# Preparation and Photocatalytic Performances of WO3/TiO2 Composite Nanofibers

Authors: Xiuping Han; Binghua Yao; Keying Li; Wenjing Zhu; Xuyuan Zhang
Journal: Journal of Chemistry
 (2020)

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## 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.


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## Body

## 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 (v : v = 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.


## 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 (v : v = 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.


## 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 Figure1, 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.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.
(a)(b)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.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.
(a)(b)


### 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].Figure 5
SEM patterns of TiO2 precursor (a), TiO2 nanofibers (b), 5% WO3/TiO2 precursor (c), and 5% WO3/TiO2 nanofibers (d).
(a)(b)(c)(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.Figure 6
EDS of TiO2 (a) and 5% WO3/TiO2 (b) nanofibers.
(a)(b)Table 1
Elemental composition and content of TiO2 and 5% WO3/TiO2 nanofibers.
SampleElementWeight percentageAtomic percentageTiO2O39.8966.02Ti60.1132.895% WO3/TiO2O37.2167.18Ti51.8731.17W10.521.64


### 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].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.
(a)(b)(c)(d)(e)


### 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 − Eg), 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 Eg 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.Figure 8
UV-Vis DRS spectra of TiO2 and 5% WO3/TiO2 nanofibers.
(a)(b)


### 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).Figure 9
N2 adsorption and desorption isotherms (a) and the BJH pore size distribution of TiO2 and 5% WO3/TiO2 nanofibers (b).
(a)(b)Table 2
Specific surface area, average pore size, and pore volume of TiO2 and 5% WO3/TiO2 nanofibers.
SampleSBET (m2/g)APS (nm)PV (cm3/g)TiO245.0674.6780.6355% WO3/TiO251.4574.7370.563SBET: 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–.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.
(a)(b)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]:(1)ECB=EVB−Eg,EVB=X−Ee+0.5Eg,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 Eg is the band gap energy of the semiconductor (EgTiO2 = 3.23 eV and EgWO3 = 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.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.
(a)(b)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].


## 3.1. XRD Analysis

XRD was employed to investigate the crystal phase structures of the samples as shown in Figure1, 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.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.
(a)(b)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.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.
(a)(b)


## 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].Figure 5
SEM patterns of TiO2 precursor (a), TiO2 nanofibers (b), 5% WO3/TiO2 precursor (c), and 5% WO3/TiO2 nanofibers (d).
(a)(b)(c)(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.Figure 6
EDS of TiO2 (a) and 5% WO3/TiO2 (b) nanofibers.
(a)(b)Table 1
Elemental composition and content of TiO2 and 5% WO3/TiO2 nanofibers.
SampleElementWeight percentageAtomic percentageTiO2O39.8966.02Ti60.1132.895% WO3/TiO2O37.2167.18Ti51.8731.17W10.521.64


## 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].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.
(a)(b)(c)(d)(e)


## 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 − Eg), 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 Eg 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.Figure 8
UV-Vis DRS spectra of TiO2 and 5% WO3/TiO2 nanofibers.
(a)(b)


## 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).Figure 9
N2 adsorption and desorption isotherms (a) and the BJH pore size distribution of TiO2 and 5% WO3/TiO2 nanofibers (b).
(a)(b)Table 2
Specific surface area, average pore size, and pore volume of TiO2 and 5% WO3/TiO2 nanofibers.
SampleSBET (m2/g)APS (nm)PV (cm3/g)TiO245.0674.6780.6355% WO3/TiO251.4574.7370.563SBET: 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–.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.
(a)(b)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]:(1)ECB=EVB−Eg,EVB=X−Ee+0.5Eg,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 Eg is the band gap energy of the semiconductor (EgTiO2 = 3.23 eV and EgWO3 = 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.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.
(a)(b)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.



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