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Facile synthesis, magnetic and optical properties of double-shelled Co3O4 hollow microspheres
Advanced Powder Technology 25 (2014) 1780–1785
Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Facile synthesis, magnetic and optical properties of double-shelled Co3O4 hollow microspheres Wenlong Hu, Liuding Wang, Qiaofeng Wu, Hongjing Wu ⇑ Department of Applied Physics, Northwestern Polytechnical University, Xi’an 710072, PR China
a r t i c l e
i n f o
Article history: Received 29 May 2014 Received in revised form 4 July 2014 Accepted 7 July 2014 Available online 17 July 2014 Keywords: Cobalt oxide Double-shelled hollow spheres Hydrothermal reaction Transmission electron microscopy
a b s t r a c t Double-shelled Co3O4 hollow spheres are successfully synthesized by chemically induced self-assembly in the hydrothermal environment. The morphology, chemical composition, and crystalline structure of the double-shelled Co3O4 hollow spheres are characterized by different techniques, such as powder X-ray diffraction (PXRD), Raman spectrum, X-ray photoelectron spectrum (XPS), field emission scanning electron microscope (FESEM), and high resolution transmission electron microscope (HRTEM) with selected area electron diffraction (SAED). Magnetic measurements and optical spectra suggest the double-shelled Co3O4 hollow spheres exhibit close to a weak ferromagnetic behaviour and enhanced photogenerated carrier separation. Since this synthetic route is simple, convenient, and ‘‘green’’, it is possible to extend this synthetic method to preparation of a wide range of the multishelled hollow spheres of metal oxides for semiconductor device applications. Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Hollow nanometer or submicrometer sized spheres and capsules have attracted great interest owing to their potential applications in catalysis, drug delivery, nanoreactors, photonic devices, chemical sensors, biotechnology, and energy conversion and storage systems [1–10]. Hollow spheres, in particular those with complex core–shell structures, have increasingly attracted attention as a result of their superior properties [11]. Up to now, single-shelled and double-shelled hollow spheres of various compositions have been synthesized by a number of methods, such as vesicles, emulsions, micelles, gas-bubble, and hard-templating methods [12–15]. More recently, increasing efforts have focused on the fabrication of hollow spheres with multiple shells, as these materials are expected to have better properties for extensive applications such as drug release with prolonged release time, heterogeneous catalysis, lithium-ion batteries, photo-catalysis, and microwave absorption aspects [16]. For example, multiple-shelled hollow spheres of Cu2O have been successfully prepared by vesicle-templating and a phase-transformation process of intermediate-templating [17,18]. Triple-shelled SnO2 hollow spheres were fabricated by chemically induced self-assembly in the hydrothermal environment which exhibited enhanced electrochemical performance [19]. ⇑ Corresponding author. Tel./fax: +86 29 8843 1664. E-mail address: [email protected] (H. Wu).
Multiple-shelled Co3O4 hollow spheres were synthesized by oriented self-assembly which exhibited excellent cycle performance and enhanced lithium storage capacity [20]. However, these methods are suitable for each specific material and cannot be applied generally to a wide range of materials [21]. Recently, Ji et al. [22,23] and Lai et al. [24] presented a straightforward and general strategy to prepare metal oxide hollow spheres with a controlled number of shells. Carbonaceous spheres were used as sacrificial templates and were saturated with a desired metal salt solution and then heated in air. Then, the carbonaceous template evaporates and templates the formation of metal oxide shells. The number of shells is controlled by the metal ion loading and the process is general for a wide range of metal oxide materials. In the present contribution, we report a novel, one-pot approach for preparation of double-shelled hollow spheres composed of nanocrystalline Co3O4 by chemically induced self-assembly in the hydrothermal environment. More importantly, this novel facile method may become a general synthetic approach for fabricating multiple-shelled hollow nanostructures of any desired materials.
2. Experimental D-Glucose monohydrate purchased from National Reagent Corp. (Shanghai, China) was used as a carbon source. Cobalt(II) nitrate hexahydrate (Co(NO3)26H2O) was obtained from Tianjin Kemiou Chemical Reagent Corp. (Tianjin, China).
http://dx.doi.org/10.1016/j.apt.2014.07.007 0921-8831/Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
W. Hu et al. / Advanced Powder Technology 25 (2014) 1780–1785
The C@Co3O4 core–shell solid spheres were successfully synthesized by a facile hydrothermal process. Typically, appropriate amounts of anhydrous glucose, cobalt (II) nitrate hexahydrate (the molar ratio is 1:2) were mixed and completely dissolved in 50 ml of deionized water. Then aforementioned aqueous solution was put into a Teflon-lined stainless-steel autoclave with 80 ml capacity, and transferred into the oven at 180 °C for 20 h. After cooling to room temperature naturally, the product was washed repeatedly with deionized water and anhydrous alcohol, and dried in the vacuum oven at 80 °C for 12 h. As following, the as-collected powders were calcined in the atmosphere of air, at 550 °C for 150 min with a velocity of 2 °C/ min from room temperature to 550 °C in a tube furnace. Then black powders were finally obtained and prepared for further characterization. The crystalline structures of the resultant samples were identified by powder X-ray diffraction (PXRD) with Cu Ka radiation. To confirm the surface component, the X-ray photoelectron spectrum (XPS) was also recorded with the excitation source of Al Ka line. And also, Raman spectrum was measured on a Nicolet Almega spectrometer. In order to identify the chemical constituent and morphology of the samples, a field emission scanning electron microscope (FESEM) and high resolution transmission electron microscope (HRTEM) with selected area electron diffraction (SAED) were used. Magnetic measurements (M–H) were done on a superconducting quantum interference device (Quantum Design Model MPMS XL-7) by varying the field up to 30 kOe at 300 K. Room temperature photoluminescence (PL) spectroscopy with the HeACd (325 nm) laser line as the exciton sources was used to know the optical properties of the synthesized Co3O4 samples in detail.
3. Results and discussion 3.1. Structure, chemical composition and morphology The crystalline structures of the prepared double-shelled Co3O4 hollow spheres were investigated by X-ray diffraction (XRD) along with the structure of as-synthesized C@Co3O4 core–shell solid spheres for comparison (Fig. 1a). All reflection peaks were indexed to a relatively pure cubic spinel Co3O4 structure (JCPDS card No. 65-3103, space group: Fd3m, a = 8.0837 Å), with no additional peaks detected. Further structure information was obtained from Raman spectra of the prepared samples, recorded at room temperature, and presented in Fig. 1b. The Raman spectrum of the phase Co3O4 (Fig. 1b) displays intense bands at 188, 458 and 665 cm 1 with bands of less intensity at 508 and 607 cm 1. The Raman band
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at 665 cm 1 is highly polarized. On the basis of the previous works by Yang and Hadjiev et al. [25,26], the bands are assigned as follows: the 665 cm 1 band to A1g mode, the 607 cm 1 the F2g mode, the band at 508 cm 1 the F2g mode, the 458 cm 1 the Eg mode, and the band at 188 cm 1 the F2g mode. No additional bands, corresponding to other phases such as CoO(OH) or cubic CoO, are observed [25], which is very consistent with the XRD result. The X-ray photoelectron spectroscopy (XPS) was also used to investigate the chemical composition of prepared samples (Fig. 2). Two characteristic broad peaks encompassing the Co2+ and Co3+ 2p3/2 states at 779.58 eV and Co2+ and Co3+ 2p1/2 states at 795.08 eV were observed (Fig. 2c); these peaks correspond to the standard Co3O4 phase [27]. Here, the species interpretation according to Gautier et al. was used [28]. The main peak consists of two species, one at 779.68 eV for Co3+ and another at 782.38 eV for Co2+. A broad satellite peak at 788.98 eV is originated from Co2+. The C1s spectra of prepared Co3O4 samples are shown in Fig. 2a to elucidate their surface compositions. The peaks centered at 286.88, 287.98 and 289.28 eV can be attributed to the CAC/C@C, CAO, and OAC@O groups, respectively. The above result suggests that as-synthesized C@Co3O4 core–shell solid spheres have a relatively large distribution of CAO and C@O functional groups in the solid spheres [29]. These functional groups can be effectively removed by air calcination as shown in Fig. 2a. Furthermore, their O1s spectra are also investigated in detail (Fig. 2b), which is particularly important to confirm or disprove the existence of oxygen vacancies. The peaks at 532.98 and 531.08 eV should be attributed to the oxygen vacancies existing in Co3O4 crystals, while the weak one at 529.58 eV corresponds to the original lattice oxygen species in Co3O4 [30]. The chemical constituents and morphologies of the samples are revealed in Fig. 3. Fig. 3a and b describes the FESEM and TEM images of the as-synthesized products obtained by the hydrothermal process. The results show that a large quantity of C@Co3O4 core–shell solid spheres (d = 1–2 lm) is made and their structures are mainly consisted of many small particles. Moreover, the morphology of cobalt oxide shell is also sustained very well after calcinations at 550 °C for 150 min in air. The FESEM and TEM images of the as-synthesized hollow spheres are displayed in Fig. 3c–e, respectively. It can be obviously seen that in Fig. 3c and a clear hollow structure emerges in these microspheres with a radius of 500–1000 nm, and these microspheres are consisted of many more tiny nanoparticles with a radius of 5–10 nm (see HRTEM image later). From the TEM image (Fig. 3d and e), the same results are acquired. Furthermore, on the basis of the analysis of each Co3O4 hollow microsphere, we determine that all Co3O4 hollow microspheres have a similar size (d = 1–2 lm) and double-shelled
Fig. 1. (a) XRD patterns and (b) Raman spectra of C@Co3O4 core–shell solid spheres and double-shelled Co3O4 spheres.
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Fig. 2. High-resolution XPS spectra of C1s, O1s and Co2p of C@Co3O4 core–shell solid spheres and double-shelled Co3O4 spheres.
Co3O4 microspheres are obtained after calcination and the yield of double-shelled Co3O4 microspheres is very high (75%). For example, in the preparation of the double-shelled Co3O4 microspheres, 75% of the product microspheres have double shells and 25% have single shell. The corresponding SAED pattern of the Co3O4 hollow sphere in Fig. 3e (inset) shows clear four diffuse diffraction rings, which can be characterized as the (2 2 2), (4 0 0), (4 4 0) and (6 2 0) planes from inner to outer and further demonstrates these hollow spheres are composed of cubic phase Co3O4 polycrystalline. In addition, the lattice fringes shown in Fig. 3f are about 0.47 nm corresponding to the Co3O4 (1 1 1) plane. Detailed structural observation by TEM imaging of each Co3O4 hollow microsphere clearly reveals nanopores inside the shells as well as the highly crystalline nature of the Co3O4 nanoparticles in the shells (Fig. 3d–f). The EDX analysis (not shown) acquired from the as-collected hollow cobalt oxide spheres shows only two substantial peaks are observed, which suggests only the existence of Co and O elements are contained in the hollow spheres and no other elements are detected, e.g., C element. It can be concluded that the carbon can be easily removed by calcination at 550 °C for 150 min in air from the C@Co3O4 core–shell solid sphere (Fig. 3a and b), and finally resulting in the formation of multishelled Co3O4 hollow spheres (Fig. 3c–e). On account of the above analysis, the facile synthesis process for the double-shelled Co3O4 hollow spheres is schematically illustrated in Fig. 4. Through a facile hydrothermal method, the C@Co3O4 core–shell solid spheres are firstly synthesized, in which the formation of the monolayered sphere shell is assumed to involve the formation of the carbon sphere by the polymerization and carbonization reaction in the first step and subsequently the adsorption of metal ions and their resulting nanoparticles near the hydrophilic shell of the carbon particles. Subsequently, cobalt oxide hydrate on the surface of the carbon spheres reacts with the OH groups of oligosaccharides formed via the glucose polymerization. After carbonization of the oligosaccharides, the second carbon shell is formed outside the primary carbon sphere. So, the well-defined carbon layer and the cobalt oxide (or
hydrate) layer are assembled one by one via the chemically induced dehydration reaction. As a consequence, the doubleshelled sphere shells with carbon and cobalt oxide layers are alternately formed. After final calcination, the double-shelled Co3O4 hollow spheres are obtained.
3.2. Magnetic properties The M–H plots for the C@Co3O4 core–shell solid spheres and double-shelled Co3O4 hollow spheres, measured at 300 K are shown in Fig. 5. The M–H curves at 300 K indicate that the two samples exhibit close to a weak ferromagnetic behaviour although bulk Co3O4 is antiferromagnetic [31]. The weak ferromagnetic behaviour at 300 K is attributed to the presence of small but finite amount of defect moments in the Co3O4 nanoparticles. From the inset, the coercive field (Hc) and the remanent magnetization (Mr) are estimated to be about 6 Oe and 0.0001 emu/g, respectively. The maximum field applied, 30 kOe does not saturate the magnetization and the magnetization at this applied field is 0.95 emu/g. The low Hc and Mr values confirm that the two samples exhibit a little ferromagnetic property. The non-saturation of the magnetization is characteristic of weak ferromagnetic ordering of the spins in the nanoparticles. The values of the remanent magnetization and coercivity of the double-shelled Co3O4 hollow spheres are higher than those of C@Co3O4 core–shell solid spheres. It is well known that the magnetization of magnetic materials is dependent on the morphology and structure of the samples [32,33]. The higher remanent magnetization and coercivity observed in the present study should be associated with the unique morphology and structure of the double-shelled Co3O4 hollow spheres. In our case, assembly of double-shelled Co3O4 hollow spheres results in an increase of the magnetic surface anisotropy compared with C@Co3O4 core–shell solid spheres, leading to the higher remanent magnetization and higher coercivity.
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Fig. 3. (a and b) FESEM and TEM images of C@Co3O4 core–shell solid spheres and (c) FESEM, (d and e) TEM and (f) HRTEM images of double-shelled Co3O4 spheres.
Fig. 4. Schematic illustration of the formation processes of the double-shelled Co3O4 sphere.
3.3. Optical properties As well known, the optical properties of materials are close relation with their structures and morphologies. In this aspect, nanomaterials exhibit more obvious than that of bulk materials. Fig. 6 shows the room temperature photoluminescence spectra of the C@Co3O4 core–shell solid sphere and double-shelled Co3O4 hollow
sphere. Generally, two bands have been appearing in the PL spectra: in the UV region, called as near band edge (NBE) emission, originating due to the recombination of free excitons through an exciton–exciton collision process and in the visible region recognized as deep level emission (DPE) caused by the impurities and structural defects in the crystal, for instance, oxygen vacancies and cobalt interstitials [34,35].
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Fig. 5. Hysteresis loops of (a) C@Co3O4 core–shell solid spheres and (b) double-shelled Co3O4 hollow spheres.
Doctorate Foundation, Doctorate Foundation, and Graduate Starting Seed Fund (No. Z2014070) of Northwestern Polytechnical University, and the Scholarship Award for Excellent Doctoral Student granted by Ministry of Education, PR China.
Intensity (a. u.)
360
a
References
b
350
400
450
500
550
600
650
Wavelength (nm) Fig. 6. Photoluminescence spectra (PL) of (a) C@Co3O4 core–shell solid spheres and (b) double-shelled Co3O4 hollow spheres.
At room temperature, the emission band for C@Co3O4 core– shell solid spheres is centered at 360 nm, which is attributed to the recombination process of self-trapped excitations, and the emission in the visible region is due to impurities and structural defects in the nanocrystal. The positions of double-shelled Co3O4 hollow spheres emission peaks are similar to C@Co3O4 core–shell structures. However, the emission intensity of the double-shelled Co3O4 hollow sphere significantly decreases. The result clearly suggests that the photogenerated electron-hole pair in the doubleshelled Co3O4 hollow spheres could be separated more efficiently compared with C@Co3O4 core–shell solid spheres. 4. Conclusions In summary, we succeed in synthesizing double-shelled Co3O4 hollow spheres in high yield with high purity by a facile hydrothermal reaction for the first time. Given the generality of this novel method, in terms of theory, it can be widely used to synthesize the multishelled hollow microspheres of other metal oxides by changing the metal oxide precursors. Because these hollow spheres are composed of nanoparticles and have stable structure and a particular inner environment, such materials will open a new avenue for the development of the next generation of semiconductor device. Acknowledgments Financial support was provided by National Nature Science Foundation of China (Nos. 50771082 and 60776822), the Excellent
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Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Facile synthesis, magnetic and optical properties of double-shelled Co3O4 hollow microspheres Wenlong Hu, Liuding Wang, Qiaofeng Wu, Hongjing Wu ⇑ Department of Applied Physics, Northwestern Polytechnical University, Xi’an 710072, PR China
a r t i c l e
i n f o
Article history: Received 29 May 2014 Received in revised form 4 July 2014 Accepted 7 July 2014 Available online 17 July 2014 Keywords: Cobalt oxide Double-shelled hollow spheres Hydrothermal reaction Transmission electron microscopy
a b s t r a c t Double-shelled Co3O4 hollow spheres are successfully synthesized by chemically induced self-assembly in the hydrothermal environment. The morphology, chemical composition, and crystalline structure of the double-shelled Co3O4 hollow spheres are characterized by different techniques, such as powder X-ray diffraction (PXRD), Raman spectrum, X-ray photoelectron spectrum (XPS), field emission scanning electron microscope (FESEM), and high resolution transmission electron microscope (HRTEM) with selected area electron diffraction (SAED). Magnetic measurements and optical spectra suggest the double-shelled Co3O4 hollow spheres exhibit close to a weak ferromagnetic behaviour and enhanced photogenerated carrier separation. Since this synthetic route is simple, convenient, and ‘‘green’’, it is possible to extend this synthetic method to preparation of a wide range of the multishelled hollow spheres of metal oxides for semiconductor device applications. Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Hollow nanometer or submicrometer sized spheres and capsules have attracted great interest owing to their potential applications in catalysis, drug delivery, nanoreactors, photonic devices, chemical sensors, biotechnology, and energy conversion and storage systems [1–10]. Hollow spheres, in particular those with complex core–shell structures, have increasingly attracted attention as a result of their superior properties [11]. Up to now, single-shelled and double-shelled hollow spheres of various compositions have been synthesized by a number of methods, such as vesicles, emulsions, micelles, gas-bubble, and hard-templating methods [12–15]. More recently, increasing efforts have focused on the fabrication of hollow spheres with multiple shells, as these materials are expected to have better properties for extensive applications such as drug release with prolonged release time, heterogeneous catalysis, lithium-ion batteries, photo-catalysis, and microwave absorption aspects [16]. For example, multiple-shelled hollow spheres of Cu2O have been successfully prepared by vesicle-templating and a phase-transformation process of intermediate-templating [17,18]. Triple-shelled SnO2 hollow spheres were fabricated by chemically induced self-assembly in the hydrothermal environment which exhibited enhanced electrochemical performance [19]. ⇑ Corresponding author. Tel./fax: +86 29 8843 1664. E-mail address: [email protected] (H. Wu).
Multiple-shelled Co3O4 hollow spheres were synthesized by oriented self-assembly which exhibited excellent cycle performance and enhanced lithium storage capacity [20]. However, these methods are suitable for each specific material and cannot be applied generally to a wide range of materials [21]. Recently, Ji et al. [22,23] and Lai et al. [24] presented a straightforward and general strategy to prepare metal oxide hollow spheres with a controlled number of shells. Carbonaceous spheres were used as sacrificial templates and were saturated with a desired metal salt solution and then heated in air. Then, the carbonaceous template evaporates and templates the formation of metal oxide shells. The number of shells is controlled by the metal ion loading and the process is general for a wide range of metal oxide materials. In the present contribution, we report a novel, one-pot approach for preparation of double-shelled hollow spheres composed of nanocrystalline Co3O4 by chemically induced self-assembly in the hydrothermal environment. More importantly, this novel facile method may become a general synthetic approach for fabricating multiple-shelled hollow nanostructures of any desired materials.
2. Experimental D-Glucose monohydrate purchased from National Reagent Corp. (Shanghai, China) was used as a carbon source. Cobalt(II) nitrate hexahydrate (Co(NO3)26H2O) was obtained from Tianjin Kemiou Chemical Reagent Corp. (Tianjin, China).
http://dx.doi.org/10.1016/j.apt.2014.07.007 0921-8831/Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
W. Hu et al. / Advanced Powder Technology 25 (2014) 1780–1785
The C@Co3O4 core–shell solid spheres were successfully synthesized by a facile hydrothermal process. Typically, appropriate amounts of anhydrous glucose, cobalt (II) nitrate hexahydrate (the molar ratio is 1:2) were mixed and completely dissolved in 50 ml of deionized water. Then aforementioned aqueous solution was put into a Teflon-lined stainless-steel autoclave with 80 ml capacity, and transferred into the oven at 180 °C for 20 h. After cooling to room temperature naturally, the product was washed repeatedly with deionized water and anhydrous alcohol, and dried in the vacuum oven at 80 °C for 12 h. As following, the as-collected powders were calcined in the atmosphere of air, at 550 °C for 150 min with a velocity of 2 °C/ min from room temperature to 550 °C in a tube furnace. Then black powders were finally obtained and prepared for further characterization. The crystalline structures of the resultant samples were identified by powder X-ray diffraction (PXRD) with Cu Ka radiation. To confirm the surface component, the X-ray photoelectron spectrum (XPS) was also recorded with the excitation source of Al Ka line. And also, Raman spectrum was measured on a Nicolet Almega spectrometer. In order to identify the chemical constituent and morphology of the samples, a field emission scanning electron microscope (FESEM) and high resolution transmission electron microscope (HRTEM) with selected area electron diffraction (SAED) were used. Magnetic measurements (M–H) were done on a superconducting quantum interference device (Quantum Design Model MPMS XL-7) by varying the field up to 30 kOe at 300 K. Room temperature photoluminescence (PL) spectroscopy with the HeACd (325 nm) laser line as the exciton sources was used to know the optical properties of the synthesized Co3O4 samples in detail.
3. Results and discussion 3.1. Structure, chemical composition and morphology The crystalline structures of the prepared double-shelled Co3O4 hollow spheres were investigated by X-ray diffraction (XRD) along with the structure of as-synthesized C@Co3O4 core–shell solid spheres for comparison (Fig. 1a). All reflection peaks were indexed to a relatively pure cubic spinel Co3O4 structure (JCPDS card No. 65-3103, space group: Fd3m, a = 8.0837 Å), with no additional peaks detected. Further structure information was obtained from Raman spectra of the prepared samples, recorded at room temperature, and presented in Fig. 1b. The Raman spectrum of the phase Co3O4 (Fig. 1b) displays intense bands at 188, 458 and 665 cm 1 with bands of less intensity at 508 and 607 cm 1. The Raman band
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at 665 cm 1 is highly polarized. On the basis of the previous works by Yang and Hadjiev et al. [25,26], the bands are assigned as follows: the 665 cm 1 band to A1g mode, the 607 cm 1 the F2g mode, the band at 508 cm 1 the F2g mode, the 458 cm 1 the Eg mode, and the band at 188 cm 1 the F2g mode. No additional bands, corresponding to other phases such as CoO(OH) or cubic CoO, are observed [25], which is very consistent with the XRD result. The X-ray photoelectron spectroscopy (XPS) was also used to investigate the chemical composition of prepared samples (Fig. 2). Two characteristic broad peaks encompassing the Co2+ and Co3+ 2p3/2 states at 779.58 eV and Co2+ and Co3+ 2p1/2 states at 795.08 eV were observed (Fig. 2c); these peaks correspond to the standard Co3O4 phase [27]. Here, the species interpretation according to Gautier et al. was used [28]. The main peak consists of two species, one at 779.68 eV for Co3+ and another at 782.38 eV for Co2+. A broad satellite peak at 788.98 eV is originated from Co2+. The C1s spectra of prepared Co3O4 samples are shown in Fig. 2a to elucidate their surface compositions. The peaks centered at 286.88, 287.98 and 289.28 eV can be attributed to the CAC/C@C, CAO, and OAC@O groups, respectively. The above result suggests that as-synthesized C@Co3O4 core–shell solid spheres have a relatively large distribution of CAO and C@O functional groups in the solid spheres [29]. These functional groups can be effectively removed by air calcination as shown in Fig. 2a. Furthermore, their O1s spectra are also investigated in detail (Fig. 2b), which is particularly important to confirm or disprove the existence of oxygen vacancies. The peaks at 532.98 and 531.08 eV should be attributed to the oxygen vacancies existing in Co3O4 crystals, while the weak one at 529.58 eV corresponds to the original lattice oxygen species in Co3O4 [30]. The chemical constituents and morphologies of the samples are revealed in Fig. 3. Fig. 3a and b describes the FESEM and TEM images of the as-synthesized products obtained by the hydrothermal process. The results show that a large quantity of C@Co3O4 core–shell solid spheres (d = 1–2 lm) is made and their structures are mainly consisted of many small particles. Moreover, the morphology of cobalt oxide shell is also sustained very well after calcinations at 550 °C for 150 min in air. The FESEM and TEM images of the as-synthesized hollow spheres are displayed in Fig. 3c–e, respectively. It can be obviously seen that in Fig. 3c and a clear hollow structure emerges in these microspheres with a radius of 500–1000 nm, and these microspheres are consisted of many more tiny nanoparticles with a radius of 5–10 nm (see HRTEM image later). From the TEM image (Fig. 3d and e), the same results are acquired. Furthermore, on the basis of the analysis of each Co3O4 hollow microsphere, we determine that all Co3O4 hollow microspheres have a similar size (d = 1–2 lm) and double-shelled
Fig. 1. (a) XRD patterns and (b) Raman spectra of C@Co3O4 core–shell solid spheres and double-shelled Co3O4 spheres.
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Fig. 2. High-resolution XPS spectra of C1s, O1s and Co2p of C@Co3O4 core–shell solid spheres and double-shelled Co3O4 spheres.
Co3O4 microspheres are obtained after calcination and the yield of double-shelled Co3O4 microspheres is very high (75%). For example, in the preparation of the double-shelled Co3O4 microspheres, 75% of the product microspheres have double shells and 25% have single shell. The corresponding SAED pattern of the Co3O4 hollow sphere in Fig. 3e (inset) shows clear four diffuse diffraction rings, which can be characterized as the (2 2 2), (4 0 0), (4 4 0) and (6 2 0) planes from inner to outer and further demonstrates these hollow spheres are composed of cubic phase Co3O4 polycrystalline. In addition, the lattice fringes shown in Fig. 3f are about 0.47 nm corresponding to the Co3O4 (1 1 1) plane. Detailed structural observation by TEM imaging of each Co3O4 hollow microsphere clearly reveals nanopores inside the shells as well as the highly crystalline nature of the Co3O4 nanoparticles in the shells (Fig. 3d–f). The EDX analysis (not shown) acquired from the as-collected hollow cobalt oxide spheres shows only two substantial peaks are observed, which suggests only the existence of Co and O elements are contained in the hollow spheres and no other elements are detected, e.g., C element. It can be concluded that the carbon can be easily removed by calcination at 550 °C for 150 min in air from the C@Co3O4 core–shell solid sphere (Fig. 3a and b), and finally resulting in the formation of multishelled Co3O4 hollow spheres (Fig. 3c–e). On account of the above analysis, the facile synthesis process for the double-shelled Co3O4 hollow spheres is schematically illustrated in Fig. 4. Through a facile hydrothermal method, the C@Co3O4 core–shell solid spheres are firstly synthesized, in which the formation of the monolayered sphere shell is assumed to involve the formation of the carbon sphere by the polymerization and carbonization reaction in the first step and subsequently the adsorption of metal ions and their resulting nanoparticles near the hydrophilic shell of the carbon particles. Subsequently, cobalt oxide hydrate on the surface of the carbon spheres reacts with the OH groups of oligosaccharides formed via the glucose polymerization. After carbonization of the oligosaccharides, the second carbon shell is formed outside the primary carbon sphere. So, the well-defined carbon layer and the cobalt oxide (or
hydrate) layer are assembled one by one via the chemically induced dehydration reaction. As a consequence, the doubleshelled sphere shells with carbon and cobalt oxide layers are alternately formed. After final calcination, the double-shelled Co3O4 hollow spheres are obtained.
3.2. Magnetic properties The M–H plots for the C@Co3O4 core–shell solid spheres and double-shelled Co3O4 hollow spheres, measured at 300 K are shown in Fig. 5. The M–H curves at 300 K indicate that the two samples exhibit close to a weak ferromagnetic behaviour although bulk Co3O4 is antiferromagnetic [31]. The weak ferromagnetic behaviour at 300 K is attributed to the presence of small but finite amount of defect moments in the Co3O4 nanoparticles. From the inset, the coercive field (Hc) and the remanent magnetization (Mr) are estimated to be about 6 Oe and 0.0001 emu/g, respectively. The maximum field applied, 30 kOe does not saturate the magnetization and the magnetization at this applied field is 0.95 emu/g. The low Hc and Mr values confirm that the two samples exhibit a little ferromagnetic property. The non-saturation of the magnetization is characteristic of weak ferromagnetic ordering of the spins in the nanoparticles. The values of the remanent magnetization and coercivity of the double-shelled Co3O4 hollow spheres are higher than those of C@Co3O4 core–shell solid spheres. It is well known that the magnetization of magnetic materials is dependent on the morphology and structure of the samples [32,33]. The higher remanent magnetization and coercivity observed in the present study should be associated with the unique morphology and structure of the double-shelled Co3O4 hollow spheres. In our case, assembly of double-shelled Co3O4 hollow spheres results in an increase of the magnetic surface anisotropy compared with C@Co3O4 core–shell solid spheres, leading to the higher remanent magnetization and higher coercivity.
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Fig. 3. (a and b) FESEM and TEM images of C@Co3O4 core–shell solid spheres and (c) FESEM, (d and e) TEM and (f) HRTEM images of double-shelled Co3O4 spheres.
Fig. 4. Schematic illustration of the formation processes of the double-shelled Co3O4 sphere.
3.3. Optical properties As well known, the optical properties of materials are close relation with their structures and morphologies. In this aspect, nanomaterials exhibit more obvious than that of bulk materials. Fig. 6 shows the room temperature photoluminescence spectra of the C@Co3O4 core–shell solid sphere and double-shelled Co3O4 hollow
sphere. Generally, two bands have been appearing in the PL spectra: in the UV region, called as near band edge (NBE) emission, originating due to the recombination of free excitons through an exciton–exciton collision process and in the visible region recognized as deep level emission (DPE) caused by the impurities and structural defects in the crystal, for instance, oxygen vacancies and cobalt interstitials [34,35].
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Fig. 5. Hysteresis loops of (a) C@Co3O4 core–shell solid spheres and (b) double-shelled Co3O4 hollow spheres.
Doctorate Foundation, Doctorate Foundation, and Graduate Starting Seed Fund (No. Z2014070) of Northwestern Polytechnical University, and the Scholarship Award for Excellent Doctoral Student granted by Ministry of Education, PR China.
Intensity (a. u.)
360
a
References
b
350
400
450
500
550
600
650
Wavelength (nm) Fig. 6. Photoluminescence spectra (PL) of (a) C@Co3O4 core–shell solid spheres and (b) double-shelled Co3O4 hollow spheres.
At room temperature, the emission band for C@Co3O4 core– shell solid spheres is centered at 360 nm, which is attributed to the recombination process of self-trapped excitations, and the emission in the visible region is due to impurities and structural defects in the nanocrystal. The positions of double-shelled Co3O4 hollow spheres emission peaks are similar to C@Co3O4 core–shell structures. However, the emission intensity of the double-shelled Co3O4 hollow sphere significantly decreases. The result clearly suggests that the photogenerated electron-hole pair in the doubleshelled Co3O4 hollow spheres could be separated more efficiently compared with C@Co3O4 core–shell solid spheres. 4. Conclusions In summary, we succeed in synthesizing double-shelled Co3O4 hollow spheres in high yield with high purity by a facile hydrothermal reaction for the first time. Given the generality of this novel method, in terms of theory, it can be widely used to synthesize the multishelled hollow microspheres of other metal oxides by changing the metal oxide precursors. Because these hollow spheres are composed of nanoparticles and have stable structure and a particular inner environment, such materials will open a new avenue for the development of the next generation of semiconductor device. Acknowledgments Financial support was provided by National Nature Science Foundation of China (Nos. 50771082 and 60776822), the Excellent
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