Thermal Undoping Induced 2D sheet Exfoliations in 1D Nanomaterial

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Thermal Undoping Induced 2D sheet Exfoliations in 1D Nanomaterial Suman Bera, Amit K. Guria, Saied Md Pratik, and Narayan Pradhan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00463 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 18, 2018

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The Journal of Physical Chemistry

Thermal Undoping Induced 2D sheet Exfoliations in 1D Nanomaterial Suman Bera, Amit K. Guria,* Saied Md Pratik,¥ and Narayan Pradhan*

Department of Materials Science and ¥Department of Spectroscopy, Indian Association for the Cultivation of Science, Kolkata 700032, India

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ABSTRACT Exfoliations leading to monolayer sheets are mostly reported in 2D materials such as graphene, WS2, MoS2 etc. However, theoretically it is established that exfoliations can also be possible for 1D materials like Sb2S3, though this has not been experimentally reported yet. Furthermore, most of reported exfoliations are carried out with physical processes and only in few cases complicated chemical pathways are also established. Keeping views on importance of both materials and methods, herein, the exfoliation of 1D Sb2S3 nanostructures were reported via a unique thermal undoping approach where annealing expelled Sn atoms from the crystal lattice of 1D Sn doped Sb2S3 nanostructures leading to 2D sheets via very intriguing 1D-2D coupled structures. Sb2S3 is a 1D material; but associated with 2D van der Walls forces and in our dopant removal approach, exfoliation was exclusively carried out in directions perpendicular to the major axis of doped nanostructures. Apart from experimental supports, DFT calculation was also carried out keeping Sn in substitutional and interstitial positions to support our claim. These results suggests that designing proper chemical process could successfully exfoliate the 1D materials and the same might be extended to other materials of same family.


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INTRODUCTION Exfoliation of nanomaterials leading to ultrathin layered structures typically widens the surface area which strongly influences their surface as well as electronic properties.1-15 However, most of such exfoliations are limited to 2D materials such as graphene, MoS2, WS2etcand carried out through thermal and mechanical approaches.1-8, 14, 16 Beyond 2D materials, layer exfoliation is also theoretically predicted for 1D material such as Bi2S3, Bi2Se3, Sb2S3 and Sb2Se3 though these have not been chemically achieved to date.17 Hence, understanding and executing the exfoliations in these materials till remain challenging. Synthesis of bulk materials and subsequent intercalation of third metal ions or incorporation of solvent molecules between layers, and stabilization of sheets by solvent remained the most feasible processes for liquid exfoliation.10-14, 18-29 For chemical exfoliation, one of the most convenient approaches is chemically weakening the interlayer interactions. Insertion of different ions in the crystal lattice is one possible method for lattice expansions. Layered structured materials are typically interacted by van der Waals interaction and hence weakening this remains the major key for the exfoliation.17, 30-31 This foreign ion incorporation or doping technique has been extensively studied in semiconductor nanostructures; but mostly for change in optical,32-43 magnetic44-50 and electronic51-55 properties. Recently, doping is also used for modifying the crystal growth and change in the shape of nanostructures.56-60 Though post synthesis approach of intercalation of Li ions has been used for exfoliations to some extent;11,

15, 18-20, 22

but the chemical doping approach during insitu growth has not been



explored. Hence, with proper manipulation and adopting direct doping approach might help to solve this chemically and result exfoliated uniform layers of nanostructures. Among the theoretically predicted semiconductors, Sb2S3, a 1D material, but associated with two dimensional van der Waals interaction within its layered structures is an ideal case for studying theexfoliation.17 Unlike group II-VI semiconductors, the chemistry of synthesis of nanostructures of this material is not also widely explored.61-62 Hence, fundamentally it is also important to understand the growth process for designing various nanostructures of this 1D material. On doping Sn, herein, the 2D lateral exfoliation is reported in 1D Sb2S3 nanostructures. Micrometer long 1D nanostructures (belt like) of Sn doped Sb2S3 were initially synthesized which on further thermal annealing led to very intriguing exfoliated 1D-2D nanostructures and finally nearly monodisperse ultra-thin sheets of Sb2S3 remained the ultimate product. Sn insertion remained here the key for both 1D nanostructures formation and also for their exfoliation. While nanostructures had ~10% Sn content, the final nanosheets were almost free from Sn. The most important observation here was the uniform 2D exfoliation along [001] and [00-1] directions of orthorhombic phased Sb2S3 and this occurred almost in all wires throughout their entire lengths. Density functional theory calculation was performed keeping Sn in both substitution and interstitial position of the crystal lattice for understanding the driving force for formation of this 1D-2D nanostructures. Results suggested that the removal of Sn from the crystal lattice during thermal annealing weakened the inter-layer van der Waals force which consequently exfoliated the layers. To our best knowledge, this is the first of such kind of report where dopants in solution process helped in separating the layer so uniformly. Entire study has


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been carefully carried out with stepwise microscopic imaging and the involved doping chemistry is elaborately discussed. EXPERIMENTAL SECTION Materials. Antimony (III) trichloride (SbCl3, ≥99.0%), tin (II) chloride (SnCl2, 98%), tin (IV) chloride pentahydrate (SnCl4.5H2O, 98%), S powder, octadecylamine (ODA, 97%), oleylamine (OAm, tech., 70%), 1-dodecanethiol (1-DDT, ≥98%) were purchased from Sigma-Aldrich, tertdodecanethiol (t-DDT; 90%) was purchased from TCI. These chemicals were used as purchased without any further purification Synthesis and 2D Exfoliation of Tin Doped Antimony Sulphide (Sn:Sb2S3) nanostructures. Tin doped Antimony sulphide (Sn:Sb2S3) nanostructures were synthesized using SnCl2. In a typical procedure, SbCl3 (0.2 mmol, 46mg), SnCl2 (0.02 mmol, 3.8 mg), 1mL 1-DDT and 2 g ODA were taken in a 25 mL three-neck flask equipped with a condenser, thermocouple, septum, and containing a magnetic stir bar. The reaction flask was sealed and placed in a heating mantle on a stir plate. After the flask was degassed with N2 bubbling for 15 minutes at 80 oC, it was heated to 180 oC. In a vial, powder S (0.3 mmol, 9.6 mg), 1 mL t-DDT and 2 mL OAm were mixed and deaerated at room temperature for 15 min followed by stirring for 15 min at 40 oC. The resulting faint yellow solution was injected into the reaction flask through the septum using a syringe and annealed for 5 min at 180 oC temperature. Then the temperature was set at 215 o

C.2 min time was taken to attain 215 oC from 180 oC. The mixture was vigorously stirred at this

temperature for 5 min. To get the belt like nanostructures, the hot reaction mixture was collected into ethanol and the nanocrystals were then precipitated by centrifugation. To



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exfoliate the belt nanostructures, after annealing at 215 oC, temperature was set at 235 oC. 1 min 20 sec time was taken to attain 235 oC from 215 oC. The reaction mixture was annealed for 5 min at that temperature and then quickly cooled down to room temperature. The nanocrystals were purified using hot ethanol as non-solvent and chloroform as dispersing solvent. Note. Three neck Flask = 25 mL, Magnetic Bar = 1 mm, Injection syringe = 5 mL and 1 mL and needle 18 mm. Sample collection syringe = 1 mL and needle 18 mm. Magnetic stirrer used= Remi India make 1 mL. Centrifuge speed=1400 rpm, time = 1 min. 0.5 mL crude sample was collected in 3 mL of non-solvent. Characterization. X-ray diffraction (XRD) analysis of the nanocrystals was performed using a Bruker D8 Advance powder diffractometer, using Cu Kα (λ = 1.54 Å) as the incident radiation. Transmission electron microscopy (TEM), high resolution TEM (HR-TEM), and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images were captured on an ultra-high resolution field emission electron gun-TEM, JEOL JEM-2100F electron microscope using a 200kV electron source. Elemental mapping and Energy Dispersive X-ray spectroscopy (EDS) measurement were performed using same instrumentation. Specimens were prepared by placing a drop of nanocrystal solution in chloroform on a carbon-coated copper grid, and the grid was subsequently dried under ambient conditions. X-ray Photoelectron Spectroscopy (XPS) measurements were performed at room temperature using Al Ka source (10 mA, 15 kV) on a laboratory

based

commercial

X-ray

photoelectron

Nanotechnology.


spectrometer

from

Omicron

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Computational Details. The plane-wave DFT calculations for the Sb2S3 crystal were performed using the PWSCF (plane-wave self-consistent field) code within local density approximation (LDA) for the exchange and correlations as implemented in the QUANTUM ESPRESSO Package.63 The interactions between the ion cores and the valence electrons were taken into account by norm-conserving Bachelet-Hamann-Schlueter scalar-relativistic pseudopotentials.64 The crystal was fully relaxed using a k−point mesh of 4×8×4 with a kinetic energy cutoff of 40 Ry, until all the forces were less than 10-3a.u. Dispersion interactions were considered using DFT–D2 empirical formalism of Grimme and co-workers.65 The initial crystal structure of Sb2S3was generated from the experimental CIF (crystallographic information file). We considered two possibilities namely, deposition of Sn atom(s) between interlayers to study the effect of Sn at interstitial position and by replacing one Sb with Sn to explore the substitutional effect. In both cases, the volume as well as the geometry relaxation was performed within unit cell by following the same methodology. In the model 1D nanoribbon, large crystallographic a and c axis were set to avoid unwanted interaction whereas the periodicity was preserved along b axis. Herein, the optimization was also performed by using DFT—D2 method at same level of theory as mention for the crystal. The inter nanoribbon energy (ΔE) was calculated as (Efull/2 – Enanoribbon). RESULTS AND DISCUSSION Thermally Controlled Exfoliations of 1D nanostructures. Figure 1 shows the microscopic images of initially synthesized micrometer long Sn doped Sb2S3 (Sn:Sb2S3) 1D belt like nanostructures. These were synthesized in one pot approach where elemental sulfur (in amine)



was injected to a mixture of Sn(II) chloride, Sb(III) chloride, alkylthiol and oleylamine at180 oC. Rising the temperature and on subsequent annealing for 5minutes at 215 oC, these crystalline doped nanostructures were formed. Figure 1a-1b and Figure S1 of the Supporting Information (SI) present the TEM images, and Figure 1c presents HAADF image of these nanostructures in different resolution. Figure 1d depicts the HRTEM image of a nanostructure showing these were grown along [010] direction. There nanostructures are in micrometer long with ~10-12 nm width. From Figure 1b and 1c, it was also observed these structures are belt like shape rather than nanowires. Being these were grown in one direction, these were referred as 1D nanostructures. From the elemental analysis, ~10% Sn was observed in these nanostructures (Figure S2). Figure S3 shows the HAADF-STEM image and mapping for the elements Sn, Sb and S; confirming Sn retained in these ultra-long nanostructures. While synthesis of Sb2S3 nanowires is reported,66 we intentionally incorporated Sn for studying the dopant induced exfoliation effects.

Figure 1. (a-b) TEM images of Sn:Sb2S3 1D nanostructures in different resolution. (c) HAADFSTEM image and (d) HRTEM image of the doped nanostructures. Images in panel (b) and (b) show the widths of these 1D structures became narrowed at the curve places suggesting these nanostructures would be belt like rather than nanowires.


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Figure 2. (a-f) TEM images showing wide area two directional sheet exfoliations from parent nanostructures. (g) TEM image and (h) HAADF-STEM image of a single nanostructure exfoliation. (i) TEM image, (j,k) HAADF-STEM images and (l) HRTEM image of early stage sample showing beginning of the layers exfoliation. The exciting result here was the transformation of this 1D belt like nanostructures to 2D sheets during thermal annealing. Figure 2a-h shows the microscopic images of the intermediate 1D-2D nanostructures which were obtained at 235oC and 1min annealing. Ultra-thin sheets of Sb2S3 were exfoliated in perpendicular directions to the 1D belt nanostructure growth. These were monolayers and were observed in each nanostructure and in the entire length. In some cases (Figure 2d, e as example), multiple exfoliated nanostructures were also combined. The formation process was also seen in early sample where sheets were observed coming out from these nanostructures. Figure 2i presents the TEM and Figure 2j-k the HAADF images of these intermediate samples collected after the reaction temperature just reached at 230 oC. The HRTEM image of one such structure is also shown in Figure 2l. Thus, while in the earlier sample



collected at 230 oC, such exfoliations were in preliminary stage, sample obtained at 235 oC after 1 minute of annealing acquired the layer exfoliated structure. These successive shape variations during annealing confirmed that exfoliations were indeed thermally induced and with prolonged heating these were elongated and finally turned to exclusive sheets(TEM images in Figure S4)of Sb2S3 (Powder X-ray diffraction, PXRD in Figure S5). Step Analysis in the Exfoliations. For understanding this exfoliation process, the 1D-2D intermediate exfoliated structures were further analyzed. PXRD pattern of these exfoliated structures obtained from the belt like structures is shown in Figure 3a and the pattern is also compared with the parent belt nanostructures and bulk orthorhombic Sb2S3 (peaks labeled in Figure S5). In both cases, the peak positions remained identical confirming the exfoliation process did not change the orthorhombic phase. However, peaks were broadened for the exfoliated structures as their dimensions changed. Figure 3b shows the HAADF image and the elemental mapping of Sb, S and Sn. Results show that Sn still retained in these nanostructures though from EDS analysis (See Figure S6 in SI) only ~5% Sn was observed. From the HRTEM image shown in Figure 3c, the crystal orientations of the stem belt and the exfoliated sheets were investigated. Unfortunately, due to very thin nature, a clear HRTEM could not be obtained from the sheet area. Hence, from FFT pattern, the HRTEM is simulated and shown in upper inset. This also confirmed monolayers and multi-layers under same plane might have shown moiré patterns and these were not observed in any places of the TEM grid. Similarly, the FFT pattern obtained from the center of the belt is presented in bottom inset. The viewing axis here is [100], the belt is grown along [010] and the two exfoliation directions are [001] and [00-1]. Figure 3d presents the X-ray photoelectron spectra (XPS) for the intermediate samples for the 10 ACS Paragon Plus Environment

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exfoliated nanostructures. Peak positions at 496.3 and 487.7eV signified for the presence of Sn(IV) as the doped ions in the host Sb2S3 lattice.67-70

Figure 3. (a) PXRD patterns of Sn doped Sb2S3 nanostructures (below) and exfoliated intermediate 1D-2D nanostructures (above). Standard XRD peaks for bulk orthorhombic Sb2S3 (PDF no. 073-0393) is provided. (b) HAADF-STEM image of the 1D-2D exfoliated intermediate structure and the elemental mapping of Sb, S and Sn. Scale bars equal to 100 nm. (c) HRTEM image of the 1D-2D exfoliated intermediate structure showing presence of both stem wire/belt and sheets. Selected area FFT patterns (1) and (2) shown in insets are from the sheet (area 1) and wire (area 2) areas respectively. Inset shows the simulated HRTEM from the FFT pattern of sheet area. (d) High resolution XPS of Sn 3d3/2 and 3d5/2 of the exfoliated intermediate showing 11 ACS Paragon Plus Environment


the presence of Sn(IV) only. Black line: experimental data, Red line: fitting, Orange line: baseline, Green line: Sn(IV). Undoping and the Exfoliation Mechanism. Further understanding this exfoliation process, the role of the dopant is thoroughly investigated. Increase in the amount of Sn-precursor used in synthesis did not alter the structural transformation; but total amount retained in 1D nanostructures remained same (~10%) (see Figure S2 in SI). During exfoliation it is reduced to ~5%(see Figure S6) and in exclusive sheets negligible amount of Sn (~1.5%) (see Figure S7) was observed. This confirmed that limited amount of Sn could be retained in the crystal lattice of doped belt nanostructures and their removal is related to exfoliation. For confirming this hypothesis, DFT calculation21, 28-29 was performed keeping Sn in the substitutional as well as interstitial lattice positions of Sb2S3.Individual nanoribbons of Sb2S3is shown in Figure 4a. The ground state optimized unit cell for the Sb2S3 crystal using the plane-wave DFT-D2 method is shown in Figure 4b. The crystal of Sb2S3 contains 20 atoms in its unit cell and is build up the one dimensional arrangement of the individual nanoribbons with a formula unit of (Sb2S3)2n. The calculated interaction energy (ΔE) between the inter 1D nanoribbons is found to be ~ -3.1 eV that clearly demonstrated that the formation of Sb2S3 crystal from 1D nanoribbons is an energetically favorable process. The computed inter-nanoribbon Sb−S distances arise from 3.14 Å, which also reveals the resulting 3D Sb2S3 crystalis a consequence of van der Waals interactions between the 1D nanoribbons that is in good agreement with the previous report.17Interestingly, Sn substitution of Sb did not alter much to the unit cell. The interlayer distance along [100] axis remained ~3.14Å (Figure 4c). However, while Sn is placed interstitially (Figure 4d), the interlayer distance along [100] directions increased with maximum upto ~


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4.63Å. The optimized lattice parameters with and without Sn dopant for Sb2S3 crystal are provided in Table S1 of SI. With the support of these theoretical calculations and experimental observations, we proposed that on thermal annealing at higher temperature dopant Sn leaves the substitutional position. When it comes to interstitial site, layers are exfoliated due to weakening of van der Waals forces. All these are summarized and our proposed mechanism of exfoliation is shown with atomic model in Figure S8. These layered structure atomic arrangements with Sn at substitutional and interstitial positions of the crystal lattice are viewed along [010] direction. The interlayer distances along [100] and [-100] directions are shown to be increased when Sn is moved to inter-layer position. As a consequence layers were slipped or removed along [001] and [00-1] directions.

Figure 4.The optimized crystal structures of (a) 1D nanoribbon of Sb2S3 (b) Sb2S3 (c) Sn doped at interstitial position in Sb2S3 and (d) one Sb replaced by Sn in Sb2S3, obtained by using plane wave DFT−D2 calculations. The inter-nanoribbon Sb−S (and Sn−S) distances are shown. Sb2S3 is a one dimensional material and has layered structures associated with van der Waals forces in two directions. The substitution doping favors Sn(IV) as its size matches with Sb(III) ion (the ionic radii of Sb(III), Sn(IV) and Sn(II) are respectively, 0.62, 0.71 and 1.12 Å).71 The presence of Sn(IV) in intermediate nanostructures was also confirmed. However, while the 13 ACS Paragon Plus Environment


parent Sn:Sb2S3 nanostructures had ~10% Sn content, the final samples contained insignificant amount of Sn which suggested the undoping process exfoliated the nanostructures. Moreover, even not doped, some amount of Sn would be detected as those would be precipitated along with final sheets. DFT calculations also confirmed that limited amount of Sn could be retained in the crystal lattice of doped nanostructures and removal of Sn from substitutional positions to interstitial positions is related to exfoliation. As this process triggered with thermal annealing at higher temperature, it could be stated here that increase of temperature annealed out Sn which indeed exfoliated the layered material. Hence, the driving force for this exfoliation is the thermal energy. Only when the reaction temperature was increased such beautiful structures were obtained and on further heating or annealing, only sheets were the final product. Hence, it can be concluded that the undoping process triggered the exfoliations. CONCLUSION In conclusion, we report a wet chemical synthesis of dopant induced 2D exfoliation of nanosheets from 1D belt nanostructures of 1D nanomaterials. Here we have studied the thermal exfoliation of Sn doped Sb2S3 nanostructures along the minor axes. This has been observed to be very uniform and in entire micrometer long 1D nanostructures. Results suggested that, the removal of Sn from the substitution position into the interstitial place in the crystal lattice during thermal annealing weakened the inter-layer van der Waals force which consequently exfoliated the layers. To our best knowledge, this is the first of such kind of report where dopants in solution processed nanocrystals helped in separating the layers so uniformly. Entire study has been carefully carried out with stepwise microscopic imaging and the involved


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doping chemistry is elaborately discussed. While exfoliations has been widely seen in 2D materials and theoretically predicted it would be possible for 1D materials; here this was experimentally shown in 1D Sb2S3. This is a beginning of the extension of designing technologically important layered materials. Hence, this will opens up a new searching whether the doping can also be extended to other matrix and would help in fabrication of wide variety of materials.

■ASSOCIATED CONTENT Supporting Information Additional figures (TEM and HAADF-STEM images, PXRD, EDS spectra) and table. This material is available free of charge via the Internet at http://pubs.acs.org.

■AUTHOR INFORMATION Corresponding Author *E-mail: (NP) [email protected], (AKG) [email protected] Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS DST of India is acknowledged for funding. ■ REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666-669. (2) Li, H.; Wu, J.; Yin, Z.; Zhang, H. Preparation and Applications of Mechanically Exfoliated SingleLayer and Multilayer MoS2 and WSe2 Nanosheets. Acc. Chem. Res. 2014, 47, 1067-1075. 15 ACS Paragon Plus Environment


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Thermal Undoping Induced 2D sheet Exfoliations in 1D Nanomaterial

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