(002) Oriented Bi 2 O 2 CO 3 Nanosheets with Enhanced Photocatalytic Performance for Toluene Removal in Air

: Layer-structured Bi 2 O 2 CO 3 is a novel photocatalyst for eliminating environmental pollutants. In this work, Bi 2 O 2 CO 3 nanosheets were synthesized by hydrothermal methods, followed by annealing in nitrogen. (002) oriented Bi 2 O 2 CO 3 nanosheets were obtained and characterized by XRD, SEM, XPS, BET and UV-Vis di ﬀ use reﬂectance spectra. Photocatalytic properties were investigated by toluene removal in air, with the assistant of Bi 2 O 2 CO 3 nanosheets under artiﬁcial irradiation. Our results show that Bi 2 O 2 CO 3 annealed in nitrogen exhibited high full-light-driven photocatalytic activity for toluene photocatalytic decomposition, which may be ascribed to facet orientation evolution during the annealing process and enhanced e ﬃ cient charge separation. The sample annealed at 150 ◦ C for 8 h (BOC-150-8 h) showed high stability and the highest toluene removal rate, which was up to 99%. The ﬁnal degradation products were detected by gas chromatography–mass spectrometer (GC-MS) and CO 2 was veriﬁed to be the primary product. Photocatalytic mineralization of toluene in air over Bi 2 O 2 CO 3 was proposed. This work may provide a foundation for application of annealed Bi 2 O 2 CO 3 in indoor air puriﬁcation.


Introduction
Volatile organic compounds (VOCs) are one of the major indoor pollutants which derive from sources including paints, varnishes, solvents, furniture, etc. Benzene and its derivatives such as toluene, xylenes are typical representative substances of VOCs. Toluene exposure may cause indisposed symptoms of lethargy, dizziness and confusion. Toluene is considered as a strong carcinogen which is heavily restricted by National Ambient Air Quality Standard [1][2][3][4]. The removal of benzene and its derivatives in air is a huge challenge for environmental restoration. Technologies such as physicochemical adsorption, gas separation, biodegradation and chemical degradation are investigated for indoor air purification. Among all the above-mentioned techniques, semiconductor photocatalysis, which is an advanced oxidation process based on oxidation-reduction reaction, demonstrated great prospect due to its green, sustainable and low-cost features. Variety of photocatalysts have been studied, including oxides, sulfide, nitride, phosphide, novel composites and delicate micro-nano structures. TiO 2 is one of the earlier photocatalysts for air purification. Though TiO 2 showed primordial activity for air pollution control, photocatalysis by semiconducting materials loaded with non-metal or noble metals is an excellent strategy for removal of gaseous toluene. By coupling atomic-scale Pt with MoS 2 as co-catalysts, TiO 2 showed superior performance than pure one [5]. Several TiO 2 based composites and non-TiO 2 -based photocatalysts, such as ZnO, Fe 2 O 3 , g-C 3 N 4, were reported for photocatalytic oxidation of VOCs and inorganic pollutants [6][7][8][9][10][11].
Bi 2 O 2 CO 3 is ascribed to the aurivillius structure and layered with alternative stacking (Bi 2 O 2 ) 2+ sheets interleaved by CO 3 2− groups in crystal structure [20][21][22]. These years, its application in photocatalysis has been explored. A variety of attempts have been investigated to enhance the photocatalytic performance of Bi 2 O 2 CO 3 , such as: doping [23], heterojunctions construction with smaller band gap semiconductors and morphology controlling with especial crystal facets exposed [24,25]. The exposed {001} facets of Bi 2 O 2 CO 3 are considered to contribute much more photocatalytic activity because the Bi-O square anti-prism with 8-coordination compressed along the c-axis supply lots of oxygen defects which more preferred to generate the electron and vacancy [26,27]. Although Bi 2 O 2 CO 3 has been studied extensively, the research and application of photocatalysis now mainly focuses on photolysis of removal NO X [28,29], water to generate hydrogen [30] and degradation of organic pollutants in water such as methyl orange (MO), rhodamine B (RhB), methylene blue (MB) and their mixed solutions [31]. The application of photocatalysis on removal toluene in air is limitedly reported [32]. Herein, we report a facile hydrothermal method to fabricate Bi 2 O 2 CO 3 nanosheets with (002) facet exposed, followed by annealed in nitrogen. Bi 2 O 2 CO 3 samples annealed at diffident temperature for various times and their photocatalytic properties for removal toluene in air under the artificial irradiation are investigated comparatively. Based on the close correlation between the structure characteristics and physicochemical properties of the material, Bi 2 O 2 CO 3 annealed at 150 • C for 8 h has proved to be most active, with removal rate up 99%. Permineralization of toluene in photocatalysis is proposed based on the characterization; CO 2 was verified to be the primary product. This work may probably extend to application of Bi 2 O 2 CO 3 in air purification.
Catalysts 2020, 10, x FOR PEER REVIEW 3 of 14 Figure 1. Patterns of as-prepared samples at different temperature.

Photocatalytic Activity
In the present work, the photocatalytic performance of BOC annealed in nitrogen was investigated by removal toluene in air. The results of removal toluene by BOC annealed in N2 at different temperature for 4h are shown in Figure 2 and Figure S2. The rate of photocatalytic degradation follows a pseudo first order kinetics based on the Langmuir-Hinshelwood model: where C0 is the equilibrium concentration, Ct is the concentration at any time, k is the first-order rate constant and t is the photocatalytic time. According to formula (1), the k values for BOC, BOC-150-4h, BOC-250-4h, BOC-350-4h, BOC-450-4h are 0.0100, 0.0105, 0.0031, 0.0016 and 0.0013min −1 , respectively, from which we could observe that the photo degradation rate constant of BOC-150-4h is larger than any others. BOC and BOC-150-4h showed almost the same impressive performance over toluene removal. BOC-250-4h had a slightly worse performance than BOC-150-4h and raw BOC nanosheets. Samples BOC-350-4hand BOC-450-4h showed negligible effectiveness for toluene removal. The content of toluene after photocatalysis in chamber showed no macroscopic different compared with that of controlled experiment. Photocatalysis test`s results revealed that Bi2O2CO3eitheruntreated BOC or annealed at low temperature-had remarkable ability in toluene removal. However, Bi2O3 showed suppressive effect.
{001} exposing facets over Bi2O2CO3have been reported to show photocatalytic activity due to efficient separation and migration of photoinduced electron-hole pairs [16]. Meanwhile, β-Bi2O3 nanosheets exposed with active {001} facets prepared through annealing Bi2O2CO3 have shown improved photocatalytic oxidation of NO [24]. However, the underlying mechanisms are still not fully understood.
In this work, exposing facets of Bi2O2CO3 crystals appeared to have relevant effect on photocatalytic activity. BOC-150-4h had the strongest diffracted intensity of (002) peak, along with almost the best photocatalytic activity.

Photocatalytic Activity
In the present work, the photocatalytic performance of BOC annealed in nitrogen was investigated by removal toluene in air. The results of removal toluene by BOC annealed in N 2 at different temperature for 4 h are shown in Figure 2 and Figure S2. The rate of photocatalytic degradation follows a pseudo first order kinetics based on the Langmuir-Hinshelwood model: where C 0 is the equilibrium concentration, C t is the concentration at any time, k is the first-order rate constant and t is the photocatalytic time.  3 have been reported to show photocatalytic activity due to efficient separation and migration of photoinduced electron-hole pairs [16]. Meanwhile, β-Bi 2 O 3 nanosheets exposed with active {001} facets prepared through annealing Bi 2 O 2 CO 3 have shown improved photocatalytic oxidation of NO [24]. However, the underlying mechanisms are still not fully understood.
In this work, exposing facets of Bi 2 O 2 CO 3 crystals appeared to have relevant effect on photocatalytic activity. BOC-150-4 h had the strongest diffracted intensity of (002) peak, along with almost the best photocatalytic activity.

Morphology and Structural Properties
To further understand the effects of annealing time and exposing facets to photocatalytic activity, BOC was annealed at 150°C for various times. Powder x-ray diffraction ( Figure 3) revealed that all diffraction peaks can be well indexed to the pure phase of Bi2O2CO3, without impurity peaks appearing in all as-prepared samples. This suggests that annealing Bi2O2CO3 in nitrogen at 150°C had not led to phase transformation. Nevertheless, the evolution of crystal facets was revealed by elaborately evaluating the diffracted intensity of special peaks.
On the basis of the above analyses, it could be speculated that during annealing, hydrothermal fabricated Bi2O2CO3nanosheets underwent a delicate evolution process of dominantly exposed facets.

Morphology and Structural Properties
To further understand the effects of annealing time and exposing facets to photocatalytic activity, BOC was annealed at 150 • C for various times. Powder X-ray diffraction ( Figure 3) revealed that all diffraction peaks can be well indexed to the pure phase of Bi 2 O 2 CO 3 , without impurity peaks appearing in all as-prepared samples. This suggests that annealing Bi 2 O 2 CO 3 in nitrogen at 150 • C had not led to phase transformation. Nevertheless, the evolution of crystal facets was revealed by elaborately evaluating the diffracted intensity of special peaks.

Morphology and Structural Properties
To further understand the effects of annealing time and exposing facets to photocatalytic activity, BOC was annealed at 150°C for various times. Powder x-ray diffraction ( Figure 3) revealed that all diffraction peaks can be well indexed to the pure phase of Bi2O2CO3, without impurity peaks appearing in all as-prepared samples. This suggests that annealing Bi2O2CO3 in nitrogen at 150°C had not led to phase transformation. Nevertheless, the evolution of crystal facets was revealed by elaborately evaluating the diffracted intensity of special peaks.
On the basis of the above analyses, it could be speculated that during annealing, hydrothermal fabricated Bi2O2CO3nanosheets underwent a delicate evolution process of dominantly exposed facets.  Compared to hydrothermal fabricated Bi 2 O 2 CO 3 , (002) facet exposed dominantly after BOC were annealed for 4 h and 8 h at 150 • C. The diffracted intensity ratio of (002)/(013) of BOC-150-4 h and BOC-150-8 h were 0.60 and 1.17, that was much larger than 0.25 of primitive BOC (JCPDS 41-1488). The ratio of BOC-150-8 h has been increased by 92%. Prolonging the annealing time further to 12 h, 24 h or 48 h, unexpectedly, the enhanced diffracted intensity ratio of (002)/(013)restored to the value of BOC.
On the basis of the above analyses, it could be speculated that during annealing, hydrothermal fabricated Bi 2 O 2 CO 3 nanosheets underwent a delicate evolution process of dominantly exposed facets.
In the following, the surface composition of the annealing BOC was further investigated by XPS technique. Figure 4 presents the detection results of the high-resolution XPS spectra. As shown in Figure 4a, the relative peaks of the C-1s, O-1s and Bi-4f elements have been successfully detected. Therein, the C-1s peak at a binding energy of 284.8 eV can be attributed to adventitious hydrocarbon; the peak observed at 289.0 eV should be ascribed to the carbon of C-O bond in Bi 2 O 2 CO 3 (Shown in Figure 4b) [38]. The O-1s binding energy of 530.3 eV in Figure 4c can be attributed to the lattice oxygen inthe (Bi 2 O 2 ) 2+ layers of Bi 2 O 2 CO 3 [39]. In Figure 4d-f, the two apparent characteristic peaks for Bi-4f located at 159.1 and 164.5 eV are attributed to Bi-4f 7/2 and Bi-4f 5/2 in Bi 2 O 2 CO 3 , which indicating the existence of Bi 3+ in the sample [40,41].
Catalysts 2020, 10, x FOR PEER REVIEW 5 of 14 In the following, the surface composition of the annealing BOC was further investigated by XPS technique. Figure 4 presents the detection results of the high-resolution XPS spectra. As shown in Figure4a, the relative peaks of the C-1s, O-1s and Bi-4f elements have been successfully detected. Therein, the C-1s peak at a binding energy of 284.8 eV can be attributed to adventitious hydrocarbon; the peak observed at 289.0 eV should be ascribed to the carbon of C-O bond in Bi2O2CO3 (Shown in Figure 4b) [38]. The O-1s binding energy of 530.3 eV in Figure 4c can be attributed to the lattice oxygen inthe (Bi2O2) 2+ layers of Bi2O2CO3 [39]. In Figure 4d,e,f, the two apparent characteristic peaks for Bi-4f located at 159.1and 164.5 eV are attributed to Bi-4f7/2 and Bi-4f5/2 in Bi2O2CO3, which indicating the existence of Bi 3+ in the sample [40,41]. SEM images were obtained to investigate the morphology of the samples，which are presented in Figure 5. According to the SEM images, samples prepared by hydrothermal method showed irregular polyhedral structures with lateral length of 300~900 nm (Figure 5a). After annealing at 150°C for various times, the appearance of BOC showed negligible changes compared to the primary sample (Figure 5b, Figure S3). The typical thickness of the annealed sample was few tens of nanometers. It appeared to be two-dimensional nanosheets [42]. It implied that a low temperature SEM images were obtained to investigate the morphology of the samples, which are presented in Figure 5. According to the SEM images, samples prepared by hydrothermal method showed irregular polyhedral structures with lateral length of 300~900 nm (Figure 5a). After annealing at 150 • C for various times, the appearance of BOC showed negligible changes compared to the primary sample ( Figure 5b, Figure S3). The typical thickness of the annealed sample was few tens of nanometers. It appeared to be two-dimensional nanosheets [42]. It implied that a low temperature annealing process had no apparent effects on the morphologies. This was further supported by BET measurement.
Catalysts 2020, 10, x FOR PEER REVIEW 2 of 14 annealing process had no apparent effects on the morphologies. This was further supported by BET measurement. The BET surface area test results of as-prepared samples are shown in Table 1. It was found that the specific surface area and pore volume of the sample BOC-150-8h were 3.556m 2 /g and 0.5319 cm 3 /g, respectively, which was much larger than the others. Moreover, the pore volume of all BOC was much larger than most of other bismuth oxides and the different morphology of Bi2O2CO3 materials. The specific surface area of BOC-150-8h is similar to plate-like Bi2O2CO3(Shown in Table S1 and S2) [43].Therefore, the larger BET surface area of BOC-150-8h may result in better catalysis performance by providing more actives sites than the other samples. The UV-Vis diffuse reflectance spectra of the samples were examined, and the results presented in Figure 6. It can be seen that the bandgap of all the samples can be decided about 2.8 eV.
The conduction band (CB) and valence band (VB) potentials of as-prepared samples were calculated using the following empirical formulae [44]: where EVB and ECB are the semiconductor VB and CB potentials, respectively, X is the absolute electronegativity of the semiconductor, E e is the energy of free electrons on the hydrogen scale (~4.5 eV), Eg is the semiconductor band gap. Whereas the absolute electronegativity is the energy difference between the vacuum level and the mid-gap position of semiconductor, the work function value is the difference between the vacuum level and the Fermi energies, which of pure Bi2O2CO3 is zero [26]. According to the above formulae, the EVB and ECB of the samples were calculated, and the band gap structures were obtained (Figure 7). It can be concluded that valence band edge position and conduction band edge position become more negative when we prolonged the annealing time in nitrogen. The BET surface area test results of as-prepared samples are shown in Table 1. It was found that the specific surface area and pore volume of the sample BOC-150-8 h were 3.556 m 2 /g and 0.5319 cm 3 /g, respectively, which was much larger than the others. Moreover, the pore volume of all BOC was much larger than most of other bismuth oxides and the different morphology of Bi 2 O 2 CO 3 materials. The specific surface area of BOC-150-8 h is similar to plate-like Bi 2 O 2 CO 3 (Shown in Tables S1 and S2) [43].Therefore, the larger BET surface area of BOC-150-8 h may result in better catalysis performance by providing more actives sites than the other samples. The UV-Vis diffuse reflectance spectra of the samples were examined, and the results presented in Figure 6. It can be seen that the bandgap of all the samples can be decided about 2.8 eV.
The conduction band (CB) and valence band (VB) potentials of as-prepared samples were calculated using the following empirical formulae [44]: where E VB and E CB are the semiconductor VB and CB potentials, respectively, X is the absolute electronegativity of the semiconductor, E e is the energy of free electrons on the hydrogen scale (~4.5 eV), E g is the semiconductor band gap. Whereas the absolute electronegativity is the energy difference between the vacuum level and the mid-gap position of semiconductor, the work function value is the difference between the vacuum level and the Fermi energies, which of pure Bi 2 O 2 CO 3 is zero [26]. According to the above formulae, the E VB and E CB of the samples were calculated, and the band gap structures were obtained (Figure 7). It can be concluded that valence band edge position and conduction band edge position become more negative when we prolonged the annealing time in nitrogen.

Photocatalytic Activity
The photocatalytic performance of annealed BOC in nitrogen for various time at 150°C are presented in Figure 8; the digital images after reaction are shown in Figure S4. All the as-prepared BOC samples showed almost the same impressive performance for toluene removal. BOC-150-8h was superior to any other samples, with removal rate of up to 99%. BOC-150-12h and BOC-150-24 had a slightly worse performance, but they were better than BOC-150-4h and BOC-150-48. BOC-150-48 was analogous with the raw BOC nanosheets.
On the basis of formula (1), k values for various samples were calculated. The specific numbers for BOC-150-4h, BOC-150-8h, BOC-150-12h, BOC-150-24h, BOC-150-48h were 0.0105, 0.0211, 0.0161, 0.0168 and 0.0116min −1 , respectively. BOC-150-8h had the largest k value, with higher ratio of (002) and (004) crystal face exposed from Figure 3, which may have contributed to the separate of photoexcited hole-electron pairs that led to the photodegradation rate constant of BOC-150-8h being larger than any others. At the same time, BOC-150-48h had a lower reaction rate than samples heated for 4h, 8h, 12 h or 24 h, which suggests longer annealing times may not always result in better performance.

Photocatalytic Activity
The photocatalytic performance of annealed BOC in nitrogen for various time at 150°C are presented in Figure 8; the digital images after reaction are shown in Figure S4. All the as-prepared BOC samples showed almost the same impressive performance for toluene removal. BOC-150-8h was superior to any other samples, with removal rate of up to 99%. BOC-150-12h and BOC-150-24 had a slightly worse performance, but they were better than BOC-150-4h and BOC-150-48. BOC-150-48 was analogous with the raw BOC nanosheets.
On the basis of formula (1), k values for various samples were calculated. The specific numbers for BOC-150-4h, BOC-150-8h, BOC-150-12h, BOC-150-24h, BOC-150-48h were 0.0105, 0.0211, 0.0161, 0.0168 and 0.0116min −1 , respectively. BOC-150-8h had the largest k value, with higher ratio of (002) and (004) crystal face exposed from Figure 3, which may have contributed to the separate of photoexcited hole-electron pairs that led to the photodegradation rate constant of BOC-150-8h being larger than any others. At the same time, BOC-150-48h had a lower reaction rate than samples heated for 4h, 8h, 12 h or 24 h, which suggests longer annealing times may not always result in better performance.

Photocatalytic Activity
The photocatalytic performance of annealed BOC in nitrogen for various time at 150 • C are presented in Figure 8; the digital images after reaction are shown in Figure S4. All the as-prepared BOC samples showed almost the same impressive performance for toluene removal. BOC-150-8 h was superior to any other samples, with removal rate of up to 99%. BOC-150-12 h and BOC-150-24 had a slightly worse performance, but they were better than BOC-150-4 h and BOC-150-48. BOC-150-48 was analogous with the raw BOC nanosheets.
On  Fourier transform infrared (FTIR) spectroscopic analysis was performed for BOC-150-12h and BOC-150-24h; the results are shown in Figure 9. All the peaks around 694, 846, 1068 and 1390 nm may be checked to be CO3 2− .The peak 553nm is the bond Bi-O, indicating that it was Bi2O2CO3 [45,46]. It can be believed that the decrease of toluene's concentration during the photocatalysis process was caused by degradation and no obvious reactant absorbed after reaction. The constituent and the content of the mixture in the chamber at end of photocatalysis process were measured by GC-MS; the results are shown in Table 2, S3, Figure 10 and FiguresS5-S9. It was found that CO2 is one of the main products of removal toluene by Bi2O2CO3 in photocatalytic degradation. Moreover, it was presumed that the other main product is H2O. However, little amount of benzene could be detected in the mixtures of BOC-150-4h and BOC-150-48h, which indicates that the benzene may have acted as one of the intermediate products during reaction. For BOC-150-48h, the rate and quantum field of the photocatalytic CO2 production were19.06 umol/g and 6.67umol, respectively, which were larger than the other samples. Furthermore, the quantum field was close to seven times of the initial quantity 1 umol, which corresponds with the mass balance of carbon.  Fourier transform infrared (FTIR) spectroscopic analysis was performed for BOC-150-12 h and BOC-150-24 h; the results are shown in Figure 9. All the peaks around 694, 846, 1068 and 1390 nm may be checked to be CO 3 2− . The peak 553 nm is the bond Bi-O, indicating that it was Bi 2 O 2 CO 3 [45,46]. It can be believed that the decrease of toluene's concentration during the photocatalysis process was caused by degradation and no obvious reactant absorbed after reaction. Fourier transform infrared (FTIR) spectroscopic analysis was performed for BOC-150-12h and BOC-150-24h; the results are shown in Figure 9. All the peaks around 694, 846, 1068 and 1390 nm may be checked to be CO3 2− .The peak 553nm is the bond Bi-O, indicating that it was Bi2O2CO3 [45,46]. It can be believed that the decrease of toluene's concentration during the photocatalysis process was caused by degradation and no obvious reactant absorbed after reaction. The constituent and the content of the mixture in the chamber at end of photocatalysis process were measured by GC-MS; the results are shown in Table 2, S3, Figure 10 and FiguresS5-S9. It was found that CO2 is one of the main products of removal toluene by Bi2O2CO3 in photocatalytic degradation. Moreover, it was presumed that the other main product is H2O. However, little amount of benzene could be detected in the mixtures of BOC-150-4h and BOC-150-48h, which indicates that the benzene may have acted as one of the intermediate products during reaction. For BOC-150-48h, the rate and quantum field of the photocatalytic CO2 production were19.06 umol/g and 6.67umol, respectively, which were larger than the other samples. Furthermore, the quantum field was close to seven times of the initial quantity 1 umol, which corresponds with the mass balance of carbon. Table 2. Constituents and content of the final products of removal toluene (%V/V). The constituent and the content of the mixture in the chamber at end of photocatalysis process were measured by GC-MS; the results are shown in Table 2, Table S3, Figure 10 and Figures S5-S9. It was found that CO 2 is one of the main products of removal toluene by Bi 2 O 2 CO 3 in photocatalytic degradation. Moreover, it was presumed that the other main product is H 2 O. However, little amount of benzene could be detected in the mixtures of BOC-150-4 h and BOC-150-48 h, which indicates that the benzene may have acted as one of the intermediate products during reaction. For BOC-150-48 h, the rate and quantum field of the photocatalytic CO 2 production were 19.06 umol/g and 6.67 umol, respectively, which were larger than the other samples. Furthermore, the quantum field was close to seven times of the initial quantity 1 umol, which corresponds with the mass balance of carbon.  The (002) plane belongs to {001} facets in Bi2O2CO3 nanosheets with a high oxygen atoms density, which can lead to great catalytic activity [17]. The exposed crystal plane of (002) in Bi2O2CO3 can not only result in higher surface area, but also help with the separation of photo excited carriers, as it leads to enhanced photocatalytic performance for toluene removal in air [25,47].
Above all, the mechanism of photocatalytic degradation of toluene is summarized in Figure 11. First of all, electrons in Bi2O2CO3 nanosheets can be excited from valence bond to conduction bond under the irradiation of light, forming photogenerated electron-hole pairs. Second, the electrons migrate to the surface of Bi2O2CO3, so that the surface-adsorbed O2 is reduced to highly active species •O2 − . Thus, highly active •O2 − oxidizes toluene to CO2, H2O and other intermediate products such as benzene, benzoic acid, benzaldehyde and benzyl alcohol [48]. Then h + oxidizes the surface-adsorbed H2O to highly active species •OH, the active species (•O2 − and •OH) oxidizes the adsorbed intermediates to CO2 and H2O, forming the final products CO2 and H2O consequently.The supposed reaction steps have listed as follows.
The (002) plane belongs to {001} facets in Bi 2 O 2 CO 3 nanosheets with a high oxygen atoms density, which can lead to great catalytic activity [17]. The exposed crystal plane of (002) in Bi 2 O 2 CO 3 can not only result in higher surface area, but also help with the separation of photo excited carriers, as it leads to enhanced photocatalytic performance for toluene removal in air [25,47].
Above all, the mechanism of photocatalytic degradation of toluene is summarized in Figure 11. First of all, electrons in Bi 2 O 2 CO 3 nanosheets can be excited from valence bond to conduction bond under the irradiation of light, forming photogenerated electron-hole pairs. Second, the electrons migrate to the surface of Bi 2 O 2 CO 3 , so that the surface-adsorbed O 2 is reduced to highly active species •O 2 − .

Preparation of Bi2O2CO3Nanosheets
All the chemical reagents used in this work were analytical-grade reagents and used without any further purification. In a typical synthesis procedure,3g urea (≥99.0%, Beijing Modern Orient Fine Chemistry Co. Ltd., Beijing, China)and 4.65g Bi2O3(99.99%, Aladdin Industrial Corporation, Shanghai, China)were added to 60mL deionized water with magnetic stirring. Then the reaction mixtures were sealed in a Teflon-lined stainless-steel autoclave and heated at the temperature 180°C under autogenous pressure for 12h. After natural cooling to room temperature, the product was filtered, washed with deionized water and ethyl alcohol and dried at 70°C in an oven. The specimen was labeled as BOC [42].

Preparation of Bi 2 O 2 CO 3 Nanosheets
All the chemical reagents used in this work were analytical-grade reagents and used without any further purification. In a typical synthesis procedure, 3 g urea (≥99.0%, Beijing Modern Orient Fine Chemistry Co. Ltd., Beijing, China) and 4.65 g Bi 2 O 3 (99.99%, Aladdin Industrial Corporation, Shanghai, China) were added to 60 mL deionized water with magnetic stirring. Then the reaction mixtures were sealed in a Teflon-lined stainless-steel autoclave and heated at the temperature 180 • C under autogenous pressure for 12 h. After natural cooling to room temperature, the product was filtered, washed with deionized water and ethyl alcohol and dried at 70 • C in an oven. The specimen was labeled as BOC [42].

Preparation of Bi 2 O 2 CO 3 Nanosheets with (002) Facet Exposed
Bi 2 O 2 CO 3 nanosheets with (002) facet exposed were prepared by annealed BOC in nitrogen. For example, 2 g BOC was sealed in tube furnace (OTF-1200, Hefei Kejing Materials Technology Co. Ltd., Hefei, China) and treated at 150 • C for 4 h in nitrogen. The nitrogen flow rate was 1.
The photocatalytic properties of the specimen were evaluated by removal toluene in air with the assistant of photocatalyst in a home-made photocatalysis equipment ( Figure S1). Before the photocatalytic test, the inter space of the unit was first substituted with nitrogen to expel the oxygen and moisture. Toluene was supplied by toluene standard gas with concentration of 50.1 ppm balanced in artificial air. When test started, 0.35 g photocatalyst was put at the bottom of the chamber. Toluene standard gas was pumped into the chamber (about 450 mL) through PUMP1. The filled unit was kept in dark for 30 min to research the adsorption equilibrium between the photocatalyst and toluene.
An incident light source (a 300 W xenon lamp) was placed above the chamber which has a quartzose cover as an upper surface. At regular time intervals, the mixture gas in chamber was analyzed by gas chromatograph (7890A, Agilent, Santa Clara, CA, USA) connecting to the chamber. The constituent and the content of the mixture in the end of photocatalysis process were measured by gas chromatograph-mass spectrometry (GCMS-QP2010, Shimadzu, Osaka, Japan). The temperature of the unit was controlled using circulating cold water passing through the interlayer continuously to prevent thermal effect during the degradation process.

Conclusions
In summary, synthesized Bi 2 O 2 CO 3 nanosheets through hydrothermal method can be proven as a relative pure phase and no other composition is found when they were annealed in nitrogen at 150 • C. When annealed for various times at 150 • C, crystal facet of (002) of Bi 2 O 2 CO 3 has been exposed. The diffracted intensity ratio of (002)/(013) of BOC-150-8 h has been increased by 92%, which can help separate the photogenerated carries and improve the efficiency of removal toluene with removal rate up 99%. Moreover, CO 2 was verified to be the primary product in the photocatalytic degradation of toluene. As a result, the photocatalytic performance of Bi 2 O 2 CO 3 for removal toluene can be optimized through facile heat treatment. This work can probably be extended to applications of Bi 2 O 2 CO 3 in air purification.
Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/10/4/389/s1, Figure S1: The photocatalysis equipment for photocatalystic removal of toluene in air; Figure S2: The colors of as-prepared samples annealed in nitrogen for four hours at different temperature after the irradiation of light; Figure S3: SEM images of BOC-150-12 h (a), BOC-150-24 h (b) and BOC-150-48 h (c); Figure S4: The colors of as-prepared samples annealed in nitrogen for various time at 150 • C after the irradiation of light; Figure S5: the total ion chromatograms (a) and the mass spectrum of CO 2 (b) and toluene (c) for BOC-150-4 h; Figure S6: the total lon chromatograms (a) and the mass spectrum of CO 2 (b) and toluene (c) for BOC-150-8 h; Figure S7: the total lon chromatograms (a) and the mass spectrum of CO 2 (b) for BOC-150-12 h; Figure S8: the total lon chromatograms (a) and the mass spectrum of CO 2 (b) and toluene (c) for BOC-150-24 h; Figure S9: the total lon chromatograms (a) and the mass spectrum of CO 2 (b) and toluene (c) for BOC-150-48 h; Table S1: N 2 physisorption results for different catalysts; Table S2: Results of BET surface areas and band gaps of Bi 2 O 2 CO 3 sample; Table S3: Rates and quantum fields of the photocatalytic CO 2 production using the as-prepared Bi 2 O 2 CO 3 samples.

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