A new approach to the synthesis of selenium based nanoparticles
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2008
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Abstract
We report the synthesis of high quality aqueous and organically soluble selenide nanoparticles by a novel greener, quick, facile, environmentally-benign and effective non-organometallic solution-based method. Briefly, the method involves the reduction of selenium powder to produce selenide ions which act as the selenide source, followed by the addition of MCI2 (M = Cd or Zn) or Zn(CH3COO)2. The nanoparticles were passivated with organic surfactants such as hexadecylamine (HDA) and tri-octylphosphine oxide (TOPO), for their solubility and stability in organic solvents, while passivation with biomolecules such as L-cysteine ethyl ester hydrochloride, methionine, ascorbic acid, starch, polyvinyl alcohol (PVA) and poly (vinylpyrroiidone) (PVP), rendered them water-soluble and aJso acted as an agent of stabilisation and facilitate conjugation with biomolecules. CdSe/ZnSe nano-composite and core-shell nanoparticles were also synthesised, using this new synthetic approach.
The high quality of the as-synthesised nanoparticles was confirmed using absorption and photoluminescence spectroscopy (PL), Fourier transform infrared spectroscopy (FT-ER), powder x-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), selected area dififractometry (SAD) and energy dispersive spectroscopy (EDS). All measurements were performed without any post-preparative size separation of the nanoparticles.
The thesis is divided into six chapters. Chapter one deals with the introduction and comprehensive review on previous works done on the synthesis of nanoparticies,
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highlighting their advantages and disadvantages. The aim and objective of the study is also stated in this chapter.
In chapter two, the systematic study of the effects of the capping group, growth time, temperature, reduction time and monomer concentration on the size, optical properties and morphology of the as-synthesized TOPO and HDA-capped CdSe nanoparticles was investigated. All the as-synthesised particles are blue-shifted in relation to the bulk band-gap of CdSe. The absorption and emission maxima are shifted to higher band-gap (lower wavelengths) as the reduction time increases from 2 to 6 hrs, indicating a decrease in particle size for the HDA-capped CdSe. The particle size ranges between, 2.3-3.2 (5-30 mins) 2.4-3.0 nm (5 -30 mins) and 2.5- 2.7 nm (15-30 mins) for the reduction time of 2,4 and 6 hrs respectively. At higher temperature (200 °C), particles with different shapes, i.e dot, rod and multi armed rods are produced. The presence of the elongated particles was attributed to the phenomenon of oriented attachment (self-assembly), due to dipole-dipole interactions between the highly charged surfaces of II-VI semiconductor nanocrystals.
The synthesis of TOPO and HDA-capped ZnSe nanoparticles were investigated in chapter three. Increasing the monomer concentration and temperature, led to a faster growth rate, increase in particle sizes and narrowing of the size distribution. XRD analysis show phase transitions from sphalerite cubic to wurtzite hexagonal phase, on increasing the monomer concentration. The mean average particle diameters are, 3.71 ± 0.40 run 3.26 ± 0.37 nm and 2.95 ± 0.32 nm for the reduction time of 2. 4 and 6 hrs respectively.
Chapter four describes the synthesis of water-soluble CdSe and ZnSe nanoparticles. The synthesis was carried-out at elevated and room temperatures, using biomolecules and polymers as passivating agents. A systematic study of the effects of parameters such as pH, reaction time, concentration, refluxing time, reactant ratio, passivating ligand and temperature on dispersibility, size, optical properties and morphology of the nanoparticles, was also investigated. At elevated temperature, the particle size decreases while the luminescence properties improve with increasing pH. Increasing the cadmium to cysteine ratio systematically from 1:10 to 1:60 at different pH, did not have any significant effect on the luminescence intensity of the nanoparticles. In contrast to the synthesis at higher temperature, particles with very sharp excitonic shoulders, absorption edges and high luminescence with narrower size distributions, are produced at room temperature. Particles with the first absorption peak at a wavelength as low as 415 nm, with a clearly resolved higher energy electronic transition, high luminescence without trap emission and particles size as small as 1.68 nm are obtained at pH 11.
The synthesis and characterisation of highly monodispersed HDA-capped CdSe-ZnSe nanocomposites and CdSe/ZnSe core/shell nanoparticles were reported in chapter five. The study of the core/shell growth at different reaction times and the effect of temperature on the optical and structural properties, was also investigated. The absorption and emission maxima of the CdSe/ZnSe core/shell nanoparticles are red-shifted in relation to the core CdSe, with a visible second absorption band at higher temperature (250 °C). The emission spectra confirmed that separate ZnSe nanoparticles are not formed in the solution. XRD analysis indicates that the diffraction is predominantly due to the CdSe core. The average particle diameter of
the CdSe/ZnSe are 3.99 ± 0.16 nm (230 °C) and 4.51 ± 0.27 run at 30 mins reaction time. The absorption and emission maxima of the CdSe-ZnSe nanocomposite are blue-shifted in relation to pure CdSe, with a small particle size (2.91 ± 0.20 nm) as compared to the core CdSe (3.00 ± 0.24 nm). The XRD analysis shows a phase transition from the hexagonal phase of the pure CdSe to the cubic phase.
Chapter six gives a short description of the experimental details. Without any further post preparation, the qualities of the particles formed by this new method, are comparable to that of the best CdSe and ZnSe nanocrystals reported in literature, synthesised by the high temperature conventional organometallic methods. This new synthetic route is safe, inexpensive, involve the use of non-toxic chemicals, reproducible and can be readily used for large scale synthesis.
We believe the knowledge gained from this work, will enable the direct green synthesis of functionalised organic and water-soluble selenide nanoparticles for large-scale production and commercial purposes, due to the economical and environmentally-benign nature of the synthetic method.
Description
A Thesis Presented by
in fulfilment of the requirement for the award of the degree of
Doctor of Philosophy
in the Department of Chemistry at the
University of Zululand, 2008.
Keywords
selenide ions, selenium powder