Insights of Diffusion Doping in Formation of Dual-Layered Material and

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials 2

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Dual Layered Material and Doped Heterostructure SnS-Sn:SbS as Efficient Energy Material: The Insights of Diffusion Doping Suman Bera, Amlan Roy, Amit K. Guria, Sagar Mitra, and Narayan Pradhan

J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00107 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 15, 2019

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The Journal of Physical Chemistry Letters

Dual Layered Material and Doped Heterostructure SnS-Sn:Sb2S3 as Efficient Energy Material: The Insights of Diffusion Doping Suman Bera, Amlan Roy,¥ Amit K. Guria,* Sagar Mitra,*¥ Narayan Pradhan* Department

of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032, India

¥Department

of Energy Science and Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India

*E-mail: (NP) [email protected], (AKG) [email protected], (SM) [email protected]

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ABSTRACT Insights of formation mechanism of a dual layered and doped heterostructure material SnIISSnIV:Sb2S3 is reported. In presence of mixed alkyl thiols, first nanotubes of Sb2S3 were formed and then introducing Sn(IV), SnIIS were deposited onto the surface of these tubular structures. Annealing further at a constant temperature, sluggish transformation resulted Sn(II)S-Sn(IV) doped Sb2S3 heterostructure which finally turned to flake like layered doped Sb2S3 nanostructures. SnS and Sb2S3, both being layered materials, were explored for the study of Naion storage, and these heterostructures were observed superior in comparison to the individual material as well as the final doped nanostructures. Details of the insight mechanism of formation of the heterostructures, the epitaxy at the junction, the diffusion doping and the dopant induced axial exfoliations leading to final doped structures were studied. Their electrochemical conversions in presence of Na-ions were also investigated and the insight mechanisms of both were reported in this letter. TOC


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Semiconductor heterostructured nanomaterials have been extensively studied as efficient energy materials for charge carrier transportation1-7 and catalysis.1,8-13 However, selecting appropriate functional materials, understanding the chemistry in placing those in single building block having effective interfacial hetero-junctions and identifying the synergistically developed new functional properties, ever remained challenging.1,3,8 Formation of epitaxial junction, the ion diffusion along the interface and retaining the identity of each material in the heterostructured matrix are the most important parameters for obtaining the robust and important heterostructured materials.1-3,5,12,14-20 From the literature reports, it is revealed that such structures composed of metal and semiconductors are studied extensively, and those were mostly applied in light harvesting and photocatalysis.7-8,10,15,21-25 However, heterostructures involving materials useful for energy storage applications, where the storage capacity of the ions would be enhanced synergistically in the coupled structure, are not widely explored.26-30 Offering importance to the materials and also to the insight chemistry in bringing those together in a single building block, herein, the complex heterostructures of two layered structured materials, SnS and exfoliated Sn doped Sb2S3(Sn:Sb2S3), ideal for Na-ion storage, is reported. This has been performed taking orthorhombic Sb2S3 and SnS semiconductors where SnS nanostructures instantaneously decorated randomly onto the preformed tubular structure of Sb2S3. During annealing, the Sn ions slowly diffused inside the Sb2S3 and helped in axially exfoliation leading to intriguing multi-component semiconductor-doped semiconductor heterostructures. The insight mechanism of these heterostructures formation, thermal ion diffusion and final exfoliation to a single material are extensively investigated. Further, as these are dual semiconductor layered heterostructured materials, study of Na-ion storage was



investigated, and the intermediate heterostructures of SnS and partially exfoliated Sn:Sb2S3 was found as the superior one than the single semiconductor materials.

Figure 1. (a) HAADF-STEM image of Sb2S3 showing tube like structure. (b) HAADF-STEM image of SnS decorated Sb2S3 nanotubes.(c) HAADF-STEM image and (d) TEM image of single nanostructure showing early stage exfoliation. (e) Elemental mapping showing Sn, Sb and S. Exfoliated parts on both ends show much lower Sn counts than Sb and S. (f) HAADF-STEM, (g) TEM image and (h) HAADF-STEM image of single nanostructure showing elongation of sheets with further annealing.(i) HAADF-STEM image of final sample which show folded sheets exfoliated and acquired needle like shape. More TEM and HAADF-STEM images of time-wise collected samples are given in Figure S4. For the synthesis of these heterostructures, Sb2S3 nanotubes were first synthesized and then Sn(IV) salt was injected for tin (II) sulfide formation. Figure 1a shows the HAADF image (Figure S1, TEM images) of typical Sb2S3 nanotubes which were prepared by injecting mixture of primary and tertiary thiols to Sb(III) chloride solution in amine at 210 oC (details in 4 ACS Paragon Plus Environment

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experimental section of supporting information). The elemental mapping of Sb2S3 nanotubes (Figure S2) confirms that the tubes are composed of Sb and S. Sn(IV) salt was used which in presence of amine partially reduced to Sn(II) in the reaction system and formed SnS (characterization is discussed later). Figure 1b shows the SnS decorated Sb2S3 heteronanostructures obtained after 10 min, and Figure S3 presents its elemental mapping showing SnS at the surface of Sb2S3 nanotube. However, without altering the reaction temperature while the reaction was further investigated, very interesting observations on exfoliations were noticed. Figure 1c-d shows the microscopic images of sample collected after 15 min from the same reaction system. These nanostructures showed evolutions of new material from both ends of nanotubes (see more images in Figure S4). Elemental mapping (Figure 1e) suggested that these new material coming out from the nanotubes contained mostly Sb and S with insignificant amount of Sn. However, Sn was noticed more concentrated on the surfaces of the initial nanotubes as observed in previous stage samples (Figure 1b). Hence, at this stage it could be assumed that Sb2S3 were separated out from SnS decorated nanotubes. Further, sample collected after 30 min (Figure 1f-h) showed further elongation of Sb2S3 from the parent material. The HAADF image in Figure 1h clearly marked the exfoliations from tube periphery. At the same time, it was also observed that the materials concentration at the center slowly decreased. Furthermore, after 60 min of annealing, the collected sample showed mostly folded nanosheets where the center part along the major axis was elongated more than the periphery (Figure S4). Figure 1i and Figure S5 present the HAADF and TEM images of the needle like nanostructure in final sample (90 min annealed) respectively (see more images in Figure S4). Elemental mapping in Figure S5 suggests the presence of Sb, S and Sn, and EDS data showed Sn retained only 1.74% (Figure S6) in the final exfoliated



nanosheets. This indicated that amount of Sn significantly reduced in the final sample where only folded sheets were the ultimate products. Hence, it could be stated here that initially formed tubes were exfoliated in the axial [010] direction during the doping and un-doping process. Moreover, as the reaction temperature was fixed and all these transformations were observed as a function of reaction time, the driving force for the shape transformation would be considered dominated with kinetic parameters that controlled the entire SnS decoration and also the exfoliations. Details of the chemistry of these transformation process and characterizations of the materials are discussed in later section. For understanding the role of Sn dopant in the exfoliations in of Sb2S3 nanotube, the intermediate structures were further characterized. Powder XRD pattern for the decorated nanotubes was observed different than the parent nanotubes or the final exfoliated nanosheets. Figure 2a shows the XRD pattern of the parent nanotube, exfoliated decorated tubes intermediates and the final sample collected, respectively, at 15 and 90 min of the reaction. While the parent nanotube and final sample had the pattern to orthorhombic Sb2S3, the intermediate had several additional peaks. Careful analysis confirmed that these peaks were from orthorhombic SnS. For comparison, standard XRD peaks for bulk orthorhombic SnS was provided (Figure 2a, Figure S7). These results confirmed that even though Sn(IV) salt was introduced to the nanotube solution, that was turned to Sn(II)S, and in final product only dopant amount of Sn retained which did not alter the original XRD pattern. These further suggested that some redox processes might also be associated in the exfoliation process. For investigating more on these oxidation states of Sn, XPS of decorated tubes intermediate heterostructures was carried out.


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Figure 2b presents the XPS spectra of exfoliated intermediate heterostructures, SnS decorated exfoliated Sb2S3 nanotubes. The spectrum obtained from this intermediate structure was broader. Analyzing the spectrum, it was concluded that this broader spectrum was contributed by both Sn(II) and Sn(IV), and hence Sn was present with both the oxidation state in these nanostructures. Peak positions at 496.3 and 487.7eV in Figure 2b signified for the presence of Sn(IV).31-35 Since the decorated materials was confirmed to be SnS, Sn(IV) was assumed to be the doped ions in the host Sb2S3 lattice.

Figure 2. Powder XRD patterns of (a) Sb2S3 nanotube (below), SnS decorated exfoliated Sb2S3 nanotube heterostructures intermediates (middle) and the final sample of axial exfoliation (above). Standard XRD peaks for bulk orthorhombic Sb2S3 (PDF no. 073-0393) and 7 ACS Paragon Plus Environment


orthorhombic SnS (PDF no. 073-1859) are provided for comparison. (b) High resolution XPS of Sn 3d3/2 and 3d5/2 of the exfoliated intermediate obtained from SnS decorated Sb2S3 nanotubes showing presence of both Sn(II) and Sn(IV). Black line: experimental data, Red line: fitting, Orange line: baseline, Green line: Sn(IV) and violet line: Sn(II). (c) HRTEM image and magnified TEM image (in inset) showing Sb2S3-SnS heterojunction. Selected area FFT patterns obtained from (d) Sb2S3 and (e) SnS regions, and these are obtained from the HRTEM image in panel (c). (f) Atomic model showing the epitaxial connection between SnS and Sb2S3. Further, HRTEM of the intermediate samples were analyzed for understanding the heterojunction and the directions of exfoliations. Figure 2c shows the HRTEM image of decorated nanostructures where the center part was Sb2S3 and outer one was SnS. Selected area FFT patterns from both areas also confirmed the same (Figure 2d and 2e respectively). Here, Sb2S3 and SnS belong to same orthorhombic phase and the HRTEM images showed the viewing direction was [01-1] for SnS and [001] for Sb2S3. Plane distances of several planes were also identical which triggered fast epitaxial connection to form their heterostructures. An atomic model showing the interface of SnS and Sb2S3 with identical d-spacing (0.19 nm) of (200) and (020) planes respectively is presented in Figure 2f. The exfoliations of Sb2S3 nanotubes were monitored with progress of time at fixed reaction temperature and hence, thermal energy might not be the exclusive factor. Heating SnS decorated Sb2S3 nanotubes at 225 oC dismantled the nanostructures and SnS were separated from the nanotube (Figure S8). This suggests increase of reaction temperature hindered the exfoliations. Analysis suggested that the key factor that triggered the exfoliations was the controlled redox process of Sn during annealing at high temperature (210 oC). Presence of both Sn(II) and Sn(IV) in intermediate samples (Figure 2b) indicated that although only Sn(IV) salts were initially taken 8 ACS Paragon Plus Environment

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some part of it transformed into Sn(II) in the reducing environment of amine and thiols. As the decorated part was SnS, Sn(IV) is assumed to be doped into Sb2S3 forming doped nanotubes because the size of Sn(IV) was compatible for substitution doping (the effective ionic radii of Sn(II), Sn(IV) and Sb(III) are respectively, 1.12, 0.71 and 0.62 Å).36 These substituted Sn(IV) had the little probability for thermal induced movement to interstitial positions as there was no additional heat was supplied. Hence, the most probable case here would be diffusion of Sn(II) ions from the decorated SnS into the interstitial layers of Sb2S3. This possibility would also enhance the inter-layer distances and trigger the exfoliation as observed in the case of previously reported Sn doped Sb2S3 nanowires. For that case, when Sn placed interstitially increment of the inter-layer (Sb-S) distances from normal ~3.14Å to a maximum up to ~ 4.63Å along [100] directions was reported, but placing Sn at substitutional position did not undergo enhancement in interlayer distance.37 Being a slow process, the annealing time continued for 90 minutes for complete exfoliations. The proposed mechanism of diffusion of Sn from SnS to interstitial positions which on exfoliation ejected in bulk solution is also supported by the diminishing Sn contents, ~17% in decorated tube to ~6% in 30 min annealed sample (Figure S9) to ~1.7% in final sample (Figure S5). Hence, it can be concluded that for diffusion induced exfoliations, reactions were controlled both thermally as well as kinetically as heating and annealing were simultaneously required. On the other hand, continuous heating to higher temperature led to fragmented structures. Accordingly, the reaction paths were schematically presented in Figure 3a which provided the control of the reaction in obtaining the Sn diffusion and shape evolutions as well as the exfoliations. As tubes were decorated with SnS, there was only possibility for the sheets to exfoliate axially which had fewer obstacles and this was also noticed from the 10 min sample (Figure 1b). 9 ACS Paragon Plus Environment


Nanotubes are nothing but rolled nanosheets with larger inter-layer separation distances and hence the proposed exfoliations were like axial unfolding of sheets. The exfoliation direction is shown in the atomic model in Figure 3b and a schematic model of tube to layer exfoliation after doping is presented in Figure 3c.

Figure 3. (a) Schematic presentation of thermal and thermal-kinetically controlled processes for the formation of different heterostructures. Increase of reaction temperature typically dismantles the heterostructures; but annealing at a certain temperature led to different exfoliated products. (b) Atomic model showing SnS attachment and direction of exfoliation. (c) Schematic presentation of dopant induced exfoliation in Sb2S3 nanotubes. As the entire process involved, Sb2S3 nanotubes, SnS-Sn(IV) doped Sb2S3 partially exfoliated heterostructure and the final Sn (IV) doped Sb2S3 exfoliated sheet like structures, all of these are further explored to study the Na+ ion storage in battery applications. For the electrochemical measurements, these synthesized nanostructures were mixed with SuperP-65 as conducting agent and PVDF, and drop casted on Cu-foil through blade casting method. Then, CR2032 type coin cells were assembled using these casted nanomaterials in an Ar-filled glove box with H2O and O2 concentration level of ∼ 0.5 ppm. Using the prepared coin 10 ACS Paragon Plus Environment

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cell as the working electrode (active material mass loading was ∼1.5-2.0 mg.cm-2) and sodium metal as the counter and/or reference electrode, electrochemical measurements were carried out. Details of the measurements provided in experimental section of supporting information.

Figure 4. (a) Cyclic voltammograms for the first three cycles of the SnS-Sn:Sb2S3 electrode at a scanning rate of 0.1 mV.s-1.(b) Galvanostatic charge-discharge profiles for selected cycles of the SnS-Sn:Sb2S3 electrode at a current density of 100mA.g-1. (c) Comparisons of cycling performances of SnS-Sn:Sb2S3, Sb2S3, and SnS electrodes. Figure 4a shows the cyclic voltammograms of the SnS-Sn:Sb2S3 electrode performed in a voltage window of 0.01–2.0 V vs. Na/Na+ at a Scanning rate of 0.1 mVS-1. The cyclic voltammograms of SnS and Sb2S3 measured under similar conditions are also provided in Figure S10. During the cathodic sweep, the peaks observed at 1.21, 0.81, 0.67, 0.42, and 0.25 were associated with the sodiation process in SnS-Sn:Sb2S3. According to literature reports, the peaks at 1.21, 0.81, 0.42 and 0.25 V were observed due to the conversion reaction in Sb2S3 (Sb2S3 to Sb and Na2S), and alloying reaction of Sb with Na to form Na3Sb.29,38-40 Similarly, the peaks at 0.67 V is associated with the conversion reaction of SnS (SnS to Sn and Na2S) as well as the alloying reaction Sn with Na.40-43 The alloying reaction between Na and Sn is known to be a multistep reaction.40-41,43-44 In the first step (at 0.67 V), Sn undergo alloying reaction with Na to form 11 ACS Paragon Plus Environment


NaSn5 and then at 0.26 V (which is overlapped with broad hump at 0.25V), more Na was incorporated to produce Na9Sn4.41,44 During the anodic sweep, the peaks observed at 0.22, 0.73, 1.0 and 1.29 V were corresponding to desodiation process in SnS-Sn:Sb2S3. According to the literature reports, the peaks at 0.73 V and 1.29 V were observed for the dealloying reaction from Na3Sb and its conversion to Sb2S3 respectively. On the otherhand, the peaks at 0.22 V and 0.68V (which is merged with intensed peak appears at 0.73 V) were due to the two step dealloying reaction from NaSnx, whereas the peak at 1.0 V was due to the formation of SnSx.41,45 Hence, the electrochemical processes in these heterostructures were observed as expected in such materials. Further, to understand the morphology and composition of the post electrochemical sample of the SnS-Sn:Sb2S3 heterostructure, SEM images of before and after 50th cycle, HRTEM and EDS during sodiation desodiation processes were performed and provided in Figure S11. The results show some morphological changes after Na incorporation; but certainly more characterization and particularly the insitu microscopic observation are required for establishing the exact path of transformation. The electrochemical performance of the electrodes was also evaluated by galvanostatic charge– discharge measurements in Figure 4b which shows galvanostatic charge–discharge profiles of SnS-Sn:Sb2S3 electrode that were carried out at constant current density of 100 mA.g-1 within the potential range of 3.0 to 0.1 V vs. Na/Na+. Initial discharge capacity obtained was 1100 mA h g-1 while a reversible capacity of 500 mA h g-1 was achieved after 50 charge–discharge cycles. The individual galvanostatic charge–discharge profiles of SnS and Sb2S3 electrode were also provided in Figure S12. The cycling performance of SnS-Sn:Sb2S3 heterostructures is also investigated under the same conditions and compared with parent Sb2S3 and SnS, shown in Figure 4c. It was found that SnS-Sn:Sb2S3 heterostructures have better cycling performance compared to their 12 ACS Paragon Plus Environment

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parent materials. The rate capability of SnS-Sn:Sb2S3 heterostructures at different current rates were also performed and compared with parent Sb2S3, provided in Figure S13. The SnSSn:Sb2S3 heterostructure exhibited the specific capacities of 502.8, 437.2, 380, 340.4, 290.2, 267.0 mA h g-1 at the current densities of 100, 200, 300, 350, 400, 450 mA g-1, respectively which was found to show the more efficient rate capability than the parent Sb2S3. The electrochemical performances of the SnS-Sn:Sb2S3 heterostructure also observed comparable to leading reports on Sb2S3 and SnS materials (shown in Table S1).26-27 Further, for understanding these differences in performances, electrochemical impedance spectroscopy (EIS) experiments were carried out for both heterostructure and the parent materials. The two zones in the obtained EIS spectra (Figure S14) reflected the charge-transfer resistance and diffusion of sodium ion throughout the electrolyte. The corresponding equivalent circuit was shown in the inset of Figure S14. From the fitted data, the charge transfer resistances (Table S2) were found to be 83.73 Ω, 205.36 Ω and 194.2 Ω for SnS-Sn: Sb2S3, SnS and Sb2S3 respectively. The lower value reflected the kinetics improvement of the hetero-junction in the SnS-Sn:Sb2S3. These results supported the faster diffusion of Na+ ions into the active material and hence, resulted better ion transport in SnS-Sn:Sb2S3 electrode than SnS or Sb2S3. In addition, exfoliation also plays a key role here for variation of cyclic performances. The comparison cyclic performances of different exfoliated samples were provided in Figure S15. The final sheet like structures also showed slightly higher cyclic performance compared to parent nanotube; but lower than the intermediate heterostructures. This might be due to generation of additional sites for Na+ ion insertion and surface achieved through exfoliation process.46 However, from these data, it is certainly clear that the dual layered heterostructured materials here showed superior cyclic performance and better for Na-ion storage capacity. 13 ACS Paragon Plus Environment


In conclusion, doped heterostructures of two layered crystal structured materials were successfully obtained in solution. The tubular structure of parent Sb2S3 were first decorated by SnS and the Sn(IV) ions were doped inside the lattice. During annealing, slowly the Sn(II) ions from SnS diffused inside and helped the exfoliation of Sn(IV) doped Sb2S3. Thermal diffusion of Sn(II), doping of Sn(IV) and axial layer exfoliation by Sn(II) intercalation leading to different complex structures having two different layered structured semiconductor materials are the new physical insights discussed here. Further, these nanostructures were explored for observing the storage capacity of Na-ion and the intermediate SnS-Sn:Sb2S3 heterostructures obtained during this process showed superior cyclic performance compared to its parent materials. Being these are new materials, we believe this would open up the new path for designing more materials for efficient battery applications.

■ ACKNOWLEDGMENTS DST of India (EMR/2016/001795) and Technical Research Center at IACS are acknowledged for funding. S.B. acknowledges CSIR for fellowship

■ SUPPORTING INFORMATION Supporting figures with additional TEM images, EDS, XRD, cyclic voltammograms, galvanostatic charge–discharge profiles, cycling performance, rate capability, electrochemical impedance spectra and tables supporting the results are provided in the Supporting Information.


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