Superior photocatalytic performance of mechanosynthesized Bi₂O₃–Bi₂WO₆ nanocomposite in wastewater treatment

Highlights

• Bi₂O₃–Bi₂WO₆ photocatalysts with different morphologies are synthesized by mechanical milling

• Broccoli type morphologies appear after 5h of milling Bi₂O₃-WO₃ mixture

• ~87% degradation of RhB is achieved within 240 min in presence of solar light

• Photodegradation rate constant is higher for the sample milled for 5h than the photolysis and others milling duration.

Abstract

The heterostructured Bi₂O₃–Bi₂WO₆ nanocomposites with superior photocatalytic performance have been synthesized by the mechanical alloying method. Both nanosheet and nanoflower-like morphologies are obtained after different durations of milling. The semiconductor-semiconductor (S–S) heterojunction formation with the NHE (normal hydrogen electrode) scale is revealed from the band positions of Bi₂O₃ and Bi₂WO₆. Detailed microstructures of all nanocomposites have been characterized by the Rietveld refinement of XRD patterns and analyzing TEM images.The morphological hierarchy and changes of all samples have been disclosed in FESEM images. The bandgap value and the visible-light-driven (VLD) photocatalytic activity of all samples have been measured from the UV–vis absorbance spectra. The Rhodamine B (RhB) dye degradation performance of the photocatalyst, its reusability, and active radical species involved in the photocatalytic reaction in all nanocomposites are obtained by analyzing UV–vis absorbance spectra.The EDX elemental mapping reveals the stability of the photocatalyst. The photocatalytic performance of all milled samples has been investigated through the degradation of RhB dye in an aqueous solution under solar light in the presence. A significant amount (~87%) of RhB degradation within 240 min has been achieved with the 5h milled Bi₂O₃-WO₃ nanocomposite_.

Introduction

Nowadays, much attention has been focused on the removal of residual organic pollutants in the aquatic environment. Different types of organic compounds effluent with water from industries as secondary products. These are widely used as organic dyes for dyeing daily necessities such as clothes, paper, cosmetics, plastics, leatherware and release the excess amount into the environment [[1], [2], [3]].

The environment causes a severe problem due to widespread and abundant organic compounds that are stable and non-bio-degradable. As per the previous report, more than 50,000 tons of dyes are lost during dyeing and released as waste, and about 10,000 tons of dyes are entering the environment and water supply [4].Thus, it is very urgent to remove these stable and toxic pollutants from effluent and wastewater through sustainable and green technologies to save our earth. The photocatalytic oxidation and reduction based on semiconductors and solar energy, green techniques are being extensively used to remove the organic pollutants from the effluents and wastewater [[5], [6], [7], [8]]. The conventional photocatalysts such as TiO₂, ZnO, SnO₂ have some limitations. They are active only in the ultraviolet region and show poor photocatalytic performances under visible light due to wide bandgap energy and high recombination rate of photogenerated electrons and holes [9]. Transition metal oxide MoO₃ has been of great interest as a photocatalyst due to its exciting features. Defect-rich α-MoO₃ (dr-MoO₃) nanoflakes with abundant exposed reactive edge sites is an excellent photocatalytic for degradation of Rhodamine B (RhB) under visible light irradiation. Different nanocomposites such as heterojunction MnO₂/MoO₃ and Fe₂O₃/MoO₃/AgBr catalysts are active in the visible light irradiation [[10], [11], [12]]. A simple steam etching process enhanced the electrocatalytic performance of MoS₂ for the hydrogen evolution reaction (HER). Single crystal of WSe₂ semiconductor was proposed as a new strategy to achieve higher electrocatalytic activity for hydrogen evolution reaction (HER), characterized by the Tafel slope [13,14].The keyword to enhance the photocatalytic performance by overcoming these drawbacks is to combine different semiconductors with their appropriate band potential positions instead of using a single semiconductor. Composite p-n junction semiconductor fabrication is a useful tool for enhancing the photocatalytic activity of wastewater treatment by eliminating toxic organic pollutants. A heterogeneous semiconductor can boost the photogenerated electron-hole separation efficiently compared to a single semiconductor and improve the photocatalytic activity [[15], [16], [17], [18], [19], [20]].The compound Bi₂WO₆ is a direct bandgap n-type semiconductor with a bandgap energy of 2.8eV and is being used as a visible-light-driven photocatalyst. It also has potential applications in solar energy conversion [21], electrode materials [22], and catalysis [23]. However, high recombination between photogenerated electron-hole pairs has suppressed its photocatalytic performance [[24], [25], [26]]. Several efforts have been made to enhance its photocatalytic activities by combining it with other semiconductors such as Bi₂S₃–Bi₂WO₆ [15], MoS₂–Bi₂WO₆ [27], Bi₂O₃–Bi₂WO₆ [[28], [29], [30], [31]], (Bi₂WO₆/MnO₂) and (Bi₂WO₆/Bi₂MoO₆) [32,33]. The present work aims to fabricate a composite p-n heterogeneous photocatalyst through matching their band potential by coupling Bi₂WO₆ with narrow bandgap Bi₂O₃. Recently, the p-n heterogeneous Bi₂O₃–Bi₂WO₆ photocatalyst with highly ordered Bi₂WO₆ nanosheets has been grown on one-dimensional (1D) Bi₂O₃ nanorods, microspheres, hollow microspheres, and flowers with improved photocatalytic activity [28,[34], [35], [36], [37], [38]].We have synthesized the p-n heterostructured Bi₂O₃–Bi₂WO₆ photocatalyst with hierarchical nanosheets by a simple one-pot mechanical alloying method. It shows an enhanced photocatalytic activity by degrading a model pollutant Rhodamine B (RhB) under direct sunlight irradiation.