Hydrogenated Cagelike Titania Hollow Spherical Photocatalysts for

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Hydrogenated Cage-like Titania Hollow Spherical Photocatalysts for Hydrogen Evolution under Simulated Solar Light Irradiation Yating Wang, Jinmeng Cai, Moqing Wu, Hao Zhang, Ming Meng, Ye Tian, Tong Ding, Jinlong Gong, Zheng Jiang, and Xingang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05777 • Publication Date (Web): 08 Aug 2016 Downloaded from http://pubs.acs.org on August 14, 2016

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ACS Applied Materials & Interfaces

Hydrogenated Cage-like Titania Hollow Spherical Photocatalysts

for

Hydrogen

Evolution

under

Simulated Solar Light Irradiation Yating Wanga,b, Jinmeng Caia,b, Moqing Wua,b, Hao Zhanga,b, Ming Menga,b, Ye Tiana,b, Tong Dinga,b, Jinlong Gonga,c, Zheng Jiangd and Xingang Li*,a,b

a

Collaborative Innovation Center for Chemical Science & Engineering, School of

Chemical Engineering & Technology, Tianjin University, Tianjin 300072 (P. R. China). b

Tianjin Key Laboratory of Applied Catalysis Science & Engineering, School of

Chemical Engineering & Technology, Tianjin University, Tianjin 300072 (P. R. China). c

Key Laboratory for Green Chemical Technology of Ministry of Education, School of

Chemical Engineering & Technology, Tianjin University, Tianjin 300072 (P. R. China). d

Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics,

Chinese Academy of Sciences, Shanghai 201204 (P. R. China).

*

Corresponding author

Email: [email protected] 1

ACS Paragon Plus Environment


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ABSTRACT: We synthesized the hydrogenated cage-like TiO2 hollow spheres through a facile sacrificial template method. After the hydrogenation treatment, the disordered surface layer and cage-like pores were generated on the shell of the hollow spheres. The spheres exhibit a high hydrogen evolution rate of 212.7 ± 10.6 µmol h-1 (20 mg) under the simulated solar light irradiation, which is ~ 12 times higher than the hydrogenated TiO2 solid spheres and is ~ 9 times higher than the original TiO2 hollow spheres. The high activity results from the unique architectures and hydrogenation. Both the multiple reflection improved by the cage-like hollow structures and the red shift of the absorption edge induced by hydrogenation can enhance the ultraviolet and visible light absorption. Additionally, the high concentration of oxygen vacancies, as well as the hydrogenated disordered surface layer, can improve the efficiency for migration and separation of generated charge carries. KEYWORDS: cage-like hollow spheres; multiple reflection; hydrogenation; photocatalysis; hydrogen production; water splitting

1. INTRODUCTION The production of hydrogen by solar energy conversion has been considered as one of the major strategies for solving the global energy problem.1 Since the discovery of the photocatalytic splitting of water in 1972,2 TiO2 has been intensively studied as a potential photocatalyst because of its environmental benignity, low cost, good chemical stability and non-toxicity.3,4 Generally, the photocatalytic efficiency is influenced by the degree of light absorption, charge separation, and surface

2


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reactivity.5 However, the weak photon absorption and poor charge transport of TiO2 lead to the low hydrogen evolution efficiency, and limit its further application in overall solar region.6,7 Construction of the specific morphology of TiO2 is a possible way to solve this problem.8 Earlier studies have revealed that the hollow structures with an appropriate inner sphere diameter can allow multiple reflection within the interior voids, which enhance the effectiveness of light utilization.9,10 For instance, cage-like hollow spheres exhibited much higher photocatalytic degradation activities than solid structures.11-13 Beyond that, the unique structure of hollow spheres provides an enhanced surface-to-volume ratio and reduced transport lengths for charge carriers, which are favourable to accelerate the migration and separation of photo-generated carriers.14 We show a schematic illustration of the comparison of light harvesting within the TiO2 solid spheres and TiO2 hollow spheres (Figure 1A). Hollow spheres (Figure 1Ab and 1Ac) allow multiple reflection of incident light within the interior cavity to obtain higher light absorption ability than solid spheres (Figure 1Aa). However, the shell will partially prevent the light passing into the hollow chamber, and diminish light multiple reflection. Thus, we suppose that the cage-like structures (Figure 1Ac) could provide even more efficient multiple reflection of light within the interior cavity and further enhance utilization of the light source. On the other hand, the large band gap (3.0-3.2 eV) of TiO2 suppresses its light absorption in the visible and near-infrared region.15 It is important to modify the electronic structure of TiO2, as well. Considerable efforts have been proposed to 3



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reduce the band gap of TiO2 to enhance its absorption of sun light, such as hydrogenation,16,17 doping TiO2 with anions18,19 or cations,20-22 and coupling TiO2 with a narrow band gap semiconductor.10 The absorption of TiO2 in the visible light region has been remarkably increased through these strategies, especially by using the hydrogenation. The hydrogenated TiO2 also exhibits the improved efficiency for migration and separation of generated charge carries, inducing the enhanced photocatalytic activity for water splitting.23 Considering the above analysis, we designed the novel hydrogenated cage-like TiO2 hollow spheres for photocatalytic water splitting. Due to both their unique architectures and hydrogenation, these materials exhibit the significantly enhanced photocatalytic activities. Figure 1B shows a synthetic strategy for the hydrogenated cage-like TiO2 hollow spheres. We chose carbonaceous microspheres (CS) as the hard template to prepare TiO2 hollow spheres, which were then loaded with 1 wt. % Pt (denoted as CST). The CST was annealed in H2 or N2 atmosphere at 750 °C, and the final products are denoted as CST-H-750 or CST-N-750, respectively. For comparison, we destroyed the structure of CST-H-750, denoted as CSTD-H-750. Additionally, the CST was reduced in H2 atmosphere at 350, 550 and 900 °C, as well, and the corresponding samples are denoted as CST-H-x, x = 350, 550 and 900 (see experimental section for more details). 2. EXPERIMENTAL SECTION 2.1 Preparation of carbonaceous microsphere Carbonaceous microspheres were synthesized through the emulsion polymerization 4


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reaction. In a typical procedure, 75 mL aqueous sugar solution (1.5 mol L-1) was filled in a 100 mL Teflon-lined stainless steel autoclave. Afterward, the autoclave was sealed and heated at 190 °C for 1.5 h. The precipitated brown powders were separate from the remaining aqueous solution by centrifugation and washed thoroughly with ethanol for several times, and finally dried at 120 °C overnight. The prepared carbonaceous microspheres were denoted as CS. 2.2 Preparation of TiO2 hollow sphere 0.2 g newly prepared carbonaceous microspheres were dispersed in 20 mL absolute ethanol with the aid of ultrasonication. After ultrasonic dispersion for 20 min, 0.5 mL TiCl4 was carefully added to the as-prepared slurry with gentle stirring in ice-water bath to avoid a drastic hydrolysis of TiCl4 in water at room temperature. The resulting suspension was aged for 5 h in ice-water, filtered, washed, and dried at 100 °C for 12 h. The powders were then heated to 500 °C in air at the rate of 1 °C min-1 for 1 h to remove the carbonaceous template. TiO2 hollow sphere was subsequently formed as a white-powder product. The TiO2 with 1 wt. % Pt loaded was synthesized by using the conventional wetness impregnation method (denoted as CST). 2.3 Preparation of cage-like TiO2 hollow sphere The CST was annealed in 8 vol. % H2/N2 or N2 atmosphere (40 mL min-1) at 750 °C for 4 h, and the final products are denoted as CST-H-750, CST-N-750, respectively. Additionally, the CST was reduced in 8 vol. % H2/N2 atmosphere (40 mL min-1) at 350, 550 and 900 °C for 4 h, as well, and the corresponding samples are denoted as CST-H-x, x = 350, 550 or 900, respectively. For comparison, we destroyed 5



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the structure of CST-H-750, denoted as CSTD-H-750. 2.4 Preparation of TiO2 solid sphere The hydrogenated solid spheres were prepared by adding 1 mL deionized water in the preparation process of cage-like TiO2 hollow sphere without any other changes. 0.2 g newly prepared carbonaceous microspheres were dispersed in 20 mL absolute ethanol and 1mL deionized water with the aid of ultrasonication. After ultrasonic dispersion for 20 min, 0.5 mL TiCl4 was added to the as-prepared slurry with gentle stirring in ice-water bath. The resulting suspension was aged for 5 h in ice-water, filtered, washed, and dried at 100 °C for 12 h. The resultant composite microspheres were heated to 500 °C in air at the rate of 1 °C min-1 for 1 h. Then, the white powders were loaded with 1 wt. % Pt by using the wetness impregnation method. The obtained dried powders were reduced in 8 vol. % H2/N2 atmosphere (40 mL min-1) at 750 °C for 4 h, and the final samples are denoted as CSTP-H-750. 2.5 Materials characterization The scanning electron microscopy (SEM) images of the samples were obtained by using a Hitachi S-4800 scanning electron microscope with an accelerating voltage of 10 kV. Before measurement, the samples were dispersed and fixed on a conducting resin. The images of transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), high-resolution transmission electron microscopy (HRTEM) and the corresponding EDS line-scanning and element mapping were taken by using JEOL-JEM-2100F electron microscope operating at 200 kV. Before 6


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measurement, the samples were ultrasonically suspended 15 min and deposited onto an ultrathin carbon film coated on the Cu grids. The UV–vis diffuse reflectance spectra (UV-vis DRS) of the catalysts were obtained on a Lambda 750S UV–vis-NIR spectrometer (PerkinElmer) equipped with an integrating sphere. BaSO4 was used as the reflectance standard. The DRS spectra were collected in 200–800 nm. The X-ray powder diffraction (XRD) patterns were obtained on an D/max 2500v/pc (Rigaku) diffraction instrument by using Cu Kα radiation source (λ = 0.15418 nm, 40 kV, 200 mA). The data of 2θ from 15 to 90 ° were collected with the step size 0.02 °. The X-ray photoelectron spectra (XPS) were recorded on a PHI-1600 ESCA spectrometer using Mg Kα excitation source (hν = 1253.6 eV). The base pressure was about 5 × 10−8 Pa. The binding energies were calibrated using C 1s peak at 284.6 eV as standard and quoted with a precision of ± 0.2 eV. The measurement of the surface area was carried out at -196 °C on Quantachrome QuadraSorb SI instrument by using the nitrogen adsorption method. The catalysts were pretreated in vacuum at 300 °C for 8 h before experiments. The surface area was determined by BET method in 0-0.3 partial pressure range. Raman measurements were performed with a Raman spectrometer (Renishaw, inVia reflex) using an excitation laser wavelength of 532 nm. The laser power is 6.0 mW. The Photoluminescence (PL) emission spectra were recored on the Fluorolog-3 photoluminescence spectrometer (Horiba Jobin Yvon, Japan) at room temperature 7



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with an excitation wavelength at 325 nm. The X-ray absorption fine structure (XAFS) spectroscopy was performed at room temperature in transmission mode on beam-line BL14W1 (the Shanghai Synchrotron Radiation Facility (SSRF), China) operating at 3.5 GeV with 250 mA. The harmonic content in monochrome beam line was reduced with a Si (111) double-crystal monochromator. The back subtracted EXAFS function was converted into k space and weighted by k3 to compensate for the diminishing amplitude. The Fourier transforming of k3-weighted EXAFS data was performed in the range of k=3-11.5 Å−1 using a Hanning function window. The EXAFS curve fitting of standard reference samples includes anatase TiO2 and rutile TiO2. 2.6 Hydrogen evolution measurements The H2 evolution experiments were performed in a Labsolar-III AG system (Beijing Perfectlight Technology Co., Ltd). In a typical photocatalytic reaction, the 20 mg catalyst powders were suspended by using a magnetic stirrer in 100 mL aqueous solution containing 50 vol. % CH3OH. The reaction temperature was maintained at 7 °C. The amount of H2 evolved was measured by an on-line gas chromatograph (Beifen 3420). The light source in the above experiments was a Xenon lamp (300 W) with an AM 1.5 G filter (Beijing Perfectlight Technology Co., Ltd).

3. RESULTS AND DISCUSSION Figure 2 shows scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the as-prepared catalysts. The SEM images of CS 8


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(Figure 2A and S1A) clearly demonstrate the uniform solid spheres with diameters of around 1.75 µm. Figure 2B shows the SEM and TEM images of CST, which is composed of uniform hollow spheres with the diameter of around 0.75 µm. Figure 2C-2F show the TEM and SEM images of hydrogenated hollow samples CST-H-350, CST-H-550, CST-H-750 and CST-H-900, respectively. The samples keep the hollow structures with no pores on the shell until the hydrogenation temperature rises up to 750 °C. After annealed at 750 °C in H2 or N2, the cage-like hollow spheres of CST-H-750 (Figure 2E and S1B) or CST-N-750 (Figure 2G) were obtained, respectively. Further increase of the annealing temperature to 900 °C leads to the shrink of the cage-like spheres (Figure 2F). Figure 2H shows the SEM image of CSTD-H-750, and the cage-like hollow structures of CST-H-750 are mostly destroyed. Figure 3 shows the X-ray powder diffraction (XRD) patterns of CST, CST-H-350, CST-H-550, CST-H-750 and CST-H-900. The XRD pattern of CST exhibits a mixed phase of anatase and rutile of TiO2, as well as CST-H-350, CST-H-550 and CST-H-750. We have calculated the weight fraction of anatase (fA) in these samples with the following equation and added the data in Table S1.24,25 fA = ( 1+1.26IR/IA)-1

(1)

where IR and IA are the intensities of the rutile (110) and anatase (101) peaks, respectively. The anatase contents of the samples are almost the same with the hydrogenated temperature up to 550 °C. The hydrogenated temperature further increased to 750 °C leads to the phase transfer from anatase to rutile and the 9



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formation of the diffraction peak at 2θ = 41.2 °, which can be indexed to the (111) crystal plane of rutile TiO2. Recent research demonstrates that the coexistence of both (110) and (111) faces in rutile TiO2 promotes the photo-induced electron transfer from (111) to (110) face.26 Thus, the (111) and (110) facets of rutile TiO2 frequently act as oxidative sites and reductive ones for trapping photo-induced holes and electrons, respectively, and the recombination of the holes and electrons will be suppressed.27 In this work, the coexistence of the (111) and (110) facets of rutile TiO2 in CST-H-750 may also inhibit the recombination of the holes and electrons, and improve the photocatalytic activity. Additionally, heterojunction of anatase/rutile is commonly reported to play a positive role in promoting the photocatalytic activity.28 The ratio of anatase to rutile in CST, CST-H-350 and CST-H-550 is ~4:1, which means that the proportion of the anatase/rutile interface of the samples might be similar. The ratio of anatase to rutile in CST-H-750 changes to ~ 1:4 (Table S1). Thus, the number of heterojunctions of anatase/rutile might be similar in this system due to the almost unchanged proportion of interface between the anatase/rutile phases. For CST-H-900, the peaks of the anatase phase nearly disappear, and only the rutile phase is present. In addition, a new peak at 40.5 ° is detected in the XRD pattern of CST-H-900, which is probably contributed by Pt3Ti alloy phase (JCPDS No. 65-3259). In addition, we measured the surface area of the samples, and the BET data are in Table S2. The specific surface area of CST, CST-H-x (x=350, 550, 750 or 900)， CST-N-750 and CSTD-H-750 is 49.4, 45.7, 40.2, 32.3, 25.7, 34.4 and 33.9 m2 g-1, respectively. It decreases with the anneal temperature increases in H2 or N2 atmosphere. The surface 10


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area of CST-H-750 is lower than that of CST, CST-H-350 and CST-H-550, and the destruction of the unique cage-like hollow architecture of CST-H-750 has little influence on the surface area. Figure 4A shows the Scanning transmission electron microscopy (STEM) image of CST-H-750. The corresponding profiles of Energy dispersive X-ray spectroscopy (EDS) line-scanning, taken across a single hollow sphere along the blue line from left to right in Figure 4A, are shown in Figure 4B. In general, the peak intensity depends on the element concentration at the corresponding location. It is clear that the peak intensity of titanium and oxygen decrease simultaneously in the area of the hollow chamber. Figure 4C and 4D show the EDS element mapping of CST-H-750. As depicted in EDS element mapping, the titanium and oxygen are homogenously distributed in the region of the shell. The EDS line-scanning and element mapping results further indicate the cage-like structure of CST-H-750. The porosity not only improves light irradiation into the interior cavity, but also accelerates the mass transfer of reactants and products.14 Figure 4E shows the high-resolution transmission electron microscopy (HRTEM) image of CST-H-750. It clearly reveals lattice fringes spacing of 2.188 Å and 1.962 Å, corresponding to the (111) plane of rutile TiO2 and (200) plane of Pt, respectively. The anatase TiO2 of the samples predominantly expose (101) low energy facets (Figure S2A).29 We observed a disordered surface layer (~ 2 nm) coating on the crystalline core in the HRTEM image of CST-H-750. However, CST-N-750 is completely crystalline, even for the surface layer of the nanocrystal (Figure S2D). This difference implies the surface disorder of CST can be induced by 11



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hydrogenation.6,23 To further investigate the disordered surface layer of CST-H-750, the Fast Fourier Transform (FFT) on the selected area marked with red dash line box was performed in Figure 4F, as well as the Inverse Fast Fourier Transform (IFFT) in Figure 4G. The IFFT image shows two different areas divided by the yellow dash line. The left part is an ordered lattice area, and the right part is a significantly disordered lattice area of destroyed lattice periodicity. It suggests the disordered nature of the surface layer on CST-H-750.17,21 The above results (SEM, TEM and HRTEM) demonstrate that the cage-like hollow spheres TiO2 with disordered surface layer have been successfully synthesized by hydrogenation. Figure 5 shows the UV–vis diffuse reflectance spectra (UV-vis DRS) of the samples. CST-N-750 exhibits stronger light harvesting ability, as compared with CST. It could mainly be attributed to the enhanced multiple reflection within the cage-like hollow chamber. As compared with CST-N-750, the light absorption of CST-H-750 can be further enhanced in visible light region, because of the disordered surface layer and the oxygen vacancies (VO) generated by hydrogenation.6,30,31 The other hydrogenated samples also exhibit higher visible light absorption and the red shift of the absorption edge, as compares with CST. Additionally, to prove the multiple reflection enhanced by cage-like hollow structure, we destroyed the structure of CST-H-750, i.e. CSTD-H-750. As we expect, the light absorption of CSTD-H-750 dramatically decrease in both the ultraviolet and visible light region. Summarily, our results provide the direct proof that the enhanced light absorption of CST-H-750 is mainly due to the unique cage-like spherical structures and hydrogenation. 12


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Figure 6A shows the valence band XPS of CST and CST-H-750. The hydrogenated sample CST-H-750 has an obvious band tail induced by the lattice disorder,32 while no tail is observed for CST. The electronic transitions from the tailed valence band and VO to the conduction band are responsible for the visible light absorption of hydrogenated samples.7,23 Therefore, both the red shift of the absorption edge and the band tail can produce more charge carriers on the samples. Figure 6B shows the photoluminescence (PL) emission spectra of CST and CST-H-750 in the wavelength range of 350-700 nm with the excitation at 325 nm. PL emission spectra are employed to evaluate the migration and separation of charge carriers, since the PL emission results from the recombination of electrons and holes.3 The emission peaks of CST and CST-H-750 at 415 nm and 427 nm are attributed to the emission of band gap transition.33 The PL intensities of CST-H-750 are much lower than that of CST. Considering that the difference in absorption cross section could be a factor resulting in the changes on PL intensities, we compared the absorbance at 325 nm for CST-H-750 and CST in Figure 5. The higher absorbance at 325 nm for CST-H-750 reveals that there are more photo-generated electrons and holes in CST-H-750, compared with CST. Therefore, the decreased PL intensities of CST-H-750 are mainly due to the inhibited recombination of photo-generated electrons and holes after hydrogenation. According to the UV-vis DRS, valence band XPS and PL results, the amount of working electrons and holes on the surface of CST-H-750 was increased by hydrogenation. Figure 7 shows the XPS survey spectra of the samples. The Ti 2p XPS survey 13



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spectra collected from the as-prepared samples are similar (Figure 7A). Their Ti 2p XPS spectra are identical with Ti 2p3/2 and Ti 2p1/2 peaks centered at binding energies of around 458.3 and 464.1 eV, which are typical for the Ti4+−O bonds in TiO2.3,32 The absence of Ti3+ in Ti 2p XPS survey spectra of CST-H-750 and CST-H-900 indicates that the Ti3+ cations are mainly located in the bulk, which will benefit the stability of the samples. In Figure 7B, the O 1s XPS spectra of the samples show a broad peak with an additional shoulder at higher binding energies and the broad peak can be deconvoluted into two peaks. The peak at around 529.7 eV is typically assigned to surface lattice oxygen (OL), while the peak at around 531.0 eV is attributed to surface Ti−OH groups (OOH).31 The ratio of OOH/(OOH+OL) of CST-H-750 is highest in Table S3, which indicates the formation of high concentration of hydroxyl groups on the TiO2 surface after hydrogenation. The bond between surface lattice oxygen and Ti is broken by the dissociated H• in the hydrogenation process, forming a hydroxyl group and VO (Scheme S1).17 Accordingly, the OOH signal can be indirectly related to the formation of VO. In addition, the high concentration of hydroxyl on the surface may stabilize the lattice disorder by passivating the dangling bonds.6 Figure 8 shows Raman scattering spectra of the samples. The Raman-active modes of anatase phase and rutile phase were detected in the investigated samples. The Raman bands located at around 143, 195, 395, 514 and 636 cm-1 are attributed to anatase phase,34 and the bands located at around 237, 447and 610 cm-1 are assigned to rutile phase.32,35-36 The change in the Eg mode can serve as the direct evidence for the presence of VO in a tetragonal system.37 VO play an essential part in acting as electron 14


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traps, when the electron-hole pair creation occurs by photoexcitation.33 These behaviours will separate the charge carriers and retard the recombination significantly. Furthermore, the generation of VO will substantially enhance the electrical conductivity, as well as the charge transfer.31 The right insert of Figure 8 is the most intense Eg peak (~ 143 cm-1) of anatase phase for the samples. The Eg peak shifts toward higher wavenumbers with the hydrogenated temperature rising from 350 to 750 °C (Table S4), indicating the increased amount of defects on the TiO2 nanoparticles. Compared with CST-N-750, the large blue-shift of the most intense Eg peak (~ 143 cm-1) for CST-H-750 demonstrates the presence of the larger number of VO in the lattice structure, further implying that the generation of the VO results from the oxygen extraction by hydrogenation.32,37-38 To gain further insights on the change of local coordination structures, we performed the experiments of extended X-ray absorption fine structures (EXAFS) (Figure 9). The first coordination shell (marked with red dash line box in Figure 9A) is attributed to the scattering pathway of the Ti−O bond. The curve fitting of the shell was performed, and Table 1 gives the relevant local coordination structural parameters.38-40 The curve-fitted results are in well accordance with the experimental data (Figure 9B).41 In Table 1, CST-H-750 possesses a Ti−O shell with a bond distance of 1.971 Å and a coordination number of 5.4, which are different from the bond distance (1.955 Å) and the coordination numbers (5.9) of CST. The length of the Ti−O bond neighboring the VO and surface hydroxyl groups will be elongated, which has been supported by the previous theoretical studies.42,43 The lengthened bond and 15



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increased coordination numbers verify the existence of the abundant defects in CST-H-750, which is consistent with the Raman results (Figure 8). However, both the length (1.964 Å) and the coordination numbers (5.9) of the Ti−O bond have little change in CST-N-750, as compared with CST. It confirms that the structure changes in CST-H-750 is mainly due to the extraction of oxygen surrounding the Ti site in the TiO6 octahedra by hydrogenation.17 Moreover, a new small peak appeared at 2.20 Å in CST-H-750 and CST-H-900 (Figure S3), which can be attributed to Ti2O3.38 It further proves the presence of Ti3+ in the bulk of CST-H-750 and CST-H-900. However, this coordination peak is absence in CST-N-750, indicating that the presence of Ti3+ mainly results from hydrogenation. Table 1. Local structural parameters in the first Ti−O shell of the Ti K-edge EXAFS of the CST, CST-H-x (x = 350, 550, 750 or 900) and CST-N-750.

Sample

CST

a)

Rb)

σ2c)

∆E0d)

Rfe)

(Å)

(×10-3Å2)

(eV)

(%)

Shell CNa)

Ti−O

5.9

1.955

4.0

-1.8

0.90

CST-H-350 Ti−O

5.9

1.963

4.7

-1.0

0.77

CST-H-550 Ti−O

5.7

1.969

4.3

-1.5

1.36

CST-H-750 Ti−O

5.4

1.971

4.5

-3.5

1.23

CST-H-900 Ti−O

5.4

1.980

4.7

-3.3

0.83

CST-N-750 Ti−O

5.9

1.964

5.0

-3.5

0.98

Coordination number; b)Coordination distance; c)Debye-Waller factor; d)Inner

Figure 10 shows the photocatalytic activity for H2 evolution under the simulated solar 16


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light irradiation. The rate of H2 evolution for CST-N-750 is 41.3 ± 2.2 µmol h-1, which is higher than that for CST (23.9 ± 1.2 µmol h-1). Considering the UV-vis DRS and the BET results, it is mainly attributed to the enhanced light absorption induced by the cage-like structures. The rate further increases with the hydrogenation temperature up to 750 °C, and a maximum value of 212.7 ± 10.6 µmol h-1 is obtained for CST-H-750. The order of the H2 evolution rate of the hydrogenated samples is CST-H-900 < CST-H-350 < CST-H-550 < CST-H-750. Nevertheless, the anatase contents and the number of heterojunctions of CST, CST-H-350 and CST-550 are almost the same (Table S1), which means that phase compositions and heterojunctions are not the main factors in this photocatalytic system. Furthermore, the surface area of CST-H-750 is lower than that of CST, CST-H-350 and CST-H-550 (Table S2), indicating that the surface area is not the key factor either. The concentration of hydroxyl groups and VO increases with hydrogenation temperature, as shown in XPS, Raman and EXAFS results, respectively. The results indicate that the high concentration of hydroxyl groups and VO is beneficial to the enhancement of the photocatalytic activities.3 In addition, coexistence of both (110) and (111) crystal faces in rutile TiO2 of CST-H-750 induced by high temperature hydrogenation will suppress the recombination of the holes and electrons and could promote the photocatalytic activity.26,27 In fact, we destroyed the structure of CST-H-750 (i.e. CSTD-H-750) to reduce the light absorption, and CSTD-H-750 shows a decreased activity of 69 %. Since CSTD-H-750 and CST-H-750 possess the similar surface areas and phase contents, the superior performance of CST-H-750 can be attributable to the 17



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effects of both the cage-like structures and hydrogenation. Surprisingly, the rate dramatically decreases for CST-H-900, even though it possesses high light absorption and cage-like structure. Probably, this deactivation is due to the complete phase transition from anatase to rutile, the sharply decreased surface areas, as well as the formation of the Pt3Ti alloy, as indicated by the XRD results (Figure 3). For comparison, we prepare hydrogenated TiO2 solid sphere by similar synthesis method with CST-H-750. However, without the unique cage-like hollow structure, the rate of H2 evolution of the as-prepared solid spheres is only 17.7 ± 0.9 µmol h-1, which is ~ 1/12 lower than that of CST-H-750 (Figure S4). In our previous study, we reported the high photocatalytic activity of the hydrogenated commercial P25 loaded with 1 wt. % Pt, i.e. Pt/P25-400 for H2 evolution under the simulated solar light irradiation (Figure S5).17 In this system, the hydrogen evolution rate of CST-H-750 is ~ 9 times higher than CST, and ~ 1.5 times higher than Pt/P25-400. It indicates that the cage-like structure plays a key role for light harvesting to induce the high photocatalytic activity. 4. CONCLUSIONS In summary, we report the high photocatalytic performance for H2 evolution over the hydrogenated cage-like TiO2 hollow spheres under simulated solar light irradiation. The original hollow spheres (CST) were synthesized with a facile sacrificial template method. The following hydrogenation treatment successfully generated the cage-like pores and disordered surface layer, as shown in SEM, TEM and HRTEM images. The rate of H2 evolution increases with the hydrogenation temperature from 350 to 750 °C, 18


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and a maximum value is obtained for CST-H-750. The high rate is ~ 12 times higher than the hydrogenated TiO2 solid spheres and is ~ 9 times higher than CST. Furthermore, the photocatalytic activity decreased 69 %, after we destroyed the structure of CST-H-750 to reduce the light absorption. The XPS, Raman and EXAFS results indicate that CST-H-750 has a high concentration of hydroxyl groups and VO introduced by hydrogenation, as compared with CST. These species will stabilize the lattice disorder and separate the charge carriers, respectively. Additionally, the Ti3+ cations are mainly located in the bulk, which will benefit the stability of the samples. We believe that the effect of the unique architectures and hydrogenation lead to the high solar light utilization and low charge carries recombination, which induce the high photocatalytic performance for H2 evolution. The catalyst design and preparation strategy in this work are expected to apply to other photocatalytic systems in the areas of energy conversion and environmental protection. ASSOCIATED CONTENT Supporting Information: The Low-magnification SEM images of CS and CST-H-750, the HRTEM images of CST-H-750 and CST-N-750, the atomic structure of TiO2 before and after hydrogenation, the EXAFS results, and the time-dependent H2 evolution over CSTP-H-750 and Pt-P25/400 under simulated solar light.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] Notes 19



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The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21476159, U1332102), the 973 program (2014CB932403), and the Natural Science Foundation of Tianjin, PR China (15JCZDJC37400). Authors are also grateful to the Program of Introducing Talents of Disciplines to China Universities (B06006). REFERENCES (1) Wang, W.; Chen, J.; Li, C.; Tian, W. Achieving Solar Overall Water Splitting with Hybrid Photosystems of Photosystem II and Artificial Photocatalysts. Nat. commun. 2014, 5, 5647. (2) Fujishima, A. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. (3) Cai, J.; Zhu, Y.; Liu, D.; Meng, M.; Hu, Z.; Jiang, Z. Synergistic Effect of Titanate-Anatase Heterostructure and Hydrogenation-Induced Surface Disorder on Photocatalytic Water Splitting. ACS Catal. 2015, 5, 1708-1716. (4) Cai, J.; Wang, Y.; Zhu, Y.; Wu, M.; Zhang, H.; Li, X.; Jiang, Z.; Meng, M. In Situ Formation of Disorder-Engineered TiO2(B)-Anatase Heterophase Junction for Enhanced Photocatalytic Hydrogen Evolution. ACS Appl. Mater. Interfaces 2015, 7, 24987-24992. (5) Xu, H.; Ouyang, S.; Liu, L.; Reunchan, P.; Umezawa, N.; Ye, J. Recent Advances in TiO2-Based Photocatalysis. J. Mater. Chem. A 2014, 2, 12642-12661. (6) Chen, X.; Liu, L.; Yu, P.; Mao, S. Increasing Solar Absorption for Photocatalysis 20


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in Hydroxylated and Reduced Rutile TiO2 (110) Surfaces. Phys. Rev. Lett. 2006, 97, 166803. (43) Cheng, H.; Selloni, A. Surface and Subsurface Oxygen Vacancies in Anatase TiO2 and Differences with Rutile. Phys. Rev. B 2009, 79, 092101.

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FIGURE CAPTIONS Figure 1. (A) Schematic illustration of the comparison of light harvesting within the TiO2 (a) solid sphere, (b) hollow sphere and (c) hollow cage-like sphere; (B) Synthetic scheme for hydrogenated cage-like TiO2 hollow spheres (CST-H-750). Figure 2. SEM images of (A) CS, (B) CST, (C) CST-H-350, (D) CST-H-550, (E) CST-H-750，(F) CST-H-900, (G) CST-N-750 and (H) CSTD-H-750, and the right insets of images (B)-(G) are the TEM images of the corresponding samples.

Figure 3. (A) The XRD patterns of the samples; (B) The regional enlarged XRD pattern marked with red dash line box in (A) of the corresponding samples.

Figure 4. (A) TEM image of CST-H-750; (B) The corresponding profile of EDS line-scanning along the blue line in (A); (C) and (D) are corresponding EDS element mapping of CST-H-750; (E) HRTEM image of CST-H-750; (F) FFT image of the selected area in (E) marked with red dash line box; (G) IFFT of image (F) performed on the red arrow pointed spots.

Figure 5. The UV-vis DRS spectra of the samples normalized at 200 nm.

Figure 6. (A) Valence band XPS spectra of the samples, and the thin black dash lines show the linear extrapolation of the curves for deriving the band edge position of the TiO2 samples; (B) The photoluminescence emission spectra of the samples. Figure 7. (A) Ti 2p and (B) O 1s XPS spectra of the samples.

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Figure 8. Raman spectra of (a) CST, (b) CST-H-350, (c) CST-H-550, (d) CST-H-750, (e) CST-H-900 and (f) CST-N-750. Inset: the most intense Eg peak of anatase TiO2. Figure 9. (A) Fourier transformed of filtered EXAFS oscillation k3•χ(k) into R space; (B) Corresponding filtered k3•χ(k) of Ti K edge of different samples, where the red lines correspond to the curve fitting results in k space.

Figure 10. Time-dependent H2 evolution over the samples under the simulated solar light.

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Table of Contents (TOC) Graphic

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Figure 1. (A) Schematic illustration of the comparison of light harvesting within the TiO2 (a) solid sphere, (b) hollow sphere and (c) hollow cage-like sphere; (B) Synthetic scheme for hydrogenated cage-like TiO2 hollow spheres (CST-H-750). 83x75mm (220 x 220 DPI)


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Figure 2. SEM images of (A) CS, (B) CST, (C) CST-H-350, (D) CST-H-550, (E) CST-H-750，(F) CST-H-900, (G) CST-N-750 and (H) CSTD-H-750, and the right insets of images (B)-(G) are the TEM images of the corresponding samples. 83x114mm (220 x 220 DPI)



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Figure 3. (A) The XRD patterns of the samples; (B) The regional enlarged XRD pattern marked with red dash line box in (A) of the corresponding samples. 470x442mm (300 x 300 DPI)


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Figure 4. (A) TEM image of CST-H-750; (B) The corresponding profile of EDS line-scanning along the blue line in (A); (C) and (D) are corresponding EDS element mapping of CST-H-750; (E) HRTEM image of CST-H750; (F) FFT image of the selected area in (E) marked with red dash line box; (G) IFFT of image (F) performed on the red arrow pointed spots. 91x126mm (220 x 220 DPI)



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Figure 5. The UV-vis DRS spectra of the samples normalized at 200 nm. 225x208mm (300 x 300 DPI)


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Figure 6. (A) Valence band XPS spectra of the samples, and the thin black dash lines show the linear extrapolation of the curves for deriving the band edge position of the TiO2 samples; (B) The photoluminescence emission spectra of the samples. 292x534mm (300 x 300 DPI)



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Figure 7. (A) Ti 2p and (B) O 1s XPS spectra of the samples. 111x202mm (300 x 300 DPI)


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Figure 8. Raman spectra of (a) CST, (b) CST-H-350, (c) CST-H-550, (d) CST-H-750, (e) CST-H-900 and (f) CST-N-750. Inset: the most intense Eg peak of anatase TiO2. 179x172mm (300 x 300 DPI)



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Figure 9. (A) Fourier transformed of filtered EXAFS oscillation k3•χ(k) into R space; (B) Corresponding filtered k3•χ(k) of Ti K edge of different samples, where the red lines correspond to the curve fitting results in k space. 470x374mm (300 x 300 DPI)


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Figure 10. Time-dependent H2 evolution over the samples under the simulated solar light. 237x219mm (300 x 300 DPI)



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Table of Contents (TOC) Graphic 81x40mm (220 x 220 DPI)


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Hydrogenated Cagelike Titania Hollow Spherical Photocatalysts for

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