Article pubs.acs.org/cm
Combination of Cation Exchange and Quantized Ostwald Ripening for Controlling Size Distribution of Lead Chalcogenide Quantum Dots Changwang Zhang,† Yong Xia,† Zhiming Zhang,† Zhen Huang,† Linyuan Lian,† Xiangshui Miao,† Daoli Zhang,† Matthew C. Beard,§ and Jianbing Zhang*,†,‡ †
School of Optical and Electronic Information, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, China § Chemical and Materials Sciences Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States ‡ Shenzhen R & D Center of Huazhong University of Science and Technology, Shenzhen, Guangdong 518057, China S Supporting Information *
ABSTRACT: A new strategy for narrowing the size distribution of colloidal quantum dots (QDs) was developed by combining cation exchange and quantized Ostwald ripening. Medium-sized reactant CdS(e) QDs were subjected to cation exchange to form the target PbS(e) QDs, and then small reactant CdS(e) QDs were added which were converted to small PbS(e) dots via cation exchange. The small-sized ensemble of PbS(e) QDs dissolved completely rapidly and released a large amount of monomers, promoting the growth and size-focusing of the medium-sized ensemble of PbS(e) QDs. The addition of small reactant QDs can be repeated to continuously reduce the size distribution. The new method was applied to synthesize PbSe and PbS QDs with extremely narrow size distributions and as a bonus they have hybrid surface passivation. The size distribution of prepared PbSe and PbS QDs are as low as 3.6% and 4.3%, respectively, leading to hexagonal close packing in monolayer and highly ordered three-dimensional superlattice.
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INTRODUCTION Colloidal quantum dots (QDs) are quantum-confined, solution-processed semiconductor nanoparticles.1 The properties of QDs strongly depend on their size; therefore, monodispersity is a basic requirement for QD synthesis. In general, Ostwald ripening governs the growth of nanoparticles in a solution.2 The theory demonstrates that large particles grow while the small ones dissolve, resulting in a broad size distribution. Therefore, synthesis of QDs with narrow size distributions is challenging. Hot-injection synthesis has been applied extensively to separate nucleation and growth of the nanoparticles for the purpose of obtaining monodispersive QDs.3 Size-selective technique has also been used to separate a series of sizes from QD ensembles, which is time-consuming and of low reproducibility.3,4 As the understanding of QD synthesis has become deeper and deeper, researchers found that monodispersive QDs can be grown using a diffusion-controlled process.5 Both experimental6 and theoretical7 studies show size focusing can occur at high oversaturation conditions, avoiding Ostwald ripening. Ostwald ripening can also result in size focusing with a broad starting size distribution.7 Peng and coworkers found interparticle diffusion played a key role in the Ostwald ripening and results in size focusing.8 Self-focusing mechanism was proposed based on the interparticle diffusion.9 © 2017 American Chemical Society
Numerical simulations show quantized Ostwald ripening is an effective method to focus the size distribution.10 The quantized Ostwald ripening occurs when two distinctly sized nanoparticles are present in the growth solution, in which the particles with radius below or close to the critical dissolution radius dissolve completely, releasing monomers for the growth and size focusing of the larger sized ensemble.10 Although the methods are different for size focusing, the fundamental mechanism is the same, that is, maintaining high oversaturation for the growth of nanoparticles. Because of different conditions, including precursors, solvents, ligands, and temperatures, in the syntheses of a variety of QDs, different strategies are needed to meet the basic rule for size distribution control. Producing QDs with superior properties and good surface passivation is another topic in addition to size/size distribution control in the synthesis of QDs. For example, PbS and PbSe QDs synthesized using lead halide as the lead precursors were found to be passivated with halide, showing higher photoluminescence quantum yields and much better stability in air compared to those synthesized from PbO.11−14 Recently, we Received: January 31, 2017 Revised: March 22, 2017 Published: March 22, 2017 3615
DOI: 10.1021/acs.chemmater.7b00411 Chem. Mater. 2017, 29, 3615−3622
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Figure 1. Evolution of absorption spectrum of PbSe QDs synthesized at different conditions after the injection of medium sized CdSe QDs: nothing (a), TOPSe (b), TBPSe (c), small CdSe QDs (d) were injected, respectively. Four injections were performed with intervals of 4 min.
developed a cation-exchange synthesis for Cd2+ and Cl− copassivated PbS and PbSe QDs.15,16 The halide passivation renders air stability and thus air-processed QD devices, and the Cd2+ repairs the traps during the fabrication of QD thin films, giving rise to better device performance.17,18 Therefore, the cation-exchanged PbS and PbSe QDs are ideal materials for optoelectronic and electronic devices. Additionally, the cation exchange is a powerful tool for nanocrystals with composition, shape, and structure that cannot be accessed by conventional synthesis.19,20 However, although injection of additional sulfur/ selenium precursors were tried to achieve the condition for size focusing, the size distribution of large PbSe and PbS QDs prepared by the cation-exchange method needed to be further improved.16 Narrow size distribution is required for enhancing the charge transport within QD solids,21 and thus QD devices with high performance. Additionally, large PbS and PbSe QDs are important for photoelectric conversion based on multiple exciton generation (MEG)22−24 and near/mid-infrared application. In the present work, we developed a strategy to effectively narrow the size distribution in the cation-exchange synthesis via quantized Ostwald ripening. Extremely small CdS(e) QDs were added frequently after the injection of medium-sized CdS(e) QDs which were converted to medium-sized PbS(e) QDs. The small CdS(e) nanoparticles were exchanged instantly upon injection to small PbS(e) particles which served as sacrificial materials that dissolved completely, releasing a large amount of monomers. Thus, the medium-sized PbS(e) QDs grew at a high oversaturation condition, resulting in size focusing, and the
size distribution decreased continuously as the frequent addition of small CdS(e) QDs.
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EXPERIMENTAL SECTION
Chemicals. Oleylamine (OLA, tech. grade, 70%) was purchased from Aldrich (St. Louis). PbCl2 (99.999%), CdO (≥99.95%), oleic acid (OA, tech. grade, 90%), 1-octadecene (ODE, tech. grade, 90%), trioctylphosphine (TOP, tech. grade, 90%), selenium powder (99.999%) and sulfur powder (99.5%) were purchased from Alfa Aesar (Ward Hill, MA). Tributylphosphine (TBP, 95%) and ammonium sulfide (40−48 wt % in H2O) were purchased from Aladdin (Shanghai, China). Tetrachloroethylene (TCE, ≥98.5%), hexane (≥97%), ethanol (≥99.7%), and acetone (≥99.5%) were purchased from Sinopharm Chemical Reagent (Shanghai, China). All the chemicals were used as received. TOPS(e) and TBPS(e) (selenium or sulfur powder dissolved in TOP and TBP, respectively) were used as molecular precursors. Synthesis of CdSe and CdS QDs. CdSe and CdS QDs with different sizes were synthesized using Peng’s and our modified methods.15,25,26 CdSe-561 QDs (the first exciton peak is at 561 nm) were used as the medium-sized reactant QDs and the diameter was determined to be 3.3 nm based on the sizing curve given by Yu et al.27 CdSe-450 QDs (1.9 nm) were used as the small reactant QDs for injections. Correspondingly, CdS-447 QDs (5.14 nm) and CdS-308 QDs (1.45 nm) were used for the synthesis of PbS QDs. Cation-Exchange Synthesis of PbSe QDs with and without Multiple Injection of Molecular Precursors. OLA (5 mL) and 1.5 mmol of PbCl2 were degassed under vacuum at 80 °C and heated to 140 °C under nitrogen and the temperature was maintained for 30 min. Then the temperature was set to 190 °C, and ∼37 mg of CdSe561 QDs in 0.3 mL of ODE was injected swiftly. Thirty seconds later, the temperature was lowered to 160 °C quickly and the temperature 3616
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Figure 2. Evolution of size (a), total particle number (b), standard deviation (c), and size distribution (d) of PbSe QDs synthesized at different conditions corresponding to those in Figure 1. The error in the particle number was estimated to be 10%.
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was maintained. An aliquot of 0.3 mL reaction solution was withdrawn for the absorption measurement. Then 0.7 mL of 0.1 M TOPSe (or TBPSe) was injected; 3 min later another 0.3 mL reaction solution was removed. The injection was repeated three more times every 4 min with an increasing amount of TOPSe (or TBPSe), that is, 1, 1.3, and 1.6 mL. An aliquot of 0.3 mL reaction solution was removed before each injection. The aliquots were dispersed in hexane with OA, washed two times, and dispersed in 3 mL of TCE for the measurement of absorption spectra. When no TOPSe or TBPSe was added, the growth solution was maintained at 160 °C and aliquots were removed at different time intervals. Cation-Exchange Synthesis of PbSe QDs with Multiple Injections of Small CdSe QDs. The synthesis was performed in the exact same manner as those when TOPSe or TBPSe were supplemented, except CdSe-450 QDs in ODE was injected instead of the molecular precursors. The concentration of CdSe-450 QDs in ODE was carefully adjusted to achieve the same Se concentration as those in TOPSe and TBPSe (0.1M). Synthesis of PbS QDs. The synthesis of PbS QDs followed the same manner as that of PbSe QDs at different conditions, except TOPS, TBPS, and CdS QDs were injected. Characterization. Optical absorption spectra were collected using a Shimadzu UV-3600 plus spectrophotometer. Transmission electron microscopy (TEM) images were obtained using a FEI Technai G2 20 microscope with a LaB6 filament operated at 200 kV. The samples were made by drop casting QD solution in hexane (for PbSe) or TCE (for PbS) onto TEM grids. X-ray photoelectron spectroscopy (XPS) data were obtained on a Kratos AXIS-ULTRA DLD and the sample was prepared by drop casting QD solution on indium tin oxide coated glass. Calculation of Dot Size, Particle Number, Size Standard Deviation, and Size Distribution. The diameter and concentration of QDs were calculated using the relationship between the optical bandgap and QD size for PbSe28 and PbS29 QDs and the relationship between QD diameter and molar extinction coefficient at the first exciton peak for PbSe QDs.28 As the volume of the growth solution was changing due to the injections of different amounts of solutions, total particle number was calculated including the withdrawn aliquots. The size standard deviation and size distribution were estimated by fitting the absorption spectra following our previous procedure.14
RESULTS AND DISCUSSION In the cation-exchange synthesis of PbS(e) QDs from CdS(e) QDs, the reaction zone width should be larger than the size of Scheme 1. Combination of Cation Exchange and Quantized Ostwald Ripening for the Control of Size Distributiona
a
First, medium-sized CdS(e) QDs were subjected to cation exchange to form PbS(e) QDs. Then small-sized CdS(e) QDs were injected frequently to promote the growth and size focusing of the PbS(e) QDs. (The dashed box represents a transient state.)
CdS(e) QDs for monodispersity. Therefore, higher reaction temperatures are needed for larger CdS(e) QDs. The Pb precursor (PbCl2−OLA) becomes unstable at temperatures above ∼200 °C; as a result, the largest size of used CdS(e) QDs and thus the largest size of PbS(e) QDs are limited.16 Therefore, monodispersive large PbS and PbSe QDs can be obtained only by further growth of the exchanged QDs. In our previous work, we have shown that the growth of PbS and PbSe in the PbCl2−OLA system follows a diffusion-controlled regime and is accompanied by dissolution of relatively small particles.14 Figure 1a shows the evolution of PbSe QDs after the complete conversion of medium-sized CdSe QDs without addition of sulfur precursor or small CdSe QDs. The first exciton peak becomes weaker and broader with the growth of these PbSe QDs. Quantitative calculation based on the absorption spectra shows a continuous decrease of particle number (Figure 2b), 3617
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Figure 3. TEM images of the PbSe-1936 QDs synthesized by the combination of cation exchange and quantized Ostwald ripening. (left) Hexagonal close packing, the inset is the size histogram; (right) three-dimensional superlattice which is approximately 4−5 QD layers thick based on TEM contrast.
2a) and size standard deviation (Figure 2c) confirm the slight effect of TOPSe on the growth of PbSe QDs, while the dissolution of particle is prohibited (Figure 2b) and the size distribution remains approximately constant (Figure 2d) after the last two injections. When TBPSe, which is more reactive than TOPSe, was used, the growth was apparently promoted as demonstrated by the increased growth rate (Figure 2a). The size deviation increases after the first two injections (Figure 2c), with the same speed as that of TOPSe, and decreases slightly after the last two injections. The size distribution is maintained and then decreases down to 5.3% (Figure 2d), indicating larger effect of TBPSe than TOPSe. However, improvement induced by TBPSe is limited as shown by the decrease in the particle number (Figure 2b) and the almost undistinguishable second and third excition peaks (Figure 1c). Because the multiple injections of TOPSe or TBPSe cannot give rise to effective control of the size standard deviation and size distribution, our goal is to find a new method to produce monodispersive QDs via cation exchange. Considering the continuous dissolution of small particles during growth of the PbS(e) QDs in the PbCl2−OLA system,14 we anticipated small PbSe QDs could serve as sacrificial materials, releasing monomers via complete dissolution. Chikan and co-worker carried out a kinetic simulation and showed it was possible to focus the size distribution via addition of small particles below the critical radius as a result of Ostwald ripening.10 Additionally, they demonstrated some advantages of the quantized Ostwald ripening compared to the multiple injections method, such as extended time required for size focusing, easier scaling up of synthesis, and reduced occurrence of renucleation that was commonly observed in multiple injections synthesis.10 The basic requirement for quantized Ostwald ripening is a large difference in size between the growing particles and the sacrificial ones. Therefore, when the growth starts from medium-sized particles, extremely small particles should be used. However, it is difficult to obtain such small PbSe (or PbS) QDs with high chemical yields. Since extremely small CdSe (or CdS) QDs can be converted to PbSe (or PbS) QDs with size similar to the original particles via cation exchange, we combined quantized Ostwald ripening and cation exchange to focus the size distribution of PbSe (or PbS) QDs. This method is facilitated by the mature synthesis of cadmium chalcogenide QDs which can be applied to synthesize CdSe (or CdS) QDs with any desired size easily. The new strategy is schematically
Figure 4. Evolution of absorption spectrum of PbSe QDs when CdSe494 QDs were injected with an interval of 6 min.
and increases of size standard deviation (Figure 2c) and size distribution (Figure 2d). Clearly, the PbSe QDs grew bigger which was accompanied by dissolution of relatively small dots. The selenium source, supporting the growth of the PbSe QDs, came from the dissolved particles, which is consistent with our previous observation.14 However, the released monomer was insufficient to maintain a high oversaturation; as a result, the size distribution broadened via Ostwald ripening. To control the size distribution, multiple or continuous injections of precursor were usually adopted.6,30−32 Here, we tried to inject TOPSe and TBPSe (lead precursor in the growth solution is in large excesses), which are commonly used selenium precursors, aimed at maintaining high oversaturation for the growth of PbSe QDs. Figure 1b shows the evolution of PbSe QDs with the injection of TOPSe, which is similar to that when nothing is added. The evolutions of growth rate (Figure 3618
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Figure 5. Evolution of the absorption spectrum of PbS QDs synthesized at different conditions after the injection of medium-sized CdS QDs: nothing (a), TOPS (b), TBPS (c), and small CdS QDs (d) were injected, respectively. The inset in (a) shows the evolution of size distribution in the different synthetic conditions.
Figure 6. TEM images of the PbS-1916 QDs synthesized by the combination of cation exchange and quantized Ostwald ripening. The inset is the size histogram.
QDs were repeated several times to further grow and narrow the size distribution. The evolution of the absorption spectrum of PbSe QDs when small CdSe QDs were injected is shown in Figure 1d. Obviously, the first exciton peak becomes sharper and higher, and the second and third exciton absorption features also evolve to sharp peaks, indicating the significant effect of the injection of small CdSe QDs as opposed to that of TBPSe and TOPSe. As the amounts of selenium in the small CdSe QDs
shown in Scheme 1. Medium-sized CdSe-561 QDs (3.3 nm) were injected into PbCl2−OLA precursor at a relatively high temperature and were converted to PbSe QDs. Then 1.9 nm CdSe-450 QDs were injected into the growth solution, which were exchanged to extremely small PbSe QDs immediately. As a result, the PbSe QDs in the growth solution was in a bimodal distribution and size-focused via quantized Ostwald ripening, producing monodispersive QDs. The injection of small CdSe 3619
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Figure 7. (a) Absorption spectra of fresh and aged PbSe-1936 QDs. The aged QDs were stored in ambient atmosphere in the form of powder. (b) Absorption spectra of fresh PbSe-1912 QDs and those (powder) heated at 80 °C for 4 h in ambient conditions.
were adjusted to be the same as that of TBPSe and TOPSe in the injections, the more remarkable improvement of the injection of small CdSe QDs is not due to different amounts of selenium. The growth rate of PbSe QDs is substantially enhanced via the injection of CdSe QDs (Figure 2a), confirming the best promotion to growth is the dissolution of smaller QDs. The particle number remains constant from the start of the reaction to the end of the growth regime in the quantized Ostwald ripening (Figure 2b), which is consistent with the theoretical study.10 Most notably, the size deviation decreases monotonously only in the growth via the combination of cation exchange and quantized Ostwald ripening (Figure 2c). Consequently, the size distribution decreases monotonously and significantly down to 3.67% (Figure 2d). Additionally, it is possible to further reduce the size distribution via more injections of small CdSe QDs. Because of the high degree of monodispersity, the PbSe QDs self-assembled into hexagonal close packing and three-dimensional superlattice as shown in Figure 3, which is consistent with previous observation.13,33 The size distribution obtained from the TEM image is 3.58% which is consistent with that extracted from the absorption spectrum. For comparison, we tried to perform the quantized Ostwald ripening in the conventional PbO/TOPSe synthesis of PbSe QDs34,35 via injection of extremely small presynthesized PbSe884 QDs (2.6 nm). However, the small PbSe QDs grew bigger instead of dissolving (Figure S3), which may due to an insufficient difference between the sizes of the two QD ensembles, indicating the quantized Ostwald ripening is difficult to be realized in the conventional synthesis. In contrast, quantized Ostwald ripening is much easier to accomplish even when relatively large CdSe QDs were added to the cationexchange synthesis. We repeated the synthesis in Figure 1d by using CdSe-494 QDs instead of CdSe-450 QDs and longer intervals (6 min) between injections were adopted to make sure the converted PbSe QDs from CdSe-494 QDs were completely dissolved. As shown in Figure 4, the first exciton peak increases and narrows continuously with the injection of CdSe-494 QDs, demonstrating the effect of quantized Ostwald ripening. The use of relatively large sacrificial QDs substantially facilitates the application of “cation exchange + Ostwald ripening” synthesis. The new strategy combining cation exchange with quantized Ostwald ripening for controlling size distribution also works
very well for PbS QDs. As shown in Figure 5, both the injections of TOPS and TBPS have little effect on the size distribution, demonstrated by the similar evolutions of the absorption spectrum. Comparatively, the injection of extremely small CdS QDs leads to much sharper first and second exciton peaks, indicating high monodispersities. Quantitative calculation shows that only the injection of CdS QDs results in a monotonous decrease in the size distribution (inset in Figure 5a), while the injections of TOPS and TBPS lead to monotonous increases. The well-packed particles in the TEM images (Figure 6) confirms the high monodispersity (4.26%) of the resulting PbS QDs. One of the advantages of cation-exchange synthesis is that the resulted QDs are copassivated with metal cation and halide anion, contributing to excellent air stability and better device performance.15 XPS measurement showed Cl− and Cd2+ contents of 52.6% and 1.1% with respect to Pb, respectively (Figure S4). Generally, PbSe QDs are very sensitive to air and show blue shift of the first exciton peak due to the decrease of the effective size.36 Additionally, lead chalcogenide QDs have a trend of increasing air sensitivity with the increase of dot size.37 Thanks to the hybrid surface passivation of Cl− and Cd2+, the PbSe QDs synthesized by our method exhibited excellent air stability even for large size (6.3 nm), demonstrated by the unchanged absorption spectrum (Figure 7a) when they were stored in air for 30 days in the form of powder. The absorption spectrum did not change even when the PbSe-1912 QD powder was heated at 80 °C for 4 h in ambient conditions (Figure 7b).
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CONCLUSION In conclusion, a combination of cation exchange and quantized Ostwald ripening was proposed as a new strategy for narrowing the size distribution. The new method was applied to synthesize PbSe and PbS QDs. Compared to the commonly used multiple injections of molecular precursors, the injection of extremely small reactant QDs for cation exchange significantly promoted the growth, leading to a constant particle number and monotonous decreases of size deviation and size distribution. The size distribution was reduced to 3.58% and 4.26% for PbSe and PbS QDs, respectively, and could be further reduced via more injections. The high degree of monodispersity was confirmed by the well-packed particles 3620
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and three-dimensional superlattice in the TEM images. Because of hybrid passivation of Cl− and Cd2+, the as-synthesized PbSe QDs show excellent stability in air even for large sizes. The new strategy will be an important tool for the control of size distribution of colloidal QDs. And the highly monodispersive, well-passivated, stable PbSe and PbS QDs produced by the present method could be ideal materials for MEG-based and infrared applications and also for investigation of self-assembled superlattice as building blocks.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00411. Absorption spectra of CdSe and CdS QDs used for the synthesis of PbSe and PbS QDs. Evolution of absorption spectrum of PbSe QDs in conventional PbO/TOPSe synthesis with the injection PbSe-884 QDs. XPS spectrum of PbSe-1936 QDs (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xiangshui Miao: 0000-0002-6801-2601 Daoli Zhang: 0000-0003-0646-1572 Matthew C. Beard: 0000-0002-2711-1355 Jianbing Zhang: 0000-0003-0642-3939 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC Grant No. 51302096), the Fundamental Research Funds for the Central Universities (No. 2015TS051), the Innovation Foundation of Shenzhen Government (JCYJ20160429182959405), and the Fundamental Research Funds of Wuhan City (No. 2016060101010075). M.C.B. acknowledges support from the Center for Advanced Solar Photophysics, an Energy Frontier Research Center funded by the Office of Science within the U.S. Department of Energy. The authors thank the Analytical and Testing Centre of Huazhong University of Science and Technology for help with measurements.
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REFERENCES
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DOI: 10.1021/acs.chemmater.7b00411 Chem. Mater. 2017, 29, 3615−3622
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DOI: 10.1021/acs.chemmater.7b00411 Chem. Mater. 2017, 29, 3615−3622
