CeO2@HNTs heterostructure for efficient photocatalytic degradation of tetracycline

Accepted Manuscript Well-dispersed nebula-like ZnO/CeO2@HNTs heterostructure for efficient photocatalytic degradation of tetracycline Zhefei Ye, Jinze Li, Mingjun Zhou, Huiqin Wang, Yue Ma, Pengwei Huo, Longbao Yu, Yongsheng Yan PII: DOI: Reference:

S1385-8947(16)30960-3 http://dx.doi.org/10.1016/j.cej.2016.07.014 CEJ 15463

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

24 February 2016 5 July 2016 5 July 2016

Please cite this article as: Z. Ye, J. Li, M. Zhou, H. Wang, Y. Ma, P. Huo, L. Yu, Y. Yan, Well-dispersed nebulalike ZnO/CeO2@HNTs heterostructure for efficient photocatalytic degradation of tetracycline, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej.2016.07.014

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Well-dispersed nebula-like ZnO/CeO2@HNTs heterostructure for efficient photocatalytic degradation of tetracycline Zhefei Yea, Jinze Lia, Mingjun Zhoua, Huiqin Wangb, Yue Maa, Pengwei Huoa,c,*, Longbao Yu a,*, Yongsheng Yana,c

a School of Chemistry & Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China b School of Environment, Jiangsu University, Zhenjiang 212013, PR China c Institute of Green Chemistry and Chemical Technology, Jiangsu University, Zhenjiang 212013, PR China

* Corresponding author

Tel.: +86 511 8879 0187

E-mail address: [email protected] 1

Fax. : +86 511 8879 1800

Abstract

A series of nebula-like ZnO/CeO2@HNTs heterostructure photocatalysts were synthesized via a novel one-step wet-calcination method for degradation of tetracycline (TC) under simulated solar irradiation. TEM, HRTEM, XRD, FT-IR, XPS, DRS, PL, PC, EIS and ESR techniques were applied for characterization of samples. And the potential structure of new photodegradation products was studied by HPLC-MS analysis. The characterization results revealed the enhanced photocatalytic activity of ZnO/CeO2@HNTs heterostructure photocatalysts due to the delayed recombination of photogenerated electron-hole pairs. Moreover, surface oxidation state analysis demonstrates that Ce3+ and Ce4+ states coexist in CeO2, which enhance the separation of electron-hole pairs though cerium oxide shifting between CeO2 and Ce2O3 under photocatalytic process. In addition, the effects of the calcinating temperature, the molar ratio of ZnO to CeO2 and the dosage of HNTs on photocatalytic activity of ZnO/CeO2@HNTs heterostructure photocatalysts were researched and the best possible values have been found. Furthermore, the photodegradation mechanism was systematically investigated by active species trapping experiment. It revealed that ·OH radicals play a little role while hole and ·O2- are the more major reactive species in the photodegradation process.

Keywords: ZnO, CeO2, Heterostructure, HNTs, Tetracycline, Wet-calcination

2

1. Introduction Currently, with modern medical technology booming and the fast development of business and economics, the usage of antibiotics is becoming increasingly frequent in very wide field of application. However, the pollution situation of antibiotics in environment is growing worse, which is mainly caused by the abuse and overuse of antibiotics for the lack of scientific guidelines [1]. Antibiotic residues in aqueous have brought potential threat to the environmental security, even in low concentrations, including antimicrobial resistance to microbes, disturbances and perturbations in ecosystems, and potential risks to people’s health through drinking water and the food chain [2-5]. There is an urgent need, therefore, to seek for effective ways to deal with the antibiotic residues. Several treatment methods have been investigated to deal with antibiotic residues, such as membrane filtration, activated carbon adsorption, advanced

oxidation

process,

microbial degradation and

electrolysis

[6,7].

Photocatalytic technology, as one kind of advanced oxidation process which has merits of both almost completed degradation and easy to operate, is a green technology to treat the antibiotic wastewater. Semiconductor nanomaterials for removal of toxic pollutants in wastewater and responding to the problem of energy crisis have becoming a hot topic over the world [8]. TiO2, ZnO, WO3 and CdS etc. are the most commonly used semiconductor nanomaterials for wastewater treatment due to their better optical and photocatalytic performances [9-13]. Zinc oxide (ZnO), a wide and direct band gap semiconductor, is used in many fields, because of their high catalytic efficiency, low cost and environmental sustainability. Nevertheless, the recombination of photogenerated charge carriers is too fast, which results in low quantum efficiencies and limits the photocatalytic potential of ZnO [14]. It is well accepted that the photoinduced electron and hole (e-/h+) pairs will be produced from the semiconductors under UV or visible light irradiation, which significantly influences the photocatalytic activity. However, the rapid recombination of photoinduced e-/h+ pairs in semiconductors would weaken the photocatalytic activity of these semiconductors. Thus, various strategies have been 3

taken to deal with these problems, including the method of coupling two semiconductors with different redox energy levels to enhance the separation of photoinduced e-/h+ pairs due to an inter particle electron transfer process (IPET) proposed by Serpone et al. [15-17]. Hence, combining ZnO with other semiconductors having suitable band structures to occur IPET process could make it possible to develop a ZnO photocatalyst with high activity. Li’s group had demonstrated a simple route to obtain ZnO/CdS nanoarrays by the combination of electrodeposition and chemical deposition, and the ZnO/CdS nanoarrays exhibited much improved photocatalytic performances compared to bare ZnO [18]. Khanchandani et al. prepared a one-dimensional ZnO/In2S3 core/shell nanostructure, finding that this nanostructure exhibited significantly enhanced photocatalytic activity due to the formation of heterojunctions between the two semiconductors [14]. However, compared with the common semiconductor oxides, sulfides are unstable and easy to resolve despite having satisfactory activity. Cerium oxide (CeO2), one of the most active oxide catalysts in the rare earth oxide series, is widely used in several key applications, including luminescence, catalysts, gas sensors, fuel cells and adsorbents [19]. Even though CeO2 can absorb light in the near UV region and slightly in the visible region, regarded as a promising material for photocatalysis, CeO2 has a wide band gap energy limiting its applications in visible region [20]. However, CeO2 has suitable valence and conduction band edges well-matching those of ZnO for the separation of photoinduced e-/h+ pairs because of the IPET process. In that context, the strategy of coupling with ZnO and CeO2 has been adopted to enhance their performance for utilization of solar light. Moreover, many researchers devote to loaded supporters and prove the importance of supporters in photocatalysts [21]. Therefore, it needs a suitable material as the carrier for prepared well-dispersed photocatalysts. In this case, Halloysite nanotubes (HNTs), a kind of naturally clay silicate minerals, have exhibited promising results as a catalyst support because of their highly specific surface area, outstanding stability, resistibility against organic solvents, ease of 4

disposal or reusability and excellent adsorptivity [22-24]. Deposited the ZnO/CeO2 nanocrystals system onto HNTs is a promising method to block their aggregation. In recent years, a number of new approaches to synthesize the ZnO/CeO2 composites have been reported, such as chemical vapor deposition [25], electro spinning [26], electro deposition [27], hydrothermal method [28], soft solution route [29], the solid-stabilized emulsion route [30], precipitation method [31] and sol-gel method [32]. HNTs being used as catalyst supports, we have used a novel wet-calcination solution for the preparation of coupled ZnO/CeO2@HNTs composites. Among various synthetic strategies, wet-calcination solution has the advantage of being controllable, simple, inexpensive and suitable for large-scale production. Compared with conventional calcination method, wet-calcination solution has not only simple process and convenient operation, but also good dispersibility and excellent ability of preparing mesoporous structures. With the directly fast heating of calcination and the rapid evaporation of water, a porous and fragmented structure could be formed on the surface of HNTs. The presence and movement of water not only provides the source of oxygen, but also guarantees the formation of uniformly distributed ZnO/CeO2 clusters and the generation of mesoporous structures. It is widely recognized that the particle morphology of ZnO can significantly impact on the reactivity of the ZnO surface [33,34]. The nebulous clusters composed of ZnO/CeO2 nanocrystals well-dispersing on the surface of HNTs provide numerous active sites for photodegradation. Herein, a series of ZnO/CeO2@HNTs heterostructure photocatalysts have been synthesized by one step process through wet-calcination method for utilization of photodegrading tetracycline (TC) under simulated solar light. It is found that these ZnO/CeO2@HNTs composites show enhanced photocatalytic activity for degrading TC due to efficient charge separation. In addition, we also demonstrated the influences of calcinating temperature, molar ratio of ZnO to CeO2, dosage of HNTs and pH values on photodegrading TC of these catalysts.

2. Experimental 5

2.1. Materials

Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, AR), Cerium(III) nitrate hexahydrate (Ce(NO3)3·6H2O, AR) were all purchased from Sinopharm Reagent Co., Ltd. Benzoquinone (BQ, AR), triethanolamine (TEOA, AR) and isopropanol (IPA, AR) were all purchased from Shanghai Chemical Reagent Co., Ltd. Halloysite nanotubes (HNTs) was purchased from Zhengzhou Jinyangguang Chinaware Co., Ltd. All reagents were used as received without further purification in this study. Tetracycline (TC) was obtained from Shanghai Shunbo Biological Engineering Co., Ltd. Deionized water was used throughout this work.

2.2. Preparation of the samples

ZnO/CeO2@HNTs

heterostructure

photocatalysts

were

prepared

using

wet-calcination method, which was modified according to the procedures of previously reported literatures [35,36]. In a typical procedure, 1.0422 g HNTs was dispersed in 10 mL deionized water, and this slurry was sonicated for 30 min. Then, 1.1161 g Zn(NO3)2·6H2O and 0.5430 g Ce(NO3)3·6H2O (atomic ratio of Zn:Ce = 3:1) were added, while the slurry was stirred. The slurry was then transferred into a porcelain crucible, calcined at 500 ℃ for 3 h to obtain the ZnO/CeO2@HNTs composite. For comparison, studies on the photocatalytic activity of ZnO/CeO2@HNTs composite have been processed by adjusting calcinating temperature (300 ℃, 400 ℃, 500 ℃, 600 ℃), the atomic ratio of Zn:Ce (10:1, 5:1, 3:1, 2:1, the total quantity of ZnO and CeO2 remains the same) and the dosage of support (0.5, 1, 1.5-fold above value). As Table 1 shows, for the sake of discussion, the samples were labeled S1 (300 ℃, 3:1, 1-fold), S2 (400 ℃, 3:1, 1-fold), S3 (500 ℃, 3:1, 1-fold), S4 (600 ℃, 3:1, 1-fold), S5 (500 ℃, 10:1, 1-fold), S6 (500 ℃, 5:1, 1-fold), S7 (500 ℃, 2:1, 1-fold), S8 (500 ℃, 3:1, 0.5-fold) and S9 (500 ℃, 3:1, 1.5-fold). The pure ZnO, CeO2, ZnO@HNTs and CeO2@HNTs were prepared via the same route. 6

Table 1 Calcination temperature and reagent dosages of different photocatalysts. 2.3. Characterization The structures and size of photocatalysts were observed via the transmission electron microscope (TEM) on a Model Tecnai 12 TEM (Philips, Holland) and the high resolution transmission electron microscope (HRTEM) on a Model Tecnai G2 F30 S-Twin TEM. Powder X-ray diffraction (XRD) measurements of the samples were performed with a D/max-RA X-ray diffractometer (Rigaku, Japan) using Cu-Kα radiation in the scanning angle range of 5-80° at a scanning rate of 5°·min-1. Fourier transform infrared spectra (FT-IR) were studied by a Nicolet Nexus 470 FT-IR spectrometer (Nicolet, USA) with KBr pellets in the 4000-400 cm-1 region. Surface electronic states were analyzed on X-ray photoelectron spectroscopy (XPS, PerkinElmer PHI 5300), and the binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. The optical properties of the products were carried out by UV-vis diffuse reflectance spectroscopy (DRS) with a Shimadzu 2450 UV-vis spectrophotometer (Shimazu, Japan) in the wavelength range of 200-800 nm and BaSO4 powder was used as the internal standard. The photoluminescence (PL) spectra for solid samples were obtained on a F4500 (Hitachi, Japan) photoluminescence detector. Electron spin resonance (ESR) was introduced to investigate the ·OH and ·O2- radicals, which carried on a Brucker A300 ESR spectrometer at room temperature, and the ESR signals of spin-trapped paramagnetic species were verified with 5,5-diamethyl-1-pyrroline N-oxide (DMPO). The spectra were recorded during a 450 W xenon lamp solar simulator irradiation at selected times (about 5 s after turning out the light). A TAS-986 (Purkinje, China) atomic absorption spectrometer was used for determination of the zinc and cerium contents of the repeated reaction solutions and photocatalysts. The sample solutions were prepared through acid dissolution according to the previously reported method [37]. The test results are shown in Table 2. 7

Table 2 Analysis results of ZnO and CeO2 contents of different photocatalysts.

2.4 Photoelectrochemical measurement

For investigating the photoelectrochemical properties of as-prepared samples, a CHI852C electrochemical station with a solar simulator (Newport 69920, 300 W Xenon lamp) was used, in which the S1, S2, S3 and S4 on FTO substrates (the active range of 1 cm2) were used as working electrodes, platinum wire as counter electrode, Ag/AgCl (saturated KCl) as reference electrode, respectively. The transient photocurrent (PC) response measurements were carried out in 0.5 M Na2SO4 aqueous solution as cathodic and anodic electrolyte. A bias potential of 0.5 V vs. SCE was applied on the anode for photoelectrochemical measurements under on-off light conditions. Electrochemical impedance spectroscopy (EIS) was performed in 0.5 M Na2SO4 solution with the frequency range from 0.1 Hz to 100 kHz at 0.5 V. The amplitude of applied sine wave potential in each case was 5 mV which was carried out using a ZENNIUM electrochemical workstation (Zahner Instruments, Germany), and all electrochemical signals were recorded by a CHI660 B electrochemical analyzer (Chen Hua Instruments, Shanghai, China).

2.5. Photocatalytic measurements

Photocatalytic activities of the as-fabricated samples were evaluated by the degradation of TC aqueous solution under simulated solar irradiation. The light source for the photocatalysis was provided by a 300 W Xe lamp equipped with an IR cut filter to remove most of IR irradiation (780-1100 nm). The artificial solar light used here is in the region of 320-780 nm, which matches well with the natural sunlight in the UV-vis region. In the experiments, 30 mg ZnO/CeO2@HNTs composites were dispersed in 100 mL 20 mg·L-1 TC solution in a cylindrical reactor with recycling 8

water to maintain temperature constant (25 ℃). Prior to simulated sunlight irradiation, the dispersions were stirred for 30 min in dark to achieve adsorption and desorption balance. At a certain time interval, about 4 mL of the dispersion was withdrawn and immediately centrifuged. The concentration of TC was determined by a UV-vis spectrophotometer at a maximum absorption wavelength of 357 nm. The blank experiments under irradiation and in the dark were performed by following the same steps without photocatalyst.

2.6. HPLC-MS procedure

Mass spectral data were obtained with an HPLC-MS instrument consisted of a Perkin-Elmer (Norwalk, CT) Series 200 HPLC coupled to a Finnigan MAT900 mass spectrometer (Bremen, GR). Analyses were carried out by using a Discovery C18 column supplied by Supelco (Agilent Technologies, Waldbronn, Germany). The separation of the compounds was carried out with a mobile phase of acetonitrile:water (50:50, v/v) at 25 ℃. The injection volume in the HPLC system was 15 µL and single online detection was carried out in PDA detector, at 270 nm. The mass spectrometer was equipped with an ESI source and operating in positive mode. The flow rate of nitrogen sheath and auxiliary gas were 40 and 5 (arbitrary units), respectively. MS experiments were performed on mass-selected precursor ions in the range of m/z 100-600. The data acquisition was carried out by using Xcalibur data system.

3. Results and discussion 3.1. TEM and HRTEM analysis Structures and size of photocatalysts were investigated by TEM and shown in Fig. 1. It is clearly seen that nebulous and well-dispersed clusters composed of ZnO/CeO2 nanocrystals with an average particle size within 10 nm were obviously loaded on the surface of the HNTs supporters which had an outer diameter of 25-100 nm, a length of about 100-600 nm and a hollow cavity of about 10-20 nm in diameter. The good 9

distribution of round ZnO/CeO2 clusters on the surface of HNTs is shown in Fig. 1 (a), indicating that the wet-calcination is a promising new method to prepare finely-dispersed ZnO/CeO2@HNTs heterojunction. It can be observed that the spheroidal ZnO/CeO2 clusters are made up of tiny ZnO/CeO2 nanocrystals, as revealed in Fig. 1 (b). This porous and fragmented structure was formed in a condition of directly fast heating of calcination, which possesses numerous active sites for photocatalyst. As the calcinating temperature rise, the loading of ZnO/CeO2 clusters has become more diffuse and the shape of ZnO/CeO2 clusters has also become irregular and unshaped, which can be seen in Fig. 1 (c), (e) and (g). It is worthwhile to note that the ZnO/CeO2 nanocrystals are intimately contact and more compacted, as shown in Fig. 1 (d), (f) and (h). Moreover, the surface of HNTs is rough and the structural dehydroxylation of HNTs has caused structure deformation after calcination at higher temperature. Meanwhile, the formed ZnO/CeO2 heterostructure was observed from HRTEM images, as shown in Fig. 2. The heterostructure was composed of polycrystal ZnO and CeO2 with an average particle size within 10 nm, which can be clearly seen in Fig. 2 (b). It is obvious that the heterostructures of ZnO/CeO2 were formed, and the interplanar distances between adjacent lattices planes for ZnO/CeO2 heterostructure were calculated and marked in Fig. 2 (c). The interplanar spacing between neighboring (102) lattice planes of ZnO nanocrystal is 0.19 nm, matching well with the (102) planes of hexagonal ZnO (JCPDS file No. 36-1451), and the one between adjacent (111) lattice planes of CeO2 nanocrystal is 0.31 nm [38]. The spheroidal ZnO/CeO2 clusters are made up of tiny ZnO/CeO2 nanocrystals because of the nature of polycrystalline, which can be further demonstrated in the fast Fourier transform patterns of Fig. 2 (d). In addition, the particle size distribution of ZnO/CeO2 nanocrystals based on HRTEM revealed that ZnO and CeO2 may have difference in the particle sizes, as shown in Fig. 3.

Fig. 1 Typical low-resolution (left) and high-magnification (right) TEM image of the (a, 10

b) S1, (c, d) S2, (e, f) S3 and (g, h) S4. Fig. 2 (a) TEM image of S3 photocatalyst, (b) HRTEM image of heterostructured ZnO/CeO2 nanocrystals in S3 photocatalyst, (c) the corresponding enlarge figure of heterostructure and (d) fast Fourier transform patterns of ZnO, CeO2 and heterostructured ZnO/CeO2 nanocrystals. Fig. 3 Particle size distribution of ZnO/CeO2 nanocrystals based on HRTEM of S3 photocatalyst.

3.2. XRD analysis The XRD patterns of HNTs, ZnO, CeO2, and a series of ZnO/CeO2@HNTs composites are exhibited in Fig. 4. Halloysite 7 Å was indentified with JCPDS file No. 29-1487, and a bit of quartz (JCPDS file No. 46-1045) also coexisted with HNTs [39]. As expected, the characteristic diffraction peaks of ZnO at 2θ values of 31.8, 34.5, 36.3, 47.5, 56.6, 62.8, 68.0 and 69.2° which corresponded to 100, 002, 101, 102, 110, 103, 112 and 201 crystalline planes of ZnO (JCPDS file No. 36-1451), respectively, could be observed in the XRD patterns of all ZnO/CeO2@HNTs composites, which are in good agreement with those published in the literature [40,41]. In addition, the diffraction peaks appeared at approximately 2θ values of 28.6, 33.1, 47.5 and 56.4° are related to the cubic fluorite phases of CeO2 (JCPDS file No. 34-0394) and can be assigned as 111, 200, 220 and 311 crystalline planes, respectively [42]. Hence, the XRD patterns revealed that the synthesized ZnO/CeO2@HNTs composites contain both ZnO and CeO2. In general, loading of semiconductors on HNTs can cause to surface dilution of the HNTs, and the high-temperature calcination can also lead to damage of HNTs crystalline structure [40]. Hence crystallinity of HNTs tends to decrease, as can be seen in Fig. 4 (a). For the XRD patterns of S3 and S4 in Fig. 4 (b), it is evident that the characteristic diffraction peaks of HNTs had partly vanished after heating to a scorching 500 ℃, which can be attributed to the dehydroxylation of Halloysite [43]. The XRD patterns of as-prepared samples obtained by regulating the molar ratio of ZnO and CeO2 are shown in Fig. 4 (c), in which the intensity of ZnO or CeO2 peaks is significantly positively related to its content. Likewise, adjusting the dosage of HNTs 11

could also affect the intensity of diffraction peaks, as shown in Fig. 4 (d). Different peaks widths were used for the estimation of particles size but the most intense peaks of ZnO (101) and CeO2 (111) were selected, and average crystallite sizes of the ZnO and CeO2 nanoparticles in ZnO/CeO2@HNTs composite were respectively estimated about 10 nm and 5 nm according to the Scherrer equation [37,44], which are consistent with the particle size distribution above.

Fig. 4 XRD patterns of (a) ZnO@HNTs, CeO2@HNTs, calcined HNTs and raw HNTs and as-prepared samples processed by adjusting the (b) calcinating temperature, (c) the molar ratio of ZnO to CeO2 and (d) the dosage of HNTs. 3.3. FT-IR spectra analysis FT-IR spectroscopy was used to examine the chemical composition of the obtained products, as showed in Fig. 5. In the spectrum of HNTs, the double peaks at 3697 cm-1 and 3622 cm-1 were due to the stretching vibrations of Al–OH groups at the surface of HNTs [39,43]. The peaks at around 1097 cm-1, 1030 cm-1 and 912 cm-1 are related to the stretch vibration of Si–O, the asymmetrical stretch vibration of Si–O–Si and the bending vibration of Al–OH, respectively [45]. One strong peak featured at 469 cm-1 corresponds to the Si–O–Si bending band, which is attributed to Si–O tetrahedral sheets [22]. For ZnO, the broad band centered at 461 cm-1 is responsible for Zn–O stretching vibration, and the peak located at 877 cm-1 are due to Zn–O bending vibration [42]. In the spectrum of CeO2, the characteristic absorption peaks of Ce–O stretching vibration and Ce–O bending vibration were observed at approximately 418 cm-1 and 850 cm-1 respectively [46]. In the case of calcinating temperatures of 300 ℃ and 400 ℃, the FT-IR spectra of S1 and S2 are very similar to that of HNTs. The characteristic absorption peaks of CeO2 showed up at 1514 cm-1 and 1390 cm-1, while the stretching vibration or bending vibration of M–O (Zn–O or Ce–O) were hidden by overlapping. When the calcination temperature higher (at 500 ℃ and 600 ℃), the above mentioned peaks of Al–OH 12

groups did not appear in the FT-IR spectra of S3 and S4, which indicated that the HNTs structure was probably destroyed due to structural dehydroxylation [45].

Fig. 5 FT-IR spectrum of HNTs, pure ZnO, CeO2, the S1, S2, S3 and S4 with different calcinating temperatures. 3.4. Surface oxidation state analysis The elemental composition and chemical status of the S3 photocatalyst were further investigated by XPS technique. Fig. 6 (a) displays the survey XPS spectrum of the as-prepared S3 photocatalyst, which mainly contains the peaks of Zn, Ce, O and C. The appearance of C 1s peak at ~284.9 eV validated the analysis under identical experimental condition. Fig. 6 (b) shows that the Zn 2p3/2 and Zn 2p1/2 are located at binding energies of 1021.2 eV and 1044.6 eV, which are consistent with the values reported recently for ZnO [47]. For the Ce 3d core level spectrum in Fig. 6 (c), the peaks labeled as * are the peaks of Ce4+ states while others are the characteristic peaks of Ce3+ states, which indicates most of Ce ions are present in the form of Ce4+ states [48]. The Ce3+ and Ce4+ states coexist in CeO2 and appear very closely to each other in the XPS pattern. Six peaks at 883.2, 888.2, 900.0, 905.4, 911.2 and 916.6 eV marked with asterisk due to three pairs of spin-orbit doublets can be ascribed to the characteristic of Ce4+ 3d final states [49]. Additionally, the existence of the Ce3+ surface states in the form of CeO2-x defects was verified by the existence of characteristic peaks at the binding energy of 880.2, 885.8, 897.0 and 902.6 eV. However, the diffraction peaks for the compounds containing Ce3+ were not clearly observed in above XRD patterns, which probably denotes that the Ce3+ compound was either amorphous or had a very low concentration in the photocatalyst [50]. The O 1s spectra was presented in Fig. 6 (d) and the broad peak at about 531.4 eV is attributed to the lattice oxygen for ZnO, CeO2, oxides in HNTs and the adsorbed oxygen on the catalyst surface. For the sake of clear illustration, the O 1s peak was deconvoluted into five peaks and the peaks of oxides in HNTs were simplified into an 13

integrated peak at about 532.6 eV [51]. The peaks with binding energies at 529.2, 530.2 and 530.8 eV respectively correspond to the Ce–O bonds in the CeO2 and Ce2O3 components and the Zn–O bond in the ZnO component as also measured earlier [50,52], while the peak at 531.6 eV is attributed to the presence of loosely bound oxygen on the surface of photocatalyst [53].

Fig. 6 XPS spectra of the S3 photocatalyst: (a) survey, (b) Zn 2p, (c) Ce 3d and (d) O 1s. 3.5. UV-vis absorption spectra UV-vis analysis was applied to measure the optical absorption properties of pure ZnO, CeO2, HNTs and the S3, as can be seen in Fig. 7. It was observed that the S3 exhibits an excellent absorption in UV region. As is known to all, ZnO and CeO2 both are the direct band gap semiconductors, allowing direct electronic transitions [40,54]. The optical band gap (Eg) of as-prepared samples is measured by the Kubelka-Munk equation [55]. And then the [F(R∞)hν]1/2 – hν curves and corresponding tangents were figured out, so that the band gap could be measured by the intersection of the tangent and x-axis. By extrapolating of the curve, the band gap energies of 2.97 and 2.99 eV were respectively estimated for the as-obtained ZnO and CeO2 which are lower than the reported values of the bulk ZnO (3.2-3.3 eV) and bulk CeO2 (3.15-3.2 eV) [40,46,56]. Consequently, the band gap of the S3 was measured to be 2.78 eV, which had slightly decreased compared with those of as-prepared ZnO (2.97 eV) and CeO2 (2.99 eV). This observation is important, as it indicates that the ZnO/CeO2@HNTs heterojunction photocatalysts can be photoexcited to generate more e-/h+ pairs under simulated solar irradiation, which could result in higher photocatalytic performance [54]. The observed narrowing of the band gaps may account for a large amount of surface defect states such as Zn interstitials and oxygen vacancies, the coexistence of Ce4+ and Ce3+ in the CeO2/ZnO heterostructure and the interplay between ZnO and CeO2 nanocrystals [16,57]. Besides, Liu et al. reported that the band gap narrowing of 14

CeO2/ZnO is mainly due to the decrease in the size of the crystal grain and the interfacial effects between the CeO2 and the ZnO grains [58]. This means the optical absorption of binary oxides could also be influenced by the size of the crystal grain.

Fig. 7 UV-vis absorption spectrum of pure ZnO, CeO2, HNTs and S3. Inset: The band gap of corresponding samples. 3.6. PL spectra Photoluminescence (PL) measurements were performed to determine the charge recombination and migration behaviors of pure ZnO and as-fabricated samples calcined at different temperatures, as shown in Fig. 8. It is common knowledge that the interface charge separation and recombination properties can strongly affect the fluorescence intensity of semiconductor [59]. The stronger fluorescence relative intensity of semiconductor, the higher the recombination rate of photogenerated electrons (e-) and holes (h+) is, which means the lower photocatalytic properties of as-detected samples. Under the condition of 408 nm excitation light source, the strong green emission observed at approximately 529 nm for pure ZnO. After the combination of ZnO and CeO2, the PL emission intensity of ZnO/CeO2@HNTs composites is notably lower than that of pure ZnO for the charge carrier transformation between two different parts of the photocatalyst, indicating that the coupled oxides can prolong electron-hole pair lifetime. This ascribed to the suppression of recombination of photoinduced e-/h+ pairs in IPET process [16]. It’s worth noting that the S3 shows the weakest emission intensity than the other three. This result indicates that the ZnO/CeO2@HNTs heterostructure photocatalyst calcined at a higher calcination temperature has a lower recombination rate of photogenerated e-/h+, which can be ascribed to the formation of ZnO/CeO2 heterojunction under the condition of directly fast heating and rapid evaporation of water.

15

Fig. 8 Room-temperature photoluminescence emission spectrum of pure ZnO, the S1, S2, S3 and S4. Inset: Expanded photoluminescence excitation spectrum of corresponding samples. 3.7. Electrochemical properties analysis The photocatalytic reactions, as well-known, are intimately relevant to the separation efficiencies of photogenerated e-/h+ pairs arisen from the excited semiconductor materials [60]. To investigate the photoelectrochemical activity of the synthesized films, the photocurrent (PC) response was carried out for the ZnO@HNTs, CeO2@HNTs, S1, S2, S3 and S4 under UV-vis light irradiation and photocurrent densities were measured after using the light on-off process with a pulse of 20 s, as shown in Fig. 9. It can be seen that ZnO/CeO2 @HNTs composites exist obviously enhanced current potential performance photocurrent toward the ZnO@HNTs or CeO2@HNTs under UV-vis irradiation with applied potential of 0.5 V vs. Ag/AgCl reference electrode and the fast and stable photocurrent responses observed in all electrodes were entirely reversible. Under UV-vis light irradiation, the CeO2@HNTs electrode shows the weakest response due to its fast recombination rate of photogenerated charge carriers. On the contrary, the photocurrent of the S3 electrode is almost 1.5 times higher than that of ZnO@HNTs electrode. The obvious enhancement of the photocurrent of synthesized photocatalysts films reveals the prolonged lifetime of the photoinduced charge carriers and the enhanced efficiency of photoinduced e-/h+ pairs separation, which can be ascribed to the coupling of ZnO and CeO2 nanocrystallites to form the heterojunction structure [61]. Moreover, it is worth noticing that the S3 electrode shows the strongest response probably because of the well-contacted nanoscale heterostructured ZnO/CeO2 IPET system formed during the annealing of 500 ℃. Furthermore, we performed electrochemical impedance spectroscopy (EIS) measurement to exam the influence of surface properties and interfacial charge carrier dynamics for the photogenerated holes of as-prepared samples can be trapped by the surface states and have recombined with electrons [62,63]. The interface charge 16

separation efficiency can also be examined by EIS Nyquist plot, which is a crucial factor for the photocatalytic activity [64]. Fig. 10 shows the EIS Nyquist plots of as-prepared samples under UV-vis light irradiation. The arc radiuses of the EIS Nyquist plots of the coupled ZnO/CeO2@HNTs composites were smaller than those of ZnO@HNTs and CeO2@HNTs under UV-vis light irradiation, which reflects the interface layer resistance occurring at the electrode surface [65,66]. More effective separation of photogenerated e–/h+ pairs and faster interfacial charge transfer is believed to have occurred on the coupled ZnO/CeO2@HNTs heterostructure photocatalysts under this condition. After the irradiation in UV-vis light, the arc radius of the S3 photocatalyst is smaller than that in the cases of dark, indicating after irradiation the photoinduced e–/h+ pairs have weakened the impedance [67]. These results clearly show that the formation of ZnO/CeO2 heterojunction could effectively enhance the separation and transfer efficiency of the photoinduced e–/h+ pairs in ZnO/CeO2@HNTs heterostructure photocatalysts. This data supports the PL and PC results.

Fig. 9 The chopped current time transient response for the ZnO@HNTs, CeO2@HNTs, S1, S2, S3 and S4 electrodes at 0.5 V vs. Ag/AgCl reference electrode in 0.5 M Na2SO4 aqueous solution under UV-vis light illumination. Fig. 10 EIS Nyquist plots of the ZnO@HNTs, CeO2@HNTs, S1, S2, S3, and S4 photocatalysts under UV-vis light illumination. Inset: EIS Nyquist plots of the S3 photocatalyst in the dark and under UV-vis light illumination. 3.8. Photocatalytic experiments Photocatalytic activity of as-synthesized catalysts was reflected by photodegrading TC under simulated solar irradiation. The concentration of obtained TC at various time intervals was detected by examining the absorption in UV-vis spectra at 357 nm. For the sake of researching the relationship between the calcinating temperatures and the photocatalytic efficiencies of as-obtained photocatalysts, Fig. 11 (a) displays the photodegradation behavior of TC catalyzed by blank, ZnO, CeO2, the S1, S2, S3 and 17

S4. The photodegradation ratio is calculated from the following expression [68]: η = (C0 – C)/C0 × 100%

(1)

where C and C0 correspond to the remaining concentration of TC after irradiation and the initial concentration of TC after adsorption, respectively. One obvious finding is that the ZnO/CeO2@HNTs composites have very good adsorptive properties and it could almost reach the adsorption equilibrium within about 30 minutes. The most interesting finding is that the S3 and S4 have better photodegradation abilities of TC, which is consistent with above characterization analyses of photoluminescence and transient photocurrent response. In the absence of photocatalyst, only a small amount of TC under simulated solar irradiation (about 4% for the blank with irradiation; no photodegradation for the blank without irradiation) was degraded which indicated that photolysis of TC under simulated solar irradiation can be almost negligible. After 60 minutes irradiation, the photodegradation efficiencies of as-obtained pure ZnO, CeO2, S1, S2, S3 and S4 at 25 ℃ were calculated to be about 66%, 52%, 76%, 82%, 87% and 85%, respectively. Furthermore, the total removal rate of TC by the S3 was reached 94%, which is calculated by including both the adsorption and photodegradation

of

photocatalysts.

The

ZnO/CeO2@HNTs

heterojunction

photocatalysts have better photodegradation abilities of TC than pure ZnO or CeO2, although pure ZnO or CeO2 has more active ingredient of photocatalysts. The results may be caused by the coupling of ZnO and CeO2 nanocrystallites and the supporting of HNTs. On the one hand, the higher calcinating temperature could promote the rapid generation of porous and fragmented structure, thus impelling the achievement of evenly distributed ZnO/CeO2 heterostructure clusters. On the other hand, the introduction of HNTs as catalyst supports is not only promising to possess satisfactory adsorptivity, but also provides the large supporting surface to improve the dispersibility of ZnO/CeO2 heterostructure clusters, thus generating more active sites to achieve excellent photocatalytic activity. In addition, B. W. Shivaraj et al. reported that annealing temperature significantly influenced crystallinity and surface roughness of the ZnO thin films, and they found that the grain sizes annealed at 500 ℃ are

18

higher than the annealed film at 400 ℃ [69]. This suggests that the structure of ZnO and CeO2 in heterostructure clusters would likely be influenced by the annealing temperature. More interesting still, ZnO/CeO2 system catalysts have been synthesized by precipitation, electrospinning, and hydrothermal method, but all these methods have usually had one thing in common: there is a need to activate the catalysts by calcination at higher temperature [42,61,70]. The effect of different molar ratios of ZnO to CeO2 on the photocatalytic activity had been studied and the results were shown in Fig. 11 (b). It can be observed that the S3 and S7 possess much better activities than the others, especially the S3. The photodegradation efficiencies of the ZnO@HNTs, CeO2@HNTs, S5, S6, S3 and S7 were calculated to be about 68%, 55%, 72%, 76%, 87% and 86%, respectively. This indicates that the molar ratio of ZnO to CeO2 of the S3 has been very close to the optimal value, which may be ascribed to the enhanced efficiency of photoinduced e-/h+ pairs separation. There are similar results reported by Sherly’s group that the synthesized coupled ZnO-CeO2 in 2:1 M ratios possesses the best photocatalytic activity due to the inter particle electron and hole transfer between ZnO and CeO2 [16]. Besides, ZnO/CeO2@HNTs heterojunction photocatalysts with different molar ratios of ZnO to CeO2 possess high photocatalytic activity on the whole, which can be attributed to the IPET process of nebulous and well-dispersed ZnO/CeO2 nanoclusters. To demonstrate the role of HNTs, the photocatalytic experiments of the 500 ℃ calcined HNTs, S8, S3 and S9 with different dosages of HNTs were carried out and the results were shown in Fig. 11 (c). It can be clearly seen that both S3 and S9 exhibited a good photoactivity, and the photodegradation efficiencies of the S8, S3 and S9 were calculated to be about 73%, 87% and 87%, respectively. The results of this study illustrate that an excess degree of using HNTs led to the decreasing of the active ingredient of photocatalysts, thus the photocatalytic activity has decreased while the adsorptivity has improved slightly, such as the S8. However, the reduction of HNTs amount may weaken the dispersibility of ZnO/CeO2 clusters and make it easier to agglomerate. It is very interesting that the S8, S3 and S9 with different dosages of 19

HNTs have similar properties of adsorption. For comparison, different doses of the 500 ℃ calcined HNTs were else experimented in parallel according to the HNTs contents of S8, S3, and S9. It can be seen that the calcined HNTs have no significant photocatalytic effect. The adsorption property of calcined HNTs has significantly reduced compared with those of S8, S3, and S9, which implied that the ZnO/CeO2 nanoclusters in ZnO/CeO2@HNTs may also have a good adsorption property. The influence of pH on the photodegradation extent of TC solution was evaluated at pH range of 2-10, as shown in Fig. 11 (d). The mediums of different pH values can lead to variations in the surface charge properties of a photocatalyst and the charge of the pollutants molecules [71]. The photodegradation efficiencies of the S3 under pH range of 2-10 were calculated to be about 26%, 44%, 74%, 86% and 70%, respectively. In the strong acidic pHs, the photocatalytic activity was obviously restrained, which may mainly due to the acid corrosion of ZnO and CeO2. The pH of point of zero charge for ZnO/CeO2@HNTs is 7.6 and the fastest degradation of TC took place at pH 8. With raising pH toward pH 10, the gradual downward trend in photocatalytic efficiency may be ascribed to the deprotonation of TC molecules and the change of catalyst surface charge [37].

Fig. 11 The decrease (C/C0) of 20 mg/L TC versus testing time for (a) the blank, S1, S2, S3 and S4 with different calcinating temperatures, (b) and the ZnO@HNTs, CeO2@HNTs, S5, S6, S3 and S7 with different molar ratios of ZnO to CeO2, (c) and the 500 ℃ calcined HNTs, S8, S3 and S9 with different dosages of HNTs, (d) and the S3 photocatalyst under different pH values.

3.9. Stability tests

Both the photocatalytic efficiency and stability are the significant factors to evaluate the property of the catalysts. The photocorrosion or photodissolution on the photocatalyst surface in the photocatalytic process is likely to reduce the photocatalytic efficiency. Therefore, we examined the stability of the S3 photocatalyst 20

by photocatalytic degradation of TC for four cycles. As can be seen from Fig. 12, TC degrading rate of the S3 after fourth repeated experiment is 75%, having dropped slightly, which could be involved with the photocorrosion of ZnO and CeO2. The photocorrosion of as-obtained S3 was detected by atomic absorption spectrometer. The zinc and cerium average contents of the repeated reaction solutions were calculated to be 2.38 and 1.34 mg·L-1, respectively, which can mainly be ascribed to the photocorrosion of ZnO and CeO2. Besides, the zinc and cerium contents of the photocatalyst before and after TC degradation for the fourth cycles were calculated by computer according to the results of atomic absorption analysis. The mass fraction of ZnO fell to 16.1% from 19.8% and the mass fraction of CeO2 was reduced to 11.7% from 13.7%. According to the above analysis, the S3 photocatalyst exhibited relatively good stability during the photodegradation process.

Fig. 12 Repeated experiments of photodegradation of 20 mg/L TC by the prepared S3.

3.10. Detection of reactive species

It has been widely acknowledged that superoxide radical anions (·O2-), holes (h+) and hydroxyl radicals (·OH) are the major factors for the photodegradation reactions. Fig. 13 displays the photocatalytic activities of ZnO/CeO2@HNTs composite in the presence of different scavenger. For investigation of ·O2-, the photodegradation experiment was started by adding 1 mmol BQ (scavenger for ·O2-) and the concentration was measured by a UV-vis spectrophotometer with the maximum absorption wavelength at 357 nm. While for the investigation of h+ and ·OH, the processes were similar with the steps of the study of ·O2-, but adding 1 mmol TEOA (scavenger for h+) and IPA (scavenger for ·OH) instead of BQ, respectively [72]. From the above results, the photocatalytic efficiency of TC was 87% without adding scavenger. When TEOA, IPA and BQ were used as scavenger, the photocatalytic efficiency of TC decreased to 57%, 71% and 67%, respectively. Hence, ·OH radicals 21

play a little role in the photodegradation process and h+ and ·O2- are the more major reactive species in the photodegradation process. In addition, the ESR measurements were used to the further quantify the presence of ·OH and ·O2- radicals of the S3 photocatalyst irradiated by the UV-vis light. As shown in Fig. 14 (a), the characteristic signal of 1:2:2:1 of DMPO-·OH adduct was detected,

indicating

the

photogenerated

holes

in

the

valence

bands

of

ZnO/CeO2@HNTs can be transformed into ·OH radicals during the photodegradation system [73]. However, the peak intensity was relatively low, compared with the intensity of DMPO-·O2- adducts. This can be rationalized by considering the extremely short life-span of ·OH, leading to the prompt quenching, which could also reflect the fact that ·OH alone has a small influence on the scavenging study [74]. For the detection of DMPO-·O2-, the characteristic signal of 1:1:1:1:1:1 could be observed within ethanol solutions, further confirming the formation of ·O2- during the photocatalytic process, as can be seen in Fig. 14 (b). The strong intensity of DMPO-·O2- adducts guarantee the high production of ·O2- which is believed to be an important active radical to oxidize the organic pollutant.

Fig. 13 Effect of different scavengers on the degradation of TC using as-synthesized S3 photocatalyst. Fig. 14 DMPO spin-trapping ESR spectra of the S3 photocatalyst under UV-vis light for (a) hydroxyl radical (·OH) in water and (b) superoxide radical (·O2-) in ethanol. 3.11. HPLC-MS spectra analysis HPLC-MS analysis became extremely important to identification of the product formed by the photodegradation of organic contaminant [75]. In order to identify the potential structure of new photodegradation products, HPLC-MS was performed. The HPLC chromatogram of TC degradation solution after 20 min, 40 min and 60 min irradiation, as can be seen in Fig. 15, revealed the occurrence of several peaks, attributable to photodegradation products. Furthermore, as shown in Fig. 16, the 22

changes of TC during the photodegradation reaction were detected by UV-vis spectra, which further demonstrated the fragmentation and abscission of TC attacked by the reactive

oxidated

species.

The

probable

molecular

structures

of

these

photodegradation products at elution time of 1.97, 2.41, 2.73, 3.04, 3.41, 3.91 and 4.49 min were speculated by analyzing the coupled tandem mass spectrometry. Fig. 17 displays the corresponding MS spectrum of those peaks in chromatogram and the respective probable structures of molecular ions. The most probable mechanistic explanation for the formation of the seven major degradation products B-H of TC is outlined in Fig. 18. The TC molecular with m/z = 445.45 were attacked by the reactive oxygen species to form the double bond oxidative opened compound B (m/z = 461.40). Then, compound C (m/z = 444.45) had been forming during the deamination reaction and esterification process. With the breaking and oxidation of the ester bond and the carbon-carbon double bond, compound C was transformed into compound D (m/z = 432.42). And then, the cyclotrione structure was oxidized to open the ring and formed the compound E (m/z = 349.47). As the photocatalytic reactions progressed, the oxidative decomposition and ring opening reactions had further occurred to form compound F (m/z = 305.53), G (m/z = 261.38) and H (m/z = 137.44).

Fig. 15 The HPLC chromatogram of TC degradation solution after 20 min, 40 min and 60 min irradiation. Fig. 16 UV-vis spectra of TC changes with reaction time. Fig. 17 The corresponding MS spectrum of peaks in the HPLC chromatogram. Fig. 18 Potential degradation pathway of TC based on HPLC-MS analysis. 3.12. Proposed mechanism of photodegradation In order to explain the enhanced photocatalytic activity mechanism, the conduction band (CB) and valence band (VB) potentials of as-prepared ZnO and CeO2 should be confirmed. For a semiconductor, the CB and VB can be calculated according to the following empirical equations: 23

ECB = χ – Ee – 0.5Eg

(2)

EVB = ECB + Eg

(3)

where ECB is the CB edge potential; EVB is the VB edge potential; χ is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms; Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV); Eg is the band gap energy of the semiconductor [8]. Based on this calculation, the main values for calculating CB and VB potentials of ZnO and CeO2 are listed in Table 3. The results showed that the CB and VB of ZnO lies below the CB and VB of CeO2, respectively. The photogenerated electrons can reduce O2 to generate ·O2- and the photogenerated holes can oxidize OH- to yield ·OH, because the CB value of ZnO is more positive than E0 (O2/·O2-) (–0.046 eV vs NHE) and the VB value of CeO2 is less positive than E0 (·OH/OH-) (2.38 eV vs NHE) [70]. As we known, both ZnO and CeO2 are of n-type semiconductors [76,77]. When tiny ZnO and CeO2 nanocrystals had grown together onto the surface of HNTs, an n-n heterojunction will be formed. The energy band diagram for isolated ZnO and CeO2 is shown in Scheme 1 (a) where ZnO and CeO2 are not yet in contact. Upon contact, the electrons keep on flowing from CeO2 to ZnO until the Fermi levels become coincident because the flatband potential of CeO2 is more negative than that of ZnO, resulting in an accumulation of negative charges in the ZnO region and a positive section in the CeO2 region in the vicinity of the junction [78-81]. The equilibrium energy band diagram of ZnO/CeO2@HNTs composite is shown in Scheme 1 (b). An internal electrostatic field directed from the CeO2 region to the ZnO region will be formed at the interface between them by the charges accumulation, generating an energy barrier for the electron transfer from ZnO to CeO2 [82]. And then the ZnO and CeO2 bands will bend due to the formation of a space charge region at the interface of the semiconductors after electron transfer [83]. Based on the discussion above, a possible mechanism has been proposed to explain the synergistic effects of CeO2 coupling on the photocatalytic activity of ZnO as shown in Scheme 2. For ZnO/CeO2@HNTs, both ZnO and CeO2 have essentially the same Fermi energy level at the interface, forming a staggered band offset near the 24

interface. Under simulated sunlight illumination, both ZnO and CeO2 are excited and the charge separation occurs simultaneously. Because the electrostatic field at the interface facilitates the separation of photogenerated e-/h+ pairs, the photogenerated electrons (e-) in CB of CeO2 can easily be transferred to the CB of ZnO where they are captured by adsorbed oxygen molecules (O2) and give rise to superoxide radical anions (·O2-). Therefore, organic pollutants in the solution can be decomposed by the produced reactive oxygen species. Simultaneously, the photoinduced holes (h+) are transferred from the VB of ZnO to the VB of CeO2, reacting with hydroxyl ions (OH-) to generate hydroxyl radicals (·OH) or directly participating in the oxidation of TC. It is well accepted that positions of VB and CB play an important role for charge separation and photocatalytic efficiency [16]. The introduction of CeO2 reduces the photoinduced e-/h+ pair recombination, and the coupling of two semiconductors results in overlapping and bending of their energy bands, prompting the formation of new energy levels which has lesser band gap [42]. Furthermore, Ce4+ is the scavenger of electron to enhance the e-/h+ pair separation that can easily trap the electron to form Ce3+ due to its varied valences and special 4f levels [58]. The generated Ce3+ will react with adsorbed O2 to form ·O2-, which makes cerium oxide shift between CeO2 and Ce2O3 under oxidizing and reducing conditions [84]. In addition to involving into the photodegradation reactions, the photogenerated electrons and holes can also recombine and consume the input energy as heat or get trapped in metastable states [85]. As sensitizing centers, the surface defect states such as intrinsic defects, Zn interstitials or oxygen vacancies in ZnO are available to trap the photogenerated electron or hole, preventing the recombination of the photoinduced e-/h+ pair [86]. The major reactions can be described in the following empirical equations: ZnO/CeO2@HNTs + hν → ZnO/CeO2@HNTs + h+ + e-

(4)

h+ + H2O → ·OH + H+

(5)

e- + O2 → ·O2-

(6)

·O2- + H+ → ·OOH

(7)

2·OOH → H2O2 + O2

(8) 25

H2O2 + e- → ·OH + OH-

(9)

e- + Ce4+ → Ce3+

(10)

Ce3+ + O2 → Ce4+ + ·O2-

(11)

h+ + TC →Degradation Products

(12)

·OH + TC → Degradation Products

(13)

·O2- + TC → Degradation Products

(14)

H2O2 + TC → Degradation Products

(15)

Table 3 Calculation of the CB and VB Potentials of ZnO and CeO2. Scheme 1 Equilibrium energy band diagrams of ZnO/CeO2@HNTs composite (a) before and (b) after the formation of n-n heterojunction: EF is the Fermi level; EC and EV are the conduction and valence band edges, respectively. Scheme 2 Schematic drawing of the proposed photocatalytic process of as-synthesized ZnO/CeO2@HNTs.

4. Conclusions

In summary, we have successfully synthesized heterojunction ZnO/CeO2@HNTs by a simple one-pot wet-calcination method. The heterostructure is composed of tiny ZnO and CeO2 nanocrystals with a mean grain size within 10 nm. The photocatalytic efficiency of ZnO is markedly improved by CeO2 coupling under simulated solar light, which can be mainly attributed to the efficient inter particle charges transfer and the transformation of cerium oxide between CeO2 and Ce2O3 under photocatalytic process. Additionally, we also found that the calcinating temperature of 500 ℃, the molar ratio of ZnO to CeO2 about 3:1 and the 1-fold dosage of HNTs may be the potential optimal values. Compared with other samples, the S3 heterojunction photocatalyst showed the highest enhancement in the photocatalytic performance. Overall, a series of nebula-like ZnO/CeO2@HNTs n-n type heterostructure photocatalysts have been synthesized through one step wet-calcination method, possessing high photocatalytic activity and good stability under simulated sunlight which directs our attention to explore it as a promising material for the treatment of organic effluents. 26

5. Acknowledgement We gratefully acknowledge the financial support of the National Natural Science Foundation of China (No.21576125), the China’s Post-doctoral Science Fund (No.2015M570416), the Natural Science Foundation of Jiangsu Province, China (BK20140532, BK20151349, BK20150484), and the Research Foundation for Advanced Talents of Jiangsu University, China (No.14JDG148).

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M.

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36

Figure captions Fig. 1 Typical low-resolution (left) and high-magnification (right) TEM image of the (a, b) S1, (c, d) S2, (e, f) S3 and (g, h) S4. Fig. 2 (a) TEM image of S3 photocatalyst, (b) HRTEM image of heterostructured ZnO/CeO2 nanocrystals in S3 photocatalyst, (c) the corresponding enlarge figure of heterostructure and (d) fast Fourier transform patterns of ZnO, CeO2 and heterostructured ZnO/CeO2 nanocrystals. Fig. 3 Particle size distribution of ZnO/CeO2 nanocrystals based on HRTEM of S3 photocatalyst. Fig. 4 XRD patterns of (a) ZnO@HNTs, CeO2@HNTs, calcined HNTs and raw HNTs and as-prepared samples processed by adjusting the (b) calcinating temperature, (c) the molar ratio of ZnO to CeO2 and (d) the dosage of HNTs. Fig. 5 FT-IR spectrum of HNTs, pure ZnO, CeO2, the S1, S2, S3 and S4 with different calcinating temperatures. Fig. 6 XPS spectra of the S3 photocatalyst: (a) survey, (b) Zn 2p, (c) Ce 3d and (d) O 1s.

Fig. 7 UV-vis absorption spectrum of pure ZnO, CeO2, HNTs and S3. Inset: The band gap of corresponding samples. Fig. 8 Room-temperature photoluminescence emission spectrum of pure ZnO, the S1, S2, S3 and S4. Inset: Expanded photoluminescence excitation spectrum of corresponding samples. Fig. 9 The chopped current time transient response for the ZnO@HNTs, CeO2@HNTs, S1, S2, S3 and S4 electrodes at 0.5 V vs. Ag/AgCl reference electrode in 0.5 M Na2SO4 aqueous solution under UV-vis light illumination. Fig. 10 EIS Nyquist plots of the ZnO@HNTs, CeO2 @HNTs, S1, S2, S3, and S4 photocatalysts under UV-vis light illumination. Inset: EIS Nyquist plots of the S3 photocatalyst in the dark and under UV-vis light illumination. Fig. 11 The decrease (C/C0) of 20 mg/L TC versus testing time for (a) the blank, S1, S2, S3 and S4 with different calcinating temperatures, (b) and the ZnO@HNTs, CeO2@HNTs, S5, S6, S3 and S7 with different molar ratios of ZnO to CeO2, (c) and the 500 ℃ calcined HNTs, S8, S3 and S9 with different dosages of HNTs, (d) and the S3 photocatalyst under different pH values. Fig. 12 Repeated experiments of photodegradation of 20 mg/L TC by the prepared S3. Fig. 13 Effect of different scavengers on the degradation of 20 mg/L TC using as-synthesized S3 photocatalyst. Fig. 14 DMPO spin-trapping ESR spectra of the S3 photocatalyst under UV-vis light for (a) hydroxyl radical (·OH) in water and (b) superoxide radical (·O2-) in ethanol. Fig. 15 The HPLC chromatogram of TC degradation solution after 20 min, 40 min and 60 min irradiation.

Fig. 16 UV-vis spectra of TC changes with reaction time. Fig. 17 The corresponding MS spectrum of peaks in the HPLC chromatogram. Fig. 18 Potential degradation pathway of TC based on HPLC-MS analysis. Scheme 1 Equilibrium energy band diagrams of ZnO/CeO2@HNTs composite (a) before and (b) after the formation of n-n heterojunction: EF is the Fermi level; EC and EV are the conduction and valence band edges, respectively. 37

Scheme 2 Schematic drawing of the proposed photocatalytic process of as-synthesized ZnO/CeO2@HNTs.

38

39

Fig. 1 Typical low-resolution (left) and high-magnification (right) TEM image of the (a, b) S1, (c, d) S2, (e, f) S3 and (g, h) S4.

Fig. 2 (a) TEM image of S3 photocatalyst, (b) HRTEM image of heterostructured ZnO/CeO2 nanocrystals in S3 photocatalyst, (c) the corresponding enlarge figure of heterostructure and (d) fast Fourier transform patterns of ZnO, CeO2 and heterostructured ZnO/CeO2 nanocrystals.

40

Fig. 3 Particle size distribution of ZnO/CeO2 nanocrystals based on HRTEM of S3 photocatalyst.

41

42

Fig. 4 XRD patterns of (a) ZnO@HNTs, CeO2@HNTs, calcined HNTs and raw HNTs and as-prepared samples processed by adjusting the (b) calcinating temperature, (c) the 43

molar ratio of ZnO to CeO2 and (d) the dosage of HNTs.

Fig. 5 FT-IR spectrum of HNTs, pure ZnO, CeO2, the S1, S2, S3 and S4 with different calcinating temperatures.

44

Fig. 6 XPS spectra of the S3 photocatalyst: (a) survey, (b) Zn 2p, (c) Ce 3d and (d) O 1s.

45

Fig. 7 UV-vis absorption spectrum of pure ZnO, CeO2, HNTs and S3. Inset: The band gap of corresponding samples.

46

Fig. 8 Room-temperature photoluminescence emission spectrum of pure ZnO, the S1, S2, S3 and S4. Inset: Expanded photoluminescence excitation spectrum of corresponding samples.

47

Fig. 9 The chopped current time transient response for the ZnO@HNTs, CeO2@HNTs, S1, S2, S3 and S4 electrodes at 0.5 V vs. Ag/AgCl reference electrode in 0.5 M Na2SO4 aqueous solution under UV-vis light illumination.

48

Fig. 10 EIS Nyquist plots of the ZnO@HNTs, CeO2 @HNTs, S1, S2, S3, and S4 photocatalysts under UV-vis light illumination. Inset: EIS Nyquist plots of the S3 photocatalyst in the dark and under UV-vis light illumination.

49

Fig. 11 The decrease (C/C0) of 20 mg/L TC versus testing time for (a) the blank, S1, S2, S3 and S4 with different calcinating temperatures, (b) and the ZnO@HNTs, CeO2@HNTs, S5, S6, S3 and S7 with different molar ratios of ZnO to CeO2, (c) and the 500 ℃ calcined HNTs, S8, S3 and S9 with different dosages of HNTs, (d) and the S3 photocatalyst under different pH values.

50

Fig. 12 Repeated experiments of photodegradation of 20 mg/L TC by the prepared S3.

51

Fig. 13 Effect of different scavengers on the degradation of 20 mg/L TC using as-synthesized S3 photocatalyst.

52

Fig. 14 DMPO spin-trapping ESR spectra of the S3 photocatalyst under UV-vis light for (a) hydroxyl radical (·OH) in water and (b) superoxide radical (·O2-) in ethanol.

53

Fig. 15 The HPLC chromatogram of TC degradation solution after 20 min, 40 min and 60 min irradiation.

54

Fig. 16 UV-vis spectra of TC changes with reaction time.

55

Fig. 17 The corresponding MS spectrum of peaks in the HPLC chromatogram.

56

Fig. 18 Potential degradation pathway of TC based on HPLC-MS analysis.

57

Scheme 1 Equilibrium energy band diagrams of ZnO/CeO2@HNTs composite (a) before and (b) after the formation of n-n heterojunction: EF is the Fermi level; EC and EV are the conduction and valence band edges, respectively.

Scheme 2 Schematic drawing of the proposed photocatalytic process of as-synthesized ZnO/CeO2@HNTs.

58

Table captions Table 1 Calcination temperature and reagent dosages of different photocatalysts. Table 2 Analysis results of ZnO and CeO2 contents of different photocatalysts. Table 3 Calculation of the CB and VB Potentials of ZnO and CeO2.

Table 1 Calcination temperature and reagent dosages of different photocatalysts. S1

S2

S3

S4

S5

S6

S7

S8

S9

ZnO@HNTs

CeO2@HNTs

T/℃

300

400

500

600

500

500

500

500

500

500

500

m[Zn(NO3 )2· 6H2O]/g

1.1161

1.1161

1.1161

1.1161

1.5723

1.3386

0.9258

1.1161

1.1161

1.9043

0

m[Ce(NO3 )3· 6H2O]/g

0.5430

0.5430

0.5430

0.5430

0.2295

0.3908

0.6757

0.5430

0.5430

0

1.3144

m(HNTs)/g

1.0422

1.0422

1.0422

1.0422

1.0422

1.0422

1.0422

0.5211

1.5633

1.0422

1.0422

Table 2 Analysis results of ZnO and CeO2 contents of different photocatalysts. S1

S2

S3

S4

S5

S6

S7

S8

S9

ZnO/wt%

19.5

19.5

19.8

19.8

27.8

23.5

16.2

29.5

CeO2 /wt%

13.6

13.7

13.7

13.9

5.83

9.83

17.0

20.6

ZnO@HNTs

CeO2@HNTs

14.8

33.4

0

10.3

0

33.2

Table 3 Calculation of the CB and VB Potentials of ZnO and CeO2. χ

Eg (eV)

ECB (eV)

EVB (eV)

ZnO

5.79

2.97

-0.195

2.775

CeO2

5.56

2.99

-0.435

2.555

59

Research Highlights 1. Nebula-like

ZnO/CeO2@HNTs

heterostructure

photocatalysts

have

been

synthesized. 2. The photocatalysts exhibit great photocatalytic activity and stability. 3+

3. The Ce

and Ce4+ states coexist in CeO2, promoting the photodegradation of TC.

4. The possible photodegradation mechanism was systematically presented. 5. The potential structure of new photodegradation products was studied.

60