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One-pot Synthesis of Ag/r-GO/TiO2 Nanocomposites with High Solar Absorption and Enhanced Anti-Recombination in Photocatalytic Applications

Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x

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In this paper, we reported a simple one-pot solvothermal approach to fabricate Ag/reduced graphene oxide (r-GO)/TiO2 composite photocatalyst under atmospheric pressure. Based on the experimental data, we concluded that the introduction of Ag into classical graphene/TiO2 system (i) efficiently enlarges the absorption range, (ii) improves the photogenerated electron separation and (iii) increases the photocatalysis reaction sites. The optimized sample exhibits prominent photocatalysis ability compared with pure TiO2 under simulated sunlight. We further proposed that besides above three advantages of Ag, different size of Ag nanoparticles is also responsible for the improved photocatalysis ability, where small size Ag nanoparticles (2~5 nm) could store photoexcited electron that generated from TiO2, while large size Ag nanoparticles could utilize visible light due to their localized surface plasmon resonance (LSPR) absorption. Our present work gives a new insights into the photocatalysis mechanism of noble metal/r-GO/TiO2 composites and provides new pathway into design of TiO2-based photocatalysts and promote their practical application in various environmental and energy issues.

1. Introduction 20

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In the past few years, photocatalysis has attracted intense attention because of its advantages of no reliance on fossil fuels and no carbon dioxide emission. In this outstanding field, many researches has been focused on environmental purification and renewable energy production.1-3 Thus, it is urgent to develop efficient and stable photocatalyst for pollutant degradation and photocatalytic hydrogen production to heal globe water pollution and renewable energy production. Currently, TiO2 is one of the most studied materials owing to its chemical stability, long-term thermodynamic stability, low cost, low-toxicity and high efficiency in the removal of pollutants in water and air as well as hydrogen generation.4-7 However, its wide band gap (~3.2 eV), which could only absorb ultraviolet region of the sunlight, restrict its applicable scope and utilizing efficiency. Besides, easy recombination of photogenerated electrons and holes could also lead to low efficiency of photocatalysis. As a result, it is of great significance to develop new strategies for more efficient TiO2based photocatalyst that could utilize the visible portion of the sunlight and facilitate electron separation. Grahpene, a two dimensional carbonaceous material, has attracted much attention recently because it can be used in many applications.8-10 Due to the high electron mobility and extended π-π conjugation structure, graphene is also an ideal electron acceptor for nanocomposites and have been intensively developed and attracted more and more attention nowadays.11 For photocatalysis application, the use of graphene could notably enhance the adsorptivity of the photocatalyst, which is largely assigned to the selective adsorption of the aromatic dye on the catalyst due to π- π stacking between aromatic dye and aromatic This journal is © The Royal Society of Chemistry [year]

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regions of the graphene.12 Based on this, many kinds of TiO2/graphene composite have been developed to improve the photocatalysis property of TiO2, which not only reduce the electron-hole recombination, but also enlarge the absorption band of pure TiO2 to visible region.12-18 In addition of this, Ag or Au nanoparticles have also been introduced to improve photocatalysis performance of TiO2.19-21 This noble metal/TiO2 composite could effectively restraint the recombination of electron-hole pair by fast transferring photogenerated electron onto Ag or Au. Besides, due to the localized surface plasmon effect (LSPR) of Ag or Au nanoparticle, noble metal/TiO2 composite could also possess visible light response ability. These methods all give notable improvement on photocatalysis property of TiO2. In this paper, we demonstrate a simple one-pot solvothermal method for fabricating Ag/r-GO/TiO2 (AGT) composite under atmospheric pressure. Our previous work has reported a one-pot synthesis of stabilizer-free Ag/r-GO using mixed solvent of dimethylacetamide (DMAc) and water, where DMAc serves as reducing agent.22 Herein, we used this mixed solvent method without introducing extra toxic reducing agent to prepare AGT composite. The reaction process of AGT is presented in Fig. 1, where AgNO3 and GO can be efficiently reduced into Ag and rGO simultaneously by this sovlothermal method. The as-prepared

Fig. 1 Schematic flowchart (illustration) of the formation of Ag/r-

75 GO/TiO2 (AGT) composite.

[journal], [year], [vol], 00–00 | 1

Nanoscale Accepted Manuscript

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Weiyin Gao,a Minqiang Wang,*a Chenxin Ran,a Xi Yao,a Honghui Yang,b Jing Liu,a Delong He,c and Jinbo 5 Baic

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2. Experimental 2.1. Reagents. GO was prepared from natural graphite (Wodetai Ltd. Co., China, 99.9%) by a classical Hummers method with some modification.23,24 Silver nitrate (A.R. ≥99.8%) was purchased from Xi’an chemical reagent factory. TiO2 (P25, 20% rutile and 80% anatase) was purchased from Degussa. Other solvents were used directly as received without further purification. The experiments were carried out at room temperature and humidity.

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2.4. Characterizations.

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2.2. Synthesis of GO.

20 First, GO was synthesized by the modified Hummers’ method. In

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a typical synthesis, 3 g graphite powder was put into an 80 °C 80 solution of 12 mL concentrated H2SO4, 2.5 g K2S2O8, and 2.5 g P2O5. Then the mixture was kept at 80 °C for 5 h in a water bath. Successively, the mixture was cooled to room temperature and diluted with 500 mL H2O and left overnight. After that, the mixture was filtered and washed with H2O using a 0.45 μm 85 Millipore filter to remove the residual acid. Then the product was dried in a vacuum oven at room temperature. This pre-oxidized graphite was then subjected to oxidation by Hummers’ method described as follows. Pre-oxidized graphite powder was put into cold (0 °C) 120 mL concentrated H2SO4. Then, 15 g KMnO4 was 90 added 1 g at a time under stirring and the temperature of the mixture was kept to be below 20 °C by cooling in ice. Successively, the mixture was stirred at 35 °C for 2 h, and then carefully diluted with 250 mL of H2O. After that, the mixture was stirred for another 2 h, and then additional 700 mL of H2O was 95 added under stirring followed by 20 mL of 30% H2O2. The resulting brilliant-yellow mixture was filtered and washed with 10 wt% HCl aqueous solution (1000 mL) to remove metal ions and washed repeatedly with H2O to remove the acid until the pH of the filtrate was neutral. The resulting GO slurry was dried in a 100 vacuum oven at 60 °C. 2.3. Synthesis of Ag/r-GO/TiO2. Ag/r-GO/TiO2 was prepared by an efficient one-pot solvothermal 105 method under atmospheric pressure. In a typical synthesis, 20 mL DMAc was added into 20 mL GO aqueous dispersion (0.5 mg/mL), where the volume ratio between DMAc and H2O was 1:1, and mixed under magnetic stirring. Meanwhile, 0.2 g of AgNO3 aqueous solution (5 wt%) and 2 g P25 powder were 110 added into the above solution and stirred for 10 min to ensure complete mixing. And then, the reaction was allowed to proceed under magnetic stirring at 150 °C in oil bath for 10 h. Finally, the product was washed with distilled water and ethyl alcohol twice and filtered through a 0.45 μm Millipore filter, the resulting 115 precipitate was then redispersed into ethyl alcohol and stored at 2 | Journal Name, [year], [vol], 00–00

Diffuse reflectance absorption spectra and ultraviolet-visible spectra were recorded on a Jasco V-570 UV/vis/NIR spectrophotometer at room temperature. X-ray photoelectron spectroscopy (XPS) (Escalab 250Xi, Thermo Fisher Co., USA) measurement was processed using an Al-Ka monochromatic Xray source (1486.6 eV). Nitrogen adsorption-desorption isotherm measurements were conducted at 77 K (SSA-4330, Builder Ltd. Co., Beijing, China). X-ray diffraction (XRD) analyses were performed on a XRD-6000 (Japan) with Cu Kα (1.5406 Å) radiation. The diffraction data was recorded for 2Ө angles between 5° and 80°. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained with a JEM-2100 transmission electron microscope (Japan Electron Optics Labortary Co., Ltd., JEOL) with an accelerating voltage of 200 kV. The sample for TEM characterization was prepared by placing a drop of prepared solution on carbon-coated copper grid and drying at room temperature. Scanning transmission electron microscope (STEM) images were acquired using a JEOL JEMARM200F microscope (Japan Electron Optics Labortary Co., Ltd., JEOL). Photoluminescence (PL) studies were done using a Gilden Photonics photoluminescence spectrophotometer under 320 nm excitation, and the slit widths at the excitation and the emission of the spectrofluorimeter were 5 and 2 nm, respectively. For the Electrochemical Impedance Spectra (EIS) measurements, P25 and AGT powders were fabricated as the film electrodes. First, the powders and ethanol were mixed homogeneously (150 mg/mL), and the obtained paste was then spread on the conducting fluorine-doped SnO2 glass substrate (FTO, 15 Ω/square) with doctor blade method. Finally, the resultant films with a ca. 10 μm thickness and 0.36 cm2 active area were calcinated at 450 °C for 2 h in N2 atmosphere. The EIS measurements were carried out on electrochemistry workstation (CHI 660D Chenhua Inc., Shanghai, China) by using threeelectrode cells. The resultant electrode served as the working electrode, with a platinum wire as the counter electrode and SCE electrode as the reference electrodes, which was performed in the presence of a 2.5 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) mixture as a redox probe in 0.1 M KCl solution. The impedance spectra were recorded with the help of ZPlot/ZView software under an ac perturbation signal of 5 mV over the frequency range of 1 MHz to 100 mHz.

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room temperature for characterization. A series of Ag/r-GO/TiO2 composites were synthesized by varying the volume of AgNO3 View Article Online and P25. The detailed reaction conditions were in Table 1. DOI:listed 10.1039/C3NR05466G

2.5. Photodegradation Experiment. The photocatalytic degradation pollutant experiment was performed by measuring the photodegradation of the Rh B solution under the simulated solar irradiation at ambient temperature. Briefly, 40 mg of the catalyst was dispersed in 80 mL of 10 mg/L Rh B solution under ultrasonication for 10 min. Before illumination, the mixture was magnetically stirred for 60 min in the dark to establish adsorption-desorption equilibrium of the pollutant with the catalyst. A solar simulator with 150 W Xe

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AGT composite exhibits outstanding photocatalysis ability on both rhodamine B (Rh B) photodegradation and photocatalytic hydrogen production. The mechanism of the photocatalysis of AGT composite was also proposed. The important role of Ag nanoparticles in the system was further demonstrated.

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Table 1. Detailed experimental donditions of AGT samples. a

Sample GO (mg) AGT-1 10 AGT-2 10 AGT-3 10 AGT-4 10 a： graphene oxide

AgNO3 (mg) 0 3 10 20

P25(g) 2 2 2 2

DMAc (g) 20 20 20 20

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5 lamp (Sciencetech Inc., SS-150) was used as the light source. The

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experimental solution was placed in a quartz bottle, 10 cm from 55 the light source. At given intervals, 5 mL of the suspension was withdrawn and centrifuged to remove the dispersed catalyst powder. The concentration of the clean transparent solution was determined by measuring the 554 nm absorbance of Rh B using a spectrophotometer (Jasco V-570, Shimadzu, Japan). For the stability test of Ag/r-GO/TiO2 in the photodegradation of Rh B under simulated solar light, six consecutive cycles were tested. At the beginning, 40 mg of Ag/r-GO/TiO2 was dispersed in 80 mL of Rh B solution (10 mg/L). Then the mixture underwent six consecutive cycles, each lasting for 100 min. Dark adsorption test was performed to compare the adsorptivity of P25, P25/r-GO and AGT. In this test, 40 mg of each photocatalyst was dispersed in 80 mL of Rh B solution (0.1 ppm) under stirring in the dark for 60 min. Then the dispersion was centrifuged and the Rh B solution was taken to the UV-visible absorption measurement. From the difference in the absorbance before and after adsorption, the amount of Rh B adsorbed by the photocatalyst could be estimated. 2.5. Photocatalytic Hydrogen Generation.

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Hydrogen generation tests were carried out in a 100 mL quartz bottle containing 40 mg of Ag/r-GO/TiO2 photocatalyst in water and methanol solution (60 mL, Vw/Vm = 2:1), where methanol serves as sacrifice agent. The bottle was degassed by bubbling N2 through the solution for 10 min at atmospheric pressure and then sealed with a rubber septum. After that, the mixture was irradiated by a 500 W Xe lamp under stirring. The distance between bottle and light was maintained at 10 cm. The hydrogen generated from the systems was measured every 30 min using an online gas chromatograph (SP-2100, HJATSCI Ltd. Co., Beijing, China) equipped with a TCD detector.

40 3. Results and discussion.

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Time (h) 10 10 10 10

Temp (°C) 150 150 150 150

corresponding to the face-centered cubic (fcc) lattice with the crystal plane.22

Fig. 2 (a)TEM and (b) HRTEM images of sample AGT. (c) STEM model of AGT. Elemental mapping of (d) Ag and (e) Ti in the same area in (c).

While the lattice fringes of 0.315 nm and 0.345 nm are for rutile

60 (101) and anatase (101) planes of TiO2, respectively.25 Elemental

3.1. Characterizations of Ag/r-GO/TiO2. Here, AGT-3 was chosen as representative on account to its best performance of photodegradation of Rh B and photocatalytic hydrogen generation (details given in later section). The morphology of AGT-3 was investigated by TEM and STEM as shown in Fig. 2. It is observed in Fig. 2a that small nanoparticles are wrapped by graphene sheets and the graphene edge was pointed out by blue arrow in Fig. 2a. HRTEM image in Fig. 2b displays clear lattice fringes of Ag and TiO2 particles. The interplanar spacing of 0.235 nm by inverse Fourier transform of the region is the d-spacing value of the Ag (111) plane,

DI (g) 20 20 20 20

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DOI: 10.1039/C3NR05466G

mapping of Ag and Ti in AGT-3 sample are given in Fig. 2d and 2e. It is observed that Ag nanoparticles disperse uniformly among TiO2 particles in the composites. Besides, it is worth noting that both large and small Ag nanoparticles can be observed, as indicated by green circles in Fig. 2c and 2d. This different size of Ag nanoparticles plays a key role in the photocatalysis performance of AGT sample, which will be discussed in the later section. As shown in Fig. 3, a typical nitrogen adsorption–desorption measurements along with the Brunauer–Emmet–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods were used. According to the IUPAC nomenclature, a type IV isotherm with a H1 hysteresis loop is presented, which was characteristic of the mesoporous material with cylindrical pore geometry present within the AGT-3, and facile connectivity between the pores.26 Based on the BJH equation from the desorption branch of the isotherm, AGT-3 showed a relatively broad pore size distribution in the range of 18–23 nm (inset of Fig. 3). A special surface area of AGT-3 was determined to be 59.4 m2g-1 based on the BET analysis, which was higher than that of the P25 (31.5 m2g-1). The parameters obtained from nitrogen desorption isotherms of different samples was given in Table 2. With increasing amount of Ag nanoparticles, surface area of the nanocomposites increased which was attributed to the increase of pore volumes. The sample AGT-4 has the highest surface area (60.1 m2g-1) which was attributed to its largest pore volume, 0.63 cm3g-1 among all the samples. Journal Name, [year], [vol], 00–00 | 3

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35 Fig. 3 Typical nitrogen adsorption-desorption isotherm of AGT-3. The inset corresponds to the pore size distribution measured by the BJH method.

5 As a result, BET tests suggested that the increased Ag content in AGT leads to the improved surface area of the composite, which would facilitate the photocatalysis of the composite due to the improved adsorbability.

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Table 2. Parameters obtained from the nitrogen desorption

10 isotherm experiments sample P25 AGT-1 AGT-2 AGT-3 AGT-4

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Mean pore size (nm) 19.1 36.3 17.5 18.5 20.9

Pore volume (cm3g-1) 0.30 0.58 0.51 0.55 0.63

Surface area (m2g-1) 31.5 32.2 50.3 59.4 60.1

In order to further prove the successful reduce of AgNO3 and GO, XPS measurement was done to analysis the chemical state of the AGT-3. Fig. 4a shows the full-scale XPS pattern of AGT-3, which indicates the existence of Ag, C, Ti and O. The binding

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energy at 284.6, 368.2, 459.5 and 530.1 eV is for C 1s, Ag 3d, Ti

20 2p and O 1s, respectively. The C 1s core level pattern of AGT-3 4 | Journal Name, [year], [vol], 00–00

is shown in Fig. 4b, 4 main types of carbon bonds centered at 284.6, 285.2, 286.6 and 288.4 eV are associated with C-C, C-OH, View Article Online 27 C-O (epoxy/alkoxy), and C=O, respectively. contrast with DOI: By 10.1039/C3NR05466G the curve of GO (Fig. S1), the obvious decrease of the oxygenated functional groups on the sheets in AGT demonstrates the reduction of GO. This results implies the successful reduction of GO into r-GO by this solvothermal reaction. The presence of Ag 3d core level XPS pattern in Fig. 4c could be deconvoluted into two peaks centered at 374.2 and 368.2 eV for Ag 3d3/2 and Ag 3d5/2, respectively. It was reported in literature that metallic Ag 3d peaks are centered at 373.9 and 367.9 eV, while the Ag iron exhibits two peaks at 375.6 and 369.4 eV.28,29 Therefore, our experimental data indicates that a small amount of Ag+ present on the surface of Ag nanoparticles, which is probably due to the electron transfer from metallic Ag to r-GO as the formation of the Ag/r-GO heterostructures.30 We further confirm the existence of Ag+ by Ag 3d pattern of AGT-4, where the curve is found to consist of two peaks which could be ascribed to the Ag and Ag+, respectively(see Fig. S2). The Ti 2p core level XPS pattern reveals two peaks at 459.0 and 464.9 eV, respectively, which are in good agreement with the reported XPS data of Ti 2p3/2 and Ti 2p1/2 in TiO2.31 In addition, all four AGT samples shows similar XRD pattern compared with pure P25 (Fig. S3). No diffraction peaks for Ag nanoparticles were observed in the composite, which might be due to the low content and relatively low diffraction intensity of them. Besides, no obvious diffraction peaks for r-GO were either observed in AGT, mainly because the regular stack of r-GO is destroyed by the intercalation of Ag and TiO2 particles. 3.2. Enlarged Light Absorption Range.

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Fig. 4 (a) XPS spectrum of AGT-3. (b) Core level XPS spectra of (b) C 1s, (c) Ag 3d and (d) Ti 2p of AGT-3.

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As discussed above, the absorption ability of photocatalyst plays an important role in the photocatalysis process. Many works improve the photocatalysis performance mainly by tuning the band gap of TiO2 into visible region. 13, 15, 32 Fig. 5 gives the diffuse reflectance absorption spectra of the pure P25 and asprepared AGT samples. It is obviously that AGT samples exhibit not only a red shift of about 90 nm (from 400 nm to almost 500 nm) in the absorption edge as the increasing dosage of AgNO3, but also a strong absorption in the visible range. Besides, the appearance of the surface plasmon resonance absorption of Ag at around 537 nm give the evidence of the existence of large Ag nanoparticles, and the presence of the absorption at 537 nm is attributed to the dipole resonance peak of large Ag nanoparticles.33 Inset of Fig. 5a displays the photo images of P25, AGT-1, AGT-2, AGT-3 and AGT-4 powder from top to bottom, respectively. It is easily observed that with the introduction of rGO, the color of AGT-1 becomes gray, while as the increased dosage of AgNO3, the product shows darker red color, which facilitates the visible absorption of the photocatalyst. A plot of the transformed Kubelka-Munk function as a function of energy of light is shown in Fig. 5b, by which the roughly estimated band gaps are 3.06, 2.69, 2.64, 2.46 and 2.21 eV corresponding to P25, AGT-1, AGT-2, AGT-3 and AGT-4, respectively. This supports the qualitative observation of a red shift in the absorption edge of AGT composites as compared to the bare P25.

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Fig. 6 Photoluminescence spectra of P25 and AGT composites with different AgNO3 content.

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Fig. 5 (a) Diffuse reflectance absorption spectra of P25 and AGT with different AgNO3 content, and inset of the corresponding image of the P25 and AGT, from top to bottom. (b) The plot of transformed Kubelka-Munk function versus the energy of light of P25 and AGT with different AgNO3 content.

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As a result of this enlarged light absorption range, AGT powder is expected to achieve more efficient utilization of the solar spectrum and show improved photocatalysis ability, which will be proved in photocatalysis experiment section.

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3.3.Efficient Photogenerated Transportation.

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15 In photocatalysis process, photogenerated electron recombination

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is one of the many aspects that are mainly responsible for the low photocatalysis efficiency. It is reported that graphene is a competitive candidate for the acceptor material due to its twodimensional π-conjugation structure,10 and has been successfully applied to TiO2-graphene system that reduces the recombination of photogenerated electron in TiO2.13, 34, 35 On the other hand, Ag is a popular noble metal nanoparticle that possess high electrical mobility and could be treated as an electron acceptor material in composite. Therefore, in Ag/r-GO/TiO2 system, r-GO serves as an electrical path for the photogenerated electrons in TiO2 while Ag can serve as electron acceptor where the photocatalysis process occurred. Thus, this system is expected to effectively suppress the electron recombination and leave more charge carriers to form reactive species to facilitate the degradation of

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Rh B and photocatalytic hydrogen generation. PL quenching effect is an efficient method to determine the charge transfer effect within the material. Therefore, it is expected that the PL spectra of P25 and as-prepared AGT samples can prove the analysis we discussed above. As shown in Fig. 6, it is observed that the PL intensity of AGT-1 decreases after the introduction of r-GO, while as the addition of Ag, further strong decrease in PL is observed. This significant PL quenching is due to the efficient electron transfer from TiO2 onto Ag through graphene sheets. Besides, the highest PL quenching happens in AGT-3, which implies that there is an optimal value for the amount of Ag in order to obtain the best efficient electron transportation. If the dosage of AgNO3 is excessive, the size of Ag nanoparticles becomes larger, and the number of active sites capturing the photoinduced electron is decreased with an increase in the size of Ag nanoparticles. Moreover, excessive Ag can cover the surface of TiO2, leading to a decrease in the concentration of photogenarated charge carrier and photocatalytic activity of photocatalyst.19 Besides, EIS results of P25 and AGT (in Fig. S5) shows that, with the introduction of graphene and Ag nanopartilces, though in small amount, the semicircle in the plot of AGT became shorter, which indicated a decrease in the solid state interface layer resistance and the charge transfer resistance on the surface. 3.4. Photocatalysis Experiment. 3.4.1. Photodegradation of Rh B

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Based on the characterization of AGT samples, we further perform the photocatalysis experiment, including photodegradation of Rh B and hydrogen generation. Rh B is one of the most important representative organic dye substances and has been widely applied in the industrial production, which often contaminates environment. Here, we choose it as pollutant to evaluate photocatalytic activity of AGT. Based on the LambertBeer Law, the concentration of Rh B was proportional to absorbance: A=εcl

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Fig. 7 Photocatalytic degradation (a) of Rh B under the simulated sun light irradiation over P25, AGT composites with different AgNO3 content. Photocatalytic degradation reaction kinetics (b) of Rh B over P25, AGT composites with different AgNO3 content.

The time dependences of the percentage of Rh B degradation was given in Fig. 7a, where commercially available P25 was used as reference. It was clear to conclude that the relative photodegradation rate of AGT hybrid composite exhibited significant improvements compared to P25. First of all, we figure out the best graphene content in r-GO/TiO2 that exhibit best photodegradation ability, that is, GO/TiO2=1:200 (See Fig. S4), and on this basis, we tuned the Ag content in Ag/r-GO/TiO2 composite to determine the influence of Ag on photodegradation performance. We can see from Fig. 7a that, under simulated solar light irradiation, ~80% of the initial pollutants were decomposed by AGT-3 after less than 1 h. Contrastingly, nearly 76% of the initial pollutant still remained in the solution after the same time period for bare P25. On the basis of Wang et al’s studies,37 the decomposition of the dye could be assigned to a pseudo-first6 | Journal Name, [year], [vol], 00–00

order kinetics reaction with a simplified Langmuir-Hinshelwood model when C0 is very small.

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ln(C/C0) = kt

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Fig. 8 Colour changing of the Rh B photodegradation over P25, AGT-1, AGT-2, AGT-3 and AGT-4.

where k is the apparent first-order kinetics rate constant, and was from a linear fit to the data as shown in Fig. 7b. Clearly, AGT-3 gave a 4 times higher rate constant of Rh B degradation than P25. The superior performance of AGT, on one hand, should be largely assigned to the effective charge transport from photogenerated electron in Rh B and TiO2 to graphene and Ag nanoparticles. On the other hand, it is also because the localized surface plasmon resonance (LSPR) of Ag NPs induces broadband optical absorption enhancement. Besides, among AGT composites, AGT-3 has the highest rate constant, which is in great consistent with the PL result that AGT-3 exhibits strongest PL quenching effect. The superior photodegradation ability of AGT-3 could be also seen from the change in color by the evolution of irradiation visually as shown in Fig. 8. Moreover, AGT-3 exhibits excellent photochemical stability after 6 cycling photodegradation tests as seen in Fig. 9.

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here A is absorbance, c is concentration of absorbing component, l is length of absorbing layer, and ε is the molar absorbing coefficient. As a result, the normalized temporal concentration changes (C/C0) of Rh B during the photodegradation were proportional to the normalized maximum absorbance (A/A0). So, the photocatalytic degradation of Rh B is monitored by the UVvis absorption spectra of Rh B in aqueous solutions. The photocatalytic performance over P25 and AGT samples have been performed under simulated solar light at room temperature and ambient pressure, and the results are shown in Fig. 7.

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Fig. 9 The cycling degradation rate for Rh B of AGT-3 under simulated sun light irradiation.

5 3.4.2. Photocatalytic

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Hydrogen Generation.

We also tested the photocatalytic hydrogen generation ability of as-prepared AGT samples. Fig. 10a shows the variation of the amount of hydrogen evolution under Xe lamp irradiation of P25, AGT-1, AGT-2, AGT-3 and AGT-4, respectively. It is observed that P25 shows a very low photocatalytic activity because of the rapid hole-electron recombination. The introduction of graphene (AGT-1) only leads to extremely limited improvement on hydrogen evolution. Surprisingly, after introducing only a small amount of Ag, the activity of the sample AGT-2 is remarkably enhanced and increased 66 times compared with P25 as shown in Fig. 10b. The photocatalytic activity of the samples further increased with increasing Ag content. The highest H2-production rate, obtained for the AGT-3 sample, is 196.87 μmol h-1 g-1. This value exceeded by 104 times that of pure TiO2. However, a further increase in Ag content leads to a reduction of the photocatalytic activity in AGT-4. It is reported by Kamat group that 2~5 nm Ag nanoparticles on TiO2 possess electron storage property and the electron storage capacity strongly depends on the size of the nanoparticle.38 This conclusion perfectly explains our results that as increased addition of AgNO3, we first observed an increased photocatalysis ability due to the increased amount of electron storage by Ag. However, at higher AgNO3 concentrations, a decrease trend in the photocatalysis ability was observed owing to low electron storage capacity and light scattering effect of larger size Ag nanoparticles. When grapheme itself was used as photocatalyst, there was no appreciable hydrogen detected which suggesting that bare r-GO is likely not active for photocatalytic H2 production.39,40 It is worth noting that Fig. 6a as well as 9b show almost similar catalytic activity of AGT-3 and AGT-4. However, PL intensity of AGT-3 is considerably weaker than that of AGT-4. The reason is that PL intensity of all samples are tested under 320 nm excitation, where it only excited the electrons in TiO2 into excited state. As a result, PL study performed here only describes the electron separation and transfer from TiO2 onto graphene and Ag, which demonstrated that charge separation in AGT-3 is more

Fig. 10 (a) Process of photocatalytic hydrogen production under sun light irradiation over P25 and AGT composites with different AgNO3 content. (b) Comparison of the photocatalytic activity of the r-GO, P25 and AGT composites with different AgNO3 content for the photocatalytic H2 production under simulated sun light irradiation. The photocatalytic activity of r-GO is obtained from ref [39,40].

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simulated sunlight irradiation, the effect of LSPR of large Ag nanoparticles also contributes to the improved photocatalysis process. From UV-vis spectra we can see that AGT-4 possesses stronger LSPR effect than AGT-3, which means the utilization of visible part of sunlight for AGT-4 is more effective than that of AGT-3. However, the photocatalytic activity of AGT-3 is higher than that of AGT-4 as shown in photocatalysis experiments. It is known that the extinction ability of Ag nanoparticles is consist of absorption and scattering. It is reported that the optical response for small metal nanoparticles (R50 nm) can scatter light more efficiently.41 So, the scattering effect, rather than LSPR effect of the large Ag nanoparticles (the case in AGT4), takes the most part of the absorbed light, which means that it may actually decrease the utilization of visible light in the form of LSPR effect. Therefore, after considering the conjunct effect of (i) utilization of both UV and visible parts and (ii) the size of Ag nanoparticles, the catalytic activity of AGT-3 and AGT-4 are almost similar, but AGT-3 is actually more effective. 3.5. Mechanism of photocatalysis of AGT.

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Based on the characterization and experimental data discussed 60 above, the high photodegradation of Rh B and hydrogen generation activity of the AGT samples under simulated sunlight irradiation can be understood as illustrated in Fig. 11. According to the literature,42-44 the work function of Rh B, excited Rh B, graphene, the conduction band (CB) of TiO2 and Ag are -5.45, - 65 3.08, -4.42, -4.40 and -4.8 eV, respectively. Normally in pure P25, photogenerated electrons are quickly recombine and only a fraction of the electrons and holes participate in the photocatalytic reaction, resulting in low reactivity.45,46 However, when P25 was modified by graphene and Ag, both UV and 70 visible region of incident light can be utilized simultaneously. Generally, it is believed that co-catalysts are usually essential besides of the main semiconductor photocatalysts. On one hand, suitable co-catalysts loaded on the surface of semiconductor photocatalysts can serve as active centers for photodegradation of 75 pollutant or H2 generation, since it can efficiently generate more reactive sites and reduce the oxidation or reduction overpotential. On the other hand, they can enhance the charge separation extent and restrain the recombination of the photo-induced electron and hole carriers in semiconductors. In our case here, both Ag 80 nanoparticles and graphene are excellent co-catalysts, and we proposed that the presence of them increases the total reaction sites of the system and synergistically contributes to the significantly improved photodegradation of Rh B and hydrogen generation. Previous works47,48 have shown that very small size metal NPs (Au or Ag, 2~5 nm) are capable of storing electrons, while another work38 reveals that as the size of nanoparticles increases, the larger size metal nanoparticles are less effective in storing electrons. So as the increase of AgNO3, the size of Ag nanoparticles is getting larger, it is observed that a strong LSRP absorption of AGT-4 around 537 nm as shown in Fig. 4a, which implies the majority content of large Ag nanoparicles that can make use of visible region of the light. Based on these works, we propose the photocatalysis mechanism of AGT samples. As shown in Fig. 11a, UV part of the incident light is absorbed by TiO2, these photogenerated electrons on the CB of TiO2 tend to transfer to small Ag (s-Ag) (2~5 nm) through graphene sheets. Here, s-Ag, which exhibits great electron storage capacity,41 acts as electron tank and facilities the charge separation. The major routes in the photocatalytic dye degradation mechanism under UV-light irradiation are described by following eqn (1-12). TiO2 + hν → TiO2 (e- + h+) (1) TiO2 (e-) + r-GO → TiO2 + r-GO (e-) (2) r-GO (e-) + s-Ag → r-GO + s-Ag (e-) (3) r-GO (e-) +O2 → r-GO + O2-· (4) s-Ag (e-) +O2 → s-Ag + O2-· (5) O2-·+ H2O → ·OH (6) TiO2 (h+) +H2O → ·OH + H+ (7) O2-· + H+ → HO2· (8) 2HO2· → H2O2 + O2 (9) H2O2+ O2-·→ OH· + OH- + O2 (10) TiO2 (h+) +OH- → TiO2 + ·OH (11) 85 ·OH + Dye → Degradation products (12) While visible region of the incident light is absorbed by LSPR of large Ag (l-Ag) as shown in Fig. 11b, plasmon-excited electrons (hot electrons) on Ag surface could efficiently transfer onto CB

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of TiO2 or r-GO, this at the same time greatly contributes to the photocatalysis process. The hot electrons that are generated by View Article Online the plasmon decay are injected from the plasmonic materials into DOI: 10.1039/C3NR05466G the conduction band of the semiconductor, while the plasmoninduced electromagnetic field creates vacancies in the conduction band of the surface of TiO2, promoting the separation of photogenerated electrons and holes.49 Besides, the visible light absorption of Rh B also provides the photogeneratated electron that further facilitates the degradation of Rh B itself. The major routes in the photocatalytic dye degradation mechanism under visible irradiation are described by following eqn (13-22). Dye + hν→ Dye* (13) Dye* + TiO2→Dye+. + TiO2 (e-) (14) l-Ag + hν→l-Ag (e- + h+) (15) l-Ag (e-) +r-GO → r-GO (e-) + l-Ag (16) r-GO (e-) + TiO2 → r-GO + TiO2 (e-) (17) r-GO (e-) +O2 → r-GO + O2-. (18) TiO2 (e-) +O2 → TiO2 + O2-· (19) O2-· + H2O → ·OH (20) (21) l-Ag (h+) + OH- → l-Ag + ·OH ·OH + Dye → Degradation products (22) So, on the one hand, s-Ag nanoparticles serve as electron tank for efficient photogenerated electron collection. On the other hand, the LSPR effect of l-Ag significantly enlarges the absorption

Fig. 11 Proposed mechanism for the photodegradation of Rh B (the red one) and photocatalytic hydrogen production (the green one) by AGT under (a) UV part and (b) visible part of simulated sunlight irradiation. sAg stands for small Ag nanoparticles, and l-Ag stands for large Ag nanoparticles.

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range of AGT and could efficiently improve the utilization rate of incident light and enhance the charge transfer process. It is worth noting that reaction species such as H2O2, ·OH or O2-· produced 60 during the photocatalytic process play an important role and is the mainly reaction species during the chemical degradation process of R hB. For hydrogen generation process, the electron generation and separation mechanism of photocatalytic hydrogen generation over 65 AGT under solar simulated light are the same as photodegradation of Rh B above. The only difference is that the chemical reaction of hydrogen generation is more simpler: photogenerated e- and h+ in the composite can directly participate in the reaction without producing any other species.32, 50 And the 70 production of H2 is brief described as follows (23-24): e- + 2H+ → H2 (23) h+ +CH3OH +6 OH· →CO2 + 5H2O (24) As a result, this Ag/r-GO/TiO2 structure has mainly two advantages: (i) making use of not only UV region of sunlight, but also visible region; (ii) leading to efficient hole-electron 75 separation in a wider range (UV and visible), which facilitates the chemical photocatalytic reaction process synergistically. Moreover, it is reported that the unique features of graphene allow photocatalytic reactions to take place not only on the surface of semiconductor catalysts, but also on the graphene sheet, 80 which could enlarge the reaction sites.40,49 Here, our AGT samples induce Ag nanoparticles into the composites, which means induced much more reaction sites into the system than TiO2/graphene system. There must be an optimal ratio between sAg and l-Ag to obtain the maximum photocatalytic activity. 85 Unfortunately, it is difficult to quantificationally determine the relative composition of s-Ag and l-Ag due to the one-pot synthesis method we used here. This issue needs to be investigated in the future. In brief, the introduction of Ag results in a red shift in the 90 absorption range and facilitates a more efficient utilization of incident light for the photocatalyst. The photogenerated electrons efficiently transfer from TiO2 onto small size Ag through graphene sheet and store in Ag nanoparticle. At the same time, large size Ag can make use of visible region of incident light, 95 which synergistically leads to an improved charge separation and efficient chemical degradation process. Besides, the introduction of graphene and Ag nanoparticles can increase the total number of reaction sites in the system. These factors all enhance the photocatalysis ability of the product.

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In conclusion, we have demonstrated a simple and efficient onepot approach to prepare Ag/r-GO/TiO2 composites using solvothermal method under atmospheric pressure, where DMAc105 serves as the reducing agent for Ag and GO reduction. On the basis of diffuse reflectance absorption spectra and photoluminesce analyses, the composite is found to possess excellent optical and photophysics property that facilities the photocatalysis property compared with pure TiO2, that is, (i)110 enlarged light absorption range; (ii) efficient photogenerated electron separation and transportation synergistically under UV and visible region of the incident light. We proposed that for the increased photocatalysis ability of the AGT composite, Ag This journal is © The Royal Society of Chemistry [year]

nanoparticles play an important role that (i) facilitate the charge separation and transfer, (ii) expand the absorption band into View Article Online visible region and (iii) increase the amount of10.1039/C3NR05466G reaction site. This DOI: work gives a clear photocatalysis mechanism of Ag/r-GO/TiO2 composite and opens up a new possibility in develop new TiO2based system photocatalyst with high photocatalysis performance in various environmental and energy issues.

Acknowledgements The authors gratefully acknowledge financial support from Natural Science Foundation of China (Grant Nos. 91123019 and 61176056). This work has been financially supported by NSFC Major Research Program On Nanomanufacturing (Grant Nos. 90923040), and the International Collaboration Program of Shaanxi Province (Grant Nos. 2013KW-12-05).

Notes and references a

Xi’an Jiaotong University, Electronic Materials Research Laboratory, Key Laboratory of Education Ministry, Xi’an, Shaanxi 710049, P.R. China. E-mail: [email protected] b Xi’an Jiaotong University, Department of Environmental Science and Technology, Xi’an, Shaanxi 710049, P.R. China. c Lab. MSSMat, UMR CNRS8579, Ecole Central Paris, 92290 Châtenay malabry, France. † Electronic Supplementary Information (ESI) available: C1s XPS spectrum of GO, XRD and EIS patterns of P25 and AGT composites, photocatalytic degradation of Rh B under the simulated sun light over P25 and r-GO/TiO2 composites, absorption of pure garphene and pure Ag and PL spectra of AGT composites with different AgNO3 content are shown in Supplementary Information. See DOI: 10.1039/b000000x/

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