Cobalt-based metal–organic framework derived Co/CoO@C under moderate temperature for improving the performance of photocatalytic degradation of organic pollutants

Abstract

This study employed the low-cost rare earth element cobalt as the metal node. By selecting Co(II) ion, N,N′-(1,4-phenylene)dinicotinamide (1,4-pdna), and 1,4-naphthalenedicarboxylic acid (1,4-H₂NDC), a novel cobalt-based metal–organic framework (Co-MOF), namely {[Co(1,4-pdna)_1.5(1,4-NDC)(H₂O)]·3H₂O}_n was designed and synthesized via a hydrothermal method. By calcinating the precursor at different temperatures, a series of materials were obtained, namely Co/CoO@C-400/600/800/1000. The characterization and analysis of Co-MOF and its derived materials were conducted in detail. The photocatalytic performance of these materials was investigated under visible light irradiation using five different dyes as degradation targets. Experimental results demonstrated that Co/CoO@C-400 exhibited the optimal degradation efficiency for gentian violet (GV), achieving a degradation rate of 96.8% after 240 min under visible-light irradiation. Mechanistic studies revealed that photogenerated holes (h⁺), hydroxyl radicals (˙OH), and superoxide anion radicals (˙O₂⁻) functioned synergistically, which was further verified through trapping experiments. The enhanced photocatalytic activity was attributed to the reduced bandgap of the material and the synergistic effect arising from the intimate interfacial coupling between Co/CoO and the carbon matrix, which facilitates efficient charge separation and transfer. The structure simultaneously accelerated interfacial charge transfer during the photocatalytic reaction. Furthermore, Co/CoO@C-400 efficiently catalyzed the degradation of various organic dyes under visible-light irradiation, demonstrating promising potential for applications in environmental remediation.

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1. Introduction

With the rapid development of textile, dyeing, and chemical industries, the discharge of dye wastewater (e.g., azo dyes and anthraquinone dyes) has become a significant global environmental pollution issue.¹ Annually, over 700 000 tons of dye pollutants enter water bodies worldwide.² These contaminants exhibit high toxicity, complex structures, and resistance to photolysis. They not only harm crop growth and yield but also pose threats to human health through the food chain and impact the sustainability of economic activities.³ It is very difficult to effectively remove these recalcitrant dye molecules using conventional wastewater treatment methods, such as chemical precipitation and membrane separation.^4,5 Consequently, the development of efficient, environmentally friendly, and stable technologies for treating dye wastewater is urgently needed. Among the potential solutions, photocatalytic degradation technology is regarded as a promising approach due to its environmental compatibility and operational simplicity.^6–9 This technology utilizes natural or ultraviolet light to irradiate semiconductor photocatalysts, thereby converting dyes into harmless substances (H₂O and CO₂).¹⁰ However, existing photocatalysts (such as TiO₂-based materials,¹¹ bismuth-based visible-light photocatalysts (e.g., AgVO₃/BiVO₄), and g-C₃N₄-based materials)¹² commonly suffer from limitations including low light utilization efficiency, poor stability, and difficult recovery, which hinder their practical application.¹³

To address the inherent limitations of conventional photocatalysts, carbon-coated metal oxide derived materials demonstrate revolutionary advantages.¹⁴ These materials typically feature a core–shell structure, wherein the metal oxide core (e.g., TiO₂ and Fe₃O₄) provides photocatalytically active sites, while the outer carbon shell performs multiple critical functions.^15,16 However, several key challenges currently hinder their development. Conventional preparation methods for such carbon-coated materials rely heavily on high-temperature thermal treatment (>600 °C). This process often induces detrimental effects including sintering of the metal oxide phase, reduced specific surface area, pore collapse, and uncontrollable pyrolysis reactions at elevated temperatures.¹⁷ For instance, sol–gel derived TiO₂@C composites frequently suffer from amorphous aggregation, random distribution of active sites, and weak interfacial coupling primarily dependent on physical encapsulation (van der Waals forces), making the carbon shell prone to delamination. This series of structural and interfacial issues ultimately compromise the overall photocatalytic performance.¹⁸

Building upon the aforementioned research, metal–organic framework (MOF) derived carbon-coated metal/metal oxide materials (Co/CoO@C) demonstrate significant advantages. Through pyrolysis-driven transformation, these materials inherit beneficial properties from their MOF precursors (such as high specific surface area and porosity)^19,20 to enhance stability and improve conductivity.^21,22 Crucially, robust interfacial coupling between the carbon layer and the metal oxide phase, formed via chemical bonds, effectively suppresses delamination and facilitates efficient interfacial electron transfer, rendering them ideal candidates for high-performance photocatalysts. The MOF-precursor-based approach provides an effective pathway for designing photocatalysts with tailored structures and properties.²³ However, certain MOFs used as precursors exhibit limitations including inherently weak catalytic activity, low conductivity, and restricted application scope, as exemplified by ZIF-8-derived ZnO which suffered from rapid recombination of photogenerated electron–hole pairs that severely hindered photocatalytic applications.²⁴ In contrast, employing Co-MOFs as precursors effectively addressed these shortcomings. In comparison, Co/CoO@C materials synthesized using composite Co-MOFs as precursors effectively address these shortcomings: Compared to conventional cobalt oxides such as CoO and Co₃O₄,²⁵ which suffer from poor electrical conductivity, Co/CoO@C composite materials enhance the overall electrical conductivity of the system by incorporating conductive carbon matrices.²⁶ The high conductivity of the carbon component provides efficient pathways for rapid electron transport. Furthermore, the diversity of cobalt oxidation states in Co/CoO@C composites, combined with their integration with the carbon matrix, offers abundant active sites and demonstrates excellent synergistic catalytic capabilities.²⁷ Research data indicate that Ma et al. synthesized Co-C1000 achieving 99.21% degradation efficiency for methyl orange (MO),²⁸ while Lu et al. developed a Co-MOF (46.248 m² g⁻¹ specific surface area) demonstrating 96.3% catalytic efficiency for basic fuchsin (BF) within 50 min with a maximum rate constant of 0.01880 min⁻¹.²⁹ Furthermore, the inherent magnetic properties of Co-MOFs enhance their recoverability and reusability. Therefore, this work aims to design and develop highly active visible-light-driven photocatalysts derived from Co-MOFs.

This study employed a single cobalt metal center for MOF construction, systematically modulating organic ligands to optimize coordination effects, with the resulting materials exhibiting excellent recyclability and maintained activity upon reuse. Innovatively adopting a moderate 400 °C calcination strategy, detrimental issues associated with high-temperature pyrolysis were effectively circumvented, including metal sintering, carbon matrix collapse, and uncontrolled reduction reactions. Moreover, the MOF derived material synthesized in this study achieved efficient dye photodegradation under visible-light irradiation, significantly enhancing solar energy utilization efficiency. MOF-derived cobalt-based materials were thus developed for aquatic organic dye remediation: (i) the Co-MOF was synthesized using CoCl₂·6H₂O, N,N′-(1,4-phenylene)dinicotinamide (1,4-pdna), and 1,4-naphthalenedicarboxylic acid (1,4-H₂NDC). (ii) MOF derivatives (Co/CoO@C-400/600/800/1000) were obtained via gradient pyrolysis (400–1000 °C) (Scheme 1). Comprehensive characterization and visible-light-driven photocatalytic evaluation revealed that Co/CoO@C-400 demonstrated optimal performance with degradation efficiencies of 96.8% for gentian violet (GV), 84.3% for Congo red (CR), and 94.1% for methyl orange (MO) within 240 min. Remarkably, cycling tests confirmed exceptional stability with negligible efficiency loss after four consecutive runs. Radical trapping experiments identified photogenerated holes (h⁺), hydroxyl radicals (˙OH), and superoxide anion radicals (˙O₂⁻) as dominant active species, ultimately informing the proposed degradation mechanism.

Scheme 1 The synthetic route of Co-MOF and derived materials.

2. Experimental section

2.1. Synthesis of the Co-MOF

CoCl₂·6H₂O (47 mg, 0.2 mmol), 1,4-pdna (30 mg, 0.1 mmol), 1,4-H₂NDC (32 mg, 0.15 mmol), NaOH (1.4 mL 0.1 mol L⁻¹), and deionized water (4 mL) were placed in a 25 mL Teflon-lined autoclave. The mixture was heated at 120 °C for 96 h, yielding orange crystals. The crystals were subsequently washed with distilled water and dried in a vacuum oven. Elemental analysis for Co-MOF (C₃₉H₃₅CoN₆O₁₁), calculated: C 56.92%, N 10.22%, H 4.26%; found: C 56.94%, N 10.24%, H 4.28%.

2.2. Synthesis of derived materials

Co/CoO@C-400: Co-MOF was placed in a quartz boat within a tube furnace. Under a nitrogen atmosphere (flow rate: 50 mL min⁻¹, purity: 99.999%), the temperature was raised to 400 °C over 40 min. The temperature was then maintained at 400 °C for 60 min under continuous N₂ flow (50 mL min⁻¹). Finally, the reactor was cooled to room temperature under N₂ protection to obtain the Co/CoO@C-400 product.

Co/CoO@C-600/800/1000: Co/CoO@C-600, Co/CoO@C-800, and Co/CoO@C-1000 were synthesized using the same procedure, but with pyrolysis temperatures of 600 °C, 800 °C, and 1000 °C, respectively.

3. Results and discussion

3.1. Crystal structure of the Co-MOF

Single crystal X-ray diffraction (SCXRD) analysis revealed that the Co-MOF crystallized in the monoclinic space group P21/n (Table S1). Each structural unit comprised one Co(II) ion, one point five 1,4-NDC anions, one 1,4-pdna ligand, one coordinated H₂O molecule and three lattice H₂O molecules. Within the {CoO₃N₃} coordination environment, the six-coordinated Co(II) ion exhibited a distorted octahedral geometry. Specifically, two O atoms (O1, O4#1; Co–O bond lengths = 2.0553(16)–2.0912(15) Å) were provided by the 1,4-NDC anions (symmetric code: #1 x + 1, y, z), while three N atoms (N1, N5, N2#2; Co–N bond lengths = 2.155(2)–2.207(2) Å) originated from the 1,4-pdna ligands (symmetric code: #2 –x + 1, –y, –z + 2) (Table S2 and Fig. 1a). The 1,4-pdna ligand connected with adjacent Co(II) ions to form a zigzag 1D “0101” [Co(1,4-pdna)]_n chain (Fig. 1b), while each oxygen atom on the carboxyl group bridged with Co ions to create another infinitely extending 1D linear [Co(1,4-NDC)]_n chain (Fig. 1c). The two kinds of chains were linked with each other to generate a 2D layer (Fig. 1d). The 1,4-pdna ligand, (1,4-pdna)₂ loop and 1,4-NDC anion can serve as linkers coordinated to Co(II) ions, thereby forming a 2D sql topological framework (Fig. 1e). To stabilize the structure, such a 2D layer and another identical 2D layer formed an interlaced structure of 2D + 2D → 3D (Fig. 1f and g).

Fig. 1 (a) The coordination environment of the Co(II) ion in Co-MOF. View of 1D chains formed by the 1,4-pdna ligand (b) and 1,4-NDC anion (c) of Co-MOF. (d) View of the 2D structure of Co-MOF. (e) View of the 2D topology structure of Co-MOF. (f) View of the 2D + 2D →3D structure of Co-MOF. (g) View of the 3D interpenetrating structure of Co-MOF.

3.2. Structural characterization of the Co-MOF

Powder X-ray diffraction (PXRD) analysis indicated that the experimentally synthesized crystal matched well the simulated one, confirming the successful synthesis of Co-MOF (Fig. S1). As shown in Fig. S2, the Fourier transform infrared (FTIR) spectrum of the pristine sample exhibited a broad and intense O–H stretching vibration band centered at 3400 cm⁻¹ (extending to 2500 cm⁻¹), accompanied by a distinct H–O–H bending vibration at 1630 cm⁻¹,³⁰ collectively confirming the presence of crystalline water. A strong absorption at 1700 cm⁻¹ was assigned to the carboxyl carbonyl (C=O) stretching vibration of the 1,4-NDC anion,³¹ while multiple peaks in the 1600–1400 cm⁻¹ region arose from aromatic skeletal vibrations (C=C/C=N) of the pyridine and naphthalene rings.^32,33 Under a N₂ atmosphere, thermogravimetric analysis (TGA) of the Co-MOF within the 20–800 °C range revealed a four-stage weight loss profile (Fig. S3). The initial stage (∼100 °C) with ∼12% mass loss corresponded to the removal of coordinated water molecules. The subsequent second and third stages (∼350 to 500 °C) exhibited significant mass loss (∼50%), which was attributed to ligand decomposition. The final gradual mass loss stage likely arises from partial gasification of amorphous carbon coupled with the reduction of cobalt oxides. Meanwhile, metallic cobalt (Co⁰), carbon, and residual cobalt oxides were likely the ultimate products.

3.3. Structural characterization of the derived materials

To elucidate the fundamental structural evolution of derived materials during calcination at different temperatures, scanning electron microscopy (SEM) was employed for morphological characterization (Fig. 2a). Carbon materials synthesized via pyrolysis using the Co-MOF as a sacrificial template were investigated, with Co/CoO@C-400 exhibiting a distinct irregular surface morphology featuring a loose and porous carbon matrix with homogeneous elemental distribution (Fig. 2b–e) compared to materials calcined at other temperatures shown in Fig. S4. Co/CoO@C-400 demonstrated relatively uniform nanoparticle aggregates, which may expose a greater density of active sites and contribute to its superior catalytic activity.

Fig. 2 (a) Typical SEM image of Co/CoO@C-400. (b)–(e) The corresponding EDX elemental mappings for Co, N, C, and O. (f) The FTIR spectra of Co/CoO@C-400/600/800/1000. (g) The PXRD patterns of Co/CoO@C-400/600/800/1000.

Following medium-to-high temperature calcination, the spectra underwent significant transformation (Fig. 2f). The complete disappearance of the band at 1630 cm⁻¹ indicated thorough removal of the water of crystallization, which was consistent with the TG data. The disappearance of the 1700 cm⁻¹ carbonyl peak confirmed thermal decomposition of the 1,4-NDC anion, while the marked attenuation of aromatic C–H (∼3100 cm⁻¹) and C=C (∼1500 cm⁻¹) vibrations revealed partial carbonization of the pyridine/naphthalene ring skeletons.³⁴ The emergence of a new strong absorption band in the low-wavenumber region (∼500 cm⁻¹) was attributed to Co–O stretching vibrations,^35,36 clearly indicating the formation of cobalt oxide phases (e.g., Co₃O₄ or CoO), as further corroborated by PXRD and X-ray photoelectron spectroscopy (XPS) analyses. These changes demonstrated a stepwise phase transformation involving dehydration, ligand decomposition, and metal oxidation within the material.

Subsequent characterization of the derived materials indicated structural degradation of the pristine Co-MOF framework at elevated temperatures, accompanied by progressive carbonization. As shown in Fig. 2g, the presence of graphitic carbon was confirmed by a diffraction peak at 2θ = 26.2°, corresponding to the (002) plane of graphite.³⁷ Diffraction peaks observed at 2θ = 44.2°, 51.5°, and 75.8° were assigned to the (111), (200), and (220) planes of metallic cobalt (Co⁰), respectively.³⁸ Peaks at 2θ = 36.6°, 61.5°, and 73.6° corresponded to the (110), (200), (220), and (311) planes of cobalt oxide (CoO).³⁹ Upon reaching 800 °C and 1000 °C, the CoO phase disappeared concomitantly with the growth of metallic Co crystallites and increased graphitization of the carbon matrix. This transformation was attributed to the reduction of CoO by reducing carbon species generated during the carbonization of the organic ligands within the precursor.

To investigate the operational mechanism of the Co/CoO@C-400 catalyst, XPS was performed, which confirmed the presence of Co, C, N, and O in Co/CoO@C-400 (Fig. S5). As shown in Fig. 3a, the C 1s spectra exhibited two peaks at 284.8 eV, 286.2 eV and 288.1 eV, assigned to C–C, C–OH and C=O bonds, respectively.^40–42 Fig. 3b displayed deconvoluted O 1s peaks at 528.8 eV and 530.7 eV, attributed to lattice oxygen (O²⁻) and oxygen vacancies.^43,44 Fig. 3c showed the high-resolution Co 2p spectrum featuring four peaks: those observed at 780.9 eV and 796.5 eV correspond to Co²⁺ species, while the satellite peaks at 786.4 eV and 802.2 eV are characteristic of CoO.^45,46 These results clearly indicated surface oxidation of cobalt during high-temperature calcination, forming a CoO overlayer.⁴⁷ The N 1s spectrum revealed a peak at 398.8 eV, assigned to C–N bonds (Fig. 3d).⁴⁸ All binding energies were calibrated against the C 1s reference peak at 284.8 eV, assigned to C–C bonds.⁴⁹

Fig. 3 High-resolution XPS spectra of C 1s (a), O 1s (b), Co 2p (c), and N 1 s (d) region for Co/CoO@C-400.

The photoluminescence (PL) emission spectra of Co-MOF and Co/CoO@C-400 were examined in order to further analyze the separation effectiveness of photogenerated electron–hole pairs. According to Fig. S6, Co/CoO@C-400 had a much lower PL intensity than Co-MOF, demonstrating the faster separation rate of electrons and holes. Overall, the photocatalytic activity could be greatly improved by the Co/CoO@C-400 composites based on the excellent separation efficiency of the photoinduced electrons and holes pairs.

3.4. Photocatalytic performance

3.4.1. Photocatalytic performance of the Co-MOF. The indiscriminate discharge of dye wastewater in recent years has emerged as a critical environmental concern, causing severe aquatic pollution with detrimental consequences for both human health and ecosystems.⁵⁰ Due to their carcinogenic properties and non-biodegradability, these pollutants pose significant threats to ecological security and public welfare. To address the issue of dye pollution and develop efficient, cost-effective photocatalysts, this study selected methyl orange (MO), gentian violet (GV), rhodamine B (RhB), Congo red (CR), and methylene blue (MB) as model dye pollutants to evaluate the photocatalytic performance of Co-MOF. As dye molecule adsorption on the catalyst surface is a prerequisite for photoinduced electron transfer between photocatalytic components, a 120-minute dark adsorption period was implemented prior to photocatalytic reactions to establish adsorption–desorption equilibrium, thereby eliminating interference from physical adsorption processes.

Blank control experiments were first conducted to evaluate the self-degradation of five dyes under visible irradiation without any photocatalyst. As shown in Fig. S7, negligible removal was observed for all five dyes under visible light-exposed conditions. Given the abundant active sites on the Co-MOF surface, its photocatalytic activity was subsequently investigated. Fig. S8 demonstrated the photocatalytic performance of the Co-MOF toward the five dyes under identical experimental conditions. The significant attenuation of characteristic UV-vis absorption peaks indicated effective photocatalytic degradation of MO, GV, and CR by the Co-MOF, achieving removal efficiencies of 79.4%, 96.5%, and 80.7% respectively. By contrast, the Co-MOF exhibited limited efficacy for RhB and MB removal (21.7% for RhB and 22.7% for MB). The PXRD patterns in Fig. S9 demonstrated that the crystal structure remained unchanged before and after degradation, confirming the structural stability of the synthesized Co-MOF.

3.4.2. Photocatalytic performance of derived materials. Given their exceptional catalytic performance and structural stability, carbon-based materials have gained extensive application in catalysis.^51–53 To leverage these advantages, this study developed porous carbon materials through direct pyrolysis of the MOF precursor modified with nonmetal dopants, systematically investigating their efficacy in enhancing the photocatalytic degradation efficiency of organic dyes.⁵⁴

Fig. 4 shows that Co/CoO@C-400 achieved removal efficiencies of 94.1% (MO), 96.8% (GV), 31.8% (RhB), 84.3% (CR), and 57.2% (MB), while Table S3 presented specific degradation data. Compared with the findings reported by Ma et al.,²⁵ the Co/CoO@C-400 material developed in this work demonstrates significantly superior photocatalytic degradation efficiency toward several dyes. Moreover, the material in this study, obtained after calcination at only 400 °C, achieves comparable performance to that of the sample calcined at 1000 °C in the aforementioned study. Consistent with previous mechanistic studies, the diminished redox activity at higher temperatures was attributed to the reduction of metal oxides to metallic cobalt. Fig. S10 presents the photocatalytic performance of porous carbon materials (Co/CoO@C-600/800/1000) synthesized via direct pyrolysis at different temperatures toward five model dyes. Comprehensive analysis of these results demonstrated that the Co/CoO@C-400 material delivered optimal photocatalytic performance. This methodology, utilizing self-assembled MOF precursors for direct pyrolysis, represented a valuable approach for fabricating highly efficient and stable photocatalysts, outperforming the pristine Co-MOF material.

Fig. 4 UV-vis spectra of aqueous solutions of MO (a), GV (b), RhB (c), CR (d), and MB (e). (f) Recorded after different durations of photocatalytic degradation using Co/CoO@C-400.

3.5. Photocatalytic mechanisms

The photocatalytic process accomplishes the conversion of photonic energy into chemical energy,⁵⁵ generating reactive oxygen species (ROS) including holes (h⁺), superoxide ions (˙O₂⁻), and hydroxyl radicals (˙OH),⁵⁶ each playing distinct critical roles in the degradation mechanism as detailed below. Radical scavenging experiments were conducted to identify the dominant reactive species in GV (50 mg L⁻¹) photodegradation by Co-MOF-derived carbon materials. Four specific scavengers were introduced: ammonium oxalate (AO) for h⁺, tert-butanol (TBA) for ˙OH, sodium iodate (NaIO₃) for electrons (e⁻), and benzoquinone (BQ) for superoxide (˙O₂⁻). Fig. 5a and b demonstrated photocatalytic activity through degradation profiles with scavengers. The degradation efficiency for GV in all samples decreased upon the addition of scavengers compared to the scavenger-free system, indicating that the photocatalytic degradation reaction by the carbon material results from the combined action of four reactive species. Furthermore, the degradation efficiency showed no significant loss after four consecutive cycles for GV, demonstrating the material's stability and recyclability (Fig. 5c). Moreover, the structural stability was further evidenced by the comparison of Co/CoO@C-400 before and after the photocatalytic reaction (Fig. S11). The spectral features exhibited no significant changes, confirming that the photodegradation process does not compromise the material's structure.

Fig. 5 The degradation rate curve (a) and bar graph (b) of GV by Co/CoO@C-400 after adding different free radical scavengers. (c) The cycling stability of the photodegradation of GV for Co/CoO@C-400.

It is generally accepted that the initial step in a photocatalytic reaction involves the separation and transfer of photogenerated electrons.³ The optical properties of the synthesized materials were analyzed by UV-visible diffuse reflectance spectroscopy (UV-vis DRS) (Fig. 6a). The valence band position (E_VB) of the synthesized materials was determined using XPS. The study demonstrated that the functionalized carbon-based Co/CoO@C-400 material exhibited enhanced visible-light utilization compared to the pristine material. The photocatalytic performance of the catalyst also depended critically on the separation efficiency of photogenerated electron–hole pairs. To investigate this, the band gap energy and conduction band potential were estimated from the Tauc plots (Fig. 6b and c) and XPS valence band spectra of the materials. Based on the UV-vis diffuse reflectance spectra, the Kubelka–Munk function was transformed into the linear absorption coefficient, enabling the calculation of the corresponding band gap values for this material:⁵⁷

αhv = A(hvE_g)_n (1)

where α, h, ν, A, and E_g represent the absorption coefficient, Planck's constant, light frequency, a constant, and the band gap, respectively. The estimated band gap values for Co-MOF (Fig. S12) and Co/CoO@C-400 were thus determined to be 2.96 eV and 1.34 eV, respectively, clearly indicating that the functionalized carbon-based semiconductor material possessed a reduced band gap, and consequently, the narrower-band-gap Co/CoO@C-400 semiconductor generated a greater number of photogenerated charge carriers during photocatalysis, thereby exhibiting superior photocatalytic performance in the degradation of organic dyes. The conduction band potential (E_CB) could thus be calculated using the formula given below:³⁰

E_CB = E_VB – E_g (2)

Fig. 6 UV-vis absorption spectrum (a) and bandgap energy (b) of Co/CoO@C-400. (c) XPS valence band spectrum.

As a carbon-based material, the sample exhibited excellent electron transport capability and can function as an efficient electron transport medium. To elucidate its photocatalytic mechanism, the valence band (VB) edge was determined at +0.69 eV (vs. NHE) through XPS valence band spectra, while the band gap of 1.34 eV was measured by UV-DRS. The conduction band (CB) edge was consequently calculated as –0.65 eV (vs. NHE). Based on these energy level analyses, the following photocatalytic pathway shown in Scheme 2 was proposed. Under visible light irradiation, electrons were excited from the valence band to the conduction band. Since the CB potential (–0.65 eV) is more negative than the reduction potential of O₂/˙O₂⁻ (–0.33 eV vs. NHE),²⁹ photogenerated electrons can react with O₂ to form superoxide radicals (˙O₂⁻). These highly oxidative species subsequently degraded dye molecules. Simultaneously, the photogenerated holes (h⁺) in the valence band directly oxidize GV. Notably, the VB potential (+0.69 eV vs. NHE) is insufficient for direct water oxidation to generate ˙OH, as it is less positive than the oxidation potential of H₂O/˙OH (+2.40 eV vs. NHE).⁵⁸ It was speculated that ˙O₂⁻ may undergo proton-coupled reactions (˙O₂⁻ + 2H⁺ → H₂O₂) to form hydrogen peroxide, which was subsequently converted to hydroxyl radicals (˙OH) within the photocatalytic system. The possible photocatalytic reaction process is as follows (formulas (3)–(7)):

Co/CoO@C-400 + visible light → e⁻_CB + h⁺_VB (3)

O₂ + e⁻_CB →˙O₂⁻ (4)

˙O₂⁻ + 2H⁺ → H₂O₂ (5)

H₂O₂ + ˙O₂⁻ → ˙OH (6)

h⁺ (VB) + ˙O₂⁻ + ˙OH + GV → Degraded products (7)

Scheme 2 Schematic diagram of GV photocatalytic mechanism over Co/CoO@C-400 under visible light irradiation.

4. Conclusions

In summary, we successfully synthesized a novel Co-MOF, which can serve as a precursor to prepare a carbon-based material to act as an efficient photocatalyst via a facile method, be identified as the optimal sacrificial template, and further be used to derive carbon-based materials capable of effectively degrading GV, MO, and CR dyes. Overall, the study demonstrated that the Co/CoO@C-400 material, obtained through moderate-temperature calcination at 400 °C, exhibited optimal photocatalytic degradation performance for dye under visible light irradiation, confirming the critical role of calcination temperature in regulating carbon material formation. Experimental data revealed that Co/CoO@C-400 achieved high degradation efficiencies of 96.8% for GV, 94.1% for MO, and 84.3% for CR, demonstrating both high efficacy and rapid reaction kinetics. Furthermore, the material maintained excellent stability over multiple cycles. Moreover, this catalyst efficiently utilized solar energy and achieved photocatalytic degradation of target organic dyes under visible light irradiation. These findings provide an efficient carbon-based photocatalytic solution for large-scale water pollution remediation, though further research is required to assess its viability for large-scale production and application.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5nj03932k.

CCDC 2481426 (Co-MOF) contains the supplementary crystallographic data for this paper.⁵⁹

Acknowledgements

This work was financially supported by the Department of Science and Technology of Liaoning Province (2024-BSLH-218) and the Department of Education of Liaoning Province (LJ212510149015).

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Footnote

† These authors contributed equally to this work.

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