Self-assembly synthesis of gallium-doped polymeric carbon nitride/Ti3C2 MXene Schottky junction for efficient photosynthesis of hydrogen peroxide

Full Length Article Self-assembly synthesis of gallium-doped polymeric carbon nitride/Ti 3 C 2 MXene Schottky junction for efficient photosynthesis of hydrogen peroxide Jingbo Ni a , Vittorio Boffa a,* , Xiaoyu Sun b , Teruhisa Ohno b , Lorenzo Sarasino c,d , Maria Cristina Paganini c , Paola Calza c a Department of Chemistry and Bioscience, Aalborg University, Aalborg, Denmark b Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology, 1-1 Sensuicho, Tobata, Kitakyushu 804-8550, Japan c Dipartimento di Chimica, Universit´ a di Torino, Via Pietro Giuria 7, 10125 Torino, Italy d Dipartimento di Chimica, Biologia e Biotecnologia, Universit` a di Perugia, via dell’Elce di sotto 8, 06123 Perugia, Italy ARTICLE INFO Keywords: Photocatalysis H 2 O 2 generation Gallium-doped Schottky-junction Polymeric carbon nitride ABSTRACT Schottky junction has been widely recognized as one promising strategy for advancing photocatalytic processes, where an internal electric field generated by the Schottky barrier ensures the efficient transfer of charge carriers and enhances the activation capabilities of active sites for reactants, thereby improving solar-to-chemicals performance. However, integration of the Schottky junction into superior photocatalytic H 2 O 2 generation remains a formidable challenge. Accordingly, here we present a novel Ga-PCN/Ti 3 C 2 Schottky junction, achieved by introducing gallium ions into tri-s-triazine repeating units of PCN and then decorating it with Ti 3 C 2 MXene through in-situ electrostatic assembly. Systematic analysis revealed that gallium atoms interlinked with nitrogen in the PCN matrix to construct Ga-N coordination sites, while metallic Ti 3 C 2 served as an excellent electron transfer mediator. This configuration simultaneously promoted the separation of photoexcited carriers and shortened charge transport distance from Ga-PCN to Ti 3 C 2 . As a result, an apparent quantum yield of 1.58 % at 400 nm together with a high H 2 O 2 production rate up to 197.6 μ mol⋅g −1 ⋅h −1 under visible light irradiation was achieved. This work highlights the synergistic effect between metal atom doping and Schottky junction construction in engineering carbon nitride/MXene-based nanocomposite catalysts for artificial photosynthesis of H 2 O 2 , providing new insights into the development of advanced high-efficient photocatalysts. 1. Introduction Hydrogen peroxide (H 2 O 2 ), has been widely utilized in various applications in sterilization[1], pharmaceutical[2], and chemical synthesis [3] owing to its intrinsic merits, including the highest active oxygen content (47.1 %w/w) and considerable theoretical output potential (3.0 mega joules (MJ)l −1 )[4]. However, conventional industrial methods for H 2 O 2 production, e.g., the anthraquinone process [5] and electrolysis [6], suffer from numerous drawbacks including severe energy consumption, generation of toxic by-products and complicated operational procedures. Photocatalysis, a straightforward and environmentally friendly technology that directly converts photon energy into usable chemical energy, with the photocatalyst serving as the central conversion medium[7], has become a major focus of research. The typical H 2 O 2 photosynthesis involves either a two-electron oxygen reduction reaction (2 e−ORR) or 2 e−water oxidation reactions (WOR)[8]. However, achieving light-driven WOR is challenging because of the uphill thermodynamics (1.76 V versus NHE)[9], where the synthesized H 2 O 2 tends to decompose under such highly oxidative conditions[9,10]. In contrast, the oxygen reduction reaction (ORR) is feasible with photocatalysts that possess a conduction band energy level more negative than the required ORR potential (0.69 V vs. NHE)[11]. Therefore, selectively promoting ORR presents a promising strategy to enhance H 2 O 2 photosynthesis. Polymeric carbon nitride (PCN), a semiconductor carbon nitride conjugated polymer, is recognized as a promising material for nextgeneration photocatalysts, due to its various advantages[12], such as malleable electronic structure (basic units are triazine ring (C 3 N 3 ) and tri-s-triazine ring/heptazine ring (C 6 N 7 ))[13], excellent chemical and thermal stability (C and N atoms are connected by sp 2 covalent bonds) and a suitable bandgap (hv <2.7 eV) for visible light absorption[12,14]. Despite these strengths, rapid recombination of photogenerated charge carriers and a dearth of active sites for ORR hinder its photocatalytic * Corresponding author. E-mail address: [email protected] (V. Boffa). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc https://doi.org/10.1016/j.apsusc.2025.163373 Received 28 February 2025; Received in revised form 24 April 2025; Accepted 25 April 2025 Applied Surface Science 704 (2025) 163373 Available online 8 May 2025 0169-4332/© 2025 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). efficiency[15]. Therefore, anchoring active sites on photocatalysts to selectively promote ORR presents a promising strategy to enhance H 2 O 2 photosynthesis. Several modification methods have been employed to tackle this issue[16]. For instance, doping the alkali metal (Na or K) altered the electronic structure of PCN by building the “ion bridge” between the perpendicular interlayers, thus promoting electron and mass transfer[17]. In addition, coupling PCN with other semiconductor materials to form a Schottky junction is also an alternative option[18]. Density functional theory (DFT) calculations suggest that a built-in electric field is created at the Schottky junction interface[19,20], which regulates the interfacial barrier, promotes electron transfer, and expands the light absorption range[16,18,21], thereby enhancing the selectivity and reaction rate of the catalytic process. MXenes are a new family member of two-dimensional transition metal carbides, nitrides, and carbonitrides with a general formula M n+1 X n T x (n =1–3), where M represents a transition metal (such as Ti, Cr, Nb, Sc, Mo, etc.), X denotes carbon or nitrogen, T refers to surfaceterminating functional groups (such as –OH, −O, and −F), and x indicates the number of surface groups per formula unit[22,23]. The first reported MXene, Ti 3 C 2 T x , exhibited semimetallic properties as theoretically predicted[23], and has been applied in supercapacitors, rechargeable batteries, and biosensors[24–27], benefiting from their high electrical conductivity, large surface area-to-volume ratio, and the incorporation of redox-active transition metal atoms[24,28]. Recent studies have also demonstrated MXenes’ effectiveness as auxiliary catalysts in enhancing the photocatalytic performance of materials such as TiO 2 [29], black phosphorus (BP)[30], and graphene-based nanosheets [31], which is attributed to MXenes’ lower Fermi energy level and the presence of a Mott-Schottky barrier at the semiconductor–metal interface[29–31], which promotes the separation of photogenerated charge carriers. In this work, we developed a gallium-doped polymeric carbon nitride (Ga-PCN) photocatalyst through blending calcination, followed by coupling with chemical etched Ti 3 C 2 MXene to synthesize a 2D/2D layered Schottky junction composite via electrostatic self-assembly, as illustrated in Fig. 1. Gallium ions present d 10 electron configuration [15,32], contributing to the formation of the conduction band in semiconductor materials through hybrid sp orbitals with large energy band dispersion, leading to high electron mobility and consequently to enhanced photocatalytic efficiency[15]. Single-atom catalysts (SACs) with a d 10 electron configuration can precisely regulate the catalytic reaction pathways and activation energies by adjusting the electronic density of the metal atoms and providing a unique coordination environment[33,34]. The conduction band potential of Ga-PCN moved towards more negative potential in comparison to pure PCN, while the narrowed bandgap extended the range of solar light absorption to an optimized doping ratio, making it ideal for H 2 O 2 photogeneration reactions. Upon coupling Ti 3 C 2 MXene, the resulting composites exhibited exceptional performance compared to individual components. This study investigates the Schottky barrier effect of two-dimensional MXene, in combination with precise band structure modulation via gallium doping, to achieve efficient spatial separation of photogenerated electron-hole pairs. Moreover, interfacial engineering is employed to enhance light absorption and surface redox kinetics, resulting in a novel photocatalytic system that exhibits both high activity and excellent stability for the green synthesis of hydrogen peroxide. 2. Experimental 2.1. Materials and reagents All chemicals were purchased from Sigma-Aldrich, and all reagents were of analytical grade and were used without further purification. The water used in all experiments was purified through a Milli-Q system (Millipore). 2.2. Synthesis of Ga-PCN Depending on the synthesis, a specific amount of gallium nitrate was dissolved in ethanol, followed by dispersing 4 g of melamine into the solution. The mixture was ultrasonicated for 60 min to ensure uniform dispersion. The solvent was then removed using a rotary evaporator under vacuum, the obtained white powder was subjected to calcination at a heating speed of 4 ◦C min −1 from 25 to 560◦C in a tube furnace with a nitrogen atmosphere. The material was maintained at 560 ◦C for 4 h to synthesize gallium ion-dispersed PCN. Pristine PCN was prepared using the same procedure, excluding the addition of the gallium precursor. 2.3. Synthesis of monolayer Ti 3 C 2 MXene The preparation followed a liquid-phase exfoliation method. First, LiF was dispersed in 9 mol L -1 concentrated hydrochloric acid and stirred Fig. 1. Illustration of the fabrication procedure for Ga 10 -PCN/Ti 3 C 2 composites. J. Ni et al. Applied Surface Science 704 (2025) 163373 2 for 10 min. Next, 1 g Ti 3 AlC 2 powder was slowly added to the acid solution to prevent overheating, and the mixture was stirred for 24 h at a 35 ◦C water bath. Afterward, the mixture was centrifuged and filtered with deionized water several times until the pH value of the suspension reached 6. The resulting black solution was ultrasonicated in an ice bath for 30 min and then centrifuged at 3500 r/min for 1 h to collect the upper dark green supernatant containing monolayer Ti 3 C 2 MXene nanosheets. The concentration of Ti 3 C 2 MXene solution was measured as follows: 10 mL of supernatant was transferred to a pre-weighed evaporation dish. After vacuum drying at 40 ◦C overnight, the final weight was recorded to calculate the concentration. 2.4. Synthesis of Ga-PCN/Ti 3 C 2 MXene heterojunction The composites were synthesized using a one-step electrostatic selfassembly method. The Ga-PCN powder and Ti 3 C 2 nanosheets were mixed in an agate mortar and ground continuously for 30 min until the powder turned light green. The mass ratios of Ti 3 C 2 to Ga-PCN were adjusted to 3 %, 5 %, 7 %, and 10 %, respectively. A detailed introduction of composition and naming convention for the experimental catalysts were provided in Table S1. 2.5. Characterization of materials The morphology of the samples was characterized by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The FESEM images were taken with the FEG-SEM TESCAN S9000G instrument and TEM was conducted by using a Titan Cubed Themis G2 300 electron microscope with a voltage of 300 kV. The microscope features a Schottky emitter as the electron source, providing a resolution of 0.7 nm at 15 keV (In-beam SE). The accelerating voltage can be adjusted from 0.2 to 30 keV. The microanalysis system used is OX-FORD −detector Ultim Max −software AZTEC. The elemental composition was analyzed through energy-dispersive X-ray spectroscopy (EDX, Titan 80–300, Philips). Structural features were assessed using powder X-ray diffraction (P-XRD) and Fourier-transform infrared spectroscopy (FT-IR). Thermo Scientific K-Alpha X-ray photoelectron spectrometer (XPS) was used to determine the oxidation states and bonding states of various elements. XPS spectra were obtained through a fixed retarding ratio mode with a bandpass energy of about 20 eV. The operating voltage was set to 12 kV, with Al K α as the X-ray photoelectron source (hv =1486.6 eV). Photoluminescence (PL) spectra were recorded with a fluorescence spectrophotometer (RF-6000plus, Shimadzu) using an excitation wavelength (λ ex ) of 350 nm[35]. Diffuse reflectance spectra (UV–Vis DRS) were measured using a UV–Visible spectrophotometer (UV-3600Plus, Shimadzu), with spectroscopic-grade BaSO 4 as the reference sample. The specific surface area and pore size distribution of the catalysts were determined by nitrogen adsorption–desorption experiments on a Micromeritics ASAP 2460 automated detector, with the samples degassed at 200◦C for 6 h prior to the measurements. Electron paramagnetic resonance (EPR) spintrapping experiments were carried out using an X-band benchtop EPR spectrometer (ADANI’s SPINSCAN X). Typical EPR conditions were as follows: sweep width of 12mT and center field at 338mT. DMPO was employed as spin trapping molecule for hydroxyl radical, superoxide and, together with formate, for hole detection. Ex-situ irradiation experiments to monitor the formation of the radicals were performed using an LED with λ ≥420 nm. The samples were prepared suspending 10 mg of photocatalyst in i) 0.5 mL of DMPO aqueous solution 0.044 M for the hydroxyl radical detection; ii) 0.5 mL of DMPO solution (0.044 M) in acetonitrile (instead of water to avoid competition with hydroxyl radicals) for the superoxide species; iii) 0.5 mL of aqueous solution buffered at basic pH containing DMPO 0.044 M and sodium formate (0.5 M) for holes detection.[36]. Photoelectrochemical testing was conducted on a CHI-660E electrochemical workstation. The prepared samples were used as the working electrode, while an Ag/AgCl electrode and a platinum sheet electrode served as the reference and counter electrodes, respectively. A 0.5 mol⋅L -1 NaSO 4 solution was used as the electrolyte. To prepare the working electrode, ITO glass (3 cm ×1.5 cm) was cut and ultrasonically cleaned sequentially in deionized water, acetone, and ethanol, sequentially. 5 mg of the synthesized catalyst was dissolved in 1 mL of a solvent mixture (water/IPA volume ratio of 3:1). Subsequently, 10 μ L of Nafion solution was added, and the mixture was thoroughly ground to form a slurry, then coated onto the ITO glass (effective area: 2.25 cm 2 ) and dried at 60 ◦C for 20 min. Transient photocurrent measurements were carried out using a 300 W xenon lamp (CEL-PF300-T8) as the light source, turning on and off the lamp at intervals of 20 s with a starting voltage of 0.5 eV[37]. The electrochemical impedance spectroscopy (EIS) measurements were performed with a frequency sweep ranging from 100 kHz to 0.01 Hz[38]. 2.6. Density functional theory calculations (DFT) Spin-polarized first-principles calculations based on the DFT +U method were conducted using the VASP package[39]. Specifically, the Perdew-Burke-Emzerhof (PBE) functional was used in conjunction with the Hubbard U correction (U =4.5 eV) to account for electron–electron exchange correlation interactions. The plane-wave energy cutoff was set at 500 eV, with a force convergence criterion of 0.02 eV/Å[40]. A kpoint grid of 2 ×2 ×1 was employed, and the energy convergence criterion for the geometric structure optimization was set at 1 ×10 -6 eV. 2.7. Photocatalytic reaction towards H 2 O 2 production Twenty milligrams of the catalyst were added to a sealed glass bottle containing a mixture of 36 mL water and 4 mL sacrificial reagent (10 % vol. ethanol, methanol, or triethanolamine). Pure O 2 gas was bubbled into the bottle for 30 min. A 500 W xenon lamp equipped with a cutoff λ >420 nm provides the simulated solar light, while the solution is magnetically stirred for 5 h. At fixed time intervals, 2 mL of the liquid is taken to measure the concentration of H 2 O 2 using the colorimetric method within a PACKTEST kit (WAKH 2 O 2 from Kyoritsu ChemicalCheck Laboratory Corp.). The concentration of H 2 O 2 was determined using a digital PACKTEST spectrometer (model ED723 from GL Sciences Inc.). 3. Results and discussion 3.1. Morphology and structural characterization TEM and SEM were utilized to visualize the microstructure and morphology of the prepared catalysts. Pure PCN exhibited a porous, wrinkled nanosheet-stacked morphology (Fig. 2a), which implies that its surface can offer anchoring sites for immobilizing active metals. The morphological characteristics of Ga-PCN showed no significant difference from PCN (Fig. S1a), indicating the loading of Ga species did not alter the interconnected nanosheet structure of PCN. Elemental mapping confirmed a uniform distribution of C, N, O, and Ga across the surface of the Ga-PCN catalyst (Fig. S1g). Notably, numerous small black dots were observed in the HRTEM images (Fig. S1e), with no additional crystalline phases detected (Fig. S1f), which means that Ga exists as highly dispersed atomic clusters on the carbon nitride support[15]. The surface of the Ti 3 C 2 MXene nanosheets appeared smooth and homogeneous (Fig. S1b), suggesting that the prepared MXene was not oxidized. TEM and HRTEM images of the Ga-PCN/Ti 3 C 2 composite revealed a distinct layered interlinking structure with complex Moir´ e patterns and clear boundary differentiation, indicating the multilayered morphology is a result of stacking multilayered PCN onto Ti 3 C 2 MXene nanosheets through intermolecular Van der Waals forces (Fig. 2d-e)[41], which is consistent with presented SEM photograph (Fig. 2b). The layered nature and atomic ordering of Ga-PCN/Ti 3 C 2 layers were further confirmed by selected area electron diffraction (SAED) pattern (Fig. 2e), where the J. Ni et al. Applied Surface Science 704 (2025) 163373 3 hexagonally-arranged atomic structure can be identified at the outermost layer. The porosity of synthesized catalysts was investigated through lowtemperature N 2 adsorption/desorption measurements. All samples exhibited a type IV isotherm with H 3 -type hysteresis loops in the P/P 0 = 0.5–1.0 region[42], reflecting the mesoporous structure of the catalysts (Fig. S2a). Furthermore, the presented Barrett-Joyner-Halenda (BJH) pore size distribution revealed that the pore sizes were mainly distributed in the 1–10 nm range (Fig. S2b). This nano-scale slit-like internal pore structure is thought to arise primarily from the stacking of nanosheets. However, the BJH Desorption average pore diameter of Ga-PCN/ Ti 3 C 2 MXene decreased down to 11.91 nm compared to that of Ga 10 -PCN (12.61 nm), likely due to the dense stacking of Ti 3 C 2 MXene nanosheets in the composite, which partially covers the surface pores of Ga-PCN. The crystal structure of materials was analyzed by XRD measurement. Both bare PCN and the Ga-PCN samples exhibited two prominent X-ray diffraction peaks at approximately 13.2◦and 27.8◦, assigned to the inter-layer structural packing of the tri-s-triazine rings (100) and interplanar stacking of the aromatic units (002)[33], respectively (Fig. 3a-b). In comparison, the crystal peaks belonging to the Ga-PCN were slightly wider than those of PCN, and the intensity of the diffraction peak at 13.1◦weakened (Fig. 3a), indicating reduced crystallinity and in-plane periodicity in PCN due to the addition of Ga. Besides, the (002) crystal planes in Ga-PCN shifted toward the high-angle direction, suggesting the narrower interlayer spacing after Ga doping, which is consistent with the SEM figures and is favorable for exciton migration between layers. In contrast, the (002) planes of pure Ti 3 C 2 MXene shifted toward the lowangle direction, which is attributed to the exfoliation of stacked sheets via HF etching. The absence of a peak at about 39◦, corresponding to the (104) plane of Ti 3 AlC 2 (Fig. S3a), confirms the complete removal of Al layers in Ti 3 AlC 2 . Notably, the Ga-PCN/Ti 3 C 2 MXene composite retained the characteristic (002) diffraction peaks of Ti 3 C 2 , with peak intensity increasing as the Ti 3 C 2 content increased in the PCN matrix, indicating the successful incorporation of Ti 3 C 2 MXene phase into PCN structure. The slight shift in the (002) diffraction peak position observed in the composite may be attributed to electronic structure adjustments within the catalyst. Steady-state PL emission spectroscopy was utilized to assess the ability of photocatalysts to separate charge carriers. The photoluminescence of semiconductor materials primarily arises from the recombination rate of internal photogenerated electrons and holes[43]. As shown in Fig. S3b, the PL intensity at a typical wavelength of 467 nm remarkably decreased with increasing gallium doping, indicating that the addition of gallium ions enhances electron separation in PCN. However, excessive gallium did not further contribute to reducing the PL intensity. Furthermore, the Ga-PCN/Ti 3 C 2 MXene composites exhibited a lower PL intensity than Ga-PCN, attributed to the superior electrical conductivity of Ti 3 C 2 MXene (Fig. 3c). Despite that, the redundancy Fig. 2. (a) SEM image of PCN and (b) Ga 10 -PCN/Ti 3 C 2 -5%, (c) EDX spectra and corresponding overlay element (inset) of Ga 10 -PCN/Ti 3 C 2 -5%, (d) TEM image, (e) HRTEM image, and (f) SEAD pattern of Ga 10 -PCN/Ti 3 C 2 -5%, (g) Elemental mapping figure of C, Ga, N, O and Ti in Ga 10 -PCN/Ti 3 C 2 -5%, respectively. J. Ni et al. Applied Surface Science 704 (2025) 163373 4 MXene led to the disorderliness and agglomeration of the composites, which negatively impacted charge carrier migration. In addition, the dynamics of photogenerated carriers in excited states of catalyst were investigated by time-resolved PL spectra (Fig. 3d). Compared with the average fluorescence lifetime ( τ Ave ) of 14.15 ns for PCN, Ga 10 -PCN and Ga 10 -PCN/Ti 3 C 2 MXene-5 % showed shorter excited state existence time, with lifetimes of 13.27 and 11.13 ns, respectively (Table S2). This phenomenon can be explained by the Ga-N interaction formed within the PCN system, anchored by gallium single atoms (details on the Ga-N interaction were further discussed in the XPS section). This interaction creates a specific non-radiative electron transfer pathway between the Ga site and the electron-donor carbon nitride. Besides, the gallium single atom site can act as an electron acceptor, thereby enhancing the photocatalytic activity. XPS analysis provided the full region element spectrum of catalysts and the individual element spectra for C 1 s, N 1 s, Ti 2p, and Ga 2p, elucidating the chemical state of the interplay between isolated metal sites and the PCN skeleton, as well as the intricate interactions between Ga-PCN and Ti 3 C 2 MXene. The binding energies of the four emerged photoelectron peaks in the Ga-PCN/Ti 3 C 2 MXene spectrum correspond to C, N, Ti, O, and Ga, respectively (Fig. S4). Among these, the O element signal likely arises from oxygen-containing functional groups on the carbon nitride surface or surface-adsorbed oxygen. The peak located in the C 1 s orbital spectrum of PCN and Ga-PCN around 288.1 eV and 284.6 eV is in accordance with the binding energies of N-C=N and surface adsorbed carbon (C-CO 2 )[44], respectively (Fig. 4a). No apparent new peak appeared, suggesting the metal atom doping preserves the chemical states of carbon in PCN and the interaction between Ga and C is nearly negligible. For the composite, the disappearance of CTi bonds at 282.2 eV associated with MXene, and the increased intensity of C–C bonds compared to Ga-PCN, suggests that the formation of composites is driven by strong C–C covalent bond interactions. The N 1 s diagrams are displayed in Fig. 4b. The two prominent peaks at 398.6 eV and 400.5 eV correspond to N-(C) 3 and C=N-C bonds[45], respectively. In particular, a distinct nitrogen peak potentially associated with the Nmetal bond appeared near 398.3 eV, and its slight shift toward higher binding energy direction after MXene loading may be attributed to the change of local atomic configuration and coordination environment of anchored Ga species. The Ti 2p spectra of single Ti 3 C 2 MXene showed in the Fig. 4c can be divided into six peaks at 455.9 eV, 457.0 eV, 459.5 eV, 461.6 eV, 463.0 eV, and 465.1 eV, corresponding to Ti-C 2p 3/2 , Ti-O 2p 3/ 2 , Ti(III) 2p 1/2, Ti-C 2p 1/2 , Ti-O 2p 1/2 and Ti-O 2p 5/2 [46], respectively. After the formation of the composite, the Ti-C bond positions moved towards lower binding energies (454.5 eV for Ti-C 2p 3/2 and 460.5 eV for Ti-C 2p 1/2 ). This highlights the presence of chemical bonding interaction and the formation of a built-in electric field between the PCN and MXene[18,46], rather than a simple physical-connected compound, the electrons were transferred from Ga-PCN to the MXene surface. In the deconvoluted Ga 2p spectrum, the peak found at 1118.2 eV was identified as the characteristic Ga-N bond and it is worth noting that the MXene did not affect the coordination of the doped Ga atoms (Fig. 4d). Thus, based on prior theoretical studies that the PCN framework can develop two-dimensional planar pores populated with six nitrogen lone electron pairs, it can be inferred that Ga atoms in PCN tend to form a coordinated structure with surrounding N atoms[15,16]. 3.2. Photo-electrochemical properties The study of band structure provides crucial insights into the light absorption range and bandgap energy of the semiconductor materials, allowing for the determination of the relative energy levels of the conduction bands (CB) and valence bands (CB)[44]. This, in turn, helps reveal the influence of Ga single-atom incorporation on the system’s electronic configuration and the delocalization of photogenerated Fig. 3. (a) XRD patterns of PCN and Ga-PCN samples, (b) XRD patterns of Ga 10 -PCN/Ti 3 C 2 MXene-X%, (c) steady-state photoluminescence spectra of Ga 10 -PCN/ Ti 3 C 2 MXene-X%, (d) time-resolved PL spectra of PCN, Ga 10 -PCN and Ga 10 -PCN/Ti 3 C 2 MXene-5% and average fluorescence lifetime (inset). J. Ni et al. Applied Surface Science 704 (2025) 163373 5 charge carriers. UV–Vis DRS spectra given in Fig. 5(a) revealed that all prepared catalysts absorb strongly to the UV-A region (315 nm <λ < 400 nm), Ga atoms doping caused slightly redshift in the absorption edges of Ga-PCN samples compared to the pristine PCN, this probably due to Ga ions altering the intrinsic orbital distribution of the planar C-N network, enhancing the delocalized π -electrons and varying degrees of bandgap transitions. Likewise, the composite showed better light absorption performance owing to the metallic properties of MXene with outstanding harvesting capabilities (Fig. 5b). Bandgap energies were calculated from the tangent of the Kubelka-Munk function to the square root of the photon energy. According to Tauc plot formula[47]: ( α h ν )1/n=B(h ν −Eg)(1) where α is the absorbance, h is the Planck constant (6.62 ×10 –34 J⋅s), and ν is the light frequency, the value of n is taken as 1/2 for PCN-based samples. The observed reduction in the bandgap of Ga-PCN is related to the formation of Ga-N bonds, which also facilitates the rapid transfer of electrons after the catalyst absorbs light energy (Fig. S5a). Furthermore, the incorporation of MXene formed a well-structured conductive network framework, promoting the efficient separation of photogenerated electron-hole pairs, leading to more effective utilization of solar energy (Fig. S5b), and contributing to improved photocatalytic performance. Mott-Schottky curve provides information about the type of semiconductor and identifies the locations of the band potential for the samples. As demonstrated in Fig. 5c-d and Figs. S5a-b, the tangential positive slope observed in all the Mott-Schottky curves represents the typical characteristics of n-type semiconductors for the synthesized PCN-based catalysts. The flat band potential of the Ga-PCN samples measured under the Ag/AgCl electrode was summarized in Figs. S6-S7. For intuitive comparison, all values were further converted to potential relative to the normal hydrogen electrode (NHE)[48]: ENHE =EAg/AgCl +Eθ Ag AgCl (2) Conduction band (ECB) and valence band (EVB) under normal hydrogen electrode (NHE, pH =7) are calculated by[48,49]: ECB =EAg/AgCl −Eθ Ag/AgCl +0.059*pH (3) EVB =ECB +Eg(4) The standard electrode potential Eθ Ag/AgCl at pH 7 is 0.197 V[50]. The unsaturated edge-constrained Ga-CN system demonstrates a more negative conduction band position, indicating that the non-uniform coordination of Ga in the Ga-CN system effectively modulates the electronic configuration of the support, thereby improving the effective separation and transfer of charge carriers. All the catalysts exhibited sufficient reduction potentials to facilitate the generation of H 2 O 2 from oxygen (ORR). Transient photocurrent response and electrochemical impedance spectroscopy (EIS) were applied to evaluate the carrier migration Fig. 4. High-resolution XPS spectra of pristine PCN, Ga 10 -PCN, Ti 3 C 2 MXene and Ga 10 -PCN/Ti 3 C 2 MXene −5%: (a) C 1 s, (b) N 1 s, (c) Ti 2p and (d) Ga 2p. J. Ni et al. Applied Surface Science 704 (2025) 163373 6 efficiency and dynamic electrochemical processes of the samples. As illustrated in Fig. S5c, the transient photocurrent responses of Ga 10 -PCN exhibited a higher photocurrent density compared to pure PCN, indicating that the incorporation of Ga single atoms promotes the separation of photogenerated electron-hole pairs within the catalyst system, consistent with PL and UV–Vis DRS results. Moreover, Ga-PCN demonstrated smaller arc radii than pure-phase PCN in the EIS test (Fig. S5d), suggesting lower impedance in these systems, which leads to higher charge carrier migration efficiency. As a result, more electrons can be effectively involved in the oxygen reduction reaction (ORR). However, an excess of Ga ions does not yield further EIS performance enhancement (Fig. S5d). This suggests that the unsaturated edge-clustered Ga single atoms with heterogeneous coordination patterns, isolated and coexisting on the surface, show a better photoelectrochemical response. MXene functionalized as a co-catalyst, leading to further improvements in photoelectrochemical performance by enhancing the dynamic migration of carriers and improving charge carrier utilization efficiency, ultimately benefiting photocatalytic hydrogen peroxide production. It is noteworthy that the photoelectrochemical properties of the composite system do not increase proportionally with the MXene loading. Despite the highest loading amount in Ga-PCN/Ti 3 C 2 -10 %, it exhibits weaker photocurrent density and larger impedance compared to Ga-PCN/Ti 3 C 2 - 5 % and Ga-PCN (Fig. 5e-f). This finding further confirms that the agglomeration of MXene at the carbon nitride edges is detrimental to the separation and transfer of photogenerated electron-hole pairs. 3.3. Evaluation of photocatalytic H 2 O 2 production H 2 O 2 photosynthesis was carried out in a batch reactor as illustrated in Fig. S8. Initially, the impact of varying gallium ion doping concentrations on photocatalytic H 2 O 2 production was investigated. Pristine PCN showed negligible catalytic activity. Among the Ga-doped PCN samples, both Ga 7 -PCN and Ga 10 -PCN exhibited the highest H 2 O 2 generation efficiency in the solution containing ethanol as a sacrificial agent at pH value 7, reaching approximately 3.5 times that of undoped PCN (Fig. S9). Therefore, the intrinsic catalytic activity of each catalyst was further evaluated by conducting parallel experiments without a sacrificial agent. The concentration of H 2 O 2 produced by Ga 10 -PCN after 5 h was 1.58 times and almost 9 times higher than that of Ga 7 -PCN and pure PCN, respectively (Fig. 6a), indicating that optimal gallium content in Ga-PCN for photocatalytic H 2 O 2 production is 10 mmol of Ga(NO 3 ) 3 in 4 g of PCN). In addition, the composites with MXene contents ranging from 3 %-10 % were prepared and tested the photocatalytic activity under identical conditions. The amounts of H 2 O 2 produced after 5 h of visible light irradiation were given in Fig. 6b. It is evident that the - addition of Ti 3 C 2 MXene remarkably improved the photocatalytic performance compared to that of bare Ga 10 -PCN. Notably, the composite with a 5 % MXene content exhibited the highest H 2 O 2 production (16.6 mg L -1 , which can also be converted to 197.6 μ mol⋅g −1 ⋅h −1 ), However, further increases in MXene content resulted in a gradual decline in H 2 O 2 production efficiency, which might be attributed to the fact that increasing opacity of the co-catalyst (dark green-colored Ti 3 C 2 MXene) obstructs light absorption as well as to the chemical heterogeneity and interfacial effects induced by the coexistence of large-area two-dimensional materials. To investigate the intermediate species involved in photocatalytic H 2 O 2 reactions, a series of radical quenching experiments were performed. Various scavengers, including p-benzoquinone (p-BQ, 1 mM), tButanol (TBA, 1 mM), and EDTA (1 mM) were added to the mixture to selectively scavenge superoxide radicals (⋅O 2 - ), hydroxyl radicals (⋅OH), and electrons (e - ), respectively. As shown in Fig. 6c and Fig. S10, the addition of TBA and EDTA made minimal impact on H 2 O 2 production, indicating that ⋅OH is not the primary reactive species in the reaction. Based on the VB potential of Ga 10 -PCN, it can be inferred that the composite photocatalyst cannot oxidize water to generate H 2 O 2 . However, the addition of p-BQ completely suppressed the H 2 O 2 yield, proving that⋅O 2 - is the inevitable intermediate for H 2 O 2 synthesis. These results demonstrated that photocatalytic H 2 O 2 generation in the system primarily occurs through a sequential two-step single-electron indirect pathway (O2→•O− 2→H2O2)rather than the direct two-electron O 2 reduction route (O2→H2O2), in which the photoexcited electrons are captured by O 2 to form⋅O 2 - as a crucial intermediate. Cycling experiments were carried out under identical conditions to assess the reproducibility and structural stability of catalysts (Fig. 6d). The concentration of H 2 O 2 remained virtually consistent over three cycles, suggesting that the composite catalyst has excellent stability. Fig. 5. (a) UV–Vis DRS spectra of pristine PCN and Ga 10 -PCN, (b) UV–Vis DRS spectra of Ga 10 -PCN/Ti 3 C 2 MXene-X%, (c) Mott-Schottky plots of PCN and (d) Ga 10 - PCN, (e) Transient photocurrent curves of PCN, Ga 10 -PCN, and Ga 10 -PCN/Ti 3 C 2 MXene-X%, (f) EIS plots of pristine PCN, Ga 10 -PCN, and Ga 10 -PCN/Ti 3 C 2 MXene-X%. J. Ni et al. Applied Surface Science 704 (2025) 163373 7 Compared with the H 2 O 2 production ability of previously published PCN-based catalysts, Ga 10 -PCN/Ti 3 C 2 MXene showed higher photocatalytic activity, suggesting that metal ions doping and Schottky junction construction are effective strategies for enhancing the photocatalytic performance of carbon nitride (Table 1). Subsequently, we examined the reaction conditions to optimize the H 2 O 2 production process. The action spectra of H 2 O 2 under different monochromatic light irradiation and their UV–Vis DRS dates were presented in Fig. 7 and Fig. S11 simultaneously. Apparent quantum efficiency (AQY) is a principal parameter for evaluating the light utilization efficiency of photocatalysts[59] and was calculated using various light bandpass filters with wavelengths of 400 ±10 nm, 420 ±10 nm, 450 ±10 nm and 470 ±10 nm[44]. AQY(%) = [NA×h×c] × [nH2O2×2] I×S×t×λ(5) Among these, N A represents Avogadro’s constant, h is the Planck constant, c is the speed of light, nH 2 O 2 refers to the amount of generated Fig. 6. (a) Amounts of H 2 O 2 produced from Ga-PCN without ethanol, (b) Photocatalytic H 2 O 2 production activities of Ga 10 -PCN/Ti 3 C 2 MXene, (c) Effect of different scavengers on the concentration of photocatalytic H 2 O 2 concentration; (d) Photochemical stability of Ga 10 -PCN/Ti 3 C 2 MXene-5%. Table 1 Comparison of photocatalytic H 2 O 2 generation performance with various photocatalysts. Photocatalyst Solvent system Light Source H 2 O 2 production ( μ mol⋅g −1 ⋅h −1 ) Ref. Defective PCN H 2 O: IPA (2:1) λ >420 nm 96 [51] Mesoporous PCN H 2 O: EtOH (9:1) λ >420 nm 83.5 [52] Ti 3 C 2 MXene/ Porous PCN H 2 O: IPA (9:1) λ >400 nm 131.71 [16] TiO 2 /Ti 3 C 2 MXene/Au H 2 O: EtOH (9:1) 360 nm <λ < 380 nm 100 [53] WO 3 /Ti 3 C 2 MXene/Au H 2 O: EtOH (9:1) λ >420 nm 110.5 [54] P-doped PCN H 2 O (100 mL) λ >420 nm 174 [55] PCN/PDI H 2 O (100 mL) λ >400 nm 21.08 [56] Ag@U-PCN-NS H 2 O (100 mL) λ >420 nm 67.5 [57] Honeycomb-likePCN H 2 O: IPA (8:1) λ >420 nm 96.80 [58] Ga-PCN/Ti 3 C 2 MXene H 2 O: EtOH (9:1) λ >420 nm 197.6 This work Fig. 7. Action spectrum and adsorption spectra of PCN, Ga 10 -PCN, and Ga 10 - PCN/Ti 3 C 2 MXene-5 % under irradiation at wavelengths of 400 nm, 420 nm, 450 nm, and 470 nm. J. Ni et al. Applied Surface Science 704 (2025) 163373 8 H 2 O 2 . S is the illuminated area, and I is the light intensity entering the reactor, measured by using an optical power meter. t represents the total irradiation duration, and λ denotes the wavelength of the monochromatic light. As summarized in Table S3, the variation in H 2 O 2 concentration is consistent with the absorption spectra of the catalysts, which means that the catalytic reaction was driven by light excitation. The AQY at 400 nm for Ga 10 -PCN/Ti 3 C 2 MXene-5 % composite reached up to 1.58 %, almost 2.89 and 1.40 times higher than that of PCN and Ga 10 -PCN, respectively. In particular, the photocatalytic activity persisted even when the wavelength was expanded to 470 nm, demonstrating excellent visible-light responsiveness. The reaction conditions were systematically investigated to optimize the photocatalytic H 2 O 2 generation using the Ga 10 -PCN/Ti 3 C 2 MXene-5 % composite. Ethanol was selected hole sacrificial agent in Fig. S12a. Keeping the same volume, the catalytic reaction with 10 % ethanol as the sacrificial agent showed the best performance, with H 2 O 2 yields 1.31 times and 6 times higher than those of the methanol and water systems, respectively. In contrast, the existence of triethanolamine (TAA) resulted in severe inhibition consequences that the yielded H 2 O 2 concentration was lower than 0.5 mg L -1 . This finding can be explained by the more negative redox potential of ethanol (−0.197 V versus NHE) compared to TAA (0.656 V vs. NHE), which means ethanol more effectively consumes holes at a rate, thereby preserving photogenerated electrons for participation in the reaction[15]. Moreover, the significant alkaline nature of TAA in aqueous solutions may lead to proton quenching, disrupting the acid-based balance. To further explore this, the effect of pH on photocatalytic H 2 O 2 production was studied and the experimental results were presented in Fig. S12b. When the solution was acidified with hydrochloric acid, H 2 O 2 generation significantly increased at pH 3 compared to. However, when sodium hydroxide was added, causing the pH to rise from 7 to 9, the H 2 O 2 concentration decreased sharply. This is due to the self-decomposition of H 2 O 2 in alkaline solutions, where it is difficult to maintain stability. Considering that, lower pH conditions are favorable for enhancing the H 2 O 2 production efficiency. Additionally, the effect of different gas atmospheres (oxygen, nitrogen and compressed air) bubbled into the solution before light irradiation was investigated to verify the formation pathway of H 2 O 2 . It was observed that the concentration of generated H 2 O 2 in air is only about half that in pure oxygen (Fig. S12c), while negligible amounts were produced in a nitrogen atmosphere. These results indicate that high-purity oxygen gas is essential for the photocatalytic H 2 O 2 generation process, which primarily occurs through the oxygen reduction reaction (ORR). The final concentration of produced H 2 O 2 is determined by the interplay between its formation and decomposition rates on the catalyst surface, reflecting a dynamic equilibrium process. To better understand the underlying principles from a kinetic perspective, appropriate kinetics models were utilized to describe the behavior of H 2 O 2 in-situ formation and decomposition. Specifically, the formation rate (K f , μ mol⋅g −1 ⋅min −1 ) adheres to zero-order kinetics, while the decomposition rate (K d , min −1 ) follows first-order kinetics[53,60]. The change in H 2 O 2 concentration as a function of time (t) can be summarized as follows[53]: [H2O2] = Kf/Kd(1−exp( − Kd τ ) ) (6) Comparing the resulting K f and K d values shown in Fig. 8, it is evident that the H 2 O 2 generation rate constant of the Ga 10 -PCN material was 3.48 times higher than that of the original PCN. Likewise, the generation rate further improved with the incorporation of MXene, proving the synergistic catalytic effects of MXene as a co-catalyst and its crucial role in the junction. However, the H 2 O 2 decomposition rate constants also increased accordingly. This phenomenon may be associated with the secondary separation of photogenerated carriers, which minimizes their interaction with the generated H 2 O 2 to a certain extent, thereby consequently accelerating decomposition. Furthermore, it is noteworthy that the increase in H 2 O 2 generation rate was even more pronounced at a pH of 3, while the decomposition rate constant decreased compared to neutral conditions. This suggests that the abundant H + ions in an acidic solution play a crucial role as essential reactants in the H 2 O 2 production process, favoring the forward reaction while simultaneously suppressing its decomposition, contributing to higher overall yields. Detailed evidence supporting this observation will be discussed in the subsequent section. 3.4. Photocatalytic mechanism In the existence of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a radical trapper, EPR spectroscopy was employed to monitor the reactive redox species generated during the photocatalytic reaction, providing more evidence for the H 2 O 2 generation pathway. As demonstrated in Fig. 9a, four distinct DMPO-O 2 •- signals appeared in the EPR spectrum of Ga 10 -PCN/Ti 3 C 2 -5 % after 5 min of visible light irradiation with the intensity gradually increasing over time. In contrast, no evident DMPOOH • or DMPO-h + signal can be observed after 30 min of irradiation, as shown in Fig. 7b-c. These results suggest that O 2 was reduced by photogenerated electrons to form O 2 •- due to their enough reduction potential, confirming that O 2 •- is a crucial and indispensable intermediate in photocatalytic H 2 O 2 production, which is in good accordance with the results of previous sacrificial agent experiment (Fig. 6c). Moreover, the signal intensity of O 2 •- of Ti 3 C 2 -decorated Ga 10 -PCN was about three times stronger than that of Ga 10 -PCN and pure PCN (Fig. 9d), demonstrating that the H 2 O 2 generation process is photoinduced, and coupling of Ti 3 C 2 MXene efficiently promoted Ga-PCN to produce more O 2 •- under light irradiation, leading to superior photocatalytic activity. Interestingly, DMPO-h + signals were only detectable in the EPR spectrum of Ga 10 -PCN, while not in those of PCN or Ga10-PCN/Ti 3 C 2 -5 % (Fig. 9f). This suggests that the incorporation of Ga atoms accelerated the generation of charge carriers, thus leading to an accumulation of h + in the VB of Ga 10 -PCN. Reported studies have confirmed that H 2 O 2 is an efficient hole scavenger[61] (h++H2O2→OH⋅+H+). However, the absence of a DMPOOH • signal in the EPR spectrum of Ga 10 -PCN revealed that no OH • is formed because of the weak oxidation potential of holes (Fig. 9e), further demonstrating that the generation of H 2 O 2 is a sequential ORR process rather than WOR reaction. Besides, the intensity of O 2 •- signals in the EPR spectrum of both PCN and Ga 10 -PCN decreased after 15 min of irradiation (Fig. S13), suggesting a reduction in O 2 •- concentration. Conversely, the O 2 •- signals exhibited by Ga 10 -PCN/Ti 3 C 2 - 5 % remained stable over 30 min, indicating improved catalytic stability Fig. 8. Formation rate constant (K f ) and decomposition rate constant (K d ) for H 2 O 2 production. J. Ni et al. Applied Surface Science 704 (2025) 163373 9