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
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 [
The core of photocatalytic oxidation technology is photocatalyst. Since Fujishima reported that TiO2 would decompose water under ultraviolet light irradiation in 1972 [
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 [
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 [
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.
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 (
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.
XRD patterns of samples were collected in the range of 10–80° using a 6100 X-ray diffract meter with Cu K
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
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
XRD was employed to investigate the crystal phase structures of the samples as shown in Figure
XRD patterns of TiO2 and WO3/TiO2 nanofibers.
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
(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
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
(a) Photodegradation performance within five cycles for 5% WO3/TiO2 nanofibers; (b) XRD patterns of 5% WO3/TiO2 nanofibers before and after photocatalysis.
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
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.
The SEM images of TiO2 and 5% WO3/TiO2 nanofibers are displayed in Figure
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
EDS of TiO2 (a) and 5% WO3/TiO2 (b) nanofibers.
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 |
The surface chemical status and elemental composition of the as-prepared 5% WO3/TiO2 nanofibers were analyzed by XPS. Figure
XPS spectra of 5% WO3/TiO2 nanofibers. (a) Survey spectra, (b) C 1s, (c) O 1s, (d) Ti 2p, and (e) W 4f.
The optical absorption property of TiO2 and 5% WO3/TiO2 nanofibers was characterized by UV-Vis diffuse reflectance technique. Figure
UV-Vis DRS spectra of TiO2 and 5% WO3/TiO2 nanofibers.
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
N2 adsorption and desorption isotherms (a) and the BJH pore size distribution of TiO2 and 5% WO3/TiO2 nanofibers (b).
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.
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
(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
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.
The band edge positions of 5% WO3/TiO2 nanofibers were theoretically calculated by the following empirical equation [
(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
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 [
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
All the data generated or analyzed during this study are included within the article.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
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).