TiO2 Hybrids as Powerful

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Yolk@Shell Nanoarchitecture of Au@r-GO/TiO2 Hybrids as Powerful Visible Light Photocatalysts Minggui Wang, Jie Han, HuiXin XIONG, and Rong Guo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b01099 • Publication Date (Web): 21 May 2015 Downloaded from http://pubs.acs.org on May 26, 2015

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Yolk@Shell Nanoarchitecture of Au@r-GO/TiO2 Hybrids as Powerful Visible Light Photocatalysts Minggui Wang,a Jie Han,*, a Huixin Xiong,b and Rong Guo*, a a

School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu,

225002, P. R. China. E-mail: [email protected]; [email protected] b

School of Environmental Science and Engineering, Yangzhou University, Yangzhou, Jiangsu,

225127, P. R. China

KEYWORDS: TiO2; graphene; Au nanoparticle; yolk@shell; visible light photocatalyst ABSTRACT: Yolk@shell nanostructures of Au@r-GO/TiO2 with mesoporous shells were prepared by a sol-gel coating processes sequentially with GO and TiO2 on Au/SiO2 core/shell spheres, followed by calcination and template removal, where the silica interlayer not only acts as a template to produce the void space, but also promoting the coating of the r-GO and TiO2 layer. Evaluation of visible light photocatalytic activities in dye decomposition and water splitting H2 production demonstrated their superior photocatalytic performances, which indicates their potential as powerful photocatalysts.

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1. INTRODUCTION Recently, yolk@shell nanostructures with void space between the interior core and the mesoporous outer shell have attracted more attention due to their special structure and properties, such as lower density, larger surface area and higher loading capacity.1, 2 Their mesoporous shell can not only guarantee fast diffusion of reactants and product, but also effectively preventing coagulation of the moveable core. The void space in the yolk@shell structures provides a unique confined space making them attractive applications in various fields, such as nanoreactors,3 drug delivery4 and energy storage.5 Ordinarily, yolk@shell nanostructures can be formed derived from core/shell nanostructures through selective etching or dissolution procedures, where the shells or cores will be partially removed to give up the void space.6-9 It has been reported that yolk@shell nanostructured materials with noble metal nanoparticle as the yolk exhibit high catalytic performances in reactions such as reduction of 4-nitrophenol, Suzuki coupling reactions, CO adsorption, etc.10-15 As a wide bandgap semiconductor material, TiO2 has attracted more attention and been frequently used in the field of energy conversion16 and photocatalysis17 due to its relatively higher photocatalytic efficiency, nontoxicity, low cost and excellent chemical stability.18 Various photocatalytic applications for TiO2 have been widely used, including CO2 reduction,19,

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organic dye degradation21 and energy storage.22 However, the application of pure TiO2 is limited because of its relatively wide band gap that can only be excited by UV light and rapid recombination of photogenerated electron-hole pairs.23 The common methodology to enhance its photocatalytic performances of TiO2 is to extend its light adsorption to the visible light region. Howbeit, how to effectively inhibit the high recombination rate of electron-hole pairs is still a challenge. Plasmonic nanostructures of noble metal nanoparticles have attracted great attention,


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because they can strongly improve the visible light irradiation absorption due to their extraordinary surface plasmon resonance. Graphene, a monolayer of carbon atoms with a tight packing of honeycomb lattice has been reported to hybridize with TiO2 to transfer electrons and therefore to lengthen the lifetime of photogenerted electrons, which can suppress the recombination of electron-hole pairs.24-26 Commonly, the improvement in catalytic activities brought by graphene is ascribed to its large surface area and superior charge carrier mobility. Based on the above considerations, designed synthesis of hybrid nanostructures composed of TiO2, noble metal nanoparticles, and graphene is an effective route to solve the low lightharvesting efficiency of TiO2, thereafter resulting in improved photocatalytic performances.27 Their improved photocatalytic performances could be attributed to the higher photon absorption efficiency in visible light by noble metal nanoparticles and good suppress of photogenerated electron-hole pairs recombination by graphene. Most recent reported hybrids consisting of TiO2, noble metal nanoparticles and graphene are shown as two-dimensional nanostructures, where graphene is often used as martix for loading TiO2 and Au.28,

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The stacking among hybrid

nanosheets and congulation of loaded TiO2 and Au nanoparticles are hard to overcome, which are believed to have negative effect towards their photocatalytic performances. Although Au@TiO2 yolk@shell nanostructures have been established to overcome the drawbacks,19 rational design of graphene incorporated Au@TiO2 yolk@shell nanostructures with wellcontrolled structure and enhanced photocatalytic performances is still a great challenge. Herein, we have reported herein the successful synthesis of Au@r-GO/TiO2 yolk@shell nanostructures and their high photocatalytic performances towards the decomposition of Rhodamine B (RhB) and water splitting H2 production have been demonstrated. In this synthesis, Au/SiO2 core/shell spheres were chosen as hard templates, followed by the loading of GO and


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TiO2. Finally, Au@r-GO/TiO2 yolk@shell nanostructures were be formed after post-treatments including calcination and template removal. Results from photocatalytic experiments suggested that Au@r-GO/TiO2 yolk@shell hybrids showed high photocatalytic performances in degradation of RhB and water-splitting H2 production under visible light irradiations, indicating their promising application as efficient visible light photocatalysts.

2. EXPERIMENTAL METHODS 2.1. Materials: P25 (20% rutile and 80% anatase), tetraethyl orthosilicate (TEOS, 99.8%) and 3aminopropyltriethosysilane (APTES) (99%) were purchased from Sigma-Aldrich. Tetrabutyl orthotitanate (TBOT, 97%) was purchased from Fluka. All other reagents were purchased from Sinopharm Chemical Reagent Co. Ltd. (China). All reagents were received without further purification. The water used was deionized purified through a Millipore system. 2.2. Synthesis of GO: 0.30 g graphite and 1.80 g KMnO4 were added into a mixture containing 4.0 mL H3PO4 and 36.0 mL concentrated H2SO4. The solution was heated to 50 °C and kept stirring for 12 h, and then 40.0 mL ice containing 0.3 mL H2O2 was slowly added into the solution. After that, the colloidal solution was washed sequentially with water, 3.4 % HCl and ethanol. Finally, the obtained GO was redispersed in ethanol (0.5 mg/mL), sonicated for 24 h for further use. 2.3. Synthesis of Au Nanoparticles: An aqueous solution of 0.30 mL HAuCl4.3H2O (0.10 mol/L) was added to 30.0 mL water and the solution was heated to the boiling point. After that, 6.0 mL sodium citrate aqueous solution (10.0 mg/ml) was added in one portion. The solution was kept refluxing for 30 minutes under stirring, and then cooled down and mixed with 0.235 mL polyvinylpyrrolidone (PVP) aqueous solution (12.8 mg/ml) overnight under stirring. Finally, Au


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nanoparticles were separated from the solution by centrifugation and redispersed in 2.0 mL water for further use. 2.4. Synthesis of Au/SiO2 Core/Shell Spheres: The above-mentioned Au colloidal solution (1.0 mL) was added into a solution containing 46.0 mL 2-propanol, 6.6 mL H2O and 1.2 mL ammonium (28%). Then, 1.0 mL TEOS was added in one portion and the solution was stirred for 6 h. The samples were purified by centrifugation and washed twice by ethanol. After that, the particles were added to a solution containing 40.0 mL 2-propanol and 0.5 mL APTES and refluxed for 3 h. The surface modified Au/SiO2 core/shell spheres were finally washed with ethanol and then dried in an oven at 60 oC for 12 h. 2.5. Synthesis of Au@r-GO/TiO2 Yolk@Shell Hybrids: 1) GO was firstly coated on surfaces of Au/SiO2 core/shell hybrids: 0.20 mL GO ethanol solution (0.5 mg/mL) was added into 40.0 mL Au/SiO2 ethanol solution (1.25 mg/mL) and the solution was refluxed for 1 h, resulting in the formation of Au/SiO2/GO hybrids. The solution was then centrifuged and redispersed in 40.0 mL 2-propanol. 2) TiO2 was coated on surfaces of Au/SiO2/GO hybrids: 0.2 mL TBOT was firstly added into 40.0 mL Au/SiO2/GO 2-propanol solution under vigorous stirring for 1 h, and then 5.0 mL water was injected into the mixture (0.5 mL/min) and the solution was stirred for 12 h. The Au/SiO2/GO/TiO2 core/shell hybrids were formed. The solution was washed twice with ethanol and dried under vacuum. Finally, Au/SiO2/GO/TiO2 core/shell hybrids were calcined at 600 oC under nitrogen for 4 h, leading to the formation of Au/SiO2/r-GO/m-TiO2 core/shell hybrids. The SiO2 layers were etched with HF solution (40%) at room temperature for 10 min. The obtained Au@r-GO/TiO2 yolk@shell hybrids were further used as photocatalysts. 2.6. Photocatalytic Activity Tests


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2.6.1. Photocatalytic Degradation of RhB: In a typical run of photocatalytic decomposition of RhB, 5.0 mg catalyst was dispersed in 25.0 mL RhB aqueous solution (2.0×10-5 mol/L) in a quartz cell and the solution was stirred in dark for 30 min to ensure adsorption equilibrium. The light sources of the visible lamp (400 W Metal Halide) with a 400 nm filter, and a sunlight simulator (350 W Xenon) were used in a commercial photoreactor system (Xujiang XPA-7). Determination of RhB concentration was measured with a UV-Vis spectrophotometry (HR2000CG-UV-NIR, Ocean Optics). 2.6.2. Water splitting H2 production: In a typical run of photocatalytic water-splitting H2 production, 10.0 mg catalyst was dispersed in 10.0 mL solution containing water and methanol (VH2O/Vmethanol = 3/1) under stirring for 20 min, and then nitrogen was bubbled for 30 min to remove the dissolved oxygen and then sealed with a parafilm. The suspension was irradiated under stirring with a Xe light source through filters with nominal cutoff wavelength of λ < 400 nm, power density 100 mW cm−2. The photocatalytic system was maintained at room temperature. The gas produced were periodically withdrawn with a syringe and examined by a gas chromatography (GC). The apparent quantum efficiency (QE) was calculated according to the following equation:

QE [%] =

number of reacted electrons × 100% number of incident photons

2.7. Instruments: Morphologies was characterized by a transmission electron microscope (TEM, JEM-2100 F) and a high resolution TEM (HRTEM, Tecnai G2 F30 S-Twin TEM, FEI). XRD patterns were recorded on a German Brucker AXS D8 ADVANCE X-ray diffractometer. The products were recorded in the 2θ range from 10° to 85.0° in steps of 0.04° with a count time of 1 s each time. The absorption measurements were performed using a UV-vis-NIR spectrophotometer (Cary 5000, Varian) with an integrating sphere. The phase composition was


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measured using an Axis Ultra X-ray photoelectron spectroscope (XPS, Kratos Analytical Ltd., UK) equipped with a standard monochromatic Al Kα source (hv = 1486.6 eV). The binding energy data were calibrated with respect to the C1s signal of the ambient hydrocarbons (C-H and C-C) at 284.8 eV. The nitrogen adsorption-desorption isotherms were performed on a Beishide 3H-2000PS2 analyzer. The surface area was calculated from the adsorption isotherm using the multi-point Brunauer-Emmett-Teller (BET) method in the pressure range of P/P0 = 0.05-0.25. The average pore size was determined by the Barrett-Joyner-Halenda (BJH) method from the adsorption isotherm. The electrochemical impedance spectroscopy (EIS) was measured on an Autolab PGSTAT30 by using three electrode cells in 0.5 mol/L Na2SO4 solution. The obtained materials were used as the working electrode, platinum wire as the counter electrode and Ag/AgCl electrode as the reference electrode.

3. RESULTS AND DISCUSSION 3.1. Morphology, Structure and Formation Mechanism of Au@r-GO/TiO2 Yolk@Shell Hybrids. Figure 1a illustrates the procedures for Au@r-GO/TiO2 yolk@shell nanostructures. Firstly, Au/SiO2 core/shell nanospheres are prepared followed by modification with a coupling agent APTES, and then a thin layer of GO is uniformly self-assembled onto the surface of the Au/SiO2 particles through electrostatic interactions. Finally, TiO2 layers are coated on Au/SiO2/GO hybrid surfaces, leading to the formation of Au@r-GO/TiO2 yolk@shell nanostructures after calcination and template removal procedures. The morphological evolutions during the procedures have been monitored and their corresponding TEM images are given in Figure S1.


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Figure 1 (a) Proposed formation scheme for Au@r-GO/TiO2 yolk@shell hybrids. (b) TEM and (d) HRTEM images of Au@r-GO/TiO2 yolk@shell hybrids. (c, e) HAADF-STEM image of Au@r-GO/TiO2 yolk@shell hybrids and (f-h) EDS maps of (f) Ti, (g) Au, and (h) C from a single particle.

Figure 1b displays the typical TEM images of Au@r-GO/TiO2 yolk@shell nanostructures, where well-defined hollow spheres with diameter of about 230 nm can be evidenced. However, the contrast between crystalline TiO2 shell and Au nanoparticle core is not clear enough to identify the yolk@shell nanostructure. In order to confirm the yolk@shell nanostructures of Au@r-GO/TiO2, the high-angle annular dark field scanning transmission electron microscopy


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(HAADF-STEM) image of Au@r-GO/TiO2 yolk@shell hybrids is shown in Figure 1c, from which the yolk@shell nanostructures can be evidently verified. Mostly, each TiO2 hollow sphere contains a single Au nanoparticle. Figure 1d gives the HRTEM image of Au@r-GO/TiO2 yolk@shell nanostructures, where the shell thickness is estimated to be 15 nm. The fringe spacing of 0.35 nm can be indexed as the lattice spacing of the (101) plane of anatase TiO2, whereas the spacing of 0.236 nm corresponding to Au (111) plane indicates the presence of Au nanoparticle (10 nm). The HAADF-STEM image of Au@r-GO/TiO2 yolk@shell nanoparticle as given in Figure 1e, together with the energy dispersive X-ray spectroscopic (EDS) elemental maps (Figures 1f-h) further confirm the expected Au@r-GO/TiO2 yolk@shell nanostructure. It can be confirmed that C is partially penetrated into TiO2 porous shells (Figure 1h), which could be explained by the crystallization and mesoporous shell formation of TiO2 during calcination process. In addition, Au nanoparticles remain the same size even after harsh calcination and etching treatment, indicating that the yolk@shell structure can effectively prevent coagulation of the Au yolk. The concentration of TBOT was found to affect the TiO2 shell integrality and thickness (Figure S2). When the added TBOT amount is 0.1 mL, Au@r-GO/TiO2 yolk@shell hybrids with incomplete shell can be found. The shell thickness is estimated to be 10 nm. It is acceptable as the thin shell is hard to preserve the completeness of the yolk@shell hybrids. Well-defined intact Au@r-GO/TiO2 yolk@shell hybrids with shell thickness of 15 nm (Figure 1d) are formed at the TBOT amount of 0.2 mL. When the TBOT amount is further increased to 0.4 mL, complete Au@r-GO/TiO2 yolk@shell hybrids with shell thickness of about 50 nm can be found. It should be noted that individual Au@r-GO/TiO2 yolk@shell hybrid is seldom seen, and most particles are cross-linked with each other.


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3.2. Characterization of Au@r-GO/TiO2 Yolk@Shell Hybrids. The crystalline and phase structures were investigated by XRD. As shown in Figure 2a, all the reflection peaks for TiO2 are assigned to anatase (JCPDS, no. 21-1272). The diffraction peaks of the TiO2 mesoporous shells are sharp and intense, confirming high crystallinity of TiO2 shell. Notably, no characteristic diffraction peaks for r-GO and Au are observed, possibly due to low amount of r-GO and Au, and strong diffraction from crystalline TiO2. In order to fully understand the structure of the Au@r-GO/TiO2 yolk@shell hybrids, Roman spectroscopy was applied to study the fine structure and crystalline phase. Figure 2b shows the Raman scattering spectra of pure TiO2 hollow spheres and Au@r-GO/TiO2 yolk@shell hybrids. The curve of TiO2 hollow spheres exhibits four peaks at 146, 397, 516, 637 cm-1 which are ascribed to the vibrations of Eg(1), B1g(1), Alg+Blg(2) and Eg(2) of anatase TiO2, respectively. As for Au@r-GO/TiO2 yolk@shell hybrids, TiO2 relaxations are blurred. In addition, the D and G bands of r-GO appeared at 1360 and 1580 cm-1 confirm the presence of r-GO in the yolk@shell hybrids. The D/G intensity ratio for Au@r-GO/TiO2 yolk@shell hybrids (ID/IG = 0.92) is higher than pure GO (ID/IG = 0.65), suggesting the successful transformation of GO into r-GO through

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(b)

G D

Eg(1)

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C=C

O-C=O Au@r-GO/TiO2

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0.10 0.08 0.06 0.04 0.02 0.00

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Figure 2 (a) XRD pattern of Au@r-GO/TiO2 yolk@shell hybrids. (b) Roman spectra of TiO2 hollow spheres and Au@r-GO/TiO2 yolk@shell hybrids. (c, e) XPS spectra of Au@r-GO/TiO2 yolk@shell hybrids: (c) C 1s, (d) Au 4f, and (e) Ti 2p. (f) Nitrogen adsorption-desorption isotherm of Au@r-GO/TiO2 yolk@shell hybrids. Inset in Figure 2f shows the pore size distribution of Au@r-GO/TiO2 yolk@shell hybrids.

In order to determine the oxidation state of surface element in materials, XPS spectroscopy was used to characterize the Au@r-GO/TiO2 yolk@shell hybrids. In the XPS spectrum of Au@rGO/TiO2 yolk@shell hybrids (Figure S3), the presence of TiO2, Au and r-GO can be verified. In comparison with C 1s peaks of GO, a high ratio of non-oxygenated C to the carbonyl C in Au@r-GO/TiO2 yolk@shell hybrids can be verified, further proving the successful


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transformation of GO into r-GO through pyrolysis during the calcination thermal treatment reduction process.31,

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The Au 4f doublet peaks of Au@TiO2 and Au@r-GO/TiO2 hybrids

(Figure 2d) both locate at binding energy of 83.0 eV and 86.7 eV with the splitting of the 4f doublet of 3.7 eV, indicating the metallic nature of Au. The Ti 2p peaks for TiO2 in TiO2 hollow spheres are located at 458.1 and 463.8 eV, whereas those in Au@r-GO/TiO2 yolk@shell hybrids locate at 458.4 and 464.1 eV, both with the splitting of the 2p doublet 5.7 eV (Figure 2e), indicating the Ti4+ state.33 In addition, the Ti 2p binding energy shows 0.3 eV shifts in the direction of low energy, indicating interactions between TiO2 and oxygen centers of r-GO. Results from TEM, XRD, Roman and XPS analysis confirm the successful formation of Au@rGO/TiO2 yolk@shell hybrids. Figure 2f shows the nitrogen adsorption-desorption isotherm of Au@r-GO/TiO2 yolk@shell hybrids. The surface area is measured to be 122.9 m2 g-1, which is much higher than that of commercial P25 (~ 50 m2 g-1). The corresponding pore size as given in Figure 2f as determined using the BJH method from the desorption branch of the isotherm is 6.0 nm. 1.6 1.4

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Figure 3 (a) UV/Vis diffuse reflectance spectra of TiO2 hollow spheres, Au@TiO2 yolk@shell hybrids and Au@r-GO/TiO2 yolk@shell hybrids. (b) The plots of the transformed KubelkaMunk function versus the energy of light.


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Figure 3a shows the UV/Vis diffuse reflectance spectra of the TiO2 hollow spheres, Au@TiO2 yolk@shell hybrids and Au@r-GO/TiO2 yolk@shell hybrids. The absorption edge near 400 nm of all samples is attributed to the band transition of anatase TiO2.34 It is clearly seen that the visible light absorption is enhanced after addition of Au. Moreover, the visible light absorption increases further for Au@r-GO/TiO2 yolk@shell hybrids, indicating the positive effect of r-GO in visible light absorption. The extending absorption of Au@r-GO/TiO2 yolk@shell hybrids into the visible light region is believed to enhance the visible light photocatalytic activity. The band gap of semiconductors is calculated according to the Kubelka-Munk method.35,

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Plots of

(αhv)1/2 versus photon energy (hv) are depicted in Figure 3b. The indirect band gap energies estimated from the intercept of the tangents to the plots are about 3.13, 3.01 and 2.92 eV, corresponding to TiO2 hollow spheres, Au@TiO2 yolk@shell hybrids and Au@r-GO/TiO2 yolk@shell hybrids, respectively.

TiO2 Au@TiO2 Au@r-GO/TiO2

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Figure 4 (a) Nyquist plots of TiO2 hollow spheres, Au@TiO2 yolk@shell hybrids and Au@rGO/TiO2 yolk@shell hybrids electrodes. (b) PL spectra of Au@TiO2 and Au@r-GO/TiO2 yolk@shell hybrids.

EIS analysis is applied to investigate the charge transfer process occurring in a three-electrode


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system. Figure 4a displays the EIS Nyquist plots of TiO2 hollow spheres, Au@TiO2 yolk@shell hybrids and Au@r-GO/TiO2 yolk@shell hybrids. The Au@r-GO/TiO2 yolk@shell hybrids show the smallest semicircle, followed by Au@TiO2 yolk@shell hybrids and TiO2 hollow spheres. Results indicate the fastest electron transfer in the solid state interface for Au@r-GO/TiO2 yolk@shell hybrids. It is reasonable as the introduction of r-GO functions as an electron collector and transporter which benefits the charge transfer and suppresses the charge recombination.37, 38 The photoluminescence (PL) spectra of Au@TiO2 and Au@r-GO/TiO2 yolk@shell hybrids excited at 365 nm are presented in Figure 4b. The PL spectra show the information about the recombination of electron-hole pairs of photoexcited photocatalysts, and their intensities can be used to determine the fate of electron-hole pairs.39 Obviously, the pure TiO2 has a high intensity (23000) emission peak at around 397 nm (Figure S7). However, the emission peak has a shift to about 500 nm after introducing Au nanoparticle with the intensity of PL signal weakened a lot (about 10500). The intensity of PL signal for Au@r-GO/TiO2 yolk@shell hybrids is much lower (about 3200). The results show that Au@r-GO/TiO2 yolk@shell hybrids have lowest recombination rate of electrons and holes, which can be explained in the way that the electrons are excited from the valence band to the conduction band of TiO2, and then transfer from TiO2 to r-GO.40 This suggests an efficient charge separation in Au@r-GO/TiO2 yolk@shell hybrids, suppressing recombination of electrons and holes, which is believed to beneficial for improvement in photocatalytic activities photocatalysts. 3.3. Photocatalytic Performances of Au@r-GO/TiO2 Yolk@Shell Hybrids. It is believed that the mesoporous shells consisting of TiO2 and r-GO with a single Au nanoparticle inside formed yolk@shell hybrids may have the potential to enhance their visible light photocatalytic performances. The catalytic activity of Au@r-GO/TiO2 yolk@shell hybrids was firstly


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determined by photodegradation of RhB under visible light irradiation. The adsorptivity of RhB on catalysts before irradiations were measured by monitoring the adsorption intensity of the characteristic RhB peak at 553 nm. As displayed in Figure 5, TiO2 hollow spheres show better adsorptivity than that of commercial P25. The introduction of Au in TiO2 shows negligible increase in adsorptivity, whereas the introduction of r-GO in TiO2 gives significant increase in adsorptivity. One side, r-GO itself processes large specific surface area and has a strong affinity to the organic dye molecules by п-п interactions. On the other hand, much more nanopores will be formed between r-GO and TiO2 after the introduction of r-GO, giving more adsorption sites for dyes. 40

Adsorptivity %

31% 30

20% 18%

20

10

A u@

Ti O 2 Au @

Ti O 2

0

rG O /T iO 2

4%

P2 5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 5 Adsorptivity of RhB in the presence of different photocatalysts.

Generally, the photocatalytic reactions by TiO2 are caused by photogenerated electrons and holes. Figure 6a shows the profiles of RhB concentration change with visible light irradiation time. For comparison, we have also prepared TiO2 hollow spheres, r-GO/TiO2 hollow hybrids and Au@TiO2 yolk@shell hybrids. Remarkably, Au@r-GO/TiO2 yolk@shell hybrids exhibit the best photocatalytic performance to degrade RhB molecules with degradation efficiency up to 99% within 100 min, while P25 can only convert about 35% of RhB under the same photocatalytic


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conditions. It has been reported that most photocatalytic reactions follow the LangmuirHinshelwood adsorption model,41,

42

which can be simplified to the following expression:

ln(C0/Ct) = kt, where C0 and Ct are the initial concentration and the concentration of RhB at the exposure time, t, respectively, and k is the apparent rate constant (Figure 6b). The k values for P25, TiO2 hollow spheres, r-GO/TiO2 hollow hybrids, Au@TiO2 yolk@shell hybrids and Au@rGO/TiO2 yolk@shell hybrids are 0.004, 0.006, 0.015, 0.018 and 0.027 min-1, respectively. The photocatalytic activities of catalysts towards RhB degradation are in the following order: Au@rGO/TiO2 > Au@TiO2 > r-GO/TiO2 > TiO2 > P25. Under simulate daylight irradiations, 99.6% RhB was degraded within 50 min by Au@r-GO/TiO2 yolk@shell hybrids (Figure 6c). The relative photocatalytic activity for degradation of RhB in the presence of catalysts follows the same orders as under visible light irradiation (Figure 6d). The photocatalytic activities of Au@rGO/TiO2 yolk@shell hybrids are superior to r-GO/AuNPs/TiO2 hybrid shells27 in RhB degradation. The QE for Au@r-GO/TiO2 yolk@shell hybrids photocatalysts under visible light and simulated daylight irradiations is calculated to be 0.95% and 3.4%, respectively. Furthermore, the TOC values of RhB solution using Au@r-GO/TiO2 yolk@shell hybrids photocatalysts under different visible light irradiation times were also conducted (Table S2). It is seen that TOC removal proceeds much more slowly than the optical color change.43 The apparent decrease in TOC values of RhB solution under visible irradiation indicates that most RhB molecules were degraded into CO2 using Au@r-GO/TiO2 yolk@shell hybrids photocatalysts.


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4

(a)

1.0

-ln(Ct/C0)

0.8

t 0 Ct/C0

0.6 0.4

P25 TiO2

0.2

k 0.00451 0.00573 0.01475 0.01801 0.02749

3 2

(b)

2

R 0.9487 0.9712 0.9916 0.9791 0.9981

1

r-GO/TiO2 Au@TiO2

0.0

Au@r-GO/TiO2

0

20

40

0

60

80

100

0

20

t / min

40

60

80

100

t / min 7

(c)

1.0

P25 TiO2 Au@TiO2 Au@r-GO/TiO2

0.6 0.4

-ln(Ct/C0)

0.8

k 0.01405 0.01430 0.04618 0.05242 0.09715

6

r-GO/TiO2

Ct/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5 4

(d)

2

R 0.98708 0.95676 0.98228 0.99501 0.96953

3 2

0.2

1

0.0

0 0

10

20

30

40

50

0

10

20

30

40

50

t / min

t / min

Figure 6 Evolution of RhB concentration with reaction time under (a) visible light and (c) simulated daylight irradiations in the presence of P25, TiO2 hollow spheres, r-GO/TiO2 hollow hybrids, Au@TiO2 yolk@shell hybrids and Au@r-GO/TiO2 yolk@shell hybrids photocatalysts. The apparent reaction rate constant with (b) visible light and (d) simulated daylight irradiation time in the presence of P25, TiO2 hollow spheres, r-GO/TiO2 hollow hybrids, Au@TiO2 yolk@shell hybrids and Au@r-GO/TiO2 yolk@shell hybrids photocatalysts.

Figure S4 reveals the photocatalytic activity variation with different TiO2 shell thickness. It is found that the best photocatalytic activity comes from Au@r-GO/TiO2 yolk@shell hybrids with complete shells in thickness of 20 nm. Increase in shell thickness from 20 to 50 nm leads to


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decreased photocatalytic activity. The migration distance of photoinduced electron-hole pairs to the catalyst surfaces will increase with TiO2 shell thickness, which results in decreased photocatalytic activity. As for Au@r-GO/TiO2 yolk@shell hybrids with broken shells in thickness of about 15 nm, the photocatalytic activity is also decreased. The incomplete TiO2 shell may lead to release of Au nanoparticles, which is not beneficial for efficient photogenerated electrons transfer between Au and TiO2. Moreover, the photocatalytic stability of Au@r-GO/TiO2 yolk@shell hybrids was examined for degradation of RhB. The photocatalytic activity under visible light irradiation is slightly attenuated after six consecutive photocatalytic experiments, but still keeps high photocatalytic performance (Figure S5). In addition, Au@r-GO/TiO2 yolk@shell hybrids were also applied as efficient visible light photocatalysts for degradation of other organic dyes, such as methylene blue and methyl orange (Figure S6). Currently, H2 has been established to be one of the most clean, sustainable, and alternative fuels because of its high energy density and environmentally-friendly product from water.44-46 Herein, Au@r-GO/TiO2 yolk@shell hybrids were used for photocatalytic H2 production under visible light irradiation in the present of methanol, a sacrificial hole scavenger. Figure 7 shows the photocatalytic H2 production performances of P25, TiO2 hollow spheres, r-GO/TiO2 hollow hybrids, Au@TiO2 yolk@shell hybrids, and Au@r-GO/TiO2 yolk@shell hybrids. As shown in Figure 7a, P25 and TiO2 hollow spheres show very weak photocatalytic activity in H2 generation. In comparison with P25 and TiO2, r-GO/TiO2 hollow hybrids and Au@TiO2 yolk@shell hybrids show dramatically enhanced photocatalytic activity. As for Au@r-GO/TiO2 yolk@shell hybrids, the H2 evolution rate increased to 309 µmol h-1 g-1 with 0.87% apparent quantum efficiency, which is much higher (1.8 times) than that of Au@TiO2 yolk@shell hybrids (167 µmol h-1 g-1),


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and is superior to most reported Au/TiO2-based photocatalysts (Table S1). In comparison with reported ternary hybrids,47 our Au@r-GO/TiO2 hybrids with unique yolk@shell structure exhibit better performance for H2 production. When the H2 production reactions are conducted under simulated daylight irradiation, most photocatalysts show enhanced photocatalytic activity in H2 production (Figure 7b). The photocatalytic H2 production activity follows the same order as in the case of visible light irradiation. The highest photocatalytic activity is obtained from Au@rGO/TiO2 yolk@shell hybrids with the H2 production rate of 462 µmol h-1 g-1 with 2.5% apparent

H2 production (umol h-1 g-1)

quantum efficiency, which is exceeded by 32 times than that of TiO2 hollow spheres.

500

300

(a)

400

(b)

300 200

200

100

20

/T iO 2 Au @ rG O

Ti O 2

0

2

10

Au @

iO 2 Au @ rG O /T

Ti O 2 Au @


Ti O 2

P2 5

0

rG O /T iO 2

Trace

rG O /T iO

Trace

30

Ti O 2

100

P2 5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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300

200

100

0 0

4

8

12

16

20

24

Time (h)

Figure 7 Comparison of the photocatalytic production of H2 from methanol aqueous solutions obtained for P25, TiO2 hollow spheres, r-GO/TiO2 hollow hybrids, Au@TiO2 yolk@shell


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hybrids and Au@r-GO/TiO2 yolk@shell hybrids under (a) visible light irradiation and (b) simulated daylight irradiations. (c) The recyclability of Au@r-GO/TiO2 yolk@shell hybrids as photocatalysts in H2 production under visible light irradiation.

Additionally, the stability of photocatalysts was tested by cycle photocatalytic H2 production experiments (Figure 7c). After the six cycles, no significant decay in the photocatalytic activity is observed in the investigation, suggesting that Au@r-GO/TiO2 yolk@shell hybrids have good stability in photocatalytic H2 production. Their excellent photocatalytic activity of Au@r-GO/TiO2 yolk@shell hybrids under visible light irradiation can be rationalized as illustrated in Scheme 1. Under visible light irradiation, Au nanoparticles are photoexcited owing to its surface plasmonic resonance, and then charge separation is accomplished, where the electrons transfer from Au nanoparticles to TiO2 conduction band leading to the generation of holes remained in Au nanoparticles.48-51 The excited electrons are captured by oxygen molecular which adsorbed on the TiO2 surface, and then produces superoxide anion radicals (·O2-). At the same time, the oxidized Au species accept electron which are generated from H2O or hydroxide ions adsorbed on the TiO2 surface or from the dye molecules in the solution, and then react with H2O or hydroxide ions produces hydroxyl radicals (·OH). Such radicals (·OH and ·O2-) are powerful oxidizing species for the degradation of RhB molecules. Moreover, the electrons on the conduction band of TiO2 further transfer to the r-GO nanosheets, due to the redox potential of graphene/graphene- is slightly lower than the conduction band level of the TiO2. The highly electron mobility on r-GO is due to its excellent electric conductivity.52-54 Therefore, both the r-GO surface and the conduction band of TiO2 can functional used as the active sites for H2 production. Consequently, the Au@r-GO/TiO2


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yolk@shell hybrids show the best photocatalytic activity in H2 production. In addition to the synergistic effect from ternary hybrids, the accessible mesoporous TiO2 shells with superior adsorption ability, which come from the novel yolk@shell structure, may also lead to increased photocatalytic activity of Au@r-GO/TiO2 yolk@shell hybrids.

Scheme 1 Possible charge transfer and separation mechanisms in Au@r-GO/TiO2 yolk@shell hybrids for photocatalytic dye degradation and H2 production under visible light irradiation.

4. CONCLUSION In summary, Au@r-GO/TiO2 yolk@shell hybrids with mesoporous shells have been successfully fabricated through sol-gel coating processes. Results from catalytic investigations of Au@r-GO/TiO2 yolk@shell hybrids towards the degradation of RhB and H2 production indicate that the introduced of r-GO and Au nanoparticles can remarkably improve the photocatalytic performances. The superior photocatalytic performances of Au@r-GO/TiO2 yolk@shell hybrids clearly indicate their potential as powerful visible light photocatalysts. It is believed that the proposed strategy gives a promising route for construction other yolk@shell structured noble metal@r-GO/TiO2 hybrids for broader applications.


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ASSOCIATED CONTENT Supporting Information. Additional figures giving detailed material characterizations. This information is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J. Han); [email protected] (R. Guo) ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (21273004 and 41472034) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. We would also like to acknowledge the technical support received at the Testing Center of Yangzhou University.


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(51) Yu, J. G.; Ran, J. R. Facile Preparation and Enhanced Photocatalytic H2-Production Activity of Cu(OH)2 Cluster Modified TiO2 Energy Environ. Sci., 2011, 4, 1364-1371. (52) Xiang, Q. J.; Yu, J. G.; Jaroniec, M. Enhanced Photocatalytic H2-Production Activity of GrapheneModified Titania Nanosheets Nanoscale, 2011, 3, 3670-3678. (53) Ding, D. W.; Liu, K.; He, S. N.; Gao, C. B.; Yin, Y. D. Ligand-Exchange Assisted Formation of Au/TiO2 Schottky Contact for Visible-Light Photocatalysis Nano Lett, 2014, 14, 6731-6736. (54) Ngaw, C. K.; Xu, Q. C.; Tan, T. T. Y.; Hu, P.; Cao, S. W.; Loo, J. S. C. A Strategy for In-Situ Synthesis of Well-defined Core-shell Au@TiO2 Hollow Spheres for Enhanced Photocatalytic Hydrogen evolution Chem. Eng. J., 2014, 257, 112-121.


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Langmuir

TOC Graphical


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TiO2 Hybrids as Powerful

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