Design and Synthesis of TiO2 Hollow Spheres with Spatially Separated Dual Cocatalysts for Efficient Photocatalytic Hydrogen Production

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.


Introduction
Semiconductor photocatalysis as a green technology has attracted much interest for the application in solving environmental pollution and energy shortage [1][2][3][4][5]. Since the photolysis of water to produce hydrogen was discovered, TiO 2 has been most investigated in photocatalysis due to the chemical stability, nontoxicity, and low price [5][6][7][8]. However, the drawbacks of TiO 2 , 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 TiO 2 with expectations of achieving novel or enhanced properties have been regarded as an efficient way to overcome the drawbacks-for instance, the fabrication of TiO 2 nanospheres, nanorods, nanowires, and nanobelts [9][10][11][12][13]. Especially, the submicron-scale hollow spheres of TiO 2 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 TiO 2 hollow spheres can be modified by loading cocatalysts [15][16][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 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 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 superior photocatalytic activity [19]. Li et al. have reported that reduction cocatalysts (MoS2, NiS, WS2, etc.) and oxidation cocatalysts (IrOx, MnOx, RuOx, etc.) can be selectively deposited on the (010) and (110) facets of BiVO4, respectively, which results in much higher photocatalytic activity compared to that with randomly distributed cocatalysts [20,21]. In general, the noble metals (Au, Ag, Pt, Pd, etc.), MoS2, and graphene exhibiting superior electron mobility often act as reduction cocatalysts to improve the efficiency of photoproduction electron migration [7,[22][23][24][25]. The cocatalysts such as RuO2, IrO2, CoOx, and MnOx can act as hole collector [26][27][28][29]. Loading the reduction and oxidation catalysts on the inner and outer surfaces of TiO2 hollow spheres can be expected to achieve enhanced photocatalytic activity.
Herein, we report a facile synthesis of TiO2 hollow spheres modified with spatially separated Ag species and RuO2 on the inner and outer surfaces of the TiO2 hollow spheres (as shown in Scheme 1), which exhibited enhanced photocatalytic hydrogen production under solar light irradiation. Scheme 1. Processes involved in the formation of dual cocatalysts co-loading on the TiO2 hollow spheres (THS).

Synthesis of Carbon Spheres (C Sphere)
In a typical synthesis of carbon spheres, glucose (6 g) was dissolved into deionized water (60 mL) to form a clear solution and then was transferred into a 100 mL Teflon-lined autoclave and was reacted at 180 °C for 12 h. The obtained brown product was collected and washed with deionized water and ethanol and then dried at 80 °C. Finally, carbon spheres (denoted as C sphere) were obtained [30].

Synthesis of TiO2 Hollow Spheres (THS)
An amount of 0.4 g of C sphere was added 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. Then, a solution consisting of 70 mL of pure ethyl alcohol, 0.2 g of hexadecyl trimethyl ammonium bromide (CTAB), and 0.6 mL of deionized water was added and stirred for 2 h. After that, 23 mL of a tetrabutyl titanate ethanol solution was added dropwise while stirring. The obtained suspended solution was transferred into the three-necked flask. After the reflux condensation at 85 °C for 100 min, the prepared product was collected, washed with deionized water and ethyl alcohol, and 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 TiO2 hollow spheres were then obtained and are denoted as THS [30]. Scheme 1. Processes involved in the formation of dual cocatalysts co-loading on the TiO 2 hollow spheres (THS).

Synthesis of Carbon Spheres (C Sphere)
In a typical synthesis of carbon spheres, glucose (6 g) was dissolved into deionized water (60 mL) to form a clear solution and then was transferred into a 100 mL Teflon-lined autoclave and was reacted at 180 • C for 12 h. The obtained brown product was collected and washed with deionized water and ethanol and then dried at 80 • C. Finally, carbon spheres (denoted as C sphere) were obtained [30].

Synthesis of TiO 2 Hollow Spheres (THS)
An amount of 0.4 g of C sphere was added 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. Then, a solution consisting of 70 mL of pure ethyl alcohol, 0.2 g of hexadecyl trimethyl ammonium bromide (CTAB), and 0.6 mL of deionized water was added and stirred for 2 h. After that, 23 mL of a tetrabutyl titanate ethanol solution was added dropwise while stirring. The obtained suspended solution was transferred into the three-necked flask. After the reflux condensation at 85 • C for 100 min, the prepared product was collected, washed with deionized water and ethyl alcohol, and 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 2 hollow spheres were then obtained and are denoted as THS [30]. 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 NaBH 4 solution (0.1 M). The obtained 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 TiO 2 hollow spheres on the inner surface were obtained and are denoted as Ag-I-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 RuO 2 -co-loaded TiO 2 hollow spheres on the inner surface and outer surface, respectively, were finally obtained and are denoted as Ag-I-RuO 2 -O-THS.

Synthesis of RuO 2 -Loaded TiO 2 Hollow Spheres on the Outer Surface (RuO 2 -O-THS)
The loading of RuO 2 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 RuO 2 -loaded TiO 2 hollow spheres on the outer surface were finally obtained and are denoted as RuO 2 -O-THS [32].

Synthesis of RuO 2 -and Ag-Co-Loaded TiO 2 Hollow Spheres on the Inner Surface and Outer Surface (RuO 2 -I-Ag-O-THS)
The loading of RuO 2 and Ag on the inner surface and outer surface of THS included three steps. In the first step, RuO 2 -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 RuO 2 -loaded carbon sphere powders (denoted as RuO 2 @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 RuO 2 @C sphere in this process. After the treatment, the RuO 2 -loaded TiO 2 hollow spheres on the inner surface were obtained and are denoted as RuO 2 -I-THS. In the final step, the above synthesized RuO 2 -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 NaBH 4 solution (0.1 M). The obtained product was washed with deionized water and ethanol and dried at 60 • C. Finally, RuO 2 -and Ag-co-loaded TiO 2 hollow spheres on the inner surface and outer surface, respectively, were obtained and are denoted as RuO 2 -I-Ag-O-THS.

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.

Photocatalytic Activity Evaluation
Photocatalytic hydrogen evolution from water-splitting reaction was carried out with powder samples to provide sufficient surface area in a glass-closed gas-circulation system and a 100 mL Pyrex glass reaction vessel. The reaction was performed by dispersing 80 mg of catalysts into an aqueous solution (80 mL) containing EDTA-2Na (0.5 g) as a sacrificial electron donor. The whole reaction system was evacuated to completely remove air before irradiation. During the experiment, a 300 W Xe lamp was employed as the light source to simulate sunlight. The incident photon flux is 3.4 × 10 18 s −1 ·cm −2 , and the irradiance intensity is 132.4 mW·cm −2 . The temperature of the reactant solution was kept at a constant temperature by a flow of cooling water during the reaction. The amount of H 2 evolution was analyzed by using an on-line gas chromatograph (Shimadzu, GC-8A, Kyoto, Japan) with a thermal conductivity detector (TCD) and using argon as the carrier gas. To evaluate the stability of the photocatalyst, the photocatalytic reactions were carried out as the similar procedure above by using 80 mg of catalysts for a total of 25 h with evacuation every 5 h.

Crystal Structure
The XRD patterns of TiO 2 hollow spheres showed a mixture of anatase and rutile TiO 2 ( Figure 1). The diffraction peak located at 25.3 • was attributed to (101) plane of anatase phase, while the diffraction peak located at 27.4 • and 36.1 • was attributed to (110) and (101) planes of rutile phase [33]. No significant peaks indicative of silver and ruthenium oxide were observed in the cocatalyst-loaded THS, which could be attributed to the very low content and/or high dispersion. 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.

Photocatalytic Activity Evaluation
Photocatalytic hydrogen evolution from water-splitting reaction was carried out with powder samples to provide sufficient surface area in a glass-closed gas-circulation system and a 100 mL Pyrex glass reaction vessel. The reaction was performed by dispersing 80 mg of catalysts into an aqueous solution (80 mL) containing EDTA-2Na (0.5 g) as a sacrificial electron donor. The whole reaction system was evacuated to completely remove air before irradiation. During the experiment, a 300 W Xe lamp was employed as the light source to simulate sunlight. The incident photon flux is 3.4 × 10 18 s −1 ·cm −2 , and the irradiance intensity is 132.4 mW·cm −2 . The temperature of the reactant solution was kept at a constant temperature by a flow of cooling water during the reaction. The amount of H2 evolution was analyzed by using an on-line gas chromatograph (Shimadzu, GC-8A, Kyoto, Japan) with a thermal conductivity detector (TCD) and using argon as the carrier gas. To evaluate the stability of the photocatalyst, the photocatalytic reactions were carried out as the similar procedure above by using 80 mg of catalysts for a total of 25 h with evacuation every 5 h.

Crystal Structure
The XRD patterns of TiO2 hollow spheres showed a mixture of anatase and rutile TiO2 (Figure 1). The diffraction peak located at 25.3° was attributed to (101) plane of anatase phase, while the diffraction peak located at 27.4° and 36.1° was attributed to (110) and (101) planes of rutile phase [33]. No significant peaks indicative of silver and ruthenium oxide were observed in the cocatalyst-loaded THS, which could be attributed to the very low content and/or high dispersion.

BET Analyses
The BET surface areas and pore structures of THS and Ag-I-RuO2-O-THS were evaluated by N2 adsorption at 77 K. The pure THS and sample Ag-I-RuO2-O-THS displayed type IV N2 adsorptiondesorption isotherms, corresponding to the mesoporous structure ( Figure 2). The BET surface area of THS and Ag-I-RuO2-O-THS samples were 34.5 m 2 /g and 8.7 m 2 /g, respectively. Compared with that

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

TEM Analyses
The

XPS Analyses
The chemical components and the states of C, Ru, Ag, O, and Ti in the Ag-I-RuO2-O-THS were investigated by XPS. As observed in Figure 4a, there are four peaks at about 284.6 eV, 285.2 eV, 286.6

TEM Analyses
The

TEM Analyses
The

XPS Analyses
The chemical components and the states of C, Ru, Ag, O, and Ti in the Ag-I-RuO2-O-THS were investigated by XPS. As observed in Figure 4a, there are four peaks at about 284.6 eV, 285.2 eV, 286.6

XPS Analyses
The chemical components and the states of C, Ru, Ag, O, and Ti in the Ag-I-RuO 2 -O-THS were investigated by XPS. As observed in Figure 4a, there are four peaks at about 284.6 eV, 285.2 eV, 286.6 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 3d 3/2 was overlapped by that of C 1s; thus, the Ru oxidation state was evaluated from the Ru 3d 5/2 . The Ru 3d 5/2 peak was located at 280.3 eV, which indicated the existence of Ru 4+ , as expected for RuO 2 [35]. Figure 4b demonstrated the high-resolution XPS spectra for Ag 3d 3/2 and Ag 3d 5/2 located at 373.3 eV and 367.3 eV, respectively, corresponding to Ag + of Ag 2 O [36]. The O 1s peak may be fitted into two peaks at 529.9 eV and 531.6 eV (Figure 4c), corresponding to the crystal lattice oxygen and the surface hydroxyl groups, respectively [37]. Meanwhile, the Ti 2p XPS spectra are deconvoluted into two peaks at 458.6 eV and 464.3 eV, corresponding to Ti 4+ in TiO 2 [38] (Figure 4d). Nanomaterials 2017, 7, 24 6 of 10 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 3d5/2 peak was located at 280.3 eV, which indicated the existence of Ru 4+ , as expected for RuO2 [35]. Figure 4b demonstrated the high-resolution XPS spectra for Ag 3d3/2 and Ag 3d5/2 located at 373.3 eV and 367.3 eV, respectively, corresponding to Ag + of Ag2O [36]. The O 1s peak may be fitted into two peaks at 529.9 eV and 531.6 eV (Figure 4c), corresponding to the crystal lattice oxygen and the surface hydroxyl groups, respectively [37]. Meanwhile, the Ti 2p XPS spectra are deconvoluted into two peaks at 458.6 eV and 464.3 eV, corresponding to Ti 4+ in TiO2 [38] (Figure 4d).

UV-Vis DRS Analyses
As shown in the UV-Vis diffuse reflectance spectra, all samples displayed a similar band edge with a value of 3.3 eV, indicating that the photo-absorption properties of TiO2 were maintained (

UV-Vis DRS Analyses
As shown in the UV-Vis diffuse reflectance spectra, all samples displayed a similar band edge with a value of 3.3 eV, indicating that the photo-absorption properties of TiO 2 were maintained ( Figure 5).   Figure 7, the photocatalytic activity of Ag-I-RuO2-O-THS increased with the increment of the photocatalytic reaction cycles. After three cycles, the hydrogen production tended to be a stable value. The increased amount of hydrogen may be due to the change of the chemical state of silver species under solar light irradiation [36,40]. As the reaction proceeds, the oxidation state of silver species can be gradually reduced to metallic silver, thus increasing photogenerated electrons mobility and significantly improving the hydrogen production. Scheme 2 shows the probable reaction mechanism for the photocatalytic water splitting reaction on Ag-I-RuO2-O-THS. The oxidation cocatalyst RuO2 and reduction cocatalyst Ag loaded on the outer and inner surface of THS can lead to the directional migration of photogenerated holes and electrons, which can prohibit the recombination of the photogenerated carriers and finally enhance the photocatalytic activity [21].    Figure 7, the photocatalytic activity of Ag-I-RuO 2 -O-THS increased with the increment of the photocatalytic reaction cycles. After three cycles, the hydrogen production tended to be a stable value. The increased amount of hydrogen may be due to the change of the chemical state of silver species under solar light irradiation [36,40]. As the reaction proceeds, the oxidation state of silver species can be gradually reduced to metallic silver, thus increasing photogenerated electrons mobility and significantly improving the hydrogen production. Scheme 2 shows the probable reaction mechanism for the photocatalytic water splitting reaction on Ag-I-RuO 2 -O-THS. The oxidation cocatalyst RuO 2 and reduction cocatalyst Ag loaded on the outer and inner surface of THS can lead to the directional migration of photogenerated holes and electrons, which can prohibit the recombination of the photogenerated carriers and finally enhance the photocatalytic activity [21].  The results demonstrated that photocatalytic hydrogen production activity was enhanced by co-loading Ag species and RuO2 on the inner and outer surfaces of THS. The stability of the photocatalysts is important for their applications. Thus, the stability of Ag-I-RuO2-O-THS was investigated. As shown in Figure 7, the photocatalytic activity of Ag-I-RuO2-O-THS increased with the increment of the photocatalytic reaction cycles. After three cycles, the hydrogen production tended to be a stable value. The increased amount of hydrogen may be due to the change of the chemical state of silver species under solar light irradiation [36,40]. As the reaction proceeds, the oxidation state of silver species can be gradually reduced to metallic silver, thus increasing photogenerated electrons mobility and significantly improving the hydrogen production. Scheme 2 shows the probable reaction mechanism for the photocatalytic water splitting reaction on Ag-I-RuO2-O-THS. The oxidation cocatalyst RuO2 and reduction cocatalyst Ag loaded on the outer and inner surface of THS can lead to the directional migration of photogenerated holes and electrons, which can prohibit the recombination of the photogenerated carriers and finally enhance the photocatalytic activity [21].

Conclusions
We have successfully synthesized TiO2 hollow spheres modified with Ag species and RuO2 on the inner and outer surfaces, respectively, via an alkoxide hydrolysis-precipitation method combined with a facile impregnation method. The as-obtained TiO2 hollow spheres exhibited enhanced photocatalytic hydrogen production activity under solar light irradiation, which is ascribed to the effective transfer and separation of the photogenerated charge carriers at the interface, resulting from the suitable spatial separation of Ag species and RuO2 cocatalysts on TiO2 hollow spheres.

Conclusions
We have successfully synthesized TiO2 hollow spheres modified with Ag species and RuO2 on the inner and outer surfaces, respectively, via an alkoxide hydrolysis-precipitation method combined with a facile impregnation method. The as-obtained TiO2 hollow spheres exhibited enhanced photocatalytic hydrogen production activity under solar light irradiation, which is ascribed to the effective transfer and separation of the photogenerated charge carriers at the interface, resulting from the suitable spatial separation of Ag species and RuO2 cocatalysts on TiO2 hollow spheres.