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Research Article pubs.acs.org/journal/ascecg

Synthesis of TiO2@WO3/Au Nanocomposite Hollow Spheres with Controllable Size and High Visible-Light-Driven Photocatalytic Activity Jiabai Cai,†,‡ Xueqing Wu,‡ Shunxing Li,*,‡,§ and Fengying Zheng‡,§ †

College of the Environment and Ecology, Xiamen University, Xiamen 361102, People’s Republic of China College of Chemistry and Environment, Minnan Normal University, Zhangzhou 363000, People’s Republic of China § Fujian Province Key Laboratory of Modern Analytical Science and Separation Technology, Minnan Normal University, Zhangzhou 363000, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: A new nanocomposite was reported as a goodperformance photocatalyst, i.e., double-shelled, positively and negatively charged, nanostructured hollow spheres with supported Au nanoparticles (NPs). TiO2, WO3, and Au NPs were coated successively onto the functionalized polystyrene (PS) template spheres. The as-synthesized product PS@ TiO2@WO3/Au nanocomposites were calcined at elevated temperature and then intact double-shelled TiO2@WO3/Au hollow spheres were obtained. The dispersity, morphology, size, and lattice of TiO2@WO3/Au hollow spheres were investigated by SEM and TEM. The presence of TiO2 hollow sphere and WO3/Au shell was proved by HAADF-STEM and XRD images. The photodegradation activity for rhodamine B and trimesic acid (i.e., color and colorless aromatic pollutants) in decreasing order were TiO2@WO3/Au, TiO2−WO3, P25. Under visible-light irradiation, the photodegradation rate of rhodamine B and trimesic acid for TiO2@WO3/Au was 94% and 95%, respectively, which exhibited a significant increase of 62% and 80% as compared with P25. The synergistic effect of coupling TiO2 hollow spheres with WO3 shell and Au NPs on photocatalytic performance was proved by this article. First, Au NPs deposited in WO3 shell and loaded on TiO2 shell separately act as electron trap site and surface plasmon resonance-sensitizer, respectively, and hence the photogenerated electron−hole separation rate was improved. Second, the visible-light absorption of TiO2 hollow spheres was increased by the coexistence of WO3 and Au and unique hierarchical mesoporous architectures of TiO2@WO3/Au. Finally, the surface charge of TiO2@WO3/ Au and rhodamine B was negative and positive, respectively, the affinity between them could be improved by electrical attractions, and then the major bottleneck in heterogeneous photocatalysis (i.e., poor affinity between pollutants and photocatalyst) could be broken. The optimal hollow sphere size of TiO2@WO3/Au was 450 nm, which was proved by the photodegradation of aromatic pollutants and photoreduction of Cr(VI). KEYWORDS: Double-shelled nanocomposite, Positively and negatively charged, Visible-light-driven photocatalysis

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INTRODUCTION As an environmentally friendly and cost-effective technology, visible-light-driven photocatalysis for organic pollutant removal has been extensively researched.1−3 The photocatalytic activity of nanostructured photocatalysts depends on their structure and morphology.4−6 The main serious obstacles in developing heterogeneous photocatalysis include poor affinity between pollutants and photocatalyst,7 rapid recombination of photogenerated electron−hole pairs,8 low-light-harvesting efficiency of photocatalyst,9 and short light residence time on/in the photocatalysts.10,11 As an efficient structure,12,13 hollow sphere has been used for metal oxide-based photocatalyst for pollution removal,14 because of its unique properties, including high surface area, low density, and good light-harvesting efficiency.15 © 2016 American Chemical Society

Besides, the size and geometrical shape of hollow sphere can be controlled by using diverse templates. When the inside and outside surface charge on hollow spheres is different, both anionic and cationic pollutants can be adsorbed onto different surfaces, respectively, i.e., the affinity between pollutants and photocatalyst can be enhanced by the electrical attraction.16 Gold nanoparticles (Au NPs) supported on semiconductors show strong visible-light absorption due to surface plasmon resonance (SPR), so supported Au materials have been used as visible-light-responding photocatalysts for organic pollutant Received: November 17, 2015 Revised: January 9, 2016 Published: January 26, 2016 1581

DOI: 10.1021/acssuschemeng.5b01511 ACS Sustainable Chem. Eng. 2016, 4, 1581−1590

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ACS Sustainable Chemistry & Engineering photodegradation.17,18 The introduction of cocatalyst (e.g.,Au NPs) into photacatalyst is an efficient way to enhance the photodegradation activity because the cocatalyst works as an active site for O2 reduction. WO3 shows better visible-light absorption due to its smaller band gap (2.7 eV).19 However, the photocatalytic activity of WO3 is not enough for oxygen reduction. Therefore, WO3 should be combined with other photocatalyst for accelerating the photogenerated electron−hole pair separation of WO3. As a photocatalyst, TiO2 has been widely used for its relatively high photocatalytic activity, wide band gap (3.0−3.2 eV), costeffectivity, low toxicity, and enough chemical stability.20−22 Therefore, WO3−TiO2 nanocomposites with various morphologies have been used for photocatalysis, including core−shelled WO3−TiO2 spheroids,23,24 WO3−TiO2 nanorod arrays,25,26 TiO2−WO3 nanotubes,27−29 WO3−TiO2 films,30,31 WO3− TiO2 nanowires,32 and single-shelled WO3−TiO2 hollow spheres.33 The isoelectric point difference between WO3 (0.4) and TiO2 (6.2) is large enough, WO3 shell and TiO2 shell can obtain positively and negatively charged, respectively, in a suitable pH range.16 Lastly, coupling of the band gap excitation of WO3 ( 420 nm, use a cutoff filter of 420 nm (UVIRCUT420, CEAULIGHT), see in Supporting Information (SI) 1582


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ACS Sustainable Chemistry & Engineering Figure S1) at room temperature (25 °C). The light was placed 12 cm above the liquid surface. A light dynamometer (TES-1333 solar power meter, China) was used to measure the photon flux reaching the reactor, which was 2.62 ± 0.05 mW cm−2. The temperature of the reaction system was maintained as 25 °C ± 2 °C with cooling air and water. After photodegradation for different time intervals, the samples were collected and filtered, followed by the pollutant concentration determination by spectrophotometry. The degradation rate was calculated using eq 2.

degradation rate =

(C0 − C) × 100% C0

(see SI Figures S3, S4, and S5), only the stretch vibration bands of W−O and O−Ti band were observed in TiO2@WO3/Au hollow spheres, so the PS core was removed after calcination at 550 °C. The nanocomposite of TiO2@WO3/Au hollow sphere was composed of TiO2 shell and WO3 shell with supported Au NPs, and the atomic ratio of Ti, W, and Au in TiO2@WO3/Au were 25.92, 8.64 and 0.29, respectively. The atomic ratio of Ti and W in single-shelled TiO2−WO3 hollow spheres were 25.10 and 16.68, respectively (see SI Figure S6). The hollow sphere size of TiO2@WO3/Au hollow spheres could be easily controlled by the choice of PS sphere size. After calcination, the ultima diameter of TiO2@WO3/Au nanocomposite was about 370, 450, and 600 nm, respectively, which could be confirmed by the SEM and the size distribution histogram (see Figure 1). TEM images confirmed a hollow in the TiO2@ WO3/Au microspheres. In agreement with SEM data (Figure 2a), the TEM images (Figure 2b,c) confirmed that the produced TiO2@WO3/Au were hollow and porous. The TEM image of TiO2@WO3/Au (Figure 2c) revealed the presence of individual nanoparticles on a thick and layered shell, and nanostructured hollow spheres with supported Au NPs. The morphology and structure of single-shelled hollow spheres of WO3, TiO2, TiO2−WO3, and double-shelled TiO2@WO3/Au hollow spheres were characterized by TEM (see SI Figure S7). The formation of the junction between TiO2 and WO3 could be confirmed by the HRTEM image (Figure 2d). Four types of lattice spacings (i.e., 0.351, 0.384, 0.376, and 0.363 nm) were showed. Corresponding to the (101) planes of anatase TiO2, the (002), (020), and (200) crystallographic planes of WO3. The polycrystalline nature was also confirmed by the SAED measurements (Figure 2e). According to the blue, orange and green colored areas in HAADF-STEM mapping image (Figure 2I), the TiO2@WO3/ Au hollow sphere was comprised of “Au”, “Ti”, and “W” enriched area, i.e., Au NPs, TiO2 shell, and WO3 shell. In the elemental mapping and line scanning images of TiO2@WO3/ Au (inset Figure 2b and Figure 3), the distribution of Au, Ti and W was hierarchical structure, revealing that TiO2@WO3/ Au was double-shelled hollow sphere. This result was quite different from the elemental mapping images of single-shelled TiO2−WO3 hollow sphere (see SI Figure S8). The hollow spheres of TiO2, TiO2−WO3, and TiO2@WO3/ Au were analyzed by powder XRD using Cu Kα radiation (see Figure 4). Six peaks, including 2θ = 25.22, 37.78, 47.94, 54.15, 54.96, and 62.69, were indexed to the (101), (004), (200), (211), (105), and (204) planes of TiO2, which corresponded to the standard pattern of anatase (JCPDS No. 21-1272) (see Figure 4). The 2θ peaks were in good agreement with the diffraction from the (002), (020), (200), (120), (112), (022), (202), (122), (222), (132), (312), (312), (004), (040), (140), (114), (024), (313), (204), and (420) crystallographic planes of hexagonal monoclinic phase of WO3 (JCPDS NO. 43− 1035). So the nanoparticles in TiO2 shell and WO3 shell were pure anatase and hexagonal monoclinic WO3 phase, respectively. The weak peak at 38.2 showed the existence of a small amount of Au (JCPDS No. 1-1172) in the sample. The XPS spectrum of TiO2@WO3/Au hollow sphere is shown in Figure 5. The XPS peaks of Ti 2p3/2 and Ti 2p1/2 were located at 458.5 and 464.2 eV with a good symmetry (Figure 5a), indicating that there was Ti4+ in octahedral coordination with oxygen.37 The peaks located at 529.8 and 531.2 eV were attributed to the oxygen in the TiO2 crystal lattice (OL) and chemisorbed water, respectively. The peak at

(2)

C0 and C are the pollutant concentrations before and after visible-light photodegradation, respectively. The first-order kinetic (eq 3) was used to fit the experimental data:

⎛C ⎞ ln⎜ 0 ⎟ = kapp × t ⎝C⎠

(3) 35

kapp is the reaction rate constant and t is the reaction time. Photoreduction of Cr(VI) Measurements. A stock solution of Cr(VI) (100 mg/L) was prepared by dissolving K2Cr2O7 in ultrapure water, which was further diluted to 5 mg/L and used for photoreduction. The photocatalysts (TiO2(370 nm)@WO3/Au, TiO2(450 nm)@WO3/Au, TiO2(600 nm)@WO3/Au, 30 mg) with Cr(VI) (4.8 μmol, pH = 4.03) in a 100 mL quartz reactor were stirred in the dark for 2 h to reach its adsorption/desorption equilibrium and then their photoreduction performances were tested using same conditions as photodegradation. The concentration of Cr(VI) in aqueous solution was determined by colorimetric method. After 1,5diphenycarbazide was added, the absorbance of color change was measured at 540 nm by using a UV−vis spectrophotometer.36 Electrochemical Performance Measurements. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were carried out on an electrochemical workstation (CHI 650D, Shanghai Chen Hua Instruments Co., China), using a conventional threeelectrode system with a bare glassy carbon electrode (GCE, Φ = 2 mm) or modified GCE, a standard saturated Ag/AgCl and a platinum wire as the working, reference and counter electrode. The EIS and CV were carried out in the frequency range of 10−2 to 105 Hz with a voltage amplitude of 10 mV at a dc bias of 0.3 V (vs the SCE) in a 1 mmol/L Fe(CN)63−/Fe(CN)64− (1:1) electrolyte solution.

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RESULTS AND DISCUSSION Morphology and Structure of Double-Shelled TiO2@ WO3/Au Hollow Spheres. Using methyl acrylate and sodium persulfate as surface active agents, smooth, anionic monodispersed PS spheres with diameters of 356, 440, and 587 nm were prepared (see SI Figure S2). The TEM images showed a complete, uniform, and smooth TiO2 shell, WO3 shell and Au NPs were formed and coated onto different sizes of PS (Scheme 2 and Figure 1). Because of abundant surface hydroxyl groups, W6+ cations from WCl6 could be absorbed and then WO3 was formed directly onto the surface of PS@TiO2 microspheres, resulting in the double-shell structure. During this transformation, Au NPs could be doped onto the WO3 shell. According to the results of TG, FTIR and Raman spectra Scheme 2. General “Template + Sol−Gel + Calcination” Method for Preparing the Double-Shell TiO2@WO3/Au

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Figure 1. SEM images,TEM images and the size distribution histogram with diverse sizes of (a, d, g) double-shelled TiO2@WO3/Au hollow spheres (370 nm), (b, e, h) double-shelled TiO2@WO3/Au hollow spheres (450 nm), (c, f, i) double-shelled TiO2@WO3/Au hollow spheres (600 nm).

Figure 2. (a) SEM images of double-shelled TiO2@WO3/Au hollow spheres, (b and c) TEM images of double-shelled TiO2@WO3/Au hollow spheres (inset: line scanning image of double-shelled TiO2@WO3/Au hollow spheres), (d) HRTEM images of double-shelled TiO2@WO3/Au hollow spheres, (e) SAED images of double-shelled TiO2@WO3/Au hollow spheres, (f) HAADF-STEM mapping image of double-shelled TiO2@ WO3/Au hollow spheres.

532.5 eV might be assigned to the oxygen in the W−O−Ti bond (Figure 5b). Figure 5c showed the peaks of W 5p3/2, W 4f3/2, and Wf7/2 from XPS, which corresponded to the WO3

layers in the TiO2@WO3/Au hollow spheres. The XPS spectra of TiO2@WO3/Au (Figure 5d) showed the peaks of Au 4f7/2 and Au 4f5/2 centered at 84.2 and 87.8 eV, respectively. The 1584


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Figure 3. TEM images and EDX of double-shelled TiO2@WO3/Au with different hollow sphere size (inset: line scanning image of double-shelled TiO2@WO3/Au hollow spheres).

electron−hole separation rate was improved, i.e., Au NPs was the crucial factor for the electron transfer. According to the diffuse reflectance UV−vis spectra of the prepared hollow sphere (Figure 6, Figure S9), the absorption edges of pure TiO2 and WO3 hollow spheres were 400 and 500 nm, respectively, corresponding to the band gap of 3.0 and 2.7 eV. These results were similar to those reported in the literatures.39,40 Because the nanoparticles of TiO2 and WO3 in the shell wall of TiO2−WO3 and TiO2@WO3/Au hollow sphere were anatase and hexagonal monoclinic WO3 phase, respectively, the absorption edge of TiO2−WO3 and TiO2@ WO3/Au hollow sphere was 430 and 470 nm, respectively. The absorption features from 500 to 600 nm were the SPR characteristics of 16 nm Au NPs encapsulated in TiO2@WO3/ Au hollow spheres. In the visible-light region, the light absorption of TiO2@WO3/Au hollow spheres was wider than TiO2 hollow sphere (see Figure 6), i.e., the visible-light harvesting efficiency of TiO2 hollow spheres was improved by the combination of WO3 shell and supported Au NPs. Evaluation of Photocatalytic and Adsorptive Activity. The specific surface area, pore volume, and average pore size of double-shelled TiO2@WO3/Au hollow spheres were 45 m2 g−1, 0.10 cm3 g−1, and 8.9 nm, respectively (see SI Figure S10 and Table S1), indicating TiO2@WO3/Au hollow spheres was a typical mesoporous material. In the pH range of 2−5, the coexistence of positive and negative surface charges on the outer and inner shell of TiO2@WO3/Au hollow spheres16 could enhance the affinity between photocatalyst and pollutants

Figure 4. XRD pattern of hollow TiO2, hollow TiO2−WO3, and hollow TiO2@WO3/Au microspheres.

spin energy separation of 3.6 eV was in good agreement with the reported data of Au 4f7/2 and Au 4f5/2 in Au NPs.38 This demonstrated that Au particles were deposited into the WO3 shell in TiO2@WO3/Au composites. Au NPs deposited in WO3 shell and loaded on TiO2 shell could separately act as electron trap site and SPR-sensitizer, hence the photogenerated 1585


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Figure 5. XPS peaks of Ti 2p (a), O 1s (b), W 5p and W 4f (c), and Au 4f (d) of TiO2@WO3/Au hollow spheres.

and double-shelled TiO2@WO3/Au hollow spheres with different hollow sphere size, were used for rhodamine B photodegradation under visible-light irradiation for 60 min. The results are shown in Figure 8, Table 1, and Table S2. For further proved the photocatalytic activity of nanomaterials, the same procedure was also performed for trimesic acid, which is a colorless aromatic pollutant. The photocatalytic activity of TiO2@WO3/Au was better than P25. The photodegradation rate of rhodamine B and trimesic acid for TiO2(370 nm)@ WO3/Au was 94% and 95%, respectively, which exhibited an increase of 62% and 80% compared with P25. Meanwhile, the linear relationship between ln(C0/C) and t indicated that the photodegradation reactions followed pseudo-first-order kinetics (see Figure 8 and Table 1) with rate constants (0.0097, 0.016, 0.0066, 0.027, and 0.047 min−1) for single-shelled hollow spheres of WO3, TiO2, P25, TiO2−WO3, and double-shelled TiO2@WO3/Au hollow spheres, respectively. A similar conclusion could be obtained by considering the total organic carbon content before and after the photodegradation (Table S3). The TOC reduction of rhodamine B and trimesic acid for TiO2(370 nm)@WO3/Au was 66% and 49%, respectively. The visible-light photodegradation rate of TiO2(370 nm)@ WO3/Au was better than that of TiO2−WO3, because Au NPs deposited in WO3 shell and loaded on TiO2 shell separately act as electron trap site and SPR-sensitizer, respectively. The recombination of photoinduced electrons and holes could be decreased by the junctions of WO3 with Au and the electronsink effect from the Au NPs.34 Comparison of the photocatalytic activity of different TiO2-based photocatalysts, the synergistic effect of coupling TiO2 hollow spheres with WO3 shell and Au NPs on photocatalytic performance was confirmed (see SI Table S4). The photocatalytic activity was in the order of TiO2(450 nm)@WO3/Au > TiO2(600 nm)@WO3/Au > TiO2(370 nm)@WO3/Au > TiO2−WO3 > P25. The photogenerated electron−hole separation could be significantly improved through the unique and hierarchical mesoporous

Figure 6. Diffuse reflectance UV−vis spectra of hollow WO3, hollow TiO2, hollow TiO2−WO3 and hollow TiO2@WO3/Au microspheres.

(including cationic and anionic pollutants) by electrostatic adsorption, and then the surface coverage rate of pollutants onto the photocatalyst was increased. The double-shelled TiO2@WO3/Au exhibited higher adsorptive activity than single-shelled TiO2−WO3, WO3, and TiO2. After adsorption for 80 min, 28%, 7%, 20%, and 6% of rhodamine B could be adsorbed onto double-shelled TiO2@WO3/Au, single-shelled TiO2−WO3, WO3, and TiO2, respectively (see Figure 7). The adsorption rate of double-shelled TiO2@WO3/Au exhibited a 20% increase compared with single-shelled TiO2−WO3, because of the coexistence of positive and negative surface charges. For comparison, the same procedure was also performed for other pollutants (see Figure 7). To investigate the influence of the photocatalyst structure on photocatalytic activity, seven kinds of photocatalysts, including single-shelled hollow spheres of WO3, TiO2, P25, TiO2−WO3, 1586


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Figure 7. Adsorption rate of (a) rhodamine B (a cationic pollutant), (b) methyl orange (an anionic pollutant), (c) 4-nitroaniline (a cationic pollutant), and (d) acid violet 43 (an anionic pollutant) on double-shelled TiO2@WO3/Au, single-shelled TiO2−WO3, TiO2, and WO3 hollow spheres under dark conditions.

Figure 8. Linear transform ln(C0/C) = f(t) of the kinetic curves of (a) rhodamine B and (c) trimesic acid by double-shelled TiO2@WO3/Au, singleshelled TiO2−WO3, TiO2, WO3 hollow spheres, or P25 under visible-light irradiation and (b) rhodamine B and (d) trimesic acid by TiO2@WO3/Au with different hollow sphere size under visible-light irradiation.

Cr(VI) was used as an electron acceptor.41 After visible-light irradiation for 4h, the photoreduction rate of Cr(VI) for TiO2(450 nm)@WO3/Au was 74%, which exhibited an increase of 8% and 10% compared with TiO2(370 nm)@ WO3/Au and TiO2(600 nm)@WO3/Au (see SI Figure S11).

hollow structure. The optimal hollow sphere size for TiO2@ WO3/Au as photocatalyst was 450 nm. The apparent quantum efficiency (AQE) of TiO2@WO3/Au with different hollow sphere size was evaluated by the photoreduction of Cr(VI) (initially 4.8 μmol) in aqueous suspensions (see Figure 9), i.e., 1587


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ACS Sustainable Chemistry & Engineering Table 1. Visible-Light Photodegradation Rate Constant k (min−1) for WO3, TiO2, P25 Single-Shelled TiO2−WO3 and Double-Shelled TiO2@WO3/Au Hollow Spheres k (min−1) sample

rhodamine B

trimesic acid

WO3 TiO2 P25 single-shelled TiO2−WO3 double-shelled TiO2(370 nm)@WO3/Au double-shelled TiO2(450 nm)@WO3/Au double-shelled TiO2(600 nm)@WO3/Au

0.0097 0.016 0.0066 0.027 0.047 0.065 0.052

0.0020 0.0067 0.0014 0.016 0.025 0.031 0.027

Figure 10. Rates of Cr(VI) photoreduction in aqueous suspensions by TiO2@WO3/Au with different hollow sphere size under visible-light irradiation.

was the minimum Therefore, a more effective separation of photogenerated electron−hole pairs and more fast interface charge transfer could be occurred in the TiO2(450 nm)@WO3/ Au hollow spheres. In addition, the CV results displayed in Figure 11b. The redox peaks of the TiO2(450 nm)@WO3/Au composite electrode increased obviously as compared to others, suggesting that the TiO2(450 nm)@WO3/Au hollow spheres provided appropriately electronic channel and accelerated the photogenerated electron−hole separation. As shown in the diffuse reflectance UV−vis spectra (see SI Figure S12 and S13), the band at 525−625 nm was likely coming from Au NPs, when Au NPs was encapsulated in TiO2@WO3/Au hollow spheres, its SPR peak was greatly red-shifted and affected by the hollow sphere size, its maximum was obtained in TiO2(450 nm)@ WO3/Au hollow spheres. The visible-light-harvesting wide was the best for 450 nm. The photoluminscence (PL) emission mainly resulted from the recombination of excited electrons and holes. Among the six photocatalysts, the PL intensity of TiO2(450 nm)@WO3/Au was the lowest (seen in Figure S14), which indicated its separation efficiency was the highest.43 Therefore, hollow sphere size of 450 nm was the optimal for TiO2@WO3/Au as photocatalyst. The stability of TiO2(450 nm)@WO3/Au as photocatalyst was enough, because no obvious decrease in photocatalytic activity on degradation of rhodamine B and trimesic acid was observed after three cycles (see SI Figure S15 and Figure S16).

Figure 9. Time courses of the amounts of Cr(VI) in aqueous suspensions by TiO2@WO3/Au with different hollow sphere size under visible-light irradiation.

The Cr(VI) photoreduction rates were 0.660, 0.756, and 0.624 μmol h−1 for TiO2(370 nm)@WO3/Au, Cr(VI) for TiO2(450 nm)@WO3/Au, and TiO2(600 nm)@WO3/Au, respectively (see in Figure 10). So, the optimal hollow sphere size of TiO2@ WO3/Au was 450 nm. Cr2O7 2 − + 14H+ + 6e− → 2Cr 3 + + 7H 2O AQE =

3 × the amount of Cr(VI) reduced × 100 the amount of incident photons

(4)

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(5)

CONCLUSIONS A new nanocomposite with double-shell, positive and negative charged, nanostructured hollow spheres, and supported Au nanoparticles (NPs) was designed as a high-performance visible-light-driven photocatalyst, using a sacrificial templates route. The synergistic effect of coupling TiO2 hollow spheres with WO3 shell and Au NPs on photocatalytic performance was proved by this paper. The heterogeneous photocatalytic activity could be improved by hollow sphere structure with suitable size, the shell−shell combination of two kinds of photocatalyst with different isoelectric point and band gap, and the addtion of cocatalyst (e.g., Au nanoparticles). The morphology of the hollow TiO2@WO3/Au was uniform and its synthesis procedure was simple. This photocatalyst structure was unique and novel, which efficient catalytic performance was demonstrated by the photocatalytic decomposition of organic pollutants.

To investigate further the influence of hollow sphere size on the surface electron transfer rate of TiO2@WO3/Au, the EIS technique and CV test were employed to study the solid/ electrolyte interfaces of electrode using TiO2@WO3/Au hollow spheres with different hollow sphere size (370, 450, or 600 nm). The impedance spectrum contained the sections of a semicircle and a straight line. Semicircle section was the high frequency zone and controlled by the kinetics. The diameter size of semicircle was equal to electron transfer impedance of the electrode surface, reflecting the characteristics of the electrode surface electron transfer, i.e., the charge transfer resistance in the solid state interface.42 Linear section was relatively low frequency area and controlled by the diffusion. As shown in Figure 11a, the diameter of the arc radius on the EIS Nyquist plot of the TiO2(450 nm)@WO3/Au composite electrode was the smallest, which suggested that the charge transfer resistance in the TiO2(450 nm)@WO3/Au interface 1588


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Figure 11. (a) Electrochemical impedance spectra (EIS) Nyquist plots and (b) cyclic voltammetry (CV) of sample electrodes TiO2@WO3/Au with different hollow sphere size in 1.0 mM [Fe(CN)6]3−/4−.

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(8) Li, R.; Zhang, F.; Wang, D.; Yang, J.; Li, M.; Zhu, J.; Zhou, X.; Han, H.; Li, C. Spatial Separation of Photogenerated Electrons and Holes Among {010} and {110} Crystal Facets of BiVO4. Nat. Commun. 2013, 4, 1432. (9) Huang, F.; Chen, D.; Zhang, X. L.; Caruso, R. A.; Cheng, Y. B. Dual-Function Scattering Layer of Submicrometer-Sized Mesoporous TiO2 Beads for High-Efficiency Dye-Sensitized Solar Cells. Adv. Funct. Mater. 2010, 20, 1301−1305. (10) Mubeen, S.; Lee, J.; Singh, N.; Krämer, S.; Stucky, G. D.; Moskovits, M. An Autonomous Photosynthetic Device in which all Charge Carriers Derive from Surface Plasmons. Nat. Nanotechnol. 2013, 8, 247−251. (11) Chen, W.; Fan, Z.; Zhang, B.; Ma, G.; Takanabe, K.; Zhang, X.; Lai, Z. Enhanced Visible-Light Activity of Titania Via Confinement Inside Carbon Nanotubes. J. Am. Chem. Soc. 2011, 133, 14896−14899. (12) Jing, L.; Zhou, W.; Tian, G.; Fu, H. Surface Tuning for OxideBased Nanomaterials as Efficient Photocatalysts. Chem. Soc. Rev. 2013, 42, 9509−9549. (13) Li, S.; Chen, J.; Zheng, F.; Li, Y.; Huang, F. Synthesis of the Double-Shell Anatase-Rutile TiO2 Hollow Spheres with Enhanced Photocatalytic Activity. Nanoscale 2013, 5, 12150−12155. (14) Cao, J.; Zhu, Y.; Bao, K.; Shi, L.; Liu, S.; Qian, Y. Microscale Mn2O3 Hollow Structures: Sphere, Cube, Ellipsoid, Dumbbell, and Their Phenol Adsorption Properties. J. Phys. Chem. C 2009, 113, 17755−17760. (15) Hu, J.; Chen, M.; Fang, X.; Wu, L. Fabrication and Application of Inorganic Hollow Spheres. Chem. Soc. Rev. 2011, 40, 5472−5491. (16) Li, S.; Cai, J.; Wu, X.; Zheng, F.; Lin, X.; Liang, W.; Chen, J.; Zheng, J.; Lai, Z.; Chen, T. Fabrication of Positively and Negatively Charged, Double-Shelled, Nanostructured Hollow Spheres for Photodegradation of Cationic and Anionic Aromatic Pollutants under Sunlight Irradiation. Appl. Catal., B 2014, 160, 279−285. (17) Heutz, N. A.; Dolcet, P.; Birkner, A.; Casarin, M.; Merz, K.; Gialanella, S.; Gross, S. Inorganic Chemistry in a Nanoreactor: Au/ TiO2 Nanocomposites by Photolysis of a Single-Source Precursor in Miniemulsion. Nanoscale 2013, 5, 10534−10541. (18) Cai, J. B.; Wu, X. Q.; Li, S.; Zheng, F.; Zhu, L.; Lai, Z. Synergistic Effect of Double-Shelled and Sandwiched TiO2@Au@C Hollow Spheres with Enhanced Visible-Light-Driven Photocatalytic Activity. ACS Appl. Mater. Interfaces 2015, 7, 3764−3772. (19) Chen, D.; Gao, L.; Yasumori, A.; Kuroda, K.; Sugahara, Y. Sizeand Shape-Controlled Conversion of Tungstate-Based InorganicOrganic Hybrid Belts to WO3 Nanoplates with High Specific Surface Areas. Small 2008, 4, 1813−1822. (20) Chen, X.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. Rev. 2007, 107, 2891−2959. (21) Shang, S.; Jiao, X.; Chen, D. Template-Free Fabrication of TiO2 Hollow Spheres and Their Photocatalytic Properties. ACS Appl. Mater. Interfaces 2012, 4, 860−865.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01511. TG data, FTIR spectra, Raman spectra, EDX data, SEM and TEM images, XRD pattern, UV−vis spectra, nitrogen adsorption−desorption isotherm, surface area, and porosity measurements of TiO2@WO3/Au hollow spheres, correlation coefficient R2 (PDF).

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AUTHOR INFORMATION

Corresponding Author

*S. Li. Tel.: +86 596 2591395. Fax: +86 596 2591395. E-mail: [email protected]. Author Contributions

Xueqing Wu and Jiabai Cai contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21175115 and 21475055, S.L), the Program for New Century Excellent Talents in University (NCET-11 0904, S.L).

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REFERENCES

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