Sonoelectrochemical synthesis of water-soluble CdTe quantum dots

Sonoelectrochemical synthesis of water-soluble CdTe quantum dots

Ultrasonics Sonochemistry xxx (2013) xxx–xxx Contents lists available at SciVerse ScienceDirect Ultrasonics Sonochemistry journal homepage: www.else...

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Ultrasonics Sonochemistry xxx (2013) xxx–xxx

Contents lists available at SciVerse ScienceDirect

Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson

Sonoelectrochemical synthesis of water-soluble CdTe quantum dots Jian-Jun Shi a,b, Sheng Wang a, Ting-Ting He a, E.S. Abdel-Halim c, Jun-Jie Zhu b,⇑ a

School of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001, China State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China c Chemistry Department, College of Science, King Saud University, Riyadh 11451, P.O. Box 2455, Saudi Arabia b

a r t i c l e

i n f o

Article history: Received 2 April 2013 Received in revised form 27 May 2013 Accepted 6 June 2013 Available online xxxx Keywords: CdTe Quantum dots Sonoelectrochemistry Synthesis Water-soluble Photoluminescence

a b s t r a c t A facile and fast one-pot method has been developed for the synthesis of CdTe quantum dots (QDs) in aqueous phase by a sonoelectrochemical route without the protection of N2. The morphology, structure and composition of the as-prepared products were investigated by high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and energy dispersive X-ray spectrometer (EDS). The influences of current intensity, current pulse width, and reaction temperature on the photoluminescence (PL) and quantum yield (QY) of the products were studied. The experimental results showed that the watersoluble CdTe QDs with high PL qualities can be conveniently synthesized without precursor preparation and N2 protection, and the PL emission wavelength and QY can be effectively controlled by adjusting some parameters. This method can be expected to prepare other QDs as promising building blocks in solar cell, photocatalysis and sensors. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Semiconductor nanocrystals and their clustered states present remarkable optical and electronical properties [1–3]. CdTe quantum dots (QDs), as one of the most important semiconductors, are envisaged as fundamental building blocks to fabricate nanostructures and nanodevices in photoelectric conversion [4,5], photocatalysis [6] and sensors [7–11]. General construction strategy is assembly of several functional components by taking advantage of the physiochemical interactions [12,13]. Obviously, it is critical to synthesize monodisperse QDs with tunable size and high QYs. Various synthetic methods of CdTe QDs with desired size, shape and surface have been developed in the past two decades. Among these, colloid chemical protocols, involving organometallic and aqueous phase routes, exhibited significant progress in QDs synthesis, while the procedure is still not facile as expected. For high temperature, it is indispensable for generating nucleus and maintaining QDs growth in organometallic routes [14,15]. Although the CdTe QDs with high PL QY can be obtained by organometallic route, the applications were limited because of hydrophobic and incompatible with the aqueous biological environment [16]. Thus, the preparation in aqueous phase has been widely studied. In brief, the water-soluble CdTe QDs were synthesized in an aqueous system by mixing tellurium and cadmium precursors in the presence of a thiolated capping agent under the protection of N2. The most ⇑ Corresponding author. Tel./fax.: +86 25 83597204. E-mail address: [email protected] (J.-J. Zhu).

popular tellurium precursor was prepared from Te powders by using NaBH4 as reducing agent. It is a time-consuming process due to the inert of the Te powder. Cathodic stripping Te electrode could also be used as the Te precursor in aqueous phase, where the generation of CdTe precursor should be heated at a water bath at 80 °C more than 6 h [17]. Na2TeO3 was used as tellurium precursor for CdTe QDs by electrodeposition for its water-soluble property. Until now, only CdTe film [18] and nanowire [19] have been obtained by electrochemical method. All above methods required the protection of N2 during the reaction to avoid oxidation of the precursors and products. Traditionally, the properties of QDs were adjusted by changing the reaction temperatures and times on the basis of optimization of the mole ratio of S to Cd. By contrast, sonoelectrochemical technique [20,21] has been successfully used as a new strategy for the controllable synthesis of nanomaterials [22–26]. As described in our previous report [27], a sonoelectrode produced a ultrasonic pulse that was triggered immediately following a current pulse in the synthetic processes repeatedly, which were more convenient to control the shape and size by simply adjusting the ultrasonic and electrochemical parameters. Despited several semiconductors nanomaterials such as PbTe [28], CdSe [29], PbSe [30] have been synthesized by this technique, to the best of our knowledge, sonoelectrochemical synthesis of QDs has not been reported, not to mention the adjusting of QYs precisely. Herein, we report a one-pot facile and controllable method to synthesize water-soluble CdTe QDs by sonoelectrochemistry without precursor preparation and N2 protection. The morphology and

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structure of the products were characterized by HRTEM, EDS and XRD. The influences of current intensity, current pulse width, and reaction temperature on the PL properties of products were studied. 2. Experimental section 2.1. Materials 3-mercaptopropionic acid (MPA) was purchased from Sigma Aldrich. Sodium tellurite (Na2TeO3), Cadmium Chloride (CdCl2) and sodium hydroxide (NaOH) were purchased from Chinese Shanghai Regent Co. All the chemicals were used as received without further purifications. All solutions were prepared with Millipore water. 2.2. Apparatus The sonoelectrochemical reactor was described previously [31,32]. The titanium horn (ultrasonic liquid processor VC-750, Sonic & Materials) acts both as the cathode and the ultrasound emitter. The electroactive part of sonoelectrode is the planar circular surface with an effective area of 1.23 cm2 at the bottom of the horn. A CHI 6301B electrochemical workstation (CH Instruments Co., USA) was employed in the pulse current regime without reference electrode. A platinum sheet (1.0  1.0 cm) was used as a counter electrode. The ultrasonic system worked at an emission frequency of 20 kHz. The characterization of the ultrasonic power was performed using a standard calorimetric method [33]. Unless otherwise indicated, the experiments were performed at 30% the maximum power. 2.3. Synthesis of CdTe QDs In a typical synthesis, 2 mL CdCl2 (50 mM), 17.6 lL 3-mercaptopropionic acid (MPA) and 1 mL Na2TeO3 (50 mM) were added into 47 mL water in turn under stirring, followed by adjusting the pH to 11.0 by dropwise adding NaOH solution (1 M). The molar ratio of 2þ Cd =MPA=TeO2 was 2:4:1. The CdTe QDs were produced in a 3 sonoelectrochemical reactor with the current density of 81.3 mA cm2, ultrasound pulse intensity of approximately 20 W and reaction time of 3000 s. The pulse-on time of the current was 0.5 s, the pulse-off time of the current was 1.5 s, and the duration of the ultrasonic pulse was 0.3 s. Various reactions were also carried out with different current densities, temperatures and pH values. 2.4. Characterization Characterization was performed via X-ray powder diffraction (XRD, Shimadzu XD-3A, with Cu Ka radiation, k = 0.15418 nm) and high resolution transmission electron microscopy (HRTEM, JEOL 2100, with a 200 kV accelerating voltage) equipped with an energy dispersive X-ray spectrometer (EDS). The UV-vis spectra of CdTe QDs were recorded by a Shimadzu UV-3600 spectrophotometer. Photoluminescence (PL) spectra were measured on a Shimadzu RF-5301PC fluorescence spectrometer at room temperature. The PL QYs were estimated by comparing the area of the fluorescence spectrum from Rhodamine 6G (kex = 475 nm, QY: 0.95) in ethanol. To investigate the electrochemical process, cyclic voltammogram (CV) measurements were carried out in the reaction precursor solution on a CHI 6301B electrochemical workstation with Pt-sheet as working electrode, Pt-wire as counter electrode, saturated calomel electrode (SCE) as reference electrode, and the scan rate was 100 mV s1.

3. Results and discussion 3.1. Characterization of CdTe QDs The HRTEM of the CdTe QDs in typical synthesis was shown in Fig. 1A. The clear lattice fringes of the particles indicated a highly crystalline structure of CdTe. The average size of 3.5 nm was consistent with the result obtained from the UV-vis absorption. The insert of Fig. 1A shows interplanar distances of 2.41 Å corresponding to the (2 2 0) planes of cubic zinc blende structure (JCPDS 15-0770). As observed in Fig. 1B, the three major peaks in the powder X-ray diffraction (XRD) spectrum can be indexed to the (1 1 1), (2 2 0) and (3 1 1) planes of the cubic zinc blende lattice of CdTe (JCPDS 15-0770). For comparison, the typical CdTe reflections (black) and cubic CdS reflections (red) are given as line-pattern below the recorded diffraction pattern. Noticeably, all of the three peaks are close to the peaks of CdS (JCPDS 89-0440). Such a consistent shift indicated the formation of CdS on the surface of CdTe, which has been observed in previous report of meracpto-capped QDs [17]. In addition, the average size of the CdTe QDs is about 3.7 nm calculated by Debye–Scherrer equation, which is consistent with the result of HRTEM. As shown in Fig. 1C, the EDS results further confirmed that the products were composed of Cd, Te and S. The typical optical properties of CdTe QDs synthesized at 70 °C in the presence of MPA ligands are shown in Fig. 1D and E. With the increase of reaction time, the solution color gradually changed from yellow to orange under visible light, and the fluorescence were observed from green to bright orange under an UV lamp with 365 nm excitation. The red-shift of PL peak and UV-vis absorbance spetra indicated the increasing particle size. The average size of CdTe QDs was estimated by UV-vis spectrometry based on the Peng’s empirical equation [34], which is consistent with the result of HRTEM and XRD. The average concentration of the obtained CdTe QDs in typical synthesis is 1.95  106 mol L1, which was calculated by previous reported formula [34]. The average yield of product is 30.83%, which was determined by using gravimetric analysis method. 3.2. Mechanism of the formation of CdTe QDs Scheme 1 shows the sonoelectrochemical formation reaction for CdTe QDs as follows: Firstly, the Cd2+ and TeO2 diffused to3 ward the surface of the sonoelectrode; Secondly, the TeO2 ions 3 were reduced to Te2 by a controlled electric pulse; The Te2 combined with Cd2+ to form CdTe nanoparticles immediately on the electrode nearby; Then the CdTe nanoparticles were removed from the electrode surface by ultrasonic pulse. With the amount of primary CdTe nanoparticles increasing in the electrolyte, the Ostwald ripening exhibited as the dominant mechanism. Namely, the CdTe nanoparticles were recrystallized and the surface defects were gradually reduced [35]. Finally, the well dispersed water-soluble CdTe QDs could be obtained by repeating above procedures. The electrochemical reduction process was further investigated by CV method. The result was shown in Fig. S1. According to the previous report [28], the peak at 1.1 V corresponds to the reduc2 tion of TeO2 , while the peak at 0.8 V, which corresponds 3 to Te to the reduction of TeO2 to Te0, was not observed. The results 3 demonstrate that the TeO2 was reduced to Te2 under the em3 ployed experimental conditions. 3.3. Influence of the reaction temperature The evolution for the synthesis of the CdTe QDs was performed at different temperatures. The fluorescence of CdTe QDs during the reaction at 60, 70 and 80 °C are shown in Fig. 2A–C, respectively.

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Fig. 1. (A) HRTEM images, (B) XRD patterns and (C) EDS analysis of CdTe QDs in a typical synthesis; (D) photographs of CdTe QDs under visible and UV light at 365 nm, sample a, b, c, d, e and f are obtained after 1000 s, 2000 s, 3000 s, 4000 s, 5000 s and 6000 s, respectively; (E) UV-vis absorption (black) and PL emission spectra (red, excited at 375 nm) of as-prepared CdTe QDs with different sizes. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Scheme 1. The formation of CdTe QDs via sonoelectrochemistry.

With the reaction time increasing, the PL peaks shifted to longer wavelengths. In the typical temperature at 70 °C, the PL peak position shifted from 509 nm to 560 nm with the reaction time increasing from 500 s to 9000 s. Meanwhile, the growth of colloidal crystals was also faster at elevated temperature. At 70 °C, it required 8000 s to reach the emission wavelength of 560 nm, whereas only 3000 s was required at 80 °C. While at 60 °C, the maximum emission wavelength was only 540 nm after the

reaction time of 9000 s. By contrast, the conventional synthesis of CdTe QDs need reflux more than 8 h to achieve a similar emission wavelength at 80 °C [36]. The influence of reaction temperature on QDs size distribution was evaluated by full-width at half-maximum (FWHM) [37]. The increased FWHM indicated the increase of the size distribution, due to the broader trap-state emission component of the fluorescence spectrum. As shown in Fig. S2, the minimum of the PL FWHM

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Fig. 2. Fluorescence characteristics of CdTe QDs synthesized at different temperatures: (A) 60 °C, (B) 70 °C, (C) 80 °C. The insets show the temporal evolution of PL peak position. (D) PL QYs of the CdTe QDs synthesized at different temperatures of 60 °C, 70 °C and 80 °C.

gradually increased while the temperature increased from 60 °C to 80 °C, because the enhanced Ostwald ripening pocess took place at the high temperature during the nanocrystal growth [38]. To avoid the negtive effect of broad size distribution on the color purity, the relative lower reaction temperature of 70 °C was employed. Furthermore, the influence of reaction temperature on the PL QY was observed in Fig. 2D. At 60 °C, the maximum PL QY reached 26% at the reaction time of 5000 s. When the temperature increased to 70 °C, the maximum PL QY increased to 36% at the reaction time of 3000 s. It is generally accepted that lower fluorescence intensity is probably due to the higher concentration of defects at the nanocrystals surface, which gives rise to a higher density of mid-gap states acting as electron and hole traps. While the temperature increased to 80 °C, the maximum PL QY decreased to 30% at the reaction time of 1000 s. It tends to obtain larger nanocrystals at higher temperature, which leads to a increased number of surface defects on the nanoparticles. It is these defects that are mainly responsible for the decreasing PL QY. The effect of reaction time on the PL QY was also investigated at different temperatures. The temporal evolution of the PL QY of CdTe QDs synthesized without any post-treatment are shown in Fig. 2D. The PL QY increased monotonically to a maximum then gradually decreased. With the increase of reaction time, the PL QY increased due to the improvement of the crystallization and annealing effect of defects. However, further reaction resulted in a decrease in PL QY due to broad distribution and relatively small surface/volume ratio of the obtained QDs. The results could be explained in terms of the machanism of Ostwald ripening and defocusing that the PL QY is very sensitive to the surface defects

state of the nanocrystals [35]. In general, 70 °C is appropriate to the synthesis of CdTe QDs, at which the growth rate is fast enough to result in the narrow FWHM and high PL QY. 3.4. Influence of sonoelectrochemical parameters The results of above mentioned experiments have shown the crystal size grow fast at high temperature due to the effects on the Ostwald ripening, maybe in combination with

Fig. 3. UV-vis spectra of the CdTe nanoparticles synthesized at different current pulse widths at low temperature of 5 °C.

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Fig. 4. (A) PL spectra and (B) PL QY of CdTe QDs synthesized at current pulse widths of 0.2 s, 0.5 s, 0.8 s and 1.0 s. The inset shows the wavelength of the PL peak plotted against current pulse width.

Fig. 5. PL spectra of CdTe QDs prepared at different current intensities (1 mA, 5 mA, 10 mA, 50 mA, 100 mA, 150 mA) after 3000 s. The inset shows the wavelength of the PL peak plotted against current intensity of the as-prepared CdTe QDs.

sonoelectrochemical process. To investigate the influence of current pulse on the size alone, 5 °C was selected in the preparation. The size of CdTe nanoparticles synthesized at different current

pulse widths of 0.2 s, 0.5 s, 0.8 s and 1.0 s was evaluated by UVvis spectra. As shown in Fig. 3, the pulse width have great effects on the size of the products. The widening and red-shift of the absorption peak indicated the broader size distribution and the increase of size. As the current pulse width of 0.2 s, the CdTe nanoparticles formed on the electrode surface is small, but the size distribution is broad. The reason is that as the pulse width is short, a part of the small CdTe nanoparticles remained on the electrode tightly, and continues to grow up during the next current pulse until they are removed by the sonication [23]. Larger CdTe nanoparticles were obtained with the increase of the current pulse width, while the size distribution was relatively uniform at 0.5 s. The PL spectra of CdTe QDs synthesized at typical temperature of 70 °C were shown in Fig. 4A. The red-shift of the PL peak with the increase of current pulse width indicated the growth of QDs. The larger current pulse width led to the higher emission wavelength, which was in good agreement with the result shown in Fig. 3. As the current pulse width of 0.2 s, the PL QY of QDs was the lowest. When the current pulse width increased to 0.5 s, the highest PL QY was obtained. However, as the current pulse width continued to increase, the CdTe nanoparticles formed on the sonoelectrode became larger but more surface defects occurred, resulting in lower PL YQ due to the production of the electronic traps.

Fig. 6. UV-vis absorption and PL spectra of CdTe QDs obtained (A) with or (B) without protection of N2.

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Acknowledgements We greatly appreciate the support of the National Natural Science Foundation of China (21121091) and the National Basic Research Program of China (2011CB933502). J.-J. Shi also thanks the support of Anhui Provincial Natural Science Foundation (1208085MB27) and the Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province (AE201107). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.ultsonch.2013.06.003. References Fig. 7. Fluorescence spectra of the CdTe QDs prepared at different pH: 10.0 (a), 10.5 (b), 11.0 (c), 11.5 (d) and 12.0 (e).

The influence of the current intensity was also taken into account. When current was not employed, no reaction took place merely under ultrasonic emission. The PL spectra of CdTe QDs prepared at different current intensity after 3000 s were shown in Fig. 5. The red-shift of PL spectra indicated that the larger current intensity led to the larger size of CdTe QDs. What is more, during the current pulse, the CdTe QDs could be obtained even without the protection of N2, because the fast reduction for Te4+ to Te2 occurred to supply the rapid nucleation of QDs on the sonoelectrode. It was further confirmed by comparison experiment with or without the protection of N2. As shown in Fig. 6, both the UV-vis absorption and PL spectra were almost similar in the peak positions and shapes, indicating that the quality of the obtaind QDs was still good under N2-saturated atmosphere.

3.5. Influence of the pH value The influence of the pH on the fluorescence properties was also studied. As seen in Fig. 7, the PL intensities reached to the maximum by adjusting the pH to 11.0, then decayed with a further increase of the pH. The fluorescence intensity of CdTe QDs could be facilely controlled by the pH value of the solution without changing the emission wavelength, which indicated that the pH value rather affected the surface than the size of nanocrystals.

4. Conclusion In summary, a convenient, fast, one-pot sonoelectrochemical method was developed for the synthesis of CdTe QDs in aqueous solution. Without precursor preparation and N2 protection, highly luminescent CdTe QDs was obtained. Ultrasonic played an important role in regenerating electrode and promoting the formation of CdTe QDs. The size, PL emission wavelength and QY can be effectively controlled by sonoelectrochemical parameters involving current intensity, pulse width, and reaction time. This method is also expected to prepare other QDs for use as promising building blocks in solar cell, photocatalysis and sensors.

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