ZnSe heterostructural microspheres with enhanced visible light photocatalytic activity

Accepted Manuscript In-situ anion exchange fabrication of porous ZnO/ZnSe heterostructural microspheres with enhanced visible light photocatalytic activity Hairui Liu, Yanchun Hu, Xia He, Husheng Jia, Xuguang Liu, Bingshe Xu PII:

S0925-8388(15)30697-6

DOI:

10.1016/j.jallcom.2015.08.001

Reference:

JALCOM 34995

To appear in:

Journal of Alloys and Compounds

Received Date: 13 March 2015 Revised Date:

31 July 2015

Accepted Date: 1 August 2015

Please cite this article as: H. Liu, Y. Hu, X. He, H. Jia, X. Liu, B. Xu, In-situ anion exchange fabrication of porous ZnO/ZnSe heterostructural microspheres with enhanced visible light photocatalytic activity, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.08.001. 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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Schematic diagram of photocatalytic mechanism ZnO/ZnSe composites under visible light irradiation.

Graphical abstract:

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Porous ZnO/ZnSe heterostructures with different ratios of the two components were fabricated and present enhance visible-light photocatalytic activity for degradation of methylene blue (MB) and 4-nitrophenol (4-NP). The enhanced photocatalytic performance is attributed to fast separation and transport of photogenerated electrons and holes derived from the coupling effect of ZnSe and

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ZnO heterostructure.

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In-situ anion exchange fabrication of porous ZnO/ZnSe heterostructural microspheres with enhanced visible light photocatalytic activity Hairui Liu1,2,3*, Yanchun Hu1, Xia He2, Husheng Jia2,3*, Xuguang Liu2, Bingshe Xu2

Materials, Xinxiang 453007, PR China.

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(1) College of Physics & Electrics Engineering, Henan Normal University, Henan Key Laboratory of Photovoltaic

(2) Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of Technology), Ministry of Education, Taiyuan, Shanxi 030024, P. R. China

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(3) College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, P. R. China.

Abstract

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Porous ZnO microspheres were fabricated by a ultrasonic irradiation technique. Subsequently, through a facile in-situ anion exchange reaction between the ZnO microsphere and sodium selenite, spherical ZnO/ZnSe heterostructures with different ratios of the two components were fabricated. The as-obtained products were characterized by field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray (EDX) spectroscopy, transmission electron

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microscopy (TEM), X-ray diffraction (XRD), and UV-vis spectrometry. The results reveal that the secondary ZnSe nanoparticles are grown on the surface of pre-grown ZnO microspheres. Compared with pure ZnO microspheres, the ZnO/ZnSe hetero-microspheres show enhance visible-light photocatalytic activity for degradation of methylene blue (MB) and 4-nitrophenol

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(4-NP). The enhanced photocatalytic performance is attributed to fast separation and transport of photogenerated electrons and holes derived from the coupling effect of ZnSe and ZnO

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heterostructure. Photoluminescent spectra further indicate that the ZnO/ZnSe heterostructures greatly suppress the charge recombination of photogenerated electron-hole pairs, which would be beneficial to improve their photocatalytic activity. Finally, the photocatalytic mechanism of the ZnO/ZnSe heterostructures is proposed.

Keywords

ZnO microspheres; ZnSe; Visible light; Photocatalysis; *Corresponding Author E-mail: [email protected] (Hairui Liu); [email protected] (Husheng Jia)

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1 Introduction Semiconductor photocatalysis has attracted growing research efforts owing to its important application in counteracting the worldwide energy shortage and environmental pollution [1-3].

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With the steady and fast growing field of nanoscience and nanotechnology, the nanostructural semiconductor metal oxides have become the promising photocatalysts in environmental

remediation, such as TiO2, ZnO, and CeO2 [4-7]. Among these nanostructural semiconductor metal oxides, zinc oxide (ZnO) nanomaterials have been recognized as excellent materials for

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photocatalytic processes owing to their high photosensitivity, high catalytic activity, unique optoelectronic properties, low cost, and environmental friendliness.

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Because photocatalytic process involves the generation of charge carriers such as electrons and holes induced by light, an ideal photocatalyst should have both a wide photo-absorption range and a low recombination rate of the photogenerated carriers. ZnO is only active under UV excitation because of its large energy band gap of 3.2 eV [8-10]. Unfortunately, the fraction of UV light is less than 5% in the total solar spectrum on the earth, which limits the further application of ZnO in visible light region [11]. Another drawback on ZnO is that the recombination rate of

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photogenerated electron-hole pairs is very high, which greatly reduces the photocatalytic efficiency and limits the industrial application of ZnO materials [9, 12-13]. To broaden the range of visible-light photo-response and promote the separation of photogenerated carriers of ZnO, various methods [14-16], such as doping, noble metal deposition

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and coupling with narrow band gap semiconductor, have been designed to enhance the absorption of ZnO photocatalysts in the visible light region. Compared with other methods, ZnO coupled

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with other narrow band gap semiconductors with matched band potentials, such as CdS, Cu2O, AgBr and In2O3 [17-20], have received tremendous interest because of the synergetic effects on

photocatalytic performance, which not only extends the absorption range to visible light region but also promotes electron-hole pair separation under light irradiation. Zinc selenide (ZnSe), as a narrow band gap semiconductor with a band gap of 2.67 eV, matches the visible light spectrum well. Also, its valence and conduction band alignments are staggered relative to those of ZnO [21-22]. When two different semiconductors are coupled together, excited electrons from small band gap semiconductor could transfer into another attached

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semiconductor in the case of proper conduction band potentials, which favors the separation of photo-induced electrons and holes and thus improves the photocatalytic efficiency of semiconductor heterostructure dramatically. Considering the band gap of ZnSe (Eg = 2.67 eV) is

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lower than that of ZnO (Eg = 3.2 eV), while the conduction band (CB) of ZnSe is higher than that of ZnO, an efficient heterostructure could be formed for the separation of photogenerated charge carriers when coupling them together [23-24]. Although a lot of researches on the ZnO/ZnSe hetero-nanorods and nanorod arrays have been reported for photovoltaic devices and

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photocatalysis [25-28], preparation of porous ZnO/ZnSe heterostructures with a tunable proportion by solution method has been investigated rarely towards degradation of organic pollutants until now.

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In this paper, porous ZnO nanostructures were synthesized by a ultrasonic irradiation technique and then converted into porous ZnO/ZnSe composites via in-situ anion exchange process using an aqueous solution containing selenium ions. By varying the solution concentrations, ZnO/ZnSe heterostructure composites with different composition were obtained. A possible formation mechanism for porous ZnO/ZnSe heterostructures was discussed and the

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optical properties and visible light photocatalytic activities of spherical ZnO/ZnSe heterostructures were also examined. Compared with the pure ZnO microspheres, the prepared ZnO/ZnSe porous heterostructure microspheres show higher photocatalytic activities towards the degradation of methylene blue (MB) and 4-nitrophenol (4-NP) under visible light irradiation. Finally, the

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mechanisms of visible photocatalysis in ZnO/ZnSe heterostructure were proposed.

2. Experimental details

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All reagents used to produce the ZnO/ZnSe heterostructures were of analytical grade and without further purification.

2.1 Synthesis of porous ZnO nanostructures First, ZnO microspheres were fabricated by an ultrasonic-assisted hydrothermal method reported in our previous study [29]. In brief, 60 ml of 1 mM zinc acetate (Zn(Ac)2) aqueous solution and 10 ml of triethanolamine (TEA) (99%) were mixed with magnetic stirring and then the mixed solution was subjected to ultrasonic agitation at a power of 200 W with a 20 MHz working frequency for 90 min. When the chemical reaction was finished, the precipitates were washed

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thoroughly with distilled water and ethanol several times by centrifugation and finally dried in a vacuum oven. 2.2 Synthesis of porous ZnO/ZnSe nanocomposites

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The as-prepared ZnO powders (0.1 g) were dispersed in 50 ml of an aqueous solution containing 0.004 M sodium selenite (Na2SeO3, 99%) and different volume of hydrazine monohydrate (N2H4H2O). The powder-dispersed solution was heated to 90 oC and kept at this temperature for 3h. The solution was then cooled, washed several times with distilled water and ethyl alcohol,and

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finally, dried in an oven at 60 oC for 6 h. In addition, for simplicity, pure ZnO microspheres are

denoted as ZS-0, the ZnO/ZnSe heterostructures prepared with the additive amount of hydrazine monohydrate (N2H4H2O) 10, 15, 20, 30 ml are denoted as ZS-1, ZS-2 ZS-3, ZS-4, respectively.

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2.3 Characterization

The morphologies and microstructures of the ZnO/ZnSe heterostructure composites were investigated by field emission scanning electron microscopy (FESEM; JSM-6700F, Japan) and high resolution transmission electron microscopy (HRTEM; JEM-2010, Japan). Chemical compositions were analyzed by X-ray energy dispersive spectroscopy (EDS) equipped to the SEM.

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The crystal structure was determined by powder X-ray diffraction (XRD) with a 0.154178 nm Cu-Kα radiation. The specific surface area of porous ZnO/ZnSe composites was obtained by auto-matic gas-adsorption micrometrics analyzer (TriStar 3000, USA). The ultraviolet-visible light (UV-vis) spectra of samples were measured on a UV-2550 spectrophotometer (Shimadzu,

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Japan). Photoluminescence (PL; Renishaw1000, UK) spectra were measured at room temperature using a He-Cd laser as the excitation light source at 325 nm.

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2.4 Photocatalytic test

The visible light photocatalytic activities of the porous ZnO/ZnSe heterostructures were evaluated by examining the decomposition of MB used as a standard system. First, 3 mg of as-prepared ZnO/ZnSe photocatalysts was ultrasonically dispersed into 25 ml of a 1*10-5 M aqueous MB solution; the mixture was magnetically stirred overnight in the dark to attain equilibrium adsorption on the catalyst surface. After that, the photocatalysis process was carried out at room temperature with a 150 W Xe lamp with a 420 nm cutoff filter as the light source. At given time intervals, 3 ml of suspension was sampled and centrifuged to remove the photocatalyst powders. The concentration of MB was then determined by measuring the absorbance at λmax 554 nm via

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UV-vis spectrophotometer (Shimadzu, UV-1800). Consequently, the degradation rate for MB was calculated according to the change of the absorbance. 4-NP is a toxic organic pollutant that does not absorb visible light, and it is difficult to be photodegraded. For its photocatalytic degradation

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experiment, 3 mg of photocatalyst was dispersed in 25 ml of 4-NP solution (10 mg/l). The residual 4-NP concentration in the treated solution was measured by the Model UV-1800

spectrophotometer monitoring the absorption maximum at λmax=317 nm. Other procedures were similar as those used in the MB photocatalytic degradation experiment.

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3. Results and discussion

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3.1. SEM images and the EDS of the ZnO/ZnSe heterostructure microspheres

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Fig. 1 SEM images of ZnO microspheres (a) and samples ZS-1(b), ZS-2(c), ZS-3(d) and ZS-4(e). The insets are magnified images. Typical EDS of the ZS-3 (f).

Fig. 1(a) is the SEM image of pure ZnO microspheres fabricated by ultrasonic-assisted hydrothermal method. It can be seen clearly that ZnO microspheres have an average diameter

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about 200 nm, and have a relatively rough surface. The morphologies for ZnO/ZnSe heterostructure composites and loading amount of secondary ZnSe nanoparticles grown on ZnO

microspheres are controlled by changing the amount of added N2H4·H2O. When the added amount

of N2H4·H2O is 10 ml, the surface becomes rougher, some holes are observable. When the amount

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of N2H4·H2O is increased to 15 ml (for ZS-2), secondary ZnSe nanoparticles significantly appear

and disperse on the surface of ZnO particles, as shown in Fig. 1(c). It is worth pointing out that the

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spherical ZnO structure is advantageous for uniform growth and distribution of ZnSe nanoparticles on the surface of ZnO. When the additive amount of N2H4·H2O is increased to 20 ml, the holes and porosity density of the product (Fig. 1(d)) are dramatically increased, and the surfaces of ZnO microspheres are almost totally covered by the ZnSe nanoparticles. As shown in Fig. 1(e), when the volume of N2H4·H2O is increased to 30 ml, the morphology of products does not change dramatically, only the porosity increases. It can be seen from Table 1 that the specific

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surface areas SBET and the average pore size gradually increase with the amount of N2H4·H2O, which is beneficial for enhancement of photocatalytic performance. From Fig. 1 it can also be found that all samples retain the microspherical morphology and the size of the microspheres does not increase with the amount of N2H4·H2O. However, the porous

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density of spherical composites and the amount of ZnSe nanoparticles increase with the volume of N2H4·H2O. The ZnSe nanoparticles are uniformly distributed on the surface of each microsphere,

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offering the high level exposure of the nanoparticles’ surface. The morphologies might also have a strong effect on the photocatalytic properties, as will be discussed in the following section. The elemental composition of each sample of ZnO/ZnSe heterostructure was deduced from

the respective energy-dispersive spectrum (EDS). Thus, the EDS of sample ZS-3 in Fig.1(f) confirm the presence of the elements Zn, O and Se in the structure. The Zn, O and Se contents of all samples are shown in Table 1. The atomic ratios of O to Se are about 14:1, 6:1, 3:1 and 1:1 for ZS-1, ZS-2, ZS-3 and ZS-4, respectively.

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Table1: The atomic percentage of elements and the BET specific surface area for samples.

Sample

Atomic %

SBET

Average pore size

Zn

O

Se

(m2/g)

(nm)

ZS-0

52.1

47.9

0

17.0

25.2

ZS-1

51.6

45.2

3.2

26.9

ZS-2

50.8.

42.4

6.8

48.4

ZS-3

49.0

38.5

12.5

67.8

ZS-4

48.8

26

25.2

75.8

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No.

27.6

30.2

32.6

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34.1

3.2. XRD patterns of ZnO/ZnSe heterostructure microspheres

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The crystalline structure of the ZnO/ZnSe heterostructure composites were analyzed using the XRD measurements. XRD patterns of all samples produced are displayed in Fig.2. The XRD pattern for pure ZnO microspheres show that all peaks can be indexed as wurtzite ZnO structures (JCPDS 36-1451). As for the XRD patterns of ZnO/ZnSe heterostructure microspheres, especially for the XRD patterns of ZS-4, there are four new characteristic peaks with 2θ values of 27.4, 45.5, 53.4 and 72.7, corresponding to (111), (220), (311), and (331) crystal planes of cubic ZnSe

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(JCPDS 37-1463), respectively. The intensities of the ZnSe characteristic peaks increase with the N2H4·H2O amount for the increased size and amount of the ZnSe nanoparticles. No impurity peaks are observed, confirming the high purity of the two components.

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♦

ZnSe

♦

♦

♦ ZnO

ZS-3

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Intensity (a.u.)

♦

* *

ZS-4

*

*

*

♦ ♦ ♦

ZS-2

ZS-1

ZS-0

20

30

40

50 60 2θ (degree)

70

80

Fig. 2 XRD patterns of (a) ZnO microspheres (ZS-0) and ZnO/ZnSe heterostructure microspheres (ZS-1, ZS-2, ZS-3, ZS-4)

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Fig. 3(a) shows the TEM and HRTEM images of ZnO microspheres. It can be found that the diameter of a typical ZnO microsphere is about 200 nm and the surface of the sphere is rather rough. It is clear that such protrusive structure on the surface of the sphere is due to the radial

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arrangement of the primary nanoparticles, which may be the nanoparticles with the side lengths of 20- 30 nm. From the high resolution TEM image (Fig. 3(a2)), no dislocations or stacking defects

are observed, revealing that the crystallites are highly crystalline with a lattice spacing of 0.26 nm, corresponding to an interlayer spacing of the (0002) planes in the wurtzite ZnO crystal lattice

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[30-31]. Also, the corresponding selected-area electron diffraction (SAED) pattern (Fig.3 (a3))

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confirms that the microsphere has a single-crystalline wurtzite structure.

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Fig.3 TEM, HRTEM images and SAED pattern of sample ZS-0(a1-a3) and ZS-3(b1-b3).

Fig. 3(b1) shows a low-magnification TEM image of sample ZS-3, the uniform contrast

indicates uniform distribution of ZnSe over individual ZnO microsphere. The Fig. 3(b2) is an HRTEM image of the interface between ZnSe and ZnO, where the fringe spacing corresponds to the cubic phase of ZnSe (111) d-spacing of 0.33 nm [25, 32]. The attached ZnSe nanoparticles are randomly oriented on the edge of the ZnO microspheres. A corresponding selected-area electron diffraction pattern in Fig. 3(b3) confirms that the nanoparticles are poly-crystalline cubic structures of ZnSe. These results confirm that the heterostructures are well formed between ZnO and ZnSe nanoparticles. Given the XRD results, the morphology and microstructure both indicate

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that ZnO/ZnSe heterostructures with different ZnSe loadings have been prepared. 3.3. UV-vis absorbance analysis To evaluate optical properties of pure ZnO microspheres and porous ZnO/ZnSe heterostructure microspheres, UV-vis absorbance spectra were collected at room temperature, as shown in Fig. 4.

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It can be seen that the pure ZnO microspheres show a strong absorbance at around 380 nm and no

significant absorbance for visible light owing to their wide energy gap (~3.2 eV), and the adsorption maxima value for pure ZnSe nanoparticles located at 450 nm, and the corresponding adsorption energy is 2.75 eV. For ZnO/ZnSe composites, all investigated samples exhibit strong

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absorption in the range of visible light originated from the interface of ZnO/ZnSe heterostructure. In comparison with ZnO, the ZnO/ZnSe composites exhibit significantly improved visible light

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absorption capability with increasing the initial weight ratio of ZnSe. Importantly, with increasing the amount of ZnSe added, a red shift of the absorption band edge to a longer wavelength is observed, indicating the bandgap narrowing of ZnO/ZnSe composite. Similar observation has also been reported for other ZnSe modified composites owing to the incorporation of ZnSe [23, 33]. The absorption in the visible-light region leads to more photogenerated electrons and holes participating in the photocatalytic reaction. Thus, the ZnO/ZnSe heterostructures exhibit

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Intensity (a.u.)

promising application as photocatalysts under visible light with respect to pure ZnO.

ZS-0

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ZS-1

ZS-2 ZS-3

ZS-4

Pure ZnSe

300

400

500

600

700

800

Wavelength (nm) Fig.4 UV-visible absorbance spectra of samples ZS-0, ZS-1, ZS-2, ZS-3, ZS-4.

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3.4 The formation mechanism of ZnO/ZnSe heterostructures

In the present study, the Zn source used to form ZnSe on the ZnO surfaces is the ZnO microspheres themselves. The main chemical reaction and possible formation mechanism of ZnO/ZnSe heterostructures are described as follows: Under alkaline conditions, hydroxyl anions

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react with the ZnO surfaces (local dissolution of ZnO), resulting in the release of Zn(OH)42(reaction 1). Thus, the concentration of Zn(OH)42- near the ZnO surfaces locally increases.

ZnOsurface + H 2O + 2OH − → Zn(OH ) 4 2− ⋯⋯⋯⋯⋯ (1)

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SeO32 − + N 2 H 4 → Se + N 2 + H 2O + 2OH − ⋯⋯⋯⋯ (2)

3Se + 6OH − → 2 Se2− + SeO32− + 3H 2O ⋯⋯⋯⋯⋯ (3)

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Se2− + Zn(OH ) 4 2− → ZnSe + 4OH − ⋯⋯⋯⋯⋯⋯ (4)

The SeO32- ions in the reaction solution are reduced first by hydrazine to Se atoms or clusters as the reaction temperature increases (reaction (2)). Highly reactive Se atoms or clusters are then further reduced, and Se2- anions are released by disproportionation reaction in the alkaline solution (reaction (3)). The Se2- ions then react with Zn(OH)42- to form ZnSe by heterogeneous nucleation

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and growth on the ZnO surfaces and as-nucleated ZnSe nanocrystallites (reaction (4)) [26, 34]. Thus, ZnO /ZnSe core-shell heterostructure is formed, as shown in Fig. 5. The growth of ZnSe accompanies a decrease in the number density of ZnO nanoparticles. That is to say that formation and growth of ZnSe nanoparticles are at the sacrifices of ZnO nanoparticles. So, the diameters of

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ZnO/ZnSe heterostructures microspheres remain basically unchanged.

Fig. 5 Schematic illustration of the growth of ZnO/ZnSe heterostructure

3.5. Photocatalytic degradation of MB

To demonstrate the photocatalytic activity of the as-obtained porous ZnO/ZnSe heterostructures for the degradation of organic pollutants, we carried out the photocatalytic degradation of

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methylene blue (MB) as a test reaction. The change of absorption spectra of MB aqueous solution shows the change of its concentration. The initial concentration (C0), final concentration (C),

D% =

C0 − C ×100%⋯⋯⋯⋯⋯ (5) C0

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and degradation rate (D%) have a mathematical expression as follows:

As shown in Fig. 6(a), the control experiments were performed under different conditions: (1) in

the presence of photocatalysts but in the dark and (2) with visible light irradiation but in the

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absence of the photocatalysts. These control experiments reveal no appreciable degradation of MB

over the ZnO/ZnSe heterostructures in the absence of visible light irradiation, indicating that the adsorption of MB on the ZnO/ZnSe heterostructures could be negligible. And, there is no

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appreciable degradation of MB after 6 h in the absence of photocatalysts.

Fig. 6(b) shows the degradation curves of MB on the samples ZS-0, ZS-1, ZS-2, ZS-3, ZS-4. Under visible light irradiation, the residual MB percentage treated by pure ZnO microspheres after 360 min of treatment was ∼70%, which confirmed that pure ZnO microspheres has very weak ability for decolorization of MB. The poor degradation ability of pure ZnO microspheres can be

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ascribe to the wide band gap, because the visible light cannot excite electrons in the valance band to the conduction band. In comparison, after visible light irradiation for 6 h, the degradation efficiency of MB is about 70%, 98%,100% and 92% for the ZS-1, ZS-2, ZS-3 and ZS-4, respectively. Obviously, the ZnO/ZnSe heterostructures show much higher photocatalytic

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activities than pure ZnO microspheres. So MB could be degraded efficiently when visible light is used as the light source in the presence of the ZnO/ZnSe heterostructures photocatalyst. The

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kinetic linear simulation curves of the photocatalytic degradation of MB over the above catalysts showed that the above degradation reactions follow a Langmuir-Hinshelwood apparent first-order kinetic model owing to the low initial concentrations of the reactants. The explanation is described as follows [35]：

r = dC / dt = κ KC / (1 + KC )⋯⋯⋯⋯⋯ (6)

where r is the degradation rate of the reactant (mg/(l.min)), C the concentration of the reactant (mg/l), t the visible light irradiation time, the reaction rate constant (mg/(l.min), and K was the adsorption coefficient of the reactant (l/mg). When the initial concentration (C0) is very low (C0=6

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mg/l for MB in the present experiment), Eq (1) is simplified to an apparent first-order model [36]:

ln C0 / C = κ Kt = κ app t ⋯⋯⋯⋯⋯ (7) where κapp is the apparent first-order rate constant (min-1).

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The determined kapp for different catalysts is summarized in Fig. 6(c). After 6 h irradiation, the order of the photocatalytic activities of ZnO/ZnSe heterostructure microspheres prepared with

different amount of ZnSe nanoparticles is: ZS-3>ZS-2>ZS-4>ZS-1>ZS-0. With the amount increase of ZnSe, the photocatalytic activities of ZnO/ZnSe heterostructure microspheres first

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increase and then decrease. The sample ZS-3 displays the highest photocatalytic activity; the MB

concentration can be reduced to around 6% in 3 h. The reason for this will be discussed in the next

(b) 100

(a) 100 80 60 Without photocatalysis 100

60 40 20

Visible light irradiation

95

ZS-3 ZS-2 ZS-4 ZS-1 ZS-0

Visible light irridiation

80

Dark

C/C0(%)

C/C0 (%)

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section.

Dark

0

5

KZS-4=0.00706 KZS-1=0.00352

4

KZS-0=0.00142

3 2 1 0 0

60

120

180

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-1

3

Time (h)

KZS-3=0.01876 KZS-2=0.01078

6

ln(C0/C)

2

4

5

6

-60

Time (min)

240

300

0

60

120

180

240

300

360

Time (min)

(d)

1nd run

100

2nd run

3nd run

80 C/C0(%)

7

1

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(c)

0

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90 -1

60 40 20

360

0

0

2

4

6 0 2 4 Time (h)

6 0

2

4

6

Fig. 6 (a)Degradation profiles of MB in the presence of the photocatalysts but in the dark and with visible light irradiation but in the absence of the photocatalysts. Degradation profiles (b) and Kinetic linear simulation curves (c) of MB photocatalytic degradation samples: ZS-0, ZS-1, ZS-2, ZS-3 and ZS-4. (d) Photocatalytic activity of the ZnO/ZnSe heterostructures (ZS-3) for MB degradation with three times of cycling uses.

Moreover, the stability of the ZnO/ZnSe heterostructures (ZS-3) was examined for degradation of MB during a three cycle experiment, which is very important for application of the ZnO/ZnSe heterostructures in environmental technology. As shown in Fig. 6(d), the photocatalytic degradation of MB over the ZS-3 under visible light irradiation is effective. More importantly, it is

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indicated that these ZnO/ZnSe heteroarchitecture photocatalysts with high photocatalytic activity could be easily separated and recovered by sedimentation, and would greatly promote their practical application to eliminate the organic pollutants from wastewater.

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It is well-known that MB could absorb visible light and may induce the photosensitization effect. To exclude the photo-sensitization effect from MB, 4-NP was also used as a target compound to evaluate the photocatalytic performance of these ZnO/ZnSe heterostructures. Fig. 7(a) shows the photocatalytic degradation curves of 4-NP by ZnO and ZnO/ZnSe composites. For

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control experiments, no adsorption of 4-NP is found without light and photocatalyst, while under

visible light irradiation, no noticeable degradation of 4-NP is observed with direct photolysis. Pure ZnO showed negligible photocatalytic degradation of 4-NP after 360 min of treatment. While, a

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significant decrease of 4-NP concentration is observed, approximate 69.7%, 85.6%, 92.5% and 83.8% of 4-NP are degraded by ZS-1; ZS-2; ZS-3 and ZS-4, respectively, and their corresponding rate constant (k) were determined at ∼0.0035, 0.0053, 0.0072 and 0.0050, respectively (as shown in Fig. 7(b)). Moreover, it is found that the highest degradation activity can be obtained with ZS-3 photocatalyst. The photocatalytic performance ranking of these ZnO/ZnSe composites on the

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degradation of MB and 4-NP was the same, which further confirms ZS-3 exhibiting the highest photocatalytic degradation rate and demonstrates that a suitable ZnSe modification can efficiently promote the photocatalytic activity of ZnO.

(a)

Visible light irridiation

60

40

20

0

-60

0

60

120

180

KZS-3=0.00717 KZS-2=0.00529 KZS-4=0.00502 KZS-1=0.00348 KZS-0=0.00038

2.5 2.0

ZS-3 ZS-2 ZS-1 ZS-4 ZS-0 No photocatalyst

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Dark

(b) 3.0

ln(C0/C)

C/C0(%)

80

EP

100

1.5 1.0 0.5 0.0

-0.5 240

300

360

Time (min)

0

60

120

180

240

300

360

Time(min)

Fig. 7 Photocatalytic degradation of 4-NP profiles (a) and Kinetic linear simulation curves (b) by samples: ZS-0, ZS-1, ZS-2, ZS-3 and ZS-4 under visible light irradiation.

The better photocatalytic activity in the case of ZnO/ZnSe heterostructures is probably attributed to (I) the larger surface area, and (II) the specific heterostructure which favors the separation of photo-induced electrons-holes pairs in ZnO/ZnSe heterostructures (Fig. 7(b)).

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Because of the spherical morphology, the surface area of the ZnO/ZnSe heterostructure microspheres is higher, and the porosity density increases with the increase of ZnSe nanoparticles, which is highly advantageous to photocatalytic activity [37-38]. In addition, the formation of

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ZnO/ZnSe heterostructure by coupling ZnSe with ZnO expands the absorption range within the visible light spectrum because of the narrow band gap of ZnSe [24, 26]. Furthermore, the conduction band (CB) bottom and the valence band (VB) top of ZnO lie at -0.4 and 2.8 eV with

respect to normal hydrogen electrode (NHE), and those of ZnSe at -0.64 and 2.16 eV [28, 39],

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respectively. As illustrated in Fig. 7(a), both the CB bottom and the VB top of ZnO lay below the

CB bottom and VB top of ZnSe, respectively. When ZnO and ZnSe are coupled together to form a heterostructure, the ZnSe could be excited under visible light irradiation and the generated

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electrons in the ZnSe then migrate to the conduction band (CB) of ZnO. Consequently, the efficient charge separation increases the lifetime of the charge carriers and enhances the efficiency of the interfacial charge transfer to the adsorbed substrates, resulting in higher activity of the ZnO/ZnSe heterostructures photocatalyst. The better separation of photogenerated electrons and holes in the ZnO/ZnSe heterostructures can also be confirmed by PL emission spectra (Fig. 7(b))

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of ZS-0, ZS-1, ZS-2, ZS-3 and ZS-4. It can be found from PL spectra that the ZnO/ZnSe heterostructures exhibit much lower emission intensity than ZS-0, and the PL intensity of ZnO/ZnSe heterostructures varies and appears the minimum intensity for the sample ZS-3. By comparison, the photocatalytic activities and PL intensities of the samples show an opposite

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variation tendency. This finding confirms the occurrence of separation effects between

(b)

ZS-0

ZS-1

ZS-4

Intensity (a.u.)

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photo-induced electrons and holes [40-42].

ZS-2

320

340

360

ZS-3

380

400

420

440

Wakelength (nm)

Fig.7 Schematic diagram of the energy band structure and electron-hole pair separation in the ZnO/ZnSe heterostructure under visible light irradiation. (b) PL emission spectra of ZS-0, ZS-1, ZS-2, ZS-3 and ZS-4.

Then, the reactions involved on the surface of ZnSe and ZnO for the photodegradation can be

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simply written as follows. Under visible light irradiation, photogenerated electrons in ZnSe move freely to the ZnO, meanwhile, the photogenerated holes are left in the valence band of ZnSe. Dissolved oxygen molecules react with ZnO surface electrons (e-) to yield superoxide radical

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anions, •O2-, while photogenerated holes can oxidize OH− to give hydroxyl radicals OH•; the latter is an extremely strong oxidant for degeneration of organic chemicals [43-44].

ZnSe + hν → ZnSe(CBe − + VBh + )⋯⋯⋯⋯⋯⋯⋯⋯ (8) ZnSe(e − ) + ZnO → ZnSe + ZnO (e − )⋯⋯⋯⋯⋯⋯⋯ (9) ZnO(e − ) + O2 → ZnO + •O2 − ⋅⋯⋯⋯⋯⋯⋯⋯⋯⋯ (10)

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h + + OH − → OH • ⋯⋯⋯⋯⋯⋯⋯⋯⋯⋯⋯⋯⋯⋯ (11) OH • + MB → deg raded min eralized prroduct ⋯ (12)

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Among samples of ZnO/ZnSe hetero-microspheres, why does ZS-3 show highest photocatalytic activity? For the ZS-1 and ZS-2, with less ZnSe nanoparticles, the contact area between the two semiconductors is smaller, which results in less photogenerated electrons and holes taking part in the photocatalytic reactions, so that their photocatalytic activity is lower. For ZS-4, its porosity density and specific surface area are highest, which should increase the accessibility of the active sites on the ZnSe nanoparticles surface, and thus enhance the photocatalytic activity, but it did not

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come out that way. Possible factors affecting photocatalytic performance of ZnO/ZnSe heterostructures are as follows: firstly, according to UV-vis absorbance spectra analysis, the samples ZS-1, ZS-2, ZS-3 and ZS-4 exhibit enhanced visible-light absorption. It is evident from

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the results that ZnO/ZnSe heterostructures absorb more visible light than pure ZnO. With the increase of ZnSe nanoparticles, surface atoms of the ZnSe nanoparticles replace ZnO and the

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specific surface area also increases, which can improve the amount of reactive adsorption/desorption sites for photocatalytic reactions. However, the photocatalytic activity of ZS-4 does not show an increase when the content of ZnSe is further increased, although the visible-light absorption is enhanced, indicating that the enhanced visible-light absorption is not the only factor influencing the activity of the heterostructures. Secondly, the molar ratios of ZnO to ZnSe also affect the photocatalytic activity. It should be particularly noted that the higher addition ratio of ZnSe would result in the deterioration of photocatalytic activities of the ZnO/ZnSe composites owing to the decreased amount of primary photoactive ingredient ZnO. The superior reactivity of the ZnO/ZnSe heterostructures is observed with appropriate molar ratios of ZnO to

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ZnSe, suggesting that there is a critical ratio for such a positive synergistic effect. Above this critical ratio, excessive ZnSe covers the active sites of ZnO and hinders the electron transfer on the interfaces of ZnO/ZnSe heterostructure microspheres, thus inhibiting the photoactivity. Last

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but not least, an effective close contact surface in the ZnO/ZnSe heterostructures has great influence on the photocatalytic activities. In this work, the in-situ anion exchange fabrication route succeeded in realizing a close contact between ZnSe nanoparticles and ZnO microspheres in ZnO/ZnSe heterostructures. However, with the amount increase of ZnSe nanoparticles

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instead of ZnO for ZS-4, the effective close contact surface between ZnO and ZnSe further

reduces, which goes against the separation of photogenerated electron-hole pairs in the ZnO/ZnSe heterostructures.

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4. Conclusions

In summary, porous ZnO/ZnSe heterostructure microspheres with enhanced photocatalytic activities for the degradation of MB dye under visible light irradiation were fabricated via the ultrasonic irradiation and in-situ anion exchange process. The obtained spherical ZnO/ZnSe heterostructures present much better visible light photocatalytic activities than pure

ZnO

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microspheres because of the large surface area and the specific heterostructure, which favors the separation of photo-induced electron-hole pairs and transport of electrons within the catalyst. Moreover, ZnO/ZnSe heterostructure microspheres obtained with different loadings of ZnSe exhibit different photocatalytic activity, the amount of ZnSe plays an important role in

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determining photocatalytic activity and an optimal value exists in a given range. It is expected that the ZnO/ZnSe heterostructure microspheres with high photocatalytic activity would greatly

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promote their practical application for the degradation of organic pollutants.

Acknowledgments

This work was supported by Program for Changjiang Scholar and Innovative Research Team in University (IRT0972); International Science &Technology Cooperation Program of China (2011DFA52290, 2012DFR50460) and the National Natural Science Foundation of China (Nos: 21402042; U1304110; 11304083).

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Highlights ► Spherical ZnO/ZnSe porous composites were fabricated by in-situ anion exchange.

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► ZnO/ZnSe composites exhibited enhanced visible-light photocatalytic activity. ►The matching band gap improve the separation of photogenerated electrons and holes.

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► ZnO/ZnSe composites showed good photostability compared with ZnO particles.