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Surface magnetism of exfoliated α-Co hydroxide nanosheets

Surface magnetism of exfoliated α-Co hydroxide nanosheets

Journal of Physics and Chemistry of Solids 107 (2017) 14–17 Contents lists available at ScienceDirect Journal of Physics and Chemistry of Solids jou...

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Journal of Physics and Chemistry of Solids 107 (2017) 14–17

Contents lists available at ScienceDirect

Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs

Surface magnetism of exfoliated α-Co hydroxide nanosheets a,⁎

a

b

b

MARK b

Zentaro Honda , Katsuki Anai , Masayuki Hagiwara , Takanori Kida , Akira Okutani , Masamichi Sakaia, Takeshi Fukudaa, Norihiko Kamataa a

Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan AHMF (Center for Advanced High Magnetic Field Science), Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 5600043, Japan b

A R T I C L E I N F O

A BS T RAC T

Keywords: Hydroxides Exfoliated nanosheets Magnetic properties

α-Co hydroxide nanosheets have been synthesized and their magnetic properties were investigated. By using a soft chemical exfoliation technique, exfoliated α-Co hydroxide nanosheets, typically with lateral dimensions of few 100 nm, were obtained in a colloidal suspension. The magnetic responses of a sample consisting of a colloidal suspension of the nanosheets indicates a ferromagnetic phase transition occurs at TC=37.8 K. The magnetization possesses a linear temperature dependence at low temperatures below TC. In addition to this observation, the magnetization is proportional to (1-T/TC)β with β=0.8 ± 0.1 near TC, which imply that the surface magnetism dominates in the exfoliated α-Co hydroxide nanosheets.

1. Introduction Nowadays, the exfoliation of layered solids is a common method to fabricate a nanosheet that is an ultrathin nanostructure with typical thicknesses ranging from a few nanometers up to hundreds of nanometers. Since the number of surface atoms relative to the number of atoms in the interior of the layered solids increases, it is expected that the surface properties are enhanced by the exfoliation of layered solids. Therefore, the exfoliated nanosheets have received significant interests due to their unusual physical and chemical properties and great potential for applications, such as electronic and magnetic devices, catalysis, and sensors. An outstanding example of the exfoliated nanosheet is graphene, which is a one-layer sheet of graphite that shows remarkable features [1,2]. Only a few classes of other nanosheets have been obtained by mechanical or soft chemical exfoliation of layered metal chalcogenides [3], layered metal oxides [4,5], layered double hydroxides [6,7], and layered α-hydroxides [8–10]. The layered αhydroxides consist of positively charged hydrotalcite like M(OH)2-x layers and charge balancing anions (e.g., Cl-, NO3-, SO42-, etc.) in the interlayer space [11,12]. It is possible to intercalate various guest organic molecules in the interlayer space through an ion exchange reaction. In such organic molecule-intercalated α-hydroxides materials, the organic molecule acts as a structural template controlling the interlayer distances. The α-hydroxide materials, which contain transition metal ions, show cooperative magnetism and are regarded as new candidates for quasi-two-dimensional (2D) magnetic materials [13,14]. Furthermore,



the exfoliation of α-hydroxides into ultrathin nanosheets with a positive charge has been realized by controlling the layer-to-layer interactions through the intercalation of organic zwitterions (e.g., p-aminobenzoic acid) into the interlayer space [10]. At high pH, the organic zwitterions can be intercalated, forming layers in the interlayer spaces of the αhydroxides. When the pH is reduced, the positive charge created on the amine end of the organic zwitterions repels the neighboring positively charged M(OH)2-x layer, and chemical exfoliation results. Repulsion between the positively charged M(OH)2-x layers leads to a stable colloidal suspension of positive charged α-hydroxide nanosheets in water. It is expected that large surface area can be realized when exfoliating the transition metal α-hydroxides into ultrathin nanosheets. Consequently, the surface magnetic interactions in the α-hydroxide nanosheets are expected to govern the magnetism. Herein, the magnetic properties and phase transition of a stable colloidal suspension of exfoliated α-Co hydroxide nanosheets are reported and are interpreted as arising from ultrathin magnetic material with large surface area. 2. Experimental section A polycrystalline sample of p-aminobenzoic-acid (abbreviated as paba) intercalated α-Co hydroxide was obtained as a blue green powder by adding 10 ml of a 0.5 M NH3 solution containing 0.9 mmol of paba to a warm (90 °C) solution of Co(NO3)2·6H2O (1 mmol) in 7 ml of water with constant stirring. The blue green precipitate that formed was immediately centrifuged, then washed with water, and subse-

Corresponding author. E-mail address: [email protected] (Z. Honda).

http://dx.doi.org/10.1016/j.jpcs.2017.03.012 Received 27 December 2016; Received in revised form 10 March 2017; Accepted 14 March 2017 Available online 15 March 2017 0022-3697/ © 2017 Elsevier Ltd. All rights reserved.

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quently dried in air over night. Exfoliation of paba-intercalated α-Co hydroxide was achieved by the following method reported in Ref. [15]. Firstly, the resultant blue green solid was dispersed into deoxidized water with continuous stirring. After few minutes, the precipitated solid tended to form a pale blue colloidal suspension. This pale blue colloidal suspension was centrifuged (4000 rpm, 5 min), and the resultant translucent colloidal supernatant, which exhibited Tyndall scattering of green laser light, was used in the TEM imaging and magnetic measurements. The crystal structure of the blue green precipitate was analyzed by using a powder X-ray diffractometer (Bruker D8 ADVANCE ECO) with Cu Kα monochromated radiation. The morphology of the as-obtained colloidal suspension of exfoliated α-Co hydroxide was characterized with a transmission electron microscope (TEM) (FEI Tecnai G2 20 microscope) operated at 200 kV. The TEM sample was prepared by placing a drop of the translucent colloidal suspension on a carbon coated grid and allowing it to evaporate under vacuum at room temperature for an hour before being placed on the sample stage of the microscope. The DC magnetization (M) of a powder sample of paba-intercalated α-Co hydroxide and a colloidal suspension of exfoliated α-Co hydroxide nanosheets were measured with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-XL). For the sample consisting of a colloidal suspension of exfoliated α-Co hydroxide, a volume of 100 μl was sealed in a NMR glass tube (5 mm diameter). Then, the tube was inserted in the sample space of the SQUID magnetometer and was cooled down to T=270 K. The M measurement was performed as a function of T from 2 K to 270 K in a magnetic field H=1 kOe under the zero-field-cooled (ZFC) and field-cooled (FC) conditions (M-T measurements). The M study was then moved to isothermal (T=2 K) measurements as a function of H (−70 to 70 kOe) (M-H measurements). The M of the colloidal suspension of exfoliated α-Co hydroxide were obtained by subtracting the M of the NMR glass tube.

3. Results and discussion Fig. 1 shows a powder XRD pattern of the as-prepared blue green precipitate. All the peaks were indexed to the reflections of paba intercalated α-M hydroxide (M=Zn, Co), M(OH)1.7(paba)0.3·H2O reported previously [15,16]. The blue green precipitate possesses a well ordered layered-structure and the interlayer space is calculated to be 1.61 nm. This result suggests the α-Co hydroxide was formed by the intercalation of organic zwitterion, paba. TEM images of the colloidal suspension of exfoliated α-Co hydroxide are shown in Fig. 2(a). The exfoliated α-Co hydroxide consists of sheets with few 100 nm in lateral dimensions. The electron transparent

Fig. 2. (a) TEM image of the exfoliated α-Co hydroxide nanosheets. (b) Expanded image of (a).

nature apparent in the TEM image of the exfoliated α-Co hydroxide suggests the very thin thicknesses of the sheets. The images also reveal that some regions near the edges of the α-Co hydroxide sheets tend to scroll and fold slightly in order to minimize the surface energy. Fig. 2(b) shows an expanded TEM image of the sample shown in Fig. 2(a). The dark area in the TEM image of a sheet suggests partial stacking of the exfoliated α-Co hydroxide sheets. Fig. 3(a) shows the T dependence of the ZFC and FC magnetization curves of the Co(OH)1.7(paba)0.3·H2O powder sample in the external magnetic field of 1 kOe. The ZFC and FC magnetization curves show an abrupt increase at about TC=36 K and deviate below 10 K. These properties of Co(OH)1.7(paba)0.3·H2O are consistent with ferromagnetic or ferrimagnetic ordering. The Curie-Weiss fit of the magnetic susceptibility (χ=M/H) versus T above 150 K gives the Curie constant C=2.77 emu K/mol, and the Weiss temperature θ=−45.6 K. This Curie constant is much larger than the spin only value (1.87 emu K/mol) for high-spin Co2+ ions, indicating the presence of high-spin Co3+ ions or an orbital contribution. Fig. 3(b) shows the M-H curve of the Co(OH)1.7(paba)0.3·H2O powder sample at T=2 K, where M is given in units of Bohr magneton μB per formula unit. The M-H curve indicates ferromagnetic or ferrimagnetic ordering with the spontaneous magnetization Ms=0.7 μB per Co2+ ion, which is smaller than the

Fig. 1. XRD pattern of the Co(OH)1.7(PABA)0.3·H2O powder sample.

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Fig. 3. (a) Temperature dependence of the zero-field-cooled (ZFC) (white circles) and field-cooled (FC) (black circles) magnetization (M) between 2 K and 300 K in a powder sample of the Co(OH)1.7(PABA)0.3·H2O sample at H=1 kOe. The inset shows an expanded view of the low temperature region. (b) M curves as a function of magnetic field (H) of a powder sample of the Co(OH)1.7(PABA)0.3·H2O. The inset shows an expanded view of the low field region.

Fig. 4. (a) Temperature dependence of the ZFC (white circles) and FC (black circles) magnetization (M) between 2 K and 270 K in a colloidal suspension of the exfoliated αCo hydroxide nanosheets at H=1 kOe. The inset shows an expanded view of the low temperature region. (b) M curves as a function of magnetic field (H) of a colloidal suspension of the exfoliated α-Co hydroxide nanosheets at T=2 K. The inset shows an expanded view of the low field region.

theoretical value of 3 μB per Co2+ ion for parallel spin ordering. As is well known, the Ms value depends on the type of spin ordering. The small value of Ms and negative Weiss constant suggest the magnetic ordering is ferrimagnetic. A hysteresis loop and relatively large coercive field Hc=4 kOe are also observed. Fig. 4(a) shows the T-dependences of the ZFC and FC magnetization of the sample made from a colloidal suspension of exfoliated α-Co hydroxide nanosheets in an external magnetic field of 1 kOe. The magnetization values for the ZFC and FC studies are negative above 40 K and positive below 40 K. The negative value of −1×10−4 emu at 270 K corresponds to the diamagnetic magnetization of water (−1×10−4 emu in H=1 kOe [17]). This result suggests the negative magnetization of −1×10−4 emu for the colloidal suspension of exfoliated α-Co hydroxide nanosheets is mainly attributed to the diamagnetism of water. At T=37.8 K, a discontinuity in M was observed. Both ZFC and FC magnetization curves show a steep increase with decreasing T below 37.8 K, suggesting the exfoliated α-Co hydroxide nanosheets possess a magnetic phase transition at finite temperature (TC=37.8 K). Below TC, the FC magnetization shows an almost linear increase down to T=5 K. The M below TC for exfoliated α-Co hydroxide nanosheets is qualitatively different from the M-T rounded curves of Co(OH)1.7(paba)0.3·H2O powder. The small bump near 60 K may be attributed to the paramagnetism of O2 that condensed in the NMR glass tube. Fig. 4(b) shows the M as a function of H for the colloidal

suspension of exfoliated α-Co hydroxide nanosheets at T=2 K, and these results suggest ferromagnetic or ferrimagnetic behavior with spontaneous magnetization and a small hysteresis loop. The coercive field of Hc =800 Oe, which is much smaller than the 4 kOe value in the Co(OH)1.7(paba)0.3·H2O powder, was also obtained from the M-H curve at T=2 K. From the magnetization measurements, we found a magnetic phase transition in a stable colloidal suspension of exfoliated α-Co hydroxide nanosheets that represent significant contribution of surface magnetism. The low temperature properties of a conventional magnet are explained by spin-wave theory. The reduction in M with T at low temperatures due to the excitation of spin-waves is given by M/M(0)~1−T3/2. This result is known as the Bloch's law and is observed in bulk magnets over a limited range of T (T < 0.5TC). Fig. 5 shows the normalized magnetization, M/M(0), as a function of normalized temperature, T/TC, for the exfoliated α-Co hydroxide nanosheets after subtracting the diamagnetic (−6.0×10−5 emu at T=40 K) and paramagnetic (CH/(T-θ), C=9.8×10−4 emu K/Oe, and θ=1.1 K) contributions attributed to the water and to a small amount of the magnetic impurities, respectively. Below 20 K (T/TC < 0.5), the data also clearly show a linear T dependence with a negative slope. The T dependence of the surface magnetization at low temperatures is known to be qualitatively different from that in a bulk magnet. The first-order spin wave theory for 2D surface spin predicts the M has a T-linear dependence with a negative slope at low temperatures [18,19]. Indeed, for 2 K≤T≤20 K, 16

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xide nanosheets show magnetic phase transition at TC=37.8 K. The linear temperature dependence, with a negative slope below 20 K, of the magnetization is associated with surface spin wave excitations in the exfoliated α-Co hydroxide nanosheets. Immediately below TC, the magnetization is proportional to (1−T/TC)0.8 ± 0.1, and this result is strikingly similar to the critical behavior arising from the surface magnetization in magnetic materials. When contrasted to the dramatically different magnetic response obtained from a Co(OH)1.7(PABA)0.3·H2O powder sample, our results enable us to identify the exfoliated α-Co hydroxide as an ultrathin magnetic material with large surface area. Acknowledgments This work was partially supported by the Grants-in-Aid for Scientific Research (No. 16K04926) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. Part of this work was conducted under the Guest Professor Program of AHMF at Osaka University. Useful comments by Prof. M.W. Meisel are gratefully acknowledged.

Fig. 5. Normalized magnetization, M/M(0), as a function of normalized temperature T/ Tc for a colloidal suspension of the exfoliated α-Co hydroxide nanosheets after subtracting the diamagnetic contribution (−6.0×10−5 emu) and a small paramagnetic contribution of magnetic impurity (CH/(T-θ) with C=9.8×10−4 emu K/Oe and θ=1.1 K). The solid and dashed lines are the fitting results discussed in the text.

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the magnetization data of the exfoliated α-Co hydroxide nanosheets exhibit this behavior that can be written as

M T ∝ 1 − SW , M (0) TC

(1)

where the zero-temperature spontaneous magnetization M(0)=0.00107 emu and the spin-wave critical temperature TCSW =45.7 K (solid line in Fig. 5). In contrast, the magnetization just below TC can be described in the context of critical phenomena followed by a scaling law, namely M~(1−T/TC)β, where β is the critical exponent for the order parameter M and 1−T/TC represents the reduced temperature. The value of the critical exponent β depends only on the spatial dimensionality and spin anisotropy in the material. Just below TC, M varies as M ∝ (1 − T /37.8)0.8 ± 0.1 (dashed line in Fig. 3) and similar T dependences of the magnetization in the critical region were reported for ferromagnetic Ni (001) surface (β=0.8 ± 0.02) [20], EuS (111) surface (β=0.72 ± 0.03) [21], and antiferromagnetic NiO (001) surface (β=0.89 ± 0.01) [22]. These results suggest that the surface magnetism dominates in the exfoliated α-Co hydroxide nanosheets. 4. Conclusions In conclusion, by using a soft chemical exfoliation technique, colloidal suspension of exfoliated α-Co hydroxide nanosheets were synthesized and their magnetic properties were investigated. The exfoliated α-Co hydro-

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