Enhanced Photocatalytic CO2 Reduction via MXene synergism: Constructing a Strong Intra-layer Electric Field Ternary Heterojunction of g-C3N4/Nb2C MXene/CsPbBr3

doi:10.21203/rs.3.rs-5012551/v1

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Research Article

Enhanced Photocatalytic CO2 Reduction via MXene synergism: Constructing a Strong Intra-layer Electric Field Ternary Heterojunction of g-C3N4/Nb2C MXene/CsPbBr3

https://doi.org/10.21203/rs.3.rs-5012551/v1

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published 06 Nov, 2024

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Slow charge kinetics and high activation energy seriously hinder the efficiency of photocatalytic CO₂.Synergies are a commonly used strategy, Nevertheless common synergies have been limited to improving catalytic results.Here, we synthesize a novel nanocomposite ternary heterojunction material, which forms a low interlayer electrostatic potential within the heterojunction through the MXene synergistic.A strong internal electric field from the outside to the inside is formed within the series layer heterojunction, which provides the inner driving force for the effective spatial separation of photoinduced electron-hole pairs. Under visible-light irradiation, the ternary heterojunction exhibited a maximum CO production rate of 53.07 μmol g^-1 h^-1, surpassing the rates of pure g-C₃N₄, CsPbBr₃ QDs, and the binary composite of g-C₃N₄/CsPbBr₃ by approximately 8.4, 10, and 2 times, respectively. Experimental results and theoretical analysis reveal the significance of 2D Nb2C MXene as an electron transporter, benefiting from lower electrostatic potential. This characteristic synergistically facilitated the rapid extraction of photoinduced electrons, enhancing the reduction ability of CO₂ to CO. This research not only provides a novel insight into MXene utilization for designing ternary heterojunction nanocomposite photocatalysts but also presents the potential of utilizing synergism ternary composites to improve solar energy conversion efficiency.

Photocatalytic

CO2 reduction

Ternary heterojunction

MXene synergism

g-C3N4/Nb2C MXene/CsPbBr3

Solar photocatalytic conversion has emerged as a promising solution to address the energy crisis and environmental challenges by directly directly converting CO₂ into CO or hydrocarbon in the new age[1].For this purpose, extensive research efforts has been done on efficient CO₂ reduction photocatalysts, including organic[2], inorganic[3], transition metal compounds[4-6], and terminal ligands[7, 8]. Among them, MXene, a novel two-dimensional layered metal carbide and metal nitride material, has gained significant attention as a co-catalyst due to its high conductivity, abundant active sites, and large surface area[9-11], making it a potential candidate for promoting photocatalytic CO₂ reduction[12]. However, the efficiency of CO₂ conversion is still unsatisfactory owing to the stable nature of CO₂ as a linear covalent molecule(high dissociation energy)[13] and its sluggish multielectron reduction kinetics[14]. How to overcome these disadvantages? One effective strategy is to construct lattice-matched morphological[15] heterojunctions using nanotubes, ultrathin nanosheets, and quantum dots. This hierarchical heterojunction design[16] offers several advantages, including charge separations at the lattice-matched interface of the two morphological components, prevention of carrier recombination, and improved photon-to-current conversion efficiency, resulting in enhanced photocurrent conversion. Taking advantage of MXene, combining it with narrow-band gap semiconductors (Eg<3eV), such as metal oxides, sulfides, III-V group compounds, and II-VI group compounds, has proven to be an excellent method. These combinations facilitate the establishment of redox abilities and the formation of Z-scheme or type-II [17, 18] heterojunction photocatalytic system. Lattice matching and heterojunction regulation are also important means for the improvement of advanced photocatalytic materials[19].

Considering the aforementioned two issues, the catalytic performance of photocatalytic materials can be significantly enhanced through the reasonable design heterogeneous interface structure. We first thought of Halogen perovskites CsPbX₃ QDs (X=Cl, Br, I) star materials[20, 21] , which have been widely used in photovoltaic and optoelectronics in recent years, due to their super optical properties, including a large extinction coefficient, a wide and tunable light absorption range, a long charge diffusion length, and an extended carrier lifetime. However, the single-component halogen perovskite materials are unsatisfactory due to low CO₂ adsorption capacity, and severe charge recombination, which greatly limits their potential for nonlinear optical properties[22] and catalytic application[23]. Moreover, their susceptibility to water instability poses a big challenge in both synthesis and practical applications. Surface passivation using organic ligands may also not effectively overcome this issue, as ligands can be easily lost during washing process. Adding protective materials is a common strategy, and the two-dimensional material g-C₃N₄ [24-26] is just right for it. It has attracted considerable attention in the water splitting and photocatalytic CO₂ reduction reactions due to its ideal bandgap with the conduction band (CB) and valence band(VB) edges at -0.59 eV and 2.36 eV vs. normal hydrogen electrode (NHE), respectively. Additionally g-C₃N₄ offers several advantages, including easy fabrication, good photoelectric chemical properties, strong adsorption capacity, thermal stability, and resistance to acid and alkali chemistry. It exhibits a favorable response to visible light, allows for band structure tuning, possesses rich surface functionality, and demonstrates good chemical stability[27, 28]. However, the rapid recombination of photogenerated holes and electrons in g-C₃N₄ limits the efficiency of light utilization, resulting in low photocatalytic performance.

To overcome the limitations of single component, researchers have explored various theoretical and experimental approaches to enhance the photocatalytic ability of CO₂, including covalent and noncovalent modifications[29], doping[30], band gap and crystalline regulation[24], and the construction of heterojunction. Compared to conventional heterojunctions, the use of n-type or p-type semiconductors with appropriate band structure can enhance visible light absorption and maintain stable reduction ability of photogenerated electrons on the interface, thereby improving the overall photocatalytic efficiency of CO₂ reduction. Therefore, delicate regulation and matching of heterogeneous are crucial to achieve optimal effect. As demonstrated in nanocomposites[31, 32], the incorporation of low-dimensional materials, such as 0D quantum dots or 1D nanowires with g-C₃N₄ nanosheets effectively enhances optoelectronic properties. Man Ou et al[33]. anchored CsPbBr₃ QDs on NH_x-rich porous g-C₃N₄ nanosheets to construct composite photocatalysts for photocatalytic CO₂ reduction, demonstrating good stability and an outstanding yield. It is important to note that their system employed acetonitrile/water and ethyl acetate/water as the reaction media, rather than common water-saturated carbon dioxide conditions. Hierarchical band structure matching and good contact between interfaces have been solved, and multiple heterojunctions have also been explored, which mainly focused on metal oxides[34-37], sulfides[38], and semiconductor materials[39]. MXene, as a typical member of two-dimensional layered material can be used as superior photothermal supports for metal nanoparticles, also tandem effects to form a porous layered multiple structure for efficient low-grade thermal energy harvest[40, 41] and energy storage[42, 43]. Ternary nanocomposite of MXene (Ti₃C₂)-Cu₂O-Fe₃O₄[44] and g-C₃N₄/ZnO/Ti₃C₂ [45] be used for photoelectrocatalytic CO₂ reduction. According to density functional theory (DFT), calculations of g-C₃N₄-based[46] heterostructured photocatalysts with good lattice matching have more stable structures and have greater potential application in the field of heterojunction catalysis, in which the lower electrostatic potential makes the ternary heterojunction of MXene[47] more attractive and promising.

In our study, Nb₂C MXene is employed to tandem heterojunction nanocomposites (g-C₃N₄/Nb₂C MXene/CsPbBr₃) as photocatalysts to enhance CO₂ reduction activity. The regulation of heterogeneous interface facilitated lattice matching and coordination among the materials. Furthermore, the combination of the three parent materials naturally created a built-in electric field, promoting efficient carrier separation and significantly improving the catalytic performance. We designed a rational ternary heterojunction structure of g‑C₃N₄/Nb₂C MXene/CsPbBr₃, Which exhibited remarkable photocatalytic activity due to the synergistic effect among the components. In addition, density functional theory (DFT) calculations provided valuable insights intothe rapid charge separation and transport process within ternary heterojunctions.To demonstrate the practical application of the ternary nanocomposite materials, the photocatalytic CO₂ reduction experiment was tested. The ternary g-C₃N₄/Nb₂C MXene/CsPbBr₃ catalyst outperformed pure g‑C₃N₄, CsPbBr₃ QDs and g-C₃N₄/CsPbBr₃ catalysts in terms of CO production.The ternary heterojunction exhibited a maximum CO production rate of 53.07 μmol g^-1 h^-1, surpassing the rates of pure g-C₃N₄, CsPbBr₃ QDs, and the binary composite of g-C₃N₄/CsPbBr₃ by approximately 8.4, 10, and 2 times, respectively. Notably, with an increase in the Nb₂C MXene content, the production of CH₄was effectively suppressed. This work provides a new insight for design of excellent ternary nanocomposite photocatalysts.

2.1 Chemicals and reagents

Cesium carbonate (CsCO₃, Alfa-Aesar), oleic acid (OA, Alfa-Aesar), oleylamine (OA, Alfa-Aesar)， Lead bromide(PbBr₂, 99.99%, Sigma-Aldrich), octadecylene (OA, Alfa-Aesar) DMSO (Alfa-Aesar), n-hexane (Alfa-Aesar), urea (Sigma-Aldrich), HF(40%), Nb₂AlC(qiyue CO.Ltd. xi’an) and acetone (Merck) were used. The pure water used throughout the experiment is deionized water. All reagents and solvents are of analytical grade and directly used without further purification.

2.2 Preparation of CsPbBr₃ QDs

The CsPbBr₃ QDs were synthesized based on the literature[48] with some subtle modification. We prepare the QDs precursors[49]. Firstly, 0.843g(0.25mmol) CsCO₃ and 30 mL octadecylene was added into a 50 mL three-necked round bottom flask, the whole process is protected by high purity nitrogen gas. The mixture was heated at 120℃ for 1 hour under Ar flow and 800r/min magnetic stirring, to remove water and impurities from the solution. At the desired temperature, we quickly heat up to 150℃, to completely dissolve cesium carbonate, and then control the temperature to 120℃, turn off the heat button and the mixture comes to room temperature, resulting in the cesium oleate precursor. Secondly, 0.367g of PbBr₂ and 20 mL octadecylene were added into a 100 mL three-necked round bottom flask. The mixture was heated at 120 ℃ for 1 hour under Ar flow and 800r/min magnetic stirring also removeing water and impurities from the solution. Subsequently, 2.5 mL of oleic acid and 2.5 mL of oleylamine were injected into the flask, vacuum it three times to prevent the effects of oxygen and fill the system with argon. A faint yellow homogeneous solution was formed when the above mixture was heated to 180 ℃. Following this, 2.5 mL of the cesium oleate precursor solution was injected into the flask, the yellow-green solution appears immediately, after 10 seconds, 10ml of n-hexane was injected into the flask, stop heating and stirring, cooled the mixture to room temperature with an ice-water bath. The yellow precipitate was centrifuged and washed with n-hexane and ethyl acetate to remove the organic residue. The mixture of yellow octadecylene suspension was centrifuged at 1000 rpm for 5 min to remove the large aggregated particles. The supernatant was again precipitated with ethyl acetate or acetone, then centrifuged at 6000 rpm for 5 min repeatedly to remove the surfactant organic molecules. The obtained CsPbBr₃ QDs were re-dispersed in n-hexane (1~2 mg/mL) for further use. The SEM elemental analysis(EDS) of the dried CsPbBr₃ QDs sample indicated that very low N contents (0.75 wt%) were remaining in the final sample, implying that surfactant organic molecules on the surface of CsPbBr₃ QDs were largely removed.

2.3 Preparation of g-C₃N₄ nanosheets

Graphitic carbon nitride(g-C₃N₄) nanosheets were prepared by direct thermal treatment of urea at 550^ᵒC in a muffle furnace for 4 hours at 5^ᵒC/min heating rate. The reaction procedure was carried out in an alumina crucible equipped with an air-tight lid to prevent any evaporation of the precursor along with associated gas loss. A pale yellow colored powdery sample was obtained which was termed g-C₃N₄. And then, the obtained g-C₃N₄ was annealed from 550^ᵒC to room temperature. However, the samples we obtained are an amorphous graphite-phase nitrogen carbide so no lattice fringes are observed in TEM experiments in our composites.

2.4 Synthesis of 2D MXene Nb₂C nanosheets

Based on the literature[50], 2D Nb₂C MXene nanosheets were synthesized by etching 2.0 g Nb₂AlC, which was placed in an oil bath constant thermal magnetic stirrer with a mixture of 40 mL of 40% HF at 60^°C, 300 rpm for 48h in a sealed plastic cup. After the reaction, the product was washed 3 to 5 times by centrifugation using anhydrous ethanol until the pH reached about 6. Then, the black Nb₂C MXene was dispersed with DMSO by ultrasonic treatment in deionized water in the glass bottle for 10 hours under nitrogen protection, the supernatant was collected, centrifuged at 3500 rpm, and the precipitate was returned to the glass bottle, then, the main component of the centrifuged supernatant was 2D Nb₂C MXene nanosheets, the content is 4mg per 100 ml. The nanosheets are dried and stored for further use in the synthesis of the complex.

2.5 Synthesis of the binary g-C₃N₄ /CsPbBr₃ and the ternary g-C₃N₄/ Nb₂C MXene /CsPbBr₃ composites

In this synthesis process[33], we developed an one-pot novel method. Firstly, 10mg g-C₃N₄ white powders were added into 20 mL octadecylene, the mixture was stirred 800r/min for 1 h and maintained at 180 ^°C under the protection of argon, vacuum a few times to remove moisture from the solution. Then, 7, 30, 43, and 55 mg of the yellow perovskite quantum dots CsPbBr₃ was added to the solution, and it was stirred for another 8 h and also maintained at 180 °C under the protection of argon. The product was washed multiple times with n-hexane and dried at 80 °C in the vacuum oven. In the composites, the g-C₃N₄ and perovskite quantum dots CsPbBr₃ can form a g-C₃N₄/CsPbBr₃ binary heterojunction, the obtained samples will be further named as CC7, CC30, CC43 CC55, respectively. We have also tried in-situ synthesis of this two-dimensional composite material and found that the results were the same.

The same synthesis method described above is used. Firstly, 10mg g-C₃N₄ white powders and 43 mg(the photocatalytic carbon dioxide reduction performance of these two compounds is better) the yellow perovskite quantum dots CsPbBr₃ were added into 20 mL octadecylene, the mixture was stirred 800r/min for 1 h and maintained at 180 °C under the protection of argon, vacuum three times guarantee to remove moisture from the solution. Then, 10, 20, 30and 40 mg of the Nb₂C MXene black nanosheets were added to the solution, and it was stirred for another 8 h and also maintained at 180 ^°C under the protection of argon. The product powder was washed multiple times with n-hexane and anhydrous ethanol, dried at 80 °C in the vacuum oven. In the composites, the perovskite quantum dots, g-C₃N₄ and Nb₂C MXene can form a g-C₃N₄/ Nb₂C MXene /CsPbBr₃ composites ternary heterojunction, and the obtained samples will be further named as CCM10, CCM20, CCM30, CCM40, respectively.

2.6 Characterization

The morphological structure and chemical properties of the as-prepared samples were characterized and tested as follows. X-ray diffraction (XRD, Cu Kα, Empyrean, Panaco, Netherlands) was used to measure the as-prepared samples structure and phase composition of samples with diffraction angles (2θ) varying from 5° to 90°. The optical absorption behavior of the samples was studied using Fourier transform infrared spectroscopy (FT-IR, IRTracer-100 Shimadzu, Japan) and Ultraviolet–visible (UV-vis) diffuse reflectance spectra (UV-3600Plus DRS, Shimadzu, Japan). Microstructure and morphology were analyzed by Scanning Electron Micrograph (NovaTM NanoSEM 450, FEI, Czech) and Field emission transmission electron microscopy (FETEM) (Talos F200S, FEI, USA) with an energy-dispersive X-ray spectrometer (EDX). Time-resolved transient PL spectra (Edinburgh FLS980, excitation wavelength of 360 nm) obtained the decay curves. X-ray Photoelectron Spectroscopy XPS(Thermo Scientific, USA) was tested by an ultrahigh vacuum VG ESCALAB 250 electron spectrometer and pass Energy 30.0 eVwith Al Kα₁ X-rays, the valence band was also measured. Photoluminescence(PL) spectra (LabRAM HR Evolution HORIBA Jobin Yvon, France) were recorded by applying a 325nm laser excitation. vacuum oven(ZDF6022 yiheng, shanghai) were used to dry the samples.

2.7 Photoelectrochemical measurements

We carried out the photoelectrochemical test using a three-electrode system (CHI 660E, Shanghai Chenhua) including a Pt foil counter electrode, a saturated Ag/AgCl reference electrode and a working electrode. Using the dip-coating method, with the same amount of photocatalyst, we got the working electrode. Following the detailed process is carried out. First, weighting 5mg of the samples, it was dispersed by ultrasound in 5 mL of n-hexane and 5 μL Nafion adhesive solution to form a uniform slurry. Second, using a Da Long pipette, measuring 10 μL of the slurry, dip-coating on the ITO conductive glass with an exposure area of 1*1 cm². And last, the completed ITO samples were dried in a vacuum oven at 80°C for 1 hour. Following, selecting the electrolyte 100 ml can be used acetonitrile solution with 0.08 mol/L of tetrabutylammonium hexafluorophosphate to avoid any moisture. Before the experiment, to remove the effects of air, the electrolyte was purged with high-purity N₂ gas. The photoinduced current density versus time (I-t) was tested at a 0.01 V bias potential under a solar simulator xenon light source (350 W) switching on and off mode. Using a frequency ranging from 105 Hz to 0.01 Hz, selecting open circuit potential, the electrochemical impedance spectroscopy (EIS) Z’-Z’’ curves can be tested.

The photocatalytic CO₂ reduction experiments were performed in a photocatalytic activity evaluation system( CEL-PAEM-D8, Beijing Zhongjiao jinyuan Technology Co. LTD), which included a closed CO₂ circulation reactor system with a top window cell(volume: 150 mL). The inside reaction temperature was controlled at 6℃ by the external circulation of the cooling water system. A 300 W Xe-lamp collocated a 420 nm UV-cut-off filter was placed on the top of the two-neck bottle as the light source passing and positioned 2 cm away from the photocatalytic reactor. The photocatalyst (10mg) was uniformly dispersed into Vessels 50 mm in diameter with n-hexane solvent 3 ml, which was sonicated, vacuum dried to remove the organic solvent and evenly dispersed in 0.0196 m² glass petri dishes. Before the photoreaction, we first test the closure of the whole photocatalytic activity evaluation system, the inner circulation reactor was evacuated vacuum throughout the system and washed three times with high-purity CO₂ gas to get rid of the air and water vapor inside. Then we start to regulate the catalytic environment, the high-purity CO₂ gas was filled into the circulation reactor to reach a pressure of 0.08 atmospheric pressure. Finally, it sits for a period of time (about, 1 hour) to form an environment saturated steam with carbon dioxide. The CO and CH₄ gaseous products at different reaction times were automatically detected by an online gas chromatograph (GC 7920-TF2A Beijing Zhongjiao jinyuan Technology Co. LTD). Then the stability of pure g-C₃N₄, CsPbBr₃, CC composite, and CCM series of samples were tested for consecutive photoreaction runs of 8 h in each run. The reactor was evacuated and refilled with 0.08 atmospheric pressure CO₂-saturated steam every time.

The synthesis mechanism of g-C₃N₄/Nb₂C MXene/CsPbBr₃ ternary heterojunction is presented in Fig. 1. The complete synthesis control details are given in the experimental section.

The XRD patterns of g-C₃N₄, CsPbBr₃, Nb₂C MXene, CC43 sample， and CCM30 sample are shown in Fig 2a. The characteristic peak patterns indicate the synthesis of pure phase g-C₃N₄, CsPbBr₃, and Nb₂C MXene, which are consistent with the corresponding standard references (Fig S1). The crystal structure of the ternary heterojunction CCM series (Fig. S1d) composites still maintain the original perovskite structure. Detailed characterization results in Fig. S1a further support the structural integrity of three parent materials. The XRD peaks of the CC(Fig. S1c) and CCM composites(Fig. S1d, Fig. S2) show similar diffraction peaks to those of g-C₃N₄, CsPbBr₃ and Nb₂C MXene, indicating that the phase structure of g-C₃N₄, CsPbBr₃ and Nb₂C MXene remain unchanged during the composite formation process. It should be noted that the peaks corresponding to g-C₃N₄ and Nb₂C MXene are relatively weak in the CCM series composites due to the amorphous nature of the g-C₃N₄ and the few layers structure of the Nb₂C MXene.

The morphologies of the prepared samples were characterized using SEM, TEM, and High-resolution TEM (HRTEM) techniques, as shown in Fig. 2c and Fig. S3-5. The SEM image (Fig. 2b, S5a) reveals a sheet structure decorated with numerous particles. TEM and HRTEM were performed to confirm the three-phase composition of the CCM30 compound (Fig. 2d). Amorphous g-C₃N₄ is dispersed in the surface layer of the sample (Fig. 2c). In Fig. 2e and 2g, lattice fringes with a d-spacing of 0.291 nm correspond to (220) panes of CsPbBr₃ QDs, while the lattice fringes with d-spacing of 0.296 nm and 0.431 nm correspond to (220) and (200) planes of Nb₂C MXene, respectively. HRTEM image with corresponding live Fast Fourier transformation (FFT) patterns (Fig. 2e and 2g) clearly displaylattice signals of CsPbBr₃ and Nb₂C MXene. The angle between them measures 28^°and 30^°, indicating a near-parallel relationship between CsPbBr₃ and Nb₂C MXene. EDS results (Fig. S5b) demonstrate that the mass fraction of each element in CCM30 composites aligns with the proportion of the synthetic materials used. Additionally, energy-dispersive X-ray spectrometer mapping data (Fig. 2i-l, S2) reveals a uniform distribution of elements (C, N, Br, Pb, Cs, Nb, and F) throughout the nanocomposite structure. Based on the above characterizations, it is evident that the g-C₃N₄/Nb₂C MXene/CsPbBr₃heterostructure has been successfully formed (Fig. 2c).

We carried out a series of optical properties to deeply understand the interaction among ternary compounds and their influence. The FTIR spectra of g-C₃N₄, CsPbBr₃ QDs, and Nb₂C MXene photocatalysts are shown in Fig. 3a and Fig. S6a-d. In the case of g-C₃N₄[51, 52], the peak at 1639 cm⁻¹, 1241 cm⁻¹correspond to C‒N and C=N stretching vibration modes, respectively. The absorption peaks at 1325 and 1245 cm⁻¹ are associated with the out-of-plane bending vibration of triazine ring. The peak at 808 cm⁻¹ is attributed to the molecule breathing modes of tris-triazine units. The peaks at 2924 cm⁻¹, 2853 cm⁻¹, and 1458 cm⁻¹ can be assigned to the asymmetric and symmetric Pb–Br stretching vibrations[53] as well as the bending vibrations of Cs-Pb of CsPbBr₃ QDs. Nb₂C MXene shows typical peaks at 621 cm^-1and 1086 cm^-1, attributing to the interlayer vibrations of C-Nb [54]. In the FTIR spectrum of the ternary compounds, in addition to the typical peaks of g-C₃N₄, CsPbBr₃ QDs, and Nb₂C MXene, a clear red shift in peak position of in C–N stretching mode and the vibrational modes of tris-s-triazine units in g-C₃N₄ is observed. The red shift indicates a strong interaction among the g-C₃N₄, CsPbBr₃ QDs, and Nb₂C MXene interface. Moreover, the typical stretching mode of aromatic C–N and C=N heterocycles in g-C₃N₄ at 1241 cm^-1 are shifted to higher values with an increasing content of Nb₂C MXene (Fig. 3d). This shift can be ascribed to a chemical interaction between the g-C₃N₄ and Nb₂C MXene surfaces, leading to an increase in the electron density of ternary heterocycles in g-C₃N₄. It is noteworthy that even with a single molar ratio of 1:1:1, all CsPbBr₃ QDs can be effectively incorporated onto the surface of two two-dimensional materials, forming a ternary heterojunction structure.

To investigate the light-harvesting property, UV-vis spectra were recorded, as shown in Fig. 3c. The absorption edge of g-C₃N₄, CsPbBr₃ QDs, CC30, and CCM30 groups are observed around 450 nm, 570 nm, 560 nm, and 550nm, respectively, corresponding to the band gap energy of approximately 2.95 eV, 2.31 eV, 2.29 eV, and 2.23 eV [33, 55] (Fig. S7). It is evident that all the binary (Fig. S6g) and ternary (Fig. S6h) samples can be excited under visible-light irradiation. Upon the addition of Nb₂C MXene, both the absorption intensity in the visible region and at the absorption edges increase, which might be correlate with the black color of Nb₂C MXene.

To further investigate the interactions among g-C₃N₄/Nb₂C MXene/CsPbBr₃ binary catalysts, photoluminescence (PL) emission spectra were recorded. The PL intensity is typically indicative of the recombination rate of photoinduced electron-hole pairs, where a lower PL emission intensity suggests a strongly suppressed recombination. As shown in Fig. 3d, it can be observed that pure g-C₃N₄ and CsPbBr₃ exhibit strong and broad PL emission at approximately 498 nm and 524 nm, respectively, which is consistent with the literature. However, the PL peak intensity of the ternary heterojunction is significantly quenched compared to the other composite samples. In particular, PL emissions of CC43 and CCM30 exhibit the lowest intensity within their respective group (Fig. S6e and S6f). It is important to note that Nb₂C MXene does no exhibit any emission peak due to its metallic characteristics. These PL results indicate that the addition of Nb₂C MXene can effectively inhibit the recombination of charge carriers within the system, leading to improved photocatalytic performance.

The charge transfer and separation behavior of the photocatalyst was investigated using time-resolved fluorescence decay technique. As shown in Fig. 3e, all samples exhibit a rapid decay within the nanosecond timescale. The average emission lifetime (Fig. 3f) of g-C₃N₄ CsPbBr₃ QDs, CC43, and CCM30 are 11.20 ns, 9.80 ns, 9.24ns, and 8.70ns, respectively. These results indicate that both binary (Fig. S6i) and ternary heterostructures (Fig. S6j) provide efficient pathways for rapid charge transfer of the photogenerated carriers. The shortened emission lifetimes suggest enhanced charge separation and reduced recombination, which are favorable for efficient photocatalytic performance

The bonding information of g-C₃N₄, Nb₂C MXene, CsPbBr₃, and their composites was investigated using X-ray photoelectron spectroscopy (XPS) (Fig. 4a,c-f and Fig. S8-9). In Fig. 4, CCM30 shows distinct binding energies ascribed to C, N, Br, Pb, Cs, and Nb, indicating the successful synthesis of the ternary composites of g-C₃N₄ / Nb₂C MXene / CsPbBr₃. After fitting the spectra, small binding energy shifts are observed in the ternary structures compared to their individual components such as C1s, N1s, Br3d, and Nd3d. These shifts suggest the presence of intimate interactions within the structure, resulting in charge redistribution. This binding energy shift indicates chemical reactions between the other parent components and g-C₃N₄. Consequently, Nb₂C MXene becomes electron-rich, while the other two components become electron-deficient. This is supported by the negative shift in the binding energy of Nb 3d in CCM30. The binding energy of Br 3d (Fig. 4f) in the CsPbBr₃ QDs is 68.4 eV and 69.5 eV, corresponding to Br 3d_5/2 and Br 3d_3/2 respectively. The shift is -0.2 eV (68.2eV) and -0.3 eV (69.5eV). The binding energy of Nb 3d (Fig. 4e) in the Nb₂C MXene shows six peaks at 203.1 eV, 205.8 eV, 206.8 eV, and 209.6eV. The shift is 0.4 eV (203.5eV), 0.5 eV (206.3eV), 0.2 eV (207.0eV), and 0.2 eV (209.8eV), indicating a strong electron deficient in Nb₂C MXene. Fig 4b shows the differential charge density of the three parent materials. According to whether there is a broken band gap at 0eV, it is obvious that g- C₃N₄ and CsPbBr₃ have obvious semiconductor properties, while Nb₂C MXene has obvious gold properties.

The valence band (VB) potential can be determined by analyzing the VB XPS spectra. As shown in Fig. S10, the energy level of the valence band maximum (VBM) for g-C₃N₄ and CsPbBr₃QDs is 2.25 eV and 1.12 eV, respectively. Based on these results, the band structures of g-C₃N₄ and CsPbBr₃ QDs can be derived, with VBM energies of 2.36 eV and 1.23 eV, respectively. Consequently, the corresponding conduction band energies are -0.59 eV and -1.08 eV respectively. Evidently, the conduction band minimum of g-C₃N₄ is 0.49 eV lower than that of CsPbBr₃ QDs. These dataprovide insights into the band structure of g-C₃N₄ and CsPbBr₃.

To further demonstrate the improved efficiency of photocarrier transfer and separation efficiency, we conducted transient photocurrent measurements for g-C₃N₄, CsPbBr₃, CC43, and CCM30 samples (Fig.5a). Among all the samples, CCM30 exhibited the highest photocurrent indicating enhanced photocarrier separation and transfer. The presence of Nb₂C Mxene is believed to facilitates the efficient separation of photoexcited electrons and holes[15, 56]. Furthermore, electrochemical impedance spectroscopy (EIS) was performed to investigate the catalytic activity of the prepared samples (g-C₃N₄, CsPbBr₃ CC43, and CCM30 samples) (Fig.5b). The arc radius of g-C₃N₄ and CsPbBr₃ samples was larger than that of CC43 and CCM30 composite samples. In contrast, CCM30 exhibited a smaller arc radius, suggesting reduced resistance and enhanced charge transfer. The presence of Nb₂C Mxene is attributed to the improved conductivity and facilitated charge transfer within the composite structure.

The photocatalytic CO₂ reduction activity (Fig. S11) of the samples was evaluated in a photocatalytic system filled with carbon dioxide-saturated water vapor under simulated sunlight[57, 58]. A controlled trial without light irradiation, CO₂ or any photocatalyst did not show any detectable production of CO, CH₄, H₂, or other hydrocarbons (Fig. S12), confirming the necessity of the photocatalyst for the CO₂ reduction process. As expected, the bare Nb₂C MXene showed no photocatalytic activity (Fig. 5c, d) because it is not a photocatalyst[59]. In our system, CO was the main product, accompanied by a small amount of CH₄. The yields of CO and CH₄ were 6.27 μmol g^-1 h^-1 and 0.15 μmol g^-1 h^-1 for pure g-C₃N₄,and 5.31 μmol g^-1 h^-1 and 0.02 μmol g^-1 h^-1 for CsPbBr₃ QDs, respectively (Fig. S13). The production of CO, significantly improved in the g-C₃N₄/CsPbBr₃ binary composite photocatalysts compared to the individual components (Fig. 5d). Furthermore, the introduction of 2D Nb₂C MXene in ternary heterojunction nanocomposites resulted in even higher performance. Among the samples, CCM30 exhibited the highest CO production rate of 53.07 μmol g^-1 h^-1 which was about 8.4, 10, and 2 times higher than that of pure g-C₃N₄, CsPbBr₃ QDs, and the binary composite g‑C₃N₄/CsPbBr₃, respectively. This significant enhancement in CO production highlights the synergistic effect of the ternary heterojunction nanocomposites.

The comparison of g-C₃N₄ and CsPbBr₃ QDs revealed that the CO yield rate in binary photocatalysts increased by more than 2 times, suggesting the formation of heterojunction structure between g-C₃N₄ and CsPbBr₃, whichfacilitates the transfer of photo-excited charges. This finding is consistent with the results obtained from EIS and photocurrent measurements, as well as with previous reports[32, 33]. By adjusting the ratio of components, slightly changes in the photocatalytic performance were observed. Among the binary photocatalysts, CC43 demonstrateed the best performance. Upon the addition of Nb₂C MXene, the CO yield rate of ternary photocatalysts initially reached a comparable level to that of the binary ones when the percentage of Nb₂C MXene was low (CCM10 and CCM20). Subsequently, the maximum performance was achieved in CCM30. However, with further increase in Nb₂C MXene (CCM40), a slight decrease in the photocatalytic ability was observed, although it still outperformed all the binary photocatalysts. This suggests that the good conductivity of Nb₂C MXene facilitates charge transfer between g-C₃N₄ and CsPbBr₃, as supported by PL and PL delay lifetime characterizations. As a result, the maximum photocatalytic performance in terms of CO production was achieved with a yield rate of 53.07 μmol g^-1 h^-1, surpassing the majority of previously the reported studies[60]. The relevant literature on previous reports is listed in Table 1. Obviously,

The CO evolution rate on CCM30 has obvious advantages.

Table 1 Comparison CO evolution rates of the prepared photocatalyst with other literature.

Photocatalyst	Light source	Solvents	CO evolution (μmol/g/h)	Ref.
PCN CsPbBr₃ CsPbX₃/g-C₃N₄.	300 W Xe-lamp (>420 nm)	CO₂ and water vapor	5.5 5.0 28.5	[32]
PCN CsPbBr₃ CPB-PCN	300 W Xe-lamp	acetonitrile/water Ethyl acetate/water	52 9.8 149	[33]
CsPbBr₃ CsPbBr₃/MXene	300 W Xe-lamp (>420 nm)	Ethyl acetate	4.13 26.3	[61]
CsPbBr₃ NCs CsPbBr₃ NCs/GO	100 W Xe lamp	Ethyl acetate	4.12 4.89	[62]
CPB-CN	300 W Xe-lamp (>420 nm)	CO₂ and water vapor	11.5	[63]
g‑C₃N₄ CsPbBr₃ g‑C₃N₄/Nb₂C MXene/CsPbBr₃	300 W Xe-lamp (>420 nm)	CO₂ and water vapor	6.27 5.30 53.07	this work

From a thermodynamic perspective, the reduction potential for converting CO₂ to CO and CH₄ (-0.53V and -0.24V) [64]. We have provided a diagram illustrating all possible pathways for CO₂ reduction to C1 products (Fig S12) The formation of CO is believed to proceed through a single elementary step, in which our ternary heterojunction provides the necessary photocatalytic energy for CO generation. On the other hand, the formation of CH₄ is thought to occur through a series of elementary steps involving the transfer of one or two electrons, with CO serving as an intermediate. It is important to note that when the reduction potential of the electrons system is low, the kinetic reaction rate of elementary elements decreases. Consequently, the subsequent catalytic reaction steps after CO formation may not proceed rapidly enough before CO desorbs from the surface of the photocatalyst. This leass to CO being the main product, with CH₄ being the secondary product.

After stability testing for eight hours, the loss in activity is only 7.1% (Fig 5d), indicating the good stability of our ternary heterojunction photocatalyst. The XRD patterns and FTIR spectra of CsPbBr₃ and CCM30 before and after the stability test were shown in Fig S17. It is evident that CsPbBr₃ undergoes noticeable crystal changes[65], transitioning from the all-orthorhombic structure to an elongated polyhedron[66, 67]. However, even after nearly 24 hours of catalytic activity, the CO production yield relatively stable. Similar structural changes can also be observed in CCM30 after the stability test, indicating that the transformation is primarily due to the change of CsPbBr₃. These results suggest that the photocatalyst exhibits goodstability under the reaction conditions.

To investigate the electronic and transport properties of the ternary heterojunction, we performed DFT calculations[68, 69]. The (110) surface orientation of three parent materials was chosen as the model to simulate the density of states (DOS) and charge density difference[70]. Since catalysis primarily occurs on the surface rather than in the inner layers, two different slabs of Nb₂C MXene were used to construct the ternary heterojunction photocatalyst (Fig 6a). In the ternary heterojunction, the conduction band is mainly occupied by Nb and Cs, consisting empty d orbitals (Fig. S14), On the other hand, the valence band is composed of orbitals from N and Br elements. This confirms that Nb₂C MXene can modify the energy band structure of ternary heterojunction. To maximizethe influence of the ternary composites on the electronic structure, Nb₂C MXene is positioned in the middle layer. The total and partial DOS of the ternary heterostructures for (110) crystal faces is shown in Fig. S13a-f. Upon formation of the heterostructure, a shift in the band gap is observed near the Fermi level, indicating improved conductivity compared to the three individual parent components. This suggests that the ternary heterojunction exhibits enhanced electronic and transport properties, which can contribute to its improved photocatalytic performance.

To gain further insights into the electronic properties at the interface between the CsPbBr₃/Nb₂C MXene and Nb₂C MXene/g‑C₃N₄, we performed calculations of the 3D charge density difference and the plane-averaged electrostatic potential drop across the interfaces of the three parent materials. As shown in Fig. 6b and c, the calculated results indicate a charge transfer from CsPbBr₃ to Nb₂C MXene and subsequently from Nb₂C MXene to g‑C₃N₄, confirming an unbalanced charge distribution at the interfaces. For the ternary heterojunction interface, CsPbBr₃/Nb₂C MXene and Nb₂C MXene/g‑C₃N₄, higher electrostatic potential and potential difference of approximately 15.83 eV and 11.93 eV, and 16.21 eV and 11.88 eV, respectively, were observed. This potential drop corresponds to the internal electric field pointing from g‑C₃N₄ and CsPbBr₃ towards Nb₂C MXene. Consequently, the internal electric field generated by the potential difference aligns with the direction of the ternary heterostructures. Due to this internal electric field, photogenerated electrons tend to drift from the surfaces of g-C₃N₄, or CsPbBr₃ QDs towards the surface of Nb₂C MXene, while photogenerated holes tend to drift in the opposite direction. This phenomenon facilitates the separation and transfer of photogenerated carrier, thereby promoting enhanced photocatalytic performance. We also compared the charge transfer and distribution ofthe binary interface g-C₃N₄/CsPbBr₃, g-C₃N₄/ Nb₂C MXene, and CsPbBr₃/ Nb₂C MXene shown in Fig. S14. The charge transfer from CsPbBr₃ to g‑C₃N₄, g‑C₃N₄ to Nb₂C MXene, and CsPbBr₃ to Nb₂C MXene reveals some unbalanced charge distribution, although less pronounced compared to the ternary heterojunction. The 3D charge density difference and plane-averaged electrostatic potential exhibit lower values, namely 7.18 eV and 2.92 eV, 12.64 eV, and 14.69 eV, respectively. These values indicate a less significant change compared to the ternary heterojunction. The total DOS of the g-C₃N₄/CsPbBr₃ binary heterojunction (Fig. S16) shows a relatively large band gap, reflecting the energy difference between the valence band and conduction band. This further supports the improved electronic properties and potential for efficient charge transfer and separation in the ternary heterojunction structure.

In summary, we have successfully synthesized a g-C₃N₄/Nb₂C MXene/CsPbBr₃ ternary heterojunction through a wet-chemical method. By regulating the heterogeneous interface, we have achieved a significant improvement in the photocatalytic performance, with a CO yield rate of 53.07 μmol g^-1 h^-1. This represents a substantial enhancement compared to single g-C₃N₄, CsPbBr₃, and binary structure. The simulation and theoretical calculations support the presence of an internal electrostatic field driven by Nb₂C MXene, which promotes the transfer of electrons. The lattice matching and enhanced interaction among the three parent materials contribute to the synergistic effect observed in the ternary heterojunction, resulting in excellent photocatalytic activity for CO₂ reduction to CO. These findings provide valuable guidance for the development of efficient ternary heterogeneous structures in various photocatalytic applications.

CRediT authorship contribution statement

Shiding Zhang: Writing main manuscript text, Data curation, Investigation. Yuhua Wang: Idea, Conceptualization, Writing - review & editing, Supervision, Project administration. Yitong Wang: Experiment, Project administration. Gaber A. M. Mersal: Methodology: VASP calculation. A. Alhadhrami, Dalal A. Alshammari, and Hassan Algadi: Experiment support. Haixiang Song：Writing - review & editing.

Declaration of Competing Interest

The authors report no declarations of interest.

Funding

This work was supported by the National Natural Science Foundation of China (Grant Nos. 11375136, 11804005, 12204014) , the Science and Technology Planning Project of Henan Province (No.232102241016), Anyang Institute of Technology University-level Scientific Research Cultivation Fund (YPY2021016). Numerical calculation is supported by High-Performance Computing Center of Wuhan University of Science and Technology. Anyang Institute of Technology energy power a new round university-level key discipline funding. The authors extend their appreciation to Taif University, Saudi Arabia for supporting this work through project number (TU-DSPP-2024-21).

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No competing interests reported.

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Enhanced Photocatalytic CO2 Reduction via MXene synergism: Constructing a Strong Intra-layer Electric Field Ternary Heterojunction of g-C3N4/Nb2C MXene/CsPbBr3

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