Surface Properties and Photocatalytic Activities of the Colloidal ZnS:Mn Nanocrystals Prepared at Various pH Conditions.

Jungho Heo,Cheong-Soo Hwang

doi:10.3390/nano5041955

Abstract

Water-dispersible ZnS:Mn nanocrystals (NC) were synthesized by capping the surface with mercaptoacetic acid (MAA) molecules at three different pH conditions. The obtained ZnS:Mn-MAA NC products were physically and optically characterized by corresponding spectroscopic methods. The UV-Visible absorption spectra and PL emission spectra showed broad peaks at 310 and 590 nm, respectively. The average particle sizes measured from the HR-TEM images were 5 nm, which were also supported by the Debye-Scherrer calculations using the X-ray diffraction (XRD) data. Moreover, the surface charges and the degrees of aggregation of the ZnS:Mn-MAA NCs were determined by electrophoretic and hydrodynamic light scattering methods, indicating formation of agglomerates in water with various sizes (50–440 nm) and different surface charge values accordingly the preparation conditions of the NCs (−7.59 to −24.98 mV). Finally, the relative photocatalytic activities of the ZnS:Mn-MAA NCs were evaluated by measuring the degradation rate of methylene blue (MB) molecule in a pseudo first-order reaction condition under the UV-visible light irradiation. As a result, the ZnS:Mn-MAA NC prepared at the pH 7 showed the best photo-degradation efficiency of the MB molecule with the first-order rate constant (kobs) of 2.0 × 10−3·min−1.

Highlights

Water-dispersible semiconductor nanocrystals were developed for various important applications such as luminescent labeling agents in biological systems [1,2,3], photocatalysts for the hydrogen gas production by water splitting [4,5,6], and environmental purification agents to remove pollutants from wastewater by photodegradation reactions [7,8,9]
Characterizations of the ZnS:Mn-mercaptoacetic acid (MAA) NC Products Prepared at Different pH Conditions
ZnS:Mn NCs can provide a novel platform to attract polar molecules such as water, they can be used for a variety of applications especially as a photocatalyst for the degradation of pollutants in wastewater or the hydrogen production by the water splitting process

Summary

Introduction

Water-dispersible semiconductor nanocrystals were developed for various important applications such as luminescent labeling agents in biological systems [1,2,3], photocatalysts for the hydrogen gas production by water splitting [4,5,6], and environmental purification agents to remove pollutants from wastewater by photodegradation reactions [7,8,9]. The complicated surface properties of the nanocrystals dispersed in water have not been investigated in detail. One should consider that there are much more complicated factors in the water-dispersed nanocrystals such as interactions between the surface capping ligands and secondary coordination by other metal ions provided during the preparation process of the nanocrystals (NCs), which majorly results in decrease of luminescence efficiency and photocatalytic activity of the NCs [16]. It is necessary to investigate specific surface properties such as coordination modes of the capping ligands, surface charges, and degree of aggregation in water to correctly understand the observed photocatalytic activities of the NC

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Results

Conclusion