Role of Acid-Base Equilibria in the Size, Shape, and Phase Control of Cesium Lead Bromide Nanocrystals

Abstract

A binary ligand system composed of aliphatic carboxylic acids and primary amines of various chain lengths is commonly employed in diverse synthesis methods for CsPbBr₃ nanocrystals (NCs). In this work, we have carried out a systematic study examining how the concentration of ligands (oleylamine and oleic acid) and the resulting acidity (or basicity) affects the hot-injection synthesis of CsPbBr₃ NCs. We devise a general synthesis scheme for cesium lead bromide NCs which allows control over size, size distribution, shape, and phase (CsPbBr₃ or Cs₄PbBr₆) by combining key insights on the acid-base interactions that rule this ligand system. Furthermore, our findings shed light upon the solubility of PbBr₂ in this binary ligand system, and plausible mechanisms are suggested in order to understand the ligand-mediated phase control and structural stability of CsPbBr₃ NCs.

Keywords: CsPbBr3; acid−base; colloidal synthesis; halide perovskites; nanocrystals.

Figure 1

Maximum reaction temperature T_max as a function of ligand concentration; at T_max, PbBr₂ precipitates from the reaction medium (33 mM PbBr₂ solution in ODE with [OA] set at a constant of 63 and 250 mM).

Figure 2

(a) Sizes of CsPbBr₃ nanocubes synthesized using various concentrations of oleylamine (OlAm) and oleic acid (OA) and different reaction temperatures (size distributions are represented as vertical bars across each symbol). (b) Image illustrating the range of monodisperse CsPbBr₃ nanocubes, nanoplatelets, and nanosheets synthesized in this work using only OlAm and OA as ligands. (c) Absorbance (black solid line) and photoluminescence (blue dashed line) spectra. (d) XRD patterns and (e–k) TEM images of the depicted samples.

Figure 3

Ligand-mediated synthetic and postsynthetic phase control of cesium lead bromide NCs (OlAm = oleylamine, OA = oleic acid). (a) Typical XRD patterns and (b) absorbance spectra of CsPbBr₃ and Cs₄PbBr₆ NCs (*COD 4510745; **ICSD 98-009-7851). (c) Scheme depicting how the interplay between the reaction temperature and ligand concentration affects the crystal structure and, hence, the composition of the obtained NCs. TEM image of Cs₄PbBr₆ NCs synthesized at (d) 165 °C and (e) 220 °C. (f) Scheme depicting the ligand-driven transformation of CsPbBr₃ NCs into Cs₄PbBr₆ NCs: in short, 100 μL of a dispersion containing CsPbBr₃ nanocubes was added to a solution (1.0 mL), kept under stirring, that contained a fixed concentration of OlAm (0.4 M) and various concentrations of OA and solvent (hexane). This transformation was followed by spectroscopy analysis, and the transformation rate was found to be modulated by the [OA]/[OlAm] ratio, as shown in the figure above. For the sake of simplicity, only the state of the systems at t = 10 min is shown. TEM images of NCs (g) before and (h) after the transformation.

Figure 4

(a) Photographs illustrating the formation of the oleylammonium oleate salt. (b) Structure of oleylamine (OlAm) and oleic acid (OA). (c) Selected regions of the ¹H NMR spectra of OlAm and OA solutions in toluene-d₈. (d) ¹H NMR spectra of a mixture of OlAm and OA in toluene-d₈, recorded at different temperatures in the range of 27 to 80 °C (upfield shifts are observed for the α-CH₂¹H NMR resonances 19 and 2, while all the other signals, e.g., 8, 11, 26, and 29, remain unchanged). All resonances identified are in agreement with previous works.^,