Facile Synthesis of Computationally Designed MgAl2O4/CeO2/Cu2O and MgAl2O4/CeO2/Ag2O Smart Heterojunction Photocatalysts for Aqueous Organic Pollutants Degradation

Abstract

Wastewater generated from different industries, such as the textile and dyeing industries, is the cardinal source of pollution for surface waters [1]. In addition, Bangladesh has placed the 2^nd exporter countries over the world as a result, toxic dyes related chemicals from the textile and dyeing industries released into river water makes the aquatic abnormalities of the ecosystem, effect on photosynthesis, aesthetically unpleasant to human health, plus severe health hazards to humans, animals, plants, as well as microorganisms [2] due to their toxic effects. It is time demand to protect the environment by developing and proliferating innovation of technologies or methods although there are some insufficient and aberrant methods, for example biological methods [3], coagulation and flocculation [4], photocatalytic processes [5], sonocatalyst processes [6,7] and the use of synthetic or natural adsorbents [8–10]. Among these, the photocatalytic degradation process has substantiated the most sustainable and effective technique due to its advantages for instance low cost, acceptance, green, chemical stability, high efficiency, and so on of which semiconductor nanoparticles (NPs) have taken the most attention of researcher for last one decade [11-13]. Secondly, the n-n or p-n junctions nano composite photocatalysts convey unique behaviour by restraining photo-induce e−-h+ recombination which remarkably increases the photostability and photocatalytic behaviour [14] regarding that inspires to design double p-n heterojunction photocatalysts.

As the n-type semiconductors Cerium oxide (CeO₂) has a wide band gap, non-toxicity, and higher stability than titania (TiO₂)-extensively and most commercially used photocatalyst in organic dye removal from aqueous environment, it introduced new antenna for dye degradation with sustainable methods. In addition, Magnesium aluminate (MgAl₂O₄) is a single-component photocatalyst [15] as well as p-type semiconductors Cuprous oxide (Cu₂O) and Silver oxide (Ag₂O) having a direct band gap of 2.0 eV and 1.3 eV respectively, and can absorb effectively visible light [16, 17]. Therefore, using P-type Ag₂O, or Cu₂O with Cerium oxide (CeO₂) and Magnesium aluminate (MgAl2O4) has been selected for the design of smart heterojunction photocatalysts. A proposed mechanism of photocatalytic action of double p-n junction type MgAl₂O₄/CeO₂/Cu₂O and MgAl₂O₄/CeO₂/Ag₂O heterojunction photocatalysts is given in Fig.1.

 

Fig. 1. Proposed Photocatalytic mechanism of double p-n junction type MgAl₂O₄/CeO₂/Cu₂O or Ag₂O heterojunction photocatalyst

To kick-off, MgAl₂O₄/CeO₂/Cu₂O and MgAl₂O₄/CeO₂/Ag₂O crystals have been selected and investigated their structural geometry, electronic structure, optical absorption, and photocatalytic activity with environmental benign fact using the Density functional theory (DFT) from Material Studio 8.0 software package in term of first principle method. From their structural geometry, the highly stable conformation has been selected for crystals (MgAl₂O₄/CeO₂/Cu₂O and MgAl₂O₄/CeO₂/Ag₂O). Secondly, these stable conformations were tested by Generalized Gradient Approximation (GGA) with Perdew Burke and Ernzerhof (PBE) and Generalized Gradient Approximation (GGA) with Revised Perdew Burke and Ernzerhof (RPBE) functionals to obtain the electronic band structure, band gap and optical absorption. From the band gap, these MgAl₂O₄/CeO₂/Cu₂O and MgAl₂O₄/CeO₂/Ag₂O smart heterojunctions convey the maximum absorption of visible light to produce the free electron for the oxidation of organic pollutant. Next, the absorption location and absorption energy were calculated. The computational investigation suggests the best fitting crystals (MgAl₂O₄/CeO₂/Cu₂O and MgAl₂O₄/CeO₂/Ag₂O) for the required photocatalytic nature have been executed in the wet lab for synthesis following the one-step polyacrylamide gel method. The obtained samples have characterized by Scanning electron microscopy (SEM), X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), Atomic force microscopy, Zeta potential analysis, and Ultraviolet–visible (UV) spectroscopy. The photocatalytic efficiency assessment of MgAl₂O₄/CeO₂/Cu₂O and MgAl₂O₄/CeO₂/Ag₂O heterojunction photocatalysts for the degradation of Methylene blue dye have been performed by a spectrophotometer using basis of the literature [15].

Wastewater treatment in developing countries, like Bangladesh, is complicated for assorted adverted situations. To triumph over the lacking, MgAl₂O₄/CeO₂/Cu₂O and MgAl₂O₄/CeO₂/Ag₂O smart heterojunction photocatalysts convey the maximum absorption the visible light, outstanding dye degradation result, and narrow band gap materials with higher chemical and thermal stability which are as well justified by computational tools from DFT functional