Synthesis of Cesium Lead Halide Perovskite Quantum Dots - Journal

Laboratory Experiment pubs.acs.org/jchemeduc

Synthesis of Cesium Lead Halide Perovskite Quantum Dots Mikhail Shekhirev,† John Goza,† Jacob D. Teeter,† Alexey Lipatov,† and Alexander Sinitskii*,†,‡ †

Department of Chemistry, University of NebraskaLincoln, Lincoln, Nebraska 68588, United States Nebraska Center for Materials and Nanoscience, University of NebraskaLincoln, Lincoln, Nebraska 68588, United States

‡

S Supporting Information *

ABSTRACT: Synthesis of quantum dots is a valuable experiment for demonstration and discussion of quantum phenomena in undergraduate chemistry curricula. Recently, a new class of allinorganic perovskite quantum dots (QDs) with a formula of CsPbX3 (X = Cl, Br, I) was presented and attracted tremendous attention. Here we adapt the synthesis of CsPbX3 QDs for implementation in inorganic chemistry laboratory class. Perovskite QDs have a number of advantages: they exhibit bright photoluminescence in the visible range of spectrum with a narrow bandwidth, and their emission wavelength can be changed by tuning both size and composition of nanoparticles. The described experiment provides a discussion point on many important concepts of inorganic chemistry, materials science, and nanotechnology, such as colloidal synthesis of nanoparticles, perovskite crystal structure, quantum size effect, as well as photovoltaics and renewable energy. KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Quantum Chemistry, Nanotechnology, Colloids, Laboratory Instruction, Hands-On Learning/Manipulatives

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INTRODUCTION Inorganic chemistry laboratory is an important course in any undergraduate chemistry program, and has been offered in numerous schools worldwide for many decades. As a result, when developing a curriculum for an inorganic chemistry laboratory course, an instructor faces a difficult choice of selecting only a few out of many excellent laboratory experiments that have been developed over the years.1 Since the number of laboratory experiments that can be offered in a single semester is limited, of particular importance are experiments that introduce not one, but several important concepts of inorganic chemistry and related disciplines, such as materials science and nanotechnology, and preferably illustrate their most recent trends. This paper describes one such laboratory experiment that can be implemented in an upperlevel undergraduate inorganic chemistry laboratory course. The laboratory experiment involves synthesis and characterization of cesium lead halide (CsPbX3, X = Cl, Br, I) quantum dots (QDs). In general, nanoscience and nanotechnology are fields relatively recently introduced into a chemistry laboratory curriculum. One of the key concepts of nanotechnology is the “size effect”, which corresponds to the change in physical properties of solids when the particle size is reduced to nanoscale dimensions. The size effect is often illustrated by QDs, nanoparticles of a semiconducting material with typical diameters in the range of 2−10 nm.2 The most common demonstration of the size effect involves solutions of QDs with different diameters that show size-dependent photoluminescence in the visible spectral range. In addition to the wide © XXXX American Chemical Society and Division of Chemical Education, Inc.

variety of applications, which include nano- and optoelectronics, photovoltaics, and bioimaging,3 QDs have also been acknowledged as valuable teaching tools due to their sizedependent optical properties directly related to the textbook “particle-in-a-box” effect.4 To date, most chemistry undergraduate laboratory experiments on QDs have revolved around metal chalcogenide nanocrystals, especially CdSe QDs.4b,c However, the described laboratory experiment on QDs based on alternative semiconductor materials, cesium halide perovskites, has a number of advantages. On one hand, the perovskite crystal structure is commonly featured in general and inorganic chemistry courses (Figure 1a). With the described experiment, an instructor has an opportunity to revisit this important crystal structure and discuss changes associated with cation and anion substitutions, which has a clear educational value. More importantly, while hundreds of compounds have perovskite crystal structures, the described experiment introduces students to the large class of organic−inorganic perovskite materials (Figure 1b) that have enjoyed a considerable amount of interest from the scientific community in recent years. Hybrid lead halide perovskites, where CH3NH3PbI3 is the most notable member of the family, have been known for decades.5 However, several recent experiments6 have started a “perovskite fever”,7 thanks to the dramatic increase in Received: February 19, 2017 Revised: June 3, 2017

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DOI: 10.1021/acs.jchemed.7b00144 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

UV−vis spectroscopy was performed using a Jasco V-670 spectrometer. Photoluminescence (PL) spectra were obtained using a Shimadzu RF-5301PC instrument. Transmission electron microscopy (TEM) studies was made using an FEI Tecnai Osiris scanning transmission electron microscope. To prepare samples for TEM measurements, one drop of a solution of QDs in hexanes was applied to a TEM grid (lacey carbon on 300 mesh, no Formvar, purchased from Ted Pella) and dried in air. XRD spectra were collected using a PANalytical Empyrean diffractometer. Procedure 1. Synthesis of CsPbX3 (X = Cl, Br, I) Quantum Dots under Nitrogen

Figure 1. (a) Scheme of the perovskite crystal structure with a general formula of ABX3. The green spheres represent X (for instance O2−), the gray spheres inside of the octahedral voids represent B (for instance Ti4+), and the red sphere in the center represents cation A, such as Ca2+. (b) Scheme of the crystal structure of organic−inorganic perovskite NH3CH3PbI3; blue and white spheres in the center represent nitrogen and carbon atoms of NH3CH3+ ion. Hydrogen atoms are not shown for the sake of clarity.

Synthesis of Cesium Oleate. A 0.203 g portion of cesium carbonate (Cs2CO3), 10 mL of octadecene (ODE), and 1.0 mL of oleic acid (OA) were added to a two-neck flask, mixed by magnetic stirring and heated to 150 °C under N2 atmosphere until Cs2CO3 completely dissolved (approximately 30 min). Cs-oleate solution was kept stirring at the temperature above 100 °C (typically 120−130 °C) in order to avoid precipitation. If needed, the solution could be prepared in advance of the experiment by instructors and used by multiple groups of students. Synthesis of CsPbX 3 Quantum Dots. Under N 2 atmosphere, 15 mL of ODE, 3 mL of OAm, 1.5 mL of OA, and 0.54 mmol of PbX2 were added to a three-neck flask and mixed with magnetic stirring. In the case of Cl/Br and Br/I mixtures, 50:50 mol ratios of halogens were used. In the case of PbCl2, 1 mL of trioctylphosphine was also added to the solution. The mixture was degassed at 100 °C for 10 min, and then stirred at 100 °C for 30 min until PbX2 dissolved. A higher temperature of 150 °C might be needed to dissolve PbCl2. Then, the solution was brought to the desired temperature (typically 170 °C), and 0.55 mL of Cs-oleate was quickly injected into the PbX2 solution via glass syringe through a septum. After 5 s, the three-neck flask was cooled down by the ice−water bath. Note! The quenching should be done only if appropriate glassware is used, for instance, PYREX, otherwise flask will crack. (See Supplementary Note 5 in SI). Alternatively, if such glassware is not available, it is recommended to slowly cool the solution without quenching the flask in the ice bath. This method might result in a broader size distribution of QDs, but it still produces QDs with bright luminescence and size/ composition dependence of the emission wavelengths. The solution of QDs was centrifuged (5000 rpm, 5 min), and the resulting supernatant and precipitate were separated; the precipitated QDs were redissolved in 8 mL of hexane. In the case of CsPbCl3 and CsPbBr3, 10 mL of acetone could be added to facilitate the precipitation. CsPbI3 was found to be unstable upon addition of the acetone and was centrifuged without any additional solvents.

perovskite solar cell power-conversion efficiency made in only a few years.8 Due to a number of unique properties, including high optical absorption, high carrier mobility, tolerance to point defects and grain boundaries,9 perovskites hold great promise for the next generation of high-performance, low-cost, and easyto-make solar cells.10 As a matter of fact, the fabrication of perovskite solar cells appears to be simple enough to be implemented as an experiment for undergraduate curricula as well.11 While this laboratory experiment does not include any student activities related to solar cell fabrication and characterization, it provides an instructor with a discussion point on photovoltaics and renewable energy in general. In addition to the multiple chemistry concepts highlighted by this laboratory experiment (possible discussion topics are described in the Classroom Testing and Teaching Objectives section), there are other advantages of introducing perovskite QDs in an undergraduate inorganic chemistry class. CsPbX3 QDs show very bright luminescence in the visible spectral region, high quantum yields, and narrow emission line widths,12 and are not very vulnerable to surface defects and the corresponding charge traps.13 Finally, safety concerns should be taken into account for the synthesis of Cd-based QDs, such as CdSe QDs, which are prepared from toxic precursors.14 While perovskite QDs still contain a heavy metal (Pb), Cd is generally considered to be more toxic than Pb.15 Encouraged by the progress in studies of perovskite QDs, we adapted their synthesis for implementation in an upper-level undergraduate chemistry laboratory. The experiment provides a good introduction to the field of quantum dots and size/ composition-dependent optical properties of nanomaterials.

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Procedure 2. Synthesis of CsPbX3 (X = Cl, Br, I) Quantum Dots in Air

EXPERIMENTAL SECTION

Cs-oleate was synthesized according to the procedure described above, but without N2 protection. It was found that Cs-oleate degrades with time at temperatures above 120 °C (the solution turns dark), which affects synthesis of QDs. We kept the solution at a temperature of approximately 100 °C without any signs of degradation or precipitation. CsPbBr3 and CsPbCl3 QDs can be synthesized in open air with no N2 flow following the same procedure. In this case, the temperature of the synthesis was decreased to 150 °C, and glass pipettes were used instead of syringes for transfer. Unfortu-

Materials and Instruments

The reagents were acquired from Sigma-Aldrich and used without any purification. In order to adapt this synthesis for teaching experiments in large-scale settings, we resorted to less expensive technical grade reagents, such as octadecene (technical grade, 90%), oleic acid (technical grade, 90%), olylamine (technical grade, 70%), trioctylphosphine (technical grade, 90%), with which perovskite QDs with bright photoluminescence could still be routinely produced by students. B




Figure 2. (a) Scheme of the synthesis. (b) Photograph of the flask with CsPbBr3 QDs immediately after the synthesis and (c) the same flask illuminated with UV light.

nately, CsPbI3 and CsPb(Br/I)3 QDs did not produce viable results and require an inert atmosphere of N2 or Ar gas.

Eg =

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HAZARDS Gloves, safety googles, and protective clothing should be used during the experiment. All the preparations should be performed in a fume hood. Octadecene and oleic acid vapors should not contact skin or eyes or be inhaled. Trioctylphosphine and oleylamine cause skin burns and eye damage. All lead-containing compounds are poisonous if inhaled or swallowed.

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hc λ

(1)

where Eg is a calculated band gap (eV), h is the Planck’s constant (4.1357 × 10−15 eV*s), c is the speed of light (299792458 m/s), and λ is a wavelength (m) extracted from PL or UV−vis spectra. The wavelengths and the corresponding band gap values for different quantum dots are presented in Table 1. Comparison of parts b and c of Figure 3 and the data from Table 1 show a slight shift of the absorption onset relative to the position of the luminescence peak. This behavior is known as a Stokes shift, which is a common phenomenon for CdSe nanocrystals and was also mentioned in a number of reports on the perovskite QDs.12a,g,16 TEM images in Figure 4 show that the perovskite QDs are nanocubes that have sizes in the range of 5−10 nm. It was previously shown that the PL spectra of a single CsPbBr3 QD and an ensemble of nanocrystals with comparable sizes are nearly identical.17 As a result, the batch-to-batch reproducibility of the QD synthesis is not very crucial for obtaining samples of perovskite nanocrystals with consistent photoluminescent properties. This makes the perovskite QDs a good study object even for students with little experience in inorganic chemistry synthesis.

RESULTS AND DISCUSSION

Procedure 1. Synthesis of CsPbX3 QDs under N2

The synthesis of perovskite QDs was adopted from ref 12a and is schematically shown in Figure 2a. In the synthesis, a hot solution of cesium oleate, prepared from cesium carbonate and oleic acid, is quickly injected into a flask containing lead halide reagent and quenched after 5 s with an ice−water bath. Immediately after injection, the solution gained a very pronounced color, which depended on the chemical composition of QDs. Figure 2b shows a photograph of CsPbBr3 QD solution with characteristic lime-green color immediately after quenching the reaction. When other lead halides are used in the synthesis, the final solution color changes from almost transparent for CsPbCl3 to red for CsPbI3. When the flask is illuminated with a UV light, the solution starts glowing with the color depending on the composition (Figure 2c). After the QDs are isolated via centrifugation and redispersed in hexane, they exhibit very bright photoluminescence under UV light, as shown in Figure 3a. The color of the QDs clearly shifts from blue to red upon halogen substitution from Cl to Br to I. The color change of the synthesized QDs is a good point to discuss energy band diagram of semiconductor materials and related optical properties (see the insets in Figure 3b,c) which are dependent on the size of the band gap. Experimentally, the band structure could be probed using two different techniques, photoluminescence (PL) and UV−vis spectroscopy (Figure 3b,c). The bright photoluminescence of perovskite QDs is characterized by the narrow line width of 10−30 nm and wide color range of 410−700 nm (Figure 3b). UV−vis spectra of the QDs’ solution show clear absorbance onsets (Figure 3c). Both techniques could be used to extract the band gap values for the QDs using the following equation:

Procedure 2. Synthesis of CsPbX3 QDs in Air

In order to further adapt the synthetic procedure for undergraduate laboratories without N2 lines, we performed syntheses of QDs under ambient atmosphere. The temperature of the synthesis was lowered to 150 °C to minimize degradation of the reagents in air. Unfortunately, we were not able to synthesize iodide-containing QDs CsPb(Br/I)3 and CsPbI3 without the N2 protection. Low stability of the Icontaining QDs is not surprising as it is generally more difficult to synthesize the metastable phase of CsPbI3.12a,18 For example, significant challenges in the fabrication of CsPbI3 QLEDs were previously reported.18 Structurally, the Br- and Cl-containing QDs that were synthesized in air showed the same properties as the samples prepared under nitrogen. Figure 5a shows a powder XRD pattern of orthorhombic CsPbBr3 QDs, which is in very good agreement with the XRD spectrum of a bulk CsPbBr 3 material.19 Optionally, synthesis of CsPbBr3 macrocrystals could be attempted in class using, for instance, a vapor diffusion method.20 The orthorhombic phase of CsPbBr3 synthesized in our experiment is different from the originally suggested cubic phase;12a however, the QDs with orthorhombic structure were previously observed, and it was noted that the C




Table 1. Band Gap Values for Different Quantum Dots Calculated from PL and UV−Vis Spectra

Compound CsPbCl3 CsPb(Cl/ Br)3 CsPbBr3 CsPb(Br/ I)3 CsPbI3

Photoluminescence Peak Position (nm)

Eg Calculated from UV− vis (eV)

Eg Calculated from PL (eV)

410 433

414 440

3.02 2.86

2.99 2.82

507 558

517 561

2.45 2.22

2.40 2.21

701

701

1.77

1.77

Absorption Onset from UV−vis (nm)

crystal structure is presented in Supplementary Note 1 in the SI. In terms of their optical properties, the air- and N2synthesized QDs are identical. Figure 5c shows PL data for CsPbX3 QDs synthesized in air, which exhibit the same composition dependence as the QDs that were prepared under N2 atmosphere (Figure 3b). For comparison, the PL spectrum of CsPbBr3 QDs synthesized at the same temperature of 150 °C under N2 is also shown in Figure 5c. Both Br-containing samples exhibit the same photoluminescence peak at 490 nm, confirming that the synthesis of CsPbBr3 QDs can be successfully performed in the ambient atmosphere. Occasionally, we observed additional peaks in PL spectra for some of the air-synthesized QDs, which might originate from defects and can be easily eliminated with an additional purification step (see Supplementary Note 2 in the SI for details). However, despite possible defects, air-synthesized QDs still possess the desired property of bright luminescence with a clear color shift upon halogen substitution, which could be used for the visual demonstration of optical properties of QDs. The described procedure can be used to synthesize the Br- and Cl-containing perovskite QDs in air if the nitrogen gas is not available in an undergraduate chemistry laboratory. Size-Dependent Optical Properties of Perovskite QDs

The data presented in Figure 3 demonstrates the control over the emission wavelength of perovskite QDs through the modulation of their chemical composition (halogen substitution). This is fundamentally different from CdSe nanocrystals, where optical properties are mainly altered by tuning the size of QDs. From our teaching experience, this difference often leads to some degree of confusion among the students, who think that only one parameter (mostly size of QDs) affects the QDs’ band gap and electronic structure. From this standpoint, it is important to demonstrate how both factors play a role in the optical properties of nanoparticles. We believe that the synthesis of perovskite QDs is ideal for this purpose because it allows control of the band structure via both the size-effect and the halogen exchange. Typically, the size of perovskite QDs is changed by tuning the temperature of the synthesis.12a However, from the teaching standpoint, performing multiple syntheses of the same nanoparticles at different temperatures may be difficult to implement because of limited time and resources. Here we propose to perform separation of QDs with different sizes via centrifugation. In this experiment, CsPbBr3 QDs were synthesized according to the procedure 1, and after centrifugation both supernatant and redissolved QDs were studied via PL and TEM. Figure 6 shows PL spectra for the supernatant and the precipitate with a clearly visible difference

Figure 3. Optical properties of the perovskite QDs. (a) Photograph of the vials with perovskite QDs with different compositions under UV light. (b) Normalized photoluminescence spectra of perovskite QDs with different compositions. (c) Normalized absorbance spectra of perovskite QDs with different compositions. The insets show the general scheme of a band diagram of a semiconductor material with absorption of a photon and formation of an electron/hole pair in panel c, and recombination of the electron and the hole with the corresponding light emission in panel b. VB − valence band; CB − conduction band.

cubic and orthorhombic phases are difficult to distinguish.17 Substitution of the bromide ions in the structure with smaller chloride ions results in the shift of the diffraction pattern, as shown in Figure 5b. If time and resources permit, perovskite QDs could be used to discuss how the crystal structure changes upon halogen substitution. Analysis of XRD patterns could also be a good exercise for students. A more detailed discussion on possible topics related to the XRD analysis and the perovskite D




Figure 4. TEM images of CsPbBr3 QDs. Scale bars: (a) 50 nm and (b) 5 nm.

Figure 5. Properties of QDs that were synthesized in air. (a) Powder XRD pattern of CsPbBr3 QDs (black) in comparison with the theoretical diffraction pattern (blue). (b) Comparison of the positions of the (0, 0, 2) reflection for CsPbCl3, CsPb(Cl/Br)3, and CsPbBr3 QDs. (c) Comparison of normalized PL spectra of QDs synthesized in air and under N2 at 150 °C.

Classroom Testing and Teaching Objectives

The perovskite QD experiment was tested in the Inorganic Chemistry Laboratory course at the University of Nebraska Lincoln for two semesters. In one laboratory period of 4 h, groups of 2−4 students were able to conduct 4−5 syntheses and characterize QDs by UV−vis and PL spectroscopy. The successful fabrication of QDs was additionally confirmed on selected samples via XRD and TEM. At the end of the course, students named the synthesis of perovskite QDs as one of the most interesting and enjoyable experiments in the course. The experiment was used for a demonstration of different approaches in nanochemistry. The discussed topics of inorganic chemistry and nanochemistry included:

Figure 6. PL spectra of the precipitate (black) and the supernatant (red) of centrifuged CsPbBr3 QDs. The insets show TEM images of the corresponding samples. TEM images were recorded at the same magnification; scale bars are 50 nm.

• Perovskite crystal structure, with an XRD method for the crystal structure determination • Quantum phenomena and size/composition-dependent electronic and optical properties of QDs • Colloidal synthesis and stabilization of nanoparticles • Air-sensitive synthesis • Photovoltaics, solar cells, and renewable energy

in the positions of the emission maxima. As expected, smaller particles in the supernatant emit at shorter wavelengths compared to larger particles in the precipitate. A comparison of TEM images of both samples at the same magnification is shown in the insets of Figure 6, confirming that the precipitate consists of larger particles. This simple experiment can be used in the chemistry laboratory to demonstrate the size-dependent optoelectronic properties of QDs.

With slight changes in the procedure, students were also able to successfully synthesize CsPbX3 nanowires.21 Occasionally, during our TEM studies we also observed nanocrystals of other shapes, such as nanoprisms, nanoplates, and nanospheres (see Supplementary Note 3 and Figure S2 in SI for details). Due to the possibility of a control over the shape of perovskite nanocrystals via surfactant/temperature modulation,16,21,22 this experiment could be further adapted for the demonstration of methods of size- and shape-controlled colloidal synthesis of nanomaterials. E



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SUMMARY We adapted the synthesis of cesium lead halide perovskite quantum dots for implementation in an upper-level undergraduate inorganic chemistry laboratory course. Perovskite QDs with bright photoluminescence can be synthesized via a relatively simple procedure even in ambient conditions. The possibility of tuning the emission wavelength of QDs by both the size-effect and the composition modulation is valuable for demonstration of how these factors affect optical properties of nanomaterials. Furthermore, the described experiment can be used for illustration and discussion of a large number of important concepts of inorganic chemistry, materials, and nanotechnology. The discussion topics may include colloidal synthesis of nanostructures, perovskite crystal structure, quantum size effect, photovoltaics and renewable energy, and many others. Finally, the experiment provides an example of cutting-edge research10 to undergraduate students, illustrating relevance of basic inorganic chemistry concepts to the recent scientific advances.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00144. Additional experimental data, student procedure, and instructor notes (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mikhail Shekhirev: 0000-0002-8381-1276 Alexander Sinitskii: 0000-0002-8688-3451 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the National Science Foundation (NSF) through CHE-1455330. The materials characterization was supported by the Nebraska Center for Energy Sciences Research (NCESR) and NSF DMR-1420645, and performed in the Nebraska Nanoscale Facility: National Nanotechnology Coordinated Infrastructure and the Nebraska Center for Materials and Nanoscience, which are supported by the NSF (ECCS-1542182), and the Nebraska Research Initiative.

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REFERENCES

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Synthesis of Cesium Lead Halide Perovskite Quantum Dots - Journal

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