Folded Nanosheets: A New Mechanism for Nanodisk Formation

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Folded nanosheets: A new mechanism for nanodisk formation Suresh Sarkar, Alice D. P. Leach, and Janet E. Macdonald Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01279 • Publication Date (Web): 28 May 2016 Downloaded from http://pubs.acs.org on June 2, 2016

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Chemistry of Materials

Folded nanosheets: A new mechanism for nanodisk formation** Suresh Sarkar,1 Alice D. P. Leach,1 and Janet E. Macdonald.1, 2*

1

Department of Chemistry, Vanderbilt University, Nashville, TN, 37235, USA.

2

Vanderbilt Institute of Nanoscale Science and Engineering, Vanderbilt University, Nashville,

TN, 37235, USA. Corresponding Author *Email: [email protected]

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ABSTRACT: Two dimensional nanostructures have generated significant interest in the scientific community due to their high surface area and remarkable optoelectronic properties. In this work, we present a simple colloidal synthesis for ultrathin nanosheets of both In2S3 and CuInS2, and discussed the detail of the crystal growth as well its transformation into 2-D nanodisks with due course of annealing. Contradictory phase designation reports exist as to the structure of In2S3 at the nanoscale, due to overlapping reflections of multiple phases in X-ray diffraction. We use high resolution transmission electron microscopy to demonstrate definitively that hexagonal γ-In2S3 is formed. Treatment of the In2S3 nanosheet with Cu+ accelerates this process of tearing of nanosheet of In2S3 and results in the formation of 1D nanoribbons of CuInS2 at a particular temperature with proper combination of solvent polarity. High-resolution TEM images and medium-angle x-ray scattering measurements indicate that these nanoribbons consist of face-to-face stacked nanodisks.


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INTRODUCTION Semiconductor nanocrystals, which exhibit size- and shape-dependent optoelectronic properties, have been the subject of intense study due to their potential applications in photovoltaics, optoelectronics, photo-catalysis and biology.1-7 Anisotropic nanostructures, especially one- or twodimensional (1D /2D) nanomaterials, have garnered considerable interest due to their superior photovoltaic performance.8,9 Furthermore, these nanostructures present the opportunity for orthogonal charge separation on photoexcitation, and have the ability to transport a charge carrier along their length, preventing unwanted carrier loss at material boundaries or interfaces.10,11 Nanoscale In2S3, a semiconductor with band gap 2.1 eV, has been reported with both 1D12,13 and 2D12,14-21 morphologies. Pradhan et al. prepared β-In2S3 nanosheets with variable thickness dependent on the rate of decomposition of a single-source precursor.22 Additionally, O’Brien et al. synthesized nanorods of β-In2S3 with < 1.0 nm diameter.12 The potential of these nanostructures in application has also been demonstrated including use as a photo-catalyst,23 a photodetector,16 in solid state batteries,24 and as a buffer layer in photovoltaics replacing toxic CdS.25 In2S3 nanostructures have also been used as a template for the growth of CuInS2 (CIS).26,27 Among the various Cu-based multinary semiconductors, CIS has been established as one of the most promising candidates28,29 owing to its direct band gap of ~ 1.5 eV, near ideal for harvesting visible light.30 Rogach et al. incorporated Cu+ into In2S3 nanoplates, resulting in CIS with tunable band gap that was used as an efficient counter electrode in dye-sensitized solar cells.26 Bottom-up approaches, in which growth is limited in one or more dimensions, are typically used to prepare nanostructures such as nanowires and nanoparticles.31-33 In contrast, 2D nanostructures can famously be formed via top-down approaches, such as the separation of 3 ACS Paragon Plus Environment


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graphene and MoS2 sheets through mechanical or chemical stress.34,35 Here, we present a curious mechanism to nanostructure growth where ultrathin, 2D nanosheets of In2S3 are synthesized in solution, using a bottom-up approach. In situ, these nanosheets fold and tear in a top-down mechanism to create disks of In2S3 with extended reactions times. Upon the addition of Cu+, the folded In2S3 nanosheets tear to form micrometer long, CIS ribbons of stacked nanodisks.

RESULTS AND DISCUSSIONS

Scheme 1. Schematic representation of the synthetic protocol for the formation of nanosheets and nanodisks of In2S3. 2D nanostructures of In2S3 were synthesized using a colloidal technique (Scheme 1). Briefly, InCl3, thiourea and oleylamine were degassed at 60°C, and then heated to 215°C under N2. Transmission Electron Microscopy (TEM) of an aliquot of the reaction taken at 4 min indicated that thin sheets of In2S3 had formed (Fig 1a, Fig S1). At early stages of the reaction (Fig 1a-c), wrinkling of the sheets was observed. Wrinkling of nanosheets has been reported before and was attributed to van der Waals forces or dipolar interactions between adjacent areas of the sheets due to the extreme thinness of the structures.22,36,37 With continued heating (Fig 1b-d, Fig S2-4), the sheets were observed to separate into hexagonally shaped nanodisks after 90 min (50 ± 10 nm, n = 120, number of particles measured) (Fig 1e, Fig S5). The UV-Vis spectra of aliquots of


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the In2S3 product collected at different time points during annealing indicate no significant change in the band edge (Fig. S6).

Figure 1. (a-e) The shape transformation of 2D nanosheets to nanodisks of In2S3 with annealing of the reaction mixture at 215°C. (f) Powder XRD of In2S3 nanosheets at 4 min with expected reflections for tetragonal β-In2S3 (JCPDS 25-0390) and hexagonal γ-In2S3 (JCPDS 33-0624) shown below. The experimental reflections are assigned to the appropriate Miller indices of the hexagonal phase.



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X-ray diffraction (XRD) measurements were performed on both the In2S3 nanosheets and nanodisks (Fig 1f, Fig S7). The resultant patterns were in good agreement with the patterns reported for other 2D In2S3 nanostructures.12,15-19 Previous work has attributed these reflections to α-In2S3 (a defect cubic structure),12,18 β-In2S3 (a tetragonal defect spinel structure),15,16,26 and γ-In2S3 (a layered hexagonal structure).17,19 These contradictory assignments arise as the most intense experimental reflections overlap with peaks characteristic of all three phases. Additionally, preferential growth of these 2D nanostructures in specific crystallographic directions with high aspect ratio leads to crystallite size broadening and even the disappearance of certain reflections from experimental XRD patterns. Thus predicted diffraction peaks from all three structures are not discernible, making definitive assignment of crystal structure difficult by XRD alone.12,15-19 In order to identify the specific crystalline phase, high-resolution TEM (HRTEM) was used in conjunction with XRD. HR-TEM of the large In2S3 nanosheets, obtained at early stages of the reaction, show the products were polycrystalline (Fig 2a) and do not show long range order. The FFT (Fig 2b) showed rings with d-spacing corresponding to two of the strongest XRD reflections (d = 0.32 Å, 27°; d = 0.19 Å, 48°) of γ-In2S3.

In contrast, the

nanodisk products were highly crystalline (Fig 2c). FFT (Fig 2d) from a HR-TEM image of an In2S3 nanodisk shows two rings of six diffraction spots each. We attribute these two rings to the {110} (d = 0.32 nm), and the {300} planes (d = 0.19 nm) of hexagonal In2S3. These each have the appropriate six-fold symmetry and 30° relationship between them when the viewing axis is the [001] of the hexagonal crystal structure. The alternative d-spacings from the tetragonal and cubic structures do not have an appropriate viewing axes that could give the 60° and 30° angles observed using FFT. Therefore, the In2S3 product is conclusively in the γ-, hexagonal phase.


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The experimental XRD patterns, for both sheet- and disk-like In2S3 products, have sharp peaks at 27.2° and 48.0° (Fig 1f, Fig S7) which are, therefore, the long crystallite dimensions. These reflections correspond to the (110) and (300) planes of the hexagonal crystal structure and match the results from HR-TEM. This indicates that nanostructures are preferentially grown in the crystallographic a- and b- directions and truncated in the c- direction.

Figure 2. (a) An HR-TEM image of an In2S3 nanosheet in a flat area; (b) FFT of the selected area in (a); (c) an HRTEM image of an In2S3 nanodisk (d) FFT of the selected area in (c); (e)



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Simulated HR-TEM for the selected area in (c); (f) The atomic model correlating the observed dspacing to atomic planes for the simulated HR-TEM in (e).

To our knowledge, the morphological transformation of nanosheet to nanodisk has not been observed before. Shen et al., observed small MgO sheets adhering to form large disks. 38

Additionally, Dubertret et al. observed the growth of CdSe nanodisks into nanosheets through

continuous external addition of molecular precursors.39 However, in this case, we observe the inverse, where nanosheets break into smaller nanodisks under prolonged heating. It appears that wrinkling of the nanosheet is an important step in the mechanism and so we chose to more carefully examine the wrinkled structures. HR-TEM of a folded region of the nanosheets (Fig 3a-b, Fig S8) shows that the thickness of each wrinkle is ~ 0.82 ± 0.05 nm (n = 120, number of particles measured), which matches the thickness of nanodisk products (Fig 3c, Fig S9). The c-axis length of the unit cell for γ-In2S3 is 1.75 nm, thus the wrinkles and disk products are ~ ½ a unit cell thick, corresponding to three S anion layers along the c-axis (Fig 3d). Accordingly, the thickness of the original sheets, taking half the width of a folded sheet, is a remarkably small ~ 0.40 nm. To our knowledge, this is the lowest reported thickness for an In2S3 sheet. Hexagonal γ-In2S3 has inversion symmetry and thus, upon folding of the nanosheets, adjacent crystallites can “click together” without atomic rearrangement. However, the poorly crystalline nanosheets become more ordered when they transform into the single crystalline disk. The recrystallization step at later stages of the reaction likely causes strain and fracture planes between crystallites that tears the sheets into hexagonal disk shapes, demonstrative of the


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hexagonal symmetry of the lattice. It is likely that initial folding of the nanosheets is caused by their high aspect ratio, which results in colloidal instability. There are several earlier reports of hexagonally shaped thin nanodisks of In2S3.15,16,26 Stacking of these disks has also been reported.15 We repeated one of these syntheses,15 collected aliquots at early time points, and found In2S3 nanosheets as an intermediate (Fig S10). It is likely that other previously reported syntheses for In2S3 nanodisks also go through mechanisms that involve the formation, folding and tearing of nanosheets. Further, In2S3 nanostructures can act as a template for Cu+ incorporation and the formation of CIS.26,27 CIS has attracted considerable interest from the scientific community, owing to its direct band gap,

30,40

high absorption coefficient,40

low toxicity,41 and wide range of

applications.26,41 ,42 Thus we treated the prepared γ-In2S3 nanostructures with Cu+ to prepare CIS.

Figure 3. (a) an HR-TEM image of a folded region of the In2S3 nanosheets. (b) a magnified HRTEM image of a folded region of the nanosheets. (c) a region of an HR-TEM image of the 9 ACS Paragon Plus Environment


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stacked nanodisks, after 60 min of heating, with c-axis perpendicular to the viewing plane (d) a schematic diagram showing the unit cell of hexagonal In2S3.

When the γ-In2S3 nanosheets were treated with CuCl at room temperature, there was an immediate color change from yellow to black. TEM indicated that large, wrinkled nanosheets had formed (Fig 4a, Fig S11). It has been shown previously that Cu+ intercalation can occur at room temperature yielding CIS nanocrystals from In2S3 nanocrystals,26 but this is the first time that nanosheets of CIS have been synthesized using a facile synthetic technique. UV-Vis indicated the formation of a new absorption onset consistent with the band gap of CIS (1.5 eV) (Fig S12). XRD confirmed that the product was CIS with either the zinc blende,43 chalcopyrite (JCPDS #38-0777) with appreciable extent of CuIn11S17 (JCPDS #34-0797) (Fig. S12). Due to the small crystallite size of the product nanostructures, resulting in broadened XRD reflections and no distinguishable lattice fringing in HRTEM, we could not determine the exact phase. However, the observed pattern was in good agreement with results from Rogach et al., who assigned their CIS product to the chalcopyrite phase.26 When CuCl, InCl3 and thiourea were heated together in a single step, small (~2 nm), spherical particles were observed (Fig S13, S14). Therefore, the formation of the ultrathin In2S3 nanosheet intermediate was identified as key in the synthesis of CIS nanosheets. When the γ-In2S3 nanosheets were treated with Cu+ and annealed at 115°C, the sheets were observed to form an aligned series of folds at 7 min (Fig S15), which by 15 min, had separated into 1D striped nanoribbons of CIS (Fig 4b-d, Fig S16). The band gap of these micrometre long CIS nanostructures was estimated to be ~ 1.6 eV from the absorbance spectrum (Fig S17), which implies that at least one crystallite dimension is small enough to cause quantum confinement.


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Photoluminescence was not observed (Fig S17). XRD of the resultant nanostructures confirms the zinc blende or chalcopyrite phase of CIS (Fig S18a). Medium-Angle X-ray Scattering (MAXS) (Fig S18b) and tilting experiments in the TEM (Fig S19), indicate that the 1D nanoribbons consist of vertically aligned, ordered, face to face stacked nanodisks with thicknesses of 2.2 nm. 1D nanostructures of CIS have been reported before, however, only in the hexagonal wurtzite phase, which is anisotropic and promotes asymmetric growth. 44,45 The higher order nanoribbons are hundreds of nanometers to a micrometer in length with width ~ 30 nm. It has been demonstrated that In2S3 nanosheets can be separated into irregular shapes via treatment with Cu+, however, 1D nanostructures and 2D nanosheets have not been formed using this mechanism.14



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Figure 4. TEM and HR-TEM images of CIS nanostructures obtained by the addition of CuCl to nanosheets of γ-In2S3 (a) at room temperature, (b-d) followed by annealing at 115°C, (e) followed by annealing at 180 °C for 15 min, and (f) followed by annealing at 115°C with excess dodecanethiol.

HR-TEM and XRD of the CIS products provide evidence that there is a major rearrangement of atoms in the γ-In2S3 folded sheets upon addition of Cu+. XRD indicates a transformation from γ-In2S3 to chalcopyrite CIS, which requires a change from hexagonal to cubic packing of S2anions. Additionally, the transition requires the In3+ ions to shift from octahedral sites in γ12 ACS Paragon Plus Environment

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In2S346to tetrahedral sites in the chalcopyrite structure.40 While the In2S3 disk products are single crystalline, HR-TEM images of the CIS product show lattice fringing consistent with multiple domains in each disk (Fig 4d). Scherrer line broadening analysis of the sharp, CIS (220)/(204) XRD reflection gives a crystallite size of 13.3 nm, smaller than the width of the nanostructures observed via TEM. The polycrystalline nature of the product disks indicates a disruption of the anion sub-lattice. Separated disks could be obtained by increasing the annealing temperature to 180°C (Fig 4e, Fig S20a,b) or tripling the DDT concentration (Fig 4f, Fig S20c,d). In general, CIS disk formation was complete at more moderate temperatures and times (115°C, 15 min) than in the pure In2S3 (215°C, 90 min). We hypothesize that Cu+ incorporation causes additional stress, promoting the tearing process. A shrinking of the crystal lattice is an unlikely source; In2S3 and CIS have similar atomic densities of 36 and 42 Å3/S2- unit, respectively, which would cause a 16% lattice expansion upon Cu+ addition. We also note that the CIS disks (2.2 nm thick, ~ 30 nm wide) are substantially thicker and wider than the In2S3 disks (0.8 nm thick, ~50 nm wide). A 2.2 nm thickness is equivalent to ~ 8 close packed S layers, over twice as many layers as an In2S3 disk. It is possible that the CIS disks originate from multiple folds of the In2S3 sheets; however, as noted above, the poly-crystallinity of the product and the change in crystal structure symmetry from hexagonal to cubic hint at a major rearrangement of atoms. The change in aspect ratio suggests there is also a migration of some of the In3+ and S2- inwards upon Cu+ incorporation. This migration is likely an additional source of strain that leads to tearing of the folded nanosheets into disks under moderate conditions.



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Figure 5. (a) HAADF image of stacked nanodisks of CIS; elemental mapping of the same region where (b), (c), (d) show elemental distribution for S, Cu and In, respectively.

High-Angle Annular Dark-Field (HAADF) imaging and EDS mapping (Fig 5a-d) show that the sulfur, copper, and indium have a homogeneous distribution across the nanoribbon and in each nanodisk, which rules out the formation of binary-binary or binary-ternary heterostructures. Revisiting the formation of the CIS nanosheets at room temperature, we note that the XRD pattern of the CIS nanosheets had very broad, low intensity peaks indicating poor long range order It is likely that there is insufficient thermal energy available to allow the extensive atomic migration, rearrangement and crystallization necessary to tear the folded CIS nanosheets into nanodisks.


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The formation of columnar stacks of nanocrystals is usually achieved through self-assembly technique. A hydrophobic colloidal dispersion of nanocrystals is destabilized by the addition of non-solvent47,48 or by the slow evaporation of solvent.49 The nanoparticles assemble into super lattices due to the attractive depletion force between them.47,48 The present case, in contrast, is not a self-assembly of separate particles; superstructures are formed directly, and the nanodisks that make up the structure may still be connected. We propose that attractive van der Waals forces between adjacent areas of the ribbon stabilize the disk assemblies after they are synthesized. Experimentally, these assemblies can be disrupted by annealing at elevated temperatures or with excess DDT to give individual disks (Fig 4e,f). It should also be noted that the chalcopyrite (112) facets, are polar as they are terminated with either cations or anions.50 Therefore, polarity driven stabilization of the superstructures must also be considered. The nanoribbons of CIS are an important discovery as they provide a simple method to produce large groupings of aligned nanocrystals. Others have shown that packed and assembled semiconductor nanocrystals can be employed to increase the efficiency of device performance by maximizing connectivity both to the supporting electrode and to each other.51,52 This synthetically facile route could be employed to achieve the same ends as more complex electrophoretic deposition53 or slow self-assembly techniques.49

CONCLUSION Ultrathin sheets of In2S3 have been synthesized. Continued heating of the system yields isolated single crystalline nanodisks. A combination of XRD and HR-TEM analysis indicate that the structures are hexagonal γ-In2S3, and which eradicate the contradictory phase recognition of the materials in the literature reports. The disks are formed by a folding and tearing of the



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nanosheet, which we hypothesize, is caused by strain during a crystal ripening process. By treating the γ-In2S3 nanosheets with a source of Cu+, sheets, disks and 1D stacks of disks of chalcopyrite CIS were produced. Disk formation for CIS occurs under milder reaction conditions than for In2S3 alone. We suggest that a lattice contraction, a change in anion sub-lattice from hexagonal to cubic packing, and an inwards migration of atoms to make thicker disks are additional sources of strain that cause the tearing of sheets into disks under more moderate conditions. 2D systems can possess radically different optoelectronic properties than the bulk. Thus, the development of a facile technique to synthesize nanosheets of both In2S3 and CIS with less than unit cell thickness is significant. Furthermore, micrometer long nanoribbons of CIS in the chalcopyrite phase have been synthesized. Each nanoribbon consists of face-to-face stacked nanodisks. The development of 1D, chalcopyrite CIS nanostructures represents an exciting advance in the study of this low toxicity semiconductor and could provide a template for the synthesis of 1D nanostructures with similar ternary materials.


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METHODS Materials Indium chloride tetrahydrate (InCl3.4H2O 99.99%), thiourea (≥ 99.0%), octadecene (ODE, 90%), diphenyl ether (99%), oleylamine (OlAm, Aldrich, 70%), 1-dodecanethiol (DDT, ≥ 98.0%) were purchased from Sigma Aldrich. Copper (I) chloride (CuCl, 99%) was purchased from Strem Chemicals. All chemicals were used as received without further purification. Synthesis of 2D nanostructures of In2S3 In a round bottom three neck flask, 0.4 mmol (117.29 mg) InCl3.4H2O was added to 4 mL OlAm and 0.4 mmol (30.44 mg) thiourea. The mixture was placed under vacuum for 45 min at 60°C, and then heated at 215°C under N2 for 4-90 min. The resultant solution was yellow and turbid. The product was then cooled to room temperature. Aliquots were collected and purified by repeated centrifugation with acetone and ethanol. The resultant nanostructures were dispersed in hexanes for spectroscopic and microscopic characterization.

Synthesis of nanostructures of CuInS2 In a round bottom three neck flask, 0.4 mmol (117.29 mg) InCl3.4H2O was added to 4 mL OlAm and 0.4 mmol (30.44 mg) thiourea. The mixture was placed under vacuum for 45 min at 60°C, and then heated at 215°C under N2 for 4 min. The reaction mixture was cooled to room temperature. When the temperature dropped below 60°C, 0.5 mL DDT, 0.4 mmol (40 mg) CuCl, 1 mL ODE and 1 mL diphenyl ether were added to the reaction mixture. The solution was placed 17 ACS Paragon Plus Environment


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under vacuum for another 15 min at room temperature (25°C), then heated to the required temperature under N2, and annealed for 15 min. The yellow reaction mixture of preformed In2S3 sheets turned black. Then, the product was cooled to room temperature. Aliquots were collected, and purified by repeated centrifugation with acetone and ethanol. The resultant nanostructures were dispersed in hexanes for spectroscopic and microscopic characterization.

Optical spectroscopy The absorption spectra of the purified samples were collected using a UV-visible spectrophotometer (Jasco V-670). Photoluminescence spectra were measured using a spectrofluorometer (Jasco FP-8300) with excitation wavelength, 348 nm. Transmission electron microscopy (TEM) TEM images were collected and energy dispersive X-ray spectroscopy (EDS) was carried out using a FEI Tecnai Osiris digital 200 kV S/TEM system. TEM samples were prepared by drop casting a dilute solution of the nanostructures dispersed in hexanes onto a carbon coated nickel grid and drying in air at room temperature. Powder X-ray diffraction (XRD) XRD measurements were performed with the purified sample in powder form using a Scintag XGEN-4000 X-ray diffractometer with a CuKα (λ = 0.154 nm) radiation source.


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ASSOCIATED CONTENT Supporting Information Additional TEM and HR-TEM images, XRD, digital pictures of reaction flasks, UV-vis and photoluminescence spectra, MAXS, and the related discussions. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This project was supported by the U.S.-Israel Binational Science Foundation grant number 2012276, the Bergmann Memorial Award, the Vanderbilt Institute of Nanoscale Science and Engineering, National Science Foundation (NSF) Career Award Sus-ChEM-1253105, and NSF EPS-1004083. We thank Jimmy Thostenson of Duke University for MAXS measurements.



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Folded Nanosheets: A New Mechanism for Nanodisk Formation

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