Improving Stability and Colloidal Dispersity of CsPbBr3@SiO2 Nanoparticles Based on In-Situ Synthesis in Entropy Ligands Functionalized SiO2 Nanoreactor

Perovskite nanocrystals (PNCs) have witnessed unprecedented development in optoelectronic fields over the past few years. However, their intrinsic ionic structural instability still dramatically hinders their practical applications. Reliably improving the stability of PNCs while retaining their colloidal dispersity remains a grand challenge. Herein, we report a new strategy whereby CsPbBr3 nanoparticles are grown in situ in an entropy ligand-functionalized SiO2 nanoreactor. Consequently, the as-obtained CsPbBr3@SiO2 NPs show outstanding stability and colloidal dispersity in various non-polar solvents and have good solution processability, which are unattainable by conventional template-assisted methods.

Typically, encapsulation treatment has been proved to be an effective strategy to enhance the stability of PNCs against environmental interference, and simultaneously preserve bright emission, such as embedding PNCs into polymers, including block copolymer micelles [19,20], poly(lauryl methacrylate) [21,22], polystyrene [23][24][25], poly(vinylidene fluoride) [26,27], or inorganic matrices (MOFs [28,29], TiO 2 [30], Al 2 O 3 [31,32], Zeolite-Y [33,34], or SiO 2 [35,36]). Among the various methods, the encapsulation of the SiO 2 matrix is considered to be one of the most common and efficient approaches due to the excellent chemical stability and mechanical properties, as well as the low price and environmentally friendliness. However, the postcoating of SiO 2 easily causes multiple PNCs to grow randomly on the shells and generate uneven or large aggregates on account of the silane hydrolysis that is difficult to control. Then, to make matters worse, the water and alcohols derived from the by-product of silane hydrolysis can decompose the PNCs. Conversely, in-situ crystallized PNCs under the nanoconfinement effect of porous templates can limit crystal growth to a specific size range and prevents secondary decomposition. However, it is worth noting that nanotemplates often tend towards aggregation in nonpolar solvents owing to their physical adsorption, which actually works against their solution processability [37]. The restriction incurs difficulties in coupling them with conventional optoelectronics platforms. Overall, it remains highly desirable for the PNCs to overcome the stability issues while maintaining excellent solution processability at the nanoscale range.
Recently, entropic ligands control has been proved to be an effective strategy for improving the monodispersity of colloidal nanocrystals [38,39]. For instance, by combining two distinguishable chain-length n-alkanoate, the solubility of the resulting colloidal nanocrystals can increase up to about six orders of magnitude, because the C-C σ-bond intramolecular entropy in the solution substantially reduces the dissolution enthalpy and facilitates its dispersion. Inspired by such entropy-enthalpy competition, herein we introduced a modified template-assisted strategy with entropy ligands for the synthesis of CsPbBr 3 nanocomposites, in which hollow siliceous nanospheres (HSNSs) templates with a high entropy surface are constructed through grafting mixed ligands (ML) with distinguishable long-chain ligands (oleoyl chloride, OCl) and short-chain bendritic ligands (trimethylbromosilane, TMBS). HSNSs-ML would serve as a nanoreactor for in situ growing of CsPbBr 3 PNCs to fabricate CsPbBr 3 @HSNSs-ML. The resulting CsPbBr 3 @HSNSs-ML nanocomposites have better stability and colloidal dispersity in comparison with either of the pure-ligand counterparts, which is favorable for solution processability. Furthermore, the other PNCs, including organic-inorganic PNCs and doped PNCs, could also be fabricated based on this method. The generalization of this strategy means it has good prospects in the field of PNC synthesis.

Experimental Section
Synthesis of the Hollow Siliceous Nanospheres (HSNSs). The HSNSs were prepared according to the previously reported method [40]. In typical synthesis, 1.0 g Pluronic F108, 5.0 mL HCl, and 25 mL H 2 O were mixed in a 50-mL glass vial and stirred vigorously to yield a homogeneous solution. Subsequently, 0.8 g TMB was introduced to the solution and was continuously stirred (1000 rpm) at 28 • C for 3 h. Then, 1.0 g TEOS was dropwise added to the system with an addition rate of 0.1 mL min −1 for 6 h, and 0.4 g DMDMS was dropwise added with a rate of 0.1 mL min −1 for 3 days. The resulting solvent was removed by rotary evaporation at 80 • C. The residual was refluxed twice in a 30 mL ethanol-HCl (29:1, v/v) solution for 12 h to remove the surfactant. The solid product was collected by centrifugation (10,000 rpm, 15 min) and repeatedly washed with ethanol to completely remove the Pluronic F108 and HCl. Finally, HSNSs were obtained by vacuum desiccation overnight at 60 • C.
Synthesis of the HSNSs-Grafted (HSNSs-OAe, HSNSs-TMe and HSNSs-ML). The HSNSs-Grafted were prepared via a nucleophilic substitution reaction at the silanol surface of the template. Before use, the HSNSs were dried overnight at 80 • C under vacuum. Then, 20.0 mg HSNSs, 3.0 mL DCM, and 90 µL TEA were mixed in a 10-mL glass vial with 60 µL of TMBS, or with a similar amount of OCl, and continuously stirred (1000 rpm) for 24 h. The solid product was repeatedly washed with ethanol by ultrasound and collected by Crystals 2021, 11, 1165 3 of 10 centrifugation (10,000 rpm, 15 min). Afterward, HSNSs-TMe or HSNSs-OCl were obtained by vacuum desiccation overnight at 60 • C. For the synthesis of HSNSs-ML, OCl was first reacted for 12 h, and then TMBS was added for another 12 h. Other procedures were performed in the same way as above.
Synthesis of CsPbBr 3 @HSNSs and CsPbBr 3 @HSNSs-Grafted NPs (including HSNSs-ML, HSNSs-Ocl and HSNSs-Tme). The CsPbBr 3 @HSNSs NPs were prepared following a template-assisted technique. Typically, the CsPbBr 3 precursor solution (0.1 M) was prepared by dissolving 0.1 mmol CsBr and 0.1 mmol PbBr 2 in 1.0 mL of DMSO. Subsequently, 50 µL of the precursor solution was thoroughly mixed with 20 mg of HSNSs and treated with sonication for 10 min. After the impregnation, the excess solution was removed by damping with filter paper. The as-obtained powder was sandwiched between two glass slides and heated up to 60 • C in a vacuum oven. Afterwards, the powder was allowed to cool to room temperature in the air. Finally, the product was collected and sealed in a centrifuge tube immediately. The synthesis of CsPbBr 3 @HSNSs-Grafted NPs was performed in an analogous way, except that the template was changed with a similar mass of HSNSs-Grafted.

Sample Characterization
Fluorescence (PL) emission spectra were collected by a Gary Eclipse (Agilent, Santa Clara, CA, USA) spectrophotometer. The PL lifetime and absolute PL quantum yields were measured using an FLS 980 (Edinburgh, Livingston, UK) spectrometer with a nanosecond flash lamp. The Specord 200 plus (Analytik Jena, Jena, Germany) was employed to obtain the UV−Vis absorption spectra in transmission mode. The morphologies were investigated by Tecnai-G2-F30 (FEI, Hillsboro, OR, USA) transmission electron microscope instruments at 200 kV. Fourier-transform infrared spectroscopy (FTIR) spectra of the compounds are recorded on Nicolet iS 10 (Thermo Fisher, Waltham, MA, USA) in the range of 500 to 4000 cm −1 . X-ray diffraction (XRD) was obtained with an Ultima IV diffractometer (Rigaku, Tokyo, Japan) with Cu Kα (1.79 Å) over the 2θ range of 10-60 • .

Results and Discussion
The synthesis of CsPbBr 3 @HSNSs-ML with a high entropy surface is illustrated in Figure 1. First, the HSNSs are synthesized via the acid catalytic hydrolysis of tetraethyl orthosilicate (TEOS) and Dimethoxydimethylsilane (DMDMS) with the aid of a surfactant. Then, long-chain ligands OCl and short-chain ligands TMBS were grafted on the silanol surface of HSNSs by stepwise reaction to form the HSNSs-ML nanoreactor. Subsequently, the CsPbBr 3 precursor solution was ultrasonically immersed into the cavity of the HSNSs-ML nanoreactor by capillary forces, followed by vacuum drying, leading to in situ crystallization of CsPbBr 3 @HSNSs-ML nanocomposites. Consequently, as a benefit of the high entropy surface of HSNSs-ML, the CsPbBr 3 @HSNSs-ML shows good colloidal dispersity and solution processability, which could be easily applied to fabricate a CsPbBr 3 @HSNSs-ML polystyrene film without any aggregation; in contrast, the CsPbBr 3 @HSNSs NPs grown within unmodified HSNSs show obvious aggregation as illustrated in Figure 1.
TEM images confirm that the as-synthesized HSNSs-ML is an exclusively hollow structure with uniform particle size distribution. An average diameter of 15.5 ± 1.1 nm is collected by directly measuring the diameters of 80 individual HSNSs-ML from the TEM images (Figure 2a,b). Besides, the pore size distributions of HSNSs and HSNSs-ML were analyed by the Barrett-Joyner-Halenda (BJH) method and results are shown in Figure 2c,d, whereby both of them demonstrated uniform pore size distributions centered at 3.3 nm and 8.6 nm, and 3.2 nm and 8.4 nm, corresponding to the surfactant-derived mesopores and center cavity, respectively. The reduced pore size distributions in HSNSs-ML are attributed to the steric hindrance of grafted alkane. Besides, HSNSs and HSNSs-ML displayed typical type-IV N 2 adsorption isotherms with obvious hysteresis and capillary condensation between the relative pressure of 0.4 and 1 (illustration in Figure 2c,d), indicating the presence of a regular pore structure. TEM images confirm that the as-synthesized HSNSs-ML is an exclusively hollow structure with uniform particle size distribution. An average diameter of 15.5 ± 1.1 nm is collected by directly measuring the diameters of 80 individual HSNSs-ML from the TEM images (Figure 2a,b). Besides, the pore size distributions of HSNSs and HSNSs-ML were analyed by the Barrett-Joyner-Halenda (BJH) method and results are shown in Figure  2c,d, whereby both of them demonstrated uniform pore size distributions centered at 3.3 nm and 8.6 nm, and 3.2 nm and 8.4 nm, corresponding to the surfactant-derived mesopores and center cavity, respectively. The reduced pore size distributions in HSNSs-ML are attributed to the steric hindrance of grafted alkane. Besides, HSNSs and HSNSs-ML displayed typical type-IV N2 adsorption isotherms with obvious hysteresis and capillary condensation between the relative pressure of 0.4 and 1 (illustration in Figure 2c,d), indicating the presence of a regular pore structure. Compared with HSNSs-ML, TEM images of CsPbBr3@HSNSs-ML NPs showed that the overall morphology and average size (15.8 ± 1.1 nm) are nearly unchanged except that the cavities disappeared (Figure 3a,b), indicating the CsPbBr3 PNCs are in situ growing in the pores. The as-synthesized CsPbBr3@HSNSs-ML emits blue-green photoluminescence (PL) under the excitation of 365 nm UV light, showing a sharp PL peak at 500 nm with a full width at half-maximum (FWHM) of 25 nm (Figure 3c). The CsPbBr3@HSNSs-ML NPs' platelet surface demonstrates high hydrophobicity with a water contact angle of 135°, which is attributed to the water-tolerance organic layer (inset in Figure 3c). In addition, as shown in Figure 3d, the X-ray diffraction (XRD) patterns of the CsP-bBr3@HSNSs-ML NPs display weak yet discernible diffraction peaks at 15  Compared with HSNSs-ML, TEM images of CsPbBr 3 @HSNSs-ML NPs showed that the overall morphology and average size (15.8 ± 1.1 nm) are nearly unchanged except that the cavities disappeared (Figure 3a which is attributed to the water-tolerance organic layer (inset in Figure 3c). In addition, as shown in Figure 3d, the X-ray diffraction (XRD) patterns of the CsPbBr 3 @HSNSs-ML NPs display weak yet discernible diffraction peaks at 15. Compared with HSNSs-ML, TEM images of CsPbBr3@HSNSs-ML NPs showed that the overall morphology and average size (15.8 ± 1.1 nm) are nearly unchanged except that the cavities disappeared (Figure 3a,b), indicating the CsPbBr3 PNCs are in situ growing in the pores. The as-synthesized CsPbBr3@HSNSs-ML emits blue-green photoluminescence (PL) under the excitation of 365 nm UV light, showing a sharp PL peak at 500 nm with a full width at half-maximum (FWHM) of 25 nm (Figure 3c). The CsPbBr3@HSNSs-ML NPs' platelet surface demonstrates high hydrophobicity with a water contact angle of 135°, which is attributed to the water-tolerance organic layer (inset in Figure 3c). In addition, as shown in Figure 3d, the X-ray diffraction (XRD) patterns of the CsPbBr3@HSNSs-ML NPs display weak yet discernible diffraction peaks at 15.1°, 21.3°, 30.4°, 34.1°, 37.5°, and 43.5°, which correspond to the (100), (110), (200), (210), (211), and (220) crystal planes of the cubic phase CsPbBr3 (standard card, ICSD#01-075-0412), respectively. The weak diffraction peaks can be ascribed to the relatively small amount of CsPbBr3 and the high content of amorphous SiO2 in the CsPbBr3@HSNSs-ML. Furthermore, the broad peak in the range between 15° and 30° can be attributed to the existence of amorphous SiO2. To verify whether the ligands are grafted on the HSNSs surface or not, Fourier-transform infrared spectroscopy (FTIR) spectra of HSNSs and HSNSs-Grafted (including HSNSs-ML, HSNSs-OCl, and HSNSs-TMe) are displayed in Figure 4a, in which we can observe that the asymmetric vibration and symmetric stretching vibration of Si-O-Si appear at about 1090 and 800 cm −1 , demonstrating the formation of cross-linked SiO2. Nonetheless, it is worth mentioning that all the FTIR results show a nearly identical signal, To verify whether the ligands are grafted on the HSNSs surface or not, Fouriertransform infrared spectroscopy (FTIR) spectra of HSNSs and HSNSs-Grafted (including HSNSs-ML, HSNSs-OCl, and HSNSs-TMe) are displayed in Figure 4a, in which we can observe that the asymmetric vibration and symmetric stretching vibration of Si-O-Si appear at about 1090 and 800 cm −1 , demonstrating the formation of cross-linked SiO 2 . Nonetheless, it is worth mentioning that all the FTIR results show a nearly identical signal, which may be ascribed to the organic chain signal overlayed by the strong silica signal. Therefore, we further characterized the 1H NMR spectrum to determine the surface condition of HSNSs-grafted. As shown in Figure 4b, the chemical shift of -CH=CH-at about 5.57 ppm in HSNSs-OCl and HSNSs-ML indicates the existence of OCl. In addition, the signal of the chemical shift of -Si-CH 3 at about −0.04 ppm in HSNSs-TMe and HSNSs-ML shows significant enhancement, indicating that TMBS was successfully grafted on the silanol interface. Moreover, the same chemical shifts of -Si-CH 3 appearing in all the templates stem from the capped silica donated by DMDMs.
The thermodynamic model states that the stability of a colloidal solution is due to the C−C σ-bond rotation/bending vibration entropy, rather than the stereoscopic barrier of the alkane chain, as traditionally believed [39]. In order to verify the solution properties regulated by entropy coefficients, control experiments were performed. First, the dispersity of the obtained CsPbBr 3 PNCs nanocrystals based on in-situ synthesis in an entropy ligandfunctionalized SiO 2 nanoreactor was fully investigated, which was evaluated via observing their sedimentation velocity for different durations under an ultraviolet lamp. First, the CsPbBr 3 @HSNSs, CsPbBr 3 @HSNSs-pure, and CsPbBr 3 @HSNSs-ML were individually mixed within cyclohexane with the same amount and sonicated for 15 min to yield a homogeneous solution. As illustrated in Figure 5a, the results showed that the sedimentation velocity is in the order of CsPbBr 3 @HSNSs > CsPbBr 3 @HSNSs-OCl > CsPbBr 3 @HSNSs-TMe > CsPbBr 3 @HSNSs-ML, demonstrating the dispersity of CsPbBr 3 @HSNSs could be feasibly improved via mix ligands grafting. The dispersity of CsPbBr 3 @HSNSs-ML reached the maximum when 40 uL OCl and TMe were used. Moreover, as shown in Figure 5c, the obtained CsPbBr 3 @HSNSs-ML could be well dispersed in a variety of non-polar solvents, including N-hexane, cyclohexane, ethyl acetate, and toluene, which is beneficial to solution processability. This result indicates the excellent properties of the entropic template and facilitates their further development in the relevant fields.  The thermodynamic model states that the stability of a colloidal solution is due to the C−C σ-bond rotation/bending vibration entropy, rather than the stereoscopic barrier of the alkane chain, as traditionally believed [39]. In order to verify the solution properties regulated by entropy coefficients, control experiments were performed. First, the dispersity of the obtained CsPbBr3 PNCs nanocrystals based on in-situ synthesis in an entropy ligand-functionalized SiO2 nanoreactor was fully investigated, which was evaluated via observing their sedimentation velocity for different durations under an ultraviolet lamp. First, the CsPbBr3@HSNSs, CsPbBr3@HSNSs-pure, and CsPbBr3@HSNSs-ML were individually mixed within cyclohexane with the same amount and sonicated for 15 min to yield a homogeneous solution. As illustrated in Figure 5a, the results showed that the sedimentation velocity is in the order of CsPbBr3@HSNSs > CsPbBr3@HSNSs-OCl > CsPbBr3@HSNSs-TMe > CsPbBr3@HSNSs-ML, demonstrating the dispersity of CsP-bBr3@HSNSs could be feasibly improved via mix ligands grafting. The dispersity of CsPbBr3@HSNSs-ML reached the maximum when 40 uL OCl and TMe were used. Moreover, as shown in Figure 5c, the obtained CsPbBr3@HSNSs-ML could be well dispersed in a variety of non-polar solvents, including N-hexane, cyclohexane, ethyl acetate, and toluene, which is beneficial to solution processability. This result indicates the excellent properties of the entropic template and facilitates their further development in the relevant fields. Furthermore, the optical properties of CsPbBr3@HSNSs-ML were also investigated. Due to the quantum confinement, the optical properties of the grown CsPbBr3 PNCs were dependent on the cavity of HSNSs. Compared with the CsPbBr3@HSNSs, we found CsPbBr3@HSNSs-ML blue shifted as illustrated in Figure 6a, though the cavity diameter Furthermore, the optical properties of CsPbBr 3 @HSNSs-ML were also investigated. Due to the quantum confinement, the optical properties of the grown CsPbBr 3 PNCs were dependent on the cavity of HSNSs. Compared with the CsPbBr 3 @HSNSs, we found CsPbBr 3 @HSNSs-ML blue shifted as illustrated in Figure 6a, though the cavity diameter of the HSNSs-ML remained almost the same after ligand grafting (about 8.4 nm). The reason might be that the TMBS penetrated into the mesopores, reacted with the silanol interface of HSNSs, and compressed the growth and crystallinity of the PNC core. As verification, the emission spectrum of CsPbBr 3 @HSNSs-TMe shows a similar blue-shift trend, while that of CsPbBr 3 @HSNSs-OCl has little change (Figure 6b,c). This is also illustrated by the weakened diffraction peak signals on the corresponding XRD patterns after ligand TMBS grafting (Figure 6d,e). However, the mechanism behind these still needs to be further explored. Furthermore, the optical properties of CsPbBr3@HSNSs-ML were also investigated. Due to the quantum confinement, the optical properties of the grown CsPbBr3 PNCs were dependent on the cavity of HSNSs. Compared with the CsPbBr3@HSNSs, we found CsPbBr3@HSNSs-ML blue shifted as illustrated in Figure 6a, though the cavity diameter of the HSNSs-ML remained almost the same after ligand grafting (about 8.4 nm). The reason might be that the TMBS penetrated into the mesopores, reacted with the silanol interface of HSNSs, and compressed the growth and crystallinity of the PNC core. As verification, the emission spectrum of CsPbBr3@HSNSs-TMe shows a similar blue-shift trend, while that of CsPbBr3@HSNSs-OCl has little change (Figure 6b,c). This is also illustrated by the weakened diffraction peak signals on the corresponding XRD patterns after ligand TMBS grafting (Figure 6d,e). However, the mechanism behind these still needs to be further explored.  The stability of the CsPbBr 3 @HSNSs and CsPbBr 3 @HSNSs-Grafted against the detrimental effects of heat and light irradiation was tested and compared with conventional hot-injected CsPbBr 3 PNCs. We fixed the concentrations of all the CsPbBr 3 PNCs by controlling the absorption intensity at the same value (at about 0.1) in their intrinsic absorption wavelength (335 nm). As a result, thermal testing shows that the remnant PL intensities of CsPbBr 3 @HSNSs NPs and CsPbBr 3 @HSNSs-ML remain over 70% after repeated heating cycling at 100 • C (Figure 7a). For comparison, the CsPbBr 3 PNCs solution was quenched rapidly within 5 cycles. Furthermore, the photostability test results show that CsPbBr 3 @HSNSs and CsPbBr 3 @HSNSs-ML still remain over 70% relative PL after continuous irradiation for 14 h, while the CsPbBr 3 PNCs had almost no performance (Figure 7b). The result suggests that the decomposition is greatly suppressed by SiO 2 shells.
Finally, to further investigate the generality of this strategy, we use HSNSs-ML for the synthesis of the other PNCs systems, including Mn-doped systems and organic-inorganic hybridized systems. (Figure 8). The experiments show that the majority of PNCs, especially organic-inorganic hybrid PNCs, have good dispersity and optical properties. tion wavelength (335 nm). As a result, thermal testing shows that the remnant PL intensities of CsPbBr3@HSNSs NPs and CsPbBr3@HSNSs-ML remain over 70% after repeated heating cycling at 100 °C (Figure 7a). For comparison, the CsPbBr3 PNCs solution was quenched rapidly within 5 cycles. Furthermore, the photostability test results show that CsPbBr3@HSNSs and CsPbBr3@HSNSs-ML still remain over 70% relative PL after continuous irradiation for 14 h, while the CsPbBr3 PNCs had almost no performance (Figure 7b). The result suggests that the decomposition is greatly suppressed by SiO2 shells. Finally, to further investigate the generality of this strategy, we use HSNSs-ML for the synthesis of the other PNCs systems, including Mn-doped systems and organic-inorganic hybridized systems. (Figure 8). The experiments show that the majority of PNCs, especially organic-inorganic hybrid PNCs, have good dispersity and optical properties.  CsPbBr3@HSNSs and CsPbBr3@HSNSs-ML still remain over 70% relative PL after continuous irradiation for 14 h, while the CsPbBr3 PNCs had almost no performance (Figure 7b). The result suggests that the decomposition is greatly suppressed by SiO2 shells. Finally, to further investigate the generality of this strategy, we use HSNSs-ML for the synthesis of the other PNCs systems, including Mn-doped systems and organic-inorganic hybridized systems. (Figure 8). The experiments show that the majority of PNCs, especially organic-inorganic hybrid PNCs, have good dispersity and optical properties.

Conclusions
In conclusion, the CsPbBr 3 @HSNSs-ML NPs have been successfully prepared by a ligand-grafted template-assisted strategy, where the templates (namely HSNSs) are functionalized by entropy ligands. As a result, the products show outstanding dispersion in various non-polar solvents. The mixed ligands with distinguishable hydrocarbon chain lengths are selected to replace the entropy ligand for verifying solution properties regulated by entropy coefficients. In addition, with the double protection of SiO 2 and the moisturetolerant organic layer, the NPs show better water resistance and higher photostability and thermal stability. Meanwhile, this work overcomes the inherent limitation of templateassisted methods in solution processability and provides a new model for monodisperse PNCs with a core/shell structure.