Effect of MXene Loaded on g-C 3 N 4 Photocatalyst for the Photocatalytic Degradation of Methylene Blue

: Photocatalytic degradation is one of the environmentally friendly methods used in treating dye wastewater. In this study, a series of MXene/g-C 3 N 4 heterostructure photocatalysts with different loading amounts of MXene (1, 4, 8, and 12 wt.%) were successfully synthesized via the wet impregnation method and their photocatalytic activity was evaluated via the degradation of methylene blue under visible-light irradiation. As such, the 1 wt.% MXene/g-C 3 N 4 heterostructure photocatalyst achieved a high degradation of methylene blue compared to the pure g-C 3 N 4 under visible-light illumination of 180 min. This signiﬁcant improvement was attributed to the intimate interfacial contact, evidently from the FESEM analysis, which allows the smooth photocharge carriers to transport between g-C 3 N 4 and MXene. Additionally, the larger BET surface area demonstrated by the 1 wt.% MXene/g-C 3 N 4 heterostructure allowed this sample to have higher adsorption of dye molecules and provided a higher number of reactive sites, which was beneﬁcial for the enhancement of the photocatalytic activity. Nevertheless, it was found that the excessive loading of MXene can substantially impede photocatalytic activity. This was attributed to the decrease in the active sites, as well as the weakened crystallinity of the MXene/g-C 3 N 4 heterostructure photocatalyst, evident from the FTIR and XRD analysis. All in all, this study has shown the potential of the MXene/g-C 3 N 4 photocatalyst as a promising photocatalyst for highly efﬁcient wastewater treatment applications. degradation mechanism for photocatalyst reaction of MXene/g-C 3 N 4 heterostructure photocatalyst.


Introduction
Nowadays, the textile industry has proven to be one of the most important industries to humankind as clothes serve as a basic need for humans. The textile industry has grown rapidly over recent years, contributing 7% of total world exports worth US $1 trillion and employing 35 million people around the globe [1]. However, the textile industry poses a serious environmental issue as it consumes a huge amount of fresh water on a daily basis and discharges 0.7 megatons of dyes products annually [2]. The released colored dye can cause several environmental issues, such as contamination of viable water sources, as well as endangering aquatic life and eventually causing harm to the human society if not controlled. Methylene blue (MB) can be commonly found inside dye waste and its natural composition is highly carcinogenic due to the presence of aromatic compounds and amines, resulting in histamine poisoning [3].
In recent years, various methods for wastewater treatment, especially for methylene blue, such as adsorption, sedimentation, filtration through a membrane, and coagulation, has been explored [4]. Even though these methods can remove most pollutants, research has also showed that the pollutants were not fully eliminated [5]. Other than that, the aforementioned environmental technologies are not very efficient for this purpose as they demand high chemical and operational costs, generation of complex sludge, and need a long time to work. It is important to explore new alternatives that are able to further degrade methylene blue with low-cost materials, as well as minimize the energy required. AOP (advanced oxidation process) is one of the viable options and is a propitious wastewater treatment activity, capable of degrading most of the pollutants as the as-generated hydroxyl radicals possess high oxidation prospects [6].
Photocatalysis is one of the applications of an advanced oxidation process, utilizing solar energy to initiate the reaction, and has become a very favorable method in treating the water pollution issues due to its sustainability and eco-friendly attributes [7]. Graphitic carbon nitride (g-C 3 N 4 ) has been a promising candidate in the photocatalytic arena as it has high thermal and chemical durability, moderate bandgap energy (~2.7 eV), non-toxicity, is economical, and has a high surface area for the reaction [8]. However, the performance of g-C 3 N 4 upon photocatalytic reaction has been limited due to its high recombination of photogenerated electrons and holes [9]. Therefore, it is very crucial for a subsequent treatment to further improve the photocatalytic reaction of pristine g-C 3 N 4 . In recent years, various strategies for the improvement of g-C 3 N 4 have been proposed, such as structural modifying, improving crystallized structure, combining with a different photocatalyst that has an appropriate band edge position, and loading with a co-catalyst [10]. Among the aforementioned strategies, loading the g-C 3 N 4 with different photocatalyst material for the formation of the heterostructure system has shown promise in expediting the separation of the photocharge carrier with respect to the formation of the Schottky junction [11].
MXene, a new type of 2D transition metal carbide, nitride, or carbonitride, which is obtained by etching the Ti 3 AlC 2 , has caught significant attention as a coupled photocatalyst material for the formation of the heterostructure system. This is attributed to its peculiar features, including excellent stability in structure, a large number of hydrophilic functional groups on its surface, great metal conductivity, and stronger redox reactivity stemming from its surface terminal Ti sites [12]. In light of this, MXene has abundant surface groups and huge exposed metal sites, which enable strong annexation with 2D photocatalyst, making it an efficient co-catalyst to expedite photocatalytic degradation of methylene blue [13,14]. With respect to the abovementioned intriguing features of MXene, it is expected that the combination of MXene and g-C 3 N 4 photocatalyst can further enhance the photocatalytic activity of the g-C 3 N 4 photocatalyst. In addition, encouraged by the advantages of MXene, which possesses an excellent metallic conductivity, the formation of the aforesaid heterostructure system will induce the development of a Schottky junction, which results in the formation of a built-in electric field. Thereinto, this phenomenon is greatly known to significantly improve the photocharge carriers' separation and migration within the heterostructure system, resulting in an augmentation in photocatalytic degradation performance.
Sparked by this inspiration, in the present work, the formation of g-C 3 N 4 /MXene heterostructure photocatalyst with Schottky junction was synthesized via a wet-impregnation method. The performance of the aforesaid heterostructure photocatalyst was evaluated using the degradation of methylene blue. Moreover, several characterization techniques were used in order to examine the properties of the designed photocatalyst, including XRD, FTIR, SAP, XPS, FESEM, HRTEM, and DR-UV-Vis analysis. The obtained photocatalyst composite presents a notable improvement in photocatalytic degradation of methylene blue. The improved photodegradation can be associated with the harmony effect of the Schottky junction formed, as well as the 2D/2D interfacial contact between both materials, enhancing the efficiency for separation and mobility of the photocharge carriers for photocatalytic reaction. On a whole, this work opens up an invigorating avenue for wastewater treatment applications, particularly on the removal of dye, in which this work offers highly efficient photocatalyst materials to be used with excellent performance.

Materials
All of the reagents and solutions used to synthesize photocatalysts and photocatalytic degradation studies were of analytical grade and were used without any further purification. Urea (purity of 99%), hydrofluoric acid (assay at 49%), Ti 3 AlC 2 (purity of 90%), and methylene blue powder were purchased from Sigma-Aldrich company (St. Louis, MO, USA). The deionized water was used in preparing all required aqueous solutions during the experiments.

Photocatalyst Preparation
The g-C 3 N 4 was synthesized using thermal polycondensation of urea by adopting a method from the previous article [15]. Alumina crucible was filled with urea powder until it reached 3 /4 of the crucible, followed by constant heating in a furnace for 120 min at 500 • C with a heating rate of 5 • C/min. After the completion of the reaction, the furnace was left to cool down to room temperature. The green-yellowish powder accumulated inside the crucible was collected and crushed with a mortar.
The formation of MXene was conducted via a chemical etching process according to the reported literature [16]. At room temperature, 1 g of Ti 3 AlC 2 was mixed with 10 mL of 49% HF while slowly stirring the solution for 24 hrs. The precipitate of the etched formate was collected and washed with deionized water and centrifuged until the pH value was close to 7. The moisture of the black powder was removed by letting it dry in the oven at 60 • C overnight.
The composites of MXene/g-C 3 N 4 heterostructure photocatalysts were synthesized by using the wet impregnation method, as shown in Figure 1. Initially, 1 g of g-C 3 N 4 was added with 0.01 g MXene (labelled as 1 wt.% MXene/g-C 3 N 4 ) and 30 mL of deionized water. The mixture then underwent sonication for 30 min at room temperature. Then, the mixture was stirred with heat for 45 min until a slurry was formed. The sample was left to dry in the oven with a temperature of 60 • C for overnight drying. The sample was then collected in a vial. The steps were repeated for the composites labelled as 4, 8, and 12 wt.% MXene/g-C 3 N 4 by changing the weight of MXene to 0.04, 0.08, and 0.12 g, respectively.

Materials
All of the reagents and solutions used to synthesize photocatalysts and photocatalytic degradation studies were of analytical grade and were used without any further purification. Urea (purity of 99%), hydrofluoric acid (assay at 49%), Ti3AlC2 (purity of 90%), and methylene blue powder were purchased from Sigma-Aldrich company (St. Louis, MO, USA). The deionized water was used in preparing all required aqueous solutions during the experiments.

Photocatalyst Preparation
The g-C3N4 was synthesized using thermal polycondensation of urea by adopting a method from the previous article [15]. Alumina crucible was filled with urea powder until it reached ¾ of the crucible, followed by constant heating in a furnace for 120 min at 500 °C with a heating rate of 5 °C/min. After the completion of the reaction, the furnace was left to cool down to room temperature. The green-yellowish powder accumulated inside the crucible was collected and crushed with a mortar.
The formation of MXene was conducted via a chemical etching process according to the reported literature [16]. At room temperature, 1 g of Ti3AlC2 was mixed with 10 mL of 49% HF while slowly stirring the solution for 24 hrs. The precipitate of the etched formate was collected and washed with deionized water and centrifuged until the pH value was close to 7. The moisture of the black powder was removed by letting it dry in the oven at 60 °C overnight.
The composites of MXene/g-C3N4 heterostructure photocatalysts were synthesized by using the wet impregnation method, as shown in Figure 1. Initially, 1 g of g-C3N4 was added with 0.01 g MXene (labelled as 1 wt.% MXene/g-C3N4) and 30 mL of deionized water. The mixture then underwent sonication for 30 min at room temperature. Then, the mixture was stirred with heat for 45 min until a slurry was formed. The sample was left to dry in the oven with a temperature of 60 °C for overnight drying. The sample was then collected in a vial. The steps were repeated for the composites labelled as 4, 8, and 12 wt.% MXene/g-C3N4 by changing the weight of MXene to 0.04, 0.08, and 0.12 g, respectively.

Physicochemical Characterization
The physicochemical properties of the as-prepared MXene/g-C3N4 photocatalysts were studied via several characterization techniques. The crystallinity behavior of the prepared photocatalysts was examined by X-ray diffraction (XRD; PANalytical model: Xpert3 powder) with Cu-Kα radiation. Fourier transform infrared spectroscopy (FTIR) was performed using Shimadzu 8400S within the range of 450 to 4000 cm −1 to identify the various functional groups in the as-prepared photocatalysts. The morphological structures

Physicochemical Characterization
The physicochemical properties of the as-prepared MXene/g-C 3 N 4 photocatalysts were studied via several characterization techniques. The crystallinity behavior of the prepared photocatalysts was examined by X-ray diffraction (XRD; PANalytical model: Xpert3 powder) with Cu-Kα radiation. Fourier transform infrared spectroscopy (FTIR) was performed using Shimadzu 8400S within the range of 450 to 4000 cm −1 to identify the various functional groups in the as-prepared photocatalysts. The morphological structures of the as-prepared MXene/g-C 3 N 4 photocatalysts were examined using HRTEM analysis (120 kV Zeiss Libra 200), FESEM analysis (Zeiss Supra 55VP), and SAP (BET: Micromeritic ASAP 2000). The bandgap energy (DR-UV-Vis) and the surface chemical properties (XPS) were measured using UV-Vis Carry 100 and Thermoscientific K-alpha, respectively.

Photocatalytic Degradation Testing
The photocatalytic degradation activities of the synthesized MXene/g-C 3 N 4 heterostructure photocatalysts were evaluated by using a prepared methylene blue solution. Firstly, 0.5 g of MXene/g-C 3 N 4 sample was mixed with 100 mL of 10 mg/L aqueous methylene blue. The pH of the reaction solution was measured in which was recorded at 8. Then, the solution was left in the dark for one hour in order to achieve the adsorption-desorption equilibrium [4]. Later, the experiment continued by irradiating the solution with a visible light source (500 W halogen lamp) with continuous stirring. The temperature of the photocatalytic reaction was maintained at room temperature by using an external tabletop fan. Subsequently, every 30 min, the methylene blue solution was extracted by using a plastic syringe attached with a 0.45 µm filter. The results were analyzed by using a UV-Vis spectrophotometer (UV-1800, Shimadzu) and the degradation activity of methylene blue solution was studied using the following equation: where Abs 0 and Abs F are the initial and final absorbance of methylene blue solution at a designated time. Figure 2 shows the detailed structures of g-C 3 N 4 , MXene, and the as-synthesized g-C 3 N 4 /MXene photocatalysts that were characterized by FTIR. Three distinguished characterized peaks were observed. The broad adsorption peaks at 3000-3600 cm −1 indicate the existence of the -NH functional group, allowing the N-H stretching vibrations, as well as the termination of OH of the adsorbed hydroxyl species [17,18]. The strong peaks in the region of 1100-1700 cm −1 indicate the C-N and C=N in the heterocycles of CN [13,19]. Other than that, the visible peak at 800 cm −1 is attributable to the breathing mode of tri-s-triazine [15]. With regard to composites of MXene/g-C 3 N 4 , the characteristics of the g-C 3 N 4 were well preserved as the primary characteristic peaks of g-C 3 N 4 were clearly observed. The results show that the growing loading amount of MXene gradually reduced the magnitude of peaks, indicating the reduction of some structures in g-C 3 N 4 through the combination of g-C 3 N 4 with Ti 3 C 2 . The addition of larger amounts of MXene facilitated more changes that occurred to the structure of MXene/g-C 3 N 4 photocatalyst.

XRD Analysis
The XRD analysis of the g-C 3 N 4 , MXene, 1 wt.% MXene/g-C 3 N 4 , 4 wt.% MXene/g-C 3 N 4 , 8 wt.% MXene/g-C 3 N 4 , and 12 wt.% MXene/g-C 3 N 4 heterostructure photocatalysts were performed in order to examine the crystallinity behavior of the as-developed materials. Figure 3 shows the XRD pattern of MXene, which is in agreement with the previously reported study (JCPDS card No. 52-0975) [20,21]. By comparing the obtained results of the as-synthesized photocatalysts, the most intense peak of MXene, which is commonly observed at 39 • of 2θ, disappeared, signifying the disappearance of Al after etching by HF. In this context, the obtained results stipulate that Al managed to be removed and MXene was successfully obtained. Additionally, the (004) peak located at 17.6 • of 2θ was monitored to be shifted to the higher angle, indicating an interaction between MXene and g-C 3 N 4 . Furthermore, the (107) peak observed at 47 • of 2θ became stronger as the amount of MXene loading was increased in the formation of the MXene/g-C 3 N 4 heterostructure photocatalyst.  On the other hand, two different diffraction peaks, located at 12.5° and 27.7° of 2θ were attributed to g-C3N4 footprints (JCPDS card no. 87-1526). The peaks at 27.7° indicate the multilayer π-π stacking of g-C3N4, while the peak of 12.5° signifies the in-plane re peated units [22,23]. The XRD patterns of MXene/g-C3N4 composites with differen amounts of MXene loaded onto the heterostructure sample show almost identical diffrac tion peaks. Nevertheless, as the amount of MXene loading was increased to 12 wt.%, the peaks located at 12.5°, 13.9°, and 35° of 2θ became weakened. Such occurrences may hap pen due to the interaction between MXene and g-C3N4, which disrupts the aromatic con ditions of g-C3N4 [13,24]. As such, the obtained XRD patterns of the as-synthesized heter ostructure photocatalyst proved the successful loading of MXene onto g-C3N4. Further more, higher loading of MXene leads to the shift of peak towards a higher angle, indicat ing stronger interaction of the two different materials, which leads to a further stacking distance of g-C3N4. Figure 4 represents the FESEM micrograph images of the as-synthesized MXene/g C3N4 heterostructure photocatalysts prepared at different amounts of MXene loadings On the other hand, two different diffraction peaks, located at 12.5 • and 27.7 • of 2θ, were attributed to g-C 3 N 4 footprints (JCPDS card No. 87-1526). The peaks at 27.7 • indicate the multilayer π-π stacking of g-C 3 N 4 , while the peak of 12.5 • signifies the in-plane repeated units [22,23]. The XRD patterns of MXene/g-C 3 N 4 composites with different amounts of MXene loaded onto the heterostructure sample show almost identical diffraction peaks. Nevertheless, as the amount of MXene loading was increased to 12 wt.%, the peaks located at 12.5 • , 13.9 • , and 35 • of 2θ became weakened. Such occurrences may happen due to the interaction between MXene and g-C 3 N 4 , which disrupts the aromatic conditions of g-C 3 N 4 [13,24]. As such, the obtained XRD patterns of the as-synthesized heterostructure photocatalyst proved the successful loading of MXene onto g-C 3 N 4 . Furthermore, higher loading of MXene leads to the shift of peak towards a higher angle, indicating stronger interaction of the two different materials, which leads to a further stacking distance of g-C 3 N 4 . Figure 4 represents the FESEM micrograph images of the as-synthesized MXene/g-C 3 N 4 heterostructure photocatalysts prepared at different amounts of MXene loadings. The FESEM images demonstrated the successful formation of layered MXene by etching the Ti 3 AlC 2 MAX using HF [25]. Generally, the Ti 3 AlC 2 structures were made up of an irregular block structure. However, after the etching process with HF, the as-prepared MXene demonstrated an obvious stratification, evincing the successful etching process of aluminum layers. Meanwhile, the presence of a crumple-sheet-like structure in the composite heterostructure photocatalysts is attributed to the morphological structure of the g-C 3 N 4 [26,27]. All of the FESEM images displayed the encapsulation and intimate contact between both g-C 3 N 4 and MXene, evincing the successful development of the heterostructure system. In addition, as both the g-C 3 N 4 and MXene form the 2D nanostructure, intimate contact between each photocatalyst was observed. In light of this, a smooth photocharge carrier transfer was postulated with respect to its well-matched geometrical structure. Thereafter, the issue of the fast recombination of the photocharge carriers within the individual photocatalyst can be mitigated. Moreover, Figure 5 represents the TEM images of the as-synthesized MXene/g-C 3 N 4 heterostructure photocatalysts. A typical 2D multilayered structure could be seen from the inset in Figure 5, where the interlayer spacing measured 0.25 nm. The aforesaid interlayer spacing corresponds to the (101) crystal plane of Ti 3 C 2 MXene [25,28,29]. After the etching process of Ti 3 AlC 2 , the structure of Ti 3 C 2 MXene with a very small spacing has been produced. Nevertheless, the lattice fringe of the pure g-C 3 N 4 photocatalyst was hardly visible, presumably due to the low crystallinity. On a whole, the FESEM and HRTEM analysis verified the successful interfacial contact between both MXene and g-C 3 N 4 particles, which will be beneficial for augmenting the photocharge carriers' separation and migration, resulting in an enhancement in the photocatalytic degradation activity. of the as-synthesized MXene/g-C3N4 heterostructure photocatalysts. A typical 2D multilayered structure could be seen from the inset in Figure 5, where the interlayer spacing measured 0.25 nm. The aforesaid interlayer spacing corresponds to the (101) crystal plane of Ti3C2 MXene [25,28,29]. After the etching process of Ti3AlC2, the structure of Ti3C2 MXene with a very small spacing has been produced. Nevertheless, the lattice fringe of the pure g-C3N4 photocatalyst was hardly visible, presumably due to the low crystallinity. On a whole, the FESEM and HRTEM analysis verified the successful interfacial contact between both MXene and g-C3N4 particles, which will be beneficial for augmenting the photocharge carriers' separation and migration, resulting in an enhancement in the photocatalytic degradation activity.

BET Surface Area and Porosity Analysis
The BET specific surface area, pore volume, and pore size analyses of the as-synthesized MXene/g-C3N4 heterostructure photocatalysts prepared at different amounts of MXene loadings are summarized in Table 1. It was found that the BET surface areas for the pure g-C3N4 and MXene were 4.64 and 6.26 m 2 /g, respectively. Upon the formation of the MXene/g-C3N4 heterostructure system, the BET surface area was substantially increased to 22.58 m 2 /g. Similarly, the pore size and pore volume of the 1 wt.% MXene/g-C3N4 heterostructure sample showed significant enhancement, at 0.123 cm 3 /g and 21.88 nm, respectively. It is worth mentioning that the high BET surface area, pore size, and pore volume are highly desirable as it is beneficial for improving the photocatalytic degradation activity of the heterostructure photocatalyst. This is explained by the fact that more pollutants can be adsorbed via the high surface area of the designed heterostructure photocatalyst. As such, a higher photocatalytic degradation performance will be achieved via the 1 wt.% MXene/g-C3N4 heterostructure sample.

BET Surface Area and Porosity Analysis
The BET specific surface area, pore volume, and pore size analyses of the as-synthesized MXene/g-C 3 N 4 heterostructure photocatalysts prepared at different amounts of MXene loadings are summarized in Table 1. It was found that the BET surface areas for the pure g-C 3 N 4 and MXene were 4.64 and 6.26 m 2 /g, respectively. Upon the formation of the MXene/g-C 3 N 4 heterostructure system, the BET surface area was substantially increased to 22.58 m 2 /g. Similarly, the pore size and pore volume of the 1 wt.% MXene/g-C 3 N 4 heterostructure sample showed significant enhancement, at 0.123 cm 3 /g and 21.88 nm, respectively. It is worth mentioning that the high BET surface area, pore size, and pore volume are highly desirable as it is beneficial for improving the photocatalytic degradation activity of the heterostructure photocatalyst. This is explained by the fact that more pollutants can be adsorbed via the high surface area of the designed heterostructure photocatalyst. As such, a higher photocatalytic degradation performance will be achieved via the 1 wt.% MXene/g-C 3 N 4 heterostructure sample.
Nevertheless, as the amount of MXene loading was increased up to 12 wt.%, the BET surface area, pore size, and pore volume of the rest of the MXene/g-C 3 N 4 heterostructure samples were decreased to 13.70 m 2 /g, 0.033 cm 3 /g, and 9.535 nm, respectively. The lower textural properties demonstrated by the 12 wt.% MXene/g-C 3 N 4 heterostructure sample are highly unfavorable from the photocatalytic viewpoint as this indicates that less available active pore sites will partake in the degradation of methylene blue. Thereafter, a substantial decrease in photocatalytic degradation activity will be monitored. Apart from that, Figure 6 shows that the as-prepared MXene/g-C 3 N 4 heterostructure samples possessed a Type IV adsorption isotherm with an H3 hysteresis pattern. In particular, the observed pattern highlights the mesoporous structure of the as-synthesized MXene/g-C 3 N 4 heterostructure samples, whereby pore size ranged from 2-50 nm. Table 1. BET surface area, pore volume, and pore size of the as-synthesized MXene/g-C 3 N 4 heterostructure photocatalysts.

Photocatalyst
BET Surface Area (m 2 /g) Pore Volume (cm 3 /g) Pore Size (nm) Nevertheless, as the amount of MXene loading was increased up to 12 wt.%, the BET surface area, pore size, and pore volume of the rest of the MXene/g-C3N4 heterostructure samples were decreased to 13.70 m 2 /g, 0.033 cm 3 /g, and 9.535 nm, respectively. The lower textural properties demonstrated by the 12 wt.% MXene/g-C3N4 heterostructure sample are highly unfavorable from the photocatalytic viewpoint as this indicates that less available active pore sites will partake in the degradation of methylene blue. Thereafter, a substantial decrease in photocatalytic degradation activity will be monitored. Apart from that, Figure 6 shows that the as-prepared MXene/g-C3N4 heterostructure samples possessed a Type IV adsorption isotherm with an H3 hysteresis pattern. In particular, the observed pattern highlights the mesoporous structure of the as-synthesized MXene/g-C3N4 heterostructure samples, whereby pore size ranged from 2-50 nm.

Optical Properties
The optical properties of the as-synthesized MXene/g-C 3 N 4 heterostructure photocatalysts prepared at different amounts of MXene loadings were examined using UV-Vis diffuse reflectance spectra (DR-UV-Vis). Figure 7a shows that the g-C 3 N 4 photocatalyst showed a light absorption capacity within the visible-light region. Similarly, the loading of the MXene onto g-C 3 N 4 photocatalyst further narrowed the light absorption capacity with a slight blue shift and red shift in accordance with the amount of MXene loading. In turn, this observation indicates the potential of the photocatalytic light absorption response up to 48% of the total solar light energy. Moreover, as the composite MXene/g-C 3 N 4 heterostructure photocatalysts possess a stronger visible-light absorption capacity, the enhancement in photothermal effect could be observed, which would be beneficial in aiding the photocatalytic activity of the aforesaid samples. Following that, Figure 7b illustrates the plotted and estimated bandgap energy using Tauc plot calculation. The bandgap energy of the pure g-C 3 N 4 was 2.61 eV. The obtained bandgap energy aligns with several previously reported works on g-C 3 N 4 photocatalysts [30,31]. As depicted in Figure 7b, the bandgap energy for the 1 wt.% MXene/g-C 3 N 4 , 4 wt.% MXene/g-C 3 N 4 , 8 wt.% MXene/g-C 3 N 4 , and 12 wt.% MXene/g-C 3 N 4 samples were 2.53, 2.09, 2.20 and 2.31 eV, respectively.

Optical Properties
The optical properties of the as-synthesized MXene/g-C3N4 heterostructure photocatalysts prepared at different amounts of MXene loadings were examined using UV-Vis diffuse reflectance spectra (DR-UV-Vis). Figure 7a shows that the g-C3N4 photocatalyst showed a light absorption capacity within the visible-light region. Similarly, the loading of the MXene onto g-C3N4 photocatalyst further narrowed the light absorption capacity with a slight blue shift and red shift in accordance with the amount of MXene loading. In turn, this observation indicates the potential of the photocatalytic light absorption response up to 48% of the total solar light energy. Moreover, as the composite MXene/g-C3N4 heterostructure photocatalysts possess a stronger visible-light absorption capacity, the enhancement in photothermal effect could be observed, which would be beneficial in aiding the photocatalytic activity of the aforesaid samples. Following that, Figure 7b illustrates the plotted and estimated bandgap energy using Tauc plot calculation. The bandgap energy of the pure g-C3N4 was 2.61 eV. The obtained bandgap energy aligns with several previously reported works on g-C3N4 photocatalysts [30,31]. As depicted in Figure 7b

XPS Analysis
The elemental and surface chemical properties of the 1 wt.% MXene/g-C3N4 heterostructure photocatalyst were examined via XPS analysis. Figure 8 delineates the XPS spectrum of C 1s, Ti 2p, O 1s, and N 1s. As displayed in Figure 8a, three significant peaks were observed in the C 1s spectrum. Accordingly, the observed peaks, located at 281.5, 284.8,

XPS Analysis
The elemental and surface chemical properties of the 1 wt.% MXene/g-C 3 N 4 heterostructure photocatalyst were examined via XPS analysis. Figure 8 delineates the XPS spectrum of C 1s, Ti 2p, O 1s, and N 1s. As displayed in Figure 8a, three significant peaks were observed in the C 1s spectrum. Accordingly, the observed peaks, located at 281.5, 284.8, and 286.3 eV, confirm the presence of the C-Ti, C-C, and C-O bonds, respectively, in the 1 wt.% MXene/g-C 3 N 4 heterostructure photocatalyst [13,24]. The reason behind the observed peaks is the presence of the g-C 3 N 4 particles in the heterostructure sample. Moreover, the peaks located at 284.8 eV are attributable to the C-C adventitious carbon that was used as the energy reference in fitting the XPS spectrum of the 1 wt.% MXene/g-C 3 N 4 heterostructure photocatalyst. Additionally, Figure 8b shows the Ti 2p spectrum, composed of two deconvoluted peaks located at 458.5 and 469.4 eV. The observed peaks were well-matched with the Ti 2p3/2 and Ti 2p1/2 of the Ti-C bond [21,32,33]. The aforesaid deconvoluted peaks demonstrated a strong peak, which was presumably due to the potential of the oxidation on the surface of the MXene, which later converted into Ti-O bonds. Furthermore, the O 1s energy spectrum in Figure 8c exhibited a peak located at 530.1 eV, which can be ascribed to the absorbed hydroxyl group. Apart from that, Figure 8d shows three peaks, deconvoluted at 399.6, 401.7, and 405.8 eV. The three peaks observed coincided well with the C-N=C, C-N-H, and C=N bonds, which originated from the parental g-C3N4 and MXene particles [34,35]. It is worthwhile to note that all of the XPS peaks showed a shift to higher and lower binding energies in comparison to the standard parental g-C3N4 and MXene peaks. The shifting phenomenon of the observed peaks signifies the presence of interfacial photocharge carriers transfer within the 1 wt.% MXene/g-C3N4 heterostructure photocatalyst, which will be beneficial for the photocatalytic degradation activity. Figure 9 shows the results of MB degradation, with different compositions of the as- Additionally, Figure 8b shows the Ti 2p spectrum, composed of two deconvoluted peaks located at 458.5 and 469.4 eV. The observed peaks were well-matched with the Ti 2p 3/2 and Ti 2p 1/2 of the Ti-C bond [21,32,33]. The aforesaid deconvoluted peaks demonstrated a strong peak, which was presumably due to the potential of the oxidation on the surface of the MXene, which later converted into Ti-O bonds. Furthermore, the O 1s energy spectrum in Figure 8c exhibited a peak located at 530.1 eV, which can be ascribed to the absorbed hydroxyl group. Apart from that, Figure 8d shows three peaks, deconvoluted at 399.6, 401.7, and 405.8 eV. The three peaks observed coincided well with the C-N=C, C-N-H, and C=N bonds, which originated from the parental g-C 3 N 4 and MXene particles [34,35]. It is worthwhile to note that all of the XPS peaks showed a shift to higher and lower binding energies in comparison to the standard parental g-C 3 N 4 and MXene peaks. The shifting phenomenon of the observed peaks signifies the presence of interfacial photocharge carriers transfer within the 1 wt.% MXene/g-C 3 N 4 heterostructure photocatalyst, which will be beneficial for the photocatalytic degradation activity. Figure 9 shows the results of MB degradation, with different compositions of the as-synthesized MXene/g-C 3 N 4 heterostructure photocatalyst. For the first hour of investigation, the degradation was commenced under dark conditions and further continued for another 3 h with the presence of light irradiation. In this study, the photocatalytic degradation of MB was determined at 664 nm due to the absence of blue shift of the main peak of MB during the photocatalytic experiment [36]. The efficiency for degradation of the as-prepared samples was calculated by using the equation stated in Section 2.4. The photocatalytic activity of the as-synthesized MXene/g-C3N4 heterostructure photocatalyst indicates that the performance of the heterostructure samples was ordered as 1 wt.% MXene/g-C3N4 sample > 4 wt.% MXene/g-C3N4 sample > 8 wt.% MXene/g-C3N4 sample > pure g-C3N4 sample > 12 wt.% MXene/g-C3N4 sample. In this regard, the 1 wt.% MXene/g-C3N4 sample exhibited the highest degradation activity within 180 min. This is explained by the fact that the aforesaid sample possessed a higher BET surface area, pore size, and pore volume, which was beneficial for improving the photocatalytic reaction. In light of this enhancement, more methylene blue compounds can be adsorbed via the 1 wt.% MXene/g-C3N4 sample, whereby more active radicals can be generated to further degrade the methylene blue molecules and eventually attain the complete removal of the recalcitrant pollutant.

Photocatalytic Degradation Performance Testing
Furthermore, the narrow bandgap energy of this heterostructure sample is highly favorable for the photocatalytic reaction as more light can be absorbed, thus generating more photocharge carriers to participate in the photocatalytic reaction. As mentioned earlier, the intimate contact between g-C3N4 and MXene particles, evident from the FESEM analysis, firmly implies that the aforesaid heterostructure system allows mobility of the smooth photocharge carriers, which significantly hindered the potential of the photocharge carriers' recombination. In turn, this phenomenon allows the MXene/g-C3N4 heterostructure photocatalyst to perform better than the pure g-C3N4. Nonetheless, a reverse scenario was demonstrated by the 12 wt.% MXene/g-C3N4 sample. The overloading of the MXene onto the g-C3N4 photocatalyst leads to a detrimental photocatalytic performance. This is consistent with the SAP analysis in Section 3.4, whereby the BET surface area, pore size, and pore volume substantially decreased as the amount of MXene loading was increased [2,37,38]. The efficiency for degradation of the as-prepared samples was calculated by using the equation stated in Section 2.4. The photocatalytic activity of the as-synthesized MXene/g-C 3 N 4 heterostructure photocatalyst indicates that the performance of the heterostructure samples was ordered as 1 wt.% MXene/g-C 3 N 4 sample > 4 wt.% MXene/g-C 3 N 4 sample > 8 wt.% MXene/g-C 3 N 4 sample > pure g-C 3 N 4 sample > 12 wt.% MXene/g-C 3 N 4 sample. In this regard, the 1 wt.% MXene/g-C 3 N 4 sample exhibited the highest degradation activity within 180 min. This is explained by the fact that the aforesaid sample possessed a higher BET surface area, pore size, and pore volume, which was beneficial for improving the photocatalytic reaction. In light of this enhancement, more methylene blue compounds can be adsorbed via the 1 wt.% MXene/g-C 3 N 4 sample, whereby more active radicals can be generated to further degrade the methylene blue molecules and eventually attain the complete removal of the recalcitrant pollutant.
Furthermore, the narrow bandgap energy of this heterostructure sample is highly favorable for the photocatalytic reaction as more light can be absorbed, thus generating more photocharge carriers to participate in the photocatalytic reaction. As mentioned earlier, the intimate contact between g-C 3 N 4 and MXene particles, evident from the FESEM analysis, firmly implies that the aforesaid heterostructure system allows mobility of the smooth photocharge carriers, which significantly hindered the potential of the photocharge carriers' recombination. In turn, this phenomenon allows the MXene/g-C 3 N 4 heterostructure photocatalyst to perform better than the pure g-C 3 N 4 . Nonetheless, a reverse scenario was demonstrated by the 12 wt.% MXene/g-C 3 N 4 sample. The overloading of the MXene onto the g-C 3 N 4 photocatalyst leads to a detrimental photocatalytic performance. This is consistent with the SAP analysis in Section 3.4, whereby the BET surface area, pore size, and pore volume substantially decreased as the amount of MXene loading was increased [2,37,38].
3.8. Possible Route for Photodegradation of Methylene Blue over MXene/g-C 3 N 4 Heterostructure Photocatalyst Figure 10 demonstrates the active site radical trapping experiments in order to verify the role of relative active species that contributed during the photocatalytic degradation of methylene blue using the 1 wt.% MXene/g-C 3 N 4 sample. The radical trapping experiment was conducted in the similar protocols used in Section 2.4. In this experiment, the methanol, 2-propanol, and AgNO 3 were used as a scavengers for •O 2 − , •OH, and e − species, respectively. In view of this radical trapping experiment, it was found that the addition of AgNO 3 as an electron scavenger had a substantial effect on the photocatalytic activity of the 1 wt.% MXene/g-C 3 N 4 sample. Meanwhile, the addition of methanol and 2-propanol had mediocre effects on the activity of the 1 wt.% MXene/g-C 3 N 4 sample. By confirming the active sites contributing to the photocatalytic reaction, the electron is likely to take a major role in degrading the methylene blue compounds, whereby the •O 2 − and •OH radicals further support the degradation process. Subsequently, the active radicals will attack the positive carbon compounds of the methylene blue, consecutively degrading methylene blue into biodegradable by-products that can be adsorbed by the designed photocatalyst [39]. Then, the initial compound of methylene blue can break into smaller and more decomposable oxidation by-products, which are relatively less toxic and even less harmless compared to their initial compounds. The intermediates develop hydroxylation by-products and are later mineralized into CO 2 and H 2 O after being attacked by the active site radicals. of methylene blue using the 1 wt.% MXene/g-C3N4 sample. The radical trapping experiment was conducted in the similar protocols used in Section 2.4. In this experiment, the methanol, 2-propanol, and AgNO3 were used as a scavengers for •O2 − , •OH, and e − species, respectively. In view of this radical trapping experiment, it was found that the addition of AgNO3 as an electron scavenger had a substantial effect on the photocatalytic activity of the 1 wt.% MXene/g-C3N4 sample. Meanwhile, the addition of methanol and 2propanol had mediocre effects on the activity of the 1 wt.% MXene/g-C3N4 sample. By confirming the active sites contributing to the photocatalytic reaction, the electron is likely to take a major role in degrading the methylene blue compounds, whereby the •O2 − and •OH radicals further support the degradation process. Subsequently, the active radicals will attack the positive carbon compounds of the methylene blue, consecutively degrading methylene blue into biodegradable by-products that can be adsorbed by the designed photocatalyst [39]. Then, the initial compound of methylene blue can break into smaller and more decomposable oxidation by-products, which are relatively less toxic and even less harmless compared to their initial compounds. The intermediates develop hydroxylation by-products and are later mineralized into CO2 and H2O after being attacked by the active site radicals. Based on the abovementioned analysis, the mechanism of the degradation of methylene blue via the 1 wt.% MXene/g-C3N4 photocatalyst was postulated and calculated as illustrated in Figure 11. The valence band (VB) and conduction band (CB) of the aforementioned photocatalyst were calculated based on the following equation: where the EVB, ECB, X, and Eg correspond to the valence band potential, conduction band potential, absolute electronegativity, and bandgap energy, respectively. More detailed ex- Based on the abovementioned analysis, the mechanism of the degradation of methylene blue via the 1 wt.% MXene/g-C 3 N 4 photocatalyst was postulated and calculated as illustrated in Figure 11. The valence band (VB) and conduction band (CB) of the aforementioned photocatalyst were calculated based on the following equation: where the E VB , E CB , X, and E g correspond to the valence band potential, conduction band potential, absolute electronegativity, and bandgap energy, respectively. More detailed explanations on the abovementioned equation have been explained in the previously reported work [8]. The value for the E e , X, and E g were 4.5, 4.64, and 2.61 eV, respectively. Thereafter, the calculated E VB and E CB of the g-C 3 N 4 were 1.445 and −1.165 eV, respectively. Thereafter, under light illumination, the g-C 3 N 4 photocatalyst absorbed the photon energy, thus initiating the generation of the electron and hole to partake in the photocatalytic reaction. As the MXene is not a visible-light absorption material, the MXene acts as an electron acceptor with respect to its well-matched band edge position, thus allowing the electrons at the conduction band of g-C 3 N 4 to quickly transfer to MXene (E F = −0.53 eV vs. NHE). Subsequently, the photoexcited electrons further attacked the methylene blue compound and further degraded it into intermediate by-products. In addition, as the E F of MXene is more negative than the standard O 2 /•O 2 − , the superoxide radicals (•O 2 − ) are readily produced upon excitation of electrons and further assist the degradation of methylene blue. On the other hand, the holes at the valence band of g-C 3 N 4 can further oxidize to produce the •OH radicals, thus attacking and degrading the methylene compounds. Thereafter, under light illumination, the g-C3N4 photocatalyst absorbed the photon energy, thus initiating the generation of the electron and hole to partake in the photocatalytic reaction. As the MXene is not a visible-light absorption material, the MXene acts as an electron acceptor with respect to its well-matched band edge position, thus allowing the electrons at the conduction band of g-C3N4 to quickly transfer to MXene (EF = −0.53 eV vs. NHE). Subsequently, the photoexcited electrons further attacked the methylene blue compound and further degraded it into intermediate by-products. In addition, as the EF of MXene is more negative than the standard O2/•O2 − , the superoxide radicals (•O2 − ) are readily produced upon excitation of electrons and further assist the degradation of methylene blue. On the other hand, the holes at the valence band of g-C3N4 can further oxidize to produce the •OH radicals, thus attacking and degrading the methylene compounds. Figure 11. The possible degradation mechanism for photocatalyst reaction of MXene/g-C3N4 heterostructure photocatalyst.

Conclusions
In conclusion, the work presented here demonstrated that an excellent photocatalytic degradation of methylene blue can be achieved via MXene/g-C3N4 heterostructure photocatalysts. The designed 1 wt.% MXene/g-C3N4 heterostructure photocatalyst exhibited the best photocatalytic performance within 180 min. Undoubtedly, the high BET surface area, stronger XRD crystallinity structures, and narrower bandgap energy play a vital role in improving the photocatalytic activity of the aforesaid heterostructure photocatalyst. Furthermore, it was found that the e − was confirmed to play a major role in the studied pho- Figure 11. The possible degradation mechanism for photocatalyst reaction of MXene/g-C 3 N 4 heterostructure photocatalyst.

Conclusions
In conclusion, the work presented here demonstrated that an excellent photocatalytic degradation of methylene blue can be achieved via MXene/g-C 3 N 4 heterostructure photocatalysts. The designed 1 wt.% MXene/g-C 3 N 4 heterostructure photocatalyst exhibited the best photocatalytic performance within 180 min. Undoubtedly, the high BET surface area, stronger XRD crystallinity structures, and narrower bandgap energy play a vital role in improving the photocatalytic activity of the aforesaid heterostructure photocatalyst. Furthermore, it was found that the e − was confirmed to play a major role in the studied photocatalytic reaction. This enhancement originated from the critical roles of MXene that helped to impede the recombination of the photocharge carriers and, thus, further augmented the photocatalytic reaction. On a whole, this work presents MXene as a new alternative that will further improve charge separation and transportation, as well as further promotes the degradation of dye in wastewater under solar irradiation.