Novel Synthesis of Plasmonic Ag / AgCl @ TiO 2 Continues Fibers with Enhanced Broadband Photocatalytic Performance

The plasmonic Ag/AgCl@TiO2 fiber (S-CTF) photocatalyst was synthesized by a two-step approach, including the sol-gel and force spinning method for the preparation of TiO2 fibers (TF), and the impregnation-precipitation-photoreduction strategy for the deposition of Ag/AgCl onto the fibers. NaOH aqueous solution was utilized to hydrolyze TiCl4, to synthesize TF and remove the byproduct HCl, and the produced NaCl was recycled for the formation and deposition of Ag/AgCl. The surface morphology, specific surface area, textural properties, crystal structure, elemental compositions and optical absorption of S-CTF were characterized by a series of instruments. These results revealed that the AgCl and Ag0 species were deposited onto TF successfully, and the obtained S-CTF showed improved visible light absorption due to the surface plasmon resonance of Ag0. In the photocatalytic degradation of X-3B, S-CTF exhibited significantly enhanced activities under separate visible or UV light irradiation, in comparison to TF.


Introduction
As a potential technology, photocatalytic oxidation is widely applied in water treatment [1,2] and energy conversion area [3,4].TiO 2 is considered to be one of the most promising semiconductor photocatalysts due to its low cost, chemical stability and non-toxicity [5].However, its large band gap (3.2 eV) and high recombination rate of photo-generation carrier [6] limit its applications as an efficient and visible-light driven photocatalyst.Numerous strategies have been explored to enhance its activity, especially doping with nonmetal elements (N and S, etc.) [7,8], coupling with other semiconductors (GO and BiVO 4 , etc.) [9,10], and depositing with noble metals (Au and Ag, etc.) [11][12][13][14].Ag 0 nanoparticles-modified TiO 2 exhibits excellent activity under visible light resulting from the surface plasmon resonance (SPR) [15,16], which is induced by the nano-size and morphology of Ag 0 [17,18].Kakuta et al. [19] observed that Ag 0 species were formed on the surface of AgBr under ultraviolet (UV) irradiation.Afterward, Huang et al. [20] found that the modification of TiO 2 with Ag/AgCl obviously improved its photocatalytic activity.However, previous research has mainly focused on the fabrication of nano-sized Ag/AgCl/TiO 2 catalysts, such as nanoparticles [21,22], nanospheres [23], nanoarrays [20,24] and nanofibers [25].These nano-sized catalysts are difficult to separate directly from post-slurry for recycling.Therefore, it is crucial to develop an easily recycled catalyst with a macroscopic shape and self-supporting structure.TiO 2 continuous fibers, fabricated be our group using sol-gel and force spinning method, is a promising strategy.
In addition, for most studies on Ag/AgCl-deposited TiO 2 , HCl [26] or cetyltrimethylammonium chloride (CTAC) [27] were employed as chlorine sources, which were costly, toxic and harmful to environment.Hence, it is necessary to explore low-cost and non-toxic alternative sources.The cheap and extensive TiCl 4 can be employed as a Ti source for TiO 2 fabrication.However, its hydrolytic process is accompanied with the formation of hazardous HCl [28,29].For this purpose, Xu et al. [30] reported a method in which NaOH was used to neutralize the produced HCl and form NaCl during the hydrolysis of TiCl 4 .This significant improvement allows us to recycle the produced NaCl, and also invites the investigation of the feasibility of using the NaCl as Cl source for Ag/AgCl deposition.
In this study, TiO 2 fibers (TF) with a macroscopic shape were fabricated by a sol-gel method according to the previously developed TiO 2 continuous fibers [31], and Ag/AgCl nanoparticles were deposited on TF by an impregnation-precipitation-photoreduction method [20].TiCl 4 served as the Ti source for TF, and NaOH aqueous solution was utilized to hydrolyze TiCl 4 , neutralize the produced HCl and form NaCl. The recycled NaCl was employed as a chlorine source for Ag/AgCl deposition.The morphology, textural properties, chemical component and optical property of Ag/AgCl@TiO 2 fibers (S-CTF) were analyzed by scanning electron microscopy (SEM), N 2 absorption and desorption isotherms, X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and UV-vis spectrophotometer (UV-vis DRS).The photocatalytic activity and stability were evaluated by brilliant red X-3B degradation under visible and UV light.

Morphology and Textural Properties
2.1.1.SEM Figure 1 presents typical SEM images of TF, AgCl@TiO 2 fibers (CTF) and S-CTF.The diameters of the fibers are about 30 µm.In Figure 1a,c,e, the surface of TF is homogeneous and smooth, while the surface of CTF and S-CTF are rough.Local enlarged images of TF, CTF and S-CTF are shown in Figure 1b,d,f, in which the porous structure that formed by the agglomeration of TiO 2 nanoparticles can be observed.AgCl particles with diameters less than 300 nm (Figure 1d) were successfully deposited on TF.However, there is no significant difference between the SEM images of CTF and S-CTF.The existence of Ag 0 cannot be observed through SEM, but will be confirmed by XRD, XPS and UV-vis DRS.

Textural Properties
The textural parameters of TF and S-CTF are presented in Table 1.After heat treatment, the Brunauer-Emmett-Teller (BET) surface area of TF increases from 16.0 of precursor fibers (PF) to 27.9 m 2 g −1 (TF), due to the removal of organic matters and the formation of porous structure.With Ag/AgCl particles deposited on TF, the surface area of S-CTF increases to 30.6 m 2 g −1 , which is due to its rough surface [32], as shown in Figure 1.

Samples
BET Surface Area (m 2 g −1 ) Pore Volume (cm N2 absorption and desorption isotherms of TF and S-CTF are shown in Figure 2a.Isotherms of TF and S-CTF belong to typical IV adsorption behavior and Type H1 hysteresis loop, according to International Union of Pure and Applied Chemistry (IUPAC) classification [33], which is consistent with pores with homogeneous and spindle cylindrical shapes.This reveals the formation of a mesoporous structure by the heat treatment.According to the pore-size distribution isotherms (Figure 2b), S-CTF shows narrower pore-size distribution (3-22 nm) and smaller average pore size (10.8 nm) than TF (5-40 nm and 16.7 nm).Hence, S-CTF (0.111 cm 3 g −1 ) possesses smaller pore volume compared to TF (0.074 cm 3 g −1 ).The decrease of average pore size and volume results from the deposition of nano-sized Ag/AgCl particles into macro-sized pores.In addition, S-CTF shows bimodal porous structure with two pore-size distribution peaks at 3.80 and 10.8 nm, which has also resulted from the pore size narrowing by AgCl deposition.The enlarged surface area and narrowed bimodal pore-size distribution obtained by S-CTF are both benefited to dye adsorption [34,35].

Textural Properties
The textural parameters of TF and S-CTF are presented in Table 1.After heat treatment, the Brunauer-Emmett-Teller (BET) surface area of TF increases from 16.0 of precursor fibers (PF) to 27.9 m 2 g −1 (TF), due to the removal of organic matters and the formation of porous structure.With Ag/AgCl particles deposited on TF, the surface area of S-CTF increases to 30.6 m 2 g −1 , which is due to its rough surface [32], as shown in Figure 1.

XPS
The XPS survey spectrum of S-CTF (Figure 4a) shows all peaks of Ti, O, Ag, Cl and C elements.The peak for C 1s at 284.6 eV is mainly from the adventitious hydrocarbon, due to the XPS instrument itself.The result of the survey of the XPS spectrum is consistent with the XRD results.
water on the surface of S-CTF) [35].In Figure 4c, two signal peaks of Cl 2p1/2 and Cl 2p3/2 at binding energy of 197.1 and 198.7 eV with a separation of 1.6 eV confirmed the Cl − from AgCl [41].The Ag 3d region is decomposed into four peaks (Figure 4e).Binding energy peaks at 367.6 and 373.6 eV are ascribed to Ag 0 , while 367.1 and 373.1 eV correspond to Ag + in AgCl particles [26,42].The difference of binding energy (0.1 eV) between CTF and S-CTF also indicates the formation of Ag 0 .According to the results of XPS, the detail atomic percentages of Ag, AgCl, and TiO2 on the surface of S-CTF were about 8.5%, 40.7% and 51.8%, respectively.Moreover, the interaction among Ag, AgCl and TiO2 [43] is further confirmed by the binding energy shifts peaks for all elements compared with standard values.
The crystal structure and chemical component of S-CTF are investigated by XRD and XPS analysis.The formation of Ag 0 and Ti 3+ would enhance the optical property of S-CTF.

UV-Vis DRS
Compared with TF, S-CTF exhibits a slight enhancement in the UV light region (<400 nm) and considerable enhancement in the visible light region (>400 nm) (Figure 5).CTF shows the highest absorption in the UV light region among the samples, even higher than S-CTF.With the photoreduction of AgCl and the formation of Ag 0 , the absorption of UV light decreases but is still higher than TF.Therefore, the enhancement of absorption in the UV light region can be ascribed to the combination of TF and AgCl with excellent UV light absorption.Because of the formation of Ti 3+ and oxygen vacancy, TF possesses localized states below the conduct bind(CB) minimum and shows expected light absorption in the visible light region [39,44,45].TF shows an absorption band edge at about 400 nm, and the absorption band edge of CTF and S-CTF (at about 420 nm) shows an expected red-shift, in comparison to TF.Compared with Ag/AgCl, S-CTF almost retains the same absorption intensity in the visible light region, illustrating that the composition of TF and Ag/AgCl has no The valence state of Ti, O, Cl and Ag in S-CTF are investigated by the high-resolution XPS spectrum (Figure 4b-e).As shown in Figure 4b, the binding energy at 458.6 and 464.6 eV of Ti 2p 1/2 and Ti 2p 3/2 are lower than those of pure TiO 2 (459.4 and 464.7 eV) [39].The curve resolution of the Ti 2p signal of S-CTF indicates four different peaks.The peaks at binding energies of 458.2 and 463.9 eV belong to Ti 2p 1/2 and Ti 2p 3/2 of Ti 3+ , while the other two peaks at 458.7 and 464.4 eV belong to Ti 2p 1/2 and Ti 2p 3/2 of Ti 4+ [40].Ti 3+ (and oxygen vacancy) in S-CTF was produced during the heat treatment by the reduction of TF (the removal of partial lattice oxygen) with organic matters in steam as a reducing atmosphere.The O 1s region is decomposed into two peaks at 530 and 531.9 eV (Figure 4b).The two kinds of oxygen contributions are ascribed to Ti-O in TiO 2 and the OH in Ti-OH (adsorbed water on the surface of S-CTF) [35].In Figure 4c, two signal peaks of Cl 2p 1/2 and Cl 2p 3/2 at binding energy of 197.1 and 198.7 eV with a separation of 1.6 eV confirmed the Cl − from AgCl [41].The Ag 3d region is decomposed into four peaks (Figure 4e).Binding energy peaks at 367.6 and 373.6 eV are ascribed to Ag 0 , while 367.1 and 373.1 eV correspond to Ag + in AgCl particles [26,42].The difference of binding energy (0.1 eV) between CTF and S-CTF also indicates the formation of Ag 0 .According to the results of XPS, the detail atomic percentages of Ag, AgCl, and TiO 2 on the surface of S-CTF were about 8.5%, 40.7% and 51.8%, respectively.Moreover, the interaction among Ag, AgCl and TiO 2 [43] is further confirmed by the binding energy shifts peaks for all elements compared with standard values.
The crystal structure and chemical component of S-CTF are investigated by XRD and XPS analysis.The formation of Ag 0 and Ti 3+ would enhance the optical property of S-CTF.

UV-Vis DRS
Compared with TF, S-CTF exhibits a slight enhancement in the UV light region (<400 nm) and considerable enhancement in the visible light region (>400 nm) (Figure 5).CTF shows the highest absorption in the UV light region among the samples, even higher than S-CTF.With the photoreduction of AgCl and the formation of Ag 0 , the absorption of UV light decreases but is still higher than TF.Therefore, the enhancement of absorption in the UV light region can be ascribed to the combination of TF and AgCl with excellent UV light absorption.Because of the formation of Ti 3+ and oxygen vacancy, TF possesses localized states below the conduct bind(CB) minimum and shows expected light absorption in the visible light region [39,44,45].TF shows an absorption band edge at about 400 nm, and the absorption band edge of CTF and S-CTF (at about 420 nm) shows an expected red-shift, in comparison to TF.Compared with Ag/AgCl, S-CTF almost retains the same absorption intensity in the visible light region, illustrating that the composition of TF and Ag/AgCl has no influence on the light absorption of Ag/AgCl.Obviously, an additional absorption for S-CTF which was observed in the region of 400-600 nm, which is attributed to the SPR of Ag 0 nanoparticles [41].The broad and enhanced absorption in the visible light region is also confirmed the formation of Ag 0 [46,47].These evidences of Ag 0 formation are consistent with the results of XPS.

Photocatalytic Activity
To evaluate the effect of the interaction between Ag/AgCl and the TF surface, Ag/AgCl/precursor fibers (S-CPF) were prepared.To investigate the influence of photosensitization and to confirm the photoactivity of TF, pure TiO2 fibers (pure-TF) were also prepared by PF through heat treatment (the heating program was the same as that used for TF preparation) in O2 atmosphere with the flow rate at 0.5 L/min.The photocatalytic activities of pure-TF, TF, CTF, S-CPF and S-CTF toward X-3B degradation under visible and UV light are shown in Figure 6.

Photocatalytic Activity
To evaluate the effect of the interaction between Ag/AgCl and the TF surface, Ag/AgCl/precursor fibers (S-CPF) were prepared.To investigate the influence of photosensitization and to confirm the photoactivity of TF, pure TiO 2 fibers (pure-TF) were also prepared by PF through heat treatment (the heating program was the same as that used for TF preparation) in O 2 atmosphere with the flow rate at 0.5 L/min.The photocatalytic activities of pure-TF, TF, CTF, S-CPF and S-CTF toward X-3B degradation under visible and UV light are shown in Figure 6.

Photocatalytic Activity
To evaluate the effect of the interaction between Ag/AgCl and the TF surface, Ag/AgCl/precursor fibers (S-CPF) were prepared.To investigate the influence of photosensitization and to confirm the photoactivity of TF, pure TiO2 fibers (pure-TF) were also prepared by PF through heat treatment (the heating program was the same as that used for TF preparation) in O2 atmosphere with the flow rate at 0.5 L/min.The photocatalytic activities of pure-TF, TF, CTF, S-CPF and S-CTF toward X-3B degradation under visible and UV light are shown in Figure 6.
After dark treatment for 30 min (Figure 6), S-CTF and CTF (about 12%) have higher X-3B adsorption than pure-TF and TF (about 4%).Nearly no adsorption is observed for S-CPF.The enhancement for X-3B adsorption is a result of the enlarged surface area and narrowed bimodal mesoporous structure after AgCl deposition.The degradation efficiencies within 120 min under visible light (Figure 6a) follow an order of pure-TF (25.6%) < TF (37.1%) < CTF (73.4%) < S-CPF (91.8%) < S-CTF (98.1%).After 120 min, the degradation efficiency by pure-TF was 25.6%, which was lower than TF (37.1%).The degradation of X-3B by pure-TF can be attributed to the photosensitization of dye, and the degradation elevation of TF can be attributed to the existence of Ti 3+ and the optical absorption of visible light.CTF shows high photoactivity due to the reduction of AgCl and the formation Ag 0 as the photoreaction progressed [32].Because of the strong SPR of Ag 0 , S-CPF and S-CTF exhibit much higher degradation efficiencies After dark treatment for 30 min (Figure 6), S-CTF and CTF (about 12%) have higher X-3B adsorption than pure-TF and TF (about 4%).Nearly no adsorption is observed for S-CPF.The enhancement for X-3B adsorption is a result of the enlarged surface area and narrowed bimodal mesoporous structure after AgCl deposition.
The degradation efficiencies within 120 min under visible light (Figure 6a) follow an order of pure − TF (25.6%) < TF (37.1%) < CTF (73.4%) < S − CPF (91.8%) < S − CTF (98.1%).After 120 min, the degradation efficiency by pure-TF was 25.6%, which was lower than TF (37.1%).The degradation of X-3B by pure-TF can be attributed to the photosensitization of dye, and the degradation elevation of TF can be attributed to the existence of Ti 3+ and the optical absorption of visible light.CTF shows high photoactivity due to the reduction of AgCl and the formation Ag 0 as the photoreaction progressed [32].Because of the strong SPR of Ag 0 , S-CPF and S-CTF exhibit much higher degradation efficiencies than TF and CTF.The highest efficiency by S-CTF is ascribed to the better separation capacity of photo-generated carriers than S-CPF.
Under UV light, the degradation efficiencies of X-3B (Figure 6b) for 120 min are displayed as following sequence: pure − TF (38.6%) < TF (44.8%) < CTF (84.5%) < S − CPF (85.7%) < S − CTF (95.1%).The higher efficiency of TF than pure-TF can be attributed to the more efficient carrier separation with oxygen vacancy in TF.CTF and S-CPF exhibit much higher activity than TF.Compared with X-3B degradation efficiency under visible light, the efficiency elevation by CTF results from the excitement of AgCl and the higher activity of TF.Since Ag 0 possesses the capacity to trap electrons [48], S-CTF shows the highest degradation efficiency.
The stability of S-CTF is further measured by a recycling experiment.After five runs, the degradation efficiencies of S-CTF under visible and UV light are still as high as 90.8% and 87.5%.The catalyst is stable enough, even considering the inevitable loss of weight during each cycle.

Photoluminescence (PL)
The recombination of the photoinduced electrons and holes is a crucial factor in a photocatalytic reaction for the decrease of quantum yield.The efficiencies of photo-generated carriers [12,49,50] are explained by PL analysis.A strong emission peak of TF is observed at about 400 nm with the excitation wavelength at 325 nm (Figure 7), which indicates the direct recombination of electrons and holes.No obvious emission peak is observed in PL spectra of Ag/AgCl and S-CTF, while S-CTF shows lower PL intensity than Ag/AgCl, indicating its better separation capacity caused by the interaction among Ag, AgCl and TiO 2 .The results of PL analysis are in accordance with the results of XRD and XPS.Otherwise, an additional peak of TF (at about 470 nm), belonging to oxygen vacancy [51], further confirms the formation of oxygen vacancy and Ti 3+ .excitement of AgCl and the higher activity of TF.Since Ag 0 possesses the capacity to trap electrons [48], S-CTF shows the highest degradation efficiency.
The stability of S-CTF is further measured by a recycling experiment.After five runs, the degradation efficiencies of S-CTF under visible and UV light are still as high as 90.8% and 87.5%.The catalyst is stable enough, even considering the inevitable loss of weight during each cycle.

Photoluminescence (PL)
The recombination of the photoinduced electrons and holes is a crucial factor in a photocatalytic reaction for the decrease of quantum yield.The efficiencies of photo-generated carriers [12,49,50] are explained by PL analysis.A strong emission peak of TF is observed at about 400 nm with the excitation wavelength at 325 nm (Figure 7), which indicates the direct recombination of electrons and holes.No obvious emission peak is observed in PL spectra of Ag/AgCl and S-CTF, while S-CTF shows lower PL intensity than Ag/AgCl, indicating its better separation capacity caused by the interaction among Ag, AgCl and TiO2.The results of PL analysis are in accordance with the results of XRD and XPS.Otherwise, an additional peak of TF (at about 470 nm), belonging to oxygen vacancy [51], further confirms the formation of oxygen vacancy and Ti 3+ .

Photocatalytic Mechanism
To better understand the photocatalytic mechanisms of S-CTF and to investigate the main oxidative species during the photodegradation of X-3B, tert-butyl alcohol (TBA), AgNO 3 , disodium ethylenediaminetetraacetate (EDTA-Na 2 ) and benzoquinone (BQ) are used as •OH, electrons, holes and •O 2 − quenchers.Under visible light irradiation, the degradation efficiency of X-3B decreases from 98.0% to 87.2% (TBA), 74.6% (AgNO 3 ), 43.9% (EDTA-Na 2 ) and 36.7% (BQ).Under UV light irradiation, the degradation efficiency of X-3B reduces from 95.1% to 85.5% (TBA), 82.2% (AgNO 3 ), 65.5% (EDTA-Na 2 ) and 40.3% (BQ).These results illustrate that the critical species under visible light are holes and The photocatalytic mechanism of S-CTF under visible light is shown in Figure 8a.Ag 0 exhibits strong SPR and generates electron-hole pairs under visible irradiation [52].TF with the existence of Ti 3+ is also excited under visible light.With the efficient interaction of Ag 0 , AgCl and TF, the plasmon-excited electrons are injected into the conduction band (CB) of TF [20,45] easily, and subsequently produce •O 2 − with O 2 .Meanwhile, the photo-excited holes are transferred to the surface of AgCl, X-3B molecules [20,32] and the value band (VB) of TF.The efficient separation of carriers among Ag, AgCl and TF is confirmed by PL spectra.Holes participate in the degradation of organic dyestuff by formation of Cl 0 and •OH.Firstly, the holes transfer to AgCl and convert Cl − ions into Cl 0 atoms [53].The Cl 0 species oxidize the X-3B molecules and form AgCl.These reactions ensure that Ag/AgCl and S-CTF remain stable during the photodegradation process [16,20,37].Secondly, the holes generate by TF yield •OH radicals or degrade organic pollutants directly.The photosensitization of X-3B [54] (generated electrons can inject into the CB of TF from excited dyes) also can be regarded as an advantage in its degradation.
Figure 8b shows the photocatalytic mechanism of S-CTF under UV light.The transformation of electron-hole pairs is different from visible irradiation.According to the CB and VB edge potentials of TiO2 (−0.29 and 2.91 eV [40]) and AgCl (0.09 and 3.16 [55]), both TiO2 and AgCl are excited under UV irradiation.The generated holes of AgCl can easily inject into the VB of TF [41], and then yield •OH with adsorbed water molecules, or directly degrade organic pollutants such as the holes generated by TF.The holes also yield Cl 0 and reduce this to Cl − via the same route under visible irradiation.The electrons generated from the CB of TF and AgCl transfer to Ag 0 , which serves as an electron trap under UV irradiation [48,56].These electrons participate in the formation of •O2 − radicals.Moreover, the oxygen vacancy in TF is also beneficial to trap electrons and promote their separation under visible or UV light irradiation [40].

Chemicals
TiCl4, acetylacetone (AcAc), triethylamine (TEA), methyl alcohol (MeOH), tetrahydrofuran (THF), NaOH and AgNO3 were purchased from Sinopharm International Co. Ltd. (Shanghai, China).Brilliant red X-3B was purchased from Shanghai Dyestuff Chemical Plant.All of the experiments in this research used deionized water.All of the chemical reagents were analytical grade, and were used without further purification.

Preparation of Catalysts
TiO2 fibers (TF) were synthesized by a sol-gel method combined with centrifugal spinning technique, followed by stage-temperature-programmed heat treatment in a steam atmosphere.The Figure 8b shows the photocatalytic mechanism of S-CTF under UV light.The transformation of electron-hole pairs is different from visible irradiation.According to the CB and VB edge potentials of TiO 2 (−0.29 and 2.91 eV [40]) and AgCl (0.09 and 3.16 [55]), both TiO 2 and AgCl are excited under UV irradiation.The generated holes of AgCl can easily inject into the VB of TF [41], and then yield •OH with adsorbed water molecules, or directly degrade organic pollutants such as the holes generated by TF.The holes also yield Cl 0 and reduce this to Cl − via the same route under visible irradiation.The electrons generated from the CB of TF and AgCl transfer to Ag 0 , which serves as an electron trap under UV irradiation [48,56].These electrons participate in the formation of •O 2 − radicals.Moreover, the oxygen vacancy in TF is also beneficial to trap electrons and promote their separation under visible or UV light irradiation [40].

Chemicals
TiCl 4 , acetylacetone (AcAc), triethylamine (TEA), methyl alcohol (MeOH), tetrahydrofuran (THF), NaOH and AgNO 3 were purchased from Sinopharm International Co. Ltd. (Shanghai, China).Brilliant red X-3B was purchased from Shanghai Dyestuff Chemical Plant.All of the experiments in this research used deionized water.All of the chemical reagents were analytical grade, and were used without further purification.

Preparation of Catalysts
TiO 2 fibers (TF) were synthesized by a sol-gel method combined with centrifugal spinning technique, followed by stage-temperature-programmed heat treatment in a steam atmosphere.The hydrolysis of TiCl 4 (10 mL in MeOH) was carried out by mixing 4 mL NaOH aqueous solution (0.5 g mL −1 ) and 25 mL MeOH under ice-cold conditions.The reaction can be expressed as: TiCl 4 +H 2 O → TiO 2 + HCl .During the reaction, NaOH neutralized the produced HCl to form NaCl, and the formed NaCl was supersaturated and precipitated due to its relatively small saturation in MeOH.Then, NaCl was filtered and collected for Ag/AgCl deposition.The TiO 2 was chelated by AcAc to form a spinning solution.After the centrifugal spinning, precursor fibers (PF) were obtained and converted to TF by the heat treatment in steam as a reducing atmosphere.The detailed heat treatment program is shown in Table 2. Ag/AgCl@TiO 2 fibers (S-CTF) were prepared by the deposition of Ag/AgCl on TF.The recycled NaCl and AgNO 3 were prepared into impregnation liquids.TF was soaked in 1 M NaCl and 0.1 M AgNO 3 solution for 10 min prior for AgCl deposition, and washed by deionized water to rinse the excess elements after each soak.This process was repeated for five trials.After being dried at 353 K, the prepared AgCl@TiO 2 fibers (CTF) were irradiated under a 1000 W xenon lamp (50 cm away from the light source) for 10 min to reduce partial Ag + to Ag 0 on the surface of the AgCl particles.

Characterization of Catalysts
To observe the structure and morphology of the prepared samples, scanning electron microscopy (SEM, JSM 6700F, JEOL, Tokyo, Japan) was employed.A Quadrasorb SI-MP system (Quantachrome, Boynton Beach, FL, USA) was used to acquire the N 2 adsorption-desorption isotherms.The Brunauer-Emmett-Teller (BET) specific surface area was detected by a multipoint BET method, and the adsorption data was acquired in the relative pressure (P/P 0 ) range of 0.05-0.3.The pore size distribution was calculated by employing the Density Functional Theory (DFT).The average pore size and pore volume data were obtained by the N 2 adsorption volume with the relative pressure at 0.991.An X-ray diffractometer (XRD, D8 Advance, Bruker AXS GmbH, Karlsruhe, Germany) was employed to determine the crystal structure of the prepared photocatalysts.An ESCALAB 250 spectrometer (Thermo Electron Corp., Cramlington, UK) with an Al KR source and a charge neutralizer was used to perform the X-ray photoelectron spectroscopy (XPS).All the spectra were calibrated to the C 1s peak at 284.6 eV.A UV-vis spectrophotometer (UV-3100, Shimadzu, Kyoto, Japan) was employed to investigate the light absorption property of the photocatalysts samples.To measure the solid-state UV-vis diffuse reflectance spectra, an integrating sphere attachment was equipped with BaSO 4 as a background in room temperature and air conditions.A fluorescence spectrometer (F-4600, Hitachi, Tokyo, Japan) was used to measure the photoluminescence (PL) spectra of all the samples.

Photoactivity Studies
The photocatalytic performance of S-CTF was evaluated by the decolorization of X-3B under visible and UV light.A 1000 W Xe lamp inserted by a glass optical filter to cut off short wavelength components (λ < 420 nm) and a 250 W high pressure mercury lamp with a maximum wavelength at 365 nm served as visible and UV light sources, respectively.Before the photocatalytic reaction, 50 mg of each sample was added to 200 mL X-3B aqueous solution (20 mg/L) in an open fixed-bed photo-reactor [57] which was cooled through the circulation of water at 293 K.To establish the adsorption/desorption equilibrium, the photo-reactor was stirred in dark conditions for 30 min.Then, the reactor was placed under visible and UV irradiation with the irradiation intensity at the surface of the dye solution being 250 W m −2 and 15 W m −2 , respectively, and cycled by a peristaltic pump with a flow rate of 20 mL min −1 .During the photocatalytic reaction, 3 mL reaction solution was taken at each interval and filtrated through a 0.45 µm syringe filter.A UV-vis 1601 spectrophotometer (Shimadzu, Japan) was used to measure the concentration of X-3B by the variation of the maximum absorption wavelength (536 nm) of filtrate which was determined.The degradation efficiency was calculated by the formula R = (1 − C/C 0 ) × 100%, in which C and C 0 stood for the initial concentration and the concentration of dye at each moment in the reaction, respectively.
Species capture experiments were performed to investigate the mechanism of TF. 10.0 mM of TBA), BQ, AgNO 3 and EDTA-Na 2 were added into X-3B solution acting as •OH, •O 2 − , electrons and holes scavengers.The reaction processes are same as that of the degradation reaction by S-CTF.

Conclusions
In summary, the plasmonic photocatalyst S-CTF was prepared by the gel-sol method to synthesize TF, and the impregnation-precipitation-photoreduction method to deposit Ag/AgCl particles on the produced TF.AgCl particles were deposited on TF with recycled NaCl as Cl sources, which were produced in the hydrolysis of TiCl 4 by NaOH aqueous solution.The improved dye adsorption compared to TF was due to the enlarged specific surface area and narrowed pore-size distribution after AgCl deposition.The increased photocatalytic efficiency resulted from the SPR of Ag 0 , enhanced light absorption, narrowed band gap and efficient charge separation.With its outstanding photocatalytic performance and high stability, S-CTF is a promising application in organic pollutant treatment and wastewater purification.Furthermore, this mild, less-toxic and lower-cost synthesis process is accordance with environmental protection concepts.

Figure 4 .
Figure 4. XPS spectra of all elements of S-CTF (a) and the band energy of Ti 2p (b), O 1s (c), Cl 2p (d) and Ag 3d (e).

Figure 4 .
Figure 4. XPS spectra of all elements of S-CTF (a) and the band energy of Ti 2p (b), O 1s (c), Cl 2p (d) and Ag 3d (e).

Figure 8 .
Figure 8. Photocatalytic mechanisms of S-CTF under visible (a) and UV (b) light irradiation.

Figure 8 .
Figure 8. Photocatalytic mechanisms of S-CTF under visible (a) and UV (b) light irradiation.

Table 2 .
Program of Segmented Heating.