Two-Dimensional Nanosheets from Redox-Active SuperatomsAnouck M. Champsaur,† Jaeeun Yu,† Xavier Roy,† Daniel W. Paley,*,†,‡ Michael L. Steigerwald,*,†
Colin Nuckolls,*,† and Christopher M. Bejger*,§
†Department of Chemistry, and ‡Columbia Nano Initiative, Columbia University, New York, New York 10027,United States§Department of Chemistry, The University of North Carolina at Charlotte, Charlotte, North Carolina 28223, United States
*S Supporting Information
ABSTRACT: We describe a new approach to synthesize two-dimensional (2D) nanosheets from the bottom-up. We func-tionalize redox-active superatoms with groups that can directtheir assembly into multidimensional solids. We synthesizedCo6Se8[PEt2(4-C6H4COOH)]6 and found that it forms a crys-talline assembly. The solid-state structure is a three-dimen-sional (3D) network in which the carboxylic acids formintercluster hydrogen bonds. We modify the self-assembly byreplacing the reversible hydrogen bonds that hold the super-atoms together with zinc carboxylate bonds via the sol-vothermal reaction of Co6Se8[PEt2(4-C6H4COOH)]6 withZn(NO3)2. We obtain two types of crystalline materials usingthis approach: one is a 3D solid and the other consists ofstacked layers of 2D sheets. The dimensionality is controlled by subtle changes in reaction conditions. These 2D sheets can bechemically exfoliated, and the exfoliated, ultrathin 2D layers are soluble. After they are deposited on a substrate, they can beimaged. We cast them onto an electrode surface and show that they retain the redox activity of the superatom building blocks dueto the porosity in the sheets.
■ INTRODUCTION
In this manuscript we connect transition metal chalcogenidemolecular clusters into three-dimensional (3D) and two-dimensional (2D) solids, as well as free-floating nanosheets.Monolayer 2D materials such as graphene and transition metaldichalcogenides show promise for next-generation electronics,yet are plagued by the occurrence of defects, and it is not easyto modify them synthetically.1,2 The type of nanosheet wedisclose here, due to the redox activity and multinuclearity of itssuperatom components, provides a new level of complexity andsynthetic sophistication to 2D materials. Our building blocksare atomically defined entities whose isolated electronic andredox properties can be incorporated into extended structuresin which the structural element is preserved. Recent theoreticalcalculations have established that polynuclear Co6Se8L6 clustersbehave as “superatoms”.3−8 We have previously used such super-atoms to form solids from two different yet electronically com-plementary building blocks; directed-layer fullerene assembliesfrom phenanthrene-decorated clusters; and covalent assembliesthrough directed ligand exchange.9−12 Redox-active M6E8
clusters (M = Re, W; E = S, Se) have previously been functional-ized with reactive ligands to generate frameworks of thesepreformed entities through cyanide and bipyridine coordinationwith transition metal ions.13−18 Others have employed a varietyof techniques to direct clusters and nanocrystals into extendedlattices.19−22
The key to unlocking both the structural utility and thefunctional solid-state infrastructure of the superatoms is theability to manipulate their surface properties at will. In thisstudy, we demonstrate a method to do so by converting theCo6Se8[PEt2(4-C6H4Br)]6 superatom into one that presents sixcarboxylic acids. We then introduce zinc carboxylate bonds viaa solvothermal reaction to produce two types of crystallinesolids, a trigonal 3D solid (Trig3D) and a tetragonal 2D solid(Tet2D) (Figure 1). Single crystal X-ray diffraction (SCXRD)reveals that Trig3D is a 3D network of superatoms held togetherwith zinc carboxylate bonds, but Tet2D forms 2D sheets thatthen stack through noncovalent forces into a 3D solid. We findit remarkable that the two-dimensionality of Tet2D is robust:individual 2D sheets can be exfoliated intact from the solid, andthese exfoliated sheets can be subsequently redeposited onarbitrary substrates. When we cast them on electrode surfacesthey retain the redox activity of the superatom building blocks.
■ RESULTS AND DISCUSSION
We previously organized these superatoms into extended vander Waals solids; our new objective was to connect the super-atoms to make extended solids through bonds. Our simple,phosphine-terminated superatoms, however, are inert in the
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sense that the phosphines (by design) chemically passivate thecluster surfaces and do not participate in the reaction chemistry.Thus, our first challenge was to create appropriately reactivesuperatom building blocks. To do so we first treated Co2(CO)8and Se with Et2P(4-C6H4Br) to give Co6Se8[PEt2(4-C6H4Br)]6(SCXRD in Figure S1) in high yield. Other than the obviousdifferences in the size, shape, and arrangement of the organiccomponents, the inorganic core of Co6Se8[PEt2(4-C6H4Br)]6 isidentical to the parent cluster, Co6Se8(PEt3)6.
12 Through a6-fold lithium/halogen exchange followed by the addition ofCO2 gas and subsequent acidification, we converted each Br inthis compound to the corresponding carboxylic acid to yield thenanosized octahedral, superatom building block Co6Se8[PEt2-(4-C6H4COOH)]6 (1). This sequence is facile and highyielding. The lithium/halogen exchange is a harsh process,and the fact that the Co6Se8 core is unchanged reveals a newmethod to easily activate and functionalize superatoms.We determined the molecular structure of 1 using SCXRD
(Figure 1a and Figure S2). 1 assembles into an organized,extended, 3D solid via extensive and ordered hydrogen bondingbetween carboxylic acids on neighboring clusters (Figure 1b).
We refer to the latter solid as 1-H. If we represent each clusteras a sphere, we see that this solid forms such that there ishydrogen-bonding between nearest neighbors (Figure 1c). Theformation of this solid-state compound is reversible: 1-Hdissolves in tetrahydrofuran to regenerate 1.We then sought to create solids from building block 1
through metal−carboxylate bonds. For example, would thesimple replacement of the two protons with a divalent metalion result in a structurally diverse family of new solids?23−40
Thus, we treated our hexatopic superatom with Zn(NO3)2 todetermine the extent to which the carboxylate−carboxylatebonds, which constitute the adhesive that stabilizes this solid,can be modified and improved. Co6Se8 superatoms are usefulbuilding 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 ispreformed and atomically defined, and thus programmable.Using the same building blocks, 1 and Zn2+, we can selec-
tively synthesize two different solids, Trig3D and Tet2D, byvarying the growth conditions. It is remarkable that the onlysignificant difference between the two reactions is the use ofmethanol versus ethanol as solvents. We obtained structures forboth solids using SCXRD (details of the refinement can befound in the Supporting Information). In both solids, all thecarboxylic acid hydrogen bonds of 1-H are replaced bycarboxylate−zinc−carboxylate nodes. SCXRD of both solidsreveals that while they have the same Zn:[Co6Se8] stoichio-metric ratio of 3:1, both the dimensionality of their extendedstructure and orientation of the cluster within the solids differsignificantly. Trig3D is a 3D network while Tet2D is a 2D struc-ture with strong in-plane bonding and comparatively weaknoncovalent interlayer interactions.We combined 1 and Zn(NO3)2 in a DMF/MeOH solvent
mixture under solvothermal conditions at 65 °C, and obtainedblack hexagonal crystals after 24 h. Figure 2 displays the crystalstructure of Trig3D. The structure is a network in which 1 iscoordinated to unusual trinuclear zinc nodes in three dimen-sions (Figure 2a). Looking down the b-axis we clearly seethe distinct pseudotrigonal layers of the solid (Figure 2b).Within each layer, the superatoms are bound to six zinc-nodes(Figure 2c). These layers are then cross-linked by a singleZn−O bond. The approximate 3-fold symmetry of the pseudo-trigonal lattice of Trig3D arises because the Co6 octahedron istilted on its face in the layer, which orients the phosphines suchthat three point up and three point down (Figure 2d). Thissymmetry mirrors that of 1-H, replacing hydrogen bonds withan organized trinuclear metal node (labeled Zn1, Zn2, and Zn3in Figure 2e). Unusual trinuclear zinc nodes have been reportedpreviously.44,45
Each zinc atom in Trig3D exhibits a different coordinationenvironment. Zn1 and Zn2 display distorted tetrahedral geo-metry and together form a three-bladed trigonal paddlewheelwith three bridging μ2-carboxylates. A solvent molecule (likelyMeOH) coordinates Zn1 axially, and Zn2 is axially coordinatedby a μ2-carboxylate, whose second oxygen coordinates Zn3. Zn3exhibits a distorted square pyramidal geometry. Each super-atom 1 within the solid contains three types of carboxylates,labeled a, b, and c in Figure 2e,f. For a, three μ2-carboxylatesform the Zn1−Zn2 paddlewheel; for b, two carboxylates coor-dinate Zn3 in an η2 fashion; for c, a μ2-carboxylate coordinatesboth Zn3 and Zn2. The latter ligand c also serves to cross-linkthe layers through its carboxylate−Zn2 bond. This bond has alength of 2.22 Å, which is a long Zn−O contact,46 and suggests
Figure 1. (a) Structure of 1 from SCXRD, Co6Se8 cluster capped with4-(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 atomsare omitted to clarify the view. (b) 1 forms a 3D hydrogen-bondnetwork, named 1-H. View of 1-H down the a-axis, showing a singlesuperatom and its six hydrogen-bonds to neighboring superatoms(in blue). (c) Representation of the view in (b) with each superatomas a sphere to emphasize the structure of the extended solid. (d) and(e) The solvothermal reaction of Co6Se8[PEt2(4-C6H4COOH)]6 withZn(NO3)2 forms two different types of extended solid, Trig3D andTet2D, depending upon solvent conditions. (d) In Trig3D the super-atoms are held together within a 2D plane to create a trigonalarrangement of superatoms, and the planes extend in three dimensionsvia further zinc carboxylate bonds. The axis of symmetry defining asuperatom within a Trig3D sheet is a C3-axis through the center of twoplanes defined by Co3 atoms. (e) In Tet2D the superatoms are heldtogether within a 2D plane to create a distorted square arrangement.The 2D layers are noncovalently stacked in the third dimension. Theaxis of symmetry defining the Tet2D plane is a C4-axis through axialatoms of a Co6 octahedron.
the interlayer carboxylate−Zn2 bond is a weaker, dative bondcompared to intralayer carboxylate−Zn bonds.The presence of weak interlayer zinc bonds in Trig3D pro-
mpted us to modify reaction conditions to eliminate interlayerbonding and synthesize 2D layers. Thus, we reacted 1 andZn(NO3)2 at 65 °C in a DMF/EtOH solvent mixture andobtained black cubic crystals after 24 h. We note that a smallfraction of Trig3D forms under these conditions but can elim-inated with the addition of “extra” protons in the form of HClin the reaction. Under these conditions, we form exclusively thenew solid-state compound, Tet2D (Figure 3). Tet2D also con-tains complete replacement of proton-nodes with metal nodes,although the types of metal nodes and dimensionality differfrom Trig3D. Distinct layers of superatoms are held togetheronly by noncovalent forces.Tet2D is a layered 2D material in which each layer is a
square arrangement of Co6Se8 superatoms with four phosphineligands residing in the 2D plane and bonding to four-bladedZn-carboxylate paddlewheels (Figure 3a). In the directionnormal to the sheet, the axial carboxylate ligands coordinate anadditional Zn2+ ion that is positioned above or below the squaresheet (Figure 3b). Figure 3c,d displays the binding interac-tion of each ligand of 1 within the solid. The zinc subunitwithin the square plane of this solid is a dinuclear four-bladedZn-carboxylate paddlewheel (Figure 3e). The combination oftwo Zn2+ ions and four bridging μ2-carboxylate groups yieldsthis Zn2 cluster with a Zn−Zn distance of 2.867(7) Å that isconsistent with other such “four-bladed” paddlewheels in zinc-based metal−organic frameworks.47 Pairs of apical phosphineson adjacent clusters that are not involved in dinuclear Znpaddlewheels within a single layer are linked via a single Znatom (in addition to their bonding via the intralayer Zn2 node)to form a mononuclear zinc complex. This complex featuresZn−O distances of 2.20 (3) and 2.36(3) Å and a carboxylate-Zn-carboxylate angle of approximately 119°. This geometry is
Figure 2. Structure of Trig3D from SCXRD: a 3D network synthesized from the solvothermal reaction of 1 and Zn(NO3)2 in a MeOH/DMF solventmixture. 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 pseudotrigonalarrays of superatoms. The blue box highlights the trinuclear zinc node, which is magnified in (e). (c) Side-on view of 1 within a pseudotrigonal layerand (d) top-down view of a single “layer” within Trig3D. (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 Trig3Dlayer surrounded by three different Zn-carboxylate binding modes. Three types of carboxylates are shown: a, b, and c.
Figure 3. Structure of Tet2D from SCXRD: square sheets in the crys-talline state. Tet2D is 2D network synthesized from the solvothermalreaction of 1 and Zn(NO3)2 in a EtOH/DMF solvent mixture. Ethylgroups are omitted for clarity. (a) Top-down view of a single layerwithin Tet2D along the b-axis. (b) Side-on view of Tet2D layers alongthe c-axis. Noncovalent forces hold the layers together in the thirddimension. (c) Single superatom in a Tet2D layer and the bindinginteraction of each carboxylate of 1. Within the 2D plane, each of thefour equatorial carboxylate ligands coordinates two Zn2+ ions, formingthe four-bladed paddlewheel upon coordination of equatorialcarboxylate ligands of three adjacent superatoms. The axial carboxylateligands coordinate an additional Zn2+ ion that lies just above or belowthe square sheet. (d) Top-down view of 1 within the 2D plane.(e) Four-bladed Zn2+ paddlewheel. Zn−Zn = 2.867(7) Å. (f) Mono-nuclear zinc complex with Zn−O distances = 2.20(3) and 2.36(3) Åand a carboxylate−Zn−carboxylate angle of 118.6(12)°.
typical of pseudotetrahedral Zn(O2R)2L2 complexes,48 but inthis case we note that the two L-type ligands (presumablyethanol or water) are disordered and could not be located. Thelayers are self-contained and stack through noncovalent inter-layer interactions in an eclipsed arrangement.The crystal packing arrangements of Trig3D and Tet2D are pro-
pagated in their macroscopic crystal morphologies. Figure 4a,bshows SEM micrographs of the crystals that form after 24 hgrowth. The black cubes (Tet2D; Figure 4a) and hexagonalplates (Trig3D; Figure 4b) reflect the tetragonal and trigonallattices of their crystalline arrangements. In Trig3D, the clusteris tilted on its side such that the symmetry is defined by aC3-axis through the offset triangular stacks of Co3, whereas inTet2D a C4-axis through the axial cobalt atoms of the Co6octahedron generates a square lattice. EDX spectra of bothsamples (Figure S3) display zinc, cobalt, and selenium as com-pared to the EDX spectrum of 1 that lacks Zn peaks. PowderXRD of each sample shows homogeneous crystalline phases(Figures S4 and S5).The 2D Tet2D crystals behave like traditional “atomic”
layered compounds such as transition metal dichalcogenides inthat we can exfoliate these materials without having the layersdisintegrate. We reasoned that since the multicoordinate Zn2+
ions in some fashion hold the layers together, a solution of aweak acid would chemically dissociate the layers of Tet2D andthat they would be stable to these conditions (having beenoriginally formed in acidic conditions). We first immersed thecubic crystals of Tet2D in a 1 mM solution of benzoic acid inDMF. SEM micrographs of immersed cubes show visiblelayered striations within the crystals (Figure 4c). Next, weimmersed the Tet2D crystals in 40 mM benzoic acid overnightand followed the transformation with powder X-ray diffraction(Figure S6). The reflections that are due to Tet2D disappear,with only low intensity peaks corresponding to trace impuritiesof Trig3D still visible. During this process, we observe a colorchange in the solution from clear to light brown upon sus-pension in the benzoic acid solution. We drop-casted this solu-tion on a silicon substrate (SiO2 on Si) and characterized the
films with optical microscopy and atomic force microscopy(AFM). Figures 4d and S7 clearly show layered 2D sheets. Thinsheets with a thickness of 7.5 nm are present throughout thesamples (Figure 4e), with step sizes between the layers cor-responding to this thickness. From the SCXRD structure ofTet2D, the expected thickness of a single sheet is 1.5 nm,corresponding to the Zn−Zn distance between stacked mono-nuclear Zn atoms in adjacent layers. Thus, 7.5 nm correspondsto five distinct superatom layers. In other images we alsoobserve 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 Tet2D oncedeposited onto a substrate are clean and flat (Figure S9,roughness of 0.3 nm).We can use these thin layers of Tet2D from solution to coat
the surface of electrodes and probe their redox activity. Forcomparison, 1 displays three reversible oxidations relative to Fc/Fc+ (Figure S10), and the bulk crystals deposited on the elec-trode show two broad, quasi-reversible oxidations (Figure S11).When we drop-cast the exfoliated sheets onto a glassy carbonelectrode, the cyclic voltammogram of the exfoliated Tet2Dsheets (Figure 4f) reveals that the redox properties of thesuperatom building block 1 persist within the sheets as theydisplay three reversible oxidations. No material is releasedinto the electrolyte solution during the cyclic voltammetry. Theredox potentials of these exfoliated materials in solution areshifted toward slightly more negative values (−0.2 V difference)relative to those of 1 in solution. We thus assign the oxidationstates of the cluster within the sheets (labeled a through d inFigure 4f) as {Co6Se8}
0 through {Co6Se8}3+, using the CV of 1
as a reference point. Another interesting feature of the CV ofthe electrodes that are covered with the 2D layers of Tet2D isthat these are permeable to the electrolyte. Analysis of bulkTet2d crystals revealed that the structure contains 43% solvent-accessible void space, predominantly in open channels orientedalong [101].49,50 The porosity of the bulk crystal is thus pre-served upon exfoliation. The important finding is that Tet2Dsheets are solution processable, porous, and redox-active.
Figure 4. SEM images of (a) Tet2D cubic crystals and (b) Trig3D hexagonal crystals as synthesized. (c) SEM images of cubic crystals of Tet2Dimmersed in a benzoic acid solution in DMF. Striations in the crystals are apparent. (d) AFM height sensor and peak force error images ofmultilayered Tet2D films after immersion in benzoic acid solution in DMF. The scale bar in an inset is 3 μm. (e) AFM topographic image ofexfoliated sheets. Sheets remain that are about 7.5 nm in thickness, with distinct step sizes apparent. (f) Solid-state cyclic voltammogram of exfoliatedTet2D 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 carbonelectrode.
■ CONCLUSIONSIn summary, we have developed the reaction chemistry tocreate the hexatopic Co6Se8[PEt2(4-C6H4COOH)]6 superatom 1.This superatom assembles into a 3D solid that is held togetherby a hydrogen bond adhesive. We can change this adhesivefrom 2H+ to Zn2+ and create extended crystalline solids Trig3Dand Tet2D. A seemingly small change in the solvent systemfrom DMF/methanol to DMF/ethanol yields remarkablechanges in crystal morphology and structure, from a 3D to a2D extended solid. Both solids are held together via zinc−carboxylate bonds. Two-dimensional Tet2D can be chemicallyexfoliated to yield ultrathin yet soluble layers. These layers canbe deposited from solution onto substrates. The sheets areredox-active, preserving the redox activity of their componentsuperatoms. These types of porous, ultrathin, and redox-activesheets will find utility in a number of other applications such asmodified electrodes for catalysis, batteries, and nanoscaleelectronic sieves.
■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acscentsci.7b00328.
ORCIDXavier Roy: 0000-0002-8850-0725Colin Nuckolls: 0000-0002-0384-5493Christopher M. Bejger: 0000-0001-9263-5414NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTSC.N. thanks Sheldon and Dorothea Buckler for their generoussupport. Support for this research was provided by the Centerfor Precision Assembly of Superstratic and Superatomic Solids,an NSF MRSEC (Award Number DMR-1420634), and the AirForce Office of Scientific Research (Award Number FA9550-14-1-0381). Single crystal X-ray diffraction was performed atthe Shared Materials Characterization Laboratory at ColumbiaUniversity, maintained using funding from Columbia Universityfor which we are grateful.
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