Mn²⁺ ion influenced optical and photocatalytic behaviour of Mn-ZnS quantum dots prepared by a microwave assisted technique

Fluorescent zinc blende structured pure and Mn²⁺ ion doped ZnS (Mn-ZnS) quantum dots were prepared via a rapid microwave assisted technique without any surfactants and their photocatalytic activity was tested for the photodegradation of methyl orange dye under UV light irradiation. They were characterized by X-ray diffraction, FE-SEM, UV-Vis and photoluminescence spectroscopies, XRF and BET surface area analysis. The Mn-ZnS quantum dots possessed a cubic blende crystal structure without any impurity phases. The absorption maximum by UV-Vis spectroscopy showed a blue shift with increasing Mn²⁺ content. Photoluminescence of Mn-ZnS was obtained as a result of combined broad blue emission from sulfur vacancies existing in the ZnS host and orange emission from the 4T₁-6A₁ transition of Mn²⁺. The relative intensity of blue to orange emissions was found to be quenched with increasing Mn²⁺ ion concentration. Methyl orange dye was photodegraded by Mn-ZnS quantum dots effectively up to a doping level of 4.28% Mn²⁺ and further increase in Mn²⁺ decreased the photocatalytic activity. Detailed mechanisms of photoluminescence and photocatalytic activity were proposed and discussed. The results suggested that the Mn-ZnS material is a potential candidate for photocatalytic applications.