Non-Stacked γ-Fe₂O₃/C@TiO₂ Double-Layer Hollow Nanoparticles for Enhanced Photocatalytic Applications under Visible Light

Abstract

Herein, a non-stacked γ-Fe₂O₃/C@TiO₂ double-layer hollow nano photocatalyst has been developed with ultrathin nanosheets-assembled double shells for photodegradation phenol. High catalytic performance was found that the phenol could be completely degraded in 135 min under visible light, due to the moderate band edge position (VB at 0.59 eV and CB at −0.66 eV) of the non-stacked γ-Fe₂O₃/C@TiO₂, which can expand the excitation wavelength range into the visible light region and produce a high concentration of free radicals (such as ·OH, ·O²⁻, holes). Furthermore, the interior of the hollow composite γ-Fe₂O₃ is responsible for charge generation, and the carbon matrix facilitates charge transfer to the external TiO₂ shell. This overlap improved the selection/utilization efficiency, while the unique non-stacked double-layered structure inhibited initial charge recombination over the photocatalysts. This work provides new approaches for photocatalytic applications with γ-Fe₂O₃/C-based materials.

1. Introduction

Phenol containing wastewater is produced in many industries, such as pharmaceuticals, polymer, dye, etc. [1]. Once phenol is released into the environment, it cannot be degraded inherently under the natural environment, and it causes harm to human health [2,3]. The phenol degradation is complex due to the high conjugated molecular system [4] and lower phenol concentration is also toxic [2]. Therefore, designing an environmentally friendly, cost-effective and highly efficient approach for phenol treatment is urgent. Photocatalysis is a simple technology among water treatment technologies [5,6,7], owing to its high mineralization and sturdy treatment efficiency [8,9].

Titanium dioxide (TiO₂) is the most-used semiconductor material in the photocatalytic field, due to its chemical/physical excellent profound mineralization capability and stability under ultraviolet light [10]. However, two significant problems need to be solved: The low specific surface area of TiO₂ (50 m²/g for P25, TiO₂ with a mixed rutile phase, and anatase phase with an average particle size of 25 nm) confines its adsorption capacity for pollutant molecules. Additionally, it has a wide energy gap and is challenging to utilize visible light. Therefore, the preparation of a TiO₂ catalyst with an extensive specific surface area and response to visible light is the bottleneck problem in applying TiO₂.

To enhance the visible light absorption capacity, the commonly used means [11] are constructing heterojunction, defect engineering, and element doping. The construction of heterojunction catalysts can enhance the absorption of visible light and enhance the separation efficiency of photogenerated carriers because photogenerated electrons and holes can migrate from one phase to another through the two-phase interface, which can effectively inhibit the recombination of the photogenerated carriers [12,13,14]. More recently, double-layer hollow microsphere semiconductor catalysts have been applied as photocatalysts [15]. By virtue of the advantages of the double-layer structure, the heterogeneous junction catalyst based on the double layer structure is explored.

The construction of a core-shell heterojunction, γ-Fe₂O₃, maybe an ideal inner material that can improve solar energy utilization efficiency, which can also simplify the separation process with magnetic properties [16]. Most of the related work has concentrated on the design of functional cores or shells, but the effect of space accumulation between the two layers has been neglected. Based on the photocatalysis mechanism [17,18,19], the specific energy barrier formed at the interface between the core and the shell can be changed by adjusting the overlap mode between the two layers, promoting the separation of photogenerated carriers. Hence, the structure of the outer TiO₂ needs a directional design. It is an urgent problem to build a non-simple stacked TiO₂ shell and increase its specific surface area.

Recently, Zhao’s group [20] developed a TiO₂ shell material composed of ultrathin nanosheets. This shell material composed of ultrathin TiO₂ nanosheets increases the specific surface area of the catalyst. It constructs a non-simple stacked core-shell structure, making the catalyst appear in a porous state, which improves the light utilization rate and the adsorption capacity of pollutant molecules. Because the structure permits light scattering and refraction, the diffusion distance of photogenerated electron-hole pairs is reduced, and the light utilization is maximized [4].

This study demonstrates a facile route to synthesize non-stacked γ-Fe₂O₃/C@TiO₂ double-layer hollow nanoparticles. The C moiety constructs a channel for photo-generated electrons between iron oxide andTiO₂, facilitating the electron transfer from the γ-Fe₂O₃ moiety to TiO₂ moiety. The double-layer structure formed by stacking nanosheets has large pores, which alleviates the transmission of reactants in the catalyst. These constructs also inhibit the separation of electron-hole pairs and achieve a high treatment effect. In this study, phenol was selected as an indicator to study the catalyst’s efficiency for photocatalytic oxidation. We found that two-step charge generation and charge transfer were matched and synergistically enhanced in the non-stacked γ-Fe₂O₃/C@TiO₂ double-layer hollow nanoparticles, which significantly improved the photodegradation of phenol.

2. Materials and Methods

2.1. Catalyst Preparation

2.1.1. Materials

Ferrocene, hydrogen peroxide (H₂O₂), acetone, phenol, BaSO₄ and ethanol anhydrous were guarantee-grade reagents from Beijing Chemicals Co. Ltd. (Beijing, China). Concentrated ammonia solution (28 wt%), ethylene glycol, sodium acetate, tetra-butyl titanate (TBOT), tetraethyl orthosilicate (TEOS), HCl, NaOH and trisodium citrate were purchased from Tianjin Chemical Corp (Tianjin, China) and were analytical grade. All the water used for all experiments was deionized.

2.1.2. Synthesis of Non-Stacked γ-Fe₂O₃/C@TiO₂ Double-Layer Hollow Nanoparticles

Figure 1 shows the synthesis approaches for the non-stacked γ-Fe₂O₃/C@TiO₂ double-layer hollow nanoparticles. In this fraction, the nanoparticles were prepared by the following steps.

(1)

Synthesis of SiO₂@γ-Fe₂O₃/C nanoparticles

Monodispersed SiO₂ nanoparticles were first synthesized through the Stöber method. A modified hydrothermal approach prepared the core-shell SiO₂/γ-Fe₂O₃/C nanoparticles. Silica nanoparticles (100 mg) and ferrocene (200 mg) were suspended in ethanol (65 mL) using ultrasonic for 30 min. Then, 2 mL H₂O₂ was dropwise counted into the mixture and cruised with energetic stirring for 1.5 h. A Teflon-lined stainless-steel autoclave(TEFIC BIOTECH CO., Xi'an, China) was used to heat the homogeneous solution at 210 °C for 48 h. The product was obtained after the autoclave cooled down to room temperature (3 h). Acetone and ethanol were used to wash the obtained products 3 times, respectively, and then the product was vacuum-dried for 12 h.

(2)

Synthesis of SiO₂@γ-Fe₂O₃/C@SiO₂ nanoparticles

The Fe₂O₃/C@SiO₂ nanoparticles (0.3 g) obtained from the last step were added into a three-neck round-bottom flask filled with the ammonia solution (5.0 mL, 28 wt%), ethanol anhydrous (280 mL), and deionized water (70 mL), ultrasonicate for 20 min. TEOS (4 mL) was added dropwise to the mixture in 10 min. The reaction mixture was mechanically stirred for 10 h at room temperature before the products separated. Then the final product was then washed 3 times, respectively, using ethanol and deionized water, and dried in vacuum.

(3)

Synthesis of SiO₂@γ-Fe₂O₃/C@SiO₂@TiO₂ nanoparticles

The core-shell SiO₂@γ-Fe₂O₃/C@SiO₂@TiO₂ nanoparticles were synthesized using the condensation and hydrolysis of TBOT. Ethanol anhydrous (200 mL) was used to disperse the obtained SiO₂@γ-Fe₂O₃/C@SiO₂ nanoparticles (0.15 g) from the above procedure. Ammonia solution (0.9 mL, 28 wt%) was counted to the mixture and ultrasonicated for 15 min. TBOT (2.0 mL) was then added dropwise to the mixture in 5 min, followed by continuous mechanical stirring for 24 h at 45 °C. The products were separated by centrifugation and washed 3 times, using ethanol and deionized water, and dried under vacuum.

(4)

Synthesis of non-stacked γ-Fe₂O₃/C@TiO₂ double-layer hollow nanoparticles

An alkaline hydrothermal etching-assisted crystallization approach was used to synthesize the final products, the SiO₂@γ-Fe₂O₃/C@SiO₂@TiO₂ nanoparticles. The SiO₂@γ-Fe₂O₃/C@SiO₂@TiO₂ nanoparticles (0.4 g) obtained above were added into a Teflon-lined stainless-steel autoclave (50 mL) filled with a NaOH aqueous solution (25 mL, 2.0 M). The autoclave was sealed and heated to 100 °C for 4 h and cooled to room temperature in 3 h before the next step. The final SiO₂@γ-Fe₂O₃/C@SiO₂@TiO₂ nanoparticles were submerged in a HCl aqueous solution (100 mL, 0.1 M) for 20 min followed by washing with deionized water until pH value was close to 7. The final nanoparticles were then dried at 60 °C for 5 h. Finally, the catalyst was annealed at 400 °C for 4 h in an oxygen-deficient environment.

2.2. Catalyst Characterization

X-ray powder diffraction (XRD) patterns with 2θ ranging from 10 to 80° (40 kV and 30 mA, D/MAX-2500, Rigaku, Japan), as well as transmission electron microscopy (TEM, JEM-2100 Felectron microscope operating at 200 kV, Tokyo, Japan) were used to obtain the crystal and morphological structure. The samples’ specific surface areas were characterized by the Brunauer–Emmett–Teller (BET) model. Quadrasorb SI analyzer was used to obtain the N₂ adsorption-desorption isotherms at 77 K. The Barrett–Joyner–Halenda (BJH) model was used to analyze the pore size and volume. X-ray photoelectron spectroscopy (XPS) was performed using a 5300 ESCA equipment (PerkinElmer PHI Co.,Hopkinton, Massachusetts, USA) with an Al Kα X-ray source (250 W) to investigate the chemical compositions of samples. A UV–vis spectrophotometer (UV-2600, Shimadzu, Tokyo, Japan) with BaSO₄ was used to acquire the UV–vis diffuse reflection spectra (UV–vis DRS).

2.3. Catalytic Activity Measurement

The photocatalytic activity was evaluated using a photochemical reactor (TG-10B, Beijing, China) under xenon lamp (300 W). In each experiment, 0.1 g catalyst was weighed in a quartz glass tube, and 30 mL phenol (20 mg L⁻¹) was added into it. Before photocatalytic reaction, a dark reaction was carried out for 30 min to achieve the equilibrium of adsorption and desorption of phenol. The absorbency of phenol was determined by a UV-VIS spectrophotometer.

According to absorbance conversion of phenol, the formula of photocatalytic reaction removal rate was shown in (1).

$$D\%=\frac{A_0-A_t}{A_0}\times 100\%$$

(1)

• A₀—the initial absorbance of phenol.
• A_t—Absorption of phenol at t min.

3. Results and Discussion

3.1. Textural Properties of Catalysts

The wide-angle XRD pattern of non-stacked γ-Fe₂O₃/C@TiO₂ is shown in Figure 2a. The characteristic peaks of the catalyst were indexed to anatase TiO₂ (JCPDS-ICDD21-1272) [21,22,23]. Other phases such as γ-Fe₂O₃ were not observed because the thick TiO₂ shell passivated the X-ray diffraction. To better analyse the phase composition of the catalyst inner layer, the precursor (hollow γ-Fe₂O₃/C) of γ-Fe₂O₃/C@TiO₂ was characterized by XRD also. As shown in Figure 2b, the diffraction peaks of γ-Fe₂O₃ (JCPDS-ICDD 25-1402) were observed in hollow γ-Fe₂O₃/C [24,25]. Figure 2c shows the Raman spectra of hollow γ-Fe₂O₃/C, stacked γ-Fe₂O₃/C@TiO₂, and non-stacked γ-Fe₂O₃/C@TiO₂. The five vibrational modes, located at 640, 545, 395, 193, and 142 cm⁻¹, represent E_g Raman active, A_1g + B_1g, B_1g, e.g., and E_g modes, indicating that the anatase is the dominant phase of the TiO₂ hollow spheres.

The N₂ adsorption-desorption was used to measure the specific surface area of the catalyst. Type IV isothermal curves and N₂ hysteresis loops were observed in Figure 3a, which indicated that the non-stacked γ-Fe₂O₃/C@TiO₂ was a mesoporous material. The pore volume and pore size distributions were also characterized. As shown in Figure 3b, the pore size was mainly distributed from 5–30 nm, which indicated the catalysts’ mesoporous nature. The hollow structure expected to endow the material with a larger specific surface area (145.328 m²/g), which has a strong ability to absorb pollution molecules so that the active species produced on its surface can directly react with pollutants and enhance the catalytic performance.

As shown in Figure 4a’, the γ-Fe₂O₃/C layer was firmly attached to the outer surface of the SiO₂ sphere, and the wrapped SiO₂@γ-Fe₂O₃/C nanoparticles exhibited relatively uniform spherical structures. Then, the thin layers of SiO₂ and TiO₂ were successfully coated on the surface of the SiO₂@γ-Fe₂O₃/C core to form uniform SiO₂@γ-Fe₂O₃/C@SiO₂@TiO₂ nanoparticles (Figure 4b’). After treatment in a hot alkaline solution, the silicon component was removed, and the TiO₂ expanded both inside and outside to form non-stacked γ-Fe₂O₃/C@TiO₂(Figure 4c’). ICP-AES results showed the nominal ratio of the components (Fe:Ti) was 3.9:1. Furthermore, from Figure 4a,b, almost all catalyst particles can maintain this particular structure. A clear lattice plane was exposed with interplanar spacings of 0.29 nm and 0.35 nm, which revealed the presence of anatase TiO₂ and γ-Fe₂O₃/C, respectively, which mainly expressed the (220) and (101) lattice planes (Figure 4d’).

To further investigate the composition distribution of the inner shell, the Fe, C, and O species located in hollow γ-Fe₂O₃/C were characterized by XPS. The typical peaks of Fe2p_3/2 and Fe2p_1/2 were located at 711.0 and 724.5 eV, respectively (Figure 5a) [26,27,28]. The presence of the characteristic satellite peak at 719.0 eV confirmed the formation of γ-Fe₂O₃, which is the key characteristic distinguishing Fe₂O₃ from Fe₃O₄. In the O1s spectra (Figure 5b), the presence of Fe-O-C indicated strong interactions between γ-Fe₂O₃ and C, which may serve as an electron pathway to facilitate electron transfer via Fe-O-C. Two small peaks at 285.9 and 288.7 eV correspond to the residual C = O and C-O species in the nano-shell after calcination (Figure 5c). The distribution of Ti and O species located in the shell of non-stacked γ-Fe₂O₃/C@TiO₂ is shown in Figure 5d,e. The peaks at binding energies of 464.9 and 458.9 eV were assigned to the Ti2p_1/2 and Ti2p_3/2 core levels of Ti⁴⁺, respectively. The two peaks located at 458.2 and 463.7 eV corresponded to the characteristic peaks of Ti2p_3/2 and Ti2p_1/2 of Ti³⁺. The O1s spectra displayed two major oxygen peaks at 530.1 and 531.7 eV, which were attributed to lattice oxygen (O_lat) and surface-absorbed oxygen (O_sur), respectively. The lattice oxygen species are nucleophilic reagents that are usually responsible for oxidation reactions.

3.2. UV-Vis Absorbance Spectra of Non-Stacked γ-Fe₂O₃/C@TiO₂

The optical absorption of TiO₂ was affected by impurities and changes in the bandgap. The impact of catalysts’ overlap modes on the absorption of visible light was analyzed using UV-vis-DRS. Figure 6a shows that the three catalysts generated the absorption band-edge around 550 nm, corresponding to the transition of electrons from the top of the valence band to the footing of the conduction band. The absorption band edge did not immediately decline to 0 from 550 to 800 nm; however, it entered an extended buffer period, corresponding to the energy of photon needed to transition electrons from the O2p level to the impurity station. This extensive absorption range reflected the main characteristics of γ-Fe₂O₃/C.

From Figure 6b, the first maximum at 2.3 eV (540 nm) is associated with point lattice defects, namely oxygen vacancies, while the subsequent growth at energies above 2.3 eV corresponds to the fundamental bandgap [29,30]. The corresponding bandgap energies of non-stacked γ-Fe₂O₃/C@TiO₂ and stacked γ-Fe₂O₃/C@TiO₂ were 1.25 and 1.32 eV (Figure 6b), which were both higher than hollow γ-Fe₂O₃/C (1.11 eV). The band locations (Figure 7) were calculated using the XPS valence spectra and bandgap energies. The γ-Fe₂O₃/C exhibited a valence band (VB) at 0.08 eV and a conduction band (CB) at −1.03 eV. The CB and VB of stacked γ-Fe₂O₃/C@TiO₂ and non-stacked γ-Fe₂O₃/C@TiO₂ were notably different. According to previous literature, the shell affects the energy band depending on the material [31,32,33,34]. The TiO₂ layer had a more positive conduction band voltage than γ-Fe₂O₃. The surface dipole layer formed by the TiO₂ shell caused the conduction band voltage of the γ-Fe₂O₃ inner layer to migrate, and the migration direction and magnitude depended on the dipole parameters due to the usage of the two materials and the different stacking modes. Non-stacked γ-Fe₂O₃/C@TiO₂ had a moderate band edge position (VB at 0.59 eV and CB at −0.66 eV). During the photodegradation of organic pollutants, the suitable band edge position enabled three active substances (·OH, ·O²⁻, holes) to function simultaneously.

The charge generation and charge transfer behaviors over non-stacked γ-Fe₂O₃/C@TiO₂ are proposed in Scheme 1. Incident light can induce the transition of γ-Fe₂O₃. Excited electrons generated in γ-Fe₂O₃ can be efficiently transmitted to the conduction band of TiO₂ through C species while holes remain in the valence band of γ-Fe₂O₃, resulting in effective electron-hole separation. Therefore, the bandgap will become smaller, and the excitation wavelength range of the TiO₂ composite was expanded into the visible light region. From Scheme 1, the ·OH, ·O²⁻ and holes will contribute to the degradation of organic matter, and the hydroxyl radicals are not obtained from hole oxidation of water but from the interaction of electrons, hydrogen ions, and superoxide radicals. And the production process of hydroxyl radical is as follows [11]:

e⁻ + O₂ = ·O₂⁻

O₂⁻ + ·O₂⁻ + 2H⁺ = H₂O₂

H₂O₂ + e⁻ = ·OH + OH⁻

3.3. Photocatalytic Degradation of Phenol

Hollow γ-Fe₂O₃/C, stacked γ-Fe₂O₃/C@TiO₂, and non-stacked γ-Fe₂O₃/C@TiO₂ photocatalysts were used to degrade phenol under visible light irradiation. As shown in Figure 8a, non-stacked γ-Fe₂O₃/C@TiO₂ showed the highest performance and thoroughly degraded phenol after 135 min of irradiation under Xe lamp with 300 W power (Figure 8b). The catalytic performance was as follows: non-stacked γ-Fe₂O₃/C@TiO₂ > stacked γ-Fe₂O₃/C@TiO₂ > hollow γ-Fe₂O₃/C. Compared with stacked γ-Fe₂O₃/C@TiO₂, the performance of non-stacked γ-Fe₂O₃/C@TiO₂ was significantly enhanced due to the unique designed morphology and structure, which can enhance the absorption capacity of visible light and improve the activity of adsorption sites. The hollow inner cavity also permitted the scattering and refraction of light; therefore, the diffusion distance of electron-hole pairs generated by light was reduced, maximizing light utilization. Simultaneously, the unique structure also expanded the specific surface area of the catalytic material. The carbon species can facilitate charge transfer from γ-Fe₂O₃ to the outer TiO₂, reducing the recombination probability of photogenerated carriers. The reusability of non-stacked γ-Fe₂O₃/C@TiO₂ was investigated (Figure S1), and the catalytic activity of the catalyst decreased by only about 5.0% after 5 catalytic cycles.

The degradation of phenol using the non-stacked γ-Fe₂O₃/C@TiO₂ was analysed by UV-Vis absorbance spectra (Figure 9a) and in situ DRIFTS (Figure 9b). Interestingly, Figure 9a shows that the absorption band (269 nm) of phenol over non-stacked γ-Fe₂O₃/C@TiO₂ substantially increased upon prolonging the irradiation time beyond 30 min, suggesting the formation of more intermediate products. When the irradiation time was prolonged, the intermediate products formed during the degradation of phenol decomposed to form small molecules, consistent with the results of in situ DRIFTS (Figure 9b). The absorption bands centered at 1410 cm⁻¹ represented the stretching vibration of -OH functional group of phenol. The characteristic peaks of phenol significantly decreased when exposed to light after 30 min. This shows that the phenolic -OH was first oxidized during phenol degradation.

To further investigate the degradation of phenol over non-stacked γ-Fe₂O₃/C@TiO₂, the intermediate products (p-dihydroxybenzene, o-dihydroxybenzene, p-benzoquinone, oxalic acid, acetic acid, and formic acid) were measured by high-performance liquid chromatography (Figure 10a,b). Under light irradiation, the holes formed by γ-Fe₂O₃/C@TiO₂ reacted with phenol to generate unstable free radicals containing phenoxy groups. The o-and p-phenolic hydroxyl groups tend to form stable o-dihydroxybenzene, p-dihydroxybenzene, which were further oxidized to p-benzoquinone. Small molecules, such as oxalic acid, acetic acid, and formic acid, were formed through ring-opening and were eventually mineralized into CO₂ and H₂O. Non-stacked γ-Fe₂O₃/C@TiO₂ had a moderate band edge position, allowing it to excite hydroxyl radicals, superoxide free radicals, and holes to oxidize organic compounds. This was ascribed to an appropriate overlap mode between the two layers. Moreover,

Table 1. Comparison of catalyst performance.

Photo Catalyst	Phenol	Characteristics	% Degradation
Er3+: YAlO3/TiO2: 0.39 g	50 mgL−1	Visible light	58% (8 h) [35]
N-doped TiO2: 0.3 g	50 mgL−1	λ = 312 nm UV	99.6% (540 min) [36]
2P-TiO2-500: 50 mg	10 mgL−1	Xenon lamp (300 W)	100% (180 min) [37]
TiO2-CdS-gCNNSs: 50 mg	10 mgL−1	Visible light	80% (300 min) [38]
TiO2/MoS2 heterostructures	-	Visible light	78% (150 min) [39]
This work: 0.1 g	20 mgL−1	Visible light	Almost 100% (135 min)

shows a comparison of the double-layer hollow nanoparticles with the other catalysts; it is found that the catalysts in this paper have better catalytic performance and can use fewer catalysts in a short time to achieve higher catalytic efficiency.

4. Conclusions

A facile route was used to synthesize non-stacked γ-Fe₂O₃/C@TiO₂ double-layer hollow nanoparticles with appropriate overlap mode between the two layers. This unique material structure reduces the catalyst’s energy band, which can broaden the light response range to the visible light absorption range, and reduces the recombination rate of photogenerated carriers owing to the C moiety facilitated electron transfer from the γ-Fe₂O₃ moiety to TiO₂. The non-stacked γ-Fe₂O₃/C@TiO₂ displayed an excellent photocatalytic performance for phenol degradation under visible light irradiation, which is attributed to the cooperativity of the existence of ·OH, ·O^2-, holes. In situ DRIFTS further explored the degradation pathway of phenol; during phenol degradation, -OH was first oxidized, and combined with the identification results of intermediate products, then the benzene rings were destroyed to form small molecule organic acid and eventually mineralized into CO₂ and H₂O. This overlap mode enhanced both charge generation and charge transfer over photocatalysts. The γ-Fe₂O₃ moiety endows the nano-shell with an ability of charge generation, while the C moiety facilitated electron transfer from the γ-Fe₂O₃ moiety to TiO₂. The unique non-stacked double-layer structure inhibited the initial charge recombination in TiO₂. The non-stacked γ-Fe₂O₃/C@TiO₂ double-layer hollow nanoparticles can make the two steps of charge generation and charge transfer well-matched and synergistically enhanced, which significantly improved the efficiency of the photodegradation of phenol. The preparation of non-stacked γ-Fe₂O₃/C@TiO₂ catalysts also provides a novel approach for the future design of a higher-efficiency photocatalytic system.