Design and Synthesis of TiO2 Hollow Spheres with Spatially ... - MDPI

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Design and Synthesis of TiO2 Hollow Spheres with Spatially Separated Dual Cocatalysts for Efficient Photocatalytic Hydrogen Production Qianqian Jiang 1 , Li Li 1 , Jinhong Bi 1,2, *, Shijing Liang 1,2, * and Minghua Liu 1 1 2

*

Department of Environmental Science and Engineering, Fuzhou University, Minhou 350108, China; [email protected] (Q.J.); [email protected] (L.L.); [email protected] (M.L.) State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou 350002, China Correspondence: [email protected] (J.B.); [email protected] (S.L.); Tel.: +86-591-2286-6070 (J.B.)

Academic Editors: Hongqi Sun and Zhaohui Wang Received: 8 November 2016; Accepted: 17 January 2017; Published: 25 January 2017

Abstract: TiO2 hollow spheres modified with spatially separated Ag species and RuO2 cocatalysts have been prepared via an alkoxide hydrolysis–precipitation method and a facile impregnation method. High-resolution transmission electron microscopy studies indicate that Ag species and RuO2 co-located on the inner and outer surface of TiO2 hollow spheres, respectively. The resultant catalysts show significantly enhanced activity in photocatalytic hydrogen production under simulated sunlight attributed to spatially separated Ag species and RuO2 cocatalysts on TiO2 hollow spheres, which results in the efficient separation and transportation of photogenerated charge carriers. Keywords: TiO2 hollow spheres; dual cocatalysts; spatial separation; hydrogen production

1. Introduction Semiconductor photocatalysis as a green technology has attracted much interest for the application in solving environmental pollution and energy shortage [1–5]. Since the photolysis of water to produce hydrogen was discovered, TiO2 has been most investigated in photocatalysis due to the chemical stability, nontoxicity, and low price [5–8]. However, the drawbacks of TiO2 , such as the low utilization of sunlight, the rapid recombination of the photogenerated charges, and few suitable active sites, extremely limit photocatalytic performance. Tuning the morphology and structure of TiO2 with expectations of achieving novel or enhanced properties have been regarded as an efficient way to overcome the drawbacks—for instance, the fabrication of TiO2 nanospheres, nanorods, nanowires, and nanobelts [9–13]. Especially, the submicron-scale hollow spheres of TiO2 are promising because they can provide large specific surface areas and enhance light scattering properties and their inner and outer surfaces can be controlled and selectively functionalized [14]. Moreover, the photocatalytic properties of TiO2 hollow spheres can be modified by loading cocatalysts [15–17], which can serve as reaction sites and provide the trapping sites for the photogenerated carriers of the surface. The internal electric field can be formed between the cocatalyst and the photocatalyst due to the different Fermi level, which can promote the directional migration of photogenerated electrons and holes and prohibit the recombination of the photogenerated carriers [18]. The space locations of the cocatalysts loaded on the photocatalytic materials can greatly affect the migration of the photogenerated carriers and then further affect the photocatalytic activity. The oxidation cocatalyst and reduction cocatalyst loaded on different spatial locations of the photocatalysts may produce spatially separated reaction sites with oxidizing and reducing abilities, respectively, which consequently lead to the directional migration of photogenerated electrons and

Nanomaterials 2017, 7, 24; doi:10.3390/nano7020024

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The space locations of the cocatalysts loaded on the photocatalytic materials can greatly affect the migration of the photogenerated carriers and then further affect the photocatalytic activity. The oxidation cocatalyst and reduction cocatalyst loaded on different spatial locations of the photocatalysts may produce spatially separated reaction sites with oxidizing and reducing abilities, respectively, which and Nanomaterials 2017, 7, 24 consequently lead to the directional migration of photogenerated electrons2 of 10 holes and thus prohibit the recombination of photogenerated carriers. Domen et al. have demonstrated that SiO2/Ta3N5 core/shell structures with spatially separated cocatalysts show holes and thus prohibit the recombination of photogenerated carriers. Domen et al. have demonstrated superior photocatalytic activity [19]. Li et al. have reported that reduction cocatalysts (MoS2, NiS, WS2, that SiO2 /Ta3 N5 core/shell structures with spatially separated cocatalysts show superior photocatalytic etc.) and oxidation cocatalysts (IrOx, MnOx, RuOx, etc.) can be selectively deposited on the (010) and activity [19]. Li et al. have reported that reduction cocatalysts (MoS2 , NiS, WS2 , etc.) and oxidation (110) facets of BiVO4, respectively, which results in much higher photocatalytic activity compared to cocatalysts (IrOx , MnOx , RuOx , etc.) can be selectively deposited on the (010) and (110) facets of BiVO4 , that with randomly distributed cocatalysts [20,21]. In general, the noble metals (Au, Ag, Pt, Pd, etc.), respectively, which results in much higher photocatalytic activity compared to that with randomly MoS2, and graphene exhibiting superior electron mobility often act as reduction cocatalysts to distributed cocatalysts [20,21]. In general, the noble metals (Au, Ag, Pt, Pd, etc.), MoS2 , and graphene improve the efficiency of photoproduction electron migration [7,22–25]. The cocatalysts such as RuO2, exhibiting superior electron mobility often act as reduction cocatalysts to improve the efficiency of IrO2, CoOx, and MnOx can act as hole collector [26–29]. Loading the reduction and oxidation catalysts photoproduction electron migration [7,22–25]. The cocatalysts such as RuO2 , IrO2 , CoOx , and MnOx on the inner and outer surfaces of TiO2 hollow spheres can be expected to achieve enhanced can act as hole collector [26–29]. Loading the reduction and oxidation catalysts on the inner and outer photocatalytic activity. surfaces of TiO2 hollow spheres can be expected to achieve enhanced photocatalytic activity. Herein, we we report report aa facile facile synthesis synthesis of TiO TiO22 hollow spheres modified with spatially separated separated Ag Herein, species and and RuO RuO22 on the inner and outer surfaces of the TiO22 hollow spheres (as shown in Scheme 1), species which exhibited exhibited enhanced enhanced photocatalytic photocatalytic hydrogen hydrogenproduction productionunder undersolar solarlight lightirradiation. irradiation. which

Scheme in the Scheme 1. 1. Processes Processes involved involved in the formation formation of of dual dual cocatalysts cocatalysts co-loading co-loading on on the the TiO TiO22 hollow hollow spheres (THS). spheres (THS).

2. 2. Experimental Experimental Section Section 2.1. Preparation of Catalysts 2.1. Preparation of Catalysts 2.1.1. Synthesis of Carbon Spheres (C Sphere) 2.1.1. Synthesis of Carbon Spheres (C Sphere) In a typical synthesis of carbon spheres, glucose (6 g) was dissolved into deionized water (60 mL) In a typical synthesis of carbon spheres, glucose (6 g) was dissolved into deionized water to form a clear solution and then was transferred into a 100 mL Teflon-lined autoclave and was reacted (60 mL) to form a clear solution and then was transferred into a 100 mL Teflon-lined autoclave and at 180 ◦ C for 12 h. The obtained brown product was collected and washed with deionized water and was reacted at 180 °C for 12 h. The obtained brown product was collected and washed with deionized ethanol and then dried at 80 ◦ C. Finally, carbon spheres (denoted as C sphere) were obtained [30]. water and ethanol and then dried at 80 °C. Finally, carbon spheres (denoted as C sphere) were obtained [30]. of TiO2 Hollow Spheres (THS) 2.1.2. Synthesis amountof of TiO 0.4 g2 Hollow of C sphere was added 2.1.2.An Synthesis Spheres (THS) to 30 mL of pure ethyl alcohol. The obtained suspended solution was stirred for 30 min and then was dispersed under ultrasonic conditions for 30 min. amountconsisting of 0.4 g of was ethyl addedalcohol, to 30 mL ethyl alcohol. obtained Then,An a solution of C 70sphere mL of pure 0.2 gofofpure hexadecyl trimethylThe ammonium suspended solution was stirred for 30 min and then was dispersed under ultrasonic conditions for 30 bromide (CTAB), and 0.6 mL of deionized water was added and stirred for 2 h. After that, 23 mL of min. Then, a solution consisting of 70 mL of pure ethyl alcohol, 0.2 g of hexadecyl trimethyl a tetrabutyl titanate ethanol solution was added dropwise while stirring. The obtained suspended ammonium (CTAB), 0.6 mL of flask. deionized wascondensation added and stirred forfor2 100 h. After solution was bromide transferred into theand three-necked After water the reflux at 85 ◦ C min, that, 23 mL of a tetrabutyl titanate ethanol solution was added dropwise while stirring. The obtained the prepared product was collected, washed with deionized water and ethyl alcohol, and dried at suspended solution was transferred into the three-necked flask. reflux condensation at 85and °C ◦ C. The dried 60 powders were further calcined at 500 ◦ C for 2 h After with athe ramping rate of 2 ◦ C/min for 100 min, the prepared product was collected, washed with deionized water and ethyl alcohol, and TiO2 hollow spheres were then obtained and are denoted as THS [30]. dried at 60 °C. The dried powders were further calcined at 500 °C for 2 h with a ramping rate of 2 °C/min and TiO hollow spheres then obtained are denoted THS [30]. 2.1.3. Synthesis of 2Ag-Loaded TiO2 were Hollow Spheres on and the Inner Surfaceas (Ag-I-THS) The loading Ag on the inner surface of THS included two steps. In the first step, Ag-loaded C sphere was prepared by an impregnation method [31]. An amount of 0.4 g of the above synthesized carbon spheres were impregnated in a 0.4 mL silver nitrate solution (10 mg/mL) and then dried at 80 ◦ C for 2 h. The resulting powders were reduced by excess NaBH4 solution (0.1 M). The obtained

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product was washed with deionized water and ethanol and dried at 60 ◦ C. Finally, the Ag-loaded carbon sphere powders (denoted as Ag@C sphere) were obtained. The second step was similar to the synthesis of THS except that 0.4 g of C sphere was replaced by 0.4 g of Ag@C sphere in this process. After the treatment, the Ag-loaded TiO2 hollow spheres on the inner surface were obtained and are denoted as Ag-I-THS. 2.1.4. Synthesis of Ag- and RuO2 -Co-Loaded TiO2 Hollow Spheres on the Inner Surface and Outer Surface (Ag-I-RuO2 -O-THS) The above synthesized Ag-I-THS (0.2 g) was impregnated in a 0.2 mL ruthenium chloride solution (10 mg/mL) and then dried at 80 ◦ C for 2 h. The resulting powders were calcined at 350 ◦ C for 1 h, and Ag- and RuO2 -co-loaded TiO2 hollow spheres on the inner surface and outer surface, respectively, were finally obtained and are denoted as Ag-I-RuO2 -O-THS. 2.1.5. Synthesis of RuO2 -Loaded TiO2 Hollow Spheres on the Outer Surface (RuO2 -O-THS) The loading of RuO2 on the outer surface of THS was also conducted by an impregnation process [31]. The above synthesized THS (0.4 g) was impregnated in a 0.4 mL ruthenium chloride solution (10 mg/mL) and then dried at 80 ◦ C for 2 h. The resulting powders were calcined at 350 ◦ C for 1 h, and RuO2 -loaded TiO2 hollow spheres on the outer surface were finally obtained and are denoted as RuO2 -O-THS [32]. 2.1.6. Synthesis of RuO2 - and Ag-Co-Loaded TiO2 Hollow Spheres on the Inner Surface and Outer Surface (RuO2 -I-Ag-O-THS) The loading of RuO2 and Ag on the inner surface and outer surface of THS included three steps. In the first step, RuO2 -loaded carbon spheres were prepared by an impregnation method [31]. An amount of 0.4 g of the above synthesized carbon spheres were impregnated in a 0.4 mL ruthenium chloride solution (10 mg/mL) and then dried at 80 ◦ C for 2 h. The resulting powders were calcined at 350 ◦ C for 1 h, and the RuO2 -loaded carbon sphere powders (denoted as RuO2 @C sphere) were finally obtained. The second step was similar to the synthesis of THS, except that the 0.4 g of carbon spheres were replaced by 0.4 g of RuO2 @C sphere in this process. After the treatment, the RuO2 -loaded TiO2 hollow spheres on the inner surface were obtained and are denoted as RuO2 -I-THS. In the final step, the above synthesized RuO2 -I-THS (0.2 g) was impregnated in a 0.2 mL silver nitrate solution (10 mg/mL) and then dried at 80 ◦ C for 2 h. The resulting powders were reduced by excess NaBH4 solution (0.1 M). The obtained product was washed with deionized water and ethanol and dried at 60 ◦ C. Finally, RuO2 - and Ag-co-loaded TiO2 hollow spheres on the inner surface and outer surface, respectively, were obtained and are denoted as RuO2 -I-Ag-O-THS. 2.2. Characterizations The as-prepared samples were characterized by powder X-ray diffraction (PXRD) on a Rigaku Mini Flex 600 X-ray diffractometer (Rigaku, Akishima, Japan) operated at 40 kV and 15 mA with Ni-filtered Cu Kα irradiation (λ = 1.5406 Å). Solid-state UV-Vis diffuse reflectance spectra (UV-Vis DRS) were obtained by using a UV-Vis spectrophotometer (Varian, Cary 500, Palo Alto, CA, USA). Barium sulfate was used as a reference. The Brunauer–Emmett–Teller (BET) surface area was measured with an ASAP2020 apparatus (Micromeritics, Atlanta, GA, USA). The transmission electron microscopy (TEM) images were recorded using a JEOL model JEM 2010 EX microscope (FEI, Hillsboro, OR, USA) at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI Quantum 2000 XPS system (Physical electronics, Portland, OR, USA) with a monochromatic Al KR source and a charge neutralizer. All binding energies were referenced to the C 1s peak (284.6 eV) of the surface adventitious carbon.


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performed on a PHI Quantum 2000 XPS system (Physical electronics, Portland, OR, USA) with a monochromatic Al KR source and a charge neutralizer. All binding energies were referenced to the Nanomaterials 2017, 7, 24 4 of 10 C 1s peak (284.6 eV) of the surface adventitious carbon. 2.3. Photocatalytic Activity Evaluation Photocatalytic hydrogen evolution from water-splitting reaction was carried out with powder area in ainglass-closed gas-circulation system and aand 100 amL Pyrex samples to toprovide providesufficient sufficientsurface surface area a glass-closed gas-circulation system 100 mL glass reaction vessel. The reaction was performed by dispersing 80 mg of catalysts into an aqueous Pyrex glass reaction vessel. The reaction was performed by dispersing 80 mg of catalysts into an solution (80 mL) containing EDTA-2Na (0.5 g) as(0.5 a sacrificial electron electron donor. The whole aqueous solution (80 mL) containing EDTA-2Na g) as a sacrificial donor. Thereaction whole system was evacuated to completely remove air beforeair irradiation. During the experiment, a 300 W reaction system was evacuated to completely remove before irradiation. During the experiment, 18 lamp was employed as the light source simulate sunlight.sunlight. The incident is 3.4flux × 10is aXe300 W Xe lamp was employed as the lighttosource to simulate The photon incidentflux photon 18 − 1 − 2 − 2 −1 −2 −2 s ·cm intensity is 132.4 mW·is cm . The temperature the reactantofsolution was 3.4 × 10, and s the ·cmirradiance , and the irradiance intensity 132.4 mW ·cm . Theof temperature the reactant kept at awas constant by a flow by of cooling during reaction. The amount of H2 solution kept attemperature a constant temperature a flow ofwater cooling water the during the reaction. The amount evolution was analyzed by using on-line gas chromatograph (Shimadzu, GC-8A,GC-8A, Kyoto, Kyoto, Japan) of H2 evolution was analyzed by an using an on-line gas chromatograph (Shimadzu, with a with thermal conductivity detector (TCD)(TCD) and using argonargon as theascarrier gas. gas. To evaluate the Japan) a thermal conductivity detector and using the carrier To evaluate stability of the photocatalyst, the photocatalytic reactions were carried out as the similar procedure the stability of the photocatalyst, the photocatalytic reactions were carried out as the similar procedure with evacuation evacuation every every 55 h. h. above by using 80 mg of catalysts for a total of 25 hh with 3. Results and Discussion 3.1. Crystal Structure Structure 3.1. Crystal The hollow spheres spheres showed showed aa mixture mixture of of anatase anatase and and rutile rutile TiO TiO22 (Figure (Figure 1). 1). The XRD XRD patterns patterns of of TiO TiO22 hollow ◦ The located at 25.3 was was attributed to (101) anatase while the diffraction The diffraction diffractionpeak peak located at 25.3° attributed toplane (101)ofplane of phase, anatase phase, while the ◦ and 36.1◦ was attributed to (110) and (101) planes of rutile phase [33]. No significant peak located at 27.4 diffraction peak located at 27.4° and 36.1° was attributed to (110) and (101) planes of rutile phase [33]. peaks indicative of indicative silver andofruthenium oxide were observed in the cocatalyst-loaded THS, No significant peaks silver and ruthenium oxide were observed in the cocatalyst-loaded which could be attributed to the very low content and/or high dispersion. THS, which could be attributed to the very low content and/or high dispersion.

Figure 1. X-ray diffraction (XRD) patterns of TiO22 hollow spheres (THS) loaded by the cocatalysts cocatalysts with different spatial spatial dispersion: dispersion: (a) (a)THS; THS;(b)(b)Ag-loaded Ag-loaded THS inner surface (Ag-I-THS); THS onon thethe inner surface (Ag-I-THS); (c) (c) RuO -loaded THS on the outer surface (RuO -O-THS); (d) Agand RuO -co-loaded THS on 2 2 2 RuO2-loaded THS on the outer surface (RuO2-O-THS); (d) Ag- and RuO2-co-loaded THS on the inner the innerand surface outer(Ag-I-RuO surface (Ag-I-RuO RuO on the inner 2 -O-THS); 2 - and Ag-co-loaded surface outerand surface 2-O-THS); (e) RuO2(e) - and Ag-co-loaded THS on THS the inner surface surface and outer surface (RuO -I-Ag-O-THS). 2 and outer surface (RuO2-I-Ag-O-THS).

3.2. BET Analyses The BET BET surface surfaceareas areasand and pore structures of THS Ag-I-RuO were evaluated pore structures of THS and and Ag-I-RuO 2-O-THS were evaluated by N2 2 -O-THS by N adsorption at 77 K. The pure THS and sample Ag-I-RuO -O-THS displayed type IV N2 adsorption at 77 K. The pure THS and sample Ag-I-RuO2-O-THS displayed type IV N2 adsorption– 2 2 adsorption–desorption isotherms, corresponding to thestructure mesoporous structure 2). The BET desorption isotherms, corresponding to the mesoporous (Figure 2). The(Figure BET surface area of 2 /g and 8.7 m2 /g, respectively. 2/g,m surface of THS and Ag-I-RuO were 34.5 THS andarea Ag-I-RuO 2-O-THS samples2 -O-THS were 34.5samples m2/g and 8.7 m respectively. Compared with that Compared with that of THS, the specific surface area of Ag-I-RuO2 -O-THS was obviously decreased, which may be because the loading of Ag and RuO2 blocked off the pores of THS [34].

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of THS, the specific surface area of Ag-I-RuO2-O-THS was obviously decreased, which may be 5 of be 10 surface area of Ag-I-RuO2-O-THS was obviously decreased, which may because the loading of Ag and RuO2 blocked off the pores of THS [34]. because the loading of Ag and RuO2 blocked off the pores of THS [34].

Nanomaterials 2017, 7, 24 of THS, the specific

Figure 2. N2 adsorption-desorption isotherms of THS and Ag-I-RuO2-O-THS. Figure2.2. N N22 adsorption-desorption adsorption-desorption isotherms isotherms of of THS THS and and Ag-I-RuO Ag-I-RuO22-O-THS. Figure

3.3. TEM Analyses 3.3. TEM TEM Analyses Analyses 3.3. The morphology of Ag-I-RuO2-O-THS and the different spatial distribution of the cocatalysts The morphology morphology of of Ag-I-RuO Ag-I-RuO22-O-THS and the the different different spatial spatial distribution distribution of the the cocatalysts cocatalysts -O-THS and wereThe investigated by TEM. TEM image of Ag-I-RuO 2-O-THS clearly elucidated theofhollow structure wereinvestigated investigatedby byTEM. TEM.TEM TEM imageofof Ag-I-RuO 2-O-THS clearly elucidated the hollow structure were Ag-I-RuO clearly elucidated hollow by 2 -O-THS by the striking contrast betweenimage the center and the edge with a diameter of the ca. 200 nmstructure and a shell by the striking contrast between the center and the edge with a diameter of ca. 200 nm and a shell the strikingofcontrast between the3a). center the edge with(Figure a diameter ca. 200clear nm and a shell thickness thickness ca. 20 nm (Figure Theand HRTEM image 3b) of showed lattice fringes. The thickness of (Figure ca. 20 nm (Figure 3a). Theimage HRTEM image (Figure 3b) showed clear lattice fringes. The of ca. 20 nm 3a). The HRTEM (Figure 3b) showed clear lattice fringes. The fringes of fringes of d = 0.25 nm and 0.32 nm matched well with that of the (101) and (110) crystallographic fringes of dand = 0.25 nm and 0.32 nm matched well with thatand of (110) the (101) and (110) crystallographic dplane = 0.25 nm 0.32 nm matched well with that of the (101) crystallographic plane of rutile of rutile TiO2 and the fringes located at 0.35 nm corresponded to that of (101) crystallographic plane of rutile TiO2 and the at fringes located at 0.35 nmtocorresponded to that of (101) plane crystallographic TiO fringes located 0.35 that (101) crystallographic ofthe anatase 2 and plane of the anatase TiO 2, which werenm incorresponded accordance with theofresult of XRD. Noteworthy, (111) plane of anatase TiO 2 , which were in accordance with the result of XRD. Noteworthy, the (111) TiO , which were in accordance with the result of XRD. Noteworthy, the (111) crystallographic plane 2 crystallographic plane of Ag2O (d = 0.27 nm) and the (111) crystallographic plane of RuO2 (d = 0.28 crystallographic of Ag 2O(111) (d = 0.27 nm) and the plane (111) crystallographic plane RuO (d = 0.28 of Agcan 0.27plane nm)on and the crystallographic ofof RuO = 0.28 nm) of can be 2observed 2 O (d 2 2(dhollow nm) be =observed the inner surface and outer surface TiO spheres, respectively, nm) can be observed on the inner surface and outer surface of TiO 2 hollow spheres, respectively, on the inner andseparated outer surface TiO2 hollow spheres, respectively, indicating spatially indicating thesurface spatially dual of cocatalysts loaded TiO2 hollow spheres has beenthe synthesized indicatingdual the spatially separated dual2 hollow cocatalysts loaded TiO 2 hollow spheres has been synthesized separated cocatalysts loaded TiO spheres has been synthesized successfully. successfully. successfully.

Figure Figure 3. 3. (a) (a) Transmission Transmission electron electron microscopy microscopy (TEM) (TEM) and and (b) (b) High High resolution resolution transmission transmission electron electron Figure 3. (a) Transmission electron microscopy (TEM) and (b) High resolution transmission electron microscopy microscopy (HRTEM) (HRTEM) images images of of Ag-I-RuO Ag-I-RuO22-O-THS. -O-THS. microscopy (HRTEM) images of Ag-I-RuO2-O-THS.

3.4. XPS Analyses 3.4. XPS Analyses The chemical components and the states of C, Ru, Ag, O, O, and and Ti Ti in in the the Ag-I-RuO Ag-I-RuO22-O-THS were The chemical components and the states of C, Ru, Ag, O, and Ti in the Ag-I-RuO2-O-THS were investigated peaks at at about 284.6 eV,eV, 285.2 eV,eV, 286.6 eV, investigated by byXPS. XPS.As Asobserved observedininFigure Figure4a, 4a,there thereare arefour four peaks about 284.6 285.2 286.6 investigated by XPS. As observed in Figure 4a, there are four peaks at about 284.6 eV, 285.2 eV, 286.6 and 288.5 eV in the C 1s spectrum corresponding to the additional carbon, the residual carbon in C sphere, tetrabutyl titanate, and CTAB, respectively. Furthermore, the binding energy of Ru 3d3/2

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eV, and 288.5 eV in the C 1s spectrum corresponding to the additional carbon, the residual carbon in C sphere, tetrabutyl titanate, and CTAB, respectively. Furthermore, the binding energy of Ru 3d3/2 was overlapped by that of C 1s; thus, the Ru oxidation state was evaluated from the Ru 3d5/2 . The Ru was overlapped by that of C 1s; thus, the Ru oxidation state was evaluated from the Ru 3d5/2. The Ru 4+ , as expected for RuO [35]. 3d5/2 peak was located at 280.3 eV, which indicated the existence of Ru4+ 3d5/2 peak was located at 280.3 eV, which indicated the existence of Ru , as expected for RuO22 [35]. Figure 4b demonstrated the high-resolution XPS spectra for Ag 3d3/2 and Ag 3d5/2 located at 373.3 eV Figure 4b demonstrated the high-resolution XPS spectra for Ag 3d3/2 and Ag 3d5/2 located at 373.3 eV + of Ag O [36]. The O 1s peak may be fitted into and 367.3 eV, respectively, corresponding to Ag + and 367.3 eV, respectively, corresponding to Ag of Ag2O2 [36]. The O 1s peak may be fitted into two two peaks at 529.9 eV and 531.6 eV (Figure 4c), corresponding to the crystal lattice oxygen and peaks at 529.9 eV and 531.6 eV (Figure 4c), corresponding to the crystal lattice oxygen and the surface the surface hydroxyl groups, respectively [37]. Meanwhile, the Ti 2p XPS spectra are deconvoluted hydroxyl groups, respectively [37]. Meanwhile, the Ti 2p XPS spectra are deconvoluted into two into two peaks at 458.6 eV and 464.3 eV, corresponding to Ti4+ in TiO2 [38] (Figure 4d). 4+ peaks at 458.6 eV and 464.3 eV, corresponding to Ti in TiO2 [38] (Figure 4d).

Figure 4. X-ray photoelectron spectroscopy (XPS) spectra of Ag-I-RuO -O-THS: (a) C 1s and Ru 3d; Figure 4. X-ray photoelectron spectroscopy (XPS) spectra of Ag-I-RuO22-O-THS: (a) C 1s and Ru 3d; (b) Ag 3d; (c) O 1s; (d) Ti 2p. (b) Ag 3d; (c) O 1s; (d) Ti 2p.

3.5. UV-Vis UV-Vis DRS DRS Analyses Analyses 3.5. As shown reflectance spectra, all all samples displayed a similar bandband edgeedge with As shown in inthe theUV-Vis UV-Visdiffuse diffuse reflectance spectra, samples displayed a similar a value of 3.3 eV, indicating that the photo-absorption properties of TiO were maintained (Figure 5). 2 2 were maintained (Figure with a value of 3.3 eV, indicating that the photo-absorption properties of TiO TiOTiO only UV light, while the RuO2 -O-THS, Ag-I-RuO2 -O-THS, and RuO -I-Ag-O-THS 2 absorbed 5). 2 absorbed only UV light, while the RuO 2-O-THS, Ag-I-RuO 2-O-THS, and RuO22-I-Ag-O-THS exhibited a stronger light absorption in the visible light region owing to the presence presence of of RuO RuO22.. exhibited a stronger light absorption in the visible light region owing to the Compared to the other samples, RuO -I-Ag-O-THS composite displayed the characteristic localized Compared to the other samples, RuO22-I-Ag-O-THS composite displayed the characteristic localized surface plasmon plasmon resonance resonance peak peak of of Ag Ag located 530 nm nm [39]. [39]. surface located at at ca. ca. 530

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Figure 5. Ultraviolet-visible diffuse reflectance spectra (UV-Vis DRS) of THS loaded by the cocatalysts Figure 5. Ultraviolet-visible diffuse reflectance spectra (UV-Vis (UV-Vis DRS) DRS) of of THS THS loaded loaded by the cocatalysts with different spatial dispersion. with with different different spatial spatial dispersion. dispersion.

3.6. Photocatalytic Activities 3.6. Photocatalytic Activities Figure 6 showed the photocatalytic activity of H2 evolution of THS loaded with different spatial Figure 66ofshowed the of H of THS with different spatial showed thephotocatalytic photocatalytic activity of2 evolution H2 2evolution of loaded THS loaded distribution the cocatalysts within 5 h.activity The original TiO hollow spheres produced awith littledifferent amount distribution of cocatalysts within 5within h.loaded The5original 2 hollow produced little amount spatial distribution of the cocatalysts h. TiO hollow a little 2 spheres of H2. When Agthe species and RuO 2 were onThe TiOoriginal 2TiO hollow spheres, the spheres activity produced of aphotocatalytic of H 2 . When Ag species and RuO 2 were loaded on TiO 2 hollow spheres, the activity of photocatalytic amount of H . When Ag species and RuO were loaded on TiO hollow spheres, the activity hydrogen production was greatly improved. The hydrogen yields of 2Ag-I-THS, RuO2-O-THS, RuO22 2 hydrogen production was greatly improved. The hydrogen yields of Ag-I-THS, RuO 2-O-THS, RuO of photocatalytic was greatly improved. hydrogen yields of Ag-I-THS, I-Ag-O-THS, and hydrogen Ag-I-RuO 2production -O-THS were 15.4, 139.0, 103.4, andThe 300.2 μmol, respectively. The H2-2 I-Ag-O-THS, and 2-O-THS were 15.4, 139.0, 103.4, and15.4, 300.2 μmol, respectively. H2 RuO RuOAg-I-RuO Ag-I-RuO were 103.4, and2-O-THS, 300.2The µmol, evolution of Ag-I-RuO 2-O-THS wasand about 19.5 times over Ag-I-THS, 2.2139.0, times over RuO and 2 -O-THS, 2 -I-Ag-O-THS, 2 -O-THS evolution of Ag-I-RuO 2 -O-THS was about 19.5 times over Ag-I-THS, 2.2 times over RuO 2 -O-THS, and respectively. The H evolution of Ag-I-RuO -O-THS was about 19.5 times over Ag-I-THS, 2.2 times 2.9 times over RuO22-I-Ag-O-THS. The results2 demonstrated that photocatalytic hydrogen production 2.9 times over RuO2and -I-Ag-O-THS. TheRuO results demonstrated photocatalytic hydrogen production over RuO 2.9 times over The results demonstrated that photocatalytic activity was enhanced by co-loading Ag species and RuO 2 that on the inner and outer surfaces of THS. 2 -O-THS, 2 -I-Ag-O-THS. activity was enhanced by co-loading Ag species and RuO 2 on the inner and outer surfaces of THS. hydrogen production activity was enhanced by co-loading Ag species and RuO on the inner The stability of the photocatalysts is important for their applications. Thus, the stability2 of Ag-I-RuO 2The stability of the photocatalysts is important for their applications. Thus, the stability of Ag-I-RuO 2and outer of THS. stability of the7,photocatalysts is important their applications. O-THS wassurfaces investigated. As The shown in Figure the photocatalytic activity for of Ag-I-RuO 2-O-THS O-THS investigated. As2 -O-THS shown in Figure 7, the photocatalytic activity Ag-I-RuO 2-O-THS Thus, thewas stability of increment Ag-I-RuO was investigated. As shown in Figure 7, theof photocatalytic activity increased with the of the photocatalytic reaction cycles. After three cycles, the hydrogen increased with the increment of the photocatalytic reaction cycles. After three cycles, the hydrogen of Ag-I-RuO -O-THS increased with the increment of the photocatalytic reaction cycles. After three cycles, production tended to be a stable value. The increased amount of hydrogen may be due to the change 2 production tended to be a stable value. The increased amount of hydrogen may be due to the change the hydrogen production tended to be under a stable value. The increased [36,40]. amountAs of the hydrogen may be due of the chemical state of silver species solar light irradiation reaction proceeds, of the chemical state silverstate species under solar light irradiation [36,40]. Assilver, the reaction proceeds, to change ofstate the chemical of silver species under solar lighttoirradiation [36,40]. As the reaction thethe oxidation ofofsilver species can be gradually reduced metallic thus increasing the oxidation state of state silver can be gradually reduced to hydrogen metallic silver, proceeds, the oxidation ofspecies silverand species can be gradually reduced to metallic silver,thus thus increasing increasing photogenerated electrons mobility significantly improving the production. Scheme 2 photogenerated electrons mobility and significantly improving the hydrogen production. Scheme photogenerated electrons mobility and significantly improving thewater hydrogen production. Scheme 2 shows shows the probable reaction mechanism for the photocatalytic splitting reaction on Ag-I-RuO 22 shows the probable reaction mechanism for the photocatalytic water splitting reaction on Ag-I-RuO 2 the probable reaction mechanism for the photocatalytic water splitting reaction on Ag-I-RuO -O-THS. O-THS. The oxidation cocatalyst RuO2 and reduction cocatalyst Ag loaded on the outer and inner 2 O-THS. The oxidation cocatalyst RuO 2 andmigration reduction onand the electrons, outer andwhich inner The oxidation cocatalyst RuO reduction cocatalyst Ag loadedAg onloaded theholes outer and inner surface of surface of THS can lead to the directional ofcocatalyst photogenerated 2 and surface THS lead to the migration directional of photogenerated holes and electrons, which THS canoflead to can the directional ofmigration photogenerated holes and electrons, which can prohibit can prohibit the recombination of the photogenerated carriers and finally enhance the photocatalytic can prohibit of the photogenerated carriersenhance and finally enhance the photocatalytic the recombination of the photogenerated carriers and finally the photocatalytic activity [21]. activity [21]. the recombination activity [21].

Figure activities of of H evolution on on THS THS loaded loaded by Figure 6. 6. Photocatalytic Photocatalytic activities H2 evolution by the the cocatalysts cocatalysts with with different different spatial dispersion. Figure 6. Photocatalytic activities of H 2 evolution on THS loaded by the cocatalysts with different spatial dispersion. spatial dispersion.

Nanomaterials 2017, 7, 24 Nanomaterials 2017, 2017, 7, 7, 24 24 Nanomaterials

8 of 10 of 10 10 88 of

Figure 7. 7. Stability Stability of of Ag-I-RuO Ag-I-RuO222-O-THS -O-THS during prolonged photocatalytic operation. -O-THS during during prolonged prolonged photocatalytic photocatalytic operation. Figure

Scheme 2. 2. The The photocatalytic photocatalytic hydrogen hydrogen production production on on Ag-I-RuO Ag-I-RuO22-O-THS. -O-THS. Scheme 2 -O-THS.

4. Conclusions Conclusions 4. Conclusions We have have successfully successfully synthesized synthesized TiO TiO22 hollow hollow spheres spheres modified modified with with Ag Ag species species and and RuO RuO22 on on We TiO 2 2 the inner innerand andouter outersurfaces, surfaces,respectively, respectively, via an alkoxide hydrolysis-precipitation method combined the inner and outer surfaces, respectively, alkoxide hydrolysis-precipitation method combined viavia anan alkoxide hydrolysis-precipitation method combined with with a facile impregnation method. The as-obtained TiO 2 hollow spheres exhibited enhanced with facile impregnation The as-obtained TiO 2 hollow spheres exhibited enhanced a facilea impregnation method. method. The as-obtained TiO2 hollow spheres exhibited enhanced photocatalytic photocatalytic hydrogen production activity under solar light light irradiation, which iseffective ascribedtransfer to the the photocatalytic hydrogen production activity under solar irradiation, ascribed to hydrogen production activity under solar light irradiation, which is ascribedwhich to theis effective transfer and separation of of the the photogenerated charge carriers at the the interface, interface, resulting from effective transfer separation photogenerated charge carriers at resulting from and separation ofand the photogenerated charge carriers at the interface, resulting from the suitable spatial the suitable spatial separation of Ag species and RuO 2 cocatalysts on TiO2 hollow spheres. the suitableofspatial separation of Ag species and cocatalysts on TiO2 hollow spheres. separation Ag species and RuO cocatalysts on RuO TiO 2 hollow spheres. 2

2

Acknowledgments: This This work work was was financially financially supported supported by by the the National National Natural Natural Science Science Foundation Foundation of of China China Acknowledgments: (51672047), the Natural Science Foundation of Fujian Province (2014J01047), and the Independent Research (51672047), the Natural Science Foundation of Fujian Province (2014J01047), and the Independent Research Project (51672047), the Natural Science Foundation of Fujian Province (2014J01047), and the Independent Research of State of Key Laboratory of Photocatalysis on Energy and Environment (2014C03). Project State Key Laboratory Laboratory of Photocatalysis Photocatalysis on Energy Energy and Environment Environment (2014C03). Project of State Key of on and (2014C03). Author Contributions: Jinhong Bi and Shijing Liang conceived and designed the experiments; Li Li performed Author Contributions: Contributions: Jinhong Jinhong Bi Bi and and Shijing Shijing Liang Liang conceived conceived and and designed designed the the experiments; experiments; Li Li Li performed performed Author the experiments; Li Li and Qianqian Jiang analyzed the data and wrote the paper; Minghua LiuLicontributed the experiments; Li Li and Qianqian Jiang analyzed the data and wrote the paper; Minghua Liu contributed reagents/materials/equipment. the experiments; Li Li and Qianqian Jiang analyzed the data and wrote the paper; Minghua Liu contributed reagents/materials/equipment. reagents/materials/equipment. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of of Interest: Interest: The The authors authors declare declare no no conflict conflict of of interest. interest. Conflicts

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