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

: The continuously growing need for clean water has increased research looking for new and e ﬃ cient 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. as catalytic activities and photocatalytic activities of nanohybrids. The catalytic activity of BFO / GNP, BGFSO-5 / GNP, BGFSO-15 / GNP, BGFSO-20 / GNP and BGFSO-25 / GNP is 32%, 48%, 44%, 56% and 51% which is due to dye adsorption on the hybrid surface under dark conditions. The photocatalytic activity of BFO / GNP, BGFSO-5 / GNP, BGFSO-15 / GNP, BGFSO20 / GNP and BGFSO-25 / GNP is 30%, 12%, 15%, 3% and 23%, respectively. The overall degradation e ﬃ ciency of pure BFO is 44% in 120 min while the degradation e ﬃ ciency for BFO / GNP is 62%, BGFSO-5 / GNP is 60%, BGFSO-15 / GNP is 59%, BGFSO-20 / GNP is 59% and BGFSO-25 / GNP is 74%, respectively. An already published work [23] showed a good CR dye removal with BGFSO-5 nanoparticles under visible light, but there was no dye removal under dark conditions. This demonstrates that catalysts become active in both light and dark, and are more favorable towards catalytic applications than already published work, due to incorporation of GNPs inside nanoparticles [23,31]. In addition, GNPs provide dye to adsorb over BGFSO / GNP nanohybrids surface actively, which further helped in the fast organic dye removal even in the absence of visible light [14]. dispersion of spherical BGFSO particles over GNP sheets. Absorption analysis showed that nanohybrids


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

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 process, was tested using UV-vis spectrophotometer for the CR concentration. Figure 1 shows the XRD results of GNPs, BGFSO nnaoparticles and BGFSO-GNP nanohybrids. In Figure 1a, the peak at 26.4 o represents the (002) plane in pristine graphite, corresponding to the JCPDS card no. 01-0646. However, the intensity and the sharpness of the peak is low and peak broadening indicates increased inter planar spacing and sharpness of the peak has been reduced and inter planar spacing (d) has been increased up to 3.37Å. The particle size was calculated using Scherrer's formula (D = Kλ/β cos θ) of x-ray diffraction analysis [17,23] and for GNP the particle size is 35 nm which shows a stack of 24 graphene sheets approximately. The pristine graphene is a single layer, but the GNPs synthesized here are indeed agglomerated clusters, composed of multilayered graphene with in a single platelet. The presence of peak (002) inside GNPs is because of bulk fragmentation of the graphite during thermal exfoliation [45,46]. The same plane (002) will also be observed inside the synthesized nanohybrids, which will correspond to GNPs loading inside the ferrite nanoparticles. Figure 1b shows the XRD traces of Gd and Sn doped BFO, with a varying concentration of Sn from 5%-25%, and a fixed Gd concentration of 10%. All the peaks present in BFO are present in the doped samples with two impurity peaks of Bi 2 O 3 and Bi 2 Fe 4 O 9 corresponding to the JCPDS card no. 20-0169. The addition of Gd distort the crystallinity of BFO structure by reducing the peak intensities. The peak intensity of impurity phase Bi 2 O 3 is increased with the increasing concentration of Sn. The addition of Sn causes the unit cell expansion due to having bigger ionic radii (1.09 Å), as compared to Fe (0.78 Å), which further changes the rhombohedral structure of BFO into orthorhombic structure and a complete structural transformation is observed by overlapping (104) and (110) peaks [17,23]. A negative peak shift is observed inside the BGFSO nanoparticles (containing Sn concentration from 15% to 25%) attributed to an increase in lattice parameter which occurs due to change in bond length as a result of tensile stress produced inside crystal lattice [17]. In addition, the BFO particle size was reduced from 46 nm to 21.5 for BGFSO-5, 21.6 for BGFSO-15, 23 nm for BGFSO-20 and 23.5 nm for BGFSO-25 because of doping. The XRD patterns for co-doped BFO have also been observed and previously published with similar kinds of planes and peaks [23,31].

Results
In Figure 1c the XRD peaks of all the BGFSO-GNP nanohybrids have been shown. A distinct sharp peak of carbon plane (002) is observed in nanohybrid structures corresponding to GNP, proving that nanohybrids contain both BGFSO particles and GNP sheets. The periodicity of perovskite lattice structure has been suppressed and peaks intensity is reduced due to the presence of GNPs inside the nanohybrids [14,16]. The peaks broadening is attributed to decreased crystallite size of nanohybrids [14]. The impurity phase Bi 2 O 3 has been completely vanished during hybrid synthesis while a minority phase Bi 2 Fe 4 O 9 is still present. A negative peak shift is also present inside the Catalysts 2020, 10, 367 4 of 14 BGFSO-5/GNP, BGFSO-15/GNP and BGFSO/GNP-20 nanohybrids, which indicates that the inter-planar distance of the samples is enhanced. The particle size of nanohybrids is further increased as compared to the BGFSO nanoparticles. The calculated particle size is 20 nm for BGFSO-5/GNP, 19 Figure 2 shows the morphological analysis of the resulted samples. Transmission electron microscopy (TEM) image of GNPs is shown containing smooth flat graphene sheets with sharp edges with lateral sheet size of around 2-3 microns and the width is 2 microns. In Figure 2b the SEM pattern of BFO/GNP nanohybrid is shown in which the darker sheet like part is representing GNPs and the lighter granular part is referring to BFO nanoparticles.     (Figure 3a). On the other hand, as the concentration of Sn is increased, an agglomeration of nanoparticles is observed. A large cluster of nanoparticles containing 25% Sn is observed in Figure 3d. The distribution of granular particles due to an ice like transparent effect can be observed over and under the graphene layers. In conclusion, the co-precipitation method helps BGFSO nanoparticles to completely diffuse inside, and over, the graphene sheets.    (Figure 3a). On the other hand, as the concentration of Sn is increased, an agglomeration of nanoparticles is observed. A large cluster of nanoparticles containing 25% Sn is observed in Figure 3d. The distribution of granular particles due to an ice like transparent effect can be observed over and under the graphene layers.

SEM Analysis
In conclusion, the co-precipitation method helps BGFSO nanoparticles to completely diffuse inside, and over, the graphene sheets.   (Figure 3a). On the other hand, as the concentration of Sn is increased, an agglomeration of nanoparticles is observed. A large cluster of nanoparticles containing 25% Sn is observed in Figure 3d. The distribution of granular particles due to an ice like transparent effect can be observed over and under the graphene layers. In conclusion, the co-precipitation method helps BGFSO nanoparticles to completely diffuse inside, and over, the graphene sheets.    Figure 4 represents the X-ray Photoelectron Spectroscopy (XPS) spectra of BGFSO/GNP nanohybrids which helps in analyzing the elemental composition of hybrid structures. The main peaks of the spectra representing bismuth (Bi), iron (Fe), gadolinium (Gd), tin (Sn), oxygen (O) and carbon (C), at different corresponding binding energies, are the clear proof of successful synthesis of BGFSO/GNP nanohybrids [47,48]. Further the deconvolution of XPS is shown in Figure 4b-g. In Figure 4b two peaks representing Bi4f are available at 159.1 eV and 164.2 eV while 2 peaks of Bi4d are available at 443 eV and 463 eV [16]. In Figure 4c three peaks of C1s are shown at binding energies corresponding to 284.8 eV, 285.9 eV and 288.7 eV. The first peak at 284.8 eV is representing the C=C (sp2) network of graphene sheets present inside GNPs while the peak at 285.9 eV is for C-C network (possibly can be of carbon tape) and the third peak at 288.7 is for O-C=O network [16,49]. Oxygen spectra in Figure 4d exhibits two peaks of O1s among which one high intensity oxygen peak is at 529.9 eV representing the oxygen atoms of BFO lattice inside the BGFSO/GNP nanohybrids [14,44] while the other is at 531.2 eV which is actually a shift in oxygen peak due to the bonding between carbon network of graphene sheets and BFO via oxy or hydroxyl group [44]. XPS spectra of each Sn and Gd can be seen in Figure 4e,f. Each of two elements have 2 separate peaks, corresponding to the core shell levels of Sn3d and Gd4d. The Figure 4g shows three peaks of iron present in BGFSO network with core shell level Fe2p. The Fe +2 spectrum is broad covering the range of 704 eV to 740 eV, and the peak present at 711 eV corresponds to 2p 3/2 , while at 730.5 eV corresponds to 2p 1/2 . Iron Fe +2 presence inside the BGFSO/GNP nanohybrids enhances the adsorption of organic dye over the surface of the catalyst by means of creating more oxygen vacancies [44]. There is peak of OKLL at 976 eV which corresponds to both, GNP and BFO as oxygen vacancies are produced inside BFO during charge compensation and inside the graphene during reduction. All elemental peaks of BGFSO nanoparticles along with the main peaks of pristine GNP [50] exist inside the nanohybrids and the shifting of some peaks (oxygen and carbon) also represent the chemical bonding in between both BGFSO nanoparticles and GNP sheets. There is no additional peak of other functional groups like C-H, COOR and C-O present inside BGFSO/GNP nanohybrid as they are eliminated during the reduction process of GNPs.

Absorption Analysis and Band-gap Calculations
The UV-vis absorption or diffusion reflectance spectra of the BFO nanoparticles and BGFSO/GNP nanohybrids is shown in Figure 5. As BFO is a UV light active photocatalyst and has a band-gap of around 2.04 eV, it has very low absorbance of visible light. While, absorption analysis of nanohybrids demonstrates that our synthesized BGFSO/GNP has absorption in both UV and visible light ranges, as the spectra is almost linear in the whole range. The maximum absorption was obtained at wavelength 496 nm [21].
Here, A represents a constant while Eg, υ and h corresponds band energy, light frequency and Planck's constant. The tauc's plot representing the curves for band-gap measurements is shown in Figure 6. The band-gap calculations showed that the band-gap values are in the range of 1.91 eV to 2.1 eV which represents that there is only a slight variation inside the band-gap of nanohybrids, as

Absorption Analysis and Band-gap Calculations
The UV-vis absorption or diffusion reflectance spectra of the BFO nanoparticles and BGFSO/GNP nanohybrids is shown in Figure 5. As BFO is a UV light active photocatalyst and has a band-gap of around 2.04 eV, it has very low absorbance of visible light. While, absorption analysis of nanohybrids demonstrates that our synthesized BGFSO/GNP has absorption in both UV and visible light ranges, as the spectra is almost linear in the whole range. The maximum absorption was obtained at wavelength 496 nm [21].
Here, A represents a constant while Eg, υ and h corresponds band energy, light frequency and Planck's constant. The tauc's plot representing the curves for band-gap measurements is shown in Figure 6. The band-gap calculations showed that the band-gap values are in the range of 1.91 eV to 2.1 eV which represents that there is only a slight variation inside the band-gap of nanohybrids, as compared to pure BFO. The variation in band-gap depends on different parameters, including dopant concentration, alteration in crystallite size and lattice parameters and the materials' morphology [52]. So the optical band-gap of our system is not much effected by all these factors. The hence prepared BGFSO/GNP nanohybrids will work actively in the whole range of ultraviolet and visible light. They are the catalyst which will perform an enhanced catalytic activity by absorbing both UV and visible light as compared to BFO.
Catalysts 2020, 10, x FOR PEER REVIEW 8 of 14 compared to pure BFO. The variation in band-gap depends on different parameters, including dopant concentration, alteration in crystallite size and lattice parameters and the materials' morphology [52]. So the optical band-gap of our system is not much effected by all these factors. The hence prepared BGFSO/GNP nanohybrids will work actively in the whole range of ultraviolet and visible light. They are the catalyst which will perform an enhanced catalytic activity by absorbing both UV and visible light as compared to BFO.  compared to pure BFO. The variation in band-gap depends on different parameters, including dopant concentration, alteration in crystallite size and lattice parameters and the materials' morphology [52]. So the optical band-gap of our system is not much effected by all these factors. The hence prepared BGFSO/GNP nanohybrids will work actively in the whole range of ultraviolet and visible light. They are the catalyst which will perform an enhanced catalytic activity by absorbing both UV and visible light as compared to BFO.

Photocatalytic Activity of Nanohybrids
As prepared BGFSO-GNP nanohybrids were further subjected to photocatalytic measurements for checking their efficiency in degrading organic dye Congo-red (CR). The photocatalytic activities of BGFSO/GNP nanohybrids are shown in Figure 7, indicating that the synthesized nanohybrids provide fast removal of organic dye compared with pure BFO, due to the presence of both BGFSO and GNP phases inside nanohybrids. The nanohybrids demonstrate dye removal ability under both dark (in the absence of light) as well as visible light. The dye removal efficiencies under dark and light are referred Catalysts 2020, 10, 367 9 of 14 as catalytic activities and photocatalytic activities of nanohybrids. The catalytic activity of BFO/GNP, BGFSO-5/GNP, BGFSO-15/GNP, BGFSO-20/GNP and BGFSO-25/GNP is 32%, 48%, 44%, 56% and 51% which is due to dye adsorption on the hybrid surface under dark conditions. The photocatalytic activity of BFO/GNP, BGFSO-5/GNP, BGFSO-15/GNP, BGFSO20/GNP and BGFSO-25/GNP is 30%, 12%, 15%, 3% and 23%, respectively. The overall degradation efficiency of pure BFO is 44% in 120 min while the degradation efficiency for BFO/GNP is 62%, BGFSO-5/GNP is 60%, BGFSO-15/GNP is 59%, BGFSO-20/GNP is 59% and BGFSO-25/GNP is 74%, respectively. An already published work [23] showed a good CR dye removal with BGFSO-5 nanoparticles under visible light, but there was no dye removal under dark conditions. This demonstrates that catalysts become active in both light and dark, and are more favorable towards catalytic applications than already published work, due to incorporation of GNPs inside nanoparticles [23,31]. In addition, GNPs provide dye to adsorb over BGFSO/GNP nanohybrids surface actively, which further helped in the fast organic dye removal even in the absence of visible light [14].
As prepared BGFSO-GNP nanohybrids were further subjected to photocatalytic measurements for checking their efficiency in degrading organic dye Congo-red (CR). The photocatalytic activities of BGFSO/GNP nanohybrids are shown in Figure 7, indicating that the synthesized nanohybrids provide fast removal of organic dye compared with pure BFO, due to the presence of both BGFSO and GNP phases inside nanohybrids. The nanohybrids demonstrate dye removal ability under both dark (in the absence of light) as well as visible light. The dye removal efficiencies under dark and light are referred as catalytic activities and photocatalytic activities of nanohybrids. The catalytic activity of BFO/GNP, BGFSO-5/GNP, BGFSO-15/GNP, BGFSO-20/GNP and BGFSO-25/GNP is 32%, 48%, 44%, 56% and 51% which is due to dye adsorption on the hybrid surface under dark conditions. The photocatalytic activity of BFO/GNP, BGFSO-5/GNP, BGFSO-15/GNP, BGFSO20/GNP and BGFSO-25/GNP is 30%, 12%, 15%, 3% and 23%, respectively. The overall degradation efficiency of pure BFO is 44% in 120 min while the degradation efficiency for BFO/GNP is 62%, BGFSO-5/GNP is 60%, BGFSO-15/GNP is 59%, BGFSO-20/GNP is 59% and BGFSO-25/GNP is 74%, respectively. An already published work [23] showed a good CR dye removal with BGFSO-5 nanoparticles under visible light, but there was no dye removal under dark conditions. This demonstrates that catalysts become active in both light and dark, and are more favorable towards catalytic applications than already published work, due to incorporation of GNPs inside nanoparticles [23,31]. In addition, GNPs provide dye to adsorb over BGFSO/GNP nanohybrids surface actively, which further helped in the fast organic dye removal even in the absence of visible light [14].  Table 1. All the nanohybrids actively worked under dark and played an efficient role in dye removal due to presence of free electrons and oxygen containing species [43,44] inside graphene sheets originated during fabrication of GNPs. In addition, they also worked under visible light for dye removal. The most active catalyst under dark is BGFSO-20/GNP and the most active photo-catalyst under visible light is BGFSO-25/GNP.  Table 1. All the nanohybrids actively worked under dark and played an efficient role in dye removal due to presence of free electrons and oxygen containing species [43,44] inside graphene sheets originated during fabrication of GNPs. In addition, they also worked under visible light for dye removal. The most active catalyst under dark is BGFSO-20/GNP and the most active photo-catalyst under visible light is BGFSO-25/GNP.

Dye Degradation Mechanism
The overall catalysis of organic dye depends on dye degradation in the absence and presence of visible light. Figure 9 illustrates the basic mechanism of dye degradation under visible light, as well as in the dark (absence of light).

Dye Degradation Mechanism
The overall catalysis of organic dye depends on dye degradation in the absence and presence of visible light. Figure 9 illustrates the basic mechanism of dye degradation under visible light, as well as in the dark (absence of light).

Dye Degradation Mechanism
The overall catalysis of organic dye depends on dye degradation in the absence and presence of visible light. Figure 9 illustrates the basic mechanism of dye degradation under visible light, as well as in the dark (absence of light).  In the presence of light, electrons travels to the conduction band of BGFSO nanoparticles, from the valence band and the donor shallow energy levels, created below the bottom of conduction band due to the doping of Sn [10]. GNPs lower the recombination rate of electron-hole charge carriers by acting as trapping site of electron [14]. These electrons (e − ) capture oxygen (O 2 ) from the environment produce superoxide radicals (O 2 − ), and further react with the CR dye molecules and form water (H 2 O) and carbon dioxide (CO 2 ) as degradation by products. The water molecule on absorbing the hole (h + ) gives OH . radicals which on further reactions produce H 2 O and CO 2 as by products. The basic steps in equate form are as follows: BGFSO/GNP nanohybrids as a whole contains both oxygen rich species (hydroxyl and oxy ions) as well as trapping sites of electrons [14,44]. The graphene sheets inside the GNPs has the ability to store the electrons inside the π−π carbon network during the reduction process [53]. The oxygen containing species, with the stored electrons, help scavenge the reactive catalytic carriers (e -) and holes (h + ), even in the absence of light, thereby further initiating the degradation phenomenon by reacting with organic dye. On the other hand, graphene facilitates quicker adsorption of organic dye due to its large surface area and due to the electrostatic interaction between the dye and graphene surface [14,16]. Therefore, nanohybrids maximum degradation occurs under dark conditions due to the high adsorption and presence of reactive carriers (e − h + ) inside the BGFSO/GNP. The catalytic congo-red degradation phenomenon has been observed before and published inside the GNP based system as well as MXene (both belonging to 2D materials family) with co-doped BFO [14,16,44]. The reported results showed the fast CR dye degradation under both light and dark conditions.

Preparation of BGFSO Nanoparticles
BGFSO nanoparticles were synthesized by using sol-gel method [23]. The amount of Gd and Sn was introduced inside the base system BFO (ABO 3 system) using the general formula A x Gd 1-x B y Sn 1-y O 3 . The amount of Gadolinium (Gd) was fixed (10%) and Tin (Sn) amount was varied (5% to 25%) using basic calculations for Bi 0.90 Gd 0. 10

Preparation of BGFSO-GNP Nanohybrids
Graphene nanoplatelets (GNPs) were dispersed in deionized (DI) water in an amount of 1 mg/mL using a bath sonicator. The BGFSO nanoparticles (0.01M) were dissolved in ethylene glycol and acetic acid (volume ratio 1:1) via sonication at 60 • C for 2 h. The GNP dispersion (1 mg/mL) and BGFSO solution were combined at room temperature and sonicated for about 15 min, followed by 60 min magnetic stirring at 85 • C. The nanohybrids settled as precipitates. The precipitates were washed three to four times with DI water, followed by vacuum filtration using Whatman 0.2 m filter paper. The filter cake was air dried for 12 h at 50 • C in a convection oven. The dried samples were named BGFSO-GNP nanohybrids.

Conclusion
In this study BFO, BGFSO nanoparticles have been prepared using sol-gel method and BGFSO/GNP nanohybrids have been fabricated through co-precipitation method and then further characterizations. XRD, SEM and TEM were carried out for structural and morphological analysis, which shows a smooth dispersion of spherical BGFSO particles over GNP sheets. Absorption analysis showed that nanohybrids absorb both UV and visible light, and act the same under both ranges. The photocatalytic application further represented that catalyst are active, not even under visible light, but also in dark conditions and proved to be more efficient catalytic material for organic dye removal during water purification. The free electrons and oxygen species inside graphene sheets played a role in oxidation/reduction reactions and helped in degradation of organic dye. The easy synthesized nanohybrids, with enhanced dye removal, proved to be more encouraging for photocatalytic applications.