Facile Synthesis of Nano-Flower β-Bi2O3/TiO2 Heterojunction as Photocatalyst for Degradation RhB

Photocatalysis is a hopeful technology to solve various environmental problems, but it is still a technical task to produce large-scale photocatalysts in a simple and sustainable way. Here, nano-flower β-Bi2O3/TiO2 composites were prepared via a facile solvothermal method, and the photocatalytic performances of β-Bi2O3/TiO2 composites with different Bi/Ti molar ratios were studied. The nano-flower Bi2O3/TiO2 composites were studied by SEM, XRD, XPS, BET, and PL. The PL result proved that the construction of staggered heterojunction enhanced the separation efficiency of carriers. The degradation RhB was applied to study the photocatalytic performances of prepared materials. The results showed that the degradation efficiency of RhB increased from 61.2% to 99.6% when the molar ratio of Bi/Ti was 2.1%. It is a mesoporous approach to enhance photocatalytic properties by forming heterojunction in Bi2O3/TiO2 composites, which increases the separation efficiency of the generated carriers and improves photocatalytic properties. The photoactivity of the Bi2O3/TiO2 has no evident changes after the fifth recovery, indicating that the Bi2O3/TiO2 composite has distinguished stability.


Introduction
Environmental pollution and destruction are becoming a vertical global crisis. Organic dye pollutants are directly discharged into water from dyeing textiles. Most dyes, such as RhB, MB, MR, and EY, are non-biodegradable and carcinogenic, which has brought damage to environment and human health [1]. Therefore, developing an environmental harmony, ecological cleanliness, and safe and energy-saving treatment technology for the problem of environment contamination is the most important immediate challenge that human beings must face [2][3][4].
Photocatalysis offers a clean, gentle reaction conditions, an easy procedure, and safe technology that can degrade the pollution under solar irradiation [4][5][6][7]. Photocatalytic technology plays a significant role in the field of environmental pollution protection [8][9][10]. Metal oxide semiconductors have garnered great attention in pollution treatment owing to their cheap and easy synthesis [11,12]. As a classic semiconductor, titanium dioxide (TiO 2 ) is the one most generally cited in photocatalytic technology in virtue of its chemical stability, nontoxicity, low price, and high activity [13]. The photocatalytic reaction system and reaction mechanism of TiO 2 were studied [14,15]. In particular, TiO 2 photocatalysts have been explored broadly in the area of environmental pollution and energy transformation [16][17][18]. However, the TiO 2 photocatalysts also expose many problems, such as the large band gap (3.2 eV), absorption narrow wavelength range light, easy recombination with photogenerated electrons, and holes, which bring about lower photocatalytic efficiency, limiting its application [19,20]. For this reason, modification of TiO 2 for enhancing its photocatalytic properties is needed and crucial. Many different means have been tried by researchers to change the surface or comprehensive performances of TiO 2 , for instance, doping metal [21], structural adjustment [22], and heterogeneous structure [23,24], thereby promoting the photocatalytic properties. In these modifications, construction of heterojunctions is a promising strategy to improve the separation efficiency of photogenerated charge carriers. TiO 2 united with narrow bandgap semiconductors that are visible-light-responsive would extend the light absorption range and decrease the combination of photogenerated charge carriers, for example, TiO 2 /ZnFe 2 O 4 [25], CdS/TiO 2 [26], g-C 3 N 4 /TiO 2 [27], and InVO 4 /TiO 2 [28].
Among these visible-light-responsive and narrow band gap semiconductors, bismuth oxide (Bi 2 O 3 ) is an up-and-coming semiconductor to construct heterojunctions with TiO 2 , in which the band gap is lower than that of TiO 2 [29]. When Bi 2 O 3 is irradiated with visible light, photogenerated holes of the valence band of Bi 2 O 3 have strong oxidation, which is beneficial to degrade the pollutions. In addition, Bi 2 O 3 has been under comprehensive study due to its excellent physical and chemical properties, low price, and non-toxicity [30][31][32]. Bi 2 O 3 has α, β, γ, δ, ε, and ω phases, i.e., six kinds of polymorphs [33]. In particular, β-Bi 2 O 3 possesses high light-absorption performances among the six polymorphs, and the band gap is about 2.3 eV [34]. Therefore, the construction of β-Bi 2 O 3 /TiO 2 heterojunction structures can be carried out to increase the light response and photocatalytic performances. However, for the case of β-Bi 2 O 3 /TiO 2 photocatalytic composites, the preparation methods are complex, including a two-step procedure, photo-deposition, deposition-reduction, and impregnation method [35,36].
Here, in this paper, the β-Bi 2 O 3 /TiO 2 heterojunction photocatalysts were prepared via a simple, one-step solvothermal method. The influences of the molar ratios of β-Bi 2 O 3 /TiO 2 on the photocatalytic degradation of RhB for β-Bi 2 O 3 /TiO 2 were assessed. In the meantime, the morphology, crystal structure, surface chemical optical state, and photoelectrochemical properties of photocatalysts were investigated.  [37]. For pure Bi 2 O 3 , the diffraction peaks are corresponding to β-Bi 2 O 3 [32]. For Bi 2 O 3 /TiO 2 composites with different molar ratios of Bi, it is evident that the diffraction peaks are similar to pure TiO 2 and correspond to the anatase TiO 2 . The diffraction peak of the Bi phase is not observed in Bi 2 O 3 /TiO 2 composites for several reasons. The size of Bi is extremely small and highly dispersed, and the doping amount of Bi is too small [36]. There are similar results in the reported literatures [38]. For example, Zhu et al. [39] found that Bi 2 O 3 /TiO 2 lacks the Bi 2 O 3 diffraction even when the Bi/Ti ratio in Bi 2 O 3 /TiO 2 was 4.1%.

Characterization of Photocatalysts
The morphologies of materials were analyzed via SEM and HRTEM, and the images are displayed in Figures 2 and 3. In Figure 2a, the pure Bi 2 O 3 reveals a stuffed, coral-like sphere with a diameter between 50-150 nm. The pure TiO 2 exhibits a flower-like structure ( Figure 2b). As shown in Figure 2c-f, the Bi 2 O 3 /TiO 2 composites in different molar ratios of Bi display a flower-like structure that is similar to the morphology of pure TiO 2 . It indicates that the incorporation of Bi 2 O 3 does not influence the topography of TiO 2 . In addition, the diameter of the flower-like structure of Bi 2 O 3 /TiO 2 composites is between 5 µm-10 µm, and the thickness diameter of the flower-like structure is about 65 nm. Meanwhile, the EDS was applied to investigate the elemental contents of 2.1% Bi 2 O 3 /TiO 2 , and the atomic ratio of Bi/Ti is about 2.9%, as displayed in Table 1. The morphologies of materials were analyzed via SEM and HRTEM, and the images are displayed in Figures 2 and 3. In Figure 2a, the pure Bi2O3 reveals a stuffed, coral-like sphere with a diameter between 50-150 nm. The pure TiO2 exhibits a flower-like structure ( Figure 2b). As shown in Figure 2c-f, the Bi2O3/TiO2 composites in different molar ratios of Bi display a flower-like structure that is similar to the morphology of pure TiO2. It indicates that the incorporation of Bi2O3 does not influence the topography of TiO2. In addition, the diameter of the flower-like structure of Bi2O3/TiO2 composites is between 5 μm-10 μm, and the thickness diameter of the flower-like structure is about 65 nm. Meanwhile, the EDS was applied to investigate the elemental contents of 2.1% Bi2O3/TiO2, and the atomic ratio of Bi/Ti is about 2.9%, as displayed in Table 1. From the TEM images of Bi2O3/TiO2 composites (Figure 3a), some nanoparticles with a diameter between 2~3 nm are evenly spread on the TiO2. The HRTEM image of Bi2O3/TiO2 composites (Figure 3b) shows that the lattice fringe is about 0.335 nm, assigned to (110) of β-Bi2O3. This indicates that β-Bi2O3 is present in the Bi2O3/TiO2 composites. Moreover, the crystal lattice fringe of 0.365 nm is corresponding to the (101) plane of anatase TiO2. This indicates that the Bi2O3 and TiO2 are closely interlinked. Due the deposition of Bi2O3 on TiO2, it could be profitable to the carriers migrate between Bi2O3 and TiO2 and enhance the separation efficiency of carriers as well as the photocatalytic properties. Furthermore, the crystallite size of Bi2O3/TiO2 composites decreased compared with Bi2O3 and TiO2 [40].  The chemical compositions and element valence state of 2.1%Bi2O3/TiO2 photocatalysts were studied through XPS. Figure 4a displays the XPS survey scan of 2.1%Bi2O3/TiO2, which demonstrates that the presence of Bi, O, and Ti elements can be seen in Bi2O3/TiO2 composites. It indicates that the Bi2O3/TiO2 composites managed to compound by sol-  The chemical compositions and element valence state of 2.1%Bi2O3/TiO2 photocatalysts were studied through XPS. Figure 4a displays the XPS survey scan of 2.1%Bi2O3/TiO2, which demonstrates that the presence of Bi, O, and Ti elements can be seen in Bi2O3/TiO2 composites. It indicates that the Bi2O3/TiO2 composites managed to compound by sol-   [40].
The chemical compositions and element valence state of 2.1%Bi 2 O 3 /TiO 2 photocatalysts were studied through XPS. Figure 4a displays the XPS survey scan of 2.1%Bi 2 O 3 /TiO 2 , which demonstrates that the presence of Bi, O, and Ti elements can be seen in Bi 2 O 3 /TiO 2 composites. It indicates that the Bi 2 O 3 /TiO 2 composites managed to compound by solvothermal method. The high-resolution XPS for Bi 4f of Bi 2 O 3 /TiO 2 composites is displayed in Figure 4b, and the two main peaks at 159.19 eV and 164.43 eV are corresponding to the Bi 4f 7/2 and Bi 4f 5/2 , respectively, which indicates the presence of Bi 3+ [41]. In additional, the peaks at 157.43 eV and 162.63 eV are attributed to the Bi 0 (metallic Bi) [42], indicating that the Bi 3+ [44]. It is worth noting that the binding energies of Ti 2p and O1s in 2.1% Bi 2 O 3 /TiO 2 are shifted towards lower binding energies compared to TiO 2 , while Bi 4f binding energy is shifted to high binding energy compared to B 2 O 3 . In conclusion, the XPS results indicate strong interactions and electron transfer between Bi 2 O 3 and TiO 2 in Bi 2 O 3 /TiO 2 .
The specific surface areas are important elements to influence the catalytic properties. Therefore, the nitrogen adsorption-desorption isotherm was employed to evaluate the BET surface area of β-Bi 2 O 3 , TiO 2 , and Bi 2 O 3 /TiO 2 photocatalysts, and the results are shown in Figure 5. The isotherms of all photocatalysts are typical type IV, and the H3 hysteresis loop in the light of IUPAC classification can be observed [45]. It indicates that the β-Bi 2 O 3 , TiO 2 , and Bi 2 O 3 /TiO 2 photocatalysts are mesoporous structures. The BET specific surface areas of β-Bi 2 O 3 , TiO 2 , and Bi 2 O 3 /TiO 2 photocatalysts are 3.1, 8.7, and 42.0 m 2 /g, respectively. Apparently, the specific surface area of Bi 2 O 3 /TiO 2 composites is considerably increased after the fusion of Bi 2 O 3 on TiO 2 compared with pure TiO 2 . With the increase of calcination temperature, Bi 5 O 7 NO 3 gradually transformed to β-Bi 2 O 3 , and the following decomposition reaction (Bi 5 O 7 NO 3 → 5/2Bi 2 O 3 +NO+3/4O 2 ) occurs at the calcination temperature. The products of NO and O 2 during the decomposition of Bi 5 O 7 NO 3 will affect the crystallization process of TiO 2 , thus making the TiO 2 structure looser and producing many pores, increasing the specific surface area of the composite. From the SEM results, it can be seen that more holes are formed after the combination of Bi 2 O 3 and TiO 2 , increasing the specific surface area. In addition, it was reported that the higher specific surface area was frequently accompanied by higher adsorption properties and active sites [46]. Therefore, the Bi 2 O 3 /TiO 2 composites have a higher adsorption capacity and active sites, indicating the improvement of photocatalytic properties.
Molecules 2023, 28, x FOR PEER REVIEW 5 of 13 energy compared to B2O3. In conclusion, the XPS results indicate strong interactions and electron transfer between Bi2O3 and TiO2 in Bi2O3/TiO2. The specific surface areas are important elements to influence the catalytic properties. Therefore, the nitrogen adsorption-desorption isotherm was employed to evaluate the BET surface area of β-Bi2O3, TiO2, and Bi2O3/TiO2 photocatalysts, and the results are shown in Figure 5. The isotherms of all photocatalysts are typical type IV, and the H3 hysteresis loop in the light of IUPAC classification can be observed [45]. It indicates that the β-Bi2O3, TiO2, and Bi2O3/TiO2 photocatalysts are mesoporous structures. The BET specific surface areas of β-Bi2O3, TiO2, and Bi2O3/TiO2 photocatalysts are 3.1, 8.7, and 42.0 m 2 /g, respectively. Apparently, the specific surface area of Bi2O3/TiO2 composites is considerably increased after the fusion of Bi2O3 on TiO2 compared with pure TiO2. With the increase of calcination temperature, Bi5O7NO3 gradually transformed to β-Bi2O3, and the following decomposition reaction (Bi5O7NO3 → 5/2Bi2O3+NO+3/4O2) occurs at the calcination temperature. The products of NO and O2 during the decomposition of Bi5O7NO3 will affect the crystallization process of TiO2, thus making the TiO2 structure looser and producing many pores, increasing the specific surface area of the composite. From the SEM results, it can be seen that more holes are formed after the combination of Bi2O3 and TiO2, increasing the specific surface area. In addition, it was reported that the higher specific surface area was frequently accompanied by higher adsorption properties and active sites [46]. Therefore, the Bi2O3/TiO2 composites have a higher adsorption capacity and active sites, indicating the improvement of photocatalytic properties.

Analysis of Optical and Photoelectrochemical Performances
The optical properties of pure Bi2O3, pure TiO2, and Bi2O3/TiO2 composites were assessed through UV-vis DRS. As displayed in Figure 6a, obviously, the absorption edge of TiO2 is about 400 nm in the UV spectrum. However, the Bi2O3 has a larger absorption range, and the absorption edge is about 500 nm. For Bi2O3/TiO2 composites, the absorption edges are red-shifted in comparison to the pure TiO2, particularly 2.1% Bi2O3/TiO2 composites. Therefore, after incorporation of Bi2O3, it is profitable to increase the light energy acquirement and visible light absorption for pure TiO2. It would help to separate the photogenerated carriers and increase the properties of degradation of RhB. Further, the Kubelka-Munk formula (αhυ = A(hυ − Eg) n/2 ) was employed to count the band-gap energy of catalysts [47]. The band gap of pure TiO2, pure Bi2O3, and 2.1% Bi2O3/TiO2 samples are about 3.1 eV, 2.72 eV, and 2.79 eV, respectively (Figure 6b).

Analysis of Optical and Photoelectrochemical Performances
The optical properties of pure Bi 2 O 3 , pure TiO 2 , and Bi 2 O 3 /TiO 2 composites were assessed through UV-vis DRS. As displayed in Figure 6a, obviously, the absorption edge of TiO 2 is about 400 nm in the UV spectrum. However, the Bi 2 O 3 has a larger absorption range, and the absorption edge is about 500 nm. For Bi 2 O 3 /TiO 2 composites, the absorption edges are red-shifted in comparison to the pure TiO 2 , particularly 2.1% Bi 2 O 3 /TiO 2 composites. Therefore, after incorporation of Bi 2 O 3 , it is profitable to increase the light energy acquirement and visible light absorption for pure TiO 2 . It would help to separate the photogenerated carriers and increase the properties of degradation of RhB. Further, the Kubelka-Munk formula (αhυ = A(hυ − Eg) n/2 ) was employed to count the band-gap energy of catalysts [47]. The band gap of pure TiO 2 , pure Bi 2 O 3 , and 2.1% Bi 2 O 3 /TiO 2 samples are about 3.1 eV, 2.72 eV, and 2.79 eV, respectively (Figure 6b).
The optical properties of pure Bi2O3, pure TiO2, and Bi2O3/TiO2 composites were assessed through UV-vis DRS. As displayed in Figure 6a, obviously, the absorption edge of TiO2 is about 400 nm in the UV spectrum. However, the Bi2O3 has a larger absorption range, and the absorption edge is about 500 nm. For Bi2O3/TiO2 composites, the absorption edges are red-shifted in comparison to the pure TiO2, particularly 2.1% Bi2O3/TiO2 composites. Therefore, after incorporation of Bi2O3, it is profitable to increase the light energy acquirement and visible light absorption for pure TiO2. It would help to separate the photogenerated carriers and increase the properties of degradation of RhB. Further, the Kubelka-Munk formula (αhυ = A(hυ − Eg) n/2 ) was employed to count the band-gap energy of catalysts [47]. The band gap of pure TiO2, pure Bi2O3, and 2.1% Bi2O3/TiO2 samples are about 3.1 eV, 2.72 eV, and 2.79 eV, respectively (Figure 6b). Photoluminescence (PL) was applied to measure the separating efficiency of photogenerated electrons and holes of prepared samples because of its high sensitivity and because it does not destroy contaminated samples. Figure 7 shows the PL results of pure TiO2, 1.3% Bi2O3/TiO2, 2.1% Bi2O3/TiO2, 2.9% Bi2O3/TiO2, 3.7% Bi2O3/TiO2, and 5% Bi2O3/TiO2, respectively. It is evident that the PL intensities of all Bi2O3/TiO2 photocatalysts are less than that of pure TiO2, implying that the construction of Bi2O3/TiO2 restrains the Photoluminescence (PL) was applied to measure the separating efficiency of photogenerated electrons and holes of prepared samples because of its high sensitivity and because it does not destroy contaminated samples. Figure 7 shows the PL results of pure

Photocatalytic Performance Analysis
The photocatalytic properties of pure TiO2, pure Bi2O3, and Bi2O3/TiO2 composites were assessed by degrading the RhB via simulation sunlight, and the consequences are displayed in Figure 8. The experiment is divided into two steps: first, in order to achieve concentration balance, the photocatalysts adsorbed the RhB for 30 min under dark reactions; then, the photocatalysts were illuminated under simulated sunlight by xenon lamp. Figure 8a presents the impact of Bi2O3 ratio on TiO2 photocatalytic properties. It is found

Photocatalytic Performance Analysis
The photocatalytic properties of pure TiO 2 , pure Bi 2 O 3 , and Bi 2 O 3 /TiO 2 composites were assessed by degrading the RhB via simulation sunlight, and the consequences are displayed in Figure 8. The experiment is divided into two steps: first, in order to achieve concentration balance, the photocatalysts adsorbed the RhB for 30 min under dark reactions; then, the photocatalysts were illuminated under simulated sunlight by xenon lamp. Figure 8a presents the impact of Bi 2 O 3 ratio on TiO 2 photocatalytic properties. It is found that the pure Bi 2 O 3 , pure TiO 2 , and Bi 2 O 3 /TiO 2 composites have lower absorption ability in the dark after 30 min. However, the photocatalytic performances of Bi 2 O 3 /TiO 2 composites with simulated sunlight increased first with the enhancement of Bi 2 O 3 content, and the 2.1% Bi 2 O 3 /TiO 2 composites exhibited exceptional photocatalytic abilities. The degradation rate of RhB can reach 99.6% under the irradiation of simulation sunlight for 60 min, which is about 0.63 of the time and six times better than pure TiO 2 and pure Bi 2 O 3 , respectively. It is mainly because of the constitution of the heterojunction between Bi 2 O 3 and TiO 2 , which restricts the combination of carriers and thus enhances the photocatalytic properties of the photocatalyst. However, the photocatalytic properties of Bi 2 O 3 /TiO 2 catalysts declined when increasing the Bi 2 O 3 content from 2.9% to 5%, which may be the result of the aggregates of the Bi 2 O 3 and the drop of active sites. On the other hand, the excessive Bi 2 O 3 can be considered as the recombination center, which results in the decrease of photocatalytic properties. The excessive Bi 2 O 3 would influence the transmission of light between the Bi 2 O 3 and TiO 2 , blocking the motivation of TiO 2 and resulting in reducing the photocatalytic efficiency of photocatalysts [48]. Moreover, the catalytic performances of different Bi 2 O 3 /TiO 2 reported in other papers were compared, and the details are listed in Table 2. It can be seen that the β-Bi 2 O 3 /TiO 2 photocatalyst in this work shows the efficient photocatalytic performance for photodegradation RhB. Meanwhile, the first-order reaction rate constant (k) determined by quasi-first-order kinetic model was applied to study the photocatalytic processes. As shown in Figure 8b, the k values of TiO2, Bi2O3, 1.3%Bi2O3/TiO2, 2.1% Bi2O3/TiO2, 2.9% Bi2O3/TiO2, 3.7% Bi2O3/TiO2, and 5% Bi2O3/TiO2 are 0.03654 h −1 , 0.0034 h −1 , 0.04315 h −1 , 0.07729 h −1 , 0.05717 h −1 , 0.05568 h −1 , and 0.05397 h −1 , respectively. It is obvious that the k value of 2.1% Bi2O3/TiO2 composites is the maximum, which is 2.1 and 22.7 times of TiO2 and Bi2O3, respectively. The result shows that the 2.1% Bi2O3/TiO2 composites display preferable photocatalytic activity compared to other catalysts. In addition, the recycling stability of the 2.1% Bi2O3/TiO2 composites was assessed. As represented in Figure 8c, the photocatalytic activity of 2.1% Bi2O3/TiO2 composites not to have obvious change after five trials, indicating the outstanding stability and repeatability. Further, in order to verify the stability of 2.1% Bi2O3/TiO2 composites, the XRD was employed to analyze the structure of 2.1% Bi2O3/TiO2 composites before and after five trials, as shown in Figure 8d. It can be seen that there is no obvious change in 2.1% Bi2O3/TiO2 composites before and after five trials, which also implies the stability of Bi2O3/TiO2 composites.   Full-spectrum light irradiation [51] Meanwhile, the first-order reaction rate constant (k) determined by quasi-first-order kinetic model was applied to study the photocatalytic processes. As shown in Figure 8b Figure 8c, the photocatalytic activity of 2.1% Bi 2 O 3 /TiO 2 composites not to have obvious change after five trials, indicating the outstanding stability and repeatability. Further, in order to verify the stability of 2.1% Bi 2 O 3 /TiO 2 composites, the XRD was employed to analyze the structure of 2.1% Bi 2 O 3 /TiO 2 composites before and after five trials, as shown in Figure 8d. It can be seen that there is no obvious change in 2.1% Bi 2 O 3 /TiO 2 composites before and after five trials, which also implies the stability of Bi 2 O 3 /TiO 2 composites.
Based on the above analysis, a potential degradation mechanism is put forward. As represented in Figure 9, when the Bi 2 O 3 /TiO 2 photocatalyst is illuminated by simulated sunlight, the photogenerated electrons (e − ) are activated and diverted from the CB of TiO 2 to the CB of Bi 2 O 3 via the interface of Bi 2 O 3 /TiO 2 composites; meanwhile, the holes (h + ) are transferred from the VB of Bi 2 O 3 to the VB of TiO 2 ; therefore, the photoinduced holes accumulate in the heterojunction interface, which is favorable for the separation of photogenerated electrons and holes. The photogenerated electron and holes could have the following reactions [ cumulate in the heterojunction interface, which is favorable for the separation of photogenerated electrons and holes. The photogenerated electron and holes could have the following reactions [52]: On the one hand, the electrons react with O2 in the solution to produce •O2 − . Then, the H+ react with the •O2 − to generate H2O2, which then reacts with electrons to transform •OH. On the other hand, the h+ on the VB of TiO2 oxides the H2O/OHto form •OH radicals for RhB degradation. Therefore, the Bi2O3/TiO2 composites have higher photocatalytic activity. Figure 9. Photocatalytic mechanism based on degradation of the RhB of the Bi2O3/TiO2 photocatalyst.

Preparation of β-Bi2O3/TiO2 Photocatalysts
Bi(NO3)3 . 5H2O, 30 mL glycerin, and 20 mL ethanol were mixed together and then stirred for 30 min and sonicated for 1 min. The mixtures were poured into a Teflon-sealed reactor. Titanium tetrachloride was added drop by drop into the solution. Then, the Teflon-sealed reactor was put into an oven and heated for 48 h at 110 °C. When the temperature dropped to 25 °C, the reaction productions were collected and washed with ethanol several times and dried at 80 °C for 6 h to obtain the material. The material was calcined at 375 °C for 4 h to obtain the product. The molar ratios of Bi/Ti in the composites were settled at 1.3%, 2.1%, 2.9%, 3.7%, and 5.0%, which were calculated by theoretical methods. The as-prepared photocatalytics were denoted as 1.3%, 2.1%, 2.9%, 3.7%, and 5.0%

Preparation of β-Bi 2 O 3 /TiO 2 Photocatalysts
Bi(NO 3 ) 3 ·5H 2 O, 30 mL glycerin, and 20 mL ethanol were mixed together and then stirred for 30 min and sonicated for 1 min. The mixtures were poured into a Teflon-sealed reactor. Titanium tetrachloride was added drop by drop into the solution. Then, the Teflonsealed reactor was put into an oven and heated for 48 h at 110 • C. When the temperature dropped to 25 • C, the reaction productions were collected and washed with ethanol several times and dried at 80 • C for 6 h to obtain the material. The material was calcined at 375 • C for 4 h to obtain the product. The molar ratios of Bi/Ti in the composites were settled at 1.3%, 2.1%, 2.9%, 3.7%, and 5.0%, which were calculated by theoretical methods. The as-prepared photocatalytics were denoted as 1.3%, 2.1%, 2.9%, 3.7%, and 5.0% Bi 2 O 3 /TiO 2 , which are based on the molar ratio of Bi/Ti. As a comparison, pure Bi 2 O 3 and TiO 2 samples were prepared with the same procedure.

Characterization
The crystal structure was studied by a X-ray diffraction (XRD, RiGdku, RINT2000 with Cu K α radiation (λ = 0.15418 nm). The morphologies of photocatalysts were studied through a field-induced emission scanning electron microscope (FESEM, JSM-6700F). The transmission electron microscopy (TEM) was employed by JEM-2010F, and the accelerating voltage was 200 kV. The Brunauer-Emmett-Teller (BET) specific surface area of prepared photocatalysts was recorded on a Quantachrome NOVA 2000e. A UV-vis scanning spectrophotometer (UV-vis/DRS, SHIMADZU UV-2450) was applied to investigate the optical properties of the prepared photocatalysts. The photoluminescence (PL) was measured by a Hitachi F4500 fluorescence spectrometer. The X-ray photoelectron spectroscopy (XPS, ESCALAB MK II) was employed to analyze the elemental compositions of the samples.

Photocatalytic Activity Analysis
The photocatalytic properties of all photocatalysts were analyzed by degrading rhodamine B (RhB) with simulation sunlight illumination. The illuminant was a 300 W Xe lamp (PLS-SXE300, Beijing Park Lay Technology Co., Ltd., Beijing, China), and the light intensity was 100 mW/cm 2 . Briefly, the photocatalyst (50 mg) was placed into 60 mL of 20 mg/L of RhB solution. Before exposure to light, the mixture solution was stirred for 30 min to obtain adsorption equilibrium. Then, 3 mL of solution was extracted and centrifuged at 20 min intervals. The RhB concentration was recorded through the UV-vis spectrophotometer. The stability of Bi 2 O 3 /TiO 2 photocatalyst was studied via five recycling experiments, and the results were the average value of three samples.

Conclusions
In this study, flower-like Bi 2 O 3 /TiO 2 photocatalysts were successfully prepared by solvothermal route, and XRD, SEM, TEM, XPS, BET, UV-vis, and PL were employed to analyze the morphology and properties of the photocatalysts. The influence of doped Bi 2 O 3 content on TiO 2 photocatalytic efficiency was determined. The XRD results implied that the presence of Bi 2 O 3 did not destroy the lattice structure of TiO 2 . The photocatalytic properties of materials were studied via RhB degradation. A significantly improvement in photoactivity was obtained when the heterojunction was created between Bi 2 O 3 and TiO 2 . Further, the 2.1% Bi 2 O 3 /TiO 2 photocatalyst has the best degradation efficiency, which is 99.6% degradation of RhB at 60 min. It is mainly because the heterojunction in Bi 2 O 3 /TiO 2 strengthens the movement and separation of carriers and then enhances the photocatalytic properties of the Bi 2 O 3 /TiO 2 .