Ternary Iodido Bismuthates and the Special Role of Copper

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Ternary Iodido Bismuthates and the Special Role of Copper Natalie Dehnhardt, Hendrik Borkowski, Johanna Schepp, Ralf Tonner,* and Johanna Heine* Department of Chemistry and Material Sciences Center, Philipps-Universität Marburg, Hans-Meerwein-Straße, 35043 Marburg, Germany S Supporting Information *

ABSTRACT: Two new, isostructural members of the title material class, [PPh4]4[Cu2Bi2I12] (1) and [PPh4]4[Ag2Bi2I12] (2), have been prepared via a facile solution route. The crystal structure of both compounds features a tetranuclear [M2Bi2I12]4− (M = Cu, Ag) anion that displays an unprecedented face-sharing mode of connection between BiI6 octahedra and MI4 tetrahedra, enabling close Bi···M contacts. The two compounds allow for a direct experimental and quantum chemical investigation of the influence of group 11 metal cations on the optical and electronic properties of ternary iodido bismuthate anions, indicating that Cu+ is a better electronic match than Ag+, resulting in a significantly lower optical band gap of the copper compound.

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INTRODUCTION Hybrid perovskites APbX3 (A = organic cation, e.g., MA, methylammonium; X = I, Br, Cl)1,2 are an emergent class of photovoltaic (PV) absorber materials,3 with power conversion efficiencies in prototype devices exceeding 20%.4,5 However, concerns over the toxicity of lead compounds6 have prompted researchers to explore other materials containing metal cations with the ns2 electron configuration as possible alternatives.7 Among these are iodido bismuthates such as (MA)3[Bi2I9],8 which feature a more favorable toxicity profile than lead compounds.9 However, while the overall optoelectronic properties of iodido bismuthates are promising, efficiencies of prototype devices are often below 2%.10−18 Explanations for this include the difference in crystal structures between plumbates and bismuthates, which typically crystallize either in vacancy ordered defect perovskite type structures (e.g., Rb3Bi2I9)19 or form compounds with well separated molecular or one-dimensional anion motifs.20−22 This produces distinctly different exciton dynamics in bismuthates23 and often results in band gaps larger than 2 eV. One way to address this is the extension toward compounds with ternary anions, formally replacing Pb2+ found in hybrid perovskites with a combination of Bi3+ and a monovalent cation. This has been applied successfully for chloro and bromo bismuthates in the double perovskite family of materials, e.g., Cs2BiAgBr6.24−26 In this work, we have applied the concept of heterovalent substitution to iodido metalates. We synthesized two new compounds, [PPh4]4[Cu2Bi2I12] (1) and [PPh4]4[Ag2Bi2I12] (2), via a solution route and uncovered an unusual mode of connection of the anions’ building blocks. Using experimental and quantum chemical methods, we investigated the influence of the group 11 metal on the optical properties of the two © XXXX American Chemical Society

isostructural compounds and discuss the general implications for compounds featuring ternary iodido bismuthate anions.

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RESULTS AND DISCUSSION Synthesis. The synthesis appears straightforward at first glance, but combining CuI or AgI with BiI3 and PPh4I in solution can lead to the formation of a number of possible products with binary anions, as outlined in Table 1. As is well Table 1. Known [PPh4]+ Salts of Binary Iodido Metalates bismuthates

cuprates

argentates

[PPh4]3[Bi2I9]29 [PPh4]3[Bi5I18]30 [PPh4]4[Bi6I22]31 [PPh4]4[Bi8I28]32

[PPh4]2[Cu2I4]33 [PPh4][Cu3I4]34 [PPh4]2[Cu4I6]35

[PPh4][Ag2I3]36 [PPh4]2[Ag6I8]37 [PPh4]4[Ag4I8]37

documented for the halogenido argentates, cuprates,27,28 and bismuthates,21 many factors, such as reaction stoichiometry and temperature, reagent concentration, and solvent, control the formation of crystals of a specific species in nontrivial ways and the cocrystallization of several compounds in one batch is a common occurrence. X-ray Crystallography. While 1 and 2 have the same composition as hypothetical double perovskites “[PPh4]2BiMI6”,, single crystal X-ray diffraction (see Table 2) shows that the two isostructural compounds are composed of well separated, molecular [M2Bi2I12]4− (M = Cu, Ag) anions, a result of using bulky [PPh4]+ cations. The anions are composed of face-sharing BiI6 and MI4 polyhedra (sees Figure Received: September 25, 2017

A

DOI: 10.1021/acs.inorgchem.7b02418 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 2. Crystallographic Data for 1 and 2 empirical formula formula weight/g·mol−1 crystal color and shape crystal size crystal system space group a/Å b/Å c/Å V/Å3 Z ρcalc/g·cm−3 μ(Mo Kα)/mm−1 measurement temp/K absorption correction type min/max transmission 2Θ range/deg no. of measured reflns no. of independent reflns R(int) no. of indep. reflns (I > 2σ(I)) no. of parameters R1 (I > 2σ(I)) wR2 (all data) S (all data) Δρmax, Δρmin/e·Å−3

1

2

C48H40BiCuI6P2 1712.66 red block 0.054 × 0.05 × 0.043 orthorhombic Pbca 21.6724(6) 18.7930(5) 24.3053(6) 9899.3(5) 8 2.298 7.819 100 multiscan

C48H40AgBiI6P2 1756.99 orange plate 0.092 × 0.079 × 0.05 orthorhombic Pbca 21.8026(7) 18.7753(7) 24.3572(9) 9970.6(6) 8 2.341 7.730 100 multiscan

0.6771/0.7452 4.338−50.548 49353 8972

0.6536/0.7452 4.32−50.6 99122 9065

0.0614 7300

0.0839 7603

523 0.0242 0.0493 1.025 0.90/−1.06

523 0.0326 0.0486 1.114 0.82/−0.78

Figure 2. Crystal structure of 1.

piperazine and 3.02 Å in [Pb(en)2]2[Ag2I6], en = ethylenediamine).54,55 Such metallophilic interactions are a well-known feature of multinuclear Cu(I) or Ag(I) complexes.56,57 Optical Properties. The nature of the group 11 metal has a striking influence on the optical properties of the two compounds. Whereas crystals of 1 are deep red, those of 2 are orange, corresponding to optical band gaps of 1.8 and 2.1 eV, as estimated from diffuse reflectance spectra. A direct comparison of the optical absorption spectra of 1, 2, [PPh4]3[Bi2I9], [PPh4]2[Cu2I4], and [PPh4]2[Ag6I8] is shown in Figure 3 along with a photograph of crystals of 1 and 2. As

1 and 2). The connection of octahedral and tetrahedral building units via direct face-sharing has not been seen before

Figure 1. (a, b) Molecular structures of the anions in 1 and 2; metal− metal distances given in Å.

in copper or silver iodido bismuthate anions,38−40 which typically display corner- or edge-sharing. This enables Cu···Bi and Ag···Bi contacts of 3.180 and 3.387 Å, respectively, well below the sum of the van der Waals radii (Cu−Bi: 4.92 Å, Ag− Bi: 5.07 Å)41 and at only 1.2 times the sum of the covalent radii (Cu−Bi: 2.63 Å, Ag−Bi: 2.79 Å).42 Similar interatomic distances have previously been reported for organobismuth complexes43−45 (e.g., [M(μ-Cl)(o-(Ph2P)C6H4)3BiCl)]2 with M−Bi distances: 3.08−3.10 Å for M = Cu, 3.17 Å for M = Ag)45 and chalcogenolate clusters.46,47 Solid state compounds such as CuBiI448 or AgBiI449 are known as well and have received some attention as photovoltaic absorbers recently.50−53 However, they feature extensive occupational disorder that makes a direct comparison of the relevant bond lengths difficult. Cu···Cu and Ag···Ag contacts of 2.78 Å in 1 and 3.05 Å in 2 are similar to distances found in compounds containing [M2I6]4− anions (e.g., 2.78 Å in [H2pipz][Cu2I6], pipz =

Figure 3. Optical absorption spectra of 1, 2, and representative [PPh4]+ salts of binary iodido bismuthate, cuprate, and argentate anions. A side-by-side photograph of dark red crystal of 1 and a yellow-orange one of 2 is shown as an inset.

expected, the iodido cuprate and argentate salts show an onset of absorption near the edge of the visible range around 400 nm. Interestingly, the optical band gap of 2 is larger than that of [PPh4]3[Bi2I9] which features a simple binuclear unit of face-sharing BiI6 octahedra. In contrast to this, the optical band gap of 1 is significantly lower, in a range that can typically only be reached with compounds featuring one-dimensionally extended iodido bismuthate anions.58 B


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Inorganic Chemistry

Table 3. Computationally (PBE-D3) Determined Structural Parameters and Band Gaps (Eg) of 1 and 2 for the Solid State Structure and the Free Anion in Comparison to Experimental Values (this Work) with and without Considering Spin−Orbit (SO) Couplingf 1 (M = Cu) d(M−Bi) d(M−M) ∠(M−M−Bi) Vb Eg (SO)c Eg (no SO)d

a

2 (M = Ag)

solid state

anion

exp.

solid state

aniona

exp.

3.091 2.640 127.0 9203.2 1.6 1.8

3.168 2.553 127.2

3.1795(7) 2.7768(13) 124.39(3) 9899.3(5) 1.8e

3.313 2.958 118.1 9254.6 2.0 2.3

3.444 2.853 115.7

3.3871(5) 3.0480(9) 118.26(2) 9970.6(6) 2.1e

2.0 2.5

2.4 2.9

a Anion extracted from solid state structure optimized without periodic boundary conditions using COSMO for charge compensation. bVolume of the unit cell in Å3. cBand gap considering spin−orbit coupling. HOMO/LUMO gap for molecular calculations. dBand gap without spin−orbit coupling. HOMO/LUMO gap for molecular calculations. eExperimental band gaps estimated from optical absorption spectra. fDistances in Å, angles in degrees, and band gaps in eV.

Preliminary photoluminescence measurements at room temperature did not yield a signal for 1; future work at low temperature will help to determine whether this is an intrinsic feature of these types of compounds. DFT Calculations. The unusual structure of the ternary anions in the material leads us to perform a detailed analysis of the structural and optical properties with state-of-the-art computational methods employing density functional theory (DFT). We optimized the structures of 1 and 2 starting from the experimental unit cell using periodic boundary condition (PBC) calculations. The resulting structures are in good agreement with the experimental findings (Table 3) and more importantly reproduce the short M···Bi contacts (Cu−Bi: 3.091 Å, Ag−Bi: 3.313 Å). The computed band gaps are close to the experimental estimates and reproduce the right trend with Eg(1) < Eg(2) by ca. 0.4 eV. The results with and without considering spin−orbital coupling (SO) already indicate a significant influence of relativistic effect on the optical properties of the material. We quantified packing effects on the structure by reoptimizing the free anion of 1 and 2 without PBC and found an overall small impact. The M−Bi distances are approximately 0.1 Å larger, while the M−M distances are smaller by a similar amount. The HOMO−LUMO gap is only a qualitative estimate for the band gap in the solid state structure but shows the right trend. Now, we used the free anion for topological analysis of the electron density (Quantum Theory for Atoms in Molecules, QTAIM) to elucidate the character of the intriguing M···Bi contacts. Figure 4 shows the Laplacian of the electron density ρ together with the molecular graph for 1 and 2. Although bond paths are found between all atoms, the small values for the electron density at the M−Bi and M−M bond critical points are in line with the usual interpretation of AIM that the respective interactions are at best weakly attractive. Thus, a strong M−Bi interaction can be excluded as the explanation of the unusual optical properties. This is supported by a projected crystal orbital Hamilton population (pCOHP) analysis of the M−Bi interactions in the solid state structure (Figure 5a,b). Both bonding and antibonding states are occupied, and the integrated pCOHP (IpCOHP) for this bond is only −0.411 eV in 1 and −0.317 eV in 2, thus underlining the weak attractive character of the bond. Key to the understanding of the optical properties of the solid state structure is the density of states (DOS) analysis in Figure 5c,d for 1 and 2. The results clearly show that the

Figure 4. Contour plot of Laplace field (∇2ρ) of the computed electron density (QTAIM) for (a) the anion of 1 and (b) the anion of 2. Solid lines indicate bond paths; dotted lines indicate the M···Bi (M = Cu (a), Ag (b)) bond paths. The electron density ρ at the bond critical points M−Bi and M−M are given in e*a0−3.

highest occupied bands in 1 consist of states located at the Cu atoms (pDOS(Cu)) and highest unoccupied states show Bi character (pDOS(Bi)). This is a strong indication that the optical transition is an excitation from Cu- to Bi-dominated states. In contrast, the pDOS(Ag) has little influence on the highest occupied bands in 2, restricting the optical transition to Bi- and I-centered states. In essence, Cu adds new states at the valence band maximum, while Ag cannot contribute to states close to the band edges, producing an anion that is electronically similar to an isolated [BiI6]3− unit. This is well in line with our experimental findings, showing an onset of absorption for 2 that is at shorter wavelength than even the binuclear unit in [PPh4][Bi2I9] (see Figure 3). The concept of a nonequivalence of structural and electronic connectivity has been recently highlighted in a related context, showing that Cs2PbPbI6 and Cs2SrPbI6 are isostructural, but differ in their electronic dimensionality, the first being 3D and C


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Inorganic Chemistry

Figure 5. (a, b) Density of states (DOS) for 1 and 2, respectively, with partial DOS (pDOS) for Cu or Ag and Bi with and without (nr-pDOS) spin−orbit coupling. (c, d) Projected crystal orbital Hamilton population (pCOHP) for the Cu−Bi bonds in 1 and the Ag−Bi bonds in 2 and integrated value (IpCOHP).

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CONCLUSION We synthesized two new iodido bismuthates [PPh4]4[Cu2Bi2I12] (1) and [PPh4]4[Ag2Bi2I12] (2) and elucidated the effect of the group 11 metal cation on the optical properties of the two isostructural compounds with experimental and quantum chemical methods, showing that Cu+ facilitates a lower band gap by adding states above the compound’s valence band. Overall, our results highlight the promising features of copper iodido bismuthates and suggest that, in looking beyond the double perovskite motif for multinary halogenido bismuthates, further attention should be directed toward this class of materials.

the second 0D, consisting of separated, molecular [PbI6] units.59 Such a disruptive influence on the electronic properties is less expected for Ag+ than for alkali or alkaline earth metals, but affirms a recent finding that illustrates the fundamental electronic mismatch between Ag+ and Bi3+ in double perovskites.60 Intriguingly, if we neglect spin−orbit coupling effects, the Bi states show up at much higher energies (nr-pDOS(Bi) in Figure 5c,d), and thus, in a nonrelativistic world, the band gap would be ca. 0.2 eV larger (Table 3). The interesting optical properties of 1 and 2 are thus a result of the strong relativistic effects on unoccupied Bi states. Similar band gap reduction is also found in semiconductor research on ternary III/V materials where the band gap tuning results from strong relativistic effects on occupied states.61

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EXPERIMENTAL SECTION

General. BiI3 was synthesized from the elements according to literature procedures.62 CuI, AgI, and PPh4I were used as supplied from commercial sources. Reactions were performed under inert conditions using 2-butanone dried over a 3 Å mole sieve to avoid the D


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Inorganic Chemistry

energy during SCF cycles converged to 10−5 eV. Density of state (DOS) computations with and without considering spin−orbit coupling used VASP 5.4.1. Smoother representation of DOS plots was achieved by a weighted moving average. The crystal orbital Hamilton computations were done with the Local Orbital Basis Suite Toward Electronic Structure Reconstruction (LOBSTER, version 2.1.0).75−79 The calculations on free anions without PBC were done with the Amsterdam Density Functional package (ADF 2016.106).80−82 The PBE functional was again used and dispersion effects considered with the DFT-D3 approach. Relativistic effects were considered with the Zeroth Order Relativistic Approximation (ZORA)83−85 and the TZ2P+ basis set was used. The high negative charge of the anions was compensated during the computations via a Conductor-like screening model (COSMO) with ε = ∞, an approach successfully used in the past.86 Topological analysis via the Quantum Theory of Atoms in Molecules (QTAIM) was done by generating wave function files with Gaussian 0987 (PBE with def2-TZVPP basis set88,89) and analyzing them with AIMALL (Version 16.10.31).90

formation of BiOI as a hydrolysis product of BiI3 during the reaction in solution. The dried products are generally air-stable. [PPh4]3[Bi2I9] and [PPh4]2[Cu2I4] used as reference compounds were synthesized according to the literature.29,33 [PPh4]2[Ag6I8] can be obtained heating stoichiometric amounts of [PPh4]I and AgI to 200 °C for 2 days in an open vessel. CHN analysis was carried out on an Elementar CHN-analyzer. Thermal analysis was conducted on a Mettler Toledo TGA/SDTA 851e from 25 to 800 °C with a heating rate of 10 °C min−1 in a constant flow of 80 mL min−1 N2. Powder patterns were recorded on a STADI MP (STOE Darmstadt) powder diffractometer, with Cu Kα1 radiation with λ = 1.54056 Å at room temperature in transmission mode. IR spectra were measured on a Bruker Tensor 37 FT-IR spectrometer equipped with an ATR-Platinum measuring unit. Optical absorption spectra were recorded on a Varian Cary 5000 UV/ vis/NIR spectrometer in the range of 300−800 nm in diffuse reflectance mode employing a Praying Mantis accessory (Harrick). For ease of viewing, raw data was transformed from %Reflectance R to Absorbance A according to A = log(1/R), which yields estimates comparable to the widely used Kubelka−Munk relation.63 Optical band gaps are estimated from the onset of absorption. Synthesis. [PPh4]4[Cu2Bi2I12] (1). A mixture of BiI3 (120 mg, 0.2 mmol), CuI (40 mg, 0.2 mmol), and PPh4I (94 mg, 0.2 mmol) was suspended in 10 mL of dry 2-butanone under an Ar atmosphere and heated at reflux for 0.5 h. Red and black powders were removed from the suspension via filtration to yield a deep red solution that was stored at room temperature. After 3 days, dark red crystals were isolated via filtration and dried in air. The yield was 76 mg (20% with respect to the limiting reagent PPh4I). Data for 1: Anal. Calcd for C48H40BiCuI6P2, (M = 1712.66 g mol−1): C, 33.66; H, 2.35%. Found: C, 33.5; H, 2.3%. [PPh4]4[Ag2Bi2I12] (2). A mixture of BiI3 (60 mg, 0.1 mmol), AgI (12 mg, 0.05 mmol), and PPh4I (47 mg, 0.2 mmol) was suspended in 5 mL of dry 2-butanone under an Ar atmosphere and heated at reflux for 0.5 h. Red and black powders were removed from the suspension via filtration to yield a deep red solution that was stored at room temperature. After 3 days, crystalline orange material was isolated via filtration and dried in air. SCXRD, PXRD analysis, and visual inspection of the bulk phase under the microscope showed that, in addition to 2, [PPh4]2[Ag6I8] is present in the form of colorless planks, strongly intergrown with orange crystals of 2, making a separation by hand or using density separation impossible for the bulk material. Variations of the reaction conditions did not improve the purity of the product. X-ray Crystallography. Single crystal X-ray determination was performed on a Bruker Quest D8 diffractometer with microfocus Mo Kα radiation and a Photon 100 (CMOS) detector. The structures were solved using direct methods, refined by fullmatrix least-squares techniques and expanded using Fourier techniques, using the Shelx software package64−66 within the OLEX2 suite.67 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were assigned to idealized geometric positions and included in structure factors calculations. Pictures of the crystal structures were created using DIAMOND.68 A number of ISOR restraints had to be used to obtain reasonable displacement parameters for some of the carbon atoms in both refinements. Computational Details. Density functional theory (DFT) based calculations for the solid state structures used the Vienna Ab initio Simulation Package (VASP 5.3.5),69,70 which uses periodic boundary conditions (PBC). The generalized gradient approximation (GGA) using the exchange-correlation functional proposed by Perdew, Burke, and Ernzerhof (PBE)71 was used in combination with a plane-wave basis set. The projector-augmented wave (PAW) method72 was used and enabled a truncation of the plane-wave basis set at a kinetic energy of 470 eV. The Brillouin zone was sampled at the Γ-point only which proved to be sufficient due to the very large unit cells. Dispersion effects were considered with the DFT-D3 scheme.73,74 Structural optimization was performed starting from the crystal structure presented in the experimental part by means of a conjugategradient or quasi-Newton algorithm for ionic positions and cell parameters until the forces converged to 5 × 10−2 eV Å−1 and the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02418. Additional crystallographic details, thermal analysis, powder diffraction, IR spectroscopy, optical properties (PDF) Accession Codes

CCDC 1571102−1571103 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.H.). *E-mail: [email protected] (R.T.). ORCID

Ralf Tonner: 0000-0002-6759-8559 Johanna Heine: 0000-0002-6795-5288 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is funded by the German Research Foundation (DFG) via the collaborative research center SFB 1083. J.H. thanks Prof. Stefanie Dehnen for her constant support. We thank Dr. Phil Rosenow for support and the HLR Stuttgart, HRZ Marburg, and LOEWE-CSC Frankfurt for computational resources.

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