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Congo Red Dye Degradation by Graphene Nanoplatelets/Doped Bismuth Ferrite Nanoparticle Hybrid Catalysts Under Dark and Light Conditions

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Congo Red Dye Degradation by Graphene Nanoplatelets/Doped Bismuth Ferrite Nanoparticle Hybrid Catalysts Under Dark and Light Conditions catalysts Article Congo Red Dye Degradation by Graphene Nanoplatelets/Doped Bismuth Ferrite Nanoparticle Hybrid Catalysts under Dark and Light Conditions 1, 2, 3, 1, Sabeen Fatima y, S. Irfan Ali y , Muhammad Z. Iqbal * and Syed Rizwan * 1 Physics Characterization and Simulations Lab (PCSL), School of Natural Sciences (SNS), National University of Sciences and Technology (NUST), Islamabad 44000, Pakistan; [email protected] 2 Shenzhen Key Laboratory of Advanced Thin Films and Applications, College of Physics and Energy, and Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China; [email protected] 3 Department of Chemical and Petroleum Engineering, United Arab Emirates University (UAEU), Al-Ain 15551, United Arab Emirates * Correspondence: [email protected] (M.Z.I.); [email protected] or [email protected] (S.R.); Tel.: +97137135398 (M.Z.I.); +9290855599 (S.R.) Sabeen Fatima and S. Irfan Ali are equal contributors. y Received: 22 February 2020; Accepted: 23 March 2020; Published: 30 March 2020 Abstract: The continuously growing need for clean water has increased research looking for new and efficient ways to treat wastewater. Due to its magnetic properties, Bismuth ferrite, a photo-catalyst, has introduced a novel field of photo-catalysis where the photo-catalytic material could easily be separated from the aqueous solution after wastewater treatment. Herein, a new type of photo-catalysts, composed of Gadolinium (Gd) and Tin (Sn), co-doped Bismuth Ferrite deposited over graphene nanoplatelet surface have been synthesized using a two-step method. In first step, Gd (fixed concentration 10%) and Sn (5%, 15%, 20% and 25%) were doped inside bismuth ferrite (BFO) host using sol-gel method (namely the BGFSO nanoparticles, abbreviated for Gd and Sn doped BFO). In the second step, BGFSO nanoparticles were introduced onto GNPs using co-precipitation method (namely the BGFSO/GNP nanohybrids). The x-ray photoelectron spectroscopy confirmed the chemical bonding between co-doped BFO and GNP sheets via oxy and hydroxyl groups. The photocatalytic activities of the nanohybrids under both, visible light and dark conditions have been increased, and the maximum degradation activity (74%) of organic dye Congo-red (CR) is obtained for 25% Sn-doped BGFSO/GNP nanohybrid. The photocatalytic activity may be attributed to enhanced adsorption capability, electron storage properties of graphene and the presence of oxygen-rich species inside nanohybrids. Based on the current overgrowing population and need for clean water, these materials present versatile potential as catalysts for wastewater treatment. Keywords: Sn and Gd co-doped BFO; graphene platelets; nanohybrids; photocatalytic activity; visible light; dark conditions 1. Introduction Photocatalysis deals with the occurrence of oxidation and reduction reactions on the surface of a solid material (photo-catalyst) due to photogenerated charge carriers (holes and electrons) [1]. For the last two decades water contamination has increased, due to over industrialization, requiring materials to treat waste water more efficiently [2]. Large organic matter (waste), which is difficult to adsorb, can be broken into smaller nonhazardous molecules using photocatalysis. Semiconductor oxides are Catalysts 2020, 10, 367; doi:10.3390/catal10040367 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 367 2 of 14 emerging as promising and efficient photo-catalysts under UV-light irradiation [1,3–5]. Among earlier photocatalysts, titanium oxide (TiO2) has been widely researched photo-catalytic material with a short coming of a wide band-gap which renders it to utilize the complete solar spectrum effectively [6]. Perovskite structures have gained great attention, due to their fascinating physical characteristics, and initiated many useful applications in different technological areas [7–9]. Perovskites are materials having chemical formula ABO3 [10] where, A is usually a bigger cation than B cation and O is oxygen (anionic specie). The key features of perovskite materials include simple and easy fabrication, huge solar adsorption, less non-radiative charge carrier recombination, high mobility of carriers for cell architectures and capitalization of dye sensitized photovoltaic cells [9]. Among perovskites, bismuth ferrite BiFeO3 (also denoted as BFO) has been enormously attractive due to its multiferroic properties. BFO belong to the class of rhombohedral perovskites with space group named R3c and possesses ferroelectric and ferromagnetic properties simultaneously [11–15]. BFO is considered to be among the highly efficient visible light-driven photocatalysts [16–18]. Upon visible light irradiation, electrons move towards conduction bands, creating holes in the valance band, leading to degradation of organic pollutants by oxidation and reduction reactions [15]. The band-gap in perovskites can be tuned by various methods for catalytic applications. Specifically, the band-gap in BFO can be tuned by doping with rare-earth or transition metals [19–23], fabricating its hybrid or composite structures with other materials such as graphene [24–27]. Co-substitution of bismuth and iron inside bismuth ferrite has been reported many times with enhanced electric, magnetic and photocatalytic properties [28–30]. Based on published reports, rare-earth metal doping, inside BFO, improved its physical and chemical properties drastically [16–19,23,31]. The doping of Sn on B site of BFO has shown a good tuned structure for catalytic and sensing applications [10]. The substitution of Gd on the A site of BFO has shown enhanced ferromagnetism and visible light catalytic effects with improved surface chemistry [21,22]. A few reports have also been published on the doping of Gadolinium (Gd) and Tin (Sn), showing enhanced photocatalytic properties of the prepared co-doped nanoparticles and hybrid systems [17,23,31]. The substitution of Gd and Sn inside BFO transforms the lattice structure from rhombohedral to distorted rhombohedral, and then to orthorhombic due to unit cell expansion, as a result of the ionic radii difference among Bi3+ (1.03 Å), Gd3+ (0.938 Å), Sn4+ (1.09 Å) and Fe3+ (0.78 Å) [17,23,31]. The addition of Sn and Gd inside BFO also tuned the band-gap of the system, which in turns, assists in fast electron-hole pair generation for higher photocatalytic activity. Graphene is a two-dimensional (2D) allotropic form of carbon with remarkable properties, such as excellent electrical conductivity, splendid absorptivity, large surface area and thermal and chemical stability [32–36]. Graphene has shown good potential for environmental applications [19,21,32,37]. Graphene, in combination with BFO, acts as an e-trapping medium that enables effective separation of electron-hole charge carriers and lowers the recombination rate accompanied by an increase in photocatalytic activity [38]. As the pristine graphene is very costly and difficult to synthesize, recently a cost-effective multilayer graphene product [39], named graphene nanoplatelets (GNP), is emerging as a good candidate for many applications, such as dye sensitized solar cells [40], supercapacitors [41], etc. GNP hybrids with MgO has be used, and has shown good photocatalytic activity in the degradation of organic dye methyl orange [42]. On the other hand, graphene hybrids with BFO and co-doped BFO has shown a good catalytic behavior with high surface area, more active sites for carriers and low band-gap [14,16,31]. All these properties play a key role in the removal of organic contaminants. Herein, Gadolinium (Gd) and Tin (Sn) co-doped bismuth ferrite (BFO) nanoparticles (BGFSO) were synthesized using sol-gel method. Graphene nanoplatelets (GNPs) were combined with the BGFSO nano-particles to form hybrids and used for photocatalytic application. BGFSO-GNP hybrid have shown good catalytic degradation under visible light, as well as dark, which is proven to be an efficient catalyst, magnetic and can perform catalytic activity without the presence of light (UV or solar light) [43,44]. Catalysts 2020, 10, 367 3 of 14 2. Characterization Details X-ray diffraction (XRD) was performed using Rigaku-2500 machine (Tokyo, Japan) having Cu-K radiation and the scanning rate was 1◦/s within 2θ range from 20◦ to 60◦. A JEOL-7001 Field emission scanning electron microscope (FESEM) (Tokyo, Japan) was used for obtaining the micro images of hybrid structures (sputter coated with platinum). The accelerated voltages in FESEM were kept from 1.5 to 3 keV. The Thermo Scientific Kα x-ray photoelectron spectrometer (Escalab-250) (Massachusetts, U.S) was used for analyzing chemical composition with binding energies. A Hitachi UV-3310 spectrophotometer (Tokyo, Japan) was used for measuring the diffused reflectance spectra (DRS) of BGFSO-GNP nanohybrids. Further for photocatalysis initially Congo red (CR) dye was dissolved in deionized water at a concentration of 100 mg per liter. The photocatalyst (100 mg) was dispersed in dye solution (100 mL) using constant stirring for 2 h under dark conditions. The stirring was performed in an ice bath to avoiding the thermal degradation of organic dye. A Xenon lamp having power of 300W with a cut-off filter of 420 nm wavelength was used as visible light source. A small amount of solution (~3 mL) was removed from the reaction flask periodically with an interval of 30 min, followed by centrifugation at 7000 RPM. The supernatant, collected after centrifugation
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