One-pot electrochemical synthesis of CdTe quantum dots in cavity cell
Graphical abstract
Introduction
Electrochemistry is an important tool for traditional organic or inorganic synthetic procedures, fairly applied in the industry. When compared with traditional chemical methods, the electrochemical method can provide advantages such as selectivity, when the reaction parameters are correctly adjusted; low operational cost and safety due to the substitution of chemical reagents by the electron; and easy automation, among other advantages [1]. These features also make the electrochemistry a green alternative, promoting cleaner and more suitable reactions, in agreement to the green chemistry principles [2].
The synthesis of semiconductor nanoparticles, also called quantum dots (QDs), has been largely explored. Their optical properties are caused by the quantum confinement phenomenon, to which the electrons are subjected [3]. Electrochemistry also has been used for the preparation of QDs as a green route for biological applications [4], [5].
In recent decades, a significant research has been focused to new nanoparticle synthetic methodologies, providing interesting luminescent properties and relevant applications: bio-imaging and biomedicine diagnostics [6], [7], [8], photovoltaic devices [9], lasers [10], light emitting diodes (LEDs) [11], and the detection of metals [12], [13], among other applications.
Currently, several QDs synthetic methods are known in aqueous or organic media [3]. Aqueous methodologies are searched due to biocompatibility and do not require additional synthesis steps to make the QDs soluble in water. Normally, thiol stabilizers as 3-mercaptopropionic acid (MPA) and cysteamine (CA) are used in aqueous synthesis, affording highly photoluminescent and chemically stable QDs. Lin et al. synthesized CdTe QDs capped by 2-thiosalicylic acid in aqueous solution, the nanocrystals showed good optical properties and potential solar cell application [14]. However, the aqueous medium methodology frequently uses reducing agents, such as NaBH4, which requires post-synthesis purification procedures for biological applications performing [15].
Engelhard et al. [16] have developed an interesting electrochemical synthetic route for H2Te preparation. H2S and H2Se preparation was also described [17]. The electrochemical preparation of these acids was subsequently applied in the synthesis of CdTe [18] and CdSe [19] QDs. Most recently, it was described an electrochemical methodology seeking a cleaner and efficient route for the synthesis of QDs in aqueous media. A classical two-compartment electrochemical cell, separated by a Nafion® membrane, was used for the chalcogen powder (Te° or Se°) reduction in basic medium (pH 13). This procedure was applied in the CdTe and CdSe preparations, furnishing nanoparticles of high luminescence and low dispersity [4], [5]. A disadvantage of the method is related to the addition step of the chalgogenide aqueous solution (Te2− or Se2−) to the cadmium/stabilizer solution, due to its air instability.
Therefore, in order to develop a clean, reproducible, versatile and simple method, we propose a new one-pot electrochemical methodology for the CdTe QDs synthesis based on a cavity cell process [20], using mercaptopropionic acid (MPA), 2-thiosalicylic acid (TSA) and cysteamine (CA) as stabilizers (Scheme 1). This methodology, rules out chemical reducing agents and post-synthesis treatment steps for the synthesized QDs.
Section snippets
Materials
Elemental tellurium (Te°) powder (99.8%, 200 mesh, Aldrich), graphite powder (particle size <20 μm, Aldrich), cadmium chloride (CdCl2, 99.999%, Aldrich), 3-mercaptopropionic acid (≥99%, Aldrich), cysteamine hydrochloride (≥98%, Aldrich), 2-thiosalicylic acid (97%, Aldrich) and NaOH (97%, Quimex) were used as purchased. The water was of Milli-Q grade.
Equipments and measurements
Controlled-current electrolyses were carried out using an Autolab PGSTAT 30 potentiostat/galvanostat and an electrochemical cavity cell (Fig. 1).
The
Eletrochemical cavity cell
Observing that the electrochemical reduction of chalcogen powder could be successfully accomplished [4], [5], a new electrochemical cell was tested in the telluride synthetic process. The cavity cell is based on the separation of the anodic and cathodic compartments, wherein one compartment is composed only by graphite powder, and the electrolyzed species can be generated in the absence of solvent, Fig. 1 [20].
For the tellurium electroreduction, 0.05 mmol Te° powder was mixed with 4.88 mmol of
Conclusion
The electrochemical cavity cell was used to produce CdTe-MPA, CdTe-TSA and CdTe-CA QDs in one-pot process and aqueous medium. By controlling the synthetic parameters, QDs were obtained with high stability and reproducibility. No purification step is necessary, making possible the direct biologic application of the synthesized QDs. Besides the great versatility, this methodology can be expanded to produce other nanocrystals in aqueous medium by modifying some reaction parameters.
Acknowledgements
The authors wish to thank granting authorities in Brazil: CNPq (302271/2014-7), CNPq-INCT-INAMI (57.3986/2008-8), FACEPE (1991-1.06/12), CAPES, LAQIS for fluorescence spectra, CETENE for XRD analysis, E.T.N. thanks FAPESP (2013/11298-0) and LNNano-CNPEM (Campinas, Brazil) for the use of the JEOL JEM-2100F TEM microscope. JMMD bolsista do CNPq—Brazil. Electronic Supplementary Material: Supplementary material is available in the online version of this article and is accessible free of charge.
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2020, Journal of LuminescenceCitation Excerpt :It can be used in laboratories and industries, and the main advantages are: control of experimental conditions, selectivity, low cost, operational safety, easy automation and elimination of toxic reagents [10–12]. The preparation of nanoparticles of semiconductors in colloidal solution has become a widespread method of synthesis, using aqueous or organic solvents as reaction media in the presence of a stabilizer agent [13–21]. Recently, Navarro et al. have developed an aqueous one-pot electrochemical procedure, using a paired electrolysis in mild conditions, making possible the simultaneous generation of cadmium (Cd2+) and chalcogenide (S2−, Se2−, Te2−) ions, during the preparation of cadmium chalcogenide QDs [21].
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