Investigation of the Photocatalytic Performance, Mechanism, and Degradation Pathways of Rhodamine B with Bi2O3 Microrods under Visible-Light Irradiation

In the present work, the photodegradation of Rhodamine B with different pH values by using Bi2O3 microrods under visible-light irradiation was studied in terms of the dye degradation efficiency, active species, degradation mechanism, and degradation pathway. X-ray diffractometry, polarized optical microscopy, scanning electron microscopy, fluorescence spectrophotometry, diffuse reflectance spectra, Brunauer–Emmett–Teller, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, UV–visible spectrophotometry, total organic carbon, and liquid chromatography–mass spectroscopy analysis techniques were used to analyze the crystal structure, morphology, surface structures, band gap values, catalytic performance, and mechanistic pathway. The photoluminescence spectra and diffuse reflectance spectrum (the band gap values of the Bi2O3 microrods are 2.79 eV) reveals that the absorption spectrum extended to the visible region, which resulted in a high separation and low recombination rate of electron–hole pairs. The photodegradation results of Bi2O3 clearly indicated that Rhodamine B dye had removal efficiencies of about 97.2%, 90.6%, and 50.2% within 120 min at the pH values of 3.0, 5.0, and 7.0, respectively. In addition, the mineralization of RhB was evaluated by measuring the effect of Bi2O3 on chemical oxygen demand and total organic carbon at the pH value of 3.0. At the same time, quenching experiments were carried out to understand the core reaction species involved in the photodegradation of Rhodamine B solution at different pH values. The results of X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and X-ray diffractometer analysis of pre- and post-Bi2O3 degradation showed that BiOCl was formed on the surface of Bi2O3, and a BiOCl/Bi2O3 heterojunction was formed after acid photocatalytic degradation. Furthermore, the catalytic degradation of active substances and the possible mechanism of the photocatalytic degradation of Rhodamine B over Bi2O3 at different pH values were analyzed based on the results of X-ray diffractometry, radical capture, Fourier-transform infrared spectroscopy, total organic carbon analysis, and X-ray photoelectron spectroscopy. The degradation intermediates of Rhodamine B with the Bi2O3 photocatalyst in visible light were also identified with the assistance of liquid chromatography–mass spectroscopy.

Recently, Bi 2 O 3 was used as a highly efficient photocatalyst to decompose RhB in the presence of different light sources due to the different reactive oxygen species (ROSs) generated by Bi 2 O 3 in the presence of light [59,60].Generally, Bi 2 O 3 can degrade RhB pollutants through the production of reactive substances (•O −2 , •OH, h + , and e − ) [61,62].Liu et al. [63] reported that Bi 2 O 3 nanoparticles exhibited good activity against RhB pollutants due to the microstructure of the Bi 2 O 3 nanoparticles and the oxygen vacancy defects of the fluorite structure.Furthermore, their capture experiments confirmed that the RhB photodegradation process was contributed to by •O −2 and h + .Meena and co-authors [64] found that very small amounts of Bi 2 O 3 nanoparticles could completely reduce RhB pollutants with an excess of NaBH 4 within 15 min of irradiation, and the results showed that e − and •O −2 played an important role in the photodegradation of RhB with Bi 2 O 3 nanoparticles.Teng et al. [65] reported that both •OH and •O −2 radicals were important reactants in the photocatalytic process of RhB (10 mg/L) at a pH value of 10 using α-Bi 2 O 3 nanoparticles as photocatalysts that were driven by sunlight.Bera et al. [66] found that RhB• + and •OH radicals produced by RhB dye might be the main degrading agents in the degradation of RhB with α-β Bi 2 O 3 as photocatalysts.As far as we know, however, there are few studies on the catalytic degradation of active substances of RhB with Bi 2 O 3 at different pH values under visible light.Moreover, the possible degradation pathways of RhB with Bi 2 O 3 microrods as photocatalysts have rarely been reported.
In this study, Bi 2 O 3 microrods was fabricated for use as a catalytic material via chemical precipitation.The photocatalytic activity of the Bi 2 O 3 microrods against RhB at different pH values was studied by using UV-vis spectroscopy.The structural characteristics, morphology, band gap values, PL spectra, surface chemical components, and degradation pathway of RhB with the synthesized Bi 2 O 3 microrods were also determined by using XRD, POM, SEM, PL, DRS, BET, XPS, FT-IR, UV-Vis, and TOC analyses.In addition, the possible mechanism of photocatalytic degradation of RhB with Bi 2 O 3 at different pH values was deduced according to the results of XRD, radical capture, FTIR, TOC, and XPS analyses.The degradation intermediates of RhB with the Bi 2 O 3 photocatalyst in visible light were also identified with the assistance of liquid chromatography-mass spectroscopy (LC-MS), and a reasonable mechanism path was drawn according to LC-MS.

Preparation of Bi 2 O 3 Microrods
In a typical synthesis process, 10 mL of 1 mol/L Bi(NO 3 ) 3 solution (0.02 mol HNO 3 solution, 98 wt%) was first transferred into a three-way flask and stirred evenly.Then, NaOH (3.0 g) was dissolved in 70 mL of distilled water, slowly dripped into the above solution in a stirred state, and heated at 70 • C for 50 min.After the reaction was completed, the as-prepared yellow products were separated through vacuum filtration and washed with ethanol and deionized water; then, the samples were dried at 60 • C for 4 h.

Photocatalytic Experiment on Bi 2 O 3 Microrods
In a typical photocatalytic experiment, under laboratory conditions, 30 mg of yellow microrod-shaped Bi 2 O 3 powders and 100 mL of 10 ppm RhB solution (pH = 3, adjusted with HCl) were added successively to a 100 mL beaker with ultrasonic dispersion for 5 min to mix the Bi 2 O 3 powders with the RhB solution to form a uniform suspension.Then, the beaker was placed in a darkroom, and the suspension was continuously stirred for 2 h.Subsequently, the beaker was placed under a 500 W iodine-tungsten lamp for illumination with stirring for 2 h.In this process, the distance from the iodine-tungsten lamp to the surface of the RhB solution was kept at 20 cm, and 2 mL of RhB solution was taken at intervals.Then, the Bi 2 O 3 powder in the RhB solution was removed with a centrifuge and reserved.

Characterization
Figure 1 shows the XRD patterns of the Bi 2 O 3 powders with Bi(NO 3 ) 3 as a precursor at 70 • C for 50 min in alkaline solutions.It was obvious that all of the diffraction peaks were matched with the standard card of as-synthesized Bi 2 O 3 powders (JCPDS No. 71-2274), and no impurities were observed, which indicated that the sample obtained was high-purity Bi 2 O 3 .These results of the XRD patterns with intense and sharp diffraction peaks indicated that the as-prepared Bi 2 O 3 sample was well crystalized.The concentration of the NaOH solution was a major factor affecting the growth rate, morphology, and size of the Bi2O3 crystals.While keeping other experimental parameters unchanged, we studied the effects of the morphology of Bi2O3 by changing the content of NaOH. Figure 2 shows POM and SEM images of the Bi2O3 microrods with different contents of NaOH while maintaining the volume of the NaOH solution at 70 mL and using a reaction time of 50 min.As Figure 2a shows, the Bi2O3 nanomaterials obtained were composed of Bi2O3 microrods at the NaOH content of 0.3 g.As the NaOH content was increased, the particle size of the Bi2O3 microrods increased (Figure 2b), and the scale uniformity of the Bi2O3 crystals decreased, while some of the crystals adhered to each other, as Figure 2c shows, which was consistent with the SEM images in Figure 2d.The SEM images (Figure 2d) clearly showed that the as-synthesized Bi2O3 had rod-like structures with a ratio of the length (~50 µm) to the diameter of about 10 (3-4 µm).In addition, as the reaction temperature increased, the rod-shaped Bi2O3 crystals became shorter, and the products consisted of microrods and irregular particles (see Supporting Information   The concentration of the NaOH solution was a major factor affecting the growth rate, morphology, and size of the Bi 2 O 3 crystals.While keeping other experimental parameters unchanged, we studied the effects of the morphology of Bi 2 O 3 by changing the content of NaOH. Figure 2 shows POM and SEM images of the Bi 2 O 3 microrods with different contents of NaOH while maintaining the volume of the NaOH solution at 70 mL and using a reaction time of 50 min.As Figure 2a shows, the Bi 2 O 3 nanomaterials obtained were composed of Bi 2 O 3 microrods at the NaOH content of 0.3 g.As the NaOH content was increased, the particle size of the Bi 2 O 3 microrods increased (Figure 2b), and the scale uniformity of the Bi 2 O 3 crystals decreased, while some of the crystals adhered to each other, as Figure 2c shows, which was consistent with the SEM images in Figure 2d.The SEM images (Figure 2d) clearly showed that the as-synthesized Bi 2 O 3 had rod-like structures with a ratio of the length (~50 µm) to the diameter of about 10 (3-4 µm).In addition, as the reaction temperature increased, the rod-shaped Bi 2 O 3 crystals became shorter, and the products consisted of microrods and irregular particles (see Supporting Information Figure S1).The concentration of the NaOH solution was a major factor affecting the growth rate, morphology, and size of the Bi2O3 crystals.While keeping other experimental parameters unchanged, we studied the effects of the morphology of Bi2O3 by changing the content of NaOH. Figure 2 shows POM and SEM images of the Bi2O3 microrods with different contents of NaOH while maintaining the volume of the NaOH solution at 70 mL and using a reaction time of 50 min.As Figure 2a shows, the Bi2O3 nanomaterials obtained were composed of Bi2O3 microrods at the NaOH content of 0.3 g.As the NaOH content was increased, the particle size of the Bi2O3 microrods increased (Figure 2b), and the scale uniformity of the Bi2O3 crystals decreased, while some of the crystals adhered to each other, as Figure 2c shows, which was consistent with the SEM images in Figure 2d.The SEM images (Figure 2d) clearly showed that the as-synthesized Bi2O3 had rod-like structures with a ratio of the length (~50 µm) to the diameter of about 10 (3-4 µm).In addition, as the reaction temperature increased, the rod-shaped Bi2O3 crystals became shorter, and the products consisted of microrods and irregular particles (see Supporting Information

Band Gap Energy Value
The optical band gap values of the Bi 2 O 3 microrods were analyzed by using UV-vis DSR and UV-vis spectra based on the Kubelka-Munk method [67,68], as illustrated in Figure 3. Figure 3a shows that the variations in the band gap energy values of the Bi 2 O 3 microrods were in the range of 2.25-3.18eV.These results were supported by the UV-DRS spectrum analysis, as depicted in Figure 3b, where the band gap energy values of 2.79 eV obtained for the Bi 2 O 3 microrods were consistent with the experimental findings [69][70][71].The outcomes demonstrated that the as-prepared Bi 2 O 3 microrods with a band gap of 2.79 eV were enough to activate photocatalysis under visible light.

Band Gap Energy Value
The optical band gap values of the Bi2O3 microrods were analyzed by using UV-vis DSR and UV-vis spectra based on the Kubelka-Munk method [67,68], as illustrated in Figure 3. Figure 3a shows that the variations in the band gap energy values of the Bi2O3 microrods were in the range of 2.25-3.18eV.These results were supported by the UV-DRS spectrum analysis, as depicted in Figure 3b, where the band gap energy values of 2.79 eV obtained for the Bi2O3 microrods were consistent with the experimental findings [69][70][71].The outcomes demonstrated that the as-prepared Bi2O3 microrods with a band gap of 2.79 eV were enough to activate photocatalysis under visible light.

PL Analysis of the Bi2O3 Microrods
The efficiency of photocatalysis was determined by the separation of e -and h + .The recombination rates of both e -and h + in the Bi2O3 microrods were studied by using photoluminescence spectroscopy (PL) with an excitation wavelength of 434 nm, voltage of 700 V, and scan speed of 240 nm/min, as depicted in Figure 4. Figure 4 shows that a strong emission was generated at 654 nm due to the probability of charge separation and recombination between the CB and VB of the as-prepared Bi2O3 microrods.In the PL spectrum, there was a significant reduction in PL intensity below 640 nm, resulting in lower charge recombination rates and better charge carrier separation, which led to higher photocatalytic efficiency [48].

PL Analysis of the Bi 2 O 3 Microrods
The efficiency of photocatalysis was determined by the separation of e -and h + .The recombination rates of both e -and h + in the Bi 2 O 3 microrods were studied by using photoluminescence spectroscopy (PL) with an excitation wavelength of 434 nm, voltage of 700 V, and scan speed of 240 nm/min, as depicted in Figure 4. Figure 4 shows that a strong emission was generated at 654 nm due to the probability of charge separation and recombination between the CB and VB of the as-prepared Bi 2 O 3 microrods.In the PL spectrum, there was a significant reduction in PL intensity below 640 nm, resulting in lower charge recombination rates and better charge carrier separation, which led to higher photocatalytic efficiency [48].

Band Gap Energy Value
The optical band gap values of the Bi2O3 microrods were analyzed by using UV-vis DSR and UV-vis spectra based on the Kubelka-Munk method [67,68], as illustrated in Figure 3. Figure 3a shows that the variations in the band gap energy values of the Bi2O3 microrods were in the range of 2.25-3.18eV.These results were supported by the UV-DRS spectrum analysis, as depicted in Figure 3b, where the band gap energy values of 2.79 eV obtained for the Bi2O3 microrods were consistent with the experimental findings [69][70][71].The outcomes demonstrated that the as-prepared Bi2O3 microrods with a band gap of 2.79 eV were enough to activate photocatalysis under visible light.

PL Analysis of the Bi2O3 Microrods
The efficiency of photocatalysis was determined by the separation of e -and h + .The recombination rates of both e -and h + in the Bi2O3 microrods were studied by using photoluminescence spectroscopy (PL) with an excitation wavelength of 434 nm, voltage of 700 V, and scan speed of 240 nm/min, as depicted in Figure 4. Figure 4 shows that a strong emission was generated at 654 nm due to the probability of charge separation and recombination between the CB and VB of the as-prepared Bi2O3 microrods.In the PL spectrum, there was a significant reduction in PL intensity below 640 nm, resulting in lower charge recombination rates and better charge carrier separation, which led to higher photocatalytic efficiency [48].

Adsorption and Degradation of RhB with Bi 2 O 3 Microrods
Figure 5 shows the N 2 adsorption-desorption isotherms, aperture distribution curve, and RhB solution photodegradation curve of the Bi 2 O 3 microrods (see Figure 2d) at different pH values in a dark room.Figure 5a shows that the Bi 2 O 3 microrods are a mesoporous material with a type IV and H 3 hysteresis loop [72].BET analysis revealed that the BET surface area, pore volume, and average pore size of the Bi 2 O 3 microrods were 4.7846 m 2 /g, 0.012 cm 3 /g, and 10.032 nm, respectively.These results confirmed that the large surface area and pore volume of the Bi 2 O 3 microrods could expose more active sites, which was beneficial for the subsequent adsorption and photocatalysis of RhB. Figure 5b shows the blank experimental results of RhB degradation when catalyzed at different pH values in a dark room.As shown in Figure 5b, in the absence of a catalyst, the degradation of RhB could be ignored in the presence of darkness for 120 min.The curves plotted in the presence of the Bi 2 O 3 microrod catalysts at the pH values of 3.0, 5.0, and 7.0 showed only 9.4%, 8.6%, and 8.1% decolorization after 120 min in the dark, respectively, confirming that the decolorization of RhB solutions was dominated by surface adsorption.

Adsorption and Degradation of RhB with Bi2O3 Microrods
Figure 5 shows the N2 adsorption-desorption isotherms, aperture distribution curve, and RhB solution photodegradation curve of the Bi2O3 microrods (see Figure 2d) at different pH values in a dark room.Figure 5a shows that the Bi2O3 microrods are a mesoporous material with a type IV and H 3 hysteresis loop [72].BET analysis revealed that the BET surface area, pore volume, and average pore size of the Bi2O3 microrods were 4.7846 m 2 /g, 0.012 cm 3 /g, and 10.032 nm, respectively.These results confirmed that the large surface area and pore volume of the Bi2O3 microrods could expose more active sites, which was beneficial for the subsequent adsorption and photocatalysis of RhB. Figure 5b shows the blank experimental results of RhB degradation when catalyzed at different pH values in a dark room.As shown in Figure 5b, in the absence of a catalyst, the degradation of RhB could be ignored in the presence of darkness for 120 min.The curves plotted in the presence of the Bi2O3 microrod catalysts at the pH values of 3.0, 5.0, and 7.0 showed only 9.4%, 8.6%, and 8.1% decolorization after 120 min in the dark, respectively, confirming that the decolorization of RhB solutions was dominated by surface adsorption.The photocatalytic properties of RhB with the Bi2O3 microrods under visible light at a pH of 3.0 are shown in Figure 6. Figure 6a summarizes the degradation efficiencies and λmax shifts (maximum wave peak displacement of RhB) of RhB with the irradiation time when using the Bi2O3 microrods in a typical photocatalytic experiment at a pH of 3.0.The maximum peak of RhB shifted blue, and the maximum absorbance gradually decreased with the increase in the illumination time, as shown in Supplementary Figure S2.After 120 min of irradiation, 97.2% degradation of RhB was achieved with the Bi2O3 microrods as photocatalysts, as shown in Figure 6a(1).Compared with hydrogen-peroxide-activated commercial P25 TiO2, the degradation efficiency of RhB with P25 TiO2 under visible light was only 55.4%, as shown in Supplementary Figure S3; therefore, the as-prepared Bi2O3 microrod catalysts were also suitable for commercial application.According to Figure 6a(2), the maximum absorption peak varied gradually from 554 to 498 nm with the prolongation of the visible-light exposure time, and the hypochromic shifts of λmax were caused by the deethylation of RhB, which was confirmed by the FTIR spectra before and after RhB degradation (see Supplementary Figure S4).As shown by the decolorization of the RhB dyes, it is possible that other colorless organic molecules were formed during the degradation process, but this was not identified in the decolorization reaction.The mineralization of RhB using Bi2O3 was confirmed by the amounts of TOC and COD remaining in the decolorized RhB solutions; these were detected using a TOC analyzer and the common volumetric method, respectively.The removal efficiencies of COD and TOC in the degraded RhB solution were 67.6% and 62.6%, respectively, after 120 min treatment, as seen in Figure 6b(1,2).According to the results of the TOC analysis (Figure 6b(2)), the TOC The photocatalytic properties of RhB with the Bi 2 O 3 microrods under visible light at a pH of 3.0 are shown in Figure 6. Figure 6a summarizes the degradation efficiencies and λ max shifts (maximum wave peak displacement of RhB) of RhB with the irradiation time when using the Bi 2 O 3 microrods in a typical photocatalytic experiment at a pH of 3.0.The maximum peak of RhB shifted blue, and the maximum absorbance gradually decreased with the increase in the illumination time, as shown in Supplementary Figure S2.After 120 min of irradiation, 97.2% degradation of RhB was achieved with the Bi 2 O 3 microrods as photocatalysts, as shown in Figure 6a(1).Compared with hydrogen-peroxide-activated commercial P25 TiO 2 , the degradation efficiency of RhB with P25 TiO 2 under visible light was only 55.4%, as shown in Supplementary Figure S3; therefore, the as-prepared Bi 2 O 3 microrod catalysts were also suitable for commercial application.According to Figure 6a(2), the maximum absorption peak varied gradually from 554 to 498 nm with the prolongation of the visible-light exposure time, and the hypochromic shifts of λ max were caused by the deethylation of RhB, which was confirmed by the FTIR spectra before and after RhB degradation (see Supplementary Figure S4).As shown by the decolorization of the RhB dyes, it is possible that other colorless organic molecules were formed during the degradation process, but this was not identified in the decolorization reaction.The mineralization of RhB using Bi 2 O 3 was confirmed by the amounts of TOC and COD remaining in the decolorized RhB solutions; these were detected using a TOC analyzer and the common volumetric method, respectively.The removal efficiencies of COD and TOC in the degraded RhB solution were 67.6% and 62.6%, respectively, after 120 min treatment, as seen in Figure 6b(1,2).According to the results of the TOC analysis (Figure 6b(2)), the TOC removal efficiency increased with the extension of the illumination time, and more than 62.6% of the carbon in the RhB solution produced CO 2 products [73,74].
removal efficiency increased with the extension of the illumination time, and more than 62.6% of the carbon in the RhB solution produced CO2 products [73,74].The concentration of H + ions in the solution is another key factor for the photodegradation of RhB dyes.Plots of the pH dependence of RhB degradation with different irradiation times are depicted in Figure 7.It can be seen in Figure 7a that the Bi2O3 microrod sample showed different photodegradation activity at different pH values.Within 120 min of irradiation, the degradation percentage of RhB in the environments of the pH value of 3.0, pH value of 5.0, and pH value of 7.0 was 97.2%, 90.6% and 50.2%, respectively.Figure 7b shows that the rate constant values (min −1 ) were 0.02761, 0.01698, and 0.00504 at the pH values of 3.0, 5.0, and 7.0 respectively.The rate constant values exhibited a maximum at the pH value of 3.0, as seen in the lower inset of Figure 7b.Greater RhB degradation at a lower pH value could be seen in this result, and this was attributed to the increased formation and accumulation of H2O2 and •OH radicals at acidic pH levels [66,75], which led to an increase in the degradation rate of RhB, resulting in a higher degradation rate higher H + concentration than those at neutral levels (pH = 7.0).The results of the pH dependence experiments showed that the as-prepared Bi2O3 microrods exhibited good photocatalytic performance for RhB removal at a higher H + concentration.The concentration of H + ions in the solution is another key factor for the photodegradation of RhB dyes.Plots of the pH dependence of RhB degradation with different irradiation times are depicted in Figure 7.It can be seen in Figure 7a that the Bi 2 O 3 microrod sample showed different photodegradation activity at different pH values.Within 120 min of irradiation, the degradation percentage of RhB in the environments of the pH value of 3.0, pH value of 5.0, and pH value of 7.0 was 97.2%, 90.6% and 50.2%, respectively.Figure 7b shows that the rate constant values (min −1 ) were 0.02761, 0.01698, and 0.00504 at the pH values of 3.0, 5.0, and 7.0 respectively.The rate constant values exhibited a maximum at the pH value of 3.0, as seen in the lower inset of Figure 7b.Greater RhB degradation at a lower pH value could be seen in this result, and this was attributed to the increased formation and accumulation of H 2 O 2 and •OH radicals at acidic pH levels [66,75], which led to an increase in the degradation rate of RhB, resulting in a higher degradation rate higher H + concentration than those at neutral levels (pH = 7.0).The results of the pH dependence experiments showed that the as-prepared Bi 2 O 3 microrods exhibited good photocatalytic performance for RhB removal at a higher H + concentration.
removal efficiency increased with the extension of the illumination time, and more than 62.6% of the carbon in the RhB solution produced CO2 products [73,74].The concentration of H + ions in the solution is another key factor for the photodegradation of RhB dyes.Plots of the pH dependence of RhB degradation with different irradiation times are depicted in Figure 7.It can be seen in Figure 7a that the Bi2O3 microrod sample showed different photodegradation activity at different pH values.Within 120 min of irradiation, the degradation percentage of RhB in the environments of the pH value of 3.0, pH value of 5.0, and pH value of 7.0 was 97.2%, 90.6% and 50.2%, respectively.Figure 7b shows that the rate constant values (min −1 ) were 0.02761, 0.01698, and 0.00504 at the pH values of 3.0, 5.0, and 7.0 respectively.The rate constant values exhibited a maximum at the pH value of 3.0, as seen in the lower inset of Figure 7b.Greater RhB degradation at a lower pH value could be seen in this result, and this was attributed to the increased formation and accumulation of H2O2 and •OH radicals at acidic pH levels [66,75], which led to an increase in the degradation rate of RhB, resulting in a higher degradation rate higher H + concentration than those at neutral levels (pH = 7.0).The results of the pH dependence experiments showed that the as-prepared Bi2O3 microrods exhibited good photocatalytic performance for RhB removal at a higher H + concentration.

XPS and FTIR Analyses of the Bi2O3 Microrods
The elements and chemical components of pre-and post-photocatalytic degradation Bi2O3 microrods at different pH values were analyzed using XPS and FTIR determination, and the results are displayed in Figure 9.The elements of C, Bi, and O can be observed in full-scan spectrum shown in Figure 9a, which indicates that these three elements coexisted in Bi2O3 before and after degradation.The presence of C may have been introduced into the environment during sample preparation [76,77].A new element, Cl, was observed in the Bi2O3 after photodegradation at a pH value of 3.0, as shown in Figure 9a(3).Highresolution XPS (HR-XPS) analysis of Bi4f in the pre-and post-photocatalytic degradation Bi2O3 samples (Figure 9b) showed that two feature peaks of the binding energies of 159.1 and 164.5 eV corresponded to Bi 4f7/2 and Bi 4f5/2 of the trivalent bismuth ion (Bi 3+ ), respectively [78,79]. Figure 9c displays the HR-XPS analysis of the post-degradation Bi2O3 microrods, from which we can see that the two highest-intensity peaks located at around 199.8 and 198.2 eV corresponded to Cl 2p1/2 and Cl 2p3/2 in the region of Cl 2p, respectively, which demonstrated that BiOCl was easily produced on the surface of Bi2O3 after degradation in the acidic environment [80].
The structural characteristics of Bi2O3 microrods pre-and post-degradation at different pH values were further determined via FTIR analysis, and the results are illustrated in Figure 9d.

XPS and FTIR Analyses of the Bi 2 O 3 Microrods
The elements and chemical components of pre-and post-photocatalytic degradation Bi 2 O 3 microrods at different pH values were analyzed using XPS and FTIR determination, and the results are displayed in Figure 9.The elements of C, Bi, and O can be observed in full-scan spectrum shown in Figure 9a, which indicates that these three elements coexisted in Bi 2 O 3 before and after degradation.The presence of C may have been introduced into the environment during sample preparation [76,77].A new element, Cl, was observed in the Bi 2 O 3 after photodegradation at a pH value of 3.0, as shown in Figure 9a(3).Highresolution XPS (HR-XPS) analysis of Bi4f in the pre-and post-photocatalytic degradation Bi 2 O 3 samples (Figure 9b) showed that two feature peaks of the binding energies of 159.1 and 164.5 eV corresponded to Bi 4f 7/2 and Bi 4f 5/2 of the trivalent bismuth ion (Bi 3+ ), respectively [78,79]. Figure 9c displays the HR-XPS analysis of the post-degradation Bi 2 O 3 microrods, from which we can see that the two highest-intensity peaks located at around 199.8 and 198.2 eV corresponded to Cl 2p 1/2 and Cl 2p 3/2 in the region of Cl 2p, respectively, which demonstrated that BiOCl was easily produced on the surface of Bi 2 O 3 after degradation in the acidic environment [80].
The structural characteristics of Bi 2 O 3 microrods pre-and post-degradation at different pH values were further determined via FTIR analysis, and the results are illustrated in Figure 9d.

Photodegradation Pathways of RhB with Bi2O3
To reveal the degradation pathway and mechanism of RhB with the Bi2O3 photocatalyst, the intermediates were determined by using LC-MS, as depicted in Figure 10.According to Figure 10  According to the intermediates analyzed with LC-MS during the reaction and in the previous literature [86,87], a possible RhB degradation pathway was proposed, as shown in Figure 11. Figure 11 displays that the degradation process of RhB mainly consisted of five steps: deethylation, decarboxylation, de-amination, ring opening, and mineralization.Initially, ethyl, carboxyl, and amino groups were removed from RhB molecules and formed multiple intermediates through ROS (•O−2 and •OH) attack, such as C22H19N2O3

Photodegradation Pathways of RhB with Bi2O3
To reveal the degradation pathway and mechanism of RhB with the Bi2O3 photocatalyst, the intermediates were determined by using LC-MS, as depicted in Figure 10.According to Figure 10  According to the intermediates analyzed with LC-MS during the reaction and in the previous literature [86,87], a possible RhB degradation pathway was proposed, as shown in Figure 11.According to the intermediates analyzed with LC-MS during the reaction and in the previous literature [86,87], a possible RhB degradation pathway was proposed, as shown in Figure 11.

Degradation Mechanism
The types of active substances formed during photodegradation and the possible mechanism of the catalytic degradation of RhB with Bi2O3 microrods at the pH values of 7.0 and 3.0 were determined with a radical-trapping experiment, and the results are displayed in Figure 12a.In general, glucose, IPA, and AC were introduced as scavengers of h + , •OH, and •O−2 during photodegradation, respectively.Figure 12a(1) shows that, on the basis of the photocatalytic experiments at a pH value of 3.0, the degradation rates of RhB were 18.5% and 68.3% after the addition of AC and IPA, respectively.However, the photodegradation of RhB had little change after adding glucose, indicating that there was a small amount of h+ during the catalytic process.The experimental results indicated that •O−2 and •OH were the effective active substances in the photodegradation process of RhB with Bi2O3 microrods at a pH value of 3.0.Figure 12a(2) displays that hole (h + ) or hydroxyl radicals (•OH) were the effective active substances in the photodegradation of

Degradation Mechanism
The types of active substances formed during photodegradation and the possible mechanism of the catalytic degradation of RhB with Bi 2 O 3 microrods at the pH values of 7.0 and 3.0 were determined with a radical-trapping experiment, and the results are displayed in Figure 12a.In general, glucose, IPA, and AC were introduced as scavengers of h + , •OH, and •O −2 during photodegradation, respectively.Figure 12b(1) shows that, on the basis of the photocatalytic experiments at a pH value of 3.0, the degradation rates of RhB were 18.5% and 68.3% after the addition of AC and IPA, respectively.However, the photodegradation of RhB had little change after adding glucose, indicating that there was a small amount of h+ during the catalytic process.According to the above discussion, a possible photocatalytic mechanism of RhB with Bi2O3 microrods at the pH values of 7.0 and 3.0 was proposed, as depicted in Figure 12b.As can been seen in Figure 12(1), the mechanism of photocatalytic degradation of RhB with Bi2O3 was summarized at a pH value of 7.0.In the presence of visible light, e − in the VB of the semiconducting Bi2O3 microrod photocatalyst was excited to CB, and VB produced photogenerated h + , which partially complexed with e − of CB; then, some photogenerated h + reacted with H2O or OH − to form •OH. Thus, RhB molecules reacted with the effective active substances of h + and •OH and formed multiple small intermediates, which were mineralized into CO2 and H2O.At the same time, a rational photocatalytic degradation mechanism of RhB with Bi2O3 microrods under acidic conditions was also proposed (pH value = 3.0), and a schematic is shown in Figure 12(2).During the initial degradation, the surface layer of the Bi2O3 microrods was dissolved by HCl, and a heterojunction of BiOCl/Bi2O3 was formed with a small amount of BiOCl on the surface of the Bi2O3 samples, which would facilitate the migration of photoinduced charge carriers [88].Furthermore, the addition of HCl made it easier for O2 to capture the e − of photocatalyst CB and produce •O−2, which could further convert •OH in the presence of H + .After multiple photodegradation cycles, the contact probability of Bi2O3 with RhB decreased with the increase in BiOCl content on the surface of Bi2O3 in the BiOCl/Bi2O3 heterostructure (Figure 8b, XRD pattern); the photocharge carrier migration was weakened, and the removal rate of RhB significantly decreased.

Conclusions
In summary, the results of the POM and SEM analyses showed that the Bi2O3 catalyst with a microrod-like structure was prepared with a chemical precipitation method.The results of the PL spectra and DRS (the band gap value of the Bi2O3 microrods is 2.79 eV) revealed that the absorption spectrum extended to the visible region, which resulted in a high separation and low recombination rate of e − and h + .The photodegradation results of Bi2O3 clearly indicated that about 97.2%, 90.6%, and 50.2% degradation of RhB dyes was observed within 120 min at the pH values of 3.0, 5.0, and 7.0, respectively.The TOC removal efficiency increased with the extension of the illumination time, and more than 62.6% of the carbon in the RhB solution produced CO2 products.The experimental results

Conclusions
In summary, the results of the POM and SEM analyses showed that the Bi 2 O 3 catalyst with a microrod-like structure was prepared with a chemical precipitation method.The results of the PL spectra and DRS (the band gap value of the Bi 2 O 3 microrods is 2.79 eV) revealed that the absorption spectrum extended to the visible region, which resulted in a high separation and low recombination rate of e − and h + .The photodegradation results of Bi 2 O 3 clearly indicated that about 97.2%, 90.6%, and 50.2% degradation of RhB dyes was observed within 120 min at the pH values of 3.0, 5.0, and 7.0, respectively.The TOC removal efficiency increased with the extension of the illumination time, and more than 62.6% of the carbon in the RhB solution produced CO 2 products.The experimental results indicated that •O −2 , •OH, and h + or •OH were the effective active substances in the degradation process of RhB with Bi 2 O 3 microrods at the pH values of 3.0 and 7.0, respectively.The results also revealed that a heterojunction of BiOCl/Bi 2 O 3 was formed with a small amount of BiOCl on the surface of Bi 2 O 3 samples based on the results of XRD, XPS, and FTIR analysis techniques.Furthermore, the effective active substances and possible mechanisms of photocatalytic degradation of Bi 2 O 3 at different pH values were analyzed based on the results of XRD, radical capture, FTIR, TOC, and XPS analyses.The degradation process of RhB mainly consisted of five steps: deethylation, decarboxylation, de-amination, ring opening, and mineralization.

Figure 2 .
Figure 2. POM images of the Bi2O3 microrods with different contents of NaOH: (a) 0.3 g, (b) 0.35 g, and (c) 0.4 g; (d) SEM images of the microrods with 0.4 g of NaOH.

Figure 2 .
Figure 2. POM images of the Bi2O3 microrods with different contents of NaOH: (a) 0.3 g, (b) 0.35 g, and (c) 0.4 g; (d) SEM images of the microrods with 0.4 g of NaOH.

Figure 2 .
Figure 2. POM images of the Bi 2 O 3 microrods with different contents of NaOH: (a) 0.3 g, (b) 0.35 g, and (c) 0.4 g; (d) SEM images of the microrods with 0.4 g of NaOH.

Figure 5 .
Figure 5. (a) N2 adsorption-desorption isotherms (the inset is their aperture distribution curve in (a)) of the Bi2O3 microrods; (b) blank experiment on the degradation of RhB solutions by Bi2O3 microrod catalysts in a dark room at a pH of 3.0-7.0.

Figure 5 .
Figure 5. (a) N 2 adsorption-desorption isotherms (the inset is their aperture distribution curve in (a)) of the Bi 2 O 3 microrods; (b) blank experiment on the degradation of RhB solutions by Bi 2 O 3 microrod catalysts in a dark room at a pH of 3.0-7.0.

Figure 6 .
Figure 6.(a) (1) Degradation efficiencies and (a) (2) λmax shifts; (b) efficiency of the removal of COD and TOC from RhB solutions with Bi2O3 microrods as photocatalysts in a typical photocatalytic experiment at a pH of 3.0.

Figure 7 .
Figure 7. (a) The degradation rate and (b) corresponding pseudo-first-order kinetic data (the variation of the rate constant is presented in the lower inset) of RhB using the as-prepared Bi2O3 microrods as a photocatalyst at the pH values of (1) 3.0, (2) 5.0, and (3) 7.0.

Figure 8
Figure 8 reveals the visible-light photodegradation percentage of RhB with the Bi2O3 microrod catalyst for six cycles and the XRD patterns of the Bi2O3 microrods after six degradation cycles at different pH values.Clearly, the degradation efficiency of RhB dropped from 10.9% and 13.9% to 86.3% and 36.3% after six repetitions at the pH values of 3.0 and

Figure 6 .
Figure 6.(a) (1) Degradation efficiencies and (a) (2) λ max shifts; (b) efficiency of the removal of COD and TOC from RhB solutions with Bi 2 O 3 microrods as photocatalysts in a typical photocatalytic experiment at a pH of 3.0.

Figure 6 .
Figure 6.(a) (1) Degradation efficiencies and (a) (2) λmax shifts; (b) efficiency of the removal of COD and TOC from RhB solutions with Bi2O3 microrods as photocatalysts in a typical photocatalytic experiment at a pH of 3.0.

Figure 7 .
Figure 7. (a) The degradation rate and (b) corresponding pseudo-first-order kinetic data (the variation of the rate constant is presented in the lower inset) of RhB using the as-prepared Bi2O3 microrods as a photocatalyst at the pH values of (1) 3.0, (2) 5.0, and (3) 7.0.

Figure 8
Figure 8 reveals the visible-light photodegradation percentage of RhB with the Bi2O3 microrod catalyst for six cycles and the XRD patterns of the Bi2O3 microrods after six degradation cycles at different pH values.Clearly, the degradation efficiency of RhB dropped from 10.9% and 13.9% to 86.3% and 36.3% after six repetitions at the pH values of 3.0 and

Figure 7 .
Figure 7. (a) The degradation rate and (b) corresponding pseudo-first-order kinetic data (the variation of the rate constant is presented in the lower inset) of RhB using the as-prepared Bi 2 O 3 microrods as a photocatalyst at the pH values of (1) 3.0, (2) 5.0, and (3) 7.0.

Figure 8
Figure 8 reveals the visible-light photodegradation percentage of RhB with the Bi 2 O 3 microrod catalyst for six cycles and the XRD patterns of the Bi 2 O 3 microrods after six degradation cycles at different pH values.Clearly, the degradation efficiency of RhB

Figure 8 .
Figure 8.(a) Cycling experiments and (b) XRD images of the Bi2O3 microrods after the sixth cycle of degradation of RhB with pH values of 3.0 and 7.0.
Figure 9d(1,2) show that the spectrum of pre-and post-degradation Bi2O3 at the pH value of 7.0 displayed two typical adsorption peaks at about 1385 and 848 cm −1 , which were related to the Bi-O bond and Bi-O-Bi bond stretching vibration and symmetrical stretching, respectively [81-83].The FTIR spectra after Bi2O3 degradation at a pH value of 3.0 shown in Figure 9d(3) demonstrate that the adsorption peaks of 1460cm−1 and 1107 cm -1 in the degraded Bi2O3 microrods were caused by O-Cl and Bi-Cl bond vibrations [84,85], respectively.In conclusion, BiOCl crystals were easily produced on the surface of the Bi2O3 crystals under acidic conditions based on the results of the XPS, FTIR, and XRD (Figure 8b) analyses of pre-and post-degradation Bi2O3.

Figure 8 .
Figure 8.(a) Cycling experiments and (b) XRD images of the Bi 2 O 3 microrods after the sixth cycle of degradation of RhB with pH values of 3.0 and 7.0.

16 Figure 9 .
Figure 9. (a-c) The results of XPS and (d) FTIR analysis of pre-and post-photodegradation Bi2O3 microrods.

Figure 10 .
Figure 10.LC-MS spectra of the RhB solution before and after photodegradation with Bi2O3 microrods as a photocatalyst under irradiation.

Figure 9 .
Figure 9. (a-c) The results of XPS and (d) FTIR analysis of pre-and post-photodegradation Bi 2 O 3 microrods.

Figure 10 .
Figure 10.LC-MS spectra of the RhB solution before and after photodegradation with Bi2O3 microrods as a photocatalyst under irradiation.

Figure 10 .
Figure 10.LC-MS spectra of the RhB solution before and after photodegradation with Bi 2 O 3 microrods as a photocatalyst under irradiation.

Figure 11 .
Figure 11.The possible degradation pathways of RhB with Bi2O3 microrods at a pH value of 3.0.

Figure 11 .
Figure 11.The possible degradation pathways of RhB with Bi 2 O 3 microrods at a pH value of 3.0.
The experimental results indicated that •O −2 and •OH were the effective active substances in the photodegradation process of RhB with Bi 2 O 3 microrods at a pH value of 3.0.Figure 12b(2) displays that hole (h + ) or hydroxyl radicals (•OH) were the effective active substances in the photodegradation of RhB with Bi 2 O 3 microrods at a pH value of 7.0.The photodegradation of RhB was hard to change after adding AC compared with that in the blank experiment (without a scavenger).The results indicated that the absence of •O −2 during the degradation of RhB and the effective active species were h + and •OH in the photodegradation of RhB with Bi 2 O 3 microrods at a pH value of 7.0.RhB with Bi2O3 microrods at a pH value of 7.0.The photodegradation of RhB was hard to change after adding AC compared with that in the blank experiment (without a scavenger).The results indicated that the absence of •O−2 during the degradation of RhB and the effective active species were h + and •OH in the photodegradation of RhB with Bi2O3 microrods at a pH value of 7.0.

Figure 12 .
Figure 12.(a) Degradation profiles of RhB with Bi 2 O 3 microrods without a scavenger and with glucose, IPA, and AC as scavengers of h + , •OH, and •O −2 .(b) Degradation mechanism of Bi 2 O 3 microrods at pH values of (1) 7.0 and (2) 3.0.According to the above discussion, a possible photocatalytic mechanism of RhB with Bi 2 O 3 microrods at the pH values of 7.0 and 3.0 was proposed, as depicted in Figure12b.As can been seen in Figure12b(1), the mechanism of photocatalytic degradation of RhB with Bi 2 O 3 was summarized at a pH value of 7.0.In the presence of visible light, e − in the VB of the semiconducting Bi 2 O 3 microrod photocatalyst was excited to CB, and VB produced photogenerated h + , which partially complexed with e − of CB; then, some photogenerated h + reacted with H 2 O or OH − to form •OH. Thus, RhB molecules reacted with the effective active substances of h + and •OH and formed multiple small intermediates, which were mineralized into CO 2 and H 2 O.At the same time, a rational photocatalytic degradation mechanism of RhB with Bi 2 O 3 microrods under acidic conditions was also proposed (pH value = 3.0), and a schematic is shown in Figure12(2).During the initial degradation, the surface layer of the Bi 2 O 3 microrods was dissolved by HCl, and a heterojunction of BiOCl/Bi 2 O 3 was formed with a small amount of BiOCl on the surface of the Bi 2 O 3 samples, which would facilitate the migration of photoinduced charge carriers[88].Furthermore, the addition of HCl made it easier for O 2 to capture the e − of photocatalyst CB and produce •O −2 , which could further convert •OH in the presence of H + .After multiple photodegradation cycles, the contact probability of Bi 2 O 3 with RhB decreased with the increase in BiOCl content on the surface of Bi 2 O 3 in the BiOCl/Bi 2 O 3 heterostructure (Figure 8b, XRD pattern); the photocharge carrier migration was weakened, and the removal rate of RhB significantly decreased.