Influence of Calcination Temperature on Photocatalyst Performances of Floral Bi2O3/TiO2 Composite

Abstract

Heterojunction photocatalytic materials show excellent performance in degrading toxic pollutants. This study investigates the influence of calcination temperature on the performances of floral Bi₂O₃/TiO₂ 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 rhodamine B (RhB) as the degradation object. The results revealed the high catalytic activity and higher photocatalytic performance of the Bi₂O₃/TiO₂ catalyst. The degradation efficiency of the Bi₂O₃/TiO₂ 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 Bi₂O₃/TiO₂ catalyst, increased its specific surface area and the active sites of catalytic reaction, and improved the separation efficiency of electrons and holes.

1. 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 (TiO₂) 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 TiO₂ 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 TiO₂/gC₃N₄ [22], CuO/TiO₂ [23], TiO₂/CeO₂ [24], and TiO₂/Bi₂WO₆ [25] heterojunctions was greatly improved under light.

Bismuth oxide (Bi₂O₃) 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-Bi₂O₃ has better photocatalytic activity than α-Bi₂O₃, 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 Bi₂O₃ 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 TiO₂ and Bi₂O₃ have been made in preparing Bi₂O₃/TiO₂ 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 Bi₂O₃/TiO₂ catalyst [32,33,34]. However, few studies have reported the effect of the calcination temperature on the photocatalytic activity of Bi₂O₃/TiO₂ composite photocatalysts.

Therefore, this study investigates the effects of different calcination temperatures on the morphology, crystal shape, and performance of Bi₂O₃/TiO₂ composite photocatalysts. The RhB was adopted as the model wastewater.

2. Experiment

2.1. 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.

2.2. Floral Bi₂O₃/TiO₂ 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 Bi₂O₃/TiO₂ samples after heat treatment at different temperatures were obtained.

For comparison, pure TiO₂ 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.

2.3. 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).

2.4. Photocatalytic Performance Test

The photocatalytic properties of the Bi₂O₃/TiO₂ 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/cm²) 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 Bi₂O₃/TiO₂ composite was isolated from the suspension by repeated centrifugation and washing. The photodegradation efficiency was calculated by Formula (2), as follows:

(1)$\mathrm{degradation} \mathrm{efficiency} \%= \frac{C0-C1}{C0}\ast 100\%$

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.

3. Results and Discussion

3.1. Crystal Structure Analysis

To explore the possible structural changes caused by calcination, the Bi₂O₃/TiO₂ prepared at gradient calcination temperatures and the pure TiO₂ calcinated at 500 °C were analyzed by XRD, as is shown in Figure 1. As the calcination temperature increases, the diffraction peak of the Bi₂O₃/TiO₂ catalyst narrows down, and the height of the peak, namely the intensity, increases accordingly. This is caused by the increase in the crystal size as the temperature rises. The XRD diagram of the Bi₂O₃/TiO₂ composite catalyst was 2 θ of 25.3°, 37.8°, 48.2°, and 54.7°, with the corresponding TiO₂ lattice planes of (101), (004), (200), and (211), typically characteristic of anatase phase titanium dioxide [35]. Only the diffraction peaks of anatase phase TiO₂ were observed in the XRD map of the Bi₂O₃/TiO₂ composite catalyst, while corresponding ones were not detected, which indicates that Bi₂O₃ did not form a separate crystallization phase. The reason can be either the even dispersion of bismuth ions in the anatase phase TiO₂ crystal, or the almost undetectable low Bi₂O₃ content (2.1 mol%) [36]. As for the pure TiO₂ 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 TiO₂ 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 Bi₂O₃/TiO₂ treated at 500 °C, which is possibly due to the Bi₂O₃ 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.

L = kλ/βcosθ(2)

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 1

XRD pattern of Bi₂O₃/TiO₂ calcined at different temperatures and TiO₂ at 500 °C.

[Figure omitted. See PDF]

3.2. Micromorphology and Analysis

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 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.

Figure 3a,b show the TEM images of the 450 °C-treated Bi₂O₃/TiO₂ composite catalyst taken at two different sites of the same sample, in which anatase phase titanium dioxide revealed a lattice stripe diameter of 0.36 nm, β phase bismuth oxide of 0.327 nm, and α phase bismuth oxide of 0.371 nm. Interestingly, the β phase bismuth oxide might have transformed to the α phase due to the temperature rise; therefore, after calcination for 4 h, both the α phase and β phase bismuth oxide might exist in the product. Figure 3c exhibits the element scanning area, and Figure 3d–f are the element distribution image of Bi, O, and Ti, respectively. All three elements can be observed clearly in the image, which revealed relatively scattered distribution.

3.3. Analysis of Light Absorption Region and Band Gap

The optical absorption capacity and response range of the Bi₂O₃/TiO₂ 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 Bi₂O₃/TiO₂ catalyst. The band gap energy of the 450 °C- and 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.

3.4. Analysis of Surface Elements

The element composition and element valence states of the Bi₂O₃/TiO₂ 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 5a shows the XPS full-survey spectrum of the Bi₂O₃/TiO₂ photocatalyst. The characteristic peaks of Bi, Ti, and O are evidently presented, which demonstrates the successful synthesis of TiO₂/Bi₂O₃ photocatalyst. The Bi4f spectral map (Figure 5b) was divided into four peaks, in which the 159.09 eV and 164.39 eV were attributed to the Bi4f 7/2 and Bi4 f 5/2 characteristic peaks, indicating the existence of Bi³⁺; the characteristic peaks of binding energy at 157.41 eV and 162.73 eV belonged to the metal Bi, indicating the simultaneous presence of Bi³⁺ and elementary Bi [42]. In Figure 5c, O1 s in 529.67 eV corresponded to the Bi-O bonds; the characteristic peaks at 530.40 eV and 531.59 eV were attributed to Ti-O bonds and hydroxyl oxygen produced in the composite possibly due to water in the chemisorption, respectively [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 Bi₂O₃/TiO₂ composite as Ti⁴⁺, 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.

3.5. 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 Bi₂O₃/TiO₂ 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 Bi₂O₃/TiO₂ composite catalysts. The specific surface areas of the Bi₂O₃/TiO₂ composite catalysts of 400 °C, 450 °C, and 500 °C were 35.2 m²/g, 40.1 m²/g, and 25.7 m²/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.

3.6. Photoluminescence Spectroscopy Analysis

A photoluminescence (PL) spectroscopy was used to study the carrier separation of the catalyst. Figure 7 is the PL profiles of Bi₂O₃/TiO₂ 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 Bi₂O₃/TiO₂ 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 TiO₂ 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.

3.7. Photocatalytic Performance Analysis

The photocatalytic activity of the Bi₂O₃/TiO₂ 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 Bi₂O₃/TiO₂ 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 Bi₂O₃/TiO₂ composite, and consequently weakened photocatalytic efficiency.

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 Bi₂O₃/TiO₂ 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 Bi₂O₃/TiO₂. The sample used in this cycle experiment was Bi₂O₃/TiO₂ (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 Bi₂O₃/TiO₂ composite samples prepared at different temperatures in this experiment had proper photocatalytic cycle stability and great potential in processing pollutants contained in sewage.

When Bi₂O₃ was involved, proven by various methods of characterization as well as experimental phenomena, the separation efficiency of photogenic electron and holes of TiO₂ 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 Bi₂O₃/TiO₂ composites. Then, Bi₂O₃/TiO₂ composites were irradiated by visible light. Subsequently, the photogenerated electrons in the VB of Bi₂O₃ transferred to the CB of Bi₂O₃. The holes on the VB of Bi₂O₃ were generated due to the electrons moved. Meanwhile, the holes on the VB of Bi₂O₃ were moved to the VB of TiO₂, and the photogenerated holes were effectively gathered in the VB of TiO₂. Finally, the holes of TiO₂ were active species, and oxidized RhBs were oxidized to other products [47,48].

4. Conclusions

Briefly, a solvothermal route was employed to synthesize the Bi₂O₃/TiO₂ 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 Bi₂O₃/TiO₂ 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 Bi₂O₃/TiO₂ photocatalyst had the best degradation efficiency, which was about 100% degradation of RhB at 60 min.

Author Contributions

Experiments, M.W. and C.L.; Analysed, B.L. and W.Q.; Writing, M.W., C.L. and B.L.; Revised, W.Q.; Funding, Y.X. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Table

Figure 2. SEM images of a Bi2O3/TiO2 composite catalyst treated at different calcination temperatures: (a) 375 °C, (b) 400 °C, (c) 425 °C, (d) 500 °C.

Figure 3. (a,b) are 450 °C Bi2O3/TiO2HRTEM images, (c) scan area, and distribution of (d) Bi, (e) O, and (f) Ti.

Figure 4. The optical diagram of (a) the UV-vis spectrum and the (b) the band gap energy spectrum of the Bi2O3/TiO2 samples with different calcination temperatures.

Figure 5. The 500 °C- versus 450 °C-treated Bi2O3/TiO2 (a) XPS full-spectrum map, high-resolution spectrum map of (b) Bi4f, (c) O1s, (d) Ti2p.

Figure 6. N2 adsorption–desorption isotherm of Bi2O3/TiO2 calcined at 400 °C, 450 °C, and 500 °C.

Figure 7. PL result of Bi2O3/TiO2 samples at different calcination temperatures.

Figure 8. (a) RhB degradation trends of Bi2O3/TiO2 composite catalyst at different calcination temperatures, (b) reaction rate constant of Bi2O3/TiO2 composite catalyst for RhB at different calcination temperatures, (c) cycle stability experiment of Bi2O3/TiO2 composite catalyst treated at 450 °C, (d) mechanism diagram of Bi2O3/TiO2 heterojunction photocatalysis.

Table 1

Specific surface area, mean pore size, and pore volume of TiO₂, Bi₂O₃, and Bi₂O₃/TiO₂ composites.

Samples	Specific Surface Area (m²/g)	Mean Pore Size (nm)	Pore Volume (cm3 g−1)
TiO2	9.1	10.5	0.1
Bi2O3	3.2	6.2	0.01
Bi2O3/TiO2 400 °C	35.2	11.9	0.15
Bi2O3/TiO2 450°C	40.1	12.4	0.21
Bi2O3/TiO2 500 °C	25.7	11.1	0.13

References

1. Tang, Q.-Y.; Yang, M.-J.; Yang, S.-Y.; Xu, Y.-H. Enhanced photocatalytic degradation of glyphosate over 2D CoS/BiOBr heterojunctions under visible light irradiation. J. Hazard. Mater.; 2020; 407, 124798. [DOI: https://dx.doi.org/10.1016/j.jhazmat.2020.124798] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33348205]

2. Luo, J.; Dai, Z.; Feng, M.; Gu, M.; Xie, Y. Graphitic carbon nitride/ferroferric oxide/reduced graphene oxide nanocomposite as highly active visible light photocatalyst. Nano Res.; 2022; pp. 1-6. [DOI: https://dx.doi.org/10.1007/s12274-022-5110-z]

3. Perumal, K.; Shanava, S.; Karthigeyan, A.; Ahamad, T.; Alshehri, S.M.; Murugakoothan, P. Hydrothermal assisted precipitation synthesis of highly stable g-C3N4/BiOBr/CdS photocatalyst with enhanced visible light photocatalytic degradation of tetracycline. Diam. Relat. Mater.; 2020; 110, 108091. [DOI: https://dx.doi.org/10.1016/j.diamond.2020.108091]

4. Tong, H.; Ji, Y.; He, T.; He, R.; Chen, M.; Zeng, J.; Wu, D. Preparation and photocatalytic performance of UIO-66/La-MOF composite. Water Sci. Technol.; 2022; 86, pp. 95-109. [DOI: https://dx.doi.org/10.2166/wst.2022.197] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/35838285]

5. Sujinnapram, S.; Wongrerkdee, S. Synergistic effects of structural, crystalline, and chemical defects on the photocatalytic performance of Y-doped ZnO for carbaryl degradation. J. Environ. Sci.; 2022; 124, pp. 667-677. [DOI: https://dx.doi.org/10.1016/j.jes.2021.11.035] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/36182172]

6. Song, X.; Su, R.; Wang, Y.; Zhang, Y.; Gao, B.; Wang, Y.; Ma, D.; Li, Q. Visible light-driven chlorite activation process for enhanced sulfamethoxazole antibiotics degradation, antimicrobial resistance reduction and biotoxicity elimination. Chem. Eng. J.; 2022; [DOI: https://dx.doi.org/10.2139/ssrn.4145257]

7. Dong, J.; Yang, F.; Wang, Q.; Jiang, C.; Wang, H.; Cao, X.; Li, Z.; Wang, Z. Preparation of porous carbon@TiO2 composites for the adsorption/sonocatalytic degradation of organic dyes. J. Mol. Liq.; 2022; 367, 120469. [DOI: https://dx.doi.org/10.1016/j.molliq.2022.120469]

8. Shi, W.; Sun, W.; Liu, Y.; Zhang, K.; Sun, H.; Lin, X.; Hong, Y.; Guo, F. A self-sufficient photo-Fenton system with coupling in-situ production H₂O₂ of ultrathin porous g-C₃N₄ nanosheets and amorphous FeOOH quantum dots. J. Hazard. Mater.; 2022; 436, 129141. [DOI: https://dx.doi.org/10.1016/j.jhazmat.2022.129141]

9. Huang, M.; Wang, X.; Zhu, C.; Zhu, F.; Liu, P.; Wang, D.; Fang, G.; Chen, N.; Gao, S.; Zhou, D. Efficient chlorinated alkanes degradation in soil by combining alkali hydrolysis with thermally activated persulfate. J. Hazard. Mater.; 2022; 438, 129571. [DOI: https://dx.doi.org/10.1016/j.jhazmat.2022.129571]

10. Fang, L.; Chu, L.; Wang, J.; Yang, Q. Treatment of polyacrylamide-containing wastewater by ionizing radiation: Efficient reduction of viscosity and degradation of polyacrylamide. Radiat. Phys. Chem.; 2023; 202, 110547. [DOI: https://dx.doi.org/10.1016/j.radphyschem.2022.110547]

11. Wei, B.; Qi, H.; Zou, J.; Li, H.; Wang, J.; Xu, B.; Ma, H. Degradation mechanism of amylopectin under ultrasonic irradiation. Food Hydrocoll.; 2020; 111, 106371. [DOI: https://dx.doi.org/10.1016/j.foodhyd.2020.106371]

12. Li, S.; Wang, X.; Li, L.; Liu, J.; Ding, Y.; Zhao, T.; Zhang, Y. Atomic-scale simulations of the deoxynivalenol degradation induced by reactive oxygen plasma species. Food Res. Int.; 2022; 162, 111939. [DOI: https://dx.doi.org/10.1016/j.foodres.2022.111939] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/36461195]

13. Fu, W.; Zhao, Y.; Wang, H.; Chen, X.; Liu, K.; Zhang, K.; Wei, Q.; Wang, B. Study on preparation, photocatalytic performance and degradation mechanism of polymeric carbon nitride/Pt/nano-spherical MoS₂ composite. J. Phys. Chem. Solids; 2022; 166, 110700. [DOI: https://dx.doi.org/10.1016/j.jpcs.2022.110700]

14. Kumar, G.; Dutta, R.K. Sunlight-induced enhanced photocatalytic reduction of chromium (VI) and photocatalytic degradation of methylene blue dye and ciprofloxacin antibiotic by Sn₃O₄/SnS₂ nanocomposite. Environ. Sci. Pollut. Res.; 2022; 29, pp. 57758-57772. [DOI: https://dx.doi.org/10.1007/s11356-022-19853-0]

15. Xiang, W.; Yuan, J.; Wu, Y.; Luo, H.; Xiao, C.; Zhong, N.; Zhao, M.; Zhong, D.; He, Y. Working principle and application of photocatalytic optical fibers for the degradation and conversion of gaseous pollutants. Chin. Chem. Lett.; 2022; 33, pp. 3632-3640. [DOI: https://dx.doi.org/10.1016/j.cclet.2021.11.074]

16. Gu, S.; Liu, X.; Wang, H.; Liu, Z.; Xing, H.; Yu, L. Preparation and characterization of TiO₂ photocatalytic composites supported by blast furnace slag fibres for wastewater degradation. Ceram. Int.; 2022; [DOI: https://dx.doi.org/10.1016/j.ceramint.2022.10.035]

17. Hu, L.; Chen, J.; Wei, Y.; Wang, M.; Xu, Y.; Wang, C.; Gao, P.; Liu, Y.; Liu, C.; Song, Y. et al. Photocatalytic degradation effect and mechanism of Karenia mikimotoi by non-noble metal modified TiO₂ loading onto copper metal organic framework (SNP-TiO₂@Cu-MOF) under visible light. J. Hazard. Mater.; 2023; 442, 130059. [DOI: https://dx.doi.org/10.1016/j.jhazmat.2022.130059]

18. Khairy, M.; Kamar, E.M.; Mousa, M.A. Photocatalytic activity of nano-sized Ag and Au metal-doped TiO₂ embedded in rGO under visible light irradiation. Mater. Sci. Eng. B; 2022; 286, 116023. [DOI: https://dx.doi.org/10.1016/j.mseb.2022.116023]

19. Liu, B.; Zhang, B.; Ji, J.; Li, K.; Cao, J.; Feng, Q.; Huang, H. Effective regulation of surface bridging hydroxyls on TiO₂ for superior photocatalytic activity via ozone treatment. Appl. Catal. B Environ.; 2022; 304, 120952. [DOI: https://dx.doi.org/10.1016/j.apcatb.2021.120952]

20. Gordanshekan, A.; Arabian, S.; Nazar, A.R.S.; Farhadian, M.; Tangestaninejad, S. A comprehensive comparison of green Bi₂WO₆/g-C₃N₄ and Bi₂WO₆/TiO₂ S-scheme heterojunctions for photocatalytic adsorption/degradation of Cefixime: Artificial neural network, degradation pathway, and toxicity estimation. Chem. Eng. J.; 2023; 451, 139067. [DOI: https://dx.doi.org/10.1016/j.cej.2022.139067]

21. Feizpoor, S.; Habibi-Yangjeh, A.; Luque, R. Design of TiO₂/Ag₃BiO₃ n-n heterojunction for enhanced degradation of tetracycline hydrochloride under visible-light irradiation. Environ. Res.; 2022; 215, 114315. [DOI: https://dx.doi.org/10.1016/j.envres.2022.114315] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/36116489]

22. Kane, A.; Chafiq, L.; Dalhatou, S.; Bonnet, P.; Nasr, M.; Gaillard, N.; Dikdim, J.M.D.; Monier, G.; Assadi, A.A.; Zeghioud, H. g-C₃N₄/TiO₂ S-scheme heterojunction photocatalyst with enhanced photocatalytic Carbamazepine degradation and mineralization. J. Photochem. Photobiol. A Chem.; 2022; 430, 113971. [DOI: https://dx.doi.org/10.1016/j.jphotochem.2022.113971]

23. Liang, C.; Li, C.; Zhu, Y.; Du, X.; Zeng, Y.; Zhou, Y.; Zhao, J.; Li, S.; Liu, X.; Yu, Q. et al. Light-driven photothermal catalysis for degradation of toluene on CuO/TiO₂ Composite: Dominating photocatalysis and auxiliary thermalcatalysis. Appl. Surf. Sci.; 2022; 601, 154144. [DOI: https://dx.doi.org/10.1016/j.apsusc.2022.154144]

24. Hou, J.; Yang, H.; He, B.; Ma, J.; Lu, Y.; Wang, Q. High photocatalytic performance of hydrogen evolution and dye degradation enabled by CeO₂ modified TiO₂ nanotube arrays. Fuel; 2022; 310, 122364. [DOI: https://dx.doi.org/10.1016/j.fuel.2021.122364]

25. Li, L.; Yang, J.; Yang, L.; Fu, F.; Xu, H.; Fan, X. Photocatalytic performance of TiO₂/Bi₂WO₆ photocatalysts with trace Fe³⁺ dopant for gaseous toluene decomposition. J. Environ. Chem. Eng.; 2022; 10, 107708. [DOI: https://dx.doi.org/10.1016/j.jece.2022.107708]

26. Xiao, X.; Hu, R.; Liu, C.; Xing, C.; Qian, C.; Zuo, X.; Nan, J.; Wang, L. Facile large-scale synthesis of  -Bi₂O₃ nanospheres as a highly efficient photocatalyst for the degradation of acetaminophen under visible light irradiation. Appl. Catal. B Environ.; 2013; 140–141, pp. 433-443. [DOI: https://dx.doi.org/10.1016/j.apcatb.2013.04.037]

27. Cheng, H.; Huang, B.; Lu, J.; Wang, Z.; Xu, B.; Qin, X.; Zhang, X.; Dai, Y. Synergistic effect of crystal and electronic structures on the visible-light-driven photocatalytic performances of Bi₂O₃ polymorphs. Phys. Chem. Chem. Phys.; 2010; 12, pp. 15468-15475. [DOI: https://dx.doi.org/10.1039/c0cp01189d]

28. Gao, J.; Rao, S.; Yu, X.; Wang, L.; Xu, J.; Yang, J.; Liu, Q. Dimensional-matched two dimensional/two dimensional TiO₂/Bi₂O₃ step-scheme heterojunction for boosted photocatalytic performance of sterilization and water splitting. J. Colloid Interface Sci.; 2022; 628, pp. 166-178. [DOI: https://dx.doi.org/10.1016/j.jcis.2022.07.112]

29. Ke, T.; Shen, S.; Yang, K.; Lin, D. In situ fabrication of Bi₂O₃/C₃N₄/TiO₂@C photocatalysts for visible-light photodegradation of sulfamethoxazole in water. Appl. Surf. Sci.; 2022; 580, 152302. [DOI: https://dx.doi.org/10.1016/j.apsusc.2021.152302]

30. Chen, J.; Tang, T.; Feng, W.; Liu, X.; Yin, Z.; Zhang, X.; Chen, J.; Cao, S. Large-Scale Synthesis of p−n Heterojunction Bi₂O₃/TiO₂ Nanostructures as Photocatalysts for Removal of Antibiotics under Visible Light. ACS Appl. Nano Mater.; 2022; 5, pp. 1296-1307. [DOI: https://dx.doi.org/10.1021/acsanm.1c03851]

31. Ren, C.; Qiu, W.; Zhang, H.; He, Z.; Chen, Y. Degradation of benzene on TiO₂/SiO₂/Bi₂O₃ photocatalysts under UV and visible light. J. Mol. Catal. A Chem.; 2015; 398, pp. 215-222. [DOI: https://dx.doi.org/10.1016/j.molcata.2014.12.007]

32. Lal, M.; Sharma, P.; Ram, C. Calcination temperature effect on titanium oxide (TiO₂) nanoparticles synthesis. Opt. Int. J. Light Electron Opt.; 2021; 241, 166934. [DOI: https://dx.doi.org/10.1016/j.ijleo.2021.166934]

33. Li, L.; Tao, R.; Liu, Y.; Zhou, K.; Fan, X.; Han, Y.; Tang, L. Co₃O₄ nanoparticles/Bi₂O₃ nanosheets: One step synthesis, high-efficiency thermal catalytic performance, and catalytic mechanism research. Mol. Catal.; 2022; 528, 112483. [DOI: https://dx.doi.org/10.1016/j.mcat.2022.112483]

34. Bao, Y.; Guo, R.; Gao, M.; Kang, Q.; Ma, J. Morphology control of 3D hierarchical urchin-like hollow SiO₂@TiO₂ spheres for photocatalytic degradation: Influence of calcination temperature. J. Alloy. Compd.; 2021; 853, 157202. [DOI: https://dx.doi.org/10.1016/j.jallcom.2020.157202]

35. Huang, Q.; Wang, Q.; Tao, T.; Zhao, Y.; Wang, P.; Ding, Z.; Chen, M. Controlled synthesis of Bi₂O₃/TiO₂ catalysts with mixed alcohols for the photocatalytic oxidation of HCHO. Environ. Technol.; 2019; 40, pp. 1937-1947. [DOI: https://dx.doi.org/10.1080/09593330.2018.1432700] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/29364059]

36. Shamaila, S.; Sajjad, A.K.L.; Chen, F.; Zhang, J. Study on highly visible light active Bi₂O₃ loaded ordered mesoporous titania. Appl. Catal. B Environ.; 2010; 94, pp. 272-280. [DOI: https://dx.doi.org/10.1016/j.apcatb.2009.12.001]

37. Shen, J.; Wang, H.; Song, Y.; Zhou, Y.; Ye, N.; Fang, L.; Wang, L. Amorphous carbon coated TiO₂ nanocrystals embedded in a carbonaceous matrix derived from polyvinylpyrrolidone decomposition for improved Li-storage performance. Chem. Eng. J.; 2014; 240, pp. 379-386. [DOI: https://dx.doi.org/10.1016/j.cej.2013.11.095]

38. Kőrösi, L.; Bognar, B.; Horvath, M.; Schneider, G.; Kovacs, J.; Scarpellini, A.; Castelli, A.; Colombo, M.; Prato, M. Hydrothermal evolution of PF-co-doped TiO₂ nanoparticles and their antibacterial activity against carbapenem-resistant Klebsiella pneumoniae. Appl. Catal. B Environ.; 2018; 231, pp. 115-122. [DOI: https://dx.doi.org/10.1016/j.apcatb.2018.03.012]

39. Korösi, L.; Pertics, B.; Schneider, G.; Bognár, B.; Kovács, J.; Meynen, V.; Scarpellini, A.; Pasquale, L.; Prato, M. Photocatalytic inactivation of plant pathogenic bacteria using TiO₂ nanoparticles prepared hydrothermally. Nanomaterials; 2020; 10, 1730. [DOI: https://dx.doi.org/10.3390/nano10091730]

40. Pelaez, M.; Nolan, N.T.; Pillai, S.C.; Seery, M.K.; Falaras, P.; Kontos, A.G.; Dunlop, P.S.M.; Hamilton, J.W.J.; Byrne, J.A.; O'Shea, K. et al. A review on the visible light active titanium dioxide photocatalysts for environmental applications. Appl. Catal. B Environ.; 2012; 125, pp. 331-349. [DOI: https://dx.doi.org/10.1016/j.apcatb.2012.05.036]

41. Hu, R.; Xiao, X.; Tu, S.; Zuo, X.; Nan, J. Synthesis of flower-like heterostructured  -Bi₂O₃/Bi₂O₂CO₃microspheres using Bi₂O₂CO₃self-sacrifice precursor and itsvisible-light-induced photocatalytic degradation of o-phenylphenol. Appl. Catal. B Environ.; 2015; 163, pp. 510-519. [DOI: https://dx.doi.org/10.1016/j.apcatb.2014.08.025]

42. Xu, D.; Hai, Y.; Zhang, X.; Zhang, S.; He, R. Bi₂O₃ cocatalyst improving photocatalytic hydrogen evolution performance of TiO₂. Appl. Surf. Sci.; 2017; 400, pp. 530-536. [DOI: https://dx.doi.org/10.1016/j.apsusc.2016.12.171]

43. Alhaddad, M.; Ismail, A.A.; Alghamdi, Y.G.; Al-Khathami, N.D.; Mohamed, R.M. Co₃O₄ Nanoparticles Accommodated Mesoporous TiO₂ framework as an Excellent Photocatalyst with Enhanced Photocatalytic Properties. Opt. Mater.; 2022; 131, 112643. [DOI: https://dx.doi.org/10.1016/j.optmat.2022.112643]

44. Hu, J.; Zhao, R.; Li, H.; Xu, Z.; Dai, H.; Gao, H.; Yu, H.; Wang, Z.; Wang, Y.; Liu, Y. et al. Boosting visible light photocatalysis in an Au@TiO₂ yolk-in-shell nanohybrid. Appl. Catal. B Environ.; 2022; 303, 120869. [DOI: https://dx.doi.org/10.1016/j.apcatb.2021.120869]

45. Duan, X.; Yang, J.; Hu, G.; Yang, C.; Chen, Y.; Liu, Q.; Ren, S.; Li, J. Optimization of TiO₂/ ZSM-5 photocatalysts: Energy band engineering by solid state diffusion method with calcination. J. Environ. Chem. Eng.; 2021; 9, 105563. [DOI: https://dx.doi.org/10.1016/j.jece.2021.105563]

46. Majhi, D.; Das, K.; Mishra, A.; Dhiman, R.; Mishra, B.G. One pot synthesis of CdS/BiOBr/Bi₂O₂CO₃: A novel ternary double Zscheme heterostructure photocatalyst for efficient degradation of atrazine. Appl. Catal. B Environ.; 2020; 260, 118222. [DOI: https://dx.doi.org/10.1016/j.apcatb.2019.118222]

47. Kőrösi, L.; Bognár, B.; Bouderias, S.; Castelli, A.; Scarpellini, A.; Pasquale, L.; Prato, M. Highly-efficient photocatalytic generation of superoxide radicals by phase-pure rutile TiO₂ nanoparticles for azo dye removal. Appl. Surf. Sci.; 2019; 493, pp. 719-728. [DOI: https://dx.doi.org/10.1016/j.apsusc.2019.06.259]

48. Bina, B.; Fatehizadeh, A.; Taheri, E.; Heydari, M.; Darvishmotevalli, M.; Bazmeh, A. Atenolol removal from aqueous solutions using Bi₂O₃/TiO₂ under UV-C and visible light irradiations. Int. J. Environ. Anal. Chem.; 2022; pp. 1-22. [DOI: https://dx.doi.org/10.1080/03067319.2022.2085045]