Carbon Quantum Dots Modified (002) Oriented Bi2O2CO3 Composites with Enhanced Photocatalytic Removal of Toluene in Air

In work, (002) oriented flower-like Bi2O2CO3(BOC) composites are synthesized by a facile chemical route and carbon quantum dots (CQDs) are modified on their surfaces through a hydrothermal method. The synthesized samples (CQD/BOC) are characterized by X-ray diffraction (XRD), SEM, X-ray photoelectron spectroscopy (XPS), UV-Vis diffuser reflectances (DRS), BET and TEM/HRTEM. The morphologies of CQD/BOC composites are the flower-like shapes, the irregular flaky structures and the fine spherical particles of CQDs attached. Photocatalytic performances were investigated in terms of removing gaseous toluene at a concentration of 94.3ppm in air, with the assistance of CQD/BOC under artificial irradiation. Our results show that CQDs modified (002) oriented Bi2O2CO3 exhibits good photocatalytic activity for toluene decomposition, which can be attributed to the enhanced efficient charge separation. A certain ratio composite photocatalyst (BOC-CQD-15) shows a toluene removal rate of 96.62% in three hours, as well as great stability. CO2 was verified to be the primary product. The oriented flower-like Bi2O2CO3 with carbon quantum dots on the surface shows great potential in the field of solar driven air purification.


Introduction
Volatile organic compounds (VOCs) are one of the major gas pollutants in indoor air, including various alcohols, aromatics (benzene, ethylbenzene, toluene, xylene, etc.), aldehydes (acetaldehyde, formaldehyde, etc.) and halocarbons, which put humans health at great risks [1][2][3]. Many technologies are used to remove gas pollutants, such as adsorption by carbon-based filter media, ionization, ultraviolet degradation, plasma technology, catalytic degradation and the photocatalysis method [4][5][6]. Among all these technologies, photocatalysis may be a promising technique for indoor air purification, because harmless CO 2 and H 2 O are the main products of pollutant degradation [7].
Bi-based photocatalysts have drawn increasing attention recently, because of their good chemical stability under visible light irradiation and unique electronic band structure. The hybridization of O 2p and Bi 6s orbitals in Bi-based photocatalysts composites results in a well-dispersed valence band (VB). In the meantime, the lone-pair distortion of Bi 6s orbitals can cause the pronounced overlap of O 2p and Bi 6s orbitals, which would increase the mobility of charge carriers and decrease the band Nanomaterials 2020, 10, 1795 3 of 12 water and ethyl alcohol and dried at 70 °C in an oven to obtain Bi2O2CO3 composites labeled as BOC.

Preparation of CQDs
The CQDs were obtained through a hydrothermal route [34]. Glucose (1 g, AR, Sinopharm Chemical Reagent Co., Ltd., Beijing, China) was dissolved into deionized water (80 mL) to obtain a homogeneous solution. Then, the solution was treated under a hydrothermal condition (180 °C, 4 h). After that, the solution was given a filter treatment (0.1 μm, nylon), and then a reddish-brown CQDs suspension was obtained.

Preparation of CQDs/BOC Composites
A total of 0.25 g BOC was added to 70 mL deionized water with ultrasonic dispersant for 15 min. Subsequently, a certain volume of CQDs suspension was dropped into above mixture. Then, the mixtures were sealed in a Teflon-lined stainless-steel autoclave and heated at the temperature 180 °C under autogenous pressure for 4h. After natural cooling to room temperature, the product was filter centrifuged, washed with deionized water and ethyl alcohol and dried at 70 °C in an oven to obtain CQD/BOC composites. To study the impact of the CQDs content on the photocatalytic performance of the composites, a series of the CQD/BOC composites were obtained by adjusting the volumes of CQDs suspension (5, 10, 15 and 20 mL). The specimens were correspondingly labeled as BOC-CQD-5, BOC-CQD-10, BOC-CQD-15, BOC-CQD-20.

Fabrication of CQD/BOC Membranes
A total of 0.025 g BOC or CQD/BOC was added to 150 mL deionized water with ultrasonic dispersant for 15 min. The mixtures were filtered at 0.1 μm nylon membrane by vacuum pump. The BOC or CQD/BOC photocatalysic thin-film on the nylon membrane was obtained. The CQD/BOC membranes were fabricated, as illustrated in Figure 1.

Characterization
The powder X-ray diffraction (XRD) patterns were obtained from a diffractometer (D8-Advance, Bruker, Karlsruhe, Germany) using monochromatized Cu Kα (λ = 1.54056 nm) radiation with scanning speed of 0.15°/s. The morphology of the samples was carried out on a scanning electron microscope (JSM-7001F, JEOL, Tokyo, Japan) operating at a 5 kV and a Chemical route for the preparation of carbon quantum dots (CQD)/Bi 2 O 2 CO 3 (BOC) membrane.

Characterization
The powder X-ray diffraction (XRD) patterns were obtained from a diffractometer (D8-Advance, Bruker, Karlsruhe, Germany) using monochromatized Cu Kα (λ = 1.54056 nm) radiation with scanning speed of 0.15 • /s. The morphology of the samples was carried out on a scanning electron microscope (JSM-7001F, JEOL, Tokyo, Japan) operating at a 5 kV and a high-resolution transmission emission electron microscope (JEM-2100F, JEOL, Tokyo, Japan). The XPS spectra measurements were conducted on an X-ray photoelectron spectroscopy (ESCALAB 250Xi, Thermo Fisher, California, USA). The specific surface area was measured on an automated gas sorption analyzer (AutosorbiQ2, Quantachrome, Florida, USA). UV-Vis diffuser reflectances (DRS) were carried out on a UV-Vis spectrometer (Lambda 950, PerkinElmer, Massachusetts, USA).

Photocatalytictest
The photocatalytic properties of the specimen were evaluated by toluene removal in air using a gas phase photocatalysis system (FPCS-1, Beijing Ferren Science & Technology Co., Ltd., Beijing, China). Before the photocatalytic test, the inter space of the reactor was first substituted with nitrogen to expel the oxygen and moisture. Toluene standard gas with concentration of 94.3 ppm in air was used as reactant. The BOC or CQD/BOC membrane was placed at the bottom of the reactor. Toluene standard gas was pumped into the chamber (about 450 mL). Then, the reactor was kept in dark for 30 min to reach the adsorption equilibrium. An incident light source (a 300W xenon lamp) was placed above the reactor which has a quartzose cover as an upper surface. At regular time intervals, the mixture gas in reactor was analyzed by gas chromatograph equipped with two flame-ionization detectors (FID). Toluene analysis was carried out with FID loaded with an Rt-Q-Bond Plot column (30 m × 0.25 mm, film thickness 10 µm), while CO 2 analysis was carried out with the other FID loaded with a packed column (TDX-01, 3 m × 3 mm), followed by a methanizer CO 2 concentration. The gas samples were fed to GC online through an automatic gas sampling valve. The temperature of the reactor was controlled using circulating cooling water to avoid thermal effect during the degradation process.

Morphology Analysis
The morphologies of the as-prepared BOC and CQD/BOC samples were observed by SEM and TEM. It is clear that the morphologies of the pure BOC were flower-like shape with a diameter of ca. 6um (Figure 2a Figure  S1c). Thus, more reactive sites could be provided, due to the higher surface-to-volume ratio of BOC and CQD/BOC. Nanomaterials 2020, 10, 1795 4 of 12 high-resolution transmission emission electron microscope (JEM-2100F, JEOL, Tokyo, Japan). The XPS spectra measurements were conducted on an X-ray photoelectron spectroscopy (ESCALAB 250Xi, Thermo Fisher, California, USA). The specific surface area was measured on an automated gas sorption analyzer (AutosorbiQ2, Quantachrome, Florida, USA).UV-Vis diffuser reflectances (DRS) were carried out on a UV-Vis spectrometer (Lambda 950, PerkinElmer, Massachusetts, USA).

Photocatalytictest
The photocatalytic properties of the specimen were evaluated by toluene removal in air using a gas phase photocatalysis system (FPCS-1, Beijing Ferren Science & Technology Co., Ltd., Beijing, China). Before the photocatalytic test, the inter space of the reactor was first substituted with nitrogen to expel the oxygen and moisture. Toluene standard gas with concentration of 94.3 ppm in air was used as reactant. The BOC or CQD/BOC membrane was placed at the bottom of the reactor. Toluene standard gas was pumped into the chamber (about 450 mL). Then, the reactor was kept in dark for 30 min to reach the adsorption equilibrium. An incident light source (a 300W xenon lamp) was placed above the reactor which has a quartzose cover as an upper surface. At regular time intervals, the mixture gas in reactor was analyzed by gas chromatograph equipped with two flame-ionization detectors (FID). Toluene analysis was carried out with FID loaded with an Rt-Q-Bond Plot column (30 m × 0.25 mm, film thickness 10 μm), while CO2 analysis was carried out with the other FID loaded with a packed column (TDX-01, 3m × 3 mm), followed by a methanizer CO2 concentration. The gas samples were fed to GC online through an automatic gas sampling valve. The temperature of the reactor was controlled using circulating cooling water to avoid thermal effect during the degradation process.

Morphology Analysis
The morphologies of the as-prepared BOC and CQD/BOC samples were observed by SEM and TEM. It is clear that the morphologies of the pure BOC were flower-like shape with a diameter of ca. 6um (Figure 2a Figure S1c). Thus, more reactive sites could be provided, due to the higher surface-to-volume ratio of BOC and CQD/BOC.   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 by chemical route were much larger than the BOC by a hydrothermal method [35]. The specific surface area and pore volume of CQD/BOC were about 50% larger than BOC. They are similar to flower-like Bi 2 O 2 CO 3 [34].Therefore, the larger BET surface area of CQD/BOC may result in better photocatalytic performance by providing more actives sites than BOC sample. The morphology of BOC-CQD-15 was also observed by TEM and HRTEM ( Figure 3 and Figure S2). As shown in Figure S2, the main morphology of sample was flower-like shape. Simultaneously, there are some fine spherical particles and irregular flaky structures, which is in accordance with SEM results. To further elucidate the element distribution of BOC-CQD-15, energy-dispersive X-ray elemental mappings were employed (Figure 3b-d), where Bi, O and C elements were uniformly distributed in the obtained sample. These mapping images correspond to the TEM image shown in Figure 3a.
To determine the C element content, the EDS spectrums of the spectrum 4 and 5 in Figure 3g were measured. The C element content of spectrum 4 was 78.68%, which was much larger than 55.38% of spectrum 5. It revealed that the deep color dots were carbon quantum dots with the diameter of ca. 5-30 nm. Additionally, in Figure 3e 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 by chemical route were much larger than the BOC by a hydrothermal method [35]. The specific surface area and pore volume of CQD/BOC were about 50% larger than BOC. They are similar to flower-like Bi2O2CO3 [34].Therefore, the larger BET surface area of CQD/BOC may result in better photocatalytic performance by providing more actives sites than BOC sample. The morphology of BOC-CQD-15 was also observed by TEM and HRTEM ( Figure 3 and Figure  S2). As shown in Figure S2, the main morphology of sample was flower-like shape. Simultaneously, there are some fine spherical particles and irregular flaky structures, which is in accordance with SEM results. To further elucidate the element distribution of BOC-CQD-15, energy-dispersive X-ray elemental mappings were employed (Figure 3b-d), where Bi, O and C elements were uniformly distributed in the obtained sample. These mapping images correspond to the TEM image shown in Figure 3a. To determine the C element content, the EDS spectrums of the spectrum 4 and 5 in Figure  3g were measured. The C element content of spectrum 4 was 78.68%, which was much larger than 55.38% of spectrum 5. It revealed that the deep color dots were carbon quantum dots with the diameter of ca. 5-30 nm. Additionally, in Figure 3e

Structure and Composition Analysis
The phase structure of the obtained BOC and CQD/BOC samples were detected by X-ray powder diffraction (XRD), and the results are shown in Figure 4. It reveals that all diffraction peaks of different samples could be well indexed to the pure phase of Bi2O2CO3 (JCPDS 41-1488), without impurity peaks. The diffracted intensity ratio of (002)/(013) in the BOC, BOC-CQD-5, BOC-CQD-10, BOC-CQD-15 and BOC-CQD-20 were 59.50%, 65.17%, 52.74%, 76.95% and 83.55%, respectively, which were much larger than 25% of the primitive BOC standard card (JCPDS 41-1488). (002) facet was exposed dominantly, which might contribute to the separation of photo-excited hole-electron pairs [36]. X-ray photoelectron spectroscopy (XPS) was conducted to investigate the chemical composition and surface electron state of the CQD/BOC samples ( Figure 5 and Figure S3). Figure 5a demonstrated the typical survey spectrum of the as-obtained samples, showing that all of the

Structure and Composition Analysis
The phase structure of the obtained BOC and CQD/BOC samples were detected by X-ray powder diffraction (XRD), and the results are shown in Figure 4. It reveals that all diffraction peaks of different samples could be well indexed to the pure phase of Bi 2 O 2 CO 3 (JCPDS 41-1488), without impurity peaks. The diffracted intensity ratio of (002)/(013) in the BOC, BOC-CQD-5, BOC-CQD-10, BOC-CQD-15 and BOC-CQD-20 were 59.50%, 65.17%, 52.74%, 76.95% and 83.55%, respectively, which were much larger than 25% of the primitive BOC standard card (JCPDS 41-1488). (002) facet was exposed dominantly, which might contribute to the separation of photo-excited hole-electron pairs [36].

Structure and Composition Analysis
The phase structure of the obtained BOC and CQD/BOC samples were detected by X-ray powder diffraction (XRD), and the results are shown in Figure 4. It reveals that all diffraction peaks of different samples could be well indexed to the pure phase of Bi2O2CO3 (JCPDS 41-1488), without impurity peaks. The diffracted intensity ratio of (002)/(013) in the BOC, BOC-CQD-5, BOC-CQD-10, BOC-CQD-15 and BOC-CQD-20 were 59.50%, 65.17%, 52.74%, 76.95% and 83.55%, respectively, which were much larger than 25% of the primitive BOC standard card (JCPDS 41-1488). (002) facet was exposed dominantly, which might contribute to the separation of photo-excited hole-electron pairs [36].  X-ray photoelectron spectroscopy (XPS) was conducted to investigate the chemical composition and surface electron state of the CQD/BOC samples ( Figure 5 and Figure S3). Figure 5a Figure 5b, the C1s peak at a binding energy of 284.8 eV can be attributed to the C-C bond with sp 2 orbital; the peak observed at 289.0 eV should be ascribed to the C-O bond in Bi 2 O 2 CO 3 [37]. The spectra of O1s can be fitted into three Gaussian-Lorenzian peaks (see Figure 5c and Figure S1). The peak located at 529.8 eV is ascribed to the lattice oxygen in Bi−O binding energy, and the peaks at 530.7 eV and 531.6 eV can be assigned to carbonate and the surface hydroxyl groups on the surface of Bi 2 O 2 CO 3 [14,38]. In Figure 5d-f, the two apparent characteristic peaks for Bi-4f located at 159.1eV and 164.4 eV are attributed to Bi-4f 7/2 and Bi4-f 5/2 in Bi 2 O 2 CO 3 , indicating the existence of Bi 3+ in the samples. The Bi-4f 7/2 and Bi-4f 5/2 of CQD/BOC have a negative shift to low binding energy compared with BOC, indicating the higher electron density around Bi elements in CQD/BOC samples, and proving CQDs modified on the surface of Bi 2 O 2 CO 3 [39,40].  Figure 5b, the C1s peak at a binding energy of 284.8 eV can be attributed to the C-C bond with sp 2 orbital; the peak observed at 289.0 eV should be ascribed to the C-O bond in Bi2O2CO3 [37]. The spectra of O1s can be fitted into three Gaussian-Lorenzian peaks (see Figure 5c and Figure S1). The peak located at 529.8 eV is ascribed to the lattice oxygen in Bi−O binding energy, and the peaks at 530.7 eV and 531.6 eV can be assigned to carbonate and the surface hydroxyl groups on the surface of Bi2O2CO3 [14,38]. In Figure 5d-f, the two apparent characteristic peaks for Bi-4f located at 159.1eV and 164.4 eV are attributed to Bi-4f7/2 and Bi4-f5/2 in Bi2O2CO3, indicating the existence of Bi 3+ in the samples. The Bi-4f7/2 and Bi-4f5/2 of CQD/BOC have a negative shift to low binding energy compared with BOC, indicating the higher electron density around Bi elements in CQD/BOC samples, and proving CQDs modified on the surface of Bi2O2CO3 [39,40].

Photocatalytic Properties
The photocatalytic properties of the as-prepared samples were investigated through removing gaseous toluene (94.3 ppm) in air under the irradiation of an incident light source. As shown in Figure 6a, the pure BOC achieved a good photocatalytic performance with a removal rate of 70%, which can be attributed to (002) crystal face exposed. However, CQD/BOCs were superior to the pure Nanomaterials 2020, 10, 1795 8 of 12 BOC, with a removal rate of up to 95%, which can be attributed to CQDs and (002) crystal face. The photocatalytic property of BOC-CQD-15 reached 96.62% after three hours irradiation.
In order to get a deeper understanding of the degradation reaction of toluene in air, the degradation products were detected using gas chromatograph (GC) with flame-ionization detectors (FID). The results are shown in Figure 6b and Figure S4. The CO 2 productivity of BOC-CQD-15 was 38.5 µmol, which is 2.4 times of the pure BOC and was the highest in all the CQD/BOCs.
On the basis of the toluene removal rate and the CO 2 productivity, the photocatalytic property of BOC-CQD-15 was the best in all the CQD/BOCs.
In order to observe the photo response range of CQD/BOC, the phototcatalytic properties of BOC-CQD-15 were investigated through its ability of toluene decomposition under the irradiation of infrared light, visible light and ultraviolet light (showed in Figure 6c). BOC-CQD-15 has not response to IR light and Vis light, but it has response to UV light. Nanomaterials 2020, 10, 1795 8 of 12

Photocatalytic Properties
The photocatalytic properties of the as-prepared samples were investigated through removing gaseous toluene (94.3 ppm) in air under the irradiation of an incident light source. As shown in Figure 6a, the pure BOC achieved a good photocatalytic performance with a removal rate of 70%, which can be attributed to (002) crystal face exposed. However, CQD/BOCs were superior to the pure BOC, with a removal rate of up to 95%, which can be attributed to CQDs and (002) crystal face. The photocatalytic property of BOC-CQD-15 reached 96.62% after three hours irradiation.
In order to get a deeper understanding of the degradation reaction of toluene in air, the degradation products were detected using gas chromatograph (GC) with flame-ionization detectors (FID). The results are shown in Figure 6b and Figure S4. The CO2 productivity of BOC-CQD-15 was 38.5 μmol, which is 2.4 times of the pure BOC and was the highest in all the CQD/BOCs.
On the basis of the toluene removal rate and the CO2 productivity, the photocatalytic property of BOC-CQD-15 was the best in all the CQD/BOCs.
In order to observe the photo response range of CQD/BOC, the phototcatalytic properties of BOC-CQD-15 were investigated through its ability of toluene decomposition under the irradiation of infrared light, visible light and ultraviolet light (showed in Figure 6c). BOC-CQD-15 has not response to IR light and Vis light, but it has response to UV light. The stability of photocatalytic degradation of toluene in air was observed by repeating the experiment for five runs under full spectrum after ultraviolet irradiation, and the result is shown in Figure 6d and Figure S5. It was clear that the photocatalytic property of BOC-CQD-15 under full spectrum was superior to its property under ultraviolet due to the outstanding up-converted photoluminescence peculiarity of carbon quantum dots modified on the surface of Bi2O2CO3, which The stability of photocatalytic degradation of toluene in air was observed by repeating the experiment for five runs under full spectrum after ultraviolet irradiation, and the result is shown in Figure 6d and Figure S5. It was clear that the photocatalytic property of BOC-CQD-15 under full spectrum was superior to its property under ultraviolet due to the outstanding up-converted photoluminescence peculiarity of carbon quantum dots modified on the surface of Bi 2 O 2 CO 3 , which further extend the photoresponse range of BOC to the near infrared light. The BOC-CQD-15 could remain a constant photocatalytic performance as high as 90% in terms of removing toluene under incident light irradiation after five recycling runs. This phenomenon revealed a good recyclability of BOC-CQD-15 for toluene removal.

Photocatalytic Degradation Mechanism
The UV-Vis diffuse reflectance spectra of the samples were examined, and the results presented in Figure 7. It can be seen that the bandgap of all the samples can be decided about (3.4~3.5) eV. The results reveal that the CQD scarcely influence the light absorption of BOC or change the band gap of samples obviously.
further extend the photoresponse range of BOC to the near infrared light. The BOC-CQD-15 could remain a constant photocatalytic performance as high as 90% in terms of removing toluene under incident light irradiation after five recycling runs. This phenomenon revealed a good recyclability of BOC-CQD-15 for toluene removal.

Photocatalytic Degradation Mechanism
The UV-Vis diffuse reflectance spectra of the samples were examined, and the results presented in Figure 7. It can be seen that the bandgap of all the samples can be decided about (3.4~3.5) eV. The results reveal that the CQD scarcely influence the light absorption of BOC or change the band gap of samples obviously. According to the characterization of chromatogram, CO2 is the primary product, and there is a little of CO during toluene degradation reaction. The CO productivity of BOC-CQD-15 was 6.33 μmol, which was 2.5 times of the pure BOC (showed in Figure S4b). No other byproducts or intermediate were detected. It indicated that the main products of toluene degradation were CO2 and H2O and there was a modicum of CO.
The photocatalytic degradation mechanism of VOCs in air is slightly different from that in aqueous solution. A possible photocatalytic mechanism of the CQD/BOC composites toward the removal of toluene under simulated sunlight irradiation is schematically depicted in Figure 8. The BOC can effectively respond to the light with the wavelength shorter than about 400 nm. When the CQD/BOC photocatalyst reacts to the photons, the electrons are excited from the valence band to the conduction band of BOC, thus producing electron-hole pairs. Simultaneously, it is generally accepted that carbon quantum dots are an outstanding up-converted photoluminescence material. The up-converted emissions are usually located at shorter wavelengths in the range of 300-650nm [38]. As a result, a part of the up-converted emissions of CQDs can in turn excite BOC to generate additional photo excited charges, further extending the photoresponse range of BOC to the NIR light. Meanwhile, CQDs can also be excited by absorbing visible light, the π electrons or σ electrons are excited to the lowest unoccupied molecular orbital (LUMO) [41,42]. The excited CQDs can act as excellent electron donors and acceptors. Consequently, the CB electrons in BOC would transfer to CQDs (π or σ orbital), which help with the separation and the migration of photo excited carriers.
The photogenerated electrons which migrate to the surface of BOC and CQDs reduce the surface-adsorbed O2 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 [43]. Then h + on the surface of BOC oxidizes the surface-adsorbed H2O to highly active species •OH, and the active species (•O2 − and •OH) oxidizes the adsorbed intermediates to CO2 and H2O, consequently forming the final products CO2 and H2O. According to the characterization of chromatogram, CO 2 is the primary product, and there is a little of CO during toluene degradation reaction. The CO productivity of BOC-CQD-15 was 6.33 µmol, which was 2.5 times of the pure BOC (showed in Figure S4b). No other byproducts or intermediate were detected. It indicated that the main products of toluene degradation were CO 2 and H 2 O and there was a modicum of CO.
The photocatalytic degradation mechanism of VOCs in air is slightly different from that in aqueous solution. A possible photocatalytic mechanism of the CQD/BOC composites toward the removal of toluene under simulated sunlight irradiation is schematically depicted in Figure 8. The BOC can effectively respond to the light with the wavelength shorter than about 400 nm. When the CQD/BOC photocatalyst reacts to the photons, the electrons are excited from the valence band to the conduction band of BOC, thus producing electron-hole pairs. Simultaneously, it is generally accepted that carbon quantum dots are an outstanding up-converted photoluminescence material. The up-converted emissions are usually located at shorter wavelengths in the range of 300-650nm [38]. As a result, a part of the up-converted emissions of CQDs can in turn excite BOC to generate additional photo excited charges, further extending the photoresponse range of BOC to the NIR light. Meanwhile, CQDs can also be excited by absorbing visible light, the π electrons or σ electrons are excited to the lowest unoccupied molecular orbital (LUMO) [41,42]. The excited CQDs can act as excellent electron donors and acceptors. Consequently, the CB electrons in BOC would transfer to CQDs (π or σ orbital), which help with the separation and the migration of photo excited carriers. The

Conclusions
In summary, (002) oriented flower-like Bi2O2CO3 composites were synthesized by a facile chemical route and carbon quantum dots were modified on their surface through a hydrothermal method. The synthesized composites (CQD/BOC) have three morphologies, which were flower-like shapes, irregular flaky structures and fine spherical particles. Through HRTEM characterization, it was proved that CQDs were modified successfully on the surface of Bi2O2CO3. Photocatalytic mineralization of toluene in air over CQD/BOC was measured. The effect of BOC-CQD-15 was optimum, and as a result, the toluene removal rate was up to 96.62% in three hours under full light irradiation, the rate was still up to 90% after five recycling runs in terms of stability. CO2 was verified to be the main product after reaction. Better performance can be contributed to (0 0 2) facet orientation evolution and CQDs modified on the surface of Bi2O2CO3, which enhance the efficient separation of photogenerated electron-holes.

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

Conclusions
In summary, (002) oriented flower-like Bi 2 O 2 CO 3 composites were synthesized by a facile chemical route and carbon quantum dots were modified on their surface through a hydrothermal method. The synthesized composites (CQD/BOC) have three morphologies, which were flower-like shapes, irregular flaky structures and fine spherical particles. Through HRTEM characterization, it was proved that CQDs were modified successfully on the surface of Bi 2 O 2 CO 3 . Photocatalytic mineralization of toluene in air over CQD/BOC was measured. The effect of BOC-CQD-15 was optimum, and as a result, the toluene removal rate was up to 96.62% in three hours under full light irradiation, the rate was still up to 90% after five recycling runs in terms of stability. CO 2 was verified to be the main product after reaction. Better performance can be contributed to (0 0 2) facet orientation evolution and CQDs modified on the surface of Bi 2 O 2 CO 3 , which enhance the efficient separation of photogenerated electron-holes.