Noninjection Synthesis of CdS and Alloyed CdSxSe1−xNanocrystals Without Nucleation Initiators
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CdS and alloyed CdSxSe1−x nanocrystals were prepared by a simple noninjection method without nucleation initiators. Oleic acid (OA) was used to stabilize the growth of the CdS nanocrystals. The size of the CdS nanocrystals can be tuned by changing the OA/Cd molar ratios. On the basis of the successful synthesis of CdS nanocrystals, alloyed CdSxSe1−x nanocrystals can also be prepared by simply replacing certain amount of S precursor with equal amount of Se precursor, verified by TEM, XRD, EDX as well as UV–Vis absorption analysis. The optical properties of the alloyed CdSxSe1−x nanocrystals can be tuned by adjusting the S/Se feed molar ratios. This synthetic approach developed is highly reproducible and can be readily scaled up for potential industrial production.
KeywordsNanocrystals CdS CdSxSe1−x Noninjection synthesis Oleic acid
Colloidal semiconductor nanocrystals have been intensively studied due to their unique physical properties. They exhibit quantum confinement effect such as size-dependent optical and electric properties for applications in optoelectronic devices [1–3] and biological fluorescence labeling [4, 5]. CdS, with a bulk band gap of ~2.42 eV (~512 nm) and exciton Bohr radius of ~3 nm, is a candidate for quantum-dot blue light-emitting diodes  and can be used in photovoltaic devices . In order to tune the optical properties of CdS quantum dots for specific applications, two schemes have been put forward. One is to change the size of the quantum dots, based on the quantum confinement effect. The other is to alloy CdS with other materials, such as CdSe, to form alloyed quantum dots whose band gap energy can vary with composition without changing size. In the past decades, significant advancement has been made in the synthesis of CdS [8–10] and CdSxSe1−x[11–13] quantum dots based on the classic hot-injection method developed by Bawendi’s group , the key part of which is the injection of room-temperature organometallic precursors into well-stirred hot organic solvents. However, these injection-based methods are not suitable for large-scale industrial preparation due to the difficulty of large mass transfer in the process of the injection.
In 2004, Cao et al.  reported a novel synthetic method for producing high-quality CdS quantum dots by simply heating the reaction mixture containing Cadmium myristate, Sulfur (S) and 1-octadecene without injection. They added two nucleation initiators (tetraethylthiuram and 2,2′-dithiobisbenzothiazole) into the reaction medium to increase the reactivity of S and found that the nucleation initiators were the key to successfully prepare the high-quality quantum dots. Ouyang et al.  extended this noninjection method to synthesize alloyed CdSxSe1−x quantum dots. Nucleation initiator 2,2′-dithiobisbenzothiazole is still applied in their work. Moreover, CdSe quantum dots have been prepared without precursor injection and nucleation initiators simultaneously by Cao’s group . Oleic acid, a natural surfactant, was chosen to stabilize the growth of the quantum dots. From this viewpoint, CdS and alloyed CdSxSe1−x quantum dots may be synthesized without nucleation initiators if oleic acid is added into the reaction mixture, which could make the synthesis greener. Recently, Li et al.  have prepared CdS magic-sized nanocrystals using OA as stabilizer and highly reactive bis(trimethylsilyl)sulfide as S source at low temperatures (90–140°C).
Herein, we report the successful synthesis of CdS and alloyed CdSxSe1−x quantum dots without injection and nucleation initiators by simply heating the reaction medium. All the source chemicals used including cadmium acetate, sulfur, selenium, oleic acid and 1-octadecene are air-stable and inexpensive relatively. The optical properties of the obtained quantum dots can be tuned by changing the OA/Cd or the S/Se feed molar ratio. The former and latter changing of feed molar ratio results in the change of the size and the composition, respectively. This synthetic approach developed is very simple and highly reproducible and thus suitable for large-scale preparation of CdS and CdSxSe1−x nanocrystals.
All of the chemicals are commercially available and were used as received. Cadmium acetate dihydrate (Cd(OAc)2·2H2O, 99.5%), sulfur (S, 99.5%) and Selenium (Se, 99.95%) were purchased from Shanghai Chemical Reagent Ltd.. Oleic acid (OA, tech. 90%) and 1-octadecene (ODE, tech. 90%) were purchased from Aldrich.
In a typical synthesis of CdS nanocrystals, a 100-ml three-neck flask containing 1 mmol Cd(OAc)2, 6 mmol OA and 15 ml ODE was heated to 120°C and was degassed under vacuum for 30 min. The flask then was filled with argon gas, and its temperature was lowed to about 30°C. A total of 0.5 mmol S was added into the flask, and then the mixture was heated to 240°C at a rate of ~20°C/min and reacted at this temperature for 60 min under a flow of argon gas. To monitor the growth of nanocrystals, small aliquots (~0.1 ml) of the reaction mixture were taken out from the flask and quenched in cold hexane (25°C) in order to stop further growth. The time was counted as 0 when the temperature reached to 240°C. At last, the quantum dots were isolated by precipitation with ethanol, then centrifuged and the resulted nanocrystals were redispersed in hexane. This process was repeated several times to wash the samples. To investigate the effect of OA, only the amount of OA was changed. For the synthesis of CdSxSe1−x nanocrystals, certain amount of S was replaced with equal amount of Se while the amount of OA was fixed at 6 mmol. The other experimental conditions were the same as those to the preparation and purification of CdS nanocrystals.
Transmission electron microscopy (TEM) and High-resolution transmission electron microscopy (HRTEM) images were collected using a Philips CM200 transmission electron microscope operating at 160 kV. A sample for TEM analysis was prepared by drying a drop of nanocrystal hexane solution on a carbon-coated copper grid and letting it dry in air. X-ray powder diffraction (XRD) was conducted on a Rigaku D/max-ga X-ray diffractometer with graphite monochromatized Cu Kα radiation (λ = 1.5418 Å). Energy-dispersive X-ray (EDX) analysis was performed using an EDAX X-ray detector attached to a Hitachi S-4800 scanning electron microscope. Ultraviolet–Visible absorption spectra were recorded on a U-4100 UV–Vis spectrophotometer. Photoluminescence (PL) spectra were taken with a He–Cd laser as the excitation source and an excitation wavelength of 325 nm.
Results and Discussions
OA has been noticed as a special ligand due to its unique configuration of hydrocarbon chains. OA is more bending than saturated fatty acid, such as myristic acid, due to the existence of a cis-double bond in the middle of the hydrocarbon chains, which makes the Cd–Oleate complex more reactive . Therefore, OA should play two roles here. First, OA accelerates the nucleation just as the nucleation initiators do. In contrast to the nucleation initiators that increase the reactivity of S , OA increases the reactivity of Cd precursors relative to myristic acid. Secondly, OA stabilizes the further growth as myristic acid does. As a result, the presence of OA separates and balances the nucleation and growth of CdS nanocrystals, leading to the successful synthesis of high-quality CdS nanocrystals without nucleation initiators.
We report the successful synthesis of CdS and CdSSe nanocrystals without injection and nucleation initiators. Highly monodispersed nanocrystals with narrow size distribution are obtained by simply heating a mixture containing Cd(OAc)2, S, Se, OA and ODE. This method is very economical and suitable for industrial production. OA plays an important role in the reaction, which increases the reactivity of Cd precursors and stabilizes the growth of nanocrystals. The effects of the OA/Cd or Se/S feed molar ratios on the optical properties of the nanocrystals have been studied. High OA/Cd molar ratios and high Se/S feed molar ratios lead to nanocrystals with small band gap.
The authors express their appreciations to the National Basic Research Program of China (973 Program) (Grant No.2007CB613403), 2008DFR50250 of MOST and PCSIRT project for the financial support.
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