Type II Heterojunction Formed between {010} or {012} Facets Dominated Bismuth Vanadium Oxide and Carbon Nitride to Enhance the Photocatalytic Degradation of Tetracycline

Both type II and Z schemes can explain the charge transfer behavior of the heterojunction structure well, but the type of heterojunction structure formed between bismuth vanadium oxide and carbon nitride still has not been clarified. Herein, we rationally prepared bismuth vanadium oxide with {010} and {012} facets predominantly and carbon nitride as a decoration to construct a core-shell structure with bismuth vanadium oxide wrapped in carbon nitride to ensure the same photocatalytic reaction interface. Through energy band establishment and radical species investigation, both {010} and {012} facets dominated bismuth vanadium oxide/carbon nitride composites exhibit the type II heterojunction structures rather than the Z-scheme heterojunctions. Furthermore, to investigate the effect of type II heterojunction, the photocatalytic tetracycline degradations were performed, finding that {010} facets dominated bismuth vanadium oxide/carbon nitride composite demonstrated the higher degradation efficiency than that of {012} facets, due to the higher conduction band energy. Additionally, through the free radical trapping experiments and intermediate detection of degradation products, the superoxide radical was proven to be the main active radical to decompose the tetracycline molecules. Therein, the tetracycline molecules were degraded to water and carbon dioxide by dihydroxylation-demethylation-ring opening reactions. This work investigates the effect of crystal planes on heterojunction types through two different exposed crystal planes of bismuth vanadate oxide, which can provide some basic research and theoretical support for the progressive and controlled synthesis of photocatalysts with heterojunction structures.


Introduction
Tetracycline (TC) is known as the second most widely used organic compound of broad-spectrum antibiotics in the world and is also applied extensively in medical and farming applications [1,2]. TC and their derivatives are stable in the environment, which tends to damage the ecosystem and affect human health, even increasing drug resistance in humans as well as in plants and animals [3]. In recent years, the abuse of TC has led to frequent detection in wastewater emissions, surfaces and groundwater, and even in soil environments [4][5][6][7][8]. Therefore, it is urgent to find an efficient and green way to solve the problem of TC pollution. Photocatalytic semiconductor technology, with the utilization of solar energy, has high catalytic oxidation activity and is environmentally friendly, which has also proven to be a very viable strategy and has attracted the attention of researchers worldwide [9][10][11][12][13].

Preparations of {012}BiVO 4 and {012}BiVO 4 /g-C 3 N 4
For the synthesis of {012}BiVO 4 , 2 mmol bismuth nitrate, 200 mg SDBS, 1.25 M nitric acid solution, and 2 mmol ammonium vanadate were added to 100 mL of deionized water and stirred for 30 min. The solution was then transferred to a polytetrafluoroethylene hydrothermal reactor and placed in an oven at 150 • C for 12 h. After completion of the reaction, the reactor was naturally cooled to room temperature, and the product was collected by filtration and washed several times with deionized water.
For the preparation of {012}BiVO 4 /g-C 3 N 4 , the prepared g-C 3 N 4 was added to the precursor solution after the stirring process, and the other steps were the same as above.

Preparation of {010}BiVO 4 and {010}BiVO 4 /g-C 3 N 4
For the synthesis of {010}BiVO 4 , 2.5 mmol of bismuth oxide and vanadium oxide were added to 25 mL of nitric acid solution (0.5 M) and subsequently stirred on a magnetic stirrer for 96 h. Then the product was collected by filtration and washed several times with deionized water.
For the preparation of {010}BiVO 4 /g-C 3 N 4 , the prepared {010}BiVO 4 was dispersed into an ethanol solution, followed by the addition of g-C 3 N 4 /ethanol solution (20 mg/mL) and stirred at room temperature for 24 h. The subsequent steps were then carried out for the preparation of {010}BiVO 4 .

Structure and Characterization of Materials
The crystal phase and purity of the catalyst were determined by X-ray diffractive apparatus (XRD, Palytical X'Pert Powder) of Panaco. The microstructure and elements of the powder samples were analyzed by field emission scanning electron microscopy (SEM, JSM-7001F) and energy dispersive X-ray spectroscopy (EDX, JSM-701F). The structural characteristics of the samples were observed through a transmission electron microscope (TEM, JSM-2100F) made by Hitachi. The surface element composition and existence state of the synthesized materials were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific K-Alpha). The chemical bonds and groups in the samples were analyzed and determined by Fourier infrared spectroscopy (FT-IR, Thermo 6700) from Thermo Field Company. Chemical bonds and groups in the samples were analyzed. The hydroxyl radical and superoxide radical of the samples were analyzed by electron paramagnetic resonance (EPR, A300) spectrometer of Brook Corporation. The comparative area of the samples was measured by the BET tester (BET, ASAP2460) of Mack Company. The intermediate products of the photocatalytic degradation of TC were tested by waters single quadrupole liquid mass spectrometry (LC-MS, ZQ2000). The optical absorption properties of the synthesized materials were characterized by UV-Vis diffuse reflectance spectroscopy (UV-VIS DRS, U-3900) from Hitachi, which scanning range was 200-800 nm, and BaSO4 powder was used as a reference.

Photocatalytic Activity Experiment
100 mL of TC aqueous solution with a concentration of 50 mg L −1 was measured and introduced into a 250 mL reactor, and 20 mg of the sample was then added. After evenly dispersed, the sample was stirred in the dark for 30 min. During this period, 3.5 mL of the above solution was taken out every 15 min. After being filtered by a microporous membrane, the clear solution was utilized as analyte, and the adsorption rate of TC was measured using Agilent's UV/Vis spectrophotometer (Carry 60). After the adsorption and desorption phases were balanced, the above solution was placed under a 300W xenon lamp and illuminated for 60 min. During this period, 3.5 mL of the above solution was taken out every 15 min and filtered, and the concentration of TC was measured by ultravio-let/visible spectrophotometer, and the light absorption intensity at 357 nm was recorded. The degradation rate of TC was calculated by Formula (1).
where C 0 is the absorbance of the TC when adsorption equilibrium is reached, and C is the absorbance of the TC measured by timed sampling.

Photoelectrochemical Tests
The electrochemical workstation (CHI920D) was used for photoelectric test. Ag/AgCl was employed as reference electrode, platinum electrode as counter electrode, and sodium sulfate (Na 2 SO 4 ) with 1 M electrolyte as electrolyte. Preparation of working electrode: 1 mg sample was dispersed in 1 mL deionized water, and ultrasound was carried out for 20 min. 100 µL of the dispersion was then measured with a pipette gun and dropped onto the cleaned FTO conductive glass. The photochemical and electrochemical properties of the samples were tested after natural air drying.

Photocatalytic Trapping Agent Experiment
Isopropyl alcohol (IPA; 0.1M), triethanolamine (TEOA; 0.1 M), and benzoquinone (BQ; 0.2 mM) were added into TC solution (100 mL; 50 mg/L) mixed with 20 mg catalyst, after adsorption equilibrium, the illumination was then performed. During this period, 3.5 mL of the above solution was taken out every 15 min, and its absorbance at 357 nm was recorded with an ultraviolet/visible spectrophotometer. Finally, the degradation rate was calculated according to Formula (1).

Characteristics
Photocatalysts of {010}BiVO 4 and {012}BiVO 4 were separately prepared through the wet chemistry method and hydrothermal method, according to the previous reports [17,32]. g-C 3 N 4 was prepared by a direct calculation method as described in the experimental section. The different loading ratios between {010}BiVO 4 and g-C 3 N 4 were via the physical hybrid approaches by long-time stirring operations; meanwhile, the {012}BiVO 4 /g-C 3 N 4 composites were fabricated by adding different amounts of the prepared g-C 3 N 4 into the precursor solutions of {012}BiVO 4 . After the synthesis, the architectures and morphologies of {010}BiVO 4 , {012}BiVO 4 , g-C 3 N 4 , {010}BiVO 4 /g-C 3 N 4 , and {012}BiVO 4 /g-C 3 N 4 were investigated by SEM and TEM. The pure {010}BiVO 4 sample displays a sheet shape with a length of ca. 1 µm (Figure 1a), while the {012}BiVO 4 depicts a near-octahedral shape with a size of about 5 µm (Figure 1b). The morphology of g-C 3 N 4 exhibits a sheet structure, as shown in Figure 1c,d. To verify the successful loading of g-C 3 N 4 onto BiVO 4 , further SEM tests of {010}BiVO 4 /g-C 3 N 4 and {012}BiVO 4 /g-C 3 N 4 were performed. Since the {010}BiVO 4 /0.2g-C 3 N 4 and {012}BiVO 4 /0.1g-C 3 N 4 equipped the highest TC degradation performances (discussed in the section of photocatalytic performance) compared to other {010}BiVO 4 /xg-C 3 N 4 and {012}BiVO 4 /xg-C 3 N 4 composites, the {010}BiVO 4 /0.2g-C 3 N 4 and {012}BiVO 4 /0.1g-C 3 N 4 were taken as instances to investigate the morphologies and structures. Therein, the x values from {010}BiVO 4 /xg-C 3 N 4 and {012}BiVO 4 /xg-C 3 N 4 represent the molar ratio of g-C 3 N 4 to BiVO 4 . As shown in Figure 1e-h, the two hybrids of {010}BiVO 4 /0.2g-C 3 N 4 and {012}BiVO 4 /0.1g-C 3 N 4 retained their flake or octahedral shapes unchanged and were almost in the same size. Magnifying the image of {010}BiVO 4 /0.2g-C 3 N 4 based on TEM tests (Figure 1i), a nanosheet shell of g-C 3 N 4 is observed to be evenly covered on the surface of {010}BiVO 4 . For {012}BiVO 4 /g-C 3 N 4 , g-C 3 N 4 is also found to be homogeneously distributed on the surface of {012}BiVO 4 with a tiny nanosheet structure (Figure 1h  The successful fabrications of {010}BiVO4/g-C3N4 and {012}BiVO4/g-C3N4 can also verified by XRD patterns and FTIR spectra. As shown in Figure 2a [33]. After integration with g-C3N4, it observed that no crystal destruction happened on both {010}BiVO4 and {012}BiVO4. N obvious g-C3N4 diffraction peaks appeared in both {010}BiVO4/0.2g-C3N4 a {012}BiVO4/0.1g-C3N4 samples, which may be ascribed to the small loading amount of C3N4 [34,35]. The structural composition of {010}BiVO4/0.2g-C3N4 and {012}BiVO4/0.1 C3N4 were explored by FTIR spectra. As illustrated in Figure 2b, {010}BiVO4 appears t vibrational peaks at 470 and 612 cm −1 , which should be attributed to the symmet bending vibration of VO4 3− and the stretching vibration of Bi-O, respectively [36,37]. Wh for {012}BiVO4, it is observed that the peak wave numbers of symmetric bending vibrati of VO4 3− and the stretching vibration of Bi-O decrease to 452.22 and 561.22 cm respectively. The different vibrations of {010}BiVO4 and {012}BiVO4 may be attributed the diversely exposed crystal planes. Additionally, the vibrational peaks of g-C3N4 fro  [33]. After integration with g-C 3 N 4 , it is observed that no crystal destruction happened on both {010}BiVO 4 and {012}BiVO 4 . No obvious g-C 3 N 4 diffraction peaks appeared in both {010}BiVO 4 /0.2g-C 3 N 4 and {012}BiVO 4 /0.1g-C 3 N 4 samples, which may be ascribed to the small loading amount of g-C 3 N 4 [34,35]. The structural composition of {010}BiVO 4 /0.2g-C 3 N 4 and {012}BiVO 4 /0.1g-C 3 N 4 were explored by FTIR spectra. As illustrated in Figure 2b, {010}BiVO 4 appears the vibrational peaks at 470 and 612 cm −1 , which should be attributed to the symmetric bending vibration of VO 4 3− and the stretching vibration of Bi-O, respectively [36,37]. While for {012}BiVO 4 , it is observed that the peak wave numbers of symmetric bending vibration of VO 4 3− and the stretching vibration of Bi-O decrease to 452.22 and 561.22 cm −1 , respectively. The different vibrations of {010}BiVO 4 and {012}BiVO 4 may be attributed to the diversely exposed crystal planes. Additionally, the vibrational peaks of g-C 3 N 4 from the {010}BiVO 4 /0.2g-C 3 N 4 6 of 15 and {012}BiVO 4 /0.1g-C 3 N 4 samples can also be detected at 1231, 1309, and 1399 cm −1 , which are attributed to the C-N aromatic ring vibrations [30]. These confirm the successful integration between BiVO 4 and g-C 3 N 4 .  Figure 3b, peaks at 159.3 and 164.6 eV correspond to Bi 3+ 4f7/2 and Bi 3+ 4f5/2 [3 respectively, which implies that element Bi is present in {010}BiVO4 at +3 valence. Wh after introducing the g-C3N4 substrate, the locations of Bi 3+ 4f7/2 and Bi 3+ 4f5/2 shift to 15 and 164.4 eV, respectively. Similar results also have happened between {012}BiVO4 a {012}BiVO4/0.1g-C3N4, where the locations of Bi 3+ 4f7/2 and Bi 3+ 4f5/2 change from 159.1 a 164.4 eV to 159.0 and 164.3 eV, respectively. These indicate that the electrons transfer fro BiVO4 to g-C3N4. The same result can also be reflected on the high-resolution XPS spec of V 2p, as shown in Figure S1a. Furthermore, The high-resolution XPS spectra of O were illustrated with two or three peaks of deconvolution in Figure S1b. Peaks at abo 530.0 and 532.0 eV are assigned to the lattice O 2-and adsorbed O 2-molecules, respective [39,40]. Interestingly, for {010}BiVO4, a characteristic peak of adsorbed O 2-was observ at about 531.4 eV, which can be attributed to the adsorbed -OH [41]. Instead, f {012}BiVO4, the characteristic peak of adsorbed O 2-is at about 532.5 eV that originates fro the C=O adsorption [37]. These different properties can ascribe to the diversely expos crystal planes, which are in accordance with the analysis of FTIR. Furthermore, after troducing g-C3N4 substrate, the binding energy of lattice O 2-decreased in bo {010}BiVO4/0.2g-C3N4 and {012}BiVO4/0.1g-C3N4, indicating the electrons transfer fro BiVO4 to g-C3N4 and corresponding to results of XPS spectra of Bi 4f and V 2p.  Figure 3b, peaks at 159.3 and 164.6 eV correspond to Bi 3+ 4f 7/2 and Bi 3+ 4f 5/2 [38], respectively, which implies that element Bi is present in {010}BiVO 4 at +3 valence. While after introducing the g-C 3 N 4 substrate, the locations of Bi 3+ 4f 7/2 and Bi 3+ 4f 5/2 shift to 159.1 and 164.4 eV, respectively. Similar results also have happened between {012}BiVO 4 and {012}BiVO 4 /0.1g-C 3 N 4 , where the locations of Bi 3+ 4f 7/2 and Bi 3+ 4f 5/2 change from 159.1 and 164.4 eV to 159.0 and 164.3 eV, respectively. These indicate that the electrons transfer from BiVO 4 to g-C 3 N 4 . The same result can also be reflected on the high-resolution XPS spectra of V 2p, as shown in Figure S1a. Furthermore, The high-resolution XPS spectra of O 1s were illustrated with two or three peaks of deconvolution in Figure S1b. Peaks at about 530.0 and 532.0 eV are assigned to the lattice O 2and adsorbed O 2molecules, respectively [39,40]. Interestingly, for {010}BiVO 4 , a characteristic peak of adsorbed O 2was observed at about 531.4 eV, which can be attributed to the adsorbed -OH [41]. Instead, for {012}BiVO 4 , the characteristic peak of adsorbed O 2is at about 532.5 eV that originates from the C=O adsorption [37]. These different properties can ascribe to the diversely exposed crystal planes, which are in accordance with the analysis of FTIR. Furthermore, after introducing g-C 3 N 4 substrate, the binding energy of lattice O 2decreased in both {010}BiVO 4 /0.2g-C 3 N 4 and {012}BiVO 4 /0.1g-C 3 N 4 , indicating the electrons transfer from BiVO 4 to g-C 3 N 4 and corresponding to results of XPS spectra of Bi 4f and V 2p.  , and g-C3N4 were observed, whi reveals the n-type semiconductor characteristics of these prepared samples [42,43]. For type semiconductors, the conduction band (ECB) is considered very close to the flat ban [44,45]. Hence, by calculating the intercept of the tangent line in Mott-Schottky plots, t conduction band energy potentials of {010}BiVO4, {012}BiVO4, and g-C3N4 were estimat as −0.45, −0.24 and −0.07 V vs. NHE at pH = 0, respectively. Since the bandgap energy h been confirmed by UV-diffuse absorption spectrum, through Formula (2), the valen band (EVB) potentials of {010}BiVO4, {012}BiVO4, and g-C3N4 were calculated to be 1.9 2.15, and 2.73 V vs. NHE at pH = 0, respectively.    4 , and g-C 3 N 4 were observed, which reveals the n-type semiconductor characteristics of these prepared samples [42,43]. For n-type semiconductors, the conduction band (E CB ) is considered very close to the flat band [44,45]. Hence, by calculating the intercept of the tangent line in Mott-Schottky plots, the conduction band energy potentials of {010}BiVO 4 , {012}BiVO 4 , and g-C 3 N 4 were estimated as −0.45, −0.24 and −0.07 V vs. NHE at pH = 0, respectively. Since the bandgap energy has been confirmed by UV-diffuse absorption spectrum, through Formula (2), the valence band (E VB ) potentials of {010}BiVO 4 , {012}BiVO 4 , and g-C 3 N 4 were calculated to be 1.96, 2.15, and 2.73 V vs. NHE at pH = 0, respectively.

Radical Species Analyses
To figure out the heterojunction types of {010}BiVO 4 /g-C 3 N 4 and {012}BiVO 4 /g-C 3 N 4 , the radical species have been analyzed. According to the previous reports [46,47], the • OH radical generation potential from H 2 O oxidation is 2.38 V vs. NHE at pH = 0 (Equation (3)), and O 2 can be reduced to O 2 •− radical by photoelectrons at −0.046 V vs. NHE at pH = 0 (Equation (4)). To reveal the practical radical species fabrications, the EPR spectra were carried out for g-C 3 N 4 , {010}BiVO 4 /0.2g-C 3 N 4 and {012}BiVO 4 /0.1g-C 3 N 4 systems under aerobic conditions. Both • OH (Figure 5a) and O 2 •− radicals (Figure 5b) were generated using g-C 3 N 4 as a photocatalyst, which is reasonable as the valence band and conduction band energy potentials of g-C 3 N 4 are more positive and negative than H 2  To reveal the practical radical species fabrications, the EPR spectra were carried o for g-C3N4, {010}BiVO4/0.2g-C3N4 and {012}BiVO4/0.1g-C3N4 systems under aerob conditions. Both • OH (Figure 5a) and O2 •− radicals (Figure 5b) were generated using C3N4 as a photocatalyst, which is reasonable as the valence band and conduction ban energy potentials of g-C3N4 are more positive and negative than H2O oxidation and reduction potentials. After the combination of {010}BiVO4 or {012}BiVO4 with g-C3N4, t concentrations of • OH radical decreased considerably, while the concentration prom tions of O2 •− radical were observed. The change trends of • OH and O2 •− radical conform the type II heterojunction. Therein, the photoelectrons from {010}BiVO4 or {012}BiV transfer into g-C3N4, elevating the O2 reduction for O2 •− radical fabrication. However, t photoholes of g-C3N4 migrate to the wrapped {010}BiVO4 or {012}BiVO4 inhibiting the H oxidation into • OH radical. Therefore, both heterojunctions of {010}BiVO4/g-C3N4 an {012}BiVO4/g-C3N4 have proven to be type II heterojunctions.

Photocatalytic Performances
The type II heterojunctions of {010}BiVO 4 /g-C 3 N 4 and {012}BiVO 4 /g-C 3 N 4 can also be proven by their photocatalytic performances. The photocatalytic performances of {010}BiVO 4 , {010}BiVO 4 /g-C 3 N 4 , {012}BiVO 4 , and {012}BiVO 4 /g-C 3 N 4 were investigated by degradations of TC under visible light irradiation. Compared with pure {010}BiVO 4 and {012}BiVO 4 , the additions of g-C 3 N 4 evidently promote the degradation performances of TC molecules. As shown in Figure 6a, after 60 min of light exposure, a maximum of 56% of TC was degraded by {010}BiVO 4 /0.2g-C 3 N 4 , while only 26% was degraded by {012}BiVO 4 /0.1g-C 3 N 4 . These different degradation performances can be explained by the type II heterogeneous structure, as the {010}BiVO 4 equips a higher conduction band than {012}BiVO 4 driving more photoelectrons into g-C 3 N 4 and promoting a larger amount of O 2 •− radicals for TC degradation. Moreover, the Z-scheme heterojunction cannot interpret the performance variation. Because, if it is the Z-scheme heterojunction, equal photoholes should remain on the valence band of g-C 3 N 4, producing the same amount of • OH radical and resulting in the same TC degradation performances between {010}BiVO 4 /g-C 3 N 4 and {012}BiVO 4 /g-C 3 N 4 . Thus, the Z-scheme heterojunction is repelled. Subsequently, as shown in Figure 6a, the further loading of g-C 3 N 4 led to the reduced degradation performances on both {010}BiVO 4 /0.3g-C 3 N 4 and {012}BiVO 4 /0.3g-C 3 N 4 , which may be attributed to the thick g-C 3 N 4 shells inhibiting the light absorption of BiVO 4 . Thus no sufficient photoelectrons were produced and transferred from BiVO 4 to g-C 3 N 4 to guarantee the high degradation performances. The kinetic analyses of these photocatalytic reactions were further explored, and the results suggested that they all conformed to the first-order reaction kinetic equations (Figure 6b). The corresponding kinetic rate constant of {010}BiVO 4 was estimated to be 0.0064 min −1 (Figure 6c), and with the continuous g-C 3 N 4 loading, the kinetic rate constant of {010}BiVO 4 /0.2g-C 3 N 4 increased to be 0.0130 min −1 . While for {012}BiVO 4 , the kinetic rate constant only achieved 0.0037 min −1 and increased to 0.0051 min −1 for {012}BiVO 4 /0.1g-C 3 N 4 . Furthermore, the cycling experiments showed that after four cycles (Figure 6d), the degradation efficiency of {010}BiVO 4 /g-C 3 N 4 exhibited merely a 3% reduction, which represents a relatively stable performance in the field of photocatalysis [48][49][50]. The minor decline in degradation efficiency may be ascribed to the attenuation of surface adsorption with recycling. As shown in Figure S3, the BET spectrum indicates that the specific surface area of {010}BiVO 4 /g-C 3 N 4 is about 9.6816 m 2 /g, which represents excellent adsorption performance and can conduce to the photocatalytic activity [51]. However, during the photodegradation of TC, some macromolecular substances should be inevitably absorbed on the surface of {010}BiVO 4 /g-C 3 N 4 (( Figure 7b) and affect the photocatalytic activity, which could not be emancipate by water clean during the recycle experiment. Therefore, a light degradation efficiency reduction emerged with the recycling of photocatalysis [52]. Health 2022, 19, x FOR PEER REVIEW 10 of C3N4 loading, the kinetic rate constant of {010}BiVO4/0.2g-C3N4 increased to be 0.01 min −1 . While for {012}BiVO4, the kinetic rate constant only achieved 0.0037 min −1 and creased to 0.0051 min −1 for {012}BiVO4/0.1g-C3N4. Furthermore, the cycling experime showed that after four cycles (Figure 6d), the degradation efficiency of {010}BiVO4/g-C exhibited merely a 3% reduction, which represents a relatively stable performance in field of photocatalysis [48][49][50]. The minor decline in degradation efficiency may be cribed to the attenuation of surface adsorption with recycling. As shown in Figure S3, BET spectrum indicates that the specific surface area of {010}BiVO4/g-C3N4 is about 9.68 m 2 /g, which represents excellent adsorption performance and can conduce to the pho catalytic activity [51]. However, during the photodegradation of TC, some macromole lar substances should be inevitably absorbed on the surface of {010}BiVO4/g-C3N4 ((Figu 7b) and affect the photocatalytic activity, which could not be emancipate by water cle during the recycle experiment. Therefore, a light degradation efficiency reduct emerged with the recycling of photocatalysis [52].

Photocatalytic Degradation Mechanism
To thoroughly and exhaustively explain the mechanism, the roles of radicals we tested. Taking benzoquinone (BQ), isopropyl alcohol (IPA), and triethanolamine (TEO as O2 •− , • OH, and photoholes capturers, the photocatalytic degradation of TC were p formed. As shown in Figure 7a, the TC degradation by {010}BiVO4/0.2g-C3N4 was reduc from 56% to 48% when O2 •− trapping agent BQ was introduced. Moreover, the degrad tion rate was almost unchanged by the addition of • OH trapping agent IPA. This indicat that O2 •− rather than • OH is the active radical species for TC degradation over BiVO4 C3N4 photocatalyst. The introduction of the hole-trapping agent of TEOA, the degradati rate increased to 83%. This is because TEOA could trap a portion of the holes, inhibiti the recombination of photogenerated carriers, thus leaving more electrons to participa in O2 •− generation for TC degradation.
The TC degradation process was speculated based on the degradation intermediat detection by LC-MS. As shown in Figures 7b and S4, the degradation intermediates of T over {010}BiVO4, {010}BiVO4/0.2g-C3N4, {012}BiVO4, and {012}BiVO4/0.1g-C3N4 are almo identical. Through the molecular debris detection with different m/z values, we specula that O2 •− first promotes the dehydroxylation and demethylation of TC to produce t products with m/z values of 410 and 427. Subsequently, they continued to undergo ca lytic ring-opening reactions, decomposing into small molecules with an m/z value of 18 which were eventually broken down completely into CO2 and H2O.

Photocatalytic Degradation Mechanism
To thoroughly and exhaustively explain the mechanism, the roles of radicals were tested. Taking benzoquinone (BQ), isopropyl alcohol (IPA), and triethanolamine (TEOA) as O 2 •− , • OH, and photoholes capturers, the photocatalytic degradation of TC were performed. As shown in Figure 7a, the TC degradation by {010}BiVO 4 /0.2g-C 3 N 4 was reduced from 56% to 48% when O 2 •− trapping agent BQ was introduced. Moreover, the degradation rate was almost unchanged by the addition of • OH trapping agent IPA. This indicates that O 2 •− rather than • OH is the active radical species for TC degradation over BiVO 4 /g-C 3 N 4 photocatalyst. The introduction of the hole-trapping agent of TEOA, the degradation rate increased to 83%. This is because TEOA could trap a portion of the holes, inhibiting the recombination of photogenerated carriers, thus leaving more electrons to participate in O 2 •− generation for TC degradation. The TC degradation process was speculated based on the degradation intermediates detection by LC-MS. As shown in Figure 7b and Figure S4, the degradation intermediates of TC over {010}BiVO 4 , {010}BiVO 4 /0.2g-C 3 N 4 , {012}BiVO 4 , and {012}BiVO 4 /0.1g-C 3 N 4 are almost identical. Through the molecular debris detection with different m/z values, we speculate that O 2 •− first promotes the dehydroxylation and demethylation of TC to produce the products with m/z values of 410 and 427. Subsequently, they continued to undergo catalytic ring-opening reactions, decomposing into small molecules with an m/z value of 186, which were eventually broken down completely into CO 2 and H 2 O.

Conclusions
In summary, to figure out the confusing heterojunction type between BiVO4 and C3N4, we rationally designed {010}BiVO4/g-C3N4 and {012}BiVO4/g-C3N4 as core-sh structures, allowing only photoelectrons or photoelectrodes to participate in oxidation reduction reactions. Through energy band establishment, radical species investigati photocatalytic TC degradation performances, and LC-MS tests, both {010}BiVO4/g-C3 and {012}BiVO4/g-C3N4 exhibit the type II heterojunction structures rather than previously reported Z-scheme heterojunctions. The O2 •− was proven as the main act radical for TC decomposition in both {010}BiVO4/g-C3N4 and {012}BiVO4/g-C3N4 system Moreover, owing to the more negative conduction band of {010}BiVO4 compared w {012}BiVO4, the {010}BiVO4/g-C3N4 demonstrated higher TC degradation efficiency th {012}BiVO4/g-C3N4. Finally, the photocatalytic mechanism of TC degradation w proposed based on the band structure, heterojunction type, free radical changes, and degradation intermediates. We expect that this work can become a reference for construction of heterogeneous complexes.

Conclusions
In summary, to figure out the confusing heterojunction type between BiVO 4 and g-C 3 N 4 , we rationally designed {010}BiVO 4 /g-C 3 N 4 and {012}BiVO 4 /g-C 3 N 4 as core-shell structures, allowing only photoelectrons or photoelectrodes to participate in oxidation or reduction reactions. Through energy band establishment, radical species investigation, photocatalytic TC degradation performances, and LC-MS tests, both {010}BiVO 4 /g-C 3 N 4 and {012}BiVO 4 /g-C 3 N 4 exhibit the type II heterojunction structures rather than the previously reported Z-scheme heterojunctions. The O 2 •− was proven as the main active radical for TC decomposition in both {010}BiVO 4 /g-C 3 N 4 and {012}BiVO 4 /g-C 3 N 4 systems. Moreover, owing to the more negative conduction band of {010}BiVO4 compared with {012}BiVO 4 , the {010}BiVO 4 /g-C 3 N 4 demonstrated higher TC degradation efficiency than {012}BiVO 4 /g-C 3 N 4 . Finally, the photocatalytic mechanism of TC degradation was proposed based on the band structure, heterojunction type, free radical changes, and TC degradation intermediates. We expect that this work can become a reference for the construction of heterogeneous complexes.

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