InBO3 Heterostructures with Enhanced Visible

Jun 4, 2014 - and Yaan Cao*. ,†. †. Key Laboratory of Weak-Light Nonlinear Photonics, Ministry of Education, TEDA Applied Physics Institute and Sc...
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Fabrication of N‑TiO2/InBO3 Heterostructures with Enhanced Visible Photocatalytic Performance Yanlong Yu,† Yue Tang,† Jixiang Yuan,† Qiang Wu,† Wenjun Zheng,*,‡ and Yaan Cao*,† †

Key Laboratory of Weak-Light Nonlinear Photonics, Ministry of Education, TEDA Applied Physics Institute and School of Physics, Nankai University, Tianjin 300457, China ‡ Department of Materials Chemistry, College of Chemistry, Nankai University, Tianjin 300457, China S Supporting Information *

ABSTRACT: A new type of heterostructured photocatalysts (N-TiO2/InBO3) were synthesized by coupling nitrogenmodified TiO2 (N-TiO2) with indium borate (InBO3) via a one-step sol−gel method. It was revealed that N-TiO2/InBO3 exhibited an improved photocatalytic performance compared with TiO2, N-TiO2, and InBO3 under both UV and visible light irradiation because of the formation of a heterostructure at the interface as well as the introduction of surface NOx species and InBO3. These results may provide a paradigm to fabricate and design the optoelectronic functional materials with high efficiency and performance.

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INTRODUCTION TiO2 has been investigated extensively as one of the most promising photocatalysts due to its high chemical stability and good photoelectric performance.1−5 However, the photocatalytic performance is still limited because of its large band gap (3.2 eV for anatase) and high recombination efficiency of photogenerated charge carriers, impeding the efficient usage of solar light in practical applications. Doping titania with metal or nonmetal elements is one of the most effective methods to extend the response into the visible region,3,6−12 and compositing TiO2 with other semiconductors to form the heterostructure can promote the separation of photogenerated electrons and holes efficiently.13−17 Thus, the combination of doping and heterostructure would become more helpful to prepare and develop TiO2-based photocatalysts with high photocatalytic performance. We recently reported that indium borate exhibited better photocatalytic capabilities than TiO2 in photodegradation of 4-chlorophenol,18 with a longer lifetime of photogenerated electrons involved in the photocatalytic process. So, we expect to combine the nitrogen-modified TiO2 (visible light response) with InBO3 (excellent long lifetime charge carriers) and synthesize the N-TiO2/InBO3 heterostructured photocatalysts, to achieve the advantages of doping and heterostructure and obtain photocatalysts with highly reactive activity under visible as well as UV light irradiation. Herein, a new type of N-TiO2/InBO3 heterostructure on nanoscale was synthesized by the sol−gel method. The molar ratios of InBO3 and N-TiO2 can be adjusted by controlling the pH value of the gel. The composition of InBO3 with N-TiO2 would extend the visible response, inhibit the recombination, and prolong the lifetime of photogenerated charge carriers. © 2014 American Chemical Society

Therefore, N-TiO2/InBO3 represents a better photocatalytic performance than N-TiO2 and InBO3 under both visible and UV light irradiation.

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EXPERIMENTAL DETAILS Catalyst Preparation. The N-TiO2/InBO3 heterostructures were synthesized via a one-step sol−gel method. All the chemicals used are of analytical grade, and water is deionized water (>18.2 MΩ cm). At room temperature, 7.1 mL of InCl3 solution (0.73 mol·L−1) was mixed with 40 mL of anhydrous ethanol. Then 240 mg of H3BO3 was added into this mixed solution under vigorous stirring for 15 min. The mixtures were added dropwise with 1 mL of concentrated HCl solution (12 mol L−1) and then 12 mL of Ti(OC4H9)4. After stirring continuously for 15 min, 3 mL of ammonia was added, and the white precipitate was observed immediately. After filtering and washing, the white precipitate was dried at 373 K after aging at room temperature for 24 h and then calcined at 723 K in a muffle for 150 min. The pH value of the gel could be easily adjusted by changing the amount of HCl solution to obtain NTiO2/InBO3 heterostructures with different molar ratios. For comparison, pure TiO2, N-TiO2, and InBO3 powders were prepared with the same procedure with relative precursors. Characterization. The XRD patterns were collected on a Rigaku D/max 2500 X-ray diffraction spectrometer (Cu Kα, λ = 1.54056 Å). Corresponding crystal size was calculated using the Scherrer equation (D = kλ/B cos θ). The high-resolution transmission electron microscopy (HRTEM) analyses were Received: December 18, 2013 Revised: June 4, 2014 Published: June 4, 2014 13545

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conducted using a Philips Tecnai G2F20 instrument at an accelerating voltage of 200 kV, for which the samples were prepared by applying a drop of ethanol suspension onto an amorphous carbon-coated copper grid and dried naturally. XPS measurements were carried out with an SECA Lab 220i-XL spectrometer using a monochromated Al Kα X-ray source (1486.6 eV), and the binding energy was calibrated according to the adventitious C 1s peak of 284.8 eV. Diffuse reflectance UV−visible (UV−vis) absorption spectra were recorded on a UV−vis spectrometer (U-4100, Hitachi). Photoluminescence (PL) spectra and the time-resolved PL decay curve were acquired by using a time-resolved spectrofluorometer with the femtosecond (fs) laser system and an intensified CCD camera (ICCD) spectrograph (LAVISION, PicoStar HR12 Camera System). The fs laser pulse (800 nm, 120 fs, 1 kHz) was amplified to a fs laser pulse (400 nm, 120 fs, 1 kHz) by a regenerative amplifier. All the measurements were carried out at room temperature, (25 ± 2) °C, unless stated otherwise. Photocatalysis. The photocatalytic activity of all samples was evaluated via the photodegradation of 4-chlorophenol (4CP). An amount of 10 mg of catalysts was suspended in the 4CP solution (5 × 10−5 mol·L−1, 40 mL, pH = 5.35) in a 100 mL photochemical reactor. A sunlamp (Philips HPA 400/30S, Belgium) was used as the light source. The visible and UV light intensity irradiated on the surface of the reactor was 0.1 and 0.2 W·cm−2, respectively. The reactor was vertical to the light beam which is located at 15 cm away from the light source. The suspensions were stirred at room temperature in the dark for 30 min before irradiation. The solution was continuously bubbled with oxygen at a flux of 5 mL·min−1 to reach an adsorption equilibrium. The concentration of the solution was detected by an UV−vis spectrometer (UV-1601PC, Shimadzu), using 4aminoantipyrine as the chromogenic reagent. The blank experiment was performed under identical conditions but without catalyst.

samples, except the peaks of anatase, the diffraction peaks at 24.3°, 31.6°, and 37.4° can be observed corresponding to (102), (104), and (110) crystal planes of InBO3, respectively. This result indicates a composite structure of N-TiO2 and InBO3 is formed in the N-TiO2/InBO3 sample. There are no other characteristic diffraction peaks (such as TiN, InCl3, B2O3, and so on) detected in the XRD patterns of N-TiO2 or NTiO2/InBO3. The XRD patterns of samples are used to calculate the lattice parameter, cell volumes, and crystal size, summarized in Table 1. The lattice parameter and cell volume Table 1. Crystallite Size and Lattice Parameter of TiO2, NTiO2, N-TiO2/InBO3, and InBO3 cell parameters (nm)

sample

a=b

c

cell volume (Å3)

TiO2 N-TiO2 N-TiO2/InBO3 InBO3

0.3792 0.3786 0.3794 0.4818

0.9499 0.9516 0.9496 1.5520

136.6 136.4 136.7 311.8

crystallite size (nm)

SBET (m2 g−1)

16.1 12.0 8.6 16.2

63.8 77.8 113.4 4.8

of N-TiO2 and N-TiO2/InBO3 are almost the same as those of TiO2, and compared with the pure TiO2, no shift is observable for the XRD peaks of N-TiO2 and N-TiO2/InBO3. According to the results of XRD, it is reasonable to infer that introduced B and In ions form the InBO3, and nitrogen may exist as a surface species. This result would be further confirmed by TEM (Figure 3), FT-IR (Figure S1, Supporting Information), and XPS (Figure 4 and Figure S3, Supporting Information). In comparison with pure TiO2, the crystallite size of N-TiO2 decreases and further decreases after the composition with InBO3. These suggest that the existence of nitrogen and InBO3 could inhibit the grain growth of the TiO2 crystal effectively because of dissimilar boundaries.11 The specific surface area of N-TiO2/InBO3 is larger than TiO2 and N-TiO2 (Table 1). The large surface area for N-TiO2/InBO3 would further contribute to the photocatalytic activity. The trend for the change of specific surface area agrees with that of crystal size. To investigate the formation of the composite material, a series of N-TiO2/InBO3 composited samples were synthesized under different pH values, and corresponding XRD patterns are shown in Figure 2. Different amounts of HCl (0.2, 0.5, 1, 1.5, 2, and 3 mL, respectively) and 3 mL of ammonia were applied to adjust the pH of the solution. No diffraction peaks of InBO3 are detected when the pH value is less than 3. It suggests InBO3 could not be formed under the strong acid circumstance, and when the pH increases to 4.65 or above, the diffraction peaks of InBO3 appear and heighten with the increase of pH value. This result indicates that InBO3 prefers to be formed by introduced In and B ions under neutral or alkaline conditions. However, the formation mechanism of composite nanoparticles (NTiO2/InBO3) still needs further investigation. The TEM and HRTEM images of N-TiO2/InBO3 are shown in Figure 3. As shown in Figure 3A, the photocatalyst consists of nanoparticles with about 20 nm. The selected area electron diffraction (SAED) patterns of N-TiO2/inBO3 (inset of Figure 3A) show a set of strong Debye−Scherrer rings as well as complicated bright spots, which indicates the coexistence of polycrystalline anatase and InBO3 crystallite structures. Two different lattices with d spaces of 0.350 and 0.367 nm can be clearly found from HRTEM images (Figure 3B), corresponding

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RESULTS AND DISCUSSION To investigate the crystal structure and the chemical state of N, In, and B in samples, XRD, TEM, and XPS technique were used. Figure 1 represents the XRD patterns of InBO3, TiO2, NTiO2, and N-TiO2/InBO3. It can be easily found that the majority of crystal phase is anatase for TiO2 and N-TiO2. InBO3 shows a calcite structure with a hexagonal lattice in the crystalline state (JCPDS, No. 17-0933).18 For N-TiO2/InBO3

Figure 1. XRD patterns of TiO2 (a), N-TiO2 (b), N-TiO2/InBO3 (c), and InBO3 (d). 13546

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Figure 4. N 1s XPS spectra of InBO3, N-TiO2, and N-TiO2/InBO3.

reflectance UV−vis absorption spectra of TiO2, N-TiO2, InBO3, and N-TiO2/InBO3 are shown in Figure 5. The strong

Figure 2. XRD patterns of N-TiO2/InBO3 synthesized under different pH values.

Figure 3. (A) TEM image and (B) HRTEM images of N-TiO2/InBO3 heterostructured photocatalyst. Inset of (A) is the electron diffraction patterns corresponding image. Figure 5. Diffuse reflectance UV−vis absorption spectra of InBO3, TiO2, N-TiO2, and N-TiO2/InBO3. The inset shows XPS valence band spectra of N-TiO2 and InBO3.

to the (1 0 1) plane of TiO2 and the (1 0 2) plane of InBO3, respectively. The InBO3 nanoparticles are tightly coupled on the surface of TiO2, implying that the compositing of nanoparticles usually keeps to coupling with the similar lattice fringes. The HRTEM image confirms N-TiO2/InBO3 behaves as a well-crystallized heterostructure on nanoscale between NTiO2 and InBO3 particles. The XPS technique is used to investigate the chemical states of In, B, and N. It could be roughly estimated from the XPS result that the molar ratio of Ti/In is about 5/2 when the addition of HCl is 1 mL (pH = 7.45). Figure 4 represents the N 1s peaks of N-TiO2 and N-TiO2/InBO3, whose broad peaks are at around 400.0 eV (from 398 to 404 eV). The broad peaks are much higher than the typical binding energy (BE) of 396.9 eV in TiN,19 implying that N atoms interact with O atoms strongly in N-TiO2 and N-TiO2/InBO3.20 Therefore, the broad peaks are attributed to the oxidized nitrogen similar to the NOx (x = 1, 2, or 3) species, meaning that the linkage of Ti−N−Ox possibly forms on the surface of N-TiO2 which has been demonstrated by a previous work.21 This result is consistent with the XRD results. Moreover, no N element is detected in InBO3 samples by XPS. The behaviors of photogenerated charge carriers and photocatalytic activity are closely related to the band structure of heterostructure. Diffuse reflectance UV−vis absorption spectra and XPS valence band spectra are used to investigate the band structure of the heterostructure. The diffuse

absorption peak around 340 nm in TiO2 and N-TiO2 is ascribed to the band-to-band transition of TiO2, corresponding to a band gap of 3.10 eV for which the absorption onset edge is about 400 nm.22,23 For InBO3, the band gap is 3.25 eV18 as the absorption onset edge is about 380 nm. Compared with TiO2, N-TiO2 presents a small hump around 450 nm tailing the visible region caused by the electron transition from the energy level of surface NOx (x = 1, 2, or 3) species (about 0.25 eV above the valence band of TiO2)24 to the conduction band of N-TiO2.25−27 It is noticed that a strong absorption ranging from 400 to 800 nm was observable for N-TiO2/InBO3, which could be ascribed to the transition from the energy level of surface NOx species to the conduction band of InBO3.18,28−30 It is also found from Figure S6 (Supporting Information) that the absorption spectra vary with the relative amount of InBO3 compared with TiO2 which further demonstrates that the enhanced absorption in the visible region is due to the electron transition from the surface state energy level of NOx species to the conduction band of InBO3. This indicates that the N-TiO2/ InBO3 heterostructure is more sensitive to the solar light than N-TiO2 or InBO3, meaning the heterojunction at the interface of N-TiO2/InBO3 is of great importance for the enhancement of visible light response. 13547

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Figure 6. (A) PL spectra of TiO2, N-TiO2, N-TiO2/InBO3, and InBO3. (B) The time-resolved PL decay curve for different samples, excited at 400 nm and monitored at 470 nm.

Table 2. Values of the Calculated Decay Time Constant τ1 and τ2 via Double Exponential Decay Fitting for the Corresponding Samples

The inset of Figure 5 shows the XPS valence band spectra for InBO3 and N-TiO2. The energy level is in alignment with the work function of the XPS instrument (4.10 eV, Fermi level). The BE of the onset edge for the O 2p peak reveals the energy gap between the Fermi level and VB maximum.14 The energy levels are +1.50 and +2.85 eV for N-TiO2 and InBO3, respectively.31 Therefore, the potential of the valence band maximum is determined to be 1.10 V (vs SHE) for N-TiO2 and 2.45 V for InBO3. Because the band gap for N-TiO2 and InBO3 is 3.10 and 3.25 eV, the potential of the conduction band for TiO2 and InBO3 is determined to be −2.00 and −0.80 V, respectively. According to the discussion above, the energy band structure ofthe heterojunction in N-TiO2/InBO3 is shown in Figure 8. The separation and recombination behavior of the photogenerated electrons and holes is closely related to the photocatalytic activity of photocatalysts.32,33 The photoinduced electrons fall into the conduction band by means of a nonradiative process. Then the electrons in the defects (oxygen vacancies) recombine with the holes in the valence band, leading to fluorescence emission. Therefore, the photoluminescence (PL) intensity and the lifetime of electrons in oxygen vacancies could be used for evaluating the photocatalytic activity,18,22 measured by PL technique (Figure 6). Figure 6A shows the PL spectra of TiO2, N-TiO2, N-TiO2/ InBO3, and InBO3. The high peaks for InBO3 are caused by the transition of the energy level for defects.18,29,30 For TiO2, NTiO2, and N-TiO2/InBO3, the PL peaks could be ascribed to the transition from oxygen vacancies with one-trapped electron and two-trapped electrons to the VB of TiO2, respectively. The time-resolved PL decay curves of TiO2, N-TiO2, N-TiO2/ InBO3, and InBO3 are plotted in Figure 6B. For metal oxide, the PL decay curve arises from a combination of a nonradiative (τ1) process and a radiative (τ2) process.25,34,35 The fast decay process (τ1) is usually attributed to the nonradiative relaxation process relevant to defects of oxide, and the slow decay range (τ2) comes from the radiative process which is relevant to the recombination of photogenerated holes and electrons.18 The values of τ1 and τ2 for samples (Figure 6B) were determined by double exponential decay fitting (Table 2). Usually, a relative decrease in the PL intensity and prolongation of the PL lifetime (the delay of recombination process) imply a decreased recombination rate and effective separation of photogenerated charge carriers, thus implying a higher photocatalytic activity.18 As a fluorescent material, InBO3 shows the highest PL intensity,29,30 and its τ2 value is 4.894 ns. Pure TiO2 exhibits a relatively high PL intensity and corresponding PL lifetime of

τ1 τ2

TiO2

N-TiO2

N-TiO2/InBO3

InBO3

0.442 1.528

0.475 2.190

0.998 4.172

1.354 4.894

1.528 ns (τ2 value). After modification with nitrogen, the PL intensity of N-TiO2 is weakened, and its τ2 value (2.190 ns) is longer than that for TiO2 (1.528 ns), which is ascribed to the capture of photogenerated holes by surface NOx species. The PL intensity for N-TiO2/InBO3 is further weakened, and the corresponding PL lifetime (4.172 ns) is much longer than that for TiO2 (1.528 ns) and N-TiO2 (2.190 ns), indicating that the recombination of photoinduced charge carriers is further inhibited, due to the contribution of a heterojunction at the interface between N-TiO2 and InBO3. The photoinduced electrons in the CB of N-TiO2 would transfer to the CB of InBO3; meanwhile, photoinduced holes on the VB of InBO3 would migrate to the valence band of N-TiO2. Therefore, the photoinduced electrons and holes are separated effectively for N-TiO2/InBO3. The photodegradation of 4-CP is used to evaluate the photocatalytic performance of catalysts. These results are shown in Figure 7 and Figure S3, Table S1, and Table S2 (Supporting Information). For all the samples, a nearly linear relationship is found for the ln(c0/c) values of 4-CP with irradiation time (Figure S3, Supporting Information), suggesting a pseudo-first-order reaction. The 4-CP can hardly be decomposed in photolysis experiment under visible light irradiation for 8 h (Figure 7A and Table S1, Supporting Information). TiO2 and InBO3 exhibit a low photocatalytic activity, and about 16.3% of 4-CP has been decomposed in the suspension of N-TiO2. For the N-TiO2/InBO3 heterostructure, about 89.1% of 4-CP has been decomposed, whose photocatalytic activity (photodegradation rate and specific photocatalytic activity) is about 5 times that for N-TiO2. The corresponding chemical oxygen demand (COD) results are consistent with these results, as shown in Figure S2 (Supporting Information). Under UV light irradiation (Figure 7B and Table S2, Supporting Information), the N-TiO2/InBO3 heterostructure still exhibits a much better photocatalytic performance than InBO3, TiO2, and N-TiO2. These results suggest that the heterojunction at the interface of N-TiO2 and InBO3 results in an enhanced photocatalytic performance under 13548

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Figure 7. Photocatalytic activity of InBO3, TiO2, N-TiO2, and N-TiO2/InBO3. (A) Under visible light irradiation for 8 h. (B) Under UV light irradiation for 60 min.

rate for charge carriers among all samples. Electrons accumulated on the conduction band of InBO3 are captured by the adsorbed O2 molecules, resulting in the formation of O2− active species that would further photodecompose the 4CP molecules.36 Meanwhile, the photoinduced holes on the VB of N-TiO2 and the energy level of surface NOx species can oxidize 4-CP adsorbed on surface active sites of photocatalysts directly. Besides that, the large surface area for the N-TiO2/ InBO3 heterostructure would also contribute to the photocatalytic reaction. Therefore, more photoinduced charge carriers would contribute to the photocatalytic process, leading to better photocatalytic performance than N-TiO2 and InBO3 under visible light irradiation. Under UV light irradiation, electron transition from the VB to the CB is allowed for every sample. TiO2 and InBO3 represent high photocatalytic activity because of the separation of photogenerated charge carriers on the CB and VB, respectively. Moreover, the lifetime of charge carriers related to the radiative process is prolonged for InBO3 compared with TiO2. Thus, InBO3 shows a better photocatalytic performance than TiO2.18 Compared with TiO2 and InBO3, N-TiO2 shows an improved photocatalytic activity due to the formation of surface NOx species. The photoinduced holes at the VB can transfer to the energy level of NOx species, which would separate the charge carriers, resulting in an enhancement of photocatalytic activity.24 Compared with pure TiO2, N-TiO2, and InBO3, the photocatalytic activity of N-TiO2/InBO3 is enhanced because of the heterojunction at the interface. As the CB and VB of InBO3 is lower than those of N-TiO2, respectively, the photoinduced electrons would transfer to the catalyst’s surface via the CB of InBO3 and N-TiO2, while the photoinduced holes at the VB of InBO3 can move to the VB of N-TiO2, part of which may further move to the energy level of surface NOx species. The photoinduced holes in both VB of TiO2 and energy level of NOx would oxidize adsorbed 4-CP molecules. This facilitates the separation of charge carriers effectively, which is a benefit for the photocatalytic reaction. Therefore, the photocatalytic activity of N-TiO2/InBO3 is also improved in comparison with TiO2, N-TiO2, and InBO3 under UV light irradiation.

both visible and UV light irradiation. The enhancement mechanism would be discussed in the following sections. The schematic diagram of the photocatalytic mechanism (Figure 8) is used to explain why heterostructured photo-

Figure 8. Schematic diagram of the photocatalytic mechanism for the N-TiO2/InBO3 heterostructure.

catalyst exhibits the best photocatalytic performance under UV and visible light irradiation, mainly attributed to energy band match of the heterostructure at the interface in N-TiO2/InBO3. Under visible light irradiation (λ > 400 nm, dashed line), InBO3 and TiO2 represent very low photocatalytic activity because of their large band gap (3.1 eV for pure TiO2; 3.25 eV for InBO3). N-TiO2 shows a restricted photocatalytic activity, which is ascribed to NOx species on the surface of the catalyst. Small amounts of electrons could be excited from the energy level of the surface NOx species to the CB of TiO2, causing a limited photocatalytic activity of 4-CP degradation. The N-TiO2/ InBO3 heterostructure exhibits a significant enhanced photocatalytic performance in comparison with the other samples, owing to the energy band match of the heterojunction at the interface as well as introduction of nitrogen and InBO3. For NTiO2/InBO3, except the electron transition from the energy level of surface NOx species to the CB of TiO2, the electron transition at the interface of the heterojunction is engendered significantly from the energy level of NOx species to the CB of InBO3, leading to a strong absorption in the visible light region (Figure 5) and an increase in the amount of photogenerated charge carriers. On the other hand, the charge carriers are separated efficiently. At the same time, the lifetime of photoinduced electrons is also prolonged effectively due to the formation of the heterostructure, compared with N-TiO2. Accordingly, N-TiO2/InBO3 presents the lowest recombination

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CONCLUSION

In summary, a new type of heterostructured photocatalyst is synthesized by coupling N-TiO2 with InBO3 nanoparticles by a simple sol−gel method. The heterostructure represents a much better photocatalytic performance than TiO2, N-TiO2, and 13549

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InBO3 under visible and UV light irradiation because of the matching of energy level of the heterojunction at the interface, introduction of surface NOx species, and the addition of InBO3. The electron transition at the interface of the heterojunction results in a significant visible light response. In the meantime, the photoinduced electrons and holes are separated efficiently under UV and visible light irradiation by reason for the heterostructure. Furthermore, the prolonged lifetime of the photogenerated electrons caused by the heterostructure is also the core reason for the efficient separation of electrons and holes. As a result, the photocatalytic performance under visible and UV light irradiation for N-TiO2/InBO3 is enhanced effectively. It was expected that the catalytic performance of the heterostructured photocatalyst could be further enhanced by optimizing the kind of the materials, heterostructure fabrication, and the doped ions. This would contribute to practical use in numerous fields, such as photocatalysis, photovoltaics, and photosynthesis.

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ASSOCIATED CONTENT

S Supporting Information *

FT-IR spectra, COD photodegradation, ln(c0/c) ∼ t, O 1s, Ti 2p, In 3d, and B 1s XPS spectra, XRD, and absorption spectra of N-TiO2/InBO3 with different molar ratios. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

* Fax: +86-22-66229310. Tel.: +86-22-66229598. E-mail: [email protected] (W.Z.). *E-mail: [email protected] (Y.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos 51072082, 21173121, and 51372120).

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REFERENCES

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