Soft chemical synthesis of HxCrS2: an antiferromagnetic material with

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Soft chemical synthesis of HxCrS2: an antiferromagnetic material with alternating amorphous and crystalline layers Xiaoyu Song, Guangming Cheng, Daniel Weber, Florian Pielnhofer, shiming Lei, Sebastian Klemenz, Yao-Wen Yeh, Kai A. Filsinger, Craig B. Arnold, Nan Yao, and Leslie M. Schoop J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07503 • Publication Date (Web): 03 Sep 2019 Downloaded from pubs.acs.org on September 3, 2019

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Journal of the American Chemical Society

Soft chemical synthesis of HxCrS2: an antiferromagnetic material with alternating amorphous and crystalline layers Xiaoyu Song,† Guangming Cheng,‡ Daniel Weber,¶ Florian Pielnhofer,§ Shiming Lei,† Sebastian Klemenz,† Yao-Wen Yeh,‡ Kai A. Filsinger,k Craig B. Arnold,k Nan Yao,‡ and Leslie M. Schoop∗,† †Department of Chemistry, Princeton University, Princeton, NJ, 08544 ‡Princeton Institute for Science and Technology of Materials, Princeton, NJ, 08544 ¶Department of Chemistry, Ohio State University, Columbus, OH, 43201 §Institute of Inorganic Chemistry, University of Regensburg, D-93040 Regensburg, Germany kDepartment of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ, 08544 E-mail: [email protected] Abstract We report a new HxCrS2-based crystalline/amorphous layered material synthesized by soft chemical methods. We study the structural nature and composition of this material with atomic resolution Scanning Transmission Electron Microscopy (STEM), revealing a complex structure consisting of alternating layers of amorphous and crystalline lamellae. Furthermore, the magnetic properties show evidence for increased magnetic frustration compared to the parent compound NaCrS2. Finally, we show that this material can be exfoliated, thus providing a facile synthesis method for chromiumsulfide based ultra-thin layers. The material reported herein can not only be a source

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of new thin TMD-related sheets for potential application in catalysis, but also be of interest for realizing new 2D magnetic materials.

Introduction Two-dimensional (2D) materials provide a way to rationally design complex multilayer systems 1 that cannot be directly synthesized by conventional high temperature solid state methods, which are governed by thermodynamic principles. 2 Many artificially designed vertical 2D heterostructures exhibit exciting physical or chemical properties, which differ from three-dimensional (3D) materials. For example, the recent discoveries of unconventional superconductivity in twisted bilayer graphene systems opened a new gateway for finding high-critical-temperature superconductors. 3 In addition, some newly developed hybrid organic/inorganic 2D heterostructures have shown enhanced mechanical 4 and energy storage abilities. 5 Among the known 2D materials, transition metal dichalcogenides (TMD) are a popular family of materials that have long been studied for their many fascinating properties, such as tunable bandgaps, 6 favourable mechanical properties 7 and as high-performing hydrogen evolution catalysts. 8,9 TMDs have general formula MX2 (where M is the transition metal (TM) and X is the chalcogenide). While all group 4 and 5 TMs form layered TMDs which can be either mechanically or chemically exfoliated down to monolayers, the middle to late 3d TMs prefer to crystallize in the cubic pyrite or macarsite structure. Thus, there is a lack of 3d-based TMD nanosheets. Such materials, if synthesized, would be of interest to a variety of applications, ranging from multivalent battery cathodes 10 to novel catalysts for water splitting. 11–13 Another heavily pursued research direction in the 2D community is the synthesis of magnetic 2D materials. 14–24 The interest in these arises from two different aspects: For one, magnetism in 2D seems to violate fundamental physical principles, 25 while such materials

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are of interests for spintronic-based application. 15,16,20 So far, a common issue with most magnetic 2D materials is their air sensitivity. Thus, the development of novel 2D magnetic materials is crucial. In the case of TMDs, this would require the incorporation of magnetic 3d TMs into the layered TMD structure. Among the 3d TMs, Cr takes a special place in forming chalcogenides – it is the only element that does not form a TMD (neither layered nor pyrite). Due to Cr’s strong desire to adopt the 3+ oxidation state (or d3 electron configuration), the most thermodynamically stable Cr chalcogenide is Cr2S3, which crystallizes in a 3D structure. 26 However, 3d TMs can exist in a ternary layered delafossite-type structure, 27 which is especially frequently adopted for Cr-based compounds. 28 Previous studies have shown that van der Waals layered CrSe2 and CrTe2 can be indirectly synthesized by oxidizing KCrSe2 and KCrTe2 with I2 in acetonitrile, 29–31 thus forming a new, magnetic TMD. However, these compounds have not yet been exfoliated and the method was not successful for synthesizing CrS2. 29 Very recent studies in creating magnetic Cr chalcogenide-based nanosheets all aimed at synthesizing thin Cr2S3 platelets. 32–34 These platelets were synthesized by chemical vapor deposition methods and maintained the three-dimensional non-van der Waals structure down to a few nanometers’ thickness.

Figure 1: An illustration of the reaction pathway for the phases reported here: Pristine NaCrS2 crystals are first treated with a HCl/ethanol solution to obtain a HxCrS2-based layered material. This phase can subsequently be treated with an aqueous alkalyammonium solution to synthesize a nanosheet suspension. (Color codes: Na cyan, fully occupied Cr red, partially occupied Cr orange, S yellow and H black.)

In this work, we adopted a soft chemical method as shown in Figure 1 which is widely used

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in chemical exfoliation of layered oxide compounds 35–37 to synthesize a HxCrS2-based layered material, where the oxidation state of Cr remains close to 3+ . We study the structural nature and composition of the new material with high-resolution STEM, revealing a complex structure consisting of alternating layers of amorphous and crystalline lamellae. The employed proton exchange reaction changes the layered structure of the parent compound NaCrS2, thus a completely new material is synthesized. We further report the magnetic properties of the new material, providing evidence for increased magnetic frustration compared to its parent NaCrS2. Finally, we show that this material can be exfoliated to ultra-thin sheets, providing a facile synthesis method for chromium-sulfide based ultra-thin sheets. Thus, the material reported herein can not only be a source of new thin TMD-related sheets for potential application in catalysis, but also be of interest for realizing new 2D magnetic materials with exotic magnetic properties.

Results and Discussion

Figure 2: (A, B) Optical microscope images of a NaCrS2 single crystal before and after the proton exchange, respectively. (C, D) Diffraction patterns of a proton-exchanged sample taken from the out-of-plane direction and on the edge of the crystal, respectively. (E) A SEM image of a proton-exchanged sample. (F) SEM-EDS spectra of the proton-exchanged sample. The Na peak decreases to almost zero while the Cr/S peaks remain constant after the proton exchange.

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The proton-exchanged phase was prepared via a two-step process. First, the NaCrS2 crystals were synthesized via high temperature solid-state methods. NaCrS2 was subsequently dispersed in a diluted HCl/ethanol solution yielding HxCrS2 in a proton exchange reaction (see Supporting Information (SI)). During the ion exchange reaction, the hexagonal crystals of NaCrS2 are slightly etched, but retain their layered morphology at macro-scale as seen in Figure 2(A), (B), (E). The crystallinity of the proton-exchanged sample was examined by X-ray diffraction, as shown in Figure 2(C) and (D). From the diffraction patterns, it is evident that the crystallinity within the layers (in-plane structure) is retained (Figure 2(C) and Figure S1), while disorder exists along the out-of-plane direction (out-of-plane structure) which result in stripes rather than diffraction spots in Figure 2(D). Additionally, the composition analysis of the sample before and after proton exchange reveals that the Cr:S ratio remains approximately 1:2, while Na+ is removed completely in the proton-exchanged sample, as shown in Figure 2(F). The elemental composition and change in the chemical environment of the constituting elements upon acid treatment of NaCrS2 was further characterized by X-ray photoelectron spectroscopy (XPS) on several samples. Representative spectra of the Na 1s, Cr 2p and S 2p transition regions are displayed in Figure S2. The presence of the Na 1s peak at a binding energy (BE) of 1071.3 eV in NaCrS2 and its absence in HxCrS2 confirm the removal of Na+ during the acid leaching process. Additionally, XPS indicates a change in the chemical environment for Cr in HxCrS2 compared to NaCrS2, as indicated in the observed shift 2p 23 from 572 to 579 eV. 38,39 Cr is present in one distinct site prior to the acid treatment while a good fit for HxCrS2 can only be obtained by considering two distinct Cr species. The S 2p 23 transition shows that in NaCrS2, the sulfur is present as a metal bound sulfide (S2–, BE 160.7-161.3 eV). 38–40 In HxCrS2, two S 2p 23 states were observed, one corresponding to metal bound sulfide, while the other originates either from disulfide (S22–) or hydrogen sulfide functional groups (HS–, BE 162.7-163.4 eV). To probe the crystal structure as well as the incorporation of H+ into the lattice, HxCrS2

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and NaCrS2 were investigated by Raman spectroscopy and the observed spectra were then compared to that of Density Functional Theory (DFT) models. In the case of NaCrS2, the observed spectrum resembles the DFT-simulated one well (see Figure S3). The Raman spectrum of the proton-exchanged phase shows a new strong vibration at 537 cm−1 . Different amounts of hydrogen loading/functionalization of a CrS2 monolayer, with H atoms sitting above and below the S atoms, were modeled via DFT. When 25 % of S is saturated with H (H0.5CrS2), the simulated Raman shifts with highest intensities was also observed in the experimental spectrum. The most intense signal at 537 cm−1 (simulated 497 cm−1 ) can be interpreted as a S-H bending vibration, while a hypothetical S-S stretching mode would be expected at a different frequency (see SI). The signal at 303 cm−1 can be assigned to a combination of Cr-S stretching and weaker S-H bending modes. It serves thus as a direct evidence that protons were incorporated into the sample and formed S-H bonds with the S atoms.

Figure 3: (A) A HAADF-STEM image showing the loosely layered structure of a protonexchanged sample viewing on top of the planes, zone axis (001). (B) A HAADF-STEM image showing the cross-sectional structure of a proton-exchanged sample, zone axis (110). Crystalline planes are clearly shown as bright stripes in the image. The inset is a magnified image showing the layered structures with periodic arrangement of amorphous and crystalline phases marked in (B). (C,D) Atomic resolution HAADF-STEM images showing the in-plane and out-of-plane structures of the proton-exchanged sample, respectively. The insets are the corresponding Fourier filtered diffraction patterns. The S, Cr and the migrated Cr atoms are indicated by the yellow, red and orange spheres, respectively. 6


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To gain a better understanding about the structural and chemical details of the new proton-exchanged phase, we investigated the atomic structures of these layered materials from both in-plane and out-of-plane directions via atomic-resolution High-Angle Annular Dark-Field STEM (HAADF-STEM), as shown in Figure 3. The HAADF-STEM image in Figure 3(A) shows the morphology of a typical layered structure from the in-plane direction, (001) zone axis. Note that the basal plane of these layered materials prefer to horizontally lie on the TEM grids if deposited directly. It can be seen that the proton-exchanged sample is composed of very thin triangular flakes with a thickness of several nanometers. Figure 3(B) shows the atomic structure of the new proton-exchanged phase from a very thin flake viewing from the out-of-plane direction, (110) zone axis. Each individual Cr atom (brighter spots, highlighted in red) is surrounded by six sulfur atoms (grey spots highlighted in yellow). Assuming the [CrS2]– layers maintain their octahedral structure and the ABCA stacking order from the parent material NaCrS2 upon Na+ deintercalation, the new proton-exchanged phase would be isostructural to the 3R-MoS2 41 (see Figure S6(A)-(B)) and the intensity of each STEM spot should be the same as the in-plane projection of each spot is the same (Figure S6(A)). The distinct intensity contrast in Figure 3(B) suggests that the new protonexchanged phase might be isostructural to 1T-MoS2 42 (Figure S6(C)-(D)), another common van der Waals-bonded layered TMD structure type for octahedrally-coordinated transition metals. However, electrochemical studies of NaCrS2 40 and NaCrO2 43 revealed that Cr cations migrate from the [CrS2]– layers to the Na vacancies during the Na+ deintercalation process which will lead to a chemically bonded three-dimensional (3D) solid. Therefore, it is important to study the nature of the interlayer structure. For this, we used a focused ion beam (FIB) to cut a thin piece from the proton-exchanged crystal (Figure 2(E)) and studied the cross-section structure and chemical composition with the high-resolution HAADF-STEM. As seen in Figure 3(C), the sodium-free phase maintains a complex layered structure, rather than transforming into a fully 3D solid. The layered phase consists of alternating layers of

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amorphous and crystalline thin lamellae, with an average thickness of 2 nm (inset of Figure 3(C)). The structure of the crystalline layer was further investigated with atomic resolution (Figure 3(D)), revealing that some Cr atoms migrate (indicated by the orange spheres) and occupy the gap between the [CrS2]– layers (Cr atoms within the [CrS2]– layers are indicated by the red spheres and S atoms are indicated by the yellow spheres). While Cr migration causes the structure to become 3D, it only appears in the crystalline lamellae of the samples, so that an overall layered structure is maintained. In addition to Cr migration, the stacking order of the crystalline phase changes to the ABA type (taking into account of the S occupation in the crystal structure), yielding a structure that is isostructural to CrS 44 with vacancies in the Cr positions (Figure S7). The chemical composition of both amorphous and crystalline layers was studied with STEM-EDS mapping (Figure S8). In both phases, the overall Cr to S composition is roughly 1:2 (Table S2, 32.8 ± 0.5 at.% Cr and 67.2 ± 0.5 at.% S to be precise). In the crystalline phases, there is a slight excess of Cr (Cr to S ratio is roughly 1:1.9), while in the amorphous lamellae there is an excess of S (Cr to S ratio roughly 1:2.4). An atomic fraction line profile along the cross-section (Figure S8(A)) is shown in Figure S8(D), which suggests that as the S content increases in the amorphous lamellae, the Cr content drops and vice versa in the crystalline lamellae. Thus, the newly synthesized material reported here is an intrinsic layered material of crystalline HxCrS1.9 with a CrS-type structure (lattice parameters: a = 0.3474 nm, c = 0.5759 nm) and amorphous HyCrS2.4 (x > 0, y ≥ 0). The amorphous phase might related to previously reported CrS3. 45 This material is different from other known Cr chalcogenide phases. There are six known chromium sulfide phases, CrS, Cr2S3, Cr3S4, Cr5S6, Cr5S8 and Cr7S8. None of these match the composition reported here and none of these are layered compounds. Thus, we here introduce the first layered Cr-S based material. This material does not only have the potential to be chemically exfoliated to thin layers, but also to be a source for an air-stable 2D magnetic material.

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Figure 4: Temperature dependent magnetic susceptibility and inverse susceptibility of NaCrS2 (A) and the proton-exchanged crystals (B), where the field is applied along the plane. ZFC data is shown in blue and FC in green. The inverse magnetic susceptibility is shown in black. The red line indicates the Curie-Weiss fit. The insets highlight the Néel transition. In the following section we would like to discuss the physical properties of the layered HxCrS2 material reported here. In order to gain an understanding about the electronic properties of the observed nanostructure, we measured the temperature dependent resistivity (Figure S10(A)). Chromium-sulfides are usually known to be either semiconducting such as CrS and Cr2S3 or metallic such as Cr3S4, Cr5S6, Cr5S8 and Cr7S8. Our material exhibits an activated electronic transport behavior in the temperature-dependent resistivity (Figure S10(A)), suggesting a semiconducting behavior. The logarithm of resistivity is more linear if plotted against T −1/4 (red) which follows a Mott variable range hopping model (Figure S10(B)); the typical model for disordered systems. As both pristine NaCrS2 and proton-exchanged HxCrS2 have structural anisotropy, it is likely that this will lead to magnetic anisotropy, a key factor needed for the existence of magnetic order in a 2D material. Bulk ACrS2 (A = Li, Na, K, Cu, Ag, Au) have previously 9



been investigated for their complex magnetism. 28,46 Thus, both the proton-exchanged material and the exfoliated sheets reported here have a high potential to exhibit complex magnetic properties. We measured the temperature and field dependent magnetic susceptibility of both pristine NaCrS2 and proton-exchanged crystals with two different field alignments (H ⊥ c and H k c). For the pristine NaCrS2 single crystals, the antiferromagnetic transition temperature, Weiss temperature and effective magnetic moment are comparable to previous studies 47 (Figure 4(A), Figure S11(A)). NaCrS2 is known to order antiferromagentically below 18 K, while having a positive Weiss temperature of approximately 30 K. The magnetic structure was determined to be a spiral spin structure within the triangular Cr-layers. 48 The proton-exchanged crystals exhibit vastly different magnetic properties as shown in Figure 4(B), Figure S11(B) and Figure S13. For one, the Néel temperature decreases from 18 K to 5 K. Further, the field-cooled magnetic susceptibility deviates from the zero-field-cooled magnetic susceptibility, which suggests that the spins are canted while antiferromagnetically aligned. The field dependent magnetic susceptibility, shown in Figure S13(C)-(D), exhibits a weak hysteresis for both applied field directions, where the coercive field is stronger for fields applied along the plane than perpendicular to the plane. This further suggests the presence of canted antiferromagnetism, which has a net ferromagnetic component. CurieWeiss fitting of the inverse zero-field-cooled magnetic susceptibility results in negative Weiss temperatures (ΘCW,ip = −83.4 K, and ΘCW,op = −109.3 K), suggesting that the antiferromagnetic interactions in the system are strong. The Weiss temperature is strongly negative, while the ordering temperature is low. This is usually an indication of magnetic frustration in the system. Based on the Curie-Weiss fits, the proton-exchanged sample has effective moments of µef f,ip =3.31-3.32 µB /Cr and µef f,op =3.57-3.58 µB /Cr, which is slightly lower than the typical moments observed for Cr3+ (3.7-3.9 µB ). This indicates that upon proton exchange part of the Cr ions are either oxidized to Cr4+ , which has a spin-only effective moment of 2.8 µB , or reduced to Cr2+ , which would have the same spin-only moment. This agrees with the Raman model, which assumes only partial hydrogen loading of CrS2. It is

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also consistent with the observation of S-H bonds in the Raman data (Figure S3), the XPS analysis (Figure S2) as well as the Electron energy loss spectroscopy (EELS) analysis (Figure S9), which suggests that the majority of Cr maintains its 3+ oxidation state. The difference between the in-plane and the out-of-plane effective magnetic moments likely stem from an anisotropic g-factor. Exfoliation of the polycrystalline proton-exchanged material was achieved by treating the powder with an alkalyammonium solution (SI). Upon shaking the material, a blackish suspension that contained sheet-like materials was formed (Figure S15(A)). The suspension was drop-casted on a Si/SiO2 wafer and investigated with AFM. Thin sheets in the AFM images (Figure 5(A)-(B)) are roughly 2-3 nm thick (Figure 5(C)). The observed height is slightly larger than the typical thickness of chemical exfoliated TMD layers (1-2 nm). While the thickness deviation could potentially be attributed to the absorbed water molecules and alkalyammonium cations, 19 the thickness perfectly fits the crystalline layers observed in the high resolution STEM studies (Figure 3(C)). Since the structure of the crystalline layers has transformed into a fully chemical-bonded structure, the typical thickness of the crystalline layers sets the limit of the nanosheets’ thickness that we could obtain via this method. Thus, we believe that we have obtained the individual crystalline layers through the chemical exfoliation method. The crystallinity of the nanosheets was confirmed by electron diffraction. The six-fold symmetry of the diffraction pattern shown in the inset of Figure 5(D) indicates that the nanosheets maintained the hexagonal structure from their parent compound. EDS analysis proves that the nanosheets are mainly made of S (Figure S15(B)) and Cr (Figure S15(C)). Thus, we successfully synthesized thin sheets of a chromium-sulfide based material. The magnetic properties of the proton-exchanged phase indicate that the exfoliated sheets might also exhibit complex magnetic properties. Further investigations of the magnetic properties of these sheets will be of interest to the 2D magnetism community.

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Figure 5: (A) An AFM image of the nanosheets from the suspension in Figure S5(A). (B) The AFM image of the selected area of the image shown in (A). (C) Line profiles of the two lines shown in the (B) showing the typical thickness of the synthesized nanosheets. (D) A HAADF-STEM image of a nanosheet. The insect is the diffraction pattern of the sheet shown in (D), proving that the sheet is crystalline.

Conclusion In conclusion, we adopted a one-step soft chemical method to successfully synthesize a HxCrS2-based crystalline/amorphous layered material from its solid-state synthesized layered parent material. Upon proton exchange, the crystal structure of the parent material changed dramatically. The resulting material opens a wide range of possible future studies. For example, amorphous materials have been shown to be better supercapacitors as compared to their crystalline counterparts. 49,50 In addition, layered crystalline/amorphous materials have also been discussed in respect to their enhanced supercapacitive properties, 5,51 offering a need for future studies. Further, it has been theoretically suggested hypothetical layered CrS2 would be a good cathode material for multivalent Mg batteries. 10 The material introduced here is closely related to CrS2. Our initial tests show that HxCrS2 intercalates Li electrochemically and shows potential to be used as a cathode material (see SI). We also showed that the proton-exchanged nanostructured samples exhibit complex magnetic properties. Finally, we showed that the material serves as a basis for the synthesis of a

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novel ultra-thin material. This points to the exciting opportunity to create thin layers with complex magnetic properties. Since the currently known magnetic 2D materials are mainly unstable in air, our work here provides the opportunity for magnetic sheets that can be studied in air.

Acknowledgement This work was supported by NSF through the Princeton Center for Complex Materials, a Materials Research Science and Engineering Center DMR-1420541. Additional funds from the Princeton Catalysis Initiative (PCI) are acknowledged. The authors acknowledge the use of Princeton’s Imaging and Analysis Center, which is partially supported by the Princeton Center for Complex Materials, a National Science Foundation (NSF)-MRSEC program (DMR-1420541). The authors thank Prof. Bettina Lotsch, Dr. Ulrich Wedig, and the Computer Service group from the Max-Planck-Institute for Solid State Research (Stuttgart, Germany) for access to CRYSTAL17 and computational facilities. The authors thank Dr. Navid Bizmark and Jason X. Liu for performing the zeta potential measurement and Prof. Robert K. Prud’homme for the access to the intrument.

Supporting Information Available The following information are available in the supplemental information free of charge. Experimental details, in-plan XRD data, Raman spectra, XPS data, additonal TEM and EELS data, electrical resistivity data, additional magnetic data, additional chemical information of the Cr-S based nanosheets, electrochemical lithium intercalation test results

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Soft chemical synthesis of HxCrS2: an antiferromagnetic material with

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