WO₃/Nb₂CT_x MXene 2D–2D heterojunction as a high performance photoanode for photoelectrochemical water splitting

Abstract

Photoelectrochemical (PEC) water splitting plays a key role in the production of green hydrogen, which is a sustainable energy source and non-exploitative to the environment. Therefore, the development of efficient photocatalysts is essential for enabling green hydrogen generation. In this work, the synergistic effect between tungsten oxide (WO₃) and Nb₂CT_x (MXene) was explored for PEC water oxidation, and a composite catalyst was prepared using a hydrothermal and sonication approach to obtain a 2D/2D WO₃/Nb₂CT_x heterojunction. WO₃ is a promising photocatalyst owing to its optimal band gap, stability, and cost-effectiveness, but its efficiency is hindered by poor charge transfer, rapid recombination, and weak visible light absorption. Integrating Nb₂CT_x, a highly conductive MXene, enhances charge separation, reduces electron–hole recombination, and strengthens photocatalytic activity. This synergy increases the number of catalytic sites, improves visible light absorption, and stabilizes WO₃, leading to a more efficient and durable material for solar energy conversion and water splitting. The Tauc plot of the composite shows a slightly lower bandgap (2.56 eV) than that of pristine WO₃ (2.74 eV), while the charge separation efficiency of the composite is confirmed by its lower photoluminescence intensity than that of pristine WO₃. Among the synthesized catalysts, WO₃@Nb₂C3 showed improved PEC-OER activity by attaining a photocurrent density of 4.71 mA cm⁻² at 1.23 V vs. RHE compared to the pristine WO₃ that attained a photocurrent density of 2.15 mA cm⁻² at the same potential. This strategy appears promising for designing catalysts for PEC water oxidation in a solar-driven hydrogen-powered future.

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1. Introduction

The environment has been severely degraded because of the widespread reliance on non-renewable energy sources, such as coal, oil, and natural gas. Burning fossil fuels as a continuous energy source has become hazardous to the environment, as it is the main cause of global warming, air contamination, extreme weather patterns, and ecosystem deterioration, making life less sustainable on the planet. Therefore, there is an urgent global need to transition to cleaner and renewable energy sources such as wind, hydropower, and solar energy.^1,2 Photoelectrochemical (PEC) water splitting is the process of generating green hydrogen via the electrolysis of water using solar energy. PEC water splitting is a great substitute for nonrenewable energy sources because it produces green hydrogen without dependence on fossil fuels; hence, it is a great technique for producing clean energy, as it assures a more environmentally friendly, sustainable, and resilient hydrogen-powered future.^3,4 Therefore, stable, abundant, and cost-effective photocatalysts are required to conduct photoelectrochemical water splitting efficiently. Some important parameters to consider when choosing an efficient photocatalyst include light harvesting efficiency, band gap energy, easy transfer of electrons from the valence band to the conduction band and less electron–hole recombination.² Various studies have been conducted on different photocatalysts such as BiVO₄,⁵ CdS,⁶ Bi₂WO₆,⁷ WO₃,⁸ and g-C₃N₄.⁹ These efficient catalysts, however, suffer from limited visible light absorption, faster charge recombination and poor stability. To overcome these problems, the formation of heterostructures has been reported, for example, with BiVO₄,^10,11 TiO₂,^12–15 Fe₂O₃,^16,17 ZnO,¹⁸ silicon-based n–i–p heterojunction¹⁹ and surface engineering for defect passivation/reduction.²⁰ WO₃ has attracted significant interest among them because it has many aspects that make it an ideal candidate for use as a photoanode material. Bulk WO₃ is an n-type semiconductor material with an indirect band gap of 2.4–2.8 eV, where the filled O 2p orbital forms the valence band, while the conduction band is formed by the empty W 5d orbital.^21,22 With visible light absorption coverage of 12% of the energy range of the solar spectrum, it offers chemical stability, straightforward synthesis, strong photocatalytic oxidizing ability, cost effectiveness, abundance, and non-toxicity.²³ However, WO₃ is susceptible to photo corrosion, and bare tungsten oxide has quite a low photocatalytic activity because of faster electron–hole recombination; there is a need to develop a strategy for overcoming these problems and enhancing the photocatalytic activity of WO₃.²² In the literature, many strategies have been employed to enhance the photocatalytic performance of WO₃, such as morphological engineering, modification through doping with various elements, oxygen vacancy studies,^23,24 and crystal phase control.²⁵ One of the strategies is to form composites with suitable semiconductor materials to create heterostructures that can result in an increased number of active sites, increased surface area and enhanced photocatalytic efficiency of tungsten oxide by an increase in the rate of charge separation and decrease in the rate of electron–hole recombination.^26–28 A class of 2D materials called MXenes are transition metal carbides or nitrides with the general formula M_n+1X_nT_x, where n is a number from 1 to 3, M is a transition metal, X is the carbide or nitride, and T_x is the surface functional group, which is usually –F, –OH, or –O. MXenes are obtained from the MAX phase by the process of etching through the removal of the “A” layer (Al, Ge, or Si).²⁹ MXene can prove to be a great choice for creating heterostructures with WO₃ because of its high conductivity, structural support and stability, photostability, hydrophilicity, and corrosion-resistant nature.^30,31 MXenes have multiple exposed metal sites, many surface groups and layered structures; all these properties agree with the formation of a heterostructure of MXene and WO₃ to improve the overall photocatalytic activity of WO₃ for PEC water oxidation. A few studies have reported composite formation using MXene with WO₃; for example, ZnO/WO₃/Ti₃C₂ has been tested for PEC water splitting.³² Another study reported the use of Ti₃C₂ MXene/WO₃ for photodegradation studies of tetracycline antibiotics, and the composite was synthesized using in situ growth of WO₃ nanosheets on the MXene surface in a one-pot solvothermal synthesis strategy.³³ In another similar study, hydrothermal and ultrasonic treatments were used to develop a 2D/2D WO₃/Ti₃C₂ heterojunction and were used for the photocatalytic degradation of methylene blue dye under visible light irradiation.³⁴ Similarly, a WO₃/MXene (Ti₃C₂T_x) composite was reported earlier using electrostatic attraction between WO₃ nanorods and MXene (Ti₃C₂T_x) for supercapacitor applications.³⁵ Few studies have been conducted on different kinds of MXene, such as niobium carbide, vanadium carbide, or molybdenum carbide, and there is still room for investigating them as PEC catalysts. In previous studies, niobium carbide MXene has been used as a photoactive material and for charge storage in the literature.³⁶ In this study, WO₃ nanorods and niobium carbide (Nb₂CT_x) MXene composites are prepared by applying the hydrothermal method, followed by sonication. The synergistic effect of WO₃ and Nb₂CT_x is explored, where both the components of the heterostructure complement the photocatalytic properties and surface characteristics of each other, resulting in enhanced photoelectrochemical OER activity. Nb₂CT_x facilitates charge extraction, while WO₃ provides an electronic structure that drives the water oxidation reaction efficiently. WO₃, which is susceptible to photo corrosion, can undergo photo corrosion during the water oxidation reaction, but it is protected by Nb₂CT_x because it stabilizes the surface of WO₃ and has a noncorrosive nature and tungsten oxide is not degraded over time and therefore has enhanced stability. Hence, the interaction of tungsten oxide with niobium carbide seems promising, as it can result in increased light absorption efficiency, enhanced charge separation, decreased electron–hole recombination, increased number of catalytic sites, prolonged stability, and enhanced interface contact surface area. This study is expected to serve as a pathway for designing highly efficient photocatalysts driven by visible light. The as-prepared samples were characterized using powder XRD, XPS, FESEM/EDX, HRTEM, and PL techniques.

2. Experimental

2.1. Materials and chemicals

Ammonium metatungstate, hydrochloric acid (HCl), sodium sulfate (Na₂SO₄), and hydrofluoric acid (HF) were purchased from Sigma-Aldrich. The MAX phase (Nb₂AlC) was purchased from Chemazone, Inc. Nanochemazone. Fluorine-doped tin oxide (FTO) plates were obtained from Sigma-Aldrich. Deionized water was used to prepare the electrolytes and reaction mixtures.

2.2. Synthesis of WO₃

WO₃ nanorods were synthesized by applying the hydrothermal method, in which 0.77 g of ammonium meta tungstate was dissolved in 12.5 ml distilled water and 0.53 ml hydrochloric acid was added to it. The homogenized mixture was then transferred into a Teflon-lined autoclave and kept at 180 °C for 4 hours. After centrifugation and washing at 5000 rpm with distilled water three times, the material was kept at 70 °C for 6 hours and then kept for annealing at 500 °C for half an hour, and yellowish powder was formed.³⁷

2.3 Synthesis of Nb₂CT_x

Two grams of Nb₂AlC were slowly added into 40 ml of HF, and the solution was magnetically stirred at room temperature for 90 hours.³⁸ After this period, the material was washed through centrifugation at 5000 rpm multiple times with DI water. The washed material was then kept for drying overnight at 60 °C.

2.4 Synthesis of WO₃/Nb₂CT_x heterojunction

WO₃/Nb₂CT_x heterojunction was prepared by applying the sonication approach. 100 mg of hydrothermally prepared WO₃ was dissolved in 10 ml of DI water, and Nb₂CT_x sheets were added to it (5, 10, and 15 weight%). The solution was kept at sonication for 12 hours; then, the resulting suspension was dried overnight at 100 °C.³⁴ The resulting materials were named WO₃@Nb₂C1, WO₃@Nb₂C2, and WO₃@Nb₂C3 for 5, 10, and 15 weight% of Nb₂CT_x, respectively. The ultrasonication approach is beneficial for synthesizing such heterojunctions because high shear forces generated by intense sound waves can be helpful in the uniform distribution of particles and can break the agglomerates of particles to ensure uniform dispersibility. As a result of ultrasonic treatment, bubbles are formed, and these bubbles rupture swiftly and hence induce local high temperature and pressure within the material, which ultimately can induce surface modification in both WO₃ and Nb₂CT_x. A schematic of the synthesis is depicted in Fig. 1.

Fig. 1 Schematic of the synthesis showing pure WO₃, Nb₂CT_x MXene and composite WO₃@Nb₂CT_x formation.

2.5 Photoelectrochemical characterization and preparation of the working electrode

Catalyst ink was prepared by applying the spin coating method, and FTO was used. For cleaning, the FTO was immersed in acetone, ethanol, and DI water for 15 minutes in each solvent. 100 mg of the prepared photocatalyst was added into 5 ml of ethanol (20 mg ml⁻¹) to prepare a slurry. This dispersion (1000 microliters) was spin coated at 1000 rpm on FTO substrates to procure WO₃ and WO₃@Nb₂C thin films, which were dried at 60 °C for 5 min. A thin uniform film was then obtained by repeating the coating and drying for 5 cycles. All PEC measurements were performed using an electrochemical workstation (Biologic VSP-300 Potentiostat) with three-electrode cell configurations: platinum wire as the counter electrode, Ag/AgCl as the reference electrode, and catalyst-coated FTO as the working electrode. The experiment was performed in a quartz cell. As the light source solar simulator, Oriel LCS-100 with a 100 W Xe lamp and 1.0 sun AM 1.5 G filter was used. The electrolyte used was a 0.5 M Na₂SO₄ solution. The Nernst equation was used for the conversion of potential to vs. RHE. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) measurements were performed in the potential range of 0.6–1.6 V vs. RHE under illumination and in the dark as well. Chopped light LSVs were also performed in the potential range of 0.6–1.6 V vs. RHE at a scan rate of 5 mV s⁻¹ using a mechanical chopper with 5 seconds light ON and 5 seconds light OFF intervals. Electrochemical impedance spectroscopy (EIS) was performed under illumination in the frequency range of 100–100 mHz at a potential of 0.8 V vs. RHE. Electrochemical surface area (ECSA) and double layer capacitance (C_dl) were calculated by taking CV cycles in the dark in the non-faradaic potential range at various scan rates. The Mott–Schottky analysis was performed to determine the flat band potential and donor density.

3 Results and discussion

3.1 Characterizations

The synthesis of all the materials was studied by powder X-ray diffraction (XRD), and the XRD was recorded in the range of 2θ 5–70°. The etching of the commercially available niobium aluminum carbide (Nb₂AlC) MAX phase to Nb₂CT_x MXene was confirmed by XRD patterns, as illustrated in Fig. S1.† The MAX phase shows a prominent diffraction pattern at the 2θ values of 12.7° (002), 33.3° (100), 38.7° (103), 39.1° (006), and 52.3° (106) of crystalline Nb₂AlC, which matches well with the standard pattern (JCPDS 30-0033) (Fig. S1†).^38,39 After treatment with HF to etching the Al layer, a characteristic broad signal was observed at a 2θ value of 8.9°, which corresponds to the (002) crystal plane of Nb₂CT_x MXene, corresponding to the d-spacing of 0.98 nm, similar to the previously reported study.⁴⁰ The appearance of this new signal as well as the substantial decrease in the intensity of the diffraction signal at 39° (006); others confirm the formation of Nb₂CT_x MXene after successful etching.^38,40 The synthesis of WO₃ is confirmed by analyzing the XRD pattern, showing the prominent signals at 2θ values of 23.1°, 23.5°, 24.4°, 33.3°, 33.6°, 34.2°, and 49.9° and it matches well with the standard pdf # 431035 (Fig. 2a) and shows the monoclinic phase of the WO₃.²² The synthesis of WO₃@Nb₂CT_x heterojunction with different ratios of Nb₂CT_x (5–15%) was also confirmed by the XRD pattern shown in Fig. 2(c–e), and the broad signal at 8.9 shows that the Nb₂CT_x is incorporated successfully to produce WO₃@Nb₂CT_x composite with varying ratios.⁴¹ The XRD of the composites shows no significant shift in WO₃ peaks, suggesting physical interfacial interaction between WO₃ and Nb₂CT_x MXene, similar to previously reported studies where composite synthesis was carried out through electrostatic attraction between WO₃ nanorods and Ti₃C₂T_x MXene for supercapacitor applications.³⁵

Fig. 2 XRD patterns of the WO₃ reference card (a), WO₃ (b), WO₃@Nb₂C1 (c), WO₃@Nb₂C2 (d), and WO₃@Nb₂C3 (e). The insets show the magnified patterns of (c)–(e).

The syntheses of WO₃ and WO₃@Nb₂C3 composites were also confirmed by analyzing the XPS survey scan. Fig. 3 shows that all the desired elements (W, O, Nb, and C) are present on their respective binding energies, which shows the successful incorporation of Nb₂CT_x MXene with WO₃. The absence of Al 2p and 2s signals at ≈74 and ≈118 eV in both survey scans also confirms successful etching.⁴² The high-resolution spectra of W 4f core levels show a well-defined doublet at binding energies at 35.6 and 37.7 eV and can be assigned to W 4f_7/2 and W 4f_5/2 for W⁶⁺ (Fig. 3b) and a weak doublet at 34.3 eV and 36.4 eV for W⁵⁺.^33,43 The high resolution XPS core level Nb 3d region shows spin orbital splitting at binding energies of 203.3 and 206.1 eV, which corresponds to 3d_5/2 and 3d_3/2 of Nb–C bonds, respectively, for both Nb₂CT_x and WO₃@Nb₂C3 composite (Fig. 3c). Another week's broad doublet at 204.9 eV and 207.4 eV shows the presence of Nb–O bonds, as reported previously for Nb₂CT_x MXene.^44,45 The core level XPS of the O 1s region of WO₃ (Fig. 3d) shows the major peak fitting at 530.5 eV and 531.4, which can be assigned to lattice oxygen and oxygen absorbed on the surface, respectively.³³ In the case of WO₃@Nb₂C3 composite, O 1s spectra show other peaks at 531.6 and 533 eV, which can be assigned to C–Nb–O_x and interlayer adsorbed water due to the presence of Nb₂CT_x MXene.^30,45

Fig. 3 XPS survey scan of WO₃ and WO₃@Nb₂C3 (a); high resolution XPS spectra of W 4f of WO₃ and WO₃@Nb₂C3 (b); high resolution XPS spectra of Nb 3d of Nb₂CT_x and WO₃@Nb₂C3 (c); and high resolution XPS spectra of O 1s of WO₃ and WO₃@Nb₂C3 (d).

Fig. S2† shows the FESEM images of the etched Nb₂CT_x MXene. It can be observed that the layered morphology of the Nb₂CT_x is present, and gaps in aluminum after etching are visible, which is a typical shape of 2D MXenes, as reported previously.^30,46 Fig. S3† shows the small plate-like structures of pristine WO₃ in the nanometer range (a) and flower-like arrangements when aggregated (b). During the composite formation, WO₃ was smoothly distributed over the surface of the delaminated layers of Nb₂CT_x to produce the WO₃@Nb₂C3 composite. The sharp edges of the nanoplate become smooth in the composite owing to sonication, as shown in Fig. 4(a) and (b). Fig. 4c shows the EDX spectrum of the WO₃@Nb₂C3 composite with all the required component elements.

Fig. 4 FESEM images of the WO₃@Nb₂C3 composite at different resolutions (a and b) and EDX spectrum of the composite (c).

Fig. 5 shows TEM images of the composite. Both components of the composite WO₃ and Nb₂CT_x MXene are visible (Fig. 5a), showing successful composite formation; the fringes on Nb₂CT_x MXene are clearly visible and are related to interlayer spacing.⁴⁰ The HRTEM (Fig. 5a) of WO₃@Nb₂C3 composite shows the fringes corresponding to plane (002) of Nb₂CT_x MXene with a d-spacing of 0.98 nm, while the WO₃ shows the fringes corresponding to plane (200) of monoclinic phase with d-spacing of 0.36 nm at the composite interface.⁴⁷

Fig. 5 TEM image of composite (a) at 20 nm and HRTEM image (b) at 5 nm.

The investigation of the optical absorption properties of WO₃ and WO₃@Nb₂C3 composite was carried out by UV-vis spectroscopy. The Tauc equation is used for the relation between the bandgap energy and absorption spectra, which is written as follows: (αhν) = k(hν − E_g)ⁿ.

Extrapolating the linear portion of the plot (hν) vs. energy (αhν)² provides the band gap (E_g) within a wavelength range of 200–800 nm. Various parameters can be employed to induce alteration in E_g values, including doping, grain size, annealing treatment, and the nature of transition (direct or indirect).⁴⁸ The bandgap energies of WO₃ and WO₃@Nb₂C3 composite were observed as 2.74 and 2.56 eV, respectively (Fig. 6a), which agrees with previously reported studies.^33,47 By the addition of Nb₂CT_x sheets to the catalyst, the band gap energy for WO₃@Nb₂C3 is lower than that of WO₃, and it can be attributed to the overlapping of electronic band structures.⁴⁷ As the band gap energy is reduced, it is beneficial for the generation of electrons and holes in visible light, and Nb₂CT_x serves to prevent the recombination of electrons and holes, which ultimately results in the enhancement of the photoelectrocatalytic efficiency of WO₃@Nb₂C3. Steady-state photoluminescence (PL) studies were conducted to interpret the behavior of photo-induced charge separation and recombination. Fig. 6b shows a broad emission peak under an excitation wavelength of 375 nm for pristine WO₃ at a signal centered at about 465 nm, which agrees with faster electron–hole pair recombination as a result of the de-excitation emission from W 4f to O 2p. Two other peaks of low intensity at 500 nm as a shoulder and 545 nm could arise owing to the band transition of the trapped electrons on defect sites to the valence band, showing the role of W⁵⁺ species in WO₃, which is evident in the XPS results.³³ It is evident that the PL intensity significantly decreased upon the composite formation of WO₃@Nb₂C3 compared to pure WO₃. This disparity in photoluminescence can be attributed to the boosted charge separation within the composite.²³ The quenching of photoluminescence intensity reveals that the assimilation of Nb₂CT_x MXene on WO₃ possibly accelerates spatial charge separation and overturns carrier recombination through the establishment of a supplementary nonradiative decay pathway across the WO₃/Nb₂CT_x MXene interface.³³ The increased charge separation and reduced recombination rate make the material highly suitable for various photocatalytic applications.²¹

Fig. 6 Spectroscopic characterization of Nb₂CT_x MXene, WO₃ and WO₃@Nb₂C3 composites: (a) Tauc plots and (b) photoluminescence.

3.2 Photoelectrochemical studies

Photoelectrochemical catalytic activity was examined using an electrochemical workstation (Biologic VSP-300 potentiostat) with a 1.0 sun AM 1.5 G solar simulator as the light source in a 0.5 M Na₂SO₄ electrolyte. This work preferred a hole scavenger-free electrolyte, as the presence of a sacrificial agent can interrupt and mask some important information about the photoelectrochemical experiment representations. LSV measurements were performed for WO₃, WO₃@Nb₂C1, WO₃@Nb₂C2, and WO₃@Nb₂C3 under illumination and in the dark as well. Fig. 7a shows that WO₃ photocatalytic activity is enhanced by forming a heterojunction with Nb₂CT_x. Among the three composites with varying ratios of Nb₂CT_x, WO₃@Nb₂C3 outperformed the other catalysts by attaining a photocurrent density of 4.71 mA cm⁻²vs. 1.23 V RHE, while WO₃, WO₃@Nb₂C1, and WO₃@Nb₂C2 attained photocurrent densities of 2.15 mA cm⁻², 3.48 mA cm⁻², and 4.56 mA cm⁻² at 1.23 V RHE, respectively. The higher photocurrents demonstrate that more charge carriers pass through the circuit for reaction with water to evolve oxygen.

Fig. 7 PEC activities of pure WO₃ and WO₃/Nb₂CT_x in 0.5 M Na₂SO₄: (a) LSV curves, (b) Nyquist plots, (c) chopped light LSV curves, (d) chopped light LSV curves for cyclic stability for WO₃, (e) chopped light LSV curves for cyclic stability for WO₃/Nb₂C3 composite, (f) double layer capacitance plots for WO₃/Nb₂C3 composite, (g) Mott–Schottky plots for WO₃ and WO₃/Nb₂C3 composite, and (h) charge carrier generation/separation and participation in water splitting.

EIS was performed under illumination in frequencies ranging from 100 kHz to 100 mHz at a potential of 0.8 V vs. RHE, investigating the charge transfer resistance. Fig. 7b shows the Nyquist plots for WO₃, WO₃@Nb₂C1, WO₃@Nb₂C2, and WO₃@Nb₂C3. WO₃@Nb₂C3 has the smallest semicircle among them; the diameter of the semicircle is directly proportional to the R_ct value. The smaller the semicircle, the smaller the resistance, the faster the charge transfer at the electrode–electrolyte interface, and hence the better the catalytic activity. Solution resistance (R_s) for WO₃, WO₃@Nb₂C1, WO₃@Nb₂C2, and WO₃@Nb₂C3 were calculated to be 29.63 Ω, 24.89 Ω, 22.99 Ω, and 21.05 Ω, respectively. R_ct values for WO₃, WO₃@Nb₂C1, WO₃@Nb₂C2, and WO₃@Nb₂C3 were determined to be 665 Ω, 589 Ω, 416 Ω, and 309 Ω, respectively. The interaction of Nb₂CT_x with WO₃ decreases charge transfer resistance by facilitating electron transfer and reducing electron–hole recombination, thereby providing better PEC-OER activity.

Chopped light LSV was performed with a mechanical chopper at an interval of 5 seconds, alternating light ON and light OFF. In PEC water oxidation, chopped light LSV can help optimize photoelectrodes because it provides information about light harvesting efficiency, generation of photocurrents, and the dynamics of charge carriers of the catalyst. WO₃@Nb₂C3 outperformed the bare WO₃ in chopped light LSV (Fig. 7c). These results agree with the LSV as well. WO₃@Nb₂C3 attained a higher photocurrent. This shows that the addition of Nb₂CT_x MXene to WO₃ helps in better charge separation, and it has reduced the electron–hole recombination as well; therefore, a higher photocurrent can be observed for the WO₃@Nb₂C3 heterojunction. The difference in the dark current when the light is OFF and the photocurrent when the light is ON is of crucial importance in evaluating the efficiency of the catalyst for the evolution of oxygen through the oxidation of water. As depicted in Fig. 6c, we can observe that the current drops to zero when there is no light and rises swiftly when the light is ON. This indicates that the reaction is mainly driven by the photo-induced charge carriers and not through the applied voltage. Chopped light LSV can also provide insights to determine the photostability of the catalyst when the catalyst is consecutively exposed to light for up to many cycles. We performed 50 consecutive chopped light LSVs to evaluate the photostability of our catalysts, and the comparison of the 10th, 20th, 30th, 40th, and 50th LSV is shown in Fig. 7d and e. WO₃@Nb₂C3 outperformed WO₃. This indicates that the WO₃@Nb₂C3 heterojunction has prevented degradation and photo corrosion. These results are considerable for analyzing the long-term stability of catalysts under real operating parameters.

Double layer capacitance (C_dl) was determined for WO₃ and WO₃@Nb₂C3 by taking multiple CV cycles in non-faradaic potential range at various scan rates of 20, 40, 60, 80, and 100 mV s⁻¹ under dark conditions to study the intrinsic electrochemical behavior of the prepared catalysts without considering the activity induced by light (Fig. S4†). Δj was plotted against the scan rate, and the slope of this plot gave the C_dl value. WO₃@Nb₂C3 exhibited a higher C_dl value (2.01 mF) than WO₃ (1.21) mF, ECSA is directly proportional to C_dl value (Fig. 7f). The higher the C_dl value, the higher the ECSA. ECSA is determined using the following formula: C_dl/C_s, where C_s is the specific capacitance value of the electrode material. WO₃@Nb₂C3 possessed a higher ECSA value (40.2 cm²) than pristine WO₃ (24.2 cm²). A higher value of ECSA for WO₃@Nb₂C3 indicates that there are more catalytic sites available on it compared to WO₃ and larger surface area availability for charge transfer and charge accumulation.

Mott–Schottky (MS) analysis was conducted for WO₃ and WO₃@Nb₂C3 heterojunction in the dark to determine the flat band potential value V_fb and donor density N_d. The Mott–Schottky equation is given below:^49,50

where C is the differential capacitance with respect to potential, ε is the dielectric constant of the semiconductor, ε₀ is the vacuum permittivity, A is the area of the electrode that is coated, e is the elementary charge, V is the applied potential, V_fb is the flat band potential, k_B is the Boltzmann constant, and T is the temperature. Following heterojunction formation, the slopes of the Mott–Schottky plot within the linear region exhibit a significant decline, indicating an increase in N_d. The flat band potential of a semiconductor film can be determined from the x-intercept of the tangent line on the potential axis in a liquid junction.^26,28Fig. 6g shows a positive slope, which clearly indicates n-type semiconductors. V_fb for WO₃@Nb₂C3 is lower than that of WO₃, the donor density for WO₃ is 0.2715 × 10¹⁶ cm⁻³, and WO₃@Nb₂C3 possesses a higher donor density of 0.36 × 10¹⁸ cm⁻³. Higher donor density and lower flat band potential confirmed that the incorporation of Nb₂CT_x to WO₃ increased charge separation, hence providing better photocatalytic activity. As depicted in Fig. 7g, it can be observed that the intercepts of MS plots reveal that a cathodic shift occurs in the V_fb values owing to the composite formation from 0.51 V to 0.46 V for WO₃ and WO₃@Nb₂C3, respectively. For an n-type semiconductor material, compared to the conduction band value, the flat band potential is supposed to be located at a more positive potential, so the conduction band value will be ∼0.1 eV lower than the flat band potential value.⁵¹ Therefore, the values for conduction band edge positions of WO₃ and WO₃@Nb₂C3 were calculated to be 0.41 and 0.36 eV, respectively. The overall band edge positions with respect to water splitting potentials were plotted by combining the band gap energy value, as shown in Fig. 7h. ESI Table 1† shows a comparison of various catalysts for photoelectrochemical water splitting in terms of the photocurrent and electrolyte used, and it is evident that the current study shows overall superior activity compared to previous studies.

Conclusion

In this work, composites of WO₃ with Nb₂CT_x MXene were successfully synthesized by applying a hydrothermal and ultrasonic approach to develop a 2D/2D WO₃@Nb₂CT_x heterojunction. The prepared catalysts were characterized by XRD, XPS, FESEM, EDX, TEM, and UV-visible spectroscopy and photoelectrochemical measurements. The photocatalytic activity of WO₃ was aimed to be enhanced by developing a heterojunction with Nb₂CT_x MXene. Nb₂CT_x is a conductive material and acts as a cocatalyst. Therefore, the composite WO₃@Nb₂C3 results in improved charge transfer and lower electron–hole recombination, which is the most common limitation of any photocatalyst. WO₃@Nb₂C3 showed a photocurrent density of 4.71 mA cm⁻² at 1.23 V vs. RHE. The heterojunction WO₃@Nb₂C3 showed enhanced photostability, lower charge transfer resistance, lower band gap (2.56 eV), lower V_fb and higher C_dl value (2.1 mF). Nb₂CT_x serves as a conductive bridge, which facilitates electron transport from WO₃ to the external circuit, thereby minimizing energy loss and increasing the availability of electrons for water oxidation. Its integration enhances the photocatalytic behavior of WO₃ by improving stability, durability, and visible light absorption while offering protection against corrosion during the oxidation process. Additionally, Nb₂CT_x optimizes charge transfer, increases catalytic sites, and expands surface area, demonstrating the synergistic effect of the heterojunction in advancing photoelectrochemical water oxidation. This study highlights a promising pathway for developing stable and sustainable photocatalysts for solar-driven green energy production.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful to the Pakistan Science Foundation for financial assistance under project no. PSF-NSFC-IV/Chem/C-QAU (27), and M. M. would like to thank the Higher Education Commission of Pakistan for indigenous PhD fellowship.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5na00345h

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