Construction of novel 3D ZnO hierarchical structure with Fe3O4 assist and its enhanced visible light photocatalytic performance

Journal Pre-proof Construction of novel 3D ZnO hierarchical structure with Fe3 O4 assist and its enhanced visible light photocatalytic performance Xiaole Zhao (Conceptualization) (Methodology) (Investigation) (Data curation) (Writing - original draft), Jiadong Li, Xinyu Cui, Yajun Bi, Xiaojun Han (Supervision) (Project administration) (Funding acquisition) (Writing - review and editing)

PII:

S2213-3437(19)30671-2

DOI:

https://doi.org/10.1016/j.jece.2019.103548

Reference:

JECE 103548

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

29 September 2019

Revised Date:

13 November 2019

Accepted Date:

14 November 2019

Please cite this article as: Zhao X, Li J, Cui X, Bi Y, Han X, Construction of novel 3D ZnO hierarchical structure with Fe3 O4 assist and its enhanced visible light photocatalytic performance, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103548

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Construction of novel 3D ZnO hierarchical structure with Fe3O4 assist and its enhanced visible light photocatalytic performance

Xiaole Zhao, Jiadong Li, Xinyu Cui, Yajun Bi, Xiaojun Han*

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State Key Laboratory of Urban Water Resource and Environment, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China

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*Corresponding author. E-mail: [email protected]

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Graphical Abstract

Highlights:    

1.First time to assemble ZnO nanorods on the surface of Fe3O4 nanosphere to obtain a novel 3D F-ZnO hierarchical structure 2.The 3D F-ZnO hierarchical structure exhibits enhanced visible light photodegradation performance toward Congo red in waste water. 3.3D F-ZnO possesses good adaptability in practical application and can be reused by magnetic field. 4. A possible synergetic effect mechanism of enhanced visible light photocatalysis property was proposed

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Abstract

In order to improve the visible light photocatalytic ability of ZnO semiconductor, a novel 3D Fe3O4/ZnO hierarchical structure (defined as 3D F-ZnO) has been

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successfully constructed by a Fe3O4 nanosphere assisted hydrothermal method. The forming mechanism was proposed. Its morphology, crystal structure, and visible light

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response property were studied via SEM, XRD, and DRS. The diameter of 3D FZnO was about ~4.5 µm and it was assembled by numerous of asymmetric cone-

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shaped ZnO nanorods on a Fe3O4 nanosphere. 3D F-ZnO structure exhibit good optical response property and enhanced visible light photodegradation ability toward Cong red

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(84%). 3D F-ZnO also displays good ability on the degradation of tetracycline (90%) and methylene blue (60%) under visible light irradiation. Influence of heavy ions, acid groups, water sources on degradation CR was investigated, whose results further

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confirm that 3D F-ZnO possesses good adaptability in practical application. The 3D FZnO can be reused by the magnet and exhibit excellent stability. At last, the enhanced

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visible light photodegradation ability was explained via its unique morphology, PL and EIS. A synergetic effect mechanism between unique morphology and Fe3O4 was proposed. The primary reactive species of ·OH, ·O2–, and h+ were also confirmed in the photodegradation process. All results suggest that this novel 3D F-ZnO hierarchical structure may have great potential in water treatment and other photocatalytic fields. Keywords: 3D hierarchical structure; ZnO; visible light; photodegradation; Fe3O4

nanosphere.

1. Introduction Nowadays, human is meeting more and more environmental and energy problems. Among them, water pollution is one of the serious challenges for the whole world. Despite many emerging contaminants were produced, dyes are still the main pollutant in the waste water. As an important pollutant, dyes have attracted much attention for harmful impacts on human health and the ecological environment since their non-

adsorption[2-7],

precipitation[8],

ion-exchange[9],

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biodegradability, toxicity, and potential carcinogenicity[1]. Several methods such as Fenton

reaction[10,

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photodegradation[12] and even biodegradation[13], have been developed for dyes

removal. Among them, the photodegradation process is considered a promising

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technology for low-energy and green reaction processes [14, 15]. Therefore, more and more photocatalysts have been developed for degradation of organic pollutants in recent

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CuO[22], and their composites[12].

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years, including ZnO[16], Ag2CO3[17, 18], Ag3PO4[19], Ca(OH)2[20], V2O5[21],

In those developed materials, ZnO is an important alternative photocatalyst for water purification considering its green properties, cheap price, excellent electron mobility

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and durability[23-28]. However, ZnO has so wide band gap (3.37 eV) leading to they absorb mainly in the UV region of sunlight[29]. So that improved performance of ZnO

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to overcome this limitation is important for accelerating its practical application for water treatment under visible light illumination. Although many methods have been

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developed to improve its visible light photocatalytic performance, such as coupling other materials[30, 31], doping elements[28, 32], making defect[33, 34] and changing morphology[35-37], it is still a big problem for application and needs to further develop new method to improve visible light performance of ZnO semiconductor. In addition, the manipulation and separation of photocatalyst from the waste water is another important problem. Normally, centrifugation[38], filtration[39] and magnetic field[40] are usually used in water treatment to reuse photocatalysts. To avoid bringing

new pollution and reduce the energy consumption of mechanical separation, the trend is to develop efficient photocatalysts with magnetic function [41, 42]. Consequently, magnetic Fe3O4, a perfect alternative, was always chosen as a coupling material with photocatalyst since its superior magnetic leading to the easy recycle of the combined photocatalysts [40, 43-45]. Furthermore, high conductivity and narrow bandgap of Fe3O4 lead to it can improve visible light photoactivity of hybrid semiconductor[46]. Hence, a lot of magnetic ZnO photocatalysts have been developed via combing with Fe3O4[47, 48]. Nevertheless, most of ZnO nanomaterials were just modified[49],

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implanted by Fe3O4[50], or simply contact each other[41]. Fe3O4 is very easy to fall out of. To our knowledge, it hasn’t been investigated that assembling ZnO nanorods on the surface of Fe3O4 nanosphere to obtain visible light responsive and magnetic

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photocatalyst.

Aiming at above problems, some relevant experiments was designed in this paper.

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Herein, we developed a Fe3O4 nanosphere assisted hydrothermal method to construct ZnO nanorods into 3D hierarchical structure. Their morphology, crystal structure and

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optical property were studied. They exhibit improved visible light photocatalytic performance on degradation CR. The influence of various conditions on degradation CR was investigated. At last, a possible photocatalytic mechanism was proposed.

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2.1 Materials

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2. Experimental Section

All the chemicals were purchased from sinopharm chemical reagent Co., Ltd (China)

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and without further purification, including ferric chloride hexahydrate (FeCl3·6H2O, ≥ 99.0%)，sodium acetate (CH3COONa, ≥ 99.0%)，ethylenediamine (C2H8N2, ≥ 99.0%)，ethylene glycol ((CH2OH)2, ≥ 99.5%)，ethanol (CH3CH2OH, ≥ 99.5%)， zinc acetate dihydrate (Zn(CH3COO)2·2H2O, ≥ 99.0%), potassium hydroxide (KOH, ≥ 85%), hexamethylenetetramine (HMT, ≥ 99.0%), sodium chloride (NaCl, ≥ 99.8%), sodium sulfate (Na2SO4, 99%), sodium nitrate (NaNO3, ≥ 99.0%), sodium molybdate

(Na2MoO4, 99%), manganese chloride (MnCl2, 99%), nickel chloride (NiCl2, ≥ 98.0%), cadmium chloride (CdCl2, 99%), sodium bicarbonate (NaHCO3, ≥ 99.5%), and disodium oxalate (Na2C2O4, ≥ 99.8%).

2.2 Synthesis of 3D F-ZnO hierarchical structure and fusiform ZnO nanorods 2.2.1. Preparation of Fe3O4 nanosphere 1.5 g FeCl3·6H2O was dissolved in 45 ml ethylene glycol. 6 g CH3COONa was

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dissolved in 5 ml ethylenediamine under vigorous stirring for 1 h. These two solutions were mixed and stirred for another 1 h. Subsequently, the mixed solution was

transferred into a Teflon stainless autoclave and heated at 200 ºC for 2 h. The product

was washed by ethanol and water three times in turn after heating. The Fe3O4

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nanosphere was dried in an oven overnight. The crystal structure and morphology of

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Fe3O4 were recorded by the XRD pattern and SEM image (Fig. S1 and Fig. S2).

2.2.2. Preparation of 3D F-ZnO hierarchical structure and fusiform ZnO

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nanorods

First step, the Fe3O4 nanosphere was modified with a ZnO seed layer on the surface.

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Simply, 0.0016 g of Fe3O4 nanoparticles was added in 100 ml of 7.5 mM zinc acetate ethanol solution, then mechanical agitation for 1h. Following that, 50 ml of 22.5 mM KOH ethanol solution was added into above solution under vigorous stirring, then

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heating for 2 h in 60 ºC condition. The above process was repeated for three times. Subsequently, the modified Fe3O4 was separated and washed by water for three times

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to use in second step. Second step (Fig. 1. Path 1), the modified Fe3O4 was added 50 ml of HMT (4.2 mM) and Zn(CH3COO)2 (4.2 mM) mixed aqueous solution and, then ultrasonic for 5 minutes. This mixed solution was heated in 72 ºC for 12 h. The product was 3D F-ZnO hierarchical structure and washed with water. Product was dried in 60 ºC oven. The fusiform ZnO nanorod was prepared in the HMT/zinc acetate aqueous solution without modified Fe3O4 nanospheres (Fig. 1. Path 2).

Fig. 1. Schematic illustration of the synthesis process of 3D F-ZnO hierarchical structure and fusiform

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ZnO nanorods

2.3 Characterizations

The morphology of the synthesized materials was observed via scanning electron

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microscopy (SEM) (Quanta 200FEG, FEI, USA). The powder X-ray diffraction (XRD) in reflection mode (Cu Kα radiation) on a diffractometer (D/Max-RB, Rigaku, Japan)

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was used to characterize the crystal structure of products. UV-Vis spectra were obtained using a Cary 60 spectrophotometer (Cary 60, Agilent, USA). PL spectra were carried

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out on a Fluorescence spectrometer (LS55, Perkin Elmer, USA). The diffuse reflectance spectra (DRS) were recorded on a UV-Vis spectrophotometer (U-4100, HITACHI, Japan). BaSO4 was used as a reference. The photocurrent response and electrochemical

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impedance spectroscopy were achieved on Autolab electrochemical workstation (PGSTAT320N, Metrohm, Switzerland) with an electrolyte solution of Na2SO4 (0.5 M),

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using three electrodes system; i.e. a Pt wire as a counter electrode, a saturated calomel electrode as a reference electrode and a modified glassy carbon electrode as working

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electrode, respectively.

2.4 Photocatalytic activity measurements of photocatalysts 2.4.1. Photodegradation of CR under visible light In order to evaluate photocatalytic activity of 3D F-ZnO and fusiform ZnO nanorods, 0.025 g of photocatalyst was added in 50 ml of 20 mg L–1 CR aqueous solution. The

suspension was stirred in the dark for 1 h. Then, the adsorption/desorption equilibrium solution system was irradiated under a metal halide lamp (500 W) equipped with an ultraviolet cutoff filter (λ ≥ 420 nm). During the irradiation process, the circulating water maintained at 25 ºC. At certain intervals, 4 ml of suspension was taken out and centrifuged to obtain supernatant. The supernatant was analyzed by a UV-Vis spectrophotometer to measure the concentration of remaining CR.

2.4.2. The influence of heavy metal ions on photodegradation CR: In order to test the influence of heavy metal ions on Congo red, 1 mM of MnCl2, NiCl2,

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and CdCl2 were dissolved in 50 ml of 20 mg L–1 CR aqueous solution, respectively.

Before irradiation, the mixed solutions were continuously stirred in dark for 1 h to reach adsorption equilibrium. Then other photodegradation processes were carried out at

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normal photodegradation conditions.

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2.4.3. The influence of acid groups on photodegradation CR:

In order to investigate the effect of acid groups on degradation CR, we prepared CR

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aqueous solution containing 1 mM of NaCl, Na2SO4, NaNO3, or Na2MoO4 respectively. The photodegradation process was carried out in 50 ml CR aqueous solution containing Cl–, SO42–, NO3–, or MoO42– respectively. Before visible light irradiation, the mixed

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solutions were continuously stirred in dark for 1 h to reach adsorption equilibrium.

2.4.4. The influence of water sources on photodegradation CR:

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To observe the influence of water sources for photodegradation on CR. 20 mg L–1 CR aqueous solution with tap water, Songhua river water (in Heilongjiang province, China),

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and distilled water was prepared, respectively. The other procedure was same as above process.

2.4.5. Photodegradation ability of 3D F-ZnO on other organic contaminates in waste water: The methylene blue (MB) and tetracycline (TC) were also chosen as the degradation targets to confirm its broad-spectrum photodegradation activity. The photodegradation

experimental processes were same as degrading single CR aqueous solution. The initial concentration of MB and TC is 10 mg L–1 containing 0.5 g L–1 F-ZnO. Before illumination, the mixed solutions were continuously stirred in dark for 1 h to reach adsorption equilibrium. Then the solution system was irradiated under a metal halide lamp (500 W) equipped with an ultraviolet cutoff filter (λ ≥ 420 nm). The other processes were same as above.

2.4.6. Stability and reusability of 3D F-ZnO hierarchical structure:

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The stability of the 3D F-ZnO powder was confirmed through repeating the photodegradation processes for five times. After each cycle, the F-ZnO power was reclaimed by a magnet and washed with ethanol and water in turn for several times and then dried at 60 ºC for the next test.

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2.4.7. Trapping experiments

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The active species produced in photocatalytic process were investigated by trapping experiment. The sodium bicarbonate (NaHCO3 4 mM, ·OH scavenger) and disodium

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oxalate (Na2C2O4 4 mM, h+ scavenger) were dissolved in CR aqueous solution before illumination. The photodegradation process was carried out under an N2 environment to demonstrate ·O2– generation. The other processes were same to above.

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3. Results and discussion

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3.1 Characteristics of photocatalyst

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3.1.1 The crystal structure analysis

Fig. 2. XRD patterns of products which were prepared by hydrothermal method with and without modified Fe3O4 nanospheres assist, respectively.

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The crystal structure of synthesized products was detected by powder X-ray diffraction

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(XRD) on a diffractometer. As seen in Fig. 2, the XRD pattern of product obtained through hydrothermal method without Fe3O4 nanosphere process is well matched with its crystal phase (JCPDS NO.99-0111), indicating that the synthesized product is ZnO

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semiconductor. Compared to XRD pattern of products synthesized in presentence of Fe3O4 nanosphere, there are apparent peaks of Fe3O4 (JCPDS NO.89-0866) except

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peaks of ZnO, indicating the product is a composite containing ZnO and Fe3O4. The difference of their peak areas reveals that there is a large quantity of ZnO and a little of

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Fe3O4 in the composite of Fe3O4/ZnO. Above results indicate that Fe3O4/ZnO composite and pure ZnO were obtained.

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3.1.2 The morphology analysis

Fig. 3. (a-b) Low and high magnification SEM images of 3D F-ZnO. (c) The model image of 3D F-ZnO. (d) The SEM image of broken 3D F-ZnO. (e-f) The partial enlarged views. (g) The energy dispersive Xray spectrum (EDX) of broken 3D F-ZnO center for Zn, Fe and O elements. (h-k) SEM images of

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fusiform ZnO nanorods produced under same condition without Fe3O4 nanosphere and corresponding EDX mapping images (O and Zn elements). Inset of (e) is Fe3O4 nanosphere.

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The morphology of Fe3O4/ZnO photocatalyst was observed via scanning electron microscopy (SEM) and results are shown in Fig. 3. Through Fe3O4 nanosphere (with a ZnO seed layer, Fig. S3) assisted hydrothermal method, we successfully obtained a large quantity of novel 3D flower-like structure which was assembled with many uniform asymmetric cone-shaped ZnO nanorods on the surface of Fe3O4 nanosphere (Fig.3 a-c). Therefore, we defined Fe3O4/ZnO hierarchical structure as 3D F-ZnO, whose diameter is approximate ~4.5 µm. Fig. 3d is a broken 3D F-ZnO, clearly

indicating the uniform length (~2.3 µm) and asymmetric cone-shaped structure of single zinc oxide nanorod. Fig. 3e is a center of broken 3D F-ZnO, we can easily find that the center is a Fe3O4 nanosphere, because its diameter is consistent with the prepared Fe3O4 (inset in Fig. 3e). On the other hands, the Fe sharp peak in EDX of this center (Fig. 3g) and the apparent Fe3O4 peak in XRD pattern (Fig. 2) strongly further demonstrate asymmetric cone-shaped zinc oxide nanorod assembled on a Fe3O4 nanosphere. The morphology of product obtained via hydrothermal process without modified Fe3O4 nanosphere was also observed by SEM and corresponding results are

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shown in Fig. 3h-k. It is interesting that a lot of fusiform ZnO nanorods were obtained if modified Fe3O4 nanosphere was absence in synthesis process. The Fig. 3i-k are corresponding EDX mapping images representing oxygen and zinc elements,

respectively. Above all observed results indicate that modified Fe3O4 nanosphere plays

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a key role to change ZnO nanorod morphology from fusiform type into asymmetric cone-shaped structure and further assist them to assemble into 3D hierarchical structure.

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3.1.3 The growth mechanism of 3D F-ZnO

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Scheme 1. The growth mechanism of 3D F-ZnO

In order to explain the growth mechanism of 3D F-ZnO, a schematic illustration was shown in scheme 1. Firstly, the prepared Fe3O4 was modified with a layer of ZnO seed on the surface according to the previous wet chemical route[51, 52]. Fe3O4 nanospheres with ZnO seed layers were depicted in TEM and EDX graph in Fig. S3. They confirmed that a ZnO seed layer was imprinted on the surface of Fe3O4 nanosphere. Previous reports also observed this similar phenomenon that diverse crystals can coalesce under

hydrothermal conditions[53-55]. Secondly, modified Fe3O4 nanospheres were distributed in HMT and Zn(CH3COO)2 aqueous solution. The ZnO nanorod was gradually formed on the surface of Fe3O4 nanosphere with increasing time under 72 ºC. ZnO seed layer on the surface of Fe3O4 nanosphere has the property to grow ZnO rod on it under a long time hydrothermal condition[51, 52, 56].

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3.1.4 The optical response analysis

Fig. 4. (a) UV-Vis diffuse reflectance absorption spectra (DRS) of 3D F-ZnO hierarchical structure and

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fusiform ZnO nanorod. (b) Plots of Kubelka-Munk function versus the energy of light, which is transformed from DRS.

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Fig. 4 displays UV-Vis diffuse reflectance absorption spectra (DRS) and corresponding transformed plots by Kubelka-Munk function (eqn. 1) of products. The Kubelka-Munk

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equation is as below:

(𝐴ℎ𝜐)2⁄𝑛 = 𝑘(ℎ𝜐 − 𝐸𝑔 )

(1)

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where A, h, υ, k and Eg are the absorption coefficient, Planck's constant, light frequency, proportionality constant and band gap, respectively.[14, 57] The value of n is 1

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corresponding to the direct adsorption of ZnO[58]. The fusiform ZnO nanorod shows an adsorption edge at 413.3 nm and its band gap is 3.0 eV which is a lower value than that well known 3.37 eV[29]. The visible light absorption intensity and range of 3D FZnO significantly increased, and an apparent red shift in comparison with the fusiform ZnO nanorods appeared. These results indicate that 3D F-ZnO hierarchical structure has excellent visible light response property. This improved optical response performance may attribute to unique morphology of 3D F-ZnO [33, 59] and coupling

Fe3O4[46]. The enhanced visible light response property of 3D F-ZnO can result in generation of more photoinduced carries under visible-light irradiation, which is benefit for improved photocatalytic activity visible-light illumination. [46]

3.2 Photocatalytic activity of photocatalysts

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3.2.1 The visible light photodegradation ability of 3D F-ZnO on CR

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Fig. 5. The visible light photodegradation ability of 3D F-ZnO and fusiform ZnO nanorod toward Congo red (CR) under visible light irradiation. (a) Photodegradation curves. (b)The histogram of removal rate. (c) Photodegradation kinetics curves. (d) The histogram of kinetics rate constant. Reaction condition: CR initial concentration is 20 mg L–1; catalyst concentration is 0.5 g L–1; volume of CR solution is 50

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ml; illumination is under visible light (metal halide lamp (500 W) cutoff filter λ ≥ 420 nm); reaction temperature is 25 ºC.

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The photocatalytic ability of 3D F-ZnO and fusiform ZnO nanorods was evaluated by degradation Congo red (CR) under visible light illumination. Specifically, the initial concentration of CR in 50 ml aqueous solution was 20 mg L–1 and the added photocatalyst was 0.025 g. Before each irradiation, the mixed solutions were continuously stirred in dark for 1 h in order to reach adsorption equilibrium (Fig. S4). The results are shown in Fig. 5. Fig. 5a is photodegradation rate curves of CR against

the time. The blue, green, and black lines represent 3D F-ZnO structures, fusiform ZnO nanorods, and absence of catalyst, respectively. As can be clearly seen in Fig. 5a, the photodegradation rate is faster and more remarkable with 3D F-ZnO structure than fusiform ZnO nanorods with increasing time. In the condition of absence catalyst, the degradation of CR is negligible compared with the condition of presence catalyst, indicating the high stability of CR under visible-light irradiation. Their corresponding removal percentage is 84%, 38% and 2%, respectively (Fig. 5b). These results apparently indicate that 3D F-ZnO structure exhibits excellent visible light

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photocatalytic ability. Following that, we studied the photocatalytic reaction kinetics and the pseudo-first-order reaction kinetics was applied to simulate reaction process. The below is the simulation equation: − ln(𝐶 ⁄𝐶0 ) = 𝑘

(2)

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where k is the pseudo-first-order rate constant, C is remaining concentration in water,

and C0 is initial concentration. As shown in Fig. 5c, it is noted that the plots of ln(C/C0)

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versus irradiation time are linear with high correlation coefficients (R2 > 0.986 for 3D F-ZnO, R2 > 0.992 for fusiform ZnO nanorods), which indicates that the

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photodegradation process obeys pseudo-first-order model very well. From the simulation results, the rate constants were obtained and displayed in Fig. 5d. The 3D F-

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ZnO photocatalyst exhibits the maximum value that is 0.0179. This value of photodegradation rate constant is 105 times than that of without catalyst (the rate constant is 0.00017) and 4.4 times than that of fusiform ZnO nanorod (the rate constant

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is 0.0041). These results further confirm that 3D F-ZnO structure has excellent visible

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light photocatalytic ability.

3.2.2 The influence of various conditions on CR and photodegradation ability of 3D F-ZnO on other contaminates Generally, in practical waste water, various organic contaminates, heavy metal ions, acid groups and different water sources must be considered when a novel photocatalyst is applied. Because whether the 3D F-ZnO can photodegrade different contaminates and whether influence of various pollutants on photodegradation efficiency of 3D F-

ZnO could be accepted are important for photocatalyst in practical application. In order to evaluate the adaptability of 3D F-ZnO in practical water treatment, the various

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conditions were simulated in our lab.

Fig. 6. (a) Influence of heavy metal ion (Mn2+, Ni2+, and Cd2+ are 1 mM, respectively) on

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photodegradation of CR under visible light irradiation. (b) Influence of acid group (Cl–, SO42–, NO3–, or MoO42– are 1 mM, respectively) on photodegradation of CR under visible light irradiation. (c) Influence of water sources for removal rate of CR under visible light irradiation. (d) Visible light photodegradation

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ability of 3D F-ZnO on tetracycline (TC, 10 mg L–1) and methylene blue (MB, 10 mg L–1) under visible light irradiation. CR initial concentration is 20 mg L–1 in a, b and c. The other conditions are same: catalyst concentration is 0.5 g L–1; volume of reaction solution is 50 ml; illumination is under visible

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light (metal halide lamp (500 W) cutoff filter λ ≥ 420 nm); reaction temperature is 25 ºC.

Heavy metal ions: In order to investigate influence of heavy metal ions on CR, we

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prepared CR aqueous solution containing Mn2+, Ni2+, or Cd2+, respectively. The other photodegradation processes were same. As results shown in Fig. 6a, photodegradation rate of CR trend to increase when metal heavy ions were added in the CR aqueous solution. The corresponding removal percentage is 100%, 92.4%, and 86.8% when Mn2+, Ni2+, and Cd2+ were in CR aqueous solution, respectively. The photodegradation process was simulated by pseudo-first-order model and the experimental data fit well to it (shown in Fig. S5a). The rate constants are 0.03296 (Ni2+), 0.02334 (Cd2+), and

0.05051 (Mn2+), respectively (Fig. S5b), which are 1.84, 1.30, and 2.82 times than that without heavy metal ions (rate constant is 0.01790). Above results indicate that photodegradation efficiency on CR is enhanced with different degree and the order is Mn2+ > Ni2+ > Cd2+. This may due to the Mn2+, Ni2+, or Cd2+ can act as a photogenerated electron-transfer site to prevent the recombination of photoinduced carriers and prolong carrier’s lifetime [60-63]. Acid groups: The influence of acid groups on photodegradation CR was also studied. The photodegradation processes were carried out in 50 ml CR aqueous solution

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containing Cl–, SO42–, NO3–, or MoO42–, respectively. As can be seen in Fig. 6b, photodegradation efficiency is a slight decrease revealed by photodegradation curves

and removal percentages (76%, 78.1%, 83.1%, 82.8%, respectively). The photodegradation reaction kinetics curves of pseudo-first-order model and

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corresponding photodegradation rate constants displayed similar trend (Fig. S6). This weak influence may be due to acid groups were competitive with dye for the adsorption

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sites on the surface of catalyst[64-66]. Thereby, the catalytic efficiency of 3D F-ZnO is

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slightly reduced. Apparently, this weak decrease of photodegradation efficiency has limited impact on 3D F-ZnO application.

Water sources: The water source is may another factor influencing on the

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photodegradation CR. Therefore, CR aqueous solutions were prepared by deionized, tap, and Songhua river (Heilongjiang province, China) water, respectively. Other

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processes were same. The results are shown in Fig. 6c. It reveals that tap water sources slightly impact photodegradation efficiency comparing with deionized water. When the

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waste water of CR was prepared with Songhua river water, the removal percentage of CR decreased from 84% to 71% in 2 h under visible light irradiation. This may due to Songhua river water is slightly muddy and dirty, leading to the light not easily irradiated into inner of waste water. In a word, the water source also can’t hinder application of 3D F-ZnO hierarchical structure. Photodegradation ability of 3D F-ZnO on tetracycline (TC) and methylene blue (MB): The photodegradation processes were same to degrade CR except replacement of CR

with MB (10 mg L–1) or TC (10 mg L–1). With 180 min visible light irradiation, the results are shown in Fig. 6d. The yellow and purple lines represent the photodegradation processes of MB and TC, respectively. As can be clearly seen in Fig. 6d, the photodegradation rates are remarkable with 3D F-ZnO structures. The removal percentages of MB and TC are up to 60% and 90%, respectively. Their photodegradation reaction kinetics was also studied. As observed in Fig. S7, the experimental data fit well to the pseudo-first-order model with high correlation coefficients (R2 > 0.958 for TC, R2 > 0.997 for MB). The photodegradation rate

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constants of 3D F-ZnO for TC and MB are 0.0038 and 0.0187, respectively. These results indicate that the 3D F-ZnO hierarchical structure can photodegrade different contaminates.

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3.2.3 Analysis of degradation products of CR

Fig. 7. The possible degradation products of CR on 3D F-ZnO after 2 h degradation.

To investigate the possible degradation pathway of CR on 3D F-ZnO, the degradation products were detected by mass spectrometry (MS) after degradation for 2 h under visible light. According to the MS result (Fig. S8) and previous reports[67-71], possible degradation products of CR are listed in Fig. 7 The peaks (m/z) from 437.2 to 188.1 in

Fig. S8 are attributed to the intermediates of A–I, respectively. Comparing structures of degradation products and undecomposed CR (Fig. S9), it is suggested that the degradation of CR on 3D F-ZnO may occur by following steps: (i) the cleavage of benzene ring, (ii) the cleavage of azo linkage (–N=N–), (iii) the cleavage of various C– C, and C–N bonds of the chromophore groups, and (iiii) the cleavage of C-S bond.

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3.2.4 Stability and reusability of 3D F-ZnO

Fig. 8. (a) The photograph of recycle via magnet. (b) Photocatalytic degradation stability test of 3D FZnO against CR under visible light irradiation. Reaction condition: CR initial concentration is 20 mg L– ; catalyst concentration is 0.5 g L–1; volume of CR solution is 50 ml; illumination is under visible light

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(metal halide lamp (500 W) cutoff filter λ ≥ 420 nm); reaction temperature is 25 ºC.

To study photocatalytic stability and reusability of 3D F-ZnO hierarchical structure,

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five recycle experiments were carried out in the same conditions. The 3D F-ZnO could be easily reused through its magnetism with a magnet (Fig. 8a). As clearly saw the

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recycle experimental results in Fig. 8b, the recovered 3D F-ZnO sample for the degradation of CR exhibits high photocatalysis stability and good recyclability since

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the removal percentage of CR is just a slight decline even after five cycling tests. According to above experimental results and analysis, 3D F-ZnO structure possesses excellent and stable visible light photocatalytic performance toward CR. Especially, it is easy to reuse via a magnet, which has great potential to reduce the cost of recycle. In order to show excellent performance of 3D F-ZnO on CR, other ZnO photocatalysts on degradation of CR was listed in table S1. From table S1, it is noted that 3D F-ZnO not only can degrade CR with a high removal rate under visible light irradiation but also

can be reused by magnetic field method which is low energy consumption. Therefore, 3D Fe3O4/ZnO possesses apparent superiority and application potential comparing with previous ZnO photocatalysts.

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3.3 The photocatalytic mechanism of 3D F-ZnO

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Fig. 9. (a) Schematic illustration of multi-absorption within 3D F-ZnO compared with fusiform ZnO nanorod and visible light absorption of ZnO tip-edge. (b) Room temperature photoluminescence (PL) spectra of samples. (c) Electrochemical impedance spectroscopy of 3D F-ZnO hierarchical structures and fusiform ZnO nanorods. (d) Effects of various scavengers on the degradation of CR over new style 3D 1

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F-ZnO. Reaction condition in d: CR initial concentration is 20 mg L–1; catalyst concentration is 0.5 g L– ; Na2C2O4 and NaHCO3 concentration are 4 mM, respectively; volume of CR solution is 50 ml;

illumination is under visible light (metal halide lamp (500 W) cutoff filter λ ≥ 420 nm); reaction

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temperature is 25 ºC.

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Fig. 10. Possible mechanism of 3D F-ZnO toward degradation pollutants under visible light irradiation.

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It is well known that excellent photocatalytic performance is determined by the number and separation of carriers. To explain this good visible light photocatalytic performance

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of 3D F-ZnO hierarchical structure, firstly, the influence of unique morphology on light

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absorption was analysed. As shown in Fig. 9a, the unique 3D hierarchical structure of 3D F-ZnO allows multiple absorption and scattering of the incident light[72-74], which can enhance light absorption. The unique tip-edge of asymmetric cone-shaped ZnO

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nanorod is also benefit for visible light absorption [33, 59]. So more photoinduced carries were produced on 3D F-ZnO surface under visible light irradiation, which lead

to

enhanced

photocatalytic

activity[46].

Furthermore,

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subsequently

photoluminescence spectra (PL) and electrochemical impedance spectrum (EIS) of

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photocatalyst were recorded to investigate recombination and separation processes of photoinduced e−/h+ pairs. Fig. 9b is the PL spectra of photocatalysts, which was excited by 365 nm laser at ambient environment. The weaker PL intensity means the lower recombination rate of photoinduced e−/h+ pairs in photocatalyst. Compared to fusiform ZnO nanorods, lower PL intensity of 3D F-ZnO confirms that recombination rate of e−/h+ pairs on 3D F-ZnO is much lower than that on fusiform ZnO nanorod. This may due to two reasons: first, the CB level of Fe3O4 (1.0 eV vs NHE)[74] is lower than that

of ZnO (– 0.31 eV vs NHE)[47] , photogenerated electrons are transferred from the ZnO CB to the Fe3O4 CB. Second, the Fe3+ of Fe3O4 nanosphere in 3D F-ZnO structure can transfer photoinduced e− to O2[50]. These two reasons both can help photoinduced e− fast transfer leading to recombination rate of e−/h+ pairs were suppressed. The low recombination rate of e−/h+ pairs results in improved photocatalytic activity of 3D FZnO structure. Fig. 9c is EIS which can study the charge transfer resistance and charge separation efficiency. As can be observed in the ESI, the charge transfer resistance of 3D F-ZnO is smaller than that of fusiform ZnO nanorods, as the Nyquist semicircle of

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3D F-ZnO is obviously smaller, demonstrating transfer efficiency of charge was enhanced. This is attribute to the high conductivity of Fe3O4 nanosphere in 3D F-ZnO structure[46]. The increased photocurrent response further signifies efficient transfer

and separation of photoinduced charge carriers in 3D F-ZnO (Fig. S10)[28]. All above

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results reasonably explain the enhanced photocatalytic activity of 3D F-ZnO.

In general, the process of photocatalytic organic pollutant degradation is involved with

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holes (h+), hydroxyl radicals (·OH), and superoxide radicals (·O2–) which usually act

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as the primary reactive species. To explore the role of these reactive species, some trapping experiments of reactive species were performed. Two different quenchers,[18] i.e., sodium oxalate (Na2C2O4 4 mM, h+ scavenger) and sodium bicarbonate (NaHCO3

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4 mM, ·OH scavenger) were added into the solution before the photodegradation of CR. The photodegradation process was carried out under an N2 environment to confirm ·O2–

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generation. Fig. 9d exhibits results that the addition of Na2C2O4, NaHCO3 and under N2 environment caused obviously decrease of the photocatalytic activity compared with

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the absence of scavenger, which prove that h+, ·OH, and ·O2– are generated during the process of photocatalysis, in particular, the ·O2– and h+ are main reactive species for the degradation of CR. According to above experimental results and theoretical analysis, a possible synergetic catalytic mechanism of 3D F-ZnO hierarchical structure in visible light degradation system was recommended, as illustrated in Fig. 10 When the 3D F-ZnO structure was added into the CR solution, it can effectively absorb and transfer more

visible light energy, resulting in the much production of photoinduced e−/h+ pairs on the surface of the 3D F-ZnO. Furthermore, the e–/h+ pairs in 3D F-ZnO could be easily separated due to Fe3O4 can act as an acceptor to fast transfer photoinduced e−. Subsequently, the electron was transferred and react with oxygen dissolved in solution to produce ·O2–. The leftover h+ interacted with H2O or hydroxyl group on the surface of 3D F-ZnO to generate the ·OH. The ·OH, ·O2–, and h+ can all efficiently degrade CR. Moreover, the 3D hierarchical structure of F-ZnO can also prevent structural aggregation, consequently to keep the large catalytically active surface area and the

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interstitial channels that permit the diffusion of chemicals into the structure, leading to further improve the efficiency of surface reactions. Thereby, in 3D F-ZnO hierarchical

structure system, this synergetic effect of morphology and Fe3O4 makes it have remarkable visible light photocatalytic property. In addition, the 3D F-ZnO hierarchical

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structure can be easily removed from the dispersion and reused via a magnet which can

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avoid bring new pollution and reduce energy consumption of mechanical separation.

4. Conclusions

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The novel 3D F-ZnO hierarchical structure was successfully synthesized by Fe3O4 nanosphere assisted hydrothermal method. The 3D F-ZnO hierarchical structure is

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assembled by many asymmetric cone-shaped ZnO nanorods on the surface of Fe3O4 nanosphere which possesses good visible light responsive performance. Their enhanced visible light photocatalytic activity for degradation of CR was confirmed. The

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photodegradation efficiency was slightly influenced by various conditions except that heavy metal ions were discovered can further enhance photocatalytic ability, indicating

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good adaptability of 3D F-ZnO in practical water treatment. 3D F-ZnO hierarchical structure can also degrade tetracycline (TC) and methylene blue (MB) under visible light irradiation. Their good visible light photocatalytic ability was due to synergetic mechanism between unique morphology and Fe3O4. All results suggest that this novel 3D hierarchical structure of F-ZnO may have a great potential in water treatment and other photocatalytic fields.

Acknowledgements This work was supported by the National Key R & D Program of China (2017YFA0207203), the National Natural Science Foundation of China (Grant No. 21773050), the Harbin Distinguished Young Scholars Fund (No. 2017RAYXJ024), State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (No. 2017DX05), and Key Laboratory of Micro-systems and Microstructures Manufacturing of Ministry of Education, Harbin Institute of Technology (No.

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2017KM006).

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