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Article pubs.acs.org/JPCC

Single-Layer [Cu2Br(IN)2]n Coordination Polymer (CP): Electronic and Magnetic Properties, and Implication for Molecular Sensors Qing Tang,†,‡ Zhen Zhou,*,† and Zhongfang Chen*,‡ †

Institute of New Energy Material Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Computational Centre for Molecular Science, Nankai University, Tianjin 300071, People's Republic of China ‡ Department of Chemistry, Institute for Functional Nanomaterials, University of Puerto Rico, Rio Piedras Campus, San Juan, PR 00931 S Supporting Information *

ABSTRACT: Inspired by the recent breakthrough in synthesizing the two-dimensional (2D) [Cu2Br(IN)2]n (IN = isonicotinato) single-layer coordination polymer (CP) (Chem. Commun. 2010, 46, 3262), we systematically investigated the structural, electronic, and magnetic properties of this periodic monolayer [Cu2Br(IN)2]n CP, as well as its possible application as molecular sensors by means of density functional theory computations. The pristine monolayer [Cu2Br(IN)2]n CP is ground-state antiferromagnetic with a band gap of 0.47 eV. Among various gas molecules (H2, O2, CO, CO2, NO, NO2, N2, and NH3), NO and NO2 have strong interactions with the metal centers and can effectively modify the electronic structure of this monolayer [Cu2Br(IN)2]n CP, suggesting the feasibility of designing 2D CP-based molecular sensors to detect NO and NO2 molecules.

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INTRODUCTION Metal−organic frameworks (MOFs), first termed by Yaghi et al. in 1995,1 are a new family of nanoporous materials composed of long-range networks constructed by organic linkers and metal ion centers. Depending on the building blocks (metal ions and ligands), MOFs exhibit a rich variety of architectures and dimensionalities, and diverse physicochemical properties.2 In particular, the extremely high surface areas and tailored pore sizes make MOFs promising in a broad range of applications,3 such as gas storage,4 catalysis,5 drug delivery6 and sensing.7 Recently, the rising of single-layer graphene8 initiated a rapidly growing field of two-dimensional (2D) materials. The excellent properties and potential technological applications of graphene stimulated the fascinating research targets toward other 2D materials.9 Actually, with intriguing topologies and great prospects in functional materials, MOFs with a one-atom thickness, here termed as a single-layer coordination polymer (CP) to differentiate it from the bulk or multilayered MOFs, may act as excellent alternatives of graphene.10 Despite the large-scale synthesis of 2D MOFs with diverse architectures in the literature, most of the as-grown 2D MOFs are assembled with layered structures, and synthesizing monolayer CPs remains a big challenge.11 Encouragingly, despite these difficulties, impressive progress has been achieved on the experimental12 and theoretical13 investigations of one-atomthick CP flakes. © 2012 American Chemical Society

Particularly, a novel single-layer CP has been extracted from crystals of [Cu2Br(IN)2]n (IN = isonicotinato),12a via a sonication method with the mechanical force to remove the interlayer interaction. Similar to other class III mixed-valence polymers,14 single-layer [Cu2Br(IN)2]n CP is assembled with pairs of Cu−Cu metal bonds, where each Cu dimer bridged by bromine is coordinated by four oxygen atoms from two IN units and two nitrogen atoms from the other two IN ligands (Figure 1), generating a unique (Cu2Br)3+ unit with mixed Cu(I)−Cu(II) metal bond formulation. Such mixed-valence Cu(I)−Cu(II) chemistry15 is of renewed interest after the discovery of binuclear “CuA” sites in cytochrome c oxidase16 and nitrous oxide reductase,17 which contain fully spindelocalized [Cu2]3+ dimers (class III) bridged by cysteine units. Gas adsorption is emerging as an attractive research field for MOFs due to an ever increasing need for high-efficient energy storage, for the treatment of pollutant gases, or for the exploration of surface chemical reactions. Particularly, MOFs have been widely employed for applications in gas storage (e.g., hydrogen,18 carbon dioxide19 and methane20), gas separation,21 or sensing,22 where the surface transition-metal centers provide the active sites for gas adsorption. As a newly isolated oneatom-thick crystalline polymer, many properties concerning single-layer [Cu2Br(IN)2]n CP remain unknown, and its Received: October 6, 2011 Revised: January 15, 2012 Published: January 17, 2012 4119

dx.doi.org/10.1021/jp211779w | J. Phys. Chem. C 2012, 116, 4119−4125

The Journal of Physical Chemistry C

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are arranged on the same side of the [Cu2Br(IN)2]n backbone (Figure 1d). To identify the possible magnetic configurations, a larger supercell containing four formula units was used (Figure 2), where the supercell of monolayer [Cu2Br(IN)2]n(I) contains 116 atoms. Here, only the magnetic configurations of monolayer [Cu2Br(IN)2]n(I) are illustrated; the FM, AFM-1, and AFM-2 states for the bulk and [Cu2Br(IN)2]n(II) are set in the same way. Our density functional theory (DFT) computations were performed by using the plane-wave technique implemented in the Vienna ab initio Simulation Package (VASP).23 The ion− electron interaction is described with the projector augmented wave (PAW) method.24 The exchange-correlation energy is described by the functional of Perdew, Burke, and Ernzerhof (PBE)25 based on the generalized gradient approximation (GGA) for both spin-polarized and spin-unpolarized cases. Note that the standard PBE functional cannot well describe the dispersion interactions among [Cu2Br(IN)2]n CP layers or between the adsorbed gas molecules and monolayer [Cu2Br(IN)2]n CP. Thus, we adopted the recently developed PBE-D2 method26 with the inclusion of dispersion energy correction. A 400 eV cutoff for the plane-wave basis set was used in all the computations. All structures were treated with periodic boundary conditions (PBC), and the supercell was large enough to ensure a vacuum spacing greater than 10 Å. Geometry optimizations were performed by using the conjugated gradient method, and the convergence threshold was set to be 10−4 eV in energy and 10−3 eV/Å in force. For geometry optimization, the Brillouin zone was sampled by 5 × 5 × 1 special k points, while larger k-point grids (25 × 25 × 1) were used for band structure computations. Because the GGAPBE functional tends to technically underestimate the band gap of semiconductors, we also used the GGA+U method27 for band gap evaluations. The adsorption energies of small molecules on monolayer [Cu2Br(IN)2]n CP are defined as Ead = E(molecule+[Cu2Br(IN)2]n) − E[Cu2Br(IN)2]n − Emolecule, where E(molecule+[Cu2Br(IN)2]n), E[Cu2Br(IN)2]n, and Emolecule are the total energies of the relaxed monolayer [Cu2Br(IN)2]n CP with molecule adsorption, monolayer [Cu2Br(IN)2]n CP, and the molecule, respectively. Hence, a negative (positive) Ead denotes an exothermic (endothermic) adsorption process.

Figure 1. (a) Detailed view of the [Cu2]3+ coordination environment, with the corresponding atoms marked by element symbols. (b) The unit cell of bulk [Cu2Br(IN)2]n MOF. (c) The unit cell of monolayer [Cu2Br(IN)2]n(I) directly derived from the bulk structure. (d) Another type of monolayer isomer, [Cu2Br(IN)2]n(II), where the protruded Br atoms are located on the same side of the [Cu2Br(IN)2]n backbone.

technical applications are still in current pursuit. For its unique topology with transition-metal centers, organic ligands, and porous structure, it is of particular interest to understand the adsorption interactions between different gas molecules and the [Cu2Br(IN)2]n monolayer. In this work, we performed systematic theoretical studies on the electronic and magnetic properties of monolayer [Cu2Br(IN)2]n CP and investigated its interactions with different gas molecules to explore the feasibility of using [Cu2Br(IN)2]n CP as molecular sensors. Particular attention is paid to understanding the modifications of electronic structures of monolayer [Cu2Br(IN)2]n CP by molecular adsorption.

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COMPUTATIONAL MODELS AND METHODS The bulk (Figure 1b) and monolayer (Figure 1c) [Cu2Br(IN)2]n were constructed according to the reported crystal information (Figure 1a).12a Its primitive cell (orthorhombic, Pbcn symmetry) includes two (Cu2Br)3+ nodes and four IN linker molecules, corresponding to two Cu2Br(C12H8N2O4)2 formula units. In this structure, the protruded Br atoms are located alternatively on both sides of the base plane. Note that the experimentally prepared iodine variant [Cu2I(IN)2]n may also adopt another configuration where all the iodine atoms are on the same side of the [Cu2I(IN)2]n backbone (Figure 1d). Thus, we considered both configurations: [Cu2Br(IN)2]n(I), the original monolayer structure (Figure 1c), in which the Br atoms are alternatively distributed on both sides, and [Cu2Br(IN)2]n(II), where all the Br atoms

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RESULTS AND DISCUSSION Structural, Electronic, and Magnetic Properties of the Bulk and Monolayer [Cu2Br(IN)2]n. We investigated both the

Figure 2. Top view of the monolayer [Cu2Br(IN)2]n(I) supercell with three magnetic configurations: (a) ferromagnetic (FM), (b) antiferromagnetic-1 (AFM-1), and (c) antiferromagnetic-2 (AFM-2). The magnetic configurations for bulk MOF and monolayer [Cu2Br(IN)2]n(II) are set in the same way. 4120



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The optimized structural parameters (lattice parameters and geometric bond lengths) of the bulk [Cu2Br(IN)2]n are well consistent with the available experimental values (Table 1).

bulk and the monolayer crystal structure of [Cu2Br(IN)2]n. For clarity, a detailed coordination arrangement of the [Cu2]3+ node is shown in Figure 1a. The unit cells of the bulk and monolayer [Cu2Br(IN)2]n as well as another monolayer isomer are presented in Figure 1, but a larger supercell is needed for the investigations of magnetic ground states (Figure 2). The monolayer [Cu2Br(IN)2]n supercell consists of four crystallographic distinct Cu atoms, which form [Cu2]3+ pairs along the monolayer open-frame network. One of the [Cu2]3+ pairs is perpendicular to the other pair in orientation (Figure 2). Those dicopper complexes, [Cu2L]3+, on the basis of the electron spin resonance (ESR) data and spectroscopic findings, are described as fully delocalized Cu−Cu bonding, indicating the equivalence and the average-valence oxidation for each Cu center. Because each [Cu2]3+ ion possesses one unpaired electron, it is important to consider the possible magnetic coupling configurations. Our computations were performed for ferromagnetic (FM), antiferromagnetic-1 (AFM-1), and antiferromagnetic-2 (AFM-2) configurations, as schematically shown in Figure 2. For both the bulk MOF and monolayer [Cu2Br(IN)2]n(I) or [Cu2Br(IN)2]n(II) CP, the AFM-2 state is most favorable thermodynamically, which is about 0.27 eV (bulk), 0.13 eV (monolayer I), and 0.12 eV (monolayer II) per supercell lower in energy than the FM state, and 0.25 eV (bulk), 0.12 eV (monolayer), and 0.11 eV (monolayerII) per supercell lower in energy than the AFM-1 state. This indicates an antiferromagnetic superexchange interaction between two adjacent [Cu2]3+−IN−[Cu2]3+ dimers mediated through IN ligands. The ground-state antiferromagnetic behavior is also consistent with the magnetic susceptibility measurements, which verified that bulk [Cu2Br(IN)2]n presents one unpaired localized electron per Cu dimer with a weak antiferromagnetic interdimer coupling.12a In the energetically most favorable AFM-2 state, the monolayer [Cu2Br(IN)2]n(II) is higher in energy than the monolayer [Cu2Br(IN)2]n(I) by about 0.92 eV per supercell, suggesting that the alternative distribution of Br atoms on both sides of [Cu2Br(IN)2]n is structurally more stable, and the isolated monolayer [Cu2Br(IN)2]n from bulk MOF should thermodynamically prefer the initial coordination configuration rather than reconstructing to form its isomerized structure. Hence, we will only concentrate on the [Cu2Br(IN)2]n(I) structure in our following discussion. On the other hand, the bulk [Cu2Br(IN)2]n is composed of layered [Cu2Br(IN)2]n(I) with the superposition of layers along the b axis. These separated [Cu2Br(IN)2]n monolayers are linked together mainly by van der Waals forces. The PBE-D2optimized interlayer distance between two adjacent [Cu2Br(IN)2]n(I) layers is 3.32 Å, in good agreement with the experimental value of 3.35 Å, which demonstrates the effectiveness of the PBE-D2 method in describing the weak interlayer interactions. The interlayer interaction energy, defined as the energy difference between the bulk [Cu2Br(IN)2]n and two separated [Cu2Br(IN)2]n monolayers, is computed to be −1.35 eV per supercell and indicates weak interactions between layers. Especially, the interlayer distance of bulk [Cu2Br(IN)2]n is very close to the interlayer separation (3.33 Å) of bulk graphite. The successful isolation of singlelayer graphene from graphite offers a pioneering work on the realization of one-atom-thick materials at the nanoscale. Similarly, the fabrication of monolayer [Cu2Br(IN)2]n(I) from bulk MOF is highly achievable.

Table 1. Selected Structural Parameters for the Bulk MOF and Monolayer [Cu2Br(IN)2]n(I) CPa parameters

computed

a/Å b/Å c/Å Cu−Cu (Å) Cu−O1, Cu−O2 (Å) Cu−N (Å) Cu−Br (Å)

lattice 14.108 (14.118) 6.750 13.869 (13.923) geometry (bond length) 2.435 (2.443) 1.969, 1.976 (2.000, 2.002) 1.996 (1.992) 2.536 (2.464)

exptl12a 14.101 6.715 13.856 2.391 1.948, 1.951 2.020 2.562

a

The results for the monolayer [Cu2Br(IN)2]n(I) CP are placed in parentheses.

Moreover, the structural parameters of monolayer [Cu2Br(IN)2]n(I) resemble significantly those of the bulk MOF, except that the monolayer structure has a slightly larger lattice parameter and longer (shorter) Cu−Cu and Cu−O (Cu−N and Cu−Br) bond lengths. In the energetically most favorable AFM-2 state of the bulk and monolayer [Cu2Br(IN)2]n(I), the optimized Cu−Cu distances (2.435 and 2.443 Å for bulk and monolayer, respectively) are comparable with the majority of [Cu2]3+ complexes determined previously.15,28,29 The [Cu2]3+ node does not spontaneously form without steric enforcement, but is achieved by the aid of ligands, which constrain the Cu bonding orbitals to approach a bonding distance (2.4 Å) in a symmetric coordination environment, where both +1 and +2 oxidation states are acceptable. The fully delocalized Cu−Cu bonding is also supported by the electron density map of monolayer [Cu2Br(IN)2]n CP (Supporting Information), which ensures a delocalized overlap of the dz2 orbital from the pair of Cu ions in a strong σ bond. The independent Br atoms are placed alternatively on both sides of the layer, which bridge the [Cu2]3+ pairs with a Cu−Br distance of 2.536 Å for the bulk and 2.464 Å for the monolayer, respectively. Furthermore, both the oxygen donor and the nitrogen donor from the same IN ligands are bonded to the [Cu2]3+ pair (the Cu−O distances are 1.969 and 1.976 Å for the bulk, and 2.000 and 2.002 Å for the monolayer; the Cu−N distance is 1.996 Å for the bulk and 1.992 Å for the monolayer), making each Cu atom adopt a trigonal-bipyramidal coordination geometry. Such a spatial conformation is particularly favorable, in view of the redox preference and the effective overlap of the dz2 orbital when collinear. The stable trigonal-bipyramidal geometries are also available in other mixed-valence [Cu2]3+ compounds under a variety of functional ligands, such as the octaazacryptand macrobicyclic ligands,30 the oxygen-donor ligands,29 and the nitrogen-donor ligands.31 The monolayer CP and bulk [Cu2Br(IN)2]n MOF also share similar electronic properties. Analyzing the band structure of the monolayer and bulk [Cu2Br(IN)2]n corresponding to the AFM-2 state (Figure 3) reveals that the spin-up and spin-down states are degenerate in their energy levels, and the energy bands above the conduction band (0−3 eV) of monolayer [Cu2Br(IN)2]n(I) are very flat along the high symmetry points. Clearly, both bulk and monolayer [Cu2Br(IN)2]n(I) are 4121



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Figure 3. Band structure of the energetically preferred AFM-2 state of the bulk (a) and monolayer (b) [Cu2Br(IN)2]n(I); the spin-up (left) and spindown (right) states are presented separately.

Figure 4. (a) Four possible adsorption sites for monolayer [Cu2Br(IN)2]n(I) CP. The most favorable configurations of the adsorbate: (b) H2, (c) O2, (d) CO, (e) CO2, (f) NO, (g) NO2, (h) N2, and (i) NH3 with 1/4 coverage on the monolayer [Cu2Br(IN)2]n(I) CP. Only the geometry around the metal sites is shown, and the shortest binding distance between the molecule and the Cu atoms are also labeled.

Table 2. Spin-Polarized Adsorption Energies (Ead), Band Gap (Eg), the GGA+U Computed Band Gap, and Partial Charge Transfer (δq) from the Gas Molecules to the Monolayer [Cu2Br(IN)2]n(I) CPa molecules

H2

O2

CO

CO2

NO

NO2

N2

NH3

Ead (eV) Eg (eV) GGA+U (eV) δq (|e|)

−0.148 0.21 0.38 −0.081

−0.913 0.42 0.58 0.425

−0.162 0.45 0.61 −0.152

−0.214 0.41 0.54 −0.186

−0.986 0.07 0.10 1.513

−1.021 0.36 0.49 1.427

−0.126 0.45 0.60 −0.128

−0.257 0.39 0.52 0.198

Negative δq values indicate charge transfer from the monolayer [Cu2Br(IN)2]n(I) CP to molecules, and positive values indicate charge transfer from molecules to the monolayer [Cu2Br(IN)2]n(I) CP. a

detriment of automotive emission; N2 is the largest single constituent of the earth’s atmosphere; and NH3 is a common industrial product in chemical engineering. There are enriched explorations on using MOFs for gas separation, storage, and sensors.7 Understanding the adsorption behaviors of these molecules on monolayer [Cu2Br(IN)2]n(I) CP is thus of both practical and fundamental interest. For each adsorbate, four possible adsorption sites are considered (Figure 4), namely, the central pore (site A), the pyridine ring (site B), the concave transition metal sites (Cu− Cu bond center and the Cu sites, site C), and the protruded Br atoms (site D). The transition-metal site is more favorable to adsorb the target molecules. When adsorbed on one of the four [Cu2]3+ sites for each supercell (with 1/4 coverage), different molecules prefer different configurations. Particularly, NO and NO2 exhibit an extremely short distance from the metal centers (1.960 and 1.984 Å, respectively). After adsorption, the N−O bond length of NO is slightly increased by 0.006 Å (to 1.171 Å), while the N−O bond length (1.229 Å) and O−N−O angle (126.8°) of NO2 deviate greatly from the isolated NO2 molecule (N−O bond length, 1.212 Å; O−N−O angle, 133.5°). The O2 molecule sits 2.537 Å above the Cu atom with an O−O bond length of 1.243 Å, longer than that of the isolated O2 molecule (1.235 Å). The adsorbed H2, CO, CO2,

semiconducting with a small band gap of 0.49 and 0.47 eV, respectively. The experimental measurement indicated that the single crystal of [Cu2Br(IN)2]n MOF shows an electrical conductivity of about (1.2−1.7) × 10−5 S cm−1, but there is currently no experimental characterization on the electronic band gap of this [Cu2Br(IN)2]n CP. Note that the GGA-PBE functional typically underestimates the band gap of semiconductors; we thus used the GGA+U method26 to reassess the band gap, which is corrected to be 0.66 and 0.62 eV for the bulk MOF and monolayer [Cu2Br(IN)2]n(I) CP, respectively. Despite the slight improvement in predicting a larger band gap by GGA+U, the qualitative results and physical basis predicted by GGA, which are our major concerns, should be reliable. Adsorption of H2, O2, CO, CO2, NO, NO2, N2, and NH3 on Monolayer [Cu2Br(IN)2]n(I) CP. Having established the validity of the PBE-D2 method in describing the interlayer interactions in bulk [Cu2Br(IN)2]n MOF (as discussed above), we employed this method to investigate the adsorption behaviors of different gas molecules on the monolayer [Cu2Br(IN)2]n(I) CP. Among the massive gas molecules, CO2 is the greenhouse gas; H2 and O2 are typical components present in the feedstock of fuel cells; NO is an air pollutant produced by combustion of substances in air, like fossil fuel in power plants; CO and NO2 are major pollutants comprising the 4122



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Figure 5. Spin-polarized band structures of monolayer [Cu2Br(IN)2]n(I) CP with gas molecule adsorption at the GGA level of theory: (a) NO and (b) NO2. The spin-up (left) and spin-down (right) states are presented separately.

some impurity states within the valence band of the spin-up state (∼0.18 eV above the Fermi level) and the conduction band of the spin-down state (∼0.22 eV above the Fermi level), resulting in a slightly narrower gap of 0.36 eV (0.49 eV at the GGA+U level). The above results indicate that the monolayer [Cu2Br(IN)2]n(I) CP under this study may serve as good sensors for NO and NO2. To further examine the performance of the monolayer [Cu2Br(IN)2]n(I) CP as NO and NO2 sensors, we examined the adsorption of these two gas molecules at higher coverage. When all the four [Cu2]3+ sites are occupied by the adsorbed NO or NO2 molecules (four molecules per supercell, Figure 6a,b), the binding energy is reduced to 0.763 and 0.847 eV per

N2, and NH3 are located further from the metal centers, with the structural changes of no more than 0.002 Å. We computed the adsorption energies (Ead) of these gas molecules on [Cu2Br(IN)2]n(I) metal centers. O2, NO, and NO2 are chemisorbed on the monolayer surface with large adsorption energies of −0.913, −0.986, and −1.021 eV, respectively, whereas H2, CO, CO2, N2, and NH3 molecules are weakly physisorbed with adsorption energies smaller than 0.3 eV (Table 2). Moreover, on the basis of the Bader32 charge analysis, O2 and NH3 molecules behave as electron donors, as indicated by the positive δq (0.425 and 0.198 |e| for each molecule, respectively), whereas H2, CO, CO2, NO, NO2, and N2 molecules behave as electron acceptors, as indicated by their negative δq (Table 2). Especially, the large charge transfer from the monolayer [Cu2Br(IN)2]n(I) CP to NO and NO2 (1.513 and 1.427 |e|, respectively) infers that these two molecules exhibit strong electron-withdrawing capabilities and form strong interactions with the transition-metal centers. To explore the feasibilities and to understand the inherent mechanisms of using this novel monolayer [Cu2Br(IN)2]n(I) CP as gas sensors to detect the target molecules, we examined the tuning effects of the adsorbed molecules on the electronic and magnetic properties of monolayer [Cu2Br(IN)2]n(I) CP. The spin-polarized band gaps computed by PBE-D2 and GGA+U (Table 2) exhibit the same trend for the gap variation. Among all the considered gas molecules, the adsorption of H2, NO, or NO2 leads to an obvious reduction of the original band gap of the monolayer, while other gases do not show much sensitivity in affecting the electronic gap of monolayer [Cu2Br(IN)2]n(I) CP. Especially, the H2 and NO adsorption greatly reduce the band gap of the pristine monolayer [Cu2Br(IN)2]n(I) CP; however, the rather weak interaction between the H2 molecule and the monolayer [Cu2Br(IN)2]n(I) CP may preclude the gas sensor application for H2 detection. Figure 5 presents the band structure of monolayer [Cu2Br(IN)2]n(I) CP with NO and NO2 adsorption (the band structures of the rest of the systems are provided in the Supporting Information). The NO (Figure 5a) adsorption introduces impurity states in the band gap of both the spin-up and spin-down states, which are hybridized with the original energy bands of the [Cu2Br(IN)2]n(I) CP, leading to a decreased band gap of 0.07 eV (0.10 eV at the GGA+U level). The NO2 molecule has a strong binding with the [Cu2Br(IN)2]n(I), and the band gap is also affected after the NO2 adsorption. The NO2 adsorption (Figure 5b) gives rise to

Figure 6. Side view of the optimized geometric structures of monolayer [Cu2Br(IN)2]n(I) CP with increased coverage of NO (a) and NO2 (b) molecules (four molecules per supercell), and the corresponding band structures for NO (c) and for NO2 (d) systems.

molecule, respectively, concomitant with the increase in binding distance d (1.991 and 2.001 Å) and the decrease in charge transfer δq (1.215 and 1.145 |e|). The increase of adsorbate coverage leads to more impurity states near the band edge, and the band gaps of the NO and NO2 complexes are enlarged to be 0.10 and 0.39 eV, respectively, which are corrected by GGA+U to be 0.16 and 0.51 eV. The correlation between the electronic conductivity of the NO and NO2 adsorbed systems to the molecule coverage suggests that this single-layer [Cu2Br(IN)2]n(I) CP is a good sensor to the NO and NO2 gases. 4123



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CONCLUSION In summary, by means of density functional theory computations, we investigated the electronic and magnetic properties of the experimentally isolated 2D monolayer [Cu2Br(IN)2]n coordination polymer (CP). The pristine monolayer [Cu2Br(IN)2]n favors the antiferromagnetic dimercoupling state and is ground-state semiconducting. The monolayer [Cu2Br(IN)2]n CP resembles significantly the bulk MOF with regard to their structures and electronic and magnetic properties. Since the band gap (0.47 eV) is small, this monolayer [Cu2Br(IN)2]n CP is electronically preferential than the graphene or hexagonal BN, due to its intrinsically zero-gap semimetal or wide-band-gap insulating character. Furthermore, the band gap of this monolayer [Cu2Br(IN)2]n CP is much smaller than that of the traditional semiconductor materials, such as silicon, GaN, ZnO, and TiO2, thus offering the prospect of band engineering for potential applications in nanoelectronics or spintronics. We also investigated the adsorption geometries and electronic structures of this monolayer [Cu2Br(IN)2]n CP with gas adsorption (H2, O2, CO, CO2, NO, NO2, N2, and NH3) and found that the NO, NO2, and O2 molecules are chemisorbed with the metal centers, whereas the other gas molecules are only weakly physisorbed. The electronic structure of monolayer [Cu2Br(IN)2]n CP can be modified substantially, which is strikingly sensitive to the NO adsorption. The NO2 adsorption can also affect the monolayer [Cu2Br(IN)2]n’s electronic structure, though less sensitive than NO. On the basis of these characteristics, the 2D monolayer [Cu2Br(IN)2]n CP can serve as NO and NO2 gas sensors. Though only one example of monolayer [Cu2Br(IN)2]n CP has been studied in this work, it should be possible to prepare the single-layer coordination polymers of other layered MOF materials as well, and these metal coordination polymers are new 2D members of a big family of hybrid inorganic−organic nanomaterials, of which more experimental and theoretical efforts are needed to explore their unique properties and find their potentials in different fields.

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ASSOCIATED CONTENT

S Supporting Information *

Electron density distribution of the single-layer [Cu2Br(IN)2]n CP and spin-polarized band structures of monolayer [Cu2Br(IN)2]n(I) CP with gas molecule adsorptions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.Z.), zhongfangchen@ gmail.com (Z.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support in China by NSFC (21073096), MOE NCET (080293), and the Innovation Team (IRT0927), and in the U.S. by NSF Grant EPS-1010094 and the NASA grants (Nos. NNX10AM80H, NNX07AO30A), is gratefully acknowledged. This work was also supported, in part, by the National Science Foundation through TeraGrid resources, and the computations were partly performed on TianHe-1(A) at the National Supercomputer Center in Tianjin. 4124



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