Synthesis of a Novel 1D/2D Bi 2 O 2 CO 3 –BiOI Heterostructure and Its Enhanced Photocatalytic Activity

: A novel 1D/2D Bi 2 O 2 CO 3 –BiOI heterojunction photocatalyst with high-quality interfaces was synthesized through a hydrothermal method by using Bi 2 O 2 CO 3 nanorods and KI as raw materials. Two-dimensional (2D) BiOI nanosheets uniformly and vertically grow on the 1D porous Bi 2 O 2 CO 3 rods. Bi 2 O 2 CO 3 –BiOI heterojunctions exhibit better photocatalytic activity than pure Bi 2 O 2 CO 3 nanorods and BiOI nanosheets. Cr(VI) (30 mg/L), MO (20 mg/L) and BPA (20 mg/L) can be completely degraded in 8–15 min. The superior photocatalytic performance of 1D/2D Bi 2 O 2 CO 3 – BiOI heterojunction is ascribed to the synergistic effects: (a) vertical 2D on 1D multidimensional structure; (b) the formation of the Bi 2 O 2 CO 3 –BiOI p–n heterojunction; (c) high-quality interfaces between Bi 2 O 2 CO 3 and BiOI.


Introduction
Nowadays, one-dimensional (1D)/two-dimensional (2D) heterogeneous photocatalysts have been extensively studied for their unique dimensional advantages in environmental and sustainable energy applications [1][2][3]. A large number of studies have shown that 1D nanostructures can provide a short diffusion length perpendicular to its axis and a fast carrier transfer path along its axis [4][5][6][7]. Two-dimensional (2D) materials usually have a large surface area with good electrical conductivity and superior electron mobility [8][9][10][11]. So, compared with other heterostructures, 1D/2D heterostructures have unique advantages because of their intrinsic structural features [12][13][14][15]. Firstly, 1D/2D heterojunction has a larger surface/interface area, which can provide more photocatalytic active sites. Secondly, the photogenerated charge carriers on 2D nanosheets will transfer to 1D nanorods, which make electron-hole pairs separation more effectively and charge lifetimes longer [12,[16][17][18]. At present, there are mainly two types of 1D/2D heterojunction based on the interfacial contact and morphology: the growth of 1D material on 2D nanomaterials and the growth of 2D nanosheets on 1D nanomaterials. Among them, the vertical epitaxial growth of 2D nanosheets on 1D nanomaterial can expose most of their overall surface [19,20]. Although 1D/2D heterojunction materials have many advantages, there are very few studies on bismuth-based 1D/2D heterojunction materials at present [21][22][23]. In recent years, our group has reported the synthesis of photocatalysts which 2D nanosheets vertically loaded on 1D bismuth-based materials [19,20,24,25]. These 1D/2D bismuth-based heterojunction photocatalysts exhibit superior photocatalytic activity in the removal of organic pollutants. However, the research in this field is still insufficient. So, it is necessary to extensively explore the synthesis of this kind of 1D/2D bismuth-based heterojunction photocatalysts with excellent photocatalytic performance.
In this article, a novel 1D/2D Bi 2 O 2 CO 3 -BiOI heterojunction was synthesized through a hydrothermal method by using porous Bi 2 O 2 CO 3 nanorods as Bi source and KI as I source. BiOI nanosheets vertically grow on the Bi 2 O 2 CO 3 porous rods. This 1D/2D Bi 2 O 2 CO 3 -BiOI heterostructure displays superior photocatalytic activity for degrading Cr (VI), methyl orange (MO) and bisphenol A (BPA) under solar light irradiation, and Cr (VI) (30 mg/L) could be completely reduced in 8 min. This excellent photocatalytic performance is due to the synergistic effect of some factors: (a) Unique 1D/2D nanostructure; (b) the formation of the p-n junction; (c) the high-quality interfaces between Bi 2 O 2 CO 3 and BiOI.

Results and Discussion
1D/2D Bi 2 O 2 CO 3 -BiOI heterojunction was synthesized by hydrothermal method using porous Bi 2 O 2 CO 3 nanorods and KI as raw materials. The as-obtained Bi 2 O 2 CO 3 -BiOI heterojunctions were labelled as S1, S2, S3 and S4 when the molar ratios of Bi 2 O 2 CO 3 : KI are 10:1, 2:1,1:2 and 1:20, respectively. The SEM images and XRD pattern of the obtained samples are shown in Figure 1. Bi 2 O 2 CO 3 is 1D rod-like structure with rough surfaces and porosity (Figure 1a). Few nanosheets grow vertically on the surface of Bi 2 O 2 CO rods when the molar ratio of Bi 2 O 2 CO 3 : KI is 10:1 (Figure 1b). With the molar ratio of Bi 2 O 2 CO 3 : KI increase, more and more nanosheets are loaded on the surface of Bi 2 O 2 CO rods (Figure 1b-d). When the molar ratio of Bi 2 O 2 CO 3 : KI is 1:20, only out-of-order nanosheets can be obtained (Figure 1e). Figure 1f shows the XRD patterns of the obtained samples. It can be seen that the main diffraction peaks in S1-S3 samples are indexed to Bi 2 O 2 CO 3 (PDF#25-1464). In samples S2 and S3, a new diffraction peak (2θ = 31.65 • ) is found clearly, which is indexed to the tetragonal BiOI (PDF#10-0445). The diffraction peaks of Bi 2 O 2 CO 3 become weaker and weaker from S1 to S3. The results indicate Bi 2 O 2 CO 3 reacts with KI and form 1D/2D Bi 2 O 2 CO 3 -BiOI heterostructure. In S4 sample, only BiOI peaks can be found, indicating all the Bi 2 O 2 CO 3 nanorods is consumed completely. The Bi 2 O 2 CO 3 nanorod skeleton is disappeared, which results in the collapse of 1D structure. So, BiOI nanosheets is formed when the molar ratio of Bi 2 O 2 CO 3 :KI is 1:20.
The S2 sample was characterized by using TEM and HRTEM. As shown in Figure 2a, it can be seen that BiOI nanosheets grow vertically on the surface of Bi 2 O 2 CO 3 nanorods, which is consistent with the SEM images. In Figure 2b, the lattice spacing of 0.685 and 0.915 nm indexes to the (002) lattice plane of Bi 2 O 2 CO 3 and the (001) lattice plane of BiOI, respectively. BiOI nanosheets grow out from the Bi 2 O 2 CO 3 rod by oriented epitaxial nucleation and growth, which is beneficial for the formation of a high-quality interface [24]. The S2 sample was characterized by using TEM and HRTEM. As shown in Figure 2a, it can be seen that BiOI nanosheets grow vertically on the surface of Bi2O2CO3 nanorods, which is consistent with the SEM images. In Figure 2b, the lattice spacing of 0.685 and 0.915 nm indexes to the (002) lattice plane of Bi2O2CO3 and the (001) lattice plane of BiOI, respectively. BiOI nanosheets grow out from the Bi2O2CO3 rod by oriented epitaxial nucleation and growth, which is beneficial for the formation of a high-quality interface [24]. The full XPS spectra of S2, S4 and Bi2O2CO3 nanorods is shown in Figure 3a. Bi, C, O and I elements are co-existence in S2 sample, indicating the formation of Bi2O2CO3-BiOI. There is almost no C element in the S4 sample, confirming that S4 is pure BiOI. The high resolution XPS spectra of the Bi 4f, I3d and C1s are shown in Figure 3b-d. Two peaks centered at about 164.50 and 159.00 eV are attributed to Bi 4f7/2 and Bi 4f5/2, respectively, indicating that the Bi element in all samples is in the form of Bi 3+ ion (Figure 3b). Compared with Bi2O2CO3 nanorods and S4 (pure BiOI), the peak of Bi 4f moves to higher binding  The S2 sample was characterized by using TEM and HRTEM. As shown in Figure 2a, it can be seen that BiOI nanosheets grow vertically on the surface of Bi2O2CO3 nanorods, which is consistent with the SEM images. In Figure 2b, the lattice spacing of 0.685 and 0.915 nm indexes to the (002) lattice plane of Bi2O2CO3 and the (001) lattice plane of BiOI, respectively. BiOI nanosheets grow out from the Bi2O2CO3 rod by oriented epitaxial nucleation and growth, which is beneficial for the formation of a high-quality interface [24]. The full XPS spectra of S2, S4 and Bi2O2CO3 nanorods is shown in Figure 3a. Bi, C, O and I elements are co-existence in S2 sample, indicating the formation of Bi2O2CO3-BiOI. There is almost no C element in the S4 sample, confirming that S4 is pure BiOI. The high resolution XPS spectra of the Bi 4f, I3d and C1s are shown in Figure 3b-d. Two peaks centered at about 164.50 and 159.00 eV are attributed to Bi 4f7/2 and Bi 4f5/2, respectively, indicating that the Bi element in all samples is in the form of Bi 3+ ion ( Figure 3b). Compared with Bi2O2CO3 nanorods and S4 (pure BiOI), the peak of Bi 4f moves to higher binding The full XPS spectra of S2, S4 and Bi 2 O 2 CO 3 nanorods is shown in Figure 3a. Bi, C, O and I elements are co-existence in S2 sample, indicating the formation of Bi 2 O 2 CO 3 -BiOI. There is almost no C element in the S4 sample, confirming that S4 is pure BiOI. The high resolution XPS spectra of the Bi 4f, I3d and C1s are shown in Figure 3b-d. Two peaks centered at about 164.50 and 159.00 eV are attributed to Bi 4f 7/2 and Bi 4f 5/2 , respectively, indicating that the Bi element in all samples is in the form of Bi 3+ ion ( Figure 3b). Compared with Bi 2 O 2 CO 3 nanorods and S4 (pure BiOI), the peak of Bi 4f moves to higher binding energies, which proves that high-quality interface forms between Bi 2 O 2 CO 3 and BiOI. The same phenomenon is also observed in the I 3d spectra (Figure 3c). The peak centered at 289.08 eV is indexed to C1s (Figure 3d). The intensity of C1s in S2 sample is obviously lower than in Bi 2 O 2 CO 3 . This result confirms Bi 2 O 2 CO 3 is consumed to form BiOI during the reaction. energies, which proves that high-quality interface forms between Bi2O2CO3 and BiOI. The same phenomenon is also observed in the I 3d spectra (Figure 3c). The peak centered at 289.08 eV is indexed to C1s (Figure 3d). The intensity of C1s in S2 sample is obviously lower than in Bi2O2CO3. This result confirms Bi2O2CO3 is consumed to form BiOI during the reaction. The FTIR spectra of Bi2O2CO3, S2 and S4 samples were measured, as shown in Figure  4. The intensive peak which centered at 847.1 cm −1 is attributed to the ν2 modes of the CO3 2− group. After Bi2O2CO3 and BiOI are combined with each other, a new peak at 493. 6 cm −1 appears in S2 sample, further proving the formation of the strong interfacial junction between Bi2O2CO3 and BiOI. Depending on the increase of the loaded content of BiOI, the main characteristic peak of the CO3 2− (847.1 cm −1 ) almost disappears in the S4. This result is consistent with XRD and XPS results, verifying that the S4 sample is pure BiOI.  The FTIR spectra of Bi 2 O 2 CO 3 , S2 and S4 samples were measured, as shown in Figure 4. The intensive peak which centered at 847.1 cm −1 is attributed to the ν2 modes of the CO 3 2− group. After Bi 2 O 2 CO 3 and BiOI are combined with each other, a new peak at 493.6 cm −1 appears in S2 sample, further proving the formation of the strong interfacial junction between Bi 2 O 2 CO 3 and BiOI. Depending on the increase of the loaded content of BiOI, the main characteristic peak of the CO 3 2− (847.1 cm −1 ) almost disappears in the S4. This result is consistent with XRD and XPS results, verifying that the S4 sample is pure BiOI.  UV-vis diffuse reflectance spectroscopies (DRS) of Bi2O2CO3, S2 and S4 were performed to study their optical absorption properties. The absorption band edge of Bi2O2CO3 and BiOI are ~450 and 670 nm (Figure 5a), respectively, indicating the wider band gap of Bi2O2CO3 than that of BiOI. The absorption band edge of the S2 heterostructure red-shifts compared with Bi2O2CO3 due to the loaded-BiOI with narrower band gap (Figure 5b). The optical band gap of Bi2O2CO3 and BiOI is obtained using the following equation: UV-vis diffuse reflectance spectroscopies (DRS) of Bi 2 O 2 CO 3 , S2 and S4 were performed to study their optical absorption properties. The absorption band edge of Bi 2 O 2 CO 3 and BiOI are~450 and 670 nm (Figure 5a), respectively, indicating the wider band gap of Bi 2 O 2 CO 3 than that of BiOI. The absorption band edge of the S2 heterostructure red-shifts compared with Bi 2 O 2 CO 3 due to the loaded-BiOI with narrower band gap (Figure 5b). The optical band gap of Bi 2 O 2 CO 3 and BiOI is obtained using the following equation: where α is the absorption coefficient, h is Planck's constant, ν is the light frequency, A is the constant and Eg is the bandgap energy [49]. In our study, both Bi 2 O 2 CO 3 and BiOI possess indirect band gaps, so n = 4 [25,50]. The band gap energies are estimated to 2.96 eV for pure Bi 2 O 2 CO 3 , and 1.75 eV for pure BiOI (Figure 5b).  UV-vis diffuse reflectance spectroscopies (DRS) of Bi2O2CO3, S2 and S4 were performed to study their optical absorption properties. The absorption band edge of Bi2O2CO3 and BiOI are ~450 and 670 nm (Figure 5a), respectively, indicating the wider band gap of Bi2O2CO3 than that of BiOI. The absorption band edge of the S2 heterostructure red-shifts compared with Bi2O2CO3 due to the loaded-BiOI with narrower band gap (Figure 5b). The optical band gap of Bi2O2CO3 and BiOI is obtained using the following equation: where α is the absorption coefficient, h is Planck's constant, ν is the light frequency, A is the constant and Eg is the bandgap energy [49]. In our study, both Bi2O2CO3 and BiOI possess indirect band gaps, so n = 4 [25,50]. The band gap energies are estimated to 2.96 eV for pure Bi2O2CO3, and 1.75 eV for pure BiOI (Figure 5b).   [39,40]. From Figure S1, the reaction rate constant (k) of S2 (0.4661 min −1 ) is much higher than that of Bi 2 O 2 CO 3 , S1, S3 and S4 (0.0049, 0.0731, 0.1363 and 0.0594 min −1 ) in degrading Cr (VI), exhibiting S2 sample superior photocatalyst. The reaction rate constant (k) of S2 is also higher than that of Bi 2 O 2 CO 3 , S1, S3 and S4 in degrading MO (20 mg/L) and BPA (20 mg/L) (Figures S2 and S3). The unique 2D vertical on 1D structure endows Bi 2 O 2 CO 3 -BiOI photocatalyst with distinctive photocatalytic activity. Firstly, 2D BiOI nanosheets vertically grew on the 1D Bi 2 O 2 CO 3 nanorods which can provide almost exposure entire active sites; Secondly, 1D Bi 2 O 2 CO 3 structures provide quickly charge carriers transfer path along their axis; In addition, the high-quality interface between Bi 2 O 2 CO 3 and BiOI promotes the transfer rate of photogenerated charge carriers at junction interface, enhancing photocatalytic activity.
in degrading MO (20 mg/L) and BPA (20 mg/L) (Figures S2 and S3). The unique 2D vertical on 1D structure endows Bi2O2CO3-BiOI photocatalyst with distinctive photocatalytic activity. Firstly, 2D BiOI nanosheets vertically grew on the 1D Bi2O2CO3 nanorods which can provide almost exposure entire active sites; Secondly, 1D Bi2O2CO3 structures provide quickly charge carriers transfer path along their axis; In addition, the high-quality interface between Bi2O2CO3 and BiOI promotes the transfer rate of photo-generated charge carriers at junction interface, enhancing photocatalytic activity. Free radical capture experiment is carried out to to explore active species in photocatalytical process. tert-butanol (TBA), 1, 4-benzoquinone (BQ) and ammonium oxalate (AO) were used to trap hydroxyl radical (OH·), superoxide radical (O2 − ) and hole (H + ) for Cr(VI) (30 mg/L) degradation. As can be seen from Figure 7a, the addition of AO and BQ significantly inhibited the photocatalytic reduction of Cr(VI) (30 mg/L). However, with the addition of TBA, the photoreduction efficiency of Cr(VI) (30 mg/L) only partly changes. These results prove that the main active species are·O2 − and H + during photocatalytic process.
In order to evaluate the reusability and photostability of Bi2O2CO3-BiOI heterostructure, catalytic cycle experiment was done using S2 as photocatalyst to degrade the Cr(VI) (30 mg/L) (Figure 7b). We can see that the S2 sample had good reusability, and its photocatalytic efficiency almost remained stable after five cycles. In addition, the S2 sample after 5 cycles was characterized by using XRD and SEM, and the results are shown in Figure  7c,d, respectively. The results demonstrate S2 sample retains the original structure and morphology after five cycles, implying a good photostability of S2 sample under solar light irradiation. Free radical capture experiment is carried out to to explore active species in photocatalytical process. tert-butanol (TBA), 1, 4-benzoquinone (BQ) and ammonium oxalate (AO) were used to trap hydroxyl radical (OH·), superoxide radical (O 2 − ) and hole (H + ) for Cr(VI) (30 mg/L) degradation. As can be seen from Figure 7a The room temperature PL emission spectra of the pure Bi2O2CO3, S2 and S4 (pure BiOI) are shown in Figure 8a. The PL emission intensity of S2 is the lowest one among the three samples, which implies 1D/2D heterostructure effectively suppresses the recombination of photogenerated e-h + , and thus enhancing the photocatalytic performance [51].
The photocurrent of pure Bi2O2CO3, S2 and S4 (pure BiOI) samples are shown in Figure 8b. The photocurrent density generated by the S2 sample is obviously higher than that of pure Bi2O2CO3 and BiOI. Therefore, the PL and photocurrent measurements all demonstrate that the 1D/2D Bi2O2CO3-BiOI heterostructure can significantly promote the separation and transfer of photogenerated electron-hole pairs.  In order to evaluate the reusability and photostability of Bi 2 O 2 CO 3 -BiOI heterostructure, catalytic cycle experiment was done using S2 as photocatalyst to degrade the Cr(VI) (30 mg/L) (Figure 7b). We can see that the S2 sample had good reusability, and its photocatalytic efficiency almost remained stable after five cycles. In addition, the S2 sample after 5 cycles was characterized by using XRD and SEM, and the results are shown in Figure 7c,d, respectively. The results demonstrate S2 sample retains the original structure and morphology after five cycles, implying a good photostability of S2 sample under solar light irradiation.
The room temperature PL emission spectra of the pure Bi 2 O 2 CO 3 , S2 and S4 (pure BiOI) are shown in Figure 8a. The PL emission intensity of S2 is the lowest one among the three samples, which implies 1D/2D heterostructure effectively suppresses the recombination of photogenerated e-h + , and thus enhancing the photocatalytic performance [51].
The photocurrent of pure Bi 2 O 2 CO 3 , S2 and S4 (pure BiOI) samples are shown in Figure 8b. The photocurrent density generated by the S2 sample is obviously higher than that of pure Bi 2 O 2 CO 3 and BiOI. Therefore, the PL and photocurrent measurements all demonstrate that the 1D/2D Bi 2 O 2 CO 3 -BiOI heterostructure can significantly promote the separation and transfer of photogenerated electron-hole pairs.
In order to obtain the relative positions of the conduction band (CB) and valence band (VB) edges, VB-XPS of Bi 2 O 2 CO 3 and S4 (pure BiOI) were characterized (Figure 9). The E VB top of Bi 2 O 2 CO 3 and S4 locate at 1.78 and 1.15 eV, respectively. On the basis of the VB position and their band gaps, the CB edge potentials of Bi 2 O 2 CO 3 and BiOI are estimated to be −1.18 and −0.6 eV, respectively through the equation E CB = E VB . three samples, which implies 1D/2D heterostructure effectively suppresses the recombination of photogenerated e-h + , and thus enhancing the photocatalytic performance [51].
The photocurrent of pure Bi2O2CO3, S2 and S4 (pure BiOI) samples are shown in Figure 8b. The photocurrent density generated by the S2 sample is obviously higher than that of pure Bi2O2CO3 and BiOI. Therefore, the PL and photocurrent measurements all demonstrate that the 1D/2D Bi2O2CO3-BiOI heterostructure can significantly promote the separation and transfer of photogenerated electron-hole pairs. Schematic diagram for energy band of Bi2O2CO3-BiOI, the formation of the p-n junction and the possible charge separation is displayed in Figure10. We know that p-n junctions can be formed between (Figure 10b), The internal electric field of Bi2O2CO3-BiOI pn heterojunction promotes the migration rate of photogenerated electrons and holes, which greatly improves the photocatalytic activity. The unique vertical 2D materials on 1D structure makes BiOI nanosheets expose almost entire surface, increasing the separation and transfer rate of photo-generated electron-holes pairs. Furthermore, high-quality interface between Bi2O2CO3 and BiOI decreases the energy barrier for the photogenerated charge carriers transfer at the junction, enhancing the photocatalytic activity. Schematic diagram for energy band of Bi 2 O 2 CO 3 -BiOI, the formation of the p-n junction and the possible charge separation is displayed in Figure 10. We know that p-n junctions can be formed between (Figure 10b), The internal electric field of Bi 2 O 2 CO 3 -BiOI p-n heterojunction promotes the migration rate of photogenerated electrons and holes, which greatly improves the photocatalytic activity. The unique vertical 2D materials on 1D structure makes BiOI nanosheets expose almost entire surface, increasing the separation and transfer rate of photo-generated electron-holes pairs. Furthermore, high-quality interface between Bi 2 O 2 CO 3 and BiOI decreases the energy barrier for the photogenerated charge carriers transfer at the junction, enhancing the photocatalytic activity.
tions can be formed between (Figure 10b), The internal electric field of Bi2O2CO3-BiOI p-n heterojunction promotes the migration rate of photogenerated electrons and holes, which greatly improves the photocatalytic activity. The unique vertical 2D materials on 1D structure makes BiOI nanosheets expose almost entire surface, increasing the separation and transfer rate of photo-generated electron-holes pairs. Furthermore, high-quality interface between Bi2O2CO3 and BiOI decreases the energy barrier for the photogenerated charge carriers transfer at the junction, enhancing the photocatalytic activity.   Deionized water and anhydrous ethanol were used to wash the obtained products and then dry them at 60 • C for 6 h. The as-make products were named as S1, S2, S3 and S4 when the molar ratios of Bi 2 O 2 CO 3 and BiOI are 10:1, 2:1, 1:2 and 1:20, respectively.

Photocatalytic Activity Measurements
The photocatalytic performance of Bi 2 O 2 CO 3 -BiOI heterojunction was evaluated by degrading Cr (VI), methyl orange (MO) and bisphenol A (BPA) under solar light irradiation. The 300W Xe lamp (CER-HXF300F, Beijing China Education Au-Light Co. Ltd. Bei Jing, China) was used as light source. 30 mg photocatalyst was dispersed in 30 mL K 2 Cr 2 O 7 solution (30 mg/L), MO solution (20 mg/L) and BPA solution (20 mg/L), respectively. The suspension kept in dark place while it was stirred. Asan adsorption/desorption equilibrium was achieved, the suspension was illuminated under the solar light. Within a given time, 4 mL was collected and centrifugated. Finally, the supernatant is monitored by UV-vis spectrophotometer.

Electrochemical Impedance Spectroscopy (EIS) Measurements
Electrochemical impedance spectroscopy (EIS) measurements were tested at a frequency between 0.1 Hz and 100 kHz using the CHI760E instrument (Shanghai Chen Hua Company, Shanghai, China) Na 2 SO 4 (0.2 M) was used as detecting electrolyte. The electrode system used a three-electrode system, which platinum wire was used as the counter electrode and the saturated ampho-mercury electrode is used as a reference electrode. Bi 2 O 2 CO 3 , S2 sample and BiOI film electrodes served as the working electrodes, separately.

Conclusions
In conclusion, a novel 1D/2D Bi 2 O 2 CO 3 -BiOI p-n heterojunction was synthesized by hydrothermal method. BiOI nanosheets were uniformly and vertically grown from the interior of Bi 2 O 2 CO 3 nanorods on the basis of a crystallography-oriented epitaxial mechanism, which provides a small barrier for the transport of photogenerated electron-hole pairs through the junctions because forming high-quality interfaces between Bi 2 O 2 CO 3 and BiOI. The Bi 2 O 2 CO 3 -BiOI heterojunction photocatalyst exhibits a superior photocatalytic activity for the degradation of Cr (VI), MO and BPA. The outstanding photocatalytic performance is ascribed to the synergistic effects of unique 2D BiOI vertical on 1D Bi 2 O 2 CO 3 multidimensional structure, the formation of the p-n junction and the high-quality interfaces between Bi 2 O 2 CO 3 and BiOI. Trapping experiments show that h + and O 2 − play key roles for photodegradation.