Oxygen generation is the key step for the photocatalytic overall water splitting and considered to be kinetically more  challenging than hydrogen generation. Here, effective water oxidation catalyst of hierarchical FeTiO3-TiO2 hollow spheres  are prepared via two-step sequential solvothermal processes and followed by thermal treatment. The existence of effective  heterointerface and built-in electric field in the surface space charge region in FeTiO3-TiO2 hollow spheres plays a positive  role in promoting the separation of photoinduced electron-hole pairs. Surface photovoltage, transient-state photovoltage,  fluorescence and electrochemical characterization are used to investigate the transfer process of photoinduced charge  carriers. The photogenerated charge carriers in the hierarchical FeTiO3-TiO2 hollow spheres with a proper molar ratio  display much higher separation efficiency and longer lifetime than those in the FeTiO3 alone. Moreover, it is suggested that  the hierarchical porous hollow structure can contribute to the enhancement of light utilization, surface active sites and  material transportation through the framework walls. This specific synergy significantly contributes to the remarkable  improvement of the photocatalytic water oxidation activity of the hierarchical FeTiO3-TiO2 hollow spheres under simulated  sunlight (AM1.5).  

1. Introduction  

Recent Years, steadily growing energy crisis has directed a global  trend toward sustainable development.1-3 Access to alternative clean  energy is expected to make considerable contribution. Solar induced  water splitting has been considered as one of the most attractive  routes because of its potential for hydrogen and oxygen production  without the use of fossil fuel energy.4-6 The major challenge for  water splitting is the oxygen evolution, which is an uphill reaction  involved a four-electron transfer process, so the research on the  photocatalytic oxygen evolution has recently been intensified as it  serves as the key step for the artificial overall water splitting.7-10 Currently, most of the effective water oxidation catalysts contain  noble metals iridium and ruthenium as active species, which limits  their widespread application.11-15 Considering the practical  application, apart from simple metal oxides (Fe2O3 ,WO3 , MnOx,),  complicated binary and ternary oxides, such as, Co-g-C3N4 ,  LaTiO2N14, RTiNO2, Co3O4 /SiO2 , have also been investigated.16-18  However, until now, the efficiency of photocatalytic water oxidation  is still moderate, which extremely restricts its practical application.  Thus, tremendous efforts are still needed to improve their catalytic  performance.   In recent years, titanium based perovskite oxides such as ATiO3 (A=Zn, Sr, Co, Fe and Ni) have been identified as a kind of  significant photocatalyst for the photocatalytic degradation of toxic  pollutants, photocatalytic hydrogen and oxygen production.19-23 For  example, CoTiO3  could be regarded as visible light-driven  photocatalyst for water oxidation.24 SrTiO3  was used as photocatalyst  for photocatalytic hydrogen evolution.25,26 NiTiO3  and FeTiO3 exhibited high visible light photocatalytic activity in removing organic pollutants.27-30 But as far as we know, FeTiO3  and its  composites have not been applied for photocatalytic oxygen  evolution from water splitting. Inspired by the previous studies about  other titanium based perovskite oxides for photocatalytic water  oxidation, it is expected to explore the synthesis and application of  FeTiO3  for photocatalytic water oxidation. Because the improved  interface transfer rate of charge carriers can play a prominent role in  promoting the water oxidation rate, coupling of FeTiO3  with a wide  bandgap semiconductor (e.g. TiO2 ) with proper valence band  position may expect to construct FeTiO3 -based heterostructure  composites and improve the transfer of photogenerated holes from  FeTiO3  to the wide bandgap semiconductor, so contributing to the  enhancement of photocatalytic oxidation ability.31 It is known that  morphology and structure are important factors influencing the  photocatalytic water oxidation properties of semiconductor  photocatalyst.32 Owing to the special properties (low density, large surface area and high light-harvesting efficiency), hollow micro- /nano-structure offer some potential for photocatalytic application.33 To this end, Wang and coworkers have carried out lots of significant  works to synthesize hollow carbon nitride nanospheres to develop  functional photosynthetic structures for solar energy application.34 Motivated by the high photocatalytic performance of hollow spheres,  it is expected to explore the synthesis of hierarchical FeTiO3 -TiO2 hollow spheres to achieve enhanced photocatalytic water oxidation  activity under simulated sunlight irradiation by reducing the  recombination rate of the charge carriers, decreasing the energy  barrier of the water oxidation kinetics, enhancing the lightharvesting efficiency, and increasing surface catalytic active sites.  

In the present study, we summarized our recent efforts towards the  facile synthesis and photocatalytic application of hierarchical  FeTiO3 -TiO2  hollow spheres. Different from the previously reported  methods, hierarchical titanium-glycerolate-iron complex hollow  sphere precursors were first prepared via two-step sequential  solvothermal processes, then, after calcination, the hierarchical  FeTiO3 -TiO2 hollow spheres can be prepared. Changing the ratio of  the molar of Ti and Fe added to the solution, the hierarchical FeTiO3 hollow spheres can also be obtained. The novel hierarchical FeTiO3- TiO2  hollow spheres showed enhanced photocatalytic water  oxidation performance and excellent recycling stability. We also  demonstrated the well designed synergistic effects in the  photocatalysts, including the semiconductor heterojunction effect,  matched energy level position, as well as the special hierarchical  hollow structural advantage, could efficiently promote the  photogenerated charge carriers separation and transfer across the  interfacial domain. As expected, the optimal hierarchical FeTiO3- TiO2  hollow spheres photocatalytic system exhibited ~2-fold  enhancement in photocatalytic oxygen production as compared to  pure FeTiO3 .  

2. Experimental section

2.1 Preparation of FeTiO3 -TiO2  hollow spheres

In the typical experiment, 0.7 mL Tetrabutyl titanate (TBOT) and  5mL glycerol (Gly) were dissolved in 25 mL ethanol to form a clear  solution. Then the solution was transferred to a 50 mL Teflon-lined  stainless steel autoclave. After which the autoclave was heated to  180°C, and keeping this temperature for 24 h. After the solvothermal  reaction, the reactor was naturally cooled to room temperature. The  obtained solid product was transferred into another 50 mL Teflonlined stainless steel autoclave. Then, 0.8310 g Fe(NO3)3 ·9H2O, 3 mL  glycerol and 25 mL ethanol were added to the autoclave to well mix  the mixture. The autoclave containing mixture solution was then  heated to 180 ℃ and maintained for 12 h. Subsequently, the autoclave  was naturally cooled to room temperature. The precipitate was  washed by absolute ethanol for three times and dried at 70 °C in air.  The resulting green powders were calcined in static air at 550 °C for  6 h at a heating rate of 2 °C min-1, and FeTiO3 -TiO2  hollow spheres  were obtained. Similarly, FeTiO3  hollow spheres can be prepared by  adjusting the mole ratio of Ti and Fe.

2.2 Characterization

Powder X-ray diffraction (XRD) data of the samples were recorded  with Bruker D8 Advance using Cu Ka radiation source (40 kV, 44  mA). Scanning electron microscopy (SEM) characterizations were  performed on a Hitachi S-4800 electron probe microanalyzer.  Transmission electron microscopy (TEM) studies were carried out  with a JEOL 2100 TEM microscope operated at 200 kV. XPS (X-ray  photoelectron spectroscopy) analysis was performed on a VG  ESCALABMK II with a Mg Kaachromatic X-ray source (1253.6  eV). UV-vis diffuse reflflectance spectra (DRS) were determined by a  UV-vis spectrophotometer (ShimadzuUV-2550). The N2  adsorption– desorption isotherms of as-prepared samples were conducted by  using a Micromeritics Tristar II. The fluorescence spectra (PL) of the  samples at room temperature were characterized via the fluorescence  spectrophotometer (F-7000, Hitachi, Japan). The excitation  wavelength was 315 nm induced from a He-Cd laser source to excite  the samples. The XAFS data at the Ti and Fe K3-edge were  measured at room temperature in transmission mode at beamline  BL14W1 of Shanghai Synchrotron Radiation Facility (SSRF),  China.  

2.3 Electrochemical test  

Linear sweep voltammetry (LSV) and electrochemical impedance  (EIS) experiments were obtained with a Versa STAT3  electrochemical workstation in a conventional three-electrode cell.  FTO was the working electrode, Ag/AgCl (saturated KCl) was the  reference electrode, and a platinum wire having 2 cm 2 of surface area  served as the counter electrode. The working electrode was prepared  on FTO glass that was cleaned by sonication in water, acetone,  ethanol for 30 min respectively and dried at 333 K. Five milligrams  of catalyst was mixed with 1 mL of ethanol by sonication to give a  slurry mixture. The slurry was spread onto pretreated FTO glass.  After air drying, the working electrode was further dried at 373 K for  2 h to improve adhesion. The electrolyte was 1 M KOH aqueous  solution without additive (pH 14). The scan rate was 50 mV s-1.The  reference was calibrated against and converted to reversible  hydrogen electrode (RHE). All the tests were carried out at room  temperature (about 25 ℃).  

2.4 Photocatalytic water oxidation test

The photocatalytic O2  evolution from water was conducted in an  online photocatalytic oxygen production system (AuLight, Beijing,  CEL-SPH2N). For each reaction, 50 mg of catalyst powder was well  dispersed in an aqueous solution (100 mL) containing AgNO3  (0.01  M) as an electron acceptor. The reaction was carried out by  irradiating the suspension with light from a 300 W Xe lamp  (AuLight, CEL-HXF-300，Beijing) lamp with a working current of  15 A. The wavelength of the incident light was controlled by  applying some appropriate long-pass cutoff filters. Prior to the  reaction, the mixture was deaerated by evacuation to remove O2  and  CO2  dissolved in water. Gas evolution was observed only under  photoirradiation, being analyzed by an on-line gas chromatograph  (SP7800, TCD, molecular sieve 5 Å, N2  carrier, Beijing Keruida  Limited).  

3. Results and discussion  

3.1. Structural and composition characterization

As shown in Fig. 1A, the typical characteristic diffraction peaks  of anatase TiO2 were detected at 2θ= 25.4° (101), 37.9° (004) and  48.1° (200) when no Fe salt was added in the reaction system. With  the gradual introduction of the Fe salt, the XRD diffraction peaks of  FeTiO3  located at 2θ= 23.79° (012), 32.51° (104), 35.25° (110),  40.27 (113) and 53.02 (116) can be found in the different products.29 Meanwhile, superimposition XRD patterns of TiO2 and FeTiO3  can  be observed in the case of FeTiO3 -TiO2 samples (curve b-d), which  demonstrates the integration of these two compositions with high  purity and good crystallization. Moreover, with the decrease of the  molar ratio of Ti and Fe, the intensity ratio of XRD diffraction peaks  of TiO2  and FeTiO3 -TiO2 also gradually decreased, indcating the  increase of the FeTiO3  content in the composites. When the molar  ratio of Ti and Fe is 1:1, pure FeTiO3  can be prepared.

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