General Synthesis of Lead-Free Metal Halide Perovskite Colloidal

Aug 9, 2019 - Lead halide perovskite nanocrystals (NCs) exhibit great application potential in optoelectronic devices because of their tunable band ga...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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General Synthesis of Lead-Free Metal Halide Perovskite Colloidal Nanocrystals in 1‑Dodecanol Ming-Ming Yao,†,‡,∥ Chen-Hui Jiang,§,∥ Ji-Song Yao,†,‡ Kun-Hua Wang,†,‡ Chen Chen,‡ Yi-Chen Yin,‡ Bai-Sheng Zhu,‡ Tao Chen,†,§ and Hong-Bin Yao*,†,‡ Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, ‡Department of Applied Chemistry, CAS Center for Excellence in Nanoscience, Hefei Science Center of CAS, and §CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China

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ABSTRACT: Lead halide perovskite nanocrystals (NCs) exhibit great application potential in optoelectronic devices because of their tunable band gaps and facile colloidal synthesis, but they suffer from serious lead toxicity and instability. It is highly desirable to substitute lead with other elements to acquire nontoxic and environmentally friendly lead-free perovskite NCs for optoelectronic devices. Here, we report a general method for the colloidal synthesis of a series of bismuth/antimony-based halide perovskite NCs with various constituents and optical band gaps from 1.97 to 3.15 eV. In our proposed synthetic system, 1-dodecanol is adopted as the solvent instead of the conventionally used 1-octadecene to realize size controllability of bismuth/antimony-based metal halide perovskite NCs. It is found that 1-dodecanol can act as a surfactant to tightly adsorb on the surface of bismuth/antimony-based halide perovskite NCs, enabling their small sizes (∼2 nm) and high dispersibility. Simultaneously, the band gaps of bismuth/ antimony-based halide (A3B2X9, where A = CH3NH3, Cs, or Rb, B = Bi or Sb, and X = Cl, Br, or I) perovskite NCs can be systematically tuned by the atomic substitution of A, B, or X lattice sites. Moreover, to show the optoelectronic application potential of these lead-free halide perovskite NCs, a solar cell based on colloidal Cs3Bi2I9 perovskite NCs is demonstrated. The developed colloidal synthesis of bismuth/antimony-based halide NCs in 1-dodecanol will offer an alternative route to fabricating lead-free halide perovskite optoelectronic devices.

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two divalent Pb2+ ions with one tetravalent Pd4+ or Sn4+ cation,23,24 (4) substitution of three Pb2+ ions with two trivalent ions like Sb3+ or Bi3+.25,26 By equivalent substitution, Sn2+- or Ge2+-based halide perovskite NCs retain the threedimensional (3D) perovskite structure, but Sn2+ or Ge2+ ion is easily oxidized to the tetravalent state in air, resulting in poor stability of these kinds of NCs.18,19 The 3D double-perovskite AMM′X6 (A = Cs or CH3NH3; M = Ag or Au; M′ = Bi, Sb, or In; X = Cl, Br, or I) maintains both the 3D perovskite crystal structure and the charge neutrality via heterovalent substitution.27,28 It has been reported that the double-perovskite Cs2AgBiBr6 and Cs2AgInCl6 NCs exhibit tunable band gaps by substituting or doping strategies. However, the noble-metal Ag was used in these 3D double-perovskite NCs, which will largely increase the cost of as-fabricated optoelectronic devices.29,30 Cs2PdBr6 and Cs2SnI6 NCs have outstanding stability toward light, humidity, and heat, but tetravalent state Pd4+ or Sn4+ produces a zero-dimensional (0D) form of vacancy-ordered perovskite.23,24 On the basis of the fourth strategy, high-quality two-dimensional (2D) perovskite MA3Bi2Br9 and Cs3Sb2Br9

INTRODUCTION Metal halide perovskite nanocrystals (NCs) as an emerging new type of semiconducting nanomaterials have gained intensive attention because of their attractive optical properties and resulting broad applications in photoelectronic devices such as solar cells, light-emitting diodes, X-ray detectors, and so on.1−5 In the past few years, a series of lead halide perovskite NCs have been synthesized via various synthetic strategies and exhibited remarkable optical properties including high efficiencies for light absorption and emitting, feasible band-gap tunability, high color purity, and bright triplet excitons.6−13 However, lead halide perovskite NCs have to face the toxicity of lead and low stability to light, moisture, and heat, which will severely restrict their practical applications.14,15 Therefore, the substitution of lead with nontoxic metal in these perovskite NCs with spontaneously enhanced stability is highly desirable for commercial applications of metal halide perovskite NCs.16,17 At present, there are three typical strategies to reduce the utilization of lead in metal halide perovskite NCs: (1) substitution of Pb2+ ion with divalent Sn2+ or Ge2+,18,19 (2) substitution of two divalent Pb2+ ions with one monovalent M+ cation and one trivalent M3+ cation,20−22 (3) substitution of © XXXX American Chemical Society

Received: June 26, 2019

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DOI: 10.1021/acs.inorgchem.9b01893 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

large particles, and then the supernatant was obtained for characterization. As for the synthesis of Cs 3 Sb 2 (Cl 4 / 5 Br 1 / 5 ) 9 , Cs 3 Sb 2 (Cl 3/5 Br 2/5 ) 9 , Cs 3 Sb 2 (Cl 2/5 Br 3/5 ) 9 , Cs 3 Sb 2 (Cl 1/5 Br 4/5 ) 9 , Cs 3Sb 2 (Br 4/5 I 1/5 ) 9 , Cs 3 Sb 2 (Br 3/5 I 2/5 )9 , Cs 3 Sb 2 (Br2/5 I 3/5 )9 , and Cs3Sb2(Br1/5I4/5)9 NCs, using 0.225 mmol of a mixture of TMSCl/ TMSBr (a precursor molar ratio of 4:1, 3:2, 2:3, or 1:4) or TMSBr/ TMSI (a precursor molar ratio of 4:1, 3:2, 2:3, or 1:4) instead of single TMSX, other conditions remained unchanged. Cs3Bi2X9 NCs were prepared using the same protocol except using 0.05 mmol of Bi(ac)3 instead of Sb(ac)3. It is worth mentioning that all labeled compositions in the finally obtained NCs are the precursor ratios in our work. Synthesis of Rb3Sb2I9 and Rb3Bi2I9 NCs. Rb3Sb2I9 NCs were synthesized by the same protocol as that of Cs3Sb2I9 except using 0.075 mmol of Rb(ac) to replace Cs(ac). The product solution was centrifuged at 3000 rpm for 3 min to discard the large particles, and then the supernatant was obtained for characterization. As for the Rb3Bi2I9 NCs, the synthetic approach and characterization procedures were the same as those used for Rb3Sb2I9 NCs. Synthesis of MA3Bi2X9 (X = Cl, Br, or I) and MA3Sb2Br9 NCs. Bi(ac)3 (0.05 mmol), 5.0 mL of 1-dodecanol, and 0.5 mL of OA were loaded into a 25 mL three-necked flask and dried under vacuum at 100 °C for 40 min along with magnetic stirring. In our work, methylamine was dissolved in tetrahydrofuran (THF) for 2.0 mol/L. When the reaction mixture was dried under vacuum and turned colorless, 0.075 mmol of methylamine (37.5 μL of a THF solution) was swiftly injected under a nitrogen atmosphere. After that, the halide precursor TMSX (0.225 mmol) was swiftly injected to finish the synthesis. Then the reaction mixture immediately turned gray (X = Cl), yellow (X = Br), or red (X = I) and became turbid. After 10 s, the reaction mixture was immediately cooled to room temperature by immersion in a cold-water bath. The product solution was centrifuged at 3000 rpm (for MA3Bi2I9), 2000 rpm (for MA3Bi2Br9), or 2000 rpm (for MA3Bi2Cl9) for 3 min to discard the large particles, and then the supernatant was obtained for characterization. As for the MA3Sb2Br9 NCs, the synthetic approach and characterization procedures were the same as those used for MA3Bi2Br9 NCs. Synthesis of Cs3(SbyBi1−y)2Br9 (y = 1/4, 1/2, or 3/4) Alloying NCs. Cs(ac) (0.075 mmol), 0.05 mmol of Sb(ac)3/Bi(ac)3 (molar ratios of 3:1, 1:1, and 1:3 for Cs3(Sb3/4Bi1/4)2Br9, Cs3(Sb1/2Bi1/2)2Br9, and Cs3(Sb1/4Bi3/4)2Br9, respectively), 5.0 mL of 1-dodecanol, and 0.5 mL of OA were loaded into a 25 mL three-necked flask and dried under vacuum at 100 °C for 40 min along with magnetic stirring. The reaction mixture, which was initially turbid, gradually turned colorless. After that, neat TMSBr (0.225 mmol) was swiftly injected under a nitrogen atmosphere. After 10 s, the reaction mixture was immediately cooled to room temperature by immersion in a cold-water bath. The product solution was centrifuged at 2000 rpm for 3 min to discard the large particles, and then the supernatant was obtained for characterization. Preparation of NC Powder. In order to obtain Cs3Sb2X9 (X = ClxBryI1−x−y, where 0 ≤ x and y ≤ 1) and Cs3Bi2X9 NC powder, ethyl acetate was added to the NC solution at a volume ratio of 1:1, and then the NCs were precipitated out of the solution and separated by centrifugation at 10000 rpm for 5 min and decanting of the supernatant. The precipitate was washed with a small amount of hexane three times. Finally, the NC powder was obtained by drying in a vacuum oven at 80 °C overnight. It is worth noting that the obtained NC powders are bulklike because of reaggregation of the NCs during the washing process. Purification of Cs3Bi2I9 NCs for Solar Cells. The Cs3Bi2I9 NCs were precipitated out of the suspension at room temperature and separated by centrifugation at 12000 rpm for 5 min and decanting of the supernatant. Then the precipitate was washed with a small amount of butyl acetate three times. The final precipitate was redispersed in butyl acetate for the fabrication of solar cells. Fabrication of Cs3Bi2I9 NC-Based Solar Cells. Fluorine-doped tin oxide (FTO)-coated glass was ultrasonically cleaned by deionized water, isopropyl alcohol, acetone, and ethanol for 30 min, consecutively. After drying, the substrate was treated by UV ozone

colloidal NCs have been fabricated by a modified ligandassisted reprecipitation method in an antisolvent, but the low colloidal stability and resulting poor dispersibility might limit the applications of these NCs in the fabrication of optoelectronic devices.14,31 Very recently, a hot-injection method in a nonpolar solvent has been used to synthesize 2D perovskite M3Sb2I9 (M = Cs or Rb) NCs, in which cesium oleate was injected into a noncoordinating solvent 1octadecene (ODE) with oleylamine/oleic acid agents at elevated temperatures (180−230 °C), but control of the M3Sb2I9 NCs sizes is unsatisfactory.32 Therefore, it is necessary to develop a general approach for the colloidal synthesis of nontoxic and environmentally friendly lead-free perovskite NCs with uniform small sizes.33−35 In this study, we report a general and efficient synthetic route of lead-free bismuth/antimony halide perovskite NCs by using 1-dodecanol as both the solvent and capping agent. 1Dodecanol is a polar solvent that can act as the structuredirecting agent to largely enhance the monodispersity and size uniformity of as-synthesized bismuth/antimony halide perovskite NCs.36−38 In addition, 1-dodecanol is cheaper and more environmentally friendly than ODE.39 Moreover, trimethylsilyl halides were chosen as halide precursors in as-proposed synthetic systems, which can enable the regulation of halide compositions in as-synthesized NCs.29,40 As a result, a series of all-inorganic and organic−inorganic bismuth/antimony halide perovskite A3B2X9 (A = Cs, Rb, or CH3NH3; B = Bi or Sb; X = Cl, Br, or I) NCs can be synthesized and the band gaps of assynthesized NCs can be tuned. It is worth mentioning that the 2D perovskites MA3Sb2Br9 were synthesized in the form of colloidal NCs for the first time, demonstrating the generality of our proposed synthetic route. Moreover, using an assynthesized environmentally friendly and stable 0D perovskite Cs3Bi2I9 NC film as a light adsorption layer, a solar cell was demonstrated.

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EXPERIMENTAL SECTION

Materials and Chemicals. All reagents were used as received without further purification. Cesium acetate [Cs(ac); Aladdin, 99%], bismuth acetate [Bi(ac)3; Alfa Aesar, 99.999%], antimony acetate [Sb(ac)3; Aladin, 99.99%], rubidium acetate [Rb(ac); Alfa Aesar, 99.8%], trimethylsilyl chloride (TMSCl; Aladdin, 99.0%), trimethylsilyl bromide (TMSBr; TCI, 95%), trimethylsilyl iodide (TMSI; Aladdin, 97%), methylamine (MA; 2.0 mol/L in THF, Energyseal), 1dodecanol (SCRC; CP), 1-octadecene (ODE; Aldrich, 90%), toluene (SCRC, AR), oleic acid (OA; Alfa Aesar, 90%), oleylamine (OAm; Sigma-Aldrich, 70%), hexane (SCRC, AR), ethyl acetate (SCRC, AR), butyl acetate (Aladdin, 98.0%), tin(IV) oxide solution (SnO2; Alfa Aesar, 15 wt % in H2O colloidal dispersion), lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI; J&K, 98%), 4-tert-butylpyridine (tBP; J&K, 96%), chlorobenzene (J&K, 99.8%), and 2,2′,7,7′tetrakis(N,N-dimethoxyphenylamine)-9,9-spirobifluorene (SpiroOMeTAD; Advanced Election Technology Co., Ltd.). Synthesis of Cs3Sb2X9 (X = ClxBryI1−x−y, where 0 ≤ x and y ≤ 1) and Cs3Bi2X9 NCs. Cs(ac) (0.075 mmol), Sb(ac)3 (0.05 mmol), 5.0 mL of 1-dodecanol, and 0.5 mL of OA were loaded into a 25 mL three-necked flask and dried under vacuum for 40 min at 100 °C along with magnetic stirring. The reaction mixture, which was initially turbid, gradually turned colorless. After that, TMSCl (0.225 mmol), TMSBr (0.225 mmol), or TMSI (0.225 mmol) was swiftly injected under a nitrogen atmosphere. Then the reaction mixture immediately turned white (X = Cl), yellow (X = Br), or red (X = I) and became turbid. After 10 s, the reaction mixture was immediately cooled to room temperature by immersion in a cold-water bath. The product solution was centrifuged at 4000 rpm (for Cs3Sb2I9), 3000 rpm (for Cs3Sb2Br9), or 2000 rpm (for Cs3Sb2Cl9) for 3 min to discard the B

DOI: 10.1021/acs.inorgchem.9b01893 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic illustration of the reaction system and the synthetic process for Cs3Sb2Br9 NCs showing the size confinement effect of 1dedocanol to Cs3Sb2Br9 perovskite. (b) TEM image of as-synthesized Cs3Sb2Br9 NCs (inset: corresponding HRTEM image). (c) FTIR spectra of dried Cs3Sb2Br9 NC powder with and without hexane washing. (d) 1H NMR spectra of dried Cs3Sb2Br9 NC powder after washing by hexane and pure 1-dodecanol in CDCl3. cleaner for 15 min. The compact SnO2 electron-transport layer was prepared by spin-coating a dilute SnO2 colloid solution (2.5% in H2O) onto FTO/glass at a speed of 3000 rpm for 30 s and then baked on a hot plate in ambient air at 150 °C for 30 min.41 Subsequently, a Cs3Bi2I9 NC suspension in butyl acetate was spin-coated on SnO2 substrates at a speed of 2000 rpm for 30 s in a nitrogen-filled glovebox, followed by drying at 100 °C for 2 min. To increase the thickness of the active material layer, a Cs3Bi2I9 NC film with 275 nm thickness was obtained by 20 sequential deposition cycles. After that, the as-prepared Cs3Bi2I9 NC film was annealed at 200 °C for 30 min. For device fabrication, the hole-transport layer of Spiro-OMeTAD was spin-coated at a speed of 3000 rpm for 30 s and heated on a hot plate at 100 °C for 10 min in air. The Spiro-OMeTAD solution was prepared by mixing 36.6 mg of Spiro-OMeTAD, 14.5 μL of tBP, and 9.5 μL of a Li-TFSI solution (520 mg of Li-TFSI in 1 mL of chlorobenzene). Finally, a gold counter electrode was deposited through a shadow mask by a thermal evaporator. The active area of the device was defined as 0.12 cm2. Solar Cell Current Density−Voltage (J−V) Measurement Setup. The J−V curves were recorded using a Keithley 2400 apparatus under solar-simulated AM 1.5 sunlight (100 mW/cm2) with a standard xenon-lamp-based solar simulator (Oriel Sol 3A, Japan). The solar simulator illumination intensity was calibrated by a monocrystalline silicon reference cell (Oriel P/N 91150 V, with a KG-5 visible color filter) calibrated by the National Renewable Energy Laboratory. Characterizations. The powder X-ray diffraction (PXRD) patterns were recorded using a Philips X’Pert PRO SUPER X-ray diffractometer equipped with graphite-monochromatized Cu Kα radiation. The samples for PXRD characterization were the NC powders obtained by ethyl acetate precipitation and hexane washing. The transmission electron microscopy (TEM) images were acquired on a Hitachi HT-7700 transmission electron microscope with an accelerating voltage of 120 kV using the dispersed NCs in 1dodecanol. The high-resolution TEM (HRTEM) images were acquired on a JEM-2100F transmission electron microscope at an accelerating voltage of 200 kV. The Fourier transform infrared

spectroscopy (FTIR) studies in the solid state using a KBr disk were performed using a Nicolet 8700 FTIR spectrometer at room temperature. The NMR spectra were acquired on a Bruker Avance III 400 MHz spectrometer, which was equipped with a broad-bandinverse probe. The ultraviolet−visible (UV−vis) absorption spectra of NC suspensions were collected using a PekinElmer in transmission mode at room temperature. The UV−vis absorption spectrometry of NC powders was performed on a SOLID3700 spectrometer with an absorbance method. Thermogravimetric analysis (TGA) was carried out on powder samples using TGA Q5000IR analyzer. The samples were heated in the range of 30−800 °C at a heating rate of 10 °C/min under an inert atmosphere. Scanning electron microscopy (SEM) was examined using a FE-SEM SU 220 microscope. X-ray photoelectron microscopy (XPS) characterization was performed on a photoelectron spectrometer (ESCALAB 250, Thermo-VG Scientific).

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RESULTS AND DISCUSSION A typical colloidal synthetic procedure of Cs3Sb2Br9 NCs in an environmentally friendly and low-cost 1-dodecanol solvent is shown in Figure 1a. At first, Cs(ac) and Sb(ac)3 were dissolved and degassed in 1-dodecanol with a little OA in a three-necked flask at 100 °C for 40 min to form a transparent solution (Figure S1a,b). Subsequently, the TMSBr precursor was swiftly injected into the reaction flask under a nitrogen atmosphere at 100 °C. The bromide anions were released from TMSBr to coordinate with antimony and cesium cations, forming the 2D perovskite Cs3Sb2Br9 framework, the growth of which was heavily confined by 1-dodecanol because of polar−polar interactions or hydrogen bonding between Cs3Sb2Br9 and 1dodecanol. After quenching the reaction using an ice bath, a yellow turbid solution of Cs3Sb2Br9 NCs was finally obtained (Figure S1c). The phase of an as-synthesized NC powder was first revealed by the PXRD pattern (Figure S2a) as trigonal 2D perovskite Cs3Sb2Br9 with a space group of P3̅m1 (ICSD No. 39824), and no secondary phase appeared in the obtained C

DOI: 10.1021/acs.inorgchem.9b01893 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. (a) Photograph of a series of Cs3Sb2X9 NCs under ambient conditions. (b and c) UV−vis absorption spectra of Cs3Sb2X9 NC suspensions with Cl-to-Br and Br-to-I substitutions, respectively.

the conventional ODE system for comparison. As shown in Figure S7, only yellow flocculent products were obtained. Although the phase of the as-obtained flocculus is confirmed as the pure trigonal Cs3Sb2Br9 perovskite (Figure S7b), the morphologies of the products are uncontrolled aggregates (Figure S7c) rather than the uniform nanoparticles synthesized in 1-dedocanol, further showing the advantage of 1-dedocanol to control the size of 2D perovskite Cs3Sb2Br9 NCs. To confirm the capping agent effect of 1-dodecanol on asobtained Cs3Sb2Br9 NCs, FTIR and 1H NMR characterizations were performed. As shown in Figure 1c, the absence of a typical carbonyl stretching frequency (1710 cm−1) suggested that the conventional OA ligands did not exist on the surface of as-synthesized Cs3Sb2Br9 NCs.42−44 In contrast, a broad O−H stretching frequency (3100−3600 cm−1) and the C−O stretching frequency (1057 cm−1) of the alcohol group were observed for both unwashed and washed Cs3Sb2Br9 NCs (Figure 1c), indicating the presence of 1-dodecanol on the surfaces of Cs3Sb2Br9 NCs.39,45 The strong adsorption of 1dodecanol onto the surface of Cs3Sb2Br9 NCs can even be maintained under hexane washing, demonstrating the good capping effect of 1-dodecanol (Figure 1c). Furthermore, the 1 H NMR spectra of the Cs3Sb2Br9 NC powders also indicated the existence of 1-dodecanol (Figure 1d), demonstrating the strong capping of 1-dedocanol on the surface of Cs3Sb2Br9 NCs. The strong interaction between the NCs and 1dodecanol is considered to be hydrogen bonding between the H of the H−O bond from 1-dodecanol and the Br of Cs3Sb2Br9 NCs. The capping agent of 1-dedocanol can enable good stability and dispersibility of as-synthesized Cs3Sb2Br9 NCs, which are easily dispersed in 1-dodecanol to form a homogeneous colloidal NC suspension (Figure S8a), and this suspension can remain stable for 2 weeks without any aggregation (Figure S8b). It is noted that when using a solution-spin processing method, it is difficult to prepare highquality films because of the poor solubility of the precursors,

product. It is worth noting that the Cs3Sb2Br9 NC powder for the PXRD characterization is not the isolated NC but the bulklike powder due to reaggregation of NCs during the collection process (Figure S2b,c). The following TEM images and UV−vis spectra of the NCs are not correlated with these PXRD patterns of the bulklike material. To show the small particle sizes enabled by 1-dodecanol, the obtained yellow Cs3Sb2Br9 NCs were characterized by TEM. As shown in Figure 1b, the monodispersed Cs3Sb2Br9 NCs displayed small sizes and uniform size distribution with an average diameter of 2.79 nm (Figure S3), which are smaller than previously reported Cs3Sb2Br9 NCs.31 Furthermore, the HRTEM image revealed that the Cs3Sb2Br9 NCs are highly crystalline and display lattice fringes with a spacing distance of 1.69 Å corresponding to the lattice space of the (−2, 4, 3) planes (top right inset in Figure 1b). The influence of the reaction time and temperature on the particle size of as-synthesized Cs3Sb2Br9 NCs was investigated as well to show the size controllability of 1-dodecanol to the perovskite NCs. As shown in Figures S4 and S5, although the particle size of assynthesized Cs3Sb2Br9 NCs gradually increased with prolonged reaction time and enhancement of the reaction temperature, the sizes were still only several nanometers, demonstrating the strong size confinement effect of 1-dodecanol. Meanwhile, the corresponding UV−vis absorption spectra (Figure S6) of the samples showed that the excitonic absorption peak exhibited a little red shift with increasing reaction time from 3 s to 10 min (∼6 nm) and reaction temperature from 50 to 150 °C (∼8 nm). These results mean that the sizes of the Cs3Sb2Br9 NCs influenced their UV−vis absorption edges but displayed a very limited effect in comparison to the halide alloying effect. To maintain the consistency of the as-obtained perovskite NCs, the synthetic conditions for the following other bismuth/ antimony halide perovskite NCs were set as 100 °C for 10 s and all labeled compositions of the NCs were the precursor ratios. Furthermore, control experiments were conducted in D

DOI: 10.1021/acs.inorgchem.9b01893 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. Typical TEM images of as-synthesized (a) Cs3Sb2Cl9 NCs, (b) Cs3Sb2(Cl3/5Br2/5)9 NCs, (c) Cs3Sb2(Br3/5I2/5)9 NCs, and (d) Cs3Sb2I9 NCs. (e−h) Size distribution analysis for the sample corresponding to parts a−d, respectively.

Figure 4. Physical properties of Cs3Sb2X9 NC powders. (a) PXRD patterns of Cs3Sb2X9 NC powders and (b) a detailed comparison of the diffraction peak position of the (−2, 4, 0) plane. (c) UV−vis absorption spectra and (d) band-gap evolution trend of Cs3Sb2X9 NC powders with halide substitution from Cl to Br and then to I (insets show photographs of the corresponding NC powders under ambient conditions).

powder is also stable from room temperature up to 264 °C (Figure S9b). These results indicate that the proposed colloidal perovskite NC synthesis in 1-dodecanol is an environmentally friendly, facile, and reliable route to preparing highly monodispersed and stable lead-free halide perovskite NCs.

and using the small NCs (∼2 nm) with high dispersibility in our work is beneficial to the deposition of high-quality films. In addition, small NCs could regulate the material’s lattice and energy-band structure in accordance with the quantum confinement effect.7 The obtained Cs3Sb2Br9 NC powder is stable in air without any phase change even after 1 month of storage under ambient conditions (Figure S9a). The NC E

DOI: 10.1021/acs.inorgchem.9b01893 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (a) Photograph of a series of Cs3Bi2X9 NCs under ambient conditions. (b and c) UV−vis absorption spectra of Cs3Bi2X9 NC suspensions with Cl-to-Br and Br-to-I substitutions.

and S11, representative TEM images of Cs 3 Sb 2 Cl 9 , Cs3Sb2(Cl3/5Br2/5)9, Cs3Sb2(Br3/5I2/5)9, and Cs3Sb2I9 NCs evidenced the small particle morphologies and narrow size distribution of as-obtained perovskite NCs. The average particle size of the NCs is 1.69 nm for Cs3Sb2Cl9, 2.34 nm for Cs3Sb2(Cl3/5Br2/5)9, 2.07 nm for Cs3Sb2(Br3/5I2/5)9, and 2.31 nm for Cs3Sb2I9 (Figure 3e−h), to our best knowledge, all of which are smaller than the corresponding NCs in previous reports.25,31,32 The PXRD patterns of Cs3Sb2X9 NC powders revealed that all profiles remained the same without the appearance of new peaks (Figure 4a) except that the Bragg diffraction peaks monotonically shifted to smaller angles (Figure 4b), indicating the atomic substitution of halide anions in as-obtained NCs. In addition, the PXRD patterns of Cs3Sb2Cl9 and Cs3Sb2I9 NC powders nicely matched with the trigonal 2D perovskite structure with a space group of P3̅m1 (Cs3Sb2Cl9, ICSD No. 22075; Cs3Sb2I9, ICSD No. 39822; Figure S12a,b).25,31,32 As shown in Figure S13, the trigonal perovskites Cs3Sb2Cl9, Cs3Sb2Br9, and Cs3Sb2I9 are known as defect perovskites because of the ordered B-site vacancies of the interlayer space, which breaks apart the 3D networks of metal halide [MX6]n− octahedra, forming a 2D-layered perovskite structure with corner-connected octahedra.53 Similarly, Cs3Sb2Cl9 and Cs3Sb2I9 NC powders were reaggregated during the hexane washing process (Figure S12c,d), resulting in narrowing PXRD peaks as the bulklike material, which are not correlated to the monodisperse NC suspensions. To further track the band-gap evolution with the halide substitution, we measured the UV−vis absorption of Cs3Sb2X9 NC powders and used Tauc’s plot to calculate their optical band gaps. As shown in Figure 4c, the steep absorption edges of Cs3Sb2X9 NC powders were observed at around 410 nm for Cs3Sb2Cl9, 437 nm for Cs3Sb2(Cl4/5Br1/5)9, 468 nm for Cs3Sb2(Cl3/5Br2/5)9, 488 nm for Cs3Sb2(Cl2/5Br3/5)9, 500 nm for Cs3Sb2(Cl1/5Br4/5)9, 518 nm for Cs3Sb2Br9, 545 nm for

Taking the advantages of colloidal synthesis of perovskite NCs in 1-dodecanol, we systematically studied atomicsubstitution-induced band-gap engineering of as-obtained NCs, which is attractive for optoelectronic applications.27,46−48 In the A3B2X9 perovskite, atomic substitution is a primary method for band-gap engineering with all lattice sites of A, B, and X available for substitution. We first tried to substitute X lattice sites of the colloidal Cs3Sb2Br9 NCs by simply tuning the relative molar ratio of reaction precursors TMSCl, TMSBr, and TMSI. The colors of as-synthesized Cs3Sb2X9 NCs changed from white to yellow and then to orange red when the halide anions in Cs3Sb2X9 perovskite NCs were gradually varied from Cl to Br and then to I (Figure 2a). The optical properties of as-obtained Cs3Sb2X9 NC suspensions were first revealed by UV−vis absorption spectroscopy. As shown in Figure 2b, the UV−vis spectra exhibited that the excitonic absorption peaks of Cs3Sb2X9 NCs have a red shift with the halide substitution process from Cl to Br, owing to the change of the band gap in which the valence band maximum (VBM) of Cs3Sb2X9 is mainly comprised of Sb s and halide p orbitals, and the conduction band minimum (CBM) is mainly derived from Sb p orbitals.14,49 These samples (Figure 2b) have two absorption peaks corresponding to the proposed n = 1 and 2 excitonic transitions, respectively, which are attributed to the split of the energy levels in these low-dimensionality perovskites. The excitonic binding energy can be approximately estimated as the difference between the maxima of the exciton peak and the leading edge of the subsequent plateau,50 which for Cs3Sb2X9 is roughly 200−170 meV from Cl to Br (Figure S10), which is commonly found in similar 2D-layered perovskites.51,52 The Cs3Sb2X9 NCs also demonstrated a gradual red shift of the absorption edges with X-lattice-site substitution from Br to I (Figure 2c). To show good particle size control by 1-dedocanol during the X-site atomic-substitution process, several typical Cs3Sb2X9 NCs were characterized by TEM. As shown in Figures 3a−d F

DOI: 10.1021/acs.inorgchem.9b01893 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Typical TEM images of (a) Cs3Bi2Cl9 NCs, (b) Cs3Bi2Br9 NCs, and (c) Cs3Bi2I9 NCs (inset: corresponding HRTEM image). (d−f) Size distribution analysis for the sample corresponding to parts a−c, respectively.

Figure 7. Physical properties of Cs3Bi2X9 NC powders. (a) PXRD patterns of Cs3Bi2X9 NC powders and (b) a detailed comparison to show the peak shift. (c) UV−vis absorption spectra and (d) band-gap evolution trend of Cs3Bi2X9 NC powders with halide substitution from Cl to Br and then to I (insets show photographs of the corresponding NC powders under ambient conditions).

Cs3Sb2(Br4/5I1/5)9, 578 nm for Cs3Sb2(Br3/5I2/5)9, 603 nm for Cs3Sb2(Br2/5I3/5)9, 630 nm for Cs3Sb2(Br1/5I4/5)9, and 652 nm for Cs3Sb2I9. This gradual red shift of the absorption edge from Cl to I is corresponding to the color change of the NC powders from white to dark red (top inset in Figure 4d). On the basis of the obtained UV−vis absorption spectra of NC

powders, the band-gap evolution was analyzed according to the Tauc plot using the equation as (αhν)1/n = C(hν − Eg), where α is the absorption coefficient, h is Planck’s constant, C is the proportionality constant, ν is the frequency of light, Eg is the band gap, and n is 1/2 or 2, expressing direct- or indirect-bandgap materials, respectively.31 For the uniformity of comparison, G

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The PXRD patterns of Cs3Bi2X9 NC powders (Figure 7a) also showed similar trends in Cs3Sb2X9 NC powders (Figure 4a) in which the main Bragg diffraction peaks monotonically shifted to smaller angles with the substitution of X from Cl to Br and then to I (Figure 7b). However, the PXRD pattern profiles changed as well with the halide substitution process, showing the appearance of several new diffraction peaks (Figure 7a). In order to explain this phenomenon, the PXRD patterns of pure Cs3Bi2Cl9, Cs3Bi2Br9, and Cs3Bi2I9 NC powders were carefully compared. As shown in Figure S19, the phase of Cs3Bi2Cl9 NC powders matched well with the orthorhombic perovskite with a space group of Pmcn (ICSD No. 2067), but the phase of Cs3Bi2Br9 NC powders nicely matched with the trigonal perovskite with a space group of P3̅m1 (ICSD No. 1142) and the phase of Cs3Bi2I9 NC powders was a hexagonal perovskite with a space group of P63/mmc (ICSD No. 1448). To gain more insight into the structures of these perovskites, their unit cells and extended lattice structures were drawn out and are shown in Figure S20. The orthorhombic perovskite Cs3Bi2Cl9 actually consists of one-dimensional (1D) chains of corner-connected octahedra (Figure S20a,b),57 which is different from the 3D perovskite CsPbBr3, which shares its Pnma space group.58 The trigonal perovskite Cs3Bi2Br9 consists of 2D bilayers with cornerconnected octahedra and is known as a defect perovskite because of the ordered B-site vacancies of the interlayer space (Figure S20c,d),54 while the hexagonal perovskite Cs3Bi2I9 is 0D with molecular face-sharing Bi2I9 dimers and is not a proper perovskite because of the absence of corner-connected octahedra (Figure S20e,f).53,59 The structural difference among Cs3Bi2Cl9, Cs3Bi2Br9, and Cs3Bi2I9 NCs indicates that the perovskite structure of Cs3Bi2X9 NCs would change with halide substitution, and therefore the profiles of PXRD pattern varied during the halide substitution process. It should be pointed out that the mixed Cl−Br compositions have dramatic PXRD peak shifts from Cs3Bi2Cl9 to Cs3Bi2(Cl4/5Br1/5)9, but after this, the shifts are quite gradual (Figure 7a). We performed XPS characterization to study the real compositions of Cs3Bi2X9 NC powders with the halide substitution process from Cl to Br. As shown in Figure S21 and Table S1, the intensities of the Cl 2p peaks dramatically decrease from Cs3Bi2Cl9 to Cs3Bi2(Cl4/5Br1/5)9, but the intensities of the Cl 2p peaks gradually decrease from Cs3Bi2(Cl4/5Br1/5)9 to Cs3Bi2Br9, which implies that the ratio of Cl has a large change and accounts for the dramatic peak shifts from Cs3Bi2Cl9 to Cs3Bi2(Cl4/5Br1/5)9. In addition, the related quantitative analysis in Table S1 shows that the Cl:Br ratio is 5:1.8, 4.2:4, 2.2:4.7, and 1.3:6.8 corresponding to 4:1, 3:2, 2:3, and 1:4 of the starting stoichiometry, respectively, which suggests that the Cl−Br products are Br-rich relative to the starting stoichiometry. For the mixed halide series Cs3Bi2X9 in the final product, the peak of PXRD in Figure 7a remains similar to the halide substitution process from Cs3Bi2Cl9 to Cs3Bi2(Cl2/5Br3/5)9, while some peaks of Cs3Bi2(Cl1/5Br4/5)9 split into two peaks. This indicates that the Cs3Bi2X9 NCs kept the 1D structure of Cs 3 Bi 2 Cl 9 from Cs 3 Bi 2 Cl 9 to Cs3Bi2(Cl2/5Br3/5)9, while the Cs3Bi2(Cl1/5Br4/5)9 NCs appeared to have mixed phases of the orthorhombic 1D structure of Cs3Bi2Cl9 and the trigonal 2D structure of Cs3Bi2Br9 because of the sharp increase of Br content in the product of Cs3Bi2(Cl1/5Br4/5)9. Similarly, the PXRD peaks of a series of Cs3Bi2X9 NC powders remain similar to those of the halide substitution process from Cs3Bi2Br9 to Cs3Bi2(Br2/5I3/5)9, while

we took the bismuth/antimony-based halide perovskite A3B2X9 as the direct-band-gap material to draw the Tauc plots of Cs3Sb2X9 NC powders, which are shown in Figure S14, and the band gaps of NC powders were determined by the crossing points of the prolonged tangent of the absorption edge to the photon energy axis. The variation trend of the band gaps of Cs3Sb2X9 NC powders with the halide substitution is plotted in Figure 4d. With the substitution process from Cl to Br and then to I, the band gaps of the as-obtained NC powders show a monotonic decrease from 3.15 to 1.97 eV, accompanied by an obvious color change from white to yellow and then to dark red (top inset in Figure 4d). Moreover, the thermal stability of the as-synthesized Cs3Sb2X9 NC powders was investigated by TGA. The powder is stable up to 264 °C for Cs3Sb2Br9, which is higher than Cs3Sb2Cl9 for 221 °C and lower than Cs3Sb2I9 for 266 °C (Figures S9b and S15a,b). Besides the antimony-based halide perovskite NCs, we further demonstrated the synthesis of colloidal Cs3Bi2X9 NCs by using Bi(ac)3 to replace Sb(ac)3 and also systematically investigated halide anion substitution in Cs3Bi2X9 NCs. The photograph of a series of Cs3Bi2X9 NC suspensions shows that the colors of the as-obtained Cs3Bi2X9 NCs changed from white to yellow and then to red with the halide substitution process from Cl to Br and then to I (Figure 5a). The UV−vis absorption spectra of Cs3Bi2X9 NCs were also collected to show variation of their optical properties with halide substitution. As shown in Figure 5b, the UV−vis spectra of Cs3Bi2X9 NCs exhibited a red shift in the excitonic peak energy with the halide substitution process from Cs3Bi2Cl9 to Cs3Bi2Br9, owing to the change of the band gap in which the VBM of Cs3Bi2X9 is mainly comprised of Bi s and halide p orbitals and the CBM is mainly derived from Bi p orbitals.54 Such features are common in the Cs3Bi2X9 perovskites with the halide substitution process from Cs3Bi2Br9 to Cs3Bi2(Br1/5I4/5)9 but not for Cs3Bi2I9 (Figure 5c), which may be due to the change of the perovskite structure. The excitonic binding energy is roughly 200 meV for Cs3Bi2Cl9, 530 meV for Cs3Bi2Br9, and 180 meV for Cs3Bi2I9 (Figure S16), which is lower than the corresponding bulk materials.54,55 Typical TEM images of as-obtained Cs3Bi2Cl9, Cs3Bi2Br9, and Cs3Bi2I9 NCs also evidenced the small particle sizes (Figures 6a−c and S17a−c) and narrow size distributions with average sizes of 2.88 nm for Cs3Bi2Cl9, 3.83 nm for Cs3Bi2Br9, and 2.30 nm for Cs3Bi2I9, respectively (Figure 6d−f). In addition, as the inset in Figure 6a shows, the lattice fringe with a spacing distance of 1.9 Å corresponds to the lattice space of the (4, 0, 0) planes of Cs3Bi2Cl9. The HRTEM image of Cs3Bi2Br9 NCs (inset in Figure 6b) displays a lattice fringe with a spacing distance of 2.0 Å corresponding to the lattice space of the (−2, 2, 4) plane. The HRTEM image of the Cs3Bi2I9 NC (inset in Figure 6c) shows a lattice fringe with a spacing distance of 3.2 Å corresponding to the lattice space of the (2, 0, 3) plane. In addition, all of the NC morphologies of Cs3Bi2Cl9, Cs3Bi2Br9, and Cs3Bi2I9 have similar quasi-spherical shape and are independent of the intrinsic perovskite structures (Figure S17a−c), which is consistent with a previous literature report.56 Furthermore, the NC morphologies of halide alloyed Cs3Bi2(Cl4/5Br1/5)9, Cs3Bi2(Cl2/5Br3/5)9, and Cs3Bi2(Cl1/5Br4/5)9 maintain a quasi-spherical shape with slight changes of the sizes (Figure S18a−c). These results mean that the crystal structure of A3B2X9 has a very limited influence on the final morphologies of as-synthesized NCs. H

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Figure 8. Typical TEM images of (a) MA3Bi2Cl9 NCs, (b) MA3Bi2Br9 NCs, and (c) MA3Bi2I9 NCs. PXRD patterns of (d) MA3Bi2Cl9, (e) MA3Bi2Br9, and (f) MA3Bi2I9 NC powders.

the trigonal perovskite structure (space group P3̅m1, ICSD No. 110575) and that of the MA3Bi2I9 NC powders exhibited a monoclinic perovskite phase (space group P1211, ICSD No. 242182). The details of unit cells and extended lattice structures of these perovskites are shown in Figure S24. The orthorhombic perovskite MA3Bi2Cl9 is formed by removal of every third Bi layer along the ⟨110⟩ direction of the parent 3D structure, and the (001) planes also need to be sliced, leaving two-thirds of the Bi atoms and resulting in the final 1D structure (Figure S24a,b).14 The trigonal perovskite MA3Bi2Br9 consists of 2D bilayers with corner-connected octahedra (Figure S24c,d), which is different from the monoclinic perovskite MA3Bi2I9 (Figure S24e,f).14,60 The optical properties of MA3Bi2X9 perovskite NCs were revealed by UV−vis absorption spectra in both NC suspension and solid powder forms. As shown in Figure S25a, we observed a long absorption tail up to 750 nm for MA3Bi2I9 NC suspensions, which has also been observed in some lead-based perovskite NCs and leadfree perovskite NCs because of sub-band-gap absorptions.61−64 In addition, the absorption spectra of NC suspensions show obvious excitonic absorption peaks at about 371 nm for MA3Bi2Cl9, 438 nm for MA3Bi2Br9, and 521 nm for MA3Bi2I9, respectively (Figure S25a). On the basis of the absorption spectra of MA3Bi2X9 NC solid powders (Figure S25b), we calculated their optical band gaps by Tauc’s plot, with a band gap of 3.17 eV for MA3Bi2Cl9, 2.65 eV for MA3Bi2Br9, and 2.07 eV for MA3Bi2I9 (Figure S26a−c). Furthermore, it is worth mentioning that we synthesized MA3Sb2Br9, Rb3Bi2I9, and Rb3Sb2I9 in the form of colloidal NCs as well (Figures S27− S30), demonstrating the generality of our proposed synthetic route. To further demonstrate the effectiveness of our synthesis route for band-gap engineering, we synthesized a series of colloidal B-site alloying Cs3(SbyBi1−y)2Br9 (y = 1/4, 1/2, and 3 /4) NCs by tuning the chemical compositions from Cs3Sb2Br9 to Cs3Bi2Br9. When using Bi to substitute the Sb position in Cs3Sb2Br9, the profile of all PXRD patterns always remained the same without new peak appearances and original peak disappearances due to the structure of the same trigonal 2D perovskite (Figure 9a). In addition, the PXRD patterns of

some PXRD peaks of Cs3Bi2(Br1/5I4/5)9 split into two peaks, which also means that the Cs3Bi2X9 NCs kept the trigonal 2D structure of Cs3Bi2Br9 from Cs3Bi2Br9 to Cs3Bi2(Br2/5I3/5)9, while the Cs3Bi2(Br1/5I4/5)9 NCs appeared to have mixed phases of trigonal 2D Cs3Bi2Br9 and hexagonal 0D Cs3Bi2I9. The steep absorption edges of Cs3Bi2X9 NC powders also show red shifts (Figure 7c), which is at about 410 nm for Cs3Bi2Cl9, 421 nm for Cs3Bi2(Cl4/5Br1/5)9, 422 nm for Cs3Bi2(Cl3/5Br2/5)9, 450 nm for Cs3Bi2(Cl2/5Br3/5)9, 453 nm for Cs3Bi2(Cl1/5Br4/5)9, 478 nm for Cs3Bi2Br9, 571 nm for Cs3Bi2(Br4/5I1/5)9, 587 nm for Cs3Bi2(Br3/5I2/5)9, 606 nm for Cs3Bi2(Br2/5I3/5)9, 622 nm for Cs3Bi2(Br1/5I4/5)9, and 623 nm for Cs3Bi2I9. This is consistent with the color change of Cs3Bi2X9 NC powders from white to red (top inset in Figure 7d). The band-gap evolution of Cs3Bi2X9 NC powders displayed a reduction trend with halide substitution variation from Cl to Br and then to I (Figures 7d and S22), but there is an obvious dramatic decrease of the bandgaps where the halide anion substitution changed from Cl to I, which is consistent with the perovskite structural change revealed by the PXRD patterns (Figure 7a). The results show that we can tune the band gap from 3.09 to 2.05 eV for Cs3Bi2X9 NC powders. Moreover, the thermal stability of the as-synthesized Cs3Bi2X9 NC powders was investigated by TGA. As shown in Figure S23a−c, Cs3Bi2Br9 is stable up to 410 °C, higher than Cs3Bi2Cl9 for 398 °C and Cs3Bi2I9 for 379 °C, which also indicates that Cs3Bi2X9 NC powders show higher thermal stability than that of Cs3Sb2X9 (Figures S9b and S15a,b). Moreover, we can extend our synthetic protocol to change A lattice sites in A3B2X9 perovskite NCs by using MA to substitute Cs cations. Typical TEM images of a series of assynthesized MA3Bi2Cl9, MA3Bi2Br9, and MA3Bi2I9 NCs (Figure 8a−c) indicate that the sizes of the NCs were retained at small scales (∼1.3 nm) regardless of the change of the A-site component. All phases of as-synthesized MA3Bi2X9 perovskite NC powders were confirmed as corresponding pure perovskite phases, as shown in Figure 8d−f. The phase of MA3Bi2Cl9 NC powders matched well with the orthorhombic perovskite corresponding to the space group of Pnma (ICSD No. 109710), while that of the MA3Bi2Br9 NC powders matched I

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Figure 9. PXRD patterns of (a) Cs3(SbyBi1−y)2Br9 alloying NC powders and (b) a detailed comparison showing the peak shift. (c) UV−vis absorption spectra of Cs3(SbyBi1−y)2Br9 alloying NC suspensions.

that Cs3Bi2I9 NCs are promising low-toxic materials for optoelectronic applications.

alloying NC powders revealed that Bragg diffraction peaks monotonically shifted to smaller angles from Cs3Sb2Br9 to Cs3Bi2Br9 (Figure 9b), owing to the unit cell expansion. The optical properties of Cs3(SbyBi1−y)2Br9 alloying NC suspensions were revealed by UV−vis absorption spectroscopy. As shown in Figure 9c, the excitonic absorption peaks of alloying NCs show a little blue shifting from Cs3Sb2Br9 to Cs3Bi2Br9, which is consistent with the band gap of Cs3Sb2Br9 and Cs3Bi2Br9, as previously mentioned.31,56 We also tested the stability of the Cs3Bi2I9 perovskite NC films via the PXRD patterns and UV−vis absorption spectroscopy under different conditions. The PXRD patterns (Figure S31a) show that the Cs3Bi2I9 NC films are stable after annealing at 200 °C for 1 h under a nitrogen atmosphere. From UV−vis absorption spectra (Figure S31b), we can observe that the light absorption spectra only showed a very small change after 1 month of storage under ambient conditions, indicating that the Cs3Bi2I9 NCs are more stable than most used lead halide perovskites in solar cells. Furthermore, we fabricated solar cells using Cs3Bi2I9 NC films as photoactive layers (Figure S32). The J−V characterization of an as-fabricated solar cell exhibits a low power conversion efficiency of 0.0103% (Figure S32d). The solar cell efficiency depends on the nonradiative recombination induced by, for example, defect states in the band gap, which may seriously reduce the performance of the solar cell.49,65 Therefore, the performance of the as-fabricated solar cell will further be improved by reducing nonradiative recombination in the NC films in future.

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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.9b01893. Synthetic process of Cs3Sb2Br9 NCs, photographs, PXRD patterns, SEM, TEM, and HRTEM images, size distributions, UV−vis absorption spectra, optical images, thermogravimetric analysis, exciton binding energies, unit cell structures, Tauc plots, XPS spectra, physical properties, fabrication example, and current density− voltage curve (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.-B.Y.). ORCID

Tao Chen: 0000-0002-3483-8341 Hong-Bin Yao: 0000-0002-2901-0160 Author Contributions ∥

These authors contributed equally to this work.

Notes

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The authors declare no competing financial interest.

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CONCLUSION In summary, we have demonstrated a general and efficient route for the synthesis of all-inorganic and organic−inorganic hybrid lead-free metal halide perovskite A3B2X9 (A = Cs, Rb, or CH3NH3; B = Bi or Sb; X = Cl, Br, or I) NCs with high colloidal stability and phase purity. Our approach adopted a cheap and environmentally friendly 1-dodecanol as the solvent, which can serve as ligands to control the particle size of assynthesized A3B2X9 NCs. In addition, we systematically and finely tuned the band gaps of as-synthesized A3B2X9 NCs by atomic substitutions of A, B, and X lattice sites, which demonstrates the versatility of our proposed synthetic method. Finally, we used the low-cost, environmentally friendly, and stable Cs3Bi2I9 perovskite NCs to fabricate a solar cell, showing

ACKNOWLEDGMENTS We acknowledge funding support from the National Natural Science Foundation of China (Grants 51571184, 21501165, and 21875236), the National Key R&D Program on Nano Science & Technology (Grants 2016YFA0200602 and 2018YFA0208702), the Fundamental Research Funds for the Central Universities (Grants WK2060190085 and WK2340000063), the Joint Funds from Hefei National Synchrotron Radiation Laboratory (Grant KY2060000111). H.-B.Y. is thankful for support by “the Recruitment Program of Thousand Youth Talents”. We are thankful for support from the USTC Center for Micro and Nanoscale Research and Fabrication. J

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DOI: 10.1021/acs.inorgchem.9b01893 Inorg. Chem. XXXX, XXX, XXX−XXX