Two-Dimensional Nanosheets from Redox-Active Superatoms.

We describe a new approach to synthesize two-dimensional (2D) nanosheets from the bottom-up. We functionalize redox-active superatoms with groups that can direct their assembly into multidimensional solids. We synthesized Co6Se8[PEt2(4-C6H4COOH)]6 and found that it forms a crystalline assembly. The solid-state structure is a three-dimensional (3D) network in which the carboxylic acids form intercluster hydrogen bonds. We modify the self-assembly by replacing the reversible hydrogen bonds that hold the superatoms together with zinc carboxylate bonds via the solvothermal reaction of Co6Se8[PEt2(4-C6H4COOH)]6 with Zn(NO3)2. We obtain two types of crystalline materials using this approach: one is a 3D solid and the other consists of stacked layers of 2D sheets. The dimensionality is controlled by subtle changes in reaction conditions. These 2D sheets can be chemically exfoliated, and the exfoliated, ultrathin 2D layers are soluble. After they are deposited on a substrate, they can be imaged. We cast them onto an electrode surface and show that they retain the redox activity of the superatom building blocks due to the porosity in the sheets.

Introduction

In this manuscript we connect transition metal chalcogenide molecular clusters into three-dimensional (3D) and two-dimensional (2D) solids, as well as free-floating nanosheets. Monolayer 2D materials such as graphene and transition metal dichalcogenides show promise for next-generation electronics, yet are plagued by the occurrence of defects, and it is not easy to modify them synthetically.[1,2] The type of nanosheet we disclose here, due to the redox activity and multinuclearity of its superatom components, provides a new level of complexity and synthetic sophistication to 2D materials. Our building blocks are atomically defined entities whose isolated electronic and redox properties can be incorporated into extended structures in which the structural element is preserved. Recent theoretical calculations have established that polynuclear Co6Se8L6 clusters behave as “superatoms”.[3−8] We have previously used such superatoms to form solids from two different yet electronically complementary building blocks; directed-layer fullerene assemblies from phenanthrene-decorated clusters; and covalent assemblies through directed ligand exchange.[9−12] Redox-active M6E8 clusters (M = Re, W; E = S, Se) have previously been functionalized with reactive ligands to generate frameworks of these preformed entities through cyanide and bipyridine coordination with transition metal ions.[13−18] Others have employed a variety of techniques to direct clusters and nanocrystals into extended lattices.[19−22]

The key to unlocking both the structural utility and the functional solid-state infrastructure of the superatoms is the ability to manipulate their surface properties at will. In this study, we demonstrate a method to do so by converting the Co6Se8[PEt2(4-C6H4Br)]6 superatom into one that presents six carboxylic acids. We then introduce zinc carboxylate bonds via a solvothermal reaction to produce two types of crystalline solids, a trigonal 3D solid (Trig) and a tetragonal 2D solid (Tet) (Figure ). Single crystal X-ray diffraction (SCXRD) reveals that Trig is a 3D network of superatoms held together with zinc carboxylate bonds, but Tet forms 2D sheets that then stack through noncovalent forces into a 3D solid. We find it remarkable that the two-dimensionality of Tet is robust: individual 2D sheets can be exfoliated intact from the solid, and these exfoliated sheets can be subsequently redeposited on arbitrary substrates. When we cast them on electrode surfaces they retain the redox activity of the superatom building blocks.

Figure 1

(a) Structure of 1 from SCXRD, Co6Se8 cluster capped with 4-(diethylphosphine)benzoic acid, Co6Se8[PEt2(4-C6H4COOH)]6. Carbon, black; oxygen, red; cobalt, blue; selenium, green; phosphorus, orange. Thermal ellipsoids are set at 50% probability. Hydrogen atoms are omitted to clarify the view. (b) 1 forms a 3D hydrogen-bond network, named 1-H. View of 1-H down the a-axis, showing a single superatom and its six hydrogen-bonds to neighboring superatoms (in blue). (c) Representation of the view in (b) with each superatom as a sphere to emphasize the structure of the extended solid. (d) and (e) The solvothermal reaction of Co6Se8[PEt2(4-C6H4COOH)]6 with Zn(NO3)2 forms two different types of extended solid, Trig and Tet, depending upon solvent conditions. (d) In Trig the superatoms are held together within a 2D plane to create a trigonal arrangement of superatoms, and the planes extend in three dimensions via further zinc carboxylate bonds. The axis of symmetry defining a superatom within a Trig sheet is a C3-axis through the center of two planes defined by Co3 atoms. (e) In Tet the superatoms are held together within a 2D plane to create a distorted square arrangement. The 2D layers are noncovalently stacked in the third dimension. The axis of symmetry defining the Tet plane is a C4-axis through axial atoms of a Co6 octahedron.

Results and Discussion

We previously organized these superatoms into extended van der Waals solids; our new objective was to connect the superatoms to make extended solids through bonds. Our simple, phosphine-terminated superatoms, however, are inert in the sense that the phosphines (by design) chemically passivate the cluster surfaces and do not participate in the reaction chemistry. Thus, our first challenge was to create appropriately reactive superatom building blocks. To do so we first treated Co2(CO)8 and Se with Et2P(4-C6H4Br) to give Co6Se8[PEt2(4-C6H4Br)]6 (SCXRD in Figure S1) in high yield. Other than the obvious differences in the size, shape, and arrangement of the organic components, the inorganic core of Co6Se8[PEt2(4-C6H4Br)]6 is identical to the parent cluster, Co6Se8(PEt3)6.[12] Through a 6-fold lithium/halogen exchange followed by the addition of CO2 gas and subsequent acidification, we converted each Br in this compound to the corresponding carboxylic acid to yield the nanosized octahedral, superatom building block Co6Se8[PEt2(4-C6H4COOH)]6 (1). This sequence is facile and high yielding. The lithium/halogen exchange is a harsh process, and the fact that the Co6Se8 core is unchanged reveals a new method to easily activate and functionalize superatoms.

We determined the molecular structure of 1 using SCXRD (Figure a and Figure S2). 1 assembles into an organized, extended, 3D solid via extensive and ordered hydrogen bonding between carboxylic acids on neighboring clusters (Figure b). We refer to the latter solid as 1-H. If we represent each cluster as a sphere, we see that this solid forms such that there is hydrogen-bonding between nearest neighbors (Figure c). The formation of this solid-state compound is reversible: 1-H dissolves in tetrahydrofuran to regenerate 1.

We then sought to create solids from building block 1 through metal–carboxylate bonds. For example, would the simple replacement of the two protons with a divalent metal ion result in a structurally diverse family of new solids?[23−40] Thus, we treated our hexatopic superatom with Zn(NO3)2 to determine the extent to which the carboxylate–carboxylate bonds, which constitute the adhesive that stabilizes this solid, can be modified and improved. Co6Se8 superatoms are useful building blocks in this regard because they have tunable ligands, multiple accessible redox states, significant magnetic moments, and charge transport capabilities.[21,41−43] Our building block 1 is preformed and atomically defined, and thus programmable.

Using the same building blocks, 1 and Zn2+, we can selectively synthesize two different solids, Trig and Tet, by varying the growth conditions. It is remarkable that the only significant difference between the two reactions is the use of methanol versus ethanol as solvents. We obtained structures for both solids using SCXRD (details of the refinement can be found in the Supporting Information). In both solids, all the carboxylic acid hydrogen bonds of 1-H are replaced by carboxylate–zinc–carboxylate nodes. SCXRD of both solids reveals that while they have the same Zn:[Co6Se8] stoichiometric ratio of 3:1, both the dimensionality of their extended structure and orientation of the cluster within the solids differ significantly. Trig is a 3D network while Tet is a 2D structure with strong in-plane bonding and comparatively weak noncovalent interlayer interactions.

We combined 1 and Zn(NO3)2 in a DMF/MeOH solvent mixture under solvothermal conditions at 65 °C, and obtained black hexagonal crystals after 24 h. Figure displays the crystal structure of Trig. The structure is a network in which 1 is coordinated to unusual trinuclear zinc nodes in three dimensions (Figure a). Looking down the b-axis we clearly see the distinct pseudotrigonal layers of the solid (Figure b). Within each layer, the superatoms are bound to six zinc-nodes (Figure c). These layers are then cross-linked by a single Zn–O bond. The approximate 3-fold symmetry of the pseudotrigonal lattice of Trig arises because the Co6 octahedron is tilted on its face in the layer, which orients the phosphines such that three point up and three point down (Figure d). This symmetry mirrors that of 1-H, replacing hydrogen bonds with an organized trinuclear metal node (labeled Zn1, Zn2, and Zn3 in Figure e). Unusual trinuclear zinc nodes have been reported previously.[44,45]

Figure 2

Structure of Trig from SCXRD: a 3D network synthesized from the solvothermal reaction of 1 and Zn(NO3)2 in a MeOH/DMF solvent mixture. Ethyl groups are omitted for clarity. (a) View of the network along the a-axis. (b) View along the b-axis of the cross-linked pseudotrigonal arrays of superatoms. The blue box highlights the trinuclear zinc node, which is magnified in (e). (c) Side-on view of 1 within a pseudotrigonal layer and (d) top-down view of a single “layer” within Trig (e) View of the metal node geometry and different coordination environments around Zn1, Zn2, and Zn3. The blue box around this trinuclear zinc node is the same as in the inset in (b). (f) Top-down view of a single cluster within a Trig layer surrounded by three different Zn-carboxylate binding modes. Three types of carboxylates are shown: a, b, and c.

Each zinc atom in Trig exhibits a different coordination environment. Zn1 and Zn2 display distorted tetrahedral geometry and together form a three-bladed trigonal paddlewheel with three bridging μ2-carboxylates. A solvent molecule (likely MeOH) coordinates Zn1 axially, and Zn2 is axially coordinated by a μ2-carboxylate, whose second oxygen coordinates Zn3. Zn3 exhibits a distorted square pyramidal geometry. Each superatom 1 within the solid contains three types of carboxylates, labeled a, b, and c in Figure e,f. For a, three μ2-carboxylates form the Zn1–Zn2 paddlewheel; for b, two carboxylates coordinate Zn3 in an η2 fashion; for c, a μ2-carboxylate coordinates both Zn3 and Zn2. The latter ligand c also serves to cross-link the layers through its carboxylate–Zn2 bond. This bond has a length of 2.22 Å, which is a long Zn–O contact,[46] and suggests the interlayer carboxylate–Zn2 bond is a weaker, dative bond compared to intralayer carboxylate–Zn bonds.

The presence of weak interlayer zinc bonds in Trig prompted us to modify reaction conditions to eliminate interlayer bonding and synthesize 2D layers. Thus, we reacted 1 and Zn(NO3)2 at 65 °C in a DMF/EtOH solvent mixture and obtained black cubic crystals after 24 h. We note that a small fraction of Trig forms under these conditions but can eliminated with the addition of “extra” protons in the form of HCl in the reaction. Under these conditions, we form exclusively the new solid-state compound, Tet (Figure ). Tet also contains complete replacement of proton-nodes with metal nodes, although the types of metal nodes and dimensionality differ from Trig. Distinct layers of superatoms are held together only by noncovalent forces.

Figure 3

Structure of Tet from SCXRD: square sheets in the crystalline state. Tet is 2D network synthesized from the solvothermal reaction of 1 and Zn(NO3)2 in a EtOH/DMF solvent mixture. Ethyl groups are omitted for clarity. (a) Top-down view of a single layer within Tet along the b-axis. (b) Side-on view of Tet layers along the c-axis. Noncovalent forces hold the layers together in the third dimension. (c) Single superatom in a Tet layer and the binding interaction of each carboxylate of 1. Within the 2D plane, each of the four equatorial carboxylate ligands coordinates two Zn2+ ions, forming the four-bladed paddlewheel upon coordination of equatorial carboxylate ligands of three adjacent superatoms. The axial carboxylate ligands coordinate an additional Zn2+ ion that lies just above or below the square sheet. (d) Top-down view of 1 within the 2D plane. (e) Four-bladed Zn2+ paddlewheel. Zn–Zn = 2.867(7) Å. (f) Mononuclear zinc complex with Zn–O distances = 2.20(3) and 2.36(3) Å and a carboxylate–Zn–carboxylate angle of 118.6(12)°.

Tet is a layered 2D material in which each layer is a square arrangement of Co6Se8 superatoms with four phosphine ligands residing in the 2D plane and bonding to four-bladed Zn-carboxylate paddlewheels (Figure a). In the direction normal to the sheet, the axial carboxylate ligands coordinate an additional Zn2+ ion that is positioned above or below the square sheet (Figure b). Figure c,d displays the binding interaction of each ligand of 1 within the solid. The zinc subunit within the square plane of this solid is a dinuclear four-bladed Zn-carboxylate paddlewheel (Figure e). The combination of two Zn2+ ions and four bridging μ2-carboxylate groups yields this Zn2 cluster with a Zn–Zn distance of 2.867(7) Å that is consistent with other such “four-bladed” paddlewheels in zinc-based metal–organic frameworks.[47] Pairs of apical phosphines on adjacent clusters that are not involved in dinuclear Zn paddlewheels within a single layer are linked via a single Zn atom (in addition to their bonding via the intralayer Zn2 node) to form a mononuclear zinc complex. This complex features Zn–O distances of 2.20 (3) and 2.36(3) Å and a carboxylate-Zn-carboxylate angle of approximately 119°. This geometry is typical of pseudotetrahedral Zn(O2R)2L2 complexes,[48] but in this case we note that the two L-type ligands (presumably ethanol or water) are disordered and could not be located. The layers are self-contained and stack through noncovalent interlayer interactions in an eclipsed arrangement.

The crystal packing arrangements of Trig and Tet are propagated in their macroscopic crystal morphologies. Figure a,b shows SEM micrographs of the crystals that form after 24 h growth. The black cubes (Tet; Figure a) and hexagonal plates (Trig; Figure b) reflect the tetragonal and trigonal lattices of their crystalline arrangements. In Trig, the cluster is tilted on its side such that the symmetry is defined by a C3-axis through the offset triangular stacks of Co3, whereas in Tet a C4-axis through the axial cobalt atoms of the Co6 octahedron generates a square lattice. EDX spectra of both samples (Figure S3) display zinc, cobalt, and selenium as compared to the EDX spectrum of 1 that lacks Zn peaks. Powder XRD of each sample shows homogeneous crystalline phases (Figures S4 and S5).

Figure 4

SEM images of (a) Tet cubic crystals and (b) Trig hexagonal crystals as synthesized. (c) SEM images of cubic crystals of Tet immersed in a benzoic acid solution in DMF. Striations in the crystals are apparent. (d) AFM height sensor and peak force error images of multilayered Tet films after immersion in benzoic acid solution in DMF. The scale bar in an inset is 3 μm. (e) AFM topographic image of exfoliated sheets. Sheets remain that are about 7.5 nm in thickness, with distinct step sizes apparent. (f) Solid-state cyclic voltammogram of exfoliated Tet sheets in 0.1 M TBAPF6 in tetrahydrofuran with a 50 mV/s scan rate. The solution of exfoliated sheets was dropcast onto a glassy carbon electrode.

The 2D Tet crystals behave like traditional “atomic” layered compounds such as transition metal dichalcogenides in that we can exfoliate these materials without having the layers disintegrate. We reasoned that since the multicoordinate Zn2+ ions in some fashion hold the layers together, a solution of a weak acid would chemically dissociate the layers of Tet and that they would be stable to these conditions (having been originally formed in acidic conditions). We first immersed the cubic crystals of Tet in a 1 mM solution of benzoic acid in DMF. SEM micrographs of immersed cubes show visible layered striations within the crystals (Figure c). Next, we immersed the Tet crystals in 40 mM benzoic acid overnight and followed the transformation with powder X-ray diffraction (Figure S6). The reflections that are due to Tet disappear, with only low intensity peaks corresponding to trace impurities of Trig still visible. During this process, we observe a color change in the solution from clear to light brown upon suspension in the benzoic acid solution. We drop-casted this solution on a silicon substrate (SiO2 on Si) and characterized the films with optical microscopy and atomic force microscopy (AFM). Figures d and S7 clearly show layered 2D sheets. Thin sheets with a thickness of 7.5 nm are present throughout the samples (Figure e), with step sizes between the layers corresponding to this thickness. From the SCXRD structure of Tet, the expected thickness of a single sheet is 1.5 nm, corresponding to the Zn–Zn distance between stacked mononuclear Zn atoms in adjacent layers. Thus, 7.5 nm corresponds to five distinct superatom layers. In other images we also observe smaller step sizes of 3.8 and 5.3 nm (Figure S8), corresponding by SCXRD to three layers and four layers, respectively. These chemically exfoliated sheets of Tet once deposited onto a substrate are clean and flat (Figure S9, roughness of 0.3 nm).

We can use these thin layers of Tet from solution to coat the surface of electrodes and probe their redox activity. For comparison, 1 displays three reversible oxidations relative to Fc/Fc+ (Figure S10), and the bulk crystals deposited on the electrode show two broad, quasi-reversible oxidations (Figure S11). When we drop-cast the exfoliated sheets onto a glassy carbon electrode, the cyclic voltammogram of the exfoliated Tet sheets (Figure f) reveals that the redox properties of the superatom building block 1 persist within the sheets as they display three reversible oxidations. No material is released into the electrolyte solution during the cyclic voltammetry. The redox potentials of these exfoliated materials in solution are shifted toward slightly more negative values (−0.2 V difference) relative to those of 1 in solution. We thus assign the oxidation states of the cluster within the sheets (labeled a through d in Figure f) as {Co6Se8}0 through {Co6Se8}3+, using the CV of 1 as a reference point. Another interesting feature of the CV of the electrodes that are covered with the 2D layers of Tet is that these are permeable to the electrolyte. Analysis of bulk Tet crystals revealed that the structure contains 43% solvent-accessible void space, predominantly in open channels oriented along [101].[49,50] The porosity of the bulk crystal is thus preserved upon exfoliation. The important finding is that Tet sheets are solution processable, porous, and redox-active.

Conclusions

In summary, we have developed the reaction chemistry to create the hexatopic Co6Se8[PEt2(4-C6H4COOH)]6 superatom 1. This superatom assembles into a 3D solid that is held together by a hydrogen bond adhesive. We can change this adhesive from 2H+ to Zn2+ and create extended crystalline solids Trig and Tet. A seemingly small change in the solvent system from DMF/methanol to DMF/ethanol yields remarkable changes in crystal morphology and structure, from a 3D to a 2D extended solid. Both solids are held together via zinc–carboxylate bonds. Two-dimensional Tet can be chemically exfoliated to yield ultrathin yet soluble layers. These layers can be deposited from solution onto substrates. The sheets are redox-active, preserving the redox activity of their component superatoms. These types of porous, ultrathin, and redox-active sheets will find utility in a number of other applications such as modified electrodes for catalysis, batteries, and nanoscale electronic sieves.