Improved Crystallinity of Zinc Sulfide Nanoparticles in Aqueous Environment
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Improved Crystallinity of Zinc Sulfide Nanoparticles in Aqueous Environment Navendu Goswami and P. Sen School of Physical Sciences, Jawaharlal Nehru University, New Delhi-10067, India. ABSTRACT Zinc sulfide nanoparticles, prepared employing a non-equilibrium route, are investigated for surface related effects. Water has been shown to induce a structural transformation in nanoparticles prepared this way, which is not related to their particle size. Employing Fourier transform infrared spectroscopy and x-ray powder diffraction, we show here the importance of S-H interaction in the buildup to the final ZnS structure of these nanoparticles. These particles hold promise as water sensors. INTRODUCTION Of late there have been reports on the reduction of surface free energy of ZnS nanoparticles without resorting to changes in particle size [1, 2]. The attendant structural reordering in different solvent environments (water, methanol) suggested a possible use of ZnS nanoparticles as environmental sensors. Strong water interaction with the surface of ZnS was proposed to be the driving force behind the increased crystalline behaviour at low water coverages, consistent with the surface chemistry of hydrated ZnS [1-3]. The thermodynamic behaviour of small particles differ from those of bulk materials due to the surface free energy term γA – the product of the surface (or interfacial) free energy and the surface (or interfacial) area. The surface contribution to the total energy becomes increasingly important as the particle size decreases [4-6] and, for very fine particles, may help in determining the final structure. Phase stability crossovers have been experimentally observed when the surface area is very large, due to differences in the surface free energy of polymorphs [7-10]. Alternately, surface free energy modifications should significantly perturb the total energy of very small particles and could lead to structural transformations without change in particle size. Recently Zhang et al.[1] have described such a nanoparticle system that undergoes structural changes in response to changes in the surface environment rather than particle size. The binding of water to these asformed particles at room temperature was shown to lead to a structural modification, reducing distortions of the surface and interior leading to a more crystalline nanostructured ZnS [1]. Employing molecular dynamics (MD) simulation to interpret their extended x-ray absorption fine structure (EXAFS) and wide angle x-ray scattering (WAXS) data, these authors predict strong interaction between water and the ZnS nanoparticle surface which provides a large stabilization effect. The simulation further goes on to show that the polar water molecules orient to permit hydrogen and oxygen bonding to the terminating S2- and Zn2+ ions. In this paper we provide evidence for remarkable water induced structure stabilization in pure free-standing ZnS nanoparticles employing an absolutely novel approach to prepare the particles. Pure ZnS particles in the size rang
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