J. Phys. Chem. A 2011, 115, 35–38
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Probing Structural Transition and Guest Dynamics of Hydroquinone Clathrates by Temperature-Dependent Terahertz Time-Domain Spectroscopy Eui Su Lee,† Kyu Won Han,‡ Ji-Ho Yoon,*,‡ and Tae-In Jeon*,† DiVision of Electrical and Electronics Engineering, Center for SubwaVelength Optics, and Department of Energy & Resources Engineering, Korea Maritime UniVersity, Busan 606-791, Korea ReceiVed: September 24, 2010; ReVised Manuscript ReceiVed: NoVember 18, 2010
The structural transition from hydroquinone clathrates to crystalline R-form hydroquinone was observed up to the range of 3 THz frequency as a function of temperatures. We found that all three hydroquinone clathrates, CO2-, CH4-, and CO2/CH4-loaded hydroquinone clathrates, transform into the R-form hydroquinone at around 102 ( 7 °C. The resonance peak of the CO2-loaded hydroquinone clathrate at 2.15 THz decreases with increasing temperature, indicating that CO2 guest molecules are readily released from the host framework prior to the structural transformation. This reveals that the hydroquinone clathrates may transform into the stable R-form hydroquinone via the metastable form of guest-free clathrate, which depends on guest molecules enclathrated in the cages of the host frameworks. A strong resonance of the R-form hydroquinone at 1.18 THz gradually shifts to the low frequency with increasing temperature and shifts back to the high frequency with decreasing temperature. Introduction The terahertz (THz) region ranging from 0.1 to 10 THz contains a lot of information on low-frequency vibration motion of small gas molecules, large biological molecules and condensed phase materials.1-9 The THz time-domain spectroscopy (THz-TDS) allows us to examine both intramolecular and intermolecular vibrations related to the hydrogen bonding networks and the interactions between molecules in the framework structure. This suggests the potential of THz-TDS in studying the dynamics of the host-guest interaction of inclusion compounds composed of gaseous guest molecules and solidstate host frameworks.10 Hydroquinone (HQ) clathrate is one of the inclusion compounds that are formed from two or more molecular species and associated with the enclosure of guest molecules within a suitable structure formed by the host molecules. A variety of guest species such as Ar, Kr, Xe, SO2, CH4, and methanol has been reported in the literature.11-13 The ideal molar ratio of HQ to guest molecules expected for HQ clathrates is 3:1. Powell14 investigated the gas release from the HQ host framework at 130 °C. A recent MD study indicates that hydrogen molecules can be spontaneously loaded into the open networks of the HQ clathrates.15 This requires high gaseous hydrogen pressures around 200 MPa.16 X-ray diffraction, nuclear magnetic resonance, and Raman spectroscopy have been used to study the characteristics of the HQ clathrates.11,12,16,17 In our previous work,10 we have shown that the THz-TDS provides a convenient and simple way to characterize various clathrate compounds by identifying the differences in the intermolecular bonding. It was found that the resonances of HQ clathrates could readily be defined in the THz frequency range. In this study, the in situ measurements of the THz spectra of CO2-, CH4-, and CO2/ CH4-loaded HQ clathrates with a variation of temperature are * To whom correspondence should be addressed. E-mail: (J.-H.Y.)
[email protected]; (T.-I.J.)
[email protected]. † Division of Electrical and Electronics Engineering, Center for Subwavelength Optics. ‡ Department of Energy & Resources Engineering.
Figure 1. A schematic diagram of setup with hydroquinone pellets for the THz time-domain spectroscopy.
performed. We report that the temperature-dependent THz-TDS can probe the structural transition and guest dynamics of the HQ clathrates, which significantly depends on the guest species encaged in the HQ clathrate framework. Experimental Section To prepare the HQ clathrates, pure HQ was allowed to react with CO2 and CH4 pure gases and CO2/CH4 (50/50) gas mixture, respectively, in high pressure reactors. Before loading, HQ powders were ground and filtered to particle sizes less than 100 µm to promote the reaction with the gas. After loading the fine HQ powders, the reactor was purged with gas at least three times to remove residual air and pressurized with gas up to 6-12 MPa. The pressurized mixture was kept for 5 days at room temperature. The sample products were then collected by slowly releasing pressure. The chemical analysis indicates that the cages of the HQ clathrates could preferentially be filled by the guest molecules, leading to 0.74CO2 · 3HQ, 0.69CH4 · 3HQ, and 0.47CO2 · 0.2CH4 · 3HQ for CO2-, CH4-, and CO2/CH4-loaded HQ clathrates, respectively. Figure 1 shows a schematic diagram of the experimental setup of THz-TDS system used in this study.18,19 A Ti:Sapphire laser
10.1021/jp109119n 2011 American Chemical Society Published on Web 12/13/2010
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Figure 2. Measured THz pulses (left) transmitted through the hydroquinone clathrate pellets and their corresponding amplitude spectra (right) obtained by numerical Fourier transforms. Blue and red lines stand for as-synthesized HQ clathrates and HQ samples at 26 °C after transformation by the heating process, respectively. (a,d) CO2-loaded hydroquinone clathrate. (b,e) CH4-loaded hydroquinone clathrate. (c,f) CO2/CH4-loaded hydroquinone clathrate. The dotted lines are the reference spectra by fitting the lines to outside the sample’s spectra.
provided 800 nm, 60 fs laser excitation pulses at a 83 MHz repetition rate with an average power of 12 mW for transmitter and 16 mW for receiver. When the laser pulses arrive at the transmitter or receiver chip, the THz pulses are generated or detected, respectively. The THz system is in an enclosure maintained at a relative humidity of less than 2% to mitigate the effects of water vapor. The samples were prepared into pellets (diameter of 16.2 mm and thickness of about 750 µm) and were placed between two aluminum plates with a 3 mm diameter aperture at the center as shown in Figure 1. Two thermoelectric modules were attached on the outside of the plate surface to control the temperature. The THz beams propagate only through the samples by the aperture because the THz beam cannot pass through the highly conductive metal plate. The temperature of each sample was increased from 26 to 122 °C by controlling the current of the thermoelectric module, followed by decreasing to 26 °C by turning off the current. The total time durations of increasing and decreasing temperature for each sample were 77 ( 3 and 48 ( 4 min, respectively. Results and Discussion The THz pulses and spectra of HQ clathrates before and after the heating process up to 122 °C are shown in Figure 2. The THz pulses are measured in 12 ps to avoid multiple reflections from the sample. The zero padding to the measured data is performed to increase a frequency resolution up to 2.6 GHz. The THz transmission spectra of the CO2- and CH4-loaded HQ clathrates show two strong vibrational lines whereas the CO2/ CH4-loaded HQ clathrates show only one vibrational line. We note that after the heating process up to 122 °C, the vibrational lines of HQ samples between 0 and 3 THz are very different from those of the as-synthesized HQ clathrates, indicating the phase transition to a new stable structure. The positions of all vibrational lines including two strong lines at 1.18 and 1.68
Lee et al.
Figure 3. (a) Temperature-dependent THz absorbance of CO2-loaded hydroquinone clathrate. For clarity, the absorbance is vertically shifted. The time sequence of the heating up to and through the phase transition starts at 26 °C (the lowest absorbance curve shown in the figure) then monotonically increases up to the phase transition temperature at 95 °C and continues up to the highest temperature of 122 °C. Upon reaching 122 °C, the temperature is then monotonically reduced to 26 °C for which the measured absorbance curve is shown at the top of the figure. The monotonic heating cycle takes about 77 min and the cooling cycle takes about 48 min. (b) Comparison of the absorbance of the sample at 26, 95, and 26 °C after the cooling process. (c) Relative magnitude of THz absorbance at 1.85 and 2.15 THz (as shown in inset with thick vertical lines) as a function of temperature.
THz are identical to those of crystalline R-form HQ.10 This indicates that all HQ clathrates transform into crystalline R-form HQ through the heating process. The difference in the amplitude between the HQ clathrates and R-form HQ can be attributed to the scattering of THz pulses by the surface roughness of R-form HQ due to the release of guests during the structural transformation. However, comparing to the results in our previous work,10 we confirm that the distinct vibration lines of R-form HQ through the heating process and structural transformation are identical to those of pure crystalline R-form HQ without any treatments. Figure 3a shows the THz absorbance of CO2-loaded HQ clathrate as a function of temperature. The first and second vertical dotted lines at 1.18 and 1.68 THz represent two main THz resonance frequencies of R-form HQ. The resonance peak at 1.18 THz is the second biggest and isolated one from its neighboring resonances. The resonance increases drastically between 95 and 122 °C during the heating process, whereas there is a minute increase of the resonance between 122 and 26 °C during the cooling process. The position of the resonance at 1.18 THz gradually shifts to the low frequency with increasing temperature and shifts back to the high frequency with decreasing temperature. The third vertical dotted line at 1.85 THz stands for the dominant THz resonance frequency of HQ clathrates. As the temperature increases up to 95 °C, the shape of the main resonance peak does not change significantly. At temperatures between 95 and 109 °C, there is a clear change in the patterns of the THz spectra, indicating their structural transformation to crystalline R-form HQ. Therefore, the midpoint of the temperature variations, 102 ( 7 °C, should be considered as the transition temperature of the HQ clathrates to the crystalline R-form HQ. When the temperature decreases back to 26 °C, the crystalline R-form HQ could not be recovered to the HQ clathrates. This indicates that the crystalline R-form HQ is the most stable solid at ambient condition. Interestingly, the intensity
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Figure 4. Temperature-dependent THz absorbance of (a) CH4-loaded hydroquinone clathrate and (b) CO2/CH4-loaded hydroquinone clathrate. For clarity, the absorbance is vertically shifted. Comparison of the absorbance of (c) CH4-loaded hydroquinone clathrate and (d) CO2/CH4-loaded hydroquinone clathrate at 26, 95, and 26 °C after the cooling process.
of the second resonance of HQ clathrates at 2.15 THz decreases nonlinearly with the increasing temperature and then minimized around 95 °C where the HQ CO2-loaded clathrate starts to transform to the crystalline R-form HQ. Moreover, it is found that a small resonance at around 1.85 THz comes out with the increasing temperature up to 95 °C. Figure 3b shows the absorbance patterns at 26, 95, and 26 °C (after the cooling process) for comparison. The resonance at 2.15 THz decreases with the increasing temperature, whereas the resonance at 1.85 THz increases in the range of 67-95 °C, as shown in Figure 3c. The chemical analysis and X-ray diffraction measurements show that the CO2-loaded HQ clathrate kept at 80 °C for 1 day does not contain CO2 molecules even though it still retains the clathrate structure. From these results and temperature-dependent THz-TDS patterns, we conclude that the CO2 molecules encaged in the CO2-loaded HQ clathrate are gradually released from the clathrate frameworks during the heating process. This also represents that the structural transition of the CO2-loaded HQ clathrate to the crystalline R-form HQ is started at 95 °C where most of CO2 guest molecules are released from the clathrate framework. It should be noted, therefore, that the resonances at 2.15 and 1.85 THz for the CO2-loaded HQ clathrate are a crucial factor to examine the dynamics of CO2 guests in the cages of the HQ clathrates. The CH4- and CO2/CH4-loaded HQ clathrates show similar transition behaviors as shown in Figure 4. The third vertical dotted lines are the dominant THz resonances of the CH4- and CO2/CH4-loaded HQ clathrates at 1.95 and 1.98 THz, respectively. We note that the dominant resonance of the CH4-loaded HQ clathrates shows the double peaks at 1.9 and 1.95 THz. In our earlier paper,10 we measured the THz signals until 67 ps including the THz multiple reflections by the CH4-loaded HQ samples. When we used the HQ samples with the thickness of 500 µm, the first multiple reflection occurred at 5.3 ps after the main THz pulse. These multiple reflections lead to periodic oscillations in the spectrum domain. The periodic oscillation does not allow us to observe the double peaks in a narrow range of the THz spectra, resulting in the single resonance at 1.9 THz as shown in our previous measurements.10 To remove the effect of the multiple reflections by the samples on the THz signals, we used the HQ clathrate samples with the thickness of 750
µm in this work. Therefore, the double peaks of the absorbance curve at around 1.9 THz are clearly shown in Figure 4. The phase transition of the CH4- and CO2/CH4-loaded HQ clathrates to the crystalline R-form HQ is also found to be at around 102 ( 7 °C. The first and second resonances at 1.9 and 1.95 THz of CH4-loaded HQ clathrate are sharper than those of the CO2loaded HQ clathrate. For the CH4-loaded HQ clathrate, the intensity of the second peak decreases with a variation of temperature in the range of 26-95 °C, indicating that the CH4 molecules gradually released from the host frameworks during the heating process. When the second resonance almost disappears at 95 °C as shown in Figure 4c, the structural transition to the crystalline R-form HQ is started. As a result, they are released from the clathrate frameworks only at the structural transition over 95 °C, which is identical to the results from the temperature-dependent synchrotron X-ray diffraction coupled with the quadrupole mass spectrometric detection.17 A relatively broad THz resonance of the CO2/CH4-loaded HQ clathrate around 1.98 THz at 26 °C is observed as shown in Figure 4d. This is attributed to the inclusion of both CO2 and CH4 molecules in the cages of the HQ clathrate frameworks. As expected, the transition temperature of CO2/CH4-loaded HQ clathrate is between 95 and 109 °C, which is consistent with those of CO2- and CH4-loaded HQ clathrates. The stability of the HQ clathrates is of particular interest in view of the weak hydrogen bond that plays an important role in the framework structure. The clathrate structure encaging the guest molecules is very stable at ambient conditions. However, the HQ clathrates are somewhat temperature sensitive, as HQ molecules readily sublimes at temperatures below its melting point, 170 °C.20 Thus, at high temperatures the stability of the HQ clathrates only depends on the power of the hydrogenbonding networks of HQ clathrates, rather than the interactions between host (HQ) and guest molecules. This allows all HQ clathrates to show the same transition temperature at 102 °C. Consequently, we argue that there is no chemical potential difference between the guest-free HQ clathrate and the crystalline R-form HQ at the transition temperature.
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Conclusions In summary, we have in situ measured the structural transition of the CO2-, CH4-, and CO2/CH4-loaded HQ clathrates using the temperature-dependent THz-TDS. For all HQ clathrates, the transition temperature is found to be around 102 ( 7 °C. The distinct molecular dynamics of CO2 and CH4 guests in the cages of the HQ clathrate frameworks could be probed by observing the resonance spectra at 2.15 and 1.95 THz, respectively, as a function of temperature. It was also found that all resonance frequencies in the rage of THz of the crystalline R-form HQ as well as the HQ clathrates show the strong temperature-dependent characteristics. These results suggest that the temperaturedependent THz-TDS provides a useful guide to probe the structural transition and the guest dynamics of clathrate inclusion compounds, leading to identification of thermodynamic stability of the host-guest frameworks. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) (SRC: R112008-095-01000-0, M10755020001-08N552-00110), the Grant of the Korean Health Technology R&D Project, Ministry for Health, Welfare & Family Affairs, Republic of Korea (A101954), the Korea Research Foundation Grant funded by the Korean Government (KRF-2008-313-D01285, NRF-2009-0087440) and the Ministry of Knowledge Economy by the Regional Industry Technology Development (Grant 70004607).
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