Unveiling the Role of Defects on Oxygen Activation and

Nov 14, 2018 - Huang, Kong, Westerman, Xu, Coffin, Cochran, Liu, Richardson, Schlenk, and Dionysiou. 2018 52 (21), pp 12697–12707. Abstract: This st...
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Cite This: Environ. Sci. Technol. 2018, 52, 13879−13886

Unveiling the Role of Defects on Oxygen Activation and Photodegradation of Organic Pollutants Mengjiao Xu,†,∥ Yao Chen,†,∥ Jiangtao Qin,† Yawei Feng,† Wei Li,‡ Wei Chen,§ Jian Zhu,† Hexing Li,*,† and Zhenfeng Bian*,† †

Environ. Sci. Technol. 2018.52:13879-13886. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/14/19. For personal use only.

The Education Ministry Key Lab of Resource Chemistry and Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Normal University, Shanghai 200234, P. R. China ‡ Department of Chemistry, Laboratory of Advanced Materials, Shanghai Key Lab of Molecular Catalysis and Innovative Materials and iChEM, Fudan University, Shanghai 200433, P. R. China § Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore S Supporting Information *

ABSTRACT: The status of defects of TiO 2 are of fundamental importance in determining its physicochemical properties. Here we report a simple chemical deposition method for controllable synthesis of defective anatase TiO2 nanocrystals under various calcination atmospheres. XPS and ESR analysis reveals that both the oxygen vacancies (VO) and the trivalent titanium (Ti3+) defects exist in TiO2 after N2 treatment (N-TiO2). Meanwhile, mainly VO defects can be obtained in TiO2 with air calcination (A-TiO2). ESR spectra for reactive oxygen species determination, clearly show that the visible light catalytic activity is mainly caused by the efficient activation of oxygen molecules to •O2− species for ATiO2, which play an important role in hindering the accumulation of intermediates during p-chlorophenol (4-CP) photodegradation process. However, the oxygen molecules cannot be activated for N-TiO2 even with superior visible light absorption and thus the photogenerated electron are reductant, which participated in the transformation of BQ to HQ via electron shuttle mechanism. Moreover, A-TiO2 exhibits higher separation efficiency of photogenerated carriers than that of N-TiO2, showing the critical role of VO with a suitable concentration in transferring photogenerated charges.

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bombardment, vacuum thermal treatment, and flame reduction.15 Furthermore, the distribution of defects in TiO2 is another important hot research topic, which shows great influence on the photocatalytic performance. For instance, Kong et al. reported that improvement of the ratio between surface defects and bulk defects could greatly enhance the separation efficiency of photogenerated carries and photocatalytic activity. 16 Yu et al. observed that the TiO 2 hydrogenated at the temperature over 873 K exhibited much higher photocatalytic activity ascribed to the diffusion of defects from the bulk to the surface.17 Zhang et al. demonstrated that the surface defects were more favorable for photocatalytic hydrogen evolution than the bulk defects.18 Despite the fact that great progress has been made on defect engineering of TiO2, some of the main issues are still under debate. One is that the role played by defects in producing

INTRODUCTION TiO2 is considered as an ideal photocatalyst in organic pollutants degradation, photocatalytic hydrogen generation, dye-sensitized solar cells and lithium-ion batteries because of its nontoxicity, chemical stability, and low cost.1−4 However, its practical applications are still limited by fast recombination of photoinduced charges and the poor absorbance of visible lights. To date, enormous strategies have been expended to improve its quantum efficiency and light harvesting, such as ions doping,5,6 surface sensitization,7,8 crystal structure,9 and morphology control.10,11 Both experimental results and theoretical prediction demonstrate that defect states can profoundly influence the physicochemical properties of TiO2, including the optical behavior, dissociative adsorption performance, electronic structure, and catalytic efficiencies.12,13 Chen et al. reported that the black titanium dioxide prepared under high pressure hydrogen atmosphere could remarkably enhance solar-driven photocatalytic activity.14 Since then, various methods are being employed to fabricate defected TiO2, such as chemical reduction approach (e.g., H2, NaBH4, Al, Carbon), electrochemical reduction, high energy particle © 2018 American Chemical Society

Received: Revised: Accepted: Published: 13879

July 10, 2018 November 9, 2018 November 14, 2018 November 14, 2018 DOI: 10.1021/acs.est.8b03558 Environ. Sci. Technol. 2018, 52, 13879−13886

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Environmental Science & Technology

precursor powder was separated (washed for several times using water and ethanol), and dried (353 K). Finally, the dried powder was calcined in air or N2 at 923 K (5 K/min, 2 h) to obtain TiO2 with different defects, named as A-TiO2 and NTiO2, respectively. Characterization. X-ray diffraction spectra (XRD, Rigaku D/MAX-2000, Cu Kα source) were operated at 40 kV and 20 mA, scanned at a rate of 5° min−1, Raman spectra (Jobin Yvon, SuperLabRam II), field emission scanning electron microscopy (FESEM, HITACHI S4800, 50 kV), transmission electronic micrograph (TEM, JEOL JEM-2010, 200 kV), X-ray photoelectron spectroscopy (XPS, PerkinElmer PHI 5000C, Al Kα), UV−vis diffuse reflectance spectra (Hitachi UH4150), nitrogen sorption (Micromeritics Tristar II 3020, at 77 K). Thermal gravimetric analyzer (TGA, DTG-60H, and heating speed of 5 K/min) was used to analyze the impurities of the precursor of TiO2. The photoluminescence (PL) spectra were obtained using a FLS 980 fluorometer (Edinburgh Instruments, Ltd.). Photocatalytic Activity Test. Photocatalytic performance of the samples was tested by the decomposition of 4chlorophenol (4-CP). Typically, 10 mg catalyst was dispersed in 50 mL of 4-CP aqueous solution (10 ppm), which was irradiated by a 400 W high-pressure Hg lamp. The wavelengths >420 nm was cut by a glass filter (JB-420). The solution was continuously stirred for 30 min in the dark before irradiation to conform the adsorption−desorption equilibrium. The residual 4-CP was analyzed by a UV−vis spectrophotometer (UV 7504/PC). ESR Test. Electron spin resonance (ESR) spectra were recorded on a JEOL JES-FA200 model spectrometer (9.05 GHz). Oxygen vacancy and Ti3+ defects were detected in a 5 mm Suprasil quartz tube at 77 K in vacuum. Samples were irradiated for 12 min before •O2− and •OH trapping tests (300 W xenon lamp >420 nm), which were operated at room temperature in methanol and water respectively in a capillary quartz tube, using 5,5-dimethyl-1-pyrroline-N-oxide (50 mM) as the spin-trapping agent. Analyses. The concentration of 4-CP was quantified by HPLC (Agilent, Zorbax Eclipse XDB-C8 column (4.6 × 150 mm2, 5 μm)), using an Infinity quaternary pump (Agilent 1260) and a fluorescence detector (Agilent 1260). Methanol and water (50:50, v/v) was used as the mobile phase (0.4 mL/ min). The wavelength employed for detecting 4-CP, BQ and HQ was 280 nm, 250 and 290 nm, respectively. Photoelectrochemical Test. Photoelectrochemical measurements were used a single-compartment quartz cell in a conventional three-electrode system (CHI 660E, platinum sheet (10 × 20 mm2) counter electrode, a saturated calomel reference electrode (SCE) and the TiO2 film working electrode). A TiO2 film electrode was prepared on a FTO conducting glass substrate (sheet resistance ≤8 Ω). Using Scotch tape as a spacer (2 × 1 cm2), colloidal TiO2 solution was deposited on the substrate. The excess suspension was raked off with a glass rod. Then the film was dried and heated at 573 K for 2 h in air (5 K/min). Na2SO4 aqueous solution (0.50 M) was used as the electrolyte. A bias voltage of 0.5 V was applied to drive the migration of photoinduced electrons. The irradiation source was 300 W xenon lamp with a cutoff filter (λ > 420 nm) and positioned 8 cm above the reaction cell. The surface photovoltage (SPV) spectra were measured on a homemade system consisting of xenon lamp (Omni-5007, Zolix), amplifier (SR830-DSP) with a light chopper (SR540) at the chopping frequency of 23 Hz.

reactive species for photocatalysis is still not clearly understood. It was recognized that capturing the photoinduced electrons by adsorbed molecular oxygen on the surface was the rate-determining step for photo-oxidation processes, which was much slower than that of capturing hole by the surface absorbed H2O or organic pollutants.19,20 The interaction between oxygen and photocatalysts is so vital that many strategies have been explored. Jing’s group developed a method of surface modification with inorganic acids to promote O2 adsorption.19 Malwadkar et al. declared that deposited Au nanoparticles on TiO2 was efficiently contributed to the adsorption and activation of oxygen.21 Specially, defect sites are shown to be active for the adsorption and activation of O2. It was demonstrated that O2 did not adsorb on fully oxidized, perfect TiO2 surface, while it adsorbed both in a molecular and an irreversible dissociative manner on the defective surface with excess negative charges.22,23 Additionally, only a few researches involved the mismatch between the superior light absorption and photoactivity. Hu et al. demonstrated that the enhanced photocatalytic hydrogen evolution from water splitting was mainly due to the incident photon-to-electron conversion efficiency (IPCE) increased in the UV region instead of the dramatic improvement in visible light absorbance for hydrogenated TiO2.24 Leshuk et al. revealed that the high-temperature hydrogenated TiO2 with strong visible-light absorption exhibited much poor photocatalytic activity in comparison with the untreated TiO2.25 They claimed that the decrease in photocatalytic activity was due to the increase in the concentration of VO defects in the TiO2 lattice. In contrast, Liu et al. found that the photocatalytic activity sharply decreased with increased content of VO defects though they exhibited enhanced visible light absorption.26 Herein, we report a successful fabrication of defective anatase TiO2 with controllable concentration of VO and Ti3+ defects using chemical deposition method. Hydrazine hydrate was employed as both the depositing and the reducing agent. Different calcination atmospheres were applied to control the defect states in TiO2. Experimental evidence clearly demonstrate the presence of mainly oxygen vacancies (VO) in the TiO2 after air-calcinations (A-TiO2). However, both VO and Ti3+ defects coexisted in the TiO2 after N2-calcination (NTiO2). It is found that the concentration of defects has different impacts on the formation of free radical species and the activity in visible light photocatalysis. The high photocatalytic performance of the A-TiO2 is attributed to the fast photogenerated charge transfer and in particular the efficient activation of oxygen molecules. N-TiO2 with a superior visible light absorption cannot activate oxygen molecules because of a relatively high concentration of defects, resulting in a dramatic decrease in visible light photocatalytic performance.

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EXPERIMENTAL SECTION Chemicals. Titanium tetrafluoride (TiF4, 99%, J&K Scientific, Ltd.), Hydrazine hydrate (NH2NH2·H2O, 85%, sinopharm chemical reagent Co., Ltd.), used without further purification. Deionized water was used in the whole experimental process. Synthesis of Defected TiO2 Nanoparticles. The defected TiO2 nanoparticles were prepared via a simple chemical precipitation method. In a typical procedure, 0.05 mol TiF4 was resolved in 200 mL deionized water under magnetic stirring. Then, 1.6 mL NH2NH2·H2O added into the solution at room temperature (stirring for 2 h). The white 13880

DOI: 10.1021/acs.est.8b03558 Environ. Sci. Technol. 2018, 52, 13879−13886

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Environmental Science & Technology

Figure 1. XRD patterns (a) and Raman spectra (b) of A-TiO2 and N-TiO2 sample, respectively. TEM micrographs of A-TiO2 (c) and N-TiO2 (d) (The insert is SAED).

Figure 2. XPS spectra of (a) Ti 2p and (b) O 1s of A-TiO2 and N-TiO2.

XAFS Study. Ti K-edge X-ray absorption near edge spectra (XANES) were collected at the BL 14W1 of Shanghai Synchrotron Radiation Facility using a double Si (111)-crystal monochromator at transmission mode. Energy of the electron beam in storage ring was 3.5 GeV. Demeter was used for data analyze.27 Theoretical phase and amplitude functions were obtained from the program FEFF 9.0.28 The k3-weighted FTEXAFS from 3 to 12 Å−1and an R window of 1−4 Å was applied for date fitting.

The insert selected area electron diffractions (SAED) in Figure 1c, d demonstrate the polycrystal characteristics of both ATiO2 and N-TiO2. The N2 adsorption−desorption isotherms (Figure S2) demonstrate that both A-TiO2 and N-TiO2 are present as Langmuir isotherm structure. The specific surface area (SBET) and pore volume (VP) of the A-TiO2 are determined to be 40.6 m2·g−1 and 0.225 cm3·g−1, similar to those of N-TiO2 (SBET = 39.6 m2 g−1 and VP = 0.223 cm3 g−1). Thermogravimetric measurements (TG, Figure S3) indicate that the precursor can be totally converted into pure TiO2 under either air or N2 calcinations at 923 K. No significant N and F were detected by XPS spectra (Figure S4), further confirming the complete removal of impurities on the TiO2 surface. As shown in Figure 2a, the XPS spectrum of A-TiO2 displayed two typical Ti4+ peaks with binding energies around 464.3 and 458.5 eV in Ti 2p1/2 and Ti 2p3/2 levels.29 The NTiO2 exhibits slightly negative shift (0.1 eV) in comparison with A-TiO2, indicating the presence of Ti3+ species. The fine scan spectra of O 1s are asymmetric both in A-TiO2 and NTiO2 (Figure 2b). The main peak of O 1s located at 529.8 eV corresponds to lattice oxygen of TiO2.30 While, a shoulder peak at a higher binding energy of 531.8 eV can be ascribed to the VO defect.31 According to the peak area ratio of VO, we

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RESULTS AND DISCUSSION XRD patterns in Figure 1a display several strong diffraction peaks with similar intensity characteristic of anatase phase (JCPDS, No. 21-1272). On the basis of Scherrer equation, the average crystallite size of A-TiO2 and N-TiO2 is estimated as both about 41 nm. The absorption at 146 cm−1 in Raman spectra (Figure 1b) also indicates the anatase crystalline structure. No significant difference in either XRD or Raman spectra has been observed, suggesting the similar crystal phase, crystallinity, and particle size of the A-TiO2 and N-TiO2 obtained under different atmosphere-calcinations. TEM micrographs (Figure S1 of the Supporting Information, SI) further reveal that A-TiO2 and N-TiO2 both show the similar morphology and the grain size ranging from 34 to 44 nm. 13881

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the enhanced visible light absorption.12 The Ti3+ defects form a shallow donor level below the conduction band (CB), which can also increase the visible light absorption.42 On the basis of the above results, we can deduce that the higher concentration of defects in TiO2 results in more visible light absorbance. Visible-light-driven photocatalytic performance was carried out (Figures 4 and S6). All spectra show characteristic absorption peaks at 228 nm (4-CP) in Figure 4. For A-TiO2, the absorption intensity of 4-CP drops monotonically under visible-light irradiation. The increased absorption intensity at 289 nm in the first 2 h during photocatalytic 4-CP degradation is due to the generation of hydroquinone (HQ).43,44 Then the formed organic intermediates undergo a further degradation rapidly suggested by the decreased absorption intensity of HQ. By contrast, benzoquinone (BQ) and higher concentrations of HQ are generated for N-TiO 2 , and the number of intermediates reaches the maximum value after irradiation for 4 h, then gradually decreased. 4-CP undergoes a more complex process of destruction at the initial stage, almost no change within the next 2 h, and slower destruction in the final 2 h. Quantification of 4-CP, BQ, and HQ made chromatographically as shown in Figure 4c, d. To obtain further insight into the roles played by different defect stages, reactive oxygen species (ROS) involved under visible light irradiation were determined by ESR. As shown in Figure 5, •OH species are detected for both A-TiO2 and NTiO2. However, only •O2− species are determined for A-TiO2. It has been demonstrated that a low concentration of defects creates a miniband below the conduction band edge, while adequately high concentration leads to the bandwidth increases and induces a continuous vacancy band of electronic states to overlap with the conduction band.45−47 Namely, a strong tail of conduction band can be formed for N-TiO2 and the generated defect level is deeper compared to A-TiO2 because of a higher density of defects for N-TiO2.15,18 When the sample is irradiated by visible light, electrons can be excited into the acceptor below the conduction band minimum (CBM). It is known that the lower energy level, the weaker of the reduction ability of located electrons. As a result, the

estimate the content of VO as 8.9% in A-TiO2 and 14.8% in NTiO2, respectively. Clearly, the concentration of VO defects in N-TiO2 is much higher than that in A-TiO2. The existence of defects in A-TiO2 and N-TiO2 is also confirmed by ESR spectroscopy. As shown in Figure 3, an

Figure 3. ESR spectra of samples at 77 K in vacuum under visible light irradiation.

intense sharp resonance signal appears at g = 2.003, which is assigned to the shallow donor states due to VO defects in TiO2.32−36 Besides, the resonance signals at g = 1.983 and 2.023 were ascribed to Ti3+ defects in TiO2.37−41 There is only one ESR signal at g = 2.003 observed in A-TiO2, suggesting the absence of Ti3+ defects. Therefore, from the above results, the N-TiO2 contains both Ti3+ and VO defects, while A-TiO2 contains only VO defects with a much lower concentration (see Scheme S1). Since VO and Ti3+ defects are significant for the optical property, optical absorption spectroscopy of either A-TiO2 or N-TiO2 has been detected. The UV−vis absorption and diffuse reflectance spectra (Figure S5) reveal that there is very little difference in UV region absorption corresponding to the intrinsic band gap absorption. However, spectra in the visible region are quite different. N-TiO2 exhibits significantly enhanced absorption (Figure S5a) and decreased diffuse reflectance (Figure S5b) compared with A-TiO2. It is widely accepted that the color center can be induced by VO, leading to

Figure 4. UV−vis absorbance spectra changes during 4-CP photodecomposition by A-TiO2 (a) and N-TiO2 (b). Photodegradation of 4-CP and the formation of BQ and HQ as a function of time for A-TiO2 (c) and N-TiO2 (d). 13882

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Figure 5. ESR test for reactive oxygen species (ROS) under visible light (a) •OH species and (b) •O2− species).

photoexcited electrons cannot reduce O2 to •O2−, thus there is no •O2− generation by N-TiO2. From the derivation of the Mott−Schottky eq (Figure S7), the flat-band potential was determined to be −0.74 V and −0.66 V (versus SCE) for ATiO2 and N-TiO2, respectively. The generated defect level in A-TiO2 is more positive than N-TiO2, which indicates that ATiO2 has a stronger reduction ability than N-TiO2. The reduction ability using MV2+ to estimate their energy level have also been tested as shown in (Figure S8). Much more MV2+ can be reduced to MV+. for A-TiO2, which indicates that photogenerated electrons for A-TiO2 have a much stronger reduction ability compared with N-TiO2. On the basis of the equilibrium potential of MV2+/ MV+• (−0.69 V vs SCE) and the above results, we estimated the energy level for A-TiO2 is above −0.69 V and below −0.69 V for N-TiO2. It is consistent with the result of flat band potential test. Thus, the different photocatalytic 4-CP degradation process can be explicated as illustrated in Scheme 1. Attacked by •OH

leading to the fast photocatalytic degradation of 4-CP by impeding the accumulation of BQ and HQ.44 Therefore, the higher performance for A-TiO2 is not only ascribed to the fast charges transfer, more importantly, it can activate oxygen molecular effectively. 4-CP degradation of A-TiO2 under N2 atmosphere have been performed to suppress the formation of superoxide anions (•O2−) (Figure S9). An inhibited performance for A-TiO2 under N2 atmosphere suggests •O2− are indeed one of the active oxidative species in the degradation process. When BQ selected as a parent compound (Figure S10), the concentration of BQ gradually decreased under visible-light irradiation, and the concentration of HQ reached the maximum value after irradiation for 1 h for A-TiO2. While as for N-TiO2, the concentration of BQ decreased apparently within the first 1 h, and almost no change within the next 5 h. HQ reached the maximum value after irradiation for 2 h, then gradually decreased. The different photocatalytic BQ degradation process is due to the electron shuttle mechanism existence between HQ and BQ. To make clear why the photoactivity does not match their visible light absorption, transfer efficiency of photogenerated charges is investigated by determining the photocurrent by visible light irradiation (Figure S11a). A-TiO2 exhibits higher photocurrent than N-TiO2 at steady state, implying the efficient photocharge migration which could diminish the recombination of photo carries. This could be further confirmed by examining the photoinduced current profile (Figure S11a). It could be seen that the instant photocurrent of N-TiO2 was much higher than that of A-TiO2, which is consistent with its stronger absorption for visible lights to generate photocharges.51 The photocurrent decreases rapidly due to the easy photoelectron−hole recombination. And thus, the distinct spike-like current signals generate. The charge transfer property is also characterized by SPV spectroscopy (Figure S11b). Both A-TiO2 and N-TiO2 exhibit obvious response under 400−600 nm visible light irradiation, which is due to the band to sub-band charge transition of TiO2.52 Moreover, the SPV response of A-TiO2 is stronger than that of N-TiO2, which can also confirm the high separation efficiency of photogenerated charge carriers. PL spectra are used to understand the correlation of the recombination behavior to various defect stages.53 The emission spectra of A-TiO2 and N-TiO2 were obtained by 345 nm laser at 298 K (as shown in Figure S12), which exhibit similar emissions peaks. The strong peak located at 398 nm (3.18 eV) is caused by the emission of bandgap transition, corresponding to the bandgap energy of anatase.54 Meanwhile, there are three main emission peaks located in the range from 450 to 500 nm, which are mainly resulted from defects. In addition, the peaks at 450 nm (2.75 eV) and 468 nm (2.65 eV)

Scheme 1. Proposed Mechanism of Photocatalytic Process under Visible Light Irradiation for A-TiO2 and N-TiO2

radicals in the para position and a following dichlorination, HQ is formed.43,48,49 It is reported that a fast electron shuttle mechanism existence between HQ and BQ.43,50 Namely, HQ can be oxidized to BQ by oxygen molecule (eq 1), whereas BQ can be rapidly reduced to HQ by electrons (eq 2), leading to the consuming of oxygen molecules and photoinduced electrons in an ineffective manner and consequently inhibits the mineralization process for N-TiO2.50 HQ + 1/2O2 → BQ + H 2O

(1)

BQ + 2e− + 2H+ → HQ + H 2O

(2)

A-TiO2, •O2−

As for originates from the efficient activation of O2 molecule. BQ, a common scavenger for •O2− in photocatalysis, can be decomposed effectively by •O2−, thus 13883

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Figure 6. (a) Normalized X-ray absorption near-edge spectra and (b) Fourier Transform and corresponding fits at the Ti K-edge of samples.

intermediates are of fundamental importance for the practical application of photocatalysis. Since 4-CP and VOCs are fundamental and toxic organic compounds, the results are useful for deepening the understanding of photodegradation mechanism of organic materials and the impact of defects on photocatalytic performance. Besides, it is also instructive for the design and development of highly efficient visible light response photocatalysts.

are ascribed to free band-edge excitons. The last peak located at 485 nm (2.55 eV) is due to bound excitations.53 The NTiO2 displays higher PL peak than A-TiO2, indicating a high recombination rate of photocarriers,55 which, in agreement with the experimental results. Ti K-edge XANES chiefly provides a signal of the bulk phase and is sensitive to the environment of the surrounding atoms.56 The intensity of the pre-edge transitions increases as the environment is distorted.45 As shown in Figure 6 a, the prepeak A1 is the result from a t2g bandlike state. While A2 and A3 prepeaks are due to eg bandlike states.57 The Ti 4s and/or O 2p orbitals are hybridized with Ti 4p characteristics, and is used to transition B. C Prepeaks, corresponding to transitions of core electron to O 2p states hybridized with Ti 4p states and Prepeaks D is the result from higher lying p AOs.57 The post edge region from 4.98 to 5.02 keV involves many poorly resolved 3s-np dipole-allowed transitions.58 The slightly shift of the absorption edge for sample A-TiO2 implies the presence of VO in the bulk. Besides, an increased intensity of A3 peak verifies the existence of TiO6 octahedron distorted structures. Figure 6 b shows the radial structure functions (RSFs) of the samples. It is seen that both samples exhibit three peaks at approximately 1.56, 2.50, 3.46 Å. The first peak (shell) arises from the Ti−O bonding with a distance of 1.56 Å, the second peak (shell) arises from the Ti−Ti bonding with a distance of 2.50 Å and the third peak (shell) arises from a mixture of Ti− Ti and Ti−O bonding with a distance of 3.46 Å, which is consistent with the previous results.56,59 Table S1 shows the best fitting results, which indicates that the surface and bulk of both samples are distorted, and Ti atom coordinated with fewer O atoms in each central. The coordination number of Ti-1Ti and Ti−O for the sample A-TiO2 is lower than that of N-TiO2 (3.3 ± 0.6 vs 3.9 ± 0.6 for Ti-1Ti and 3.1 ± 0.8 vs 4.6 ± 0.5 for Ti−O). Low coordinated Ti will be more favorable sites for reactants in photocatalytic reactions.12 Defective anatase TiO2 nanocrystals is successfully synthesized. XPS and ESR spectra indicate that oxygen vacancies (VO) and trivalent titanium (Ti3+) defects exist simultaneously in the N2-calcinated sample, while mainly VO defects are present in the air-calcinated sample. Investigations on photocurrent response, SPV and PL reveal that A-TiO2 has a higher separation efficiency of photogenerated carriers. Moreover, A-TiO2 with a low coordinated Ti can effectively activate oxygen molecular, leading to the superior photocatalytic activity for deterioration of organics under visible light irradiation. It is observed that various intermediates can be formed and accumulate in the reaction system before their complete mineralization, which may be more toxic than their parent. Therefore, a better insight into the formation of these

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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/acs.est.8b03558.

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TEM images, N2 adsorption−desorption isotherms, thermal analysis profiles of precursor, XPS spectra, UV−vis absorbance spectra, optical reflectance spectra, proposed sample lattice structure, photocatalytic performance, photocurrent spectra, photoluminescence spectra, SPV spectra of A-TiO2 and N-TiO2, and their structural parameters (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wei Li: 0000-0002-4641-620X Wei Chen: 0000-0002-1131-3585 Zhenfeng Bian: 0000-0001-7552-8027 Author Contributions ∥

These two authors made equal contributions to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSF of China (21761142011, 21522703, 21876114, 21603036, 21377088), Shanghai Government (13SG44, 15520711300), Shanghai Rising-Star Program, and Singapore National Research Foundation under the grant of NRF2017NRF-NSFC001-007. W.L. is thankful for the support from the Major State Basic Research Development Program (2016YFA0204000). Research is also supported by The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning and Ministry of Education of China (PCSIRT_IRT_16R49), and the International Joint Laboratory on Resource Chemistry of China (IJLRC). 13884

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DOI: 10.1021/acs.est.8b03558 Environ. Sci. Technol. 2018, 52, 13879−13886