Continuous Microwave-Assisted Gas–Liquid Segmented Flow Reactor

Communication pubs.acs.org/crystal

Continuous Microwave-Assisted Gas−Liquid Segmented Flow Reactor for Controlled Nucleation and Growth of Nanocrystals Ki-Joong Kim,†,∥ Richard P. Oleksak,†,∥ Eric B. Hostetler,†,∥ Daniel A. Peterson,‡,∥ Padmavathi Chandran,‡,∥ David M. Schut,§,⊥,∥ Brian K. Paul,*,‡,∥ Gregory S. Herman,*,†,∥ and Chih-Hung Chang*,†,∥ †

School of Chemical, Biological & Environmental Engineering, and ‡School of Mechanical, Industrial and Manufacturing Engineering, Oregon State University, Corvallis, Oregon 97331, United States ∥ Oregon Process Innovation Center, Microproducts Breakthrough Institute, Corvallis, Oregon 97330, United States § Nanophotonic Materials and Devices, Voxtel, Inc., 1241 University of Oregon, Eugene, Oregon 97403, United States S Supporting Information *

ABSTRACT: Hot-injection techniques are currently the state-ofthe-art method for the synthesis of high-quality colloidal nanocrystals (NCs) but have typically been limited to small batch reactors. The nature of this method leads to local fluctuations in temperature and concentration where inhomogeneity due to mixing makes precise control of reaction conditions very challenging at a large scale. Therefore, development of methods to produce high-quality colloidal NCs with highthroughput is necessary for many technological applications. Herein, we report a high-quality and high-throughput NC synthesis method via a continuous microwave-assisted flow reactor where separation of nucleation and growth is demonstrated. A significant issue of microwave heating in a single-phase continuous flow microwave reactor is the deposition of in situ generated NCs on the inner wall of the reactor in the microwave zone. This deposited material leads to significantly enhanced microwave absorption and rapid heating and can result in sparking in the reactor. A gas−liquid segmented flow is used to avoid this problem and also results in improved residence time distributions. The use of this system allows for finely tuned parameters to achieve a high level of control over the reaction by separating the nucleation and growth stages.

C

some lower temperature for NCs growth.12 However, the complex mixing and thermal characteristics of the typical hotinjection technique make it difficult to obtain reproducible NC synthesis, especially when forming ternary (or greater multinary) compositions due to differences in the rates of nucleate formation during the synthesisforming heterogeneous or multiphasic materials. These problems are magnified as the reaction vessel size increases due to changes in heat and mass transfer rates. Therefore, batch hot-injection methods have a fundamental limitation in scalability, and other methods may be required for synthesis of high-quality NCs for commercial applications. Among the alternatives to batch type hot-injection methods, there have been many reports of flow reactors used for synthesizing NCs with high production rates.13−17 Furthermore, the use of microwaves for scaling up the synthesis of colloidal NCs has promise due to the potential for uniform

olloidal nanocrystals (NCs), exhibiting a wide range of size- and shape-dependent properties, have gained attention for a number of applications including light emitting diodes,1,2 lasers,3,4 and solar cells.5−9 Each of these applications depends critically on the ability to synthesize “high-quality” NCs as judged by their uniformity in size, shape, composition, and crystal structure.9,10 The development of a high-throughput method to produce colloidal NCs with the aforementioned properties is also a key step in the commercialization process. NCs are generally synthesized by thermal decomposition of precursors in a mixture of solvents and coordinating ligands. One of the key factors for synthesizing high-quality NCs is the temporal separation of nucleation and growth stages of the reaction which is most commonly accomplished using a hotinjection technique.11 In a typical hot-injection reaction, a solution containing one or more precursors at lower temperature is rapidly injected into a hot (i.e., above nucleation temperature) solution containing the remaining precursor(s), resulting in burst nucleation. The mixed solution naturally decreases rapidly in temperature (thus confining nucleation to a short time period following injection) and is finally held at © 2014 American Chemical Society

Received: June 30, 2014 Revised: September 6, 2014 Published: October 9, 2014 5349

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Figure 1. (A) Schematic diagram of CMAFR system separating nucleation and growth stage developed in this work. (B) The effect of segmented flow in tubing (details are shown in Supporting Information).

Table 1. Experimental Conditions Used in This Work, Measured Cu/In/Se Compositions and Band-Gap of the CuInSe2 NCs Synthesized Using a CMAFR System flow single

segment

temperature (°C)a

residence time (min)b

precursor composition (atomic % Cu/In/Se)

product composition (atomic % Cu/In/Se)c

band-gap (eV)d

w/oe 201 247 247 247 247

20 20 20 20 20 10

1:1:2 1:1:2 1:1:2 1:1:2 1:1:1 1:1:1

1.00:0.21:1.25 1.00:0.23:1.17 1.00:0.49:1.70 1.00:0.97:2.22 1.00:1.01:2.26 1.00:0.15:1.08

1.15 1.09 2.15

a

Temperatures obtained using the thermal model (see Supporting Information) at the microwave heating zone. bResidence times at growth zone maintained at 210 °C. cCompositions measured by EDS (EDS spectra were collected from 4 randomly selected areas on a Si substrate and the average compositions were calculated). dOptical band-gaps were determined by extrapolating the linear region of a plot of the squared absorbance versus the photon energy. eWithout microwave heating (i.e., heating bath only).

degree of control. In a typical synthesis CuCl, InCl3 and Se in oleylamine (OLA) were continuously pumped into a micro Tmixer for rapid initial mixing of the precursors prior to microwave heating. The solution then flowed through a microwave heating zone for burst nucleation followed by a constant temperature bath for NC growth. A gas−liquid segmented flow system was created by introducing Ar gas prior to microwave heating (see Supporting Information for more experimental details). Temperatures outside the microwave heating zone were measured using a fiber optic sensor. In order to approximate the temperature of the processing fluid within the microwave heating (nucleation) zone, a heat transfer balancing model was developed (seeSupporting Information for more details). The different reaction conditions used in the synthesis of CuInSe2 NCs are summarized in Table 1. To evaluate the effects of microwave heating on NC synthesis, reactions were conducted with and without (i.e., heating bath only) microwave heating. Unstable NC solutions were obtained without microwave heating as shown in Figure 2A. Energy dispersive spectroscopy (EDS) results (Table 1) reveal an In and Se deficiency (Cu1.00In0.21Se1.25) for this reaction indicating that longer reaction time or higher reaction temperature is required to obtain CuInSe2 NCs with the desired stoichiometry of 1:1:2. A transmission electron microscopy (TEM) image (Figure 2B)

volumetric heating, very rapid heating rates, and nonthermal effects.18 Most microwave synthesis techniques have relied on simple batch schemes where all reagents are present in the starting reaction mixture and heated to a target temperature, thus typically yields NCs of quality lower than state-of-the-art samples from the hot-injection method. Producing colloidal NCs via a continuous microwave method has the potential to more precisely control reaction conditions while reducing the required production time and lowering the cost per mass of synthesized colloidal NCs. A number of papers have been reported recently using continuous microwave methods to generate unary (Ni,19 Cu,20 and Ag21−23) and binary (Fe2O323 and PbSe24) NCs. However, none of these prior publications report the use of a separated microwave-assisted zone for burst nucleation followed by a growth zone controlled by a constant temperature bath. Herein a high-quality colloidal NCs synthesis method was developed using a continuous microwave-assisted flow reactor (CMAFR) where separation of nucleation and growth is demonstrated (Figure 1A). Ternary CuInSe2 was used as a model system in this work. This material is regarded as a potential candidate for next generation photovoltaic applications.25 Through this system, the rate of nucleation of a thermodynamically dominated step was enhanced to match that of a kinetically dominated step to enable the formation of high-quality ternary CuInSe2 NCs with an unprecedented 5350

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The above reactions were conducted using single phase flow, and the temperature profiles by microwave heating of these reactions are shown in Figure S2A. The temperatures were rapidly ramped to the set point in just a few seconds. However, microwave heating at 247 °C after 20 min led to extremely high temperatures. This was due to deposition of in situ generated NCs on the inner wall of the tubing (Figure S2B), which resulted in sparking (Figure S2C), indicating that some adaptation to the CMAFR system was required in order to perform continuous syntheses. Herein, we applied a gas−liquid segmented flow system to solve this issue. Argon gas was supplied continuously between the micro T-mixer and microwave heating zone leading to gas−liquid segmented flow, which is shown schematically in Figure 1B. In addition to solving the issue of material deposition, the segmented flow provided significant improvement in reaction uniformity. In single-phase laminar flow, a parabolic velocity profile exists causing NCs near the tubing wall to spend a longer time in the tubing than those in the center, leading to a large distribution of residence times. On the contrary, segmentation introduces recirculation (as shown in Figure 1B) within the liquid bringing synthesized NCs from the tubing wall to the center, which facilitates rapid mixing and eliminates the effect of dispersion in residence times.34 It also more rapidly consumes the available precursors in solution, allowing for better size control and smaller size distributions. Gas−liquid segmented reactors have already proven suitable for the synthesis of II−VI semiconductor NCs such as CdSe or CdS quantum dots in microfluidic reactors.35,36 Here a segmented flow is utilized for the ternary CuInSe2 NC synthesis in a CMAFR to eliminate issues with reactor wall fouling and improve the overall reaction uniformity. A sample synthesized with segmented flow microwave heating to 247 °C without further growth from 1:1:1 ratios of Cu/In/Se precursor was analyzed to investigate early stages of the NC reaction pathway. A TEM image (Figure 3A) revealed the presence of NCs, and the structure determined from high-resolution TEM (HRTEM) and composition from EDS indicates NCs synthesized via 5 s of microwave heating with no further heating consists of CuInSe2 quantum dots (QDs). Figure 3B shows the optical absorbance spectrum of the OLA-capped CuInSe2 QDs and was found to have a band

Figure 2. (A) Photographs and TEM images of NCs synthesized (B) without microwave (w/o), (C) microwave heating to 201 °C, and (D) microwave heating to 247 °C. Inset in panel D shows enlarged TEM image of the NCs with an average diameter of 18 nm and a coefficient of variation (COV) of 31.7%. Residence time is 20 min at 210 °C of bath heating.

shows the aggregated nature of NCs synthesized without microwave heating. On the contrary, it can be clearly seen that solution-stable NCs were synthesized when microwave heating was incorporated (Figure 2A) and that the degree of aggregation decreases significantly with increasing microwave heating temperature as shown in the TEM images in Figure 2C,D. The colors of the solutions appear darker with increasing microwave heating temperature, indicating increased conversion of precursors into NCs. In particular, NCs synthesized at higher (247 °C) microwave heating temperature are very solution stable. Several results have been reported in the literature for the synthesis of CuInSe2 by microwave heating in batch reactor,26−33 but all syntheses seemingly resulted in unstable solutions and poor uniformity in size, shape, and composition.

Figure 3. (A) TEM image of CuInSe2 QDs with an average diameter of 2.6 ± 0.4 nm (inset shows an HRTEM image of single QDs with lattice spacings of 0.34 nm corresponding to the (112) plane of CuInSe2). EDS analysis confirms the presence of Cu, In, and Se and shows the composition to be Cu1.00In0.98Se2.19). (B) Absorption spectrum of the QDs synthesized using microwave heating at 247 °C for 5 s without further growth from 1:1:1 ratios of Cu/In/Se precursor. The linear extrapolation in Figure 4B of the absorption spectrum indicates a band gap of 1.59 eV (or 780 nm). 5351

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Figure 4. (A−C) Low and HR-TEM images and (D−F) histogram of particle size distributions of NCs synthesized at different reaction conditions with segmented flow microwave heating of 180 °C and growth at 247 °C. (A) Cu/In/Se = 1:1:2, growth time = 20 min. (B) Cu/In/Se = 1:1:1, growth time = 20 min. (C) Cu/In/Se = 1:1:1, growth time = 10 min. All syntheses used 247 °C in the microwave zone and 210 °C in the growth zone.

Figure 5. (A) XRD patterns and (B) absorbance spectra of the CuInSe2 NCs synthesized at different reaction conditions with segmented flow microwave heating of 247 °C. (Black) Cu/In/Se = 1:1:2, growth time = 20 min; (red) Cu/In/Se = 1:1:1, growth time = 20 min; (blue) Cu/In/Se = 1:1:1, growth time = 10 min. The box in panel A shows an expanded XRD pattern in the 2θ scan range from 34° to 37° to enhance the (211) peak located at 35.5°.

Segmented flow reactions incorporating both microwave heating (nucleation) and heating bath (growth) were investigated. All NCs synthesized with this complete CMAFR system yielded highly monodisperse NCs (Figure 4A−C) with very narrow size distributions, as indicated by the histograms (Figure 4D−F). The NCs grown at 247 °C for 20 min from 1:1:2 ratios of Cu/In/Se precursor shows an average diameter of 16.6 ± 3.2 nm and COV of 19.3%, and had a mixture of trigonal, hexagonal, and spherical shapes as shown in Figure 4A. Surprisingly, NCs exhibited a nearly perfect trigonal shape with

edge at 780 nm (1.59 eV) determined at linear region of the absorption spectrum x-intercept.37 The band edge is blueshifted by approximately 0.54 eV in relation to bulk CuInSe2 (1.04 eV). This is due to the quantum confinement when size decreases below the Bohr exciton radius of the CuInSe2 (10.6 nm), and the shift is in good agreement with that expected for QDs of this size according to DFT calculations.37−40 The rapid synthesis of uniform, rather small diameter CuInSe2 QDs with only 5 s of microwave heating is an intriguing result and is discussed further below. 5352

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Figure 6. Free precursor concentration profiles as a function of reaction time (or distance along the reactor) showing the different stages of nucleation and growth for (A) typical hot-injection technique and (B) the synthesis of high-quality NCs in the CMAFR system according to the LaMer model.

agreement with that of bulk chalcopyrite CuInSe2 (1.05 eV).43,46,47 The formation of CuInSe 2 NCs in OLA has been demonstrated by multiple groups, including a detailed spectroscopic study investigating the reaction pathway of this system.48 It was suggested that disordered (sphalerite) CuInSe2 forms primarily via the reaction of binary CuSe and InSe, whereas ordered (chalcopyrite) CuInSe2 forms primarily via the reaction of CuSe and dissolved InCl3. Thus, due to the lower required temperature and faster kinetics of CuSe nucleation (relative to InSe), slow heating of all dissolved precursors results primarily in chalcopyrite CuInSe2 whereas hot-injection of Se into the high temperature solution containing both metals results primarily in sphalerite CuInSe2. Interestingly, after only 5 s of microwave heating with no subsequent growth bath the primary reaction product is CuInSe2 QDs < 3 nm in diameter. This suggests the rapid and volumetrically uniform heating offered by the microwave zone accelerates the formation of CuInSe2 at very early stages of the reaction, although formation of one or both of the binary selenides (such as CuSe and InSe) likely precedes CuInSe2 formation. CuInSe2 QDs are formed during microwave heating consuming a fraction of dissolved precursors and are sufficiently coordinated with OLA ligands. Incorporating the heating bath following nucleation by microwave heating allows for growth of the stabilized CuInSe2 QDs via diffusion of dissolved Cu, In, and Se ions and/or controlled Ostwald ripening while simultaneously nucleating CuSe, which is common to slow heating procedures using these chemistries.48 In contrast, using only the heating bath results in unstable (or aggregative) NCs in solutions consisting primarily of CuSe intermediates (see EDS in Table 1) due to poor reaction uniformity from lack of distinct nucleation events. Thus, precursors subjected to microwave heating prior to the growth zone resulted in CuInSe2 while those without microwave resulted primarily in CuSe intermediate. Furthermore, the reaction containing stoichiometric precursor concentrations (Cu/In/Se = 1:1:2) resulted primarily in sphalerite CuInSe2 while the reaction containing a Se deficiency in precursor concentration (Cu/In/ Se = 1:1:1) resulted in the ordered chalcopyrite phase. In the case of stoichiometric precursors, the reaction likely proceeds via the reaction of binary CuSe and InSe leading to the formation of sphalerite CuInSe2 according to the formation pathway reported by Kar.48 In the case of Se deficiency in the precursors, formation of CuInSe2 is more likely to occur via reaction of CuSe with dissolved InCl3 and thus leads to

an even higher level of monodispersity (average diameter of 17.1 ± 2.6 nm and a COV of 15.2%) when the Cu/In/Se precursor ratio is changed to 1:1:1 (Figure 4B). The Fourier transform image (inset of Figure 4B) showed well-formed single crystals indexed to the chalcopyrite (tetragonal) phase (JCPDS 40-1487). When the growth time of this 1:1:1 reaction solution is reduced to 10 min, the NCs shows an average diameter of 14.3 nm ±3.0 nm and a COV of 20.9% (Figure 4C). This reaction appears similar to that shown in Figure 4A, consisting of multiple shapes and a similar size variance but a slightly smaller average size, as expected due to the decreased reaction time. The crystal structures of these NCs were investigated using X-ray diffraction (XRD) (Figure 5A). The XRD patterns of the NCs synthesized at growth times of 20 min, with 1:1:2 and 1:1:1 ratios of Cu/In/Se precursors, show peaks at 2θ = 26.6°, 44.2°, and 52.4° corresponding to the (112), (204)/(220), and (312)/(116) planes of CuInSe2. Close inspection of the XRD pattern for the 1:1:1 Cu/In/Se 20 min reaction reveals weaker peaks at 35.5°, which correspond to the (211) planes, consistent with cation ordered tetragonal (chalcopyrite) CuInSe2 (JCPDS No. 40-1487).41 For the 1:1:2 Cu/In/Se 20 min reaction these unique peaks are absent, yet EDS results reveal the both NCs consist of Cu, In, and Se with the desired stoichiometry of 1:1:2 with a slight excess of Se (Table 1). This suggests that the trigonal-shaped NCs (Figure 4B) are chalcopyrite CuInSe2 phase, whereas cation disordered (sphalerite) CuInSe2 NCs have other shapes (Figure 4A) in this study. The trigonal shape of chalcopyrite CuInSe2 NCs is consistent with prior studies.42,43 XRD pattern of the shorter growth time materials (10 min) can be indexed to the tetragonal CuSe phase (JCPDS No. 27-0185) in addition to CuInSe2. The composition of these NCs analyzed by EDS shows an In and Se deficiency, in agreement with XRD results and indicative of the intermediate CuSe phase. Figure 5B shows the optical absorption spectra of these NCs suspended in toluene. A broad and intense absorbance peak for the NCs synthesized at growth time of 10 min was observed at 1050 nm, which is attributed to the indirect band gap of copper selenide.44,45 This peak disappears with an increase in growth time to 20 min, indicating the conversion to CuInSe2 in agreement with TEM, XRD, and EDS data. The estimated direct band gap values for each reaction condition are consistent with a decrease in copper selenide (Table 1, Figure S3), yielding a value of 1.09 eV for 1:1:1 ratios of Cu/In/Se precursor at growth times of 20 min, which is in a good 5353

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using the CMAFR within 5 s. We believe this system will be applicable to a wide range of multinary colloidal NCs and may provide a promising route to large-scale synthesis.

chalcopyrite CuInSe2. This data suggest microwave nucleation plays a critical role in the formation of high-quality NCs and that this process may be suitable for formation of other complex multinary NC systems which are very difficult to produce with significant uniformity via batch type hot-injection. The different stages of nucleation and growth for the precise control of kinetics for NCs in the CMAFR system is adapted from LaMer model49,50 and is compared with typical hotinjection technique as shown in Figure 6. Ideally, the initial nucleation events partially relieve the supersaturation condition, preventing nucleation of new NCs. During the final stage, slower and more controlled NC growth continues only from existing nuclei resulting in increased NC uniformity over time. As shown, the rapid hot-injection of precursors instantly raises the concentration above the critical nucleation threshold (stage II in Figure 6A). This supersaturation is then partially relieved by a short intense burst of nucleation, as well as a rapid temperature decrease causing the precursor concentration to fall below the critical threshold. In most cases, precipitation of NCs occurs by growth of existing nuclei during the remaining reaction time. If the growth of NCs continues in the solution, secondary growth by Ostwald ripening can occur because some smaller NCs are consumed and eventually disappear, and thus NCs are still growing leading to large size distributions.51 In contrast, Figure 6B clearly shows the different concentration profile of the CMAFR system as a function of reaction time (or distance along the reactor). Note that the initial precursors were already mixed homogeneously prior to microwave heating (stage I in Figure 6B) since the reactions could be separated into nucleation and growth zone in the CMAFR system. The NCs are formed having a burst nucleation event in the microwave heating (stage II in Figure 6B), diminish the temperature and concentration gradients within the reactor by uniform volumetric heating and very rapid heating rates, then the concentration of precursors quickly decreases over time until it reaches the level at which the nucleation rate is zero. After this, the system enters the growth stage (stage III in Figure 6B), sufficient to provide the thermal activation necessary for continued growth of existing nuclei, but insufficient to allow for any appreciable nucleation to occur. Thus, nucleation and growth are successfully divided, providing the optimum conditions needed to produce monodisperse and narrow size distribution of NCs. The results presented here show that the CMAFR system is very suitable for synthesizing high-quality NCs with good control over NCs size and can reproducibly and reliably generate large quantities of uniform NCs. In conclusion, we demonstrated the first example of ternary CuInSe2 NCs synthesis using a CMAFR system. The synthesized NCs are of very high quality, which is largely attributed to a distinct separation between nucleation and growth stages for this reaction method. A key obstacle during microwave heating was deposition of the synthesized NCs on the reactor tube wall resulting in sparking. This issue was solved by introducing Ar gas prior to microwave heating, resulting in segmented flow which minimized deposition of the NCs on the wall surface and improved residence time distributions. The segmented flow also provided a recirculation motion in the liquid layer to enhance the overall uniformity of the reaction. CMAFR has been demonstrated to provide better control of the sizes and shapes of ternary NCs in comparison with more conventional synthetic methods. Uniform, rather small CuInSe2 NCs that show quantum confinement could be synthesized

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures, characterization of the NCs and details of the thermal model are included. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(C.-H.C.) E-mail: [email protected]. *(B.K.P.) E-mail: [email protected]. *(G.S.H.) E-mail: [email protected]. Present Address ⊥

(D.M.S.) Shoei Electronic Materials, Inc., Eugene, Oregon 97402, United States.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the Oregon Nanoscience & Microtechnologies Institute (ONAMI) GAP program for funding this project. This work was also performed at the Oregon Process Innovation Center (OPIC) for Sustainable Solar Cell Manufacturing, an Oregon BEST signature research facility.

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REFERENCES

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dx.doi.org/10.1021/cg500959m | Cryst. Growth Des. 2014, 14, 5349−5355