Inﬂuence of Calcination Temperature on Photocatalyst Performances of Floral Bi 2 O 3 /TiO 2 Composite

: Heterojunction photocatalytic materials show excellent performance in degrading toxic pollutants. This study investigates the inﬂuence of calcination temperature on the performances of ﬂoral Bi 2 O 3 /TiO 2 composite photocatalyst crystal, which was prepared with glycerol, bismuth nitrate, and titanium tetrachloride as the major raw materials via the solvothermal method. XRD, SEM/TEM, BET, Uv-vis, and XPS were employed to analyze the crystal structure, morphology, speciﬁc surface area, band gap, and surface chemical structure of the calcined temperature catalysts. The calcination temperature inﬂuence on the catalytic performance of composite photocatalysis was tested with rho-damine B (RhB) as the degradation object. The results revealed the high catalytic activity and higher photocatalytic performance of the Bi 2 O 3 /TiO 2 catalyst. The degradation efﬁciency of the Bi 2 O 3 /TiO 2 catalyst to RhB was 97%, 100%, and 91% at 400 ◦ C, 450 ◦ C, and 500 ◦ C calcination temperatures, respectively, in which the peak degradation activity appeared at 450 ◦ C. The characterization results show that the appropriate calcination temperature promoted the crystallization of the Bi 2 O 3 /TiO 2 catalyst, increased its speciﬁc surface area and the active sites of catalytic reaction, and improved the separation efﬁciency of electrons and holes.


Introduction
Due to rapid industrial development, pollution has become an ever-growing issue [1]. In particular, water pollution as a major basic environmental factor affects and restricts human survival, safety, and social development worldwide [2]. In recent years, a tremendous number of industrial pollutants, phenolic pesticides, fertilizers, dyes, and organic pollutants have been discharged into waters [3]. Even the low concentration of the pollutants remaining in the wastewater can still affect plants, aquatic animals, and humans due to their non-degradability and toxicity. Therefore, organic wastewater purification is necessary for sustainable development and environmental protection. At present, a variety of techniques have been used for the degradation of organic pollutants in water, such as photocatalysis [4][5][6], adsorption [7], the Fenton method [8], hydrolysis [9], ionizing radiation [10], ultrasonic radiation [11], and oxygen plasma treatment [12]. Among them, photocatalysis can completely eliminate organic pollutants with its outstanding reaction rate, low processing cost, moderate operating conditions, and environment-friendly nature, making it one of the most effective methods in the removal of organic pollution [13][14][15]. Photocatalytic degradation is to mineralize and purify water for organic pollutants with solar energy and is especially suitable for wastewater that contains a small amount of refractory organic matter; thus, it has a promising application prospect.
Due to the nature of the stable physical and chemical structure, high oxidizing capacity, and being low-cost and non-toxic, titanium dioxide (TiO 2 ) has a broad application prospect in photocatalytic volatile organic compounds [16]. Nevertheless, defects such as large width of the forbidden band (3.2 eV), narrow absorption light wavelength range, and a tendency to merge with photoelectrons and holes often lead to the reduction of photocatalytic efficiency [17]. The methods to improve the photocatalytic efficiency of TiO 2 mainly include precious metal doping [18], structure regulation [19], and heterojunction construction [20,21]. Among them, heterojunction is widely used to achieve the spatial separation of redox reaction sites and to retain the high redox capability of charge carriers, while extending the spectral response to wider wavelengths. For example, it has been reported that the photocatalytic performance of TiO 2 /gC 3 N 4 [22], CuO/TiO 2 [23], TiO 2 /CeO 2 [24], and TiO 2 /Bi 2 WO 6 [25] heterojunctions was greatly improved under light.
Bismuth oxide (Bi 2 O 3 ) is an important metal oxide p-type semiconductor. It has four main crystal types, namely, α, β, γ, and δ, in which the α phase is stable at low temperature, δ is stable at high temperature, and the others are metastable phases [26]. In general, β-Bi 2 O 3 has better photocatalytic activity than α-Bi 2 O 3 , as it has lower band gap energy and higher visible light absorption efficiency [27]. Bismuth oxide is easy to synthesize, with the advantages of low quantum efficiency, appropriate band gap (2.58~2.8 eV), and that it can be activated by visible light [28]. However, the rapid recombination of photocarriers may occur in pure Bi 2 O 3 material; therefore, it is often used to construct heterojunctions with other semiconductors in order to effectively improve the photocatalytic performance [29]. Attempts to improve the photocatalytic activity of TiO 2 and Bi 2 O 3 have been made in preparing Bi 2 O 3 /TiO 2 heterojunctions, which not only enhanced the photogenerated charge separation, but also increased the light absorption range [30,31]. Since the phase transitions of titanium dioxide and bismuth oxide are accompanied by temperature changes, the calcination temperature is an important factor in determining the photocatalytic activity in the preparation of the Bi 2 O 3 /TiO 2 catalyst [32][33][34]. However, few studies have reported the effect of the calcination temperature on the photocatalytic activity of Bi 2 O 3 /TiO 2 composite photocatalysts.
Therefore, this study investigates the effects of different calcination temperatures on the morphology, crystal shape, and performance of Bi 2 O 3 /TiO 2 composite photocatalysts. The RhB was adopted as the model wastewater.

Reagent
Titanium tetrachloride was purchased from Shanghai Nuotai Chemical Co., Ltd., Shanghai, China. Nitric acid, bismuth nitrate pentahydrate, and sodium bicarbonate were from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Anhydrous ethanol was from Aladdin Bio-Chem Technology Co., Ltd., Shanghai, China. Sodium hydroxide and propylene glycol were from Xilong Chemical Co., Ltd., Guangzhou, China. RhB was purchased from Shanghai Yuanye Biotechnology Co., Ltd., Shanghai, China. All reagents were analytically pure without any further processing.

Floral Bi 2 O 3 /TiO 2 Preparation
To start with, 30 mL of glycerol, 50 ml bismuth nitrate (Bi/Ti mol of 2.1%, the concentration of bismuth nitrate was 4.16 g/L), and 20 mL of absolute ethanol were poured into a 100 mL beaker, stirred for 30 min, and ultrasonic processed for 1 min. Subsequently, the mixture was poured into a Teflon-sealed reactor. Then, titanium tetrachloride was added in dropwise, and the mixture was reacted into an autoclave at 110 • C for 48 h. Afterwards, the suspension was cooled down to room temperature, centrifuged, and washed with absolute ethanol, then vacuum-dried at 80 • C for 8 h. The product was divided into five portions, calcined at 375 • C, 400 • C, 425 • C, 450 • C, and 500 • C for 4 h, respectively, and then cooled to room temperature. Thus, the Bi 2 O 3 /TiO 2 samples after heat treatment at different temperatures were obtained.
For comparison, pure TiO 2 was also prepared. Firstly, 30 mL glycerol and 20 mL ethanol were added into a 100 mL beaker with continuous stirring for 30 min and ultrasounding for 1 min at room temperature. Then, the mixed solution was added into the Teflon reactor. Next, 1 mL titanium tetrachloride solution was added into the above solution drop by drop in the fume hood. Then, the solution was transferred into the oven and heated at 110 • C for 48 h. After cooling down to room temperature, the materials were washed and centrifuged repeatedly with ethanol and then dried at 80 • C for 8 h. Finally, the samples were obtained after calcinating at 500 • C for 4 h.

Characterization
X-ray diffraction (XRD; RiGdku, RINT2000, CuK α emission line of λ = 0.15418 nm) was used to detect the crystal structure of the sample, and the sample morphology was identified using a field-emission scanning electron microscope (FESEM) in secondary electron scattering mode of JSM-6700F at 15 kV. High-resolution TEM mapping of the lattice structure and lattice stripes of the prepared samples was obtained via JEM-2010F transmission electron microscope at an accelerating voltage of 200 kV. The optical properties of the samples were tested by UV-visible spectrophotometer (DRS; Spec-3700 DUV Shimadzu). The specific surface area and porosity of the samples were measured using the Brunauer-Emmett-Teller (BET) method (Micromeritics, ASAP2020) at 77 K and N2 adsorption and desorption. The UV-visible diffuse reflection spectrum (UV-vis/DRS) of the samples was measured with a Shimadzu UV-2450 spectrometer. The steady-state photoluminescence spectrogram (PL) was tested by a single Hitachi F4500 fluorescence spectrometer at an excitation wavelength of 320 nm. Finally, the surface composition of the samples and the chemical state of the containing elements were measured through X-ray photoelectron spectroscopy (XPS; ESCALAB MK II).

Photocatalytic Performance Test
The photocatalytic properties of the Bi 2 O 3 /TiO 2 composites were tested through the degradation of RhB, an organic dye, under a xenon lamp. In this process, the filter was employed to filter out light with a wavelength of less than 420 nm. The power of the xenon lamp's (PLS-SXE300, Beijing Park Fei Lay Technology Co., LTD, Beijing, China. light intensity of 100 mW/cm 2 ) was set for 300 W. The distance between RhB and the xenon lamp was 5 cm. The experiment was divided into five groups (five temperature-treated samples); steps of each group were as follows: Firstly, 50 mg prepared catalyst was mixed with 60 mL 20 mg/L RhB and stirred for 30 min, during which any light source was completely cut off in order to create the adsorption equilibrium. Secondly, 3 mL mixture was extracted through a syringe and placed in darkness as the initial sample, then another 3 mL sample was extracted from the mixture after every 20 min, and the residual catalyst separated with a filter. The entire catalytic process was performed under a xenon lamp equipped with an optical filter, with an on-going stirrer throughout the illumination period. Finally, the absorbance intensity of all collected samples was measured at the corresponding wavelength via UV spectrophotometer (the maximum absorption wavelength of RhB is 554 nm). In recycling experiments, the Bi 2 O 3 /TiO 2 composite was isolated from the suspension by repeated centrifugation and washing. The photodegradation efficiency was calculated by Formula (2), as follows: where C0 is the initial concentration of the solution before the reaction, respectively; C1 is the concentration of the solution after a period of reaction.
namely the intensity, increases accordingly. This is caused by the increase in the crystal size as the temperature rises. The XRD diagram of the Bi 2 O 3 /TiO 2 composite catalyst was 2 θ of 25.3 • , 37.8 • , 48.2 • , and 54.7 • , with the corresponding TiO 2 lattice planes of (101), (004), (200), and (211), typically characteristic of anatase phase titanium dioxide [35]. Only the diffraction peaks of anatase phase TiO 2 were observed in the XRD map of the Bi 2 O 3 /TiO 2 composite catalyst, while corresponding ones were not detected, which indicates that Bi 2 O 3 did not form a separate crystallization phase. The reason can be either the even dispersion of bismuth ions in the anatase phase TiO 2 crystal, or the almost undetectable low Bi 2 O 3 content (2.1 mol%) [36]. As for the pure TiO 2 treated at 500 • C, in addition to the diffraction peak containing anatase, a distinct characteristic peak at 2 θ = 27.4 • for rutile phase titanium dioxide (JCPDS 75-1749) was captured, indicating that TiO 2 had a phase transition into an anatase and rutile mixed phase at 500 • C [37]. However, the characteristic peak of the rutile phase was not observed for the Bi 2 O 3 /TiO 2 treated at 500 • C, which is possibly due to the Bi 2 O 3 inhibition of the transition from the anatase phase to the anatase/rutile mixed phase.
According to the previous studies, it was found that the crystallinity of titania samples was of crucial importance to the photocatalytic efficiency [38,39]. The Scherrer's equation was employed to calculate the crystallite size (L in nm) [40]. The crystallite size was 17.72 nm.
where k is a constant and it is equivalent to 1. λ-X-ray wavelength: its value is 0.15418 nm. θ-half angle diffraction, which is 25.294. β-full width at maximum half intensity (FWHM) and the value is 0.0089 (Rad).  Figure 2 shows the SEM images of the Bi2O3/TiO2 composite, catalyzing at different calcination temperatures. It can be clearly seen that the morphology of Bi2O3/TiO2 is a petal-like spheroid. The petals are composed of many small particles, and the surface is relatively rough. The temperature increase did not change the texture of the composite. The figures indicate that the diameter of the petal at 425 °C was 54.2 nm, and 88.3 nm at   Figure 2 shows the SEM images of the Bi 2 O 3 /TiO 2 composite, catalyzing at different calcination temperatures. It can be clearly seen that the morphology of Bi 2 O 3 /TiO 2 is a petal-like spheroid. The petals are composed of many small particles, and the surface is relatively rough. The temperature increase did not change the texture of the composite. The figures indicate that the diameter of the petal at 425 • C was 54.2 nm, and 88.3 nm at 500 • C. The crystallinity of titanium dioxide increased due to the temperature rise, and thus the size of small petal particles increased. This is consistent with the conclusion of XRD. The sample treated at 425 • C had relatively intact shapes, whereas some petals at 500 • C were fragmented.

Analysis of Light Absorption Region and Band Gap
The optical absorption capacity and response range of the Bi2O3/TiO2 catalyst w different calcination temperatures were studied through UV-visible diffuse reflect spectroscopy (UV-Vis DRS). The results are shown in Figure 4a. From the figure, all sa ples exhibited high absorption capacity within the visible light range. Relatively obvio absorption initiation changes could be seen near the wavelength of 410 nm, and the li absorption performance of the 450 °C-treated sample was slightly better than that of others. The band gap of catalytic material can be calculated by the formula: α h ν = A ν-Eg) n/2 , where the α, h, ν, and Eg represent the absorption coefficient, the Planck consta the photon frequency, and the band gap of the semiconductor, respectively [41]. Fig  4b shows the band gap energy of the Bi2O3/TiO2 catalyst. The band gap energy of the °C-and 500 °C-treated samples were 2.7 eV and 2.86 eV, respectively. It is evident that 450 °C-treated sample has a lower band gap and more enhanced light absorption capac compared to the 500 °C-treated one.

Analysis of Light Absorption Region and Band Gap
The optical absorption capacity and response range of the Bi 2 O 3 /TiO 2 catalyst with different calcination temperatures were studied through UV-visible diffuse reflection spectroscopy (UV-Vis DRS). The results are shown in Figure 4a. From the figure, all samples exhibited high absorption capacity within the visible light range. Relatively obvious absorption initiation changes could be seen near the wavelength of 410 nm, and the light absorption performance of the 450 • C-treated sample was slightly better than that of the others. The band gap of catalytic material can be calculated by the formula: α h ν = A (h ν-Eg) n/2 , where the α, h, ν, and Eg represent the absorption coefficient, the Planck constant, the photon frequency, and the band gap of the semiconductor, respectively [41]. Figure 4b shows the band gap energy of the Bi 2 O 3 /TiO 2 catalyst. The band gap energy of the 450 • Cand 500 • C-treated samples were 2.7 eV and 2.86 eV, respectively. It is evident that the 450 • C-treated sample has a lower band gap and more enhanced light absorption capacity compared to the 500 • C-treated one. the photon frequency, and the band gap of the semiconductor, respectively [41]. Fi 4b shows the band gap energy of the Bi2O3/TiO2 catalyst. The band gap energy of th °C-and 500 °C-treated samples were 2.7 eV and 2.86 eV, respectively. It is evident tha 450 °C-treated sample has a lower band gap and more enhanced light absorption cap compared to the 500 °C-treated one.

Analysis of Surface Elements
The element composition and element valence states of the Bi 2 O 3 /TiO 2 samples synthesized at 450 • C and 500 • C were analyzed in comparison via high-resolution XPS mapping, the results of which are shown in Figure 5. Figure [43]. In the spectra of Ti2p (Figure 5d), the binding energy of Ti 2p 1/2 and Ti 2p 3/2 spin-orbit splitting photoelectrons were 464.38 eV and 458.53 eV, respectively, reflecting the presence of Ti in the Bi 2 O 3 /TiO 2 composite as Ti 4+ , which tetrahedral coordinated with oxygen [44]. Notably, when comparing the element maps of Ti, O, and Bi in Bi 2 O 3 /TiO 2 at 500 • C and 450 • C, it was found that the element characteristic peaks at 500 • C shifted right to those of Ti2p and Bi4f at 450 • C, while the O1s peak shifted towards lower binding energy. This may be caused by an increased crystallinity at a higher calcination temperature. ence of Ti in the Bi2O3/TiO2 composite as Ti 4+ , which tetrahedral coordinated with oxygen [44]. Notably, when comparing the element maps of Ti, O, and Bi in Bi2O3/TiO2 at 500 °C and 450 °C, it was found that the element characteristic peaks at 500 °C shifted right to those of Ti2p and Bi4f at 450 °C, while the O1s peak shifted towards lower binding energy This may be caused by an increased crystallinity at a higher calcination temperature.

Analysis of the Specific Surface Area
The surface area of the photocatalyst is a crucial factor in terms of creating active sites that associate with the enhancement of photocatalytic degradation efficiency. According

Analysis of the Specific Surface Area
The surface area of the photocatalyst is a crucial factor in terms of creating active sites that associate with the enhancement of photocatalytic degradation efficiency. According to previous studies, photocatalysts with a large surface area provide more surface reaction sites. The porosity and structural properties of catalytic materials were systematically investigated using the N2 adsorption-desorption isotherm. Figure 6 shows the adsorption and desorption curve of the Bi 2 O 3 /TiO 2 composite catalyst at 400 • C, 450 • C, and 500 • C. Typical type IV isotherms of type H3 hysteresis loop can be observed for all samples at different temperatures, indicating the mesoporous structure of the prepared Bi 2 O 3 /TiO 2 composite catalysts. The specific surface areas of the Bi 2 O 3 /TiO 2 composite catalysts of 400 • C, 450 • C, and 500 • C were 35.2 m 2 /g, 40.1 m 2 /g, and 25.7 m 2 /g, as shown in Table 1, respectively, in which is the maximum value was at 450 • C, consistent with the results covered in the document [45]. This may be attributed to the crystalline grain growth of bismuth oxide, alongside phase transition, as calcination temperature increases [26,35]. The TEM image indicates that when it exceeded 450 • C, bismuth oxide underwent a phase transition and size expansion; at 500 • C, anatase phase titanium dioxide also transformed into rutile phase titanium dioxide. As can be seen from the XRD pattern, the grain size of titanium dioxide increased and its effective specific surface area decreased. ered in the document [45]. This may be attributed to the crystalline grain growth muth oxide, alongside phase transition, as calcination temperature increases [26, TEM image indicates that when it exceeded 450 °C, bismuth oxide underwent transition and size expansion; at 500 °C, anatase phase titanium dioxide also trans into rutile phase titanium dioxide. As can be seen from the XRD pattern, the grai titanium dioxide increased and its effective specific surface area decreased.  Table 1. Specific surface area, mean pore size, and pore volume of TiO2, Bi2O3, and Bi2O3/T posites.

Photoluminescence Spectroscopy Analysis
A photoluminescence (PL) spectroscopy was used to study the carrier separ the catalyst. Figure 7 is the PL profiles of Bi2O3/TiO2 when calcined at 375 °C, 400 °C, 450 °C, and 500 °C, respectively. All samples had a strong emission peak at resulting from the energy released from the electrons rapidly combining the hole

Photoluminescence Spectroscopy Analysis
A photoluminescence (PL) spectroscopy was used to study the carrier separation of the catalyst. Figure 7 is the PL profiles of Bi 2 O 3 /TiO 2 when calcined at 375 • C, 400 • C, 425 • C, 450 • C, and 500 • C, respectively. All samples had a strong emission peak at 470 nm, resulting from the energy released from the electrons rapidly combining the holes in the valence band after the transition from valence band to conduction band. As we can see, the PL peak intensity of Bi 2 O 3 /TiO 2 calcined at 450 • C was significantly lower than that of the other samples. To our knowledge, the lower the PL peak intensity, the higher the separation efficiency of the catalyst photogenerated electron-hole pairs to a certain extent, and the more efficient the corresponding photocatalysis [46]. PL peak intensity gradually decreased as the calcination temperature increased up to 450 • C, indicating that temperature rise favors the separation of the electron-hole pairs. When it further increased to 500 • C, peak intensity reached the maximum, indicating that excess temperature accelerates the composite rate of the electron-hole pairs, which is possibly due to the phase transition of TiO 2 in the composite induced by high temperature. The results above are consistent with the XRD and the photocatalytic efficiency. In summary, among all prepared samples, the 450 • C calcined one showed the lowest PL peak intensity and had higher electron-hole separation efficiency, with the best photocatalytic performance. Furthermore, the fluctuation of separation efficiency also demonstrates the presence of heterojunctions.
the composite rate of the electron-hole pairs, which is possibly due to of TiO2 in the composite induced by high temperature. The results a with the XRD and the photocatalytic efficiency. In summary, among al the 450 °C calcined one showed the lowest PL peak intensity and h hole separation efficiency, with the best photocatalytic performance fluctuation of separation efficiency also demonstrates the presence of

Photocatalytic Performance Analysis
The photocatalytic activity of the Bi2O3/TiO2 composite was tested radation under visible light, which was conducted in two steps: (1) d first 30 min to reach concentration equilibrium where the samples ad using Xenon lamp to simulate sunlight. Figure 8a is a curve graph of th trends of Bi2O3/TiO2 samples treated at a different temperature. It ca min, the degradation rate of the 450 °C-treated sample was almost 100 °C-and 500 °C-treated samples showed 97% and 91% degradation Based on the TEM results, in the samples calcined above 450 °C, some verted into the α phase, the photocatalytic activity of which was very light, as previously mentioned. This is likely to be one of the reasons f tocatalytic efficiency of the 500 °C-treated sample. In addition, accord ysis, sample grains treated at 500 °C had a smaller specific surface ar below 450 °C led to insufficient crystallinity of the Bi2O3/TiO2 composit weakened photocatalytic efficiency.
To present the catalytic performance of each sample more intuit that all photocatalytic processes follow a quasi-first-order kinetic mo Figure 8b, the reaction rate constant of Bi2O3/TiO2 was k(375°C) > k(400 k(500°C). Among them, k(450 °C) (0.11316) is more than twice larger than k(5 is possibly due to the transition from anatase phase titanium dioxide

Photocatalytic Performance Analysis
The photocatalytic activity of the Bi 2 O 3 /TiO 2 composite was tested through RhB degradation under visible light, which was conducted in two steps: (1) dark reaction for the first 30 min to reach concentration equilibrium where the samples adsorbed the dye; (2) using Xenon lamp to simulate sunlight. Figure 8a is a curve graph of the RhB degradation trends of Bi 2 O 3 /TiO 2 samples treated at a different temperature. It can be seen that at 60 min, the degradation rate of the 450 • C-treated sample was almost 100%, whereas the 400 • C-and 500 • C-treated samples showed 97% and 91% degradation rates, respectively. Based on the TEM results, in the samples calcined above 450 • C, some bismuth oxide converted into the α phase, the photocatalytic activity of which was very weak under visible light, as previously mentioned. This is likely to be one of the reasons for the reduced photocatalytic efficiency of the 500 • C-treated sample. In addition, according to the BET analysis, sample grains treated at 500 • C had a smaller specific surface area, while treatment below 450 • C led to insufficient crystallinity of the Bi 2 O 3 /TiO 2 composite, and consequently weakened photocatalytic efficiency.
to the CB of Bi2O3. The holes on the VB of Bi2O3 were generated due to the electrons m Meanwhile, the holes on the VB of Bi2O3 were moved to the VB of TiO2, and the pho erated holes were effectively gathered in the VB of TiO2. Finally, the holes of TiO active species, and oxidized RhBs were oxidized to other products [47,48].

Conclusions
Briefly, a solvothermal route was employed to synthesize the Bi2O3/TiO2 pho lysts, and the influence of calcination temperature was studied systematically. The of this study implied that the calcination temperature had an effect on the perform To present the catalytic performance of each sample more intuitively, it is assumed that all photocatalytic processes follow a quasi-first-order kinetic model. As is shown in Figure 8b, the reaction rate constant of Bi 2 O 3 /TiO 2 was k (375 • C) > k (400 • C) > k (425 • C) > k (450 • C) > k (500 • C) . Among them, k (450 • C) (0.11316) is more than twice larger than k (500 • C) (0.05482), which is possibly due to the transition from anatase phase titanium dioxide to rutile induced by temperature increase, and the consequent catalytic efficiency reduction. The temperature rises also led to the transition from the β phase to α phase bismuth oxide, and thus photocatalysis was reduced. Figure 8c shows the cycle stability experiment of Bi 2 O 3 /TiO 2 . The sample used in this cycle experiment was Bi 2 O 3 /TiO 2 (450 • C), and the RhB was used for photodegradation. The experimental procedure was the same as the photocatalytic experiment, with a sampling time of every 20 min, thus, 60 min in total. As is shown in Figure 8c, in all five cycles, the degradation efficiency of RhB was almost unchanged, which remained at around 98%. This shows that the flower-shaped Bi 2 O 3 /TiO 2 composite samples prepared at different temperatures in this experiment had proper photocatalytic cycle stability and great potential in processing pollutants contained in sewage.
When Bi 2 O 3 was involved, proven by various methods of characterization as well as experimental phenomena, the separation efficiency of photogenic electron and holes of TiO 2 increased, indicating the formation of heterojunctions in composite. Furthermore, the possible mechanism is displayed in Figure 8d. First, RhB molecules were adsorbed on the surface and cavity of Bi 2 O 3 /TiO 2 composites. Then, Bi 2 O 3 /TiO 2 composites were irradiated by visible light. Subsequently, the photogenerated electrons in the VB of Bi 2 O 3 transferred to the CB of Bi 2 O 3 . The holes on the VB of Bi 2 O 3 were generated due to the electrons moved. Meanwhile, the holes on the VB of Bi 2 O 3 were moved to the VB of TiO 2 , and the photogenerated holes were effectively gathered in the VB of TiO 2 . Finally, the holes of TiO 2 were active species, and oxidized RhBs were oxidized to other products [47,48].

Conclusions
Briefly, a solvothermal route was employed to synthesize the Bi 2 O 3 /TiO 2 photocatalysts, and the influence of calcination temperature was studied systematically. The results of this study implied that the calcination temperature had an effect on the performances of the Bi 2 O 3 /TiO 2 photocatalysts. The XRD results implied that 450 • C was a suitable temperature. The photocatalytic performances of catalysts were assessed through degradation RhB. When the calcination temperature was 450 • C, the Bi 2 O 3 /TiO 2 photocatalyst had the best degradation efficiency, which was about 100% degradation of RhB at 60 min.

Conflicts of Interest:
The authors declare no conflict of interest.