Atomic Structural Studies on Thin Single-Crystalline Misfit-Layered

Jul 30, 2015 - Various nanotubes from ternary misfit compounds have been reported in recent years. In the present work, the detailed atomic structure ...
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Atomic Structural Studies on Thin Single-Crystalline Misfit-Layered Nanotubes of TbS-CrS2 Leela S. Panchakarla,†,∥ Luc Lajaunie,‡,∥ Reshef Tenne,*,† and Raul Arenal*,‡,§ †

Department of Materials and Interfaces, Weizmann Institute of Science, 76100 Rehovot, Israel Laboratorio de Microscopías Avanzadas, Instituto de Nanociencia de Aragón, Universidad de Zaragoza, 50018 Zaragoza, Spain § ARAID Foundation, 50018 Zaragoza, Spain ‡

S Supporting Information *

ABSTRACT: Various nanotubes from ternary misfit compounds have been reported in recent years. In the present work, the detailed atomic structure and chemical configuration of misfit-layered nanotubes based on the TbS-CrS2 are reported. These analyses have been developed via different transmission electron microscopy techniques, including highresolution scanning transmission electron microscopy, electron diffraction, and electron energy loss spectroscopy. These structural analyses show that two different kinds of nanotubes can be produced: a “regular” nanotube and a “wavy” one. Both kinds of nanotubes show the alternating arrangements of the TbS and CrS2 subsystems; however, the wavy ones present a nearly periodically deficiency in terbium. In addition to the structural investigation, the chemical analyses have proved that the outer layer of both kinds of nanotubes is composed of the elements Cr and S. All these findings helped to understand the growth mechanism during the sulfurization reaction taking place in the synthesis process.

1. INTRODUCTION Though misfit layered compounds (MLC) have been known for some time,1 the interest in these materials has increased in recent years. Their unique structure allows them to behave like electron crystals and show a phonon glass behavior, thus exhibiting superior thermoelectric properties.2−4 Misfit layered compounds can be considered as intergrown materials with the general formula [(MX)1+x]m[TX2]n, where M is rare earths, Pb, Sb, etc.; T is Ti, V, Cr, Nb, etc., and X is S, Se.1,5,6 Alternating slabs of each MX and TX2 show different crystal structures, where a two-atom thick MX layer can be viewed as a molecular slice cut from the (001) plane of a rock-salt structure. On the other hand, the three-atom thick TX2 layer can be considered as a slice cut from the lattice with a pseudohexagonal structure. Alternatively, they can be regarded as intercalation compounds. Here, a molecular layer of a compound with a distorted rocksalt structure (O) is intercalated in between two slabs of a layered compound TX2 (T). The chalcogen atoms in the TX2 layer (when T = V or Cr) are arranged in a hexagonal (octahedral) configuration around the metal atom. An alternating O-T arrangement of the two sublattices in MLC is usually observed. More complex arrangements of the layers, such as O-O-T, O-T-T, and O-T-O-T-T are known, though.7 The in-plane periodicities of the MX and TX2 layers in misfit compounds are different along at least one direction (usually aaxis), thus making the structure of MLC mutually incommensurate. The mutually incommensurate structure of the individual components in misfit chalcogenides induces © XXXX American Chemical Society

structural distortions from their corresponding bulk phases (often called “structural modulation” in the literature).1 One way of relaxation of the misfit stress associated with different lattice parameters is by bending of the layers, thus facilitating scrolling of the entire structure.8−10 In addition, the reactivity of the rim atoms on the layers helps close the structure seamlessly into nanotubes.10,11 TbS-CrS2 is a misfit compound with an orthorhombic structure.12 CrS2 is a metastable phase and is not known in pristine form as a bulk phase.1,12 However, the metastable CrS2 can be stabilized in a hexagonal layered structure upon intercalating with alkali, Cu, or Au atoms, with the six S atoms octahedrally coordinated to the Cr atom.1 For CrS2 based misfit layered structures, the stability of the CrS2 is gained mostly through charge transfer,1 from the intercalated layer (in this case TbS). The symmetry and the crystalline structure of the CrS2 layer are mostly dependent on the intercalating layer. For instance, CrS2 adopts a triclinic structure upon intercalating LaS, whereas it adopts an orthorhombic structure on intercalation of GdS or TbS.10 It has been predicted that, with decreasing dimensionality of a material, the thermoelectric properties are improved.13,14 On Special Issue: Kohei Uosaki Festschrift Received: June 17, 2015 Revised: July 28, 2015

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HRTEM) were performed by using an FEI Titan Cube microscope working at 300 kV. Finally, the rest of the TEM studies, and in particular SR-EELS chemical analyses, have been performed using an FEI Titan Low-Base microscope equipped with a Cs probe corrector, a monochromator, an ultrabright XFEG electron source and a Gatan Tridiem ESR 865 EELS spectrometer. This microscope was operated at two different accelerating voltages: 300 kV and 80 kV. The higher accelerating voltage was used for the sole purpose of imaging while the lower voltage was used for SR-EELS analyses and imaging. Decreasing the accelerating voltage for EELS analyses, which required longer exposure times, has been found mandatory for this sample in order to avoid electron beam damage. These latter being mainly related in this case to knockon damage.21,29 HR-STEM imaging was performed by using annular dark field (ADF) and annular bright field (ABF) detectors. The inner and outer angles at 300 kV for ABF imaging were 8 and 22 mrad, respectively and 22 and 123 mrad for the ADF imaging. With this setup, the contrast of light elements is enhanced in the ABF micrographs.31,32 This is particularly welcomed in the case of sample showing a mixture of light and heavier elements (such as TbS-CrS2 nanotubes), for which other HR-STEM methods (in particular high-angle annular dark-field) failed to image properly lighter elements.32,33 During SR-EELS analyses, the energy resolution was about 1 eV. The spectra were collected in STEM mode, using spectrum image and line scan modes.21,24,29 For most of the EELS experiments, the convergence and collection angles were 25 and 31 mrad, respectively. The EELS spectra data sets were then denoised with the open-source program Hyperspy by using principal component analysis routines.34,35

the other hand, heterostructures may decouple the electrical conductivity and the Seebeck coefficient due to electron filtering, which could result in a high figure of merit (ZT).15 These observations make the study of nanotubes from misfit compounds, such as TbS-CrS2, highly warranted. However, making these structures in the nanosize, particularly as 1D (one-dimensional) nanostructures, is rather challenging. Nanotubes of various MLC were recently reported.9,10,12,16−18 Different synthetic strategies and growth mechanisms of these nanostructures were discussed in some of the reports.10 Most of the misfit nanotubes which are reported in the earlier reports have wall thicknesses of more than a few dozen nanometers. Therefore, the structural and compositional information which can be retrieved using high resolution transmission electron microscopy (HRTEM) analysis of such nanostructures is limited. This limitation is further exacerbated by the curved slabs of the nanotube, which, in contrast to the flat crystallite, lacks 2D translational periodicity. Nonetheless, using a multifocal reconstruction series, of aberration-corrected high resolution transmission electron microscopy (Cs-HRTEM) and modeling, detailed structural analysis of WS2 nanotubes with a few atomic layers has been realized.19 Importantly, this detailed structural analysis permitted suggesting a plausible growth mechanism for such nanotubes. Other TEM techniques,20 in particular electron energy loss spectroscopy (EELS),21−30 offer the possibility to have access to the local atomic configuration of 1D-2D layered nanostructures.22,24,25 These analyses also provided very rich information about the growth mechanism of such nanostructures.20,26,29 In the current work, nanotubes based on the TbS-CrS2 misfit compound, a few layers thick, are reported and their detailed structure and chemical composition are elucidated by different TEM techniques, including high-resolution scanning TEM (HR-STEM) and spatially resolved EELS (SR-EELS). Surprisingly, structural modulation and composition variation along the thin nanotubes axis were observed. In light of this new structural and chemical information, a growth mechanism for these nanotubes is proposed.

3. RESULTS AND DISCUSSION 3.1. Structural and Chemical Analyses. Figure 1 shows high (a) and low (b) magnification SEM images of as-

2. MATERIALS AND METHODS The synthesis of these nanostructures was done in two steps:12 First, the respective hydroxide precursors were synthesized by ammonolysis of the respective metal salts. In the second step, the precipitate was sulfurized in a high temperature reaction. The precursor was synthesized as follows: Tb(NO3)3·6H2O and CrCl3·6H2O were dissolved 1:1 mol ratio in milli Q (Millipore) water. A hydroxide mixture of Tb(OH)3 and Cr(OH)3 was precipitated from the solution by addition of NH4OH. This precursor was used to synthesize the TbS-CrS2 nanotubes. In a typical reaction, the hydroxide mixture was transferred to a fused silica boat and placed in a horizontal furnace. The furnace was heated up to 950 °C with a heating rate of 10 °C/min and a continuous flow of 40 sccm (standard cubic centimeters per minute) of 5% H2 + 95% N2 and 5 sccm of H2S. The furnace was kept at 950 °C for 1 h before it was allowed to cool down to room temperature naturally in the presence of N2. The nanotubes were observed to grow only above 900 °C. A Zeiss model Ultra scanning electron microscope (SEM) model V55 was used for imaging. Conventional HRTEM and selected area electron diffraction (SAED) analyses were done using a FEI Tecnai F30-UT high-resolution operated at 300 kV. In addition, high-resolution analyses with a Cs image corrector (Cs-

Figure 1. High (a) and low (b) magnification SEM images of TbSCrS2 nanotubes.

synthesized TbS-CrS2 nanotubes. The nanotubes/nanoscrolls grow vertically from the substrate. The diameters and lengths of the nanotubes are typically in the range of 25−75 nm and 200− 700 nm, respectively. Much longer and slender nanotubes were observed to grow from cracks in the oxide cake or from the edges of the crystallites (Figure 1b). Analyzing several SEM and TEM micrographs, it can be concluded that the nanotubes are more abundant compared to the nanoscrolls in the TbS-CrS2 synthesis. On the other hand, nanoscrolls were found to be in excess of nanotubes in compounds with triclinic structure, such as LaS-CrS2. It has been already observed that symmetry plays a major role in determining the ratio between nanotubes and nanoscrolls besides the reaction conditions.10 As TbS-CrS2 B

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Figure 2. (a) Cs-HRTEM image of a TbS-CrS2 nanotube. The insets display the filtered image, obtained by using the radial difference filter from HREM Research Inc.,36 and the intensity profile obtained from the area delimited by the red arrow. The blue arrows highlight the pinning points of the bent layers. (b) SAED pattern taken from another wavy nanotube as shown in the inset. The tube axis and the basal reflections are marked with green and white arrows, respectively. Spots corresponding to the same interplanar spacing are marked by segmented circles and measured values, with the corresponding Miller indices indicated. The blue and orange colors indicate TbS and CrS2 subsystems, respectively. The red color indicates spots originating from both subsystems.

Figure 3. (a) HR-STEM ADF micrograph of a wavy NT. The blue arrows highlight the radial strikes at the pinning points of the bent layers (b) lowmagnification image. (c) HR-STEM ADF micrograph of the NT surface. (d) Tb/Cr ratio obtained from SR-EELS elemental quantification performed on the spectrum-image which has been recorded on the area delimited by the red square in part c. (e) Intensity profile taken on the area delimited by the green arrow in part c. The purple arrows highlight the weaker intensity features corresponding to the CrS2 subsystem.

TbS-CrS2 nanotubes were characterized by HRTEM and CsHRTEM. A typical Cs-HRTEM image of a TbS-CrS2 nanotube which was synthesized at 950 °C is shown in Figure 2a. The diameter of this NT is equal to 55 nm, and its length is larger than 1 μm. A superstructure of alternating double layers with a periodicity of 1.05 nm is clearly visible at the surface of the NT. The double layers are slightly bent, with the pinning points

presents an orthorhombic structure; there are less boundary conditions to satisfy for seaming the nanotubes, compared to, e.g., triclinic structures. In the same vein, the complex unit cell of V2O5 did not prevent the seaming of the plane edges along one axis to form nanotubes.19 However, folding of the V2O5 planes along two axes to form seamless quasi-spherical nanostructures was found to be elusive.19 C

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Figure 4. (a) HR-STEM ADF and (b) ABF micrographs of the NT shown in the (c) low-magnification image. For visualization purposes, the contrast of the ABF image has been inverted. The two insets display the filtered images obtained by using the radial difference filter. (e) Survey image of the (d) SR-EELS line scan spectra. For clarity only the spectra close to the surface have been displayed and were normalized in order to display the same intensity around the 200 eV spectral region.

attributed to a strain relaxation mechanism, as discussed by Figovsky and co-workers.37 Figure 3 shows HR-STEM micrographs of a telescopic wavy nanotube whose length is around 800 nm and which seems open-ended on one side (Figure 3a). Its internal diameter remains constant (around 9 nm) along the entire length while its smaller and bigger external diameters are 41 and 48 nm, respectively. As seen in Figure 3c, the atomic layers are bent in a nearly periodic manner and present similar curvature radii. In addition, radial strips with darker contrast are highlighted at the pinning points of the bent layers (blue arrows in Figure 3a). The darker contrast of the strips on the ADF micrographs points out a local decrease of the mass density and/or a local decrease of the thickness. The radial strips are visible along the entire length of the NT and are nearly periodically spaced with a period around 5 ± 1 nm. This description matches perfectly the description of the wavy misfit NTs based on the SnS-SnS2 system already reported in the literature.37 It has been suggested that the occurrence of wavy NTs is higher in misfit NTs with O-T alternating layers whereas the presence of additional layers in other arrangements, such as O-T-O-T-T, is believed to prevent the bending.37 The alternating O-T arrangement is confirmed by the analysis of the ADF micrograph and the corresponding intensity profile (Figures 3c,e, respectively). The double (O-T) layers with a 1.10 nm spacing along the radial direction of the nanotube are clearly visible, and the interlayer spacing between each sublayer is 0.33 nm. These double layers display the highest intensity in the profile of the corresponding ADF micrograph (Figure 3e) and correspond thus to TbS layers. Between the TbS double layers, weaker intensity features (highlighted by purple arrows on Figure 3e), which correspond to the CrS2 layers, can be distinguished. The alternating O-T arrangement with 1.1 nm

(highlighted by blue arrows in Figure 2a) being nearly periodically spaced with a distance of about 9 nm. This last point confirms the wavy nature of this nanotube. Selected area electron diffraction (SAED) along with a low-magnification image of another wavy TbS-CrS2 nanotube is presented in Figure 2b. SAED also confirms the superstructure periodicity. The tubular axis is given by the green arrow. The SAED patterns recorded from the TbS-CrS2 nanotube show eight pairs each of {110} and {220} reflections of the TbS sublayers, distributed on the blue segmented circles with interplanar spacings of 3.81 and 1.91 Å, respectively. The multiplicity factor of these planes is four, indicating the presence of two types of TbS layers in the tube with different rolling vectors. Further splitting of each spot into two with an angle of 2.4° results from the chirality of the TbS layer, which corresponds to a chiral angle of 1.2°. There are four pairs of TbS {200} reflections with d-spacing of 2.67 Å. The {020} reflections of both TbS and CrS2, which appear in the same place (commensurate direction), with a d-spacing of 2.81 Å, are marked by red arrows in the SAED pattern. The [020] direction is observed to be oriented with an azimutal angle of 15° with respect to the tubular direction in this case. A family of eight CrS2 {110} planes (multiplicity factor is four for this plane) with a d-spacing of 2.81 Å is also present. There are four pairs of 200 reflections and eight pairs of 130 reflections from the CrS2 appearing with d-spacing of 1.67 and 1.64 Å, respectively. The chiral angle in the CrS2 sublayer is ∼0.9° in this case. The wavy nature of this NT is responsible for splitting of the basal reflections in the electron diffraction pattern. A wavy nature is commonly observed in thin walled nanotubes, whereas it is quintessentially not present in thick walled nanotubular structures (Figure S1 in the Supporting Information). The wavy nature of the misfit nanotubes was D

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Figure 5. (a) Schematic model of the growth mechanism of misfit naotubes/nanoscrolls. Misfit stress between adjacent layers induces curling in misfit compounds. Depending on reaction conditions and symmetry, it further converts the folded nanosheet into either as a nanoscroll or nanotube. Schematic model of CrS2 on top of LaS (b) and TbS (c), where CrS2 feels compressive strain whereas TbS feels tensile strain which is responsible for rippling the structure.

made of nearly 20 TbS double layers which can be well distinguished close to the surface of the nanotubes (red arrows in Figure 4a). As previously observed, each sublayer is separated by 0.33 nm and each TbS doublet is separated by 1.10 nm. In addition, the atomic columns corresponding to the CrS2 subsystem can be well-distinguished between the TbS doublets, in particular in the ABF micrograph (blue arrows in Figure 4b). However, the TbS doublets do not constitute the most external layer and another layer of weak contrast can be seen on the HR-STEM micrographs at the surface of nearly all the NTs (see also Figure 3c). The knowledge of the nature of the outer layer is crucial because it will determine most of the chemical and physical properties of the nanotube.18 In order to get more insight on this outer layer, EELS analyses were performed at the surface of the nanotube (Figure 4e,d). As clearly seen from Figure 4d, there is a striking difference between the EELS responses coming from the surface and the one coming from areas closest to the NT center. Close to the surface, only two main spectral features can be highlighted: the S-L2,3 and the CrL2,3 edges. The S-L2,3 edge presents a delayed-maxima shape which starts around 160 eV and extends itself over several dozen electronvolts. The Cr-L2,3 edge presents the characteristic white-line shape and is situated roughly between 575 and 595 eV. Going deeper in the NT, a sharp peak situated between 145 and 164 eV (maximum at 156.4 eV), which belongs to the Tb−N4,5 edge, appears, and its intensity gradually increases while the one of the Cr-L2,3 edge decreases. A comparison of the fine structures of the S-L2,3 and Tb-N4,5 edges shows that the terbium is completely absent from the outermost layer of the nanotube (Figure S3 in the Supporting Information). Thus, this layer is only composed of the combination of Cr and S elements. EELS elemental quantification of the outer layer yields an S/Cr ratio nearly equal to 1, although it is important to keep in mind that precise and accurate EELS quantification requires comparison with reference materials.23 It is noteworthy that this CrS outer layer was observed in nearly all the nanotubes investigated by EELS. The outer layer was also imaged on Cs-HRTEM micrographs (Figure S4 in the Supporting Information), which highlights its crystalline nature. The thickness of the outer layer goes from less than 1 nm to 2− 3 nm, for different nanotubes. For this nanotube, no oxygen signal was observed during the EELS analyses. However, few of the investigated nanotubes were found to be prone to oxidation. The oxygen concentration was around 4% or

spacing was highlighted by EELS analyses (Figure S2 in the Supporting Information). In order to get more chemical information on the radial strips, SR-EELS spectrum images were recorded at the pinning points of the bent layers, and Figure 3d shows the Tb/Cr ratios obtained from EELS elemental quantification. On the left and right sides of the spectrum-image, alternating layers with richer Tb and Cr concentrations spaced by about 1.1 nm can be delineated. The result of the quantification in these areas is similar to the elemental quantification shown in Figure S2a. As detailed in the Supporting Information, extracting absolute quantitative information from TbS-CrS2 nanotubes via EELS analysis is challenging due to the proximity between the Tb−N4,5 and SL2,3 edges, which triggers an overestimation of the Tb/Cr ratio. However, the variations of this ratio, and in particular spatial variations, are undoubtedly indicative of chemical variations at the local scale. In this regard, the striking feature of Figure 3d lies at the center of the spectrum-image which corresponds to the localization of the radial strips. In this area a decrease of roughly 15% of the Tb/Cr ratio is observed with respect to the areas on the left and right sides of the spectrum image. This observation has been confirmed for all the radial strips investigated. Thus, the darker contrast of the radial strips in the ADF image does not result only from thickness variation of the goffered NT but also from a deficiency in Tb. This suggests that one (or more) Tb atom is missing in one row at every edge point where the two arcs coalesce. A discontinuity in the TbS layer during the formation of the NT might then be at the origin of the formation of wavy NT. The compounds (LnS)1+x-CrS2, where Ln = rare earth, are semiconductors.1 There are x electrons remaining on the LnS layer considering that each CrS2 gets one electron per formula unit from the LnS layer. These excess remaining electrons should make (LnS)1+xCrS2 metallic, which is in contrast to the measured semiconductor behavior. It was suggested that cation vacancies in the rock-salt layer are responsible for the discrepancy in the expected and measured physical properties of these misfit layered compounds.38 However, these vacancies cannot be easily detected by HRTEM. Nonetheless, in this work the Tb-vacancies could be clearly mapped in the wavy TbS-CrS2 nanotubes (see Figure 3). Figure 4 shows HR-STEM micrographs of a regular nanotube whose length, external diameter, and internal diameter are 1.2 μm, 30 nm, and 10 nm, respectively. It is E

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visible along the nanotube, especially at curved regions, which is confirmed by SR-EELS analyses and HR-STEM measurements.

below. Its variation follows the Cr distribution and is higher close to the NT’s surface (Figure S2). This observation might indicate the oxidation of the first external CrS2 layers. 3.2. Growth Model. It is suggested that the growth of the nanotubes during the high temperature annealing follows different steps as follows. In the first few minutes, the mixture of hydroxides heats up and loses water. Above 500 °C the oxide starts reacting with the sulfide and is converted to the TbS-CrS2 at elevated temperatures. As the densities of the misfit sulfide are smaller compared to respective oxides, there is a huge volumetric change upon conversion of the oxides into sulfides. Since the thermal expansion is higher at higher temperatures, the strain between the oxide and the incipient sulfide nanostructures increases. One way of releasing the strain is by propelling sulfide nanowhiskers growth perpendicular to the surface across the interface. Simultaneously, the strain between the two misfit sulfides increases, leading to scrolling and eventually formation of nanotubes (see Figure 5a).10 The average length and diameters of the TbS-CrS2 nanotubes are smaller compared to other lanthanide based nanotubes synthesized earlier.10,12 The atomic weight of the Tb is higher compared to other lanthanides, such as La, Ce, Nd, and Gd; thus, the rate of mass transport in the vertical direction is slower with Tb compared to the earlier lanthanides. Thus, the growth rate of TbS-CrS2 is lower and their thickness is smaller compared to nanotubes of LnS-CrS2 (Ln = La, Ce, Nd, and Gd) under given reaction conditions. Interestingly, the nanotubes were observed to grow from a chromium-enriched precursor (compared to the chromium concentration in the nanotubes) whereas the growth was not observed from a precursor which shows a depletion of chromium relative to the nanotube stoichiometry. Furthermore, extensive HR-STEM/SR-EELS analyses of the product revealed that nearly all of the nanotubes terminate with a chromium sulfide layer. The following hypothesis can be made on the early stages of the nanotubes growth: The Cr−O bonds are sulfurized first followed by conversion of Tb−O to Tb−S at a later stage of the sulfurization reaction.39,40 This may be due to two reasons: (a) converting Tb−O bonds to Tb−S requires a higher energy than converting Cr−O to Cr−S, and (b) the thermal expansion decreases with increasing bond energy; thus, the lattice thermal expansion of the CrS2 sublayer is higher compared to that of the TbS sublayer. The excessive strain produced by the Cr−S layer leads to the “sprouting” of nanotubes from the reacting precursor film. Nanotubes with fewer numbers of layers were found to be wavy in nature. Contrarily, thicker walled TbS-CrS2 nanotubes are found to have straight walls (see Figure S1 in the SI). Strain generated in the nanotube (apart from misfit stress) might be responsible for this wavy nature. A general observation has been made in the lanthanide based misfit compounds, i.e. that wavy nature is barely observed in nanotubes of LaS-CrS2 or CeS-CrS2 but is more evident in the case of TbS-CrS2. The lattice parameter of TbS is smaller compared to LaS or CeS. Thus, when LaS is replaced by TbS, CrS2 feels larger compressive strain to accommodate itself in a smaller area. This extra compression strain may force rippling of the CrS2 layers (see Figure 5b,c), and the tensile strain felt by TbS might be responsible for the periodic deficiency of Tb in the nanotubes. The rock-salt TbS seems to be less flexible compared to the more covalent natured CrS2 layers. The TbS layer is found to be discontinuous. The discontinuities are

4. CONCLUSIONS In summary, a complete analysis of the structure and chemical configuration of misfit-layered nanotubes based on the TbSCrS2 has been developed via different TEM measurements. Two different kind of nanotubes, “regular” and “wavy” (socalled goffered) nanotubes, have been reported. Alternating arrangements of the TbS and CrS2 subsystems are observed for both kinds of nanotubes. However, a nearly periodic deficiency in terbium is highlighted for the wavy ones. The local (close to atomic level) chemical analyses carried out in these systems have determined that the outer layer of both kinds of nanotubes is composed of Cr and S elements with 1:1 stoichiometry. From all these structural and chemical results, we have proposed a growth mechanism for these nanostructures which is based on the strain release during the sulfurization reaction in the synthesis process. All these findings and the improvement of the knowledge of these nanotubes are crucial not only for the potential scale-up of their production, but also for the study of their physical and chemical properties and their future applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05811. HRTEM image of a thick walled TbS-CrS2 nanotubular structure, elemental quantification of a radial EELS line highlighting the O-T alternating system and the surface oxidation of one wavy NT, EELS analyses of the surface of one nanotube highlighting the CrS termination, and Cs-HRTEM micrograph of the surface of a TbS-CrS2 nanotube showing the presence of an outer crystalline layer (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ∥

L.S.P. and L.L. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Ronit Popovitz-Biro for help in HRTEM measurements. The HR-STEM-EELS and CsHRTEM works have been conducted in the Laborario de ́ Avanzadas (LMA) at the Instituto de NanoMicroscopias ciencia de Aragón (INA) - Universidad de Zaragoza (Spain). Some of the research leading to these results has received funding from the European Union Seventh Framework Program under Grant Agreement 312483 - ESTEEM2 (Integrated Infrastructure Initiative - I3) and under Grant Agreement 604391 Graphene Flagship. R.A. also acknowledges funding from the Spanish Ministerio de Economia y Competitividad (FIS2013-46159-C3-3-P). R.T. acknowledges the support of the Israel Science Foundation; the Israel National Nano-Initiative through its Focal Technology Areas F

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The Journal of Physical Chemistry C

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program; the H. Perlman Foundation; the Irving and Azelle Waltcher Foundations in honor of Prof. M. Levy; and the Irving and Cherna Moskowitz Center for Nano and Bio-Nano Imaging. L.S.P. would like to thank the PBC Program of the Government of Israel and the Dean of the chemistry faculty, Weizmann Institute of Science, for a postdoctoral fellowship. I wish to express my deepest appreciation and friendship to Prof. Oasaki on his 65th birthday. I wish him many more years of active research, health and happiness (R.T.).

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DOI: 10.1021/acs.jpcc.5b05811 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C (40) Haibin, Y.; Ohta, M.; Hirai, S.; Nishimura, T.; Shimakage, K. Preparation of Terbium Sesquisulfide and Holmium Sesquisulfide by Sulfurization of Their Oxide Powders Using CS2 Gas. J. Rare Earths 2004, 22, 759−762.

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DOI: 10.1021/acs.jpcc.5b05811 J. Phys. Chem. C XXXX, XXX, XXX−XXX