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Article Cite This: Inorg. Chem. 2018, 57, 13594−13605

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Synthesis, Crystal Structure, and Liquid Exfoliation of Layered Lanthanide Sulfides KLn2CuS6 (Ln = La, Ce, Pr, Nd, Sm) Tatiana A. Pomelova,† Tatiana Yu. Podlipskaya,† Natalia V. Kuratieva,†,‡ Alexander G. Cherkov,§,‡ Nadezhda A. Nebogatikova,§ Maxim R. Ryzhikov,†,‡ Arthur Huguenot,⊥ Reǵ is Gautier,⊥ and Nikolay G. Naumov*,†,‡ †

Nikolaev Institute of Inorganic Chemistry SB RAS, 3, Akad. Lavrentiev Ave., 630090 Novosibirsk, Russian Federation Novosibirsk State University, 2, Pirogova Str., 630090 Novosibirsk, Russian Federation § Rzhanov Institute of Semiconductor Physics SB RAS, 13, Acad. Lavrentyev Ave., 630090 Novosibirsk, Russian Federation ⊥ Univ Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR−UMR 6226, F-35000, Rennes, France

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‡

S Supporting Information *

ABSTRACT: Among the great amount of known lanthanide nanoparticles, reports devoted to chalcogenide ones are deficient. The properties of such nanoparticles remain almost unknown due to the lack of simple and proper synthetic methods avoiding hydrolysis and allowing preparation of oxygen-free lanthanide nanoparticles. A liquid exfoliation method was used to select the optimum strategy for the preparation of quaternary lanthanide sulfide nanoparticles. Bulk KLn2CuS6 (Ln = La−Sm) materials were obtained via a reactive flux method. The crystal structures of three new members of the KLn2CuS6 series were determined for Pr, Nd, and Sm as well as for known KLa2CuS6. KLn2CuS6 (Ln = La, Pr, Nd) compounds crystallize in the monoclinic C2/c space group, whereas KSm2CuS6 crystallizes in the orthorhombic Fddd space group. The analysis of their electronic structures confirms that the main bonding interactions occur within the anionic {Ln2CuS6}− layers. Due to their layered structure, exfoliation of these compounds is possible using ultrasonic treatment in appropriate solvents with the formation of colloidal solutions. Colloidal particles show a plate-like morphology with a lateral size of 100−200 nm and a thickness of 2−10 nm. Highly negative or positive charges found in isopropanol and acetonitrile dispersions, respectively, are associated with high stability and concentration of the dispersions.

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layers.22,24 Consequently, vast numbers of potential 2D materials can be produced by LPE. Considering all of the advantages resulting from the diversity of 2D materials, the increasing number of 2D materials that can be exfoliated will undoubtedly expand the number of potential applications of such materials. Many chalcogenides composed of both f- and d-elements exhibit interesting structural and physical properties resulting from the presence of the covalent nature of the transitionmetal−chalcogen bonds with the more ionic character of the lanthanide−chalcogen bonds. Most of them resemble channellike or layered structures and display semiconducting25 or semimetal26 properties. A lot of reports concerning preparation of different ternary and quaternary lanthanide chalcogenides are known to date.27−30 Nanochemistry based on sophisticated materials such as quaternary rare-earth chalcogenides is a challenging task. According to the hard and soft acid and base (HSAB) theory,31

INTRODUCTION Recently, two-dimensional (2D) materials have become a hot spot in nanochemistry. A wide range of such materials are known to date, from graphene1 to its “inorganic analogies” such as BN, transition metal chalcogenides and oxides,2−4 black phosphorus,5 silicene,6 and more complex layered double hydroxides.7 Thus, their broad variety is a key to success, as the many different types of 2D materials result in a very broad palette of properties and potential uses such as in sensors,8,9 optoelectronic devices,10,11 catalysis,12 etc. One of the most versatile preparation methods for 2D materials is liquid phase exfoliation (LPE).13 This method is an effective way to produce few and single layered nanoplates by ultrasonic treatment or shearing14 in an appropriate solvent3,15 as well as surfactant solutions16 or polymers.17 This approach is applicable for a variety of layered compounds with van der Waals gap, including graphene and its derivatives,13 BN,18 transition metal chalcogenides,19,20 transition metal carbides and carbonitrides (MXenes),11,21 clays,22 and Zintl phases.23 Recent investigations demonstrated that it was also possible to apply this method to layered compounds with charged © 2018 American Chemical Society

Received: August 5, 2018 Published: October 24, 2018 13594

DOI: 10.1021/acs.inorgchem.8b02213 Inorg. Chem. 2018, 57, 13594−13605

Article

Inorganic Chemistry lanthanide cations Ln3+ are considered to be hard acids, while chalcogenide anions Q2− act as soft bases. It is therefore complex to tune the preparation conditions to prevent hydrolysis. The most explored rare-earth chalcogenide nanoparticles are EuQ (Q = S, Se, Te),32 with divalent europium; meanwhile, for Ln3+, reliable examples of chalcogenide nanoparticles are limited to a few examples of quantum dots of Ln2Q3,33,34 Sm2S3 nanowires,35 polymer-stabilized cerium sulfide nanoparticles,36 NaLnS2 nanocubes,37 LnSMS2 misfit nanotubes,38 LnSe2 nanoplates,39 and several materials obtained as thin films.40,41 Such scarcity (especially in comparison to the plethora of known nanostructured lanthanide oxides, carbonates, etc.) demonstrates the limitations of conventional bottom-up synthetic methods in the case of rare-earth chalcogenides. Since approaches involving self-assembly in water solutions that are widely applied are not efficient enough for lanthanide chalcogenide nanoparticles, a top-down approach where an initially bulk sample is delaminated to nanoparticles appears more appropriate. In order to examine the applicability of LPE to quaternary layered lanthanide sulfides, the KLn2CuS6 (Ln = La, Ce, Pr, Nd, Sm) series was considered. Three new members (Ln = Pr, Nd, Sm) were prepared for the first time using a K2Sx mixture as the reactive flux. It was found that La, Pr, and Nd analogues crystallized in the C2/c monoclinic space group, whereas KSm2CuS6 crystallized in the orthorhombic Fddd space group. The electronic structures of these compounds were studied using density functional theory (DFT) calculations and topological analysis of the density. The successful formation of nanoparticles by ultrasonic treatment in an appropriate solvent was shown for new compounds as well as already known KLa2CuS6 and KCe2CuS6.

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with distilled water and drying in a vacuum. The yield was quantitative. The purity of products was confirmed by PXRD. To grow single crystals suitable for crystal structural determination, a larger excess of S and K2Sx (5−6 mol to 1 mol of Ln2S3) was used. The reaction mixture was prepared as described above, placed into ampules, and sealed. The heating profile was as follows: increased the temperature to 400 °C at a rate of 1 °C/min, held for 120 h, cooled at a rate of 2 °C/h to 125 °C, and then quenched at 50 °C. After cooling, the product was washed in the manner described above. The yellow (1, 3, 4) or orange (5) needles and plate crystals were obtained. The K/Ln/Cu/Q ratio determined by the EDX method (Hitachi Tabletop TM 3000 Scanning. Electron Microscope with Quantax 70 Energy Dispersive X-ray Spectrometer) was close to 1:2:1:6 in all cases. Dispersion Preparation. For a typical preparation procedure of colloidal suspensions to determine the most suitable solvents, 20 mg of KLn2CuS6 and 20 mL of solvent was placed in a glass vial, and after that vial was closed, it was sonicated using a Sapphire ultrasonic bath (ultrasonic power of 150 W, frequency of 35 kHz) for 8 h three times with a gap of 16 h between. To prevent the thermal restacking of colloidal particles, samples were thermostated at 20−22 °C during ultrasonication. The resulting mixture was centrifuged for 10 min at 2000 rpm to settle down large particles, and the upper 4/5 of the supernatant solution was selected for further analysis. For the preparation of films of nanoparticles, 100 mL of solvent and 200 mg of KLn2CuS6 were placed in a conical flask and sonicated as described above, followed by filtration through a “Vladipore” membrane with a 50 nm pore size or Whatman Anodisc membrane with a 20 nm pore size. Characterization Techniques. The PXRD analysis of the samples was performed on a Shimadzu XRD7000 diffractometer (CuKα radiation, Ni filter, 2θ = 10°−70°). Raman spectra were recorded with a Spex Triplemate spectrometer. The concentrations of the dispersions were determined by a Thermo Scientific iCAP-6500 inductively coupled plasma-atomic emission spectrometer ICP-AES). For analysis, 3 mL of dispersion was dissolved in aqua regia (HCl/HNO3 = 3:1), diluted to 50 mL, and then analyzed by AES for the corresponding lanthanide. UV−vis spectra of dispersions and diffuse reflectance of powdered bulk samples were recorded with a UV-3101 PC Shimadzu spectrometer in the range of 200−800 nm. The Kubelka−Munk function was used to convert diffuse reflectance data to absorption spectra. The dynamic light scattering (DLS) measurements were carried out in a 1 cm glass cuvette in a NanoBrook Omni spectrometer (Brookhaven Inst., USA). The scattering angle was 90°, and for each measurement the photon accumulation time was 10 s and the temperature was 20 °C (accuracy 0.1 °C). Autocorrelation was performed by spectrometer software with the cumulant method for monomodal analysis and the NNLS (non-negatively constrained leastsquares) algorithm for polymodal analysis. The Z-averaged (by the intensity from 60 to 100 measurements) hydrodynamic diameter Dhz was calculated by the Stokes−Einstein equation43 for spherical particles. The electrokinetic potentials (ζ-potential) of the particles in solutions were recorded with a NanoBrook Omni spectrometer (Brookhaven Inst., USA) by a laser electrophoresis method with phase analysis light scattering (PALS). Photons scattered by the particles were detected at the angle of 15°. Measurements were carried out at 20 °C in a special SRR2 cell (Brookhaven, USA) resistant to organic solutions equipped with parallel plate palladium electrodes with a surface of ∼45 mm2; the gap between the electrodes was 3.45 mm. Measurements were carried out in the manual mode, and the ζ-potentials were determined by the Smoluchowski model from 10 to 20 measurements. The solutions were measured without dedusting. The method error did not exceed 5%. High-resolution transmission electron microscopy (HRTEM) micrographs were obtained by a Jeol JEM-2200FS Cs-corrected transmission electron microscope with a point-image resolution of 1.9 Å and an acceleration voltage of 200 kV. Analysis of the local elemental composition (atomic %) was carried out using an energydispersive EDX spectrometer.

EXPERIMENTAL METHODS

Solvents were dried before usage to remove residual water and to prevent the possible hydrolysis of the nanoparticles. Isopropanol (IPA) was distilled over Mg, acetonitrile (ACN) over P2O5, ethanol (EtOH) over Na, pyridine (Py) over KOH, dimethylformamide (DMF) over BaO under low pressure, and N-methyl pyrrolidone (NMP) over Na2SO4 under low pressure. Acetone was kept over molecular sieves of 4 Å. Isoamyl alcohol (iAmA) and Nmethylformamide (NMF) were used as purchased. Synthesis. Potassium Polysulfide (K2Sx). A mixture of alkali metal polysulfides was prepared by a similar method as Ln2S3.42 K2CO3 (high purity) was used as the starting material, and NH4SCN vapor was used as the sulfurization agent. For further details, see the Supporting Information. We repeated the sulfurization process at 250 °C, and after each iteration, we performed a chemical test with a BaCl2 water solution. The presence of the white precipitate of BaCO3 in a test tube indicated the unfinished formation of a K2Sx mixture, while a clear yellow solution in the test tube meant that all carbonates were substituted by polysulfide groups. An orange hydroscopic powdered mixture consisting of K2Sx (x = 4−6) according to powder X-ray diffraction (PXRD) was obtained. KLn2CuS6 (Ln = La (1), Ce (2), Pr (3), Nd (4), Sm (5)). For a typical synthesis, 0.25 mmol of Ln2S3 and 0.13 mmol of Cu2S were ground together in an agate mortar and then placed in a glass ampule with 1.7 mmol of S and 0.53 mmol of K2S5 (as the main ingredient in the polysulfide mixture of K2Sx). In the case of compound 5, the amount of S and K2Sx was doubled, as a smaller amount of flux leads to the formation of a mixture of KSm2CuS6 and KSm2CuS4 phases. The ampule was then evacuated to 30 mV in absolute magnitude, except for KLa2CuS6 in ACN). These surface charges result in electrostatic repulsion between nanoparticles and ensure the stability of the colloid. Differences in charges in the two investigated solvents seem to arise from specific nanoparticle− solvent interactions. However, as we cannot determine the exact exfoliation mechanism, the origin of these charges is still unknown. The UV−vis spectra of the resulting dispersions (Figure 7) show several bands in the 200−500 nm region. These characteristic bands can be used for the calculation of the KLn2CuS6 concentration in the colloid solutions according to the Beer−Lambert−Bouguer law: A/l = αC, where A is the absorbance, l is the cell length, α is the extinction coefficient, and C is the concentration of the dispersion. Calculated

concentrations derived from both methods (determination from UV−vis spectra using extinction coefficients and direct iCP-AES measurements of dispersions) are in good agreement. The morphology, size, and crystalline structure of the nanoparticles were determined by TEM and AFM methods. According to typical TEM (Figures 8 and S3) images, KLn2CuS6 nanoparticles exist as irregular shaped nanoplates. We estimated the lateral sizes of the nanoparticles of a large number of flakes as the largest dimension of width and length; the results are sketched in Figure 8D−I. HRTEM images and corresponding selected area electron diffraction (SAED) patterns (Figures 8B,C and S3) demonstrate the single crystalline nature of the nanoparticles and indicate that the crystal structure of the particles remains the same after exfoliation. Lattice spacings in a HRTEM image (Figure 8B) were measured to be 0.579 and 0.528 nm; they were indexed to (113) and (008), respectively. The thicknesses of the nanoparticles in IPA and ACN colloidal solutions were measured by AFM. As for lateral sizes, we evaluated the distribution function of the thickness of the nanoparticles of IPA and ACN dispersions. We found that the thickness of particles is considerably larger in IPA than in ACN for all compounds (Figure 9). In ACN dispersions, most of the particles turn out to be 2−3 nm thick. Since one layer of KLn2CuS6 corresponds to ca. 0.8 nm according to their crystal structure, the particles in acetonitrile dispersions are based on 2−4 layers. DLS Analysis. Dynamic light scattering (DLS) measurements were carried out to determine the size of the particles direct in colloidal solutions. Table 5 contains the Z-averaged (by intensity) hydrodynamic diameters Dhz of IPA and ACN dispersions. According to microscopic (TEM, AFM) data, there are some diversity of particles’ sizes; thus, use of a polymodal analysis of DLS is reasonable. For all investigated systems, the polymodal analysis of Z-averaged diameter Dhz resulted in bimodal distribution (Table 6). Z-averaging is known to overestimate the values of hydrodynamic diameters in the case of polydispersity because the most contribution in light scattering belongs to the larger particles. In our case Naveraging (by number) seems more appropriate. Polymodal analysis of N-averaged diameters Dhn indicated only one mode distributions, which means that the amount of the large particles is small. Considering that the particles could be qualify as nanoplates, we could use disc approximation for more precise calculation of the hydrodynamic diameters:69

Dhn

−1 ÄÅ É ÄÅ É1/2 ÉÑÑÑ iÄÅ Å 2 Ñ1/2 2Ñ ÅÅ ÑÑ ÑÑ h zyz ij h yz ÑÑÑÑ i y 3 jjjjÅÅÅÅ d ÅÅÅÅ h h z Å Ñ = djjÅÅ1 + jj zz ÑÑ + lnÅÅ + ÅÅÅ1 + jjj zzz ÑÑÑ ÑÑÑ − zzz 2 jjÅÅÅÇ h ÅÅÅ d ÅÅÅÇ k d { ÑÑÑÖ k d { ÑÑÑÖ ÑÑÑÑ d zz ÅÇ Ö k { (1)

where d is disc diameter and h is thickness of the disc, estimated from AFM data. The obtained values of d (Table 6) lying in the range of 80−150 nm are consistent with the TEM data (Figure 8). Films of KLn2CuS6. The dispersions were filtered onto alumina (ACN) or cellulose acetate (IPA) membranes. The typical pore size of these membranes is 20 or 50 nm. As the dispersed flakes are typically 80−150 nm in size, they tend to deposit on the surface of the membrane. Thickness of the film made from flakes could be controlled by the amount of the filtered material, determined by concentration and volume of the dispersion. 13602

DOI: 10.1021/acs.inorgchem.8b02213 Inorg. Chem. 2018, 57, 13594−13605

Inorganic Chemistry

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XRD patterns of the films prepared from IPA and ACN dispersions (Figure 10A,C,E) in comparison to bulk sample and theoretical calculations from crystal structure demonstrate phase purity of the films. The films are textured along the direction of layers’ packing (0k0); it is obvious from difference in the ratio of intensity of the peaks. The Raman spectra of KLn2CuS6 are similar for different Ln. They consist of a strong mode located at 472−479 cm−1 corresponding to S−S stretching vibrations. For the Pr compound, we observed an additional S−S vibrational mode at approximately 410 cm−1. The presence or absence of this mode depends on the crystal orientation during measurement.70 Other modes have weak intensities and correspond to Ln−S vibrations. The characteristic bands in the Raman spectra of the films correspond to the ones of bulk samples of KLn2CuS6 (Figure 10B,D,F).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tatiana A. Pomelova: 0000-0002-6563-9943 Nikolay G. Naumov: 0000-0002-7531-6291 Funding

We thank Federal Agency for Scientific Organizations for funding. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.G. is indebted to the Région Bretagne for a Ph.D. grant (to A.H.). R.G. acknowledges support of the French Agence Nationale de la Recherche (ANR) under grant MASSCOTE (ANR-15-CE05-0027). The authors gratefully acknowledge financial support from LIA-CNRS CLUSPOM.

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CONCLUSION In summary, this work reports the successful preparation of quaternary lanthanide sulfides by a reactive flux method. Crystal structure investigations carried out on KLn2CuS6 (Ln = La, Pr, Nd, Sm) show that compounds with Ln = La, Pr, and Nd are isostructural to KCe2CuS6 and crystallize in the monoclinic C2/c space group, whereas the crystal structure of KSm2CuS6 is similar to KEu2CuS6. All obtained compounds possess a layered structure with charged layers of {LnS8} twocapped prism connected with {CuS4} tetrahedra, and K+ cations lie into the interlayer space. First principle calculations confirm the strong covalent interactions within the anionic Ln2CuS6− layers. For the first time, it has been demonstrated that bulk KLn2CuS6 can be obtained in the colloidal state by ultrasonic treatment in isopropanol and acetonitrile. The concentration of isopropanol dispersions was found to be the largest for all investigated Ln compounds (up to 180 mg/L). The analysis of TEM, AFM, and DLS data shows that KLn2CuS6 exists in the dispersions as charged nanoplates with the thickness of the particles ranging from 2 to 10 nm and lateral size between 100 and 200 nm. The texture along 0k0 direction films of the nanoparticles can be prepared by filtering. PXRD and Raman investigations indicate that after exfoliation, the nanoparticles retain their phase identity. This research contributes to the growing number of reports concerning the preparation of colloids of layered compounds and confirms the wide applicability of LPE for the preparation of complex nanoparticles, including layered compounds with charged layers.

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Article

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ABBREVIATIONS LPE, liquid phase exfoliation; HSAB, hard and soft base and acid theory; IPA, isopropanol; ACN, acetonitrile; EtOH, ethanol; DMF, dimethylformamide; Py, pyridine; iAOH, isoamyl alcohol; XRD, X-ray powder diffraction; DLS, dynamic light scattering; PALS, phase analysis light scattering; TEM, transmission electron microscopy; HRTEM, high-resolution transmission electron microscopy; AFM, atomic force microscopy; ELF, electron localization function

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02213. Precursor preparation description, additional DOS and TEM images, magnetic measurements (PDF) Accession Codes

CCDC 1824144−1824147 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. 13603

DOI: 10.1021/acs.inorgchem.8b02213 Inorg. Chem. 2018, 57, 13594−13605

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