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. 2026 Jul 3;16(35):36454–36463. doi: 10.1039/d6ra03278h

Synergistic effect in heterojunction TiO2/SnO2 nanofibers: boosting photocatalytic performance for methylene blue degradation

Jiaqi Xu a, Feng Ye a, Yeqian Ge a,b,c,d,✉
PMCID: PMC13330779  PMID: 42405064

Abstract

Titanium dioxide (TiO2) is widely applied in water treatment, air purification, self-cleaning surfaces, and other fields owing to its strong oxidizing capacity, excellent chemical stability, low cost, and non-toxicity. However, its practical application as a photocatalyst is severely limited by the extremely low solar light utilization efficiency. To address this defect and enhance the photocatalytic performance of TiO2 under ultraviolet (UV) and nature light irradiation, TiO2/SnO2 composite nanofibers were fabricated via electrospinning combined with calcination processes. SnO2 possesses high electron mobility as well as stable chemical and thermal properties, the construction of a heterojunction between TiO2 and SnO2 is conducive to improving the separation efficiency of photogenerated electron–hole pairs, thereby enhancing the overall photocatalytic activity. The morphology, crystal structure, and photocatalytic properties of the prepared composite nanofibers were systematically characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and photocatalytic degradation experiments with methylene blue (MB) as the target pollutant. The results demonstrated that the TiO2/SnO2 nanofibers prepared at a TiO2 : SnO2 molar ratio of 10 : 1 and a calcination temperature of 450 °C exhibited the optimal photocatalytic activity under UV light, achieving a MB degradation rate of 99% within 60 min. Additionally, the photocatalytic performance of TiO2/SnO2 nanofibers under nature light was significantly improved with the calcination temperature of 650 °C: the MB degradation rates after 3 h of irradiation were 42.39%, 66.15%, and 74.06% at 450 °C, 550 °C, and 650 °C, respectively, which represented increments of 12.26%, 36.02%, and 43.93% compared with pure TiO2. This work provides a feasible strategy for the modification of TiO2-based photocatalysts and offers technical support for the efficient treatment of dyeing and printing wastewater.

Titanium dioxide (TiO2) is widely applied in water treatment, air purification, self-cleaning surfaces, and other fields owing to its strong oxidizing capacity, excellent chemical stability, low cost, and non-toxicity.graphic file with name d6ra03278h-ga.jpg

1. Introduction

In the textile production process, the treatment of dyeing and printing wastewater constitutes a critical technical issue that directly relates to environmental sustainability and industry development. The textile industry is inherently a water-intensive sector: the production of one ton of textile products typically consumes a substantial volume of water, and the wastewater generated during dyeing and finishing processes accounts for a considerable proportion of global industrial wastewater discharge. With the continuous expansion of dye and pigment varieties and the steady growth of production scales, textile wastewater is characterized by complex composition, high color intensity, strong toxicity, and high salinity, containing high concentrations of dyes, auxiliaries, heavy metals, and refractory organic compounds. In China, the textile industry discharges large amounts of wastewater each year, posing serious threats to aquatic ecosystems, including reduced photosynthesis of aquatic plants and alterations in the habitats of aquatic organisms. Excessively high dye concentrations in water bodies can also decrease dissolved oxygen levels, thereby affecting the biological activities of aquatic organisms. Furthermore, numerous dyes exhibit toxic, carcinogenic, and mutagenic properties, and most of them are difficult to degrade through conventional wastewater treatment technologies. Therefore, the development of low-cost and high-efficiency technologies for treating dyeing and printing wastewater has become an important research focus in the fields of environmental engineering and textile industry sustainability.1

Among existing photocatalytic materials, semiconductor photocatalysts have garnered extensive research attention in wastewater treatment due to their unique capability to generate highly oxidative reactive oxygen species (ROS) under light irradiation. Among these semiconductor materials, TiO2 stands out as one of the most representative and widely studied photocatalysts, attributed to its favorable chemical stability, low toxicity, and excellent photocatalytic activity for the degradation of refractory organic pollutants.

TiO2 is widely used in water treatment,2 air purification,3 self-cleaning surfaces, and other fields due to its strong oxidizing ability, excellent chemical stability and low cost.4 However, the practical application of pure titanium dioxide is limited by several factors. Its wide bandgap (about 3.2 eV for the anatase phase) limits light absorption primarily to the ultraviolet region, which accounts for only 4–5% of the solar spectrum. In addition, the rapid recombination of photogenerated electron–hole pairs leads to low quantum efficiency.5 Moreover, commonly used powdered titanium dioxide is difficult to recycle, raising concerns about secondary pollution.6

To address these shortcomings, various modification strategies have been explored, including metal/non-metal doping, dye sensitization, and the construction of heterostructures.7–10 Among them, coupling TiO2 with another suitable semiconductor to form a heterojunction has been proven to be an effective approach. Commonly used semiconductor materials include g-C3N4, CdS, SiO2, Bi2O3, SnO2, etc.11–15 Aldama-Huerta et al.16 synthesized silica–titanium dioxide fibers through a sol–gel method and electrospinning technology, and evaluated their photocatalytic activity using methyl orange, chrome black T, and methylene blue. These fibers (average diameter: 214 nm) exhibit a band gap of 3.2 eV and can almost completely remove dyes under ultraviolet light irradiation within 8 h. The reaction follows Langmuir–Hinshelwood kinetics, achieving a degradation efficiency of 98%. Wang et al.17 prepared TiO2/WO3 hollow spheres via hydrothermal and calcination methods. The 10% TiO2/WO3 showed the best photocatalytic activity, degrading 87.8% of MB in 120 min under visible light. The heterojunction facilitates charge separation by transferring holes to TiO2 and electrons to WO3. Sun et al.18 used a hydrolysis-precipitation method to prepare nanocellulose-templated CeO2–TiO2 (CeO2–TiO2–NCC). This composite completely removed RhB and MO within 70 and 50 min, respectively, and reduced Cr(vi) within 60 min, due to its smaller grain size, larger surface area, and lower recombination rate. Wang et al.19 synthesized nanoflower-like β-Bi2O3/TiO2via a solvothermal method. At a Bi/Ti molar ratio of 2.1%, the RhB degradation rate increased from 61.2% to 99.6%. The heterojunction improves carrier separation, and the material showed good stability after multiple cycles.

Tin dioxide (SnO2) is an n-type semiconductor with a wide bandgap (approximately 3.6 eV), high conductivity, and excellent chemical stability, making it an ideal candidate for constructing such composite materials. When SnO2 comes into contact with TiO2, a type II heterojunction (staggered band alignment) is typically formed. Since the conduction band (CB) and valence band (VB) of SnO2 are positioned more positively relative to TiO2, an internal electric field is formed at the interface. This electric field promotes the migration of photogenerated electrons from the CB of TiO2 to the CB of SnO2, while simultaneously driving the migration of holes from the VB of SnO2 to the VB of TiO2. This spatial separation of charge carriers significantly inhibits electron–hole recombination, thereby enhancing the availability of electrons and holes in surface redox reactions.20

In this study, TiO2/SnO2 composite nanofibers were prepared using nano-SnO2 and TBT as precursors, and polyvinylpyrrolidone as a polymer template. The photocatalytic performance of the obtained material was evaluated by degrading MB under ultraviolet and nature light irradiation.

2. Experimental

2.1. Material

Polyvinylpyrrolidone (PVP, average molecular weight 1.3 × 106), tetrabutyl titanate (TBT, ≥99.0%), nano-SnO2 (Shanghai Aladdin Biochemical Technology Co., Ltd), anhydrous ethanol (analytical pure, Huzhou Shuanglin Chemical Technology Co., Ltd), glacial acetic acid (analytical pure, Hangzhou Shuanglin Chemical Reagent Co., Ltd), methylene blue (MB, 99%, Sinopharm Chemical Reagent Co., Ltd). All chemicals were used without further purification.

2.2. Preparation of TiO2/SnO2 nanofibers

In this experiment, TiO2/SnO2 composite nanofibers were synthesized through electrospinning followed by a calcination process. First, PVP was dissolved in a mixture of ethanol and glacial acetic acid with a mass ratio of 7 : 1 to prepare a 10 wt% PVP solution. TBT was then added at a PVP-to-TBT mass ratio of 1 : 3, followed by the addition of nano-SnO2 powder in different masses according to the molar ratios of TBT to SnO2 (5 : 1, 10 : 1, and 15 : 1). The resulting solutions were labeled as TBT/SnO2(5 : 1), TBT/SnO2(10 : 1), and TBT/SnO2(15 : 1), respectively.

Each solution was stirred thoroughly before being subjected to electrospinning. The electrospinning process was carried out under the following conditions: a voltage of 18 kV, a distance of 15 cm between the needle and the drum collector, a drum rotation speed of 150 rpm, and a solution flow rate of 1 mL h−1. The as-spun nanofiber membranes were placed in a BTF-1400C-II dual-temperature zone tube furnace and calcined in an air atmosphere. The temperature was raised from room temperature to 450 °C, 550 °C, 650 °C at a heating rate of 5 °C min−1 and held for 240 min. Finally, after cooling to room temperature inside the furnace, the resulting TiO2/SnO2 nanofibers were obtained and labeled as TiO2/SnO2(5 : 1)-450, TiO2/SnO2(10 : 1)-450, TiO2/SnO2(15 : 1)-450, TiO2/SnO2(10 : 1)-550, TiO2/SnO2(10 : 1)-650.

2.3. Performance testing and characterization

2.3.1. Morphology and structural characterization

The morphology of the composite nanofiber membranes was measured by a SU3800 scanning electron microscope (SEM) for observational investigation. The diameters of 40 randomly selected fibers were measured, and the diameter distribution of the fibers was calculated. Thermal behavior of the nanofibers was investigated with a TG/DTA 6300 comprehensive thermal analyzer in an air atmosphere, with a heating rate of 10 °C min−1 over a temperature range of 25–700 °C. The crystal structure of the samples was characterized using an X-ray diffractometer (XRD) with a 2θ scanning range of 10°–80°. The TiO2/SnO2 nanofibers were tested by JEM-2100F high resolution transmission electron microscope (TEM-EDS), and the distribution and crystallization of Ti, Sn, O and other elements in TiO2/SnO2 nanofibers were analyzed.

2.3.2. Photocatalytic performance test

The photocatalytic performance of the TiO2/SnO2 nanofibers was investigated by simulating ultraviolet and nature light degradation of methylene blue (MB) solution in mercury lamp (175 W) and xenon lamp (300 W). MB solutions of different concentrations were prepared (5 mg L−1, 10 mg L−1, 15 mg L−1). For each test, 100 mL of the dye solution was placed in a photoreactor, and a certain amount of TiO2/SnO2 nanofiber photocatalyst was added. The mixture was stirred continuously to ensure uniform dispersion. Before irradiation, the mixture was stirred in the dark for 30 min to achieve adsorption–desorption equilibrium. After taking the initial sample, the UV lamp was turned on, and the circulating water system was started. Subsequent samples were taken every 10 min until the MB solution was completely decolorized.

The absorbance of the samples was measured using a UV-2600 UV-vis spectrophotometer, and the absorbance values at various time points during the catalytic degradation were recorded. The photocatalytic degradation efficiency (η) was calculated as follows:212.3.2.where A0 is the absorbance of the dye solution after adsorption equilibrium, and At is the absorbance of the dye solution after a certain period of photocatalytic reaction.

3. Result and discussion

3.1. Morphological analysis

Fig. 1(a, c and e) shows the SEM images and corresponding diameter distribution maps of TBT/SnO2 nanofibers with different proportions. The nanofiber membrane before calcination has good film-forming properties and no obvious adhesion between fibers. Comparing the diameter distribution of TBT/SnO2 nanofiber membranes with different ratios, it was found that as the amount of nano tin dioxide added increased, the diameter of the nanofiber membrane decreased. This may be due to the dispersed nano tin dioxide particles during spinning reducing the entanglement density of polymer chains, making the spinning solution more easily stretched and thinned in an electric field.22,23Fig. 1(b, d and f) shows SEM images and corresponding diameter distribution maps of TiO2/SnO2 nanofibers with different ratios. After calcination, the fiber diameter decreases and a small amount of fracture occurs, which is attributed to the decomposition of PVP and the oxidation of tetrabutyl titanate to titanium dioxide. And after calcination, the fiber diameter still showed the same trend as before calcination, which may be due to the role of tin dioxide in the nanofibers as a skeleton, inhibiting the decrease in fiber diameter during calcination and maintaining the same trend of change. Fig. 1(d, g and h) shows the fiber morphology and diameter distribution of TiO2/SnO2 nanofibers at different calcination temperatures. It can be observed that the fiber surface is relatively smooth without large adhesion. With the increase of calcination temperature, the diameter of nanofiber membrane increased gradually, and the fibers were partially broken after calcination, but still maintained good fiber morphology. Compared with powder materials, it can provide larger specific surface area and more reaction sites for subsequent photocatalytic experiments, so as to improve the catalytic effect.

Fig. 1. SEM images of TiO2/SnO2 nanofiber membranes with Ti to Sn molar mass ratios of 5 : 1, 10 : 1 and 15 : 1. (a) TBT/SnO2(5 : 1), (b) TiO2/SnO2(5 : 1)-450, (c) TBT/SnO2(10 : 1), (d) TiO2/SnO2(10 : 1)-450, (e) TBT/SnO2(15 : 1), (f) TiO2/SnO2(15 : 1)-450, (g) TiO2/SnO2(10 : 1)-550, (h) TiO2/SnO2(10 : 1)-650.

Fig. 1

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3.2. EDS analysis

Fig. 2(a–c) displays the EDS spectra of TiO2/SnO2 nanofibers with varying composite ratios. The results indicate that Ti and Sn elements are clearly detected in all samples, and the content of Sn element exhibits a consistent trend with changes in the Sn/Ti composite ratio, indicating that SnO2 has been successfully incorporated into the nanofiber system, which is in good agreement with the variation in the weight percentage of Sn element shown in Table 1. Further analysis in conjunction with XRD results reveals that the samples consist solely of SnO2 and anatase phase TiO2, with no detection of any impurity phases or peaks. The EDS elemental composition analysis and XRD phase analysis results mutually corroborate, jointly demonstrating the successful composite of SnO2 and TiO2 from both the elemental distribution and crystal phase structure perspectives. Additionally, the incorporation of nano-sized SnO2 does not introduce new impurities, providing a structural foundation for subsequent photogenerated carrier transport/heterojunction formation.

Fig. 2. EDS images of TiO2/SnO2 nanofiber. (a) TiO2/SnO2(5 : 1)-450, (b) TiO2/SnO2(10 : 1)-450, (c) TiO2/SnO2(15 : 1)-450, (d) TiO2/SnO2(10 : 1)-550, (e) TiO2/SnO2(10 : 1)-650.

Fig. 2

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Table 1. Weight percentage of O, Ti, and Sn elements in TiO2/SnO2 nanofiber.

O Ti Sn
TiO2/SnO2(5 : 1)-450 31.73 68.08 0.19
TiO2/SnO2(10 : 1)-450 43.02 56.85 0.13
TiO2/SnO2(15 : 1)-450 40.35 59.60 0.05
TiO2/SnO2(10 : 1)-550 43.52 56.36 0.12
TiO2/SnO2(10 : 1)-650 43.97 55.93 0.10
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Fig. 2(b, d and e) shows the element distribution of TiO2/SnO2(10 : 1) nanofibers at different calcination temperatures. Combined with Table 1, it can be observed that there is no significant difference in the content of tin element at 450 °C and 550 °C, but as the temperature increases to 650 °C, the content of tin decreases, which is consistent with the results of XRD. The reason may be that as the temperature increases, SnO2 in the nanofibers partially evaporates.

3.3. TG analysis

Fig. 3 shows the thermogravimetric curves of TBT/SnO2 nanofibers with different ratios. It can be observed that three different proportions of composite nanofiber membranes exhibit consistent weight loss curves, with two distinct weight loss intervals. The first interval appears before 100 °C, mainly due to the volatilization of residual solvents ethanol and acetic acid in the nanofiber membrane. The second weight loss interval occurs after 300 °C, when the weight of the nanofiber membrane rapidly decreases. At this time, the high polymer template in the fiber membrane gradually begins to decompose at high temperatures, while the tetrabutyl titanate in the fiber membrane gradually decomposes and reacts with oxygen in the air to gradually generate TiO2 and form heterojunctions with nano-SnO2. The quality of the nanofiber membrane gradually stabilizes. Compared with the non composite TiO2 nanofiber membrane, the addition of nano-SnO2 has a certain impact on the weight loss curve of the composite membrane, with TBT/SnO2(10 : 1) reaching weight loss equilibrium earliest. The final quality retention rate of nanofiber membrane shows a consistent change with the composite ratio.

Fig. 3. TG images of TiO2/SnO2 nanofiber membranes with different proportions.

Fig. 3

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3.4. XRD analysis

As illustrated in Fig. 4(a), the XRD patterns of TiO2/SnO2 nanofibers with different composite ratios reveal that no additional phases are formed in the three prepared samples. The peaks observed at approximately 25.5° and 48° correspond to the (101) and (200) crystal planes of anatase TiO2,24 respectively. Meanwhile, the peaks around 26.6°, 33.8°, and 51.8° are attributed to the (110), (101), and (211) planes of SnO2. A composite peak appears near 38°, which is associated with the (004) plane of TiO2 and the (200) plane of SnO2. Furthermore, the incorporation of SnO2 does not induce any change in the crystal structure of TiO2.25 The TiO2/SnO2 heterostructure, fabricated under the premise of preserving the crystal phase stability of TiO2, enables the formation of efficient interfacial band coupling between the two components. This structural feature not only promotes the separation and migration of photogenerated electron–hole pairs but also suppresses carrier recombination, thereby laying a crucial structural and phase foundation for the subsequent enhancement of photocatalytic performance.

Fig. 4. XRD images of TiO2/SnO2 nanofiber. (a) TiO2/SnO2 nanofibers with different composite ratios, (b) TiO2/SnO2(10 : 1) nanofibers with different calcination temperatures.

Fig. 4

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Fig. 4(b) presents the XRD patterns of TiO2/SnO2(10 : 1) nanofibers synthesized at various calcination temperatures. It can be observed that as the calcination temperature increases, the diffraction peaks of anatase TiO2 gradually sharpen, accompanied by the emergence of rutile TiO2 characteristic peaks at 650 °C. This phenomenon indicates that the crystal phase of TiO2 undergoes a gradual transformation from anatase to rutile with increasing temperature. Notably, the intensity of SnO2 characteristic peaks decreases significantly at this temperature (650 °C), which is consistent with the results obtained from energy-dispersive X-ray spectroscopy (EDS) analysis.

3.5. Photocatalytic performance analysis

3.5.1. Photocatalytic degradation performance under different reaction conditions

As shown in Fig. 5(a), under the condition of 10 mg L−1 MB concentration and 0.5 mg mL−1 catalyst concentration, the catalytic degradation performance of TiO2, TiO2/SnO2(5 : 1)-450, TiO2/SnO2(10 : 1)-450 and TiO2/SnO2(15 : 1)-450 for MB was compared. The results demonstrate that TiO2/SnO2(10 : 1)-450 nanofibers exhibit the best catalytic degradation performance, achieving 44.19% degradation after 10 min and 99% after 60 min. By comparing the catalytic degradation curves of TiO2 and TiO2/SnO2(10 : 1)-450 nanofibers, it is evident that the composite nanofibers show better catalytic degradation efficiency than pure TiO2. At 10 min, the degradation rate of TiO2/SnO2(10 : 1)-450 nanofibers is 5.2% higher than that of TiO2 nanofibers and significantly higher than that of SnO2 nanoparticles.

Fig. 5. Catalytic degradation curves of TiO2/SnO2 nanofibers under different reaction conditions. (a) Different proportions, (b) different MB concentrations, (c) different catalyst concentrations, (d) different pH condition.

Fig. 5

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Fig. 5(b) presents the photocatalytic degradation curves of MB at different initial concentrations (5 mg L−1, 10 mg L−1, and 15 mg L−1) using TiO2/SnO2(10 : 1)-450 nanofibers at a catalyst concentration of 0.5 mg mL−1. The results indicate that, with increasing initial MB concentration, the degradation rate at each time point gradually decreases. Moreover, by comparing the degradation rates over the same time intervals, it can be observed that higher MB concentrations lead to slower degradation kinetics.

Fig. 5(c) shows the photocatalytic degradation curves at a fixed MB concentration of 10 mg L−1 under different catalyst concentrations (0.5, 1.0, and 1.5 mg mL−1). As the catalyst concentration increases, the degradation rate of MB first decreases and then increases. However, the enhancement in degradation efficiency gradually becomes less pronounced.

Fig. 5(d) shows the degradation curves of TiO2/SnO2(10 : 1)-450 nanofibers under different pH conditions when the dye concentration is 10 mg L−1 and the catalyst concentration is 0.5 mg mL−1. When pH = 7, the catalytic effect of TiO2/SnO2(10 : 1)-450 nanofibers is the best. At 10 min, the degradation rate reached 44.19%, the degradation rate approaches 100% at 90 min. When pH = 4 or pH = 10, the catalytic effect decreases significantly, while pH = 4, it was 47.99% at 90 min, and at pH = 10, it was only 34.93% at 90 min. In conclusion, acidic or alkaline environments will inhibit the catalytic effect of TiO2/SnO2(10 : 1)-450 nanofibers. The underlying reason is that under excessively acidic or alkaline conditions, the surface charge distribution of TiO2/SnO2 nanofibers is altered, which hinders the effective adsorption of MB molecules and the participation of photogenerated carriers, thereby inhibiting the photocatalytic degradation process. In contrast, under neutral conditions, the reaction environment on the catalyst surface becomes more stable, facilitating the generation and transfer of reactive species and thus resulting in enhanced photocatalytic performance.26

3.5.2. Photocatalytic degradation performance under different calcination temperatures

Fig. 6(a) shows the catalytic degradation curve of TiO2/SnO2 nanofibers prepared at different calcination temperatures under UV light. The dye concentration was set at 10 mg L−1 and the concentration of TiO2/SnO2 nanofibers was 0.5 mg mL−1. The results showed that the catalytic degradation rates of TiO2/SnO2(10 : 1)-450, TiO2/SnO2(10 : 1)-550 and TiO2/SnO2(10 : 1)-650 were 44.19%, 46.42% and 24.09% respectively at 10 min. The catalytic degradation rate reached 98.97%, 96.91% and 70.33% respectively at 60 min. It can be found that when the calcination temperature increases to 550 °C, the catalytic effect of TiO2/SnO2 nanofibers under UV light is not significant, but when the calcination temperature increases to 650 °C, the catalytic effect decreases significantly. Based on the XRD analysis results, it can be seen that when the temperature rises to 650 °C, the crystal phase of TiO2 transforms from anatase to rutile. The rutile phase TiO2 is unfavorable for the effective separation and migration of photogenerated charge carriers, thereby weakening its photocatalytic degradation performance under UV light conditions.

Fig. 6. Catalytic degradation curves of TiO2/SnO2 nanofibers under UV and nature light conditions at different calcination temperatures. (a) UV light, (b) nature light.

Fig. 6

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Fig. 6(b) shows the catalytic degradation curve of TiO2/SnO2 nanofibers prepared at different calcination temperatures under nature light. The concentration of MB was set at 10 mg L−1 and the concentration of TiO2/SnO2 nanofibers was set at 0.5 mg mL−1. The results showed that the catalytic degradation rates of TiO2/SnO2(10 : 1)-450, TiO2/SnO2(10 : 1)-550 and TiO2/SnO2(10 : 1)-650 nanofibers were 8.61%, 15.61% and 17.60% respectively at 0.5 h. The catalytic degradation rate of nanofibers calcined at 450 °C was only 42.39% at 3 h, while the catalytic degradation rates of TiO2/SnO2(10 : 1)-550 and STiO2/SnO2(10 : 1)-650 nanofibers for MB were 42.00% and 48.83% at 1.5 h. The catalytic degradation rates of them in 3 h were 66.15% and 74.06%. Compared with TiO2/SnO2(10 : 1)-450 nanofibers, the catalytic degradation rate increased by 23.76% and 31.67%, respectively It can be found that the catalytic effect of TiO2/SnO2 nanofibers under nature light has been greatly improved with the increase of calcination temperature. The reason for the above difference may be that the bandgap of the rutile phase is approximately 3.0 eV, which is smaller than that of the anatase phase (3.2 eV). This means that the rutile phase has a broader photoresponse range than the anatase phase and can utilize longer-wavelength visible light.

To sum up, in order to ensure that the prepared TiO2/SnO2 nanofibers have excellent photocatalytic activity under UV light and can effectively improve their catalytic activity under nature light, we selected TBT : SnO2 molar ratio of 10 : 1 as the best composite ratio, and the best calcination temperature is 550 °C.

3.6. Recoverability of TiO2/SnO2 nanofibers

Fig. 7(a) shows the degradation curves of the TiO2/SnO2 nanofibers over multiple cycles. It can be observed that the TiO2/SnO2 nanofibers still exhibit excellent catalytic activity after recycling, achieving a degradation rate of over 95% for methylene blue under UV light within 90 minutes after three cycles of use. Fig. 7(b) compares the recycling performance of commercial nano-TiO2 and TiO2/SnO2 nanofibers. The mass loss here includes the portion consumed for degradation rate testing. The results show that when both commercial nano-TiO2 and TiO2/SnO2 nanofibers are used in an amount of 65 mg, the commercial one cannot be recycled after being used twice in a row, and its recovery amount after the first use is only 26.2 mg (40.15%). However, the recovery amount of TiO2/SnO2 nanofibers after the first use is 53.9 mg (82.92%), and after being used three times, it can still be recycled to 25.4 mg (39.08%). This indicates that TiO2/SnO2 nanofibers prepared through electrospinning and calcination processes have structural advantages. Their use not only enhances the catalytic activity of TiO2 under ultraviolet light and nature light but also avoids the difficulty in recycling powdered TiO2, thereby improving usability to a certain extent and reducing usage costs.

Fig. 7. Cyclic properties of TiO2/SnO2 nanofibers. (a) Changes in degradation rate after cyclic use, (b) comparison of commercial nano-TiO2 and TiO2/SnO2 nanofibers recycling performance.

Fig. 7

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3.7. Mechanism of photocatalytic degradation of TiO2/SnO2 nanofibers

Fig. 8 illustrates the proposed mechanism for the enhanced photocatalytic activity of the TiO2/SnO2 nanofibers. As illustrated in the schematic, under light irradiation, the TiO2/SnO2 heterojunction is photoexcited to generate electron–hole pairs. Owing to the suitable band alignment between TiO2 and SnO2, the conduction band of TiO2 is located at a higher energy level than that of SnO2, and its valence band is also higher than that of SnO2. Consequently, the photogenerated electrons migrate from the conduction band of TiO2 to that of SnO2, while the holes in the valence band of SnO2 transfer to the valence band of SnO2, resulting in effective spatial separation of charge carriers and suppression of electron–hole recombination. During the photocatalytic process, the electrons accumulated on the SnO2 surface react with adsorbed O2 to generate reactive oxygen species such as ˙O2−, whereas the holes enriched on the TiO2 surface can directly oxidize MB or react with H2O or OH− to produce ˙OH radicals. These highly oxidative reactive species synergistically degrade MB molecules and ultimately mineralize them into CO2 and H2O, thereby significantly enhancing the photocatalytic degradation performance of the TiO2/SnO2 system.27,28

Fig. 8. The photocatalytic reaction mechanism of TiO2/SnO2 nanofibers.

Fig. 8

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4. Conclusion

In this paper, TiO2/SnO2 nanofibers were prepared by electrospinning and calcination, and the optimal composite ratio of SnO2 was determined. When the molar ratio of TiO2 to SnO2 was 10 : 1 and the calcination temperature was 550 °C, the comprehensive catalytic activity of TiO2/SnO2 nanofibers under UV and nature light is the best. Under UV light, the TiO2/SnO2 nanofibers calcined at 450 °C have the best catalytic activity. With the increase of calcination temperature, the catalytic performance of TiO2/SnO2 nanofibers under nature light was improved. The catalytic degradation rates of MB at 450 °C, 550 °C and 650 °C for 3 h were 42.39%, 66.15% and 74.06%, which were increased by 12.26%, 36.02% and 43.93% compared with TiO2.

Conflicts of interest

No potential conflict of interest was reported by the authors.

Data availability

All data included in the study are available upon request by contact with the corresponding author.

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Associated Data

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Data Availability Statement

All data included in the study are available upon request by contact with the corresponding author.

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