Influence of the isothermal annealing time on structural morphological, and photocatalytic characters of BiT/ZnWO 4 composite

The abbreviated composite Bi 4 Ti 3 O 12 /ZnWO 4 (BIT/ZW) was produced using the molten salt technique at 800°C. The impact of isothermal annealing time, in the second step, on the structural, morphological, and photocatalytic properties was investigated. X-ray diffraction (XRD) confirmed the formation of composites with varying proportions (76.8% BIT, 20.2% ZW), (71% BIT, 22.8% ZW), and (56.4% BIT, 36.6% ZW) at annealing times of 4, 6, and 8 hours at 950°C, respectively. A minor amount of pyrochlore phases (<7%) was observed. Increasing the annealing time affected the crystallinity of the composites. The crystallite size and grain size of BIT and ZnWO 4 decreased with longer annealing times, leading to a reduction in the band gap from 2.95 to 2.83 eV. The composite (76.8% BIT, 20.2% ZW) exhibited photocatalytic activity that was two times higher for degrading methylene blue (MB) in the dark compared to the other composites, but showed an improvement in degradation under solar irradiation from 70% to 72.18%. The results indicate that annealing isothermal time can effectively modify the properties of the composites, creating a proven heterojunction between BIT and ZnWO 4 . (


Introduction
Over the past few years, researchers have been confronted with a substantial obstacle in tackling the crisis of water pollution caused by human activities.This pollution not only endangers human health but also poses a threat to the environment [1][2][3].Among the various technologies available, photocatalytic technology utilizing environmentally friendly semiconductors has emerged as the most promising solution to combat pollution caused by heavy metals [1][2][3], inorganic pollutants [4][5][6][7], organic pollutants [8][9][10][11], and even bacteria [12].
The objective of this study is to explore new composite materials based on ZnWO 4 that could be competitive for potential photocatalytic applications.To achieve this, we have chosen to synthesize a molten salt mixture of Aurivillius Bi 4 Ti 3 O 12 and ZnWO 4 , with variations in the sintering time.The aim is to investigate the effect of isothermal treatment time on the phase ratio (BIT and ZnWO 4 ) of the composite and to study the photocatalytic properties of the resulting compounds on the sunlight degradation of pollutant methylene blue.

Catalyst preparation
It is well known that synthesis parameters such as precursor selection, calcination and sintering time, heating and cooling rate, pH, and concentration play a crucial role in determining the size, shape, and surface properties of nanomaterials [40][41][42].Therefore, we chose the molten salt method to synthesize the Bi 4 Ti 3 O 12 /ZnWO 4 composite, abbreviated as BIT-ZW.Stoichiometric amounts of high-purity precursors of the BIT compound, Bi 2 O 3 (99.9%)and TiO 2 (99.9%), were mixed with a (1:1) ratio of (NaCl : KCl) for 4 hours.The reaction mixture was calcined at 800°C for 4 hours with a heating rate of (2°C/min).Subsequently, the calcined mixture was ground with stoichiometric amounts of ZnO (99.5%) and WO 3 (99.9%)for 4 hours.The resulting powder was then subjected to a second calcination at 950°C for 4, 6, and 8 hours, with the same heating rate of (2°C/min).The material was washed multiple times with hot water until the chlorinated salts were completely removed.
After drying at 80°C for 12 hours, the structural analysis was performed using X-ray diffraction (XRD), morphological analysis was conducted using scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR), and the degradation kinetics of methylene blue under sunlight irradiation was studied using UV-visible spectroscopy.

Photocatalytic test
In all photocatalytic activity experiments, catalysts (100 mg) were placed in a solution of MB dye (100 ml, 1 mg.l -1 , normal pH = 6.04) under solar irradiation in the month of June in the Biskra region (Algeria), while maintaining a temperature of 25 °C using a closed water circuit.The dye was analyzed using a UV-vis-NIR spectrophotometer (Perkin-Elmer, Lambda 850) at λ= 664 nm.At 15-minute intervals of solar irradiation, samples (4 mL) of the reaction solution were taken, centrifuged, and filtered.Finally, the filtrates were analyzed.The degradation efficiency was calculated using following equation (1): where ; C 0 (mg/l) represents the initial concentration of MB, and C t (mg/l) is the concentration of the collected quantities after centrifugation.The Scheme 1 illustrates the stepwise preparation and characterization of a composite photocatalyst under the effect of isothermal time annealing.

Scheme 1. Stepwise illustration of Molten-salts reaction of (BIT/ZW) under the effect of isothermal time
annealing.

XRD analysis
Fig. 1 illustrates the results of the X-ray diffraction (XRD) spectral analysis of the samples after thermal treatment.Firstly, the peaks of Bi 4 Ti 3 O 12 were indexed to the BIT prototype lattice (ICSD No. 98-002-4738), indicating an orthorhombic structure with the Fmmm space group.The lattice parameters for pure Aurivillius BIT were calculated using HighScore Plus software, resulting in a = 5.402 Å, b = 5.454 Å, c = 32.594Å, α = β = γ = 90°.The crystallite size, determined by the D. Scherrer method using the highest peak of the three-layer Aurivillius [43], the (hkl) plane (117), is in the nanometer range (41.23 nm) (Table 1).The addition of one mole of zinc oxide and tungsten, followed by recalcination at 950°C for 4 hours, 6 hours, and 8 hours, resulted in a composite (x% BIT, y% ZW) with varying proportions : (76.8% BIT, 20.2% ZW), (71.2% BIT, 8% ZW), and (56.4% BIT, 36.6%ZW).All the peaks of the ZnWO 4 compound can be attributed to the monoclinic crystal structure of the card (ICSD no.98-016-9214) with a space group of P12/C1.A minor amount of pyrochlore, approximately 3%, was observed to form through the reaction Bi 2 O 3 + 2 TiO 2 → Bi 2 Ti 2 O 7 at 4 hours in 950°C.However, this phase increased to 7% with a decrease in the BIT phases, indicating the instability of the Aurivillius phase with a change in the isothermal processing time.This resulted in the formation of the pyrochlore phase through the reaction: Bi 4 Ti 3 O 12 → Bi 2 Ti 2 O 7 + Bi 2 O 3 + TiO 2 .Therefore, since the secondary phase does not exceed 7%, the formed composites can be considered biphasic [44].

FTIR analysis
Fig. 2 shows the FTIR spectrum of the (x% BIT-y% ZW) samples sintered at 950°C.The (x% BIT -y% ZW) composites are a two-phase mixture consisting of an Aurivillius BIT three-layer phase and ZnWO 4 wolframite phase.Most of the vibrations of the considered bonds are consecutive in nature.A wide band observed at [583-663] cm -1 corresponds to the vibration of the oxygen atoms in the octahedron [TiO 6 ] 6 of Aurivillius BIT [45][46][47].The bands at 820 and 880 cm -1 are associated to the vibration of the W-O bonds, while the bands at 600 and 700 cm -1 are attributed to the Zn-O-W bonds of ZnWO 4 [48,49].Additionally, the band at 1600 cm -1 is linked to the presence of absorbed water in the composite or the KBr used to dilute the samples in the analyte pellets [48].

SEM/X-EDX analysis
The analysis of the samples using scanning electron microscopy (SEM) of the solid solution allowed for determining the surface condition of the different samples.This technique provides an estimation of grain distribution and average grain size after sintering at 950°C [50][51][52].The micrographs of the composites in Fig. 3 (a, b, and c), at a scale of 20 μm, confirm the formation of two phases, one appearing as plates (BIT) and the other as spheres (ZW), while the pyrochlore phase exhibits a pyramidal shape.The three histograms in Fig. 3 (d, e, and f) show a slight variation in the average grain size of the ZnWO 4 compound with changes in the holding time at 950°C, ranging from 1.87 to 1.6 μm.On the other hand, the BIT grains have a plate-shaped form with a slight decrease in size.The following table 1 presents the results of calculating the average grain size obtained using « image J » analysis software.

Fig. 3. (a-c) SEM images of (x% BIT-y% ZW) composites (20µm scale), (d-f) Histograms grain size of ZnWO 4 in the composites.
The X-EDS analysis was conducted concurrently with SEM analysis on the surfaces of the sintered samples to obtain comprehensive insights.The X-EDS spectrum presented in Figure 4(ad) illustrates the elemental composition of the (x% BT-y% ZW) samples sintered at 950°C.The spectrum confirms the presence of all constituent elements of the biphasic compound, including Bismuth (Bi), Tungsten (W), Titanium (Ti), Zinc (Zn), and oxygen (O).

Photocatalytic study
Since the discovery of Aurivillius Bi 4 Ti 3 O 12 and ZnWO 4 , they have been considered as powerful photocatalysts due to their ability to rapidly decompose and mineralize a wide range of natural organic matter and organic pollutants, attributed to their narrow band gaps ranging from 2.7 to 3.2 eV [53-55] and 3.05 to 4.7 eV [14][15][16][17][18], respectively.We utilized the direct transition model proposed by Wood and Tauc [56][57][58] to determine the optical band gap of the sintered (x% BIT-y% ZW) composites at 950°C.This was accomplished by plotting the absorbance curve (fig.5(a)) and (αhν)² versus (hν) (fig.5(b)).By extrapolating a linear fit to the plot to zero in Fig. 5(b), we obtained estimated energy gaps of 2.95, 2.89, and 2.83 eV.There is a slight decrease in the band gap energy values due to changes in crystallite size and scattering phenomena [59], indicating the presence of a heterojunction between Aurivillius and ZnWO 4 .Fig. 6 (a, b) shows the efficiency of methylene blue (MB) photodegradation under solar irradiation using the composite catalyst (x% BIT, y% ZW).It was observed that the catalysts (76.8% BIT, 20.2% ZW), (71.3% BIT, 22.8% ZW), and (56.4% BIT, 36.6%ZW) successively degraded methylene blue by 57.47%, 32.28%, and 24.43% after 30 minutes of dark adsorption, and 70.35%, 72.15%, and 70.18% after 75 minutes of photocatalysis under solar irradiation.
In most cases, the degradation kinetics of many organic molecules are described as a firstorder reaction following the Langmuir-Hinshelwood kinetic model at low concentration [60].Therefore, according to the following equation ( 2): where V is the photocatalytic degradation rate (mg•l -1 •min), k app. is the apparent degradation constant (min -1 ), C is the concentration of MB dye in solution (mg•l -1 ), and t is the irradiation time (min).Integrating equation ( 2) (with the initial condition: C = C 0 at t = 0) leads to the following Equation(3): The regression coefficient and the kinetic constant were estimated by linear fitting of Ln(C t /C 0 ) versus t (Fig. 4 (c, d, e)) and listed in Table 2.This implies that the half-life of dye degradation will occur at t 1/2 = 0.693/k app. at 74, 78, and 96 min.
Based on the analysis results, which are consistent with previous studies on composite semiconductor nanoparticles based on BIT or ZnWO 4 [19,49,51,53,55,61], we propose a possible photocatalytic mechanism illustrated in Fig. 6(f), confirming that photon absorption allows for the excitation of e -to the highest conduction band (CB) of BIT to the lower (CB) of ZnWO 4 , resulting in the generation of oxidizing or reducing species on the semiconductor surface.These species, mostly highly oxidizing .OH radicals, are formed near the catalyst from water and oxygen in the air.They attack the chemical bonds of most organic compounds (pollutants) and partially or completely destroy them.The short half-life of the radicals prevents them from being transported far from the active surface [61].
The values of the conduction band (CB) and valence band (VB) potentials of BIT and ZnWO 4 were calculated using the following equations (4 and 5) [61,62]: where χ is the absolute electronegativity of the semiconductor, E e is the energy of free electrons on the hydrogen scale (approximately 4.5 eV), and Eg is the experimental energy of the band gap for BIT (2.86 eV) and ZnWO 4 (3.25 eV).

Conclusion
In summary, Bi 4 Ti 3 O 12 /ZnWO 4 composites were successfully synthesized using the molten salt method, followed by an isothermal treatment at 950°C for 4, 6, and 8 hours.X-ray diffraction (XRD) analysis and the ICSD data of the pure synthesized Bi 4 Ti 3 O 12 (BIT) using the same method indicated the formation of composites (x% BIT, y% ZW%) with varying proportions, along with a minor pyrochlore phase below 7%.Furthermore, FT-IR spectroscopy confirmed these structures as most of the considered bond vibrations were detected.The UV-Vis spectra revealed a decrease in the Eg values with increasing isothermal treatment time.The photocatalytic performance for the degradation of the MB dye was studied for all composites, and the results confirm that it is influenced by changes in the isothermal time at 950°C.

Fig. 6 .
Fig. 6. (a, b) Photocatalytic degradation efficiency curves of MB under solar irradiation in the presence of the photocatalyst (x% BIT/ y% ZW); ( c-e) first-order kinetic curves of the three composites ; (f) proposed mechanism for MB degradation.

Table 2 .
Photocatalytic characteristics and kinetics of MB dye degradation in the presence of catalysts (x% BIT, y% ZW) under solar irradiation.