Steering Charge Directional Separation in MXenes/Titanium Dioxide for Efficient Photocatalytic Nitrogen Fixation

Abstract

Photocatalytic nitrogen fixation has attracted much attention because of its ability to synthesize ammonia under mild conditions. However, the ammonia yield is still greatly limited by the sluggish charge separation and extremely high N₂ dissociation energy. Herein, two-dimensional Ti₃C₂ MXene ultrathin nanosheets were introduced to construct Ti₃C₂/TiO₂ composites via electrostatic adsorption for photocatalytic nitrogen fixation. The photocatalytic activity experiments showed that after adding 0.1 wt% Ti₃C₂, the ammonia yield of the Ti₃C₂/TiO₂ composite reached 67.9 μmol L⁻¹ after 120 min of light irradiation, nearly 3 times higher than that of the monomer TiO₂. XPS, DRS, LSV, and FTIR were used to explore the possible photocatalytic nitrogen fixation mechanism. Studies showed that a close interfacial contact has been formed via the bonding mode of =C-O between the Ti₃C₂ and TiO₂ samples. The formed =C-O bond boosts an oriented photogenerated charge separation and transfer in the Ti₃C₂/TiO₂ composite. This work provides a promising idea for constructing other efficient MXene-based composite photocatalysts for artificial photosynthesis.

1. Introduction

Ammonia plays a vital role in the national economy due to its foundation in the national nitrogen fertilizer industry and is an indispensable part of chemical raw materials [1,2,3]. Since the beginning of the last century, ammonia synthesis has been highly dependent on conventional industrial production technology and the “Haber-Bosch” process, which uses N₂ and H₂ as raw materials, and metal Fe as a catalyst [4,5]. However, it brings about huge energy consumption for the extremely harsh reaction requirements of high temperature and high pressure, simultaneously releasing massive amounts of CO₂ gas during ammonia synthesis. Therefore, seeking a mild and sustainable technology for green ammonia production becomes extremely urgent.

In 1977, Schrauzer and Guth discovered that TiO₂ semiconductor materials can be applied for ammonia production under UV light irradiation at ambient temperature and pressure [6]. Since then, photocatalytic N₂ conversion to ammonia using water and N₂ as raw materials has received widespread attention [7,8,9]. The core of photocatalytic N₂ fixation technology is the design and preparation of highly efficient photocatalytic materials. At present, a variety of photocatalysts have been reported for N₂ fixation, such as Ni₂P, Bi₃O₄Br, TiO₂, and so on [10,11]. Among them, titanium dioxide (TiO₂) has attracted much attention in the field of photocatalysis because of its notable structural stability and high catalytic performance. However, the band gap of TiO₂ in the range from 3.0 to 3.2 eV is too wide and not conducive to the timely separation of photogenerated charge carriers, thus restricting the photocatalytic N₂ fixation activity of TiO₂ materials [12,13,14,15]. Therefore, how to further improve the carrier separation rate to optimize the N₂ photoreduction performance of TiO₂ materials has important research value.

Transition metal carbide/nitride (MXene) materials are novel two-dimensional materials which were prepared by etching out the “A” group in the MAX phase (A: H, Al, Ga, In, Si, Ge, Sn, Pb, P, or As) [16]. In recent years, MXene has received widespread attention in the research fields of energy storage, water treatment, catalysis, and electromagnetic shielding [17,18]. Ti₃C₂ as one of the typical MXene materials has great potential applications in the photocatalytic hydrogen evolution, CO₂ reduction, and nitrogen fixation owing to its excellent metal conductivity, hydrophilicity, electrical conductivity, chemical stability, and photostability [19,20,21]. In addition, the terminal on the surface of Ti₃C₂ contains the abundant functional groups (-OH, -O, and -F), providing adequate active sites for the formation of a close contact interface with other semiconductors [22]. Moreover, thanks to the strong electron transfer characteristics of Ti₃C₂, a Schottky barrier could be formed between the interface of Ti₃C₂ and the other semiconductor materials, which plays a crucial role in inhibiting the recombination of photogenerated electrons and holes [23]. Zhao et al. found that a special built-in electric field was formed by combing Ti₃C₂ with MIL-100(Fe), and thus boosted the charge transfer rate and the photocatalytic nitrogen fixation activity [24]. Hence, the fabrication of novel Ti₃C₂-based composite photocatalytic systems has a good research prospect [25].

In this work, composite photocatalysts of TiO₂ nanoparticles modified Ti₃C₂ ultrathin nanosheets (Ti₃C₂/TiO₂) were synthesized via an ultrasound-assisted method for photocatalytic nitrogen fixation. The yield of ammonia over the Ti₃C₂/TiO₂ materials and pure TiO₂ was evaluated in the absence of any sacrifice. Photocatalytic activity experiments revealed that the optimum addition of ultrathin Ti₃C₂ nanosheets was 1.0 wt%, and the NH₄⁺ yield of the Ti₃C₂/TiO₂-2 composite in pure water was 3.25 times that of the pure TiO₂. The results show that the =C-O bond that formed between the ultrathin Ti₃C₂ nanosheets and the TiO₂ nanoparticles induced the directional migration of interfacial charges, greatly improving the utilization efficiency of photogenerated electrons. Thereby the enhanced photocatalytic nitrogen fixation activity of the Ti₃C₂/TiO₂ composite materials was realized.

2. Results and Discussion

The Ti₃C₂/TiO₂ composite materials were synthesized via electrostatic adsorption with different Ti₃C₂ addition contents of 0.5 wt%, 1.0 wt%, and 2.0 wt%, respectively. The transmission electron microscope (TEM) technique was used to explore the morphology of TiO₂, Ti₃C₂, and Ti₃C₂/TiO₂ samples. TEM images in Figure 1a,b show that the pure TiO₂ presents a uniform nanoparticle morphology with a diameter around 25 nm, while Ti₃C₂ is a large-size ultrathin nanosheet structure. Figure 1c shows a TEM image of the as-synthesized Ti₃C₂/TiO₂-2 composite, which distinctly reveals that a strong interfacial contact has been formed between the TiO₂ nanoparticles and the Ti₃C₂ ultrathin nanosheets. The HRTEM image of Ti₃C₂/TiO₂-2 in Figure 1d further verifies this result. The lattice fringes of 0.358 and 0.364 nm correspond to the (101) crystal plane of the TiO₂ monomer [26]. A distinct boundary can be observed between the TiO₂ nanoparticles and Ti₃C₂ ultrathin nanosheets, demonstrating the successful construction of the Ti₃C₂/TiO₂-2 composite system.

To further explore the crystal structures of the obtained TiO₂ and Ti₃C₂/TiO₂-x (x = 1, 2 and 3) samples, the materials were characterized using an X-ray powder diffractometer (XRD). As can be seen in Figure 2a, the obvious characteristic peaks at 25.3°, 37.8°, 48.1°, 53.9°, and 55.1° correspond well with the (101), (004), (200), (105), and (211) crystalline faces in the anatase phase TiO₂, respectively. For the Ti₃C₂/TiO₂ composites, the crystal phases of these composites still exactly match the anatase TiO₂ after different amounts of Ti₃C₂ were introduced [27]. It can be observed in Figure S1 (in the Supplementary Materials) that the characteristic peak (101) in the Ti₃C₂/TiO₂ composites tend toward smaller diffraction angles compared to the pure TiO₂ with the increased amount of added Ti₃C₂, indicating the existence of interaction force between Ti₃C₂ and TiO₂ samples. Moreover, no other diffraction peaks belonging to Ti₃C₂ can be found in the XRD patterns of the Ti₃C₂/TiO₂ composites, which is attributed to the small amounts of Ti₃C₂ added to the composites. FT-IR spectra were collected to further verify the coexistence of Ti₃C₂ and TiO₂, and are shown in Figure 2b. The characteristic peaks at 511 and 700 cm⁻¹ in the monomer TiO₂ and Ti₃C₂/TiO₂ materials are assigned to the telescopic vibration of Ti-O-Ti [28]. A newly emerged peak at 1051 cm⁻¹ can be observed in the Ti₃C₂/TiO₂ materials, and the peak intensity enhances gradually as the amount of added Ti₃C₂ increases, which belongs to the vested in tensile vibration of =C-O [29]. It could be reasonably speculated that a linkage between C and O has been established in the Ti₃C₂/TiO₂ composite after the combination of TiO₂ and Ti₃C₂. The same phenomenon also occurs in the Raman spectra of TiO₂ and Ti₃C₂/TiO₂. In Figure S2a (in the Supplementary Materials), the series of characteristic peaks at 143.8, 196.1, 397.2, 516.3, and 639.0 cm⁻¹ are attributed to the E_g, E_g, B_1g, A_1g, and E_g modes of the anatase structures, respectively. Meanwhile, the strongest peak at 143.8 cm⁻¹ is derived from the extended vibration of anatase, indicating the anatase crystal phase of the obtained TiO₂ [30]. In Figure S2b (in the Supplementary Materials), the characteristic peaks at 1433 and 1558 cm⁻¹ are attributed to the characteristic peaks of Ti₃C₂. Simultaneously, with the increase in the added amount of Ti₃C₂, the corresponding characteristic peak intensity of Ti₃C₂ becomes more obvious. Raman spectra further demonstrate the successful construction of the Ti₃C₂/TiO₂ composite, which is consistent with the results of HRTEM, XRD, and FTIR.

X-ray photoelectron spectroscopy (XPS) was used to investigate the elemental composition and surface chemistry of the obtained TiO₂ and Ti₃C₂/TiO₂-2 samples. In the Ti 2p high-resolution XPS spectra (Figure 3a), two peaks of 457.2 and 463.0 eV in the pure TiO₂ correspond to the Ti 2p_3/2 and Ti 2p_1/2 orbitals of Ti⁴⁺ [31]. Significantly, the Ti 2p characteristic peaks in the Ti₃C₂/TiO₂-2 shift to 457.0 and 462.8 eV in the direction of lower binding energy, illustrating that the charge density on the surface of TiO₂ has changed. It can be legitimately speculated that a charge transfer behavior occurs in the Ti₃C₂/TiO₂ composite thanks to the close contact between the TiO₂ and Ti₃C₂. In the O 1s high-resolution XPS spectra (Figure 3b), the peaks at 528.3 eV in the pure TiO₂ and 528.5 eV in Ti₃C₂/TiO₂-2 are attributed to the lattice oxygen of O-Ti-O. The peak at 530.1 eV of pure TiO₂ and at 530.1 eV of Ti₃C₂/TiO₂-2 belong to the adsorbed oxygen [32]. For the Ti₃C₂/TiO₂ composite material, the O 1s peak also shifts to the lower binding energy, which may be caused by the mutative electron density around O atoms arising from the formation of =C-O bonds between O and C. This result is consistent with the FT-IR spectra analyses.

To delve into the effect on the photocatalytic N₂ fixation activity after the introduction of Ti₃C₂ ultrathin nanosheets, the ammonia yields of the prepared Ti₃C₂/TiO₂ composite materials and pure TiO₂ nanoparticles were systematically evaluated. Each of the photocatalytic activities were obtained from three parallel experiments. The nitrogen fixation experiments were performed under the constant temperature of 25 °C with a 300 W Xe lamp as the light source. None of the sacrifice agents or cocatalysts were introduced into the photocatalytic system throughout the process. In Figure 4, it could be found that all of the prepared materials exhibit no N₂-fixing activity in darkness. After irradiation under a Xe lamp for 120 min, the NH₄⁺ yield of pure TiO₂ was determined to be 20.9 μmol L⁻¹. The photocatalytic N₂ reduction activities of the Ti₃C₂/TiO₂ composites significantly increase after the addition of different Ti₃C₂ contents. Among these samples, the Ti₃C₂/TiO₂-2 composite shows the highest NH₄⁺ production performance of 67.9 μmol L⁻¹, more than thrice of that pure TiO₂. Therefore, it can be confirmed that 1.0 wt% of Ti₃C₂ is the optimal additive amount for the construction of the Ti₃C₂/TiO₂ composite photocatalytic system. The boosted N₂ photo-fixation activity in the Ti₃C₂/TiO₂ composites might be ascribable to the unique ultrathin structure of Ti₃C₂. A charge interaction via the formed =C-O linkage in the Ti₃C₂/TiO₂ composites decreases the adsorption activation of N₂, while accelerating the separation and transfer of photogenerated charge carriers. Moreover, a couple of condition experiments have been carried out, and the corresponding results are displayed in Figure S3 (in the Supplementary Materials). Activity results revealed that the production of NH₄⁺ was extremely scarce without the devotion of the photocatalysts; thus, Ti₃C₂/TiO₂ is an indispensable condition for the photocatalytic N₂ reduction. Under dark conditions, the absence of NH₄⁺ production indicates that light is an indispensable driver supplying abundant electrons for the adsorption and activation of N₂. Moreover, almost no NH₄⁺ can be detected after replacing N₂ with Ar, further attesting that N₂ is the source of NH₄⁺ in this reaction.

The optical absorption properties of Ti₃C₂/TiO₂ composite materials were investigated in detail using UV-visible diffuse reflectance spectra (DRS) to reveal the role of Ti₃C₂ in the Ti₃C₂/TiO₂ composite materials (Figure 5a). The maximum absorption band edge of the monomer TiO₂ nanoparticles is estimated to be 369 nm, indicating that the TiO₂ can only be excited in the ultraviolet light region. When the amount of Ti₃C₂ is introduced, the light response range of the synthesized composite materials tend to expand to the visible light, which also proves that the construction of the composites is conducive to enhancing the visible light harvesting ability, thereby promoting the photocatalytic N₂ fixation reaction. However, it can be seen that the Ti₃C₂/TiO₂-3 composite exhibit the strongest light absorption ability among the synthesized photocatalysts. The decreased ammonia productivity of Ti₃C₂/TiO₂-3 might be caused by the diminished intrinsic absorption of the TiO₂, due to the massive Ti₃C₂ ultrathin nanosheet coating on the surface of TiO₂ nanoparticles. The N₂ adsorption–desorption isotherms shown in Figure 5b are used to explore the changes in the specific surface area of the achieved TiO₂ and Ti₃C₂/TiO₂ composite materials. According to the Brunauer–Emmett–Teller (BET) surface area data of TiO₂ and Ti₃C₂/TiO₂ composite materials, it can be observed that the surface area of the original TiO₂ was 45.96 m² g⁻¹. The specific surface areas of Ti₃C₂/TiO₂-1, Ti₃C₂/TiO₂-2, and Ti₃C₂/TiO₂-3 composites were also determined to be 45.67, 48.46, and 48.18 m² g⁻¹, respectively. The result shows that no significant changes have occurred in the specific surface area of the Ti₃C₂/TiO₂ composites, indicating that minimal addition of Ti₃C₂ had little effect on the specific surface area of the composite materials. BET results revealed that the specific surface area is not a crucial factor affecting photocatalytic activity.

Meanwhile, the band structure of the prepared TiO₂ nanoparticles has also been explored via different means of characterization. From the DRS measurements (Figure S4, in the Supplementary Materials), the bandgap (E_g) for the TiO₂ nanoparticles was estimated to be ≈ 3.05 eV [33]. Figure S5 (in the Supplementary Materials) is the valence band spectrum and the Mott–Schottky plot of the TiO₂ material. According to the Mott–Schottky plot, the intersection point of the curve and the X-axis in the figure is the flat-band potential (E_fb) of the TiO₂ material, which is calculated to be −0.13 V (vs. NHE) after conversion. The band gap between the valence band potential (E_VB), and the Fermi level of the TiO₂ material is 1.25 V, so the band gap between E_VB and E_fb is 1.25 V. Therefore, E_VB of the TiO₂ material can be estimated to be 1.12 V. The conduction band potential (E_CB) of the resulting TiO₂ material is −1.92 V according to the conduction band calculation formula E_CB = E_VB − E_g. According to the reduction potential of N₂ molecules, the E_CB of the monomer TiO₂ is sufficiently negative to drive the N₂ reduction reaction. Therefore, this photocatalytic system is feasible in thermodynamics [34,35].

Generally, the separation and migration efficiency of photogenerated charge carriers has a decisive effect on the photocatalytic activity of semiconductors. Therefore, the charge separation behavior of the obtained photocatalysts was further studied using the photocurrent curves and electrochemical impedance spectroscopy (EIS). In Figure 6a, the transient photocurrent response tests of the prepared composite materials and pure TiO₂ were carried out under Xe lamp illumination to explore the separation of photogenerated electron–hole pairs. However, the photocurrent response value is barely detected in the pure TiO₂, indicating the low separation efficiency of the photogenerated carriers. The current values of Ti₃C₂/TiO₂ composite materials dramatically increase after combining with the Ti₃C₂, among which the Ti₃C₂/TiO₂-2 composite material possesses the highest current values [36]. The apparently enhanced current signals are attributed to the formation of close contact between Ti₃C₂ and TiO₂, advancing the separation rate of the photogenerated electrons [37]. However, a sudden decrease in the photocurrent value occurs in the Ti₃C₂/TiO₂-3 composite, which is thanks to the excess Ti₃C₂ hindering the light absorption of TiO₂. EIS tests of the monomer TiO₂ and Ti₃C₂/TiO₂ composite materials were further performed to reveal the migration ability of the photogenerated carriers, and the results are shown in Figure 6b [38]. Compared with the monomer TiO₂ material, the arc radii of the Nyquist circle of Ti₃C₂/TiO₂ composite materials are significantly reduced, indicating that the migration efficiency of photogenerated carriers of Ti₃C₂/TiO₂-x composites is improved after recombination with Ti₃C₂. Among them, the Ti₃C₂/TiO₂-2 material has the smallest arc radius, demonstrating that a higher photogenerated carrier migration ability has been achieved, which is consistent with the results of the measured photocurrent curves. The linear sweep voltammogram curves (LSV) of the pure TiO₂ and Ti₃C₂/TiO₂ composite materials were used to further explore the N₂ activation ability. The working electrode in the electrochemical test is a TiO₂ or Ti₃C₂/TiO₂-2 modified ITO electrode, and the obtained results are shown in Figure 6c,d. In the saturated N₂ atmosphere, the current densities of the pure TiO₂ and Ti₃C₂/TiO₂ composite materials are −0.42 and −0.61 mA cm⁻², respectively. After illuminating the working electrode for 1 min, the current densities of TiO₂ and Ti₃C₂/TiO₂ composite materials increased to −0.44 and −0.64 mA cm⁻², respectively. With the extension of illumination time to 5 min, the current densities of TiO₂ and Ti₃C₂/TiO₂ composite materials respectively continue to increase to −0.46 and −0.68 mA cm⁻², indicating that more photogenerated electrons have been produced in the Ti₃C₂/TiO₂ composite to participate in the N₂ fixation reaction. Significantly, the current densities of TiO₂ and Ti₃C₂/TiO₂ materials in the Ar atmosphere are much smaller than that of the N₂ atmosphere. Meanwhile, there is no significant change to be found after light illumination. The above data show that the photogenerated electrons generated by the constructed Ti₃C₂/TiO₂ composite materials can effectively reduce N₂, exhibiting excellent photocatalytic ammonia production activity.

Based on the experimental results, a possible photocatalytic N₂ fixation mechanism for Ti₃C₂/TiO₂ was proposed (Figure 7). First of all, under illumination, the electrons located on the valence band of TiO₂ material are excited to transition to the conduction band, and Ti₃C₂ material can be used as a photogenerated electron reservoir due to its unique layered structure and the formed linkage =C-O in the Ti₃C₂/TiO₂, so that the photogenerated electrons can be further transferred to Ti₃C₂, and the photogenerated carrier recombination is inhibited, thereby effectively improving the photogenerated charge separation efficiency. In addition, Liao et al. calculated the adsorption capture capacity of TiO₂ and Ti₃C₂ materials for N₂ through DFT. The results demonstrated that the adsorption energy of Ti₃C₂ was 2.713 eV, much higher than that of TiO₂ (0.170 eV) [39]. In addition, the N=N bond lengths for the adsorbed N₂ were 1.351 Å for Ti₃C₂ and 1.113 Å for TiO₂, respectively. It could be verified that the Ti₃C₂ materials possess an excellent N₂ adsorption activation capacity. Therefore, Ti₃C₂ ultrathin nanosheets in the Ti₃C₂/TiO₂ composites could be used as active sites for adsorbing a large amount of N₂ and transforming it into an active state during the photocatalytic nitrogen fixation reaction. The photogenerated electrons enriched on the surface of Ti₃C₂ materials have a sufficient negative reduction potential to effectively reduce the adsorbed N₂ to ammonia. This work provides new ideas for the preparation of MXene-based materials.

3. Materials and Methods

3.1. Materials and Characterizations

Ti₃AlC₂ of 98% purity was obtained from Forsman; hydrofluoric acid (HF), dimethyl sulfoxide (DMSO), tetrabutyl titanate, ethanol and ammonia were all fractional pure concentrations obtained from Sinopharm Chemical Reagent; potassium sodium tartrate (>99%) was obtained from Sigma-Aldrich in Shanghai, China; and Knott’s reagent (13.61% NaOH, 5.07% KI, 9.49% HgI₂) was obtained from of HACH.

X-ray diffraction (XRD) patterns were tested with an XRD-6100 from Shimadzu corporation (Kyoto, Japan) with a PANalytical X’Pert Pro MPD diffractometer (Almelo, The Netherlands) using an X’Celerator RTMS detector with a Cu source at 30 KV in the range of 2θ = 10–80°; the scan rate is 7°/min. Scanning electron microscopy (SEM) was performed using a JSM-7001F from HITACHI (Tokyo, Japan). Transmission electron microscopy (TEM) was performed using a JSM-2010 from JEOL (Tokyo, Japan). Diffuse reflection spectra (DRS) of the samples were conducted on a UV–2450 spectrophotometer from SHIMADZU in the range of 200–800 nm. XPS spectra were investigated using an American VG scientific K-Alpha electron spectrometer. The surface properties were characterized by X-ray photoelectron spectroscopy (XPS, VG Scientific, Waltham, MA, USA). The binding energies were calibrated with respect to the C 1s peak (284.8 eV).

3.2. Synthesis of the TiO₂ Material

Firstly, 100 mL of anhydrous ethanol and 4 mL of tetrabutyl titanate were mixed uniformly, and then 1 mL of distilled water was added to the mixture, stirred continuously for 2 h and then ripened for 12 h. The obtained product was washed with anhydrous ethanol three times and dried at 60 °C. Next, 0.4 g of the dried product was weighed and mixed with 10 mL of anhydrous ethanol, 5 mL of water, and 0.5 mL of ammonia, stirring for 0.5 h; the resulting dispersion was transferred to a high-pressure reactor, heated under the condition of 200 °C for 16 h. Subsequently, the resulting product was centrifuged and washed three times with distilled water and anhydrous ethanol, and then dried in a vacuum to obtain the TiO₂ precursor. Finally, 0.35 g of the TiO₂ precursor was weighed with a porcelain boat and calcined in a muffle furnace at 500 °C for 120 min, with a temperature increase rate of 10 °C min⁻¹.

3.3. Synthesis of the Ti₃C₂ Ultrathin Nanosheets

This sample was synthesized based on our previous work [40]. First, 2.0 g of Ti₃AlC₂ was weighed and 30 mL of 49% hydrofluoric acid solution was added to it and stirred at 50 °C for 24 h. The resulting product was centrifuged at 5000 rpm and washed with deionized water until the pH of the solution was greater than 6, and then centrifuged at 5000 rpm for 10 min to obtain product A. Subsequently, 1.0 g of product A was weighed and added to a solution made of 5 mL dimethyl sulfoxide (DMSO). Subsequently, 1.0 g of product A was weighed and 5 mL of dimethyl sulfoxide (DMSO) was added to form a solution, which was pumped to vacuum and stirred at room temperature for 24 h. The product was centrifuged at 5000 rpm to remove the upper layer of DMSO. The upper layer of DMSO was obtained as intermediate product B. Next, 300 mL of deionized water was added to product B, Ar was passed through it as a protective gas, and the product was ultrasonicated in an ice-water bath for 6 h. Finally, the product was centrifuged at 3500 rpm for 30 min, and the precipitate was removed. The upper layer of the solution was taken, filtered, and freeze-dried in vacuo to obtain Ti₃C₂ powder. The powder was prepared into a Ti₃C₂ solution with a concentration of 1 g L⁻¹ and kept as a reserve.

3.4. Synthesis of the Ti₃C₂/TiO₂ Composite Materials

An amount of 0.099 g of final TiO₂ product was weighed in a 100 mL round-bottomed flask to which 0.5 wt%, 1 wt%, and 2 wt% Ti₃C₂ solution were added and recorded as Ti₃C₂/TiO₂-1, Ti₃C₂/TiO₂-2 and Ti₃C₂/TiO₂-3, respectively. Next, 50 mL of ethylene glycol was added to the solution, and a glass stopper was coated with vacuum grease and pumped to vacuum. To the solution, 50 mL of ethylene glycol was added, and the round-bottomed flask was evacuated to vacuum after the glass stopper was coated with vacuum grease. After stirring for 30 min at room temperature and ultrasonication for 5 h, the TiO₂ and Ti₃C₂ were compounded, and the obtained products were centrifuged at high speed, washed three times each with distilled water and anhydrous ethanol, and dried under vacuum at 60 °C to obtain Ti₃C₂/TiO₂ products.

3.5. Photocatalytic N₂ Fixation Activity Experiments

First, 50 mg of the prepared photocatalytic materials was weighed and dispersed in 100 mL of ammonia-free water. Then, N₂ was continuously passed into the photocatalytic nitrogen fixation system at a rate of 60 mL min⁻¹ for 30 min under dark conditions to saturate it. The temperature was maintained at 25 °C in a circulating water bath, and a 300 W Xenon lamp was used as a light source, with a distance of 12 cm from the photocatalytic vial. The light intensity of 300 W Xenon lamp in this work was determined to be 0.046 W/cm² using a FZ-A radiometer. Every 1 h after the light was turned on, 7 mL of reaction solution was taken from the reaction system, centrifuged, and filtered through 0.22 μm filter membrane. Then, 5 mL of the clear solution was pipetted into a cuvette, 100 μL of potassium sodium tartrate and 100 μL of Nessler’s agent were added into the cuvette, and the solution was shaken and dispersed and then left to be dispersed for 10 min. Next, 3 mL of the solution was pipetted into the cuvette and measured at the absorbance of 420 nm with an ultraviolet spectrophotometer. The concentration of ammonia in the solution was obtained by referring to the standard curve that had been developed. Each set of data was tested more than three times to ensure the accuracy of the data.

3.6. Electrochemical Tests

Electrochemical tests of the prepared photocatalytic materials were carried out in a standard three-electrode system, in which the reference electrode was a saturated Ag/AgCl electrode and the counter electrode was a platinum wire electrode. The working electrode was prepared by dispersing a certain amount of photocatalytic material in a solvent and coating it on conductive glass (ITO, 0.5 × 1 cm²). The specific preparation method was as follows: during the photocurrent and impedance test, 2 mg of the material was ultrasonically dispersed in 200 μL of ethylene glycol, and 20 μL of the dispersed solution was added dropwise onto the conductive surface of ITO and dried with infrared lamp. For the linear voltammetry and Mott–Schottky curve tests, 5 mg of material was ultrasonically dispersed in 990 μL of isopropanol solution (isopropanol/water = 3:1) and 10 μL of naphthol mixture, and 20 μL of the dispersion was applied to the conductive surface of the ITO and dried with an infrared lamp. The electrolyte for photocurrent testing was 0.1 mol L⁻¹ phosphate buffer salt solution (pH = 7); the electrochemical impedance solution was 5 mmol L⁻¹ Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ in 0.1 M KCl solution. The electrolyte for testing the linear voltammetric and Mott–Schottky curves was 0.2 mol L⁻¹ sodium sulfate solution. A 300 W Xenon lamp (current 15 A) was used as a light source.

4. Conclusions

In summary, the Ti₃C₂/TiO₂ composite materials were successfully prepared at room temperature by introducing the Ti₃C₂ ultrathin nanosheets into the TiO₂ nanoparticles using ethylene glycol as a solvent through a simple ultrasonic reaction. Without the addition of any sacrifices, the NH₄⁺ yield of the Ti₃C₂/TiO₂-2 composite was 3.25 times than greater that of the pure TiO₂. Various studies have demonstrated that the unique layered structure of the MXene ultrathin nanosheets provides a larger specific surface area and abundant exposed active sites for the high dispersion of TiO₂ nanoparticles. Moreover, the formed =C-O linkage in the Ti₃C₂/TiO₂ composites promotes the effective separation rate and directed movement of photogenerated charge carriers, thus greatly boosting the adsorption and activation of N₂ molecules. This work provides a feasible idea for the design of MXene-based composites with high performance for artificial nitrogen-fixing.