CsPbCl3 → CsPbI3 Exchange in Perovskite Nanocrystals Proceeds through a Jump-the-Gap Reaction Mechanism

Halide exchange is a popular strategy to tune the properties of CsPbX3 nanocrystals after synthesis. However, while Cl → Br and Br → I exchanges proceed through the formation of stable mixed-halide nanocrystals, the Cl ⇌ I exchange is more elusive. Indeed, the large size difference between chloride and iodide ions causes a miscibility gap in the CsPbCl3–CsPbI3 system, preventing the isolation of stable CsPb(ClxI1–x)3 nanocrystals. Yet, previous works have claimed that a full CsPbCl3 → CsPbI3 exchange can be achieved. Even more interestingly, interrupting the exchange prematurely yields a mixture of CsPbCl3 and CsPbI3 nanocrystals that coexist without undergoing further transformation. Here, we investigate the reaction mechanism of CsPbCl3 → CsPbI3 exchange in nanocrystals. We show that the reaction proceeds through the early formation of iodide-doped CsPbCl3 nanocrystals covered by a monolayer shell of CsI. These nanocrystals then leap over the miscibility gap between CsPbCl3 and CsPbI3 by briefly transitioning to short-lived and nonrecoverable CsPb(ClxI1–x)3 nanocrystals, which quickly expel the excess chloride and turn into the chloride-doped CsPbI3 nanocrystals found in the final product.

Table S1.Conditions for the exchange experiments discussed in Figure 1.All exchanges were performed on 6 ml aliquots of CsPbCl3 NCs with constant concentration (see Methods).The concentration of Cl in the pristine sample was estimated indirectly based on ICP-OES to determine lead ([Pb] = 6.3 mM), and assuming a stoichiometric ratio of Pb:Cl=1:3 ([Cl] = 18.9 mM).The actual concentration of Cl after each addition was therefore calculated based on the starting volume of the sample and the volume of the PbI2 stock solution added for the exchange.The concentration of I in the exchanged solution was calculated based on the moles of PbI2 in the stock solution and the volume of stock solution added for each experiment.The final halide composition of the exchanged NCs was measured experimentally post-exchange by EDX, as detailed in the Main Text.When the ionic radii difference is small (high y-values), the system can form a continuous solid solution (i.e., Br/Cl and I/Br pairs).However, when the difference is substantial (low y-values), a miscibility gap is formed, and each individual CsPbX3 compound can only accept a limited concentration of the other halide (as is the case of Cl/I) due to the significant lattice strain this induces.In such case, an increase in the concentration of the second halide will force the system to leap over the miscibility gap, as reported in this work.

Figure S2 .
Figure S2.Size distribution histograms.a) Pristine CsPbCl3 NCs.b-e) CsPbCl3 NCs reacted with increasing amounts of PbI2 from partial to nearly full conversion to CsPbI3 NCs.f) Pristine CsPbI3 NCs.The NCs size distribution histograms were estimated using the Ilastik 1 software.

Figure S3 .
Figure S3.Cl→I halide exchange on DDACl-capped CsPbCl3 NCs using PbI2 as the halide source.a) ABS (grey) and PL (colored) spectra of pristine and exchanged CsPbCl3 NCs with increasing amounts of PbI2.b) XRD patterns and (c) BF-TEM images of the same samples.

Figure S4 .
Figure S4.Cl→I exchange on oleate-and DDACl-capped CsPbCl3 NCs using benzoyl iodide as the halide source.a) ABS (grey) and PL (colored) spectra, and b) TEM images of both pristine and exchanged oleate capped CsPbCl3 NCs.c,d) Analogous exchange and characterization techniques applied for DDACl capped CsPbCl3 NCs.

Figure S5 .
Figure S5.Cl→I exchange on oleate-and DDACl-capped CsPbCl3 NCs using TMSI as the halide source.a) ABS (grey) and PL (colored) spectra, TEM images (insets), and b) XRD patterns of both pristine and exchanged oleate capped CsPbCl3 NCs.The anion exchanged sample transformed to the non-perovskite δ-CsPbI3 polymorph during the XRD measurement, due to its intrinsic instability and the exposure to air and moisture.The remaining sharp peaks not compatible with the δ-CsPbI3 are attributed to the minary compounts CsI and PbI2.c,d) Similar experiments performed on DDACl capped CsPbCl3 NCs.

Figure S6 .
Figure S6.Multiple halide exchanges inducing several halides ratios (expressed by Xcl) before and after the appearance of the CsPbI3 phase.a) ABS (grey) and PL (colored) spectra of pure-halide CsPbCl3 and I -doped CsPbCl3 NCs, and of exchanged CsPbCl3 NCs with increasing amounts of PbI2.b) XRD patterns of the same samples.

Figure S7 .
Figure S7.Lattice parameters extraction from CsPbCl3 and CsPbI3 NCs.a) XRD pattern of one of the exchanged CsPbCl3NCs (Xcl = 54.5%).Circled in grey are the (211) pseudocubic peaks of both CsPbCl3 and CsPbI3 NCs, which were chosen for the extraction of lattice parameters because they were the only high-angle peaks with limited overlap with other reflections.In this case, we preferred not to perform a full-profile (e.g., Rietveld or Le Bail) because the overlap of many reflections within each experimental peak, broadened by finite-size effects, would make the fit converge to unreliable results.b) Lattice parameters as a function of xCl (tuned via the amount of added PbI2, and measured experimentally after the exchange).The analysis indicates that some iodide is incorporated within the volume of CsPbCl3 NCs, and vice versa.The samples used for the analysis are presented in Figures1 and S6.The sample with Xcl = 92.5% was not included in the plot (b) to avoid the overlap of the experimental points with the Xcl = 94% sample, as their lattice parameters are very close.

Figure S8 .
Figure S8.Morphology and composition of partially exchanged CsPbCl3 NCs upon different halides ratios (i.e.various Xcl %).HAADF-STEM image with the respective EDX mapping for each partially exchanged case (Xcl = 28.6% at the bottom row, Xcl = 54.5% at the middle row, and Xcl = 74.4% at the upper row).EDX mapping refers to Cs (green), Pb (purple), Cl (blue), and I (red).

Figure S9 .
Figure S9.Intensity profile analysis of Figure 2f.a) Reproduction of Figure 2f.b) Magnified area of the CsPbCl3 NC surface, indicating the presence of a Cs-I surface layer (Cs + = Green, Cl − = blue, I − = red, and [PbCl6] 4− octahedra = light blue).The dashed arrows indicate the atomic planes chosen to extract the cross-sections shown in panel (c).c) Cross-section of the intensity profiles along both Cs-Cl (upper panel) and Pb-Cl (lower panel) planes, indicated in panel (b) by the purple and cyan colored arrows, respectively.The cross sections demonstrate an increase of signal intensity compared to the corresponding halide positions in the NCs bulk, which is attributed to a Cl→I replacement limited to the NC surface.

Figure S10 .
Figure S10.Examples of CsPbCl3 NCs showing the formation of Cs-I surface layers.b, c) Atomic-resolution images of CsPbCl3 NCs after partial exchange, with a, d) their magnified areas of the NCs' surface, respectively.Notably, in the first case (panel a, b), only one Cs-I plane is observed on the CsPbCl3 NC, whereas in the latter one (panel c, d), also a second Cs-I plane shifted by halfcell is preset, giving rise to a structural motif characteristic of the all-inorganic Ruddlesden-Popper phase Cs2PbCl2I2.

Figure S11 .
Figure S11.In-situ PL spectra.The spectra were collected every 4 seconds starting after the addition of OA/OLA/ODE in a solution of pristine CsPbCl3 NCs.The incorporation of OA/OLA leads to a fast increase in the PL intensity of the NCs immediately after the injection.

Figure S12 .
Figure S12.Absorption spectrum of the PbI2 precursor solution.

Figure S13 .
Figure S13.Miscibility gap represented in a halide-ratio / relative ionic radii phase diagram.The miscibility of two CsPb(A)3 and CsPb(B)3 lead halides perovskites (where A, B = Cl, Br, I and A ≠ B) can be represented in binary phase diagram where the vertical coordinate indicates the of A and B in ionic radii (higher y values = closer radii).When the ionic radii difference is small (high y-values), the system can form a continuous solid solution (i.e., Br/Cl and I/Br pairs).However, when the difference is substantial (low y-values), a miscibility gap is formed, and each individual CsPbX3 compound can only accept a limited concentration of the other halide (as is the case of Cl/I) due to the significant lattice strain this induces.In such case, an increase in the concentration of the second halide will force the system to leap over the miscibility gap, as reported in this work.

Table S2 . Average NCs compositions estimated by Vegard's law.
Compositions of I-doped CsPbCl3 NCs and Cl-doped CsPbI3 NCs estimated by applying the Vegard's law 2 [dmixed-halide NCs = xCl • dpure Cl NCs + (1 -xCl) • dpure I NCs]. 2or the purpose of the analysis, the structure of CsPbI3 was approximated as cubic.We chose the lattice constants of as-synthesized pure-halide NCs as reference values to minimize errors related to the small lattice expansion of NCs if compared to their relative bulk references.