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Journal Pre-proofs Research paper Highly Luminescent CsPbI3 Quantum Dots and Their Fast Anion Exchange at Oil/Water Interface Kun He, Yanqing Zhu, Zhuoneng Bi, Xiaoli Chen, Xiudi Xiao, Gang Xu, Xueqing Xu PII: DOI: Reference:
S0009-2614(20)30011-7 https://doi.org/10.1016/j.cplett.2020.137096 CPLETT 137096
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Chemical Physics Letters
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17 November 2019 1 January 2020 7 January 2020
Please cite this article as: K. He, Y. Zhu, Z. Bi, X. Chen, X. Xiao, G. Xu, X. Xu, Highly Luminescent CsPbI3 Quantum Dots and Their Fast Anion Exchange at Oil/Water Interface, Chemical Physics Letters (2020), doi: https://doi.org/10.1016/j.cplett.2020.137096
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Highly Luminescent CsPbI3 Quantum Dots and Their Fast Anion Exchange at Oil/Water Interface Kun Hea,b, Yanqing Zhua*, Zhuoneng Bia,b, Xiaoli Chena, Xiudi Xiaoa, Gang Xua， Xueqing Xua,b* a
Guangzhou Institute of Energy Conversion, Key Laboratory of Renewable Energy,
Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Chinese Academy of Sciences, Guangzhou 510640, PR China b
Center of Materials Science and Optoelectronics Engineering, University of Chinese
Academy of Sciences, Beijing 100049, PR China * Corresponding authors. E-mail addresses: [email protected] (Xueqing Xu), [email protected](Yanqing Zhu)
1
Abstract： In this paper, the highly emissive CsPbI3 perovskite quantum dots (QDs) with good stability are successfully synthesized with trioctylphosphine (TOP) as surface ligands. Furthermore, a facile and fast anion exchange is realized at the interface of nonpolar CsPbX3 solution and aqueous hydrohalic acid. Perovskite QDs can react with halide ions at room temperature swiftly to adjust their ratio of halogens and then the emission. The photoluminescence (PL) can be tuned over wide spectral region (423.0670.5 nm). And this work also provides a fast avenue to tune the emission of halide perovskite QDs. Keywords：CsPbX3; quantum dots; anion exchange; hot-injection; photoluminescence
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1 Introduction Nowadays, inorganic metal-halide CsPbX3 (X=Cl, Br, I) perovskite materials have gained significant attention owing to their excellent properties. Because of the absence of small organic groups, CsPbX3 perovskite materials possess better thermal and chemical stability compared with hybrid organic-inorganic perovskite counterparts [1]. Besides, CsPbX3 perovskite materials are low-cost and easy to be synthesized [2]. Above all, CsPbX3 perovskite materials possess excellent photoelectric properties, including satisfactory photoluminescence quantum yields (PLQY), narrow emission bandwidths, strong absorption coefficients and tunable band gaps [2-4]. These excellent properties make these semiconducting materials be considered as a promising candidate for solid-state lighting, laser gain media, solar light absorption layer and so on [5-8]. CsPbX3 perovskite QDs are developing nanocrystals with excellent luminescent properties and they can be synthesized by simple synthetic methods such as hotinjection and supersaturated recrystallization [2, 9]. However, CsPbX3 QDs face a big challenge of poor stability due to weak binding of surface ligands via traditional synthetic method. It increases the density of trapping defects, quench luminescence and break the crystal structure after losing surface ligands [3]. Some superior surface ligands, such as alkyl phosphinic acid, 2, 2’-iminodibenzoic acid, TOP and so on, are tried to improve crystal quality, optical properties and stability [3, 10-12]. Ideal surface ligands not only decrease trapping defects, but also increase crystal stability. Among them, TOP is an appealing choice because of its lone pair electrons and strong coordination ability. It is reported that PLQY of CsPbI3 QDs passivated with TOP is up 3
to 100% previously [3]. However, the long reaction time about 7 days and poor repeatability for organolead compound trioctylphosphine-PbI2 impede their further research [3]. Improvement scheme is necessary to be put forward to decrease reaction period to synthesize CsPbX3 perovskite QDs with TOP as surface ligands. Another prominent property of CsPbX3 QDs is their ability to undergo a postsynthesis anion exchange to tune the emission. The high anionic diffusion and the dynamic surface ligands binding make it possible to realize anion exchange [13]. For anion exchange, the halogen sources are diverse, including metal halides MX2 (M=Pb, Zn, Mg or Cu; X=Cl, Br or I), oleylammonium halides (OAmX, X= Cl, Br or I), organometallic Grignard reagents (MeMgX, X= Cl, Br or I) and so on [1, 4, 13-17]. Normally, the anion exchange time is a little bit long due to the lack of halogen ions [4, 15]. But rapid reaction is advantageous in some special situation, for instance, halogen ions detection. It is well-known that halogen ions are easy to be acquired in polar solvent while the integrity of crystal structure for CsPbX3 QDs tends to be destroyed when they disperse in polar solvent because of their intrinsic ionic property [18, 19]. To solve this problem, we choose an immiscible system. It not only supplies enough halogen ions, but also avoids CsPbX3 QDs contact polar solvent directly. In this work, we optimize the hot-injection method reported before to synthesize CsPbI3 perovskite QDs [20]. TOP is chosen to replace traditional oleic acid and oleylamine as surface ligands and then CsPbI3 perovskite QDs with high PLQY up to 98% and satisfying stability are synthesized successfully. Subsequently, we choose usual hydrobromic acid (HBr) and hydrochloric acid (HCl) as the source of halogen 4
ions. A facile anion exchange is realized via an oil/water two-phase immiscible system to tune the emission spectra of CsPbX3 QDs from 423.0 to 670.5 nm.
2 Experiment section 2.1 Materials Cesium carbonate (Cs2CO3, Aladdin, AR, 99%), lead iodide (PbI2, TCI, 99.99%), 1-octadecene (ODE, Aladdin, 90%), oleic acid (OA, Aladdin, 90%), oleylamine (OLA, Aladdin, 80-90% ), trioctylphosphine (TOP, Aladdin, 90%), hydroiodic acid (HI, Aladdin, 55.0-58.0%), hydrobromic acid (HBr, Aladdin, 48%), hydrochloric acid (HCl, Guangzhou Chemical Reagent Factory, 36.0-38.0%),. n-hexane (C6H14,Tianjin FuYu, ≥98%), and methyl acetate (C3H6O2, Aladdin, 98%). 2.2 Synthesis of CsPbI3 perovskite QDs CsPbI3 perovskite QDs were synthesized according to previous Pradhan’s work with some modifications [20].Two kinds of precursor solution, including Cs-oleate and OLA-HI, were synthesized in advance. Cs-oleate was synthesized by 0.6 mmol Cs2CO3 and 1 ml OA in 9 ml ODE. The mixture was scrubbed with N2 for 1h at 120 °C and annealed at 150 °C for 20 min. OLA-HI was obtained by reaction between 1ml of HI and 10 ml of OLA at 120 °C for 2h along with purging with N2. For the synthesis of CsPbI3, the mixture of 0.2 mmol PbI2 and 5 ml ODE were heated at 130 °C along with purging with N2 in a round bottomed flask. 1ml of TOP (or 0.5ml OA and 0.5ml OLA) was injected, following by gas scrubbing for 1h. Then the pre-heated precursor of OLA-HI (0.5 ml) was injected and the solution was kept for 10 min. The solution was heated to 160-240 °C (220°C for OA and OLA). Subsequently, 5
0.5 ml Cs-oleate was injected. After the annealing time of 1h, the system cooled down naturally. The QDs were dispersed in hexane and purified with C3H6O2 via centrifugation for 3 times and the parameter was 8000rpm for 5 min. For the synthesis of CsPbI3 using traditional hot-injection, 0.2 mmol PbI2 and 5 ml ODE were heated at 120°C. 0.5ml OA and 0.5ml OLA were injected following by gas scrubbing for 1h. Then it was heated at 160°C, following by injecting 0.5 ml of Csoleate. 5s later, the system cooled down in an ice bath and purified the QDs as mentioned above. 2.3 Preparation of CsPbBrXI3-X and CsPbClXBr3-X perovskite QDs For the CsPbBrXI3-X, 1ml of CsPbI3 solution was loaded in centrifugal tubes and then different amounts of HBr from 0 to 15µl were added. Subsequently, the solution was shaken lightly to obtain colorful solutions and the reaction time was less than one minute. For the CsPbClXBr3-X, the CsPbI3 solution was mixed with excess HBr to synthesize CsPbBr3 solution. Subsequently 0-50 µl of HCl was added in 1 ml CsPbBr3 solution. 2.4 Characterization The morphology, crystal structure and composition of these samples were analyzed by Transmission Electron Microscopy (TEM, JEOL JEM-2100F), Selected Area Electron Diffraction (SAED), X-ray diffraction measurement (XRD, Rigaku SmartLab3KW) and Energy Dispersive Spectrometer (EDS, JEOL JEM-2100F). UVVis spectra, PL spectra and PL decay curve was recorded by using PerkinElmer 6
Lambda750 spectrophotometer, USA PerkinElmer ls 55 spectrometer and Edinburgh instruments FLS980 respectively. PLQY was obtained by using Absolute PLQY Spectrometer (c11347, Hamamatsu). The excitation wavelength of CsPbBrxI3-x and CsPbClxBr3-x was 450 nm and 350 nm, respectively. The FTIR spectra were recorded using the BRUKER TENSOR27 instrument by drop casting the dispersed solution on KBr pallet. For stability test, these CsPbX3 QDs were dispersed in hexane. Thin films were prepared via solvent evaporation under low air pressure at 80 °C and photographed in the atmosphere.
3 Results and discussion
Fig. 1. (a) XRD patterns, (b) Absorption spectra, (c) normalized PL spectra and (d) optical stability of CsPbI3 perovskite QDs synthesized at different temperature. (e) Time-resolved PL decay curve of CsPbI3 perovskite QDs synthesized at 200 °C. TEM images a of CsPbI3 perovskite QDs synthesized at (f) 160 °C, (g) 180 °C, (h) 200 °C, (i) 220 °C and (j) 240°C. In this typical synthesis, it is important to form organolead compound. It is worth 7
mentioning that only when reaction temperature is 200 °C does the solution become clear. XRD patterns of the resulting QDs synthesized at 160 to 240 °C can be well indexed to the cubic CsPbI3 (ISCD: 161481) as shown in Fig. 1a. All the resulting QDs synthesized at different temperatures possess analogical absorption spectra and PL spectra as shown in Fig. 1b and 1c. It indicates that all resulting QDs have same electron structure of cubic CsPbI3. PL spectra show a slight red-shift along with increasing temperature because of quantum size effect [20]. It is of crucial importance for injection temperature to synthesize highly luminescent CsPbI3 QDs. PLQY of these CsPbI3 QDs differs at different temperatures, form 65 to 98%, and the best injection temperature is 200 °C. Such high PLQY means that detrimental nonradiative recombination centers are almost eliminated [3, 20]. The solution is turbid when temperature is not 200 °C, so there exists PbI2 without coordination in the reaction system. It is difficult to form [PbI6]2- octahedron for PbI2 and the density of crystal defects will increase. Besides, some lead without coordination on the surface tends to fall off and it increases surface trapping [20]. Surface ligands are also related to the stability and Fig. 1d shows the resulting CsPbI3 QDs synthesized in the range of 180~240 °C possess satisfactory stability. The PL kinetics of radiative relaxation of excitons is also investigated in Fig. 1e and PL decay curve indicates CsPbI3 QDs synthesized at 200 °C possess fast emission with average radiative lifetimes of 50.01ns [9]. The detailed morphologies of these CsPbI3 QDs synthesized at 160~240 °C shows cubic shape as shown in Fig. 1f to 1j. However, some particles synthesized at 160 °C are destroyed as detected in Fig 1f and the degradation may account for the rapid decline in PLQY in Fig 1d. 8
Fig. 2. Optical stability of CsPbI3 QDs obtained from the reaction in presence of (a) OA and OLA at 160 °C, (b) OA, OLA and OLA-HI at 220 °C and (c) TOP and OLAHI at 200 °C. FTIR spectra of corresponding CsPbI3 QDs obtained from the reaction in the presence of (d) OA and OLA at 160 °C, (e) OA, OLA and OLA-HI at 220 °C and (f) TOP and OLA-HI at 200 °C. To investigate the stability of CsPbI3 QDs with different surface passivation, PL stability of CsPbI3 QDs with difference surface ligands is contrasted with successive time. The PL stability of CsPbI3 QDs with conventional approach is disappointing as shown in Fig. 2a and these CsPbI3 QDs suffer severe quenching because conventional ligands, OA and OLA, tend to fall off [20]. For comparison, CsPbI3 QDs synthesized at 220 °C in the presence of OLA-HI behave better in PL stability as shown in Fig. 2b. The surface binding of the ammonium ions with QDs is not dynamic and this strong binding strength contributes to high stability [20]. It is inspiring that CsPbI3 QDs with TOP as surface ligands possess the best stability among them as shown in Fig 2c. Such good stability is attributed to the firm bonds of ammonium ions with QDs and TOP 9
with lead. We carry out first-principle calculations for coordination binding energy via Materials Studio. It is found that the coordination binding energy of Pb2+ and TOP is 107.0 kJ/mol and such firm binding indicates that TOP is difficult to lose. Surface ligands are investigated according to FTIR spectra in Fig. 2d to 2f. COO binding (1551 cm-1) and N-H binding (1551 and 1640 cm-1) can be detected in both conventional hotinjection and hot-injection in the presence of OLA-HI. P-C bond at 1437 cm-1 can be detected for CsPbI3 with TOP as ligands. The results of FTIR spectra confirm all these surface ligands are binding to CsPbI3 QDs.
Fig. 3. (a) Absorption spectra, PL spectra, (b) PLQY and (c) XRD patterns of CsPbX3 QDs after anion exchange. (d) Photographs of CsPbX3 QDs under the UV-light of 365 nm. SAED patterns of (e) CsPbI3, (f) CsPbBr3 and (g) CsPb(Cl/Br)3 QDs. Changing halide reagents is a fine choice to tune emission, but the extremely low binding between TOP and PbBr2 (or PbCl2) makes it difficult to conduct the synthesis 10
[3]. Herein, a post-synthesis anion exchange is conducted to widen the emission. As shown in Fig. 3a, The PL emission can be tuned from 670.5 to 423.0 nm and absorption spectrum also shows blue-shift along with change in halogen ions. As for Brsuperseding I- in CsPbI3, The emission does not change after 12 µl HBr reacts with 1ml CsPbI3 solution even though more HBr is added. The PLQY drastically declines from 98% (CsPbI3) to 58% (CsPbBr3) after anion exchange in Fig. 3b. It is inevitable to generate some internal crystal defects or take off ligands after anion exchange and thus weaken luminance [16]. As for further tuning the emission, it is noticeable that HCl cannot react with CsPbI3. Because the formation energy of CsPbIxCl3-x is much higher than CsPbBrxCl3-x [21]. Furthermore, the miscibility gap temperature of 625 K for CsPbIxCl3-x indicates that hybrid CsPbIxCl3-x perovskite is hard to be formed at room temperature. [22]. And 40 µl HCl is enough exactly or excess slightly for 1 ml CsPbBr3 solution synthesized via anion exchange. The PLQY decreases further and becomes 11% after Cl- superseding Br- adequately (Fig. 3b). Fig. 3d clearly illustrates the different emission of CsPbX3 QDs. XRD patterns are also analyzed to investigated changes in lattice in Fig. 3c. The cubic CsPbI3 perovskite structure (ISCD: 161481) changes to orthorhombic perovskite structure of CsPbBr3 (ISCD: 28312) gradually. After chlorine superseding bromine in CsPbBr3, XRD pattern is indexed to the orthorhombic CsPbCl3 (ISCD: 243734), but the diffraction angle is less than theoretical value. It is likely that chlorine cannot supersede all the bromine and the bigger bromine expands the lattice [14]. To further confirm the crystal structure, SAED patterns are analyzed. There exist three obvious circles observed with radius of 2.23, 3.09 and 4.32 1/nm in reciprocal 11
space as shown in Fig 3e. They stand for 4.48 Å, 3.24 Å and 2.31 Å in real space and match cubic perovskite crystal of CsPbI3 (ISCD: 161481). Analyzed in the same way, CsPbBr3 QDs accord with orthorhombic structure (ISCD: 28312) in Fig. 3f. And after chlorine superseding bromine in CsPbBr3, the SAED pattern shows that further structure changes to orthorhombic structure of CsPbCl3 (ISCD: 243734), but the interplanar spacing calculated by Fig. 3g is about 6% bigger than theoretical value.
Fig. 4. TEM images of (a) CsPbI3, (b) CsPbI1.5Br1.5, (c) CsPbBr3 (d) CsPbBr2Cl1 and (e) CsPbCl1.7Br1.3 perovskite QDs. The insets are corresponding HRTEM images and size distribution. EDS patterns of these CsPbX3 perovskite QDs after anion exchange. To investigate morphology changes of these CsPbX3 perovskite QDs, TEM images are analyzed in Fig. 4. All CsPbX3 perovskite QDs have analogous morphology of squares, indicating anion exchange does not change the pristine morphology [23]. But the size of these QDs varies a little as counted in insets. The average size of CsPbI3 QDs is 8.48 nm. After CsPbI3 reacts with HBr, the average size increases to 8.55 nm (reacted with 6µl HBr, Fig 4b) and further to 9.05 nm (reacted with 12µl HBr, Fig 4c). 12
The increase in size is related to the volume of unit cell of cubic CsPbI3 and orthorhombic CsPbBr3 (249 Å3 for cubic CsPbI3 and 751 Å3 for orthorhombic CsPbBr3). After chlorine superseding bromine, the size decreases to 8.93nm (reacted with 20µl HCl, Fig 4d) and further to 8.58 nm (reacted with 40µl HCl, Fig 4e). It is owing to lattice contraction after smaller chlorine superseding bromine partially. These HRTEM images show all these QDs are highly crystalline. The extent of anion exchange are investigated according to EDS patterns as shown in Fig 4f. After anion exchange, iodine in CsPbI3 is almost replaced while chlorine cannot supersede all the bromine in CsPbBr3. The final ratio of chlorine to bromine is about 1.7:1.3 after sufficient reaction.
Fig. 5. PL stability of (a) CsPbBr3 and (b) CsPbBrxCl3-x QDs after anion exchange to the full extent. (c) Thermal stability of CsPbX3 perovskite QDs when fabricated as thin films. (d) Reaction mechanism of anion-exchange reaction. Stability of these CsPbX3 QDs is also investigated after anion exchange. After anion exchange, CsPbBr3 and CsPbCl1.7Br1.3 perovskite QDs keep photoluminescence 13
after four weeks when stored in hexane as shown in Fig 5a and 5b. And CsPbIxBr3-x shows better thermal stability than CsPbBrxCl3-x QDs when fabricated as thin films in Fig 5c. At last, a rational mechanism has been put forward to describe the anion exchange in Fig. 5d. When HBr is added into CsPbI3 solution, an immiscible system is formed owing to a striking difference of polarity. Proton and Br- are dispersed in water randomly and CsPbI3 QDs are dispersed in hexane. A part of Br- traverses the interface to replace iodine ions on the surface of grains. The Br- on the surface of grains moves to the internal of crystals along I-I bond of [PbI6]2- octahedron and I- migrates to aqueous phase until all iodine ions in CsPbI3 are superseded [24]. As for chlorine superseding bromine, the chemical mechanism is also related to diffusion of halogen ions. After CsPbI3 changed to CsPbBr3 perovskite QDs, the [PbX6]2- octahedron tilts and diffusion is affected. Furthermore, the binding energies indicate that superseding bromine in perovskite crystals is more difficult than iodine (the binding energy of bromine is 40 meV while iodine is 20 meV in CsPbX3 perovskite [2]). Hence all the bromine is not superseded and it is CsPb(Cl/Br)3 that be synthesized.
4 Conclusion In summary, owing to the combined effect of OLA-HI and TOP, surface defects of CsPbI3 perovskite QDs are passivated and the crystalline integrity is retained. CsPbI3 QDs passivated with TOP possess high PLQY of 98% and high stability. Such high PLQY exhibits potential application in LEDs. Subsequently, fast anion exchange is realized through an immiscible system to tune the emission in the range from 423.0 to 670.5 nm. Moreover, the reaction mechanism of halogen diffusion is adopted to 14
illustrate how anion exchange proceeds. Halogen ions traverse the interface of the two immiscible solutions and replace other halogen ions. This work provides a fast and efficient way to tune the emission of CsPbX3 QDs at room temperature and it exhibits potential application in detecting halogen content in wastewater. Acknowledgements This work was supported by the Key Project on Synergy Collaborative Innovation of Guangzhou City [201704030069], Project on the Collaborative Innovation and Environmental Construction Platform of Guangdong Province. [2018A050506067] National Key R&D Program of China [No.2018YFD0700200], Project of Science and Technology Service Network initiative, Chinese Academy of Sciences [KFJ-STSQYZD-010], Youth Innovation Promotion Association CAS [No.2017400], Special Plan of Guangdong Province [2015TQ01N714], Natural Science Foundation of Guangdong Province [2018A0303130146]
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Credit Author Statement Manuscript title: Highly Luminescent CsPbI3 Quantum Dots and Their Fast Anion Exchange at Oil/Water Interface.
The article has been written by the stated authors who are all aware of its content and approve its submission. This manuscript and its contents in some other form, have not been published previously by any of the authors, and also have not under consideration for publication in any other journal. No conflict of interest exists. If accepted, the article will not be published elsewhere in the same form, in any language, without the written consent of the publisher.
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Declaration of Interest Statement There is no conflicts of interest in this work.
21
Graphical Abstract
CsPbI3 perovskite quantum dots with TOP as surface ligands possess good optical property such as high PLQY and excellent stability of crystal structure when stored in hexane. And CsPbI3 perovskite quantum dots can transfer to CsPbBr3 and further CsPbCl1.7Br1.3 to tune the emission via anion exchange. The difference in solution polarity results in an immiscible system and this system not only supplies enough halide ions, but also prevents CsPbX3 perovskite QDs from contacting polar solvent directly.
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Highlights
1. TOP is chosen as surface ligands and highly emissive CsPbI3 QDs are synthesized. 2. CsPbI3 QDs with TOP as surface ligands possess good stability. 3. Anion exchange is realized via an immiscible system to tune the emission.
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S0009-2614(20)30011-7 https://doi.org/10.1016/j.cplett.2020.137096 CPLETT 137096
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
17 November 2019 1 January 2020 7 January 2020
Please cite this article as: K. He, Y. Zhu, Z. Bi, X. Chen, X. Xiao, G. Xu, X. Xu, Highly Luminescent CsPbI3 Quantum Dots and Their Fast Anion Exchange at Oil/Water Interface, Chemical Physics Letters (2020), doi: https://doi.org/10.1016/j.cplett.2020.137096
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Highly Luminescent CsPbI3 Quantum Dots and Their Fast Anion Exchange at Oil/Water Interface Kun Hea,b, Yanqing Zhua*, Zhuoneng Bia,b, Xiaoli Chena, Xiudi Xiaoa, Gang Xua， Xueqing Xua,b* a
Guangzhou Institute of Energy Conversion, Key Laboratory of Renewable Energy,
Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Chinese Academy of Sciences, Guangzhou 510640, PR China b
Center of Materials Science and Optoelectronics Engineering, University of Chinese
Academy of Sciences, Beijing 100049, PR China * Corresponding authors. E-mail addresses: [email protected] (Xueqing Xu), [email protected](Yanqing Zhu)
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Abstract： In this paper, the highly emissive CsPbI3 perovskite quantum dots (QDs) with good stability are successfully synthesized with trioctylphosphine (TOP) as surface ligands. Furthermore, a facile and fast anion exchange is realized at the interface of nonpolar CsPbX3 solution and aqueous hydrohalic acid. Perovskite QDs can react with halide ions at room temperature swiftly to adjust their ratio of halogens and then the emission. The photoluminescence (PL) can be tuned over wide spectral region (423.0670.5 nm). And this work also provides a fast avenue to tune the emission of halide perovskite QDs. Keywords：CsPbX3; quantum dots; anion exchange; hot-injection; photoluminescence
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1 Introduction Nowadays, inorganic metal-halide CsPbX3 (X=Cl, Br, I) perovskite materials have gained significant attention owing to their excellent properties. Because of the absence of small organic groups, CsPbX3 perovskite materials possess better thermal and chemical stability compared with hybrid organic-inorganic perovskite counterparts [1]. Besides, CsPbX3 perovskite materials are low-cost and easy to be synthesized [2]. Above all, CsPbX3 perovskite materials possess excellent photoelectric properties, including satisfactory photoluminescence quantum yields (PLQY), narrow emission bandwidths, strong absorption coefficients and tunable band gaps [2-4]. These excellent properties make these semiconducting materials be considered as a promising candidate for solid-state lighting, laser gain media, solar light absorption layer and so on [5-8]. CsPbX3 perovskite QDs are developing nanocrystals with excellent luminescent properties and they can be synthesized by simple synthetic methods such as hotinjection and supersaturated recrystallization [2, 9]. However, CsPbX3 QDs face a big challenge of poor stability due to weak binding of surface ligands via traditional synthetic method. It increases the density of trapping defects, quench luminescence and break the crystal structure after losing surface ligands [3]. Some superior surface ligands, such as alkyl phosphinic acid, 2, 2’-iminodibenzoic acid, TOP and so on, are tried to improve crystal quality, optical properties and stability [3, 10-12]. Ideal surface ligands not only decrease trapping defects, but also increase crystal stability. Among them, TOP is an appealing choice because of its lone pair electrons and strong coordination ability. It is reported that PLQY of CsPbI3 QDs passivated with TOP is up 3
to 100% previously [3]. However, the long reaction time about 7 days and poor repeatability for organolead compound trioctylphosphine-PbI2 impede their further research [3]. Improvement scheme is necessary to be put forward to decrease reaction period to synthesize CsPbX3 perovskite QDs with TOP as surface ligands. Another prominent property of CsPbX3 QDs is their ability to undergo a postsynthesis anion exchange to tune the emission. The high anionic diffusion and the dynamic surface ligands binding make it possible to realize anion exchange [13]. For anion exchange, the halogen sources are diverse, including metal halides MX2 (M=Pb, Zn, Mg or Cu; X=Cl, Br or I), oleylammonium halides (OAmX, X= Cl, Br or I), organometallic Grignard reagents (MeMgX, X= Cl, Br or I) and so on [1, 4, 13-17]. Normally, the anion exchange time is a little bit long due to the lack of halogen ions [4, 15]. But rapid reaction is advantageous in some special situation, for instance, halogen ions detection. It is well-known that halogen ions are easy to be acquired in polar solvent while the integrity of crystal structure for CsPbX3 QDs tends to be destroyed when they disperse in polar solvent because of their intrinsic ionic property [18, 19]. To solve this problem, we choose an immiscible system. It not only supplies enough halogen ions, but also avoids CsPbX3 QDs contact polar solvent directly. In this work, we optimize the hot-injection method reported before to synthesize CsPbI3 perovskite QDs [20]. TOP is chosen to replace traditional oleic acid and oleylamine as surface ligands and then CsPbI3 perovskite QDs with high PLQY up to 98% and satisfying stability are synthesized successfully. Subsequently, we choose usual hydrobromic acid (HBr) and hydrochloric acid (HCl) as the source of halogen 4
ions. A facile anion exchange is realized via an oil/water two-phase immiscible system to tune the emission spectra of CsPbX3 QDs from 423.0 to 670.5 nm.
2 Experiment section 2.1 Materials Cesium carbonate (Cs2CO3, Aladdin, AR, 99%), lead iodide (PbI2, TCI, 99.99%), 1-octadecene (ODE, Aladdin, 90%), oleic acid (OA, Aladdin, 90%), oleylamine (OLA, Aladdin, 80-90% ), trioctylphosphine (TOP, Aladdin, 90%), hydroiodic acid (HI, Aladdin, 55.0-58.0%), hydrobromic acid (HBr, Aladdin, 48%), hydrochloric acid (HCl, Guangzhou Chemical Reagent Factory, 36.0-38.0%),. n-hexane (C6H14,Tianjin FuYu, ≥98%), and methyl acetate (C3H6O2, Aladdin, 98%). 2.2 Synthesis of CsPbI3 perovskite QDs CsPbI3 perovskite QDs were synthesized according to previous Pradhan’s work with some modifications [20].Two kinds of precursor solution, including Cs-oleate and OLA-HI, were synthesized in advance. Cs-oleate was synthesized by 0.6 mmol Cs2CO3 and 1 ml OA in 9 ml ODE. The mixture was scrubbed with N2 for 1h at 120 °C and annealed at 150 °C for 20 min. OLA-HI was obtained by reaction between 1ml of HI and 10 ml of OLA at 120 °C for 2h along with purging with N2. For the synthesis of CsPbI3, the mixture of 0.2 mmol PbI2 and 5 ml ODE were heated at 130 °C along with purging with N2 in a round bottomed flask. 1ml of TOP (or 0.5ml OA and 0.5ml OLA) was injected, following by gas scrubbing for 1h. Then the pre-heated precursor of OLA-HI (0.5 ml) was injected and the solution was kept for 10 min. The solution was heated to 160-240 °C (220°C for OA and OLA). Subsequently, 5
0.5 ml Cs-oleate was injected. After the annealing time of 1h, the system cooled down naturally. The QDs were dispersed in hexane and purified with C3H6O2 via centrifugation for 3 times and the parameter was 8000rpm for 5 min. For the synthesis of CsPbI3 using traditional hot-injection, 0.2 mmol PbI2 and 5 ml ODE were heated at 120°C. 0.5ml OA and 0.5ml OLA were injected following by gas scrubbing for 1h. Then it was heated at 160°C, following by injecting 0.5 ml of Csoleate. 5s later, the system cooled down in an ice bath and purified the QDs as mentioned above. 2.3 Preparation of CsPbBrXI3-X and CsPbClXBr3-X perovskite QDs For the CsPbBrXI3-X, 1ml of CsPbI3 solution was loaded in centrifugal tubes and then different amounts of HBr from 0 to 15µl were added. Subsequently, the solution was shaken lightly to obtain colorful solutions and the reaction time was less than one minute. For the CsPbClXBr3-X, the CsPbI3 solution was mixed with excess HBr to synthesize CsPbBr3 solution. Subsequently 0-50 µl of HCl was added in 1 ml CsPbBr3 solution. 2.4 Characterization The morphology, crystal structure and composition of these samples were analyzed by Transmission Electron Microscopy (TEM, JEOL JEM-2100F), Selected Area Electron Diffraction (SAED), X-ray diffraction measurement (XRD, Rigaku SmartLab3KW) and Energy Dispersive Spectrometer (EDS, JEOL JEM-2100F). UVVis spectra, PL spectra and PL decay curve was recorded by using PerkinElmer 6
Lambda750 spectrophotometer, USA PerkinElmer ls 55 spectrometer and Edinburgh instruments FLS980 respectively. PLQY was obtained by using Absolute PLQY Spectrometer (c11347, Hamamatsu). The excitation wavelength of CsPbBrxI3-x and CsPbClxBr3-x was 450 nm and 350 nm, respectively. The FTIR spectra were recorded using the BRUKER TENSOR27 instrument by drop casting the dispersed solution on KBr pallet. For stability test, these CsPbX3 QDs were dispersed in hexane. Thin films were prepared via solvent evaporation under low air pressure at 80 °C and photographed in the atmosphere.
3 Results and discussion
Fig. 1. (a) XRD patterns, (b) Absorption spectra, (c) normalized PL spectra and (d) optical stability of CsPbI3 perovskite QDs synthesized at different temperature. (e) Time-resolved PL decay curve of CsPbI3 perovskite QDs synthesized at 200 °C. TEM images a of CsPbI3 perovskite QDs synthesized at (f) 160 °C, (g) 180 °C, (h) 200 °C, (i) 220 °C and (j) 240°C. In this typical synthesis, it is important to form organolead compound. It is worth 7
mentioning that only when reaction temperature is 200 °C does the solution become clear. XRD patterns of the resulting QDs synthesized at 160 to 240 °C can be well indexed to the cubic CsPbI3 (ISCD: 161481) as shown in Fig. 1a. All the resulting QDs synthesized at different temperatures possess analogical absorption spectra and PL spectra as shown in Fig. 1b and 1c. It indicates that all resulting QDs have same electron structure of cubic CsPbI3. PL spectra show a slight red-shift along with increasing temperature because of quantum size effect [20]. It is of crucial importance for injection temperature to synthesize highly luminescent CsPbI3 QDs. PLQY of these CsPbI3 QDs differs at different temperatures, form 65 to 98%, and the best injection temperature is 200 °C. Such high PLQY means that detrimental nonradiative recombination centers are almost eliminated [3, 20]. The solution is turbid when temperature is not 200 °C, so there exists PbI2 without coordination in the reaction system. It is difficult to form [PbI6]2- octahedron for PbI2 and the density of crystal defects will increase. Besides, some lead without coordination on the surface tends to fall off and it increases surface trapping [20]. Surface ligands are also related to the stability and Fig. 1d shows the resulting CsPbI3 QDs synthesized in the range of 180~240 °C possess satisfactory stability. The PL kinetics of radiative relaxation of excitons is also investigated in Fig. 1e and PL decay curve indicates CsPbI3 QDs synthesized at 200 °C possess fast emission with average radiative lifetimes of 50.01ns [9]. The detailed morphologies of these CsPbI3 QDs synthesized at 160~240 °C shows cubic shape as shown in Fig. 1f to 1j. However, some particles synthesized at 160 °C are destroyed as detected in Fig 1f and the degradation may account for the rapid decline in PLQY in Fig 1d. 8
Fig. 2. Optical stability of CsPbI3 QDs obtained from the reaction in presence of (a) OA and OLA at 160 °C, (b) OA, OLA and OLA-HI at 220 °C and (c) TOP and OLAHI at 200 °C. FTIR spectra of corresponding CsPbI3 QDs obtained from the reaction in the presence of (d) OA and OLA at 160 °C, (e) OA, OLA and OLA-HI at 220 °C and (f) TOP and OLA-HI at 200 °C. To investigate the stability of CsPbI3 QDs with different surface passivation, PL stability of CsPbI3 QDs with difference surface ligands is contrasted with successive time. The PL stability of CsPbI3 QDs with conventional approach is disappointing as shown in Fig. 2a and these CsPbI3 QDs suffer severe quenching because conventional ligands, OA and OLA, tend to fall off [20]. For comparison, CsPbI3 QDs synthesized at 220 °C in the presence of OLA-HI behave better in PL stability as shown in Fig. 2b. The surface binding of the ammonium ions with QDs is not dynamic and this strong binding strength contributes to high stability [20]. It is inspiring that CsPbI3 QDs with TOP as surface ligands possess the best stability among them as shown in Fig 2c. Such good stability is attributed to the firm bonds of ammonium ions with QDs and TOP 9
with lead. We carry out first-principle calculations for coordination binding energy via Materials Studio. It is found that the coordination binding energy of Pb2+ and TOP is 107.0 kJ/mol and such firm binding indicates that TOP is difficult to lose. Surface ligands are investigated according to FTIR spectra in Fig. 2d to 2f. COO binding (1551 cm-1) and N-H binding (1551 and 1640 cm-1) can be detected in both conventional hotinjection and hot-injection in the presence of OLA-HI. P-C bond at 1437 cm-1 can be detected for CsPbI3 with TOP as ligands. The results of FTIR spectra confirm all these surface ligands are binding to CsPbI3 QDs.
Fig. 3. (a) Absorption spectra, PL spectra, (b) PLQY and (c) XRD patterns of CsPbX3 QDs after anion exchange. (d) Photographs of CsPbX3 QDs under the UV-light of 365 nm. SAED patterns of (e) CsPbI3, (f) CsPbBr3 and (g) CsPb(Cl/Br)3 QDs. Changing halide reagents is a fine choice to tune emission, but the extremely low binding between TOP and PbBr2 (or PbCl2) makes it difficult to conduct the synthesis 10
[3]. Herein, a post-synthesis anion exchange is conducted to widen the emission. As shown in Fig. 3a, The PL emission can be tuned from 670.5 to 423.0 nm and absorption spectrum also shows blue-shift along with change in halogen ions. As for Brsuperseding I- in CsPbI3, The emission does not change after 12 µl HBr reacts with 1ml CsPbI3 solution even though more HBr is added. The PLQY drastically declines from 98% (CsPbI3) to 58% (CsPbBr3) after anion exchange in Fig. 3b. It is inevitable to generate some internal crystal defects or take off ligands after anion exchange and thus weaken luminance [16]. As for further tuning the emission, it is noticeable that HCl cannot react with CsPbI3. Because the formation energy of CsPbIxCl3-x is much higher than CsPbBrxCl3-x [21]. Furthermore, the miscibility gap temperature of 625 K for CsPbIxCl3-x indicates that hybrid CsPbIxCl3-x perovskite is hard to be formed at room temperature. [22]. And 40 µl HCl is enough exactly or excess slightly for 1 ml CsPbBr3 solution synthesized via anion exchange. The PLQY decreases further and becomes 11% after Cl- superseding Br- adequately (Fig. 3b). Fig. 3d clearly illustrates the different emission of CsPbX3 QDs. XRD patterns are also analyzed to investigated changes in lattice in Fig. 3c. The cubic CsPbI3 perovskite structure (ISCD: 161481) changes to orthorhombic perovskite structure of CsPbBr3 (ISCD: 28312) gradually. After chlorine superseding bromine in CsPbBr3, XRD pattern is indexed to the orthorhombic CsPbCl3 (ISCD: 243734), but the diffraction angle is less than theoretical value. It is likely that chlorine cannot supersede all the bromine and the bigger bromine expands the lattice [14]. To further confirm the crystal structure, SAED patterns are analyzed. There exist three obvious circles observed with radius of 2.23, 3.09 and 4.32 1/nm in reciprocal 11
space as shown in Fig 3e. They stand for 4.48 Å, 3.24 Å and 2.31 Å in real space and match cubic perovskite crystal of CsPbI3 (ISCD: 161481). Analyzed in the same way, CsPbBr3 QDs accord with orthorhombic structure (ISCD: 28312) in Fig. 3f. And after chlorine superseding bromine in CsPbBr3, the SAED pattern shows that further structure changes to orthorhombic structure of CsPbCl3 (ISCD: 243734), but the interplanar spacing calculated by Fig. 3g is about 6% bigger than theoretical value.
Fig. 4. TEM images of (a) CsPbI3, (b) CsPbI1.5Br1.5, (c) CsPbBr3 (d) CsPbBr2Cl1 and (e) CsPbCl1.7Br1.3 perovskite QDs. The insets are corresponding HRTEM images and size distribution. EDS patterns of these CsPbX3 perovskite QDs after anion exchange. To investigate morphology changes of these CsPbX3 perovskite QDs, TEM images are analyzed in Fig. 4. All CsPbX3 perovskite QDs have analogous morphology of squares, indicating anion exchange does not change the pristine morphology [23]. But the size of these QDs varies a little as counted in insets. The average size of CsPbI3 QDs is 8.48 nm. After CsPbI3 reacts with HBr, the average size increases to 8.55 nm (reacted with 6µl HBr, Fig 4b) and further to 9.05 nm (reacted with 12µl HBr, Fig 4c). 12
The increase in size is related to the volume of unit cell of cubic CsPbI3 and orthorhombic CsPbBr3 (249 Å3 for cubic CsPbI3 and 751 Å3 for orthorhombic CsPbBr3). After chlorine superseding bromine, the size decreases to 8.93nm (reacted with 20µl HCl, Fig 4d) and further to 8.58 nm (reacted with 40µl HCl, Fig 4e). It is owing to lattice contraction after smaller chlorine superseding bromine partially. These HRTEM images show all these QDs are highly crystalline. The extent of anion exchange are investigated according to EDS patterns as shown in Fig 4f. After anion exchange, iodine in CsPbI3 is almost replaced while chlorine cannot supersede all the bromine in CsPbBr3. The final ratio of chlorine to bromine is about 1.7:1.3 after sufficient reaction.
Fig. 5. PL stability of (a) CsPbBr3 and (b) CsPbBrxCl3-x QDs after anion exchange to the full extent. (c) Thermal stability of CsPbX3 perovskite QDs when fabricated as thin films. (d) Reaction mechanism of anion-exchange reaction. Stability of these CsPbX3 QDs is also investigated after anion exchange. After anion exchange, CsPbBr3 and CsPbCl1.7Br1.3 perovskite QDs keep photoluminescence 13
after four weeks when stored in hexane as shown in Fig 5a and 5b. And CsPbIxBr3-x shows better thermal stability than CsPbBrxCl3-x QDs when fabricated as thin films in Fig 5c. At last, a rational mechanism has been put forward to describe the anion exchange in Fig. 5d. When HBr is added into CsPbI3 solution, an immiscible system is formed owing to a striking difference of polarity. Proton and Br- are dispersed in water randomly and CsPbI3 QDs are dispersed in hexane. A part of Br- traverses the interface to replace iodine ions on the surface of grains. The Br- on the surface of grains moves to the internal of crystals along I-I bond of [PbI6]2- octahedron and I- migrates to aqueous phase until all iodine ions in CsPbI3 are superseded [24]. As for chlorine superseding bromine, the chemical mechanism is also related to diffusion of halogen ions. After CsPbI3 changed to CsPbBr3 perovskite QDs, the [PbX6]2- octahedron tilts and diffusion is affected. Furthermore, the binding energies indicate that superseding bromine in perovskite crystals is more difficult than iodine (the binding energy of bromine is 40 meV while iodine is 20 meV in CsPbX3 perovskite [2]). Hence all the bromine is not superseded and it is CsPb(Cl/Br)3 that be synthesized.
4 Conclusion In summary, owing to the combined effect of OLA-HI and TOP, surface defects of CsPbI3 perovskite QDs are passivated and the crystalline integrity is retained. CsPbI3 QDs passivated with TOP possess high PLQY of 98% and high stability. Such high PLQY exhibits potential application in LEDs. Subsequently, fast anion exchange is realized through an immiscible system to tune the emission in the range from 423.0 to 670.5 nm. Moreover, the reaction mechanism of halogen diffusion is adopted to 14
illustrate how anion exchange proceeds. Halogen ions traverse the interface of the two immiscible solutions and replace other halogen ions. This work provides a fast and efficient way to tune the emission of CsPbX3 QDs at room temperature and it exhibits potential application in detecting halogen content in wastewater. Acknowledgements This work was supported by the Key Project on Synergy Collaborative Innovation of Guangzhou City [201704030069], Project on the Collaborative Innovation and Environmental Construction Platform of Guangdong Province. [2018A050506067] National Key R&D Program of China [No.2018YFD0700200], Project of Science and Technology Service Network initiative, Chinese Academy of Sciences [KFJ-STSQYZD-010], Youth Innovation Promotion Association CAS [No.2017400], Special Plan of Guangdong Province [2015TQ01N714], Natural Science Foundation of Guangdong Province [2018A0303130146]
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Credit Author Statement Manuscript title: Highly Luminescent CsPbI3 Quantum Dots and Their Fast Anion Exchange at Oil/Water Interface.
The article has been written by the stated authors who are all aware of its content and approve its submission. This manuscript and its contents in some other form, have not been published previously by any of the authors, and also have not under consideration for publication in any other journal. No conflict of interest exists. If accepted, the article will not be published elsewhere in the same form, in any language, without the written consent of the publisher.
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Declaration of Interest Statement There is no conflicts of interest in this work.
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Graphical Abstract
CsPbI3 perovskite quantum dots with TOP as surface ligands possess good optical property such as high PLQY and excellent stability of crystal structure when stored in hexane. And CsPbI3 perovskite quantum dots can transfer to CsPbBr3 and further CsPbCl1.7Br1.3 to tune the emission via anion exchange. The difference in solution polarity results in an immiscible system and this system not only supplies enough halide ions, but also prevents CsPbX3 perovskite QDs from contacting polar solvent directly.
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Highlights
1. TOP is chosen as surface ligands and highly emissive CsPbI3 QDs are synthesized. 2. CsPbI3 QDs with TOP as surface ligands possess good stability. 3. Anion exchange is realized via an immiscible system to tune the emission.
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