Understanding the role of surfactants on the preparation of ZnS nanocrystals

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Abstract

We have synthesized surface modified ZnS nanoparticles of size 2–3 nm using non-ionic surfactant-stabilized reverse emulsions. The non-ionic surfactants in the Span series, i.e. sorbitan monolaurate (Span 20) and sorbitan monooleate (Span 80) of hydrophilic–lipophilic balance (HLB) values of 8.6 and 4.3, respectively, have been used for the stabilization of emulsions. The role of these surfactants in controlling the size and properties of the ZnS nanoparticles has been discussed. The triethylamine (TEA) has been proved to be the effective surface modifying (capping) agent for the preparation of free-standing ZnS nanoparticles. The Span 20 with the higher HLB value of 8.6 has been found to be highly suitable in synthesizing TEA-capped ZnS nanoparticles of smaller size and higher photophysical characteristics compared to that of the Span 80 of lower HLB value of 4.3. A mechanism for the formation of TEA-capped ZnS nanoparticles from the surfactant-stabilized reverse emulsions has been proposed.

Graphical abstract

Photoluminescence spectra of ZnS nanoparticles using (a) Span 80 and (b) Span 20.

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Introduction

Semiconductor nanocrystals have attracted a lot of interest in recent years owing to their special electronic and optical properties due to quantum confinement effect [1], [2], [3], [4], [5]. Various wet chemical methods have been developed for the synthesis of these nanoparticles [6], [7], [8], [9], [10]. Synthesis of these nanoparticles with narrow size distribution is an essential requirement for practical purposes. Further, the optical properties are strongly dependent on the technique adopted for the chemical synthesis of these nanoparticles. Among other methods, reverse emulsion method is a well-accepted method for the synthesis of these nanoparticles due to several advantages. In our previous study, we have already seen the effect of non-ionic surfactants in tailoring the size of the alumina microspheres. In the present study, we will examine the role of these surfactants in synthesizing ZnS nanoparticles and their effects on photophysical properties which is not yet well studied. Keeping the above points in view, in the present investigation we report a reverse emulsion technique for the synthesis of triethylamine (TEA)-capped ZnS nanoparticles. In the present investigation, the two immiscible liquids, i.e. for the w/o in oil type emulsions (reverse emulsion), i.e. dispersion of the water phase (aqueous Zn2+ in this study) in the oil phase (cyclohexane) are used. It is known that thermodynamically, the increase in surface area (ΔA) of the dispersed phase associated with a free energy (ΔG) can be represented as [12]ΔG=γΔA, where γ is the interfacial tension. The equation indicates that a low interfacial tension favours droplet disruption. In fact, the high interfacial tension between the dispersed water phase and continuous oil phase is reduced by the addition of a system compatible amphiphilic surface active agent or surfactant (emulsifier in the present case), i.e. the molecule with a polar head and a non-polar tail [12]. The surfactant molecules orient themselves according to the polarities of the involved chemical constituents. Thus, due to the high polarity of water, the polar heads of the surfactant molecules at the water–oil interface are oriented towards the water droplets and get adsorbed on their surfaces and prevent the coalescence of the droplets by steric hindrance occurring between two droplets (steric stabilization). Therefore, the emulsifier added to the system: (i) lowers the interfacial tension between two immiscible liquids followed by droplet disruption and (ii) prevents re-coalescence of droplets by adsorbing on their surfaces, thus producing a stable emulsion. In the present investigation, Span 80 and Span 20 stabilized reverse emulsions comprising aqueous Zn2+ solutions and cyclohexane as the water and oil phases, respectively, were used for the synthesis of ZnS nanoparticles. The surfactants in the Span series, i.e. Span 80, and Span 20 in the present case, are basically the fatty acid esters of anhydro sorbitols which are good oil soluble emulsifying agents [12]. A high (hydrophilic–lyophobic balance) HLB value of the surfactant indicates strongly hydrophilic character while a low value is an indication of a strong hydrophobic nature. Considering these points, non-ionic surfactants, i.e. Span 80 and Span 20 with HLB values of 4.3 and 8.6, respectively, were selected for the present study. Figs. 1a and 1b depict the molecular structures of the amphiphilic Span 80 and Span 20, respectively, [11] where the hydrophilic sorbitan group acts as a ‘polar head’ and the hydrophobic fatty acid group acts as the ‘non-polar tail.’ The effect of the type of the surfactants on the photophysical properties of the nanocrystals is discussed. A mechanism for the separation free-standing ZnS nanocrystals from the stabilized emulsion is suggested.

Section snippets

Synthesis of TEA-capped ZnS nanoparticles

In the present investigation, water-in-oil (w/o) type emulsions were used for the synthesis of ZnS nanoparticles. For such synthesis, zinc acetate, dihydrate (A.R., Glaxo, India) and thioacetamide [CH3(CS)NH2)] (G.R., Loba Chemie, India) were used as the starting materials. An aqueous solution of zinc acetate with Zn2+ concentration of 0.05 M and thioacetamide of concentration of 2.5 M was prepared by dissolving the respective salts in deionized water. The two solutions were mixed under

Formation of ZnS nanoparticles

The metal ions in the emulsified aqueous microdroplets were precipitated as sulphides by H2S, in situ generated at 80±1°C via thermal decomposition of the thioacetamide [8]. The particle size in the microreactor was tailored by controlling the reaction temperature, ageing time and the concentration of Zn2+ ions in the aqueous solutions. Based on the experimental results, the ageing time of 10 min at the reaction temperature of 80±1°C and the concentration of Zn2+ ions of 0.05 M were found to be

Summary

We have reported a soft chemical method for in situ synthesis of monodispersed, TEA-capped ZnS nanoparticles using two different surfactant-stabilized w/o type reverse emulsions. Two types of non-ionic surfactants in the Span series, i.e. Span 80 and Span 20 with the HLB values of 4.3 and 8.6, respectively, have been used for the stabilization of emulsions in the present study. A mechanism for the generation of TEA-capped ZnS nanoparticles from stabilized emulsions has been suggested. The role

Acknowledgements

Authors thank Dr. H.S. Maiti, Director of CGCRI for his constant encouragement and active co-operation to carry out the work. This work is partially financial supported by the Department of Science & Technology (DST), New Delhi (Project No. III.5(1)/2000-ET).

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