Carbon Nanofiber Membranes Loaded with MXene@g-C3N4: Preparation and Photocatalytic Property

In this study, a Ti3C2 MXene@g-C3N4 composite powder (TM-CN) was prepared by the ultrasonic self-assembly method and then loaded onto a carbon nanofiber membrane by the self-assembly properties of MXene for the treatment of organic pollutants in wastewater. The characterization of the TM-CN and the C-TM-CN was conducted via X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectrometer (FTIR) to ascertain the successful modification. The organic dye degradation experiments demonstrated that introducing an appropriate amount of Ti3C2 MXene resulted in the complete degradation of RhB within 60 min, three times the photocatalytic efficiency of a pure g-C3N4. The C-TM-CN exhibited the stable and outstanding photocatalytic degradation of the RhB solution over a wide range of pH values, indicating the characteristics of the photodegradation of organic pollutants in a wide range of aqueous environments. Furthermore, the results of the cyclic degradation experiments demonstrated that the C-TM-CN composite film maintained a degradation efficiency of over 85% after five cycles, thereby confirming a notable improvement in its cyclic stability. Consequently, the C-TM-CN composite film exhibits excellent photocatalytic performance and is readily recyclable, making it an auspicious eco-friendly material in water environment remediation.


Introduction
With the development of human society since the first industrial revolution in the 18th century, global production and manufacturing industries have rapidly developed, leading to environmental damage and pollution to human production and living.So far, millions of organic chemicals, particularly organic dyes, drugs, personal care products, and pesticides, have been discovered, synthesized, and produced.Once organic dyes enter into water, it is no longer safe to drink, and it is sometimes difficult to completely treat such contaminated water [1,2].Organic dyes are common pollutants in the textile dyeing, plastics, and paper industries, and many of them are aromatic compounds with potential carcinogenic and mutagenic effects [3].Various wastewater treatment techniques have been developed [4][5][6][7][8][9][10].However, compared with other wastewater treatment technologies, photocatalytic technology does not require secondary pollutants after treatment.Moreover, photocatalytic technology does not require secondary pollutants after treatment and also has a series of advantages such as high efficiency, stability, and easy operation, which has a broad application prospect in energy development and environmental remediation [11].However, most semiconductor photocatalytic materials typically exist in the form of powders, but powder photocatalysts are often prone to aggregation in water, which reduces their light absorption capacity and utilization efficiency.In addition, powder materials are challenging to recycle and reuse, leading to high application costs, thereby limiting the practical application of semiconductor photocatalysts [12,13].The photocatalytic membrane developed by combining photocatalysts and membrane materials is one of the most effective methods for solving photocatalysts' recycling utility.
Since Liu and Cohen predicted, in 1989, that carbon nitride compounds had the potential to become superhard materials [14], research on carbon nitride materials has become increasingly in depth.Graphitic carbon nitride (g-C 3 N 4 , CN) is known as a potential photocatalyst because of its outstanding thermal and chemical stability, nontoxicity, easy preparation, and low cost [15][16][17][18][19][20].Kroke [21], through density functional theory (DFT) calculations, found that graphitic carbon nitride (g-C 3 N 4 ) built by tri-s-triazine units was considered to be the most stable homolog in the environment among various isomers.As a photoelectric catalyst, g-C 3 N 4 can separate holes and electrons, and its 2.7 eV bandgap enables it to absorb sunlight.However, the photocatalytic activity of g-C 3 N 4 has intrinsic shortcomings, such as high charge recombination efficiency, inhibiting conductivity, and insufficient surface area caused by self-aggregation [22][23][24][25][26][27].Researchers have explored many strategies to overcome these intrinsic deficiencies, such as creating new forms, chemical blending, auxiliary catalysis loading, modifying defects, controlling crystal structures, and constructing heterojunctions.
In recent years, two-dimensional transition metal carbides, nitrides, and carbonitrides have developed rapidly and gradually become a research hotspot for photocatalytic material development.MXene, as a new emerging two-dimensional material, has excellent properties such as a typical layered structure, tunable element composition, excellent conductivity, persistent structure stability, adjustable surface functional groups, element abundance, and large surface area, making them a powerful robust candidate material for various applications in photocatalysis [28,29].As a photocatalyst carrier or auxiliary catalyst, or in combination with other two-dimensional semiconductor materials to improve photocatalytic efficiency, MXene materials can promote light-induced carrier separation by various actions, thereby enhancing the photocatalytic activity of other photocatalysts, such as strongly supporting the homogeneous growth and fine dispersion of other photocatalysts, effectively replacing precious metals, and enhancing the adsorption of reactants.In 2011, the earliest MXene materials discovered were Ti 3 C 2 Tx, followed by the preparation of about 20 different unusual MXene materials.Ti 3 C 2 was a widely studied MXene material with excellent properties.As a two-dimensional (2D) transition metal carbide, Ti 3 C 2 MXene has a conductivity of 4600 ± 1100 S cm −1 with highly anisotropic carrier mobility, which facilitates the material for electron trapping and transfer [30][31][32].Secondly, the Ti 3 C 2 surface has end groups that can provide active sites and also provide strong interfacial solid contact between the co-catalyst and the photocatalyst [33][34][35][36].In addition, its 2D lamellar structure has a large specific surface area [33] and a suitable figure of merit.These properties are very promising for the co-construction of heterojunction-structured photocatalytic materials with other semiconductor materials, which can improve the photocatalytic performance of single catalysts [37].Under visible light irradiation, the electron-hole in a photocatalytic reaction undergoes a jump to form an electron (e − ) and hole (h + ) in its conduction and valence bands, respectively [35].Appropriately designed heterojunction photocatalysts were shown to be capable of higher photocatalytic activity because of the spatial separation of electron-hole pairs generated by photoluminescence [36].Since electrons have reducing power and holes have oxidizing power, they can react with OH-, O 2 , and H 2 O adsorbed on the surface of the material to form radicals with strong oxidizing activity, such as -OH, -O 2 − , -OOH, and other radicals with strong oxidizing activity [37].Combining MXene, an excellent metal conductive material, with g-C 3 N 4 can effectively provide shorter electron transport channels and accelerate the rapid movement of electrons [38].
To begin with, an ultrasonic self-assembly measure was applied to form TM-CN in this study, and TM-CN was loaded over carbon fibers via the Ti 3 C 2 MXene self-assembly films.The organic dye degradation test indicates that an appropriate amount of Ti 3 C 2 MXene can significantly improve the photocatalytic activity of g-C 3 N 4 .After calculations, g-C 3 N 4 and Mxene have a staggered energy band structure, promoting the rapid separation of photogenerated electron-hole pairs, reducing the degree of electron-hole complexation, and improving the photocatalytic degradation of pollutants.Surface-loaded TM-CN carbon nanofiber membranes exhibited excellent photocatalytic degradation of organic dyes under various pH conditions.Moreover, powder photocatalysts loaded on carbon nanofiber membranes improved the problem of the difficult recycling of powder materials, and C-TM-CN maintained excellent photocatalytic effects in five photocatalytic cycle degradation experiments.

Experimental Materials
Polyvinyl pyrrolidone, isopropyl alcohol (IPA), and LiF were procured from Shanghai Aladdin Bio-Chem Technology Co., Ltd.(Shanghai, China) N,N-Dimethylformamide (DMF) was obtained from Tianjin Bohua Chemical Reagent Co., Ltd.(Tianjin, China) Carbamide and HCl were obtained from Tianjin Fengchuan Chemical Reagent Co., Ltd.(Tianjin, China) Polyacrylonitrile was purchased from Spectrum Chemical (Shanghai) Co., Ltd.(Shanghai, China) Rhodamine B (RhB), methylene blue (MB), and methyl Orange (MO) were procured from Beijing Solarbio Science & Technology Co., Ltd.(Beijing, China) Benzoquinon, ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), and NaOH were obtained from Tianjin Kermel Chemical Reagent Co., Ltd.(Tianjin, China) Ti 3 AlC 2 was obtained from Ningbo Jinlei Nano Material Technology Co., Ltd.(Ningbo, China) All chemicals used in this study without particular states were of analytical grade and used as received.All solutions were prepared with deionized water.

Preparation of g-C 3 N 4
This study chose a conventional thermal polymerization method to prepare the g-C 3 N 4 powder.Melamine powder was chosen as the raw material, and it was placed in a covered ceramic crucible and heated in a muffle furnace to 550 • C (with a ramp rate of 3 • C min −1 ) and held for 4 h to synthesize bulk g-C 3 N 4 .After this, the bulk g-C 3 N 4 was ground in a mortar and pestle to produce g-C 3 N 4 powder.

Preparation of Ti 3 C 2 MXenes
Firstly, 1 g lithium fluoride (LiF) powders were dispersed in 20 mL of 9 M hydrochloric acid and then mixed for 30 min at room temperature, forming a LiF/HCl mixture.Afterwards, 1 g of Ti 3 AlC 2 was added to the LiF/HCl mixture for another 24 h, blending at 40 • C to remove the Al etch.The final suspension was repeatedly rinsed with deionized water and underwent centrifugation at 4000 rpm for five minutes until the pH was 6.At last, the black swelling clay-like sediment mounted in an ice salt bath underwent consecutive ultrasonic treatment for 2 h and then centrifugation for 1 h.The supernatant from the final centrifugation was used to ensure that Ti 3 C 2 T x had separated (See Figure 1).

Preparation of TM-CN
To begin with, 10 mg g-C 3 N 4 was dispersed in 100 mL of deionized water and then ultrasonic processed for 2 h, after which 100 µg of the resulting solution was removed and mixed with 10 mL of deionized water for another 15 min of ultrasonic treatment to attain an even distribution.Ti 3 C 2 solution was slowly dripped into the g-C 3 N 4 suspension, and the mixture was ultrasonically processed for 4 h, forming TM-CN.

Preparation of TM-CN
To begin with, 10 mg g-C3N4 was dispersed in 100 mL of deionized water and then ultrasonic processed for 2 h, after which 100 µg of the resulting solution was removed and mixed with 10 mL of deionized water for another 15 min of ultrasonic treatment to attain an even distribution.Ti3C2 solution was slowly dripped into the g-C3N4 suspension, and the mixture was ultrasonically processed for 4 h, forming TM-CN.

Preparation of TM-CN-Loaded Carbon Fibers
To begin with, 1 mg of TM-CN (powders) was dispersed in 4 mL of deionized water, after which the carbon nanofiber membrane was soaked in a 15 µg mL −1 Ti3C2 solution for fifteen minutes.The membrane was removed and baked in an oven until the surface became viscous.Next, the well-distributed TM-CN suspension was dripped over the surface of the composite membranes.The impregnated composite membranes were baked in an oven for 1 h, forming the C-TM-CN (See Figure 2).

Preparation of TM-CN-Loaded Carbon Fibers
To begin with, 1 mg of TM-CN (powders) was dispersed in 4 mL of deionized water, after which the carbon nanofiber membrane was soaked in a 15 µg mL −1 Ti 3 C 2 solution for fifteen minutes.The membrane was removed and baked in an oven until the surface became viscous.Next, the well-distributed TM-CN suspension was dripped over the surface of the composite membranes.The impregnated composite membranes were baked in an oven for 1 h, forming the C-TM-CN (See Figure 2).

Preparation of TM-CN
To begin with, 10 mg g-C3N4 was dispersed in 100 mL of deionized water and then ultrasonic processed for 2 h, after which 100 µg of the resulting solution was removed and mixed with 10 mL of deionized water for another 15 min of ultrasonic treatment to attain an even distribution.Ti3C2 solution was slowly dripped into the g-C3N4 suspension, and the mixture was ultrasonically processed for 4 h, forming TM-CN.

Preparation of TM-CN-Loaded Carbon Fibers
To begin with, 1 mg of TM-CN (powders) was dispersed in 4 mL of deionized water, after which the carbon nanofiber membrane was soaked in a 15 µg mL −1 Ti3C2 solution for fifteen minutes.The membrane was removed and baked in an oven until the surface became viscous.Next, the well-distributed TM-CN suspension was dripped over the surface of the composite membranes.The impregnated composite membranes were baked in an oven for 1 h, forming the C-TM-CN (See Figure 2).

Characterization
A scanning electron microscope (HITACHI S4800, Japan Hitachi Limited, Tokyo, Japan) was used to study the micromorphology, size, and composition based on SEM images.The XRD measurement (D8 Discover, Bruker company, Bremen, Germany) was to observe the crystal structure, while XPS (D8 Discover, Bruker company, Germany) was used to measure the constituent elements and valence state.Moreover, a UV-visible spectrometer (UV2600, Ltd. echcomp Science and Technology, Shanghai, China) was used to measure the photo absorption of catalysts.The electrochemical workstation (CHI 660E, Shanghai Chen Hua Electric Furnace Co., Ltd., Shanghai, China) was utilized to measure the photocurrent response and electrochemical impedance of samples.

Photocatalytic Property Characterizations of C-TM-CN
Rhodamine B (RhB) serves as the target pollutant and, as such, examines the photocatalytic properties of the C-TM-CN and the TM-CN.This measurement involved the main reactive species in the photocatalytic response system.Before the dark reactions, 1 mM of isopropyl alcohol (IPA), 1 mM of potassium iodate (KIO 3 ), and 1 mM of ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) were added to RhB (50 mL, 10 mg L −1 ) to capture •OH, •O 2 − , and h + separately.After the 30 min dark reaction, the adsorption equilibrium between the photocatalyst and the pollutant was attained, followed by the conduction of the photocatalytic reaction.The suspension was removed every other (a certain length of) time, and the ultraviolet-visible spectrophotometer (UV2600 Ltd. echcomp Science and Technology, Shanghai, China) measured the absorbance of RhB, thereby computing the photocatalytic degradation efficiency.Next, the C-TM-CN was the photocatalyst in the photocatalytic cyclic decomposition test.In each cycle, forceps were used to remove the membrane that was to be rinsed with deionized water to remove the residual dye, and samples were then baked in an oven at 60 • C for 2 h.Finally, the bake-dried C-TM-CN was once again added to the RhB solution to repeat the next cycle of the photocatalytic decomposition experiment.

SEM Analysis
The micromorphology of C-TM-CN nanofiber composite membranes was observed using scanning electron microscopy (SEM).Figure 3a

XRD Pattern Analysis and FTIR Spectra Analysis
Figure 4a shows the XRD pattern of Ti3C2, g-C3N4, and the TM-CN.Based on the XRD patterns, carbon fiber nanofiber membranes (C-NM) exhibit a strong diffraction peak that corresponds to the crystal plane (002).After the loading of Ti3C2, the peak (002) shifts toward a higher angle, which indicates that the two molecules are strongly interacted, which in turn affects the overlapped distance of g-C3N4.The findings prove that the heterojunc-

XRD Pattern Analysis and FTIR Spectra Analysis
Figure 4a shows the XRD pattern of Ti 3 C 2 , g-C 3 N 4 , and the TM-CN.Based on the XRD patterns, carbon fiber nanofiber membranes (C-NM) exhibit a strong diffraction peak that corresponds to the crystal plane (002).After the loading of Ti 3 C 2 , the peak (002) shifts toward a higher angle, which indicates that the two molecules are strongly interacted, which in turn affects the overlapped distance of g-C 3 N 4 .The findings prove that the heterojunction is formed between Ti 3 C 2 and g-C 3 N 4 .In addition, there is a strong absorption peak present at 27.7 • according to the X-ray powder diffraction pattern of the C-TM-CN.Meanwhile, C-M-C 3 N 4 nanofiber membranes exhibit diffraction peaks that separately correspond to carbon fiber (22 • ), g-C 3 N 4 (27.4 • ), and Ti 3 C 2 (5.0 • ).In Figure 4a, two prominent diffraction peaks were detected at 13.2 • and 27.6 • , which were indexed to the (100) plane, corresponding to the inplane packing of heptazine units, and the (200) plane, resulting from the characteristic stacking of conjugated aromatics, respectively [39].After combination with the MXene and carbon nanofiber membrane, the characteristic diffraction peaks of TM-CN and C-TM-CN appeared at the same positions, indicating that the typical layered structure of polymeric g-C 3 N 4 was well preserved.In addition, the C-TM-CN showed the characteristic diffraction peaks of graphene-like carbon nitride, which was derived from the MXene.Moreover, the typical peak for the Mxene material revealed the successful preparation of Ti

XPS Analysis
XPS is used to characterize the components of C-TM-CN nanofiber composite membranes.In Figure 5a, the signature peaks of N, C, and Ti in the C-TM-CN are observed, suggesting the presence of the four elements above in the C-TM-CN.As for the C 1s in Figure 5b, 284.8 eV, 288.1 eV, and 281.5 eV appear as three corresponding binding energy peaks, which represent the interaction between sp 2 C and Ti-C from the adsorb carbon species of (C-C) and N-C=N, respectively [48,49].As for the N 1s XPS spectra in Figure 5c, the four binding energy peaks at 397.6 eV, 398.1 eV, and 399.2 eV correspond to N-Ti, C-N=C, and N-C3.A new Ti-N peak indicates the heterostructure between the two components.In addition, the peak at 404.1 eV is attributed to the charging effect of the heterocycle [19].As for the Ti 2p XPS spectra of C-TM-CN in Figure 5d, the peaks of Ti-C 2p3/2, Ti-C 2p1/2, and Ti-O 2p3/2 represent the binding energy at 455.6 eV, 461.1 eV, and 456.6 eV, respectively.In the meanwhile, the binding energy at 458.4 eV is ascribed to Ti-F [20].The preparation of C-TM-CN was confirmed to be complete based on XPS analyses.The presence of peaks at 810 cm −1 can be attributed to the 3-s-Triazine unit characteristic peak of g-C3N4 [18,43].The peaks in the range 1135-1629 cm −1 were ascribed to the stretching vibrations of heterocycles (C-N/C=N).Furthermore, the broad peak at around 3137 cm −1 was assigned to uncondensed terminal amino groups [44][45][46].This indicated that g-C 3 N 4 nanomaterials were successfully synthesized.Moreover, the Fourier transform infrared spectroscopy analysis proves the formation of g-C 3 N 4 over nanofiber composite membranes.As shown in Figure 4b, the firm peaks at 3435 cm −1 and 1624 cm −1 in the FTIR spectrum of MXene were assigned to the stretching vibrations of the OH and C=O groups, the peak at 620 cm −1 belongs to Ti-O group, and the peak at 1432 cm −1 was attributed to O-H group bending [47].As a result, through the self-build membrane capacity of Ti 3 C 2 MXenes, TM-CN powders are successfully loaded over the C-TM-CN nanofiber composite membranes.

XPS Analysis
XPS is used to characterize the components of C-TM-CN nanofiber composite membranes.In Figure 5a, the signature peaks of N, C, and Ti in the C-TM-CN are observed, suggesting the presence of the four elements above in the C-TM-CN.As for the C 1s in Figure 5b, 284.8 eV, 288.1 eV, and 281.5 eV appear as three corresponding binding energy peaks, which represent the interaction between sp 2 C and Ti-C from the adsorb carbon species of (C-C) and N-C=N, respectively [48,49].As for the N 1s XPS spectra in Figure 5c, the four binding energy peaks at 397.6 eV, 398.1 eV, and 399.2 eV correspond to N-Ti, C-N=C, and N-C 3 .A new Ti-N peak indicates the heterostructure between the two components.In addition, the peak at 404.1 eV is attributed to the charging effect of the heterocycle [19].As for the Ti 2p XPS spectra of C-TM-CN in Figure 5d, the peaks of Ti-C 2p 3/2 , Ti-C 2p 1/2 , and Ti-O 2p 3/2 represent the binding energy at 455.6 eV, 461.1 eV, and 456.6 eV, respectively.In the meanwhile, the binding energy at 458.4 eV is ascribed to Ti-F [20].The preparation of C-TM-CN was confirmed to be complete based on XPS analyses.

UV-Vis Diffuse Reflection and PL Photoluminescence
The UV-Vis diffuse reflectance spectra of TM-CN, g-C3N4, Ti3C2, and C-TM-CN were examined for photo absorption, as illustrated in Figure 6a.The g-C3N4 exhibited a powerful absorption spectrum at 460 nm.Similarly, TM-CN exhibited an intense absorption spectrum at 450 nm.Compared to pure g-C3N4, TM-CN has an absorption margin that blue shifts, which can be attributed to the heterostructure caused by Ti3C2 and g-C3N4.In addition, Ti3C2 MXene has a strong light absorption capability, and its solar spectral response ranges from UV to visible [50,51].Thus, the loading of Ti3C2 MXene significantly extends the light absorption range of the C-TM-CN so that it can utilize most of the light

UV-Vis Diffuse Reflection and PL Photoluminescence
The UV-Vis diffuse reflectance spectra of TM-CN, g-C 3 N 4 , Ti 3 C 2 , and C-TM-CN were examined for photo absorption, as illustrated in Figure 6a.The g-C 3 N 4 exhibited a powerful absorption spectrum at 460 nm.Similarly, TM-CN exhibited an intense absorption spectrum at 450 nm.Compared to pure g-C 3 N 4 , TM-CN has an absorption margin that blue shifts, which can be attributed to the heterostructure caused by Ti 3 C 2 and g-C 3 N 4 .In addition, Ti 3 C 2 MXene has a strong light absorption capability, and its solar spectral response ranges from UV to visible [50,51].Thus, the loading of Ti 3 C 2 MXene significantly extends the light absorption range of the C-TM-CN so that it can utilize most of the light in the 200-800 nm range.In summary, the C-TM-CN is a photocatalytic composite membrane with visible light responsive properties.In the meantime, photoluminescence (PL) spectra are measured to assess the properties of the g-C 3 N 4 , TM-CN, and C-TM-CN nanofiber composite membranes.Photoluminescence (PL) spectroscopy has been regarded as an effective tool for revealing the separation and recombination rates of photogenerated charge carriers in excited photocatalysts.Figure 6b illustrates that the three groups exhibit a photoluminescence range of 400-550 nm.The main peak in the PL spectra of g-C 3 N 4 , TM-CN, and C-TM-CN (Figure 6a) appeared at 450 nm.Still, the photoelectric emission peak of g-C 3 N 4 was higher than that of TM-CN, indicating that the photogenerated electron-hole complexation rate of CM-TN after the addition of Ti 3 C 2 MXene material was lower compared to that of g-C 3 N 4 .This result is the same as that shown by the photocurrent results (Figure 7a) and electrochemical impedance response (Figure 8a), which proved that the addition of Ti 3 C 2 MXene is tightly bonded to g-C 3 N 4 because of electrostatic interaction to form a practical interface.Moreover, the peak value of the C-TM-CN is significantly lower, indicating that it has the lowest photoluminescence intensity compared to g-C 3 N 4 and TM-CN.

Electrochemical Impedance
Electrochemical impedance spectroscopy (EIS) is a valuable technique for studying the electrical behavior of photocatalytic systems.It is commonly employed to assess the electron-hole pairs' transport capacity of photocatalysts.The smaller the arc radius, the smaller the transfer resistance of photogenerated electron-hole pairs enabling the faster surface charge velocity transfer [52,53].Comparison of the fitted EIS Nyquist plots in Figures S1 and S2 shows that the g-C 3 N 4 impedance value is smaller than the TM-CN impedance value [54], indicating that the TM-CN has a stronger conductivity, the direct photogenerated electron-hole separation becomes more efficient, and the interfacial charge separation rate is accelerated.Compared to other samples, the C-TM-CN has the smallest impedance arc radius (see Figures 7 and S3).This indicates that the electrons generated by the C-TM-CN nanofibrous membrane can be transported rapidly, promoting the effective separation of the photogenerated electron-hole pairs and prolonging the existence of the photogenerated electron-hole pairs.A higher separation efficiency promotes the photocatalytic activity of the C-TM-CN.

Photocurrent Response
Figure 8a shows the photocurrent response of the TM-CN and g-C 3 N 4 .During the experiment, the assembly is mounted in a camera obscura with an Xe lamp as the light source turned on and off every thirty seconds.The primary voltage was set to 0.5 V.According to the image, under dark conduction, the g-C 3 N 4 has generated a weak photocurrent of 0.003 µA, and the TM-CN has generated a photocurrent of 0.02 µA.Under light conduction, the TM-CN group has a photocurrent that soars and efficiently arrives at a stable value of about 0.069 µA, which is significantly greater than the g-C 3 N 4 (0.048 µA).With the Xe lamp being turned off, the dark current rapidly descends to a stable value.Figure 8b shows the photocurrent response of C-TM-CN; with the Xe lamp off, the C-TM-CN generated a photocurrent of approximately 55 µA.With exposure to light, the photocurrent of the C-TM-CN groups swiftly increased to reach a stable value of 75.5 µA.The photocurrent response cycles were repeated three times to ensure the stability of the photocurrent response.The results suggested that the C-TM-CN obtained a better separation efficiency of photogenerated electron-holes because of the electrical conductivity of carbon fibers and Ti 3 C 2 MXene.

Photocatalytic Properties and Mechanism
To investigate the photocatalytic performance of the samples against organic dyes, the photocatalytic degradation of RhB by several materials under visible light conditions was tested.Figure 9a shows that TM-CN reaches 100% degradation efficiency after 60 min, whereas g-C 3 N 4 takes 180 min to perform the same task, which proves that the photo-catalytic performance of TM-CN was greatly improved.Figure 9b illustrates that the photocatalytic degradation constant of TM-CN is 5.91 times higher than that of g-C 3 N 4 .When TM-CN was attached to the carbon nanofiber membrane, the removal rate of 99% was still achieved after 120 min of sunlight irradiation.Compared to the carbon nanofiber membrane loaded with g-C 3 N 4 (C-C 3 N 4 ), the photocatalytic effect of TM-CN exhibited a notable advantage.The photodegradation of RhB is achieved by two competing processes [55]: deethylation at the N-site and disruption of the conjugated structure.Under sun light irradiation, the photocatalyst produces active substances •OH,•O 2 − and h + , which first attack the central carbon atom in the structure of RhB, further decolorize and degrade the dye molecule through the process of deethylation at the N-site and gradually break its conjugated structure, and finally completely mineralize RhB into CO 2 and H 2 O.It was also observed that the RhB solution became colorless and transparent when the degradation rate reached 99% by TM-CN.The C-TM-CN was also tested for the photocatalytic degradation of two organic dyes, methylene blue (MB) and methyl orange (MO).Figure 10a illustrates that the photocatalytic degradation efficiencies of C-TM-CN for MB and MO were 90% and 73%, respectively, after 120 min of light irradiation.This indicated that C-TM-CN has excellent photocatalytic degradation effects on various organic dyes.Furthermore, the photocatalytic degradation of RhB by C-TM-CN was tested under different pH conditions (3)(4)(5)(6)(7)(8)(9)(10)(11).Among these, the highest photocatalytic degradation efficiency was exhibited by C-TM-CN at pH 3, as shown in Figure 10b.This is because superoxide, the principal active substance in the photocatalytic degradation reaction, is highly effective in degrading RhB under acidic conditions.Furthermore, studies have demonstrated that the acid effect plays a positive role in the photocatalytic degradation of RhB [59][60][61][62][63].In contrast, the photocatalytic activity was significantly reduced when the pH was at 11, which may be attributed to electrostatic repulsion.In conclusion, the C-TM-CN has a broad range of application conditions and is suitable for acidic and alkaline wastewater treatment processes.
The stability of the C-TM-CN is another crucial point in the photodegradation of dyeing wastewater.Accordingly, the C-TM-CN was examined for stability for practical use via the five cycles of photodegrading RhB with exposure to visible light, as in Figure 10c.Notably, the C-TM-CN is easily transferred and recycled.In every cycle, the photodegra- As illustrated in Figure 9e, this study demonstrates a comparable performance to other studies on preparing photocatalytic membrane materials [56][57][58].The C-TM-CN achieved a superior degradation effect in a shorter period of time than PVDF membrane materials with the addition of pure g-C 3 N 4 and composite MCU-g-C 3 N 4 and g-C 3 N 4 /Ag 3 PO 4 photocatalysts.In addition, the carbon nanofiber membrane used in this work is more environmentally friendly compared to the PVDF membrane.Compared with another CeO 2 /BiWO 6 membrane, C-TM-CN, with the addition of the TM-CN heterostructured photocatalyst, has a great advantage for the degradation of RhB in an aqueous environment by visible light irradiation.
The C-TM-CN was also tested for the photocatalytic degradation of two organic dyes, methylene blue (MB) and methyl orange (MO).Figure 10a illustrates that the photocatalytic degradation efficiencies of C-TM-CN for MB and MO were 90% and 73%, respectively, after 120 min of light irradiation.This indicated that C-TM-CN has excellent photocatalytic degradation effects on various organic dyes.Furthermore, the photocatalytic degradation of RhB by C-TM-CN was tested under different pH conditions (3)(4)(5)(6)(7)(8)(9)(10)(11).Among these, the highest photocatalytic degradation efficiency was exhibited by C-TM-CN at pH 3, as shown in Figure 10b.This is because superoxide, the principal active substance in the photocatalytic degradation reaction, is highly effective in degrading RhB under acidic conditions.Furthermore, studies have demonstrated that the acid effect plays a positive role in the photocatalytic degradation of RhB [59][60][61][62][63].In contrast, the photocatalytic activity was significantly reduced when the pH was at 11, which may be attributed to electrostatic repulsion.In conclusion, the C-TM-CN has a broad range of application conditions and is suitable for acidic and alkaline wastewater treatment processes.
catalytic degradation against the RhB solution.Next, 1 mM EDTA-2Na, 1 mM (KIO3) potassium iodate, or 1 mM isopropyl alcohol (IPA) is individually added to the RhB solution as the scavengers to remove H + ,•O2 − , and •OH.Exposed to a 500 W Xe lamp, the photocatalytic degradation against the RhB solution of samples is evaluated.Figure 10d exhibits the trapping experiment results for the reactive species.With isopropyl alcohol (IPA) as a trapping agent, the photocatalytic degradation efficiency is reduced to 51.79%, indicating a significant drop in the degradation efficiency.Notably, with KIO3 as a trapping agent, the RhB solution exhibits a remarkably reduced absorbance at a maximal light-absorbed wavelength of 552 nm.However, the solution discolors from reddish-purple to yellow, instead of from reddish-purple to transparent.Because the superoxide radical is restricted, the RhB solution fails to have a complete degradation.Similarly, the ethyl group of the benzene nucleus is only partially oxidized rather than wholly oxidized.When EDTA-2Na is used as a trapping agent, the result is the same as the phenomenon with KIO3.Therefore, superoxide radicals and cavities interact to a certain extent in the system.To summarize, •OH, •O2 − , and H + participate in photocatalytic reactions.Superoxide radicals have a certain service life and can diffuse freely, suggesting that vicarious contact is also greatly involved with photocatalytic degradation.The stability of the C-TM-CN is another crucial point in the photodegradation of dyeing wastewater.Accordingly, the C-TM-CN was examined for stability for practical use via the five cycles of photodegrading RhB with exposure to visible light, as in Figure 10c.Notably, the C-TM-CN is easily transferred and recycled.In every cycle, the photodegradation against RhB retains greater than 85%, which suggests that the C-TM-CN does not exhibit distinct photocatalytic activity loss.Hence, the prepared C-TM-CN features excellent recycling and photodegradation cycles, which address the problems of powder catalysts, as in the difficulty of recycling and ease of agglomeration while preventing the powders from causing secondary pollution to the water environment.In addition, the chemical composition of C-TM-CN before and after the photocatalytic experiment was characterized by FTIR.As shown in Figure 10d, it can be seen that the samples have absorption peaks belonging to MXene and g-C 3 N 4 before and after cycling, which proved that the material has good chemical stability during the long cycling test [64].
To study the photocatalytic reaction mechanism of the C-TM-CN, a quenching experiment is conducted as related to the primary active matter in the photodegradation response system (see Figure 10e).As the photocatalyst, the C-TM-CN is used for photocatalytic degradation against the RhB solution.Next, 1 mM EDTA-2Na, 1 mM (KIO 3 ) potassium iodate, or 1 mM isopropyl alcohol (IPA) is individually added to the RhB solution as the scavengers to remove H + ,•O 2 − , and •OH.Exposed to a 500 W Xe lamp, the photocatalytic degradation against the RhB solution of samples is evaluated.Figure 10d exhibits the trapping experiment results for the reactive species.With isopropyl alcohol (IPA) as a trapping agent, the photocatalytic degradation efficiency is reduced to 51.79%, indicating a significant drop in the degradation efficiency.Notably, with KIO 3 as a trapping agent, the RhB solution exhibits a remarkably reduced absorbance at a maximal light-absorbed wavelength of 552 nm.However, the solution discolors from reddish-purple to yellow, instead of from reddish-purple to transparent.Because the superoxide radical is restricted, the RhB solution fails to have a complete degradation.Similarly, the ethyl group of the benzene nucleus is only partially oxidized rather than wholly oxidized.When EDTA-2Na is used as a trapping agent, the result is the same as the phenomenon with KIO 3 .Therefore, superoxide radicals and cavities interact to a certain extent in the system.To summarize, •OH, •O 2 − , and H + participate in photocatalytic reactions.Superoxide radicals have a certain service life and can diffuse freely, suggesting that vicarious contact is also greatly involved with photocatalytic degradation.
The excellent organic dye removal performance of the C-TM-CN nanofiber composite membrane is mainly attributed to the enhancement in the light absorption performance by the heterostructure.The close contact between MXene and g-C 3 N 4 promoted the rapid transfer and separation of the photoelectron-hole pair, which enhanced the photocatalytic performance.Based on Figure 6a's UV-Vis diffuse reflectance spectra (DRS), the Eg of the g-C 3 N 4 and TM-CN were calculated as 2.66 eV and 2.57 Ev, used Equation ( 1), and made into Figure 11a.The Eg of TM-CN after adding MXene was reduced compared to that of pure g-C 3 N 4 , which implied a superior utilization of light absorption by TM-CN.Therefore, the TM-CN photocatalytic degradation efficiency of RhB under visible light irradiation was significantly improved compared to pure g-C 3 N 4 (see Figure 9a).The positions of the conduction and valence bands of the g-C 3 N 4 were calculated according to Equations ( 2) and (3).The ECB position g-C 3 N 4 was calculated as −1.055 eV.Then, the EVB corresponds to 1.515 eV.Thus, the band structure [48,65] of g-C 3 N 4 after contact with MXene is shown in Figure 11b.The results showed a staggered energy band structure between the g-C 3 N 4 and Ti 3 C 2 .Consequently, the carbon fiber membrane C-TM-CN loaded with TM-CN had more muscular photocatalytic activity and higher efficiency in degrading RhB than the carbon fiber membrane C-C 3 N 4 loaded with pure g-C 3 N 4 (see Figure 9b).The excellent organic dye removal performance of the C-TM-CN nanofiber composite membrane is mainly attributed to the enhancement in the light absorption performance by the heterostructure.The close contact between MXene and g-C3N4 promoted the rapid transfer and separation of the photoelectron-hole pair, which enhanced the photocatalytic performance.Based on Figure 6a's UV-Vis diffuse reflectance spectra (DRS), the Eg of the g-C3N4 and TM-CN were calculated as 2.66 eV and 2.57 Ev, used Equation (1), and made into Figure 11a.The Eg of TM-CN after adding MXene was reduced compared to that of pure g-C3N4, which implied a superior utilization of light absorption by TM-CN.Therefore, the TM-CN photocatalytic degradation efficiency of RhB under visible light irradiation was significantly improved compared to pure g-C3N4 (see Figure 9a).The positions of the conduction and valence bands of the g-C3N4 were calculated according to Equations ( 2) and (3).The ECB position g-C3N4 was calculated as −1.055 eV.Then, the EVB corresponds to 1.515 eV.Thus, the band structure [48,65] of g-C3N4 after contact with MXene is shown in Figure 11b.The results showed a staggered energy band structure between the g-C3N4 and Ti3C2.Consequently, the carbon fiber membrane C-TM-CN loaded with TM-CN had more muscular photocatalytic activity and higher efficiency in degrading RhB than the carbon fiber membrane C-C3N4 loaded with pure g-C3N4 (see Fig- Under visible light irradiation, the electron and hole pair jumps on the material's surface, forming photoelectron-hole pairs in its conduction and valence bands, respectively.Since electrons are reductive and holes are oxidative, they can react with OH-, O2, and H2O adsorbed on the surface of the C-TM-CN nanofiber membrane to generate free radicals with oxidative solid activity.These free radicals can be directly oxidized to remove organic dye molecules in solution.However, if the electron transfer efficiency is low and the photoelectron-hole pair complexation efficiency is high, it will reduce the photocatalytic degradation of pollutants.In contrast, the tight binding of MXene and g-C3N4 provided more efficient electron transfer conditions, accelerating the photocatalytic degradation of organic dyes.
Figure4ashows the XRD pattern of Ti 3 C 2 , g-C 3 N 4 , and the TM-CN.Based on the XRD patterns, carbon fiber nanofiber membranes (C-NM) exhibit a strong diffraction peak that corresponds to the crystal plane (002).After the loading of Ti 3 C 2 , the peak (002) shifts toward a higher angle, which indicates that the two molecules are strongly interacted, which in turn affects the overlapped distance of g-C 3 N 4 .The findings prove that the heterojunction is formed between Ti 3 C 2 and g-C 3 N 4 .In addition, there is a strong absorption peak present at 27.7 • according to the X-ray powder diffraction pattern of the C-TM-CN.Meanwhile, C-M-C 3 N 4 nanofiber membranes exhibit diffraction peaks that separately correspond to carbon fiber (22 • ), g-C 3 N 4 (27.4 • ), and Ti 3 C 2 (5.0 • ).In Figure4a, two prominent diffraction peaks were detected at 13.2 • and 27.6 • , which were indexed to the (100) plane, corresponding to the inplane packing of heptazine units, and the (200) plane, resulting from the characteristic stacking of conjugated aromatics, respectively[39].After combination with the MXene and carbon nanofiber membrane, the characteristic diffraction peaks of TM-CN and C-TM-CN appeared at the same positions, indicating that the typical layered structure of polymeric g-C 3 N 4 was well preserved.In addition, the C-TM-CN showed the characteristic diffraction peaks of graphene-like carbon nitride, which was derived from the MXene.Moreover, the typical peak for the Mxene material revealed the successful preparation of Ti 3 C 2 material and removal of Al layers in the Ti 3 AlC 2 [40-42].To sum up, Ti 3 C 2 and g-C 3 N 4 generate a heterojunction that successfully grows over carbon fiber matrices.Nanomaterials 2024, 14, x FOR PEER REVIEW 7 of 17

Figure
Figure4bshows the FTIR spectrogram for g-C 3 N 4 , Ti 3 C 2 , and C-TM-CN nanofiber composite membranes.C-TM-CN nanofiber composite membranes exhibit a broader absorption band at 1200-1800 cm −1 , which corresponds to the C(sp 3 )-N and C(sp 2 )=N in the CN heterocycle of g-C 3 N 4 , but the broader absorption band shifts to the left compared to the CN heterocycle of g-C 3 N 4 .The presence of peaks at 810 cm −1 can be attributed to the 3-s-Triazine unit characteristic peak of g-C3N4[18,43].The peaks in the range 1135-1629 cm −1 were ascribed to the stretching vibrations of heterocycles (C-N/C=N).Furthermore, the broad peak at around 3137 cm −1 was assigned to uncondensed terminal amino groups[44][45][46].This indicated that g-C 3 N 4 nanomaterials were successfully synthesized.Moreover, the Fourier transform infrared spectroscopy analysis proves the formation of g-C 3 N 4 over nanofiber composite membranes.As shown in Figure4b, the firm peaks at 3435 cm −1 and 1624 cm −1 in the FTIR spectrum of MXene were assigned to the stretching vibrations of the OH and C=O groups, the peak at 620 cm −1 belongs to Ti-O group, and the peak at 1432 cm −1 was attributed to O-H group bending[47].As a result, through the

Nanomaterials 2024 ,
14,  x FOR PEER REVIEW 9 of 17 tightly bonded to g-C3N4 because of electrostatic interaction to form a practical interface.Moreover, the peak value of the C-TM-CN is significantly lower, indicating that it has the lowest photoluminescence intensity compared to g-C3N4 and TM-CN.
Nanomaterials 2024, 14, x FOR PEER REVIEW 9 of 17 tightly bonded to g-C3N4 because of electrostatic interaction to form a practical interface.Moreover, the peak value of the C-TM-CN is significantly lower, indicating that it has the lowest photoluminescence intensity compared to g-C3N4 and TM-CN.

Figure 8 .
Figure 8. Photocurrent response curves of (a) g-C 3 N 4 and TMCN as well as (b) C-TM-CN nanofiber composite membranes.

Nanomaterials 2024 ,
14, x FOR PEER REVIEW 11 of 17achieved a superior degradation effect in a shorter period of time than PVDF membrane materials with the addition of pure g-C3N4 and composite MCU-g-C3N4 and g-C3N4/Ag3PO4 photocatalysts.In addition, the carbon nanofiber membrane used in this work is more environmentally friendly compared to the PVDF membrane.Compared with another CeO2/BiWO6 membrane, C-TM-CN, with the addition of the TM-CN heterostructured photocatalyst, has a great advantage for the degradation of RhB in an aqueous environment by visible light irradiation.

Figure 9 .
Figure 9. Photocatalytic degradation (a) efficiency and (b) rate constants of g-C3N4 and TM-CN on RhB.Photocatalytic degradation (c) efficiency and (d) rate constants of TM-CN, C-C3N4, and C-TM-CN on RhB.(e) Comparison of the performance of different photocatalytic materials for RhB degradation.

Figure 9 .
Figure 9. Photocatalytic degradation (a) efficiency and (b) rate constants of g-C 3 N 4 and TM-CN on RhB.Photocatalytic degradation (c) efficiency and (d) rate constants of TM-CN, C-C 3 N 4, and C-TM-CN on RhB.(e) Comparison of the performance of different photocatalytic materials for RhB degradation.

Figure 10 .
Figure 10.Under visible light irradiation, the photocatalytic degradation of C-TM-CN nanofiber composite membranes' (a) plot of MB solution and MO solution, (b) plot of concentration change in degraded RhB under different pH conditions, (c) plot of photocatalytic cycle of RhB, (d) FTIR

Figure 10 .
Figure 10.Under visible light irradiation, the photocatalytic degradation of C-TM-CN nanofiber composite membranes' (a) plot of MB solution and MO solution, (b) plot of concentration change in degraded RhB under different pH conditions, (c) plot of photocatalytic cycle of RhB, (d) FTIR spectroscopy before and after photocatalytic degradation, and (e) plot of photocatalytic performance of RhB about scavenger.

Figure 11 .
Figure 11.(a) The band gap diagram of g-C3N4, TM-CN, and (b) relative energy band positions and charge transfer mechanism between MXene and g-C3N4.

Figure 11 .
Figure 11.(a) The band gap diagram of g-C 3 N 4 , TM-CN, and (b) relative energy band positions and charge transfer mechanism between MXene and g-C 3 N 4 .