Interphase Synthesis of Zinc Oxide Nanoparticles in a Droplet Flow

Jun 7, 2017 - Liang-Jun Pan , Jia-Wei Tu , Hao-Tian Ma , Yu-Jun Yang , Zhi-Quan Tian , Dai-Wen Pang , Zhi-Ling Zhang. Lab on a Chip 2018 18 (1), 41-56...
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Interphase Synthesis of Zinc Oxide Nanoparticles in a Droplet Flow Reactor Brian G. Zukas and Nivedita R. Gupta* Department of Chemical Engineering, University of New Hampshire, Durham, New Hampshire 03824, United States S Supporting Information *

ABSTRACT: We present a technique for the continuous interphase synthesis of metal oxide nanoparticles using a droplet flow reactor. Conducting the synthesis reaction inside droplets of controlled volume offers several advantages, such as eliminating temperature and concentration gradients inside the reactor as well as preventing reactor fouling. The synthesis reagents are initially located in separate phases, and reagent addition is accomplished through diffusion from the bulk phase to the droplet phase. In this work, the technique is demonstrated by synthesizing zinc oxide nanoparticles inside aqueous droplets containing zinc acetate in a bulk stream of sodium hydroxide in 1-octanol. This flow synthesis of zinc oxide nanoparticles provides more control of the nanoparticle morphology and has a narrow particle size distribution as compared to that of a batch reactor. The size and morphology of the nanoparticles are shown to be easily controlled from 41 to 62 nm by varying zinc acetate and sodium hydroxide concentration, reaction temperature, and residence time. Spherical as well as platelike shapes of zinc oxide nanoparticles are seen for the ranges of parameters studied. This technique can be used to synthesize a variety of metal oxide nanoparticles of controllable size.

1. INTRODUCTION Conducting nanoparticle synthesis in droplet flow reactors has numerous advantages over both batch reactors and continuous single-phase laminar flow reactors. Using droplets to contain the synthesis reaction prevents reactor fouling,1 increases heat transfer,2 and reduces internal concentration gradients through internal droplet convection.3,4 While there has been a significant amount of work investigating the synthesis of nanoparticles in droplets where all the synthesis reagents exist in the same droplet phase, very minimal work has been done investigating nanoparticle synthesis where the reagents are initially located in two separate phases.5 Because the droplet volume in a droplet flow reactor can be precisely controlled by initially separating the reagents into separate phases, the rate of reagent addition can be controlled through mass transfer. This also eliminates the need for some form of droplet merging that is used in many single-phase droplet reactors to combine reagents.6 Droplet merging is often possible over only a tight range of flow rates, may require specialized geometries, and can be difficult if any surfactants are used.7 Zinc oxide nanoparticles can be synthesized using solutionbased techniques where a zinc salt solution is used as the source of zinc ions. In the presence of hydroxide ions, different zinc hydroxo complexes are formed, which depends on the temperature and molar ratio of [OH−]:[Zn2+].8 Under slightly basic conditions, the main zinc species formed are Zn(OH)2 and ZnO, whereas under highly basic conditions (pH > 12), Zn(OH)42− is the preferred species. The formation of ZnO is favored at high temperatures because the decomposition of © XXXX American Chemical Society

solid Zn(OH)2 to ZnO is endothermic at room temperature. At low temperatures, an excess of hydroxide ions is required to solubilize solid Zn(OH)2 to form higher-order zinc hydroxo complexes which further decompose to ZnO.8,9 Zinc oxide nanoparticles have been produced in both continuous laminar flow reactors and in continuous droplet flow reactors. Laminar flow reactors have been used to synthesize zinc oxide nanoparticles in supercritical ethanol,10 to produce zinc oxide quantum dots,11 and to investigate how the presence of Dean vortices affects zinc oxide nanoparticle aggregation.12 The reactor dimensions have been on both the milli- and microscale.13 Flow reactors have also been used to premix reagent streams that are then impinged onto hot silicon wafers as a method of producing zinc oxide nanoparticles.14,15 Zinc oxide nanoparticles have also been formed in a microfluidic droplet flow reactor by forming droplets from a premixed stream of reagents in an inert bulk flow of tetradecane.16,17 Interphase synthesis of nanoparticles in a flow reactor has been investigated to a limited extent. Barium sulfate nanoparticles have been produced in a two-phase system using sulfuric acid dissolved in a bulk alcohol phase and a droplet phase containing aqueous BaCl2.18 A coaxial flow reactor has been used to synthesize TiO2 nanoparticles in a stable annular flow.19 Interphase synthesis of zinc oxide has been limited to batch Received: Revised: Accepted: Published: A

January 30, 2017 May 2, 2017 June 7, 2017 June 7, 2017 DOI: 10.1021/acs.iecr.7b00407 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. Schematic of the experimental setup for the two-phase droplet flow synthesis of zinc oxide nanoparticles.

2.3. Flow Synthesis of Zinc Oxide Nanoparticles. The reactor used for the two-phase flow synthesis of zinc oxide nanoparticles comprised four parts, as shown in Figure 1. First, a coflow drop formation apparatus was used to form droplets of aqueous zinc acetate solution in pure 1-octanol. The inset shows an image of a droplet of zinc acetate forming at the nozzle. Aqueous zinc acetate was pumped by a syringe pump at a flow rate of 0.015 mL/min through a 27 gauge stainless steel needle. The needle was concentrically placed in a cylindrical polydimethylsiloxane (PDMS) channel, ID = 800 μm, that contained the flow of pure octanol at 0.075 mL/min. The stream of droplets exited the drop formation setup through a PTFE tube, ID = 812 μm. Second, a stream of sodium hydroxide dissolved in 1-octanol was introduced to the droplet stream using a T-junction. The sodium hydroxide in 1-octanol solution was pumped by a syringe pump at a flow rate of 0.075 mL/min. Third, the PTFE tubing containing the droplet reactors continued into a hot water bath where zinc oxide nanoparticles were synthesized at a set temperature between 25 and 80 °C. Two tubing lengths were used to control the residence time of the reactor while keeping the total flow rate constant. The 10 min residence time reactor had a tubing length of 3.18 m, while the 20 min residence time reactor had a length of 6.36 m. Finally, the reactor products were collected in an ice-cooled round-bottom flask containing 50 mL of 190 proof ethanol. For each trial at the set reaction conditions, the reactor products were collected for 30 min in the flask before they were prepared for analysis. 2.4. Nanoparticle Washing. After 30 min of collection, the flask contents were split into four 50 mL centrifuge tubes and were centrifuged at 4000 rpm for 20 min in an Eppendorf 5702 centrifuge. The supernatant was decanted until approximately 1 mL remained. The zinc oxide nanoparticles were then dispersed into the remaining liquid and centrifuged at 13 000 rpm for 10 min in an Eppendorf Minispin Plus centrifuge. The supernatant was decanted and replaced with fresh ethanol. The zinc oxide nanoparticles were then dispersed into the ethanol using an ultrasonic bath. The centrifugation and washing steps were repeated four times. After the last centrifugation, the supernatant was decanted and the tubes were placed in a 60 °C oven to evaporate any remaining liquid. 2.5. Size and Morphology Analysis. Images of the zinc oxide nanoparticles were taken using a Tescan Lyra3 scanning electron microscopy (SEM) instrument using the in-beam secondary electron detector at an accelerating voltage of 6.0 kV. The zinc oxide samples used in the SEM instrument were first

systems. Interphase batch synthesis has been done using zinc oleate dissolved in decane and sodium hydroxide dissolved in either water or ethanol.20 Another interphase, batch, miniemulsion synthesis was conducted using triethylamine dissolved in n-decane to synthesize zinc oxide in aqueous droplets containing zinc acetate.9 Noninterphase, batch emulsion synthesis of zinc oxide has also been investigated with aqueous droplets of sodium hydroxide and zinc sulfate dispersed in ndecane. Reagent combination was accomplished through the forced coalescence of the aqueous sodium hydroxide and zinc sulfate droplets.21 In this work, we have synthesized zinc oxide nanoparticles in a millifluidic continuous droplet reactor using a bulk phase of sodium hydroxide in 1-octanol and a droplet phase of aqueous zinc acetate. Reagent combination is accomplished through mass transfer of the sodium hydroxide from the bulk phase to the droplets. The effects of temperature and reagent concentrations and ratios on the size and morphology of the nanoparticles were investigated. In this reactor configuration, the ZnO nanoparticles are synthesized inside the droplets which are separated from the channel walls by thin films of 1octanol resulting in less fouling of the reactor. Convective mixing within the droplets and the bulk flow segments is known to enhance rates of heat transfer,2,22 interfacial mass transfer,23,24 as well as reaction rate.24 An advantage of conducting an interphase reaction is that as the hydroxide ions are consumed inside the droplet, they are continuously supplied from the bulk by mass transfer across the interface. The supply of the hydroxide ions in the bulk octanol flow can be controlled by changing the bulk sodium hydroxide concentration as well as the droplet size and spacing, which in turn affects the interfacial area available for mass transfer.

2. MATERIALS AND METHODS 2.1. Reagents. Zinc acetate dihydrate (Certified, Fisher Scientific), sodium hydroxide (ACS grade, Fisher Scientific), 1octanol (99% pure, Acros Organics), and 190 proof ethanol (ACS grade, Pharmco-Aaper) were used as received without further purification. 2.2. Solution Preparation. The required mass of zinc acetate was dissolved in 100 mL of ultrapure water (Milli-Q, 18.2 MΩ·cm) at room temperature. The required mass of sodium hydroxide was dissolved in 100 mL of 1-octanol. The sodium hydroxide solution was agitated with a magnetic stir bar for 24 h at room temperature to fully dissolve the sodium hydroxide pellets. B

DOI: 10.1021/acs.iecr.7b00407 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. SEM micrographs showing the effect of temperature on the morphology of zinc oxide nanoparticles. Zinc acetate concentration was 0.033 M, and sodium hydroxide concentration was 0.1 M.

transitioning from a droplet shape to a plug shape. This droplet size has been shown in other two-phase systems to produce a local maximum in the nanoparticle size.18 In the following sections, we study the effect of changing reaction temperature, reactant concentrations, and the residence time on the size, shape, and particle size distribution of the synthesized nanoparticles. Unless otherwise noted, the reaction parameters used are the zinc acetate concentration, [Zn2+] = 0.033 M; the sodium hydroxide concentration, [OH−] = 0.1 M; total flow rate, Q = 0.165 mL/min; residence time, τ = 10 min; and temperature, T = 60 °C. 3.1. Effect of Reaction Temperature. The two-phase synthesis of zinc oxide shows clear dependence on the reaction temperature. The reaction temperature directly affects both the rate of mass transfer from the bulk phase and the synthesis kinetics. Figure 2 shows the effects of reaction temperature on the nanoparticle morphology. A reaction temperature of 25 °C produces only platelike particles with lengths that can be over 500 nm. Increasing the temperature to 40 °C changes the morphology to a larger prominence of more spherical particles; however, some platelike particles are still present in the collected product. Reaction temperatures of 60 and 80 °C produce very similar morphologies, and at both temperatures, no platelike particles could be found in the SEM micrographs. The sizes of the particles synthesized at 25 °C were not analyzed because the highly aggregated and overlapping placement of the plates made image analysis extremely difficult. Figure 3 shows the particle size distributions for the reaction temperatures 40 °C and above, and Table 1 provides the size distribution statistics. The size measurements for the reaction temperature at 40 °C were not taken on any platelike particles; only the more spherical particles were measured. Increasing the temperature from 40 to 60 °C produces an increase in the average particle diameter. The reaction kinetics increases as the reaction temperature increases, which promotes faster nucleation and the formation of spherical particles.8 The platelike morphology produced from reaction temperatures at 25 °C has been reported previously in aqueous batch systems but under very different operating conditions. Platelike particles have been reported after a batch reaction time of 15 min using zinc nitrate as a precursor at a pH of 10.5.26 Reaction times of 2 h converted the platelike particles into star shapes. Using a pH of 11, zinc sulfate as a precursor, and a [Zn2+]:[OH−] ratio of 1:4, another report still produced platelike particles after a reaction time of 2 h.8 The morphology of the product produced at 25 °C appears to be very sensitive to the system used. Figure 4a shows the thermogravimetric analysis of the zinc oxide nanoparticles synthesized at the different temperatures. As seen in Figure 4a, mass loss occurring between room

dispersed in acetone using an ultrasonic bath and then deposited on silicon wafer squares. Before viewing in the SEM instrument, the samples were sputter-coated with a gold− palladium layer to improve conductivity. Size measurements were made only for samples with nearly spherical particles by importing the SEM micrographs into Microsoft Visio. Nanoparticles with clear edges were manually selected from different regions of the micrograph, and the longest axis of each particle was measured and recorded as the particle size. The particle size distribution, mean size, and the standard deviation for any sample for a collection of 200 nanoparticles is reported. 2.6. Energy Dispersive Spectroscopy. Elemental analysis was conducted using an EDAX energy dispersive spectroscopy (EDS) system on the Lyra3 SEM instrument with an acceleration voltage of 20 kV and a magnification of 211,000X. 2.7. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) was conducted using a Mettler-Toledo TGA/ DSC 1 STARe system. The temperature range was room temperature to 600 °C, and a heating rate of 5 °C/min was used. The analysis was conducted under an argon atmosphere. 2.8. X-ray Diffraction. X-ray diffraction (XRD) of the nanoparticle samples was conducted on a Burker D2 Phaser diffractometer. The diffractometer was equipped with a copper X-ray tube, and Cu Kα1 X-rays were used for the analysis (λ = 1.5406 Å). The tube was operated at a voltage of 30 kV and a current of 10 mA.

3. RESULTS AND DISCUSSION A coflow geometry was used to produce the aqueous droplets in the millifluidic reactor. The introduction of the coaxial aqueous stream into the bulk 1-octanol stream causes the formation of droplets due to the viscous forces of the 1-octanol stream acting to pinch the aqueous stream into droplets, as seen in the inset in Figure 1. The resulting droplets are formed continuously and with an essentially monodisperse size. The synthesis of zinc oxide begins when this stream of droplets in 1octanol is then mixed with a stream of sodium hydroxide in 1octanol. The mass transfer of the sodium hydroxide to the droplet phase initiates the synthesis of zinc oxide. Because the droplets are monodisperse, the rate of mass transfer can be controlled by either changing concentrations or droplet size. As the droplet volume decreases, the surface area to volume ratio of the droplet increases as does the surface area available for mass transfer, causing an increase in the rate of mass transfer.23 Faster mass transfer leads to faster reaction rates, causing faster nucleation and a smaller average particle size.25 All the results presented in this study are for an initial drop droplet volume of 0.25 μL. The 0.25 μL droplet volume has a droplet radius approximately equal to the channel radius and is at the cusp of C

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Figure 3. Particle size distributions for nanoparticles synthesized at 40, 60, and 80 °C (n = 200).

Table 1. Particle Size Distribution Statistics for Nanoparticles Synthesized at 40, 60, and 80 °C (n = 200) temperature (°C)

mean (nm)

std. dev. (nm)

CV

40 60 80

45.4 54.0 51.9

9.39 9.50 10.5

0.207 0.176 0.202

temperature and approximately 120 °C is attributed to the loss of unbounded water. Mass loss occurring between approximately 120 and 150 °C is typically attributed to the conversion of Zn(OH)2 to ZnO, and additional mass loss at higher temperatures is due to the loss of surface hydroxyl groups.8 The samples from all reaction temperatures show no significant mass loss from 100 to 200 °C. This suggests that all the synthesized nanoparticles are predominantly ZnO. The XRD scans of the synthesized nanoparticles at all four temperatures seen in Figure 4b also confirm peaks consistent with the wurtzite crystal structure for ZnO. Figure 5a shows the EDS spectrum for the ZnO nanoparticles synthesized at 60 °C, while Figure 5b shows the quantitative results for the particles analyzed at all four temperatures. The EDS X-ray spectrum shows that the product is composed of zinc and oxygen. The excess oxygen can be attributed to the presence of oxygenbearing surface species whose presence is shown in the thermogravimetric analysis as well. 3.2. Effect of Zinc Acetate Concentration. The concentration of the zinc acetate solution in the drop phase was changed from 0.025 to 0.075 M at a reaction temperature of 60 °C and a bulk phase sodium hydroxide concentration of 0.1 M. The SEM micrographs for all the synthesized nanoparticles for the various zinc acetate concentrations in

Figure 4. (a) Thermogravimetric and (b) XRD data for zinc oxide nanoparticles synthesized at reaction temperatures 25, 40, 60, and 80 °C.

Figure 6 show spherically shaped particles. The corresponding particle size distribution statistics are shown in Table 2 (particle size distribution is available in Figure S1 in the Supporting Information). A zinc acetate concentration of 0.025 M yielded smaller particles on average, 49.5 nm, while a concentration of 0.075 M yielded the largest particles, 61.9 nm, of all the trials conducted with a residence time of 10 min. While concentrations of 0.033 and 0.050 M produced similarly sized particles, using 0.033 M zinc acetate produced particles with a narrower size distribution. The small mean particle size obtained from the zinc acetate concentration of 0.025 M is most likely caused by the more rapid and complete consumption of the Zn2+ which would prevent any subsequent growth from occurring after nucleation.27 A high ratio of OH−:Zn2+ drives the reaction toward the formation of ZnO; the lower the ratio, the more favorable it is for Zn(OH)2 to be produced.8 The XRD measurements for all zinc acetate concentrations studied show that the synthesized nanoparticles D

DOI: 10.1021/acs.iecr.7b00407 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. (a) EDS X-ray spectrum for zinc oxide synthesized at 60 °C and (b) elemental concentrations at reaction temperatures 25, 40, 60, and 80 °C.

Figure 6. SEM micrographs showing the effect of zinc acetate concentration on nanoparticle morphology. The reaction temperature was 60 °C, and the sodium hydroxide concentration was 0.1 M.

increasing the sodium hydroxide concentration from 0.1 to 0.5 M on the synthesized nanoparticles with a zinc acetate concentration of 0.033 M and a reaction temperature of 60 °C. Increasing the sodium hydroxide concentration has drastic effects on the morphology of the resulting particles. As shown in Table 2, using a sodium hydroxide concentration of 0.1 M produces spherically shaped particles with an average size of 55.0 nm. Increasing the sodium hydroxide to 0.3 M produces many platelike particles, while some spherical-shaped particles are present. Increasing the concentration further to 0.5 M appears to produce primarily platelike particles however these particles are smaller than those produced at 0.3 M. The appearance of platelike zinc oxide particles as the concentration of sodium hydroxide increases has been attributed in previous work to an increase in the reaction kinetics allowing for more time for particle coarsening to occur.8 In addition to the reaction rate, the hydroxide ions can adsorb onto the surface of nanoparticles, encouraging anisotropic particle growth.28

Table 2. Particle Size Distribution Statistics for Nanoparticles Synthesized from Zinc Acetate Concentrations of 0.025, 0.033, 0.050, and 0.075 M (n = 200) [Zn2+] (M)

mean (nm)

std. dev. (nm)

CV

0.025 0.033 0.050 0.075

49.5 54.0 55.0 61.9

9.73 9.49 10.9 16.3

0.197 0.176 0.198 0.263

do not contain Zn(OH)2, showing peaks that are consistent with the zinc oxide wurtzite structure (see Figure S3). 3.3. Effect of Hydroxide Concentration. Changing the sodium hydroxide concentration in the bulk flow of 1-octanol increases the mass-transfer driving force between the bulk and droplet phase and also increases the reaction kinetics of the zinc oxide nanoparticle synthesis. Figure 7 shows the effect of E

DOI: 10.1021/acs.iecr.7b00407 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 7. SEM micrographs showing the effect of sodium hydroxide concentration on zinc oxide nanoparticle morphology. The reaction temperature was 60 °C, and the zinc acetate concentration was 0.05 M.

Figure 8. SEM micrographs showing the effect of reactor residence time on zinc oxide nanoparticle morphology. The reaction temperature was 60 °C; the zinc acetate concentration was 0.033 M, and sodium hydroxide concentration was 0.1 M; the total flow rate was 0.165 mL/min.

3.4. Effect of Residence Time. The effect of residence time was investigated at a constant total flow rate of 0.165 mL/ min by using three different reactor lengths. Figure 8 and Table 3 show the effect of changing residence time on the particle

millifluidic synthesis displays a number of advantages. For the three different zinc acetate concentrations used, Figure 9 shows that the flow reactor produces nanoparticles with a much more homogeneous morphology. The flow reactor produces mostly spherical particles as opposed to the batch reactor which produces a mixture of spherical and platelike particles, especially at higher zinc acetate concentrations. The flow reactor also produces particles with a narrower particle size distribution compared to batch reactor, as seen in Figure 10 and Table 4.

Table 3. Particle Size Distribution Statistics for Nanoparticles Synthesized in a Reactor Using Either a 5, 10, or 20 min residence time (n = 200) τ (min)

mean (nm)

std. dev. (nm)

CV

5 10 20

42.2 54.0 49.6

10.85 9.49 8.02

0.257 0.176 0.162

4. CONCLUSIONS We have demonstrated the two-phase millifluidic synthesis of zinc oxide by using an aqueous droplet phase containing zinc acetate and a 1-octanol bulk phase containing sodium hydroxide. When the synthesis of zinc oxide nanoparticles is conducted in a two-phase millifluidic reactor, a wider degree of control can be exerted over the reaction conditions. We have demonstrated control over the particle size and morphology by controlling both reagent concentrations and reactor parameters. Increasing reactor temperature was seen to promote full conversion of the reagents to zinc oxide and produce smaller, spherical nanoparticles. Increasing zinc acetate concentration in the droplet produced an increase in the mean nanoparticle size. Increasing the concentration of sodium hydroxide in the bulk phase was seen to produce platelike particles. The nanoparticle size as well as morphology could be changed by changing the residence time of the reactor. In general, the millifluidic reactor produced a more monodisperse particle size distribution and a more homogeneous particle morphology when compared to

morphology and size distribution for a reaction temperature of 60 °C, zinc acetate concentration of 0.033 M, and sodium hydroxide concentration of 0.1 M. Particle size distribution is shown in Figure S2. As the residence time increases from 5 to 10 min, the mean particle size increases from 42.2 to 54 nm because of increased time for particle growth. As the residence time is increased from 10 to 20 min, the mean particle size decreases and the size distribution narrows as seen in Figure 8 and Table 3. 3.5. Batch Reactor Comparison. Finally, we compare the zinc oxide nanoparticles synthesized using the droplet flow reactor with a single-phase batch reactor with a residence time of 10 min, sodium hydroxide concentration of 0.1 M, and varying zinc acetate concentrations. A comparison of the particle morphology and size distributions is seen in Figures 9 and 10 and Table 4. When compared to the single-phase aqueous batch synthesis of zinc oxide nanoparticles, the F

DOI: 10.1021/acs.iecr.7b00407 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 9. SEM micrographs comparing zinc oxide nanoparticles produced from the droplet flow reactor and a batch reactor at various zinc acetate concentrations. The reaction temperature was 60 °C, and the sodium hydroxide concentration was 0.1 M.

Figure 10. Particle size distributions for nanoparticles synthesized in either the batch reactor or the droplet flow reactor using various zinc acetate concentrations. The reaction temperature was 60 °C, and the sodium hydroxide concentration was 0.1 M (n = 200).

single-phase batch reactions. For the reactor and reaction parameters chosen in this study, particles in the size range of 40−60 nm were synthesized, which are useful in applications such as water treatment and ultraviolet photodetectors. The reactor diameter and length, droplet size and spacing, as well as

presence of surfactants can be used to control the overall size of the synthesized ZnO nanoparticles. We believe this work on two-phase millifluidic zinc oxide nanoparticles synthesis offers a new approach to synthesize other metal oxide nanoparticles. G

DOI: 10.1021/acs.iecr.7b00407 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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(9) Fricke, M.; Voigt, A.; Veit, P.; Sundmacher, K. Miniemulsionbased process for controlling the size and shape of zinc oxide nanoparticles. Ind. Eng. Chem. Res. 2015, 54, 10293−10300. (10) Ilin, E. S.; Marre, S.; Jubera, V.; Aymonier, C. Continuous supercritical synthesis of high quality UV-emitting ZnO nanocrystals for optochemical applications. J. Mater. Chem. C 2013, 1, 5058−5063. (11) Schejn, A.; Fregnaux, M.; Commenge, J.-M.; Balan, L.; Falk, L.; Schneider, R. Size Controlled synthesis of ZnO quantum dots in microreactors. Nanotechnology 2014, 25, 145606. (12) Choi, C.-H.; Su, Y.-W.; Chang, C. Effects of fluid flow on the growth and assembly of ZnO nanocrystals in a continuous flow microreactor. CrystEngComm 2013, 15, 3326−3333. (13) Kang, H. W.; Leem, J.; Yoon, S. Y.; Sung, H. J. Continuous synthesis of zinc oxide nanoparticles in a microfluidic system for photovoltaic application. Nanoscale 2014, 6, 2840−2846. (14) Jung, J. Y.; Park, N.-K.; Han, S.-Y.; Han, G. B.; Lee, T. J.; Ryu, S. O.; Chang, C.-H. The growth of the flower-like ZnO structure using a continuous flow microreactor. Curr. Appl. Phys. 2008, 8, 720−724. (15) Han, S.-Y.; Paul, B. K.; Chang, C. Nanostructured ZnO as biomimetic anti-reflective coatings on textured silicon using a continuous solution process. J. Mater. Chem. 2012, 22, 22906−22912. (16) Li, S.; Günther, P. M.; Köhler, J. M. Micro segmented-flow technique for continuous synthesis of different kinds of ZnO nanoparticles in aqueous and in DMSO solution. J. Chem. Eng. Jpn. 2009, 42, 338−345. (17) Li, S.; Gross, G. A.; Günther, P. M.; Köhler, J. M. J. M. Hydrothermal micro continuous-flow synthesis of spherical, cylinder-, star- and flower-like ZnO microparticles. Chem. Eng. J. 2011, 167, 681−687. (18) Li, S.; Xu, J.; Wang, Y.; Luo, G. Controllable Preparation of Nanoparticles by Drops and Plugs Flow in a Microchannel Device. Langmuir 2008, 24, 4194−4199. (19) Takagi, M.; Maki, T.; Miyahara, M.; Mae, K. Production of titania nanoparticles by using a new microreactor assembled with same axle dual pipe. Chem. Eng. J. 2004, 101, 269−276. (20) Vorobyova, S. A.; Lesnikovich, A. I.; Mushinskii, V. V. Interphase synthesis and characterization of zinc oxide. Mater. Lett. 2004, 58, 863−866. (21) Winkelmann, M.; Grimm, E.-M.; Comunian, T.; Freudig, B.; Zhou, Y.; Gerlinger, W.; Sachweh, B.; Petra Schuchmann, H. Controlled droplet coalescence in miniemulsions to synthesize zinc oxide nanoparticles by precipitation. Chem. Eng. Sci. 2013, 92, 126− 133. (22) Fischer, M.; Juric, D.; Poulikakos, D. Large convective heat transfer enhancement in microchannels with a train of coflowing immiscible or colloidal droplets. J. Heat Transfer 2010, 132, 112402. (23) Xu, J. H.; Tan, J.; Li, S. W.; Luo, G. S. Enhancement of mass transfer performance of liquid−liquid system by droplet flow in microchannels. Chem. Eng. J. 2008, 141, 242−249. (24) Burns, J. R.; Ramshaw, C. The intensification of rapid reactions in multiphase systems using slug flow in capillaries. Lab Chip 2001, 1, 10−15. (25) Wang, Y.; He, J.; Liu, C.; Chong, W. H.; Chen, H. Thermodynamics versus kinetics in nanosynthesis. Angew. Chem., Int. Ed. 2015, 54, 2022−2051. (26) Oliveira, A. P. A.; Hochepied, J.-F.; Grillon, F.; Berger, M.-H. Controlled precipitation of zinc oxide particles at room temperature. Chem. Mater. 2003, 15, 3202−3207. (27) Nguyen, T.-D. From formation mechanisms to synthetic methods toward shape-controlled oxide nanoparticles. Nanoscale 2013, 5, 9455−9482. (28) Xu, X.; Pang, H.; Zhou, Z.; Fan, X.; Hu, S.; Wang, Y. Preparation of multi-interfacial ZnO particles and their growth mechanism. Adv. Powder Technol. 2011, 22, 634−638.

Table 4. Particle Size Distribution Statistics for Nanoparticles Synthesized in Either the Batch Reactor or the Droplet Flow Reactor Using Various Zinc Acetate Concentrationsa flow [Zn2+] (M)

mean (nm)

std. dev. (nm)

0.025 0.033 0.050

49.5 54.0 55.0

9.73 9.49 10.9

batch CV

mean (nm)

std. dev. (nm)

CV

0.197 0.176 0.198

58.7 50.74 77.5

19.5 12.7 25.2

0.332 0.250 0.325

a

The reaction temperature was 60°C, and the sodium hydroxide concentration was 0.1 M (n = 200).

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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.iecr.7b00407. Additional particle size distributions and XRD measurements for nanoparticles synthesized in a reactor using different zinc acetate concentrations and residence times (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 603-862-3655. Fax: 603862-3747. ORCID

Nivedita R. Gupta: 0000-0002-6742-8445 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Xiaowei Teng of the Chemical Engineering Department for helpful discussions and suggestions. The authors thank Nancy Cherim of the University Instrumentation Center for assistance with the EDS measurements. The authors also thank Shao-Liang Zheng of the Harvard X-ray Laboratory for assistance with the XRD data collection.

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REFERENCES

(1) Nightingale, A. M.; deMello, J. C. Segmented flow reactors for nanocrystal synthesis. Adv. Mater. 2013, 25, 1813−1821. (2) Urbant, P.; Leshansky, A.; Halupovich, Y. On the forced convective heat transport in a droplet-laden flow in microchannels. Microfluid. Nanofluid. 2008, 4, 533−542. (3) Tice, J. D.; Song, H.; Lyon, A. D.; Ismagilov, R. F. Formation of droplets and mixing in multiphase microfluidics at low values of the Reynolds and the capillary numbers. Langmuir 2003, 19, 9127−9133. (4) Teh, S.-Y.; Lin, R.; Hung, L.-H.; Lee, A. P. Droplet microfluidics. Lab Chip 2008, 8, 198−220. (5) Phillips, T. W.; Lignos, I. G.; Maceiczyk, R. M.; deMello, A. J.; deMello, J. C. Nanocrystal synthesis in microfluidic reactors: where next? Lab Chip 2014, 14, 3172−3180. (6) Trivedi, V.; Doshi, A.; Kurup, G. K.; Ereifej, E.; Vandevord, P. J.; Basu, A. S. A modular approach for the generation, storage, mixing, and detection of droplet libraries for high throughput screening. Lab Chip 2010, 10, 2433−2442. (7) Baroud, C. N.; Gallaire, F.; Dangla, R. Dynamics of microfluidic droplets. Lab Chip 2010, 10, 2032−2045. (8) Moezzi, A.; Cortie, M.; McDonagh, A. Aqueous pathways for the formation of zinc oxide nanoparticles. Dalton Trans. 2011, 40, 4871− 4878. H

DOI: 10.1021/acs.iecr.7b00407 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX