Ti3C2@g-C3N4/TiO2 Ternary Heterogeneous Photocatalyst for Promoted Photocatalytic Degradation Activities

Abstract

Ternary heterojunction photocatalysts can improve the transport and separation of photogenerated electrons and holes, which could promote their reduction and oxidation properties for environmental and energy applications. In this research, the ternary photocatalyst Ti₃C₂@TiO₂/g-C₃N₄ was successfully synthesized via direct electrostatic self-assembly during hydrothermal process. Ti₃C₂ MXene was used to optimize the interfacial carrier transport and separation between the interfaces. The obtained ternary heterostructured photocatalyst had a higher photocatalytic degradation performance for removing rhodamine B (RhB) and 4-chlorophenol (4-CP). The synergistic effect of heterojunction between g-C₃N₄ and TiO₂ and Schottky barrier presented among TiO₂ and Ti₃C₂ suppressed the recombination of the photogenerated electron–hole pairs. Moreover, the Ti₃C₂ can serve as an active site for the adsorption and activation of organic pollutants resulting from sufficient functional groups (F⁻ here).

1. Introduction

Industrial wastewater seriously affects the environment due to its high concentrations of organic pollutants and substantial toxicity [1,2,3,4]. Semiconductor photocatalysis has been widely used to treat industrial wastewater because of its high efficiency, low consumption and environmental friendliness [5,6,7,8,9,10]. The photocatalysis reaction process involves the excitation, separation, transport and recombination of electron-hole pairs [11,12]. Constructing heterogeneous structures can separate photogenerated carriers, prolonging the lifetime of photogenerated electron–hole pairs with increased reduction and oxidation activities [13,14,15,16,17,18].

Two-dimensional (2D) materials own unique layered structures and excellent chemical stability, and many researchers have focused on them [19,20,21,22,23]. Among all 2D layered materials, g-C₃N₄ has demonstrated excellent activity in the photolytic aqua-hydrogen and photocatalytic degradation of organic pollutants due to other excellent optical absorption capabilities and suitable band positions [24,25,26,27]. However, the spontaneous recombination of generated electron-hole pairs significantly limits the applications of g-C₃N₄ in practical photocatalysis [28]. Therefore, the fabrication of heterogeneous photocatalysts with other semiconductors can effectively prevent the recombination of generated electron-hole pairs leading to the promoted overall efficiency [29,30,31]. Various g-C₃N₄-based heterogeneous nanostructures have been synthesized to improve the separation efficiency of photogenerated electron-hole pairs through rapid charge transfer at the interfaces [32,33,34,35,36,37].

MXene, as a new 2D transition metal carbide or carbon-nitride, was obtained by etching the A-layer in MAX using hydrofluoric acid to obtain the MXene phase [38,39,40,41]. Ti₃C₂, the most common MXene, possesses a two-dimensional graphene-like structure, leading to its capabilities such as excellent light absorption, electrical conductivity and good hydrophilicity [42,43,44]. Thus, it has been used as a cocatalyst for photocatalysis reactions. Ti₃C₂ can also effectively enhance light absorption and promote the separation of photogenerated carriers by forming Schottky junctions [45,46,47]. MXene-based photocatalysts have been investigated in many different photocatalytic applications, such as the degradation of organic contaminants [48,49], water splitting [50,51], CO₂ reduction [52,53], NO_x removal [54] and N₂ fixation [55,56,57].

Our previous research showed TiB₂-TiO₂@g-C₃N₄ (TBCN) ternary heterojunction composites with promoted photocatalytic degradation performances for RhB and 4-CP removal [58]. Herein, new Ti₃C₂@TiO₂/g-C₃N₄ (TC-TBCN) ternary heterostructured photocatalysts were synthesized through the direct electrostatic self-assembly of TBCN with Ti₃C₂ via a hydrothermal process; the addition of Ti₃C₂ could effectively enhance light absorption and promote the transfer of photogenerated carriers by forming Schottky junctions with TBCN. The morphology, pore structure, phase composition, optical properties and photocatalytic performances of the TC-TBCN photocatalysts were investigated. The obtained ternary Ti₃C₂@TiO₂/g-C₃N₄ photocatalysts could effectively improve the separation and migration efficiency of photogenerated charges, and its large surface active sites and effective interfacial charge transfer showed a better photodegradation performance for both rhodamine B (RhB) and 4-chlorophenol (4-CP), providing an essential idea for degrading pollutants and treating organic wastewater. Finally, we propose the mechanism of the photocatalytic degradation of such ternary photocatalysts.

2. Materials and Methods

2.1. Chemicals

Titanium aluminum carbide (Ti₃AlC₂) powders were purchased from Laizhou Kai Ceramic Materials Co., Ltd. (Yantai, China). Hydrofluoric acid (HF, 40%), titanium boride (TiB₂), melamine (C₆H₆N₆, ≥ 99.0%), anhydrous ethanol (C₂H₅OH), rhodamine B (RhB), 4-chlorophenol (4-CP), silver nitrate (AgNO₃), isopropyl alcohol (IPA), benzoquinone (BQ) and disodium ethylenediaminetetraacetate (EDTA-2Na) were all purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China). All chemical reagents were of analytical grade (AG) and used without other treatments in our experiments.

2.2. Synthesis of Catalysts

2.2.1. Synthesis of TBCN

Our previous research reported on TiB₂-TiO₂@g-C₃N₄ (TBCN) composites with core-shell structures, the synthesis process of which is presented in Figure 1 [58]. The mass ratio of TiB₂ to melamine was 1:100 in this experiment. Firstly, 0.1 g TiB₂ and 10 g melamine powders were ground for 10 min to form a uniform mixed powder. Then, the obtained gray powder was transferred to a crucible with a cover and wrapped using aluminum foil, and then it was calcinated at 550 °C for 5 h in a muffle furnace in air. g-C₃N₄ was grown in situ on the TiB₂ surface, making a g-C₃N₄ coating on the TiB₂ core, and the products were obtained after washing and drying, here named TBCN.

2.2.2. Synthesis of Ti₃C₂

As shown in Figure 1, 1.0 g of Ti₃AlC₂ powder (MAX phase) was put in a PTFE reactor; then, 20 mL of a 40% HF solution was added for etching the Al layer. The solution was stirred for 3 days at room temperature. Then, the pH of the obtained suspension could be tuned to pH ≥ 6 using lots of deionized (DI) water. Finally, the resulting black Ti₃C₂F_x powder was dried in an oven after washing with ethanol through centrifugation for use in the next step.

2.2.3. Synthesis of TC-TBCN

The TC-TBCN was synthesized via a hydrothermal process, as shown in Figure 1, where 0.2 g of TBCN powder and a given amount of Ti₃C₂F_x powder were added into 60 mL of DI water under stirring, followed by ultrasonication to obtain the dispersed suspension. Then, the suspension was transferred to a Teflon-lined stainless-steel autoclave and maintained at 120 °C for 12 h. A solid yellow powder deposited at the bottom was collected and washed thoroughly with deionized water after the autoclave cooled down. The final TC-TBCN products were obtained after drying in a vacuum drying oven. The mass ratios of Ti₃C₂F_x and TBCN were 0 wt%, 1 wt%, 2.5 wt%, 5 wt% and 10wt%, and were designated as 1-TBCN, 1TC-TBCN, 2.5TC-TBCN, 5TC-TBCN and 10TC-TBCN, respectively.

2.3. Characterization

The morphologies of the obtained products were conducted on scanning electron microscopy (SEM, FEI Quanta FEG 250, Hillsboro, OR, USA) and transmission electron microscopy (TEM, JEOL JEM-2100, Tokyo, Japan). An X-ray diffraction spectrometer (XRD, Bruker D8 ADVANCE, Karlsruhe, Germany) was used to determine the crystal phases. The synthesized photocatalysts’ surface composition and elemental chemistry were measured on an X-ray photoelectron spectrometer (XPS, Thermo Scientific K-Alpha+, Waltham, MA, USA) equipped with a monochromatic Al K α X-ray source (1486.6 eV). The XPS spectra were calibrated with the C1s peak of amorphous carbon (284.6 eV) and fitted using XPSPEAK 4.1 software. The UV-Vis absorption spectra were obtained with a UV-Vis spectrophotometer (UV-Vis, Shimadzu UV2600, Kyoto, Japan). A specific surface area analyzer (JW-BK200B, Beijing JWGB Sci&Tech Co., Ltd., Beijing, China) was used to determine the Nitrogen adsorption-desorption isotherms, and the specific surface area and pore size distribution were obtained based on the Brunauer-Emmett-Teller (BET) method and Barret-Joyner-Halenda (BJH) method. Fourier transform infrared spectra (FTIR) were obtained using a FTIR spectrophotometer (Thermo Scientific Nicolet iS50, Waltham, MA, USA), equipped with an ATR accessory.

2.4. Photocatalytic Performance Measurements

RhB and 4-CP were used as target pollutants to evaluate the photocatalytic ability of the obtained photocatalysts in a Pyrex reactor with a volume of 57.5 mL. The detailed experiment was as follows: In total, 20 mg of the photocatalyst was dispersed in 36 mL DI water under sonication for 2 min; then, 4 mL of 100 ppm RhB or 4-CP solution was added and stirred in the dark for 30 min to establish an adsorption-desorption equilibrium of the pollutants. The light source for the photoreaction was a 300 W Xe lamp equipped with a VisREF filter (350 nm–780 nm). The reacted solution was magnetically stirred during the photocatalytic reaction. In total, 1.0 mL sample suspensions were taken from the reactor intermittently in specific time intervals and filtered through a 0.22 µm pore-sized PTFE syringe filter to determine the removal rate of the pollutants. A UV–Vis spectrophotometer (Shimadzu UV2600, Kyoto, Japan) was used to determine the absorbance of the resulting RhB solution at the characteristic wavelength (554 nm). The changes in absorbance values quantitatively were used to calculate the degradation percentage. In contrast, the 4-CP concentration was determined on a high-performance liquid chromatography system (HPLC, LC-2030Plus, Shimadzu, Kyoto, Japan) equipped with a Shim Pack C18 column using a UV-Vis detector (measurement details: column temperature: 35 °C; mobile phase: acetonitrile-0.1 vol. % acetic acid 60:40 (v:v); flow rate: 1.0 mL/min; detect wavelength: 280 nm). Incidentally, the Cl⁻ was also determined using the ion chromatograph (IC, Shine CIC-D120, Qingdao, China) equipped with a Shine SH-AC-3 column with conductivity detection (measurement details: mobile phase: column temperature: 35 °C; 2.4 mM Na₂CO₃ + 1.0 mM NaHCO₃ aqueous solution; flow rate: 1.0 mL/min; current: 30 mA;). Finally, photocatalytic efficiency was calculated according to the concentration changing (C/C₀), where C and C₀ are the measured and initial concentrations of RhB and 4-CP, respectively. The final degradation efficiency of RhB could be calculated according to the Lambert-Beer law.

3. Results

3.1. Structure Characterization

3.1.1. Structure and Composition of Ti₃C₂

Scanning electron microscopy (SEM) was used to investigate the morphologies of the MAX and MXene phases. As shown in Figure 2, the Ti₃AlC₂ powders were measured with any other treatments. As shown in Figure 2a, the original Ti₃AlC₂ MAX possessed an irregular blocky layered structure. After HF etching, the Ti₃C₂ MXene exhibited a nanosheet-like layer structure, with some thin nanosheets clearly shown in Figure 2b, which was significantly different from the MAX phase, indicating that the HF etching could remove the Al atom layer sandwiched in between the MAX layers [59].

Figure 3a shows the XRD patterns of the Ti₃AlC₂ before and after being etched in the HF solution. The diffraction peaks of the Ti₃AlC₂ located at 9.5°, 19.2°, 35.9°, 38.8° and 41.7° belonged to the different crystal planes (002), (004), (101), (104) and (105), respectively. The characteristic peaks of Ti₃AlC₂ located at 9.5° and 19.2° shifted to lower angles, and the intensity was decreased after HF etching [60]. The highest diffraction peak at 38.8° from the (104) crystal plane disappeared, indicating the successful preparation of Ti₃C₂ via a complete Al layer etching [61]. In addition, the red curve was the XRD pattern of the Ti₃C₂F_x MXene sample. The peaks matched well with those of Ti₃C₂F_x synthesized by others [62].

An XPS measurement was used to investigate the elemental composition and surface chemical environment of the Ti₃C₂ MXene sample. Figure 3b depicts the XPS survey spectra of Ti₃C₂, which confirmed the presence of C, Ti, O and F elements. The presence of F and O in Ti₃C₂ obtained from the HF etching indicated surface termination (Ti₃C₂(OH)₂, Ti₃C₂F_x), because the enhanced surface activity of Ti₃C₂ after the Al layer etching led to reactions with the surrounding −O or −F [63]. Figure 3c shows the high-resolution Ti 2p XPS spectra, the peaks were corresponding to Ti−C, indicating that the Ti−C binding signal resulted from the Ti atoms in the interior of the MXene layers. As shown in Figure 3d, the C1s spectra mainly exhibited two peaks ascribed to the C−C (284.6 eV) and C−Ti (281.3 eV) bonds [61]. As shown in Figure 3e, the O1s spectra showed three peaks ascribed to Ti−O (529.9 eV), C−O−Ti (530.9 eV) and C−Ti−OH_x (531.9 eV) [64]. The F 1s spectra in Figure 3f showed the presence of Ti−F−C bonds located at 684.4 eV and Ti−F−Ti bonds located at 685.6 eV [65]. Overall, the surface of Ti₃C₂ contained −O and −F surface functional groups.

3.1.2. Structure and Composition of TC-TBCN

Figure 4 shows the morphologies of the TC-TBCN samples. All samples were irregularly shaped powders, and the size of the powders was not uniform, but fine granular materials were coated on the surface. As shown in Figure 4a,b, the size of the 0TC-TBCN sample was smaller than that of TBCN, which indicated the partial decomposition and recrystallization of TBCN during the hydrothermal process. Compared with TBCN, the roughness of the obtained TC-TBCN samples significantly increased after the hydrothermal treatment. Ti₃C₂ will be partially decomposed and recrystallized during the hydrothermal process. Thus the accordion-like Ti₃C₂ could not be observed in the low-content samples (1TC-TBCN and 2.5TC-TBCN), as shown in Figure 4c,d. In addition, the further increased Ti₃C₂ content significantly affected the composites’ morphology. The accordion-like structures could be observed in the 5TC-TBCN and 10TC-TBCN samples, as shown in Figure 4e,f. C₃N₄ and TiO₂ were coated on Ti₃C₂. Figure 5 shows the TEM images of the 5TC-TBCN sample. The small crystal particles with clear TiO₂ lattice fringes were grown on the surface by transforming Ti₃C₂ into TiO₂ through a hydrothermal reaction.

An XPS measurement was also used to investigate the surface chemical environment of the 5TC-TBCN sample. Figure 6a depicts the XPS survey spectra of 5TC-TBCN, which confirmed that the elements (C, N, Ti, O and F) existed in the sample. The high-resolution Ti 2p XPS spectra in Figure 6b were shown to possess two peaks of Ti 2p_3/2 and Ti 2p_1/2 that were deconvoluted into two components, Ti−C and Ti−O, indicating the presence of Ti₃C₂ and TiO₂ in the TC-TBCN sample. The high-resolution C1s spectrum shown in Figure 6c mainly exhibited four peaks. The peak located at 281.3 eV could be ascribed to the C−Ti of Ti₃C₂, and the peaks at 284.6 eV, 286.4 eV and 288.6 eV were assigned to the C−C, C−NH₂ and N−C = N bonds in the aromatic skeleton rings of g-C₃N₄, respectively. For the N1s spectrum in Figure 6d, the spectrum was fitted into three peaks, the peak at 400.9 eV was ascribed to the C−N−H functional groups, the peaks detected at a binding energy of 399.60 eV was ascribed to tertiary N−(C)₃ and the peak observed at 398.51 eV corresponded to the C−N = C coordination from the sp²-bonded N in the triazine rings of g-C₃N₄ [58]. Moreover, the intensities of these peaks differed from those of g-C₃N₄ obtained via calcination of melamine, which also indicated that the decomposition and recrystallization of TBCN happened during the hydrothermal process. As shown in Figure 6e, the O1s spectrum showed two peaks at 529.9 eV and 531.9 eV, ascribed to Ti−O and H−O−H from the H₂O/O₂ adsorption, respectively. The Ti−O bonds confirmed the formation of TiO₂, which was consistent with the TEM data. No obvious peak in the B 1s XPS spectrum (Figure 6f) indicated that the TiB₂ remaining in the TBCN sample would react with water during the hydrothermal process. The F 1s XPS spectrum is shown in Figure 6f, with peak v at 685.3 eV corresponding to the Ti−F binding energy. The F 1s spectrum in Figure 3f showed the presence of Ti−F bonds at 685.5 eV, confirming the presence of Ti₃C₂ remaining in the TC-TBCN sample.

Figure 7a shows the XRD patterns of the obtained products. The diffraction peaks at 26.5° and 13.2° were consistent with the (002) and (001) planes of g-C₃N₄ (PDF#87–1526). Such typical characteristics of interlayer stacking structures indicated the presence of g-C₃N₄ in the TBCN and TC-TBCN samples. The characteristic diffraction peaks of TiB₂ were located at 34.1°, 44.4°, 61.1°, 68.1° and 68.3°, corresponding to the (100), (101), (110), (102) and (111) planes of TiB₂ (PDF#35-0741), respectively. The peaks located at 27.4°, 54.3° and 56.6° corresponded to the (110), (211) and (220) planes of rutile (PDF#21-1276), indicating that TiB₂ could be partially oxidized into rutile TiO₂ during the thermal treatment at 550 °C. It should be noted here that the most prominent peak located at approximately 26~28° was the mixed peak from the (002) plane of g-C₃N₄, the (001) plane of TiB₂ and the (110) plane of rutile TiO₂. Incidentally, the peak at approximately 18.0° was related to the carbon phase through the calcination of melamine. All the peaks of g-C₃N₄, TiB₂ and rutile TiO₂ decreased with the Ti₃C₂ addition increasing, while the intensities of the peaks located at 25.3°, 36.9°, 48.0° and 53.9° corresponded to anatase TiO₂ (PDF#21-1272) increased with the Ti₃C₂ addition increasing, indicating that the Ti₃C₂ or TiB₂ remaining in TBCN were partially transformed into anatase TiO₂ during the hydrothermal process.

3.1.3. N₂ Sorption Isotherms and Pore Size Distributions

Figure 7b shows the products’ N₂ adsorption-desorption isotherm and pore size distribution plots. The hysteresis loop was of type A, consistent with cylindrical pores. The isotherm’s desorption branch could be used to determine the pore size distribution using the Barrett-Joyner-Halenda (BJH) method [58]. The detailed BET surface areas, total pore volumes and average pore diameters of the obtained samples are shown in Table 1. As shown in Table 1, adding TiB₂ brought along the growing site for g-C₃N₄, enabling the TBCN samples to process larger surface areas than g-C₃N₄. The addition of Ti₃C₂ slightly affected the surface areas, and the recrystallization of TBCN led to a shell coated on Ti₃C₂. Thus, the appropriate amount of Ti₃C₂ addition brought about proper growing sites, enabling an increased large surface area. 5TC-TBCN had a specific surface area of approximately 22.034 m²/g, and the smallest average pore size among all the samples.

3.1.4. Optical Absorption Performances of the Samples

The photocatalytic performance of photocatalysts is highly dependent on the light absorption properties of the materials. Figure 7c shows the diffuse reflection absorption spectra of different products. The absorption in 500~1400 nm was enhanced with the increase of Ti₃C₂ content, thus, improving the utilization of visible light during the photocatalytic degradation. However, the Ti₃C₂ addition did not change the absorption edge of TBCN. Combined with the SEM and XPS results analysis, the added Ti₃C₂ only partially converted to TiO₂, and some of them remained as the TC-TBCN products, resulting in an enhanced absorption in 500~1400 nm. Meanwhile, more Ti₃C₂ would be present in the final product with the increase in Ti₃C₂ addition, and, thus, the light absorption abilities in the visible region increased. Moreover, Ti₃C₂ increased the light absorption and created a charge transfer channel in the composite photocatalysts. Furthermore, the optical band gap was estimated by applying Tauc’s equation. The curves in Figure 7d were obtained using Tauc’s formula [(Ahν) = B(hν − Eg)ⁿ] (n = 2 for indirect transitions). The values of optical band gaps were 1.16, 2.78, 2.78 and 2.80 eV for Ti₃C₂, TBCN, 5TC-TBCN and C₃N₄, respectively.

3.2. Photocatalytic Activity

The photocatalytic performance was evaluated from the degradation of RhB and 4-CP under light irradiation in the solution. The data are presented in Figure 8a, with the photocatalysts with different additions showing some differences after 120 min of light irradiation, and their photocatalytic degradation activities increased first and then decreased with the increased Ti₃C₂ content. Figure 8b shows the corresponding kinetic fitting curves, which conformed to the pseudo-first-order kinetics. The degradation rate constants (k) of RhB could be calculated from the curves and are shown in Table 2. The 5TC-TBCN displayed the highest k value of RhB (0.01575 min⁻¹). Thus, the addition of Ti₃C₂ was favorable for the photodegradation of RhB. As expected, the photocatalytic activity of the TC-TBCN sample was higher than that of the absolute g-C₃N₄ and TBCN samples due to the introduction of Ti₃C₂, resulting in an enhanced light absorbance and a centered electron acceptor. The cyclic stability of the photocatalysts during practical application was also essential. The cycling degradation of RhB was conducted to measure the recyclability and stability of 5TC-TBCN. As shown in Figure 8c, RhB could maintain a high removal rate, and no significant decrease could be observed after five cycles under the same conditions. Generally, photogenerated holes and electrons can be transferred to the catalyst surface or interface to accede to the oxidation and reduction reactions in the photocatalytic process. Furthermore, to prove the contribution of different reactive groups to the photocatalytic degradation of RhB, isopropyl alcohol (IPA), benzoquinone (BQ), disodium ethylenediaminetetraacetate (EDTA-2Na) and silver nitrate served as trapping agents for the hydroxyl groups (OH), superoxide anion (O₂₋), photogenerated holes (h⁺) and electrons (e⁻), respectively. As shown in Figure 8d, the removal efficiency of RhB decreased to 14%, 23%, 23%, 34% and 40% in the presence of IPA, EDTA-2Na, N₂, BQ and AgNO₃, respectively. The degradation rate of RhB was limited in the presence of these scavengers.

Furthermore, the stability of the sample was also confirmed by using FTIR. The samples were obtained from a hydrothermal process, which indicated their excellent stability in water, even hot water. Therefore, FTIR was better for checking the surface change after the photocatalytic reaction. The FTIR spectra are shown in Figure 9. The spectral patterns in 1100–1750 cm⁻¹ corresponded to the aromatic C−N stretching vibration and C=N stretching vibrations from C₃N₄ [33,66]. Furthermore, the tri-s-triazine unit band was observed at approximately 810 cm⁻¹, and the N−H stretching and O−H stretching broad vibration bands from the adsorbed H₂O were observed at 2800–3350 cm⁻¹ [67]. There were no significant differences in the curves, indicating no great changes after the photocatalytic reactions.

A similar photocatalytic performance was also conducted in photocatalytic degradation (oxidation) of 4-CP, and the time profiles of the concentrations of 4-CP and Cl⁻ are presented in Figure 10. The measurements began after continuously stirring for 30 min under dark conditions to reach the adsorption equilibrium. HPLC and IC were used to determine the removal of 4-CP, and the data from HPLC and IC were consistent [68]. The TC-TBCN samples showed a similar photocatalytic degradation (oxidation) performance with RhB degradation, and 5TC-TBCN also showed the highest degradation efficiency of 4-CP, indicating that adding Ti₃C₂ was also helpful for the photodegradation of 4-CP. As shown in Figure 10, 4-CP and Cl⁻ concentrations were very stable after 120 min of light irradiation alone without the presence of photocatalysts as the blank experiment, showing that 4-CP was hardly self-degradable [10].

4. Discussion

The photocatalyst mechanism was investigated by adding different scavengers during the RhB photodegradation process. The TC-TBCN samples obtained from the hydrothermal treatment of Ti₃C₂ and TBCN showed excellent photocatalytic degradation activity for RhB and 4-CP. Finally, we proposed the mechanism of TC-TBCN for photocatalysis based on the characterization and photocatalytic performances, as shown in Figure 11 [58,69]. Both g-C₃N₄ and TiO₂ could produce electrons and holes under light irradiation, and the addition of Ti₃C₂ enhanced the light absorbance capability of the TC-TBCN samples. Figure 10 delineates that the photogenerated electrons on the conduction band (CB) of g-C₃N₄ would be delivered to the CB of TiO₂ and further transferred to the Ti₃C₂. Ti₃C₂ could act as an intermediate electron acceptor for photogenerated electrons [70,71]. The accumulated electrons on Ti₃C₂ could be associated with the adsorbed oxygen reaction to induce the •O₂⁻ active groups for involvement in RhB and 4-CP removal. However, some photogenerated holes on the valence band (VB) of g-C₃N₄ would combine with the accumulated electrons on Ti₃C₂. Moreover, the electric field of Ti₃C₂ and TiO₂ caused a space charge layer near the Ti₃C₂/TiO₂ interface, resulting in a bending ‘upward’ VB and CB in the ternary photocatalysts. The Schottky barrier at the interfaces between TiO₂ and Ti₃C₂ inhibited the electron-hole pair recombination, resulting in prolonged electron lifetimes. Thus, the amounts of holes at the VB of TiO₂ would react with H₂O to form hydroxyl radicals (OH) on the heterogenous photocatalyst’s surface, which could efficiently oxidize organic pollutants via the holes at the VB of TiO₂ into small intermediates, or directly into end products (CO₂ and H₂O) [41,61]. The g-C₃N₄/TiO₂ heterojunction and Schottky barrier between TiO₂ and Ti₃C₂ boosted the transfer and separation of the photogenerated carriers, thus, suppressing their recombination [49]. Moreover, the sufficient functional groups on Ti₃C₂F_x t could provide active sites for adsorbing and activating organic pollutants. The enhanced charge separation efficiency could prolong the lifetime of the photogenerated carriers and produce more active species; the abundant functional groups of the 5TC-TBCN sample provided more active sites, thus, efficiently degrading RhB and 4-CP [54].

5. Conclusions

In this research, ternary heterogeneous Ti₃C₂@TiO₂/g-C₃N₄ photocatalysts were synthesized through electrostatic self-assembly using a hydrothermal method. The obtained ternary photocatalysts presented a higher photodegradation performance for RhB and 4-CP. The results showed that Ti₃C₂ could construct n-type Schottky heterojunctions between g-C₃N₄ and TiO₂, accepting the photogenerated electrons from the CB of g-C₃N₄; the multiple built-in electric fields also enhanced the charge transfer and suppressed the recombination of electron-hole pairs, resulting in an enhanced charge separation efficiency. The Schottky barrier between TiO₂ and Ti₃C₂ could also boost the transfer and suppress the recombination. Moreover, the sufficient functional groups in Ti₃C₂F_x could serve as active sites, which was beneficial for the adsorption and activation of organic pollutants. Thus, the 5TC-TBCN sample process enhanced the photocatalytic degradation of RhB and 4-CP. Ternary heterojunctions such as the one in this study could be efficient photocatalysts in degrading pollutants from organic wastewater.

Author Contributions

Conceptualization, J.C. (Jing Chen), J.C. (Jingcai Chang) and Z.H.; supervision, J.C. (Jingcai Chang) and Z.H.; project administration, J.C. (Jing Chen) and Z.H.; writing—original draft preparation, Y.B., J.C. (Jing Chen) and Z.H.; methodology, Y.B. and S.X.; validation, Y.B., J.C. (Jing Chen) and S.X.; investigation, Y.B., S.X. and Z.H.; data curation, Y.B., S.X. and Z.H.; writing—review and editing, X.S., S.Z., J.C. (Jing Chen), X.S., S.Z., J.C. (Jingcai Chang) and Z.H.; funding acquisition, J.C. (Jing Chen) and Z.H. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors thank Sen Wang from The State Key Laboratory of Microbial Technology, Shandong University, for the assistance in SEM analysis. The authors also thank Fanping Zhu from the Analysis and Test Center of the School of Environmental Science and Engineering, Shandong University, for the assistance in HPLC and UV–Vis analysis.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. Schematic of the synthesis process for TC-TBCN composite.

Figure 2. SEM images of Ti3AlC2 before (a) and after (b) HF etching for 72 h.

Figure 3. (a) XRD patterns of the MAX phased Ti3AlC2 before and after being etched in HF solution; XPS spectra of Ti3C2: (b) survey; (c) Ti2p; (d) C1s; (e) O 1s; (f) F 1s.

Figure 4. SEM images of different samples: (a) TBCN, (b) 0TC-TBCN, (c) 1TC-TBCN, (d) 2.5TC-TBCN, (e) 5TC-TBCN and (f) 10TC-TBCN.

Figure 5. TEM images of 5TC-TBCN at different magnifications: (a) 4000×; (b) 8000×; (c–d) 500,000×.

Figure 6. XPS spectra of 5TC-TBCN: (a) survey; (b)Ti 2p; (c) C 1s; (d) N 1s.; (e) O 1s; (f) B 1s and F 1s.

Figure 7. (a) XRD patterns, (b) N2 adsorption/desorption isotherms and corresponding pore size distribution curves; (c) UV–Vis DRS spectra; (d) the estimated bandgap energies for the different samples.

Figure 8. (a) Time profiles of the degradation of RhB and (b) the corresponding kinetic fitting curves with irradiation time; (c) photocatalytic stability and (d) photodegradation of RhB over 5TC-TBCN in the presence of different reactive species scavengers. [Catalyst] = 0.5 g/L; [RhB]0 = 10 ppm.

Figure 9. FTIR spectra of 5TC-TBCN before and after the reactions.

Figure 10. (a,b) Time profiles of 4-CP and Cl− concentration during 4-CP removal in the presence of different photocatalysts under light irradiations: [catalyst] = 0.5 g/L; [4-CP]0 = 10 ppm.

Figure 11. Schematic photocatalytic degradation mechanism over 5TC-TBCN.

Table 1

BET surface areas, total pore volumes and average pore sizes of the obtained products.

Sample	BET Surface Area(m2/g)	Total Pore Volume(cm3/g)	Average Pore Size(nm)
g-C3N4	10.937	0.096	5.223
TBCN	17.384	0.138	3.650
0TC-TBCN	16.829	0.129	3.626
1TC-TBCN	17.231	0.140	3.788
2.5TC-TBCN	16.156	0.137	3.728
5TC-TBCN	22.034	0.106	3.447
10TC-TBCN	15.828	0.142	3.659

Table 2

Degradation rate constant (k) values of RhB for different photocatalysts.

Sample	k
g-C3N4	0.0104
TBCN	0.01312
0TC-TBCN	0.00974
1TC-TBCN	0.01202
2.5TC-TBCN	0.01291
5TC-TBCN	0.01575
10TC-TBCN	0.00653

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