Enhanced Photo-Fenton Activity of SnO₂/α-Fe₂O₃ Composites Prepared by a Two-Step Solvothermal Method

Abstract

The x-SnO₂/α-Fe₂O₃ (x = 0.04, 0.07, and 0.1) heterogeneous composites were successfully prepared via a two-step solvothermal method. These composites were systematically characterized by the X-ray diffraction technique, field emission scanning electron microscopy, an energy dispersive spectrometer, X-ray photoelectron spectroscopy and a UV–visible spectrometer. It was found that SnO₂ nanoparticles were uniformly decorated on the surface of α-Fe₂O₃ particles in these heterogeneous composites. A comparative study of methylene blue (MB) photodegradation by α-Fe₂O₃ and x-SnO₂/α-Fe₂O₃ composites was carried out. All x-SnO₂/α-Fe₂O₃ composites showed higher MB photodegradation efficiency than α-Fe₂O₃. When x = 0.07, the MB photodegradation efficiency can reach 97% in 60 min. Meanwhile, the first-order kinetic studies demonstrated that the optimal rate constant of 0.07-SnO₂/α-Fe₂O₃ composite was 0.0537 min⁻¹, while that of pure α-Fe₂O₃ was only 0.0191 min⁻¹. The catalytic mechanism of MB photodegradation by SnO₂/α-Fe₂O₃ was examined. The SnO₂ can act as a sink and help the effective transfer of photo-generated electrons for decomposing hydrogen peroxide (H₂O₂) into active radicals. This work can provide a new insight into the catalytic mechanism of the photo-Fenton process.

1. Introduction

As an important advanced oxidation process, the heterogeneous photo-Fenton system has been considered a promising method for the removal of stubborn organic dyes [1,2,3,4]. In this process, iron-based catalysts are generally applied to activate H₂O₂ in order to generate strong oxidative hydroxyl radicals (·OH) [5,6,7,8]. Among the iron-based catalysts, α-Fe₂O₃ is one of the most promising Fenton candidates due to its stable structure, low cost, wide absorption of visible light, and environmental benignity [9,10]. However, several adverse factors seriously reduce the reaction activity of α-Fe₂O₃, such as the high recombination rate of photoelectrons and holes, and a weak activation in alkaline environments. To remedy these drawbacks, various measures have been studied, such as porous regulation [11,12,13,14,15,16], facet engineering [17,18,19,20], and composite construction [21,22,23,24,25,26]. Among these methods, integrating α-Fe₂O₃ with other catalysts has caught the attention of many because it can effectively separate photo-generated electron–hole pairs. Liu et al. [27] reported the synthesis of α-Fe₂O₃ anchored to a graphene oxide nanosheet. The graphene oxide was considered to accelerate the transfer of photo-generated electrons and to enhance the absorption to methylene blue (MB) through electro-static interaction and π–π stacking. Deng et al. [28] constructed an advanced TiO₂/Fe₂TiO₅/Fe₂O₃ heterojunction structure, and the abundant phase interfaces improved both the migration and separation of charges.

SnO₂ has a high photochemical property and stability, and is extensively studied in the fields of Li-ion batteries [29,30,31], dye sensitized solar cells [32,33,34], and gas sensors [35,36,37]. Additionally, many studies have shown that the SnO₂/α-Fe₂O₃ heterogeneous catalyst has an excellent photodegradation activity. Wang et al. [38] synthesized SnO₂-encapsulated α-Fe₂O₃ nanocubes by annealing Prussian blue microcubes, and revealed the important contribution of SnO₂ cubic shells for improving photocatalytic performance. Tian et al. [39] prepared a tube-like SnO₂/α-Fe₂O₃ heterostructure by using an anion-assisted hydrothermal route and studied the effective separation of photo-generated carriers. Niu et al. [40] synthesized branched SnO₂/α-Fe₂O₃ composites by a hydrothermal system of Sn(OH)₆²⁻ dilute aqueous solution, and investigated their photocatalytic activity. The synthesis methods can significantly influence the morphology of SnO₂/α-Fe₂O₃ composites. First, the SnO₂ and α-Fe₂O₃ were prepared by a sol–gel method, then their composite (SnO₂–α-Fe₂O₃) systems were synthesized by combining SnO₂ with α-Fe₂O₃ in various weight percent ratios, and finally, the photocatalytic activity was investigated [41]. A necklace-like SnO₂/α-Fe₂O₃ hierarchical heterostructure was fabricated by the chemical vapor deposition method, using SnO₂ nanowires with the preferential growth direction of [1] as a template, and then the photocatalytic property was studied [42]. Therefore, it is interesting to explore the new synthesis methods as well as establish the relationship between morphology and internal catalytic mechanism of SnO₂/α-Fe₂O₃ composites.

In this study, we synthesized the x-SnO₂/α-Fe₂O₃ (x = 0.04, 0.07, and 0.1) heterogeneous catalysts via a two-step hydrothermal method. The materials were systematically characterized by using the X-ray diffraction (XRD) technique, field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), and a UV–visible (UV–vis) spectrometer. The XRD analysis indicated the successful synthesis of purity and well-crystalline of SnO₂/α-Fe₂O₃ powders. FE-SEM images showed that SnO₂ nanoparticles were homogeneously decorated on the surface of the peach-like α-Fe₂O₃. XPS measurement demonstrated Fe and Sn elements in the 0.07-SnO₂/α-Fe₂O₃ sample to be trivalent and tetravalent, respectively. The UV–vis spectra revealed the strong visible light absorption ability of SnO₂/α-Fe₂O₃ powders. The photodegradation of MB over different catalysts was tested, and the first-order kinetic analysis was performed to get the photodegradation rate. Free radical trapping experiments and hydroxyl radical quantitative experiments were carried out to explore the mechanism of photocatalytic reaction.

2. Experimental Section

2.1. Materials and Chemicals

Ferric chloride hexahydrate (FeCl₃·6H₂O) (Macklin, Shanghai, China), tin chloride pentahydrate (SnCl₄·5H₂O) (Macklin, Shanghai, China), urea (CO(NH₂)₂) (Macklin, Shanghai, China), polyethylene glycol (PEG) (Sinopharm, Shanghai, China), sodium hydroxide (NaOH) (Sinopharm, Shanghai, China), ethanol (C₂H₅OH) (Sinopharm, Shanghai, China), and methylene blue (MB) (Sinopharm, Shanghai, China) were used in the experiment. All the reagents were analytical grade and used as received without further purification. Deionized water was used throughout the experiment.

2.2. Synthesis

2.2.1. Synthesis of α-Fe₂O₃

The peach-like α-Fe₂O₃ powders were prepared by a facile hydrothermal method, as shown in the upper section of Scheme 1. First, 1.623 g of FeCl₃·6H₂O, 0.6 g of polyethylene glycol, and 0.6 g of NaOH were dissolved in 60 mL of deionized water. Secondly, the mixture solution was stirred for 30 min and transferred into a 100 mL Teflon-lined stainless steel autoclave (H-100ml, Zhuoran Company, Zhengzhou, China). The thermal treatment was performed at 180 °C for 5 h. After the autoclave was naturally cooled down to room temperature, the precipitate was continually washed with deionized water and absolute ethanol. Finally, the α-Fe₂O₃ was achieved after being dried out at 60 °C for 2 h.

2.2.2. Synthesis of SnO₂/α-Fe₂O₃

The SnO₂/α-Fe₂O₃ composites were prepared via a two-step hydrothermal method. In this method, the first step is used to prepare α-Fe₂O₃ precursors as in the above description. The second step is to grow SnO₂ on the surface of α-Fe₂O₃ precursors as follows (shown in the bottom of Scheme 1). The x g (x = 0.04, 0.07, and 0.1, which denoted the mass of tin chloride pentahydrate) SnCl₄·5H₂O, 0.5 g of CO(NH₂)₂ were dispersed in a mixed solvent consisting of 30 mL of deionized water and 20 mL of absolute ethanol. Then, 0.1 g of the α-Fe₂O₃ precursor was added into the above solution. Subsequently, the mixture solution was stirred for 10 min before transferred into a 100 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and heated at 170 °C for 10 h and then naturally cooled down to room temperature. The obtained SnO₂/α-Fe₂O₃ photocatalysts were washed several times with deionized water and absolute ethanol before they were dried at 60 °C for 12 h. The final products were termed as x-SnO₂/α-Fe₂O₃ (x = 0.04, 0.07, and 0.1). The precise control of reaction temperatures is very important to obtain target samples with good crystal structures and homogeneous particle size [43]. Pure SnO₂ can also be obtained by the process in the bottom of Scheme 1 without adding the α-Fe₂O₃.

2.3. Characterization

Phase structures of powders were analyzed by XRD using Japan Rigaku (Tokyo, Japan) with a Cu target. The morphologies and chemical compositions of samples were characterized by FE-SEM (JEM-7001F, JEOL, Tokyo, Japan), equipped with an energy dispersive spectroscope (EDS). X-ray photoelectron spectroscopy of SnO₂/α-Fe₂O₃ was detected using Thermo Fisher 250-XI (Thermofisher, Waltham, MA, USA) with an Al Kα. The optical properties and photocatalytic activities were measured using a UV–vis spectrophotometer (Shanghai Metash, UV-9000S, Shanghai, China). The molar ratios of composites were checked by an inductive coupled plasma emission spectrometer (Agilent ICP-MS 7500a, Santa Clara, CA, USA).

2.4. Photodegradation Experiment

The photo-Fenton activity of the samples was investigated by the degradation of MB dyes under visible light irradiation. The light source used was a 300 W xenon lamp with a 420 nm cutoff filter (Microsolar 300, PerfectLight, Beijing, China). In a typical procedure, 20 mg of as-prepared samples was dispersed into 100 mL of MB solution (20 mg·L⁻¹) with the assistance of ultrasonic for 2 min. Afterwards, the reaction mixture was magnetically stirred in the dark for 30 min to ensure absorption equilibrium. The reaction was initiated by adding 1 mL hydrogen peroxide solution (H₂O₂, 30 wt%, Sinopharm, Shanghai, China) after the xenon light was stable. During the irradiation, 4 mL solution was sampled at 10 min intervals. The whole process took place under stirring, while the circulating cooling water worked at the same time. After removing the catalysts from each sample by centrifugation, the degree of photodegradation was calculated by measuring the absorbance of MB at 664 nm where the solution had the maximum absorption.

3. Results and Discussion

The crystal structure of as-prepared α-Fe₂O₃ precursors, SnO₂, and x-SnO₂/α-Fe₂O₃ (x = 0.04, 0.07, and 0.1) powders were measured by XRD. As shown in Figure 1, all the peaks appearing in the α-Fe₂O₃ powders are sharp and well indexed to a pure rhombohedral structure of hematite (JCPDS No. 33-0664). This indicates the high purity and good crystallinity of the prepared α-Fe₂O₃ sample. The widths of peaks of SnO₂ are much larger than that of α-Fe₂O₃, which might be attributed to the poor crystallinity and small particle size of SnO₂. After being decorated by SnO₂, all the extra diffraction peaks of x-SnO₂/α-Fe₂O₃ were indexed to (110), (101), and (211) planes of rutile phase of SnO₂ (JCPDS No. 41-1445). This illustrates the successful synthesis of x-SnO₂/α-Fe₂O₃ composites. The particle size was estimated using the Scherrer equation [44]: $D=\frac{K\lambda }{\beta COS \theta }$ where D is the grain size, K is the Scherrer’s constant, λ is the X-ray wavelength (0.154 nm), β is the FHWM, and θ is the diffraction angle. The grain sizes of SnO₂, α-Fe₂O₃, and 0.07-SnO₂/α-Fe₂O₃ are about 15 nm, 203 nm, and 212 nm, respectively.

The morphologies of α-Fe₂O₃ and 0.07-SnO₂/α-Fe₂O₃ were measured by FE-SEM. As shown in Figure 2a, α-Fe₂O₃ is peach-like, and the inwardly concave symmetry curve can be clearly observed on the surface of α-Fe₂O₃. The α-Fe₂O₃ particle is homogeneous, with a diameter of around 220 nm. Interestingly, many small spots are decorated on the outer surface of α-Fe₂O₃ after adding Sn, as shown in Figure 2b. Moreover, those spots, each of around 20 nm, are uniformly distributed. The particle sizes essentially agree with the results from the above XRD, which were calculated using the Scherrer’s equation. Figure 2c indicates the EDS spectra of 0.07-SnO₂/α-Fe₂O₃. It clearly identifies that the composite is composed of Fe, O, and Sn elements. Considering the XRD of 0.07-SnO₂/α-Fe₂O₃, it is reasonable to assume that these small spots are SnO₂. The chemical compositions of x-SnO₂/α-Fe₂O₃ (x = 0.04, 0.07, and 0.1) were checked by ICP-MS. The mass contents of Fe₂O₃ and SnO₂ in 0.04-SnO₂/α-Fe₂O₃, 0.07-SnO₂/α-Fe₂O₃, and 0.1-SnO₂/α-Fe₂O₃ are 87.3% and 12.7%, 78.5% and 21.5%, and 70.6% and 29.4%, respectively.

To further demonstrate the compositions of surface dots and the oxidation states of metal elements of 0.07-SnO₂/α-Fe₂O₃ powders, an XPS measurement was taken. Figure 3a shows the full-scale XPS spectrum, and the presence of Sn, Fe, O, and C was confirmed without any other element. The characteristic peak of C 1s found at 284.8 eV was from adventitious carbon. Figure 3b is the high-resolution XPS spectrum of Fe 2p. The binding energy peaks at both 710.5 and 724.5 eV were ascribed to Fe 2p_3/2 and Fe 2p_1/2, respectively. The peak for Fe 2p_3/2 at 710.5 eV is sharper than that for Fe 2p_1/2 due to the spin–orbit coupling [45]. The appearance of satellite peaks at 712.8 and 733.5 eV confirms that the Fe element in 0.07-SnO₂/α-Fe₂O₃ is trivalent. A high intensity peak at the binding energy of 716.6 eV is due to the presence of Sn 3p_3/2. Figure 3c shows the high-resolution XPS spectrum of Sn 3d. The peaks at 487.2 and 495.5 eV were attributed to Sn 3d_5/2 and Sn 3d_3/2, respectively. This could support the +4 oxidation states of SnO₂. Thus, the results clearly confirm that these heterogeneous catalysts are composed of 0.07-SnO₂/α-Fe₂O₃, agreeing with the XRD results in Figure 1.

In order to study the visible light absorption properties of α-Fe₂O₃, SnO₂, and 0.07-SnO₂/α-Fe₂O₃ samples, their UV–vis absorption spectra were measured, as shown in Figure 4. The present SnO₂ could hardly absorb visible light (λ > 420 nm) in the UV–vis diffuse reflection spectrum, agreeing with the literature [46,47]. Interestingly, the 0.07-SnO₂/α-Fe₂O₃ composite exhibited much higher photo absorption ability compared with pure α-Fe₂O₃. This might contribute to the improvements in the photocatalytic activities of 0.07-SnO₂/α-Fe₂O₃.

The photo-Fenton activities of the present samples were studied using a degrading MB experiment under visible light for 60 min. One milliliter of H₂O₂ was added to the MB solution to activate the Fenton reaction. Figure 5a shows the visible light Fenton degradation of MB under different catalysts, including α-Fe₂O₃, SnO₂, and x-SnO₂/α-Fe₂O₃ (x = 0.04, 0.07, and 0.1) heterogeneous catalysts. Self-degradation of MB is limited. In the dark stage, SnO₂ shows excellent adsorption of dye molecules, while α-Fe₂O₃ presents poor adsorption. The adsorption capacity of the x-SnO₂/α-Fe₂O₃ composite was improved due to the decoration of SnO₂. More adsorption means a closer contact between the catalyst and the dye molecules, which might contribute to the photo-Fenton reaction. Under visible light irradiation, 66% of the MB was degraded within 60 min in the presence of α-Fe₂O₃. Interestingly, the MB degradation of x-SnO₂/α-Fe₂O₃ was much faster than that of α-Fe₂O₃. Moreover, with the increase in SnO₂ dosage, the degradation efficiency increased and then decreased, reaching the optimal efficiency of 97% for the 0.07-SnO₂/α-Fe₂O₃ sample.

With a low concentration of MB, the degradation followed the pseudo-first-order kinetics and the reaction constant of photodegradation was determined by the following equation [28]:

$$\ln ({\mathrm{C}}_0/\mathrm{Ct})=\mathrm{kt}$$

where C₀ is the initial dye concentration which reached adsorption–desorption equilibrium in the dark, C_t is the dye concentration at given time t during the Fenton process, and k is the reaction rate constant. As shown in Figure 5b, the plots ln(C₀/Ct) versus irradiation time are almost linear, which indicates that the photocatalytic degradation of MB solution agrees with the pseudo-first-order kinetic model. The optimal reaction rate constant was obtained for the 0.07-SnO₂/α-Fe₂O₃ sample (0.0537 min^–1), which was more than 2.8 times higher than that of α-Fe₂O₃ (0.0191 min^–1). Thus, it can be concluded that SnO₂ shows positive effects on the MB Fenton degradation of α-Fe₂O₃ [46,47,48]. Previous studies have reported that H₂O₂ could capture the photo-generated electrons of a semiconductor and decompose itself into ·OH for the dye’s degradation [49,50]. In this study, all the degradation efficiencies are enhanced with the assistance of H₂O₂ and the degradation efficiency of α-Fe₂O₃ is much higher than that of SnO₂. These might be ascribed to the photo-Fenton reaction of Fe²⁺ and H₂O₂.

To assess the contribution of reactive radicals and further explore catalytic mechanisms, control experiments were carried out with or without scavengers over the 0.07-SnO₂/α-Fe₂O₃ catalyst shown in Figure 6. Isopropanol (IPA) and 1, 4-benzoquinone (BQ) were used as hydroxyl radicals (·OH) and superoxide radical (·O₂) scavengers, respectively. As shown in Figure 6, the catalytic degradation of MB was limited by scavengers. This indicates that both the ·O₂ and ·OH have significant effects on the MB degradation of SnO₂/α-Fe₂O₃.

In addition, experiments on the amount of ·OH generation under α-Fe₂O₃ and 0.07-SnO₂/α-Fe₂O₃ were performed to further explore the effect of SnO₂ on the photo-Fenton reaction. The amount of ·OH in the heterogeneous photo-Fenton reaction was measured based on the fluorescence intensity of hydroxy terephthalic acid. As shown in Figure 7, the concentration of ·OH obviously increases after the introduction of SnO₂, suggesting that the presence of SnO₂ could accelerate the generation of ·OH. This is consistent with the results of the Fenton activity test.

It has been reported that Fe³⁺ could act as an acceptor of photo-generated electrons from a semiconductor during the photocatalytic process to suppress electron–hole recombination [26,28,51,52]. The E_CB of SnO₂ (0.4 eV) is more positive than the Fe³⁺/Fe²⁺ (0.77 eV) redox potential. Therefore, the photoelectrons from CB of SnO₂ could transfer to the Fe³⁺ that are located at the abundant heterogeneous interfaces, and reduce Fe³⁺ to Fe²⁺. The Fe²⁺ could further react with H₂O₂ and produce more ·OH.

According to the above results, the catalytic mechanism was illustrated in Figure 8. Under visible light irradiation, the photoelectrons (e^–) of the α-Fe₂O₃ are excited, and transition into the conduction band (CB), leaving the same amount of holes (h⁺) in the valence band (VB) [40]. The h⁺ and e⁻ can react with H₂O₂ to generate ·O₂ and ·OH. The separation of h⁺ and e⁻ can be promoted owing to the presence of numerous H₂O₂ and Fe³⁺. Meanwhile, the generation of ·O₂ and ·OH can also be accelerated [49,50]. Importantly, the CB of SnO₂ could act as a sink for the generated electrons from the α-Fe₂O₃ and the excited MB molecules. Since the CB position of SnO₂ is more negative than the Fe³⁺ redox potential, these electrons would be captured by the Fe³⁺ on the abundant interface of the SnO₂/α-Fe₂O₃ heterogeneous catalyst, which can accelerate the cycle of Fe³⁺/Fe²⁺ and the photo-Fenton reaction for the generation of ·OH [51,52,53]. In this way, more ·OH and ·O₂ radicals can be produced in SnO₂/α-Fe₂O₃ compared with the α-Fe₂O₃, resulting in the significant enhancement of photocatalytic properties.

4. Conclusions

In summary, the x-SnO₂/α-Fe₂O₃(x = 0.04, 0.07, and 0.1) heterogeneous catalysts were successfully prepared using a straightforward two-step hydrothermal strategy. The MB photodegradation investigation showed that the SnO₂/α-Fe₂O₃ composites exhibited an excellent photodegradation ability, with the addition of H₂O_2. The rate constant of 0.07-SnO₂/α-Fe₂O₃ composite (0.0537 min⁻¹) is 2.8 times higher than that of pure α-Fe₂O₃ powder (0.0191 min⁻¹). This remarkable enhancement is attributed to the effective transfer of photo-generated electrons for decomposing hydrogen peroxide into active radicals. The catalytic mechanism of the SnO₂/α-Fe₂O₃ heterogeneous catalyst can provide a new insight into the catalytic mechanism of the photo-Fenton process.