Pressure-Driven Topological Transformations of Iodine Confined in

Article pubs.acs.org/JPCC

Pressure-Driven Topological Transformations of Iodine Confined in One-Dimensional Channels Mingguang Yao, Tianyi Wang, Zhen Yao, Defang Duan, Shuanglong Chen, Zhaodong Liu, Ran Liu, Shuangchen Lu, Ye Yuan, Bo Zou, Tian Cui, and Bingbing Liu* State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China S Supporting Information *

ABSTRACT: The behavior of molecules and molecular chains confined in 1D nanochannels imposed by external interactions is a problem of fundamental interest. Here, we report structural manipulation of iodine confined inside zeolite (AFI) nanochannels by the application of high pressure. Structural transformations of the confined iodine under pressure have been unambiguously identified by polarized Raman spectroscopy combined with theoretical simulation. The length of the iodine chains and the orientation and intermolecular interaction of the confined iodine have been tuned at the molecular level by applied pressure. Almost all the confined iodine can be tuned into an axially oriented state upon compression, favoring the formation of long chains. The long iodine chains can be preserved to ambient pressure when released from intermediate pressures.

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requires further manipulation.17 For this purpose, understanding the transformation dynamics of the confined iodine imposed by external interactions becomes very important. High pressure serves as a powerful tuning parameter that has been used to tune the intermolecular interaction and structure of bulk materials.18−23 In the confined environment, the configuration of the material is expected to be modified not only by the applied pressure but also by the evolution of the confinement volume of the host matrix.21 The transformations of the host matrix under pressure have been studied during the past years. Kim et al. recently report compression behavior of SSZ-24, which is a zeolite framework containing hollow channels that can accommodate guest materials. They found SSZ-24 was among the most highly compressible zeolites, which is probably suited to inducing changes in confined guests under pressure.24 Our recent studies on empty zeolite AlPO4-5 (AFI) and N2 filled AFI show that the framework stability of the hosting AFI filled with nitrogen is significantly enhanced compared to the empty AFI,23 while how pressure affects the confined guest structure is not clear. On the other hand, in the case of pressure treatment of iodine-filled SWNTs, the arrangement of the iodine is found to be changed. Nevertheless, the intercalation sites for iodine in nanotube bundles are quite complex (both interstitial channels and the tube pores), and the confined iodine can take up several configurations due to the diameter inhomogenity in nanotube bundles.15,16 This makes

INTRODUCTION Studies of the control and manipulation of atoms/molecules and their assemblies generate remarkable new insights into how physical and chemical systems function. They permit direct observation of molecular behavior that can be obscured by ensemble averaging and enables the study of important problems ranging from fundamental physics to the design of nanoscale electro-optical devices. In particular, much effort has been focused on the control of atomic/molecular chains due to their potential application as quantum wires.1−9 By using simultaneous STM and TEM (transmission electron microscopy), gold nanowires composed of several atomic chains have been fabricated and show quantum conductance behavior.2 Later, thinner nanowires have been fabricated in high vacuum with an electron beam thinning technique but the stability becomes lower with decreasing size and the length is still limited to a few nanometers ( 75°) at 0 GPa, in a direction forming a 45° angle with the channel at 5 GPa and to be oriented in quasi-parallel to the channel at 10 GPa. Such a pressure-induced rotation of the iodine molecules is consistent with our experimental observations. At ambient pressure, the intensity of the Raman peak at 210 cm−1 is noticeably stronger than that of the peak at 206 cm−1, suggesting that the population of perpendicular I2 molecules is larger than that of the parallel ones, that is, the nonchained I2 prefers to be oriented perpendicular to the walls. Upon compression, the AFI channels are squeezed and the interior space contracts; thus, the molecules rotate to lie along the channels. Such a rotation of the I2 molecules together with the compression of the c-axis (i.e., the channel length) in turn leads to a concomitant decrease of the distance between neighboring molecules such that the iodine molecules come sufficiently close together to form (I2)n wires (polymers). Thus, in the experiment a significant increase of the intensity ratio between the 166 cm−1 line due to (I2)n chains and the 210 cm−1 line due to I2 molecules and an enhancement of the overtone band at 334 cm−1 have been observed under pressure (with a maximum at 2−3 GPa; Figure 1). At about 5 GPa, the parallel iodine chains (166 cm−1) and molecules (206 cm−1) both undergo transitions into another parallel state with a new, strong peak at 197 cm−1 and a different pressure evolution at higher pressure (Figure 1). With

Figure 4. Raman spectra for iodine@AFI at ambient pressure and released from 7.2 and 15.1 GPa, respectively.

spectra of the as-prepared iodine@AFI and the samples decompressed from 7.2 and 15.1 GPa. Decompressed from medium pressure, the AFI framework is recovered, and the Raman intensity from (I2)n chains becomes much higher than that from either parallel or perpendicular I2 molecules in the channels relative to that of as-prepared iodine@AFI. This suggests that the (I2)n wires grow in length at the expense of free (parallel and perpendicular) I2 molecules in the channels and that the elongated chains are preserved upon decompression. However, when decompressed from the higher pressure of 15.1 GPa the peak intensity from (I2)n chains is significantly lower than those from the free I2 molecules, suggesting that (I2)n chains were decomposed into I2 molecules 25055

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Figure 5. Models of parallel (a) and perpendicular (c) iodine arrays inside the AFI channels; calculated intramolecular vibration frequency for parallel (b) and perpendicular (d) iodine as a function of pressure. The experimental data are plotted for comparison. The grid area indicates the transition point.

patterns show that some diffracted peaks disappear or slope change in the pressure dependence at around 5 GPa, suggesting that the AFI undergoes a transition. Similar changes have also been observed in our previous high pressure studies on nitrogen filled AFI and discussed within the framework of the ovalization of AFI channels,23 which possibly leads to either a lower symmetry of the starting crystal structure (hexagonal P6cc) or partial phase transition of AFI, into a orthorhombic structure similar to that of AlPO4-11 (AEL) with elliptic channels. The obvious contraction of the AFI channels above 5 GPa in turn results in a significant enhancement of the interaction between the channels and the confined iodine species. Such an effect is more pronounced for the vibrations of the remaining perpendicular I2 molecules in the channel due to the spatial confinement, and thus the slope of this mode become obviously steeper above 5 GPa (Figure 5d). The changes in channel dimensions above 5 GPa can also affect the Raman vibration frequencies of the parallel iodine molecules and chains due to changes in the interaction. In fact, the frequency of an (I2)n chain has been reported to show a significant upshift to 196 or 199 cm−1 when it is confined inside elliptic channels of silicaferrierite (0.42−0.54 nm, 10-member ring channels)11 and AEL (0.44−0.67 nm).12 It should be noted here that the elliptic channels can be taken as a 2D confinement environment compared with the size of iodine molecule, in which the rotation of I2 can only occur along the long-axis of the elliptic cross section of the channel. Thus, the vibration of the iodine molecule is different from that in a round channel of AFI, due to different confinement effect. Also, for our sample decompressed from 15.1 GPa, the intense and broad Raman peak at ∼200 cm−1 probably also contains a Raman scattering contribution from the chains confined in the deformed channels (Figure 4). On the other hand, a possible change in the bonding character of the parallel iodine molecules/(I2)n

after the compression. This can be explained by the irreversible deformation of the AFI channels deduced from XRD measurement. We further calculated the phonon spectra for parallel and perpendicular iodine molecules confined in AFI channels under pressure. The curves for the calculated intramolecular Ag stretching mode of I2 with the two configurations as a function of pressure are plotted in Figure 5 and compared with the experimental results. The ambient frequencies calculated for the I2 molecule in the channels depend on the filling density and always show a redshift compared to the value calculated for I2 in the gas phase, which is consistent with experimental observations.10,13 We can see that the pressure dependences of the frequencies for the confined iodine from the simulations show an evolution similar to that found in the experiment, in which the slope of frequency versus pressure for the two simulated modes changed noticeably at around 5 GPa. Let us now discuss the origin of the observed transitions at ∼5 GPa for the confined iodine species. The simulations show that the AFI lattice is compressed gradually and above 5 GPa a higher compressibility is observed along the a-axis but a lower compressibility along the c-axis (Figure S6). Also, the a-axis becomes slightly more compressible than the b-axis above this pressure, suggesting a gradual ovalization of the AFI channel. These transitions of AFI are consistent with our high pressure synchrotron X-ray diffraction measurements. In the high pressure XRD measurements on our iodine@AFI sample, the observed diffracted peaks are mainly from AFI crystal, while the diffraction from 1D iodine chains probably is too weak to be observed. The recorded diffraction patterns are shown in Figure S7 and compared with that of our published data on empty AFI and N2-filled AFI.23 It is clear that the framework stability of the hosting AFI filled with iodine is much higher than that of empty AFI. This is due to the support of the filled species to the AFI channels, similar to that observed in N2@AFI. The recorded 25056

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wires due to the increase of the intermolecular interaction at above 5 GPa can not be excluded, for example, into an atomic chain, because the observed pressure evolution for parallel iodine molecules is probably different from the simulated configuration above 5 GPa. However, this question requires further investigation, to shed light on the synthesis of atomic chains and the metallization of diatomic molecules in a nanochannel under pressure.

CONCLUSION In summary, the existence of structural transformations of confined iodine inside the 1D channels of single crystal AlPO45 under pressure has been demonstrated. The transformations include the growth of (I2)n chains and structural transitions for both parallel and perpendicular iodine species due to changes in the interaction with the channel walls. In particular, the parallel iodine molecules and chains transform into a new parallel state with a different pressure evolution. These transitions can be rationalized by a pressured-induced rotation of iodine molecules with increasing pressure, leading to a population increase for parallel iodine molecules at the expense of a population decrease for perpendicular I2 in the channels. The elongated iodine chains can be preserved to ambient conditions when released from medium pressure. The observed structural transformations in single iodine chains can be considered as a model example for both experimental and theoretical studies of the behavior of a 1D molecular chain confined in a nanochannel, while the rotation of single iodine molecules can be taken as an example of the behavior of diatomic molecules confined in a nanochannel. In both cases, the system can be manipulated at a molecular level by applying pressure. The present methodology can be extended to study other similar confinement systems under pressure. ASSOCIATED CONTENT

S Supporting Information *

SEM images of the iodine@AFI crystals, fitting analysis of the Raman spectra of the sample at pressures, theoretical and experimental results of XRD measurements on the sample under pressure, and polarized Raman measurements of iodine@AFI at different pressures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 86-431-8516825. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2011CB808200), the NSFC (10979001, 51025206, 51032001, 21073071, 11104105), and the Cheung Kong Scholars Programme of China. The authors would like to thank Prof. Jihong Yu, Prof. Jiyang Li and Dr. Jiancheng Di from State Key Laboratory of Inorganic Synthesis and Preparative Chemistry in Jilin University for the synthesis of AFI single crystals. The authors also thank Prof. Bertil Sundqvist for useful discussions. 25057

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