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2007, 111, 2-5 Published on Web 12/14/2006
Bulk Modulus and Structural Phase Transitions of Wurtzite CoO Nanocrystals J. F. Liu,† Y. He,† W. Chen,† G. Q. Zhang,†,‡ Y. W. Zeng,†,§ T. Kikegawa,| and J. Z. Jiang*,† Laboratory of New-Structured Materials, Department of Materials Science and Engineering, Zhejiang UniVersity, Hangzhou 310027, P. R. China, Key Laboratory of AdVanced Textile Materials and Manufacturing Technology, Zhejiang Sci-Tech UniVersity, Ministry of Education, Hangzhou 310018, P. R. China, Analysis and Testing Centre, Zhejiang UniVersity, Hangzhou, 310027, P. R. China, and Photon Factory, Institute for Materials Structure Science, High Energy Accelerator Organization, 1-1, Oho, Tsukuba 305-0801, Japan ReceiVed: NoVember 7, 2006; In Final Form: NoVember 29, 2006
High-pressure behaviors of wurtzite-type hexagonal CoO nanocrystals were investigated by in situ highpressure synchrotron radiation X-ray diffraction measurements up to 57.4 GPa at ambient temperature. It is found that bulk modulus of the hexagonal CoO phase is about 115 GPa at zero pressure. During compression, the hexagonal CoO phase transfers into rocksalt-type cubic phase in the pressure range of 0.8-6.9 GPa. The volume collapse accompanied by the transition was estimated to be about 20%. This is irreversibly phase transformation; that is, the cubic CoO phase remains after pressure release. Based on the data of peak width vs pressure, a cubic-to-rhombohedral phase transition was detected for the nanocrystalline cubic CoO phase with the transition pressure of about 36 GPa, lower than 43 GPa for bulk cubic CoO phase. The bulk modulus of the nanocrystalline cubic CoO phase of about 258 GPa is larger than 180 GPa for the corresponding bulk cubic CoO phase.
Transition metal oxides constitute a well-studied class of materials due to their many interesting properties and numerous applications.1-3 CoO nanocrystals are promising functional materials for applications owing to their catalytic, magnetic, and gas-sensing properties.4-6 CoO typically crystallizes in a stable phase of rocksalt-type cubic structure (space group Fm-3m, marked as c-CoO) with octahedral Co2+. Very recently, Seo et al.7 used the thermal decomposition of Co(acac)3 (acac ) acetylacetonate) in oleylamine to synthesize wurtzite-type hexagonal CoO (marked as h-CoO). However, in order to extend its application, the stability of the new-structured CoO nanocrystals needs to be investigated in details. Liu et al.8 investigated the formation of h-CoO nanocrystals under hydrothermal conditions and phase transformation in terms of temperature and time. To the best of our knowledge, no study for pressure effect of structural stability on h-CoO has been reported. In this work, we report an in situ high-pressure synchrotron radiation X-ray diffraction study of h-CoO sample with an average grain size of 50 nm up to 57.4 GPa at ambient temperature. For c-CoO, considerable efforts have been devoted to the pressured-induced phase transitions. Cohen et al.9 predicted the high-spin-to-low-spin magnetic transition for c-CoO obtained from first-principles calculations at pressure 90 GPa. Noguchi et al.10 revealed two phase transitions at about 80 and 120 GPa under static high-pressure and one phase transition at 81 GPa * Corresponding author. E-mail:
[email protected]. Phone: +86 571 8795 2107. Fax: +86 571 8795 1171. † Laboratory of New-Structured Materials, Department of Materials Science and Engineering, Zhejiang University. ‡ Zhejiang Sci-Tech University. § Analysis and Testing Centre, Zhejiang University. | Institute for Materials Structure Science.
10.1021/jp067354i CCC: $37.00
from the measurements of shock compression curve. Guo et al.11 performed in situ X-ray diffraction experiments using diamond anvil cell and He pressure medium and found two phase transitions of CoO from cubic-to-rhombohedral structure at 43 GPa without significant volume change and from the lowdensity rhombohedral-to-high-density rhombohedral phase at 90 GPa. Atou et al.12 observed a drastic decrease in the electrical resistance (8 orders of magnitude) in the pressure range of 4363 GPa and a small decrease in the electrical resistance at about 90 GPa. These results were consistent with the phase transitions reported by Guo et al.11 Rueff et al.13 revealed that c-CoO shows a magnetic collapse by the abrupt decrease of the satellite intensity in the Kβ emission lines at about 80 and 100 GPa. The oleylamine-capped h-CoO nanocrystals used in this study was synthesized using a method reported in ref 8. A green slurry of Co(acac)3 (0.10 g, 0.14 mmol) and oleylamine (7.51 g, 28.07 mmol, 200 equiv) in a 100 mL Schlenk flask connected to a bubbler was heated at 135 °C for ca. 5 min under an argon atmosphere to give a clear green solution. Immediately after dissolution, the reaction was quickly heated up to 200 °C with a rate of about 1 K/s. After the solution was annealed at 200 °C for 1 h, the reaction mixture was allowed to cool to roomtemperature giving a green suspension. The light yellow supernatant was removed by centrifugation at 3000 rpm for 10 min. h-CoO nanocrystals were washed with ethanol. Transmission electron microscopy (TEM) measurements were performed using TEM-2010HR, by which crystalline structure, average crystallite size and particle morphology were observed. Specimens for TEM measurements were prepared by dipping liquid containing nanocrystals onto a carbon-coated copper grid. It is found that the prepared CoO nanocrystals have a hexagonal © 2007 American Chemical Society
Letters
Figure 1. Low- and high-resolution TEM images of hexagonal CoO nanocrystals (a) hexagonal pyramid-shaped CoO nanocrystals with 50 nm in side edge length and 25 nm in basal edge length; (b) HRTEM image of the hexagonal pyramid-shaped CoO nanocrystalline, lattice fringe images, d-spacing ) 2.80 Å, from the (100) plane of highly crystalline CoO.
structure, pyramid shape, and an average grain size of about 50 nm, as shown in Figure 1. An HRTEM image of an isolated hexagonal pyramid-like CoO nanocrystal is shown in Figure 1b, in which a d-spacing of 2.80 Å of (100) plane of the h-CoO phase is detected. X-ray diffraction measurements using Cu KR radiation of the sample confirm the hexagonal structure of the studied sample.8 For high pressure measurements, h-CoO powders were mounted in a 300-µm-diameter hole of the T301 stainless-steel gasket in a Mao-Bell diamond-anvil cell. In situ high-pressure angle-dispersive XRD measurements up to about 57.4 GPa were performed using synchrotron radiation source and an imaging plate detector at KEK Synchrotron Radiation Facility in Japan at ambient temperature. The monochromatized X-ray (0.61651 Å) was collimated 50 × 50 µm and irradiated to the center of the sample. A pressure transmitting medium of 16:3:1 methanol: ethanol:water solution was used. The sample pressure was measured by ruby fluorescence method. Figure 2 shows some selected high-pressure synchrotron radiation angle-dispersive X-ray diffraction patterns for the h-CoO sample at room temperature from 0 to 57.4 GPa and decompression from 57.4 to 1.0 GPa. From the XRD patterns in Figure 2a, it is found that the end pressure for the hexagonal-
J. Phys. Chem. C, Vol. 111, No. 1, 2007 3 to-cubic phase transformation is at about 6.9 GPa for the h-CoO sample, whereas the onset pressure for the transition is difficult to be estimated exactly from the limited pressure points. The (111), (200), and (220) peaks for the cubic phase can be observed with increasing pressure up to 57.4 GPa in Figure 2b. According to the work for bulk cubic CoO by Guo et al.,11 a cubic-to-rhombohedral phase transformation was reported at about 43 GPa with almost no volume change when the c-CoO phase was used as a starting material. During this transition, Guo et al.11 observed that the (111) peak in the cubic phase splits into (003) and (101) two peaks in the rhombohedral phase, whereas the (200) peak in the cubic phase overlaps with the (102) peak in the rhombohedral phase. In our experiments, no peaks become asymmetrical in Figure 2b in the pressure range of 6.9-57.4 GPa, whereas the relationship of peak widths vs pressure for (111) and (200) peaks in the cubic phase differs from each other. Thus, we plot the data of (fwhm(111)/fwhm(200) - 1)2 vs pressure, similar to the plot in ref 14, as shown in Figure 3, where fwhm is the peak width at the half-maximum. It is clear that above 30 GPa the width for the (111) peak becomes broader than (200), indicating that the cubic-torhombohedral phase transition might also occur in the present nanocrystalline CoO sample. By extrapolating data, we assign the transition pressure to about 36 GPa. It should be mentioned that the (003) and (101) peaks in the rhombohedral phase are very close to the (111) peak in the cubic phase.11 Thus, for a nanocrystalline sample, in which Bragg peaks become broader as compared with the polycrystalline sample, the (003) and (101) peaks in the rhombohedral phase strongly overlap with the (111) peak in cubic phase. Consequently, no peak splitting during cubic-to-rhombohedral phase transformation could occur for our nanocrystalline c-CoO. The CoO decompression experiments in Figure 2c reveal that this hexagonal-to-cubic phase transformation is irreversible, since only peaks related to the cubic CoO phase are detected in the sample after pressure release. Figure 4 shows the volume-compression curve of the CoO sample. The estimated volume collapse during the wurtzite-tocubic phase transformation at 6.9 GPa is about 20%. The compression data can be used to determine the bulk modulus (B0) at zero pressure of the hexagonal and cubic CoO phase, respectively. B0 is defined as the reciprocal of the volume compressibility at zero pressure. The P-V data were fitted using a Birch-Murnaghan equation of state: P ) 1.5B0[(V/V0)-7/3 - (V/V0)-5/3] [1 - 0.75(B0′ - 4)[1 - (V/V0)-2/3], where B0′ is the pressure derivative at zero pressure. In the first fitting procedure, two fitting parameters B0 and B0′ were used. The data can be fitted well. It is known that B0 and B0′ are correlated to each other. In order to compare present values with those published in the literature, we then fit the compression data by fixing B0′ ) 4. Data can also be fitted well, as shown in Figure 4. To fit the equation of state of the cubic CoO phase, we also consider the free parameter, the volume V0 at zero pressure (i.e., to fit the compression data of the cubic CoO phase by fixing B0′ ) 4 and varying V0 and B0 in Birch-Murnaghan equation of state). Theoretical bulk modulus of hexagonal and cubic CoO without considering spin of Co ions were calculated using Wien2K package within the generalized gradient approximation, listed in Table 1. It is found that the experimental bulk modulus of CoO phases are lower than values obtained from theoretical calculation, which might be due to the spin effect of Co ions. The bulk modulus of the h-CoO phase experimentally deduced here is about 115 GPa with B0′ ) 2.8 and about 112 GPa with B0′ ) 4 (fixed), whereas the bulk modulus of the cubic CoO phase experimentally deduced here is 270 GPa with B0′ ) 3.4,
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Figure 2. XRD patterns for CoO at room temperature with compression from 0 to 6.9 GPa (a) and from 6.9 to 57.4 GPa (b). (c) The XRD patterns for CoO at room temperature with decompression from 57 to 1 GPa.
Figure 3. Data of (fwhm(111)/fwhm(200) - 1)2 vs pressure for the cubic CoO phase. Solid lines are extrapolating curves.
about 258 GPa with B0′ ) 4 (fixed), and 252 GPa with B0′ ) 4 (fixed) and free V0. The obtained B0 values for the cubic phase are larger than the data reported in refs 9 and 10. The differences might be related to at least two factors: (1) in this work, c-CoO phase was obtained at about 6.9 GPa via the hexagonal-to-cubic phase transformation, in which defects and stress exist in the c-CoO crystallites and could be higher than c-CoO phases used as starting sample in refs 9 and 10. Consequently, the deformed cubic CoO phase might have high bulk modulus as compared to the undeformed cubic phase; and (2) the grain size effect. Liu et al.8 reported that, by increasing temperature or time, the h-CoO phase with a pyramid-like shape and grain size of 50 nm transfers into cubic-structured CoO grains with an average grain size of 25 nm and square-like and rhombus-like shapes. The average grain size of the CoO sample decreases during the phase transformation. In this work, the average grain size of h-CoO, used here as the starting material, is 50 nm estimated from TEM. Here we roughly estimated the average grain size of the transferred c-CoO phase by diffraction peak width. First, we estimated instrument broadening from the XRD pattern recorded at zero pressure for the 50 nm h-CoO phase. Then, the average grain size of the c-CoO phase at 6.9 GPa was estimated by the Scherrer equation from 200 diffraction peak to be about 9 nm, which is underestimated because of no consideration of stress. Tolbert and Alivisatos15 reported that
Figure 4. Relative volume-compression data for CoO at room temperature during compression and decompression. Solid lines are BM (“BM” indicates the results calculated from the Birch-Murnaghan equation of state) simulation data. The uncertainties of experimental data are smaller than the symbols’ size.
the domain size of crystalline CdSe decreases as the system undergoes subsequent transition. Jiang et al.16 reported that for nanometer-sized γ-Fe2O3 particles the bulk modulus is higher than that for bulk materials. Thus, it might be also the case for c-CoO; that is, the smaller the grain size, the higher the bulk modulus. Nanometer-sized materials exhibit particles size dependence of high-pressure behavior. An enhancement of transition pressure in nanocrystals Si,17 CdSe,15 PdS,18 ZnO,19 and SnO220 compared with corresponding bulk materials. However, a reduction of transition pressure was also reported in nanocrystals Fe2O3,16 TiO2,21 and CeO2.22 In our present results, the pressure of the phase transition of c-CoO from facecentered cubic B1 to rhombohedral structure was lower than 43 GPa reported by Guo et al.11 More investigation is required to clarify the grain-size effect on the high pressure behaviors of h- and c-CoO phases. In conclusion, wurtzite-type hexagonal CoO nanocrystals were prepared by thermal decomposition of a single molecular precursor Co(acac)3 in oleylamine. High-pressure behavior of hexagonal CoO was investigated by in situ high-pressure
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J. Phys. Chem. C, Vol. 111, No. 1, 2007 5
TABLE 1: Bulk Modulus, B0, at Zero Pressure and Its Pressure Derivative, B0′, for Both Hexagonal and Cubic CoO Phases Obtained from Compression Experiments Using the Birch-Murnaghan Equation of State, Theoretical Calculations, Together with Some Data for Cubic CoO Phase Published in the Literature CoO samples hexagonal CoO BM (first fit) BM (second fit) theory 1 theory 2 cubic CoO BM (first fit) BM (second fit) BM (V0 varied) theory 1 theory 2 ref 10 ref 9
B0 (GPa)
B0′
115 112 242 208
2.8 4 4 5.7
270 258 252 307 281 180 179
3.4 4 4 4 5.4 3.8 3.7
synchrotron radiation X-ray diffraction measurements up to 57.4 GPa at ambient temperature. It is found that bulk modulus of the hexagonal CoO phase is about 115 GPa at zero pressure and it transfers into cubic phase in the pressure range of 0.86.9 GPa. The volume collapse accompanied by the transition was estimated to be about 20%. This is irreversibly phase transformation (i.e., presence of cubic CoO phase on unloading). Based on the peak width vs pressure, it is found that the nanocrystalline cubic CoO phase has a phase transition from cubic to rhombohedral phase while the transition pressure is about 36 GPa, lower than 43 GPa for the bulk cubic CoO phase. The bulk modulus of the nanocrystalline cubic CoO phase of about 258 GPa is larger than 180 GPa for the corresponding bulk cubic CoO phase. Acknowledgment. The authors thank Dr. H. K. Mao for fruitful discussion and HASYLAB in Hamburg, Germany, KEK and SPring8 in Japan, BSRF in Beijing, and NSRL in Hefei P. R. China for use of the synchrotron radiation facilities. Financial support from the National Natural Science Foundation of China
(Grant Nos. 50341032 and 50425102), the Ministry of Science and Technology of China (Grant Nos. 2004/249/37-14 and 2004/ 250/31-01A), the Ministry of Education of China (Grant Nos. 2.005E+10 and 2005-55), and Zhejiang University is gratefully acknowledged. References and Notes (1) Zeng, H.; Li, J.; Liu, J. P.; Wang, Z. L.; Sun, S. Nature 2002, 420, 395. (2) Tarascon, J.-M.; Armand, M. Nature 2001, 414, 359. (3) Raj, K.; Moskowitz, R. J. Magn. Magn. Mater. 1990, 85, 233. (4) Skumryev, V.; Stoyanov, S.; Zhang, Y.; Hadjipanayis, G.; Givord, D.; Nogues, J. Nature 2003, 423, 850. (5) Lin, H.-K.; Chiu, H.-C.; Tsai, H.-C.; Chien, S.-H.; Wang, C.-B. Catal. Lett. 2003, 88, 169. (6) Koshizaki, N.; Yasumoto, K.; Sasaki, T. Sens. Actuators B 2000, 66, 122. (7) Seo, W. S.; Shim, J. H.; Oh, S. J.; Lee, E. K.; Hur, N. H.; Park, J. T. J. Am. Chem. Soc. 2005, 127, 6188. (8) Liu, J. F.; Yin, S.; Wu, H. P.; Zeng, Y. W.; Hu, X. R.; Wang, Y. W.; Lv, G. L.; Jiang, J. Z. J. Phys. Chem. B 2006, 110, 21588. (9) Cohen, R. E.; Mazin, I. I.; Isaak, D. G. Science 1997, 275, 654. (10) Noguchi, Y.; Atou, T.; Kondo, T.; Yagi, T.; Syono, Y. Jpn. J. Appl. Phys. 1999, 38, L7. (11) Guo, Q. Z.; Mao, H.-K.; Hu, J. Z.; Shu, J. F.; Hemley, R. J. J. Phys.: Condens. Matter 2002, 14, 11369. (12) Atou, T.; Kawasaki, M.; Nakajima, S. Jpn. J. Appl. Phys. 2004, 43, L1281. (13) Rueff, J.-P.; Mattila, A.; Badro, J.; Vanko, G.; Shukla, A. J. Phys.: Condens. Matter 2005, 17, S717. (14) Hemley, R. J.; Shu, J.; Carpenter, M. A.; Hu, J.; Mao, H. K.; Kingma, K. Solid State Commun. 2000, 114, 527. (15) Tolbert, S. H.; Alivisatos, A. P. J. Chem. Phys. 1995, 102, 4642. (16) Jiang, J. Z.; Olsen, J. S.; Gerward, L.; Morup, S. Europhys. Lett. 1998, 44, 620. (17) Tolbert, S. H.; Alivisatos, A. P. Science 1994, 265, 373. (18) Qadri, S. B.; Yang, J.; Ratna, B. R.; Skelton, E. F.; Hu, J. Z. Appl. Phys. Lett. 1996, 69, 2205 (19) Jiang, J. Z.; Olsen, J. S.; Gerward, L.; Frost, D.; Rubie, D.; Peyronneau, J. Europhys. Lett. 2000, 50, 48 (20) He, Y.; Liu, J. F.; Chen, Y.; Wang, Y.; Wang, H.; Zeng, Y. W.; Zhang, G. Q.; Wang, L. N.; Liu, J.; Hu, T. D.; Hahn, H.; Gleiter, H.; Jiang, J. Z. Phys. ReV. B 2005, 72, 212102. (21) Olsen, J. S.; Gerward, L.; Jiang, J. Z. J. Phys. Chem. Solids 1999, 60, 229. (22) Rekhi, A.; Saxana, S. K.; Lazor, P. J. Appl. Phys. 2001, 89, 1968.
