WS₂-intercalated Ti₃C₂T_x MXene/TiO₂-stacked hybrid structure as an excellent sonophotocatalyst for tetracycline degradation and nitrogen fixation

Highlights

• •Ti₃C₂ MXene/TiO₂-WS₂ integrated heterostructure was prepared hydrothermally.
• •In-situ generated TiO₂ on MXene-WS₂ exhibited excellent sonophotocatalytic activity.
• •Excellent sonophotocatalytic NRR rate (526 μmol) is achieved by TiO₂ interface on heterostructure.
• •Surface oxidized Ti-O strengthens the interface and promotes electron transfer path.

Abstract

Designing a heterostructure nanoscale catalytic site to facilitate N₂ adsorption and photogenerated electron transfer would maximize the potential for photocatalytic activity and N₂ reduction reactions. Herein, we have explored the interfacial TiO₂ nanograins between the Ti₃C₂T_x MXene-WS₂ heterostructure and addressed the beneficial active sites to expand the effective charge transfer rate and promote sonophotocatalytic N₂ fixation. Benefiting from the interfacial contact and dual heterostructure interface maximizes the photogenerated carrier separation between WS₂ and MXene/TiO₂. The sonophotocatalytic activity of the MXene@TiO₂/WS₂ hybrid, which was assessed by examining the photoreduction of N₂ with ultrasonic irradiation, was much higher than that of either sonocatalytic and photocatalytic activity because of the synergistic sonocatalytic effect under photoirradiation. The Schottky junction between the MXene and TiO₂ on the hybrid MXene/TiO₂-WS₂ heterostructure resulted in the sonophotocatalytic performance through effective charge transfer, which is 1.47 and 1.24 times greater than MXene-WS₂ for nitrogen fixation and pollutant degradation, respectively. Under the sonophotocatalytic process, the MXene/TiO₂-WS₂ heterostructure exhibits a decomposition efficiency of 98.9 % over tetracycline in 90 min, which is 5.46, 1.73, and 1.10 times greater than those of sonolysis, sonocatalysis, and photocatalysis, respectively. The production rate of NH₃ on MXene/TiO₂-WS₂ reached 526 μmol g⁻¹ h⁻¹, which is 3.17, 3.61, and 1.47 times higher than that of MXene, WS₂, and MXene-WS₂, respectively. The hybridized structure of MXene-WS₂ with interfacial surface oxidized TiO₂ nanograins minimizes the band potential and improves photocarrier use efficiency, contributing directly to the remarkable catalytic performance towards N₂ photo fixation under visible irradiation under ultrasonic irradiation. This report provides the strategic outcome for the mass carrier transfer rate and reveals a high conversion efficiency in the hybridized heterostructure.

1. Introduction

Global environmental and energy concerns have matured into severe issues in modern civilization that pose a grave danger to human life. Ammonia (NH₃ production) is highly impactful for producing artificial fertilizers and sustainable carbon–neutral development progress [1], [2]. On an industrial scale, hydrogen and N₂ gas are combined to generate NH₃ by the traditional Haber–Bosch reaction process while being subjected to high temperatures and pressures in the presence of a catalyst [3]. This process would require a huge amount of fossil fuel resources, producing a high volume of greenhouse gases during the reaction that would impact global climate change [4]. To overcome this issue and explore a non-harmful route for NH₃ production from the air, the N₂ fixation of plants has inspired many researchers to develop the photocatalytic N₂ fixation, which is a clean and non-harmful mild process and cost-effective process [5], [6]. Direct photo fixation of N₂ at room temperature using a photocatalytic nitrogen reduction reaction (NRR) induced by renewable solar energy has the potential to synthesize NH₃ in an environmentally friendly route [7]. In addition to the photocatalytic process, sonophotocatalytic technology has been studied extensively as a green technology for the degradation of pollutants and N₂ fixation to promote the catalytic activity through the synergistic effect of the sonocatalytic and photocatalytic activity [8], [9]. Apart from the catalytic activity, the construction of innovative nanostructured materials leveraged the power of sonochemical approaches by generating heat and pressure in a short period of time [10], [11]. In a streamlined reaction environment with minimal energy usage, the ultrasonic process may be used to regulate the morphology and size of the nanostructures [12], [13], [14]. Sonochemical methods are widely used in the creation of nanostructures and in the removal of organic contaminants [15], [16]. An appealing strategy would be to take advantage of ultrasonication and visible, responsive catalytic activity that promotes the carrier density on the heterostructure interface by building a partially occupied band between the conduction band (CB) and valence band (VB) of a semiconductor catalyst.

Developing distinctive and efficient heterojunction photocatalysts for highly effective solar-to-chemical energy conversion is still challenging. The typical catalytic activity of the catalytic heterostructures is associated primarily with the band function and their effective carrier separation efficiency [17], [18], [19]. Among the various semiconductors, metal oxides, transition metal chalcogenides, and carbon derivatives are photocatalysts for degrading organic pollutants and N₂ fixation activity [20], [21]. TiO₂-based materials are the most stable catalytic network among the various semiconductor systems. On the other hand, their rapid carrier recombination and the poor visible response have limited their catalytic activity in the broad solar spectrum [22]. Inducing the doping functionality or tagging the other semiconductor as a heterostructure network has strategically improved the photocatalytic activity [23]. On the other hand, N₂ reduction in an aqueous media is highly challenging through photocatalytic systems without sacrificial agents. The two major prerequisites are required for the effective carrier separation rate with high redox potential and reductive active sites. The ideal interface of the staggered band configuration of the heterostructure for effective charge carrier separation improvises selectively towards photoinduced N₂ fixation [24], [25]. MXene, a metal carbide, is a contender in catalytic reactions because of its band configuration and high electrical conductivity, large surface area, and rich surface chemistry with tunable structural morphology [26]. Ti-based Ti₃C₂T_x MXene has attracted significant interest through its superior layered structure and considerable interest in photocatalysis, CO₂ reduction, and water splitting through its band configuration [27], [28]. The three-layered Ti covering the two-layered C promoted the structural advantages of MXene for faster electron transportation [29]. On the other hand, MXene encounters several issues related to its easy aggregation through the weak van der Waals interrelation, oxidation, and surface defect formation in the aqueous solution phase, which limits their applicability for future development [30]. These issues have been addressed by conjugating the MXene with different carbon functionalities, polymers, and transition metal dichalcogenides to promote the functionality of MXene in vast applications [31], [32], [33]. Designing the MXene with the surface oxidized TiO₂ nanograins influences the significant band functions with the Schottky junction. They promote the application towards photocatalysis in the form of Ti₃C₂/Ti³⁺-TiO₂ where the contact interface of Ti₃C₂-TiO₂ maximizes the photoinduced charge transfer and minimizes the carrier recombination [34]. In addition, MXene can partially oxidize to form the TiO₂ on the MXene surface without titanium addition, leading to the Schottky barrier at the interface, and the heterostructure assembly would maximize the photoexcited carrier separation efficiency [35]. Inducing the defective sites and fluoride functionalities on the TiO₂ surface promoted the activation of the catalytic surface for the N₂ adsorption that maximizes the NH₃ production rate as 206 μmol h⁻¹ g⁻¹, which is approximately 10 times larger than pure TiO₂ [24]. Designing the TiO₂ on the MXene surface as a hybrid structure (Ti₃C₂T_x/TiO₂-400) exhibits the remarkable N₂ photo fixation with the NH₃ formation production rate as 422 μmol h⁻¹ g⁻¹ at room temperature because of the oxygen vacancy sites on the TiO₂ phase that operate as active foci for N₂ adsorption and activation.

Among the heterostructure interface of MXene, hexagonally packed transition metal atoms with two layered chalcogen atoms have significant advantages in energy and catalytic applications by offering distinctive optical, mechanical, and electrical behavior [36]. Controlling the morphology, highly active sites, and structural stability with high carrier mobility are the key drivers to maximize the functional advantage of the hybrid structure. Growing lamellar structures on the stacked MXene surface will fill up the space that inhibits restacking and exhibits the hierarchical structural assembly that promotes the structural advantages in photocatalytic and electrochemical applications [37]. Coupling the layered features of ZnIn₂S₄ in the MXene surface processes the porous scaffold hybrid configuration that reduces the electron transfer pathway and maximizes the photocatalytic activity [38], [39]. Engineering the stacked MXene-WS₂ heterostructure with the surface promotes defect functionalities, strategically improvises the electron transfer rate at the heterostructure interface, and promotes the carrier migration path. Tagging WS₂ over the MXene as a hierarchical structural assembly exhibited a significant improvement in electrochemical performances, which maximizes the specific capacitance of 1111F. g⁻¹ @ 2 A g⁻¹ with high energy and power density and evidence the electrocatalytic hydrogen evolution reaction at alkaline and acidic medium with ultralong stability of 24 h. The exhibition of the metallic 1 T phase and semiconducting 2H phase on WS₂ with octahedral coordination has improved the carrier transfer rate, maximizing the photocatalytic efficiency through the improvised conductivity [40]. Enabling the heterostructure interface of WS₂–TiO₂ induces the type II band function that improves the carrier separation efficiency and the presence of defect coordinated that maximizes the photocatalytic conversion efficiency of N₂ to NH₃ at 1.39 mmol h⁻¹ g⁻¹ [41]. Hybridizing the multi-compound heterostructure feature to maximize seamless contact interface and minimize the charge transfer distance with less light shielding effectivity influences the carrier migration and promotes visible, responsive activity.

Inspired by previous reports, MXene-WS₂ hierarchical heterostructures were constructed with oxygen-defected TiO₂-based interfacial contact on the MXene surface to enrich the N₂ concentration and promote the adsorption of N₂ and catalytic active sites. The photocatalytic and sonocatalytic activity and selectivity were improved for the NRR-based NH₃ synthesis by modulating the interfacial contact and the heterostructure assembly with the tunable surface interfacial states. The electronic configuration with the dual MXene/TiO₂-WS₂-based heterostructure interface maximizes the active area for the effective activation and adsorption of N₂ owing to the interfacial contact of TiO₂ through thermal oxidization. The thermally oxidized (300 °C) TiO₂ on MXene-WS₂ achieved the maximum sonophotocatalytic efficiency of 89.1 % under 60 min of visible irradiation, which was 2.47 and 1.24 times higher than MXene and MXene-WS₂. Importantly, the superior sonophotocatalytic under visible light for the MXene/TiO₂-WS₂ evidences the NH₃ production rate of 526 μmol·g⁻¹·h⁻¹, which is 2.55 and 1.44 times higher than sonolysis and photocatalysis. As an additional advantage, MXene/TiO₂-WS₂ also delivers high stability by maintaining > 94 % of its initial activity even after ten successive catalytic reactions. The local electron density of adsorbed N₂ was tuned by tuning the local electronic states of oxygen vacancies by the thermally oxidized MXene surface, and the energy barrier for hydrogenation of N₂ was minimized to promote the sonophotocatalytic N₂ fixation. This study provides additional insights on the interfacial contact interface in designing the catalytic with multiple active sites for the photoresponsive NRR, and it reaffirms the electronic structural assembly to tune the catalytic activity.

2. Materials and methods

2.1. Chemicals

Ti₃AlC₂ powder was purchased from JILIN 11 Technology Co. LTD (China). Hydrofluoric acid (HF, 37 %), sodium tungstate dihydrate (Na₂WO₄·2H₂O, 99 %), oxalic acid (C₂H₂O₄, 98 %), thioacetamide (CH₃CSNH₂, 98 %), hydrochloric acid (HCl), benzoquinone (BQ, 98 %), silver nitrate (AgNO₃, 99 %), tret-butanol (t-BuOH), disodium ethylenediaminetetraacetate (Na₂-EDTA, 99 %), methanol (CH₃OH), ethanol (C₂H₅OH), tetracycline (TC), sodium hydroxide (NaOH, 98 %), were obtained from Sigma–Aldrich. All the chemicals purchased were of analytical grade and used as received.

2.2. Synthesis of stacked MXene and oxidized MXene/TiO₂ heterostructure

Stacked Ti₃C₂T_X MXene was fabricated by acidic etching [42]. As a part of the etching strategy, 40 mL of a 37 % HF solution was taken in a Teflon vessel and stirred for 5 min, and 2 g of Ti₃AlC₂ MAX powder was added slowly to the acid solution to carve the Al from the MAX surface. The temperature of the solution was maintained at 30 °C for 72 h under constant stirring. The resulting solution was centrifuged, and the precipitate was washed with deionized (DI) water until the solution reached the desired pH of > 5. The resulting residue was collected and freeze-dried. Fig. 1a shows a schematic diagram of the process. MXene was oxidized by heat treating the as-prepared Ti₃C₂T_X powder sample at 100, 200, 300, and 400 °C for 30 min in both ends closed tubular furnace to gain the TiO₂ interface on the MXene surface. The samples were called MXene/TiO₂(1 0 0), MXene/TiO₂(2 0 0), MXene/TiO₂, and MXene/TiO₂(4 0 0). After thermal annealing, the surface of MXene was decomposed to TiO₂, which minimized the structural quality of the stacked network and produced a Schottky junction between the MXene and TiO₂.

Fig. 1.

(a) Schematic synthesis route for fabricating the MXene/TiO₂-WS₂ hybrid heterostructure. (b) X-ray diffraction pattern and (c) FTIR spectra of MXene, MXene/TiO₂, WS₂, MXene-WS₂, and MXene/TiO₂-WS₂ nanostructures.

2.3. Synthesis of WS₂ nanoflakes

For the growth of WS₂ nanoflakes, 5 mmol of thioacetamide and 2.5 mmol of Na₂WO₄·2H₂O were dispersed in 30 mL of DI water and stirred homogeneously to attain a clear solution. Subsequently, 4 mmol of oxalic acid was used as a catalyst to control the pH of the solution as < 2, and the solution was stirred for 10 min to generate a homogeneous dispersion. The prepared solution was transferred into a Teflon-lined stainless-steel autoclave (50 mL) and heated for 12 h at 200 °C. After the reaction, the solution was naturally cooled to room temperature. The precipitate was collected, centrifuged with ethanol and DI water, and dried in vacuum annealing for 6 h at 60 °C.

2.4. Synthesis of MXene-WS₂ and MXene/TiO₂-WS₂ hybrid structure

First, 40 mg of as-prepared MXene or MXene/TiO₂(3 0 0) was dispersed homogeneously in the 30 mL of DI water and sonicated for 5 min at room temperature. The agitated suspension was added with the above 5 mmol of thioacetamide and 2.5 mmol Na₂WO₄·2H₂O precursors and homogeneously stirred for 30 min. The mixed solution was placed into a Teflon-lined stainless-steel autoclave (50 mL) and heated for 12 h at 200 °C. The precipitate was recovered after cooling, centrifuged with ethanol and DI water, and dried in vacuum annealing for 6 h at 60 °C. Furthermore, the supporting information presents the detailed characterization, photocatalytic and sonophotocatalytic TC degradation, sonophotocatalytic N₂ fixation measurements, and photoelectrochemical measurements. Based on the yield of WS₂ synthesized through hydrothermal, the loading ratio of WS₂ on MXene was varied, and their respective sonocatalytic properties were investigated. In addition, the WS₂ was grown on different temperature-based oxidized MXene, and the catalytic activity of the composites was studied and compared.

2.5. Material characterization

X-ray diffraction (XRD, PANalytical X'Pert Pro multifunctional) was performed using Cu K irradiation (λ = 0.15406 nm) to characterize the structural properties of nanostructures. Fourier transform infrared (FTIR, JASCO FTIR 6600) spectroscopy was conducted to assess the surface functionalities of composites between 400 and 4000 cm⁻¹. High-resolution scanning electron microscopy (HRSEM, S-4800, Hitachi), high-resolution transmission electron microscopy (FETEM, JEM-2100F, JEOL Japan), and selected area electron diffraction (SAED) were used to examine the morphologic and structural characterization of the produced composites. The specific surface areas and pore size distributions of composites were measured using a Tristar ASAP 2020 to measure the Brunauer–Emmett–Teller (BET) surface area and N₂ adsorption at 77 K. A Thermo Scientific spectrometer with Al K (1486.6 eV) source were used for X-ray photoelectron spectroscopy (XPS). The UV-DRS measurements acquired with a UV–vis spectrophotometer (JASCO V-600) were used to estimate the optical adsorption and band gaps of the composites.

3. Results and discussion

As shown schematically in Fig. 1a, the MXene-WS₂ hybrid heterostructure with an oxygen-deficient TiO₂ interface was prepared using a three-step optimization process: (i) Initially, MXene was prepared by the selective etching of Al under the HF etching process. (ii) The MXene was oxidized thermally to induce the TiO₂ nanograins on the layered surface with high oxygen defect states. (iii) Finally, the staked MXene surface as a hybrid heterostructure was decorated with WS₂ nanoflakes through a hydrothermal process. Unless stated further, the discussion focuses on the characteristics of 300 °C of thermally oxidized TiO₂ nanograins over MXene-WS₂ heterostructure, which exhibits the best sono-photoreduction of NH₃. Fig. 1b presents the corresponding XRD pattern of MXene and their respective compositions. The peaks associated with the (0 0 2), (0 0 6), and (0 0 8) crystal plains were evidence of the formation of MXene after the HF-based etching of MAX. With thermal oxidization, a broad peak at 25.2° was attributed to the anatase (1 0 1) plane of TiO₂ nanograins on the MXene surface. Pure WS₂ exhibits the (0 0 2), (0 0 4), (1 0 0), (1 0 3), and (1 0 5) lattice plains evidence of the hexagonal structure of WS_2, which perfectly matches with JCPDS card no: 08–0237. Furthermore, the prepared MXene and oxidized MXene had the characteristic of (0 0 2) plane at 9.1°, which was shifted to 7.2° on the growing WS₂ on the MXene surface, indicating the intercalation of MXene due to the growth of WS₂ between the layers, which increased the d-spacing. Similarly, there was a mild shift in the WS₂-based XRD peaks, evidencing the structural interaction of the heterostructure surface. The average crystalline size of the WS₂ along the (0 0 2) plane for the MXene-WS₂ and MXene/TiO₂-WS₂ samples was calculated as 18.2 and 17.6 nm, respectively, through the Scherrer formula [43], [44] showcases that there is no huge variation on the structural nature of the WS₂ on the MXene surface through the inclusion of nanograin TiO₂ features. The structural quality of the MXene and TiO₂ was increased during the oxidization process, which resulted in intense peak signals. The FTIR spectra in Fig. 1c revealed the surface functional states of the heterostructure samples. The peak position between 3300 cm⁻¹ to 3500 cm⁻¹ was evidence of the stretching and bending vibrations of − OH functionalities, and the peak at approximately 1600 cm⁻¹ revealed the bending vibrations of C–OH interaction on the MXene surface. Furthermore, the broadband at 482 cm⁻¹, 576 cm⁻¹, and 733 cm⁻¹ were assigned to the Ti–C, Ti–O, and Ti–F interaction on the MXene samples, respectively. Although the surface was oxidized, the host layer of MXene was preserved with the Ti–C and Ti–F functionalities. The peaks at 1388 cm⁻¹ and 1117 cm⁻¹ were assigned to the O–H bending and C–F stretching vibrations, respectively. The vibrations of the –CH₂, –C = C–, C–H, and C–O bonds were observed at 2920 cm⁻¹, 1560 cm⁻¹, 1458 cm⁻¹, and 1120 cm⁻¹, respectively. For WS₂, the intense peaks from 623 cm⁻¹ and 1080 cm⁻¹ represent the W–S and S–S bonds, and the bands at 1396 cm⁻¹ and 1632 cm⁻¹ were attributed to the stretching vibrations of hydroxyl functionalities. The peaks at 3100 cm⁻¹, 1200 cm⁻¹, and 2100 cm⁻¹ were assigned to the stretching and bending vibrations of W–S functionality. The specific surface area of the WS₂, MXene, and MXene/TiO₂-WS₂ nanostructures was evaluated via the Brunauer–Emmett–Teller (BET) method, and the MXene/TiO₂-WS₂ heterostructure showed a higher surface area (5.84 m²/g) than MXene (2.43 m²/g) and WS₂ (4.56 m²/g) nanostructures (Fig. S1). Intercalating of WS₂ on the stacked MXene surface increases the surface area by creating high rough textured features on the stacked surface. The hierarchical assembly of nanoflakes on the stacked surface played an efficient role in improvising the surface area of the heterostructure assembly.

FESEM was conducted to examine the morphology and surface characteristics of the prepared heterostructure. Fig. 2 (a-c) shows FESEM images of the stacked MXene structures, which highlights the packed sheet assembly. This packed layered structure was formed due to the purposeful carving of Al from the MAX phase by the HF-based etching process. After thermally treating the MXene surface, small nano grain features appeared on the MXene surface due to TiO₂ formation (Fig. 2 (d-f)). The grain assembly was disturbed uniformly throughout the stacked layer surface, indicating a TiO₂-based heterostructure interface on the MXene surface. The hydrothermally prepared WS₂ was evidence of a thin layered nanoflake-like assembly ((Fig. 2 g-i). In contrast, the growth of WS₂ on the MXene surface resulted in the hierarchical hybrid heterostructure ((Fig. 2 j-l).

Fig. 2.

FESEM images of (a-c) MXene, (d-f) MXene/TiO₂, (g-i) WS₂, and (j-l) MXene/TiO₂-WS₂ nanostructures in different magnifications.

During the hydrothermal process, the layered WS₂ nanoflakes were grown epitaxially on the stacked MXene, resulting in a hierarchical sandwich heterostructure with the dual heterostructure interface between MXene, TiO₂, and WS₂. No significant morphological change in the heterostructure assembly was observed with and without the TiO₂ formation on the MXene surface (Fig. S2). The loading density of WS₂ on the MXene surface was increased as the WS₂ concentration in the growth process was increased (Fig. S3). The high-magnified images of the heterostructures revealed the porous structure of the layered features with a nonuniform surface nature, which promotes the highly active sites and surface interaction during the catalytic reaction. The atomic level interaction and the structural assembly MXene, MXene/TiO₂, WS₂, and MXene/TiO₂-WS₂ were analyzed by TEM. Fig. 3 (a-c) shows TEM images of MXene at different magnifications. A smooth surface finish with the thin layered assembly was revealed from the high-magnified images. The derived SAED profile (Inset of Fig. 3b) indicated the polycrystalline nature of the MXene sheets. The MXene surface became rougher on thermally treating the samples at 300 °C with a dispersed nanograin assembly (Fig. 3 d-e) due to surface oxidization, which formed the TiO₂ on the MXene surface. The HRTEM image of the MXene/TiO₂ displayed two different sets of fringes with an interplanar spacing of 0.228 nm and 0.243 nm, representing the (1 0 3) plain of MXene and the (1 0 4) plain of TiO₂ on the MXene/TiO₂ sample. The co-existence of the TiO₂ on the MXene surface was consistent with the XRD results. WS₂ was exhibited as a thin sheet-like assembly, resulting in a lattice spacing of 0.621 nm representing the (0 0 2) crystallographic plains of WS₂ (Fig. 3 g-i). Fig. 3(j-l) shows the hierarchical structure of the MXene-WS₂ without an interlayer spacing between the MXene and WS₂, evidence of the tightly coupled heterostructure interface. The HRTEM images of MXene/TiO₂-WS₂ heterostructure evidence the 0.262 nm lattice space corresponding to the (1 0 0) crystal plane of WS₂. The coupling interface resulted in the lattice spacing of 0.228 nm representing the (1 0 3) crystal plane of MXene, showing the integrity of MXene/TiO₂ and the strong contact interface of MXene/TiO₂ and WS₂. The EDAX profile of MXene/TiO₂-WS₂ shows the combined elemental peaks composed of 1.03, 49.16, 5.84, 0.33, 14.93, and 28.71 at.% of Ti, C, O, F, W, and S, respectively, which confirms the MXene/TiO₂-WS₂ nanocomposite (Fig. S4). The inset in Fig. 3k presents the SAED pattern of MXene/TiO₂-WS₂, showing the mixed crystal quality of polycrystalline WS₂ and the crystalline network of MXene. Mapping analysis of MXene/TiO₂-WS₂ revealed the layered distribution of Ti, C, and O with the surface-loaded W and S elemental composition with a uniform distribution.

Fig. 3.

Typical TEM and HRTEM images of (a-c) MXene; (d-f) MXene/TiO₂; (g–i) WS₂ and (j-l) MXene/TiO₂-WS₂ heterostructure. (m) EDAX mapping images of the MXene/TiO₂-WS₂ heterostructure. The inset shows the SAED pattern of the respective images.

The electronic states and elemental composition of MXene, WS₂, MXene-WS₂, and MXene/TiO₂-WS₂ were examined by X-ray photoelectron spectroscopy (XPS). Fig. S5 presents the survey spectrum of the MXene, WS₂, MXene-WS₂, and MXene/TiO₂-WS₂, showing C, Ti, O, W, and S. Fig. 4a shows the C 1 s spectra of MXene deconvoluted into multiple peaks at 281.7, 284.6, 285.4, 288.4, and 288.9 eV, which were assigned to the Ti–C, C–C, C–O, C O, and C–F bonds, respectively [45]. The Ti 2p peaks of the pristine MXene peaks fitted with the Gaussian function at 454.9, 455.8, 456.9, and 458.8 eV were attributed to Ti–C (Ti⁺), Ti–X (Ti²⁺), Ti–X (Ti³⁺), and Ti–O, respectively [46]. Fig. 4c and 4d show the W 1 s and S 2p core level spectra of the pristine WS₂ nanoflakes. The W 1 s spectra of WS₂ nanoflakes show multiple Gaussian-fitted peaks attributed at 32.0, 34.1, and 38.6 eV, which corresponds to the binding energies of W 4f_7/2, W 4f_5/2, and W 5p_3/2 that well matched with the W⁴⁺ species of WS₂ [47]. On the other hand, an emerging peak at 35.4 eV was assigned to the W–O bond, revealing the significant oxidization of W on the nanoflakes [48]. The surface oxidization of W formed a part of unreacted W that was capped on the WS₂ surface exposed to air. Surface oxidization will not influence the quality of the sample because the stochiometric of W:S is very close to the ideal value of 2. The doublet in S 2p spectra at 162.3 and 163.2 eV were attributed to S 2p_3/2 and S 2p_1/2 peaks of 1 T-WS₂, representing S^2- species. Fig. 4 (e-h) and 4(i-l) show the C1s, Ti 2p, W 4f, S 2p XPS spectra of the MXene-WS₂ and MXene/TiO₂-WS₂ heterostructures. The oxidized sample has higher structural stability of the Ti–C phase after the hydrothermal process than the unoxidized sample. The durability of the Ti–C phase was increased by forming high crystalline TiO₂ nanograins as a surface protector to minimize in-situ oxidization during the hydrothermal processes. The presence of interfacial TiO₂ on the heterostructure surface is evidence of the dominant TiO₂ vibration compared to the Ti–C and Ti³⁺ energy states. Unoxidized MXene-WS₂ heterostructures also have the strong vibration of Ti–O, but the structural quality of MXene was poor because of the partial oxidization of MXene during the hydrothermal process. In addition, while growing the WS₂ on the MXene surface, the rate of W oxidization was increased, which was observed from the exhibition of the intense peak at 35.4 eV on the W 4f peaks of heterostructure samples. The expanded W⁶⁺ states on the MXene-WS₂ and MXene/TiO₂-WS₂ heterostructures indicated the interaction of the tungsten atom with distinct oxygen moieties from the environment [49]. In addition, there was no significant change in the S 2p spectra for the heterostructure samples, showing the good structural quality of WS₂ on the MXene surface. From the observation, inducing thermal oxidization preserved the structural quality of the MXene surface during the hydrothermal process compared to the untreated MXene surface. The O 1 s spectra of the MXene/TiO₂-WS₂ heterostructure resulted in peaks at 529.6 and 531.3 eV, contributing to the Ti–O–Ti and Ti–O–H bonds, respectively (Fig. S6a). The peak at 684.8 eV was attributed to the Ti–F bonds, showing the F interaction on the MXene surface, as shown in Fig. S6b. The F functionality of the MXene remained even after decorating the WS₂.

Fig. 4.

High-resolution XPS profile: (a) C 1s and (b) Ti 2p of MXene; (c) W 4f and (d) S 2p of WS₂; (e) C 1s, (f) Ti 2p, (g) W 4f, and (h) S 2p of MXene-WS₂ heterostructure; (i) C 1s, (j) Ti 2p, (k) W 4f, and (l) S 2p of MXene/TiO₂-WS₂ heterostructure.

The optical absorption properties of MXene-based heterostructures were investigated through the UV–vis DRS spectrum to address the role of adsorption properties with the function of different heterostructure assemblies. Fig. S7 presents the visible light adsorption behavior on the heterostructure samples, and MXene/TiO₂-WS₂ showed high visible adsorption. All the samples exhibited high adsorption over the 280–750 nm range, with prominent plasmonic adsorption around 450 nm. The band edge adsorption moved toward the IR region in all the samples, which exhibited wide visible adsorption. The band gap of the materials was calculated using the Kubelka–Munk (K–M) plot. The estimated band gap of the WS₂, MXene, MXene-WS₂, and MXene/TiO₂-WS₂ heterostructures were 1.62, 1.59, 1.56, and 1.53 eV, respectively. The band gap decreased after inducing the TiO₂ functionality as an interfacial layer on the heterostructure assembly.

3.1. Photocatalysis and sonophotocatalysis

The tetracycline (TC) removal efficiency was documented to address the sonophotocatalytic activity of the MXene/TiO₂-WS₂ hybrid heterostructure. Fig. 5a shows the UV–vis absorbance spectra of TC under different reaction conditions in 90 min. The effects of sonophotocatalytic properties were assessed by examining the sonolysis, adsorption, sonocatalysis, and photocatalysis properties of MXene/TiO₂-WS₂. Comparing the ultrasonic irradiation with and without MXene/TiO₂-WS₂, the adsorption peak intensity was reduced drastically, demonstrating the sonocatalytic activity of the heterostructure. In photo-irradiation, the rate of TC decomposition in solution was higher than the sonocatalytic activity, highlighting the promising effect of photo-responsive radical production on the catalytic surface. Combining the sonocatalytic and photocatalytic effects, the photoinduced reactive oxygen species oxidation and the pyrolysis caused by the cavitation effects from ultrasound promoted the degradation rate. MXene/TiO₂-WS₂ hybrid heterostructure showed 98.9 % TC decomposition efficiency in 90 min, which is significantly higher than that of adsorption (14.1 %), sonolysis (18.1 %), sonocatalysis (57.1 %), and photocatalysis (89.9 %) (Fig. 5b). The synergistic effects of the visible irradiation and ultrasonic contribution maximized the decomposition efficiency. In the dark, MXene/TiO₂-WS₂ exhibited a removal efficiency of 14.2 % due to the static interaction of surface charges and the presence of defect states that adsorb the pollutant ions. The higher separation efficiency of photogenerated carriers and reactive oxygen species produced during the light-induced sonophotocatalytic activity degraded the TC and finally mineralized organic pollutants into H₂O and CO₂. Owing to the high photoresponsive behavior, the MXene/TiO₂-WS₂ heterostructure catalyst resulted in high photocatalytic efficiency towards the TC compared to all other heterostructure assemblies (Fig. S8). Fig. 5c presents the sonophotocatalytic decomposition of TC over different catalysts. The sonophotocatalytic activity of the MXene/TiO₂-WS₂ resulted in a high removal efficiency, which was 2.48, 1.66, 1.98, and 1.24 times more elevated than that of MXene, MXene/TiO₂, WS₂, and MXene-WS₂ after 60 min of irradiation. The sonophotocatalytic degradation efficiency of MXene, MXene/TiO₂, WS₂, MXene-WS₂, and without catalyst were 36.2, 54.1, 45.3, 72.1, and 13.1 %, respectively, in 60 min of irradiation. From the observation, MXene/TiO₂-WS₂ showed high sonophotocatalytic degradation efficiency, and the sonophotocatalytic activity was observed in the following order: MXene/TiO₂-WS₂ > MXene-WS₂ > WS₂ > MXene/TiO₂ > MXene. Tagging the WS₂ nanoflakes as a heterostructure assembly on the MXene surface improved the sonophotocatalytic activity. The interfacial TiO₂ nanograins between the MXene-WS₂ interface minimized the conduction band position between the MXene and WS₂. The TiO₂ nanograins promoted the carrier separation efficiency between the heterostructure interface that helps maximize the catalytic efficiency. A degradation kinetic study of TC followed pseudo-first-order kinetics with correlation coefficient (R²) values > 0.99 (Fig. S9). The synergetic effect of MXene/TiO₂-WS₂ during the sonophotocatalytic process was addressed using the synergetic indices. The synergetic indices were calculated using eq. (1) from the estimated pseudo-first-order rate constant (k_app). The k_app values during sonolysis, adsorption, sonocatalysis, photocatalysis, and sonophotocatalysis for TC using MXene/TiO₂-WS₂ were 0.0019, 0.0011, 0.0078, 0.0171, and 0.0341 min⁻¹, respectively (Fig. 5d). Degradation turnover (dTON) was proposed to quantify the catalytic degradation activity to compare catalysts regardless of pollutant concentration or catalyst quantity. The dTON for the control experiment was calculated using the equation below [50], [51].

$$\mathit{dTON}=\frac{\left[M_i\right]-[M_f]}{t\times [Cat]}$$

Fig. 5.

UV–Vis spectra of TC with a catalyst under different reaction conditions in 90 min. (b) the degradation efficiency of TC with MXene/TiO₂-WS₂ catalyst by absorption, sonolysis, sonocatalysis, photocatalysis, and sonophotocatalysis. (c) Effects of different MXene compositions on the sonophotocatalytic degradations of TC. (d) Synergy factors for the sonophotocatalytic degradations of TC by MXene/TiO₂-WS₂. Sonophotocatalytic efficiency over (e) different WS₂ loading densities and (f) different oxidization temperatures of MXene/TiO₂ (100–400 °C). Operational conditions: [MXene/TiO₂-WS₂] = 1 g L⁻¹, TC = 10 mg L⁻¹, and neutral pH.

M_i and M_f are the pollutant concentrations before and after the sonocatalytic process, respectively; t is the reaction time (h); and Cat signifies the amount of catalyst (g). The dTON of MXene/TiO₂-WS₂ was estimated as 1.57 µmol h⁻¹ g_cat⁻¹ for TC, which is significantly higher than that of MXene/TiO₂ (0.95 µmol h⁻¹ g_cat⁻¹) and MXene-WS₂ (1.28 µmol h⁻¹ g_cat⁻¹). Introducing the interfacial TiO₂ nanograins at the heterostructure interface of MXene-WS₂ significantly boosted the sonophotocatalytic activity of heterostructure through the effective photogenerated charge carrier mobility. In addition, with increasing WS₂ loading density on the MXene surface, the sonophotocatalytic performance of MXene/TiO₂-WS₂ was enhanced initially and decreased (Fig. 5e). The optimized growth rate of MXene/TiO₂-WS₂ composition showed high decomposition efficiency and excessive WS₂ on the MXene had a shielding effect that led to a decrease in the sonophotocatalytic performance of MXene/TiO₂-WS₂. The sonophotocatalytic efficiency was improved by increasing the oxidization temperature of MXene before WS₂ growth (Fig. 5f). The oxidization temperature was varied from 100 to 400 °C. Increasing the oxidization temperature to ≤ 300 °C increased the degradation rate, possibly because of the influence of possible oxygen defect functionalities on the TiO₂ nanograins that promote the catalytic activity by inducing the defect band level on the heterostructure network. Although increasing the oxidization temperature to 400 °C, possible defect density was reduced compared to the low-temperature oxidization that promotes the carrier recombination rate that would minimize the catalytic efficiency. The effectiveness of the MXene/TiO₂-WS₂ composition over the organic pollutant degradation is compared and summarized in Table S1. The Ti-base MAX and layered MXene-based nanostructures were evidence of the significant sonocatalytic activity over the degradation of pharmaceutical pollutants [52], [53]. The production rate of reactive oxygen species and boosted H₂O₂ production has maximized the degradation rate with respect to time. In contrast, the MXene/TiO₂-WS₂ heterostructure sped up the reaction and promoted the degradation efficiency of 98.9 % in 90 min.

The sonophotocatalytic fixation of N₂ to NH₃ was studied under ambient conditions for 130 min under visible light irradiation. Fig. 6a presents the sonophotocatalytic synthesis of nitrogen fixation over the different samples. Among the all-other catalysts, the MXene/TiO₂-WS₂ heterostructure exhibited a higher N₂ fixation ability than the WS₂, MXene, and MXene/TiO₂. The rate of NH₃ production increased through the influence of the TiO₂ interfacial layer, and the 300 °C thermal treatment reached a high NH₃ production rate. This may be because of the high photo-responsive nature and promotive carrier separation efficiency. The rate of NH₃ formation over the MXene, MXene/TiO₂, WS₂, MXene-WS₂, and MXene/TiO₂-WS₂ catalysts were 169, 256, 146, 356, and 526 μmol h⁻¹ g⁻¹, respectively. Fig. 6b compares the effects of the ultrasonic condition, photocatalytic activity, and their dual effect with different operation conditions with MXene/TiO₂-WS₂ as a catalyst. The sonophotocatalytic nitrogen fixation (Ultra + light + cat + N₂) efficiency of MXene/TiO₂-WS₂ (526 μmol h⁻¹ g⁻¹) is higher than the photocatalysis (Light + cat + N₂) (366 μmol h⁻¹ g⁻¹) and sonocatalysis (Ultra + cat + N₂) (206 μmol h⁻¹ g⁻¹), which is 1.44 and 2.55 times higher than that of photocatalysis and sonocatalysis, respectively. While comparing with the sonophotocatalysis nitrogen fixation, the photocatalytic active activity in the presence of stirring did not show any significant N₂ fixation activity compared to photocatalysis, which was attributed to the hot spot effect through ultrasonic irradiation. The sonophotocatalytic experiments were performed under an Ar atmosphere instead of N₂ to assess the influential role of N₂ in the catalytic reaction. The results revealed suppressed NH₃ formation, showing that N₂ is the nitrogen source for NH₃ production. Under photo-irradiation, the photogenerated electrons are the main active species for N₂ fixation. Moreover, the photogenerated electrons and hole were well separated and effectively contributed to the reduction process, which maximizes the catalytic activity due to the high visible adsorption and ternary band function. Fig. 6d shows the impact of ultrasonic power on sonophotocatalytic N₂ fixing. The enhanced cavitation effect increasing the NH₃ formation rate was due to the extended sonication power from 90 W to 180 W. Increasing the sonication power to 210 W did not increase NH₃ formation significantly compared to 180 W. This may be because increasing the sonication power tends to increase the generation of bubbles too large to continue the cavitation [54], which may minimize the N₂ production rate. In addition, the sonophotocatalytic performance, catalytic usability, and stability are the concern parameters for the catalyst for the practical application. The rate of NH₃ production has been monitored as 498 μmol h⁻¹ g⁻¹ in 120 min in the fourth cycle, indicating a stable catalyst (Fig. 6e). The loss in the catalytic activity may be due to some mass loss of catalyst during the reusable process. In addition, XRD showed that the structural nature of the MXene/TiO₂-WS₂ was preserved, even after four reusable cycles, and the morphology of the reused catalyst did not show any significant change (Fig. 6f). The sonophotocatalytic N₂ fixation properties of MXene/TiO₂-WS₂ heterostructure were compared with the recently reported heterostructure catalysts (Table S2). The production rate of NH₃ is higher for the ternary MXene/TiO₂-WS₂ heterostructure as compared with most of the heterostructure catalysts. The higher efficiency of the catalytic system can be attributed to the highly photo-responsive nature, structure of the heterostructure interface, and decrease in the resistance of carrier transfer by creating the MXene-WS₂ heterostructure with TiO₂ nanograin interface. The photoluminescence (PL) properties of the MXene and their heterostructures were studied to investigate the charge transfer effect in the heterostructure and recombination effect. Fig. 7a presents the PL spectra of MXene heterostructure under the excited wavelength of 375 nm. Compared to the MXene and WS₂ samples, the heterostructure samples had a low PL intensity, suggesting a reduced recombination rate for electron and hole pairs produced by photosynthesis. Under light irradiation, the photoinduced electrons were separated from the CB of WS₂ to the MXene via the CB of TiO₂, which promotes separation efficiency by minimizing the recombination rate. The possible charge transfer mechanism was assessed by obtaining the XPS valance band spectra and Mott–Schottky curve to study the band details of WS₂ and MXene-WS₂ and MXene/TiO₂-WS₂.

Fig. 6.

(a)Yield of NH₃ concentration over different catalysts during the sonophotocatalytic process respective to time. (b) NH₃ concentration and (c) NH₃ production rate under different conditions with the MXene/TiO₂-WS₂ catalyst. (d) sonication power on the formation of NH₃, (e) cycling experiment on MXene/TiO₂-WS₂ catalyst. (f) XRD and SEM image of the MXene/TiO₂-WS₂ catalyst after four reusable catalytic cycles.

Fig. 7.

(a) Photoluminescence spectra of the as-prepared MXene-based heterostructures. (b) XPS valance band spectra of WS₂, MXene-WS₂, and MXene-TiO₂-WS₂. (c) Mott Schottky plot of the WS₂, MXene/TiO₂, MXene-WS₂, and MXene/TiO₂-WS₂. (d) Transition photocurrent response of WS₂, MXene-WS₂, and MXene/TiO₂-WS₂ under visible irradiation using three-electrode measurements. (e) Electrochemical impedance spectrometry (EIS) spectra of the MXene-based heterostructure. (f) Sonophotocatalytic nitrogen fixation of MXene/TiO₂-WS₂ in the presence of alcohol hole scavengers (methanol, ethanol, and tert-butanol) and electron scavengers (AgNO₃).

From the XPS VB (Fig. 7b) spectra, the VB of the WS₂, MXene-TiO₂, and MXene/TiO₂-WS₂ were estimated to be 0.60, 1.94, and 0.86 eV, respectively. The Mott–Schottky curve (Fig. 7c) was analyzed over the heterostructure catalyst to calibrate the band structure. The flat band potential WS₂, MXene-WS₂, and MXene/TiO₂-WS₂ were estimated to be − 0.63, −0.49, and − 0.54 eV (vs. Ag/AgCl) based on the linear potential intersects at Cs² = 0. The positive slope of the WS₂, MXene-WS_2, and MXene/TiO₂-WS₂ confirmed the n-type behavior of the nanostructures. The transition photocurrent of the as-prepared samples was analyzed under visible irradiation with 50 sec on/off cycles. Compared to WS₂, the MXene-WS₂ and MXene/TiO₂-WS₂ heterostructures have displayed higher photocurrent density because of the improved charge separation efficiency of the photoinduced charge carrier. The photocurrent density of MXene/TiO₂-WS₂ was more than 10 times higher than the WS₂ nanoflakes, addressing the significant heterostructure interface and carrier migration efficiency. Furthermore, the flat band potential of the n-type materials was equal to the Fermi level, where CB was very close to the Fermi level. Moreover, the VB position was calculated from the XPS VB spectra. The calculated band gap from the UV–Vis DRS results was correlated with the VB position, and the CB was calculated. As shown in Fig. 7d, the photocurrent density of MXene-WS₂ and MXene/TiO₂-WS₂ samples were spiked immediately under visible irradiation and dropped under dark conditions. The photocurrent density of MXene/TiO₂-WS₂ was approximately two times higher than that of MXene-WS₂. The charge transfer resistance between the electrolyte and the catalyst was studied by EIS. Fig. 7e shows the Nyquist plot attained from the catalyst without visible light irradiation. The arc radius of MXene/TiO₂-WS₂ was smaller than the other catalysts, highlighting the low charge transfer resistance. The electrolyte resistance (R_s) and charge transfer resistance R_ct were attributed to the internal charge transfer resistance of the catalytic nanostructures. Fig. 7e shows the Nyquist plot attained from the catalyst without visible light irradiation. The arc radius of MXene/TiO₂-WS₂ was smaller than the other catalysts, highlighting the low charge transfer resistance. The electrolyte resistance (R_s) and charge transfer resistance R_ct were attributed to the internal charge transfer resistance of the catalytic nanostructures. Fig. 7e shows that MXene/TiO₂-WS₂ has the R_ct value of 236 Ω, which is smaller than the MXene-WS₂ and MXene/TiO₂ nanostructures. The result shows that tagging the WS₂ on the MXene surface minimizes the charge transfer resistance. The in-situ generation of TiO₂ facilitates the charge transfer and promotes the photogenerated carrier across the electrode and electrolyte interface. Fig. 7f shows the scavenger experiment to understand the reaction mechanism of the MXene/TiO₂-WS₂ sonophotocatalyst during the photo fixation process of N₂. AgNO₃ and alcohol (methanol, tert-butanol, and ethanol) were trapping agents for e⁻ and h⁺, respectively. The N₂ fixation process was suppressed by adding AgNO₃, suggesting that the photogenerated electrons are the promising active species in the photo fixation of N₂. The possible sonophotocatalytic N₂ fixation process was explained as follows:

H₂O + ultrasound → •OH + •H

MXene/TiO₂-WS₂ + photo + sono energy → e⁻ + h⁺

e⁻ + O₂ →.O₂⁻

h⁺ + OH⁻ →.OH

2H₂O + 4 h⁺ → O₂ + 4H⁺

N₂ + 6H⁺ + 6 e⁻ → 2NH₃

NH₃ + H₂O → NH₃·H₂O ↔ NH₄⁺ + OH⁻

Fig. 8 shows the proposed possible sonophotocatalytic N₂ fixation over MXene/TiO₂-WS₂. The synergistic function of the sonocatalysis and photocatalysis promotes the sonophotocatalytic functionality of the MXene/TiO₂-WS₂ heterostructure. The band position of WS₂ was calculated using the XPS valance band position and Mott–Schottky plot. The valance band positions of WS₂ and MXene/TiO₂ were 0.60 eV (0.51 vs. NHE) and 1.94 eV (1.85 vs. NHE), respectively. According to the UV DRS results, the calculated CB position was − 1.11 eV vs. NHE for WS₂. TiO₂ formed over the MXene would form a Schottky junction, and the work function of MXene was greater than that of WS₂, resulting in the electron in WS₂ flowing through the metallic MXene, leading to the effective carrier separation. In addition, the work function of MXene would vary depending on the surface functionality where the hydrothermal growth of WS₂ could induce the − O or − OH functionalities [55]. The strong interfacial contract of TiO₂ and MXene established the Schottky contact, and the surface-loaded WS₂ resulted in the type II band function between the WS₂ and in-situ-generated TiO₂ nanograins. To understand the insight of MXene/TiO₂-WS₂ heterostructure, the VB and CB position of WS2 is calculated as 0.6 eV and −1.02 eV from the XPS VB spectra and their respective bandgap calculated from UV-DRS spectra. The VB and CB positions of TiO₂ were determined as 2.45 V and − 0.63 V vs. NHE, according to a previous report [56]. Under photoirradiation, electrons from WS₂ were excited from the VB to CB. Because the CB of WS₂ is more negative than the TiO₂ nanograin, photogenerated electrons migrated from the CB of WS₂ to TiO₂, and transferred to the MXene, which is more positive than the TiO₂ and WS₂ when the contact among the TiO₂, WS₂, and MXene is so tight [41].

Fig. 8.

Sonophotocatalytic nitrogen fixation mechanism of the MXene/TiO₂-WS₂ heterostructure.

The presence of oxygen vacancy states on the TiO₂ will trap the electrons in the CB and inject them to the empty antibonding orbitals (π*) of the N₂ molecule, which promotes the formation of NH₃. Hence, the highly crystalline TiO₂ on the MXene surface has a co-catalytic effect that could extract the photogenerated electrons rapidly for the fixation of N₂ to boost NH₃ production. Cavitation bubble formation caused by acoustic cavitation by the sonication can generate hot spots that can pyrolyze the water to generate •OH. The influence of ultrasonication emitted the sonoluminescence that excited the e⁻ that further reduce the N₂ to NH₃. In the absence of catalysis, the sonolysis can explain the role of the ultrasonic wave that influences the cavitation effect for NH₃ production. The oxidization of MXene induced TiO₂ functionality on the surface with high oxygen-based defect states, causing extended visible light adsorption. When forming the heterostructure interface of WS₂ on the MXene/TiO₂ surface, the electrons are photoexcited to the CB of WS₂ and CB of defect TiO₂ nanograins under light irradiation. With the type II band function, the photogenerated electrons were moved towards the CB of TiO₂ and transferred to the MXene surface through the Schottky junction interface. The hole was well separated in the VB of WS₂, which minimized carrier recombination. Because the CB position of MXene was more negative than N₂/NH₃ redox potential (−0.092 V vs. NHE), which effectively reduces the N₂ to NH₃. The rate of photoinduced electron separation was influenced by photogenerated holes collected on the VB of WS₂ and TiO₂, where the VB holes oxidized the H₂O to provide H⁺ for NH₃ production. Sonophotocatalytic generated electrons and holes might react with surface adsorbed OH⁻ and O₂ to produce •OH and •O₂⁻ radicals that decompose the organic contaminants into H₂O and CO₂. The as-prepared MXene/TiO₂-WS₂ heterostructure shows enhanced N₂ fixation performances through the influence of sonication in the traditional photocatalytic system.

4. Conclusion

In summary, a highly active hybrid hierarchical MXene/TiO₂-WS₂ heterostructure sonophotocatalyst was designed and fabricated through the thermal treatment of MXene followed by the hydrothermal process. This strategy endows the in-situ generation of TiO₂ nanograins on the stacked MXene surface that decorated by the hierarchical growth of WS₂ nanoflakes. During photo fixation of N₂, MXene/TiO₂-WS₂ achieved the maximum NH₃ production rate of 526 μmol·g⁻¹·h⁻¹ under a visible, responsive sonophotocatalytic reaction, which was 2.06 and 1.47 times higher than MXene/TiO₂ and MXene-WS₂. Further, the sonophotocatalytic decomposition efficiency of TC was 2.19 and 1.32 times higher than the sonocatalytic and photocatalytic degradation rates. The defect-enriched TiO₂-based interfacial layer on the MXene enriched the electronic configuration with the Schottky junction, and the dual MXene/TiO₂-WS₂ heterostructure interface promoted the effective activation/adsorption of N₂. It was found that, MXene/TiO₂-WS₂ heterostructure present a wide adsorption range with favorable band potential for effective charge separation rate that increases the radical production under light irradiation. Furthermore, the rate of NH₃ production by the MXene/TiO₂-WS₂ heterostructure retained 94 % after four cycles, indicating the durability and reusability of the sonophotocatalytic process. The hybridized MXene/TiO₂-WS₂ heterostructure improves photocarrier usage and leads to superior catalytic performance for N₂ photo fixation and TC decomposition. The present finding focuses on developing new active catalytic through the ratio design of catalytic sites to increase the adsorption of N₂ concentration on the catalytic surface and effective carrier separation efficiency to achieve high N₂ conversion efficiency.

CRediT authorship contribution statement

Kugalur Shanmugam Ranjith: Investigation, Formal analysis, Conceptualization, Writing – original draft. Seyed Majid Ghoreishian: Software, Formal analysis. Reddicherla Umapathi: Formal analysis. Ganji Seeta Rama Raju: Formal analysis. Hyun Uk Lee: Formal analysis. Yun Suk Huh: Supervision, Writing – review & editing. Young-Kyu Han: Supervision, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following are the Supplementary data to this article:

Data availability

Data will be made available on request.