Colloidal synthesis of lead-free all-inorganic Cs3Sb2BrxI9-x nanocrystals

Environment-friendly lead-free perovskite nanocrystals (NCs) are highly desirable in terms of optoelectronic applications. Reported herein is the synthesis of perovskite-related Cs3Sb2Br9 NCs through a simple hot-injection approach. The morphologies and optical properties of NCs were controlled and optimized by varying the reaction temperature, time, and volume of surface ligands. Meanwhile, the Cs3Sb2Br9 NCs showed bandgap emission in the blue region centered at 470 nm, and 63 nm full width at half maximum (FWHM). In addition, the Cs3Sb2Br9 NCs displayed high stability when immersed in a polar solvent like ethanol. It was also found that the bandgaps of the Cs3Sb2BrxI9-x NCs could be adjusted by regulating the composition of halogen.


Introduction
The lead halide perovskite ABX 3 (A = Cs + , B = Pb 2+ , and X = Cl − , Br − , and I − ) has attracted much attention and has been the subject of in-depth research because it has a high photoluminescence quantum yield (PLQY, ∼ 100%), narrower full width at half maximum (FWHM, 12-35 nm), high intrinsic carrier mobility, and high defect tolerance, making it a good potential material for use in efficient optoelectronic devices [1][2][3][4][5][6][7]. The toxicity of lead, however, is a great hindrance to their practical application. To solve this problem, significant efforts have been made of late to develop lead-free colloidal perovskite nanocrystals (NCs) [8,9].
There are two typical strategies for obtaining lead-free metal halide perovskites. One is by replacing Pb 2+ with lead-free cations, which are isoelectronic with Pb 2+ , such as Sn(II), Ge(II), Bi(III), and Sb(III) [10,11]. The other is based on the replacement of two divalent Pb 2+ ions with one monovalent M + and one trivalent M 3+ cation, generating quaternary A 2 M + M 3+ X 6 double-perovskite compounds, which maintain a three-dimensional (3D) perovskite structure and charge neutrality [12,13]. Among the aforementioned lead-free metal halide perovskites, Sb-based perovskites show a nearly direct bandgap, a low trap state density, and long carrier diffusion as well as an electronic configuration and a phase structure similar to those of lead halide perovskites [14,15]. Therefore, Sb-based perovskites are highly expected to be promising alternatives to lead-free perovskite materials in the future. To date, however, only the solution phase synthesis of Cs 3 Sb 2 X 9 (X = Cl, Br, I) quantum dots (QDs) and the colloidal synthesis of A 3 Sb 2 X 9 (X = Cl, I, A = Cs, Rb) have been reported [16][17][18]. The Cs 3 Sb 2 Br 9 colloidal QDs prepared through the solution phase method have a 46% PLQY and 41 nm FWHM. Unfortunately, this method cannot effectively control the morphology of NCs. For this, the colloidal route was used to synthesize uniform Cs 3 Sb 2 Cl 9 nanowires (NWs) up to several microns and change the aspect ratio of nanorods by turning precursors and ligands. Meanwhile, nanoplatelets and nanorods of Cs 3 Sb 2 I 9 and spherical Rb 3 Sb 2 I 9 NCs were prepared through the colloidal route, and the photoelectric properties were also characterized. There have been few reports, however, on the colloidal synthesis of Cs 3 Sb 2 Br 9 NCs and their optical properties.
Reported herein is the one-pot synthesis of colloidal Cs 3 Sb 2 Br 9 NCs with good control over the morphologies of the final product by turning the reaction conditions, including the temperature, time, and volume of surface ligands, so that pure-phase Cs 3 Sb 2 Br 9 NCs, which have NC and NW morphologies, can be synthesized. The NCs exhibited blue emission at 470 nm, with a 0.6% PLQY and 63 nm FWHM, on account of the radiative recombination caused by excitons. In the same approach, Cs 3 Sb 2 Br x I 9−x samples were prepared by regulating different proportions of SbBr 3 and SbI 3 in a precursor solution to adjust the optical properties. The synthesized Cs 3 Sb 2 Br 9 NCs showed excellent ambient and ethanol stability. These results can contribute to the future application of lead-free perovskites in optoelectronic devices [14,18].

Preparation of Cs-Oleate
Cesium oleate was prepared by dissolving Cs 2 CO 3 (2.0 mmol) with 1-octadecene (20.0 mL, ODE) and oleic acid (2.5 mL) in a three-neck flask. The mixture was first degassed under vacuum at 120°C for 1 h, and then heated up to 150°C under N 2 flux with vigorous stirring for 3 h, until the solution became clear. The resulting product was cooled to room temperature and was stored in a vial under air.

Synthesis of Cs 3 Sb 2 Br 9 and Cs 3 Sb 2 Br x I 9−x NCs
In a typical synthesis, a mixture of 10.0 mL ODE, 0.4 mL OAm, 1.0 mL OnA, and 0.1 mmol SbBr 3 was loaded into a three-neck flask and was degassed under vacuum at 80°C for 1 h. After further being heated up to 180°C under a nitrogen atmosphere, 0.3 mL of the as-prepared Cs-oleate solution was swiftly injected under vigorous stirring. The transparent solution instantaneously turned yellow green, indicating the formation of the target NCs. After a specific reaction time (e.g. 10, 30, 60, and 300 s), the reaction mixture was cooled with an ice water bath. The final solution was centrifuged at 7800 rpm for 5 min to separate the precipitates and the supernatant. After adding 5.0 mL hexane, the precipitates and the supernatant solution were centrifuged at 12,500 rpm for 5 min to obtain Cs 3 Sb 2 Br 9 NCs and Cs 3 Sb 2 Br 9 NWs, respectively. The as-obtained Cs 3 Sb 2 Br 9 NCs and Cs 3 Sb 2 Br 9 NWs were finally dispersed into hexane for further characterization. No argon-filled glovebox was needed during the synthesis and purification processes.

Characterization
The X-ray diffraction (XRD) patterns were identified through the laboratory powder X-ray diffraction system (Rigaku D/MAX 2200 VPC) at a scanning rate of 10°/minute in the 2θ range from 10 to 60°, with Cu Kα1 radiation (λ = 1.5405 Å) at 40 kV and 26 mA. The morphologies and structures of the as-prepared products were inspected using transmission electron microscopy (TEM, FEI Tecnai G2 Spirit). The collection time during the energy-dispersive X-ray (EDX) was 30 s. The infrared (IR) spectra were obtained through Fourier transform infrared spectrometry (FTIR) coupled with infrared microscopy (EQUINOX 55). The time-resolved photoluminescence (PL) decay and emission spectra were recorded by a steady-state fluorescence spectrometer (Edinburgh Instruments FLS1000) equipped with a 450W xenon lamp as the excitation source and an RP928 photodetector.

Results and discussion
Cs 3 Sb 2 X 9 is known to have an ordered-vacancy perovskite structure consisting of bioctahedral Sb 2 X 3− 9 dimer-like units surrounded by cesium cations ( Figure  1a). The Cs 3 Sb 2 Br 9 -perovskite-related NCs were synthesized via the hot-injection method ( Figure 1b). OAm and OnA serve as ligands in this system, which can synergistically control the crystallization kinetics of Cs 3 Sb 2 Br 9 NCs. Specifically, OnA can form a chelate with Sb 3+ metal ions whereas OAm can stabilize the resulting NC and NW formation. The morphology of Cs 3 Sb 2 Br 9 NCs in terms of NCs and NWs can be tuned by varying the proportion of ligands and the selective precipitation [19]. FTIR spectroscopy showed the presence of OAm, OnA, and OA on the surfaces of the Cs 3 Sb 2 Br 9 NCs (Figure 1c). The presence of C-H stretching frequencies (3037 and 2913 cm −1 ) and C-O stretching frequency peaks (1403 cm −1 ) in the Cs 3 Sb 2 Br 9 NCs indicates that OA and OnA were successfully bound onto the surfaces of the NCs. The presence of N-H bending vibration (1618 cm −1 ) and N-H stretching vibration (3399 cm −1 ) indicates that OAm interacted with the surfaces of the NCs [20].
Additionally, it is worth mentioning that unlike in the hot-injection synthesis of CsPbBr 3 , an appropriate condition contributes to the formation of a specific morphology. The corresponding XRD patterns for the NCs synthesized at different temperatures while keeping the same ratio of ligands and a fixed synthesis time are shown in Figure 1d. The impurities' peaks that appeared at 140°C are ascribed to CsBr while the products showed poor crystallinity at > 180°C, which may have been due to the volatilization of some reactants [21]. For the samples prepared within the 160-220°C temperature range, their XRD patterns were consistent with that of the pure-phase Cs 3 Sb 2 Br 9 (standard card PDF#77-1055). The obvious difference in XRD pattern of the samples prepared at 160 and 180°C is that the first strongest peak changed from 32°(160°C) to 27.5°(180°C), which may have been due to the change in the crystal plane growth direction as the temperature increased. The EDX data in TEM show that the atomic ratio of Cs/Sb/Br is 3:1.67:8.94, which is close to the chemical composition of Cs 3 Sb 2 Br 9 ( Figure S1). The temperature-dependent TEM micrographs (Figure 2a-c, S2) indicate that as the temperature increases, the morphology of the nanoparticles (NPs) becomes irregular. Considering the results of the XRD and TEM data analysis, 180°C was chosen as the optimal temperature for synthesizing high-quality NCs.
The XRD patterns of the NCs synthesized at different reaction times while keeping the same ratio of ligands and fixing the synthesis temperature at 180°C are shown in Figure 1e. The XRD pattern of the sample prepared at 180°C for 10 s was mainly consistent with the impurity phase of Sb 2 O 4 (standard card PDF#17-0620) as well as a tiny target phase of Cs 3 Sb 2 Br 9 NCs (standard card PDF#77-1055). As the reaction time increased to above 30 s, the second and third strongest peaks of standard card PDF#17-0620 at 30.6 and 31.0°disappeared, suggesting that the impurity phase of Sb 2 O 4 did not exist in the samples. Moreover, the XRD patterns of all the samples prepared at 180°C for 30, 60, and 300 s were consistent with that of the pure-phase Cs 3 Sb 2 Br 9 . Similar to what is shown in Figure 1d, the first strongest peak changed from 32°(30 and 60 s) to 27.5°(300 s), which may have been due to the change in the crystal plane growth direction as the reaction time increased. To track the morphology evolution and growth process, the TEM micrographs were measured to monitor the reaction process (Figure 2d-f). The results revealed that the Cs 3 Sb 2 Br 9 NCs experienced fast nucleation and slow growth processes. At the same time, the size distribution of the NCs gradually broadened and reunited. At 10 s (Figure 2d), small NPs were already formed, indicating fast nucleation. At 30 and 60 s (Figure 2e and f), some small NPs gradually grew, forming regular NCs ( ∼ 25 nm in diameter). At 300 s, most of the particles were transformed into larger irregular NCs ( ∼ 100 nm in diameter), which was due to Ostwald ripening ( Figure  S3) [22].
Furthermore, the effects of the different volume ratios of OAm and OnA (OAm = 0.2, 0.4, 0.6, 0.8 mL; OnA = 1 mL) were also examined. As shown in Figure  2f, when OAm was less than 0.2 mL, no pure Cs 3 Sb 2 Br 9 NCs were obtained, and the XRD patterns (Figure 2f) showed that CsBr and Cs 3 Sb 2 Br 9 were present. When OAm was more than 0.8 mL, only CsBr was present. The possible reason for this is that the increasing amount of OAm slows down the release of free Sb ions in the reaction, especially at a higher OAm concentration and a shorter reaction time (1 min), because OAm as a hard base can readily coordinate with Sb ions, which control the crystallization process of Cs 3 Sb 2 Br 9 NCs. These finally result in the presence only of CsBr in the samples when OAm was more than 0.8 mL. Sb ions may exist in solution in the form of soluble Sb-OAm complex cations [23][24][25]. The checking of the TEM of the final NCs (Figure 2g-i) revealed that when OAm was kept at 0.4-0.6 mL, NCs were found in the precipitation while NWs were found in the supernatant. These results suggest the amount of OAm that can contribute to the formation of ligands on the surfaces of the NCs and that will result in the self-assembly of the NCs to form a regular morphology. In addition, the reaction conditions showed effects on the absorption properties of the assynthesized samples ( Figure S4). There was an absorption tail on the longer-wavelength sides of all the NCs with different reaction conditions, which may have been due to the existence of some defect states [26][27][28].
The PL properties of Cs 3 Sb 2 Br 9 NCs are rarely investigated, but these are the subject of ongoing research. Excited wavelength-dependent PL spectra (Figure 3a) show Cs 3 Sb 2 Br 9 NCs exhibiting band edge emission with a peak at 470 nm, which gives a blue shift compared to the Cs 3 Sb 2 Br 9 single crystal (bandgap: 2.36 eV; crystal structure: P3m1 (no. 164)) [17,[29][30][31]. Meanwhile, the tails in the PL spectra ranging from 550 to 700 nm suggest a defect-related contribution [16]. Unfortunately, the Cs 3 Sb 2 Br 9 NCs showed very weak emission, with only a ∼ 0.6% PLQY, similar to the other antimony halide perovskite NCs synthesized through hot injection [15][16][17].  The PL decay curves (Figure 3b) were fitted with a biexponential decay model (equation 1), which gave a short lifetime τ 1 and a long lifetime τ 2 , respectively. The τ 1 was estimated to be 1.23 ns, with a 79% percentage, and the τ 2 was 8.37 ns, with a 25% percentage. It is believed that the non-recombination decay associated with shorter lifetimes ∼ 1 ns comes from the deep or vacancy defects that lead to poor PL efficiency. Therefore, reducing the defect density is strongly expected to improve the luminescence performance in the future [32].
Additionally, a series of Cs 3 Sb 2 Br x I 9−x (x = 0, 3,6,8,9) NCs was prepared via the one-pot hot-injection method. Typical images of Cs 3 Sb 2 Br x I 9−x (x = 0, 3,6,8,9) NCs under daylight are shown in Figure S5. The absorption of the NCs in this study is shown in Figure S6b. The absorption onset showed red shifts when the x changed from 9 to 3, while it slightly shifted back (sky blue line) with the x at 0. It is well known that the alloying of I in the original Br-terminated perovskites commonly leads to a red shift in absorption in the same phase structure [33]. Therefore, the obtained results imply that Cs 3 Sb 2 Br x I 9−x NCs are halide alloy NCs. The NCs' phase structure is shown in Figure S6a. The results showed that they had the same structure as space group P3m1. With the x decreasing from 9 to 0, the (201) peak shifts gradually became smaller, which can be ascribed to the alloying of iodine ions with bromide with a larger ion radius. The Cs 3 Sb 2 Br x I 9−x NCs actually showed no visible emission. Nonetheless, they had the highest crystalline quality and an adjustable bandgap, implying their potential application in photovoltaics.
It is well known that the stability of perovskite materials is a crucial issue especially in their practical applications [34]. Therefore, the stability of Cs 3 Sb 2 Br 9 NCs was carefully studied in this work. As shown in Figure 4, after Cs 3 Sb 2 Br 9 NCs were soaked in ethanol for 12 h, their phase structure remained the same, and the fresh sample and bandgap absorption around 450 nm were unchanged, indicating that they have excellent stability in ethanol. To evaluate the water resistance properties of Cs 3 Sb 2 Br 9 NCs, they were kept suspended in air for 30 days. As can be seen in Figure 4c-d, the intensity of the peak at around 27.5°decreased and that of the peak at around 32°increased while both became narrower. These results indicate that the colloidal NCs may gradually grow along the (022) plane (32°) and turn into large particles while being kept under long-term air conditions with moisture. After exposure of the NCs to air for 30 days, the resulting extra absorption tail in the 500-700 nm wavelength might have been caused by some defects or by the scattering of the generated large particles [35][36][37]. Figure  S7 shows the TGA data of the Cs 3 Sb 2 Br 9 NCs within the 25-800°C temperature range. Obviously, weight loss started after 350°C, and the NCs decomposed into CsBr and SbBr 3 . It can thus be said that Cs 3 Sb 2 Br 9 NCs have outstanding thermal stability. In summary, the Cs 3 Sb 2 Br 9 NCs showed widely tunable absorbance and excellent ambient and thermal stability compared to lead-halidebased perovskites, suggesting that they are promising for use in Pb-free perovskite optoelectronic devices in the future.

Conclusions
In conclusion, in this study, lead-free, air-stable, allinorganic Cs 3 Sb 2 Br 9 nanocrystals (NCs) were synthesized using colloidal synthesis for the first time. The factors affecting the morphology and size of such NCs were systematically studied by controlling the temperature, time, and volume of the surface ligands, and their optical properties were characterized. The Cs 3 Sb 2 Br 9 NCs formed NC and nanowire (NW) shapes when the temperature, time and ratio of the ligands were controlled. The growth mechanism was found to have similarities to Ostwald ripening. Cs 3 Sb 2 Br x I 9−x NCs were used to effectively adjust the bandgap by controlling the anion composition. It is firmly believed that the widely tunable absorbance and excellent ambient stability of Cs 3 Sb 2 Br 9 NCs make them highly promising materials for use in stable and efficient Pb-free perovskite optoelectronic devices.

Associated content
Supporting information. The supporting information (EDX, TEM images, absorption spectra, optical images, TGA graph (PDF)) is available free of charge at the ACS Publications website of DOI.

Disclosure statement
No potential conflict of interest was reported by the authors.

Notes on contributors
Weijiang Gan is a Ph.D. candidate at the School of Chemistry of Sun Yat-Sen University. He received his B.S. Applied Chemistry degree from Beijing University of Chemical Technology, China in 2017. His current research is focused on the synthesis and understanding of the luminescent and structural properties of leadfree halide perovskites to stabilize them for optoelectronic applications. In 2014, he was promoted to professor. His research concerns nanoscience and optical functional materials for display, lighting, bioimaging, and solar energy (luminescent materials, phosphors, glass-ceramic, nano-probe, persistent luminescence, rare earth, quantum dots, perovskite, etc.).