Revealing the Role of Defect in 3D Graphene‐Based Photocatalytic Composite for Efficient Elimination of Antibiotic and Heavy Metal Combined Pollution

Defect engineering can give birth to novel properties for adsorption and photocatalysis in the control of antibiotics and heavy metal combined pollution with photocatalytic composites. However, the role of defects and the process mechanism are complicated and indefinable. Herein, TiO2/CN/3DG was fabricated and defects were introduced into the tripartite structure with separate O2 plasma treatment for the single component. We find that defect engineering can improve the photocatalytic activity, attributing to the increase of the contribution from h+ and OH. In contrast to TiO2/CN/3DG with a photocatalytic tetracycline removal rate of 75.2%, the removal rate of TC with D‐TiO2/CN/3DG has increased to 88.5%. Moreover, the reactive sites of tetracycline can be increased by adsorbing on the defective composites. The defect construction on TiO2 shows the advantages in tetracycline degradation and Cu2+ adsorption, but also suffers significant inhibition for the tetracycline degradation in a tetracycline/Cu2+ combined system. In contrast, the defect construction on graphene can achieve the cooperative removal of tetracycline and Cu2+. These findings can provide new insights into water treatment strategies with defect engineering.


Introduction
Antibiotics have been widely applied in the treatment and prevention of animals and human beings from microorganism infection since their public appearance in the 1930s. [1]The discovery of antibiotics no doubt plays a crucial role in saving lives and keeping healthy, but the over-reliance on antibiotics also drives up consumption in recent decades, resulting in serious antibiotic pollution to the eco-environment. [2,3]t is known that even if the concentration of antibiotics is very low in the environment, it can also become the incentive for the generation of antibiotic-resistant genes (ARGs) and antibiotic-resistant bacteria (ARBs). [4]Wastewater treatment plants (WWTPs) are the terminations for domestic sewage and industrial effluents and serve the purification process for pollutants in water before entering into the environment.However, the microbial technique as the main procedure for the conventional process cannot be capable of efficiently removing the antibiotics from water for their high toxicity to microorganisms. [5,6][9] They can form combined pollution with antibiotics, thereby posing a serious threat to the microbial treatment process and increasing the ecological risk of the effluent. [10,11][17][18][19] Moreover, compared with a single TiO 2 or g-C 3 N 4 catalyst, the combination of TiO 2 and g-C 3 N 4 into TiO 2 /g-C 3 N 4 heterojunction is found to present an optimized band structure for higher photocatalytic activity, which can conduct more efficient purification process taking advantage of solar light. [20,21]24] In this case, the construction of 3D graphene-based TiO 2 /g-C 3 N 4 composite is proven to hold efficient removal of heavy metal and antibiotics in the combined pollution.In addition, the composite shows a porous structure, which is easy to recover in practice, [25,26] and also provides a high specific surface area for catalyst loading besides adsorption. [27,28]efect engineering can give birth to novel properties for adsorption and photocatalysis in the control of antibiotics and heavy metal combined pollution with photocatalytic composites.However, the role of defects and the process mechanism are complicated and indefinable.Herein, TiO 2 /CN/ 3DG was fabricated and defects were introduced into the tripartite structure with separate O 2 plasma treatment for the single component.We find that defect engineering can improve the photocatalytic activity, attributing to the increase of the contribution from h + and OH.In contrast to TiO 2 /CN/3DG with a photocatalytic tetracycline removal rate of 75.2%, the removal rate of TC with D-TiO 2 /CN/3DG has increased to 88.5%.Moreover, the reactive sites of tetracycline can be increased by adsorbing on the defective composites.The defect construction on TiO 2 shows the advantages in tetracycline degradation and Cu 2+ adsorption, but also suffers significant inhibition for the tetracycline degradation in a tetracycline/Cu 2+ combined system.In contrast, the defect construction on graphene can achieve the cooperative removal of tetracycline and Cu 2+ .These findings can provide new insights into water treatment strategies with defect engineering.
Nonetheless, it should also be noted that the amount of active sites of the immobilized catalysts can be reduced compared with that of the suspending catalysts. [29,30]On the other hand, the spatial orientation for the adsorption behavior in a 3D graphene-based photocatalytic composite is vague and imprecise, and taking control of the adsorption behavior of different pollutants in a composite structure is still extremely hard.These lead to difficulties in establishing a cooperation mechanism for antibiotics/heavy metal combined pollution.Defect engineering has been developed to tune the microstructure and bring novel properties for crystal materials in recent years. [31,32]It is found that the defect in photocatalysts can create new active sites and change the electronic structure. [33,34]Additionally, the defect sites in graphene show excellent performance for anchoring heavy metal ions, which can induce oriented adsorption. [35]Considering the function of defect sites, it can be believed that the introduction of defects in 3D graphene-based photocatalytic composite may establish an efficient mechanism for the elimination of antibiotics/heavy metal combined pollution.Up to now, the investigations of defects are mainly focused on single-crystal materials, the role of defects in a composite material is rarely discussed for the complexity of the structure.Herein, 3D graphene-based TiO 2 /g-C 3 N 4 composite is fabricated for the treatment of antibiotics and antibiotics/heavy metal combined pollution.Tetracycline and Cu 2+ are chosen as model pollutants for antibiotics and heavy metal since they are the most common pollutants and can easily form combined pollution in livestock wastewater.Defects are introduced into the different components by plasma etching.The influence of defects on the removal of antibiotics and antibiotics/heavy metal pollutants is evaluated by the analysis of the adsorption and photocatalytic process.The mechanism for the influence of defect on the removal performances is revealed with the combination of experimental and calculation results.

Characterization of Pristine TiO 2 /CN/3DG and Defective Samples
The hierarchical structure of TiO 2 /CN/3DG is first confirmed by SEM.It can be seen that the 3DG presents an alveolate structure formed by the cross-link of graphene oxide nanosheets in a hydrothermal environment and wrinkles can be found on the surface of 3DG flakes (Figure 1a).Differing from the graphene flakes, the obtained CN are smaller nanosheets with jagged edges, and the CN/TiO 2 composite presents a similar structure to the CN with TiO 2 nanoparticles or nanoclusters distributed on the surface (Figure S1, Supporting Information).The integration of CN and 3DG shows that CN nanosheets can be immobilized on the framework of 3DG and create a more luxuriant microstructure (Figure 1b).Since the CN and TiO 2 are pre-mixed, the morphology of TiO 2 /CN/3DG follows the characters of CN/3DG, meanwhile TiO 2 can be found on CN nanosheets (Figure 1c).The results of EDS mapping also confirmed the composition and architectural feature (Figure 1d-g and Table S1, Supporting Information).TEM is used to further confirm the structural properties.It is found that smaller nanosheets are distributed on the large graphene flake (Figure 1h).Amplifying the image, nanoparticles can be seen on the nanosheet with a polycrystalline form confirmed by the SAED pattern (Figure 1i).The characteristic lattice spacing of TiO 2 (0.35 nm) and CN (0.33 nm) is observed for the nanoparticle and nanosheet, respectively (Figure 1j).XRD pattern also confirmed the crystal form of CN and TiO 2 on the surface of TiO 2 /CN/3DG (Figure S2, Supporting Information).The defective TiO 2 /CN/3DG is fabricated based on the defect-engineered single component with plasma.It is observed that atomic scale defects can be formed on the surface of 3DG, CN, and TiO 2 (Figure 1k-m).Compared with TiO 2 /CN/3DG, the specific surface area of the defective TiO 2 /CN/3DG samples shows no significant changes, but the average pore size and total pore volume have been increased (Figure S3 and Table S2, Supporting Information).
The chemical composition and features of TiO 2 /CN/3DG and the defective samples are further confirmed by FTIR and XPS.FTIR spectra show the characteristic peak at 3160 cm −1 , the characteristic peaks in the region of 1200-1750 cm −1 , and the characteristic peak at 807 cm −1 for all the samples (Figure S4, Supporting Information).The peak at 3160 cm −1 is attributed to the -OH groups, indicating the hydrophily of the composites. [19]The peaks in the range of 1200-1750 cm −1 are the typical characteristics for CN nanosheets, which are ascribed to the aromatic C-N stretching. [36]The peak at 807 cm −1 is due to the Ti-O bond in the TiO 2 crystal. [37]XPS survey spectra confirmed the elemental existence of C, N, Ti, and O (Figure S5, Supporting Information), which is in accord with EDS.Compared with TiO 2 / CN/3DG, a significant increase in the C-O binding is observed in the C 1s and O 1s XPS because of the defect construction in 3DG or CN (Figure 2a,b). [38]On the other hand, the defect engineering in CN can result in the reduction of N-(C) 3 and C-NH 2 in the N 1s XPS, [39] which indicates the formation of N-deficient sites (Figure 2c).For the Ti 2p XPS of TiO 2 /CN/3DG, the Ti 2p 3/2 and Ti 2p 1/2 locate at 459.2 and 464.9 eV with a distance of 5.7 eV, which is identified as the typical Ti 4+ form in the TiO 2 crystal.As the defects were created in TiO 2 , a new peak at 461.3 eV emerged, indicating the appearance of Ti 3+ as a result of the oxygen vacancies (Figure 2d). [40]The O-vacancies can lead to new binding forms for C and N, and also induce the enhanced adsorption of H 2 O for the formation of a new peak with high binding energy in the O 1s XPS, which demonstrates the improvement in hydrophily. [41]The UV-vis DRS shows that TiO 2 /CN/3DG and the defective TiO 2 /CN/3DG composites present narrow bandgap in contrast to the conventional photocatalytic semiconductors, and the defect engineering in TiO 2 /CN/3DG can effectively promote the absorption of light energy (Figure S6, Supporting Information).Among the defective TiO 2 /CN/3DG composites, D-TiO 2 /CN/3DG holds the lowest response in PL spectrum and the highest photocurrent response, indicating high photocatalytic activity (Figures S7 and S8, Supporting Information). [42,43]

Defect Tuning Induced Influences on the Removal of Tetracycline
As a photocatalytic composite with well-developed porous structure, TiO 2 /CN/3DG can achieve an adsorption removal rate of 38.0% for TC in a dark environment and a photocatalytic removal rate of 75.2% following 30 min pre-adsorption.The introduction of defects can improve the adsorption performance of TiO 2 /CN/3DG, as well as the photocatalytic process.It can be seen that D-TiO 2 /CN/3DG exhibits the most significant improvement in adsorption with a removal rate of 49.5%.Compared with TiO 2 /CN/3DG, the photocatalytic performance with D-TiO 2 /CN/3DG has achieved 88.5% (Figure 3a,b).It is found that the position difference of defects on TiO 2 /CN/3DG can affect the photocatalytic process.Generally, the defect construction can promote Energy Environ.Mater.2024, 7, e12616 the production of OH and O À 2 .Among the different defective composites, the defect engineering in TiO 2 can greatly increase the production of OH, while the defect engineering in CN can induce the maximum improvement in the formation ofÁO À 2 (Figure S9, Supporting Information).The order for the photocatalytic performance is identified as D-TiO 2 /CN/3DG > TiO 2 /CN/D-3DG > TiO 2 /D-CN/3DG > TiO 2 / CN/3DG.The k app value of D-TiO 2 /CN/3DG is nearly twice than that of TiO 2 /CN/3DG (Figure S10 and Table S3, Supporting Information).In the view of TC degradation products, it is found that the cleavage of the functional groups in TC molecules and the breakdown of the aromatic nucleus are the main degradation reactions.Compared with TiO 2 /CN/3DG, the creation of defects on TiO 2 /CN/3DG can induce more degradation pathways and result in more by-products with smaller molecular weight (Figure 3c and Tables S4-S7, Supporting Information), which indicates that the defective composites have higher efficiency for the photocatalytic degradation of TC.

The Analysis of Effective Radicals in the Removal Process
In the conventional photocatalytic process, OH, h + , andÁO À 2 are considered as the main radicals for the degradation of pollutants. [44]In order to reveal the contribution of different radicals in the TC degradation process, IPA (1.0 mM), TEOA (1.0 mM), and BQ (1.0 mM) are applied as the scavengers for OH, h + , O À 2 , respectively. [45,46]It is found thatÁO À 2 is the major radical contributing to the TC degradation with TiO 2 /CN/3DG, while the contribution from h + and OH is not significant (Figure 4a).However, the defect construction on TiO 2 /CN/3DG is deemed to increase the contribution of h + and OH, thereby boosting the degradation of TC.Specifically, the defects on graphene and TiO 2 can significantly improve the contribution of both h + and OH, while the defects on CN prefer to promote the contribution of OH rather than h + (Figure 4b-d).The difference in the reactive radicals may be ascribed to band structure tuning induced by different defects.

The Influence of Defect Sites on the Removal of Tetracycline/Cu 2+ Process
Tetracycline/Cu 2+ pollution is considered as representative combined pollution of antibiotics and heavy metal, which is deemed to be an intractable problem in wastewater treatment. [47]Although the adsorption-photocatalysis process can achieve the removal of the combined pollutants, there still exists complicated competition between adsorption removal and photocatalytic degradation.Defect construction on the photocatalytic composite is able to tune the adsorption and photocatalytic process for the treatment of the antibiotics/ heavy metal pollution in expectation of figuring out the intractable problem.The adsorption and photocatalytic performances of TiO 2 / CN/3DG and the defective TiO 2 /CN/3DG composites were investigated for the removal of Cu 2+ and TC.It is found that D-TiO 2 /CN/ 3DG presents more efficient performance for the adsorption of Cu 2+ in the dark condition in contrast to other photocatalytic composites (Figure S11, Supporting Information).As for the tetracycline/Cu 2+ combined pollutants, the adsorption of Cu 2+ has been greatly inhibited for TiO 2 /CN/3DG and the defective TiO 2 /CN/3DG composites.Compared with the high adsorption rate of 93.0% for the single pollutant of Cu 2+ , the adsorption rate of Cu 2+ in the combined pollutants has been reduced to 14.3% with D-TiO 2 /CN/3DG.TiO 2 /CN/3DG and the defective TiO 2 /CN/ 3DG composites present close adsorption rate in the combined pollutants (Figure 5a).However, the adsorption performance for TC in the combined pollutants shows slight inhibition in contrast to the adsorption performance for TC in the single pollutant solution (Figure 5b).This indicates that TiO 2 /CN/ 3DG and the defective TiO 2 /CN/3DG composites prefer to adsorb TC rather than Cu 2+ in the case of tetracycline/Cu 2+ combined pollutants.As for the photocatalytic removal for the combined pollutants, it is found that TiO 2 / CN/3DG presents a higher removal rate for Cu 2+ , though it exhibits poor performance for Cu 2+ adsorption in the single pollutant system.Therefore, it is implied that the strong adsorption capacity can inhibit the photocatalytic reduction of Cu 2+ .Among the defective TiO 2 /CN/ 3DG composites, the defect construction on TiO 2 displays more excellent capacity in the removal of Cu 2+ than other engineered constructions whatever the pollutant system or the reacting condition (Figure 5c).On the other hand, it should be noted that D-TiO 2 /CN/ 3DG suffers from more inhibition for TC degradation in the combined pollutants system.Compared with the removal rate of TC in the single pollutant system, D-TiO 2 /CN/3DG suffers 7.0% reduction in the removal rate of TC in the combined pollutants system, on the contrary, TiO 2 /CN/D-3DG goes through 1.3% increase for TC removal in the combined pollutants system with only slight decrease in k app value (Figure 5d, Figure S12 and Table S3, Supporting Information).It indicates that the defect construction on graphene shows great potential in mediating the conflict of photocatalytic removal of TC and Cu 2+ .As for the products of TC degradation, it is found that compared with the degradation process for only TC, the coexistence of Cu 2+ can lead to more diversified pathways for TC degradation, but the mineralization of TC has been reduced (Figure 5e and Tables S8-S11, Supporting Information).This is ascribed to the creation of new reactive sites and the competition in the photocatalytic active sites.

Theoretical Calculation and Mechanism Analysis
The atomic charges and frontier electron densities (FEDs) of TC molecules are further calculated to analyze the reactive sites for TC degradation with TiO 2 /CN/3DG and the defective TiO 2 /CN/3DG composites.For free TC molecule, the negatively charged 30(N) has the highest FED 2 HOMO þ FED 2 LUMO value of 0.62, followed by the positively charged 17(C) with a value of 0.22.Although 30(N) has notable reactivity, 17 (C) is considered as a more important reactive site because it possesses negative charge, thereby preferring to be attacked by O À 2 , which has been proven to be the key radical in TC degradation according to the experimental results.Meanwhile, 30(N) shows the highest 2FED 2 HOMO value of 1.21, and this value is much more than that contributed by the second choice of atom 8 (C) with 0.09.Therefore, 30(N) is considered as the unique site preferred to be oxidized by h + , corresponding to the C-N-C fragment in the TC molecule (Figure 6a). [44,48]owever, the reactive sites can be affected if the TC molecule is adsorbed on the photocatalytic composite.For the TC molecule adsorbed on TiO 2 /CN/3DG, the FED 2 HOMO þ FED 2 LUMO value and 2FED 2 HOMO value of the main reactive sites such as 30(N) and 17(C) are found to decrease to some extent, indicating the reduction in reactivity in contrast with the free TC.On the other hand, it is interestingly found that the defect construction on TiO 2 /CN/3DG can improve the reactivity of the adsorbed TC.The TC adsorbed on the defective TiO 2 /CN/3DG presents higher FED 2 HOMO þ FED 2 LUMO values for the main positively charged carbon atoms, indicating the enhanced reactivity with the negatively chargedÁO À 2 .In addition, the defect-engineered composites can result in new reactive sites for the adsorbed TC and these reactive sites have negative charges and higher 2FED 2 HOMO values, which can be attacked by h + and OH (Figure 6b-e and Table S12, Supporting Information).It is also implied that the defective TiO 2 /CN/3DG composites can achieve more efficient degradation for TC and the contribution of h + and OH radicals in TC degradation will be promoted, which is in accord with the experimental results.
Cu 2+ adsorption is considered as a vital process for the catalyst materials in the tetracycline/Cu 2+ system that can affect TC degradation.The Cu 2+ adsorption configurations are optimized for TiO 2 /CN/3DG, the defective TiO 2 /CN/3DG composites and the single component of TiO 2 /CN/3DG with defects, and the corresponding adsorption energies are calculated.It is found that the adsorption energies are negative for all the composites and single components with defects, implying the spontaneous adsorption of Cu 2+ on the material surface.TiO 2 /CN/ 3DG shows a weak adsorption energy of −0.17 eV with Cu 2+ on the surface; however, the defective TiO 2 /CN/3DG composites show more negative adsorption energies with Cu 2+ , indicating an easier adsorption behavior for Cu 2+ .Among the defective materials, whatever the composite or single components, the defect construction on TiO 2 exhibits the most powerful capacity for Cu 2+ adsorption (Figure 7 and Figure S13, Supporting Information).
The PDOS and energy gap of the components in the photocatalytic composites are calculated and shown in Figure 8a-f.It can be seen that the defect engineering in the graphene substrate cannot form a gap in the material.As for the photocatalytic components, the defect construction can alter the positions of conduction band bottom (CBM) and the valence band maximum (VBM, equals to Fermi energy), thus narrowing the bandgap of the semiconductors.This also indicates that the defective composites are more easily activated by light irradiation and produce photo-generated hole-electron pairs.On the other hand, the charge density differences for TiO 2 /CN/3DG and the defective TiO 2 /CN/3DG composites are calculated and shown in Figure S14 and Figure 8g-j, Supporting Information.It is verified that the CN layer prefers to obtain electrons from the adjacent TiO 2 and graphene layer by forming the inner electric field for TiO 2 /CN/3DG, TiO 2 /CN/D-3DG, and TiO 2 /D-CN/3DG.As for D-TiO 2 /CN/3DG, the inner electric field between CN and the graphene layer is weak, and the CN layer mainly obtains electrons from the adjacent TiO 2 .The Cu 2+ adsorption on the material surface cannot impose significant influence on the charge density differences of TiO 2 /CN/3DG, TiO 2 /CN/D-3DG, and TiO 2 /D-CN/3DG, but can induce an obviously disturbed charge density in the defected TiO 2 layer, which can increase the recombination of photo-generated holes and electrons, thus restraining the photocatalytic activity. [49,50]he electron transport mechanism for TiO 2 /CN/3DG and the defective TiO 2 /CN/3DG composites are summarized and shown in Figure 9.For TiO 2 /CN/3DG, both TiO 2 and CN can be activated by visible light to produce h + and e − , but the activation of TiO 2 is weak for the wide bandgap. [51]The electrons can be excited to the CB of TiO 2 and CN, while h + can be formed on the VB for TiO 2 and CN.Since the existence of an inner electric field in the composite, the electrons on the CB of TiO 2 will transfer to the CB of CN, meanwhile the electrons on the graphene layer will transfer to the VB of CN, where the recombination of h + and e − happens.The electrons on the VB of CN can further react with O 2 to produce O À 2 and there is still few h + existing on the VB of TiO 2 to produce OH.Therefore, the photocatalytic degradation of TC is mainly attributed to O À 2 , which is in line with the experimental result.When the defects are constructed on the graphene, the band location of the graphene shifts to a higher position.The band position of the defective graphene is higher than the CB of CN.In this case, the inner electric field can drive the electrons on the defective graphene to the CB of CN, thereby decreasing the recombination of photogenerated h + and e − on the VB of CN.As a consequence, h + and OH can contribute more to the photocatalytic degradation, which agrees with the results of radical scavenging experiments.The defect engineering in CN can narrow the bandgap of CN, thus improving the utilization efficiency for light energy and producing more h + and e − . [52]Although the e − can transfer from the graphene layer to the VB of D-CN, the contribution of h + and OH for photocatalytic degradation is still promoted in contrast to TiO 2 /CN/3DG.As for the D-TiO 2 / CN/3DG composite, the bandgap of TiO 2 can be significantly reduced to 1.33 eV by defect engineering, which can greatly enhance the production of photo-generated carriers.The inner electric field between D-TiO 2 and CN can drive the electrons on the CB of D-TiO 2 to the CB of CN, thus promoting the separation of photo-generated carriers on D-TiO 2 .On the other hand, compared with other composites, the inner electric field between CN and the graphene layer is weak for D-TiO 2 /CN/3DG, in this case, the electrons on the CB of CN can be further transferred to the graphene layer, thereby inhibiting the recombination of h + and e − on CN.

Conclusions
In this study, the TiO 2 /CN/3DG photocatalytic composite was fabricated with a hydrothermal method, and the defects were introduced into the TiO 2 /CN/3DG composite by the plasma treatment of TiO 2 , CN, or graphene oxide before the hydrothermal fabrication.Plasma can introduce O-defect in TiO 2 , N-defect in CN, and C-defect in graphene, further fabricating different defective TiO 2 /CN/3DG composites.Results show that the defect construction on TiO 2 /CN/3DG can significantly improve the photocatalytic performance for TC degradation.Radical scavenging experiments and theoretical calculations indicate that O À 2 is the major radical to induce photocatalytic degradation with TiO 2 /CN/3DG.Defect engineering can alter the band location of the corresponding component in TiO 2 /CN/3DG.On the one hand, the bandgap is narrowed with defects.On the other hand, defects can change the inner electric field to promote the separation of photo-generated carriers, thereby increasing the contribution of h + and OH in the photocatalytic process.In addition, the defect engineering in TiO 2 /CN/3DG can increase the reactive sites for TC molecule adsorbed on the material surface.Among the different defective composites, D-TiO 2 /CN/3DG has the best performance for photocatalytic degradation, but it also shows the advantage in Cu 2+ adsorption and suffers great inhibition from Cu 2+ in the tetracycline/Cu 2+ combined system.In contrast, although TiO 2 /CN/D-3DG presents a lower photocatalytic activity for TC degradation, it exhibits excellent performance for the cooperative removal of TC and Cu 2+ .

Experimental Section
Preparation of TiO 2 /CN/3DG and defective TiO 2 /CN/3DG composites: The TiO 2 /CN/3DG was prepared with a hydrothermal method.Firstly, 276 mg asprepared g-C 3 N 4 (CN for short in this work and the preparation method can be seen in Supporting Information) and 120 mg TiO 2 were fully ground into the TiO 2 /CN composite.The obtained TiO 2 /CN composite was mixed with 240 mg graphene oxide (GO) in 120 mL deionized water with stirring.The mixture was then transferred to a Teflonlined hydrothermal reactor and heated at 140 °C for 12 h to obtain 3D structured hydrogel.The hydrogel was washed several times with deionized water and then freeze-dried for 24 h to obtain the TiO 2 /CN/3DG composite.The defective components (D-TiO 2 , D-CN, and D-GO) were obtained by plasma etching (TS-VPM02, Tonson Tech., Shenzhen) for 5 min in an oxygen atmosphere.Correspondingly, the defective TiO 2 / CN/3DG composites were prepared by replacing the pristine component (TiO 2 , CN, or GO) with the defective material with the above procedures.
Photocatalytic experiments: The adsorption and photocatalytic experiments were conducted in a 150 mL quartz reactor with a condensing surface.A 300 W Xe lamp was used as the visible light source with a cutoff filter (λ > 400 nm) at a distance of 2 cm from the reactor.Typically, 100 mg as-prepared catalysts were added into 100 mL of tetracycline solution (50 mg/L) or tetracycline/Cu 2+ solution (50 mg/L + 1 mg/L) with magnetically stirred at 200 rpm.For an adsorption experiment, the whole process was conducted in a dark environment, while for a photocatalytic experiment, the light was turned on following a 30 min pre-adsorption process in a dark environment.In the radical scavenging experiments, p-benzoquinone (BQ, 1 mM), triethanolamine (TEOA, 1 mM), and isopropyl alcohol (IPA, 1 mM) were selected as the scavengers of superoxide radical, photo-generated hole, and hydroxyl radical, respectively.The scavenger was added to the solution to form the specific concentration before the reaction.The sample was taken out from the sampling connection at given time intervals and filtered with 0.22 μm nylon membrane.The concentration of tetracycline was measured with a UV-vis spectrophotometer at 358 nm and the concentration of Cu 2+ was detected after microwave digestion with ICP-MS.

Figure 1 .
Figure 1.Morphology of TiO 2 /CN/3DG and defective samples.SEM images of a) 3DG, b) CN/3DG, c) TiO 2 /CN/3DG.EDS mapping of the image c) for the elements of d) C, e) N, f) O, and g) Ti.HRTEM images of h-j) TiO 2 /CN/3DG, k) defective 3DG, l) defective CN, and m) defective TiO 2 .The inset for i) is the SAED pattern of the sample.

Figure 6 .
Figure 6.The optimized structures and HOMO and LUMO electron densities of a) free TC molecule and b-e) adsorbed TC molecule on different composites.The atomic charges and FEDs of representative atoms in TC molecule are shown in the inserted tables.