Unveiling the potential of MXene-fabricated catalysts: an effective approach for H₂ generation from water splitting

Abstract

Hydrogen has enough potential and can be successfully used as an alternative to the conventional fuel. It can be successfully produced from water that is not only a sustainable source but exists everywhere on earth. Additionally, its combustion releases water that is quite safe and environment friendly. The current project was designed to generate hydrogen from catalytic water splitting on TiO₂@Ti₃C₂T_x catalysts. To obtain the required catalytic characteristics, titania was engineered on Ti₃C₂T_x surfaces in situ using an ethanol-assisted solvothermal approach. After careful recovery, the catalysts were characterized and assessed for the photoreaction. All photoreactions were performed in a quartz reactor (150 mL), where hydrogen evolution activities were monitored on GC-TCD (Shimadzu-JP). The comparative activities indicated that TiO₂@C and TiO₂@Ti₃C₂T_x catalysts deliver 9.37 and 18.57 mmol g⁻¹ h⁻¹ of hydrogen, respectively. The higher activities of TiO₂@Ti₃C₂T_x were attributed to the existence of higher active sites (charge trapping centres) on the multilayer MXene that progressively promote and facilitate redox reactions. Reason is that existence of titania on MXene interfaces develops heterojunctions that rectify the charge transfer; hence reduce the charge recombination (i.e., back reaction). On the basis of encouraging activities, it has been concluded that the aforementioned approach holds promise to replace the costly and conventional hydrogen generation technologies.

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Introduction

Without doubt, fossil fuels are the main sources of the world's energy, but pollution generated by their burning is enough to ruin life on earth. If the current speed of fossil fuel consumption continues, it will definitely bring a severe change in the earth's climate.¹ Unpredictable weather, sudden hailstorms and rainfalls are undeniable evidence of climate change. Thus, for life on earth, an alternative and green source is urgently needed.² That is why the scientific community is focusing on new and sustainable sources that not only meet the energy demand but also protect our earth and atmosphere. Additionally, excessive growth in the human population and industrialization increase these challenges. Due to high costs of conventional energy sources, developing countries are struggling to shift from fossil fuels to green and sustainable sources.³

Hydrogen is an ideal fuel due to its (i) high calorific value (150 kJ g⁻¹), (ii) zero carbon emissions, and (iii) renewable nature.⁴ Water is a stable molecule, and it needs 237 kJ mol⁻¹ of energy to split into hydrogen and oxygen. However, the activation energy of the water splitting reaction can be reduced with the use of stable and effective catalysts.⁵ For water splitting, commonly used photocatalysts are TiO₂, ZrO₂, ZnO, SrTiO₃, BaTiO₃, ZnS, CeVO₄, CuO, CdS, CdZnS, g-C₃N_4, ZnIn₂S₄, and Zn₃V₂O₈.^6–14 However, these catalysts have been restricted due to the following drawbacks: (i) fast charge recombination; (ii) poor optical response; (iii) photocorrosion; (iv) stability and durability; and (v) improper band gaps and structural defects.¹⁵ Due to the aforementioned drawbacks, these catalysts cannot split water efficiently; therefore, for progressive water splitting, improvements in the catalytic system have become necessary. Thus, modifications using co-catalysts like MXene or graphene have gained more interest due to their excellent conductivity.¹⁶

In recent decades, MXene (Ti₃C₂T_x) has opened new pathways for developing stable and durable catalysts. The reason is that MXenes naturally exhibit surface terminations like –F, =O, and –OH that enhance the surface reaction ability of catalysts.¹⁷ Among the MXenes family, titanium carbide (Ti₃C₂T_x) has gained significant attention for energy conversion and storage applications. Due to tuneable characteristics, increased conductivity, and intrinsic features, it facilitates more active sites during photoreactions.¹⁸

Among co-catalysts, MXene and graphene have been identified as exceptional co-catalysts that can effectively promote surface charges to the active centres (i.e., redox sites). Additionally, these co-catalysts have substantial selectivity and can only perform in optimized catalytic reactions. However, the fabrication of MXene with TiO₂ for targeted applications (i.e., water splitting) is quite challenging having relatively high mechanical strength.¹⁹ Because the dispersion of MXene matrix on semiconductor's surfaces can be problematic, this causes agglomeration during the developmental process. Hence, the synthesis of TiO₂/Ti₃C₂T_x and TiO₂/C has been found significant factor for excellent photocatalytic activities.²⁰ The advantage of MXene support is that it can become the inherent source of TiO₂ growth due to Ti–C–Ti–C–Ti structural arrangements.²¹ Additionally, the complete oxidation of Ti₃C₂T_x produces the carbide-derived carbon (CDC), which also acts as co-catalyst and promotes the photon absorption ability of semiconductor systems.²² Thus, in situ developments of metal oxide, i.e., TiO₂, on Ti₃C₂T_x and CDC are quite important to enhance the overall catalytic efficiency of the photoreaction.

In this work, we have extended the utilization of MXene using in situ-engineered TiO₂ (i.e., TiO₂@Ti₃C₂T_x and TiO₂@C) via an ethanol-assisted solvothermal approach. The significant advantage of in situ growth of TiO₂ has the following advantages for catalytic water splitting: (i) superior conductivity and charge transportation; (ii) formation of heterojunctions to reduce back reactions; (iii) extension of the internal optical response during photoreaction; (iv) enhanced stability and durability; (v) enhanced pool of reactive charges on the redox sites. The findings of the current project deliver encouraging examples of renewable energy systems that can be further be improved for large-scale implementation.

Experimental

The details of the chemicals and instruments used in current work have been discussed in the ESI† section.

Catalysts preparation

The scheme that was employed to design the MXene and derived catalysts is illustrated in Fig. 1. In this study, Ti₃C₂T_x, i.e., titanium carbide, has been developed by the etching process, which was designed and selectively performed with HF (ca. 40%) [20]. Typically, 500 mg of Ti₃AlC₂ was transferred to 40 mL of the aforementioned HF in a 100 mL Teflon reactor. Note that the reaction process was completed in a safe environment, i.e., under a fume hood. To ensure the complete removal of aluminium, the product was vigorously stirred under gentle sonication [21]. The chemical reaction involved in this process is given in eqn (1).

Ti₃AlC₂ + 6HF → AlF₃ + Ti₃C₂T_x + 3H₂ (1)

Fig. 1 Graphical illustration for the synthesis of Ti₃C₂T_x, TiO₂@Ti₃C₂T_x and TiO₂@C catalysts.

To isolate Ti₃C₂T_x from the HF solution, the suspension underwent some important steps: (i) centrifugation at 5000 rpm; (ii) careful washing six times to eliminate impurities; and (iii) vacuum-drying at 85 °C.

In this study, TiO₂@Ti₃C₂T_x and TiO₂@C catalysts were synthesized via an ethanol-assisted solvothermal approach.²³ The Ti₃C₂T_x was used as a precursor material because it has an inherent titanium source for the growth of TiO₂ due to Ti–C–Ti–C–Ti structural arrangements.²⁴ Typically, 200 mg of the aforementioned precursor support was dispersed in 40 mL of mixture solvents (with 80 : 20 water/ethanol ratios) and sonicated for 10 min. For the in situ growth of TiO₂ on Ti₃C₂T_x, this mixture was transferred to a Teflon-lined autoclave reactor for 2 h at 180 °C. The reaction was continued for a further 4 h to achieve the complete oxidation of MXene so that it is successfully transformed into the high-textured TiO₂@C product. It has been noted that optimized conditions are necessary to obtain the complete oxidation of MXene for developing high-quality TiO₂ catalyst textures in situ on the surfaces of carbide.²⁵ After hydrothermal reaction, catalysts were recovered by vacuum filtration. To remove the impurities or unreacted precursors, catalysts were thoroughly rinsed with the ethanol/acetone mixture (80 : 20 v/v ratios). To eliminate the moisture from layered structures, catalysts were vacuum dried at 95 °C for 5 h.

Photocatalytic activities

Photocatalytic water splitting reactions for hydrogen generation were carried out using a PLS-SXE300 lamp. The photoreactions were performed in a 150 mL quartz reactor, where a 20 mg dose of catalysts was optimised for 50 mL of pure water. H₂ and O₂ generation rates were quantified by taking gas samples (0.5 mL per syringe) from the headspace of the photoreactor. For the accurate analysis, high-purity argon served as the carrier gas. A specialized molecular sieve capillary column was used for gas separation. The gases were quantified using the internal calibration curves of the gas chromatography system (GC-TCD, Shimadzu, 2010).²⁶ Quantum efficiencies (QEs) were calculated using eqn (2). Note that these measurements were executed under controlled conditions.

Results and discussions

The synthesis scheme of Ti₃C₂T_x, TiO₂@Ti₃C₂T_x, and TiO₂@C catalysts is shown in Fig. 1. Various characterization techniques have been employed to study the structural, morphological, optical, magnetic, and chemical properties of the synthesized catalysts, which have been discussed below.

XRD analyses

The XRD patterns of Ti₃AlC₂ (MAX), Ti₃C₂T_x (MXene), TiO₂@Ti₃C₂T_x and TiO₂@C are described in Fig. 2a. XRD is an excellent tool that can accurately provide significant insights about structural transformations.²⁷ Results of this study indicate that the XRD pattern of pristine Ti₃AlC₂ closely matches JCPDS No. 52-0875, validating its purity with hexagonal crystalline structure.²⁸ Notably, the XRD patterns of Ti₃C₂T_x display broader lattice planes (002) as compared to Ti₃AlC₂. It is worth mentioning that shift of the (002) planes towards smaller angles along with the disappearance of the characteristic peak at 39° (i.e., corresponds to (104) planes) indicate the complete removal of aluminium atoms (in the form of AlF₃).²⁹ These results confirm the phase purity and successful transformation of MAX to Ti₃C₂T_x structures. The TiO₂@Ti₃C₂T_x catalysts exhibit the inherent diffractions of TiO₂ (anatase) along with the Ti₃C_xT_x XRD patterns. The changes in diffraction pattern confirm the in situ growth of TiO₂ over Ti₃C₂T_x surfaces. On the other hand, the peak positions of TiO₂@C confirm the anatase structure and are exactly matched with PDF#73-1764.³⁰ Additionally, removal of Ti₃C₂T_x diffraction peaks confirms the complete conversion (i.e., oxidation) of Ti–C–Ti–C–Ti to the TiO₂@C.³¹ XRD pattern and their corresponding hkl values for as reported catalysts have been tabulated in Table S1.† The structural changes observed in XRD results provide valuable insights into catalyst performance.

Fig. 2 (a) XRD pattern; (b) FTIR results of Ti₃AlC₂, Ti₃C₂T_x, TiO₂@Ti₃C₂T_x, and TiO₂@C; (c) Raman spectroscopy results of Ti₃C₂T_x, TiO₂@Ti₃C₂T_x, and TiO₂@C; (d) TGA of Ti₃C₂T_x.

FTIR analyses

The FTIR analysis (Fig. 2b) of Ti₃AlC₂ revealed the bending vibrations of the Ti–Al–C bond at 505 cm⁻¹. The etching of aluminium was confirmed by the removal of the 505 cm⁻¹ peak, whereas new vibration patterns are attributed to surface termination (T_x =O, –F, –OH).³² FTIR results indicate that the vibrations observed at 566 cm⁻¹, 706 cm⁻¹, 855 cm⁻¹, and 922 cm⁻¹ are attributed to the Ti–C/Ti–O stretching vibrations. The O-terminations on the Ti₃C₂T_x surface caused Ti–O vibrations at 617 cm⁻¹. F-terminations are observed at 1057 and 1097 cm⁻¹, whereas O–H terminations are detected at 1399 and 1437 cm⁻¹.³³ The vibrations at 1229 and 1343 cm⁻¹ are attributed to adsorb/atmospheric CO₂.³⁴ These results confirm the formation of a multilayer Ti₃C₂T_x structure. In the FTIR results of TiO₂@Ti₃C₂T_x catalysts, the strong vibration at 480 cm⁻¹ confirms the formation of TiO₂ over Ti₃C₂T_x. The aforementioned results also demonstrate the surface terminations (=O and –OH) along with Ti–C vibrations of Ti₃C₂T_x. For TiO₂@C, the Ti–O peak was detected at a frequency of 480 cm⁻¹. The presence of adsorbed water molecules on the catalyst surface was observed at 1628 (bending vibrations) and 3550 cm⁻¹ (stretching vibrations).³⁵ The absence of any Ti–C peaks confirms that all Ti–C–Ti–C–Ti bonds were completely converted into TiO₂@C, which underwent heterostructures to enhance the activity performances of catalysts during photoreactions.

Raman analyses

The Raman results for Ti₃C₂T_x, Ti₃C₂T_x@TiO₂, and TiO₂@C were obtained in the 100–800 cm⁻¹ region (Fig. 2c). MXenes (Ti₃C₂T_x) belong to the D_3d point group, and their vibrations can be described using Mulliken symbols as 2(N − 2) E_g + (N − 2) A_1g + 2(N − 2) E_u + (N − 2) A_2u, where N represents the number of atoms per unit cell (i.e., five in the case of Ti₃C₂).³⁶ The E_g and E_u are doubly degenerate modes (mostly E_g is used for representation), while E_g and A_1g are the only Raman-active vibration modes. In MXene (Ti₃C₂T_x), four Raman modes are active: E_g and A_1g signify the in-plane and out-of-plane vibration of Ti atoms, along with two additional vibration modes of C atoms (Fig. S1†).³⁷ The Raman frequency of 208 cm⁻¹ (E_g mode) is attributed to in-plane vibrations of Ti and C atoms. The carbon layer vibrations appear at higher frequencies (500–800 cm⁻¹). The presence of surface terminations (=O, –F, –OH) causes more vibrations due to the increased atoms in the unit cell. The Raman results of Ti₃C₂T_x show three regions: lower frequencies for entire flake vibrations, mid-frequency for surface terminations, and higher frequencies for carbon and surface terminations (Fig. S1†).³⁶ The E_g corresponds to in-plane vibrations, and A_1g corresponds to out-of-plane vibrations. The Raman vibrations around 600 cm⁻¹ (E_g mode) were attributed to the C–O vibrations. As compared to the pristine Ti₃C₂T_x, the Raman peaks of TiO₂@Ti₃C₂T_x indicate the formation of titanium dioxides, with the vibrations at 147 cm⁻¹ that correspond to the anatase phase. Moreover, the Raman spectrum of TiO₂@C exhibits Raman vibrations at 148 cm⁻¹ (E_g), 397 cm⁻¹ (B_1g), 510 cm⁻¹ (A_1g) and 620 cm⁻¹ (E_g). These vibrations were attributed to the oxygen atoms of O–Ti–O (correspond to the anatase phase).³⁸

TGA analysis

Thermal gravimetric analysis (TGA) of Ti₃C₂T_x in an oxygen atmosphere was conducted to observe changes as the temperature increased up to 900 °C.³⁹ The results (Fig. 2d) demonstrate four weight-change steps. The first weight change was due to the loss of water adsorbed on the surface of Ti₃C₂T_x. In the second step, the surface terminations and bonded water were released up to 319 °C.⁴⁰ Further increases in temperature led to the oxidation of Ti₃C₂T_x, resulting in a subsequent 19.98% increase in mass. The oxidation of Ti₃C₂T_x started at 320 °C and finally began to decompose at 729 °C.⁴¹ TGA results revealed the thermal stability of Ti₃C₂T_x in an oxygen atmosphere and TiO₂ formation (oxidation) on its surface.

SEM studies

Field-emission scanning electron microscopy (FESEM) was utilized to analyse the surface features and characteristics of catalyst.⁴² The SEM results of Ti₃AlC₂, Ti₃C₂T_x, TiO₂@Ti₃C₂T_x, and TiO₂@C are demonstrated in Fig. 3a–d. The SEM results of commercial Ti₃AlC₂ powder (Fig. 3a) demonstrate the solid block structure with smooth surfaces. The etching of Ti₃AlC₂ produces accordion-like multilayer nanosheets loosely bound together,⁴³ as shown in Fig. 3b. The MXene layers are clearly separated as compared to the MAX powder that confirms the successful exfoliation (i.e., aluminium removal). The expansive 2D structure of Ti₃C₂T_x nanosheets offers an abundance of active sites and terminations (i.e., –O, –OH, –F) that improve the interactions with reactants for hydrogen generation.⁴⁴ The in situ growth of anatase TiO₂ over the Ti₃C₂T_xvia hydrothermal reactions is revealed in Fig. 3c. The result illustrates the uniform distribution of TiO₂ particles over the Ti₃C₂T_x surfaces and within the structure of Ti₃C₂T_x.⁴⁵ The complete oxidation of MXene produced TiO₂@C catalysts having uniform dispersion of TiO₂ across the material. Fig. 3d illustrates a significant difference when compared to pristine anatase (see Fig. S2†). The TiO₂ derived from Ti₃C₂T_x showcases significantly higher crystallinity compared to other forms of TiO₂.⁴⁶

Fig. 3 (a) SEM results of MAX (Ti₃AlC₂); (b) MXene (Ti₃C₂T_x); (c) TiO₂@Ti₃C₂T_x; (d) TiO₂@C.

AFM analysis

Atomic force microscopy (AFM) provides detailed insights into the surface features, morphology, and size distribution of TiO₂@Ti₃C₂T_x catalysts at nearly atomic scale.⁴⁷ The detailed 2D topographical images are exhibited in Fig. 4a and b. A scan area of 2.17 × 2.17 μm was utilized to assess the height (thickness) distribution within the range of 0.8 μm. The recorded average thickness of 0.445 μm is validated in Fig. 4c. Moreover, Fig. 4d and e reveals three-dimensional results that provide insights to the surface features and roughness of the TiO₂@Ti₃C₂T_x photocatalysts.⁴⁸

Fig. 4 AFM results of the TiO₂@Ti₃C₂T_x photocatalysts: (a and b) 2D-tapping modes, (c) thickness distribution, and (d and e) 3D images.

XPS analyses

The oxidation states and chemical compositions of as-synthesized catalysts were studied by X-ray photoelectron spectroscopy (XPS).⁴⁹ The comparative results (survey scans) of Ti₃C₂T_x, TiO₂@Ti₃C₂T_x, and TiO₂@C are illustrated in Fig. 5a. The XPS results of Ti₃C₂T_x demonstrate the existence of fluorine terminations due to the use of HF during its synthesis.⁵⁰ The binding energies of titanium 2p_3/2 and 2p_1/2 were observed at 455.1 and 461.3 eV (see Fig. 5b), which confirms the existence of Ti–C bonds along with various other surface terminations (i.e., =O, –OH).⁵¹ This spectroscopic study provides a detailed material's composition as well as its surface features. In TiO₂@Ti₃C₂T_x catalysts, the formation of TiO₂ over the Ti₃C₂T_x surfaces was confirmed by the sifting of 2p_3/2 and 2p_1/2 peaks to 459.3 eV and 465 eV, respectively.⁵² The results depict that complete oxidations of Ti₃C₂T_x cause the removal of surface termination. Note: termination that exists on MXene layered structure. Fig. 5c demonstrates O 1s peaks for the Ti₃C₂T_x, TiO₂@Ti₃C₂T_x, and TiO₂@C catalysts. The Ti₃C₂T_x exhibits C–Ti–(OH)_x, C–Ti–O_x and TiO₂ peaks at binding energies of 532.98, 530.78, and 529.18 eV, respectively.⁵³ The increased intensity of Ti–O bonds and decreased intensity of C–Ti peaks indicate the in situ growth of TiO₂ on the Ti₃C₂T_x surface. Results depict that oxygen terminal groups were transformed into TiO₂. The C 1s XPS results demonstrate four peaks fitted at 281.88, 284.88, 286.58, and 289.18 eV, which were assigned to the C–Ti, C=C, C–O, and C–F bonds, respectively (Fig. 5d). The intensities of the C–Ti and C–F peaks decrease in TiO₂@Ti₃C₂T_x and TiO₂@C catalysts (as oxidation progresses), which converts the Ti–C bonds into Ti–O bonds.⁵⁴

Fig. 5 (a) Survey scans, (b) Ti 2p, (C) O 1s, and (d) C 1s XPS results of Ti₃C₂T_x, TiO₂@Ti₃C₂T_x, and TiO₂@C.

UV-vis/DRS and PL studies

The optical properties of as-synthesized catalysts were obtained using UV-vis diffuse reflectance absorption spectra as shown in Fig. 6a. The UV-vis/DRS results demonstrate the ability of TiO₂, TiO₂@C, TiO₂@Ti₃C₂T_x, and Ti₃C₂T_x to absorb photons, and then bandgap energy values were estimated using the Tauc plot method (Fig. S3†).⁵⁵ TiO₂ (anatase) displays absorption in the UV region with a bandgap energy of 3.1 eV, whereas Ti₃C₂T_x (MXene) exhibits photon absorption across the entire visible and UV region. The in situ growth of TiO₂ over Ti₃C₂T_x displays an increase in photon absorption.⁵⁶ The increase in light absorption is due to the presence of black-coloured MXene and carbide-derived carbon (CDC).⁵⁷

Fig. 6 (a) UV-vis/DRS, (b) PL, (c) photocurrent response, and (d) EIS results of TiO₂, TiO₂@C, and TiO₂@Ti₃C₂T_x catalysts.

The charge separation, transfer, and recombination were studied via the photoluminescence (PL) technique. The PL intensity of pristine TiO₂ indicates higher recombination of photoexcited charges (back reaction).^13,58 The PL intensities of synthesized catalysts suggest that TiO₂@Ti₃C₂T_x has less charge recombination and can deliver relatively higher H₂ production; see Fig. 6b. The improvements in electron transport and charge separation are attributed to the development of an in situ heterojunction.⁵⁹

TPC and EIS studies

Electrochemical impedance spectroscopy (EIS) and transient photocurrent (TPC) responses were used to study the charge transfer efficiency. The TPC responses of TiO₂, TiO₂@Ti₃C₂T_x and TiO₂@C were analysed with on/off cycles (Fig. 6c). The results demonstrate that TiO₂@Ti₃C₂T_x catalysts exhibit a higher photocurrent response as compared to TiO₂ and TiO₂@C. The higher photoresponse is attributed to the higher charge separation.⁶⁰

EIS measurements for TiO₂, TiO₂@Ti₃C₂T_x and TiO₂@C catalysts are shown in Fig. 6d. TiO₂@Ti₃C₂T_x shows a smaller semicircle diameter, which suggests improved interface carrier transfer and decreased interface resistance. TiO₂@Ti₃C₂T_x catalysts exhibit improved charge separation efficiency and improved charge transfer across interfaces. The lower interface resistance indicates the improved photocatalytic efficiencies.⁵²

BET surface area analyses

The surface area of the prepared photocatalysts was determined through nitrogen adsorption–desorption isotherms using BET measurements, as depicted in Fig. 7a–c. All catalysts exhibit typical IV isotherms and H₃ hysteresis loops according to Brunauer–Deming–Deming–Teller (BDDT) classifications.⁶¹ The surface areas of Ti₃C₂T_x, TiO₂@Ti₃C₂T_x, and TiO₂@C are 8.12, 32.57 and 28.17 m² g⁻¹, respectively. The larger surface area of TiO₂@Ti₃C₂T_x is due to the in situ distributions of TiO₂ particles over Ti₃C₂T_x; hence, the possibility of agglomeration between TiO₂ particles becomes quite low.⁶² Pore size distributions were calculated and are depicted in Fig. 7a–c, revealing very close pore size distributions with a peak centered at ca. 5.0 nm. The TiO₂@Ti₃C₂T_x and TiO₂@C catalysts are approximately twice as mesoporous compared to pristine Ti₃C₂T_x.⁶³ Therefore, the in situ growth of TiO₂ over Ti₃C₂T_x provides a larger and more mesoporous surface area for enhanced H₂ and O₂ evolution reactions.

Fig. 7 BET results of (a) Ti₃C₂T_x, (b) TiO₂@Ti₃C₂T_x, and (c) TiO₂@C catalysts, and (d) VSM hysteresis loop of Ti₃C₂T_x, TiO₂@Ti₃C₂T_x and TiO₂@C catalysts.

EPR and VSM studies

The presence and properties of oxygen vacancies were investigated utilizing electron paramagnetic resonance (EPR) spectroscopy.⁶⁴ It is evident from the results (Fig. S4†) that no EPR signal appeared in the pristine Ti₃C₂T_x sample. On the other hand, a distinct signal featuring a G factor of 2.005 was observed in both the TiO₂@C and TiO₂@Ti₃C₂T_x catalysts, indicating the trap and transfer of unpaired electrons. Notably, a higher signal was observed in the TiO₂@Ti₃C₂T_x catalysts as compared to the TiO₂@C catalysts.⁶⁴ Some recent reports suggest that the existence of oxygen vacancies is more challenging for photocatalytic H₂ evolution reactions.⁶⁵

The magnetic properties of the as-synthesized catalysts were examined via the vibrating-sample magnetometry (VSM) technique (Fig. 7d). This analysis provides crucial insights into saturated magnetic intensity (Ms), residual magnetic intensity (Mr), coercivity (Hc), and squareness ratio.⁶⁶ The TiO₂@Ti₃C₂T_x and TiO₂@C photocatalysts exhibit Mr values of 0.064 and 0.071 emu g⁻¹ respectively. By using the VSM plots, the magnetic parameters (Ms, Mr, Hc, and squareness ratio) of the as-synthesized catalysts are tabulated in Table S2.†

Water splitting activities

The hydrogen and oxygen evolution rates have been measured to evaluate the catalytic performances. Fig. 8a and b indicate the activities in mmol g⁻¹ and mmol g⁻¹ h⁻¹, respectively. The results demonstrate that H₂ and O₂ generation increases linearly with the irradiation time for each photocatalytic reaction. It has been observed that TiO₂@C and TiO₂@Ti₃C₂T_x catalysts exhibit 9.37 and 18.57 mmol g⁻¹ h⁻¹ of hydrogen, respectively. The O₂ production rate was approximately half that of the H₂ production rate. It is worth mentioning that the activity in the case of TiO₂@C is two times higher, while for TiO₂@Ti₃C₂T_x, it is five times higher than pristine TiO₂. The comparison of H₂ and O₂ evolution rates of TiO₂, TiO₂@C and TiO₂@Ti₃C₂T_x catalysts and their %QE are tabulated in Table S3.† The higher catalytic performances of TiO₂@Ti₃C₂T_x are attributed to the existence of multilayer structures that contribute relatively higher active sites to promote water reduction reactions. Additionally, during the photoreaction, the formation of heterojunctions and Schottky barriers at the interfaces enhances the charge transfer to promote and drag water splitting reactions.⁶⁷ Additionally, the higher surface area also has a considerable impact on the photocatalytic activity. In TiO₂@C catalysts, carbide-derived carbon (CDC) serves as an electron acceptor and drags charges to improve H₂ and O₂ production rates.⁶⁸ Comparatively, TiO₂@Ti₃C₂T_x is a promising catalyst for water splitting reactions due to its layered structures, higher charge transfer, and large number of active sites available for redox reactions.

Fig. 8 Hydrogen and oxygen production activities of TiO₂, TiO₂@C, and TiO₂@Ti₃C₂T_x (a) mmol g⁻¹ and (b) mmol g⁻¹ h⁻¹; (c) recyclability and (d) XRD results of fresh and used TiO₂@Ti₃C₂T_x catalysts.

Photocatalyst stability and its reusability are crucial factors in accessing the catalyst's practicability. The recycling studies were carried out for five consecutive cycles using TiO₂@Ti₃C₂T_x catalysts. It is noteworthy that the photocatalytic activity remained steady even after five cycles (Fig. 8c). The slight decrease in activity was caused by catalyst deposition on the inner surface of the reactor wall.³⁵ The XRD analysis of both fresh and used TiO₂@Ti₃C₂T_x catalysts was compared, and it was found that there was no significant difference in crystallinity, as shown in Fig. 8d. These findings suggest that TiO₂@Ti₃C₂T_x catalysts have exceptional stability, which is due to the in situ-grown TiO₂ over the MXene structures.⁶⁹

Charge transfer and water splitting mechanisms

In a photocatalytic reaction, photons have greater energy than the band gaps, i.e., ≥ E_G, when the excitation of charges occurs on the surfaces of semiconductors. During excitation, electrons start to move from the valence band (VB) to the conduction band (CB) of the semiconductor, leaving behind the electrons deficient charges, i.e., holes.⁷⁰Fig. 9a demonstrates the water splitting process during the photocatalytic reaction. It has been found that TiO₂@Ti₃C₂T_x exhibits higher catalytic efficiencies relative to the TiO₂@C catalysts. Higher H₂ and O₂ evolution efficiencies in the case of TiO₂@Ti₃C₂T_x are attributed to the presence of electron promoters, i.e., MXene. It is obvious that MXene's exceptional conductivity contributes to the transportation of electrons to active sites, where a water reduction reaction occurs to generate hydrogen.⁷¹ Additionally, due to the layered structure of MXene, electron trapping and transfer become relatively high, where electrons can stay on redox sites for further utilization/consumption.¹⁸ During the photoreaction, a dynamic equilibrium is established between MXene and TiO₂, which aligns the Fermi energy (E_F) of the TiO₂@Ti₃C₂T_x photocatalyst to develop a new quasi-Fermi state, .⁷² Moreover, due to the higher work function of MXene (Ti₃C₂T_x), the feasibility of electron population on the TiO₂@Ti₃C₂T_x system increases, which is why these catalysts contribute relatively higher rates of hydrogen evolution in photoreaction.⁷³ MXene develops heterojunctions with TiO₂; hence, it enhances the feasibility to quench and transfer the charges during photoreaction. That is why TiO₂@Ti₃C₂T_x effectively promotes electron transfer to the of TiO₂.⁷⁴ It is worth mentioning that due to heterojunctions, the Schottky barrier at the interface suppressed the charge recombination (i.e., back reaction).

Fig. 9 (a) Charge transfer on TiO₂@Ti₃C₂T_x catalysts; (b) mechanism of water splitting over TiO₂@Ti₃C₂T_x catalysts.

On the other hand, during catalytic water splitting, oxygen-carrying moieties, i.e., ⁻OH, or peroxide ⁻O₂, undergo oxidation with holes.⁷⁵ During photoreactions, MXene layers favour the generation of O vacancies when they become exposed to or come into contact with water.⁷⁶ The hydrogen evolution process over TiO₂@Ti₃C₂T_x catalysts is initiated with the protonation of –O terminations during photoreaction (photon absorptions). It is worth mentioning that in photoreaction protons inherently adsorb on the –O sites instead of Ti sites to in situ develops the TiO₂@Ti₃C₂(OH)_x structures (see eqn (3)–(5)). Subsequently, the adjacent Ti site undergoes protonation. This protonation then favours the enhancement and promotion of catalytic hydrogen.⁷⁷ The catalytic reaction is demonstrated in eqn (3)–(5).

Ti–O–Ti + H⁺ + e⁻ → Ti–OH*–Ti (3)

Ti–OH*–Ti + H⁺ + e⁻ → Ti–OH*–Ti–H* (4)

Ti–OH*–Ti–H* → Ti–O–Ti + H₂ (5)

These above chemical reactions prove the process of protonation and charge transfer involved in hydrogen generation.⁷⁷Fig. 9b illustrates the photoreaction and catalytic hydrogen and oxygen generation over TiO₂@Ti₃C₂T_x catalysts. In the case of TiO₂@C catalysts (see Fig. S5†), photogenerated electrons are quenched by the CDC layers to drag the water splitting reactions.⁶⁸

Conclusions

TiO₂@Ti₃C₂T_x catalysts have been successfully synthesised and employed for water splitting reactions. In this project, an ethanol-assisted solvothermal approach was introduced to achieve the required morphologies and catalytic characteristics. Typically, TiO₂@Ti₃C₂T_x and TiO₂@C catalysts were optimized and assessed for comparative activities. The morphology and optical characteristics were assessed via XRD, FTIR, Raman spectroscopy, TGA, SEM, AFM, UV-vis/DRS, XPS, PL, TPC, EIS, BET, EPR, and VSM. The results confirm the morphology, excellent optical response, and charge transfer that indicate the speciality of the current work. The results show that TiO₂@Ti₃C₂T_x exhibits higher catalytic activity, i.e., 18.57 mmol g⁻¹ h⁻¹ of H₂ and 8.84 mmol g⁻¹ h⁻¹ of O₂, which is almost double the activity of TiO₂@C. It is worth mentioning that the higher activity of TiO₂@Ti₃C₂T_x is attributed to the existence of titania on MXene multilayers that offer more reactive sites and redox centres. The results indicate that heterojunctions between titania and Ti₃C₂T_x executed the rectification and charge transfer to active sites (i.e., redox centres). Additionally, heterojunctions rectify and reduce the back flow of charges during the photoreaction. This significant factor anticipates the consequences of higher catalytic efficiencies. Based on results and activities, it has been anticipated that the current study holds promise to replace costly and conventional catalysts. Although there are many challenges ahead, the described approach has great potential to deliver sustainable hydrogen for the proper implementation of green technologies.

Data availability

The data and necessary protocols of this study have been included as part of the ESI.†

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This project was funded by the Higher Education Commission (HEC) of Pakistan with grant No. 377/IPFP-II/Batch-I/SRGP/NAHE/HEC/2020/27 and ASIP/R&D HEC/2023/5/22696/124. The completion of the project is fully accredited to the Inorganic Material Laboratory 52 s, Institute of Chemistry-IUB Pakistan. Dr E. H. thanks Carnegie Mellon University of USA and the University of Karachi (HEJ–Centre) for facilitating instrumental access and support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4na00754a

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