A novel route for microplastic mineralization: visible-light-driven heterogeneous photocatalysis and photothermal Fenton-like reaction

Abstract

Microplastic pollution has attracted considerable attention. Here, we develop a visible-light-driven photocatalysis and photothermal Fenton-like reaction method for their removal, utilizing an α-Fe₂O₃ nanoflower on TiO₂ with a hierarchical structure of inverse opal-like layer/nanotube arrays (α-Fe₂O₃/TiO₂HNTAs film). Without external H₂O₂ dosage, nearly 100% degradation of 310 nm polystyrene (PS) spheres is obtained after 4 h at 75 °C induced by visible-light irradiation. The light initiation and photothermal effect are essential for achieving high degradation efficiency of plastics on α-Fe₂O₃. A distinctive melting phenomenon of PS under mild conditions due to the synergistic effect is observed for the first time. The catalyst system also works well for PS spheres with large particle size (2.0–2.9 μm) and real PS plastic foam. We believe that this work provides a novel strategy and new insights for removing microplastics and bulk plastics.

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Environmental significance

Microplastic pollution has attracted considerable attention due to its potential adverse effect on the environment and human health. Microplastic removal with solar energy is of significance in terms of the ecological environment and energy utilization. This study reports a synergic mode of heterogeneous photocatalysis and photothermal Fenton-like reaction for highly efficient microplastic degradation.

1. Introduction

Nearly 400 million tons of plastics were produced worldwide in 2020.¹ Most of them are discarded as waste and could persist for hundreds or even thousands of years, causing severe environmental issues and a potential threat to public health.^2,3 For example, microplastics either from industrial production or decomposition of large plastics distribute ubiquitously in water, land, air and oceans.^3–5 These small plastics can be easily ingested by various organisms which eventually results in unexpected accumulative and adverse effects on the food chain.^3–5 Therefore, removing microplastics from the natural environment is highly desirable.

Advanced oxidation processes (AOPs) are widely utilized for microplastic degradation. Photocatalysis is a branch of AOPs and various photocatalysts have been developed for plastic conversion,^2,3,5,6 including TiO₂ and ZnO based-semiconductors. The Fenton/Fenton-like reaction is another efficient route for mineralizing microplastics.^5,7 However, peroxide dosage such as H₂O₂ is necessary for the thermal Fenton system and acidic conditions are required to inhibit the formation of iron hydroxides.⁸ Moreover, relatively high temperatures are always adopted for a high efficiency. For example, 140 °C should be applied to drive microplastic decomposition.⁹

Numerous studies have been conducted on heterogeneous photo-assisted Fenton/Fenton-like oxidation for removing organic pollutants.^10,11 Besides the normal photo-activated process (eqn (1) and (2)), in such a system, ≡Fe³⁺ in Fe₂O₃ or FeOOH could be reduced by the photogenerated electrons, forming the active ≡Fe²⁺ centers for H₂O₂ activation (eqn (4)). Although sometimes external H₂O₂ is added to improve the efficiency,^12–16 its consumption is lowered since in situ H₂O₂ is generated during the photocatalytic process (eqn (3)). There are still several challenges that need further endeavors: enhance the separation efficiency of h⁺/e⁻ pairs and electron transfer rate; promote the redox–oxidation cycle of ≡Fe³⁺/Fe²⁺; and accelerate in situ H₂O₂ formation and activation. To date, the photo-assisted Fenton reaction on Fe₂O₃ for plastic degradation has not been reported yet. There are only a few reports utilizing iron(III) salts¹⁷ and composites of 1 wt% or 2 wt% iron(II) phthalocyanine on TiO₂.¹⁸

In the organic synthesis field, the coupling of photocatalytic- and photothermal-effects exhibits superior properties to individual effects.^19–22 It is promising to achieve such a synergic mode between photocatalysis and photothermal Fenton-like reaction for plastic degradation. Then, the synergistic effect could not only promote the photocatalytic generation of reactive oxidation species h⁺/˙OH/O₂˙⁻/H₂O₂ (eqn (1)–(3)) but also accelerate the thermal Fenton reaction, namely, the limiting step of the redox cycle and activation of H₂O₂ (eqn (4)).

α-Fe₂O₃ + hv → h⁺ + e⁻ (1)

h⁺ + H₂O(air) → ˙OH + H⁺ (2a)

e⁻ + O₂(air) → O₂˙⁻ (2b)

e⁻ + H₂O₂ → ˙OH + OH⁻ (2c)

In situ H₂O₂ formation:

2h⁺ + H₂O(air) → H₂O₂ + 2H⁺ (3a)

2H⁺ + O₂(air) + 2e⁻ → H₂O₂ (3b)

Fenton-like reaction:

≡Fe³⁺ + e⁻ → ≡Fe²⁺ (4a)

≡Fe²⁺ + H₂O₂ → ≡Fe³⁺ + ˙OH + OH⁻ (4b)

Thus, in this work, we fabricated an α-Fe₂O₃ nanoflower on TiO₂ film with a hierarchical structure of inverse opal-like layer/nanotube arrays (denoted as α-Fe₂O₃/TiO₂HNTAs). Through the synergic oxidation method (Table 1), the degradation of polystyrene (PS) spheres with diameters of 310 nm and 2.0–2.9 μm and real PS plastic foam is investigated and excellent performance is achieved under visible-light illumination. The study of light intensity and temperature demonstrates that light initiation and the photothermal effect are indispensable in our catalytic systems. The melting of PS at temperature much lower than its melting point due to the synergic mode is discovered for the first time. Experimental results also show that TiO₂HNTAs is not the catalytic site but it could promote the PS degradation efficiency on α-Fe₂O₃.

Table 1 Heterogeneous catalytic degradation of microplastics or plastics through various routes reported

Plastic type	Catalyst	Methods	Reaction conditions	Degradation efficiency	Ref.
Small pieces (<0.6 mm, 5.0 g L^−1)	Mn@NCNTs	Thermal catalysis	PMS, 120 °C	41% after 8 h	23
PS small pieces (<1 mm, 1.0 g L^−1)	Fe3O4	Adsorption and thermal catalysis	H2O2, ≈200–250 °C	100% after 12 h	24
PVC (100–200 μ m, 0.1 g L^−1)	TiO2/graphite cathode	Electrocatalysis	pH = 3, −0.7 V vs. Ag/AgCl, 100 °C	56% after 6 h	25
PS film	Bacteria + homogeneous Fe(III)	Microbial Fenton	pH = 7, 25 °C	6.1% after 14 d	7
PS, PE, PP (<100 μm, 1.0 g L^−1)	Homogeneous Fe(II)	Thermal Fenton	pH ≤ 1, H2O2, 140 °C	>90% after 16 h	9
PE sphere (200–250 μm, 1.0 g L^−1)	BiOCl–X	Photocatalysis	Visible-light (250 W Xe lamp)	5.4% after 5 h	26
PVC film	Dye-ZnO		UV light, 356 nm	20% after 2 h	27
PE film	TiO2		UV light, 254 nm	86% after 300 h	28
PE small pieces (<210 μm, 4.0 g L^−1)	C, N–TiO2		pH = 3, visible-light (50 W LED lamp)	71.8% after 50 h	29
PS sphere (400 nm)	TiO2 film		UV light, 365 nm	98.4% after 12 h	30
PS film	FePc–TiO2		UV light, 254 nm	82% after 20 d	18
PE film	Goethite		UV light, 254 nm	16% after 300 h	31
PS film	CuPc–TiO2		UV-visible-light, 310–750 nm	16% after 250 h	32
PS sphere (310 nm)	α-Fe2O3/TiO2HNTAs film	Photocatalysis and photothermal Fenton-like reaction	In air, visible-light (0.5 W cm^−2, halogen lamp), 75 °C induced by light irradiation (catalyst surface temperature is 80–90 °C)	Almost completely degrade after 4 h to 12 h	This work
PS sphere (2.0–2.9 μm)
PS plastic foam

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2. Materials and methods

2.1. Chemicals and materials

A titanium sheet with a thickness of 0.2 mm was cut into slices (1 cm × 2.5 cm). All chemicals including ethylene glycol (EG, 99%), ammonium fluoride (NH₄F, 98%), iron nitrate (Fe(NO₃)₃·9H₂O, 98%), carboxylated polystyrene micro-spheres (310 nm, Aladdin Biochemical Technology Co.), carboxylated polystyrene (2.0–2.9 μm, Shanghai Macklin Biochemical Co.), and polystyrene plastic foam were used as received. All aqueous solutions were prepared using ultrapure water with a resistivity of 18.2 MΩ cm.

2.2. Catalyst preparation

Hierarchical TiO₂ consisting of a top photonic crystal layer and a bottom tubular structure (TiO₂HNTAs) was prepared by a two-step anodic oxidation process:³³ the Ti slices were firstly sonicated in acetone, deionized water, and ethanol for 10 min respectively. After drying with N₂ flow, anodic oxidation was carried out on a DC power supply utilizing the Ti slice (1 cm × 1 cm working area) as an anode and a platinum foil (1 cm × 1 cm) as a cathode. 0.3 wt% ammonium fluoride, 2% v/v water in a solution of ethylene glycol, was used as the electrolyte. In the first step of the anodic oxidation process, a potential of 50 V was applied for 3 h. After anodization, the obtained films were immediately immersed in ethanol for 5 min and then sonicated in ultrapure water for 10 min to remove the formed nanotubes, leaving hexagonal nanoimprints on the Ti surface. In the second anodization step, a potential of 50 V was applied for 1 h. The prepared TiO₂HNTAs samples were then immersed in ethanol immediately for 5 min and rinsed with ethanol. Finally, the samples were annealed at 450 °C for 3 h to obtain anatase titanium dioxide with high crystallinity.

Preparation of α-Fe₂O₃/TiO₂HNTAs: 1.01 g of iron nitrate nonahydrate was ultrasonically dissolved in 50 mL water and then sealed with a heat-resistant sealing film. The TiO₂HNTAs was vertically immersed into the solution through the slit in the middle of the sealed film. Then the solution was stirred at 90 °C for a certain time (1 h, 2 h or 3 h). The obtained sample slice was taken out and rinsed with ethanol and finally calcined at 550 °C for 1 h to obtain α-Fe₂O₃/TiO₂HNTAs. α-Fe₂O₃/TiO₂HNTAs-1, α-Fe₂O₃/TiO₂HNTAs-2 and α-Fe₂O₃/TiO₂HNTAs-3 refer to the hydrothermal time of 1 h, 2 h and 3 h, respectively.

2.3. Characterization

The X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku Ultima IV diffractometer using Cu Kα radiation (λ = 1.5418 Å). The light absorption spectra of the samples were determined using a UV-2600 spectrophotometer (Shimadzu). Fourier transform infrared (FTIR) spectroscopy was recorded using an iS50 infrared spectrometer (Thermo) with wavenumber of 4000–400 cm⁻¹. To investigate the valence state of Fe and the local environment of C and O, X-ray photoelectron spectroscopy (XPS) measurements were performed (Thermo Scientific Escalab 250Xi). The morphology of the samples was investigated by FE-SEM (Merlin Compact).

2.4. Activity test

PS sphere degradation: 1.0 mL of polystyrene microspheres (2.5 wt% in DI water, 310 nm) was diluted with 9.0 mL ultrapure water. After ultrasonication, 100 μL of the mixture was dropped and coated on the prepared α-Fe₂O₃/TiO₂HNTAs via a spin-coating method with a rotational speed of 1660 rpm for 40 s. After spin-coating, it was heated at 90 °C on an electric heating plate for 10 min to prevent the PS spheres from falling off the catalyst surface. The photocatalytic process was conducted utilizing a halogen lamp as the visible-light source with a light intensity of 0.5 W cm⁻² from 2 h, 3 h to 4 h. The environmental temperature was 75 °C, induced by light irradiation, and the surface temperature of the catalyst monitored using a thermometer (JK804, Changzhou Jinailian Electronic Technology Co.) was 80–90 °C, if not specified. The degradation of monodispersed PS spheres was characterized with SEM. The mineralization product CO₂ was characterized by gas chromatography. The reaction in the dark was conducted in an oven at 75 °C for 4 h. A Hg lamp (365 nm, 0.5 W cm⁻²) was utilized as the light source for the experiments under UV light irradiation.

The experimental procedure for PS spheres with large particle size is the same as above.

Real PS plastic foam: 0.03 g PS plastic foam was first dissolved in 1 mL dichloromethane. Then 100 μL of the solution was dropped uniformly on the prepared α-Fe₂O₃/TiO₂HNTAs and kept still, forming a transparent film after 5 h. The film on α-Fe₂O₃/TiO₂HNTAs was irradiated with visible-light for 12 h under the same experimental conditions as above.

Liquid product test with ¹H NMR: during the photocatalytic reaction for a certain time, the sample was immersed in dichloromethane and then the solution was taken for an NMR test. The solvent for the NMR test was CDCl₃.

Radical scavenger experiments: the experimental process is the same as the PS sphere degradation experiment except that 0.05 g DMPO was dissolved in 1.0 mL of the diluted PS sphere solution.

3. Results and discussion

3.1. Characterization of catalysts and degradation of PS spheres

The catalyst preparation process is given in Fig. 1a and the crystal planes of the α-Fe₂O₃ phase and TiO₂ anatase phase could be confirmed by the XRD pattern in Fig. 1b.^12,34 As shown in Fig. 1c, both the TiO₂HNTAs film and its scratched powder exhibit visible-light absorption, which is further enhanced for the former. This is caused by F doping formed during the anodization process and by the hierarchical structure of inverse opal-like layer/nanotube arrays (Fig. 1d), which has been reported in the literature and our former works.³³ The absorption edge extends to 700 nm for the deposited α-Fe₂O₃ samples (Fig. 1c).

Fig. 1 (a) Process for the catalyst preparation. (b) XRD pattern (*: Ti foil, T: TiO₂, F: α-Fe₂O₃) and (c) UV-vis spectra of α-Fe₂O₃/TiO₂HNTAs (deposited for 1 h, 2 h, 3 h) and TiO₂HNTAs. The samples were scratched off from the Ti foil for the UV-vis test. The UV-vis adsorption spectrum of TiO₂HNTAs film is also provided. Corresponding SEM images and digital photos (inset) for TiO₂HNTAs (d) and α-Fe₂O₃/TiO₂HNTAs (e).

For the TiO₂HNTAs film, the average pore size for the inverse opal-like layer is 120 nm according to the pore size distribution analysis (Fig. S1a†), while the average length of the bottom nanotube array is 4.3–4.4 μm according to the cross-section image and length distribution analysis (Fig. S1b†). After deposition, the α-Fe₂O₃ nanoflower is formed on TiO₂HNTAs (Fig. 1e) and its average thickness increases from 260 nm, 310 nm to 330 nm for deposition time of 1 h, 2 h, and 3 h, respectively (Fig. S2†). Compared with other catalysts, the sample with a deposition time of 2 h achieved nearly 100% PS degradation (Fig. S3†) and was utilized as a model photocatalyst for the following investigation, marked as α-Fe₂O₃/TiO₂HNTAs.

We first compare the PS (310 nm) degradation on TiO₂HNTAs and α-Fe₂O₃/TiO₂HNTAs. The visible-light intensity is 0.5 W cm⁻² and the reaction temperature is 75 °C, induced by light irradiation. As shown in Fig. 2a, although TiO₂HNTAs could be excited by visible-light, there is no degradation of PS on TiO₂HNTAs after irradiating for 4 h even up to 12 h. For α-Fe₂O₃/TiO₂HNTAs, the PS melted after 2 h (Fig. 2b), which is probably caused by the stretching and opening of carbon chains as well as the decomposition of the polymer forming reaction intermediates. The corresponding SEM-EDS mapping is given in Fig. S5.† This is a totally different phenomenon compared with TiO₂HNTAs under UV-light irradiation, where only the size of the PS sphere becomes smaller and smaller from 6 h to 12 h until it disappears after 24 h (Fig. S6†). The melting of PS could also result in a large surface area of interfacial contact between PS and α-Fe₂O₃ and in turn accelerate the degradation rate. Complete degradation is achieved on α-Fe₂O₃/TiO₂HNTAs after 4 h (Fig. 2d). ¹H NMR results also confirm that the intermediate species disappeared after complete degradation (Fig. S8†). During the photocatalytic reaction, the real temperature of the catalyst surface remained at 80–90 °C due to light irradiation (Fig. 2e).

Fig. 2 The degradation of PS on TiO₂HNTAs after irradiating for (a) 4 h to 12 h; on α-Fe₂O₃/TiO₂HNTAs after irradiating for 2 h (b), 3 h (c), and 4 h (d). The SEM images at 1 μm scale for (a–d) are provided in Fig. S4.† (e) The real-time temperature monitoring of the catalyst surface during reaction with a thermometer. The environmental temperature was 75 °C induced by visible-light irradiation, 0.5 W cm⁻².

An XPS test was also conducted for α-Fe₂O₃/TiO₂HNTAs and the sample coated with PS spheres before and after photodegradation for 2 h. The analysis of C1s and O1s is given in Fig. 3, while Fe2p and full spectra are provided in Fig. S7.† For C1s, the peak at 284.8 eV ascribed to the C–C bond increased a lot after coating of PS spheres when compared with pure α-Fe₂O₃/TiO₂HNTAs. The new small peak at 288.9 eV and the extended peak at 291.5 eV are ascribed to C=O and –COO groups of carboxylated PS spheres, respectively. After light illumination for 2 h, the content of the C–C bond decreased dramatically, indicating the degradation of the carbon chain in PS. For O1s, the binding energy at 530.0 eV is assigned to the lattice oxygen, while that at 531.7 eV is assigned to the typical oxygen groups such as carbonyl and OH.²⁸ After coating of carboxylated PS spheres, a new peak at 533.3 eV, which comes from the –COOH group, is observed and the peak area increases after illuminating for 2 h. The increase of this peak is probably induced by the formation of carboxylic acids or carboxylates as intermediate products during the degradation process.

Fig. 3 XPS analysis of C1s and O1s for (a) α-Fe₂O₃/TiO₂HNTAs and (b and c) PS sphere coated α-Fe₂O₃/TiO₂HNTAs before and after photodegradation. The degradation time is 2 h. The Fe2p and full spectra are given in Fig. S7.†

3.2. Influence of light and temperature

Utilizing α-Fe₂O₃/TiO₂HNTAs as a catalyst, we investigate the light effect on the PS photodegradation efficiency. The degradation rate increases with the increase of light intensity and the results are listed in Fig. 4. In the absence of light (0 W cm⁻²), no degradation was observed. The size and shape of PS spheres remained the same when the reaction system was heated to 90 °C in the dark. Even up to 130 °C, the PS on α-Fe₂O₃/TiO₂HNTAs are still capable of keeping the spherical structure with only a slight deformation (Fig. S9†). These results demonstrate that light initiation is essential for the reaction. With light irradiation at 0.33 W cm⁻², a melting phenomenon and obvious degradation of PS could be observed (Fig. S10†). Nearly 100% mineralization is obtained when the light intensity increased to 0.5 W cm⁻². The influence of light wavelength on the activity was then studied while keeping the light intensity and reaction temperature unchanged. The PS spheres melt and degrade even with the light wavelength over 660 nm (Fig. S11†) and the performance is in the order of 400–800 nm > 550–800 nm > 660–800 nm.

Fig. 4 The influence of light intensity and temperature on PS degradation over α-Fe₂O₃/TiO₂HNTAs. Under light irradiation, the temperature was controlled by the heating or cooling system. In the absence of light, the experiment was conducted in an oven at certain temperature.

Moreover, the temperature under visible-light irradiation (0.5 W cm⁻²) is varied by cooling systems. When the temperature is down to 65 °C, the degradation efficiency decreases but the melting process still occurs. With a continuous decrease of temperature to 55 °C, only a slight reduction of PS size from 310 nm to ∼296 nm is observed and the size distribution is shown in Fig. S12.† It can be concluded that the high temperature induced by light irradiation is also required for the high degradation rate. The above bare light system at low temperature and bare heating experiments in the dark indicate that there is a synergistic effect between light initiation and Fenton-like reaction (thermal catalysis) to overcome the activation energy and drive the reaction.

3.3. Large size of PS spheres

Large size PS spheres were studied. We purchased PS spheres with a diameter of 2.0–2.9 μm as a model. The experimental procedure was the same as the PS with a diameter of 300 nm. After 4 h of irradiation, the PS spheres are almost completely degraded (Fig. 5a and b), which demonstrates that the catalytic system works well for nano-microplastics with large size. The EDS mapping in Fig. 5b and c confirms that the particle is the remaining part of the PS sphere rather than the surface Fe₂O₃ particle. Compared with the complete degradation of PS with a diameter of 300 nm, the results are consistent with the fact that larger size plastics require a longer degradation time than smaller ones.

Fig. 5 SEM images for PS spheres with a diameter of 2.0–2.9 μm before (a) and after photocatalysis for 4 h (b), and the corresponding EDS mapping results (c). The environmental temperature was 75 °C induced by visible-light irradiation, 0.5 W cm⁻².

3.4. Mineralization of real PS plastic foam

Real plastic foam was also studied with our catalyst system. The polystyrene plastic foam was first dissolved in dichloromethane and then coated onto the surface of α-Fe₂O₃/TiO₂HNTAs to form a transparent thin film. The SEM image of the film is shown in Fig. 6a. After visible-light irradiation for 12 h, more than 90% of the film disappeared and the exposed surface of α-Fe₂O₃/TiO₂HNTAs was left (Fig. 6b). Such a performance is among the highest active systems that have been reported (Table 1). The corresponding FT-IR characterization is conducted and given in Fig. 6c. The major bands at 2917 cm⁻¹ and 2849 cm⁻¹ are the typical hydrocarbon stretching,³⁵ or can be ascribed to asymmetric and symmetric stretching vibrations of the CH₂ group.^9,25 The peak at 1595 cm⁻¹ is from the C=C group and the major peak at 1445 cm⁻¹ is from C–H.³⁵ These peaks for hydrocarbons including C–H, CH₂, and C=C dramatically decrease after photodegradation. Compared with the undegraded film, new peaks at 1732 cm⁻¹ and 1630 cm⁻¹ assigned to C=O of carboxylic acid or ketone groups appeared, which are reaction intermediates of PS degradation in air.^9,25,35,36 These are consistent with the observed results from SEM images. The possible final liquid product and gas product were detected. A liquid product is not observed according to the result of the NMR test (Fig. 6d and e), while the mineralization product CO₂ is confirmed by gas chromatography in a sealed system and only CO₂ was detected as the gas product (Fig. 6f). These further confirm the complete degradation of the plastic to CO₂.

Fig. 6 The degradation of real plastic waste on α-Fe₂O₃/TiO₂HNTAs. (a) The plastic film was prepared from PS plastic foam via the solvent dissolution and dropping method. (b) Photodegradation after 12 h under visible-light irradiation, 75 °C, 0.5 W cm⁻². (c) Corresponding FT-IR spectra for (a) and (b). (d–f) The NMR and GC spectra for the liquid product and gas product. (d and e) ¹H NMR spectra for the PS plastic film before and after photodegradation on the catalyst. (f) GC test for the mineralization product CO₂ in a sealed system for the sample and no other gas product was observed (CO₂, CH₄, C₂H₄, C₂H₆ standard gas were used as reference, high concentration of CO₂ will lead to the left shift of retention time).

3.5. Mechanism investigation

Amylopectin and KI were used to confirm the formation of H₂O₂ for the light-excited α-Fe₂O₃ powder system in PS sphere solution (75 °C, 0.5 W cm⁻²). The reaction solution was filtered after the reaction. After adding amylopectin and KI, the color of the filtrate obviously changed (Fig. 7a, ii). Without light irradiation, no color change was observed at 75 °C (Fig. 7b). As also shown in Fig. 7b, for the experiment at 55 °C with a light intensity of 0.5 W cm⁻², there was much less color change than that at 75 °C with the same light intensity (Fig. 7a-ii). This demonstrates that light irradiation and the thermal effect by light illumination are essential to produce a high concentration of H₂O₂. Under the same experimental conditions as in Fig. 2d, the presence of radical scavenger DMPO dramatically decreases the mineralization rate of PS (Fig. 7c and d). All these experiments confirm that the synergistic effect of photocatalysis and the photothermal effect contributes to H₂O₂ production and the high degradation rate of PS.

Fig. 7 (a and b) Control experiments for H₂O₂ formation in the filtrate after light irradiation with α-Fe₂O₃ as a catalyst: (a) i-KI, ii-amylopectin and KI, iii-amylopectin, visible-light irradiation (75 °C); (b) amylopectin and KI, visible-light irradiation (55 °C) and in the dark (75 °C). (c and d) SEM image after reaction with radical scavenger DMPO on α-Fe₂O₃/TiO₂HNTAs. Reaction conditions: visible-light irradiation for 4 h, 75 °C, 0.5 W cm⁻².

The role of α-Fe₂O₃ was further investigated. We deposited Cu₂O on TiO₂HNTAs and applied it in PS degradation. The results are given in Fig. S13.† Although both Cu₂O and Fe₂O₃ possess strong visible-light absorption bands, the activity of Cu₂O is much lower than that of α-Fe₂O₃, probably caused by the Fenton-like reaction process of ≡Fe³⁺/Fe²⁺ in eqn (4) and (4b). Based on the investigation above, we proposed the possible mechanism for PS degradation via photocatalysis and photothermal Fenton-like reaction on α-Fe₂O₃. As shown in Scheme 1, the α-Fe₂O₃ semiconductor is excited by visible-light irradiation and generates electron/hole pairs. The electrons could not only activate O₂ producing in situ H₂O₂, but also reduce ≡Fe³⁺ in Fe₂O₃ forming the active ≡Fe²⁺ centers. Then, ≡Fe²⁺ reacts with H₂O₂ and ·OH forms. At the same time, ≡Fe²⁺ returns back to its original ≡Fe³⁺ state. This photo-assisted Fenton-like reaction process is promoted by the photothermal effect (red color). The holes generated in the valence band of α-Fe₂O₃ are also able to oxidize the moisture in air and generate ·OH. The ·OH with high oxidizing power attacks the PS on the α-Fe₂O₃ surface, oxidizing PS to intermediate species and then to the final product CO₂. This oxidation process is dramatically accelerated by the melting phenomenon due to the close and large contact area at the PS/catalyst solid–solid interface. To further investigate the possible radical species, an EPR test was conducted for the methanol solution with the addition of DMPO after immersing the α-Fe₂O₃/TiO₂HNTAs film under light irradiation. According to ref. 37 and 38 and the calculated splitting constant (Fig. S14†), the formation of O₂˙⁻ and ·OH was confirmed.

Scheme 1 Proposed mechanism for the degradation of PS via photocatalysis and photothermal Fenton-like reaction on α-Fe₂O₃. CB – conduction band, VB – valence band.

When utilizing α-Fe₂O₃ deposited on Ti foil as a catalyst, the melting of PS and a high degradation rate are observed (Fig. S15†) which indicates lower activity than α-Fe₂O₃/TiO₂HNTAs. Thus, we believe that α-Fe₂O₃ is the catalytic site and plays a major role in PS mineralization, while TiO₂HNTAs makes a minor contribution and could promote the degradation efficiency of PS on α-Fe₂O₃. The enhanced efficiency is either because visible-light-excited TiO₂HNTAs could populate electrons to the conduction band of α-Fe₂O₃ or the light adsorption energy of the TiO₂HNTAs film matches with the energy level of α-Fe₂O₃ generating h⁺/e⁻ pairs in α-Fe₂O₃.

3.6. Recyclability

To illustrate the stability of the photocatalyst, we repeat the photodegradation process three times. It can be seen from Fig. 8a and b that excellent degradation efficiency could still be obtained and there is no morphology change for the α-Fe₂O₃/TiO₂HNTAs catalyst. There is also no crystal phase change of α-Fe₂O₃/TiO₂HNTAs after recycling (Fig. 8c).

Fig. 8 Repeating PS degradation on α-Fe₂O₃/TiO₂HNTAs three times. (a and b) The SEM images and (c) XRD of the catalyst after the 3rd cycle.

4. Conclusion

In summary, we demonstrate that the synergistic effect between heterogeneous photocatalysis and photothermal Fenton-like reaction could dramatically accelerate the PS degradation rate on the α-Fe₂O₃/TiO₂HNTAs surface. The light initiation and photothermal effect are essential for the melting phenomenon of PS driven at a much lower temperature than its melting point. PS spheres with nanosize and microsize diameters as well as real PS plastic foam almost completely degrade after 4 h to 12 h, under visible-light (0.5 W cm⁻², halogen lamp) irradiation and at 75 °C induced by light irradiation (catalyst surface temperature is 80–90 °C).

Author contributions

Z. H. Xue: investigation, methodology, validation, data curation. X. Yu: investigation, validation. X. B. Ke: writing – reviewing & editing. Jian Zhao: supervision, conceptualization, writing – original draft and reviewing & editing.

Conflicts of interest

There is no conflict of interests.

Acknowledgements

The National Natural Science Foundation of China (NSFC, 21703154) and the Natural Science Foundation of Tianjin (18JCYBJC43100) are gratefully acknowledged.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3en00642e

‡ The authors contribute equally to this work.

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