Solvent-Free Mechanosynthesis of Composition-Tunable Cesium

Letter pubs.acs.org/JPCL

Solvent-Free Mechanosynthesis of Composition-Tunable Cesium Lead Halide Perovskite Quantum Dots Zhi-Yuan Zhu, Qi-Qi Yang, Lin-Feng Gao, Lei Zhang, An-Ye Shi, Chun-Lin Sun, Qiang Wang,* and Hao-Li Zhang* State Key Laboratory of Applied Organic Chemistry (SKLAOC), Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, Lanzhou University, Lanzhou 730000, China S Supporting Information *

ABSTRACT: A facile and green mechanosynthesis strategy free of solvent and high reaction temperature was developed to fabricate highly emissive cesium lead halide perovskite (CsPbX3) quantum dots (QDs). Their composition can be adjusted conveniently simply through mechanically milling/grinding stoichiometric combinations of raw reagents, thereby introducing a broad luminescence tunability of the product with adjustable wavelength, line width, and photoluminescence quantum yield. Desired CsPbX3 QDs “library” can thus be readily constructed in a way like assembling Lego building blocks. Hence, the method offered new avenues in the preparation of multicomponent cocrystals, adding one appealing apparatus to the tool box of perovskite-type QDs synthesis. Intriguingly, photoinduced dynamic study revealed the hole-transfer process of the as-prepared QDs toward electron donors, indicative of their potential in chargetransfer-based applications such as light-harvesting devices and photocatalysis.

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organic frameworks (MOFs),18,19 organohalide perovskites,20,21 graphene,22 black phosphorus (BP), and carbon hybrid materials.23,24 The mechanochemical method cleverly harnesses the cocrystal formation process of readily accessible individual components by simply grinding/milling them together.25 In our case, CsPbX3 QDs can be conveniently constructed by stoichiometrically ball-milling or grinding CsX and PbX2 salts (X = Cl, Br, I, or their mixture) at room temperature without dissolving them in solution first. The halide composition can be tuned flexibly through various combinations of Cl, Br, and I content, thereby introducing a broad luminescence tunability of the product with adjustable wavelength, line width, and photoluminescence (PL) quantum yield. A desired CsPbX3 product “library” can thus be readily constructed in a way like assembling Lego building blocks. Hence, the method offered new avenues in the preparation of multicomponent cocrystals, adding one appealing apparatus to the tool box of perovskitetype QDs synthesis. Moreover, a photoinduced dynamic study revealed the hole-transfer process of the as-prepared QDs toward electron donors such as triethylamine (TEA), indicative of their potential in charge-transfer-based applications such as light-harvesting devices and photocatalysis. The synthesis strategy is depicted in Figure 1a. The metal salts of CsX and PbX2 were used as the immediate raw materials and led to the products directly in one single step simply after milling/grinding at ambient temperature (Section

ll-inorganic perovskite-type quantum dots (QDs) with the structural formula of CsPbX3 (X = ClxBryI1−x−y, 0 ≤ x, y ≤ 1) have emerged as a promising complement to their hybrid organic−inorganic counterpart of CH3NH3PbX3 (X are the same single or mixed halides), which have been a rising star in lighting, display, and solar cell field, in particular, but plagued by chemical instability issues.1−5 CsPbX3 QDs, on the contrary, exhibit much more robustness while maintaining high fluorescence quantum yield (QY), high photostability, and widely tunable spectral range and hence have found potentials in broad optoelectronic devices like light-emitting diodes (LEDs),6,7 nanolasers,8−10 photodetectors,11 solar cells,12 and even biological imaging.13 However, for both types of QDs current synthesis strategies are mostly limited to tedious and toxic solution synthesis methods, in which volatile organic solvent, inert gas protection, and high reaction temperature are usually routine and severely stymie their practical use (Section S1, Supporting Information).7 Several recent pieces of literature proposed new methods to improve the synthesis procedures. For instance, Akkerman et al. exploited short propionic acid ligands and isopropanol, butylamine, and hexane-solventrealized room-temperature synthesis. 12 Other attractive achievements on low/room-temperature reaction7,14 or polarsolvent free synthesis15 have also appeared. However, a onestep synthesis simultaneously with low room temperature and complete solvent-free environment is still beyond reach. With this limitation in mind, we note that mechanosynthesis is a solid-state synthesis method capable of bypassing solubility issues,16 potentially for scalable production, and has been successfully employed to fabricate nanoparticles,17 metal− © XXXX American Chemical Society

Received: February 21, 2017 Accepted: March 22, 2017 Published: March 22, 2017 1610

DOI: 10.1021/acs.jpclett.7b00431 J. Phys. Chem. Lett. 2017, 8, 1610−1614

Letter

The Journal of Physical Chemistry Letters

Figure 1. (a) Schematic of mechanosynthesis of CsPbX3 QDs (with CsPbBr3 as one representative). (b) Photographs of the sample color changes and fluorescence under UV light irradiation during ball milling.

CsPbCl2Br1, CsPbBr1.5I1.5, or similar combinations, to maintain a perovskite crystal structure. We characterized the mechanosynthesized CsPbBr3 QDs (written as “1”) as the representative. As displayed by Figure 2a, the QDs exhibit cubic structures with average lateral size of

S2, Supporting Information). The rationale is that the mechanical mixing, especially high-speed milling, induces heat, supplying reaction energy in a similar way as heating or ultrasonication needed for the cocrystal formation.19,25 Unlike the solution synthesis of nanoparticles, where the researchers have accumulated deep understanding toward the growth mechanism and harnessed the knowledge to construct rather complicated hybrid assemblies,26−30 the mechanochemical reactions under a microscopic perspective remain much to be explored.16 Kaupp once proposed a three-step mechanism to explain the molecular solid reactivity:19,31 First, reactants diffused through a mobile phase like amorphous solid and chemical reaction occurred; second, the product phase underwent nucleation and growth; finally, the product separated and fresh reactant surface was exposed for further reaction cycles. These basic processes may also apply to the reaction of CsX and PbX2 herein. Take the mechanosynthesis of CsPbBr3 QDs as an example. Within a few minutes, the color of the initial CsBr and PbBr2 mixture transformed into yellow, indicative of the rapid formation of CsPbBr3 QDs or bulk products (Figure 1b). The result was verified by XRD measurements, where the main diffraction peaks of the products were in good accordance with that of CsPbBr3 (ICSD 97851) but distinct from those of pure CsBr and/or PbBr2 metal salts (Figure S1). However, to obtain highly luminescent QDs, further grilling/milling and, even more important, surface treatment of the grinding perovskite product with appropriate ligands were still necessary.14,32,33 In this case, we used a small amount of oleylamine (OAm), with the assistance of which the surface of the QDs was functionalized and would demonstrate strong fluorescence (Figure 1b). A detailed discussion of the role of the OAm ligands is in the Supporting Information (Section S3.2 and Figures S2−S6). We then systematically compared the influence of milling conditions such as rotation speed and milling duration on the QDs formation (Section S3.3 and Figures S7−S11, Supporting Information). The method provides a rich toolbox for “cooking” a broad range of perovskite QDs. One can actually adjust the “recipe” freely by choosing different raw material combinations stoichiometrically to obtain CsPbX3 QDs, where X represents single Cl, Br, I, or mixed halides such as Cl and Br, Br and I, and so on. In any case, the halide stoichiometric number remains to be 3 in the formula, for example,

Figure 2. TEM images of CsPbBr3 QDs 1 (a) and 2 (b) and corresponding HR-TEM images are shown as insets. (c) XRD patterns of CsPbBr3 QDs 1 and 2. (d) Normalized single- (λex = 390 nm) and two- (λex = 780 nm) photon excited fluorescence spectra of CsPbBr3 QDs 1 and 2.

∼9.7 nm, and the statistical histogram particle size distribution was provided in Figure S12. The inset was the high-resolution TEM (HR-TEM), from which a lattice space of ∼0.58 nm corresponding to the (110) lattice face of orthorhombic perovskite crystal phase was observed.34−36 In comparison, Figure 2b showed the morphology of CsPbBr3 QDs (written as “2”) developed in our lab with trioctylphosphine (TOPO), OAm as the ligands, and 1-octadecene (ODE) as the solvent, which demonstrated similar size of ∼8.5 nm, cubic shape, and lattice space of ∼0.58 nm (inset) as that of 1. Figure S12 also displayed the statistical histogram particle size distribution of 2. Lattice space of ∼0.41 nm was also found for both types of QDs (Figure S13), which could be assigned to the (020) lattice face and further supported the assignment of the orthorhombic 1611

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The Journal of Physical Chemistry Letters phase (ICSD 97851).35 The XRD patterns of the two types of QDs are mostly the same (Figure 2c), with the peaks at 15.2, 21.5, and 30.7° attributed to the lattice space of (110), (020), and (220), respectively, according to previous literature.34,35 The diffraction patterns, especially the narrow split of diffraction peaks at ∼15° (see the enlarged inset in Figure 2c) and ∼21°, reflected orthorhombic phase of the perovskite QDs,7,34 consistent with that of TEM results. Clearly, the mechanochemical method produced high-crystalline-quality QDs comparable to those prepared by conventional hot-inject protocol but with great advantages of facile and green procedures. The most attractive properties of perovskite QDs lie in their strong photoluminescence. The intrinsic single photon excited fluorescence spectra of 1 and 2 are displayed in Figure 2d, with same emission maximum around 518 nm and similar narrow full width at half-maximum (fwhm) of ∼21 nm and very high absolute QY of 44 and 38%, respectively, determined by using an integrating sphere (Section S4.1, Supporting Information). The strong luminescence characteristics endow these QDs with potential in lighting and display field. As a proof-of-concept, the milled CsPbBr3 QDs can be fabricated into films to coat commercial UV LEDs and emitted strongly under current (inset in Figure 2d). Besides, both 1 and 2 also exhibited attractive two-photon excited fluorescence properties, the spectra of which nearly overlapped with that of single-photon excited fluorescence (Figure 2d), in agreement with previous literature.8,37 The mechanosynthesis strategy was then expanded to fabricate perovskite QDs with varied halide compositions/ ratios. Figure 3 displayed the characterization of a series of representative perovskite QDs with tunable composition and emission peaks within a broad range. There were several interesting trends that could be extracted from these data. First, using CsPbCl3, CsPbBr3, and CsPbI3 as boundaries, one could clearly observe that the first excitonic absorption peaks of the QDs gradually moved from the shortest 353 nm of CsPbCl3 to

longer wavelengths with increasing heavier halide content, all the way to 645 nm of CsPbI3 (Figure 3a and Table S1). The photoluminescence of the samples was also controllable in the same way. Upon light irradiation, a broad emission range spanned the whole visible region from emission maximum of 409 nm for CsPbCl3 to 518 nm of CsPbBr3 and 658 nm of CsPbI3 (Figure 3b), covering the full colors of blue, green, yellow, and red. More comprehensive comparisons for the representative QDs are shown in the Supporting Information (Table S1, Figures S14 and S15) Fluorescence lifetime measurement was further employed to explore the exited-state behaviors of the QDs. As displayed in Figure 4a, with CsPbBr3 QDs as the representative, three

Figure 4. Fluorescence decay profiles (a) and steady-state fluorescence quenching (b) of CsPbBr3 QDs with gradual addition of TEA (indicated by the arrows). (c) Schematic electronic energy-level diagram for the hole transfer from CsPbBr3 QDs to TEA. (d) Stern− Volmer plots (QDs concentration: 0.293 O.D. at 390 nm).

lifetime components of 1.7, 8.3, and 41.0 ns were extracted from the corresponding decay profile (Table S2, Supporting Information). The short (1.7 and 8.3 ns) and the long (41.0 ns) lifetimes were expected to stem from bound and free excitons, respectively.7,38 The lifetime was averaged to be 10.1 ns, which was intriguingly shortened when TEA, a frequently used reducing agent (“sacrificial reagent”) during photocatalysis, was present.39,40 Likewise, during the steady-state fluorescence titration, the emission intensity of QDs also gradually weakened with the stepwise addition of TEA (Figure 4b). Both observations suggested a possible occurrence of charge-transfer process between the species (Figure S16). We then carried out ultraviolet photoelectron spectroscopic (UPS) measurement, which gave a valence band (VB) edge of −6.61 eV for the QDs (Figure S17). Tauc plot was adopted to extract the optical bandgap of 2.41 eV (Figure S18),41,42 and then conduction band (CB) edge of −4.20 eV could be deducted for the QDs. Meanwhile, the HOMO and LUMO levels of −5.13 and −3.82 eV for TEA were obtained via electrochemical measurements (Figure S19). On the basis of the information, the electronic energy-level diagram could be drawn as Figure 4c. A large driving force of 1.48 eV substantiated that hole transfer from the QD valence band to the HOMO level of TEA was energetically favored. The above observed charge transfer behavior was in agreement with previous works where Lian’s group reported the charge-transfer phenomena of perovskite QDs with electron

Figure 3. (a) Stacked UV−vis absorption spectra, (b) normalized fluorescence spectra, and photographs without (upper c) and with (lower c) UV light irradiation of a series of mechanosynthesized CsPbX3 QDs in toluene. The arrows show the samples in the order of CsPbCl 3 . 0 , CsPbCl 2 . 0 Br 1 . 0 , CsPbCl 1 . 5 Br 1 . 5 , CsPbCl 1 . 0 Br 2 . 0 , CsPbCl0.5Br2.5, CsPbBr3.0, CsPbBr2.0I1.0, CsPbBr1.5I1.5, CsPbBr1.0I2.0, CsPbBr0.5I2.5 and CsPbI3.0. 1612

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acceptors/donors.43 Ultrafast electron transfer rate of ∼65 ps to benzoquinone and hole transfer rate of ∼49 ps to phenothiazine were deduced, where static quenching occurred as the adsorbates bound on the QD surface due to electrostatic interaction. The latest report by Begum et al. also found ultrafast interfacial electron transfer between Bi3+ ion-doped CsPbBr3 QDs and tetracyanoethylene (TCNE) and PC61BM on the picosecond time scale.42 However, in our case, there was no identifiable spectral variation when TEA was mixed with QDs, implying that static quenching would be negligible (Figure S20). The quantitative Stern−Volmer analysis, where the averaged lifetime component (TCSPC measurement) or the integrated emission intensity (steady-state emission measurement) changes of QDs was plotted versus increased TEA concentration (i.e., τ0/τ or F0/F vs cTEA), was employed to extract the charge-transfer rate (Figure 4d).39,40 The near overlap between the two linear fittings verified a dynamic quenching with a large charge-transfer rate constant of (∼1.1 to 1.2) × 1010 M−1 s−1 within the diffusion-controlled limit.44,45 The efficient charge-transfer behaviors between perovskite QDs with electron donor/acceptors may spur their applications in photocatalysis and optoelectronic conversion devices such as LEDs and solar cells.43 In this study, we opened a new avenue to fabricate inorganic perovskite-type QDs by mechanochemically grinding/milling them into luminescent multicomponent cocrystals. The facile and green strategy bypassed the routine use of toxic organic and inorganic solvent, high reaction temperature, and inert gas protection, enabling the construction of a CsPbX3 QD “library” with adjustable composition and photoluminescence properties as convenient as assembling Lego building blocks. The asprepared QDs demonstrated rapid charge-transfer properties with molecular electron donors upon photoexcitation, a merit desirable for optoelectronic conversion devices and photocatalytic applications based on charge-transfer dynamics. Hence the work provided not only fresh ammunition to the arsenal of CsPbX3 QD fabrication but also useful insights into their remarkable luminescence properties and excited-state dynamics.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (NSFC 21673106, 51525303, 21233001), 111 Project, and the Fundamental Research Funds for the Central Universities (lzujbky-2016-k08).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b00431. Experimental details of synthesis, characterization methods, experiments of milling effects on QDs formation, supplemental TEM, XRD, absorption and emission data, PLQY calculation procedures, fitting methods for fluorescence lifetime measurements, and UPS and CV measurements. (PDF)

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Letter

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.W.) *E-mail: [email protected] (H.-L.Z.). ORCID

Qiang Wang: 0000-0003-4008-5144 Notes

The authors declare no competing financial interest. 1613

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