Chem. Res. Chin. Univ., 2016, 32(1), 16―19

doi: 10.1007/s40242-015-5279-8

Microwave-assisted Synthesis of CdTe Quantum Dots Using 3-Mercaptopropionic Acid as Both a Reducing Agent and a Stabilizer HUANG Yantao, LAN Yuwei, YI Qilei, HUANG Hualin, WANG Yilin* and LU Jianping Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, P. R. China Abstract In this paper, a facile synthetic approach to prepare CdTe quantum dots(QDs) with high luminescence via a one-pot microwave irradiation reaction route using 3-mercaptopropionic acid(MPA) as both a sodium tellurite reducer and a capping molecule was described, and the mechanism of the formation of CdTe QDs was elucidated. In this approach, CdTe QDs with six different emission wavelengths of 553, 567, 577, 595, 608 and 615 nm were obtained via changing the refluxing time and the quantum yields(QY) of these QDs were 40.6%, 55.3%, 63.6%, 43.4%, 37.4% and 29.7%, respectively. The characterization results of X-ray powder diffraction(XRD) and transmission electron microscopy(TEM) indicate that the obtained QDs have a pure cubic zinc blended structure with a spherical shape. No toxic gases were released during the preparation process, indicating that the method is relatively fast, cheap and environmentally friendly. Keywords Cadmium telluride; Quantum dot; Photoluminescence

1

Introduction

Quantum dots(QDs) have attracted tremendous attention in both theoretical studies and practical application due to their unique electrical and optical properties[1―3]. Up to now, a multitude of synthetic methods have been developed[4―6]. Based on these researches, the synthesis of QDs can be summarized via two chemical routes, nonaqueous technique[7] and aqueous technique, a method using different thiols as stabilizing agents in aqueous solutions[8]. It is well-known that QDs with excellent optical properties are often achieved by the former method. However, these QDs cannot be directly used in bioapplications due to their hydrophobic characteristics. Compared to the nonaqueous synthesis, the aqueous synthesis is relatively simpler, cheaper and less toxic, and the as-prepared samples are more water-soluble and bio-compatible. However, QDs with poor quality and low photoluminescence quantum yield(PLQY) are generally obtained. In recent years, tremendous efforts[9―11] have been devoted to improving the optical properties of QDs directly prepared in the aqueous phase. With comparison to the conventional oil-bath heating route, the microwave-assisted synthesis of QDs has gradually gained popularity because microwave irradiation can provide a rapid and homogeneous heating for the entire sample, enhancing the reaction rates, and facilitating the formation of uniform nucleation centers. The

microwave assisted synthesis route of CdS QDs was first reported in 2002[12], and recently has been employed to prepare water-dispersed QDs, such as CdTe[13], CdSe[14], CdTe/CdS[15] and CdSe/CdS/ZnS[16] with high crystallinity and photoluminescence quantum yield. On the other hand, for most aqueous routes, either NaBH4[17] or N2H4·H2O[18] is often used as a reducing agent for the preparation of NaHTe(or NaHSe) precursors. 3-Mercaptopropionic acid(MPA) is a weak acid with reductive and complexing properties, and can be used as a complexing agent rather than a reducing agent to prepare metal selenide crystals, particularly nanorods and fractal nanocrystals[19―21]. Recently, Hodlur and coworkers[22] reported the synthesis of CdSe QDs using SeO2 as Se source and MPA as both a reducing agent and a capping molecule in an aqueous medium. In the present work, we described a facile synthetic approach to prepare CdTe QDs via a one-pot microwave irradiation reaction route using MPA as both a sodium tellurite reducer and a capping molecule.

2

Experimental

0.4567 g(2 mmol) of CdCl2·2.5H2O was dissolved in 100 mL of deionized water in a 250-mL three-neck flask, and 400 μL(4.5 mmol) of MPA was added under stirring. The pH value of the solution was then adjusted to 10.0 by the dropwise

——————————— *Corresponding author. E-mail: [email protected] Received July 17, 2015; accepted October 8, 2015. Supported by the Dean Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, China(No.2013K05), the Scientific Research Foundation of Guangxi University, China(No.XBZ120723) and the Foundation of College Student Experimental Skills and Innovation Ability Training of Guangxi University, China (No.SYJN20130311). © Jilin University, The Editorial Department of Chemical Research in Chinese Universities and Springer-Verlag GmbH

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addition of 1.0 mol/L NaOH solution. Under stirring, 0.0222 g(0.1 mmol) of Na2TeO3 was added to the above solution. Finally, the flask was heated on an MCR-3 microwave-assisted heating system and refluxed for different time to control the sizes of CdTe QDs. Ultraviolet-visible(UV-Vis) absorption spectra were obtained on a UV-2102 spectrometer. The photoluminescence(PL) spectra were monitored on an RF-5301 fluorescence spectrophotometer and an excitation wavelength of 365 nm was used in the process. Fourier transform infrared(FTIR) spectra were recorded on a Nicolet IS50 infrared spectrometer in the wavenumber range of 4000―400 cm–1. XRD patterns were obtained on a Rigaku/Dmax-2500 X-ray diffractometer with Cu Kα radiation(λ=0.15406 nm). The solution of QDs was spread on an ultra-thin carbon-coated film with 200 mesh copper grids to dry in air. A sample was then visualized at 300 kV by an FEI-TF30 transmission electron microscopy equipped with an energy dispersive X-ray(EDX) spectroscopy.

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oil-bath heating route. It could be explained that the microwave heating process is different from the conventional oil-bath one because the heat is produced in a local domain through exciting atoms or molecules rather than heat conduction, which only heats the surface of molecules. That is to say, in the microwave assisted process, molecules are heated thoroughly inside and outside. Thus, the microwave heating is able to accelerate chemical reactions.

Results and Discussion

It has been demonstrated that Cd2+ can form 1:1 and 1:2 complexes with MPA[23]. However, in our experiment, the molar ratio of MPA to Cd was 4.5:2, which was larger than 2:1. According to the Ostwald ripening mechanism, the formation of QDs in solution involves two stages: nucleation and growth. In this method, MPA was used as both a complexing agent and a reducing agent. The reaction process of the formation of CdTe QDs is proposed as follows: Cd2++2HSCH2CH2COOH=Cd2+-(SCH2CH2COO−)2 (1) TeO32−+HSCH2CH2COO−=Te2−+−OOCCH2CH2SO3− (2) Cd2+-(SCH2CH2COO−)2+Te2−=CdTe-(SCH2CH2COO−)2 (3) nCdTe-(SCH2CH2COO−)2=[Cd2+-(SCH2CH2COO−)2]n (4) First, the Cd2+-(SCH2CH2COO−)2 complex was formed under a basic condition[Eq.(1)]. Then TeO32− was reduced by excess MPA molecules to produce Te2− ions[Eq.(2)], which can combine with Cd2+-(SCH2CH2COO−)2 complex to form CdTe nucleus[Eq.(3)]. With the CdTe nucleus growing under refluxing, CdTe QDs with different emission wavelengths were formed at last[Eq.(4)]. Fig.1(A) shows the absorption spectra of a series of CdTe QDs with different sizes. The spectra of the as-prepared CdTe colloidal solutions were measured, which were taken from the refluxing reaction mixture at different intervals of time and diluted with deionized water. It can be observed that CdTe QDs have a wide range of absorption and a peak corresponding to 1s-1s electronic transitions can be observed in the absorption spectra. The first absorption peaks of the QDs are at 514, 533, 559, 566, 576 and 587 nm, respectively. Compared with those of the CdTe bulk material(827 nm), the first absorption peaks of QDs have obviously blue shift, showing the quantum confinement effect. According to the empirical formula of the previous reports[24,25], the particle sizes were estimated to be 2.4, 2.5, 2.7, 2.8, 3.0 and 3.2 nm, respectively. The diameter of the CdTe QDs reached to 3.2 nm after reaction for 240 min, which indicated that the growth rate of QDs prepared via microwave-mediated heating route was faster than that prepared via

Fig.1

UV-Vis absorption(A) and photoluminescence(B) spectra of CdTe QDs prepared for different reaction time

Refluxing time/min for a―f : 20, 40, 60, 120, 180 and 240, respectively.

Fig.1(B) displays the PL spectra of CdTe QDs prepared for different reaction time. As is shown, the CdTe QDs mainly exhibit bandgap emission with no observable deep trap emissions. After 20 min of refluxing, the green-yellow QDs with the smallest size show emission peak at 553 nm. As the reaction time increases, the emission peak in the PL spectra shifts systematically to longer wavelengths, which demonstrates the growth of nanocrystals. It has been found that the PL peaks red-shift about 62 nm(from λ=553 nm to 615 nm) during a period of 220 min, the full width at half maximum(FWHM) keeps at around 55―60 nm. As we know, the QY of QDs synthesized by the aqueous route is quite poor when compared to that of the QDs obtained by the organic phase synthesis. Here, the QY of CdTe QDs was measured using rhodamine 6G as a standard(QY is taken to be 95%) according to the procedure in Ref.[26]. The calculated QYs for the six samples are 40.6%, 55.3%, 63.6%, 43.4%, 37.4% and 29.7%, respectively, which suggests that the luminescent efficiency of the as-prepared QDs is comparable with that of the QDs synthesized in organic phases[27]. This may be contributed to the fact that microwaves can penetrate the reaction solution, leading to thorough and fast heating. Thus, a successive process of epitaxy crystal growth can be realized and the nanocrystals can be formed sufficiently in a short time during the Ostwald ripening stage, which is

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extraordinarily beneficial for reducing the surface defects of QDs. Therefore, microwave technique is an excellent strategy to prepare high-quality QDs. Fourier transform infrared(FTIR) spectrum of the CdTe QDs was measured to confirm the formation of CdTe QDs and to investigate the interaction between MPA and CdTe QDs. As shown in Fig.2, a broad absorption band at around 3400 cm−1 is assigned to the O―H vibration, and the band at 2918 cm−1 is attributed to the stretching vibration of the alkyl chains of MPA molecules. The absence of S―H stretching band around 2560 cm−1 clearly indicates that thiolates coordinate with the Cd2+ sites on the CdTe QDs surface via sulphur atom of organic molecule. The sharp band at around 1561 cm−1 can be assigned to the vibration of carboxylate anion of MPA molecule. The band at 1410 cm−1 can be attributed to the symmetric stretching vibration of C―O. These results strongly suggest that the thiol groups of MPA coordinate with Cd2+ ions on the QDs surface, and the carboxylate anions faced outward, which results in a net negative charge on the surface of CdTe, preventing QDs from coagulation and stabilizing colloids of these QDs.

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The images obviously show that the particles are agglomerated, which is common for many aqueous solutions of thiol capped QDs. The existence of lattice fringes(seen from HRTEM) confirms the good crystallinity of the CdTe QDs and the selected area electron diffraction(SAED) pattern of CdTe QDs in Fig.4(E) can also be indexed to the cubic phase of CdTe.

Fig.4

Fig.2 FTIR spectrum of MPA-capped CdTe QDs The powder XRD pattern of the CdTe QDs solid sample precipitated from aqueous solution with an excess of 2-propanol was recorded and presented in Fig.3. The diffraction of CdTe QDs is quite close to that of bulk cubic CdTe(JCPDS No. 65-1046). The diffraction peaks at 2θ=24.5°, 40.8° and 48.3° can be readily assigned to the (111), (220) and (311) planes, respectively. These peaks are comparatively wider than those of the bulk materials due to the finite crystalline size.

TEM images of CdTe QDs refluxed for 40(A), 120(B) and 240 min(C), and HRTEM image(D) and SAED pattern[inset of (D)] of CdTe QDs refluxed for 240 min The result of the EDX measurement(Fig.5) indicates the presence of Cd, Te, O, S and C, further confirming that the sample is MPA capped CdTe QDs. Among these elements, O, S and C are from the stabilizer(MPA), which is as the capping ligand covered on the surface of QDs. The surfaces of the QDs are rich of cadmium, which is resulted from the atomic ratio of Cd to Te, calculated from the EDX to be 2.27:0.26.

Fig.5

4 Fig.3 XRD pattern of CdTe QDs refluxed for 240 min Fig.4(A)―(C) show the TEM images of CdTe QDs refluxed for 40, 120 and 240 min, respectively. The TEM results confirm that the size distribution of the prepared CdTe QDs is nearly monodisperse in the range of 2.0―3.0 nm, which is close to that calculated from the UV-Vis absorption spectra.

EDX pattern of CdTe QDs refluxed for 240 min

Conclusions

A facile synthetic approach of preparing CdTe QDs using MPA as both a sodium tellurite reducer and a capping molecule via a one-pot microwave irradiation reaction route was reported. The as-prepared QDs were highly luminescent and the PL QY of one of the QDs reached up to 63.6%. No toxic gases were released during the preparation process, which was relatively fast, cheap and environmentally friendly.

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[14] Xuan T. T., Wang X. J., Zhu G., Li H. L., Pan L. K., Sun Z., J. Alloy.

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