TiO2-In2S3: A photocatalysis composite with enhanced photocatalytic activity

Accepted Manuscript The visible-light-driven type III heterojunction H3PW12O40/TiO2-In2S3: A photocatalysis composite with enhanced photocatalytic activity Huimin Heng, Qiang Gan, Pengcheng Meng, Xia Liu PII:

S0925-8388(16)33586-1

DOI:

10.1016/j.jallcom.2016.11.116

Reference:

JALCOM 39612

To appear in:

Journal of Alloys and Compounds

Received Date: 18 September 2016 Revised Date:

5 November 2016

Accepted Date: 8 November 2016

Please cite this article as: H. Heng, Q. Gan, P. Meng, X. Liu, The visible-light-driven type III heterojunction H3PW12O40/TiO2-In2S3: A photocatalysis composite with enhanced photocatalytic activity, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.11.116. 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.

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Graphical Abstract

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The visible-light-driven Type III heterojunction H3PW12O40/TiO2-In2S3 : a

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photocatalysis composite with enhanced photocatalytic activity

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Huimin Henga, Qiang Ganb, Pengcheng Menga, Xia Liua*

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a. College of Science, China Agricultural University, Beijing 100193, China.

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b. State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology,

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Beijing 100081, China.

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H3PW12O40/TiO2-In2S3, a visible-light-driven Type III photocatalysis composite

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heterojunction, is synthesized by typical sol-gel method. The X-ray diffraction

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(XRD)，inductively coupled plasma-atomic emission spectrometer (ICP-AES) and

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energy dispersive X-ray spectra (EDS) analysis show that HPW which is anchored to

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the surface of TiO2-In2S3 makes the crystallite sizes of HPW/TiO2-In2S3 reduce to

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9.7nm. X-ray photoelectron spectroscopy (XPS) analysis confirm that HPW acts as the

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bridge assisting in the transfer of electronics from TiO2 to In2S3 in H3PW12O40/TiO2-

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In2S3. The optical properties of synthesized composites are investigated by UV-Vis

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diffused reflection spectra (UV-Vis DRS) and photoluminescence (PL) spectra. The

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lattice defects of TiO2 lead to the existence of oxygen-vacancies and impurity energy

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levels which make TiO2 respond to visible light. And the type-III heterojunction

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restrains the recombination of photo-generated carriers effectively. Under visible light

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irradiation

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degradation activity of imidacloprid (82.7%), comparing with H3PW12O40/TiO2 (26.7%),

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(λ≥400nm),

H3PW12O40/TiO2-In2S3

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displays

higher

photocatalytic

ACCEPTED MANUSCRIPT TiO2-In2S3 (20.6%) and TiO2 (16.0%). The pseudo-first-order degradation rate constant

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of HPW/TiO2-In2S3 is 6, 7 and 13 times higher than that of HPW/TiO2, TiO2-In2S3 and

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TiO2, respectively. Moreover, the photogenerated holes and ·OH radicals are proved as

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the main active species in degradation process.

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Keywords: Visible-light-driven; Type III heterojunction; H3PW12O40/TiO2-In2S3;

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Photocatalysis composite;

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1. Introduction The photocatalysts have been widely studied since the first research paper was

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published in 1972[1]. So far, photocatalysis have infiltrated into many fields, such as

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waste-water purification[2], water-splitting[3] and organic synthesis[4]. However, the

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large-scale practical application of photocatalysis is confined, because of the weak

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response to visible light. Much efforts have been devoted to the development of novel

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photocatalysts, for instance, g-C3N4[5], Bi2WO6[6], and Zn2V2O7[7]. But the pristine

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photocatalysts still have inevitable shortcomings, such as the high recombination

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efficiency of photogenerated carrier and the low quantum yield. In addition, the

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photocatalysis composite heterojunctions can make the photocatalysts response to

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visible light and simultaneously achieve a high separation efficiency of photogenerated

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carriers. So the construction of photocatalysis composite heterojunctions is regarded as

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the more efficient approach and attracts numerous research interests[8, 9].

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Overall, there are three typical categories of the photocatalysis composite

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heterojunctions, including type I, type II, type III[10]. Among the three categories, type

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III heterojunctions (Z-scheme) is better than the others for its wide response range, high

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separation efficiency of photogenerated carriers and retained redox ability[11]. Many

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works about the type III heterojunction have been reported[12-14]. For example, Jia et

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al. developed direct Z-scheme CdS/CdWO4 by solvothermal method. CdS/CdWO4

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showed 18 times higher H2 evolution activity under visible-light, comparing with CdS

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[15]. Wang et al. synthesized Z-scheme AgI/WO3 by a facile precipitation method. The

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photocatalytic degradation efficiency of tetracycline hydrochloride over 20%-AgI/WO3

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was as 4.3 and 25 times high as that of pure AgI and WO3[16]. Rong et al. studied the

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photocatalytic performance of Z-scheme Bi2S3-WO3 synthesized by a combination of

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hydrothermal and postcalcination method. When the Bi2S3-WO3 and WO3 were used for

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photocatalytic degradation of Rhedamine B, the photocatalytic degradation efficiency of

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Bi2S3-WO3 was 3.5 times higher than that of WO3[17]. As the incipient photocatalyst, titanium dioxide (TiO2) has attracted much attention

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owing to its good stability, eco-friendliness and low cost. But TiO2 can only be excited

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by ultraviolet (UV) light for its wide band gap (3.2eV of anatase). To broaden the

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response range of TiO2, many methods have been applied, such as surface

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modification[18], element doping[19], metal deposition[20], and heterojunction

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composite[21]. It is worth noting that the TiO2 based type III heterojunctions are

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presented less, because it is difficult to construct type III heterojunctions between TiO2

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and other semiconductors for their unmatched band gap position. Up to now, only

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TiO2/C3N4[22],

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anatase/rutile TiO2[26] are proposed. Depending on the results, these heterojunctions all

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show higher photocatalytic activity than pure TiO2. In especially, the construction of

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these type III heterojunctions enables TiO2 based photocatalysts to achieve both high

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separation efficiency of photogenerated carriers and high redox ability.

TiO2-Ag-Cu2O[24],

CdS-Ag-TiO2[25]

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TiO2/WO3[23],

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In2S3 is a typical narrow band gap semiconductor (2.0eV) which has been

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investigated as a visible-light-driven photocatalyst or a sensitizer for wide band gap

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semiconductor photocatalysts. Gao et al. reported the synthesis of the tetragonal In2S3

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which possessed broad-spectrum photocatalytic activity under UV, visible, and near-

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infrared irradiation[27]. Wang et al. fabricated a visible-light-driven heterogeneous

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photocatalysts In2S3/Pt-TiO2. In2S3 was proved to be a good sensitizer for TiO2,

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enabling hydrogen generation under visible light[28]. However, In2S3 rarely appears in

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type III heterojunctions system, as far as our information goes.

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ACCEPTED MANUSCRIPT Many substances with similar properties of semiconductor materials are also

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concerned. Polyoxometalates (POMs) are transition metal oxide clusters of d0 or d1

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metal ions bridged via oxygen atoms[29]. As one of the typical POMs, phosphotungstic

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acid (H3PW12O40, HPW) is used in many fields such as acid catalyst[30], wastewater

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decontamination[31], desulfurization[32], and sensor[33], because of its adjustable

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structures, well redox properties and the ability of electronic transmission. Owing to the

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weak response to visible light and the water-solubility, so many works are devoted to

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the sensitization and heterogenization of HPW. Similar to In2S3, HPW is seldom applied

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to construct the type III heterojunction.

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Herein, we designed a visible-light-driven type III photocatalysis composite

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heterojunction H3PW12O40/TiO2-In2S3 (HPW/TiO2-In2S3) and synthesized it by typical

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sol-gel method. The advantages, such as the broad-spectrum response of In2S3, the

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electronic transmission ability of HPW and the retained redox ability of type III

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heterojunction, are combined to obtain the enhanced photocatalytic activity of

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HPW/TiO2-In2S3 by this way. Meanwhile, the photocatalytic activity evaluation of

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composites was conducted via photocatalytic degradation of imidacloprid under visible

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light (λ≥400nm). Depending on the experimental results and the band gap position of

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heterojunction, the photoexcitation process of H3PW12O40/TiO2-In2S3, transfer path of

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photon-generated carriers and degradation mechanism of imidacloprid are conjectured.

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2. Experimental

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2.1 Materials

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Tetrabutyl titanate is chemically pure. Indium nitrate, thiocarbamide, phosphotungstic

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acid, hydrochloric acid and ethanol are analytic grade. All the chemicals were procured

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from Sinopharm Chemical Reagent Co.,Ltd, and used directly without further

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purification. Imidacloprid (98%) was provided by Institute of Plant Protection, Chinese

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Academy of Agricultural Sciences.

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HPW/TiO2-In2S3 was synthesized by typical sol-gel method[34]. Ti(OC4H9)4 (8.20

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mL) was dispersed in ethanol (20mL), then diluted hydrochloric acid (0.78mL,

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12mol·L-1) was added as hydrolysis inhibitor. A mixture of water (2.00mL), ethanol

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(2mL), In(NO3)3·4.5H2O (0.235g) and thiocarbamide (0.070g) was added into solution

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dropwise, following stirred for 2h. HPW (0.400g) dispersed in ethanol (3mL) was

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mixed with above solution quickly under vigorous stirring. The sol was aged for 24h at

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room temperature, and dried at 60℃. The solid was ground into powder and washed

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with distilled water. The photocatalyst was obtained after calcination at 450℃ for 3h

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with a heating rate of 10℃·min-1 in air. The content of In2S3 and HPW in HPW/TiO2-

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In2S3 was 3.79wt% and 16.10wt% detected by ICP-AES. By similar procedure, TiO2-

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In2S3 was synthesized without HPW added, HPW/TiO2 was synthesized with the

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absence of In(NO3)3·4.5H2O and thiocarbamide.

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2.3 Photocatalytic activity measurements

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The photocatalytic activity measurement was implemented by the photocatalytic

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degradation of imidacloprid under visible light. Before irradiation, a suspension of

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imidacloprid solution (50mL, 8.0mg·L-1) and photocatalysis composite (2.0g·L-1) was

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stirred in the dark for 0.5h. Then the suspension was vertically irradiated using a Xe

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lamp (Beijing Perfectlight Technology Co.Lt, PLS-SXE300UV, 300W) with a UV-cut

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ACCEPTED MANUSCRIPT filter (λ≥400nm). The lamp was positioned 10cm on top of the reactor (Fig. S1). All

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experiments were implemented in recirculating cooling water system under constant

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stirring. 1.00mL sample, filtered through a cylinder membrane filter (0.22µm*13mm),

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was obtained at set intervals (0.5h) for subsequent measurement. Afterwards, the

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changes in concentration of imidacloprid were analyzed with liquid chromatography

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(LC). The initial concentration (C0), the residual concentration (Ct) and the degradation

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efficiency (D) was illustrated by the mathematical expression as follows,

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D = (1-Ct/C0) × 100%.

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2.5 Recyclability tests and active species scavenger experiments After photocatalytic experiment, the used photocatalysis composites were collected

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by centrifugation. Before the recyclability tests, the used photocatalysis composites

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were washed with distilled water, dried at 80℃ for 12h.

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Active species scavenger experiments were conducted by adding 10mmol edetate

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disodium (EDTA-2Na), potassium bromate (KBrO3) or tertiary butanol (BuOH) as

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scavengers of h+, e- and ·OH into imidacloprid solution (50mL, 8.0mg·L-1) before the

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photocatalytic tests.

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2.6 Characterization methods

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The accurate compositions of samples were detected by ICP-AES with a inductively

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coupled plasma atomic emission spectrometer (Leeman Prodigy Spec, America). Using

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a X' Pert PRO MPD diffractometer (PANalytical, Netherlands), the crystal phase of

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samples were analyzed by XRD with Cu Kα radiation in the 2θ range from 10° to 80°.

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FT-IR spectra of samples were recorded in wavenumber range of 4000~400cm-1 with a

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ACCEPTED MANUSCRIPT Tensor 27 Fourier transform infrared spectrometer. SEM and EDS were applied to

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observe the surface morphology and composition elements of samples on a field

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emission scanning electron microscopy (Hitachi S-4800, Japan). The TEM micrographs

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were obtained with a JEM 1200EX transmission electron microscope (JEOL Ltd.,

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Japan). More information about composition elements of samples were analyzed by

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XPS on a X-ray Photoelectron Spectrometer (Thermo escalab 250Xi, America) using Al

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Kα as the radiation source. UV-vis DRS was used to analyze the spectrum response

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range of samples on a UV-visible spectrophotometer (Hitachi U-4100, Japan) equipped

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an integrating sphere, using BaSO4 as reference. The photoluminescence intensity of

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samples were analyzed by PL on a fluorescence spectrophotometer (Fluorolog-Tau-3,

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America), excited at 350nm. The concentration of imidacloprid was monitored by LC

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(Agilent series 1200, America) equipped with an Agilent Eclipse XDB-C18 column

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(4.6mm×150mm×5µm) and a VWD detector (270nm). The mobile phase was a mixture

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of acetonitrile and water (45:55, v/v) with a flow rate of 1.0mL·min−1, the injection

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volume was 20µL.

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3. Results and discussion

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3.1 Sample characterization Fig. 1 presents the XRD patterns of HPW/TiO2-In2S3, HPW/TiO2 and TiO2-In2S3. It

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can be found that all samples have similar diffraction peaks. Based on the JCPDS card

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of TiO2 (No. 21-1272)(black line in Fig. 1), all diffraction peaks of samples could be

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indexed to anatase crystalline phase. The peaks at 2θ of 25.3°, 38.1°, 48.2° and 62.8°

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are corresponding to (101), (004), (200) and (204) planes, respectively[35]. No

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characteristic diffraction peaks of HPW or In2S3 are observed in samples, comparing

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with the JCPDS card of HPW (No. 50-0304)(pink line in Fig. 1) and In2S3 (No. 32-

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0456)(orange line in Fig. 1). This phenomenon is ascribed to the less content (3.79wt.%)

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of In2S3 and the high dispersion of amorphous HPW[36]. Using the Scherrer equation

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with the (101) diffraction peak, the crystallite sizes of HPW/TiO2-In2S3, HPW/TiO2 and

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TiO2-In2S3 are calculated to be 9.7nm, 10.3nm and 10.8nm, respectively. The decreased

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crystallite size of HPW/TiO2-In2S3 implies the existence of HPW and In2S3 in synthesis

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process has a negative effect on the crystallinity of TiO2, as Kumbar reported[37]. The

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negative effect caused by HPW is especially stronger than that by In2S3. In addition, the

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broad diffraction peak also reflects the same conclusion.

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FT-IR spectra of all samples are shown in Fig. 2. The peaks at 1130cm-1 and

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1054cm-1 are found in HPW/TiO2-In2S3 and HPW/TiO2, which belong to the

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stretching vibrations of P-O bonds and W=O bonds in HPW. The results suggest

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that the HPW in HPW/TiO2-In2S3 still retains its major Keggin structure[38]. In

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comparison with the standard characteristic peaks of HPW[39], the detected

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peaks in HPW/TiO2-In2S3 shift to red. This shift indicates that HPW interacts

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with TiO2[40]. This deduction is consistent with the XRD analysis. Other

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characteristic absorption peaks of HPW, such as W-O-W bonds, are covered by

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the peak of Ti-O stretching vibration (500cm-1~800cm-1)[41]. Less content

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(3.79wt.%) of In2S3 conduces to the result that no peaks of In2S3 are observed. The visualized morphology of HPW/TiO2-In2S3 is displayed in Fig. 3. From the SEM

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pictures, it can be observed that the sample is made up of nanoparticles as the form of

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granular aggregate. The EDX analysis is conducted to confirm the elements (Ti, O, P,

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W, In and S) existed on the surface of HPW/TiO2-In2S3, as shown in Fig. 3D. The

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specific data of EDX and ICP-AES are listed in Table S1. The contrast of the EDX

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results and the ICP-AES results could be used for the inference of the dispersion of

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HPW and In2S3 in the surface or inside of sample. About W, the data of EDX and ICP-

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AES are similar. However, the data of In are different. The result indicates that all of

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HPW and a part of In2S3 exist on the surface of HPW/TiO2-In2S3. Combining the XRD

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and IR analysis with the dispersion of HPW, it can be deduced that all of HPW is

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anchored to the surface of TiO2-In2S3 through interactions. As Rengifo-Herrera

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suggested, the interaction between HPW and TiO2 is electrostatic attraction[34]. In

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addition, the terminal W-O groups of the HPW’s Keggin units could interact with the

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surface ≡Ti-OH groups of TiO2 via W–O–Ti covalent bonds, according to the research

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of Li Kexin[42]. So the hydrogen bonds and the formation of the W–O–Ti covalent

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bonds could suppress the crystal growth of TiO2 and damage the the crystallinity of

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TiO2, shown in Fig. S2. Therefore, the negative effect on the crystallinity of TiO2 which

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caused by HPW is strong. Simultaneously, because of the suited ionic radius (0.081nm

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for In3+, 0.061nm for Ti4+), In3+ may be incorporated into the lattice of TiO2 in the

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synthesis process. And the other part of In2S3 only is present in the bulk of HPW/TiO2-

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In2S3. Depending on the TEM pictures of HPW/TiO2-In2S3, it could be found that the

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nanoparticles are roughly spherical in shape with a diameter ranging from 5~15nm. The

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value accords with the crystallite sizes calculated with XRD results. More information about the surface chemical compositions and the valence states of

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elements in HPW/TiO2-In2S3, HPW/TiO2 and TiO2-In2S3 are presented in Fig. 4 and Fig.

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S3, provided by XPS analysis. All specific data of binding energy are listed in Table S2.

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The survey spectrum of HPW/TiO2-In2S3 reveals the existence of Ti, O, In and W (in

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Fig. 4A). The peaks at 458.98eV and 464.88eV of Ti 2p (Fig. 4B) corresponds to Ti

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2p3/2 and Ti 2p1/2, demonstrating the existence of Ti4+ in HPW/TiO2-In2S3[43]. However,

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the binding energy values of Ti 2p in HPW/TiO2-In2S3 are higher than that in TiO2-

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In2S3. The result is caused by the loss of electrons on TiO2. In consideration of the XRD

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and IR analysis, the betatopic state of TiO2 is connected with the interaction between

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HPW and TiO2. Fig.4C displays the spectrum of In 3d, in which the peaks at 444.98eV

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and 452.48eV are indexed to In 3d5/2 and In 3d3/2, respectively. The presence of In3+ in

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HPW/TiO2-In2S3 is reason for this phenomenon[44]. Noteworthy, the binding energy

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values of In 3d in HPW/TiO2-In2S3 are lower than that in TiO2-In2S3. This phenomenon

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reveals that In2S3 in HPW/TiO2-In2S3 gains electrons after the introduction of HPW.

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Depending on Fig. 4D, the peaks at 35.78eV and 37.88eV are ascribed to W 4f7/2 and W

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4f5/2[34]. It could be found that HPW in HPW/TiO2-In2S3 loses electrons after the

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introduction of In2S3, compared the binding energy values of W 4f in HPW/TiO2-In2S3

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with that in HPW/TiO2. According to the XPS analysis, the electron transfer between

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the components on the surface of HPW/TiO2-In2S3 could be deduced. HPW acts as the

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bridge assisting in the transfer of electronics from TiO2 to In2S3.

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The optical absorption ability is an important factor to evaluate the photocatalytic

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activity of visible-light-driven photocatalysis composites. Fig. 5A shows the UV-Vis

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ACCEPTED MANUSCRIPT DRS results of HPW/TiO2-In2S3, HPW/TiO2, TiO2-In2S3 and TiO2. In addition, the band

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gap energy of samples can be estimated by extrapolating and intersecting the linear

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portion of (Fhν)2 to hν (the Kubelka-Munk equation)[45], as shown in Fig.5B~E. The

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band gap energy of pure TiO2 is 3.22eV which is agreement with the theoretical

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value[46]. Therefore, TiO2 can only absorb UV light (λ<400nm). About TiO2-In2S3, the

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absorption edge extends to visible light region (λ≤650nm) and the band gap energy

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decreases to 2.96eV. This result is ascribed to the introduction of narrow band gap

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semiconductor In2S3 into TiO2[44]. The reason for the blue-shifted absorption edge and

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the increased band gap energy of HPW/TiO2 (3.36eV) is the introduction of HPW as a

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broad band gap semiconductor into TiO2.[47] It could be observed in Fig. 5(A),

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HPW/TiO2-In2S3 (the band gap energy is 3.32eV) has higher absorption in visible light

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region than that of HPW/TiO2, TiO2-In2S3 and TiO2. As Rengifo-Herrera reported, the

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interaction between HPW and TiO2 is responsible for this phenomenon, taking above

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analysis into account[34]. The UV-Vis DRS results could provide an evidence for the

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photocatalytic activity evaluation of samples under visible light.

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The PL is used to detect the recombination of photo-generated carriers. Fig. 6

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presents the PL spectra of HPW/TiO2-In2S3, HPW/TiO2, TiO2-In2S3 and TiO2. All

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samples exhibit the peaks at 469nm and 564nm which are related to the recombination

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of the binding excitons. This result implies that the oxygen-vacancies or impurity

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energy levels exist in samples as the recombination sites[48]. Considering the above

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analysis, the existence of oxygen-vacancies and impurity energy levels may be

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attributed to the lattice defects of TiO2 which caused by the interaction between HPW

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and TiO2 and the existence of In3+ incorporated in TiO2 lattice. In addition, the PL

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intensity order is TiO2 > HPW/TiO2 > TiO2-In2S3 > HPW/TiO2-In2S3, shown in Fig. 6.

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ACCEPTED MANUSCRIPT It is because that different transfer processes of photogenerated carrier occur on the

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samples excited at 350nm. HPW/TiO2 and TiO2-In2S3 are type-II heterojunction which

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can separate the photo-generated carriers. However, the complete transfer processes of

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photogenerated carrier on TiO2-In2S3 could suppress the recombination more effectively

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than the incomplete transfer processes on HPW/TiO2 (Fig. S4). Noteworthily, the

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HPW/TiO2-In2S3 exhibits the lowest PL intensity, implying that the type-III

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heterojunction separates the photogenerated carriers effectively[49].

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8 3.2 Photocatalytic degradation

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The structure of imidacloprid makes it be non-volatile, hydrolytically stable and

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toxic[50]. The photocatalytic degradation of imidacloprid under visible light is operated

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to evaluate the photocatalytic activity of synthesized photocatalysis composites. Fig. 7A displays the variation of imidacloprid degradation efficiency under different

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conditions. Depending on the data in Fig. 7A, the photolysis of imidacloprid under

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visible light is weak. And the decrease in concentration of imidacloprid without

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irradiation is caused by adsorption of HPW/TiO2-In2S3. Hence, it could be inferred that

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the presences of visible light and photocatalysis composites are equal necessary to

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photocatalysis. Under visible light for 5h, HPW/TiO2-In2S3 shows the best

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photocatalytic activity (the degradation efficiency is 82.7%), but HPW/TiO2, TiO2-In2S3

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and TiO2 display low photocatalytic activity with degradation efficiency of 26.7%,

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20.6% and 16.0%, respectively. Those results prove that the constructed type III

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photocatalysis composite heterojunction could availably enhance photocatalytic activity

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and reconform above UV-Vis DRS and PL results. It's worth noting that the type II

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heterojunction also could improve the photocatalytic activity slightly. The distinction

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ACCEPTED MANUSCRIPT between type II and type III comes from that type III heterojunction could retain redox

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ability[11]. In addition, HPW/TiO2 exerts higher photocatalytic activity than TiO2-In2S3.

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The higher absorption in visible light region of HPW/TiO2 is responsible for this

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phenomenon.

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The kinetic study of heterogeneous photocatalytic degradation process could be

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implemented using the Langmuir-Hinshelwood model[51]. The kinetic non-linear

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fitting curves of HPW/TiO2-In2S3, HPW/TiO2, TiO2-In2S3 and TiO2 are shown in Fig.

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6B, conducting with the pseudo-first-order rate model. Ct=C0·exp(-k·t)

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where Ct is the residual concentration, C0 is the initial concentration, k is the pseudo-

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first order rate constant, t is the reaction time.

Based on the kinetic parameters summarized in Table 1, it can be deduced that all the

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degradation processes follow the Langmuir-Hinshelwood pseudo-first-order model. The

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pseudo-first-order degradation rate constant of HPW/TiO2-In2S3 is 6, 7 and 13 times

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higher than that of HPW/TiO2, TiO2-In2S3 and TiO2, respectively. For practical

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wastewater treatment, recyclability test of HPW/TiO2-In2S3 is also needed to be taken

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into account. The results shown in Fig.7C demonstrate that HPW/TiO2-In2S3 has a

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steady photocatalytic activity after 3 cycles.

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3.3 The possible mechanism of photocatalytic degradation

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The analysis of the photocatalytic degradation mechanism is conducive to explain the

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reason for the better photocatalytic activity of visible-light-driven Type III

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photocatalysis composite heterojunction HPW/TiO2-In2S3.

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shown in Fig.8. Based on the band gap energy of HPW(3.7eV)[52], TiO2(3.2eV) and

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In2S3(2.0eV), when HPW/TiO2-In2S3 is excited by visible light, only In2S3 in

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HPW/TiO2-In2S3 can be excited directly (solid straight line in Fig. 8). However, TiO2 in

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HPW/TiO2-In2S3 could be excited partly and indirectly (dash straight line in Fig. 8), due

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to the existence of oxygen-vacancies and impurity energy level. Following the

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excitation process of HPW/TiO2-In2S3, the photo-generated electrons (e-) jump from

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valence band (VB) to conduction band (CB), while the photogenerated holes (h+) are

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respectively generated on VB of TiO2 and In2S3, respectively. Then the photon-

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generated carriers move onto the surface of TiO2 and In2S3, respectively. At photogenerated carriers transfer stage, the potential values of CB bottom and VB

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top of HPW, TiO2 and In2S3 (the ECB is 0.22V, -0.25V and -0.78V vs. NHE, and the EVB

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is 3.70V, 2.95V and 1.18V vs. NHE, respectively)[52, 53] as shown in Fig.8, should be

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considered. It can be inferred that the e- on the CB of In2S3 could react with O2 to

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produce ·O2- radicals, because of the fact that the ECB of In2S3 is more negative than the

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potential of O2/·O2-(-0.33V vs. NHE). Then, the e- transfer from the CB of TiO2 to the

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CB of HPW(dash curve in Fig. 8), while h+ still stay on the VB of TiO2 and In2S3[54].

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Because the EVB of TiO2 is more positive than the potential of ·OH/OH- (2.38V vs.

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NHE) and ·OH/H2O (2.27V vs. NHE)[55], so the h+ on the VB of TiO2 could oxidize

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H2O or OH- to produce ·OH. The e- on the CB of HPW tend to recombine with the h+

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on the VB of In2S3 (blue dash line in Fig. 8)[56]. This deduced process is consistent

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with the XPS analysis results.

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According to the visible light excitation and the photogenerated carriers transfer

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process which happened on HPW/TiO2-In2S3, the superiority of Type III heterojunction

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respond to visible light, as the UV-Vis DRS results. Secondly, the photogenerated

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carriers are separated efficiently, which lengthens the lifetime of photogenerated

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carriers, as revealed by PL results. Thirdly, the intrinsic redox ability of each component

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could be retained to enhance the photocatalytic activity, as the results of the

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photocatalytic degradation tests.

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Depending on the above analysis of the photogenerated carriers transfer process,

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the ·O2- radicals, ·OH radicals and h+ would be conjectured as the main active species

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for degradation of imidacloprid. To verify the conjectures, edetate disodium (EDTA-

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2Na), potassium bromate (KBrO3) and tertiary butanol (BuOH) are added into reaction

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system as scavengers of h+, e- and ·OH, respectively[57]. Then according to variations

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of photocatalytic degradation efficiency, the effects of different active species on the

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photocatalytic reaction could be deduced. Fig. 9 shows that all these additions present

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negative effects on the photocatalytic degradation efficiency. So it can be inferred from

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the results that all of h+, e- and ·OH are the active species in this degradation process of

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imidacloprid, in accordance with above conjectures. What’s more, the decreases of

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degradation efficiency with EDTA-2Na and BuOH as scavengers of h+ and ·OH are

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more than that with KBrO3 as scavengers of e-. That is to say, the h+ and ·OH are the

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main active species. This phenomenon may be caused by the less content (3.79wt.%) of

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In2S3. Depending on above photon-generated carriers transfer process, only the e- on the

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CB of In2S3 can react with O2 to finally produce ·OH which could oxidize imidacloprid.

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However the less content of In2S3 makes the less content of e- show weak influence on

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photocatalysis. Based on the above analysis, the photocatalytic degradation process of

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imidacloprid could be concluded as following.

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ACCEPTED MANUSCRIPT + − HPW / TiO 2 - In 2S3 + hν → h（ TiO 2）+ e（ In 2S3）.......（1） − e（ In2S3）+ O 2 → •O 2 .......... .......... .......... .......... .........（2） -

• O 2 + H 2 O → •OOH + OH - .......... .......... .......... .........（3） -

• OOH + H 2 O → H 2 O 2 + •OH........... .......... .......... .....（4） H 2 O 2 → 2 • OH........... .......... .......... .......... .......... ........（5）

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h + (TiO 2 ) + OH - → •OH........... .......... .......... .......... .....（6） h + (TiO 2) + H2O → •OH + H + .......... .......... .......... ........( 7 )

• OH + Imidaclopr id → Degraded Products ........... .....（8）

+ h（ TiO 2）+ Imidaclopr id → Degraded Products.. .......（9）

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1 4. Conclusion

HPW/TiO2-In2S3 is synthesized by typical sol-gel method and used as a visible-light-

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driven Type III photocatalysis composite heterojunction for the degradation of

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imidacloprid under visible light. Depending on the characterization results, HPW/TiO2-

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In2S3 has higher response to visible light and lower fluorescence intensity than

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HPW/TiO2, TiO2-In2S3 and TiO2. HPW/TiO2-In2S3 displays better photocatalytic

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activity and good recyclability via degradation of imidacloprid, among all the

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synthesized photocatalysis composites. The construction of Type III photocatalysis

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composite heterojunction enhances the photocatalytic activity, because of the effective

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separation of photo-generated carriers and the retention of intrinsic redox ability.

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Fig. 1. XRD patterns of HPW/TiO2-In2S3, HPW/TiO2 and TiO2-In2S3.

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Fig. 2. FT-IR spectra of HPW/TiO2-In2S3, HPW/TiO2, TiO2-In2S3 and TiO2.

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Fig. 3. (A~C) SEM pictures of HPW/TiO2-In2S3, (D) EDX diagram of HPW/TiO2-

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In2S3, (E~F) TEM pictures of HPW/TiO2-In2S3.

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Fig. 4. XPS spectra of HPW/TiO2-In2S3, (A) survey, (B) Ti 2p, (C) In 3d, (D) W 4f.

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Fig. 5. UV-Vis DRS spectra (A) and Kubelka-Munk reflection plots (B~E) of

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HPW/TiO2-In2S3, HPW/TiO2, TiO2-In2S3 and TiO2.

Fig. 6. PL spectra of HPW/TiO2-In2S3, HPW/TiO2, TiO2-In2S3 and TiO2.

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Fig. 7. (A) Photocatalytic degradation of imidacloprid, (B) the kinetic non-linear fitting curves, (C) recyclability test for HPW/TiO2-In2S3.

Fig. 8. Mechanism schematic for the excitation of HPW/TiO2-In2S3 under visible light.

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Fig. 9. Photocatalytic degradation of imidacloprid with HPW/TiO2-In2S3 as

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photocatalyst and the effects of different scavengers to photocatalytic

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degradation reactions.

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ACCEPTED MANUSCRIPT 1 Table 1 The kinetic parameters of photocatalytic degradation

Catalysts

Equation

t1/2 / h

k / h-1

R2

TiO2

Ct=C0·exp(-0.02691t)

25.76

0.02691

0.9711

TiO2-In2S3

Ct=C0·exp(-0.04897t)

14.15

HPW/TiO2

Ct=C0·exp(-0.06041t)

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HPW/TiO2-In2S3

Ct=C0·exp(-0.34829t)

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Highlights (1) The Type III heterojunction HPW/TiO2-In2S3 was designed and synthesized.

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(2) HPW/TiO2-In2S3 showed high and steady photocatalytic activity under visible light.

(3) The advantages of each part in HPW/TiO2-In2S3 were simultaneously highlighted.

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(4) The mechanism of photoexcited process and photocatalytic degradation were

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