Journal of Chemistry
Volume 2020 (2020), Article ID 2390486, 12 pages
https://doi.org/10.1155/2020/2390486

Research Article

Preparation and Photocatalytic Performances of WO₃/TiO₂ Composite Nanofibers

Xiuping Han,1,2 Binghua Yao,¹ Keying Li,¹ Wenjing Zhu,² and Xuyuan Zhang³

¹Xi’an University of Technology, Xi’an 710048, China
²School of Science, Hainan University, Haikou 570228, China
³Hainan Vocational University of Science and Technology, Haikou 571126, China

Correspondence should be addressed to Binghua Yao; bhyao@xaut.edu.cn

Received 3 July 2020; Accepted 23 July 2020; Published 30 August 2020

Guest Editor: Tifeng Jiao

Copyright © 2020 Xiuping Han et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The use of sunlight for photocatalytic oxidation is an ideal strategy, but it is limited by factors such as insufficient light absorption intensity of the photocatalyst and easy recombination of photogenerated electron holes. TiO₂ is favored by researchers as an environment-friendly catalyst. In this paper, TiO₂ is combined with WO₃ to obtain a nanofiber with excellent catalytic performance under sunlight. The WO₃/TiO₂ composite nanofibers were synthesized by using the electrospinning method. The X-ray diffraction (XRD) analysis indicated that WO₃ was successfully integrated onto the surface of TiO₂. The photodegradation performance and photocurrent analysis of the prepared nanofibers showed that the addition of WO₃ really improved the photocatalytic performance of TiO₂ nanofibers, methylene blue (MB) degradation rate increased from 72% to 96%, and 5% was the optimal composite mole percentage of W to Ti. The scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse reflectance spectra (UV-Vis DRS), and Brunauer-Emmett-Teller (BET) analysis further characterized the properties of 5% WO₃/TiO₂ nanofibers. The H₂ generation rate of 5% WO₃/TiO₂ nanofibers was 107.15 μmol·g⁻¹·h⁻¹, in comparison with that of TiO₂ nanofibers (73.21 μmol·g⁻¹·h⁻¹) under the same condition. The 5% WO₃/TiO₂ produced ·OH under illumination, which played an important role in the MB degradation. Also, the enhanced photocatalytic mechanism was also proposed based on the detailed analysis of the band gap and the active species trapping experiment. The results indicated that the effective separation of Z-scheme photogenerated electron-hole pairs and transfer system constructed between TiO₂ and WO₃ endowed the excellent photocatalytic activity of 5% WO₃/TiO₂ nanofibers.

1. Introduction

Photocatalytic oxidation is a green, environmentally friendly, inexpensive, and efficient wastewater treatment technology. The efficient degradation of toxic and hazardous substances in wastewater is the goal that researchers have always sought and has achieved very good results. For example, Jiao and coworkers have prepared a series of environmentally friendly composite hydrogel photocatalysts, which can efficiently degrade the toxic and harmful substances nitrophenol and nitroaniline that are difficult to degrade in wastewater [1–3].

The core of photocatalytic oxidation technology is photocatalyst. Since Fujishima reported that TiO₂ would decompose water under ultraviolet light irradiation in 1972 [4], TiO₂ has attracted considerable attention in photocatalysis because of its superior photocatalytic activity, good chemical stability, nontoxicity, low cost, and no secondary pollution [5]. However, several disadvantages of TiO₂ photocatalyst severely blocked its practical application such as the limited visible light response, low specific surface area, and high recombination efficiency of charge carriers, which lead to the fact that the photocatalytic activity of TiO₂ was relatively unsatisfactory [6]. Numerous efforts such as morphology designing [7], noble metal [8, 9], or nonmetal doping [10], ion doping [11, 12], composite material forming [13, 14], and heterojunction fabricating [15] had been devoted to improving its photocatalytic performance. Among the abovementioned methods, heterojunction fabricating had been considered as an efficient strategy for broadening spectrum response range and effectively improving the separation efficiency of photogenerated charge carriers. For instance, Li and coworkers [16] fabricated TiO₂/MoS₂ nanoheterojunctions by electrospinning and hydrothermal method and demonstrated that as-prepared samples exhibited super photocatalytic hydrogen evolution activity of 171.24 μmol·g⁻¹·L⁻¹. Chen et al. [17] also reported that coupling LaNiO₃ with TiO₂ would remarkably enhance the photocatalytic activity of TiO₂ and explained that the heterojunction formed between them played an important role in improving the photocatalytic performance. Recently, TiO₂-C₃N₄ [18], CdS/TiO₂ [19], TiO₂/NiO [20], and TiO₂/Cu₂O [21] had been reported and shown enhanced photocatalytic performance for organic pollutant degradation and H₂ evaluation.

WO₃ as an n-type semiconductor with an energy band gap of 2.7 eV has also been considered as an efficient candidate for the formation of heterojunction with other photocatalysts due to its excellent physiochemical stability and strong visible light response [22, 23]. Thus, fabricating WO₃ with TiO₂ to form heterojunction might be an effective strategy to enhance the photocatalytic performance of TiO₂. For example, Pan et al. prepared WO₃-coated TiO₂ catalysts via alcohothermal synthesis; thus, the WO₃-coated TiO₂ heterostructure exhibited excellent photocatalytic performance compared with anatase TiO₂ [24]. Hunge et al. synthesized WO₃/TiO₂ thin films by the two-step spray pyrolysis method, and because a junction is formed between the WO₃ and TiO₂, their photoelectrocatalytic activity was enhanced [25]. Khan et al. fabricated hybrid TiO₂/WO₃ samples, and TiO₂ and WO₃ act in synergy effectively separated holes and electrons and improved photocatalytic performance [26].

The photocatalytic performance of the catalyst was closely related to its micromorphology and porous structure. The fibrous photocatalyst had a large specific surface area and more exposed active sites, which ensured its higher adsorption capacity and better photocatalytic activity [27]. Moreover, the special fibrous structure was beneficial to transfer photogenerated electrons and holes from the bulk phase to the catalyst surface and inhibited the recombination of electron-hole pairs. Therefore, fibrous photocatalysts could generate higher photocatalytic performance in comparison with traditional materials [28, 29]. Hu et al. synthesized TiO₂/WO₃ nanofibers by electrospinning technique and clarified that the heterojunction formed after the addition of WO₃ in TiO₂ fibers increased H₂ production rate [30]. Gao et al. prepared TiO₂/WO₃ nanofibers where the hydrogen production rate is 27.73 μmol/h and clarified the formation of band bending and artificial Z-scheme when there is a WO₃ contact with TiO₂ [31]. Soares and Alves synthesized TiO₂/WO₃ fibers by electrospinning which increased the photocatalytic performance due to the addition of H₂WO₄ and degraded 51% of the dye in 135 min [32].

In this work, the fibers TiO₂ and WO₃/TiO₂ fibers were successfully synthesized through a one-step electrospinning process and were characterized by using XRD, SEM, XPS, UV-Vis DRS, and BET techniques. The photocatalytic performance and stability of as-prepared samples were estimated by photocatalytic degradation of MB and hydrogen evolution under UV-Vis light irradiation. Finally, the possible enhanced photocatalytic mechanism of WO₃/TiO₂ heterojunction nanofibers was proposed.

2. Experimental

2.1. Synthesis by Electrospinning

The nanofibers of TiO₂ were obtained from the precursor solution made by mixing 2.50 g of tetrabutyl titanate, 9.0 mL of glacial acetic acid, 1.10 g of polyvinylpyrrolidone (PVP), and 10 mL of ethanol solution containing N,N-dimethylformamide ( :  = 1 : 1) as the solvent. The WO₃/TiO₂ precursor solution was prepared by mixing the abovementioned reactants plus different amounts of (NH₄)₂WO₄. These solutions were continuous stirring for 12 h and then transferred into an electrospinning device. The electrospinning parameters were optimized such as flow rate for solution 2.7 mL/h, an applied voltage of 14 kV, and the tip to collector distance of 11 cm. The samples collected on aluminum-foil collectors were designated as precursors.

Finally, the precursors prepared were heated up to 520°C at a heating rate of 1°C/min for four hours in a tube furnace, and the nanofibers of TiO₂ and WO₃/TiO₂ were obtained. We prepared a set of WO₃/TiO₂ nanofibers by varying (NH₄)₂WO₄ molar percent at 1%, 5%, and 10% where all other parameters remained unchanged.

2.2. Characterization of As-Prepared Samples

XRD patterns of samples were collected in the range of 10–80° using a 6100 X-ray diffract meter with Cu Kα radiation. The morphologies of photocatalysts were characterized by VTGA3 SBH scanning electron microscopy. XPS analysis was performed using a K-alpha photoelectron spectroscope. UV-Vis DRS of photocatalysts were analyzed by UV-2102, UV-Vis spectrophotometers with BaSO₄ as the internal reflectance standard. The BET surface area of samples was carried out using the JK-BK122W apparatus. The photocurrent was measured using a CHI-660b electrochemical analyzer.

2.3. Evaluation of Photocatalytic Activity

2.3.1. Photocatalytical Degradation of MB

The photocatalytic performance was estimated by the degradation of MB and was performed. In a typical process, 50 mg of as-prepared samples was dispersed in 50 mL 20 mg·L⁻¹ MB and stirred in the dark for 30 min before irradiation to reach the adsorption/desorption equilibrium. Then, the solution was illuminated with a 250 W Xenon lamp. 5 mL of the sample was collected sequentially at every 20 min and then centrifuged, and the supernatant dye solution was analyzed by TU-1901 spectrophotometer at 664 nm. The degradation efficiency was calculated according to the equation of D% = (A₀ − At)/A₀ × 100%. A₀ and At were the were the absorbance of MB at an initial time and time t, respectively.

2.3.2. Photocatalytic Hydrogen Evolution

The photocatalytic hydrogen evolution experiments were carried out by a DS-GHX-V system with a 100 W Mercury lamp as the light source. In short, 30 mg of as-prepared photocatalysts was added to a sealed 100 mL quartz tube containing a mixture of 60 mL of 0.25 mol·L⁻¹ Na₂S and 0.35 mol·L⁻¹ Na₂SO₃ aqueous solution as a sacrificial agent. Before irradiation, the reaction solution was purged with N₂ for 30 min to exhaust the air in the quartz tube. The quartz tube was kept in a circulating cooling water system to maintain the temperature at 25°C and stirred continuously to make an even dispersed solution. A Thermo Trace 1300 gas chromatograph equipped was adopted to determine the amount of hydrogen production on an 80/100 PORAPAK N molecule column. The temperature of the thermal conductivity detector, column box, and filament was assigned at 200, 150°C, and 300°C, respectively, and the gas flow rate is 10 mL/min in the constant pressure mode. The holding pressure is 60.0 kPa and the ion mode is in negative ion mode. After preheating the gas chromatography, 300 μL of air for zero adjustments was injected. After each irradiation for one hour, 300 μL of gas was taken out to analyze the quantity of hydrogen according to the peak areas with the retention time around 0.625–0.630 min through the external standard method.

3. Results and Discussion

3.1. XRD Analysis

XRD was employed to investigate the crystal phase structures of the samples as shown in Figure 1, and the characteristic peaks of pure TiO₂ corresponding to both anatase and rutile were observed. The peaks at 2θ of 25.3°, 37.9°, 48.2°, and 62.7° were indexed to (101), (004), (200), and (204) planes of anatase TiO₂ (PDF NO. 21–1272), and those at 2θ of 27.4°, 36.1°, 41.2°, and 54.3° corresponded to (110), (101), (111), and (211) planes of rutile TiO₂ (PDF NO. 21–1276), respectively [33]. For WO₃/TiO₂ nanofibers, the diffraction peaks of were similar to those of TiO₂ nanofibers and the peak intensities gradually decreased with the increase of WO₃ content and characteristic peaks corresponding to rutile phase diminished in 10% WO₃/TiO₂ samples, indicating that the introduction of WO₃ into TiO₂ affected the crystal structure of TiO₂. In addition, no diffraction peaks related to WO₃ were detected which might be due to its high dispersion or low content [34].

Figure 1: XRD patterns of TiO₂ and WO₃/TiO₂ nanofibers.

3.2. Photocatalytic Degradation Performance

The photocatalytic performance of TiO₂ and WO₃/TiO₂ nanofibers was assessed by degrading MB under Xenon lamp illumination. The experimental results are displayed in Figure 2(a). Apparently, as shown in Figure 2(a), 5% WO₃/TiO₂ exhibited the highest photocatalytic activity with the elimination rate of 96.2% for MB within 150 min, in comparison with that of TiO₂ nanofibers (72.0%), 1% WO₃/TiO₂ (91.5%) nanofibers, and 10% WO₃/TiO₂ nanofibers (79.0%) under the same condition. This result indicates that the content of WO₃ in WO₃/TiO₂ composites had a significant effect on the degradation efficiency of MB. The reasons could be attributed to the following: the lower the amount of WO₃ is, the less the electron transfer center can be formed, leading to the low photocatalytic activity; on the contrary, too much amount of WO₃ might change the electron transfer centers to the electron-hole recombination centers; in addition, the W⁶⁺ could scramble electrons from TiO₂; all of these lead to the low photocatalytic activity. In terms of the charge layer thickness, excessive doping of WO₃ can make the depth of incident light less than the surface charge layer thickness, decreased potential difference, and reduced the electron-hole migration force. Thus, the recombination of photogenerated electrons and holes becomes more easily. Moreover, the larger specific surface area of 5% WO₃/TiO₂ nanofibers could supply more magnificent active sites, resulting in the promoted photocatalytic performance.

(a)

(b)

Figure 2: (a) Decomposition of MB by TiO₂ and WO₃/TiO₂ nanofibers; (b) UV-Vis absorption spectra of MB during the photodegradation process by 5% WO₃/TiO₂ nanofibers.

The photocatalytic activity of 5% WO₃/TiO₂ nanofibers is shown in Figure 2(b). With the photocatalytic degradation, the absorption peak at 664 nm blueshifts and turns broadened at the same time, which may be caused by the gradual deethylation of MB [35]. The strong chromophore and conjugated structure of MB were destroyed during degradation [36]. Those characteristic peaks disappeared after 150 min irradiation, indicating that the aromatic rings and the conjugated π bond in the MB molecules were completely broken [34]. Obviously, 5% WO₃/TiO₂ nanofibers could degrade MB completely under illumination.

The significant stability of the photocatalyst was very substantial for its practical applications. To confirm the stability of 5% WO₃/TiO₂ nanofibers, recycling degradation tests were conducted by successive batches degradation of MB as shown in Figure 3. It was clear that at the end of the 5th cycle, 5% WO₃/TiO₂ nanofibers could still degrade 93% MB within 140 min (shown in Figure 3(a)) under Xenon lamp illumination, indicating the high stability of 5% WO₃/TiO₂ nanofibers. The crystal structure of used 5% WO₃/TiO₂ nanofibers was also determined to be stable as shown in Figure 3(b). In summary, 5% WO₃/TiO₂ nanofibers showed high photocatalytic activity and stability in the catalytic process.

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(b)

Figure 3: (a) Photodegradation performance within five cycles for 5% WO₃/TiO₂ nanofibers; (b) XRD patterns of 5% WO₃/TiO₂ nanofibers before and after photocatalysis.

3.3. Photocurrent Analysis

The photocurrent test is one of the important means to characterize the response intensity of carriers to illumination and the difficulty of carrier separation. A common method for detecting photocurrent is to use a photocatalyst as a working electrode and a saturated calomel electrode as a reference electrode. When light is irradiated, due to the photoelectric effect, the movement of the electrons emitted by the electrode will form a photocurrent. The intensity of the generated photocurrent is closely related to the nature of the photocatalyst, and the intensity of the photocurrent is directly proportional to the separation efficiency of photogenerated carriers.

The TiO₂ and WO₃/TiO₂ nanofibers were coated on the glass plate as the working electrode (coating amount is 3 mg), and the working electrode was irradiated by the 125 W Mercury lamp every 10 s, and the cycle was repeated. The results are shown in Figure 4. It can be seen from Figure 4 that the photocurrent response intensity of WO₃/TiO₂ nanofibers is higher than that of TiO₂ nanofibers (0.7 × 10⁻⁶ A/cm³), which indicates that the doping of WO₃ improves the separation efficiency of photogenerated carriers. 5% WO₃/TiO₂ nanofibers have the highest photocurrent response intensity, reaching 3.0 × 10⁻⁶ A/cm³, indicating that the light-induced electron-hole pairs are effectively separated in the 5% WO₃/TiO₂ nanofiber system, and the recombination trend is lower.

Figure 4: Photocurrent response curves of TiO₂ and WO₃/TiO₂ nanofibers.

Based on the above analysis results, we believe that 5% WO₃/TiO₂ nanofibers are the best composite percentage samples, and their comprehensive performance is the best. Therefore, 5% WO₃/TiO₂ nanofibers were further characterized and their hydrogen production performance was studied.

3.4. SEM and EDS Analysis

The SEM images of TiO₂ and 5% WO₃/TiO₂ nanofibers are displayed in Figure 5. From Figures 5(a) and 5(c), it was clear that the TiO₂ precursor and 5% WO₃/TiO₂ precursor were composed of smooth fibers with about 200 nm in diameter and 20–30 μm in length. After calcination, these smooth fibers fractured to short fibers with several hundred nanometers in length and the surface became rough as shown in Figures 5(b) and 5(d), which might be resulting from the oxidative decomposition of precursor fibers. As we all know, a rough surface possessed a larger surface area and is conducive to the adsorption reaction, which is helpful to improve the photocatalytic activity [37].

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(b)

(c)

(d)

Figure 5: SEM patterns of TiO₂ precursor (a), TiO₂ nanofibers (b), 5% WO₃/TiO₂ precursor (c), and 5% WO₃/TiO₂ nanofibers (d).

To further identify the chemical composition of both TiO₂ nanofibers and 5% WO₃/TiO₂ nanofibers, EDS analysis was conducted, and the results are illustrated in Figure 6. As expected, TiO₂ nanofibers were composed of Ti, O elements as shown in Figure 6(a). Three Ti, O, and W elements were detected in 5% WO₃/TiO₂ nanofibers as shown in Figure 6(b), implying the successful incorporation of WO₃ into TiO₂ nanofibers. Table 1 shows the element contents of TiO₂ and 5% WO₃/TiO₂ samples, which were basically consistent with the raw material ratio in the precursor.

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(b)

Figure 6: EDS of TiO₂ (a) and 5% WO₃/TiO₂ (b) nanofibers.

Table 1: Elemental composition and content of TiO₂ and 5% WO₃/TiO₂ nanofibers.

Sample
Element
Weight percentage
Atomic percentage

TiO₂
O
39.89
66.02

Ti
60.11
32.89

5% WO₃/TiO₂
O
37.21
67.18

Ti
51.87
31.17

W
10.52
1.64

3.5. XPS Analysis

The surface chemical status and elemental composition of the as-prepared 5% WO₃/TiO₂ nanofibers were analyzed by XPS. Figure 7(a) demonstrates the survey XPS spectra of sample, which indicated that 5% WO₃/TiO₂ nanofibers consisted of Ti, W, and O elements, in consistent with EDS results (Figure 6(b)). As indicated in Figure 7(b), the typical peak at 285.49 eV could be ascribed to the carbon contamination in the XPS measurements [38] or organic residues from raw material PVP while calcination [23]. Figure 7(c) displays the high-resolution XPS spectra of O1s with the binding energy at 530.69 eV, which could be attributed to O atom in Ti-O bond and W-O; they share the orbital O1s [26, 39]. The binding energies of Ti 2p3/2 and Ti 2p1/2 at 459.3 and 464.8 eV in Figure 7(d) confirmed the existence of Ti⁴⁺ in 5% WO₃/TiO₂ [40]. The peaks centered at 37.52 and 35.78 eV in Figure 7(e) were attributed to W 4f5/2 and W 4f7/2, which fitted well with the characteristic peaks of W⁶⁺ [30].

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(e)

Figure 7: XPS spectra of 5% WO₃/TiO₂ nanofibers. (a) Survey spectra, (b) C 1s, (c) O 1s, (d) Ti 2p, and (e) W 4f.

3.6. UV-Vis DRS Analysis

The optical absorption property of TiO₂ and 5% WO₃/TiO₂ nanofibers was characterized by UV-Vis diffuse reflectance technique. Figure 8(a) indicated that the light absorption ability of 5% WO₃/TiO₂ nanofibers was slightly enhanced compared with TiO₂ nanofibers. The band gaps of TiO₂ and 5% WO₃/TiO₂ nanofibers were evaluated according to the equation [41] (αhν)² = A(hν − ), where A is a constant, h is Plank’s constant, ν is the photon frequency, α is the absorption coefficient (replaced with absorbance in the calculation), and is the band gap energy. Figure 8(b) displays the plots of (αhν)² versus (hν) and the band gap energy of TiO₂ and 5% WO₃/TiO₂ nanofibers was estimated to be 3.23 eV and 3.16 eV, respectively, by extrapolating the linear portion of the graphs to energy axis.

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Figure 8: UV-Vis DRS spectra of TiO₂ and 5% WO₃/TiO₂ nanofibers.

3.7. BET Analysis

N₂ adsorption-desorption measurements were conducted to characterize the specific surface areas and pore size distributions of TiO₂ and 5% WO₃/TiO₂ nanofibers. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, the N₂ adsorption-desorption isotherms of both TiO₂ and 5% WO₃/TiO₂ nanofibers in Figure 9(a) all corresponded to type IV with the obvious H3-type hysteresis loops, indicating the existence of large mesopores. Figure 9(b) shows the Barret-Joyner-Halenda (BJH) pore size distribution curves of TiO₂ and 5 % WO₃/TiO₂ nanofibers. The pore size of TiO₂ and 5% WO₃/TiO₂ nanofibers was mainly distributed in the range of 2–10 nm, further confirming the existence of mesopores. In addition, the BET specific surface areas and detailed BJH pore size of TiO₂ and 5% WO₃/TiO₂ nanofibers are listed in Table 2. The 5% WO₃/TiO₂ nanofibers had a larger surface area (51.457 m²/g) than that of TiO₂ (45.067 m²/g), implying that the coupling of WO₃ with TiO₂ boosts the specific surface areas and provided abundant active sites for adsorption and surface reactions, which could be attributed to their rough surface as shown in Figure 5(d).

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(b)

Figure 9: N₂ adsorption and desorption isotherms (a) and the BJH pore size distribution of TiO₂ and 5% WO₃/TiO₂ nanofibers (b).

Table 2: Specific surface area, average pore size, and pore volume of TiO₂ and 5% WO₃/TiO₂ nanofibers.

Sample
SBET (m²/g)
APS (nm)
PV (cm³/g)

TiO₂
45.067
4.678
0.635

5% WO₃/TiO₂
51.457
4.737
0.563

SBET: specific surface area; APS: average pore size; PV: pore volume.

3.8. Photocatalytic Hydrogen Production Performance

Hydrogen production performance of 5% WO₃/TiO₂ nanofibers by water splitting was evaluated under 100 W Mercury lamp irradiation. The Na₂S solution and Na₂SO₃ solution were added to splitting water to capture photogenerated holes and accelerate the outward migration of photogenerated electrons, which is conducive to H₂ generation. Figure 10(a) shows the photocatalytic H₂ evolution capabilities of WO₃, TiO₂, and 5% WO₃/TiO₂ nanofibers. It was obvious that WO₃ exhibits no H₂ generation activity, perhaps due to the higher positive CB potential (0.79 eV) [42] than H⁺/H₂O (−0.33 eV), while TiO₂ nanofibers display a relatively low photocatalytic H₂ production activity with 73.32 μmol·g⁻¹·h⁻¹, due to the rapid recombination of electrons in CB and holes in VB. For the 5% WO₃/TiO₂ nanofibers, the H₂ yield rate was 107.15 μmol·g⁻¹·h⁻¹ which was approximately 1.5 times that of TiO₂ nanofibers. This result can be attributed to the formation of WO₃/TiO₂ Z-scheme heterostructure, which can effectively separate useful electrons and holes, and the relatively useless electrons and holes are compounded nearby. In other words, the photoinduced electrons were injected onto the CB of TiO₂, while the holes were migrated onto the VB of WO₃. Thus the recombination of photogenerated charge carries was effectively inhibited. As a result, the reduction of H⁺ to H₂ could easily be achieved on 5% WO₃/TiO₂ nanofibers system by electrons, while the holes were reacted with S²⁻ and SO₃2–.

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Figure 10: (a) H₂ production rate over WO₃, TiO₂, and 5% WO₃/TiO₂ nanofibers. (b) H₂ production rate from different sacrificial agents over 5% WO₃/TiO₂ nanofibers.

Some photocatalytic materials can only oxidize (electron acceptor) or reduce (electron donor) water to generate oxygen or hydrogen with the participation of sacrificial agents (including electron acceptor and electron donor). Therefore, sacrificial agents are widely used to verify the photocatalytic properties of the materials. The selection of the sacrificial agent has a great influence on hydrogen production. In this experiment, Na₂S/Na₂SO₃, triethanolamine, and ethanol were selected as electron donors, which are combined with photogenerated holes to promote hydrogen production and prevent photocorrosion. From Figure 10(b), it was apparent that the mixture of Na₂S/Na₂SO₃ was the best sacrificial agents for hydrogen evolution over 5% WO₃/TiO₂ photocatalyst.

Therefore, the reason why 5% WO₃/TiO₂ nanofibers have high hydrogen production activity may be that, on the one hand, the structure of the catalyst is Z-scheme heterojunction, which reduces the recombination probability of photogenerated electrons and holes; on the other hand, the addition of Na₂S and Na₂SO₃ hole trapping agents further inhibits the recombination of photogenerated electrons and holes.

3.9. Photocatalytic Mechanism Analysis

The band edge positions of 5% WO₃/TiO₂ nanofibers were theoretically calculated by the following empirical equation [43]:

where X is the electronegativity of the semiconductor (XTiO2 = 5.81 eV and XWO3 = 6.59 eV [44]), Ee is the energy of free electrons on the hydrogen scale (≈4.5 eV), and is the band gap energy of the semiconductor ( = 3.23 eV and  = 2.6 eV). According to this empirical expression, the calculated CB (ECB) and VB (EVB) edge positions for WO₃ and TiO₂ are displayed in Figure 11(a). The CB potential of WO₃ (+0.79 eV) is more negative than the VB of TiO₂ (−0.31 eV) but more positive than the CB of TiO₂ (+2.93 eV); therefore, a Z-scheme charges transportation system is formed between WO₃ and TiO₂. The photoinduced electrons on the CB of TiO₂ can efficiently inject onto the CB of WO₃ and the holes on VB of WO₃ can transfer onto the VB of TiO₂. As a result, the highly effective separation and migration system are generated for boosting the photocatalytic performance of 5% WO₃/TiO₂ composite nanofibers.

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Figure 11: (a) Z-scheme charge transfer and surface redox reactions for 5% WO₃/TiO₂ nanofibers. (b) Catalytic degradation of MB over 5% WO₃/TiO₂ nanofibers with different quenchers.

To further elucidate the photocatalytic mechanism and identify the main reactive species in the MB degradation process, the active species trapping experiment was conducted. The detailed free radical capture experiment processes were similar to the photocatalytic activity experiments. The reactive free species in MB photocatalytic process over 5% WO₃/TiO₂ nanofibers were identified by using isopropanol (IPA), benzoquinone (BQ), sodium ethylenediaminetetraacetic acid (Na₂EDTA), and potassium bromate (KBrO₃) as scavengers of ·OH, ·O₂⁻, h⁺, and e⁻, respectively. As indicated in Figure 11(b), with the addition of IPA, BQ, and Na₂EDTA, the degradation efficiency of MB declined to 60.3%, 42.3%, and 60.5%, indicating that H₂O and/or OH⁻ trap h⁺ and turn into ·OH in the system. The addition of BQ did not show a significant impact on the photocatalytic activity, indicating that no ·O₂⁻ radicals involved in the process. The comparison results confirmed that ·OH, h⁺, and e⁻ are the dominant reactive radicals in the MB photodegradation process. ·OH with strong oxidation ability (the standard redox potential is +2.8 eV) can oxidize most organic dyes. Due to the formation of Z-scheme heterojunction, the photogenerated electrons and holes were transferred onto the CB of TiO₂ and VB of WO₃, respectively. As the CB level of TiO₂ (−0.31eV) is close to the potential value of ·O₂⁻/O₂ (−0.33 eV) [45], ·O₂⁻ could not be formed on TiO₂. Since holes are accumulated on VB of WO₃ with more positive potential (+3.39 eV) than the redox potential of ·OH/OH⁻ (+2.27eV) [45], therefore, large amounts of ·OH are generated in 5% WO₃/TiO₂ composite nanofibers.

The highest photocurrent response intensity of 5% WO₃/TiO₂ nanofibers further confirmed that intimate contact between TiO₂ and WO₃ could efficiently separate the photogenerated charge carriers and accelerate efficient charge transfer [34, 46].

4. Conclusions

In this paper, highly efficient WO₃/TiO₂ nanofibers were prepared by the electrospinning method. The mole percentage of W and Ti in the composite fibers has an important influence on their performance. The optimized 5% WO₃/TiO₂ nanofibers exhibited tremendous enhanced photocatalytic degradation capability for MB solution with a 96.2% removal rate under the Xenon lamp irradiation. In addition, under the irradiation of Mercury lamp, the photocatalytic H₂ production rate over 5% WO₃/TiO₂ nanofibers is 107.15 μmol·g⁻¹·h⁻¹, which is about 1.5 times of TiO₂ nanofibers. The enhanced activity may be due to the formation of Z-scheme heterojunction, which is conducive to the effective separation and transfer of photoinduced carriers. It could be seen from the experimental results of free radical trapping that ·OH was the main oxidant for MB degradation. Our work provides a useful reference for the preparation of other TiO₂ based photocatalysts towards both solar energy conversion and wastewater recovery.

Data Availability

All the data generated or analyzed during this study are included within the article.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (21767008), Research initiation fund of Hainan University (KYQD(ZR)1993), and Hainan Natural Science Foundation (218QN187).

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