TiO2 hierarchical composites: Synthesis, characterization and application on photocatalysis

Accepted Manuscript Title: Novel BiOCl/TiO2 hierarchical composites: Synthesis, characterization and application on photocatalysis Author: Wei Li Yi Tian Huan Li Chenhui Zhao Baoliang Zhang Hepeng Zhang Wangchang Geng Qiuyu Zhang PII: DOI: Reference:

S0926-860X(16)30061-8 http://dx.doi.org/doi:10.1016/j.apcata.2016.02.006 APCATA 15765

To appear in:

Applied Catalysis A: General

Received date: Revised date: Accepted date:

19-10-2015 29-11-2015 5-2-2016

Please cite this article as: Wei Li, Yi Tian, Huan Li, Chenhui Zhao, Baoliang Zhang, Hepeng Zhang, Wangchang Geng, Qiuyu Zhang, Novel BiOCl/TiO2 hierarchical composites: Synthesis, characterization and application on photocatalysis, Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2016.02.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1

Novel BiOCl/TiO2 hierarchical composites: Synthesis, characterization

and application on photocatalysis

Wei Li,a,b Yi Tian,a,b Huan Li,a,b Chenhui Zhao,a,b Baoliang Zhang,a,b Hepeng Zhang,a,b Wangchang Geng,*a,b Qiuyu Zhang*a,b

a

Department of Applied Chemistry, College of Science, Northwestern Polytechnical University,

Xi’an 710072, China b

Key Laboratory of Space Physics and Chemistry, Ministry of Education, Northwestern

Polytechnical University, P. O. Box 624, Xi’an 710072, China

1

Phone: +86-029-88431675, Fax: +86-029-88431675, E-mail: [email protected] (Qiuyu Zhang), [email protected] (Wangchang Geng). 1

Graphical Abstract

Highlights ● A BiOCl/TiO2 hierarchical composite was synthesized at the presence of PVP. ● The PVP plays an important role for enhancing the photoactivity. ● This hierarchical composite exhibits enhanced photoactivity and photostability. ● This hierarchical composite exhibits rapid migration of the interface charges. ● This BiOCl/TiO2 heterojunction exhibits superior visible-light absorption ability.

2

Abstract: In this paper, a novel BiOCl/TiO2 hierarchical composite was successfully synthesized by a facile one-pot solvothermal synthesis with the aid of polyvinylpyrrolidone (PVP). Then, the as-prepared composite was characterized by field-emitting scanning electron microscope (FESEM), transmission electron microscopy (TEM), N2 adsorption/desorption, X-ray power diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectra (DRS) and Electrochemical impedance spectroscopy (EIS), respectively. Research shows that this novel composite exhibits superior visible-light response, enhanced photoactivity and photostability for the formation of the BiOCl/TiO2 heterojunctions. Especially, the rate constant of the photocatalyst with the molar ratio of anatase TiO2 about 30% for decomposing RhB, can reach about 0.1510 min-1, is far more superior than commercial P25, as-prepared anatase TiO2 nanoparticles and tetragonal BiOCl alone. In addition, the superoxide radicals (•O2-) and photogenerated holes (h+) were demonstrated to be the main active radicals in this photodegradation process. This study further reveals that the enhanced photoactivity of this photocatalyst should be ascribed to the successful formation of BiOCl/TiO2 heterojunctions and the rapid migration of the interface charges.

Keywords: photocatalysis; visible-light response; heterojunction; rhodamine B; hierarchical structure

3

1. Introduction Photocatalysis technology was first found by Fujishima and his co-workers in 1967.[1] In the past few decades, the development of photocatalysis has been explored by many pioneers, and much work has been reported recently in this field.[2-6] Currently, a considerable amount of research has being focused on the exploitation of novel high-efficient photocatalyst with favourable visible-light response and fast interfacial charge migration ability,[7-11] and the central issue in all these studies is to effectively restrain the recombination of photoactivated electron-hole pairs under the completion of wider light absorption. For instance, Li et al.[12] prepared a TiO2/In2O3 composite photocatalyst by using a facile ultrasonic aerosol spray-assisted method. Ansari et al.[13] synthesized a visible-light-active Ag/TiO2@Pani nanocomposite film via a simple biogenic-chemical route. Li et al.[14] fabricated a Al2O3/g-C3N4 heterojunction photocatalysts through ultrasonic dispersion method. No doubt, all above photocatalysts exhibited the enhanced photoactivity under visible-light irradiation, and are very significant and valuable in photocatalysis field. Among numerous novel photocatalysts, bismuth oxyhalide (BiOX, X = Cl, Br, I), with a unique layered tetragonal structure, were found to be an efficient photocatalysts.[15-17] Therefore, this subject has been extensively explored and it is still under investigation as well in methodological aspects as in concrete applications. For instance, Liu et al.[18] reported a novel photoelectrocatalytic approach using an I-BiOCl/BPM sandwich structure for water splitting. Huang et al.[19] fabricated a band-gap-broadening Br- substituted BiOI photocatalyst via a facile chemical precipitation route. Dong et al.[20] synthesized a BiOI hollow microspheres for the photocatalytic removal of NO under visible-light irradiation. Wang et al.[21] synthesized a novel BiOI/Ag3VO4 bifunctional material with high adsorption-photocatalysis using a facile solvothermal method followed by the chemical precipitation. Although a lot of effort is being spent on improving the photoactivities of present photocatalysts, more research is still required before final goal of the synthesis of high-efficient photocatalyst with superior visible-light response can be completed. To date, none of the methods developed is perfect and all are far from ready to be used in commercial systems. In this paper, a novel BiOCl/TiO2 bifunctional photocatalyst was synthesized via a facile solvothermal method along with the use of different surfactants as additives. In this trial, the titanium source was introduced to generate the surface complex (TiO2 nanoclusters). Surface complex, an 4

important active site and light adsorption site for photocatalysis, is believed to impact the surface reactivity of semiconductor. It should be caused by that the introduced surface complex can broaden the optical absorption of photocatalyst from the ultraviolet light to the visible light.[22] Besides, the formation of the heterojunction may also is an important role for enhancing the photoactivity of the semiconductor. To illustrate the enhanced photoactivity of our synthesized BiOCl/TiO2 bifunctional photocatalyst, both the photodegradation tests and radical trapping tests were carried out, and the possible photocatalysis mechanism was also proposed.

2. Experimental Section 2.1. Reagents and Materials. All chemicals were purchased from J﹠K Chemical and were used without further treatment.

2.2. Synthesis of hierarchical BiOCl/TiO2 composite. The BiOCl/TiO2 hierarchical composite was synthesized by the simple one-pot solvothermal method. Briefly, 10 mmol of Bi(NO3)3·5H2O and 10 mmol of TiCl4 were dissolved in 80 mL of ethylene glycol containing 10 mmol of KCl and 0.2 g of PVP with the aid of ultrasound to form a white milky liquid. Then, aforementioned system was transferred into a teflon-lined stainless steel autoclave with a capacity of 50 mL and kept for 8 h at 180℃. After the reaction finished, the resulting precipitates were collected and separated by centrifugation and washed for several times with distilled water and absolute ethanol followed by drying via the vacuum freeze-drying technology. In addition, a series of BiOCl/TiO2 composites with different molar ratio (0%, 30%, 50%, 70%, 90% and 100%) of anatase TiO2 and different PVP addition (0 g, 0.2 g, 0.4 g, 0.6 g, 0.8 g and 1.0 g) were also synthesized under the same synthesis conditions. Beyond that, PVP was respectively replaced by cetyltrimethylammonium bromide (CTAB, 0.8 g) and sodium dodecyl sulfate (SDS, 0.8 g), and the corresponding BiOCl/TiO2 composite was also synthesized under the same synthesis conditions.

2.3. Characterization. X-ray power diffraction (XRD) analysis was performed on a Bruker AXS D8-advance X-ray diffractometer with Cu Kα radiation. The morphologies and sizes of the samples were characterized using the field-emitting scanning electron microscope (FESEM, 5

JEOL-JSM-6700F) and JEOL JEM-2100F field emission transmission electron microscopy (FETEM). X-ray photoelectron spectroscopy (XPS) datas were collected to examine the chemical states of the multi-component photocatalyst with an Axis Ultra instrument (Kratos Analytical, Manchester, U.K.) under ultrahigh vacuum condition (<10-6 Pa) and using a monochromatic Al Kα X-ray source (1486.6 eV). UV-vis diffuse reflectance spectra (DRS) were obtained using a Shimadzu UV-3600 spectrometer by using BaSO4 as a reference at room temperature. N2 adsorption/desorption isotherms were obtained on a TriStar II 20 apparatus. The Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface area based on the adsorption branches. Electrochemical impedance spectroscopy (EIS) was performed on a ZENNIUM electrochemical workstation (Zahner Instruments, Germany). Electrochemical signals were recorded using a CHI660 B electrochemical analyzer (Chen Hua Instruments, Shanghai, China). The concentration of total organic carbon (TOC) was analyzed by a TOC-VCPN Analyzer (TOC-VCPN, Shimadzu Corporation, Japan).

2.4. Photocatalytic Performance Tests. The photocatalytic activities of the as-prepared BiOCl/TiO2 hierarchical composite photocatalysts were evaluated by catalyzing the photodegradation of RhB in aqueous solution at room temperature under visible-light irradiation with one 500 W xenon lamp (CHF-XM500, light intensity = 600 mW/cm2) located at 20 cm away from the reaction solution. To make sure that the photocatalytic reaction was really driven by visible-light, all the UV lights with the wavelength less than 420 nm were removed by a glass filter (JB-420). In a typical reaction, 0.1 g of as-prepared photocatalyst was dispersed into 100 mL of RhB aqueous solution (20 mg/L). Before light irradiating, the suspension was stirred for 30 min in dark to reach adsorption equilibrium of dye molecules on the surface of photocatalyst. Then, 10 mL of reaction solution was extracted to determine the concentration of the dye in aqueous solution by UV-vis spectroscopy. In this study, each of BiOCl and TiO2, both of which was synthesized by the same method, was used as the reference catalyst to catalyze the photodegradation of RhB under the same condition as above operation. At the same time, the commercial P25 photocatalyst was also chosen as the contrast photocatalyst, and its photoactivity under visible-light irradiation was also investigated. After the experiment finished, the catalyst was collected by centrifugation separation and washed several times with distilled 6

water, then the recovered catalyst was undergo the further purification treatment in a dialysis bag for 12 h. At last, the photodegradation experiment catalyzed by the recovered catalyst was carried out repeatedly according to aforementioned operation steps.

2.5. Electrochemical Measurements. The measurements were carried out according to the method in reference.[23] Prior to modification, the GCE was firstly polished with sand paper followed by 1.0, 0.3, and 0.05 mm alumina slurry, respectively, and then sonicated in water to remove any residues. The procedure for the preparation of the modified electrodes was described as follows: 2.0 mg of sample was dispersed in 1 mL of ultrapure water to make a homogeneous suspension, then 6 μL of this suspension was cast onto the pretreated GCE surface and dried in air at room temperature to form the measured sample modified GCE. The sample-GCE was rinsed with water several times prior to use.

2.6. Radical Trapping Experiments. For detecting the active species during photocatalytic reactivity, hydroxyl radicals (•OH), superoxide radical (•O2-), photogenerated electrons (e-) and holes (h+) were investigated by adding 1.0 mM isopropanol (IPA, a quencher of •OH),[24] 1.0 mM 1,4-benzoquinone (BQ, a quencher of •O2-),[25] 1.0 mM tert-Butanol (TBA, a scavenger of e-)[26] and 1.0 mM potassium iodide (KI, a scavenger of h+),[27] respectively. The method was similar to the former photocatalytic activity test. 3. Results and Discussion 3.1. Characterization of Materials The crystallographic phases of as-synthesized samples were detected by XRD. As shown as Fig.1a, the as-synthesized BiOCl can be indexed to a standard tetragonal structure (JCPDS 06-0249) for no other diffraction peaks were detected. The as-synthesized TiO2 can be indexed to a standard anatase phase structure (JCPDS 21-1272). It is obvious that the BiOCl/TiO2 composites with different molar ratio of TiO2 almost exhibit similar XRD patterns. With the increase of the amount of anatase TiO2, some diffraction peaks would broaden to some extend, and the diffraction peaks at 33.6, 38.2 and 64.8º corresponding to the planes of (102), (112) and (005) of tetragonal BiOCl would gradually disappear, and the diffraction peaks at 25.2, 37.8, 47.9, 53.9, 54.9 and 62.6º corresponding to the 7

planes of (101), (004), (200), (105), (211) and (204) of anatase TiO2 would gradually appear, revealing the coexistence of tetragonal BiOCl and anatase TiO2. Fig.1b displays the XRD patterns of the BiOCl/TiO2 composites synthesized under the different PVP addition. It shows that the crystallinity of the BiOCl/TiO2 composite synthesized under the PVP addition of 0.2 g is the highest one for its sharp and clear diffraction peaks. Subsequently, the diffraction peaks would be more and more wide and fuzzy with the increase of the amount of PVP. Especially, the diffraction peaks of (102) plane of tetragonal BiOCl and (200) plane of anatase TiO2 would gradually disappear, indicating that the PVP indeed plays an important role to the crystal growth of the BiOCl/TiO2 composite. To the synthesis of BiOCl/TiO2 composite, the reaction temperature was also an important factor to the crystal growth. Fig.1c displays the XRD patterns of the BiOCl/TiO2 composites synthesized under different temperature, and it shows that the diffraction peaks would be more sharp and narrow with the increase of the synthesis temperature. Furthermore, the characteristic diffraction peaks corresponding to the anatase TiO2 would be more distinct at the higher synthesis temperature. It indicates that the BiOCl/TiO2 composite would exhibit more perfect crystal structure with the increase of the synthesis temperature, and the appropriate synthesis temperature may be conducive to the formation of the heterojunction. Beyond that, the reaction time is also essential to be investigated, and Fig.1d displays the XRD patterns of the BiOCl/TiO2 composites synthesized within different reaction time. Obviously, the characteristic diffraction peaks corresponding to the anatase TiO2 would not be observed when the BiOCl/TiO2 composite was synthesized within short reaction time (2 h and 4 h). Only when the reaction time was increased to 8 h, the obvious characteristic diffraction peaks corresponding to the anatase TiO2 would be observed. It well demonstrates that the reaction time also plays an important role to the formation of BiOCl/TiO2 composites. In the initial stage of the reaction, the BiOCl nanosheets would be generated firstly, then the anatase TiO2 nanoparticles would be generated on the surface of BiOCl nanosheets with the reaction time extending. To investigate the effect of the introduction of anatase TiO2 to the resultant samples, other reaction conditions were kept constant, then the size and morphology were examined by SEM, and the corresponding SEM images were displayed in Fig.2. As shown as Fig.2a, the pure BiOCl photocatalyst prepared in our method exhibits the regular flower-like spherical morphology, and its size distribution is very uniform and is about 1.8 μm. When 30% of anatase TiO2 was introduced into 8

the sample in the synthetic process, the BiOCl/TiO2 composite would exhibit nonuniform flower-like spherical morphology with rough surface for the introduction of anatase TiO2, and its size is about 0.9~1.8 μm (Fig.2b). Subsequently, the flower-like spherical morphology would become more and more misty until it disappears entirely with the increase of the molar ratio of anatase TiO2, and the corresponding particle size would also decrease gradually (as Fig.2c-e). Especially, when the molar ratio of anatase TiO2 reached to 100%, the flower-like morphology could not be observed any more, and its particle size is only 20~30 nm (as Fig.2f). According to the above results, it is easy to understand that the introduction of anatase TiO2 would play an important role to the morphology of the as-synthesized BiOCl/TiO2 photocatalyst, and it should be caused by the different distribution of anatase TiO2 in the samples. Some reports had demonstrated that the addition of PVP in the synthesis process would play an important role to the morphology and photoactivity of the as-prepared photocatalyst,[28,29] so the effect of different addition amounts of PVP was also investigated. As shown as Fig.2g, when the PVP is absent, the irregular flower-like spherical structure can be observed. Remarkably, when the molar ratio of anatase TiO2 is 30%, the BiOCl/TiO2 composite shows the most regular flower-like spherical morphology (Fig.2b). However, the flower-like spherical morphology would also become more and more misty until it disappears entirely with the increase of the addition amount of PVP (Fig.2h-k). Aforementioned conclusion demonstrates that the addition of PVP indeed plays an important role to the morphology of the resultant BiOCl/TiO2 composite, which is mainly caused by the different distribution and adsorption of PVP on the surface of BiOCl/TiO2 composite. It has been demonstrated that the PVP adsorbed on the surface of the crystals can effectively adjust its crystal growth.[30,31] Of course, the existence of PVP in the synthetic system can also control the distribution of BiOCl and TiO2 in BiOCl/TiO2 composite, which may lead to the different photoactivities of the resultant photocatalysts for the formation of the different heterojunction system. At the same time, the BiOCl/TiO2 photocatalyst with the molar ratio of anatase TiO2 about 30% and PVP addition about 0.8 g was also characterized by field emission transmission electron microscopy (FETEM), and Fig.2l displays the corresponding HRTEM image and TEM image (as inset). The inset shows that the BiOCl/TiO2 composite with the molar ratio of anatase TiO2 about 30% and PVP addition about 0.8 g indeed exhibits the representative hierarchical structure. The obvious crystal lattices with the lattice spacing of d = 0.2755 nm and d = 0.234 nm can be observed in Fig.2l, which 9

can be attributed to the (110) plane of the tetragonal BiOCl and (112) plane of the anatase TiO2. It demonstrates that the BiOCl/TiO2 heterojunctions were successfully generated. The reaction time is also an important role to synthesize the BiOCl/TiO2 hierarchical composite photocatalyst, so a series of BiOCl/TiO2 composite photocatalysts were synthesized at different reaction time, and Fig.S3 displays the corresponding SEM images. In the initial stage of the synthetic process, the BiOCl/TiO2 composite nanosheets would be first generated (Fig.S3a), then the self-assembly of the BiOCl/TiO2 composite nanosheets would happen during the ripening process (Fig.S3b-c) to form the ultimate flower-like spherical structure (Fig.2b). Similarly, the reaction temperature would also influence the synthesis of the BiOCl/TiO2 composite photocatalyst, and the corresponding SEM images were displayed in Fig.S4. As shown as Fig.S4a-b, both the BiOCl/TiO2 composite synthesized at 140℃ and 160℃ do not exhibit the flower-like spherical structure. The reason is that the low temperature would lead to the insufficient reaction of the precursors, so it would not fulfill the ideal self-assembly of BiOCl/TiO2 composite nanosheets for the absence of the effective ripening process. Only when the reaction time and temperature respectively reached to 8 h and 180℃, the regular flower-like spherical structure can be obtained. Beyond that, the effect of the different additives to the synthesis of BiOCl/TiO2 composite was also investigated. As displayed in Fig.3a, when CTAB was added into the system in the synthesis process, the obvious flower-like structure and hollow structure can be observed. The former is BiOCl species, and the latter is TiO2 species. It means that the composite of BiOCl species and TiO2 species did not perform well at the presence of CTAB, which should be caused by the weak interactions among CTAB, BiOCl and TiO2 species. As shown as Fig. 3b, when SDS was added into the system in the synthesis process, only the non-spherical flower-like structure can be observed. What the reason is that the SDS can not supply the effective oriented effect to the crystal growth of BiOCl/TiO2 composite, thus it would lead to the undesired distribution of BiOCl species and TiO2 species. Aforementioned results demonstrate that the PVP is a superior additive for synthesizing the BiOCl/TiO2 hierarchical composite photocatalyst with the flower-like spherical structure. XPS was employed to further investigate the chemical states of surface elements of the as-prepared BiOCl/TiO2 hierarchical composite photocatalyst. The survey spectrum (Fig.4a) shows the Bi, Ti, O, Cl and C signals, indicating that the surface of the sample is composed of Bi, Ti, O, Cl and C elements. The presence of carbon mainly comes from the PVP added in the synthetic process. As 10

displayed in Fig.4b, two well resolved peaks located at 159.3 eV and 164.6 eV are observed in the Bi 4f spectrum, which are in good agreement with the characteristic of Bi3+ in the BiOCl.[32] Fig.4c shows two major peaks at 197.8 and 196.2 eV, which can be attributed to Cl 2p1/2 and Cl 2p3/2 of Cl-, respectively. Fig.4d shows the XPS spectrum of Ti 2p, where the chemical binding energies located at 458.7, 459.5, 464.8 and 464.5 eV are attributed to Ti4+ of TiO2,[17] indicating the presence of TiO2 in our sample. Compared with the peak location of pure BiOCl and pure TiO2 (as Fig.4b and Fig.4d), the weak shift can be observed in both XPS spectra. It indicates that the interaction between BiOCl and TiO2 in the BiOCl/TiO2 composite had happened, which should be caused by the formation of the BiOCl-TiO2 heterojunction. Additionally, four peaks at 533.4, 531.7, 530.9 and 530.2 eV can be observed in Fig.4e, which belongs to O 1s of O2- in BiOCl and TiO2, respectively. Furthermore, Four peaks at 289.1, 287.0, 286.1 and 285.2 eV in Fig.4f should be attributed to the characteristic peaks of C1s in PVP adsorbed on the surface of the photocatalyst. Beyond that, Fig.S1 displays the UV-vis spectra of the BiOCl/TiO2 composites synthesized under different condition. It shows that all samples exhibit good absorption ability to visible-light (λ > 420 nm) except the anatase TiO2 nanoparticles, indicating that these photocatalysts can be used under visible-light response. To further investigate the visible-light response of the samples, Fig.5a shows the UV-vis DRS of the samples. Obviously, the BiOCl/TiO2(30%) hierarchical composite photocatalyst exhibits the strongest adsorption ability to visible-light among all samples. Then, Eqn (1) was employed to evaluate the Eg values of the as-synthesized samples:[33] 𝛼ℎ𝜈 = A(ℎ𝜈 − 𝐸𝑔 )𝑛/2

(1)

In above formula, α, ν and A is absorption coefficient, light frequency and proportionality constant, respectively. Where n is 4 for the indirect transition, and it is 1 for direct transition. Based on the equation, the Eg of the samples can be deduced from the tangent lines that are extrapolated to (αhν)1/2 = 0, and it is 3.17, 2.93 and 2.42 eV for TiO2, BiOCl and BiOCl/TiO2(30%) (as in Fig.S5). Apparently, the BiOCl/TiO2(30%) hierarchical composite photocatalyst exhibits the narrowest band gap, revealing that it can be easily excited by visible-light. The narrow band gap should be attributed to the formation of the BiOCl-TiO2 heterojunction. Electrochemical impedance spectroscopy (EIS) measurements were also employed to investigate the charge transfer resistance and the separation efficiency between the photogenerated electrons and 11

holes. Using Fe(CN)63-/4- as the electrochemical probe,[23] the Nyquist plots of different electrodes were obtained (Fig.5b). Obviously, the charge transfer resistance (Rct) at all the BiOCl/TiO2 composites is smaller than that of the anatase TiO2 and the tetragonal BiOCl alone. Generally, the smaller the arc radius on the EIS Nyquist plot, the lower the charge transfer resistance. Therefore, it indicates that the composite between tetragonal BiOCl and anatase TiO2 can effectively improve the charge transfer ability of the materials, and it would further effectively inhibit the recombination of the photoexcited electron-hole pairs. 3.2. Photocatalytic Performance When the addition amount of PVP is 0.8 g, the photocatalytic performances of the BiOCl/TiO2 composites with different molar ratio of anatase TiO2 were evaluated in terms of the photodegradation of RhB by the photocatalyst under visible-light irradiation. The P25, is the most efficient commercially available photocatalyst, was also chosen as the contrast photocatalyst to decompose RhB under visible-light irradiation. Furthermore, the photolysis of RhB under visible-light were also investigated at the absence of any photocatalyst. Fig.6a displays the corresponding saturated adsorption capacities of RhB on the surface of P25 and BiOCl/TiO2 composites with different molar ratio of anatase TiO2 under the dark. Obviously, the BiOCl/TiO2 hierarchical composite with the molar ratio of anatase TiO2 about 30% shows the strongest adsorption ability to RhB molecules, and its saturated adsorption capacity is about 61.23%. Amony them, the pure anatase TiO2 nanoparticles synthesized with the same method shows the lowest adsorption ability to RhB molecules, and their saturated adsorption capacity is about 0.72%. When visible-light was supplied, the photodegradation of RhB would be initiated, and the corresponding fitted Langmuir-Hinshelwood models of the photodegradation kinetic curves were displayed in Fig.6c. Obviously, the photolysis of RhB under visible-light irradiation can be neglected for the unobvious change of the kinetic curve (as Fig.b), and this conclusion had also been demonstrated by our previous research [34]. Each of the pure BiOCl photocatalyst, anatase TiO2 nanoparticles and P25 photocatalyst exhibit very poor photoactivity, which is mainly due to their weak response to visible-light. It is easy to see that all the BiOCl/TiO2 composites exhibit the stronger photodegradation ability to RhB than both of tetragonal BiOCl and anatse TiO2 alone. Particularly, the BiOCl/TiO2 hierarchical composite with the molar ratio of anatase TiO2 about 30% exhibits the highest photoactivity (k = 0.1510 min-1) to the degradation of RhB among these BiOCl/TiO2 12

composites with different molar ratio of anatase TiO2, and the RhB molecules would be decomposed entirely within 30 min (as Fig.6b). Subsequently, a series of BiOCl/TiO2 composites were synthesized under the different addition amounts of PVP, and the effect of PVP to the adsorption ability and photoactivity of the BiOCl/TiO2 composites was also investigated. Fig.6d displays the corresponding saturated adsorption capacities of RhB on the surface of the BiOCl/TiO2 composites synthesized under the different addition amounts of PVP. It is easy to see that the BiOCl/TiO2 composite synthesized under the addition amounts of PVP about 0.8 g exhibits the strongest adsorption ability to RhB among these composites, which indicates that the appropriate addition of PVP in the synthesis process would imrpove the adsorption ability of the BiOCl/TiO2 composite for the improvement of the surface affinity. Then, the visible-light was supplied, and the photodegradation of RhB was initiated. It can be seen from Fig.6f that the BiOCl/TiO2 composite synthesized under the addition amounts of PVP about 0.8 g exhibits more remarkable photoactivity to the degradation of RhB under visible-light irradiation than other BiOCl/TiO2 composites synthesized under the different addition amounts of PVP, and it is about 1.9 times of that (k = 0.0799 min-1) of BiOCl/TiO2 composite synthesized under the absence of PVP. At the same time, the degradation kinetic curves also demonstrate this conclusion (as Fig.6e). It reveals that the appropriate addition amounts of PVP in the synthesis process would significantly improve the photoactivity of the BiOCl/TiO2 composite. However, both the higher and lower addition amounts of PVP would not exhibit the positive effect to the photoactivity of the BiOCl/TiO2 composites. Therefore, the superior photoactivity to the photodegradation of RhB can be attributed to the enhanced adsorption ability of the BiOCl/TiO2 composite and the synergetic effects among BiOCl/TiO2 and PVP. Furthermore, the BiOCl/TiO2 composites were also synthesized under the different temperature and different addition of additives, and the effect of the synthesis temperature and different additives to the adsorption ability and photoactivity of the BiOCl/TiO2 composite was also investigated. Fig.6g displays the corresponding saturated adsorption capacities of RhB on the surface of the BiOCl/TiO2 composites synthesized under the different temperature and different addition of additives. It is obvious that the adsorption ability of the BiOCl/TiO2 composite to RhB molecules would be enhanced with the rise of the synthesis temperature (from ～1.41% to ～61.23%). It should be 13

attributed to that the increased synthesis temperature would lead to more regular assembly, and the SEM images in Fig.S4a and Fig.S4b can also demonstrate this conclusion. In addition, the BiOCl/TiO2 composite synthesized under the addition of SDS exhibited the highest adsorption ability to RhB molecules (～85.68%), and the BiOCl/TiO2 composite synthesized under the addition of CTAB also exhibited the superior adsorption ability to RhB molecules (～60.89%). It indicates that SDS can significantly enhance the adsorption ability of the BiOCl/TiO2 composite, which should be attributed to the increased surface area and affinity of the BiOCl/TiO2 composite to RhB molecules (Fig.S2). Subsequently, the photodegradation of RhB by these BiOCl/TiO2 composites were investigated under visible-light response, and the corresponding fitted Langmuir-Hinshelwood models of the degradation kinetic curves were displayed in Fig.6i. It is obvious that the photoactivity of the BiOCl/TiO2 composites synthesized under different synthesis temperature from high to low is 180℃>160℃>140℃, and the rate constant is about 0.1510, 0.1472, and 0.1256 min-1, respectively. Combining with the SEM images in Fig.2b, FigS4a,b, it can be concluded that the higher synthesis temperature would lead to the perfect ripening process, and more regular assembly and larger surface areas of the BiOCl/TiO2 composite would be obtained, so the BiOCl/TiO2 composite synthesized under the higher temperature exhibited better photoactivity. It can be seen from Fig.6i that the BiOCl/TiO2 composites synthesized under the addition of CTAB (k = 0.1205 min-1) and SDS (k = 0.1384 min-1) did not exhibit the better photoactivity than that synthesized under the addition of PVP (k = 0.1510 min-1) although both of them exhibited more superior adsorption ability to RhB molecules. It well demonstrates the presence of the synergetic effects among BiOCl/TiO2 and PVP. The absorption spectra changes of RhB (20 mg/L, 100 mL) in the presence of the BiOCl/TiO2 hierarchical photocatalyst with molar ratio of anatase TiO2 about 30% under the visible-light irradiation also demonstrates the high photocatalytic efficiency of the catalyst (in Fig.7a). To demonstrate the successful photodegradation of the RhB by the BiOCl/TiO2 hierarchical photocatalyst with molar ratio of anatase TiO2 about 30% under the visible-light irradiation but not only decolorization, the concentration of total organic carbon (TOC) for RhB was evaluated, and the results are illusteated in Fig.S6. The TOC value of RhB is decreased to about 29.58% after 40 min of visible-light irradiation, which can provide the evidence of the decomposition of RhB into CO2. In addition, the stability of the photoactivity of the BiOCl/TiO2 hierarchical composite photocatalyst 14

was also investigated, and Fig.7b displays its recycled experiments for decomposing RhB molecules in water system. Encouragingly, the photoactivity of this novel hierarchical photocatalyst did not show significant decrease after being used several times, revealing that this novel hierarchical photocatalyst exhibits very robust photocatalytic stability. 3.3. Photocatalytic Mechanism In general, photocatalytic decompositions of dyes are oxidative processes in which several reactive intermediate species may be involved such as electrons (e-), holes (h+), superoxide radicals (•O2-) and hydroxyl radicals (•OH).[12,35,36] In this study, several different scavengers isopropanol (IPA), tert-Butanol (TBA), KI and 1,4-benzoquinone (BQ) was selected to quench the •OH, e-, h+ and •O2radicals generated in the RhB degradation process, respectively. Fig.8 displays the corresponding degradation efficiency. Obviously, when IPA or TBA was added into the degradation system, the tiny decrease (～2.45% and ～5.28%) of the degradation efficiency could be observed, revealing that the •OH and e- are not the main active radicals in the photodegradation process of RhB. When KI existed in the degradation system, the decrease (～21.97%) of the RhB degradation efficiency could be observed, indicating that the h+ participates in the photodegradation of RhB under the catalysis of BiOCl/TiO2 hierarchical composite photocatalyst. Specifically, when BQ was added into the degradation system, the obvious influence could be observed, and the RhB molecules almost could not be decomposed by BiOCl/TiO2 hierarchical composite photocatalyst under visible-light irradiation, indicating that the •O2- radicals are the main active radicals for decomposing the RhB molecules by BiOCl/TiO2 hierarchical composite photocatalyst under visible-light irradiation. In this photodegradation system, the possible photocatalytic mechanism is proposed as: 𝐵𝑖𝑂𝐶𝑙/𝑇𝑖𝑂2 + ℎ𝜈 → 𝐵𝑖𝑂𝐶𝑙/𝑇𝑖𝑂2 (ℎ+ + 𝑒 − )

(1)

O2 + 𝑒 − →• O2−

(2)

𝑅ℎ𝐵 + ℎ+ → 𝑅ℎ𝐵+

(3)

𝑅ℎ𝐵+ +• 𝑂2− → ⋯ → 𝑑𝑒𝑔𝑟𝑎𝑑𝑎𝑡𝑖𝑜𝑛 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠

(4)

The proposed photocatalytic mechanism of the BiOCl/TiO2 hierarchical composite photocatalyst under visible-light irradiation was displayed in Fig.9. The photogenerated electrons (e-) and holes (h+) were produced when the BiOCl/TiO2 heterojunctions was illuminated by visible-light (Eqs.(1)). For the formation of the heterojunctions, the photogenerated electrons (e-) would quickly transfer to 15

the conduction band (CB) of BiOCl from the conduction band (CB) of TiO2 through the interfaces of the heterojunctions, then O2 molecules would react with e- to generate •O2- active radicals during this process (Eqs.(2)). Furthermore, the corresponding photogenerated holes (h+) would quickly transfer to the valence band (VB) of TiO2 from the valence band (VB) of BiOCl through the interfaces of the heterojunctions, then RhB would react with h+ to generate RhB+ active reactants during this process (Eqs.(3)). Finally, the generated RhB+ active reactants would react with •O2- active radicals to generate the ultimate degradation products (Eqs.(4)).

4. Conclusion In this paper, a novel BiOCl/TiO2 hierarchical composite photocatalyst was successfully synthesized by a facile one-pot solvothermal synthesis with the aid of PVP. Research shows that this novel composite exhibits superior visible-light response, enhanced photoactivity and photostability for the formation of the BiOCl/TiO2 heterojunctions. Especially, the rate constant of the photocatalyst with the molar ratio of anatase TiO2 about 30% for decomposing RhB, can reach about 0.1510 min-1, is far more superior than commercial P25, as-prepared anatase TiO2 nanoparticles and tetragonal BiOCl alone. In addition, the superoxide radicals (•O2-) and photogenerated holes (h+) were demonstrated to be the main active radicals in this photodegradation process. This study further reveals that the enhanced photoactivity of this photocatalyst should be ascribed to the successful formation of BiOCl/TiO2 heterojunctions and the rapid migration of the interface charges.

Acknowledgment The authors are grateful for the financial support provided by the National Natural Science Foundation of China (No. 51173146 and NO. 21201140), Basic Research Fund of Northwestern Polytechnical University (3102014JCQ01094, 3102014ZD), NPU Foundation for Graduate Innovation.

16

References [1] A. Fujishima, K. Honda. Nature 238 (1972) 37–38. [2] S. Chen, S. Shen, G. Liu, Y. Qi, F. Zhang, and C. Li. Angew. Chem. Int. Ed. 54 (2015) 3047–3051. [3] K. Rekab, C. Lepeytre, M. Dunand, F. Dappozze, J.-M. Herrmann, C. Guillard. Appl. Catal. A-Gen. 488 (2014) 103–110. [4] X. Zhang, G. Ji, Y. Liu, X. Zhou, Y. Zhu, D. Shi, P. Zhang, X. Cao and B. Wang. Phys. Chem. Chem. Phys. 17 (2015) 8078–8086. [5] G. Liu, T. Wang, S. Ouyang, L. Liu, H. Jiang, Q. Yu, T. Kako and J. Ye. J. Mater. Chem. A 3 (2015) 8123–8132. [6] W. Peng, X. Wang and X. Li. Nanoscale 6 (2014) 8311–8317. [7] M. Myilsamy, V. Murugesan, M. Mahalakshmi. Appl. Catal. A-Gen. 492 (2015) 212–222. [8] R. Asahi, T. Morikawa, H. Irie, and T. Ohwaki. Chem. Rev. 114 (2014) 9824−9852. [9] H. Du, X. Xie, Q. Zhu, L. Lin, Y.-F. Jiang, Z.-K. Yang, X. Zhou and A.-W. Xu. Nanoscale 7 (2015) 5752–5759. [10] Z. Shen, S. Sun, W. Wang, J. Liu, Z. Liu and J. C. Yu. J. Mater. Chem. A 3 (2015) 3285–3288. [11] T. Kato, Y. Hakari, S. Ikeda, Q. Jia, A. Iwase, and A. Kudo. J. Phys. Chem. Lett. 6 (2015) 1042−1047. [12] C. Li, T. Ming, J. Wang, J. Wang, J. C. Yu, S. Yu. J. Catal. 310 (2014) 84–90. [13] M. O. Ansari, M. M. Khan, S. A. Ansari, K. Raju, J. Lee, and M. H. Cho. ACS Appl. Mater. Interfaces 6 (2014) 8124−8133. [14] F. Li, Y. Zhao, Q. Wang, X. Wang, Y. Hao, R. Liu, D. Zhao. J. Hazard. Mater. 283 (2015) 371–381. [15] Z.-F. Huang, J. Song, L. Pan, X. Jia, Z. Li, J.-J. Zou, X. Zhang and L. Wang. Nanoscale 6 (2014) 8865–8872. [16] S. Shenawi-Khalil, V. Uvarov, E. Menes, I. Popov, Y. Sasson. Appl. Catal. A-Gen. 413 (2012) 1–9. [17] X.-X. Wei, C.-M. Chen, S.-Q. Guo, F. Guo, X.-M. Li, X.-X. Wang, H.-T. Cui, L.-F. Zhao and W. Li. J. Mater. Chem. A 2 (2014) 4667–4675. [18] X. Liu, H. Yang, H. Dai, X. Mao and Z. Liang. Green Chem. 17 (2015) 199–203. [19] H. Huang, X. Li, X. Han, N. Tian, Y. Zhang and T. Zhang. Phys. Chem. Chem. Phys. 17 (2015) 3673–3679. [20] G. Dong, W. Ho, L. Zhang. Appl. Catal. B-Environ. 168 (2015) 490–496. [21] S. Wang, Y. Guan, L. Wang, W. Zhao, H. He, J. Xiao, S. Yang, C. Sun. Appl. Catal. B-Environ. 168 (2015) 448–457. [22] F. Dong, Q. Li, Y. Sun, and W.-K. Ho. ACS Catal. 4 (2014) 4341−4350. [23] X. Yan, X. Wang, W. Gu, M.M. Wu, Y. Yan, B. Hu, G. Che, D. Han, J. Yang, W. Fan, W. Shi. Appl. Catal. B-Environ. 164 (2015) 297–304. 17

[24] Y.-R. Jiang, H.-P. Lin, W.-H. Chung, Y.-M. Dai, W.-Y. Lin, C.-C. Chen. J. Hazard. Mater. 283 (2015) 787–805. [25] M.-H. Hsu, C.-J. Chang. J. Hazard. Mater. 278 (2014) 444–453. [26] J. Di, J. Xia, S. Yin, H. Xu, L. Xu, Y. Xu, M. He and H. Li. J. Mater. Chem. A 2 (2014) 5340–5351. [27] L. Zhang, W. Wang, S. Sun, Y. Sun, E. Gao, Z. Zhang. Appl. Catal B-Environ. 148 (2014) 164–169. [28] X. Shi, X. Chen, X. Chen, S. Zhou, S. Lou, Y. Wang, L. Yuan. Chem. Eng. J. 222 (2013) 120–127. [29] W. Zhang, T. Zhou, J. Zheng, J. Hong, Y. Pan, and R. Xu. ChemSusChem 8 (2015) 1464–1471. [30] K. Zhang, J. Liang, S. Wang, J. Liu, K. Ren, X. Zheng, H. Luo, Y. Peng, X. Zou, X. Bo, J. Li, and X. Yu. Cryst. Growth Des. 12 (2012) 793−803. [31] X. Zhang, X.-B. Wang, L.-W. Wang, W.-K. Wang, L.-L. Long, W.-W. Li, and H.-Q. Yu. ACS Appl. Mater. Interfaces 6 (2014) 7766−7772. [32] J. Hu, W. Fan, W. Ye, C. Huang, X. Qiu. Appl. Catal. B-Environ. 158 (2014) 182–189. [33] F. Li, Q. Wang, J. Ran, Y. Hao, X. Wang, D. Zhao and S. Z. Qiao. Nanoscale 7 (2015) 1116–1126. [34] W. Li, X. Jia, P. Li, B. Zhang, H. Zhang, W. Geng, and Q. Zhang. ACS Sustainable Chem. Eng. 3 (2015) 1101−1110. [35] X. Wang, W. Yang, F. Li, Y. Xue, R. Liu, and Y. Hao. Ind. Eng. Chem. Res. 52 (2013) 17140−17150. [36] M. Zhou, D. Han, X. Liu, C. Ma, H. Wang, Y. Tang, P. Huo, W. Shi, Y. Yan, J. Yang. Appl. Catal. B-Environ. 172 (2013) 174–184.

18

Figure1

a

b PVP1.0 PVP0.8

90%

Intensity (a.u.)

Intensity (a.u.)

100%

70% 50%

30% 0%

PVP0.6

PVP0.4

PVP0.2 PVP0

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2θ (degree)

2θ (degree)

d

c

160

8h

℃

Intensity (a.u.)

Intensity (a.u.)

180

℃

140

6h

4h

2h ℃

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

2θ (degree)

2θ (degree)

Fig.1 XRD patterns of BiOCl/TiO2 photocatalyst prepared under different reaction conditions: different molar ratio of TiO2 (The PVP addition is 0.8 g, and the reaction temperature and time is 180℃ and 8 h, respectively.) (a), different addition amounts of PVP (The molar ratio of anatase TiO2 is 30%, and the reaction temperature and time is 180 ℃ and 8 h, respectively.) (b), different temperature (The reaction time is 8 h, and the molar ratio of anatase TiO2 and the addition of PVP is 30% and 0.8 g, respectively. ) (c), different reaction time (The reaction temperature is 180℃, and the molar ratio of anatase TiO2 and the addition of PVP is 30% and 0.8 g, respectively. ) (d).

19

Figure2

Fig.2 SEM images of BiOCl/TiO2 hierarchical photocatalyst with different molar ratios of anatase TiO2: 0% (a), 30% (b), 50% (c), 70% (d), 90% (e), and 100% (f). The addition of PVP is 0.8 g, and the reaction temperature and time is 180℃ and 8 h, respectively. SEM images of BiOCl/TiO2 photocatalyst synthesized under different addition of PVP: 0 g (g), 0.4 g (h), 0.6 g (i), 0.8 g (j) and 1.0 g (k) (The molar ratio of anatase TiO2 is 30%, and the reaction temperature and time is 180℃ and 8 h, respectively.). (l) HRTEM image of BiOCl/TiO2 hierarchical photocatalyst of panel b, and the inset is the corresponding TEM image.

20

Figure3

Fig.3 SEM images of BiOCl/TiO2 hierarchical photocatalyst prepared under the addition of CTAB (a) and SDS (b), respectively. The molar ratio of anatase TiO2 is 30%, and the reaction temperature and time is 180℃ and 8 h, respectively.

21

Figure4 O1s

b

Bi4f

BiOCl BiOCl/TiO2 164.6

800

600

400

164.5

Intensity (cps)

Intensity (cps)

Cl2p

1000

200

0

168

166

Binding Energy (eV)

d

Cl2p3/2

159.2

Bi4f5/2

Ti2p

1200

c

159.3 Bi4f7/2

C1s

164

Intensity (cps)

a

162

160

158

Cl2p1/2

156

200

Binding Energy (eV)

Ti2p

458.1

198

196

194

192

Binding Energy (eV)

O1s

e

C1s

f

285.2

530.2

458.7 TiO2

464.8 466.8

Intehsity (cps)

463.9

Intensity (cps)

Intensity (cps)

531.7 533.4

530.9

286.1 287.0 289.1

459.5 BiOCl/TiO2

470

468

466

464

462

460

458

Binding Energy (eV)

456

454

452

542

540

538

536

534

532

530

Binding Energy (eV)

528

526

295

290

285

280

Binding Energy (eV)

Fig.4 XPS spectra of BiOCl/TiO2 hierarchical composite photocatalyst with the molar ratio of anatase TiO2 about 30% (The addition of PVP is 0.8 g, and the reaction temperature and time is 180℃ and 8 h, respectively): (a) survey XPS spectrum, (b) Bi 4f, (c) Cl 2p, (d) Ti 2p, (e) O 1s and (f) C 1s core-level spectra.

22

Figure5

a

2500

TiO2 BiOCl/TiO2(30%) BiOCl/TiO2(50%) BiOCl/TiO2(70%) BiOCl/TiO2(90%) BiOCl

b 2000

Abs

-Z″(ohm)

1500

1000

TiO2 BiOCl/TiO2(30%) BiOCl/TiO2(50%) BiOCl/TiO2(70%) BiOCl/TiO2(90%) BiOCl

500

0

200

300

400

500

600

700

800

0

1000

2000

3000

4000

5000

6000

7000

Z′/ohm

Wavelength (nm)

Fig.5 UV-vis diffuse reflection spectra (a) and EIS (b) of the samples. The inset of panel b is the corresponding equivalent circuit.

23

Figure 6 70

a

61.23

b

100

TiO2 0% TiO2 30% TiO2 50% TiO2 70% TiO2 90% TiO2 100% P25

5

4

60

3

blank TiO2 0% TiO2 30% TiO2 50% TiO2 70% TiO2 90% TiO2 100% P25

40

30 20

20

12.24

11.47

0

10

5.03

3.19

0.72

0

0

0%

30%

50%

90%

70%

100%

10

2

0 30

40

50

0

5

e

4

80

51.09

-ln(ct/c0)

c/c0

60

40 33.69

40

28.81

20

PVP 0 PVP 0.2 PVP 0.4 PVP 0.6 PVP 0.8 PVP 1.0

20

10

20

25

30

35

40

f

PVP 0 PVP 0.2 PVP 0.4 PVP 0.6 PVP 0.8 PVP 1.0

5

50

30.41

15

Time (min)

100

56.42

30

10

Time (min)

61.23 60

c

1

20

P25

d

70

-ln(ct/c0)

40

1.31

Adsorption Capacity (%)

80

50

c/c0

Adsorption Capacity (%)

60

3

2

1

0

0 0

0 PVP 0

PVP 0.4

PVP 0.6

PVP 0.8

10

15

20

25

30

40

0

10

15

20

25

30

35

40

Time (min)

PVP 140 PVP 160 PVP 180 CTAB 180 SDS 180 ℃

100

PVP 140 PVP 160 PVP 180 CTAB 180 SDS 180

5

℃

℃

℃

4

℃

80

i

℃

℃

℃

℃

℃

61.23

5

Time (min)

PVP 1.0

85.68

g

35

60.89

-ln(ct/c0)

60 60

c/c0

Saturated Adsorption (%)

80

PVP 0.2

5

40

40

3

2

30.16 1 20

20

h

0

0

1.41

0

0

PVP140

℃

PVP160

℃

PVP180

℃

CTAB180

℃

SDS180

℃

5

10

15

20

25

Time (min)

30

35

40

0

5

10

15

20

25

30

35

40

Time (min)

Fig.6 The saturated adsorptions, photodegradation kinetic curves and corresponding fitted Langmuir-Hinshelwood models of the photodegradation of RhB on the surface of the BiOCl/TiO2 composites with different molar ratio of anatase TiO2 (0~100%) and P25 (a-c), under different addition amount of PVP (0~1.0 g) (The molar ratio of anatase TiO2 is 30%) (d-f) and under different addition of additives (CTAB, PVP and SDS) and different synthesis temperatrure (140℃, 160℃ and 180℃) (The molar ratio of anatase TiO2 is 30%) (g-i).

24

Figure 7 2.5

a

2.0

c/c0 (%)

black 30 min light 10 min

1.0

0.5

Third

Fourth

80

120

Fifth

Sixth

60

40

20

light 80 min 0.0 400

Second

80

1.5

Abs

First

100

original

b

0

450

500

550

600

650

700

Wavelength (nm)

0

40

160

200

Time (min)

Fig.7 Absorption spectra changes of RhB (20 mg/L, 100 mL) in the presence of the BiOCl/TiO2 hierarchical photocatalyst with molar ratio of anatase TiO2 about 30% under the visible-light irradiation (a), and its reusability investigation (b).

25

Figure 8 Degradation Efficiency (%)

100

98.78

96.33

93.5

76.81

80

60

40

20

4.69 0 No scavenger

IPA

t-Butanol

KI

BQ

Fig.8 Effects of different scavengers on degradation of RhB in the presence of BiOCl/TiO2 hierarchical composite photocatalyst with molar ratio of anatase TiO2 about 30% under visible-light irradiation.

26

Figure 9

Fig.9 Schematic illustration of the proposed photocatalystic mechanism of the BiOCl/TiO2 hierarchical composite photocatalyst with molar ratio of anatase TiO2 about 30% under visible-light irradiation.

27