Taoran Han, a Yajie Chen, a Guohui Tian,*a,b Jian-Qiang Wang, c Zhiyu Ren, a Wei Zhoua and Honggang Fu*

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 Teflon lined 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 reflectance 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).

Apparent quantum yield (A.Q.Y.) was measured using a 420 nm, 450 nm and 520 nm band-pass filter and an irradiate-meter, and calculated according to the following equation:

2.5 Photocatalytic degradation test

The photodegradation experiments were performed in a slurry reactor containing 100 mL of 50 mg L-1 2, 4-dichlorophenol and 0.05 g of catalyst. A 300 W xenon lamp (Institute of Electric Light Source, Beijing) was used as the solar-simulated light source, and visible light was achieved by 420 nm cutoff filter. Prior to light irradiation, the suspension was kept in the dark under stirring for 30 min to ensure the establishing of an adsorption/desorption equilibrium. Adequate aliquots (5 mL) of the sample were withdrawn after periodic interval of irradiation, and centrifuged at 10000 rpm for 5 min, then filtered through a Milipore filter (pore size 0.22 mm) to remove the residual catalyst particulates for analysis.

In order to detect the active species during the photocatalytic reaction, isopropanol (IPA), ammonium oxalate (AO), benzoquinone (BQ) and AgNO3 were added into the 2, 4-dichlorophenol solution dispersed with the FeTiO3 -TiO2 heterostructures photocatalyst to capture hydroxyl radicals (•OH), holes (h+ ), superoxide radicals (•O2 − ), and the electrons (e- ), respectively, followed by the photocatalytic activity test.

The electron spin resonance (ESR) technique (with DMPO) was used to detect the •O2 − radical species over the catalyst on a Bruker EMX-8/2.7 spectrometer by accumulating three scans at a microwave frequency of 9.85 GHz and a power of 6.35 mW. Before testing, DMPO was added to the suspension system, and then the system was irradiated by visible light using a halogen tungsten lamp with a UV cutoff filter (λ＞400 nm).

A total organic carbon (TOC) analyzer (Analytik Jena, Multi N/C 2100S, Germany) was employed for mineralization degree analysis of the dye solutions. Prior to injection into the TOC analyzer, the samples were filtrated with a Millipore filter. All experiments were carried out at least in duplicate. The reported values were within the experimental error range of ±3%.

3. Results and discussion

3.1. Structural and composition characterization

Fig. 1 XRD spectra of the products prepared from different molar ratios of Ti and Fe, (a) 1:0, (b) 1:0.25, (c) 1: 0.5, (d) 1:0.75, (e) 1:1.

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.

Fig. 2 A, B, C are the SEM, TEM, HRTEM images of the FeTiO3, respectively; D, E, F are the SEM, TEM, HRTEM images of the FeTiO3-TiO2 (Ti:Fe=1:0.75), respectively.

The morphology and microstructure of the FeTiO3 and FeTiO3 - TiO2 (Ti:Fe=1:0.75) composite were investigated by SEM and TEM. Fig. 2A and B shows the SEM and TEM images of the FeTiO3 . As expected, these spheres are hollow structure with particle diameters of about 0.5-1 µm. Meanwhile, as revealed in Fig. 2D, E, the obtained FeTiO3 -TiO2 also kept the hierarchical hollow structure, which clearly demonstrates the hollow structure having a shell consisting of interconnected nanoflakes with a thickness of about 15 nm. The nanopores in the shell proved to be interconnected from Fig. 2B and 2E. The specific surface area as large as about 45-65 m2 g-1 and a broad pore size distribution in hollow spheres measured from the N2 adsorption–desorption curves (Fig. S1 and Table S1) also supported the TEM observation. The corresponding HRTEM image in Fig. 2C shows lattice fringes corresponding to the interplanar distance of 0.253 nm is ascribed to the (110) plane of rhombohedral FeTiO3 . 29 Fig. 2F depicts the spacings of adjacent lattice planes, which are consistent with the interplanar distance (0.253 nm) of the (110) plane of rhombohedral phase of FeTiO3 and the interplanar distance (0.352 nm) of the (101) plane of anatase TiO2 , respectively, indicating the formation of heterojunction, which can benefit better charge separation and transfer within the hybrid structure compared with pure FeTiO3 and TiO2.

In order to disclose the morphology evolution and growth mechanism of the hierarchical FeTiO3 -TiO2 hollow spheres, the SEM images of the FeTiO3 -TiO2 precursors obtained at different reaction stages are shown in Fig. S2. The morphology evolution from nanoflowers to hierarchical hollow spheres can be clearly seen. Based on the above results, the possible growth pattern and formation mechanism of hierarchical FeTiO3 -TiO2 hollow sphere precursors is shown in Scheme 1. The formation process includes two-step sequential solvothermal reactions. In the first solvothermal reaction process, amorphous titanium oxyhydrate nanospheres first formed through alcoholysis reactions between TBOT and ethanol, then glycerol gradually reacted with the formed titanium oxyhydrate to form titanium glycerolate (Ti-Gly) flower-like complexes by replacing the hydroxyls in titanium oxyhydrate. This is a dissolutionrecrystallization growth process. With the reaction going on, the growth of thorn-like Ti-Gly complexes continued at the expense of the gradual dissolution of the solid spheres, and finally the product was entirely composed of the hierarchical flower-like nanostructure. The XRD pattern of the precursor is shown in Fig.S3a, which is in agreement with our previous report. 35 In the following solvothermal process, with the addition of iron salt, further solvothermal reaction led to the formation of Ti-Gly-Fe complexes from outer surface to inside, and the formed complexes diffused spontaneously toward the outer surface. The XRD patterns of the Fe-Gly-Ti complex in Fig. S3b,c exist several sharp peaks at low angle region and several weak ones at high angle region, which is similar to that of the reported metal glycerolate complexes.35 Continuation of this process gradually dissolved the core spheres, sequentially formed hollow sphere structure. As a result from Ostwald ripening process, small nanoflakes initially formed on the surface of the hollow spheres and gradually grew up into ultrathin nanoflakes which are interlaced each other with thin edges and perpendicular to the surface of the hollow spheres. Finally, the hierarchical hollow sphere precursors were formed. After calcination, these precursors changed into FeTiO3 -TiO2 composites without changing the hierarchical hollow sphere structure. Changing the molar ratio of Ti and Fe to 1:1, pure hierarchical FeTiO3 hollow spheres can also be obtained.