Controllable Magnetism of CoO Nanoparticles Modified by the

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The Controllable Magnetism of CoO Nanoparticles Modified by the Reduced Graphene Oxide Xiaofeng Bi, Wentao Liu, Qingsong Huang, and Jinghua Pang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07151 • Publication Date (Web): 05 Oct 2015 Downloaded from http://pubs.acs.org on October 10, 2015

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The Journal of Physical Chemistry

The

Controllable

Magnetism

of

CoO

Nanoparticles

Modified by the Reduced Graphene Oxide Xiaofeng Bi1, Wentao Liu1, Qingsong Huang1*, and Jinghua Pang2 1

School of Chemical Engineering, Sichuan University, Chengdu 610065, China

2

School of Materials Science and Engineering, Sichuan University, Chengdu 610065,

China *Email: [email protected]

Phone: 0086-15882289926

1

ACS Paragon Plus Environment


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ABSTRACT Rocksalt CoO nanoparticles (CNPs) have been prepared with a facile solvothermal method. To control the magnetism of CNPs, the reduced graphene oxide (RGO) is adopted to engineer the CNPs. Enwrapped by the atomic-layered carbon sheets, the CNPs can be reduced locally. The CoO/RGO composites were prepared by one-pot and two-pot synthetic methods respectively. Compared with the CoO/RGO raw composite that have no magnetization hysteresis loop, the weak and the strong hysteresis loops emerged successively via heating the particles at different temperatures, and maintaining their nano scales. The magnetism can be adjusted by controlling the temperature via two routes. By the one-pot synthetic route, the CNPs can be reduced sharply, and an obvious hysteresis is available around 400 °C, which is ascribed to the appearance of Co nano clusters decorated in the corners of CNPs. By the two-pot synthetic route, a weak and gradually enhanced hysteresis can be observed, and its magnetic properties should be ascribed to the topological defects of RGO. Our findings have opened a new way to acquire nanoparticles with controllable magnetism.

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1. INTRODUCTION Recently, the magnetic nanoparticles have attracted extensive investigations since their exotic properties. Magnetic nanoparticles offer some attractive possibilities in the magnetic storage and the applications in biomedical field, such as the cell separation, the magnetic resonance imaging, and the targeted drug delivery.1-5 Lots of nano-scaled magnetic materials under room temperature have been reported, such as γ-Fe2O3, superparamagnetic nano Fe3O4,6 ferromagnetic-like ZnO etc.7 Cobaltous oxide (CoO) is a kind of transition metal oxide with potential applications towards catalysts, anodic material of lithium-ion battery, and magnetic data storage devices.8 Bulk rocksalt CoO is a collinear antiferromagnet with the Neel temperature (TN) ~298 K. However, according to the suggestion of Neel, the nano-sized antiferromagnetic particles usually exhibit paramagnetism or weak ferromagnetism because of the uncompensated number of spins on the surface of nanoparticles.9,10 As a result, Zhang et al studied the relationship between the size of CoO and magnetism.11 Meanwhile, the magnetism of CoO is not temperature sensitive, and its magnetism property can be kept from decaying sharply, sustaining in a specific order of magnitude.9 At present, numerous fabrication means of CNPs have been reported. Zhang et al synthesized CoO nanoparticles within 10-80 nm via sol-gel method11 and Ghosh et al within 4.5-18 nm by decomposition of Co(ΙΙ) cupferronate in Decalin at 270 °C.9 An et al prepared a kind of pencil-shaped and uniformly sized CoO nanorods by the thermal decomposition of a coablt-oleate complex.2 Other methods, e.g. the calcination of Co(OH)2 under N2 atmosphere and the laser vaporization controlled condensation 3



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from high purity cobalt metal, proceed with complicated operations.12,13 Yang et al explored different cobalt salts as the precursor to synthesize CNPs by a simple solvothermal route and found that only cobalt acetate can be used as cobalt source.14 Ye et al made a further exploration about synthesizing CNPs via solvothermal method and explained the specific reaction mechanism.15 Generally, the solvothermal method provides a practical and feasible way to synthesize CNPs. As one kind of anti-ferromagnetic materials, the magnetic properties of CoO are not controllable. Although the magnetic properties of CoO seem size-dependent

9,11

and are also affected by Co metal impurity during the process of synthesis.19 However, both of the methods can’t control the magnetic properties of single-phase CoO effectively. To control the magnetic properties of CNPs, RGO is adopted to break the balance of the anti-ferromagnetic structure of CoO. The reduced CNPs decorated by Co nano clusters, display different magnetic hysteresis and saturation magnetization, depending on the reduction degree of nanoparticles. Furthermore, modifying magnetism properties of the CoO is performed under room temperature to realize its potential application widely. In addition, because the Neel temperature of the CoO is around 298 K, when the temperature is approaching the Neel temperature, its magnetic susptibility reaches the maximum, which seems conducive to its application. Meanwhile, modifying the CoO under room temperature can obtain specific structure towards wide applications, because the antiferromagnetism materials usually own specific property at the Neel point temperature. In this paper, we synthesized rocksalt CNPs via the solvothermal way. 4


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Subsequently, a one-pot and a two-pot synthetic route were presented to obtain CNPs wrapped by RGO sheets prepared via the modified Hummers method. The as-prepared CoO/RGO composites via both routes show no hysteresis loop, but their magnetic properties can be controlled by annealing under vacuum. The CoO/RGO composite prepared by the one-pot route displays a strong hysteresis after a proper annealing process, because of the chemical contact between CNPs and RGO in the raw composite. By the two-pot synthetic route, a weak hysteresis can be controllably obtained, by changing the annealing temperature because of the physical contact between CNPs and RGO in the raw composite.

2. EXPERIMENTAL SECTION Synthesis of CNPs. Cobalt acetate (Co(CH3COO)2•4H2O), anhydrous ethanol (CH3CH2OH) and polyvinylpyrrolidone (PVP) are analytic grade reagents and purchased without further treatment. In a typical reaction, Co(CH3COO)2•4H2O around 0.6 g, PVP around 0.2 g and CH3CH2OH around 60 mL were added into a beaker, and then the mixture was ultrasonicated until the red particles dispersed well in the solution followed by being transferred into a 100 mL Teflon lined stainless steel autoclave. Finally, the sealed autoclave was put into a drying oven at 165 °C for 6.5 h. After the autoclave cooled down naturally, the resulting yellowish reaction mixture was diluted by ethanol and centrifuged at a speed of 9000 r/min for 5 min. The supernatant was removed and the residue was washed by deionized water and centrifuged at a speed of 11000 r/min for 15 min. Finally, the yellow black CNPs powder can be obtained after a freeze-drying process. 5



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Synthesis of RGO. Graphene oxide (GO) was prepared from purified graphite with the modified Hummers method.20 Firstly, graphite around 1 g and NaNO3 around 0.5 g were added into a 250 mL beaker and then concentrated H2SO4 around 24 mL was poured into the beaker under ice bath. After one hour bath, KMnO4 powder around 3 g was gently added into the beaker followed by a 2 h-reaction. Then the reaction system was transferred to a 250 mL round-bottom flask and oil bath was carried out at 38 °C for 30 min. The temperature was scheduled to increase to 96 °C slowly and maintained for 30 min with constant stirring. During the process of temperature increasing, the deionized water around 80 mL was slowly added into the flask. After reacting at high temperature fully, an additional batch of deionized water around 60 mL was instantly added to stop the reaction. After that, the 30% H2O2 around 15 mL was added to remove the remnants of KMnO4. Finally, the solution turned golden yellow. Then a diluted HCl solution was added and the mixture was washed by deionized water until the PH was close to 7. The sediment was redispersed in deionized water and ultrasonicated for 3 h followed by centrifugation at a speed of 3000 r/min. The GO powder can be obtained after a freeze-drying process of supernatant. GO around 0.1 g was dispersed into 100 mL deionized water followed by adding the hydrazine hydrate around 1.5 mL to the solution and then the mixture was heated for 24 h at 98 °C in an oil bath under a water-cooled condenser. The black suspension was collected by centrifugation at a speed of 11000 r/min for 15 min and washed by deionized water. The loose black RGO powder can be obtained after freeze-drying. 6


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Synthesis of CoO/RGO composite. One-pot synthetic method: The RGO-wrapped CNPs were fabricated in a Teflon lined stainless steel autoclave by the solvothermal route. RGO around 0.011 g was mixed with Co(CH3COO)2•4H2O around 0.6 g and PVP around 0.2 g in ethanol. The successive procedures follow the synthesis of CNPs. Two-pot synthetic method: The CNPs were fabricated via the solvothermal method. After that, RGO dispersed in ethanol after untrasonicated was mixed with the CNPs suspension. The successive procedures follow the synthesis of CNPs.

Heat-treatment of CoO/RGO composite. The CoO/RGO composite sample was placed in a quartz boat and heated in a pipe furnace under vacuum at 300 °C and 400 °C for 20 min, respectively.

Characterization. The morphology was examined by a field emission transmission electron microscopy, FE-TEM, (FEI, Tecnai, G2 F20S, America) and field emission scanning electron microscopy, FE-SEM, (Hitachi, S-4800, Japan). X-ray diffraction (XRD) patterns were obtained using Cu Kα radiation (λ = 0.1542 nm) at voltage of 40 kV and tube current 30 mA (Philips, X’pert Pro MPD, Netherlands). Data were collected by scanning 2θ from 10° to 90° with a step width of 0.03°. Raman spectra were recorded with the excitation wavelength of 632.8 nm using a high-resolution Raman spectrometer (HR800). X-ray photoelectron spectroscopy, XPS, (Kratos, XSAM800, England) was also used to analyze the samples. The as-prepared samples were subject to the magnetic characterization under room-temperature by using a vibration sample magnetometer, VSM, (Lake Shore 7



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7410).

3. RESULTS AND DISCUSSION Figure 1 shows the nanoparticles synthesized at different conditions. The formation mechanism of CNPs is based on the esterification reaction between CH3CH2OH and Co(CH3COO)2 under solvothermal conditions.15 The nanoparticles look irregular with angular existence (Fig 1a), and most of the particles are tetragonal projected shapes. The contour profile of CNPs is octahedron as shown in HADDF-STEM image (Figure S1, Supporting Information). When the reaction time is reduced to 6.5 h, the profile of nanoparticles sustains from changing (Fig 1b), but the size of nanoparticles decreases with the reaction time shortening from 16 h (mean size: ~ 42.4 nm) to 6.5 h (mean size: ~ 25.4 nm). The five sharp peaks of XRD pattern (Fig 2c) at 36.40, 42.28, 61.42, 73.60, 77.47 can be assigned to (111), (200), (220), (311), (222) planes (ISCD card no. 09-0402) and match very well with the standard data of rocksalt CoO and no other peak of impurity is spotted, demonstrating the reaction has been fully carried out after 6.5 h. According to Scherrer equation, the mean size of the CNPs is 22.5 nm, estimated from the (200) peak. The volume of autoclave was increased from 50 mL to 100 mL without any other adjustment except for the doubled volume of reagents. The sizes of CNPs, as shown in Fig 1d, become smaller (mean size: ~ 16.2 nm) than before. The selected area electron diffraction (SAED) shown in Fig 1e is exactly identical to the cubic lattice structure of CoO (space group: Fm3m, the lattice parameters a0 = b0 = c0 = 0.426 nm; JCPDS card no.71-1178), which is in consistent

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with the XRD results. Furthermore, the lattice spacing (Fig 1f) along [111] direction is estimated to be 2.55 Å which is close to the standard data of rocksalt CoO (2.46 Å).

Fig 1. Characterization of CNPs. (a, b, d) TEM image of as-synthesized CNPs obtained under different conditions (reaction time and volume of the autoclave) and the inserts show the histograms of CNPs size distributions. (a) 16 h, 50 mL. (b) 6.5 h, 50 mL. (d) 6.5 h, 100 mL. (c) The XRD pattern of CNPs as in (b). (e) SAED pattern of CNPs as in (d). (f) HR-TEM image of CNPs as in (d).

Fig 2 shows the characterization of RGO and GO obtained with a modified Hummers method. The RGO is a transparent and ultrathin 2D film as shown in Fig 2a, which can provide a large surface area, and serves as a reduction agent. The successful preparation of RGO from graphite was confirmed by the Raman spectrum (Fig 2b). The Raman spectrum of the GO exhibits that the D band is even stronger than G band, proving that there are significant structural disorder and many defects in 9



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GO.21 The G peak position RGO (1578 cm-1) is much close to the well-organized free-standing graphene, and has undergone an obvious red shift during the reduction of GO. Although many functional groups are removed by the reduction process, the intensity ratio of IG/ID on RGO becomes smaller than that on GO, which is

Fig 2. (a) SEM image of RGO. (b) Raman spectra of GO and RGO. (c) The C 1s XPS spectrum of GO. (d) The C 1s XPS spectrum of RGO.

much different from the CVD process, because the domain size in the RGO powder by the redox process is smaller than the original counterpart in the GO referring to G (sp2).22,23 Both Raman spectra of GO and RGO display 2D peaks, suggesting that the crystallized sheets are more than amorphous crystals.24-26 The successful synthesized RGO was further verified by XPS, which reveals the significant changes of structure from GO to RGO. The deconvoluted components of C 1s XPS spectrum of GO (Fig 10


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2c) clearly show a strong peak at 286.5 eV ascribed to C-O bond due to a considerable degree of oxidation, suggesting that lots of functional groups were suspended on the surface of carbon framework. The C-C peak of RGO evolves in the main peak after reduction (Fig 2d), which indicates the removal of many groups. But an additional peak at 285.3 eV corresponding to C-N bond is partially introduced due to the hydrazine.27 Two kinds of CoO/RGO composites, CoO/RGO-1 and CoO/RGO-2, were prepared via a one-pot and a two-pot synthetic route respectively. During one-pot synthetic process, RGO as an ideal template was added into the autoclave to make the CoO crystal nucleate directly on its surface. The SEM image (Fig 3a) shows the CoO nano-particles are well-dispersed on the RGO sheets, displaying an octahedral morphology for most of the nanoparticles, and a uniform size distribution is observed with TEM (Fig 3b). The XRD pattern of CoO/RGO-1 (Fig S2, Supporting Information) is almost the same as that of the single-phase CoO, and no other new peaks appear due to the small amount of RGO. Thus the chemical contact between RGO and CNPs can be inferred. The CoO/RGO-2 shows lots of CNPs are wrapped by RGO and disperse uniformly on the surface of RGO (Fig 3c), despite it’s just a simple mixture of CNPs and RGO. The interaction between RGO and CNPs is inferred as the physical contact. Both CoO/RGO-1 and CoO/RGO-2 seem no hysteresis loop at room temperature owing to the uncompensated spins on the surface of CoO nanoparticles (Fig 4a, 4b).14 After the composites were annealed at low temperature under vacuum, their magnetic 11



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properties changed and the composites prepared via different methods exhibit different magnetic properties. The weak hysteresis, the strong hysteresis emerged successively after CoO/RGO-1 was annealed at low temperatures (Fig 4a). The magnetic properties of CoO/RGO-1 shows a weak hysteresis after heat-treatment at 300 °C as shown in Fig 4a and its insert. When the temperature reaches 400 °C, a

Fig 3. (a) SEM image of CoO/RGO-1. (b) TEM image of CoO/RGO-1. (c) SEM image of CoO/RGO-2.

strong hysteresis comes up. However, no hysteresis can be found when the temperature is about 350 °C (Fig S3, Supporting Information). The XRD results shows no other peak except for that of single-phase CoO after the heat-treatment at 300 °C (Fig S2, Supporting Information). According to the spectrum of XPS in the Co 12


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2p region after heat-treatment at 300 °C (Fig 4c), the two predominant peaks at 780.2 eV and 796.0 eV are ascribed to Co(ΙΙ) 2p3/2 and 2p1/2. In addition, their satellite shake-ups are located at 786.5 eV and 803.1 eV. No other peak of impurity emerges. The intensity of saturation magnetization is in the same order of magnitude as that of RGO,28,29 so the appearance of weak hysteresis origins from RGO, which can also be verified by the XPS. Fig 4c (black line) illustrates no peak representing Co clusters. The magnetic properties of RGO depend on the edge chirality and defects. During the process of heat-treatment, topological defects with unpaired electrons were introduced into the RGO sheets.28 On the other hand, the zigzag-edge comes into being during the process of heat-treatment. Due to the special geometry of zigzag-edge RGO, where the polarized electron spins are ferromagnetically aligned along the edges, a weak ferromagnetic structure is possible.30-32 Provided the reaction proceeds, the zigzag-edge structure should be broken and the density of defects reduced at 350 °C, so the CoO/RGO-1 shows no hysteresis again. As for the appearance of strong hysteresis, it is mainly attributed to the appearance of Co clusters reduced by RGO and the assumption is certified by HR-TEM (Fig 4e) and XPS (Fig 4c, red line). The CoO lattice on the surface of CNPs is partially reduced to Co lattice after heat-treated at 400 °C and the CNPs remain their size despite of somewhat agglomeration (Fig 4d). As shown in Fig 4e, the lattice-spacing matches well with Co (200). The XRD results (Fig S2, Supporting Information) are all almost the same after heat-treatment and show no other peaks except for that of single-phase CoO, suggesting only trace amount of Co in nano cluster decorated on the surface of CNPs. In comparison with 13



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the XPS spectrum in the Co 2p region after heat-treatment at 300 °C, a weak peak at about 778.1 eV emerges after heat-treatment at 400 °C, which is ascribed to the appearance of Co metal. The CoO/RGO-2 demonstrates a weak hysteresis after heat-treatment at 300 °C and

Fig 4. (a, b) Magnetization hysteresis loops for CoO/RGO-1 and CoO/RGO-2 after heat-treatment at different temperatures. (c) The Co 2p XPS spectra of CoO/RGO-1 after heat-treatment at 300 °C and 14


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400 °C. (d) SEM image of CoO/RGO-1 after heat-treatment at 400 °C. (e) HR-TEM image of CoO/RGO-1 after heat-treatment at 400 °C. (f) SEM image of CoO/RGO-2 after heat-treatment at 400 °C.

400 °C (Fig 4b), and the weak hysteresis of CoO/RGO-2 at 400 °C is stronger than that at 300 °C. In comparison with CoO/RGO-1, the controllable weak magnetic properties can be realized by heat-treatment of the CoO/RGO-2 under a low temperature, because of the weak physical contact between the RGO and the CNPs. Whereas the strong magnetic properties of CNPs can be obtained by the CoO/RGO-1 route. Because of the chemical contact between the RGO and CNPs, the reduction process can proceed more easily and severely than that by a physical contact. As for the CoO/RGO-2, the CNPs remain their nano scales after heat-treatment at 400 °C (Fig 4f). In general, the magnetic properties of CNPs can be controlled by the modification of RGO.

4. CONCLUSIONS In summary, we synthesized CoO nanoparticles through the solvothermal method and developed two routes to form RGO-wrapped CoO. The obtained composite materials can be treated into nanoparticles with different magnetic properties. Heat-treatment of CoO/RGO can control their magnetic properties. Strong magnetism can be obtained by one-pot route and weak magnetism can be obtained by the modification of CNPs via a two-pot route under a low temperature. Modification of metal oxide by graphene has opened a new way to obtain controllable magnetic nanoparticles.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51472170).

Supporting Information Available

HADDF-STEM of CoO nanoparticles, XRD patterns of CoO/RGO-1 after heat-treatment at different temperatures and magnetization hysteresis loop. This information is available free of charge via the Internet at http://pubs.acs.org.

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Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Synthesis of Graphene-Based Nanosheets via Chemical Reduction of Exfoliated Graphite Oxide. Carbon 2007, 45, 1558-1565. [23] Tuinstra, F.; Koenig, J. L.; Raman Spectrum of Graphite. J. Chem. Phys. 1970, 53, 1126-1130. [24] Schwan, J.; Ulrich, S.; Batori, V.; Ehrhardt, H. Raman Spectra on Amorphous Carbon Films. J. Appl. Phys. 1996, 8, 440-447. [25] Ferrari, A. C.; Robertson, J. Interpretation of Raman Spectra of Disordered and Amorphous Carbon. Phys. Rev. B 2000, 61, 14095-14107. [26] Eckmann, A.; Felten, A.; Mishchenko, A.; Britnell, L.; Krupke, R.; Novoselov, K. S.; Casiraghi, C. Probing the Nature of Defects in Graphene by Raman Spectroscopy. Nano Lett. 2012, 12, 3925-3930. [27] Becerril, H. A.; Mao, J.; Liu, Z.; Stoltenberg, R. M.; Bao, Z.; Chen, Y. Evaluation of Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors. ACS Nano 2008, 2, 463-470. [28] Wang, Y.; Huang, Y.; Song, Y.; Zhang, X.; Ma, Y.; Liang, J.; Chen, Y. Room-Temperature Ferromagnetism of Graphene. Nano Lett. 2009, 9, 220-224. [29] Wang, C.; Diao, D. Magnetic Behavior of Graphene Sheets Embedded in Carbon Film Originated from Graphene Nanocrystallite. Appl. Phys. Lett. 2013, 102, 052402. [30] Radovic, L. R.; Bockrath, L. On the Chemical Nature of Graphene Edges: Origin of Stability and Potential for Magnetism in Carbon Materials. J. AM. Chem. Soc. 2005, 127, 5917-5927. [31] Wu, W.; Zhang, Z.; Lu, P.; Guo, W. Electronic and Magnetic Properties of Zigzag 19



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Graphene Nanoribbons with Periodic Protruded Edges. Phys. Rev. B 2010, 82, 085425. [32] Xu, B.; Yin, J.; Xia, Y. D.; Wan, X. G.; Jiang, K.; Liu, Z. G. Electronic and Magnetic Properties of Zigzag Graphene Nanoribbon with One Edge Saturated. Appl. Phys. Lett. 2010, 96, 163102.

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Controllable Magnetism of CoO Nanoparticles Modified by the

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