Study of the aqueous synthesis, optical and electrochemical characterization of alloyed ZnxCd1-xTe nanocrystals

doi:10.1016/j.matchemphys.2016.04.076

Materials Chemistry and Physics

Volume 178, 1 August 2016, Pages 104-111

https://doi.org/10.1016/j.matchemphys.2016.04.076 Get rights and content

Highlights

•
ZnCdTe quantum dots were obtained by aqueous synthesis.
•
Nature of capping ligand was the most relevant factor.
•
Optical and electrochemical band gaps were well correlated.

Abstract

The effects of experimental factors such as initial reaction pH, capping ligand, and heating method on the optical and electrochemical properties of aqueous alloyed Zn_xCd_1-xTe nanocrystals were evaluated. Here the type of capping ligand (glutathione GSH and 3-mercaptopropionic acid MPA) was found to be the most significant factor in controlling the range of photoluminescence emission. Also a pronounced pH effect on the emission wavelength has been verified in the presence of GSH, in contrast to MPA for which only a minor pH effect was observed. The heating method (microwave or hydrothermal) was found to be irrelevant for the emission wavelength at the conditions studied. The electrochemical characterization in aqueous medium (cyclic voltammetry and differential pulse voltammetry) evidenced a good correlation between electrochemical and optical band gap values and allowed estimation of band edge positions.

Graphical abstract

Introduction

Semiconductor nanocrystals have long been established as important photoluminescent materials in view of their large potentialities for diverse applications such as diagnostics, chemical/biochemical analysis, optoelectronics, etc. For some of those applications, the stability and preservation of the properties under aqueous medium are crucial requisites fulfilled by materials obtained directly in water [1], [2]. In this context CdTe is probably the most suitable composition for preparation in aqueous medium, since the resulting nanocrystals generally exhibit relatively high quantum yields and satisfactory size control [3]. Also, photoluminescence (PL) emissions can be achieved over almost the entire visible range, as the nanocrystals sizes can be controllably changed. On the other hand previous reports have shown that alloying two or more semiconductors may change the band gap energy [4], [5]. The dependence of band gap energy on the alloy composition opens the possibility of band gap engineering, as the introduction of a wider band gap semiconductor increases the alloy band gap in a composition dependent fashion. For instance the alloying of CdSe and ZnSe may lead to Zn_xCd_1-xSe systems emitting blue light which is hardly obtained with binary compositions [6].

Formation of semiconductor alloys affects the energy band structure and minimum energy gap, lattice parameters, mechanical constants, optical and electronic conduction properties, and many other aspects of the resulting material [7], [8]. In this context, the formation of semiconductor alloys generally aims to change specific optical or electronic properties. The composition effect can be explained as result of a strong dependence of electronic energies on the effective exciton mass, which should be modulated by the alloy composition [4]. The band edges and effective mass parameters of a ternary alloy A_1-xB_xC can be interpolated between the values of pure semiconductors AB and AC. In this context Leite and co-workers calculated effective mass parameters from first-principles for Zn_xCd_1-xTe alloys varying the composition x, showing that the effective masses have an almost linear dependence on x [9], [10], [11]. Thus Zn_xCd_1-xTe alloys can be rationally designed to exhibit emission in the visible range, with a gradual shift of emission wavelength with composition.

Concerning the synthetic approach, most of the works on ternary alloyed quantum dots reported the classical rapid injection of organometallic precursors into high boiling point solvents, which yields hydrophobic nanocrystals unsuitable for biological applications. Currently the alloying of CdTe and ZnTe is attracting increasing attention and contributions on the aqueous synthesis of the resulting ternary semiconductors have been reported [12], [13], [14], [15], [16], [17]. Teng and co-workers reported the first aqueous synthesis of TGA-capped Zn_xCd_1-xTe and observed that the increase in nominal Zn:Cd proportion causes a blue shift of emission peak, while increasing reaction time and temperature caused a red shift –related to particle growth with fixed composition [12]. Glutathione-capped ternary Zn_xCd_1-xTe nanocrystals were reported using reflux up to 20 h, obtaining quantum yields up to 75% [13] and microwave heating [14], observing a reduced cytotoxicity in comparison to GSH-capped CdTe. Other capping agents were also evaluated such as N-acetyl-l-cysteine under hydrothermal conditions [15]. An evaluation of the effect of reaction time as well as Zn/Cd ratio on the optical properties of aqueous MPA-capped CdZnTe revealed that the maximum quantum yield was obtained after 24 h reflux, for the lowest Zn/Cd ratio studied [16]. We recently evaluated the bioconjugation of glutathione-capped ZnCdTe nanocrystals to Concanavalin-a by spectroscopic methods [17]. It is clear that the optimization of synthesis conditions is still demanding research efforts, thus here we compared the effect of two different capping agents (MPA and GSH) in the preparation of Zn_xCd_1-xTe under hydrothermal and microwave heating. We also evaluated the effects of different synthesis pH's. Samples were studied by TEM (transmission electron microscopy), absorption and photoluminescence spectroscopy being as well as by cyclic voltammetry measurements.

Section snippets

Materials

Here we used Te powder (Sigma, ∼200 mesh, 99.8%), CdCl₂·H₂O (Sigma, 98%), ZnCl₂ (Sigma, 99%), reduced glutathione (GSH, Sigma, ≥99%) and 3-mercaptopropionic acid (MPA, Sigma, ≥99%). All experiments were carried out with ultrapure MILLI Q water.

Preparation of Zn_xCd_1-xTe quantum dots

First we studied the effects of heating method (hydrothermal and microwave), thiol capping agent (glutathione (GSH) and mercaptopropionic acid (MPA)) and reaction pH (9.0, 10.0, 11.0, 11.5). In this set of experiments we used a 1.0:1.0:2.0 Zn:Cd:thiol

Results and discussion

The effects of initial synthesis pH, capping agent, and heating method were evaluated by photoluminescence, UV visible spectra, and morphological data. Fig. 1 shows PL and UV/visible absorption spectra, both exhibiting bands related to electron transitions involving the semiconductor band gap. Absorption and PL spectra show bands related respectively to excitation of electrons from the valence to conduction band and to the radiative recombination of the electron-hole pairs formed. For quantum

Conclusions

The most important effect observed here was the type of capping ligand, since depending on the ligand used additional relevant effects such as pH could be observed or not. The formation of glutathione capped nanocrystals was found to be strongly dependent on pH while only a bare pH effect was found for MPA-capped ones. All effects have been discussed in terms of their influence on the nanocrystal composition. Electrochemical characterization of nanocrystals evidenced allowed determining the

Acknowledgments

The authors are grateful for the support provided by CNPq, Capes (Proc. 7915-13-6), Fapitec, the Brazilian Synchrotron Light Laboratory (LNLS: TEM-MSC-15049), and CMNano-UFS (Project 82). L. P. Costa acknowledges a grant from CNPq (229901/2013-1/PDE/CNPq).

References (38)

Y. Wang et al.
J. Cryst. Growth
(2007)
J. Cheng et al.
J. Alloys Compd.
(2014)
J. Cheng et al.
J. Alloys Compd.
(2014)
M. Xue et al.
Talanta
(2011)
J. Li et al.
J. Electroanal. Chem.
(2009)
K.G. Deepa et al.
Mater. Sci. Eng. B
(2012)
N. Gaponik et al.
Phys. Chem. Chem. Phys.
(2010)
N. Gaponik et al.
J. Phys. Chem. B
(2002)
V. Lesnyak et al.
Chem. Soc. Rev.
(2013)
R.E. Bailey et al.
J. Am. Chem. Soc.
(2003)

P. Samokhvalov et al.

Chem. Eur. J.

(2013)

M.D. Regulacio et al.

Acc. Chem. Res.

(2009)

A. Rockett

The Materials Science of Semiconductors

(2008)

S. Glutsch

Excitons in Low-dimensional Semiconductors: Theory, Numerical Methods and Applications

(2004)

R. Paiva et al.

Braz. J. Phys.

(2002)

P.P. Ingole et al.

J. Phys. Chem. C

(2013)

N.P. Gurusinghe et al.

J. Phys. Chem. C

(2008)

W. Li et al.

J. Mater. Chem.

(2010)

J. Du et al.

J. Mater. Chem.

(2010)

Cited by (8)

Electrosynthesis and characterization of alloyed CdS<inf>x</inf>Se<inf>1−x</inf> ternary quantum dots
2023, Journal of Alloys and Compounds
CdS, CdSe and CdS_xSe_1−x (x = 0.15, 0.35, 0.5, 0.65, 0.85) ternary quantum dots (QDs) were electrosynthesized in a cavity cell and aqueous medium, using 3-mercaptopropionic acid (MPA) as stabilizer. The electrochemical procedure was carried out by a paired electrolysis, where elemental sulfur and selenium were reduced in a graphite powder macroelectrode while a sacrificial cadmium rod was oxidized, giving the simultaneous generation of S^2-, Se^2- and Cd²⁺ ions for efficient production of the ternary QDs, in the S^2-/Se^2- ratio of choice. The CdS_xSe_1−x QDs were characterized by XRD, HRTEM, UV-Vis absorption and emission spectroscopies, and voltammetric analyses. XRD analysis showed lattice parameters of the cubic CdS_xSe_1−x nanoalloys with a linear drop tendency when the sulfide molar ratio increased in the crystalline structure. HRTEM analysis was used to determine nanoparticle sizes: 3.94 ± 0.70 nm (CdS), 4.23 ± 1.00 nm (CdS_0.5Se_0.5), and 4.12 ± 0.78 nm (CdSe), with interplanar distances of 0.348 nm, 0.341 nm, and 0.339 nm, respectively. The band gap energy (E_{g opt}) of the CdS_xSe_1−x QDs were determined by UV-Vis absorption spectroscopy: 2.42, 2.47, 2.53, 2.66, 2.63, 2.73 and 2.84 eV, respectively, showing a linear increasing according to the S^2-/Se^2- ratio. The same behavior was observed for the E_{g Elect} determined by cyclic and linear voltammetry analysis, presenting a nanoalloy nature of the ternary QDs electrosynthesized.
Quantum dot amplified impedimetric aptasensor for interferon-gamma
2023, Electrochimica Acta
An aptasensor for the detection of interferon-gamma, a Tuberculosis (TB) biomarker was developed with metal dichalcogenide-tin telluride selenide quantum dots. The stability and solubility of the QD was improved by functionalizing the surface with short-chained L-cysteine. The optical properties of the QD were studied using UV–Vis and photoluminescence (PL) analysis which gave an absorption and emission band at 430 and 500 nm. The absorption band was associated to a bandgap value of 3.5 eV. Furthermore, Fourier transform infrared spectroscopy (FTIR) studies revealed the presence of the capping agent, L-cysteine on the surface of the QD and displayed its hydrophilic properties due to the carboxyl and hydroxyl groups present on the QD. High-resolution transmission electron microscopy (HRTEM) studies showed that the QD were crystalline, well-dispersed and spherical in shape with an average diameter of 3.8 nm. The response of the aptasensor to detecting interferon-gamma was studied using electrochemical impedance spectroscopy (EIS). The electrochemical aptasensor proved to be sensitive to interferon-gamma, obtaining a detection limit of 0.151 pg/mL. The aptasensor also displayed good selectivity to interferon-gamma in the presence of interfering agents. When evaluated in real sample, the aptasensor obtained a good recovery range of 98 – 105% indicating its ability for use in infectious disease monitoring.
Shape-controlled synthesis of aqueous-based metallic nanocrystals and their catalytic applications
2022, Industrial Applications of Nanocrystals
Aqueous-based nanocrystals have become some of the most comprehensively studied nanomaterials because of their unique physicochemical and electronic properties. Specifically, crystalline one-dimensional metal nanocrystals fabricated in aqueous solution have received significant research interest owing to their resulting thermodynamic stability and nearly spherical morphology. This informs their use in various fields such as biomedicine and catalysis. Generally referred to as green nanocrystals, aqueous nanocrystals are environmentally friendly and benign because of the lesser use of toxic solvents during synthesis, which is in line with the principles of green chemistry. In this chapter, we begin with an introduction to the theoretical framework for the formation of nanocrystals in solution, followed by the unique features and synthesis of single and binary nanocrystals. We also highlight advances in the morphological characterization of synthesized nanocrystals. At the end, we examine applications of aqueous-based nanocrystals as green catalysts in the environment and biomedicine.
Concepts behind the Redox Photocatalysis with Quantum Dots
2024, ChemCatChem
Quantum Dot Amplified Impedimetric Aptasensor for Interferon-Gamma
2023, SSRN
Highly luminescent ZnCdTeS nanocrystals with wide spectral tunability for efficient color-conversion white-light-emitting-diodes
2021, Journal of Physics D: Applied Physics

View all citing articles on Scopus

View full text

Study of the aqueous synthesis, optical and electrochemical characterization of alloyed ZnxCd1-xTe nanocrystals

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Preparation of ZnxCd1-xTe quantum dots

Results and discussion

Conclusions

Acknowledgments

J. Cryst. Growth

J. Alloys Compd.

J. Alloys Compd.

Talanta

J. Electroanal. Chem.

Mater. Sci. Eng. B

Phys. Chem. Chem. Phys.

J. Phys. Chem. B

Chem. Soc. Rev.

J. Am. Chem. Soc.

Chem. Eur. J.

Acc. Chem. Res.

The Materials Science of Semiconductors

Excitons in Low-dimensional Semiconductors: Theory, Numerical Methods and Applications

Braz. J. Phys.

J. Phys. Chem. C

J. Phys. Chem. C

J. Mater. Chem.

J. Mater. Chem.

Study of the aqueous synthesis, optical and electrochemical characterization of alloyed Zn_xCd_1-xTe nanocrystals

Preparation of Zn_xCd_1-xTe quantum dots