Structural and Optical Properties of Bi12NiO19 Sillenite Crystals: Application for the Removal of Basic Blue 41 from Wastewater

This article covers the structural and optical property analysis of the sillenite Bi12NiO19 (BNO) in order to characterize a new catalyst that could be used for environmental applications. BNO crystals were produced by the combustion method using Polyvinylpyrrolidone as a combustion reagent. Different approaches were used to characterize the resulting catalyst. Starting with X-ray diffraction (XRD), the structure was refined from XRD data using the Rietveld method and then the structural form of this sillenite was illustrated for the first time. This catalyst has a space group of I23 with a lattice parameter of a = 10.24 Å. In addition, the special surface area (SSA) of BNO was determined by the Brunauer-Emmett-Teller (BET) method. It was found in the range between 14.56 and 20.56 cm2·g−1. Then, the morphology of the nanoparticles was visualized by Scanning Electron Microscope (SEM). For the optical properties of BNO, UV-VIS diffusion reflectance spectroscopy (DRS) was used, and a 2.1 eV optical bandgap was discovered. This sillenite′s narrow bandgap makes it an effective catalyst for environmental applications. The photocatalytic performance of the synthesized Bi12NiO19 was examined for the degradation of Basic blue 41. The degradation efficiency of BB41 achieved 98% within just 180 min at pH ~9 and with a catalyst dose of 1 g/L under visible irradiation. The relevant reaction mechanism and pathways were also proposed in this work.


Introduction
Traditional methods of water treatment cannot effectively eliminate pollutants from wastewaters and could cause great harm to the environment [1][2][3][4]. On the other hand, it has been shown that photocatalysis is a promising approach for the degradation of nonbiodegradable compounds in water [5,6]. It is based on a photocatalyst that can be activated by light such as sunlight and using this energy to remove various types of pollutants [7]. Among these pollutants, basic blue 41 (BB41) has been identified as one of the most problematic dyes. It is present in industrial effluents and commonly used in acrylic, nylon, silk, cotton, and wool dyeing [8][9][10]. This dye is potentially fatal to living organisms. It is effective as a strainer for identifying avian leukocytes, blood, and bone marrow cells. It can also induce short periods of fast or difficult breathing when inhaled, and it can also cause nausea, vomiting, excessive perspiration, mental disorientation, and methemoglobinemia when consumed through the mouth [11]. Therefore, its detection and elimination are challenging goals. Based on our previous studies, photocatalysis-using catalysts have successfully removed both organic and inorganic pollutants in wastewater [12,13]. A novel catalyst with high photocatalytic activity and a narrow bandgap must be developed and tested in order to achieve this goal [14]. The increase in research in recent years for new material categories allowed new studies on structural and optical properties [15,16]. Among these materials-despite being relatively new-sillenites have attracted the attention of many researchers due to their unusual crystal formations, their peculiar electronics, their interesting photochromic, photorefractive, electro-optic, piezoelectric, dielectric properties, and promising optical activity [17][18][19]. They are used in various industrial applications, such as image amplification, phase conjugation, real-time and multiwavelength holography, optical memories for data storage, optical communications, signal processing and a variety of photocatalytic applications [20,21]. Sillenite crystals with a general formula of Bi 12 MO 20x (BMO) have a body-centered cubic crystal structure crystallized in the I23 space group [22]. Their overall structure is described by the atom of bismuth surrounded by seven oxygen atoms that share corners with other similar Bi polyhedrons and with MO 4 which represents a tetravalent ion or a combination of ions that exist both at the cube center and on the corners in the BMO structure [23,24]. There are a significant number of new sillenites that are used as photo-catalysts in previous research, such as Bi 12 TiO 20 [25], Bi 12 GeO 20 [21], Bi 12 PbO 19 [22], Bi 12 CoO 20 [26], Bi 12 SiO 20 [21] , Bi 12 MnO 20 [17], Bi 12 FeO 20 [27] and Bi 12 ZnO 20 [28]. Among the sillenite crystals, bismuth nickelate Bi 12 NiO 19 (BNO) has not been used as a photocatalyst yet, although it has recently gained considerable attention because of its tiny bandgap, its high photoconductivity, ease of separation of photogenerated electron-hole pairs and ease of recycling [29][30][31]. BNO is a single-phase multiferroic, which co-exists with ferroelasticity and has a magnetic character. It is also a lead-free and environmentally friendly material, which makes it very promising for photocatalytic applications. Due to its novelty in the photocatalyst field and its interesting proprieties, we selected BNO as a photocatalyst for this work.
We report in this study the synthesis of Bi 12 NiO 19 sillenite by the sol-gel method using polyvinylpyrrolidone (PVP) as a combustion reagent. Firstly, the phase of the crystals was identified by X-ray diffraction (XRD); then, the structure and lattice constants of the phase were refined using the Rietveld method. The special surface area (SSA) of BNO was determined by the Brunauer-Emmett-Teller (BET) method. Then, the morphology of the nanoparticles was investigated by Scanning Electron Microscope (SEM). After that, the BNO's optical properties were investigated using UV-VIS diffusion reflectance spectroscopy (DRS), and the obtained bandgap was discussed. The photocatalytic activity of the sillenite Bi 12 NiO 19 was tested for photodegradation of basic blue 41 dye. O] was dissolved in water using stoichiometric amounts (12:1 ratio). In order to improve the solubility of the solutions, nitric acid was added. The reaction was carried out according to the following equation: calcination, the obtained powder was homogenized by grinding in an agate mortar; it was calcined then in an air oven for 6 h at 800 • C. The calcination step was done to increase the crystallinity and to remove all carbonated waste left after the combustion reaction. All the synthesis process was summarized and illustrated in Figure 1. Then, the sample was subjected to phase identification, structural characterization and optical study. 2021, 1, FOR PEER REVIEW 3 After total solubilization, PVP K30 as a complexing agent was added to the reaction solution with a concentration of 15% w/w to obtain its complexing role [32]. The obtained solution was dehydrated by evaporation on a hot plate until it turned to a gel, then burned to form Xerogel, an amorphous powder, and after that denitrified at 600 °C for 3 h. Before calcination, the obtained powder was homogenized by grinding in an agate mortar; it was calcined then in an air oven for 6 h at 800 °C. The calcination step was done to increase the crystallinity and to remove all carbonated waste left after the combustion reaction. All the synthesis process was summarized and illustrated in Figure 1. Then, the sample was subjected to phase identification, structural characterization and optical study.

Characterization
X-ray diffraction (XRD) was conducted using a Phillips PW 1730. With the MAUD software (version 2.9.3), the Rietveld technique was used to measure structural properties, refinement and crystallinity. VESTA (version 3.4.0) was used to create the structure illustration. With an electron scanning microscope FEI Quanta 650, crystal pictures were captured. The sample's UV-visible diffuse reflection (DRS) was measured using a Cary 5000 UV-Vis spectrophotometer.

Photocatalysis Test
The photocatalytic degradation of Basic Blue 41 was carried out in a double-walled reactor with a magnetic stirring and cooling system. A 1 g/L dose of catalyst was suspended in a solution of BB41 aqueous solution with a concentration of 15 mg/L and pH ~9, which was found to be optimal conditions. The pH was modified by adding small amounts of HCl and NaOH. The adsorption experiment was performed in the absence of irradiation for 120 min before the photocatalysis test to reach the adsorption equilibrium. After the adsorption equilibrium, the reactor was exposed to visible irradiation using a tungsten lamp supplied by Osram (200 W). The temperature of the solution was maintained at a nearly constant 25 °C during the photocatalytic experiments using a thermostatic bath and a double-walled reactor as a cooling system. The concentration of BB41 was followed by measuring the absorbance in the wavelength (610 nm) with a UV-Vis spectrophotometer (OPTIZEN, UV-3220UV). The degradation efficiency was calculated from the relation: where and are the initial absorbance of BB41 and absorbance after time t, respectively.

Characterization
X-ray diffraction (XRD) was conducted using a Phillips PW 1730. With the MAUD software (version 2.9.3), the Rietveld technique was used to measure structural properties, refinement and crystallinity. VESTA (version 3.4.0) was used to create the structure illustration. With an electron scanning microscope FEI Quanta 650, crystal pictures were captured. The sample's UV-visible diffuse reflection (DRS) was measured using a Cary 5000 UV-Vis spectrophotometer.

Photocatalysis Test
The photocatalytic degradation of Basic Blue 41 was carried out in a double-walled reactor with a magnetic stirring and cooling system. A 1 g/L dose of catalyst was suspended in a solution of BB41 aqueous solution with a concentration of 15 mg/L and pH~9, which was found to be optimal conditions. The pH was modified by adding small amounts of HCl and NaOH. The adsorption experiment was performed in the absence of irradiation for 120 min before the photocatalysis test to reach the adsorption equilibrium. After the adsorption equilibrium, the reactor was exposed to visible irradiation using a tungsten lamp supplied by Osram (200 W). The temperature of the solution was maintained at a nearly constant 25 • C during the photocatalytic experiments using a thermostatic bath and a double-walled reactor as a cooling system. The concentration of BB41 was followed by measuring the absorbance in the wavelength (610 nm) with a UV-Vis spectrophotometer (OPTIZEN, UV-3220UV). The degradation efficiency was calculated from the relation: where abs ad and abs are the initial absorbance of BB41 and absorbance after time t, respectively.

Phase Identification and Structural Investigation
To explore the formation of sillenite crystals Bi 12 NiO 19 , X-ray diffraction (XRD) was used ( Figure 2). All diffraction peaks are attributed to the sillenite phase of Bi 12 NiO 19 (JCPDS card, PDF No. 43-0448) [33]. This indicates good crystallization at 800 • C. The XRD data and I23 cubic structure were used to conduct Rietveld refinement, experimental findings and theoretical data determined by Maud are shown in Figure 2 as points and red lines, respectively. According to the results, the findings estimated using the Rietveld technique matched well with the experimental X-ray diffraction pattern. The Rietveld refinement results gave identical results compared to other known methods of structural properties. [34,35]. Figure 1 also showed the purity and good crystallinity of our sillenite phase through the great congruence between the results of the experiment and the theoretical data. It was determined that the refined values represented a cubic form with a space group of I23 and a lattice parameter of a = 10.24 Å. The Rietveld refined parameters such as reliability factors R p , R exp , R wp and Sig with the cell parameter (a) and atomic position (x,y,z) are presented in Table 1.

Phase Identification and Structural Investigation
To explore the formation of sillenite crystals Bi12NiO19, X-ray diffraction (XRD) was used ( Figure 2). All diffraction peaks are attributed to the sillenite phase of Bi12NiO19 (JCPDS card, PDF No. 43-0448) [33]. This indicates good crystallization at 800 °C. The XRD data and I23 cubic structure were used to conduct Rietveld refinement, experimental findings and theoretical data determined by Maud are shown in Figure 2 as points and red lines, respectively. According to the results, the findings estimated using the Rietveld technique matched well with the experimental X-ray diffraction pattern. The Rietveld refinement results gave identical results compared to other known methods of structural properties. [34,35]. Figure 1 also showed the purity and good crystallinity of our sillenite phase through the great congruence between the results of the experiment and the theoretical data. It was determined that the refined values represented a cubic form with a space group of I23 and a lattice parameter of a = 10.24 Å. The Rietveld refined parameters such as reliability factors Rp, Rexp, Rwp and Sig with the cell parameter (a) and atomic position (x,y,z) are presented in Table 1.  The structural representation of the sillenite BNO was illustrated in Figure 3 by Vesta using the structural parameters in Table 1. The atoms colors for Ni, Bi and O are green, yellow and red, respectively. As can be seen, the phase is a cubic structure (space group I23). The atoms in the crystal structure show shared occupancy between bismuth and nickel while bismuth atoms are dominant at around 92% due to the ratio of atoms being 12:1 for bismuth and nickel. The structural representation of the sillenite BNO was illustrated in Figure 3 by Vesta using the structural parameters in Table 1. The atoms′ colors for Ni, Bi and O are green, yellow and red, respectively. As can be seen, the phase is a cubic structure (space group I23). The atoms in the crystal structure show shared occupancy between bismuth and nickel while bismuth atoms are dominant at around 92% due to the ratio of atoms being 12:1 for bismuth and nickel. The crystallite size, X-ray density, and special surface area were calculated from the following equations [36]: Where D is the regular phase crystallite, K is the Scherener constant, β is the full width at the highest half of the phase, is the angle Braggs, is Avogadro's number, is the Molecular mass of BNO (2870.45 g·mol −1 ), is the density of X-ray, is the count of model units in a cell (Z = 2 for sillenites [26]) and is the unit cell volume (1083.428335 Å 3) and S is the specific surface area.
The crystallite size was calculated from the main peaks of the XRD diffractogram, and it was found in the range between 33.20 and 33.20 nm. The density of X-rays was The crystallite size, X-ray density, and special surface area were calculated from the following equations [36]: where D is the regular phase crystallite, K is the Scherener constant, β is the full width at the highest half of the phase, θ is the angle Braggs, N A is Avogadro's number, M is the Molecular mass of BNO (2870.45 g·mol −1 ), ρ is the density of X-ray, Z is the count of model units in a cell (Z = 2 for sillenites [26]) and V is the unit cell volume (1083.428335 A 3) and S is the specific surface area. The crystallite size was calculated from the main peaks of the XRD diffractogram, and it was found in the range between 33.20 and 33.20 nm. The density of X-rays was found to be 8.79 g·cm −3 . The special surface area (SSA) of the particular BNO was estimated by the Brunauer-Emmett-Teller (BET) method in Equation (4), and it was found in the range between 14.56 and 20.56 cm 2 ·g −1 .

Morphology Investigation
In order to analyze the morphology of the BNO crystals, a Scanning Electron Microscopy (SEM) was used. Figure 4 shows typical SEM images of the BNO crystals. A small agglomeration can be seen due to the ultrafine nature of the sample [37]. This leads to a non-uniform distribution of crystals of different shapes and a noticeable porosity in the sample [28]. The viscosity of the suspension in the synthesis route plays an important role in the development of the porous structure [1,38]. The viscosity of suspension during synthesis causes the porous structure to get compacted. The sol-gel method is well-known for its great viscosity because it forms a gel during the synthesis. Due to this, we have significant porosity in our sample. agglomeration can be seen due to the ultrafine nature of the sample [37]. This leads to a non-uniform distribution of crystals of different shapes and a noticeable porosity in the sample [28]. The viscosity of the suspension in the synthesis route plays an important role in the development of the porous structure [1,38]. The viscosity of suspension during synthesis causes the porous structure to get compacted. The sol-gel method is well-known for its great viscosity because it forms a gel during the synthesis. Due to this, we have significant porosity in our sample.

Optical Study
To determine the catalyst's performance, knowledge about the gap is highly necessary. The optical properties of BNO crystals were investigated using UV-vis diffuse reflectance spectroscopy (DRS). The bandgap energy (Eg) is determined by plotting the Tauc plot ( Figure 5), which is the dependence of the absorption coefficient (α) on the photon energy (h). It is expressed in the following relationship [37,39]: where α is the absorption coefficient, K is a proportionality constant, and the exponent n= 2 or 1/2 refers to the nature of the transition. The direct bandgap was estimated by the interception of the linear plot (αh) and the h axis. The band-gap energy (Eg) of NBO crystals was about 2.1 ± 0.1 eV which is a smaller bandgap than of TiO2 and ZnO (3.2 eV) [40,41]. This shows that BNO has a significant absorption level for both UV and visible light from wavelength 200 to 800 nm where the fraction of the light irradiance is converted into electrical and/or chemical energy. This tends to be more effective than the photocatalysts ZnO and TiO2 and leads in visible light irradiation to an increase in the formation of pairs of electron holes. This sillenite's narrow bandgap makes it a new promising catalyst for photocatalysis applications for environmental aquatic pollution.

Optical Study
To determine the catalyst's performance, knowledge about the gap is highly necessary. The optical properties of BNO crystals were investigated using UV-vis diffuse reflectance spectroscopy (DRS). The bandgap energy (E g ) is determined by plotting the Tauc plot ( Figure 5), which is the dependence of the absorption coefficient (α) on the photon energy (hv). It is expressed in the following relationship [37,39]: where α is the absorption coefficient, K is a proportionality constant, and the exponent n= 2 or 1/2 refers to the nature of the transition. The direct bandgap was estimated by the interception of the linear plot (αhv) 2 and the hv axis. The band-gap energy (E g ) of NBO crystals was about 2.1 ± 0.1 eV which is a smaller bandgap than of TiO 2 and ZnO (3.2 eV) [40,41]. This shows that BNO has a significant absorption level for both UV and visible light from wavelength 200 to 800 nm where the fraction of the light irradiance is converted into electrical and/or chemical energy. This tends to be more effective than the photocatalysts ZnO and TiO 2 and leads in visible light irradiation to an increase in the formation of pairs of electron holes. This sillenite's narrow bandgap makes it a new promising catalyst for photocatalysis applications for environmental aquatic pollution.

Photocatalytic Activity
To test the photocatalytic activity of this catalyst, BB41 was selected as a topical example of organic pollutants. The photolysis effect of irradiation must be taken into account. An irradiation test for the elimination of the effects of photolysis was carried with-

Photocatalytic Activity
To test the photocatalytic activity of this catalyst, BB41 was selected as a topical example of organic pollutants. The photolysis effect of irradiation must be taken into account. An irradiation test for the elimination of the effects of photolysis was carried without the Catalyst BNO. Photolysis showed only a small percentage of BB41 degradation, not even above 5% percent. The effects of photolysis could consequently be overlooked. After that, the test was performed in the presence of the catalyst BNO. Before illumination of the photocatalytic reactor, it is important to eliminate the adsorption effect in a dark condition, the adsorption has not a significant removal for BB41.
After that, photocatalysis tests were started by the effect of pH, as the pH played a major role in the degradation. The tests were done in different pH mediums with a concentration of 15 mg/L and a catalyst dose of 1 g/L for 3 h, the results are shown in Figure 6. As can be seen, pH 9 was the optimal pH condition, because BB41 is a cationic dye, its photodegradation is favored in the basic medium, and the photodegradation performance of BNO for BB41 is boosted in the basic condition.

Photocatalytic Activity
To test the photocatalytic activity of this catalyst, BB41 was selected as a topical example of organic pollutants. The photolysis effect of irradiation must be taken into account. An irradiation test for the elimination of the effects of photolysis was carried without the Catalyst BNO. Photolysis showed only a small percentage of BB41 degradation, not even above 5% percent. The effects of photolysis could consequently be overlooked. After that, the test was performed in the presence of the catalyst BNO. Before illumination of the photocatalytic reactor, it is important to eliminate the adsorption effect in a dark condition, the adsorption has not a significant removal for BB41.
After that, photocatalysis tests were started by the effect of pH, as the pH played a major role in the degradation. The tests were done in different pH mediums with a concentration of 15 mg/L and a catalyst dose of 1 g/L for 3 h, the results are shown in Figure  6. As can be seen, pH 9 was the optimal pH condition, because BB41 is a cationic dye, its photodegradation is favored in the basic medium, and the photodegradation performance of BNO for BB41 is boosted in the basic condition. After selecting the optimal pH, a test with a kinetic study was carried out under optimal conditions. The results are illustrated in Figure 7; Figure 7a shows the evolution of BB41 degradation as a function of time, where Figure 7b shows the associated UV-vis After selecting the optimal pH, a test with a kinetic study was carried out under optimal conditions. The results are illustrated in Figure 7; Figure 7a shows the evolution of BB41 degradation as a function of time, where Figure 7b shows the associated UVvis spectra for each point and the absorption peaks of BB41 at 610 nm can be observed decreasing with time. As can be observed, the degradation efficiency of BB41 achieved 98% within just 180 min at pH~9 and 25 • C. This result can be explained by the gap energy of the catalyst (2.1 eV), which offers a higher absorption in both UV and visible areas from 200 to 800 mm [42].
It was already demonstrated in our previous works [13,14] that the degradation of the dye as an organic compound in the photocatalytic process is mainly due to the reactive oxidative species (ROS) such as superoxide radicals (O 2 •− ) and hydroxyl radicals ( • OH), where the electrons of Bi 12 NiO 19 conduction band are degraded the Basic Blue 41 by reducing the absorbed O 2 to the super-radical anion O 2 •− . The oxidation occurs concomitantly by radicals • OH through a valence band that reacts with H 2 O [43]. In order to investigate the active species for the degradation of BB41, isopropanol and benzoquinone were chosen as scavenger agents to capture • OH and O 2 •− , respectively. Figure 8 shows the degradation efficiency with and without the presence of scavengers. spectra for each point and the absorption peaks of BB41 at 610 nm can be observed decreasing with time. As can be observed, the degradation efficiency of BB41 achieved 98% within just 180 min at pH ~9 and 25 °C. This result can be explained by the gap energy of the catalyst (2.1 eV), which offers a higher absorption in both UV and visible areas from 200 to 800 mm [42]. It was already demonstrated in our previous works [13,14] that the degradation of the dye as an organic compound in the photocatalytic process is mainly due to the reactive oxidative species (ROS) such as superoxide radicals (O2 •− ) and hydroxyl radicals ( • OH), where the electrons of Bi12NiO19 conduction band are degraded the Basic Blue 41 by reducing the absorbed O2 to the super-radical anion O2 •− . The oxidation occurs concomitantly by radicals • OH through a valence band that reacts with H2O [43]. In order to investigate the active species for the degradation of BB41, isopropanol and benzoquinone were chosen as scavenger agents to capture • OH and O2 •− , respectively. Figure 8 shows the degradation efficiency with and without the presence of scavengers.  It was already demonstrated in our previous works [13,14] that the degradation of the dye as an organic compound in the photocatalytic process is mainly due to the reactive oxidative species (ROS) such as superoxide radicals (O2 •− ) and hydroxyl radicals ( • OH), where the electrons of Bi12NiO19 conduction band are degraded the Basic Blue 41 by reducing the absorbed O2 to the super-radical anion O2 •− . The oxidation occurs concomitantly by radicals • OH through a valence band that reacts with H2O [43]. In order to investigate the active species for the degradation of BB41, isopropanol and benzoquinone were chosen as scavenger agents to capture • OH and O2 •− , respectively. Figure 8 shows the degradation efficiency with and without the presence of scavengers.  As can be seen, adding benzoquinone and isopropanol to the solution reduced the BB41 degradation efficiency, indicating that both O 2 •− and • OH were important active species in the photocatalytic activity. The photocatalytic reactions that occur at the solid/liquid interface can be bonded according to the following reaction mechanism: Dye + • OH / O 2 •− → Degradation products (10)

Conclusions
The main objective of this work was devoted to the study of the structural and optical properties of Bi 12 NiO 19 and its application as a photocatalyst for the disposal of the basic blue 41 dye, used in the textile industry, under visible irradiation. BNO nanoparticles were synthesized using the combustion method using Polyvinylpyrrolidone as a combustion reagent. Different approaches were used to characterize the resulting catalyst. Starting with X-ray diffraction (XRD), the Rietveld method was used to refine the structure based on XRD data. which showed the purity and good crystallinity of our sillenite phase. Then, the structural form of this sillenite was illustrated for the first time. This catalyst has a space group of I23 with a lattice parameter of (a = 10.24 Å) for this catalyst. The special surface area (SSA) of BNO was determined by the Brunauer-Emmett-Teller (BET) method, it was found in the range between 14.56 and 20.56 cm 2 ·g −1 . Then, the morphology of the nanoparticles was visualized by Scanning Electron Microscope (SEM). Finally, the optical properties of BNO were determined by UV-VIS diffusion reflectance spectroscopy (DRS), a 2.1 eV optical bandgap was discovered. This narrow bandgap and the good crystallinity allow this sillenite to be a promising and effective catalyst for photocatalytic applications in the environmental field such as the treatment of polluted water under visible light radiation. That was confirmed by the application of this sillenite by performing the decomposition of the basic blue 41 dye under visible irradiation where a total degradation was obtained at pH~9 and 25 • C in less than 180 min. For future research, we will test the effects of this sillenite on the degradation and reduction of organic and inorganic pollutants, as they are both hazardous contaminants to the environment.