One-pot electrochemical synthesis of CdTe quantum dots in cavity cell

doi:10.1016/j.electacta.2016.01.016

Electrochimica Acta

Volume 190, 1 February 2016, Pages 689-694

https://doi.org/10.1016/j.electacta.2016.01.016 Get rights and content

Highlights

•
CdTe nanocrystals were prepared by a new electrochemical method.
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Mercaptopropionic acid, 2-thiosalicylic acid and cysteamine stabilizers.
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An electrochemical cavity cell was used to generate Te²⁻ ions.
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Te° was reduced in the graphite powder cathode cavity.
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Te²⁻ ions migrate to aqueous medium generating CdTe QDs in one-pot process.

Abstract

A novel one-pot electrochemical methodology was developed to produce CdTe quantum dots (QDs) stabilized by mercaptopropionic acid (MPA), 2-thiosalicylic acid (TSA) and cysteamine (CA). A graphite powder cavity cell was used to obtain Te²⁻ and Te₂²⁻ ions in the absence of solvent, which migrate to the intermediate compartment of the cell, containing the Cd²⁺/stabilizer complex in aqueous solution. The electrochemical cavity cell is of facile assembly and produces QDs of high quality (10 min) by controlling the electrolysis parameters for the different stabilizers. A rapid growth of the nanocrystals is observed by heating treatment (90 °C), presenting light emission from green (520 nm) to orange (576 nm). XRD characterization determined a zinc blend crystalline structure for the synthesized QDs. Nanoparticle medium sizes were determined by high-resolution TEM (60 min heating treatment): CdTe-MPA 3.1 nm, CdTe-TSA 3.9 nm and CdTe-CA 2.5 nm.

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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One-pot electrochemical synthesis of CdTe quantum dots in cavity cell

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Equipments and measurements

Eletrochemical cavity cell

Conclusion

Acknowledgements

Chem. Biol.

Nanomedicine

Sensor Actuat. B-Chem.

Thin Solid Films

Mat. Sci. Semicon. Proc.

Electrochim. Acta

Electrochim. Acta

Int. J. Electrochem. Sci.

Mater. Sci. Semicond. Process.

Organic Electrochemistry

Organic electrosynthesis: a promising green methodology in organic chemistry

Green Chem.

Quantum dots and Nanocomposite

Wiley Interdiscip. Rev.: Nanomed. Nanobiotechnol.

Electrochemical synthetic route for preparation of CdTe quantum-dots stabilized by positively or negatively charged ligands

Green Chem.

Electrochemical synthesis of TGA-capped CdTe and CdSe quantum dots

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The targeted delivery of multicomponent cargos to cancer cells by nanoporous particle-supported lipid bilayers

Nat. Mater.

Photo-induced charge transfer dynamics in efficient TiO2/CdS/CdSe sensitized solar cells

Energy Environ. Sci.