One‑Pot Synthesis of Zn(II)‑Porphyrin‑Sensitized TiO₂ Hollow Nanoboxes for Synergistic Visible‑Light Degradation of Organic Dyes

Abstract

We report a simple, single‑step solvothermal route to fabricate Zn(II) meso‑tetra(4‑carboxyphenyl)porphyrinato (ZnTCP)‑sensitized three‑dimensional TiO₂ hollow nanoboxes (TiO₂‑HNBs). The ZnTCP is covalently linked to the TiO₂ surface through ester bonds, forming a robust electron‑transfer bridge that dramatically improves visible‑light responsiveness. Under simulated sunlight (λ > 420 nm), the ZnTCP/TiOF₂ mass ratio of 2 % (T‑2p) achieves a 99 % Rhodamine B (RhB) degradation in 2 h, a 3.6‑fold rate increase relative to bare TiO₂‑HNBs. The enhanced performance originates from a cooperative generation of •OH, •O₂⁻, and ¹O₂, the latter arising uniquely from the biomimetic porphyrin. The composite exhibits excellent recyclability, attributable to the strong chemical bonding between ZnTCP and TiO₂‑HNBs. Cyclic voltammetry confirms that the ZnTCP center remains unchanged during photocatalysis, underscoring the role of the porphyrin ring in electron transfer.

Background

Industrial dyes are ubiquitous in textile, leather, and paper manufacturing, yet their persistence and toxicity threaten aquatic ecosystems and human health. Conventional removal techniques—adsorption, coagulation, ion exchange, and membrane filtration—only relocate contaminants without degrading them. Biological methods are slow and sensitive to pollutant concentrations, limiting scalability. Advanced oxidation processes (AOPs) that generate reactive oxygen species (ROS) offer a more effective solution, yet many rely on harsh conditions or non‑renewable reagents.

Biomimetic catalysis, inspired by natural enzymes, combines high selectivity with mild operational conditions. Metalloporphyrins, particularly zinc‑based variants, have attracted attention due to their strong light absorption, non‑toxic nature, and facile synthesis. Unlike iron‑porphyrins, which require stringent preparation conditions, ZnTCP can be produced under moderate temperatures and remains chemically stable. The peripheral carboxyl groups on ZnTCP not only enhance water dispersibility but also provide anchoring sites for covalent attachment to TiO₂ surfaces.

TiO₂ is a benchmark photocatalyst, but its wide bandgap (3.2 eV) limits activity to the UV range. Strategies to shift its absorption into the visible spectrum include heterojunction formation, doping, and surface sensitization. Porphyrin sensitization is particularly attractive because of the porphyrin’s high molar extinction coefficient and long triplet lifetime, enabling efficient electron transfer to TiO₂’s conduction band.

Recent advances have demonstrated TiO₂ hollow nanostructures (nanotubes, microspheres, hollow nanoboxes) that offer high surface area and efficient charge separation. However, creating non‑spherical, facet‑controlled hollow nanoboxes with dominant {001} facets remains challenging. Here, we present a one‑pot topological transformation of TiOF₂ nanocubes into ZnTCP‑sensitized TiO₂ hollow nanoboxes with exposed high‑energy facets, providing a template for visible‑light photocatalysis.

Methods

Materials

All reagents were reagent‑grade (Wuhan Guoyao Chemical Reagent Co. Ltd.) and used without further purification. Deionized water was employed for all solutions.

Synthesis of ZnTCP‑Sensitized TiO₂‑HNBs (ZnTCP@TiO₂‑HNBs)

Preparation of TiOF₂ Nanocube Template

In a typical protocol, 30 mL of acetic acid, 5 mL of HF, and 15 mL of tetrabutyl titanate were added dropwise to a 100 mL PFA vessel and stirred for 25 min. The mixture was transferred to a 100 mL Teflon‑lined autoclave and heated at 200 °C for 12 h. After cooling, the white precipitate was washed with ethanol and dried at 60 °C under vacuum overnight to yield TiOF₂ nanocubes.

Synthesis of ZnTCP

Porphyrin (TCP) was prepared via a one‑step condensation of 4‑carboxybenzaldehyde and pyrrole in propionic acid at 140 °C. The product was isolated by butanol extraction, yielding a purple solid (M = 614.14 g mol⁻¹). Zinc insertion was achieved by refluxing 0.544 g of TCP with 0.20 g zinc acetate in DMF at 150 °C for 2 h, followed by precipitation in water and drying to give ZnTCP (M = 678.11 g mol⁻¹). Structural confirmation was obtained by ¹H NMR, IR, UV‑vis, mass spectrometry, and XPS.

One‑Pot Formation of ZnTCP@TiO₂‑HNBs

300 mg TiOF₂, 70 mL ethanol, and 3 mg ZnTCP were mixed in a Teflon‑lined autoclave and heated at 200 °C for 48 h. The resulting product was washed, concentrated, and dried at 60 °C to yield T‑1p (ZnTCP/TiOF₂ = 1 %). By varying ZnTCP amounts, samples T‑0p, T‑1p, T‑2p, T‑3p, and T‑5p were prepared.

Characterization

Fourier‑transform infrared (FT‑IR) spectra were recorded with a NeXUS 470 spectrometer (KBr pellets). Powder X‑ray diffraction (XRD) was performed on a Bruker D8 Advance (Cu‑Kα). X‑ray photoelectron spectroscopy (XPS) used a VG Multilab 2000 (Mg‑Kα). Morphology was examined by SEM (Hitachi) and TEM (Tecnai G² 20). UV‑vis diffuse reflectance (DRS) employed a Shimadzu UV‑2600; the Kubelka‑Munk function was used to estimate band gaps. Photoluminescence (PL) spectra were collected on a Hitachi F‑7000. Cyclic voltammetry (CV) was measured on a three‑electrode cell with a Pt counter, Ag/AgCl reference, and glassy carbon working electrode (ZnTCP dissolved in DMF, 0.1 M TBAP).

Photocatalytic Measurements

Photodegradation of RhB (1 × 10⁻⁵ mol L⁻¹) was carried out in a 50‑mL cylindrical vessel containing 50 mg of catalyst. After adsorption equilibrium in the dark, the suspension was irradiated with a 210 W xenon lamp (λ > 420 nm) under continuous water cooling. Aliquots (3.5 mL) were withdrawn at regular intervals, centrifuged, and analyzed by UV‑vis spectroscopy. Degradation efficiency was calculated as C/C₀, where C is the residual RhB concentration.

Active Species Detection

Hydroxyl radicals (•OH) were monitored using coumarin, superoxide radicals (•O₂⁻) with 4‑chloro‑7‑nitrobenzo‑2‑oxa‑1,3‑diazole (NBD‑Cl), and singlet oxygen (¹O₂) via 1,3‑diphenylisobenzofuran (DPBF). The samples were irradiated under the same visible‑light conditions, and fluorescence spectra were recorded to quantify radical production.

Results and Discussion

Morphology and Crystallinity

TiOF₂ nanocubes displayed a uniform cubic morphology with an average side length of ~250 nm (Fig. 1a). After solvothermal treatment, TiO₂‑HNBs retained the cubic shape but became hollow, composed of six ordered nanosheets (Fig. 1b). TEM images of the ZnTCP‑sensitized samples (T‑1p, T‑2p, T‑3p, T‑5p) confirmed the hollow box‑like architecture with surface “hairy” ZnTCP tentacles (Fig. 1c‑f). HRTEM revealed lattice fringes of 0.235 nm, corresponding to the anatase TiO₂ {001} facet (Fig. 1g). XRD patterns confirmed the anatase phase; TiOF₂ peaks vanished while a sharp {101} peak appeared at 2θ = 25.37°, indicating complete topotactic transformation (Fig. 4). The crystallite sizes (~260 nm) derived from the Scherrer equation matched the TEM measurements, and ZnTCP incorporation did not alter the TiO₂ lattice parameters.

Optical Properties

UV‑vis absorption of ZnTCP in DMF showed a B‑band at 432 nm and Q‑bands at 553/598 nm (Fig. 5a). Upon sensitization, the Q‑band red‑shifted to 660 nm, evidencing strong electronic coupling with TiO₂. TiO₂‑HNBs alone displayed a steep absorption edge at 400 nm, whereas ZnTCP‑sensitized samples exhibited a pronounced visible‑light tail (Fig. 5b). Kubelka‑Munk analysis yielded a reduced band gap of 2.83 eV for T‑2p versus 3.08 eV for TiO₂‑HNBs, confirming that ZnTCP introduces mid‑gap states that facilitate visible‑light excitation.

Chemical Bonding

FT‑IR spectra revealed the disappearance of the –COOH stretch at 1720 cm⁻¹ and the emergence of a new ester peak at 1713 cm⁻¹ in T‑2p, confirming covalent ester formation between ZnTCP carboxyl groups and TiO₂ surface hydroxyls (Fig. 6). XPS showed a shift of Ti 2p peaks toward lower binding energy (0.84 eV), and the O 1s lattice peak also shifted, indicating electron donation from ZnTCP to Ti⁴⁺ sites. The C 1s spectrum displayed a prominent C=O peak at 284.6 eV, further supporting ester linkage. Zn 2p signals matched those of ZnTCP, confirming its presence on the surface.

Photocatalytic Performance

Under visible‑light irradiation, T‑2p achieved 99 % RhB degradation in 2 h, a 3.6‑fold rate enhancement over TiO₂‑HNBs (Fig. 8a). The apparent first‑order rate constant for T‑2p (0.7139 h⁻¹) far exceeded that of the other samples and bare TiO₂‑HNBs (0.198 h⁻¹). Excessive ZnTCP (T‑5p) slightly reduced activity, likely due to surface recombination centers. The composite maintained >90 % degradation efficiency after five recycling runs (Fig. 8c), and XRD after reuse showed unchanged anatase crystallinity (Fig. 8d).

Active Species and Mechanism

Fluorescence probes revealed the generation of •OH, •O₂⁻, and ¹O₂ during photocatalysis with T‑2p (Fig. 9). Radical scavenger experiments (EDTA, p‑benzoquinone, iso‑propyl alcohol, DPBF) confirmed that all four species contribute to RhB degradation, with ¹O₂ being predominant for ZnTCP alone and a synergistic effect observed in ZnTCP‑sensitized TiO₂ (Fig. 11). The proposed mechanism involves (1) photoexcitation of ZnTCP generating singlet and triplet states that produce ¹O₂; (2) electron transfer from ZnTCP to TiO₂ conduction band, followed by O₂ reduction to •O₂⁻; and (3) TiO₂‑generated holes oxidizing water or hydroxide to •OH (Fig. 12). The covalent linkage ensures efficient charge separation and prolongs charge carrier lifetimes, as evidenced by reduced PL intensity.

Electrochemical Insight

CV curves of ZnTCP and T‑2p displayed identical redox peaks, indicating that the ZnTCP center remains chemically unchanged during photocatalysis and that the electron‑transfer process occurs through the porphyrin ring (Fig. 10). This further supports the role of the ester bond as an electron bridge.

Proposed Photocatalytic Pathway

The cooperative generation of •OH, •O₂⁻, and ¹O₂ by ZnTCP‑sensitized TiO₂‑HNBs leads to rapid oxidation of RhB, yielding mineralization products such as CO₂ and H₂O. The combination of high‑energy {001} facets, porous hollow architecture, and strong ZnTCP anchoring results in superior visible‑light utilization and charge separation, enabling the observed high degradation rates.

Conclusions

We have developed a scalable, one‑pot solvothermal method to synthesize ZnTCP‑sensitized TiO₂ hollow nanoboxes with exposed {001} facets. The covalent ester linkage between ZnTCP and TiO₂ facilitates efficient electron transfer and broadens light absorption into the visible region. The resulting composite exhibits remarkable RhB photodegradation (99 % in 2 h, 3.6‑fold faster than bare TiO₂‑HNBs) and excellent recyclability. This strategy provides a versatile platform for designing biomimetic, visible‑light photocatalysts applicable to wastewater treatment and environmental remediation.

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