Facile Phosphine-Free Synthesis of CdSe/ZnS Core/Shell Nanocrystals Without Precursor Injection
© to the authors 2008
Received: 31 March 2008
Accepted: 3 June 2008
Published: 24 June 2008
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© to the authors 2008
Received: 31 March 2008
Accepted: 3 June 2008
Published: 24 June 2008
A new simple method for synthesis of core/shell CdSe/ZnS nanocrystals (NCs) is present. By adapting the use of cadmium stearate, oleylamine, and paraffin liquid to a non-injection synthesis and by applying a subsequent ZnS shelling procedure to CdSe NCs cores using Zinc acetate dihydrate and sulfur powder, luminescent CdSe/ZnS NCs with quantum yields of up to 36% (FWHM 42–43 nm) were obtained. A seeding-growth technique was first applied to the controlled synthesis of ZnS shell. This method has several attractive features, such as the usage of low-cost, green, and environmentally friendlier reagents and elimination of the need for air-sensitive, toxic, and expensive phosphines solvent. Furthermore, due to one-pot synthetic route for CdSe/ZnS NCs, the approach presented herein is accessible to a mass production of these NCs.
Colloidal semiconductor nanocrystals (NCs) are of great interest for both fundamental studies [1, 2] and technical applications such as light-emitting devices [3, 4], lasers [5, 6], and fluorescent labels [7–9]. CdSe nanocrystals have become the most extensively investigated NCs due to their size-dependent photoluminescence tunable across the visible spectrum . Recently, much experimental work has been devoted to molecular surface modification to improve the luminescence efficiency [11, 12] and photostability of the NCs or to develop a reliable processing chemistry [13, 14]. To attain the ends, overcoating NCs with another wide band gap semiconductor is a well-established method [11, 15–17]. In particular, for CdSe NCs the particles were covered with ZnS [15, 17] to establish a core/shell system, where the band gap of the core lies energetically within the band gap of the shell material and the photogenerated electrons and holes are mainly confined inside the CdSe. Such core/shell NCs have widespread applications in biological and biomedical research [7–9, 18] due to their better stability and processibility. In addition, with the growing interest in applications based on these nanoscale materials comes a need for their large-scale synthesis.
So far, the methods for synthesis of NCs, described in the literature, are too many and variable. The most successful and widely used nanocrystal synthesis relies on rapid precursor injection [19–21]. Meanwhile, non-injection-based synthesis method has also been employed for mass production of NCs in recent years. One noteworthy study was the one-pot synthesis of CdS NCs reported by Cao et al. . Also, continued research on synthesis of NCs without precursor injection has successfully achieved making CdSe and CdTe NCs . This strategy mainly focused on synthesis of core NCs. In fact, in the case of core/shell NCs, one-pot synthetic approach proposed by Weller et al., where both core and shell can grow controllably in the same reaction mixture, has been applied to fabricate CdSe/CdS NCs . For the synthesis of CdSe/ZnS NCs, numerous approaches based on one-pot synthetic method have been exploited, such as ultrasonic baths , the use of single molecular precursors , and microwave-supported synthesis . However, until recently it was not possible to eliminate phosphines as coordinating solvents from the synthesis procedure. For example, trioctylphosphine (TOP) or tributylphine (TBP) was ubiquitously used to dissolve Se precursor or shell precursors in the process of synthesis of NCs. In fact, the costs of large-scale synthesis of NCs are still very high because of using expensive solvents such as TOP (or TBP). Additionally, TOP (TBP) is hazardous, unstable, and not an environmentally friendly solvent . In this article, we are interested in the preparation of this core/shell NCs from inexpensive, stable starting materials of low toxicity via a greener chemistry route. Here a one-pot synthesis of CdSe/ZnS NCs without phosphines is reported, using zinc acetate dihydrate and sulfur powder as shell precursor and paraffin liquid as solvent, based on a modified non-injection method for the synthesis of CdSe core NCs. Compared with the former methods [24–26], the precursors we used are low-cost, clean, and air/water-stable. Moreover, we adopt a seeding-growth technique to mediate the growth of ZnS shell. So, controllable shell growth can be favorably achieved without coordinating solvents such as TOP. The seeding-growth technique has been applied to synthesize CdS NCs by Ji and An et al. . In this article, it is first used for synthesis of core/shell NCs. The CdSe/ZnS NCs reported here exhibit considerable improvement of PL and relative monodispersity. Furthermore, the CdSe/ZnS NCs synthesized by the new route have a different crystal structure from those made by precursor injection methods [19–21] (zinc blande vs wurtzite, respectively), which would make these NCs possess some special properties.
All chemicals were used as received without further purification. Cadmium oxide (CdO, Aldrich, 99.5%), selenium powder (Se, Aldrich, 95%), oleylamine (OA, Aldrich, 70%) were used in the preparations described here. Paraffin liquid (chemical grade), stearic acid (analytical grade), Zinc acetate dihydrate (Zn(OAc)2 · 2H2O, analytical grade), sulfur powder (S, analytical grade),n-hexane (analytical grade), methanol (analytical grade), and acetone (analytical grade) were obtained from Sinopharm chemical Reagent Co., Ltd, China. Rhodamine B was purchased from Alfa Aesar.
Cadmium stearate was prepared by heating the mixture of CdO and Stearic acid at 220 °C for 12 min. The crude product was recrystallized twice from toluene and then used for further reaction. A typical synthesis procedure of CdSe NCs is given briefly below. Se powder (0.0039 g, 0.05 mmol) and cadmium stearate (0.0679 g, 0.1 mmol) were added into a three-neck flask with 8 mL paraffin liquid. The mixture was degassed in vacuum for 15 min at room temperature and then heated under N2to 225 °C with oil-bath heating for reaction. Aliquots were taken from the reaction mixture to monitor the growth of core NCs. After 2.5 min, 1.5 mL OA was added dropwise into the mixture to stabilize the growth of the NCs. The mixture was cooled naturally to ambient temperature when NCs with desirable size were obtained. Because Cadmium stearate was solidified from paraffin liquid at room temperature, the crude core solution was only simply purified by direct centrifugation at a low speed. Cadmium stearate separated from the solution can be used for synthesis of CdSe NCs.
Typically, the resulting core solution (reaction for 3 min), Zn(OAc)2 · 2H2O (0.085 mmol) and S powder (0.085 mmol) were mixed together in the reaction vessel. The reaction volume was adjusted to 15 mL by adding paraffin liquid. Next, with stirring, the mixture was degassed at 80 °C for 20 min. Afterward, temperature was set to 145 °C for the shell growth under N2 atmosphere. To monitor the reaction, aliquots were taken at different times. The reaction mixture was cooled to room temperature after 50 min. To grow shell ZnS with different thicknesses around a CdSe core, a seeding-growth technique  was applied. The same amount of Zn and S precursors as the first shell precursors as added to the reaction mixture and further shelling was done. This shell growth cycle was repeated until desired core–shell NCs were obtained. Finally, the CdSe/ZnS NCs were precipitated by the addition of acetone, then separated, and finally redispersed for further processing.
In this equation, I (sample) and I s (standard) are the integrated emission peak areas, upon 480 nm excitation; A (sample) and A s (standard) are the absorption at 480 nm; n (sample) and n s (standard) are the refractive indices of the solvents; and Φ and Φs are the PL QYs for the sample and the standard, respectively.
As mentioned earlier, the formation of a semiconductive shell over a NCs core structure greatly enhances the photophysical properties of the NCs. ZnS has a wide band gap (3.68 eV) as compared to the CdSe (band gap of 1.7 eV) at 300 K . So ZnS is the commonly used capping agent for CdSe NCs.
PL FWHM, peak site, and QYs of CdSe/ZnS NCs for varying shell growth cycles
Cycles of reaction (times)
Peak site (nm)
Figure 4 displays TEM and HRTEM images of the CdSe/ZnS core/shell NCs. It can be observed that the NCs have a narrow size distribution. Furthermore, the ZnS shell thickness can be estimated by the subtraction of the core size (observed in Fig. 2b) from that of the present prepared core/shell particles. The average diameters of CdSe/ZnS core/shell NCs, as measured by HRTEM, are 4.5 nm. Therefore, the average shell thickness is about 1.7 nm, which corresponds to 2.7 monolayer shells estimated using the distance (0.31 nm) between the adjacent lattice fringes along the ZnS (111) plane. The clear lattice plane observations on the HRTEM are indicative of the good crystallinity. Although the mismatch of the lattice constants between core (CdSe) and shell (ZnS) is 12%, it is impossible to allow the resolution of the core and shell individually via the difference in the lattice orientations using TEM. This phenomenon has also been reported in many other types of core/shell NCs [15, 17, 24].
In summary, we have developed a low-cost and non-phosphine-based approach for growing CdSe/ZnS core/shell NCs with zinc blende crystalline structure. We have found that a little water contained in Zn precursors play an important role in the preparation of the resulting NCs. By using a seeding-growth technique, the relative monodispersed CdSe/ZnS NCs were prepared. Compared with the traditional route, this method can be performed at relatively low temperature without the need for precursor injection. In addition, this approach may be applied to the synthesis of other core/shell NCs. The structure of CdSe/ZnS core/shell NCs is confirmed by TEM, XRD, and XPS. The as-prepared core/shell with enough high QYs is used potentially for bio-applications. Moreover, further work for digging out the properties and applications of such NCs is under way.
The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 20575002) and the Natural Science Foundation of Anhui Province (No. 070416239).