Research article Received: 11 December 2014,

Revised: 26 February 2015,

Accepted: 28 February 2015

Published online in Wiley Online Library

(wileyonlinelibrary.com) DOI 10.1002/bio.2909

Simple synthesis of luminescent CdSe quantum dots from ascorbic acid and selenium dioxide Yilin Wang,a* Meihua Yu,b Kun Yang,a Jianping Lua and Linqing Chena ABSTRACT: A simple, low-cost and convenient method was developed for the synthesis of highly luminescent CdSe quantum dots (QDs) in an aqueous medium. Compared with previous methods, this synthesis was carried out in one pot using ascorbic acid (C6H8O6) to replace NaBH4 or N2H4 · H2O as a reductant, and selenium dioxide to replace selenium or its other hazardous, expensive and unstable compounds as a precursor. The mechanism of CdSe QDs formation was elucidated. The influence of various experimental variables, including refluxing time, Cd/MSA and Cd/Se molar ratios, on the luminescent properties of the QDs were systematically investigated. X-Ray powder diffraction and transmission electron microscopy characterization indicated that the QDs had a pure cubic zinc-blended structure with a spherical shape. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: cadmium selenium; nanocrystalline materials; selenium dioxide; luminescence

Introduction Because of their unique size-dependent optical properties, quantum dots (QDs) are of great interest in both fundamental research and technical applications. The excellent photoluminescence (PL) and electroluminescence (EL) properties of QDs make them an attractive alternative to organic molecules in applications such as light-emitting diodes (LEDs) (1,2), solar cells (3,4), biological fluorescence labels (5,6), and so on. To date, two chemical routes, organic (7) and aqueous syntheses (8), have been suggested for the preparation of QDs, and each has its own advantages and disadvantages. Compared with organic synthesis, aqueous synthesis is more reproducible and cheaper, and the as-prepared samples are more water-soluble and biocompatible. However, the main disadvantage of this route is the poor crystallinity and broad size distribution of the products due to the mild synthetic conditions, particularly temperature. CdSe QDs are one of the most important group II–VI semiconductor nanomaterials. Recently, several improvements to the conventional aqueous synthetic route of thiol-capped CdSe QDs have been reported (9–11). In these aqueous routes, either NaBH4 (12) or N2H4 · H2O (13) is used as a reductant in the preparation of NaHSe from Se powder or other Se compounds. When NaBH4 is used as a reductant, it must react with Se powder or other Se compounds to first produce the NaHSe precursor (an unstable compound due to its spontaneous oxidation in the presence of oxygen). Thus, the synthetic procedure involves two steps, making a selenium precursor (NaHSe), and then synthesizing CdSe QDs. Moreover, maintaining an inert atmosphere is necessary during the synthesis. Even though volatile N2H4 · H2O as a reductant may be in option of preparing QDs, it is hypertoxic. Considering the complexity caused by the inert atmosphere and the risk caused by N2H4 · H2O, a stable and nontoxic reductant is needed for the synthesis of QDs. A water-soluble substance such as ascorbic acid (C6H8O6) that is inexpensive and nontoxic compared with NaBH4 or N2H4 · H2O might be an alternative reductant in the synthesis of QDs. However, application of this reductant has not been reported in the preparation of QDs.

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In this work, we report a simple one-pot method of preparing CdSe QDs capped with mercaptosuccinic acid (MSA) using ascorbic acid as the reductant and selenium dioxide (SeO2) as the Se source. The main advantages of this method are the process of making the selenium precursor using a stable and cheap reductant, no toxic gases are released, and it is suitable for large-scale CdSe QDs preparation.

Experimental Reagents and materials All chemicals were used as received. Double-deionized water was used throughout the experiment. MSA (98%, Aladdin Chemistry Co. Ltd, Shanghai, China), CdCl2 · 2.5H2O (99%, Tianjin Damao Chemical Reagent Factory, Tianjin, China), SeO2 (99.9%, Aladdin Chemistry Co. Ltd) and ascorbic acid (99%, Aladdin Chemistry Co. Ltd) were used for the preparation of CdSe. Rhodamine 6G with a photoluminescence quantum yield of 95% (Aladdin Chemistry Co. Ltd) was used to determine the quantum yields of QDs. Preparation of CdSe QDs For a typical synthesis of CdSe QDs, in a 250-mL three-neck flask, 0.4567 g of CdCl2 · 2.5H2O (2 mmol) and 0.4805 g MSA (3.2 mmol) were mixed in 100 mL of deionized water. A solution of 1.0 mol/L NaOH was used to adjust the pH of the mixture to 11.8. Under * Correspondence to: Y. Wang, Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China. E-mail: [email protected] a

Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China

b

Ministry–Province Jointly-Constructed Cultivation Base for the State Key Laboratory of Processing Non-Ferrous Metal and Featured Materials, Guangxi Zhuang Autonomous Region, Guangxi University, Nanning 530004, China

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Y. Wang et al. vigorous stirring, 0.3522 g of ascorbic acid (C6H8O6) and 0.0222 g of SeO2 (0.2 mmol) were added to the above solution, and the pH of the resultant solution was readjusted to 11.8. The flask was then attached to a condenser and refluxed at 100 °C under atmospheric conditions. Characterization

Results and discussion Mechanism of CdSe QDs formation The aqueous synthesis of QDs is based on the Ostwald ripening phenomenon, and the formation of QDs in a solution involves two stages: nucleation and growth (14). In this synthesis, the formation of CdSe QDs is illustrated in Figure 1: first, Cd2+–MSA complexes were formed under basic conditions. The ascorbic acid (C6H8O6) then reacted with sodium selenite (Na2SeO3), a product from selenium dioxide (SeO2), to form Se2 ions that combined with Cd2+–MSA complexes to generate CdSe nuclei. Because the CdSe nuclei grew under reflux, the CdSe QDs were formed. Optical properties The normalized UV/vis absorption and PL spectra of the CdSe QDs prepared at different reflux times are shown in Figure 2. All samples show well-resolved absorption maxima at the first electronic transition. With the increase in reflux time, the absorption edge shifts to lower energy, suggesting an increase in the CdSe nanoparticle size. The absorption onsets for the five samples appear at 524 nm (2.37 eV), 536 nm (2.31 eV), 560 nm (2.21 eV), 567 nm (2.19 eV) and 575 nm (2.16 eV), respectively, which is obviously blue shifted compared with bulk CdSe absorption at 716 nm (1.74 eV) (15). The particle size is estimated using a simple model, in which the geometry of a semiconductor is assumed as a sphere,

Figure 2. Temporal evolution of normalized UV/vis absorption and PL spectra of 2+ 2 CdSe QDs prepared at a Cd : Se : MSA molar ratio of 2: 0.2: 3.2.

  h2 1 1 via an equation ΔEg ¼ 8a (16), where ΔEg = (Egn – Egb) 2 m þm e h is the difference between the band gaps of nanoparticles (Egn) and the bulk (Egb) (CdSe Egb = 1.74 eV), h is Planck’s constant, α is the diameter of a particle, and me and mh are the effective masses of electrons and holes, respectively. The calculated sizes for the five samples are 2.44, 2.57, 2.83, 2.89 and 2.99 nm. The particle size increases from 2.44 to 2.99 nm over a period of 10 h, which indicates that the growth kinetics is slow. This is because the preparation was carried out at low temperature, highlighting an important aspect of QD growth. As observed in Figure 2, the PL bands of the CdSe QDs locate close to the absorption thresholds (so-called band-edge or ’excitonic’ PL), which are significantly narrow (full width at halfmaximum, FWHM, is ~ 35 nm). This is probably a result of the strong capping of Cd2+ ions with MSA molecules (one -SH and two -COOH groups), indicating an important aspect of the QDs with a narrow size distribution. The band-gap emission may be attributed to the fact that MSA is an effective passivating agent compared with other thiols (such as thioglycolic acid and mercaptopropionic acid). Thus, the dangling bonds or traps on 1.4 1.2

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Ultraviolet/visible (UV/vis) absorption and photoluminescence (PL) spectra of all samples diluted to 1: 25 were recorded with a UV-2102PC spectrophotometer and a RF-5301 fluorescence spectrophotometer, respectively. The as-prepared QDs were precipitated for 10 min by adding a three-fold volume of isopropyl alcohol, and a sediment was collected after centrifugation at 4000 rpm for 10 min. After drying, the powder was used for XRD analysis. The sample powder was placed onto a sample holder plate, and the XRD spectra was conducted using a Rigaku/Dmax-2500 X-ray diffractometer with CuKα radiation (λ = 0.15406 nm). The QDs solution 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 using an FEI-TF30 transmission electron microscopy equipped with an energy dispersive X-ray (EDX) spectroscopy.

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Figure 3. Normalized absorption and PL spectra of CdSe QDs solution (sample 5): solid lines and long dash lines are the measurements conducted initially and four months later.

Copyright © 2015 John Wiley & Sons, Ltd.

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Figure 4. PL spectra of CdSe QDs prepared at various Cd: Se molar ratios: (a) 2: 0.2, (b) 2: 0.3, (c) 2: 0.4 and (d) 2: 0.5.

the particle surface can be effectively removed. The PL maximum of the smallest (2.44 nm) CdSe QDs locates at 514 nm ( green emission); whereas that of the largest (2.99 nm) CdSe QDs is at 561 nm (yellow–green emission). It is well known that the PL quantum yield (QY) of semiconductor QDs synthesized by an aqueous route is quite poor compared those synthesized via an organic route. Therefore, it is important to estimate the parameter of QDs. Here, we measured the QY of CdSe QDs using the procedure given in Liu et al. (17). The PL spectra of the sample solution and the standard solution (Rhodamine 6G) were recorded at an excitation wavelength of 365 nm. QY was then calculated according to the  As following equation Yu ¼ Ys Fu Fs  Au , where Yu and Ys (Ys = 95%) are the PLQYs for the sample solution and the standard solution, Au and As are the absorbances of the sample solution and the standard solution at an excitation wavelength of 365 nm, and Fu and Fs are the areas under the PL curves of the sample solution and the

standard solution. For PLQY measurements, the concentrations of the solutions were adjusted so that the absorbances were < 0.1, to minimize the error arising from inner filter effects (18). The PLQYs of as-prepared CdSe QDs, dependent on the reflux time, are calculated to be 1.9, 5.3, 10.8, 16.3 and 22.7%, respectively. The CdSe QDs exhibit a maximum PLQY of 22.7% at an emission wavelength of 561 nm, which is higher than those of CdSe QDs synthesized using other thiols as a stabilizer (19–21). Furthermore, these QDs were very stable in the dark under ambient conditions. The CdSe QDs absorption and PL spectra showed almost no change after four months’ storage (Figure 3), and no obvious precipitation was observed. Using a fixed Cd: MSA molar ratio of 2: 3.2 and a pH of 11.8, the influence of various Cd/Se molar ratios on the PL of CdSe QDs was investigated, and the experimental results are shown in Figure 4. As the reflux time increases, continuous growth of CdSe results

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in the PL peak position shifting to longer wavelengths. Meanwhile, the PL intensity of CdSe QDs is enhanced with increasing reflux time. This may be attributed to the fact that the CdSe QDs, formed at the initial growth stage, contain a lot of surface traps that act as a nonradiative centre for excitons. As shown in Figure 4, the CdSe QDs show distinguished PL intensity, depending on the stoichiometric ratio of Cd to Se precursors in the preparation. Compared with the CdSe QDs obtained with the other three Cd: Se molar ratios, CdSe QDs prepared at a Cd: Se molar ratio of 2: 0.2 maintain a high PL intensity. This contributes to the two stages, nucleation and growth, in the formation of QDs in solution. At the nucleation stage, the number of CdSe nuclei is dependent on the concentration of the Se precursor. When the amount of Cd and MSA is kept constant and the Se precursor concentration is low, fewer nuclei are formed, and more Cd monomers remain for the growth of nuclei. Thus, these leftover Cd monomers probably cause the effective passivation of particle surface defects, resulting in the strong PL intensity. The passivation of surface sites by ligand stabilizer molecules results in the formation of a favourable structure for removing the dangling bonds from the surface, which can improve the PL property of QDs. Therefore, the thiol plays an important role in the synthesis of thiol-capped QDs. In our experiments, when Cd: MSA molar ratios were changed from 2: 2.4 and 2: 2.8 to 2: 3.2 with a fixed Cd: Se molar ratio (2: 0.2) and pH (11.8), the influence of various Cd: MSA molar ratios on the PL property of CdSe QDs was investigated. In Figure 5, it is observed that the decrease in the Cd: MSA ratio leads to an increase in the PL intensity of CdSe QDs. The highest PL intensity of CdSe QDs occurs at a Cd Ú MSA ratio of 2: 3.2, showing that more MSA probably causes the effective passivation of particle surface defects, generating the strong luminescence. Crystal structure and morphology characterization The crystallinity and structure of CdSe QDs were characterized by X-ray powder diffraction (XRD), as shown in Figure 6. Three distinct peaks of (1 1 1), (2 2 0) and (3 1 1) at 2θ = 26°, 43° and 50° are observed. They are the reflection characteristics of the zincblended phase of CdSe nanocrystallites. The absence of reflections at 2θ = 35° and 46°, due to (1 0 2) and (1 0 3), is an indication that the wurtzite CdSe structure does not exist and the QDs possess a pure zinc-blended structure (22). A cubic zinc-blended phase structure is preferable when thiol-stabilized cadmium chalcogenides QDs are prepared via an aqueous route in a low temperature regime (100–260 °C). However, a hexagonal wurtzite phase forms

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at high temperatures (>300 °C) using TOP/TOPO-capping molecules in an organic route (23). The main reason for the formation of the zinc-blended structure is that our reaction occurred at a comparatively low temperature (100 °C). The broad peaks indicate that the nanocrystal size is very small. The diffraction peaks become narrower as the reflux time increases, suggesting a larger size of the corresponding nanocrystals. Transmission electron microscopy (TEM) measurements were carried out to study the size and morphology of the nanoparticles. Figure 7(A) shows TEM and high-resolution transmission electron microscopy (HRTEM) images of CdSe QDs refluxed for 10 h as an example of the samples. It can be seen that the CdSe appears as spherical particles with excellent monodispersity. The estimated average particle size in this case is ~ 2.9 nm, very close to that calculated from the UV/vis absorption spectra. The inset in Figure 7 shows a high-resolution image of a single CdSe QD whose lattice fringes are clearly visible, demonstrating its high crystallinity. Elemental analysis was performed using an energy-dispersive X-ray (EDX) technique, and the results are shown in Figure 7(b). The characteristic peaks of C, O, S, Cd and Se are observed, showing the components of the CdSe QDs. Among these elements, C, O and S are from the stabilizer (MSA). The results show that CdSe QDs, capped with MSA, are formed.

Conclusions A new route for obtaining a precursor of selenium ions, via reduction of selenium dioxide using ascorbic acid, was proposed, and highly luminescent CdSe QDs capped with mercaptosuccinic acid were prepared in an aqueous solution. It was found that CdSe QDs prepared using our method were homogeneous, spherical and of small size. The CdSe QDs exhibited a strong PLQY (up to 22.7%) and a narrow spectral bandwidth (FWHM, 35 nm) at room temperature. The method has the advantage that both the reducing agent and the selenium precursor are stable and cheap, and no toxic gases are released during the preparation process. This method is suitable for large-scale synthesis of CdSe QDs in terms of its low-cost and green chemistry. Acknowledgements This work was supported by the Scientific Research Fund of the Education Department of Guangxi Zhuang Autonomous Region (2013YB014) and the Opening Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology (2013 K007)

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Simple synthesis of luminescent CdSe quantum dots

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Simple synthesis of luminescent CdSe quantum dots from ascorbic acid and selenium dioxide.

A simple, low-cost and convenient method was developed for the synthesis of highly luminescent CdSe quantum dots (QDs) in an aqueous medium. Compared ...
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