Self-Assembly 2D Ti3C2/g-C3N4 MXene Heterojunction for Highly Efficient Photocatalytic Degradation of Tetracycline in Visible Wavelength Range

An ultrathin 2D Ti3C2/g-C3N4 MXene (2D-TC/CN) heterojunction was synthesized, using a facile self-assembly method; the perfect microscopic-morphology and the lattice structure presented in the sample with a 2 wt% content of Ti3C2 were observed by the field-emission scanning electron microscopy (SEM) and transmission electron microscope (TEM). The optimized sample (2-TC/CN) exhibited excellent performance in degrading the tetracycline (TC), and the degradation rate reached 93.93% in the conditions of 20 mg/L, 50 mL of tetracycline within 60 min. Except for the increased specific-surface area, investigated by UV-vis diffuse reflectance spectra (UV-vis DRS) and X-ray photoelectron microscopy (XPS) valence spectra, the significantly enhanced photocatalytic activity of the 2-TC/CN could also be ascribed to the formation of Ti-N bonds between Ti3C2 and g-C3N4 nanosheets, which reduced the width of the band gap through adjusting the position of the valence band, thus resulting in the broadened light-absorption. Furthermore, the facilitated electron transmission was also proved by time-resolved photoluminescence (TRPL) and electrochemical impedance spectroscopy (EIS), which is effective in improving the quantum efficiency of photo-generated electrons. In addition, the resulting radical-capture experiment suggested that superoxide radicals have the greatest influence on photodegradation performance, with the photodegradation rate of TC reducing from 93.16% to 32.08% after the capture of superoxide radicals, which can be attributed to the production of superoxide radicals only, by the 2-TC/CN composites with a high conduction-band value (−0.62 eV). These facilely designed 2D Ti3C2/g-C3N4 composites possess great application potential for the photodegradation of tetracycline and other antibiotics.


Introduction
With the rapid development of human society, the increasing abuse of antibiotics is becoming serious. Those antibiotics which originate from large-scale farms, hospitals, pharmaceutical factories, and domestic sewage have caused tremendous pollution in natural waterbodies [1]. Tetracycline, a broad-spectrum antibiotic, has become a common detection indicator in water. Even at low levels (<1.0 ppm), long-term exposure to residual tetracycline may induce antibiotic resistance and harm human health [2,3]. In addition, the residual tetracycline will not only affect the growth and metabolism of microorganisms, but also the microbial-community structure of the ecosystem [4]. Therefore, it is of great urgency to develop an efficient and economical strategy to degrade residual tetracycline in water.
Therefore, based on the synergistic effect of the 2D materials and the heterostructure, the new composite named 2D Ti 3 C 2 /g-C 3 N 4 MXene composite was synthesized by introducing 2D-Ti 3 C 2 MXene onto the surface of 2D g-C 3 N 4, via a facile liquid self-assembly method. The introduction of 2D-Ti 3 C 2 MXene not only improves the electronic-transmission performance of 2D g-C 3 N 4 , but also broadens the wavelength range of visible-light absorption waves in the photocatalytic-degradation process of tetracycline. Moreover, 2D-Ti 3 C 2 MXene also reduces the recombination of photogenerated electrons of 2D g-C 3 N 4 and improves its recyclability and degradation efficiency during the removal of tetracycline from wastewater. Experimental results evidence that the 2D Ti 3 C 2 /g-C 3 N 4 MXene composite shows excellent application prospects for the photodegradation of tetracycline.

Preparation of Protonated 2D-C 3 N 4 Nanosheet
The typical synthesis method of protonated g-C 3 N 4 involves the calcination of melamine, following the above steps. Step 1: 3 g melamine powder was put into a crucible with a cover, and then calcinated at 550 • C for 4 h with a heating rate of 5 • C/min in a muffle furnace, under an air atmosphere.
Step 2: after being naturally cooled down, the yellow bulk C 3 N 4 powder was obtained after careful grinding.
Step 3: the g-C 3 N 4 nanosheets were dispersed into a 100 mL 0.5 M HCl, and ultrasound was performed for 0.5 h. Step 4: the obtained g-C 3 N 4 HCl solution was washed with deionized water several times until the solution pH was 4, and the protonated g-C 3 N 4 was obtained.

Preparation of 2D-Ti 3 C 2 MXene
Step 1: 1.6 g of LiF was added into 20 mL 12 M of HCl, and stirring started at 500 r/min for 10 min, to completely dissolve LiF, finally forming an etching solution. Then, 1.0 g Ti 3 A1C 2 was gradually added to the etching reagent, and the reaction temperature was set at 45 • C, with a reaction time of 45 min. Step 2: after the etching, the black liquid was centrifuged at 3000 r/min. The original etching reagent was poured out, and the bottom precipitation was collected. The collected precipitation was washed again with 1M HCl several times to absorb the excess LiF and other impurities, until the superstratum was colorless and transparent.
Step 3: the precipitation was washed with deionized water for 3-5 times. The upper layer of the liquid gradually became cloudy and stratified, at which point the MXene began to stratify. Due to the low concentration of this part of the solution, further centrifugation was required, to obtain the bottom precipitated portion. Finally, water continued to be added, and it was centrifuged until the pH reached approximately 6.
Step 4: the lower layer of precipitation was ultrasonically treated for 30 min, during which inert protective gas was added and ice was added to the ultrasonic groove, to avoid irreversible damage to the structure of the material during ultrasonic heating. After the ultrasound, the material was centrifuged for 40 min at a speed of 10,000 r/min, and the precipitation was freeze-dried for 48 h at −50 • C, to obtain powder 2D-Ti 3 C 2 MXene.

Synthesis of 2D Ti 3 C 2 /g-C 3 N 4 MXene Composite
Step 1: the protonated 2D-C 3 N 4 was dispersed into 100 mL deionized water and stirred magnetically for 10 min. Then, 1 mL 2 g/L of single-layer Ti 3 C 2 MXene solution was added into 100 mL protonated 2D-C 3 N 4 suspension, drop by drop, and stirred for 30 min after dropping; the suspension was then static for 1 h. Step 2: The suspension containing Ti 3 C 2 /C 3 N 4 composite was centrifuged, and the Ti 3 C 2 /C 3 N 4 precipitation was obtained by pouring out the supernatant.
Step 3: the Ti 3 C 2 /C 3 N 4 precipitation was placed in a vacuum drying oven and dried at 80 • C for 24 h, to obtain a 2D Ti 3 C 2 /g-C 3 N 4 composite with a Ti 3 C 2 content of 2 wt%, which can be represented as 2-TC/CN. The TC/CN composites with Ti 3 C 2 content of 1 wt%, 3 wt% and 5 wt% were prepared using the same method, which was expressed as 1-TC/CN, 3-TC/CN and 5-TC/CN, respectively. The synthesis process is illustrated in Scheme 1.
water continued to be added, and it was centrifuged until the pH reached approximately 6.
Step 4: the lower layer of precipitation was ultrasonically treated for 30 min, during which inert protective gas was added and ice was added to the ultrasonic groove, to avoid irreversible damage to the structure of the material during ultrasonic heating. After the ultrasound, the material was centrifuged for 40 min at a speed of 10,000 r/min, and the precipitation was freeze−dried for 48 h at −50 °C, to obtain powder 2D−Ti3C2 MXene.

Synthesis of 2D Ti3C2/g−C3N4 MXene Composite
Step 1: the protonated 2D−C3N4 was dispersed into 100 mL deionized water and stirred magnetically for 10 min. Then, 1 mL 2 g/L of single−layer Ti3C2 MXene solution was added into 100 mL protonated 2D−C3N4 suspension, drop by drop, and stirred for 30 min after dropping; the suspension was then static for 1 h. Step 2: The suspension containing Ti3C2/C3N4 composite was centrifuged, and the Ti3C2/C3N4 precipitation was obtained by pouring out the supernatant.
Step 3: the Ti3C2/C3N4 precipitation was placed in a vacuum drying oven and dried at 80 °C for 24 h, to obtain a 2D Ti3C2/g−C3N4 composite with a Ti3C2 content of 2 wt%, which can be represented as 2−TC/CN. The TC/CN composites with Ti3C2 content of 1 wt%, 3 wt% and 5 wt% were prepared using the same method, which was expressed as 1−TC/CN, 3−TC/CN and 5−TC/CN, respectively. The synthesis process is illustrated in Scheme 1. Scheme 1. Schematic diagram of the preparation process of 2D Ti3C2/g−C3N4 MXene composite materials.

Characterization
The microscopic morphology of the prepared samples was analyzed using field−emission scanning electron microscopy (FESEM) (Hitachi, S−4800, Tokyo, Japan). The lattice structure and energy−dispersive X−ray spectroscopy (EDS) images were conducted on a transmission electron microscope (TEM), (Philips, JEOL JEM−2010F, Nieuw-Vennep, The Netherlands), with an accelerating voltage of 100 kV. The crystal structure of the as−synthesized samples was studied by X−ray diffraction (XRD) analysis (Rigaku, SMARTLAB3KW, Tokyo, Japan) on the PERSEE XD3 diffractometer system from Bruker with Cu−Kα radiation (λ = 1.5420 Å), scanning−angle range purview 5-80° at 45 kV and 200 mA. The analysis of the functional group of as−prepared samples was conducted on the infrared diffuse reflectance (FT−IR) spectrum (Thermo Fisher Scientific, IS50, Scheme 1. Schematic diagram of the preparation process of 2D Ti 3 C 2 /g-C 3 N 4 MXene composite materials.

Characterization
The microscopic morphology of the prepared samples was analyzed using fieldemission scanning electron microscopy (FESEM) (Hitachi, S-4800, Tokyo, Japan). The lattice structure and energy-dispersive X-ray spectroscopy (EDS) images were conducted on a transmission electron microscope (TEM), (Philips, JEOL JEM-2010F, Nieuw-Vennep, The Netherlands), with an accelerating voltage of 100 kV. The crystal structure of the as-synthesized samples was studied by X-ray diffraction (XRD) analysis (Rigaku, SMART-LAB3KW, Tokyo, Japan) on the PERSEE XD3 diffractometer system from Bruker with Cu-Kα radiation (λ = 1.5420 Å), scanning-angle range purview 5-80 • at 45 kV and 200 mA. The analysis of the functional group of as-prepared samples was conducted on the infrared diffuse reflectance (FT-IR) spectrum (Thermo Fisher Scientific, IS50, Waltham, MA, USA) with an in situ cell. The chemical state and valence-band spectra of the prepared samples were characterized using X-ray photoelectron microscopy (XPS) (Thermo Fisher Scientifc, ESCALAB 250Xi, Waltham, MA USA) spectra, using a monochromated Al X-ray resource, at 1486.6 eV. The electrical properties and the and capture of superoxide radicals and hydroxyl radicals were measured by the electron spin resonance (ESR) spectra (JEOL, JES-FA200, Tokyo, Japan). Photoluminescence (PL) spectral measurements for analyzing the photogenerated electron-hole complexation rate of the materials were performed on a Fluorolog-3 fluorescence spectrophotometer with excitation wavelength of 464 nm (Hitachi, F-2700, Tokyo, Japan). The values of the energy gap of the as-synthesized samples were evaluated by UV-vis diffuse reflectance spectra (UV-vis DRS) using a UV-vis spectrophotometer with excitation wavelength of 365 nm (Shimadzu, UV-3600, Kyoto, Japan). The separation efficiency and transient photocurrent of the samples were tested by time-resolved photoluminescence (TRPL) spectra on a fluorescence lifetime spectrophotometer (FLS1000, Edinburgh, UK). The enhancement of conductivity of the composites was measured by electrochemical impedance spectroscopy (EIS) on an electrochemical workstation (CHI660E, Shanghai Chenhua, China).

Working Electrode Preparation Procedure
The working electrode was prepared by a dip-coating method. Specifically, 10 mg of the sample and 500 µL of 5 wt% Nafion solution were dispersed in 5 mL of a mixed solution of water and ethanol (V water /V ethanol = 1/4), to form a homogeneous slurry, which was subsequently dropped and coated onto a 1.0 cm × 1.0 cm ITO glass substrate and then exposed to air for 6 h, to dry.

Photoelectrochemical Measurement
Photoelectrochemical measurements were carried out on an electrochemical workstation in 0.5 M Na 2 SO 4 aqueous solution with the standard three-electrode system. The saturated calomel electrode (SCE) and platinum wire served as the reference electrode and counter electrode, respectively. The working electrode was prepared by the doctorblade technique on FTO conductive glass (MTI Corporation, Baiye Center, China), which was described in our previous work. The visible light was provided by a 300 W xenon lamp (Zhongjiao Jinyuan Technology Co., Ltd., CEL-HX300, Beijing, China) equipped with a cutoff filter (>420 nm). Prior to measurement, the suspension was purged with N 2 several times, to remove the residual air.

Evaluation of Photocatalytic Degradation of Tetracycline
The photocatalytic activity of the samples was evaluated by the decomposition of the liquid tetracycline (TC, 20 mg/L, 50 mL) under a 300 W xenon lamp equipped with a cutoff filter (>420 nm). Prior to irradiation, the mixed suspension was stirred in the dark for 30 min, to achieve the adsorption-desorption equilibrium. At given time-intervals, 3 mL liquor was extracted, then high-speed centrifuged (17,000 r/min) for 10 min, to remove the catalyst completely. The concentration of TC was analyzed using a UV-Vis spectrophotometer (Shimadzu, UV-Vis 2700, Kyoto, Japan).
Free radical trapping tests: to further investigate the photocatalytic oxidation pathway, 2-TC/CN sample was taken as a photodegradation catalyst. Tert-Butyl alcohol (TBA, 10 mM, 5 mL) as •OH radical scavenger, EDTA-2Na (0.1 g) as hole scavenger, and p-Quinone as •O 2− scavenger were chosen, respectively, to participate in the RhB photodegradation test. The process was the same as that of the photocatalytic degradation of the TC above, except for the extra added corresponding scavengers.

Synthesis Process of 2D Ti 3 C 2 /g-C 3 N 4 MXene Composite
The formation process of the 2D Ti 3 C 2 /g-C 3 N 4 MXene composite was described in Scheme 1. The 2D-Ti 3 C 2 was etched from the starting sample Ti 3 AlC 2 MAX by the following equations: Based on the SEM images, we can see that the Ti 3 AlC 2 MAX with bulk morphology ( Figure S1) changed into accordion fold ( Figure 1a) after being etched by the concentrated hydrochloric acid solution of lithium fluoride, which proved the efficient removal of aluminum ions from the MAX phase. The morphology and EDS-mapping of the composite (2-TC/CN) is shown in Figure S2. As shown in Figure 1b, the accordion fold-like Ti 3 C 2 MXene was changed into monolayer Ti 3 C 2 MXene after the dispersion of the aqueous solution and ultrasonic vibration, which can be evidenced by the Tyndall phenomenon exhibited in the illustration in Figure 1b. As shown in Figure 1c, the morphology of the g-C 3 N 4 maintained a sheet structure after ultrasonic treatment in aqueous HCl solution. The morphology of the 2D-TC/CN composites is exhibited in Figure 1d, where we can see that the morphology of protonated g-C 3 N 4 with a 2% content of monolayer Ti 3 C 2 MXene was not distinguished from that of the protonated g-C 3 N 4 . This could be attributed to the thin-flake morphology of both the 2D-TC/CN composites and the protonated g-C 3 N 4 , and it is hard to distinguish from the SEM images. The SEM images of Figure 1d show that the 2-TC/CN composites exhibited a more perfect two-dimensional layered morphology than composites with other contents of the monolayer Ti 3 C 2 ( Figure S3). To throw light on the morphology and microstructure of the 2D−TC/CN composites, the 2−TC/CN sample was chosen to be further investigated, using TEM and HRTEM (Figure 2). The SEM image of the 2D−TC/CN in Figure 1a shows that the 2D−TC/CN composites still maintained two−dimensional structures consisting of flat, regularly shaped nanosheets of 2D/2D structures. Figure 2b exhibits clear lattice−fringes and a disordered area, between which an intimate boundary arises. The disordered area was assigned to the g−C3N4 nanosheets, while the clear lattice−fringe was the Ti3C2 flakes, and the obvious lattice−spacing of 1.18 nm was attributed to the (0 0 2) facet of Ti3C2 [42]. The intimate boundary suggested the successful synthesis of the 2D−TC/CN composite, which is in accordance with the EDS analysis in Figure 2c. Figure 2c evidences the fact that the elements C, N, O, and F were uniformly distributed on the composites, and that the Ti element only existed in the 2D−Ti3C2 section, creating a clear boundary between the distribution of other To throw light on the morphology and microstructure of the 2D-TC/CN composites, the 2-TC/CN sample was chosen to be further investigated, using TEM and HRTEM ( Figure 2). The SEM image of the 2D-TC/CN in Figure 1a shows that the 2D-TC/CN composites still maintained two-dimensional structures consisting of flat, regularly shaped nanosheets of 2D/2D structures. Figure 2b exhibits clear lattice-fringes and a disordered area, between which an intimate boundary arises. The disordered area was assigned to the g-C 3 N 4 nanosheets, while the clear lattice-fringe was the Ti 3 C 2 flakes, and the obvious Nanomaterials 2022, 12, 4015 7 of 18 lattice-spacing of 1.18 nm was attributed to the (0 0 2) facet of Ti 3 C 2 [42]. The intimate boundary suggested the successful synthesis of the 2D-TC/CN composite, which is in accordance with the EDS analysis in Figure 2c. Figure 2c evidences the fact that the elements C, N, O, and F were uniformly distributed on the composites, and that the Ti element only existed in the 2D-Ti 3 C 2 section, creating a clear boundary between the distribution of other elements, which corresponded to the results in Figure 2b. This intimate boundary was beneficial to the transfer of photoinduced electrons from g-C 3 N 4 to Ti 3 C 2, due to the excellent electronic conductivity of Ti 3 C 2 .

Structure Characterization of 2D Ti3C2/C3N4 MXene Composite
The phase structure of the as−synthesized samples was characterized by the X−ray diffraction method. As shown in Figure 3a, the XRD pattern with intense peaks at 9.6° and 39°, which respectively correspond to the (0 0 2) and (1 0 4) facets, is attributed to the Ti3AlC2 MAX phase [43]. After etching treatment with the concentrated hydrochloric acid solution of lithium fluoride, the (0 0 2) facet at 9.6° and (0 0 4) facet at 19.3° of Ti3AlC2 shifted to lower angles in the XRD pattern of Ti3C2. Moreover, the most intense peak at 39° disappeared after etching treatment. According to previous studies [44,45], those changes that appeared in Figure 3a suggest the successful conversion from Ti3AlC2 to Ti3C2. The XRD patterns of g−C3N4 and 2D−TC/CN composites in Figure 3a,b show that the two distinct diffraction peaks at 13.1° and 27.5° corresponded to the (1 0 0) and (0 0 2) facets of g−C3N4, respectively, and corresponding to the intralayer packing of the polymeric melon units and the interlayer π−π stacking of g−C3N4, respectively [46]. When the content of Ti3C2 > 10 wt%, two new diffraction peaks at 6.6° and 60.4° gradually appeared in the TC/CN composites, which corresponded to (the 0 0 2) and (1 1 0) facets of Ti3C2 respectively. No changes occurred on the site of the two peaks of g−C3N4, even after loading Ti3C2, suggesting the combination of g−C3N4 and Ti3C2 belonged to an interfacial self−assembly between monolayers.

Structure Characterization of 2D Ti 3 C 2 /C 3 N 4 MXene Composite
The phase structure of the as-synthesized samples was characterized by the X-ray diffraction method. As shown in Figure 3a, the XRD pattern with intense peaks at 9.6 • and 39 • , which respectively correspond to the (0 0 2) and (1 0 4) facets, is attributed to the Ti 3 AlC 2 MAX phase [43]. After etching treatment with the concentrated hydrochloric acid solution of lithium fluoride, the (0 0 2) facet at 9.6 • and (0 0 4) facet at 19.3 • of Ti 3 AlC 2 shifted to lower angles in the XRD pattern of Ti 3 C 2 . Moreover, the most intense peak at 39 • disappeared after etching treatment. According to previous studies [44,45], those changes that appeared in Figure 3a suggest the successful conversion from Ti 3 AlC 2 to Ti 3 C 2 . The XRD patterns of g-C 3 N 4 and 2D-TC/CN composites in Figure 3a,b show that the two distinct diffraction peaks at 13.1 • and 27.5 • corresponded to the (1 0 0) and (0 0 2) facets of g-C 3 N 4 , respectively, and corresponding to the intralayer packing of the polymeric melon units and the interlayer π-π stacking of g-C 3 N 4 , respectively [46]. When the content of Ti 3 C 2 > 10 wt%, two new diffraction peaks at 6.6 • and 60.4 • gradually appeared in the TC/CN composites, which corresponded to (the 0 0 2) and (1 1 0) facets of Ti 3 C 2 , respectively. No changes occurred on the site of the two peaks of g-C 3 N 4 , even after loading Ti 3 C 2, suggesting the combination of g-C 3 N 4 and Ti 3 C 2 belonged to an interfacial self-assembly between monolayers. ure 3g, the Ti 2p spectrum was deconvoluted into four components, including peaks at 454.8 eV, 457.2 eV, 460.7 eV and 463.4 eV. The peaks at 454.8 eV and 460.7 eV were assigned to Ti−C 2p3/2 and Ti−C 2p5/2 [55], while peaks at 457.2 eV and 463.4 eV were attributed to Ti−O 2p3/2 and Ti−O 2p5/2 [56]. From the high−resolution spectra of g−C3N4 and 2−TC/CN in Figure 3e-g, we can see that the binding energy of C−NHX, C−N=C and C−N=C in the 2−TC/CN composite exhibited a positive shift, indicating the low electrondensity of g−C3N4 in 2−TC/CN and the strong interaction between g−C3N4 and Ti3C2. Through the electron density of the C−NHX and the newly generated Ti−N bond in the 2−TC/CN, it can be deduced that Ti3C2 showed self−assembly formation on the g−C3N4 through Ti−NHX interaction, as shown in Scheme 1.  The FTIR results of the prepared samples are exhibited in Figure 3c. The broad peaks between 3000 and 3500 cm −1 represent N-H stretching vibrations [40], and the intensity of the peak was slightly reduced after loading Ti 3 C 2 in the protonated g-C 3 N 4, which suggests that the chemical N-H bond might act as "positive bridge" to attract electronegative Ti 3 C 2 through electrostatic self-assembly. The several peaks at 1200-1700 cm −1 suggest the typical stretching modes of CN heterocycles, such as C-N and C=N heterocycles [47]. The peak at approximately 810 cm −1 belonged to the out-of-plane breathing vibration of the triazine units [48]. From Figure 3c, we can see that the 2D-TC/CN composites with different contents of Ti 3 C 2 almost process the same FTIR spectra, indicating that the composites maintain a stable structure during the compounding process.
The chemical states and compositions of g-C 3 N 4 and 2-TC/CN were explored by XPS. As shown in Figure 3d, the elements of C 1s, N 1s, O 1s, F 1s and Ti 2p, which are clearly presented in g-C 3 N 4 and Ti 3 C 2 , suggest the successful synthesis of the 2D-Ti 3 C 2 /C 3 N 4 composites. The high resolutions of C 1s, N 1s and Ti 2p between g-C 3 N 4 and Ti 3 C 2 are shown in Figure 3e-g. For C 1s high resolutions, the binding energy at 284.7 eV was attributed to C-C on the g-C 3 N 4 and 2-TC/CN surface [49], the peaks at 286.3 eV (2-TC/CN) and 286.2 eV (g-C 3 N 4 ) were attributed to C-NH X [50] and the peaks at 288.1 eV (2-TC/CN) and 287.9 eV (g-C 3 N 4 ) were assigned to C-N=C [51]. For N 1s in Figure 3f, the high resolutions of N 1s in 2-TC/CN can be divided into five peaks, while N 1s in g-C 3 N 4 can be divided into four peaks. The peaks at 398.6 eV (2-TC/CN) and 398.4 eV (g-C 3 N 4 ) were attributed to N-C=N (triazine rings), the peak at 400 eV (both in 2-TC/CN and g-C 3 N 4 ) was assigned to (C) 3 -N, the peaks at 401.1 eV (2-TC/CN) and 401 eV (g-C 3 N 4 ) were attributed to C-NH X [52,53], the peak at 404.3 eV (both in 2-TC/CN and g-C 3 N 4 ) was attributed to charging effects or positive-charge location in heterocycles [50]. Notably, the newly generated peak at 397.6 eV presented in 2-TC/CN was assigned to Ti-N interaction [54]. For Ti 2p in Figure 3g, the Ti 2p spectrum was deconvoluted into four components, including peaks at 454.8 eV, 457.2 eV, 460.7 eV and 463.4 eV. The peaks at 454.8 eV and 460.7 eV were assigned to Ti-C 2p 3/2 and Ti-C 2p 5/2 [55], while peaks at 457.2 eV and 463.4 eV were attributed to Ti-O 2p 3/2 and Ti-O 2p 5/2 [56]. From the highresolution spectra of g-C 3 N 4 and 2-TC/CN in Figure 3e-g, we can see that the binding energy of C-NHX, C-N=C and C-N=C in the 2-TC/CN composite exhibited a positive shift, indicating the low electrondensity of g-C 3 N 4 in 2-TC/CN and the strong interaction between g-C 3 N 4 and Ti 3 C 2 . Through the electron density of the C-NH X and the newly generated Ti-N bond in the 2-TC/CN, it can be deduced that Ti 3 C 2 showed self-assembly formation on the g-C 3 N 4 through Ti-NH X interaction, as shown in Scheme 1.

Optical and Electrochemical Properties of 2D Ti 3 C 2 /g-C 3 N 4 MXene Composite
The light-absorption capacity of the as-prepared samples was evaluated by the UVvis diffuse reflectance spectra. It can be seen from Figure 4a that the absorption edge of the g-C 3 N 4 sample was 472.69 nm. The optical-absorption properties of the TC/CN composites gradually improved optical absorption in the range of visible light (510-551 nm), accompanied by the increased content of Ti 3 C 2 from 1 wt% to 5 wt%. This change may be attributed to the dark slate-gray color of Ti 3 C 2, which possessed full-spectrum absorption. The improved optical absorption can be conducive to converting the light quantum into heat energy and enhancing the photocatalytic reaction, due to the efficient photothermalconversion characteristics [57]. The valence-band values (Ev) of g-C 3 N 4 and 1-TC/CN, 2-TC/CN, 3-TC/CN, and 5-TC/CN were measured using XPS valence-band spectra. As described in Figure 4b, the valence-band values of g-C 3 N 4 and 1-TC/CN, 2-TC/CN, 3-TC/CN, and 5-TC/CN were 2.03 eV, 1.91 eV, 1.71 eV, 1.82 eV and 1.74 eV, respectively. Based on the valence-band values, the lowest value (1.71 eV) represented the optimal valence-band value appearing in the 2-TC/CN sample. Moreover, based on the results in the diffuse-reflectance spectra, the band-gap (Eg) value of g-C 3 N 4 was 2.62 eV, which could be calculated using the Tauc plot method (Equation (5)). Using the same method, the band-gap (Eg) values of 1-TC/CN, 2-TC/CN, 3-TC/CN, 5-TC/CN were 2.30 eV, 2.33 eV, 2.43 eV and 2.23 eV, respectively, and the computation procedure is listed in Table S1. Based on the values of the band gap, we can conclude that the band gap of the composites reduced when compared to that of g-C 3 N 4 , resulting in the excellent absorption edge of the light. Thus, according to the results in Figure 4a, the introduction of Ti 3 C 2 into g-C 3 N 4 not only enhanced the light-absorption capacity, but also improved the oxidant capacity of the composites, and these two changes synergistically improved the photocatalytic performance of the materials. The conduction-band (CB) values of g-C 3 N 4 and 1-TC/CN, 2-TC/CN, 3-TC/CN, and 5-TC/CN can be calculated using Equation (6) and the results are presented in Figure 4c. The expression of Equations (5) and (6) are as follows: where γ (nm) represents the absorption edge of the samples, E g (eV) is the value of the band gap, and E c (eV) is the value of the conduction band. The introduction of Ti 3 C 2 into g-C 3 N 4 greatly impacted the electrical properties of the TC/CN composites, and the intrinsic reason was explored using the room-temperature EPR measurement. From Figure 4d, we can see that a single Lorentzian line is present at a g-value of 2.003 in all samples, which originates from the π-excitation of the triazine units in the g-C 3 N 4 [58]. The intensities of (1-5)-TC/CN were higher than that of g-C 3 N 4, and the intensity order was 1-TC/CN > 2-TC/CN > 5-TC/CN > 0.5-TC/CN > g-C 3 N 4 . The enhanced intensities may be attributed to the new formation of the Ti-N bond, which increased the asymmetry of unpaired electrons around the π bond of the triazine units. The enhanced asymmetry of the unpaired electrons was beneficial in accelerating the electron mobility in the π bond of the triazine units in the 2D-TC/CN composites, which may be due to the delocalization of the valence electrons and the excellent electroconductibility of Ti 3 C 2 MXene.

Photodegradation Performance and Mechanism of Tetracycline
To evaluate the photocatalytic properties of the composites, a typical tetracycline was selected as the target pollutant. In this study, the main target was to thoroughly investigate the intrinsic degradation−mechanism of tetracycline on the composites, so the degradation tests were designed at the best conditions for the target pollutant. As shown in Figure 5a, the residual rate of pure g−C3N4 was 83.92%, while the residual rate showed a marked decline with the addition of Ti3C2 into g−C3N4, and the best photodegradation performance of tetracycline was presented in the composites with a 2 wt% content of Ti3C2, which can be denoted as 2−TC/CN. The residual rate of tetracycline reached 6.05% within 60 min, corresponding to 93.95% of the degradation rate. These photodegradation results powerfully suggest the composites possess an excellent photodegradation performance for the degradation of tetracycline. Moreover, the reusability of the 2−TC/CN after tetracycline removal using bare adsorption and adsorption-photocatalysis synergism modes was compared, as presented in Figure S4. It is evident that the degradation rate of tetracycline shows no apparent loss after five cycles of experiments, indicating that the prepared 2−TC/CN is stable, and difficult to decompose during the process. We also made a detailed comparison of the photocatalytic activity of the degrading tetracycline of the prepared 2−TC/CN with that of the photocatalysts prepared by other groups in the literature. Through an extensive literature research, the intensify and wavelength of incident light,

Photodegradation Performance and Mechanism of Tetracycline
To evaluate the photocatalytic properties of the composites, a typical tetracycline was selected as the target pollutant. In this study, the main target was to thoroughly investigate the intrinsic degradation-mechanism of tetracycline on the composites, so the degradation tests were designed at the best conditions for the target pollutant. As shown in Figure 5a, the residual rate of pure g-C 3 N 4 was 83.92%, while the residual rate showed a marked decline with the addition of Ti 3 C 2 into g-C 3 N 4, and the best photodegradation performance of tetracycline was presented in the composites with a 2 wt% content of Ti 3 C 2, which can be denoted as 2-TC/CN. The residual rate of tetracycline reached 6.05% within 60 min, corresponding to 93.95% of the degradation rate. These photodegradation results powerfully suggest the composites possess an excellent photodegradation performance for the degradation of tetracycline. Moreover, the reusability of the 2-TC/CN after tetracycline removal using bare adsorption and adsorption-photocatalysis synergism modes was compared, as presented in Figure S4. It is evident that the degradation rate of tetracycline shows no apparent loss after five cycles of experiments, indicating that the prepared 2-TC/CN is stable, and difficult to decompose during the process. We also made a detailed comparison of the photocatalytic activity of the degrading tetracycline of the prepared 2-TC/CN with that of the photocatalysts prepared by other groups in the literature. Through an extensive literature research, the intensify and wavelength of incident light, the distance between light source and reactor, and the addition amount of the photocatalyst are usually different, and are listed as follows (shown in Table S2). It is obvious that the prepared 2-TC/CN also exhibits the excellent photocatalytic activity of degrading tetracycline among the prepared g-C 3 N 4 -based photocatalysts. togenerated electrons, photogenerated hole and superoxide radical, were tested in th section, and the results are listed in Figure 5b. Figure 5b suggests that the photodegrad tion rate of tetracycline was reduced from 93.16% to 77.97%, 93.16% to 56.07%, and 93.16 to 32.08%, respectively, after adding IPA (photogenerated−electrons scavenge EDTA−2Na (photogenerated−hole scavenger) and quinone (superoxide−radical scave ger). Based on the results, it can be concluded that superoxide radicals have the great influence on photodegradation performance, followed by photogenerated holes and nally photogenerated electrons. Moreover, these results also corresponded to the fact th the introduction of Ti3C2 into g−C3N4 could enhance the transmission performance of ph togenerated electrons and the electron−trapping ability, and reduce the recombination photogenerated holes, resulting in the least impact on the photodegradation tetrac−cline from the capture of photogenerated electrons. For the purpose of gradually revealing the degradation mechanism of tetracycline by the composite materials, the basic units affecting photocatalytic performance, such as photogenerated electrons, photogenerated hole and superoxide radical, were tested in this section, and the results are listed in Figure 5b. Figure 5b suggests that the photodegradation rate of tetracycline was reduced from 93.16% to 77.97%, 93.16% to 56.07%, and 93.16% to 32.08%, respectively, after adding IPA (photogenerated-electrons scavenger), EDTA-2Na (photogenerated-hole scavenger) and quinone (superoxide-radical scavenger). Based on the results, it can be concluded that superoxide radicals have the greatest influence on photodegradation performance, followed by photogenerated holes and finally photogenerated electrons. Moreover, these results also corresponded to the fact that the introduction of Ti 3 C 2 into g-C 3 N 4 could enhance the transmission performance of photogenerated electrons and the electron-trapping ability, and reduce the recombination of photogenerated holes, resulting in the least impact on the photodegradation of tetrac-cline from the capture of photogenerated electrons.
The EPR technique was performed to throw light on the •O 2 − and •OH radical production of 2D-TC/CN composites. Figure [60]. Thus, in line with previous studies [61,62], DMPO − •O 2 peaks may be formed by the further reduction of •O 2 − following the process: This process can also be verified by the above fact that the effect of photogenerated holes on degradation performance was less than that of the superoxide ions. Based on the results in Figures 4c and 5c,d, the degradation process of tetracycline can be shown in the following mechanism diagram (Scheme 2): omaterials 2022, 12, x 13 o [59]. Figure 5d exhibits the typical characteristic peaks of the DMPO − •O2 radicals in g−C3N4 and TC/CN composites, which suggests the presence of •OH radical in the sa ples. Similarly to the formation of •O2 − radical, the highest number of generated •O radical species also presented in the 2−TC/CN. However, the VB potential (1.71 eV) of 2−TC/CN was not positive enough to form •OH radical through water (2.37 eV) or droxyl (1.99 eV) oxidation [60]. Thus, in line with previous studies [61,62], DMPO − • peaks may be formed by the further reduction of •O2 − following the process: This process can also be verified by the above fact that the effect of photogenera holes on degradation performance was less than that of the superoxide ions. Based on results in Figures 4c and 5c,d, the degradation process of tetracycline can be shown in following mechanism diagram (Scheme 2): Scheme 2. Diagram of photodegradation mechanism of tetracycline.

Transfer Mechanism of Photogenerated Carrier
According to the above results, the photodegradation performance of the composi mainly depends on the generated photogenerated−electrons. Thus, the transfer mec nism of the photogenerated carriers should be clarified. Steady−state photoluminescen (PL) was adopted to reveal the recombination of electrons and holes. As is known, weaker peak−intensity in the PL spectra suggested a lower recombination of electrons a Scheme 2. Diagram of photodegradation mechanism of tetracycline.

Transfer Mechanism of Photogenerated Carrier
According to the above results, the photodegradation performance of the composites mainly depends on the generated photogenerated-electrons. Thus, the transfer mechanism of the photogenerated carriers should be clarified. Steady-state photoluminescence (PL) was adopted to reveal the recombination of electrons and holes. As is known, the weaker peak-intensity in the PL spectra suggested a lower recombination of electrons and holes. Figure 6a exhibits the strongest fluorescence intensity, which appeared on the g-C 3 N 4, and the intensity sharply decreased with the addition of Ti 3 C 2 ; this could be ascribed to the rapid electron-transfer of metallic Ti 3 C 2 to reduce the recombination of the photogenerated carriers. The lowest PL intensity presented in the 2-TC/CN sample, which corresponded to the results in Figure 4. To further investigate the transfer of photogenerated carriers, transient fluorescence-emission spectra were used to test the lifetime of the photogenerated carriers. As illustrated in Figure 6b (the calculation procedure is listed in Table S3), the lifetime (4.38 ns, 2-TC/CN) of the photogenerated carriers of the 2D-TC/CN samples remarkably increased, compared with that (2.56 ns) of the pure g-C 3 N 4 , indicating the sluggish recombination and effective transfer of photogenerated carriers at the interface of the 2D-TC/CN samples, which was a considerable reason for the enhanced photocatalytic activity [63]. Moreover, transient photocurrent response and electrochemical impedance spectra (EIS) were applied to further confirm the excellent electron-transport properties of the composites. As shown in Figure 6c, the photocurrent of the 2-TC/CN sample was approximately six times larger than that of the pristine g-C 3 N 4 , suggesting the better chargetransfer efficiency within the ultrathin 2D-TC/CN composites. In particular, the 2-TC/CN processed the largest photocurrent, which might be due to its better crystallization, higher light-harvesting ability and more intimate heterojunction-structure between the g-C 3 N 4 and Ti 3 C 2 , resulting in more photoinduced electrons and holes produced, and more efficient separation and immigration of photoexcited carriers. The enhanced conductivity of the 2-TC/CN composites was also verified by EIS. Figure 6d shows that the smallest Nyquist plots radius was found in the 2-TC/CN composites, indicating the best conductivity and lowest recombination-probability of the 2-TC/CN, which was in agreement with the fact that the semicircle diameter in the EIS Nyquist plot is relevant to the electron-transfer resistance [64]. interface of the 2D−TC/CN samples, which was a considerable reason for the enhanced photocatalytic activity [63]. Moreover, transient photocurrent response and electrochemical impedance spectra (EIS) were applied to further confirm the excellent electrontransport properties of the composites. As shown in Figure 6c, the photocurrent of the 2−TC/CN sample was approximately six times larger than that of the pristine g−C3N4, suggesting the better charge−transfer efficiency within the ultrathin 2D−TC/CN composites. In particular, the 2−TC/CN processed the largest photocurrent, which might be due to its better crystallization, higher light−harvesting ability and more intimate heterojunctionstructure between the g−C3N4 and Ti3C2, resulting in more photoinduced electrons and holes produced, and more efficient separation and immigration of photoexcited carriers. The enhanced conductivity of the 2−TC/CN composites was also verified by EIS. Figure  6d shows that the smallest Nyquist plots radius was found in the 2−TC/CN composites, indicating the best conductivity and lowest recombination−probability of the 2−TC/CN, which was in agreement with the fact that the semicircle diameter in the EIS Nyquist plot is relevant to the electron−transfer resistance [64].

Conclusions
The preparation process of the 2D Ti 3 C 2 /g-C 3 N 4 composites suggested that selfasembly was an effective method to realize the combination of g-C 3 N 4 and Ti 3 C 2, due to the electrostatic interaction between the electropositive g-C 3 N 4 and the electronegative Ti 3 C 2 . The new Ti-N bond emerging in the 2D Ti 3 C 2 /g-C 3 N 4 composites evidenced the fact that the composite was formed by the bonding of titanium and nitrogen. The experimental study of the photodegradation mechanism showed that the degradation of tetracycline was carried out in two ways: on the one hand, superoxide ions generated by photogeneratedelectron excitation, and on the other hand, hydroxyl radicals, formed by oxidizing water molecules or hydroxyl ions through superoxide ions. The combination of superoxide and hydroxyl radicals in the degradation of tetracycline showed the practical application value of the 2D Ti 3 C 2 /g-C 3 N 4 composites in the field of photodegradation of antibiotics. The generation and transmission of photoelectrons are the decisive factors for the efficient degradation of tetracycline. This study evidenced the fact that that the introduction of 2D-Ti 3 C 2 MXene into g-C 3 N 4 not only enhanced the light-absorption capacity in the visiblelight range, but also improved the transmission property of the photo-induced electrons. The improvement of light-absorption performance is due to the introduction of titanium carbide, which changes the color of the composites, while the improvement of the electrontransport performance can be attributed to the excellent metal conductivity of 2D-Ti 3 C 2, which resulted in more photoinduced electrons and holes produced, and more efficient separation and immigration of photoexcited carriers. Moreover, the results of densityfunctional theory calculations further revealed the transfer of the photogenerated electrons from the 2D-C 3 N 4 to 2D-Ti 3 C 2 , which evidenced the important synergistic effect of the 2D-Ti 3 C 2 in improving the catalytic activity of 2D-C 3 N 4 . In short, 2D Ti 3 C 2 /g-C 3 N 4 composites possess great application prospects for the photodegradation of tetracycline and even the antibiotics field, based on the results of the photodegradation experiment and degradation-mechanism analysis. Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano12224015/s1, Figure S1. SEM image of Ti 3 AlC 2 MAX phase; Figure S2. The morphology and EDS-maping of the pure sheet MXene (Ti 3 C 2 ); Figure S3. SEM images of 1-TC/CN, 3-TC/CN, 5-TC/CN; Figure S4. The the reusability of 2-TC/CN MXene composite; Figure S5. UV-vis spectra of the pure Mxene (Ti 3 C 2 ); Table S1. Valence band, band gap and conduction band computation procedure of the samples; Table S2. Comparison of the photocatalytic activity of degrading tetracycline of the prepared 2-TC/CN with that of photocatalysts in literature; Table S3. The calculation procedure of the lifetime of photogenerated carriers. References [65][66][67]

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.