Improving the Photocatalytic Activity of Ti3C2 MXene by Surface Modification of N Doped

Methyl orange dye (MO) is one of the azo dyes, which is not only difficult to degrade but also hazardous to human health, therefore, it is necessary to develop an efficient photocatalyst to degrade MO. In this paper, a facile and low-cost elemental doping method was used for the surface modification of Ti3C2 MXene, i.e., nitrogen-doped titanium carbide was used as the nitrogen source, and the strategy of combining solvent heat treatment with non-in situ nitrogen doping was used to prepare N-Ti3C2 MXene two-dimensional nanomaterials with high catalytic activity. It was found that the catalytic efficiency of N-Ti3C2 MXene materials was enhanced and improved compared to the non-doped Ti3C2 MXene. In particular, N-Ti3C2 1:8 MXene showed the best photo-catalytic ability, as demonstrated by the fact that the N-Ti3C2 1:8 MXene material successfully degraded 98.73% of MO (20 mg/L) under UV lamp irradiation for 20 min, and its catalytic efficiency was about ten times that of Ti3C2 MXene, and the N-Ti3C2 photo-catalyst still showed good stability after four cycles. This work shows a simplified method for solvent heat-treating non-in situ nitrogen-doped Ti3C2 MXene, and also elaborates on the photo-catalytic mechanism of N-Ti3C2 MXene, showing that the high photo-catalytic effect of N-Ti3C2 MXene is due to the synergistic effect of its efficient charge transfer and surface-rich moieties. Therefore, N-Ti3C2 MXene has a good prospect as a photo-catalyst in the photocatalytic degradation of organic pollutants.


Introduction
With the progressive increase in population and the rapid expansion of business over the past decade, various types of domestic sewage and industrial wastewater have rapidly increased, making the water pollution problem more serious and causing serious damage to the ecological environment [1][2][3]. In particular, azo dyes are discharged from wastewater due to their anti-biodegradability. Excessive amounts of azo dyes are teratogenic, mutagenic, and carcinogenic, posing a highly toxic hazard to plants, animals, and humans [4,5] Therefore, there is an urgent need for efficient and cost-effective water treatment technologies. So far, many methods have been developed, such as adsorption, redox photochemical degradation, membrane filtration, and photocatalysis [6,7] have been widely used to remove pollutants from water. Among them, photocatalytic technology is gaining popularity owing to its several benefits, which include environmental friendliness, high efficiency, stability, and a high degradation rate [8,9].
MXenes [10], a novel two-dimensional substance, has several applications in photocatalysis [11] and high electrical conductivity owing to its enormous specific surface area [12], many active sites [13], and the presence of numerous functional groups (-O, Materials 2023, 16, 2836 2 of 15 -OH, -F, etc.) with good hydrophobicity and controllable interlayer distances [14][15][16][17]. MXenes follow the general formula of M n+1 X n T x , where n+1 (n = 1, 2, 3) is the early transition metal layer, and M is intercalated with n layers of carbon or nitrogen X. Since the first report of MXene in 2011 [18], more than 30 MXene such as Ti 3 C 2 , Ti 3 CN, TiNbC, V 2 C, Mo 2 C, Nb 2 C, and Y 2 CF 2 have been successfully prepared [19][20][21]. The general MXene is obtained by the selective removal of atoms from the MAX phase by treatment with hydrofluoric acid or other fluorine sources. The precursors of MXene MAX phase are a collective term for a ternary layered ceramic material, where M is a pre-transition metal (e.g., Ti, Sr, V, Nb, Cr, Ta, Zr, Mo, etc.), A is the main group II or IV element, and X is C or N, etc. [22][23][24][25][26][27]. Ti 3 C 2 , a member of the MXenes family, has superior electrical conductivity, a large specific surface area, and many active sites, which may result in the separation and transmission of photoexcited electron-hole pairs. At the same time, it is simple to create composite materials using Ti 3 C 2 and other photo-catalysts because of their excellent flexibility and distinctive layered structure. In the case of Ti 3 C 2 composites, Ti 3 C 2 not only increases the absorption of light by its blackbody but also causes rapid carrier migration and inhibits its compounding [28]. Therefore, Ti 3 C 2 is often utilized as an alternative noble metal [29] co-catalyst to boost the complexes' photo-catalytic activity [30][31][32][33]. We note that it has been reported in the literature that Ti 3 C 2 with -F and -OH end groups are a narrow band gap semiconductor [34], and considering that Ti 3 C 2 exfoliated from HF acid liquid phase will have -F and -OH end groups and may also have photo-catalytic properties. Therefore, we investigated the photocatalytic activity of Ti 3 C 2 . Unfortunately, the photosynthesis performance of Ti 3 C 2 itself was not satisfactory.
Considering that Ti 3 C 2 can be improved by changing the elemental composition and adjusting the surface functional groups to improve the performance of two-dimensional accordion-like Ti 3 C 2 nanomaterials, we used the surface modification of Ti 3 C 2 utilizing introducing heteroatoms to investigate its photocatalytic properties. It has been discovered that ecologically friendly urea (CH 4 N 2 O) in high-concentration nitrogen alcohol is an effective source of liquid nitrogen doping and that the liquid N doping source can retain excellent contact with Ti 3 C 2 to achieve high N doping without stacking N-Ti 3 C 2 nanospheres [35,36]. The modulation of nitrogen-doped Ti 3 C 2 nanomaterials is greatly enhanced in terms of carrier density, surface energy, and surface reactivity.
Currently, there are many research reports on N-Ti 3 C 2 in the fields of batteries [37], capacitors [38], and electrocatalysis [39], however, there are fewer reports on the application of N-Ti 3 C 2 in the field of photocatalysis. Thus, in this paper, urea-saturated alcohol solution as a liquid nitrogen source, and a simple and controllable strategy combining solvent thermal treatment and non-in situ nitrogen doping were used to prepare N-Ti 3 C 2 MXene 2D nanomaterials with high catalytic activity. Transforming the usual way of N-Ti 3 C 2 nanomaterials, we applied it as the main catalyst for the first time in the field of photocatalysis and confirmed that it could significantly improve the photocatalytic degradation of MO. In addition, the morphological and physicochemical properties of the materials were thoroughly examined, and the reaction mechanism of the photo-catalysts was discussed through experimental and theoretical studies.
The synthetic roadmap of the N-Ti 3 C 2 catalyst is shown in Figure 1, and its structure is schematically shown in Figure 2. and then dried for 12 h at 60 °C in a drier. Next, take the intercalated Ti3C2 and urea in the ratio of 8:1 put it into 60 mL anhydrous ethanol for mixing, put it into the ultrasonic machine for 30 min to mix well, and transfer it to the stainless-steel autoclave lined with polytetrafluoroethylene for solvent heat reaction at 180 °C for 12 h. Finally, the material in the reaction kettle was collected by centrifugation, washed with deionized water and anhydrous ethanol until the pH was about neutral, and put into a dryer for 12 h at 60 °C to obtain N-doped Ti3C2 powder.  and then dried for 12 h at 60 °C in a drier. Next, take the intercalated Ti3C2 and urea in the ratio of 8:1 put it into 60 mL anhydrous ethanol for mixing, put it into the ultrasonic machine for 30 min to mix well, and transfer it to the stainless-steel autoclave lined with polytetrafluoroethylene for solvent heat reaction at 180 °C for 12 h. Finally, the material in the reaction kettle was collected by centrifugation, washed with deionized water and anhydrous ethanol until the pH was about neutral, and put into a dryer for 12 h at 60 °C to obtain N-doped Ti3C2 powder.  Synthesis of Ti 3 C 2 : Ti 3 C 2 was synthesized by etching off the aluminum element in Ti 3 AlC 2 . Firstly, 2 g Ti 3 AlC 2 powder was mixed with 20 mL of 30% hydrofluoric acid at room temperature for 5 h. Secondly, the etched material was centrifuged by centrifuge to collect the black powder and washed with deionized water to a pH of about 6-7, and placed in a dryer for 12 h at 60 • C.
The synthesis route of N-doped Ti 3 C 2 : Firstly, 0.5 g of the above-dried Ti 3 C 2 was placed in 15 mL of dimethyl sulfoxide (DMSO) and agitated for 24 h for intercalation. The powder was recovered by centrifugation, washed several times with deionized water, and then dried for 12 h at 60 • C in a drier. Next, take the intercalated Ti 3 C 2 and urea in the ratio of 8:1 put it into 60 mL anhydrous ethanol for mixing, put it into the ultrasonic machine for 30 min to mix well, and transfer it to the stainless-steel autoclave lined with polytetrafluoroethylene for solvent heat reaction at 180 • C for 12 h. Finally, the material in the reaction kettle was collected by centrifugation, washed with deionized water and anhydrous ethanol until the pH was about neutral, and put into a dryer for 12 h at 60 • C to obtain N-doped Ti 3 C 2 powder.

Materials Characterization
By using energy-dispersive X-ray spectroscopy (EDS) detectors and a field emission scanning electron microscope (FESEM, SU5000, Hitachi, Tokyo, Japan), researchers were able to examine the morphological characteristics and elemental composition of the samples. X-ray photoelectron spectroscopy (XPS, ESCALAB-250XI) analyzed the valence and chemical composition of the elements. X-ray diffraction spectroscopy (XRD, Bruker D8 Advanced) measured the crystal structure of the samples. Fourier infrared spectroscopy (FTIR) was used to measure the functional groups and chemical bonds on the surface of the samples. Optical properties were tested by UV-Vis spectrophotometer (DRS, Shimadzu Lambda 750, Tokyo, Japan).

Electrochemical Measurements
The prepared sample coated on FTO glass served as the working electrode, platinum wire served as the counter electrode, the Hg/Hg 2 Cl 2 /KCl (saturated) electrode served as a reference, and 0.5 mol/L aqueous solution served as the electrolyte, photochemical measurements were carried out using an electrochemical workstation (CHI860B). The produced photocurrent was measured while a solar simulator was irradiated with a 10-s lamp on/off cycle. The irradiation intensity was 1000 W/m 2 and the effective area of the sample was 1 cm 2 .

Photocatalytic Performance
The photocatalytic performance of N-Ti 3 C 2 was tested by methyl orange dye MO degradation. The sample numbers obtained at different scales will be used in this experiment, and the light source used is a 1000 W mercury lamp (Wavelength range 320 nm-390 nm, main peak value 365 nm) to emit ultraviolet light. Take 50mL of methyl orange dye with a concentration of 20 mg/L mixed with 50 mg N-Ti 3 C 2 and pour it into the test tube, sonicate for some time to make it mix evenly, and then put it into a photochemical reactor (BL-GHX-1D) and stir under dark conditions for 30 min to achieve adsorption resolution equilibrium, turn on the light source and take 3-4 mL every 10 min. The resulting solutions were detected with a UV-Vis spectrophotometer for absorbance at 464 nm (characteristic wavelength of MO) to determine the degree of degradation.

SEM and EDS Analysis
In the chemical etching of aluminum in Ti 3 AlC 2 as shown in Figure 3a,b, SEM can observe that Ti 3 C 2 MXene shows a unique accordion-like multilayer structure, revealing the effective production of Ti 3 C 2 through HF etching of Ti 3 AlC 2 . It is well known that at a certain temperature and high pressure, due to the low boiling point and high mobility of ethanol, it may easily lead to the diffusion of urea molecules into the interlayer space during the solvent heat treatment. The morphology of the nitrogen-doped Ti 3 C 2 MXene is shown in Figure 3c,d was also well preserved after the solvent thermal reaction, in which the spacing between some of the layers became larger, the surface of the sample was rougher and more complex, and the layer surface was attached with uniform and fine nanoparticles. To further elucidate the structure of N-Ti 3 C 2 MXene nanomaterial, EDS elemental mapping of individual elements in N-Ti 3 C 2 MXene nanomaterial was performed (Figure 3e), and X-ray spectra (EDS) showed that Ti, C, N, F, and O elements were uniformly distributed in the prepared samples. These characterizations strongly demonstrate the successful preparation of N-Ti 3 C 2 MXene 2D nanomaterial. The formation of N-Ti 3 C 2 expands the interlayer distance, resulting in an increased specific surface area and more active sites, which is more favorable for carrier transport and separation.

XRD and FTIR Analysis
The X-ray powder diffraction was used to examine the crystalline and phase structure of the material (XRD). As shown in Figure 4a, the typical four prominent peaks of Ti3AlC2 (JCPDS No. 52-0875) are distributed at 9.517°, 19.154°, 39.037°, and 60.257°, corresponding to the four crystallographic planes (002), (004), (104), and (110), respectively. The strongest XRD diffraction peak of Ti3AlC2 after HF etching (104) crystalline plane disappears and (002) and (004) crystalline planes are shifted to lower angles compared to Ti3C2. The results show that the layered Ti3C2 MXene was successfully prepared. with the addition of the nitrogen dopant, the diffraction peaks of the (002) crystalline plane corresponding to N-Ti3C2 1:2, N-Ti3C2 1:5, N-Ti3C2 1:8 and N-Ti3C2 1:10 shifted from 9.517° to 6.94°, 6.8°, 6.79° and 6.75° relative to Ti3C2 is particularly obvious. This indicates that the interlayer distance of N-Ti3C2 MXene increases with the doping of nitrogen, which gives it a larger specific surface area and exposes more active sites. In addition, we found diffraction peaks (101) and (211) of titanium dioxide based on XRD diffraction patterns, indicating that a small portion of titanium dioxide was also formed during the nitrogen doping process [40]. The surface functional groups and bonds of the sample were measured by Fourier infrared spectroscopy (FTIR), as shown in Figure 4b, and some surface -F groups of Ti3C2 were substituted by hydroxyl groups through anhydrous ethanol due to the abundance of hydroxyl groups in anhydrous ethanol, i.e., the sharp characteristic peaks near 3606 and 3738 cm −1 represent free -OH. Since the N atom forms polar covalent bonds with the H atom and is easily adsorbed to the Ti3C2 surface, the characteristic peaks near 3130 and 3109 cm −1 are the N-H vibrational peaks of the bonding, the vibrational peaks at 1350-1550 cm −1 can be attributed to the vibrational stretching of -CH3 and the C-N stretching vibrational peaks in the urea molecule, and the characteristic peaks near 580 cm −1 corresponds to the stretching vibration of Ti-C. The presence of these func-

XRD and FTIR Analysis
The X-ray powder diffraction was used to examine the crystalline and phase structure of the material (XRD). As shown in Figure 4a, the typical four prominent peaks of Ti 3 AlC 2 (JCPDS No. 52-0875) are distributed at 9.517 • , 19.154 • , 39.037 • , and 60.257 • , corresponding to the four crystallographic planes (002), (004), (104), and (110), respectively. The strongest XRD diffraction peak of Ti 3 AlC 2 after HF etching (104) crystalline plane disappears and (002) and (004) crystalline planes are shifted to lower angles compared to Ti 3 C 2 . The results show that the layered Ti 3 C 2 MXene was successfully prepared. with the addition of the nitrogen dopant, the diffraction peaks of the (002) crystalline plane corresponding to N-Ti 3 C 2 1:2, N-Ti 3 C 2 1:5, N-Ti 3 C 2 1:8 and N-Ti 3 C 2 1:10 shifted from 9.517 • to 6.94 • , 6.8 • , 6.79 • and 6.75 • relative to Ti 3 C 2 is particularly obvious. This indicates that the interlayer distance of N-Ti 3 C 2 MXene increases with the doping of nitrogen, which gives it a larger specific surface area and exposes more active sites. In addition, we found diffraction peaks (101) and (211) of titanium dioxide based on XRD diffraction patterns, indicating that a small portion of titanium dioxide was also formed during the nitrogen doping process [40]. The surface functional groups and bonds of the sample were measured by Fourier infrared spectroscopy (FTIR), as shown in Figure 4b, and some surface -F groups of Ti 3 C 2 were substituted by hydroxyl groups through anhydrous ethanol due to the abundance of hydroxyl groups in anhydrous ethanol, i.e., the sharp characteristic peaks near 3606 and 3738 cm −1 represent free -OH. Since the N atom forms polar covalent bonds with the H atom and is easily adsorbed to the Ti 3 C 2 surface, the characteristic peaks near 3130 and 3109 cm −1 are the N-H vibrational peaks of the bonding, the vibrational peaks at 1350-1550 cm −1 can be attributed to the vibrational stretching of -CH 3 and the C-N stretching vibrational peaks in the urea molecule, and the characteristic peaks near 580 cm −1 corresponds to the stretching vibration of Ti-C. The presence of these functional groups can promote electron transfer and enhance electrical conductivity, while the doping of N atoms can effectively improve the electron transport network and ion transport channels of Ti 3 C 2 MXene materials [41]. tional groups can promote electron transfer and enhance electrical conductivity, while the doping of N atoms can effectively improve the electron transport network and ion transport channels of Ti3C2 MXene materials [41].

XPS Analysis
X-ray photoelectron spectroscopy was used to investigate the chemical makeup and surface state of N-Ti3C2 MXene (XPS). As illustrated in Figure 5a. The obvious peaks of Ti, O, C, and F elements could be seen by the full scan spectrum, compared to the XPS curve of N-Ti3C2 material with an extra minor peak of N element. In addition, N-Ti3C2 materials have higher peak O element intensities than Ti3C2 materials, and the atomic concentration of O increases from 18.33% to 29.15%, indicating that part of Ti3C2 is converted to TiO2 in the hydrothermal process. Among them, it should be noted that the atomic concentration of F is 11.9%, indicating the presence of a considerable amount of F termination groups on the surface of N-Ti3C2 MXene. Figure 5b represents the original Ti3C2 MXene with peak I at 685.0 eV representing the C-Ti-F functional group and peak II at 686.5 eV representing AlFx [42], with a shift in the peak position after nitrogen doping, probably due to a change in the elemental valence state and the appearance of a new diffraction peak, i.e., peak III at 689.18 eV, representing F-C [43]. Similarly, peak I at 529.9 eV in the O 1s region of the spectrum in Figure 5c represents the C-Ti-O functional group, peak II at 531.1 eV represents the C-O functional group, and peak III at 533.0 eV represents the C-Ti-OH functional group [44]. As shown in Figure 5d, peaks I at 455.1 eV and 461.1 eV in the Ti 2p region of the XPS spectrum of Ti3C2 MXene refer to the C-Ti functional group, whereas the second expanded peak II at 456.6 eV alludes to the C-Ti-F functional group [42]. In addition, the XPS spectrum of Ti 2p in N-Ti3C2 MXene has a new pair of peaks III (Ti 3p/2 and Ti p/2) appearing at 459.2 eV and 464.88 eV, respectively, inferred to be possibly from the oxidation product TiO2 formed during the hydrothermal treatment [45], which corresponds to the (101) and (211) diffraction peaks found in the XRD diffraction energy spectrum corresponding to the formation of TiO2, both confirming the formation of TiO2. From Figure 5e, it can be seen that the XPS spectra of C 1s found in the pristine Ti3C2 MXene have three diffraction peaks with different valence states, which may originate from external functional groups; peak I is 282.0 eV, representing the C-Ti-(O/OH/F) functional group; peak II is 284.8 eV, representing the C-C functional group; and peak III is 286.5 eV, representing the C-Hx/C-O functional group.

XPS Analysis
X-ray photoelectron spectroscopy was used to investigate the chemical makeup and surface state of N-Ti 3 C 2 MXene (XPS). As illustrated in Figure 5a. The obvious peaks of Ti, O, C, and F elements could be seen by the full scan spectrum, compared to the XPS curve of N-Ti 3 C 2 material with an extra minor peak of N element. In addition, N-Ti 3 C 2 materials have higher peak O element intensities than Ti 3 C 2 materials, and the atomic concentration of O increases from 18.33% to 29.15%, indicating that part of Ti 3 C 2 is converted to TiO 2 in the hydrothermal process. Among them, it should be noted that the atomic concentration of F is 11.9%, indicating the presence of a considerable amount of F termination groups on the surface of N-Ti 3 C 2 MXene. Figure 5b represents the original Ti 3 C 2 MXene with peak I at 685.0 eV representing the C-Ti-F functional group and peak II at 686.5 eV representing AlFx [42], with a shift in the peak position after nitrogen doping, probably due to a change in the elemental valence state and the appearance of a new diffraction peak, i.e., peak III at 689.18 eV, representing F-C [43]. Similarly, peak I at 529.9 eV in the O 1s region of the spectrum in Figure 5c represents the C-Ti-O functional group, peak II at 531.1 eV represents the C-O functional group, and peak III at 533.0 eV represents the C-Ti-OH functional group [44]. As shown in Figure 5d, peaks I at 455.1 eV and 461.1 eV in the Ti 2p region of the XPS spectrum of Ti 3 C 2 MXene refer to the C-Ti functional group, whereas the second expanded peak II at 456.6 eV alludes to the C-Ti-F functional group [42]. In addition, the XPS spectrum of Ti 2p in N-Ti 3 C 2 MXene has a new pair of peaks III (Ti 3p/2 and Ti p/2) appearing at 459.2 eV and 464.88 eV, respectively, inferred to be possibly from the oxidation product TiO 2 formed during the hydrothermal treatment [45], which corresponds to the (101) and (211) diffraction peaks found in the XRD diffraction energy spectrum corresponding to the formation of TiO 2 , both confirming the formation of TiO 2 . From Figure 5e, it can be seen that the XPS spectra of C 1s found in the pristine Ti 3 C 2 MXene have three diffraction peaks with different valence states, which may originate from external functional groups; peak I is 282.0 eV, representing the C-Ti-(O/OH/F) functional group; peak II is 284.8 eV, representing the C-C functional group; and peak III is 286.5 eV, representing the C-Hx/C-O functional group. One more diffraction peak can be seen in the XPS spectrum of C 1s in N-Ti 3 C 2 compared to Ti 3 C 2 after doping with nitrogen, i.e., peak IV is 289.1 eV, representing the C-N functional group (visible in the Annex) [45]. atoms to form mainly N-Ti (nitrides), which indicates that the N atom reacts not only with the C atom, but also bonds with the Ti atom. In contrast, the two peaks located near 400.18 eV and 401.98 eV originate from the -N functional group and surface adsorption, respectively [35]. It can be inferred that nitrogen is mainly present in the form of surface adsorption and pyrrole nitrogen (N-5), and the new peak found near 289.1 eV in the carbon elemental analysis spectrum also indicates the formation of the -N functional group.

Transient Photocurrent Response
Today, photocurrent is widely considered to be the most effective evidence for charge separation in heterogeneous structured photo-catalysts [42,47,48]. Typically, the value of photocurrent indirectly reflects the ability to generate and transfer photo-excited charge carriers under irradiation, which correlates with photo-catalytic activity.
Using the use of transient photocurrent response, the migration and separation traits of the photo-generated electron-hole pairs of the samples were examined. The transient photocurrent curves of N-Ti3C2 are shown in Figure 6. Among all the samples, the pho-  Figure 5f shows the XPS spectra of N 1s. Based on the high-resolution spectra of N 1 s, it can be shown that nitrogen is successfully doped into Ti 3 C 2 and three peaks are identified near 396.58 eV, 400.18 eV, and 401.98 eV, which correspond to three nitrogen functional groups [15,35,46]: (i) nitride, abbreviated as N-Ti (396.58 eV), (ii) pyrrole nitrogen, abbreviated as N-5 (400.18 eV) and (iii) quaternary nitrogen, abbreviated as N-Q (401.98 eV). The peak at 396.58 eV originates from the substitution of nitrogen for carbon atoms to form mainly N-Ti (nitrides), which indicates that the N atom reacts not only with the C atom, but also bonds with the Ti atom. In contrast, the two peaks located near 400.18 eV and 401.98 eV originate from the -N functional group and surface adsorption, respectively [35]. It can be inferred that nitrogen is mainly present in the form of surface adsorption and pyrrole nitrogen (N-5), and the new peak found near 289.1 eV in the carbon elemental analysis spectrum also indicates the formation of the -N functional group.

Transient Photocurrent Response
Today, photocurrent is widely considered to be the most effective evidence for charge separation in heterogeneous structured photo-catalysts [42,47,48]. Typically, the value of photocurrent indirectly reflects the ability to generate and transfer photo-excited charge carriers under irradiation, which correlates with photo-catalytic activity.
Using the use of transient photocurrent response, the migration and separation traits of the photo-generated electron-hole pairs of the samples were examined. The transient photocurrent curves of N-Ti 3 C 2 are shown in Figure 6. Among all the samples, the photocurrent of Ti 3 C 2 was the highest, while the photocurrent of N-Ti 3 C 2 1:8 was the lowest, and the photocurrent of Ti 3 C 2 was much larger. The photocurrent magnitudes of Ti 3 C 2 and N-Ti 3 C 2 1:8 are not consistent with their photo-catalytic activities. It may occur as a result of the possibility that oxygen molecules adsorbed on the surface of N-Ti 3 C 2 can interact with unbound electrons to form negatively charged O 2 − . abundant carrier density [50]. The transient photocurrent density of different samples was measured with three electrodes, and the order of the magnitude of photocurrent was Ti3C2 > N-Ti3C2 1:2 > N-Ti3C2 1:10 > N-Ti3C2 1:5 > N-Ti3C2 1:8, which is the opposite order of photo-catalytic activity, so it can be seen that there is no absolute relationship between the magnitude of photocurrent and photocatalytic activity.

Photocatalytic Degradation Activity and Cycling Experiments of MO
The photocatalytic ability of nitrogen-doped Ti3C2 MXene two-dimensional material was investigated by photocatalytic degradation of MO. The degradation efficiency was defined as = (C0 − Ct)/C0, where C0 is the initial concentration of methyl orange and Ct is the concentration of methyl orange at time t. Blank experiments with degradant solutions under the same conditions without a catalyst were also performed for comparison. In Figure 7a, Ti3C2 MXene materials with nitrogen doping ratios of 1:2, 1:5, 1:8, and 1:10 were tested. Compared to pure Ti3C2 MXene, N-Ti3C2 2D nanomaterials with high surface area and large pore volume due to N-Ti3C2 1:2, N-Ti3C2 1:5, N-Ti3C2 1:8 and N Ti3C2 1:10 two-dimensional nanomaterials all exhibited better photo-catalytic ability. Especially, N-Ti3C2 1:8 had the best photo-catalytic ability to degrade MO under UV light, and it degraded 98.73% of the methyl orange dye after 20 min of irradiation, and the removal rate of MO was 99.93% at 40 min. Figure 7b shows the pseudo-first-order kinetic plot of MO degradation with the equation: −ln(Ct/C0) = kt. Figure 7c depicts the kinetic rate constant (k) estimated using the pseudo-first-order kinetic model, as well as the variation of k for all samples visually, N-Ti3C2 1:2, N-Ti3C2 1:5, N-Ti3C2 1:8 and N-Ti3C2 1:10 for The kinetic rate constants for MO removal by 2D nanomaterials were 0.00574, 0.02387, It is known that the current is proportional to the mobility (µ) and density (n) of the charge carriers [49].
where q is the electron charge, F is the electric field, and A is the cross-sectional area. F and A were the same across all samples included in the investigation. This shows the great difference in material carrier mobility and that high photocurrent does not imply an abundant carrier density [50]. The transient photocurrent density of different samples was measured with three electrodes, and the order of the magnitude of photocurrent was Ti 3 C 2 > N-Ti 3 C 2 1:2 > N-Ti 3 C 2 1:10 > N-Ti 3 C 2 1:5 > N-Ti 3 C 2 1:8, which is the opposite order of photo-catalytic activity, so it can be seen that there is no absolute relationship between the magnitude of photocurrent and photocatalytic activity.

Photocatalytic Degradation Activity and Cycling Experiments of MO
The photocatalytic ability of nitrogen-doped Ti 3 C 2 MXene two-dimensional material was investigated by photocatalytic degradation of MO. The degradation efficiency was defined as η = (C 0 − C t )/C 0 , where C 0 is the initial concentration of methyl orange and C t is the concentration of methyl orange at time t. Blank experiments with degradant solutions under the same conditions without a catalyst were also performed for comparison. In Figure 7a, Ti 3 C 2 MXene materials with nitrogen doping ratios of 1:2, 1:5, 1:8, and 1:10 were tested. Compared to pure Ti 3 C 2 MXene, N-Ti 3 C 2 2D nanomaterials with high surface area and large pore volume due to N-Ti 3 C 2 1:2, N-Ti 3 C 2 1:5, N-Ti 3 C 2 1:8 and N Ti 3 C 2 1:10 two-dimensional nanomaterials all exhibited better photo-catalytic ability. Especially, N-Ti 3 C 2 1:8 had the best photo-catalytic ability to degrade MO under UV light, and it degraded 98.73% of the methyl orange dye after 20 min of irradiation, and the removal rate of MO was 99.93% at 40 min. Figure 7b shows the pseudo-first-order kinetic plot of MO degradation with the equation: −ln(C t /C 0 ) = kt. Figure 7c depicts the kinetic rate constant (k) estimated using the pseudo-first-order kinetic model, as well as the variation of k for all samples visually, N-Ti 3 C 2 1:2, N-Ti 3 C 2 1:5, N-Ti 3 C 2 1:8 and N-Ti 3 C 2 1:10 for The kinetic rate constants for MO removal by 2D nanomaterials were 0.00574, 0.02387, 0.14669, 0.18717, and 0.02959 min −1 . The above results show that the N-Ti 3 C 2 1:8 material shows the best photocatalytic performance with abundant active sites. The main reason may be that the nitrogen-doped materials have more abundant surface functional groups, which improve the charge transfer efficiency and enhance the electrical conductivity of the materials.
In addition, the reusability of N-Ti3C2 1:8 2D nanomaterial was carried out to assess the stability. As seen in Figure 7d, the catalytic activity of the N-Ti3C2 1:8 2D nanomaterial for the degradation of methyl orange dye still reached 94.87% after four cycles. The catalytic degradation efficiency decreased only slightly after 4 cycles, and the slight decrease might be due to the loss of photocatalyst during the recovery process. The results demonstrate that the N-Ti3C2 1:8 two-dimensional nanomaterials have good chemical stability.

Effect of Dye Concentration on Degradation Rate
The experimental dye concentration gradient ranged from 20-60 mg/L, and it can be seen from Figure 8 that the photocatalytic degradation rate decreases sharply with the increase in dye concentration. There are two reasons for this: Firstly, at a certain amount of catalyst, the greater the concentration of dye, the slower the decrease in chromaticity, and the longer the time required for complete degradation [51]. Secondly, the solution light transmittance decreases with increasing dye concentration, the radiant energy absorbed by the catalyst decreases, and the photocatalytic efficiency becomes poor [52,53]. In addition, we also compared different two-dimensional material-based composite photocatalysts listed in Table 1. In addition, the reusability of N-Ti 3 C 2 1:8 2D nanomaterial was carried out to assess the stability. As seen in Figure 7d, the catalytic activity of the N-Ti 3 C 2 1:8 2D nanomaterial for the degradation of methyl orange dye still reached 94.87% after four cycles. The catalytic degradation efficiency decreased only slightly after 4 cycles, and the slight decrease might be due to the loss of photocatalyst during the recovery process. The results demonstrate that the N-Ti 3 C 2 1:8 two-dimensional nanomaterials have good chemical stability.

Effect of Dye Concentration on Degradation Rate
The experimental dye concentration gradient ranged from 20-60 mg/L, and it can be seen from Figure 8 that the photocatalytic degradation rate decreases sharply with the increase in dye concentration. There are two reasons for this: Firstly, at a certain amount of catalyst, the greater the concentration of dye, the slower the decrease in chromaticity, and the longer the time required for complete degradation [51]. Secondly, the solution light transmittance decreases with increasing dye concentration, the radiant energy absorbed by the catalyst decreases, and the photocatalytic efficiency becomes poor [52,53]. In addition, we also compared different two-dimensional material-based composite photocatalysts listed in Table 1.

Optical Properties
The UV-vis absorption properties and band gap of the materials were investigated by UV-vis diffuse reflectance spectroscopy. From Figure 9a, it can be seen that the absorption range of N-Ti3C2 is wider, and the absorption edge extends to 800 nm. The absorption of N-Ti3C2 is higher than that of Ti3C2, which is related to the doping of nitrogen elements. Moreover, a new absorption band appears at the position of the peak around 400 nm after nitrogen doping. Considering that the previous XRD and XPS tests showed the appearance of TiO2 in the sample after nitrogen doping, this absorption should be caused by TiO2. It can be seen from Figure 9b that nitrogen doping reduces the band gap, and the band gap of the material after N doping is reduced from 1.376 eV to 1.145 eV.

Optical Properties
The UV-vis absorption properties and band gap of the materials were investigated by UV-vis diffuse reflectance spectroscopy. From Figure 9a, it can be seen that the absorption range of N-Ti 3 C 2 is wider, and the absorption edge extends to 800 nm. The absorption of N-Ti 3 C 2 is higher than that of Ti 3 C 2 , which is related to the doping of nitrogen elements. Moreover, a new absorption band appears at the position of the peak around 400 nm after nitrogen doping. Considering that the previous XRD and XPS tests showed the appearance of TiO 2 in the sample after nitrogen doping, this absorption should be caused by TiO 2 . It can be seen from Figure 9b that nitrogen doping reduces the band gap, and the band gap of the material after N doping is reduced from 1.376 eV to 1.145 eV.

Proposed Degradation Pathway and Photocatalytic Mechanism
In Figure 10, to further explore the photo-catalytic active substance, is to conduct a capture experiment of the active substance. In the degradation of MO by N-Ti3C2 1:8 2D nanomaterial, ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) as a hole scavenger (h + ), isopropyl alcohol (IPA) as a hydroxyl radical (•OH) scavenger, sodium iodate (NaIO3) as an electron (e − ) scavenger and p-benzoquinone (BQ) as superoxide radical (•O2 − ) scavengers were analyzed for the active substances that play a major role in the photocatalytic degradation of methyl orange dye. The catalytic activity of N-Ti3C2 1:8 2D nanomaterial for MO degradation was found to be unaffected by the presence of EDTA-2Na, showing that the photo-catalytic active component was not h + . And the catalytic activity decreased significantly after the addition of IPA, NaIO3, and BQ, which proved that •OH, e − , and •O2 − were the photo-catalytic active substances that played the main role in the UV degradation of MO experiments. Organic pollutants first diffuse from the contaminated solution to the surface of the photocatalyst and then desorb to the outer surface of the photocatalyst through effective adsorption and products of redox reactions. Based on the analysis of active substances, a

Proposed Degradation Pathway and Photocatalytic Mechanism
In Figure 10, to further explore the photo-catalytic active substance, is to conduct a capture experiment of the active substance. In the degradation of MO by N-Ti 3 C 2 1:8 2D nanomaterial, ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) as a hole scavenger (h + ), isopropyl alcohol (IPA) as a hydroxyl radical (•OH) scavenger, sodium iodate (NaIO 3 ) as an electron (e − ) scavenger and p-benzoquinone (BQ) as superoxide radical (•O 2 − ) scavengers were analyzed for the active substances that play a major role in the photocatalytic degradation of methyl orange dye. The catalytic activity of N-Ti 3 C 2 1:8 2D nanomaterial for MO degradation was found to be unaffected by the presence of EDTA-2Na, showing that the photo-catalytic active component was not h + . And the catalytic activity decreased significantly after the addition of IPA, NaIO 3 , and BQ, which proved that •OH, e − , and •O 2 − were the photo-catalytic active substances that played the main role in the UV degradation of MO experiments.

Proposed Degradation Pathway and Photocatalytic Mechanism
In Figure 10, to further explore the photo-catalytic active substance, is to conduct a capture experiment of the active substance. In the degradation of MO by N-Ti3C2 1:8 2D nanomaterial, ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) as a hole scavenger (h + ), isopropyl alcohol (IPA) as a hydroxyl radical (•OH) scavenger, sodium iodate (NaIO3) as an electron (e − ) scavenger and p-benzoquinone (BQ) as superoxide radical (•O2 − ) scavengers were analyzed for the active substances that play a major role in the photocatalytic degradation of methyl orange dye. The catalytic activity of N-Ti3C2 1:8 2D nanomaterial for MO degradation was found to be unaffected by the presence of EDTA-2Na, showing that the photo-catalytic active component was not h + . And the catalytic activity decreased significantly after the addition of IPA, NaIO3, and BQ, which proved that •OH, e − , and •O2 − were the photo-catalytic active substances that played the main role in the UV degradation of MO experiments. Organic pollutants first diffuse from the contaminated solution to the surface of the photocatalyst and then desorb to the outer surface of the photocatalyst through effective adsorption and products of redox reactions. Based on the analysis of active substances, a Organic pollutants first diffuse from the contaminated solution to the surface of the photocatalyst and then desorb to the outer surface of the photocatalyst through effective adsorption and products of redox reactions. Based on the analysis of active substances, a speculative method to explain the photocatalytic activity of N-Ti 3 C 2 material for the degradation of pollutants is proposed. As shown in Figure 11, Ti 3 C 2 is recognized as a material with excellent electrical conductivity, which can effectively separate electrons and holes and inhibit their complexation, and N doping further enhances this property. Photogenerated electrons and holes are generated on the surface of N-Ti 3 C 2 MXene under the photoexcitation of UV light. And some of the photogenerated electrons on the conduction band of N-Ti 3 C 2 MXene after photoexcitation are directly involved in the degradation of MO as a major active substance. Another part of photogenerated electrons reacts with O 2 to produce the active substance superoxide radical (•O 2 − ) to participate in the degradation process of MO. The other part of photogenerated electrons reacts with O 2 to produce the reactive substance superoxide radical (•O 2 − ) to participate in the degradation process of MO [60]. The photogenerated holes left in the valence band react with adsorbed hydroxide ions (OH − ) and H 2 O to produce hydroxyl radicals (•OH), allowing the oxidation of organic pollutants by •OH [61,62]. These highly reactive species (•O 2 − , •OH, and e − ) enable rapid photodegradation of MO under UV light irradiation. This is in agreement with our active species capture experiment Figure 10. speculative method to explain the photocatalytic activity of N-Ti3C2 material for the degradation of pollutants is proposed. As shown in Figure 11, Ti3C2 is recognized as a material with excellent electrical conductivity, which can effectively separate electrons and holes and inhibit their complexation, and N doping further enhances this property. Photo-generated electrons and holes are generated on the surface of N-Ti3C2 MXene under the photoexcitation of UV light. And some of the photogenerated electrons on the conduction band of N-Ti3C2 MXene after photoexcitation are directly involved in the degradation of MO as a major active substance. Another part of photogenerated electrons reacts with O2 to produce the active substance superoxide radical (•O2 − ) to participate in the degradation process of MO. The other part of photogenerated electrons reacts with O2 to produce the reactive substance superoxide radical (•O2 − ) to participate in the degradation process of MO [60]. The photogenerated holes left in the valence band react with adsorbed hydroxide ions (OH − ) and H2O to produce hydroxyl radicals (•OH), allowing the oxidation of organic pollutants by •OH [61,62]. These highly reactive species (•O2 − , •OH, and e − ) enable rapid photodegradation of MO under UV light irradiation. This is in agreement with our active species capture experiment Figure 10.

Conclusions, Ongoing Challenges, and Perspective
In this work, N-Ti3C2 MXene two-dimensional nanomaterials were prepared by a facile and controllable strategy combining solvent heat treatment and non-in situ nitrogen doping using urea-saturated alcohol solution as a liquid nitrogen source. Its methyl orange dye removal rate was 98.73% after 20 min of UV lamp irradiation, and it still had good photo-catalytic activity after four cycles. N-doped Ti3C2 MXene would not only provide an effective electron transfer pathway but also increase the interlayer distance of MXene and expose more active sites, thus showing extraordinary photo-catalytic performance in pollutant degradation. Moreover, it further demonstrates that the electrical conductivity of N-Ti3C2 MXene can effectively transfer photo-generated electrons, resulting in a large amount of photo-catalytically active substances (•OH, e − , and •O2 − ). Therefore, N-Ti3C2 MXene two-dimensional nanomaterial can be used as an effective and promising photo-catalyst for wastewater purification.

Conclusions, Ongoing Challenges, and Perspective
In this work, N-Ti 3 C 2 MXene two-dimensional nanomaterials were prepared by a facile and controllable strategy combining solvent heat treatment and non-in situ nitrogen doping using urea-saturated alcohol solution as a liquid nitrogen source. Its methyl orange dye removal rate was 98.73% after 20 min of UV lamp irradiation, and it still had good photo-catalytic activity after four cycles. N-doped Ti 3 C 2 MXene would not only provide an effective electron transfer pathway but also increase the interlayer distance of MXene and expose more active sites, thus showing extraordinary photo-catalytic performance in pollutant degradation. Moreover, it further demonstrates that the electrical conductivity of N-Ti 3 C 2 MXene can effectively transfer photo-generated electrons, resulting in a large amount of photo-catalytically active substances (•OH, e − , and •O 2 − ). Therefore, N-Ti 3 C 2 MXene two-dimensional nanomaterial can be used as an effective and promising photocatalyst for wastewater purification.
For the present, there are some limitations to this work. The excitation light source used during the experiments is UV light, which is more energetic, and the organic pollutants degraded in the photo-catalytic study are relatively single. The cycling experiments have not yet reached efficiency saturation. There is a need to further explore the photocatalytic activity of N-Ti 3 C 2 MXene under sunlight or visual light to degrade a variety of organic pollutants, such as antibiotics. And the photo-catalytic cycle of the material needs to be further investigated until the cycling efficiency is saturated. In addition, a toxicity assessment of the treated solution is needed to ensure that the discharged wastewater will reduce the adverse environmental impact.