Continuous In-Line Synthesis and Surface Passivation of Perovskite Nanocrystals via a Hybrid Multistep Flow Reactor

Abstract

Perovskite nanocrystals (PeNCs) are promising photoactive materials for next-generation display applications owing to their strong absorption, narrow emission bandwidth, and tunable band gap. However, batch-based hot-injection synthesis is limited by discontinuous operation and difficulties in scale-up, while post-synthetic ligand exchange adds extra processing steps that can destabilize the labile PeNC surface. In this study, we developed a two-zone hybrid continuous-flow reactor (h-CFR) that integrates CsPbBr₃ PeNC synthesis with in-line aromatic ligand exchange. A room-temperature liquid-phase Cs precursor was formulated to enable continuous feeding through stainless steel tubing, and three static mixer geometries were compared to evaluate the effect of precursor mixing on emission bandwidth. The vortex mixer produced CsPbBr₃ PeNCs with a photoluminescence (PL) peak at 519 nm and a full width at half-maximum (FWHM) of 18.4 nm. Subsequent in-line exchange using benzoic acid and 4-phenylbutylamine increased the photoluminescence quantum yield (PLQY) from 69 to 95% while maintaining a narrow FWHM of 18.6 nm. The optimized process was further operated continuously for 3 h, yielding 1.8 L of a PeNC dispersion and 6.12 g of purified powder. These results demonstrate a practical continuous strategy for integrating PeNC formation and surface passivation in a single-flow process.

Introduction

Perovskite nanocrystals (PeNCs) possess excellent optoelectronic properties, with their superior light absorption and emission characteristics garnering significant attention. , Compared to conventional quantum dot emitters, they exhibit a 10-fold higher blue light absorption and a narrow full width at half-maximum (FWHM) of approximately 20 nm for emission, which is 50% narrower than that of InP quantum dots, enabling excellent color purity. , Furthermore, band gap control can be achieved through both halide-content control and particle size control, securing a tunable band gap in the 410–700 nm range. , These properties have attracted considerable interest from both academic and industrial researchers, leading to ongoing studies for their application as next-generation photoactive materials in solar cells, X-ray sensors, and displays. −

Typically, ligand-assisted reprecipitation and hot-injection methods are used to synthesize PeNCs. While ligand-assisted reprecipitation is advantageous for rapid screening via small-scale synthesis, it has limitations in scaling up to an industrial scale. , The hot-injection method is relatively favorable for scale-up; however, it requires a long time from batch reactor preparation to synthesis completion. This limits rapid screening and the ability to secure diverse material candidates. ,

The surface of synthesized pristine PeNCs is capped with long-chain aliphatic ligands such as oleic acid and oleylamine. Although these pristine ligands provide dispersibility in hydrophobic solvents, their excessively long carbon chains degrade electrical properties, acting as a limitation in improving the device characteristics of PeQLEDs. , Moreover, unlike conventional quantum dots, PeNCs have a low binding energy between the surface and ligands, causing ligands to dissociate easily. This increases the surface defect ratio, leading to a decrease in photoluminescence quantum yield (PLQY). To improve such opto-electrical degradation, surface modification of PeNCs is required as a post-treatment; however, this adds process steps during PeNC material development, resulting in relatively long development times. , Recent reviews and surface-chemistry studies further emphasize that ligand binding, surface termination, and ligand-solvent interactions are central factors governing the colloidal stability and optical properties of lead-halide PeNCs. −

Recent microfluidic and continuous-flow studies have demonstrated the value of rapid mixing and controlled residence time for PeNC synthesis, but the practical integration of high-temperature synthesis, clogging-mitigated precursor delivery, and immediate in-line ligand exchange remains less explored. − To address this gap, a continuous fluidic reactor (CFR) was applied to simultaneously perform the synthesis and surface modification of PeNCs. The CFR enables the rapid development and effective scale-up of PeNCs, QDs, and various nanomaterials, making it an area of active research as a next-generation reactor. −

There are various types of CFRs, including patterned plate types, line-types, and stirred tank reactors. The patterned plate type offers excellent mixing efficiency and enables reactions at high temperatures over 300 °C, but if precursor precipitation or solidification occurs, reusing the reactor is difficult. , Line-type reactors handle clogging issues easily but require bent lines to increase mixing efficiency. Since this necessitates Teflon-based tubing, its application at temperatures above 250 °C is limited. , Stirred tank reactors exhibit excellent mixing efficiency and can proceed with high-temperature reactions but require a large volume of solution per experiment, meaning a significant amount of chemicals is needed to test various synthesis conditions. −

This study employed a hybrid-CFR (h-CFR) utilizing a static mixer and SUS tube to maximize mixing efficiency while enabling high-temperature reactions. To improve the synthesis efficiency of PeNCs using the h-CFR, synthesis and surface modification studies were conducted applying three types of static mixers: a Y-junction type (without an internal mixer structure), a spiral mixer, and a vortex mixer. For the Y-junction type, after precursor injection, a laminar flow is initially formed by the injected solution flow without an internal mixer, but Dean flow generated as it flows through the coil-shaped tubing was utilized. , For the spiral mixer, a helical element was inserted inside the Y-junction tubing to repeatedly divide and rotate the injected solution, accelerating mixing. , In the case of the vortex mixer, injected solutions formed a strong swirling flow within the chamber due to unaligned inlets and an internal round structure, aiming to accelerate mixing. − The detailed geometries of the Y-junction, spiral, and vortex static mixers are shown in Figure a.

3.

(a) Schematic illustrations of the Y-junction (without internal mixer), spiral, and vortex mixers. (b) UV–Vis absorption and (c) PL spectra of CsPbBr₃ PeNCs synthesized using three types of static mixers.

To simultaneously perform the synthesis and surface modification of PeNCs, the h-CFR was divided into a 2-zone system. The reactor was designed so that PeNC synthesis is performed in the first zone and ligand exchange in the second zone, ensuring the final product comprises PeNCs with modified surface ligands. Changes in emission characteristics depending on the presence or absence of surface modification were compared and analyzed. This configuration was used to clarify the distinct roles of mixer-assisted nanocrystal formation and ligand exchange-assisted surface passivation without introducing a separate batch post-treatment step.

Results and Discussion

Figure presents a detailed schematic illustration of the two-zone h-CFR. The term hybrid-CFR is used here because the reactor integrates multiple functional reactor elements into a single continuous platform, including static mixers, heated stainless steel (SUS) tube reactor modules, an in-line ligand injection section, a real-time optical analysis module, and a collector. The precursor and ligand solutions were injected by using high-pressure precision syringe pumps. In the first zone, the Cs and Pb precursor streams are first mixed using the first static mixer and then introduced into the first heated SUS tube reactor module, where CsPbBr₃ PeNCs formation occurs. After PeNC formation, the aromatic ligand solution is introduced downstream and immediately passes through the second static mixer before entering the second SUS tube reactor module. The second static mixer promotes rapid mixing between the freshly formed PeNC dispersion and the aromatic ligand solution, while the second SUS tube reactor module provides an additional residence time for in-line ligand exchange. Unlike a conventional single-zone CFR or simple tubing-based flow reactor, this configuration separates PeNC formation and surface ligand exchange into two connected reaction zones, allowing nanocrystal formation, surface passivation, real-time optical monitoring, and product collection to be connected sequentially within one continuous process. The SUS tube was selected because it provides thermal and mechanical stability under high-temperature flow conditions and enables stable residence time control in the heated reactor modules. This configuration was designed to reduce local ligand concentration gradients and to promote more homogeneous surface modification during continuous operation.

1.

Detailed schematic illustration of the hybrid continuous fluidic reactor (h-CFR) for the in-line synthesis and surface ligand exchange of CsPbBr₃ PeNCs. The h-CFR consists of precursor solution inlets, a first static mixer for precursor mixing, a first heated SUS tube reactor module for PeNC formation, a downstream ligand solution inlet, a second static mixer for ligand mixing, a second SUS tube reactor module for in-line ligand exchange, a real-time optical analysis module, and a collector.

Figure a shows the digital photographs of cesium (Cs) precursors with various ligand combinations, captured 8 h after synthesis to evaluate their phase stability at room temperature (RT). For a fixed condition of 0.25 mmol of cesium carbonate, three different ligand compositions were introduced: 1.25 mmol of OA, a mixture of 2.5 mmol of OA and 0.4 mmol of stearic acid (SA), and 2.5 mmol of OA. It was confirmed that only the 2.5 mmol of OA condition maintained a stable liquid phase at RT, whereas the other conditions transitioned into a waxy solid phase. This liquid-phase stability is crucial for ensuring a continuous supply of precursors without clogging the h-CFR system. For a more detailed comparative analysis of the Cs-precursor characteristics according to the ligand composition and ratio, additional experiments were conducted and are provided in the Supporting Information.

2.

(a) Digital photographs of Cs precursors with various ligand combinations at room temperature. The color-shaded regions are used as visual annotations to indicate the physical state of the Cs precursor and do not represent intrinsic color differences of the samples. (b) UV–Vis absorption and (c) photoluminescence (PL) spectra of CsPbBr₃ PeNCs synthesized using ligand-controlled Cs precursors.

Figures S1 and S2 show the preparation process of the liquid-phase precursor and the analysis of the optical properties of PeNCs synthesized using the liquid-phase precursor. While the PbBr₂–OA/OAm precursor successfully maintained a liquid phase at room temperature, allowing direct application to the h-CFR, the Cs precursor exhibited a phase transition from liquid to a waxy solid phase due to a temperature decrease. To resolve this, control over the ligand composition and ratio was performed in Figures S1 and S2. To analyze differences in the properties of PeNCs synthesized according to the Cs precursor ligand composition, pre-synthesis tests were conducted using the hot-injection method before applying the precursors with oleic acid (OA) only and a mixture of OA and stearic acid (SA) to the h-CFR. As seen in Figure S1, the precursor exhibiting an all-liquid phase represents a condition where OA was added in a 10-fold excess relative to Cs. Conditions with less OA or with added SA, as shown in the Figure S2 results, possessed a waxy solid state phase at room temperature (25 °C, RT), making them difficult to apply to the h-CFR.

To compare the optical properties for each composition, UV–Vis absorbance and photoluminescence (PL) measurements were conducted as shown in Figure to compare the band gap and optical properties of PeNCs synthesized using different precursor compositions. In the UV–Vis absorbance spectra shown in Figure b, similar peak shapes were observed for all three conditions, and the 1st excitonic peak position was located at 483 nm. This confirmed that changes in the absorption characteristics due to ligand composition and ratio control were insignificant.

The shape of the PL spectrum in Figure c varied depending on the ligand composition and ratio. The 1.25 mmol OA condition exhibited a single peak (FWHM: 31.5 nm) with a peak position of 508 nm. However, when OA was doubled, the proportion of the 508 nm peak decreased, while the proportions of the 492 and 478 nm peaks significantly increased (FWHM: 37.2 nm). This is considered to be a result of a decrease in particle size and particle size uniformity. Nevertheless, when SA was mixed, peak multiplication did not occur despite OA being 2.5 mmol, and it was confirmed that the shoulder peak ratio was lower (FWHM: 29.6 nm) even compared to the OA 1.25 mmol condition. Looking at PLQY in Table , values of 62, 66, and 68% were obtained, confirming no significant difference based on ligand ratio and composition. This PL peak shape variation depending on the ligand conditions is attributed to the difference in the carbon chain shapes of OA and SA. When OA is added in excess, steric hindrance caused by its unique kinked chain structure induces non-uniform growth and secondary nucleation, leading to multipeak emission around 492 nm. , Conversely, it has been reported that when a small amount of SA, a straight-chain saturated fatty acid, is added, a high-density ligand shell is formed on the surface of the nanocrystals, suppressing surface defects and non-uniform growth. Because of these characteristics, a mixed ligand composition of OA and SA is generally used for synthesizing CsPbX₃-based nanocrystals. , However, the introduction of SA, a straight-chain saturated fatty acid, makes it difficult to maintain a stable liquid phase at room temperature, which can lead to catastrophic clogging within the CFR. Therefore, in this study, we utilized a precursor with an excess of OA (2.5 mmol) to ensure a completely liquid state at room temperature.

1. PL Spectra and PLQY Results of CsPbBr₃ PeNCs Synthesized Using Ligand-Controlled Cs Precursors.

sample (ligand, mmol)	peak (nm)	FWHM (nm)	PLQY (%)
OA:1.25	508	31.5	62
OA:2.5, SA:0.4	509	29.6	66
OA:2.5, SA:0	492	37.2	68

In Figure , PeNC synthesis was performed using the liquid-phase precursor and the h-CFR equipped with static mixers and SUS tubing. To compare the optical properties of PeNCs according to the type of static mixer, experiments were conducted by applying three types of static mixers as shown in Figure a. The reaction conditions applied a flow rate of 10 mL/min and a reaction temperature of 200 °C. The Y-junction type mixer, an empty tube structure, was introduced for comparison, and two types of static mixers with internal spiral and vortex structures were used to compare PeNC characteristics under each condition.

As a result of comparing the UV–Vis absorbance in Figure b, the difference in absorbance according to the static mixer type was not significant. The first excitonic peak position was confirmed to be 510 nm under all three conditions, indicating that the overall particle sizes of the synthesized PeNCs are similar.

In the PL spectra of Figure c peak broadening occurred depending on the shape of the static mixer. According to Table , the peak positions were 516 nm for the Y-junction, 520 nm for the spiral, and 519 nm for the vortex, indicating that the particles grew relatively smaller in the empty tubing compared to when mixer structures were introduced inside the tube. Regarding FWHM, the Y-junction was 22.9 nm, the spiral was 19.2 nm, and the vortex was 18.4 nm. The use of spiral and vortex mixers generates strong Dean vortices and swirling flows within the tubing, which rapidly dissipate local concentration gradients. ,, This enhanced mixing is expected to suppress secondary nucleation and non-uniform growth even in the presence of excess OA, resulting in narrower emission from the nanocrystals. Notably, the vortex mixer showed a 4.5 nm decrease of FWHM value compared to the Y-junction. This suggests that inserting a mixer structure inside the tube improves the precursor mixing efficiency, thereby narrowing the emission bandwidth and reducing growth heterogeneity. In the Y-junction-type mixer with an empty interior, the injected precursor forms a laminar flow in the initial state, causing mixing only by the free diffusion of the precursor solution. Subsequently, when entering the coil-structured zone of the aging part, Dean flow is formed, and the solution is mixed. ,,

2. PL Spectra and PLQY Results of the Y-Junction (without Internal Mixer), Spiral, and Vortex Mixers.

mixer type	peak (nm)	FWHM (nm)	PLQY (%)
Y-junction	516	22.9	68
spiral	520	19.2	66
vortex	519	18.4	67

The changes in optical properties of PeNCs according to temperature and flow rate in the Y-junction condition were verified in Figure S3. As temperature increased, there was a tendency to shift from a multipeak shape to a single-peak shape. As flow rate increased, a red shift occurred, and particle size uniformity improved, resulting in a relatively decreased FWHM. At 220 °C, the size uniformity was the best with an FWHM of 21 nm, but the PLQY tended to decrease to 54%, confirming that increasing the temperature did not simultaneously improve FWHM and PLQY.

In contrast, the spiral mixer utilized a helical element inside the tube to induce repetitive division and rotation of the injected solution, aiming to accelerate precursor solution mixing. , In Figure S4, changes in PeNC optical properties depending on temperature and flow rate under the spiral mixer condition were confirmed. Compared to the Y-junction structure, multiple peaks did not form even at low temperatures, which is deemed a result of relatively improved mixing efficiency. While FWHM tended to decrease as the reaction temperature increased, a slight increase in FWHM was observed in the section above 200 °C as the flow rate increased.

The vortex mixer has a spherical structure inside the tube and an unaligned inlet above it, causing the injected solution to flow along the spherical structure to form a strong swirling flow, accelerating mixing. − Figure S5 confirms the changes in PeNC optical properties based on the temperature and flow rate under the vortex mixer condition. Compared to the spiral mixer, a significant formation of multiple peaks was observed at low temperatures, but it transitioned to a single-peak shape as the reaction temperature increased. Furthermore, in the case of the vortex mixer, the improvement in FWHM with increasing flow rate was much greater than that of the spiral mixer, rendering it a suitable mixer structure for high flow rate conditions for scale-up.

Taken together, the static mixer comparison indicates that the mixer geometry mainly influences the emission bandwidth rather than the PLQY. The PLQY values of the Y-junction, spiral, and vortex conditions were similar, whereas the FWHM decreased from 22.9 to 18.4 nm. This suggests that enhanced convective mixing primarily suppresses concentration-gradient-driven heterogeneous nucleation and growth, while surface-related non-radiative recombination must be addressed separately through ligand exchange.

To improve the emission characteristics of CsPbBr₃ PeNCs, in situ surface ligand exchange was performed as shown in Figure . Aromatic ligands, benzoic acid and 4-phenylbutylamine were applied for the ligand exchange. To improve the aromatic ligand exchange efficiency, a ligand solution mixing a benzene solvent with aromatic ligands was used. In the case of pristine PeNCs, they are composed of OA- and OAm-based ligands, but due to weak binding strength, surface defect sites increase via ligand self-diffusion, leading to a tendency for PLQY to decrease in alkyl solvents such as octane. In contrast, aromatic ligands have shorter chain lengths than long-chain ligands, making them advantageous for surface packing, and their low solubility under alkyl solvent conditions can reduce the ratio of diffusing out, which is effective in maintaining or improving PLQY.

4.

(a) UV–Vis absorption, (b) PL spectra, and (c) FT-IR spectra of CsPbBr₃ PeNCs at various ligand flow rates. (d) Digital photographs of CsPbBr₃ PeNC solutions synthesized at different ligand flow rates (0, 0.1, 0.15, and 0.2 mL/min).

Figure a shows that the changes in UV–Vis absorption of PeNCs that underwent ligand exchange were measured by injecting pristine and aromatic ligand solutions. The first excitonic peak position was confirmed to be identical at 509 nm across the three conditions. However, under the pristine condition (0 mL/min) and 0.15 mL/min condition, baseline elevation due to scattering occurred, whereas this baseline elevation was relatively suppressed in the 0.1 mL/min condition. This means that the colloidal stability at 0 and 0.15 mL/min was relatively lower compared with the 0.1 mL/min condition. For the pristine condition, it is judged that colloidal stability decreased as oleic acid and oleylamine diffused out. At 0.15 mL/min, as the ratio of short-chain ligands increased, the dispersibility in the alkyl solvent began to decrease, resulting in reduced colloidal stability. Conversely, the 0.1 mL/min condition is believed to lower the probability of aggregation relatively by intercalating short-chain ligands among the long-chain ligands that maintain the colloidal stability.

Figure b and Table present the analysis of PL spectra and PLQY according to PeNC ligand exchange conditions. The PL spectra exhibited no change across ligand exchange conditions, indicating no change in particle size due to ligand exchange. However, the PLQY was 69% for the pristine condition, but it increased to 95% when the ligand solution was injected at 0.1 mL/min and 92% at 0.15 mL/min, confirming a maximum PLQY difference of 26% following ligand exchange. On the basis of these results, the aromatic ligands appear to reduce surface-related non-radiative recombination even under in situ ligand exchange conditions. Because the PL peak position was nearly unchanged after ligand injection, the PLQY enhancement is attributed mainly to the modification of the surface environment rather than a change in the nanocrystal core size.

3. PL Spectra and PLQY Results of CsPbBr₃ PeNC Solutions Synthesized at Different Ligand Flow Rates (0, 0.1, 0.15, and 0.2 mL/min).

ligand flow (mL/min)	peak position (nm)	FWHM (nm)	PLQY (%)
0	519	18.8	69
0.1	519	18.6	95
0.15	519	19.1	92
0.2	N/A (precipitated)	N/A (precipitated)	N/A (precipitated)

Figure c shows the results of Fourier transform infrared spectroscopy (FT-IR) analysis of pristine and ligand-exchanged CsPbBr₃ PeNCs. The 2840–2950 cm^–1 region is attributed to the asymmetric and symmetric stretching vibrations of the −CH₂ and −CH₃ groups of oleic acid and oleylamine. , The decrease in peak intensity in the 2840–2950 cm^–1 region as the aromatic ligand solution injection rate increases means that the long-chain aliphatic ligands desorbed and were replaced by the short-chain 4-phenylbutylamine and aliphatic chain-free benzoic acid, reducing the ratio of C–H bonding. , As the injection amount of the aromatic ligand solution increases, the peak ratio at the 1466 cm^–1 position decreases, while the peak ratio at the 1393 cm^–1 position simultaneously tends to increase. The 1466 cm^–1 peak is a characteristic peak of the scissoring vibration of the −CH₂ group in OA and OAm, and the 1393 cm^–1 peak corresponds to the symmetric stretching of the carboxylate group in the benzoate ligand and the CC vibration mode of the aromatic ring. These FT-IR results support partial replacement of OA/OAm-based aliphatic ligands by the aromatic ligands, benzoic acid, and 4-phenylbutylamine. Together with the PLQY enhancement, these results indicate that surface ligand exchange can be incorporated as an in situ process during PeNC synthesis using the h-CFR.

To further support the change in the surface-bound organic ligand environment after in-line aromatic ligand exchange, thermogravimetric analysis (TGA) was performed for pristine and ligand-exchanged CsPbBr₃ PeNCs, as shown in Figure S6. The residual weight at 500 °C increased from 29.2% for the pristine PeNCs to 42.1 and 52.6% for the samples prepared at ligand injection rates of 0.1 and 0.15 mL min^–1, respectively. The pristine PeNCs are mainly capped with long-chain OA/OAm ligands; therefore, the larger weight loss of the pristine sample is attributed to the thermal desorption/decomposition of these aliphatic ligands. After in-line aromatic ligand injection, the increased residual weight suggests that a portion of the long-chain OA/OAm ligands was replaced by shorter aromatic ligands, benzoic acid, and 4-phenylbutylamine, resulting in a systematic change in the surface-bound organic fraction. These TGA results are consistent with the FT-IR evidence for partial ligand replacement and provide additional support for the formation of a modified surface ligand environment after in-line ligand exchange.

Figure d shows the digital photographs of the dispersions according to the surface modification conditions of CsPbBr₃ PeNCs. When the injection rate of the ligand solution increased to 0.15 mL/min, it was observed that the color of the solution became brighter due to the increase in the PLQY. However, from the flow rate conditions of 0.15 mL/min and above, the ratio of short-chain ligands replacing long-chain ligands, which directly affects colloidal stability, increased, showing a tendency for decreased dispersity. Synthesizing these results with the PL characterization results, the optimal ligand exchange condition for PeNCs applied with h-CFR was determined to be 0.1 mL/min.

Figure S7 presents the X-ray diffraction (XRD) patterns used to analyze the crystal structures of pristine and surface ligand-exchanged perovskite nanocrystals. Both samples were synthesized via an h-CFR, and in situ ligand exchange was conducted with a ligand injection rate of 0.1 mL/min. The XRD results show that both pristine and modified PeNCs exhibit diffraction peaks corresponding to the cubic phase of the CsPbBr₃. These observations suggest that the in-line aromatic ligand exchange step does not measurably disturb the perovskite crystal lattice, while the FT-IR results support changes in the surface ligand environment discussed above. Although both pristine and ligand-exchanged PeNCs exhibited diffraction peaks corresponding to cubic CsPbBr₃, the relative intensities of the (200) and (211) reflections changed after the aromatic ligand injection. Because no additional diffraction peaks or obvious peak shifts were observed, this change was not attributed to the formation of a secondary crystalline phase or significant lattice distortion. Instead, it is likely associated with a change in nanocrystal packing or preferred orientation during drying and XRD sample preparation, which can be affected by the surface ligand environment. Partial replacement of long-chain OA/OAm ligands by shorter aromatic ligands may alter interparticle interactions and drying-induced assembly behavior, leading to different relative diffraction intensities. Similar orientation-dependent XRD features have been reported for CsPbBr₃ nanocrystal superlattices, where the preferential alignment and structural coherence of nanocrystal assemblies affect the observed diffraction features. Therefore, the XRD results indicate that the CsPbBr₃ crystal phase was retained after ligand exchange, while the relative intensity variation suggests that the modified surface ligand environment influences the nanocrystal assembly.

Figure S8 shows the additional documentation of the 3 h continuous synthesis and purification process using the h-CFR equipped with a vortex static mixer. The synthesis was performed at 200 °C with a total flow rate of 10 mL min^–1. After 3 h of continuous operation, 1.8 L of crude CsPbBr₃ PeNC dispersion was recovered. For purification, 300 mL of the crude dispersion was mixed with 200 mL of acetonitrile as an antisolvent in a 500 mL centrifuge tube and centrifuged at 4500 rpm for 10 min. The supernatant containing residual solvent, unreacted precursors, and excess ligands was discarded, and the precipitated PeNCs were collected and dried under vacuum to remove residual solvent and antisolvent. The final mass of the dried PeNC powder obtained from the 1.8 L crude dispersion was 6.12 g, corresponding to an isolated PeNC concentration of 3.4 mg mL^–1 in the crude dispersion and a laboratory-scale productivity of 2.04 g h^–1. Because the isolated solid contains CsPbBr₃ PeNCs capped with surface ligands and the purification process removes residual solvent, unreacted precursors, and excess ligands, the isolated amount is reported as the recovered dried PeNC mass and isolated PeNC concentration rather than as a theoretical conversion yield. These results provide quantitative mass-balance information for the gram-scale feasibility of the h-CFR process under the present laboratory-scale continuous-flow conditions.

To further evaluate the product consistency and temporal stability of the h-CFR process during the 3 h continuous synthesis, PeNC samples were collected at 1, 60, and 180 min after the crude dispersion began to elute from the collector, and the results are summarized in Figure S9. The representative TEM image of the 1 min sample showed relatively broad particle size dispersion, with apparent particle sizes ranging from approximately 5 to 11 nm, which is attributed to incomplete stabilization of precursor mixing and flow uniformity at the very early stage of operation. In contrast, the samples collected at 60 and 180 min exhibited more uniform cubic PeNCs with apparent particle sizes of approximately 12–13 nm, indicating that the reactor reached a more stable mixing and growth regime during continuous operation. The corresponding PL spectra showed the same emission peak at 518 nm for all sampling times. The FWHM decreased from 20.1 nm at 1 min to 18.5 nm at both 60 and 180 min, and the PLQY values remained high at 90, 92, and 91%, respectively. These results support the formation of PeNCs with consistent morphology and optical properties after the initial stabilization period of the h-CFR process.

Overall, these results indicate that the h-CFR provides a process-level advantage by linking three functions in one continuous pathway: liquid-phase precursor delivery, static mixer-assisted nanocrystal formation, and immediate aromatic ligand exchange. Compared with previous microfluidic or continuous-flow PeNC studies that primarily focused on rapid synthesis, residence time mapping, or dual-reactor production, − the present configuration emphasizes the combined control of emission bandwidth and surface passivation within a single in-line process. The current results should therefore be interpreted as a gram-scale feasibility demonstration rather than full industrial validation; longer continuous operation and statistical batch-to-batch reproducibility tests will be required in future work.

Conclusion

In conclusion, a two-zone h-CFR was applied to the continuous synthesis and in-line surface ligand exchange of CsPbBr₃ PeNCs. A room-temperature liquid-phase Cs precursor was selected to enable continuous feeding through SUS tubing and to mitigate precursor solidification under flow conditions. The comparison of static mixer geometries showed that the vortex mixer narrowed the emission bandwidth most effectively, producing CsPbBr₃ PeNCs with a PL peak at 519 nm and an FWHM of 18.4 nm at 200 °C and 10 mL/min. In-line aromatic ligand exchange with benzoic acid and 4-phenylbutylamine increased the PLQY from 69 to 95% while maintaining a narrow FWHM of 18.6 nm. FT-IR analysis supported partial replacement of long-chain OA/OAm ligands by aromatic ligands, and XRD indicated that the CsPbBr₃ crystal phase was retained after ligand exchange. A 3 h continuous run yielded 1.8 L of PeNC dispersion and 6.12 g of purified powder, demonstrating gram-scale feasibility under laboratory-scale continuous operation. Additional TGA and time-dependent TEM/PL analyses further supported the modified surface ligand environment and the optical consistency of PeNCs during the 3 h continuous operation. These results show that the h-CFR can integrate PeNC formation and surface passivation in one flow process, while extended long-term operation and statistical reproducibility tests will be needed for further process validation.

Experimental Methods

Precursor and Ligand Solution Preparation

A stock solution of the lead bromide precursor was prepared by reacting a PbBr₂ (2.28 mmol), oleic acid (19.2 mmol), and oleylamine (18 mmol) in 1-octadecene­(60 mL, ODE) under vacuum with vigorous mixing and heating to 120 °C for drying. When the temperature of the solution reached 120 °C, the solution was purged with Ar, and the solution was heated to a 160 °C for fully dissolution of the lead bromide salt. The stock solution of the cesium carboxylate precursor was prepared by reacting cesium carbonate (0.25 mmol), oleic acid (0.75–2.5 mmol), and stearic acid (0.4 mmol) in ODE (4 mL) under vacuum with vigorous mixing and heating to 120 °C for drying and dissolution. When the solution was tuned to transparent phase, the solution was purged with Ar and remove the heating source. The aromatic ligand solution was prepared by mixing a benzoic acid (0.4 mmol) and 4-phenylbutylamine (0.25 mmol) in octane and benzene co-solvent (10 mL, octane/benzene = 9:1).

Synthesis of CsPbBr₃ PeNCs with a Flow Reactor

The stock solution of the Cs-carbonate, lead bromide, and aromatic ligand solution were loaded at the SUS syringe. Before injection, the SUS tubing temperature was heated to target temperature (160–220 °C for PeNCs and room temperature for ligand exchange). After pre-heating, the precursor loaded syringe was pushed for a target flow rate (0.2–0.8 mL/min for Cs precursor, 2.5–10 mL/min for PbBr precursor, and 0.1–0.2 mL/min for aromatic ligand solution).

Characterization of the CsPbBr₃ PeNCs

The optical properties of the CsPbBr₃ PeNCs were evaluated by UV–Vis spectroscopy (Ocean Insight, QE Pro), photoluminescence spectroscopy (Ocean Insight, QE Pro), and photoluminescence quantum yield measurements (Otsuka, QE-2100). The surface ligand environment and crystal structure of the CsPbBr₃ PeNCs were analyzed by FT-IR spectroscopy and XRD (Rigaku D/MAX 2500 V/PC; Cu Kα radiation source, α = 0.15418 nm). Thermogravimetric analysis (TGA) was performed using a TA Instruments TGA Q500 to evaluate the thermal decomposition behavior and residual weight of pristine and ligand-exchanged CsPbBr₃ PeNCs. The samples were loaded in an alumina pan and heated to 500 °C under a nitrogen atmosphere at a heating rate of 10 °C min^–1. The nitrogen balance and sample purge gas flow rates were 40 and 60 mL min^–1, respectively.

Glossary

Abbreviations

PeNCs

perovskite nanocrystals

h-CFR

hybrid continuous-flow reactor

FWHM

full width at half-maximum

PL

photoluminescence

PLQY

photoluminescence quantum yield

FT-IR

Fourier transform infrared spectroscopy

XRD

X-ray diffraction

OA

oleic acid

SA

stearic acid

OAm

oleylamine

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c05786.

• Liquefaction process of Cs precursors and absorbance and photoluminescence spectra according to the precursor types; effects of the h-CFR’s static mixer types and process conditions on the absorbance and photoluminescence spectra; and results of XRD analysis and large-scale synthesis (PDF)

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Y.-H.S. designed the project and conducted most of this work and wrote the paper. S.H.L., I.-G.K., Y.K.H., and S.-W.B. contributed the PeNC characterization, PeNC scale-up, and data analysis.

The authors declare no competing financial interest.