Synthesis of BiF 3 and BiF 3 -Added Plaster of Paris Composites for Photocatalytic Applications

: A BiF 3 powder sample was prepared from the purchased Bi 2 O 3 powder via the precipitation route. The photocatalytic performance of the prepared BiF 3 powder was compared with the Bi 2 O 3 powder and recognized as superior. The prepared BiF 3 powder sample was added in a plaster of Paris (POP) matrix in the proportion of 0%, 1%, 5%, and 10% by wt% to form POP–BiF 3 (0%), POP–BiF 3 (1%), POP–BiF 3 (5%), and POP–BiF 3 (10%) composite pellets, respectively, and activated the photocatalytic property under the UV–light irradiation,in the POP. In this work, Resazurin (Rz) ink was utilized as an indicator to examine the photocatalytic activity and self-cleaning performance of POP–BiF 3 (0%), POP–BiF 3 (1%), POP–BiF 3 (5%), and POP–BiF 3 (10%) composite pellets. In addition to the digital photographic method, the UV–visible absorption technique was adopted to quantify the rate of the de-colorization of the Rz ink, which is a direct measure of comparative photocatalytic performance of samples.


Introduction
Recent development has focused on creating newly sustainable, low-cost photocatalytic materials with a superior performance than the traditional semiconductor photocatalysts such as TiO 2 and ZnO for self-cleaning applications [1]. In this direction, a huge potential is observed for the Bi-based semiconductors and their complexes [2][3][4][5][6][7][8][9][10][11]. This group of materials possess a direct band gap of a wide range from 2.5 eV to 3.2 eV and is severally reported for the fast rate of the creation of photo-induced charge carriers [12,13]. Moreover, most of all the Bi-based compounds show layered structures with polar surfaces and are found responsible for accelerating the separation of photo-generated carriers; consequently, reducing the recombination efficiency and showing a better photocatalytic activity even under the exposure of low intensity of light irradiation [13][14][15][16][17]. Among them, Bi 2 O 3 has been studied extensively and found to be superior photocatalytic, highly photoconductive, and nontoxic in nature, having a narrow band gap of about 2.8 eV [15,16,18]. Bi 2 O 3 has four kinds of polymorphs which are designated as α for the monoclinic structure, β for the tetragonal structure, γ for the body-centered cubic structure, and δ is for the face-centered cubic structure [19,20]. In several cases, Bi 2 O 3 has been reported for dye degradation, the photosynthesis of organic compounds, and water splitting for hydrogen generation [21][22][23]. To enhance the photocatalytic performance further, the structure and the surfaces of the Bi 2 O 3 compound are tailored extensively. For example, the β phase of Bi 2 O 3 is doped with Ti, which improves the photocatalytic activity comparatively [24]. In some cases, Bi 2 O 3 has been further modified to form Bi 2 O 3 -based complexes with several other materials to create heterojunctions, which further supported the creation and separation of the photo-induced charge carriers in the heterointerface [25][26][27][28]. In previous reports, Singh et al. demonstrated the modification of Bi 2 O 3 or Bi-based compounds through halogenations which lead to the formation of BiOCl and BiOF compounds, respectively, with a huge advancement in photocatalytic and self-cleaning properties [29,30].
Similarly, in the present work, Bi 2 O 3 powder is used as the initial material and processed further with HF treatment and completely modified into the β phase of BiF 3 . Usually, BiF 3 exists in two structures, cubic and orthorhombic, depicted as the α phase and β phase, respectively [31]. The α phase of BiF 3 and its applications are reported most commonly for simple synthesis methods and low cost with high photo-activity [32][33][34]. In a reported research work by Chenkai Feng et al., the α phase of the BiF 3 sample is prepared and then the photocatalytic performance is compared with the commercially available TiO 2 powder sample [32]. Interestingly, the α phase of BiF 3 is found 2.1 times superior to the TiO 2 powder sample [32]. However, the preparation and applications of BiF 3 having β phase are less explored. Therefore, in this work, after the preparation of the β phase of BiF 3 , we analyzed its photocatalytic property and compared it with the initially purchased Bi 2 O 3 powder. In order to explore the possible utilization of BiF 3 for commercial application, it is important to look into a sustainable strategy. One of the methods could be in composite paints and coatings. BiF 3 can be physically mixed with any well-known and widely used materials, such as cement-based paints and other ceramic coatings. Plaster of Paris is known for the aesthetics and decoration material. It is also used in medicine to make casts for broken bones. To explore photocatalysis-based effects in plaster of Paris, it may be used for air cleaning as well as antibacterial properties. Hence, BiF 3 embedded in a plaster of Paris (POP) matrix is fabricated by varying the BiF 3 amounts from 0% to 10 wt%. Further, these POP-BiF 3 (%) compositions are tested for the photocatalytic response on Resazurin (Rz) ink. Rz ink is used as a prototype carcinogenic pollutant and an indicator of photocatalytic performance.

Materials and Methods
In the process of making POP-BiF 3 (%) composites, first, we prepared the powder of the BiF 3 sample. We obtained a precipitation technique to prepare the BiF 3 sample. The fixed amount (5 g) of pure Bi 2 O 3 powder of AR grade (Hi-media, ≥99%) was dissolved in the hydrofluoric (HF) acid (Qualikems 40%, Vadodara, India) solution (30 mL) and stirred for 30 min. Under the stirring with HF solution, the yellow color of Bi 2 O 3 gradually changed into white-color powder. The product of white powder was washed in distilled water several times followed by acetone and dried at 80 • C for 24 h in a vacuum oven. A fraction of the sample was collected for testing, named sample Bi 2 O 3 -HF-1. Again, the remaining product of white powder of Bi 2 O 3 -HF-1 sample was dissolved in the concentrated HF solution and stirred for 30 min. The output product obtained after the second treatment from HF solution was followed with the same procedures of washing, filtering, and drying as for the sample Bi 2 O 3 -HF-1, and we procured the test sample2 named as sample Bi 2 O 3 -HF-2. Similarly, a test sample3 was procured and named as sample Bi 2 O 3 -HF-3 for further testing.
Next, by using the Bi 2 O 3 -HF-3 powder sample, we prepared POP-BiF 3 (%) composite pellets. In the composites, we maintained the concentration of Bi 2 O 3 -HF-3 powder in the POP matrix in accordance with 0, 1, 5, and 10 by wt%. With respect to each composition of POP-BiF 3 (%) composites, the calculated amount of Bi 2 O 3 -HF-3 powder sample and POP were mixed rigorously to prepare a homogeneous mixture, separately. The paste of each mixture of different POP-BiF 3 (%) composites was obtained by adding an equal amount of distilled water. Individually, the paste of different POP-BiF 3 (%) composites was transferred into cylindrical molds of 20 mm diameter and 10 mm height to prepare the pellets of each composition, respectively. Finally, the pellets were left to naturally dry fortwo days.
The structural analysis of Bi 2 O 3 , HF-treated Bi 2 O 3 test samples, and POP-BiF 3 (%) composite pellets was performed through X-ray diffraction (XRD) (Rigaku), having 9 kW rotating anode and Cu Kα source. Microstructure analysis was obtained from FE-SEM (Inspect™S50). Optical property and photocatalytic performance were tested via UVvisible spectrophotometer of double beam (Thermo Scientific, Evolution 220, Waltham, MA, USA). In addition, to carry out the photocatalytic reaction, we used a box inbuilt with a lamp (Hitachi FL8BL-Blight) as a UV light source having maximum emission~355 nm wavelengths. The distance between the lamps and the samples was adjusted such that the intensity falling on the samples was maintained at about~3200 lx.

Results and Discussion
The systematic XRD results of   [32]. In this method, the constituent of Bi was obtained from Bi 2 O 3 while the element of F was attained from the NH 4 F solution [32]. Zhao et al. reported the evolution of the BiF 3 nanocrystals in various shapes from monodispersed nano-plates to nano-rods and then to nanowires via the novel acid-base couple extraction route and tuning the molar ratio of F vs. Bi [31]. Sarkar et al. used Poly (vinyl pyrrolidone) (PVP) for the encapsulation and formation of cubic nanocrystals of BiF 3 via the hydrothermal method [35]. In another method, by using a novel ion-exchange approach, Kan et al. produced pure BiF 3 (JCPDS: 73-1988). Here, the NH 4 F solution was used for the constituent of F while BiOCl was utilized to attain the element of Bi in accordance with the molar ratio (RF = F/Bi) of 8:1 [36]. Below to the molar ratio of 8:1 (RF), the final product consisted of a small amount of Bi 7 F 11 O 5 as an impurity phase [36].
Contrary to the above reported studies, in the present method, the ratio of O/F was controlled via a chemical bath of Bi 2 O 3 in a concentrated HF solution. The constituent of Bi was extracted from the powder of Bi 2 O 3 , while for the element of F in a concentrated HF solution was utilized. Bi 2 O 3 powder was washed several times from the concentrated HF solution, which may have led to two types of products, as follows in the reaction mechanisms one and two given below: Multiple washing from HF solution increased the constituent of F and led towards the formation of BiF 3 from Bi 2 O 3 powder. Normally, if the Bi 2 O 3 powder is washed from the concentrated HF solution, the positively charge Bi 3+ ion reacts with F − ion and forms BiF 3 and follows reaction mechanism one. However, in the HF solution, some water content always remains present; therefore, in the case of the 1st washing, some of the Bi 2 O 3 powder converted into BiF 3 according to reaction mechanism one, while some of the Bi 2 O 3 powder reacted with HF as well as the water content, followed reaction mechanism2 and formed an intermediate product of the BiOF family, which was recognized as BiO 0.51 F 1.98 in the present case. Further, the 2nd and 3rd washing provided more and more F − ion in the solution which again reacted with the intermediate product of BiO 0.51 F 1.98 and converted it to the final product of the BiF 3 sample. Multiple washing and high concentrations of HF increased the F concentration in the O/F ratio and led toward the cubic-αBiF 3 phase (PDF-073-1988) from the Bi 2 O 3 powder.
Further, the BiF 3 powder was investigated for its photocatalytic performance and compared with the Bi 2 O 3 powder sample. A total of0.05 g powder of both Bi 2 O 3 and BiF 3 were sonicated in 50-50 mL water solutions of the hazardous dye of Methylene blue (MB), separately, as test solutions. Before the photocatalytic investigation of Bi 2 O 3 and BiF 3 samples, to neglect the effect of the adsorption-desorption of the dye over the surfaces of these powders, first both the test solutions of Bi 2 O 3 and BiF 3 in MB were sonicated under dark for a 30 min duration to achieve the adsorption-desorption equilibrium. Then, both the test solutions were transferred under the UV light irradiation of a 355 nm wavelength. To probe the photocatalytic activity of the Bi 2 O 3 and BiF 3 powder samples, 1-1 mL of the MB was collected in a separate Eppendorf from both the test solutions at fixed time intervals. Finally, the absorption study of the collected samples was carried out by using a UV-visible spectrophotometer. After the fixed time of reaction, both the solutions were sonicated for five minutes to maintain the homogeneity and, then, 1-1 mL of the MB was collected. Each collected sample was centrifuged to remove the segregated residual from the solutions. At last, the photocatalytic decomposition of MB for each collected sample from both the set of solutions immersed with Bi 2 O 3 and BiF 3 powders were tested from UV-visible spectroscopy, respectively.
The adsorption-desorption reaction analysis for both the samples under the dark environment showed an insignificant change in the concentration of MB solutions (absorbance results probed by UV-visible spectroscopy is not shown here). After adsorption-desorption, the photolysis and photocatalytic results of decomposition, as well as the kinetic rate of the decomposition of MB solutions under UV exposure, were plotted and represented in Figure 2a Figure 2d,e, the Co stands for initial concentration, while Ct represents the time-dependent concentration of the MB solution after the photocatalytic decomposition. Using the parameters Co and Ct and their relation with time, i.e., ln(C/Co) = k f t, the decomposition rate constant of MB solutions due to the photocatalytic reaction for both the powder samples were calculated and compared. Here, k f represents the rate constant of photocatalytic reactions, which was calculated as 0.0011 and 0.0103 min −1 for the Bi 2 O 3 and BiF 3 powders, respectively. Thus, the photocatalytic degradation of the BiF 3 powder sample was confirmed to be multiple times higher than the Bi 2 O 3 powder sample. A comprehensive study of photocatalysis is shown in Table 1. Four different composite pellets of POP-BiF 3 (%) were obtained. The BiF 3 power was varied in the POP matrix in accordance with 0, 1, 5, and 10 by wt%. Here, the pellets of POP-BiF 3 (%) composites were named as POP-BiF 3 (0%), POP-BiF 3 (1%), POP-BiF 3 (5%), and POP-BiF 3 (10%), respectively. The composite formation and purity of POP-BiF 3 (0%), POP-BiF 3 (1%), POP-BiF 3 (5%), and POP-BiF 3 (10%) pellets were checked and verified through XRD (not shown here).
Again, the absorption of UV-visible radiation for POP-BiF 3 (0%), POP-BiF 3 (1%), POP-BiF 3 (5%), and POP-BiF 3 (10%) pellets was probed and utilized for the bandgap calculations according to the Kubelka-Munk model [37]. Results are shown in Figure 3a,b. Figure 3a demonstrates the gradual increase in the absorption from the visible to UV region for the POP sample. Adding BiF 3 in the POP matrix improved the absorption towards the visible range, relatively. The results of the absorption coefficient (α) vs. incident photon energy E (hν) obtained from Figure 3a were extrapolated according to the Kubelka-Munk model to calculate the direct bandgap of POP and POP-BiF 3 (%) composites, as shown in Figure 3b. Results showed that the bandgap tended to decrease with the addition of BiF 3 in the POP matrix from 3.69 eV to 3.26 eV, respectively. The variation in morphology of the POP-BiF 3 (%) composite pellets along with the change of BiF 3 concentration in the POP matrix were probed through SEM and are shown in Figure 4a-d. The surface image of POP-BiF 3 (0%) pellet stands for the pure POP sample and shows a homogeneous rod-shape microstructure. These rods were entangled with each other and made net-like porous surfaces. One end of the rod was defused in the surface while the other end seemed to emerge from the surface and dangle. Such entangled rods over the surface of the POP-BiF 3 (0%) pellet were due to water treatment and the continuous hydration of POP which led to the continuous nucleation and growth of rods in random orientations. SEM images of the BiF 3 -added POP-BiF 3 (0%), POP-BiF 3 (1%), POP-BiF 3 (5%), and POP-BiF 3 (10%) composite pellets revealed almost similar surfaces to the POP-BiF 3 (0%) pellet accompanied with several entangled random oriented rods forming the net-like porous structure. However, with the addition of BiF 3 in the POP matrix, the porosity of the surface reduced in POP-BiF 3 (1%), POP-BiF 3 (5%), and POP-BiF 3 (10%) composite pellets, relatively, as compared to the surface of the POP-BiF 3 (0%) pellet. The addition of BiF 3 may have filled the pores of POP and, therefore, led to a smooth and glassy surface in the POP-BiF 3 (10%) composite pellet.
Further, to check the photocatalytic performance of BiF 3 in the POP matrix, the Rz ink was prepared by using a known reported method [38]. Rz ink is an indicator of self-cleaning/photocatalysis, which changes its color if coated over any surface of photocatalytic material under the exposure of UV irradiation. Therefore, an equal amount of ink was pasted over each surface of POP-BiF 3 (0%), POP-BiF 3 (1%), POP-BiF 3 (5%), and POP-BiF 3 (10%) composite pellets. These pellets were subjected to UV light (355 nm) illumination with equal exposures and monitored with time on several fixed intervals for 180 min of time duration. Each time, surface images of POP-BiF 3 (0%), POP-BiF 3 (1%), POP-BiF 3 (5%), and POP-BiF 3 (10%) composite pellets were obtained via a high-quality digital camera. For comparison purposes, all captured images on certain time intervals were compiled together and shown in Figure 5. In correlation to Figure 5, ∆RGB't vs. time is plotted and shown as Figure 6a,b, which represents the change in blue as Figure 6a and change in red color as Figure 6b due to photo-reduction in the Rz indicator ink under the UV light irradiation on POP-BiF 3 (%) composites, respectively.    In the present case, under the exposure of UV light on the POP-BiF 3 (0%) composite pellet, which had no BiF 3 content, showed no color change on the surface up to the 30 min of time duration, while a slight change after 60 min to 180 min was monitored. This slight change from a blue to purple color was due to the photolysis of the Rz ink. For POP-BiF 3 (1%), POP-BiF 3 (5%), and POP-BiF 3 (10%) composite pellets, the color of the Rz ink readily changed in proportion to the BiF 3 content in POP from royal blue to pink and then into colorless ink. The rate of change in the color of the Rz ink was systematic and increased for POP-BiF 3 (0%), POP-BiF 3 (1%), POP-BiF 3 (5%), and POP-BiF 3 (10%) composite pellets, respectively. The color of the Rz ink over the surface of the POP-BiF 3 (10%) composite pellet appeared almost colorless within the 180 min time period.
Usually, the rate of the change of color of the Rz ink indicates the rate of photocatalytic reduction. The reaction mechanism of photocatalytic reduction and the color-change of the Rz ink are mentioned as under Equations (3)- (6).
At the same time of the image capture, the POP-BiF 3 (0%), POP-BiF 3 (1%), POP-BiF 3 (5%), and POP-BiF 3 (10%) composite pellets were again tested through UV-visible spectroscopy for a quantitative analysis of the photocatalytic reduction in the Rz ink coated over the surfaces. Absorption peak intensities corresponding to the photo catalytic reduction were monitored at two different wavelengths, i.e., 630 nm and 581 nm. The absorption spectra obtained from the UV-visible spectroscopy is illustrated in Figure 7a-d. The peak intensity monitored at the 630 nm wavelength directly correlated to the blue color of the Rz ink and with the exposure of UV light (355 nm). The peak intensity of absorbance decreased at the wavelength of 630 nm along with the color change from blue to pink, resulting in the formation of resorufin (Rf) as a byproduct. The intensity decay of another peak at 581 nm represented the photobleaching of the Rf molecule as and when the color changed from pink to colorless. Under the effect of UV light irradiation, a negligible change in the intensity of characteristic absorption of the Rz ink was observed for the POP-BiF 3 (0%) composite pellet; however, the absorption intensity of the Rz ink decreased consistently for all the other samples just in accordance with the color change observed in the digital photographs as shown in Figure 5. The absorption results are shown in Figure 7a-d was further utilized to extract the kinetic rate of photocatalytic reduction and photo mineralization of intermediates of the Rz ink monitored for both the wavelengths at 630 nm and 581 nm, respectively, shown in Figure 8. In Figure 8, Co represents the initial (t = 0) absorbance of the Rz ink and C is the absorbance of the Rz ink, which varied with time t. The Co and C and kinetic rate of photocatalytic reduction was calculated corresponding to the absorption spectra monitored for both the wavelengths of absorbance, i.e., 581 nm and 630 nm of Rz ink. For the POP-BiF 3 (0%) sample, the results showed that the intensity decay, as well as the kinetic rate of reaction due to the photocatalytic degradation of the Rz ink corresponding to the wavelengths monitored at 581 nm and 630 nm for 180 min of time duration, was less. Generally, the photocatalytic reaction rate was monitored to correspond faster to the absorbance at 630 nm wavelengths in comparison to the absorbance around the 581 nm wavelength for all samples. As compared to POP-BiF 3 (0%), the POP-BiF 3 (1%), POP-BiF 3 (5%), and POP-BiF 3 (10%) composite pellets showed relatively faster kinetic rate of photocatalytic reduction in the Rz ink, respectively.  For simplicity, the photoreduction due to UV light irradiation of Rz ink, which was coated on POP-BiF 3 composite pellets, is shown in Figure 9. Under the illumination of UV light irradiation, the catalyst material BiF 3 present in POP generated sacrificial donor electrons on its surface which reacted with the photo-induced holes of the Rz ink. Therefore, the glycerol present in the Rz ink utilized this sacrificial donor electron and generated an •OH radicle along with glyceric acid as a by-product. At the same time, these intermediate •OH radicles reduced the blue color of the Rz ink into Rf of the pink color after the longtime illumination of UV light irradiation, resulting in the reduction in the Rf molecule into a colorless product. Similar to the Rz ink, the harmful pollutants may have also reduced into non-harmful products with the aid of hydroxyl radicals generated on the surface of the photocatalytic BiF 3 in the POP matrix. In the present work, we obtained a maximum content of BiF 3 only up to 10% in the POP matrix. The Further addition of BiF 3 in POP may have led to the decay in the mechanical strength of the architecture. The concentration of BiF 3 inside the POP matrix along with an optimized photocatalytic performance and superior strength of the structures are a further matter of research. Thus, the above results demonstrate the successful photocatalytic application of BiF 3 inside the POP matrix. POP is an important cementitious material used for the making of several building constructions and sculptures. We propose here for the addition of BiF 3 as an activated photocatalytic material in the side of any cementitious material for self-cleaning of building constructions and a reduction in environmental pollution.

Conclusions
Successfully, theBiF 3 powder sample was prepared via the precipitation route simply by washing the purchased powder of Bi 2 O 3 several times into the concentrated HF solution. After several washes through the concentrated HF solution, the Bi 2 O 3 powder was systematically transformed into BiF 3 powder in conjunction with each washing. An intermediate phase of Bi 1.2 F 2.4 O 0.6 was identified for the 1st and 2nd washing of the Bi 2 O 3 powder, which completely vanished after the 3rd washing and converted into the BiF 3 powder sample. The photocatalytic performance of the as-prepared BiF 3 powder was tested and compared with the Bi 2 O 3 powder on a hazardous industrial waste solution of MB dye. The BiF 3 powder rapidly decomposed the MB solution under the visible light illumination as compared to the purchased Bi 2 O 3 powder sample. Usually, POP is used vastly in the construction as well as the building of sculptures and, therefore, is always exposed to water as well as air pollutants. Therefore, to check the self-cleaning activity from such pollutants and introduce the photocatalytic properties in POP, BiF 3 powders were added according to 0%, 1%, 5%, and 10% by wt% in the POP matrix. The photocatalytic and self-cleaning properties of these POP-BiF 3 composites were demonstrated by using a well-known photocatalysis indicator ink of Rz under visible light exposure. Due to the photocatalytic effect of POP-BiF 3 composites under the solar illumination, the blue color of the Rz ink turned into Rf of the pink color. The long-time illumination of visible light resulted from the reduction in the pink color Rf molecules into the colorless product. Photocatalytic performances were improved linearly for the incorporation of BiF 3 up to 10% of the concentration in the POP matrix.