Subscriber access provided by University of Sunderland
Article
General Synthesis Principles for Ruddlesden-Popper Hybrid Perovskite Halides from a Dynamic Equilibrium Iain W. H. Oswald, Alexandra A. Koegel, and James R. Neilson Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03817 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 5, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
General Synthesis Principles for Ruddlesden-Popper Hybrid Perovskite Halides from a Dynamic Equilibrium Iain W. H. Oswald, Alexandra A. Koegel, James R. Neilson* *corresponding author: J. R. N. (
[email protected]) Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States.
Abstract Two-dimensional (2D) Ruddlesden-Popper hybrid perovskites are a homologous series of compounds with the formula A’2An–1BnX3n+1 (A’ = bulky organic cation; A = small organic cation; B = Pb2+ or Sn2+, X = Cl–, Br–, I–; n is an integer) composed of inorganic octahedra layers separated by organic spacer cations. The octahedral layer thickness is modified by the stoichiometry of the A site cation, but limited methods exist for controlled and discriminating synthesis for all compositions. We report a general synthesis method and its principles that yield phase-pure 2D hybrid perovskites; the chemistry operates within a dynamic equilibrium established by the molar solubility of the compounds within the homologous series. A solventantisolvent (HI-acetic acid) pair and the common-ion effect provide selective control over the molar solubility to precipitate phase-pure compounds, as is supported by simple and predictive calculations. Here, this approach is demonstrated in detail with A’ = n-butylammonium, A = methylammonium, and n ≤ 3 and is applied to the synthesis and discovery of materials with other bulky ammonium cations (e.g., isobutylammonium and benzylammonium).
Introduction Layered hybrid organic-inorganic perovskites (HOIPs) have recently emerged as a new class of semiconducting materials with many desirable properties for photovoltaic, radiation detection, lasing, and light emission applications.1-12 These compounds, like the prototypical perovskite structure with the formula ABX3 (A is methylammonium (MA+), formamidinium (FA+), or Cs+; B is Sn2+ or Pb2+; X is Cl-, Br-, or I-), are composed of corner-sharing [BX6] octahedra with large cations occupying the A-site void.13-16 These layered materials, often referred to as “Ruddlesden-Popper” type perovskites form a homologous series with the generic formula A’2An–1BnX3n+1 where A’ is a bulky organic cation, A is small organic or inorganic cation, B is a main-group cation such as Sn2+ or Pb2+, X is Cl-, Br-, or I-, and n is an integer.2, 5, 17 The bulky organic cations segregate the layers of inorganic octahedra; synthetic tuning yields variable layer thicknesses (Figure 1).2
1 ACS Paragon Plus Environment
Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Although many synthesis protocols exist for targeting compounds with a single inorganic [PbI6] layer (the n = 1 member of a homologous series), few report the synthesis of higher n-members.6-7, 18-27 The synthesis of 2D phase-pure, bulk samples with purity in ‘n’ (i.e., without irreversible co-precipitation of n = ∞ or n ± 1) has remained technically challenging, and in many cases, lacks detailed understanding of the chemistry that drives the synthesis. Additionally, few examples of syntheses of multiple homologous series with different A’ site cations exist. Typical strategies revolve around supersaturating stoichiometricallycontrolled solutions through slow cooling3, 5, 17 or slow evaporation of solvent.8 In the case of slow cooling from a supersaturated solution, phase-pure compounds up to n = 4 have been successfully grown by using the spacer cation, n-butylammonium, as the limiting reagent.17 This method avoids unwanted precipitation of the n = “∞”, three-dimensional perovskite, MAPbI3, by exploiting its higher solubility in the solvent medium at elevated temperatures, allowing the less soluble layered phases to selectively precipitate upon cooling. A follow-up report showed that the n = 5 compound can also be synthesized through this same protocol.28 Slow evaporation of solvent was used for the synthesis of the n = 3 compound (PEA)2(MA)2Pb3I10 (PEA = phenylethylammonium).8 The solvent pair, acetone and nitromethane, were used in conjunction with solid NaI to increase solubility of the solid starting materials, which after 6 days yielded single crystals. Yet another report to obtain phase pure (nBA)2(MA)Pb2I7 and (nBA)2(MA)2Pb3I10 used nitromethane as an antisolvent to induce precipitation of the compounds from a DMF solution containing the starting reagents.29 When targeting n = 2 compounds, stoichiometric amounts of reagents were used, but off-stoichiometric ratios were used when targeting n = 3. This therefore presents a challenge for understanding and adapting a reproducible, predictive, and stoichiometrically-discriminating synthesis. Furthermore, synthesis of bulk, phase-pure compounds is essential for determination of intrinsic properties, structure, and dynamics. Here we present a chemical synthesis and the synthetic design principles which guide and enable phasepure Ruddlesden-Popper (RP) 2D perovskites up to n = 3. By using a HI(aq):acetic acid::solvent:antisolvent pair, the molar solubility of the target product of a homologous series is separated from other n-values and is controllably lowered to yield phase-pure compounds and small single crystals. Acetic acid is an ideal antisolvent, which we attribute to its miscibility with HI(aq) (and thus ability to form a dynamic equilibrium with the products in a homogeneous solution) as well as the high solubility of starting reagents that prevents their unintended precipitation. We find that the ideal initial concentrations of the reagents in HI(aq) are highly dependent on the molar solubility of the easily-synthesized n = 1 member. This solubility limit determines the optimal working concentrations for a given homologous series up to n = 3 and can be tuned by addition of acetic acid to yield phase-pure n-members. The compounds (A’)2PbI4 (n = 1), (A’)2(MA)Pb2I7 (n = 2), and (A’)2(MA)2Pb3I10 (n = 3) (A’ = n-butylammonium (nBA), iso-butylammonium (iBA), and benzylammonium (BZA)) are synthesized using this strategy (Figure 1); these ammonium cations are used 2 ACS Paragon Plus Environment
Page 2 of 22
Page 3 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
to illustrate the synthesis principles across a wide range of molar solubilities of their respective n = 1 members and to illustrate that these principles apply to cations with distinct molecular characteristics (e.g., linear, branched, aromatic). Furthermore, we find that the series containing BZA has a competing Pb-rich, non-perovskite phase in higher n-members that is suppressed using the common-ion effect. This synthesis yields a microcrystalline precipitate that undergoes rapid color changes, indicative of a solid-to-solute-tosolid transformation occurring by dissolution and reprecipitation of solids in a sequential manner. These results highlight a new method and general rules for synthesizing phase-pure RP phases that are universally applied to other chemical compositions.
Figure 1. Top: Crystal structures of the homologous Ruddlesden-Popper type 2D HOIPs, (nBA)2(MA)n— 1PbnI3n+1, with n = 1, 2, and 3 (left to right) showing inorganic [PbI6] octahedra, separated by bulky nbutylammonium cations in the A’ sites. The n = 2 and 3 products (middle and right) have methylammonium cations occupying the cuboctahedral voids. Bottom: Chemical structure of the spacer cations used in this study.
Experimental. Synthesis. All manipulations were conducted in air and in a well-ventilated fume hood. All reagents were purchased from commercial vendors and used as received. Abbreviations used: MA = methylammonium, nBA = nbutylammonium, iBA = iso-butylammonium, BZA = benzylammonium. We note that the use of PbI2 as the Pb2+ source versus PbO or MAI as opposed to MACl leads to the same products, indicating the synthesis is robust to changes in precursors (Figures S12 and S13). MAPbI3 (n = ∞). A 0.48 M Pb2+ solution in stabilized HI (57 wt% in H2O,
