Room Temperature In-Situ Synthesis of Inorganic Lead Halide Perovskite Nanocrystals Sol Using Ultraviolet Polymerized Acrylic Monomers as Solvent and Their Composites with High Stability

: As a kind of promising optoelectrical material, all-inorganic perovskite nanocrystals CsPbX 3 (X = Cl, Br, I) have attracted much attention, due to their excellent optoelectrical characteristics, in recent years. However, their synthesis approaches require rigorous conditions, including high temperature, eco-unfriendly solvent or complex post-synthesis process. Herein, to overcome these issues, we reported a novel facile room temperature in-situ strategy using ultraviolet polymerizable acrylic monomer as solvent to synthesis CsPbX 3 nanocrystals without a complex post-synthesis process. In this strategy, adequate soluble precursors of Cs, Pb and Br and reaction terminating agent 3-aminopropyltriethoxysilane (APTES) were used. The obtained CsPbBr 3 nanocrystals showed a high photoluminescence quantum yields (PLQY) of 87.5%. The corresponding polymer composites, by adding light initiator and oligomer under ultraviolet light radiation, performed excellent stability in light, air, moisture and high temperature. The reaction process and the e ﬀ ect of the reaction terminating agent have been investigated in detail. This strategy is a universal one for synthesizing CsPbX 3 nanocrystals covering visible light range by introducing HCl and ZnI 2 .

Acrylic monomers (IBOMA/IBOA/LMA/MMA, 20 mL), OA (1 mL) and TOAB (0.3 mmol) were added into a 100 mL beaker to obtain a TOAB solution. The TOAB solution was swiftly injected into above prepared Cs-oleate and Pb-oleate solution (5 mL) and 60 s later APTES (0.3 mmol) was injected. It was stirred for 4 h in air and then preserved in the reagent bottle for further use.
For the preparation of block, blended 25 mL CsPbBr 3 solution (prepared in IBOMA) with light initiator 2-hydroxy-2-methyl-1-acetone (0.58 g) and then added them into a 50 mL beaker. After stirred for 5 min, the solution was poured into a template and then cured by a 395 nm UV lamp. The thickness of the block can be changed according to depth of the template. For the preparation of film, 25 mL CsPbBr 3 solution (prepared in IBOMA), 0.58 g light initiator 2-hydroxy-2-methyl-1-acetone and 75 g oligomer have been used.

Anion-Exchange Process
Certain amounts of HCl or ZnI 2 (Table 1) as the anion source were mixed with IBOMA (10 mL) in a beaker, and stirred until they were dissolved completely. When TOAB-IBOMA solution was injected into the Cs-oleate and Pb-oleate IBOMA solution, the CsPbBr 3 nanocrystals were obtained, and the sol showed bright green under the ultraviolet light (395 nm). The HCl/ZnI 2 -IBOMA solution was added into CsPbBr 3 sol, and 60 s later the APTES (0.3 mmol) was injected. After stirring for 4 h in air, the anion-exchanged samples were obtained.

Characterization Methods
UV-VIS absorption spectra were collected using a 950 UV-Visible Spectrophotometer (Shimadzu Corporation, Kyoto, Japan) with absorption mode. Photoluminescence (PL) spectra were measured by a Fluorolog-3-P UV-VIS-NIR fluorescence spectrophotometer (Jobin Yvon, Longjumeau, France) with a 450 W Xe lamp as the excitation source. PLQY was estimated according to standard procedure, using Rhodamine B (95% of the yield in ethanol) as reference. For samples and rhodamine ethanol solution, an excitation wavelength of 450 nm was used. Powder X-ray diffraction (XRD) patterns were collected using a Bruker AXS D8 Advance diffractometer (Bruker AXS Ltd., Karlsruhe, Germany) with a step width of 0.02 • (Voltage 50 kV, current 40 mA, Cu-Ka). The CsPbBr 3 acrylic sols (after adding APTES) were precipitated by adding 1-butyllactone, and then centrifuged in a centrifuge for 10 min (10,000 r/min). The precipitate was freeze-dried and ground to produce the yellow powder samples for XRD test. The morphology and particles size of the samples were examined by a JEM-2100 (JEOL Ltd.,Tokyo, Japan) transmission electron microscopy (TEM). High-resolution TEM (HRTEM) images were recorded using a JEOL JEM-2100 microscope (JEOL Ltd., Tokyo, Japan) operated at 200 kV. After adding 2-hydroxy-2-methylpropiophenone (0.58 g) as a photoinitiator, CsPbBr 3 blocks were prepared via an ultraviaolet polymerization method. The CsPbBr 3 composite sample for TEM measurement was obtained by means of ultrathin frozen section. Fourier transform infrared (FTIR) spectra in the region of 400~4000 cm −1 were recorded on a Thermo Fisher Scientific Nicolet 6700 Spectrometer (Thermo Nicolet Corporation, Basingstoke, United Kindom) by pressing the mixture of the powder sample (obtained after centrifugation and freeze-drying) and KBr to a sheet.

Results and Discussion
As schematically shown in Figure 1a, the synthetic process was performed in two steps: the formation of CsPbBr 3 nanocrystals and size controlling by adding APTES. Details of the process have been described in the Experimental section. The sol sample showed bright green under the ultraviolet light (395 nm) (Figure 1c), and CsPbBr 3 nanocrystals/polyarcylic composites can be prepared via an ultraviaolet polymerization method (Figure 1b,c).
Appl. Sci. 2020, 10, x 4 of 11 diffraction (XRD) patterns were collected using a Bruker AXS D8 Advance diffractometer (Bruker AXS Ltd., Karlsruhe, Germany) with a step width of 0.02° (Voltage 50 kV, current 40 mA, Cu-Ka). The CsPbBr3 acrylic sols (after adding APTES) were precipitated by adding 1-butyllactone, and then centrifuged in a centrifuge for 10 min (10000 r/min). The precipitate was freeze-dried and ground to produce the yellow powder samples for XRD test. The morphology and particles size of the samples were examined by a JEM-2100 (JEOL Ltd.,Tokyo, Japan) transmission electron microscopy (TEM). High-resolution TEM (HRTEM) images were recorded using a JEOL JEM-2100 microscope (JEOL Ltd., Tokyo, Japan) operated at 200 kV. After adding 2-hydroxy-2-methylpropiophenone (0.58 g) as a photoinitiator, CsPbBr3 blocks were prepared via an ultraviaolet polymerization method. The CsPbBr3 composite sample for TEM measurement was obtained by means of ultrathin frozen section. Fourier transform infrared (FTIR) spectra in the region of 400~4000 cm −1 were recorded on a Thermo Fisher Scientific Nicolet 6700 Spectrometer (Thermo Nicolet Corporation, Basingstoke, United Kindom) by pressing the mixture of the powder sample (obtained after centrifugation and freeze-drying) and KBr to a sheet.

Results and Discussion
As schematically shown in Figure 1a, the synthetic process was performed in two steps: the formation of CsPbBr3 nanocrystals and size controlling by adding APTES. Details of the process have been described in the Experimental section. The sol sample showed bright green under the ultraviolet light (395 nm) (Figure 1c), and CsPbBr3 nanocrystals/polyarcylic composites can be prepared via an ultraviaolet polymerization method (Figure 1b Figure 2a shows the absorption and fluorescence spectra of CsPbBr3 nanocrystals prepared in IBOMA. An emission peak at 510 nm was observed, and the corresponding absorption peak located at 485 nm. According to the formula proposed by Tauc and Mott, the calculated bandgap of as-prepared CsPbBr3 nanocrystals is 2.25 eV [39,40]. However, red shift can be observed in the emission spectra of CsPbBr3 prepared in other three acrylic monomers, indicating the particle size increasing [41] ( Figure S1 in Supplementary Materials). It was noted that the PLQY of CsPbBr3 nanocrystals prepared in IBOMA reached 87.5%, much higher than the samples prepared in other acrylic monomers (Table S1 in Supplementary Materials). In order to study the crystal structure of the samples, the XRD patterns of CsPbBr3 powder samples are shown in Figure 2b and Figure S1, and CsPbBr3 nanocrystals possessing a cubic structure (PDF # 54-0752) were synthesized in IBOMA. However, a small impurity peak ( Figure S1) identified as PbBr2 existing in the pattern of CsPbBr3 nanocrystals synthesized in IBOA. The XRD results demonstrated that cubic CsPbBr3 nanocrystals can be synthesized at room temperature by this method.  Figure 2a shows the absorption and fluorescence spectra of CsPbBr 3 nanocrystals prepared in IBOMA. An emission peak at 510 nm was observed, and the corresponding absorption peak located at 485 nm. According to the formula proposed by Tauc and Mott, the calculated bandgap of as-prepared CsPbBr 3 nanocrystals is 2.25 eV [39,40]. However, red shift can be observed in the emission spectra of CsPbBr 3 prepared in other three acrylic monomers, indicating the particle size increasing [41] ( Figure S1 in Supplementary Materials). It was noted that the PLQY of CsPbBr 3 nanocrystals prepared in IBOMA reached 87.5%, much higher than the samples prepared in other acrylic monomers (Table S1 in Supplementary Materials). In order to study the crystal structure of the samples, the XRD patterns of CsPbBr 3 powder samples are shown in Figure 2b and Figure S1, and CsPbBr 3 nanocrystals possessing a cubic structure (PDF # 54-0752) were synthesized in IBOMA. However, a small impurity peak ( Figure S1) identified as PbBr 2 existing in the pattern of CsPbBr 3 nanocrystals synthesized in IBOA. The XRD results demonstrated that cubic CsPbBr 3 nanocrystals can be synthesized at room temperature by this method. Appl. Sci. 2020, 10, x 5 of 11  The CsPbBr3-IBOMA nanocrystals sol exhibited excellent stability. After being placed in air without any protection for 2 weeks, the particle size remained nearly unchanged (Figure 4a), which was attributed to the capping of hydrolyzed APTES ligand [32]. APTES hydrolyzes on the surface of nanocrystals, forming a network structure to prevent the desorption of ligands and to subsequently prohibit size increases, which enhanced the stability of nanocrystals [37,42]. To further prove the effect of APTES, CsPbBr3-IBOMA nanocrystals without APTES were synthesized, and their morphology and size were determined by TEM (Figure 4b). The size (45.7 ± 7.7 nm) was obviously larger than the sample using APTES (8.4 ± 1.3 nm). The reaction with and without APTES revealed a large difference (Video S1). Because of unrestricted increasing of particle size, the APTES-free sample rapidly turned to yellow after mixing two precursors, and finally the sample subsided [43]. Meanwhile, the sample with APTES remained unchanged and no aggregation was observed, suggesting that the addition of APTES effectively prevents the size increasing of CsPbBr3 nanocrystals. FTIR spectrum was used to demonstrate the capping of APTES, as shown in Figure 4c, in which it exists a N-H vibration absorption peak at 3448 cm −1 , originating from the -NH2 group of APTES. The peak at 948 cm −1 was ascribed to the vibration absorption of the -Si-OH group of hydrolyzed APTES. The peak at 1125 cm −1 originated from the vibration absorption of the -Si-O-Sigroup, owing to the hydrolysis condensation between the -Si-OH groups or the -Si-OH and -SiOC2H5 groups [37]. Therefore, the mechanism of CsPbBr3 nanocrystals with APTES can be summarized as follows: the polar -NH2 group of APTES closely connects to CsPbBr3 nanocrystals and three ethoxyl siloxane groups hydrolyze to form -Si-OH, following a dehydration process between the -SiOH groups or -SiOH and -SiOC2H5, resulting in the formation of an Si-O-Si  The CsPbBr3-IBOMA nanocrystals sol exhibited excellent stability. After being placed in air without any protection for 2 weeks, the particle size remained nearly unchanged (Figure 4a), which was attributed to the capping of hydrolyzed APTES ligand [32]. APTES hydrolyzes on the surface of nanocrystals, forming a network structure to prevent the desorption of ligands and to subsequently prohibit size increases, which enhanced the stability of nanocrystals [37,42]. To further prove the effect of APTES, CsPbBr3-IBOMA nanocrystals without APTES were synthesized, and their morphology and size were determined by TEM (Figure 4b). The size (45.7 ± 7.7 nm) was obviously larger than the sample using APTES (8.4 ± 1.3 nm). The reaction with and without APTES revealed a large difference (Video S1). Because of unrestricted increasing of particle size, the APTES-free sample rapidly turned to yellow after mixing two precursors, and finally the sample subsided [43]. Meanwhile, the sample with APTES remained unchanged and no aggregation was observed, suggesting that the addition of APTES effectively prevents the size increasing of CsPbBr3 nanocrystals. FTIR spectrum was used to demonstrate the capping of APTES, as shown in Figure 4c, in which it exists a N-H vibration absorption peak at 3448 cm −1 , originating from the -NH2 group of APTES. The peak at 948 cm −1 was ascribed to the vibration absorption of the -Si-OH group of hydrolyzed APTES. The peak at 1125 cm −1 originated from the vibration absorption of the -Si-O-Sigroup, owing to the hydrolysis condensation between the -Si-OH groups or the -Si-OH and -SiOC2H5 groups [37]. Therefore, the mechanism of CsPbBr3 nanocrystals with APTES can be summarized as follows: the polar -NH2 group of APTES closely connects to CsPbBr3 nanocrystals and three ethoxyl siloxane groups hydrolyze to form -Si-OH, following a dehydration process between the -SiOH groups or -SiOH and -SiOC2H5, resulting in the formation of an Si-O-Si The CsPbBr 3 -IBOMA nanocrystals sol exhibited excellent stability. After being placed in air without any protection for 2 weeks, the particle size remained nearly unchanged (Figure 4a), which was attributed to the capping of hydrolyzed APTES ligand [32]. APTES hydrolyzes on the surface of nanocrystals, forming a network structure to prevent the desorption of ligands and to subsequently prohibit size increases, which enhanced the stability of nanocrystals [37,42]. To further prove the effect of APTES, CsPbBr 3 -IBOMA nanocrystals without APTES were synthesized, and their morphology and size were determined by TEM (Figure 4b). The size (45.7 ± 7.7 nm) was obviously larger than the sample using APTES (8.4 ± 1.3 nm). The reaction with and without APTES revealed a large difference (Video S1). Because of unrestricted increasing of particle size, the APTES-free sample rapidly turned to yellow after mixing two precursors, and finally the sample subsided [43]. Meanwhile, the sample with APTES remained unchanged and no aggregation was observed, suggesting that the addition of APTES effectively prevents the size increasing of CsPbBr 3 nanocrystals. FTIR spectrum was used to demonstrate the capping of APTES, as shown in Figure 4c, in which it exists a N-H vibration absorption peak at 3448 cm −1 , originating from the -NH 2 group of APTES. The peak at 948 cm −1 was ascribed to the vibration absorption of the -Si-OH group of hydrolyzed APTES. The peak at 1125 cm −1 originated from the vibration absorption of the -Si-O-Si-group, owing to the hydrolysis condensation between the -Si-OH groups or the -Si-OH and -SiOC 2 H 5 groups [37]. Therefore, the mechanism of CsPbBr 3 nanocrystals with APTES can be summarized as follows: the polar -NH 2 group of APTES closely connects to CsPbBr 3 nanocrystals and three ethoxyl siloxane groups hydrolyze to form -Si-OH, following a dehydration process between the -SiOH groups or -SiOH and -SiOC 2 H 5 , resulting in the formation of an Si-O-Si cross-linking network structure (Figure 4d). In the whole process, a small amount of water in air and solvent advanced the hydrolysis reaction slowly [44]. cross-linking network structure (Figure 4d). In the whole process, a small amount of water in air and solvent advanced the hydrolysis reaction slowly [44]. The present strategy can be expanded to synthesize blue and red emission CsPbX3 by adding HCl or ZnI2 to adjust the ratio of halide ions. Details of the process have been described in the Experimental section. The XRD patterns of the CsPbBr3 nanocrystals and anion-exchanged samples showed the retention of cubic perovskite structure, and the shift of the XRD reflections was observed due to the difference of anion radius ( Figure S3 in Supplementary Materials). Their PL spectra were adjustable from 430 to 620 nm ( Figure S4 in Supplementary Materials) with narrow FWHM (10−40 nm), indicating this is a universal strategy.
It is well known that the high sensitivity to the oxygen, moisture, light and temperature etc. leads to the poor stability of perovskites nanocrystals [13,45,46]. In order to enhance the stability for further application, in previous work, CsPbBr3 nanocrystals were often combined with polymer by complicated processing [12]. Herein, by a facile UV curing process, CsPbBr3-IBOMA sol, can be cured directly into various shapes, because of the polymerization of the IBOMA monomer. The morphology of the CsPbBr3 nanocrystals block sample was studied by TEM. As shown in Figure 5a, the CsPbBr3 nanocrystals were uniformly distributed in the polymer matrix without phase separation or aggregation as expected, owing to the in-situ method. The particle size (11.6 ± 2.2 nm) showed a slight increase compared with the sample before curing, because of the UV irradiation [47]. Furthermore, the emission spectrum was also unchanged, and only the emission intensity slightly reduced (Figure 5b), suggesting that the polymerization process of IBOMA does not affect the performance of nanocrystals. The present strategy can be expanded to synthesize blue and red emission CsPbX 3 by adding HCl or ZnI 2 to adjust the ratio of halide ions. Details of the process have been described in the Experimental section. The XRD patterns of the CsPbBr 3 nanocrystals and anion-exchanged samples showed the retention of cubic perovskite structure, and the shift of the XRD reflections was observed due to the difference of anion radius ( Figure S3 in Supplementary Materials). Their PL spectra were adjustable from 430 to 620 nm ( Figure S4 in Supplementary Materials) with narrow FWHM (10−40 nm), indicating this is a universal strategy.
It is well known that the high sensitivity to the oxygen, moisture, light and temperature etc. leads to the poor stability of perovskites nanocrystals [13,45,46]. In order to enhance the stability for further application, in previous work, CsPbBr 3 nanocrystals were often combined with polymer by complicated processing [12]. Herein, by a facile UV curing process, CsPbBr 3 -IBOMA sol, can be cured directly into various shapes, because of the polymerization of the IBOMA monomer. The morphology of the CsPbBr 3 nanocrystals block sample was studied by TEM. As shown in Figure 5a, the CsPbBr 3 nanocrystals were uniformly distributed in the polymer matrix without phase separation or aggregation as expected, owing to the in-situ method. The particle size (11.6 ± 2.2 nm) showed a slight increase compared with the sample before curing, because of the UV irradiation [47]. Furthermore, the emission spectrum was also unchanged, and only the emission intensity slightly reduced (Figure 5b), suggesting that the polymerization process of IBOMA does not affect the performance of nanocrystals. High stability is very crucial for the practical application of CsPbBr3 nanocrystals. Therefore, the light, air, moisture and thermal stability of as-synthesized nanocrystals were investigated. The PL spectra of the CsPbBr3-IBOMA block sample was measured at 5 h intervals under continuous irradiation with 395 nm UV LEDs (3W). As shown in Figure 6a, the shape and peak position of the High stability is very crucial for the practical application of CsPbBr 3 nanocrystals. Therefore, the light, air, moisture and thermal stability of as-synthesized nanocrystals were investigated. The PL spectra of the CsPbBr 3 -IBOMA block sample was measured at 5 h intervals under continuous irradiation with 395 nm UV LEDs (3W). As shown in Figure 6a, the shape and peak position of the emission spectra did not change significantly with the increase of irradiation time, indicating that the particle size of CsPbBr 3 nanocrystals was not affected by UV irradiation. However, the luminous intensity of the sample showed a decreasing trend (Figure 6b). It increased in the initial 10 h, and then went through slight decreasing. Finally, it decreased only by 9.7%, to 0.903 after 70 h. The results exhibited the excellent light stability of CsPbBr 3 nanocrystals, due to the encapsulation of IBOMA and the coating of APTES.

Conclusions
In summary, we have introduced a controllable one step in-situ preparation method at room temperature in acrylic monomer. The cubic phase CsPbBr3 nanocrystals with outstanding luminous performance have been successfully synthesized. The photoluminescence quantum yield is up to 87.5%. Isobornyl methacrylate as solvent can be cured directly by ultraviolet light to form transparent solid materials, such as block and film. In this way, not only can a large amount of organic waste liquid be avoided, but also the acrylic polymer encapsulates the nanocrystals and limits further growth. The CsPbBr3-IBOMA nanocrystals block showed great stability under the conditions of air, continuous irradiation and water. In addition, the ligands APTES adsorbing on the surface of CsPbBr3 nanocrystals effectively controlled its particle size at 8.4 ± 1.3 nm, which ensured its excellent luminous performance.
Author Contributions: L.Z. prepared the samples and wrote this manuscript. J.C., C.L. and L.C. processed and To explore the stability in air, the CsPbBr 3 -IBOMA nanocrystals block sample was placed in the air without any protection and the emission spectrum was measured every 4 days under laboratory conditions (K = 298 K, 57% RH). The emission peak of the CsPbBr 3 sample kept at about 510 nm steadily without any shift (Figure 6c), indicating unchanged particle size. In addition, the fluorescence intensity maintained a high level of 89.5% at 30 days (Figure 6d), compared with the previous work [48].
To evaluate the water and thermal stability of CsPbBr 3 nanocrystals, we kept the CsPbBr 3 -IBOMA nanocrystals block sample immersed in water and heated to 348.15 K. The fluorescence intensity of the emission spectrum was tested at 15 h intervals. As shown in Figure 6e, the emission spectra shape and peak position of the sample nearly did not change, while the luminous intensity decreased gradually (Figure 6f). Within the initial 15 h, it went through a slight rise and then decreased slowly. Finally, after 135 h, the luminous intensity reached 75.6%. The result is comparable with previous reported CsPbBr 3 nanocrystals [29].

Conclusions
In summary, we have introduced a controllable one step in-situ preparation method at room temperature in acrylic monomer. The cubic phase CsPbBr 3 nanocrystals with outstanding luminous performance have been successfully synthesized. The photoluminescence quantum yield is up to 87.5%. Isobornyl methacrylate as solvent can be cured directly by ultraviolet light to form transparent solid materials, such as block and film. In this way, not only can a large amount of organic waste liquid be avoided, but also the acrylic polymer encapsulates the nanocrystals and limits further growth. The CsPbBr 3 -IBOMA nanocrystals block showed great stability under the conditions of air, continuous irradiation and water. In addition, the ligands APTES adsorbing on the surface of CsPbBr 3 nanocrystals effectively controlled its particle size at 8.4 ± 1.3 nm, which ensured its excellent luminous performance.
Author Contributions: L.Z. prepared the samples and wrote this manuscript. J.C., C.L. and L.C. processed and analyzed the signal data. S.Y., H.T., H.Z., and Q.C. conceived the research and provided many suggestions for the research method and revised the article completely. All authors have read and agreed to the published version of the manuscript.