Electrochemical Intercalation and Exfoliation of CrSBr into Ferromagnetic Fibers and Nanoribbons

Recent studies dedicated to layered van der Waals crystals have attracted significant attention to magnetic atomically thin crystals offering unprecedented opportunities for applications in innovative magnetoelectric, magneto‐optic, and spintronic devices. The active search for original platforms for the low‐dimensional magnetism study has emphasized the entirely new magnetic properties of two dimensional (2D) semiconductor CrSBr. Herein, for the first time, the electrochemical exfoliation of bulk CrSBr in a non‐aqueous environment is demonstrated. Notably, crystal cleavage governed by the structural anisotropy occurred along two directions forming atomically thin and few‐layered nanoribbons. The exfoliated material possesses an orthorhombic crystalline structure and strong optical anisotropy, showing the polarization dependencies of Raman signals. The antiferromagnetism exhibited by multilayered CrSBr gives precedence to ferromagnetic ordering in the revealed CrSBr nanostructures. Furthermore, the potential application of CrSBr nanoribbons is pioneered for electrochemical photodetector fabrication and demonstrates its responsivity up to 30 µA cm−2 in the visible spectrum. Moreover, the CrSBr‐based anode for lithium‐ion batteries exhibited high performance and self‐improving abilities. This anticipates that the results will pave the way toward the future study of CrSBr and practical applications in magneto‐ and optoelectronics.


Introduction
Layered van der Waals (vdW) crystals have dramatically changed our lives since they began to be employed in various electronic devices.Nowadays, 2D materials comprise a large family of DOI: 10.1002/smtd.202300609conductors, semiconductors, and insulators, which are widely used for the creation of vdW-heterostructures with a broad range of mechanical, electrical, and optical properties. [1,2]However, magnetic atomically thin crystals have remained veiled fellows for the 2D materials family.Only a few recent studies utilizing magneto-optical Kerr effect microscopy have revealed long-range ferromagnetic ordering in ultrathin exfoliated Cr 2 Ge 2 Te 6, [3] and CrI 3, [4] vdW magnets.These groundbreaking discoveries instigated further research into magnetotransport and magnetic anisotropy in 2D layered magnets. [5] Furthermore, theoretical calculations classify CrSBr as an intrinsic semiconductor with a direct bandgap of ∼1.5 eV in bulk and up to ≈2.11 eV in monolayer. [7,10]Since the exceptionally large, reported bandwidth in few-layered CrSBr [11] can lead to high carrier mobility, [12] it is highly anticipated that this 2D material can be employed in energy conversion and storage.The crystal lattice of CrSBr incorporates the vdW layers consisting of two buckled rectangular planes of CrS terminated by Br atoms along the caxis.The vdW stacking and interlayer spacing of ≈8 Å facilitate the easy cleavage of CrSBr layers along the ab-planes. [10]][10][11][12] Although mechanical exfoliation remains a stable and robust technique for the production of 2D nanosheets from a bulk crystal, this method is hindered by limited scale, expensive production, and lack of reproducibility, which renders it unsuitable to meet the requirements for macroscopic quantities of 2D materials. [13,14]On the other hand, liquid phase exfoliation (LPE) has been proven to be an efficient way to produce high-quality 2D nanosheets in large quantities.This top-down approach has been applied to exfoliate inorganic van der Waals crystals such as graphite, [15] h-BN, [16] transition metals dichalcogenides (MoS 2 , www.small-methods.comWS 2 , MoSe 2 ), [17,18] MXenes, [19] pnictogens [20,21] as well as organic semiconductors, [22] and layered polymers. [23]Among the exfoliation methods, electrochemical exfoliation is a promising approach to preparing uniform nanosheets under mild conditions.Driven by an external force, the intercalation of species (cations or anions) between the layers of a crystal overcomes the vdW forces between layers to eventually produce the 2D counterpart. [24]The review of Yang et al. [25] reports multiple approaches to the electrochemical exfoliation of vdW materials, where cathodic exfoliation is highlighted as the most common way to treat layered crystals.The diversity of organic solvents utilized for the cathodic exfoliation of various vdW materials suggests that polar aprotic dimethyl sulfoxide (DMSO) is a convenient electrolyte due to its large potential window and low viscosity, which facilitates the flow of ionic species in an electrolyte. [25,26]erein, we present the electrochemical intercalation and exfoliation of layered vdW-crystal CrSBr in DMSO using tetrabutylammonium hexafluorophosphate (TBAPF 6 ).Followed by the structural expansion of the bulk crystal, the intercalation with quaternary ammonium salt revealed CrSBr nanoribbons with a thickness of single and few CrSBr layers.Interestingly, electrochemically intercalated CrSBr exhibits increased interlayer spacing and ferromagnetic transition with a T c ≈130 K.The structural and morphological characterization of the exfoliated material confirmed that the crystalline structure remained within the exfoliated sample, granting it the optical anisotropy of the parent crystal and the bandgap of a CrSBr monolayer.Together with the cleavage perpendicular to the c-direction, the weak interlayer hybridization and an in-plane structural anisotropy caused the simultaneous delamination along the a-direction.Stabilized in the suspension, CrSBr nanoribbons were employed to fabricate a photoelectrochemical (PEC) photodetector, where the nanomaterial demonstrated high photoresponsivity in the visible spectrum region.Furthermore, the exfoliated CrSBr was tested in lithiumion batteries as a high-performance anode showing promising self-improving abilities over battery cycling.

Morphological and Structural Characterization
The fabrication of CrSBr nanoribbons was carried out by the electrochemical exfoliation of the correspondent bulk crystal used as a cathode.Polar aprotic DMSO was employed as an electrolyte since it exhibits suitable electrochemical stability by having a wide electrochemical window. [26]Furthermore, it exhibits a high ability to dissolve salts to form dissociated ions.The ionic radius of the intercalant plays a key role in the cathodic exfoliation performance.Considering the fact that for successful exfoliation, the size of the intercalant should exceed the interlayer distance, tetrabutylammonium (TBA + ) cations appear to fit this requirement for the exfoliation of CrSBr.[27] TBA + diameter is 8.3 Å, which exceeds the size of the interlayer distance in CrSBr.
The exfoliation itself requires two stages at different potentials.A starting potential of −1 V was applied to the cathode to initially pretreat the bulk material and subsequently raised to −2 V to facilitate further cation intercalation.The intercalation of the TBA + ions into CrSBr interlayers began at a minimum potential of −1.9 V, followed by further exfoliation of the crystal as evidenced by a clear visual expansion of the crystal.[18][19][20][21][22][23] In this case, a period of 1 h was sufficient, since the crystal started to swiftly expand forming visible fiber-shape structures of intercalated CrSBr, which led to eventual crystal breakdown and exfoliation (Figure 1A).
Initially, we analyzed the crystallinity of the bulk CrSBr by X-ray diffraction (XRD), as shown in Figure 1B.The pattern of the bulk material displayed sharp diffraction peaks and confirmed a successful synthesis of CrSBr (PDF 04-010-7030).The anisotropic crystal has an orthorhombic Pmmn space group with a larger interplanar spacing along the b-direction (4.80 Å) than the a-direction (3.49Å).The majority of the bulk crystal peaks occur along the <00l> direction.We carried out the analysis of a visible fiber-shape structure of intercalated CrSBr besides.The X-ray diffractogram of CrSBr intercalated with TBA + cations presented in Figure S1 (Supporting Information) confirms the interlayer spacing is enlarged to 13.16 Å.The parent non-intercalated material persists in its crystallinity showing well-defined peaks primarily along <00l> direction.The electrochemically exfoliated CrSBr maintains its orthorhombic structure, with the most intense peaks being along the c-axis (001, 003, 004) as well as a sharp reflection at ≈37.7°, which corresponds to the 020 plane.The crystal structure of CrSBr is formed by layers of hexacoordinated chromium bonded to the nearest chromium through sulfur and bromine along the a-direction, while along the b-direction the nearest chromium atoms are linked together just by sulfur (Figure 2A).Furthermore, the CrSBr electronic anisotropy suggests weakened interlayer hybridization within the orthorhombic crystal, [10] which induces cleavage along the a-direction.Such an exotic behavior was described as well for hexagonal boron nitride nanotubes, fibrous red phosphorus, and the group of transition-metal trichalcogenides (TiS 3 , ZrS 3 , and TaSe 3 ). [28]It is thus not surprising that the (001) surface energy reflecting the bond cleavage along the z-axis as obtained from our DFT (GGA+U) calculations attains a relatively low value, 144 mJ m −2 .
The in-plane lattice anisotropy is also reflected in the observed anisotropic Raman peaks.Presented in Figure 2B, the Raman spectrum of bulk CrSBr shows three main phonon out-ofplane atomic displacement modes A 1 g (115 cm −1 ), A 2 g (242 cm −1 ), and A 3 g (344 cm −1 ), which correspond to bromine, sulfur, and chromium out-of-plane lattice vibrations, respectively, as shown in the inset. [29]Anisotropic structural morphology persists within the exfoliated CrSBr, for which the maximum intensity of A 2 g mode was observed with polarization along the a-direction.The A 1 g and A 3 g modes, on the other hand, exhibit an intensity maximum along the b-direction.The prominent vibrational modes of the bulk CrSBr are strongly present in the Raman spectrum of the nanoribbons, even though the peaks were broadened, which occurred due to the exfoliation.The fact that the A 2 g mode exhibits a maximum along the a-direction was confirmed by the polarization angle-dependent intensity of the A 2 g phonon mode (242 cm −1 ) for the individually chosen nanoribbon (Figure 2C).A polar plot collected from the marked region of the nanoribbon, presented in Figure 2D, shows that the A 2 g mode is polarized with the maximum intensity at 90 and 270 degrees which correspond to the a-direction.However, for the additional nanoribbons, the maximum intensity of A 2 g mode was observed at 120 and 300 degrees as well as at 60 and 240 degrees (Figure S2, Supporting Information), which is the result of the random orientations of the dropcast nanoribbons on the substrate.
After confirming the successful synthesis and subsequent exfoliation of CrSBr, we proceeded with the evaluation of the optical characterization by measuring the absorption spectra of the exfoliated CrSBr.The UV-vis spectrum shows that strong absorption occurs in the visible spectral region (Figure 3A).Considering the absorbance spectra, we evaluated the bandgap of elec-trochemically exfoliated CrSBr combining the Tauc plot with the Kubelka−Munk function (inset graph) using direct allowed transition.The estimated band gap value of ≈2.26 eV exceeds the predicted value of the CrSBr monolayer by 0.15 eV, which is plausibly attributed to the structural defects introduced during the redox reactions of the host CrSBr layers. [10]uring the electrochemical intercalation and exfoliation procedure, we obtained the fiber-like material with a dark reddish-brown color (Figure 3B).Therefore, we performed subsequent centrifugation at 800 rpm for 15 min to separate the exfoliated material (Figure 3C) and the intercalated CrSBr fibers (Figure 3D).Images obtained by the optical microscopy of supernatant deposited on a silicon substrate (Figure 3E,F) revealed that the  substance contains numerous multi-and few-layered transparent nanoribbons with a high aspect ratio.
The finest details of the morphology and the structure of electrochemically exfoliated CrSBr were assessed by transmission electron microscopy (TEM) and presented in Figure 4.
Figure 4A shows the high-angle annular dark-field scanning TEM (STEM HAADF) image of a representative material with a ribbon-like structure, which is shown in the bright-field TEM image (Figure 4C) as well.It is worth noting that a more concentrated sample contains a large number of bundle-like nanostructures with uniform shapes and high aspect ratios, as illustrated on a representative bright-field TEM image (Figure 4B) and observed by optical microscopy.The STEM HAADF image (Figure 4D) represents a well-arranged lattice structure even after the exfoliation; the red lines show an atomic distance between two S/Br atoms of 0.476 nm along the b direction.This value corresponds closely with the b-cell parameter of 0.480 nm gained from XRD.This proves the b-direction is perpendicular to the long axis of the nanoribbon and cleavage occurred along the adirection leading to the formation of nanoribbons.The selected area electron diffraction (SAED) in Figure 4E clearly demonstrates the orthorhombic structure of the investigated sample with registered [020], [200], and [110] plane directions.The energy-dispersive X-ray (EDX) elemental mapping (Figure 4F) and corresponding EDX spectrum (Figure S3, Supporting Information) ensure the presence and homogeneous distribution of Cr, S, and Br elements within the CrSBr nanoribbons.
Wide-scan X-ray photoelectron spectra (XPS) were obtained to examine the surface composition.Figure 5 shows the survey spectrum (Figure 5A) along with the detailed high-resolution spectra of Cr, S, and Br of the electrochemically exfoliated CrSBr.The peak of oxygen originates from adsorbed species and possible surface oxidation.Furthermore, the wide-survey spectrum shows the surface is not contaminated with fluorine, which points out to the complete purification process from the electrolyte used for the exfoliation process.Analysis of the Cr 3d region (Figure 5B) revealed two spin-orbit components separated by 9.3 eV where each was deconvoluted into two peaks.The peak at 575.9 eV is attributed to Cr III originating from CrSBr, [30] while the peaks at 578.6 originate from loss features.The distinct feature in the S 2p (Figure 5C) was deconvoluted with a single pair of peaks corresponding to S 2p3/2 and S 2p1/2 at 161.9 and 163.1 eV, respectively.The Br 3d region (Figure 5D) shows a single state deconvoluted in two peaks corresponding solely to CrSBr that are located at 69.2 and 70.2 eV assigned to Br 3d5/2 and Br 3d3/2, respectively.Additionally, the surface is barely contaminated by nitrogen, N 1s region shown in Figure 5E can be treated as a single component positioned at 400.4 eV and assigned to the adsorbed acetonitrile molecules used as solvent.We further probed the surface composition of the intercalated CrSBr, Figure S4 (Supporting Information) shows the survey spectrum (Figure S4a, Supporting Information) along with the detailed high-resolution spectra of Cr, S, Br, and N. The peaks of carbon and oxygen originate from the salt of quaternary ammonium cation used for the intercalation and adsorbed species on the surface.The Cr 3d region (Figure S4b, Supporting Information) was deconvoluted into two peaks corresponding to CrIII and loss features at 575.1 and 584.5 eV, respectively.Analysis of the S 2p region (Figure S4c, Supporting Information) revealed three pairs of peaks corresponding to the following oxidation states: S −II from CrSBr and two oxidized states S IV and S VI with concentrations of 82%, 9.2%, and 8.8%, respectively.The S IV state is more likely to be found on the defect site in the basal plane, whereas the S VI state could be found on the edges.The Br 3d core line is displayed in Figure S4d (Supporting Information) shows a single state Br −I from CrSBr deconvoluted in two peaks assigned to Br  3d5/2 and Br 3d3/2 at 68.5 and 69.5 eV, respectively.The N 1s region is displayed in Figure S4e (Supporting Information).Compared with the exfoliated CrSBr, the intercalated material shows a novel peak with higher binding energy at 403.0 eV from quaternary ammonium TBA + accompanied by a peak with core level at 400.6 eV is corresponding to the adsorbed N atoms from the solvent. [31]dditionally, we performed atomic force microscopy (AFM) in order to evaluate the topology of the exfoliated CrSBr sample, which is predominantly cos4mposed of thin and long ribbon-like structures overlapping with each other, as shown in Figure 6A.The corresponding height profile in Figure 6B illustrates the single strand with a thickness of ≈4 nm attached to the additional nanoribbon exhibited a thickness of ≈2 nm, which corresponds to the five-and bi-layered nanoribbons, respectively.Interestingly, apart from the distinguishable few-layered nanostructures, we observed concealed CrSBr monolayers (Figure S5, Supporting Information).A height distribution histogram in Figure 6C indicates that the majority of the nanoribbons possess an average length of ≈2 μm and a ≤2 nm thickness, which corresponds to mono-and few-layered (mostly ≤3 layers) material.Additionally, there is a noticeable, proportional correlation between the lateral size and the thickness of the nanoribbons.
The magnetic susceptibility measurement of the bulk CrSBr confirmed the antiferromagnetic ordering with the Néel temperature T N = 132 K (Figure 7A) and the total spin per formula unit S = 1.46-3/2 evaluated from the slope of  −1 versus T linear dependence in the paramagnetic (PM) region.As the cation Cr 3+ -Cr 3+ separation between the edge-sharing octahedra is relatively large, 2.13 Å, the prevailing magnetic interaction within the layers is the 90°Cr-S-Cr or Cr-Br-Cr ferromagnetic (FM) coupling, as confirmed by DFT calculations yielding the higher energies of two different AFM arrangements within the layer (by 2 and 4 kJ mol −1 as compared to the FM arrangement).The overall magnetic ordering is thus of A-type with an antiferromagnetic (AFM) interaction between the layers.As mentioned, the bonding interaction between the layers is relatively weak, as manifested by a quite low (001) surface energy and zero energy difference between the FM and AFM structures as obtained from DFT calculations for the room temperature crystal structure.
By contrast, the expansion of the interlayer distance leads to an alteration of the electronic band structure of the intercalated material in comparison with bulk.The intercalated CrSBr exhibits a broad but clear FM transition with a Tc around 130 K.The spin value S≈0.64 per formula unit was obtained from the linear  −1 versus T. Similar phenomena was previously discussed by Wang et al. [32] for Cr 2 Ge 2 Te 6 intercalated with the TBA + cation, where the organic ion intercalation propagated electron-doping and altering of ground state with subsequent double-exchange ferromagnetism interaction.The transition to the FM state for the intercalated sample is also evident from the magnetization curve recorded at 5 K (Figure 7B) revealing a sharp saturation within the range 0-1 T with an additional slowly saturating component superimposed on the FM part.Nonetheless, the additional AFM/PM component exhibits similar behavior as the magnetization curve acquired at 140 K, corresponding to a PM phase just above the transition temperature.
It is important to note that the VSM measurement was carried out on a sample with a low mass, due to the challenges faced in separating exfoliated CrSBr.Additionally, the actual molar mass of the sample might have deviated from that of pure CrSBr, due to various moieties attached to the exfoliated layers.Despite these factors, the successful conversion from an AFM bulk CrSBr to FM intercalated CrSBr has been unambiguously demonstrated.

Photodetection Performance
Produced by means of electrochemical exfoliation, the mono-and few-layered CrSBr nanoribbons clearly possess the parent bulk orthorhombic crystallinity, optical anisotropy, and band-gap of ≈2.26 eV as explained in detail above.The CrSBr nanoribbons stabilized in the suspension are fundamentally suitable for the applications requiring the solution-based deposition of nanomaterials, e.g. the fabrication of various films and electrodes for energy storage and generation.Since the absorption spectroscopy measurement of CrSBr nanoribbons displayed their broadband absorption from the UV to IR region, we first investigated the photoresponse behavior of exfoliated CrSBr evaluating its photodetection performance.Figure 8A shows the I-V curve measured via linear sweep voltammograms (LSV) with a scanning rate of 10 mV s −1 under the chopped -on and -off illumination of a 420 nm LED light.Upon illumination, the photoanode demonstrates an increasing photocurrent with an onset potential of approximately 1 V versus SCE for oxygen evolution reaction, which is the reason behind selecting the potential of 0.7 V test the CrSBr-based photodetector.
We further investigated the performance of the electrochemically exfoliated CrSBr by power-dependent photocurrent measurements to determine its photodetecting capabilities.The chronoamperometry in 1 M KOH at 0.7 V versus SCE was performed with a continuous "on-off" switching under 420, 460, and 532 nm light irradiation changing the power from 200 to 1000 mW, the results of these measurements are presented in Figure 8B-D.PEC performance of electrochemically exfoliated CrSBr shows the highest photoresponse under 420 nm illumination achieving a maximum PEC value increasing from 9.5 to 26.8 μA cm −2 with an increase in power ranging from 200 to 1000 mW.Under different illumination wavelengths (blue  = 460 nm, green  = 532 nm) the results presented prominent and repeatable responses over identical cycles, thus, the photocurrent density gradually increased from 6.3 to 20.1 μA cm −2 under 460 nm LED illumination and from 1.3 to 4.6 μA cm −2 under 532 nm.Since the material shows the ultimate absorption in the violet-blue region, the results are in agreement with the aforementioned absorbance spectrum.
The value of responsivity (R) provides an estimation of PEC performance considering the generated photocurrent and irradiance power density: where R is the responsivity of the photodetector, ΔI is the difference between photocurrent under light illumination and dark current, and P is the irradiance power intensity per surface area of detector S. [32] Figure 8E shows the dependence of the photoresponsivity towards the irradiance power intensity (I-P) with different wavelengths under 200, 600 and 1000 mW light power illumination.Clearly, with increasing light intensity the responsivity decreases.The number of photo-generated carriers is proportional to the number of absorbed photons; thus, at low light intensity, the rate of recombination is reduced which results in higher responsivity. [31]Notably, the responsivity of the photodetector under 200 mW light power illumination shows nearly the same values under the 420 and 460 nm light illumination (≈200 and 187 μAW −1 , respectively) keeping that trend for higher power illumination.Therefore, the photons with high energy efficiently excite exfoliated CrSBr nanomaterial with the aforementioned band gap of ≈2.26 eV.The responsivity of the photodetector becomes gradually weaker when the wavelength of light is longer, which is reflected in the decreased responsivity value of ≈100 μAW −1 under 532 nm light illumination.The relationship between photocurrent density (I) and light intensity (P) follows the power law: where the exponent  is a factor determining the response of the photocurrent to light intensity.The absence of charge recombination and trapping processes led to a unity value for , while the larger the component defect density, the smaller the  value. [33,34]igure 8F represents the I-P power function where the power non-linear equation (dashed lines) fits to the experimental data (solid spheres) with correspondent exponent  between 0.77 and 0.92, which indicates efficient kinetics of the photo-generated carriers with negligible charge recombination and trapping processes, thus, it consequently reports the small defect density.The single-and few-layer nanoribbons CrSBr provide high electrochemically accessible surface area and ease the transfer between photogenerated charges and the catalytic surface area, which is favorable for PEC photodetector performance. [35,36]

CrSBr-Based Anode for Lithium-Ion Batteries
Exploring advanced anode materials beyond graphite has been a key subject in improving the overall energy density of lithiumion batteries (LIBs). [37]Compared to the theoretical capacity of graphite (372 mAh g −1 ), CrSBr can deliver a relatively higher capacity of 490 mAh g −1 through the following reaction: Thus, to examine the practical application of the exfoliated CrSBr, the material was investigated as an anode material in lithium-ion batteries.To prepare the free-standing electrode, exfoliated CrSBr was combined with 20 wt.% multi-walled carbon nanotubes (MWCNTs), which therefor was assembled in a 2032type coin cell.In order to assess the lithium storage process of CrSBr material, the CV measurements were conducted for both pure MWCNTs and CrSBr/MWCNTs electrodes.As shown in Figure 9A, three reduction peaks in the initial negative scan can be ascribed to the combination of lithium with Br (R1), the further reaction between lithium with sulfur (R2), and the formation of solid electrolyte interphase (SEI) (R3). [38]The SEI peak did not appear in the subsequent cycling process, indicating its stability, which can effectively prevent the electrode material from a side reaction with the electrolyte.Upon cycling, two reversible redox peaks (Redox 1 and Redox 2) and lithiation below 0.3 V dominate the following reaction processes.In addition, the Nyquist plots of CrSBr/MWCNTs electrodes before and after experiencing electrochemical activation (Figure 9B) also confirmed the formation of SEI, delivering two different equivalent diagrams (inset Figure 9B).
The galvanostatic discharge/charge analysis of exfoliated CrSBr in LIBs was evaluated further.Prior to conducting the longterm cycling measurements, the current density of 40 mA g −1 was applied to the battery for 3 cycles to perform a complete electrochemical reaction.Interestingly, the battery exhibits a selfimproving ability (Figure 10A).The value of a specific capacity gradually decreases from 390 mAh g −1 in the second cycle to 267 mAh g −1 after 32 cycles.Strikingly, the specific capacity after 200 cycles gradually increases to a high value of 397 mAh g −1 , along with a Coulombic efficiency (the ratio of charge capacity to discharge capacity) approaching 100%.The increase in capacity can be ascribed to the electrochemically driven phase transformation or the structural collapse. [39]igure 10B represents the performance of a pure MWCNTs electrode.Clearly, the pure electrode delivers a stable but relatively lower capacity, indicating the capacity contribution of the CrSBr/MWCNTs electrode mainly comes from the CrSBr.Furthermore, the electrode was tested by the rate performance tests, which results are pictured in Figure 10C with corresponding discharge/charge curves, shown in Figure 10D.While current densities were reaching 40, 80, 200, 400, 800, 2000, and 4000 mA g −1 , the average charge capacities of 516, 425, 321, 242, 194, 143, and 110 mAh g −1 were exhibited, respectively.Comparing that performance with the reported capability of graphite anodes, [40] CrSBr has promising potential to be used as the new anode for lithiumion batteries.

Conclusion
In summary, we have reported the electrochemical exfoliation of layered vdW crystal CrSBr, which triggered in-plane and outof-plane bulk delamination, disclosing numerous few-layered CrSBr nanoribbons.We have confirmed that based on a series of analyses, exfoliated CrSBr possesses the orthorhombic structure and optical anisotropy like its bulk counterpart, while intercalated CrSBr exhibits the ferromagnetic order with a Tc around 130 K.The analysis of PEC-type photodetector based on CrSBr nanoribbons revealed their high charge carrier mobility and extraordinary responsivity up to 30 μA cm −2 .In addition, the performance of exfoliated CrSBr in lithium-ion batteries showed its self-improving ability.We believe that, when stabilized as a dispersion, CrSBr nanoribbons will develop the study of that layered crystal and inspire the investigation of CrSBr-based highperformance digital devices.
CrSBr Synthesis: CrSBr bulk single crystal was synthesized by a chemical vapor transport method.Chromium, sulfur, and bromine elements with a stoichiometry of 1:1:1 were combined and sealed in a quartz tube under high vacuum.The precursors in ampoule were first reacted in crucible furnace at 700°C for 10 h keeping one end of ampoulecold to awoid formation of high internal pressure.Then the ampoule was placed into a two-zone tube furnace for crystal growth.First the source zone was keept at 700°C and growth zone were kept at 850 and 900°C, respectively.After 25 h, the temperature gradient was reversed, and the source zone temperature gradually increased from 880 to 930°C for 5 days period while keeping growth zone at 800°C.Finally for other 5 days was kept thermal gradient of 930°C for source zone and 800°C for growth zone.After cooling on room temperautre the high-quality CrSBr single crystals were removed from the ampule in an Ar glovebox.
Structural and Morphological Characterization: X-ray powder diffraction data were collected under the ambient conditions on Bruker D8 Discoverer (Bruker, Germany) powder diffractometer with parafocusing Bragg-Brentano geometry using CuK radiation ( = 0.15 418 nm, U = 40 kV, I = 40 mA).Data were scanned over the angular range 5-90°(2) with a step size of 0.019°(2).The acquired data were analyzed by using HighScore Plus 3.0 software.Raman measurements were performed by a confocal Raman microscope (WiTec alpha 300 R, WiTec GmbH, Ulm, Germany) equipped with a 20×, 50×, 100 × Zeiss EC Epiplan-Neofluar Dic objectives and polarizer, using a green laser (532 nm) at 1.5 mW laser power.The spectrum and optical images were collected using Project 6.0 software (WiTec GmbH).UV-vis spectroscopy measurements were performed using a LAMBDA 850+ UV-vis spectrophotometer (PerkinElmer, USA) inside an integrating sphere in a scan range between 250 and 800 nm.High-resolution X-ray photoelectron spectroscopy (XPS) was performed using an ESCAProbeP spectrometer (Omicron Nanotechnology Ltd, Germany) with a monochromatic aluminum X-ray radiation source (1486.7 eV).The samples were placed on a conductive carrier made from a high-purity silver bar.An electron gun was used to eliminate sample charging during measurement (1-5 V).The AFM measurements were carried out on an Ntegra Spectra from NT-MDT, and the sample suspension (1 mg mL −1 ) was dropcast on freshly cleaved mica substrate.The surface scans were performed in a tapping (semi-contact) mode.A cantilever with a strain constant of 1.5 kN m −1 equipped with a standard silicon tip with a curvature radius lower than 10 nm was used for all measurements.The magnetic susceptibility as a function of temperature (applied magnetic field 1 kOe) and the magnetic field dependence of magnetization (at a temperature 5 K) were measured on Physical Properties Measurement System (PPMS) EverCool-II (Quantum Design, USA) using the Vibrating Sample Magnetometry (VSM) option.
DFT calculations were performed using MedeA-VASP software by applying GGA+U functional (with on-site Coulomb repulsion potential U = 4 eV imposed on Cr-3d states) and considering AFM-bulk and FM-bulk.
STEM imaging was conducted with a probe-corrected Thermo Fisher Scientific Themis Z G3 60-200 kV S/TEM.The probe size of the aberrationcorrected electron beam (at 200 kV and 17 mrad convergence angle) is sub-Angstrom (0.6−1 Å).A collection semi-angle of 63-200 mrad was used for STEM-HAADF imaging.A beam energy of 200 kV and a beam current of 40 pA was used.The data were collected using Velox software (Thermo Fisher) with a frame size of 1024 × 1024 pixels and a dwell time of 500 ns/pixel.
To provide the image in Figure 4B image, TEM was carried out using the EFTEM Jeol 2200 FS microscope (Jeol, Japan).An acceleration voltage of 200 keV was used for the measurement.The exfoliated sample suspension was dropcast on a TEM grid (Cu, 200 mesh, Formvar/carbon) and dried at 60°C.
Electrochemical Intercalation and Exfoliation: The synthesized bulk crystal was fixed PTFE-based holder with a platinum plate and served as a working electrode together with the platinum plate of about 1 × 2 cm 2 size was used as the counter electrode and Ag/Ag + (0.01 m AgNO 3 in acetonitrile) as a reference electrode.The nonaqueous solution of TBAPF 6 (0.01 m) in DMSO was employed as the electrolyte.The whole procedure was carried out in an oxygen-free atmosphere by continuous argon purging into the electrochemical cell.The following three main procedures were carried out to intercalate and exfoliate CrSBr electrochemically: namely, 0 to −1 V with a scan rate of 0.01 mV s −1 and kept at −1 V for 2 min; −1 to −2 V with a scan rate 0.01 mV s −1 and kept at -2 V for 2 min; then −2 to −5 V with the same scan rate and kept at −5 V for 1 h.The obtained material was collected and sonicated for 15 min in an ice bath.Then, the material was centrifuged at 800 rpm for 10 min to separate non-exfoliated crystal.The collected supernatant was centrifuged at 8000 rpm for 15 min to remove the residual salt and separate the intercalated fibers and CrSBr nanoribbons.Taken supernatant and intercalated CrSBr fibers were separately washed with acetonitrile and centrifuged at 8000 rpm with subsequent redispersion of the precipitant in acetonitrile.The final suspension was purged with Ar for 5 min and then sonicated for 10 min in an ice bath and stored in the glovebox for the subsequent use.
Electrochemical Measurements: A glassy carbon electrode with a 3 mm active radius area was ultrasonically cleaned for 10 min using acetone, isopropanol, and deionized water for 10 min and then dried by nitrogen flow.To fabricate photoelectrode, the suspension of exfoliated CrSBr was sonicated for 5 min, and then 10 μL of the exfoliated sample was dropcast onto the glassy carbon surface and dried in a vacuum oven at 65°C for 1 h.Electrochemical measurements were carried out at room temperature in a three-electrode cell using an Autolab PGSTAT204 workstation (Utrecht, Netherlands, NOVA Version 2.1.4).Photoelectrochemical measurements were performed in an alkaline (1 M KOH) solution under an inert atmosphere at room temperature using a three-electrode configuration cell.The saturated calomel electrode (SCE) served as a reference electrode and Pt wire as the counter electrode.A glassy carbon electrode loaded with CrSBr nanoribbons was employed as the working electrode.The purple ( = 420 nm), blue ( = 460 nm), and green ( = 532 nm) LEDs were used as a light source, and the light intensity was controlled by a source meter-controlled LED driver.Linear sweep voltammograms (LSV) were measured at a scan rate of 50 mV s −1 .
Preparation of the Free-Standing CrSBr/MWCNT Electrode: The MWC-NTs were preliminarily purified and slightly oxidized by reaction with KMnO 4 in the presence of concentrated sulfuric acid.To obtain the target electrode for LIBs, at first, 0.01 mg MWCNTs was sonicated and dispersed for 1.5 h in acetonitrile (30 mL).Then, exfoliated CrSBr (0.09 mg) was introduced into the above dispersion with mild sonication for 30 min.This was followed by vacuum-filtration through a polyamide filter (0.45 μm pore size) and drying in vacuo.The as-obtained CrSBr/MWCNT hybrid film was punched into a diameter of 10 mm as an anode for LIBs.
Lithium-Ion Battery Assembly and Performance Characterization: The prepared electrode was assembled into a CR2032 coin cell utilizing lithium foil as a counter electrode and porous polypropylene membrane (Celgard 2400) as a separator.The fabrication of lithium-ion battery was carried out in an argon-filled glovebox.1 M LiPF 6 in a mixture of ethylene carbonate and dimethyl carbonate (1: 1 v/v) served as an electrolyte.The discharge/charge performance analysis was conducted on a Neware battery test system (Neware BTX 7.6, Shenzhen, China) in a fixed potential window from 0.001 to 3.0 V (vs Li + /Li).The cyclic voltammetry (CV) measurements and electrochemicalimpedance spectroscopy (EIS) measurements were performed using an Autolab PGSTAT204 (Eco Chemie, Utrecht, The Netherlands) workstation.

Figure 1 .
Figure 1.A) Experimental linear sweep voltammetry curve recorded during the electrochemical exfoliation of CrSBr.Optical images show the bulk crystal used for the electrode fabrication (right) and a dropcast exfoliated sample (left).B) The X-ray diffractogram of bulk (red) and electrochemically exfoliated (blue) CrSBr.

Figure 2 .
Figure 2. A) Schematic crystal structure of CrSBr along a-, b-, and c-directions.B) Raman spectra of bulk (red) and exfoliated CrSBr nanoribbons (blue, green) at 1.5 mW laser power and excitation energy of 2.33 eV, the atomic displacements of the three Raman active Ag modes are given in the inset.C) Polar plot of polarized Raman spectra ranging from 0°to 360°, where the angles of 0, 180, and 360 degrees correspond to the b-direction while 90 and 270 degrees correspond to the a-direction.The lobe fits the sine square function.D) Optical microscopy image of CrSBr nanoribbon on the golden substrate with marked area exposed for polar plot measurement.

Figure 3 .
Figure 3. A) Absorption spectra of electrochemically exfoliated CrSBr and respective Tauc plot for a direct transition bandgap, presented in the inset.The individual suspension of CrSBr supernatant was taken for the investigation.Individual suspensions of B) obtained material right after theelectrochemical performance, C) supernatant of exfoliated material taken after the centrifugation, D) bulky fibers of intercalated CrSBr.E,F) Optical microscopy images of dropcast on the silicon substrate exfoliated CrSBr at different magnifications.

Figure 4 .
Figure 4. TEM analysis of the CrSBr after electrochemical exfoliation in 0.01 M TBAPF 6 /DMSO: A) STEM-HAADF image of the individual nanoribbon.B) Representative bright-field TEM image of assembled nanoribbons.C) Bright-field TEM image and E) corresponding SAED pattern.D) STEM-HAADF image showing the bright S/Br atomic columns and the dimmer Cr columns in between.The two parallel red lines measure the distance between two S/Br atoms along the b-direction.F) High-angle annular dark-field scanning TEM image (left) and elemental mapping images of Cr, S, Br.

Figure 5 .
Figure 5. XPS survey spectrum A) and high-resolution spectra of the B) Cr 2p, C) S 2p, D) Br 3d states and N 1s states E) of exfoliated CrSBr.

Figure 7 .
Figure 7. A) Temperature dependence of magnetic susceptibility of the bulk (○) and intercalated (□) CrSBr.The reciprocal susceptibility revealing the linear behavior in the paramagnetic region above the critical temperature is shown in the inset.B) Magnetization curves (magnetic moment in Bohr magnetons versus the applied field) of the intercalated sample above Tc (T = 140 K, ∆) and below Tc (T = 5 K, □).

Figure 8 .
Figure 8. PEC photodetector performance.A) I-V curve measured via linear sweep voltammograms (LSV) with a scanning speed of 10 mV s −1 under the 10 s chopped-on and -off illumination of a 420 nm LED light.Chronoamperometry measurement of photocurrent density response under LED irradiation B) 420 nm C) 460 nm D) 532 nm at different irradiation power (200 to 1000 mW).E) The responsivity of photodetectors in 1 M KOH solution under 100 mW light power illumination as a function of power density upon  = 420, 460, 532 nm illumination wavelengths.F) I-P measurements illuminated by the 420 nm LED at different lasers at different irradiation power with plotted power law fitting.

Figure 10 .
Figure 10.Galvanostatic discharge/charge performance of A) CrSBr/MWCNTs and B) MWCNTs electrodes, C) rate performance at various current densities, and D) the corresponding discharge/charge curves.
university research A2 FCHT 2022 076 and A2 FCHT 2023 077.J.K. acknowledges support by the Alexander von Humboldt Foundation.A.S. was supported by project 956813 from Horizon call H2020-MSCA-ITN-2020. F.M.R. acknowladges funding from the U.S. Department of Energy, Office of Basic Energy Science, Division of Materials Sciences and Engineering under Award DE--SC0019336 for STEM characterization.This work was performed in part through the use of microscopy facilities at MIT nano.