**1. Introduction**

2

Recent development has focused on creating newly sustainable, low-cost photocatalytic materials with a superior performance than the traditional semiconductor photocatalysts such as TiO2 and ZnO for self-cleaning applications [1]. In this direction, a huge potential is observed for the Bi-based semiconductors and their complexes [2–11]. 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 [13–17]. Among them, Bi2O3has 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]. Bi2O3 has four kinds of polymorphs which are designated as α for the monoclinic structure, βfor the tetragonal structure, γ for the body-centered cubic structure, and δ is for the face-centered cubic structure [19,20]. In several cases, Bi2O3has been reported for dye degradation, the photosynthesis of organic compounds, and water splitting for hydrogen generation [21–23]. To enhance the photocatalytic performance further, the structure and the surfaces of the Bi2O3 compound are tailored extensively. For example, the β phase of

**Citation:** Singh, V.P.; Kumar, M.; Sharma, M.; Mishra, D.; Seong, K.-S.; Park, S.-H.; Vaish, R. Synthesis of BiF3 and BiF3-Added Plaster of Paris Composites for Photocatalytic Applications. *Energies* **2021**, *14*, 5159. https://doi.org/10.3390/en14165159

Academic Editor: Wasim Khan

Received: 29 June 2021 Accepted: 14 August 2021 Published: 20 August 2021

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Bi2O3 is doped with Ti, which improves the photocatalytic activity comparatively [24]. In some cases, Bi2O3 has been further modified to form Bi2O3-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 [25–28]. In previous reports, Singh et al. demonstrated the modification of Bi2O3 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, Bi2O3 powder is used as the initial material and processed further with HF treatment and completely modified into the β phase of BiF3. Usually, BiF3 exists in two structures, cubic and orthorhombic, depicted as the α phase and β phase, respectively [31]. The α phase of BiF3 and its applications are reported most commonly for simple synthesis methods and low cost with high photo-activity [32–34]. In a reported research work by Chenkai Feng et al., the α phase of the BiF3 sample is prepared and then the photocatalytic performance is compared with the commercially available TiO2 powder sample [32]. Interestingly, the α phase of BiF3 is found 2.1 times superior to the TiO2 powder sample [32]. However, the preparation and applications of BiF3 having β phase are less explored. Therefore, in this work, after the preparation of the β phase of BiF3, we analyzed its photocatalytic property and compared it with the initially purchased Bi2O3powder. In order to explore the possible utilization of BiF3 for commercial application, it is important to look into a sustainable strategy. One of the methods could be in composite paints and coatings. BiF3 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, BiF3 embedded in a plaster of Paris (POP) matrix is fabricated by varying the BiF3 amounts from 0% to 10 wt%. Further, these POP-BiF3(%) 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.

### **2. Materials and Methods**

In the process of making POP-BiF3(%) composites, first, we prepared the powder of the BiF3 sample. We obtained a precipitation technique to prepare the BiF3 sample. The fixed amount (5 g) of pure Bi2O3 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 Bi2O3 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 Bi2O3-HF-1. Again, the remaining product of white powder of Bi2O3-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 Bi2O3-HF-1, and we procured the test sample2 named as sample Bi2O3-HF-2. Similarly, a test sample3 was procured and named as sample Bi2O3-HF-3 for further testing.

Next, by using the Bi2O3-HF-3 powder sample, we prepared POP-BiF3(%) composite pellets. In the composites, we maintained the concentration of Bi2O3-HF-3 powder in the POP matrix in accordance with 0, 1, 5, and 10 by wt%. With respect to each composition of POP-BiF3(%) composites, the calculated amount of Bi2O3-HF-3 powder sample and POP were mixed rigorously to prepare a homogeneous mixture, separately. The paste of each mixture of different POP-BiF3(%) composites was obtained by adding an equal amount of distilled water. Individually, the paste of different POP-BiF3(%) 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 Bi2O3, HF-treated Bi2O3 test samples, and POP-BiF3(%) 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 UV– visible 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.

### **3. Results and Discussion**

The systematic XRD results of Bi2O3 and the samples obtained from the successive fluorination of Bi2O3 via HF solution are shown in Figure 1. The step-wise fluorinated samples are denoted as Bi2O3-HF-1, Bi2O3-HF-2, and Bi2O3-HF-3, respectively. XRD of the purchased Bi2O3 sample was compared with the JCPDS file no 76–1730 and matched with the monoclinic phase. XRD results of the Bi2O3-HF-1 sample revealed that after the 1st washing of Bi2O3 powder, new diffraction peaks appeared in the X-ray diffraction pattern. A set of these new diffraction peaks was related to the intermediate phase of Bi1.2F2.4O0.6(PDF-36-0457), which are marked as '\*'. The other set of remaining peaks with less intensity belongs to the orthorhombic structure of BiF3 (PDF-70-2407). XRD results of the Bi2O3-HF-2 sample for a 2nd consecutive washing of the Bi2O3 powder showed the relative increase in the intensity of diffraction peaks belonging to the BiF3 (PDF-70-2407) phase structure at the expense of the diffraction peaks belonging to an intermediate phase of Bi1.2F2.4O0.6 (PDF-36-0457), relatively. The almost pure phase of BiF3 (PDF-70-2407) appeared after the 3rd consecutive washing of Bi2O3 powder in addition to a very small quantity of an unidentified impure phase which is marked as '#'. Thus, multiple washing of Bi2O3 powder through the concentrated HF solution led towards the formation of the almost pure orthorhombic structure of the BiF3 powder sample.

**Figure 1.** X-ray diffraction patterns of purchased Bi2O3 and Bi2O3 after three successive HF treatments depicted as Bi2O3-HF-1, Bi2O3-HF-2, Bi2O3-HF-3, and Bi2O3-HF-4, respectively.

Only a few solution techniques have been used for the formations of various phases of the BiF3 sample through different methods. For example, Feng et al. reported the formation of BiF3 (JCPDS: 51-0944) by a simple water-bath method, where they kept the molar ratio of Bi and F above 1:3, otherwise the impurity of Bi2O3 and BiOF remained present [32]. In this method, the constituent of Bi was obtained from Bi2O3 while the element of F was attained from the NH4F solution [32]. Zhao et al. reported the evolution of the BiF3 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 BiF3 via the hydrothermal method [35]. In another method, by using a novel ion-exchange approach, Kan et al. produced pure BiF3 (JCPDS: 73-1988). Here, the NH4F 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 Bi7F11 O5 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 Bi2O3 in a concentrated HF solution. The constituent of Bi was extracted from the powder of Bi2O3, while for the element of F in a concentrated HF solution was utilized. Bi2O3 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:

$$2\text{Bi}^{3+} + 6\text{F}^- \rightarrow 2\text{BiF}\_3 \rightarrow \text{(Concentrated HF)}\tag{1}$$

$$2\text{Bi}^{3+} + 2\text{H}\_2\text{O} + 6\text{F}^- \rightarrow 2\text{BiOF}\_4 + 4\text{HF} \text{ (Diluted HF)}\tag{2}$$

Multiple washing from HF solution increased the constituent of F and led towards the formation of BiF3 from Bi2O3 powder. Normally, if the Bi2O3 powder is washed from the concentrated HF solution, the positively charge Bi3+ ion reacts with F− ion and forms BiF3 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 Bi2O3 powder converted into BiF3 according to reaction mechanism one, while some of the Bi2O3 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 BiO0.51F1.98 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 BiO0.51F1.98 and converted it to the final product of the BiF3 sample. Multiple washing and high concentrations of HF increased the F concentration in the O/F ratio and led toward the cubicαBiF3 phase (PDF-073-1988) from the Bi2O3 powder.

Further, the BiF3 powder was investigated for its photocatalytic performance and compared with the Bi2O3 powder sample. A total of0.05 g powder of both Bi2O3 and BiF3were sonicated in 50-50 mL water solutions of the hazardous dye of Methylene blue (MB), separately, as test solutions. Before the photocatalytic investigation of Bi2O3 and BiF3 samples, to neglect the effect of the adsorption–desorption of the dye over the surfaces of these powders, first both the test solutions of Bi2O3 and BiF3 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 Bi2O3 and BiF3 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 Bi2O3 and BiF3 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–c. The decomposition of the MB solution probed via absorbance data of UV–visible spectroscopy confirmed a fast degradation of MB solution for the BiF3 powder sample as compared to the Bi2O3 powder. For comparison purposes, and to remove the artifacts during the photocatalytic reaction, the photolysis of the blank MB solution was obtained for the same parameters and for the same time scale as maintained for the photocatalytic decomposition of MB solutions for the Bi2O3 and BiF3 powder samples. A control reaction of photolysis of blank MB solution under the exposure of 355 nm light radiation showed a negligible change in the concentration. In 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) = kf t, the decomposition rate constant of MB solutions due to the photocatalytic reaction for both the powder samples were calculated and compared. Here, kf represents the rate constant of photocatalytic reactions, which was calculated as 0.0011 and 0.0103 min−<sup>1</sup> for the Bi2O3 and BiF3 powders, respectively. Thus, the photocatalytic degradation of the BiF3 powder sample was confirmed to be multiple times higher than the Bi2O3 powder sample. A comprehensive study of photocatalysis is shown in Table 1.

**Figure 2.** Absorption vs. wavelength spectra for (**a**) the photolysis of Methylene blue (MB), and the photocatalysis of MB under the exposure of UV light of 355 nm wavelength due to (**b**) Bi2O3powder and (**c**) BiF3 powder (**d**) respective C/Co vs. time plots, and (**e**) ln(C/Co) vs. time plots to obtain the pseudo-first-order reaction rate constant for the photolysis of MB and the photocatalysis of MB.

Four different composite pellets of POP–BiF3(%) were obtained. The BiF3 power was varied in the POP matrix in accordance with 0, 1, 5, and 10 by wt%. Here, the pellets of POP–BiF3(%) composites were named as POP–BiF3(0%), POP–BiF3(1%), POP–BiF3(5%), and POP–BiF3(10%), respectively. The composite formation and purity of POP–BiF3(0%), POP–BiF3(1%), POP–BiF3(5%), and POP–BiF3(10%) pellets were checked and verified through XRD (not shown here).

Again, the absorption of UV–visible radiation for POP–BiF3(0%), POP–BiF3(1%), POP– BiF3(5%), and POP–BiF3(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 BiF3 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–BiF3(%) composites, as shown in Figure 3b. Results showed that the bandgap tended to decrease with the addition of BiF3 in the POP matrix from 3.69 eV to 3.26 eV, respectively.

**Figure 3.** (**a**) Absorption spectra and (**b**) α2(hν)<sup>2</sup> vs. hν plots for the band gap (Eg) calculation of POP–BiF3 (%) composites with the variation of BiF3 concentration 0%, 1%, 5%, and 10% in POP matrix.

The variation in morphology of the POP–BiF3(%) composite pellets along with the change of BiF3 concentration in the POP matrix were probed through SEM and are shown in Figure 4a–d. The surface image of POP–BiF3(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–BiF3(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 BiF3-added POP–BiF3(0%), POP–BiF3(1%), POP–BiF3(5%), and POP–BiF3(10%) composite pellets revealed almost similar surfaces to the POP–BiF3(0%) pellet accompanied with several entangled random oriented rods forming the net-like porous structure. However, with the addition of BiF3 in the POP matrix, the porosity of the surface reduced in POP–BiF3(1%), POP–BiF3(5%), and POP– BiF3(10%) composite pellets, relatively, as compared to the surface of the POP–BiF3(0%) pellet. The addition of BiF3 may have filled the pores of POP and, therefore, led to a smooth and glassy surface in the POP–BiF3(10%) composite pellet.

Further, to check the photocatalytic performance of BiF3 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–BiF3(0%), POP–BiF3(1%), POP–BiF3(5%), and POP–BiF3(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–BiF3(0%), POP–BiF3(1%), POP–BiF3(5%), and POP–BiF3(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, ΔRGB'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–BiF3(%) composites, respectively.

**Figure 4.** Typical SEM images of POP–BiF3 (%) composites along with the concentration variation of BiF3 for (**a**) 0%, (**b**) 1%, (**c**) 5%, and (**d**) 10% in POP matrix.



**Figure 5.** Photo-reduction in Rz indicator ink for 3 h reaction timeline on POP–BiF3 (%) composites having the variation of BiF3 0%, 1%, 5%, and 10% by wt., respectively, under the exposure of 355 nm wavelength of UV light irradiation.

**Figure 6.** ΔRGB't vs. time plots for (**a**) blue and (**b**) red color change due to photo-reduction inRz indicator ink on POP–BiF3(%) composites having BiF3 0%, 1%, 5%, and 10% by wt., respectively, under the UV light irradiation.

In the present case, under the exposure of UV light on the POP–BiF3(0%) composite pellet, which had no BiF3 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– BiF3(1%), POP–BiF3(5%), and POP–BiF3(10%) composite pellets, the color of the Rz ink readily changed in proportion to the BiF3 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–BiF3(0%), POP–BiF3(1%), POP–BiF3(5%), and POP–BiF3(10%) composite pellets, respectively. The color of the Rz ink over the surface of the POP–BiF3(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).

$$BiF\_3 \stackrel{\text{luf}}{\rightarrow} BiF\_3(e^-, h^+) \tag{3}$$

$$\text{Gylcerool} \stackrel{h^+}{\rightarrow} OH + \text{Glycercic acid} \tag{4}$$

$$\text{Map}\_{\text{comm}}^{\mathcal{K}} \text{Cat}\_{\bullet} \xrightarrow[]{\text{Gan}} \text{Cat}\_{\bullet} \text{Cat}\_{\bullet} \text{Cat}\_{\bullet} \tag{5}$$

$$BiF\_3 \* \left(\mathcal{e}^-, h^+\right) \to BiF\_3 \tag{6}$$

At the same time of the image capture, the POP–BiF3(0%), POP–BiF3(1%), POP– BiF3(5%), and POP–BiF3(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–BiF3(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–BiF3(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– BiF3(0%), the POP–BiF3(1%), POP–BiF3(5%), and POP–BiF3(10%) composite pellets showed relatively faster kinetic rate of photocatalytic reduction in the Rz ink, respectively.

**Figure 7.** Absorption vs. wavelength spectra for the photocatalysis of Rz ink on the surface of POP–BiF3(%) composites with the variation of BiF3 concentration (**a**) 0%, (**b**) 1%, (**c**) 5%, and (**d**) 10% in POP matrix.

**Figure 8.** The C/Co vs. time plots for photocatalytic degradation of Rz ink for POP–BiF3(%) composites monitored at: (**a**) 581 nm and (**b**) 630 nm wavelength, and respective percentage of photocatalytic degradation for (**c**) 581 nm and (**d**) 630 nm wavelength.

For simplicity, the photoreduction due to UV light irradiation of Rz ink, which was coated on POP–BiF3 composite pellets, is shown in Figure 9. Under the illumination of UV light irradiation, the catalyst material BiF3 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 BiF3 in the POP matrix. In the present work, we obtained a maximum

content of BiF3 only up to 10% in the POP matrix. The Further addition of BiF3 in POP may have led to the decay in the mechanical strength of the architecture. The concentration of BiF3 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 BiF3 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 BiF3 as an activated photocatalytic material in the side of any cementitious material for self-cleaning of building constructions and a reduction in environmental pollution.

**Figure 9.** The schematic representation of the proposed mechanism for the photocatalytic degradation of pollutants under UVlight irradiation over POP–BiF3 composite pellet surfaces.
