Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Reciprocating Compression of ZnO Probed by X‑ray Diffraction: The Size Effect on Structural Properties under High Pressure Qiming Wang,†,‡ Shourui Li,† Qiang He,† Wenjun Zhu,*,† Duanwei He,‡ Fang Peng,‡ Li Lei,‡ Leilei Zhang,‡ Qiang Zhang,‡ Lijie Tan,‡ Xin Li,‡ and Xiaodong Li§ †
National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, Chinese Academy of Engineering Physics, Mianyang 621900, China ‡ Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China § Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: Zinc oxide, ZnO, an important technologically relevant binary compound, was investigated by reciprocating compress the sample in a diamond anvil cell using in situ high-pressure synchrotron X-ray diffraction at room temperature. The starting sample (∼200 nm) was compressed to 20 GPa and then decompressed to ambient condition. The quenched sample, with average grain size ∼10 nm, was recompressed to 20 GPa and then released to ambient condition. The structural stability and compressibility of the initial bulk ZnO and quenched nano ZnO were compared. Results reveal that the grain size and the fractional cell distortion have little effect on the structural stability of ZnO. The bulk modulus of the B4 (hexagonal wurtzites structure) and B1 (cubic rock salt structure) phases for bulk ZnO under hydrostatic compression were estimated as 164(3) and 201(2) GPa, respectively. Importantly, the effect of pressure in atomic positions, bond distances, and bond angles was obtained. On the basis of this information, the B4-to-B1 phase transformation was demonstrated to follow the hexagonal path rather than the tetragonal path. For the first time, the detail of the intermediate hexagonal ZnO, revealing the B4-to-B1 transition mechanism, was detected by experimental method. These findings enrich our knowledge on the diversity of the size influences on the high-pressure behaviors of materials and offer new insights into the mechanism of the B4-to-B1 phase transition that is commonly observed in many other wurzite semiconductor compounds.
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INTRODUCTION Zinc oxide (ZnO) crystallizes in the wurzite (hexagonal, B4, space group P63mc) phase at ambient conditions, is one of the most important inorganic compounds, and has numerous technological applications in the fields of electronics, optoelectronics, catalysis, chemical sensors, and conductive solar cell window layers. As a promising semiconductor and a typical mineral, high-pressure experimental data on ZnO are fundamental to condensed matter physics and geophysical physics. Following the first report of the structural transition of ZnO under high pressure by Bates et al. in 1962,1 the highpressure behaviors of ZnO, including its structural stability, compressibility, and transformation mechanism, have been extensively investigated from both experimental2−7 and theoretical8−12 points of view. Exploring the unique properties of nanomaterials has been a long-standing topic of interest for materials science and engineering.13−15 Semiconductor nanocrystals are novel materials with properties being controlled by size and shape. In that view, the study of size effects16 and morphology effects17−21 on the properties of ZnO under high pressure provides a basis for probing the potential physical and © XXXX American Chemical Society
chemical mechanisms that command the structure−property relationships. However, to our knowledge, the available studies about the high-pressure behavior of ZnO are still inconclusive and controversial. The first point is the large difference of the bulk modulus (K0) of the two phases as shown in Table 1, ranging from 135 to 183 GPa for the B4 phase and from 177 to 229 GPa for the B1 phase (cubic, rock salt, space group Fm3m). In addition, while the transition pressures of the forward and backward phase transitions for nanocrystalline ZnO have been studied,16 little is currently known about its equation of state (EOS).22 Moreover, the B4-to-B1 phase transformation mechanism has not been fully understood. On the basis of the first-principles calculations, the wurtzite structure can continuously transform to the rock salt structure following the “hexagonal” (i-H) path or the “tetragonal” (i-T) path. Through theoretical computation, Saitta et al. indicated that ZnO transforms from B4 phase to B1 phase along the tetragonal Received: February 7, 2018
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DOI: 10.1021/acs.inorgchem.8b00357 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 1. Summary of the Main Experimental Results of High-Pressure Studies of ZnO B4 to B1 transition pressure (GPa)
size morphology bulk bulk bulk bulk bulk bulk bulk bulk bulk bulk bulk bulk nanocrystalline nanocrystalline nanocrystalline nanodots nanodots nanowires nanoflowers nanotube nanorods nanorods nanobelts
diameter (d)
0.2 μm
200 nm 200 nm 10 nm 50 nm 12 nm 4.5 nm 7.3 nm 50−100 nm 10−70 nm 150 nm 10 nm 10−30 nm
lengtha (l) / / / / / / / / / / / / / / / / / 3−4 μm / bulk 12 μm 80 nm /
Pib
Pcc
9.0 8.8 11.1 9.3 9.0 8.9 7.0 9.3 9.3
11.0 15.5
9.2 10.7 9.4 10.5 100 nm) ZnO, the first-cycle quenched (denoted by Q1) and second-cycle quenched (denoted by Q2) nano ZnO samples were shown in Figure 1. High-pressure experiments were performed using symmetric diamond anvil cell (DAC) with diamond culets of 300 μm. The sample was loaded into a 120 μm diameter hole of tungsten (W) gasket. In the main experiments of this study, no pressure transmitting medium (PTM) was used, and a very small ruby ball30 was placed in the central position of the sample chamber. Both the starting bulk sample and the quenched sample were compressed to ∼20 GPa and then decompressed to atmospheric pressure. A parallel experiment was performed to acquire the Q1 for TEM test. In the compared
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RESULTS AND DISCUSSION In Figure 3a, we show the variations of the angle-dispersive synchrotron XRD patterns of the bulk ZnO to ∼21 GPa under C
DOI: 10.1021/acs.inorgchem.8b00357 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
between 10.1 and 10.7 GPa, and the completion pressure Pc lies between 14.6 and 15.9 GPa. When the external pressure was released, the diffraction pattern representing the B4 phase reappears at P < 3 GPa, and a fraction of the B1 phase is retained when the pressure is totally released. At 0.2 GPa, the Rietveld refinement shows that the fractions for B4 phase and B1 phase are 94.7% and 5.3%, respectively. As mentioned in the Introduction, high pressure has been demonstrated to be an effective method for the synthesis of new materials and a powerful approach to regulating the morphology of materials. In this study, ZnO nanocrystalline (Q1) with diameters of 8−12 nm were synthesized by high-pressure treatment of bulk counterpart at room temperature, as shown in Figure 1b. The lattice parameters of B4 phase for Q1 are a = 3.2535(6) Å and c = 5.1853(15) Å at 0.2 GPa. Notice that this unit cell is a little distorted, squashed along c axis, with c/a = 1.5937 after decompression compared with the starting one with c/a = 1.6020. To directly acquire the structural stability and compressibility of nanocrystalline ZnO, XRD measurements were performed on the quenched (Q1) sample on compression up to 20 GPa followed by decompression. Figure 3b depicted the selected XRD patterns versus pressure. The peak intensity for B4 phase gradually decreased with increasing pressure. As the reflections for B4 phase have nearly disappeared at the pressure above 14.1 GPa, the intensity of (200) B1 phase increased at P > 9.4 GPa. During decompression, the B4 phase was observed at 1.0 GPa. Both phases are discerned when the pressure is completely released in the cell chamber, with the fractions for B4 phase and B1 phase being 95.5% and 4.5%, respectively. Previous studies, as summarized in Table 1, demonstrate that the initial transition pressure of bulk ZnO lies between 7.0 and 11.1 GPa, and the completion pressure lies between 11.0 and 15.5 GPa. Then, on the one side, the Pi of 50 nm ZnO nanocrystalline,22 nanotube,18 nanobelts,19 and nanorods17 are all identical to that of bulk polycrystalline ZnO, whereas the Pi of 12 nm ZnO,16 nanodots,28 nanowires,20 and nanoflowers29 are enhanced compared to that of bulk ZnO. On the other side, the Pc of nanotube and nanorods are higher than that of bulk ZnO.3 The elevated completion pressure Pc is perhaps attributable to the pressure-induced morphology tuning effect as explained by Hou et al.18 There is a premise for this explanation, that is, the smaller the size (dimensions in any direction), the higher the transition pressure. As elaborated by Jiang et al., the variation of the transition pressure in nanocrystals may arise from three factors: the surface energy difference, the ratio of the volume collapses, and the internal energy difference. And they inferred that the enhancement of transition pressure in 12 nm ZnO nanocrystals as compared with the corresponding bulk material is mainly caused by the surface energy difference between the phase involved.16 However, the way they judge the Pi and Pc is different from most reported researches. The transition pressure Pi is determined based on the pressure evolution of the intensity ratio of the 200 (B1) to 100 (B4) peaks (I200(B1)/I100(B4)). It is difficult to identify the phase transition when the intensity for 200 (B1) peak is less or equal than that for 100 (B4) peak. Actually, the XRD in the literature indicated the coexistence of the B4 and B1 phases at 13.2 GPa, and the I200(B1)/I100(B4) is ∼2. Then the transition pressure is less than 13.2 GPa. Moreover, in view of the differences in the experiment and sample details for different studies as shown in Table 1, it is still
Table 4. Bond Lengths and Bond Angles of ZnO at Various Pressures bulk ZnO pressure (GPa) Zn−O−Zn (deg) Zn−O D1 (Å) Zn−O D2 (Å)
0.3 109.5 (1) 1.936 (4) 1.987 (2)
12.2 97.9 (1) 2.300 (5) 1.859 (1)
nano ZnO 0.2 115.9 (1) 1.680 (4) 2.085 (2)
12.3 93.7 (2) 2.667 (8) 1.858 (1)
Figure 2. Typical Rietveld refinement by GSAS of bulk ZnO under nonhydrostatic compression at (a) 7.3, (b) 11.1, and (c) 17.4 GPa. (○) Experimental data points. (red lines on data) Refinements. (blue lines) Residue between the observed and calculated patterns. (short vertical lines) Peaks of corresponding samples. (insets) Twodimensional diffraction patterns).
nonhydrostatic compression at room temperature. The unit cell parameters of the B4 phase ZnO at ambient pressure are a = 3.2467(1) Å and c = 5.2012(2) Å, which are in good agreement with the data reported previously.8 While the hexagonal wurtzite structure persisted to 14.6 GPa, the (200) reflection of the cubic rock salt phase was discerned at pressures beyond 10.7 GPa. The two phases (B4 and B1) coexisted in the pressure range from 10.7 to 14.6 GPa. Above 15.9 GPa, only the rock salt B1 phase could be identified. The rock salt phase was proved to be stable up to 202 GPa before the occurrence of another phase transition.5 Hence, no further compression was performed. Then, the initial pressure of phase transition Pi lies D
DOI: 10.1021/acs.inorgchem.8b00357 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 3. Selected XRD patterns of starting bulk ZnO (a) and quenched nano ZnO (b) up to ∼20 GPa without PTM.
Figure 4. (a) Pressure dependence of volume per formula unit for ZnO without PTM. (blue) starting bulk and (red) quenched nano ZnO. Filled and open symbols represent data for compression and decompression, respectively. (b) Pressure dependence of volume per formula unit for ZnO with argon as PTM. Error bars are smaller than symbol sizes.
hard to draw a conclusion that the grain-size reduction enhanced the structure stability for nanocrystalline ZnO under compression. In this study, the high-pressure experiments for the bulk ZnO and nano ZnO were conducted in the same way, and the grain sizes for the two initial samples were all detected by TEM. As shown in Table 1 and Figure 1, the phase transformation pressure for nano ZnO and bulk ZnO lies in the pressure range of that for bulk ZnO obtained by previous works (7.0−11.1 GPa). Therefore, we deduce that the grain-size reduction has little effect on the structural stability of ZnO. In
future work, more attention should be paid to the essential microstructure information that relates to the morphology and grain size,33 such as the lattice distortion, defect and so on, to investigate the morphology or size effect on the high-pressure properties for ZnO. Figure 4a displays the pressure-induced changes of the volume per formula unit for the starting bulk ZnO and quenched nano ZnO under compression and decompression. For the bulk ZnO, a slight break in the compression curve for both B4 and B1 phases is observed when the B4-to-B1 E
DOI: 10.1021/acs.inorgchem.8b00357 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 5. (a) Pressure evolution of c/a ratio for rock salt B4 phase. The data for nanotube are derived from ref 18. (b) The pressure evolution of the internal structural parameter u of wurzite structure ZnO. Data for (blue) starting bulk and (red) quenched nano ZnO.
transition is starting at ∼10.7 GPa and ending at ∼15.9 GPa. Then, the bulk modulus was estimated by fitting the P−V data to the third-order Birch−Murnaghan equation of state (BMEOS) before 10.7 GPa and after15.9 GPa for B4 and B1 phase, respectively. The bulk modulus K0 for B4 phase is calculated to be 180(13) GPa with K0′ = 5.4(18) and V0 = 23.87(2) Å3, where V0 is the unit cell volume at ambient pressure. To facilitate the comparison with previous studies with K0′ is fixed to 4.0 as shown in Table 1; we also adopt K0′ = 4.0, and the bulk modulus K0 obtained is 187(3) GPa with V0 = 23.87(1) Å3/f.u. For B1 phase, very few data points preclude us from performing BM-EOS analysis with K0′ unfixed. Then, the bulk modulus K0 of B1 phase is estimated to be 227(1) GPa with K0′ = 4 (fixed) and V0 = 19.70(4) Å3/f.u. For nano ZnO, however, the compression curve is irregular and does not permit reliable fitting before the B4-to-B1 transition. Since the nano Q1 ZnO possesses a dual-phase composite structure, which the fraction for B4 phase and B1 phase is 94.7% and 5.3%, the unusual compression behavior for 10 nm ZnO probably due to the nano arching effect.15,34 At pressure above the phase transition, a BM-EOS fit to the nano B1-phase ZnO data yields a zero-pressure bulk modulus K0 = 223(2) GPa with K0′ = 4(fixed) and V0 = 19.63(4) Å3/f.u. This value is close to that of the bulk ZnO in this study. The value of K0 under non-hydrostatic compression conditions will be overestimated compared with that under hydrostatic or better quasi-hydrostatic conditions. However, it is still meaningful to compare the relative values of K0 between nano and bulk samples, because they underwent almost the same compression conditions.19 Therefore, in spite of the apparent grain size difference in the initial samples, their compression curves for B1 phase at P > 10 GPa almost coincided with each other. This result may be due to the grain sizes for the resultant B1 phase after the phase transition are all in nanoscale ( 10 GPa, both the angle and the Zn−O distance D2 decrease with increasing pressure, leading the cell parameter a to contract. At the same time, the Zn−O G
DOI: 10.1021/acs.inorgchem.8b00357 Inorg. Chem. XXXX, XXX, XXX−XXX
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time to obtain the detailed information for the B4-to-B1 transition path by experiment.
CONCLUSIONS In conclusion, we have investigated the high-pressure behavior of ZnO by reciprocating compressed the sample in the diamond anvil cell using in situ synchrotron XRD at room temperature. The starting bulk sample was compressed to 20 GPa and then decompressed to atmospheric pressure. The quenched sample (Q1), with average grain size of ∼10 nm, was recompressed up to 20 GPa and then released to room condition. The structure stability and compressibility for the starting bulk ZnO and quenched nano ZnO have been compared. Results reveal that the grain size and the subtle cell distortion have little effect on the structure stability of ZnO. And their compression curve for the B1 phase at P > 10 GPa was almost coincided with each other. However, for Q1, the compressibilities for B4 and B1 phases at P < 10 GPa were unusual resulting from the dual-phase structure. The bulk modulus of the B4 and B1 phases for bulk ZnO under hydrostatic compression were estimated as 164(3) GPa and 201(2) GPa, respectively. The B4-to-B1 transition path for the bulk ZnO and the nano ZnO prefer the hexagonal model, as is evidenced by the cell distortions under high pressure. These results are of fundamental importance to understand the mechanism of the B4-to-B1 phase transition that frequently occurs in many other wurzite semiconductor compounds. ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00357. The grain size of ZnO versus pressure (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Wenjun Zhu: 0000-0002-8001-6605 Author Contributions
Q.M.W. and S.R.L. conducted this research work in collaboration with W.J.Z., D.W.H., F.P., and L.L.; Q.H. performed part of the data processing. L.L.Z., Q.Z., L.J.T., and X.L. performed the synchrotron X-ray experiments at high pressures. X.D.L. was responsible for the beamline 4W2 of the Beijing Synchrotron Radiation Facility. All authors reviewed the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the China Postdoctoral Science Foundation (Grant No. 2015M572645XB), the National Natural Science Foundation of China (Grant No. 11504353), and the joint fund of the National Natural Science Foundation of China and Chinese Academy of Sciences Fund (Grant No. U1332104). H
DOI: 10.1021/acs.inorgchem.8b00357 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.8b00357 Inorg. Chem. XXXX, XXX, XXX−XXX
