Scaling up the charge transfer on Pd@Ti₃C₂T_x–TiO₂ catalysts: a sustainable approach for H₂ generation via water splitting

Abstract

The increasing energy demands have led modern societies to search for green and renewable alternatives that can replace traditional energy sources (i.e. fossil fuels). The aim of this project is to generate sustainable hydrogen from renewable sources. For this purpose, we synthesized highly active MXene-supported catalysts (Pd@Ti₃C₂T_x–TiO₂). In this work, Pd²⁺ ions were directly reduced on Ti₃C₂T_x surfaces without using an additional reducing agent. Morphology and optical characteristics were evaluated using XRD, UV-vis/DRS, SEM, TEM, Raman, and FT-IR techniques. Chemical compositions and surface terminations of the catalysts were examined via EDX and XPS techniques. It was found that low Fermi levels of Pd@Ti₃C₂T_x effectively quench photogenerated electrons and facilitate H₂ evolution without using any sacrificial reagents. This innovative approach enhanced the stability and ability of TiO₂ to effectively transfer the photoinduced charges on the Pd@Ti₃C₂T_x cocatalysts. The role of Pd was elucidated and demonstrated for hydrogen generation progress. The results suggest that the presence of Pd metal over Ti₃C₂T_x surfaces boosted H₂ generation. Overall, Pd@Ti₃C₂T_x–TiO₂ catalysts delivered 35.11 mmol g⁻¹ h⁻¹ of hydrogen. Thus, it was concluded that the as-synthesized catalysts hold promise to replace costly and conventional hydrogen generation technologies.

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Introduction

Energy shortages have become a real challenge, specifically for the developing countries. However, this issue is less common in developed countries because of multiple alternatives (i.e., nuclear, wind, and hydropower sources). In the world, the most common and conventional sources of energy are fossil fuels (coal, petroleum, and natural gas). The combustion of fossil fuel is becoming a serious threat to our environment and climate,¹ of which the climate change is proof. Moreover, increase in human population and industrialization is becoming a source of torment and curse.² Undoubtedly if the environment is not safe, we should be ready for the ruin and end of the world.

To develop energy sources other than fossil fuels, it is essential to explore green and renewable energy sources.³ Therefore, it is urgent to generate hydrogen energy that can not only meet energy demands but also protect our climate and environment. Generally, the production of hydrogen via water splitting is hectic and challenging because water is an extremely stable molecule, and its splitting needs ∼237 KJ mol⁻¹ of energy. Although various methods have been reported for hydrogen generation, such as electrolysis, photolysis, thermolysis, and photoelectrolysis,^4,5 they have many drawbacks. The use of solar radiation combined with photocatalysts is believed to have the potential to replace conventional fossil fuels. The conversion of solar energy into chemical energy is not only reliable but also a practically sustainable approach.^6,7 Thus, the utilization of advanced, reliable, and stable catalysts can be practically helpful in generating hydrogen energy from renewable sources, i.e. water and biomass.⁸

Since the last few decades, water splitting has attracted significant attention as a sustainable and eco-friendly approach for renewable energy production.⁹ Up to now, various transition metal oxides (i.e. TiO₂, CuO, and ZnO^10–13), mixed metal oxides (i.e. Zn₃V₂O₈ and SrTiO₃¹⁴), metal sulphides (i.e. CdS, ZnS, CuS, and CdZnS^15–17), perovskites and chalcogenides,^18,19 and metal and non-metal doped catalysts have been employed for water reduction.^20,21 Similarly, graphene and g-C₃N₄ supported metal or non-metal fabricated catalysts^22–25 have been extensively utilized for H₂ generation. It has been found and well justified that the ability of TiO₂ to split water can be extended by the use of cocatalysts.²⁶ The employment of cocatalysts on TiO₂ has been found to be more effective for sustainable water splitting.^27,28 Cocatalysts with the ability to quench the electrons from semiconductor systems can be effective for excellent charge transportation, specifically in water-splitting reactions.^29,30 The higher charge separation ability of the cocatalyst is attributed to its chemical properties (i.e., conductivity and stability). Furthermore, cocatalysts certainly enhance active sites and the life of the photoreaction.^30,31 Numerous noble metal cocatalysts like Pd, Pt, Au, Ru, Rh, Os and Ag are considered to be crucial for enhancing photocatalytic performances.^32–34

MXenes (Ti₃C₂T_x), where ‘T_x’ represents surface terminations, ‘T’ corresponds to functional groups like –F, –OH, =O, etc., while ‘x’ denotes variability.³⁵ MXenes have distinct chemical characteristics and can be employed as a cocatalyst with other semiconductor systems.³⁶ Due to two-dimensional structures, a substantial surface area, and chemical properties, Ti₃C₂T_x enhances the electrical conductivity of the host semiconductor.³⁷ These unique characteristics make MXenes attractive cocatalysts for higher charge transfer to accelerate the photocatalytic water-splitting reactions.³⁸ The utilization of MXenes as a cocatalyst has been found to be very successful in enhancing the photocatalytic performances of semiconductor systems. Several reports have been found very supportive for the use of Ti₃C₂ as a cocatalyst. Examples of such reports are CdZnS@Ti₃C₂,³⁹ CdS@Ti₃C₂,⁴⁰ BiVO₄@Ti₃C₂,⁴¹ g-C₃N₄@Ti₃C₂⁴² and TiO₂@Ti₃C₂.⁴³ For example, g-C₃N₄ with Ti₃C₂ cocatalysts exhibited higher hydrogen generation efficiencies (seven times higher than bare g-C₃N₄).⁴⁴ Similarly, bare Cd_0.5Zn_0.5S is less stable and a bad semiconductor due to charge recombination during photoreaction, but when coupled with Ti₃C₂, its photocatalytic efficiency was remarkably enhanced.⁴⁵

Surprisingly, the work function (ϕ) of Ti₃C₂ is (5.78 eV)⁴⁶ is comparable to the noble metals,^47,48 and thus it has an inherent ability to quench free electrons from semiconductor systems. The finding of this study depicts that the stability, conductivity, and electron quenching ability of Ti₃C₂ can be enhanced by fabricating it with noble metals.⁴⁹ Among transition metals, palladium (Pd) exhibits a higher electron quenching ability for various photocatalytic reactions.⁵⁰ The fabrication of Ti₃C₂ surface with Pd can provide additional active sites for sustainable H₂ evolution reactions.^51,52

This work aims to explore the effectiveness of Pd on MXenes (Ti₃C₂T_x) with higher charge transport for efficient H₂ evolution. This study further demonstrates the novel design and sustainable approach for green fuel production (hydrogen energy) via water-splitting reactions. In addition, the results of this study depict and justify the mutual role of TiO₂ (semiconductor) and Pd@Ti₃C₂T_x cocatalyst. For the very first time, the mechanistic insights have been explored and explained with evidence. The work reported herein demonstrates and justifies the synergistic effects between TiO₂ and Pd@MXenes cocatalysts that offer excellent insights about the design of catalyst synthesis, optimized role of semiconductor systems and cocatalysts, and progress of photoreaction. In brief, this work contributes to the fundamental understanding of catalyst engineering and its effectiveness for sustainable hydrogen energy systems.

Experimental

Chemicals

The following chemicals were used to prepare the catalysts: Degussa P25 (Evonik Germany), Ti₃AlC₂ (Forsman Scientific Beijing), Pd(O₂CCH₃)₂(99.0% Sigma-Aldrich), 40% HF Solution (MERK, 48%), absolute ethanol (99.0% Sigma-Aldrich), and distilled water (PAEC-PK).

Catalyst synthesis

Ti₃C₂T_x was prepared by exfoliating the Al layers from MAX (Ti₃AlC₂).⁵³ Briefly, 2 g of Ti₃AlC₂ was gradually transferred to 60 mL HF solution (40%) in a 100 mL Teflon reactor (Techinistro/China). This precursor was continuously stirred for 8 h under ambient conditions.⁵⁴ After obtaining the slurry (dull black), progressive slurry was separated via multiple centrifugations (at 5000 rpm, 20 min for each) and a similar process was repeated multiple times (seven-eight times) for washing with distilled water. Note: at this stage, careful washing is mandatory to leach and eliminate HF properly. The washing of the MXene slurry was continued until the pH of the waste liquid approached 7 to 7.2. Finally, the product was dried under vacuum conditions at 70 °C for 10 h.⁵⁵ The protocol employed for the synthesis of Pd@Ti₃C₂T_x–TiO₂ is demonstrated in Fig. 1.

Fig. 1 Synthesis scheme of the Pd@Ti₃C₂T_x–TiO₂ catalyst.

To develop and fabricate the Ti₃C₂T_x surfaces with palladium contents, typically, 500 mg of Ti₃C₂T_x is dispersed in 100 mL of deoxygenated water (i.e., water obtained after 20 min purging with high-purity dry N₂) and sonicated for 25 min. Then, a nominal amount of Pd(O₂CCH₃)₂ was added to the Ti₃C₂T_x slurry to maintain the Pd²⁺ to Ti₃C₂T_x ratios (2 : 98). After vigorous stirring for 5 h, Pd@Ti₃C₂T_x was thoroughly washed first with deionized water and then with ethanol (analytical grade). Finally, the 2%-Pd@Ti₃C₂T_x powder was dried under vacuum conditions at 70 °C for 10 h.⁵⁶ Various Pd@Ti₃C₂T_x ratios (1%, 2%, and 3%) were prepared under similar conditions to examine the impact of Pd loading over the Ti₃C₂T_x surfaces.

To develop Pd@Ti₃C₂T_x–TiO₂, appropriate amounts of Pd@Ti₃C₂T_x (2.5, 5.0 7.5, and 10% W/W) were dispersed (via sonication) in 40 mL of distilled water. Then, TiO₂ (200 mg) was dispersed in 30 mL of deoxygenated water. To produce a uniform mixture, the Pd@Ti₃C₂T_x slurry was gradually transferred to TiO₂ suspensions and the mixture was stirred/sonicated repeatedly for 3 h. After confirming the phase transformation, the precipitates were rinsed with distilled water. Using vacuum atmosphere, catalysts were dried at 70 °C for 10 h and preserved for characterisations and photoreactions.

Characterizations

To get insights into their atomic arrangements and crystallographic properties, an XRD instrument (Bruker D2 Phaser) was used in the 2θ range from 5–80° with a 2° min⁻¹ scan rate. To study the structural characteristics of synthesized materials, Raman spectra were recorded using a Raman spectrometer RMP-500/JASCO. An infrared spectrometer Bruker Alpha Platinum (ATR mode) was used to measure the Fourier transform infrared (FTIR) results collected in the 400–4000 cm⁻¹ range. To determine the morphology and elemental composition, FEI-Nova-450/Nano SEM equipped with an EDX detector was employed. A JEOL model JEM 2100 F instrument was used to investigate images generated via TEM and high-resolution TEM (HRTEM). To investigate the surface morphology at the nanometer scale, atomic force microscopy (AFM) was performed using an AFM-5500/Agilent-USA. The oxidation states and terminations were examined using an X-ray photoelectron spectrometer (Thermo ESCALAB 250Xi) coupled with an Al Ka X-ray source (1486.6 eV). To measure the optical properties via diffuse reflectance spectroscopy, UV-2550/Shimadzu was used for the powder sample. Photoluminescence (PL) spectra were achieved using LS-45, PerkinElmer spectrometer. A Micromeritics-Tristar II 3020 analyzer was employed to examine the specific surface areas based on the Brunauer–Emmett–Teller method. Solartron impedance analyzer-1260 was employed for EIS analysis, whereas the photo-responses were monitored on a conventional three-electrode system (λ > 420 nm).

Experiments for water-splitting reactions

The photocatalytic H₂ production activities were performed using a solar simulator (94043A/450W Xenon lamp). For photoreactions, a source with an AM1.5 filter and output power of 100 mW cm⁻² was used for all catalysts. For each photoreaction, 25 mg of photocatalysts were dispersed in 40 mL of deionized water in a 100 mL Pyrex reactor. Note: in order to remove dissolved oxygen contents, the reaction mixture was purged using high-purity argon (Ar) for 25 min prior to starting the photocatalytic reactions. During the photoreaction, gas samples were collected from the headspace of the photoreactor using a syringe (0.5 mL sizes) at regular intervals, and hydrogen (H₂) generation was monitored on GC-TCD (Shimadzu 2010/Japan). It is important to declare that high-purity argon was used as the carrier gas to the GC-TCD outfitted with a molecular sieve capillary column. The H₂ gas produced was quantified using the system's internal calibration curves, and the results for each photoreaction were compared. Units of mmol g⁻¹ and mmol g⁻¹ h⁻¹ were used to measure the rates of H₂ generation. The apparent quantum yield (AQY) for each photoreaction was calculated by employing the formula represented in eqn (1).^57,58

n _H2 represents the quantity of hydrogen produced (mol), N_A is Avogadro's number (mol⁻¹), P is the power density of light (W m⁻²), h is Planck's constant (J s), A is the irritation area (m²), c is the speed of light (m s⁻¹), t is time (sec), and λ is the wavelength (m).⁵⁹

Results and discussion

Structural characteristics

The structural characteristics of synthesized catalysts were obtained using XRD, Raman, and FT-IR techniques. The results are illustrated in Fig. 2. It has been found that the XRD pattern of the precursor material Ti₃AlC₂ (MAX phase) was well matched to JCPDS No. 52-0875, which confirms its purity and crystalline structure.⁶⁰ The extraction of Al atoms from Ti₃AlC₂ is evident from the shifting of the (002) peak towards smaller angles. The XRD (002) peaks of Ti₃C₂T_x display broader profiles as compared to Ti₃AlC₂. Additionally, the complete depletion of Al is further confirmed by the absence of a peak at 39° (104). These significant changes confirm the MXene's phase purity achieved in the current work.⁶¹ It has been further justified that due to the deposition of Pd over the Ti₃C₂T_x surfaces, the intensity of the peak (002) decreased. In the XRD pattern, no additional peaks corresponding to Pd were observed, as shown in Fig. 2a.⁶²

Fig. 2 (a) XRD patterns of Ti₃AlC₂, Ti₃C₂T_x and Pd@Ti₃C₂T_x. (b) FT-IR results of Ti₃AlC₂ and Ti₃C₂T_x. (c) XRD diffraction patterns of TiO₂, Ti₃C₂T_x–TiO₂ and Pd@Ti₃C₂T_x–TiO₂. (d) Raman spectra of TiO₂ and Pd@Ti₃C₂T_x–TiO₂.

The FT-IR results (Fig. 2b) of MAX phases exhibit vibrations located at 498 cm⁻¹ that correspond to the Ti–Al–C bond (bending vibrations). The conversion of MAX to MXene was confirmed by the removal of the peak primarily observed at 498 cm⁻¹. The appearance of new peak patterns in FT-IR results was attributed to Ti–O, C–F, C–O, and C–OH bonds, confirming the formation of Ti₃C₂T_x (i.e., MXene).⁶³ In the FTIR spectra of Ti₃C₂T_x, the vibrations at 567 cm⁻¹, 707 cm⁻¹, 854 cm⁻¹, and 921 cm⁻¹ represent the Ti–C bond stretching vibrations.⁶⁴ It has been reported that the interaction between atoms of Ti and the O–H groups on the surface of Ti₃C₂T_x causes deformation vibrations of the Ti–O bond at 617 cm⁻¹.⁶⁵ The vibrations at 1057 and 1097 cm⁻¹ indicate the existence of C–F functional groups, whereas the peaks at 1229 cm⁻¹ and 1343 cm⁻¹ were attributed to oxygen-containing (C–O) functional groups whereas the O–H terminations were detected at 1399 and 1437 cm⁻¹.⁶⁶ The existence of OH is due to the absorbed external water through absorption peaks at 1636 and 3437 cm⁻¹.⁶⁷ The results confirm the formation of a multilayer Ti₃C₂T_x structure. During the etching, the elimination of the Al layers will cause the beginning of surface terminations (T_x) like hydroxyl (–OH), oxygen (=O), or fluorine (–F) groups.

The powder XRD patterns of TiO₂, Ti₃C₂T_x–TiO₂, and Pd@Ti₃C₂T_x–TiO₂ have been revealed in Fig. 2c. The peaks of rutile and anatase are distinctive in the pattern; however, the anatase phase of TiO₂ dominates due to higher percentage.⁶⁸ The additional peaks are due to the existence of as-synthesized Ti₃C₂T_x–TiO₂ and Pd@Ti₃C₂T_x–TiO₂ catalysts. The additional peaks at 2 theta 9.9° (002) and 19.9° (006) correspond to the Ti₃C₂T_x diffraction peaks.⁶⁹ Note: neither the peak positions of anatase nor the peak of rutile changed, which indicates the stable morphologies of as-developed catalysts. Moreover, the deposition of Pd@Ti₃C₂T_x on TiO₂ did not alter the TiO₂ crystal lattice or phase transition between anatase and rutile structures.⁵⁶

Raman spectroscopy was employed to investigate the vibrational and rotational modes of catalysts. This technique provides valuable information about the structure, chemical composition, and physical properties.⁷⁰ The Raman spectroscopy results of TiO₂ and Pd@Ti₃C₂T_x–TiO₂ photocatalysts are depicted in Fig. 2(d). In the Raman spectrum, the TiO₂ sample shows prominent peaks at 144.5 cm⁻¹ (E_g), 396.1 cm⁻¹ (B_1g), 452.2 cm⁻¹ (A_1g), 518.5 cm⁻¹ (B_1g), and 637.8 cm⁻¹ (E_g). The E_g mode of vibrations corresponds to the oxygen in the O–Ti–O bond.⁷¹ The modes (A_1g and B_1g) are associated with the anti-symmetric and symmetric bending vibrations of the O–Ti–O bonds in anatase TiO₂. A weak rutile peak is observed at 452.2 cm⁻¹ due to its low concentration. The fabrication of TiO₂ with Pd@Ti₃C₂T_x shifts the E_g peak from 144.5 cm⁻¹ to 147.2 cm⁻¹. This shifting was attributed to interactions of Pd@Ti₃C₂T_x with TiO₂ catalyst.⁷²

Morphological analysis

The surface features and structural morphology of catalysts were determined with scanning electron microscopy (SEM). The schematic illustration, digital photograph, and SEM results of the starting material Ti₃AlC₂, as well as Ti₃C₂T_x and Pd@Ti₃C₂T_x, are demonstrated in Fig. 3a–i. The scheme used herein to achieve the required morphology of catalysts clarifies the transformation process from Ti₃AlC₂ to Ti₃C₂T_x and the subsequent deposition of Pd onto Ti₃C₂T_x (Pd@Ti₃C₂T_x). The digital photographs (Fig. 3b, e and h) provide evidence of the apparent changes occurring during the conversion of Ti₃AlC₂ to Ti₃C₂T_x and the fabrication of Pd metal over Ti₃C₂T_x.⁷³Fig. 3c represents the SEM micrograms of commercial Ti₃AlC₂ particles. Fig. 3f suggests that the etching of Ti₃AlC₂ produced the unsymmetrical accordion-like multilayer structures, which justify the Ti₃C₂T_x developments.⁷⁴ The SEM results (Fig. 3i) reveal the distribution and morphology of the deposited Pd particles over the Ti₃C₂T_x surfaces.

Fig. 3 (a) Schematic illustration, (b) digital photograph, and (c) SEM image of MAX (Ti₃AlC₂). (d) Schematic illustration, (e) digital photograph, (f) SEM image of MXene (Ti₃C₂T_x). (g) Schematic illustration, (h) digital photograph, and (i) SEM image of Pd@MXene (Pd@Ti₃C₂T_x).

The SEM images of Pd@Ti₃C₂T_x–TiO₂ are demonstrated in Fig. 4a and b. The results elucidate the uniform dispersion of TiO₂ particles having exfoliated Pd@Ti₃C₂T_x structures. The absence of agglomeration is due to the layered structure of Ti₃C₂T_x that reduces the TiO₂ accumulations.⁷⁵ It has been noted that the intimate contact between TiO₂ (101) and Ti₃C₂T_x facets in the photoreaction facilitates the transfer of photogenerated electrons to the active sites where the rate of water reduction is remarkably enhanced. Surprisingly, Pd nanoparticles located on Ti₃C₂T_x remain un-oxidized (i.e., exist in a metallic state) due to the free electrons of MXene.⁵⁶ The lattice images in Fig. 4c display the interplanar spacing of Pd (111) at 0.22 nm. TEM results confirm the presence of Pd particles on the Ti₃C₂T_x surfaces. The SEM, TEM, and XPS further confirm the high degree of dispersion and metallic state of palladium. It is well accepted that Pd, in its metallic state, quenches more electrons and is utilized in the H⁺ reductions in photoreactions.^68,76

Fig. 4 SEM images at (a) 1 μm and (b) 200 nm, (c) HR-TEM, (d) TEM diffraction pattern, and (e) EDX analysis of Pd@Ti₃C₂T_x–TiO₂.

The HRTEM result clearly justifies the high crystalline nature of metallic Pd (Fig. 4c and d). Moreover, the existence of crystalline structures of palladium can be revealed by concentric rings.⁷⁷ The TEM results provide important information about the structural conformations of as-fabricated catalysts. The EDX (Fig. 4e) results confirm the purity and existence of elements in the Pd@Ti₃C₂T_x–TiO₂ catalysts.

Atomic force microscopy (AFM) provides essential insights into the surface roughness, morphology, and size distribution of catalysts at high resolutions.⁷⁸ In this study, AFM (5500/Agilent-USA) was employed to examine the Pd@Ti₃C₂T_x–TiO₂ photocatalysts, and the resulting 2D topographical results are presented in Fig. 5a and b. The scrutinized area of 2.35 × 2.35 μm was covered to evaluate the particle size distribution in 3 to 10 nm ranges. The average particle size was 9.5 nm; see Fig. 5c for the size distribution. Additionally, Fig. 5d and e display three-dimensional images, revealing the surface roughness and morphology of Pd@Ti₃C₂T_x–TiO₂ photocatalysts.⁷⁹

Fig. 5 AFM results of Pd@Ti₃C₂T_x–TiO₂ photocatalysts: (a) and (b) 2D-tapping modes, (c) size distribution and (d) and (e) 3D images.

Terminations and chemical compositions

XPS was employed to examine the chemical composition and oxidation states of elements present in Pd@Ti₃C₂T_x and Pd@Ti₃C₂T_x–TiO₂ catalysts (Fig. 6). The results confirm the existence of Ti, O, C, and with various terminal groups (i.e. Ti–C, Ti–X, Ti_x–O_y, C–Ti–(OH)_x, C–Ti–O_x, C–F, C–O and C–Ti).⁸⁰ To get deeper insights into the formation and terminal characteristics of Pd@Ti₃C₂T_x–TiO₂ catalysts, individual analysis for Ti, O, Pd, and C was performed, and the results are shown in Fig. 6a–d. The Ti 2p peaks for Pd@Ti₃C₂T_x and Pd@Ti₃C₂T_x–TiO₂ catalysts are illustrated in Fig. 6a; the Ti 2p_3/2 peak in Pd@Ti₃C₂T_x have components at 454.78, 455.78, 457.28, and 458.98 eV, corresponding to Ti–C, Ti–X, Ti_x–O_y, and TiO₂, respectively. In Pd@Ti₃C₂T_x–TiO₂ catalysts, the TiO₂ peak intensity has been increased while the intensity of other peaks has decreased, demonstrating the successful incorporation of Pd@Ti₃C₂T_x in TiO₂ structures. Fig. 6b shows O 1s peaks for the Pd@Ti₃C₂T_x and Pd@Ti₃C₂T_x–TiO₂ catalysts. The Pd@Ti₃C₂T_x cocatalyst 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. The increase in the intensity of Ti–O bonds decreases in the C–Ti peak, confirming the successful fabrication of palladium on the Ti₃C₂T_x cocatalyst into TiO₂.⁸¹ Numerous oxygen terminal groups were established during the conversion and transformations of cocatalysts. The C 1s spectrum shows four distinct peaks fitted at 281.78, 284.78, 286.68, and 289.08 eV, which were allocated to C–Ti, C=C, C–O, and C–F bonds, respectively (Fig. 6c). The C–Ti peak's intensity declines due to the conversion of surface and edge Ti-C bonds into Ti-O bonds. The peak locations of Pd 3d_5/2 and Pd 3d_3/2 are situated at 335.68 eV and 340.88 eV, respectively, as illustrated in Fig. 6d.^82,83

Fig. 6 XPS results of Pd@Ti₃C₂T_x and Pd@Ti₃C₂T_x–TiO₂: (a) Ti 2p, (b) O 1s, (c) C 1s, and (d) Pd 3d.

Optical properties

The UV-Vis/DRS results of TiO₂, Ti₃C₂T_x, Ti₃C₂T_x–TiO₂, and Pd@Ti₃C₂T_x–TiO₂ catalysts are illustrated in Fig. 7a; TiO₂ exhibits excellent absorption in the UV-region owing to its large bandgap (3.2 eV). Ti₃C₂T_x exhibits absorption throughout the entire UV-visible region due to its black colour, whereas the absence of an apparent absorption edge indicates its highly conductive nature.⁸⁴ It has been found that in the case of Ti₃C₂T_x–TiO₂ catalysts, Ti₃C₂T_x increases the absorption and impacts on the band edges. Thus, Ti₃C₂T_x expresses its ability for red shift (extended absorption) when it combines with TiO₂. However, in comparison with Ti₃C₂T_x–TiO₂ and Pd@Ti₃C₂T_x–TiO₂, the presence of Pd metal resulted in further red shift.⁸⁵ Moreover, due to inherent d-d transitions, Pd exhibits its absorption at 550 nm.⁸⁶Fig. 7b illustrates the band gaps of TiO₂, Ti₃C₂T_x–TiO₂, and Pd@Ti₃C₂T_x–TiO₂, which are 3.10 eV, 2.99 eV, and 2.95 eV, respectively.

Fig. 7 (a) The UV-vis/DRS and (b) energy diagram of TiO₂, Ti₃C₂T_x, Ti₃C₂T_x–TiO₂ and Pd@Ti₃C₂T_x–TiO₂. (c) and (d) PL and photocurrent density of TiO₂, Ti₃C₂T_x–TiO₂, and Pd@Ti₃C₂T_x–TiO₂.

The photoluminescence (PL) results provide valuable insights about the excitations, charge traps, and transportations.⁸⁷ The PL demonstrates the emission spectrum due to de-excitations in TiO₂, Ti₃C₂T_x–TiO₂, and Pd@Ti₃C₂T_x–TiO₂.⁸⁸ The result (indicated in Fig. 7c) for pristine TiO₂ indicates the back reaction (higher recombination of photoexcited charges). However, Ti₃C₂T_x–TiO₂ catalyst effectively reduces charge recombination by promoting the electron transfer to Ti₃C₂T_x surfaces.⁸⁹ Meanwhile, the presence of Pd over Ti₃C₂T_x further enhances the charge transport and enhances the active sites. The comparison of PL results indicates that Pd@Ti₃C₂T_x–TiO₂ delivers comparatively higher H₂ production activities that are attributed to higher charge transfer.

The electrochemical measurements (i.e., photocurrent and EIS) were conducted to examine the dynamics of charge carrier generation, recombination, and charge transportation reactions.⁹⁰ The photocurrent responses of Ti₃C₂T_x, Ti₃C₂T_x–TiO₂, and Pd@Ti₃C₂T_x–TiO₂ under on/off cycles were analyzed (Fig. 7d). The results clearly indicate that Pd@Ti₃C₂T_x–TiO₂ photocatalysts exhibit higher photocurrent density compared to Ti₃C₂T_x–TiO₂ and Pd@Ti₃C₂T_x–TiO₂, which is linked with their stability and durability for photoreaction. These higher photocatalytic performances were attributed to the higher charge separation offered by Pd@Ti₃C₂T_x cocatalysts. The existence of Pd on the surfaces of MXene promotes the charge transfer dynamics. Moreover, the low Fermi level of Pd suppresses the recombinations between charges and promotes catalytic performances.¹¹ The EIS studies further provide evidence about the effectiveness of cocatalysts. Comparatively, the smallest arc EIS radius of Pd@Ti₃C₂T_x–TiO₂ as compared to Ti₃C₂T_x–TiO₂ or pristine TiO₂ indicates reduced interfacial charge resistance (Fig. 8a). Evidence obtained from PL, photocurrent, and EIS results are completely in accordance with higher hydrogen evolution performances of Pd@Ti₃C₂T_x–TiO₂ catalysts.

Fig. 8 (a) EIS results of TiO₂, Ti₃C₂T_x–TiO₂ and Pd@Ti₃C₂T_x–TiO₂. (b) BET results, (c) Mott–Schottky and (d) calculated band positions of TiO₂ and Pd@Ti₃C₂T_x–TiO₂.

Surface area and electronic band structures

N₂ adsorption–desorption study was carried out to examine the specific surface area of synthesised catalysts.⁹¹ The specific surface area of Pd@Ti₃C₂T_x–TiO₂ was found (68.24 m² g⁻¹), which is greater than that of TiO₂ (52.65 m² g⁻¹) (Fig. 8b). This increased surface area was attributed to the addition of Pd@Ti₃C₂T_x cocatalysts. Ti₃C₂T_x being a two-dimensional material contributes a relatively high surface. When Ti₃C₂T_x combined with TiO₂, it increased the overall specific surface area⁹² of the composite material. Obviously, due to fabrication with MXene, the distribution of TiO₂ particles increased; hence, the chances of agglomeration between TiO₂ particles became quite low.

Positions of the energy bands (i.e., VB and CB) of Pd@Ti₃C₂T_x–TiO₂ and TiO₂ catalysts were calculated using the Mott–Schottky (M–S) technique (Fig. 8c). The study clearly demonstrates that the flat-band potential of TiO₂ was at −0.36 eV, whereas it is −0.51 eV for Pd@Ti₃C₂T_x–TiO₂.⁹³ The valence band positions were calculated with the help of UV-VIS/DRS and M–S techniques as reported,¹⁵ indicated by eqn (2) and Fig. 8d. It is worth mentioning that the M–S results were highly supportive in elucidating the electron transfer mechanism discussed in this work.

E_VB = E_CB− E_G (2)

E _CB and E_VB represent the conduction as well as valence band potentials, respectively. The E_VB values of TiO₂ and Pd@Ti₃C₂T_x–TiO₂ were calculated as 2.74 and 2.44 eV, respectively.⁹⁴

Comparative study of photocatalyst activities

The comparative H₂ evolution rates of as-designed photocatalysts are illustrated in Fig. 9. The hydrogen evolution rate on pristine TiO₂ (P25) was found to be 0.68 mmol g⁻¹ h⁻¹, which is considerably low due to the rapid recombination of charges. To enhance its photocatalytic performance, Ti₃C₂T_x was employed as a co-catalyst. In order to fix the optimal ratio of Ti₃C₂T_x over TiO₂, various weight percentages, i.e., 2.5, 5, 7.5, and 10%, were designed. It was found that by increasing the Ti₃C₂T_x ratio, the H₂ evolution rate remarkably increased. The maximum H₂ evolution rate was observed with 7.5% Ti₃C₂T_x on TiO₂, i.e., 12.04 mmol g⁻¹ h⁻¹ (see results in Table 1 and Fig. 9a). The improved activity was attributed to the lowering of Fermi levels for rapid electron transfer to active sites.⁹⁵ Subsequently, a concentration of MXene higher than 7.5% causes a decline in the activities. This drawback was attributed to the blocking of light penetrations. To overcome this drawback, Pd metal was fabricated with Ti₃C₂T_x and then employed on a TiO₂ support. The introduction of Pd@Ti₃C₂T_x on TiO₂ progressively enhances the active sites where actual water reduction occurs. Because of the inherent higher work function,^10,68 Pd quenches more electrons to utilize for water reduction reactions even in the absence of a sacrificial agent.⁹⁶ In this work, 2% Pd with 7.5% Ti₃C₂T_x on TiO₂ was found to be the most active catalyst that delivered 35.11 mmol g⁻¹ h⁻¹ of hydrogen (see results in Fig. 9b). Surprisingly, this activity is 50 times higher than bare TiO₂. The higher activities on Pd@Ti₃C₂T_x–TiO₂ compared to bare TiO₂ were attributed to the higher electron transfer progress during the photoreactions. It has been found that in the absence of Pd@Ti₃C₂T_x cocatalyst, TiO₂ is unable to resist the recombination of charges.

Fig. 9 (a) Optimization of Ti₃C₂T_x over TiO₂. (b) Optimization of Pd over Ti₃C₂T_x–TiO₂; (c) comparison of optimised catalysts; (d) corresponding scheme of catalysts.

Table 1 Comparison of the H₂ evolution rate for the optimization of Pd and Ti₃C₂T_x (wt%) over TiO₂

Sr. No.	Photocatalyst	Ti3C2Tx (wt%)	Pd (wt%)	H2 (mmol g^−1)	H2 (mmol g^−1h^−1)	AQY (%)
1	TiO2	0	0	4.12	0.68	0.7
2	2.5-Ti3C2Tx–TiO2	2.5	0	24.69	4.11	4.19
3	5-Ti3C2Tx–TiO2	5.0	0	46.54	7.75	7.9
4	7.5-Ti3C2Tx–TiO2	7.5	0	72.24	12.04	12.6
5	10-Ti3C2Tx–TiO2	10	0	63.04	10.54	10.7
6	Pd@TiO2	0	2.0	90.95	15.15	15.4
7	1-Pd@Ti3C2Tx–TiO2	7.5	1.0	134.16	22.35	22.8
8	2-Pd@Ti3C2Tx–TiO2	7.5	2.0	213.06	35.11	35.8
9	3-Pd@Ti3C2Tx–TiO2	7.5	3.0	216.88	36.14	36.8

---

The higher hydrogen evolution results justify the electron-promoting behaviour of Pd@Ti₃C₂T_x cocatalysts. The simultaneous enhancement of hydrogen evolution was attributed to the higher active sites available on Pd@Ti₃C₂T_x–TiO₂ as compared to TiO₂ and Ti₃C₂T_x–TiO₂. The other important role of Pd is contributing to a higher work function during the photoreactions.⁹⁷ Similarly, the effective role of MXenes has become prominent in the presence of Pd on TiO₂ support. MXenes, due to their conducting nature, facilitate the more reactive and labile electrons available for anticipating the higher water reductions (i.e., more hydrogen evolution). To make this work more distinctive, hydrogen generation experiments were also done for Pd on TiO₂ to elucidate the individual role of Titania and MXene. It has been found that Pd@TiO₂ produces 15.15 mmol g⁻¹ h⁻¹ of hydrogen in the absence of MXene. The comparative performances of hydrogen generation on TiO₂, Ti₃C₂T_x–TiO₂, and Pd@Ti₃C₂T_x–TiO₂ were evaluated in terms of mmol g⁻¹, as depicted in Fig. 9c. The activities calculated in mmol g⁻¹ h⁻¹ are included in the supporting information section; See results in Fig. S1 (ESI†). The challenging task of this work was to affix the optimised ratio of the metal and MXene on TiO₂, which is less common in the literature. The development and design of catalysts have proven to be a big advantage compared to other conventional catalysts utilized for water splitting.⁹⁸ The comparison of related reported catalysts is tabulated in Table S1 (ESI†).

Recyclability

Monitoring of the recyclability of any catalysts is an important factor.⁹⁹ This study emphasizes experiments to check the recyclability or reusability of as-designed and prepared catalysts. For the comparative study, Pd@Ti₃C₂T_x–TiO₂ and Ti₃C₂T_x–TiO₂ were chosen to perform the recyclability tests. It has been found that Pd@Ti₃C₂T_x–TiO₂ catalysts show no significant loss in photoactivity (Fig S2, ESI†) after three cycles.¹⁰⁰ Meanwhile, Ti₃C₂T_x–TiO₂ exhibits a 22% loss in activity after three consecutive runs. The results obtained during the recyclability tests clearly demonstrate the stability of Pd@Ti₃C₂T_x–TiO₂, which is higher than that of other catalysts of this project. The phase transformations, structural changes, and surface chemical states during the photoreaction were assessed through XRD and XPS analyses.⁹⁶ It has been found that the XRD of Pd@Ti₃C₂T_x–TiO₂ exhibits no phase transformations even after photoreactions (see results in Fig S3, ESI†). The XPS of the used Pd@Ti₃C₂T_x–TiO₂ catalysts demonstrate that the oxidation states remain unchanged (illustrated in Fig S4, ESI†). Moreover, the MXene F-terminations have been reduced (i.e., replaced by −OH terminations during photoreactions).¹⁰¹ Surprisingly, the morphology of Ti₃C₂T_x–TiO₂ was masked due to little loss of crystallinity (absence of metallic Pd site). The other reason is that due to the presence of Pd, more electrons were utilized for water reduction occurring on the metal centres. The extra stability was in accordance with the design of the catalyst, i.e., Pd@Ti₃C₂T_x–TiO₂.

Charge transportation and hydrogen evolution

Insights into understanding the mechanism is one of the crucial factors for any photocatalytic process.^102,103 It has been reported that the progress of a photocatalytic reaction depends upon the bandgap and redox potentials of catalysts.¹⁰⁴ The bandgap of TiO₂ and Pd@Ti₃C₂T_x–TiO₂ were measured to be 3.10 eV and 2.95 eV, respectively (see UV-Vis/DRS results in Fig. 7b).¹⁰⁵ The major drawback linked to TiO₂ is its over potential and poor charge transportation ability, due to which it cannot control the back reaction between surface charges. When catalyst surfaces are exposed to light, the electrons are excited and move to the conduction band, leaving the holes at the valence band.¹⁰⁶ However, by fabricating TiO₂ with Ti₃C₂T_x, the transfer of charges towards active sites was significantly enhanced. Due to the conducting nature of Ti₃C₂T_x, TiO₂ becomes more progressive for water reduction reactions.¹⁰⁷ Thus, due to the higher charge conduction and transfer ability, MXene cocatalysts dramatically reduce the overpotential of TiO₂. Another chemical property of MXene is the possession of lower Fermi levels compared to TiO₂, which holds promise to lower the recombination of charges (i.e., back reaction between e⁻/h⁺). This lowering of charge recombination during photoreactions is responsible for higher catalytic performances. Fig. 10a elucidated the mechanism of hydrogen generation on Pd@Ti₃C₂T_x–TiO₂. It has been found that Pd@Ti₃C₂T_x–TiO₂ exhibits the highest rate of H₂ production that is attributed to the presence of higher active sites.¹⁰⁸ Note: it has been predicted that the presence of Pd@Ti₃C₂T_x at the interfaces of TiO₂ acted as electron sinks to facilitate water reduction (i.e., redox sites) (see Fig. 10b). Moreover, it is worth mentioning that the work function of Pd@Ti₃C₂T_x is higher than TiO₂. The higher work function of Pd@Ti₃C₂T_x led to the formation of a space charge layer between the interfaces (TiO₂ and Pd@Ti₃C₂T_x cocatalysts) that causes alteration in the potentials for new energy bands (electron rich maxima) on a semiconductor.^68,109 This alteration in the potentials enhances the developments of Schottky barriers during the photoreaction.¹¹⁰ These Schottky barriers hold promise to stop the backflow of electrons (recombination). The presence of Pd further promotes Schottky barriers to trap and transfer more electrons from the conduction bands and boost hydrogen generation reaction on metal active sites.⁴⁵ Pd has the ability to quench the electrons and then release these electrons due to low electron affinities. Due to its high metallic character, Pd metal enhances the conductivity and electron transfer in the semiconductor system. Thus, the presence of Pd@Ti₃C₂T_x on the TiO₂ system holds promise as a novel approach for researchers to design the catalysts for water-splitting reactions.

Fig. 10 Mechanism of (a) H₂ generation. (b) Charge transfer via Pd@Ti₃C₂T_x–TiO₂.

Conclusions

In the current study, Pd@Ti₃C₂T_x–TiO₂ photocatalysts were synthesized and employed for hydrogen generation reactions. The phase transformations from MAX (i.e., Ti₃AlC₂) to MXene (i.e., Ti₃C₂T_x 2D structures) were obtained by HF etching in a high-grade Teflon reactor (Techinistro/China). Reduction of Pd²⁺ to Pd⁰ on the Ti₃C₂T_x surfaces was successfully achieved without a reducing agent, i.e., in situ reduction by the interaction of localized/free electrons of Ti₃C₂T_x. The fabrication of Pd@Ti₃C₂T_x on TiO₂ was achieved via in situ phase transformation reactions. The phase transformations from MAX to MXene were confirmed with XRD, FT-IR, SEM and XPS results. The chemical compositions and surface terminations of Pd@Ti₃C₂T_x–TiO₂ were confirmed and evaluated with EDX and XPS analyses. The nature of the chemical bonding of catalysts was investigated via FT-IR and Raman spectroscopes. The presence of Pd on Ti₃C₂T_x was determined and confirmed using HRTEM. The electron traps and transfer from supports to the cocatalyst (Pd@Ti₃C₂T_x) were investigated via PL, EIS, and photocurrent results. Hydrogen generation activities were monitored on GC-TCD (Shimazdu-2010/Japan). The higher photocatalytic progress was attributed to the higher charge transfer, alteration in Fermi levels, and higher work function of cocatalysts. The ability of cocatalysts to effectively quench more electrons to promote and facilitate higher hydrogen generation has been investigated. In addition, the results depict that under similar conditions, Pd@Ti₃C₂T_x–TiO₂ delivers higher hydrogen as compared to bare TiO₂. Overall, 35.11 mmol g⁻¹ h⁻¹ of hydrogen was obtained on optimized catalysts (i.e., Pd@Ti₃C₂T_x–TiO₂). Based on the results, it can be anticipated that fabrications of TiO₂ with Pd@Ti₃C₂T_x cocatalysts hold promise as an attractive approach to designing new catalysts for hydrogen energy applications.

Conflicts of interest

The author declares no competing financial interest.

Acknowledgements

This work was financially supported by the Higher Education Commission of Pakistan award No. 377/IPFP-II/Batch-I/SRGP/NAHE/HEC/2020/27 and ASIP/R&D HEC/2023/5/22696/124 grants. Experimental work was conducted at the Institute of Chemistry, The Islamia University of Bahawalpur (Inorganic Material Laboratory (52s). Dr E.H. thanks the Carnegie Mellon University of USA for providing access to advanced instruments that were mandatory for catalyst characterizations.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ma00710c

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