Pressure and Photo-Induced Phase Transitions of Elemental Sulfur

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C: Physical Processes in Nanomaterials and Nanostructures

Pressure and Photo-Induced Phase Transitions of Elemental Sulfur Confined by Open-End Single-Wall Carbon Nanohorns Bo Li, Yanli Nan, Yun Hu, Xiang Zhao, Xiaolong Song, Haining Li, and Lei Su J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00378 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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The Journal of Physical Chemistry

Pressure and Photo-Induced Phase Transitions of Elemental Sulfur Confined by Open-End Single-Wall Carbon Nanohorns Bo Li,1,2,a) Yanli Nan,1 Yun Hu,3 Xiang Zhao,2 Xiaolong Song,1,b) Haining Li,4 and Lei Su5,c) 1

State Key Laboratory for Mechanical Behavior of Materials, School of

Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China 2

Institute for Chemical Physics & Department of Chemistry, School of

Science, Xi’an Jiaotong University, Xi’an 710049, China 3

College of Optoelectronic Technology, Chengdu University of

Information Technology, Chengdu 610225, China. 4

Center for High Pressure Science and Technology Research,

Zhengzhou University of Light Industry, Zhengzhou 450002, China 5

Key Laboratory of Photochemistry, Institute of Chemistry, Chinese

Academy of Sciences, Beijing 100080, China.

a)

Electronic mail: [email protected].

b)

Electronic mail: [email protected].

c)

Electronic mail: [email protected]

1

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

Confined molecules in tubular nanospaces of nanocarbons, for example, carbon nanotubes (CNTs) and nanohorns (CNHs), lead to extraordinary behavior and properties different to their bulk analogues. Here, we investigated the confinement effect of CNHs on high-pressure elastic and vibrational properties of sulfur via diamond anvil cell (DAC) technique. X-ray diffraction measurements up to 40 GPa demonstrate two phase transitions of S-I → amorphous → S-II. A Fit of equation of state yields bulk modulus of ~24.8 GPa, about 70% higher than that of soft sulfur. Different to previous Raman studies, laser with red light wavelength (694.8 nm) and high laser density (~2 mW µm-2) was employed under the threshold for generating C-S bonds. We observed a similar photo-induced transition of S-I to amorphous sulfur at 4-6 GPa compared with results taken from blue, and green light excitation and low laser density (e.g. ＜28 µW µm-2), showing enhanced photothermal stability of sulfur by the aid of SWCNHs.

2


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Introduction Confinement of materials in nanospaces lead to distinct behavior and properties different to their bulk analogues. Owing to rich nanocavities, carbon nanotubes (CNTs) and nanohorns (CNHs) are extremely suitable for molecule encapsulation and storage 1

, for chemical reaction as a nanoreactor

2-3

, or to stabilize metastable structure using

tubular confinement, for example, linear carbon 4-5. For CNTs, various guest materials have been studied over a broad range from gases to solids for different purposes, e.g., argon 6, water 7, cobalt 8, iron 9, and fullerene

10-11

. When subjected to high pressure,

filled CNTs show extraordinary physical and chemical properties compared with their empty analogues, both experimentally and theoretically. In contrast, CNHs are more complex than CNTs in structure, featuring with a disordered graphene core surrounded by thousands of single-wall carbon nanohorns in the periphery 12-13. More importantly, dual physical natures, such as nanomechanical 14, electrical 15, spin lattice relaxation

16

and electromagnetic behaviors

17

, have been found for the external and

internal sections. Recent high-pressure Raman and X-ray diffraction (XRD) studies on CNHs exhibit a similar behavior of phase transition compared with SWCNTs, and, when cold compression reached 35 GPa, a post-graphite phase having both sp2 and sp3 bonds formed via structural reconstruction induced by topologically defective carbon rings within nanohorn’s lattice

14, 18

. These topological defects could also be

potential sites for bond interconnection to yield superhard carbon allotrope under higher pressure, for example, the so-called V-carbon synthesized by cold compressed C70 peapods

10

. On the other hand, the tubular confinement effect of CNHs, as 3



discovered by Urita et al

Page 4 of 33

19

, could lead to the growth of high-pressure phase of KI

crystal at ambient condition via a quasi-high-pressure effect. Thus, knowing the confinement effect of such nanohorns on the behavior and properties of filled molecules at both ambient and extreme conditions are of great importance. In this work, we focused on the high pressure behavior of sulfur encapsulated by open-end single-wall CNHs (S@SWCNHs) based on two points of motivations. On one hand, sulfur is a member of group VIa elements and exists in a large number of allotropes under pressure, probably only less than H2O for the most common materials 20. The stable structure at ambient conditions comprises covalently-bonded S8 rings arranged in orthorhombic structure, also known as α-S8 (S-I phase). Upon compression at room temperature, previous XRD studies show that the orthorhombic form undergoes a series of structural transitions: monoclinic structure (P ~5GPa)

21

,

amorphous structure (~18-26.5 GPa) 21-22, trigonal or tetragonal structure (S-II phase, ~37-75 GPa) GPa)

22-27

, base-centered orthorhombic (bco) structure (S-III phase, ~83-86

22, 24

, and rhombohedral β-Po structure (S-IV phase, ~162-212 GPa, so far the

highest experimental pressure) 24. A further compression may generate a β-Po to bcc transition at 550 GPa according to ab initio calculations

28

. Along with complex

structural changes, more remarkable results caused by pressure is the varying electrical properties, namely, an insulator-metal transition for an experimental investigation at critical pressure Pc ˃ 95 GPa)

29

, and, further to be superconductive

above 550 GPa as discovered by a theoretical study sulfur and its compounds, e.g., liquid sulfur 30, H2S

28

. In addition, several types of

31

, CS2

4


32-34

, and C60S16 35, have

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also been investigated under extreme conditions. In particular, CS2 being highly compressed above 50 GPa exhibits an insulator-to-metal transition as well

32, 34

. This

transition towards metal-like behavior is due to a formation of highly disordered CS4 phase, rather than CS2 decomposition or elemental sulfur, plus, the Pc of metallization is much lower than that for elemental sulfur since C-S bonding reaction. Besides, the stoichiometric ratio, sulfur content and structural configuration of C-S bonds in such a binary system may also lead to varying structural properties according to theoretical studies

36-37

. Raman spectroscopy is also a powerful tool to investigate phase

transitions of sulfur under pressure. However, the interpretations of Raman results have not yet reached a consensus, and even self-contradictory, due to strong photothermal-induced phenomena that taking place during light exposure and excitation

38-44

. Some Raman investigations tentatively concluded a photo-induced

transition sequence as follows: α-S8 → first photo-induced amorphous phase (a-S) → second photo-induced phase (p-S) → S6, and the determination of phase diagram is closely dependent on pressure, laser energy, and power density

39-43

. On the other

hand, nanocarbon/sulfur hybrids have been the subject of intensive research efforts in fields such as nano energy engineering

45

. In order to get more insights into the

confinement effect on filled molecules encapsulated in nanospaces of CNHs, as well as into the pressure-induced phase transition and photothermal behavior of nanocarbon/sulfur hybrids, the S@SWCNHs is designed and is measured using X-ray diffraction and Raman spectroscopy upon cold compression. Our results show not only the pressure- and photo-induced transitions but remarkably enhanced structural 5



and thermal properties for confined sulfur. Furthermore, results obtained in this work may also be applicable for other heavier VIa elements, Se and Te, due to their isostructural transition behavior under pressure 23. Experimental Methods The samples of raw CNHs were produced by improved arc discharge method 46. One-step post-treatment for purification and tip-opening were carried out by air annealing. Then the samples were filled by sulfur with a purity of 99.999% by means of thermal infusion. (See details in supporting information). A typical morphology of S@SWCNHs is shown in Figure 1a. Figure 1b presents the surface morphology and corresponding selected-area electron diffraction (SAED) pattern, showing a polycrystalline structure. Figure 1c shows the crystal stripes of a random magnified area of the periphery section in Figure 1b. According to the fast Fourier transform (FFT) pattern and the d spacing of 0.38 nm for (222) reflection, such filled sulfur maintains the orthorhombic structure consisted of S8 molecules. High pressure XRD experiments were carried out at the 4W2 High-Pressure Station of Beijing Synchrotron Radiation Facility (BSRF) in China. The monochromatic 0.6199 Å radiation with a 10 × 31 µm2 spot was used for data collection. The scattered angle and the incident wavelength were calibrated by a standard CeO2 polycrystalline. Silicone oil is selected as the pressure transmitting medium (PTM). And the two-dimensional (2D) data were converted to one-dimensional intensity data versus 2θ by the software FIT2D. 6


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The excitation of Raman measurement was achieved by using a solid-state laser operating at 694.8 nm (1.78 eV) with a laser power of about 30 mW and a laser focal spot with diameter of about 2-3 µm. Argon was employed as PTM in Raman measurements. Great attention was given on the laser power, measured outside DAC at different ratios, so that we could ensure high quality signals and minimal thermal damage on the sample (see Figure S3, SI). At 50% of 30 mW laser power, a strong vibration for C-S bonds is shown between the D and G bands of CNHs, indicating a strong photo-induced bonding reaction. Thus the Raman experiments inside DAC were performed by an appropriate laser power approaching 50% attenuation with the aims not only to avoid sample damage but to enhance signal quality. Furthermore, scattering signals were gathered repeatedly in several runs in order to increase the signal-to-noise ratio. Even so, the signals became weak very quickly with increasing pressures, and thus the pressurization ended up to about 8 GPa.

Figure 1 (a) Typical morphology of sulfur-encapsulated SWCNHs (S@SWCNHs), embedded with a schematic illustration indicative of open-end SWCNHs; (b) Surface morphology and its SAED pattern; (c) Lattice fringes of S@SWCNHs show a d-spacing of ~0.38 nm for sulfur (222). The inset FFT image indicates the 7



orthorhombic structure of S-I consisted of S8 molecules.

Results and discussion Observed diffraction patterns to 40 GPa suggested two structural phase transitions. Figure 2a presents the XRD pattern of pristine S@SWCNHs at ambient conditions outside DAC. The vertical bars represent positions of bulk sulfur (S-I) at ambient conditions. The strongest peak located at 2θ ~9.4o is indicative of S (222) with a d-spacing of 3.76 Å, agreeing with the HRTEM observation. The original orthorhombic lattice could be followed up at least to 20 GPa with a continuous transition, showing a disagreement with the result reported by Luo, et al., i.e., an orthorhombic to monoclinic transition at ~5 GPa

21

. However, other authors did not

confirm this monoclinic form in their reports, such as Akahama, et al. 22, and Hejny, et al

23

. The intensities of diffraction lines, with increasing pressure, gradually

decreased and at 23.5 GPa no sharp line could be recognized in the pattern, implying the pressure-induced amorphization (PIA) of sulfur (Figure 2b), which was also observed in some works

21-22

. Some of these messy peaks with large widths may

derive from the disordered arrangements of sulfur molecules or creation of defects under compression, rather than background signals. But it was not observed by Hejny et al., instead of a direct transition from S-I to S-II at 37.5 GPa 23. Luo et al. proposed two possible mechanisms of PIA

21

: 1) the system attempts to transform into new

phase but remains trapped in disordered state before finishing the transition since insufficient mobility of atoms; 2) amorphization is triggered by intramolecular bond 8


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breaking. Sanloup et al. 47 discussed a phenomenon of low-density amorphous (LDA) to high-density amorphous (HDA) polyamorphic transition in sulfur above 65 GPa at 40 K, and they suggested that the LDA and HDA forms might correspond to their crystalline counterparts, that is, polymetric S-III and metallic S-IV. However, they also thought that the experimental data might be related with the creation of small sulfur nanocrystals, which cannot be distinguished from the genuine amorphous form within the resolution of XRD technique (so-called XRD amorphization). A theoretical study by Plašienka et al. revealed that, at room temperature and pressure of 20 GPa a transformation from S-I phase to monoclinic phase where half of the molecules develop a different conformation48. Upon further compression, the monoclinic phase undergoes PIA into an amorphous phase. The amorphous form they found appears to correspond to the experimentally observed LDA form. Based on the review on both experimental and theoretical studies, we therefore tentatively conclude that the observed PIA phenomenon in this case is probably a LDA form of confined sulfur, and the amorphization would not be expected for a truly amorphous structure, even if its local structure is closely related to the nanocrystalline phase. At 25-27 GPa, one sharp peak appeared at ~16.4o could be considered as graphitic structure of nanohorns18, but no signals for crystallized sulfur were observed. When pressure reached 30.5 GPa, several intense peaks arise in the vicinity of the former graphitic peak, located at 14.8o (d=2.40 Å), 17.8o (1.99 Å), and 19.5o (1.92 Å), respectively, indicating a recrystallization of encapsulated sulfur (Figure 2c). This phase could be assigned as a S-II phase with a spiral chain structure as observed in many high 9



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pressure XRD experiments of sulfur. But some key issues should be addressed. The S-II phase observed by Luo et al, and Akahama et al, was very likely due to the preferred orientation of large sulfur crystallites, as pointed out by Degtyareva et al 27. Degtyareva et al also carefully determined the crystal structure of S-II phase and confirmed a trigonal structure with space group P3221, and parameters a=6.91 Å, c=4.26 Å, Vatom= 19.54 Å3

26-27

. In contrast, a different conformation of S-II was

proposed by Fujihisa et al., and they considered the S-II as tetragonal crystal with space group I41/acd, and parameters a=7.84 Å, c=3.10 Å, Vatom= 11.91 Å3 25. In this work, the recrystallized phase has insufficient diffraction lines due to limited applied pressure, and therefore the existed peaks only matched well with the positions of S-II phase measured by Akahama and reported by Fujihisa et al 31. We also noted that, S-II phase exhibits richer diffraction characteristics at higher pressure of 75-82.8 GPa 22, 24, due to a complete transformation to spiral chains from rings

25

. Up to 40 GPa, a

shoulder peak split from the second strongest peak of S-II phase was observed together with an obvious weakening of the strongest peak (Figure 2d). During whole compression, all sulfur peaks shifted to higher angles, and some of them became weak, broad or divided, while the graphitic peak as marked by the asterisk seems more or less unchanged. Upon decompression of the samples from the S-II phase, the diffraction characteristics of such high pressure S-II could be maintained down to 7.6 GPa (Figure 2e), but totally disappeared when pressure was released to ambient conditions (Figure 2f). The pattern of fully recovered sample with no observable sulfur signals implies a permanent breakdown or dissociation of chain-like molecules. 10


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Figure 2 Angle-dispersive XRD patterns of S@SWCNHs under (a) ambient condition, (b, c, d) compression and (e, f) decompression at different pressures. The signals as indicated by the asterisks derive from the graphitic structure of SWCNHs. The ambient diffraction pattern gives lattice parameters for confined sulfur, a=10.13 Å (10.46 Å), b=12.53 Å (12.87 Å), c=25.21 Å (24.49 Å), β=90o, and Vunit cell=

3199.87 Å3 (3299.46 Å3). The values in brackets are parameters of bulk sulfur

(amcsd no. 0010058). Obviously, nanoscale encapsulation leads to a slight distortion of sulfur cells along c-axis and a reduction of cell volume. This is also responsible for the angle shifting between the diffractions of our sample and bulk sulfur. The relation of the reduced cell volume V/V0 as a function of pressure is plotted in Figure 3, along with the fit of third-order Birch-Murnaghan equation of state (EOS) to the data points. For comparison, relative data for elemental sulfur is also presented as taken from literature. The zero pressure isothermal bulk modulus K0, and its pressure derivate K0’ of our sample were fitted to be 24.8 GPa, and 3.6, respectively. Interestingly, the K0 11



of S@SWCNHs is obviously larger than both orthorhombic (14.5 GPa) and monoclinic forms (17.3 GPa) of bulk sulfur

21

. These results suggest that sulfur

confined in nanospaces is roughly twice less compressible than bulk sulfur. As mentioned above, two possible structures of S-II have been proposed by different groups. Therefore, the P-V/V0 relations for S-II are calculated based on both trigonal and tetragonal forms. The original volume of unit cell V0 for S-I is still used for S-II. Take tetragonal structure for example, a unit cell consists of 16 atoms arranged by a manner of spiral chains with both 41 and 43 screws, thus its cell volume is ~190.6 Å3. Furthermore, the three strongest peaks of S-II phase (Figure 2c) could be assigned as (201), (400) and (311) reflections from low to high angles. Thus the V/V0 – P relation for tetragonal S-II is plotted as those diamonds in the range of 30-40 GPa. Similarly, data for trigonal S-II structure are added as well. During the limited range, both tetragonal and trigonal forms of S-II phase confined in SWCNHs exhibit very low compressibilities according to second-order Birch-Murnaghan EOS fitting (~0.0054 GPa-1 for tetragonal, and ~0.023 GPa-1 for trigonal). The diamonds with black edges at 7.6 and 25 GPa denote data on pressure decrease. Both tetragonal and trigonal structures could be maintained to a very low pressure and the hysteresis of reversible transition is calculated to be 15.4 % and 6.8 % volume changes compared with the staring unit cell of S-II, respectively.

12


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Figure 3 Plot of the reduced cell volume V/V0 vs pressure for S@SWCNHs (red diamonds). For comparison, the data for bulk S are also taken from literature21, 23, 25 (blue circles). S8 molecular sulfur crystallizes in an orthorhombic form (Fddd (D2h24)), with four molecules per unit cell occupying C2 sites. Normal coordinate analysis for free S8 molecule (D4d) generates 18 fundamental modes, and 7 of them are doubly degenerate 49

. The correlation of molecular point group (D4d) with the crystal factor group (D2h)

predicts 48 active Raman modes consisting of 36 internal and 12 external modes. As shown in Figure 4, the resolution of our spectra allows us to inspect 1-3 of external modes (librational (L), translational (T)), and 11 of internal modes (bond bending and stretching) during compression. Specifically, the external modes usually locate at 55-80 cm-1 observed in this work can be assigned as T modes. No L mode was observed in this work. Above this range, the internal modes of S8 ring-like species dominate; as divided by the vertical lines in Figure 4, ν9, ν8, ν6, and ν2 modes belong to bending modes up to 250 cm-1, while the stretching modes of ν10, and ν1 fall within 13



Page 14 of 33

the range of 430-480 cm-1, leaving a wide phonon bandgap between 250-430 cm-1. In high-frequency region, the broad bands started at ~1586 cm-1 and subsequently blue-shifted indicate the evolution of E2g mode for graphitic structure. The sharp bands near 1330 cm-1 derive from the strong scattering of diamond anvil. In Figure S3, we show strong vibration of C-S bonds of our sample induced by photothermal effect as a dramatic lineshape changes between D and G bands (additional peak ~1427 cm-1). In fact, similar behavior for confined sulfur, for instance, in few-layer graphene

50

,

was also observed. By increasing pressure, we observed remarkable changes as regards frequency, intensity and merging of degenerate Raman bands. T modes, together with most bending modes seem to disappear along with a drastic pressure-induced changes for stretching modes at 5.4 GPa or higher. Further pressurization led to a complete vanishment of scattering signals for both sulfur and carbon, hence the data collection ended up to 7.8 GPa. Eckert et al. reported three photo-induced transitions following a sequence of S8 → amorphous → p-S → S6, in which the crystal-to-amorphous transition appears at 3-4, 5-6, 10, and 12-13 GPa for laser wavelengths 488, 524.5, 600, 632.8 nm, respectively, within laser power density of 16-28 µW µm-2

39

. At elevated pressures and increasing laser power, amorphous

phase further transforms to a recrystallized structure composed of S6 molecules. Similar observations of S8 → amorphous → p-S → S6 transitions were also reported for 488, 514.5 and 590 nm laser wavelength by Yoshioka et al 51. But the presence of S6 was only observed by means of laser powers of 800, 1000 mW, independent of laser energy. Based on a survey on literature, the anomaly occurred at 4-5 GPa in this work 14


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can be safely concluded as S8 → amorphous transition derived from the breakdown of S8 molecules. As pressure increased, Raman signals become broad and weak, in particular, the stretching mode ν1 for sulfur and E2g for SWCNHs tend to be flat, and no new bands take place during further pressurization. This is contrary to previous Raman studies, i.e., a recrystallized form (e.g. S6) followed amorphization appears at higher pressures. Eckert et al suggested a transition to p-S took place only if the excitation light had a power density above a threshold of about 30 µW µm-2. The power density used in this work (~2 mW µm-2), however, is about tens of times higher than that value, but no p-S and S6 are observed. Thus it is reasonable to suggest that the nanohorn host enhances the photothermal stability of sulfur due to CNHs themselves possess high thermal stability. We also observed the in situ thermal behavior of SWCNHs in air up to 800 oC via environmental transmission electron microscope (H-9500, Hitachi), providing direct evidence for their high thermal stability at least up to 400 oC (see Figure S4). Finally, low intensities and low symmetry of lineshapes for both ν1 and E2g modes probably result from a highly disordered structure of sulfur whose further recrystallization is restricted by nanohorns’ lattices.

15



Figure 4. High pressure Raman spectra of S@SWCNHs for pressures up to 7.8 GPa. Figure 5 shows the pressure dependence of the most characteristic vibrational frequencies of our samples up to 7.5 GPa. Albeit with different dependences for each mode, we observe a general mode hardening with increasing pressure. Some modes are shown smooth increases upon pressure, such as T translational mode and ν9 (ag, b1g), ν6 (b1g), ν2 (ag, b1g), and ν1 (ag) vibrational modes. Above 5.4 GPa, several modes disappeared while the slopes (dωi/dP) of some modes were changed where the discontinuities could be clearly seen. This demonstrates that a structural transition takes place at such pressure. Due to very limited data points, a polynomial fit as widely used in literature seems inappropriate, and thus a further analysis based on linear fit gives a data comparison with literature. The linear coefficients of some key modes, ν8 (ag), ν2 (b1g, ag), and ν1 (ag) are calculated to be 4.26, 1.83, 2.99 and 2.59 cm-1 GPa-1, showing a somewhat similarity to the results of 5.7, 1.9, 4.3 and 2.9 cm-1 GPa-1 by Andrikopoulos et al. 38, and to Eckert et al.’s data of NULL, 1.6, 3.0, and 1.7 cm-1 GPa-1

39

. Our data seems closer to those from Andrikopoulos et al., probably 16


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resulting from a similar excitation wavelength of 694.8 nm used in this work. By the way, the laser employed by Andrikopoulos et al., and Eckert et al., were 752.5 nm, and a group of 488, 514.5, and 600 nm, respectively. The determination of mode Grüneisen parameter for each mode is another concern. The value of mode Grüneisen parameter, γi, is defined by the following equation:

γi = −

d ( lnν i ) V dν i 1 d ( lnν i ) =− = d ( ln V ) ν i dV κ dp

where νi denotes the frequency of mode i, κ is the isothermal compressibility, V the crystal volume, and p the applied pressure. The γi value is derived as the slope of ln(νi) versus ln (V) plot. Using the isothermal bulk modulus K0 as discussed above, the κ value, which is inversely proportional to K0, is about 0.04 GPa-1. A summary of mode Grüneisen parameters for observed modes are presented in Table 1, also compared with results from literature.

17



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Figure 5 Pressure dependence of external and internal modes for S8, and of E2g mode for graphitic structure. Table 1 Mode Grüneisen parameters of sulfur

Mode

Zallen 44

Häfner, et al 42

This work

T

1.7

1.95

1.98

ν9 (b1g)

1.05

1.49

0.99

ν9 (ag)

1.2

1.57

0.99

ν8 (ag)

-

0.54

0.64

ν6 (b1g)

-

0.70

0.55

ν2 (b1g)

0.15

0.35

0.20

ν2 (ag)

0.23

0.41

0.25

ν10 (ag)

0.1

0.28

0.25

ν1 (ag)

0.67

0.19

0.13

Conclusions In conclusion, the orthorhombic sulfur confined by open-end SWCNHs shows different behavior compared with bulk sulfur under pressure. First, the confinement of sulfur inside nanohorns leads to slight distortion of unit cells along c-axis and a reduction of cell volume. Second, XRD results exhibit that two phase transitions occurred at pressures up to 40 GPa, following a sequence of S-I → PIA (onset: 23.5 GPa) → S-II (onset: 30.5 GPa). No monoclinic sulfur is observed in this work. Upon 18


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releasing pressure, the diffractions for S-II phase could be maintained down to 7.6 GPa, and further disappear at ambient conditions, while the signals indicative of nanohorn host can persist from 40 GPa to ambient. These results could be interpreted as being due to the dissociation of sulfur chains and high structural stability of SWCNHs, and this idea is also supported by the results of higher bulk modulus for confined sulfur by fitting EOS. Fits of Birch-Murnaghan EOS for the first transition yield the parameters K0 and K0’ for confined sulfur to be 24.8 GPa, and 3.6, respectively. Third, Raman investigation suggests an obvious photo-induced phase transition by red light excitation (laser wavelength ~694.8 nm) and high laser density (~2 mW µm-2). Although the laser density is hundreds to thousands of times larger than those used in previous studies, such as low laser densities (e.g. ~1.5-28 µW µm-2) reported by Andrikopoulos et al.38 and Eckert et al.39, a similar phase transition of S-I to amorphous phase is observed at 4-6 GPa, and the signals tend to become flat up to 8 GPa, indicating an enhanced thermal stability of confined sulfur by the aid of CNHs. Further analyses on the pressure dependences and mode Grüneisen parameters of the Raman modes also confirm the vibrational difference between confined and free sulfur. Finally, this work not only presents a facile method to effectively prepare CNH-based nanocomposites using confinement effect, but further shed lights on the structural and vibrational properties of confined molecular crystals under pressure.

Supporting Information Description

19



Experimental details on sample preparation and post-treatments. Figure S1. Schematic of Preparation of S@SWCNHs; Figure S2. BET and HRTEM results for tip-opening treatment; Figure S3. Raman spectra of samples at different ratios of laser power of 30 mW; Figure S4. In situ thermal behavior of empty SWCNHs.

Author Information

Corresponding authors: E-mails: a) [email protected] b) [email protected] c) [email protected] Notes: The authors declare no competing financial interest.

Acknowledgements The authors would like to acknowledge the financial support from the Fundamental Research Funds for the Central Universities.

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