Interfacial Engineering of CN-B/Ti₃C₂ MXene Heterojunction for Synergistic Solar-Driven CO₂ Reduction

Abstract

Photocatalytic CO₂ reduction holds great potential for sustainable solar fuel production, yet its practical application is often limited by inefficient charge separation and poor product selectivity. The photothermal effect presents a viable strategy to address these challenges by reducing activation energies and accelerating reaction kinetics. In this work, we report a rationally designed CN-B/Ti₃C₂ heterojunction that effectively leverages photothermal promotion for enhanced CO₂ reduction. The black carbon nitride (CN-B) framework, synthesized via a one-step calcination of urea and Phloxine B, exhibits outstanding photothermal conversion, reaching 131.4 °C under 300 mW cm⁻² illumination, which facilitates CO₂ adsorption and charge separation. Coupled with Ti₃C₂ MXene, the optimized composite (3:1) achieves remarkable CO and CH₄ production rates of 80.21 and 35.13 μmol g⁻¹ h⁻¹, respectively, without any cocatalyst—representing a 2.9-fold and 8.8-fold enhancement over CN-B and g-C₃N₄ in CO yield. Mechanistic studies reveal that the improved performance stems from synergistic effects: a built-in electric field prolongs charge carrier lifetime (3.15 ns) and reduces interfacial resistance, while localized heating under full-spectrum light further promotes CO₂ activation. In situ Fourier transform infrared (FTIR) spectroscopy confirms the accelerated formation of key intermediates (*COOH and *CO). The catalyst also maintains excellent stability over 24 h. This study demonstrates the promise of combining photothermal effects with heterojunction engineering for efficient and durable CO₂ photoreduction.

1. Introduction

The escalating atmospheric CO₂ concentration has precipitated a global climate crisis, necessitating urgent innovations in solar-driven carbon capture and conversion technologies [1,2,3]. Photocatalytic CO₂ reduction represents a promising route to mitigate emissions while advancing renewable energy economies, yet conventional semiconductors face intrinsic limitations in solar spectrum utilization (limited visible-to-infrared photon utilization efficiency), rapid charge recombination, and sluggish surface reaction kinetics, restricting their practical deployment [4,5,6]. Graphitic carbon nitride (g-C₃N₄), a visible-light-active semiconductor with a tunable bandgap (~2.7 eV), has garnered significant attention due to its low cost, non-toxicity, and robustness. Nevertheless, pristine g-C₃N₄ suffers from inherent limitations, including a narrow light absorption edge (~450 nm), high exciton binding energy, and weak CO₂ adsorption, leading to low quantum efficiency and poor product selectivity [7,8,9]. While established strategies like elemental doping and morphology engineering have significantly improved its performance, many have not fully explored the utilization of infrared photons (>700 nm), which constitute nearly half of the solar spectrum. Additionally, the deliberate integration of photothermal effects remains a relatively underexplored avenue for further enhancing catalytic efficiency through reduced activation barriers [10,11,12].

Recent advances in photothermal-assisted photocatalysis have revolutionized this field by synergizing light absorption and thermal activation [13,14]. Lin et al. developed a sulfur-vulcanized Ti₃C₂ MXene that achieves efficient CO₂-to-C₂⁺ fuel conversion (C₂H₄: 3.55 mmol g⁻¹ h⁻¹) via full-spectrum photothermal-photocatalysis, setting a new benchmark with 0.045% solar-to-fuel efficiency [15]. This approach leverages materials’ ability to convert underutilized infrared photons into localized heat, which weakens the C=O bond (≈750 kJ mol⁻¹) and accelerates electron injection into CO₂ antibonding orbitals, thereby enabling kinetically hindered multi-electron reduction pathways [16]. Concurrently, two-dimensional transition metal carbides (MXenes), exemplified by Ti₃C₂, have emerged as game-changing cocatalysts [17,18,19]. Ti₃C₂ MXene exhibits metallic conductivity, broad-spectrum light absorption (UV to NIR), and exceptional photothermal conversion efficiency. Its hydrophilic surface terminations (-O, -OH) enhance CO₂ adsorption capacity, while layered structures provide abundant redox-active sites [20,21]. When integrated into Schottky junctions, Ti₃C₂’s higher work function creates built-in electric fields that drive directional electron transfer, substantially reducing recombination losses [22,23,24,25]. Despite these merits, existing MXene-modified g-C₃N₄ systems lack infrared-specific sensitization, limiting full-spectrum utilization, and fail to unify thermal and electronic optimization in a single architecture.

Herein, we engineer a novel CN-B/Ti₃C₂ Schottky heterojunction to synergize broadband light harvesting, photothermal catalysis, and interfacial charge dynamics. The CN-B matrix is synthesized via the one-step calcination of g-C₃N₄ with phloxine B dye—a red organic sensitizer that induces partial heptazine ring opening, extending visible-light absorption to 800 nm and amplifying photothermal conversion (ΔT > 35 °C under 1-sun illumination) [26]. Ti₃C₂ MXene is integrated via hydrothermal assembly, serving dual roles: as a Schottky booster facilitating electron-hole separation via built-in electric fields, and as a photothermal amplifier converting infrared photons into localized heat to thermally excite CO₂ molecules and lower activation barriers for multi-electron reduction [27,28,29]. This dual functionality creates a dynamic reaction microenvironment where thermal energy synergizes with optimized charge separation to promote •CO₂⁻ intermediate formation, a critical step for selective CH₄ generation [30,31].

The resultant heterojunction achieves excellent CO₂ reduction performance, CO and CH₄ yields of 80.21 μmol g⁻¹ h⁻¹ and 35.13 μmol g⁻¹ h⁻¹, respectively—representing 2.9-fold and 3.2-fold enhancements over pristine CN-B. In situ DRIFTS data confirm thermally accelerated C=O dissociation and stabilization of •CO₂⁻ intermediates, while the system maintains >90% activity over 24 h due to the prevention of oxidative corrosion by hydrophobic MXene layers. This work pioneers three innovations: phloxine B sensitization for full-spectrum infrared harvesting, Ti₃C₂-enabled thermo-electronic synergy, and waste-free design avoiding noble metals. By unifying photothermal heating, carrier dynamics, and targeted CO₂ activation, this study establishes a scalable blueprint for solar-powered carbon neutrality technologies.

2. Results and Discussion

2.1. Photocatalyst Characterization

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses reveal the distinct morphologies and interfacial characteristics of the synthesized materials. As shown in Figure 1a, CN-B exhibits a porous, layered stacking structure, where the open pore architecture facilitates active site exposure. Multilayered Ti₃C₂ nanosheets, obtained via HF etching of Ti₃AlC₂, display a characteristic two-dimensional layered morphology (Figure 1b). The SEM image of the CN-B/Ti₃C₂ (3:1) heterostructure (Figure 1c) demonstrates CN-B intimately attached to the layered surface of Ti₃C₂, forming a well-integrated interface. This unique microstructure leverages the inherent advantages of MXene materials—high specific surface area and excellent conductivity—while the heterojunction formation promotes efficient separation and transport of photogenerated charge carriers, providing an ideal platform for enhanced photocatalytic activity [31,32].

TEM analysis further elucidates the fine structural details. Figure 1e clearly shows the typical layered structure of Ti₃C₂ with ordered stacking. Notably, the CN-B/Ti₃C₂ (3:1) composite (Figure 1f) retains the characteristic curved, porous nanosheet morphology of CN-B (Figure 1d) while achieving effective integration of both phases. Elemental mapping (Figure 1g–j) confirms the spatial distribution of characteristic elements: C and N signals correspond precisely to the CN-B nanosheet structure, while Ti signals localize exclusively to the Ti₃C₂ regions. The elements exhibit a uniform, interpenetrating 3D spatial distribution, with overlapping C/N and Ti signal clouds forming a continuous, non-segregated coverage across the composite surface. This indicates intimate atomic-scale interfacial coupling between CN-B and Ti₃C₂ nanosheets. Such uniform dispersion prevents agglomeration of individual components and maximizes heterointerface contact area, thus enabling efficient cross-phase charge carrier migration. From a materials chemistry perspective, this microscale elemental homogeneity provides direct structural evidence for stable heterojunction interface formation, underpinning subsequent analyses of band alignment, charge transfer kinetics, and synergistic catalytic site interactions [33,34].

2.2. Surface Composition and Photoelectric Analysis

X-ray diffraction (XRD) was employed to characterize the crystallographic phases of the photocatalysts. Figure 2a displays XRD patterns of CN-B, Ti₃C₂, and CN-B/Ti₃C₂ composites with varying mass ratios. CN-B exhibits two distinct diffraction peaks at 13.1° and 27.2°, corresponding to the (001) plane (reflecting in-plane triazine ring ordering) and interlayer stacking along the c-axis, respectively [35]. Multilayered Ti₃C₂ obtained via HF etching shows characteristic peaks at 18.28°, 25.8°, 36°, and 59.2°, indexed to the (004), (006), (008), and (110) planes. The composite patterns confirm successful hybridization, retaining CN-B peaks at 13.1°/27.2° and Ti₃C₂ peaks at 18.28–59.2°. Notably, increasing the CN-B/Ti₃C₂ ratio progressively narrows the full-width at half-maximum (FWHM) of the dominant diffraction peaks, correlating with enhanced crystallinity. The optimal CN-B/Ti₃C₂ (3:1) composite demonstrates the sharpest peaks, indicating superior crystallinity [36]. Fourier-transform infrared (FTIR) spectra (Figure 2b) reveal consistent chemical bonding across CN-B and all composites. Characteristic absorption bands include: 810 cm⁻¹ (out-of-plane breathing vibration of triazine units), 1200–1700 cm⁻¹ (aromatic C-N stretching modes of heterocycles), and 3000–3500 cm⁻¹ (N-H stretching vibrations). The spectral fidelity confirms structural integrity of CN-B within the composites [37].

X-ray photoelectron spectroscopy (XPS) was employed to probe the elemental composition and chemical states of CN-B and the CN-B/Ti₃C₂ (3:1) composite. The survey spectrum (Figure 3a) confirms the co-presence of C, N, and Ti in the composite, indicating successful hybridization. In the high-resolution N 1s spectrum (Figure 3c), the peaks are deconvoluted into three components at 395.65 eV, 397.52 eV, and 401.60 eV, corresponding to sp² C-N═C, sp³ N-(C)₃, and C-N-H species, respectively. A positive binding energy shift in N 1s relative to pristine CN-B suggests electron transfer from CN-B to Ti₃C₂, which enhances charge separation efficiency. Similarly, the C 1s spectrum (Figure 3b) of the composite exhibits peaks at 284.8 eV (C-C), 286.1 eV (C-N-H), and 288.1 eV (N-C═N), along with additional features at 281.7 eV, attributed to C-Ti bonding, and at 288.6 eV, assigned to C-O species. These also display a noticeable shift toward higher binding energies, further supporting electron migration toward Ti₃C₂. The Ti 2p spectrum (Figure 3d) shows doublet peaks at 455.3 eV (Ti-C 2p_3/2), 458.7 eV (Ti-O 2p_3/2), 461.1 eV (Ti-C 2p_1/2), and 464.2 eV (Ti-O 2p_1/2). A consistent redshift in these Ti 2p peaks, compared to pure Ti₃C₂, reflects an altered electronic environment and strong interfacial coupling. This synergy establishes Ti₃C₂ as an effective electron acceptor, facilitating charge transfer across the heterojunction. The built-in electric field induced by interfacial band alignment significantly suppresses electron-hole recombination, thereby optimizing the photocatalytic CO₂ reduction performance [38].

The light absorption properties of the photocatalysts were investigated by UV-vis diffuse reflectance spectroscopy (DRS). As shown in Figure 4a, both CN-B and CN-B/Ti₃C₂ exhibit broad absorption across the visible to near-infrared range (450–800 nm). Incorporation of Ti₃C₂ induces a notable red-shift in the absorption edge, indicating enhanced light-harvesting capability. The absorption intensity increases systematically with higher CN-B content, reaching a maximum at the optimal CN-B/Ti₃C₂ mass ratio of 3:1. The bandgap energies were determined from Tauc plots (Figure 4b) using the equation (αhυ)² = k(hυ − Eg). CN-B and CN-B/Ti₃C₂ (3:1) exhibit direct bandgaps of 2.88 eV and 2.82 eV, respectively. The reduced bandgap of the composite facilitates greater visible-light-driven electron generation. XPS valence band spectra further reveal a shift in the valence band maximum from 1.55 eV (CN-B) to 1.46 eV (CN-B/Ti₃C₂), indicating enhanced reducibility. Using the equation E_VB = E_CB + Eg, the calculated conduction band positions are −1.33 eV for CN-B and −1.36 eV for CN-B/Ti₃C₂. The more negative conduction band potential of the composite demonstrates superior electron reduction capacity, consistent with its improved photocatalytic performance.

Nitrogen adsorption–desorption isotherms (Figure 5a) reveal type-IV hysteresis loops in the P/P₀ range of 0.5–1.0 for CN-B, Ti₃C₂, and the CN-B/Ti₃C₂ (3:1) composite, confirming their mesoporous structures. Pore size distribution curves further exhibit prominent peaks within 1–20 nm, consistent with mesoporous characteristics. The interconnected porous network significantly increases active site accessibility, thereby enhancing catalytic reaction kinetics. The composite material demonstrates markedly optimized surface properties, exhibiting a specific surface area of 60.454 m² g⁻¹—nearly double that of pristine CN-B (31.39 m² g⁻¹). This expansion in surface area not only improves CO₂ physisorption capacity but also provides abundant reactive sites for photogenerated charge carriers. The increased density of surface active sites effectively captures and utilizes charge carriers, substantially reducing electron-hole recombination. Furthermore, the hierarchical pore architecture shortens diffusion pathways for reactants through localized concentration gradients, synergistically enhancing reaction efficiency via confined spatial effects and improved charge transport.

To elucidate the charge transfer properties of the photocatalysts, photoluminescence (PL) spectroscopy was employed to characterize CN-B and the CN-B/Ti₃C₂ (3:1) composite. The steady-state PL spectrum (Figure 6a) of CN-B exhibits a distinct emission peak at 450 nm, originating from the rapid recombination of photogenerated electrons and holes. In contrast, the CN-B/Ti₃C₂ (3:1) composite shows significantly quenched PL intensity, indicating that heterojunction formation establishes efficient charge transfer pathways, thereby suppressing non-radiative recombination. Time-resolved transient fluorescence decay spectra (Figure 6b) further reveal carrier dynamics. The average charge carrier lifetime of CN-B/Ti₃C₂ (3:1) is prolonged to 3.15 ns, notably longer than that of CN-B (2.07 ns). This prolonged lifetime confirms that the heterojunction facilitates directed electron transfer, enabling rapid migration of photogenerated electrons to surface reactive sites for CO₂ reduction. The optimized charge separation mechanism provides a kinetic foundation for enhanced photocatalytic performance, allowing more high-energy electrons to participate in CO₂ activation and conversion processes.

To further validate the enhanced charge separation in CN-B/Ti₃C₂ nanocomposites, transient photocurrent response and electrochemical impedance spectroscopy (EIS) measurements were conducted using a standard three-electrode system. As depicted in Figure 7a, the EIS Nyquist plots of CN-B/Ti₃C₂ (x:1) composites and CN-B—under both light and dark conditions—reveal significantly reduced arc radii for the composites compared to CN-B, indicating decreased charge transfer resistance. This reduction is attributed to facilitated charge separation and improved electrical conductivity resulting from effective heterojunction formation. Notably, the CN-B/Ti₃C₂ (4:1) sample exhibits increased resistance, which can be ascribed to excessive CN-B content leading to reduced conductivity, weakened heterojunction effects, impeded charge transport pathways, and increased interfacial defects. Among all composites, CN-B/Ti₃C₂ (3:1) demonstrates the smallest arc radius, confirming optimal charge transfer characteristics at the electrode/electrolyte interface. The high impedance observed under dark conditions in EIS measurements is characteristic of the semiconducting CN-B material, reflecting its limited charge-carrier generation in the absence of light. Photocurrent was measured without applied bias to evaluate the intrinsic photocatalytic activity under simulated solar irradiation, mimicking realistic solar-driven conditions. The photocurrent response under repeated on/off illumination cycles (Figure 7b) shows that the CN-B/Ti₃C₂ (3:1) electrode generates the highest photocurrent density, indicative of superior light absorption and charge separation efficiency. The reproducible photocurrent during cycling further underscores the catalyst’s excellent operational stability. In summary, the CN-B/Ti₃C₂ heterojunction catalyst exhibits enhanced light absorption, a narrower bandgap, improved charge separation, and superior charge transport properties compared to CN-B, collectively contributing to its outstanding photocatalytic CO₂ reduction performance.

2.3. Photocatalytic Activity

The photocatalytic CO₂ reduction activity of the samples was evaluated under simulated solar irradiation (Xe lamp) without any sacrificial agents, following the experimental details provided in Section 3.3. As illustrated in Figure 8a,b, pristine CN-B exhibited a CO production rate of 27.56 μmol·g⁻¹·h⁻¹. Multilayer Ti₃C₂ nanosheets alone showed limited activity, generating CO and CH₄ at rates of 12.16 μmol·g⁻¹·h⁻¹ and 5.01 μmol·g⁻¹·h⁻¹, respectively. In the CN-B/Ti₃C₂ nanocomposite series, the CO₂ reduction performance increased with the CN-B-to-Ti₃C₂ mass ratio up to 3:1. The optimal composite, CN-B/Ti₃C₂ (3:1), achieved a total CO yield of 320.84 μmol·g⁻¹ over 4 h, along with a CH₄ yield of 140.2 μmol·g⁻¹. The CO production rate of this composite was 2.9 times higher than that of pure CN-B (110.24 μmol·g⁻¹ under identical conditions). Further increasing the CN-B ratio to 4:1 resulted in decreased activity, due to reduced electrical conductivity [39], impaired charge separation, the aggregation of CN-B, and decreased active site availability.

Notably, the CN-B/Ti₃C₂ (3:1) composite maintained high and stable production rates of both CO and CH₄ during a 48 h longevity test (Figure 8c). This consistent performance is attributed to the chemically robust heterojunction interface between CN-B and Ti₃C₂, which promotes structural integrity and sustained catalytic activity throughout prolonged photoreaction. Figure 8d displays the product selectivity distribution during the photocatalytic CO₂ reduction over different catalysts. The CN-B/Ti₃C₂ (3:1) composite exhibits a slightly higher selectivity toward CH₄ compared to other catalysts, indicating its enhanced ability to drive the multi-electron reduction pathway. As illustrated in Figure 8e, the apparent quantum yield (AQY) of CN-B/Ti₃C₂ (3:1) for CO₂ reduction was measured at designated wavelengths (please refer to

Table 1. Parameters to calculate AQYs of photoreduction CO₂ over the CN-B/Ti₃C₂ (3:1) under different wavelengths.

Wavelength (nm)	CH4 Evolution (μmol g−1)	Light Intensity (mW cm−2)	Irradiation Time (h)	Irradiation Area (cm2)	AQY (%)
380	0.447	20	1	4	0.33
405	0.4505	20	1	4	0.335
460	0.2485	20	1	4	0.185
520	0.142	20	1	4	0.105
600	0.1055	20	1	4	0.08
700	0.088	20	1	4	0.065
850	0.0625	20	1	4	0.045

for more detailed data), showing a gradual decrease in efficiency as the wavelength increases, which is consistent with the typical photo-response behavior of semiconductor-based photocatalysts. Furthermore, XRD analysis was performed on the CN-B/Ti₃C₂ (3:1) sample after a 48 h cycling test and compared with the pristine material (Figure 8f). The results reveal no significant changes in the crystal structure of the post-reaction sample, demonstrating the robust durability of the heterojunction system under prolonged photocatalytic conditions.

The CO₂ reduction performance of the catalysts was evaluated under simulated solar irradiation (300 W Xe lamp) without sacrificial agents or photosensitizers. As shown in Figure 9a, in the presence of water vapor and without external temperature control, the CN-B/Ti₃C₂ (3:1) composite exhibited CO and CH₄ production rates of 80.2 μmol·g⁻¹·h⁻¹ and 35.05 μmol g⁻¹ h⁻¹, respectively—2.9 times higher than that of CN-B (27.2 μmol g⁻¹ h⁻¹). However, when the temperature was maintained at 20 °C using a circulator chiller, the activity decreased significantly for all samples (Figure 9b), underscoring the important role of photothermal heating in reaction kinetics. Notably, while the thermal effect contributes to the reaction kinetics by providing localized heating, it alone is not the primary driver of the performance enhancement; instead, photocatalysis remains the dominant pathway, with the photothermal effect acting synergistically to reduce the activation energy barriers for CO₂ dissociation and intermediate formation. This synergy arises from the integration of photoexcited charge carriers—which drive the redox reactions—with localized thermal energy that weakens C=O bonds and accelerates surface processes, thereby optimizing the overall reaction efficiency without supplanting the essential role of light-induced charge separation (Figure 9d,e).

Control experiments were conducted to confirm the origin of CO and CH₄ (Figure 9c). No products were detected when CO₂ was replaced by N₂, when water vapor was omitted, in the absence of catalyst, or without light irradiation, confirming that both CO₂ and H₂O are essential reactants and that the process is photo-driven [40,41].

The photothermal effect was further investigated by monitoring the surface temperature under 300 mW·cm⁻² irradiation (Figure 9f–h). The temperature of CN-B/Ti₃C₂ (3:1) rapidly increased from 22.7 °C to 233.9 °C within 150 s, substantially higher than that of CN-B (reaching 26.0 °C from 131.4 °C). This enhanced photothermal conversion stems from the composite’s black color and broad-spectrum absorption, which efficiently converts photon energy into heat via lattice vibrations, thereby accelerating reaction dynamics [15].

The heterojunction formed between CN-B and Ti₃C₂ facilitates charge separation through a built-in electric field: electrons migrate to Ti₃C₂, while holes remain in CN-B [42]. This not only suppresses recombination but also releases additional energy during charge transfer, further enhancing photothermal effects. The synergy between photocatalysis and photothermal conversion significantly improves overall CO₂ reduction efficiency.

2.4. Mechanism of CO₂ Photoreduction

Figure 10a presents the in situ FTIR spectra of CO₂ adsorption on CN-B/Ti₃C₂. The gradually increasing peaks at 2359 cm⁻¹, corresponding to C=O stretching vibrations, confirm the adsorption of CO₂ on the catalyst surface [43]. Using the adsorption equilibrium state as the background, the dynamic interfacial interactions during photocatalysis are revealed in Figure 10b. Under 15 min of illumination, multiple characteristic absorption bands emerge: bicarbonate (HCO₃⁻; δ(C=O-H⁺) at 1067 cm⁻¹), monodentate carbonate (m-CO₃²⁻; ν(C-O) at 1337 cm⁻¹), and bidentate carbonate (b-CO₃²⁻; ν(C-O) at 1266 cm⁻¹), and the peaks at 1430 cm⁻¹ were considered to be the CO₃²⁻, all originating from CO₂ hydration [44,45]. Additionally, A nascent positive peak emerged at 2368 cm⁻¹, adjacent to the declining CO₂ stretching vibrations, suggesting the formation of a protonated CO₂ adspecies (O=C=O-H⁺), and absorption band at 1511 cm⁻¹ is attributed to carboxylate (COOH*) species, indicating the formation of key intermediates via reaction between adsorbed CO₂ and H⁺ [46]. The consumption of water and CO₂ is evidenced by peaks at 1634 cm⁻¹ and 2338 cm⁻¹, highlighting the essential role of H₂O in the reduction process [47]. These results collectively demonstrate that CN-B/Ti₃C₂ facilitates CO₂ reduction through efficient adsorption, proton-assisted activation, and hydrolysis-mediated intermediate formation.

Based on the above results and analysis, Figure 11 illustrates the proposed mechanism for photocatalytic CO₂ reduction over the CN-B/Ti₃C₂ (3:1) composite. Under light irradiation, the CN-B/Ti₃C₂ heterostructure functions as a Schottky junction, where metallic Ti₃C₂ forms a directional electron-extraction interface with semiconducting CN-B, markedly enhancing charge separation and photothermal-coupled catalytic activity [48]. The difference in Fermi levels between CN-B and Ti₃C₂ drives electron transfer from CN-B to Ti₃C₂, establishing a built-in electric field that promotes the spatial separation of photogenerated carriers: electrons migrate from the conduction band of CN-B to Ti₃C₂, while holes move from the valence band of Ti₃C₂ to CN-B. This directed charge flow significantly reduces recombination probability. The high electrical conductivity of Ti₃C₂ enables rapid electron transport to surface active sites, where CO₂ is reduced stepwise to CO and CH₄. Meanwhile, holes accumulated in the valence band of CN-B oxidize water molecules, facilitating proton generation and O₂ evolution [49].

In situ infrared spectroscopy under illumination identifies key intermediate species, including adsorbed CO₂* (bent at ~2359 cm⁻¹) and COOH* (1710 cm⁻¹), confirming efficient CO₂ activation and proton-coupled reduction pathways. The overall process involves: (i) photoexcitation and thermal effects, where visible and near-infrared light generate electron–hole pairs while photothermal heating (enhanced by the black matrix and Ti₃C₂ plasmonics) elevates the local temperature, promoting hot carrier generation and weakening C=O bonds; (ii) efficient charge dynamics across the Schottky barrier, with electrons rapidly injected into Ti₃C₂ and suppressed recombination; and (iii) surface reduction processes wherein CO₂ is converted via •CO₂⁻ → COOH* → CO* → CO/CH₄, with protons supplied through water oxidation [46,50]. The intimate interfacial contact and synergistic electronic–thermal effects enable continuous solar-driven CO₂ reduction, positioning CN-B/Ti₃C₂ as an advanced system for efficient and stable solar-to-chemical energy conversion. The CO₂ photoreduction process over CN-B/Ti₃C₂ can be inferred as follows:

CN-B/Ti₃C₂ + hv → CN-B/Ti₃C₂ (e⁻ + h⁺)

(1)

CO₂ → CO₂*

(2)

CO₂* + e⁻ + H⁺ → COOH*

(3)

COOH* + e⁻ + H⁺ → CO* + H₂O

(4)

CO* → CO (g) & CO* + 6e⁻ + 6H⁺ → CH₄ (g) + H₂O

(5)

H₂O + h⁺ → OH* + H⁺

(6)

OH* + h⁺ → 1/2 O₂ (g) + H⁺

(7)

3. Experimental Section

3.1. Methods

3.1.1. Materials and Chemicals

All reagents were used as received without further purification: urea, phloxine B, absolute ethanol, potassium ferricyanide, potassium ferrocyanide, oleic acid, polyvinylpyrrolidone (PVP), and barium sulfate (BaSO₄) of analytical reagent (AR) grade were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Potassium bromide (KBr) and sodium sulfate (Na₂SO₄) (AR grade) were purchased from Aladdin Industrial Co. (Shanghai, China). Hydrofluoric acid (HF, 40%, AR grade) and titanium aluminum carbide (Ti₃AlC₂, 98%, 200 mesh) were obtained from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China) with HF handled in PTFE containers under strict safety protocols due to its high toxicity.

3.1.2. Synthesis of CN-B

CN-B photocatalysts were synthesized via a one-step calcination method. Briefly, 10 g of urea and xmg of phloxine B (x = 25, 35, 45) were homogenized by mortar grinding for 30 min. The mixture was transferred to an alumina crucible and heated in static air at 5 °C min⁻¹ to 540 °C, followed by a 2 h isothermal treatment. After cooling to room temperature naturally, the product was alternately washed with deionized water and absolute ethanol via centrifugation (8000 rpm, 10 min, three cycles) to remove impurities. The resulting solid was dried at 60 °C for 12 h, yielding CN-B powders. Samples were labeled CN-B-0.025, CN-B-0.035, and CN-B-0.045 based on phloxine B mass ratios (0.25–0.45 wt%). For comparison, pristine g-C₃N₄ (denoted CN) was synthesized identically without phloxine B addition [26].

3.1.3. Synthesis of Ti₃C₂ MXene

Multilayered Ti₃C₂ nanosheets were synthesized via selective etching of aluminum from Ti₃AlC₂ MAX phase. Briefly, 2 g of Ti₃AlC₂ powder (200 mesh, 98%) was slowly added to 40 mL of 40 wt% hydrofluoric acid (HF) in a PTFE container over 10 min to prevent localized overheating. The mixture was maintained at 35 °C under vigorous magnetic stirring (800 rpm) for 24 h to ensure complete Al removal. The resulting suspension was centrifuged at 3500 rpm for 10 min, followed by repeated washing cycles with deionized water until the supernatant reached pH ≈ 7. The collected sediment was vacuum-dried at 60 °C for 12 h, yielding accordion-like Ti₃C₂ MXene [27].

3.1.4. Synthesis of CN-B/Ti₃C₂ Composites

CN-B/Ti₃C₂ heterojunctions were fabricated via a hydrothermal self-assembly strategy. First, 0.1 g CN-B powder was dispersed in 100 mL deionized water under sonication (40 kHz, 20 min) to form a homogeneous suspension. Subsequently, 0.1 g Ti₃C₂ MXene was added, followed by additional sonication (30 min) and magnetic stirring (600 rpm, 6 h) to ensure interfacial contact. The mixture was transferred to a 150 mL Teflon-lined autoclave and hydrothermally treated at 120 °C for 12 h. After natural cooling, the product was collected by centrifugation (3500 rpm, 10 min), washed alternately with deionized water and ethanol (3–5 cycles), and vacuum-dried at 60 °C for 12 h. To optimize the heterojunction, composites with varying CN-B:Ti₃C₂ mass ratios (1:1, 2:1, 3:1, 4:1) were synthesized by adjusting CN-B mass (0.1–0.4 g) while maintaining constant Ti₃C₂ mass (0.1 g). The resulting materials were labeled CN-B/Ti₃C₂ (n:1), where n denotes the CN-B:Ti₃C₂ mass ratio.

3.2. Characterizations

The XRD patterns were performed on a Shimazu-6100 powder X-ray diffractometer (Shimadzu, Kyoto, Japan) with Cu Kα radiation. At 2θ, the scanning range was 10–80° and the scanning speed 10°/min. The XPS data was obtained using a Shimadzu/Krayos AXIS Ultra DLD (Shimadzu, Kyoto, Japan). The HRTEM images were taken on a field emission electron microscope (Tecnai G2 F20, FEI Company, Hillsboro, OR, USA) with an acceleration voltage. The scanning electron microscopy (SEM) images were characterized on a field emission scanning electron microscope (FE-SEM, JSM-7001F, JEOL, Tokyo, Japan). The ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) was carried out on a Shimadzu UV-3600 spectrometer (Shimadzu, Kyoto, Japan). The photoluminescence (PL) spectra for solid power were investigated by an F4500 (Hitachi, Tokyo, Japan) and the xenon (Xe) lamp with an excitation wavelength of 375 nm. In situ Fourier transform infrared spectroscopy (in situ FTIR) was carried out to research the CO₂ photoreduction process (Thermo Fisher Nicolet iS-10, Thermo Fisher Scientific, Waltham, MA USA). Thermal imaging temperature photos were obtained using an Infrared Thermal Imagers (FLIR Systems, Wilsonville, OR, USA). Photoelectrochemistry experiments including electrochemical impedance spectroscopy (EIS), and transient photocurrent responses (TPR) were studied.

The XRD patterns were performed on a Shimadzu-6100 powder X-ray diffractometer (Shimadzu, Kyoto, Japan) with Cu Kα radiation. At 2θ, the scanning range was 10–80° and the scanning speed 10°/min. The XPS data was obtained using a Shimadzu/Krayos AXIS Ultra DLD (Shimadzu, Kyoto, Japan). The HRTEM images were taken on a field emission electron microscope (Tecnai G2 F20, FEI Company, Hillsboro, OR, USA) with an acceleration voltage. The scanning electron microscopy (SEM) images were characterized on a field emission scanning electron microscope (FE-SEM, JSM-7001F, JEOL, Tokyo, Japan). The ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) was carried out on a Shimadzu UV-3600 spectrometer (Shimadzu, Kyoto, Japan). The photoluminescence (PL) spectra for solid powder were investigated by an F4500 spectrometer (Hitachi, Tokyo, Japan) using a xenon (Xe) lamp with an excitation wavelength of 375 nm. In situ Fourier transform infrared spectroscopy (in situ FTIR) was carried out to research the CO₂; photoreduction process using a Thermo Fisher Nicolet iS-10 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). Thermal imaging temperature photos were obtained using an Infrared Thermal Imager (FLIR Systems, Wilsonville, OR, USA). Photoelectrochemistry experiments including electrochemical impedance spectroscopy (EIS) and transient photocurrent responses (TPR) were studied.

3.3. CO₂ Photoreduction Experiments

The CO₂ photoreduction is conducted in a sealed 100 mL quartz homemade reactor using a 300 W Xe lamp (1000 mW cm⁻², Beijing China Education Au-light Co., Ltd., Beijing, China) as the white light source. In a typical procedure, 10 mg of catalyst was dispersed in 3 mL of deionized water. The resulting mixture was dropped on quartz glass (area of 4 cm²) and then dried at 60 °C. The dried catalyst was placed in the quartz reactor with 1 mL of H₂O. CO₂ was then introduced into the reactor for 30 min to completely remove the air Gas products were detected by a gas chromatography (GC 5890N, Agilent Technologies, Santa Clara, CA, USA) equipped with hydrogen flame ionization detector (FID, Agilent Technologies, Santa Clara, CA, USA).

3.4. Photo-Electrochemical Measurements

The preparation of working electrodes began by mechanically grinding the CN-B and CN-B/Ti₃C₂ (x:1) powders into fine particles. Subsequently, 0.05 g of each powdered sample was uniformly dispersed in separate 3.0 mL ethanol solutions. To this dispersion, 0.03 mL of oleic acid and 0.01 g of polyvinylpyridone (PVP) were added as modifiers to enhance electrode film formation. The resulting homogeneous mixture was spin-coated onto a 1 × 1 cm² fluorine-doped tin oxide (FTO) glass substrate, followed by air-drying at ambient temperature to form the working electrode. For electrochemical characterization, a conventional three-electrode setup was employed, consisting of a platinum (Pt) wire counter electrode, a saturated calomel electrode (SCE) as the reference, and 0.5 M Na₂SO₄ aqueous solution as the electrolyte. Electrochemical measurements were performed using a Chen Hua electrochemical workstation, applying a 5 mV sinusoidal potential waveform. All resulting data were recorded and analyzed using a CHI660 B electrochemical analyzer (Chen Hua Instruments, Shanghai, China).

3.5. Calculation of AQYs Parameters for Photoreduction of Carbon Dioxide by CN-B/Ti₃C₂ (3:1) at Different Wavelengths

When wavelength λ = 380 nm (CN-B/Ti₃C₂ (3:1)):

The number of incident photons:

$$N={\displaystyle \frac{E\lambda }{hc}}={\displaystyle \frac{20\times 4\times {10}^{-3}\times 1\times 3600\times 450\times {10}^{-9}}{6.626\times {10}^{-34}\times 3\times {10}^8}}=6.52\times {10}^{20}$$

AQY:

$$\begin{array}{l}\mathit{A}\mathit{Q}\mathit{Y}={\displaystyle \frac{8\times \mathit{t}\mathit{h}\mathit{e} \mathit{n}\mathit{u}\mathit{m}\mathit{b}\mathit{e}\mathit{r} \mathit{o}\mathit{f} \mathit{e}\mathit{v}\mathit{o}\mathit{l}\mathit{v}\mathit{e}\mathit{d} {\mathit{C}\mathit{H}}_4 \mathit{m}\mathit{o}\mathit{l}\mathit{e}\mathit{c}\mathit{u}\mathit{l}\mathit{e}\mathit{s}}{\mathit{N}}}\\ ={\displaystyle \frac{8\times 6.02\times {10}^{23}\times 0.447\times {10}^{-6}}{6.52\times {10}^{20}}}=0.33\%\end{array}$$

4. Conclusions

In this study, we successfully synthesized a series of CN-B/Ti₃C₂ (x:1) composite photocatalysts via a hydrothermal method. The optimal sample, CN-B/Ti₃C₂ (3:1), exhibited exceptional photocatalytic CO₂ reduction performance without the use of cocatalysts, achieving remarkable CO and CH₄ production rates of 80.21 μmol·g⁻¹·h⁻¹ and 35.13 μmol·g⁻¹·h⁻¹, respectively—2.9 times higher than that of pristine CN-B. Furthermore, the composite demonstrated outstanding stability, maintaining high activity over a 24 h cyclic test. The significantly enhanced performance is attributed to the synergistic effects of the well-designed heterojunction: efficient charge separation and transfer across the CN-B/Ti₃C₂ interface substantially reduces electron–hole recombination, while the high conductivity of Ti₃C₂ facilitates rapid electron migration and CN-B stabilizes holes, collectively boosting photocatalytic efficiency. Moreover, the composite exhibits superior photothermal conversion capability, rapidly reaching 233.9 °C under light irradiation. This photothermal effect provides additional thermal energy that lowers the activation barrier, promotes carrier generation and separation, and accelerates reaction kinetics. This work highlights the importance of integrating heterojunction engineering with photothermal effects for designing highly efficient and stable photocatalysts. The CN-B/Ti₃C₂ composite represents a promising candidate for solar-driven CO₂ conversion, offering valuable insights for developing advanced catalytic systems toward artificial carbon cycling and renewable energy utilization.