Easy conversion of BiOCl plates to flowers like structure to enhance the photocatalytic degradation of endocrine disrupting compounds

Endocrine disrupting chemicals (EDCs) are exogenous agents that interfere with the synthesis, secretion, transport, binding action and elimination of the natural hormones in our body. Bisphenol A (BPA), one of the well-known emerging pollutants, is widely used as an industrial ingredient in polycarbonate plastic products. BPA can be released into aquatic environments through domestic and industrial wastewater discharge. Development of cost-effective technologies for the removal of BPA is currently a priority R&D area. BiOCl microstructures materials were successfully prepared via a hydrothermal method and evaluated for the photocatalytic degradation of Bisphenol A via under UV-B light. The prepared BiOCl microstructures were characterised using XRD, DRSUV–vis, SEM, EDS and XPS analyses. SEM results demonstrated that the morphology of these BiOCl nanostructured materials could easily be modified from microplates to microflowers by altering the solution stirring speed during synthesis. XRD analysis showed that the product could be indexed to the tetragonal phase of BiOCl. The band gap energies determined by DRUV–vis were estimated to be 3.36 eV and 3.32 eV for microflowers and microplates respectively. EDS analysis confirmed the presence of three elements bismuth, oxygen and chlorine in the BiOCl. The microflowers exhibited enhanced photocatalytic performance because of the higher adsorption capacity and presence of more active sites compared to BiOCl microplates.


Introduction
Providing clean water to satisfy human needs in an era of the growing world population and massive industrial development is a grand challenge of the 21st century [1]. Water pollution due to the release of harmful substances from various anthropogenic processes has resulted in some footprints of these substances in water bodies throughout the world [2]. Consumption of polluted water can prompt ailments and sicknesses that have negative health impacts on humans and other living organisms [3]. Endocrine disrupting chemicals (EDCs) are among the exogenous chemicals capable of interfering and mimicking the hormonal action of living organisms. Bisphenol A (BPA) is one of the most common and well-known endocrine disrupting chemicals. 5-6 and is widely used as an intermediate in the production of polycarbonate and epoxy resins. BPA ends up in a variety of consumer goods such as plastic bottles and canned food liners [4,5].
Traditional physical wastewater treatment methods have the major disadvantage of transferring the pollutants from one phase to another, which require subsequent treatment or disposal. On the other hand, biological treatments take a long time for the effluent to reach the required standards and produce a large quantity of sludge, which normally cannot be reused [6,7]. Advanced oxidation processes (AOPs) have emerged as more effective and sustainable alternative techniques for the complete removal of pollutants in wastewater [8][9][10][11][12]. Heterogeneous photocatalysis is an AOP that is based on the generation highly reactive hydroxyl radicals (·HO) upon irradiation of a semiconductor catalyst [13][14][15][16][17].
Besides traditional semiconductor photocatalysts such as TiO 2 and ZnO, new materials such as BiOX (X=halogen) are slowly gaining increased attention as alternatives due to their easy synthesis, stability and possibility of activation by low energy visible light [18,19]. BiOX microstructures exist as stratified Bi-based oxides consisting of Bi 2 O 2 2+ slabs inserting into the double halogen atom slabs. This special structure offers the unit of BiOX enough space to polarize the related atoms and orbital exciting the formation of an internal electric field between Bi 2 O 2 2+ slabs and halogen slabs [20,21]. In addition, the induced dipole will expeditiously separate the electron/hole pairs, resulting in superior photocatalytic activity.
In general, the photocatalytic activity of BiOX microstructures is dependent on their morphologies and microstructures [22][23][24][25][26]. Different routes have been proposed for the synthesis of BiOX structures with various morphologies [27,28]. In a recent study, BiOX photocatalysts were used for the photodegradation of synthetic estrogens. The catalysts were more effective at degrading selected endocrine disrupting compounds compared to traditional catalysts such as TiO 2 [29]. In another study, hierarchical BiOX microspheres were successfully synthesized by a microwave-assisted solvothermal method using bismuth (III) nitrate pentahydrate. The flowerlike BiOI microsphere structures showed superior performance in the photocatalytic production of hydrogen under visible light irradiation [30]. Hierarchically structured bismuth oxyiodide (BiOI) with tunable ratios of (110) and (001) facets were fabricated through a combination of solid-state reaction and subsequent hydrolysis at room temperature. The hierarchical BiOI structures exhibited excellent visible-light photocatalytic performance for phenol degradation based on the optimisation of the ratio of (001) and (110) surfaces [31].
In this study, two different morphological forms of BiOCl were synthesised through a hydrothermal method. The microstructures were evaluated for their photocatalytic performance towards the degradation of Bisphenol A under UV-B irradiation.

Materials
Analytical grade Bismuth nitrate pentahydrate Bi(NO 3 ) 3 .5H 2 O, 98%, Nitric acid HNO 3 , 99%, Potassium chloride KCl, 99%, were purchased from Sigma-Aldrich and were employed as received without additional purification. The chemical sample bisphenol was obtained from the Sigma-Aldrich and used without further purification. The molecular structure of bisphenol is shown in scheme 1. Deionized, doubly distilled water was used for the preparation of all solutions.

Synthesis of BiOCl microstructures
In a typical procedure for the synthesis of BiOCl microflowers and microplates, of Bismuth nitrate pentahydrate Bi(NO 3 ) 3 ·5H 2 O (0.2425 g) was dissolved in of HNO 3 (50 ml) and dropwise added to of potassium chloride (0.0821 g) dissolved in of distilled water (20 ml) using a burette with fast stirring. This was followed by a nonstirring period of 2 h. The resulting solution was transferred into an autoclave and placed in an oven at 160°C for 12 h. After that, the white powder obtained was dried at 80°C for 12 h and then collected for characterization.

Characterization of the BiOCl microstructures
The crystalline properties of BiOCl microcrystals were studied by x-ray diffraction (XRD) using a Bench x-ray diffractometer (Model: MiniFlex 600) x-ray diffractometer equipped with graphite monochromatised CuK α radiation (λ=1.540 Å) and NaI(T1) detector. The absorption spectra of the as-prepared samples were recorded on a DRUV-vis diffuse reflectance spectroscopy (Jasco V670) in the wavelength range of 200-1000 nm, using BaSO 4 as a reference. The morphological features were examined by field-emission scanning electron microscope (JEOL, Japan (Model: JSM 7800F). The maximum working voltage of 30 kV was achieved at a maximum resolution of 0.8 nm, and a working distance of 10 mm was used during the measurements. The chemical states and relative surface compositions of the samples were studied using multi-probe x-ray photoelectron spectroscopy (XPS) (Omicron Nanotechnology, Germany) and a software package (Casa Software Ltd) was used to analyse the XPS data. The binding energies of the obtained spectra were calibrated with respect to the intrinsic carbon C1 peak at 284.6 eV.

Photocatalytic degradation studies
All photoreaction experiments were carried out in a photocatalytic reactor system which consists of a cylindrical borosilicate glass reactor vessel with an effective volume of 250 ml, a cooling water jacket, and a UV-B lamp (8 W medium-pressure mercury lamp (Institute of Electric Light Source, Beijing) positioned axially at the centre as a visible light. The reaction temperature was kept at 25°C by circulating the cooling water. A special glass frit as an air diffuser was fixed at the reactor to disperse air into the solution uniformly. Photocatalytic activities of the prepared samples were examined by the degradation of Bisphenol A under UV-B light irradiation. For each run, the slurry was freshly prepared by adding 0.250 and 0.500 g of catalyst into 250 ml of 10 ppm aqueous BPA. The slurry was stirred in the dark for about an hour to establish the adsorption-desorption equilibrium. Aliquots were then sampled at fixed time intervals and filtered using syringe filter (0.45 μm) to remove any suspended particles. The filtrate was analysed on a UV-vis spectrophotometer.

XRD analysis
The crystalline properties of prepared BiOCl samples were investigated through x-ray diffraction analysis figure 1. According to the JCPDS card No.006-0249, both samples were indexed to the tetragonal crystal phase. In addition, the grain sizes and other crystal parameters of the synthesized BiOCl were measured using the Maud Software with Cif card number 1011175 [32]. The space groups and lattice parameters of the samples were identified as P4/nmm, a=b=3.89 Å, c=7.37 Å and α=β=γ=90°where the grain sizes were 94.6 nm for microplates and 296.3 nm for microflower structures. The increase in grain size was attributed to low molecular diffusion rate (without stirring) in the case of microflowers that allowed the growth of the grain size compared to the microplate, which was vigorously stirred allowing more seeds to form with small grain sizes.   where α, ν, A, and Eg are the absorption coefficient, light frequency, proportionality constant and band gap, respectively. The band gap energy is estimated from the extrapolation of the straight region of the αhv 1/2 versus energy graph to the x-axis. The band gap values for the microplate and microflower structures were 3.41 and 3.43 eV, respectively. The small difference in the band gap energy values was ascribed to the difference in the morphology of the synthesised materials. Therefore, any enhancement in the photocatalytic performance will be due to the morphology, surface area and surface defects differences [35].

SEM and EDS analysis
The morphologies of BiOCl microstructures were evaluated by scanning electron microscopy (SEM). Figure 3 shows the SEM images of the BiOCl obtained with vigorous stirring (figure 3(a)) and without stirring ( figure 3(b)). A vigorous stirring of the precursor solution results in the formation of microplates while microflower structures were observed without stirring. The formation of microflower structures is attributed to low number of nucleation sites formed in the absences of stirring movement allowing the microplate structure to grow further. In general, the Van der Waals forces through Cl atoms in the atomic layers of [Cl-Bi-O-Bi-Cl] allow these sheets to packed on each other to form a 3D flower-like construction [36]. This microflower structure is expected to enhance the photocatalytic performance of synthesized BiOCl sample by providing more active sites to degrade the BPA [37]. The elemental composition of the microflower sample was confirmed through EDS analysis ( figure 4). The sample was composed of three main elements; bismuth, oxygen and chlorine with an atomic ratio of Bi:O:Cl of 64.8:5.7:7.6 wt%. These results further confirmed the high purity of the synthesised BiOCl nanoflowers.  The Bi 4f doublet appears as two strong peaks with the splitting energy Δ=5.2 eV, which can be assigned to the Bi 4f 7/2 and Bi 4f 5/2 of Bi 3+ ( figure 6(b)). The Bi 4d doublet showed a peak split splitting energy, Δ of around 25 eV, again confirming the presence of Bi 3+ ( figure 6(b)). The O and Cl peaks were also observed at binding energy value around 535 and 203.6 eV (figures 6(c) and (d)). These binding energies values are well matched with previously reported findings [38,39].

Photocatalytic activity
Bisphenol A is known as a hard-to-degrade endocrine-disrupting chemical and was therefore selected as a model pollutant to determine the photocatalytic activity of the synthesised catalysts. The performance of the semiconductor photocatalytic microplates and microflowers was studied with the help of UV-vis absorption spectra by observing changes in the maximum absorbance of BPA at 278 nm ( figure 7). The band intensity at 278 nm gradually declines with an increase in the irradiation time, indicating the degradation of the BPA with BiOCl microplates (figure 7(a)). The degradation was faster at higher photocatalyst loading of 500 mg with near  complete degradation being realised after 240 min of light exposure ( figure 7(b)). These findings signify the importance of catalyst dosage optimization in photocatalytic experiments. The degradation efficiency is calculated using the equation: where, C 0 is the initial concentration of BPA and C is the concentration of BPA after treatment at time, t. The maximum degradation efficiency BPA achieved after 90 min of illumination with UV-B light was 80.12% for the BiOCl microflowers compare to 60.7% for the BiOCl microplates at a catalyst dosage of 500 mg ( figure 8). The experimental results showed enhancement photocatalytic activity for the microflowers, and this can be  attributed to the flower-like morphology, which offers more surface active sites and high adsorption capacity for the BPA. Photolysis showed barely any change in the concentration of the BPA, confirming the photostability of the pharmaceutical compound. The photocatalytic degradation mechanism was proposed based on the discussion on the performance of the microflowers ( figure 9). Under UV-B light irradiation, electrons are excited from the valance band to the conduction band of BiOCl simultaneously leaving holes in the valence band of the microflowers. The holes can scavenge H 2 O molecules to form highly reactive OH· radicals that can oxidise the BPA to the degradation byproducts. The electrons, on the other hand, also scavenge O 2 to form superoxide radicals (O 2 ·) that can equally oxidise the BPA. The high surface area of microflowers provides active sites for these reactions to occur. BPA molecules adsorb onto the surface of BiOCl by electrostatic attraction and get mineralised by non-selective radicals.

Conclusion
A simple approach to fabricate BiOCl microstructures using the hydrothermal method was demonstrated. Plate-and flower-like microstructures were obtained by varying the precursor solution stirring rate. The XRD analysis confirmed the tetragonal phase of BiOCl. The band gap calculated through Tauc plots was estimated to be 3.36 and 3.32 eV for microflowers and microplates, respectively. EDS and XPS analyses revealed that the  samples were of high purity. Tuning the morphology of the BiOCl enhances the photocatalytic performance as the microflowers exhibited higher photocatalytic activity compared to the microplates. A degradation efficiency of 80% was observed within 4 h of UV-B irradiation using the BiOCl microflowers,. Therefore, the adsorption of the target molecule on BiOCl surface is a crucial step toward efficient degradation of pollutants in water.