Luminescent properties of CdTe quantum dots synthesized using 3-mercaptopropionic acid reduction of tellurium dioxide directly
© Shen et al.; licensee Springer. 2013
Received: 3 August 2012
Accepted: 22 April 2013
Published: 29 May 2013
A facile one-step synthesis of CdTe quantum dots (QDs) in aqueous solution by atmospheric microwave reactor has been developed using 3-mercaptopropionic acid reduction of TeO2 directly. The obtained CdTe QDs were characterized by ultraviolet–visible spectroscopy, fluorescent spectroscopy, X-ray powder diffraction, multifunctional imaging electron spectrometer (XPS), and high-resolution transmission electron microscopy. Green- to red-emitting CdTe QDs with a maximum photoluminescence quantum yield of 56.68% were obtained.
KeywordsCdTe quantum dots 3-mercaptopropionic acid TeO2 Microwave irradiation Photoluminescence
In recent years, water-soluble CdTe luminescent quantum dots (QDs) have been used in various medical and biological imaging applications because their optical properties are considered to be superior to those of organic dyes [1–4]. Up to now, in most of the aqueous approaches, Te powder was used as the tellurium source and NaBH4 as the reductant, which needs a pretreatment to synthesize the unstable tellurium precursor. The process of preparing CdTe QDs requires N2 as the protective gas at the initial stage [5–10]. Even though Na2TeO3 as an alternative tellurium source can also be used for preparing CdTe QDs [11–15], it is toxic and expensive. Therefore, it is very necessary to hunt for a novel tellurium source for the synthesis of CdTe QDs. Compared with Na2TeO3, TeO2 has the same oxidation state of Te and is stable, cheap, and less toxic. Recently, TeO2 was explored as the Te source for synthesis of CdTe QDs, but the reduction of TeO2 by NaBH4 in ambient conditions requires a long reaction time and easily produces a black precipitate of CdTeO3[16–20]. Here, we proposed a new facile synthetic approach for preparing CdTe QDs with tellurium dioxide as a tellurium source. 3-mercaptopropionic acid was explored as both reductant for the reduction of TeO2 and capping ligand for CdTe QDs. Such synthetic approach eliminates the use of NaBH4 and allows facile one-pot synthesis of CdTe QDs.
Tellurium dioxide (TeO2, 99.99%), cadmium chloride hemi(pentahydrate) (CdCl2 · 2.5H2O, 99%), and 3-mercaptopropionic acid (MPA, 99%) were purchased from Aldrich Corporation (MO, USA). All chemicals were used without additional purification. All the solutions were prepared with water purified by a Milli-Q system (Millipore, Bedford, MA, USA).
Synthesis of CdTe QDs
In our experiments, 2 mmol CdCl2 · 2.5H2O was dissolved in 100 mL of deionized water in a breaker, and 5.4 mmol MPA was added under stirring. The pH of the solution was then adjusted to 10.0 by dropwise addition of 1 mol/L NaOH solution. Under stirring, 0.5 mmol TeO2 was added to the original solution. The typical molar ratio of Cd2+/Te2−/MPA was 1:0.25:2.7. The monomer was heated in a XO-SM100 microwave-assisted heating system (XO-SM100 Microwave and Ultrasonic combination response system, MW-50%; Xianou Company, Nanjing, China) and refluxed at different times to control the size of the CdTe QDs. The particles were extracted by precipitation with the addition of 2-propanol to the solution. Then, the resulting powders were dried at room temperature.
The absorption and photoluminescence (PL) spectra were measured using a UV-2501PC spectrometer (Shimadzu Corporation, Tokyo, Japan) and CARY ECLIPSE (Agilent Technologies, Santa Clara, USA) fluorescence spectrometer, respectively. The PL quantum yield was determined using Rhodamine 6G as fluorescence standard. X-ray powder diffraction (XRD) analysis was performed using a Dmax-2500 (CuKα = 1.5406 Å; Rigaku Corporation, Tokyo). The morphology of the QDs was characterized using JEM-2100 transmission electron microscopy (HR-TEM; Jeol Ltd., Tokyo). X-ray photoelectron spectra (XPS) were recorded by Thermo ESCALAB 250XI X-ray photoelectron spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA) with nonmonochromatized Al Kα radiation as excitation source.
Results and discussion
In strong alkali solutions, TeO2 was firstly dissolved and formed TeO32- anion. Meanwhile, Cd2+ is complexed by RSH (MPA) and forms Cd(RS)+. In the presence of excess MPA, tellurite is first slowly formed to RS-Te-SR (3), and then the RS-Te-SR is further reduced by MPA into RS-TeH/RS-Te− (4) and H2Te/HTe−/Te2− (5). The CdTe QDs were obtained by the reaction between HTe− and Cd2+ in the presence of MPA, according to reaction (6).
In summary, a facile synthetic route for the preparation of water-soluble CdTe QDs has been proposed using 3-mercaptopropionic acid reduction of TeO2 directly. Since the raw materials are cheap and easy to be obtained, the synthesis process is simple, fast, and mild. The as-synthesized CdTe QDs were highly luminescent, which ensures its promising future applications as biological labels.
The authors gratefully acknowledge the support for this research from Zhejiang Provincial Natural Science Foundation of China under grant no. LQ12B03002 and from the National Natural Science Foundation of China under grant no. 21207095, as well as the State Key Laboratory of Chemical Resources Engineering under grant no. CRE-2012-C-303.
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