Ti 3 C 2 @g-C 3 N 4 /TiO 2 Ternary Heterogeneous Photocatalyst for Promoted Photocatalytic Degradation Activities

: Ternary heterojunction photocatalysts can improve the transport and separation of photogenerated electrons and holes, which could promote their reduction and oxidation properties for environmental and energy applications. In this research, the ternary photocatalyst Ti 3 C 2 @TiO 2 /g-C 3 N 4 was successfully synthesized via direct electrostatic self-assembly during hydrothermal process. Ti 3 C 2 MXene was used to optimize the interfacial carrier transport and separation between the interfaces. The obtained ternary heterostructured photocatalyst had a higher photocatalytic degradation performance for removing rhodamine B (RhB) and 4-chlorophenol (4-CP). The synergistic effect of heterojunction between g-C 3 N 4 and TiO 2 and Schottky barrier presented among TiO 2 and Ti 3 C 2 suppressed the recombination of the photogenerated electron–hole pairs. Moreover, the Ti 3 C 2 can serve as an active site for the adsorption and activation of organic pollutants resulting from sufficient functional groups (F − here).

Two-dimensional (2D) materials own unique layered structures and excellent chemical stability, and many researchers have focused on them [19][20][21][22][23].Among all 2D layered materials, g-C 3 N 4 has demonstrated excellent activity in the photolytic aqua-hydrogen and photocatalytic degradation of organic pollutants due to other excellent optical absorption capabilities and suitable band positions [24][25][26][27].However, the spontaneous recombination of generated electron-hole pairs significantly limits the applications of g-C 3 N 4 in practical photocatalysis [28].Therefore, the fabrication of heterogeneous photocatalysts with other semiconductors can effectively prevent the recombination of generated electron-hole pairs leading to the promoted overall efficiency [29][30][31].Various g-C 3 N 4 -based heterogeneous nanostructures have been synthesized to improve the separation efficiency of photogenerated electron-hole pairs through rapid charge transfer at the interfaces [32][33][34][35][36][37].
MXene, as a new 2D transition metal carbide or carbon-nitride, was obtained by etching the A-layer in MAX using hydrofluoric acid to obtain the MXene phase [38][39][40][41].
Our previous research showed TiB 2 -TiO 2 @g-C 3 N 4 (TBCN) ternary heterojunction composites with promoted photocatalytic degradation performances for RhB and 4-CP removal [58].Herein, new Ti 3 C 2 @TiO 2 /g-C 3 N 4 (TC-TBCN) ternary heterostructured photocatalysts were synthesized through the direct electrostatic self-assembly of TBCN with Ti 3 C 2 via a hydrothermal process; the addition of Ti 3 C 2 could effectively enhance light absorption and promote the transfer of photogenerated carriers by forming Schottky junctions with TBCN.The morphology, pore structure, phase composition, optical properties and photocatalytic performances of the TC-TBCN photocatalysts were investigated.The obtained ternary Ti 3 C 2 @TiO 2 /g-C 3 N 4 photocatalysts could effectively improve the separation and migration efficiency of photogenerated charges, and its large surface active sites and effective interfacial charge transfer showed a better photodegradation performance for both rhodamine B (RhB) and 4-chlorophenol (4-CP), providing an essential idea for degrading pollutants and treating organic wastewater.Finally, we propose the mechanism of the photocatalytic degradation of such ternary photocatalysts.

Synthesis of TBCN
Our previous research reported on TiB 2 -TiO 2 @g-C 3 N 4 (TBCN) composites with coreshell structures, the synthesis process of which is presented in Figure 1 [58].The mass ratio of TiB 2 to melamine was 1:100 in this experiment.Firstly, 0.1 g TiB 2 and 10 g melamine powders were ground for 10 min to form a uniform mixed powder.Then, the obtained gray powder was transferred to a crucible with a cover and wrapped using aluminum foil, and then it was calcinated at 550 • C for 5 h in a muffle furnace in air.g-C 3 N 4 was grown in situ on the TiB 2 surface, making a g-C 3 N 4 coating on the TiB 2 core, and the products were obtained after washing and drying, here named TBCN.

Synthesis of Ti 3 C 2
As shown in Figure 1, 1.0 g of Ti 3 AlC 2 powder (MAX phase) was put in a PTFE reactor; then, 20 mL of a 40% HF solution was added for etching the Al layer.The solution was stirred for 3 days at room temperature.Then, the pH of the obtained suspension could be tuned to pH ≥ 6 using lots of deionized (DI) water.Finally, the resulting black Ti 3 C 2 F x powder was dried in an oven after washing with ethanol through centrifugation for use in the next step.

Synthesis of Ti3C2
As shown in Figure 1, 1.0 g of Ti3AlC2 powder (MAX phase) was put in a PTFE reactor; then, 20 mL of a 40% HF solution was added for etching the Al layer.The solution was stirred for 3 days at room temperature.Then, the pH of the obtained suspension could be tuned to pH ≥6 using lots of deionized (DI) water.Finally, the resulting black Ti3C2Fx powder was dried in an oven after washing with ethanol through centrifugation for use in the next step.

Synthesis of TC-TBCN
The TC-TBCN was synthesized via a hydrothermal process, as shown in Figure 1, where 0.2 g of TBCN powder and a given amount of Ti3C2Fx powder were added into 60 mL of DI water under stirring, followed by ultrasonication to obtain the dispersed suspension.Then, the suspension was transferred to a Teflon-lined stainless-steel autoclave and maintained at 120 °C for 12 h.A solid yellow powder deposited at the bottom was collected and washed thoroughly with deionized water after the autoclave cooled down.The final TC-TBCN products were obtained after drying in a vacuum drying oven.The mass ratios of Ti3C2Fx and TBCN were 0 wt%, 1 wt%, 2.5 wt%, 5 wt% and 10wt%, and were designated as 1-TBCN, 1TC-TBCN, 2.5TC-TBCN, 5TC-TBCN and 10TC-TBCN, respectively.

Characterization
The morphologies of the obtained products were conducted on scanning electron microscopy (SEM, FEI Quanta FEG 250, Hillsboro, OR, USA) and transmission electron microscopy (TEM, JEOL JEM-2100, Tokyo, Japan).An X-ray diffraction spectrometer (XRD, Bruker D8 ADVANCE, Karlsruhe, Germany) was used to determine the crystal phases.The synthesized photocatalysts' surface composition and elemental chemistry were measured on an X-ray photoelectron spectrometer (XPS, Thermo Scientific K-Alpha+, Waltham, MA, USA) equipped with a monochromatic Al K α X-ray source (1486.6 eV).The XPS spectra were calibrated with the C1s peak of amorphous carbon (284.6 eV) and fitted using XPSPEAK 4.1 software.The UV-Vis absorption spectra were obtained with a UV-Vis spectrophotometer (UV-Vis, Shimadzu UV2600, Kyoto, Japan).A specific surface area analyzer (JW-BK200B, Beijing JWGB Sci&Tech Co., Ltd., Beijing, China) was used to determine the Nitrogen adsorption-desorption isotherms, and the specific surface area and

Synthesis of TC-TBCN
The TC-TBCN was synthesized via a hydrothermal process, as shown in Figure 1, where 0.2 g of TBCN powder and a given amount of Ti 3 C 2 F x powder were added into 60 mL of DI water under stirring, followed by ultrasonication to obtain the dispersed suspension.Then, the suspension was transferred to a Teflon-lined stainless-steel autoclave and maintained at 120 • C for 12 h.A solid yellow powder deposited at the bottom was collected and washed thoroughly with deionized water after the autoclave cooled down.The final TC-TBCN products were obtained after drying in a vacuum drying oven.The mass ratios of Ti 3 C 2 F x and TBCN were 0 wt%, 1 wt%, 2.5 wt%, 5 wt% and 10wt%, and were designated as 1-TBCN, 1TC-TBCN, 2.5TC-TBCN, 5TC-TBCN and 10TC-TBCN, respectively.

Characterization
The morphologies of the obtained products were conducted on scanning electron microscopy (SEM, FEI Quanta FEG 250, Hillsboro, OR, USA) and transmission electron microscopy (TEM, JEOL JEM-2100, Tokyo, Japan).An X-ray diffraction spectrometer (XRD, Bruker D8 ADVANCE, Karlsruhe, Germany) was used to determine the crystal phases.The synthesized photocatalysts' surface composition and elemental chemistry were measured on an X-ray photoelectron spectrometer (XPS, Thermo Scientific K-Alpha+, Waltham, MA, USA) equipped with a monochromatic Al K α X-ray source (1486.6 eV).The XPS spectra were calibrated with the C1s peak of amorphous carbon (284.6 eV) and fitted using XPSPEAK 4.1 software.The UV-Vis absorption spectra were obtained with a UV-Vis spectrophotometer (UV-Vis, Shimadzu UV2600, Kyoto, Japan).A specific surface area analyzer (JW-BK200B, Beijing JWGB Sci&Tech Co., Ltd., Beijing, China) was used to determine the Nitrogen adsorption-desorption isotherms, and the specific surface area and pore size distribution were obtained based on the Brunauer-Emmett-Teller (BET) method and Barret-Joyner-Halenda (BJH) method.Fourier transform infrared spectra (FTIR) were obtained using a FTIR spectrophotometer (Thermo Scientific Nicolet iS50, Waltham, MA, USA), equipped with an ATR accessory.

Photocatalytic Performance Measurements
RhB and 4-CP were used as target pollutants to evaluate the photocatalytic ability of the obtained photocatalysts in a Pyrex reactor with a volume of 57.5 mL.The detailed experiment was as follows: In total, 20 mg of the photocatalyst was dispersed in 36 mL DI water under sonication for 2 min; then, 4 mL of 100 ppm RhB or 4-CP solution was added and stirred in the dark for 30 min to establish an adsorption-desorption equilibrium of the pollutants.The light source for the photoreaction was a 300 W Xe lamp equipped with a VisREF filter (350 nm-780 nm).The reacted solution was magnetically stirred during the photocatalytic reaction.In total, 1.0 mL sample suspensions were taken from the reactor intermittently in specific time intervals and filtered through a 0.22 µm pore-sized PTFE syringe filter to determine the removal rate of the pollutants.A UV-Vis spectrophotometer (Shimadzu UV2600, Kyoto, Japan) was used to determine the absorbance of the resulting RhB solution at the characteristic wavelength (554 nm).The changes in absorbance values quantitatively were used to calculate the degradation percentage.In contrast, the 4-CP concentration was determined on a high-performance liquid chromatography system (HPLC, LC-2030Plus, Shimadzu, Kyoto, Japan) equipped with a Shim Pack C18 column using a UV-Vis detector (measurement details: column temperature: 35 • C; mobile phase: acetonitrile-0.1 vol.% acetic acid 60:40 (v:v); flow rate: 1.0 mL/min; detect wavelength: 280 nm).Incidentally, the Cl − was also determined using the ion chromatograph (IC, Shine CIC-D120, Qingdao, China) equipped with a Shine SH-AC-3 column with conductivity detection (measurement details: mobile phase: column temperature: 35 • C; 2.4 mM Na 2 CO 3 + 1.0 mM NaHCO 3 aqueous solution; flow rate: 1.0 mL/min; current: 30 mA;).Finally, photocatalytic efficiency was calculated according to the concentration changing (C/C 0 ), where C and C 0 are the measured and initial concentrations of RhB and 4-CP, respectively.The final degradation efficiency of RhB could be calculated according to the Lambert-Beer law.004), ( 101), ( 104) and (105), respectively.The characteristic peaks of Ti 3 AlC 2 located at 9.5 • and 19.2 • shifted to lower angles, and the intensity was decreased after HF etching [60].The highest diffraction peak at 38.8 • from the (104) crystal plane disappeared, indicating the successful preparation of Ti 3 C 2 via a complete Al layer etching [61].In addition, the red curve was the XRD pattern of the Ti 3 C 2 F x MXene sample.The peaks matched well with those of Ti 3 C 2 F x synthesized by others [62].

Structure and Composition of TC-TBCN
Figure 4 shows the morphologies of the TC-TBCN samples.All samples were irregularly shaped powders, and the size of the powders was not uniform, but fine granular materials were coated on the surface.As shown in Figure 4a,b, the size of the 0TC-TBCN sample was smaller than that of TBCN, which indicated the partial decomposition and recrystallization of TBCN during the hydrothermal process.Compared with TBCN, the roughness of the obtained TC-TBCN samples significantly increased after the hydrothermal treatment.Ti3C2 will be partially decomposed and recrystallized during the hydrothermal process.Thus the accordion-like Ti3C2 could not be observed in the low-content samples (1TC-TBCN and 2.5TC-TBCN), as shown in Figure 4c,d.In addition, the further increased Ti3C2 content significantly affected the composites' morphology.The accordionlike structures could be observed in the 5TC-TBCN and 10TC-TBCN samples, as shown in Figure 4e,f.C3N4 and TiO2 were coated on Ti3C2.Figure 5 shows the TEM images of the 5TC-TBCN sample.The small crystal particles with clear TiO2 lattice fringes were grown on the surface by transforming Ti3C2 into TiO2 through a hydrothermal reaction.[63].Figure 3c shows the high-resolution Ti 2p XPS spectra, the peaks were corresponding to Ti−C, indicating that the Ti−C binding signal resulted from the Ti atoms in the interior of the MXene layers.As shown in Figure 3d, the C1s spectra mainly exhibited two peaks ascribed to the C−C (284.6 eV) and C−Ti (281.3 eV) bonds [61].As shown in Figure 3e, the O1s spectra showed three peaks ascribed to Ti−O (529.9 eV), C−O−Ti (530.9 eV) and C−Ti−OH x (531.9 eV) [64].The F 1s spectra in Figure 3f Coatings 2023, 13, 655 6 of 16 showed the presence of Ti−F−C bonds located at 684.4 eV and Ti−F−Ti bonds located at 685.6 eV [65].Overall, the surface of Ti 3 C 2 contained −O and −F surface functional groups.

Structure and Composition of TC-TBCN
Figure 4 shows the morphologies of the TC-TBCN samples.All samples were irregularly shaped powders, and the size of the powders was not uniform, but fine granular materials were coated on the surface.As shown in Figure 4a,b, the size of the 0TC-TBCN sample was smaller than that of TBCN, which indicated the partial decomposition and recrystallization of TBCN during the hydrothermal process.Compared with TBCN, the roughness of the obtained TC-TBCN samples significantly increased after the hydrothermal treatment.Ti 3 C 2 will be partially decomposed and recrystallized during the hydrothermal process.Thus the accordion-like Ti 3 C 2 could not be observed in the low-content samples (1TC-TBCN and 2.5TC-TBCN), as shown in Figure 4c,d.In addition, the further increased Ti 3 C 2 content significantly affected the composites' morphology.The accordion-like structures could be observed in the 5TC-TBCN and 10TC-TBCN samples, as shown in Figure 4e,f.C 3 N 4 and TiO 2 were coated on Ti 3 C 2 .Figure 5 shows the TEM images of the 5TC-TBCN sample.The small crystal particles with clear TiO 2 lattice fringes were grown on the surface by transforming Ti 3 C 2 into TiO 2 through a hydrothermal reaction.An XPS measurement was also used to investigate the surface chemical environment of the 5TC-TBCN sample.Figure 6a  For the N1s spectrum in Figure 6d, the spectrum was fitted into three peaks, the peak at 400.9 eV was ascribed to the C−N−H functional groups, the peaks detected at a binding energy of 399.60 eV was ascribed to tertiary N−(C) 3 and the peak observed at 398.51 eV corresponded to the C−N = C coordination from the sp 2 -bonded N in the triazine rings of g-C 3 N 4 [58].Moreover, the intensities of these peaks differed from those of g-C 3 N 4 obtained via calcination of melamine, which also indicated that the decomposition and recrystallization of TBCN happened during the hydrothermal process.As shown in Figure 6e, the O1s spectrum showed two peaks at 529.9 eV and 531.

N2 Sorption Isotherms and Pore Size Distributions
Figure 7b shows the products' N2 adsorption-desorption isotherm and pore size distribution plots.The hysteresis loop was of type A, consistent with cylindrical pores.The isotherm's desorption branch could be used to determine the pore size distribution using the Barrett-Joyner-Halenda (BJH) method [58].The detailed BET surface areas, total pore volumes and average pore diameters of the obtained samples are shown in Table 1.As shown in Table 1, adding TiB2 brought along the growing site for g-C3N4, enabling the TBCN samples to process larger surface areas than g-C3N4.The addition of Ti3C2 slightly affected the surface areas, and the recrystallization of TBCN led to a shell coated on Ti3C2.Thus, the appropriate amount of Ti3C2 addition brought about proper growing sites, enabling an increased large surface area.5TC-TBCN had a specific surface area of approximately 22.034 m 2 /g, and the smallest average pore size among all the samples.

N 2 Sorption Isotherms and Pore Size Distributions
Figure 7b shows the products' N 2 adsorption-desorption isotherm and pore size distribution plots.The hysteresis loop was of type A, consistent with cylindrical pores.The isotherm's desorption branch could be used to determine the pore size distribution using the Barrett-Joyner-Halenda (BJH) method [58].The detailed BET surface areas, total pore volumes and average pore diameters of the obtained samples are shown in Table 1.As shown in Table 1, adding TiB 2 brought along the growing site for g-C 3 N 4 , enabling the TBCN samples to process larger surface areas than g-C 3 N 4 .The addition of Ti 3 C 2 slightly affected the surface areas, and the recrystallization of TBCN led to a shell coated on Ti 3 C 2 .Thus, the appropriate amount of Ti 3 C 2 addition brought about proper growing sites, enabling an increased large surface area.5TC-TBCN had a specific surface area of approximately 22.034 m 2 /g, and the smallest average pore size among all the samples.The photocatalytic performance of photocatalysts is highly dependent on the light absorption properties of the materials.Figure 7c shows the diffuse reflection absorption spectra of different products.The absorption in 500~1400 nm was enhanced with the increase of Ti 3 C 2 content, thus, improving the utilization of visible light during the photocatalytic degradation.However, the Ti 3 C 2 addition did not change the absorption edge of TBCN.Combined with the SEM and XPS results analysis, the added Ti 3 C 2 only partially converted to TiO 2 , and some of them remained as the TC-TBCN products, resulting in an enhanced absorption in 500~1400 nm.Meanwhile, more Ti 3 C 2 would be present in the final product with the increase in Ti 3 C 2 addition, and, thus, the light absorption abilities in the visible region increased.Moreover, Ti 3 C 2 increased the light absorption and created a charge transfer channel in the composite photocatalysts.Furthermore, the optical band gap was estimated by applying Tauc's equation.The curves in Figure 7d were obtained using Tauc's formula [(Ahν) = B(hν − Eg) n ] (n = 2 for indirect transitions).The values of optical band gaps were 1.16, 2.78, 2.78 and 2.80 eV for Ti 3 C 2 , TBCN, 5TC-TBCN and C 3 N 4 , respectively.

Photocatalytic Activity
The photocatalytic performance was evaluated from the degradation of RhB and 4-CP under light irradiation in the solution.The data are presented in Figure 8a, with the photocatalysts with different additions showing some differences after 120 min of light irradiation, and their photocatalytic degradation activities increased first and then decreased with the increased Ti 3 C 2 content.Figure 8b shows the corresponding kinetic fitting curves, which conformed to the pseudo-first-order kinetics.The degradation rate constants (k) of RhB could be calculated from the curves and are shown in Table 2.The 5TC-TBCN displayed the highest k value of RhB (0.01575 min −1 ).Thus, the addition of Ti 3 C 2 was favorable for the photodegradation of RhB.As expected, the photocatalytic activity of the TC-TBCN sample was higher than that of the absolute g-C 3 N 4 and TBCN samples due to the introduction of Ti 3 C 2 , resulting in an enhanced light absorbance and a centered electron acceptor.The cyclic stability of the photocatalysts during practical application was also essential.The cycling degradation of RhB was conducted to measure the recyclability and stability of 5TC-TBCN.As shown in Figure 8c, RhB could maintain a high removal rate, and no significant decrease could be observed after five cycles under the same conditions.Generally, photogenerated holes and electrons can be transferred to the catalyst surface or interface to accede to the oxidation and reduction reactions in the photocatalytic process.Furthermore, to prove the contribution of different reactive groups to the photocatalytic degradation of RhB, isopropyl alcohol (IPA), benzoquinone (BQ), disodium ethylenediaminetetraacetate (EDTA-2Na) and silver nitrate served as trapping agents for the hydroxyl groups (OH), superoxide anion (O 2− ), photogenerated holes (h + ) and electrons (e − ), respectively.As shown in Figure 8d, the removal efficiency of RhB decreased to 14%, 23%, 23%, 34% and 40% in the presence of IPA, EDTA-2Na, N 2 , BQ and AgNO 3 , respectively.The degradation rate of RhB was limited in the presence of these scavengers.
Furthermore, the stability of the sample was also confirmed by using FTIR.The samples were obtained from a hydrothermal process, which indicated their excellent stability in water, even hot water.Therefore, FTIR was better for checking the surface change after the photocatalytic reaction.The FTIR spectra are shown in Figure 9.The spectral patterns in 1100-1750 cm −1 corresponded to the aromatic C−N stretching vibration and C=N stretching vibrations from C 3 N 4 [33,66].Furthermore, the tri-s-triazine unit band was observed at approximately 810 cm −1 , and the N−H stretching and O−H stretching broad vibration bands from the adsorbed H 2 O were observed at 2800-3350 cm −1 [67].There were no significant differences in the curves, indicating no great changes after the photocatalytic reactions.Furthermore, the stability of the sample was also confirmed by using FTIR.The samples were obtained from a hydrothermal process, which indicated their excellent stability in water, even hot water.Therefore, FTIR was better for checking the surface change after the photocatalytic reaction.The FTIR spectra are shown in Figure 9.The spectral patterns in 1100-1750 cm −1 corresponded to the aromatic C−N stretching vibration and C=N stretching vibrations from C3N4 [33,66].Furthermore, the tri-s-triazine unit band was observed at approximately 810 cm −1 , and the N−H stretching and O−H stretching broad vibration bands from the adsorbed H2O were observed at 2800-3350 cm −1 [67].There were no significant differences in the curves, indicating no great changes after the photocatalytic reactions.Furthermore, the stability of the sample was also confirmed by using FTIR.The samples were obtained from a hydrothermal process, which indicated their excellent stability in water, even hot water.Therefore, FTIR was better for checking the surface change after the photocatalytic reaction.The FTIR spectra are shown in Figure 9.The spectral patterns in 1100-1750 cm −1 corresponded to the aromatic C−N stretching vibration and C=N stretching vibrations from C3N4 [33,66].Furthermore, the tri-s-triazine unit band was observed at approximately 810 cm −1 , and the N−H stretching and O−H stretching broad vibration bands from the adsorbed H2O were observed at 2800-3350 cm −1 [67].There were no significant differences in the curves, indicating no great changes after the photocatalytic reactions.A similar photocatalytic performance was also conducted in photocatalytic degradation (oxidation) of 4-CP, and the time profiles of the concentrations of 4-CP and Cl − are presented in Figure 10.The measurements began after continuously stirring for 30 min under dark conditions to reach the adsorption equilibrium.HPLC and IC were used to determine the removal of 4-CP, and the data from HPLC and IC were consistent [68].The TC-TBCN samples showed a similar photocatalytic degradation (oxidation) performance with RhB degradation, and 5TC-TBCN also showed the highest degradation efficiency of 4-CP, indicating that adding Ti 3 C 2 was also helpful for the photodegradation of 4-CP.As shown in Figure 10, 4-CP and Cl − concentrations were very stable after 120 min of light irradiation alone without the presence of photocatalysts as the blank experiment, showing that 4-CP was hardly self-degradable [10].
TBCN 0.01312 0TC-TBCN 0.00974 1TC-TBCN 0.01202 2.5TC-TBCN 0.01291 5TC-TBCN 0.01575 10TC-TBCN 0.00653 A similar photocatalytic performance was also conducted in photocatalytic degradation (oxidation) of 4-CP, and the time profiles of the concentrations of 4-CP and Cl − are presented in Figure 10.The measurements began after continuously stirring for 30 min under dark conditions to reach the adsorption equilibrium.HPLC and IC were used to determine the removal of 4-CP, and the data from HPLC and IC were consistent [68].The TC-TBCN samples showed a similar photocatalytic degradation (oxidation) performance with RhB degradation, and 5TC-TBCN also showed the highest degradation efficiency of 4-CP, indicating that adding Ti3C2 was also helpful for the photodegradation of 4-CP.As shown in Figure 10, 4-CP and Cl − concentrations were very stable after 120 min of light irradiation alone without the presence of photocatalysts as the blank experiment, showing that 4-CP was hardly self-degradable [10].

Discussion
The photocatalyst mechanism was investigated by adding different scavengers during the RhB photodegradation process.The TC-TBCN samples obtained from the hydrothermal treatment of Ti3C2 and TBCN showed excellent photocatalytic degradation activity for RhB and 4-CP.Finally, we proposed the mechanism of TC-TBCN for photocatalysis based on the characterization and photocatalytic performances, as shown in Figure 11 [58,69].Both g-C3N4 and TiO2 could produce electrons and holes under light irradiation, and the addition of Ti3C2 enhanced the light absorbance capability of the TC-TBCN samples.Figure 10 delineates that the photogenerated electrons on the conduction band (CB) of g-C3N4 would be delivered to the CB of TiO2 and further transferred to the Ti3C2.Ti3C2 could act as an intermediate electron acceptor for photogenerated electrons [70,71].The accumulated electrons on Ti3C2 could be associated with the adsorbed oxygen reaction to induce the •O2 − active groups for involvement in RhB and 4-CP removal.However, some photogenerated holes on the valence band (VB) of g-C3N4 would combine with the accumulated electrons on Ti3C2.Moreover, the electric field of Ti3C2 and TiO2 caused a space charge layer near the Ti3C2/TiO2 interface, resulting in a bending 'upward' VB and CB in

Discussion
The photocatalyst mechanism was investigated by adding different scavengers during the RhB photodegradation process.The TC-TBCN samples obtained from the hydrothermal treatment of Ti 3 C 2 and TBCN showed excellent photocatalytic degradation activity for RhB and 4-CP.Finally, we proposed the mechanism of TC-TBCN for photocatalysis based on the characterization and photocatalytic performances, as shown in Figure 11 [58,69].Both g-C 3 N 4 and TiO 2 could produce electrons and holes under light irradiation, and the addition of Ti 3 C 2 enhanced the light absorbance capability of the TC-TBCN samples.Figure 10 delineates that the photogenerated electrons on the conduction band (CB) of g-C 3 N 4 would be delivered to the CB of TiO 2 and further transferred to the Ti 3 C 2 .Ti 3 C 2 could act as an intermediate electron acceptor for photogenerated electrons [70,71].The accumulated electrons on Ti 3 C 2 could be associated with the adsorbed oxygen reaction to induce the •O 2 − active groups for involvement in RhB and 4-CP removal.However, some photogenerated holes on the valence band (VB) of g-C 3 N 4 would combine with the accumulated electrons on Ti 3 C 2 .Moreover, the electric field of Ti 3 C 2 and TiO 2 caused a space charge layer near the Ti 3 C 2 /TiO 2 interface, resulting in a bending 'upward' VB and CB in the ternary photocatalysts.The Schottky barrier at the interfaces between TiO 2 and Ti 3 C 2 inhibited the electron-hole pair recombination, resulting in prolonged electron lifetimes.Thus, the amounts of holes at the VB of TiO 2 would react with H 2 O to form hydroxyl radicals (OH) on the heterogenous photocatalyst's surface, which could efficiently oxidize organic pollutants via the holes at the VB of TiO 2 into small intermediates, or directly into end products (CO 2 and H 2 O) [41,61].The g-C 3 N 4 /TiO 2 heterojunction and Schottky barrier between TiO 2 and Ti 3 C 2 boosted the transfer and separation of the photogenerated carriers, thus, suppressing their recombination [49].Moreover, the sufficient functional groups on Ti 3 C 2 F x t could provide active sites for adsorbing and activating organic pollutants.The enhanced charge separation efficiency could prolong the lifetime of the photogenerated carriers and produce more active species; the abundant functional groups of the 5TC-TBCN sample provided more active sites, thus, efficiently degrading RhB and 4-CP [54].radicals (OH) on the heterogenous photocatalyst's surface, which could efficiently oxidize organic pollutants via the holes at the VB of TiO2 into small intermediates, or directly into end products (CO2 and H2O) [41,61].The g-C3N4/TiO2 heterojunction and Schottky barrier between TiO2 and Ti3C2 boosted the transfer and separation of the photogenerated carriers, thus, suppressing their recombination [49].Moreover, the sufficient functional groups on Ti3C2Fx t could provide active sites for adsorbing and activating organic pollutants.The enhanced charge separation efficiency could prolong the lifetime of the photogenerated carriers and produce more active species; the abundant functional groups of the 5TC-TBCN sample provided more active sites, thus, efficiently degrading RhB and 4-CP [54].

Conclusions
In this research, ternary heterogeneous Ti3C2@TiO2/g-C3N4 photocatalysts were synthesized through electrostatic self-assembly using a hydrothermal method.The obtained ternary photocatalysts presented a higher photodegradation performance for RhB and 4-CP.The results showed that Ti3C2 could construct n-type Schottky heterojunctions between g-C3N4 and TiO2, accepting the photogenerated electrons from the CB of g-C3N4; the multiple built-in electric fields also enhanced the charge transfer and suppressed the recombination of electron-hole pairs, resulting in an enhanced charge separation efficiency.The Schottky barrier between TiO2 and Ti3C2 could also boost the transfer and suppress the recombination.Moreover, the sufficient functional groups in Ti3C2Fx could serve as active sites, which was beneficial for the adsorption and activation of organic pollutants.Thus, the 5TC-TBCN sample process enhanced the photocatalytic degradation of RhB and 4-CP.Ternary heterojunctions such as the one in this study could be efficient photocatalysts in degrading pollutants from organic wastewater.

Conclusions
In this research, ternary heterogeneous Ti 3 C 2 @TiO 2 /g-C 3 N 4 photocatalysts were synthesized through electrostatic self-assembly using a hydrothermal method.The obtained ternary photocatalysts presented a higher photodegradation performance for RhB and 4-CP.The results showed that Ti 3 C 2 could construct n-type Schottky heterojunctions between g-C 3 N 4 and TiO 2 , accepting the photogenerated electrons from the CB of g-C 3 N 4 ; the multiple built-in electric fields also enhanced the charge transfer and suppressed the recombination of electron-hole pairs, resulting in an enhanced charge separation efficiency.The Schottky barrier between TiO 2 and Ti 3 C 2 could also boost the transfer and suppress the recombination.Moreover, the sufficient functional groups in Ti 3 C 2 F x could serve as active sites, which was beneficial for the adsorption and activation of organic pollutants.Thus, the 5TC-TBCN sample process enhanced the photocatalytic degradation of RhB and 4-CP.Ternary heterojunctions such as the one in this study could be efficient photocatalysts in degrading pollutants from organic wastewater.

Figure 1 .
Figure 1.Schematic of the synthesis process for TC-TBCN composite.

Figure 1 .
Figure 1.Schematic of the synthesis process for TC-TBCN composite.

Figure
Figure 3a shows the XRD patterns of the Ti3AlC2 before and after being etched in the HF solution.The diffraction peaks of the Ti3AlC2 located at 9.5°, 19.2°, 35.9°, 38.8° and 41.7° belonged to the different crystal planes (002), (004), (101), (104) and (105), respectively.The characteristic peaks of Ti3AlC2 located at 9.5° and 19.2° shifted to lower angles, and the intensity was decreased after HF etching [60].The highest diffraction peak at 38.8° from the (104) crystal plane disappeared, indicating the successful preparation of Ti3C2 via

Figure 2 .
Figure 2. SEM images of Ti 3 AlC 2 before (a) and after (b) HF etching for 72 h.

Figure
Figure3ashows the XRD patterns of the Ti 3 AlC 2 before and after being etched in the HF solution.The diffraction peaks of the Ti 3 AlC 2 located at 9.5 • , 19.2 • , 35.9 • , 38.8 • and 41.7 •

Figure 3 .
Figure 3. (a) XRD patterns of the MAX phased Ti 3 AlC 2 before and after being etched in HF solution; XPS spectra of Ti 3 C 2 : (b) survey; (c) Ti 2p; (d) C 1s; (e) O 1s; (f) F 1s.An XPS measurement was used to investigate the elemental composition and surface chemical environment of the Ti 3 C 2 MXene sample.Figure 3b depicts the XPS survey spectra of Ti 3 C 2 , which confirmed the presence of C, Ti, O and F elements.The presence of F and O in Ti 3 C 2 obtained from the HF etching indicated surface termination (Ti 3 C 2 (OH) 2 , Ti 3 C 2 F x ), because the enhanced surface activity of Ti 3 C 2 after the Al layer etching led to reactions with the surrounding −O or −F[63].Figure3cshows the high-resolution Ti 2p XPS spectra, the peaks were corresponding to Ti−C, indicating that the Ti−C binding signal resulted from the Ti atoms in the interior of the MXene layers.As shown in Figure3d, the C1s spectra mainly exhibited two peaks ascribed to the C−C (284.6 eV) and C−Ti (281.3 eV) bonds[61].As shown in Figure3e, the O1s spectra showed three peaks ascribed to Ti−O (529.9 eV), C−O−Ti (530.9 eV) and C−Ti−OH x (531.9 eV)[64].The F 1s spectra in Figure3f
depicts the XPS survey spectra of 5TC-TBCN, which confirmed that the elements (C, N, Ti, O and F) existed in the sample.The high-resolution Ti 2p XPS spectra in Figure 6b were shown to possess two peaks of Ti 2p 3/2 and Ti 2p 1/2 that were deconvoluted into two components, Ti−C and Ti−O, indicating the presence of Ti 3 C 2 and TiO 2 in the TC-TBCN sample.The high-resolution C1s spectrum shown in Figure 6c mainly exhibited four peaks.The peak located at 281.3 eV could be ascribed to the C−Ti of Ti 3 C 2 , and the peaks at 284.6 eV, 286.4 eV and 288.6 eV were assigned to the C−C, C−NH 2 and N−C = N bonds in the aromatic skeleton rings of g-C 3 N 4 , respectively.

Figure 8 .
Figure 8.(a) Time profiles of the degradation of RhB and (b) the corresponding kinetic fitting curves with irradiation time; (c) photocatalytic stability and (d) photodegradation of RhB over 5TC-TBCN in the presence of different reactive species scavengers.[Catalyst] = 0.5 g/L; [RhB]0 = 10 ppm.

Figure 9 .
Figure 9. FTIR spectra of 5TC-TBCN before and after the reactions.

Figure 8 .
Figure 8.(a) Time profiles of the degradation of RhB and (b) the corresponding kinetic fitting curves with irradiation time; (c) photocatalytic stability and (d) photodegradation of RhB over 5TC-TBCN in the presence of different reactive species scavengers.[Catalyst] = 0.5 g/L; [RhB] 0 = 10 ppm.

Figure 8 .
Figure 8.(a) Time profiles of the degradation of RhB and (b) the corresponding kinetic fitting curves with irradiation time; (c) photocatalytic stability and (d) photodegradation of RhB over 5TC-TBCN in the presence of different reactive species scavengers.[Catalyst] = 0.5 g/L; [RhB]0 = 10 ppm.

Figure 9 .
Figure 9. FTIR spectra of 5TC-TBCN before and after the reactions.Figure 9. FTIR spectra of 5TC-TBCN before and after the reactions.

Figure 9 .
Figure 9. FTIR spectra of 5TC-TBCN before and after the reactions.Figure 9. FTIR spectra of 5TC-TBCN before and after the reactions.

Table 1 .
BET surface areas, total pore volumes and average pore sizes of the obtained products.

Table 1 .
BET surface areas, total pore volumes and average pore sizes of the obtained products.

Table 2 .
Degradation rate constant (k) values of RhB for different photocatalysts.