MXene-derived C-doped TiO2/Ti3C2 heterojunction as a high-performance visible-light photocatalyst

MXene-derived C-doped TiO2/Ti3C2 heterojunction as a high-performance visible-light photocatalyst | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article MXene-derived C-doped TiO 2 /Ti 3 C 2 heterojunction as a high-performance visible-light photocatalyst Kai Chen, Kang Yan, Qun Xie, Hui Zhu, Xuanke Li, Zhijun Dong, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-1806462/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 7 You are reading this latest preprint version Abstract Fabrication TiO 2 with conductive two-dimensional materials is an effective strategy to improve its photocatalytic activity. Herein, a well-defined carbon doped TiO 2 /Ti 3 C 2 heterojucntion is constructed via in-situ controllable oxidation of Ti 3 C 2 MXene in carbon dioxide. The formed carbon doped TiO 2 nanoparticles as the photocatalyst uniformly disperse on the surface of Ti 3 C 2 MXene and generate electrons and holes under the irradiation. The two-dimensional Ti 3 C 2 MXene, ascribing to its excellent conductivity, acts as the electron transport channels and accelerates the separation of photo-generated electrons and holes. Meanwhile, due to its large specific surface area and good solubility, Ti 3 C 2 MXene may facilitate to enhance the adsorption of pollutant on the photocatalyst as well as the absorption of photocatalyst for visible light. Therefore, the unique properties of TiO 2 and Ti 3 C 2 MXene are integrated in a complementary way and significantly improve the photocatalytic performance of the composites. The proper content of Ti 3 C 2 MXene and TiO 2 in the composite is crucial for enhancing the photocatalytic performance, which can be effectively tuned by varying the oxidation temperature. In this work, C-TiO 2 /Ti 3 C 2 oxidized at 400 o C presents the optimum photocatalytic performance. Ti3C2 MXene TiO2/Ti3C2 Photocatalysis In-situ oxidation Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Introduction MXene is a new type of two-dimensional (2D) transition metal carbide, nitride and carbonitride nanomaterials. It has aroused special attention in the fields of energy storage, adsorption and catalysis due to its fantastic structure and properties, including large specific surface area, rich layer structure, excellent conductivity, tunable optical and electronic property and reversible embedding performance of metal cations[ 1 – 4 ]. Ti 3 C 2 MXene, as the most important member of MXene family is generally obtained by etching the Al atoms from Ti 3 AlC 2 with hydrofluoric acid or a mixture of fluoride and hydrochloric acid, the surface of which is loaded with O, OH or F groups[ 5 – 7 ]. It is worthy to note that its intrinsic characteristics, such as zero band gap and lower photoelectric conversion rate, make bare metallic Ti 3 C 2 hard to be an ideal photocatalyst[ 8 ]. However, Ti 3 C 2 MXene can act as a good candidate for photocatalyst because of its high conductivity, large surface area, hydrophilicity and modifiable bandgap with variable terminating functional groups[ 9 – 12 ]. Photocatalysis technique has always been considered as one of the most promising approaches to relieve the environmental pollution and energy problems[ 13 – 15 ]. The combination of TiO 2 with 2D materials such as graphene can greatly improve the performance of TiO 2 photocatalyst. Zhang et al. [ 16 ] synthesized a series of TiO 2 /graphene composites with different graphene contents by sol method, which greatly ameliorated the photocatalytic activity of TiO 2 . Considering the similar two-dimensional structure of Ti 3 C 2 MXene with graphene, the integration of Ti 3 C 2 MXene and TiO 2 is promising to construct high-performance photocatalyst. Naguib and his co-workers reported TiO 2 –disordered graphitic carbon hybrid structures by flashing oxidation of 2D Ti 3 C 2 in air[ 17 ]. The oxidation processes in CO 2 and pressurized water were also discussed. But the transient flashing oxidation process is difficult to control, and the sample powder may be ignited during the transient oxidation process. The TiO 2 /Ti 3 C 2 composites were also prepared by hydrothermal oxidation of Ti 3 C 2 MXene, which showed higher catalytic activity than pure Ti 3 C 2 and TiO 2 [ 18 , 19 ]. Ahmed et al. oxidized Ti 2 C MXene with hydrogen peroxide solution to grow TiO 2 nanocrystals on the surface of Ti 2 C sheets[ 20 ]. The oxidation was exceedingly rapid and the small product was hard to be collected due to the strong oxidizing ability of hydrogen peroxide solution. Yuan et al. modulated Ti 3 C 2 MXene at high temperatures (above 700 o C) to form C/TiO 2 Hybrids[ 21 ]. The result showed that band gap of C/TiO 2 hybrids was slightly narrower than that of pure TiO 2 . Unfortunately, oxidation at higher temperature will destroy the layered structure of Ti 3 C 2 MXene and result in most of Ti 3 C 2 transferring to amorphous carbon, which undoubtedly will weaken the positive effects of Ti 3 C 2 and reduce the conductivity of the material. Moreover, carbon monoxide gas is generated by oxidation at high temperature, which makes the experiment dangerous. Consequently, controlling the oxidation of Ti 3 C 2 MXene at low temperature in CO 2 atmosphere is an appealing approach to get TiO 2 /Ti 3 C 2 composite materials with variable components, meeting their requirements as photocatalysts by combination the merits of TiO 2 and Ti 3 C 2 . In this paper, we focus on the controlling oxidation of Ti 3 C 2 in CO 2 at relative lower temperature to prepare TiO 2 /Ti 3 C 2 composites. The components and surface microstructure of TiO 2 and Ti 3 C 2 in the composites can be tuned by varying the oxidation temperature. Correspondingly, Ti 3 C 2 MXene can effectively retain and contribute its auspicious effects in photocatalytic reactions. TiO 2 crystals formed by oxidation Ti 3 C 2 MXene can produce photogenerated hole-electron pairs, and the unique two-dimensional layered structure can provide electron transport channels and improve the separating efficiency of electrons and holes. Moreover, carbon doping generated by the transformation of Ti 3 C 2 MXene may play positive roles for enhancing the activity of TiO 2 . The effects of different oxidation temperatures on structure of TiO 2 /Ti 3 C 2 composites and photocatalytic degradation for Rhodamine B solution were systematically investigated. It may lay a foundation for the research of Ti 3 C 2 MXene in the field of photocatalysis and provide a reference for studying other similar materials. Experimental Preparation of Ti 3 C 2 MXene Ti 3 C 2 MXene multilayer materials was prepared by chemical etching of Ti 3 AlC 2 powder (400 mesh) in hydrofluoric acid solution following a typical procedure as previously described[ 22 ]. A total of 1 g Ti 3 AlC 2 powder was gradually added into 20 mL of hydrofluoric acid (HF, 49%), magnetically stirring for 24 h at 30°C. The mixture was then centrifuged, and the sediment was washed several times with deionized water until the pH reached about 6. Then the final product was dried in a vacuum oven at 60 ° C for 24 h to get Ti 3 C 2 MXene powder. Preparation of TiO 2 /Ti 3 C 2 composites 0.2 g of as-prepared Ti 3 C 2 MXene powder was put into corundum crucible and placed in a horizontal tube furnace. After purging by argon gas for 20 minutes, the temperature of the tube furnace was raising to the target temperature at a heating rate of 10 o C min − 1 . Subsequently, CO 2 (gas flow rate of 50 sccm) was used as an oxidant to replace argon gas and the target temperature was kept constant for 1 h, where the target temperatures were 300 o C, 400 o C, 500 o C, 600 o C, and 700 o C, respectively. After oxidation process, argon gas was maintained to cool down the furnace to room temperature. The as-prepared products were labeled as TiO 2 /Ti 3 C 2 - T , where T presented the oxidation temperature. The preparation process of TiO 2 /Ti 3 C 2 composites is illustrated in Fig. 1 . Characterization of materials The phase composition of the material was analyzed by X-ray diffraction (XRD) measurement using a Bruker A X'Pert PRO MPD type target X-ray powder diffractometer. Scanning electron microscopy (SEM) was used to investigate the morphology on Nova 400 NanoSEM with the operating voltage of 30 kV. The microstructure of the materials was characterized by field emission transmission electron microscope (FE-TEM, FEI Tecnai G2 F20 S-TWIN) with an accelerating voltage of 120 kV. Raman spectroscopy was recorded by Raman spectrometer (LabRam HR Evolution) to testify the molecular structure of materials. X-ray photoelectron spectroscopy (XPS) was performed to study the elemental composition and chemical states on ThermoFischer ESCALAB 250XI. ASAP 2020 physical absorber was carried out to measure the specific surface area. UV-visible diffuse reflectance spectroscopy was performed on the Shimadzu UV-2550PC UV-Vis spectrophotometer to determine the light absorption properties of the photocatalyst. The separating efficiency of electrons and holes of the photocatalyst was analyzed by fluorescence spectroscopy with Hitachi F-7000 fluorescence spectrophotometer. Photocatalytic activity measurement In a typical process, 0.1 g photocatalyst was added into 100 mL of Rhodamine B solution (30 mg L − 1 ). Before irradiated under visible light, the solution was stirring in a dark environment for 3 h until the adsorption/desorption equilibrium. A 300 W Xenon lamp with a cut-off filter (λ ≥ 420 nm) was used as the irradiation source. During the photodegradation, 4 mL suspension was collected at a certain interval and centrifuged to remove the residual photocatalyst. Then the solution was analyzed on X3PC UV-Vis spectrophotometer to determine the degradation rate for Rhodamine B. Results And Discussion Phase and morphology analysis of Ti 3 C 2 MXene The XRD patterns of Ti 3 AlC 2 and Ti 3 C 2 MXene are illustrated in Fig. 2 . The sharp diffraction peak at 2 θ ≈ 9.5° belongs to the characteristic diffraction peak of Ti 3 AlC 2 , corresponding to the (002) crystal plane of Ti 3 AlC 2 . The other diffraction peaks at 2 θ ≈ 34.0°, 39.0°, 41.8° and 60.2° are ascribed to the (101), (104), (105) and (110) plane of Ti 3 AlC 2 (JCPDS:00-052-0875), respectively. A weak diffraction peak at 2 θ ≈ 35.9 o corresponds to the (111) crystal plane of TiC (JCPDS: 00-031-1400), which is due to trace amounts of TiC residual during the synthesis of Ti 3 AlC 2 . After hydrofluoric acid etching, the diffraction peaks of Ti 3 AlC 2 disappear, and new diffraction peaks appear at 2 θ ≈ 8.5°, 18.2°, 27.5°, 33.1°, 42.1° and 61.4°, corresponding to the (002), (006), (008), (0010), (0012) and (110) crystal planes of Ti 3 C 2 MXene, respectively. It confirms that Ti 3 AlC 2 has transformed to Ti 3 C 2 MXene after etching in hydrofluoric acid solution. This is due to that the Ti-C bonds in Ti 3 AlC 2 are mainly covalent bonds and ionic bonds; while the Ti-Al and Al-Al bonds are mainly metal bonds, and the strength of which is weaker than that of Ti-C bond. So Ti 3 AlC 2 would be more likely to strip off the Al atoms after being treated with hydrofluoric acid. Moreover, the diffraction peak of Ti 3 AlC 2 at 2 θ ≈ 9.5°shifts to lower angle after the etching, indicating the increasing of the interlayer spacing of the material. Figure 3 is SEM images of Ti 3 AlC 2 and Ti 3 C 2 MXene. As can be seen from Fig. 3 , the as-synthesized Ti 3 AlC 2 shows a typical ternary layered structure of the MAX phase. After treatment with hydrofluoric acid, the morphology of Ti 3 AlC 2 changes significantly. The ternary layered structure of Ti 3 AlC 2 is peeled off and manifests the typical accordion-like morphology with stacked layers and smooth surface. Composition and structure analysis of TiO 2 /Ti 3 C 2 MXene The XRD patterns of TiO 2 /Ti 3 C 2 MXene after oxidation in CO 2 at different temperatures are presented in Fig. 4 . It is obvious that when the oxidation temperature is lower than 700 o C, the diffraction peaks are mainly attributed to Ti 3 C 2 MXene, accompanied by the weak diffraction peak (2 θ ≈ 25.5 o ) of the crystal plane of TiO 2 . This implies the co-existence of Ti 3 C 2 MXene and ultrafine TiO 2 . With the increase of oxidation temperature, the intensities of the diffraction peaks of Ti 3 C 2 MXene gradually decrease, while the intensities of the diffraction peaks (2 θ ≈ 25.5 o , 37.9 o , 38.6, 48.1 o , 54.0 o , 55.1 o , 62.7 o , 68.8 o , 70.4 o and 75.1 o ) of anatase TiO 2 (JCPDS: 01-084-1286) enhance. When the oxidation temperature reaches 700°C, obvious and sharp diffraction peaks ascribed to anatase TiO 2 appear. Meanwhile, the observable diffraction peaks of Ti 3 C 2 MXene (2 θ ≈ 9.0 o , 18.3 o , 27.7 o , 35.2 o , 41.5 o and 60.8 o ) still maintain, confirming the coexistence of Ti 3 C 2 and TiO 2 in the final product. This is different with the case of oxidation in CO 2 under higher temperature where the XRD peaks of MXene disappeared[ 21 ]. Figure 5 shows the SEM images of TiO 2 /Ti 3 C 2 oxidized at different temperatures. Compared with the pristine Ti 3 C 2 MXene (Fig. 3 b), the surface of TiO 2 /Ti 3 C 2 -300 is basically smooth, and a small number of oxidized particles can be observed at the edge of the sheet, indicating its lower oxidation degree. With the increase of oxidation temperature, the surface of the sample becomes rougher and more nanoparticles can be observed. Correspondingly, the samples show visual expansion in sheet structure and increase interlayer spacing. This is attributed to the increasing formation and growth of TiO 2 nanoparticles as the oxidation temperature increases. However, when the temperature reaches 700°C, the layer structure occurs remarkably swollen, and larger nanoparticles embed into the sheets. It implies the oxidation degree is deep, consistent with the XRD results in Fig. 4 . This is due to that the concentration of carbon dioxide diffused into the compact sheets is relatively low at the beginning, so the degree of oxidation of the edge and the surface is higher than that of interlayers. As the oxidation process progresses, the interlayer spacing increases, and carbon dioxide is more likely to spread into the deep sites of the interlayer, causing the oxide particles to gradually grow. The layer spacing is further enlarged and consequently the degree of oxidation is further deepened. Figure 6 presents TEM images of Ti 3 C 2 MXene and TiO 2 /Ti 3 C 2 oxidized at 400°C and 700°C, respectively. It can be clearly seen from Fig. 6 a that Ti 3 C 2 MXene exhibits a typical accordion layered structure, which is consistent with the morphology observed in the SEM image of Fig. 2 b. From the HRTEM image of Ti 3 C 2 MXene (Fig. 6 b), the interlayer spacing of Ti 3 C 2 MXene is about 0.971 nm, which is larger than that of Ti 3 AlC 2 , indicating that the Al element in the Ti 3 AlC 2 phase has been removed. Furthermore, there are obvious nanoparticles at the edge of the TiO 2 /Ti 3 C 2 -400 sheets (Fig. 6 c). The lattice spacing of these nanoparticles measured from the HRTEM image of TiO 2 /Ti 3 C 2 -400 (Fig. 6 d) is 0.35 nm, which is ascribed to the (101) crystal plane of anatase TiO 2 . After the oxidation of Ti 3 C 2 MXene at 700°C (shown in Fig. 6 e), the sheet structure become thinner, and numerous nanoparticles emerge between the sheets. The measured lattice fringe from HRTEM image of TiO 2 /Ti 3 C 2 -700 (Fig. 6 f) is also 0.35 nm which corresponds to the (101) crystal plane of anatase TiO 2 . In addition, amorphous carbon emerges at the edge of the nanoparticles after oxidation at 700°C, indicating that Ti 3 C 2 MXene transforms to TiO 2 nanoparticles and amorphous carbon, which can be further confirmed by Raman spectra. In the HRTEM images of TiO 2 /Ti 3 C 2 -700, we detected that Ti 3 C 2 MXene formed amorphous carbon after oxidation, but because of the low content and low crystallinity of amorphous carbon, it is difficult to identify it through XRD. Raman spectroscopy was used to determine the TiO 2 and amorphous carbon formation after Ti 3 C 2 MXene oxidation at 700 o C. Figure 7 shows the Raman spectra of Ti 3 C 2 MXene oxidized at 400°C and 700°C. The peaks at 144 cm − 1 , 198 cm − 1 , 397 cm − 1 , 513 cm − 1 and 638 cm − 1 are ascribed to the anatase TiO 2 , indicating that Ti 3 C 2 MXene has transformed to anatase TiO 2 during oxidation. TiO 2 /Ti 3 C 2 -700 also presents additional two peaks at 1350 cm − 1 and 1590 cm − 1 , corresponding to the D and G peaks of carbon, respectively. The D peak is derived from the defects of the C atomic lattice and the G peak is due to the sp 2 hybrid in-plane stretching vibration of carbon atoms[ 23 , 24 ]. And the high intensity of D band and the wide bands of D and G indicate the amorphous characteristics of carbon in TiO 2 /Ti 3 C 2 -700. By contrast, weak peaks of anatase TiO 2 appear, suggesting its lower crystallinity. Meanwhile, the D and G band are also hard to be observed. In order to compare the variation of chemical bonds in Ti 3 C 2 MXene during oxidation process, X-ray photoelectron spectroscopy (XPS) was performed. Figure 8 exhibits the XPS spectra of Ti 3 C 2 MXene, TiO 2 /Ti 3 C 2 -400 and TiO 2 /Ti 3 C 2 -700, respectively. As shown in Fig. 8 a, the surface of Ti 3 C 2 MXene obtained by etching with hydrofluoric acid was loaded with abundant oxygen-containing groups and absorbed water. After oxidizing by CO 2 gas, the peak intensity of O-H bond significantly weakened while the peak intensity of the Ti-O bond enhanced (presented in Fig. 8 d and g). This is due to the transformation of Ti 3 C 2 MXene to TiO 2 and the reduction of the terminal surface functional groups during oxidation process. By comparing the Ti 2p spectra, it is observable Ti 2p 3/2 (454.5 eV) and Ti 2p 1/2 (460.3 eV) peaks assigned to Ti-C bond of Ti 3 C 2 gradually transformed to Ti 2p 3/2 (458.9 eV) and Ti 2p 1/2 (465.0 eV) peaks ascribed to Ti-O band of TiO 2 with the increasing of oxidation temperature[ 25 , 26 ]. Meanwhile, as shown in Figures c, f and i, the C-Ti (281.9eV) bond of Ti 3 C 2 MXene gradually faded away after oxidation. It should be noted that both Ti-O and Ti-C bonds can be clearly found in TiO 2 /Ti 3 C 2 -400, confirming coexist of TiO 2 and Ti 3 C 2 MXene. Most interesting, it exhibits abnormal Ti 2p spectra, where the Ti-O bond presented a broaden peak and higher binding energy than pure Ti-O bond. Correspondingly, the peaks ascribed to Ti-C bonds appear at lower binding energy. Therefore, it may be presumed the formation of O-Ti-C bonds, due to that oxygen is more electronegative than carbon, which results in the inhomogeneous distribution of electron cloud in O-Ti-C bond. The higher electronegativity of oxygen causes higher electron density of O-Ti than Ti-C bond, correspondingly slightly increasing the bonding energy of O-Ti bond and decreasing that of Ti-C bond. In term of XPS results, it is confirmed that part of Ti 3 C 2 MXene successfully converts to TiO 2 and carbon doping states simultaneously occur in TiO 2 /Ti 3 C 2 composites. Table 1 Specific surface areas of Ti 3 C 2 MXene and TiO 2 /Ti 3 C 2 oxidized at different temperatures Sample Ti 3 C 2 MXene TiO 2 /Ti 3 C 2 -300 TiO 2 /Ti 3 C 2 -400 TiO 2 /Ti 3 C 2 -500 TiO 2 /Ti 3 C 2 -600 TiO 2 /Ti 3 C 2 -700 S BET m 2 /g 4.9 5.5 6.8 9.6 22.7 43.1 Table 1 shows the specific surface areas of Ti 3 C 2 MXene and its oxidation products at different temperatures. Compared with Ti 3 C 2 MXene, the specific surface area of TiO 2 /Ti 3 C 2 obtained at lower temperature only has a slightly increase, which is mainly due to the formation of TiO 2 nanoparticles. As the oxidation temperature increases, the specific surface areas of the oxidation products are also gradually enhanced. When the oxidation temperature rises to 700°C, the specific surface area reaches 43.1 m 2 g − 1 . This is attributed to the expansion of Ti 3 C 2 MXene sheets and growth of a large amount of TiO 2 nanoparticles. Optical absorption properties of Ti3C 2 MXene and TiO 2 /Ti3C 2 composites Generally, pure TiO 2 is unable to absorb visible light due to its large band gap, which is also the biggest obstacle in photocatalysis. Inversely, it can be seen from Fig. 9 that TiO 2 /Ti 3 C 2 composites show strong absorption in the visible light region, which is due to the excellent absorption performance of Ti 3 C 2 MXene. However, the absorption value of TiO 2 /Ti 3 C 2 -700 in the visible light region remarkably declines. This is ascribed that most of Ti 3 C 2 transforms to TiO 2 and the aggregation of TiO 2 nanoparticles on the surface of Ti 3 C 2 MXene will inhibit the absorption of Ti 3 C 2 MXene for visible light. The fluorescence spectra are mainly generated by the recombination of photo-generated electrons and holes in the semiconductor, and the recombination efficiency of the electron-hole pair has a great influence on the photocatalytic performance. Therefore, fluorescence spectra are carried out to analyze the efficiency of charge carrier capture, transport and separation of samples. Figure 10 is the fluorescence spectra of Ti 3 C 2 MXene, TiO 2 /Ti 3 C 2 oxidized at different temperatures and commercial TiO 2 (P25). It can be clearly seen that the intensity of P25 emission peak is much stronger than that of Ti 3 C 2 MXene and TiO 2 /Ti 3 C 2 composites. The emission peak positions of TiO 2 /Ti 3 C 2 oxidized at different temperatures are nearly invariable, but the intensity gradually enhances with the increase of oxidation temperature. This indicates that the electron-hole pair recombination efficiency of TiO 2 /Ti 3 C 2 is much lower than that of pure TiO 2 , and the electron-hole pair combination efficiency of TiO 2 /Ti 3 C 2 increases with the increase of oxidation temperature. This is because the high-conductivity of Ti 3 C 2 MXene material contributes to effectively separate photo-generated electrons and holes of TiO 2 . However, with the increase of oxidation temperature, the content of Ti 3 C 2 MXene in TiO 2 /Ti 3 C 2 composites gradually decreases, leading to the conductivity decline of the composite and the corresponding increase in recombination efficiency of electrons and holes. Photocatalytic activity and mechanism of TiO 2 /Ti 3 C 2 composites The photocatalytic activities of Ti 3 C 2 MXene and TiO 2 /Ti 3 C 2 were evaluated by degradation for Rhodamine B solution (30 mg L − 1 ) under visible light irradiation. It can be seen from Fig. 11 that Rhodamine B is almost not degraded on original Ti 3 C 2 MXene, P25 and no catalyst. After oxidation in CO 2 , the degradation rates of TiO 2 /Ti 3 C 2 are greatly improve and are greatly higher than that of P25. When the oxidation temperature is 400°C, the photodegradation rate of the TiO 2 /Ti 3 C 2 composite reaches the maximum (~ 94%). However, as the oxidation temperature further increases, the photodegradation rate conversely decrease. The proposal mechanism of TiO 2 /Ti 3 C 2 composite is illustrated in Fig. 12 . In TiO 2 /Ti 3 C 2 system, TiO 2 nanoparticles as the main photocatalyst can produce photo-generated electrons and holes under visible light irradiation. On the other hand, Ti 3 C 2 MXene as the co-catalyst can improve electron transfer ability, promote the electron-hole separation owing to its excellent conductivity. When the oxidation temperature is too low, the amount of TiO 2 generated is less, resulting in insufficient photo-generating electrons and holes. On the contrary, when the temperature is exceedingly high, the content of Ti 3 C 2 MXene in the composites greatly decays, resulting in weakening electron transport efficiency and decrease of photocatalytic activity. As a result, the synergetic effects of Ti 3 C 2 MXene and TiO 2 will be critical to improve the photocatalytic activity. Consequently, the key approach is to adjust the proper contents of Ti 3 C 2 MXene and TiO 2 in Ti 3 C 2 /TiO 2 composites by controlling the oxidation temperature. Moreover, the incorporation of carbon-doping states may also facilitate to enhance the absorption of TiO 2 for visible light and improve the photocatalytic activity. Conclusions In summary, carbon doped TiO 2 /Ti 3 C 2 composite photocatalysts were prepared via one-step oxidation of Ti 3 C 2 MXene at relatively low temperature in CO 2 atmosphere. Rhodamine B solution was used as the target pollutants to evaluate the photocatalytic activity of TiO 2 /Ti 3 C 2 composites under visible light. TiO 2 /Ti 3 C 2 composites exhibit greatly enhanced photocatalytic performance than original Ti 3 C 2 MXene and commercial P25. In addition, the oxidation temperature plays key roles for the composition, structure and photocatalytic activity. Increasing the oxidation temperature will promote the transformation of Ti 3 C 2 MXene to TiO 2 and facilitate to enhance the photo-generated electrons and holes. Inversely, the exceedingly high temperature will result in the declining of Ti 3 C 2 MXene and deteriorate the separation of electrons and holes. Therefore, the proper content of Ti 3 C 2 MXene and TiO 2 will achieve efficient synergistic effects to improve the photocataltic activity of the composites. Especially, TiO 2 /Ti 3 C 2 composite oxidized at 400°C provides the highest degradation rate (~ 94%) for Rhodamine B, which has a significant improvement over Ti 3 C 2 MXene and P25. Declarations Ethics approval and consent to participate ：The research described in this paper did not involve human participants and/or animals. Consent for publication: Not applicable. Availability of data and materials: All data generated or analyzed during this study are included in this published article. Competing interests: The authors (Kai Chen, Kang Yan, Qun Xie, Hui Zhu, Xuanke Li, Zhijun Dong, Guanming Yuan, Jiang Zhang and Ye Cong) declare that they have no conflict of interest. Funding: This work was supported by the National Natural Science Foundation of China (grant number 52002296) and supported by China University of Petroleum Beijing State Key Laboratory of Heavy Oil Processing. Authors' contributions: All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Kang Yan and Kai Chen. The first draft of the manuscript was written by Kai Chen and Kang Yan, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. Acknowledgements: We appreciate the financial supports of the National Natural Science Foundation of China (grant number 52002296) and China University of Petroleum Beijing State Key Laboratory of Heavy Oil Processing. Authors' information: All authors’ affiliation is Hubei Province Key Laboratory of Coal Conversion and New Carbon Materials, Wuhan University of Science and Technology. Chen Kai and Kang Yan are contributed to equal this work. Corresponding authors and E-mail address: Ye Cong ( [email protected] ) and Hui Zhu ( [email protected] ). References D. Wang, G. Yu, Y. Liu, Y. Gogotsi, Y. Wei, J. Mater. Chem. A 5, 24720 (2017) T. Xu, J. Wang, Y. Cong, S. Jiang, Q. Zhang, H. Zhu, Y. Li, X. 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Chachuli, M. N. Hamidon, M. Ertugrul, M. S. Mamat, O. Coban, F. N. Tuzluca, Y. O. Yesilbag, N. H. Shamsudin, J. Alloy. Compd. 882, 160671 (2021) P. Zhang, R. A. Soomro, Z. Guan, N. Sun, B. Xu, Energy Stor. Mater. 29, 163 (2020) Y. Wang, Y. Cui, D. Kong, X. Wang, B. Li, T. Cai, X. Li, J. Xu, Y. Li, Y. Yan, H. Hu, M. Wu, Q. Xue, Z. Yan, L. Zhao, W. Xing, Carbon 180, 118 (2021) Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Major revision 03 Aug, 2022 Reviews received at journal 24 Jul, 2022 Reviewers agreed at journal 10 Jul, 2022 Reviewers invited by journal 09 Jul, 2022 Editor assigned by journal 01 Jul, 2022 Submission checks completed at journal 01 Jul, 2022 First submitted to journal 29 Jun, 2022 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-1806462","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":117893729,"identity":"3812f3d5-9705-4b80-a6bf-62867b432aa5","order_by":0,"name":"Kai Chen","email":"","orcid":"","institution":"Wuhan University of Science and Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Kai","middleName":"","lastName":"Chen","suffix":""},{"id":117893730,"identity":"1be891f0-48ef-4081-8cbf-edb3d687c326","order_by":1,"name":"Kang Yan","email":"","orcid":"","institution":"Wuhan University of Science and Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Kang","middleName":"","lastName":"Yan","suffix":""},{"id":117893731,"identity":"9dee6bbc-88b2-4278-b89b-b56f1bc31afb","order_by":2,"name":"Qun Xie","email":"","orcid":"","institution":"Wuhan University of Science and Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Qun","middleName":"","lastName":"Xie","suffix":""},{"id":117893732,"identity":"068862d8-3c54-4b6e-b6ce-2479145c53c1","order_by":3,"name":"Hui Zhu","email":"","orcid":"","institution":"Wuhan University of Science and Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Hui","middleName":"","lastName":"Zhu","suffix":""},{"id":117893733,"identity":"7224d54c-0878-4674-b135-260d6e207e67","order_by":4,"name":"Xuanke Li","email":"","orcid":"","institution":"Wuhan University of Science and 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Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Jiang","middleName":"","lastName":"Zhang","suffix":""},{"id":117893737,"identity":"9c918614-f419-480d-a17b-4d3ed1be20c2","order_by":8,"name":"Ye Cong","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAzElEQVRIiWNgGAWjYPACGyjNRryWNCBmJk3LYRK08Lf3Hn7NU3E+ccP58wcYPpQdZuCf3YBfi8SZc2nWPGduJ264kczAOOPcYQaJOwfwazGQyDEz5m27nbvhBjMDM2/bYaBIAjFa/p3L3XD+MAPzXyK1GD/mbTiQu+FAMgMzIzFaJM6cMWOccyy5fuaNZIODPefSeSRuENDC395j/OFNjZ0x3/mDDx/8KLOW459BQAsQsEnxQFkHgJgHj0o4YP74gxhlo2AUjIJRMHIBADSCRDb7V65nAAAAAElFTkSuQmCC","orcid":"","institution":"Wuhan University of Science and Technology","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Ye","middleName":"","lastName":"Cong","suffix":""}],"badges":[],"createdAt":"2022-06-29 06:59:12","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-1806462/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-1806462/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":23713988,"identity":"b331fc8a-ee3c-400f-810a-20f9bec88559","added_by":"auto","created_at":"2022-07-11 14:59:25","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":110408,"visible":true,"origin":"","legend":"\u003cp\u003eScheme diagram of preparation of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composites\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-1806462/v1/5e7ec73653da35757372331e.png"},{"id":23713201,"identity":"01282aba-569e-475d-bc05-0f1d22415942","added_by":"auto","created_at":"2022-07-11 14:54:25","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":19214,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e and Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig02.png","url":"https://assets-eu.researchsquare.com/files/rs-1806462/v1/70f634f0dc406a808b1a6393.png"},{"id":23714661,"identity":"3cc31a5e-3bfa-47ec-8936-ba6951dc091d","added_by":"auto","created_at":"2022-07-11 15:04:25","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":288600,"visible":true,"origin":"","legend":"\u003cp\u003eSEM image of (a) Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e and (b) Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-1806462/v1/175db1e576c328a23d7972c9.jpeg"},{"id":23713984,"identity":"c96aa5d7-d4a3-4ab9-b62c-cad905638380","added_by":"auto","created_at":"2022-07-11 14:59:25","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":17200,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e oxidized at different temperatures\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig04.png","url":"https://assets-eu.researchsquare.com/files/rs-1806462/v1/2b0be77493a34099f09694ad.png"},{"id":23713991,"identity":"dc5afe8a-e1b1-49c9-8459-1a8b5759bec3","added_by":"auto","created_at":"2022-07-11 14:59:25","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":177549,"visible":true,"origin":"","legend":"\u003cp\u003eSEM image of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2 \u003c/sub\u003eoxidized at different temperatures (a) 300 °C, (b) 400 °C, (c) 500 °C, (d) 700 °C\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-1806462/v1/ac090bc6517224a4c6d0c7ee.jpeg"},{"id":23713983,"identity":"f46c6638-5878-47eb-93ab-9107fa7b0a2d","added_by":"auto","created_at":"2022-07-11 14:59:25","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":375052,"visible":true,"origin":"","legend":"\u003cp\u003eTEM images of (a, b) Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene, (c, d) TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-400 and (e, f) TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-700\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-1806462/v1/c3881f9eb1cf2f876faffcf6.jpeg"},{"id":23714668,"identity":"ee086a5a-f5c7-4f97-8811-1fcb9cdc3430","added_by":"auto","created_at":"2022-07-11 15:04:25","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":14731,"visible":true,"origin":"","legend":"\u003cp\u003eRaman spectra of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-400 and TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-700\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig07.png","url":"https://assets-eu.researchsquare.com/files/rs-1806462/v1/c4078af06d6d890989f75242.png"},{"id":23713210,"identity":"2602f97b-2f70-4222-9b55-7e7dffe49ecb","added_by":"auto","created_at":"2022-07-11 14:54:25","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":35165,"visible":true,"origin":"","legend":"\u003cp\u003eThe XPS spectra of (a,b,c) Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2 \u003c/sub\u003eMXene, (d,e,f) TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-400 and (g,h,i)TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-700\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig08.png","url":"https://assets-eu.researchsquare.com/files/rs-1806462/v1/fb647f70ecf5c87b4d16f898.png"},{"id":23713211,"identity":"94039830-4044-4ef0-a8ad-585e17847cf0","added_by":"auto","created_at":"2022-07-11 14:54:25","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":9220,"visible":true,"origin":"","legend":"\u003cp\u003eUV-vis diffuse reflection spectra and of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene, TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-400 and TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-700\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig09.png","url":"https://assets-eu.researchsquare.com/files/rs-1806462/v1/ab8cc0da5397635c38b310a3.png"},{"id":23713205,"identity":"84d3ecbc-e25c-45b1-b55f-3c5321baab4c","added_by":"auto","created_at":"2022-07-11 14:54:25","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":27618,"visible":true,"origin":"","legend":"\u003cp\u003eFluorescence spectra of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e oxidized at different temperatures, (a) Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene, (b)300 \u003csup\u003eo\u003c/sup\u003eC, (c) 400 \u003csup\u003eo\u003c/sup\u003eC, (d) 500 \u003csup\u003eo\u003c/sup\u003eC, (e) 600 \u003csup\u003eo\u003c/sup\u003eC, (f) 700 \u003csup\u003eo\u003c/sup\u003eC\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig10.png","url":"https://assets-eu.researchsquare.com/files/rs-1806462/v1/5cd53dc4007c2a59eb3d90ee.png"},{"id":23713212,"identity":"4cc537e5-a2e0-41bd-8c42-0840c0b0d330","added_by":"auto","created_at":"2022-07-11 14:54:25","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":9543,"visible":true,"origin":"","legend":"\u003cp\u003ePhotodegradation rate for Rhodamine B on (a) Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene, (b) TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-300, (c) TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-400, (d) TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-500, (e) TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-600, (f) TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-700, (g) P25, and (h) no catalyst\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig11.png","url":"https://assets-eu.researchsquare.com/files/rs-1806462/v1/8f1ce8704a7c1e6399ae4f7b.png"},{"id":23713208,"identity":"19758196-913a-41ca-b901-75df2cf3bda2","added_by":"auto","created_at":"2022-07-11 14:54:25","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":288580,"visible":true,"origin":"","legend":"\u003cp\u003eMechanism diagram of photocatalytic degradation for Rhodamine B solution on TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composite\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig12.png","url":"https://assets-eu.researchsquare.com/files/rs-1806462/v1/1fa952bdac8573219288b5b7.png"},{"id":23714669,"identity":"cbd25f9b-b0d0-4b8a-b5f3-d0c8a1c80331","added_by":"auto","created_at":"2022-07-11 15:04:28","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":700794,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-1806462/v1/bcc01cea-4b0f-4b49-8555-67dc82028b5b.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003eMXene-derived C-doped TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e heterojunction as a high-performance visible-light photocatalyst\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eMXene is a new type of two-dimensional (2D) transition metal carbide, nitride and carbonitride nanomaterials. It has aroused special attention in the fields of energy storage, adsorption and catalysis due to its fantastic structure and properties, including large specific surface area, rich layer structure, excellent conductivity, tunable optical and electronic property and reversible embedding performance of metal cations[\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene, as the most important member of MXene family is generally obtained by etching the Al atoms from Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e with hydrofluoric acid or a mixture of fluoride and hydrochloric acid, the surface of which is loaded with O, OH or F groups[\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. It is worthy to note that its intrinsic characteristics, such as zero band gap and lower photoelectric conversion rate, make bare metallic Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e hard to be an ideal photocatalyst[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. However, Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene can act as a good candidate for photocatalyst because of its high conductivity, large surface area, hydrophilicity and modifiable bandgap with variable terminating functional groups[\u003cspan additionalcitationids=\"CR10 CR11\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003ePhotocatalysis technique has always been considered as one of the most promising approaches to relieve the environmental pollution and energy problems[\u003cspan additionalcitationids=\"CR14\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. The combination of TiO\u003csub\u003e2\u003c/sub\u003e with 2D materials such as graphene can greatly improve the performance of TiO\u003csub\u003e2\u003c/sub\u003e photocatalyst. Zhang et al. [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] synthesized a series of TiO\u003csub\u003e2\u003c/sub\u003e/graphene composites with different graphene contents by sol method, which greatly ameliorated the photocatalytic activity of TiO\u003csub\u003e2\u003c/sub\u003e. Considering the similar two-dimensional structure of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene with graphene, the integration of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and TiO\u003csub\u003e2\u003c/sub\u003e is promising to construct high-performance photocatalyst. Naguib and his co-workers reported TiO\u003csub\u003e2\u003c/sub\u003e\u0026ndash;disordered graphitic carbon hybrid structures by flashing oxidation of 2D Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e in air[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. The oxidation processes in CO\u003csub\u003e2\u003c/sub\u003e and pressurized water were also discussed. But the transient flashing oxidation process is difficult to control, and the sample powder may be ignited during the transient oxidation process. The TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composites were also prepared by hydrothermal oxidation of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene, which showed higher catalytic activity than pure Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e and TiO\u003csub\u003e2\u003c/sub\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Ahmed et al. oxidized Ti\u003csub\u003e2\u003c/sub\u003eC MXene with hydrogen peroxide solution to grow TiO\u003csub\u003e2\u003c/sub\u003e nanocrystals on the surface of Ti\u003csub\u003e2\u003c/sub\u003eC sheets[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. The oxidation was exceedingly rapid and the small product was hard to be collected due to the strong oxidizing ability of hydrogen peroxide solution. Yuan et al. modulated Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene at high temperatures (above 700 \u003csup\u003eo\u003c/sup\u003eC) to form C/TiO\u003csub\u003e2\u003c/sub\u003e Hybrids[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. The result showed that band gap of C/TiO\u003csub\u003e2\u003c/sub\u003e hybrids was slightly narrower than that of pure TiO\u003csub\u003e2\u003c/sub\u003e. Unfortunately, oxidation at higher temperature will destroy the layered structure of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and result in most of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e transferring to amorphous carbon, which undoubtedly will weaken the positive effects of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e and reduce the conductivity of the material. Moreover, carbon monoxide gas is generated by oxidation at high temperature, which makes the experiment dangerous. Consequently, controlling the oxidation of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene at low temperature in CO\u003csub\u003e2\u003c/sub\u003e atmosphere is an appealing approach to get TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composite materials with variable components, meeting their requirements as photocatalysts by combination the merits of TiO\u003csub\u003e2\u003c/sub\u003e and Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003eIn this paper, we focus on the controlling oxidation of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e in CO\u003csub\u003e2\u003c/sub\u003e at relative lower temperature to prepare TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composites. The components and surface microstructure of TiO\u003csub\u003e2\u003c/sub\u003e and Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e in the composites can be tuned by varying the oxidation temperature. Correspondingly, Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene can effectively retain and contribute its auspicious effects in photocatalytic reactions. TiO\u003csub\u003e2\u003c/sub\u003e crystals formed by oxidation Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene can produce photogenerated hole-electron pairs, and the unique two-dimensional layered structure can provide electron transport channels and improve the separating efficiency of electrons and holes. Moreover, carbon doping generated by the transformation of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene may play positive roles for enhancing the activity of TiO\u003csub\u003e2\u003c/sub\u003e. The effects of different oxidation temperatures on structure of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composites and photocatalytic degradation for Rhodamine B solution were systematically investigated. It may lay a foundation for the research of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene in the field of photocatalysis and provide a reference for studying other similar materials.\u003c/p\u003e"},{"header":"Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003ePreparation of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene\u003c/h2\u003e \u003cp\u003eTi\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene multilayer materials was prepared by chemical etching of Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e powder (400 mesh) in hydrofluoric acid solution following a typical procedure as previously described[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. A total of 1 g Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e powder was gradually added into 20 mL of hydrofluoric acid (HF, 49%), magnetically stirring for 24 h at 30\u0026deg;C. The mixture was then centrifuged, and the sediment was washed several times with deionized water until the pH reached about 6. Then the final product was dried in a vacuum oven at 60 \u0026deg; C for 24 h to get Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene powder.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003ePreparation of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composites\u003c/h2\u003e \u003cp\u003e0.2 g of as-prepared Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene powder was put into corundum crucible and placed in a horizontal tube furnace. After purging by argon gas for 20 minutes, the temperature of the tube furnace was raising to the target temperature at a heating rate of 10 \u003csup\u003eo\u003c/sup\u003eC min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Subsequently, CO\u003csub\u003e2\u003c/sub\u003e (gas flow rate of 50 sccm) was used as an oxidant to replace argon gas and the target temperature was kept constant for 1 h, where the target temperatures were 300 \u003csup\u003eo\u003c/sup\u003eC, 400 \u003csup\u003eo\u003c/sup\u003eC, 500 \u003csup\u003eo\u003c/sup\u003eC, 600 \u003csup\u003eo\u003c/sup\u003eC, and 700 \u003csup\u003eo\u003c/sup\u003eC, respectively. After oxidation process, argon gas was maintained to cool down the furnace to room temperature. The as-prepared products were labeled as TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-\u003cem\u003eT\u003c/em\u003e, where \u003cem\u003eT\u003c/em\u003e presented the oxidation temperature. The preparation process of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composites is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eCharacterization of materials\u003c/h2\u003e \u003cp\u003eThe phase composition of the material was analyzed by X-ray diffraction (XRD) measurement using a Bruker A X'Pert PRO MPD type target X-ray powder diffractometer. Scanning electron microscopy (SEM) was used to investigate the morphology on Nova 400 NanoSEM with the operating voltage of 30 kV. The microstructure of the materials was characterized by field emission transmission electron microscope (FE-TEM, FEI Tecnai G2 F20 S-TWIN) with an accelerating voltage of 120 kV. Raman spectroscopy was recorded by Raman spectrometer (LabRam HR Evolution) to testify the molecular structure of materials. X-ray photoelectron spectroscopy (XPS) was performed to study the elemental composition and chemical states on ThermoFischer ESCALAB 250XI. ASAP 2020 physical absorber was carried out to measure the specific surface area. UV-visible diffuse reflectance spectroscopy was performed on the Shimadzu UV-2550PC UV-Vis spectrophotometer to determine the light absorption properties of the photocatalyst. The separating efficiency of electrons and holes of the photocatalyst was analyzed by fluorescence spectroscopy with Hitachi F-7000 fluorescence spectrophotometer.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003ePhotocatalytic activity measurement\u003c/h2\u003e \u003cp\u003eIn a typical process, 0.1 g photocatalyst was added into 100 mL of Rhodamine B solution (30 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). Before irradiated under visible light, the solution was stirring in a dark environment for 3 h until the adsorption/desorption equilibrium. A 300 W Xenon lamp with a cut-off filter (λ\u0026thinsp;\u0026ge;\u0026thinsp;420 nm) was used as the irradiation source. During the photodegradation, 4 mL suspension was collected at a certain interval and centrifuged to remove the residual photocatalyst. Then the solution was analyzed on X3PC UV-Vis spectrophotometer to determine the degradation rate for Rhodamine B.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results And Discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003ePhase and morphology analysis of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe XRD patterns of Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e and Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene are illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The sharp diffraction peak at 2\u003cem\u003eθ\u003c/em\u003e\u0026thinsp;\u0026asymp;\u0026thinsp;9.5\u0026deg; belongs to the characteristic diffraction peak of Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e, corresponding to the (002) crystal plane of Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e. The other diffraction peaks at 2\u003cem\u003eθ\u003c/em\u003e\u0026thinsp;\u0026asymp;\u0026thinsp;34.0\u0026deg;, 39.0\u0026deg;, 41.8\u0026deg; and 60.2\u0026deg; are ascribed to the (101), (104), (105) and (110) plane of Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e (JCPDS:00-052-0875), respectively. A weak diffraction peak at 2\u003cem\u003eθ\u003c/em\u003e\u0026thinsp;\u0026asymp;\u0026thinsp;35.9\u003csup\u003eo\u003c/sup\u003e corresponds to the (111) crystal plane of TiC (JCPDS: 00-031-1400), which is due to trace amounts of TiC residual during the synthesis of Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e. After hydrofluoric acid etching, the diffraction peaks of Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e disappear, and new diffraction peaks appear at 2\u003cem\u003eθ\u003c/em\u003e\u0026thinsp;\u0026asymp;\u0026thinsp;8.5\u0026deg;, 18.2\u0026deg;, 27.5\u0026deg;, 33.1\u0026deg;, 42.1\u0026deg; and 61.4\u0026deg;, corresponding to the (002), (006), (008), (0010), (0012) and (110) crystal planes of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene, respectively. It confirms that Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e has transformed to Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene after etching in hydrofluoric acid solution. This is due to that the Ti-C bonds in Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e are mainly covalent bonds and ionic bonds; while the Ti-Al and Al-Al bonds are mainly metal bonds, and the strength of which is weaker than that of Ti-C bond. So Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e would be more likely to strip off the Al atoms after being treated with hydrofluoric acid. Moreover, the diffraction peak of Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e at 2\u003cem\u003eθ\u003c/em\u003e\u0026thinsp;\u0026asymp;\u0026thinsp;9.5\u0026deg;shifts to lower angle after the etching, indicating the increasing of the interlayer spacing of the material.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e is SEM images of Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e and Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene. As can be seen from Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the as-synthesized Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e shows a typical ternary layered structure of the MAX phase. After treatment with hydrofluoric acid, the morphology of Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e changes significantly. The ternary layered structure of Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e is peeled off and manifests the typical accordion-like morphology with stacked layers and smooth surface.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003eComposition and structure analysis of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe XRD patterns of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene after oxidation in CO\u003csub\u003e2\u003c/sub\u003e at different temperatures are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. It is obvious that when the oxidation temperature is lower than 700 \u003csup\u003eo\u003c/sup\u003eC, the diffraction peaks are mainly attributed to Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene, accompanied by the weak diffraction peak (2\u003cem\u003eθ\u003c/em\u003e\u0026thinsp;\u0026asymp;\u0026thinsp;25.5\u003csup\u003eo\u003c/sup\u003e) of the crystal plane of TiO\u003csub\u003e2\u003c/sub\u003e. This implies the co-existence of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and ultrafine TiO\u003csub\u003e2\u003c/sub\u003e. With the increase of oxidation temperature, the intensities of the diffraction peaks of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene gradually decrease, while the intensities of the diffraction peaks (2\u003cem\u003eθ\u003c/em\u003e\u0026thinsp;\u0026asymp;\u0026thinsp;25.5\u003csup\u003eo\u003c/sup\u003e, 37.9\u003csup\u003eo\u003c/sup\u003e, 38.6, 48.1\u003csup\u003eo\u003c/sup\u003e, 54.0\u003csup\u003eo\u003c/sup\u003e, 55.1\u003csup\u003eo\u003c/sup\u003e, 62.7\u003csup\u003eo\u003c/sup\u003e, 68.8\u003csup\u003eo\u003c/sup\u003e, 70.4\u003csup\u003eo\u003c/sup\u003e and 75.1\u003csup\u003eo\u003c/sup\u003e) of anatase TiO\u003csub\u003e2\u003c/sub\u003e (JCPDS: 01-084-1286) enhance. When the oxidation temperature reaches 700\u0026deg;C, obvious and sharp diffraction peaks ascribed to anatase TiO\u003csub\u003e2\u003c/sub\u003e appear. Meanwhile, the observable diffraction peaks of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene (2\u003cem\u003eθ\u003c/em\u003e\u0026thinsp;\u0026asymp;\u0026thinsp;9.0\u003csup\u003eo\u003c/sup\u003e, 18.3\u003csup\u003eo\u003c/sup\u003e, 27.7\u003csup\u003eo\u003c/sup\u003e, 35.2\u003csup\u003eo\u003c/sup\u003e, 41.5\u003csup\u003eo\u003c/sup\u003e and 60.8\u003csup\u003eo\u003c/sup\u003e) still maintain, confirming the coexistence of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e and TiO\u003csub\u003e2\u003c/sub\u003e in the final product. This is different with the case of oxidation in CO\u003csub\u003e2\u003c/sub\u003e under higher temperature where the XRD peaks of MXene disappeared[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the SEM images of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e oxidized at different temperatures. Compared with the pristine Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb), the surface of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-300 is basically smooth, and a small number of oxidized particles can be observed at the edge of the sheet, indicating its lower oxidation degree. With the increase of oxidation temperature, the surface of the sample becomes rougher and more nanoparticles can be observed. Correspondingly, the samples show visual expansion in sheet structure and increase interlayer spacing. This is attributed to the increasing formation and growth of TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles as the oxidation temperature increases. However, when the temperature reaches 700\u0026deg;C, the layer structure occurs remarkably swollen, and larger nanoparticles embed into the sheets. It implies the oxidation degree is deep, consistent with the XRD results in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. This is due to that the concentration of carbon dioxide diffused into the compact sheets is relatively low at the beginning, so the degree of oxidation of the edge and the surface is higher than that of interlayers. As the oxidation process progresses, the interlayer spacing increases, and carbon dioxide is more likely to spread into the deep sites of the interlayer, causing the oxide particles to gradually grow. The layer spacing is further enlarged and consequently the degree of oxidation is further deepened.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e presents TEM images of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e oxidized at 400\u0026deg;C and 700\u0026deg;C, respectively. It can be clearly seen from Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea that Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene exhibits a typical accordion layered structure, which is consistent with the morphology observed in the SEM image of Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb. From the HRTEM image of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb), the interlayer spacing of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene is about 0.971 nm, which is larger than that of Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e, indicating that the Al element in the Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e phase has been removed. Furthermore, there are obvious nanoparticles at the edge of the TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-400 sheets (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec). The lattice spacing of these nanoparticles measured from the HRTEM image of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-400 (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed) is 0.35 nm, which is ascribed to the (101) crystal plane of anatase TiO\u003csub\u003e2\u003c/sub\u003e. After the oxidation of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene at 700\u0026deg;C (shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee), the sheet structure become thinner, and numerous nanoparticles emerge between the sheets. The measured lattice fringe from HRTEM image of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-700 (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef) is also 0.35 nm which corresponds to the (101) crystal plane of anatase TiO\u003csub\u003e2\u003c/sub\u003e. In addition, amorphous carbon emerges at the edge of the nanoparticles after oxidation at 700\u0026deg;C, indicating that Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene transforms to TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles and amorphous carbon, which can be further confirmed by Raman spectra.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn the HRTEM images of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-700, we detected that Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene formed amorphous carbon after oxidation, but because of the low content and low crystallinity of amorphous carbon, it is difficult to identify it through XRD. Raman spectroscopy was used to determine the TiO\u003csub\u003e2\u003c/sub\u003e and amorphous carbon formation after Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene oxidation at 700 \u003csup\u003eo\u003c/sup\u003eC. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows the Raman spectra of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene oxidized at 400\u0026deg;C and 700\u0026deg;C. The peaks at 144 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 198 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 397 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 513 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 638 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are ascribed to the anatase TiO\u003csub\u003e2\u003c/sub\u003e, indicating that Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene has transformed to anatase TiO\u003csub\u003e2\u003c/sub\u003e during oxidation. TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-700 also presents additional two peaks at 1350 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1590 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, corresponding to the D and G peaks of carbon, respectively. The D peak is derived from the defects of the C atomic lattice and the G peak is due to the sp\u003csup\u003e2\u003c/sup\u003e hybrid in-plane stretching vibration of carbon atoms[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. And the high intensity of D band and the wide bands of D and G indicate the amorphous characteristics of carbon in TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-700. By contrast, weak peaks of anatase TiO\u003csub\u003e2\u003c/sub\u003e appear, suggesting its lower crystallinity. Meanwhile, the D and G band are also hard to be observed.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn order to compare the variation of chemical bonds in Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene during oxidation process, X-ray photoelectron spectroscopy (XPS) was performed. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e exhibits the XPS spectra of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene, TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-400 and TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-700, respectively. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea, the surface of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene obtained by etching with hydrofluoric acid was loaded with abundant oxygen-containing groups and absorbed water. After oxidizing by CO\u003csub\u003e2\u003c/sub\u003e gas, the peak intensity of O-H bond significantly weakened while the peak intensity of the Ti-O bond enhanced (presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ed and g). This is due to the transformation of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene to TiO\u003csub\u003e2\u003c/sub\u003e and the reduction of the terminal surface functional groups during oxidation process. By comparing the Ti 2p spectra, it is observable Ti 2p\u003csub\u003e3/2\u003c/sub\u003e(454.5 eV) and Ti 2p\u003csub\u003e1/2\u003c/sub\u003e (460.3 eV) peaks assigned to Ti-C bond of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e gradually transformed to Ti 2p\u003csub\u003e3/2\u003c/sub\u003e(458.9 eV) and Ti 2p\u003csub\u003e1/2\u003c/sub\u003e(465.0 eV) peaks ascribed to Ti-O band of TiO\u003csub\u003e2\u003c/sub\u003e with the increasing of oxidation temperature[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Meanwhile, as shown in Figures c, f and i, the C-Ti (281.9eV) bond of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene gradually faded away after oxidation. It should be noted that both Ti-O and Ti-C bonds can be clearly found in TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-400, confirming coexist of TiO\u003csub\u003e2\u003c/sub\u003e and Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene. Most interesting, it exhibits abnormal Ti 2p spectra, where the Ti-O bond presented a broaden peak and higher binding energy than pure Ti-O bond. Correspondingly, the peaks ascribed to Ti-C bonds appear at lower binding energy. Therefore, it may be presumed the formation of O-Ti-C bonds, due to that oxygen is more electronegative than carbon, which results in the inhomogeneous distribution of electron cloud in O-Ti-C bond. The higher electronegativity of oxygen causes higher electron density of O-Ti than Ti-C bond, correspondingly slightly increasing the bonding energy of O-Ti bond and decreasing that of Ti-C bond. In term of XPS results, it is confirmed that part of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene successfully converts to TiO\u003csub\u003e2\u003c/sub\u003e and carbon doping states simultaneously occur in TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composites.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSpecific surface areas of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e oxidized at different temperatures\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTi\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-300\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-400\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eTiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-500\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eTiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-600\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eTiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-700\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eS\u003c/em\u003e\u003csub\u003eBET\u003c/sub\u003e m\u003csup\u003e2\u003c/sup\u003e/g\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e9.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e22.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e43.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the specific surface areas of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and its oxidation products at different temperatures. Compared with Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene, the specific surface area of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e obtained at lower temperature only has a slightly increase, which is mainly due to the formation of TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles. As the oxidation temperature increases, the specific surface areas of the oxidation products are also gradually enhanced. When the oxidation temperature rises to 700\u0026deg;C, the specific surface area reaches 43.1 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. This is attributed to the expansion of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene sheets and growth of a large amount of TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eOptical absorption properties of Ti3C\u003csub\u003e2\u003c/sub\u003e MXene and TiO\u003csub\u003e2\u003c/sub\u003e/Ti3C\u003csub\u003e2\u003c/sub\u003e composites\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eGenerally, pure TiO\u003csub\u003e2\u003c/sub\u003e is unable to absorb visible light due to its large band gap, which is also the biggest obstacle in photocatalysis. Inversely, it can be seen from Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e that TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composites show strong absorption in the visible light region, which is due to the excellent absorption performance of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene. However, the absorption value of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e-700 in the visible light region remarkably declines. This is ascribed that most of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e transforms to TiO\u003csub\u003e2\u003c/sub\u003e and the aggregation of TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles on the surface of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene will inhibit the absorption of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene for visible light.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe fluorescence spectra are mainly generated by the recombination of photo-generated electrons and holes in the semiconductor, and the recombination efficiency of the electron-hole pair has a great influence on the photocatalytic performance. Therefore, fluorescence spectra are carried out to analyze the efficiency of charge carrier capture, transport and separation of samples. Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e is the fluorescence spectra of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene, TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e oxidized at different temperatures and commercial TiO\u003csub\u003e2\u003c/sub\u003e (P25). It can be clearly seen that the intensity of P25 emission peak is much stronger than that of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composites. The emission peak positions of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e oxidized at different temperatures are nearly invariable, but the intensity gradually enhances with the increase of oxidation temperature. This indicates that the electron-hole pair recombination efficiency of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e is much lower than that of pure TiO\u003csub\u003e2\u003c/sub\u003e, and the electron-hole pair combination efficiency of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e increases with the increase of oxidation temperature. This is because the high-conductivity of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene material contributes to effectively separate photo-generated electrons and holes of TiO\u003csub\u003e2\u003c/sub\u003e. However, with the increase of oxidation temperature, the content of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene in TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composites gradually decreases, leading to the conductivity decline of the composite and the corresponding increase in recombination efficiency of electrons and holes.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003ePhotocatalytic activity and mechanism of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composites\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe photocatalytic activities of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e were evaluated by degradation for Rhodamine B solution (30 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) under visible light irradiation. It can be seen from Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e that Rhodamine B is almost not degraded on original Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene, P25 and no catalyst. After oxidation in CO\u003csub\u003e2\u003c/sub\u003e, the degradation rates of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e are greatly improve and are greatly higher than that of P25. When the oxidation temperature is 400\u0026deg;C, the photodegradation rate of the TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composite reaches the maximum (~\u0026thinsp;94%). However, as the oxidation temperature further increases, the photodegradation rate conversely decrease.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe proposal mechanism of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composite is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e. In TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e system, TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles as the main photocatalyst can produce photo-generated electrons and holes under visible light irradiation. On the other hand, Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene as the co-catalyst can improve electron transfer ability, promote the electron-hole separation owing to its excellent conductivity. When the oxidation temperature is too low, the amount of TiO\u003csub\u003e2\u003c/sub\u003e generated is less, resulting in insufficient photo-generating electrons and holes. On the contrary, when the temperature is exceedingly high, the content of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene in the composites greatly decays, resulting in weakening electron transport efficiency and decrease of photocatalytic activity. As a result, the synergetic effects of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and TiO\u003csub\u003e2\u003c/sub\u003e will be critical to improve the photocatalytic activity. Consequently, the key approach is to adjust the proper contents of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and TiO\u003csub\u003e2\u003c/sub\u003e in Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e composites by controlling the oxidation temperature. Moreover, the incorporation of carbon-doping states may also facilitate to enhance the absorption of TiO\u003csub\u003e2\u003c/sub\u003e for visible light and improve the photocatalytic activity.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn summary, carbon doped TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composite photocatalysts were prepared via one-step oxidation of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene at relatively low temperature in CO\u003csub\u003e2\u003c/sub\u003e atmosphere. Rhodamine B solution was used as the target pollutants to evaluate the photocatalytic activity of TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composites under visible light. TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composites exhibit greatly enhanced photocatalytic performance than original Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and commercial P25. In addition, the oxidation temperature plays key roles for the composition, structure and photocatalytic activity. Increasing the oxidation temperature will promote the transformation of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene to TiO\u003csub\u003e2\u003c/sub\u003e and facilitate to enhance the photo-generated electrons and holes. Inversely, the exceedingly high temperature will result in the declining of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and deteriorate the separation of electrons and holes. Therefore, the proper content of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and TiO\u003csub\u003e2\u003c/sub\u003e will achieve efficient synergistic effects to improve the photocataltic activity of the composites. Especially, TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e composite oxidized at 400\u0026deg;C provides the highest degradation rate (~\u0026thinsp;94%) for Rhodamine B, which has a significant improvement over Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and P25.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e：The research described in this paper did not involve human participants and/or animals.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication:\u003c/strong\u003e Not applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials:\u003c/strong\u003e All data generated or analyzed during this study are included in this published article.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests:\u003c/strong\u003e The authors (Kai Chen, Kang Yan, Qun Xie, Hui Zhu, Xuanke Li, Zhijun Dong, Guanming Yuan, Jiang Zhang and Ye Cong) declare that they have no conflict of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding:\u003c/strong\u003e This work was supported by the National Natural Science Foundation of China (grant number 52002296) and supported by China University of Petroleum Beijing State Key Laboratory of Heavy Oil Processing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026apos; contributions:\u0026nbsp;\u003c/strong\u003eAll authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Kang Yan and Kai Chen. The first draft of the manuscript was written by Kai Chen and Kang Yan, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements:\u0026nbsp;\u003c/strong\u003eWe appreciate the financial supports of the National Natural Science Foundation of China (grant number 52002296) and China University of Petroleum Beijing State Key Laboratory of Heavy Oil Processing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026apos; information:\u0026nbsp;\u003c/strong\u003eAll authors\u0026rsquo; affiliation is Hubei Province Key Laboratory of Coal Conversion and New Carbon Materials, Wuhan University of Science and Technology. Chen Kai and Kang Yan are contributed to equal this work. Corresponding authors and E-mail address: Ye Cong ([email protected]) and Hui Zhu ([email protected]).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eD. Wang, G. Yu, Y. Liu, Y. Gogotsi, Y. Wei, J. Mater. Chem. A 5, 24720 (2017)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eT. Xu, J. Wang, Y. Cong, S. Jiang, Q. Zhang, H. Zhu, Y. Li, X. Li, Chinese Chem. Lett. 31, 1022 (2020)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eY. Guan, S. Jiang, Y. Cong, J. Wang, Z. Dong, Q. Zhang, G. Yuan, Y. Li, X. Li, 2D Mater. 7, 025010 (2020)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eY. Guan, R. Zhao, Y. Cong, K. Chen, J. Wu, H. Zhu, Z. Dong, Q. Zhang, G. Yuan, Y. Li, J. Zhang, X. Li, Chem. Eng. J. 433, 133582 (2022)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eK Chen, Y. Guan, Y. Cong, H. Zhu, K. Li, J. Wu, Z. Dong, G.Yuan, A. Zhang, X. Li, J. Alloy. 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Xing, Carbon 180, 118 (2021)\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"research-on-chemical-intermediates","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"rint","sideBox":"Learn more about [Research on Chemical Intermediates](http://link.springer.com/journal/11164)","snPcode":"11164","submissionUrl":"https://submission.nature.com/new-submission/11164/3","title":"Research on Chemical Intermediates","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Ti3C2 MXene, TiO2/Ti3C2, Photocatalysis, In-situ oxidation","lastPublishedDoi":"10.21203/rs.3.rs-1806462/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-1806462/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eFabrication TiO\u003csub\u003e2\u003c/sub\u003e with conductive two-dimensional materials is an effective strategy to improve its photocatalytic activity. Herein, a well-defined carbon doped TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e heterojucntion is constructed via in-situ controllable oxidation of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene in carbon dioxide. The formed carbon doped TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles as the photocatalyst uniformly disperse on the surface of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and generate electrons and holes under the irradiation. The two-dimensional Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene, ascribing to its excellent conductivity, acts as the electron transport channels and accelerates the separation of photo-generated electrons and holes. Meanwhile, due to its large specific surface area and good solubility, Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene may facilitate to enhance the adsorption of pollutant on the photocatalyst as well as the absorption of photocatalyst for visible light. Therefore, the unique properties of TiO\u003csub\u003e2\u003c/sub\u003e and Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene are integrated in a complementary way and significantly improve the photocatalytic performance of the composites. The proper content of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e MXene and TiO\u003csub\u003e2\u003c/sub\u003e in the composite is crucial for enhancing the photocatalytic performance, which can be effectively tuned by varying the oxidation temperature. In this work, C-TiO\u003csub\u003e2\u003c/sub\u003e/Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e oxidized at 400 \u003csup\u003eo\u003c/sup\u003eC presents the optimum photocatalytic performance.\u003c/p\u003e","manuscriptTitle":"MXene-derived C-doped TiO2/Ti3C2 heterojunction as a high-performance visible-light photocatalyst","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2022-07-11 14:54:23","doi":"10.21203/rs.3.rs-1806462/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major revision","date":"2022-08-03T13:28:03+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2022-07-24T09:29:42+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"d53a0046-14ad-4d6c-88d6-43df4dd71a8b","date":"2022-07-10T04:57:25+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2022-07-09T14:16:27+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2022-07-01T14:07:54+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2022-07-01T14:07:53+00:00","index":"","fulltext":""},{"type":"submitted","content":"Research on Chemical Intermediates","date":"2022-06-29T06:46:41+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"research-on-chemical-intermediates","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"rint","sideBox":"Learn more about [Research on Chemical Intermediates](http://link.springer.com/journal/11164)","snPcode":"11164","submissionUrl":"https://submission.nature.com/new-submission/11164/3","title":"Research on Chemical Intermediates","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"a181188f-2b07-438a-b4b1-43284cd4c6b3","owner":[],"postedDate":"July 11th, 2022","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2022-09-01T10:44:26+00:00","versionOfRecord":[],"versionCreatedAt":"2022-07-11 14:54:23","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-1806462","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-1806462","identity":"rs-1806462","version":["v1"]},"buildId":"cBFmMYwuxLRRLfASyISRj","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}