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silica composite
Journal of Physics and Chemistry of Solids 66 (2005) 21–30 www.elsevier.com/locate/jpcs
Phase transition behavior and proton conduction mechanism in cesium hydrogen sulfate/silica composite Junichiro Otomoa,*, Hitoshi Shigeokab, Hidetoshi Nagamotoa, Hiroshi Takahashib a
Department of Environmental Chemical Engineering, Faculty of Engineering, Kogakuin University, Nakanomachi 2665-1, Hachioji, Tokyo 192-0015, Japan b Department of Chemical System Engineering, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan Received 15 January 2004; revised 30 March 2004; accepted 30 July 2004
Abstract Proton conduction and crystal structure in CsHSO4/SiO2 composite composed of polycrystalline CsHSO4 and mesoporous silica particles were investigated based on conductivity measurement and characterizations using Raman spectroscopy, XRD, and differential thermal analysis. The conductivity of pure CsHSO4 abruptly changes at around 414 K (superprotonic phase transition), being accompanied with the structural transformation from a monoclinic phase to a tetragonal phase, while the conductivity of CsHSO4/SiO2 composite is significantly larger by over three orders of magnitude than that of pure CsHSO4 below the critical temperature of the superprotonic phase transition (353– 414 K). Raman spectroscopy and XRD indicate that this remarkable conductivity-enhancement in the composite is not due to the stabilization of the tetragonal phase (superprotonic phase) below its critical temperature. The line-broadening of the internal modes in the Raman spectra suggests that the rapid reorientational motion of the HSOK 4 ion, which leads to superprotonic conduction, is induced in the composite even below the critical temperature. The reorientational motion of the HSOK 4 ion below the critical temperature will occur at the interfacial phase which is structurally disordered and forms between CsHSO4 and SiO2 in the mesopores and/or on the surfaces of silica particles. Proton transfer will be accelerated via the interfacial conduction-pathway in the composite. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Inorganic compounds; C. Raman spectroscopy; C. X-ray diffraction; D. Electrical conductivity; D. Phase transitions
1. Introduction Cesium hydrogen sulfate (CsHSO4) is one of the proton conductors having hydrogen bonds [1]. CsHSO4 shows a superprotonic phase transition, which means that its conductivity steeply increases by four orders of magnitude at 414 K and reaches 10K2 S/cm (superprotonic phase) [1]. The superprotonic phase transition of CsHSO4 occurs with a structural transformation from a monoclinic phase to a tetragonal phase [2]. In the superprotonic phase, it is considered that the reorientational motion of a SO4 tetrahedron, which induces a disordering of the hydrogen bond network, facilitates proton transfer [3]. On the other hand, heterogeneous doping by the dispersion of oxide particles in ionic salt has been
* Corresponding author. Tel./fax: C81 426 28 4523. E-mail address: [email protected] (J. Otomo). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.07.006
extensively studied as an effective approach to enhance ionic conductivity. Heterogeneous systems that consist of ionic conductors and chemically inert insulators such as Al2O3, SiO2, and ZrO2, etc. involve interface interaction between ionic salts and oxide particles. In the heterogeneous ionic conduction systems, interfaces play an important role in ionic conductivity-enhancement. As is well known, AgC ionic conductor-based composites have been intensively studied, e.g. AgI/A (AZAl2O3 [4–7], SiO2 [7], ZrO2 [8]), AgCl/Al2O3, [6,9] AgBr/Al2O3 [10]. The remarkable increase in their conductivities has been explained by an increase in the point defect concentration within the interface region. The formation of defects at the interface can promote ionic conduction via interface regions, and the conduction at the interface governs mainly the conductivity of composites [11,12]. Thus, a variation of structure among bulky ionic salt, the interface, and the insulator characterizes heterogeneous systems. Several ionic conduction models have been proposed,
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i.e. a space-charge model based on the generation of point defects at interfaces [9,13,14], defect-induced order– disorder phase transition (or defect-induced sublattice melting) [15–17], and formation of a structurally disordered phase [18,19] that may induce defect formation. In the cases of proton conductive composite materials that consist of solid acids and silica particles such as CsHSO4/SiO2 and CsH2PO4/SiO2, significant enhancements of their conductivities have been observed [20–23]. In those composite systems, the characteristic phenomenon is the significant increase of the conductivity below the critical temperature of the superprotonic phase transition [20,23]. As for CsHSO4/SiO2 composite, Ponomareva et al. [21] have reported that the conductivity-enhancement is strongly influenced by silica properties such as molar ratio, surface area, and pore size. They have also suggested that a structurally disordered interface such as an amorphous phase forms between CsHSO4 and SiO2 in the composite [20,21]. Thus, the conductivities of composites composed of solid acid and silica will be affected by the physical and chemical characteristics of their bulk and interfaces. In the previous reports, however, observation of the crystal structures of those composites was limited, and no definite conclusions have been obtained [24]. In the present study, we focus on the protonic conduction of CsHSO4/SiO2 composite, and discuss its protonic conduction mechanism in terms of in situ measurements for the structure and dynamics in the composite as a function of temperature using Raman spectroscopy, X-ray diffraction, and differential thermal analysis. In our preliminary experiment, CsHSO4/SiO2 composite in which silica particles were highly dispersed was successfully synthesized with an evaporation-to-dryness method [22], and its conductivity was further improved in comparison with the results of previous reports [20,21]. By using this sample, the structural transformation of CsHSO4, phase stability, and the reorientational motion of a SO4 tetrahedron as a function of temperature are discussed, which will clarify the protonic conduction in the CsHSO4/SiO2 composite.
pellet was about 0.95 after having calcined at 473 K. CHS/SiO2 composite was prepared by the evaporation-todryness method as described below. First, an aqueous suspension containing equimolar amounts of Cs2SO4 and H2SO4, and a relevant amount of mesoporous amorphous silica particles was prepared. The mesoporous amorphous silica particles were provided by Mitsubishi Chemical. The properties of the silica particle are described as follows according to the information from Mitsubishi Chemical: average particle size, 5.1 mm, specific surface area, 297 m2/g, average pore volume, 1.3 ml/g, and average pore size, 16 nm. The suspension was then stirred constantly for 120 min under a reduced pressure condition in order to remove the air in the mesopores of the silica particles and to allow the solution to easily infiltrate into the mesopores. The suspension was evaporated at 333 K, and then dried at 363 K for 24 h. After additional heating at 443 K for 1 h in air in order to minimize the influence of adsorbed water, the resultant sample was mechanically ground for 30 min, and pressed at 3 t/cm2 to form pellets. Finally, the pellet of CHS/SiO2 composite was calcined at 473 K for 1 h in air as in the case of pure CHS. The molar ratio of CHS and SiO2 was 1:1, which corresponded to approximately 30% of the volume ratio for silica in the composite. The relative density of CHS/SiO2 composite pellet was about 0.93 after having calcined at 473 K. 2.2. Proton conductivity measurement Conductivity measurements were performed by means of ac impedance spectroscopy using a Hewlett–Packard 4192A impedance analyzer in the frequency range from 10 MHz to 10 Hz with an applied voltage of 50 or 500 mV. Gold electrodes were applied on the both sides of the pellets using gold paste after gold-deposition. The sample was pinched between gold meshes that were connected to the ac impedance analyzer with gold wires, and it was set in a glass tube. Data were collected in both heating and cooling processes, changing the temperature stepwise between 353 and 463 K under a dry atmospheric Ar flow-condition. 2.3. Raman spectroscopic measurement
2. Experimental 2.1. Sample preparations Polycrystalline CsHSO4 (designated as CHS hereafter) was obtained from an aqueous solution containing equimolar amounts of Cs2SO4 (Soekawa Chemical, purityO 99%) and H2SO4 (Wako Pure Chemical). The solution was evaporated, and then the resultant CHS residue was dried at 363 K for 24 h. Powdered CHS sample was prepared by grinding in an agate mortar for 30 min, and pressed at 3 t/cm2 to form dense pellets (diameter: 10 mm, thickness ca. 1.5 mm). The resultant pellet of pure CHS was then calcined at 473 K for 1 h in air. The relative density of CHS
The pellet-samples were set in a sealed glass cell. The cell was introduced into a furnace in order to observe their in situ Raman spectrum as a function of temperature that was changed stepwise between 293 and 446 K. After the sample was maintained for 10 min at each temperature, spectroscopic measurement was performed in order to collect data in equilibrium. A continuous wave ArC ion laser beam at 488 nm of 0.5 W was used as the exciting line, which was focused into the sample through a glass window using a 2.0 cm focal length lens. The backscattered light was led inversely through the lens to a spectrometer (JASCO NRS2000) and detected by a CCD detector. The data collection at each temperature was carried out at several random
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positions on the pellet so as to verify the reproducibility. The wavelength was calibrated using Hg lines in a fluorescent lamp within a precision of 2 cmK1, and the typical bandwidth was 4 cmK1. 2.4. Characterizations The X-ray diffraction (XRD) patterns of powdered samples of CHS and its silica composite were investigated from room temperature to 433 K in air. XRD was carried out using MAC Science MXP18 employing a Cu Ka source operating typically at 40 kV and 300 mA. The XRD patterns were measured at a scan rate of 0.1 degree/min with an angular resolution of 0.0068. The peak profiles were obtained from Cu Ka line-corrected data. As well, thermogravimetric measurement (TG) and differential thermal analysis (DTA) were performed using a Rigaku TAS200-TG8110D from room temperature up to 513 K in a dry Ar flow.
3. Results and discussions 3.1. Conductivity measurements in pure CHS and CHS/SiO2 composite The proton conductivities, s, of polycrystalline CsHSO4 (CHS) and its silica composite (molar ratio, CHS: SiO2Z 1:1) were measured stepwise between 353 and 463 K in heating–cooling cycles under a dry-Ar condition. Each sample was kept for 30 min at each temperature so as to be in equilibrium, and the ac impedance measurement was then carried out. The s values were calculated according to the procedure explained in our previous reports [22,23]. The results of natural logarithmic function, ln(sT), versus temperature, T, in the heating–cooling cycles are shown in Fig. 1. The results regarding pure CHS showed that the conductivity suddenly jumped up by four orders of magnitude at around 413–423 K (superprotonic transition), and it changed reversibly during the heating–cooling cycle. It has been known that the abrupt change of conductivity in CHS from a low conduction phase to a high conduction one is closely related to a structural phase transition in CHS [1,2]. Baranov et al. reported that such superprotonic phase transition in CHS occurred reversibly at 414 K, being accompanied with the structural phase transition from the monoclinic phase to the tetragonal phase [1,2]. In the case of CHS/SiO2 composite, a significant improvement in conductivity was observed, especially, at the low conduction phase. At temperatures below the surperprotonic transition of CHS (353–414 K), the conductivity of the composite was larger by over three orders of magnitude than that of pure CHS. Apparently, the low conduction phase disappeared, and the high conduction phase was stabilized. This conduction behavior was observed not only in the heating process but also in the cooling one, and it was confirmed to
Fig. 1. Temperature dependences of the conductivities of pure CHS and CHS/SiO2 composite. Filled symbols: heating process; open symbols: cooling process. C and B: pure CHS; : and 6: CHS/SiO2 composite.
be reproducible. This remarkable conductivity-enhancement is evidently due to the presence of silica particles. Next, we will show Raman spectra, XRD patterns, and DTA curves concerning CHS/SiO2 composite in order to clarify the relationship of the conductivity-enhancement and the crystal structure of the CHS/SiO2 composite. 3.2. Observations of structural phase transitions of pure CHS and CHS/SiO2 composite 3.2.1. Raman spectroscopy The Raman spectra of the internal modes of polycrystalline CsHSO4 (CHS) and its silica composite (molar ratio, CHS:SiO2Z1:1) were recorded between 270 and 1340 cmK1 at various temperatures. Fig. 2 shows the data collected in the heating process. In the cases of both pure CHS and its silica composite, stretching modes between 800 and 1300 cmK1, and bending modes between 400 and 600 cmK1 were observed, and each line shifted and broadened with an increase in temperature. We will discuss the line-shift and line-broadening in detail later in this report. The Raman spectroscopy of pure CHS has been well studied, and thus each band was well assigned [25–27]. The observed Raman bands at selected temperatures (293, 369, and 427 K) and their assignments according to the previous studies [25–27] are summarized in Table 1. The observed spectra were consistent with the previous studies [25–27]. As well, the crystal phase of CHS at each temperature can be identified by line positions. The assignment of pure CHS indicates that different phases exist at the three temperatures, i.e. phase I (monoclinic, space group: P21/c [28]) at room temperature (293 K), phase II (monoclinic, space group: P21/c) at 369 K, and phase III (tetragonal, space group: I41/amd) at 427 K, which is also consistent with
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Fig. 2. Raman spectra for pure CHS and CHS/SiO2 composite as a function of temperature between 293 and 427 K. (a) Polycrystalline CHS pellet sample, (b) CHS/SiO2 composite pellet sample.
the previous reports [25–27] (we employed the notation concerning the crystal phase in CHS by Pham-Thi et al. [25]. Additionally, the crystal structure of phase I has been corrected: monoclinic, space group: P21/m [29]/monoclinic, space group: P21/c [28]). As for CHS/SiO 2 composite, each line and crystal phase were also well identified according to the assignment of pure CHS. The crystal phases of CHS in the composite at 369 and 427 K were phase II and phase III, respectively, which were the same as those in pure CHS. At room temperature (293 K), however, the crystal phase of CHS in the composite was phase II, which was different that of pure CHS. The Raman spectra were again recorded at room temperature following the data-collection in the heating process. Fig. 3a and b show the Raman spectra of pure CHS and CHS/SiO2 composite at room temperature, respectively, which were recorded with the passage of time. Immediately after cooling to room temperature, pure CHS as well as CHS/SiO2 composite maintained phase II (Fig. 3a1 and b1). As shown in Fig. 3a, it took about 2 days to make a transition from phase II to I in pure CHS, thus demonstrating that pure CHS undergoes the phase transition from phase II to I slowly. On the other hand, it was observed that phase II maintained its stability in CHS/SiO2 composite even after 14 days (Fig. 3b2), suggesting that phase II was further stabilized by silica additives. It has been reported that the phase stability between phase I and II in pure CHS is changed by sample conditions, treatments, and circumstances such as grain size, grinding, humidity, and stress [30,31]. In the CHS/ SiO2 composite, CHS would infiltrate into the mesopores of silica and among the silica particles. Therefore, there is
a possibility that strains of CHS crystal generate in the mesopores and among the silica particles, although any external stress is not given to the samples during the measurements of Raman spectroscopy and XRD. Table 1 Wavenumbers (cmK1) in the Raman spectra for pure CHS and CHS/SiO2 composite at three temperatures 293 K CHS phase I
1253 m
369 K
427 K
CHS/ SiO2 phase II
CHS phase II
CHS/ SiO2 phase II
1255 w 1231 w
1255 w 1230 w
1254 w 1230 w
CHS phase III
CHS/ SiO2 phase III
n3SO4 1228 w
1230 w nSO2
1180 w 1176 w
1173 w
1177 w 1069 w
573 m 475 w 420 s
dOH
1177 w
1163 w 1057 w 998 vs 860 s 615 m 602 s 588 s
Assignment
1178 w
1070 w n(S–O)
1022 vs 855 s
1023 vs 851 vs
1024 vs 850 m
597 m
596 sh
594 sh
587 s 579 m
588 s 579 sh
588 m 580 sh
428 s 415 m
426 s 415 m
426 m 415 m
1030 vs 829 m
1032 vs 829 m
n(S–O) n(S–OH) n4SO4 n3SO4
586 m
587 m
416 w
416 w
vs, very strong; s, strong; m, medium; w, weak; sh, shoulder.
d(S–OH) n2SO4
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not only silica additives but also responds to sample conditions such as powdering and pelletizing. This discrepancy may be caused by grain size, grinding, humidity, and stress as stated above [30,31]. After measurement at room temperature, XRD patterns were recorded as the temperature was increased to 433 K. In pure CHS, the signal of phase I disappeared over 383 K, and phase II appeared in its stead (Fig. 4a). With further increase of the temperature, phase III (I41/amd) was observed at 423 K in pure CHS. In the case of CHS/SiO2 composite, the phase transition from phase II to III was observed at 423 K (Fig. 4b), similarly to that of pure CHS. During the cooling process, the phase transition from phase III to II occurred immediately under 414 K in both the cases of pure CHS and CHS/SiO2 composite. After cooling to room temperature, phase II was observed in pure CHS, and it was also observed in the composite. Those results concerning XRD were consistent with the observations of the Raman spectra. They also suggest that the structural phase transition between phase I and II is affected by silica additives, while that between phases II and III is not influenced by silica additives. Additionally, the FWHM values of the XRD signals at phases II and III were evaluated (2q scan range: 25–328, temperature: 383– 433 K). The FWHM values of CHS/SiO2 composite were larger by 10–30% than those of pure CHS, thus suggesting that the CHS crystal structure in the composite was partly disordered in comparison with the pure CHS polycrystalline sample. Fig. 3. Raman spectral changes in pure CHS and CHS/SiO2 composite at room temperature with the passage of time after heating at 446 K. (a) CHS pellet sample: (1) immediately after cooling down, (2) after 2 days, (3) after 6 days, (b) CHS/SiO2 composite pellet sample: (1) immediately after cooling down, (2) after 14 days.
The strain in the CHS/SiO2 composite will influence on the phase stability between phase I and II because CHS crystal has ferroelastic characteristics in phases I and II [32–34]. 3.2.2. X-ray diffraction XRD measurements of powdered CHS and CHS/SiO2 composite were conducted under atmospheric condition, as plotted in Fig. 4a and b, respectively. The same pelletsamples as prepared in the Raman spectroscopic measurement were mechanically powdered, and then they were analyzed. The detected peaks all originated from CHS, and other observable peaks were not found. Initial measurement at room temperature (303 K) showed that only phase I (P21/c) was observed in pure CHS, while phase II (P21/c) and phase I coexisted in CHS/SiO2. Thus phase II was partly stabilized at room temperature by silica additives. Taking into consideration the Raman spectrum of CHS/SiO2 at room temperature, the structural transformation between phases I and II is dependent on
3.2.3. Differential thermal analysis (DTA) and thermogravimetric (TG) The DTA and TG curves of pure CHS and CHS/SiO2 composite in dry Ar flow are shown in Fig. 5a–c. Powdered samples were prepared by the mechanical grinding of the same pellet samples as prepared for the Raman spectroscopic measurement, and then pretreated by heating at 373 K. Immediately after cooling to room temperature, the temperature was raised to 513 K at various heating rates (1, 3, 5, and 10 K/min). The heat flow was normalized by the weight of CHS in the contents (Fig. 5a,b1 and b2). Fig. 5a shows the DTA curves of CHS and CHS/SiO2 composite at the heating rate of 10 K/min. In pure CHS, the endothermic heat flow associated with the phase transition from phase II to III was observed at around 414 K (peak 1), and that associated with the melting point (peak 2) was observed at around 483 K. In the CHS/SiO2 composite, the line shape at the phase transition from II to III (peak 1 0 ) significantly broadened, which suggested that a structural change took place below the phase transition temperature. Two endothermic peaks appeared in the composite at around the melting point. The first peak (peak 2 0 ) was lower by about 30 K, and second one (peak 3 0 ) was lower by about 10 K than that of pure CHS. Similar results have been observed via differential scanning calorimetry in CHS/SiO2 composite prepared by the mechanical mixing of CHS and
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Fig. 4. XRD patterns of powdered samples of (a) pure CHS, (b) CHS/SiO2 composite. Temperature was raised from room temperature to 433 K.
SiO2 [21]. Additionally, Fig. 5b1 and b2 show the DTA curves of phase transition from phase II to III for pure CHS and CHS/SiO2 composite at various heating rates of 1, 3, 5, and 10 K/min. The onset temperature of phase transition from II to III for pure CHS was 406 K (T1 in Fig. 5b1), and that for CHS/SiO2 composite was 391 K (T1 in Fig. 5b2). The onset temperature of melting for CHS/SiO2 composite (T2 in Fig. 5b2) was ca. 435 K, suggesting that the melting in a high temperature region (O435 K) should be considered concerning conductivity-enhancement in the composite. In fact, the conductivity of CHS/SiO2 composite was found to be slightly higher than that of CHS at around 450 K (Fig. 1). On the other hand, the significant enhancement of conductivity in the composite was observed even at the temperatures lower than 435 K, although CHS in the composite was solid under 435 K. We discuss the mechanism of conductivity-enhancement under 435 K in Section 3.3. TG curves at the heating rate of 10 K/min showed that the weight change fraction of pure CHS was within K0.3% at 513 K in comparison with that at room temperature, and that of CHS/SiO2 composite were K1.4G 0.2% at 513 K, as shown in Fig. 5c. In the composite system, such a weight loss will be due mostly to water adsorbed on silica particles from the atmosphere. In fact, we confirmed that the weight change fraction of CHS/SiO2 powder prepared by mechanical mixing with CHS powder and SiO2 particles was K1.1G0.2% at 513 K, which was almost the same as that of CHS/SiO2 prepared using the present evaporation-to-dryness method. Though the conductivity of CHS/SiO2 composite can be influenced to some extent by
adsorbed water on silica, we believe that other factors are involved with this significant enhancement of protonic conduction. As stated above, in regard to pure CHS, it was confirmed that the superprotonic transition from the low conduction phase to the high conduction one at around 414 K attends with the structural phase transition from phase II (monoclinic phase) to phase III (tetragonal phase), which agrees with Baranov’s reports [2]. However, it can be concluded that the remarkable enhancement of the conductivity below the phase transition temperature in CHS/SiO2 composite is not related to phase III, because the results of the Raman spectra and XRD patterns indicate that phase III does not form below the phase transition temperature (414 K). Although phase II was stabilized below the critical temperature, ca 333 K, with the addition of silica, phase III was not stabilized by the addition of silica below the critical temperature, 414 K. The Raman spectroscopy and XRD measurements, therefore, indicate that other reasons for the significant enhancement of the conductivity in the composite should be considered. On the other hand, the DTA results show that the onset temperatures of the endothermic heat flow associated with the phase transition from phase II to III and the melting significantly decrease in CHS/SiO2 composite, suggesting that CHS crystalline structure will be partly disordered. The line-broadening in XRD patterns of the CHS/SiO2 composite also provides evidence that the CHS crystalline structure is partly disordered. It is possible that this structurally disordered part of CHS in the composite can
J. Otomo et al. / Journal of Physics and Chemistry of Solids 66 (2005) 21–30
Fig. 5. DTA and TG profiles of powdered samples of pure CHS and CHS/SiO2 composite in dry Ar flow. (a) Heat flows versus temperature for pure CHS and CHS/SiO2 composite at a heating rate of 10 K/min. (b1) DTA curves of pure CHS at various heating rates of 1, 3, 5, and 10 K/min for the phase transition from phase II to III. (b2) DTA curves of CHS/SiO2 composite at various heating rates of 1, 3, 5, and 10 K/min for the phase transition from phase II to III. T1 and T2 show the onset temperatures of phase transition from phase II to III and melting, respectively. (c) Weight change fractions versus temperature for pure CHS and CHS/SiO2 composite at a heating rate of 10 K/min.
play an important role in conductivity enhancement. Next we will discuss the protonic conduction mechanism, particularly concerning a new phase that can enhance protonic conduction even below the phase transition temperature. 3.3. Lineshift and line-broadening vs. temperature in the Raman spectra of pure CHS and CHS/SiO2 composite Generally, it is considered that an interface between ionic salt and oxide plays an important role in the enhancement of conductivity [24]. In present CHS/SiO 2 composite,
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mesoporous silica particles will disperse in polycrystalline CHS (we confirmed that mesoporous silica particles well dispersed in polycrystalline CHS by SEM-EDX), and CHS will infiltrate into the mesopores of silica particles. Thus, the new phase, which is observed by DTA and XRD to be structurally disordered, is expected to form in the mesopores. This new phase will form on the silica particle’s surface in addition to mesopores, and it should be denominated as an interfacial phase. Therefore, it is expected that the CHS/SiO2 composite consists of bulky polycrystalline CHS, an interfacial phase, and silica particles. The structural and dynamical information regarding the CHS crystal at the interfacial phase should be reflected in the results of the present Raman spectroscopy, though the data represent an average structure, including bulk and interface. Thus, by comparing the differences between pure CHS bulk and its composite, we can discuss the characteristics of the interface. As shown in Fig. 2, the internal modes of the CHS showed the spectral shifts and the broadenings of the linewidths with increasing temperature. The Raman spectrum reflects a micro-structural change, and the analyses of the line shapes in the Raman spectrum have been widely studied in terms of the dynamical motion of ions and molecules in solid states [35–38]. For example, concerning the NOK 2 anion in NaNO2 [35], the water molecule in betaine potassium iodine dihydrate [36], the 2K SeO2K 4 anion in CsH(SO4)0.76(SeO4)0.24 [37], and the SO4 anion in K3D(SO4)2 [38], the reorientational motions (or librations) of those ions and that molecule were analyzed in terms of the linewidths of the internal/external modes in those crystals as a function of temperature. Concerning CHS, Pham-Thi et al. also discussed the reorientational motion of the SO2K anion, based on the temperature 4 dependence of the linewidths of the internal S–OH stretching and bending modes and the librational mode of the HSOK 4 anion [25,30]. In the present study, we focused our attention on the shift and broadening of a stretching mode, n(S–OH), at ca. 850 cmK1 as a function of temperature, because the n(S–OH) line seems to consist of a single line, thus the change of the line shape with temperature can be detected accurately. In addition, it is expected that this vibration mode is sensitive to SO4 tetrahedron dynamics [25], because CHS contains the hydrogen-bonded chains of the SO4 tetrahedra [29], and SO4 tetrahedron dynamics will occur with the break of the hydrogen bond. The spectral shifts of n(S–OH) in pure CHS and CHS/SiO2 composite are plotted versus temperature in Fig. 6. As for pure CHS, the red-shift of n(S–OH) mode was observed with the increase in temperature, and besides, steep changes of n(S–OH) occurred twice at around 333 and 414 K. As already mentioned above, it has been reported that there are three phases; phases I–III [2,29,39]. Thus, the steep changes of the spectral shifts at the two temperatures of ca. 333 and ca. 414 K are accompanied by the structural phase transition from phase I to II, and from phase II to III,
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from phase II to III (414 K), respectively. In contrast, in the case of CHS/SiO2 composite, there were no discrete changes observed at the phase transition temperature from phase I to II. It was important to note that the linewidth of the composite in the n(S–OH) mode was obviously larger than that of pure CHS at each temperature between 350 and 420 K. At phase III, the linewidth of the composite was almost the same as that of pure CHS at each temperature. The excess value of the linewidth of the composite at each temperature between 350 and 420 K strongly suggests that the HSOK anion in the composite is orientationally 4 disordered far below the phase transition temperature from phase II to III, which will be discussed below in the context of protonic conduction in CHS/SiO2 composite. Fig. 6. Spectral shifts of n(S–OH) in pure CHS and CHS/SiO2 composite plotted against temperature from 293 to 446 K in the heating process. Filled circle (C): pure CHS, open circle (B): CHS/SiO2 composite.
respectively. In contrast, no steep change of the spectral shift at around 333 K was observed in the CHS/SiO2 composite, which suggested that phase II was stabilized even below the critical temperature (ca. 333 K) in the composite. At around 414 K, the steep change of the spectral shift according to the phase transition from phase II to III took place in the composite as well as pure CHS. Additionally, at around the critical temperature from phase II to III, the line position of n(S–OH) in the composite was almost the same as pure CHS. The above results show that the effects of silica additives on the stabilities of phases I, II, and III are different from each other. The FWHM (full width at half maximum) values of the internal n(S–OH) stretching mode in pure CHS and its silica composite are plotted against temperature in Fig. 7. In the case of pure CHS, the linewidth of n(S–OH) mode increased with the increase of temperature, and the discrete changes of the linewidth were observed at the phase transition temperature from phase I to II (333 K), and that
Fig. 7. FWHM values of S–OH stretching mode of pure CHS and CHS/SiO2 composite plotted against temperature from 293 to 446 K in the heating process. Filled circle (C): pure CHS, open circle (B): CHS/SiO2 composite.
3.4. Proposed mechanism of conductivity enhancement in CHS/SiO2 composite Raman spectroscopy can provide information on the local structures of SO4 tetrahedra and its hydrogen bond, which has been well studied and discussed in the previous literature concerning pure CHS [25]. Phase I consists of infinite hydrogen-bonded chains of SO4 tetrahedra running along the b-axis one-dimensionally; (HSOK 4 )n, and SO4 tetrahedra are loosely packed by Cs atoms [29]. The n(S–OH) at about 850 cmK1 was assigned to be in the proton donor mode, while n(S–O) at about 1020 cmK1 was assigned to be in the proton acceptor mode [25]. Thus, the line position of the n(S– OH) can be influenced by the displacement of protonic position along S–OH/O–S [25]. As for pure CHS, it is considered the abrupt red-shift of the n(S–OH) according to the phase transition from I to II is due to a lengthening of the S–OH bond, which suggests a weakening of the hydrogen bond of (HSOK 4 )n. Additionally, the discrete broadening (FWHMw4 cmK1) of n(S–OH) at the phase transition from I to II suggests an orientational disorder of the HSOK 4 ion [39]. This rearrangement from I to II has been interpreted as a structural change from the infinite chain, (HSOK 4 )n to a cyclic dimer, (HSOK ) , with the subsequent weakening of the 4 2 hydrogen-bond [25]. At the phase transition from phase II to III in pure CHS, an abrupt red-shift and a line-broadening again occur, which suggests that further weakening of the hydrogen bond and increase of the orientational disorder of the HSOK 4 ion will be induced. Also, the abrupt change of the linewidth of the S–OH stretching mode is significantly amplified at the phase transition from II to III in pure CHS. Therefore, in phase III, the HSOK 4 ion can be characterized by rapid reorientational motion [25]. It is considered that this HSOK 4 ion-dynamic realizes fast proton diffusion in the superprotonic phase [3,39–41]. At first, the displacement of a proton occurs along the hydrogen bond closer to SO4 tetrahedron forming this bond. Next, the longer half of the hydrogen bond is broken, and then the HSOK 4 ion rotates to a new position. Through this reorientation process, a new hydrogen bond forms between neighboring SO4 tetrahedra so that the proton transfers along the newly formed hydrogen
J. Otomo et al. / Journal of Physics and Chemistry of Solids 66 (2005) 21–30
bond. This process is repeated, and thus, the superprotonic conduction is realized [3,39–41]. The above statements suggest that the proton-conductivity increases as the linewidth broadens. In CHS/SiO2 composite, the spectral shift of the n(S–OH) as a function of temperature accords with that of pure CHS at phases II and III. Thus, the displacement of the protonic position of S–OH is not strongly induced by silica additives. On the other hand, the fact that the linewidth of the n(S–OH) of the composite surpasses that of pure CHS between 350 and 420 K suggests that the reorientational motion of the HSOK 4 ion is partly enhanced in the composite even far below the critical temperature, 414 K. This dynamical motion of the HSOK 4 ion will be the origin of the remarkable enhancement of conductivity at the low conduction phase in the composite. The present evaporation-to-dryness method allows the CHS to infiltrate well into the mesopores of silica. The new interfacial phase that has a disordered structure is generated in the mesopores and/or on the surfaces of silica particles. In fact, the present silica particles can hold 75% of the CHS in the mesopores, taking into account the molar ratio of the composite and pore volume of silica. Thereby, the reorientational motion of the HSOK 4 ion below the critical temperature will be induced mostly in the interfacial phase, which results in high conductivity. The above description is also consistent with the DTA result. The observed broad peak for endothermic heat flow under the critical temperature (391–414 K) in the composite probably originates from a hydrogen-bond break that subsequently induces reorientational motion of the HSOK 4 ion at the interface. At the interface, a structurally disordered phase can form due to an interfacial interaction between CHS and silica. The interfacial affinity probably contributes to disordering of the structure at the interfacial phase and the reorientational motion of the HSOK 4 ion even below the critical temperature, leading to the enhancement of proton conductivity. The protonic conduction process in the CHS/SiO2 composite can be conclusively described as follows. CHS in composite has two main phases, i.e. a bulky phase and an interfacial phase. The interfacial phase having a disordered crystal structure forms in the mesopores and/or on the surface of silica particles. The relevant amount of silica particles and their sufficient dispersion in the composite system will form protonic conduction pathways via the interfacial phase from electrode to electrode. Even below the critical temperature of the superprotonic phase transition, the reorientational motion of the HSOK 4 ion will be induced in the interfacial phase to enhance protonic conduction, which is comparable to the superprotonic conduction, whereas the bulky phase maintains the low conduction phase. When the temperature surpasses the critical temperature, neither phase can be distinguished in terms of conductivity. The conductivities of the both phases reach the superprotonic realm.
29
4. Conclusion CsHSO4/SiO2 composite prepared by means of the evaporation-to-dryness method showed remarkable improvement of the conductivity by more than three orders of magnitude even below the superprotonic phase transition temperature in comparison with that of pure CsHSO4 (353–414 K). In view of the structural transformation of CsHSO4 in the composite, Raman spectroscopy and X-ray diffraction measurements suggest that the tetragonal phase (phase III) that corresponds to the superprotonic conduction phase is not stabilized in CsHSO 4/SiO2 composite below its critical temperature, ca. 414 K, although the monoclinic phase (phase II) is stabilized in the composite even below its critical temperature. Thus, the reasons for conductivity-enhancement other than the stabilization of phase III should be considered. Internal vibration study of Raman spectroscopy was performed in the 293–446 K range, with attention focused on the spectral shift and the linewidth of the S–OH internal stretching mode. There are no apparent differences in the spectral shift between the composite and pure CsHSO4 around the critical temperature of the superprotonic phase transition. On the other hand, the fact that the bandwidth of the composite is apparently larger than that of pure CsHSO4 in the region of 350–420 K suggests that the reorientational motion of the HSOK 4 ion is induced even below the critical temperature of the superprotonic phase transition. This dynamical motion of the HSOK 4 ion probably accounts for the conductivityenhancement. The DTA results also support the HSOK 4 ion dynamics described above, namely, the endothermic heat flow occurring below the critical temperature in the composite. It is concluded that the rapid reorientational motion of the HSOK 4 ion is induced even below the critical temperature in the interfacial phase that forms in mesopores and/or on the surfaces of silica particles, and thus the protonic conduction is enhanced especially below the critical temperature of the superprotonic phase transition.
Acknowledgements The authors are very grateful to Mitsubishi Chemical for providing the SiO2 particles. The authors also acknowledge the financial support by a Grant-in-Aid for Young Scientists (B), No. 14750614, from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.
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Phase transition behavior and proton conduction mechanism in cesium hydrogen sulfate/silica composite Junichiro Otomoa,*, Hitoshi Shigeokab, Hidetoshi Nagamotoa, Hiroshi Takahashib a
Department of Environmental Chemical Engineering, Faculty of Engineering, Kogakuin University, Nakanomachi 2665-1, Hachioji, Tokyo 192-0015, Japan b Department of Chemical System Engineering, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan Received 15 January 2004; revised 30 March 2004; accepted 30 July 2004
Abstract Proton conduction and crystal structure in CsHSO4/SiO2 composite composed of polycrystalline CsHSO4 and mesoporous silica particles were investigated based on conductivity measurement and characterizations using Raman spectroscopy, XRD, and differential thermal analysis. The conductivity of pure CsHSO4 abruptly changes at around 414 K (superprotonic phase transition), being accompanied with the structural transformation from a monoclinic phase to a tetragonal phase, while the conductivity of CsHSO4/SiO2 composite is significantly larger by over three orders of magnitude than that of pure CsHSO4 below the critical temperature of the superprotonic phase transition (353– 414 K). Raman spectroscopy and XRD indicate that this remarkable conductivity-enhancement in the composite is not due to the stabilization of the tetragonal phase (superprotonic phase) below its critical temperature. The line-broadening of the internal modes in the Raman spectra suggests that the rapid reorientational motion of the HSOK 4 ion, which leads to superprotonic conduction, is induced in the composite even below the critical temperature. The reorientational motion of the HSOK 4 ion below the critical temperature will occur at the interfacial phase which is structurally disordered and forms between CsHSO4 and SiO2 in the mesopores and/or on the surfaces of silica particles. Proton transfer will be accelerated via the interfacial conduction-pathway in the composite. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Inorganic compounds; C. Raman spectroscopy; C. X-ray diffraction; D. Electrical conductivity; D. Phase transitions
1. Introduction Cesium hydrogen sulfate (CsHSO4) is one of the proton conductors having hydrogen bonds [1]. CsHSO4 shows a superprotonic phase transition, which means that its conductivity steeply increases by four orders of magnitude at 414 K and reaches 10K2 S/cm (superprotonic phase) [1]. The superprotonic phase transition of CsHSO4 occurs with a structural transformation from a monoclinic phase to a tetragonal phase [2]. In the superprotonic phase, it is considered that the reorientational motion of a SO4 tetrahedron, which induces a disordering of the hydrogen bond network, facilitates proton transfer [3]. On the other hand, heterogeneous doping by the dispersion of oxide particles in ionic salt has been
* Corresponding author. Tel./fax: C81 426 28 4523. E-mail address: [email protected] (J. Otomo). 0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.07.006
extensively studied as an effective approach to enhance ionic conductivity. Heterogeneous systems that consist of ionic conductors and chemically inert insulators such as Al2O3, SiO2, and ZrO2, etc. involve interface interaction between ionic salts and oxide particles. In the heterogeneous ionic conduction systems, interfaces play an important role in ionic conductivity-enhancement. As is well known, AgC ionic conductor-based composites have been intensively studied, e.g. AgI/A (AZAl2O3 [4–7], SiO2 [7], ZrO2 [8]), AgCl/Al2O3, [6,9] AgBr/Al2O3 [10]. The remarkable increase in their conductivities has been explained by an increase in the point defect concentration within the interface region. The formation of defects at the interface can promote ionic conduction via interface regions, and the conduction at the interface governs mainly the conductivity of composites [11,12]. Thus, a variation of structure among bulky ionic salt, the interface, and the insulator characterizes heterogeneous systems. Several ionic conduction models have been proposed,
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i.e. a space-charge model based on the generation of point defects at interfaces [9,13,14], defect-induced order– disorder phase transition (or defect-induced sublattice melting) [15–17], and formation of a structurally disordered phase [18,19] that may induce defect formation. In the cases of proton conductive composite materials that consist of solid acids and silica particles such as CsHSO4/SiO2 and CsH2PO4/SiO2, significant enhancements of their conductivities have been observed [20–23]. In those composite systems, the characteristic phenomenon is the significant increase of the conductivity below the critical temperature of the superprotonic phase transition [20,23]. As for CsHSO4/SiO2 composite, Ponomareva et al. [21] have reported that the conductivity-enhancement is strongly influenced by silica properties such as molar ratio, surface area, and pore size. They have also suggested that a structurally disordered interface such as an amorphous phase forms between CsHSO4 and SiO2 in the composite [20,21]. Thus, the conductivities of composites composed of solid acid and silica will be affected by the physical and chemical characteristics of their bulk and interfaces. In the previous reports, however, observation of the crystal structures of those composites was limited, and no definite conclusions have been obtained [24]. In the present study, we focus on the protonic conduction of CsHSO4/SiO2 composite, and discuss its protonic conduction mechanism in terms of in situ measurements for the structure and dynamics in the composite as a function of temperature using Raman spectroscopy, X-ray diffraction, and differential thermal analysis. In our preliminary experiment, CsHSO4/SiO2 composite in which silica particles were highly dispersed was successfully synthesized with an evaporation-to-dryness method [22], and its conductivity was further improved in comparison with the results of previous reports [20,21]. By using this sample, the structural transformation of CsHSO4, phase stability, and the reorientational motion of a SO4 tetrahedron as a function of temperature are discussed, which will clarify the protonic conduction in the CsHSO4/SiO2 composite.
pellet was about 0.95 after having calcined at 473 K. CHS/SiO2 composite was prepared by the evaporation-todryness method as described below. First, an aqueous suspension containing equimolar amounts of Cs2SO4 and H2SO4, and a relevant amount of mesoporous amorphous silica particles was prepared. The mesoporous amorphous silica particles were provided by Mitsubishi Chemical. The properties of the silica particle are described as follows according to the information from Mitsubishi Chemical: average particle size, 5.1 mm, specific surface area, 297 m2/g, average pore volume, 1.3 ml/g, and average pore size, 16 nm. The suspension was then stirred constantly for 120 min under a reduced pressure condition in order to remove the air in the mesopores of the silica particles and to allow the solution to easily infiltrate into the mesopores. The suspension was evaporated at 333 K, and then dried at 363 K for 24 h. After additional heating at 443 K for 1 h in air in order to minimize the influence of adsorbed water, the resultant sample was mechanically ground for 30 min, and pressed at 3 t/cm2 to form pellets. Finally, the pellet of CHS/SiO2 composite was calcined at 473 K for 1 h in air as in the case of pure CHS. The molar ratio of CHS and SiO2 was 1:1, which corresponded to approximately 30% of the volume ratio for silica in the composite. The relative density of CHS/SiO2 composite pellet was about 0.93 after having calcined at 473 K. 2.2. Proton conductivity measurement Conductivity measurements were performed by means of ac impedance spectroscopy using a Hewlett–Packard 4192A impedance analyzer in the frequency range from 10 MHz to 10 Hz with an applied voltage of 50 or 500 mV. Gold electrodes were applied on the both sides of the pellets using gold paste after gold-deposition. The sample was pinched between gold meshes that were connected to the ac impedance analyzer with gold wires, and it was set in a glass tube. Data were collected in both heating and cooling processes, changing the temperature stepwise between 353 and 463 K under a dry atmospheric Ar flow-condition. 2.3. Raman spectroscopic measurement
2. Experimental 2.1. Sample preparations Polycrystalline CsHSO4 (designated as CHS hereafter) was obtained from an aqueous solution containing equimolar amounts of Cs2SO4 (Soekawa Chemical, purityO 99%) and H2SO4 (Wako Pure Chemical). The solution was evaporated, and then the resultant CHS residue was dried at 363 K for 24 h. Powdered CHS sample was prepared by grinding in an agate mortar for 30 min, and pressed at 3 t/cm2 to form dense pellets (diameter: 10 mm, thickness ca. 1.5 mm). The resultant pellet of pure CHS was then calcined at 473 K for 1 h in air. The relative density of CHS
The pellet-samples were set in a sealed glass cell. The cell was introduced into a furnace in order to observe their in situ Raman spectrum as a function of temperature that was changed stepwise between 293 and 446 K. After the sample was maintained for 10 min at each temperature, spectroscopic measurement was performed in order to collect data in equilibrium. A continuous wave ArC ion laser beam at 488 nm of 0.5 W was used as the exciting line, which was focused into the sample through a glass window using a 2.0 cm focal length lens. The backscattered light was led inversely through the lens to a spectrometer (JASCO NRS2000) and detected by a CCD detector. The data collection at each temperature was carried out at several random
J. Otomo et al. / Journal of Physics and Chemistry of Solids 66 (2005) 21–30
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positions on the pellet so as to verify the reproducibility. The wavelength was calibrated using Hg lines in a fluorescent lamp within a precision of 2 cmK1, and the typical bandwidth was 4 cmK1. 2.4. Characterizations The X-ray diffraction (XRD) patterns of powdered samples of CHS and its silica composite were investigated from room temperature to 433 K in air. XRD was carried out using MAC Science MXP18 employing a Cu Ka source operating typically at 40 kV and 300 mA. The XRD patterns were measured at a scan rate of 0.1 degree/min with an angular resolution of 0.0068. The peak profiles were obtained from Cu Ka line-corrected data. As well, thermogravimetric measurement (TG) and differential thermal analysis (DTA) were performed using a Rigaku TAS200-TG8110D from room temperature up to 513 K in a dry Ar flow.
3. Results and discussions 3.1. Conductivity measurements in pure CHS and CHS/SiO2 composite The proton conductivities, s, of polycrystalline CsHSO4 (CHS) and its silica composite (molar ratio, CHS: SiO2Z 1:1) were measured stepwise between 353 and 463 K in heating–cooling cycles under a dry-Ar condition. Each sample was kept for 30 min at each temperature so as to be in equilibrium, and the ac impedance measurement was then carried out. The s values were calculated according to the procedure explained in our previous reports [22,23]. The results of natural logarithmic function, ln(sT), versus temperature, T, in the heating–cooling cycles are shown in Fig. 1. The results regarding pure CHS showed that the conductivity suddenly jumped up by four orders of magnitude at around 413–423 K (superprotonic transition), and it changed reversibly during the heating–cooling cycle. It has been known that the abrupt change of conductivity in CHS from a low conduction phase to a high conduction one is closely related to a structural phase transition in CHS [1,2]. Baranov et al. reported that such superprotonic phase transition in CHS occurred reversibly at 414 K, being accompanied with the structural phase transition from the monoclinic phase to the tetragonal phase [1,2]. In the case of CHS/SiO2 composite, a significant improvement in conductivity was observed, especially, at the low conduction phase. At temperatures below the surperprotonic transition of CHS (353–414 K), the conductivity of the composite was larger by over three orders of magnitude than that of pure CHS. Apparently, the low conduction phase disappeared, and the high conduction phase was stabilized. This conduction behavior was observed not only in the heating process but also in the cooling one, and it was confirmed to
Fig. 1. Temperature dependences of the conductivities of pure CHS and CHS/SiO2 composite. Filled symbols: heating process; open symbols: cooling process. C and B: pure CHS; : and 6: CHS/SiO2 composite.
be reproducible. This remarkable conductivity-enhancement is evidently due to the presence of silica particles. Next, we will show Raman spectra, XRD patterns, and DTA curves concerning CHS/SiO2 composite in order to clarify the relationship of the conductivity-enhancement and the crystal structure of the CHS/SiO2 composite. 3.2. Observations of structural phase transitions of pure CHS and CHS/SiO2 composite 3.2.1. Raman spectroscopy The Raman spectra of the internal modes of polycrystalline CsHSO4 (CHS) and its silica composite (molar ratio, CHS:SiO2Z1:1) were recorded between 270 and 1340 cmK1 at various temperatures. Fig. 2 shows the data collected in the heating process. In the cases of both pure CHS and its silica composite, stretching modes between 800 and 1300 cmK1, and bending modes between 400 and 600 cmK1 were observed, and each line shifted and broadened with an increase in temperature. We will discuss the line-shift and line-broadening in detail later in this report. The Raman spectroscopy of pure CHS has been well studied, and thus each band was well assigned [25–27]. The observed Raman bands at selected temperatures (293, 369, and 427 K) and their assignments according to the previous studies [25–27] are summarized in Table 1. The observed spectra were consistent with the previous studies [25–27]. As well, the crystal phase of CHS at each temperature can be identified by line positions. The assignment of pure CHS indicates that different phases exist at the three temperatures, i.e. phase I (monoclinic, space group: P21/c [28]) at room temperature (293 K), phase II (monoclinic, space group: P21/c) at 369 K, and phase III (tetragonal, space group: I41/amd) at 427 K, which is also consistent with
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Fig. 2. Raman spectra for pure CHS and CHS/SiO2 composite as a function of temperature between 293 and 427 K. (a) Polycrystalline CHS pellet sample, (b) CHS/SiO2 composite pellet sample.
the previous reports [25–27] (we employed the notation concerning the crystal phase in CHS by Pham-Thi et al. [25]. Additionally, the crystal structure of phase I has been corrected: monoclinic, space group: P21/m [29]/monoclinic, space group: P21/c [28]). As for CHS/SiO 2 composite, each line and crystal phase were also well identified according to the assignment of pure CHS. The crystal phases of CHS in the composite at 369 and 427 K were phase II and phase III, respectively, which were the same as those in pure CHS. At room temperature (293 K), however, the crystal phase of CHS in the composite was phase II, which was different that of pure CHS. The Raman spectra were again recorded at room temperature following the data-collection in the heating process. Fig. 3a and b show the Raman spectra of pure CHS and CHS/SiO2 composite at room temperature, respectively, which were recorded with the passage of time. Immediately after cooling to room temperature, pure CHS as well as CHS/SiO2 composite maintained phase II (Fig. 3a1 and b1). As shown in Fig. 3a, it took about 2 days to make a transition from phase II to I in pure CHS, thus demonstrating that pure CHS undergoes the phase transition from phase II to I slowly. On the other hand, it was observed that phase II maintained its stability in CHS/SiO2 composite even after 14 days (Fig. 3b2), suggesting that phase II was further stabilized by silica additives. It has been reported that the phase stability between phase I and II in pure CHS is changed by sample conditions, treatments, and circumstances such as grain size, grinding, humidity, and stress [30,31]. In the CHS/ SiO2 composite, CHS would infiltrate into the mesopores of silica and among the silica particles. Therefore, there is
a possibility that strains of CHS crystal generate in the mesopores and among the silica particles, although any external stress is not given to the samples during the measurements of Raman spectroscopy and XRD. Table 1 Wavenumbers (cmK1) in the Raman spectra for pure CHS and CHS/SiO2 composite at three temperatures 293 K CHS phase I
1253 m
369 K
427 K
CHS/ SiO2 phase II
CHS phase II
CHS/ SiO2 phase II
1255 w 1231 w
1255 w 1230 w
1254 w 1230 w
CHS phase III
CHS/ SiO2 phase III
n3SO4 1228 w
1230 w nSO2
1180 w 1176 w
1173 w
1177 w 1069 w
573 m 475 w 420 s
dOH
1177 w
1163 w 1057 w 998 vs 860 s 615 m 602 s 588 s
Assignment
1178 w
1070 w n(S–O)
1022 vs 855 s
1023 vs 851 vs
1024 vs 850 m
597 m
596 sh
594 sh
587 s 579 m
588 s 579 sh
588 m 580 sh
428 s 415 m
426 s 415 m
426 m 415 m
1030 vs 829 m
1032 vs 829 m
n(S–O) n(S–OH) n4SO4 n3SO4
586 m
587 m
416 w
416 w
vs, very strong; s, strong; m, medium; w, weak; sh, shoulder.
d(S–OH) n2SO4
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not only silica additives but also responds to sample conditions such as powdering and pelletizing. This discrepancy may be caused by grain size, grinding, humidity, and stress as stated above [30,31]. After measurement at room temperature, XRD patterns were recorded as the temperature was increased to 433 K. In pure CHS, the signal of phase I disappeared over 383 K, and phase II appeared in its stead (Fig. 4a). With further increase of the temperature, phase III (I41/amd) was observed at 423 K in pure CHS. In the case of CHS/SiO2 composite, the phase transition from phase II to III was observed at 423 K (Fig. 4b), similarly to that of pure CHS. During the cooling process, the phase transition from phase III to II occurred immediately under 414 K in both the cases of pure CHS and CHS/SiO2 composite. After cooling to room temperature, phase II was observed in pure CHS, and it was also observed in the composite. Those results concerning XRD were consistent with the observations of the Raman spectra. They also suggest that the structural phase transition between phase I and II is affected by silica additives, while that between phases II and III is not influenced by silica additives. Additionally, the FWHM values of the XRD signals at phases II and III were evaluated (2q scan range: 25–328, temperature: 383– 433 K). The FWHM values of CHS/SiO2 composite were larger by 10–30% than those of pure CHS, thus suggesting that the CHS crystal structure in the composite was partly disordered in comparison with the pure CHS polycrystalline sample. Fig. 3. Raman spectral changes in pure CHS and CHS/SiO2 composite at room temperature with the passage of time after heating at 446 K. (a) CHS pellet sample: (1) immediately after cooling down, (2) after 2 days, (3) after 6 days, (b) CHS/SiO2 composite pellet sample: (1) immediately after cooling down, (2) after 14 days.
The strain in the CHS/SiO2 composite will influence on the phase stability between phase I and II because CHS crystal has ferroelastic characteristics in phases I and II [32–34]. 3.2.2. X-ray diffraction XRD measurements of powdered CHS and CHS/SiO2 composite were conducted under atmospheric condition, as plotted in Fig. 4a and b, respectively. The same pelletsamples as prepared in the Raman spectroscopic measurement were mechanically powdered, and then they were analyzed. The detected peaks all originated from CHS, and other observable peaks were not found. Initial measurement at room temperature (303 K) showed that only phase I (P21/c) was observed in pure CHS, while phase II (P21/c) and phase I coexisted in CHS/SiO2. Thus phase II was partly stabilized at room temperature by silica additives. Taking into consideration the Raman spectrum of CHS/SiO2 at room temperature, the structural transformation between phases I and II is dependent on
3.2.3. Differential thermal analysis (DTA) and thermogravimetric (TG) The DTA and TG curves of pure CHS and CHS/SiO2 composite in dry Ar flow are shown in Fig. 5a–c. Powdered samples were prepared by the mechanical grinding of the same pellet samples as prepared for the Raman spectroscopic measurement, and then pretreated by heating at 373 K. Immediately after cooling to room temperature, the temperature was raised to 513 K at various heating rates (1, 3, 5, and 10 K/min). The heat flow was normalized by the weight of CHS in the contents (Fig. 5a,b1 and b2). Fig. 5a shows the DTA curves of CHS and CHS/SiO2 composite at the heating rate of 10 K/min. In pure CHS, the endothermic heat flow associated with the phase transition from phase II to III was observed at around 414 K (peak 1), and that associated with the melting point (peak 2) was observed at around 483 K. In the CHS/SiO2 composite, the line shape at the phase transition from II to III (peak 1 0 ) significantly broadened, which suggested that a structural change took place below the phase transition temperature. Two endothermic peaks appeared in the composite at around the melting point. The first peak (peak 2 0 ) was lower by about 30 K, and second one (peak 3 0 ) was lower by about 10 K than that of pure CHS. Similar results have been observed via differential scanning calorimetry in CHS/SiO2 composite prepared by the mechanical mixing of CHS and
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Fig. 4. XRD patterns of powdered samples of (a) pure CHS, (b) CHS/SiO2 composite. Temperature was raised from room temperature to 433 K.
SiO2 [21]. Additionally, Fig. 5b1 and b2 show the DTA curves of phase transition from phase II to III for pure CHS and CHS/SiO2 composite at various heating rates of 1, 3, 5, and 10 K/min. The onset temperature of phase transition from II to III for pure CHS was 406 K (T1 in Fig. 5b1), and that for CHS/SiO2 composite was 391 K (T1 in Fig. 5b2). The onset temperature of melting for CHS/SiO2 composite (T2 in Fig. 5b2) was ca. 435 K, suggesting that the melting in a high temperature region (O435 K) should be considered concerning conductivity-enhancement in the composite. In fact, the conductivity of CHS/SiO2 composite was found to be slightly higher than that of CHS at around 450 K (Fig. 1). On the other hand, the significant enhancement of conductivity in the composite was observed even at the temperatures lower than 435 K, although CHS in the composite was solid under 435 K. We discuss the mechanism of conductivity-enhancement under 435 K in Section 3.3. TG curves at the heating rate of 10 K/min showed that the weight change fraction of pure CHS was within K0.3% at 513 K in comparison with that at room temperature, and that of CHS/SiO2 composite were K1.4G 0.2% at 513 K, as shown in Fig. 5c. In the composite system, such a weight loss will be due mostly to water adsorbed on silica particles from the atmosphere. In fact, we confirmed that the weight change fraction of CHS/SiO2 powder prepared by mechanical mixing with CHS powder and SiO2 particles was K1.1G0.2% at 513 K, which was almost the same as that of CHS/SiO2 prepared using the present evaporation-to-dryness method. Though the conductivity of CHS/SiO2 composite can be influenced to some extent by
adsorbed water on silica, we believe that other factors are involved with this significant enhancement of protonic conduction. As stated above, in regard to pure CHS, it was confirmed that the superprotonic transition from the low conduction phase to the high conduction one at around 414 K attends with the structural phase transition from phase II (monoclinic phase) to phase III (tetragonal phase), which agrees with Baranov’s reports [2]. However, it can be concluded that the remarkable enhancement of the conductivity below the phase transition temperature in CHS/SiO2 composite is not related to phase III, because the results of the Raman spectra and XRD patterns indicate that phase III does not form below the phase transition temperature (414 K). Although phase II was stabilized below the critical temperature, ca 333 K, with the addition of silica, phase III was not stabilized by the addition of silica below the critical temperature, 414 K. The Raman spectroscopy and XRD measurements, therefore, indicate that other reasons for the significant enhancement of the conductivity in the composite should be considered. On the other hand, the DTA results show that the onset temperatures of the endothermic heat flow associated with the phase transition from phase II to III and the melting significantly decrease in CHS/SiO2 composite, suggesting that CHS crystalline structure will be partly disordered. The line-broadening in XRD patterns of the CHS/SiO2 composite also provides evidence that the CHS crystalline structure is partly disordered. It is possible that this structurally disordered part of CHS in the composite can
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Fig. 5. DTA and TG profiles of powdered samples of pure CHS and CHS/SiO2 composite in dry Ar flow. (a) Heat flows versus temperature for pure CHS and CHS/SiO2 composite at a heating rate of 10 K/min. (b1) DTA curves of pure CHS at various heating rates of 1, 3, 5, and 10 K/min for the phase transition from phase II to III. (b2) DTA curves of CHS/SiO2 composite at various heating rates of 1, 3, 5, and 10 K/min for the phase transition from phase II to III. T1 and T2 show the onset temperatures of phase transition from phase II to III and melting, respectively. (c) Weight change fractions versus temperature for pure CHS and CHS/SiO2 composite at a heating rate of 10 K/min.
play an important role in conductivity enhancement. Next we will discuss the protonic conduction mechanism, particularly concerning a new phase that can enhance protonic conduction even below the phase transition temperature. 3.3. Lineshift and line-broadening vs. temperature in the Raman spectra of pure CHS and CHS/SiO2 composite Generally, it is considered that an interface between ionic salt and oxide plays an important role in the enhancement of conductivity [24]. In present CHS/SiO 2 composite,
27
mesoporous silica particles will disperse in polycrystalline CHS (we confirmed that mesoporous silica particles well dispersed in polycrystalline CHS by SEM-EDX), and CHS will infiltrate into the mesopores of silica particles. Thus, the new phase, which is observed by DTA and XRD to be structurally disordered, is expected to form in the mesopores. This new phase will form on the silica particle’s surface in addition to mesopores, and it should be denominated as an interfacial phase. Therefore, it is expected that the CHS/SiO2 composite consists of bulky polycrystalline CHS, an interfacial phase, and silica particles. The structural and dynamical information regarding the CHS crystal at the interfacial phase should be reflected in the results of the present Raman spectroscopy, though the data represent an average structure, including bulk and interface. Thus, by comparing the differences between pure CHS bulk and its composite, we can discuss the characteristics of the interface. As shown in Fig. 2, the internal modes of the CHS showed the spectral shifts and the broadenings of the linewidths with increasing temperature. The Raman spectrum reflects a micro-structural change, and the analyses of the line shapes in the Raman spectrum have been widely studied in terms of the dynamical motion of ions and molecules in solid states [35–38]. For example, concerning the NOK 2 anion in NaNO2 [35], the water molecule in betaine potassium iodine dihydrate [36], the 2K SeO2K 4 anion in CsH(SO4)0.76(SeO4)0.24 [37], and the SO4 anion in K3D(SO4)2 [38], the reorientational motions (or librations) of those ions and that molecule were analyzed in terms of the linewidths of the internal/external modes in those crystals as a function of temperature. Concerning CHS, Pham-Thi et al. also discussed the reorientational motion of the SO2K anion, based on the temperature 4 dependence of the linewidths of the internal S–OH stretching and bending modes and the librational mode of the HSOK 4 anion [25,30]. In the present study, we focused our attention on the shift and broadening of a stretching mode, n(S–OH), at ca. 850 cmK1 as a function of temperature, because the n(S–OH) line seems to consist of a single line, thus the change of the line shape with temperature can be detected accurately. In addition, it is expected that this vibration mode is sensitive to SO4 tetrahedron dynamics [25], because CHS contains the hydrogen-bonded chains of the SO4 tetrahedra [29], and SO4 tetrahedron dynamics will occur with the break of the hydrogen bond. The spectral shifts of n(S–OH) in pure CHS and CHS/SiO2 composite are plotted versus temperature in Fig. 6. As for pure CHS, the red-shift of n(S–OH) mode was observed with the increase in temperature, and besides, steep changes of n(S–OH) occurred twice at around 333 and 414 K. As already mentioned above, it has been reported that there are three phases; phases I–III [2,29,39]. Thus, the steep changes of the spectral shifts at the two temperatures of ca. 333 and ca. 414 K are accompanied by the structural phase transition from phase I to II, and from phase II to III,
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from phase II to III (414 K), respectively. In contrast, in the case of CHS/SiO2 composite, there were no discrete changes observed at the phase transition temperature from phase I to II. It was important to note that the linewidth of the composite in the n(S–OH) mode was obviously larger than that of pure CHS at each temperature between 350 and 420 K. At phase III, the linewidth of the composite was almost the same as that of pure CHS at each temperature. The excess value of the linewidth of the composite at each temperature between 350 and 420 K strongly suggests that the HSOK anion in the composite is orientationally 4 disordered far below the phase transition temperature from phase II to III, which will be discussed below in the context of protonic conduction in CHS/SiO2 composite. Fig. 6. Spectral shifts of n(S–OH) in pure CHS and CHS/SiO2 composite plotted against temperature from 293 to 446 K in the heating process. Filled circle (C): pure CHS, open circle (B): CHS/SiO2 composite.
respectively. In contrast, no steep change of the spectral shift at around 333 K was observed in the CHS/SiO2 composite, which suggested that phase II was stabilized even below the critical temperature (ca. 333 K) in the composite. At around 414 K, the steep change of the spectral shift according to the phase transition from phase II to III took place in the composite as well as pure CHS. Additionally, at around the critical temperature from phase II to III, the line position of n(S–OH) in the composite was almost the same as pure CHS. The above results show that the effects of silica additives on the stabilities of phases I, II, and III are different from each other. The FWHM (full width at half maximum) values of the internal n(S–OH) stretching mode in pure CHS and its silica composite are plotted against temperature in Fig. 7. In the case of pure CHS, the linewidth of n(S–OH) mode increased with the increase of temperature, and the discrete changes of the linewidth were observed at the phase transition temperature from phase I to II (333 K), and that
Fig. 7. FWHM values of S–OH stretching mode of pure CHS and CHS/SiO2 composite plotted against temperature from 293 to 446 K in the heating process. Filled circle (C): pure CHS, open circle (B): CHS/SiO2 composite.
3.4. Proposed mechanism of conductivity enhancement in CHS/SiO2 composite Raman spectroscopy can provide information on the local structures of SO4 tetrahedra and its hydrogen bond, which has been well studied and discussed in the previous literature concerning pure CHS [25]. Phase I consists of infinite hydrogen-bonded chains of SO4 tetrahedra running along the b-axis one-dimensionally; (HSOK 4 )n, and SO4 tetrahedra are loosely packed by Cs atoms [29]. The n(S–OH) at about 850 cmK1 was assigned to be in the proton donor mode, while n(S–O) at about 1020 cmK1 was assigned to be in the proton acceptor mode [25]. Thus, the line position of the n(S– OH) can be influenced by the displacement of protonic position along S–OH/O–S [25]. As for pure CHS, it is considered the abrupt red-shift of the n(S–OH) according to the phase transition from I to II is due to a lengthening of the S–OH bond, which suggests a weakening of the hydrogen bond of (HSOK 4 )n. Additionally, the discrete broadening (FWHMw4 cmK1) of n(S–OH) at the phase transition from I to II suggests an orientational disorder of the HSOK 4 ion [39]. This rearrangement from I to II has been interpreted as a structural change from the infinite chain, (HSOK 4 )n to a cyclic dimer, (HSOK ) , with the subsequent weakening of the 4 2 hydrogen-bond [25]. At the phase transition from phase II to III in pure CHS, an abrupt red-shift and a line-broadening again occur, which suggests that further weakening of the hydrogen bond and increase of the orientational disorder of the HSOK 4 ion will be induced. Also, the abrupt change of the linewidth of the S–OH stretching mode is significantly amplified at the phase transition from II to III in pure CHS. Therefore, in phase III, the HSOK 4 ion can be characterized by rapid reorientational motion [25]. It is considered that this HSOK 4 ion-dynamic realizes fast proton diffusion in the superprotonic phase [3,39–41]. At first, the displacement of a proton occurs along the hydrogen bond closer to SO4 tetrahedron forming this bond. Next, the longer half of the hydrogen bond is broken, and then the HSOK 4 ion rotates to a new position. Through this reorientation process, a new hydrogen bond forms between neighboring SO4 tetrahedra so that the proton transfers along the newly formed hydrogen
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bond. This process is repeated, and thus, the superprotonic conduction is realized [3,39–41]. The above statements suggest that the proton-conductivity increases as the linewidth broadens. In CHS/SiO2 composite, the spectral shift of the n(S–OH) as a function of temperature accords with that of pure CHS at phases II and III. Thus, the displacement of the protonic position of S–OH is not strongly induced by silica additives. On the other hand, the fact that the linewidth of the n(S–OH) of the composite surpasses that of pure CHS between 350 and 420 K suggests that the reorientational motion of the HSOK 4 ion is partly enhanced in the composite even far below the critical temperature, 414 K. This dynamical motion of the HSOK 4 ion will be the origin of the remarkable enhancement of conductivity at the low conduction phase in the composite. The present evaporation-to-dryness method allows the CHS to infiltrate well into the mesopores of silica. The new interfacial phase that has a disordered structure is generated in the mesopores and/or on the surfaces of silica particles. In fact, the present silica particles can hold 75% of the CHS in the mesopores, taking into account the molar ratio of the composite and pore volume of silica. Thereby, the reorientational motion of the HSOK 4 ion below the critical temperature will be induced mostly in the interfacial phase, which results in high conductivity. The above description is also consistent with the DTA result. The observed broad peak for endothermic heat flow under the critical temperature (391–414 K) in the composite probably originates from a hydrogen-bond break that subsequently induces reorientational motion of the HSOK 4 ion at the interface. At the interface, a structurally disordered phase can form due to an interfacial interaction between CHS and silica. The interfacial affinity probably contributes to disordering of the structure at the interfacial phase and the reorientational motion of the HSOK 4 ion even below the critical temperature, leading to the enhancement of proton conductivity. The protonic conduction process in the CHS/SiO2 composite can be conclusively described as follows. CHS in composite has two main phases, i.e. a bulky phase and an interfacial phase. The interfacial phase having a disordered crystal structure forms in the mesopores and/or on the surface of silica particles. The relevant amount of silica particles and their sufficient dispersion in the composite system will form protonic conduction pathways via the interfacial phase from electrode to electrode. Even below the critical temperature of the superprotonic phase transition, the reorientational motion of the HSOK 4 ion will be induced in the interfacial phase to enhance protonic conduction, which is comparable to the superprotonic conduction, whereas the bulky phase maintains the low conduction phase. When the temperature surpasses the critical temperature, neither phase can be distinguished in terms of conductivity. The conductivities of the both phases reach the superprotonic realm.
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4. Conclusion CsHSO4/SiO2 composite prepared by means of the evaporation-to-dryness method showed remarkable improvement of the conductivity by more than three orders of magnitude even below the superprotonic phase transition temperature in comparison with that of pure CsHSO4 (353–414 K). In view of the structural transformation of CsHSO4 in the composite, Raman spectroscopy and X-ray diffraction measurements suggest that the tetragonal phase (phase III) that corresponds to the superprotonic conduction phase is not stabilized in CsHSO 4/SiO2 composite below its critical temperature, ca. 414 K, although the monoclinic phase (phase II) is stabilized in the composite even below its critical temperature. Thus, the reasons for conductivity-enhancement other than the stabilization of phase III should be considered. Internal vibration study of Raman spectroscopy was performed in the 293–446 K range, with attention focused on the spectral shift and the linewidth of the S–OH internal stretching mode. There are no apparent differences in the spectral shift between the composite and pure CsHSO4 around the critical temperature of the superprotonic phase transition. On the other hand, the fact that the bandwidth of the composite is apparently larger than that of pure CsHSO4 in the region of 350–420 K suggests that the reorientational motion of the HSOK 4 ion is induced even below the critical temperature of the superprotonic phase transition. This dynamical motion of the HSOK 4 ion probably accounts for the conductivityenhancement. The DTA results also support the HSOK 4 ion dynamics described above, namely, the endothermic heat flow occurring below the critical temperature in the composite. It is concluded that the rapid reorientational motion of the HSOK 4 ion is induced even below the critical temperature in the interfacial phase that forms in mesopores and/or on the surfaces of silica particles, and thus the protonic conduction is enhanced especially below the critical temperature of the superprotonic phase transition.
Acknowledgements The authors are very grateful to Mitsubishi Chemical for providing the SiO2 particles. The authors also acknowledge the financial support by a Grant-in-Aid for Young Scientists (B), No. 14750614, from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.
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