Manos et al. 10.1073/pnas.0711528105. |
Fig. 5. Thermogravimetric analysis (TGA) (N2 atmosphere, heating rate ~10°C/min) and DTG curves for a sample KMS-1 prepared with solid-state reaction and purified with dimethyl-sulfoxide. Two steps of mass loss were observed until 600°C. The first step (25-206°C) is assigned to removal of ~2 H2O molecules.
Fig. 6. Powder x-ray diffraction (PXRD) patterns for a sample of pristine compound KMS-1 (prepared with solid-state reaction and purified with dimethyl-sulfoxide) and its TGA residue (N2 atmosphere, 600°C). (Inset) Enlarged view of the high angle region of these patterns. The similarity between the patterns of the pristine and its TGA residue is obvious.
Fig. 7. PXRD patterns of a sample of KMS-1 freshly prepared and the same sample after its exposure in atmosphere for ~2 months. (Inset) Enlarged view of the (003) and (006) peaks showing the expansion of the interlayer spacing of KMS-1 and the broadening of the (006) peak due to hydration from the atmosphere.
Fig. 8. Mn2p peak in the x-ray photoelectron spectroscopy (XPS) spectrum of the Sr2+-exchanged KMS-1. The continuous and dashed lines represent the overall fitting and the den-convoluted peaks, respectively. The dotted lines represent the fitting of the background.
Fig. 9. The Mn2p peak in the XPS spectrum of KMS-1. The continuous and dashed lines represent the overall fitting and the den-convoluted peaks, respectively. The dotted lines represent the fitting of the background. The assignment of the manganese and tin oxidation states of pristine and strontium-exchanged materials is shown in SI Table 2.
Fig. 10. X-ray powder diffraction patterns for pristine and Ca2+, Sr2+-exchanged materials. The c-axes for Ca2+ and Sr2+ exchanged materials were found to be 35.04 and 35.34 Å, respectively.
Fig. 11. TGA (N2 atmosphere, heating rate ~10°C/min) curves for the alkaline earth (Ca2+, Sr2+)-exchanged materials with assignment of their mass losses until 400°C.
Fig. 12. PXRD pattern of pristine vs. those of exchanged materials isolated at various reaction times (t = 0, 15 min, 2 h, and 24 h). In these reactions, the pristine material was treated with a highly alkaline solution (pH ~14) containing ~2 ppm Sr2+ and 5 M Na+. It can be seen that a shift of the (003) and (006) Bragg peaks to lower 2q values was observed after 15 min of reaction time. This is due to the complete exchange of the K+ ions with the Na+ ones being in extremely large excess in the solution. Sr2+ was also absorbed by the material, but it was in very small concentration compared with the sodium ions, and of course it could replace only a very small fraction of the K+ ions. After 2 h of reaction, the (003) and (006) Bragg peaks of the exchanged material were still apparent, indicating that the layered structure was largely preserved. However, some loss of crystallinity and extra peaks in the PXRD pattern of the material are apparent. After 24 h of reaction, the (003) and (006) Bragg peaks were disappeared, which is consistent with the collapse of the layered structure of KMS-1.
Table 2. X-ray photoelectron spectroscopy data for KMS-1 and Sr2+-exchanged products
Compound | Mn2p, eV (Mn2+/Mn3+ ratio of integrals) | Sn3d5/2, eV | Assignment | Standards from literature (Mn)* | Standards from literature (Sn)* |
Pristine KMS-1 | 640.4, 639.4 | 486 | Mn2+, Sn4+ | MnS (639.9-640.4 eV) | SnS2 (486.2 eV) |
Sr2+-exchanged | 640.1, 641.4 (1/4.2) | 486.2 | Mn2.8+, Sn4+ | MnS/Mn2O3 (641.1-641.3 eV) | SnS2 (486.2 eV) |
*Information for the electron binding energies of all elements can be found at www.lasurface.com/database/liaisonxps.php.
SI Materials and Methods
Synthesis of K2xMnxSn3-xS6·yH2O (x = 0.5-0.95; y = 2-5) (KMS-1). Method A.
Solid-state synthesis: A mixture of Sn (1.9 mmol, 226 mg), Mn (1.1 mmol, 60 mg), K2S (2 mmol, 220 mg), and S (16 mmol, 512 mg) was sealed under vacuum (10-4 Torr) in a silica tube and heated (50°C/h) to 500°C (or 400°C) for 60 h, followed by cooling to room temperature at 50°C/h. The excess flux was removed with H2O or dimethyl-sulfoxide (DMF; Aldrich) to reveal dark-brown polycrystalline material (0.4 g, ~ 80% yield based on Sn). Energy dispersive spectroscopy (EDS) analysis gave the average formula "K1.4MnSn2.5S5.5." More accurate determination of the Mn, K, and S content by inductively coupled plasma (ICP)-atomic emission (AES) analysis is consistent with the formula K2xMnxSn3-xS6 with x being in between 0.5 and 0.95 depending on batch. A small quantity of MnS (green powder, £5-10%), found as impurity in some preparations, can be picked out by hand under microscope. Thermogravimetric analysis (TGA) data revealed the existence of two to five water molecules per formula unit.Method B.
Hydrothermal synthesis (i): K2S (0.40 mmol, 0.044 g), MnCl2 (0.20 mmol, 0.025 g), Sn (0.40 mmol, 0.024 g), and S (0.40 mmol, 0.014 g) were combined and loaded in a 3/8-inch Pyrex tube along with 0.30 ml of water under nitrogen atmosphere in a glovebox. The tube was then evacuated to <3 ´ 10-3 torr, flame-sealed, and kept in an oven at ~220°C for 14 d. The products were isolated in air by filtration and washed with deionized water, ethanol, and ether. Under microscopic observation, the product consisted of dark-red/black hexagonal plate-like crystals plus unidentified white material. The yield for manually separated crystals was ~15-30%. The formula for the product of the hydrothermal reaction determined by single-crystal diffraction measurements was K1.9Mn0.95Sn2.05S6.Method C.
Hydrothermal synthesis (ii): Elemental Sn (60 mmol, 7.140 g), Mn (30 mmol, 1.656 g), S (180 mmol, 5.784 g), K2CO3 (30 mmol, 4.157 g), and water (40 ml) were mixed in a 125-ml Teflon-lined stainless-steel autoclave. The autoclave was sealed and placed in a box furnace with a temperature of 200°C. The autoclave remained undisturbed at this temperature for 4 d. Then, the autoclave was allowed to cool at room temperature. A brown polycrystalline product was isolated by filtration (14.30 g, yield »81%), washed several times with water, acetone, and ether (with this order), and dried under vacuum. The composition of the product was determined with EDS and ICP-AES. TGA data showed the existence of two water molecules per formula unit.Single-Crystal X-Ray Crystallography.
A Siemens SMART Platform CCD diffractometer operating at room temperature and using graphite-monochromatized Mo Ka radiation was used for data collection. The data were collected at 293(2) K over a full sphere of reciprocal space, up to 27-28° in q. Cell refinement and data reduction were carried out with the program SAINT (Siemens Analytical X-Ray Instruments). An empirical absorption correction was done to the data by using SADABS (Siemens Analytical X-Ray Instruments). The intensities were extracted by the program XPREP (G. M. Sheldrick, SHELXTL, Crystallographic Software Package, SHELXTL, Version 5.1, Bruker-AXS). The structure was solved with direct methods by using SHELXS and least-square refinement were done against Fobs2 using routines from SHELXTL software. The K atom positions in the structures of K1.90Mn0.95Sn2.05S6 were modeled as split sites.Powder X-Ray Diffraction (PXRD).
The samples were examined by PXRD for identification purposes and to assess phase purity. Powder patterns were obtained by using a CPS 120 INEL x-ray powder diffractometer with Ni-filtered Cu Ka radiation operating at 40 kV and 20 mA and equipped with a position-sensitive detector. Samples were ground and spread on a glass slide. The purity of phases was confirmed by comparison of the x-ray powder diffraction patterns with ones calculated from single-crystal data using the NIST Visualize 1.0.1.2 software.ICP-AES (Optical Emission) [ICP-AES(OES)/MS] Analyses.
The determination of the content of K, Sn, Zn, and S of compounds KMS-1 was performed on diluted aqua regia (HCl:HNO3 = 3:1) solutions of KMS-1 by ICP-AES, using a VISTA MPX CCD SIMULTANEOUS ICP-OES instrument. Standards of the ions of interest were prepared by diluted commercial (Aldrich or GFS chemicals) ~1,000-ppm ICP-standards of these ions. The calibration was linear with an error of 5%. The samples were also diluted before the measurements so that their concentrations could fall within the range of calibration. The ICP-AES intensity was the result of three (30 sec) exposures. For each sample, three readings of the ICP-AES intensity were recorded and averaged. The standards were reanalyzed after analysis of the samples.The determination of the Sr2+ content of the solutions after the ion exchange processes were also conducted by ICP-AES. Standards in the range 0.010-2 ppm were prepared. The calibration was also linear with an error of 5%. Strontium is characterized by very strong emission and thus solutions with extreme low concentrations (even less than 5 ppb) can be analyzed with ICP-AES.
EDS (Energy Dispersive Spectroscopy) Analyses.
The analyses were performed with a JEOL JSM-6400V scanning electron microscope (SEM) equipped with a Tracor Northern EDS detector. Data acquisition was performed with an accelerating voltage of 25 kV and a 40-sec accumulation time.XPS (X-Ray Photoelectron Spectroscopy) Analysis.
X-ray photoelectron spectroscopy was performed on a Perkin-Elmer Phi 5400 ESCA system equipped with a Mg Ka x-ray source. Samples were analyzed at pressures between 10-9 and 10-8 torr with a pass energy of 29.35 eV and a take-off angle of 45°. All peaks were referred to the signature C1s peak for adventitious carbon at 284.6 eV. Fitting of the peaks has been made by using the software XPSPEAK41. The measurements were performed at the XPS facility of the Chemical Engineering Department at Michigan State University.Thermal Analysis.
TGA was carried out with a Shimadzu TGA 50. Samples (10 ± 0.5 mg) were placed in an aluminum crucible and heated from ambient temperature to 600°C in a 20 ml/min flow of N2 or air. Heating rate of 10°C/min was used and continuous records of sample temperature, sample weight, and its first derivative (DTG) were taken.