CRYSTAL GROWTH & DESIGN
Shape Evolution and Magnetic Properties of Cobalt Sulfide Shu-Juan Bao,† Yibin Li,‡ Chang Ming Li,*,† Qiaoliang Bao,† Qing Lu,† and Jun Guo§ School of Chemical and Biomedical Engineering and Center for AdVanced Bionanosystems, Nanyang Technological UniVersity, Nanyang AVenue, Singapore 639798, Center for Composite Materials, School of Astronautics, Harbin Institute of Technology, P.O. Box 3010, Harbin 150001, P.R. China, and School of Materials Science and Engineering, Nanyang Technological UniVersity, Nanyang AVenue, Singapore 639798
2008 VOL. 8, NO. 10 3745–3749
ReceiVed April 13, 2008; ReVised Manuscript ReceiVed June 13, 2008
ABSTRACT: In this work, well-defined uniform octahedrons of Co3S4 are synthesized via a simple biomolecule-assisted hydrothermal process for the first time. Evolution of the crystal-structure and the shape of the samples are examined systematically by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM). The formation mechanism of the octahedral Co3S4, possibly due to a ripening-splitting crystal growth process, and the effect of microstructure and morphology on their physical properties are investigated. This work may not only render a simple approach to synthesize highly symmetric, microstructured materials but also could provide fundamental insights of the effect of crystal structure transformation on physical properties. Introduction Unique size- or shape-dependent properties displayed by nanocrystals have fueled up intense research on new materials.1,2 Nowadays, the research interests have been expanded into controlling the shape of materials and understanding the correlations between the materials properties and their microstructure and morphology as well.3-5 Successes in shape control of desired architectural nanocrystals have been reported including rods (CdSe),6 stars (PbS),7 tetrahedrons (Pt, CdS),8-10 pentapods (MnO2),11 and snowflakes (Co, BiS).12,13 However, the shape controlled synthesis of new nanocrystals for novel physical properties is surely a great challenge in both experimental design and fundamental theory. As semiconductor compounds, cobalt sulfides are important materials because of their unique catalytic, electrical, and magnetic properties and their potential applications for hydrodesulfurization and hydrodearomatization in many industrial fields.14,15 It is also one of the most complicated metal sulfides, which has a number of phases and chemical compositions.15 Although the shape-control studies are actively being pursed on the semiconductor and metal nanocrystals, only a little work has been done on cobalt sulfides. The challenges for shapecontrolled synthesis of cobalt sulfides can be caused by the stoichiometry of cobalt sulfides, which is much more complicated than that of cobalt oxides because of the coexistences of strongly reducible cobalt ion and oxidizable sulfide ion. In addition, cobalt ion has a very strong affinity to oxygen. It is difficult to exclude cobalt oxide or cobalt hydroxide impurities from the resultant materials. Another but not the last challenge is that the reaction temperature needs to be well-controlled because of the complicated phase diagram of cobalt sulfides. Recently, synthesized CoS and CoS2 nanoparticles, 16,17 Co3S4 nanotubes,15 and hollow structured Co9S818 have been reported; however, few of them have well-defined morphology. We have synthesized CoS nanospheres and nanowires assembled by rice-like rods and small flowers, respectively, * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +65 67904485. Fax: +65 67911761. † School of Chemical and Biomedical Engineering and Center for Advanced Bionanosystems. ‡ Harbin Institute of Technology. § School of Materials Science and Engineering.
Figure 1. XRD patterns of the samples obtained by different reaction times (a, 12 h; b, 48 h; c, 96 h; d, 240 h).
through adjusting the polarity of the precursor solution.19 In this work, a well-defined octahedral Co3S4 is synthesized by a simple and efficient biomolecule-assisted hydrothermal process for the first time. The effect of reaction time on the microstructure and morphology of the product was thoroughly investigated, and the photoluminescence (PL) and magnetic properties of the different structured cobalt sulfides were studied systematically as well. The crystal structure and morphology was found to have remarkable effects on their physical properties. The forming mechanism of the different microstructured cobalt sulfides was proposed in terms of the experimental results. Experimental Section Synthesis Method. All chemical reagents were of analytical grade and used as received. The synthesis of cobalt sulfides was performed with a hydrothermal process, in which 3 mmol L-cysteine and 3 mmol CoCl2 · 6H2O were dissolved in 45 mL of deionized water, respectively. Then, the CoCl2 solution was added into the L-cysteine solution under constant and vigorous stirring. After 15 min stirring, the resulting mixture was transferred into a Teflon-lined stainless steel autoclave, and then the autoclave was sealed and maintained at 200 °C for different reaction time followed by cooling down to room temperature. The prepared products were washed with deionized water and ethanol for three times, respectively, and then placed in a vacuum oven at 50 °C for overnight.
10.1021/cg800381e CCC: $40.75 2008 American Chemical Society Published on Web 09/05/2008
3746 Crystal Growth & Design, Vol. 8, No. 10, 2008
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Figure 2. Low and high magnification FESEM of the samples obtained by different reaction times (a and b, 12 h; c and d, 48 h; e and f, 96 h; g and h, 240 h). Characterization. The crystal structure of the product was characterized by X-ray diffraction (XRD, Bruker AXS X-ray diffractometer). The morphology and microstructure of the synthesized materials were investigated by field emission scanning electron microscopy (FESEM, JSM-6700F, Japan) and high-resolution transmission electron microscopy (HRTEM, JEM-2100F, Japan). The chemical element states of the shallow surface of materials were examined by using X-ray photoelectron spectroscopy (XPS, Kratos AXIS Ultra spectrometer). PL spectra were recorded at room temperature with a Jobin-Yvon Horiba Fluorolog-3 spectrofluorimeter. The magnetic properties of the synthesized samples were studied by using a Lakeshore 7300 Vibrating Sample Magnetometer (VSM).
Results and Discussion The synthesized new materials were first checked with XRD. All of the diffraction peaks in Figure 1a,b can be indexed to a hexagonal phase of CoS, which match well with the standard XRD pattern (ICSD No. 029305), suggesting pure CoS is
obtained through the hydrothermal reaction process. With increase of the hydrothermal reaction time, peaks of cubic phase Co3S4 (ICSD No. 024212) along with the hexagonal phase CoS are observed. Although with the increase of reaction time, lesser symmetric CoS transformed into higher symmetric Co3S4 gradually, but a highly pure phase Co3S4 still could not be obtained even after a 240 h hydrothermal reaction. A higher symmetric Co3S4 may be obtained at higher temperature. Unfortunately, a higher temperature (>200 °C) could not be achieved in our present reaction system because of the instrumental constraints. To fully understand the effect of reaction time on the microstructure and morphology of the synthesized samples, FESEM examination was conducted. The product obtained by a 12 h hydrothermal reaction illustrates uniform spheres (Figure 2a) constructed by interleaving rice-like rods (Figure 2b). When the reaction time increases to 48 h, the sample mainly remains
Shape and Magnetic Properties of Cobalt Sulfide
Figure 3. FESEM images of the sample obtained by hydrothermal reaction 240 h (a, b, c, different profiles of the sample; d, ideal octahedral structure).
Figure 4. TEM (a) and HRTEM (b) image of octahedral structured Co3S4.
Crystal Growth & Design, Vol. 8, No. 10, 2008 3747
as spherical shapes (Figure 2c) and some octahedral particles begin to appear on the surface of the spheres (Figure 2d). With a reaction time of 96 h, more octahedral particles are observed (Figure 2e), but most of them are undergrown in their shape (Figure 2f). When the reaction time further prolongs to 240 h, the octahedral structure develops further and becomes the dominant product with good crystallization and regular morphology (Figure 2g,h). It demonstrates that a long reaction time is required to anneal the amorphous structure for well-defined crystalline geometries. However, in the samples obtained by reaction for 240 h there still are some particles with amorphous forms. It indicates that only increasing the reaction time is not enough to obtain higher symmetric Co3S4, which is consistent with the result of XRD analysis. It is known that a single-crystal particle usually has a specific shape because it must be enclosed by crystallographic facets that have lower energy. To understand more fundamental insights about the octahedral cobalt sulfides in our work, the different profiles of the samples obtained by the hydrothermal reaction for 240 h were examined. As shown in Figure 3a, 3b and 3c, the synthesized octahedrons are fairly perfect with a highly symmetric regular shape, which is similar to the ideal octahedron (Figure 3d) enclosed by (111) planes. The microstructure of the octahedral Co3S4 was further studied by using TEM and HRTEM. Figure 4a demonstrates the polyhedral shape of the sample obtained by hydrothermal reaction for 240 h. Figure 4b displays the HRTEM image of the polyhedrons shown in Figure 4a. It illustrates clear lattice fringes, which confirm single-crystallinity of the sample. The lattice spacing of 0.53 nm between adjacent lattice planes in the image corresponds to the distance between two (111) crystal planes of cubic phase Co3S4. The results combined with above XRD and FESEM analysis reveal that the octahedral structured sample is the Co3S4 crystal. On the basis of the above evolution of the time-dependent crystallinity and morphology, an “Ostwald ripening pro-
Figure 5. XPS spectra of Co2p (a), S2p (b), C1s (c), and N1s (d) for the samples (I, the sample obtained from 12 h; II, the sample obtained from 240 h).
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Figure 6. PL spectra of samples obtained by different hydrothermal reaction times (a, 12 h; b, 48 h; c, 96 h; d, 240 h).
cess” 4,20-22 could be used to explain the formation of different structures with different reaction times. During the reaction procedure, a large number of nuclei were formed in a short time through a known “Ostwald ripening process” at first, followed by a slow crystal growth. With the reaction going on, the aggregate continuously grew in size and density to form spheres through the interaction between L-cysteine molecules and the nanoparticles by van der Waals forces and intermolecular hydrogen bonds. This stage might last for a long time, and then the assembled nanoparticles with a relatively large size could serve as the crystal nucleus. In succession, the particles with small size could dissolve and recrystallize at the site of the large particles because of their much higher surface free energy compared with those of the large ones. Consequently, some octahedral particles appeared on the surface of the spheres. A
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subsequent increase of the hydrothermal dwell time not only led to small particles composed of spheres disappearing gradually but also increased the size of the octahedral particles. The crystallinity of the synthesized samples increased with the reaction time (shown in the XRD analysis), confirming that Ostwald ripening (crystallites grow at the expense of the smaller ones) is the mechanism in the octahedron’s forming process.20 During the reaction process, the functional groups, such as -NH2, -COOH and -SH of L-cysteine have a strong tendency to coordinate with inorganic cations and metals.13,19 In a homogeneous system composed of L-cysteine and CoCl2 solution, Co2+ can coordinate with the cysteine molecule to form a complex. With the increase of the reaction time, the strong coordination bonds between the hydrosulfide group and Co2+ could weaken the S-H bond and further break it to form the cobalt sulfide nanoparticle because of the high reaction temperature. The changes of the chemical state of the elements constituting cobalt sulfide were investigated by XPS to understand the evolution process of the crystal structure and the shape of cobalt sulfides. The XPS results recorded from the samples obtained by different reaction times are shown in Figure 5, in which after 12 h reaction, the Co2p XPS profile could be deconvoluted into three couples of spin-orbit splitting components (labeled in Figure 5a (I)) assigned to Co-S, COO-Co, and Co-NH of the L-cysteine-Co complex, respectively. However, when the reaction time prolongs to 240 h, most of the Co-O, Co-N bonds in the cysteine-Co complex and S-thiolate are broken and Co-S is formed, which is confirmed by the Co2p profile II where the Co-S binding peak dominates. More solid support can be given by analysis of the S2p signal as shown in Figure 5b. For the sample obtained by 12 h (S2p profile I), the sulfur peak at 161.5 eV corresponds to Co-S,
Figure 7. Magnetization vs applied magnetic field of the as-obtained samples (a, b, c, and d, the sample obtained by 12, 48, 96, and 240 h, respectively).
Shape and Magnetic Properties of Cobalt Sulfide
whereas the emission peak at 167 eV is ascribed to Co-S-thiolate binding.25 It is worth noticing that after 240 h reaction, as shown in S2p profile II, the peak at the high energy side decreases significantly, indicating a dramatic decomposition of L-cysteine. At the same time, the peak at the low energy side splits into two peaks, possibly because of the formation of Co3S4, and the binding energy of Co-S in Co3S4 and CoS is different. With increase of the reaction time, the C1s and N1s emission peaks ascribed to a related functional group decrease significantly. This further confirms that after a long reaction time, more Co-Sthiolate bonds are broken gradually and the samples transformed from CoS to Co3S4 correspondingly. Figure 6 shows the PL spectra of the differently microstructured cobalt sulfides excited at λex ) 280 nm, which display a strong luminescence peak at 355 nm and two weak/broad peaks at 665 and 715 nm. In comparison with the spectra of the flowerlike cobalt sulfide sphere, the intensity of the emission peak of the octahedral cobalt sulfides is much weaker. This difference is related to the special crystal structure and surface properties of the octahedral Co3S4 and is consistent with the reported literature.26 The magnetic properties of nanomaterials with different microstructure and shape can provide great insight into the fundamentals of nanomagnetism. The field dependences of magnetization on different shapes and microstructures of the samples are shown in Figure 7. A representative hysteresis loop for the flowerlike cobalt sulfide sphere obtained by 12 h illustrates its ferromagnetic properties. At 300 K, the magnetization value of the prepared flowerlike cobalt sulfide sphere is 0.375 emu · g-1 (at 1 T), and the coercivity value (Hc) is 175 G. With the increase of reaction time, the corresponding values of the samples decrease gradually. A nearly linear magnetization versus magnetic field curve in Figure 7d reveals that the sample obtained by 240 h of hydrothermal reaction is not ferromagnetic. The experimental results indicate that the magnetic properties of the synthesized cobalt sulfides drastically changed with the evolution of their crystal structure and morphology, which may be very significant for fundamental research and practical applications.
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tion of crystal structure and shape of the samples, their PL and magnetic properties change correspondingly. Acknowledgment. This work is financially supported by Center of Advanced Bionanosystems of Nanyang Technological University in Singapore.
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(5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21)
Conclusion In summary, an environmentally friendly biomolecule-assisted hydrothermal method was developed for the synthesis of different nanostructured cobalt sulfides. Well-defined octahedral Co3S4 single crystals were obtained for the first time by simply controlling the reaction time. Although highly pure Co3S4 octahedrons could not be obtained in this work, it still renders a new approach to synthesize highly symmetric, microstructured materials. The different structural samples were formed by a ripening-splitting crystal growth process. With the transforma-
(22) (23) (24) (25) (26)
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CG800381E
