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Cs Oleate Precursor Preparation for Lead Halide Perovskite Nanocrystal Synthesis: The Influence of Excess Oleic Acid on Achieving Solubility, Conversion, and Reproducibility. Chang Lu, Marcus W. Wright, Xiao Ma, Hui Li, Dominique S. Itanze, Jake A. Carter, Corey A. Hewitt, George L. Donati, David L. Carroll, Pamela M. Lundin, and Scott M. Geyer Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04876 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 13, 2018
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Chemistry of Materials
Cs Oleate Precursor Preparation for Lead Halide Perovskite Nanocrystal Synthesis: The Influence of Excess Oleic Acid on Achieving Solubility, Conversion, and Reproducibility. Chang Lu†, Marcus W. Wright†, Xiao Ma†, Hui Li†, Dominique S. Itanze†, Jake A. Carter†, Corey A. Hewitt‡, George L. Donati†, David L. Carroll‡, Pamela M. Lundin§, Scott M. Geyer†,* †Department
of Chemistry, Wake Forest University, Winston-Salem, North Carolina 27109, USA
‡Center
for Nanotechnology and Molecular Materials, Department of Physics, Wake Forest University, WinstonSalem, North Carolina 27109, USA §Department
of Chemistry, High Point University, High Point, North Carolina 27268, USA
ABSTRACT: In the colloidal synthesis of inorganic perovskite materials, cesium oleate (CsOL) is the most commonly used Cs precursor. Yet despite its ubiquitous use in literature, CsOL has been observed to be insoluble at room temperature and leads to surprisingly inconsistent results in CsPbX3 nanocrystal synthesis depending on the Cs salt from which the precursor is derived. We show that under the conditions used in most reports, the amount of oleic acid (OA) added, while stoichiometrically sufficient, still leads to incomplete conversion of the Cs salts to CsOL. This results in a mixture of Cs sources being present during the reaction, causing decreased homogeneity and reproducibility. When a 1:5 Cs:OA ratio is used, complete conversion is readily obtained even under mild conditions, resulting in a precursor solution that is soluble at room temperature and yields identical synthetic results regardless of the initial Cs source. Further, 1H nuclear magnetic resonance (NMR) of solutions prepared using varying Cs:OA ratios shows that the maximum ratio of Cs:OA obtainable in solution is 1:5, with any excess Cs present in the precipitate. We believe the use of a soluble, fully converted CsOL reagent will improve reproducibility for Cs-based perovskite synthesis and directly benefit synthetic methods based on microfluidics.
The colloidal synthesis of halide perovskites nanocrystals (NCs) has seen a remarkable development recently1,2, with protocols for hybrid organic-inorganic3 and fully inorganic4 perovskite NCs having been reported. In particular, synthetic routes for Pb-based systems with narrow size distributions as well as narrow emission line widths have been developed for CsPbX3 (X = Cl, Br, I or combination) and methylammonium (MAPbX3) NCs5. Moreover, for the fully inorganic perovskite family, a series of doped6 and lead-free systems have also been developed, including not only, CsPb(Mn)X37, CsPb(Sn)X38, and CsSnX39, but also double perovskite Cs2AgBiX610, double perovskite variant Cs2SnI611, and perovskite alternative Cs3Sb2X912,13. Owing to the immense compositional diversity and unique properties of the fully inorganic perovskite family, a variety of optoelectronic applications have been explored, such as solar cells14 and light emitting diodes15. For Cs-based halide perovskites, Cs oleate (CsOL) is the most commonly used Cs source. This precursor is synthesized by combining oleic acid with either cesium carbonate (Cs2CO3) or cesium acetate (CsOAc), and the long alkyl chain of CsOL is expected to impart solubility in the non-polar solvents used in synthesis, similar to
Pb(oleate)2 (Pb(OL)2). Despite the simplicity of this procedure, several problems persist. Previous reports16,17 showed that the optical spectra of CsPbBr3 NCs obtained using CsOL derived from Cs2CO3 and CsOAc under the same conditions yield a 20-nm difference in emission peak position17. Given that the products of CsOL formation for both precursors are expected to be readily removed under vacuum at elevated temperature (CO2 and H2O for Cs2CO3, acetic acid for CsOAc), the source of this discrepancy is not clear. Further, a difference in solubility has been observed for the CsOL derived from Cs2CO3 versus that from CsOAc17,18. Surprisingly, the CsOL precursor as typically prepared in literature is not soluble when cooled to room temperature. Methods such as successive ion layer adsorption and reaction (SILAR)19 and microfluidic technology20, use capillary-based injection that requires high solubility to avoid clogging are not accessible. These inconsistencies and limitations are a challenge for establishing consistent results across literature and the ultimate development of large-scale synthesis for perovskite device manufacturing21,22. In this communication, we show that the vast majority of procedures for Cs-based NCs use insufficient OA to obtain complete conversion to CsOL under typical
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synthetic conditions. Fully converted CsOL prepared from both Cs2CO3 and CsOAc yields NCs with no discernable difference, regardless of reaction temperature. This contrasts with the ratios typically used in literature, which lead to clear spectral differences. These results are understood in the context of the monovalent nature of Cs, which leads to a permanent dipole in Cs oleate reducing solubility. When properly prepared, the precursor exhibits room temperature solubility and can be used for slow addition synthesis as demonstrated by the synthesis of high quantum yield blue emissive CsPbBr3 NCs. Figure 1(a) shows a series of CsOL samples made with varied Cs:OA ratios. Each sample was stirred under vacuum at 110 °C for 3 hours, consistent with the typical literature synthesis (see Table S1 and S2 for detailed recipe). The ratio used in literature is typically less than 1:2, both CsOAc and Cs2CO3 form precipitates after cooling to room temperature. The amount of precipitate decreases with increasing OA content, which is quantified by the % weight of the precipitate in Figure S1. At a ratio of 1:4 the CsOAc sample shows good solubility (only 0.30 wt% precipitate) while the Cs2CO3 counterpart still produces a small amount (3.54 wt%) of precipitate. The quantitative results are in agreement with the discussion in literature that CsOAc derived CsOL (CsOL(OAc)) has better solubility17. However, this is true only over a very narrow range. Once the Cs:OA ratio is 1:5 or beyond, both systems show complete solubility (0 wt% observed), consistent with the established ability of long chain oleic acid to bring cations into solution in non-polar solvents.
Figure 1. (a) CsOL made by Cs2CO3 or CsOAc mixing with oleic acid (OA) under different Cs:OA ratios; (b) pure CsOL powder; (c) 0.1 M pure CsOL powder in ODE at 150 °C, still as a slurry; (d) 0.4 M CsOL ODE solution made with 4 molar parts of OA to 1 molar part of pure CsOL (no heat treatment). To fully eliminate any effect of the conversion process from CsOAc and Cs2CO3 on the solubility and reactivity, we synthesized pure CsOL powders from CsOH and OA. Pure CsOL is white to yellow as shown in Figure 1(b), and the NMR spectrum is shown in Figure S(2-4) The dispersion of CsOL powder in ODE (0.1 M) is not soluble even under 150 °C (Figure 1(c)). Yet when 4 molar parts of
OA is added to 1 part of pure CsOL to form a 1:5 Cs:OL mixture, the CsOL is readily dissolved under room temperature as shown in Figure 1(d). In Tables S3 and S4 we summarize high impact perovskite or perovskite-like synthesis papers in which the ratio of Cs:OA could be determined from the experimental procedure. The vast majority of these works use a Cs:OA ratio of less than 1:3, with most less than 1:2. For the works marked with an asterisk, the CsOL solution is heated prior to injection, which is typically done due to insolubility at room temperature and indicates that the solubility issue observed here is universal for CsOL prepared using an insufficient amount of OA. A ratio sufficient for solubility was only used in two works13,23, in the one case due to complete omission of the ODE solvent23. Upon full conversion, the CsOL precursor derived from either Cs salt source should behave identically. This was tested by using the CsOL(1:5) solutions prepared from both CsOAc and Cs2CO3, following a modified hot injection method based on the work by Protesescu et al. (details in SI)16. Unlike the previous results17, Figure 2 shows consistent absorption and emission spectrum of CsPbBr3 NCs synthesized by both CsOL(OAc) and CsOL(CO3) at 170 °C. The spectra clearly agree with each other, with only a 1 nm difference in the emission peak position. The quantum yield (QY) is nearly the same as well, 72% for CsOL(OAc) and 74% for CsOL(CO3). The X-ray diffraction (XRD) patterns show the same phase (Figure 2(b)) and the transmission electron microscope (TEM) images in Figures 2(c-d) confirm the nearly identical cubic shape and size.
Figure 2. CsPbBr3 NCs made using CsOL(OAc) and CsOL(CO3) with 1:5 Cs:OA ratio at 170°C. (a) Absorption and emission spectrum; (b) XRD pattern; (c)-(d) TEM image of CsPbBr3 NCs made by CsOL(OAc) and CsOL(CO3), respectively.
The difference between solutions prepared with complete and incomplete conversion is further demonstrated in Figure 3, which compares the emission spectrum between NCs synthesized using CsOL(CO3) and CsOL(OAc) at different temperatures (for absorption spectra see Figure S5, for TEM see Figure S6). In all cases using the 1:5 ratio, nearly identical results are obtained with only small variations attributable to batch to batch
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Chemistry of Materials inconsistency. This is the expected result when working with a pure CsOL solution, for which the original source should not matter. A similar consistency is observed for NCs synthesized from the pure CsOL with OA added to 1:5 Cs:OA ratio (Figure S7). For the 1:2 ratio, the distinction between the actual components of the nominal ‘CsOL’ leads to significant differences in emission. These are attributed to variations in kinetics caused by having multiple Cs-organic precursors present with differing reactivity during the reaction. NCs synthesized below 150 °C using CsOL(CO3) emit at shorter wavelengths, indicating a smaller size compared with NCs synthesized by CsOL(OAc). While this is consistent with previous reports that CsOAc and Cs2CO3 can lead to different results17, this is only true when insufficient oleic acid is present leading to incomplete conversion. Further, we emphasize that slight differences in preparation procedure such as temperature and time will lead to varying degrees of conversion, causing discrepancies in the literature even for the same Cs salt. From these results, it is clear that the identity of the nominal ‘CsOL’ precursor is not well established for the typical oleate-deficient synthesis found in literature. Fourier-transform infrared spectroscopy (FTIR) was used to identify the species present in both the supernatant and the precipitate for the different Cs:OA ratios. Figure 4(a) shows the spectra obtained for the 1:4 CsOL(CO3) sample. For reference, these are compared to the spectrum of Cs2CO3 powder and the fully soluble CsOL solution obtained for a 1:5 Cs:OA ratio. At lower wavenumbers (
