because of its favourable crystalline structure and its tendency to form particles with a 2D morphology. At elevated temperatures, γ-FeO (OH) particles can transform by thermal dehydroxilation to the spinel maghemite without changing their morphology. During the topotactic
⁎ Corresponding author. Tel.: +386 14773579; fax: +386 1 2519 385. E-mail address: [email protected] (D. Makovec). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.11.009
transformation to maghemite, γ-FeO(OH) particles preserve their shape [5]. Among the most common methods that yield the γ-FeO(OH) precursor are the following: the oxidative precipitation of an aqueous ferrous salt solution near neutral conditions [6], the hydrolysis of Fe3+ cations [7], and the oxidation of tetrapyridino-ferrous chloride [8]. We used the last method to study the synthesis of 2D maghemite nanoparticles. The method excludes the formation of magnetite as an intermediate phase, and for this reason it avoids a sensitive heat treatment that might lead to the formation of hematite. 2. Experimental In a typical procedure, 2 mmol of FeCl2∙4H2O was dissolved in 100 mL of mQ oxygen-free water, which was previously deoxygenated under an Ar flow. The solution was stirred under a flow of Ar (300 mL/min) for a period of 30 min, after which 2 mL of pyridine was added. The colour of the solution changed from yellowgreen to green-blue. The solution was allowed to stand for 1 h in an argon flow. After that the oxygen gas was vigorously bubbled through the reaction mixture with a flow of 300 mL/min for 15 min, followed by 3 h with an oxygen flow of 50 mL/min. The solution immediately turned orange as the pyridine complex oxidized to γ-FeO(OH). The solid product was centrifuged and washed several times with distilled water. The dehydroxilation of the γ-FeO(OH) was performed in a furnace at 265 °C for 1 h in air. The samples were characterized by x-ray diffractometry (XRD) and high-resolution transmission electron microscopy (HRTEM) using a field-emission electron-source TEM (JEOL 2010 F) operated at 200 kV. For the TEM investigations the materials were deposited on a copper-grid-supported perforated transparent carbon foil. The
440
M. Drofenik et al. / Materials Letters 65 (2011) 439–441
magnetizations of the samples were measured using a vibratingsample magnetometer (Lake Shore 7307 VSM). 3. Results and discussion For the synthesis of 2D maghemite nanoparticles the synthesis route based on tetrapyridino-ferrous chloride (Fe(py)4Cl2) was selected [7]. The synthesized γ-FeO(OH) precursor was then used for the synthesis of maghemite nanoparticles with a 2D morphology. The x-ray diffractometry of the material obtained by the oxidation of Fe(py)4Cl2 showed the single phase lepidocrocite, γ-FeO(OH). The particle size was estimated to be approximately 5 nm from the XRD-peak broadening. The TEM (Fig. 1a) revealed that the synthesized γ-FeO(OH) precursor is in the form of very thin, sheet-like nanoparticles, i.e., nanosheets. The edges of the nanosheets, which are several hundreds of nm wide, are usually curled and bent. Thus, the edges of the nanosheets are frequently oriented edge-on, parallel with the electron beam, showing a small thickness, below 10 nm. These nanosheets frequently bend and they usually exfoliate at the edges. The electron diffraction pattern (Fig. 1a) corresponds to nanocrystalline lepidocrocite, γ-FeO(OH). The size of the nanosheets observed with the TEM is much larger than the crystallite size estimated from the XRD, most probably because of the poor crystallinity, also observed by HREM imaging. At least partially, the poor crystallinity observed by the HREM might be a consequence of the instability of the material in the high vacuum (below 2 × 10−5 Pa) and under the electron beam in the TEM. Intense irradiation of the material with the electron beam resulted in the gradual transformation of the lepidocrocite into the spinel, as can be seen from following the change in the electron diffraction pattern. When the HREM images were taken using a low beam intensity, the lattice fringes corresponding to the
Fig. 1. TEM image (a) and HREM image (b) of as-synthesized γ-FeO(OH). The corresponding electronic diffraction pattern (inset of Fig. 2a) was indexed according to the lepidocrocite structure.
(200) planes of the lepidocrocite orthorhombic structure were visible in the part of the nanosheets lying parallel to the electron beam (Fig. 1(b)). The γ-FeO(OH) precursor was dehydroxylatized by heating at 265 °C. The process is associated with a topotactic transformation in the maghemite, where the morphology of the γ-FeO(OH) crystallites is completely preserved. The XRD spectrum of the sample after the dehydroxilation of γ-FeO(OH) by heating was very diffuse due to the poor crystallinity of the product; however, the electron diffraction (Fig. 2a) confirmed that the dehydroxylation of the γ-FeO(OH) resulted in the formation of a phase with the spinel structure, i.e., maghemite. The TEM image in Fig. 2a shows the morphology of the maghemite, obtained with the dehydroxylation of γ-FeO(OH). The morphology of the maghemite product is similar to that of the γ-FeO(OH) precursor. Also, after the dehydroxylation the particles retained the form of thin nanosheets. The HREM analysis of these maghemite nanosheets (Fig. 2b) revealed that they are nanocrystalline and composed of smaller nanostructured domains, from a few nm up to several tens of nm wide. γ-FeO(OH) is orthorhombic, containing four formula units per unit cell, with a = 1.252 nm, b = 0. 387 nm and c = 0.307 nm [9]. The structure consist of ccp anions (O2−/OH−) stacked along the [150] direction with Fe3+ ions occupying the octahedral interstices [10]. The [150] direction of the orthorhombic unit cell corresponds to the [111]
Fig. 2. (a) TEM image of the sample obtained with dehydroxylation of the γ-FeO(OH) during heating for 1 h at 265 °C. The corresponding electron diffraction pattern (inset) was indexed according to the spinel structure of maghemite. (b) HREM image shows that the nanosheets of material are nanocrystalline, composed of smaller nanostructured domains (marked with broken lines).
M. Drofenik et al. / Materials Letters 65 (2011) 439–441
441
nanosheets display only approximately 25% of the bulk magnetization. The main reason for this low magnetisation is the small size of the nanostructured domains. The largest domains observed with HREM imaging are a few tens of nm wide, but very thin, with a thickness below 6 nm. However, they have a broad size distribution and a significant proportion of the nanostructured domains are much smaller. Due to the small size, the proportion of the nonmagnetic surface layer, i.e., the magnetically “dead” layer, is large. In addition, the nanostructured domains are separated by incoherent grain boundaries, which provide an additional source of magnetic disorder and frustration through internal stresses. 4. Conclusions
Fig. 3. Room-temperature hysteresis of the maghemite nanosheets, synthesized with the dehydroxylation of γ-FeO(OH) during heating for 1 h at 265 °C (a) are compared with the hysteresis loop for typical isotropic maghemite nanoparticles with an average size of 14.5 nm (b) (data from ref. 11).
direction of a distorted cubic cell, and this relationship facilitates the dehydroxylation to the spinel phase. γ-FeO(OH) consists of double chains of Fe(O,OH)6 octahedra running parallel to the c-axes. These double chains share edges with adjacent double chains and each chain is displaced by half, with respect to the neighbour, thus forming corrugated sheets of octahedra. These sheets are stacked perpendicular to the [100] direction and are separated by double rows of empty octahedral sites. The sheets are held together by hydrogen bonds [5]. The dehydroxylation of the Fe(O,OH)6 sheets in the γ-FeO(OH) structure, which are bonded together with hydrogen bonds, results in the formation of the maghemite structure. The magnetic hysteresis loop of the maghemite in the form of nanosheets is shown in Fig. 3. It is clear that the hysteresis curve does not show any coercivity, indicating superparamagnetic behaviour at room temperature. The magnetisation of the sample at room temperature and a magnetic field of 1 T reaches 20 emu/g. This value is much lower than that of the bulk, and much lower than that of isotropic maghemite nanoparticles of comparable size. Isotropic particles with sizes around 10 nm typically show magnetizations of approximately 80% of the bulk value [11]. In contrast, the maghemite
γ-FeO(OH) in the form of thin, 2D nanoparticles, i.e., nanosheets, was synthesized by precipitation from an aqueous solution of tetrapyridino-ferrous chloride (Fe(py)4Cl2) with pyridine, followedby oxidation using oxygen bubbling. The dehydroxylation of the γ-FeO (OH) precursor with heating for 1 h at 265 °C in air resulted in a topotactic transformation into maghemite, without losing the nanosheet morphology of the nanoparticles. Acknowledgements The support by the Ministry of Higher Education, Science and Technology of the Republic of Slovenia within the National Research Program is gratefully acknowledged. References [1] Sun C, Lee JSH, Zhang M. Adv Drug Deliv Rev 2008;60:1252–65. [2] Ennas G, Marongiu G, Mosinu A, Falqui A, Ballirano P, Caminiti R. Mater Res 1999;14:1570–5. [3] Dorman JL, Fiorani D, Tronc E. Adv Chem Phys 1997;98:273–6. [4] Kodama RH, Berkowitz AE. Phys Rev 1999;59:6321–36. [5] Cornell RM, Schwertmann U. The iron Oxides. Weinheim: Wiley-VCH; 2003. [6] Hibst H, Schwab E. Magnetic recording media. In: Cahn RW, Hassen P, Kramer EJ, editors. Materials Science and Technology, 3B. Weinheim: VCH; 1994. p. 212–6. [7] Frini A, El. Maaoui MJ. Colloid Interface Sci 1997;190:269–77. [8] Budisch O, Hartung WH. Inorg Synth 1939;1:184–9. [9] Christensen H, Norlund-Christensen A. Acta Chim Scand 1978;32:87–8. [10] Fasiska EJ. Corros Sci 1967;7:833–9. [11] Makovec D, Čampelj S, Bele M, Maver U, Zorko M, Drofenik M, et al. Colloids Surf A 2009;334:74–9.