Nanoscaled Bi2O4 confined in firework-shaped TiO2 microspheres with enhanced visible light photocatalytic performance

Accepted Manuscript Title: Nanoscaled Bi2 O4 confined in firework-shaped TiO2 microspheres with enhanced visible light photocatalytic performance Authors: Yaya Ma, Cuiqing Zhang, Chengyu Li, Feng Qin, Lin Wei, Changyuan Hu, Quanhong Hu, Shuwang Duo PII: DOI: Article Number:

S0927-7757(19)30745-9 https://doi.org/10.1016/j.colsurfa.2019.123757 123757

Reference:

COLSUA 123757

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

5 June 2019 21 July 2019 5 August 2019

Please cite this article as: Ma Y, Zhang C, Li C, Qin F, Wei L, Hu C, Hu Q, Duo S, Nanoscaled Bi2 O4 confined in firework-shaped TiO2 microspheres with enhanced visible light photocatalytic performance, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), https://doi.org/10.1016/j.colsurfa.2019.123757 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.

Nanoscaled Bi2O4 confined in firework-shaped TiO2 microspheres with enhanced visible light photocatalytic performance

Yaya Ma a, b, Cuiqing Zhang a, b, Chengyu Lia, b, Feng Qina, b, Lin weia, b, Changyuan Hua, b, *,

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Quanhong Hua, b, Shuwang Duoa, b

Key Laboratory of Surface Engineering, Jiangxi Science and Technology Normal

University, Nanchang 330013, P. R. China

of Materials and Mechanical & Electrical Engineering, Jiangxi Science and Technology

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Normal University, Nanchang 330013, P. R. China

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*Corresponding author. Tel/Fax: + 86-791-83831266. E-mail: [email protected] (C. Hu)

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Graphical abstract

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A space-confined strategy is developed to restrict the size of Bi2O4 down to nanoscale level and as-

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constructed Bi2O4/TiO2 heterojunctions display superior photocatalytic performance than pure Bi2O4.

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Abstract

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Bi2O4 is a newly found semiconductor photocatalyst with visible-light response property; however, due to its large particle size, the photocatalytic efficiency is greatly limited. Reducing the

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particle size down to nanoscale is an efficient way to enhance the photocatalytic ability, but still challenging. Herein, the firework-shaped hierarchical TiO2 microsphere was used as the framework to spatially confine the growth of Bi2O4. SEM characterization results reveal nanosized Bi2O4 particles are successfully inserted into the TiO2 microsphere, forming enormous Bi2O4/TiO2 heterojunctions. Benefitted from the nanoscaled Bi2O4 and the formation of Bi2O4/TiO2 type II

heterojunction, both the separation efficiency of charge carriers and the quantum yield are improved. As a result, Bi2O4/TiO2 heterojunctions show much higher photocatalytic activity than that of pristine Bi2O4 in the degradation of methyl orange and tetracycline under visible light. Radical capture experimental results imply that hole is the dominant reactive species for the degradation of methyl orange. The present work paves the way for designing nanosized photocatalyst via the space-confined effect.

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KEYWORDS: nanosized Bi2O4, spatially confinement, photocatalytic, firework-shaped TiO2 1. Introduction

Photocatalysis, an advanced technology that using sustainable solar light as resource

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decomposes various contaminants is considered as the promising route toward environmental

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remediation [1-5] Photocatalyst, that plays significant role in the photocatalysis process, should

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feature the merits of wide spectral response range and high charge carriers’ separation efficiency [6]. Bi2O4 presents a suitable bandgap (~2.0 eV), which appears to be a good candidate as visible-

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light-responsive photocatalyst [7-9]. However, like many other photocatalysts, Bi2O4 alone suffers from the rapid recombination of photoinduced electron-hole pairs. Up to date, enormous efforts

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were dedicated to construct Bi2O4-based heterostructured photocatalyst (such as g-C3N4/Bi2O4 [10, 11], Bi2O2CO3/Bi2O4 [12], Fe3O4/Bi2O4 [13] and TiO2/Bi2O4 [14, 15]) for extending spectral

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response range and improving charge carriers’ separation efficiency. As a result, the superior photocatalytic performance was observed for above heterostructures in comparison with pure Bi2O4. Despite these progress, submicron-rod shaped Bi2O4 (500 nm in diameter and 2-3 μm in length) in above mentioned work is the main drawback that hinders the further increase of photocatalytic efficiency. On one hand, the large particle size could prolong the transfer pathways

of photogenerated charge carriers and increase the recombination possibility. On the other hand, the submicron-rod shape of Bi2O4 possesses relatively small surface areas and less exposed active sites, which impairs the photocatalysis activity. Therefore, how to regulate its microstructure and decrease its size is urgent for improving the photocatalysis efficiency of Bi2O4. Nanoscaling has been proven as a promising strategy to exploit highly efficient photocatalysts, because of increased surface area, shortened paths for electron and hole transport, and boosted

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quantum yield [16]. Space-confinement strategy is an effective way to prepare nanosized active materials. For example, atomically thin platy Fe2O3 nanoparticles were synthesized via the confinement of the interlayer nanospace in 2D layered silicate, benefiting from which the

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enhanced charge separation efficiency were realized and thus the photocatalytic activity of

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oxidative decomposition of formic acid was improved. [17]. By virtue of the 3-dimentonal cubic

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mesoporous structure, KIT-6 could serve as the framework to improve the dispersion and limit the particle size of Au by solid-state reduction strategy [18]. Additionally, V. Polshettiwar et al

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reported the utilization of fibrous nanosilica (KCC-1) as template to confine the growth of TiO2 during atomic layer deposition (ALD) process [19]. Through the confinement effect of the fibrous

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nanosilica, small TiO2 nanoparticles (12 nm) with high dispersion were created, which can overcome the fast recombination of charge carriers. Inspired by the previous works, hierarchical

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firework-shaped TiO2 microspheres with enriched pore structure, similar to the KCC-1 substrate, seem to be the ideal scaffold to confine the size of Bi2O4 [20, 21]. Not only as the skeleton that limits the growth of Bi2O4, hierarchical firework-shaped TiO2 microsphere itself but also serves as a component to form type II heterojunction with Bi2O4, which can promote charge carriers separation. It is anticipated that the physical dimension of Bi2O4 would decrease to nanocale level

and enormous Bi2O4/TiO2 heterojunctions would be created simultaneously, through the spatialconfinement effect of hierarchical firework-shaped TiO2 microsphere. Benefitting from the shortened transfer pathways of charge carriers, improved quantum yield and increased number of exposed active sites, photocatalytic performance of Bi2O4 will be promoted significantly. As far as we know, there are no reports of confining Bi2O4 within hierarchical firework-shaped TiO2. Herein, three-dimensional TiO2-Bi2O4 heterojunctions with different weight percentages of

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Bi2O4 were fabricated by a two-step hydrothermal method. Firstly, firework-shaped hierarchical TiO2 microsphere is prepared by a solvothermal method and then utilized to space-confined growth of Bi2O4 via a hydrothermal method. Due to the enlarged specific surface area and

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improved charge carriers separation efficiency derived from the nanosized Bi2O4 and Bi2O4/TiO2

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type II heterojunction, the visible light photocatalytic activity toward methyl orange (MO) and

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tetracycline (TC) are obviously enhanced. The reaction constant of the optimal Bi2O4/TiO2 heterojunction toward the MO and TC photocatalytic degradation under visible light are 0.0162

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min-1 and 0.00553 min-1, about 2.5 and 1.53 times higher as that of pristine Bi2O4, respectively. Radical capture experiment reveals that the main reactive species for the degradation of MO is

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hole. Our work may open up a new pathway to design and construct Bi2O4–based photocatalyst with high catalytic performance.

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2. Experimental section

2.1 Materials and chemicals Titanium butoxide (reagent grade, 97%), hydrochloric acid (ACS reagent, 99.7%), oleic acid (reagent grade, 97%), acetone, ethyl alcohol, Sodium bismuthate (NaBiO3·2H2O), isopropyl alcohol (IPA), sodium sulfate (Na2SO4), sodium oxalate (NaC2O4), potassium bromate (KBrO3),

triethanolamine (TEA), benzoquinone (BQ), methylene Orange (MO) and tetracycline (TC) were purchased from Sigma Aldrich. Ultrapure water (18.2 MΩ cm-1) was used for all the experiments. 2.2.1 Synthesis of TiO2 microflowers The three-dimensional firework-shaped TiO2 microspheres were fabricated through hydrothermal method with tetrabutyl titanate (TBOT), hydrochloric acid and oleic acid as the starting reactants. In general, TBOT (4mL) and hydrochloric acid (2mL) were mixed and stirred

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for 10 minutes in a beaker to form a beige transparent solvent. Then oleic acid (20 mL) was added into the solvent drop by drop, followed by magnetically stirring for 30 min. The resulting uniform

mixed solvent was transferred into a 50mL Teflon-lined stainless steel autoclave and kept at 180 ℃

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for 4 h, and then cooling to room temperature naturally. Finally, white precipitate was obtained

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after acetone/ethanol washing several times and air-dried at 60 ℃ overnight [22].

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2.2.2 Preparation of Bi2O4/TiO2 –x composites

A certain proportion of sodium bismuthate and TiO2 microspheres were dispersed in deionized

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water (60 mL) and stirred at room temperature for 10 min to form a suspension. The resulting homogeneous suspension was loaded into a Teflon-lined stainless steel autoclave with a capacity

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of 100 mL and maintained at 160 ℃ for 12 h. After cooling, the resultant product was washed three times with deionized water and ethanol, respectively, and then dried overnight in an oven at

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60 ℃.

2.3 Characterization The crystalline phases of as-prepared samples were identified by X-ray diffraction (Japan, XRD6100) using Cu Kα radiation scanning from 10 to 80°. The morphology and microstructure of samples were investigated by field emission scanning electron microscopy (FESEM, Zeiss,

Sigma). UV-visible absorbance spectra were obtained for the dry-pressed disk samples with a UVvisible spectrophotometer (UV-2550, Shimadzu, Japan). BaSO4 was used as a reflectance standard in a UV-visible diffuse reflectance experiment. The X-ray photoelectron spectroscopy (XPS) measurements were performed on a KRATOS Analytical AXISHSi spectrometer with a monochromatized Al Ka X-ray source (1486.6 eV photons). N2 adsorption-desorption isotherms was carried out by using Quantachrome ASIQM000-200-6 automated gas sorption analyzer,

(EPR) of the B/T-4 sample before and after visible light irradiation. 2.4 Photocatalytic activity measurement

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Electron paramagnetic resonance spectrometer was used to detect the electronic spin resonance

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The photocatalytic activities of as-prepared photocatalysts were determined by the

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photodegradation of methyl orange (MO) and tetracycline (TC) under the visible light irradiation.

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In details, 24 mg as-prepared catalysts powders were dispersed in a 58 ml MO (30 mg/L) or TC (25 mg/L) aqueous solutions. Before light irradiating, the suspensions were stirred in the dark to

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achieve adsorption–desorption equilibrium. At regular time intervals, the suspensions were collected and then filtered with microporous filter (0.22 µm) to remove the catalysts. Then the

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concentration changes of MO and TC were measured by using a UV-vis spectrophotometer at 464.5 and 358.05 nm, respectively.

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2.5. Radical trapping and EPR experiments In order to verify the active species in the photocatalytic process, hydroxyl radicals (•OH),

superoxide radical (•O2−), holes (h+) and electrons (e-) were investigated by adding 2 mM IPA (a scavenger of (•OH)，Ar (a scavenger of •O2−), 2 mM Na2C2O4/ TEA (a scavenger of h+), 0.2 mM BQ and 2 mM KBrO3 (a scavenger of e-), respectively. The method is similar to the former

photocatalytic performance testing. Furthermore, 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) was used as the scavenger of •OH and•O2− radicals to verify their formation during the photocatalytic reaction system under visible light (λ> 420 nm). In details, 50 mg samples were dispersed in 0.5 mL deionized water (DMPO-•OH) or 0.5 mL methanol (DMPO-•O2−) with ultrasonic, and after that DMPO was added with continuous ultrasonic for 10 min. 2.6 Photoelectrochemical measurement

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The as-prepared Bi2O4, TiO2 and Bi2O4/TiO2-x composites coated on the FTO were worked as the working electrode. The detailed process to fabricate the working electrodes are as follows: 1)

The as-prepared samples mixed with appropriate water and alcohol were grounded for 2 hours to

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obtain an evenly slurry; 2) Then, the FTO glasses were cleaned via alcohol and coated with resin

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in order to obtain an exposed area of 1cm2; 3) After that, the slurry was coated on the surface of

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FTO glasses and dried at room temperature. The photoelectrochemical behaviors were measured with a standard three-electrode cell in 0.5 M Na2SO4 electrolyte using platinum wire as the

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counter and saturated calomel electrode (SCE) as the reference electrode, respectively. The working electrode was coated with Bi2O4 and Bi2O4/TiO2-x composites on the FTO. The

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electrochemical system (CHI-660C instruments) was used to record the measured photoelectrochemical experiment results. The photoresponses of visible light (>420 nm)

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photocatalysts on and off were measured at 0.0 V. 3. Results and discussion 3.1. Morphology and microstructure characterizations Fig. 1 Fig. 2

Table 1

To reveal the morphology and microstructure of as-prepared catalysts, SEM was conducted. As shown in Fig. 1a, as-synthesized TiO2 sphere exhibits firework shape with diameter about 3µm, which stems from the self-assembly of homogeneous TiO2 nanorods [22]. There are abundant interspaces between neighbored nanorods in the firework-shaped TiO2 sphere (Fig. 1a-b), which

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could not only provide the space but also play the role of spatial limitation for the subsequent growth of Bi2O4. The sizes of interspaces were quantitative analyzed from the magnified SEM

micrograph (Fig. 1b), by measuring two hundred and one individual gaps. Fig. S1 is the histogram

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of pore size distribution of as-synthesized TiO2. As can be seen therein, the sizes of interspaces

range from 25 to 150 nm, which indicates that TiO2 microspheres coexist with mesoporous and

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macropores. To make a contrast, the pristine Bi2O4 was also characterized and exhibits micro-rod

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morphology with diameter of about 300-500 nm and length of about 2-4 μm (Fig. 1c). This microsized Bi2O4 may suffer from the prolonged charge transfer distances and will lead to the

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recombination of photoexcited electrons-holes.

To overcome this drawback, nano-sized Bi2O4 is fabricated via a feasible dual–step process,

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namely, synthesis of firework-shaped TiO2 sphere with plenty of mesopores and macropores at

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first and then preparation of nanoscaled Bi2O4 by the space limitation of firework-shaped TiO2 sphere (as shown in the scheme 1 in the supporting information). As shown in Fig. S2a-2b and Fig. S2d-2e, when small amount of Bi2O4 (B/T-X, X=0.1 and 0.25) were incorporated into the TiO2 sphere, negligible morphology variation can be viewed compared with pristine TiO2. However, the corresponding EDS mapping results (Fig. S2c and Fig. S2f) confirm the successful formation of Bi2O4-TiO2 composites. After the amount of Bi2O4 increased to 0.5, obviously

different morphology of B/T-0.5 composite can be found from Fig. 1d and Fig.S3a, in which Bi2O4 nanoparticles (NPs) insert into the interspaces of TiO2 sphere, leading to coarse and loose surface of B/T-0.5 composite，compared with the parent TiO2 sphere. It should be noted that there are still some visible small gaps on the surface of B/T-0.5 composite because of the low mass loading of Bi2O4. With the increase of mass ratio of Bi2O4(x=1, 2) in the hybrid, the density of Bi2O4 NPs on surface gradually increased (Fig. 1(e,f), and S3(b,c)), however, the relatively

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loose feature of the surface still remain. Further increasing the Bi2O4 mass ratio to 4, the fireworkshaped TiO2 sphere is densely wrapped by Bi2O4 NPs, forming a solid sphere (Fig.1g and S13d). Importantly, Large Bi2O4 particles and free Bi2O4 micro-rods begin to appear on the surface of

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composite when the mass ratio of Bi2O4 reach to 6 (Fig.1h and S3e), which means beyond the

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spatial-limitation capability of firework-shaped TiO2 sphere, due to the excessive addition of

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Bi2O4 precursor. Besides, the energy dispersive spectrum (EDS) mapping in Fig. 1i depicts the well-dispersion of Bi element in the whole TiO2 sphere, demonstrating the uniform growth of

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Bi2O4 NPs in the firework-shaped TiO2 sphere. These results suggest that firework-shaped TiO2 sphere could work as the confined-template to down the size of Bi2O4 to nanoscale particles.

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To further verify this confined effect of firework-shaped TiO2 sphere, nitrogen adsorption−desorption isotherms were carried out to further uncover the structural features of

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firework-shaped TiO2 microspheres, pristine Bi2O4 and B/T-X composites (X=0.5-6). As shown in Fig.2(a), the isotherms of TiO2 and all B/T composites are between II-type and IV-type, which also combined of H2 and H3 hysteresis loop, suggesting the existence of mesoporos and microporos[23-27]. The firework-shaped TiO2 microspheres displayed relative high specific surface area of 30.9 m2g−1 and large total pore volume of 0.101cm3g-1 (Table 1), demonstrating the

promising structure for the confined growth of Bi2O4. B/T-X composites exhibit decreased specific surface area and pore volume, meanwhile, the higher amount of Bi2O4, the smaller specific surface area and pore volume (Table 1). These results indicate that the nanoscaled Bi2O4 particles are embedded into the interspaces of neighbored nanorods of firework-shaped TiO2, which is consistent well with the results of SEM. Based on the above characterization results, it can be concluded that the size of Bi2O4 was successfully decreased to nanoscale through the spatial-

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limitation effect of firework-shaped TiO2 microsphere and abundant nano-heterostructured

interfaces between Bi2O4 and TiO2 were formed simultaneously by our two-step strategy，which

3.2 Phase structure and surface elemental composition

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Fig. 3

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can promote greatly the transfer and separation of photoinduced charge carriers.

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XRD was conducted to identify the phase structure and crystalline of as-prepared TiO2, Bi2O4, and Bi2O4/TiO2-x heterojunctions as shown in Fig. 3(a). For bare TiO2 and Bi2O4, all the

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diffraction can match well with the rutile phase (JCPDS NO.21-1276) and monoclinic phase (JCPDS NO.83-0410), respectively [12,13]. With respect to the Bi2O4/TiO2-x (x=0.5, 1)

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heterojunctions, both the characteristic diffraction peaks of Bi2O4 and (110) crystal plane of TiO2 are observed, revealing the successful formation of Bi2O4/TiO2-x composites. However, with the

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increase of mass fraction of Bi2O4 in the hybrid, the characteristic peaks intensities of TiO2 weaken gradually and the characteristics peaks intensities of Bi2O4 strengthen gradually. Once the proportion of Bi2O4 is over 2, it is difficult to find the characteristic peak of TiO2 from the XRD patterns. This phenomenon is ascribed to the high content of Bi2O4 in the heterojunctons and the much higher diffraction peaks intensity of Bi2O4.

The surface compositions and elemental chemical status of the TiO2, Bi2O4 and all B/T-X composites (X=0.5-6) were characterized by XPS, wherein carbon, titanium, oxygen and bismuth species are detected in Fig. S4(a), which implies the formation of the Bi2O4/TiO2 heterostructure. The C 1s signal comes from a carbon-based contaminant in the atmosphere or vacuum system [28]. The comparison of Bi 4f high resolution spectra between Bi2O4 and all B/T composites were displayed in Fig. S4(b) and all B/T composites show obvious shift of binding energy, especially

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for the B/T-0.5 sample. As expected, the Ti 2p high resolution spectra (Fig. S4(c)) also

demonstrate the existence of binding energy shift in all B/T hybrids. To deeply investigate this

shift of binding energy, further XPS spectra comparison between Bi2O4 and B/T-4 were executed.

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Fig. 3(c) shows that the Bi 4f7/2 (or 4f5/2) peak in the B/T-4 at binding energies of 158.4 and

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159.1 eV (or at 163.7 and 164.4 eV) can be deconvoluted well into two bimodal peaks. According

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to the reports in the related literature [29], it is apparent that these peaks are consistent with the Bi(III) and Bi(V) spin-orbital peaks of Bi2O4, which confirms the presence of Bi2O4 in B/T-4. In

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comparison with bare Bi2O4, the Bi 4f 7/2 (or 4f 5/2) peaks of B/T-4 heterojunction shift toward higher binding energy, indicating the decrease of electron cloud density. This shift of binding

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energy not only proves the successful formation of Bi2O4/TiO2-x heterojunctions, but also means the existence of the strong electron interactions at the interfaces of Bi2O4 and TiO2 in the

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heterojunction [30]. In detail, the photogenerated electrons transfer from the CB of Bi2O4 to the CB of TiO2. From the Fig. 3(d), it is obvious that the Ti2p3/2 peak of B/T-4 shifts toward lower binding energy compared with that of pure TiO2, indicating the strong electronic interaction between TiO2 and Bi2O4 again[31].The results of these XPS indicate that there is electron transfer at the interface between Bi2O4 and TiO2, which is beneficial to improve the efficiency of

photoinduced charges separation and reduce the recombination probability of photogenerated carriers in the process of photocatalysis. It is anticipated that the photocatalytic activity of B/T-X heterostructure will be improved. 3.3. Optical and Photoelectrochemical performance Fig. 4 The optical absorption of as-prepared samples was analyzed with the UV–vis absorption spectra

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in the Fig. 4(a). As we can see that the absorption edge of pristine firework-shaped TiO2 sphere

locates at the UV range, and from which the band energy is estimated about 3.0 eV, agreed well with the previous report [32]. For pure Bi2O4, the absorption edge is extended to 595 nm,

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equivalent to the band energy about 2.1 eV [28]. Compared to firework-shaped TiO2, Bi2O4/TiO2

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heterostructures have obvious red shift phenomenon, confirming the successful incorporation of

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Bi2O4 into firework-shaped TiO2 sphere. Additionally, with the increase of mass ratio of Bi2O4 in the heterostructure, more notable band edge red shift and visible absorption intensity are

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visualized. When the mass ratio is up to 4, optimal absorption edge and absorption intensity are achieved. In general, the wider visible light response range and higher visible light absorption

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intensity, the better visible light photocatalytic activity of photocatalyst [33]. In order to investigate the charge excitation and transfer of pristine Bi2O4 and B/T-X

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heterojunction, the photoelectrochemical (PEC) measure was employed. Fig. 4(b) presents the transient photocurrent response ability of the samples with light on−off cycles under visible light (λ > 420 nm). With the increase of Bi2O4 contents, the photocurrent density increase first and then decrease. Apparently, the highest photocurrent density of B/T-4 (0.034 µA/cm2) is visualized, compared with pristine Bi2O4 (0.016 µA/cm2), which directly supports the accelerated charge

transfer and separation efficiency in B/T-X heterojunction [34]. This can be explained by following two aspects. On one hand, nanoscaled Bi2O4 embedded in firework-shaped TiO2 sphere could shorten the charges migration distance, leading to the decreased recombination probability of electron-hole pairs. On the other hand, enormous heterostructured interfaces between Bi2O4 NPs and TiO2 nanorods could promote the electron migration, resulting in improved charges separation efficiency. Furthermore, as evidenced by the decrease of semicircular diameter in the

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electrochemical impedance spectroscopy (EIS) profiles of Fig. 4(c), the B/T-X exhibit smaller resistance than pristine Bi2O4, among them, the resistance of B/T-4 is the smallest, which is

consistent well with the result from Fig. 4(b) [35]. Therefore, it can be concluded that embedding

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nanoscaled Bi2O4 into firework-shaped TiO2 sphere can yield enhanced photoinduced charge

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3.4. Photocatalytic performance

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carriers separation efficiency, which promise the boosted photocatalytic activity.

Fig. 5

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Table 2

To investigate the photocatalytic activities of Bi2O4/TiO2 heterojunctions, photodegradation of

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MO was firstly selected as the target reaction under visible light irradiation. It is clear that B/T-X heterostructures show superior adsorption capability compared with pure Bi2O4 for MO molecule

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(Fig (5a)). As shown in Fig. 5(b), for the visible-light-responded Bi2O4, the removal rate of methyl orange (MO) is about 60% within 150min, which suggests it has good visible light photocatalytic activity. Moreover, it can be clearly seen that most of the Bi2O4/TiO2 heterostructures exhibit higher photocatalytic activity for degradation of MO under visible light than Bi2O4. This result confirms that constructing Bi2O4/TiO2 heterojunction can improve greatly the photocatalytic

activity of Bi2O4. Significantly, with the increase of mass ratio of Bi2O4, the photocatalytic performance of Bi2O4/TiO2 heterostrucutres improves gradually, and achieves the optimizing efficiency at the ratio of 4. To obviously illustrate the photodegradation efficiency of these photocatalysts, the preudo-first-order reaction kinetics is taken (Fig. 5(c)) and the corresponding rates constant are given (Table2). As seen in Table 2, B/T-4 displays the highest rate constant of 0.0162 min-1, which is 2.5 times higher than that of Bi2O4. This can be well understood since B/T-

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4 exhibited better optical absorption performance and higher photogenerated charges separation efficiency than pure Bi2O4, as verified by the above optical and photoelectrochemical characterization results.

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Colorless TC was also chosen as the objective pollutant to further confirm the excellent visible-

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light photocatalytic performance of Bi2O4/TiO2-x heterojunctions. As displayed in Fig. 5(d), Bi2O4

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and Bi2O4/TiO2-x heterojunctions exhibit favorable adsorption ability towards TC molecules and about 25% TC molecules were adsorbed onto the surface of photocatalysts in dark within 2 hours.

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With respect to the photocatalytic process, 55% TC can be removed within 150 min under visible light irradiation for pristine Bi2O4, revealing its favorable degradation efficiency toward TC.

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Similarly, the photocatalytic activities of Bi2O4/TiO2 heterostructures are superior to pristine Bi2O4. Contrary to the degradation trend of MO, Bi2O4/TiO2 heterojunctions with relatively low

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content of Bi2O4 possess superior photocatalytic activity toward TC. In a series of heterojunctions, B/T-1 has the highest degradation efficiency for TC. As shown in Table 1, the rate constant of B/T1 is 1.53 times higher than that of pure Bi2O4. It is clear that incorporation of Bi2O4 NPs into firework-shaped TiO2 sphere to form heterojunction through the spatial limitation effect can effectively promote the efficiency of visible light photocatalytic degradation of organic pollutants.

According to various characterization results, the improved photocatalytic performance of Bi2O4/TiO2 hybrid should be related to the following aspects. Firstly, taking advantage of the space-confined effect of firework-shaped TiO2 sphere, the size of Bi2O4 is decreased to nanoscale level, which contributes to improve the surface active site concentration of Bi2O4/TiO2 heterostructures and thus the photocatalytic behavior of Bi2O4/TiO2 heterojunctions. Secondly, nanoscaled Bi2O4 particles shorten the transfer distance of photogenerated charges from interior to

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surface and then enhance the separation efficiency of charge carriers as well as prolong the life time of charges. Thirdly, the intimate contact between nanoscale Bi2O4 particles and TiO2

nanorods would form enormous nano-heterojunctions, which could accelerate charge carriers’

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separation and suppress their recombination. In this way, more charge carriers can participate in

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activity of Bi2O4/TiO2 heterostructure.

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the photocatalytic degradation of MO and TC, which greatly improves the visible light catalytic

Fig. 6

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In the actual water environment, many factors such as inorganic anions and pH could affect the photocatalytic activity of catalysts. To reveal the impact of inorganic anions on the

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photocatalytic performance toward TC, the sodium salts of NO3-, Cl- and HCO3- (0.05 M) were introduced to the reaction system (B/T-1 sample as the catalyst). As shows in Fig. 6(a), compared

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to the photocatalytic performance of blank reaction system, the introduction of NO3- and Cl- has negligible influence on the photodegradation efficiency of TC. However, when the HCO3- were introduced to the reaction system, an obvious potocatalytic performance promotion is discerned, which probably ascribed to the increased pH value after the addition of HCO3-. To further explore the influence of pH value on the catalytic activity, the photocatalytic activity of TC for B/T-1

under different pH values (pH=4 and pH=10) were conducted (Fig. 6b). As can be seen, pH values have great effect on the degradation efficiency. When the pH of the reaction system is 4, an apparent performance decay is observed, which is probably related to the weaker adsorption of photocatalyst for TC molecules at such pH value (Fig. 6b). However, a strong contrast can be found at pH=10. Namely, enhanced photodegradation rate was observed, which agrees well with the result from the addition of HCO3- to the reaction system. These results imply that introduction

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of neutral anions (NO3- and Cl-) has negligible influence on the photocatalytic performance toward TC degradation, while pretreatment of wastewater to alkalinity environment is helpful for increasing the photodegradation efficiency of TC.

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The stability and reusability of a photocatalyst are very important for practical applications. The

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recycling runs of MO degradation over B/T-4 sample were performed to evaluate its stability. As

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shown in Fig. S5, the degradation rate over B/T-4 decreased from 90% to 35% under visible light irradiation after three cycles. This result suggests the photo-corrosion of Bi2O4 induced by long

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time irradiation is still the urgent problem to address. Further study via optimizing the nanostructure and composition should ongoing to improve the photostability of Bi2O4.

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3.5. Photocatalytic mechanismFig. 7

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In order to further understand the photocatalytic process, EPR characterization and the free

radicals trapping experiments were tested, and the results are shown in Fig. 7. Obviously, the characteristic signals of the DMPO-•O2− (Fig. 7a) and DMPO-•OH (Fig. 7b) were detected under the visible light irradiation (5 and 10 min) compared with B/T-4 in the dark. Therefore, it can be concluded that •O2− and •OH radicals play a certain roles in photodegradation process. In addition,

the signals of the DMPO-•O2− (Fig. 7a) are stronger than those of DMPO-•OH, which suggest that more •O2− radicals were formed during the reaction process. Meanwhile, the trapping experiments were further conducted, as shown in Fig. 7c and 7d in the system of MO and TC, respectively. In the system of MO, when sodium oxalate [36] as a holes (h+) scavenger was added, the degradation of MO over B/T-4 nearly inhibits completely, proving that h+ plays a significant role in the photocatalytic degradation process. Removal of dissolved oxygen by Ar purging [37], the

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photodegradation efficiency of MO decreases to some extent, which may be due to the anaerobic environment prohibits the formation of •O2- radicals. When isopropyl alcohol [38] was acted as •

OH scavenger, the photodegradation efficiency of MO drops slightly, indicating that • OH radical

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is not the main active species. In addition, e- radical(KBrO3) [39] have no effect on the reaction

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process. These results suggest that only h+ is the main oxidative species in the photocatalysis

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reaction process of MO.

In the system of TC, the degradation behavior of TC decreases quickly after adding BQ [40],

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indicating that •O2− radical plays a vital roles in the photocatalytic process. When IPA and TEA [41] were added, the TC degradation rate decreases slightly, which indicates that •OH and h+ are

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not the main active species for the degradation of TC. The introduction of KBrO3 has no effect on the photodegradation efficiency of TC. Thus, it can be deduced that •O2− is the main active species

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in the photocatalytic degradation of TC. On account of aforementioned characterization results, a probable mechanism was posed and

discussed to reveal the enhanced performance of Bi2O4/TiO2 heterojunctions. Based on the previous experimental and calculation results, the maximum valance band (VBM) and minimum conduction band (CBM) of rutile TiO2 are identified at 2.80 eV and -0.20 eV, while the

counterparts of Bi2O4 are located at 1.68 eV and -0.42 eV [42,43], respectively. Benefitting from the staggered edge position between TiO2 and Bi2O4, Type II heterojunction would be formed after spatial-confined growth of Bi2O4 within the hierarchical TiO2 as shown in Fig. 8. In this promising heterojunction, visible light-responsed Bi2O4 absorbs visible light and generates charge carriers under visible light illumination. The photogenerated electrons transfer from the CB of Bi2O4 to the CB of TiO2, in which electrons reduce dissolved oxygen to form •O2- radicals. Then these •O2-

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radicals would be involved in the photocatalytic degradation reaction [44]. The left equal amount of holes on the VB of Bi2O4 will react with the pollutants directly and decompose them into CO2 and H2O at last [45]. This spatial separation of electrons and holes, which has been supported by

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XPS, photocurrent and EIS results, could greatly increase the photocatalytic performance of

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Bi2O4/TiO2 heterojunctions. On the other hand, the nanoscaled Bi2O4 particles confined into the

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hierarchical TiO2 have higher specific surface area, which is very beneficial for improving the surface active site concentration [46]. Moreover, due to the dramatically shortened charges

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transfer pathways for nanosized Bi2O4, the photogenerated charge carriers transport from interior to surface will be promoted greatly, decreasing the recombination rate of charge carriers [47].

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Furthermore, the intimate contact between nanosized Bi2O4 particles and TiO2 nanorods contributes to construct enormous heterojunctions and enlarge interfacial contract areas, which

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could accelerate the separation of charge carriers and inhibit their recombination, which is also an important factor for the enhancement of photocatalytic performance [48]. In a word, the enhanced photocatalytic performance of Bi2O4/TiO2 heterojunctions is originated from the synergistic effect of the nanosized Bi2O4 and type II heterojunctions. Fig. 8

Conclusion In summary, Bi2O4/TiO2 heterostructured photocatalysts have been successfully fabricated through the space-confined strategy of firework-shaped TiO2 microspheres, in which the size of Bi2O4 is decreased down to nanoscale level and nanosized Bi2O4 particles are grown within the TiO2 microspheres. Firework-shaped TiO2 microspheres with rich pore structure play an important role in regulating the microstructure of Bi2O4 and constructing enormous type Ⅱ Bi2O4/TiO2

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heterojunctions. As-prepared Bi2O4/TiO2 heterostructured photocatalysts have better activity in the photocatalytic degradation of MO and TC under visible light irradiation than pristine micro-rod

Bi2O4. The improved photocatalytic performance of Bi2O4/TiO2 heterojunction is derived from the

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following aspects: (i) nanoscaled Bi2O4 with higher surface active sites concentration, (ii)

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nanosized Bi2O4 with higher charges migration and separation efficiency from interior to surface,

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(iii) enormous intimated heterostructured interfaces between Bi2O4 nanoparticles and TiO2 nanorods accelerating the electrons transfer from Bi2O4 to TiO2. This study provides a new

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pathway to construct high-performance photocatalyst via the spatial confinement strategy. Acknowledgements

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The authors would like to express their thanks for the support of National Natural Science Foundation of China (No. 21663012), Natural Science Foundation of Jiangxi province

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(20181BAB203009), Scientific &Technological Project of Jiangxi Science and Technology Normal University (No. 2015CXTD003) and Graduate Innovation Foundation of Jiangxi Science and Technology Normal University (No. YC2018-X05), P.R. China.

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Figure Caption

Fig. 1. SEM images of pristine TiO2(a,b), Bi2O4(c), B/T-0.5(d), B/T-1(e), B/T-2 (f), B/T-4(g) and B/T-6(h), (i) The mapping distribution of O, Ti, and Bi of B/T-4. Fig. 2. N2 adsorption-desorption isotherms and corresponding pore size distributions (inset) of TiO2 and B/T-X samples. Fig. 3. (a) XRD patterns of the as-synthesized samples, (b) Full survey XPS spectra of Bi2O4 and

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B/T-4, (c) high-resolution XPS Bi 4f spectra of Bi2O4 and B/T-4, (d) high-resolution XPS Ti 2p spectra of TiO2 and B/T-4.

Fig. 4. (a) UV–vis spectra of TiO2, Bi2O4 and Bi2O4/TiO2-X samples (b) Transient photocurrent

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nm) irradiation at 0 V versus SEC. [Na2SO4]=0.5M

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responses and (c) electrochemical impedance spectra of samples under the visible light (λ>420

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Fig. 5. Adsorption and photocatalysis of MO (a,b) and TC(d,e) under visible light irradiation over Bi2O4 and Bi2O4/TiO2-x samples The preudo-first-order reaction kinetics of the prepared samples

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for MO(c) and TC (f) degradation.

Fig. 6. The impact of (a) inorganic anions and (b) pH on the TC photocatalytic

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degradation efficiency in the presence of B/T-1 Fig. 7. (a) and (b) ESR spectra of B/T-4 in water and in methanol aqueous dispersion

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with or without visible light irradiation (λ > 420 nm). (c) Degradation of MO over B/T-4 composite under visible light in the presence of different scavengers, (d) Degradation of TC over B/T-1 composite under visible light in the presence of different scavengers. Fig. 8. Proposed the charge transfer and separation in the Bi2O4/TiO2 heterojunctions under visible

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na

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-p

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light irradiation

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Fig. 1. SEM images of pristine TiO2(a,b), Bi2O4(c), B/T-0.5(d), B/T-1(e), B/T-2 (f), B/T-4(g) and

60

0.30 0.25

TiO2

0.20 0.15

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50

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70 Pore volume(cm3g-1nm-1)

Volume adsorbed (cm3g-1)

B/T-6(h), (i) The mapping distribution of O, Ti, and Bi of B/T-4.

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0.05 0.00

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30

0.10

B/T-0.5 B/T-1 B/T-2 B/T-4 B/T-6

20

5

10

15

Pore radicus(nm)

20

10 0

0.0

Fig. 2. N2

0.2

0.4

0.6

0.8

1.0

Relative pressure/p/p0 Relative pressure/p/p0 distributions (inset) of adsorption-desorption isotherms and corresponding pore size

TiO2 and B/T-X samples.

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Fig. 3. (a) XRD patterns of the as-synthesized samples, (b) Full survey XPS spectra of Bi2O4 and B/T-4, (c) high-resolution XPS Bi 4f spectra of Bi2O4 and B/T-4, (d) high-resolution XPS Ti 2p

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spectra of TiO2 and B/T-4.

Fig. 4. (a) UV–vis spectra of TiO2, Bi2O4 and Bi2O4/TiO2-X samples (b) Transient photocurrent

responses and (c) electrochemical impedance spectra of samples under the visible light (λ>420

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na

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nm) irradiation at 0 V versus SEC. [Na2SO4]=0.5M

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Fig. 5. Adsorption and photocatalysis of MO (a,b) and TC(d,e) under visible light irradiation over Bi2O4 and Bi2O4/TiO2-x samples The preudo-first-order reaction kinetics of the prepared samples for MO(c) and TC (f) degradation.

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na

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Fig. 6. The impact of (a) inorganic anions and (b) pH on the TC photocatalytic degradation efficiency in the presence of B/T-1

Fig. 7. (a) and (b) ESR spectra of B/T-4 in water and in methanol aqueous dispersion with or without visible light irradiation (λ > 420 nm). (c) Degradation of MO over B/T-4 composite under visible light in the presence of different scavengers, (d)

Degradation of TC over B/T-1 composite under visible light in the presence of

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different scavengers.

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Fig. 8. Proposed the charge transfer and separation in the Bi2O4/TiO2 heterojunctions under visible

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light irradiation

Table 1 Summary of the textural properties of the samples Samples

TiO2

B/T-0.5

B/T-1

B/T-2

B/T-4

B/T-6

Bi2O4

SBET(m2g-1)

30.918

22.281

17.561

14.763

7.555

4.864

3.5

Pore volume

0.101

0.072

0.065

0.048

0.031

0.025

0.010

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(cm3g-1)

Table 2 Pseudo-first-order rate constants for MO and TC degradation over different photocatalyst Rate constant(min-1)

B/T-0.5

B/T-1

MO

0.0066

000.0115

0.0133

TC

0.00362

0.00475

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B/T-2

B/T-4

B/T-6

0.0137

0.0162

0.0109

0.00436

0.00403

0.00358

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Bi2O4

-p

Samples

0.00553