Synthesis, Structure, and Tunability of Zero-Dimensional Organic–Inorganic Metal Halides Utilizing the m-Xylylenediammonium Cation: MXD2PbI6, MXDBiI5, and MXD3Bi2Br12·2H2O

Over the past decade, the efficiency of photovoltaic devices based on CH3NH3PbI3 have dramatically increased. This has driven research efforts in all areas, from the discovery of materials to film processing to long-term device stability studies. Here, we report the synthesis and structure of three new “zero dimensional” organic–inorganic metal halides which use the meta-xylylenediammonium (MXD) cation: MXD2PbI6, MXDBiI5, and (MXD)3Bi2Br12·2H2O. The different structures of the new materials lead to compounds with a range of band gaps with MXDBiI5 having the lowest at 2.15 eV. We have explored the tunabilty of MXDBiI5 through halide substitution by preparing a series of samples with composition MXDBiI5–xBrx and determined the halide content using energy dispersive X-ray spectroscopy. A large range of solid solution is obtained in MXDBiI5–xBrx, resulting in the formation of single-phase materials for bromine contents from x = 0 to 3.71 (iodine contents from 1.29 to 5). This highlights the fact that zero-dimensional organic–inorganic halides are highly tunable, in a similar manner to the higher-dimensional perovskite counterparts. Such new materials open up the opportunity for further studies of the physics and optoelectronic properties of these materials.


■ INTRODUCTION
In 2009, Miyasaka and co-workers first tested CH 3 NH 3 PbI 3 and CH 3 NH 3 PbBr 3 in photovoltaic devices. 1 Although the initial efficiencies were modest, the resulting optimization of all aspects of device fabrication has led to groundbreaking photovoltaic power conversion efficiencies. 2−4 Power conversion efficiencies of 25.7% have now been reported for single junction photovoltaics based on CH 3 NH 3 PbI 3 and 29.8% for tandem photovoltaics, based on a combination of silicon and a perovskite layer. 5 CH 3 NH 3 PbI 3 adopts the perovskite structure, (ABX 3 ) where A = CH 3 NH 3 + , B= Pb 2+ , and X = I − (or another halide). The structure consists of corner-sharing PbI 6 octahedra, with the organic cation being disordered on the perovskite A-site at room temperature. 6 As the size of the organic ammonium cation is increased, different perovskite-related structure types can form. 7 In particular, layered perovskites based on the Ruddlesden−Popper and Dion−Jacobson structure types have received considerable interest. A notable breakthrough came when a power conversion efficiency of 12.52% was achieved for (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 ) 3 Pb 4 I 13 , which adopts the Ruddlesden−Popper structure. 8 Very recently, the "Memory Seed Effect" has been reported, which involves dissolving presynthesized crystals in solvents such as DMF, which enables the preparation of high-quality thin films for a variety of layered perovskites. 9 The solutions have been shown to retain "memory seed" crystallites, which facilitates the production of phase-pure layered perovskites. 9 The use of this "Memory Seed Effect" has led to solar cells with power conversion efficiencies of 17.1% for (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 ) 3 Pb 4 I 13 . 9 Organic−inorganic halide perovskites exhibit great compositional flexibility. 4,10 The A, B, and X sites can be doped in order to fine-tune the electronic structure and band gap for a range of optoelectronic applications. 3,10,11 In particular, as the electronic structure of organic−inorganic metal halides depends on the B-site cation and halide, halide substitution is an excellent way of tuning the band gap of these materials. 12,13 For example, halide substitution in FAPbI 3-x Br x (FA = formamidinium) resulted in the band gap being tuned from 1.48 to 2.23 eV. 3 Unfortunately, photoinduced halide segregation has been reported in CH 3 NH 3 PbI 3−x Br x , yielding iodine-and bromine-rich domains in the thin films. 14 Nevertheless, such tunability of the materials is of interest, particularly in the fields of indoor photovoltaics or tandem solar cells, which can utilize two perovskites with different compositions and band gaps. 4 One method of preventing halide segregation is to use multidentate ligands which create interfaces with a low defect content. 15 Recently, several new families of layered perovskites have been prepared which incorporate methylammonium in addition to cations which contain aromatic rings such as phenylethylammonium (PEA), 4-aminomethylpyridinium (4AMPY), 3-aminomethylpyridinium (3AMPY), or metaphenylenediammonium (mPDA). 16−18 Several of these perovskites adopt the Dion−Jacobson structure. 16,17 Interestingly, the position of the aminomethyl group on the pyridinium ring in 4AMPY and 3AMPY influences the stacking of the inorganic layer containing lead iodide octahedra and results in a decrease in band gap when going from (4AMPY)(CH 3 NH 3 ) n-1 Pb n I 3n+1 to (3AMPY)(CH 3 NH 3 ) n-1 Pb n I 3n+1 . 16 (3AMPY)(CH 3 NH 3 ) 3 -Pb 4 I 13 exhibited a power conversion efficiency of 9.20%. In the (mPDA)(CH 3 NH 3 ) n-1 Pb n I 3n+1 system, fabrication of phase pure thin films has been challenging for some values of n. 17 (PEA) 2 (CH 3 NH 3 ) 2 Pb 3 I 10 has been shown to have enhanced moisture stability with respect to CH 3 NH 3 PbI 3 . 18 The term "zero-dimensional perovskite" has been coined for organic−inorganic metal halides which consist of isolated clusters of metal halide octahedra which are separated by organic cations, although strictly speaking these materials do not possess the perovskite structure. Depending on the nature of the metal and halide, different connectivities of octahedra may exist in the cluster, such as isolated MX 6 (where M = metal cation and X = halide) or edge-sharing octahedra to make M 2 X 10 or face-sharing octahedra to make M 2 X 9 . Zerodimensional materials are already showing interesting properties; for example, (1,3-propanediammonium) 2 Bi 2 I 10 ·2H 2 O has been tested as a photodetector, and reproducible photocurrents could be drawn from this material. 19 (CH 3 NH 3 ) 3 Bi 2 I 9 shows highly anisotropic photoluminescence and has shown evidence of quantum cutting. 20 In addition, isovalent doping in the zero-dimensional (CH 3 NH 3 ) 3 Bi 2−x Sb x I 9 has shown a bandbowing phenomena. 21 Zero-dimensional all-inorganic halides such as A 4 PbX 6 and Cs 4 SnX 6 (A = K, Cs, Rb and X = Cl, Br, I) are also of interest, particularly for white light emission, and work by Mohammed et al. has recently found that the local octahedral distortions enabled the formation of self-trapped states. 22 Motivated by reports of enhanced moisture stability when aromatic organic ammonium cations are used, 18 along with the huge compositional and structural diversity of organic− inorganic halides, we have explored the synthesis and tunability of new organic−inorganic halides which use the meta-

■ EXPERIMENTAL SECTION
The synthesis of all materials was based on modifications of the method reported by Poglitsch and Weber for the preparation of CH 3 NH 3 PbI 3 . 23 The appropriate metal oxide was dissolved in 5 mL of HX and heated to 80°C. Separately, 0.5 mL of HX was added to 0.4 mL m-xylylenediamine, resulting in the formation of crystals. These crystals were heated until dissolved. The two solutions were mixed and stirred for 30 min. The solution was cooled to room temperature, and the resulting crystals were filtered. The crystals were dried in a vacuum oven at 80°C for 2 h.
For the synthesis of MXDBiI 5−x Br x , 0.47 g Bi 2 O 3 was added to a mixture of HX acid (X = Br, I). The volumetric ratio of HI:HBr used in the reactions are given in Table 1. The reaction mixture was then placed on a hot plate and heated to 80°C, with stirring, until all reactants were dissolved. Then, an equimolar quantity of mxylylenediamine (0.14 mL) was added to the solution, and the mixture was stirred for 30 min. After cooling to room temperature, the crystals were filtered off and left to dry in the fume hood.
Powder X-ray diffraction data were collected on a Panalytical Empyrean Diffractometer using CuK α1 radiation in Bragg−Brentano geometry. Data were collected from 5°to 70°with a step size of 0.017°and a time per step of 0.94 s. PXRD data were analyzed using Topas Academic ver. 6. 24 Single crystal diffraction data were recorded at either 173 or 293 K using a Rigaku FR-X Ultrahigh brilliance Microfocus RA generator/ confocal optics and Rigaku XtaLAB P200 diffractometer [Mo Kα radiation (λ = 0.71073 Å)]. Intensity data was collected using ω steps accumulating area detector images spanning at least a hemisphere of reciprocal space (CrystalClear). 25 The data was processed using CrysAlisPro software. 26 Structure solution was carried out using SHELXT, 27 and structure refinement by full matrix least-squares against F 2 was carried out with SHELXL (2018/3). 28 Non-hydrogen atoms were refined anisotropically, and carbon-bound hydrogens were refined using a riding model. Ammonium hydrogens were located from the difference Fourier map and refined isotropically subject to distance restraints. Selected crystallographic data are presented in Tables 2 and 3. Deposition numbers 2151572−2151574 contains the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.
Scanning electron microscopy studies were carried out using a JSM IT200 equipped with a 25 mm 2 Jeol DrySD EDS detector and a Jeol JSM 5600. UV−vis diffuse reflectance spectra were collected on a Jasco V650 spectrophotometer equipped with an integrating sphere, in the wavelength range 190−900 nm. BaSO 4 was used as a reference.
■ RESULTS AND DISCUSSION Crystal Structure. The reactions of MXD with either Bi 2 O 3 and HI or HBr, along with PbO and HI, produced the crystalline products MXDBiI 5 , MXD 3 Bi 2 Br 12 ·2H 2 O, and MXD 2 PbI 6 . Their structures were determined from single-  Crystal Growth & Design pubs.acs.org/crystal Article crystal X-ray diffraction data, and selected parameters are tabulated in Table 2.
The structure of MXD 2 PbI 6 consists of isolated PbI 6 octahedra separated by MXD cations (Figure 1). Although the structure is not strictly a layered perovskite, the structure can be thought of as a "pseudolayered" material, as the PbI 6 octahedra form layers in the crystal structure but are too far apart to form the corner-sharing-octahedral connectivity required in the perovskite structure. It is likely that this is driven by the ammonium groups from the large MXD cation, which protrude into the inorganic layer. The Pb−I bond lengths range from 3.20774(17) to 3.2443(2) Å ( Table 3). The I−Pb−I bond angles range from 81.808(4)°to 98.192(4)°for cis I−Pb−I angles, while the trans I−Pb−I are 180°. This deviation away from the ideal octahedral angles indicates that the octahedra show some distortion. Such a distortion can be quantified in terms of the bond angle variance and the bond length distortion, as originally reported by Robinson et al. 29 Here, we calculate the bond length distortion to be 0.258 and bond angle variance to be 31.63. The bond angle variance here is much smaller than the 234.2 reported for (mPDA)PbI 4 recently reported. 17 The aromatic core of the MXD cation is not directly above adjacent MXD cations but is offset. The orientations of the adjacent MXD cations result in NH 3 groups pointing in three different directions to give a Y-shaped arrangement of the NH 3 groups, with one pair of NH 3 groups being almost eclipsed (Figure 1c). Hydrogen bonds between NH 3 hydrogens and iodine from the PbI 6 octahedra range from 3.546(2) to 3.718(2) Å. Figure S1 shows the PXRD data of the bulk sample. Pawley fits were carried out using the unit cell parameters and space group obtained from single-crystal diffraction. Twelve Chebyshev background parameters, sample displacement, and profile parameters were also refined. The resulting fit is shown in Figure S1, and this confirms the phase purity of the sample.
In contrast to the structure of MXD 2 PbI 6 , which contains isolated PbI 6 octahedra, the structure of MXDBiI 5 consists of BiI 6 octahedra, which share edges to form Bi 2 I 10 dimers (Figure 2). This kind of structural motif has also been observed in other organic bismuth halides. 30 17,31,32 Hydrogen bonds between NH 3 hydrogens and iodine in the Bi 2 I 10 units ranged from 3.712(5) to 3.812(6) Å. Figure  S2 shows the PXRD data of the bulk MXDBiI 5 sample. Pawley fits were carried out using the cell parameters and space group obtained from single crystal diffraction MXDBiI 5 and the same nonstructural parameters that were described for MXD 2 PbI 6 . The resulting fit is shown in Figure S2, and this confirms the phase purity of the sample. The structure of MXD 3 Bi 2 Br 12 ·2H 2 O consists of isolated BiBr 6 octahedra, with the asymmetric unit containing two inequivalent BiBr 6 octahedra ( Figure 3). For the Bi (1) octahedron, the Bi−Br bond lengths range from 2.7942(8) to 2.9389(8) Å, while for the Bi(2) octahedron, the Bi−Br bond lengths range from 2.7722(8) to 2.9350(8) Å (Table 3). In addition, the bond angles of Bi (1) (1) showing the greater angular variance than the Bi(2), while Bi(1) shows less distortion of bond lengths than Bi(2) ( Table  3). The bond angle variance of the BiBr 6 octahedra are much smaller than that of MXD 2 PbI 6 , which also contains isolated octahedra (PbI 6 ) in the structure. They are also significantly smaller than the bond angle variance reported for (mPDA)-PbI 4 , but are comparable to those reported for some quinoline and isoquinoline lead halides. 17,33 The shortest hydrogen bonds are formed between the NH 3 and H 2 O, with N···O distances of 2.757(12) Å and 2.822(10) Å. Figure S3 shows the PXRD data of the bulk MXD 3 Bi 2 Br 12 ·2H 2 O sample. Pawley fits were carried out using the cell parameters and space group obtained from single crystal diffraction and parameters described for MXD 2 PbI 6 . The resulting fit is shown in Figure  S3, and this confirms the phase purity of the sample. SEM images of MXD 2 PbI 6 , MXDBiI 5 , and MXD 3 Bi 2 Br 12 ·2H 2 O are shown in Figure 4.
SEM images of MXD 2 PbI 6 , MXDBiI 5 , and MXD 3 Bi 2 Br 12 · 2H 2 O are shown in Figure 4. MXD 2 PbI 6 displays aggregates of crystals, with a needle-like morphology, of dimensions of approximately 600 μm by 40 μm. Crystallites of MXDBiI 5 also show a needle-like morphology, although the crystals are much shorter than those reported for the MXD 2 PbI 6 , with dimensions of approximately 50−75 μm by 20 μm. The MXD 3 Bi 2 Br 12 ·2H 2 O sample consists of much thinner crystallites with a narrow, plate-like morphology. A range of crystallite sizes can be observed, with typical dimensions being around 50 μm by 100−200 μm. Although the control of morphology was not considered in this study, this will be of interest in the future, when manufacturing these materials into thin films, as any pinholes in thin films as a result of surface morphology can influence factors such as device performance in photovoltaics or LEDs.
Halide Substitution. It is well-known that the valence band and conduction bands in organic−inorganic halides comprise orbital contributions from both the halide and the inorganic cation. 34 Therefore, isovalent doping on the anion site, e.g., replacing I − with Br − , is a particularly useful technique to tune the band gap of the CH 3 NH 3 PbI 3 perovskites. 13,35 This is often termed halide substitution and results in the creation of materials with compositions such as CH 3 NH 3 PbI 3−x Br x . Zero-dimensional inorganic−organic halides commonly exhibit a similar electronic structure to the 3D analogues, but with greater orbital decoupling; 36 therefore, we   decided to probe halide substitution in MXDBiI 5−x Br x , in order to determine the doping limit of Br − in the MXDBiI 5 structure type. A photograph of the MXDBiI 5−x Br x samples is shown in Figure 5 and shows that the color of the samples can be varied from yellow (Br-rich samples) to dark red (I-rich samples). As these samples were synthesized using solution based-routes, the ratio of I to Br was determined using energy dispersive spectroscopy (EDS) using the SEM. The I:Br ratio in a sample is not the same as the I:Br ratio in its precursor solutions, but across the I:Br ratios studied, the ratio in the sample can be related directly to its ratio in solution ( Figure S5). 36 We note that similar phenomena have been reported when two different cations are used in the synthesis of Ruddlesden−Popper phases, as nonstoichiometric ratios of reagents must be used to isolate the phase pure product. 37 In addition, in the synthesis of Cs 2 SnX 6 (X = Cl, Br, I) mixed halides, the products were found to be richer in the Cl or Br than would be expected given the ratios in the precursors, and this has been attributed to the difference in solubility of halides in solution. 38 The PXRD data of MXDBiI 5−x Br x samples are shown in Figure 6, and a representative SEM image of the polycrystalline sample of MXDBiI 4.11 Br 0.89 is shown in Figure S5. As can be seen, the 100% Br sample (i.e., "x = 5") exhibits a completely different PXRD pattern to the other samples prepared in this series, which indicates that it adopts a completely different structure type, MXD 3 Bi 2 Br 12 ·2H 2 O (vide supra). However, with only a small amount of HI in the precursor solution (see Table 1 and Figure S4), the MXDBiI 5 structure type is adopted, and all peaks can be indexed to a monoclinic unit cell in space group I2/m. This structure type is adopted for all MXDBiI 5−x Br x compositions which have a bromine content, x, between 0 and 3.71 (i.e., having iodine contents from 1.29 up to 5.0). In order to determine the extent of the solid solution in MXDBiI 5−x Br x , Pawley refinements were carried out. During the refinements, 12 Chebyshev polynomial terms were used to fit the background, and in addition, unit cell parameters, profile parameters, and specimen displacement were all refined. The resulting variation of unit cell volume with iodine content is shown in Figure 6b, and the corresponding unit cell parameters are plotted in the Supporting Information ( Figure S6). As can be seen from Figure 6b, there is a linear relationship between bromine content (x) and unit cell volume, in agreement with Vegard's law, with the exception of a slight leveling off at the lowest bromine contents. This shows that there is a large region of solid solution in the MXDBiI 5−x Br x system. This region of solid solution ranges from a bromine content, x, of 0 to 3.71, and it is likely that a full solid solution could be obtained with further optimization of synthetic conditions. As the bromine content increases, the β angle increases, but there is a leveling off at the highest iodine contents (which corresponds to the lowest bromine contents). The a, b, and c unit cell parameters all show a linear relationship with bromine content and exhibit a smaller leveling off at the lowest bromine contents. The decrease in unit cell parameters with increasing bromine content is expected due to the smaller size of Br − (1.96 Å) with respect to I − (2.20 Å). 39 The Kubelka−Munk transformations of the UV−vis diffuse reflectance spectra of the MXDBiI 5−x Br x samples and MXD 3 Bi 2 Br 12 ·2H 2 O are shown in Figure 7, while the MXD 2 PbI 6 spectrum is shown in Figure S7. Two absorption features are observed in the spectra for MXDBiI 5−x Br x . For MXDBiI 5−x Br x , the first feature is in the 2.36−2.06 eV range (corresponding to 525−600 nm) and the second feature is centered around 1.9 eV (∼650 nm), while for MXD 2 PbI 6 , the peak occurs at ∼500 nm (2.5 eV) We note that dual features have been observed in both UV−vis spectroscopy and photoluminescence measurements for both 2D, layered materials, and zero-dimensional materials. 40−42 For example, Nag et al. noticed dual-emission in photoluminescence studies of single crystals of (PEA) 2 SnI 4 (PEA = phenylethylammonium), which was also accompanied by two absorption features in UV−vis absorption spectra of the same material. 41 A similar behavior was also observed for (4-AMP)SnI 4 (4-AMP = (4aminomethyl)piperidinium). 41 Dual PL emission has also been observed for the zero-dimensional TPA 2 SbCl 5 (TPA = tetrapropylammonium). 42 To date, the presence of dual features in the photoluminescence or absorption spectra has been attributed to differences in the edge or bulk of the crystals  Crystal Growth & Design pubs.acs.org/crystal Article or self-trapped excitons. 41,42 As the bromide content is increased in MXDBiI 5−x Br x , the absorption edge shifts to larger energies, which is in agreement with an increase in band gap with increasing Br content. The resulting band gaps were estimated using Tauc plots and are listed in Table 4. The lowest band gaps were obtained for the iodine-rich compositions, with MXDBiI 5 exhibiting the lowest band gap in the series (2.15 eV) and MXD 3 Bi 2 Br 12 ·2H 2 O the largest (2.86 eV). In contrast, the band gap of MXD 2 PbI 6 was determined to be 2.33 eV. The variation of band gap with bromine content is shown in Figure 7b and shows an increase in band gap with increasing bromine content, although this trend is not linear. We also note that the band gap of MXDBiI 5−x Br x can be tuned over a similar range to that observed for the FAPbI 3−x Br x perovskites, which allowed tuning of the band gap from 1.48 to 2.25 eV. 3 The high crystallinity of these samples warrants further investigation into the photostability of MXDBiI 5-x Br x , as in the mixed cation, mixed halide perovskites, Cs y FA 1−y PbI 3−x Br x , a high level of crystallinity was found to suppress halide segregation. 43

■ CONCLUSIONS
Here, we have reported the synthesis and characterization of three new organic−inorganic metal halides, using the metaxylylenediammonium (MXD) cation: MXDBiI 5 , MXD 3 Bi 2 Br 12 · 2H 2 O, and MXD 2 PbI 6 . MXDBiI 5 has short intercluster I−I distances and π−π stacking between the MXD cations, while also possessing the lowest band gap of the materials studied here. We explored the tunability of MXDBiI 5 through halide substitution and found that a large region of solid solution exists in the MXDBiI 5−x Br x system (where x = 0 to 3.71). This work highlights the fact that zero-dimensional organic− inorganic halides are also highly tunable semiconductors and opens up the way for further studies of the physics and the long-term stability of these materials.
■ ASSOCIATED CONTENT * sı Supporting Information