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Size-controlled synthesis of ZnO quantum dots in microreactors
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Nanotechnology Nanotechnology 25 (2014) 145606 (9pp)
doi:10.1088/0957-4484/25/14/145606
Size-controlled synthesis of ZnO quantum dots in microreactors Aleksandra Schejn1 , Mathieu Frégnaux1 , Jean-Marc Commenge1 , Lavinia Balan2 , Laurent Falk1 and Raphaël Schneider1 1
Université de Lorraine, Laboratoire Réactions et Génie des Procédés (LRGP), UMR 7274, CNRS, 1 rue Grandville, BP 20451, F-54001 Nancy Cedex, France 2 Institut de Science des Matériaux de Mulhouse (IS2M), LRC 7228, 15 rue Jean Starcky, F-68093 Mulhouse, France E-mail: [email protected] Received 31 October 2014, revised 30 January 2014 Accepted for publication 13 February 2014 Published 14 March 2014
Abstract
In this paper, we report on a continuous-flow microreactor process to prepare ZnO quantum dots (QDs) with widely tunable particle size and photoluminescence emission wavelengths. X-ray diffraction, electron diffraction, UV–vis, photoluminescence and transmission electron microscopy measurements were used to characterize the synthesized ZnO QDs. By varying operating conditions (temperature, flow rate) or the capping ligand, ZnO QDs with diameters ranging from 3.6 to 5.2 nm and fluorescence maxima from 500 to 560 nm were prepared. Results obtained show that low reaction temperatures (20 or 35 ◦ C), high flow rates and the use of propionic acid as a stabilizing agent are favorable for the production of ZnO QDs with high photoluminescence quantum yields (up to 30%). Keywords: quantum dots, zinc oxide, microreactor, surface ligand, fluorescence S Online supplementary data available from stacks.iop.org/Nano/25/145606/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
microchip) and preparation of large quantities of nanocrystals through a continuous process. Among nanocrystals, semiconductor quantum dots (QDs) are of great interest for both fundamental research and industrial applications due to their size-dependent optical properties originating from quantum size effects [4, 5]. During the last couple of decades, QDs achieved many and various applications as biological fluorescent probes [6, 7], sensors [8], or in optoelectronic devices [9, 10]. QDs like CdS [11, 12], CdSe [13–18], CdTe [14, 17, 19], InP [20], and their core/shell counterparts were successfully prepared in microreactors and the quality of the nanocrystals obtained was reported to be higher than what can be produced by bench top methods. Due to its wide band gap (3.37 eV) and large exciton binding energy (60 meV at room temperature), zinc oxide (ZnO) is an important member of II–VI semiconductor. Indeed, it is abundant, stable, of low toxicity [21] and has shown great potential for use in ultraviolet laser devices [22], biomedical labels [23, 24], photocatalysis, varistors, sensors, and
A microreactor is a device with microscale channels which allows chemical reactions to be performed in a reaction volume several orders of magnitude smaller than conventional batch reactors. The potential advantages of using a microreactor, rather than a conventional batch reactor, include enhanced heat and mass transfer, high speed mixing, precise and independent control of operating parameters, better homogeneity in the chemical environment, improved safety and improved yields [1]. Microreactors have for a long time only been considered for organic reactions. Since ten years, it has been demonstrated that they may also offer a great variety of advantages for the synthesis of nanoparticles [2, 3]. Among them, efficient mixing allows the preparation of monodispersed nanoparticles, and microfluidic devices provide potential for automating multi-step process (analysis, surface functionalization and purification in a single 0957-4484/14/145606+09$33.00
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piezoelectric transducers [25–27]. ZnO QDs display interesting photoluminescent properties in the near UV or in the visible spectrum ranges. The near UV emission corresponds to the excitonic emission and is based on the direct recombination of electron–hole pairs [28]. The visible green–yellow emission centered at about 520 nm originates from the electronic transition from the conduction band edge to a trap level and the singly ionized oxygen vacancies on the QD surface have been assumed to be the recombination centers [29, 30]. Micro continuous-flow synthesis of ZnO particles has recently received a great interest. Supercritical hydrothermal syntheses using either homogeneous phase reaction or segmented flow can produce cylinder-, star-, or flower-like ZnO particles [31] or ZnO nanorods [32]. Syntheses in supercritical methanol using oleic acid (OA) as ligand afford ZnO nanocrystals with diameters varying between 10 and 30 nm [33]. The use of more coordinating ligands such as tri-n-octylphosphine or tri-n-octylphosphine oxide allows ZnO QDs with sizes of about 3.7 nm to be prepared [34, 35]. Noteworthy is that the high operating temperature (250 ◦ C) used in these syntheses affords nanocrystals with excitonic emission located at about 375 nm (3.30 eV), thus valuable in applications such as UV-leds and lasers. Photoluminescence (PL) in the visible range (500–600 nm) is much more interesting for bio-imaging applications [23, 36, 37] but the synthesis of ZnO QDs exhibiting this feature has never been reported through a microfluidic process. Among the wet chemical routes recently developed for the preparation of ZnO QDs emitting in the visible range (polyol, organometallic, pyrolysis,. . . ) [38–41], the sol–gel process is by far the most popular due to its simplicity but it generates QDs with relatively low photoluminescence quantum yields (generally about 10–15%) [21, 23, 26, 36, 42, 43]. Moreover, if the dots are not efficiently stabilized, they tend to undergo Ostwald ripening in solution and to aggregate. After the nucleation stage, the ZnO QDs photoluminescence shifts gradually to the red and their quantum yields decrease [44, 45]. Therefore, the synthesis of stable, equisized, and well-dispersible ZnO QDs is still a challenge in material science. This paper concentrates on the preparation of ZnO QDs through a low temperature sol–gel method because wet chemical routes involving an appropriate choice of precursors and surfactants generally provide a much better tuning of the particles shape and size distribution. Since surface modifications have a significant impact on the optical properties of ZnO QDs [40, 46, 47], designing and controlling their surface chemistry appears a very attractive means to tune their photoluminescence (PL) properties. In the last part of the paper, the influence of surface modification by various organic acids (oleic acid (OA), benzoic acid (BA), propionic acid (PA) and formic acid (FA)) on the luminescence of ZnO QDs is described. The molecular structures of these ligands are schematically presented in figure 1. Organic acids were used as capping ligands because carboxylates are known to strongly interact with metal oxides surfaces and allow controlling their size, their shape, and their dispersity [48–50]. Moreover, organic acids should be favorable ligands for the synthesis of
Figure 1. Molecular structures of the surface ligands used in this
work.
high quality ZnO microcrystals in microreactors since they exhibit low melting points, a high packing density, a low reactivity in air and should thus facilitate the synthesis of high quality nanocrystals. Finally, these ligands are no hurdle to the penetration of molecules through the surfactant shell to the surface of the nanocrystals, thus allowing catalytic reactions [41, 51]. A serpentine microchannel was applied to achieve narrow residence time distributions and high mixing efficiency under fast flow rates. The tailored kinetic control achieved by varying residence times, temperature and surface ligands allowed high quality ZnO QDs with PL quantum yields (PL QYs) up to 30% to be prepared. The structure of these nanocrystals was confirmed by UV–visible and fluorescence spectroscopic analysis, powder x-ray diffraction, electron diffraction, and transmission electron microscopy (TEM). 2. Experimental section 2.1. Reagents and materials
Anhydrous zinc acetate, Zn(OAc)2 , (99.99%, Aldrich), oleic acid, OA (90%, Aldrich), benzoic acid, BA (99%, Merck), propionic acid, PA (>99%, Aldrich), formic acid, FA (99%, Carlo Erba), tetramethylammonium hydroxide, TMAH (98%, Aldrich), anhydrous toluene and ethanol (HPLC grade) were used as received without additional purification. 2.2. Preparation of stock solutions
The zinc acetate/acid stock solution was prepared by gently heating around 50 ◦ C 40 ml of a 0.06 M anhydrous Zn(OAc)2 (440 mg, 2.4 mmol) dispersion in ethanol with OA (140 µl, 0.44 mmol), BA (53.7 mg, 0.44 mmol), PA (33.2 µl, 0.44 mmol) or FA (16.6 µl, 0.44 mmol) until a clear solution was obtained. This solution was cooled to room temperature before use. The 0.78 M TMAH stock solution was prepared by dissolving TMAH (720 mg, 3.98 mmol) in 10 ml ethanol. 2.3. Synthesis of ZnO QDs
The experimental set-up (figure 2) included two syringe pumps, a micromixer with total interior volume of about 7.85 µl with two inlet tubes of cylindrical shape (inner diameter: Ø = 500 µm), and an outlet tube (inner diameter: Ø = 1 mm, length = 1 cm). The growth of QDs was carried out in a flowing microsystem made of Teflon (inner diameter: 2
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3. Results and discussion
The continuous-flow microreactor consisted of a miniature convective tangential mixer followed by a heated serpentine channel maintained at a constant temperature (20, 35, 50, 65, 80 or 90 ◦ C). The Zn(OAc)2 /acid and TMAH precursor solutions were delivered in two separate flows and combined in the micromixer before they attained the heated reaction section (figure 2). Small ZnO clusters are probably formed in the mixer because Zn(OAc)2 is known to react with hydroxide ions at 0 ◦ C or at room temperature and grow in the serpentine channel. The volume of the mixer was chosen so that the chamber was long enough to ensure complete hydrolysis of Zn(OAc)2 but short enough to avoid growth of the QDs. A serpentine channel was used because it is known to provide efficient mixing conditions at the microscale. Indeed, the local fluctuations of velocity was achieved by the recirculation around the turns, mainly due to the continuously ‘stretching’ and ‘refolding’ of solute volumes induced by the variation in channel geometries [53]. These periodic fluctuations allow the dead volume near the channel wall to be reduced and a narrow residence time distribution in the whole serpentine channel to be achieved. The channel length and diameter have been designed to ensure sufficient residence time, with appropriate mixing and heat-transfer conditions, while minimizing the total pressure drop. Finally, the stability of this microreactor was demonstrated by injecting the two precursor solutions in continuous flow. Absorption spectra recorded from the QDs solutions sampled throughout this experiment were indistinguishable from each other. In a first set of experiments conducted in ethanol as solvent and using OA as capping ligand, the temperature around the serpentine channel was increased from 20 to 80 ◦ C while maintaining a given precursor composition and a constant flow rate (4.5 ml min−1 for the Zn(OAc)2 /OA solution and 1 ml min−1 for the TMAH solution, residence time of 10 min). The UV–vis spectra of the different samples prepared are shown in figure 3(a). As can be observed, the UV–vis absorbance excitonic peak redshifts when the temperature in the serpentine channel increases, indicating thus an increasing average diameter of the ZnO cores. From the UV–vis data, the band gap values for a direct band gap semiconductor like ZnO can be determined by the Tauc plot, (abs × hυ)2 versus hυ, where abs is the absorbance, h the Planck’s constant, and υ is the frequency of the photon [54]. The value of Eg was obtained by extrapolating the tangent of the curve to (abs × hυ)2 = 0 (figure 3(b)). The values obtained for the five samples prepared are listed in table 1 and confirm that the bandgaps of ZnO QDs decrease with increasing the particle size. Equation (2) shows the relationship between the radius R of spherical nanocrystals and their band gap E based on the effective mass model approximation [55], h¯ 2 π 2 1 1 1.8e2 E = Eg + + − (2) εR 2R 2 m e m h
Figure 2. Laboratory experimental set-up using a microreactor unit.
φ = 1.2 mm, length = 2.5 m). Heating was provided by a water bath at temperatures comprised between 20 and 90 ◦ C. For reaction carried out at 90 ◦ C, the pressure was controlled with a back-pressure regulator downstream. The tubes and the injector were compatible with the used chemicals and high pH. ZnO QDs were prepared at residence times of 10 or 20 min. For the 10-min residence time, flow rates of 4.5 ml min−1 and 1 ml min−1 were used for zinc acetate/acid solution and for TMAH solution, respectively. For 20-min reaction time, flow rates were decreased to 2.25 and 0.5 ml min−1 , respectively. QDs were collected in an ice-cooled ethanol solution, isolated by centrifugation (4000 rpm for 15 min), and twice washed with ethanol. Nanocrystals were dispersed in toluene for further characterization. 2.4. Instrument
Transmission electron microscopy (TEM) images were taken by placing a drop of the particles dispersion in toluene onto a carbon-film-supported copper grid. Samples were studied using a Philips CM20 instrument operating at 200 kV. The x-ray powder diffraction (XRD) diagrams of all samples were measured using Panalytical X’Pert Pro MPD diffractometer using Cu Kα radiation. The XRD data were collected from an X’Pert MPD diffractometer (Panalytical AXS) with a goniometer radius 240 mm, fixed divergence slit module (1/2◦ divergence slit, 0.04 rad Sollers slits) and an X’Celerator as a detector. The powder samples were placed on zero background quartz sample holders and the XRD patterns were recorded at room temperature using Cu Kα radiation (λ = 0.15418 nm). Absorption spectra were recorded on a Perkin-Elmer (Lambda 2) UV–vis spectrophotometer. Fluorescence spectra were recorded on a fluorolog-3 spectrofluorimeter F222 (Jobin Yvon) using a 450 W xenon lamp. The PL QYs were determined from the following equation: ! n 2sample Fsample Aref QY(ref) (1) QY (sample) = Fref Asample n 2ref where F, A and n are the measured fluorescence (area under the emission peak), absorbance at the excitation wavelength and refractive index of the solvent, respectively. PL spectra were spectrally corrected and QYs were determined relative to Rhodamine 6 G in water (QY = 95%) using the procedure recently developed [52].
where Eg is the band gap of the bulk material (3.37 eV), h¯ is the reduced Plank’s constant, e is the charge of the electron, ε is the semiconductor dielectric constant, m e and m h are effective 3
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Table 1. Band gaps, particles diameters and PL QYs determined from oleate-capped ZnO QDs by varying the temperature in the
microreactor. Reaction temperature (◦ C)
Band gapa (eV)
Particles diameterb (nm)
PL QYs (%)
Particles diameterc (nm)
20 35 50 65 80
3.75 3.66 3.60 3.58 3.52
4.1 4.6 5.0 5.2 5.4
24.5 19.7 16.7 15.6 12.9
3.7 ± 0.4 4.3 ± 0.5 4.4 ± 0.6 4.6 ± 0.5 4.8 ± 0.5
a Calculated using UV–vis absorption spectra and the corresponding Tauc plots. b Calculated using Brus formula. c Determined by TEM.
Figure 4. (a) PL excitation and PL emission spectra of
oleate-capped ZnO QDs versus reaction temperature and (b) digital photograph of the QDs under UV light excitation.
h
Figure 3. (a) UV–vis absorption spectra versus reaction temperature and (b) the corresponding (abs × hυ)2 versus energy plot to determine the bandgap of ZnO QDs prepared at a flow rate of 4.5 ml min−1 for the Zn(OAc)2 /oleic acid solution and 1.0 ml min−1 for the TMAH solution.
particles diameter increase, the emission wavelength shifts from blue–green (about 511 nm) to yellow (about 538 nm). This size-induced redshift in the PL emission of ZnO QDs is consistent with relevant theoretical considerations [57]. The broad blue–green to yellow emission in the visible region observed in figure 4 is generally considered as originating from intrinsic defects of nanocrystals such as oxygen vacancies, zinc interstitials, zinc vacancies, antisite oxygen, donor–acceptors pairs, and surface defects [58, 59]. With the reduction of the ZnO QDs diameters and the increase of their surface area, particles with more surface defect are produced. Thus, the decrease in PL QY from 24.5 to 12.9% with increasing the particle size is therefore in good accordance with the decrease of effective luminescence centers at the surface of the dots (table 1). Typical XRD patterns of the samples are shown in figure 5. The (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes, without any peaks corresponding to Zn
masses of electrons and holes, respectively, and m o is the free electron mass. With the effective masses of electrons, m e = 0.26m o and holes m h = 0.59m o [56], the radius and the diameters of nanocrystals were calculated. The results are listed in table 1 and confirm that increasing the temperature allows the average diameter of QDs to be tuned to a certain extent (from 4.1 to 5.4 nm). The PL excitation and PL emission spectra of these samples are shown in figure 4. All the PL excitation spectra display a similar shape and exhibit a strong absorption below about 365 nm. This corresponds to the transition between the valence and the conduction band which has previously been observed in the absorption spectra (figure 3(a)). As the 4
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the TEM micrographs of the ZnO QDs, and the insets show the size distributions (top left corner) and the SAED patterns (top right corner). TEM analyses indicate that ZnO QDs have average diameters between 3.7 ± 0.4 and 4.8 ± 0.5 nm (table 1), which is in good accordance with the particle size determined by UV–vis absorption spectroscopy and XRD (vide supra). Well-separated spheroid/ellipsoid particles can be discerned on TEM micrographs. All the diffraction rings can be ascribed to ZnO with a wurtzite structure and confirm a high crystallinity of the samples. A second parameter, the residence time, was controlled in the microreactor via the flow rate. An important result observed during these experiments is the increase of nanoparticles diameter with increasing the residence time for syntheses conducted at low temperatures (20 or 35 ◦ C). Figure 7(a) shows absorption spectra of oleate-capped ZnO QDs prepared at 20 ◦ C when the flow rate was divided per two in respect to the first experiment (2.25 ml min−1 for the Zn(OAc)2 /OA solution and 0.5 ml min−1 for the TMAH solution, reaction time of 20 min). The Brus formula (equation (2)) was used to estimate the diameter of nanocrystals produced. At the high flow rate and at 20 ◦ C, the first absorption band located at 313 nm, corresponds to the formation of ZnO QDs of about 4.1 nm in diameter. With the increase of the residence time, this absorption shifted to a longer wavelength (329 nm), corresponding to ZnO nanocrystals of 5.5 nm in diameter. The differences in absorption spectra are less pronounced for reactions conducted at temperatures above 50 ◦ C (figure 7(b)), the excitonic peaks being located at 335 and 337 nm for syntheses conducted at 80 ◦ C at low and high flow rates, respectively. Figure 8 shows UV–vis and PL emission spectra of oleate-capped ZnO QDs prepared using a 20-min residence time. The sizes derived from bandgaps at increasing temperatures are 5.5 nm (20 ◦ C), 5.6 nm (35 ◦ C), 5.8 nm (50 ◦ C),
Figure 5. XRD patterns of ZnO QDs prepared at 20, 35, 50, 65, and
80 ◦ C.
impurities, were observed. The position and intensity ratios of these peaks agree well with the hexagonal wurtzite structure of bulk ZnO (space group P63 mc, a = b = 0.325 39 nm, c = 0.520 98 nm, JCPDS File No. 36-1451). The broadening of XRD peaks is a clear indication of the formation of nanosized ZnO particles. The Rietveld refinements using the fundamental parameters approach built in Topas application (Bruker XAS) was used to estimate ZnO QDs diameters. A particle size of 4.5 ± 1.0 nm with a standard deviation (anisotropy) of 0.17 nm was found for all ZnO nanocrystals. In order to examine the microstructure of ZnO QDs, TEM and selected-area electron diffraction (SAED) experiments were conducted on the five samples prepared. Figure 6 shows
Figure 6. TEM micrographs, size distributions, and electron diffraction patterns of oleate-capped ZnO QDs prepared at (a) 20 ◦ C, (b) 35 ◦ C, (c) 50 ◦ C, (d) 65 ◦ C, and (e) 80 ◦ C. 5
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Figure 7. Absorption spectra of oleate-capped ZnO QDs prepared at
high flow rate (black line) and low flow rate (red line). Synthesis conducted at (a) 20 ◦ C and (b) 80 ◦ C.
Figure 9. (a) UV–vis absorption and (b) PL emission spectra of propionate-capped ZnO QDs prepared at 20, 35, 50, 65, or 80 ◦ C,
(c) digital photograph of the dots under UV light irradiation.
6.1 nm (65 ◦ C), and 6.4 nm (80 ◦ C). The PL emission wavelengths of the dots can easily be tuned between 500 and 530 nm by simply varying the reaction temperature (figure 7(b)). In the meantime, the PL QYs of the particles decreased continuously from 15.7% (reaction at 20 ◦ C) to 8.4% (reaction at 80 ◦ C) due to an increase of their size. It is finally worth to mention that when the reaction is conducted in pressurized ethanol at 90 ◦ C (figure 8), the diameter of the dots increased ultimately to 9.3 nm (3.38 eV), a value close to that of bulk material (3.37 eV). Finally, we sought to investigate the role of the carboxylic acid used as capping agent on the properties of QDs properties. These ligands dynamically adsorb on/desorb from the surface of ZnO QDs at the synthesis temperature in order to allow for growth while the nanocrystals are stabilized against aggregation. Ligands play a crucial role on controlling the electronic and optical properties of the nanoparticles. For example, for ZnO QDs prepared under electrochemical conditions, it has been demonstrated that the use of FA as capping ligand affords nanocrystals with enhanced near
Figure 8. (a) UV–vis absorption spectra and (b) PL emission spectra versus reaction temperature of oleate-capped ZnO QDs prepared at a flow rate of 2.25 ml min−1 for the Zn(OAc)2 /oleic acid solution and 0.5 ml min−1 for the TMAH solution. 6
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Figure 10. TEM micrographs, size distributions, and diffraction patterns of propionate-capped ZnO QDs prepared at (a) 20 ◦ C, (b) 35 ◦ C, (c) 50 ◦ C, (d) 65 ◦ C, and (e) 80 ◦ C. Table 2. Band gaps, particles diameters and PL QYs determined from propionate-capped ZnO QDs by varying the temperature in the
microreactor. Reaction temperature (◦ C)
Band gapa (eV)
Particles diameterb (nm)
PL QYs (%)
Particles diameterc (nm)
20 35 50 65 80
3.79 3.85 3.70 3.60 3.57
3.9 3.7 4.4 5.0 5.3
25.6 30.9 26.5 23.1 24.7
4.2 ± 0.4 4.2 ± 0.6 4.6 ± 0.5 4.7 ± 0.6 5.1 ± 0.5
a Calculated using UV–vis absorption spectra and the corresponding Tauc plots. b Calculated using Brus formula. c Determined by TEM.
band edge emission (3.38 eV) as compared with QDs capped with larger aromatic acids [60]. In a last set of experiments conducted at the high flow rate, we used BA, PA or FA as passivating ligands for ZnO QDs. Their influence on the surface of the nanocrystals was evaluated through changes in their optical properties and their sizes. With BA, PA and FA, similar trends as those described for OA were observed. Increasing the heating temperature systematically induced a shift of the absorption and PL emission to longer wavelengths, which is a clear indication of nanocrystals growth. With BA and FA, PL QYs slightly decreased compared to those measured with OA-capped ZnO QDs (see figures S1 and S2 in the supplementary material available at stacks.iop.org/Nan o/25/145606/mmedia), while PA-capped ZnO QDs exhibited markedly enhanced PL QYs compared to those of OA-, BAor FA-capped QDs. Increasing the temperature from 20 to 80 ◦ C while maintaining a constant flow rate (4.5 ml min−1 for the Zn(OAc)2 /OA and 1 ml min−1 for the TMAH solution) resulted in a decrease of PL QYs of ZnO@oleate QDs from 24.5 to 12.9%, which is consistent with previous reports [36, 61]. On the contrary, PL QYs of ZnO QDs engineered with PA remained almost constant (about 24%), the highest QY (30.9%) being obtained for the reaction conducted at 35 ◦ C. It
is also worth mentioning that the emission of OA-, BA- and FAcapped ZnO QDs spans over slightly extended wavelengths window (from 500 to 536 nm, from 525 to 560 nm, and from 517 to 546 nm for OA, BA and FA, respectively) than that of PA-capped QDs which is restricted between 499 and 524 nm. As can be seen from figures 9 and 10 and table 2, the dots produced with PA and OA have similar diameters. Particles diameters derived from bandgaps or calculated from TEM size distributions show that the ZnO QDs produced exhibit monodisperse size with average diameters between 4 and 5 nm (figure 9). The electron diffraction patterns indicate also that ZnO nanocrystals have a wurtzite structure. Interestingly, under these optimized experimental conditions, about 3 g per hour of PA-capped ZnO can be synthesized. Figures 9 and 10 clearly show that both the size and optical properties were not significantly affected by extending the reaction time, thus demonstrating the scaled-up synthesis of PA-capped ZnO QDs. 4. Conclusions
A continuous-flow microreactor process was developed to synthesize carboxylate-capped ZnO QDs with diameters rang7
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ing from about 3.6 to 5.2 nm and emission maxima from about 500 to 560 nm. Low reaction temperatures (20 or 35 ◦ C), high flow rates, and the use of propionic acid as capping ligand allow the generation of ZnO QDs with narrow size distribution and optimal fluorescence quantum yields (up to 30%). The continuous synthesis ZnO QDs emitting in the blue–green to yellow range with high PL QYs can be achieved by the online adjustment of temperature or flow rate or by exchange of the capping ligand. The experimental results obtained emphasize the flexibility of microreactors in efficiently tuning physical properties of semiconductor nanocrystals such as ZnO. At midpoint, this work should also help designing future metal oxide syntheses and should provide useful means to control both the size and the size distribution of these nanocrystals.
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Acknowledgments
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[45] Spanhel L 2006 Colloidal ZnO nanostructures and functional coatings: a survey J. Sol–Gel Sci. Technol. 39 7–24 [46] Balazcs A C, Emrick T and Russell T P 2006 Nanoparticle polymer composites: where two small worlds meet Science 314 1107–10 [47] Bauermann L P, Bill J and Aldinger F 2006 Bio-friendly synthesis of ZnO nanoparticles in aqueous solution at near-neutral pH and low temperature J. Phys. Chem. B 110 5182–5 [48] Hidov T, Schroeter M K, Khodeir L, Chemseddine A, Muhler M and Fischer R A 2006 Selective photo-deposition of Cu onto the surface of monodisperse oleic acid capped TiO2 nanorods probed by FT-IR CO-adsorption studies Phys. Chem. Chem. Phys. 8 1550–5 [49] Kahn M L, Monge M, Colli`ere V, Senocq F, Maisonnat A and Chaudret B 2005 Size- and shape-control of crystalline zinc oxide nanoparticles: a new organometallic synthetic method Adv. Funct. Mater. 15 458–68 [50] Yin M, Gu Y, Kuskovsky I L, Andelman T, Zhu Y, Neumark G F and O’Brien S 2004 Zinc oxide quantum rods J. Am. Chem. Soc. 126 6206–7 [51] Astruc D, Lu F and Aranzaes J R 2005 Nanoparticles as recyclable catalysts: the frontier between homogeneous and heterogeneous catalysis Angew. Chem. Int. Edn 44 7852–72 [52] Grabolle M, Spiles M, Lesnyak V, Gaponik N, Eychm¨uller A and Resch-Genger U 2009 Determination of the fluorescence quantum yield of quantum dots: suitable procedures and achievable uncertainties Anal. Chem. 81 6285–94 [53] Song H, Tice J D and Ismagilov R F 2003 A microfluidic system for controlling reaction networks in time Angew. Chem. Int. Edn 42 768–72 [54] Shchukin D G, Skorb E, Belova V and M¨ohwald H 2011 Ultrasonic cavitation at solid surfaces Adv. Mater. 23 1922–34 [55] Brus L E 1986 Electronic wave-functions in semiconductor clusters—experiment and theory J. Phys. Chem. 90 2555–60 [56] Fu Y S, Du X Y, Kulinich S A, Qiu J S, Qin W J, Li R and Liu J 2007 Stable aqueous dispersion of ZnO quantum dots with strong blue emission via simple solution route J. Am. Chem. Soc. 129 16029–33 [57] Li J W et al 2009 Photoluminescence and photoabsorption blueshift of nanostructured ZnO: skin-depth quantum trapping and electron–phonon coupling Appl. Phys. Lett. 95 031906 [58] Reschikov M A, Morkoc H, Nemeth B, Nause J, Xie J, Hertog B and Osinsky A 2007 Luminescence properties of defects in ZnO Physica B 401/402 358–61 [59] Zhang L, Yin L, Wang C, Iun N, Qi Y and Xiang D 2010 Origin of visible photoluminescence of ZnO quantum dots: defect-dependent and size-dependent J. Phys. Chem. C 114 9651–8 [60] Lenz A, Selegard L, S¨oderling F, Larsson A, Holz P O, Urdal K, Ojamae L and K¨all P O 2009 ZnO Nanoparticles functionalized with organic acids: an experimental and quantum-chemical study J. Phys. Chem. C 113 17332–41 [61] Shi H Q, Li W N, Sun L W, Liu Y, Xiao H M and Fu S 2011 Synthesis of silane surface modified ZnO quantum dots with ultrastable, strong and tunable luminescence Chem. Commun. 47 11921–3
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Size-controlled synthesis of ZnO quantum dots in microreactors
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Nanotechnology Nanotechnology 25 (2014) 145606 (9pp)
doi:10.1088/0957-4484/25/14/145606
Size-controlled synthesis of ZnO quantum dots in microreactors Aleksandra Schejn1 , Mathieu Frégnaux1 , Jean-Marc Commenge1 , Lavinia Balan2 , Laurent Falk1 and Raphaël Schneider1 1
Université de Lorraine, Laboratoire Réactions et Génie des Procédés (LRGP), UMR 7274, CNRS, 1 rue Grandville, BP 20451, F-54001 Nancy Cedex, France 2 Institut de Science des Matériaux de Mulhouse (IS2M), LRC 7228, 15 rue Jean Starcky, F-68093 Mulhouse, France E-mail: [email protected] Received 31 October 2014, revised 30 January 2014 Accepted for publication 13 February 2014 Published 14 March 2014
Abstract
In this paper, we report on a continuous-flow microreactor process to prepare ZnO quantum dots (QDs) with widely tunable particle size and photoluminescence emission wavelengths. X-ray diffraction, electron diffraction, UV–vis, photoluminescence and transmission electron microscopy measurements were used to characterize the synthesized ZnO QDs. By varying operating conditions (temperature, flow rate) or the capping ligand, ZnO QDs with diameters ranging from 3.6 to 5.2 nm and fluorescence maxima from 500 to 560 nm were prepared. Results obtained show that low reaction temperatures (20 or 35 ◦ C), high flow rates and the use of propionic acid as a stabilizing agent are favorable for the production of ZnO QDs with high photoluminescence quantum yields (up to 30%). Keywords: quantum dots, zinc oxide, microreactor, surface ligand, fluorescence S Online supplementary data available from stacks.iop.org/Nano/25/145606/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
microchip) and preparation of large quantities of nanocrystals through a continuous process. Among nanocrystals, semiconductor quantum dots (QDs) are of great interest for both fundamental research and industrial applications due to their size-dependent optical properties originating from quantum size effects [4, 5]. During the last couple of decades, QDs achieved many and various applications as biological fluorescent probes [6, 7], sensors [8], or in optoelectronic devices [9, 10]. QDs like CdS [11, 12], CdSe [13–18], CdTe [14, 17, 19], InP [20], and their core/shell counterparts were successfully prepared in microreactors and the quality of the nanocrystals obtained was reported to be higher than what can be produced by bench top methods. Due to its wide band gap (3.37 eV) and large exciton binding energy (60 meV at room temperature), zinc oxide (ZnO) is an important member of II–VI semiconductor. Indeed, it is abundant, stable, of low toxicity [21] and has shown great potential for use in ultraviolet laser devices [22], biomedical labels [23, 24], photocatalysis, varistors, sensors, and
A microreactor is a device with microscale channels which allows chemical reactions to be performed in a reaction volume several orders of magnitude smaller than conventional batch reactors. The potential advantages of using a microreactor, rather than a conventional batch reactor, include enhanced heat and mass transfer, high speed mixing, precise and independent control of operating parameters, better homogeneity in the chemical environment, improved safety and improved yields [1]. Microreactors have for a long time only been considered for organic reactions. Since ten years, it has been demonstrated that they may also offer a great variety of advantages for the synthesis of nanoparticles [2, 3]. Among them, efficient mixing allows the preparation of monodispersed nanoparticles, and microfluidic devices provide potential for automating multi-step process (analysis, surface functionalization and purification in a single 0957-4484/14/145606+09$33.00
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piezoelectric transducers [25–27]. ZnO QDs display interesting photoluminescent properties in the near UV or in the visible spectrum ranges. The near UV emission corresponds to the excitonic emission and is based on the direct recombination of electron–hole pairs [28]. The visible green–yellow emission centered at about 520 nm originates from the electronic transition from the conduction band edge to a trap level and the singly ionized oxygen vacancies on the QD surface have been assumed to be the recombination centers [29, 30]. Micro continuous-flow synthesis of ZnO particles has recently received a great interest. Supercritical hydrothermal syntheses using either homogeneous phase reaction or segmented flow can produce cylinder-, star-, or flower-like ZnO particles [31] or ZnO nanorods [32]. Syntheses in supercritical methanol using oleic acid (OA) as ligand afford ZnO nanocrystals with diameters varying between 10 and 30 nm [33]. The use of more coordinating ligands such as tri-n-octylphosphine or tri-n-octylphosphine oxide allows ZnO QDs with sizes of about 3.7 nm to be prepared [34, 35]. Noteworthy is that the high operating temperature (250 ◦ C) used in these syntheses affords nanocrystals with excitonic emission located at about 375 nm (3.30 eV), thus valuable in applications such as UV-leds and lasers. Photoluminescence (PL) in the visible range (500–600 nm) is much more interesting for bio-imaging applications [23, 36, 37] but the synthesis of ZnO QDs exhibiting this feature has never been reported through a microfluidic process. Among the wet chemical routes recently developed for the preparation of ZnO QDs emitting in the visible range (polyol, organometallic, pyrolysis,. . . ) [38–41], the sol–gel process is by far the most popular due to its simplicity but it generates QDs with relatively low photoluminescence quantum yields (generally about 10–15%) [21, 23, 26, 36, 42, 43]. Moreover, if the dots are not efficiently stabilized, they tend to undergo Ostwald ripening in solution and to aggregate. After the nucleation stage, the ZnO QDs photoluminescence shifts gradually to the red and their quantum yields decrease [44, 45]. Therefore, the synthesis of stable, equisized, and well-dispersible ZnO QDs is still a challenge in material science. This paper concentrates on the preparation of ZnO QDs through a low temperature sol–gel method because wet chemical routes involving an appropriate choice of precursors and surfactants generally provide a much better tuning of the particles shape and size distribution. Since surface modifications have a significant impact on the optical properties of ZnO QDs [40, 46, 47], designing and controlling their surface chemistry appears a very attractive means to tune their photoluminescence (PL) properties. In the last part of the paper, the influence of surface modification by various organic acids (oleic acid (OA), benzoic acid (BA), propionic acid (PA) and formic acid (FA)) on the luminescence of ZnO QDs is described. The molecular structures of these ligands are schematically presented in figure 1. Organic acids were used as capping ligands because carboxylates are known to strongly interact with metal oxides surfaces and allow controlling their size, their shape, and their dispersity [48–50]. Moreover, organic acids should be favorable ligands for the synthesis of
Figure 1. Molecular structures of the surface ligands used in this
work.
high quality ZnO microcrystals in microreactors since they exhibit low melting points, a high packing density, a low reactivity in air and should thus facilitate the synthesis of high quality nanocrystals. Finally, these ligands are no hurdle to the penetration of molecules through the surfactant shell to the surface of the nanocrystals, thus allowing catalytic reactions [41, 51]. A serpentine microchannel was applied to achieve narrow residence time distributions and high mixing efficiency under fast flow rates. The tailored kinetic control achieved by varying residence times, temperature and surface ligands allowed high quality ZnO QDs with PL quantum yields (PL QYs) up to 30% to be prepared. The structure of these nanocrystals was confirmed by UV–visible and fluorescence spectroscopic analysis, powder x-ray diffraction, electron diffraction, and transmission electron microscopy (TEM). 2. Experimental section 2.1. Reagents and materials
Anhydrous zinc acetate, Zn(OAc)2 , (99.99%, Aldrich), oleic acid, OA (90%, Aldrich), benzoic acid, BA (99%, Merck), propionic acid, PA (>99%, Aldrich), formic acid, FA (99%, Carlo Erba), tetramethylammonium hydroxide, TMAH (98%, Aldrich), anhydrous toluene and ethanol (HPLC grade) were used as received without additional purification. 2.2. Preparation of stock solutions
The zinc acetate/acid stock solution was prepared by gently heating around 50 ◦ C 40 ml of a 0.06 M anhydrous Zn(OAc)2 (440 mg, 2.4 mmol) dispersion in ethanol with OA (140 µl, 0.44 mmol), BA (53.7 mg, 0.44 mmol), PA (33.2 µl, 0.44 mmol) or FA (16.6 µl, 0.44 mmol) until a clear solution was obtained. This solution was cooled to room temperature before use. The 0.78 M TMAH stock solution was prepared by dissolving TMAH (720 mg, 3.98 mmol) in 10 ml ethanol. 2.3. Synthesis of ZnO QDs
The experimental set-up (figure 2) included two syringe pumps, a micromixer with total interior volume of about 7.85 µl with two inlet tubes of cylindrical shape (inner diameter: Ø = 500 µm), and an outlet tube (inner diameter: Ø = 1 mm, length = 1 cm). The growth of QDs was carried out in a flowing microsystem made of Teflon (inner diameter: 2
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3. Results and discussion
The continuous-flow microreactor consisted of a miniature convective tangential mixer followed by a heated serpentine channel maintained at a constant temperature (20, 35, 50, 65, 80 or 90 ◦ C). The Zn(OAc)2 /acid and TMAH precursor solutions were delivered in two separate flows and combined in the micromixer before they attained the heated reaction section (figure 2). Small ZnO clusters are probably formed in the mixer because Zn(OAc)2 is known to react with hydroxide ions at 0 ◦ C or at room temperature and grow in the serpentine channel. The volume of the mixer was chosen so that the chamber was long enough to ensure complete hydrolysis of Zn(OAc)2 but short enough to avoid growth of the QDs. A serpentine channel was used because it is known to provide efficient mixing conditions at the microscale. Indeed, the local fluctuations of velocity was achieved by the recirculation around the turns, mainly due to the continuously ‘stretching’ and ‘refolding’ of solute volumes induced by the variation in channel geometries [53]. These periodic fluctuations allow the dead volume near the channel wall to be reduced and a narrow residence time distribution in the whole serpentine channel to be achieved. The channel length and diameter have been designed to ensure sufficient residence time, with appropriate mixing and heat-transfer conditions, while minimizing the total pressure drop. Finally, the stability of this microreactor was demonstrated by injecting the two precursor solutions in continuous flow. Absorption spectra recorded from the QDs solutions sampled throughout this experiment were indistinguishable from each other. In a first set of experiments conducted in ethanol as solvent and using OA as capping ligand, the temperature around the serpentine channel was increased from 20 to 80 ◦ C while maintaining a given precursor composition and a constant flow rate (4.5 ml min−1 for the Zn(OAc)2 /OA solution and 1 ml min−1 for the TMAH solution, residence time of 10 min). The UV–vis spectra of the different samples prepared are shown in figure 3(a). As can be observed, the UV–vis absorbance excitonic peak redshifts when the temperature in the serpentine channel increases, indicating thus an increasing average diameter of the ZnO cores. From the UV–vis data, the band gap values for a direct band gap semiconductor like ZnO can be determined by the Tauc plot, (abs × hυ)2 versus hυ, where abs is the absorbance, h the Planck’s constant, and υ is the frequency of the photon [54]. The value of Eg was obtained by extrapolating the tangent of the curve to (abs × hυ)2 = 0 (figure 3(b)). The values obtained for the five samples prepared are listed in table 1 and confirm that the bandgaps of ZnO QDs decrease with increasing the particle size. Equation (2) shows the relationship between the radius R of spherical nanocrystals and their band gap E based on the effective mass model approximation [55], h¯ 2 π 2 1 1 1.8e2 E = Eg + + − (2) εR 2R 2 m e m h
Figure 2. Laboratory experimental set-up using a microreactor unit.
φ = 1.2 mm, length = 2.5 m). Heating was provided by a water bath at temperatures comprised between 20 and 90 ◦ C. For reaction carried out at 90 ◦ C, the pressure was controlled with a back-pressure regulator downstream. The tubes and the injector were compatible with the used chemicals and high pH. ZnO QDs were prepared at residence times of 10 or 20 min. For the 10-min residence time, flow rates of 4.5 ml min−1 and 1 ml min−1 were used for zinc acetate/acid solution and for TMAH solution, respectively. For 20-min reaction time, flow rates were decreased to 2.25 and 0.5 ml min−1 , respectively. QDs were collected in an ice-cooled ethanol solution, isolated by centrifugation (4000 rpm for 15 min), and twice washed with ethanol. Nanocrystals were dispersed in toluene for further characterization. 2.4. Instrument
Transmission electron microscopy (TEM) images were taken by placing a drop of the particles dispersion in toluene onto a carbon-film-supported copper grid. Samples were studied using a Philips CM20 instrument operating at 200 kV. The x-ray powder diffraction (XRD) diagrams of all samples were measured using Panalytical X’Pert Pro MPD diffractometer using Cu Kα radiation. The XRD data were collected from an X’Pert MPD diffractometer (Panalytical AXS) with a goniometer radius 240 mm, fixed divergence slit module (1/2◦ divergence slit, 0.04 rad Sollers slits) and an X’Celerator as a detector. The powder samples were placed on zero background quartz sample holders and the XRD patterns were recorded at room temperature using Cu Kα radiation (λ = 0.15418 nm). Absorption spectra were recorded on a Perkin-Elmer (Lambda 2) UV–vis spectrophotometer. Fluorescence spectra were recorded on a fluorolog-3 spectrofluorimeter F222 (Jobin Yvon) using a 450 W xenon lamp. The PL QYs were determined from the following equation: ! n 2sample Fsample Aref QY(ref) (1) QY (sample) = Fref Asample n 2ref where F, A and n are the measured fluorescence (area under the emission peak), absorbance at the excitation wavelength and refractive index of the solvent, respectively. PL spectra were spectrally corrected and QYs were determined relative to Rhodamine 6 G in water (QY = 95%) using the procedure recently developed [52].
where Eg is the band gap of the bulk material (3.37 eV), h¯ is the reduced Plank’s constant, e is the charge of the electron, ε is the semiconductor dielectric constant, m e and m h are effective 3
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Table 1. Band gaps, particles diameters and PL QYs determined from oleate-capped ZnO QDs by varying the temperature in the
microreactor. Reaction temperature (◦ C)
Band gapa (eV)
Particles diameterb (nm)
PL QYs (%)
Particles diameterc (nm)
20 35 50 65 80
3.75 3.66 3.60 3.58 3.52
4.1 4.6 5.0 5.2 5.4
24.5 19.7 16.7 15.6 12.9
3.7 ± 0.4 4.3 ± 0.5 4.4 ± 0.6 4.6 ± 0.5 4.8 ± 0.5
a Calculated using UV–vis absorption spectra and the corresponding Tauc plots. b Calculated using Brus formula. c Determined by TEM.
Figure 4. (a) PL excitation and PL emission spectra of
oleate-capped ZnO QDs versus reaction temperature and (b) digital photograph of the QDs under UV light excitation.
h
Figure 3. (a) UV–vis absorption spectra versus reaction temperature and (b) the corresponding (abs × hυ)2 versus energy plot to determine the bandgap of ZnO QDs prepared at a flow rate of 4.5 ml min−1 for the Zn(OAc)2 /oleic acid solution and 1.0 ml min−1 for the TMAH solution.
particles diameter increase, the emission wavelength shifts from blue–green (about 511 nm) to yellow (about 538 nm). This size-induced redshift in the PL emission of ZnO QDs is consistent with relevant theoretical considerations [57]. The broad blue–green to yellow emission in the visible region observed in figure 4 is generally considered as originating from intrinsic defects of nanocrystals such as oxygen vacancies, zinc interstitials, zinc vacancies, antisite oxygen, donor–acceptors pairs, and surface defects [58, 59]. With the reduction of the ZnO QDs diameters and the increase of their surface area, particles with more surface defect are produced. Thus, the decrease in PL QY from 24.5 to 12.9% with increasing the particle size is therefore in good accordance with the decrease of effective luminescence centers at the surface of the dots (table 1). Typical XRD patterns of the samples are shown in figure 5. The (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes, without any peaks corresponding to Zn
masses of electrons and holes, respectively, and m o is the free electron mass. With the effective masses of electrons, m e = 0.26m o and holes m h = 0.59m o [56], the radius and the diameters of nanocrystals were calculated. The results are listed in table 1 and confirm that increasing the temperature allows the average diameter of QDs to be tuned to a certain extent (from 4.1 to 5.4 nm). The PL excitation and PL emission spectra of these samples are shown in figure 4. All the PL excitation spectra display a similar shape and exhibit a strong absorption below about 365 nm. This corresponds to the transition between the valence and the conduction band which has previously been observed in the absorption spectra (figure 3(a)). As the 4
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the TEM micrographs of the ZnO QDs, and the insets show the size distributions (top left corner) and the SAED patterns (top right corner). TEM analyses indicate that ZnO QDs have average diameters between 3.7 ± 0.4 and 4.8 ± 0.5 nm (table 1), which is in good accordance with the particle size determined by UV–vis absorption spectroscopy and XRD (vide supra). Well-separated spheroid/ellipsoid particles can be discerned on TEM micrographs. All the diffraction rings can be ascribed to ZnO with a wurtzite structure and confirm a high crystallinity of the samples. A second parameter, the residence time, was controlled in the microreactor via the flow rate. An important result observed during these experiments is the increase of nanoparticles diameter with increasing the residence time for syntheses conducted at low temperatures (20 or 35 ◦ C). Figure 7(a) shows absorption spectra of oleate-capped ZnO QDs prepared at 20 ◦ C when the flow rate was divided per two in respect to the first experiment (2.25 ml min−1 for the Zn(OAc)2 /OA solution and 0.5 ml min−1 for the TMAH solution, reaction time of 20 min). The Brus formula (equation (2)) was used to estimate the diameter of nanocrystals produced. At the high flow rate and at 20 ◦ C, the first absorption band located at 313 nm, corresponds to the formation of ZnO QDs of about 4.1 nm in diameter. With the increase of the residence time, this absorption shifted to a longer wavelength (329 nm), corresponding to ZnO nanocrystals of 5.5 nm in diameter. The differences in absorption spectra are less pronounced for reactions conducted at temperatures above 50 ◦ C (figure 7(b)), the excitonic peaks being located at 335 and 337 nm for syntheses conducted at 80 ◦ C at low and high flow rates, respectively. Figure 8 shows UV–vis and PL emission spectra of oleate-capped ZnO QDs prepared using a 20-min residence time. The sizes derived from bandgaps at increasing temperatures are 5.5 nm (20 ◦ C), 5.6 nm (35 ◦ C), 5.8 nm (50 ◦ C),
Figure 5. XRD patterns of ZnO QDs prepared at 20, 35, 50, 65, and
80 ◦ C.
impurities, were observed. The position and intensity ratios of these peaks agree well with the hexagonal wurtzite structure of bulk ZnO (space group P63 mc, a = b = 0.325 39 nm, c = 0.520 98 nm, JCPDS File No. 36-1451). The broadening of XRD peaks is a clear indication of the formation of nanosized ZnO particles. The Rietveld refinements using the fundamental parameters approach built in Topas application (Bruker XAS) was used to estimate ZnO QDs diameters. A particle size of 4.5 ± 1.0 nm with a standard deviation (anisotropy) of 0.17 nm was found for all ZnO nanocrystals. In order to examine the microstructure of ZnO QDs, TEM and selected-area electron diffraction (SAED) experiments were conducted on the five samples prepared. Figure 6 shows
Figure 6. TEM micrographs, size distributions, and electron diffraction patterns of oleate-capped ZnO QDs prepared at (a) 20 ◦ C, (b) 35 ◦ C, (c) 50 ◦ C, (d) 65 ◦ C, and (e) 80 ◦ C. 5
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Figure 7. Absorption spectra of oleate-capped ZnO QDs prepared at
high flow rate (black line) and low flow rate (red line). Synthesis conducted at (a) 20 ◦ C and (b) 80 ◦ C.
Figure 9. (a) UV–vis absorption and (b) PL emission spectra of propionate-capped ZnO QDs prepared at 20, 35, 50, 65, or 80 ◦ C,
(c) digital photograph of the dots under UV light irradiation.
6.1 nm (65 ◦ C), and 6.4 nm (80 ◦ C). The PL emission wavelengths of the dots can easily be tuned between 500 and 530 nm by simply varying the reaction temperature (figure 7(b)). In the meantime, the PL QYs of the particles decreased continuously from 15.7% (reaction at 20 ◦ C) to 8.4% (reaction at 80 ◦ C) due to an increase of their size. It is finally worth to mention that when the reaction is conducted in pressurized ethanol at 90 ◦ C (figure 8), the diameter of the dots increased ultimately to 9.3 nm (3.38 eV), a value close to that of bulk material (3.37 eV). Finally, we sought to investigate the role of the carboxylic acid used as capping agent on the properties of QDs properties. These ligands dynamically adsorb on/desorb from the surface of ZnO QDs at the synthesis temperature in order to allow for growth while the nanocrystals are stabilized against aggregation. Ligands play a crucial role on controlling the electronic and optical properties of the nanoparticles. For example, for ZnO QDs prepared under electrochemical conditions, it has been demonstrated that the use of FA as capping ligand affords nanocrystals with enhanced near
Figure 8. (a) UV–vis absorption spectra and (b) PL emission spectra versus reaction temperature of oleate-capped ZnO QDs prepared at a flow rate of 2.25 ml min−1 for the Zn(OAc)2 /oleic acid solution and 0.5 ml min−1 for the TMAH solution. 6
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Figure 10. TEM micrographs, size distributions, and diffraction patterns of propionate-capped ZnO QDs prepared at (a) 20 ◦ C, (b) 35 ◦ C, (c) 50 ◦ C, (d) 65 ◦ C, and (e) 80 ◦ C. Table 2. Band gaps, particles diameters and PL QYs determined from propionate-capped ZnO QDs by varying the temperature in the
microreactor. Reaction temperature (◦ C)
Band gapa (eV)
Particles diameterb (nm)
PL QYs (%)
Particles diameterc (nm)
20 35 50 65 80
3.79 3.85 3.70 3.60 3.57
3.9 3.7 4.4 5.0 5.3
25.6 30.9 26.5 23.1 24.7
4.2 ± 0.4 4.2 ± 0.6 4.6 ± 0.5 4.7 ± 0.6 5.1 ± 0.5
a Calculated using UV–vis absorption spectra and the corresponding Tauc plots. b Calculated using Brus formula. c Determined by TEM.
band edge emission (3.38 eV) as compared with QDs capped with larger aromatic acids [60]. In a last set of experiments conducted at the high flow rate, we used BA, PA or FA as passivating ligands for ZnO QDs. Their influence on the surface of the nanocrystals was evaluated through changes in their optical properties and their sizes. With BA, PA and FA, similar trends as those described for OA were observed. Increasing the heating temperature systematically induced a shift of the absorption and PL emission to longer wavelengths, which is a clear indication of nanocrystals growth. With BA and FA, PL QYs slightly decreased compared to those measured with OA-capped ZnO QDs (see figures S1 and S2 in the supplementary material available at stacks.iop.org/Nan o/25/145606/mmedia), while PA-capped ZnO QDs exhibited markedly enhanced PL QYs compared to those of OA-, BAor FA-capped QDs. Increasing the temperature from 20 to 80 ◦ C while maintaining a constant flow rate (4.5 ml min−1 for the Zn(OAc)2 /OA and 1 ml min−1 for the TMAH solution) resulted in a decrease of PL QYs of ZnO@oleate QDs from 24.5 to 12.9%, which is consistent with previous reports [36, 61]. On the contrary, PL QYs of ZnO QDs engineered with PA remained almost constant (about 24%), the highest QY (30.9%) being obtained for the reaction conducted at 35 ◦ C. It
is also worth mentioning that the emission of OA-, BA- and FAcapped ZnO QDs spans over slightly extended wavelengths window (from 500 to 536 nm, from 525 to 560 nm, and from 517 to 546 nm for OA, BA and FA, respectively) than that of PA-capped QDs which is restricted between 499 and 524 nm. As can be seen from figures 9 and 10 and table 2, the dots produced with PA and OA have similar diameters. Particles diameters derived from bandgaps or calculated from TEM size distributions show that the ZnO QDs produced exhibit monodisperse size with average diameters between 4 and 5 nm (figure 9). The electron diffraction patterns indicate also that ZnO nanocrystals have a wurtzite structure. Interestingly, under these optimized experimental conditions, about 3 g per hour of PA-capped ZnO can be synthesized. Figures 9 and 10 clearly show that both the size and optical properties were not significantly affected by extending the reaction time, thus demonstrating the scaled-up synthesis of PA-capped ZnO QDs. 4. Conclusions
A continuous-flow microreactor process was developed to synthesize carboxylate-capped ZnO QDs with diameters rang7
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ing from about 3.6 to 5.2 nm and emission maxima from about 500 to 560 nm. Low reaction temperatures (20 or 35 ◦ C), high flow rates, and the use of propionic acid as capping ligand allow the generation of ZnO QDs with narrow size distribution and optimal fluorescence quantum yields (up to 30%). The continuous synthesis ZnO QDs emitting in the blue–green to yellow range with high PL QYs can be achieved by the online adjustment of temperature or flow rate or by exchange of the capping ligand. The experimental results obtained emphasize the flexibility of microreactors in efficiently tuning physical properties of semiconductor nanocrystals such as ZnO. At midpoint, this work should also help designing future metal oxide syntheses and should provide useful means to control both the size and the size distribution of these nanocrystals.
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Acknowledgments
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