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Nanostructured Mechanochemically Prepared Hybrid Perovskites Based on PbI2 and Alkylammonium Halides for Optoelectronic Applications Oleg Yu. Posudievsky, Natalia V. Konoshchuk, Anatoliy Shkavro, Volodymyr Karbivskiy, Vyacheslav G. Koshechko, and Vitaly Pokhodenko ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00881 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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Nanostructured Mechanochemically Prepared Hybrid Perovskites Based on PbI2 and Alkylammonium Halides for Optoelectronic Applications Oleg Yu. Posudievsky,1* Natalia V. Konoshchuk,1Anatoliy G. Shkavro,2 Volodymyr Karbivskiy,3 Vyacheslav G. Koshechko,1 and Vitaly D. Pokhodenko1 1

L.V. Pisarzhevsky Institute of Physical Chemistry of the National Academy of Sciences of Ukraine, prospekt Nauki 31, Kyiv 03028, Ukraine; tel./fax:+038044 5256672; e-mail: [email protected] 2 T. Shevchenko National University of Kyiv, Institute of High Technologies, prospekt Glushkova 4g, Kyiv 03022, Ukraine 3 G.V. Kurdyumov Institute for Metal Physics of the National Academy of Sciences of Ukraine, Academician Vernadsky Boulevard 36, Kyiv 03142, Ukraine

ABSTRACT The preparation of hybrid perovskites (HPs) with small and large organic cations with a solventless mechanochemical treatment of the mixture of lead iodide and alkylammonium halides is presented. X-ray diffraction and Electron microscopy show that the use of the mechanochemical approach for synthesis of HPs with small cation (CH3NH3PbI2X, X=I, Br, Cl) leads to the formation of crystalline nanoparticles with uniform shapes and sizes that do not exceed 70 nm. At the same time, the mechanochemical synthesis of HPs with large organic cations, (C6H13NH3)2PbI2Х2, induces the nanostructuration with the formation of 2D structures regardless the stoichiometry of the initial reaction mixture. We show that our mechanochemically prepared HPs exhibit a photoluminescence whose colors depend on the size of the organic cations and the type of halide ions that could be used for different optoelectronic applications, for improving color rendering index of garnet based white light-emitting diodes in particular.

Keywords: hybrid perovskites, solventless mechanochemical synthesis, organic cation size, stoichiometry, 2D structure

1.

INTRODUCTION

Hybrid organic-inorganic perovskites (HPs) based on lead halides and organic amine hydrohalides have recently become a subject of intense research due to their promising inherent semiconductor characteristics (luminescence, variation of the energy gap depending on the composition, charge transport, etc.) that determine their applications in various future optoelectronic devices.1–3 Despite the fact that the efficiency of ACS Paragon Plus Environment

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2 the first CH3NH3PbI3 based solar cell did not exceed 4%,4 further optimization of the methods for preparation of HPs and configuration of solar cells allowed to achieve an external quantum yield above 20%.5,6 Additionally, HPs can show an effective luminescence in the visible spectral range, which makes them attractive materials for use in light-emitting devices 3. Light-emitting diodes with luminous efficiency above 17 cd/A, a power efficiency of 30 lm/W, and an external quantum yield of ~13% are already known.7,8 It is now established that the efficiency of HP-based optoelectronic devices depends not only on the HP’s structure, but also on the degree of their crystallinity, because it has a significant influence on the charge diffusion length.9 Moreover, due to the asphericity of the organic cations, HPs possess an inherent nanoscale structural disorder and a tendency of "amorphization" that can induce the coexistence of a significant portion of the amorphous component and the crystal structure. It was shown that morphology, size, shape, crystallinity, and a number of other important characteristics of HP particles are largely determined by their preparation methods.1,10–12 So the development of new effective methods for synthesis of HP is a priority which focuses the attention of many researchers.13 The formation of HPs occurs due to the layered structure of lead halides and the possibility of intercalation of various organic molecules into the interlayer space.14–16 The size of the organic cations and the nature of halide anions affect the band gap and the photophysical properties of the resulting HP, which in turn determines the color and luminescence quantum yield of these materials.3,17 Chemical synthesis of HPs is typically carried out in solution,3,4,6,7,10,12,13 and the nature of the solvent has a significant impact on the quality and characteristics of the prepared HP. In particular, in ref.

18

, the

effect of a solvent polarity on the structure of HPs and the formation of defects was established. Therefore, the creation of alternative routes for preparation of HPs is an urgent goal to obtain improved characteristics. Solid-state mechanochemical synthesis is one of the most convenient and environmentally acceptable methods for preparing various inorganic and organic compounds as well as nanocomposite materials, such as conducting conjugated polymers19 and their host-guest nanocomposites with a layered vanadium oxide xerogel,20 or various 2D materials (graphene with varying degree of oxidation,21–24 molybdenum disulfide,25 boron nitride26). Until recently, however, this method has not been widely used for preparation of HP. It was known that even a simple hand grinding of the equimolar mixture of lead iodide and methyl ammonium chloride in a mortar leads to the formation of a small amount of HPs with different composition.27 The mechanochemical synthesis of CH3NH3PbI3 was carried out for the first time by D. Prochowicz et al. in ref. 28

using a vibrating ball mill. However, it should be noted that the authors of that study dedicated the work

only to one object (CH3NH3PbI3). The resulting product consisted of relatively large (100–450 nm) particles and was characterized by a great number of structural defects.28 During the last years several papers devoted

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3 to the possibility of the mechanochemical synthesis of HPs in the form of 1D-, 2D- and 3D-structures as well as the study of the synthesis mechanism were published. 29,30 The aim of this work is the study of the possibility of a mechanochemical preparation of HPs with small and large organic cations (OC = CH3NH3, C6H13NH3) and different halide anions (X = Cl, Br, I) suitable for different optoelectronic applications. We will present also the influence of the preparation method, organic cation size and the nature of the halide anion on the nanostructure, morphology and optical properties of the prepared materials.

2.

EXPERIMENTAL SECTION

2.1. Synthesis of perovskites

Lead acetate trihydrate (Khimlaborreraktiv), potassium iodide (Khimlaborreraktiv), methylamine solution (40% in H2O, Merck), hexylamine (99%, Acros Organics), concentrated aqueous solutions of acids (57% HI, 48% HBr and 37% HCl from Acros Organics) were used as received without further purification. All solvents (acetonitrile, 99.5%, ethanol, 96.0%, diethyl ether, 99.5% from Carl Roth) were purified and dried by standard methods from literature. 31 Lead iodide (PbI2) was prepared according to standard procedure presented in the literature. 32. The XRD pattern of the synthesized PbI2, with intense reflections at 12.7, 38.7° and 52.5° corresponding to (00l) reflexes, indicates the presence in the sample of a large number of crystals oriented in the (001) direction. Indexing reflexes of the diffraction pattern and calculations of the interlayer distances, conducted in accordance with ref.33, showed that the obtained PbI2 has a layered structure with a hexagonal crystal lattice (a = 0.449 nm, c = 0.695 nm), which is consistent with the data for a bulk lead iodide from ref. 14. Halide salts of alkylamines were prepared by adding an equimolar amount of the appropriate acid to a solution of methylamine in water or hexylamine in ethanol under vigorous magnetic stirring.11 The salt was precipitated by the addition of diethyl ether, filtered and recrystallized from ethanol/diethyl ether mixture. Hybrid perovskites were prepared by mechanochemical treatment of the dry mixture of lead iodide and the corresponding alkylammonium halide in the 80 mL silicon nitride grinding bowl with 30 (10 mm in diameter) balls using a planetary ball mill "Pulverizette 6" (Fritsch) at the rotating rate of 300 rpm (Scheme 1). The molar ration between the components OC–X:PbI2 was equal to 1:1 and 2:1 (the last only for mixtures with hexylammonium halides). The treatment time ranged from 1 to 3 hours. The resulting material was used for further studies, particularly in the production of films, without further purification.



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4 Films of the prepared HPs used for characterization by UV-Vis, IR and photoluminescence spectroscopy, were deposited by drop-casting a definite quantity of HP solution in acetonitrile (with concentration of 25 mg/ml) on silicon or glass substrate, preliminarily washed in an ultrasonic bath with hexane, ethanol and distilled water.

Scheme 1. Solventless mechanochemical preparation of hybrid perovskites.

2.2. Characterization

The structure and morphology of the prepared materials were studied by X-Ray Diffraction (XRD), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). XRD patterns of powder samples were recorded on a D8 ADVANCE (Bruker) diffractometer using the CuKα filtered radiation in the range 2θ = 2–70° with an increment of 0.05°. These data were used to evaluate the size of the perovskite particles with the Scherrer equation. TEM images and SAED patterns of the synthesized HPs were obtained using a 125K (Selmi) microscope operating at a potential of 100 kV. The samples were prepared by ultrasound disintegration (SONOPULS 2070) in dimethylformamide and deposited on a copper grid coated with a carbon film. IR spectra were recorded with a SPECTRUM ONE (Perkin Elmer) spectrometer in 400– 4000 cm-1 frequency range with a resolution of 1 cm-1. UV-vis spectra of the films were obtained using a double beam spectrophotometer 4802 (UNICO) in the 350–850 nm range with a resolution of 0.5 nm. Diffuse reflection spectra of HP powders were recorded using a Specord 210 spectrophotometer (Analytic Jena AG) equipped with an integrating sphere in the range of 380–1100 nm and with a resolution of 0.5 nm. Photoluminescence spectra of (C6H13NH3)2PbI2Х2 films in the visible range of the spectrum were registered ACS Paragon Plus Environment

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5 using a LS55 (Perkin Elmer) spectrometer at room temperature with a xenon lamp as an excitation light source. Photoluminescence spectra of CH3NH3PbI2X films in the near infrared region were recorded using a computerized unit based on MDR-2 monochromator (LOMO), a grating with 600 lines/mm which covers the spectral range of 400–1250 nm and ensures a resolution of 0.1 nm, a cooled photoelectron amplifier FEU-79 and a blue (405 nm) solid state laser (Dangler). XPS spectra were registered by XPS-9200 (JEOL) with aluminum anode (10 kV, 15 mA). For the determination of the content of iodide and chloride ions in the HP samples, I3d5/2 and Cl2p3/2 signals were used (in the last case the value of Cl2p spin-orbit splitting is rather small and the numerical procedure was applied to decompose the spectrum into the components).

3.

RESULTS AND DISCUSSIONS

Optimal conditions for mechanochemical synthesis of HP were determined by varying the time of the mechanochemical treatment with registration of XRD patterns of the prepared products obtained after 1, 2 and 3 hours. It was established that the reaction mixture changes the color – from yellow to black for CH3NH3X/PbI2 or orange for C6H13NH3X/PbI2 initial mixtures (Figure S1 and Figure S2) – after the first hour of the treatment with initial reagents. The reflexes of PbI2 and OC–X (their diffraction patterns are shown in Figure 1a and Figure S3) disappear and new peaks emerge in the diffraction patterns (Figure 1a). The position of these peaks coincides with the corresponding reflexes of the HPs synthesized in aqueous and non-aqueous solutions.34 These diffraction patterns indicated the formation of HPs as a result of the solventless mechanochemical process and the absence of diffraction reflexes of the original PbI2 showing its complete conversion into HPs.

Figure 1. (a) XRD patterns of PbI2 (red line) and CH3NH3PbI2Br (black lines) depending on the time of the mechanochemical treatment of the equimolar starting reaction mixture CH3NH3Br/PbI2 (the intensity of the



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6 HP’s diffractograms was increased five times for a best convenience); (b) Dependence of the perovskite particle size versus the time of the mechanochemical treatment.

It should be noted that the intensity of the diffraction peaks of the synthesized HPs increased and their width decreased (Figure 1a) with increasing time of the mechanochemical treatment that is obviously connected with an increase of the crystallinity degree and the size of the perovskite particles (Figure 1b). Therefore, we used a 3 hours mechanochemical treatment for the preparation of all mentioned HPs. The XRD pattern for the products of the mechanochemical treatment of the equimolar CH3NH3I/PbI2 and CH3NH3Br/PbI2 mixtures is shown in Figure 2a. It is shown that in the case of CH3NH3I/PbI2 mixture, the diffractogram contains intense peaks about 14.00, 19.90, 28.45, 31.80 and 40.45°. They could be attributed to (110), (112), (220), (114) and (400) reflexes of CH3NH3PbI3 with a tetragonal structure (lattice constants а = 0.885 nm, с = 1.273 nm) usually formed after application of the liquid-phase preparation of this HP.35,36 The presence of the splitted peak about 28.2 and 28.45° additionally is due to the tetragonal lattice for the mechanochemically prepared CH3NH3PbI3,37,38 even if (00l) reflexes are practically absent in the diffractogram (Figure 2a). Contrary to CH3NH3PbI3, the XRD pattern for the product of the mechanochemical treatment of equimolar CH3NH3Br/PbI2 mixture contains peaks at 14.35, 20.35, 24.95, 28.90 and 32.40°, which could be assigned to (100), (110), (111), (200) and (210) reflexes of CH3NH3PbI2Br HP with cubic lattice (а = 0.614 nm).39,40 The change in the crystal lattice is obviously connected with the substitution of one iodine atom by bromine. The absence of the peaks in these diffractograms connected with the presence of other products could think that just CH3NH3PbI2Х (Х – Br or I) HPs are mechanochemically formed. The XRD pattern for the product of the mechanochemical treatment of the equimolar CH3NH3Cl/PbI2 mixture differs significantly from the one of CH3NH3PbI3 and CH3NH3PbI2Br (Figure 2a). It should be noted that there is no consensus on the structure and stoichiometry of CH3NH3PbI2Cl HP in the literature. Researchers, who tried to synthesize it in dimethylformamide solutions by one- or two-step processes at various ratios between the starting components (CH3NH3Cl/PbI2 or CH3NH3I/PbCl2), pointed out that the small number or total absence of chlorine in the composition of the resulting compounds.41,42 Some authors43 believed that instead of CH3NH3PbI2Cl, a mixture of HPs with a predominant content of CH3NH3PbI3 is formed, while others41,42 emphasized the formation of CH3NH3PbI3 only, explaining by the volatility of CH3NH3Cl. Our studies presented in Figure 2a show that the XRD pattern of the product of the mechanochemical treatment of equimolar CH3NH3Cl/PbI2 mixture contains reflexes characteristic of the mechanochemically prepared CH3NH3PbI3 as well as sufficiently intense reflexes (denoted by asterisk) which could be connected with CH3NH3PbCl3 HP with cubic lattice (а = 0.55 nm) in agreement with ref.43-45. ACS Paragon Plus Environment

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Figure 2. XRD patterns of the mechanochemically synthesized CH3NH3PbI2X (a) and (C6H13NH3)2PbI2X2 prepared with OC–X/PbI2 molar ratio 1:1 (b) and 2:1 (c). Vertical lines show the position of PbI2 reflexes. Reflexes connected with CH3NH3PbCl3 are denoted by ’*’. Reflexes connected with (C6H13NH3)2PbI4 are denoted by ‘#’. ACS Paragon Plus Environment


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8 That allowed us to assume that a mixture of CH3NH3PbI3 and CH3NH3PbCl3 is formed as a result of the mechanochemical treatment under the stated conditions of the equimolar CH3NH3Cl/PbI2 mixture, but the exact ratio between these products is not established at the moment (the absence of CH3NH3Cl in the product follows from Figure S4). As the formula of CH3NH3PbI2Cl probably does not reflect the correct composition of the mechanochemically prepared product and is corresponding to the stoichiometry of the starting components only, it is further denoted as CH3NH3PbI3-хClх and is commonly used in literature. It should be noted that in agreement with the XPS data (Figure S5) the ratio between iodide and chloride ions in the prepared chloride-containing HPs is equal to 2.3 for OC = CH3NH3 and 1.9 for OC = C6H13NH3 (see below). So, it is close to 2 considering the 10% accuracy of the XPS method in this case. The proposed mechanochemical method opens up the possibility of successful intercalations in PbI2 of more volumetric alkylammonium halides and hexylammonium halides in particular. XRD patterns for the products fabricated with a mechanochemical treatment of the equimolar C6H13NH3Х/PbI2 mixtures contained intense peaks in the small angle region at 4.60°, 4.95° and 5.40° for Х= Cl, Br and I, respectively (Figure 2b). The emergence of this reflex in the diffractograms of the samples indicated an expansion of the interlayer space in the original structure of PbI2 that is obviously associated with an increase in the OC size in the transition from CH3NH3 to C6H13NH3, since it is known that OCs in the structure of HPs are arranged perpendicular to the layers of the inorganic matrix.46,47 Such changes in the structure of HPs (significant increase in the distance between PbI2 layers up to ~1.8 nm) were previously observed only for the HPs with a molar ratio OC–X:PbI2 = 2:1,46,47 whereas we used the equimolar ratio of the reagents for the mechanochemical synthesis. It should be also noted that there is an increase of the lattice constant c from 3.27 nm to 3.57 nm and 3.80 nm for (C6H13NH3)2PbI2X2 type of HPs with Х = I, Br and Cl respectively. At the same time, it is known in the literature39,41 that the replacement of iodide ions by less volumetric bromide and chloride ones leads to the decrease of the lattice constant, as it was also observed for the CH3NH3PbI2Х type of HPs considered above. It is known that the possibility of HPs nanostructuration – transition from bulk (3D) to lowdimensional (2D, 1D, 0D) structures – is due not only to the increase of OC content in their composition, but also due to an increase in the size of OC.1,48,49 (It should be noted that the formation of HPs with an intermediate dimension, when one organic layer can be alternated with multiple PbI2 layers – so-called quasi2D structures1,50, is also possible). The dimension of the HP with the same stoichiometry can be also reduced by reducing the particle size due to the effect of the spatial confinement.51,52 For example, the synthesis of HPs using various mesoporous oxide matrices allows the preparation of 2-D and 1-D materials whose particle size ranges from 5 to 20 nm.4,51


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9 XRD patterns for the products fabricated with the mechanochemical treatment of equimolar C6H13NH3Х/PbI2 mixtures contained intense reflexes (4.60°, 9.25°, 14.50°, 23.32°, 28.03°; 4.95°, 9.93°, 15.09°, 19.92°, 25.01°; 5.40°, 10.85°, 16.30°, 21.75°, 27.30° for Х = Cl, Br and I respectively), which could be attributed to (002l) planes of 2D HPs with a composition of (C6H13NH3)2PbI2X2 and an orthorhombic crystal lattice (Tables S1 and S2).47 Taking this result into account, it is obvious that the peaks of residual PbI2 should be present in the diffractograms of the products prepared using the equimolar initial stoichiometry. Really, these peaks are seen in the obtained diffractogram patterns (Figure 2b), but there intensity is rather small due to the delamination and the decrease of the PbI2 particle size during the mechanochemical treatment as evidenced from Figure S6. It is well known from the literature that a 2D structure in HPs is formed when the stoichiometry between organic and inorganic components is 2:1.10,46–48,53,54 In order to elucidate the effect of the reaction mixture composition on the structure of the resulting products, the mechanochemical synthesis of HPs at the molar ration between C6H13NH3Х and PbI2 components equal to 2:1 was carried out. As it is shown in the Figure 2, b and c, XRD patterns of the powder samples prepared for different molar ratio between C6H13NH3Х and PbI2 (1:1 and 2:1) are very close. So, in the case of X = Br or I, (C6H13NH3)2PbI2Х2 type of HPs are produced as a result of the mechanochemical synthesis regardless the stoichiometry of the starting reaction mixtures, while in the case of all considered C6H13NH3Cl/PbI2 mixtures, in addition to the peaks stated above, the diffractograms of the products (Figure 2b and c) contain the low intense reflexes corresponding to the mechanochemically synthesized (C6H13NH3)2PbI4. We conclude that the mechanochemical treatment of the C6H13NH3Cl/PbI2 mixture leads to the formation of several products – (C6H13NH3)2PbCl4 and (C6H13NH3)2PbI4. The precise ratio between them is not established at the moment and therefore this material is denoted as (C6H13NH3)2PbI4-xClx. The observed XRD patterns of both (C6H13NH3)2PbX4 samples prepared from different reagent mixtures (1:1 and 2:1 for OC–X:PbI2) reflect the formation of 2D structures during mechanochemical synthesis. Such nanostructuration of the prepared HPs is also evidenced by the diffractogram of the HP based film due to its identity to that of the powder sample (Figure 3 and Figure S7). As follows from the Figure 3a the diffraction pattern of CH3NH3PbI3 film was almost identical to that of the powder sample. At the same time, the XRD pattern of the film, prepared from the product of the mechanochemical treatment of the equimolar C6H13NH3I/PbI2 mixture, differed significantly from that of the corresponding powder sample – only reflexes (002l) were observed (Figure 3b and Figure S7). This type of diffraction pattern could serve as an additional argument in favor of delamination of the prepared HP to separate layers under the effect of the organic solvent. ACS Paragon Plus Environment


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10 For additional identification of the prepared materials, we used IR spectroscopy, which showed the presence of all major bands characteristic of alkylamines in HPs (Figure S8 and Figure S9), in particular the bands about 1110–1140 cm-1 (stretching vibrations of C–N bond), 1460–1470 cm-1 (symmetric deformation vibration of N–H bonds), 1515–1570 cm-1 (anti-symmetrical deformation vibrations of N–H bonds) and 3490–3500 cm -1 (symmetric stretching vibrations of N–H bonds), which are consistent with results obtained for HPs synthesized in solution.55-59

Figure 3. XRD patterns for powders and films of the mechanochemically prepared CH3NH3PbI3 (a) and (C6H13NH3)2PbI4 prepared at the equimolar OC–X/PbI2 ratio (b). The TEM observations of the prepared HPs are presented in Figure 4. As it can be seen from the Figure 4, CH3NH3PbI2Х type HPs are composed of fairly homogeneous particles with a size in the range from 30 to 70 nm, which is 5–10 times less than CH3NH3PbI3 particles prepared mechanochemically in ref.28. This distinction in particle sizes could be due to the difference in the mechanochemical synthesis procedures – the planetary ball mill in our case and the vibrating ball mill in the mentioned reference. Besides, the occurrence of the spot SAED pattern for the prepared HPs showed a high degree of their crystallinity (Figure 4). The size of the particles for HPs with a large OC ((C6H13NH3)2PbI2Х2), according to the data of electron microscopy, reached 1.0−1.5 µm that is one order greater than in the case of the materials with a small OC (Figure 5, Figure S10 and Figure S11). For such HPs the presence of particles with an obvious layered structure (Figure 5) was observed. We consider this feature reflects a 2D nanostructuration and a high degree of crystallinity (Figure 3, Figure 5).


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Figure 4. TEM image and SAED pattern of the mechanochemically prepared CH3NH3PbI3. We evaluated the thickness of the layers in (C6H13NH3)2PbI2X2 particles using XRD data and a Scherrer equation. We obtained the values for X=I, Br and Cl approximately equal to 80, 60 and 50 nm respectively. It is probably worthwhile to mention that these values are correlated with the increase of the lattice parameter c discussed above.

Figure 5. TEM image and SAED pattern of (C6H13NH3)2PbI4-xClx (a) and (C6H13NH3)2PbI4 (b) mechanochemically prepared at the equimolar and 2:1 OC–X/PbI2 ratio correspondingly. We studied the electron absorption spectra for the powders of the synthesized HPs and the starting PbI2, using the corresponding diffusion reflectance spectra according to the Kubelka-Munk equation.60 The results of these studies are presented in Figure 6. As it can be seen on the figure, the starting PbI2 had an absorption edge in the yellow-green region of the spectrum, whereas in CH3NH3PbI2Х and (C6H13NH3)2PbI2Х2 it was in the range of 700−800 nm (Figure 6a) and 510−550 nm (Figure 6b) respectively. It should be noted that for CH3NH3PbI2Br and CH3NH3PbI2Cl there is a hypsochromic shift of the absorption edge with respect to its position in CH3NH3PbI3, and its value in the case of the Br-containing HP is much larger (~100 nm) than for the Cl-containing material (~20 nm) (Figure 6a). ACS Paragon Plus Environment


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12 An analogous shift was also observed in the absorption spectra of (C6H13NH3)2PbI2Х2 type HPs (Figure 6b). Additionally, in the spectrum of (C6H13NH3)2PbI4 there was another, more energetic absorption band that is characteristic of 2D-nanostructured perovskites.48 The appearance of this band is consistent with the X-ray diffraction data presented above, and could serve as an additional argument in favor of our assumption that mechanochemical synthesis promotes the nanostructuration of HPs.

Figure 6. Absorption spectra for powders of CH3NH3PbI2X (а) and (C6H13NH3)2PbI2X2 mechanochemically prepared at the equimolar OC–X/PbI2 ratio (b). The results concerning the optical properties of the prepared HP films are shown in Figure 7. As it can be shown from Figure 7a, the absorption spectra of the films based on CH3NH3PbI2X slightly differ from those of the powders (Figure 6a). At the same time, in the spectra of (C6H13NH3)2PbI2Х2 based films, unlike CH3NH3PbI2Х type HPs, there is an intense narrow absorption band at 509 nm ((C6H13NH3)2PbI4-xClx), 480 nm ((C6H13NH3)2PbI2Br2) and 515 nm ((C6H13NH3)2PbI4) for products prepared from the equimolar reagent mixtures (Figure 7b) and absorption band at 514 nm ((C6H13NH3)2PbI4-xClx), 480 nm ((C6H13NH3)2PbI2Br2) and 522 nm ((C6H13NH3)2PbI4) for products synthesized from the mixtures with 2:1 molar ratio between the reagents (Figure 7c). This is characteristic of excitons confined in inorganic layers53 that could be connected with the formation of 2D structures in (C6H13NH3)2PbI2Х2 type HPs during the mechanochemical processes of their preparation. It also follows from these spectra that in the case of films (similar to Figure 6 for powders) the position of the absorption maximum is shifted to the red region of the spectrum in a series X = Br, Cl, I regardless of the size of the OC. It should be noted that in the spectra of (C6H13NH3)2PbI2Х2 type HPs, having two different halide ions in the structure, a blue shifted band with a maximum about 411 nm (for (C6H13NH3)2PbI4-xClx) and 433 or 448 nm (for (C6H13NH3)2PbI2Br2 prepared from the 1:1 and 2:1 reagent ACS Paragon Plus Environment

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13

Figure 7. Absorption and photoluminescence spectra for films of CH3NH3PbI2X (а) and (C6H13NH3)2PbI2X2 prepared at OC–X/PbI2 molar ratio 1:1 (b) and 2:1 (c).



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14 mixtures) is observed, and this band could be associated with the appearance of two types of excitons due to different Pb−X transitions (with participation of Pb−I and Pb−Br or Pb−Cl bonds).57,61 Based on the electronic absorption spectra obtained for the powders of PbI2 and synthesized HPs the values of the band gap (Eg) were estimated using the Tauc equation for the direct allowed interband transitions: F(R)E∼ (E - Eg)1/2.43,60 The results of the calculations are presented in the Table 1. It follows from the calculations that the value of Eg for CH3NH3PbI3 is close to the values of the optical gap for analogous samples synthesized in solution (1.55–1.57 eV in ref. 34,60,62), but it is larger than the ones (1.49 eV) for the similar material prepared mechanochemically in ref. 28. The authors in this study attributed the difference to the influence of the defects. So considering these results, the higher value we obtained for our sample could indicate a higher degree of orderliness (crystallinity), which may be due to different conditions of their mechanochemical synthesis. The value of Eg for CH3NH3PbI2Br practically coincides with the ones obtained for this HP synthesized in solution.63 At the same time, as it follows from the data presented in Table 1, the gap in Clcontaining HPs is less than in Br-containing analogues. The decrease in the width of the optical gap in CH3NH3PbI2Cl compared to that of CH3NH3PbI2Br was observed earlier in ref.39,43, where it was shown that in mixed HPs, the nature of the second halide ion affects their structure and, accordingly, electronic properties. In the case of Br/I mixed HPs, bromide ion can occupy both the equatorial and axial positions, whereas for Cl/I mixed HPs, due to a large difference in the size of iodide and chloride ions, the latter can occupy only the axial position in the PbX62– octahedron. This occupation leads to a decrease in the size of the cavity in which the OC is located, and is accompanied by a change in the HP’s structure and finally leads to the decrease in the width of the optical gap (due to the specific interaction between organic and inorganic components). It is known that PbI2 based HPs possess a photoluminescence (PL) at room temperature due to the radiative recombination of excitons localized in inorganic layers.12 The color and intensity of PL depend on the method of their synthesis, the size of OC, the nature of halide anion, availability of different halide anions in their structure, as well as other factors.55,60-62,64,65 In particular, CH3NH3PbI3, depending on the preparation method, exhibits PL in the range from 700 to 800 nm, and the variation of composition of the halide anions in CH3NH3PbX3 allows to adjust the photoluminescence emission in a wide spectral interval, up to ~400 nm.12,13,64 In the layered crystals of PbX2, excitons with the lowest energy are generated during transitions between the bottom of the conduction band and the top of the valence band. In the case of the presence of several types of halide anions, and therefore several types of Pb–X (Pb–I, Pb–Br and Pb–Cl) bonds in the HPs, the conduction band is associated with Pb(6p) orbitals and the valence band incorporates Pb(6s) and I ACS Paragon Plus Environment

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15 (5p), Br (4p) or Cl (3p) orbitals. Due to the difference in energy of the orbitals based on two different halide anions, several types of excitons could be expected in the spectra of (OC)PbI2X.54,59 This physical picture was only observed for (C6H13NH3)2PbI2Br2 and (C6H13NH3)2PbI4-xClx films due to the presence of two peaks in their electronic absorption spectra(Figure 7b and c). However, in the corresponding PL spectra, shown in Figure 7, there was only one emission band regardless of the nature of OC and X. This band is most likely due to the excitons connected with Pb–I bonds that could be caused by the more rapid relaxation of excitons of other types.57 We think that Figure 7 is consistent with the presence of only one phase in the corresponding HP samples, because in the case of the coexistence of two phases, namely (C6H13NH3)2PbI4 and (C6H13NH3)2PbBr4 in particular, there should be an additional PL peak about 400 nm analogously to emission spectra of (OC)2PbBr4 samples with different OC in ref. 56. Our proposal is also confirmed by the excitation spectrum of (C6H13NH3)2PbI2Br2 (Figure S13) which also contains two peaks in accordance with the absorbance spectrum in Figure 7. Our studies revealed that mechanochemically synthesized CH3NH3PbI2X type HPs exhibit PL of red (CH3NH3PbI2Br) and magenta (CH3NH3PbI2Cl, CH3NH3PbI3) color. The PL intensity is sharply reduced for a transition from CH3NH3PbI2Cl to CH3NH3PbI2Br and CH3NH3PbI3 (Figure 7a). At the same time, (C6H13NH3)2PbI2Х2 type HPs were characterized by the emission in the green region of the spectrum (Figure 7b and c). The intensity of this emission is considerably higher than PL of HPs with small OC (CH3NH3PbI2X). In this case the nature of the halide anion did not have a significant impact on the position of the PL maximum (519 (527), 511 (514) and 527 (527) nm for (C6H13NH3)2PbI4-xClx, (C6H13NH3)2PbI2Br2 and (C6H13NH3)2PbI4, for 1:1(2:1) OC–X/PbI2 ratio respectively). The Stokes shift between the peaks of the exciton absorption and emission for the prepared (C6H13NH3)2PbI2Х2 type HPs was 47, 166 and 55 meV for (C6H13NH3)2PbI4-xClx, (C6H13NH3)2PbI2Br2 and (C6H13NH3)2PbI4, respectively. These values are 2–7 times greater than the value of kT ~ 25 meV at room temperature, and indicate the localization of excitons. In particular, it could be associated with defective levels.53 However, we believe that it is not observed in the present case, since the data of structural studies demonstrated above, conclude in favor of an enough high ordering in (C6H13NH3)2PbI2Х2. In our view, the localization of excitons in such HPs is due to their nanostructuration, i.e. the presence of isolated inorganic layers in their structure.66 It should be also mentioned that a great value of the Stokes shift was earlier found for other 2D HPs.56,61

4.

CONCLUSIONS



Page 16 of 40

16 In summary, we have showed the possibility to prepare CH3NH3PbI2Br, CH3NH3PbI3-xClx and (C6H13NH3)2PbI2Х2 (X = Cl, Br, I) hybrid perovskites by a solventless mechanochemical treatment of the mixture of lead iodides and alkylammonium halides. The study of the prepared materials by X-ray diffraction and electron microscopy shows that the use of the mechanochemical approach for synthesis of CH3NH3PbI2X leads to the formation of HPs consisting of crystalline nanoparticles with an uniform shape and size that does not exceed 70 nm. At the same time, the mechanochemical synthesis of HPs with large organic cations such as (C6H13NH3)2PbI2Х2 promotes their nanostructuration with the formation of 2D structures. It was shown that the mechanochemically prepared HPs exhibited a photoluminescence and the color of the emission depends on the size of the organic cation and the type of halide ion that could be used for different optoelectronic applications, for improving color rendering index of garnet based white lightemitting diodes in particular.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI : images of the prepared HPs in the form of powders and films; XRD patterns of CH3NH3X (X = Cl, Br, I); high resolution XPS spectra I3d and Cl2p of CH3NH3PbI3-xClx and (C6H13NH3)2PbI4-xClx; XRD patterns of the initial PbI2 and self-milled PbI2; XRD patterns of (C6H13NH3)2PbI4 film and powders and powders of CH3NH3Cl and CH3NH3PbI2Cl; FTIR spectra of CH3NH3PbI2Br, CH3NH3PbI3, (C6H13NH3)2PbI2Br2 and (C6H13NH3)2PbI4; indexed SAED patterns of different particles of particles (C6H13NH3)2PbI4-xClx, XRD data for (C6H13NH3)2PbI2X2 in Tables S1 and S2.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Targeted Comprehensive Fundamental Research Program of the National Academy of Sciences of Ukraine “Fundamental problems of creating new nanomaterials and nanotechnologies” as well as the Targeted Research Program of the National Academy of Sciences of Ukraine “New functional substances and materials of chemical production”. ACS Paragon Plus Environment

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Table 1. Optical gap (Eg) of the starting PbI2 and mechanochemically prepared HPs. Eg, eV Sample PbI2 CH3NH3PbI3-xClx CH3NH3PbI2Br CH3NH3PbI3 (C6H13NH3)2PbI4-xClx (C6H13NH3)2PbI2Br2 (C6H13NH3)2PbI4

Present work

Data from literature for HPs

2.35 1.61 1.79 1.57 2.36 2.43 2.32

2.23–2.4 14,15 1.55 63 1.78–1.80 40,59 1.49–1.57 28,34,59,62 -



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24 TOC Graphic


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Scheme 1. Solventless mechanochemical preparation of hybrid perovskites. 75x47mm (600 x 600 DPI)



Figure 1. (a) XRD patterns of PbI2 (red line) and CH3NH3PbI2Br (black lines) depending on the time of the mechanochemical treatment of the equimolar starting reaction mixture CH3NH3Br/PbI2 (the intensity of the HP’s diffractograms was increased five times for a best convenience); (b) Dependence of the perovskite particle size versus the time of the mechanochemical treatment. 64x52mm (600 x 600 DPI)


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Figure 1. (a) XRD patterns of PbI2 (red line) and CH3NH3PbI2Br (black lines) depending on the time of the mechanochemical treatment of the equimolar starting reaction mixture CH3NH3Br/PbI2 (the intensity of the HP’s diffractograms was increased five times for a best convenience); (b) Dependence of the perovskite particle size versus the time of the mechanochemical treatment. 64x52mm (600 x 600 DPI)



Figure 2. XRD patterns of the mechanochemically synthesized CH3NH3PbI2X (a) and (C6H13NH3)2PbI2X2 prepared with OC–X/PbI2 molar ratio 1:1 (b) and 2:1 (c). Vertical lines show the position of PbI2 reflexes. Reflexes connected with CH3NH3PbCl3 are denoted by ’*’. Reflexes connected with (C6H13NH3)2PbI4 are denoted by ‘#’. 70x61mm (600 x 600 DPI)


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Figure 3. XRD patterns for powders and films of the mechanochemically prepared CH3NH3PbI3 (a) and (C6H13NH3)2PbI4 prepared at the equimolar OC–X/PbI2 ratio (b). 68x59mm (600 x 600 DPI)



69x60mm (600 x 600 DPI)


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Figure 4. TEM image and SAED pattern of the mechanochemically prepared CH3NH3PbI3. 69x60mm (300 x 300 DPI)



Figure 5. TEM image and SAED pattern of (C6H13NH3)2PbI4-xClx (a) and (C6H13NH3)2PbI4 (b) mechanochemically prepared at the equimolar and 2:1 OC–X/PbI2 ratio correspondingly. 80x40mm (300 x 300 DPI)


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Figure 6. Absorption spectra for powders of CH3NH3PbI2X (а) and (C6H13NH3)2PbI2X2 mechanochemically prepared at the equimolar OC–X/PbI2 ratio (b). 71x63mm (600 x 600 DPI)



68x59mm (600 x 600 DPI)


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Figure 7. Absorption and photoluminescence spectra for films of CH3NH3PbI2X (а) and (C6H13NH3)2PbI2X2 prepared at OC–X/PbI2 molar ratio 1:1 (b) and 2:1 (c). 77x37mm (600 x 600 DPI)



79x39mm (600 x 600 DPI)


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79x39mm (600 x 600 DPI)



34x15mm (600 x 600 DPI)


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Nanostructured Mechanochemically Prepared ... - ACS Publications

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