Letter pubs.acs.org/NanoLett
Disappearance of the Superionic Phase Transition in Sub‑5 nm Silver Iodide Nanoparticles Takayuki Yamamoto,† Hirokazu Kobayashi,†,‡ Loku Singgappulige Rosantha Kumara,§ Osami Sakata,*,§ Kiyofumi Nitta,∥ Tomoya Uruga,∥ and Hiroshi Kitagawa*,†,⊥,# †
Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § Synchrotron X-ray Station at SPring-8, Research Network and Facility Services Division, National Institute for Materials Science (NIMS), 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan ∥ Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan ⊥ Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan # INAMORI Frontier Research Center, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ‡
S Supporting Information *
ABSTRACT: Bulk silver iodide (AgI) is known to show a phase transition from the poorly conducting β/γ-phases into the superionic conducting α-phase at 147 °C. Its transition temperature decreases with decreasing the size of AgI, and the α-phase exists stably at 37 °C in AgI nanoparticles with a diameter of 6.3 nm. In this Letter, we investigated the atomic configuration, the phase transition behavior, and the ionic conductivity of AgI nanoparticles with a diameter of 3.0 nm. The combination of pair distribution function (PDF) analysis and reverse Monte Carlo (RMC) modeling based on highenergy X-ray diffraction (XRD) revealed for the first time that they formed the β/γ-phases with atomic disorder. The results of extended X-ray absorption fine structure (EXAFS) analysis, differential scanning calorimetry (DSC), and AC impedance spectroscopy demonstrated that they did not exhibit the superionic phase transition and their ionic conductivity was lower than that of crystalline AgI. The disappearance of the superionic phase transition and low ionic conductivity in the very small AgI nanoparticles originates from their small size and disordered structure. KEYWORDS: Silver iodide, phase transition, pair distribution function analysis, reverse Monte Carlo modeling, EXAFS analysis
S
Pair distribution function (PDF) analysis7 has been a powerful tool to determine structures of poorly crystalline materials such as amorphous8,9 and even liquid materials.10,11 Recently, PDF analysis has also been applied to nanomaterials such as metal nanoparticles12,13 and quantum dots.14,15 Moreover, the combination of PDF analysis and reverse Monte Carlo (RMC) modeling16 allows us to determine atomic configurations in nanomaterial from the local structure data obtained by high-energy X-ray diffraction (XRD) or extended X-ray absorption fine structure (EXAFS). Although there are some reports on PDF/RMC analysis for singlecomponent metal nanoparticles such as ruthenium17,18 or gold,19,20 there are few reports on nanomaterials of bimetallic alloys21 or ionic crystals.22
ilver iodide (AgI) has been widely investigated from the viewpoints of both fundamental science and applications. Because of its similar crystal structure to ice, AgI has been used for cloud seeding.1 AgI is also a direct bandgap semiconductor, and its optical properties have been studied.2 Moreover, AgI undergoes a structural phase transition into the α-phase above 147 °C, which shows superionic conductivity due to a sublattice melting of silver ions.3 Recently, it has been revealed that the hysteresis of the transition temperatures in the heating and cooling processes becomes larger with decreasing particle size of AgI.4,5 It is reported that the disappearance temperature of the α-phase is 37 °C in AgI nanoparticles with a diameter of 6.3 nm, showing a large hysteresis of more than 100 °C,5 whereas the hysteresis is only 2.5 °C in bulk AgI.6 It is very interesting to investigate the structural phase-transition behavior of very small AgI nanoparticles of less than 5 nm. However, the structure of such small AgI nanoparticles has not been fully investigated due to broad diffraction patterns because of their small crystal size. © XXXX American Chemical Society
Received: April 12, 2017 Revised: August 9, 2017
A
DOI: 10.1021/acs.nanolett.7b01535 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters Here we report for the first time the atomic configuration of AgI nanoparticles with a diameter of 3 nm determined by means of PDF/RMC based on high-energy XRD and EXAFS analysis based on the RMC-generated model; the very small AgI nanoparticles formed β/γ-phases with atomic disorder, and the structural phase transition into the α-phase disappeared. The AgI nanoparticles used in this work were prepared using the low-temperature liquid-phase method as previously reported.23 The liquid-phase reaction of silver nitrate and sodium iodide in the presence of poly(N-vinyl-2-pyrrolidone) (PVP) as the protecting agent at −4 °C and the following centrifugation provided a yellow powder. From transmission electron microscopy (TEM), the mean diameter and size distribution of the synthesized AgI nanoparticles were estimated to be 3.0 ± 0.7 nm, as shown in Figure 1, which is the smallest particle size in AgI obtained as a solid state. The energy-dispersive X-ray analysis indicated that the ratio of Ag to I is 1:1 (Figure S1).
further investigations, we performed PDF analysis and RMC modeling based on the high-energy XRD patterns. A spherical particle consisting of 205 Ag atoms and 205 I atoms was used in the simulation box. Figure 2b shows the experimental and RMC-generated X-ray total structure factor S(Q), and Figure 2c shows the RMC-generated model. The experimental Fourier transformed total pair distribution function G(r) shown in Figure 2d indicates that the first and second nearest neighbor atoms are located at 2.8 and 4.8 Å, respectively. According to the RMC-generated partial pair correlation function g(r) shown in Figure 2e, the first nearest neighbor peak at r = 2.8 Å derives mainly from Ag−I and Ag−Ag correlations, and the contribution of I−I correlation is relatively small. On the other hand, these three kinds of correlations equally contribute to the second nearest neighbor peak at r = 4.8 Å. Considering the perfect crystalline β/γ-phases, only the Ag−I correlation with r = 2.81 Å should appear at the first nearest neighbor, and Ag−Ag and I−I correlations with r = 4.59 Å should appear at the second nearest neighbor. In the case of the α-phase, although I−I correlation will be located at r = 4.36 Å, Ag−I and Ag−Ag correlations are expected to have wide distributions because there are so many stable sites of silver ion in the crystal lattice (6 octahedral, 12 tetrahedral and 24 triangular sites).24 Here large contributions of Ag−I at 2.8 Å, Ag−Ag and I−I at 4.8 Å suggest that the structure of the AgI nanoparticles is similar to the β/γ-phases, and additional Ag−Ag and I−I correlations at the first nearest neighbor imply that the AgI nanoparticles contain atomic disorder. Such disorder is distributed in the whole range of the particle and reduces the crystallinity. In other words, the AgI nanoparticles form a disordered amorphous structure based on the β/γ-phases. We calculated the coordination number distributions in the AgI nanoparticles as shown in Figure 2f. The center of distribution was located at around two for all kinds of bonds, which is smaller than for bulk AgI with a coordination number of four for both Ag−I and I−Ag. The smaller coordination number in the AgI nanoparticles is considered to contribute to the larger number of surface atoms and/or amorphous component in nanosized materials.
Figure 1. (a) TEM image and (b) size distribution of the AgI nanoparticles.
Figure 2a shows a high-energy XRD intensity I(Q) of the AgI nanoparticles measured at beamline BL04B2 of SPring-8. The AgI nanoparticles are suggested to form the β/γ-phases by comparing the simulated patterns of the β-phase (wurtzite structure) and the γ-phase (zincblende structure), which are obtained as stable phases at room temperature in bulk AgI. For
Figure 2. (a) High-energy XRD intensity I(Q), (b) total structure factor S(Q), (c) RMC-generated model (chemical bonds with a length less than 3.5 Å are shown), (d) Fourier transformed total pair distribution function G(r), (e) RMC-generated partial pair correlation function g(r), and (f) coordination number distribution of the AgI nanoparticles. B
DOI: 10.1021/acs.nanolett.7b01535 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
and Table S1). The slight difference between two analyses is considered to originate from a phonon anharmonicity in AgI Moreover, the coordination numbers of Ag−I, Ag−Ag, and I− Ag are estimated to be 2.3, 2.2, and 1.8, respectively, at room temperature, also showing good agreement with the results of PDF/RMC. The bond lengths of Ag−I and Ag−Ag monotonically decreased with increasing temperature due to the thermal contraction as shown in Figure S4, suggesting no structural change up to 200 °C. The ionic conductivity of the AgI nanoparticles was investigated by AC impedance spectroscopy using a quasifour-probe method. Typical Nyquist plots are shown in Figure S5, and the temperature dependence of the ionic conductivity is plotted in Figure 5. In contrast to bulk AgI, whose ionic
We performed differential scanning calorimetry (DSC) with a scanning rate of 5 K/min to investigate the phase-transition behavior of the AgI nanoparticles. As shown in Figure 3, in bulk
Figure 3. DSC thermograms of the AgI nanoparticles (red), bulk AgI (black), and PVP (gray).
AgI, sharp peaks deriving from a phase transition between the β/γ- and α-phases were observed on both heating and cooling processes with a small hysteresis. In the case of the AgI nanoparticles, no phase transition of AgI was observed up to 240 °C. The broad peaks observed around 200 °C are derived from a glass transition of PVP (see Figure S2). To investigate the phase-transition behavior from a structural aspect, variable-temperature synchrotron X-ray absorption spectroscopy was performed at beamline BL01B1 of SPring-8. Figure 4 shows the EXAFS oscillations and Fourier transforms
Figure 5. Temperature dependence of the ionic conductivity of the AgI nanoparticles on the heating (red) and cooling (blue) cycles.
conductivity shows a discontinuous change at 150 °C due to the phase transition as shown in Figure S6, linear dependence and no hysteresis behavior on the heating and cooling cycles clearly demonstrated that the AgI nanoparticles do not show a phase transition below 260 °C. The results of DSC, EXAFS analysis, and AC impedance spectroscopy revealed that the AgI nanoparticles do not show any structural phase transition up to 260 °C, whereas bulk AgI shows a phase transition from the β/γ- to the α-phase at 147 °C. This unique behavior should originate from their small size and disordered amorphous structure. It is well-known that nanomaterials show unique phase-transition behavior due to their high surface-to-volume ratio and high surface energy.25,26 Moreover, amorphous materials also show different phase behavior from crystalline materials. It is reported that an amorphous thin film of LaAlO3 does not show a phase transition up to the crystallization temperature of 864 °C, although crystalline bulk LaAlO3 undergoes a structural phase transition from orthorhombic to cubic structure at 420 °C.27 Very recently, similar phase behavior was suggested by only DSC in 4 nm AgI nanoparticles,28 but the details are still unclear due to the poor structural characterization and measurement on ionic conductivity for the AgI nanoparticles. It should also be noted that the ionic conductivity is strongly affected by the crystallinity of the structure because the ionic conduction path is an important factor in ionic conduction. In the case of our AgI nanoparticles, the ionic conductivity is 8.5 × 10−9 S/cm at room temperature, which is much lower than that of bulk AgI (Figure S6) and crystalline AgI nanoparticles4 with 7.7 × 10−6 and 1.5 × 10−2 S/cm, respectively. This low ionic conductivity originates from their amorphous structure, that is,
Figure 4. (a) Ag K-edge and (b) I K-edge EXAFS oscillation of the AgI nanoparticles at room temperature (blue) and 200 °C (red). (c) Ag K- and (d) I K-edge radial distribution function of the AgI nanoparticles at room temperature (blue) and 200 °C (red).
of the AgI nanoparticles at room temperature and 200 °C. Both Ag K-edge and I K-edge spectra could be well fitted with the model generated by RMC modeling based on the high-energy XRD (Figure 2c). The bond lengths of Ag−I and Ag−Ag were estimated to be 2.77 and 3.02 Å, respectively, at room temperature from EXAFS analysis, which are generally consistent with the results of PDF/RMC analysis (Figure S3 C
DOI: 10.1021/acs.nanolett.7b01535 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
(6) Binner, J. G. P.; Dimitrakis, G.; Price, D. M.; Reading, M.; Vaidhyanathan, B. J. Therm. Anal. Calorim. 2006, 84, 409−412. (7) Toby, B. H.; Egami, T. Acta Crystallogr., Sect. A: Found. Crystallogr. 1992, A48, 336−346. (8) Cargill, G. S., III Solid State Phys. 1975, 30, 227−320. (9) Fujiwara, T.; Ishii, Y. J. Phys. F: Met. Phys. 1980, 10, 1901−1911. (10) Yarnell, J. L.; Katz, M. J.; Wenzel, R. G.; Koenig, S. H. Phys. Rev. A: At., Mol., Opt. Phys. 1973, 7, 2130−2144. (11) Posada-Amarillas, A.; Garzon, I. L. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 53, 8363−8368. (12) Oxford, S. M.; Lee, P. L.; Chupas, P. J.; Chapman, K. W.; Kung, M. C.; Kung, H. H. J. Phys. Chem. C 2010, 114, 17085−17091. (13) Ma, R.; Levard, C.; Marinakos, S. M.; Cheng, Y.; Liu, J.; Michel, F. M.; Brown, G. E., Jr.; Lowry, G. V. Environ. Sci. Technol. 2012, 46, 752−759. (14) Petkov, V.; Gateshki, M.; Choi, J.; Gillan, E. G.; Ren, Y. J. Mater. Chem. 2005, 15, 4654−4659. (15) Masadeh, A. S.; Bozin, E. S.; Farrow, C. L.; Paglia, G.; Juhas, P.; Billinge, S. J. L.; Karkamkar, A.; Kanatzidis, M. G. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 115413. (16) McGreevy, R. L.; Pusztai, L. Mol. Simul. 1988, 1, 359−367. (17) Gereben, O.; Petkov, V. J. Phys.: Condens. Matter 2013, 25, 454211. (18) Kumara, L. S. R.; Sakata, O.; Kohara, S.; Yang, A.; Song, C.; Kusada, K.; Kobayashi, H.; Kitagawa, H. Phys. Chem. Chem. Phys. 2016, 18, 30622−30629. (19) Petkov, V.; Ren, Y.; Shan, S.; Luo, J.; Zhong, C. J. Nanoscale 2014, 6, 532−538. (20) Bedford, N. M.; Hughes, Z. E.; Tang, Z.; Li, Y.; Briggs, B. D.; Ren, Y.; Swihart, M. T.; Petkov, V. G.; Naik, R. R.; Knecht, M. R.; Walsh, T. R. J. Am. Chem. Soc. 2016, 138, 540−548. (21) Petkov, V.; Wanjala, B. N.; Loukrakpam, R.; Luo, J.; Yang, L.; Zhong, C. J.; Shastri, S. Nano Lett. 2012, 12, 4289−4299. (22) Kompch, A.; Sahu, A.; Notthoff, C.; Ott, F.; Norris, D. J.; Winterer, M. J. Phys. Chem. C 2015, 119, 18762−18772. (23) Yamamoto, T.; Kobayashi, H.; Kitagawa, H. Chem. Lett. 2014, 43, 1355−1356. (24) Nield, V. M.; Keen, D. A.; Hayes, W.; McGreevy, R. L. Solid State Ionics 1993, 66, 247−258. (25) Buffat, P.; Borel, J.-P. Phys. Rev. A: At., Mol., Opt. Phys. 1976, 13, 2287−2298. (26) Ohkoshi, S.; Tsunobuchi, Y.; Matsuda, T.; Hashimoto, K.; Namai, A.; Hakoe, F.; Tokoro, H. Nat. Chem. 2010, 2, 539−545. (27) Lu, X.; Liu, Z.; Wang, Y.; Yang, Y.; Wang, X.; Zhou, H.; Nguyen, B. J. Appl. Phys. 2003, 94, 1229−1234. (28) Xu, B.; Wang, X. Small 2011, 7, 3439−3444. (29) Minami, K.; Hayashi, A.; Tatsumisago, M. J. Ceram. Soc. Jpn. 2010, 118, 305−308.
the ionic conduction path may be blocked by the disordered amorphous structure. It is reported that the superionicconducting crystalline Li7P3S11 can be obtained by annealing the glass state with low ionic conductivity.29 In summary, we have first investigated the atomic configuration of very small AgI nanoparticles with a diameter of 3.0 ± 0.7 nm using the PDF/RMC method based on highenergy XRD. Their amorphous structure based on the β/γphases was maintained during heating up to 260 °C in contrast to crystalline bulk AgI, which shows a structural phase transition at 147 °C. The ionic conductivity of the AgI nanoparticles measured by AC impedance spectroscopy was lower than that of crystalline AgI. The disappearance of the superionic phase transition and the low ionic conductivity in the AgI nanoparticles originate from their small particle size and amorphous structure. We hope that the new findings in this report will contribute to the clarification of the origin associated with the size effect on the phase-transition behaviors.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b01535. Experimental details, energy-dispersive X-ray spectroscopy (EDX), phase transition of poly(N-vinyl-2-pyrrolidone) (PVP), comparison of pair distribution function (PDF)/reverse Monte Carlo (RMC) analysis with extended X-ray absorption fine structure (EXAFS) analysis, temperature dependence of parameters determined by EXAFS analysis, and ionic conductivity (PDF)
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Loku Singgappulige Rosantha Kumara: 0000-0001-9160-6590 Osami Sakata: 0000-0003-2626-0161 Hiroshi Kitagawa: 0000-0001-6955-3015 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by Core Research for Evolutional Science and Technology (CREST) and ACCEL from the Japan Science and Technology Agency (JST) and Grants-in-Aid for JSPS Fellows (27-1603) from the Japan Society for the Promotion of Science (JSPS). Synchrotron XRD measurements were supported by the Japan Synchrotron Radiation Research Institute (JASRI) (proposal no. 2014A1321 and 2014B1554).
■
REFERENCES
(1) Marcolli, C.; Nagare, B.; Welti, A.; Lohmann, U. Atmos. Chem. Phys. 2016, 16, 8915−8937. (2) Cardona, M. Phys. Rev. 1963, 129, 69−78. (3) Tubandt, C.; Lorenz, F. Z. Phys. Chem. 1914, 87, 513−542. (4) Makiura, R.; Yonemura, T.; Yamada, T.; Yamauchi, M.; Ikeda, R.; Kitagawa, H.; Kato, K.; Takata, M. Nat. Mater. 2009, 8, 476−480. (5) Yamasaki, S.; Yamada, T.; Kobayashi, H.; Kitagawa, H. Chem. Asian J. 2013, 8, 73−75. D
DOI: 10.1021/acs.nanolett.7b01535 Nano Lett. XXXX, XXX, XXX−XXX
