Synthesis and characterization of Ag doped ZnS quantum dots for enhanced photocatalysis of Strychnine as a poison: Charge transfer behavior study by electrochemical impedance and time-resolved photoluminescence spectroscopy

Abstract

In this study, the photocatalytic degradation of Strychnine was investigated by ZnS quantum dots and doped with silver in UV systems. ZnS and Ag-ZnS quantum dots were synthesized by chemical method and characterized by powder X-ray diffraction, transmission electron microscopy, UV–vis spectra and photoluminescence. The charge transfer process on the semicon-ductor/electrolyte interface was investigated via electrochemical impedance spectroscopy (EIS) and time-resolved photoluminescence. The average diameters of ZnS and Ag doped ZnS QDs were 3.0–5.0 nm and 3.0–5.3 nm, respectively. The band gap of ZnS and Ag-ZnS QDs was computed as 3.47 and 3.1 eV, respectively. The surface area values of ZnS and Ag-ZnS QDs have been found as 78.25 and 89.54 m²/g, respectively. The influences of key operating parameters such as initial pH, catalyst dosage, UV radiation intensity, reaction time as well as the effect of initial Strychnine concentration on mineralization extents were studied. The results of the study showed that the maximum removal efficiency of Strychnine had been achieved by un-doped and Ag-doped ZnS QDs at radiation intensity of 100 W/m², at time of 60 min, pH of 3 and initial Strychnine concentration of 20 mg/ml. Also the observations clearly showed that the photocatalysis process with Ag doped ZnS QDs are more effective than un-doped ZnS QDs.

Introduction

Strychnine is an indole alkaloid obtained from the seeds of the Indian tree Strychnos nux-vomica. Strychnine-containing baits are currently labeled for below-ground use and are intended for the control of pocket gophers [1]. In the past, strychnine has been used as a pesticide to control rats, moles, gophers, and coyotes. Strychnine is highly toxic to humans and many other animals. Its oral LD₅₀ in humans is 16 mg/kg, and in child is lowest. Strychnine is a white, odorless, bitter crystalline powder that can be taken by mouth, inhaled (breathed in), or mixed in a solution and given intravenously (injected directly into a vein) [2]. Strychnine is a strong poison; only a small amount is needed to produce severe effects in people. Strychnine poisoning can cause extremely serious adverse health effects, including death [3]. The common methods to eliminate pollution from aqueous solutions include adsorption; biological procedures, ozonation, photolysis, Fenton and membrane process that each one having its own advantages and disadvantages. One of methods of advanced oxidation is photocatalysis which is a process based on absorption of light energy by nanoparticles. In this process, nanomaterial as catalyst, absorb UV spectrum's photons of higher energy and consequently the active chemical substances like hydroxyl radical are formed. The catalysts such as semiconductor quantum dots have been used for photocatalytic oxidation of the pollutants existing in the water [4], [5], [6], [7], [8], [9], [10], [11]. Light emitting semiconductor nanocrystals, otherwise known as quantum dots (QDs), have been widely investigated during the last two decades in view of their size-tunable optical properties, wide range of excitations, emission color purity, high quantum efficiency, and applications as a light emitting source in various optoelectrical devices, imaging, solar cells, environment, remediation, and therapeutics and in biological applications [12]. ZnS is the most studied II–VI compound with band gap energy of 3.6 eV and has sufficiently intrigued an enormous amount of researchers to devote themselves to researching in this hot field for its excellent properties [13]. The doping of ZnS semiconductor nanocrystals by the incorporation of atomic impurities is routinely used to modify their electrical and optical properties [14], [15], [16], [17], [18], [19]. Hence, in this present work, we have attempted to employ photocatalysis and electrochemical impedance spectroscopy in evaluating ZnS and Ag doped ZnS QDs for degrading Strychnine. The morphology and structural of the samples was characterized using different instruments. The samples were tested for its charge transfer resistance in an electrolyte under UV irradiation. An examination of the formation of OH radicals can be providen a quantitative data for charge carriers in the Strychnine degradation.