Synthesis of mesoporous TiO₂-coupled Fe₂O₃ as efficient visible nano-photocatalysts for degrading colorless pollutants

Abstract

In this paper, a feasible route is developed to greatly enhance the visible photocatalytic activities of α-Fe₂O₃ nanoparticles for degrading colorless pollutants by coupling a suitable amount of mesoporous nanocrystalline anatase TiO₂. Based on the atmosphere-controlled steady-state surface photovoltage spectra, transient-state surface photovoltage responses, photoelectrochemical measurements and O₂ temperature-programmed desorption, it is suggested that the coupled TiO₂ with a high-level conduction band bottom would act as a high-level platform, could facilitate the uncommon spatial transfers of visible-excited energetic electrons from Fe₂O₃ to TiO₂, and the mesoporous structure of TiO₂ could promote the capture of the energetic electrons by O₂ by enhancing the O₂ adsorption. This is favorable to prolong the carrier lifetime and to promote the charge separation, leading to the obviously-enhanced visible photoactivities. The developed strategy would provide us with a simple and feasible idea to greatly improve the photocatalytic performance of visible-response oxide-based nano-semiconductors.

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Introduction

Photocatalysis using solar energy is highly expected to be an ideal “green” technology for resolving problems of environmental pollution, where an active photocatalyst is definitely an important key. Among the metal oxides investigated,^1–11 α-Fe₂O₃ (hematite) has received much attention due to its favorable optical band gap (∼2.0 eV, corresponding to visible light range), excellent chemical stability, natural abundance, and low cost.^12–22 However, it always exhibits notoriously low activities (or efficiencies), which is mainly attributed to its very short lifetime of photogenerated charge carriers (<10 ps) and short hole diffusion length (∼2–4 nm).²³ Besides, its low-level position of conduction band (CB) bottom is thermodynamically unfavorable for photogenerated electrons to induce reduction reactions with O₂,^24,25 which is considered as the rate-determining step of photocatalytic degradation of pollutants. In fact, lots of energetic electrons of Fe₂O₃ could be produced under visible illumination. For the energetic electron, its energy level is higher than the CB bottom. Although the produced energetic electrons are thermodynamically allowed, they usually relax so quickly to the CB bottom that their possibilities for inducing reduction reactions with O₂ would be decreased greatly.

Generally speaking, constructing the heterojunctional composite is an effective way to promote photogenerated charge separation.^26–29 In our previous work,³⁰ the rutile TiO₂–Fe₂O₃ nanocomposite was fabricated and exhibits better visible photocatalytic activity for degrading organic pollutants than either Fe₂O₃ or rutile TiO₂ alone, and it is preliminary attributed to the visible-photogenerated energetic electron transfers from Fe₂O₃ to rutile TiO₂. Meanwhile, this unexpected charge transfer was supported by our another work,³¹ in which coupling BiVO₄ with a proper amount of TiO₂ could improve its visible activity for H₂ evolution by promoting the charge carrier separation. Obviously, it is completely different from the frequently accepted view that the excited electrons of a semiconductor with high-level CB would transfer to another one with a low CB level.^32–34 Therefore, it is much meaningful to further clarify the unexpected transfers.

Since anatase TiO₂ has similar band structure to rutile TiO₂, the unexpected visible-charge transfer is expected to be happened in anatase TiO₂ coupled Fe₂O₃, which is mentioned in the previous work,³⁰ but detailed study and discussion are lacking. And, compared to rutile TiO₂, anatase TiO₂ is easy to synthesize and control the structure, implying anatase TiO₂–Fe₂O₃ has advantages in practical application. In addition, promoting the O₂ adsorption is an effective way to promote the photogenerated charge separation by accelerating the capture of electrons by O₂. Mesoporous TiO₂ has larger surface area than the non-mesoporous one, and is expected has stronger O₂ adsorption than the latter. In other words, coupling Fe₂O₃ with mesoporous TiO₂ would be benefit for the utilization of the visible-photogenerated charge more than that with non-mesoporous TiO₂. Naturally, this would further promote the charge separation, leading to greatly enhanced photocatalytic activity.

Herein, we have developed a feasible route to greatly enhance the visible photocatalytic activities of α-Fe₂O₃ for degrading colorless phenol and acetaldehyde by coupling with a proper amount of mesoporous nanocrystalline anatase TiO₂. Interestingly, it is suggested that the greatly enhanced activities are attributed to the spatial transfers of the visible-excited energetic electrons from α-Fe₂O₃ to TiO₂ and the promoted capture of the energetic electrons by adsorbed O₂ on TiO₂, which is favorable to prolong the lifetimes and to promote the separation of photogenerated charge carriers.

Experimental section

Synthesis

All of the reagents were of analytical grade and used as received without further purification, and deionized water was employed throughout.

Mesoporous nanocrystalline (Nc) TiO₂ was synthesized by a sol-hydrothermal method. A typical process was as follows: 5 mL Ti(OBu)₄ was dissolved in 5 mL anhydrous ethanol in a dry atmosphere, and then the mixed solution was added dropwise into another mixture consisting of 20 mL anhydrous ethanol, 5 mL water and 1 mL 70% HNO₃ at room temperature under vigorously stirring, forming a yellowish transparent sol. And a certain amount (0.5 g, 1 g, 3 g, 5 g) of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (P123) was added to the formed sol under stirring. Subsequently, 30 mL of resulting sol was kept at 160 °C for 6 h in a Teflon-lined stainless-steel vessel to carry out hydrothermal reactions, and afterwards cooled naturally to room temperature. After washed with deionized water and absolute ethanol in turn, and dried at 80 °C in air, mesoporous Nc-TiO₂ was obtained. By contrast, Nc-TiO₂ was prepared through a similar process without the addition of P123. Nc-Fe₂O₃ was prepared by a water–organic two-phase separated hydrolysis-solvothermal (HST) method.³⁵

To construct different mass ratios of TiO₂–Fe₂O₃ nanocomposites, we took the desired-amount as-prepared Nc-TiO₂ or mesoporous Nc-TiO₂ and Nc-Fe₂O₃ to put them together into a 50 mL 50% ethanol solution under vigorously stirring for 1 h. Subsequently, the mixture was heated to 80 °C and kept the temperature under stirring. After evaporated out, the residual mixture was dried at 80 °C in air. Finally, different ratios of TiO₂–Fe₂O₃ nanocomposites were obtained by thermal treatment of the dried mixture at 450 °C for 2 h. The obtained nanocomposite is defined as XT-F or YM-XT-F, in which X indicates the mass content percentage of TiO₂ in the composite and Y indicates the mass (g) of added P123 in the preparation of mesoporous Nc-TiO₂, T is Nc-TiO₂, M-T is mesoporous Nc-TiO₂ and F means Fe₂O₃.

The film electrodes for photoelectrochemical (PEC) measurements were fabricated by the doctor blade method, which has been described in the previous work.³⁰ Similar to the above nanocomposite powder, the resulting film is named by XT-FF or YM-XT-FF, in which the last F means film. The FTO glass covered by the nanocomposite film was cut into 1.5 cm × 3.0 cm pieces with nanocomposite film surface area of 1.0 cm × 1.0 cm. To make a photoelectrode, an electrical contact was made with FTO substrate by using silver conducting paste connected to a copper wire, which was then enclosed in a glass tube. The working geometric surface area of nanocomposite was 0.5 cm × 0.5 cm, where the remaining area was covered by epoxy resin.

Characterization

The materials were characterized by X-ray powder diffraction (XRD) with a Rigaku D/MAX-rA powder diffractometer (Japan), using Cu_Kα radiation (α = 0.15418 nm), and an accelerating voltage of 30 kV and an emission current of 20 mA were employed. The ultraviolet-visible diffuse reflectance spectra (UV-Vis DRS) of the samples were recorded with a Model Shimadzu UV2550 spectrophotometer. Transmission electron microscopy (TEM) observation was carried out on a JEOL JEM-2100EX instrument operated at 200 kV accelerating voltage. Scanning electron microscopy (SEM) observation was carried out on a Philips XL-30-ESEM-FEG operated at an accelerating voltage of 20 kV. The specific surface area was determined according to the Brunauer–Emmett–Teller (BET) method using a Tristar II 3020 surface area and porosity analyzer (micromeritics).

The steady-state surface photovoltage spectroscopy (SS-SPS) measurement of the sample was carried out with a home-built apparatus that has been described elsewhere.^2,36 Monochromatic light was obtained by passing light from a 500 W xenon lamp (CHFXQ500W, Global Xenon Lamp Power, made in China) through a double-prism monochromator (Hilger and Watts, D 300, made in England). A lock-in amplifier (SR830, made in U.S.A), synchronized with a light chopper (SR540, made in U.S.A.), was employed to amplify photovoltage signal. The powder sample was sandwiched between two ITO glass electrodes, and the sandwiched electrodes could be arranged in an atmosphere-controlled container with a quartz window for transmitting light.

The transient-state surface photovoltage (TS-SPV) response measurements were performed with a self-assembled device in air atmosphere at room temperature,^2,37 in which the sample chamber is connected to an ITO glass as the top electrode and to a steel substrate as the bottom electrode, and an about 10 μm thick mica spacer was placed between the ITO glass and the sample to decrease the space charge region at the ITO-sample interface. The samples were excited by a 532 nm laser radiation with 10 ns pulse width from a second harmonic Nd:YAG laser (Lab-130-10H, Newport, Co.). The laser intensity was modulated with an optical neutral filter and measured by a high energy pyroelectric sensor (PE50BF-DIF-C, Ophir Photonics Group). The signals were registered by a 1 GHz digital phosphor oscilloscope (DPO 4104B, Tektronix) with an amplifier at the aid of a computer.

PEC experiment was performed in a glass cell with a quartz window using a 500 W xenon light with a stabilized current power supply as the illumination source with a 420-cutoff filter and a pH 13.6 NaOH solution as the electrolyte. The working electrode was the TiO₂–Fe₂O₃ nanocomposite film (0.25 cm² area), illuminated from the FTO glass side. A platinum plate (99.9%) was used as the counter electrode, and a saturated-KCl Ag/AgCl electrode (SSE) was used as the reference electrode. All the potentials in the paper were referred to the SSE. Oxygen-free nitrogen gas was used to bubble through the electrolyte before and during the experiment. Applied potentials were controlled by a commercial computer-controlled potentiostat (LK2006A made in China). For comparison, the current was also measured on dark condition. Electrochemical impedance data were performed in a three electrode configuration with the Princeton Applied Research Versa STAT 3.

Temperature-programmed desorption (TPD) of oxygen was also measured using a home-built facility. Sample powder (50 mg) was pretreated in a Pyrex tube (i.d. 6 mm) at 270 °C for 30 min by an ultra-high-pure He flow, and then the temperature was cooled to 25 °C. For O₂ adsorption saturation, the sample was continuously blown with ultra-high-pure O₂ for 90 min at 25 °C. After O₂ adsorption, the sample was flushed in ultrahigh-purity He flow to remove the physically adsorbed O₂ on the powders. An O₂-TPD profile of the sample was recorded by increasing the temperature from 30 to 600 °C at a heating rate of 10 °C min⁻¹ under 20 mL min⁻¹ of ultra-high-purity He flow. The desorbed O₂ was analyzed by a gas chromatograph (GC-2014, SHIMADZU) with a TCD detector.

Analysis of hydroxyl radical

0.02 g of desired-ratio TiO₂–Fe₂O₃ powder was dispersed in 50 mL of 1 × 10⁻³ M coumarin aqueous solution in a quartz reactor. After being irradiated by the visible light (175 W xenon light with a 420-cutoff filter) for 1 h, a certain amount of the solution was transferred into a Pyrex glass cell for the fluorescence measurement of 7-hydroxycoumarin at around 456 nm under the light excitation of 332 nm with a spectrofluorometer (PerkinElmer LS 55).

Visible photocatalytic activities

The photocatalytic degradation of liquid-phase phenol was carried out in a 100 mL of open photochemical glass reactor, and irradiation was provided from a side of the reactor by using a 150 W GYZ220 high-pressure xenon lamp made in China with a 420-cutoff filter, which was placed at about 10 cm from the reactor. During the photocatalytic degradation of phenol, 0.2 g photocatalyst and 60 mL 10 mg L⁻¹ phenol solution were mixed by a magnetic stirrer for 0.5 h to reach the adsorption saturation and then begin to irradiate. After photocatalytic reaction for 1 h, the phenol concentration was analyzed by the colorimetric method of 4-aminoantipyrine at the characteristic optical adsorption of 510 nm with a Model Shimadzu UV2550 spectrophotometer after centrifugation.³⁸

For the photocatalytic degradation of gas-phase acetaldehyde, it was conducted in a 640 mL of cylindrical quartz reactor with 3 mouths for introducing a planned amount of photocatalyst powders and a planned concentration of acetaldehyde gas.³⁵ The reactor was placed horizontally and irradiated from the top side by using a 150 W xenon lamp with a 420-cutoff filter. In a typical process, 0.2 g photocatalyst was used, and a premixed gas system, which contained 810 ppm acetaldehyde, 20% of O₂, and 80% of N₂, was introduced into the reactor. To reach adsorption saturation, the mixed gas continuously moved through the reactor for half an hour prior to the irradiation. The determination of acetaldehyde concentration at different time interval in the photocatalysis was performed with a gas chromatograph (GC-2014, Shimadzu) equipped with a flame ionization detector.

Results

The XRD patterns (Fig. 1A) and UV-Vis DRS spectra (Fig. S1A†) demonstrate that the nanocomposites with different mass ratios of anatase TiO₂ to α-Fe₂O₃ are formed, and the introduction of TiO₂ does not change the optical absorption of α-Fe₂O₃ across bang-gap energy. And also, one can see from the TEM photographs (Fig. S1B†) that the nanocrystalline α-Fe₂O₃ is spherical-like, and its size is about 20 nm. Notably, the anatase TiO₂ crystallite with about 6 nm size is dispersed on resulting α-Fe₂O₃ (Fig. 1B). And also, the intimate heterojunctions between TiO₂ and Fe₂O₃ are formed by means of the HRTEM image as an inset (Fig. 1B), in which the facet distances of 0.35 and 0.25 nm correspond to TiO₂ (101)³⁹ and α-Fe₂O₃ (110),⁴⁰ respectively. Naturally expected, the formed intimate heterojunction is favorable for charge transfer.

Fig. 1 XRD patterns (A) and TEM image of 20T-F (B), (a: F, b: 10T-F, c: 20T-F, d: 30T-F, e: 50T-F, f: T, and the same elsewhere unless stated), the inset in the (B) is the HRTEM image.

The steady-state surface photovoltage spectroscopy (SS-SPS), with its very high sensitivity, is a well-suitable and direct method to explore the properties of photogenerated charges of solid semiconducting materials.^2,36 As shown in Fig. 2A, it is noticed that the SPS response of TiO₂–Fe₂O₃ nanocomposite gradually becomes strong with increasing the amount of TiO₂, and it is the strongest for the 20T-F one, even 20 times larger than that of the F at 425 nm wavelength. According to the SPS principle,³⁶ the strong SPS response corresponds to the high photogenerated charge separation. However, if the amount of TiO₂ is continuously increased, the SPS response begins to go down. This should be attributed to the excess TiO₂ unfavorable for visible light absorption. Notably, the SPS response of Nc-TiO₂ is only within the UV range, while that of TiO₂–Fe₂O₃ nanocomposite could appear at the visible range, similar to that of Nc-Fe₂O₃. In addition, it is found that the SPS response edges of Nc-Fe₂O₃ and TiO₂–Fe₂O₃ nanocomposite are obvious blue shift compared to the corresponding DRS one (>620 nm). For Fe₂O₃, its SPS signal results from the adsorbed O₂ capturing excited electrons.³⁷ Fe₂O₃ and TiO₂–Fe₂O₃ show negligible SPS signal at 620 nm, indicating the excited electrons of Fe₂O₃ at the CB bottom could not be captured by the adsorbed O₂. In other words, for the observed SPS blue shift, it is interesting to verify the low CB bottom level of Fe₂O₃.

Fig. 2 SS-SPS in air (A) and I–V (A) curves. PEC performance was measured in the 1 M NaOH electrolyte solution. A three-electrode cell was used with the testing film as the working electrode, an Ag/AgCl (saturated KCl solution) electrode as the reference, and a Pt plate as the counter electrode.

The current density of excited semiconductor usually associates with its charge carrier separation. As seen from XRD patterns (Fig. S2†), SEM images (Fig. S3†), and I–V curves in the dark (Fig. S4†), the introduction of Nc-TiO₂ does not change the crystal phase, crystallinity, and surface morphology of α-Fe₂O₃ films, and it could slightly influence the dark current. One can see from Fig. 2B that the photocurrent density of the FF is rather small, and even neglectable, indicating that its charge carrier separation rate is very low. Interestingly, as the amount of coupled Nc-TiO₂ is increased, the photocurrent density is improved gradually. For the 20T-FF nanocomposite, it displays the largest photocurrent density among the fabricated ones, and its photocurrent density (0.2 mA cm⁻²) is about 12 times larger than that of the FF (0.016 mA cm⁻²) at the same applied voltage (0.4 V). If the amount of used TiO₂ is excess, the corresponding photocurrent is decreased, like 30T-FF and 50T-FF. As expected, the TF displays negligible photocurrent under visible illumination. In addition, 20T-FF nanocomposite shows smaller arc radii in Nyquist plot than FF in the dark or under visible irradiation (Fig. S5†), indicating it has small charge transfer resistance, which is in good agreement with the TS-SPV signals.

These results are further supported by the measurements of formed hydroxyl radical amounts on different samples after visible irradiation through the wide-used coumarin fluorescent method, which is a highly sensitive technique to detect the ˙OH amount. As shown in Fig. S6,† compared with Fe₂O₃, TiO₂–Fe₂O₃ samples exhibit large ˙OH amount, and the 20T-F sample produces the largest amount among those samples. As an important kind of active intermediate species, the ˙OH amount formed is associated with the charge carrier separation rate and the final photocatalytic activity. Therefore, it is suggested that the TiO₂–Fe₂O₃ samples would show good performance for pollutant degradation.

Based on the SS-SPS and TS-SPV responses, PEC performance and hydroxyl radical amount mentioned above, it is deduced that the photogenerated charge separation of α-Fe₂O₃ under visible illumination is greatly enhanced by coupling with a proper amount of Nc-TiO₂. Thus, it is anticipated that the fabricated TiO₂–Fe₂O₃ nanocomposites should exhibit high visible photocatalytic activities for degrading pollutants compared with Fe₂O₃. Phenol as a typical recalcitrant contaminant without sensitizing as a dye and acetaldehyde as a kind of volatile organic compound widely existing in industrial production are harmful to human health and natural environment. Thus, phenol and acetaldehyde were chosen as liquid-phase and gas-phase representative pollutants to evaluate the photocatalytic activity of the fabricated TiO₂–Fe₂O₃ nanocomposite samples under visible irradiation, respectively. From Fig. 3, one can note that the photocatalytic activities for degrading colorless liquid-phase phenol and gas-phase acetaldehyde of Fe₂O₃ are obviously improved by coupling with a proper amount of Nc-TiO₂. As expected, the 20T-F sample exhibits the highest activity among the investigated nanocomposites, which is consistent with the above results. Therefore, it is concluded that the fabricated TiO₂–Fe₂O₃ nanocomposites could exhibit much high visible photocatalytic activities for degrading colorless pollutants, compared with Fe₂O₃.

Fig. 3 Photocatalytic degradation rates of phenol (A) and acetaldehyde (B) under visible irradiation. K means the corresponding degradation reaction rate constant.

Discussion

Given the results above, it is concluded that the visible-excited charge separation of α-Fe₂O₃ could be greatly enhanced by coupling with a proper amount of anatase TiO₂, leading to the obvious increase in the photocatalytic activities for degrading colorless pollutants. As described above, it is suggested that the obviously enhanced charge separation is mainly attributed to the uncommon transfers of visible-excited energetic electrons of Fe₂O₃ to TiO₂.

Interestingly, the assumption is supported by the results of the SPS measurements in different atmospheres. As shown in Fig. S7,† 20T-F shows the detectable SPS response in N₂, while it is negligible for Fe₂O₃, suggesting the transfers have happened.³¹ And, the assumption is also supported by the results that the noble Pt could be deposited on TiO₂ in the fabricated nanocomposite after visible irradiation for 1 h based on the HRTEM image (Fig. S8†), clearly indicating that the visible-excited electrons of Fe₂O₃ have transferred to TiO₂.

As mentioned above, mesoporous TiO₂ would show good O₂ adsorption due to the large surface area. It is expected that replacing the non-mesoporous TiO₂ with mesoporous TiO₂ in the TiO₂–Fe₂O₃ nanocomposite would improve the photocatalytic activity by promoting the capture of energetic electron, which could further support the assumption. As shown in Fig. S9,† the substitution of TiO₂ does not change the structural characters of nanocomposite, and there are some pores with 3 nm size in the anatase TiO₂. Fig. 4 shows the N₂ adsorption–desorption isotherms and the corresponding BJH pore size distribution plots of 20T-F and 3M-20T-F samples. From that it can be seen the isotherms of both 20T-F and 3M-20T-F samples are type IV, indicating the presence of mesoporous structure according to the IUPAC classification.⁴¹ And the average pore size of them is approximately 20 nm according to the corresponding BJH pore-size distributions (insets of Fig. 4), which is formed by the accumulation of Fe₂O₃. It is worth noting that some pores sized of 3 nm could be seen in 3M-20T-F sample (insets of Fig. 4B), which is from the mesoporous TiO₂. These pores obviously increase the surface area (from 47.28 m² g⁻¹ to 53.76 m² g⁻¹), though the mesoporous TiO₂ take a small part in the nanocomposite.

Fig. 4 N₂ adsorption–desorption isotherms of 20T-F (A) and 3M-20T-F (B), the inset is the corresponding BJH pore size distribution plot.

In our previous work, it has been demonstrated that the large surface area corresponds to the high amount of O₂ adsorption.⁴² As expected, as shown in Fig. 5A, 3M-20T-F sample desorders more O₂ than 20T-F sample based on the O₂ temperature-programmed desorption measurements, indicating the presence of mesoporous greatly enhances the O₂ adsorption. Fig. 5B shows the TS-SPV responses, from that it is can be seen 20T-F sample exhibits a high charge separation compared to F, which is attributed to the visible-photogenerated charge transfer from Fe₂O₃ to anatase TiO₂. And interestingly, 3.0M-20T-F shows a longer carrier lifetime of ∼2 ms and higher charge separation than 20T-F. As indicated above, the mesoporous structure could enhance the O₂ adsorption by increasing the surface area so as to promote the capture of photogenerated charge. Therefore, it is reasonable to attribute the improved carrier properties to the promoted untilization of visible-photogenerated charge, which is supported by the SS-SPS response (Fig. S10A†) and further certify the assumption of the uncommon charge transfers. In addition, T sample exhibits a negligible TS-SPV response due to the wide bandgap, which cannot be excited under visible irradiation. As a result, the mesoporous TiO₂ coupled Fe₂O₃ samples show much higher visible photocatalytic activity for degradation of acetaldehyde than 20T-F sample, and 3.0M-20T-F sample is the highest one among them (Fig. S10B†), which is attributed the highest charge separation rate (Fig. S10A†). Further increase the amount of pores, the activity would decrease, may be attributed to the new formed recombination centers.

Fig. 5 Curves of O₂ temperature-programmed desorption (A) and TS-SPV responses in air (B).

Conclusions

In conclusion, we have successfully developed a feasible route to greatly enhance the visible photocatalytic activities of α-Fe₂O₃ for degrading colorless pollutants by coupling a proper amount of mesoporous Nc-TiO₂. By means of atmosphere-controlled SS-SPS responses, TS-SPV responses, PEC measurements and O₂ temperature-programmed desorption, it is suggested that TiO₂ could act as high energy level platform to receive the visible-excited energetic electrons of Fe₂O₃, and the mesoporous structure could promote the capture of the energetic electrons by O₂ by enhancing O₂ adsorption of TiO₂. This is favorable to prolong the carrier lifetime and then to promote the charge separation. Consequently, the fabricated TiO₂–Fe₂O₃ nanocomposite exhibits the obviously enhanced photocatalytic activities for degrading liquid-phase phenol and gas-phase acetaldehyde, compared to the Fe₂O₃ alone. The developed strategy would provide us with a simple and feasible idea to greatly enhance photogenerated charge separation of α-Fe₂O₃, which is also applicable to other visible-response oxide-based semiconductors, even non-oxide-based ones.

Acknowledgements

We are grateful for financial support from the NSFC (21071048), the National Key Basic Research Program of China (2014CB660814), the Program for Innovative Research Team in Chinese Universities (IRT1237), the Research Project of Chinese Ministry of Education (213011A), the Specialized Research Fund for the Doctoral Program of Higher Education (20122301110002), the Chang Jiang Scholar Candidates Programme for Heilongjiang Universities (2012CJHB003), and the Science Foundation for Excellent Youth of Harbin City of China (2014RFYXJ002).

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Footnote

† Electronic supplementary information (ESI) available: DRS spectra and TEM image (Fig. S1), XRD patterns (Fig. S2), SEM images (Fig. S3), I–V curves (Fig. S4), EIS spectra (Fig. S5), the formed hydroxyl radical amount (Fig. S6), SS-SPS responses (Fig. S7), HRTEM image (Fig. S8), XRD patterns, DRS spectra and TEM images (Fig. S9), and SS-SPS response and photocatalytic degradation rates (Fig. S10). See DOI: 10.1039/c4ra08451a

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