Transforming Layered to Nonlayered Two-Dimensional Materials

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Letter

Transforming Layered to Non-layered Two-dimensional Materials: Cation Exchange of SnS to CuSnS 2

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Yuanxing Wang, Yurii V. Morozov, Maksym Zhukovskyi, Rusha Chatterjee, Sergiu Draguta, Pornthip Tongying, Barry Bryant, Sergei Rouvimov, and Masaru Kuno ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00117 • Publication Date (Web): 27 May 2016 Downloaded from http://pubs.acs.org on May 27, 2016

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Transforming Layered to Non-Layered Two-Dimensional Materials: Cation Exchange of SnS2 to Cu2SnS3 Yuanxing Wang,† Yurii V. Morozov,† Maksym Zhukovskyi,† Rusha Chatterjee,† Sergiu Draguta,† Pornthip Tongying,† Barry Bryant,† Sergei Rouvimov,‡ and Masaru Kuno*,† †

University of Notre Dame, Department of Chemistry and Biochemistry, 251 Nieuwland

Science Hall, Notre Dame, IN, 46556, USA. ‡

University of Notre Dame, Department of Electrical Engineering and Notre Dame Integrated

Imaging Facility, 233 Stinson-Remick Hall, Notre Dame, IN, 46556, USA.

Corresponding Author *[email protected]

ACS Paragon Plus Environment

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Abstract We demonstrate the chemical transformation of layered, two-dimensional (2D) SnS2 to nonlayered Cu2SnS3 via cation exchange. Resulting Cu2SnS3 nanosheets (NSs) retain the overall starting morphology of their parent, few-layer SnS2 templates.

Specifically, they possess

micron-sized dimensions and have controlled thicknesses dictated by the number of initial SnS2 layers. Our demonstration shows that existing layered compounds can serve as templates for difficult-to-synthesize non-layered 2D specimens with cation exchange providing a bridge between families of layered and non-layered materials.

New 2D systems are therefore

accessible, opening the door to future explorations of low dimensional nanostructure anisotropic optical and electrical properties.

TOC GRAPHICS


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Since the isolation of graphene, 1 two-dimensional (2D) nanomaterials have attracted considerable attention within the context of fundamental and applied research.2 On the applied side, large surface areas and strong one-dimensional carrier confinement make 2D specimens useful for electronic, optoelectronic, and energy-harvesting devices.3,4,5 This has led to growing interest in developing robust approaches for their production. 6 Existing strategies include mechanical exfoliation,7 chemical vapor deposition,7 and liquid-phase exfoliation.2 Despite their proven utility, these approaches are restricted to the production of single and few-layered specimens of materials having weak interlayer van der Waals interactions. In tandem, the development of direct solution chemistries for 2D materials is often challenging due to the lack of an intrinsic driving force for promoting anisotropic growth.8 While surface ligand chemistries have been exploited to restrict growth in certain crystallographic directions,9,10 these approaches are generally system-specific and lack universal applicability. Additionally, successes in the realm of 2D colloidal growth have largely consisted of producing nanoplatelets (NPLs) with lateral dimensions ranging from 20-700 nm.9,10,11,12,13,14,15,16 Few examples of micron-sized nanosheets (NSs) exist.17,18 This makes the production of large area 2D specimens one of the more outstanding issues in the bottom up, chemical growth of 2D materials.

Consequently, a need exists for developing alternative means of producing 2D

nanomaterials possessing large lateral dimensions. Cation exchange19 represents one such approach. It involves substituting cations of a parent crystalline nanostructure with different metal ions. 19 , 20 Under appropriate conditions, the exchange is fast and results in new materials which conserve the anion sublattice21 along with the overall size and shape of the parent nanostructure.21

Successful demonstrations of cation

exchange have included the synthesis of Ag2Se and Cu2-xSe nanoparticles,19,22 as well as PbS and


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ZnS nanorods.23,24 For 2D materials, cation exchange has proven effective in producing ultrathin ZnS,25 PbS,25 as well as thickness-controlled Cu2-xSe NPLs26 with lateral dimensions between 50 – 200 nm. These latter successes suggest that cation exchange may offer an approach towards producing large area 2D specimens as well as new, previously inaccessible materials. One constraint for applying cation exchange to 2D systems is the availability of suitable starting materials. In this regard, layered metal chalcogenides are promising candidates. Their layered morphology represents a natural 2D template with strong lateral bonding coupled to weak, interlayer van der Waal interactions facilitating the formation of large area 2D sheets.3 Cation exchange on these materials should therefore yield correspondingly large area NSs and may, more broadly, represent a general strategy for synthesizing 2D materials.

To our

knowledge, the cation exchange of 2D layered metal chalcogenides has never been carefully investigated.27,28 In this study, we demonstrate the successful conversion of tin disulfide (SnS2), a layered metal dichalcogenide, 29 into non-layered Cu2SnS3 NSs via cation exchange.

Cu2SnS3 is a

potentially useful absorber material for thin film solar cells, 30 given its non-toxic, abundant elements as well as large absorption coefficients.30 Of note in the experiment is that the size and shape of the parent, few-layer SnS2 is preserved in resulting Cu2SnS3 NSs. In effect, what has been achieved is the conversion of a layered compound into a non-layered 2D specimen. Resulting Cu2SnS3 sheets possess large, micron-scale lateral dimensions as well as controlled nanometer thicknesses. Although the fabrication of Cu2SnS3 nanoparticles31,32 and films30,33 has previously been reported, there have been no reports on the synthesis of their 2D analogues. The NSs in this study thus represent the first large area Cu2SnS3 NSs made to date. The study


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additionally establishes a precedent for using cation exchange to produce heretofore unavailable, compositionally-complex, 2D materials using existing layered specimens. Starting materials for the cation exchange reaction were obtained by mechanically exfoliating bulk SnS2. Figure 1a shows a representative optical micrograph of resulting SnS2 layers on a Si wafer with a 285 nm SiO2 capping layer. Large area specimens with lateral dimensions ranging from 2 – 20 µm were obtained. To establish exfoliated SnS2 thicknesses, confocal Raman spectroscopy was employed. Figure 1b shows the distinct A1g Raman resonance of few-layer SnS2 at 315 cm-1. 34,35 The intensity of this feature, relative to the Si zone-center optical phonon at 521 cm-1, scales directly with layer thickness (Figure 1b).35 This property has enabled us to construct a calibration curve (Figure S1), in conjunction with independent atomic force microscopy (AFM) measurements [Figures S (2,3)], which establishes SnS2 layer thicknesses using direct results of Raman imaging. Additional information about these measurements can be found in the Supporting Information (SI).


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Figure 1. (a) Representative optical micrograph of exfoliated few-layer SnS2 on a SiO2/Si wafer. (b) Raman spectra of bilayer to multilayer SnS2 sheets, normalized to the intensity of the 521 cm-1 Si resonance. Spectra offset for clarity. (c) Representative low-magnification TEM image of few-layer SnS2 on a SiN TEM grid obtained by wedging transfer of the area shown in (a). (d) Representative high-magnification TEM image of wedging transferred few-layer SnS2. Inset: Associated SAED pattern obtained along the [001] zone axis.

Exfoliated few-layer SnS2 specimens were subsequently transferred onto silicon nitride (SiN) transmission electron microscopy (TEM) grids using wedging transfer.36 A detailed description of the process can be found in the SI. A comparison between the optical micrograph in Figure 1a and the representative low-magnification TEM image of the corresponding wedging transferred specimen in Figure 1c, demonstrates that the process preserves the size and shape of the originally exfoliated SnS2 sheets. Clear lattice fringes in post-transfer, high-magnification TEM images (Figure 1d) reveal that the transferred sheets retain their high degree of crystallinity. The associated [001] zone axis selected area electron diffraction (SAED) pattern (inset, Figure 1d) reveals (100) and (110) d-spacings of 3.17 Å and 1.81 Å respectively. These values are consistent with d-spacings reported for bulk SnS2 [(100): 3.17 Å and (110): 1.81 Å,


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JCPDS card No. 23-0677]. Figure S4 of the SI provides additional information about few-layer SnS2. Cation exchange of SnS2 with Cu(I) was carried out as described in the SI. Figures 2 (a-c) show low-magnification TEM images of resulting, micrometer-scale Cu2SnS3 NSs from the cation exchange of 11-, 7- and 5-layer SnS2. Figures S5 of the SI shows TEM images of corresponding SnS2 specimens. As expected, the exchange largely preserves the 2D morphology of the parent material. Observed wrinkles likely arise from lattice expansion during exchange given different SnS2 and Cu2SnS3 lattice parameters (SnS2: ܽௌ௡ௌమ = 3.65 Å, ܿௌ௡ௌమ =5.90 Å, JCPDS card no. 23-0677; Cu2SnS3: ܽ஼௨మ ௌ௡ௌయ = 3.90 Å, ܿ஼௨మ ௌ௡ௌయ = 17.27 Å32).

AFM

measurements to establish thickness differences between parent few-layer SnS2 and resulting Cu2SnS3 NSs were performed and clearly reveal that Cu2SnS3 thicknesses are defined by the number of starting SnS2 layers. This is discussed in greater detail below.


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Figure 2. (a-c) Low- and (d-f) high-magnification TEM images of Cu2SnS3 NSs resulting from the cation exchange of 11-, 7- and 5-layer SnS2. Dashed blue lines in low-magnification images indicate areas of interest for successive high-magnification and electron diffraction measurements. Insets in d-f show associated SAED patterns from the outlined regions in (a-c).

Figures 2 (d-f) show clear lattice fringes in resulting Cu2SnS3 NSs, which indicate their crystallinity. Reflections in associated SAED patterns [insets, Figures 2 (d-f)] can additionally be indexed to hexagonal, non-layered Cu2SnS3. This is further corroborated by extracted (101) and (110) d-spacings of 3.37 ± 0.01 Å and 1.94 ± 0.01 Å. The experimental values again agree with those reported in the literature [(101) d-spacing: 3.314 Å; (110) d-spacing: 1.948 Å].32 Beyond the lattice-resolved TEMs, the partial rings and smaller side features next to the primary spots in acquired SAED patterns indicate polycrystallinity of the resulting Cu2SnS3 NSs. Corresponding TEM-based energy-dispersive X-ray spectroscopy (EDXS) measurements (Figure

S6)

yield

atomic

ratios

of

Cu2.58±0.33SnS2.55±0.14,

Cu3.45±0.65SnS2.52±0.44

and

Cu3.95±0.38SnS2.68±0.37 for 11-, 7- and 5-layer cation-exchanged products. Observed Sn/S ratios differ from the 1:2 stoichiometry of few-layer SnS2, suggesting substitutional incorporation of Cu(I) into the lattice. At the same time, observed Cu/Sn atomic ratios are higher than that expected for Cu2SnS3 (2:1). This likely arises due to the presence of residual copper on the surface of Cu2SnS3 NSs, stemming from the solution phase disproportionation of Cu(I) to Cu(0). In support of this, Figure S7 shows TEM images of Cu2SnS3 NSs, decorated with surfacedeposited Cu(0) nanoparticles.37 Such samples result when exchange reactions are not carried out under the strictest air and moisture free conditions. Figure 3a schematically depicts the exchange reaction.

Given that Cu2S exists as a

monoclinic crystal at the reaction temperature, 38 significant reorganization of the anion framework from hexagonal to monoclinic prevents complete conversion of SnS2 to Cu2S.20,39 The greater resemblance of hexagonal Cu2SnS3’s anion framework to initial SnS2 templates thus


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makes it the favored product of cation exchange. Additionally, the smaller solubility of SnS2 relative to Cu2S (SnS2 pKsp = 70.8; Cu2S pKsp = 52.4),40 as well as the restricted diffusion of Sn4+ due to its higher charge density,41 hinders complete exchange. This is borne out by experiment where above TEM-based EDXS measurements show that the stoichiometry of the cationexchanged products is effectively that for Cu2SnS3, not Cu2S.

Figure 3. (a) Schematic illustration for the transformation of few-layer SnS2 into Cu2SnS3 NSs. The unit cells of SnS2 and Cu2SnS3 are also depicted. (b) Top view and (c) side view of the (001) faces of SnS2 and Cu2SnS3.

Figure 3 highlights this by showing top views of the (001) basal planes of SnS2 and Cu2SnS3 (Figure 3b). A comparison between the two crystal faces shows a small ~7% in-plane distortion of the S2- anion sub-lattice (SnS2: ܽௌ௡ௌమ = 3.65 Å and Cu2SnS3: ܽ஼௨మ ௌ௡ௌయ = 3.90 Å32) upon exchange. This distortion is small and hence does not strongly influence the efficacy of cation exchange.20 Anion displacements along the z-direction, however (Figure 3c), are much larger with an average S-S displacement of ~14%.

This likely impacts the exchange efficiency,


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limiting it but not preventing it. Hence, reactions terminate at Cu2SnS3 rather than Cu2S under the mild reaction conditions employed here. As suggested earlier, the large lattice distortion is also likely responsible for the wrinkling seen in earlier TEM images (Figure 2). Beyond this, Figure 3 suggests that three layers of SnS2 are converted into one monolayer of Cu2SnS3. Namely, the Cu2SnS3 unit cell c lattice parameter (ܿ஼௨మ ௌ௡ௌయ = 17.27 Å32) is nearly three times that of SnS2 (ܿௌ௡ௌమ =5.90 Å) (Figure 3a).

This suggests that a simple linear

relationship exists between the number of initial SnS2 layers (n) and the thickness (t) of resulting ௡

Cu2SnS3 NSs, (i.e. ‫ ݐ‬ൌ ܿ஼௨మ ௌ௡ௌయ ). ଷ

Figure 4. Experimental AFM-derived heights of Cu2SnS3 NSs (blue circles) plotted atop predicted thicknesses given the number of initial SnS2 layers (dashed black line).

Figure 4 compares observed Cu2SnS3 NS thicknesses from the cation exchange of few layer SnS2 (blue circles, 22-, 13-, 10-, and 6-layer SnS2) with that expected from the above linear relationship (dashed black line). Figures S (8-11) show AFM images of the exchanged 22-, 13-, 10-, and 6-layer SnS2 specimens. The comparison reveals good agreement with expected values from the structure conserving cation exchange depicted in Figure 3. Slight ~15% deviations


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between observed/predicted Cu2SnS3 thicknesses likely arise from the presence of expansionrelated wrinkles in resulting NSs, which affect area-averaged AFM height measurements. All of our data at this point suggest the successful cation exchange of few-layer SnS2 into non-layered Cu2SnS3 NSs. To be definitive, though, Cu(0) intercalation must be excluded.27 Towards establishing this, Figure 5a shows the Raman spectrum of a representative 14-layer exchanged product. The data shows observed resonances at 287 and 335 cm-1, which are known Cu2SnS3 Raman features42 and which are distinct from that of SnS2 (315 cm-1) (inset, Figure 5a).

Figure 5. (a) Raman spectrum of Cu2SnS3 NSs obtained from the cation exchange of 14-layer SnS2 (Dashed lines indicate fit results) Inset: Raman spectrum of initial 14-layer SnS2. (b) EELS spectrum of the corresponding Cu2SnS3 NS.

Additional electron energy loss spectroscopy (EELS) measurements of the 14-layer exchanged product show that cation exchange results in Cu incorporation as Cu(I) -not Cu(0) as expected for intercalation.27,37 Specifically, Figure 5b shows a distinct Cu-L3 edge along with a slightly smaller Cu-L2 edge. Both originate from 2p-d electron transitions in Cu(I).43 These sharp Cu-L2,3 edges distinguish themselves from the significantly broader Cu-L2,3 edges typically observed for Cu(0).37 All of our measurements (i.e. TEM, SAED, EDXS, Raman, AFM, and EELS) thus allow us to conclude that substitutional incorporation of Cu(I) occurs into parent SnS2 templates during cation exchange.


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To compare the optical properties of initial SnS2 and cation-exchanged Cu2SnS3 NSs, pristine SnS2 layers were first wedge transferred from SiO2/Si wafers onto microscope coverslips. Representative absorption spectra of individual few-layer SnS2 specimens are shown in Figure 6a. SnS2 is an indirect-gap material and possesses a reported bandgap of ~2.3 eV.34,35 This appears to be borne out in the data where a clear rise of the absorption is seen at ~2.34 eV (Figure S12a).

Corresponding absorption coefficients (α) have been calculated from

experimental transmittance measurements, using ‫ ܫ‬ൌ ‫ܫ‬଴ ݁ ିఈ௫ with I (I0) the transmitted (incident) light intensity and x the specimen thickness. Following this, SnS2 specimens were cation exchanged. Absorption spectra of corresponding Cu2SnS3 NSs are shown in Figure 6b. Notable changes seen include an apparent absorption edge at ~1.55 eV (Figure S12b), which suggests a change in bandgap. In this regard, the literature suggests that Cu2SnS3 has a direct optical gap between 0.9-1.8 eV.42,44 Additionally supporting this, the magnitude of α changes dramatically following exchange, increasing by nearly an order of magnitude (α ~105 – 106 cm-1). Apart for corroborating successful cation exchange, these large α-values further suggest that Cu2SnS3 NSs may represent promising absorber materials for thin film solar cells44, given that their sizable absorption originates from just a few monolayers of material.


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Figure 6. (a) Absorption spectra of individual few-layer SnS2. (b) Absorption spectra of corresponding Cu2SnS3 NSs. Values obtained using an average of 11- and 4-layer SnS2 specimens before and after cation exchange. Dashed blue lines are guides to the eye.

In summary, we have demonstrated the successful use of cation exchange to produce nonlayered 2D materials beginning with corresponding layered templates. Specifically, few-layer SnS2 specimens have been converted to Cu2SnS3 NSs while maintaining their initial 2D morphology and crystallinity. Notable outcomes of the work include (a) the first microscale (i.e. lateral dimensions ~5 µm) Cu2SnS3 NSs made to date, (b) the production of 2D NSs with complex compositions, (c) the production of NSs with controlled thicknesses via control over the number of starting SnS2 layers, and (d) the first in-depth study of a layered to non-layered conversion of 2D specimens via cation exchange.

This work thus contributes to existing

approaches for producing 2D materials and illustrates a general method for creating both large area and compositionally-complex 2D systems, which would be difficult to obtain otherwise.

ASSOCIATED CONTENT Supporting Information. Experimental methods, Raman spectra of few-layer SnS2, AFM images of few-layer SnS2, EDXS spectrum of few-layer SnS2, TEM images of few-layer SnS2,


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EDXS spectra of Cu2SnS3 NSs, TEM images and EDXS spectrum of Cu2SnS3 NSs decorated with copper nanoparticles, AFM images of Cu2SnS3 NSs, Tauc plot of few-layer SnS2 and Cu2SnS3 NSs, and optical images of few-layer SnS2.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ACKNOWLEDGMENT This research has been supported by the Army Research Office (W911NF-12-1-0578) and the Center for Sustainable Energy at Notre Dame (cSEND). M. Z. thanks cSEND for partial financial support via the ND Energy Postdoctoral Fellowship Program. We additionally thank the Notre Dame Integrated Imaging Facility (NDIIF) as well as the Notre Dame Radiation Laboratory/Department of Energy (DOE), Office of Basic Energy Sciences for use of their facilities and equipment. We thank Norcada for providing us discounted SiN TEM grids.

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Transforming Layered to Nonlayered Two-Dimensional Materials

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