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. 2026 Aug 3;11(32):47638–47653. doi: 10.1021/acsomega.6c02479

Template-Assisted Galvanic Replacement Synthesized Pt/Co3O4/TiO2 for Efficient Visible-Light Degradation of Tetracycline and Methylene Blue

Yanchi Liu †, Mohan Sun †, Chengchao Qian †, Yabo Fu †, Junwei Qiao ‡, Diangang Wang §, Jiazi Shi †,*, Gaimei Zhang †, Xinlin Zhang †, Hui Liu †
PMCID: PMC13491513  PMID: 42626147

Abstract

To overcome the poor visible light response and high recombination rate of photogenerated carriers in bare TiO2, a Pt/Co3O4/TiO2 photocatalyst was fabricated via a template-assisted spontaneous galvanic replacement route. The as-prepared material contains anatase–rutile mixed-phase TiO2, Co3O4, and metallic Pt and retains a dodecahedral morphology with a high specific surface area of 122.64 m2/g. Under visible light irradiation for 120 min, its degradation rates for tetracycline and methylene blue reached 85% and 93%, respectively, which are significantly better than those of pure TiO2 and Pt/Co3O4. The composite also exhibited excellent stability and recyclability. Its superior performance is ascribed to the synergistic effects of the mixed-phase TiO2, the p–n heterojunction between TiO2 and Co3O4, the Schottky barrier at the Pt/TiO2 interface, the localized surface plasmon resonance of Pt, and the high adsorption capacity of the porous material. This work offers a new strategy for treating dye- and antibiotic-containing wastewater.

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1. Introduction

The pollution of water by dyes and antibiotics is a major environmental concern. Dyes such as methylene blue (MB) are common water pollutants frequently found in the industrial effluents of textile, papermaking, paint, and printing industries. If these toxic and carcinogenic organic dyes are discharged directly into the environment along with industrial wastewater, they can cause severe harm to both the environment and human health. Tetracycline (TC) is a low-cost, highly effective, and structurally stable antibiotic that is widely used in both the medical and livestock industries. Because of incomplete metabolism, the vast majority of TC is excreted in an active form, entering wastewater treatment plants and eventually natural waters through effluent discharge and agricultural runoff. Accumulated tetracycline in the environment not only poses a long-term threat to ecosystems but also accelerates the emergence and spread of antibiotic-resistant bacteria. , The coexistence of dye and antibiotic residues in aquatic ecosystems poses synergistic ecological risks, including photosynthesis inhibition, antibiotic resistance gene proliferation, and endocrine disruption. Therefore, the treatment and removal of these pollutants are the foremost priority for preventing environmental contamination.

Various technologies have been explored in recent years to remove these pollutants, including adsorption, biodegradation, ion exchange, membrane filtration, and photocatalytic degradation. Among these, photocatalytic degradation is regarded as the most advantageous method because of its low cost, high efficiency, mild reaction conditions, and the absence of secondary pollution. Titanium dioxide (TiO2) is widely employed in photocatalysis owing to its strong redox capability and stable physical properties. Compared with other photocatalysts, TiO2 offers several advantages, such as excellent chemical corrosion resistance, photochemical stability, nontoxicity, and high photocatalytic activity. Nevertheless, single-phase TiO2 suffers from drawbacks that restrict its application, including poor visible-light response, a wide bandgap, the tendency of charge carriers to agglomerate, and facile electron–hole recombination. − These limitations can be overcome by preparing mixed-phase TiO2, − TiO2 modified with metal oxides, and TiO2 decorated with noble metals. −

One effective strategy for enhancing TiO2 photocatalytic efficiency is to engineer a mixed-phase (rutile/anatase) structure. Studies have shown that the photocatalytic performance of mixed-phase rutile/anatase TiO2 is superior to that of either single-phase TiO2, likely due to a synergistic effect between rutile and anatase TiO2 arising from interfacial charge transfer. − Charge carriers in mixed-phase TiO2 are spatially separated and migrate to distinct phases upon photoexcitation, which suppresses electron–hole recombination and extends the carrier lifetime. , Another approach to enhancing the photocatalytic performance of TiO2 is to couple it with narrow-band-gap semiconductors such as Co3O4. Studies have shown that integrating TiO2 with magnetic Co3O4 not only enhances light utilization but also facilitates its recovery and reuse. To further enhance the visible-light response of photocatalysts, noble metals can be employed as cocatalysts to accelerate charge transfer across the semiconductor interface. − Depositing high work function noble metals (Pt or Pd) onto the semiconductor surface to form Schottky junctions drives photoexcited electrons from the TiO2 conduction band into the metal, thereby sustaining long-term spatial separation of electrons and holes. , Moreover, Pt nanoparticles anchored on the TiO2 surface can enhance visible-light absorption of the material via localized surface plasmon resonance (LSPR). Therefore, the combination of mixed-phase rutile/anatase TiO2 with Co3O4 and noble metal Pt is expected to markedly alter the electronic structure and physicochemical properties of the resulting composite, thereby enhancing its photocatalytic performance. Furthermore, composite materials prepared using metal–organic frameworks (MOFs) as templates possess abundant porous structures and large specific surface areas, which can promote pollutant adsorption to enhance the photodegradation efficiency. , Nevertheless, the challenge lies in integrating these components in a way that maximizes activity, which requires a distinctive synthetic strategy.

In this work, a Pt/Co3O4/TiO2 composite containing mixed-phase TiO2, Co3O4, and Pt was successfully designed and fabricated through a template-assisted approach, combined with a spontaneous galvanic replacement reaction. Using a Co-based metal–organic framework, ZIF-67, as a template, amorphous TiO2 was uniformly coated onto the ZIF-67 surface via a tetrabutyl titanate (TBT) sol–gel process. The resulting precursor was then calcined under a nitrogen atmosphere to obtain the Co/mixed-phase TiO2 composite. The Pt/Co3O4/TiO2 composite was prepared by a galvanic replacement between Co/mixed-phase TiO2 and chloroplatinic acid. The photocatalytic performance was evaluated against MB and TC as the model pollutants. The Pt/Co3O4/TiO2 composite shows superior photocatalytic activity compared to TiO2 and Pt/Co3O4 through the synergistic effects of mixed-phase TiO2, p–n junction, Schottky barrier, Pt LSPR, and strong adsorption. Furthermore, the demonstrated cycle stability with less than 2% activity loss, combined with the magnetic recoverability conferred by Co3O4, facilitates catalyst separation and recycling, further underscoring the operational feasibility of this system. The preparation method of the Pt/Co3O4/TiO2 composite provides an important reference for the subsequent development and optimization of highly efficient photocatalysts.

2. Experimental Section

2.1. Materials

Cobalt nitrate hexahydrate (Co­(NO3)2•6H2O), 2-methylimidazole (2MIM), methanol, ethanol, tetrabutyl titanate (98%), chloroplatinic acid (H2PtCl6), tetracycline, and methylene blue (C16H18N3SCl) were purchased from Macklin Company. Ammonia (25–28%) was supplied by Aladdin Biochemical Technology Company.

2.2. Synthesis

2.2.1. Pt/Co3O4/TiO2

Synthesis of the Pt/Co3O4/TiO2 composite is illustrated in Figure . ZIF-67 was prepared according to the literature. 100 mg of ZIF-67 was dispersed in 50 mL of ethanol containing 900 μL of 28 wt % ammonia by ultrasonication. While stirring, 4 mL of TBT was introduced, and the sol was aged at 45 °C for 24 h. The solid was separated, washed three times with ethanol, and dried at 50 °C for 6 h. The resulting powder was calcined at 500 °C for 2 h under N2 to afford Co/TiO2. Then, 30 mg Co/TiO2 powder was added to 1 mL H2PtCl6 solution (0.01931 mol/L). The mixture was sonicated to obtain a homogeneous suspension and then left to stand at room temperature for a period of time to allow the spontaneous displacement reaction between H2PtCl6 and Co/TiO2 to proceed to completion. The powder was collected by centrifugation, washed three times with ethanol, and dried at 50 °C for 6 h to obtain the Pt/Co3O4/TiO2 composite. Additionally, reference materials TiO2 and Pt/Co3O4 were prepared for comparison.

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Synthetic route of the Pt/Co3O4/TiO2 composite.

2.2.2. TiO2

TBT was introduced into an ethanolic ammonia medium, and the sol was aged at 45 °C for 24 h under continuous agitation. The resulting solid was collected by centrifugation, purified by triple washing with ethanol, and dried at 50 °C for 6 h. Subsequent calcination at 500 °C for 2 h under N2 to obtain TiO2.

2.2.3. Pt/Co3O4

ZIF-67 was inserted into a tube furnace, and pyrolyzed under flowing N2 at 500 °C for 2 h to obtain Co/C. The 30 mg Co/C powder was added to 1 mL chloroplatinic acid solution (0.01931 mol/L). After ultrasonic dispersion, the mixture was left to react at room temperature. The precipitate was collected by centrifugation, washed three times with ethanol, and dried at 50 °C for 6 h to obtain Pt/Co3O4 composite material.

2.3. Characterization

X-ray diffraction (XRD) patterns were collected with a Bruker D8 ADVANCE using Cu–Kα or Co–Kα radiation. A Hitachi SU-8020 provided Scanning Electron Microscopy (SEM) images, while High-Resolution Transmission Electron Microscopy (HRTEM) images were obtained using a Tecnai G2 F30. Elemental surface analysis was carried out by X-ray Photoelectron Spectroscopy (XPS) on a Thermo Escalab 250XI with monochromatic Al–Kα radiation. In situ irradiated XPS spectra were subsequently acquired on the same instrument, using a 300 W xenon lamp to simulate visible light, and the data were collected after 20 min of irradiation. Optical properties were examined with a Scinco S-4100 spectrophotometer operating in diffuse-reflectance mode over the Ultraviolet–Visible (UV–vis) range. The total organic carbon (TOC) contents of the samples were quantified with a TOC Torch analyzer (multi N/C UV HS).

Photoluminescence (PL) emission was captured with an Edinburgh Instruments FLS980 system under 325 nm photoexcitation. A Bruker A300–10/12 spectrometer was employed to record DMPO-trapped electron paramagnetic resonance (EPR) signals at room temperature. A Bio-Logic SP-50 potentiostat served for the electrochemical impedance spectra (EIS) and Mott–Schottky analysis in a standard three-electrode setup. The specific surface area and pore structure of the material were determined at −195.8 °C using a Micromeritics ASAP 2460 N2 adsorption–desorption analyzer.

2.4. Photocatalytic Activity

Photocatalytic activity was evaluated by degrading TC and MB under visible light. Thirty milligrams of photocatalyst was dispersed in 30 mL of a 20 mg L–1 TC solution and stirred in the dark for 30 min to reach adsorption–desorption equilibrium. A 50 mL MB solution (50 mg L–1) containing 100 mg of photocatalyst was magnetically stirred in the dark for 30 min to establish adsorption–desorption equilibrium before being irradiated by a 300 W xenon lamp (400–780 nm). The pollutant concentration was determined using an Agilent Cary 60 UV–vis spectrophotometer. The residual concentration (C) was normalized to its initial value (C0). eq was employed to quantify the photocatalytic removal efficiency.

Degradation%=(C0‐C)/C0×100% 1

To evaluate recyclability, the photocatalyst was reused for three successive visible light irradiation experiments toward TC and MB degradation.

3. Results and Discussion

3.1. Characterization

The schematic illustration of the Pt/Co3O4/TiO2 composite synthesis is shown in Figure . Using ZIF-67 as a template, an amorphous titania layer was coated onto its surface via a sol–gel method. The resulting core–shell structure ZIF-67@TiO2 was then calcined at high temperature under a nitrogen atmosphere to yield a porous-structured Co/TiO2 composite. Upon subsequent addition of H2PtCl6, a spontaneous galvanic replacement reaction between PtCl6 2– and Co0 was initiated, yielding the Pt/Co3O4/TiO2 composite. In addition, the schematic illustration of the Pt/Co3O4 composite synthesis is shown in Figure S1. ZIF-67 was pyrolyzed at high temperature to afford a Co/C composite, which then underwent a galvanic replacement reaction with H2PtCl6 to yield a Pt/Co3O4 composite.

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Schematic illustration of the synthesis of the Pt/Co3O4/TiO2 composite.

The phase structures of TiO2, Co/C, Co/TiO2, Pt/Co3O4 and Pt/Co3O4/TiO2 were determined by XRD, as shown in Figure S2 and Figure . TiO2 was measured using Cu–Kα radiation (λ = 1.5406 Å), while all other materials were measured using Co–Kα radiation (λ = 1.7903 Å) due to their magnetic properties. As illustrated in Figure S2, the diffraction peaks correspond to the lattice planes of anatase TiO2 (PDF#21–1272). The results demonstrate that amorphous TiO2 derived from TBT via the sol–gel route, crystallizes into pure anatase after calcination at 500 °C under N2. The XRD pattern of the Co/C composite obtained after carbonizing ZIF-67 at 500 °C is shown in Figure a. Two sharp diffraction peaks are observed, which match the metallic cobalt (PDF#15–0806). The XRD pattern of the Co/TiO2 composite obtained after ZIF-67@TiO2 calcination at 500 °C is shown in Figure b. In addition to the diffraction peaks of metallic cobalt, reflections corresponding to both anatase and rutile phases are observed. This is likely due to the inhibitory effect of the carbon on TiO2 grain growth at high temperatures. The XRD patterns of the Pt/Co3O4 composite are displayed in Figure c. The diffraction peaks at 2θ = 22.1°, 36.4°, 43.0°, 45.0°, 52.5°, 65.6°, 70.1°, and 77.5°could be attributed to the (111), (220), (311), (222), (400), (422), (511), and (440) reflections of Co3O4 (PDF#73–1701), respectively. No obvious diffraction peaks of metallic Pt are detected in the pattern, probably because of the very low Pt loading. XRD patterns of the Pt/Co3O4/TiO2 composite are shown in Figure d. It can be seen that the peaks located at 2θ = 31.9° and 64°, correspond to rutile TiO2 nanocrystals for planes of (110) and (211), respectively (PDF#21–1276). The peak at 29.5° corresponds to the (101) plane of anatase TiO2 (PDF#21–1272). The peaks at 2θ = 36.5° and 43.1° are assigned to the (220) and (311) crystal planes of Co3O4 (PDF#73–1701). Two typical crystal peaks at 2θ = 46.5° and 54.2°, corresponding to Pt metal, are for planes of (111) and (200), respectively (PDF #04–0802). XRD confirms the Pt/Co3O4/TiO2 composite to contain anatase–rutile TiO2, Co3O4 and metallic Pt. The Pt/Co3O4/TiO2 composite obtained from the reaction between Co/TiO2 and H2PtCl6 exhibited diffraction peaks corresponding to metallic Pt and Co3O4, but no diffraction peaks attributable to metallic Co were detected, indicating that Co had been completely converted to Co3O4 after the reaction.

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XRD patterns of (a) Co/C, (b) Co/TiO2, (c) Pt/Co3O4 and (d) Pt/Co3O4/TiO2.

The surface chemistry of the Pt/Co3O4/TiO2 composite was further examined by XPS. The wide-scan spectrum presented in Figure a reveals signals originating from C, Ti, O, Co, Pt, and residual carbonaceous contamination. The C 1s XPS spectrum (Figure b) was deconvoluted into three peaks at 284.8, 286.1, and 288.1 eV, which are assigned to C–C, C–N, and CO, respectively. The C–C peak at 284.8 eV comprises graphitic carbon from ZIF-67 pyrolysis. The C–N bonds are derived from thermal pyrolysis and rearrangement of imidazole rings in ZIF-67, and the CO peak at 288.1 eV arises from surface-oxidized carbon species. As shown in Figure c, Ti 2p peaks at 458.3 and 464.0 eV reveal the Ti 2p3/2 and Ti 2p1/2 doublet of TiO2 in Pt/Co3O4/TiO2. , Figure d presents the Pt 4f XPS spectrum. The peaks at 70.8 and 74.4 eV are ascribed to metallic Pt0, whereas those at 75.9 and 71.8 eV correspond to Pt2+ 4f5/2 and Pt2+ 4f7/2, respectively. The XPS spectrum of the Co 2p is shown in Figure e. The Co 2p3/2 peaks at 780.8 and 783.1 eV correspond to octahedral Co3+ and tetrahedral Co2+, while the Co 2p1/2 peaks at 796.4 and 798.8 eV are attributed to the same species. Two shake-up satellite peaks at 786.7 and 802.8 eV further confirm the Co3O4 phase. The absence of Co0 signals in the XPS spectra further confirms that metallic cobalt has been completely converted to Co3O4. High-resolution O 1s (Figure f) yields three peaks at 529.7, 531.1, and 532.1 eV, assigned to Oβ, Oα and Oα′ species, respectively. Oβ denotes lattice oxygen ions, Oα originates from chemisorbed oxygen species, and the Oα′ component is ascribed to surface hydroxyl groups. The presence of Co3O4 can be further verified by the O 1s XPS peak at 529.7 eV. The XPS results confirm the successful formation of Pt/Co3O4/TiO2, consistent with the XRD data. Figure S3 displays the XPS survey of the Pt/Co3O4 composite. The Co 2p spectrum exhibits three main peaks at 779.7, 780.5, and 795.3 eV, assigned to Co 2p3/2 and Co 2p1/2 of Co2+ and Co3+, confirming the formation of Co3O4. In addition, the peaks at 71.3 and 74.5 eV can be attributed to metallic Pt0, while the peaks appearing at 76.5 and 71.4 eV correspond to Pt2+ 4f5/2 and Pt2+ 4f7/2 respectively. Combined XRD and XPS results confirm that the Pt/Co3O4 composite is composed of Pt and Co3O4. Figure S4 XPS confirms TiO2, consistent with XRD.

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(a) Survey XPS spectrum of Pt/Co3O4/TiO2; High-resolution XPS spectra of C 1s (b), Ti 2p (c), Pt 4f (d), Co 2p (e), and O 1s (f) of Pt/Co3O4/TiO2.

The SEM images of TiO2, Co/C, Co/TiO2, Pt/Co3O4, and Pt/Co3O4/TiO2 are shown in Figure S5 and Figure a. The image in Figure S5a shows that the synthesized TiO2 consists of spherical particles with uneven particle sizes and obvious agglomeration. As shown in Figure S5b-d and Figure a, Co/C, Co/TiO2, Pt/Co3O4, and Pt/Co3O4/TiO2 all retain the dodecahedral morphology of the ZIF-67 template, and this regular framework effectively suppresses the aggregation of the TiO2 nanoparticles. Meanwhile, a comparison of the microstructures of Co/C and Co/TiO2 reveals that Co/TiO2 exhibits a rougher surface, likely arising from the reaction between amorphous TiO2 and the ZIF-67-derived carbon framework during its formation. Analysis of the specific surface area and pore structure of Co/C and Co/TiO2 (Figure S6 and Table S1) reveals that Co/TiO2 exhibits a lower BET surface area than Co/C, accompanied by the formation of abundant macropores. Consequently, compared with Co/C, the metallic Co in Co/TiO2 reacts more readily with H2PtCl6. As a result, the Pt/Co3O4/TiO2 composite displays a rougher surface decorated with distinct granular species. The saturation magnetization data further corroborate the occurrence of the spontaneous galvanic replacement reaction between Co/TiO2 and H2PtCl6. As shown in Figure S7, the magnetic features of Co/TiO2 and Pt/Co3O4/TiO2 have been studied using the vibrating sample magnetometry (VSM) technique. The saturation magnetization (Ms) values of Co/TiO2 and Pt/Co3O4/TiO2 were 4 emu/g and 0.53 emu/g, respectively. This is mainly attributed to the continuous consumption of metallic cobalt, which acts as the reducing agent during the galvanic replacement reaction. HRTEM was used to resolve the fine microstructures of Pt/Co3O4/TiO2 and Pt/Co3O4, as shown in Figure b–d and Figure S8. Both Pt/Co3O4/TiO2 and Pt/Co3O4 exhibit dodecahedral morphology. Accordingly, the HRTEM analysis confirms their highly crystalline nature. Lattice fringes of 0.352 and 0.325 nm match the (101) anatase and (110) rutile planes, respectively. , Moreover, the interplanar distance of 0.244 nm was attributed to the (311) planes of Co3O4. Meanwhile, the lattice fringe spacing of 0.227 nm is assigned to the (111) plane of Pt. The selected area electron diffraction (SAED) results demonstrated multiple diffraction rings in good correspondence with the crystal planes of TiO2, Co3O4, and Pt. These findings are consistent with the XRD analysis results. Subsequently, the Pt/Co3O4/TiO2 composite was characterized by HRTEM imaging combined with element mapping based on energy-dispersive spectroscopy (EDS). In Figure e, EDS demonstrates that Pt, Co, Ti, N, O, and C elements are uniformly distributed. Moreover, compared with element mapping of Pt/Co3O4 (Figure S8), the Pt/Co3O4/TiO2 composite exhibits a higher Pt content. As shown in Tables S2 and S3, EDS analysis results reveal that the Pt loading of Pt/Co3O4/TiO2 is 9.21 wt %, which is approximately 6.7 times that of Pt/Co3O4 (1.37 wt %). As a result, the absence of detectable Pt diffraction peaks in Pt/Co3O4 (Figure c) is primarily attributed to its low Pt content below the XRD detection limit, whereas the pronounced Pt peaks in Pt/Co3O4/TiO2 (Figure d) reflect both the higher Pt loading and improved crystallinity. These findings collectively indicate that the abundant macropores in Co/TiO2 facilitate Pt deposition during the galvanic replacement reaction.

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(a) SEM images, (b) (c)­HRTEM images, (d) SAED patterns, and (e) EDS mapping of Pt/Co3O4/TiO2.

BET analyses were conducted to quantify the specific surface area and pore-size distribution of TiO2, Pt/Co3O4, and Pt/Co3O4/TiO2, as shown in Figure . Pt/Co3O4 and Pt/Co3O4/TiO2 exhibit nearly overlapping Type I isotherms. Pt/Co3O4/TiO2 exhibits the maximum specific surface area (122.64 m2/g), which is higher than those of Pt/Co3O4 (121.81 m2/g) and TiO2 (9.37 m2/g). The Pt/Co3O4/TiO2 composite, prepared via the template-assisted approach combined with the spontaneous galvanic replacement reaction, exhibits high specific surface areas, which are mainly attributed to the porous skeleton structure formed by pyrolysis of the ZIF-67 template. On this basis, the uniformly formed Pt nanoparticles through the replacement reaction further construct abundant pore architectures. Further examination of the pore size distribution curve reveals mesoporous characteristic peaks in the 2–4 nm range for Pt/Co3O4 and Pt/Co3O4/TiO2. Notably, the intensity of the mesoporous peak for Pt/Co3O4/TiO2 exhibits a significant increase compared with that of Pt/Co3O4. It is evident that the Pt/Co3O4/TiO2 composite possesses a large specific surface area and a desirable porous structure, which endow it with excellent pollutant adsorption performance. This enables sufficient contact between pollutants and the photocatalyst, thereby effectively improving photocatalytic efficiency.

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(a) Nitrogen adsorption–desorption isotherms and (b) pore size distributions of Pt/Co3O4/TiO2, Pt/Co3O4 and TiO2.

3.2. Photoelectrochemical Analysis

UV–vis diffuse-reflectance spectra were collected to assess light-harvesting capability and extract the optical bandgaps of the materials. Figure a shows that both Pt/Co3O4/TiO2 and Pt/Co3O4 absorb visible light far more strongly than TiO2, evidencing their improved harvesting capability. Moreover, the bandgap energy of a semiconductor photocatalyst serves as a pivotal optical parameter that governs the photogeneration and subsequent migration of charge carriers under illumination. The optical bandgap (Eg) was extracted from the Tauc plot (Ahν)= B­(hν – Eg)n/2, with A, h, ν, and B denoting the absorption coefficient, Planck constant, photon frequency, and proportionality constant, respectively. The absorption edge of Pt/Co3O4/TiO2, Pt/Co3O4 and TiO2 is about 325 nm, 342 nm, and 337 nm, corresponding to their bandgaps of about 2.73 eV, 2.78 eV, and 2.83 eV, as shown in Figure b. Compared with Pt/Co3O4 and TiO2, the composite exhibits the narrowest bandgap, indicating the strongest light-harvesting capability and thus potentially the highest photocatalytic activity. This can be attributed to the modification of the electronic structure by coupling TiO2 with narrow-bandgap Co3O4, while the LSPR effect of Pt further extends the visible-light absorption range.

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(a) UV–vis diffuse reflectance spectra, (b) Kubelka–Munk model, (c) PL spectra, and (d) EIS spectra of Pt/Co3O4/TiO2, Pt/Co3O4 and TiO2.

Furthermore, the PL spectra for Pt/Co3O4/TiO2, Pt/Co3O4, and TiO2 were plotted as revealed (Figure c). PL intensity provides insight into charge trapping efficiency, migration dynamics of photogenerated electron–hole pairs, and the extent of carrier recombination. Pt/Co3O4/TiO2 displays superior charge-transfer properties, manifested by a weaker PL intensity than either Pt/Co3O4 or TiO2. This suppression of radiative recombination signals enhanced carrier separation and prolonged lifetime, thereby boosting photocatalytic performance. − Interfacial charge-transfer characteristics of TiO2, Pt/Co3O4, and Pt/Co3O4/TiO2 were compared by electrochemical impedance spectroscopy (EIS). In EIS analysis, a smaller arc radius corresponds to lower charge-transfer resistance. As shown in Figure d, Pt/Co3O4/TiO2 exhibits the smallest arc radius compared with Pt/Co3O4 or TiO2, indicating lower interfacial charge-transfer resistance and faster photogenerated carrier transport. This can be ascribed to the localized surface plasmon resonance (LSPR) effect of Pt, which facilitates carrier separation and migration.

3.3. Photocatalytic Activity

The catalytic activities of TiO2, Pt/Co3O4, and Pt/Co3O4/TiO2 were evaluated by monitoring the degradation of TC and MB under visible-light irradiation. Dark equilibration was established by suspending the photocatalyst in the TC or MB solution for 30 min, ensuring that adsorption–desorption equilibrium was achieved prior to the photoreaction. As shown in Figure S9, the adsorption curves of the Pt/Co3O4/TiO2 photocatalyst in TC and MB solutions under dark conditions show that equilibrium is reached within 30 min, with adsorption efficiencies of 42% and 58%, respectively. This is ascribed to the large specific surface area and desirable porous structure of Pt/Co3O4/TiO2. Enhanced contact between photocatalysts and pollutants via adsorption thus favors higher photocatalytic efficiency. Photocatalytic activities of the three photocatalysts were assessed through monitoring TC and MB decomposition. Under visible-light irradiation, degradation of both contaminants occurred over all catalysts tested, as presented in Figure and Figure S10. The Pt/Co3O4/TiO2 photocatalyst showed the highest degradation rate, with 85% removal within 120 min. Whereas, the degradation of TC by Pt/Co3O4 and TiO2 was about 70% and 60%, respectively (Figure a). The degradation efficiencies of MB in 120 min for Pt/Co3O4/TiO2, Pt/Co3O4, and TiO2 are 93%, 67%, and 65%, respectively (Figure b). The degradation efficiency of MB by the Pt/Co3O4/TiO2 photocatalyst reaches 96% within 180 min, as shown in Figure S11. The photocatalytic performances of Pt/Co3O4/TiO2 and Pt/Co3O4 with approximately the same Pt loading were also compared. As shown in Figure S12, Pt/Co3O4/TiO2 exhibited significantly superior performance compared to Pt/Co3O4.

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Photocatalytic activity of the materials in the degradation of (a) TC and (b) MB; kinetics studies of (c) TC and (d) MB.

By contrast, bare TiO2 exhibits the poorest photocatalytic degradation capability, primarily ascribed to its negligible visible light absorption and the rapid recombination of photogenerated electrons and holes, which result in inferior photocatalytic performance. The exceptional photocatalytic performance of Pt/Co3O4/TiO2 is likely attributable to the LSPR effect of Pt and the formation of the Co3O4/TiO2 heterojunction, which enhances carrier migration and separation while suppressing electron–hole recombination, thereby improving degradation efficiency.

Furthermore, the reaction kinetics were quantified with a pseudo-first-order model to extract the rate constant, k (eq ).

−ln(C/C0)=kt 2

Herein, C and C 0 represent the residual and initial concentrations of TC or MB, respectively, t represents time, and k corresponds to the degradation rate constant. The kinetic profiles for TC degradation are presented in Figure c. The k values of Pt/Co3O4/TiO2, Pt/Co3O4 and TiO2 were 0.01153 min–1, 0.00715 min–1and 0.00575 min–1, respectively. The kinetic curves of MB degradation are shown in Figure d. The k values of Pt/Co3O4/TiO2, Pt/Co3O4 and TiO2 were 0.01890 min–1, 0.00726 min–1 and 0.00678 min–1, respectively. Kinetic analysis revealed that Pt/Co3O4/TiO2 exhibits the highest degradation rate constant, confirming its remarkable degradation efficiency and excellent photocatalytic activity.

The as-obtained results were compared with those of relevant studies to evaluate the effectiveness of Pt/Co3O4/TiO2 as a photocatalyst for pollutant degradation. The data in Table S4 summarize the photocatalytic degradation efficiencies of various photocatalysts, particularly TiO2 composite materials, for the removal of TC and MB. These comparisons confirm that Pt/Co3O4/TiO2 prepared in this work exhibits superior photocatalytic performance, indicating its promising potential for practical applications in pollutant treatment. The reusability and stability of Pt/Co3O4/TiO2 are major factors for cost reduction in pollutant control. To evaluate reusability, the recovered photocatalyst was washed and reused for three identical adsorption-photodegradation cycles under the same conditions. Figure a shows TC degradation efficiencies of 85%, 85%, and 83% over three successive cycles, demonstrating the stable reusability of Pt/Co3O4/TiO2 for TC removal. Repeatable photodegradation tests were also performed on MB solutions (Figure b). It can be seen that the photocatalytic reaction was repeated three times, and the degradation efficiencies of methylene blue were 93%, 92%, and 91% in that order, indicating that the degradation efficiency remained unchanged at about 91% for three repetitions. Furthermore, the postreaction XRD analysis (Figure c) reveals no appreciable change from the original pattern, confirming the exceptional phase stability under photocatalytic conditions. At the same time, the SEM images (Figure d,e) of the material after repeated degradation of the two pollutants were examined, and it was found that there was no significant difference from the initial topography. These results demonstrate that Pt/Co3O4/TiO2 exhibits excellent stability and recyclability during photocatalysis, which underscores its practical potential for pollutant degradation.

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Recyclability of the Pt/Co3O4/TiO2 photocatalyst for (a) TC and (b) MB degradation; (c) XRD patterns of photocatalysts after three photocatalytic degradation cycles; SEM images of photocatalysts after three photocatalytic degradation cycles: (d) TC and (e) MB.

3.4. Mechanism for the Photocatalytic Activity of Pt/Co3O4/TiO2

In order to investigate the degradation mechanism of Pt/Co3O4/TiO2, the primary reactive species under visible-light irradiation were systematically identified by selective scavenging of holes and radicals under otherwise identical conditions. Benzoquinone (BQ) was employed to scavenge superoxide radicals (·O2 –), whereas methanol (MeOH) and isopropanol (IPA) served as scavengers for holes (h+) and hydroxyl radicals (·OH), respectively. The Pt/Co3O4/TiO2 photocatalyst achieved 85% removal of TC within 2 h. As shown in Figure a, the TC degradation efficiency decreased only marginally from 85% to 55% upon the addition of MeOH. In sharp contrast, the efficiency plummeted to 21% and 42% after the introduction of BQ and IPA, respectively. These trapping results confirm that ·O2 – and ·OH radicals are the key oxidative species. Unlike TC degradation, the addition of MeOH and IPA lowered the MB removal rates to 65% and 68%, respectively (Figure b), suggesting the involvement of h+ and ·OH. However, the degradation was most significantly suppressed by BQ, with the efficiency dropping from 93% to 46%, indicating that ·O2 – is the dominant reactive species for MB degradation. Thus, all three species participate in the photocatalytic process, with ·O2 – playing the primary role.

10.

10

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Degradation rates of Pt/Co3O4/TiO2 with the addition of radical scavengers: (a) TC and (b) MB; EPR spectra of (c) DMPO-·OH and (d) DMPO-·O2 – by Pt/Co3O4/TiO2 under dark and light.

EPR spectroscopy was used to verify the formation of ·O2 – and ·OH radicals over Pt/Co3O4/TiO2 under illumination, with DMPO as the spin-trapping agent, and the results are displayed in Figure c-d. In the dark, no discernible characteristic signals were detected, indicating that Pt/Co3O4/TiO2 cannot generate superoxide or hydroxyl radicals without light. Under visible-light irradiation, the characteristic four-line spectra of DMPO-·O2 – and DMPO-·OH adducts were clearly observed. Prolonging the illumination from 5 to 10 min progressively intensified the signals, confirming the concurrent generation of ·O2 – and ·OH radicals over Pt/Co3O4/TiO2. The synergistic action of photogenerated superoxide (·O2 –) and hydroxyl (·OH) radicals serves as the primary driving force for the oxidative degradation of organic pollutants.

In situ irradiation XPS enables the inference of the direction of electron transfer at the heterojunction in Pt/Co3O4/TiO2 by comparing the photoelectron energy spectra under light and dark conditions. , Figure shows the XPS spectra of C 1s, Ti 2p, Pt 4f, Co 2p, and O 1s after illumination. When compared with the XPS spectra under dark conditions (Figure ), all exhibit varying degrees of binding energy shifts before and after illumination. Under visible-light irradiation, the Co 2p and Pt 4f peaks shift toward lower binding energies, indicating an increase in electron density around the Co3O4 and Pt. Conversely, the Ti 2p and C 1s peaks simultaneously shift toward higher binding energies, signifying a decrease in electron density at the TiO2 and carbon. On the basis of these findings, it can be inferred that electrons are transferred from TiO2 to Co3O4 and Pt at the heterojunction interface. ,

11.

11

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In situ XPS spectra of Pt/Co3O4/TiO2 under visible-light illumination: (a) survey spectrum, (b) C 1s, (c) Ti 2p, (d) Pt 4f, (e) Co 2p, and (f) O 1s.

Figure illustrates the mechanism by which the Pt/Co3O4/TiO2 photocatalyst degrades MB and TC under visible-light irradiation. Photocatalysis is a green technology driven by light-induced charge separation. Upon illumination, photons excite electrons (e–) from the valence band (VB) to the conduction band (CB) of both TiO2 and Co3O4, leaving behind holes (h+) in their respective VBs. The photogenerated electrons reduce surface-adsorbed O2 to superoxide radical anions (·O2 –), while the holes oxidize H2O or OH– to hydroxyl radicals (·OH). These reactive species collectively oxidize organic pollutants into smaller molecules and eventually mineralize them to CO2, H2O, and inorganic ions, thus achieving the effect of degrading pollutants. , As shown in Table S5, the TOC measurement results directly verify this deep mineralization process. The TOC removal efficiency of TC is approximately 69%, while that of MB reaches nearly 80%. The high TOC removal rates indicate that a large number of organic carbon skeletons are destroyed by reactive radicals in the system. Most organic pollutants are not merely decolorized or simply cleaved. Instead, they are continuously converted into inorganic small molecules such as CO2 and H2O, which fully demonstrates that the photocatalytic system is capable of thorough mineralization of organic pollutants. ,

12.

12

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Photocatalytic degradation mechanism of TC and MB over Pt/Co3O4/TiO2.

The TiO2 component of Pt/Co3O4/TiO2 is a mixed-phase material composed of rutile and anatase. Because the VB and CB edges of rutile lie above those of anatase, an internal electric field is established at their interface when the two polymorphs are in intimate contact. This field drives photogenerated electrons from rutile into anatase, markedly enhancing charge-carrier separation. , Additionally, TiO2 is an n-type semiconductor, whereas Co3O4 is p-type; consequently, a p-n heterojunction forms at their interface that further suppresses electron–hole recombination. ,, Electrons that migrate to the Co3O4 surface reduce O2 to ·O2 – radicals, while holes transferring to the TiO2 surface oxidize H2O to ·OH radicals, both of which degrade MB and TC. Meanwhile, Pt contacts TiO2 to form a Schottky junction, across which electrons are injected from the semiconductor into the metal. Owing to the high work function and low Fermi level of Pt, electrons are injected from the CB of TiO2 into the metal Pt, inhibiting recombination. Under visible light, the LSPR effect of Pt induces strong electron oscillations, enabling the nanoparticles to trap additional electrons and generate hot-plasmonic carriers. , These hot electrons are directly captured by surface-adsorbed O2, producing activated ·O2 – species that rapidly degrade MB and TC. Additionally, the large specific surface area and mesoporous architecture of the Pt/Co3O4/TiO2 photocatalyst significantly boost pollutant adsorption, intensify visible-light harvesting, and furnish a high density of active sites, all of which are key origins of its superior photocatalytic efficiency. − Accordingly, the exceptional photocatalytic activity of Pt/Co3O4/TiO2 stems from the synergistic contributions of: (i) the mixed-phase TiO2 junction, (ii) the p-n heterojunction between TiO2 and Co3O4, (iii) the Schottky barrier formed at the Pt/TiO2 interface, (iv) the LSPR effect of noble metal Pt, and (v) the pollutant-adsorption enrichment provided by the high-surface-area support.

4. Conclusion

In this study, a Pt/Co3O4/TiO2 photocatalyst was designed and prepared by using ZIF-67 as a template through a spontaneous galvanic displacement reaction. The Pt/Co3O4/TiO2 photocatalyst is composed of anatase–rutile mixed-phase TiO2, Co3O4, and metallic Pt. The synergistic effect of the mixed-phase TiO2, the p-n heterojunction between TiO2 and Co3O4, and the Schottky barrier formed at the Pt/TiO2 interface effectively suppresses electron–hole recombination. The LSPR effect of Pt further broadens the visible-light response range. In addition, the composite possesses a large specific surface area and a desirable porous structure, which endow it with excellent pollutant adsorption capacity. This enables sufficient contact between the catalyst and pollutants, thereby effectively improving photocatalytic degradation efficiency. Under visible-light irradiation, Pt/Co3O4/TiO2 achieves degradation efficiencies of 85% for TC and 93% for MB, which are significantly higher than those of TiO2 and Pt/Co3O4. In addition, Pt/Co3O4/TiO2 exhibits excellent stability and recyclability. The synergistic effects proposed in this study provide a universal approach to designing efficient, stable, and recyclable visible-light photocatalysts, offering broad prospects for treating wastewater containing refractory organic pollutants in industries such as textile dyeing and pharmaceuticals.

Supplementary Material

ao6c02479_si_001.pdf (1.1MB, pdf)

Acknowledgments

This study was supported by the National Natural Science Foundation of China (12475254), the Lab of Intelligent and Green Flexographic Printing (ZBKT2502), and the Fundamental Research Funds for the Municipal Universities of Beijing (Ee202511).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c02479.

  • Synthetic schematic diagram; XRD patterns; XPS spectra; SEM images; pore structure data; magnetic curves; HRTEM-EDS mapping; quantitative EDS elemental results; dark adsorption curves; time-dependent UV–vis degradation spectra; decolorization photos; photocatalytic activity comparisons; TOC analysis (PDF)

The authors declare no competing financial interest.

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