Ultrasonication‑Assisted Anion Exchange of CsPbBr₃ Nanocrystals: A Green, High‑Yield Route to Full‑Visible‑Spectrum Emission

Background

All‑inorganic CsPbX₃ (X = Cl, Br, I) perovskite nanocrystals have attracted intense interest due to their record‑high PLQY, sub‑20 nm emission linewidths, intrinsic defect tolerance, and the ability to tune their band‑gaps via composition or size control. These attributes have spurred investigations into light‑emitting diodes, photodetectors, lasers, and photovoltaics. A key enabler of this versatility is the facile anion‑exchange capability inherent to the perovskite lattice, which allows the post‑synthetic modification of halide composition through simple mixing with reactive anion precursors. However, many established protocols rely on highly reactive, often toxic, organometallic halides (e.g., OLAM‑X, TBA‑X) and demand inert, anhydrous conditions, limiting their practicality for large‑scale synthesis. In contrast, aqueous halide salts offer a greener alternative but typically exhibit sluggish exchange kinetics due to their low solubility in non‑polar solvents.

Building on recent reports that leverage interfacial reactions between hexane‑dispersed CsPbX₃ NCs and aqueous CsX solutions, we introduce a method that couples this interfacial approach with ultrasonic agitation. The high concentration of CsX in water (up to 1865 g L⁻¹ for CsCl and 440 g L⁻¹ for CsI) provides a strong thermodynamic driving force for halide substitution, while ultrasonication enhances mass transport and accelerates the exchange process.

Methods

Synthesis and Purification of CsPbBr₃ NCs

CsPbBr₃ NCs were synthesized following the protocol of Protesescu et al. Briefly, 0.8 g of Cs₂CO₃ (99.9 %, Aldrich), 2.5 mL oleic acid (OA, 90 %, Aldrich), and 30 mL octadecene (ODE, 90 %, Aldrich) were degassed for 30 min and then dried under Ar at 120 °C for 1 h. Separately, 0.136 g PbBr₂ (99.9 %, Aldrich), 2 mL oleylamine (OALM, 80–90 %), 1.5 mL OA, and 8 mL ODE were prepared, degassed, and heated to 120 °C for 30 min before raising to 180 °C for 10 min. A 1 mL aliquot of Cs‑oleate was injected, followed by a 10 s ice‑bath quench. The resulting NCs were precipitated with acetone, centrifuged, and redispersed in hexane.

Anion Exchange Reactions

For exchange, 5 mL of aqueous CsX (1 M or 0.2 M, X = Cl, I) was transferred to a 25 mL glass bottle, and 3 mL of CsPbBr₃ NCs/hexane (4.5 mM in Br⁻) was added dropwise. The mixture was sonicated in a 50 W bath (KQ‑50B) for a user‑defined duration, then left undisturbed for 5 min. The organic phase was collected, and the aqueous phase was purified for reuse. The product was centrifuged at 2500 rpm for 5 min to remove any precipitate.

Characterization

Phase purity was verified by X‑ray powder diffraction (D8 Advance, Bruker) and high‑resolution TEM (JEM 2100F, JEOL, 200 kV). UV‑vis absorption spectra were recorded on a Shimadzu UV3600 spectrophotometer.

Photoluminescence Measurements

PL spectra were collected using a PTI QM/TM/NIR spectrophotometer with a 914‑photomultiplier and a 75 W xenon lamp. Excitation wavelengths were 400 nm for all samples, except 360 nm for CsPb(Br/Cl)₃ NCs. PLQY was calculated following the method of Prato et al.: PLQY (%) = [(SEM – BEM)/(BEX – SEX)] × 100. Solutions were sufficiently dilute to neglect reabsorption effects.

Stability Test

NCs dispersed in hexane were sealed in glass vials and stored at ambient conditions. Absorption and PL spectra were recorded every 7 days over several weeks to monitor stability.

Results and Discussion

Figure 2 illustrates the evolution of absorption and emission spectra of CsPbBr₃ NCs during ultrasonication in CsI and CsCl aqueous solutions. Exchange with CsI produces a pronounced red‑shift, stabilizing at 675 nm (absorption) and 685 nm (emission) after 30 min. Conversely, CsCl induces a blue‑shift, reaching 405 nm (absorption) and 411 nm (emission) within 45 min. These shifts confirm the successful incorporation of I⁻ or Cl⁻ into the lattice and the ability to tune the band‑gap across the visible range. The full width at half maximum (FWHM) of CsPb(Br/I)₃ NCs widens from 20 nm to 39 nm, while CsPb(Br/Cl)₃ NCs exhibit a modest narrowing from 20 nm to 10 nm, indicating that size dispersion remains largely unchanged.

Figure 3 shows the color change of the NCs under 365‑nm illumination: the emission transitions smoothly from green to blue with CsCl exchange and to red with CsI exchange. TEM images confirm that the cubic morphology and average sizes (≈ 9–11 nm) are preserved post‑exchange. Selected‑area electron diffraction and HR‑TEM further corroborate the retention of the cubic perovskite structure (space group Pmar{3}m) and the successful halide substitution, as evidenced by lattice constant shifts (0.56 nm for CsPbCl₃ and 0.615 nm for CsPbI₃).

Energy‑dispersive X‑ray spectroscopy (EDX) data (Table 1) reveal that the Br⁻ to Cl⁻ substitution ratio can reach 93 % and the Br⁻ to I⁻ ratio up to 90 %. PLQY values peak at 85 % for CsPbBr₂.₃Cl₀.₇, then decline sharply for higher Cl content; CsPb(Br/I)₃ NCs show a monotonic decrease from 76 % to 31 % as I⁻ content rises. These trends mirror those reported for directly synthesized CsPbCl₃ and CsPbI₃ NCs, confirming that the aqueous exchange does not compromise optical performance. Despite the aqueous exposure, the NCs exhibit reasonable room‑temperature stability when stored in hexane, with only a 30 % drop in PL intensity over 4 weeks for CsPb(Br/Cl)₃ NCs and 5 % for CsPb(Br/I)₃ NCs after 20 min sonication.

The rapidity and efficiency of the exchange are attributed to the high solubility of CsX in water, which creates a strong thermodynamic gradient across the hexane‑water interface. Control experiments with lower CsX concentrations (0.2 M) demonstrate that while the early‑stage kinetics remain similar, the final halide composition is dictated by the aqueous concentration, enabling precise tuning of the band‑gap.

Conclusions

We have established a simple, green, and scalable ultrasonication‑assisted anion‑exchange protocol for CsPbBr₃ NCs using aqueous CsX solutions. The method achieves >90 % halide substitution while preserving nanocrystal morphology and delivers emission spanning the entire visible spectrum with high PLQY and respectable stability. This approach offers a versatile route for tailoring the chemical composition and optical properties of halide perovskite nanocrystals, paving the way for their integration into advanced optoelectronic devices.

Abbreviations

EDX

Energy‑dispersive X‑ray spectroscopy

NCs

Nanocrystals

OA

Oleic acid

OALM

Oleylamine

ODE

Octadecene

PL

Photoluminescence

TBA

Tetrabutylammonium

TEM

Transmission electron microscope

XRD

X‑ray diffraction