MXene-derived TiO­2 nanoparticles coated with C/N shell for photocatalytic hydrogen generation under solar light

MXene-derived TiO­2 nanoparticles coated with C/N shell for photocatalytic hydrogen generation under solar light | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article MXene-derived TiO­ 2 nanoparticles coated with C/N shell for photocatalytic hydrogen generation under solar light Daria Baranowska, Bartosz Środa, Tomasz Kędzierski, Zhang Bowen, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5866238/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 08 Jun, 2025 Read the published version in Advanced Composites and Hybrid Materials → Version 1 posted 7 You are reading this latest preprint version Abstract Photocatalytic hydrogen production offers a sustainable and innovative solution to address environmental challenges and global energy shortages by leveraging solar energy. Developing highly efficient photocatalysts is pivotal for advancing photocatalysis technology and facilitating its practical applications. In this study, Ti 3 C 2 T X MXene was used as a precursor of TiO 2 nanoparticles coated with a carbon/nitrogen (C/N) shell for photocatalytic hydrogen generation under simulated solar light. The fabrication strategy was based on a straightforward one-step annealing process. The photoactivity of the sample was optimized through: (1) tuning the ratio of precursors MXene:gCN calcinated in the air at 550 ℃, and (2) controlling the temperature of the annealing process of the sample which indicated the most outstanding hydrogen evolution yield in strategy 1° (MXene:gCN = 1:19). The optimized sample, C/N@TiO 2 , demonstrated an exceptional H 2 production rate of 37.66 mmol/g (37 660 µmol/g), approximately 655 times and 37 times higher than those of gCN (57 µmol/g), and TiO 2 derived from pristine MXene (1024 µmol/g), respectively. This remarkable photocatalytic performance is attributed to the formation of a carbon/nitrogen (C/N) shell, which made TiO 2 extraordinarily robust in the experimental conditions, promoting charge separation, suppressing electron-hole recombination, and enhancing visible light absorption. Additionally, Density Functional Theory (DFT) calculations revealed that the C/N layer serves as an electron-rich active site, further promoting efficient photocatalytic hydrogen generation. This study provides a facile and cost-effective pathway to advancing green hydrogen production technologies. The findings underscore the potential of photocatalytic systems for sustainable energy development, paving the way for scalable renewable energy solutions. photocatalytic hydrogen generation MXene MXene-derived TiO2 TiO2 graphitic carbon nitride mechanism Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction The rapid advancement of modern industry and urbanization has undoubtedly brought immense benefits to global society. However, these developments have also given rise to two pressing challenges: energy shortages and environmental degradation. Tackling these issues is crucial to achieving long-term sustainable growth and preserving the health of our planet. Among the various strategies proposed, photocatalysis has emerged as a promising and innovative approach due to its ability to utilize solar energy for eco-friendly applications, including pollutant degradation and renewable energy generation. By effectively converting solar energy into chemical energy, photocatalytic materials offer a dual benefit – enabling clean energy production, such as hydrogen generation, while simultaneously addressing environmental remediation [1]. This versatile technology holds significant potential to contribute to a more sustainable and energy-secure future. Titanium dioxide (TiO 2 ), one of the most extensively studied photocatalysts, has attracted significant attention since its groundbreaking application in water splitting by Fujishima and Honda in 1972 [2]. TiO 2 is particularly appealing for photocatalytic applications due to its excellent stability, low cost, and environmentally friendly nature [1]. However, its practical performance is limited by its wide bandgap of 3.2 eV, which causes its light absorption predominantly to the ultraviolet region, with only ~ 5% of the solar spectrum [3]. Furthermore, the rapid recombination of photogenerated electron-hole pairs significantly reduces its photocatalytic efficiency, especially under visible light irradiation [1, 3]. To address these challenges, extensive research efforts have been dedicated to improving the photocatalytic performance of TiO 2 -based materials. Key strategies include advanced nanomaterial synthesis [4, 5], surface modifications [5], doping with various elements [3, 5], and the development of heterojunctions to facilitate efficient charge separation [1, 3, 5]. These approaches have collectively paved the way for enhancing the effectiveness of TiO 2 -based photocatalysts in a wide range of photocatalytic applications. In recent years, MXenes – a family of two-dimensional (2D) materials derived from the selective etching of MAX phases – have garnered significant attention as promising precursors for the fabrication of TiO 2 nanoparticles. MXenes, such as Ti 3 C 2 T X , exhibit a unique combination of metallic conductivity, tunable surface chemistry, and catalytic properties, making them highly attractive for photocatalytic applications. Through the thermal conversion of Ti 3 C 2 T X MXenes into TiO 2 , researchers have harnessed the layered and customizable structure characteristics of MXene to develop more efficient photocatalysts [6–9]. For instance, X. Han et al. [4] reported a synthesis of carbon-doped TiO 2 derived from Ti 3 C 2 T X MXene integrated with graphitic carbon nitride (gCN). This photocatalyst achieved an exceptional visible-light-driven H 2 production rate of 1409 µmol/h/g. Similarly, Q. Liu et al. [10] fabricated MXene-derived TiO 2 @C/gCN heterojunctions, demonstrating remarkable photocatalytic performance in nitrogen reduction, with an NH 3 production rate of 250 µmol/g/h under visible light. Beyond hydrogen generation and nitrogen reduction, MXene-derived TiO 2 heterostructures have shown great potential in diverse applications, including gas sensing, pollutant removal, and CO 2 reduction [11, 12]. These findings underscore the versatility and promise of MXene-derived TiO 2 in advancing photocatalytic technologies. Thus, the novelty of our work lies in employing the MXene and 2D graphitic carbon nitride as the precursors of a 3D hybrid system based on TiO 2 nanoparticles coated by carbon and nitrogen at high temperature allowing for high crystallinity of the sample. The unique composition of the resulting C/N shell acts as a protective and conductive layer surrounding the accordion-like TiO 2 , effectively facilitating charge separation and transfer. This structure enhances photocatalytic performance by suppressing charge recombination and improving light absorption, ultimately leading to superior photocatalytic activity. In this study, we propose a fabrication route of a highly efficient photocatalyst in the form of TiO 2 nanoparticles coated with a carbon/nitrogen (C/N) shell derived from Ti 3 C 2 T X MXene and graphitic carbon nitride. What is more, the proposed strategy is facile and cost-effective - a one-step annealing process. The photoactivity of the material was verified in the reaction of hydrogen generation under simulated solar light. The optimal sample, C/N@TiO 2 , achieved an extraordinary H 2 production rate of 37.66 mmol/g (37 660 µmol/g), which is ~ 655 and 37 times higher than those of gCN (57 µmol/g) and TiO 2 derived from pristine MXene (1024 µmol/g), respectively. These results outperform the efficiency of the TiO 2 -based photocatalysts reported in the current state of the art. The remarkable photocatalytic performance of C/N@TiO 2 is attributed to the presence of the carbon/nitrogen (C/N) shell on TiO 2 particles, which promotes effective charge separation, suppresses electron-hole recombination, and enhances visible light absorption. Additionally, DFT calculation revealed that the C/N layer serves as an electron-rich active site, further promoting efficient photocatalytic hydrogen generation. This study provides valuable insights into the design of MXene-derived TiO 2 with the potential to catalyze various photocatalytic reactions. Material and methods Preparation of MXene To synthesize Ti 3 C 2 T X MXene, 590 mg of Ti 3 AlC 2 MAX phase (American Elements) was slowly and carefully added to 10 mL of a 40% hydrofluoric acid (HF) solution (Sigma-Aldrich). The mixture was stirred continuously at room temperature for 24 hours to allow for selective etching of aluminum from the Ti 3 AlC 2 structure. After the etching process, the mixture was diluted with deionized water. The solid Ti 3 C 2 T X MXene product was separated from the solution by centrifugation. The precipitate was subjected to multiple wash cycles with deionized water and ethanol until the pH of the supernatant was neutral. The purified MXene was dried in a vacuum oven at 50 ℃ overnight and stored at 5 ℃ to prevent oxidation and ensure long-term stability. Preparation of TiO_MXene The as-synthesized MXene was further annealed under ambient air conditions at two different temperatures: 550 or 600 ℃, for 4 hours to obtain reference samples for photocatalytic hydrogen evolution reactions. The resulting samples were labeled as TiO 2 _MXene_550 and TiO 2 _MXene_600, respectively. For clarity and simplicity, the TiO 2 _MXene_600 is referred to as TiO 2 _MXene through the main manuscript. Preparation of graphitic carbon nitride (gCN) To synthesize graphitic carbon nitride (gCN), 8 g of melamine powder (Sigma-Aldrich) was placed in a covered corundum crucible. The crucible was then positioned inside a muffle furnace under ambient air conditions. The heating process was initiated with a temperature ramp rate of 2 ℃min − 1 , gradually increasing to a final temperature of 550 ℃. The material was held at 550 ℃ for 4 hours to allow complete polymerization and condensation of the melamine into gCN. After cooling naturally to room temperature, the resulting yellow powder was collected for further use. Preparation of C/N@TiO In the next step, the previously synthesized MXene and gCN were mixed in different mass ratios using a mortar. After achieving a homogeneous powder blend, the mixture of MXene and gCN was transferred to a crucible and placed in a muffle furnace under ambient air conditions. The temperature was increased at a ramp rate of 2 ℃min − 1 until reaching a target range of 500–650 ℃. The mixture was then annealed at the set temperature for 4 hours. Samples were labeled as C/N@TiO 2 _X_T, where X denotes the MXene content, while T represents the annealing temperature. For instance, the sample labeled C/N@TiO 2 _5_600 was prepared by combining 5wt% MXene with 95wt% gCN, followed by mortaring and heating the mixture at 600 ℃ for 4 hours under air conditions in a muffle furnace. For clarity and brevity, the C/N@TiO 2 _5_600 is referred to simply as C/N@TiO 2 through the main manuscript. The effect of varying the ratio of the components and the specific annealing temperature on the photocatalytic activity for hydrogen evolution from water splitting is described in detail in the Supplementary Material . Characterization The morphology of the studied samples was characterized using the Scanning Electron Microscope (SEM, Apreo S2) and the Transmission Electron Microscope (TEM, Spectra 300, Thermo Fisher Scientific). X-ray Powder Diffraction (XRD) patterns were recorded using an Aeris diffractometer (Malvern Panalytical) with CuKα radiation. Raman spectra were acquired using a Renishaw InVia spectrometer with a laser wavelength of 785 nm. The specific surface area was measured through N 2 adsorption/desorption isotherm analysis using a Micromeritics ASAP 2460 instrument. The optical absorption spectra of samples were obtained using a JASCO V-770 UV-vis spectrophotometer in diffuse reflectance mode (DRS), with BaSO 4 serving as a white standard for baseline correction. Mott-Schottky measurements were conducted using a BioLogic VMP-3 potentiostat in a three-electrode configuration. The reactor was equipped with a temperature-controlled bath, maintained at 25 ℃. A mercury sulfate electrode (MSE, HgǀHg 2 SO 4 in 1 M H 2 SO 4 , RE-2CP) was employed as the reference electrode, while a platinum wire served as the counter electrode. The working electrode was a 5 mm diameter glassy carbon disk in a PEEK polymer case. The electrode surface was prepared by polishing with 0.05 µm alumina slurry, followed by washing with acetone. To prepare the active material for the working material, a dispersion was prepared at a concentration of 2 mg/mL in a solution of isopropanol and water (1:3 by volume) with 0.05% Nafion solution (Sigma-Aldrich) acting as a binder. The measurements were carried out in 0.5 M Na 2 SO 4 . Mott-Schottky analysis was performed via Electrochemical Impedance Spectroscopy (EIS). The measurements were recorded at frequencies of 1, 10, 20, 50, and 100 kHz with an AC signal amplitude of 5 mV, capturing 11 points per decade. The potential was swept from 1.0 to -1.0 V vs MSE in 26 discrete steps. The room temperature photoluminescence (PL) spectra were recorded using a Hitachi F-7000 spectrophotometer with an excitation wavelength of 350 nm, with samples suspended in isopropanol during the measurements. Electrochemical characterization, including Chronoamperometry (CA) and Electrochemical Impedance Spectroscopy (EIS), was conducted using a three-electrode configuration with an Autolab PGSTAT302N potentiostat. CA tests were performed at 0.50 V vs. MSE under 388 nm light excitation, while EIS measurements were carried out at 0.15 V vs. MSE in the dark. The setup consisted of a platinum plate as the counter electrode, an MSE as the reference electrode, and 0.50 M H 2 SO 4 as the electrolyte. The active material dispersion was prepared at a concentration of 1 mg/mL in a solution of isopropanol and water (1:3 by volume) with 0.05% Nafion solution (Sigma-Aldrich) as a binder. The mixture was sonicated to ensure uniform suspension. Subsequently, 50 µL of the suspension was drop-cast onto fluorine-doped tin oxide (FTO) coated glass (Sigma-Aldrich), serving as the working electrode for electrochemical tests. Photocatalytic hydrogen generation Photocatalytic hydrogen evolution experiments were carried out in a three-neck Pyrex glass reactor under an argon (Ar) atmosphere. To prepare the reaction mixture, 10 mg of photocatalyst powder was dispersed in 20 mL of deionized water and 4 mL of triethanolamine (TEOA, Sigma-Aldrich), used as a sacrificial agent. Before each experiment, the reactor system was sealed and purged with Ar gas for 30 minutes to remove any residual oxygen and establish an inert environment. Subsequently, the photocatalytic reaction was initialed by illuminating the reactor with a 150 W Xe lamp fitted with an A.M. 1.5G filter to simulate solar light conditions. During the experiment, the composition of the gas phase was monitored using a gas chromatograph (Young Lin 6500), equipped with a 5 Å molecular sieve capillary column (Merck) and thermal conductivity detector (TCD). For each analysis, a 100 µL gas sample was injected into the chromatograph, and the hydrogen content was determined using a pre-calibrated curve based on known standards. Multiple consecutive reaction cycles were conducted under identical experimental conditions to assess the stability and recyclability of the photocatalyst with the highest efficiency. DFT calculations Density functional theory calculations with the Perdew-Burke-Ernzerhof (PBE) functional [13] were carried out with Quantum ESPRESSO version 7.2 [14]. Standard solid-state pseudopotentials [15] were used. The kinetic energy cutoff for plane-wave wavefunctions was set to 50 Rydberg, and the kinetic energy cutoff for charge density and potential was set to 450 Rydberg. Gaussian smearing with a width of 0.02 Rydberg for Brillouin-zone integration was applied. Grimme’s D3 dispersion correction [16] with Becke-Johnson damping [17] was included. Monkhorst-Pack [18] 4x4x1 K-point grids were used. A vacuum space of 20 Å along the z direction was used between adjacent layers. Charge transfer was quantified by Bader charge analyses [19]. Results and discussion The synthesized C/N@TiO 2 _X_T were thoroughly characterized to evaluate their structural, morphological, optical, and electrochemical properties. Figure 1 a illustrates a schematic overview of the experimental process, highlighting the one-step calcination method employed to fabricate MXene-derived TiO 2 coated with a carbon/nitrogen (C/N) shell. This innovative photocatalyst was carefully designed to optimize its performance for photocatalytic hydrogen production. The morphology of graphitic carbon nitride (gCN), MXene annealed at 600 ℃ in air (TiO 2 _MXene) and C/N@TiO 2 was systematically investigated using Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM), as depicted in Fig. 1 b-g, and Fig. S1 . Graphitic carbon nitride (gCN) ( Fig. S1 ) exhibits a characteristic flat, two-dimensional (2D) plate-like structure composed of smooth-surfaced, multilayered nanosheets. The interconnected carbon and nitrogen networks demonstrate a disordered arrangement over long distances, consistent with its amorphous nature [20]. TiO 2 _MXene (Fig. 1 b) features a layered, accordion-like structure with distinct interlayer spacing, which is characteristic of MXene materials [21]. Notably, the lamellar structure remains well-preserved even after annealing (Fig. 1 c). The morphology of C/N@TiO 2 reflects the retained accordion-like structure of MXene-derived TiO 2 with additional modifications attributed to the incorporation of carbon and nitrogen on its surface. These structural changes of C/N@TiO 2 suggest that gCN may act as a source of the coating of the nanoparticles, interacting with the MXene-derived TiO 2 surface during the annealing process. Energy-dispersive X-ray Spectroscopy (EDX) analysis (Fig. 1 c ’ ) further supports this conclusion, revealing an enriched carbon and nitrogen content on the surface of C/N@TiO 2 . TEM imaging provides additional insights into the material structure. TiO 2 _MXene (Fig. 1 d) and C/N@TiO 2 (Fig. 1 f) both exhibit irregularly shaped nanoparticles with well-defined crystallinity. A High-resolution TEM (HRTEM) image of C/N@TiO 2 (Fig. 1 g) reveals lattice fringes indicative of the high crystallinity of MXene-derived TiO 2 . The observed lattice spacing of 3.51 Å corresponds to the (101) plane of the tetragonal anatase phase of TiO 2 (ICDD No. 01-073-1764). Additional lattice spacings of 2.16, 2.34, 2.55, and 3.57 Å are assigned to the (006), (112), (104), and (201) planes of Al 2 O 3 (ICDD No. 04-008-4095 and 04-005-4213), respectively. TEM-EDX elemental mapping (Fig. 1 h) provides further evidence of the successful fabrication of C/N@TiO 2 , showing the formation of a visually discernible carbon/nitrogen (C/N) shell coating the surface of MXene-derived TiO 2 . The coating is expected to enhance photocatalytic performance by improving charge separation and surface reactivity. The XRD patterns of gCN, TiO 2 _MXene, and C/N@TiO 2 are shown in Fig. 1 i. The graphitic carbon nitride (gCN) exhibits two characteristic diffraction peaks at 13.1 and 27.7°, corresponding to the (001) and (002) planes, respectively, as indexed by ICDD No. 00-066-0813. For the TiO 2 _MXene and C/N@TiO 2 samples, distinct diffraction peaks at 25.4, 37.1, 37.9, 38.7, 48.2, 54.1, 55.2 62,3, 62.9, 68.9, 70.5, 74.3, 75.3, and 76.2° are observed. These peaks correspond to the (101), (103), (004), (112), (200), (105), (211), (213), (204), (116), (220), (107), (215), and (301) planes of tetragonal anatase phase of TiO 2 (ICDD No. 01-073-1764), confirming the complete transformation of MXene into the anatase phase during calcination [4]. This observation demonstrates the effective oxidation of the Ti atoms in Ti 3 C 2 T X at high temperatures [10]. Additionally, several peaks with low intensity corresponding to Al 2 O 3 are identified in the XRD patterns of TiO 2 _MXene and C/N@TiO 2 . These include peaks at 31.2, 31.6, 32.8, 36.7, 40.1, and 44.8°, which correspond to the (400), ( \(\:\stackrel{-}{4}\) 01), (002), (111), ( \(\:\stackrel{-}{3}11\) ), and ( \(\:\stackrel{-}{1}12\) ) planes of monoclinic Al 2 O 3 (ICDD No. 04-008-4095). Additional peaks at 35.2, 43.4, 52.6, 57.6, 66.6, and 68.3° are attributed to the (104), (113), (024), (116), (214), and (300) planes of hexagonal Al 2 O 3 (ICDD No. 04-005-4213). The presence of Al 2 O 3 in TiO 2 _MXene is caused by the incomplete etching of aluminum from the Ti 3 AlC 2 MAX phase. Interestingly, the C/N@TiO 2 sample does not exhibit any discernible peaks corresponding to graphitic carbon nitride, suggesting that the material predominantly consists of TiO 2 and a small amount of Al 2 O 3 phases. However, an additional peak at 14.3°, was observed and attributed to (002) planes of carbon (ICDD No. 04-013-3397) formed during gCN decomposition. The Raman spectra of gCN, TiO 2 _MXene, and C/N@TiO 2 are shown in Fig. S2a . The spectrum of gCN displays several characteristic peaks at 485, 706, 749, 1232, 1480, 1556, and 1615 cm − 1 , which are attributed to the vibration modes of graphitic carbon nitride. Specifically, the band at 1615 cm − 1 is attributed to C = N stretching vibration and is commonly referred to as the graphitic (G) band, indicative of sp 2 -hybridized carbon structure. The peaks at 1556 and 1480 cm − 1 correspond to symmetric N-C-N stretching and asymmetric C = N stretching, respectively. The band at 1232 cm − 1 is attributed to the in-plane bending of sp 2 -hybridized = C-N groups within the heptazine or triazine framework [22]. In the TiO 2 _MXene and C/N@TiO 2 samples, five prominent Raman bands are observed at 143, 195, 395, 514, and 637 cm − 1 , corresponding to the E g , E g , B 1g , A 1g, and E g vibrational modes characteristic of the anatase phase of TiO 2 , respectively [10, 23]. Notably, the Raman spectrum of C/N@TiO 2 exhibits an additional feature in the 600–1300 cm − 1 region, highlighted in the insert. These bands are absent in the TiO 2 _MXene but present in both gCN and C/N@TiO 2 and are attributed to C-N stretching and ring-breathing vibrations of triazine/heptazine units [10]. Finally, the results obtained from SEM-EDX, TEM-EDX, XRD and Raman confirm the successful deposition of carbon and nitrogen species on the C/N@TiO 2 surface. The nitrogen adsorption-desorption isotherms of gCN, TiO 2 _MXene, and C/N@TiO 2 are presented in Fig. S2b . All studied materials exhibit a type IV isotherm with an H3-shaped hysteresis loop. The specific surface areas, determined via Brunauer-Emmett-Teller (BET) analysis, were found to be 12.73 m 2 /g for gCN, 19.78 m 2 /g for TiO 2 _MXene, and 25.26 m 2 /g for C/N@TiO 2 . The total pore volumes were measured as 5.41, 8.12, and 10.34 mm 3 /g for gCN, TiO 2 _MXene, and C/N@TiO 2 , respectively. An increased total pore volume is advantageous for designing effective photocatalysts due to several factors, including increased surface area, and improved mass transport [20, 22]. According to the Density Functional Theory (DFT) spectra shown in Fig. S2c , all synthesized materials exhibit microporous structures, with median pore widths of 10.71, 12.33, and 11.40 Å for gCN, TiO 2 _MXene, and C/N@TiO 2 , correspondingly. Notably, C/N@TiO 2 displays higher pore volume and larger pore width than its counterparts, particularly in the 25–50 Å range. This characteristic enhances the diffusion of reactants and products, improving catalytic efficiency by providing greater surface accessibility and enabling better light penetration during the photocatalytic hydrogen evolution process. The optical absorption properties of gCN, TiO 2 _MXene, and C/N@TiO 2 were investigated using UV-Vis spectroscopy, as shown in Fig. 2 a. All the studied catalysts exhibit absorption within the solar light spectrum. The pristine gCN shows an absorption edge at about 468 nm, while TiO 2 _MXene and C/N@TiO 2 display strong absorption across a broad spectral range from UV to visible light, with absorption edges at approximately 395 nm [10]. Notably, the C/N@TiO 2 sample exhibits a slightly red-shifted absorption edge compared to TiO 2 _MXene, indicating an enhanced response in the visible light region. To further analyze the optical properties, the corresponding Kubelka-Munk function was employed (Fig. 2 b) to estimate the band gaps of the materials. The calculated band gap energies were 2.65, 3.17, and 3.13 eV for gCN, TiO 2 _MXene, and C/N@TiO 2 , respectively. The slight improvement in visible light absorption for C/N@TiO 2 is attributed to the incorporation of carbon and nitrogen, which enhances its optical performance in the visible region. Further insights into the electronic structure of the studied materials were obtained through Mott-Schottky analysis (Fig. 2 c and Fig. S3 ). The positive slope in the linear regions confirms that all materials are n-type semiconductors [23]. Notably, C/N@TiO 2 exhibits a significantly lower slope compared to gCN and TiO 2 _MXene, indicating a reduced charge carrier density [24]. This characteristic is advantageous for photocatalysis as it reduces electron-hole recombination and facilitates prolonged charge carrier lifetimes. Additionally, the flat band potentials, estimated from the Mott-Schottky plots, were found to be -0.82, -1.40, and − 1.63 V for gCN, TiO 2 _MXene, and C/N@TiO 2 , respectively. For an n-type semiconductor, the flat band potential approximates the Conduction Band (CB) edge due to the proximity of the Fermi level to the CB [25]. The notable shift in a flat band potential for C/N@TiO 2 , along with its reduced slope, suggests significant modifications in its electronic structure. These changes are indicative of enhanced charge separation and transfer efficiency, which are crucial for improved photocatalytic performance [23]. The enhanced electronic properties of C/N@TiO 2 can be attributed to the fabrication of an efficient MXene-derived TiO 2 photocatalyst, where the TiO 2 surface is coated with a carbon/nitrogen (C/N) shell. This interaction not only shifts the flat band potential but also optimizes carrier concentration, ultimately contributing to superior photocatalytic activity. The result highlights that the tailored electronic structure of C/N@TiO 2 enhances its ability to drive photocatalytic reactions effectively. By integrating data from DRS UV-Vis and Mott-Schottky analyses, the Valance Band (VB) positions were determined to be 1.83, 1.77, and 1.50 V for gCN, TiO 2 _MXene, and C/N@TiO 2 , respectively. The schematic energy band diagram (Fig. 2 d) illustrates that the Conduction Bands (CBs) of all three materials are above the H + /H 2 reduction potential (0 V vs. RHE), confirming their feasibility for hydrogen production, while the valance bands (VBs) are below the O 2 /H 2 O oxidation potential (1.23 V vs. RHE), enabling the oxidation half-reaction necessary for water splitting [26]. Among the tested materials, C/N@TiO 2 exhibits the most negative CB potential, highlighting its superior reducing ability. This feature is particularly beneficial for enhanced photocatalytic hydrogen production, as it promotes efficient electron transfer to drive the H + /H 2 reduction reaction [1]. Furthermore, the band alignment of C/N@TiO 2 is optimized to support effective charge carrier separation and utilization in both hydrogen and oxygen evolution reactions. The unique band structure of C/N@TiO 2 underscores its potential for efficient photocatalysis. This innovative photocatalyst enables simultaneous reduction and oxidation processes, thereby achieving enhanced photocatalytic activity. The combination of favorable CB and VB positions, coupled with the intrinsic properties of the C/N@TiO 2 system, positions it as a promising candidate for high-performance photocatalytic applications. The Photoluminescence (PL) spectra of gCN, TiO 2 _MXene, and C/N@TiO 2 (Fig. 2 e) provide crucial insight into the electronic properties and charge carrier dynamics of the materials. The PL spectrum of gCN exhibits a broad emission band in the range of 410–580 nm, with a prominent peak at 448 nm. This peak is attributed to the radiative recombination of electron-hole pairs within the material. The observed emission arises from the n-π* electronic transition [20]. In contrast, the PL intensity of TiO 2 _MXene is significantly lower, indicating a reduced electron-hole recombination rate due to its modified electronic structure. Notably, the PL intensity of C/N@TiO 2 is further suppressed compared to TiO 2 _MXene and gCN, suggesting the most efficient spatial charge separation among the studied samples [27]. This pronounced reduction in PL intensity demonstrates an enhancement in the electronic properties of C/N@TiO 2 , which is expected to improve photocatalytic performance by minimizing energy losses due to the recombination process. The electrochemical properties of gCN, TiO 2 _MXene, and C/N@TiO 2 were evaluated through Chronoamperometry (CA) (Fig. 2 f) and Electrochemical Impedance Spectroscopy (EIS) (Fig. 2 g). The periodic photocurrent spikes observed for all samples during light on/off cycles demonstrate a rapid and consistent response to light exposure indicating efficient light-driven charge separation and transfer. Among the tested materials, C/N@TiO 2 exhibits the highest photocurrent response, reflecting its significantly enhanced efficiency in separating and migrating photogenerated charges [20]. This improvement is attributed to the successful fabrication of MXene-derived TiO 2 coated with a carbon/nitrogen (C/N) shell. The Nyquist plot further supports this conclusion, as C/N@TiO 2 shows the smallest semicircle diameter compared to the reference materials, confirming its superior ability to separate photogenerated electron-hole pairs and facilitate charge transfer at the solid/liquid interface [20]. This behavior suggests that the incorporation of carbon and nitrogen onto the MXene-derived TiO 2 surface significantly reduces transfer resistance, thereby optimizing photocatalytic performance. The results of the EIS analysis align perfectly with the findings from CA measurements, collectively demonstrating that C/N@TiO 2 exhibits superior charge separation and transfer properties. This can be attributed to its tailored electronic structure and the successful fabrication of MXene-derived TiO 2 . Ultimately, these electrochemical studies, combined with the optical characterization of the materials, underscore the effectiveness of the MXene-derived TiO 2 photocatalysts in enhancing photoinduced charge carrier behavior, making it a promising material for advanced photocatalytic applications. The photocatalytic hydrogen generation performance of graphitic carbon nitride (gCN), MXene, TiO 2 _MXene_550, TiO 2 _MXene_600, and C/N@TiO 2 _X_T were systematically evaluated. The fabricated MXene-derived TiO 2 photocatalysts were optimized in two directions: (i) influence of MXene to gCN ratio calcinated in the air at 550 ℃, and (ii) temperature dependence of a 5wt% MXene and 95wt% gCN mixture calcinated in the air at different temperatures ranging from 500–650 ℃. The results are illustrated in Fig. 2 h and Fig. S4 . The photocatalytic experiments were conducted under simulated solar light irradiation, using TEOA as a sacrificial agent, and notably, without the addition of platinum as a co-catalyst. Pristine gCN exhibited an H 2 evolution rate of 57.5 µmol/g, while pristine MXene generated 36.9 µmol/g of H 2 , likely due to the superior optical properties of gCN over MXene. Calcination of MXene at 550 ℃ (TiO 2 _MXene_550) and 600 ℃ (TiO 2 _MXene_600 or TiO 2 _MXene) resulted in significantly improved photocatalytic activity, highlighting that the transformation of MXene into MXene-derived TiO 2 is an effective strategy to enhance its efficiency. Initially, the MXene-to-gCN ratio was optimized ( Fig. S4ab ). It was observed that the increase in H 2 evolution was not linear upon the increase of MXene loading. The highest efficiency was achieved for C/N@TiO 2 _5_550, with an H 2 evolution rate of 6 825 µmol/g. Therefore, the next step involved calcination of the mixture of 5wt% MXene and 95wt% gCN at various temperatures in the range of 500 and 650 ℃ to reveal the optimal photocatalyst ( Fig. S4cd ). Among all tested samples, C/N@TiO 2 _5_600 (or C/N@TiO 2 ) achieved an extraordinary H 2 production rate of 37.66 mmol/g (37 660 µmol/g), which is about 655 times higher than gCN (57 µmol/g), and 37 times higher than TiO 2 _MXene (1024 µmol/g) (Fig. 2 h). The remarkable photocatalytic activity of C/N@TiO 2 is attributed to several synergistic factors, including (i) successful fabrication of MXene-derived TiO 2 coated with a carbon/nitrogen (C/N) shell, (ii) increased surface area, (iii) enhanced visible light absorption, (iv) reduced electron-hole recombination, and (v) improved charge separation and migration efficiency. These findings demonstrate the effectiveness of optimizing the MXene-derived TiO 2 photocatalysts to achieve superior photocatalytic hydrogen generation. The superior photocatalytic activity of C/N@TiO 2 may also be influenced by the presence of the small amount of Al 2 O 3 in the sample resulting from incomplete Al etching from the MAX phase using 40% HF. The positive influence of residual Al 2 O 3 can be attributed to its role as an energy barrier that suppresses the charge recombination, despite its insulating nature (energy band gap of 8.7 eV) [28–30]. Furthermore, insulators play a crucial role in optimizing the optical path and enhancing light-scattering efficiency, thereby improving photon capture in insulator-based photocatalysts and promoting charge carrier generation. In semiconductor-insulator composites, TiO 2 is often combined with insulators such as Al 2 O 3 and SiO 2 . Among these, Al 2 O 3 is widely used as a catalyst support due to its well-structured porosity and high surface area, which prevent catalyst agglomeration at elevated temperatures. Studies have shown that composite insulators optimize the surface structure and light absorption, ultimately improving photocatalytic activity. Both Al 2 O 3 and SiO 2 contribute to an increase in the number of catalytic sites by reducing TiO 2 particle agglomeration and enhancing dispersion during calcination [31]. In this term, an additional study of photocatalytic hydrogen evolution over C/N@TiO 2 , in which precursor Ti 3 C 2 T X MXene was produced via etching with 48% HF, was tested (details provided in Supporting Information ). The results revealed ( Fig. S5 ) that C/N@TiO 2 – 40% HF exhibited 12-hold higher hydrogen production in comparison to the sample composed of only a trace amount of Al 2 O 3 (C/N@TiO 2 – 48% HF). Furthermore, the photocatalytic performance of C/N@TiO 2 was evaluated against similar photocatalysts reported in the current state of the art ( Table S1 ). Notably, C/N@TiO 2 exhibits one of the highest H 2 production rates under simulated solar light, suppressing most graphitic carbon nitride/TiO 2 -based photocatalysts. However, it is important to consider that direct comparisons of photocatalytic activity can be influenced by variations in reaction conditions, including light source, light intensity, the presence of co-catalysts and sacrificial agents, etc.). To assess the stability and durability of C/N@TiO 2 , five consecutive photocatalytic cycles were performed, spanning a total of 20 hours (Fig. 3 a). The cyclic experiments, conducted under identical reaction conditions, revealed a noticeable decrease in photocatalytic H 2 generation during the second cycle (23.16 mmol/g), followed by stabilization in subsequent cycles. Interestingly, after the 4th cycle, the additional portion of the sacrificial agent (TEOA) was introduced into the reaction system. However, this replenishment did not result in any substantial improvement in photocatalytic efficiency. This suggests that factors beyond the availability of the sacrificial agent, such as changes in electronic, optical, or electrochemical properties of the photocatalyst, might be responsible for the observed decline. Post-reaction analyses were conducted on the recovered C/N@TiO 2 material, which was filtered, dried, and subjected to a series of characterization techniques, including SEM, TEM, TEM-EDX, XRD, nitrogen adsorption/desorption isotherms, UV-Vis spectroscopy, PL, CA, and EIS. SEM (Fig. 3 b) and TEM (Fig. 3 c) revealed that the morphology of the material remained intact, indicating good structural durability after the photocatalytic reaction. TEM-EDX (Fig. 3 d) mapping further confirmed the preserved microstructure. However, the absence of a carbon layer on the surface of the MXene-derived TiO 2 was observed, suggesting partial removal of carbon during the photocatalytic process. The XRD diffractogram (Fig. 3 e) of the post-reaction material displayed more pronounced peaks compared to the pre-reaction sample, indicating an increase in crystallinity. Nitrogen adsorption/desorption isotherms and pore size distribution analyses ( Fig. 3fg ) revealed an increase in surface area, total pore volume, and median pore width to 43.30 m 2 /g, 16.36 mm 3 /g, and 15.92 Å, respectively, of the material after the reaction. These changes are consistent with the removal of amorphous carbon, which likely exposes the crystalline TiO 2 surface, as corroborated by the microscopy, XRD, and Raman data. The optical properties of C/N@TiO 2 before and after the reaction were evaluated using UV-Vis spectroscopy (Fig. 3 h), and the corresponding Kubelka-Munk transformation (Fig. 3 i). The material after the reaction exhibited a higher energy band gap of 3.22 eV, compared to an initial 3.13 eV, which reduces visible light absorption during cycling tests. Moreover, a slight increase in the recombination rate of electron-hole pairs was observed (Fig. 3 j), contributing to the lower photocatalytic efficiency. Electrochemical measurements ( Fig. 3kl ) demonstrate a reduction in the photocurrent response and an increase in the charge transfer resistance of the material after the reaction, both of which adversely affect the photocatalytic performance. In summary, while C/N@TiO 2 maintains good structural integrity and durability during repeated photocatalytic cycles, the partial removal of the carbon layer, changes in optical properties, and increased recombination of electron-hole pairs to the reduced efficiency in hydrogen generation over time. Density functional calculations were performed to reveal the mechanistic insight for the enhanced photocatalytic performance of C/N@TiO 2 . The charge density difference map (Fig. 4 ) reveals charge redistribution upon interaction between the C/N layer and MXene-derived TiO 2 . Broader charge analysis indicates a net charge transfer of 0.11 e⁻ from the TiO 2 layer (bottom) to the C/N layer (top), suggesting electron donation from TiO 2 to C/N. This charge transfer enhances charge separation, which is beneficial for photocatalysis. Furthermore, the calculated density of states (DOS) and band structure show a narrowed band gap for MXene-derived TiO 2 upon the introduction of the C/N layer, favoring electron-hole separation. This is further supported by a charge density difference map, which indicates the electron transfer from TiO 2 to the C/N layer. Consequently, the C/N layer serves as an electron-rich active site on the surface, promoting efficient photocatalytic hydrogen generation. Conclusions In summary, this study successfully developed an MXene-derived TiO 2 photocatalyst coated with a carbon/nitrogen (C/N) shell using a one-step calcination method. This innovative photocatalyst was meticulously designed and optimized to enhance its photocatalytic hydrogen generation performance. Among all tested samples, C/N@TiO 2 emerged as the most efficient photocatalyst, achieving an extraordinary H 2 production rate of 37.66 mmol/g (37 660 µmol/g), approximately 655 times greater than gCN (57 µmol/g), and 37 times higher than TiO 2 _MXene (1024 µmol/g). The photocatalytic mechanism of C/N@TiO 2 was analyzed in depth: the Tauc plot estimated its band gap at 3.13 eV, while Mott-Schottky analysis confirmed its n-type semiconductor nature with a conduction band position of -1.63 V. The superior photocatalytic activity of C/N@TiO 2 is attributed to several synergistic factors: (i) the successful fabrication of MXene-derived TiO 2 coated with a carbon/nitrogen (C/N) shell, (ii) an increased surface area, (iii) enhanced visible light absorption, (iv) suppression of electron-hole recombination, and (v) improved charge separation and migration efficiency. Additionally, DFT calculation revealed that the C/N layer serves as an electron-rich active site, further promoting efficient photocatalytic hydrogen generation. Overall, this study provides valuable insights into the design of MXene-derived TiO 2 photocatalysts, demonstrating their potential for efficient photocatalytic hydrogen generation under solar light. Declarations Acknowledgments : Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) analyses have been conducted at the Center for Advanced Materials and Manufacturing Process Engineering (CAMMPE). The Ti 3 C 2 T X MXenes used in this study were obtained with support from the National Science Center (Poland) through the PRELUDIUM BIS 2 program (Grant No: 2020/39/O/ST5/01340). The raw data will be placed publicly available in an open-access repository. Funding : This research received no external funding. Competing interests : The authors declare no competing interests. References B. Zhang, B. Sun, F. 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Supplementary Files gcnmxeneSIfinalrevised.docx Cite Share Download PDF Status: Published Journal Publication published 08 Jun, 2025 Read the published version in Advanced Composites and Hybrid Materials → Version 1 posted Editorial decision: Revision requested 06 May, 2025 Reviews received at journal 24 Apr, 2025 Reviewers agreed at journal 22 Apr, 2025 Reviewers agreed at journal 17 Apr, 2025 Reviewers invited by journal 17 Apr, 2025 Submission checks completed at journal 10 Apr, 2025 First submitted to journal 07 Apr, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5866238","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":444496727,"identity":"2a8f167b-30f9-4783-8caf-c0da3d0e9110","order_by":0,"name":"Daria Baranowska","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABHklEQVRIie3PMUvDQBTA8XcUMsVmTUilXyEh0A6l9KvkCCTLKbhlcDgpxKWQ9QY/REHofOFAlytZO3SwCJky1M0hFlODg5KoOIncn3twHPx4HIBK9Rfj76fJB6O59I5Df0YsCog2BHUS+EAc/g3p3693/Bm2w/H1OnvUq3Do5XL8APEEU/uqlVgycrIFFO6NPA+8k4S4Ky5dCjLCdJC1EoeHwHUQiAEZ2Wc0Rqts4VKUCExN3E7yArIKxIwZ5cgmVTy7nes1OXxBNiGIegtmZr2FaAQvtSOh3cTaFCAGUATMLD3vJQkDJrUL5t9FXtLxl34e9p5K2E6ZQdwdq4Jpmorlfn85OU3tOW8jb6HD5xe/Hs3sBJ39gqhUKtW/7BVJLWy8l2FZ/QAAAABJRU5ErkJggg==","orcid":"","institution":"West Pomeranian University of Technology in Szczecin","correspondingAuthor":true,"prefix":"","firstName":"Daria","middleName":"","lastName":"Baranowska","suffix":""},{"id":444496730,"identity":"1f1e7935-4db4-410b-bb4d-efe7f4876a87","order_by":1,"name":"Bartosz Środa","email":"","orcid":"","institution":"West Pomeranian University of Technology in Szczecin","correspondingAuthor":false,"prefix":"","firstName":"Bartosz","middleName":"","lastName":"Środa","suffix":""},{"id":444496734,"identity":"5b217546-1cf3-4b67-a59f-4f4d527d1484","order_by":2,"name":"Tomasz Kędzierski","email":"","orcid":"","institution":"West Pomeranian University of Technology in Szczecin","correspondingAuthor":false,"prefix":"","firstName":"Tomasz","middleName":"","lastName":"Kędzierski","suffix":""},{"id":444496738,"identity":"a1c3ff92-488a-4b7c-b69b-2d8630ae39db","order_by":3,"name":"Zhang Bowen","email":"","orcid":"","institution":"Shandong Agricultural University","correspondingAuthor":false,"prefix":"","firstName":"Zhang","middleName":"","lastName":"Bowen","suffix":""},{"id":444496742,"identity":"61be8da2-9691-4666-8a97-8319bc67af8e","order_by":4,"name":"Liu Xiaoguang","email":"","orcid":"","institution":"Shandong Agricultural University","correspondingAuthor":false,"prefix":"","firstName":"Liu","middleName":"","lastName":"Xiaoguang","suffix":""},{"id":444496744,"identity":"7ede14fb-e720-4a2e-8fb9-d23944d0b42c","order_by":5,"name":"Ewa Mijowska","email":"","orcid":"","institution":"West Pomeranian University of Technology in Szczecin","correspondingAuthor":false,"prefix":"","firstName":"Ewa","middleName":"","lastName":"Mijowska","suffix":""},{"id":444496748,"identity":"51ce825a-dcb3-4366-958b-d4f9e2e02fc9","order_by":6,"name":"Beata Zielińska","email":"","orcid":"","institution":"West Pomeranian University of Technology in Szczecin","correspondingAuthor":false,"prefix":"","firstName":"Beata","middleName":"","lastName":"Zielińska","suffix":""}],"badges":[],"createdAt":"2025-01-20 13:23:04","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5866238/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5866238/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s42114-025-01342-w","type":"published","date":"2025-06-08T15:57:36+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":80914195,"identity":"c438d7c0-9bdf-479c-8791-89ca5a6cab3c","added_by":"auto","created_at":"2025-04-18 17:11:14","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":676412,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e(a) Schematic illustration of the fabrication of C/N@TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2 \u003c/em\u003e\u003c/sub\u003e\u003cem\u003efor photocatalytic hydrogen production. SEM of (b) TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e_MXene, and (c) C/N@TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e. SEM-EDX of (b’) TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e_MXene, and (c’) C/N@TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e. TEM of (d) TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e_MXene. HAADF image of (e) TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e_MXene with displayed elements: (e’) Ti, (e’’) O, (e’’’) Al, and (e’’’) C. TEM of (f) C/N@TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e. (g) HRTEM image of C/N@TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e. HAADF image of (h) C/N@TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e with displayed elements: (h’) Ti, (h’’) O, (h’’’) N, (h’’’) Al, and (h’’’’’) C. (i) XRD patterns of gCN, TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e_MXene, and C/N@TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-5866238/v1/4f937e05881d4033e9dd668e.png"},{"id":80914170,"identity":"212716b3-0397-4ea9-a509-41da3accadb7","added_by":"auto","created_at":"2025-04-18 17:11:13","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":207533,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e(a) UV-Vis spectra with (b) corresponding Kubelka-Munk function, (c) Mott-Schottky plots with determined flat bands potentials, (d) schematic energy band diagram, (e) PL, (f) CA, and (g) EIS of gCN, TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e_MXene, and C/N@TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e. (h) Hydrogen evolution from water splitting catalyzed by gCN, TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e_MXene, and C/N@TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-5866238/v1/a006446e4b4398902da2e980.png"},{"id":80914173,"identity":"e4dab94f-e006-45b4-90bf-ae2ff6245b9e","added_by":"auto","created_at":"2025-04-18 17:11:13","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":601941,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e(a) Stability test over C/N@TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e. Comparison of C/N@TiO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2 \u003c/em\u003e\u003c/sub\u003e\u003cem\u003ebefore and after stability test using (b) SEM, (c) TEM, (d) HAADF, (e) XRD, (f) nitrogen adsorption/desorption isotherms, (g) pore size distribution, (h) UV-Vis spectra with (i) corresponding Kubelka-Munk function, (j) PL, (k) CA, and (l) EIS.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-5866238/v1/e17dac6565c2c79c34fae16b.png"},{"id":80914179,"identity":"f6adff55-da2a-4323-ac70-28dc91c83e07","added_by":"auto","created_at":"2025-04-18 17:11:13","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":442999,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eCharge density difference map (isovalue = 0.001). Cyan: accumulation. Yellow: depletion.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-5866238/v1/5e9f74c756d93956eccad568.png"},{"id":84242613,"identity":"5ed7e007-6b65-4113-bcca-649885641b8a","added_by":"auto","created_at":"2025-06-09 16:10:16","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2716391,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5866238/v1/8cb11b79-1222-4fae-8918-57cc6821dbdd.pdf"},{"id":80914166,"identity":"cbba2f33-000e-491b-9161-2d15f5903398","added_by":"auto","created_at":"2025-04-18 17:11:13","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":923008,"visible":true,"origin":"","legend":"","description":"","filename":"gcnmxeneSIfinalrevised.docx","url":"https://assets-eu.researchsquare.com/files/rs-5866238/v1/5c200af994a82e080c1396b1.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003eMXene-derived TiO­\u003csub\u003e2\u003c/sub\u003e nanoparticles coated with C/N shell for photocatalytic hydrogen generation under solar light\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe rapid advancement of modern industry and urbanization has undoubtedly brought immense benefits to global society. However, these developments have also given rise to two pressing challenges: energy shortages and environmental degradation. Tackling these issues is crucial to achieving long-term sustainable growth and preserving the health of our planet. Among the various strategies proposed, photocatalysis has emerged as a promising and innovative approach due to its ability to utilize solar energy for eco-friendly applications, including pollutant degradation and renewable energy generation. By effectively converting solar energy into chemical energy, photocatalytic materials offer a dual benefit \u0026ndash; enabling clean energy production, such as hydrogen generation, while simultaneously addressing environmental remediation [1]. This versatile technology holds significant potential to contribute to a more sustainable and energy-secure future.\u003c/p\u003e \u003cp\u003eTitanium dioxide (TiO\u003csub\u003e2\u003c/sub\u003e), one of the most extensively studied photocatalysts, has attracted significant attention since its groundbreaking application in water splitting by Fujishima and Honda in 1972 [2]. TiO\u003csub\u003e2\u003c/sub\u003e is particularly appealing for photocatalytic applications due to its excellent stability, low cost, and environmentally friendly nature [1]. However, its practical performance is limited by its wide bandgap of 3.2 eV, which causes its light absorption predominantly to the ultraviolet region, with only\u0026thinsp;~\u0026thinsp;5% of the solar spectrum [3]. Furthermore, the rapid recombination of photogenerated electron-hole pairs significantly reduces its photocatalytic efficiency, especially under visible light irradiation [1, 3]. To address these challenges, extensive research efforts have been dedicated to improving the photocatalytic performance of TiO\u003csub\u003e2\u003c/sub\u003e-based materials. Key strategies include advanced nanomaterial synthesis [4, 5], surface modifications [5], doping with various elements [3, 5], and the development of heterojunctions to facilitate efficient charge separation [1, 3, 5]. These approaches have collectively paved the way for enhancing the effectiveness of TiO\u003csub\u003e2\u003c/sub\u003e-based photocatalysts in a wide range of photocatalytic applications.\u003c/p\u003e \u003cp\u003eIn recent years, MXenes \u0026ndash; a family of two-dimensional (2D) materials derived from the selective etching of MAX phases \u0026ndash; have garnered significant attention as promising precursors for the fabrication of TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles. MXenes, such as Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003eX\u003c/sub\u003e, exhibit a unique combination of metallic conductivity, tunable surface chemistry, and catalytic properties, making them highly attractive for photocatalytic applications. Through the thermal conversion of Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003eX\u003c/sub\u003e MXenes into TiO\u003csub\u003e2\u003c/sub\u003e, researchers have harnessed the layered and customizable structure characteristics of MXene to develop more efficient photocatalysts [6\u0026ndash;9]. For instance, X. Han et al. [4] reported a synthesis of carbon-doped TiO\u003csub\u003e2\u003c/sub\u003e derived from Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003eX\u003c/sub\u003e MXene integrated with graphitic carbon nitride (gCN). This photocatalyst achieved an exceptional visible-light-driven H\u003csub\u003e2\u003c/sub\u003e production rate of 1409 \u0026micro;mol/h/g. Similarly, Q. Liu et al. [10] fabricated MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e@C/gCN heterojunctions, demonstrating remarkable photocatalytic performance in nitrogen reduction, with an NH\u003csub\u003e3\u003c/sub\u003e production rate of 250 \u0026micro;mol/g/h under visible light. Beyond hydrogen generation and nitrogen reduction, MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e heterostructures have shown great potential in diverse applications, including gas sensing, pollutant removal, and CO\u003csub\u003e2\u003c/sub\u003e reduction [11, 12]. These findings underscore the versatility and promise of MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e in advancing photocatalytic technologies. Thus, the novelty of our work lies in employing the MXene and 2D graphitic carbon nitride as the precursors of a 3D hybrid system based on TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles coated by carbon and nitrogen at high temperature allowing for high crystallinity of the sample. The unique composition of the resulting C/N shell acts as a protective and conductive layer surrounding the accordion-like TiO\u003csub\u003e2\u003c/sub\u003e, effectively facilitating charge separation and transfer. This structure enhances photocatalytic performance by suppressing charge recombination and improving light absorption, ultimately leading to superior photocatalytic activity.\u003c/p\u003e \u003cp\u003eIn this study, we propose a fabrication route of a highly efficient photocatalyst in the form of TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles coated with a carbon/nitrogen (C/N) shell derived from Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003eX\u003c/sub\u003e MXene and graphitic carbon nitride. What is more, the proposed strategy is facile and cost-effective - a one-step annealing process. The photoactivity of the material was verified in the reaction of hydrogen generation under simulated solar light. The optimal sample, C/N@TiO\u003csub\u003e2\u003c/sub\u003e, achieved an extraordinary H\u003csub\u003e2\u003c/sub\u003e production rate of 37.66 mmol/g (37 660 \u0026micro;mol/g), which is ~\u0026thinsp;655 and 37 times higher than those of gCN (57 \u0026micro;mol/g) and TiO\u003csub\u003e2\u003c/sub\u003e derived from pristine MXene (1024 \u0026micro;mol/g), respectively. These results outperform the efficiency of the TiO\u003csub\u003e2\u003c/sub\u003e-based photocatalysts reported in the current state of the art. The remarkable photocatalytic performance of C/N@TiO\u003csub\u003e2\u003c/sub\u003e is attributed to the presence of the carbon/nitrogen (C/N) shell on TiO\u003csub\u003e2\u003c/sub\u003e particles, which promotes effective charge separation, suppresses electron-hole recombination, and enhances visible light absorption. Additionally, DFT calculation revealed that the C/N layer serves as an electron-rich active site, further promoting efficient photocatalytic hydrogen generation. This study provides valuable insights into the design of MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e with the potential to catalyze various photocatalytic reactions.\u003c/p\u003e"},{"header":"Material and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003ePreparation of MXene\u003c/h2\u003e \u003cp\u003eTo synthesize Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003eX\u003c/sub\u003e MXene, 590 mg of Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e MAX phase (American Elements) was slowly and carefully added to 10 mL of a 40% hydrofluoric acid (HF) solution (Sigma-Aldrich). The mixture was stirred continuously at room temperature for 24 hours to allow for selective etching of aluminum from the Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e structure. After the etching process, the mixture was diluted with deionized water. The solid Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003eX\u003c/sub\u003e MXene product was separated from the solution by centrifugation. The precipitate was subjected to multiple wash cycles with deionized water and ethanol until the pH of the supernatant was neutral. The purified MXene was dried in a vacuum oven at 50 ℃ overnight and stored at 5 ℃ to prevent oxidation and ensure long-term stability.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003ePreparation of TiO_MXene\u003c/h3\u003e\n\u003cp\u003eThe as-synthesized MXene was further annealed under ambient air conditions at two different temperatures: 550 or 600 ℃, for 4 hours to obtain reference samples for photocatalytic hydrogen evolution reactions. The resulting samples were labeled as TiO\u003csub\u003e2\u003c/sub\u003e_MXene_550 and TiO\u003csub\u003e2\u003c/sub\u003e_MXene_600, respectively. For clarity and simplicity, the TiO\u003csub\u003e2\u003c/sub\u003e_MXene_600 is referred to as \u003cb\u003eTiO\u003c/b\u003e\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e\u003cb\u003e_MXene\u003c/b\u003e through the main manuscript.\u003c/p\u003e\n\u003ch3\u003ePreparation of graphitic carbon nitride (gCN)\u003c/h3\u003e\n\u003cp\u003eTo synthesize graphitic carbon nitride (gCN), 8 g of melamine powder (Sigma-Aldrich) was placed in a covered corundum crucible. The crucible was then positioned inside a muffle furnace under ambient air conditions. The heating process was initiated with a temperature ramp rate of 2 ℃min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, gradually increasing to a final temperature of 550 ℃. The material was held at 550 ℃ for 4 hours to allow complete polymerization and condensation of the melamine into gCN. After cooling naturally to room temperature, the resulting yellow powder was collected for further use.\u003c/p\u003e\n\u003ch3\u003ePreparation of C/N@TiO\u003c/h3\u003e\n\u003cp\u003eIn the next step, the previously synthesized MXene and gCN were mixed in different mass ratios using a mortar. After achieving a homogeneous powder blend, the mixture of MXene and gCN was transferred to a crucible and placed in a muffle furnace under ambient air conditions. The temperature was increased at a ramp rate of 2 ℃min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e until reaching a target range of 500\u0026ndash;650 ℃. The mixture was then annealed at the set temperature for 4 hours. Samples were labeled as C/N@TiO\u003csub\u003e2\u003c/sub\u003e_X_T, where X denotes the MXene content, while T represents the annealing temperature. For instance, the sample labeled C/N@TiO\u003csub\u003e2\u003c/sub\u003e_5_600 was prepared by combining 5wt% MXene with 95wt% gCN, followed by mortaring and heating the mixture at 600 ℃ for 4 hours under air conditions in a muffle furnace. For clarity and brevity, the C/N@TiO\u003csub\u003e2\u003c/sub\u003e_5_600 is referred to simply as \u003cb\u003eC/N@TiO\u003c/b\u003e\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e through the main manuscript. The effect of varying the ratio of the components and the specific annealing temperature on the photocatalytic activity for hydrogen evolution from water splitting is described in detail in the \u003cem\u003eSupplementary Material\u003c/em\u003e.\u003c/p\u003e\n\u003ch3\u003eCharacterization\u003c/h3\u003e\n\u003cp\u003eThe morphology of the studied samples was characterized using the Scanning Electron Microscope (SEM, Apreo S2) and the Transmission Electron Microscope (TEM, Spectra 300, Thermo Fisher Scientific). X-ray Powder Diffraction (XRD) patterns were recorded using an Aeris diffractometer (Malvern Panalytical) with CuKα radiation. Raman spectra were acquired using a Renishaw InVia spectrometer with a laser wavelength of 785 nm. The specific surface area was measured through N\u003csub\u003e2\u003c/sub\u003e adsorption/desorption isotherm analysis using a Micromeritics ASAP 2460 instrument. The optical absorption spectra of samples were obtained using a JASCO V-770 UV-vis spectrophotometer in diffuse reflectance mode (DRS), with BaSO\u003csub\u003e4\u003c/sub\u003e serving as a white standard for baseline correction. Mott-Schottky measurements were conducted using a BioLogic VMP-3 potentiostat in a three-electrode configuration. The reactor was equipped with a temperature-controlled bath, maintained at 25 ℃. A mercury sulfate electrode (MSE, HgǀHg\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e in 1 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, RE-2CP) was employed as the reference electrode, while a platinum wire served as the counter electrode. The working electrode was a 5 mm diameter glassy carbon disk in a PEEK polymer case. The electrode surface was prepared by polishing with 0.05 \u0026micro;m alumina slurry, followed by washing with acetone. To prepare the active material for the working material, a dispersion was prepared at a concentration of 2 mg/mL in a solution of isopropanol and water (1:3 by volume) with 0.05% Nafion solution (Sigma-Aldrich) acting as a binder. The measurements were carried out in 0.5 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e. Mott-Schottky analysis was performed via Electrochemical Impedance Spectroscopy (EIS). The measurements were recorded at frequencies of 1, 10, 20, 50, and 100 kHz with an AC signal amplitude of 5 mV, capturing 11 points per decade. The potential was swept from 1.0 to -1.0 V vs MSE in 26 discrete steps. The room temperature photoluminescence (PL) spectra were recorded using a Hitachi F-7000 spectrophotometer with an excitation wavelength of 350 nm, with samples suspended in isopropanol during the measurements. Electrochemical characterization, including Chronoamperometry (CA) and Electrochemical Impedance Spectroscopy (EIS), was conducted using a three-electrode configuration with an Autolab PGSTAT302N potentiostat. CA tests were performed at 0.50 V vs. MSE under 388 nm light excitation, while EIS measurements were carried out at 0.15 V vs. MSE in the dark. The setup consisted of a platinum plate as the counter electrode, an MSE as the reference electrode, and 0.50 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e as the electrolyte. The active material dispersion was prepared at a concentration of 1 mg/mL in a solution of isopropanol and water (1:3 by volume) with 0.05% Nafion solution (Sigma-Aldrich) as a binder. The mixture was sonicated to ensure uniform suspension. Subsequently, 50 \u0026micro;L of the suspension was drop-cast onto fluorine-doped tin oxide (FTO) coated glass (Sigma-Aldrich), serving as the working electrode for electrochemical tests.\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003ePhotocatalytic hydrogen generation\u003c/h2\u003e \u003cp\u003ePhotocatalytic hydrogen evolution experiments were carried out in a three-neck Pyrex glass reactor under an argon (Ar) atmosphere. To prepare the reaction mixture, 10 mg of photocatalyst powder was dispersed in 20 mL of deionized water and 4 mL of triethanolamine (TEOA, Sigma-Aldrich), used as a sacrificial agent. Before each experiment, the reactor system was sealed and purged with Ar gas for 30 minutes to remove any residual oxygen and establish an inert environment. Subsequently, the photocatalytic reaction was initialed by illuminating the reactor with a 150 W Xe lamp fitted with an A.M. 1.5G filter to simulate solar light conditions. During the experiment, the composition of the gas phase was monitored using a gas chromatograph (Young Lin 6500), equipped with a 5 \u0026Aring; molecular sieve capillary column (Merck) and thermal conductivity detector (TCD). For each analysis, a 100 \u0026micro;L gas sample was injected into the chromatograph, and the hydrogen content was determined using a pre-calibrated curve based on known standards. Multiple consecutive reaction cycles were conducted under identical experimental conditions to assess the stability and recyclability of the photocatalyst with the highest efficiency.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eDFT calculations\u003c/h3\u003e\n\u003cp\u003eDensity functional theory calculations with the Perdew-Burke-Ernzerhof (PBE) functional [13] were carried out with Quantum ESPRESSO version 7.2 [14]. Standard solid-state pseudopotentials [15] were used. The kinetic energy cutoff for plane-wave wavefunctions was set to 50 Rydberg, and the kinetic energy cutoff for charge density and potential was set to 450 Rydberg. Gaussian smearing with a width of 0.02 Rydberg for Brillouin-zone integration was applied. Grimme\u0026rsquo;s D3 dispersion correction [16] with Becke-Johnson damping [17] was included. Monkhorst-Pack [18] 4x4x1 K-point grids were used. A vacuum space of 20 \u0026Aring; along the z direction was used between adjacent layers. Charge transfer was quantified by Bader charge analyses [19].\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cp\u003eThe synthesized C/N@TiO\u003csub\u003e2\u003c/sub\u003e_X_T were thoroughly characterized to evaluate their structural, morphological, optical, and electrochemical properties. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea illustrates a schematic overview of the experimental process, highlighting the one-step calcination method employed to fabricate MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e coated with a carbon/nitrogen (C/N) shell. This innovative photocatalyst was carefully designed to optimize its performance for photocatalytic hydrogen production.\u003c/p\u003e \u003cp\u003eThe morphology of graphitic carbon nitride (gCN), MXene annealed at 600 ℃ in air (TiO\u003csub\u003e2\u003c/sub\u003e_MXene) and C/N@TiO\u003csub\u003e2\u003c/sub\u003e was systematically investigated using Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM), as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb-g, and \u003cb\u003eFig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/b\u003e. Graphitic carbon nitride (gCN) (\u003cb\u003eFig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/b\u003e) exhibits a characteristic flat, two-dimensional (2D) plate-like structure composed of smooth-surfaced, multilayered nanosheets. The interconnected carbon and nitrogen networks demonstrate a disordered arrangement over long distances, consistent with its amorphous nature [20]. TiO\u003csub\u003e2\u003c/sub\u003e_MXene (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) features a layered, accordion-like structure with distinct interlayer spacing, which is characteristic of MXene materials [21]. Notably, the lamellar structure remains well-preserved even after annealing (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). The morphology of C/N@TiO\u003csub\u003e2\u003c/sub\u003e reflects the retained accordion-like structure of MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e with additional modifications attributed to the incorporation of carbon and nitrogen on its surface. These structural changes of C/N@TiO\u003csub\u003e2\u003c/sub\u003e suggest that gCN may act as a source of the coating of the nanoparticles, interacting with the MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e surface during the annealing process. Energy-dispersive X-ray Spectroscopy (EDX) analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec\u003cb\u003e\u0026rsquo;\u003c/b\u003e) further supports this conclusion, revealing an enriched carbon and nitrogen content on the surface of C/N@TiO\u003csub\u003e2\u003c/sub\u003e. TEM imaging provides additional insights into the material structure. TiO\u003csub\u003e2\u003c/sub\u003e_MXene (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed) and C/N@TiO\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef) both exhibit irregularly shaped nanoparticles with well-defined crystallinity. A High-resolution TEM (HRTEM) image of C/N@TiO\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eg) reveals lattice fringes indicative of the high crystallinity of MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e. The observed lattice spacing of 3.51 \u0026Aring; corresponds to the (101) plane of the tetragonal anatase phase of TiO\u003csub\u003e2\u003c/sub\u003e (ICDD No. 01-073-1764). Additional lattice spacings of 2.16, 2.34, 2.55, and 3.57 \u0026Aring; are assigned to the (006), (112), (104), and (201) planes of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (ICDD No. 04-008-4095 and 04-005-4213), respectively. TEM-EDX elemental mapping (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eh) provides further evidence of the successful fabrication of C/N@TiO\u003csub\u003e2\u003c/sub\u003e, showing the formation of a visually discernible carbon/nitrogen (C/N) shell coating the surface of MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e. The coating is expected to enhance photocatalytic performance by improving charge separation and surface reactivity.\u003c/p\u003e \u003cp\u003eThe XRD patterns of gCN, TiO\u003csub\u003e2\u003c/sub\u003e_MXene, and C/N@TiO\u003csub\u003e2\u003c/sub\u003e are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ei. The graphitic carbon nitride (gCN) exhibits two characteristic diffraction peaks at 13.1 and 27.7\u0026deg;, corresponding to the (001) and (002) planes, respectively, as indexed by ICDD No. 00-066-0813. For the TiO\u003csub\u003e2\u003c/sub\u003e_MXene and C/N@TiO\u003csub\u003e2\u003c/sub\u003e samples, distinct diffraction peaks at 25.4, 37.1, 37.9, 38.7, 48.2, 54.1, 55.2 62,3, 62.9, 68.9, 70.5, 74.3, 75.3, and 76.2\u0026deg; are observed. These peaks correspond to the (101), (103), (004), (112), (200), (105), (211), (213), (204), (116), (220), (107), (215), and (301) planes of tetragonal anatase phase of TiO\u003csub\u003e2\u003c/sub\u003e (ICDD No. 01-073-1764), confirming the complete transformation of MXene into the anatase phase during calcination [4]. This observation demonstrates the effective oxidation of the Ti atoms in Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003eX\u003c/sub\u003e at high temperatures [10]. Additionally, several peaks with low intensity corresponding to Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e are identified in the XRD patterns of TiO\u003csub\u003e2\u003c/sub\u003e_MXene and C/N@TiO\u003csub\u003e2\u003c/sub\u003e. These include peaks at 31.2, 31.6, 32.8, 36.7, 40.1, and 44.8\u0026deg;, which correspond to the (400), (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\stackrel{-}{4}\\)\u003c/span\u003e\u003c/span\u003e01), (002), (111), (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\stackrel{-}{3}11\\)\u003c/span\u003e\u003c/span\u003e), and (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\stackrel{-}{1}12\\)\u003c/span\u003e\u003c/span\u003e) planes of monoclinic Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (ICDD No. 04-008-4095). Additional peaks at 35.2, 43.4, 52.6, 57.6, 66.6, and 68.3\u0026deg; are attributed to the (104), (113), (024), (116), (214), and (300) planes of hexagonal Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (ICDD No. 04-005-4213). The presence of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e in TiO\u003csub\u003e2\u003c/sub\u003e_MXene is caused by the incomplete etching of aluminum from the Ti\u003csub\u003e3\u003c/sub\u003eAlC\u003csub\u003e2\u003c/sub\u003e MAX phase. Interestingly, the C/N@TiO\u003csub\u003e2\u003c/sub\u003e sample does not exhibit any discernible peaks corresponding to graphitic carbon nitride, suggesting that the material predominantly consists of TiO\u003csub\u003e2\u003c/sub\u003e and a small amount of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phases. However, an additional peak at 14.3\u0026deg;, was observed and attributed to (002) planes of carbon (ICDD No. 04-013-3397) formed during gCN decomposition.\u003c/p\u003e \u003cp\u003eThe Raman spectra of gCN, TiO\u003csub\u003e2\u003c/sub\u003e_MXene, and C/N@TiO\u003csub\u003e2\u003c/sub\u003e are shown in \u003cb\u003eFig. S2a\u003c/b\u003e. The spectrum of gCN displays several characteristic peaks at 485, 706, 749, 1232, 1480, 1556, and 1615 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which are attributed to the vibration modes of graphitic carbon nitride. Specifically, the band at 1615 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is attributed to C\u0026thinsp;=\u0026thinsp;N stretching vibration and is commonly referred to as the graphitic (G) band, indicative of sp\u003csup\u003e2\u003c/sup\u003e-hybridized carbon structure. The peaks at 1556 and 1480 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e correspond to symmetric N-C-N stretching and asymmetric C\u0026thinsp;=\u0026thinsp;N stretching, respectively. The band at 1232 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is attributed to the in-plane bending of sp\u003csup\u003e2\u003c/sup\u003e-hybridized\u0026thinsp;=\u0026thinsp;C-N groups within the heptazine or triazine framework [22]. In the TiO\u003csub\u003e2\u003c/sub\u003e_MXene and C/N@TiO\u003csub\u003e2\u003c/sub\u003e samples, five prominent Raman bands are observed at 143, 195, 395, 514, and 637 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, corresponding to the E\u003csub\u003eg\u003c/sub\u003e, E\u003csub\u003eg\u003c/sub\u003e, B\u003csub\u003e1g\u003c/sub\u003e, A\u003csub\u003e1g,\u003c/sub\u003e and E\u003csub\u003eg\u003c/sub\u003e vibrational modes characteristic of the anatase phase of TiO\u003csub\u003e2\u003c/sub\u003e, respectively [10, 23]. Notably, the Raman spectrum of C/N@TiO\u003csub\u003e2\u003c/sub\u003e exhibits an additional feature in the 600\u0026ndash;1300 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e region, highlighted in the insert. These bands are absent in the TiO\u003csub\u003e2\u003c/sub\u003e_MXene but present in both gCN and C/N@TiO\u003csub\u003e2\u003c/sub\u003e and are attributed to C-N stretching and ring-breathing vibrations of triazine/heptazine units [10]. Finally, the results obtained from SEM-EDX, TEM-EDX, XRD and Raman confirm the successful deposition of carbon and nitrogen species on the C/N@TiO\u003csub\u003e2\u003c/sub\u003e surface.\u003c/p\u003e \u003cp\u003eThe nitrogen adsorption-desorption isotherms of gCN, TiO\u003csub\u003e2\u003c/sub\u003e_MXene, and C/N@TiO\u003csub\u003e2\u003c/sub\u003e are presented in \u003cb\u003eFig. S2b\u003c/b\u003e. All studied materials exhibit a type IV isotherm with an H3-shaped hysteresis loop. The specific surface areas, determined via Brunauer-Emmett-Teller (BET) analysis, were found to be 12.73 m\u003csup\u003e2\u003c/sup\u003e/g for gCN, 19.78 m\u003csup\u003e2\u003c/sup\u003e/g for TiO\u003csub\u003e2\u003c/sub\u003e_MXene, and 25.26 m\u003csup\u003e2\u003c/sup\u003e/g for C/N@TiO\u003csub\u003e2\u003c/sub\u003e. The total pore volumes were measured as 5.41, 8.12, and 10.34 mm\u003csup\u003e3\u003c/sup\u003e/g for gCN, TiO\u003csub\u003e2\u003c/sub\u003e_MXene, and C/N@TiO\u003csub\u003e2\u003c/sub\u003e, respectively. An increased total pore volume is advantageous for designing effective photocatalysts due to several factors, including increased surface area, and improved mass transport [20, 22]. According to the Density Functional Theory (DFT) spectra shown in \u003cb\u003eFig. S2c\u003c/b\u003e, all synthesized materials exhibit microporous structures, with median pore widths of 10.71, 12.33, and 11.40 \u0026Aring; for gCN, TiO\u003csub\u003e2\u003c/sub\u003e_MXene, and C/N@TiO\u003csub\u003e2\u003c/sub\u003e, correspondingly. Notably, C/N@TiO\u003csub\u003e2\u003c/sub\u003e displays higher pore volume and larger pore width than its counterparts, particularly in the 25\u0026ndash;50 \u0026Aring; range. This characteristic enhances the diffusion of reactants and products, improving catalytic efficiency by providing greater surface accessibility and enabling better light penetration during the photocatalytic hydrogen evolution process.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe optical absorption properties of gCN, TiO\u003csub\u003e2\u003c/sub\u003e_MXene, and C/N@TiO\u003csub\u003e2\u003c/sub\u003e were investigated using UV-Vis spectroscopy, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea. All the studied catalysts exhibit absorption within the solar light spectrum. The pristine gCN shows an absorption edge at about 468 nm, while TiO\u003csub\u003e2\u003c/sub\u003e_MXene and C/N@TiO\u003csub\u003e2\u003c/sub\u003e display strong absorption across a broad spectral range from UV to visible light, with absorption edges at approximately 395 nm [10]. Notably, the C/N@TiO\u003csub\u003e2\u003c/sub\u003e sample exhibits a slightly red-shifted absorption edge compared to TiO\u003csub\u003e2\u003c/sub\u003e_MXene, indicating an enhanced response in the visible light region. To further analyze the optical properties, the corresponding Kubelka-Munk function was employed (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb) to estimate the band gaps of the materials. The calculated band gap energies were 2.65, 3.17, and 3.13 eV for gCN, TiO\u003csub\u003e2\u003c/sub\u003e_MXene, and C/N@TiO\u003csub\u003e2\u003c/sub\u003e, respectively. The slight improvement in visible light absorption for C/N@TiO\u003csub\u003e2\u003c/sub\u003e is attributed to the incorporation of carbon and nitrogen, which enhances its optical performance in the visible region.\u003c/p\u003e \u003cp\u003eFurther insights into the electronic structure of the studied materials were obtained through Mott-Schottky analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec and \u003cb\u003eFig. S3\u003c/b\u003e). The positive slope in the linear regions confirms that all materials are n-type semiconductors [23]. Notably, C/N@TiO\u003csub\u003e2\u003c/sub\u003e exhibits a significantly lower slope compared to gCN and TiO\u003csub\u003e2\u003c/sub\u003e_MXene, indicating a reduced charge carrier density [24]. This characteristic is advantageous for photocatalysis as it reduces electron-hole recombination and facilitates prolonged charge carrier lifetimes. Additionally, the flat band potentials, estimated from the Mott-Schottky plots, were found to be -0.82, -1.40, and \u0026minus;\u0026thinsp;1.63 V for gCN, TiO\u003csub\u003e2\u003c/sub\u003e_MXene, and C/N@TiO\u003csub\u003e2\u003c/sub\u003e, respectively. For an n-type semiconductor, the flat band potential approximates the Conduction Band (CB) edge due to the proximity of the Fermi level to the CB [25]. The notable shift in a flat band potential for C/N@TiO\u003csub\u003e2\u003c/sub\u003e, along with its reduced slope, suggests significant modifications in its electronic structure. These changes are indicative of enhanced charge separation and transfer efficiency, which are crucial for improved photocatalytic performance [23]. The enhanced electronic properties of C/N@TiO\u003csub\u003e2\u003c/sub\u003e can be attributed to the fabrication of an efficient MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e photocatalyst, where the TiO\u003csub\u003e2\u003c/sub\u003e surface is coated with a carbon/nitrogen (C/N) shell. This interaction not only shifts the flat band potential but also optimizes carrier concentration, ultimately contributing to superior photocatalytic activity. The result highlights that the tailored electronic structure of C/N@TiO\u003csub\u003e2\u003c/sub\u003e enhances its ability to drive photocatalytic reactions effectively.\u003c/p\u003e \u003cp\u003eBy integrating data from DRS UV-Vis and Mott-Schottky analyses, the Valance Band (VB) positions were determined to be 1.83, 1.77, and 1.50 V for gCN, TiO\u003csub\u003e2\u003c/sub\u003e_MXene, and C/N@TiO\u003csub\u003e2\u003c/sub\u003e, respectively. The schematic energy band diagram (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed) illustrates that the Conduction Bands (CBs) of all three materials are above the H\u003csup\u003e+\u003c/sup\u003e/H\u003csub\u003e2\u003c/sub\u003e reduction potential (0 V vs. RHE), confirming their feasibility for hydrogen production, while the valance bands (VBs) are below the O\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003eO oxidation potential (1.23 V vs. RHE), enabling the oxidation half-reaction necessary for water splitting [26]. Among the tested materials, C/N@TiO\u003csub\u003e2\u003c/sub\u003e exhibits the most negative CB potential, highlighting its superior reducing ability. This feature is particularly beneficial for enhanced photocatalytic hydrogen production, as it promotes efficient electron transfer to drive the H\u003csup\u003e+\u003c/sup\u003e/H\u003csub\u003e2\u003c/sub\u003e reduction reaction [1]. Furthermore, the band alignment of C/N@TiO\u003csub\u003e2\u003c/sub\u003e is optimized to support effective charge carrier separation and utilization in both hydrogen and oxygen evolution reactions. The unique band structure of C/N@TiO\u003csub\u003e2\u003c/sub\u003e underscores its potential for efficient photocatalysis. This innovative photocatalyst enables simultaneous reduction and oxidation processes, thereby achieving enhanced photocatalytic activity. The combination of favorable CB and VB positions, coupled with the intrinsic properties of the C/N@TiO\u003csub\u003e2\u003c/sub\u003e system, positions it as a promising candidate for high-performance photocatalytic applications.\u003c/p\u003e \u003cp\u003eThe Photoluminescence (PL) spectra of gCN, TiO\u003csub\u003e2\u003c/sub\u003e_MXene, and C/N@TiO\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee) provide crucial insight into the electronic properties and charge carrier dynamics of the materials. The PL spectrum of gCN exhibits a broad emission band in the range of 410\u0026ndash;580 nm, with a prominent peak at 448 nm. This peak is attributed to the radiative recombination of electron-hole pairs within the material. The observed emission arises from the n-π* electronic transition [20]. In contrast, the PL intensity of TiO\u003csub\u003e2\u003c/sub\u003e_MXene is significantly lower, indicating a reduced electron-hole recombination rate due to its modified electronic structure. Notably, the PL intensity of C/N@TiO\u003csub\u003e2\u003c/sub\u003e is further suppressed compared to TiO\u003csub\u003e2\u003c/sub\u003e_MXene and gCN, suggesting the most efficient spatial charge separation among the studied samples [27]. This pronounced reduction in PL intensity demonstrates an enhancement in the electronic properties of C/N@TiO\u003csub\u003e2\u003c/sub\u003e, which is expected to improve photocatalytic performance by minimizing energy losses due to the recombination process.\u003c/p\u003e \u003cp\u003eThe electrochemical properties of gCN, TiO\u003csub\u003e2\u003c/sub\u003e_MXene, and C/N@TiO\u003csub\u003e2\u003c/sub\u003e were evaluated through Chronoamperometry (CA) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef) and Electrochemical Impedance Spectroscopy (EIS) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg). The periodic photocurrent spikes observed for all samples during light on/off cycles demonstrate a rapid and consistent response to light exposure indicating efficient light-driven charge separation and transfer. Among the tested materials, C/N@TiO\u003csub\u003e2\u003c/sub\u003e exhibits the highest photocurrent response, reflecting its significantly enhanced efficiency in separating and migrating photogenerated charges [20]. This improvement is attributed to the successful fabrication of MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e coated with a carbon/nitrogen (C/N) shell. The Nyquist plot further supports this conclusion, as C/N@TiO\u003csub\u003e2\u003c/sub\u003e shows the smallest semicircle diameter compared to the reference materials, confirming its superior ability to separate photogenerated electron-hole pairs and facilitate charge transfer at the solid/liquid interface [20]. This behavior suggests that the incorporation of carbon and nitrogen onto the MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e surface significantly reduces transfer resistance, thereby optimizing photocatalytic performance. The results of the EIS analysis align perfectly with the findings from CA measurements, collectively demonstrating that C/N@TiO\u003csub\u003e2\u003c/sub\u003e exhibits superior charge separation and transfer properties. This can be attributed to its tailored electronic structure and the successful fabrication of MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e. Ultimately, these electrochemical studies, combined with the optical characterization of the materials, underscore the effectiveness of the MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e photocatalysts in enhancing photoinduced charge carrier behavior, making it a promising material for advanced photocatalytic applications.\u003c/p\u003e \u003cp\u003eThe photocatalytic hydrogen generation performance of graphitic carbon nitride (gCN), MXene, TiO\u003csub\u003e2\u003c/sub\u003e_MXene_550, TiO\u003csub\u003e2\u003c/sub\u003e_MXene_600, and C/N@TiO\u003csub\u003e2\u003c/sub\u003e_X_T were systematically evaluated. The fabricated MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e photocatalysts were optimized in two directions: (i) influence of MXene to gCN ratio calcinated in the air at 550 ℃, and (ii) temperature dependence of a 5wt% MXene and 95wt% gCN mixture calcinated in the air at different temperatures ranging from 500\u0026ndash;650 ℃. The results are illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh and \u003cb\u003eFig. S4\u003c/b\u003e. The photocatalytic experiments were conducted under simulated solar light irradiation, using TEOA as a sacrificial agent, and notably, without the addition of platinum as a co-catalyst. Pristine gCN exhibited an H\u003csub\u003e2\u003c/sub\u003e evolution rate of 57.5 \u0026micro;mol/g, while pristine MXene generated 36.9 \u0026micro;mol/g of H\u003csub\u003e2\u003c/sub\u003e, likely due to the superior optical properties of gCN over MXene. Calcination of MXene at 550 ℃ (TiO\u003csub\u003e2\u003c/sub\u003e_MXene_550) and 600 ℃ (TiO\u003csub\u003e2\u003c/sub\u003e_MXene_600 or TiO\u003csub\u003e2\u003c/sub\u003e_MXene) resulted in significantly improved photocatalytic activity, highlighting that the transformation of MXene into MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e is an effective strategy to enhance its efficiency. Initially, the MXene-to-gCN ratio was optimized (\u003cb\u003eFig. S4ab\u003c/b\u003e). It was observed that the increase in H\u003csub\u003e2\u003c/sub\u003e evolution was not linear upon the increase of MXene loading. The highest efficiency was achieved for C/N@TiO\u003csub\u003e2\u003c/sub\u003e_5_550, with an H\u003csub\u003e2\u003c/sub\u003e evolution rate of 6 825 \u0026micro;mol/g. Therefore, the next step involved calcination of the mixture of 5wt% MXene and 95wt% gCN at various temperatures in the range of 500 and 650 ℃ to reveal the optimal photocatalyst (\u003cb\u003eFig. S4cd\u003c/b\u003e). Among all tested samples, C/N@TiO\u003csub\u003e2\u003c/sub\u003e_5_600 (or C/N@TiO\u003csub\u003e2\u003c/sub\u003e) achieved an extraordinary H\u003csub\u003e2\u003c/sub\u003e production rate of 37.66 mmol/g (37 660 \u0026micro;mol/g), which is about 655 times higher than gCN (57 \u0026micro;mol/g), and 37 times higher than TiO\u003csub\u003e2\u003c/sub\u003e_MXene (1024 \u0026micro;mol/g) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh). The remarkable photocatalytic activity of C/N@TiO\u003csub\u003e2\u003c/sub\u003e is attributed to several synergistic factors, including (i) successful fabrication of MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e coated with a carbon/nitrogen (C/N) shell, (ii) increased surface area, (iii) enhanced visible light absorption, (iv) reduced electron-hole recombination, and (v) improved charge separation and migration efficiency. These findings demonstrate the effectiveness of optimizing the MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e photocatalysts to achieve superior photocatalytic hydrogen generation.\u003c/p\u003e \u003cp\u003eThe superior photocatalytic activity of C/N@TiO\u003csub\u003e2\u003c/sub\u003e may also be influenced by the presence of the small amount of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e in the sample resulting from incomplete Al etching from the MAX phase using 40% HF. The positive influence of residual Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e can be attributed to its role as an energy barrier that suppresses the charge recombination, despite its insulating nature (energy band gap of 8.7 eV) [28\u0026ndash;30]. Furthermore, insulators play a crucial role in optimizing the optical path and enhancing light-scattering efficiency, thereby improving photon capture in insulator-based photocatalysts and promoting charge carrier generation. In semiconductor-insulator composites, TiO\u003csub\u003e2\u003c/sub\u003e is often combined with insulators such as Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and SiO\u003csub\u003e2\u003c/sub\u003e. Among these, Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e is widely used as a catalyst support due to its well-structured porosity and high surface area, which prevent catalyst agglomeration at elevated temperatures. Studies have shown that composite insulators optimize the surface structure and light absorption, ultimately improving photocatalytic activity. Both Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and SiO\u003csub\u003e2\u003c/sub\u003e contribute to an increase in the number of catalytic sites by reducing TiO\u003csub\u003e2\u003c/sub\u003e particle agglomeration and enhancing dispersion during calcination [31]. In this term, an additional study of photocatalytic hydrogen evolution over C/N@TiO\u003csub\u003e2\u003c/sub\u003e, in which precursor Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003eX\u003c/sub\u003e MXene was produced via etching with 48% HF, was tested (details provided in \u003cem\u003eSupporting Information\u003c/em\u003e). The results revealed (\u003cb\u003eFig. S5\u003c/b\u003e) that C/N@TiO\u003csub\u003e2\u003c/sub\u003e \u0026ndash; 40% HF exhibited 12-hold higher hydrogen production in comparison to the sample composed of only a trace amount of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (C/N@TiO\u003csub\u003e2\u003c/sub\u003e \u0026ndash; 48% HF).\u003c/p\u003e \u003cp\u003eFurthermore, the photocatalytic performance of C/N@TiO\u003csub\u003e2\u003c/sub\u003e was evaluated against similar photocatalysts reported in the current state of the art (\u003cb\u003eTable \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/b\u003e). Notably, C/N@TiO\u003csub\u003e2\u003c/sub\u003e exhibits one of the highest H\u003csub\u003e2\u003c/sub\u003e production rates under simulated solar light, suppressing most graphitic carbon nitride/TiO\u003csub\u003e2\u003c/sub\u003e-based photocatalysts. However, it is important to consider that direct comparisons of photocatalytic activity can be influenced by variations in reaction conditions, including light source, light intensity, the presence of co-catalysts and sacrificial agents, etc.).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo assess the stability and durability of C/N@TiO\u003csub\u003e2\u003c/sub\u003e, five consecutive photocatalytic cycles were performed, spanning a total of 20 hours (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). The cyclic experiments, conducted under identical reaction conditions, revealed a noticeable decrease in photocatalytic H\u003csub\u003e2\u003c/sub\u003e generation during the second cycle (23.16 mmol/g), followed by stabilization in subsequent cycles. Interestingly, after the 4th cycle, the additional portion of the sacrificial agent (TEOA) was introduced into the reaction system. However, this replenishment did not result in any substantial improvement in photocatalytic efficiency. This suggests that factors beyond the availability of the sacrificial agent, such as changes in electronic, optical, or electrochemical properties of the photocatalyst, might be responsible for the observed decline.\u003c/p\u003e \u003cp\u003ePost-reaction analyses were conducted on the recovered C/N@TiO\u003csub\u003e2\u003c/sub\u003e material, which was filtered, dried, and subjected to a series of characterization techniques, including SEM, TEM, TEM-EDX, XRD, nitrogen adsorption/desorption isotherms, UV-Vis spectroscopy, PL, CA, and EIS. SEM (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb) and TEM (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec) revealed that the morphology of the material remained intact, indicating good structural durability after the photocatalytic reaction. TEM-EDX (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed) mapping further confirmed the preserved microstructure. However, the absence of a carbon layer on the surface of the MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e was observed, suggesting partial removal of carbon during the photocatalytic process. The XRD diffractogram (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee) of the post-reaction material displayed more pronounced peaks compared to the pre-reaction sample, indicating an increase in crystallinity. Nitrogen adsorption/desorption isotherms and pore size distribution analyses (\u003cb\u003eFig.\u0026nbsp;3fg\u003c/b\u003e) revealed an increase in surface area, total pore volume, and median pore width to 43.30 m\u003csup\u003e2\u003c/sup\u003e/g, 16.36 mm\u003csup\u003e3\u003c/sup\u003e/g, and 15.92 \u0026Aring;, respectively, of the material after the reaction. These changes are consistent with the removal of amorphous carbon, which likely exposes the crystalline TiO\u003csub\u003e2\u003c/sub\u003e surface, as corroborated by the microscopy, XRD, and Raman data. The optical properties of C/N@TiO\u003csub\u003e2\u003c/sub\u003e before and after the reaction were evaluated using UV-Vis spectroscopy (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eh), and the corresponding Kubelka-Munk transformation (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ei). The material after the reaction exhibited a higher energy band gap of 3.22 eV, compared to an initial 3.13 eV, which reduces visible light absorption during cycling tests. Moreover, a slight increase in the recombination rate of electron-hole pairs was observed (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ej), contributing to the lower photocatalytic efficiency. Electrochemical measurements (\u003cb\u003eFig.\u0026nbsp;3kl\u003c/b\u003e) demonstrate a reduction in the photocurrent response and an increase in the charge transfer resistance of the material after the reaction, both of which adversely affect the photocatalytic performance. In summary, while C/N@TiO\u003csub\u003e2\u003c/sub\u003e maintains good structural integrity and durability during repeated photocatalytic cycles, the partial removal of the carbon layer, changes in optical properties, and increased recombination of electron-hole pairs to the reduced efficiency in hydrogen generation over time.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eDensity functional calculations were performed to reveal the mechanistic insight for the enhanced photocatalytic performance of C/N@TiO\u003csub\u003e2\u003c/sub\u003e. The charge density difference map (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) reveals charge redistribution upon interaction between the C/N layer and MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e. Broader charge analysis indicates a net charge transfer of 0.11 e⁻ from the TiO\u003csub\u003e2\u003c/sub\u003e layer (bottom) to the C/N layer (top), suggesting electron donation from TiO\u003csub\u003e2\u003c/sub\u003e to C/N. This charge transfer enhances charge separation, which is beneficial for photocatalysis. Furthermore, the calculated density of states (DOS) and band structure show a narrowed band gap for MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e upon the introduction of the C/N layer, favoring electron-hole separation. This is further supported by a charge density difference map, which indicates the electron transfer from TiO\u003csub\u003e2\u003c/sub\u003e to the C/N layer. Consequently, the C/N layer serves as an electron-rich active site on the surface, promoting efficient photocatalytic hydrogen generation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn summary, this study successfully developed an MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e photocatalyst coated with a carbon/nitrogen (C/N) shell using a one-step calcination method. This innovative photocatalyst was meticulously designed and optimized to enhance its photocatalytic hydrogen generation performance. Among all tested samples, C/N@TiO\u003csub\u003e2\u003c/sub\u003e emerged as the most efficient photocatalyst, achieving an extraordinary H\u003csub\u003e2\u003c/sub\u003e production rate of 37.66 mmol/g (37 660 \u0026micro;mol/g), approximately 655 times greater than gCN (57 \u0026micro;mol/g), and 37 times higher than TiO\u003csub\u003e2\u003c/sub\u003e_MXene (1024 \u0026micro;mol/g). The photocatalytic mechanism of C/N@TiO\u003csub\u003e2\u003c/sub\u003e was analyzed in depth: the Tauc plot estimated its band gap at 3.13 eV, while Mott-Schottky analysis confirmed its n-type semiconductor nature with a conduction band position of -1.63 V. The superior photocatalytic activity of C/N@TiO\u003csub\u003e2\u003c/sub\u003e is attributed to several synergistic factors: (i) the successful fabrication of MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e coated with a carbon/nitrogen (C/N) shell, (ii) an increased surface area, (iii) enhanced visible light absorption, (iv) suppression of electron-hole recombination, and (v) improved charge separation and migration efficiency. Additionally, DFT calculation revealed that the C/N layer serves as an electron-rich active site, further promoting efficient photocatalytic hydrogen generation. Overall, this study provides valuable insights into the design of MXene-derived TiO\u003csub\u003e2\u003c/sub\u003e photocatalysts, demonstrating their potential for efficient photocatalytic hydrogen generation under solar light.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e: Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) analyses have been conducted at the Center for Advanced Materials and Manufacturing Process Engineering (CAMMPE).\u003c/p\u003e\n\u003cp\u003eThe Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003eX\u003c/sub\u003e MXenes used in this study were obtained with support from the National Science Center (Poland) through the PRELUDIUM BIS 2 program (Grant No: 2020/39/O/ST5/01340). The raw data will be placed publicly available in an open-access repository.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e: This research received no external funding.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e: The authors declare no competing interests.\u0026nbsp;\u003c/p\u003e\n"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eB. Zhang, B. Sun, F. Liu, T. Gao, G. Zhou, TiO2-based S-scheme photocatalysts for solar energy conversion and environmental remediation, Sci. China Mater., 2024, 67, 424-443, 10.1007/s40843-023-2754-8.\u003c/li\u003e\n\u003cli\u003eA. Fujishima, K. Honda, Electrochemical Photolysis of Water at a Semiconductor Electrode, Nature, 1972, 238, 37-38, 10.1038/238037a0.\u003c/li\u003e\n\u003cli\u003eS. S. Mani, S. Rajendran, T. Mathew, C. S. 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Zheng, Preparation and Photocatalytic Properties of Al2O3\u0026ndash;SiO2\u0026ndash;TiO2 Porous Composite Semiconductor Ceramics, Molecules, 2024, 29, 4391, 10.3390/molecules29184391.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"advanced-composites-and-hybrid-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"achm","sideBox":"Learn more about [Advanced Composites and Hybrid Materials](https://link.springer.com/journal/42114)","snPcode":"42114","submissionUrl":"https://submission.nature.com/new-submission/42114/3","title":"Advanced Composites and Hybrid Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"photocatalytic hydrogen generation, MXene, MXene-derived TiO2 , TiO2, graphitic carbon nitride, mechanism","lastPublishedDoi":"10.21203/rs.3.rs-5866238/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5866238/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003ePhotocatalytic hydrogen production offers a sustainable and innovative solution to address environmental challenges and global energy shortages by leveraging solar energy. Developing highly efficient photocatalysts is pivotal for advancing photocatalysis technology and facilitating its practical applications. In this study, Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003eX\u003c/sub\u003e MXene was used as a precursor of TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles coated with a carbon/nitrogen (C/N) shell for photocatalytic hydrogen generation under simulated solar light. The fabrication strategy was based on a straightforward one-step annealing process. The photoactivity of the sample was optimized through: (1) tuning the ratio of precursors MXene:gCN calcinated in the air at 550 ℃, and (2) controlling the temperature of the annealing process of the sample which indicated the most outstanding hydrogen evolution yield in strategy 1\u0026deg; (MXene:gCN\u0026thinsp;=\u0026thinsp;1:19). The optimized sample, C/N@TiO\u003csub\u003e2\u003c/sub\u003e, demonstrated an exceptional H\u003csub\u003e2\u003c/sub\u003e production rate of 37.66 mmol/g (37 660 \u0026micro;mol/g), approximately 655 times and 37 times higher than those of gCN (57 \u0026micro;mol/g), and TiO\u003csub\u003e2\u003c/sub\u003e derived from pristine MXene (1024 \u0026micro;mol/g), respectively. This remarkable photocatalytic performance is attributed to the formation of a carbon/nitrogen (C/N) shell, which made TiO\u003csub\u003e2\u003c/sub\u003e extraordinarily robust in the experimental conditions, promoting charge separation, suppressing electron-hole recombination, and enhancing visible light absorption. Additionally, Density Functional Theory (DFT) calculations revealed that the C/N layer serves as an electron-rich active site, further promoting efficient photocatalytic hydrogen generation. This study provides a facile and cost-effective pathway to advancing green hydrogen production technologies. The findings underscore the potential of photocatalytic systems for sustainable energy development, paving the way for scalable renewable energy solutions.\u003c/p\u003e","manuscriptTitle":"MXene-derived TiO­2 nanoparticles coated with C/N shell for photocatalytic hydrogen generation under solar light","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-18 17:11:08","doi":"10.21203/rs.3.rs-5866238/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-05-06T16:10:56+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-04-24T16:41:00+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"184085451226445630779610076866091871526","date":"2025-04-23T02:57:13+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"12441489606259279926011309621114486887","date":"2025-04-17T15:43:34+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-04-17T15:39:33+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-04-10T23:29:12+00:00","index":"","fulltext":""},{"type":"submitted","content":"Advanced Composites and Hybrid Materials","date":"2025-04-07T20:30:54+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"advanced-composites-and-hybrid-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"achm","sideBox":"Learn more about [Advanced Composites and Hybrid Materials](https://link.springer.com/journal/42114)","snPcode":"42114","submissionUrl":"https://submission.nature.com/new-submission/42114/3","title":"Advanced Composites and Hybrid Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"c6b90887-4471-42a2-b153-710ebf10e4cc","owner":[],"postedDate":"April 18th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-06-09T16:03:10+00:00","versionOfRecord":{"articleIdentity":"rs-5866238","link":"https://doi.org/10.1007/s42114-025-01342-w","journal":{"identity":"advanced-composites-and-hybrid-materials","isVorOnly":false,"title":"Advanced Composites and Hybrid Materials"},"publishedOn":"2025-06-08 15:57:36","publishedOnDateReadable":"June 8th, 2025"},"versionCreatedAt":"2025-04-18 17:11:08","video":"","vorDoi":"10.1007/s42114-025-01342-w","vorDoiUrl":"https://doi.org/10.1007/s42114-025-01342-w","workflowStages":[]},"version":"v1","identity":"rs-5866238","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5866238","identity":"rs-5866238","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}