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Improving the Stability and Size Tunability of Cesium Lead Halide Perovskite Nanocrystals using Trioctylphosphine Oxide as the Capping Ligand Linzhong Wu, Qixuan Zhong, Di Yang, Min Chen, Huicheng Hu, Qi Pan, Haiyu Liu, Muhan Cao, Yong Xu, Baoquan Sun, and Qiao Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02963 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017

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Improving the Stability and Size Tunability of Cesium Lead Halide Perovskite Nanocrystals using Trioctylphosphine Oxide as the Capping Ligand Linzhong Wu, Qixuan Zhong, Di Yang, Min Chen, Huicheng Hu, Qi Pan, Haiyu Liu, Muhan Cao, Yong Xu, Baoquan Sun,* Qiao Zhang*

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, P. R. China KEYWORDS Perovskite; Cesium lead halide; Nanocrystal; Stability; TOPO

ABSTRACT. Recently, all-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) nanocrystals (NCs) have drawn wide attention because of their excellent optoelectronic properties and potential applications. However, one of the most significant challenges of such NCs is their low stability against protonic solvent. In this work, we demonstrate that by incorporating a highly-branched capping ligand, trioctylphosphine oxide (TOPO), into the traditional oleic acid/oleylamine system, monodisperse CsPbX3 NCs with excellent optoelectronic properties can be achieved at elevated temperatures (up to 260 oC). The size of such NCs can be varied in a relatively wide range. The capping of TOPO on NCs has been verified through FTIR

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measurement. More importantly, the presence of TOPO can dramatically improve the stability of CsPbX3 NCs against ethanol treatment. After ethanol treatment for 100 min, the emission intensity of TOPO-capped sample dropped only 5%, while that of non-TOPO capped NCs dropped up to 86%. This work may shed some light on the preparation and application of CsPbX3 NCs with higher stability.

Introduction Recently, lead halide perovskite materials with formula of APbX3 (where A = Cs+, CH3NH3+, HC(NH2)2+, or C6H5CH2CH2NH3+; X = Cl−, Br−, and I−) have attracted enormous attentions because of their excellent optoelectronic properties

and

potential

applications

in

various

fields,

including

photovoltaics,1-3 light emitting diode,4-6 lasing,7-10 photocatalysis11,

12

and

photo-detector,13 and so on. Although organic-inorganic hybrid perovskites show great promise regarding photoluminescence quantum yield (PLQY) and colour tunability, their intrinsic instability against moisture, oxygen, and light illumination, has limited their practical applications.14,

15

As a result, much

effort has been directed to the synthesis and applications of all-inorganic perovskite nanocrystals (CsPbX3 NCs) over the past several years.1,

9, 16-19

These all-inorganic perovskite NCs have shown distinguished photophysical properties, including composition- and size-controlled photoluminescence (PL) property, high PLQY of luminescence, narrow emission widths, and short radiative lifetime.5, 20 2 ACS Paragon Plus Environment

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Synthesis of CsPbX3 NCs can be achieved through solution- and gas-phase based protocols, such as hot-injection,18, temperature

reprecipitation,27

post-treatment,30,

31

21

anion exchange,22-26 room

solvothermal,28

ultrasonication,29

microwave-assisted32 and chemical vapour deposition

(CVD) methods.33-37 The size and shape of CsPbX3 NCs can be readily controlled by tuning the reaction parameters, including temperature, capping ligands, reaction duration, and so forth. Various shaped CsPbX3 NCs, such as nanocubes,9,

24, 30

nanoplates,38,

39

nanowires,40-43 and nanorods,44 have been

prepared. Despite the great progress in the preparation of CsPbX3 NCs, many challenges still remain. For example, although many methods have been developed to improve the stability of CsPbX3 NCs, it is still very difficult to retain their PL properties in the presence of protonic solvent, such as alcohol and water.45 It is therefore highly desired to develop some methods that can prepare CsPbX3 NCs with tuneable morphology and enhanced stabilities. Additionally, it will be great if the size of CsPbX3 NCs could be tuned in a relatively wide range.31 There are several methods that can be used to improve the stability of crystalline semiconductor NCs, such as encapsulation of NCs into a more robust shell,46, 47 higher temperature treatment to improve the crystallinity,48 and stronger binding capping ligands49 that can effectively protect NCs from aggregation and corrosion.50, 51 So far, oleic acid (OA) and oleylamine (OAm) are the most widely used capping ligands for the preparation of CsPbX3 NCs. 3 ACS Paragon Plus Environment

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The combination of OA and OAm can not only help to dissolve the precursors, but also act as the capping ligands to protect the NCs.

52

However, the

interaction between OA/OAm and CsPbX3 NCs is not very strong. As a result, the prepared CsPbX3 NCs are suffering from the loss of their excellent optoelectronic properties during the post-purification process.4 Branched capping ligands are usually regarded as good capping ligands for improving the stability of NCs because they can provide strong steric effect. For example, Luo et al. developed a branched capping strategy with (3-aminopropyl) triethoxysilane and improved the stability of CH3NH3PbBr3 perovskite NCs in polar solvent successfully.41 Huang and co-workers reported that after treated with a polyhedral oligomeric silsesquioxane (POSS), CsPbX3 NCs can be very stable in water for several months.53 As an important capping ligand, trioctylphosphine oxide (TOPO) has been widely used in the synthesis of semiconductor crystals, e.g., II-VI group NCs.54-57 Because of the highly branched molecular structure and relatively strong coordination ability of P=O group, TOPO is featured by high stability against high temperature treatment and effective protecting effect originated from the strong steric effect.58-60 Herein, by incorporating the branched capping ligand, TOPO, into the reaction system, monodisperse CsPbX3 NCs with enhanced stability have been successfully prepared. Because of the strong interaction between TOPO and CsPbX3 NCs surface, uniform NCs can be obtained at high temperature (up to 260 oC). In a striking contrast, only large agglomeration is obtained at such a 4 ACS Paragon Plus Environment

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high temperature in the absence of TOPO. The size of such NCs can be tuned in a relatively wide range. More importantly, the stability of CsPbBr3 NCs has been dramatically improved by capping with TOPO. Compared to CsPbBr3 NCs capped with OA and OAm, the TOPO-capped CsPbBr3 NCs are more stable against cleaning and corrosion of ethanol.

Experimental Materials Chemicals: lead halide (PbX2, ultradry 99.999%, Alfa), cesium carbonate (Cs2CO3, ≥99.0%, Sigma-Aldrich), trioctylphsphine oxide (TOPO, 99%, Sigma-Aldrich), cyclohexane (>99.5%, TCI), ethanol (AR, Sinopharm) were used as received without purification. 1-octadecene (ODE, tech. 90%, Alfa), oleic acid (OA, tech. 90%, Alfa), and oleylamine (OAm, 80-90%, Alfa), were degassed for 1 hour under vacuum at 110 oC and kept in glovebox before use. Synthesis of TOPO-capped CsPbX3 NCs. CsPbX3 (X = Cl, Br, I and mixed Cl/Br, Br/I) NCs were synthesized as described by Protesescu et al.9 with some minor modifications. To prepare cesium oleate (CsOL), 0.49 mmol of Cs2CO3 was loaded into a 25 mL three-neck flask along with 5 mL of dried ODE and 0.5 mL of dried OA. The reaction system was dried under vacuum at 120 oC for 0.5 h, and then heated under N2 to 150 oC until all Cs2CO3 were dissolved. In another three-neck flask, ODE (5 mL), OA (1.17 mmol), OAm (1.23 mmol) and TOPO (0.85 mmol) were loaded and dried under vacuum for 0.5 h at 120 5 ACS Paragon Plus Environment

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o

C. Then PbX2 (0.188 mmol) was added into the solution. After the complete

dissolving of PbX2 salt, the reaction temperature was raised to 160 oC-260 oC. Subsequently, the prepared CsOL solution (0.4 mL) was quickly injected into the PbX2 solution. A few seconds (5 s for 160 oC, 3 s for 220 oC and above) later, the reaction mixture was rapidly cooled down by using an ice-water bath. The purification was conducted after the solution was cooled down to room temperature. Purification of TOPO-capped CsPbX3 NCs. The products from the crude solution were separated by centrifugation. 3 mL of cyclohexane was added into the crude solution, and then the mixture was centrifuged at 7500 rpm for 6 min to remove the residual reactants. The supernatant was discarded. The precipitate was dispersed in 5 mL cyclohexane and centrifuged at 4000 rpm for 5 min. The supernatant was kept for further characterization. Stability test of CsPbBr3 NCs. CsPbBr3 NCs (160 oC without TOPO, 160 oC with TOPO, 220 oC without TOPO, 220 oC with TOPO) were dispersed in cyclohexane. The concentration of CsPbBr3 NCs was tuned by testing the intensity of ultraviolet-visible (UV-Vis) absorption of the solution until the position at 335 nm exhibited the same intensity. Then, 2.4 mL of solution was added into a cuvette and the PL measurements were conducted. After loading 10 µL ethanol into the cuvette containing CsPbBr3 NCs solution, the UV-Vis and PL were recorded at different time.


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Characterization. Powder X-ray diffraction (XRD) patterns were measured on a desktop diffractometer (D2 PHASER, Bruker, Germany) with a Cu Kα source. Transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) data were collected at 200 kV on a TECNAI G2 F20 from FEI, USA. All the UV−Vis spectra were recorded in the range of 300−800 nm by using the UV spectrometer (SPECORD S 600). The PL measurements were collected by a fluorescence spectrophotometer (FLUOROMAX-4).

Results and discussion

Figure 1. (a-e) TEM images of (a) CsPbBr3, (b) CsPbCl3, (c) CsPb(Cl/Br)3, (d) CsPb(Br/I)3, and (e) CsPbI3 NCs, obtained at 160 oC in the presence of TOPO. In a typical experiment, Cs-oleate (CsOL) was first prepared by dissolving Cs2CO3 in ODE in the presence of OA. Then, PbX2, TOPO, OA and OAm were 7 ACS Paragon Plus Environment

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dissolved in ODE in a three-neck flask and heated to desired temperature. The formation of CsPbX3 NCs was initiated by the rapid injection of CsOL solution. The reaction system was quickly terminated by immersing the flask into an ice-bath. To investigate the influence of TOPO, the concentration of TOPO was set as 0.12 M and the reaction temperature was set as 160 oC. As shown in Figure 1, monodisperse CsPbX3 NCs were obtained in all cases. Figure 1a presents a typical transmission electron microscope (TEM) image of CsPbBr3 NCs. The obtained NCs are cube-like structure with an average edge length of ~ 9.8 nm, which is similar to the NCs obtained in the absence of TOPO. When bromide ions were replaced by chloride or iodide ions, cube-like structures could still be obtained (Figure 1b-1e). From this point of view, the presence of TOPO has no significant effect on the morphology of CsPbX3 NCs. A unique feature of the all-inorganic cesium lead halide perovskite NCs is their composition-dependent PL properties. In the presence of TOPO, colourful CsPbX3 nanocrystal solutions have been prepared. As shown in Figure 2a, the colour of solution can be steadily tuned from violet to blue, green, orange and red by varying the anions from chloride to bromide and iodide. Because of the highly uniform size distribution of CsPbX3 NCs, the products show sharp absorption and emission peaks, as plotted in Figure 2b. We take CsPbBr3 nanocrystal as the example. The prepared NCs have a sharp absorption and emission peak at 506 nm and 516 nm, respectively, which are consistent with the reported values.9 It is worth noting that the full width at half maximum 8 ACS Paragon Plus Environment

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(FWHM) of emission spectrum is only 22 nm, further confirming the high uniformity of prepared NCs. The emission peak of such CsPbX3 NCs can be readily tuned between 405 nm (CsPbCl3) and 688 nm (CsPbI3), covering the entire visible-light range. As a result, the NCs may be used for LEDs and display applications. CsPbCl3, CsPb(Cl/Br)3, CsPb(Br/I)3 and CsPbI3 NCs show sharp emission peaks at 405, 445, 577 and 688 nm with a narrow FWHM of 13, 12, 27, and 37 nm, respectively.

Figure 2. (a) Photographs of CsPbX3 NCs in cyclohexane under UV light (λ = 365 nm). (b) Absorption (solid line) and PL (dash line) spectra of CsPbX3 NCs (λexc= 400 nm for all except CsPbCl3 sample (excited at 350 nm)). (c) XRD patterns of CsPbX3 (X = Cl, Br, I, and mixed Cl/Br, Br/I) NCs. (d) FTIR spectroscopy of CsPbBr3 samples capped with OA/OAm (black line), and with TOPO (red line), and pure TOPO (blue line). 9 ACS Paragon Plus Environment

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The crystal structures of obtained NCs have been characterized by powder X-ray diffraction (XRD). As shown in Figure 2c, all samples can be indexed as cubic phase (JCPDS card No. 54-0752). With the anion changing from chloride to bromide and iodide, the characteristic peaks shift gradually to lower 2θ range. This phenomenon can be interpreted by the fact that the radii of halide ions increased from chloride to bromide and iodide.9,

24

TOPO anchoring on the

crystal surface has been confirmed by the Fourier transform infrared spectroscopy (FTIR) study. The FTIR spectra of OA/OAm capping CsPbBr3, as-prepared CsPbBr3 NCs with additional TOPO and pure TOPO are shown in Figure 2d. To remove excessive TOPO in the solution, the CsPbBr3 sample has been washed with hexane twice. C-H stretching vibrations (from 2850 to 3100 cm-1) and in-plane bending vibration (1465 cm-1) can be clearly observed.61 The weak peak at 1155 cm-1 is C-P stretching, which is a characteristic band for TOPO, confirming the presence of TOPO. In the OA/OAm capped cases, the absorptions at 910 and 1655 cm-1 can be assigned to C-N and C=O stretching vibrations in OAm and OA, respectively.21, 62, 63 It is well known that higher reaction temperature may help to improve the crystallinity of NCs, and thus improve the stability. However, with higher reaction temperature, NCs tend to agglomerate and form large particles if the protection from the capping ligands is not strong enough. Since the interaction between OA/OAm and CsPbX3 NCs is weak, the reaction system is usually kept below 200 oC. Figure 3 shows the representative TEM images of CsPbX3 10 ACS Paragon Plus Environment

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NCs obtained at 220 oC in the presence of OA and OAm. Instead of uniform cube-like structures obtained at 160 oC, only large aggregated particles can be prepared in all cases, suggesting poor protective effect of capping ligands. This might be attributed to the weak interaction between OA/OAm and CsPbX3 NCs, leading to the loss of ligands and irregular aggregation of small nanoparticles at elevated temperatures. Correspondingly, the emission spectra become broad. As plotted in Figure 3f, a sharp peak at 410 nm and a small peak at 460 nm can be observed in the sample of CsPbCl3 NCs. The peak position of other CsPbX3 NCs is similar to the typical CsPbX3 NCs. The corresponding peaks of CsPb(Cl/Br)3, CsPbBr3, CsPb(Br/I)3 and CsPbI3 NCs are located at 466, 517, 597, and 678 nm, respectively.

Figure 3. (a-e) TEM images of CsPbX3 NCs prepared at 220 oC in the presence of OA and OAm: (a) CsPbCl3, (b) CsPb(Cl/Br)3, (c) CsPbBr3, (d) CsPb(Br/I)3, and (e)


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CsPbI3. (f) PL emission spectra of CsPbX3 NCs (λexc= 400 nm for all except CsPbCl3 (excited at 350 nm)).

In a striking contrast, the presence of TOPO can dramatically improve the uniformity against high-temperature treatment. Figure 4 shows the morphology of CsPbX3 NCs obtained at 220 oC in the presence of TOPO, from which one can clearly see that the uniform cubic structure has been maintained in all samples. Figure 4a shows a typical TEM image of the as-prepared CsPbBr3 NCs, which are cube-like structure with an average edge length of ~11.2 nm. The size is ~1.4 nm larger than that of the NCs obtained at 160 oC. Inset high-resolution TEM (HRTEM) image in Figure 4a shows that the lattice space of the prepared NCs is 0.58 nm, which is in good agreement with the (100) plane of cubic perovskite CsPbBr3 phase. Interestingly, compared to NCs at 160 oC, other CsPbX3 NCs are also cube-like structure with uniform size distribution. As shown in Figure 4b-4e, the average edge length of CsPbCl3, CsPb(Cl/Br)3, CsPb(Br/I)3, and CsPbI3 NCs is 14.0, 12.2, 18.0, and 20.7 nm, respectively, which are slightly larger than the corresponding ones obtained at 160 oC. It is worth pointing out that all samples show strong PL emission. As plotted in Figure 4f, the corresponding emission peaks are 411 (CsPbCl3), 463 (CsPbBr3), 518 (CsPb(Cl/Br)3), 572 (CsPb(Br/I)3), and 693 nm (CsPbI3) with a narrow FWHM of 10, 15, 18, 25 and 33 nm, respectively. Since the presence of TOPO can dramatically improve the quality of product at high temperature, a series of control experiments have been conducted. The required concentration 12 ACS Paragon Plus Environment

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of TOPO is studied first, in which the reaction is conducted at 220 oC and the concentration of TOPO is varied with the other parameters kept constant. It is found that the minimum concentration of TOPO is about 0.05 mol/L (data not shown here).

Figure 4. Representative TEM images of (a) CsPbBr3, (b) CsPbCl3, (c) CsPb(Cl/Br)3, (d) CsPb(Br/I)3, (e) CsPbI3, obtained at 220 oC in the presence of TOPO. Inset in (a) is an HRTEM image of CsPbBr3 NC. (f) PL spectra of CsPbX3 NCs (λexc= 400 nm for all but 350 nm for CsPbCl3 samples). (g) Typical optical absorption and PL spectra of all samples.

TOPO has been widely used in the preparation of II-VI group semiconductor NCs at very high temperature (over 300 oC), verifying the high stability of TOPO itself against high temperature treatment. Here, we demonstrate that CsPbX3 NCs can be prepared at higher temperature. As shown in Figure 5a, when the reaction temperature is raised to 240 oC, CsPbBr3 nanocubes can still 13 ACS Paragon Plus Environment

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be obtained. The average edge length of nanocubes is ~13.0 nm. The product has a very sharp PL emission peak at 517 nm (Figure 5c). Further raising the reaction temperature to 260

o

C doesn’t have a significant effect on the

morphology and emission spectrum. As shown in Figure 5b, the average edge length of CsPbBr3 nanocubes is about 14.8 nm, which emits light at 517 nm (Figure 5d). Based on the above-mentioned results, we can conclude that the highly branched capping ligand, TOPO, can help to obtain monodisperse CsPbX3 NCs. This phenomenon might be attributed to the fact that the interaction between TOPO and CsPbX3 NCs is stronger than that between OA/OAm and CsPbX3 NCs. More importantly, the highly-branched structure may give a more effective protection because of the steric effect. It is worth pointing out that the working window for this system should be limited below 300 oC. As shown in Figure S1, only irregular and large crystals could be obtained when the reaction temperature was raised to 300 oC.


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Figure 5. (a, b) TEM images and (c, d) the corresponding emission spectra of CsPbBr3 NCs (λexc= 400 nm) obtained at different temperature: (a, c) 240 oC, (b, d) 260 oC in the presence of TOPO.

Due to the weak interaction between OA/OAm and CsPbX3 NCs, post-purification process using anti-solvent, such as ethanol, usually deteriorate the original properties of CsPbX3 NCs.

4, 64

For example, the PL would

gradually disappear when ethanol was added into a CsPbX3 nanocrystal dispersion. Severe agglomerated NCs can also be observed. The stability of TOPO-caped CsPbX3 NCs against cleaning has been dramatically improved. Here, we demonstrate the enhanced stability by adding certain amount of ethanol into CsPbBr3 NCs. Four samples, prepared at 160 oC (with and without TOPO) and 220 oC (with and without TOPO), were used to investigate the tolerance of CsPbBr3 NCs against ethanol treatment. To ensure the same 15 ACS Paragon Plus Environment

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concentration of CsPbBr3 NCs, the intensity of UV-Vis absorption at 335 nm was kept as the same, as shown in Figure 6a. Into a 2.4 mL of solution, 10 µL of ethanol is added rapidly. We first took the sample prepared at 160 oC in the absence of TOPO as the example. As shown in Figure 6b, upon the injection of ethanol, the emission intensity decreased dramatically within the first 2 min. With the prolonged time, the intensity decreased gradually and the peak position red-shifted from 516 nm to 520 nm, suggesting the deterioration of NCs. As plotted in Figure 6c, the intensity dropped about 86% within 100 min. The corresponding photograph of the sample is shown in Figure 6d (before) and 6e (after), from which one can clearly see that the bright green emission became very light after ethanol treatment for 100 min. The same trend has been observed in the sample obtained at 200 oC without TOPO (blue line in Figure 6c). After treated with the same amount of ethanol, the PL emission intensity dropped about 60%, which is slightly better than that of sample obtained at 160 o

C. It is worth noting that the position of both PL and absorption peaks did not

shift during the treatment, implying higher stability. The TOPO-capped NCs showed much better stability against ethanol treatment. As shown in Figure 6c and 6d, both TOPO-capped samples showed much better performance against ethanol treatment. After 100 min, the intensity dropped 20 % and 5% for the sample prepared 220 oC and 160 oC, respectively. No peak shift (Figure S2c and S2a) was observed for both samples. The lower stability of sample prepared at 220 oC might be attributed to the larger size of NCs, which could 16 ACS Paragon Plus Environment

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aggregate easily upon the addition of ethanol. For the TOPO-capped sample obtained at 160 oC, the addition of ethanol caused a rapid drop in the PL emission intensity, which then gradually recovered. This phenomenon might be interpreted by the aggregation of NCs caused by the addition of ethanol. Because of the strong steric effect, when ethanol diffused into the whole system, the agglomeration of NCs disappear and PL emission was recovered.

Figure 6. Stability of CsPbBr3 NCs against ethanol treatment. (a) UV-Vis absorption of samples. (b) Monitoring the PL spectra change against ethanol treatment. CsPbBr3 NCs were prepared at 160 oC using OA/OAm as the capping ligands. (c) The intensity of PL emission as a function of treating time. (d-e) Photographs of CsPbBr3 NCs before (d) and after (e) treated with ethanol for 2 h. From left to right, the samples are (1) 160 oC without TOPO; (2) 160 oC with TOPO; (3) 220 oC without TOPO; and (4) 220 oC with TOPO. The excitation wavelength of all PL spectra was 400 nm.

Conclusions


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In summary, by utilizing a highly-branched capping ligand, TOPO, we demonstrated that monodisperse CsPbX3 NCs could be obtained at elevated temperatures (up to 260 oC). The reaction temperature has been varied to test the influence on the morphology and quality of CsPbX3 NCs. The size of CsPbX3 NCs can be tuned in a relatively broad range by varying the reaction temperature. With higher reaction temperature, monodisperse CsPbX3 NCs with larger particle size can be obtained. TOPO-capped CsPbX3 NCs display excellent stability against protonic solvent (ethanol) attack. The emission intensity dropped only 5% after treated with ethanol for 2 h. In a striking contrast, the emission intensity dropped 86% if the sample is capped with OA/OAm. This result may shed some light on the preparation and applications of CsPbX3 NCs because of the higher stability against ethanol washing. ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: The following files are available free of charge. Additional TEM images, UV/vis absorption and PL emission spectra (PDF)

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (B. S.);

*Email: [email protected] (Q.Z) 18 ACS Paragon Plus Environment

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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the Ministry of Science and Technology of the People's Republic of China (2016YFE0129600), National Natural Science Foundation of China (21401135, 21673150). We acknowledge the financial support from the 111 Project, Collaborative Innovation Center of Suzhou Nano Science and Technology (NANO-CIC), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and SWC for Synchrotron Radiation Research.

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