FeOx Derived from an Iron-Containing Polyoxometalate Boosting the

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23135−23143

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FeOx Derived from an Iron-Containing Polyoxometalate Boosting the Photocatalytic Water Oxidation Activity of Ti3+-Doped TiO2 Yifan Wang,† Xiaohu Cao,† Qiyu Hu,† Xiangming Liang,† Tian Tian,† Junqi Lin,† Meie Yue,‡ and Yong Ding*,†,§

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†

State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China ‡ College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China § State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China S Supporting Information *

ABSTRACT: The development of efficient and stable catalyst systems using lowcost, abundant, and nontoxic materials is the primary demand for photocatalytic water oxidation. Distinguishing the true active species in a heterogeneous catalytic system is important for construction of efficient catalytic systems. Herein, hydrothermally synthesized Ti3+ self-doped TiO2, labeled as Ti3+/TiO2, was first used as a light absorber in a powder visible light-driven photocatalytic water oxidation reaction. When an iron-containing polyoxometalate Na27[Fe11(H2O)14(OH)2(W3O10)2(αSbW9O33)6] (Fe11) was used as a cocatalyst, an amorphous layer of active species was wrapped outside the initial Ti3+/TiO2 nanorod and the in situ formed composite was labeled as F/Ti3+/TiO2. When the composite F/Ti3+/TiO2 was tested as a photocatalytic water oxidation catalyst, dramatically improved oxygen evolution performance was achieved. The composite F/Ti3+/TiO2 showed an oxygen evolution rate of 410 μmol/g/h, which was about 11-fold higher than that of prism Ti3+/TiO2. After 24 h of illumination, an O2 yield of 36.4% was achieved. The contrast experiments, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy characterization demonstrated that FeOx is the true cocatalyst that enhanced the oxygen evolution activity of TiO2. A recycling experiment proved that the composite F/Ti3+/TiO2 has favorable stability in the oxygen production process. KEYWORDS: Ti3+-doped TiO2, polyoxometalate, water oxidation, semiconductor, cocatalyst

1. INTRODUCTION Using solar energy to split water into oxygen and hydrogen has been recognized as one of the optimal approaches to resolve future energy shortage and environment pollution problems.1−3 Water oxidation that involves a four-proton coupled with four-electron transfer process is more sluggish in kinetics and has been recognized as the bottleneck for the watersplitting process.4,5 Many semiconductors have been applied in photocatalytic water splitting since TiO2 was first reported as a photoanode for water-splitting reaction in 1972.6,7 TiO2 have many advantages including low cost, high stability, good biocompatibility, and high photocatalytic activity under UV light. It has been widely used in many photocatalytic reactions, such as pollution degradation, water splitting, and CO2 reduction.8,9 However, the big band gap of TiO2 (3.2 eV for anatase) confines its absorption in the UV region, which leads to low solar energy utilization efficiency. Many strategies have been proposed to extend the optical adsorption of TiO2 to the visible light region, such as metal (Au, Pt, Pd, and Ag) nanocluster grafting, and nonmetal element (N, B, S, F, and I) doping, metal oxide compositing, formation of disordered layers on the surface, Ti3+ self-doping, and so forth.10−13 Among them, doping Ti3+ into the TiO2 lattice by different © 2019 American Chemical Society

methods can introduce impure energy levels below the conduction band, which produce visible light absorption.10,14 Ti3+ self-doped TiO2 has shown great photocatalytic hydrogen evolution reaction activity.11,15−17 However, its activity in photocatalytic oxygen evolution still needs to be explored. Besides the narrow light absorption, the fast recombination of photoelectrons and holes and slow surface reaction dynamics also restrict the photocatalytic water oxidation activity of titanium dioxide.13,18,19 A general solution for these problems is to construct a junction with other semiconductors or load cocatalysts on titania, which can enhance carrier separation or accelerate surface reaction dynamics.20−22 Many noble and transition-metal oxides have been attached on TiO2 to facilitate oxygen evolution,19,20,23,24 such as using a simple impregnation and calcination process; nanoparticles of CoOx with favorable dispersion have been loaded on rutile TiO2.20 The composite presented superior water oxidation activity and showed charge transfer from the valence band of CoOx to the conduction band of TiO2.20 Received: February 28, 2019 Accepted: June 5, 2019 Published: June 5, 2019 23135

DOI: 10.1021/acsami.9b03714 ACS Appl. Mater. Interfaces 2019, 11, 23135−23143

Research Article

ACS Applied Materials & Interfaces

TiO2 (labeled as F/Ti3+/TiO2) was collected for photocatalytic reaction. 2.5. Photocatalytic Water Oxidation Reaction. For photocatalytic water oxidation reaction, 10 mg of F/Ti3+/TiO2 as the light absorber and different concentrations of NaIO3 as the electron acceptor were dispersed in 15 mL of acetic buffer. The photocatalytic reaction was carried out in a 32 mL flask which was sealed with a rubber stopper. Before photoreaction, the air in the reaction flask was removed by evacuation and purged with Ar gas several times. After the air was totally removed, the photocatalytic reaction was started by irradiating the system with a 420 nm LED light (100 mW/cm2). The gas in the top of the reaction flask was withdrawn by an SGE gastight syringe and analyzed by gas chromatography after a fixed time of illumination. The gas was separated by passing a sieves 5 Å column with Ar carrier gas and analyzed by a thermal conductivity detector. 2.6. Photoelectrochemical Measurement. All photoelectrochemical measurements were carried out on a CHI 660D workstation (CH Instruments Co.) under a 420 nm LED lamp irradiation (100 mW/cm2). The fabricated electrode, Pt electrode, and Ag/AgCl electrode (3.5 M KCl) were used as working, counter, and reference electrodes, respectively. The preparation method of the working electrodes is described as follows. First, 5 mg of the sample was dispersed in 1 mL of ethanol, and then 100 μL of solution was dropped onto the surface of the conductive fluorine-doped tin oxide (FTO) glass. The FTO was dried at 70 °C for 30 min; 10 μL 0.5% Nafion was dropped on the electrode to avoid the potential loss of the catalyst powders. 0.1 M sodium acetate buffer (NaAc electrolyte, pH = 4) was used as an electrolyte after saturation with Ar gas for 30 min. All measured potentials were converted to V versus RHE (ERHE = EAg/AgCl + 0.208 + 0.059 pH). 2.7. Characterization. All the chemicals and salts used for synthesizing the catalysts were obtained from a chemical company and used without further purification. Ultrapure water (18.2 MΩ·cm) for the preparation of solutions was attained from a molecular lab water purifier. Crystalline structures were determined by X-ray diffraction (XRD) using a Rigaku D/MAX 2400 diffractometer (Japan) with Cu Kα radiation (k = 1.5418 Å) operating at 40 kV and 40 mA. X-ray photoelectron spectroscopy (XPS) spectra were measured by ESCALAB 250xi with X-ray monochromatization. The binding energy of each element was corrected by the C 1s peak (284.8 eV) from residual carbon. The surface morphologies of the samples were characterized by scanning electron microscopy (SEM, S4800) and transmission electron microscopy (TEM, FEI Talos F200S). The optical properties of the samples were measured using a Shimadzu UV-2600 UV−vis diffuse reflectance spectrophotometer. PL spectra were recorded on a FLS920 fluorescence spectrometer (Edinburgh Instruments) with excitation at 370 nm. The electron paramagnetic resonance (EPR) measurements were carried out at the microwave power of 24.6 mW and frequency of 9.44 GHz at 150 K.

However, there is rarely work concern about FeOx as a water oxidation cocatalyst for TiO2.25 Polyoxometalates (POMs) with all inorganic ligands are considered as efficient water oxidation catalysts as they can undergo a fast reversible multielectron transfer process.26,27 Using POMs as a precursor to construct efficient catalytic systems has been reported in many works. 28−31 A Na 27 [Fe 11 (H 2 O) 14 (OH) 2 (W 3 O 10 ) 2 (α-SbW 9 O 33 ) 6 ] POM (Fe11) consisting of eleven-FeIII-substituted antimoniotungstate has shown superior water oxidation activity in a molecular photocatalytic system.32 The composite system of Fe11 coupled with BiVO4 demonstrated excellent water oxidation activity,28 which makes Fe11 a potential cocatalyst for water oxidation reaction of Ti3+-doped TiO2. Herein, by a simple hydrothermal method, TiO2 doped with Ti3+ was synthesized and first used in a powder photocatalytic water oxidation process. An amorphous FeOx layer derived from a molecular water oxidation catalyst Fe11 was in situ photodeposited on the surface of Ti3+-doped TiO2. After 3 h of illumination, the composite F/Ti3+/TiO2 showed an oxygen evolution rate of 410 μmol/g/h, which was about 11-fold higher than that of prism Ti3+/TiO2. The cocatalyst of FeOx efficiently improved the water oxidation performance of Ti3+/ TiO2. Photoelectron chemical measurement and photoluminescence (PL) results indicated that the charge carriers can be efficiently separated when combined with a cocatalyst, which led to the enhanced water oxidation performance of F/ Ti3+/TiO2.

2. EXPERIMENT 2.1. Preparation of the Na9[α-SbW9O33] Precursor. Na9[α-SbW9O33] was prepared by dissolving 40 g (121 mmol) of Na2WO4 in 80 mL of boiling water, in which 1.96 g (6.72 mmol) of Sb2O3 dissolved in concentrated hydrochloric acid (10 mL) was dropwise added. The solution was refluxed for 1 h and cooled to room temperature naturally. Colorless crystals can be collected after one-third of the solution was evaporated. 2.2. Preparation of Na27[Fe11(H2O)14(OH)2(W3O10)2(αSbW9O33)6] (Fe11). First, 1.16 g (7.2 mmol) of FeCl3 was dissolved in 80 mL of deionized H2O; then, 9.45 g (3.3 mmol) of Na9[α-SbW9O33] was dissolved in the solution. After adjusting the pH of the solution to 3.0 by 4.0 M NaOH, the solution was heated at 90 °C for 1 h and filtered after cooling to room temperature. Slow evaporation of the filtrate at room temperature resulted in yellow crystals that are suitable for Xray diffraction within 8−9 days. 2.3. Preparation of Ti3+/TiO2. Ti3+/TiO2 was synthesized according to a published work.33 Titanium powder (0.3 g) was dispersed in 10 mL of 2 M HCl, after vigorous stirring for about 30 min; the mixture was transferred to a Teflon-lined stainless-steel autoclave (25 mL capacity) and hydrothermally treated for 12 h under 220 °C. The Ti3+/TiO2 can be obtained after washing the precipitate with deionized water and ethanol several times and drying overnight. 2.4. Photodeposition Process. The photodeposition process was conducted as follows: 100 mg of Ti3+/TiO2 was dispersed in a mixture solution composed of 5 mM NaIO3 as the sacrificial electron acceptor and different concentrations of Fe11 in 100 mL (100 mM pH = 4) of acetic buffer. The solution was illuminated by a 420 nm light-emitting diode (LED) light source for 3 h. After illumination, the composite was collected by centrifugation and washed several times with deionized water. After drying, the composite of Fe11-deposited

3. RESULTS AND DISCUSSION The Fe11 POM that consists of six trilacunary [α-SbW9O33]9− fragments linked by an electrophilic [Fe11(H2O)14(OH)2(W3O10)2]27+ cluster was prepared and confirmed by IR (Figure S2) according to our previous published works.28,32 The hydrolysis stability of Fe11 POM 23136


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Figure 1. (a) Powder XRD spectra of F/Ti3+/TiO2, Ti3+/TiO2, and the JCPDS of rutile no. 89-4920. (b) Raman spectra of F/Ti3+/TiO2 and Ti3+/ TiO2. (c) UV−vis diffuse reflectance spectra of Ti3+/TiO2 (black) and P25 (blue). (d) Low-temperature EPR of Ti3+/TiO2 operated at 150 K.

Figure 1d, the prepared TiO2 shows a strong EPR signal at g = 1.96, which could be assigned to Ti3+.33 It is worth noting that the EPR result indicates the absence of Ti3+ on the surface of TiO2 because surface Ti3+ can trap O2 and generates O2− whose EPR signal is around g = 2.02.9 The absence of such a signal in our sample reveals that no Ti3+ exists on the surface of TiO2, which is important for the stability of Ti3+-doped TiO2. The synthesized titania was labeled as Ti3+/TiO2. The bare Ti3+/TiO2 sample was tested for photocatalytic water oxidation using NaIO3 as a sacrificial electron acceptor. After 3 h of illumination, only about 1 μmol of O2 was produced. Then, the Fe11 POM as a cocatalyst was added to the Ti3+/TiO2 photocatalytic water oxidation system. It was found that Fe11 can promote oxygen evolution of Ti3+/TiO2. The oxygen evolution amount in the presence of Fe11 is 4.5 times higher than that of bare Ti3+/TiO2 (Figure 2). When Fe11 was used as a cocatalyst in the photocatalytic process, pH = 4 showed the best oxygen evolution activity (Figure S4). After the photocatalytic process, the suspended solid was collected and reused in the photocatalytic process. If Fe11 works as a true molecular water oxidation cocatalyst in the water oxidation process, the recovered powder should perform the same water oxidation kinetics as original Ti3+/ TiO2. However, when the recovered powder was retested in the photocatalytic system, it shows better activity compared with the water oxidation system of Fe11 + Ti3+/TiO2 (Figure 2). As the recovered powder composite shows a different oxygen evolution behavior compared with the Ti3+/TiO2 system, it can be concluded that active specie derived from Fe11 was loaded on titanium dioxide. The samples after photodeposition were selected as the main objects for the following investigation. To obtain the best photodeposition condition, a series of optimization experiments involving pH and Fe11 concentrations were conducted. When the recovered powder was used

was investigated by an aging experiment. As shown in Figure S3, the UV−vis absorption spectra of Fe11 did not change for 4 h of aging in acetic buffer solution (pH = 4, 100 mM), indicating the Fe11 POM is hydrolytically stable in our reaction condition. The visible-light-responsive TiO2 was prepared by hydrothermal reaction between titanium powder and hydrochloric acid at 220 °C for 12 h.33 After the autoclave was cooled to room temperature, light blue powder was collected by washing with water and ethanol several times. The crystalline phase of synthesized Ti3+-doped TiO2 is confirmed as rutile [Joint Committee on Power Diffraction Standards (JCPDS) no. 894920] by XRD analysis (Figure 1a). According to published works, rutile is considered to be more beneficial for photocatalytic water oxidation than anatase and brookite TiO2.34,35 The Raman spectrum of the synthesized TiO2 showed typical Raman bands of rutile, which appeared at 143, 235, 447, and 612 cm−1 (Figure 1b),36 which further proves that the crystal form of synthesized TiO2 is rutile. Unlike the general TiO2 which shows white color, the synthesized Ti3+/TiO2 exhibits a light blue appearance. The optical responses of commercial P25 and Ti3+/TiO2 were detected by UV−vis diffuse reflectance spectra and are shown in Figure 1c. The Ti3+-doped TiO2 exhibits a stronger absorption than the commercial TiO2 in the ultraviolet and visible regions. In addition, the absorption edge of Ti3+/TiO2 moves to the longer wavelength. According to published works,37,38 the Ti3+ inside the bulk of TiO2 could induce a vacancy band of electronic states just below the conduction band, which is responsible for the red shift of the absorption edge.33 The existence of the vacancy band explains why the absorbance of the prepared sample is stronger than that of P25 in the visible range.33 The presence of Ti3+ in the as-prepared TiO2 can be proved by low-temperature EPR operated at 150 K. As illustrated in 23137


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Figure 2. Oxygen evolution activities of F/Ti3+/TiO2, Fe11 + Ti3+/ TiO2, and Ti3+/TiO2. Reaction conditions: 10 mg of composite, 5 mM NaIO3, 15 mL of acetic acid buffer (pH = 4, 100 mM), illumination with a 420 nm LED (100 mW/cm2), and vigorous agitation using a magnetic stirrer.

in photocatalytic system, the composite obtained from pH 4 demonstrated the best water oxidation activity (Figure S5), so the optimal deposition pH was confirmed as pH 4. The sample obtained from 10 μM Fe11 showed the best photocatalytic activity, yielding 12.3 μmol of O2 after 3 h of photoreaction (Figure S6). Therefore, the optimal condition for photodeposition can be determined as pH 4 and the Fe11 concentration as 10 μM. The composite obtained from the optimal condition was labeled as F/Ti3+/TiO2. After the photodeposition process, the F/Ti3+/TiO2 shows identical diffraction peaks with those of Ti3+/TiO2 (Figure 1a). That is understandable when considering that only a small amount of active species were deposited on Ti3+/TiO2 during the photodeposition process. By inductively coupled plasma atomic emission spectrometry, the amounts of Fe, Sb, W deposited on Ti3+/TiO2 in the optimal F/Ti3+/TiO2 were determined as 0.09, 0.04, and 0.5 wt %, respectively, which were too little to be detect by XRD. After photodeposition, the F/Ti3+/TiO2 showed a Raman spectrum identical to that of Ti3+/TiO2 (Figure 1b), which may also be due to the small deposition amount of active species. The morphologies of Ti3+/TiO2 and F/Ti3+/TiO2 were observed by SEM and TEM. As shown in Figure 3a, the initial Ti3+/TiO2 showed a tetragonal prism appearance with two tetragonal pyramid ends. The main exposure crystal facets of Ti3+/TiO2 were {110} for the tetragonal prisms and {111} for the tetragonal pyramids, respectively. According to the firstprinciples calculation, the crystal facet with the lowest surface energy is {110} for rutile.33 In our condition, Ti3+/TiO2 crystals grow along the [001] direction, which makes the Ti3+/TiO2 expose more of the {110} facet and show nanorodlike morphology. In the HRTEM of Ti3+/TiO2 (Figure 3d), the lattice fringe spacing of 0.248 nm corresponds to the (101) crystal facet of rutile Ti3+/TiO2.39 After photodeposition treatment, the SEM of F/Ti3+/TiO2 (Figure 3e) does not show any obvious changes compared with that of Ti3+/TiO2. From the TEM of F/Ti3+/TiO2 (Figure 3g), a dense layer with a thickness of about 8−9 nm could be observed (Figure 3g,h), which encloses the Ti3+/TiO2 nanorods. This dense layer, which is absent in Ti3+/TiO2 (Figure 3c), resulted from the photodeposition process. In the HRTEM of F/Ti3+/TiO2, lattice fringes with lattice spacing of 0.248 and 0.324 nm are indexed to the (101) facet and (110) facet of the rutile phase Ti3+/TiO2, respectively. No obvious lattice fringes were

Figure 3. (a) SEM and (b,c) TEM and (d) HRTEM images of Ti3+/ TiO2. (e) SEM, (f,g) TEM, and (h) HRTEM images of F/Ti3+/TiO2. The coating amorphous layer was separated from Ti3+/TiO2 using a dashed line in (g,h).

detected in the coating layer, indicating the cocatalyst layer is amorphous.40 The compact contact between titanium dioxide and the coating layer facilitates electron transfer between the semiconductor of Ti3+/TiO2 and the cocatalyst,21,41,42 thus endowing the composite with outstanding water oxidation activity. XPS analyses were further conducted to investigate the surface state and composition of Ti3+/TiO2 and F/Ti3+/TiO2. The survey-scale XPS spectra for Ti3+/TiO2 and F/Ti3+/TiO2 are shown in Figure S7; Ti, O, C can be detected in the Ti3+/ TiO2 sample. After photodeposition, besides the previous elements that existed in Ti3+/TiO2, the signals of Fe, Sb, and W are observed in the survey scale of F/Ti3+/TiO2. The Ti 2p curves show binding energy located at 464.1 and 458.2 eV corresponding to Ti4+ 2p1/2 and Ti4+ 2p3/2, respectively (Figure 4a).43 No signal corresponding to Ti3+ can be detected on the surface of all the samples, which should appear at 463.2 and 457.9 eV for Ti3+ 2p1/2 and Ti3+ 2p3/2.44 The absence of Ti3+ on the surface is important for the stability of Ti3+/TiO2 as Ti3+ is easily oxidized by oxygen in air, which will reduce the catalyst activity of partly reduced TiO2.45 After photodeposition, Ti 2p of F/Ti3+/TiO2 does not show obvious changes compared with Ti3+/TiO2. The presence of FeOx is verified by an XPS plot of Fe 2p (Figure 4b).The highresolution XPS spectrum of Fe 2p shows two peaks at 712 and 725 eV, which are assigned to Fe 2p3/2 and Fe 2p1/2 of 23138


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Figure 4. XPS spectra of (a) Ti 2p of F/Ti3+/TiO2 and Ti3+/TiO2. (b) Fe 2p spectrum of F/Ti3+/TiO2.

Figure 5. (a) O2 production activities of F/Ti3+/TiO2 (red) and FC/Ti3+/TiO2 (blue) and Ti3+/TiO2 (black) and P25 (dark cyan). (b) Amounts of O2 evolution in the photocatalytic system using different composites. Reaction conditions: 10 mg of composite, 5 mM NaIO3, 15 mL of acetic acid buffer (pH = 4, 100 mM), illumination with 420 nm LED (100 mW/cm2), vigorous agitation using a magnetic stirrer.

Fe3+.46,47 The peak position of O 1s shifts from 529.6 eV in Ti3+/TiO2 to 530.7 eV in F/Ti3+/TiO2 (Figure S8), which may be affected by the coating layer outside the Ti3+/TiO2 nanorod.48,49 After the photodeposition process, the Sb 3d and W 4f spectra of F/Ti3+/TiO2 were detected and are shown in Figures S9 and S10. The valence state of Sb and W are confirmed as +3 and +6 according to the peak position.50,51 Because of the overlapping of the Sb 3d and the O 1s core peak, the valence state of Sb is confirmed as +3 according to the peak at 539.2 eV.50 In the photocatalytic water oxidation process, the optimal amount of used F/Ti3+/TiO2 has been confirmed as 10 mg (Figure S11). The O2 evolution activities of F/Ti3+/TiO2 were investigated using different sacrificial electron acceptors and the result is shown in Figure S12. The oxygen evolution activity of NaIO3 is better than those of AgNO3 and Na2S2O8. Under the optimal condition of pH 4 and NaIO3 concentration of 5 mM (Figures S13 and S14), the oxygen evolution rate was 410 μmol/g/h. The composite catalyst F/Ti3+/TiO2 produced 12.3 μmol of oxygen after 3 h of illumination, which was about 11 times more than that of pure Ti3+/TiO2 (only 1.1 μmol O2) under the same condition (Figure 5a). When a simple Fe salt (FeCl3) was used as the reference deposition source (the composite after deposition was labeled as FC/Ti3+/TiO2), a smaller oxygen production amount (3.4 μmol after 3 h of illumination) was obtained. When commercial Fe2O3 was used in the photocatalytic water oxidation as a cocatalyst, only 1.3 μmol of oxygen was generated after 3 h of illumination, which is smaller than the activity achieved by F/Ti3+/TiO2 and FC/ Ti3+/TiO2(Figure 5b). In a word, the Fe11 POM works as a

better precursor in enhancing the oxygen evolution activity of Ti3+/TiO2. The SEM and TEM of the composite FC/Ti3+/TiO2 are shown in Figure S15. After photodeposition, nanoparticles were deposited on the surface of the Ti3+/TiO2 nanorod. From the HRTEM of FC/Ti3+/TiO2 (Figure S16), the lattice fringes with lattice spacing of 0.324 and 0.329 nm are detected, corresponding to the (110) facet of Ti3+/TiO2 and the (110) facet of FeOOH (JCPDS no. 89-6096). Different from the compact and even contact between the cocatalyst layer and the Ti3+/TiO2 nanorod in F/Ti3+/TiO2, the uneven contact between Ti3+/TiO2 and the cocatalyst in FC/Ti3+/TiO2 may influence the charge transfer between the light absorber and the cocatalyst.52 In situ coating FeOx derived from allinorganic Fe11 POM on Ti3+/TiO2 demonstrates the unique advantages. To rule out the impact of deposited Sb and W on photocatalytic activity of Ti3+/TiO2, Ti3+/TiO2 deposited with Na9[α-SbW9O33] was investigated in a water oxidation process as a contrast. The TiO2 deposited with Na9[αSbW9O33] (SW/Ti3+/TiO2) shows a similar oxygen evolution amount to that of Ti3+/TiO2 (Figure 5b). Although there were Sb and W decorated on Ti3+/TiO2, these species did not work as cocatalysts. FeOx derived from Fe11 is the real cocatalyst that promotes the oxygen evolution of Ti3+/TiO2. As a contrast, Fe11 in situ decorated on P25 (labeled as F/P25) was prepared and used in photocatalytic water oxidation (Figure 5b). The F/P25 produces negligible O2 compared with F/ Ti3+/TiO2, indicating the superiority of Ti3+-doped TiO2 in response to visible-light. 23139


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Figure 6. (a) Kinetics of oxygen evolution in the photocatalytic system using different titania. (b) Cycling photocatalytic water oxidation of the sample F/Ti3+/TiO2. Photocatalytic condition: 10 mg of composite, 15 mL of pH = 4 acetic acid buffer (100 mM), 420 nm LED light (100 mW/ cm2), 5 mM NaIO3 as the sacrificial electron acceptor.

amount of Ti3+ increases during photodeposition. In order to confirm that the increase of signal intensity of Ti3+ is attributed to the increase of the amount of Ti3+ rather than deposited FeOx, we carried out the EPR test of the F/Ti3+/TiO2 treated by HCl. As shown, the signal of Fe3+ disappears owing to the dissolution of FeOx and the signal of F/Ti3+/TiO2 treated by HCl is almost the same as that of untreated F/Ti3+/TiO2, further confirming that the extra generated Ti3+ should be responsible for the increase of the signal intensity of Ti3+. After photodeposition, the color of titania changed from light blue (Ti3+/TiO2) to light gray for the sample of F/Ti3+/ TiO2 (Figure S17). The UV−vis spectrum of the composite presents enhanced absorption in the visible light region compared with that of Ti3+/TiO2 (Figure 8a). According to the XPS and elementary analysis characterization, the amorphous coating layer contains the species of FeOx. The property of the amorphous layer is somewhat similar to the semiconductor, which can be used as a light-harvesting material.55 Furthermore, the EPR of sample F/Ti3+/TiO2 indicates that the concentration of Ti3+ increases in F/Ti3+/ TiO2. As the visible light response of Ti3+/TiO2 is ascribed to the existence of Ti3+, the increased Ti3+ concentration can lead to enhanced absorption of F/Ti3+/TiO2 in visible light.56 On the basis of the above analysis, the enhanced absorption of F/ Ti3+/TiO2 should be a combination effect of light absorption of the coating layer and increased Ti3+ amount in F/Ti3+/TiO2. Besides light absorption, increasing the charge separation efficiency is a key factor to further improve the photocatalytic activity of a semiconductor.57,58 PL spectra as powerful tools were obtained to investigate the carrier separation efficiency of Ti3+/TiO2 and F/Ti3+/TiO2 (Figure 8b). Compared with the poor carrier separation in Ti3+/TiO2, the composite F/Ti3+/ TiO2 shows an obvious weaker PL signal, which indicates efficient electron−hole separation after photoabsorption. F/ Ti3+/TiO2 exhibits relatively a low electrons and holes recombination, so photogenerated holes are more likely to transfer to the cocatalyst and participate in the photocatalytic water oxidation process, which is one of the reasons for the improved water oxidation activity of F/Ti3+/TiO2. In addition to the light absorption and carrier separation efficiency, surface reaction dynamics is another important factor affecting photocatalytic efficiency. The electrochemical impedance spectroscopy (EIS) was conducted to gain a deeper insight into the interfacial charge transfer rate and the separation efficiency of photoinduced hole−electron pairs. F/ Ti3+/TiO2 gives a smaller arc radius of Nyquist plots in

In order to estimate the effect of FeOx on oxygen evolution activity of F/Ti3+/TiO2, long-period photocatalytic water oxidation experiments were conducted and the results are shown in Figure 6a. Although the O2 evolution rate of composite F/Ti3+/TiO2 is slowing down with the increase of illumination time, F/Ti3+/TiO2 still demonstrates much better water oxidation activity than FC/Ti3+/TiO2 and Ti3+/TiO2. After 24 h of illumination, the sample F/Ti3+/TiO2 produces 41 μmol of O2 and the oxygen chemical yield is 36.4%. The quantum yield was measured as 22% for the composite catalyst F/Ti3+/TiO2. As the stability of a catalyst is important for its practical application,53 the stability of F/Ti3+/TiO2 was evaluated in the photocatalytic process. It can be observed from Figure 6b that the photocatalyst F/Ti3+/TiO2 shows nearly the same activity over four consecutive cycles, suggesting that F/Ti3+/TiO2 is stable during the photocatalytic water oxidation processes. The EPR spectra of composite F/Ti3+/TiO2 and simple Ti3+/TiO2 are demonstrated in Figure 7. After the deposition

Figure 7. EPR data of Ti3+/TiO2, F/Ti3+/TiO2, and F/Ti3+/TiO2 washed with hydrochloric acid. The measurements were operated at 150 K.

of FeOx, the EPR signal located at a g value around 1.96 remains unchanged, suggesting that the chemical environment of Ti3+ in F/Ti3+/TiO2 is the same as that of Ti3+/TiO2. In addition, an extra signal with a g value of around 2.0 arose, which should be assigned as Fe3+54 with high symmetry coordination environment in deposited FeOx on the surface of the F/Ti3+/TiO2 samples. The intensity of the signal of Ti3+ increases compared with that of Ti3+/TiO2, indicating that the 23140


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Figure 8. (a) UV−vis diffuse reflectance spectra of F/Ti3+/TiO2 (red), Ti3+/TiO2 (black), and P25 (blue). (b) PL spectra of Ti3+/TiO2 and F/ Ti3+/TiO2.

Figure 9. (a) EIS measurements of Ti3+/TiO2 and F/Ti3+/TiO2 (inset: equivalent circuit). (b) Reproducible variation of photocurrent of F/Ti3+/ TiO2 and Ti3+/TiO2 under intermittent illumination of a 420 nm LED lamp.

impedance spectra compared with Ti3+/TiO2. The EIS results were fitted with a model circuit (see inset in Figure 9a), in which R1 is the interfacial resistance between the FTO substrate and the photocatalyst, R2 is the bulk resistance inside the photocatalyst, and R3 is the interfacial resistance between the photocatalyst and electrolyte.59 The fitted values are listed in Table S1. F/Ti3+/TiO2 and Ti3+/TiO2 have similar R1 values but rather different R2 and R3 values. The R2 and R3 values of F/Ti3+/TiO2 are significantly lower than those of Ti3+/TiO2, indicating the superior charge transfer efficiency of F/Ti3+/ TiO2. Besides photocatalysis, the photoelectrochemical activities of these materials were further monitored and are shown in Figure 9b. Variations of photocurrents of Ti3+/TiO2 and F/ Ti3+/TiO2 were recorded by chopping the light on and off at a constant potential of 1.23 VRHE. The photocurrent comes into being immediately upon illumination and F/Ti 3+ /TiO2 generates a photocurrent response approximately two times more than that of Ti3+/TiO2. Such improvement in photocurrent density should be attributed to the combined effect of smaller charge transfer resistance and reduced photoelectrons and holes recombination.

layer of active species was formed on the surface of initial Ti3+/ TiO2 nanorods, which is the true cocatalyst for the improved water oxidation activity. The composite F/Ti3+/TiO2 showed an oxygen evolution rate of 410 μmol/g/h, which was about 11-fold higher than that of prism Ti3+/TiO2. The O2 yield and quantum yield of the composite catalyst F/Ti3+/TiO2 were 36.4 and 22.0%, respectively. Furthermore, the composite exhibits favorable stability during the photocatalytic water oxidation process. The coating cocatalytic layer would enhance carrier separation and reduce charge-transfer resistance in the composite. The combination of these factors endows the composite F/Ti3+/TiO2 with excellent photocatalytic water oxidation activity. This work provides a good insight into the design of a stable and efficient photocatalytic water oxidation catalyst based on inexpensive semiconductors.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b03714.

4. CONCLUSIONS In summary, Ti3+-doped TiO2 was prepared with titanium powder and hydrochloric acid under hydrothermal conditions, which is responsive to visible light. However, this photocatalyst only produced lesser oxygen evolutions. When the cocatalyst of Na27[Fe11(H2O)14(OH)2(W3O10)2(α-SbW9O33)6] (Fe11) was used, the oxygen evolution was greatly enhanced. Unfortunately, this molecular catalyst was unstable and an amorphous

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Calculation of O2 yield and quantum yield; polyhedral and ball-and-stick representation of Fe11 POM; additional O2 production experiments; SEM and TEM plots; XPS plots; and other additional data (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yong Ding: 0000-0002-5329-8088 23141


Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (grant no. 21773096), Fundamental Research Funds for the Central Universities (lzujbky-2018-k08), and the Natural Science Foundation of Gansu (17JR5RA186).

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FeOx Derived from an Iron-Containing Polyoxometalate Boosting the

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