Effective Electron Transfer Pathway of the Ternary TiO 2 / RGO / Ag Nanocomposite with Enhanced Photocatalytic Activity under Visible Light

Mesoporous TiO2/reduced graphene oxide/Ag (TiO2/RGO/Ag) ternary nanocomposite with an effective electron transfer pathway is obtained by an electrostatic self-assembly method and photo-assisted treatment. Compared with bare mesoporous TiO2 (MT) and mesoporous TiO2/RGO (MTG), the ternary mesoporous TiO2/RGO/Ag (MTGA) nanocomposite exhibited superior photocatalytic performance for the degradation of methylene blue (MB) under visible light, and the degradation rate reached 0.017 min−1, which was 3.4-times higher than that of MTG. What is more, the degradation rate of MTGA nanocomposite after three cycle times is 91.2%, and the composition is unchanged. In addition, we found that the OH•, h+ and especially O2 contribute to the high photocatalytic activity of MTGA for MB degradation. It is proposed that Ag nanoparticles can form the local surface plasmon resonance (LSPR) to absorb the visible light and distract the electrons into MT, and RGO can accept the electrons from MT to accelerate the separation efficiency of photogenerated carriers. The establishment of MTGA ternary nanocomposite makes the three components act synergistically to enhance the photocatalytic performance.

Graphene, a promising material with the traits of large surface area, high conductivity, good transparency and high stability, has attracted great recent attention in various fields [16][17][18].Combining TiO 2 with graphene could enhance the photocatalytic capacity, which facilitates the charge separation and transportation and suppresses the recombination of photogenerated electrons and holes [19][20][21].Besides graphene, the decoration of noble metal Ag could enhance the photocatalytic activity of TiO 2 photocatalysts, due to the local surface plasmon resonance (LSPR) effect for optimizing the use of solar energy and improving the transfer efficiency of photogenerated carriers [13,[22][23][24][25][26].
In recent years, constructing a ternary structure has turned into an effective tactic to enhance the absorbing of solar light and photocatalytic capability.Xian et al. reported that the obtained TiO 2 nanotube array (TNT)-graphene (GR)-CdS quantum dot (TNT-GR-CdS) composite film revealed higher photoelectric response and photocatalytic activities than other bare TNT, TNT-CdS and TNT-GR [27].Myilsamy et al. synthesized by the sol-gel route indium and cerium co-doped mesoporous TiO 2 nanocomposite with a shifted light absorption band-edge position to the visible region and the efficiently suppressed electron-hole recombination [28].Li and his co-workers prepared graphene/Fe 3+ -doped TiO 2 nanowire composites; it was found that Fe 3+ doping could keep the more direct contact between TiO 2 and graphene and improved the response of TiO 2 nanowires [29].To date, more similar ternary structures have appeared in pace with the higher requirement for the photocatalytic capability of materials, e.g., TiO 2 /InVO 4 /reduced graphene oxide (RGO) [30], C, N co-doping mesoporous Au/TiO 2 [31] and Ag-AgBr/TiO 2 [32].
Accordingly, combining Ag and graphene with TiO 2 to build the ternary skeleton can enhance the photocatalytic performance of TiO 2 due to the collective effect of Ag nanoparticles and graphene.
In 2011, Wen et al. came up with a new nanocomposite consisting of anatase TiO 2 , Ag and graphene serving as a visible light-activated photocatalyst [33].Then, more reports about TiO 2 , Ag and graphene nanocomposites appeared, mainly focusing on synthetic methods, morphologies and applications.The synthetic methods include the microwave-assisted with hydrothermal, one-step electrochemical deposition method, the sol-gel/solvothermal method and single-step solvothermal synthesis [25,[34][35][36].In terms of morphologies, Zhang et al. fabricated surface-coarsened TiO 2 nanobelts enwrapped with monodispersed Ag nanoparticles and GR nanosheets, but the improved photocatalytic performances still remained in UV light [37]; Gou et al. prepared TiO 2 nanofibers/reduced graphene oxide/Ag nanoparticle nanocomposite photocatalysts with UV/Vis response properties, and the electron transfer path was Ag→RGO→TiO 2 under visible light [38].In another aspect, Vasilaki et al. carried out a comprehensive comparison between TiO 2 /Ag/RGO and other photocatalysts including bare TiO 2 , TiO 2 /RGO and even TiO 2 /Ag [39]; Zhang et al. utilized the one-pot solvothermal method to synthesize Ag-TiO 2 /reduced graphene oxide (r-GO) nanocomposite with excellent adsorbent capacity, and the Ag-TiO 2 /r-GO nanocomposite displayed enhanced performance toward removal of rhodamine B (Rh B) dyes and reduction of CO 2 [40].However, few reports formally proposed "the electron-transfer mechanism"; we put forward that a feasible electron transfer route is that Ag nanoparticles deliver the electrons to the conduction band (CB) of mesoporous TiO 2 (MT), following RGO nanosheets that can receive electrons to effectively separate electrons and holes.
In this work, we utilized an electrostatic self-assembly approach and a photo-assisted reduction process to complete the decoration of Ag nanoparticles and RGO nanosheets on the basis of mesoporous TiO 2 .Compared with MT and mesoporous TiO 2 /RGO (MTG), the ternary mesoporous TiO 2 /RGO/Ag (MTGA) nanocomposite exhibited superior photocatalytic performance for the degradation of methylene blue (MB) under visible light irradiation, and the degradation rate reached 0.017 min −1 , which was 3.4-times that of MTG.What is more, the degradation rate of MTGA nanocomposite after three cycle times is 91.2%, and the composition is unchanged.In addition, we found that the OH•, h + and especially O 2 •− contribute to the high photocatalytic activity of MTGA for MB degradation.
More importantly, the enhancement of the photocatalytic activity could be ascribed to the large specific surface area, the extensive photo response regions and the highly efficient separation and transmission of the photo-induced charge carriers caused by the feasible electron-transfer pathway.The resulting Raman spectra of MT, MTG and MTGA nanocomposites shown in Figure 2 highlights four particular peaks owed to anatase-phase TiO2 located at 146 (Eg), 399 (B1g), 517 (A1g) and 639 cm −1 (Eg), emerging in all of the samples [41].Besides that, the inset of Figure 2 shows the Raman spectrum of GO with two manifest peaks at 1351 cm −1 (D-band) and 1605 cm −1 (G-band).The D-band originates from the disorder with structural defects caused in the process of the reduction of GO.Meanwhile, the G-band corresponds to the first order scattering of the E2g photon of sp2 C atoms of graphene [21,29].Therefore, the intensity ratio between D-band and G-band (ID/IG) for GO was 0.88, signifying the existence of plentiful sp3 in the sample.The D-and G-bands of MTG and MTGA were approximately in the same position with GO.However, the ID/IG of MTG and MTGA decreased to 0.69 and 0.79, respectively, demonstrating the reduction of GO via the photo-assisted process [42,43].The resulting Raman spectra of MT, MTG and MTGA nanocomposites shown in Figure 2 highlights four particular peaks owed to anatase-phase TiO 2 located at 146 (Eg), 399 (B1g), 517 (A1g) and 639 cm −1 (Eg), emerging in all of the samples [41].Besides that, the inset of Figure 2 shows the Raman spectrum of GO with two manifest peaks at 1351 cm −1 (D-band) and 1605 cm −1 (G-band).The D-band originates from the disorder with structural defects caused in the process of the reduction of GO.Meanwhile, the G-band corresponds to the first order scattering of the E2g photon of sp2 C atoms of graphene [21,29].Therefore, the intensity ratio between D-band and G-band (I D /I G ) for GO was 0.88, signifying the existence of plentiful sp3 in the sample.The D-and G-bands of MTG and MTGA were approximately in the same position with GO.However, the I D /I G of MTG and MTGA decreased to 0.69 and 0.79, respectively, demonstrating the reduction of GO via the photo-assisted process [42,43].The pore structure and the Brunauer-Emmett-Teller (BET) surface area of MT, MTG and MTGA nanocomposites were evaluated by the nitrogen adsorption-desorption curves shown in Figure 3.All of the isotherms belong to type IV with the H2 hysteresis loop, which verify that the pore structure is made up of ink-bottle pores [43].The MTGA ternary nanocomposite has a large specific surface area and suitable pore size, which are investigated respectively to be 119 m 2 g −1 and 6.55 nm, shown in Table S1.In a sense, the pore structure and large surface area would become principal factors for heightening the photocatalytic performance of MTGA nanocomposite.The surface elemental compositions and chemical states were inspected by X-ray photoelectron spectroscopy (XPS).Figure 4 shows the existence of the elements Ti, Ag, O and C in the MTGA ternary nanocomposite.In particular, as shown in Figure 4a, Ti 2p exhibits two typical peaks situated at 459.2 and 464.9 eV, noting the state of Ti 4+ in anatase-phase TiO2 [44].In Figure 4b, the two peaks at 368.35 eV and 374.1 eV for Ag 3d5/2 and Ag 3d3/2 can be assigned to metallic silver (Ag) [45].The XPS spectrum of O 1s is divided into three peaks (532.25,533.65 and 535.25 eV) attributed to the Ti-O bond The pore structure and the Brunauer-Emmett-Teller (BET) surface area of MT, MTG and MTGA nanocomposites were evaluated by the nitrogen adsorption-desorption curves shown in Figure 3.All of the isotherms belong to type IV with the H2 hysteresis loop, which verify that the pore structure is made up of ink-bottle pores [43].The MTGA ternary nanocomposite has a large specific surface area and suitable pore size, which are investigated respectively to be 119 m 2 g −1 and 6.55 nm, shown in Table S1.In a sense, the pore structure and large surface area would become principal factors for heightening the photocatalytic performance of MTGA nanocomposite.The pore structure and the Brunauer-Emmett-Teller (BET) surface area of MT, MTG and MTGA nanocomposites were evaluated by the nitrogen adsorption-desorption curves shown in Figure 3.All of the isotherms belong to type IV with the H2 hysteresis loop, which verify that the pore structure is made up of ink-bottle pores [43].The MTGA ternary nanocomposite has a large specific surface area and suitable pore size, which are investigated respectively to be 119 m 2 g −1 and 6.55 nm, shown in Table S1.In a sense, the pore structure and large surface area would become principal factors for heightening the photocatalytic performance of MTGA nanocomposite.The surface elemental compositions and chemical states were inspected by X-ray photoelectron spectroscopy (XPS).Figure 4 shows the existence of the elements Ti, Ag, O and C in the MTGA ternary nanocomposite.In particular, as shown in Figure 4a, Ti 2p exhibits two typical peaks situated at 459.2 and 464.9 eV, noting the state of Ti 4+ in anatase-phase TiO2 [44].In Figure 4b, the two peaks at 368.35 eV and 374.1 eV for Ag 3d5/2 and Ag 3d3/2 can be assigned to metallic silver (Ag) [45].The XPS spectrum of O 1s is divided into three peaks (532.25,533.65 and 535.25 eV) attributed to the Ti-O bond The surface elemental compositions and chemical states were inspected by X-ray photoelectron spectroscopy (XPS).Figure 4 shows the existence of the elements Ti, Ag, O and C in the MTGA ternary nanocomposite.In particular, as shown in Figure 4a, Ti 2p exhibits two typical peaks situated at 459.2 and 464.9 eV, noting the state of Ti 4+ in anatase-phase TiO 2 [44].In Figure 4b, the two peaks at 368.35 eV and 374.1 eV for Ag 3d5/2 and Ag 3d3/2 can be assigned to metallic silver (Ag) [45].The XPS spectrum of O 1s is divided into three peaks (532.25,533.65 and 535.25 eV) attributed to the Ti-O bond in the TiO 2 nanocrystal, the Ti-OH group on the surface of TiO 2 and the -COOH species from CO 2 adsorption, respectively [46].Moreover, the fact that GO was reduced to RGO is proven by the Figure 4d-f.The XPS spectrum in Figure 4d highlights the abundance of diverse oxygen-containing functional groups, mainly including C-OH, C-O and O=C-O on the GO surface, corresponding to the peaks position at 286, 287.4 and 289 eV [47].The attenuation of the oxygen-containing functional groups in Figure 4e,f of MTG and MTGA nanocomposites is remarkable, which certified the reduction of GO after light illumination [48][49][50].

Results and Discussion
in the TiO2 nanocrystal, the Ti-OH group on the surface of TiO2 and the -COOH species from CO2 adsorption, respectively [46].Moreover, the fact that GO was reduced to RGO is proven by the Figure 4d-f.The XPS spectrum in Figure 4d highlights the abundance of diverse oxygen-containing functional groups, mainly including C-OH, C-O and O=C-O on the GO surface, corresponding to the peaks position at 286, 287.4 and 289 eV [47].The attenuation of the oxygen-containing functional groups in Figure 4e,f of MTG and MTGA nanocomposites is remarkable, which certified the reduction of GO after light illumination [48][49][50].The morphology of the MTGA nanocomposite can be characterized by field emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM).Figure 5 shows SEM images of MT, MTG and a series of MTGA nanocomposites with different amounts of Ag nanoparticles.MT in Figure 5a is agglomerated into a block, and no other particles are attached to its surface.When combining MT with RGO nanosheets, as shown in Figure 5b, we find that MT is intimately enwrapped by RGO nanosheets, suggesting efficient self-assembly of RGO on the surface of MT via electrostatic interactions.What is more, Ag nanoparticles are deposited onto the binary photocatalyst by photo-assisted reduction.When the deposition amount of Ag nanoparticles is 3%, only a few Ag nanoparticles on the surface of MT are observed (Figure 5c).With the amount of Ag increasing, Figure 5d presents that more and more Ag nanoparticles decorate the MT.As the amount progressed to 8% in Figure 5e, we can see that MT is partially wrapped by RGO nanosheets, and Ag nanoparticles uniformly distribute on the other naked surface of MT.When continuing to add the amount of Ag, Ag nanoparticles have agglomerated easily into nanoclusters, and a similar phenomenon can also seen from Figure S1.The morphology of the MTGA nanocomposite can be characterized by field emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM).Figure 5 shows SEM images of MT, MTG and a series of MTGA nanocomposites with different amounts of Ag nanoparticles.MT in Figure 5a is agglomerated into a block, and no other particles are attached to its surface.When combining MT with RGO nanosheets, as shown in Figure 5b, we find that MT is intimately enwrapped by RGO nanosheets, suggesting efficient self-assembly of RGO on the surface of MT via electrostatic interactions.What is more, Ag nanoparticles are deposited onto the binary photocatalyst by photo-assisted reduction.When the deposition amount of Ag nanoparticles is 3%, only a few Ag nanoparticles on the surface of MT are observed (Figure 5c).With the amount of Ag increasing, Figure 5d presents that more and more Ag nanoparticles decorate the MT.As the amount progressed to 8% in Figure 5e, we can see that MT is partially wrapped by RGO nanosheets, and Ag nanoparticles uniformly distribute on the other naked surface of MT.When continuing to add the amount of Ag, Ag nanoparticles have agglomerated easily into nanoclusters, and a similar phenomenon can also seen from Figure S1.Ultraviolet-visible diffuse reflectance spectrum (UV/Vis DRS) measurements were performed to study the optical properties of the samples.As shown in Figure 7a, the pronounced absorption peak between 200 and 400 nm for all of the samples is ascribed to the electron excitation from the valence band to the conduction band of TiO2 [45].In comparison, the MTG nanocomposite exhibits a broadened, but weak absorbance within the visible-light region from 400 to 500 nm, which may come from the band-edge absorption of graphene.Notably, the absorption edge of MTGA exhibits an obvious red shift, and the absorption range is extended to the visible-light district because of the LSPR effect of Ag nanoparticles [22].Moreover, a plot obtained via the conversion in view of the Kubelka-Munk function versus the energy of light is shown in Figure 7b, from which the band gap energy (Eg) of MT, MTG and MTGA nanocomposites is roughly estimated to be about 3.16 eV, 2.8 eV and 2.51 eV, respectively.This suggests that the decoration of Ag nanoparticles and RGO components with the TiO2 have a significant impact on the absorption of visible light.The photocatalytic performance of MT, MTG and MTGA was detected by the decomposition of MB dyes.As shown in Figure 8a, in the dark adsorption 30-min phase, with the increasing of silver content, the adsorption quantity is increasing until the amount reaches 8%.Following, under the condition of visible light, MB dye has self-degradation.Excluding the self-degradation, MT basically has no the ability of degradation.Nevertheless, by contrast, MTG has a heightened photocatalytic activity to a certain extent, and MTGA presents the prominently highest photocatalytic activity, whereby when the illumination time is 120 min, the degradation rate of MTGA-8 reaches 93%, and Ultraviolet-visible diffuse reflectance spectrum (UV/Vis DRS) measurements were performed to study the optical properties of the samples.As shown in Figure 7a, the pronounced absorption peak between 200 and 400 nm for all of the samples is ascribed to the electron excitation from the valence band to the conduction band of TiO 2 [45].In comparison, the MTG nanocomposite exhibits a broadened, but weak absorbance within the visible-light region from 400 to 500 nm, which may come from the band-edge absorption of graphene.Notably, the absorption edge of MTGA exhibits an obvious red shift, and the absorption range is extended to the visible-light district because of the LSPR effect of Ag nanoparticles [22].Moreover, a plot obtained via the conversion in view of the Kubelka-Munk function versus the energy of light is shown in Figure 7b, from which the band gap energy (Eg) of MT, MTG and MTGA nanocomposites is roughly estimated to be about 3.16 eV, 2.8 eV and 2.51 eV, respectively.This suggests that the decoration of Ag nanoparticles and RGO components with the TiO 2 have a significant impact on the absorption of visible light.Ultraviolet-visible diffuse reflectance spectrum (UV/Vis DRS) measurements were performed to study the optical properties of the samples.As shown in Figure 7a, the pronounced absorption peak between 200 and 400 nm for all of the samples is ascribed to the electron excitation from the valence band to the conduction band of TiO2 [45].In comparison, the MTG nanocomposite exhibits a broadened, but weak absorbance within the visible-light region from 400 to 500 nm, which may come from the band-edge absorption of graphene.Notably, the absorption edge of MTGA exhibits an obvious red shift, and the absorption range is extended to the visible-light district because of the LSPR effect of Ag nanoparticles [22].Moreover, a plot obtained via the conversion in view of the Kubelka-Munk function versus the energy of light is shown in Figure 7b, from which the band gap energy (Eg) of MT, MTG and MTGA nanocomposites is roughly estimated to be about 3.16 eV, 2.8 eV and 2.51 eV, respectively.This suggests that the decoration of Ag nanoparticles and RGO components with the TiO2 have a significant impact on the absorption of visible light.The photocatalytic performance of MT, MTG and MTGA was detected by the decomposition of MB dyes.As shown in Figure 8a, in the dark adsorption 30-min phase, with the increasing of silver content, the adsorption quantity is increasing until the amount reaches 8%.Following, under the condition of visible light, MB dye has self-degradation.Excluding the self-degradation, MT basically has no the ability of degradation.Nevertheless, by contrast, MTG has a heightened photocatalytic activity to a certain extent, and MTGA presents the prominently highest photocatalytic activity, whereby when the illumination time is 120 min, the degradation rate of MTGA-8 reaches 93%, and The photocatalytic performance of MT, MTG and MTGA was detected by the decomposition of MB dyes.As shown in Figure 8a, in the dark adsorption 30-min phase, with the increasing of silver content, the adsorption quantity is increasing until the amount reaches 8%.Following, under the condition of visible light, MB dye has self-degradation.Excluding the self-degradation, MT basically has no the ability of degradation.Nevertheless, by contrast, MTG has a heightened photocatalytic activity to a certain extent, and MTGA presents the prominently highest photocatalytic activity, whereby when the illumination time is 120 min, the degradation rate of MTGA-8 reaches 93%, and the apparent reaction rate constant of the ternary material in Table S2 is 0.017 min −1 , which is almost 3.4-times the degradation rate of MTG; when silver content is added up to 10%, the degradation rate does not rise, rather being reduced because too much deposition of Ag nanoparticles themselves can become the recombination center that suppresses the carrier separation [51].the apparent reaction rate constant of the ternary material in Table S2 is 0.017 min −1 , which is almost 3.4-times the degradation rate of MTG; when silver content is added up to 10%, the degradation rate does not rise, rather being reduced because too much deposition of Ag nanoparticles themselves can become the recombination center that suppresses the carrier separation [51].The stability of MTGA-8 is exhibited in Figure 9a; after three cycle times, the degradation rate reduces to 91.2%.Furthermore, the XRD spectrum of MTGA-8 after three cycle times shown in Figure 9b is also surveyed to determine whether the catalyst is deteriorated after the photocatalytic redox reaction.Compared to original MTGA-8, MTGA-8 after three cycle times remains as the typical anatase-phase TiO2, and no peak of the other valence state of Ti appears.Furthermore, the peaks of Ag nanoparticles still are located at 38.1° and 44.2°, and the intensity of peaks is slightly weaker.What is more, XPS spectra for the MTGA nanocomposite in Figure S5 can also prove that the MTGA nanocomposite retains its composition after the photoreaction.Therefore, it is confirmed that MTGA exhibits an excellent photocatalytic capacity and high stability.In consideration of the above results, the boosted photocatalytic performance of MTGA ternary nanocomposite in the MB's decomposition can be attributed to the following factors: firstly, the decoration of Ag nanoparticles can form the LSPR effect to enhance the absorbance of the TiO2 to The stability of MTGA-8 is exhibited in Figure 9a; after three cycle times, the degradation rate reduces to 91.2%.Furthermore, the XRD spectrum of MTGA-8 after three cycle times shown in Figure 9b is also surveyed to determine whether the catalyst is deteriorated after the photocatalytic redox reaction.Compared to original MTGA-8, MTGA-8 after three cycle times remains as the typical anatase-phase TiO 2 , and no peak of the other valence state of Ti appears.Furthermore, the peaks of Ag nanoparticles still are located at 38.1 • and 44.2 • , and the intensity of peaks is slightly weaker.What is more, XPS spectra for the MTGA nanocomposite in Figure S5 can also prove that the MTGA nanocomposite retains its composition after the photoreaction.Therefore, it is confirmed that MTGA exhibits an excellent photocatalytic capacity and high stability.the apparent reaction rate constant of the ternary material in Table S2 is 0.017 min −1 , which is almost 3.4-times the degradation rate of MTG; when silver content is added up to 10%, the degradation rate does not rise, rather being reduced because too much deposition of Ag nanoparticles themselves can become the recombination center that suppresses the carrier separation [51].The stability of MTGA-8 is exhibited in Figure 9a; after three cycle times, the degradation rate reduces to 91.2%.Furthermore, the XRD spectrum of MTGA-8 after three cycle times shown in Figure 9b is also surveyed to determine whether the catalyst is deteriorated after the photocatalytic redox reaction.Compared to original MTGA-8, MTGA-8 after three cycle times remains as the typical anatase-phase TiO2, and no peak of the other valence state of Ti appears.Furthermore, the peaks of Ag nanoparticles still are located at 38.1° and 44.2°, and the intensity of peaks is slightly weaker.What is more, XPS spectra for the MTGA nanocomposite in Figure S5 can also prove that the MTGA nanocomposite retains its composition after the photoreaction.Therefore, it is confirmed that MTGA exhibits an excellent photocatalytic capacity and high stability.In consideration of the above results, the boosted photocatalytic performance of MTGA ternary nanocomposite in the MB's decomposition can be attributed to the following factors: firstly, the decoration of Ag nanoparticles can form the LSPR effect to enhance the absorbance of the TiO2 to In consideration of the above results, the boosted photocatalytic performance of MTGA ternary nanocomposite in the MB's decomposition can be attributed to the following factors: firstly, the decoration of Ag nanoparticles can form the LSPR effect to enhance the absorbance of the TiO 2 to visible light and offer a large number of electrons scattering to the CB of MT; secondly, TiO 2 with a mesoporous structure possesses a large specific surface area, increasing the adsorption of dyes; additionally, RGO nanosheets serve as the electron acceptors that effectively accelerate the separation efficiency of carriers and provide more active sites for the degradation reaction [52]; there is a potential and inseparable synergistic effect among MT, RGO and Ag that can facilitate the charge transfer and separation in MTGA nanocomposite.
To clarify the separation ability of the photogenerated carriers of MTGA, an electrochemical impedance spectroscopy (EIS) Nyquist plot has been performed to discuss electrons' separation and transfer [53,54].As shown in Figure 10a, the typical electrochemical impedance spectra are presented as Nyquist plots, and it is observed that the semicircle of MTGA-8 is shorter than MTG's, which demonstrates the decrease of the solid state interface layer resistance and the charge transfer resistance on the surface [55].Therefore, the construction of the ternary structure could suppress the charge recombination, and thereby, the MTGA nanocomposite exhibits the enhanced photocatalytic performance.
Catalysts 2017, 7, 156 9 of 15 visible light and offer a large number of electrons scattering to the CB of MT; secondly, TiO2 with a mesoporous structure possesses a large specific surface area, increasing the adsorption of dyes; additionally, RGO nanosheets serve as the electron acceptors that effectively accelerate the separation efficiency of carriers and provide more active sites for the degradation reaction [52]; there is a potential and inseparable synergistic effect among MT, RGO and Ag that can facilitate the charge transfer and separation in MTGA nanocomposite.
To clarify the separation ability of the photogenerated carriers of MTGA, an electrochemical impedance spectroscopy (EIS) Nyquist plot has been performed to discuss electrons' separation and transfer [53,54].As shown in Figure 10a, the typical electrochemical impedance spectra are presented as Nyquist plots, and it is observed that the semicircle of MTGA-8 is shorter than MTG's, which demonstrates the decrease of the solid state interface layer resistance and the charge transfer resistance on the surface [55].Therefore, the construction of the ternary structure could suppress the charge recombination, and thereby, the MTGA nanocomposite exhibits the enhanced photocatalytic performance.On the other hand, to determine the dominating active species and expose the potential reaction mechanism of the MTGA ternary nanocomposite, a series of experiments adding different radical scavengers was carried out.In those experiments, tert butyl alcohol (TBA) as a hydroxyl radical (OH•) scavenger, ethylenediamine tetraacetic acid disodium salt (EDTA-Na2) as a hole (h + ) scavenger and benzoquinone (BQ) as a superoxide radical (O2 •− ) scavenger were joined into the photocatalytic system of the MTGA nanocomposite.Degradation efficiency of MB dye under these scavengers is shown in Figure 10b.In the dark adsorption 30-min phase, the adsorption efficiency of MB dye decreased as the reactive species scavenger was added, which may be on account of the competitive adsorption between MB and the additives [56].When OH• scavenger (TBA) or h + scavenger (EDTA-Na2) was added to the reaction system, the photocatalytic activity of MTGA obviously decreased from 93.4% down to 83.5% and 61.2%, respectively.When the O2 •− scavenger (BQ) was added, eliminating the role of dark adsorption and self-degradation, MTGA towards degradation of the dye is basically ineffective.The above results illustrate that the OH•, h + and especially O2 •− contribute to the high photocatalytic activity of MTGA for MB degradation.The most active radical is O2 •− , which may derive from the dissolved oxygen in the solution reacting with photoexcited electrons.
Based on the above analyses, a feasible photocatalytic mechanism over the MTGA ternary nanocomposite was proposed as depicted in Scheme 1.Under irradiation with visible light, Ag nanoparticles can work as an electron emitter to send out electrons (e − ) to the CB of MT.The electrons On the other hand, to determine the dominating active species and expose the potential reaction mechanism of the MTGA ternary nanocomposite, a series of experiments adding different radical scavengers was carried out.In those experiments, tert butyl alcohol (TBA) as a hydroxyl radical (OH•) scavenger, ethylenediamine tetraacetic acid disodium salt (EDTA-Na 2 ) as a hole (h + ) scavenger and benzoquinone (BQ) as a superoxide radical (O 2 •− ) scavenger were joined into the photocatalytic system of the MTGA nanocomposite.Degradation efficiency of MB dye under these scavengers is shown in Figure 10b.In the dark adsorption 30-min phase, the adsorption efficiency of MB dye decreased as the reactive species scavenger was added, which may be on account of the competitive adsorption between MB and the additives [56].When OH• scavenger (TBA) or h + scavenger (EDTA-Na 2 ) was added to the reaction system, the photocatalytic activity of MTGA obviously decreased from 93.4% down to 83.

Synthesis of GO
GO was produced from natural graphite fakes by a modified Hummers method [58][59][60].In a typical experiment, 1 g graphite powder and 1 g NaNO3 were added to 33 mL concentrated H2SO4 in a 3 °C ice-bath.Then, 6 g ground KMnO4 was slowly added while stirring.Then, the mixture was stirred for 1.5 h at 35 °C, with slowly adding 40 mL distilled water to the mixture.The obtained mixture was stirred for a further 35 min at 90 °C.In the end, 100 mL distilled water were added to the mixture and 30 wt % H2O2 dropped gradually into it until the mixture turned bright yellow.Then, the mixture was washed with 150 mL 5% aqueous HCl to get rid of the metal ions, followed by washing with distilled water to effectively remove any remaining metal ions and acids.Finally, the

Synthesis of GO
GO was produced from natural graphite fakes by a modified Hummers method [58][59][60].In a typical experiment, 1 g graphite powder and 1 g NaNO 3 were added to 33 mL concentrated H 2 SO 4 in a 3 • C ice-bath.Then, 6 g ground KMnO 4 was slowly added while stirring.Then, the mixture was stirred for 1.5 h at 35 • C, with slowly adding 40 mL distilled water to the mixture.The obtained mixture was stirred for a further 35 min at 90 • C. In the end, 100 mL distilled water were added to the mixture and 30 wt % H 2 O 2 dropped gradually into it until the mixture turned bright yellow.Then, the mixture was washed with 150 mL 5% aqueous HCl to get rid of the metal ions, followed by washing with distilled water to effectively remove any remaining metal ions and acids.Finally, the resultant mixture was put into a vacuum drying oven at 60 • C.After 24 h, the brown exfoliated graphite oxide was obtained.

Synthesis of MT
MT was obtained via the template method based on Fan's work with modifications [61].In a typical sol-gel process, 1.6 g of F-127 was dissolved in 30 mL of ethanol by stirring for 30 min, and then 2.3 mL HOAc, 1.8 mL HNO 3 and 3.4 mL TBOT were added stepwise.The mixed solution was stirred continuously for 1 h and then poured into a 120-mm Petri dish.Then, the Petri dish was placed in an oven at 40 • C for 12 h to form a transparent membrane, followed by ageing at 65 • C for 24 h.Finally, the as-prepared membranes were calcined at 350 • C in an air atmosphere for 5 h (heating rate 2 • C min −1 ) to eventually obtain MT.

Synthesis of MTG and MTGA Nanocomposites
The MTGA nanocomposites were prepared by a photo-assisted reduction method [29,58].Typically, 0.1 g MT were dispersed into 20 mL ethanol.Then, 1 mL of 3-aminopropyl-trimethoxysilane (APTES) was added, and the suspension was refluxed at 80 • C for 4 h.Subsequently, the suspension was washing three times with ethanol.Afterwards, the APTES-modified MT was added into 5.26 mg GO suspension after ultrasonication with stirring for 30 min to obtain a meso-TiO 2 /GO suspension.The as-obtained meso-TiO 2 /GO suspension and AgNO 3 were dispersed in ethanol and transferred to a glass reactor.Subsequently, the glass reactor containing the suspension kept at room temperature was exposed to the simulated solar light for 2 h.Then, the suspension was rinsed with ethanol and water a few times.Finally, the resultant was dried at 60 • C overnight.Similarly, MTG was synthesized by a photo-reduction in which the meso-TiO 2 /GO suspension was dispersed in ethanol, and the above steps were repeated.Finally, we obtained the MTG and MTGA-x (x = 3, 5, 8 and 10), where x indicates the mass ratio of Ag loaded onto MT.

Characterization
The as-obtained catalysts were examined by an X-ray diffractometer (XRD) with Cu Kα radiation.The Raman spectra were surveyed by laser confocal micro-Raman spectroscopy with an excitation wavelength of 532 nm.X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Thermo ESCALAB 250 spectrometer with a hemisphere detector at an energy resolution of 0.1 eV offered by an Al Kα radiation source.Field emission scanning electron microscope (FESEM) images were obtained by a Hitachi SU8010 machine.Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained by a JEOL Model JEM-2100F instrument operated at 200 kV.The Brunauer-Emmett-Teller (BET) specific surface areas of the sample powders were obtained through nitrogen adsorption-desorption, measured on a JW-BK132F.Ultraviolet-visible (UV/Vis) absorption spectra were surveyed using a UV/Vis spectrometer (UV-2500, Shimadzu, Kyoto, Japan), in which BaSO 4 was chosen as the reference standard.The electrochemical analysis was investigated by a conventional three-electrode system measured on a CHI660E.

Photocatalytic Activity
The photocatalytic degradation reaction is based on a semiconductor photocatalyst that oxidizes the target contaminants to convert to H 2 O and CO 2 under visible light irradiation.In this experiment, an amount of photocatalyst was added to 10 mg/L MB solution under normal temperature and pressure.After a few minutes, the dark reaction was carried out for 30 min to allow the photocatalyst to reach the desorption equilibrium and then transferred to the photocatalytic reaction device.

Figure 3 .
Figure 3. Nitrogen sorption isotherms and the corresponding pore size distribution curves for MT, MTG and MTGA.

Figure 3 .
Figure 3. Nitrogen sorption isotherms and the corresponding pore size distribution curves for MT, MTG and MTGA.

Figure 3 .
Figure 3. Nitrogen sorption isotherms and the corresponding pore size distribution curves for MT, MTG and MTGA.

Figure 6
Figure6presents TEM and high-resolution transmission electron microscopy (HRTEM) images of MTGA nanocomposite, from which we can see that the TiO2 nanocrystals modified by the Ag nanoparticles adhere to the chiffon-like RGO nanosheets, besides that TiO2 possesses the mesoporous structure proven by FigureS2.The HRTEM image in Figure6bunequivocally reveals that the specific interplanar spacings are 0.35 nm and 0.24 nm, respectively, belonging to the (101) facet of anatase TiO2 and the (111) plane of a cubic phase Ag, which prove the coexistence of TiO2 and Ag.Furthermore, FigureS4a-d displays the elemental mapping images based the selected area (FigureS3) on behalf of the Ti, O, C and Ag elements; it is further side evidence that Ag nanoparticles are deposited successfully by the photo-assisted process.

Figure 6
Figure6presents TEM and high-resolution transmission electron microscopy (HRTEM) images of MTGA nanocomposite, from which we can see that the TiO 2 nanocrystals modified by the Ag nanoparticles adhere to the chiffon-like RGO nanosheets, besides that TiO 2 possesses the mesoporous structure proven by FigureS2.The HRTEM image in Figure6bunequivocally reveals that the specific interplanar spacings are 0.35 nm and 0.24 nm, respectively, belonging to the (101) facet of anatase TiO 2 and the (111) plane of a cubic phase Ag, which prove the coexistence of TiO 2 and Ag.Furthermore, FigureS4a-d displays the elemental mapping images based the selected area (FigureS3) on behalf of the Ti, O, C and Ag elements; it is further side evidence that Ag nanoparticles are deposited successfully by the photo-assisted process.

Figure 7 .
Figure 7. Ultraviolet-visible diffuse reflectance spectrum (UV/Vis DRS) (a) and a plot of transformed Kubelka-Munk function versus the energy of light (b) for MT, MTG and MTGA.

Figure 7 .
Figure 7. Ultraviolet-visible diffuse reflectance spectrum (UV/Vis DRS) (a) and a plot of transformed Kubelka-Munk function versus the energy of light (b) for MT, MTG and MTGA.

Figure 7 .
Figure 7. Ultraviolet-visible diffuse reflectance spectrum (UV/Vis DRS) (a) and a plot of transformed Kubelka-Munk function versus the energy of light (b) for MT, MTG and MTGA.

Figure 8 .
Figure 8. Photodegradation of methylene blue (MB) (a) and the apparent reaction rate constants k (b) in the presence of visible light of MT, MTG and MTGA (with different Ag loading values).

Figure 9 .
Figure 9.The cycle time of MTGA for degrading MB under visible light (a) and the XRD spectra of MTGA-8 and MTGA-8 after three cycle times (b).

Figure
Figure Photodegradation of methylene blue (MB) (a) and the apparent reaction rate constants k (b) in the presence of visible light of MT, MTG and MTGA (with different Ag loading values).

Figure 8 .
Figure 8. Photodegradation of methylene blue (MB) (a) and the apparent reaction rate constants k (b) in the presence of visible light of MT, MTG and MTGA (with different Ag loading values).

Figure 9 .
Figure 9.The cycle time of MTGA for degrading MB under visible light (a) and the XRD spectra of MTGA-8 and MTGA-8 after three cycle times (b).

Figure 9 .
Figure 9.The cycle time of MTGA for degrading MB under visible light (a) and the XRD spectra of MTGA-8 and MTGA-8 after three cycle times (b).

Figure 10 .
Figure 10.Electrochemical impedance spectroscopy (EIS) Nyquist impedance plots of MTG and MTGA-8 under visible light irradiation (a); photodegradation efficiency of MB with different scavengers under visible light irradiation (b).

Figure 10 .
Figure 10.Electrochemical impedance spectroscopy (EIS) Nyquist impedance plots of MTG and MTGA-8 under visible light irradiation (a); photodegradation efficiency of MB with different scavengers under visible light irradiation (b).

Scheme 1 .
Scheme 1. Proposed mechanism diagram illustrating the photocatalytic redox reactions with the ternary MTGA catalyst in the presence of visible light.

Scheme 1 .
Scheme 1. Proposed mechanism diagram illustrating the photocatalytic redox reactions with the ternary MTGA catalyst in the presence of visible light.