The commonly used benzophenone-3 (BP-3) as ultraviolet filter ingredients is an endocrine-disrupting chemical that has received particular attention owing to its environmental ubiquity, and it poses a threat to aquatic biota and human health. In this study, novel α-Bi2O3@g-C3N4 nanocomposites with different α-Bi2O3 contents and enhanced photocatalytic activity were synthesized by a mixing calcination method. The as-synthesized photocatalysts were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, ultraviolet–visible diffuse reflectance spectroscopy, N2 adsorption/desorption isotherm analysis, electrochemical impedance spectroscopy, photoluminescence spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. The 1 wt% α-Bi2O3@g-C3N4 composite exhibited the highest rate constant of 0.42 h-1 for photocatalytic degradation of BP-3, which was up to 6.3 times higher than that of g-C3N4 (0.07 h-1). The enhanced photocatalytic activity might be due to the enhanced separation of photogenerated electron-hole (e--h+) charge pairs and suppression of e--h+ recombination. Scavenging experiments suggested that •OH, h+ and •O2- worked together in the α-Bi2O3@g-C3N4 photocatalytic process. The EPR spectra demonstrated that the α-Bi2O3@g-C3N4 composites generated considerably more •O2- and •OH than g-C3N4. Finally, cyclic degradation experiments showed the reusability of 1 wt% α-Bi2O3@g-C3N4 for BP-3 removal.
Benzophenone-3 ((2-hydroxy-4-methoxyphenyl) phenylmethanone, BP-3) has received special attention owing to its extensive use, ubiquity, persistence as a contaminant, potential ecotoxicity to aquatic biota and health risks to humans [1, 2]. BP-3 is widely used in cosmetics as a sunscreen ingredient, industrial products and food contact materials; it is commonly used to protect skin from sunburn and materials from possible UV light-induced photochemical degradation [3, 4]. BP-3 may be introduced to aquatic environments directly through human recreational activities such as swimming and indirectly by wastewater treatment plant (WWTP) effluents, which indicates that conventional wastewater treatment processes do not effectively remove BP-3 [5, 6].
Several studies have shown the incomplete removal of BP-3 in conventional WWTPs. For example, Liu et al. reported BP-3 concentrations up to 2086 ng L-1 in the influent and up to 153 ng L-1 in the effluent, with a removal efficiency of 92% , while Wick et al. reported maximum BP-3 concentrations up to 720 ng L-1 in the influent , and Langford et al. reported maximum BP-3 concentrations up to 1915 ng L-1 in the effluent . Similarly, Li et al. reported BP-3 concentrations up to 722 ng L-1 in the influent and up to 664 ng L-1 in the effluent, with a removal efficiency of only 9%, and Tsui et al. mentioned maximum BP-3 concentrations up to 371.3 ng L-1 in the influent and up to 115.8 ng L-1 in the effluent, with a removal efficiency of 69% [10, 11].
Advanced oxidation processes (AOPs) involving heterogeneous photocatalysts can effectively remove BP-3 from WWTP effluent. AOPs utilize chemical oxidation based on the in-situ generation of free radicals (hydroxyl radical (•OH), superoxide radical (•O2-), etc.) that nonselectively react with emerging organic contaminants and are capable of completely degrading them into carbon dioxide and water [12, 13]. Heterogeneous photocatalysts have received particular interest owing to their low cost, nontoxicity, efficiency, lack of secondary pollution and environmental friendliness [14,15,16]. Wang et al. reported the photocatalytic degradation of BP-3 using PbO@TiO2 and Sb2O3@TiO2 composites and showed optimal removal efficiencies of up to 87% and 80%, respectively . Zuniga-Benitez et al. reported the photodegradation of BP-3 using UV + TiO2 treatment with an optimal remnant efficiency of 62% .
Recently, metal-free graphitic carbon nitride (g-C3N4) has been considered for use as a metal-free π-conjugated photocatalyst with an appropriate bandgap energy (~2.7 eV). It has attracted considerable attention for organic pollutant degradation, hydrogen production, carbon dioxide reduction and atmospheric purification owing to its high thermochemical stability, 2D optical structure, suitable electronic properties and inexpensive synthesis [19, 20]. The more negative conduction potential edge of g-C3N4 (-1.22 eV vs. normal hydrogen electrode (NHE)) is enough for the reduction of the adsorbed oxygen (O2/•O2-) redox couple (-0.33 eV vs. NHE) to produce •O2-. However, the photocatalytic efficiency of pristine g-C3N4 catalysts is low because of the fast recombination of photoinduced e- and h+, limited delocalized conductivity, low quantum efficiency and narrow visible light absorption range [21, 22]. Consequently, to improve the photocatalytic performance, the application of various strategies, such as heterojunction formation, surface modification, metal or anion doping and use of dye-sensitized photocatalysts, is important. Among the aforementioned strategies, heterojunction formation efficiently suppresses charge carrier recombination, leads to excellent e- and h+ separation and promotes the redox capacity of photogenerated carriers, thus enhancing the photocatalytic performance of the materials [23,24,25].
Herein, two or more semiconductors with appropriate bandgap energies are combined to fabricate type II heterojunction photocatalysts. Such photocatalysts can be formed by photogenerated electrons being transferred from the conduction band of photosystem I (PSI) to the valence band (VB) of photosystem II (PSII), while the hole is transferred in the opposite direction due to the potential difference between the conduction band and the VB edges. This potential difference improves the charge separation efficiency and suppresses recombination, which is favorable for photocatalytic performance. However, these merits are at the expense of the reduction abilities of PSI and oxidation abilities of PSII [26,27,28].
Regarding a Z-scheme photocatalytic system, the photoexcited electrons in the conduction band of PSII, which stem from visible light irradiation, may recombine with holes in the VB of PSI. Therefore, the conduction band edge of PSII should be lower than that of PSI, and the VB edge of PSI should be higher than that of PSII. Thus, strong oxidative holes and reductive electrons can be generated in the two different bands, thereby resulting in enhanced photocatalytic activity [25, 29].
Currently, a number of bismuth-containing semiconductors, such as bismuth oxide (Bi2O3), BiVO4, Bi2WO6 and BiOX (X = Cl, Br, I), are available. As a preeminent semiconductor, Bi2O3, which has a band gap of ∼2.8 eV, has received increasing attention owing to its nontoxicity, thermal stability, environmental friendliness, outstanding visible light absorption and strong oxidation when used as a photocatalyst. It has been reported that Bi2O3 exhibits six polymorphs, α, β, ϒ, δ, ε and ω phases. Among them, α-Bi2O3 has a monoclinic structure and is stable under ambient pressure and temperature conditions [27, 30, 31]. The more positive valence potential edge of α-Bi2O3 (2.89 eV vs. NHE) is enough for the oxidation of the water H2O/•OH redox couple (2.72 eV vs. NHE) and hydroxyl ion OH-/•OH redox couple (2.40 eV vs. NHE) to produce •OH radicals. Therefore, considering the matching band gap alignments and appropriate band edge potentials of semiconductors, α-Bi2O3 is a promising candidate for integration with g-C3N4 to construct a direct Z-scheme heterojunction.
In the present study, we synthesized α-Bi2O3@g-C3N4 photocatalysts with different weight percentages of α-Bi2O3 through a mixing-calcination method. The objective of our study was to investigate the photocatalytic activity of the as-synthesized samples for the degradation of BP-3 and the effects of α-Bi2O3 doping on the photocatalytic properties of g-C3N4 catalysts. The as-synthesized samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy, (ultraviolet–visible diffuse reflectance spectroscopy (UV-Vis DRS), N2 adsorption/desorption isotherm analysis, electrochemical impedance spectroscopy (EIS) and photoluminescence (PL) spectroscopy. Furthermore, a possible reaction mechanism was proposed based on the energy band positions and electron paramagnetic resonance (EPR) spectroscopy and trapping experimental findings. Finally, the reusability of the best-performing photocatalyst in the photocatalytic process was also investigated.
2 Materials and methods
All chemicals, including melamine powder (C3H6N6, 99.5%), bismuth (III) nitrate pentahydrate (Bi (NO3)3·5H2O), isopropanol (IPA, C3H8O, 99.9%), superoxide dismutase (SOD, 99%), ammonium oxalate (AO, 99.9%), BP-3 (98%), barium sulfate (BaSO4, 99%), and methanol (CH3OH, 99.9%), were purchased from Sigma–Aldrich. All reagents and precursors were used as received without further purification, and deionized water was used in this study.
2.2 Synthesis of the α-Bi2O3@g-C3N4 photocatalyst
g-C3N4 was synthesized according to a previously reported method . In a typical synthesis procedure, 5 g of melamine (C3H6N6) was placed in a crucible with a cover. The crucible was heated to 520 °C under nitrogen in a tube furnace at a heating rate of 10 °C min-1 and held for 2 h. Further deammoniation treatment was performed at 540 °C for 2 h. After the reaction, the crucible was cooled to room temperature. The yellow product was collected and ground into a powder.
α-Bi2O3 was synthesized according to a previously reported method . Bismuth (III) nitrate pentahydrate (Bi (NO3)3·5H2O) was dried at 60 °C for 2 h to obtain bismuth (III) nitrate (Bi (NO3)3) powder without crystal water. Thereafter, 5 g of Bi (NO3)3 was placed in a crucible, which was first heated in a tube furnace with a nitrogen atmosphere at 600 °C for 2 h and further heated at 620 °C for 2 h at a heating rate of 10 °C min-1. After the reaction, the crucible was allowed to cool naturally to room temperature. The yellow product was collected and ground into a powder.
α-Bi2O3@g-C3N4 composites were synthesized according to a previously reported method  with some modification. In detail, 10 g of C3H6N6 was first dissolved in 50 mL of a methanol solution by adding different amounts of Bi (NO3)3. The suspension was continuously stirred for 2 h at room temperature and then transferred to a vacuum drying oven at 80 °C for 24 h. Next, the white precursor mixture was ground and added to a crucible with a cover. Finally, the precursor mixture was heated in a tube furnace with a nitrogen atmosphere for 2 h, reaching a temperature of 520 °C at a heating rate of 10 °C min-1. Further treatment was performed at 540 °C for 2 h. When 0.05, 0.1, 0.3 and 0.5 g Bi (NO3)3 were used, the obtained yellow solids were ground into powders and labeled 0.5 wt% α-Bi2O3@g-C3N4, 1 wt% α-Bi2O3@g-C3N4, 3 wt% α-Bi2O3@g-C3N4 and 5 wt% α-Bi2O3@g-C3N4. A schematic illustration of the synthesis method is shown in Fig. S1 of Supplemental Materials.
2.3 Material characterization
2.3.1 Physicochemical characterization
Physicochemical characterization of the as-synthesized samples was obtained using SEM, SEM-EDS, TEM, XRD, XPS and FT-IR spectroscopy, and N2 adsorption and desorption isotherms. The crystal structure and phase composition of the as-synthesized samples were studied by XRD using a Bruker D2 Phaser diffractometer (Japan). The X-ray powder diffractometer was operated at 10 kV and used a Cu Kα radiation source with a wavelength of 1.5406 Å. The morphologies and elemental composition of the samples were studied by field-emission SEM (FE-SEM; JEOL JSM-7600F, Tokyo, Japan), in which the microscope was equipped with energy dispersive spectroscopy (EDS) and operated at an accelerating voltage of 15 kV, and TEM (JEOL JEM 1400Plus electron microscope, Tokyo, Japan). XPS measurements were performed with a Thermo VG Scientific XPS (ESCALAB 250, England). Moreover, all the binding energies were calibrated with respect to the C 1s peak of the adventitious carbon at 284.9 eV. FT-IR spectra of the as-synthesized samples were recorded on a JASCO-4200 spectrometer at room temperature with KBr as a reference sample. N2 adsorption and desorption isotherm curves were obtained with a Micromeritics system at 77.35 K (Autosorb-Iq-MP, Quantachrome Instruments).
2.3.2 Photoelectrochemical characterization
Photoelectrochemical characterization of the as-synthesized samples was obtained using UV-Vis spectrophotometry, PL spectroscopy, EIS analysis and EPR spectroscopy. The absorbance, diffuse reflectance spectra and optical bandgap of each sample were assessed using a UV-Vis spectrophotometer (JASCD 760) with BaSO4 as a reference sample. PL emission spectra were measured at room temperature on a JASCD FB-8500 fluorescence spectrophotometer. EIS was performed using an Autolab PGSTAT302N electrochemical test system (Metrohm Autolab B.V., Netherlands). EIS was performed for the as-synthesized samples with a standard three-electrode system that utilized Ag/AgCl and platinum strips as the reference and counter electrodes, respectively. The working electrode was prepared by mixing 8 mg of the as-synthesized samples with 1 mg of polyvinylidene fluoride and 1 mg of carbon black, and then, 100 μL n-methyl-2-pyrrolidone was added. Thereafter, the slurry was coated on a 1 × 3 cm piece of nickel foam and dried at 60 °C for 12 h in a vacuum oven. The EIS spectra were recorded from 0.1 to 100 kHz at a current amplitude of 10 mV and with a 0.5 M Na2SO4 aqueous solution as the electrolyte.
2.3.3 The measurement of reactive oxygen species (ROS)
A Bruker EPR spectrometer was used to measure the ROS in the presence of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to further verify the generation of ROS in the photocatalytic systems. The samples were prepared as follows: 20 mg of the as-synthesized samples was dispersed in 1 mL of deionized water, and then, 80 μL of DMPO was introduced with a 3 min ultrasonic treatment, which was used to trap hydroxyl radicals (DMPO-•OH). The superoxide radicals (DMPO-•O2-) were determined by the same process except that the deionized water was replaced with methanol.
2.4 Photocatalytic degradation of BP-3
The photocatalytic activities of the as-synthesized g-C3N4 and α-Bi2O3@g-C3N4 composites were evaluated by using BP-3 as the organic contaminant. Briefly, the process was performed by dispersing 0.15 g of the as-prepared sample into 100 mL of a BP-3 solution at an initial concentration of 2 mg L-1 and with the pH of the pristine solution. The formed suspension was stirred in the dark for half an hour to establish an adsorption/desorption equilibrium. Next, the solution was exposed to 24 W m-2 visible light irradiation (Philips CLEO HPA 400S, λ > 420 nm), and the suspension was continuously stirred magnetically at 450 rpm. As an illustration, the apparatus used for the photocatalytic experiment is presented in Fig. S2. During the photodegradation process, a 4 mL aliquot was removed at a predetermined time, and the complete removal of the catalyst particles was achieved by centrifugation and filtration through a 0.2-μm Millipore filter. The supernatant was analyzed by UV-Vis spectrophotometry to monitor both the absorbance and absorption spectra of the remaining BP-3. To further confirm the photodegradation of BP-3, aliquots were collected and analyzed using high-performance liquid chromatography (HPLC). Eq. (1) was used to determine the removal efficiency (η%):
where Ci is the initial BP-3 concentration in mg L-1 before irradiation and Cf is the BP-3 concentration in mg L-1 after illumination time t.
2.5 Influence of ROS scavengers on the photocatalytic process
An active species trapping experiment was performed by using the 1 wt% α-Bi2O3@g-C3N4 catalyst for the photodegradation of BP-3. The degradation process was observed by adding different active species quenchers. To do this, AO (5 mg), SOD (5 mg) and IPA (5 mL) were chosen as the quenchers of h+, •O2- and •OH, respectively. The utilized method was similar to that of the previous photocatalytic activity test except when treated with the respective quenchers in the presence of BP-3.
2.6 Photocatalyst reusability test
To assess the reusability of the photocatalyst, 0.15 g of the 1 wt% α-Bi2O3@g-C3N4 catalyst was dispersed in 100 mL of a 2 mg L-1 BP-3 solution and then illuminated for 300 min to complete the first BP-3 degradation cycle. Thereafter, the used catalysts in solution were centrifuged, repeatedly washed with water and alcohol, dried at 60 °C for 12 h in a vacuum drying oven, and then used again in the next experiment. The same procedure was repeated for all the remaining cycles.
3 Results and discussion
3.1 Characterization of the catalysts
The surface morphologies of the as-synthesized g-C3N4, α-Bi2O3 and 1 wt% α-Bi2O3@g-C3N4 composite samples were studied by FE-SEM. According to Fig. 1a, g-C3N4 exhibits an aggregated structure, and clearly, the surface structure of α-Bi2O3 in Fig. 1b exhibits thin sheet hierarchical microspheres. However, in Fig. 1c, Figs. S3a, S4a and S5a of the 1, 0.5, 3 and 5 wt% α-Bi2O3@g-C3N4 composite samples show the aggregated g-C3N4 structure covered by the thin sheet hierarchical microspheres of the α-Bi2O3 particles, confirming the formation of the α-Bi2O3@g-C3N4 composites [27, 35]. Small pores were observed on the surface structure of the α-Bi2O3@g-C3N4 composite; these pores might be due to the release of small molecular gases, specifically NO2 and H2O, during the mixing-calcination process. The elemental compositions of the 1, 0.5, 3 and 5 wt% α-Bi2O3@g-C3N4 composites were determined by EDS, and the results are shown in Fig. 1d, Figs. S3b, S4b and S5b. The EDS spectra reveal that the as-synthesized samples contain carbon (C), nitrogen (N), bismuth (Bi), and oxygen (O). Therefore, the EDS results confirm the presence of Bi, O, C, and N in the 1, 0.5, 3 and 5 wt% α-Bi2O3@g-C3N4 composites, further indicating the presence of both α-Bi2O3 and g-C3N4. Additionally, the elemental maps of the as-synthesized samples in Fig. 2a-d, Figs. S6, S7 and S8 confirm the presence of C, N, Bi, and O in the 1, 0.5, 3 and 5 wt% α-Bi2O3@g-C3N4 composites, respectively.
TEM was used for structural analysis and microstructural characterization of the as-synthesized g-C3N4, α-Bi2O3 and 1 wt% α-Bi2O3@g-C3N4 composite samples. Figure 3a and b shows that the pure g-C3N4 sample displays a typical smooth surface with a layered sheet structure , and the pure α-Bi2O3 sample shows a dark image of nanoparticles, as demonstrated in Fig. 3c. The deposition of α-Bi2O3 on the surface of g-C3N4 is clearly observed in the TEM images of 1 wt% α-Bi2O3@g-C3N4, as shown in Fig. 3e. Numerous dark, particle-like structures are observed on the surface of g-C3N4. Consequently, the dark particles can be assigned to α-Bi2O3, and the gray part and layered sheet are assigned to g-C3N4. Since Bi has a higher atomic mass than C and N, the electron beam can penetrate g-C3N4 more easily than α-Bi2O3. High-resolution TEM was further performed to investigate the d-spacing of the as-synthesized catalysts, and the results are displayed in Fig. 3b, d and f. The 0.376 nm d-spacing of the lattice fringes corresponds to the (002) lattice plane of g-C3N4 (Fig. 3b), while the 0.401 nm d-spacing of the lattice fringes corresponds to the (021) lattice plane of α-Bi2O3 (Fig. 3d). In addition, Fig. 3f displays the presence of α-Bi2O3 and g-C3N4 phases and an interplanar spacing of 0.406 nm, which correspond to the (021) crystal plane of α-Bi2O3, further confirming the formation of heterojunctions in the as-synthesized sample.
The XRD patterns of the as-synthesized g-C3N4, α-Bi2O3, 0.5 wt% α-Bi2O3@g-C3N4, 1 wt% α-Bi2O3@g-C3N4, 3 wt% α-Bi2O3@g-C3N4 and 5 wt% α-Bi2O3@g-C3N4 catalysts are shown in Fig. 4a. The characteristic peaks of the g-C3N4 sample are indexed to the corresponding orthorhombic phase (PDF #89-8491) with lattice parameters of a = 4.048 Å, b = 4.885 Å and c = 6.495 Å. The g-C3N4 catalyst shows two diffraction peaks at 2θ = 13.62 and 27.44°, which can be indexed as the (001) and (002) planes, respectively. The weak peak at 13.62° corresponds to an interlayer d-spacing of 0.649 nm, which is related to the order of the tri-s-triazine in-plane structural repeating unit, and the strong peak at 27.44°, which corresponds to an interlayer d-spacing of 0.367 nm, is attributed to the long-range interlayer stacking of aromatic systems and/or graphitic sheets [36, 37]. The peaks of the α-Bi2O3 sample are related to the corresponding monoclinic phase (PDF #02-0498) with lattice parameters of a = 5.830 Å, b = 8.140 Å and c = 7.480 Å. The diffraction peaks located at 2θ = 26.03, 27.59, 33.53 and 46.53° are attributed to the (002), (121), (202) and (041) planes, respectively. The peaks of α-Bi2O3, especially the peak at 27.59°, are very sharp, which suggests good crystallinity. The intensity of the XRD peaks of the α-Bi2O3@g-C3N4 composites decreases as the α-Bi2O3 content increases from 0.5 to 5 wt%, and the peak of α-Bi2O3 that is ascribed to the monoclinic phase is not identified. Therefore, the α-Bi2O3 particles are confirmed to be homogeneously dispersed on the surface of g-C3N4, which results in a significant decrease in the intensity of the (002) peaks of the composite catalysts. In addition, no new diffraction peaks are observed in the composite, which confirms that the composite contains only g-C3N4 and α-Bi2O3 [38, 39]. The crystallite sizes of g-C3N4, α-Bi2O3 and the 1 wt% α-Bi2O3@g-C3N4 composite are estimated to be 6.73, 14.18 and 4.47 nm, respectively.
The chemical bonding of the as-synthesized g-C3N4, α-Bi2O3 and α-Bi2O3@g-C3N4 composites with different weight percentages of α-Bi2O3 was investigated by FT-IR spectroscopy. The spectra are depicted in Fig. 4b. Regarding g-C3N4, the sharp peak centered at 808 cm-1 can be assigned to the out-of-plane bending modes of the s-triazine units . Moreover, the peaks located at 1560, 1412, 1321 and 1242 cm-1 correspond to aromatic C–N stretching vibrations, whereas the peak located at 1641 cm-1 is assigned to the C=N stretching vibration mode . Additionally, the broad peak centered at 3000-3300 cm-1 is associated with the stretching vibration mode of NH2 or N-H groups, and the bending vibration of the O-H band resembles that of adsorbed water [37, 41]. The peak of α-Bi2O3 located at approximately 520 cm-1 corresponds to the typical stretching vibration mode of the Bi–O bands of BiO6 units . Regarding the four α-Bi2O3@g-C3N4 composites, the peak of α-Bi2O3 centered at 520 cm-1 is not observed in all composite samples, which might be due to its low content and weak vibrations. Similar trends have been reported by Huang et al.  and Wang et al.  regarding CeO2@g-C3N4 and sulfur-doped@g-C3N4, respectively. However, strong peak intensities from g-C3N4 are observed in the 1240-1640 cm-1 region when the α-Bi2O3 content in the composites is increased from 0.5 to 5 wt%.
The surface chemical compositions of g-C3N4, α-Bi2O3 and the 1 wt% α-Bi2O3@g-C3N4 composite were analyzed by XPS. Figure S9a shows the full survey XPS spectra of α-Bi2O3, g-C3N4 and 1 wt% α-Bi2O3@g-C3N4. Clearly, Bi 4f, O 1s, C 1s and N 1s signals are observed for all samples. Figure 5a-d depicts the Bi 4f, O 1s, C 1s and N 1s XPS spectra of α-Bi2O3, g-C3N4 and 1 wt% α-Bi2O3@g-C3N4. Figure 5a shows the Bi 4f high-resolution spectra of α-Bi2O3 and 1 wt% α-Bi2O3@g-C3N4. The two peaks centered at 159.3 eV and 164.3 eV represent the 4f7/2 and 4f5/2 orbitals of trivalent Bi (Bi3+) in α-Bi2O3, respectively . However, the binding energies of the Bi 4f7/2 and Bi 4f5/2 orbitals in 1 wt% α-Bi2O3@g-C3N4 shift to 158.3 eV and 163.3 eV, respectively, which indicates the formation of a heterojunction between g-C3N4 and α-Bi2O3 in the composite . Figure 5b shows two strong peaks at 529.9 eV and 530.4 eV, which are the binding energies of the O 1s core level; the former peak is attributed to the lattice O2- species of the Bi-O bonds, while the latter peak can be assigned to surface-absorbed hydroxyl oxygen . Compared with that of α-Bi2O3, the O 1s signal in of 1 wt% α-Bi2O3@g-C3N4 shows a positive shift, which also confirms the formation of a heterojunction between g-C3N4 and α-Bi2O3 in the composite.
The C 1s spectrum shown in Fig. 5c shows two peaks and binding energies at 284.9 eV and 288.6 eV, which correspond to the carbon species adsorbed on the sample surface from the instrument and the sp2-hybridized bonded carbon in the heterocyclic aromatic ring (N–C=N) of the g-C3N4 lattice, respectively . As shown in Fig. 5c, the signal in the C 1s spectrum shifts to a lower binding energy after α-Bi2O3 loading, which further confirms the formation of heterojunctions. The binding energy of the adventitious carbon peak (284.9 eV) does not change after heterojunction formation.
The N 1s spectrum of g-C3N4 in Fig. 5d shows three peaks centered at 398.5, 399.3 and 399.5 eV, which are assigned to the tertiary N bonded to carbon atoms in the form of N-(C)3 units, the sp2-hybridized aromatic N atoms bonded to carbon in the form of C-N=C, and amino groups with a hydrogen atom (C-N-H) and charging effects, respectively [19, 37]. As shown in Fig. 5d, the N 1s binding energy of 1 wt% α-Bi2O3@g-C3N4 exhibits a blueshift, which also suggests a possible interaction between g-C3N4 and α-Bi2O3 in the composite. Generally, this kind of interaction promotes charge carrier separation and interparticle electron transfer, leading to the enhanced photocatalytic performance of 1 wt% α-Bi2O3@g-C3N4.
To investigate the surface physicochemical properties of the as-synthesized samples with respect to the surface area and pore size distribution, nitrogen adsorption-desorption isotherms were obtained. As shown in Fig. S9c, g-C3N4, α-Bi2O3 and the α-Bi2O3@g-C3N4 composites exhibit type IV isotherms with a type H3 hysteresis loop, as defined by International Union of Pure and Applied Chemistry; these results are characteristic of slit-like mesoporous materials .
Furthermore, the BET surface areas, pore volumes and pore diameters of the g-C3N4, α-Bi2O3, 0.5 wt% α-Bi2O3@g-C3N4, 1 wt% α-Bi2O3@g-C3N4, 3 wt% α-Bi2O3@g-C3N4 and 5 wt% α-Bi2O3@g-C3N4 samples are summarized in Table 1. The results suggest that α-Bi2O3 loading reduces the BET surface area of the composite samples, possibly due to the particle aggregation that occurs on the surface of g-C3N4. The BET surface area of 1 wt% α-Bi2O3@g-C3N4 is larger than that of 0.5 wt% α-Bi2O3@g-C3N4, 3 wt% α-Bi2O3@g-C3N4 and 5 wt% α-Bi2O3@g-C3N4. This phenomenon could be due to the different pore size distributions of the as-synthesized samples, as shown in Fig. S9d. Therefore, the high BET surface area of 1 wt% α-Bi2O3@g-C3N4 offers the possibility for efficient diffusion of the pollutant on the catalyst surface, which is beneficial for the photocatalytic reaction .
3.2 UV-Vis DRS analysis
The optical properties of the as-synthesized samples were measured by UV-Vis DRS. Figure 6a shows the UV-Vis DRS spectra of the as-synthesized samples. The Kubelka-Munk equation (Eq. (2)) is used to calculate the optical bandgap energy of semiconductors .
where α is the absorption coefficient, h is Planck’s constant, ν is the light frequency, A is a constant, Eg is the bandgap energy, and n depends on the nature of the transition in a semiconductor (n = 1 for a direct transition and n = 4 for an indirect transition). The intrinsic Eg values were determined by extrapolating the steepest portion of (αhν)2 = 0. The light absorption onset edges of g-C3N4, α-Bi2O3, 0.5 wt% α-Bi2O3@g-C3N4, 1 wt% α-Bi2O3@g-C3N4, 3 wt% α-Bi2O3@g-C3N4 and 5 wt% α-Bi2O3@g-C3N4 are approximately 450, 437, 453, 455, 459 and 463 nm, respectively, with Eg values of 2.72, 2.8, 2.70, 2.69, 2.66 and 2.64 eV, respectively (Fig. 6b). The absorption spectra of the α-Bi2O3@g-C3N4 composites show a slight redshift, which is triggered by the α-Bi2O3 doped on the porous surface of g-C3N4, confirming the formation of heterojunctions between g-C3N4 and α-Bi2O3. Figure S9b shows that the optical bandgaps of the α-Bi2O3@g-C3N4 composites linearly decrease with an increasing concentration of α-Bi2O3, which may result from the interaction between g-C3N4 and α-Bi2O3.
3.3 Photocatalytic activity evaluation
Figure S10a shows that the time-dependent absorption spectra obtained for the direct photolysis of BP-3 are unchanged without the addition of a catalyst. This result confirms that BP-3 is highly photostable owing to the strong intramolecular hydrogen bonds between the carbonyl oxygen and hydrogen atoms of the ortho hydroxyl group . However, BP-3 can undergo rapid photocatalytic decomposition over α-Bi2O3@g-C3N4 composites, as demonstrated in Figs. S10 and S11. Figures S11b, 11c, 11d, 10b and 10c show the photocatalytic degradation of BP-3 by g-C3N4 and the 0.5, 1, 3 and 5 wt% α-Bi2O3@g-C3N4 catalysts, respectively.
Figure 7a shows that approximately 20% of BP-3 is removed by adsorption in the dark, and the photocatalytic activity of pristine g-C3N4 is relatively low, with only 55% of BP-3 being photocatalytically degraded. However, the α-Bi2O3@g-C3N4 composites show higher photocatalytic activity than pristine g-C3N4, and the 1 wt% α-Bi2O3@g-C3N4 composite shows the optimal removal efficiency, decomposing 92% of BP-3. Notably, the photocatalytic efficiencies for the photodegradation of BP-3 decrease by 5 and 14% for 3 wt% α-Bi2O3@g-C3N4 and 5 wt% α-Bi2O3@g-C3N4, respectively. This phenomenon may be due to the excess loading of α-Bi2O3 decreasing the BET areas and the photoinduced charge transfer efficiency (as shown in the EIS results), thus leading to poor photocatalytic performance.
The Langmuir–Hinshelwood (L–H) kinetic model equation (Eq. (3)) is widely used to describe photocatalytic kinetics:
where r is the photocatalytic reaction rate, kL-H is the reaction rate constant, KL-H is the adsorption constant of BP-3 on the surface of a photocatalyst, and C is the concentration of BP-3. Due to the low concentration of BP-3, i.e., the value of KL-HC is << 1, the photocatalytic degradation of BP-3 can be simplified to the common pseudo-first-order kinetics equation (Eq. (4)) shown below :
where kapp (h-1) is the apparent photocatalytic rate constant or pseudo-first-order rate constant. This constant can be determined from the slope of -ln (Ct/C0) versus the irradiation time plot (Fig. 7b), in which C0 is the initial concentration of BP-3 in the aqueous solution at time zero and Ct is the concentration of BP-3 at irradiation time t (h). The apparent photocatalytic rate constants of different photocatalysts for the degradation of BP-3 are summarized in Fig. 7b. Accordingly, the highest kapp is found with 1 wt% α-Bi2O3@g-C3N4. The synergistic effect between the binary phases in 1 wt% α-Bi2O3@g-C3N4 results in better photogenerated electron-hole (e--h+) charge separation, thus inhibiting e- and h+ recombination.
3.4 PL and EIS spectral analysis
To examine the recombination processes of the photoinduced e- and h+ of the as-synthesized photocatalysts, PL spectroscopy was employed, and the spectra are shown in Fig. 8a. Herein, the PL spectra of bare g-C3N4, α-Bi2O3 and the α-Bi2O3@g-C3N4 composites were obtained with an excitation wavelength of 300 nm (~4.08 eV) to elucidate the recombination processes of the photogenerated e--h+ pairs. As shown in Fig. 8a, pristine g-C3N4 has the highest PL peak intensity, which is due to its faster rate of e--h+ recombination; therefore, it shows the lowest photocatalytic activity . The PL intensity of bare α-Bi2O3 is the lowest and is almost negligible. Hence, the PL peak intensity of the composite is lower than that of pristine g-C3N4, which confirms the formation of heterojunctions and indicates efficient charge separation from the CB of g-C3N4 to the CB of the α-Bi2O3 nanocluster. Therefore, the recombination process of the photoinduced charge carriers in the α-Bi2O3@g-C3N4 composites is significantly hampered, which might be favorable for improving the photocatalytic activity of the composites .
To further demonstrate the photoinduced charge transfer efficiency and recombination processes of the as-synthesized catalysts, EIS measurements were performed. Figure 8b shows the Nyquist impedance plots of g-C3N4, α-Bi2O3 and the α-Bi2O3@g-C3N4 composites. A smaller arc radius indicates a lower charge transfer resistance (Rt) and a more facile charge transfer process at the electrode/electrolyte interface . Notably, 1 wt% α-Bi2O3@g-C3N4 exhibits a smaller arc radius than the other as-synthesized samples. This result suggests that 1 wt% α-Bi2O3@g-C3N4 has a better photoinduced charge transfer efficiency and a lower recombination rate of photoinduced charge carriers [52, 53]. This scenario could be beneficial for the photocatalytic performance of 1 wt% α-Bi2O3@g-C3N4.
3.5 Possible photocatalytic mechanisms
The CB and VB potential edge positions of semiconductor materials determine their oxidation and reduction abilities; these positions can be calculated by empirical equations (Eqs. (5) and (6)) 
Herein, the EVB and ECB are the VB and CB potentials, respectively; x is the absolute electronegativity of the corresponding semiconductor material, which can be obtained by determining the arithmetic mean of the electron affinity and first ionization potential of the constituent atoms (5.99 eV for α-Bi2O3 and ~4.64 eV for g-C3N4) [42, 54]; Ee is the free electron energy on the hydrogen scale (~4.5 eV vs. NHE); and Eg is the bandgap energy of the semiconductors. The calculated values of the EVB and ECB edge positions are estimated to be ~1.50 and ~2.89 eV for the as-synthesized g-C3N4 and ~-1.22 and ~0.09 eV for the as-synthesized α-Bi2O3, respectively.
When the 1 wt% α-Bi2O3@g-C3N4 is illuminated, both g-C3N4 and α-Bi2O3 can produce a photoexcited e--h+ pair (Eq. (7)). The photoexcited e- in the CB of α-Bi2O3 and photoinduced h+ in the VB of g-C3N4 can recombine, causing the accumulation of e- in the CB of g-C3N4 and h+ in the VB of α-Bi2O3; this behavior results in strong oxidation and reduction for the photodegradation of BP-3 (Eq. (8)).
The reduction potential of CB in g-C3N4 is ~-1.22 eV vs. NHE, which is more negative than the standard redox potential of O2/•O2- (~-0.33 eV vs. NHE) . The photoexcited electrons in the CB of g-C3N4 can reduce adsorbed oxygen molecules to •O2- (Eq. (10)) or cause •O2- to react with hydrogen ions and e- to produce H2O2 (Eq. (11)), which can be further reduced to generate •OH (Eq. (12)). In addition, the oxidation potential of the VB in α-Bi2O3 (~2.89 eV vs. NHE) is more positive than the standard redox potential of •OH/H2O (~2.72 eV vs. NHE) . Thus, photoinduced h+ in the VB of α-Bi2O3 on the surface of the catalyst can react with H2O to generate •OH (Eq. (9)). Additionally, the remaining h+ in the VB of α-Bi2O3 on the surface of the catalyst directly oxidizes BP-3.
To further elucidate the mechanism of the photocatalytic reaction and verify the reactive species responsible for the degradation of BP-3, reactive species scavenging experiments were conducted. The results of the scavenging study with the 1 wt% α-Bi2O3@g-C3N4 composite are shown in Fig. 9. The figure shows that when IPA, AO and SOD are used as quenchers, the photodegradation rate of BP-3 decreases from 92% under normal conditions to 51, 56 and 78%, respectively, and the kapp for the photocatalytic degradation of BP-3 by the 1 wt% α-Bi2O3@g-C3N4 composite decreases from 0.42 h-1 in the absence of scavengers to 0.06, 0.07 and 0.14 h-1 in the presence of IPA, AO and SOD, respectively (Fig. 9b). Thus, the presence of IPA, AO and SOD significantly affects the photocatalytic process. Therefore, the generated •OH, h+ and •O2- work together to degrade BP-3 (Eq. (13)).
To ascertain the Z-scheme heterojunction and further confirm the involvement of ROS (•OH and •O2-) in the photocatalytic system, EPR measurements were performed. As depicted in Fig. 10a and b, the characteristic peaks of the •OH and •O2- radicals were not detected prior to light exposure, whereas the typical characteristic peaks of •O2- and •OH radicals were observed when the sample was illuminated. This result suggests that •O2- and •OH radicals play significant roles in the photocatalytic process. As shown in Fig. 10a, the typical characteristic peak of •OH radicals for g-C3N4 does not appear under dark and illuminated conditions owing to the photogenerated holes on g-C3N4 having an inadequate oxidation potential for producing OH radicals. This result reveals that the VB potential edge of g-C3N4 is 1.50 eV, which is lower than the ~2.72 eV standard redox potential of •OH/H2O. However, g-C3N4 can facilitate the generation of •OH radicals on the surface of 1 wt% α-Bi2O3@g-C3N4. As seen in Fig. 10b, the •O2- EPR signal intensity of 1 wt% α-Bi2O3@g-C3N4 is stronger than that of g-C3N4. This result shows that more •O2- radicals are generated on the surface of 1 wt% α-Bi2O3@g-C3N4 than on the surface of g-C3N4. Additionally, Figs. S12a, S13a and S14a show the 0.5 wt% α-Bi2O3@g-C3N4, 3 wt% α-Bi2O3@g-C3N4, and 5 wt% α-Bi2O3@g-C3N4 composites dispersed in methanol (for DMPO-•O2-), while Figs. S12b, S13b and S14b show the 0.5, 3, and 5 wt% α-Bi2O3@g-C3N4 composites dispersed in water (for DMPO-•OH). Thus, it can be inferred that the photocatalysis of the as-synthesized sample follows the Z-scheme photocatalytic mechanism, which not only delays charge carrier recombination but also preserves strong redox potentials to effectively degrade pollutants.
The main steps of the possible photocatalytic reaction mechanism are proposed as follows:
From the obtained experimental and characterization results, a schematic illustration showing the possible photodegradation mechanism is proposed in Fig. 11.
To further confirm the photodegradation of BP-3, HPLC measurements were conducted. Figure S10d shows the HPLC chromatograms of BP-3 before and after photocatalytic degradation by the 1 wt% α-Bi2O3@g-C3N4 composite. The flow rate was 0.8 mL min-1, and the injection volume was 20 μL. Two major peaks are recorded for the 300 min photocatalytic degradation process. The peaks centered at 4.68 and 8.21 min correspond to BP-3 and an intermediate byproduct, respectively. The peak intensities of BP-3 and the intermediate byproduct significantly decrease during the photodegradation process, which indicates that both BP-3 and the intermediate byproducts degrade during photocatalysis. Generally, BP-3 is metabolized to 2,4-dihydroxybenzophenone and dioxybenzone via demethylation and hydroxylation routes, respectively .
3.6 Photocatalyst reusability test
The reusability of catalysts is a very important parameter for their practical application. Herein, we investigated the reusability of the as-synthesized 1 wt% α-Bi2O3@g-C3N4 by performing four successive recycling runs, and the results are shown in Fig. 12. The removal efficiencies observed after the second, third and fourth recycling runs for BP-3 photodegradation decrease slightly by 4%, 6% and 8%, respectively. This decrease might be due to the loss of some catalyst during the washing, centrifugation and drying procedures and the active sites of the catalyst surface being covered by reactive chemicals. However, the apparent rate constant of 1 wt% α-Bi2O3@g-C3N4 after the fourth recycling test indicated a higher photocatalytic efficiency than that after the other runs. Therefore, the as-synthesized 1 wt% α-Bi2O3@g-C3N4 has good stability and favorable reusability, which promotes its practical applicability.
In this study, novel α-Bi2O3@g-C3N4 composites with different weight percentages (0.5, 1, 3 and 5 wt%) of α-Bi2O3 were synthesized by a mixing-calcination strategy. Among these photocatalysts, 1 wt% α-Bi2O3@g-C3N4 showed higher photocatalytic efficiency than the other photocatalysts for BP-3 degradation, exhibiting an optimal rate constant of 0.42 h-1, which is up to 6.3 times higher than that of pristine g-C3N4. The improved photocatalytic performance can be attributed mainly to the enhanced photogenerated e--h+ charge separation and suppressed e- and h+ recombination, which are dominant factors for improving the photocatalytic activity. More importantly, radical trapping experiments and EPR findings confirm that the active species •OH, h+ and •O2- work together to enhance the photocatalytic activity of the composite. The migration pattern of photogenerated carriers suggests a Z-scheme photocatalytic mechanism rather than a type II mechanism. The method presented in this study might be considered a promising approach for the removal of emerging organic pollutants.
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This work was supported by the Ministry of Science and Technology of Taiwan (MOST 108-2621-M-010-002). The authors are grateful to the Electron Microscopy Facility at National Yang Ming Chiao Tung University for providing the TEM, SEM and EDS images. They are also grateful to the National Taiwan University of Science and Technology for providing the XRD and XPS measurements. We also appreciate the EIS and measurements provided by National Tsing Hua University (NTHU) and the BET and BJH data obtained from the facility at National Chung Hsing University. The authors would like to thank the Instrumentation Center at NTHU for providing the EPR measurement.
This work was supported by the Ministry of Science and Technology of Taiwan (MOST 108-2621-M-010-002).
Authors and Affiliations
International Ph.D Program in Environmental Science and Technology, University System of Taiwan, Taipei, 11221, Taiwan
Abiyu Kerebo Berekute & Kuo-Pin Yu
Institute of Environmental and Occupational Health Sciences, National Yang Ming Chiao Tung University, Taipei, 11221, Taiwan
Abiyu Kerebo Berekute & Kuo-Pin Yu
Institute of Environmental Engineering, National Yang Ming Chiao Tung University, Hsinchu City, 30010, Taiwan
Abiyu Kerebo Berekute conducted the experiments and wrote the study. Kuo-Pin Yu and Yi-Hsueh Brad Chuang are Abiyu Kerebo Berekute’s advisor and coadvisor, respectively. They provided research ideas and guidance for this study. All authors read and approved the final manuscript.
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Berekute, A.K., Yu, KP. & Chuang, YH.B. Enhanced photocatalytic activity of novel α-Bi2O3@g-C3N4 composites for the degradation of endocrine-disrupting benzophenone-3 in water under visible light.
Sustain Environ Res32, 17 (2022). https://doi.org/10.1186/s42834-022-00129-8