Solvent Polarity: How does this Influence the Precursor Activation

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Letter

Solvent Polarity: How does this Influence the Precursor Activation, Reaction Rate, Crystal Growth and Doping in Perovskite Nanocrystals? Anirban Dutta, Rakesh Kumar Behera, and Narayan Pradhan ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00443 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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Solvent Polarity: How does this Influence the Precursor Activation, Reaction Rate, Crystal Growth and Doping in Perovskite Nanocrystals?

Anirban Dutta,§ Rakesh Kumar Behera§ and Narayan Pradhan* School of Materials Sciences, Indian Association for the Cultivation of Science, Kolkata 700032, India

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Abstract: A minute difference in solvent polarity in hydrophobic solvents can drastically alter the rate of formation of cesium lead halide perovskite nanocrystals. This has been observed for room temperature and in single solvent synthesis routes for CsPbBr3 and CsPbCl3 nanocrystals. Exploring Pb-thiolate as the active precursor and using seven different hydrophobic solvents, while their polarity was varied from chloroform to hexane, the phase of CsPbBr3 changed from cubic to orthorhombic. In contrary, for CsPbCl3 the phase retained cubic in all solvents, but their shapes turned from cube to platelets. Being the rate of the reaction was controlled here, doping of Mn(II) was performed for all cases and efficient doping was observed for the slower reaction in hexane medium. Systematically, the reaction chemistry and the physical processes associated in these formation processes were analyzed and reported in this letter. TOC:


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The chemistry of formation of lead halide perovskite nanocrystals which are recently emerged as one of the most efficient energy materials has been widely studied in recent years.1-8 Tremendous affords have been put forwarded for understanding the surface ligands binding, precursors activation, the crystal growth for size/shape/phase variations, doping of various impurity ions for inducing new materials properties and stabilizing these materials for their implementations in different platforms.9-25 Literature reports revealed that these nanocrystals and also the doping of various metal ions are typically synthesized both at room temperature and high temperature reactions.11, 18, 26-32 However, while the crystal growth understanding for high temperature hot injection approaches are extensively studied; the chemistry of formation of these nanocrystals at room temperature is largely unexplored. The first room temperature approach reported by Dong's research group involved a dual solvent approach for triggering the formation of perovskite nanocrystals and the method also further modified and extended by other groups.33-35 This method is primarily dominated by Ligand Assisted Reprecipitation Approach (LAPR) where all reagents were dissolved in polar solvent and the mixture was added in a non-polar solvent for triggering supersaturation.26,

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This led to burst nucleations and

subsequent formation of the perovskite nanocrystals. However, for understanding the fundamentals crystal growth, control of the chemical process for the crystal formation of various perovskites is important as this would help in understanding the composition, phase, shape and the doping process. From various literature reports, it is revealed that for Mn doping, simply Mn precursors were introduced along with other precursors or added after the synthesis of perovskite host nanocrystals for anion exchange mediated Mn insertion.31,

37-41

However, doping is typically a kinetically controlled 3

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process when slow reaction helps for the adsorption of the dopants42-44 and hence, for better understanding the doping process, more information and particularly slowing down the formation process of the host nanocrystals is critically important. Keeping all these in mind, a precursor activation approach was adopted here as the major controlling parameter for tuning the reaction kinetics of the formation of cesium lead bromide and chloride perovskite nanocrystals at room temperature. This activation also varied with the tuning of the polarity of the solvent medium. Hence, exploring seven different hydrophobic solvents ranging the polarity from chloroform to hexane, the precursor activation was controlled and this helped in tuning the crystal phase, shape, the Mn doping efficiency and the yield in the formed nanocrystals. This was possible by using Pb-thiolate as Pb precursor which controllably released Pb2+ in different reaction medium for the perovskite formation. For CsPbBr3, while in chloroform led to nearly monodisperse nanocubes of orthorhombic phase; in hexane, these were turned to non-emitting trigonal phase Cs4PbBr6 nanostructures. However, for CsPbCl3 while all solvents led to the cubic phase, but their shape tuned from cube in chloroform to platelets in hexane. Being different solvents followed different rate of Pb release, the doping of Mn was also performed for observing the doping efficiency. Interestingly, the doping was observed more efficient for low yield and slow reaction in hexane medium showing 56 % quantum yield; but only retained 11% in chloroform. Using seven different organic solvents and analyzing the key thio-esterification reaction for the Pb activations, details of the reaction chemistry involved for these room temperature reactions in controlling the crystal phase, shape and doping are studied and reported in this article.


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Thiols are established as a bad reagent for perovskite synthesis for their strong and selective affinity towards Pb which triggers Pb depletion and changes the phase from CsPbBr3 to Cs4PbBr6.20, 45 In contrary, Pb-thiolate was observed here as the efficient and controlled Pb release precursor. The precursor was distinguished from the alkyl chain interdigitations packing and this was reflected from the periodicity of the peak positions (Figure 1a) in their powder Xray diffraction (XRD) pattern. An interval of 2.52 degree (2Ɵ) were observed in the powder XRD pattern of this thiolate aggregate and this corresponded to the interspacing distance equivalent to 3.5 nm. Even though several metal thiolate complexes of similar XRD patterns are reported; but fortunately, herein, well distinguished and intense XRD patterns were observed for the associated complex.46-49A schematic presentation of the polymeric Pd-thiolate association is represented in Figure 1b. Transmission electron microscopy (TEM) images showed these as thick sheet like morphology and Electron disperse X-ray spectroscopy(EDS) data confirmed the presence of lead and sulfur in the aggregate (Figure S1). For the reactions, hydrophobic solvents with little differences in their dielectric constants were chosen as the medium. The reaction protocols using Pb-thiolate in different solvents (Chloroform, o-Xylene, Toluene, m-Xylene, Benzene, p-Xylene and Hexane) and the phase/shape selective products for both CsPbBr3 and CsPbCl3 are shown schematically in Figure 1c. For CsPbBr3, contrasting results were observed while the reactions were carried out in solvent chloroform and hexane. Where the former led to phase-pure orthorhombic phase (CsPbBr3) cube shaped nanocrystals, the later led to phase-pure trigonal phase (Cs4PbBr6) polyhedral shaped nanostructures. However, for all other solvents which had dielectric constants in between chloroform and hexane led to mixed phase and mixed shape 5 ACS Paragon Plus Environment


nanostructures. In contrary, for CsPbCl3, only cubic phase nanocrystals were obtained in all solvents; but importantly, chloroform led to cube shape, hexane to platelets and other solvents resulted mixed shape products.

Figure 1. (a) Powder XRD pattern of Pb-thiolate aggregates. (b) Schematic presentation of interdigitations of the alkylthiol ligands and the spacing between two layers. In this case 1-DDT was used for synthesizing lead thiolate. (c) Schematic presentation of the reaction carried out in seven hydrophobic solvents for the formation of cesium lead bromide and chloride nanocrystals. For cesium lead bromide, chloroform led to phase pure orthorhombic phase, hexane to phase pure trigonal phase and other solvents to mixed phase of cesium lead bromide nanostructures. The dielectric constants are provided for each solvent.50 For cesium lead chloride, all solvents led to cubic phase; but chloroform led to cube shape and hexane to platelets. 6 ACS Paragon Plus Environment

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The entire process adopted here involves only one solvent where all previous room temperature synthesis reports followed dual solvent approach.33 For initiating the Pb-release, alkyl carboxylic acid was used which showed different kinetics in different solvents and controlled the growth process of the resulting perovskite nanocrystals. In addition, Cs-oleate was used as Cs and oleylammonium halides were used as halide source (see Experimental Section in Supporting Information) for carrying out the reactions.

Figure 2. Powder X-ray diffraction patterns of the nanocrystals obtained from the room temperature reactions carried out in solvents chloroform, toluene, benzene, o-, m- and pxylenes and hexane for both cesium lead halides (Br and Cl).



Figure 2 shows the powder XRD patterns of the products from the reactions carried out using all these seven solvents under identical conditions for both bromide and chloride systems. As marked in dashed lines for cesium lead bromide perovskites, the decrease of the intensities of (002) and (110) peaks of orthorhombic phase of CsPbBr3 and appearance with successive increase of peak intensity of (012) peaks of trigonal phase of Cs4PbBr6 clearly reflected the phase tuning of these nanostructures obtained from the solvent variation reactions. This also reflected that in chloroform and hexane phase pure CsPbBr3 and Cs4PbBr6 nanocrystals were obtained where in all other solvents led to mixed phase nanocrystals. However, for CsPbCl3, phase retained cubic for all solvents; but the intensities of (100) and (110) peaks were varied as per the shape variations from cube in chloroform to platelet in hexane. Figure 3a and 3b present the absorption and corresponding photoluminescence (PL) spectra for the obtained CsPbBr3 and Cs4PbBr6 nanostructures. The absorption spectra showed CsPbBr3 nanocrystals obtained from chloroform medium has absorption in visible region; but Cs4PbBr6 remained in UV region. Similarly, CsPbBr3 showed the emission at 510 nm; but for Cs4PbBr6, almost no emission was observed. These two materials were also viewed under electron microscope where CsPbBr3 retained the expected cube shape; but Cs4PbBr6 has mostly polyhedral shape (Figure 3c and Figure 3d). The PLQY for the cube shape nanostructures obtained in chloroform solvent remained ~68%. The absorption and the PL spectra of the nanocrystals, and respective TEM images obtained from other solvents are presented in Figure S2-S9 in Supporting Information. Their nature also supports the mixer of both phase and shape of nanostructures. Figure 3e and 3f presents the absorption and PL spectra of CsPbCl3 nanostructures obtained in chloroform and hexane respectively. Platelets being thinner, these 8 ACS Paragon Plus Environment

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showed the blue shifting in the optical spectra because of quantum confinement.32, 51 The PLQY for these nanocrystals remained less than 10%.

Figure 3. (a) Absorption spectra for cesium lead bromide nanocrystals obtained from the reactions carried out in solvent chloroform and hexane and (b) corresponding PL spectra. (c) TEM image of CsPbBr3 nanocrystals obtained from chloroform medium and (d) TEM image of Cs4PbBr6 nanocrystals obtained from hexane medium. (e) Absorption spectra and (f) corresponding PL spectra of cesium lead chloride nanocrystals carried out in chloroform and hexane respectively. (g) and (h) Representative TEM images of CsPbCl3 showing cube and platelets shapes.



Figure 3g and 3h showed the TEM images of respective samples; from chloroform cubes were formed and hexane led to platelet nanostructures. Optical spectra and TEM images obtained from other solvents are presented in Figure S10-S17 in supporting information.

Figure 4. Successive (a) absorption and (b) corresponding PL spectra of a typical reaction of formation of CsPbBr3 nanocrystals and carried out using thiolate activation process in chloroform solvent. Beyond the above representative data, the successive optical spectral evolutions during a typical CsPbBr3 nanocrystals formation reaction were also recorded and presented in Figure 4. The absorption spectra in Figure 4a show the red-shifting and enhancement of the absorption with the progress of the reaction. Similarly, the PL spectra in Figure 4b also showed the intensity enhancement and also red tuning the emission. However, the red-shifting observed here was in too small window; but confirmed the successive growth of the nanostructures. The completion of this reaction took approximately 5 min and certainly this could be treated as the


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slower reaction in comparison to the previously reported dual solvent or even with the hot injection approaches.11, 33

Figure 5. (a-b) Absorption and corresponding PL spectra of Mn doped CsPbCl3 nanocubes and nanoplatelets obtained from the reactions carried out in chloroform and hexane medium respectively. Inset of panel 6b shows the digital image of the cuvette under illumination (excitation wavelength 365 nm). (c) and (d) corresponding TEM images showing cube and platelet/sheet shapes of the doped nanocrystals respectively. (e)Excited state decay lifetime plot of Mn doped CsPbCl3nanocrystals prepared in hexane (em= 600nm). Inset showing the parameters for the fitting of decay curve. (f) PLQY histogram of Mn d-d emission for the doped nanocrystals obtained from the reactions performed in seven different solvent medium. (g) and (h) Electron paramagnetic resonance spectra of Mn doped CsPbCl3 cubes and platelets obtained in solvents chloroform and hexane respectively.



Being these were comparatively slower reactions, doping of Mn2+ were also carried out for these reactions in all solvents for both cesium lead bromide and chloride nanocrystals. Having high bandgap, CsPbCl3 on doping emitted the orange Mn d-d emission. Cube shapes obtained here could dope

Solvent Polarity: How does this Influence the Precursor Activation

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