Green Synthesis of Ag Nanoparticles for Plasmon-Assisted Photocatalytic Degradation of Methylene Blue

: Green synthesis of metal nanoparticles has been gaining great interest due to the increasing need for eco-friendly manufacturing and application of nanomaterials. Metal nanoparticles, especially silver nanoparticles, are widely used in water treatments and as environmental remedy. Here we present a method to synthesize silver nanoparticles at room temperature using green tea extract under visible light irradiation, along with an application for enhanced photocatalytic degradation on methylene blue (MB) dye. Ag nanoparticles were synthesized under different photoreduction times and then further characterized. The photocatalytic rate of synthesized nanoparticles was also investigated and compared with TiO 2 nanowires under UV and visible light irradiations. The results showed that Ag nanoparticles can directly degrade MB dye through plasmonic excitation and electron transferring under visible light, and Ag nanoparticles can further enhance TiO 2 photonic degradation by enhanced e-h separation with UV and/or a wide band light, including UV light. Ag nanoparticles under visible light photoreduction for 0.5 h presented better behavior for two kinds of plasmonic enhanced photodegradation; the average size of the nanoparticles is about 30 nm. Therefore, the green synthesized Ag nanoparticles exhibit promising prospects in chemical and biological pollutant treatment.


Introduction
Noble metal nanoparticles are being widely used in diverse areas such as microelectronics, medicine, and environment remedies due to both their unique electrical, optical and biological properties and special applications in areas such as catalysis, plasmonic devices, and biosensing [1,2]. Conventional synthesis of Ag NPs such as chemical reduction methods [3,4], often use toxic chemicals that may cause environmental risks [3,5]. Therefore, green synthesis of Ag NPs has many great advantages because of its facile and bio-friendly processes. In fact, over the past decade, a number of biologic procedures and/or syntheses with nontoxic organic compounds have been investigated for eco-friendly synthesis of silver nanoparticles. Microbe-mediated synthesis of Ag NPs was first reported by using Pseudomonas stutzeri AG259, which was isolated from silver mines, showing the ability to produce Ag NPs [6]. Diversity of bacteria (both gram-positive and gram-negative) were also employed for synthesizing of Ag NPs [7]. Besides bacteria, fungi also gained wide attention for synthesis of Ag NPs [8]. For example, the capping protein on the surface of white-rot fungus Anamorphous Bjerkandera sp. R1 played a determining role in the synthesis of Ag NPs [9]. Penicillium oxalicum 1-208 has proved useful for rapid extracellular biosynthesis of Ag NPs under light radiation [10]. Recently, plant-mediated synthesis was found to be a better choice for Ag NPs synthesis since microbe-mediated methods required the maintenance of highly aseptic conditions and preservation of microbe cultures [11].
The mechanism of plasmonic enhanced photonic degradation under visible light was demonstrated. The current results are beneficial in developing green chemistry to optimize photocatalytic degradation for water treatment.

Results and Discussion
2.1. UV-Vis Spectroscopy and X-ray Diffraction Figure 1 shows the UV-Vis spectra of Ag NPs synthesized by heating and visible light irradiation for an hour, respectively. Since there is no other reducing agent in the system, the formation of Ag NPs can be attributed to the direct redox between tea extract and Ag + . The SPR spectra of Ag NPs synthesized by heating and visible light radiation were obtained at 456 nm and 489 nm, respectively, due to silver clusters with different sizes and oxidation states. Ag NPs typically having plasmonic resonance wavelength (λ max values) in the visible range of 400-500 nm [33]. SPR absorbance is extremely sensitive to the size and shape of the nanoparticles and surrounding media [34]. The shift to longer wavelength and broadening of peak plasmon resonance indicate the increasing diameters of Ag NPs, and broad SPR can be attributed to different particle sizes and the formation of anisotropic particles [35]. In principle, spherical nanoparticles contribute a single SPR, whereas shape variation leads to an additional SPR bands at a longer wavelength [36]. Figure 1 shows a single SPR band, indicating that the spherical shape of synthesized Ag NPs is dominant, which was further confirmed by SEM observations (presented later).
to demonstrate enhanced photocatalytic activities. With different photonic deg light sources, the degradation rate was also evaluated with or without TiO2 na The mechanism of plasmonic enhanced photonic degradation under visible li demonstrated. The current results are beneficial in developing green chemistry mize photocatalytic degradation for water treatment. Figure 1 shows the UV-Vis spectra of Ag NPs synthesized by heating and vis irradiation for an hour, respectively. Since there is no other reducing agent in the the formation of Ag NPs can be attributed to the direct redox between tea extract The SPR spectra of Ag NPs synthesized by heating and visible light radiation w tained at 456 nm and 489 nm, respectively, due to silver clusters with different s oxidation states. Ag NPs typically having plasmonic resonance wavelength (λma in the visible range of 400-500 nm [33]. SPR absorbance is extremely sensitive to and shape of the nanoparticles and surrounding media [34]. The shift to longe length and broadening of peak plasmon resonance indicate the increasing diam Ag NPs, and broad SPR can be attributed to different particle sizes and the form anisotropic particles [35]. In principle, spherical nanoparticles contribute a sin whereas shape variation leads to an additional SPR bands at a longer wavelen Figure 1 shows a single SPR band, indicating that the spherical shape of synthes NPs is dominant, which was further confirmed by SEM observations (presented  Figure 2 shows the XRD pattern of synthesized Ag NPs by visible light irr for an hour. The peaks at 38.26° and 64.8° are assigned to diffraction from (111) a planes of face centered cubic silver (JCPD S No. 4-783). The XRD pattern illustr the synthesized Ag NPs are crystalline in nature. To avoid the sintering of silver n ticles during drying, the solution is relatively diluted. Therefore, the XRD patte markably noisy. According to the Scherrer formula = βcosθ ⁄ , where K stand Scherrer's constant, γ corresponds to the wavelength of Cu Kα irradiation, β is th diffraction angle (rad), and θ corresponds to the half width of the diffraction pea sample; thus, the average size of the synthesized Ag NPs is 34.6 nm.  Figure 2 shows the XRD pattern of synthesized Ag NPs by visible light irradiation for an hour. The peaks at 38.26 • and 64.8 • are assigned to diffraction from (111) and (220) planes of face centered cubic silver (JCPD S No. 4-783). The XRD pattern illustrates that the synthesized Ag NPs are crystalline in nature. To avoid the sintering of silver nanoparticles during drying, the solution is relatively diluted. Therefore, the XRD pattern is remarkably noisy. According to the Scherrer formula D = Kγ/β cos θ, where K stands for the Scherrer's constant, γ corresponds to the wavelength of Cu Kα irradiation, β is the Bragg diffraction angle (rad), and θ corresponds to the half width of the diffraction peak of the sample; thus, the average size of the synthesized Ag NPs is 34.6 nm.

SEM Studies
SEM was employed to analyze the morphology of Ag NPs and particle size distribution. The size distribution was evaluated by the statistical measurement of 100 arbitrarily chosen Ag NPs. Figure S1a showed the morphology of Ag NPs synthesized by heating with green tea extract as a reduction agent. The particle size was about 70 nm. Some of the Ag NPs were anisotropic particles, attributed to the broadened SPR. These results are consistent with previous results with sodium citrate as a reduction reagent [4]. Figure S1b-e showed the structure of synthesized Ag NPs sample 2-5 by visible light irradiation for different times (0.5 h, 1 h, 1.5 h, 2 h) and the statistic distribution of particle size. After 0.5 h visible light irradiation, the average size of synthesized Ag NPs sample 2 is about 25.3 nm, while the average size of sample 3 reaches 32.9 nm after 1 h irradiation. And as the radiation time increases, the Ag NPs grow larger. The average size of samples 4 and 5 reaches 33.2 nm and 40.7 nm, respectively, after 1.5 h and 2 h irradiation. Obviously, the silver nanoparticles remarkably coarsen with increasing irradiation time.

IR Spectra and XPS Analysis of Organic Shells
High resolution SEM images in Figure 3 show that the Ag NPs synthesized by visible light irradiation are covered by an organic layer. The thickness of this surface layer is about 8 nm. This organic layer keeps the nanoparticle agglomerated. The composite of this outer layer was further characterized by IR spectra and XPS analysis.

SEM Studies
SEM was employed to analyze the morphology of Ag NPs and particle size distribution. The size distribution was evaluated by the statistical measurement of 100 arbitrarily chosen Ag NPs. Figure S1a showed the morphology of Ag NPs synthesized by heating with green tea extract as a reduction agent. The particle size was about 70 nm. Some of the Ag NPs were anisotropic particles, attributed to the broadened SPR. These results are consistent with previous results with sodium citrate as a reduction reagent [4]. Figure S1b-e showed the structure of synthesized Ag NPs sample 2-5 by visible light irradiation for different times (0.5 h, 1 h, 1.5 h, 2 h) and the statistic distribution of particle size. After 0.5 h visible light irradiation, the average size of synthesized Ag NPs sample 2 is about 25.3 nm, while the average size of sample 3 reaches 32.9 nm after 1 h irradiation. And as the radiation time increases, the Ag NPs grow larger. The average size of samples 4 and 5 reaches 33.2 nm and 40.7 nm, respectively, after 1.5 h and 2 h irradiation. Obviously, the silver nanoparticles remarkably coarsen with increasing irradiation time.

IR Spectra and XPS Analysis of Organic Shells
High resolution SEM images in Figure 3 show that the Ag NPs synthesized by visible light irradiation are covered by an organic layer. The thickness of this surface layer is about 8 nm. This organic layer keeps the nanoparticle agglomerated. The composite of this outer layer was further characterized by IR spectra and XPS analysis.

SEM Studies
SEM was employed to analyze the morphology of Ag NPs and particle size distribution. The size distribution was evaluated by the statistical measurement of 100 arbitrarily chosen Ag NPs. Figure S1a showed the morphology of Ag NPs synthesized by heating with green tea extract as a reduction agent. The particle size was about 70 nm. Some of the Ag NPs were anisotropic particles, attributed to the broadened SPR. These results are consistent with previous results with sodium citrate as a reduction reagent [4]. Figure S1b-e showed the structure of synthesized Ag NPs sample 2-5 by visible light irradiation for different times (0.5 h, 1 h, 1.5 h, 2 h) and the statistic distribution of particle size. After 0.5 h visible light irradiation, the average size of synthesized Ag NPs sample 2 is about 25.3 nm, while the average size of sample 3 reaches 32.9 nm after 1 h irradiation. And as the radiation time increases, the Ag NPs grow larger. The average size of samples 4 and 5 reaches 33.2 nm and 40.7 nm, respectively, after 1.5 h and 2 h irradiation. Obviously, the silver nanoparticles remarkably coarsen with increasing irradiation time.

IR Spectra and XPS Analysis of Organic Shells
High resolution SEM images in Figure 3 show that the Ag NPs synthesized by visible light irradiation are covered by an organic layer. The thickness of this surface layer is about 8 nm. This organic layer keeps the nanoparticle agglomerated. The composite of this outer layer was further characterized by IR spectra and XPS analysis. IR measurements were carried out to identify the potential functional groups of the surface layer, as shown in Figure 4. The band at 1632 cm −1 in the spectra correspond to C=C groups or aromatic rings or C=O stretching in the carboxyl group [16,37]. The band at 1384 cm −1 correspond to -NO 2 aliphatic nitro groups [38]. The band at 1034 cm −1 corresponds to -CN-stretching of amines. The XPS analysis in Figure 5 showed that the composite cover on Ag NPs consists of the following elements: C, Na, O, Ca, K and N. Thereinto, C, O and N were elements detected from components of tea extract such as methylxanthines, polyphenols, theaflavins, amino acids and alkaloids, which contain C=C, C=O and -CN-. These assignments are consistent with the IR results [39]. Na, K and Ca are trace elements which have been found in tea extract [40]. Si in the XPS spectra can be attributed to the Si substrate. Therefore, the surface organic layer can be attributed to the residual tea extract. Both polyphenols and methylxanthines could act as reducing agents for Ag reduction while other organic groups in the tea extract act like a capping agent for Ag NPs synthesis [41]. Due to the presence of a variety of plant constituents in tea extract, there are various alternative routes to reduce Ag + to Ag 0 , but the polyphenols play a better role as a reducing agent in tea extract. The main part of tea polyphenols is cianidanol, which contains numbers of the -OH group. It can easily release electrons to reduce Ag + to Ag 0 on absorption of visible light. As shown in Figure 6, molecular changes are induced by charge transfer during the reduction of Ag + to Ag 0 , and the enol form is converted to the quinonoid form. This transformation releases two electrons by removing two hydrogen atoms from the enol form, thus reducing two Ag + to Ag 0 . These reduced silver atoms further form Ag NPs. Binding energy of Ag 3d ( Figure S2) showed that the sample was entirely made of Ag 0 nanoparticles, with binding energy of 368.3 eV, consistent with M. Romand's results [42]. IR measurements were carried out to identify the potential functional groups of th surface layer, as shown in Figure 4. The band at 1632 cm −1 in the spectra correspond to C=C groups or aromatic rings or C=O stretching in the carboxyl group [16,37]. The band at 1384 cm −1 correspond to -NO2 aliphatic nitro groups [38]. The band at 1034 cm −1 corre sponds to -CN-stretching of amines. The XPS analysis in Figure 5 showed that the com posite cover on Ag NPs consists of the following elements: C, Na, O, Ca, K and N. There into, C, O and N were elements detected from components of tea extract such a methylxanthines, polyphenols, theaflavins, amino acids and alkaloids, which contain C=C, C=O and -CN-. These assignments are consistent with the IR results [39]. Na, K and Ca are trace elements which have been found in tea extract [40]. Si in the XPS spectra can be attributed to the Si substrate. Therefore, the surface organic layer can be attributed to the residual tea extract. Both polyphenols and methylxanthines could act as reducing agents for Ag reduction while other organic groups in the tea extract act like a capping agent for Ag NPs synthesis [41]. Due to the presence of a variety of plant constituents in tea extract, there are various alternative routes to reduce Ag + to Ag 0 , but the polyphenol play a better role as a reducing agent in tea extract. The main part of tea polyphenols i cianidanol, which contains numbers of the -OH group. It can easily release electrons to reduce Ag + to Ag 0 on absorption of visible light. As shown in Figure 6, molecular change are induced by charge transfer during the reduction of Ag + to Ag 0 , and the enol form i converted to the quinonoid form. This transformation releases two electrons by removing two hydrogen atoms from the enol form, thus reducing two Ag + to Ag 0 . These reduced silver atoms further form Ag NPs. Binding energy of Ag 3d ( Figure S2) showed that th sample was entirely made of Ag 0 nanoparticles, with binding energy of 368.3 eV, con sistent with M. Romand's results [42].

Photocatalytic Degradation of MB
Ag NPs possess lower loss and higher quality plasmon resonance than other meta NPs, under LSPR hot electrons transfer from Ag NPs to oxygen absorbed on the Ag NP which result in a negatively charged state of the oxygen-absorbed Ag nanostructure, an the hot electrons enhanced the catalytic oxidation reaction of plasmonic Ag NPs [43 Green synthesized Ag NPs has been proven efficient for the degradation of MB under fu spectrum sunlight [44], the calculated percentage of degradation efficiency was 95.3% a ter 72 h sunlight irradiation, but the photocatalytic degradation under visible light sti remains unexplored. In this study, the visible light sensitized photocatalytic activity o synthesized Ag NPs was evaluated by degradation of MB under a sunlight simulator wit a UV filter cut-off at 450 nm. The absorption peak of MB was observed to be around 66 nm. The reduction progress of the reaction was monitored by UV-Vis spectroscopy t track the changes in the absorption intensity at 664 nm at different time intervals [17]. Th photocatalytic degradation can be described using a pseudo first-order kinetic mod (Equation (1)) and its integrated form (Equation (2)): where C0 is the initial concentration, C is the concentration at time t, kap is the apparent rat constant. A versus t plot and a line of best fit yields kap [45].
Figure 7a-e showed the UV-Vis spectra of degradation of MB under visible light i radiation using Ag NPs sample 1-5 and their degradation rate K (min −1 ) was calculated All the samples reached equilibrium before irradiation. According to Figure 7 a,e, , th photolysis of MB under visible light irradiation can be negligible. The relationship be tween degradation rate K and synthesis method along with average particle size is sum

Photocatalytic Degradation of MB
Ag NPs possess lower loss and higher quality plasmon resonance than other meta NPs, under LSPR hot electrons transfer from Ag NPs to oxygen absorbed on the Ag NPs which result in a negatively charged state of the oxygen-absorbed Ag nanostructure, and the hot electrons enhanced the catalytic oxidation reaction of plasmonic Ag NPs [43] Green synthesized Ag NPs has been proven efficient for the degradation of MB under ful spectrum sunlight [44], the calculated percentage of degradation efficiency was 95.3% af ter 72 h sunlight irradiation, but the photocatalytic degradation under visible light stil remains unexplored. In this study, the visible light sensitized photocatalytic activity of synthesized Ag NPs was evaluated by degradation of MB under a sunlight simulator with a UV filter cut-off at 450 nm. The absorption peak of MB was observed to be around 664 nm. The reduction progress of the reaction was monitored by UV-Vis spectroscopy to track the changes in the absorption intensity at 664 nm at different time intervals [17]. The photocatalytic degradation can be described using a pseudo first-order kinetic mode (Equation (1)) and its integrated form (Equation (2)): where C0 is the initial concentration, C is the concentration at time t, kap is the apparent rate constant. A versus t plot and a line of best fit yields kap [45].
Figure 7a-e showed the UV-Vis spectra of degradation of MB under visible light irradiation using Ag NPs sample 1-5 and their degradation rate K (min −1 ) was calculated All the samples reached equilibrium before irradiation. According to Figure 7 a,e, , the photolysis of MB under visible light irradiation can be negligible. The relationship between degradation rate K and synthesis method along with average particle size is summarized in Table 1. The table showed that by increasing irradiation time, the Ag NPs grow

Photocatalytic Degradation of MB
Ag NPs possess lower loss and higher quality plasmon resonance than other metal NPs, under LSPR hot electrons transfer from Ag NPs to oxygen absorbed on the Ag NPs, which result in a negatively charged state of the oxygen-absorbed Ag nanostructure, and the hot electrons enhanced the catalytic oxidation reaction of plasmonic Ag NPs [43]. Green synthesized Ag NPs has been proven efficient for the degradation of MB under full spectrum sunlight [44], the calculated percentage of degradation efficiency was 95.3% after 72 h sunlight irradiation, but the photocatalytic degradation under visible light still remains unexplored. In this study, the visible light sensitized photocatalytic activity of synthesized Ag NPs was evaluated by degradation of MB under a sunlight simulator with a UV filter cut-off at 450 nm. The absorption peak of MB was observed to be around 664 nm. The reduction progress of the reaction was monitored by UV-Vis spectroscopy to track the changes in the absorption intensity at 664 nm at different time intervals [17]. The photocatalytic degradation can be described using a pseudo first-order kinetic model (Equation (1)) and its integrated form (Equation (2)): where C 0 is the initial concentration, C is the concentration at time t, k ap is the apparent rate constant. Aln C C 0 versus t plot and a line of best fit yields k ap [45]. Figure 7a-e showed the UV-Vis spectra of degradation of MB under visible light irradiation using Ag NPs sample 1-5 and their degradation rate K (min −1 ) was calculated. All the samples reached equilibrium before irradiation. According to Figure 7a,e, the photolysis of MB under visible light irradiation can be negligible. The relationship between degradation rate K and synthesis method along with average particle size is summarized in Table 1. The table showed that by increasing irradiation time, the Ag NPs grow larger while the degradation rate K decreases. On the hand, with further shortening the illumination time, the concentration of silver nanoparticles is also significantly reduced. Therefore, to obtain better photocatalytic activity, the average size of Ag NPs should be optimized. larger while the degradation rate K decreases. On the hand, with further shortening the illumination time, the concentration of silver nanoparticles is also significantly reduced. Therefore, to obtain better photocatalytic activity, the average size of Ag NPs should be optimized.    The plasmon-assisted photocatalytic activity of mixture of Ag NPs and TiO 2 nanowires was also evaluated by degradation of MB under UV and visible light. TiO 2 nanowires without Ag NPs were used for degradation of MB as blank control. TiO 2 absorbed photo energy and produced valance band holes and conduction band electrons, the valence band holes can react with water and produce •OH radicals, which dominate the degradation of MB. Since the band gap energy of anatase phase TiO 2 is 3.2 eV, it can only absorb UV irradiation. Thus, as shown in Figure 8a, under visible light, TiO 2 nanowires have no photocatalytic activity. Different modification methods have been investigated to improve the visible light response of TiO 2 . Iodine doped TiO 2 showed a considerable improvement in degradation of aqueous bisphenol A under visible light irradiation [46]. Similarly, nitrogen doped TiO 2 also proved efficient for degradation of methyl orange (MO), the degradation efficiency was enhanced from 34 to 78% after 8 h sunlight irradiation [47]. The presence of Ag NPs on the surface of TiO 2 and N-TiO 2 further improved the photo degradation percentage from 40 to 95%. The degradation efficiency of MB under visible light was also investigated, chemical reduced Ag NPs and TiO 2 presented 30% degradation after 180 min visible light irradiation [48]. In Figure 8b, the green synthesized Ag NPs showed better photocatalytic activity, with 90 min visible light irradiation, the degradation percentage of MB reached about 30%, and the calculated degradation rate was 0.0044. The increasing photocatalytic activity of TiO 2 can be attributed to two major roles of Ag NPs, promoting charge separation through the formation of Schottky junction and enhancing local electric through the LSPR effect [49]. The built-in potential in Schottky junction can promote the separation of photo-generated electron-hole pairs, which could strongly affect the degradation of MB. The high electronegativity of Ag NPs promotes the separation of photoinduced electron-hole pairs, the photogenerated electrons react directly with the oxygen adsorbed on the surface of TiO 2 to produce oxygen radicals •O 2 − , and the photogenerated holes can react with H 2 O to produce •OH radicals. These reactions decreased the high recombination rate of TiO 2 nanowires, therefore improving the photocatalytic activity of TiO 2 under visible light irradiation [50]. While under sunlight, UV irradiation also played an important role in degradation of MB, Ag NPs also enhanced the photocatalytic activities of TiO 2 by both trapped electrons and holes. Figure S3 showed the UV-Vis spectra and degradation rate of the mixture under UV light irradiation. After adding Ag NPs into the TiO 2 and MB solution, the degradation rate increased from 0.09819 to 0.121, which is consistent with experiment results using visible light irradiation of a previous study (Figure 8). It can be proposed that silver plasmonic photocatalysts working under a full solar spectrum can be obtained by preparation of various sizes of Ag NPs. To further understand the photocatalytic mechanism of the mixture of Ag NPs an TiO2 nanowires, the main reactive species such as h + , •OH and •O2 − were investigate during the photocatalytic oxidation process. Triethanolamine (TEOA), isopropanol (IPA and p-benzoquinone (BQ) were used as scavengers for h + , •OH and •O2 − , respectively The degradation of MB with no quenchers under UV irradiation was used as the blan control, and the degradation efficiency was 99.5% after 1 h irradiation, as shown in Figur 9. It was found that when 1 mL IPA quencher was added, the degradation efficiency wa significantly decreased to 32.6%, while in the presence of TEOA and BQ, the degradatio efficiencies were not quenched obviously. The result suggested that the degradation o MB by the mixture of Ag NPs and TiO2 could mainly be attributed to •OH, while h + an •O2 − only played a relatively minor role in the whole degradation process. This is con sistent with previous studies [17,18]. The generated main reactive species can be describe as follows:  To further understand the photocatalytic mechanism of the mixture of Ag NPs and TiO 2 nanowires, the main reactive species such as h + , •OH and •O 2 − were investigated during the photocatalytic oxidation process. Triethanolamine (TEOA), isopropanol (IPA) and p-benzoquinone (BQ) were used as scavengers for h + , •OH and •O 2 − , respectively. The degradation of MB with no quenchers under UV irradiation was used as the blank control, and the degradation efficiency was 99.5% after 1 h irradiation, as shown in Figure 9. It was found that when 1 mL IPA quencher was added, the degradation efficiency was significantly decreased to 32.6%, while in the presence of TEOA and BQ, the degradation efficiencies were not quenched obviously. The result suggested that the degradation of MB by the mixture of Ag NPs and TiO 2 could mainly be attributed to •OH, while h + and •O 2 − only played a relatively minor role in the whole degradation process. This is consistent with previous studies [17,18]. The generated main reactive species can be described as follows: During the reaction, pH of the solution was monitored using a pH meter at 10 min interval. Deionized water, pure Ag NPs and TiO2 nanowires were also measured under the same irradiation for comparison. As shown in Figure 10, pH values of the solution decreased as the reaction time got longer. The reaction pH was generally consistent with the mixture of TiO2 and Ag NPs separately. The decreasing pH of the reaction solution indicated the increase in •OH radicals.
As mentioned earlier, the main mechanism of Ag NPs degradation is dominated by free electrons from LSPR, the electron transfer is enhanced by the surface organic groups (as shown in Figure 3) around the Ag NPs and further generates •O2 − radicals, as shown in Equation (4), but this direct plasmonic degradation is not observed by Ag NPs synthesized by a conventional chemical method, such as using sodium citrate as a reducing agent [4]. According to Equation (5), the holes trapped by Ag nanoparticles can also attribute to the •OH radical generation in TiO2 photocatalytic degradation, as shown in Figure 11. The degradation mechanism of mixture of Ag NPs and TiO2 nanowires thus involves two kinds of plasmonic enhanced photodegradation, one is the direct degradation of MB by electron transfer between organic groups and Ag NPs surface plasmonic electrons and the second is the enhancement of TiO2 degradation due to the e-h separation by Ag nanoparticles. Through SPR effect the conduction electrons of Ag was stimulated [51]. Dissolved oxygen molecules captured the excited surface electrons and produced hydroxyl radicals [52], which leaves positively charged Ag nanoparticles and is attracted to anionic MB dye. MB molecules can be oxidized by the created hydroxyl radicals. Furthermore, as an electron conductor Ag NPs can retard the recombination of electron-hole pairs [20,26], along with the LSPR it thus improved the degradation rate of MB. This evidence proved that the degradation mechanism of MB under visible light irradiation with Ag nanoparticle-enhanced TiO2 nanowires is dominated by •OH radicals. During the reaction, pH of the solution was monitored using a pH meter at 10 min interval. Deionized water, pure Ag NPs and TiO 2 nanowires were also measured under the same irradiation for comparison. As shown in Figure 10, pH values of the solution decreased as the reaction time got longer. The reaction pH was generally consistent with the mixture of TiO 2 and Ag NPs separately. The decreasing pH of the reaction solution indicated the increase in •OH radicals.

Materials
Silver nitrate (AgNO3), methylene blue (MB), commercial P25, sodium hydroxide (NaOH), and hydrochloric acid (HCl, 70%) was purchased from Fuchen Tianjin Chemical As mentioned earlier, the main mechanism of Ag NPs degradation is dominated by free electrons from LSPR, the electron transfer is enhanced by the surface organic groups (as shown in Figure 3) around the Ag NPs and further generates •O 2 − radicals, as shown in Equation (4), but this direct plasmonic degradation is not observed by Ag NPs synthesized by a conventional chemical method, such as using sodium citrate as a reducing agent [4]. According to Equation (5), the holes trapped by Ag nanoparticles can also attribute to the •OH radical generation in TiO 2 photocatalytic degradation, as shown in Figure 11. The degradation mechanism of mixture of Ag NPs and TiO 2 nanowires thus involves two kinds of plasmonic enhanced photodegradation, one is the direct degradation of MB by electron transfer between organic groups and Ag NPs surface plasmonic electrons and the second is the enhancement of TiO 2 degradation due to the e-h separation by Ag nanoparticles. Through SPR effect the conduction electrons of Ag was stimulated [51]. Dissolved oxygen molecules captured the excited surface electrons and produced hydroxyl radicals [52], which leaves positively charged Ag nanoparticles and is attracted to anionic MB dye. MB molecules can be oxidized by the created hydroxyl radicals. Furthermore, as an electron conductor Ag NPs can retard the recombination of electron-hole pairs [20,26], along with the LSPR it thus improved the degradation rate of MB. This evidence proved that the degradation mechanism of MB under visible light irradiation with Ag nanoparticleenhanced TiO 2 nanowires is dominated by •OH radicals. Figure 10. PH variation during a reaction with UV light irradiation. Figure 11. The main mechanism of Ag/TiO2 plasmonic enhanced photocatalytic degradation.

Materials
Silver nitrate (AgNO3), methylene blue (MB), commercial P25, sodium hydroxide (NaOH), and hydrochloric acid (HCl, 70%) was purchased from Fuchen Tianjin Chemical Reagents Factory, China. All chemicals were used without further purification. Green tea (Ti Kuan Yin) was purchased from a local market. Unless otherwise stated, deionized water was used in all of the experiments. 1 g of green tea was washed using deionized water and boiled with 1 L of deionized water at 373 K for an hour. The extract was then filtered and stored at 4℃ in a fridge for further experimentation.

Synthesis of Ag NPs
Aqueous solution of AgNO3 was prepared using deionized water at a concentration of 100 mmol/L. 1 mL AgNO3 solution was added into 20 mL freshly prepared tea extract, and then made the mixture up to 100 mL with deionized water. Five copies of this solution was made and labeled sample 1-5, Sample 1 was put in a water bath at 80 °C for an hour, samples 2-5 were irradiated under a 500 W xenon lamp (PL-XQ 500W, Beijing, China) with a UV filter for 0.5 h, 1 h, 1.5 h and 2 h respectively, the distance between the light and the sample was 20 cm. A brown-yellow solution was formed indicating the formation of Ag NPs. The solution was concentrated in a centrifugal machine at 8000 rps for 10 min, the extra tea extract, was then removed the sediments washed with deionized water and the solution concentrated again. The sediments were dried under 50 °C to obtain the Ag NPs. Figure 11. The main mechanism of Ag/TiO 2 plasmonic enhanced photocatalytic degradation.

Materials
Silver nitrate (AgNO 3 ), methylene blue (MB), commercial P25, sodium hydroxide (NaOH), and hydrochloric acid (HCl, 70%) was purchased from Fuchen Tianjin Chemical Reagents Factory, China. All chemicals were used without further purification. Green tea (Ti Kuan Yin) was purchased from a local market. Unless otherwise stated, deionized water was used in all of the experiments. 1 g of green tea was washed using deionized water and boiled with 1 L of deionized water at 373 K for an hour. The extract was then filtered and stored at 4 • C in a fridge for further experimentation.

Synthesis of Ag NPs
Aqueous solution of AgNO 3 was prepared using deionized water at a concentration of 100 mmol/L. 1 mL AgNO 3 solution was added into 20 mL freshly prepared tea extract, and then made the mixture up to 100 mL with deionized water. Five copies of this solution was made and labeled sample 1-5, Sample 1 was put in a water bath at 80 • C for an hour, samples 2-5 were irradiated under a 500 W xenon lamp (PL-XQ 500W, Beijing, China) with a UV filter for 0.5 h, 1 h, 1.5 h and 2 h respectively, the distance between the light and the sample was 20 cm. A brown-yellow solution was formed indicating the formation of Ag NPs. The solution was concentrated in a centrifugal machine at 8000 rps for 10 min, the extra tea extract, was then removed the sediments washed with deionized water and the solution concentrated again. The sediments were dried under 50 • C to obtain the Ag NPs.

Characterization
The synthesized Ag NPs were dispersed in deionized water for UV-Visible absorption test using UV-Visible spectroscopy (METASH UV-9000s, Shanghai, China). The spectra were recorded using an UV-Vis Analyst from 200 to 800 nm. Deionized water was used as the baseline. The X-ray diffraction (XRD) analysis of Ag NPs was carried out by the D8 Advances Brucker instrument in transmission mode with Cu Kα radiation (λ = 0.154 nm) (D8 Advances Brucker, Germany). Scanning was performed in the region of 2θ from 20 • to 80 • . The morphology and size of the synthesized Ag NPs were investigated by Scanning Electron Microscopy (Hitachi S-4800, Tokyo, Japan). The XPS data were taken on an AXIS-Ultra instrument from Kratos Analytical using monochromatic Al Kα radiation (225 W, 15 mA, 15 kV) and low-energy electron flooding for charge compensation (Kratos Analytical, Manchester, UK). To compensate for surface charges effects, binding energies were calibrated using C 1 s hydrocarbon peak at 284.80 eV. SEM and XPS samples were prepared by dropping the nanoparticle solution onto silicon wafers and dried at room temperature prior to measurement.

Photocatalytic Degradation of Dye
The photocatalytic activity of the synthesized Ag NPs was evaluated by degradation of MB under UV and visible radiation respectively. 0.02 mmol/L of MB was added to 1000 mL deionized water maintaining a concentration of 0.02 mmol/L. About 5 mg of synthesized Ag NPs sample 1-5 was added to 100 mL of the MB solution respectively. Before irradiation, the suspensions were magnetically stirred for 30 min in the dark to make sure the solutions reached equilibrium. Then the solutions with sample 1-5 were put under a sunlight simulator with a UV filter cut-off at 450 nm for 60 min or more. MB concentration was monitored by UV-Vis absorption spectra. About 5 mg of Ag NPs was mixed with 100 mg TiO 2 nanowires and stirred in 100 mL MB solution in the dark for 30 min. TiO 2 nanowires were prepared by the hydrothermal method using commercial P25 [53]. The detailed synthesize process was demonstrated in Supplementary Materials. Then the solutions were irradiated by UV and visible light respectively, and the UV-Vis spectra were recorded to monitor concentration of MB. During the reaction, pH was also monitored by a pH meter (INESA PHS-3C, Shanghai, China), and all the measurements of the solution were carried out at a constant temperature of 25 • C. The solution without Ag NPs, deionized water, and solution without MB were also measured as a blank control.

Conclusions
In this study, Ag NPs was synthesized by heating up to 80 • C and by visible light irradiation using tea extract as a reduction agent. When the reduction was under visible light for different times, it would affect the particle size and photocatalytic activity. The longer they were under visible light irradiation, the larger the particle size was, and the lesser the photocatalytic activity was. Ag NPs can also improve the photocatalytic ability of TiO 2 nanowires, with higher degradation rate K under both UV and visible irradiations. The degradation rate reached 0.121 under UV irradiation. The catalytic enhancement mechanism of TiO 2 nanowires under visible light irradiation was demonstrated. The high yield of •OH radicals plays a dominate role in the plasmonic enhanced photodegradation of dye molecules. Therefore, Ag NPs mixed with TiO 2 nanowires may be used as highly stable and effective photocatalysts. The green synthesized Ag NPs showcase a highly significant advancement towards the development of an eco-friendly method.