Electrospinning Fabricating Au/TiO2 Network-like Nanofibers as Visible Light Activated Photocatalyst

Exploiting photocatalysts with characteristics of low cost, high reactivity and easy recovery offer great potentials for complete elimination of toxic chemicals and environmental remediation. In this work, Au/TiO2 network-like nanofibers were fabricated using a facile electrospinning technique followed by calcinations in air. Photocatalytic tests indicate that the Au/TiO2 network-like nanofibers possess an excellent photodegradation rate of rhodamine B (RB) under UV, visible and natural light radiation. The enhanced photocatalytic activity can be attributed to the plasmonic resonance absorption of Au nanoparticles, and photogenerated electrons and holes are effectively separated by the Au/TiO2 heterojunction structures. Furthermore, the three-dimensional network structure can provide a large number of active sites for RB degradation.

(2019) 9:8008 | https://doi.org/10.1038/s41598-019-44422-w www.nature.com/scientificreports www.nature.com/scientificreports/ For these concerns above mentioned, in this work, we fabricated nanofibers composed of Au NPs and TiO 2 by electrospinning method. The prepared nanofibers presented a macropores 3D network-like structure. The elementary composition and morphology of Au/TiO 2 nanocomposite were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The as-prepared 3D network-like nanofibers showed good photocatalytic performance in the degrading RB under visible and UV light because of the efficient charge separation between the formed heterojunction between Au and TiO 2 NPs and high efficient utilization of photocatalysts. Besides, because of the long length of the nanofibers, the photocatalysts can be simply recycled by centrifuging, washing and drying with little damage of the photocatalytic performance. experimental section Chemicals and materials. All chemical reagents used in this study were analytical grade without further purification. Tetrabutyl titanate (C 16 H 36 O 4 Ti) and acid glacial (C 2 H 4 O 2 ) were purchased from Chengdu Kelon Chemical Reagent Factory. Polyvinylpyrrolidone (PVP) was purchased from Shanghai Macklin Biochemical Co., Ltd. Ethanol (CH 3 CH 2 OH) was purchased from Chongqing Sichuan East Chemical Co., Ltd. Gold chloride solution (HAuCl 4 ) and sodium citrate (C 6 H 5 Na 3 O 7 ·2H 2 O) were purchased from Shanghai Aladdin biochemical technology Co., Ltd.
preparation of Au nanoparticles. The synthetic method is that 2 mL of 50 mM HAuCl 4 and 98 mL of deionized water were added in flask. The flask was heated in oil bath at 120 °C until the solution boiling. Then, 10 mL of 38.3 mM sodium citrate was added in flask quickly. Finally, keeping heating for 20 min under magnetic stirring, gold nanoparticles were synthesized. In the whole experiment process, the mouth of flask covered a layer of plastic wrap for reducing the evaporation of the water.
preparation of Au/tio 2 nanofibers. In this paper, Au/TiO 2 nanofibers were prepared by using electrostatic spinning. The electrostatic spinning set-up was composed by an injector, a spinneret (a 15-gauge stainless steel needle), a high voltage power supply (18 kV), and a grounded aluminum foil used as collector. The precursor solution was consisted of tetrabutyl titanate (1 mL), acetic acid (1.5 mL), ethanol (5 mL) and polyvinylpyrrolidone (PVP) solution (10 mL). The as-prepared precursor solution was stirred at room temperature for about 12 h vigorously. Subsequently, the as-synthesized Au NPs solution with the amount of 5 μL, 10 μL, and 15 μL were added to the precursor solution, respectively. The precursor solution was stirred for 30 min at room temperature after adding Au. Then this mixture was put in a plastic syringe with a 15-gauge spinneret. The whole device was carried out under an electric voltage of 18 kV and was controlled the distance between the tip of the needle and the collector was 15 cm. The injection rate was set at 1.0 mL/h during electrospinning. The obtained fibers were then transferred into the muffle furnace for annealing at 500 °C for 12 h in air with a heating rate of 5 °C/min to produce Au/nanofibers. The as-prepared Au/TiO 2 nanofiber with different Au NPs contents were marked as Au(x)/TiO 2 , where X represented the volume of milliliters of the Au solution that was added to the precursors solution. For comparison, pure TiO 2 nanofibers were also made by the same process without adding Au NPs to the precursor.
Characterization. X-ray diffraction (XRD) analysis for the crystal structures of the samples was executed by X-ray diffractometer (Rigaku D/MAX2500PC) with Cu Kα radiation. The morphologies and size of the samples were tested by scanning electron microscopy (SEM, TESCAN MIRA) and transmission electron microscopy (TEM, FEI TECNAI G2 F20). UV-vis diffuse reflectance spectra of the samples were measured by a UV3600 spectrophotometer (Shimadzu, Japan) and BaSO 4 was used as a reference. photocatalytic test. The photocatalytic performance of these Au/TiO 2 nanofibers were measured by detecting the quantities of the RB degradation in water. An internal 350 W Mercury lamp was adopted as UV light source and a 300 W Xe lamp was equipped with an optical filter as visible light (λ > 420 nm). To cool the lamps, a circulating water system was used. Firstly, the initial concentration of the 50 mL RB solution was 10 mg L −1 and solid catalyst (0.02 g) was added and the solution was stirred without UV-Vis light for 30 min to obtain a good dispersion for establishing adsorption-desorption balance between the catalyst surface and the organic molecules. At given intervals of irradiation (time interval was 5 min in UV irradiation and 30 min in visible light), the reaction solutions (3 ml) as samples were extracted and centrifuged. Then the filtrates were analyzed by a spectrophotometer to calculated the decreased amount in the concentration of RB solution. The morphologies of the as-prepared nanofibers were measured by SEM and shown in Fig. 2. Figure 2a shows that the pure TiO 2 fibers are randomly aligned with the diameter ranging from 100 to 200 nm, and the randomly orientated nanofibers form a 3D network with macropores. SEM image ( Fig. 2b) with higher magnification reveals that these TiO 2 nanofibers have a glossy and homogeneous surface. After adding Au nanoparticles, the Au/TiO 2 composite remained as a randomly aligned fiber network-like morphology, as shown in Fig. 2c. Because of the small size and low concentration of Au NPs, the surface of Au/TiO 2 nanofibers remained unchanged (inset of Fig. 2c).

Results and Discussion
www.nature.com/scientificreports www.nature.com/scientificreports/ For purpose of further studying the microstructure of the Au/TiO 2 nanofibers, the TEM and HRTEM images were measured. The low magnification TEM image of the Au/TiO 2 samples also shows network nanofibers inlaid with particles (Fig. 2d). Meanwhile, a high-resolution image of the Au/TiO 2 fiber (Fig. 2e) indicates that the nanofibers are composed of many granular particles in size of 10-20 nm. The lattice spacing of 0.235 nm is observed on the HRTEM image (Fig. 2f), corresponding to the (111) planes of Au, which indicates Au nanoparticles were successfully embedded with TiO 2 particles.
UV-vis diffuse reflectance spectra. Figure 3 indicates the UV-Vis absorption spectra of the pure TiO 2 , Au(5)/TiO 2 , Au(10)/TiO 2 , Au(15)/TiO 2 nanofibers. It can be observed that the pure TiO 2 nanofiber has a sharp absorption edge at 420 nm, which is corresponding to TiO 2 band gap excitation. The Au/TiO 2 nanofibers were fabricated by incorporation of as-prepared Au nanoparticles during the electrospinning of TiO 2 nanofibers and subsequent calcination in air. The main problem in this method maybe lead to the uneven distribution of Au nanoparticles and the Au nanoparticles were embedded in the TiO 2 nanofibers, which showed no obvious absorption  www.nature.com/scientificreports www.nature.com/scientificreports/ peak from Au nanoparticles in Au/TiO 2 nanofibers. Furthermore, as shown in Table 1, the actual Au contents of Au(x)/TiO 2 nanofibers is not high, which maybe lead to no obvious absorption peak as well.
However, the absorption edges of Au/TiO 2 nanofibers show slightly red-shift, comparing with pure TiO 2 nanofiber, which indicates an enhanced light absorption for Au/TiO 2 nanofibers through incorporation of Au nanoparticles. photocatalytic activity. The photocatalytic performance of these Au/TiO 2 nanofibers were measured by detecting the quantity of the RB degradation in water and the degradation effect of the Au/TiO 2 nanofiber catalysts was labeled as C/C 0 , where C and C 0 were marked the remainder and initial concentration of RB, respectively. The pure TiO 2 nanofibers were acted as a photocatalytic performance reference. As shown in Fig. 4, The control experimental designs of different conditions were as follows: (1) the RB solution with photocatalysts without UV-Vis light irradiation; (2) the RB solution with UV-Vis light irradiation without photocatalysts; (3) the RB solution with photocatalysts with UV light irradiation; (4) the RB solution with photocatalysts with visible light irradiation. After 6 h without UV-Vis light irradiation or without nanofiber photocatalysts, the results (Fig. 4a) showed that there is barely degradations of RB.
TiO 2 is well known as a very efficient UV light active photocatalyst. In this work, we firstly examined the photocatalytic performance of our prepared TiO 2 and Au/TiO 2 nanofibers under UV light irradiation of 100 mw/ cm 2 . As expected, all the TiO 2 and Au/TiO 2 nanofibers exhibited good photocatalytic activity for degrading RB solution. The TiO 2 nanofibers could degrade almost 100% RB solution in 30 min under UV irradiation, and Au/ TiO 2 nanofibers even exhibited faster degrading rate than pure TiO 2 nanofibers. It is worth mentioning that this degrading rate of our Au/TiO 2 nanofibers can surpass most of the reported TiO 2 and other UV-activated photocatalysts, as listed in Table 2.
Exploring TiO 2 as a visible light activated photocatalyst is of great importance for potential applications. We further evaluated the photocatalytic activity of our prepared nanofibers under visible light irradiation of 300 mw/ cm 2 . Not surprising, pure TiO 2 nanofibers showed a bad degradation rate in visible light, with only degradation rate 5% in 120 min, as shown in Fig. 4b. However, Au/TiO 2 nanofiber photocatalysts exhibited much enhanced photocatalytic activity in visible light, and the degradation effect of RB was about 35, 42 and 9% after 120 min for the sample of Au(5)/TiO 2 , Au(10)/TiO 2 and Au(15)/TiO 2 nanofibers, respectively. The kinetic analysis of degradation of RB which illustrates the photocatalytic efficiency was also evaluated. Because of the initial concentration of RB solution was low (C 0 = 10 mg/L) in our experiment, and the kinetics linear emulation curve of the photocatalytic performance of these Au/TiO 2 nanofibers followed the first order kinetics model of Langmuir-Hinshelwood. The explanation is depicted below 36 :   www.nature.com/scientificreports www.nature.com/scientificreports/ mg) and K app means the apparent first-order rate constant (min −1 ). The determined K app for four catalysts is summarized in Fig. 4c. It is revealed that the photocatalytic performances followed the order: Au(10)/TiO 2 nanofibers >Au(5)/TiO 2 nanofibers >Au(15)/TiO 2 nanofibers >TiO 2 nanofibers.
The photocatalytic activity of the as-prepared pure TiO 2 and Au(10)/TiO 2 nanofibers were further compared by RB degradation under natural light irradiation (15 mw/cm 2 ). It can be seen that the Au(10)/TiO 2 photocatalysts exhibited superior photocatalytic performance for degrading RB solution compared with the pure TiO 2 nanofibers. As shown in Fig. 5a, the degradations of RB by using Au(10)/TiO 2 nanofibers as photocatalysts reached almost 100% in 270 min irradiation. In the same condition, the degradation of RB by using pure TiO 2 nanofibers was just 40%.
Stability is an important factor of the catalyst in the practical application. In order to test the stability of the Au(10)/TiO 2 nanofibers, three recycling experiments were carried under identical conditions. As shown in Fig. 5b, after a three cycle experiment in UV irradiation, the photocatalytic degradation efficiency barely changes, with the degradation rate of about 93% during the third experiment. possible mechanism on the photocatalytic activity. As      www.nature.com/scientificreports www.nature.com/scientificreports/ mechanism, ammonium oxalate (AO), isopropanol (IPA) and benzoquinone (BQ) were chosen as quenching agents for h + , •OH and •O 2 − , respectively [37][38][39] . The experimental results (Fig. 6a) revealed that with the addition of AO and BQ, the photodegradation efficiency of RB decreases from 40% to 27.0% and 28.3%, implying that hole (h + ) and •O 2 − act as the main reactive oxygen species in the process of photodegradation. Based on the aforementioned experimental results, a feasible scheme (Fig. 6b) is proposed. In our case, the Au/TiO 2 nanofibers were irradiated by UV and visible light, respectively. As shown in Scheme 1 for the situation under visible light irradiation, photo-generated electron-hole pairs are appeared at Au NPs because of surface plasmon resonance (SPR) 40 . The conduction band energy of TiO 2 is lower that the Femi level of Au, but the sprayed electrons coming from gold can transfer to the conduction band of TiO 2 . In our work, The TiO 2 nanofibers have anatase/rutile phases. O 2 reduction by the photo-induced electrons on the rutile surface is inefficient but the anatase is more active for O 2 reduction. As a result, the electrons prefer transfer from rutile to anatase can effectively suppresses the recombination of photogenerated electron-hole pairs and accelerates the photodegradation procedure 41 . The photogenerated electrons transfer to the rutile of TiO 2 and further transfer to anatase to initiate reacting with the dissolved oxygen and the holes (·Au + ) react with H 2 O or OH − , avoiding the recombination of electron-hole pairs, which can enhance the photocatalytic effect of TiO 2 . The migration of photogenerated electrons is very fast on TiO 2 , also indicating that the photogenerated electron-hole pairs can be effectively separated 42 . In other words, the combination of gold and TiO 2 is supposed to generate a charge separation condition with relatively mild oxidation (positive gold) and same reduction (TiO 2 conduction band) potentials as TiO 2 43 . The process of RB degradation can be further illustrated as following: the photo-induced electrons are injected into the TiO 2 conduction band (Eqs (2) and (3)). The electrons can combine with the dissolved oxygen molecules and produce •O 2 − (Eq. (4)), then the HOO• are produced by protonation (Eq. (5)), the HOO• and captured electrons react to generate H 2 O 2 (Eq. (6)), and finally •OH are produced (Eq. (7)). At the same time, the h + can combine with OH − or H 2 O in the solution to form •OH (Eqs (8) and (9) (10) and (11)).

Conclusion
In summary, using a facile electrospinning method followed by calcinations, Au/TiO 2 nanofibers were successfully fabricated. The nanofibers presented a network-like three-dimensional (3D) structures with macropores and Au particles with a size of 10-20 nm were well dispersed on the TiO 2 fibers. The prepared Au/TiO 2 nanofibers exhibited much enhanced photocatalytic activity by degradation of RB under UV, Vis and natural light irradiation. It is believed that the enhanced photocatalytic performance is due to the high utilization of Au particles on the fibers with a three-dimensional network structures which worked as a framework for providing high available Au active sites for degrading RB, and to the efficient charge separation through Au/TiO 2 heterojunction structure. This study highlights the potential use of electrospinning technique to fabricate TiO 2 nanofibers as noble metal supports for photcatalysis.

Data Availability
The authors declare that data in our manuscript are available.