Home
Search
Collections
Journals
About
Contact us
My IOPscience
Phase-controlled synthesis and magnetic properties of cubic and hexagonal CoO nanocrystals
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2016 Nanotechnology 27 455602 (http://iopscience.iop.org/0957-4484/27/45/455602) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 207.162.240.147 This content was downloaded on 12/10/2016 at 06:17
Please note that terms and conditions apply.
You may also be interested in: Shape-related optical and catalytic properties of wurtzite-type CoO nanoplates and nanorods Aolin Lu, Yuanzhi Chen, Deqian Zeng et al. Synthesis and photocatalytic properties of multi-morphological AuCu3-ZnO hybrid nanocrystals Deqian Zeng, Yuanzhi Chen, Jian Peng et al. Controlled synthesis and anomalous magnetic properties of relatively monodisperse CoOnanocrystals Hai-Tao Zhang and Xian-Hui Chen Granular Fe3 xO4-CoO hetero-nanostructures produced by in situ seed mediated growth in polyol: magnetic properties and chemical stability R Sayed Hassan, T Gaudisson, N Yaacoub et al. Facile synthesis of near-monodisperse Ag@Ni core–shell nanoparticles and their applicationfor catalytic generation of hydrogen Huizhang Guo, Yuanzhi Chen, Xiaozhen Chen et al. Room temperature ferromagnetism in CoO nanoparticles obtained from sonochemicallysynthesized precursors Dimple P Dutta, Garima Sharma, P K Manna et al.
Nanotechnology Nanotechnology 27 (2016) 455602 (9pp)
doi:10.1088/0957-4484/27/45/455602
Phase-controlled synthesis and magnetic properties of cubic and hexagonal CoO nanocrystals Qiongqiong Qi, Yuanzhi Chen, Laisen Wang, Deqian Zeng and Dong-Liang Peng Department of Materials Science and Engineering, Collaborative Innovation Center of Chemistry for Energy Materials, College of Materials, Xiamen University, Xiamen 361005, People’s Republic of China E-mail: [email protected] and [email protected] Received 4 July 2016, revised 28 August 2016 Accepted for publication 19 September 2016 Published 11 October 2016 Abstract
We report facile solution approaches for the phase-controlled synthesis of rock-salt cubic CoO (c-CoO) and wurtzite-type hexagonal CoO (h-CoO) nanocrystals. In the syntheses, the cobalt precursor cobalt (II) stearate is decomposed in 1-octadecene at 320 °C, and the crystalline phase of synthesized products depend critically on the amounts of H2O. While the presence of small amounts of H2O promotes the generation of c-CoO, h-CoO is obtained in the absence of H2O. The as-prepared c-CoO nanocrystals exhibit a multi-branched morphology with several short rods growing on the 〈100〉 direction interlaced together whereas the h-CoO nanocrystals show a multi-rod structure with several rods growing on the same base facet along the c-axis. The formation mechanisms are discussed on the basis of FTIR spectrometry data and color changes of the reaction mixture. Finally the magnetic properties of as-prepared CoO nanocrystals are measured and the results show that c-CoO nanocrystals are intrinsically antiferromagnetic with a Néel temperature of about 300 K but the antiferromagnetic ordering is not distinct for the h-CoO nanocrystals. Weak ferromagnetic contributions are also observed for both c-CoO and h-CoO nanocrystals with obvious magnetic hysteresis at 5 and 300 K. The uncompensated spins that can be induced by crystalline defects such as cation-vacancy may account for the observed weak ferromagnetism. S Online supplementary data available from stacks.iop.org/NANO/27/455602/mmedia Keywords: cobalt oxides, nanocrystals, solution synthesis, crystal growth, magnetic properties (Some figures may appear in colour only in the online journal) to the lattice energy calculations, face-centered cubic (fcc) structure is more stable than the hexagonal close-packed structure by 0.27 eV per CoO [7], so that the hexagonal phase is readily converted into the cubic phase under some conditions, such as high temperature, high pressure, charge-discharge recycle, annealing and refluxing process [8–10]. Up to now, many chemical and physical chemical methods have been used to prepare cobalt oxides nanocrystals, such as spray pyrolysis [11], chemical vapor deposition [12], pulsed laser deposition [13], electrospinning technique [14] and surfactant-free aqueous synthesis [15]. For the synthesis of CoO nanocrystals with well-controlled shape and
1. Introduction Cobalt oxide nanocrystals have attracted considerable interest because of their wide range of applications in lithium ion battery materials [1, 2], solid state sensors [2], magnetic data storage [3], catalysts [4, 5] and electrode material in supercapacitors [6]. Among various cobalt oxides, cobalt monoxide (CoO) is of particular interest. CoO usually crystallizes in two crystalline phases: rock-salt CoO (c-CoO, space group Fm3¯ m) with the Co2+ cations occupying the octahedral sites and wurtzite-type hexagonal CoO (h-CoO, space group P63mc) with Co2+ cations occupying the tetrahedral sites. According 0957-4484/16/455602+09$33.00
1
© 2016 IOP Publishing Ltd Printed in the UK
Nanotechnology 27 (2016) 455602
Q Qi et al
crystalline phase, organic solution approaches are usually employed [8–10, 16–28]. For example, Chaudret et al synthesized c-CoO nanoparticles with a size of ∼2 nm by solidstate oxidation of 1.6 nm metallic Co nanoparticles [16]. Rao et al synthesized c-CoO nanoparticles with sizes ranging from 4–18 nm by decomposing Co(II) cupferronate in decalin under solvothermal conditions [17]. On the other hand, for the synthesis of h-CoO nanocrystals, Seo et al reported the synthesis of h-CoO nanorods and nanopyramids by thermal deposition of cobalt (III) acetylacetonate (Co(acac)3) in oleylamine, and found that a prolonged reaction time was a crucial factor [8]. Seshadri et al reported a synthetic approach wherein Co(acac)2 was decomposed in refluxing benzyl ether to generate h-CoO [18]. Moreover, Hyeon and co-workers reported that h-CoO nanorods could be formed in a short pencil shape under thermal decomposition of a Co(II)-oleate complex [19]. Although a large number of synthetic methods for CoO nanocrystals with different shapes and crystalline phases have been documented in the literature, it is still interesting to explore facile and cost-effective synthetic approach to synthesize CoO nanocrystals with specific shape and crystalline phase which are very important for their practical applications such as catalysis. Another important aspect that deserves to be investigated is the magnetic properties of nanoscale CoO materials. It is well known that bulk CoO is antiferromagnetic in nature; however CoO nanocrystals may show unexpected superparamagnetism or weak ferromagnetism with varying the size. There are many reports involved in the magnetism of c-CoO nanocrystals [15–17, 23, 29–33]. For example, Zhang et al reported that CoO nanosheets exhibited superparamagnetic features, which are different from their bulk counterparts [15]. Dutta et al found that c-CoO nanoparticles exhibited room-temperature ferromagnetism that can be explained by the uncompensated spins on the surface [29]. On the other hand, the report on the magnetism of h-CoO nanocrystals is relatively scarce and the results are perplexing. Electronic structure calculations indicated that h-CoO has antiferromagnetic ground state [18, 19]; however, experimental data revealed no long range magnetic ordering presenting in the structure [8, 18]. Shi et al found that the h-CoO nanoparticles in the size range of 38–93 nm did not exhibit a distinct antiferromagnetic transition around 300 K but instead showed hysteresis at 5 K [34]. Recently, Zhong et al found that 45 nm wurtzite CoO nanocrystals have a Néel temperature (TN) of 245 K [35]. Coey et al reported that pure h-CoO thin film prepared via physical method were paramagnetic and showed no trace of ferromagnetism [13]. It seems that the magnetic properties of CoO nanocrystals correlate closely with the preparation methods that eventually determine the size, shape, crystalline phase and surface status of CoO nanocrystals. It is therefore deserve to initiate further studies on the synthesis and magnetic properties of CoO nanocrystals. In this study, we report the selective synthesis of c-CoO and h-CoO nanocrystals. We will show that the crystalline phases of CoO nanocrystals can be controlled by adjusting the content of H2O. Moreover, the synthesized c-CoO
nanocrystals exhibit an interesting multi-branched morphology with a 〈100〉 growth direction on the rod branch, which is different from commonly reported 〈111〉 growth direction. Furthermore, we investigate the magnetic properties of asprepared c-CoO and h-CoO nanocrystals, and the results show that while the c-CoO nanocrystals exhibit unambiguous antiferromagnetic characteristics with a Néel temperature of about 300 K, the h-CoO nanocrystals do not show distinct antiferromagnetic ordering, and a combination of paramagnetic and weak ferromagnetic characteristics is observed.
2. Experimental section 2.1. Synthesis 2.1.1. Synthesis of c-CoO nanocrystals with multi-branched morphology. In a typical synthesis, 15 ml of 1-octadecene
(ODE, 90%, Acros), 1.5 mmol of cobalt (II) stearate (Co(st)2, 95%, Acros) and 0.5 ml of distilled water were placed into a flask and the mixed solution was heated to 130 °C under vigorous stirring for 30 min. Then, the mixed solution was heated to 320 °C for 60 min before it was cooled down to room temperature. The whole process was conducted under a flow of high-purity argon gas. Finally, the products were washed three times by hexane and ethanol, separated by centrifugation and dried in air.
2.1.2. Synthesis of h-CoO nanocrystals with multi-rod morphology. In a typical synthesis, 15 ml of ODE was
placed into a flask and then the mixed solution was heated to 130 °C under vigorous stirring for 60 min. During this process, the condenser pipe up was removed for removing water. Then, 1.5 mmol of Co(st)2 was added into the solution and the mixed solution was heated to 320 °C and kept at this temperature for another 60 min, after which, it was cooled down to room temperature. The whole process was conducted under a flow of high-purity argon gas. The resulting products were purified via centrifugation plus washing with hexane and ethanol three times, and then dried in air.
2.2. Characterization
X-ray diffraction (XRD) patterns were collected using a Bruker D8-Advance x-ray diffractometer with Cu Kα radiation. Transmission electron microscopy (TEM) images and selective area electron diffraction (SAED) patterns were recorded on JEM-1400, TECNAI F-30 and JEM-2100 transmission electron microscopes. Fourier transform infrared (FTIR) spectra were obtained with an IS10 FTIR spectrometer. X-ray photoelectron spectroscopy (XPS) analyses were performed using a PHI Quantum 2000 scanning ESCA Microprobe spectrometer using an Al Kα photon source. Magnetic measurements were carried out with a Physical Property Measurement System (PPMS-9, Quantum Design) in the temperature ranging from 5 to 350 K. 2
Nanotechnology 27 (2016) 455602
Q Qi et al
Figure 1. Low-magnification TEM image (a), HRTEM images ((b) and (c)) and SAED pattern (d) of c-CoO nanocrystals. The inset in
figure 1(c) shows the TEM image of a multi-branched nanocrystal from which the HRTEM image is obtained.
ends. The SAED pattern (figure 1(d)) recorded from randomly distributed nanocrystals also exhibits diffraction rings that correspond well to rock-salt cubic CoO. In addition, the (200) diffraction ring exhibits a stronger intensity than the (111) ring, further confirming the 〈100〉 growth direction of rod structure. Figures 2(a) and (b) show the typical TEM images of the as-prepared h-CoO nanocrystals which have a multi-rod structure with an average width of about 30 nm and a length of about 70 nm (30×70 nm). From the morphology of figure 2(b), most nanocrystals split into three or four rods, which may be due to the relatively high growth rate along the rod direction. This rod morphology is similar to that of h-CoO nanorods synthesized by decomposing Co(acac)3 in benzylamine and o-dichlorobenzene [1]. The HRTEM image in figure 3(c) exhibits distinct lattice fringes with a lattice spacing of 0.520 nm, corresponding well to the (002) plane of wurtzite-type hexagonal CoO. It also reveals a growth direction of [001], namely, the nanorods grow along the caxis. The recorded SAED pattern (figure 2(d)) shows multiple concentric diffraction rings that can be indexed to wurtzitetype hexagonal CoO. The (002) diffraction ring appears much
3. Results and discussion 3.1. Morphology and structure characterization
Figure 1(a) shows the low-magnification TEM image of the as-prepared c-CoO nanocrystals which have a multi-branched morphology with several short rods interlaced together from different directions. These nanocrystals have a typical size varying from 15 to 20 nm. The HRTEM images (figures 1(b) and (c)) recorded from a single nanocrystal reveal a highly crystalline structure with a lattice spacing of 0.213 nm which can be indexed to the (200) plane of rock-salt cubic CoO. A growth direction of 〈100〉 along the long axis of rod is also revealed. In previous studies on the c-CoO nanocrystals with rod-like morphology, a growth direction of [111] was usually reported [25, 26]. However, in our case, the growth direction on rod branch is 〈100〉, indicating a different growing mechanism. This distinct structure indicates that the rod branch may be developed from the assembly of CoO nanocubes with {100} facets connecting together. This assumption is supported by the TEM image (inset in figure 1(c)) in which the rod branches seem to consist of many small cubes with flat 3
Nanotechnology 27 (2016) 455602
Q Qi et al
Figure 2. Low-magnification TEM images ((a) and (b)), HRTEM image (c) and SAED pattern (d) of h-CoO nanocrystals.
Typical XRD patterns of the as-prepared CoO nanocrystals are shown in figure 3. The diffraction peaks in pattern (a) are in good agreement with the standard crystallographic data for rock-salt cubic CoO while the seven obvious diffraction peaks in pattern (b) can be assigned to the (100), (002), (101), (102), (110), (103) and (112) planes of wurtzitetype hexagonal CoO. The anisotropic structure of the asprepared CoO nanocrystals is also revealed in the XRD patterns. For c-CoO, the (200) diffraction peak is much stronger than the (111) peak, indicating a 〈100〉-orientated anisotropic growth, whereas for h-CoO, the (002) peak intensity is enhanced, which coincides with their growth direction along the c-axis. Obviously, there is no evidence in the XRD patterns for the existence of crystalline impurities such as Co and Co3O4. The chemical characteristics of two types of CoO nanocrystals were further studied by XPS analysis. Figure 4(a) shows the XPS spectra of Co 2p regions. There is no obvious difference between the c-CoO and h-CoO, indicating that they are both composed of Co ions in the same valence state. The main peak at 780.2 eV with the shakeup satellite at 786.2 eV can be assigned to Co(II) 2p3/2, while the peak at 796.2 eV with the shakeup satellite at 802.4 eV can be attributed to
Figure 3. XRD patterns of h-CoO (a) and c-CoO (b) nanocrystals.
brighter than (100) and (101) rings, implying preferential growth along [001] direction. It is known that a kinetically controlled condition generally enhances the growth rate along the c-axis in wurtzite crystal structures, and thus it ensures the growth of anisotropic nanorods [1]. 4
Nanotechnology 27 (2016) 455602
Q Qi et al
Figure 4. XPS spectra of Co 2p (a) and O 1s (b) regions for c-CoO and h-CoO nanocrystals.
at 2852.5 and 2922.9 cm−1 are due to the symmetric and asymmetric CH2 stretching modes. The peaks from 1000 to 1348 cm−1 can be assigned to the C=O stretching on stearic acid [31]. In comparison with pure Co(st)2, the peak at 3440 cm−1, which represents O–H stretching of H2O [37], is observed in Co(st)2 exposed to air. Thus, we infer that the Co(st)2 exposed in air may adsorb moisture. In order to confirm our assumption, 0.5 ml of H2O was added into Co(st)2 whose FTIR transmission spectrum is plotted as the red curve in figure 5. The peak at 3440 cm−1 representing O–H stretching of H2O is clearly present. Similarly, we synthesized c-CoO nanocrystals by adding H2O, while h-CoO was obtained instead by using dry Co(st)2. Therefore, H2O plays a critical role in selecting the phase of CoO nanocrystals. Previously, Shim et al also found that H2O is critical in the synthesis of CoO nanoparticles via decomposing Co (acac)2 in amine solution. The h-CoO was obtained through two different pathways: solvolysis with C–C bond cleavage and direct condensation by amine, while the c-CoO was synthesized by strong nucleophilic attack of hydroxide ions from water and subsequent C–C bond breaking [24]. Similarly, we suspect that the source of oxygen for the formation of c-CoO may be from the strong nucleophilic hydroxide ions in water, while that for h-CoO may be from stearate ion. This different source of oxygen gave rise to different phases. The influences of different amounts of water on the morphology and phase of CoO are shown in online supplementary figures S1 and S2. In the absence of H2O, the products are h-CoO nanorods. When 0.25 ml of H2O is added, the nanocrystals have already changed into fcc phase (see the XRD pattern in online supplementary figure S2). Discrete nanocrystals with near-cubic shape along with interlaced nanorods are the major component of the products. Further increasing the amount (1 ml) of H2O, the crystalline phase remains fcc phase; however, the morphology further changes from interlaced nanorods to discrete nanocrystals with a near-cubic morphology. Thus it can be seen that the amount of H2O has a
Figure 5. FTIR transmission spectra of pure Co(st)2 (top),Co(st)2
exposed in air (middle) and Co(st)2 containing H2O (bottom).
Co(II) 2p1/2 energy levels. The presence of both peaks at 780.2 and 796.2 eV and their obvious satellite peaks prove that Co exists as Co2+ ions [17]. The absence of feature peak of metallic Co at about 778 eV suggests that the samples do not have Co impurities, which agree well with the XRD and SAED results. The O 1s regions is shown in figure 4(b). The peaks at about 529 eV are attributed to the lattice oxygen (OL) while the peaks at about 531 eV are due to the chemisorbed and dissociated oxygen species (OC) [4, 36]. The fact that OC peak of h-CoO was lower than that of c-CoO can be ascribed to the smaller size of c-CoO nanocrystals which can have larger specific surface area. 3.2. Formation mechanism discuss
In the syntheses, we accidentally found that c-CoO was obtained when the precursor Co(st)2 which was exposed to air for a time was employed. We further performed FTIR spectrum analyses on the samples of pure Co(st)2 and Co(st)2 exposed to air for three days. As shown in figure 5, the peaks 5
Nanotechnology 27 (2016) 455602
Q Qi et al
Figure 6. Schematic illustration of the formation of c-CoO and h-CoO nanocrystals.
significant influence on the phase and morphology of CoO nanocrystals. The schematic illustration of the formation different phases of CoO is shown in figure 6. The color change in the reaction mixture is a good indicator of reaction progress. For the formation of c-CoO with multi-branched morphology, when the mixture was heated to 130 °C, the color was purple and the solution was in a gel state, which indicated that the stearate ligand was tightly bound to the Co2+ metal center under the action of H2O and the density of solution was increased [5]. With increasing temperature, the colloidal mixed purple solution turned to transparent blue, then transformed to dark blue after 40 min at 320 °C, which was an indication of the seed formation. TEM observation on the products taken out from the reaction solution at this stage confirmed the seed formation. As shown in online supplementary figure S3(a), most of the crystal seeds have a nearcubic shape and a few of seeds have grown into interlaced rods. After holding 10 min, the color turned yellow-green, accounting for the growth of seeds. In this slow nucleation, thermodynamically controlled condition yields the stable c-CoO nanocrystals. The c-CoO nanocrystals may further develop into interlaced rod morphology via self assembly by connecting {100} facets. On the other hand, for the synthesis of h-CoO, when the mixture was maintained at 130 °C, the color was transparent purple. After heated to 320 °C, the transparent blue solution appeared, and then turned to dark blue in 10 min at 320 °C in the absence of H2O, indicating the formation of crystal seeds (see online supplementary figure S3(b)). After 15 min warming, the dark blue became green rapidly. This sudden kinetically controlled condition preferred to generate metastable h-CoO. The formation mechanism of a rod shape is similar to that of ZnO whiskers [38].
When come down to the synthetic condition for preparing CoO nanocrystals with different phases, Nam et al found that a flash heating of the reaction mixture (185 °C for 2 h) belonged to the kinetically controlled condition, and h-CoO nanocrystals were synthesized, whereas a prolonged heating at a relatively low temperature (130 °C for 12 h) pertained thermodynamically controlled condition, and c-CoO nanocrystals were obtained [1]. In our study, the existence of water in the reaction mixture played a key role in the phase control of CoO nanocrystals. Because the hydroxide ions of H2O are a relatively strong nucleophilic Lewis base, more energy is needed for Co ions to get rid of their constraint under this strong nucleophilic attack, which results in lower system energy [24]. Accordingly, more stable phase of c-CoO is obtained in this process. On the contrary, Co ions are easier to shake off stearic acid ions in the absence of H2O, reach reaction equilibrium quickly and thus generate metastable h-CoO. 3.3. Magnetic properties
The zero-field-cooled (ZFC) and field-cooled (FC) magnetization versus temperature plots measured at an applied field of 100 Oe are shown in figure 7. It is apparent that the ZFC curve of c-CoO shows a peak at about 7.5 K. However, there is no such a peak in the ZFC curve of h-CoO nanocrystals. There is also a delicate kink around 300 K for c-CoO, which can be ascribed to the Néel temperature that correlates with antiferromagnetism. Again there is no such a kink in the ZFC curve of h-CoO nanocrystals, indicating different magnetic properties. It is also observed that bifurcation between the ZFC and FC curves (i.e., irreversibility) occurs at a temperature well above room-temperature for both c-CoO and h-CoO nanocrystals, which means that the nanocrystals are 6
Nanotechnology 27 (2016) 455602
Q Qi et al
Figure 7. ZFC and FC magnetization versus temperature curves of c-CoO (a) and h-CoO (b) nanocrystals.
Figure 8. Temperature dependence of the high-field (3 T) magnetization of c-CoO (a) and h-CoO (b) nanocrystals.
The hysteresis loops of CoO nanocrystals measured at 5 and 300 K are shown in figure 9. The c-CoO nanocrystals exhibit almost a linear increase of the magnetization with increasing the magnetic field, which is associated with the antiferromagnetic structure. In addition, a coercivity of about 641 Oe is also observed, indicating that ferromagnetic contribution is also present. The h-CoO nanocrystals also show hysteresis loops composed of curved and linear portions; however the ferromagnetic contribution is obviously stronger than that of c-CoO. Since the results of XRD and XPS analyses do not confirm the presence of metallic Co which can bring about ferromagnetic contribution, the observed magnetic hysteresis in both of CoO samples may mainly arise from the uncompensated spins induced by cation-vacancy defects. Like other transition metal oxide such as FeO, NiO and MoO, CoO is also a non-stoichiometric compound, which usually exhibit cation efficiency under normal ambient conditions. The transition from +2 to +3 valence of Co ions to maintain electroneutrality may produce
still in a ferromagnetic state. The peak at 7.5 K in the ZFC curve of c-CoO could be due to the weakly exchange coupled to the antiferromagnetic lattice like those associated with the defects or the amorphous phase inside the CoO nanoparticles, which often characterized by a spin-glass behavior at low temperatures [31, 32]. The bifurcation between the ZFC and FC curves at a temperature well above room-temperature may be caused by uncompensated spins that are strongly exchange coupled to the antiferromagnetic lattice like those seated on regular lattice sites or surfaces of CoO. To clearly reveal the antiferromagnetic properties, we further increased the applied field to 3 T, and the temperature dependence of the high-field magnetization curves are shown in figure 8. A characteristic maximum at about 300 K is clearly observed for c-CoO, which is close to the Néel temperature of bulk antiferromagnetic CoO, whereas no such a peak is present for h-CoO nanocrystals. The temperaturedependent high-field magnetic data confirm that as-prepared c-CoO nanocrystals are intrinsically antiferromagnetic. 7
Nanotechnology 27 (2016) 455602
Q Qi et al
Figure 9. Hysteresis loops of c-CoO (a) and h-CoO (b) nanocrystals.
unbalanced spins. Wdowik et al have demonstrated that cation vacancies can account for the ferromagnetic properties of CoO nanostructures based on the first-principles calculation [39]. The absence of obvious antiferromagnetic characteristics of h-CoO nanocrystals is related to its crystalline structure. The rock-salt cubic structure of c-CoO exhibits the Co2+–O2−–Co2+ 180° superexchange interactions which favorite antiferromagnetic coupling [22]. However, the wurtzite structure of h-CoO consists of sheets with hexagonal rings of alternating Co2+ and O2− ions in an ABAB stacking sequence. The tilted Co2+–O2−–Co2+ angle of 110° frustrates the antiferromagnetic superexchange in each individual layer, which make it difficult for the spins to satisfy all the interactions simultaneously, and therefore result in the absence of magnetic ordering [13, 22].
that can be strongly exchange-coupled to the antiferromagnetic lattice.
Acknowledgments This work was partially supported by the National Natural Science Foundation of China (Grant Nos. 51471137 and 51371154) and the National Basic Research Program of China (Grant No. 2012CB933103).
References [1] Nam K M, Shim J H, Han D-W, Kwon H S, Kang Y-M, Li Y, Song H, Seo W S and Park J T 2010 Syntheses and characterization of wurtzite CoO, rocksalt CoO, and spinel Co3O4 nanocrystals: their interconversion and tuning of phase and morphology Chem. Mater. 22 4446–54 [2] Li W Y, Xu L N and Chen J 2005 Co3O4 nanomaterials in lithium-ion batteries and gas sensors Adv. Funct. Mater. 15 851–7 [3] Shima H, Takano F, Muramatsu H, Akinaga H, Tamai Y, Inque I H and Takagi H 2008 Voltage polarity dependent low-power and high-speed resistance switching in CoO resistance random access memory with Ta electrode Appl. Phys. Lett. 93 113504 [4] Lu A, Chen Y, Zeng D, Li M, Xie Q, Zhang X and Peng D L 2014 Shape-related optical and catalytic properties of wurtzite-type CoO nanoplates and nanorods Nanotechnology 25 035707 [5] Lu A, Chen Y, Jin J, Yue G-H and Peng D-L 2012 CoO nanocrystals as a highly active catalyst for the generation of hydrogen from hydrolysis of sodium borohydride J. Power Sources 220 391–8 [6] Wang G, Shen X, Horvat J, Wang B, Liu H, Wexler D and Yao J 2009 Hydrothermal synthesis and optical, magnetic, and supercapacitance properties of nanoporous cobalt oxide nanorods J. Phys. Chem. C 113 4357–61 [7] Ravindra A V, Behera B C, Padhan P, Lebedev O I and Prellier W 2014 Tailoring of crystal phase and Néel
4. Conclusion Rock-salt c-CoO and wurtzite-type h-CoO nanocrystals have been selectively synthesized via facile chemical solution approaches in which cobalt precursor Co(st)2 is decomposed in 1-octadecene at different conditions. The synthesized c-CoO nanocrystals have a multi-branched morphology with several short rods growing on the 〈100〉 direction interlaced together while the h-CoO nanocrystals exhibit a multi-rod structure with several rods growing on the same base facet along c-axis. The presence of small amounts of H2O promotes the formation of cubic phase whereas the absence of H2O is essential to the growth of hexagonal phase of CoO. The magnetism measurements reveal an intrinsic antiferromagnetism with a Néel temperature of about 300 K for the c-CoO nanocrystals. However the antiferromagnetic ordering is not distinct for the h-CoO nanocrystals. Both c-CoO and h-CoO nanocrystals exhibit weak ferromagnetic contributions which may arise from the uncompensated spins 8
Nanotechnology 27 (2016) 455602
[8] [9]
[10]
[11] [12]
[13] [14]
[15] [16]
[17] [18] [19] [20]
[21] [22]
[23]
Q Qi et al
[24] Shim J H, Nam K M, Seo W S, Song H and Park J T 2011 The role of water for the phase-selective preparation of hexagonal and cubic cobalt oxide nanoparticles Chem. Asian J. 6 1575–81 [25] Zhang Y, Zhu J, Song X and Zhong X 2008 Controlling the synthesis of CoO nanocrystals with various morphologies J. Phys. Chem. C 112 5322–7 [26] Zhang Y, Zhong X, Zhu J and Song X 2007 Alcoholysis route to monodisperse CoO nanotetrapods with tunable size Nanotechnology 18 195605 [27] Buck M R, Biacchi A J and Schaak R E 2014 Insights into the thermal decomposition of Co(II) oleate for the shapecontrolled synthesis of wurtzite-type CoO nanocrystals Chem. Mater. 26 1492–9 [28] Chen C-J, Chiang R-K and Wang S-L 2013 Controllable organic-phase synthesis of cuboidal CoO mesocrystals and their magnetic properties Cryst. Eng. Comm. 15 9161–9 [29] Dutta D P, Sharma G, Manna P K, Tyagi A K and Yusuf S M 2008 Room temperature ferromagnetism in CoO nanoparticles obtained from sonochemically synthesized precursors Nanotechnology 19 245609 [30] Yang G, Gao D, Shi Z, Zhang Z, Zhang J, Zhang J and Xue D 2010 Room temperature ferromagnetism in vacuumannealed CoO nanospheres J. Phys. Chem. C 114 21989–93 [31] Dai Q and Tang J 2013 Magnetic properties of CoO nanocrystals prepared with a controlled reaction atmosphere RSC Adv. 3 9228–33 [32] Dai Q and Tang J 2013 The optical and magnetic properties of CoO and Co nanocrystals prepared by a facile technique Nanoscale 5 7512–9 [33] Deori K and Deka S 2013 Morphology oriented surfactant dependent CoO and reaction time dependent Co3O4 nanocrystals from single synthesis method and their optical and magnetic properties Cryst. Eng. Comm. 15 8465–74 [34] He X and Shi H 2011 Synthesis and anomalous magnetic properties of hexagonal CoO nanoparticles Mater. Res. Bull. 46 1692–7 [35] He X, Zhong W, Yan S, Liu C, Shi H, Au C-T and Du Y 2014 Transition temperature of wurtzite CoO nanocrystals as revealed in comprehensive magnetic characterization J. Phys. Chem. C 118 13898–903 [36] Zhang H, Ling T and Du X-W 2015 Gas-phase cation exchange toward porous single-crystal CoO nanorods for catalytic hydrogen production Chem. Mater. 27 352–7 [37] Mitsuzuka A, Fujii A, Ebata T and Mikamia N 1996 Infrared spectroscopy of OH stretching vibrations of hydrogenbonded tropolone-(H2O)n (n = 1–3) and tropolone(CH3OH)n (n = 1 and 2) clusters J. Chem. Phys. 105 2618–27 [38] Hu J Q, Li Q, Wong N B, Lee C S and Lee S T 2002 Synthesis of uniform hexagonal prismatic ZnO whiskers Chem. Mater. 14 1216–9 [39] Wdowik U D and Parlinski K 2008 Electronic structure of cation-deficient CoO from first principles Phys. Rev. B 77 115110
temperature of cobalt monoxides nanocrystals with synthetic approach conditions J. Appl. Phys. 116 033912 Seo W S, Shim J H, Oh S J, Lee E K, Hur N H and Park J T 2005 Phase- and size-controlled synthesis of hexagonal and cubic CoO nanocrystals J. Am. Chem. Soc. 127 6188–9 He X, Li Z, Zhang X, Qiao W, Song X, Yan S, Zhong W and Du Y 2015 Effects of Ar/H2 annealing on the microstructure and magnetic properties of CoO nanoparticles RSC Adv. 5 69948–54 Liu J F, Yin S, Wu H P, Zeng Y W, Hu X R, Wang Y W, Lv G L and Jiang J Z 2006 Wurtzite-to-rocksalt structural transformation in nanocrystalline CoO J. Phys. Chem. B 110 21588–92 Shinde V R, Mahadik S B, Gujar T P and Lokhande C D 2006 Supercapacitive cobalt oxide (Co3O4) thin films by spray pyrolysis Appl. Surf. Sci. 252 7487–92 Bahlawane N, Ngamou P H, Vannier V, Kottke T, Heberle J and Kohse-Hoinghaus K 2009 Tailoring the properties and the reactivity of the spinel cobalt oxide Phys. Chem. Chem. Phys. 11 9224–32 Alaria J, Cheval N, Rode K, Venkatesan M and Coey J M D 2008 Structural and magnetic properties of wurtzite CoO thin films J. Phys. D: Appl. Phys. 41 135004 Barakat N A M, Khil M S, Sheikh F A and Kim H Y 2008 Synthesis and optical properties of two cobalt oxides (CoO and Co3O4) nanofibers produced by electrospinning process J. Phys. Chem. C 112 12225–33 Zhang W, Han M, Jiang Z, Song Y, Xie Z, Xu Z and Zheng L 2007 Controllable synthesis of CoO nanosheets and their magnetic properties Chem. Phys. Chem. 8 2091–5 Verelst M, Ely T O, Amiens C, Snoeck E, Lecante P, Mosset A, Respaud M, Broto J M and Chaudret B 1999 Synthesis and characterization of CoO, Co3O4, and mixed Co/CoO nanoparticules Chem. Mater. 11 2702–8 Ghosh M, Sampathkumaran E V and Rao C N R 2005 Synthesis and magnetic properties of CoO nanoparticles Chem. Mater. 17 2348–52 Risbud A S, Snedeker L P, Elcombe M M, Cheetham A K and Seshadri R 2005 Wurtzite CoO Chem. Mater. 17 834–8 An K et al 2006 Synthesis, characterization, and self-assembly of pencil-shaped CoO nanorods J. Am.Chem. Soc. 128 9753–60 Deng X, Yang D, Tan G, Li X, Zhang J, Liu Q, Zhang H, Mellors N J, Xue D and Peng Y 2014 Bimagnetic h-Co/hCoO nanotetrapods: preparation, nanoscale characterization, three-dimensional architecture and their magnetic properties Nanoscale 6 13710–8 Wang D, Ma X, Wang Y, Wang L, Wang Z, Zheng W, He X, Li J, Peng Q and Li Y 2010 Shape control of CoO and LiCoO2 nanocrystals Nano Res. 3 1–7 He X, Song X, Qiao W, Li Z, Zhang X, Yan S, Zhong W and Du Y 2015 Phase- and size-dependent optical and magnetic properties of CoO nanoparticles J. Phys. Chem. C 119 9550–9 Zhang H T and Chen X H 2005 Controlled synthesis and anomalous magnetic properties of relatively monodisperse CoO nanocrystals Nanotechnology 16 2288–94
9
Search
Collections
Journals
About
Contact us
My IOPscience
Phase-controlled synthesis and magnetic properties of cubic and hexagonal CoO nanocrystals
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2016 Nanotechnology 27 455602 (http://iopscience.iop.org/0957-4484/27/45/455602) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 207.162.240.147 This content was downloaded on 12/10/2016 at 06:17
Please note that terms and conditions apply.
You may also be interested in: Shape-related optical and catalytic properties of wurtzite-type CoO nanoplates and nanorods Aolin Lu, Yuanzhi Chen, Deqian Zeng et al. Synthesis and photocatalytic properties of multi-morphological AuCu3-ZnO hybrid nanocrystals Deqian Zeng, Yuanzhi Chen, Jian Peng et al. Controlled synthesis and anomalous magnetic properties of relatively monodisperse CoOnanocrystals Hai-Tao Zhang and Xian-Hui Chen Granular Fe3 xO4-CoO hetero-nanostructures produced by in situ seed mediated growth in polyol: magnetic properties and chemical stability R Sayed Hassan, T Gaudisson, N Yaacoub et al. Facile synthesis of near-monodisperse Ag@Ni core–shell nanoparticles and their applicationfor catalytic generation of hydrogen Huizhang Guo, Yuanzhi Chen, Xiaozhen Chen et al. Room temperature ferromagnetism in CoO nanoparticles obtained from sonochemicallysynthesized precursors Dimple P Dutta, Garima Sharma, P K Manna et al.
Nanotechnology Nanotechnology 27 (2016) 455602 (9pp)
doi:10.1088/0957-4484/27/45/455602
Phase-controlled synthesis and magnetic properties of cubic and hexagonal CoO nanocrystals Qiongqiong Qi, Yuanzhi Chen, Laisen Wang, Deqian Zeng and Dong-Liang Peng Department of Materials Science and Engineering, Collaborative Innovation Center of Chemistry for Energy Materials, College of Materials, Xiamen University, Xiamen 361005, People’s Republic of China E-mail: [email protected] and [email protected] Received 4 July 2016, revised 28 August 2016 Accepted for publication 19 September 2016 Published 11 October 2016 Abstract
We report facile solution approaches for the phase-controlled synthesis of rock-salt cubic CoO (c-CoO) and wurtzite-type hexagonal CoO (h-CoO) nanocrystals. In the syntheses, the cobalt precursor cobalt (II) stearate is decomposed in 1-octadecene at 320 °C, and the crystalline phase of synthesized products depend critically on the amounts of H2O. While the presence of small amounts of H2O promotes the generation of c-CoO, h-CoO is obtained in the absence of H2O. The as-prepared c-CoO nanocrystals exhibit a multi-branched morphology with several short rods growing on the 〈100〉 direction interlaced together whereas the h-CoO nanocrystals show a multi-rod structure with several rods growing on the same base facet along the c-axis. The formation mechanisms are discussed on the basis of FTIR spectrometry data and color changes of the reaction mixture. Finally the magnetic properties of as-prepared CoO nanocrystals are measured and the results show that c-CoO nanocrystals are intrinsically antiferromagnetic with a Néel temperature of about 300 K but the antiferromagnetic ordering is not distinct for the h-CoO nanocrystals. Weak ferromagnetic contributions are also observed for both c-CoO and h-CoO nanocrystals with obvious magnetic hysteresis at 5 and 300 K. The uncompensated spins that can be induced by crystalline defects such as cation-vacancy may account for the observed weak ferromagnetism. S Online supplementary data available from stacks.iop.org/NANO/27/455602/mmedia Keywords: cobalt oxides, nanocrystals, solution synthesis, crystal growth, magnetic properties (Some figures may appear in colour only in the online journal) to the lattice energy calculations, face-centered cubic (fcc) structure is more stable than the hexagonal close-packed structure by 0.27 eV per CoO [7], so that the hexagonal phase is readily converted into the cubic phase under some conditions, such as high temperature, high pressure, charge-discharge recycle, annealing and refluxing process [8–10]. Up to now, many chemical and physical chemical methods have been used to prepare cobalt oxides nanocrystals, such as spray pyrolysis [11], chemical vapor deposition [12], pulsed laser deposition [13], electrospinning technique [14] and surfactant-free aqueous synthesis [15]. For the synthesis of CoO nanocrystals with well-controlled shape and
1. Introduction Cobalt oxide nanocrystals have attracted considerable interest because of their wide range of applications in lithium ion battery materials [1, 2], solid state sensors [2], magnetic data storage [3], catalysts [4, 5] and electrode material in supercapacitors [6]. Among various cobalt oxides, cobalt monoxide (CoO) is of particular interest. CoO usually crystallizes in two crystalline phases: rock-salt CoO (c-CoO, space group Fm3¯ m) with the Co2+ cations occupying the octahedral sites and wurtzite-type hexagonal CoO (h-CoO, space group P63mc) with Co2+ cations occupying the tetrahedral sites. According 0957-4484/16/455602+09$33.00
1
© 2016 IOP Publishing Ltd Printed in the UK
Nanotechnology 27 (2016) 455602
Q Qi et al
crystalline phase, organic solution approaches are usually employed [8–10, 16–28]. For example, Chaudret et al synthesized c-CoO nanoparticles with a size of ∼2 nm by solidstate oxidation of 1.6 nm metallic Co nanoparticles [16]. Rao et al synthesized c-CoO nanoparticles with sizes ranging from 4–18 nm by decomposing Co(II) cupferronate in decalin under solvothermal conditions [17]. On the other hand, for the synthesis of h-CoO nanocrystals, Seo et al reported the synthesis of h-CoO nanorods and nanopyramids by thermal deposition of cobalt (III) acetylacetonate (Co(acac)3) in oleylamine, and found that a prolonged reaction time was a crucial factor [8]. Seshadri et al reported a synthetic approach wherein Co(acac)2 was decomposed in refluxing benzyl ether to generate h-CoO [18]. Moreover, Hyeon and co-workers reported that h-CoO nanorods could be formed in a short pencil shape under thermal decomposition of a Co(II)-oleate complex [19]. Although a large number of synthetic methods for CoO nanocrystals with different shapes and crystalline phases have been documented in the literature, it is still interesting to explore facile and cost-effective synthetic approach to synthesize CoO nanocrystals with specific shape and crystalline phase which are very important for their practical applications such as catalysis. Another important aspect that deserves to be investigated is the magnetic properties of nanoscale CoO materials. It is well known that bulk CoO is antiferromagnetic in nature; however CoO nanocrystals may show unexpected superparamagnetism or weak ferromagnetism with varying the size. There are many reports involved in the magnetism of c-CoO nanocrystals [15–17, 23, 29–33]. For example, Zhang et al reported that CoO nanosheets exhibited superparamagnetic features, which are different from their bulk counterparts [15]. Dutta et al found that c-CoO nanoparticles exhibited room-temperature ferromagnetism that can be explained by the uncompensated spins on the surface [29]. On the other hand, the report on the magnetism of h-CoO nanocrystals is relatively scarce and the results are perplexing. Electronic structure calculations indicated that h-CoO has antiferromagnetic ground state [18, 19]; however, experimental data revealed no long range magnetic ordering presenting in the structure [8, 18]. Shi et al found that the h-CoO nanoparticles in the size range of 38–93 nm did not exhibit a distinct antiferromagnetic transition around 300 K but instead showed hysteresis at 5 K [34]. Recently, Zhong et al found that 45 nm wurtzite CoO nanocrystals have a Néel temperature (TN) of 245 K [35]. Coey et al reported that pure h-CoO thin film prepared via physical method were paramagnetic and showed no trace of ferromagnetism [13]. It seems that the magnetic properties of CoO nanocrystals correlate closely with the preparation methods that eventually determine the size, shape, crystalline phase and surface status of CoO nanocrystals. It is therefore deserve to initiate further studies on the synthesis and magnetic properties of CoO nanocrystals. In this study, we report the selective synthesis of c-CoO and h-CoO nanocrystals. We will show that the crystalline phases of CoO nanocrystals can be controlled by adjusting the content of H2O. Moreover, the synthesized c-CoO
nanocrystals exhibit an interesting multi-branched morphology with a 〈100〉 growth direction on the rod branch, which is different from commonly reported 〈111〉 growth direction. Furthermore, we investigate the magnetic properties of asprepared c-CoO and h-CoO nanocrystals, and the results show that while the c-CoO nanocrystals exhibit unambiguous antiferromagnetic characteristics with a Néel temperature of about 300 K, the h-CoO nanocrystals do not show distinct antiferromagnetic ordering, and a combination of paramagnetic and weak ferromagnetic characteristics is observed.
2. Experimental section 2.1. Synthesis 2.1.1. Synthesis of c-CoO nanocrystals with multi-branched morphology. In a typical synthesis, 15 ml of 1-octadecene
(ODE, 90%, Acros), 1.5 mmol of cobalt (II) stearate (Co(st)2, 95%, Acros) and 0.5 ml of distilled water were placed into a flask and the mixed solution was heated to 130 °C under vigorous stirring for 30 min. Then, the mixed solution was heated to 320 °C for 60 min before it was cooled down to room temperature. The whole process was conducted under a flow of high-purity argon gas. Finally, the products were washed three times by hexane and ethanol, separated by centrifugation and dried in air.
2.1.2. Synthesis of h-CoO nanocrystals with multi-rod morphology. In a typical synthesis, 15 ml of ODE was
placed into a flask and then the mixed solution was heated to 130 °C under vigorous stirring for 60 min. During this process, the condenser pipe up was removed for removing water. Then, 1.5 mmol of Co(st)2 was added into the solution and the mixed solution was heated to 320 °C and kept at this temperature for another 60 min, after which, it was cooled down to room temperature. The whole process was conducted under a flow of high-purity argon gas. The resulting products were purified via centrifugation plus washing with hexane and ethanol three times, and then dried in air.
2.2. Characterization
X-ray diffraction (XRD) patterns were collected using a Bruker D8-Advance x-ray diffractometer with Cu Kα radiation. Transmission electron microscopy (TEM) images and selective area electron diffraction (SAED) patterns were recorded on JEM-1400, TECNAI F-30 and JEM-2100 transmission electron microscopes. Fourier transform infrared (FTIR) spectra were obtained with an IS10 FTIR spectrometer. X-ray photoelectron spectroscopy (XPS) analyses were performed using a PHI Quantum 2000 scanning ESCA Microprobe spectrometer using an Al Kα photon source. Magnetic measurements were carried out with a Physical Property Measurement System (PPMS-9, Quantum Design) in the temperature ranging from 5 to 350 K. 2
Nanotechnology 27 (2016) 455602
Q Qi et al
Figure 1. Low-magnification TEM image (a), HRTEM images ((b) and (c)) and SAED pattern (d) of c-CoO nanocrystals. The inset in
figure 1(c) shows the TEM image of a multi-branched nanocrystal from which the HRTEM image is obtained.
ends. The SAED pattern (figure 1(d)) recorded from randomly distributed nanocrystals also exhibits diffraction rings that correspond well to rock-salt cubic CoO. In addition, the (200) diffraction ring exhibits a stronger intensity than the (111) ring, further confirming the 〈100〉 growth direction of rod structure. Figures 2(a) and (b) show the typical TEM images of the as-prepared h-CoO nanocrystals which have a multi-rod structure with an average width of about 30 nm and a length of about 70 nm (30×70 nm). From the morphology of figure 2(b), most nanocrystals split into three or four rods, which may be due to the relatively high growth rate along the rod direction. This rod morphology is similar to that of h-CoO nanorods synthesized by decomposing Co(acac)3 in benzylamine and o-dichlorobenzene [1]. The HRTEM image in figure 3(c) exhibits distinct lattice fringes with a lattice spacing of 0.520 nm, corresponding well to the (002) plane of wurtzite-type hexagonal CoO. It also reveals a growth direction of [001], namely, the nanorods grow along the caxis. The recorded SAED pattern (figure 2(d)) shows multiple concentric diffraction rings that can be indexed to wurtzitetype hexagonal CoO. The (002) diffraction ring appears much
3. Results and discussion 3.1. Morphology and structure characterization
Figure 1(a) shows the low-magnification TEM image of the as-prepared c-CoO nanocrystals which have a multi-branched morphology with several short rods interlaced together from different directions. These nanocrystals have a typical size varying from 15 to 20 nm. The HRTEM images (figures 1(b) and (c)) recorded from a single nanocrystal reveal a highly crystalline structure with a lattice spacing of 0.213 nm which can be indexed to the (200) plane of rock-salt cubic CoO. A growth direction of 〈100〉 along the long axis of rod is also revealed. In previous studies on the c-CoO nanocrystals with rod-like morphology, a growth direction of [111] was usually reported [25, 26]. However, in our case, the growth direction on rod branch is 〈100〉, indicating a different growing mechanism. This distinct structure indicates that the rod branch may be developed from the assembly of CoO nanocubes with {100} facets connecting together. This assumption is supported by the TEM image (inset in figure 1(c)) in which the rod branches seem to consist of many small cubes with flat 3
Nanotechnology 27 (2016) 455602
Q Qi et al
Figure 2. Low-magnification TEM images ((a) and (b)), HRTEM image (c) and SAED pattern (d) of h-CoO nanocrystals.
Typical XRD patterns of the as-prepared CoO nanocrystals are shown in figure 3. The diffraction peaks in pattern (a) are in good agreement with the standard crystallographic data for rock-salt cubic CoO while the seven obvious diffraction peaks in pattern (b) can be assigned to the (100), (002), (101), (102), (110), (103) and (112) planes of wurtzitetype hexagonal CoO. The anisotropic structure of the asprepared CoO nanocrystals is also revealed in the XRD patterns. For c-CoO, the (200) diffraction peak is much stronger than the (111) peak, indicating a 〈100〉-orientated anisotropic growth, whereas for h-CoO, the (002) peak intensity is enhanced, which coincides with their growth direction along the c-axis. Obviously, there is no evidence in the XRD patterns for the existence of crystalline impurities such as Co and Co3O4. The chemical characteristics of two types of CoO nanocrystals were further studied by XPS analysis. Figure 4(a) shows the XPS spectra of Co 2p regions. There is no obvious difference between the c-CoO and h-CoO, indicating that they are both composed of Co ions in the same valence state. The main peak at 780.2 eV with the shakeup satellite at 786.2 eV can be assigned to Co(II) 2p3/2, while the peak at 796.2 eV with the shakeup satellite at 802.4 eV can be attributed to
Figure 3. XRD patterns of h-CoO (a) and c-CoO (b) nanocrystals.
brighter than (100) and (101) rings, implying preferential growth along [001] direction. It is known that a kinetically controlled condition generally enhances the growth rate along the c-axis in wurtzite crystal structures, and thus it ensures the growth of anisotropic nanorods [1]. 4
Nanotechnology 27 (2016) 455602
Q Qi et al
Figure 4. XPS spectra of Co 2p (a) and O 1s (b) regions for c-CoO and h-CoO nanocrystals.
at 2852.5 and 2922.9 cm−1 are due to the symmetric and asymmetric CH2 stretching modes. The peaks from 1000 to 1348 cm−1 can be assigned to the C=O stretching on stearic acid [31]. In comparison with pure Co(st)2, the peak at 3440 cm−1, which represents O–H stretching of H2O [37], is observed in Co(st)2 exposed to air. Thus, we infer that the Co(st)2 exposed in air may adsorb moisture. In order to confirm our assumption, 0.5 ml of H2O was added into Co(st)2 whose FTIR transmission spectrum is plotted as the red curve in figure 5. The peak at 3440 cm−1 representing O–H stretching of H2O is clearly present. Similarly, we synthesized c-CoO nanocrystals by adding H2O, while h-CoO was obtained instead by using dry Co(st)2. Therefore, H2O plays a critical role in selecting the phase of CoO nanocrystals. Previously, Shim et al also found that H2O is critical in the synthesis of CoO nanoparticles via decomposing Co (acac)2 in amine solution. The h-CoO was obtained through two different pathways: solvolysis with C–C bond cleavage and direct condensation by amine, while the c-CoO was synthesized by strong nucleophilic attack of hydroxide ions from water and subsequent C–C bond breaking [24]. Similarly, we suspect that the source of oxygen for the formation of c-CoO may be from the strong nucleophilic hydroxide ions in water, while that for h-CoO may be from stearate ion. This different source of oxygen gave rise to different phases. The influences of different amounts of water on the morphology and phase of CoO are shown in online supplementary figures S1 and S2. In the absence of H2O, the products are h-CoO nanorods. When 0.25 ml of H2O is added, the nanocrystals have already changed into fcc phase (see the XRD pattern in online supplementary figure S2). Discrete nanocrystals with near-cubic shape along with interlaced nanorods are the major component of the products. Further increasing the amount (1 ml) of H2O, the crystalline phase remains fcc phase; however, the morphology further changes from interlaced nanorods to discrete nanocrystals with a near-cubic morphology. Thus it can be seen that the amount of H2O has a
Figure 5. FTIR transmission spectra of pure Co(st)2 (top),Co(st)2
exposed in air (middle) and Co(st)2 containing H2O (bottom).
Co(II) 2p1/2 energy levels. The presence of both peaks at 780.2 and 796.2 eV and their obvious satellite peaks prove that Co exists as Co2+ ions [17]. The absence of feature peak of metallic Co at about 778 eV suggests that the samples do not have Co impurities, which agree well with the XRD and SAED results. The O 1s regions is shown in figure 4(b). The peaks at about 529 eV are attributed to the lattice oxygen (OL) while the peaks at about 531 eV are due to the chemisorbed and dissociated oxygen species (OC) [4, 36]. The fact that OC peak of h-CoO was lower than that of c-CoO can be ascribed to the smaller size of c-CoO nanocrystals which can have larger specific surface area. 3.2. Formation mechanism discuss
In the syntheses, we accidentally found that c-CoO was obtained when the precursor Co(st)2 which was exposed to air for a time was employed. We further performed FTIR spectrum analyses on the samples of pure Co(st)2 and Co(st)2 exposed to air for three days. As shown in figure 5, the peaks 5
Nanotechnology 27 (2016) 455602
Q Qi et al
Figure 6. Schematic illustration of the formation of c-CoO and h-CoO nanocrystals.
significant influence on the phase and morphology of CoO nanocrystals. The schematic illustration of the formation different phases of CoO is shown in figure 6. The color change in the reaction mixture is a good indicator of reaction progress. For the formation of c-CoO with multi-branched morphology, when the mixture was heated to 130 °C, the color was purple and the solution was in a gel state, which indicated that the stearate ligand was tightly bound to the Co2+ metal center under the action of H2O and the density of solution was increased [5]. With increasing temperature, the colloidal mixed purple solution turned to transparent blue, then transformed to dark blue after 40 min at 320 °C, which was an indication of the seed formation. TEM observation on the products taken out from the reaction solution at this stage confirmed the seed formation. As shown in online supplementary figure S3(a), most of the crystal seeds have a nearcubic shape and a few of seeds have grown into interlaced rods. After holding 10 min, the color turned yellow-green, accounting for the growth of seeds. In this slow nucleation, thermodynamically controlled condition yields the stable c-CoO nanocrystals. The c-CoO nanocrystals may further develop into interlaced rod morphology via self assembly by connecting {100} facets. On the other hand, for the synthesis of h-CoO, when the mixture was maintained at 130 °C, the color was transparent purple. After heated to 320 °C, the transparent blue solution appeared, and then turned to dark blue in 10 min at 320 °C in the absence of H2O, indicating the formation of crystal seeds (see online supplementary figure S3(b)). After 15 min warming, the dark blue became green rapidly. This sudden kinetically controlled condition preferred to generate metastable h-CoO. The formation mechanism of a rod shape is similar to that of ZnO whiskers [38].
When come down to the synthetic condition for preparing CoO nanocrystals with different phases, Nam et al found that a flash heating of the reaction mixture (185 °C for 2 h) belonged to the kinetically controlled condition, and h-CoO nanocrystals were synthesized, whereas a prolonged heating at a relatively low temperature (130 °C for 12 h) pertained thermodynamically controlled condition, and c-CoO nanocrystals were obtained [1]. In our study, the existence of water in the reaction mixture played a key role in the phase control of CoO nanocrystals. Because the hydroxide ions of H2O are a relatively strong nucleophilic Lewis base, more energy is needed for Co ions to get rid of their constraint under this strong nucleophilic attack, which results in lower system energy [24]. Accordingly, more stable phase of c-CoO is obtained in this process. On the contrary, Co ions are easier to shake off stearic acid ions in the absence of H2O, reach reaction equilibrium quickly and thus generate metastable h-CoO. 3.3. Magnetic properties
The zero-field-cooled (ZFC) and field-cooled (FC) magnetization versus temperature plots measured at an applied field of 100 Oe are shown in figure 7. It is apparent that the ZFC curve of c-CoO shows a peak at about 7.5 K. However, there is no such a peak in the ZFC curve of h-CoO nanocrystals. There is also a delicate kink around 300 K for c-CoO, which can be ascribed to the Néel temperature that correlates with antiferromagnetism. Again there is no such a kink in the ZFC curve of h-CoO nanocrystals, indicating different magnetic properties. It is also observed that bifurcation between the ZFC and FC curves (i.e., irreversibility) occurs at a temperature well above room-temperature for both c-CoO and h-CoO nanocrystals, which means that the nanocrystals are 6
Nanotechnology 27 (2016) 455602
Q Qi et al
Figure 7. ZFC and FC magnetization versus temperature curves of c-CoO (a) and h-CoO (b) nanocrystals.
Figure 8. Temperature dependence of the high-field (3 T) magnetization of c-CoO (a) and h-CoO (b) nanocrystals.
The hysteresis loops of CoO nanocrystals measured at 5 and 300 K are shown in figure 9. The c-CoO nanocrystals exhibit almost a linear increase of the magnetization with increasing the magnetic field, which is associated with the antiferromagnetic structure. In addition, a coercivity of about 641 Oe is also observed, indicating that ferromagnetic contribution is also present. The h-CoO nanocrystals also show hysteresis loops composed of curved and linear portions; however the ferromagnetic contribution is obviously stronger than that of c-CoO. Since the results of XRD and XPS analyses do not confirm the presence of metallic Co which can bring about ferromagnetic contribution, the observed magnetic hysteresis in both of CoO samples may mainly arise from the uncompensated spins induced by cation-vacancy defects. Like other transition metal oxide such as FeO, NiO and MoO, CoO is also a non-stoichiometric compound, which usually exhibit cation efficiency under normal ambient conditions. The transition from +2 to +3 valence of Co ions to maintain electroneutrality may produce
still in a ferromagnetic state. The peak at 7.5 K in the ZFC curve of c-CoO could be due to the weakly exchange coupled to the antiferromagnetic lattice like those associated with the defects or the amorphous phase inside the CoO nanoparticles, which often characterized by a spin-glass behavior at low temperatures [31, 32]. The bifurcation between the ZFC and FC curves at a temperature well above room-temperature may be caused by uncompensated spins that are strongly exchange coupled to the antiferromagnetic lattice like those seated on regular lattice sites or surfaces of CoO. To clearly reveal the antiferromagnetic properties, we further increased the applied field to 3 T, and the temperature dependence of the high-field magnetization curves are shown in figure 8. A characteristic maximum at about 300 K is clearly observed for c-CoO, which is close to the Néel temperature of bulk antiferromagnetic CoO, whereas no such a peak is present for h-CoO nanocrystals. The temperaturedependent high-field magnetic data confirm that as-prepared c-CoO nanocrystals are intrinsically antiferromagnetic. 7
Nanotechnology 27 (2016) 455602
Q Qi et al
Figure 9. Hysteresis loops of c-CoO (a) and h-CoO (b) nanocrystals.
unbalanced spins. Wdowik et al have demonstrated that cation vacancies can account for the ferromagnetic properties of CoO nanostructures based on the first-principles calculation [39]. The absence of obvious antiferromagnetic characteristics of h-CoO nanocrystals is related to its crystalline structure. The rock-salt cubic structure of c-CoO exhibits the Co2+–O2−–Co2+ 180° superexchange interactions which favorite antiferromagnetic coupling [22]. However, the wurtzite structure of h-CoO consists of sheets with hexagonal rings of alternating Co2+ and O2− ions in an ABAB stacking sequence. The tilted Co2+–O2−–Co2+ angle of 110° frustrates the antiferromagnetic superexchange in each individual layer, which make it difficult for the spins to satisfy all the interactions simultaneously, and therefore result in the absence of magnetic ordering [13, 22].
that can be strongly exchange-coupled to the antiferromagnetic lattice.
Acknowledgments This work was partially supported by the National Natural Science Foundation of China (Grant Nos. 51471137 and 51371154) and the National Basic Research Program of China (Grant No. 2012CB933103).
References [1] Nam K M, Shim J H, Han D-W, Kwon H S, Kang Y-M, Li Y, Song H, Seo W S and Park J T 2010 Syntheses and characterization of wurtzite CoO, rocksalt CoO, and spinel Co3O4 nanocrystals: their interconversion and tuning of phase and morphology Chem. Mater. 22 4446–54 [2] Li W Y, Xu L N and Chen J 2005 Co3O4 nanomaterials in lithium-ion batteries and gas sensors Adv. Funct. Mater. 15 851–7 [3] Shima H, Takano F, Muramatsu H, Akinaga H, Tamai Y, Inque I H and Takagi H 2008 Voltage polarity dependent low-power and high-speed resistance switching in CoO resistance random access memory with Ta electrode Appl. Phys. Lett. 93 113504 [4] Lu A, Chen Y, Zeng D, Li M, Xie Q, Zhang X and Peng D L 2014 Shape-related optical and catalytic properties of wurtzite-type CoO nanoplates and nanorods Nanotechnology 25 035707 [5] Lu A, Chen Y, Jin J, Yue G-H and Peng D-L 2012 CoO nanocrystals as a highly active catalyst for the generation of hydrogen from hydrolysis of sodium borohydride J. Power Sources 220 391–8 [6] Wang G, Shen X, Horvat J, Wang B, Liu H, Wexler D and Yao J 2009 Hydrothermal synthesis and optical, magnetic, and supercapacitance properties of nanoporous cobalt oxide nanorods J. Phys. Chem. C 113 4357–61 [7] Ravindra A V, Behera B C, Padhan P, Lebedev O I and Prellier W 2014 Tailoring of crystal phase and Néel
4. Conclusion Rock-salt c-CoO and wurtzite-type h-CoO nanocrystals have been selectively synthesized via facile chemical solution approaches in which cobalt precursor Co(st)2 is decomposed in 1-octadecene at different conditions. The synthesized c-CoO nanocrystals have a multi-branched morphology with several short rods growing on the 〈100〉 direction interlaced together while the h-CoO nanocrystals exhibit a multi-rod structure with several rods growing on the same base facet along c-axis. The presence of small amounts of H2O promotes the formation of cubic phase whereas the absence of H2O is essential to the growth of hexagonal phase of CoO. The magnetism measurements reveal an intrinsic antiferromagnetism with a Néel temperature of about 300 K for the c-CoO nanocrystals. However the antiferromagnetic ordering is not distinct for the h-CoO nanocrystals. Both c-CoO and h-CoO nanocrystals exhibit weak ferromagnetic contributions which may arise from the uncompensated spins 8
Nanotechnology 27 (2016) 455602
[8] [9]
[10]
[11] [12]
[13] [14]
[15] [16]
[17] [18] [19] [20]
[21] [22]
[23]
Q Qi et al
[24] Shim J H, Nam K M, Seo W S, Song H and Park J T 2011 The role of water for the phase-selective preparation of hexagonal and cubic cobalt oxide nanoparticles Chem. Asian J. 6 1575–81 [25] Zhang Y, Zhu J, Song X and Zhong X 2008 Controlling the synthesis of CoO nanocrystals with various morphologies J. Phys. Chem. C 112 5322–7 [26] Zhang Y, Zhong X, Zhu J and Song X 2007 Alcoholysis route to monodisperse CoO nanotetrapods with tunable size Nanotechnology 18 195605 [27] Buck M R, Biacchi A J and Schaak R E 2014 Insights into the thermal decomposition of Co(II) oleate for the shapecontrolled synthesis of wurtzite-type CoO nanocrystals Chem. Mater. 26 1492–9 [28] Chen C-J, Chiang R-K and Wang S-L 2013 Controllable organic-phase synthesis of cuboidal CoO mesocrystals and their magnetic properties Cryst. Eng. Comm. 15 9161–9 [29] Dutta D P, Sharma G, Manna P K, Tyagi A K and Yusuf S M 2008 Room temperature ferromagnetism in CoO nanoparticles obtained from sonochemically synthesized precursors Nanotechnology 19 245609 [30] Yang G, Gao D, Shi Z, Zhang Z, Zhang J, Zhang J and Xue D 2010 Room temperature ferromagnetism in vacuumannealed CoO nanospheres J. Phys. Chem. C 114 21989–93 [31] Dai Q and Tang J 2013 Magnetic properties of CoO nanocrystals prepared with a controlled reaction atmosphere RSC Adv. 3 9228–33 [32] Dai Q and Tang J 2013 The optical and magnetic properties of CoO and Co nanocrystals prepared by a facile technique Nanoscale 5 7512–9 [33] Deori K and Deka S 2013 Morphology oriented surfactant dependent CoO and reaction time dependent Co3O4 nanocrystals from single synthesis method and their optical and magnetic properties Cryst. Eng. Comm. 15 8465–74 [34] He X and Shi H 2011 Synthesis and anomalous magnetic properties of hexagonal CoO nanoparticles Mater. Res. Bull. 46 1692–7 [35] He X, Zhong W, Yan S, Liu C, Shi H, Au C-T and Du Y 2014 Transition temperature of wurtzite CoO nanocrystals as revealed in comprehensive magnetic characterization J. Phys. Chem. C 118 13898–903 [36] Zhang H, Ling T and Du X-W 2015 Gas-phase cation exchange toward porous single-crystal CoO nanorods for catalytic hydrogen production Chem. Mater. 27 352–7 [37] Mitsuzuka A, Fujii A, Ebata T and Mikamia N 1996 Infrared spectroscopy of OH stretching vibrations of hydrogenbonded tropolone-(H2O)n (n = 1–3) and tropolone(CH3OH)n (n = 1 and 2) clusters J. Chem. Phys. 105 2618–27 [38] Hu J Q, Li Q, Wong N B, Lee C S and Lee S T 2002 Synthesis of uniform hexagonal prismatic ZnO whiskers Chem. Mater. 14 1216–9 [39] Wdowik U D and Parlinski K 2008 Electronic structure of cation-deficient CoO from first principles Phys. Rev. B 77 115110
temperature of cobalt monoxides nanocrystals with synthetic approach conditions J. Appl. Phys. 116 033912 Seo W S, Shim J H, Oh S J, Lee E K, Hur N H and Park J T 2005 Phase- and size-controlled synthesis of hexagonal and cubic CoO nanocrystals J. Am. Chem. Soc. 127 6188–9 He X, Li Z, Zhang X, Qiao W, Song X, Yan S, Zhong W and Du Y 2015 Effects of Ar/H2 annealing on the microstructure and magnetic properties of CoO nanoparticles RSC Adv. 5 69948–54 Liu J F, Yin S, Wu H P, Zeng Y W, Hu X R, Wang Y W, Lv G L and Jiang J Z 2006 Wurtzite-to-rocksalt structural transformation in nanocrystalline CoO J. Phys. Chem. B 110 21588–92 Shinde V R, Mahadik S B, Gujar T P and Lokhande C D 2006 Supercapacitive cobalt oxide (Co3O4) thin films by spray pyrolysis Appl. Surf. Sci. 252 7487–92 Bahlawane N, Ngamou P H, Vannier V, Kottke T, Heberle J and Kohse-Hoinghaus K 2009 Tailoring the properties and the reactivity of the spinel cobalt oxide Phys. Chem. Chem. Phys. 11 9224–32 Alaria J, Cheval N, Rode K, Venkatesan M and Coey J M D 2008 Structural and magnetic properties of wurtzite CoO thin films J. Phys. D: Appl. Phys. 41 135004 Barakat N A M, Khil M S, Sheikh F A and Kim H Y 2008 Synthesis and optical properties of two cobalt oxides (CoO and Co3O4) nanofibers produced by electrospinning process J. Phys. Chem. C 112 12225–33 Zhang W, Han M, Jiang Z, Song Y, Xie Z, Xu Z and Zheng L 2007 Controllable synthesis of CoO nanosheets and their magnetic properties Chem. Phys. Chem. 8 2091–5 Verelst M, Ely T O, Amiens C, Snoeck E, Lecante P, Mosset A, Respaud M, Broto J M and Chaudret B 1999 Synthesis and characterization of CoO, Co3O4, and mixed Co/CoO nanoparticules Chem. Mater. 11 2702–8 Ghosh M, Sampathkumaran E V and Rao C N R 2005 Synthesis and magnetic properties of CoO nanoparticles Chem. Mater. 17 2348–52 Risbud A S, Snedeker L P, Elcombe M M, Cheetham A K and Seshadri R 2005 Wurtzite CoO Chem. Mater. 17 834–8 An K et al 2006 Synthesis, characterization, and self-assembly of pencil-shaped CoO nanorods J. Am.Chem. Soc. 128 9753–60 Deng X, Yang D, Tan G, Li X, Zhang J, Liu Q, Zhang H, Mellors N J, Xue D and Peng Y 2014 Bimagnetic h-Co/hCoO nanotetrapods: preparation, nanoscale characterization, three-dimensional architecture and their magnetic properties Nanoscale 6 13710–8 Wang D, Ma X, Wang Y, Wang L, Wang Z, Zheng W, He X, Li J, Peng Q and Li Y 2010 Shape control of CoO and LiCoO2 nanocrystals Nano Res. 3 1–7 He X, Song X, Qiao W, Li Z, Zhang X, Yan S, Zhong W and Du Y 2015 Phase- and size-dependent optical and magnetic properties of CoO nanoparticles J. Phys. Chem. C 119 9550–9 Zhang H T and Chen X H 2005 Controlled synthesis and anomalous magnetic properties of relatively monodisperse CoO nanocrystals Nanotechnology 16 2288–94
9
