Engineering the Photoluminescence of CsPbX3 (X = Cl, Br, and I

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Functional Nanostructured Materials (including low-D carbon)

Engineering the Photoluminescence of CsPbX3 (X= Cl, Br, and I) Perovskite Nanocrystals across the Full Visible Spectra with the Interval of 1 nm Huiwen Liu, Zhaoyu Liu, Wenzhe Xu, Liting Yang, Yi Liu, Dong Yao, Daqi Zhang, Hao Zhang, and Bai Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01930 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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ACS Applied Materials & Interfaces

Engineering the Photoluminescence of CsPbX3 (X= Cl, Br, and I) Perovskite Nanocrystals across the Full Visible Spectra with the Interval of 1 nm Huiwen Liu,†,§ Zhaoyu Liu,†,§ Wenzhe Xu,† Liting Yang,† Yi Liu,† Dong Yao,*,† Daqi Zhang,*,‡ Hao Zhang,*,† and Bai Yang†

†State

Key Laboratory of Supramolecular Structure and Materials, College of Chemistry,

Jilin University, Changchun 130012, P. R. China.

‡Department

of Thyroid Surgery, China-Japan Union Hospital, Jilin University, Changchun

130033, P. R. China.

ACS Paragon Plus Environment

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ABSTRACT: Fluorescent CsPbX3 (X=Cl, Br, I) perovskite nanocrystals (NCs) are compelling candidates for illumination and display applications because of the high photoluminescence quantum yields (PLQYs), narrow PL emission spectra, and in particular the potential to tune the emission spectra in the entire visible range. However, limited by the current preparation strategy, the successive adjustment of PL emission across the full visible spectral range with very small interval like conventional semiconductor quantum dots is still challenging. In this work, we demonstrate the capability to tune the PL emission of CsPbX3 NCs in the full visible range with the interval of 1 nm on the basis of a modified anion-exchange route. Highly luminescent CsPbCl3 NCs with PLQY up to 34.2% are foremost prepared using alkanoyl chlorides as the chlorine source and further employed to perform anion-exchange. A successive and accurate adjustment of the PL emission is achieved with the addition of ZnX2 (X=Br and I) aqueous solution and assisted by ultrasound to improve the reactivity of halogens in the anion-exchange. Besides the accurately tunable PL emission position, the as-


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prepared CsPbX3 NCs exhibit good phase/chemical stability, high PLQY, and narrow PL emission spectra.

KEYWORDS: Perovskites, CsPbX3, photoluminescence, anion-exchange, metal halides.


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Introduction CsPbX3 (X=Cl, Br, and I) perovskite nanocrystals (NCs) are ones of the most competitive candidates for the next-generation illumination and display applications due to their extraordinary optoelectronic properties.1-8 As the alternatives to well-developed semiconductor quantum dots (QDs),9-12 CsPbX3 NCs show higher photoluminescence quantum yields (PLQYs) up to 100%, narrower full-width-at-half-maximum (FWHM) down to 20 nm, and the tunable PL emission position covering the entire visible spectra.13-23 Unlike conventional semiconductor QDs, CsPbX3 NCs usually possess cube morphology with average size over 10 nm beyond the diameter of Bohr exciton, which makes it hardly possible to perform accurate control of PL emission by tailoring their size.13-15,24-33 The common strategy to achieve emission control of CsPbX3 NCs is anion-exchange, which refers to the exchange of halide ions in the as-prepared CsPbX3 NCs to realize broad halide composition on the basis of their high ion mobility.34-37 However,

the

conventional

anion-exchange

routes

suffer

from

complicated

pretreatment, inert reaction environment, excess ligand-induced NCs degradation and non-quantitative halide-exchange process, usually causing unpredictable PL shift rather


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than specific emission control.38,39 Although droplet-based microfluidic platform,40,41 assembly-induced contraction strategy,42 and thermodynamic size control method have successfully achieved good emission control of perovskite NCs in limited spectral range,43 the continuous emission adjustment in the full visible spectra with the interval of 1 nm is still challenging. Note that the accurate control of the PL emission of CsPbX3 NCs is actually the quantitative control of the amount of halide ions. The most commonly used anionexchange reagents, such as alkylammonium halides and surfactant-containing lead halide salts, are mainly soluble in non-polar organic solvents like toluene or hexane.3437,39

The incomplete ionization of halide sources makes it hard to accurately control the

amount of halide ions. When PbX2 (X=Br and I) solution is used as halide source, the lead and halide content are fed with fixed ratio, which is really hard to mention the accurate emission control by controlling the stoichiometric ratio of them.34,36 In addition, the reaction activity of most ligand-assisted anion-exchange reagents is relatively low, making the further anion-exchange process uncontrolled.39 Most importantly, the current methods for performing anion-exchange reactions are not perfect to maintain the crystal


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phase stability and therefore the emission of CsPbX3 NCs, which are sensitive to the management of reagents, solvents, and ligands.38,39 For example, CsPbBr3 NCs will convert partially or fully to Cs4PbBr6 NCs if excess oleylamine (OLA) is employed.44 The addition of alkylammonium bromide for anion-exchange process can also cause the unexpected evolution into CsPb2Br5 perovskite phase.45 In this respect, the key to perform precise emission control of CsPbX3 NCs is to establish a programmed and accurate anion-exchange route with high halide ions reactivity for preparing pure phase CsPbX3 NCs. Some efforts have been devoted to enhance the reactivity of halides in the anionexchange process. Benzoyl halides and trimethylsilyl halides are electrophilic reagents, which show high reactivity in the anion-exchange reaction by breaking halogen bond, releasing halide ions and promoting the substitution of the halide component in CsPbX3 NCs, though these reagents are toxic.38,39 Environmentally friendly metal halide solids MX2 (M=Zn, Mg, and Cu) are also used as halide sources to realize a fast anionexchange at room temperature, where inorganic halide salts greatly simplify the reaction process and speed up the reaction rate.46-48 Besides, the treatment of CsPbX3 NCs by


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excess MX2 (M=Zn or Cd) solution enhances the PLQY by decreasing the X ions defect on the surface of NCs.22,23 Although these salts are hardly soluble in non-polar solvents, which lower the efficiency of anion-exchange reaction, they are good for improving halide ions activity and accurate modulating the anion-exchange kinetics. Based on the Arrhenius equation, precursor concentration and activity, activation energy and reaction temperature are the determined parameters in the anion-exchange reaction. The concentration and activity of MX2 can be improved by introducing polar solvents, such as water and alcohol, into the reaction system to accelerate the ionization, because MX2 salts are strong electrolytes. In this scenario, water and alcohol are immiscible with toluene or n-octane. Additional ultrasound or microwave irradiation is required to promote the dispersion of MX2 solution droplets in organic phase, which lowers the activation energy by increasing the contact area of MX2 droplets and organic phase. Ultrasound and microwave also increase the temperature of reaction system, which further benefit the reaction kinetics of anion-exchange. In addition, as the host to perform anion-exchange, pristine CsPbCl3 NCs usually suffer from chlorine-deficiency problem, which leads to poor stability and the low PLQY


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below 5%.13,22 In this context, CsPbCl3 NCs are usually prepared using PbCl2 as both lead and chlorine source.13 Limited by the fixed cation-to-anion ratio in PbCl2, it is not possible to obtain chlorine-rich NCs and therefore a good emission property.13,22 Other methods, that employ alkylammonium chlorine to introduce additional ions, are also limited by the poor reactivity and fail to work with excess chlorine ions.49,50 Recently, Imran et. al proposed the use of benzoyl halides as halide precursors, which can be simply injected into the reactive mixtures containing lead cations and proper ligands to immediately trigger the nucleation and the growth of highly luminescent CsPbCl3 NCs.38 Although benzoyl halides are highly toxic, this method undoubtedly gives a novel route for improving the reactivity of chlorine sources and in particular the PLQYs of asprepared CsPbCl3 NCs after selecting less toxic alternatives. The host CsPbCl3 NCs with improved PLQYs will be certainly helpful to obtain highly luminescent bromide or iodine-containing NCs via anion-exchange. In this study, a successive and accurate adjustment of the PL emission peak position of CsPbX3 NCs in the full visible spectral range with the interval of 1 nm is demonstrated. Chloride-rich and highly luminescent CsPbCl3 NCs with PLQY up to


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34.2% are foremost prepared using alkanoyl chlorides as the chlorine source and employed to perform anion-exchange. ZnX2 (X=Br and I) aqueous solution is further employed as the halide sources and assisted by ultrasound to improve the reactivity of halogens in the anion-exchange. Besides the successive adjustment of the PL emission, the as-prepared CsPbX3 NCs exhibit good phase/chemical stability, high PLQY, and narrow FWHM of PL emission spectra.

EXPERIMENTAL SECTION Experimental details can be found in the Supporting Information.

RESULTS AND DISCUSSION Preparation of CsPbCl3 NCs with alkanoyl chlorides. In a typical preparation, Cs2CO3 and Pb(Ac)2 are dissolved and degassed in OLA, oleic acid (OA), and octadecene (ODE) at 120 °C in a three-neck flask. Subsequently, the solution is heated up to 190 °C, and oleoyl chloride (OCD) is rapidly injected into the reaction flask, triggering the nucleation and growth of NCs (see Experimental Section). The NCs products are centrifuged for purification and then dissolved with toluene. As revealed by transmission electron microscopy (TEM) observation, the as-prepared CsPbCl3 NCs show cubic


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morphology with the size of 9.4 ± 1.2 nm (Figure 1a). The cubic shape is similar to the CsPbCl3 NCs prepared by conventional hot-injection method, but the size is much smaller with a narrower size distribution (Figure S1 and S2).51-54 It also reveals that the CsPbCl3 NCs have legible crystal lattices and the corresponding interplanar distance is 3.96 Å with high-resolution TEM (HRTEM) observation (Figure 1b), which corresponds to the (101) plane of the tetragonal bulk CsPbCl3 (PDF#18-0366).49 Besides, it clearly shows that the NCs possess (101) and (200) planes of the tetragonal crystal structure on the basis of the selected area electron diffraction (SAED) pattern (Figure 1c). It further verifies that the CsPbCl3 NCs have the tetragonal bulk CsPbCl3 crystalline structure by the detection of X-ray diffraction (XRD) (Figure 1e).49 The as-prepared NCs indicate good dispersibility in non-polar solvents, such as noctane and toluene. The toluene solution of the NCs shows an absorption peak at 397 nm (Figure 1d), and the NCs solution exhibits a bright purple emission under 365 nm ultraviolet (UV) irradiation (Figure 1a inset). Besides, the CsPbCl3 NCs exhibit a narrow FWHM of 13 nm with the PL emission at 408 nm (Figure 1d). The PLQY of CsPbCl3 NCs is measured to be as high as 34.2%. Note that such a high PLQY is achieved only


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when a large excess of Cl precursors is employed. The molar feed ratio of OCD to Pb(Ac)2 is 6:1 (see Experimental Section). Energy-dispersive X-ray spectroscopy (EDX) and corresponding mapping images further prove the existence of Cs, Pb, and Cl in the as-prepared CsPbCl3 NCs with the molar ratio of 1:1:3.4 (Figure S3 and Figure 1f-i), confirming the composition of NCs as CsPbCl3 and the high Cl content. In contrast, when CsPbCl3 NCs are prepared using a lower amount of OCD, the products exhibit a weaker PL emission with PLQY lower than 7% (Figure S4), further indicating the contribution of excess Cl on the PL enhancement. In addition, the EDX and PLQY measurements of the CsPbCl3 NCs respectively prepared with PbCl2, small amount of OCD and excessive OCD as the Cl source further indicate that the PLQY positively correlates with the Cl content, which confirms the contribution of excess Cl on the PL enhancement (Figure S1, S3, S4 and Table S1). The enhanced PLQY is attributed to the self-passivation effect of excess Cl. Namely, the excess Cl atoms on CsPbCl3 NCs surface is capable to bind with Cs and Pb cations, thus inhibiting the trapping of excited electrons by eliminating surface defects and improving the efficiency of exciton


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recombination.17,55 This effect is similar to the surface passivation and PL enhancement of conventional semiconductor QDs by halogen ions.17,55 CsPbCl3 NCs are also prepared with other alkanoyl chlorides, including nonanoyl chloride (NCD) and decanoyl chloride (DCD) (Figure S5). As revealed by TEM (Figure S6a-c), PL emission and UV-vis absorption spectra (Figure S6d), XRD patterns (Figure S6e), the morphology, size, crystal phase and optical properties are same to those of the CsPbCl3 NCs prepared using OCD. Anion-exchange with ZnBr2 and ZnI2 aqueous solution. Conventional anion-exchange reaction usually involves complicated pretreatment under inert atmosphere.34-37 For example, OLA halides have been prepared overnight with OLA and HX under nitrogen gas and the products have to be in vacuum for evaporation.13 The anion source of PbX2 should be dissolved in ODE through a heat treatment.34,35 In comparison, our modified anion-exchange reaction is simply carried out at room temperature in open environment with the addition of specific amounts of ZnX2 aqueous solution into the n-octane or toluene solution of CsPbX3 NCs. It should be mentioned that both ZnBr2 and ZnI2 possess the highest solubility in water than other solvents, such as methanol, ethanol


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and toluene etc, which are 446g/100g H2O and 432/100g H2O at room temperature. The high solubility facilitates the formation of highly concentrated solution, permitting to introduce minimum amount of water into the reaction system. In contrast, the commonly used PbX2 salts fail to replace ZnX2 salts as the halogen source for anion-exchange, because of the much lower solubility in water. For example, the room-temperature solubility of PbBr2 is only 0.86g/100g H2O. The slight solubility of PbBr2 demands for excessive H2O, which may degrade the as-prepared NCs. As a result, in the current anion-exchange method, ZnX2 salts are more favorable. The water-to-toluene volume ratio is only 1:4000~1:130 in the current anion-exchange reaction. If excess water is introduced, the decomposition of perovskite NCs will occur and generate a number of intermediate phases.56 After 30 s ultrasound treatment, the NCs solution changes from colorless to green for ZnBr2 and green to red for ZnI2 (Scheme 1). In this context, water and toluene are immiscible. The aqueous solution of ZnX2 not only provides Br- or I- to support the anion-exchange reaction at water/toluene interface, but also enhances the reactivity of halide ions with fully ionization in aqueous media. The excess ZnX2 aqueous solution can be easily separated from the toluene solution by centrifugation


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after preparation. With the aid of ultrasound, the aqueous solution of ZnX2 is dispersed into many small droplets, increasing the contact area with toluene phase. Thus, the binding of Br- or I- with OLA and/or OA in toluene phase and therefore the anionexchange reaction is facilitated. Without ultrasound, the color change, which represents the anion-exchange, is rather slow. As indicated in Figure 2a-f, the as-prepared halide-exchanged CsPbX3 NCs show cubic shapes and remain unchanged after the exchange with Br- or I- anions. The XRD patterns further verify that the NCs possess the monoclinic CsPbBr3 (PDF#18-0364) crystalline structure (Figure 2h).37 The XRD diffraction peak gradually shifts to small angles with the halide contents changing from Cl to I, which is contributed to the lattice expansion by the substitution of larger I atoms for the smaller Cl atom.33,37 EDX analysis indicates the corresponding stoichiometric ratio of Cs, Pb, and X (Table S2, Figure S3 and S7). Under the detection limit of EDX measurement (Zn/Pb

Engineering the Photoluminescence of CsPbX3 (X = Cl, Br, and I

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