Facile Synthesis of Monodisperse CdS Nanocrystals via Microreaction
© to the authors 2009
Received: 2 July 2009
Accepted: 25 September 2009
Published: 13 October 2009
CdS-based nanocrystals (NCs) have attracted extensive interest due to their potential application as key luminescent materials for blue and white LEDs. In this research, the continuous synthesis of monodisperse CdS NCs was demonstrated utilizing a capillary microreactor. The enhanced heat and mass transfer in the microreactor was useful to reduce the reaction temperature and residence time to synthesize monodisperse CdS NCs. The superior stability of the microreactor and its continuous operation allowed the investigation of synthesis parameters with high efficiency. Reaction temperature was found to be a key parameter for balancing the reactivity of CdS precursors, while residence time was shown to be an important factor that governs the size and size distribution of the CdS NCs. Furthermore, variation of OA concentration was demonstrated to be a facile tuning mechanism for controlling the size of the CdS NCs. The variation of the volume percentage of OA from 10.5 to 51.2% and the variation of the residence time from 17 to 136 s facilitated the synthesis of monodisperse CdS NCs in the size range of 3.0–5.4 nm, and the NCs produced photoluminescent emissions in the range of 391–463 nm.
The strong quantum confinement effect of colloidal semiconductor nanocrystals (NCs) has made these materials the subject of extensive research [1–5]. In the past decades, binary semiconductor NCs, identified as groups II–VI and III–V, have been studied intensively because of their size-dependent photoluminescence (PL) covering the spectrum range from ultraviolet to near infrared [6–10]. The importance of CdS NCs is manifested in their potential application as fluorescent materials with UV–blue PL emission [11–17]. The typical synthesis recipes for CdS NCs are based on a process that involves the injection of Cd-oleic acid (OA) into batch reactors. Nearly, monodispersed CdS NCs have been prepared using octadecene (ODE) as the non-coordinating solvent, and subsequent treatment of these NCs resulted in considerable improvement of PL efficiency. In these cases, the inefficient heat and mass transfer of batch reactors constrained the economical synthesis of CdS NCs. In order to overcome the energy barrier to form active monomers, a high reaction temperature (250–300°C) is generally required. Meanwhile, the temperature and concentration gradients involved in large reactors lead to poor quality and low process reproducibility. Furthermore, according to the International Commission on Illumination chromaticity diagram, the ideal wavelength for blue light is 460–480 nm. To achieve CdS NCs with blue PL emission, large CdS NCs with diameters greater than 5.2 nm are required. However, the preparation of these CdS NCs is limited by the high reaction temperature and long reaction time, where the advent of Ostwald ripening leads to wide size distributions.
The enhanced heat and mass transfer properties in the microenvironment could provide highly homogeneous conditions for the chemical synthesis of NCs [18–24]. Compared with the batch reactor, a microreactor offers a controllable way to synthesize NCs continuously in a steady fashion [9, 20–23]. Meanwhile, the incorporation of novel channel structures and flow patterns facilitated the synthesis of monodisperse NCs in an accelerated manner. However, the limited research conducted to date on the microfluidic synthesis of CdS NCs has been based on aqueous processes, and the resulting NCs generally had wide size distributions and deep-trap luminescence that overwhelmed the visible range [19, 25–27]. Droplet microfluidics has been demonstrated as a powerful tool for achieving efficient mixing, while realizing highly homogeneous environment for the synthesis of monodisperse CdS NCs , but its low throughput limits its use to commercial-scale synthesis.
In this study, a capillary microreactor was used for the accelerated synthesis of monodisperse CdS NCs. In order to obtain high-quality samples, systematic investigations of temperature and residence time requirements were conducted. Also, a quantitative investigation of the influence of OA on size distribution and growth kinetics was conducted to achieve the size-controlled synthesis of CdS NCs. The influence of OA on the reaction kinetics was evidenced by the collected absorption spectra. In addition, the high quality of the resulting CdS NCs was confirmed by the PL spectra, powder X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM).
Cadmium oxide (CdO, SCR, 99.9%), sulfur (S, SCR, 99.5%), 1-octadecene (ODE, Fisher, 90%), oleic acid (OA, SCR, 90%), and analytic grade acetone and chloroform (SCR) were used directly without further processing.
Synthesis of CdS NCs
CdS NCs were prepared using a recipe similar to the one previously reported by Yu et al. . Typically, 1 mmol CdO, 4.5 mmol OA, and ODE (5 ml in total) were mixed together and heated to 150°C under a nitrogen atmosphere for 1 h with vigorous stirring to prepare a clear, yellow, cadmium-precursor solution. Meanwhile, a stock solution of sulfur was prepared by dissolving 1 mmol S powder in ODE (5 ml in total) at 150°C with magnetic stirring for 1.5 h.
UV–vis absorption spectra were recorded on a Cary 50 UV–vis spectrometer (Varian, USA) at room temperature. PL spectra were measured at room temperature with a Cary Eclipse spectrofluorometer (Varian, USA) for colloidal solutions with an optical density of less than 0.2 at an excitation wavelength of 330 nm. Quantum yields (QY) of PL was obtained by comparing the integrated PL intensities of the NCs with the organic dye (Rhodamine 6G) . To obtain samples for TEM and XRD characterization, the formed CdS NCs were precipitated by adding acetone into their chloroform solution; the NCs were isolated and purified by repeated centrifugation and decantation. XRD patterns were obtained using a D/max-2550 diffraction meter (Rigaku, Japan) with a Cu anode. During the preparation of samples for XRD analysis, an Si wafer was used as a support to minimize background noise. The morphologies and dimensions of the as-formed NCs were observed by a JEM-2100F HRTEM (JEOL, Japan). The samples for HRTEM observations were obtained by dipping a carbon–copper grid in a dilute, chloroform-dispersed solution of NCs.
Results and Discussion
In batch reactions, the long response time for temperature stabilization and the vibration at reaction conditions make it difficult to achieve precise control of the sizes of CdS NCs. On the contrary, microreaction provides a reproducible tool for synthesizing NCs in a highly controllable manner. Furthermore, the enhanced heat and mass transfer in a microreactor greatly improves the concentrations of reactive monomers, resulting in reductions in the reaction temperature and residence time required to form high-quality NCs.
In order to achieve a balance between process stability and the reactivity of precursors, the reaction temperature and residence time were systematically investigated.
FWHM of PL is an indirect measure for the size distribution of NCs. In Fig. 2c, it can be seen that FWHM of the NCs tended to decrease when the temperature was changed from 180 to 220°C. Further increases in the temperature led to wider FWHM values. During the diffusion-controlled synthesis of spherical NCs, the burst of nucleation that separated from the growth is the prerequisite for achieving monodisperse products . Temperatures below 180°C cannot overcome the energy barrier to form the reactive monomer species, which restricted the burst of nucleation. The nucleation in the whole reaction phase led to poor size distribution. With the increase in reaction temperature from 180 to 220°C, the continuously increased reactivity of the monomer allowed the burst of nucleation, leading to gradual narrowing of the size distribution. The narrowest FWHM was observed at 220°C, indicating a balance of nucleation and growth at this temperature, which is a threshold after which the widened FWHM was induced by further increases in temperature.
The underlying kinetics for the growth of NCs played an important role in the observed FWHM in Fig. 2c. For the diffusion-controlled growth of spherical NCs, the size(r)-dependent growth rate (dr/dt) of NCs has a maximum at a critical radius r cr. When r >r cr, the smaller NCs grow faster than the larger ones, resulting in a narrower size distribution. When r <r cr, the smaller NCs grow slower than the larger ones or even dissolve in the solution (Ostwald ripening), which led to a broad size distribution of the final products. In our experiment, temperatures above 220°C resulted in the rapid depletion of monomers, and the low concentration of monomers led to a larger critical size. As a result, large NCs grew from the monomers formed by the dissolution of small NCs, and broad FWHM was observed with the increase in temperature.
The microfluidic reaction facilitated the process of optimization using a minimal amount of precursors. In this study, the optimization process for the reaction temperature was achieved within an hour, while consuming less than 1 ml of the precursors. Furthermore, the enhanced heat and mass transfer in the microreactor resulted in a reduction in the temperature required to achieve the synthesis of high-quality NCs. In our work, monodisperse CdS NCs were synthesized at a fairly low temperature of 220°C, which is among the lowest values reported based on other studies using similar recipes .
In batch reactions, CdS NCs prepared using high OA concentrations generally demonstrated wide size distribution. However, monodisperse (HWHM~11 nm) CdS NCs can be obtained with a high volume ratio of OA of 51.2%, and the size of the CdS NCs were in the range of 4.5–5.3 nm in the microreaction. The efficient mixing and heat transfer, as well as uniform reaction conditions, achieved by the microreactor contributed to the formation of the high-quality products. In addition, the narrow diffusion length along the radius of the microchannels created homogeneous reaction conditions for the uniform growth of CdS NCs, even at rapid growth rate that was induced by high OA concentrations.
In conclusion, a capillary microreactor was used for the continuous synthesis of CdS NCs, and the influences of reaction temperature and residence time were investigated systematically to obtain the optimal synthesis parameters. The enhanced mass and heat transfer involved in microchannels facilitated the rapid synthesis of high-quality CdS NCs at the low reaction temperature of 220°C. The combined effect of RTD and Ostwald ripening resulted in the optimal residence time as 68 s. The continuous operation and low sample consumption involved in microreaction allowed an accelerated study of the kinetics involved. Variations of OA concentration and residence time were demonstrated as an effective way to control the size of CdS NCs produced. With an increase in residence time from 17 to 136 s and an increase in the volume percentage of OA from 10.5 to 51.2%, CdS NCs in the size range from 3.0 to 5.4 nm were obtained, with corresponding PL peak wavelength from 391 to 463 nm. With the careful selection of reaction parameters, a moderate QY as 14.5% was obtained. Furthermore, excellent size distributions were obtained over a wide range of OA concentrations, and narrow HWHM absorption peaks (from 11.6 to 14.0 nm) were maintained during the entire process.
Authors appreciated the financial supports from the NSFC (50772036), The Focus of Scientific and Technological Research Projects (109063) and the State Key Laboratory of Chemical Engineering at ECUST (SKL-ChE-08C09).
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