Two-dimensional Acetate-based Light Lanthanide Fluoride

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Two-Dimensional Acetate-Based Light Lanthanide Fluoride Nanomaterials (F-Ln, Ln=La, Ce, Pr and Nd): Morphology, Structure, Growth Mechanism and Stability Leitao Zhang, Weimin Kang, Qiang Ma, Yingfang Xie, Yunling Jia, Nanping Deng, Yuzhong Zhang, Jing Ju, and Bowen Cheng J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b05355 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on July 30, 2019

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Journal of the American Chemical Society

Two-Dimensional Acetate-Based Light Lanthanide Fluoride Nanomaterials (F-Ln, Ln=La, Ce, Pr and Nd): Morphology, Structure, Growth Mechanism and Stability Leitao Zhang,† Weimin Kang,† Qiang Ma,† Yingfang Xie,† Yunling Jia,‡ Nanping Deng, † Yuzhong Zhang,*,† Jing Ju,*,‡ Bowen Cheng*,† † State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, China. ‡ Beijing National Laboratory for Molecular Sciences (BNLMS), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. ABSTRACT: To discover novel two-dimensional (2D) materials is of fundamental importance but remains challenging. In this work, we design a simple and facile bottom-up approach to fabricate a new family of two-dimensional acetate-based light lanthanide fluoride nanomaterials (F-Ln, Ln=La, Ce, Pr, Nd) at room temperature and atmosphere pressure, for the first time. Various characterization techniques confirm that as-synthesized F-Ln exhibit an ultrathin morphology with thickness up to 1.45 nm and lateral dimensions up to several hundred nanometers. Microstructure analysis demonstrates that F-Ln are a series of defect-rich 2D nanomaterials, which are consisted of nanocrystals with sub-10 nm domains. Structure characterization of F-Ce, a typical example, infers that BN-like F-Ce one-atom-layers sandwiched by intercalated acetate anions stack alternately along [001] direction to form nanocrystal building blocks of F-Ce. The study of growth mechanism suggests that three procedures are involved in the formation of F-Ce: hydrolysis reaction of cerium (III) acetate, structure transformation induced by fluorine ions and assembly process guided by acetate anions. The as-prepared nanosheets show excellent stability with respect to environment stimulis such as air, heat, solvent and high-energy electron beam. This study enriches the library of 2D materials and paves the way for future application of such 2D materials in areas such as catalysis, adsorption, separation and energy storage/conversion.

INTRODUCTION The past decades have witnessed much advance in twodimensional materials that start with the first demonstration of the extraordinary properties of graphene in 2004,1 and have been extended to other 2D materials, such as transition metal dichalcogenides, metal oxides / carbides / nitrides, metalorganic frameworks, polymers, perovskite and so on.2-5 The promise of 2D materials is largely based on their unique advantages as material platforms to explore fundamental chemistry and physics at the limit of single- or few-atom thickness and their huge potential in creating new technological opportunities beyond the reach of existing materials.6 This excellent prospect in turn stimulates keen interest in developing novel 2D materials to enrich the library. The fluoride host materials, especially light lanthanide fluorides (Ln= La, Ce, Pr and Nd), which feature low vibrational energies and long-lived intermediate energy levels,7 have captured substantial attention and shown a wide range of applications in optics, bioimaging, energy, catalysis and tribology.8,9 Although diverse synthetic protocols have been developed to fabricate nanostructured light lanthanide fluorides with controllable morphology including zerodimensional (0D) nanocrystals,10 one-dimensional (1D) nanowires/rods11 and three-dimensional (3D) assemblies,12 little focus is on the synthesis and application of 2D light lanthanide fluoride with ultrathin morphology.13,14

Generally speaking, the discovery of a new family of 2D crystals usually depends on known structural prototypes.15,16 To develop 2D light lanthanide fluorides nanomaterials, it is imperative to have a close inspection of the crystal structure of fluoride host materials. Take Ce-based fluoride host materials as an example, the possibility for the preparation of 2D ceriumbased fluoride nanomaterials is explored. As illustrated in figure 1a, bulk CeF3 belongs to the space group P-3c1 (a=b= 7.126Å, c=7.225Å, α=β=90°, γ=120°) and there exists six cerium atoms and eighteen fluorine atoms in one unit-cell (C6F18). All the cerium atoms are located in same structural sites and every three atoms are in a same plane to construct a one-cerium-atom layer (light blue rectangle in figure 1a). Eighteen fluorine atoms can be classified into three kinds: twelve atoms (dark green, F1) lie in the interlayers of onecerium-atom layers; four atoms (orange, F2) deviate slightly from the one-cerium-atom layer; two atoms (pink, F3) are in the plane of one-cerium-atom layer. Therefore, one-unit-cell chemical formula of CeF3 can be expressed as {Ce6 (F3)2 (F2)4} (F1)12 and further simplified as (CeF) F2, which accounts for two thirds of fluorine atoms intercalated within the gaps between one-cerium-atom layers. Bastnasite (CeFCO3) has the largest proven reserve among all rare earth minerals in the world,17 which adopts the space group P-62c18 (a=b=7.1438Å, c=9.808Å, α=β=90°, γ=120°) and one-unitcell composition is Ce6F6C6O18 (figure 1b). Similarly, three in six cerium atoms make up a one-cerium-atom layer (light blue

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rectangle in figure 1b), while four fluorine atoms (orange, F2) lie in the slightly deviated sites of one-cerium-atom plane and two fluorine atoms (pink, F3) appear in the one-cerium-atom layer. Likewise, the one-unit-cell formula of bastnasite can be noted as {Ce6 (F3)2 (F2)4} (CO32-)6 and simplified as (CeF) CO3, indicating the inserted carbonate within the interlayers between one-cerium-atom layers. Moreover, if these cerium atoms in CeF3 or CeFCO3 are bonded with F2 and F3, a BNlike F-Ce one-atom-layer with alternate arrangement of F and Ce atoms can be observed along the [001] axis (figure 1c). Accordingly, if F1 in CeF3 or CO32- in CeFCO3 were replaced by some chain-like monodentate ligands, such as acetate anions (Ac), it would have a chance to expand the interlayer spacing and create novel 2D materials with a unique structure where F-Ce one-atom-layers sandwiched by chain-like monodentate ligands might be freestanding or stack alternately along a specific direction. Such 2D material shares particular structure similarity with surface functionalized graphene oxide by long-chain molecules19 or 2D organic-inorganic perovskite which features a well-aligned inorganic slabs sandwiched by intercalated bulky organic ions.20,21

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prepared nanosheets with respect to environment stimulis such as air, heat, solvent and high-energy electron beam is claimed experimentally.

RESULTS AND DISCUSSION Morphology characterization In this study, 2D acetate-based light lanthanide fluoride (FLn, Ln=La, Ce, Pr, Nd) nanomaterials could be synthesized at room temperature and atmosphere pressure by introducing fluorine ions into the pre-hydrolyzed aqueous solution of light lanthanide metal-acetate salts. Fluorine ions are introduced in two ways: drop-by-drop addition of sodium fluoride (NaF) aqueous solution and chemical release from potassium fluoborate (KBF4). During the drop-by-drop addition of NaF aqueous solution, fluorine ions can be released into water from NaF droplets at the highest rate. The high release rate of fluorine ions results in the formation of disc-like nanosheets, which are named as H-F-Ln. In chemical release process, fluorine ions are derived from the hydrolysis reaction of KBF4. The equilibrium constant of KBF4 in water is only 2.5 × 10-10, implying that its hydrolysis reaction is a rather slow process, i.e., the release rate of fluorine ions is extremely low.22 The low release rate keeps fluorine ions in an ultralow concentration, which may be helpful to the nucleation and growth of trigonal or hexagonal nanosheets denoted as L-F-Ln.

Figure 1. The ball-and-stack structure model of CeF3 and CeFCO3. (a) Structure model of CeF3 with space group P-3c1; (b) Structure model of CeFCO3 with space group P-62c; (c) Structure model of a BN-like F-Ce one-atom-layer, which can be gained by cleaving surface along [001] direction of CeF3 or CeFCO3 model.

Herein, inspired by the above idea, we report a facile and scalable bottom-up approach to a new family of 2D acetatebased light lanthanide fluoride nanomaterials (F-Ln, Ln=La, Ce, Pr, Nd) at room temperature and atmosphere pressure. With the presence of light lanthanide metal, 2D unary, binary, trinary metal and even metal-doped fluoride nanosheets can be synthesized. The as-synthesized F-Ln exhibit an ultrathin morphology with thickness up to 1.45 nm and lateral dimensions up to several hundred nanometers. Microstructure analysis infers that F-Ln are a series of defect-rich 2D nanomaterials, which are consisted of nanocrystals with sub-10 nm domains. The structure characterization demonstrates that nanocrystal building blocks of F-Ce adopt a unique structure where BN-like F-Ce one-atom-layers sandwiched by acetate anions alternately stack along [001] direction. A plausible growth mechanism for F-Ln is proposed by tracking the formation process of F-Ce. At last, the excellent stability of as-

Figure 2. Morphology characterization of H-F-Ln. (a) SEM image, (b) low-magnification and (c) representative cross-section TEM image of H-F-Ce; (d) SEM image, (e) low-magnification, (f) AFM image and corresponding line-profiles height of H-F-Pr; (g) SEM, (h) TEM image and (i) cross-section TEM image of HF-Ce0.5Nd0.5.

As a typical example, H-F-Ce with disc-like morphology were successfully synthesized with dropwise addition of NaF aqueous solution into the mixed Ce3+-Ac aqueous solution. As revealed by SEM image (figure 2a), TEM image (figure 2b) and scanning electron microscopy/energy dispersive X ray spectrometry (SEM/EDS) mapping in figure S2 and EDS lineprofile intensity in figure S3. Based on the same synthetic procedure, H-F-La (figure S1a-S1b), H-F-Pr (figure 2d-2f and figure S4), H-F-Nd (figure S1c-S1d) could be obtained by means of the replacement of cerous (Ce3+) with trivalent


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lanthanum (La3+), praseodymium (Pr3+) and neodymium (Nd3+) cations, respectively. As well, this methodology is also suitable for the preparation of binary, trinary metal fluoride nanosheets through the arbitrary combination of multiple light lanthanide ions (see the experimental section), such as H-FCe0.5Nd0.5 (figure 2g-2h), H-F-Ce0.3Pr0.7 (figure S1e-S1f) and H-F-La0.4Pr0.3Nd0.3 (figure S1g and S1h). Besides the light lanthanide elements, 2D nanosheets containing other lanthanide elements could be fabricated via elemental doping strategy. For instance, the preparation of Sm/Er-doped H-FCe (figure S5 and S6) can be achieved by virtue of doping Sm3+/Er3+ into the Ce3+―Ac aqueous solution (see the experimental section). Unfortunately, the proposed strategy cannot be used for the fabrication of other unary metal 2D fluoride nanosheets except light lanthanide elements, which may be associated with the atomic configuration difference of referenced fluorides where light lanthanide trifluoride adopts the space group P-3c1 while other metal fluoride shares the space group FM-3M, PNMA or others. The lateral size and thickness distribution of H-F-Ln (Ln=La, Ce, Pr, Nd) are measured through TEM and AFM techniques. The Gaussian fitting results with at least 100 nanosheets in TEM figures (figure S7) exhibit that the lateral size is 85.12 ± 13.92 nm for H-F-La, 119.36 nm ± 30.82 nm for H-F-Ce, 244.69 ± 60.74 nm for H-F-Pr and 310.56 ± 204.11 nm for H-F-Nd. The lateral size distribution of 2D nanosheets exhibits a negative correlation with ionic radius (0.103 nm for La3+, 0.102 nm for Ce3+, 0.099 nm for Pr3+ and 0.098 nm for Nd3+), which may result from that the decreased ionic radius is conducive to the enhanced electrostatic attraction between lanthanide cations with fluorine ions in preparation process. According to the cross-section TEM images, the thickness of H-F-Ce (figure 3c) and H-F-Ce0.5Nd0.5 (figure 3i) is calculated to be ca.1.45 nm and 1.48 nm, respectively. Tapping-mode AFM image and corresponding line-profile height of resultant H-F-Pr deposited on a mica substrate confirms its ultrathin morphology with an average thickness of ca. 1.6 nm, as depicted in figure 2f. The reason for higher height measured by AFM than cross-section TEM is that acetate anions anchored on the surface of H-F-Ln is detected in AFM tests but it is omitted in TEM technique due to excessively low massthickness contrast.23 The successful synthesis of L-F-Ln was achieved by chemically releasing fluorine ions from KBF4 into the mixed Ln3+-Ac aqueous solution. Take L-F-Ce as an example, after stirring for 24h at 30°C, the aqueous solution containing KBF4 and Ce(Ac)3 chemicals became turbid and exhibited obvious “Tyndall effect”, indicating the occurrence of nanomaterials in aqueous solution. TEM (figure 3a) and HAADF (figure 3b) characterizations confirm the successful fabrication of L-F-Ce with trigonal or hexagonal sheet-like morphology. The sideview TEM image of L-F-Ce sample in figure 3c demonstrates its ultrathin morphology with thickness up to ~ 1.45nm. With use of the same synthesis protocol, a variety of 2D acetatebased light lanthanide fluoride nanosheets, including unary and binary metal nanosheets, could be synthesized through the replacement of Ce3+ with other unary or binary metal ions. As described in the TEM image (figure 3d and 3e), L-F-Nd present trigonal or hexagonal morphology. The thickness of LF-Nd in figure 3f is measured to be ~ 1.44 nm. The HAADF-

EDS elemental mappings (figure 3g to figure 3i) reveal the homogeneous distribution of F and Nd elements in the L-FNd. The ultrathin feature of binary metal fluoride L-FCe0.5Nd0.5 is disclosed by TEM characterization in figure S8 and HAADF-EDS mappings prove that F, Ce and Nd element distributes evenly in the nanosheets. Nevertheless, not all asprepared L-F-Ln keeps trigonal or hexagonal morphology (figure S9). According to the statistics of L-F-Nd, it is found that the shape of about 40 % nanosheets is regular sheet-like triangle or hexagon.

Figure 3. The morphology characterization of L-F-Ln. (a) TEM image, (b) HAADF image and (c) cross-section TEM image of LF-Ce; (d) TEM image, (e) HAADF image and (f) side-view TEM image of L-F-Nd; (g) HAADF image and EDS-mapping of L-FNd: (h) F element and (i) Nd element.

In addition, we performed extensive experiments to investigate the growth parameters in the formation process of H-F-Ce. The aim is to get optimal growth parameters for the synthesis of F-Ln with controllable morphology. Detailed analysis is presented in S3 part and optimal parameters are listed as follow: 7°C to 50°C for synthesis temperature, 6.00 to 9.00 for the aqueous pH of cerium acetate, 0.20 to 0.79 for the molar ratio of fluorine ions to cerous ion (F/Ce ratio) and 2.00 to 3.00 for the molar ratio of acetate anions to cerous ions (Ac/Ce ratio).

Microstructure analysis Based on TEM technique, the microstructure analysis of FLn is performed. As shown in figure S17a, the typical highresolution transmission electron microscopy (HRTEM) image taken from an individual edge-jagged H-F-Ce (figure 4a) clearly shows that H-F-Ce is consisted of nanocrystals with sub-10 nm nanodomains. The appearance of periodic diffraction spots in selected area electron diffraction (SADE) pattern (figure 4b) indicates that nanocrystals are assembled into highly oriented superstructures. Additionally, it is also observed that the periodic diffraction spots corresponding to d2 spacing are elongated along the circular direction, meaning



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Figure 4. The microstructure analysis of F-Ce. (a) TEM image of an individual disc-like H-F-Ce and (b) corresponding SADE pattern; (c) The typical HRTEM image of H-F-Ce and (d) corresponding fast Fourier transform (FFT) pattern; (e) TEM image of an individual trigonal L-F-Ce and (f) corresponding SADE pattern; (g) The typical HRTEM image of L-F-Ce and (h) corresponding FFT pattern. The small circles in figure 4b and figure 4f suggest the occurrence of low-intensity diffraction spots and corresponding d1-spacing value is 0.334nm; The high-intensity diffraction spots in figure 4b and figure 4f correspond to the lattice plane with d2=0.202nm.

that the majority of nanocrystal are in a perfect crystallographic orientation24,25 while the minority of those have a small misorientation26 deviating from the perfect crystallographic orientation. In other words, nanocrystal building blocks of H-F-Ce share an imperfect crystallographic orientation.26,27 The imperfect orientation of nanocrystals is evidenced furtherly by the HRTEM image in figure 4c and corresponding FFT pattern in figure 4d, where a rotation angle of 4.2° between two adjacent nanocrystals is observed. Compared to H-F-Ce, trigonal L-F-Ce in figure 4e is also consisted of nanocrystals with sub-10 nm domains, as evidenced by the appearance of interrupted crystalline fringes in corresponding HRTEM image (figure S17b). The SADE pattern in figure 4f displays a single-crystal-like SADE pattern, but nanocrystal building blocks of L-F-Ce still share an imperfect crystallographic orientation, as confirmed by HRTEM image in figure 4g and corresponding FFT pattern in figure 4h, where one nanocrystal involves a rotation angle of 3.6° relative to the other one. Similarly, this misorientation of the minority of nanocrystals is also observed in H-F-Nd and LF-Nd as depicted in figure S18 and S19. Moreover, it is evident that a larger elongation of diffraction spots along the circular direction corresponding to d2 spacing is in H-F-Ce than that in L-F-Ce, meaning that more nanocrystals in H-F-Ce have a larger crystallographic misorientation. This result is consistent with XRD analysis presented in figure S22d where H-F-Ce show poorer crystallinity due to its increased half-peak width. As a result, nanocrystals with sub-10 nm domains are assembled into F-Ln where the majority of nanocrystals share a perfect crystallographic orientation but the minority of nanocrystals preferably deviate from this perfect orientation. Of particular note, this imperfect orientation of nanocrystals in 2D materials is fully different from the previous literature reports, in which perfect 2D single-crystal nanosheets28,29 or supperlatices30,31 assembled from nanoparticles could be prepared via oriented attachment32 while 2D quasi-

nanosheets33,34 consisted of aligned randomly nanoparticles could be synthesized through template- or surfactantmediated strategy. Interestingly, this imperfect orientation of nanocrystals in 2D nanomaterials induces the generation of nano-pores or potholes and interparticle defects. As revealed in figure S20, the appearance of low-contrast bright dots in TEM image (figure S20a) and dark dots in STEM-HAADF image (figure S20b) demonstrates the pore- / pothole-rich feature of H-FCe. Furtherly, HRTEM analysis confirms incontrovertibly the generation of nano-pores (figure S20c) and potholes (figure S20d) in H-F-Nd. In addition, it is easy to observe some dislocations (white arrowheads in figure S20e), grainboundaries (white lines in figure S20e) and atomic terraces (black arrowheads in figure S20e and S20f). These results suggest the defect-rich feature of H-F-Ln. Similarly, although low release rate of fluorine ions derived from KBF4 is beneficial to the growth of acetate-based light lanthanide fluoride nanomaterials, L-F-Nd with trigonal morphology hold numerous nano-pores and potholes, as evidenced by the existence of many low-constrast bright dots in TEM image (figure S21a). The zoomed-in HRTEM images provide a clear evidence for the formation of nanopores (figure S21b) and potholes (figure S21c) in the nanosheet. Meanwhile, the defect-rich feature is claimed by the occurrence of dislocations (white arrowheads in figure S21d) and grain interfaces (white lines in figure S21e). In short, F-Ln are a series of defect-rich 2D materials, which are consisted of nanocrystals with sub-10 nm domains. Given the fact that nanopores, nano-potholes or defects usually dominate the performance in the application of nanomaterials,35,36 it is reasonable to propose F-Ln as a powerful material platform to advance various research fields, especially catalysis, adsorption, separation, energy storage and conversion and so on.


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Figure 5. The structure analysis of F-Ce. (a) The high-resolution Ce3d XPS spectra of H-F-Ce; (b) The high-resolution C1s XPS spectra of H-F-Ce and rinsed H-F-Ce; (c) The infrared spectra of H-F-Ce, rinsed H-F-Ce, sodium acetate and CeF3; (d) Raman spectra of H-F-Ce, bulk CeF3 and CeFCO3; (e) The optical images of H-F-Ce and bulk CeF3 under visible and UV irradiation; (f) The excitation and emission spectra of bulk CeF3 and H-F-Ce; (g) [001] view of a BN-like F-Ce one-atom-layer (4×4×1 supercell); (h) [110] view and [1-10] view of the proposed 2×2×1 supercell structure model. Note: in figure 5g, acetate anions are omitted for clarity; θ is the tilted angle of acetate anions relative to the plane of F-Ce one-atom-layer

Sturcture characterization To determine the structure of F-Ln, X-ray powder diffraction (XRD) experiments were performed. The results in figure S22 show all as-synthesized samples are isostructural due to their identical diffraction peak positions. Therefore, in this part, H-F-Ce is selected as the representative example to determine the structure of F-Ln. The elementary compositions of H-F-Ce were identified using energy-dispersive x-ray spectroscopy (EDS). The EDS mappings in figure S2 and element line-profile intensity in figure S3 confirm the existence of C, O, F and Ce elements in the H-F-Ce. The mean value of F/Ce and F/C atomic ratio with at least 20 EDS experiments is estimated to be 0.969 ± 0.286 and 0.250 ± 0.055, respectively. The quantitative elemental analysis by an element analyzer exhibits that the atomic ratio of C: H: O is 1.05:1.43:1, close to the stoichiometric atomic ratio (1: 1.5:1) of acetate anion, strongly implying the presence of acetate anions in H-F-Ce. In addition, the valence state of cerium elements is analyzed

based on the obtained core-level high-resolution Ce3d XPS data. As exhibited in figure 5a, the binding energies at 884ev, 887ev and 902ev, 905ev can be assigned to Ce (III) 3d5/2 and 3d3/2 peaks, respectively.37 This result rules out the possibility of Ce4+ and provides a solid evidence that only Ce3+ is in the resultant samples. The presence of acetate anions in H-F-Ce is evidenced furtherly by other characterization techniques, such as high resolution C1s XPS spectrum, infrared spectrum and Raman spectrum. High resolution XPS C1s signal in figure 5b can be deconvolved into two peaks at binding energies of 284.80 eV and 288.56 eV. The strong peak centered at 284.80 eV may originate from aliphatic carbons of acetate anions.38 The relatively weak peak at a binding energy of 288.56 eV is assigned to carbons bound to oxygen (O-C=O) in acetate chains.39 In infrared (IR) spectrum of H-F-Ce (figure 5c), a weak band at 2940 cm-1 (red arrows) is attributed to the asymmetric vibrations of C-H in acetate anions, while two strong vibrations centered at 1567 cm-1 and 1444cm-1 (red



arrows) correspond to asymmetric and symmetric stretching vibrations of C=O in acetate chains, respectively.40 As well, there appear two bands indicated by the red arrows in Raman spectrum (figure 5d). The strong but narrow band from 900 cm-1 to 985 cm-1 is dominated by the symmetric C-C stretching vibration of acetate anions and a weak but broad vibration region from 450 cm-1 to 710 cm-1 is comprised mainly of O=CO in-plane and out-of-plane bending vibrations.41 To eliminate the undesired signals derived from surface-adsorbed acetate anions, copious pure water was used to dissolve the surfacebounded acetate anions and the rinsed samples were again subject to XPS (figure 5b) and IR analysis (figure 5c). The results show that rinsed samples hold identical peak position with primary ones, demonstrating that XPS and IR signals come from the acetate anions in crystal framework but not that on the surface of H-F-Ce. In a word, these spectroscopic data draw a conclusion that acetate anion is the indispensable component of H-F-Ce. Based on above analysis, the molecular formula of H-F-Ce can be written as CeF(CH3COO)2.

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to-blue color change of bulk CeF3 results from the photoluminescence induced by the electron allowed transitions of Ce3+ from the lowest components of 5d level to the spin-orbit components of 2F5/2 ground state,22 as confirmed by the broad emission band with a maxima at 323 nm in figure 5f. The violet color of H-F-Ce is the result of diffuse reflection to ultraviolet light due to the complete disappearance of excitation and emission spectra in ultraviolet region (figure 5f). The photoluminescence quenching may be caused by the higher vibrational energy (1567 cm-1) of C=O in acetate anions, in line with the photoluminescence quenching caused by intercalated CO32- in the host lattice of LaFCO3.47 Therefore, it can be believed that acetate anions are intercalated in the interlayers of F-Ce one-atom-layers in the structure of H-F-Ce.

Structure correlation is studied based on IR and Raman spectroscopy. In IR spectrum (figure 5c), some characteristic IR vibrations of bulk CeF3 (encircled by a dotted rectangle) appear in H-F-Ce sample, suggesting the presence of particular structure relationship between H-F-Ce and bulk CeF3. In Raman spectrum (figure 5d), H-F-Ce, CeF3 and CeFCO3 share a similar vibration region with maximum peak at ca.250nm indicated by an encircled dotted rectangle. The vibration region is associated with electron transition of Ce3+ from 4f level to 5d level42 or the Ce-F stretching vibrations43. Hence, it can be inferred that H-F-Ce may adopt a locally similar structure with bulk CeF3 or CeFCO3.

Combined with the above analysis and three-dimensional rotation electron diffraction (RED)48 data of L-F-Ce (figure S24), a plausible structure model (detailed analysis in S5 part) is proposed to determine the structure of F-Ce, as presented in figure 5g, 5h and 5i. In the propositional structure model (figure 5g), F atoms and Ce atoms are packed alternately into a BN-like one-atom-thickness hexagonal honeycomb lattice, namely F-Ce one-atom-layer. Four F-Ce one-atom-layers sandwiched by intercalated acetate anions stacks along [001] direction to form the nanocrystal building blocks of F-Ce (figure 5h and 5i). The reasonability of the proposed structure model is verified by checking the simulated XRD and HRTEM data with experimental counterpart. Figure S26 shows that the atomic arrangement in the simulated HRTEM image is in good arrangement with that in the experimental HRTEM image. Figure S27 manifests that the diffraction peak positions in the simulated XRD pattern matches well with that in experimental XRD pattern. Of note, even though these data provide some experimental evidence for the proposed structure model to determine the structure of F-Ce, the model might be not accurate due to the lack of powerful characterization techniques to locate intercalated acetate anions in the interlayers. The experiments for structure analysis is going on.

Structure difference is analyzed by investigating UV-Vis adsorption spectrum and photoluminescence spectrum. The UV-Vis spectrum of H-F-Ce shows three adsorption peaks with the maxima at 258nm, 243 nm and 228 nm, whereas there are three adsorption bands centered at 249 nm, 235 nm and 219 nm in the adsorption spectrum of bulk CeF3 (figure S23). Compared to bulk CeF3, a slight red shift of adsorption peaks occurs in H-F-Ce. Given that acetate anion is an electrondonor ligand while fluorine ion is an electron-acceptor ligand,44 if partial fluorine ions of Ce3+ coordination environment in CeF3 are replaced by acetate anions, crystalfield splitting energy of Ce3+ tends to increase and electron transition becomes easy from 5d orbits of Ce3+ to the corresponding ligand orbits, and eventually a red-shift phenomenon occurs.45,46 Thus, it may be deduced that acetate anions substitute partial fluorine ions in CeF3 to coordinate with Ce3+. In order to confirm furtherly the coordination of acetate anions with Ce3+, the photoluminescence spectra of HF-Ce and bulk CeF3 were recorded. As shown in figure 5e, two materials appear white color under visible irradiation, but the color of bulk CeF3 changes from white to blue while H-F-Ce becomes violet upon exposure to ultraviolet light. The white-

In this part, H-F-Ce is selected as the representative example to shed light on the formation mechanism of F-Ln. The growth process of H-F-Ce is divided into two stages: dissolution of cerium (III) acetate in pure water and drop-by-drop addition of sodium fluoride into aqueous solution. In first stage, after cerium (III) acetate completely dissolved in pure water, the pH of mixed solution was measured to be ca. 6.85. According to the literatures, Ce3+ in neutral aqueous solution tends to form Ce3(OH)54+ oligomers.49,50 With the coexistence of Ce3+ and Ac in aqueous solution, Sonesson51 and Powell52 pointed out that one or two acetate anions prefer to coordinate with one Ce3+ to form Ce-Ac complexes via electrostatic interaction.53 Thus, the molecular formula of hydrolysates can be noted as Ce3(OH)5(Ac)n(4-n)+, n=1~2. In addition, the hydrolysates were studied by ex situ HRTEM technique. As presented in figure S30a, an individual nanocrystal displays clear lattice fringes with d-spacing values of 0.311 nm and a dihedral angle of 71.89°, an explicit evidence of CeO2. Presumably, CeO2 should derive from the hydrolysate aggregates, which are sensitive to electron beam irradiation and preferably transformed into CeO2 once they suffer from

In figure 1, a conjecture is proposed that the insertion of chain-like monodentate anions into the interlayers of F-Ce one-atom-layers in CeF3 have a chance to create a new family of 2D materials. Here, the structure correlation and difference between H-F-Ce and bulk CeF3 or CeFCO3 is captured to support the proposed conjecture and determine the structure of H-F-Ce.

Growth mechanism


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Figure 6. Schematic illustration of the grow mechanism for F-Ce.

the irradiation of high-energy electrons. The appearance of CeO2 in turn provides a convincing evidence about the formation of Ce-Ac hydrolysates in hydrolysis reaction. However, as discussed in structure analysis, H-F-Ce is consisted of nanocrystals, which feature a stacked structure of F-Ce one-atom-layers sandwiched by acetate anions. Therefore, hydrolysates are not the very nanocrystal building blocks of H-F-Ce.

disappears and aggregates into large-size nanoparticles immobilized on the surface of nanosheets (figure S30e and S30f). Combined with a previously claimed mechanism that acetate anions40,55 or other monodentate carboxylate ligands56 acted as capping reagent or soft template57 to control the size and shape of nanomaterials, it is safe to conclude that acetate anions play a vital role in guiding the assembly process of CeF(Ac)2 nanocrystals for the growth of H-F-Ce.

In second stage of drop-by-drop addition of sodium fluoride, nanosheets would not appear unless fluorine ions were introduced (F/Ce ratio is 0.2, figure S14a), demonstrating that the addition of fluorine ions is a prerequisite for the formation of H-F-Ce. About the working mechanism of fluorine ions, an assumption is that bridging oxygen atoms in Ce3(OH)5(Ac)n(4n)+ might be replaced by fluorine atoms and this leads to the structure transformation of Ce3(OH)5(Ac)n(4-n)+ into CeF(Ac)2 nanocrystals. This assumption can be supported by the study of Donald B54, where they put forward that bridging oxygen atoms could be substituted by fluorine atoms in polymerized melts. Furthermore, a theoretical calculation is carried out to verify this assumption. As described in figure S31, total energy of molecular configuration for F-substituted hydrolysate, an presumptive intermediate that eventually transforms into CeF(Ac)2 nanocrystal, is lower than that for hydrolysate, proving that the supposed replacement of bridging oxygen atoms by fluorine atoms is a thermodynamically favorable. As a result, it is reasonable to assume that fluorine ions induce the structure transformation of hydrolysates into the very nanocrystal building blocks for the growth of H-F-Ce.

For L-F-Ce, some CeF(Ac)2 nanocrystals could be noticed in TEM images (figure S32) and adventitious acetate anions could give rise to the quick aggregation and precipitation, suggesting that H-F-Ce and L-F-Ce share a same growth mechanism. Only difference is the addition way of fluorine ions. In the formation process of H-F-Ce, drop-by-drop addition of NaF aqueous solution means that fluorine ions diffuse into the pre-hydrolyzed Ce3+-Ac solution and have access to hydrolysates at the highest release rate. This leads to the fast structure transformation and the generation of quite a lot of CeF(Ac)2 nanocrystals in a short time. The formed nanocrystals, with the guidance of acetate anions, can be promptly assembled into H-F-Ce. The quick assembly process may result in the larger crystallographic misorientation of more nanocrystals. In the formation process of L-F-Ce, the sluggish release rate of fluorine ions derived from KBF4 results in a slow “birth” of CeF(Ac)2 nanocrystals, which in turn causes a slow assembly process guided by acetate anions. The slow process is conducive to the formation of L-F-Ce where less nanocrystals have a smaller crystallographic misorientation.

Acetate anion is another essential ingredient for the preparation of H-F-Ce. As stated above, low Ac/Ce ratio (0.50) in growth process leads to the formation of a few nanosheets surrounded by abundant nanoparticles in resultant product (figure S16a). The nanoparticles are usually observed in TEM tests (figure S30b) and parts of them are the nanocrystal building blocks of H-F-Ce according to HRTEM analysis in figure S30c and S30d. Excessive addition of Ac (Ac/Ce ratio >11) causes quick aggregation and precipitation of as-synthesized nanosheets (figure S16d). These experiments infer that acetate anions are able to function as a “glue” to guide the assembly process of nanocrystals. To gain further insight into the role of acetate anions in the assembly process, several experiments were performed that adventitious acetate anions were introduced into the mixed aqueous solution containing H-F-Ce. With the addition of adventitious acetate anions (sodium acetate), the mixed solution destabilizes instantaneously and nanosheets precipitate spontaneously in seconds. Moreover, it is very intriguing to notice that nanoparticles around H-F-Ce (figure S30b)

Based on above analysis, a suppositional growth mechanism for F-Ce is presented in figure 6. Three procedures is involved in the formation process of F-Ce. Procedure 1 is the hydrolysis reaction of Ce(Ac)3 in pure water, where the hydrolysates form. Procedure 2 involves in the structure transformation of hydrolysates into CeF(Ac)2 nanocrystals due to the introduction of F-1 derived from NaF aqueous solution or KBF4. In procedure 3, with the guide of acetate anions, the formed CeF(Ac)2 nanocrystals are assembled into defect-rich F-Ce.

Stability In general, physicochemical stability of a new material is the fundamental prerequisite for its practical application.58 In this part, the excellent stability of F-Ln is evidenced experimentally with respect to environment stimulis such as air, heat, solvent and high-energy electron beam. Long-term stability towards air of F-Ln is evaluated based on the structure comparison of H-F-Ce and as-stored H-F-Ce in air for one year. As revealed by figure S33, after one-year storage in air, the morphology, crystal structure and cerium



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valence state of H-F-Ce remains unchanged, confirming the excellent stability towards air. Thermal stability of H-F-Ce is investigated via time-dependent XRD patterns, TEM technique and thermogravimetry (TG). Temperaturedependent XRD (figure S34a) patterns prove that H-F-Ce can hold original crystal structure with sheet-like morphology (figure S34b) with calcination at 200 °C for 3h in air, but a higher calcination temperature leads to the structure collapse and transformation of H-F-Ce into CeF3 and CeO2 (figure S34c). TG curve (figure S34d) exhibits that thermal decomposition of H-F-Ce happens at ca.300 °C, indicating the superior thermal stability of H-F-Ce compared to common organic-inorganic perovskites.59 Solvent-tolerant experiments of H-F-Nd were carried out at room temperature and atmosphere pressure. After one-month soakage in acetone, dimethylsulfoxide (DMSO), dimethyl formamide (DMF) and glacial acetic acid, TEM images in figure S35 show that H-FNd still maintain sheet-like morphology and identical crystal structure (SADE pattern along [001] direction) with primary H-F-Nd. Furthermore, little change in lateral size according to the statistics with over 100 nanosheets provides another solid support for the strong stability towards solvent. The excellent stability towards high-energy electron beam is evaluated based on continuous TEM electron-beam irradiation experiments. As shown in figure S36, a prolonged irradiation time up to 120 min does not give rise to any variation in morphology and structure of the trigonal L-F-Ce. A twin experiment with H-FNd presented in figure S37 exhibits that there is not any change in domain size and structure within 50min electron irradiation.

various characterization techniques and the results show that F atoms and Ce atoms are packed alternately to form 2D honeycomb F-Ce one-atom-layers, which are sandwiched by intercalated acetate anions and stack alternately along [001] direction to form nanocrystal building blocks of F-Ce. In the formation process of such materials, three procedures are involved in the formation of F-Ce: hydrolysis reaction of cerium (III) acetate, F-1 induced structure transformation of hydrolysates into CeF(Ac)2 nanocrystals and acetate-mediated assembly process of CeF(Ac)2 nanocrystals into 2D defect-rich nanosheets. The stability is experimentally investigated and the results exhibit that as-synthesized nanosheets have excellent stability towards environment stimulis such as air, heat, solvent and high-energy electron beam. This work provides an emerging two-dimensional material platform to advance various research fields such as catalysis, adsorption, separation, energy storage and conversion and so on.

In a word, the above experiments give a distinct evidence that as-prepared 2D acetate-based light lanthanide fluoride materials (F-Ln) have excellent stability and this will lay a solid foundation for their practical application.

Yuzhong Zhang: 0000-0003-4181-5743 Jing Ju: 0000-0001-9540-2165 Bowen Cheng: 0000-0001-7470-8077

CONCLUSION In conclusion, we develop a simple and facile bottom-up methodology to fabricate a series of 2D acetate-based light lanthanide fluoride nanomaterials (F-Ln, Ln=La, Ce, Pr, Nd) at room temperature and atmosphere pressure, for the first time. In this methodology, fluorine ions are introduced into the pre-hydrolyzed aqueous solution of light lanthanide metalacetate salts in two ways: drop-by-drop addition of NaF aqueous solution and chemical release from KBF4. Dropwise addition of NaF solution results in the formation of disc-like nanosheets due to high release rate of fluorine ions, while low chemical-release rate of fluorine ions derived from KBF4 is beneficial to the growth of trigonal or hexagonal nanosheets. Extensive experiments prove that, with the presence of light lanthanide metal, 2D unary, binary, trinary metal and even metal-doped fluoride nanosheets can be synthesized. The assynthesized F-Ln exhibit an ultrathin morphology with thickness up to 1.45 nm and lateral dimensions up to several hundred nanometers. Microstructrue analysis based on TEM technique demonstrates that nanocrystals with sub-10 nm domains are assembled to form defect-rich nanosheets, where a large proportion of nanocrystals adopt a perfect crystallographic orientation but a small proportion of nanocrystals deviate from the perfect orientation. Compared to L-F-Ce, more nanocrystals adopt a larger crystallographic misorientation in H-F-Ce. The structure of F-Ce is studied by

ASSOCIATED CONTENT Supporting Information. Experimental details, morphology and EDS characterization, growth parameters, microstructure analysis, structure characterization, growth mechanism, stability and so on.

AUTHOR INFORMATION Corresponding Author *Yuzhong Zhang: [email protected] *Jing Ju: [email protected] *Bowen Cheng: [email protected]

ORCID

Author Contributions These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21676201, 51678411, 51673148), National Key Research and Development Plan (2017YFC0404001), National Key Technology Support Program (2015BAE01B03) and Science and Technology Plans of Tianjin (No.17PTSYJC00040, 18PTSYJC00180, 18JCQNJC06800, 17PTSYJC00050, 18PTSYJC00190).

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Two-dimensional Acetate-based Light Lanthanide Fluoride

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