Rational Design of PCN/Ce-MOF S-Scheme Heterojunction for Highly Efficient Synergistically Photocatalytic H2 Evolution and Tetracycline Degradation

TL;DR

This study creates a new photocatalyst that efficiently produces hydrogen and breaks down tetracycline under visible light.

Contribution

A novel S-scheme heterojunction of PCN and Ce-MOF is designed for synergistic hydrogen evolution and tetracycline degradation.

Findings

The PCN/Ce-MOF composite achieves a hydrogen production rate of 495.7 μmol g−1 h−1.

Tetracycline removal efficiency reaches 78% due to effective hole capture and reduced electron-hole recombination.

Abstract

Catalytic systems that couple pollutant degradation with hydrogen evolution have attracted significant attention due to their potential to simultaneously address environmental and energy issues. In this study, an S-scheme heterojunction composed of lamellar polymeric carbon nitride (PCN) anchored with a rod-like cerium metal–organic framework (Ce-MOF) was successfully synthesized via a facile one-step oxidation method, enabling efficient visible-light-driven photocatalytic hydrogen evolution and simultaneous tetracycline degradation. The optimized PCN/Ce-MOF composite delivers a hydrogen production rate of 495.7 μmol g−1 h−1 and achieves a tetracycline removal efficiency of 78%. Such excellent performance is attributed to the charge transfer mechanism of the S-scheme heterojunction in the PCN-Ce-MOF composite during the reaction process, while retaining the intrinsic redox capabilities of both materials. Meanwhile, mechanistic studies reveal that tetracycline can effectively capture holes during its efficient degradation, inhibit electron–hole recombination, and promote proton reduction to generate hydrogen. This investigation provides valuable insights for the rational design of S-scheme heterojunction photocatalysts, aiming to achieve efficient and stable photocatalytic hydrogen production and synergistic degradation of organic pollutants.

Figures

Full text

1. Introduction

The escalating global energy crisis and worsening environmental degradation pose serious challenges to sustainable development and ecosystem preservation. Solar-driven hydrogen production coupled with photocatalytic pollutant degradation emerges as a promising strategy to mitigate these dual crises [1,2,3,4]. However, most current hydrogen-evolution systems depend on sacrificial agents (e.g., methanol, lactic acid, triethanolamine (TEOA), etc.) to facilitate oxidative half-reactions; their irreversible consumption both wastes resources and leaves photogenerated holes underutilized [5,6]. Moreover, organic contaminants such as antibiotics can function as alternative sacrificial agents to enhance photocatalytic hydrogen production. Replacing conventional sacrificial reagents with antibiotics in dual-functional photocatalytic systems—thereby enabling concurrent antibiotic wastewater degradation and hydrogen generation—holds substantial practical and environmental significance and aligns with efforts to achieve a carbon peak and carbon neutrality [7,8].

Simultaneous photocatalytic degradation of antibiotic-laden wastewater and concurrent hydrogen production has recently garnered significant attention. A range of bifunctional catalysts—such as Pt/TiO_2_, TiO_2_/g-C_3_N_4_, ZnInS_4_/RGO/BiVO_4_, UiO-66@Zn_0.5_Cd_0.5_S, and g-C_3_N_4_/BiVO_4_—have been reported [9,10,11,12,13], and among them, polymeric carbon nitride (PCN) stands out for its straightforward synthesis, mechanical robustness, low cost, and environmental compatibility [14,15]. Nevertheless, pristine PCN often falls short of the demanding redox performance required for coupled oxidation–reduction reactions; rapid recombination of photogenerated electron–hole pairs substantially limits its photocatalytic efficiency [16]. This limitation can be mitigated by constructing heterojunction composites that pair carbon nitride with other photocatalysts possessing suitably aligned energy bands and strong oxidizing power [17,18,19,20]. For example, Nie et al. [21] prepared a graphene quantum dots/Mn-NTiO_2_/g-C_3_N_4_ heterojunction photocatalyst that enabled concurrent photocatalytic degradation of p-nitrophenol (4-NP), ciprofloxacin (CIP), and diethyl phthalate (DEP) alongside hydrogen evolution. Density functional theory calculations coupled with liquid chromatography–mass spectrometry analysis were employed to rationalize the observed lower photocatalytic hydrogen production rate in the 4-NP solution relative to CIP and DEP. By combining theoretical calculations with experimental observations, this study establishes a robust basis for translating photocatalytic technologies into real-world wastewater treatment applications. Cerium metal–organic frameworks (Ce-MOFs) are particularly promising for photocatalytic oxidation owing to the redox flexibility of Ce^3+^/Ce^4+^ pairs and pronounced ligand-to-metal charge-transfer (LMCT) effect [22]. When integrated with PCN, such a material is expected to form an efficient heterojunction that not only increases oxidative power but also substantially improves spatial charge separation [23,24]. However, current research on PCN/Ce-MOF composite materials mostly focuses on single-pollutant degradation or hydrogen production reactions. The reaction kinetics, the fundamental electron-transfer pathways, and the mechanism by which these two processes mutually enhance one another during the coupled antibiotic degradation and hydrogen production reaction remain unresolved; systematic, in-depth studies addressing these aspects are still lacking.

Guided by these considerations, we synthesized a series of PCN/Ce-MOF S-scheme heterojunctions via a straightforward partial-oxidation protocol and deployed them in a coupled system for photocatalytic tetracycline degradation and H_2_ production. Tetracycline functions here both as the target pollutant and as the sacrificial agent for photocatalytic hydrogen generation. The optimized PCN/Ce-MOF specimen delivers an H_2_ evolution rate of 495.7 μmol g^−1^ h^−1^ and achieves 78% synergistic tetracycline degradation. Comprehensive characterization indicates the emergence of strong chemical coupling and a built-in electric field at the heterojunction interface, which together create efficient charge-separation channels. Mechanistic investigations further show that the coupled process proceeds along a bifurcated pathway: photogenerated holes and reactive oxygen species (h^+^ and •O_2_^−^) chiefly drive the mineralization and breakdown of tetracycline, whereas photogenerated electrons are channeled into proton reduction to form hydrogen. The two reactions not only proceed without mutual interference, but the organic intermediates formed during degradation can also serve as hole sacrificial agents, thereby further enhancing hydrogen production efficiency and realizing genuine “synergistic efficiency”.

2. Results and Discussion

2.1. Characterisations

The morphologies and microstructures of PCN, MVCM, and CNCM composites were examined comprehensively by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Thermal polymerization produces PCN with a highly curled, lamellar architecture (Figure 1b), while MVCM adopts a rod-like morphology with relatively uniform thickness (Figure 1c). Among all CNCM composites (Figure 1d and Figure S1), both nanorod and nanosheet morphologies can be observed, and the number of nanorods increases accordingly with a higher proportion of MVCM composites. Furthermore, Figure 1e clearly shows rod-shaped MVCM adsorbed on the nanosheet-structured PCN. High-angle annular dark-field (HAADF) imaging (Figure 1f) together with energy dispersive spectrometer (EDS) mappings (Figure 1g) further confirm that nanorod-shaped MVCM is deposited on the surfaces of PCN nanosheets. Collectively, the SEM, TEM, and EDS data corroborate the successful synthesis of the PCN-MVCM composite.

Powder X-ray diffraction (XRD) was used to determine the phase composition and crystallographic features of the catalyst samples. As shown in Figure 2a, pure PCN exhibits two characteristic reflections at 2θ = 13.1° and 27.3°, attributable to in-plane periodic stacking of the tri-s-triazine units (100) and to the typical interlayer aromatic stacking (002), respectively [25,26]. There are also two weaker XRD peaks at 18° and 21.6°, which mainly result from incomplete conversion during the thermal condensation of urea into PCN, representing characteristic diffraction peaks of non-ideal crystalline PCN [27]. Additionally, the diffraction peak at 57.5° represents the (004) direction in the PCN crystal lattice [28]. MVCM preserves the crystalline framework of the parent Ce-MOF (Figure S2), adopting a monoclinic phase with space group Cc, in agreement with prior reports [29]. In the CNCM composites, diffraction peaks from both PCN and MVCM are present; their relative intensities increase progressively with higher MVCM loading, indicating the growing contribution of the MVCM phase.

The Fourier-transform infrared (FT-IR) spectra of pristine PCN, MVCM, and CNCM composites with different ratios are presented in Figure 2b. For pristine PCN, the band at 810 cm^−1^ is assigned to the stretching vibration of the tri-s-triazine ring. The absorptions between 1650 and 1220 cm^−1^ are attributed to C=N and C-N stretching modes within the same tri-s-triazine units. A broad feature spanning 2900–3300 cm^−1^ is ascribed to N-H vibrations of uncondensed amino groups and O-H vibrations from surface-adsorbed H_2_O [30,31]. In MVCM, bands at 1615–1550 cm^−1^ and 1433–1370 cm^−1^ are assigned to asymmetric stretching vibrations of carboxylate groups, while a broad band near 3400 cm^−1^ and a band at 530 cm^−1^ are attributed to O-H and Ce-O stretching vibrations, respectively [29]. The composites exhibit the characteristic bands of both PCN and MVCM, confirming the successful formation of the PCN/Ce-MOF composites.

Nitrogen adsorption–desorption measurements were used to determine the specific surface area and porosity of the as-prepared PCN, MVCM, and CNCM(1:1), as shown in Figure S3a–c. All samples display type IV isotherms with H3-type hysteresis loops (IUPAC classification), indicative of mesoporous and macroporous structures [32]. The specific surface areas of PCN, MVCM, and CNCM(1:1) are 74.2, 32.5, and 68.7 m^2^·g^−1^, respectively. Pore size distribution analysis (Figure S3d–f) reveals that the pores of PCN are primarily distributed within the range of 18–50 nm, whereas the pores of MVCM are concentrated in the 3–10 nm range. Evidently, CNCM(1:1) contains pores of both components, further demonstrating the successful synthesis of PCN/MVCM composite materials

2.2. Photocatalytic Activity Evaluation

Photocatalytic hydrogen evolution was evaluated for the as-synthesized materials using TC as a sacrificial agent under the irradiation of a 300 W xenon lamp equipped with a 420 nm cutoff filter. Figure 3a summarizes the average H_2_ evolution rates measured after 2h illumination for all samples. MVCM produces virtually no hydrogen, which is likely due to the MVCM conduction band potential being more positive than the H^+^/H_2_ redox potential. Pure PCN exhibits a modest H_2_ evolution rate of 22.3 µmol·g^−1^·h^−1^, consistent with rapid recombination of photogenerated electrons and holes. In contrast, the CNCM composites show a pronounced improvement in photocatalytic activity. The CNCM(1:1) composite delivers the highest H_2_ evolution rate of 495.7 µmol·g^−1^·h^−1^, more than 21 times that of pure PCN. A similar pattern emerges for TC degradation (Figure 3b); the TC degradation efficiencies of PCN, CNCM(1:2), CNCM(1:1), CNCM(2:1), and MVCM are 7%, 45%, 78%, 51%, and 59%, respectively. A notable synergistic interaction is observed between photocatalytic hydrogen production and tetracycline degradation in the CNCM composites. The CNCM composite’s enhanced performance in concurrent hydrogen evolution and TC removal arises chiefly from its S-scheme heterojunction architecture, which boosts redox capability and promotes more efficient charge-carrier separation and transport [33,34]. The stability and recyclability of CNCM(1:1) were assessed in a 10 h redox reaction test. As displayed in Figure 3c,d, both photocatalytic hydrogen evolution and simultaneous TC degradation remain largely consistent across five cycles, demonstrating the material’s reasonable reusability and photostability. Complementary XRD and FTIR analyses of CNCM(1:1) after the photocatalytic runs reveal no appreciable structural changes (Figure S4), further corroborating its structural stability.

To identify the principal reactive species responsible for TC degradation, we employed specific scavengers: isopropanol (IPA) for hydroxyl radicals (•OH), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) for photogenerated holes(h^+^) [35], potassium bromate (KBrO_3_) for electrons (e^−^) [36], and ascorbic acid (ASA) for superoxide radicals (•O_2_^−^) [37]. As illustrated in Figure 4a, the addition of KBrO_3_ or IPA produces no appreciable change in the tetracycline degradation rate, indicating that neither electrons nor •OH are the dominant reactive species in this system. However, upon the introduction of EDTA-2Na and ASA, the degradation efficiency of TC drops sharply from 78% to 22% and 8%, respectively. This indicates that •O_2_^−^ serves as the dominant reactive species, while h^+^ contributes to a lesser extent. The involvement of h^+^ and •O_2_^−^ in TC photodegradation was further corroborated by electron spin resonance (ESR) spectroscopy using 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as spin-trapping agents. The addition of TEMPO, a scavenger for h^+^, produces the characteristic three-line ESR pattern with a 1:1:1 intensity ratio attributable to the TEMPO-h^+^ adduct, observed both in the dark and under illumination (Figure 4b). The intensity of this signal decreases upon light exposure, confirming light-induced generation of h^+^. Likewise, DMPO trapping shows no discernible signal in the dark (Figure 4c), whereas a DMPO-•O_2_^−^ adduct signal emerged after 5 min of irradiation and intensified substantially after 10 min, providing clear evidence for the formation of •O_2_^−^ under illumination.

2.3. Photocatalytic Degradation Pathway

To elucidate the TC degradation pathway in this system, liquid chromatography-mass spectrometry (LC-MS) was used to identify degradation products before and after the photocatalytic hydrogen production reaction. TC possesses three electron-rich, oxidation-prone functional groups—conjugated double bonds, hydroxyl groups, and amino groups—that are particularly vulnerable to reactive oxidative species [38,39]. Based on the predicted active sites and detected degradation products (Figure S6), the possible degradation pathways for TC are proposed. Figure 5 illustrates the two main degradation pathways of TC, among which pathway I gradually oxidizes the dimethylamino group on the TC side chain through active substances (•O_2_^−^, •OH), generating demethylation product A_I_(m/z = 416), followed by a further demethylation and ring opening reaction [38,39]. Pathway II proceeds via a h^+^-mediated attack: initial dehydroxylation of the TC scaffold is followed by deamination and deacetylation, then ring-opening and cleavage accompanied by carboxylation, gradually forming product D_II_(m/z = 237) and other low-molecular-weight species. Eventually, the produced small molecules or organic compounds are further mineralized to CO_2_ and H_2_O [38,39,40]. At the same time, due to the capture of photogenerated holes by some intermediates, the recombination of photogenerated electrons and holes is suppressed, promoting the utilization of electrons and thus improving the photocatalytic hydrogen production performance of the catalyst.

2.4. Mechanism Considerations

We characterized the optical properties of the samples by UV–visible diffuse reflectance spectroscopy (UV–Vis DRS) and steady-state photoluminescence (PL) spectroscopy. As shown in Figure 6a, pristine PCN and MVCM display intrinsic absorption onsets at 446 nm and 400 nm, respectively. The corresponding optical band gaps (Eg) were determined using the Tauc relation (αhν) ^n^ = A(hν − Eg), where α is the absorbance, h is Planck’s constant, ν is the photon frequency, A is a proportionality constant, and the exponent n reflects the nature of the electronic transition. Because PCN is an indirect-bandgap semiconductor, n = 1/2, whereas MVCM, as a direct-bandgap material, uses n = 2. From the Tauc plot (Figure 6b), the band gaps of PCN and MVCM are estimated to be 2.6 eV and 3.10 eV, respectively. Additionally, we used room-temperature photoluminescence spectroscopy with an excitation wavelength of 300 nm to analyze the separation and recombination of photogenerated carriers in the samples. As shown in Figure 6c, PCN displays the strongest band-to-band emission, with a peak near 450 nm. In the CNCM composites, a secondary emission appears near 390 nm—assigned to interband transitions in MVCM—superimposed on the principal 450 nm peak, a result that aligns with the DRS data. Notably, increasing MVCM loading progressively reduces the overall fluorescence intensity. Since the decrease in photoluminescence intensity usually indicates lower radiative recombination of photogenerated charge carriers, this trend suggests an improvement in photocatalytic performance.

To further elucidate the separation efficiency of photogenerated electrons and holes, we carried out a series of electrochemical measurements. In the electrochemical impedance spectroscopy (EIS) experiment (Figure 7a), CNCM(1:1) shows the smallest ESR semicircle compared with PCN and MVCM, indicating that the construction of the composite material enhances the charge separation efficiency. The obtained semicircle can be simulated using an equivalent circuit model, as shown in the inset part of Figure 7a. The charge transfer impedance (Rct) of CNCM(1:1) through AC impedance spectra is notably low at only 241 Ω, sharply contrasting with both PCN (354 Ω) and MVCN (421 Ω). The reduced impedance value of CNCM(1:1) indicates that it has higher conductivity, which is more conducive to the migration of photogenerated charges. Transient photocurrent measurements (Figure 7b) further confirm this trend; the photocurrent density of CNCM(1:1) is substantially higher than that of PCN and MVCM, demonstrating that heterojunction formation markedly improves the kinetics of photogenerated charge transport. Together, these results indicate enhanced carrier mobility in the CNCM(1:1) composite, consistent with the observed increases in hydrogen production and pollutant degradation activity. Mott–Schottky plots (Figure 7c,d) display positive slopes for both PCN and MVCM, identifying them as n-type semiconductors. Their flat-band potentials are −1.41 V and 0.12 V versus the normal hydrogen electrode (NHE), respectively. For n-type materials, the flat-band potential lies approximately 0.1 eV above the conduction band (CB), which yields CB values of −1.31 eV for PCN and 0.22 eV for MVCM [41]. The corresponding valence band (VB) positions are 1.29 eV for PCN and 3.32 eV for MVCM.

In situ-irradiated XPS provides an effective means to probe elemental chemical states and electron flow [42,43]. For CN, the C 1s spectrum in Figure 8a exhibits three diagnostic peaks at 284.8, 286.3, and 288.3 eV, assignable to C-C, C-O, and N-C=N environments, respectively. Relative to pristine PCN, the 288.3 eV feature of the CNCM(1:1) composite shifts to 288.5 eV in the dark and reverts to 288.2 eV under illumination, implying that the corresponding carbon sites behave as electron donors in the dark but as electron acceptors when illuminated. A comparable effect is observed in the high-resolution N 1s spectra (Figure 8b). The N 1s envelope of PCN can be deconvoluted into peaks at 398.7, 400.4, and 401.4 eV, which correspond to sp^2^-hybridized nitrogen (C=N-C), bridging nitrogen (N-(C)3), and terminal amino groups (C-N-H), respectively. Notably, the binding energy of the first two N atoms in the CNCM(1:1) composite is higher than that of pristine PCN in the dark, but shifts toward lower values under illumination. This behavior implies that these N sites act as electron donors in the absence of light and become electron acceptors when illuminated. As shown in Figure 8c, the O1s spectrum of MVCM exhibits two distinct peaks at 529.6 and 531.6 eV, corresponding to lattice oxygen (Ce-O) and surface-adsorbed oxygen species, respectively. Importantly, the lattice oxygen binding energy in the CNCM(1:1) composite shifts to a lower value in the dark and then increases under illumination relative to pure MVCM. The Ce 3d spectrum in Figure 8d can be deconvoluted into ten components, confirming the coexistence of Ce^3+^ and Ce^4+^ oxidation states. Compared with pure MVCM, intimate contact between MVCM and PCN produces a shift in the Ce binding energies: the signals at 887, 897.8, 903.1, and 916.27 eV shift to lower values in the dark and to higher values upon illumination. This behavior implies that, in the composite, Ce species function as electron acceptors under dark conditions and as electron donors under light. All XPS results demonstrate that, prior to illumination, electrons transfer from the PCN component to the MVCM component across the MVCM/PCN interface, indicating the establishment of a heterojunction between MVCM and PCN, which is consistent with the previous band structure analysis. Upon light excitation of the MVCM/PCN heterojunction, the direction of photogenerated electron flow reverses (from MVCM to PCN), consistent with an S-scheme migration pathway for the photogenerated electrons.

Synthesizing the foregoing results and analysis, we propose an S-scheme charge-transfer mechanism illustrated in Figure 9. Prior to contact, the conduction band (CB), valence band (VB), and Fermi level (EF) of PCN lie at higher energies than the corresponding levels of MVCM. Upon intimate contact, electrons migrate from PCN, which possesses the higher EF, into MVCM with the lower EF until the two materials reach a common Fermi energy. At equilibrium, this charge redistribution produces an electron-accumulation layer on the MVCM surface and an electron-depletion layer on the PCN surface, thereby establishing an internal electric field (IEF) directed from PCN toward MVCM. Concurrently, the PCN CB and VB bend upward while the MVCM bands bend downward, creating a potential barrier at the interface. Under illumination, the internal electric field (IEF) drives photogenerated electrons from the conduction band (CB) of MVCM toward the valence band (VB) of PCN. An interfacial energy barrier simultaneously impedes electron transfer from PCN to MVCM and hole transfer from MVCM to PCN, which enhances charge separation. In addition, the IEF, together with Coulomb interactions, promotes recombination between PCN-photogenerated holes and MVCM-photogenerated electrons. As a result, electrons accumulate in the CB of the PCN component and reduce H^+^ to produce H_2_, while holes build up in the VB of MVCM and oxidatively degrade TC, enabling concurrent hydrogen evolution and TC degradation.

3. Materials and Methods

3.1. Materials

Urea, cerium nitrate (Ce(NO_3_)3·6H_2_O), trimesic acid (H_3_BTC), tetracycline, N, N-dimethylformamide (DMF), ethanol, isopropanol, disodium ethylenediaminetetraacetate, ascorbic acid, potassium bromate, sodium hydroxide, and hydrogen peroxide (30 wt%) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China) in analytical grade. All chemicals were used as received without further purification. Deionized water was employed throughout the experiments.

3.2. The Preparation of Polymeric Carbon Nitride

Polymeric carbon nitride (PCN) was prepared by conventional thermal polycondensation of urea. In brief, urea was heated in a muffle furnace at 2 °C·min^−1^ to 550 °C and held at that temperature for 4 h. The resulting yellow bulk PCN was subsequently ground to a fine powder.

3.3. The Preparation of Ce-MOF

Ce-MOF was prepared by a facile, low-temperature solvothermal method using Ce(NO_3_)3·6H_2_O as the cerium source, following the reported procedure [26]. In brief, 10 mmol of Ce(NO_3_)3·6H_2_O was dissolved in 45 mL of deionized water (solution A), and 10 mmol of H_3_BTC was dissolved in 15 mL of a 1:1 (v/v) water–ethanol mixture (solution B). Solution A was then added dropwise to solution B under vigorous magnetic stirring at 60 °C, and the resulting mixture was held at 60 °C for 1 h. The white precipitate was collected by centrifugation, washed thoroughly with deionized water and ethanol, and dried overnight at 70 °C.

3.4. The Preparation of PCN/Ce-MOF

As shown in Figure 1a, PCN/Ce-MOF (CNCM) was prepared by a simple and rapid in situ partial-oxidation procedure. In a typical synthesis, 400 mg of Ce-MOF and X mg (X = 200, 400, 600) of PCN were dispersed in 80 mL of deionized water and ultrasonicated for 20 min to produce a uniform suspension. A freshly prepared NaOH/H_2_O_2_ solution (0.5 mL, prepared by mixing 9.5 mL of 2.5 M NaOH with 0.5 mL of 30 wt% H_2_O_2_) was then added to the suspension, which was shaken vigorously for several minutes. The resulting yellow product was washed repeatedly with deionized water until the supernatant reached neutral pH and was dried overnight at 70 °C. Based on the mass ratio of Ce-MOF to PCN, the final samples are denoted CNCM(1:2), CNCM(1:1), and CNCM(2:1). The partially oxidized Ce-MOF (MVCM) was synthesized identically, omitting PCN from the procedure.

3.5. Catalyst Characterization

The X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer using Cu-Kα radiation (λ = 1.5406 Å). Fourier transform infrared (FTIR) spectra were acquired on a Nicolet iS 50 spectrometer (Thermo, Waltham, MA, USA). The morphology of the samples was characterized by field-emission scanning electron microscopy (FESEM, Hitachi SU-8010, Hitachi High-Tech, Tokyo, Japan) and transmission electron microscopy (TEM, FEI Talos F200x G2, Thermo, Waltham, MA, USA). Nitrogen adsorption–desorption isotherms were measured at 77 K on a Micromeritics ASAP 2460 instrument to determine the specific surface area and porosity. Ultraviolet-visible (UV–Vis) diffuse reflectance spectra (DRS) were obtained using a Varian Cary 500 UV–Vis spectrophotometer with BaSO_4_ as the reference. X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB250 instrument with a monochromatized Al Kα line source (200 W) (Thermo Scientific, Waltham, MA, USA). Photoluminescence (PL) spectra were recorded on an Edinburgh FLS920 spectrophotometer (Edin-burgh, Livingston, West Lothian, UK). The electrochemical measurements were performed using a Shanghai Chenhua CHI760E system (Chenhua, Shanghai, China) in a standard three-electrode cell, with a silver/silver chloride electrode as the reference electrode and a Pt plate as the counter electrode. The DMPO-•O_2_^−^ and TEMPO-h^+^ signals were analyzed using Bruker EMXplus-6/1 electron spin harmonic spectroscopy (EPR) (Billerica, MA, USA), and the possible degradation products of tetracycline were analyzed using TSO Fortis Liquid Chromatograph-Mass Spectrometer (Thermo, Waltham, MA, USA).

3.6. Photocatalytic Test

Photocatalytic hydrogen evolution reactions (HER) were carried out in a Pyrex top-irradiation reactor fitted with a quartz window and coupled to a closed gas-circulation system. For each experiment, 50 mg of catalyst powder was dispersed in 100 mL of tetracycline solution (20 mg/L). Platinum (3 wt%) was deposited on the catalyst surface by in situ photodeposition using chloroplatinic acid as the precursor. The reaction vessel was sealed, and residual gases were evacuated by repeated pumping to vacuum. Illumination was provided by a 300 W xenon lamp (CEL-HXF300-T3, Ceaulight, Beijing, Chinaequipped with a 420 nm cutoff filter. Reaction temperature was held at 5 °C by circulating coolant through a water jacket surrounding the reactor. Gaseous products were monitored in real time by online gas chromatography (GC) with a thermal conductivity detector (TCD). After the reaction, the absorbance of the TC solution at 357.4 nm was measured using a UV–Vis spectrophotometer.

4. Conclusions

In summary, a series of S-scheme PCN/Ce-MOF hybrid catalysts was synthesized via a facile one-step oxidation method. The optimized CNCM(1:1) nanocomposite displayed superior performance, achieving an H_2_ evolution rate of 495.7 μmol g^−1^ h^−1^ and TC degradation efficiency of 78%, substantially surpassing the activities of pristine PCN and MVCM. Free-radical quenching and EPR measurements indicate that photogenerated h^+^ and •O_2_^−^ species contribute positively to both TC degradation and hydrogen evolution. Band-structure analysis combined with in situ irradiation XPS reveals the formation of an interfacial internal electric field directed from PCN toward MVCM. Under illumination, charge transfer from the CB of MVCM to the VB of PCN establishes an S-scheme heterojunction that preserves strong redox potentials while promoting effective separation of photogenerated carriers, thereby enhancing photocatalytic performance. This work presents a viable design strategy for multifunctional photocatalytic systems that concurrently address clean energy generation and wastewater remediation.