Density Functional Theory and - American Chemical Society

Sep 22, 2016 - College of Science, Liaoning University of Technology, Jinzhou, ... Institute of New Energy, Bohai University, Jinzhou, Liaoning 121000...
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Effect of High Pressure on the Transformations of Ferrocene-Filled, Single-Wall, Carbon Nanotubes: Density Functional Theory and Raman Spectroscopy Studies Zhen Yao,† Jing Zhang,‡ Ming-Guang Yao,*,‡ Shuang-Long Chen,§ and Bing-Bing Liu*,‡ †

College of Science, Liaoning University of Technology, Jinzhou, Liaoning 121001, China State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun, 130012, China § Institute of New Energy, Bohai University, Jinzhou, Liaoning 121000, China ‡

ABSTRACT: Metallocene-filled single-walled carbon nanotubes (SWCNTs) have tunable electronic properties and a large potential in creating new structures due to their tunable electron doping and unique chemical reaction in a nanoscale confinement environment. Here we study the effect of high pressure on the transformations of ferrocene (FeCp2)-filled SWCNTs (FC@SWCNTs) by theoretical simulation and Raman spectroscopy. It is found that the filling of FeCp2 into carbon nanotube leads to higher transition pressures for the nanotube cross section changes and the intertubular bonding compared with the unfilled nanotubes. These results can be explained by the different charge distribution on the nanotubes due to charge transfer and the effect from host−guest interactions. Band structure analysis shows a pressure-induced decrease of band gap below 11 GPa, a transformation into a semimetal structure at 11 GPa, and subsequently, an abrupt increase of band gap due to the formation of intertube bonding. The increased host−guest interaction also leads to a decomposition of FeCp2 and the formation of a new 3D zeolite-like structure, which is quenchable to ambient conditions. Our preliminary experimental results confirm that the encapsulation of FeCp2 affects the transformations of nanotubes, which can be explained by the host−guest interactions, as demonstrated by our simulations.

I. INTRODUCTION Encapsulation of molecules inside single-wall carbon nanotubes (SWCNTs) is of great research interest, which generates unique one-dimensional molecular nanostructures that have a large potential to exhibit novel electronic properties.1,2 Among the filling materials reported, metallocene is in the focus of attention due to their large variation of the chemical properties depending on the metal atom species.3,4 Theoretical investigations have shown that encapsulation of different metallocenes (C5H5)2M, where M is a metal center, leads to electron doping of SWCNTs that depends on their chirality and the type of metal atom.5−10 It has also been found that filling metallocene into SWCNTs is a very effective way to modify the electronic structure of nanotubes and to observe unique chemical reaction in a nanoscale confinement environment.11 Experimental studies proved that electron doping was observed for SWCNTs encapsulating ferrocene,12 nickelocene,13 and cerocene.14,15 It has also been reported that cobaltoceneencapsulated SWNTs (denoted CoCp2@SWNTs) exhibited a negative-type (n-type) semiconducting behavior, which is an air-stable and flexible thermoelectric material with high conversion efficiency.16 Furthermore, upon heat-treatment (500−700 °C), organometallic molecules encapsulated inside SWCNTs can be reacted to form metallic particles that subsequently catalyze the formation of inner carbon nanotubes in a confined tubular environment.17−20 It is found that such © XXXX American Chemical Society

reactions inside the nanotubes can lead to changes in the electron doping of SWCNTs to lower levels. Recently, chemical reaction in nickelocene-encapsulated SWNTs has been systematically studied at high temperature, which results in the formation of nickel carbides, nickel and inner carbon nanotubes.13 As far as we know, most of these studies are carried out at ambient pressure and high temperature,21−24 while little attention has been paid to the study of metallocenefilled SWCNTs under high pressure. As one of the independent thermodynamic parameters apart from those usually studied, such as temperature and composition, pressure plays an important role in tuning the structure and properties of materials. Applying pressure is an effective way to shorten atomic distance, enhance orbital overlap, and consequently modify material structures and tune the interactions.25−28 Thus, it is very promising to tune the structure and properties of metallocene-encapsulated SWNTs by using high pressure technique, giving new insight into the host−guest interactions between carbon nanotubes and filler molecules. This may also reveal the high-pressure-driven reactions of encapsulated molecules in a nanoscale confinement Received: May 28, 2016 Revised: September 21, 2016


DOI: 10.1021/acs.jpcc.6b05398 J. Phys. Chem. C XXXX, XXX, XXX−XXX


The Journal of Physical Chemistry C

relaxing both the lattice constants and the atomic positions in the unit cell simultaneously. The first-principles total energy and electronic structure calculations were performed by using the pseudopotential plane wave method within the generalized gradient approximation (GGA). In order to obtain the decompression structure, we carried out molecular dynamics simulation in the microcanonical ensemble (NPT: constant number of particles, pressure, and temperature). The pressure and temperature is set at 0 GPa and 298 K, respectively. For the filling experiment, single-walled carbon nanotubes with diameter distributed from 1.3−1.6 nm and length of several micrometers have been used. The purified SWCNT samples were heated at 420 °C in air for 1 h to open the ends and then degassed at 450 °C for 2 h under dynamic vacuum condition (10−3Pa) to remove any functional groups adsorbed on the nanotube surface. Stoichiometric amounts (2:1 in weight) of FeCp2 and SWCNTs were inserted into two separate parts of a U-shaped glass tube and sealed under vacuum condition (10−3 pa). The samples were heated at an optimized filling temperature of 300 °C for 48 h. Finally, the obtained samples were washed with diethyl ether for several times to remove the FeCp2 absorbed on SWCNT out surface. The prepared FeCp2 filled SWCNT sample was characterized by Raman spectroscopy (Renishaw in Via) and IR spectroscopy. High pressure experiments were carried out in a diamond anvil cell with silicone oil as pressure transmitting medium (PTM). It should be noted that our experiments were performed at room temperature and thus the silicon oil molecules were unlikely to be inserted into the nanotubes because of their large size and the strong adhesive interactions between the oil molecules. The R1-line emission of a tiny ruby chip was used for pressure calibration. The hole drilled in the gasket served as the sample chamber. For a direct comparison, we loaded both FC@SWCNTs and empty SWCNTs samples into one sample chamber (Figure 2). The maximum pressure reached in our experiments is 15 GPa.

environment and thus create new structures, providing parallel results as those obtained from high temperature experiments. In this work, we studied the effects of high pressure on the transformations of ferrocene-filled Single wall carbon nanotubes by using density functional theory and Raman spectroscopy measurement. It is found that the filling of FeCp2 obviously affects the structural transformation of nanotubes upon compression. Band structure analysis on FC@SWNTs shows a pressure-induced decrease of band gap below 11 GPa and subsequently an abrupt increase of the band gap due to the intertube bonding. The different charge distribution on the nanotubes due to electron doping is supposed to be responsible for the higher transition pressure for the intertubular bonding compared with the unfilled nanotubes. The increased host− guest interaction leads to a decomposition of FeCp2 molecule into Fe atoms and C5H5 molecules and subsequently the formation of a new 3D zeolite-like structure which is quenchable to ambient conditions. Our preliminary experimental results confirm that the encapsulation of FeCp2 affect the structural transformations of the nanotubes, which can be explained by the host−guest interactions as demonstrated by our theoretical simulations.

II. THEORETICAL SIMULATION AND EXPERIMENT METHOD In order to simulate infinitely long nanotube systems and study the influence of interaction (FeCp2-SWCNT and FeCp2FeCp2) and the filling density of FeCp2 molecules on the high-pressure behavior of the hybrid peapod, we construct two one-dimensional (1-D) periodic models. Model I includes three primitive unit cells for the zigzag tubes with the lattice parameters a (b) and c, which are 13.54 and 12.85 Å, respectively; model II includes two primitive unit cells for the zigzag tubes with the lattice parameters a (b) and c, which are 13.54 and 8.58 Å, respectively (see Figure 1). Each cell contains

Figure 1. Sketch map for FC@SWCNTs with cell parameters of a = b = 13.54 Å, c = 12.85 Å (I), and a = b = 13.54 Å, c = 8.58 Å (II). Figure 2. Photographic image of SWCNTs and FC@SWCNTs samples loaded in the sample chamber in a diamond anvil cell.

a FeCp2 molecule for which its 5-fold axis coincides with the tube axis. Thus, the center−center distances of adjacent molecules are 12.85 and 8.58 Å, respectively. In model I, only the FeCp2−SWCNT interaction has been taken into account, while the FeCp2−FeCp2 interaction is ignored due to their lager intermolecular distance. In model II, both the FeCp2−SWCNT and FeCp2−FeCp2 interactions were considered. Based on the constructed model I and II of FC@ SWCNTs, density functional theory within the VASP code is employed to study the structural transitions and the electronic structure under high pressure. The optimized structures with minimum energy at each pressure point were obtained by

III. RESULTS AND DISCUSSION For the theoretical simulation, we first analyzed the structural evolution of the host SWCNTs and guest FeCp2 molecules by calculating their volumes as functions of pressure. The results are shown in Figure 3. For comparison, the high-pressure structures of unfilled SWCNTs have also been simulated and presented in Figure 3. The slope changes in the plotted volume-pressure curves can be correlated with the structural deformation and inter/intratubular interaction changes of FC@ B

DOI: 10.1021/acs.jpcc.6b05398 J. Phys. Chem. C XXXX, XXX, XXX−XXX


The Journal of Physical Chemistry C

Figure 4. Deformation of empty tube (a) and FC@SWCNT under pressure. Intermolecular distances of adjacent ferrocene molecules for model I (b) and model II (c) are 12.85 and 8.58 Å, respectively.

to a smaller deformation at the intertubular bonding position of nanotube, which might cause a higher pressure for the intertubular bonding. It is noted that FeCp2−SWCNT constructed in modes I and II exhibits different pressure responses when pressure is higher than 12 GPa. It is clear that the filling of FeCp2 into SWNTs leads to an obvious widening of the transition pressure range of the host nanotube compared with the unfilled nanotube, and the widening of the transition pressure range depends on the filling density. This is consistent with our experimental observations (see below). For model I, as the host nanotube deforms, the encapsulated FeCp2 molecule decomposes into two C5H5 fragments and an iron atom when the pressure is increased to 14 GPa (see Figures 4 and 5). Such FeCp2 decomposition is observed at 23 GPa in higher density filling (model II). The transformation and decomposition of FeCp2 molecules is a progressive process, that is, the FeCp2 molecule begins to distort around the tube axis direction at pressure higher than 12 GPa. As shown in Figure 6, we plotted the twist angle between the two opposite pentagonal rings of one FeCp2 molecule, which increases from 0 to 17° when the pressure increased from 12 to 22 GPa for the model II. While the corresponding transformation of FeCp2 molecules in model I is finished with the pressure up to 14 GPa. The different high pressure behaviors of SWCNTs as well as the encapsulated FeCp2 molecules between mode I and mode II at above 12 GPa also suggest that the interactions of both the FeCp2−SWCNT and FeCp2−FeCp2 molecules play an important role in their structural deformation. By the comparative study, we conclude that the FeCp2−SWCNT interaction plays a main role for the structural transition of FC@SWCNT at pressure below 12 GPa, while the FeCp2−FeCp2 intermolecular interactions cannot be ignored as the pressure increases further. Thus, the filling density also plays important roles on the structural transition of the hybrid peapods. Similar effect that filling density of fillers such as C70s in SWNTs strongly affects the transformations of nanotubes under pressure has also been reported in previous work.33 More interestingly, our molecular dynamics simulation suggests that the novel structure formed at high pressure can be preserved after the pressure is removed. As

Figure 3. Volumes of the constructed unit cell as functions of pressure for (a) the empty tube, (b) FC@(13,0)SWCNTs (model I), and (c) FC@(13,0)SWCNTs (model II).

SWCNTs. We also present the optimized structures of both materials at selected pressures in Figure 4. It can be seen that the cross section of unfilled SWCNT changes from circle to ellipse and then to flattened ellipse shape with pressure increased to ∼3 and 9 GPa, respectively. Intertubular bonding between SWCNTs is formed at above 8 GPa. The transitions observed in our nanotubes are generally consistent with previous studies in theory and experiment.29−32 For the FeCp2 filled nanotube peapods, two models with different filling densities of FeCp2 molecules (models I and II) have been studied. The results from both models show that the circle cross section of the hosting nanotube starts to deform at around 2 GPa, as evidenced by a slight drop in the nanotube volume in the volume-pressure curve. As pressure increases further, the cross section of host nanotube changed to an ellipse shape. It can be seen that the filling of FeCp2 molecules induces a lower circle-ellipse transition pressure (2 GPa) of SWCNT compared with the unfilled tubes (3 GPa). Then the filled nanotube changes into a walnut shape and starts to form intertubular bonding at above 12 GPa. This bonding pressure is higher than that for the unfilled SWCNTs. It can be seen that the supporting role of FeCp2 molecules to the nanotube leads C

DOI: 10.1021/acs.jpcc.6b05398 J. Phys. Chem. C XXXX, XXX, XXX−XXX


The Journal of Physical Chemistry C

(see below). As shown in Figure 8, the band gap of FC@ SWCNTs decreases with pressure increasing. At 11 GPa the hybrid peapod transforms into a semimetal structure with a very small band gap of 0.0146 eV. Interestingly, as pressure increases further, an obvious increase in the band gap is observed in the compressed FC@SWCNT peapod due to the intertube bonding between SWCNTs. Such interesting compression behavior in FC@SWCNT peapod may promote its application in electronics devices under extreme conditions. In order to clarify the effect of charge transfer on the structural evolution and band structure of the peapod under pressure, we also performed an electron localization function (ELF) analysis. As shown in Figure 9, we can see that only small electronic overlapping occurs between the FeCp2 and the nanotube at ambient pressure (Figure 9a), indicating a small charge transfer, which is consistent with our band structure analysis. Upon compression, the overlap is more and more obvious with the increasing of pressure, such as the ELF in Figure 9b,c. At the pressure of 23 GPa, where the FeCp2 decomposed and the nanotube clearly deformed, we can see that electron is mainly distributed on the C5H5 molecule and no electron distribution is found around the iron atom. This confirmed our above structural simulation on the hybrid peapod. To verify the theoretical simulations, we further performed high pressure Raman experiments on FC@SWCNTs and unfilled SWCNTs. The encapsulation of the FeCp2 molecules into SWCNTs has been confirmed by our Raman and IR spectroscopy measurements at room temperature. The spectra collected from the unfilled SWCNTs (a), pristine FeCp2 crystals (b), and FC@SWCNTs (c) samples are shown in Figures 10 and 11, respectively. For the Raman spectrum shown in Figure 10a, the intensive G band at 1592 cm−1 corresponds to an in-plane oscillation of carbon atoms in the graphene sheet. The weak D band located at 1342 cm−1 indicates a low level of defects or dangling bonds in the unfilled nanotubes. The high ratio of IG/ID indicates a high quality of our carbon nanotubes studied in the experiments.12 The Raman peaks at low frequency region (165 cm−1, 172 and 185 cm−1) are the radial breathing mode (RBM) of the nanotubes.23,34 For the Raman spectrum of FeCp2 (Figure 10b), both the 304 and 1103 cm−1 peaks are the A1g modes, which correspond to the ring-metal stretch and ring breathing mode, respectively. The 390 and 1408 cm−1 peaks are E1g modes, originated from the symmetry ring tilt and C−C stretch, respectively. The 1058 and 1178 cm−1 peaks (E2g

Figure 5. Structure of the quenched phase of FC@SWCNT from 25 GPa to ambient conditions. Images (a), (b), and (c) containing 1 × 2 × 2 supercell are viewed from x−z, x−y, and y−z planes, respectively.

shown in Figure 5, it can be seen that the quenched supercell structure constitutes of a two-dimension (2D) planar mesh skeleton structure, which is similar to the zeolite-like (such as AFI) structure, in which the elliptical channel contains periodic (C5H5)2 and iron atom. Such a structure might exhibit new mechanical and electronic properties, which deserves a further study. We further calculated the energy band structure of unfilled (13,0) SWCNT and filled FC@(13,0)SWCNT, both at ambient conditions and under pressure. As shown in Figure 7a, (13,0) SWCNT is semiconducting with a band gap of 0.6682 eV. When filled with FeCp2 molecules, the hosting nanotube does not show obvious electronic overlapping and still exhibit semiconducting character, while the Fermi level upshifts compared with the unfilled tubes due to the introduced d orbital electron of Fe atom, leading to a decrease of the band gap of the hybrid structure (0.2618 eV). This also indicates that small amount of charge transfer occurs from FeCp2 molecule to the nanotube. The charge transfer might be responsible for the higher transition pressure of filled SWCNTs observed above

Figure 6. Images of deformed ferrocene molecule encapsulated in the nanotube (a) and the plotted curve for the distortion angle of the two opposite C5H5 units as functions of pressure (b). D

DOI: 10.1021/acs.jpcc.6b05398 J. Phys. Chem. C XXXX, XXX, XXX−XXX


The Journal of Physical Chemistry C

Figure 7. Energy band structure of (13,0) SWCNTs (a) and FC@(13,0)SWCNTs (b) at ambient conditions.

Figure 8. Band gap as a function of pressure for FC@(13,0)SWCNTs.

Figure 10. Raman spectra of unfilled SWCNTs (a), pristine ferrocene powder (b), and FC@SWCNTs (c), excited by 514 nm laser at ambient conditions.

Figure 9. Electron localization functions (ELF) of FC@(13,0)SWCNTs molecule at pressures of 0 (a), 11 (b), 12 (c), and 23 GPa (d), respectively. The yellow, purple, and green atoms correspond to C, Fe, and H atoms, respectively.

modes) correspond to the vibration of C−H bending (⊥) and C−H bending (∥), respectively.35 After the filling of ferrocene into nanotubes, we can see that the G band (1585 cm−1) exhibits a red-shift of 7 cm−1 and becomes weak, while a blue shift happens to the D band (1350 cm−1) in the spectrum (Figure 10c). It is interesting to note that a new peak occurs at 1484 cm−1 after filling FeCp2 into the nanotubes. This peak should be related to the encapsulated ferrocene molecules in the nanotubes. As we know, each pristine ferrocene molecules contains two pentagonal carbon rings, which may exhibit the Raman vibration at around 1480 cm−1 in a confinement environment. Similar Raman peak has also been reported in C60 filled SWCNTs peapod system, in which the Ag(2) vibrational mode (1474 cm−1) is found to be from the vibration of pentagonal carbon rings of C60 molecules.36 The downshift in

Figure 11. IR spectra of unfilled SWCNTs (a), pristine ferrocene (b), and FC@SWCNTs (c), recorded at ambient conditions.

G-band should be related to the charge transfer effect between FeCp2 molecules and nanotubes. The analysis of band structure and electron localization functions (ELF) supports the charge transfer in FC@SWCNTs. It is also worth noting that the intensity increase of D band of FC@SWCNT sample compared with the empty tubes can be interpreted as the increment of defects on the tubes in the heat treatment of filling progress23 and also probably due to the charge transfer effect in FC@ SWCNTs. All these results confirm a successful encapsulation in our experiment. E

DOI: 10.1021/acs.jpcc.6b05398 J. Phys. Chem. C XXXX, XXX, XXX−XXX


The Journal of Physical Chemistry C As shown in Figure 11a, all the IR peaks in the spectrum of empty tubes belong to E1u modes, which correspond to the C− C stretching vibration but in different models.18,37 For the pure FeCp2 molecules, nine IR peaks are observed in the spectrum (Figure 11b).38 The A2″ modes located at 475, 816, 1105, and 1632 cm−1 are originated from the C−H stretch vibration. The other five absorption bands belong to E1′ modes and the peaks at 491 cm−1, 1001 and 1047 cm−1, and 1383 and 1408 cm−1 correspond to the vibration of ring tilt, C−C−H bend, and asymmetric stretch of C−C:Cp, respectively. As shown in Figure 11c, some vibrations of ferrocene molecules, such as the peaks at 475 and 491 cm−1, are submerged after the filling. No obvious shift can be observed in the IR bands with peak position at 1001, 1105, 1383, and 1632 cm−1 for ferrocene molecule or 1624 cm−1 for SWCNTs after the filling. An evident downshift of 808 cm−1 peak in spectrum (c) compared with the peak at 816 cm−1 of spectrum (b) indicates that the vibration frequency of C−H stretch is decreased by an elongation of bond length due to the π−electrons interaction of the nanotbue and the ferrocene.18 This guest−host interaction also induces a downshift (from 1047 to 1032 cm−1) of IR peak from the nanotubes. Similar downshift of C− H vibration has also been observed in the case of encapsulating H8Si8O12 molecules into SWCNTs, for which a 15 cm−1 red shift of Si−H vibrations was rationalized by an elongation of Si−H bonds due to the interaction between the inner molecules and nanotube walls.39 High pressure Raman spectroscopy measurement has been performed on both SWCNTs and FC@SWCNTs samples to compare their structural evolutions upon compression. The Raman spectra recorded from both samples at selected pressures are shown in Figure 12. We can see that the

Figure 13. Frequency of the TMs with maximum intensity of (13,0)SWCNTs (a) and FC@SWCNTs (b) exhibited with the various pressure conditions.

Table 1. Calculated Results Presented Here Are the Slope of Raman Frequency of SWCNTs and FC@SWCNTs for the Different Pressure Regions mode (A1g; cm−1)

pressure (GPa)

K (cm−1 GPa−1)