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Continuous synthesis of zinc oxide nanoparticles in a microfluidic system for photovoltaic application Hyun Wook Kang, Juyoung Leem, Sang Youl Yoon and Hyung Jin Sung* This study describes the synthesis of zinc oxide nanoparticles (ZnO NPs) using a microfluidic system. A continuous and efficient synthetic process was developed based on a microfluidic reactor in which was implemented a time pulsed mixing method that had been optimized using numerical simulations and experimental methods. Numerical simulations revealed that efficient mixing conditions could be obtained over the frequency range 5–15 Hz. This system used ethanol solutions containing 30 mM sodium hydroxide (NaOH) or 10 mM dehydrated zinc acetate (Zn(OAc)2) under 5 Hz pulsed conditions, which provided the optimal mixing performance conditions. The ZnO NPs prepared using the microfluidic synthetic system or batch-processed system were validated by several analytical methods, including transmission electron microscopy (TEM), energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD), UV/VIS NIR and zeta (z) potential analysis. Bulk-heterojunction organic photovoltaic
Received 19th November 2013 Accepted 27th November 2013
cells were fabricated with the synthesized ZnO NPs to investigate the practicability and compared with batch-process synthesized ZnO NPs. The results showed that microfluidic synthesized ZnO NPs had
DOI: 10.1039/c3nr06141h
good preservability and stability in working solution and the synthetic microfluidic system provided a
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low-cost, environmentally friendly approach to the continuous production of ZnO NPs.
Introduction Zinc oxide nanoparticles (ZnO NPs) are widely used as n-type semiconductor materials in the preparation of useful nanoscale devices that depend on a direct wide band gap (3.37 eV), a highly selective sensitivity for certain chemical species, piezoelectric characteristics, electrical and optical properties, and a large (60 meV) excitation binding energy.1,2 The unique collection of properties displayed by ZnO NPs makes the material useful in transparent conducting electrodes for photovoltaic devices,3 photonic sensors,4 and chemical sensors,5,6 and the NPs provide excellent seed materials in ZnO nanowire synthesis.7–10 ZnO NPs may be synthesized by a variety of methods, including coprecipitation,11 sol–gel,12 plasma reaction,13 and solution-based methods.14,15 Among these methods, the solution-based methods are most amenable to integration with applications because they are low-temperature, rapid, and environmentally friendly processes. The cost-effectiveness of ZnO NP production may be improved by simplifying the synthetic process, as in a continuous microuidic manufacturing system. Microuidic devices have been developed in the context of physics, biology, chemistry, and engineering applications. Most applications rely on control over small volumes of uids on the microliter scale. From miniaturized fuel cells to DNA and labon-a-chip devices, microuidic systems provide special
Department of Mechanical Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea. E-mail: [email protected]
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functions and a high efficiency.16–19 As an example of a microuidic chemical synthesis application, a micro-reaction system was designed to enable reactions involving multiple chemical reagents, thereby offering continuous operation, enhancing heat and mass transfer, reducing the reaction time and volumes, and protecting the reaction from air and moisture in a microscale device with a closed reaction environment.20–23 In such systems, microuidic mixing increased the rate of diffusion between two reagent delivery solutions. The benets derived from rapid diffusion have motivated progress toward device miniaturization and mass production through the integration of microscale devices into a large plant system. Microuidic mixing can proceed either actively or passively. Active mixing involves the application of external forces, such as acoustic,24 ultrasonic,25 electrokinetic,26 magnetic,27 or thermal28 elds, to disturb the sample reagents during a mixing process. In addition to these mixing enhancement methods, the timed pulsing of reagent ows has been explored. Several numerical simulation and experimental studies have examined mixing enhancement using time pulsing techniques. Glasgow and coworkers investigated the parameters associated with pulsed ow mixing using computational uid dynamics and ow visualization schemes.29,30 They showed that a higher Strouhal number (ratio of the ow characteristic time scale to the pulsing time period) and pulse volume ratio (ratio of the volume of a pulsed uid to the volume of an inlet intersection) induced better mixing. The pulse waveform and Reynolds number were found to have relatively insignicant effects on the
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mixing performance of a pulsed mixing system. Lim and coworkers developed a pulsed ow approach using electroosmotic ow as the external driving force and a T-shaped microchannel.31 They showed that mixing could be enhanced by increasing the pulsed ow frequency up to an optimal frequency of 4–6 Hz. Subsequent increases in the pulsed ow frequency reduced the homogeneity of mixing. Wang and coworkers demonstrated the preparation of a magnetic particledriven micromixer, which was also characterized by an optimal frequency that depended on the dimensions of the microchannel.32 In this case, the pulsed ow was driven by magnetic forces acting on magnetic particles present in the working uid, which, unfortunately, prevents the application of this approach to general chemical analysis. Here, we report the design of a continuous ZnO NPs synthetic process using a microuidic system. The system was optimized for rapid synthesis through numerical simulation and experimental time pulsed mixing studies. The microuidic synthetic ZnO NPs were compared with batch-process synthetic ZnO NPs to investigate the advantages of the microuidic synthetic system. For more applications, organic photovoltaic cells were fabricated and analyzed. The process described here presents an environmentally friendly, fast, and low-cost mass production system that may potentially be integrated into a large chemical plant.
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microchannels is shown in Fig. 1b. The synthetic process proceeded by injecting chemical reagents into the inlet ports directly connected to the preheating region. The 92.5 mm-long preheating region functioned both to preheat and to stabilize the reagents. The preheated reagent ows converged at the conuence of the mixing region. Aer completion of mixing, the mixed reagents owed into a 341.5 mm-long synthesis region where the chemicals reacted under uniform heating conditions. Aer reacting, the reaction solution was then collected in a reservoir via the outlet port. Chemical preparation The chemicals were prepared according to the method described by Pacholski, with modications.15 Ethanol served as a working uid and as the solvent. The sodium hydroxide (NaOH, Sigma Aldrich) and zinc acetate dehydrate (Zn(OAc)2, Sigma Aldrich) solutions were prepared to have concentrations of 30 mM and 10 mM, respectively. The solutions were injected into the microchannel through Teon microtubing using a syringe pump (Nemesys, Cetoni). The injected volume ratio of the two solutions was 1 : 1.924 NaOH : Zn(OAc)2. During the synthetic process, the microchannel was heated at 60 C (K type thermocouples) using a lm heater controlled by a proportional integral derivative (PID) controller. The synthesized ZnO NPs were cooled to room temperature (25 C) and stored in a glass bottle.
Experimental Microuidic system
Photovoltaic device fabrication
A microuidic system for ZnO NPs synthesis was fabricated by applying deep reactive ion etching (DRIE) techniques to a silicon substrate. The microchannels were 200 mm wide and 200 mm deep. The initial DRIE process generated the 200 mm deep channels. The silicon substrates containing the microchannels were then anodically bonded to a glass coverslip. As shown in Fig. 1a, the channels formed three distinct regions to allow for preheating, mixing, and synthesis, with two inlet ports and one outlet port. A photograph of the fabricated
The bulk-heterojunction organic photovoltaic cells were fabricated by depositing the following thin layers onto the indium tin oxide coated glass (15 U sq1): 80 nm of ZnO NPs (spin coated with 1000 rpm for 30 s aer drying at 150 C for 10 min), 400 nm of P3HT:PCBM, 1 mm of PEDOT:PSS, and 1 mm of Ag (printed by screen printer). The fabricated cell area was 1 cm1. Measurements The transmission electron microscopy (TEM) images and energy dispersive spectrometry (EDS) data were acquired using an FE-TEM (Tecnai F20, Philips). X-ray diffraction (XRD) measurements were collected using a thin lm X-ray diffractometer (D/MAX-RC, Rigaku) by q–2q scanning. Zeta (z) potential measurements were performed using a zeta analyzer (ELS-22, Otsukael) based on Smoluchowski's theory. Specular transmission spectra were obtained using a UV/VIS NIR spectrophotometer (V-570, Jasco). The current density–voltage curves were obtained using a solar simulator (IVT Solar) under 100 mW cm2 of AM 1.5 global solar illumination with a calibrated light source by a standard Si photodiode. Numerical simulations
Fig. 1 (a) Schematic diagram of the microfluidic channel used for ZnO NPs synthesis. Microfluidic channels included preheating, mixing, and synthesis regions, with two inlets and one outlet. (b) A deep reactive ion etched microfluidic channel with an anodic bonded glass cover.
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The computational domain of the simulation is shown in Fig. 2. The T-shape microchannel indicates the conuence at the mixing region. The system coordinates were positioned such that the origin was located at the center of the two inlet branches, the streamwise direction of the outlet branch was
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Results and discussion
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Numerical simulations Effective mixing conditions were identied in the numerical simulations by measuring the degree of mixing based on the mass fractions of the synthetic solutions. The degree of mixing was dened as Computational domain of the numerical simulation. The xcoordinate corresponds to the streamwise direction of the outlet branch, the y-coordinate corresponds to the direction perpendicular to the outlet branch, and the z-coordinate corresponds to the lateral direction of the microchannel. Fig. 2
oriented along the X-axis, the direction perpendicular to the outlet branch was oriented along the Y-axis, and the lateral direction of the channel was oriented along the Z-axis. The y-directional distance between the two inlets was 3 mm, and the x-directional distance from the conuence and the outlet was 7 mm. The width and height of the microchannel were each 200 mm. The ow rates of the NaOH and Zn(OAc)2 solutions were 0.855 mL min1 and 1.645 mL min1, respectively. Aer conuence, the total ow rate was 2.5 mL min1, with a mean velocity of 10.42 mm s1. This ow rate was determined by calculating the maximum theoretical ow rate under an expected total pressure drop (DPtotal) of less than 20 kPa for a stable process. The total pressure drop was calculated from the sum of the pressure drops in the rst preheating region DPph1, second preheating region DPph2, mixing region DPmixing, and synthesis region DPsynthesis as follows, DPtotal ¼ DPph1 + DPph2 + DPmixing + DPsynthesis.
(1)
Each pressure drop was calculated using the pressure drop equation described by Bahrami,33 DP ¼ 16p2m uIPLA3
(2)
is where m is the dynamic viscosity (0.604 103 kg m1 s), u the mean velocity (m s1), L is the length of the streamwise direction (m), A is the cross-sectional area (m2), and IP is the polar moment of inertia (m4), IP ¼ 0.25pwh(w2 + h2),
(3)
where w and h are the dimensions of the channel width and height (m), respectively. The ow rate at each inlet could be expressed in the form of a sine function, 0.855(1 + sin( f 2pt + p)) mL min1 and 1.645(1 + sin( f 2pt)) mL min1 for the NaOH and Zn(OAc)2 solutions, respectively (t: time, f: frequency). The phase difference between the pulse injection proles at the two inlets was explored as a function of frequency. The diffusion constant (D) was D ¼ 109 m2 s1, which is typical of ion diffusion in aqueous solutions. The time step in the numerical simulation was set to 50 steps per pulse cycle. Data were recorded every 10 steps and analyzed according to the mass fraction of the selected regions to evaluate the effectiveness of mixing.
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" Degree of mixing ¼ 1
n X
#0:5 2
ðfi fÞ ðyi ymean Þ=n
f1 ;
i¼1
(4) where n is the number of cells in a selected region, yi is the velocity of the ith cell, ymean is the mean velocity in the selected region, fi is the mass fraction of the ith cell, and f is the mass fraction assuming equal mixing between the two mixed solutions. Due to differences in the ow rates of each solution, f was dened as . f ¼ qNaOH qNaOH þ qZnðOAcÞ2 for NaOH solution . (5) f ¼ qZnðOAcÞ2 qNaOH þ qZnðOAcÞ2 for ZnðOAcÞ2 solution NaOH is the mean ow rate of the NaOH solution and where q Zn(OAc)2 is the ow rate of the Zn(OAc)2 solution. The mixing q efficiency was quantied based on the degree of mixing over the range [0 to 1], where a value of 0 indicated no mixing and 1 indicated full mixing. To avoid round-off errors from the numerical simulation, full mixing was dened as a degree of mixing greater than 0.995. In addition to the degree of mixing, the full mixing time (tf) and length (‘f) were calculated to form a basis for effective mixing comparisons. Full mixing was dened as the point aer which the degree of mixing always exceeded 0.995. The full mixing time was dened as the time required for the uid initially at the conuence (t ¼ 0) to reach the rst point at which full mixing was observed. The full mixing length was dened as the distance from the conuence point to the full mixing point. Fig. 3 shows the degree of mixing as a function of time for various mixing frequencies (1, 3, and 5 Hz) at a cross section of X ¼ 7.0 mm. From 0 to 0.4 s, no mixing was observed because the synthetic solutions had not yet owed. The degree of mixing increased signicantly aer 0.4 s. The initial rate of mixing was most rapid in the system that underwent 1 Hz pulsed mixing; however, the degree of mixing in this system did not reach the full mixing region and uctuated over the range 0.6–0.9. The system that underwent 3 Hz pulsed mixing tended to uctuate over the range 0.9–0.99. On the other hand, the system that underwent 5 Hz pulsed mixing showed a stable increase in the degree of mixing up to the full mixing region. Full mixing was obtained at 1.4 s, and no signicant uctuations were observed thereaer. The effects of pulsed ow are apparent from the spatial proles of the mass fractions for various pulse frequencies, as shown in Fig. 4. The contours of the Zn(OAc)2 mass fraction are shown at several perpendicular cross sections corresponding to Z ¼ 100 mm, X ¼ 0, 1.5, 3.0, 5.0, and 7.0 mm. As shown in Fig. 4a and b, the large plug stream prevented
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facilitated efficient mixing at the interface of the mixing solutions. In this case, full mixing was observed prior to the end of the mixing channel. The optimal frequency range for achieving effective mixing was identied based on a productivity factor (h), dened as the ratio of the full mixing times (at X ¼ 7.0 mm) and lengths under pulsed or continuous ow mixing (no pulse), h = (tf‘f/tclc) 1
Fig. 3 The degree of mixing depended on the mixing time and mixing frequency (1, 3, and 5 Hz). The full mixing region indicates a 0.995 degree of mixing.
(6)
where tc and ‘c are the full mixing time and length under continuous ow mixing, respectively. Numerical simulations were used to measure the full continuous ow mixing time and length, found to be tc ¼ 1.62 sec and ‘c ¼ 6.75 mm, respectively. Fig. 5 plots the productivity factor as a function of the mixing frequency. Mixing was not enhanced at frequencies below 4 Hz (the poor mixing region). Efficient mixing was obtained in the frequency range 4–15 Hz (the mixing-enhanced region). A frequency of 5 Hz yielded the best mixing enhancement. Productivity factors at frequencies beyond 15 Hz asymptotically approached the continuous ow limit value. The validation between simulation and experiment should be made. However, it is difficult in the present system to visualize the mixing phenomena due to the opaque silicon based microuidic channel and the transparent synthetic solutions (ethanol).
Synthesis of ZnO NPs ZnO NPs were synthesized in a batch process for comparison purposes. Sodium hydroxide (NaOH) and zinc acetate dehydrate (Zn(OAc)2) were dissolved in ethanol (C2H5OH) to prepare 30 mM and 10 mM solutions, respectively. The solutions were heated in an Erlenmeyer ask immersed in a silicon oil bath maintained at 60 C. Subsequently, 25 mL of the Zn(OAc)2 solution were poured into an Erlenmeyer ask and incubated for 30 min to ensure uniform heating conditions and
Numerical simulations of the pulse mixing system. (a) 1 Hz, (b) 3 Hz, and (c) 5 Hz. The contours indicate the mass fraction of the Zn(OAc)2 solution.
Fig. 4
effective mixing between two solutions. Cross-sectional views of the streamwise mass fraction distribution show that effective mixing was not achieved in these cases, even up to the end of the mixing region. By contrast, the sample that underwent 5 Hz pulsed ow mixing displayed effective mixing at the end of the mixing region, as shown in Fig. 4c. The small plug stream
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Productivity factor as a function of the mixing frequency. The mixing enhancement region indicates regions in which mixing was more efficient under pulse mixing conditions than under continuous mixing conditions.
Fig. 5
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stabilization. Finally, 13 mL of the NaOH solution were slowly added to the Zn(OAc)2 solution, and the reaction was stirred for 2 hours to ensure full reaction. The synthesized ZnO NPs were cooled to room temperature and stored in a glass bottle. Fig. 6a shows a TEM image of the NPs produced from the batch process. ZnO NPs were 3–5 nm in diameter and were crystalline in structure, as shown in the inset of Fig. 6a. Red ellipses indicate the single ZnO NPs. ZnO NP synthesis was conducted in a microuidic system according to the optimal system parameters identied in the numerical simulations described above. A 5 Hz pulse frequency was applied using the pulse injection mode of a syringe pump. The mean ow rates were 0.855 mL min1 for the NaOH and 1.645 mL min1 for the Zn(OAc)2 solutions. The pulsed ow could be described by the expressions: 0.855(1 + sin(10pt + p)) and 1.645(1 + sin(10pt)) mL min1 for the NaOH and Zn(OAc)2 solutions, respectively. The Reynolds number was 2.72 and the Strouhal number was 0.1. As an initial condition, the microchannel was lled with pure ethanol and preheated to 60 C using a thin lm heater. The 30 mM NaOH and 10 mM Zn(OAc)2 solutions were injected from separate syringes and delivered to the inlet ports via Teon microtubes. The injected solutions were heated in the 92.5 mm preheating region. The NaOH and Zn(OAc)2 solutions remained in the preheating region for 26 s and 13.5 s, respectively, and were fully heated to 60 C. The solutions subsequently reached the conuence point marking the start of the mixing region. A uniform temperature across both solutions avoided abnormal ZnO NP synthetic conditions. In the mixing region, the injected solutions converged and mixed under the differential pulse phase conditions. Full mixing between the two solutions was achieved in the mixing region. The mixed solution subsequently entered the synthetic
Transmission electron microscopy (TEM) images of (a) the batch process-synthesized zinc oxide nanoparticles (ZnO NPs) and (b) the microfluidic system-synthesized ZnO NPs under 5 Hz pulse mixing. The insets indicate a magnified view (individual ZnO NPs are highlighted in the red ellipses). Electron dispersive spectrometry (EDS) data for (c) the batch process-synthesized ZnO NPs and (d) the microfluidic system-synthesized ZnO NPs.
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region to synthesize the ZnO NPs. The solution remained in the synthesis region for 35 s and was uniformly heated to 60 C. The ZnO NPs synthesized in ethanol were collected in an enclosed reservoir via Teon microtubing connected to the outlet port. Fig. 6b shows a TEM image of ZnO NPs. The ZnO NPs were 3–5 nm in diameter, crystalline in structure, and morphologically indistinguishable from the NPs prepared in the batch synthetic process. The EDS results (Fig. 6c and d) showed the presence of ZnK (elemental zinc) and OK (elemental oxygen) peaks for NPs synthesized by either method, indicating a zinc and oxygen elemental composition without other chemical components. The peaks corresponding to C and Cu were introduced from the TEM grid. The NP compositions were compared quantitatively based on the XRD data. Fig. 7 shows the XRD proles for the ZnO NPs synthesized via batch or microuidic systems. The XRD data were collected along the q–2q angles and the powder diffraction le (PDF) was analyzed. The peaks of both samples were coincident at 31.75, 34.20, 36.20, 46.63, and 56.58. The PDF revealed the presence of crystalline ZnO, indicating that the ZnO NPs synthesized using a pulse ow mixing microuidic system were indistinguishable from those synthesized using a batch synthetic process. The measured properties of microuidic/batch synthesized ZnO NPs showed the same results in the structure morphology and chemical formation. However, the preservability and dispersibility of synthesized ZnO NPs in ethanol solution showed different results depending on their synthetic conditions. In Fig. 8a and b, the transmittance results of ZnO NPs dispersed ethanol solution are presented according to the storage time
Fig. 6
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Fig. 7 X-ray diffraction (XRD) data for the (a) batch process-synthesized ZnO NPs and (b) the microfluidic system-synthesized ZnO NPs.
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and synthetic methods. The transmittance of microuidic synthesized ZnO NPs in ethanol solution showed 90.26% and 85.74% in the range of visible wavelength (390–700 nm) for storage times of 24 and 330 hours respectively. On the other hand, the transmittance of batch process-synthesized ZnO NPs ethanol solution signicantly decreased from 89.82% for a storage time of 24 hours to 52.23% for 330 hours storage with the same measurement conditions. In addition, the visible color of batch process-synthesized ZnO NPs ethanol solution slightly changed from transparent to white opaque, while the microuidic synthetic ZnO NPs ethanol solution remained transparent up to a storage time of 330 hours. These effects arose from the heat conditions of the synthetic process which affected the electrical stabilization of synthesized ZnO NPs. As described before, the microuidic system provides rapid heat and mass transfer conditions during the synthetic process that contribute to reduce the reaction time and maintain the stability of synthesized ZnO NPs. In contrast, the batch process-synthetic approach needs more time than the microuidic system to ensure the full reaction of ZnO NPs because of the low heat transfer and mixing efficiency. By these effects, the earlier synthesized ZnO NPs were held at high temperature until nishing the synthesis and cooling to room temperature, and the electrical stability of ZnO NPs became worse as they tend to easily coagulate with other ZnO NPs. For more quantitative analysis, the zeta (z) potential value was measured as presented in the tables in Fig. 8a and b. The results show that the
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measured z potential values were 31.3 eV (24 h) to 10.3 eV (330 h) and 28.7 eV (24 h) to 2.39 eV (330 h) for microuidic and batch processed syntheses respectively. A low z potential value indicates high occulation tendency in a colloid.34,35 From this point of view, the microuidic synthesized ZnO NPs have more stable electrical and storage properties than batch processsynthesized ZnO NPs; this shows the advantages of the microuidic synthetic system and their high potential for practical applications. To measure the performance difference of synthesized ZnO NPs in electrical device applications, bulkheterojunction organic photovoltaic (OPV) cells with an inverted conguration were tested with the 330 hours stored ZnO NPs as an electron transporting layer. The OPV cells were fabricated using the following built-up sequence of thin layers on the indium tin oxide (ITO) coated glass (15 U sq1): 80 nm of ZnO NPs, 400 nm of P3HT:PCBM, 1 mm of PEDOT:PSS (hole transporting), and 1 mm of Ag. In Fig. 8c, the current density– voltage characteristics of the devices under 100 mW cm2 of AM 1.5 illumination are presented. The power conversion efficiency (PCE) of the OPV with microuidic synthesized ZnO NPs was 1.67%, higher than the PCE of the OPV with batch processsynthesized ZnO NPs (0.09%). In addition to the PCE, the open circuit voltage (VOC), short circuit current (JSC), and ll factor (FF) values also show that the microuidic synthetic ZnO NPs based OPV has higher performance than the batch processsynthesized ZnO NPs based OPV. These results show that the microuidic system has advantages to maintain the characteristics of synthesized ZnO NPs and its applications, and guarantees the good preservability and performance. We anticipate that a microuidic system and ow control technique will be applicable to more functional nanomaterials synthesis and present economic and environmental benets through mass production plant systems with closed reaction conditions.
Conclusions
Fig. 8 Transmittance and zeta potential values of ZnO NPs ethanol solutions depending on the storage time after synthesis (24 or 330 hours): (a) microfluidic synthesis, and (b) batch-process synthesis. (c) Current density versus voltage results for fabricated organic photovoltaic cells with synthesized ZnO NPs.
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We successfully prepared a microuidic system for use in ZnO NP synthesis. The degree of mixing enhancement through application of time pulsing was investigated using numerical simulations. Efficient mixing was observed over the range 5–15 Hz. A pulse frequency of 5 Hz was predicted to display the best mixing enhancement. A microuidic channel was designed to allow for the mixing and reaction of two components. The microchannels included regions for preheating, mixing, and synthesis, where uniform heating at 60 C was applied using a thin lm heater. Ethanol solutions of 30 mM sodium hydroxide and 10 mM dehydrate zinc acetate were prepared for the synthesis of the ZnO NPs. The solutions were pulse owed with a 180 phase difference to achieve a uniform ow rate at the outlet. The ow rates were selected based on the system efficiency and the pressure drop across the microuidic channel. The mean ow rates were 0.855 mL min1 and 1.645 mL min1 for the NaOH and Zn(OAc)2 solutions, respectively. The synthetic results were analyzed by TEM, EDS, and XRD. ZnO NPs were synthesized by a batch process for comparison purposes. The ZnO NPs synthesized in the microuidic system were 3–5 nm in diameter, crystalline in structure, and indistinguishable from the
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batch-synthesized ZnO NPs. However, the measurements of transparency and zeta potential showed that the preservability and dispersibility in solution of synthesized ZnO NPs had better performance than batch-synthesized ZnO NPs. In terms of the performance in a practical application, the fabricated OPV cells with microuidic synthesized ZnO NPs showed better results than batch-synthesized ZnO NPs based OPV. These results were induced by the rapid heat and mass transfer effects of the microuidic system with reaction conditions closed to the environment. This useful ZnO NP synthetic microuidic system is low-cost and environmentally benign, offering a powerful means for efficient mass production.
Acknowledgements This work was supported by the Creative Research Initiatives (no. 2012-0000246) program of the National Research Foundation of Korea.
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Continuous synthesis of zinc oxide nanoparticles in a microfluidic system for photovoltaic application Hyun Wook Kang, Juyoung Leem, Sang Youl Yoon and Hyung Jin Sung* This study describes the synthesis of zinc oxide nanoparticles (ZnO NPs) using a microfluidic system. A continuous and efficient synthetic process was developed based on a microfluidic reactor in which was implemented a time pulsed mixing method that had been optimized using numerical simulations and experimental methods. Numerical simulations revealed that efficient mixing conditions could be obtained over the frequency range 5–15 Hz. This system used ethanol solutions containing 30 mM sodium hydroxide (NaOH) or 10 mM dehydrated zinc acetate (Zn(OAc)2) under 5 Hz pulsed conditions, which provided the optimal mixing performance conditions. The ZnO NPs prepared using the microfluidic synthetic system or batch-processed system were validated by several analytical methods, including transmission electron microscopy (TEM), energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD), UV/VIS NIR and zeta (z) potential analysis. Bulk-heterojunction organic photovoltaic
Received 19th November 2013 Accepted 27th November 2013
cells were fabricated with the synthesized ZnO NPs to investigate the practicability and compared with batch-process synthesized ZnO NPs. The results showed that microfluidic synthesized ZnO NPs had
DOI: 10.1039/c3nr06141h
good preservability and stability in working solution and the synthetic microfluidic system provided a
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low-cost, environmentally friendly approach to the continuous production of ZnO NPs.
Introduction Zinc oxide nanoparticles (ZnO NPs) are widely used as n-type semiconductor materials in the preparation of useful nanoscale devices that depend on a direct wide band gap (3.37 eV), a highly selective sensitivity for certain chemical species, piezoelectric characteristics, electrical and optical properties, and a large (60 meV) excitation binding energy.1,2 The unique collection of properties displayed by ZnO NPs makes the material useful in transparent conducting electrodes for photovoltaic devices,3 photonic sensors,4 and chemical sensors,5,6 and the NPs provide excellent seed materials in ZnO nanowire synthesis.7–10 ZnO NPs may be synthesized by a variety of methods, including coprecipitation,11 sol–gel,12 plasma reaction,13 and solution-based methods.14,15 Among these methods, the solution-based methods are most amenable to integration with applications because they are low-temperature, rapid, and environmentally friendly processes. The cost-effectiveness of ZnO NP production may be improved by simplifying the synthetic process, as in a continuous microuidic manufacturing system. Microuidic devices have been developed in the context of physics, biology, chemistry, and engineering applications. Most applications rely on control over small volumes of uids on the microliter scale. From miniaturized fuel cells to DNA and labon-a-chip devices, microuidic systems provide special
Department of Mechanical Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea. E-mail: [email protected]
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functions and a high efficiency.16–19 As an example of a microuidic chemical synthesis application, a micro-reaction system was designed to enable reactions involving multiple chemical reagents, thereby offering continuous operation, enhancing heat and mass transfer, reducing the reaction time and volumes, and protecting the reaction from air and moisture in a microscale device with a closed reaction environment.20–23 In such systems, microuidic mixing increased the rate of diffusion between two reagent delivery solutions. The benets derived from rapid diffusion have motivated progress toward device miniaturization and mass production through the integration of microscale devices into a large plant system. Microuidic mixing can proceed either actively or passively. Active mixing involves the application of external forces, such as acoustic,24 ultrasonic,25 electrokinetic,26 magnetic,27 or thermal28 elds, to disturb the sample reagents during a mixing process. In addition to these mixing enhancement methods, the timed pulsing of reagent ows has been explored. Several numerical simulation and experimental studies have examined mixing enhancement using time pulsing techniques. Glasgow and coworkers investigated the parameters associated with pulsed ow mixing using computational uid dynamics and ow visualization schemes.29,30 They showed that a higher Strouhal number (ratio of the ow characteristic time scale to the pulsing time period) and pulse volume ratio (ratio of the volume of a pulsed uid to the volume of an inlet intersection) induced better mixing. The pulse waveform and Reynolds number were found to have relatively insignicant effects on the
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mixing performance of a pulsed mixing system. Lim and coworkers developed a pulsed ow approach using electroosmotic ow as the external driving force and a T-shaped microchannel.31 They showed that mixing could be enhanced by increasing the pulsed ow frequency up to an optimal frequency of 4–6 Hz. Subsequent increases in the pulsed ow frequency reduced the homogeneity of mixing. Wang and coworkers demonstrated the preparation of a magnetic particledriven micromixer, which was also characterized by an optimal frequency that depended on the dimensions of the microchannel.32 In this case, the pulsed ow was driven by magnetic forces acting on magnetic particles present in the working uid, which, unfortunately, prevents the application of this approach to general chemical analysis. Here, we report the design of a continuous ZnO NPs synthetic process using a microuidic system. The system was optimized for rapid synthesis through numerical simulation and experimental time pulsed mixing studies. The microuidic synthetic ZnO NPs were compared with batch-process synthetic ZnO NPs to investigate the advantages of the microuidic synthetic system. For more applications, organic photovoltaic cells were fabricated and analyzed. The process described here presents an environmentally friendly, fast, and low-cost mass production system that may potentially be integrated into a large chemical plant.
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microchannels is shown in Fig. 1b. The synthetic process proceeded by injecting chemical reagents into the inlet ports directly connected to the preheating region. The 92.5 mm-long preheating region functioned both to preheat and to stabilize the reagents. The preheated reagent ows converged at the conuence of the mixing region. Aer completion of mixing, the mixed reagents owed into a 341.5 mm-long synthesis region where the chemicals reacted under uniform heating conditions. Aer reacting, the reaction solution was then collected in a reservoir via the outlet port. Chemical preparation The chemicals were prepared according to the method described by Pacholski, with modications.15 Ethanol served as a working uid and as the solvent. The sodium hydroxide (NaOH, Sigma Aldrich) and zinc acetate dehydrate (Zn(OAc)2, Sigma Aldrich) solutions were prepared to have concentrations of 30 mM and 10 mM, respectively. The solutions were injected into the microchannel through Teon microtubing using a syringe pump (Nemesys, Cetoni). The injected volume ratio of the two solutions was 1 : 1.924 NaOH : Zn(OAc)2. During the synthetic process, the microchannel was heated at 60 C (K type thermocouples) using a lm heater controlled by a proportional integral derivative (PID) controller. The synthesized ZnO NPs were cooled to room temperature (25 C) and stored in a glass bottle.
Experimental Microuidic system
Photovoltaic device fabrication
A microuidic system for ZnO NPs synthesis was fabricated by applying deep reactive ion etching (DRIE) techniques to a silicon substrate. The microchannels were 200 mm wide and 200 mm deep. The initial DRIE process generated the 200 mm deep channels. The silicon substrates containing the microchannels were then anodically bonded to a glass coverslip. As shown in Fig. 1a, the channels formed three distinct regions to allow for preheating, mixing, and synthesis, with two inlet ports and one outlet port. A photograph of the fabricated
The bulk-heterojunction organic photovoltaic cells were fabricated by depositing the following thin layers onto the indium tin oxide coated glass (15 U sq1): 80 nm of ZnO NPs (spin coated with 1000 rpm for 30 s aer drying at 150 C for 10 min), 400 nm of P3HT:PCBM, 1 mm of PEDOT:PSS, and 1 mm of Ag (printed by screen printer). The fabricated cell area was 1 cm1. Measurements The transmission electron microscopy (TEM) images and energy dispersive spectrometry (EDS) data were acquired using an FE-TEM (Tecnai F20, Philips). X-ray diffraction (XRD) measurements were collected using a thin lm X-ray diffractometer (D/MAX-RC, Rigaku) by q–2q scanning. Zeta (z) potential measurements were performed using a zeta analyzer (ELS-22, Otsukael) based on Smoluchowski's theory. Specular transmission spectra were obtained using a UV/VIS NIR spectrophotometer (V-570, Jasco). The current density–voltage curves were obtained using a solar simulator (IVT Solar) under 100 mW cm2 of AM 1.5 global solar illumination with a calibrated light source by a standard Si photodiode. Numerical simulations
Fig. 1 (a) Schematic diagram of the microfluidic channel used for ZnO NPs synthesis. Microfluidic channels included preheating, mixing, and synthesis regions, with two inlets and one outlet. (b) A deep reactive ion etched microfluidic channel with an anodic bonded glass cover.
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The computational domain of the simulation is shown in Fig. 2. The T-shape microchannel indicates the conuence at the mixing region. The system coordinates were positioned such that the origin was located at the center of the two inlet branches, the streamwise direction of the outlet branch was
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Results and discussion
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Numerical simulations Effective mixing conditions were identied in the numerical simulations by measuring the degree of mixing based on the mass fractions of the synthetic solutions. The degree of mixing was dened as Computational domain of the numerical simulation. The xcoordinate corresponds to the streamwise direction of the outlet branch, the y-coordinate corresponds to the direction perpendicular to the outlet branch, and the z-coordinate corresponds to the lateral direction of the microchannel. Fig. 2
oriented along the X-axis, the direction perpendicular to the outlet branch was oriented along the Y-axis, and the lateral direction of the channel was oriented along the Z-axis. The y-directional distance between the two inlets was 3 mm, and the x-directional distance from the conuence and the outlet was 7 mm. The width and height of the microchannel were each 200 mm. The ow rates of the NaOH and Zn(OAc)2 solutions were 0.855 mL min1 and 1.645 mL min1, respectively. Aer conuence, the total ow rate was 2.5 mL min1, with a mean velocity of 10.42 mm s1. This ow rate was determined by calculating the maximum theoretical ow rate under an expected total pressure drop (DPtotal) of less than 20 kPa for a stable process. The total pressure drop was calculated from the sum of the pressure drops in the rst preheating region DPph1, second preheating region DPph2, mixing region DPmixing, and synthesis region DPsynthesis as follows, DPtotal ¼ DPph1 + DPph2 + DPmixing + DPsynthesis.
(1)
Each pressure drop was calculated using the pressure drop equation described by Bahrami,33 DP ¼ 16p2m uIPLA3
(2)
is where m is the dynamic viscosity (0.604 103 kg m1 s), u the mean velocity (m s1), L is the length of the streamwise direction (m), A is the cross-sectional area (m2), and IP is the polar moment of inertia (m4), IP ¼ 0.25pwh(w2 + h2),
(3)
where w and h are the dimensions of the channel width and height (m), respectively. The ow rate at each inlet could be expressed in the form of a sine function, 0.855(1 + sin( f 2pt + p)) mL min1 and 1.645(1 + sin( f 2pt)) mL min1 for the NaOH and Zn(OAc)2 solutions, respectively (t: time, f: frequency). The phase difference between the pulse injection proles at the two inlets was explored as a function of frequency. The diffusion constant (D) was D ¼ 109 m2 s1, which is typical of ion diffusion in aqueous solutions. The time step in the numerical simulation was set to 50 steps per pulse cycle. Data were recorded every 10 steps and analyzed according to the mass fraction of the selected regions to evaluate the effectiveness of mixing.
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" Degree of mixing ¼ 1
n X
#0:5 2
ðfi fÞ ðyi ymean Þ=n
f1 ;
i¼1
(4) where n is the number of cells in a selected region, yi is the velocity of the ith cell, ymean is the mean velocity in the selected region, fi is the mass fraction of the ith cell, and f is the mass fraction assuming equal mixing between the two mixed solutions. Due to differences in the ow rates of each solution, f was dened as . f ¼ qNaOH qNaOH þ qZnðOAcÞ2 for NaOH solution . (5) f ¼ qZnðOAcÞ2 qNaOH þ qZnðOAcÞ2 for ZnðOAcÞ2 solution NaOH is the mean ow rate of the NaOH solution and where q Zn(OAc)2 is the ow rate of the Zn(OAc)2 solution. The mixing q efficiency was quantied based on the degree of mixing over the range [0 to 1], where a value of 0 indicated no mixing and 1 indicated full mixing. To avoid round-off errors from the numerical simulation, full mixing was dened as a degree of mixing greater than 0.995. In addition to the degree of mixing, the full mixing time (tf) and length (‘f) were calculated to form a basis for effective mixing comparisons. Full mixing was dened as the point aer which the degree of mixing always exceeded 0.995. The full mixing time was dened as the time required for the uid initially at the conuence (t ¼ 0) to reach the rst point at which full mixing was observed. The full mixing length was dened as the distance from the conuence point to the full mixing point. Fig. 3 shows the degree of mixing as a function of time for various mixing frequencies (1, 3, and 5 Hz) at a cross section of X ¼ 7.0 mm. From 0 to 0.4 s, no mixing was observed because the synthetic solutions had not yet owed. The degree of mixing increased signicantly aer 0.4 s. The initial rate of mixing was most rapid in the system that underwent 1 Hz pulsed mixing; however, the degree of mixing in this system did not reach the full mixing region and uctuated over the range 0.6–0.9. The system that underwent 3 Hz pulsed mixing tended to uctuate over the range 0.9–0.99. On the other hand, the system that underwent 5 Hz pulsed mixing showed a stable increase in the degree of mixing up to the full mixing region. Full mixing was obtained at 1.4 s, and no signicant uctuations were observed thereaer. The effects of pulsed ow are apparent from the spatial proles of the mass fractions for various pulse frequencies, as shown in Fig. 4. The contours of the Zn(OAc)2 mass fraction are shown at several perpendicular cross sections corresponding to Z ¼ 100 mm, X ¼ 0, 1.5, 3.0, 5.0, and 7.0 mm. As shown in Fig. 4a and b, the large plug stream prevented
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facilitated efficient mixing at the interface of the mixing solutions. In this case, full mixing was observed prior to the end of the mixing channel. The optimal frequency range for achieving effective mixing was identied based on a productivity factor (h), dened as the ratio of the full mixing times (at X ¼ 7.0 mm) and lengths under pulsed or continuous ow mixing (no pulse), h = (tf‘f/tclc) 1
Fig. 3 The degree of mixing depended on the mixing time and mixing frequency (1, 3, and 5 Hz). The full mixing region indicates a 0.995 degree of mixing.
(6)
where tc and ‘c are the full mixing time and length under continuous ow mixing, respectively. Numerical simulations were used to measure the full continuous ow mixing time and length, found to be tc ¼ 1.62 sec and ‘c ¼ 6.75 mm, respectively. Fig. 5 plots the productivity factor as a function of the mixing frequency. Mixing was not enhanced at frequencies below 4 Hz (the poor mixing region). Efficient mixing was obtained in the frequency range 4–15 Hz (the mixing-enhanced region). A frequency of 5 Hz yielded the best mixing enhancement. Productivity factors at frequencies beyond 15 Hz asymptotically approached the continuous ow limit value. The validation between simulation and experiment should be made. However, it is difficult in the present system to visualize the mixing phenomena due to the opaque silicon based microuidic channel and the transparent synthetic solutions (ethanol).
Synthesis of ZnO NPs ZnO NPs were synthesized in a batch process for comparison purposes. Sodium hydroxide (NaOH) and zinc acetate dehydrate (Zn(OAc)2) were dissolved in ethanol (C2H5OH) to prepare 30 mM and 10 mM solutions, respectively. The solutions were heated in an Erlenmeyer ask immersed in a silicon oil bath maintained at 60 C. Subsequently, 25 mL of the Zn(OAc)2 solution were poured into an Erlenmeyer ask and incubated for 30 min to ensure uniform heating conditions and
Numerical simulations of the pulse mixing system. (a) 1 Hz, (b) 3 Hz, and (c) 5 Hz. The contours indicate the mass fraction of the Zn(OAc)2 solution.
Fig. 4
effective mixing between two solutions. Cross-sectional views of the streamwise mass fraction distribution show that effective mixing was not achieved in these cases, even up to the end of the mixing region. By contrast, the sample that underwent 5 Hz pulsed ow mixing displayed effective mixing at the end of the mixing region, as shown in Fig. 4c. The small plug stream
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Productivity factor as a function of the mixing frequency. The mixing enhancement region indicates regions in which mixing was more efficient under pulse mixing conditions than under continuous mixing conditions.
Fig. 5
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stabilization. Finally, 13 mL of the NaOH solution were slowly added to the Zn(OAc)2 solution, and the reaction was stirred for 2 hours to ensure full reaction. The synthesized ZnO NPs were cooled to room temperature and stored in a glass bottle. Fig. 6a shows a TEM image of the NPs produced from the batch process. ZnO NPs were 3–5 nm in diameter and were crystalline in structure, as shown in the inset of Fig. 6a. Red ellipses indicate the single ZnO NPs. ZnO NP synthesis was conducted in a microuidic system according to the optimal system parameters identied in the numerical simulations described above. A 5 Hz pulse frequency was applied using the pulse injection mode of a syringe pump. The mean ow rates were 0.855 mL min1 for the NaOH and 1.645 mL min1 for the Zn(OAc)2 solutions. The pulsed ow could be described by the expressions: 0.855(1 + sin(10pt + p)) and 1.645(1 + sin(10pt)) mL min1 for the NaOH and Zn(OAc)2 solutions, respectively. The Reynolds number was 2.72 and the Strouhal number was 0.1. As an initial condition, the microchannel was lled with pure ethanol and preheated to 60 C using a thin lm heater. The 30 mM NaOH and 10 mM Zn(OAc)2 solutions were injected from separate syringes and delivered to the inlet ports via Teon microtubes. The injected solutions were heated in the 92.5 mm preheating region. The NaOH and Zn(OAc)2 solutions remained in the preheating region for 26 s and 13.5 s, respectively, and were fully heated to 60 C. The solutions subsequently reached the conuence point marking the start of the mixing region. A uniform temperature across both solutions avoided abnormal ZnO NP synthetic conditions. In the mixing region, the injected solutions converged and mixed under the differential pulse phase conditions. Full mixing between the two solutions was achieved in the mixing region. The mixed solution subsequently entered the synthetic
Transmission electron microscopy (TEM) images of (a) the batch process-synthesized zinc oxide nanoparticles (ZnO NPs) and (b) the microfluidic system-synthesized ZnO NPs under 5 Hz pulse mixing. The insets indicate a magnified view (individual ZnO NPs are highlighted in the red ellipses). Electron dispersive spectrometry (EDS) data for (c) the batch process-synthesized ZnO NPs and (d) the microfluidic system-synthesized ZnO NPs.
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region to synthesize the ZnO NPs. The solution remained in the synthesis region for 35 s and was uniformly heated to 60 C. The ZnO NPs synthesized in ethanol were collected in an enclosed reservoir via Teon microtubing connected to the outlet port. Fig. 6b shows a TEM image of ZnO NPs. The ZnO NPs were 3–5 nm in diameter, crystalline in structure, and morphologically indistinguishable from the NPs prepared in the batch synthetic process. The EDS results (Fig. 6c and d) showed the presence of ZnK (elemental zinc) and OK (elemental oxygen) peaks for NPs synthesized by either method, indicating a zinc and oxygen elemental composition without other chemical components. The peaks corresponding to C and Cu were introduced from the TEM grid. The NP compositions were compared quantitatively based on the XRD data. Fig. 7 shows the XRD proles for the ZnO NPs synthesized via batch or microuidic systems. The XRD data were collected along the q–2q angles and the powder diffraction le (PDF) was analyzed. The peaks of both samples were coincident at 31.75, 34.20, 36.20, 46.63, and 56.58. The PDF revealed the presence of crystalline ZnO, indicating that the ZnO NPs synthesized using a pulse ow mixing microuidic system were indistinguishable from those synthesized using a batch synthetic process. The measured properties of microuidic/batch synthesized ZnO NPs showed the same results in the structure morphology and chemical formation. However, the preservability and dispersibility of synthesized ZnO NPs in ethanol solution showed different results depending on their synthetic conditions. In Fig. 8a and b, the transmittance results of ZnO NPs dispersed ethanol solution are presented according to the storage time
Fig. 6
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Fig. 7 X-ray diffraction (XRD) data for the (a) batch process-synthesized ZnO NPs and (b) the microfluidic system-synthesized ZnO NPs.
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and synthetic methods. The transmittance of microuidic synthesized ZnO NPs in ethanol solution showed 90.26% and 85.74% in the range of visible wavelength (390–700 nm) for storage times of 24 and 330 hours respectively. On the other hand, the transmittance of batch process-synthesized ZnO NPs ethanol solution signicantly decreased from 89.82% for a storage time of 24 hours to 52.23% for 330 hours storage with the same measurement conditions. In addition, the visible color of batch process-synthesized ZnO NPs ethanol solution slightly changed from transparent to white opaque, while the microuidic synthetic ZnO NPs ethanol solution remained transparent up to a storage time of 330 hours. These effects arose from the heat conditions of the synthetic process which affected the electrical stabilization of synthesized ZnO NPs. As described before, the microuidic system provides rapid heat and mass transfer conditions during the synthetic process that contribute to reduce the reaction time and maintain the stability of synthesized ZnO NPs. In contrast, the batch process-synthetic approach needs more time than the microuidic system to ensure the full reaction of ZnO NPs because of the low heat transfer and mixing efficiency. By these effects, the earlier synthesized ZnO NPs were held at high temperature until nishing the synthesis and cooling to room temperature, and the electrical stability of ZnO NPs became worse as they tend to easily coagulate with other ZnO NPs. For more quantitative analysis, the zeta (z) potential value was measured as presented in the tables in Fig. 8a and b. The results show that the
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measured z potential values were 31.3 eV (24 h) to 10.3 eV (330 h) and 28.7 eV (24 h) to 2.39 eV (330 h) for microuidic and batch processed syntheses respectively. A low z potential value indicates high occulation tendency in a colloid.34,35 From this point of view, the microuidic synthesized ZnO NPs have more stable electrical and storage properties than batch processsynthesized ZnO NPs; this shows the advantages of the microuidic synthetic system and their high potential for practical applications. To measure the performance difference of synthesized ZnO NPs in electrical device applications, bulkheterojunction organic photovoltaic (OPV) cells with an inverted conguration were tested with the 330 hours stored ZnO NPs as an electron transporting layer. The OPV cells were fabricated using the following built-up sequence of thin layers on the indium tin oxide (ITO) coated glass (15 U sq1): 80 nm of ZnO NPs, 400 nm of P3HT:PCBM, 1 mm of PEDOT:PSS (hole transporting), and 1 mm of Ag. In Fig. 8c, the current density– voltage characteristics of the devices under 100 mW cm2 of AM 1.5 illumination are presented. The power conversion efficiency (PCE) of the OPV with microuidic synthesized ZnO NPs was 1.67%, higher than the PCE of the OPV with batch processsynthesized ZnO NPs (0.09%). In addition to the PCE, the open circuit voltage (VOC), short circuit current (JSC), and ll factor (FF) values also show that the microuidic synthetic ZnO NPs based OPV has higher performance than the batch processsynthesized ZnO NPs based OPV. These results show that the microuidic system has advantages to maintain the characteristics of synthesized ZnO NPs and its applications, and guarantees the good preservability and performance. We anticipate that a microuidic system and ow control technique will be applicable to more functional nanomaterials synthesis and present economic and environmental benets through mass production plant systems with closed reaction conditions.
Conclusions
Fig. 8 Transmittance and zeta potential values of ZnO NPs ethanol solutions depending on the storage time after synthesis (24 or 330 hours): (a) microfluidic synthesis, and (b) batch-process synthesis. (c) Current density versus voltage results for fabricated organic photovoltaic cells with synthesized ZnO NPs.
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We successfully prepared a microuidic system for use in ZnO NP synthesis. The degree of mixing enhancement through application of time pulsing was investigated using numerical simulations. Efficient mixing was observed over the range 5–15 Hz. A pulse frequency of 5 Hz was predicted to display the best mixing enhancement. A microuidic channel was designed to allow for the mixing and reaction of two components. The microchannels included regions for preheating, mixing, and synthesis, where uniform heating at 60 C was applied using a thin lm heater. Ethanol solutions of 30 mM sodium hydroxide and 10 mM dehydrate zinc acetate were prepared for the synthesis of the ZnO NPs. The solutions were pulse owed with a 180 phase difference to achieve a uniform ow rate at the outlet. The ow rates were selected based on the system efficiency and the pressure drop across the microuidic channel. The mean ow rates were 0.855 mL min1 and 1.645 mL min1 for the NaOH and Zn(OAc)2 solutions, respectively. The synthetic results were analyzed by TEM, EDS, and XRD. ZnO NPs were synthesized by a batch process for comparison purposes. The ZnO NPs synthesized in the microuidic system were 3–5 nm in diameter, crystalline in structure, and indistinguishable from the
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batch-synthesized ZnO NPs. However, the measurements of transparency and zeta potential showed that the preservability and dispersibility in solution of synthesized ZnO NPs had better performance than batch-synthesized ZnO NPs. In terms of the performance in a practical application, the fabricated OPV cells with microuidic synthesized ZnO NPs showed better results than batch-synthesized ZnO NPs based OPV. These results were induced by the rapid heat and mass transfer effects of the microuidic system with reaction conditions closed to the environment. This useful ZnO NP synthetic microuidic system is low-cost and environmentally benign, offering a powerful means for efficient mass production.
Acknowledgements This work was supported by the Creative Research Initiatives (no. 2012-0000246) program of the National Research Foundation of Korea.
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