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The titanium dioxide (TiO2) photocatalyst has been widely studied as a safe, inexpensive, and environmentally friendly photocatalyst. However, it does not exhibit high photocatalytic performances since it is only active for ultraviolet (UV) light due to its wide forbidden band gap and because the photogenerated electron–hole pairs easily recombine [ 1–4 ]. Previous studies have demonstrated that the compositing of TiO2 and silver phosphate (Ag3PO4) via the construction of heterojunctions can expand its light absorption range and suppress the recombination of photogenerated electron–hole pairs, thereby improving the photocatalytic performance. However, such a performance improvement is limited, and the composite catalysts are costly due to the expensive Ag3PO4 [ 5–8 ]. Ti3C2 MXene is a new two-dimensional material with good hydrophilicity and excellent electrical conductivity. It can be used as a co-catalyst with other semiconductors to form Schottky junctions, which can not only effectively expand the light absorption range but also promote the rapid separation of photogenerated electron–hole pairs, thus enhancing the photocatalytic performances of the composite photocatalysts [ 9–12 ].

To obtain high-performance and low-cost photocatalysts, TiO2(B)/Ti3C2 MXene/Ag3PO4 ternary composite photocatalysts were constructed in this study. First, a TiO2(B)/Ti3C2 MXene composite was constructed by electrostatic self-assembly of strip-like TiO2(B) and lamellar Ti3C2 MXene, and then Ag3PO4 quantum dots were formed on the surface of the TiO2(B)/Ti3C2 MXene by in situ growth, thereby forming the TiO2/Ti3C2 MXene/Ag3PO4 ternary composite photocatalyst. The composite photocatalyst exhibited excellent photocatalytic performances, which first increased and then decreased with the increase in the content of the co-catalyst Ti3C2 MXene. The composite photocatalyst reached the best performance when the content of Ti3C2 MXene was 20% of the mass of TiO2(B). Compared with the common TiO2 phases (anatase, brookite, and rutile), bronze TiO2 has a good electrical conductivity and a loose, porous structure, which facilitates the rapid migration of carriers and the formation of reaction sites in heterojunction photocatalysts, ultimately enhancing the performances of composite photocatalysts. Therefore, bronze TiO2 was used in this study as a component of the composite photocatalyst [ 13–16 ].

As shown in Figure 1a, the standard PDF cards of Ag3PO4, TiO2(B), and Ti3AlC2 were JCPDS#06-0505, JCPDS#35-0088, and JCPDS#015-3266, respectively. The diffraction pattern of the Ti3C2 MXene demonstrated that, after HF etching, the strongest diffraction peak of Ti3AlC2 was at 2θ = 28.99◦, corresponding to the (104) crystal plane disappearing, while the diffraction peaks at 2θ = 9.52◦ and 19.12◦ corresponding to the (002) and (004) crystal planes, respectively, were enhanced, and their positions shifted towards smaller diffraction angles. Referring to the literature [ 17–20 ], the characteristics of the above diffraction peaks indicated that the Al atomic layer of Ti3AlC2 was successfully removed via etching to obtain the Ti3C2 MXene in this study. A comparison of the XRD (X-ray diffraction) patterns of the TiO2(B)/20%Ti3C2, Ti3C2 MXene, and TiO2(B) revealed that the diffraction peaks of the TiO2(B)/20%Ti3C2 spectrum were mainly those of TiO2(B), which appeared weakly and were only observed at 2θ = 9.14◦, corresponding to the (002) crystal plane of the Ti3C2 MXene. In addition, compared with that of the TiO2(B), the half-peak width of the diffraction peak of the TiO2(B)/20%Ti3C2 spectrum increased, which may have been due to the low content of the Ti3C2 MXene and the relatively weak diffraction peaks. Figure 1b shows that the diffraction pattern of the ternary composite was mainly composed of diffraction peaks corresponding to Ag3PO4, while those corresponding to the Ti3C2 MXene and TiO2(B) were not observed, which was mainly because Ag3PO4 had good crystallinity and exhibited strong diffraction compared to the Ti3C2 MXene and TiO2(B). 1 μm 1 μm (a) ples. According to the L3angmuir–Hinshelwood model, the degradation reaction can be considered to be a first-order reaction at low RhB concentrations, and thus, the fitting of the degradation kinetics can be simplified to a linear case. The degradation reaction in this experiment satisfied the apparent first-order reaction rate equation ln)(=C−/kC·t0,)w=h−erke·t, where k is the apparent first-order reaction rate constant. Thus, ln(C/C0) is a function of the irradiation time t. The k values of the TiO2(B), TiO2(B)/20%Ti3C2 MXene, Ag3PO4 and P25 were low, with values of 0.008, 0.011, 0.085 and 0.0−4 min−1, respectively, while the k value corresponding to TTA(TiO2(B)/Ti3C2 MXene/Ag3PO4) was significantly higher, which gradually increased from kTTA-2.5 = 0−.182 min−1 to the maximum kTTA-20 =−0.345 min−1 and then decreased to kTTA-30−= 0.241 min−1.

Figure 4 shows the UV-Vis absorption spectra of the samples. Compared with TiO2(B), the visible light absorption capacity was clearly improved after forming a composite of TiO2(B) and Ti3C2 MXene, and the light absorption performance of sample TTA-20 obtained by further incorporating Ag3PO4 was even higher. The band gaps of the Ti3C2, TiO2(B), TiO2(B)/20%Ti3C2 MXene, and TTA-20 were calculated to be 1.67, 3.14, 3.23, and 3.32 eV, respectively, based on the Tauc plot. These results confirmed the band gap reduction by the formation of the composite, which is an important reason for the improved light absorption performances of the composite materials.

As shown in Figure 5, the fluorescence emission spectra as well as the transient photocurrent response and electrochemical impedance Nyquist plots of the samples were obtained to analyze the influence of the formation of composites on the recombination and migration of photogenerated electron–hole pairs in the samples. Figure 5a shows that the fluorescence of the TiO2(B)/20%Ti3C2 MXene was significantly weaker than that of the TiO2(B), and the fluorescence of TTA-20 (TiO2(B)/20%Ti3C2 MXene/Ag3PO4) was the weakest. This was because the highly conductive Ti3C2 MXene was composited with TiO2(B) to form Schottky junctions on the contact surface, which caused the fast migration of photogenerated electrons to the Ti3C2 MXene, achieving rapid separation of photogenerated electron–hole pairs and suppressing the recombination. By forming the Ti3C2 MXene and TiO2(B) composite, Ag3PO4 quantum dots were introduced on the surfaces of the Ti3C2 MXene and TiO2(B) by in situ self-growth, the Ag3PO4 and Ti3C2 MXene formed Schottky junctions, and the Ag3PO4 and TiO2(B) formed heterojunctions. These junctions created a synergistic effect with the Schottky junctions formed by the Ti3C2 MXene and TiO2(B), further promoting the rapid migration and separation of photogenerated electrons and greatly suppressing the recombination of photogenerated electron–hole pairs. Figure 5b shows the transient photocurrent responses of the samples. The photocurrents of the samples were enhanced by compositing. The photocurrent of TTA-20 was the strongest, followed by those of the TiO2(B)/20%Ti3C2 MXene, Ag3PO4, and TiO2(B), in that order. The photocurrent enhancement pattern was consistent with the fluorescence weakening pattern, and the photocurrent enhancement also indicated that the recombination of photogenerated electron–hole pairs was effectively suppressed. Thus, Figure 5b further confirms the results and analysis of Figure 5a. Figure 5c shows the Nyquist plot of the electrochemical impedances of the samples. In general, the smaller the curvature radius of the curve is, the greater the electron mobility of the sample is. The mobility was significantly enhanced by forming the composite. The electron mobility of TTA-20 was the largest, followed by that of the TiO2(B)/20%Ti3C2 MXene, and both were larger than those of the TiO2(B) and Ag3PO4.

3.1. Preparation of Materials and Samples

The following reagents were used: Rhodamine B (RhB, C28H31CIN2O3) (Solarbio, Beijing, China); titanium aluminum carbide (Ti3AlC2) (Xianfeng Nanomaterials, Nanjing, China); disodium hydrogen phosphate 12-water (Na2 HˑPO4·12H2O, Jinshan Chemical Reagent, Chengdu, China); silver nitrate (AgNO3) (Sinopharm Chemical Reagent, Shanghai, China); and sodium hydroxide (NaOH), hydrofluoric acid (HF), and nano-TiO2 (Aladdin Industrial, Shanghai, China). The deionized (DI) water used for the experiments was prepared in our laboratory. All reagents were of analytical grade and were used without further purification.

Ti3C2 MXene was prepared from Ti3AlC2 by HF etching as follows. An appropriate amount of 40% (mass fraction) Ti3AlC2 was slowly added to the HF acid solution at a ratio of 1 g/20 mL and stirred at room temperature for 24 h. The prepared product was separated by centrifugation at 4000 rpm and rinsed repeatedly with DI water to near neutrality.

The preparation of TiO2(B) was as follows. A 10 M NaOH aqueous solution was prepared. Then, raw TiO2 was added, stirred well, poured into a hydrothermal reactor, and heated up to 200 ◦C in a blast drying oven and held for 24 h. Next, the reaction product was removed, rinsed with a large amount of DI water to near neutrality, soaked with 0.1 M dilute hydrochloric acid for 72 h, rinsed again with a large amount of DI water to near neutrality, dried at 60 ◦C for 10 h, and finally calcined in a muffle furnace at 500 ◦C for 2 h to obtain TiO2(B).

The TiO2(B)/Ti3C2 MXene/Ag3PO4 ternary composite was prepared as follows. About 100 mg of TiO2(B) and 20 mg of Ti3C2 MXene were added to DI water and sonicated for 4 h to allow the formation of a composite of TiO2(B) and Ti3C2 MXene by electrostatic self-assembly, which was followed by drying in a vacuum drying oven to obtain the TiO2(B)/20%Ti3C2 MXene composite. Na2HPO4·12H2O and TiO2(B)/20%Ti3C2 MXene were added to DI water at a molar ratio of 0.8:1 (note that the number of moles of TiO2(B)/20%Ti3C2 MXene was the sum of the numbers of moles of the two components) and mixed well. A solution was prepared using AgNO3 in a molar ratio of 3:1 with Na2HPO4·12H2O, and then the solution was slowly added dropwise using a syringe to the mixture of Na2HPO4·12H2O and TiO2(B)/20%Ti3C2 MXene. The mixture was stirred continuously to form Ag3PO4 quantum dots on the surface of the TiO2(B)/20%Ti3C2 MXene via in situ self-growth. The product was rinsed with DI water and dried in the vacuum drying oven to obtain TiO2(B)/20%Ti3C2 MXene/Ag3PO4, which was labeled as TTA-20. The preparation flow chart is shown in Figure 6. Using the same method, TiO2(B)/2.5%Ti3C2 MXene/Ag3PO4, TiO2(B)/5%Ti3C2 MXene/Ag3PO4, TiO2(B)/10%Ti3C2 MXene/Ag3PO4, and TiO2(B)/30%Ti3C2 MXene/Ag3PO4 were prepared and labeled as TTA-2.5, TTA-5, TTA-10, and TTA-30, respectively. 3.2. Analytical Tests

An X-ray powder diffraction (XRD, BRUCKER D8 ADVANCE, Bruker, Bremen, Germany) was used to test and analyze the phases of the samples using the following test parameters: Cu target, wide-angle diffraction, scan range of 5◦–730◦, scan rate of 10◦/min, test wavelength of 1.5406 Å, tube voltage of 40 kV, and tube current of 40 mA. Fieldemission scanning electron microscopy (FE-SEM, Gemini SEM 300, Zeiss, Oberkochen, Germany) was used to characterize the morphologies and structures of the samples at an operating voltage of 3 kV. Field-emission transmission electron microscopy (FEI Talos F200X, FEI, Hillsboro, OR, USA), with a maximum magnification of 1.1 million times and a limit resolution of 0.12 nm, was used to observe and analyze the morphologies and structures of the samples at an operating voltage of 200 kV. An ultraviolet–3visible (UVVis) spectrophotometer (UV-3600, Shimadzu company, Kyoto, Japan), with a test range of 200–25030 nm, was used to test the photocatalytic degradation of RhB and obtain the UV-Vis absorption spectra of the samples. An electrochemical workstation (CHI-760E, Shanghai Chenhua Instrument, Shanghai, China) was used to test the photocurrents and impedances of the samples.

Two liters of 20 mg/L RhB solution were prepared, and 100 mL was taken for each experiment, added to 20 mg of the sample, and sonicated for 30 min. Then, this mixture was placed under a light source with an intensity of 600 W/m2 (at the liquid surface) in a solar simulator (Solar-500Q, Newbit, Beijing, China) to start the photocatalytic degradation experiment, which lasted for 60 min. During the experiment, 5 mL of liquid was taken from the mixture every 10 min and subjected to centrifugation at 10,000 rpm for 10 min. After centrifugation, the supernatant was taken and its absorbance An was measured by the UV-Vis spectrophotometer. There was a direct proportionality relation between the solution concentration Cn and An, Cn/C0 = An/A0, where C0 was the concentration of 20 mg/L of the RhB solution, and A0 was the absorbance of the 20 mg/L RhB solution.

In this study, the TiO2(B)/Ti3C2 MXene Schottky junction composite was first prepared by electrostatic self-assembly, and then Ag3PO4 quantum dots were generated on the surface of the TiO2(B)/Ti3C2 MXene by in situ self-growth, thereby forming Ag3PO4/TiO2(B) heterojunctions and Ag3PO4/Ti3C2 MXene Schottky junctions. Throug（h the successful construction and synergy of these functional junctions (As shown in Figure 7), the light absorption capacity of the composite photocatalyst wa3s greatly enhanced, and the recombination of photogenerated electron–hole pairs was effectively suppressed, thus substantially improving the photocatalytic degradation performance of the composite photocatalyst. It was further found that the photocatalytic performance of the TiO2(B)/Ti3C2 MXene/Ag3PO4 composite increased with the increase in the Ti3C2 MXene content, and it reached the maximum when the Ti3C2 MXene content increased to 20%, beyond which the photocatalytic performance of the composite decreased as the Ti3C2 MXene content continued to increase.

Author Contributions: Conceptualization, methodology, S.W. and Y.C.; software, Y.L. (Yong Li); validation, Y.L. (Yong Li) and Y.L. (Yanfang Liu); formal analysis, Y.L. (Yong Li) and M.Z.; investigation,4Y.L. (Yong Li), Y.L. (Yanfang Liu) and M.Z.; resources, S.W. and Y.C4.; data curation, S.W. writing—original draft preparation, Y.L. (Yong Li), M.Z. and X.L.; writing—review and editing, Y.L. (Yong Li), M.Z. and Y.L. (Yanfang Liu); visualization, Q.Z. (Qinghua Zhao) and Q.Z. (Qianyu Zhou); project administration, Y.L. (Yong Li).; funding acquisition, Y.L. (Yong Li) and S.W. All authors have read and agreed to the published version of the manuscript.

Funding: The National Natural Science Foundation of China (Grant No.: 52062045), Central Government Funds for Local Scientific and Technological Development (Grant No.: XZ202101YD0019C and XZ202201YD0026C), Natural Science Foundation of Tibet Autonomous Region (Grant No. XZ202101ZR0121G), and Everest Discipline Construction Project of Tibet University (Grant No.: ZF22004002).

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

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