The effect of manganese doping on structural, optical, and photocatalytic activity of zinc oxide nanoparticles

Abstract

In this work, polyethylene glycol-6000 (PEG-6000) capped ZnO nanoparticles (NPs) were synthesized by doping with varying levels of Mn (0, 1, 2, 3, and 4 wt%; 0% implies no doping). The crystalline sizes of the hexagonal wurtzite-structured nanoparticles, when measured with 4% Mn doping and without doping (0%), were 30 and 28 nm, respectively. The Mn doping led to a shift of the ZnO optical band gap from 3.36 to 3.51 eV. The Mn²⁺ ions from the doping agent caused tail states in the absorbance spectrum of ZnO NPs, allowing them to be used as effectual UV photocatalysts for the degradation of organic contaminants (e.g., methyl orange (MO), methylene blue (MB), and congo red (CR)). This effect was optimized when doped with 4% Mn. If the effect of Mn doping is compared between 0 and 4% results, the degradation efficiency of the three contaminants was approximated as 87/93.5 (MO), 85/88 (MB), and 86/93 (CR)%, respectively. Accordingly, Mn doping on ZnO NPs was found to be distinctive enough to enhance their photo-degradation efficiency.

Introduction

Most countries are facing the problems of environmental pollution due to huge amounts of toxic organic pollutants discharged into the environment from various man-made sources. Among them, cationic and anionic organic dyes play a very important role owing to their huge demand from various fields including the paper mill, textile, plastic, leather, food, printing, and pharmaceuticals industries [1,2]. For instance, methylene blue (MB), methyl orange (MO), and congo red (CR) are the most commonly used dyes in industries such as textiles, printing, and rubber [3]. Dyes discharged into the hydrosphere pose considerable threats to the environment due to their recalcitrant nature. As the increase in turbidity leads to the reduction of sunlight penetration and the oxygen dissolving capacity of water, such changes in environmental conditions can affect aquatic life to a large degree [4,5].

In an effort to find a solution for this problem, the use of photocatalytic oxidation approaches has recently become of great interest to the material research field. There has been an increasing demand for the development of nano-sized semiconductors due to their unique chemical, optical, electrical, and non-toxic properties [6,7]. Among the commonly used metal oxides, ZnO has attracted researchers due to its ecofriendly and nontoxic nature [8]. ZnO has emerged as a competent photocatalyst because it generates H₂O₂ more proficiently compared to other metal oxide nanostructures [9]. ZnO is considered an improved photocatalyst compared to commercialized TiO₂ due to its significant light absorption efficacy along with many active sites and high surface reactivity [10,11] Moreover, there are momentous merits to use ZnO NPS as photocatalysts such as strong absorption capability of visible light. This is due to the exceptional optical property of ZnO NPs, i.e., surface plasmon resonance (SPR). Thus, this property signifies the photocatalytic potential of nanoparticles under direct sunlight radiations which can lead to higher degradation efficiency of dyes [[12], [13], [14], [15], [16], [17]].

In general, surface area and surface defects in metal oxide-based photocatalysts play a vital role during degradation reactions. However, pristine ZnO nanoparticles are found to have certain limitations, such as fast recombination of photo-generated electron-hole pairs, which leads to a reduction in its photocatalytic efficiency [18,19](c). Thus, as a means to upgrade photocatalytic performance, the need for modification approaches such as transition metal-doping on ZnO nanoparticles has attracted a good deal of attention. Doping metal oxide with metal and/or transition metals increases the surface defects and optical absorption of light, which can ultimately lead to an increase in the efficiency of the photocatalyst [[20], [21], [22]].

Doping with 3d metals such as Mn, Ni, Fe, Co, and Cr was reported to increase the surface area while reducing the particle size of ZnO nanoparticles [23]. Among the transition elements used for doping, Mn may result in large injected spins and carrier, making the particle suitable for applications requiring a diluted magnetic semiconductor (DMSs). Doping ZnO with Mn is preferred because relative to other transition elements, Mn's d-electrons at the ?? 2?? level can easily overlap with ZnO's valence band. The optical properties of undoped ZnO nanoparticles, especially band gap tuning, can be significantly improved by the addition of an optimal amount of Mn [24,25].

To date, several synthesis routes for Mn-ZnO NPs, such as sol-gel, hydrothermal, spray pyrolysis, and co-precipitate methods, have been explored by researchers [[26], [27], [28], [29]]. Proficient Mn doping not only enhanced the optical properties of ZnO, but also lowered the particle dimensions [25]. Yet, the photocatalytic performance of Mn-ZnO is poorly characterized as is knowledge regarding the ideal amount of dopant Mn. Thus, the focus of the present work is to synthesize Mn-ZnO nanoparticles with a tunable band gap for superior photocatalytic performance.

To retain the structural characteristics of synthesized NPs, the use of capping agents is helpful. Capping agents not only hinder the aggregation of NPs, but also inhibit overgrowth [30]. For instance, polyethylene glycol-6000 (PEG-6000) acts as both a reducing and capping agent for the synthesis of ZnO [31]. In this study, Mn-doped ZnO nanoparticles were synthesized using the co-precipitation method with PEG-6000 as the capping agent. ZnO nanoparticles were doped using 0, 1, 2, 3, and 4% concentrations of Mn. Note that 0% is equivalent to no doping. Of all the samples, the 4% Mn-ZnO nano-particles exhibited the highest band gap energy. The higher band gap energy semiconductor was considerable due to the samples' superior optical and electronic properties. The samples were further evaluated for their photocatalytic performance in terms of particle size and structural, morphological, optical, and luminescence properties. This study offers valuable insights into the photo-degradation efficiency of MO, MB, and CR dyes, which can be increased by doping ZnO with Mn using a simple and cost-effective co-precipitation process.