Atomically Thin Two-Dimensional Nanosheets with Tunable Spin

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Atomically Thin Two-Dimensional Nanosheets with Tunable Spin-Crossover Properties Yang-Hui Luo, Chen Chen, Guo-Wei Lu, Danli Hong, XiaoTong He, Cong Wang, Jia-Ying Wang, and Bai-Wang Sun J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03298 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 5, 2018

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The Journal of Physical Chemistry Letters

Atomically Thin Two-Dimensional Nanosheets with Tunable Spin-Crossover Properties

Yang-Hui Luo, *,1 Chen Chen,1 Guo-Wei Lu,2 Dan-Li Hong,1 Xiao-Tong He,1 Cong Wang,1 Jia-Ying Wang,1 and Bai-Wang Sun*,1

1 School

of Chemistry and Chemical Engineering, Southeast University,

Nanjing, 211189, P.R. China. E-mail: [email protected] (LYH); [email protected] (SBW). 2

Institute of Innovative Science and Technology, Tokai University, Kanagawa 259-1292, Japan.

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Combining the fascinating advantages of ultrathin two-dimensional (2-D) nanosheets with the nano-structuration of spin-crossover (SCO) materials, represent the attractive target of controlled fabrication of SCO nano-objects at the device level. Here, we demonstrate that through a facile-operating ultrasonic force-assisted liquid exfoliation technology, the three-dimensional (3D) van der Waals SCO bulk precursors {[Fe(1,3-bpp)2(NCS)2]2 (1, 1, 3-bpp = 1, 3-di(4-pyridyl)-propane)} can be exfoliated into single-layered 2-D nanosheets (NS-1). As a consequence, the magnetisms have been tuned from complete paramagnetic (bulk precursors) to SCO transition at around 250 K (2-D nanosheets). In addition, the metal-to-ligand charge transition (MLCT), the intra-ligand π - π* transition and the color display also have been altered both in colloidal suspension and in solid-state. These dramatical change of physicalchemical properties at different forms and states can be attributed to the efficient cooperativity derived from the interlayer van der Waals interactions within the curly or vertically stacked 2-D building blocks.

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Spin-crossover (SCO), which refers to the interconversion between high-spin (HS) and low-spin (LS) electronic configurations of 3d4-3d7 transition-metal ions,1 belongs to the most promising molecular switching species that can be developed into technology devices and have enjoyed continuous attractiveness in the past two decades.2 The SCO is rationalized by the ligand-field theory and triggered by a large panel of stimuli (e.g., temperature, light, irradiation, press, guest molecules, electric/magnetic fields),3 which was accompanied by several physical/chemical changes between (at least) two (meta) stable states, including magnetic, optical, dielectric and mechanical properties, thus providing the potential to be applicated in such field as memories, data storage, sensors, switches, displays, molecule spintronics, and so on.4 From the microscopic viewpoint of crystalline structures, the SCO manifests dramatic changes in coordination bond distances and unit cell volumes, which can be propagated cooperatively in the solid state via elastic interactions from one center to another.5 As a consequence, the nature of SCO (gradual, abrupt, stepwise and/or hysteretic) is dominated by the crystalline cooperativity.6 To be practically applicated, the materials should possess strong SCO cooperativity under ambient conditions and displays large thermal hysteresis loop spanning room temperature.7 Much efforts then have been performed on controlling the characteristic temperature and nature of SCO.8 At present, two kinds of approaches occupied the mainstream: one is the molecular design and crystal engineering through the introduce of effective intermolecular interactions (e.g., 3



π…π interactions, host-guest interactions, hydrogen bonding, pH) and/or covalent linker;9 the other is nano-structuring and processing of SCO materials into nanostructures with various sizes and morphologies (e.g., monodisperse nanoparticles, thin films, micro- and nano-patterned media, hysteretic softmedia assemblies, Langmuir-Blodgett films).10 Among them, the latter approach is of ultra-most importance owing to the possible implementation of nanometric scale SCO materials into technology devices.11 On considering the fact that reduction of particle size and film thickness, loss of 3D (threedimensional) structural order, could affect profoundly the physical-chemical properties,12, 13 a precise control of the size, thickness and dimensionality of SCO nanostructures with tunable properties is therefore highly desired. It should be notated that, the nano-structuring of SCO materials into 0-D (zero-dimensional), 1-D (one-dimensional) and 3-D nanostructures have been investigated extensively,10 however, the intrinsic 2-D (two-dimensional) SCO materials, especially those composed of ultrathin 2-D nanosheets building blocks, remain uninvestigated. In general, the ultrathin 2-D nanosheets possess unique characteristics and functionalities such as adjustable structure and functionality, highly ordered pore arrays in plane, highly accessible active sites on their large surface, highly surface sensitive, and so on.14 Which thus provide a versatile platform for researchers to investigate the influences of 2-D morphologies and/or assemble of van der Waals heterostructures (vdWH,15 which refers to the stacking of ultrathin 2-D nanosheets building blocks by vdW 4


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interactions) on the intrinsic properties of their bulk counterparts. As a consequence, the correlation between morphologies and functionalities can be established, which is indispensable for the controlled fabrication and manipulation of the nanoscale SCO materials at the device level. Based on the above discussions, combining the fascinating advantages of ultrathin 2-D nanosheets with the nano-structuration of SCO materials may represent the attractive target of controlled fabrication and manipulation of the nano-objects at the device level with desired SCO properties. Indeed, deposition of SCO molecules assembled nanoparticles on the surface of graphene through chemical vapor deposition technology have been investigated, where, the spin state switching can be detected by transport measurements, attributing to the interactions between SCO nanoparticles and 2-D graphene nanosheets.16 However, these interactions were relatively weak because of long van der Waals distances. In this regard, the formation of SCO nanosheets with the central ion in close proximity to the 2-D surface would be expected to make a significant cooperativity and provide more sites for tunable properties. Hence in this work, we investigated the exfoliation of a 3-D SCO bulk precursors [Fe(1,3-bpp)2(NCS)2]2 (1, 1, 3-bpp = 1, 3-di(4-pyridyl)-propane), which was prepared in our previously work

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and composed of 2-fold

interpenetrated layer structures, into ultrathin 2-D nanosheets via a facileoperating ultrasonic force-assisted liquid exfoliation technology. As we have expected, the single-layered 2-D nanosheets (NS-1), with thickness of sub-2.0 5



nm, have obtained in high-yield. Interestingly, this kind of 2-D MOF nanosheets can provide a stable colloidal suspension in all kinds of solvent and maintained for at least one month without any sedimentation under ambient condition, demonstrating the highly stability and excellent dispersion. More importantly, the solid-state samples of NS-1, which were obtained via Freeze drying the aqueous colloidal suspension, have shown completely different magnetism from their bulk counterparts. Where, below room-temperature complete SCO transition was observed at around 250 K. Compared with the absolute highspin (HS) state characteristic of the bulk precursors 1, the promising SCO properties of NS-1 can be attributed to the efficient cooperativity derived from the interlayer van der Waals interactions within the curly or vertically stacked 2D building blocks. To the best of our knowledge, this is the first report of exfoliation 3-D van der Waals SCO materials into single-layered 2D nanosheets with promising SCO properties. The bright-yellow coloured van der Waals materials of 3-D bulk precursors 1 was initially prepared in our previously work.17 Single-crystal X-ray diffractions revealed that the 3-D framework of 1 was composed by 2-fold interpenetrated layered 2-D crystalline structure via inter-layer van der Waals interactions, which thus shown the potential to be exfoliated into single-layered 2-D nanosheets. Within the 2-D layered structure, each Fe ion is six-coordinated by four 1, 3-bpp ligands and two axial thiocyanate ligands through the terminal nitrogen atoms (Figure 1a and Figure S1), while each 1,3-bpp ligand is 6


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Figure 1. Crystal structure of layered bulk precursor 1: (a) The connectivity style for a (4, 4) Fe(1,3-bpp)2 rhombic-grid unit; (b) The topological and (c) side views of the 2-fold interpenetrated 2-D sheets; (d) The stacking style of the 2D layers into 3-D framework; and (e) SEM image of bulk precursor 1. connected by two Fe metallic nodes with a bis-monodendate connectivity, forming a distorted [2 x (4, 4)] rhombic grids network. Note that, within the 2-D layered structure, the structurally equivalent rhombic grids networks were interwoven in pairs (Figure 1b), generating a 2-fold interpenetrated 2-D 7



nanosheets with rhombus one-dimensional (1-D) channels paralleled strength along the a axis (Figure 1c and Figure S2). Where, the abundant sulfur atoms with high electronegativity was arrayed on the double surfaces of the sheets (Figure S2-S3). In the crystal, the 2-fold interpenetrated 2-D nanosheets were further stacked by an …ABAB… fashion along the (002) crystallographic direction, giving rise to a 3-D porous framework (Figure 1d). Notably, the thickness of a single layer is about 1.0 nm (Figure 1c), and the interlayer interactions were dominated by the weak C–H... S interactions, generating a loose 3-D framework with crystal voids of 1282 Å3 (Figure S4). Thus, disintegrate of the bulk precursors of 1 into ultrathin 2-D nanosheets via a facile top-down exfoliation strategy can be expected.18 What’s more, the SEM (scanning electron microscopy) images of the bulk precursors have demonstrated the lamellar structures, where, the 2-D layers were stacked together via weak van der Waals contacts into massive 3-D morphologies (Figure 1e and Figure S5), indicating again the highly potential to be exfoliated into single-layered 2-D nanosheets with large lateral area. Through an ultrasonic force-assisted liquid exfoliation method in the aqueous solution (please see the Experimental Section for detail in Supporting Information section), for the first time, the ultrathin 2-D nanosheets of SCO materials have been prepared. Compared with the bright-yellow colored crystalline bulk precursors 1, the exfoliated ultrathin 2-D nanosheets NS-1 have displayed tan colors (Figure 2a), demonstrating the profound effect of 8


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Figure 2. (a) Comparison between the optical image of bulk precursors 1 and 2-D nanosheets NS-1; (b) The Tyndall effect of the aqueous colloidal suspension; (c) the TEM image and (d) SEM images of NS-1. disintegration from packed 3-D motif to unfettered 2-D nanosheets on the electronic configuration of the SCO centers. In the aqueous solution, NS-1 can be well dispersed to form a colorless colloidal suspension with significant Tyndall effects (Figure 2b). Note that, these aqueous suspensions can stand for at least one month without any sedimentation under ambient condition, suggesting the high stability of this kind ultrathin 2D nanosheets. One thing should be stressed that: compared with the solid-state materials, the significant color change in solution-state can be attribute to the solidly absorption of water 9



molecules to the surfaces of nanosheets through the robust O-H…S hydrogen bonding contacts or suction of water molecules into the 1-D channels, which then have altered the electronic configuration of the SCO centers within the 2D nanosheets, as have been revealed by UV-vis absorption spectroscopy and thermal analysis that discussed below. The successful exfoliation of ultrathin nanosheets NS-1 have been further proved by the measurements of SEM, transmission electron microscopy (TEM), atom force microscopy (AFM), thermos-gravimetric analysis (TGA), powder Xray diffraction (PXRD). Shown in Figure 2c and 2d were the TEM and SEM images of NS-1, where, ultrathin 2-D nanosheets with uniformly size distribution and large lateral sizes have been observed. In addition, these nanosheets were stacked randomly into flower-like clusters with each nanosheet act as a petal that exhibited unambiguous outlines and somewhat curling edges, suggesting the highly flexible and easily stacking of these ultrathin 2D nanosheets upon gradual removal of media solvents on the porous TEM grid at room temperature. The morphologies of NS-1 were further revealed by AFM topological image, where, rectangle-like single-layered nanosheets with lateral size about 400 nm was observed (Figure 3a), where, the highlight regions correspond to the stacked nanosheets. These results were verified by the height profile (Figure 3b), where, the thickness of about 1.5 nm has been observed for the nonstacked region, while thickness of about 4.0 nm for the stacked region. On considering the theoretical height about 1.0 nm for the single layer in the X-ray 10


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structure model of 1, the non-stacked region belongs to single-layered 2-D nanosheets, while the stacked region corresponds to 3-4 layered stacking. This phenomenon demonstrated clearly the exfoliation into single-layered 2-D nanosheets in aqueous solution.

Figure 3. (a) Comparison between the AFM topological image of NS-1; (b) The height profile along the white lines for different regions; Comparison of (c) PXRD patterns and (d) TGA profiles between 1 and NS-1. What’s more, the PXRD patterns of the 2-D nanosheets revealed the complete disappearance of the (002) crystallographic planes (Figure 3c), indicating the layered-by-layered exfoliation mechanism from the bulk

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precursors to single-layered nanosheets. It should be noted that, the singlelayered nanosheets were re-stacked into crystalline van der Waals materials upon removal of aqueous solution during the freezing drying process, the obtained van der Waals materials have shown good crystallinity but completely different from that of bulk species. Meanwhile, despite the re-stacking, the water molecules were still absorbed on the surfaces of nanosheets, as has been demonstrated by the ≈ 4.5 % mass-loss of water molecules in temperature range 120-170 ºC upon heating the solid-state samples of NS-1(Figure 3d), demonstrating the ultrahigh surface adsorption ability. All these aforementioned results have demonstrated the successfully exfoliation of single-layered 2-D nanosheets NS-1 through the facile ultrasonic force-assisted liquid exfoliation technology. To demonstrated the exfoliation process has no damage of the 2-fold interpenetrated layered 2-D crystalline structure, IR and Raman spectroscopy as well as TEM/electron diffraction were performed. Where, 1 and NS-1 have shown almost the identical spectrum both for IR and Raman except for the peak at around 1250 cm-1 (Figure 4a and 4b), which can be attribute to the loos of 3-D structural order for the 2-D nanosheets species. In addition, the selectedarea electron diffraction images have shown annularity diffraction spots (Figure 4c and 4d), indicating the high crystallinity of the present 2-D nanosheets with highly centrosymmetric lattice that is in accordance with the bulk species.

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These results demonstrate directly the integrity of the 2-fold interpenetrated layer structures down to single-layer limit.

Figure 4. Comparison between the IR (a) and Raman spectroscopy (b) for 1 and NS-1; (c) TEM images and (d) the corresponding selected-area electron diffraction patterns for NS-1. On considering the highly surfaces sensitive of ultrathin 2-D nanosheets, NS1 is expected to shows interesting SCO properties that completely different from the bulk species. To verify the aforementioned speculation, temperaturedependent UV-vis absorption spectra measurements, both in solution and in solid-state, were carried out and compared with that of bulk precursors. As 13



shown in Figure 5a, at the room temperature (25 ºC), the solid-state bulk precursors 1 shows absorption band at around 410, 520, and 850 nm, which can be ascribed to the intra-ligand π - π* transition, metal-to-ligand charge transition (MLCT) from the d(Fe) + π (SCN) to π*(1, 3-bpp), and d-d transition of the HS state Fe(II) ions, respectively.18 While for the 2-D nanosheets NS-1, the intra-ligand π - π* transition absorption band is preserved, demonstrating again the structural integrity of nanosheets. However, the d-d transition absorption band has disappeared and MLCT absorption band has blue-shifted to around 480 nm with great increase in intensity, demonstrating the profound effect of dimensionality change on the solid-state MLCT properties. As a result, the change of electronic state for the center Fe(II) ions can be expected. More importantly, upon raising the temperature from 2o to 55 ºC, the intensity of MLCT absorption band underwent a decrease accompanied by a red-shift from 480 to 520 nm, which can be interpreted as the thermal induced distortion of the single-layered 2-D nanosheets. To further confirm the profound effects of dimensionality changing on the electronic state of 2-D nanosheets, the variable UV-vis absorption spectra of the aqueous colloidal suspension have been recorded. Results revealed the presence of intra-ligand π - π* transition (200-225 nm) and MLCT bands (252 nm) (Figure S6). Note that, the π - π* transition bands have split into two bands, which can be attributed to the adsorption of solvent molecules on the both sides of single-layered nanosheets that have affected the molecular orbital of 14


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Figure 5. Temperature-dependent UV-vis absorption spectra of 2-D MOF nanosheets NS-1 in the solid-state (a) and in aqueous solution (b-d, c and d are the enlargement of spectral change), the data of bulk precursors were presented for comparison. 1, 3-bpp.19 Upon heating, the intensity of the π - π* transition band at 202 nm has increased with a slight red-shift, while the bands around 220 nm decreased in intensity without shift (Figure 5b and 5c). This phenomenon indicates that the bands around 202 nm belongs to the solvent mediated π - π* transition, which is strengthened with the increasing of temperature, as a consequence, the nonsolvent mediated π - π* transition band has weakened. Meanwhile, the MLCT 15



bands also decrease with increasing of temperature, with a relative slight extent with that of π - π* transition (Figure 5b and 5d). These spectral changes have suggested the highly sensitive character of the interaction between adsorbed solvent molecules and single-layered 2-D nanosheets. Based on the above results, interesting magnetisms properties of NS-1 can be expected. Above-room temperature magnetic susceptibilities of solid-state samples NS-1 was recorded in temperature range 320-5K (Figure 6). From 320 K to 278 K, the χT values were nearly keep a constant of 6.01 cm3 mol-1 K, which was in good agreement with the theoretical value (6.00 cm3 mol−1 K) expected for two isolated typical HS Fe(II) ion (S=2 and g=2.0), and match well with that of bulk precursors. Upon cooling, the χT value decrease sharply from 270 K to reach a micro-step at 245 K with a value of 3.120 cm3 mol-1 K, corresponding to about half HS to LS transition to give a mixed 1:1 HS-LS state. Upon further cooling, this 1:1 HS-LS mixed spin state was decrease gradually to reach a plateau at 188 K with χT value of 1.75 cm3 mol-1 K. This value was then kept un-changed down to 25 K, after that, the χT value decreased gradually to 1.18 cm3 mol-1 K at 5 K. Note that, the gradual decrease of the χmT value below 25 K can be attribute to the intermolecular antiferromagnetic interactions and/or the contribution of spin orbit coupling of the Fe(II) ion. The aforementioned phenomenon has demonstrated clearly the SCO transition of NS-1, which was further confirmed by the optical image of the nanosheets samples at variable temperatures, where, the color of nanosheets samples can 16


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be changed reversibly from bright yellow at room-temperature to brown at liquid nitrogen temperature (Figure 6 insert), demonstrating the reversibly SCO transition.

Figure 6. Comparison between the temperature-dependent magnetic susceptibilities of 1 and NS-1, the color change at different spin-state have been highlighted. It should be stressed that, compared with the magnetic properties of bulk precursors 1, the spin-state transition for the solid-state samples of NS-1 can be attribute to the ultrahigh surface sensitivity of single-layered 2-D nanosheets. Upon removal of the media solvents, the single-layered 2-D nanosheets can acting as building blocks that assemble into curly or vertically stacked heterostructures by using interlayer van der Waals interactions. Note that, these stacked heterostructures have shown different curly or vertically packing 17



models, as a consequence, different degrees of van der Waals interactions were formed. These van der Waals interactions, although weak in nature, affect dramatically the spin-state and magnetic properties of 2-D nanosheets. And Finally, the moderate ligand-field strength and cooperativity have been generated within NS-1, which thus induced considerable SCO transition. In summary, through an ultrasonic force-assisted liquid exfoliation method, for the first time, the single-layered 2-D nanosheets from 3-D SCO van der Waals MOF, have been prepared with thickness of sub-2.0 nm. These 2-D nanosheets have shown considerable SCO transition at around 250 K. Compared with the intrinsic HS state characteristic of their bulk counterparts, the interlayer van der Waals interactions provide the 2-D nanosheets with moderate degree of cooperativity for on-demand SCO transition, which thus promote the fabrication of novel SCO device architectures. Therefore, this work offers a new design strategy to tuning the SCO properties of van der Waals materials, and may paves the way for the rational adjusting the intrinsic characteristics of all kinds of van der Waals MOF materials.

Supporting Information Experimental details, additional crystal structures, SEM image, charge distribution and crystal voids of bulk precursors 1, additional UV-Vis absorption properties of NS-1.

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ACKNOWLEDGMENT This research was supported by the Natural Science Foundation of China (Grant No. 21701023), Natural Science Foundation of Jiangsu Province (Grant No. BK20170660), Fundamental Research Funds for the Central Universities (No.3207048427) and PAPD of Jiangsu Higher Education Institutions.

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