Hierarchical ZnO Decorated with CeO2 Nanoparticles as Direct Z

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Energy, Environmental, and Catalysis Applications

Hierarchical ZnO Decorated with CeO2 Nanoparticles as Direct Z-Scheme Heterojunction for Enhanced Photocatalytic Activity Linyu Zhu, Hong Li, Pengfei Xia, Zirui Liu, and Dehua Xiong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13782 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 28, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

Hierarchical ZnO Decorated with CeO2 Nanoparticles as Direct Z-Scheme Heterojunction for Enhanced Photocatalytic Activity

Linyu Zhu,† Hong Li, *,† Pengfei Xia,† Zirui Liu,‡ and Dehua Xiong*,†

†

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of

Technology, Wuhan, 430070, P. R. China.

‡

Department of Materials Science and Engineering, University of California, Los

Angeles, CA, 90095-1595, USA.

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Development of high-efficiency heterojunction with improved photocatalytic property is regarded as a promising way to decontaminate wastewater. Herein, the direct novel Z-scheme heterojunction formed between CeO2 nanoparticles and hierarchical ZnO was synthesized through the wet chemistry method and then heat-treatment technique. The as-synthesized ZnO/CeO2 composites display highly enhanced photocatalytic Rhodamine B (RhB) degradation compared with pristine ZnO and CeO2. Specifically, ZnO/CeO2-3 (mass fraction of CeO2, 30%) shows good photostability and the best removal efficiency for photodegradated RhB, which is almost 2.5 and 1.7 times than pristine ZnO and CeO2, respectively. Based on the detailed characterizations and the degradation behavior of as-prepared samples over RhB, the formed heterojunction between the hierarchical ZnO and CeO2 nanoparticles is confirmed as the direct Z-scheme heterojunction. The heterojunction system shows fast transfer, high-efficiency separation and long lifetime of photoinduced charge carriers, as well as enhanced redox capacity. This study affords a novel approach to construct ZnO-based Z-scheme heterojunctions for the photocatalytic applications.

KEYWORDS: Zinc oxide, Z-scheme heterojunction, Cerium oxide, photocatalysis, decomposition


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INTRODUCTION In recent years, water pollution associated with industrial dyes has caused serious ecological problems since even a small quantity of organic dyes are toxic to living organisms.1 Hence, an efficient solution to treat the wastewater problem is considerably urgent for human development. Satisfactorily, semiconductor-based photocatalysis is an eco-friendly and efficient technology to tackle the problem which utilizes the renewable solar energy to remove the organic dyes without any secondary pollutants.2 Up to now, remarkable progresses in photocatalysis have been achieved and abundant photocatalysts including metal oxides, organic semiconductors, metal nitrides and metal sulfides have been exploited for photocatalytic degradation towards organic pollutants. 3-5 Zinc oxide (ZnO) has been intensively researched for photodegradation because of these noteworthy properties including low cost, non-toxicity, high photosensitivity and redox potential.6,7 With the wide bandgap (Eg=3.37 eV), the photocatalytic activity of ZnO is usually limited by inefficient exploitation of solar energy.8 Besides, the fast recombination of photoinduced electron-hole pairs is another issue to hinder the improvement of its photocatalytic activity.9,10 To tackle the abovementioned problems, researchers have made many attempts to increase the photodegradation efficiency of ZnO, including metal or nonmetal doping,11,12 deposition of noble metals,13 coupling with carbon materials14 and constructing heterojunctions.15,16 Specially, constructing direct Z-scheme heterojunction has been considered as a more



effective way than typical type-II heterojunction.17,18 Upon photoexcitation, for the Z-scheme heterojunction, the photo-generated electrons on the conduction band (CB) of one photocatalyst (PSA) would transfer to valence band (VB) of another photocatalyst (PSB). Consequently, the photogenerated electrons and holes accumulate on the VB of PSA and the CB of PSB, respectively. Hence, the Z-scheme photocatalytic heterojunction achieves high redox potential and effective spatially separation of photoinduced charge carriers.19,20 In addition, with the strong electrostatic force between electrons and holes, the charge transport of the Z-scheme system is more feasible in physical compared with that of the type-II heterojunction. Thus, ZnO-based Z-scheme heterojunctions have aroused increasing attention in removing the organic dyes from the wastewater under the light irradiation due to the aforementioned advantages. Cerium oxide (CeO2), as an active rare earth oxide, has been commonly regarded as a good electron acceptor and high oxygen storage carrier apart from these advantages of high stability, abundance, nontoxicity and low cost.21,22 In this case, CeO2 materials are widely applied in water gas shift (WGS) reactions, automotive three-way catalysts, solid oxide fuel cells and oxygen sensors.23-25 Recent researches report that CeO2 is regarded as one of the most potential photocatalysts with UV-vis light response due to the abundant oxygen vacancies and strong redox ability, and exhibits excellent photocatalytic activities in wastewater treatment and water splitting for hydrogen evolution.22,26,27 More importantly, according to the previous reports, it can be found that the energy level structure of CeO2 is well matched with that of ZnO. 18,28 Hence,


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it is reasonably anticipated that the interfacial Z-scheme heterojunction between CeO2 and ZnO would be achieved due to their suitable band structures, which is expected to be a high-efficient photocatalyst with high redox capacity and effective separation of photoinduced charge carriers. Although some ZnO/CeO2 composites have been developed for the photocatalytic applications,29,30 their photocatalytic mechanism in the removal of dye molecules, to now, is still a great challenge to be detailedly expounded and convincingly confirmed. Meanwhile, the unsatisfying photocatalytic activity and stability further limit their photocatalytic applications. In this work, a direct ZnO/CeO2 Z-scheme heterojunction constructed by hierarchical ZnO and CeO2 nanoparticles was fabricated through the wet chemistry method and then calcination process. The CeO2 nanoparticles were in-situ incorporated into the hierarchical ZnO and the intimate interface contact between them was achieved after calcination. The obtained ZnO/CeO2 composites exhibit expectedly improved photocatalytic performance and stability over the decomposition of RhB compared with pristine ZnO and CeO2. According to the detailed experimental analysis, their photocatalytic behaviors fully meet the direct Z-scheme mechanism.

EXPERIMENTAL SECTION Synthesis of ZnO/CeO2 Composites. All chemical reagents employed were of AR grade (SINOPHARM, CHINA) and no further purification. ZnO precursors were prepared by modified method based on the previous literature.31 Specifically, 0.41 g



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Zn(C2H3O2)2·2H2O, 0.45 g urea and 0.11 g C6H8O7·H2O were added into 80 mL of distilled water and stirred rapidly to achieve a homogeneous mixture. Afterwards, the o

mixture was put in a 100 mL teflon-lined reaction kettle. After reacting at 120 C for 3 h in an oven, the mixture were carefully centrifuged. Next, the obtained precipitates were fully rinsed with ethanol. Eventually, the product was dried in an oven at 60 °C for 10 h and marked as ZnO precursors. The pristine ZnO was collected by calcination of ZnO precursors at 350 °C for 2 h.

As illustrated in Figure 1, ZnO/CeO2 composites were prepared through the wet chemistry method and then calcination process. ZnO/CeO2-3 was obtained as follows: the mixture of 70 mg ZnO precursors and 30 mg Ce(NO3)2·6H2O (mass fraction of Ce(NO3)2·6H2O, 30% ) were ultrasonically dispersed in 40 mL of deionized water. After that, 0.6 mL of ammonium hydroxide was dropwise added in the abovementioned solution with rapid stirring. Afterwards, the resulting solution was placed into a water bath and retained at 40 °C for 2 h under magnetic stirring. After cooling down, the precipitates were centrifugally obtained, adequately rinsed with ethanol and then dried at 60 °C for 2 h in an oven. The as-obtained product was labeled as ZnO/CeO2-3 precursors. Finally, the ZnO/CeO2-3 composites were obtained by calcination of ZnO/CeO2-3 precursors at 350 °C for 2 h. The other ZnO/CeO2 composites were obtained under the same condition by changing the mass fraction of Ce(NO3)2·6H2O (10, 20 and 40%) and labeled as ZnO/CeO2-1,


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ZnO/CeO2-2, and ZnO/CeO2-4, respectively. For comparison, the pristine CeO2 was prepared by the above process merely without the addition of ZnO precursors.

Figure 1. Schematic diagram of forming process for ZnO/CeO2 composites.

RESULTS AND DISCUSSION Characterizations

of

ZnO/CeO2

Composites.

X-ray

diffraction

(XRD)

measurements were conducted to analyze the crystal structures of ZnO, CeO2 and ZnO/CeO2 composites. In Figure 2, the peaks of pure ZnO and CeO2 are indexed to the hexagonal wurtzite ZnO (JCPDS no.36-1451) and cubic phase CeO2 (JCPDS no.65-2975), respectively.21,32 Additionally, a weak peak located at 28.5°, can be detected on the XRD patterns of ZnO/CeO2 composites, corresponding to the (111) plane of cubic CeO2 component. The gradually increasing intensity of that diffraction peak suggests the increase of CeO2 content in the composites. The results of XRD patterns demonstrate the successful incorporation between ZnO and CeO2.



Figure 2. XRD patterns of ZnO, CeO2 and ZnO/CeO2 composites.

The morphology and structure of as-obtained hierarchical ZnO precursors, ZnO/CeO2 -3 precursors and ZnO/CeO2-3 composites were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images as presented in Figure S1 and Figure 3. Figure S1 presents the SEM images of ZnO and ZnO/CeO2-3 precursors before calcination. The ZnO precursors in Figure S1a and S1b exhibit the spherical structure assembled by nanosheets and their diameters are about 3 μm. Figure S1c shows that the surface of ZnO precursors is quit smooth. After in-situ process, the ZnO/CeO2-3 precursors still show the spherically hierarchical structure with numerous CeO2 precursors loaded on their surface (Figure S1d-f). After the calcination of ZnO/CeO2-3 precursors, the ZnO/CeO2-3 composites are obtained and still exhibit the similar morphology to its precursors in Figure 3a and b. Interestingly, Figure 3c presents that ZnO and CeO2 nanoparticles agglomerate into 2D nanosheets and further assemble into hierarchical structure after the calcination process, which can also be confirmed by the TEM images (Figure 3d, e). This


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hierarchical structure endows the composites with the structure-dependent catalytic advantages of porosity, large specific surface area and numbers of reactive sites.13 Additionally, the lattice fringes of 0.28 nm and 0.31 nm, which corresponds to the (100) and (111) planes of ZnO and CeO2, respectively, are observed in the high-resolution TEM image (HRTEM) of ZnO/CeO2-3 composites (Figure 3f). More importantly, the intimate interface between ZnO and CeO2 nanoparticles is clearly observed, implying the formation of heterojunction between these two components. The EDS in Figure S2 and elemental mapping images in Figure 3g-i further evidence that CeO2 nanoparticles are well anchored and evenly distributed on the hierarchical ZnO.



Figure 3. (a-c) SEM images, (d,e) TEM images, (f) HRTEM image and (g-i) Elemental mapping images of ZnO/CeO2-3. The X-ray photoelectron spectroscope (XPS) spectra were measured to investigate the surface chemical states and chemical compositions of ZnO, CeO2 and ZnO/CeO2-3 under the light illumination. The XPS survey spectrum of ZnO displays the peaks attributed to Zn and O elements, while that of CeO2 shows the peaks belonging to Ce and O elements (Figure 4a). As expected, the XPS survey spectrum of ZnO/CeO2-3 composites presents the co-existence of the peaks attributed to Ce, O


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and Zn elements. From Figure 4b, in the Zn 2p high-resolution XPS spectra for ZnO, two peaks at 1020.9 eV and 1044.0 eV are assigned to Zn 2p3/2 and Zn 2p1/2, respectively, which is accordance with the previous reports.7 In comparison with pristine ZnO, the two peaks in ZnO/CeO2-3 shift to higher binding energies of 1021.2 eV and1044.3 eV, respectively, revealing the construction of interfacial interaction between ZnO and CeO2. The high-resolution Ce 3d XPS spectra of CeO2 and ZnO/CeO2-3 are shown in Figure 4c. The v and u are assigned to the spin splitting orbits of 3d5/2and 3d3/2 for the Ce ion.33 Thereinto, the peaks due to Ce4+ are designated as v, v´´, v´´´and u, u´´, u´´´, while those of Ce3+ are marked as v´and u´. It can be seen that most of Ce ions in CeO2 and ZnO/CeO2-3 are in Ce4+ chemical states. Importantly, those Ce4+/Ce3+ peaks for ZnO/CeO2-3 shift to lower binding energies compared with those for CeO2, which indicates the chemical state of surface Ce ions is changed by incorporating CeO2 nanoparticles in hierarchical ZnO.28 Those results reveal the formation of heterojunction between ZnO and CeO2, with electrons migrating from ZnO to CeO2. To further investigate the heterojunction between ZnO and CeO2, the Mott-Schottky (MS) plot for ZnO/CeO2-3 was measured as shown in Figure 4d. Typically, the characteristic of the semiconductors can be uncovered by the slope in the linear portion of MS plots, namely, the negative slope corresponding to p-type semiconductor and the positive one to n-type. Besides, the extrapolation of the linear portion to the x-axis reflects the flat-band potential (Vfb) of a semiconductor.34 As for Figure 4d, the dual linear portions in the Mott-Schottky plot of the composites are



attributed to the ZnO and CeO2 component, suggesting the successful construction of the heterojunction between n-type CeO2 and ZnO.35

Figure 4. XPS spectra of ZnO, CeO2 and ZnO/CeO2-3: (a) survey spectra, (b) Zn 2p and (c) Ce 3d high-resolution spectra. (d) MS plot of ZnO/CeO2-3. Brunauer-Emmett-Teller (BET) method was employed to investigate the pore-size distributions and specific surface areas of ZnO, CeO2 and ZnO/CeO2 composites. In accordance with BDDT (Brunauer-Deming-Deming-Teller) classification, the N2 adsorption-desorption isotherms for all the samples exhibit type IV with the H3 type hysteresis loops, meaning that those samples have slit-shaped pores (Figure 5a).4,36 The pore size distribution curves of ZnO, CeO2 and ZnO/CeO2 composites were obtained by the Barrett-Joiner-Halenda (BJH) method (Figure 5b). The specific


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surface areas for the as-prepared ZnO/CeO2 composites are almost the same as those for the pristine components, implying the deposition of CeO2 nanoparticles contributes less on the improvement of the specific surface area (Table S1). From Figure 5b, all the ZnO/CeO2 composites possess more mesopore and macropore compared with pure ZnO and CeO2. Meanwhile, as seen in Table S1, the pore volumes of all the as-obtained composites are larger than that of pristine components, meaning that the absorption capacity and mass transfer of those composites would be enhanced during the photocatalytic process.

Figure 5. (a) N2 adsorption-desorption isotherms of ZnO, CeO2 and ZnO/CeO2 composites and (b) the corresponding BJH pore size distribution curves. Density functional theory (DFT) calculations of ZnO and CeO2 were conducted to research their electronic structures. The optimized structure models of ZnO and CeO2 are displayed in Figure 6a and 6c, respectively. Based on these models, their band structures and density of states (DOS) are calculated, as presented in Figure 6b and Figure 6d, respectively. Specifically, the CB of ZnO is mainly consists of Zn 3s and Zn 3p orbitals, whereas its VB mainly comprised of O 2p and Zn 3d orbitals. The CB



of CeO2 is mainly composed of Ce 4f orbital, while its VB mainly consists of O 2p orbital. More importantly, it can be seen that ZnO is the direct-gap semiconductor and its band gap is calculated to be 3.08 eV. Meanwhile, CeO2 is the indirect-gap semiconductor with the calculated band gap of 2.77 eV. The calculated band gap is slightly smaller than the measured results, which is most probably because of the limitation of DFT calculations.

Figure 6. The structural models of (a) ZnO and (c) CeO2. The calculated band structure and DOS of (b) ZnO and (d) CeO2 simulated models. Figure 7 shows the UV-vis absorption spectra of pristine ZnO, CeO2 and ZnO/CeO2 composites. The band edges of pristine ZnO and CeO2 are ~ 396 and 434 nm, respectively. After the deposition of CeO2 nanoparticles, all the ZnO/CeO2 composites


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show slightly enhanced absorption toward longer wavelength compared with pristine ZnO, which is most probably contributed to the construction of interfacial heterojunction. 20 Generally, their band gaps can be evaluated via the Kubelka-Munk function.37-39 From Figure 7b, the band gaps of ZnO and CeO2 are calculated to be 3.13 and 2.86 eV, respectively. These results indicate that the ZnO/CeO2 composites could broaden the absorption slope to the visible-light region, improving the utilization of solar spectrum, which is quite benefit to enhance the photocatalytic property.

Figure 7. (a) UV-vis absorption spectra of ZnO, CeO2 and ZnO/CeO2 composites. (b) Band gap energies of ZnO and CeO2. Photocatalytic Property. The photocatalytic performances of ZnO/CeO2 composites were estimated by the photodegradation of RhB under the light irradiation. Figure 8a-d and Figure S3 shows the absorption spectra of RhB with different concentrations during the photocatalytic reaction. For the blank experiment, the absorption intensity at 554 nm remains almost unchanged with the increasing irradiation time as shown in Figure 8a, indicating the photolysis of RhB molecules is almost negligible. As expected, with the addition of the as-prepared photocatalysts in



the photocatalytic systems, the concentration of RhB gradually decreases with the prolonged light irradiation, as presented in Figure 8c-d and Figure S3. Moreover, photodegradation efficiency of RhB over the blank experiment and as-prepared photocatalysts are recorded and presented in Figure 8e. In comparison with single component, all ZnO/CeO2 composites exhibit enhanced photocatalytic property. In particular, the ZnO/CeO2-3 composites possess the best photocatalytic activity and photocatalytic efficiency is up to 96% after 80 min irradiation, which is rather higher than pure ZnO (38%) and CeO2 (56%). Meanwhile, their kinetic behaviors towards photocatalytic RhB degradation quite conform to the pseudo-first order kinetics equation: Ln(C0/C) = Kapt.40 Kap represents the degradation rate constant, while C0 and C correspond to the initial (t = 0) and residual (t time) concentration of RhB, respectively. Figure 8f shows the photocatalytic rate curves and corresponding fitted kinetics curves of blank experiment and these as-obtained samples. Generally, the Kap value can be used to evaluate the photodegradation activity and obtained by the slope of the kinetics curve as illustrated in Figure 8f. It is apparent that all ZnO/CeO2 composites exhibit larger Kap values than pristine components, suggesting that constructing the heterojunction is favorable for strengthening the photocatalytic property. Among all the as-obtained photocatalysts, ZnO/CeO2-3 owns the largest Kap value of 0.039 min-1, which is almost 6.5 and 4.3 times than pure ZnO (0.006 min-1) and CeO2 (0.009 min-1).


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Figure 8. UV-vis absorption spectra of photoctalytic RhB degradation over (a) Blank, (b) ZnO, (c) CeO2 and (d) ZnO/CeO2-3 under the light irradiation. (e) Photodegradation efficiency towards RhB measured at 554 nm and (f) the photocatalytic rate curves and corresponding fitted kinetics curves of blank experiment, CeO2, ZnO and ZnO/CeO2 composites under the light irradiation.



To investigate the photocatalytic stability of ZnO/CeO2 composites, the cycling photocatlytic experiments for ZnO/CeO2-3 composites towards RhB photogradation were performed, as illustrated in Figure 9a, the degaradation efficiency of ZnO/CeO2-3 exhibits almost no decrease even after three cycling. Besides, no obvious change is observed on the XRD patterns of ZnO/CeO2-3 before and after photocatalysis (Figure 9b). The results suggest that the ZnO/CeO2-3 can maintain good stability and durability during the photocalytic reaction.

Figure 9. (a) Cycling experiments of photodegradation RhB over ZnO/CeO2-3. (b) XRD patterns of ZnO/CeO2-3 before and after photocatalysis.

Photocatalytic Mechanism. To research the photophysical characteristic of photoinduced charge carriers, the Time-resolved fluorescence spectra (TRPL) of ZnO, CeO2 and ZnO/CeO2-3 are obtained. As presented in Figure 10a, the decay curves from these TRPL are exponentially fitted and the corresponding fitted parameters are presented in Table S2. Hereinto, the short lifetime (τ1) and the long lifetime (τ2) reveal the radiative recombination and nonradiative relaxation process of photogenerated charge carriers, respectively.7 Their average life time (τPL) can be determined by the


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following equation:41 τPL= (A1τ12 + A2τ22)/ (A1τ1 + A2τ2)

(1)

As seen from Table S2, the τ1 of pure ZnO and CeO2 are 3.0 and 2.6 ns, respectively, while that of ZnO/CeO2-3 is increased up to 4.4 ns. The long lifetime (τ2) increases from 8.1 ns in CeO2 and 10.7 ns in ZnO to 14.5 ns in ZnO/CeO2-3. Meanwhile, the according percentages of τ1 and τ2 in the composites only change a little compared with those in the ZnO and CeO2. The resulting average life time (τPL) of ZnO/CeO2-3 is 12.2 ns, which is quite longer than 6.9 ns for CeO2 and 8.6 ns for ZnO. The results suggest that the lifetime of photoinduced charge carriers for ZnO/CeO2 heterojunction are prolonged, which boosts the possibility of their involvement in the photodegradation process, leading to the improvement of photocatalytic property. 42



Figure 10.

(a) Time-resolved fluorescence spectra (TRPL), (b) Photocurrent

transient curves and (c) electrochemical impedance spectroscopy (EIS) of ZnO, CeO2 and ZnO/CeO2-3.

The photocurrent transient curves and EIS were recorded to probe the separation and transfer dynamics of photoexcited charge carriers. In Figure 10b, the photocurrent transient curves of ZnO, CeO2 and ZnO/CeO2-3 were obtained with several turn on-off cycles. Clearly, the transient photocurrent density of ZnO/CeO2-3 is far higher than those of pristine ZnO and CeO2, meaning the photoinduced charge carriers on this composites are effectively separated, leading to relatively low recombination rate.43 Moreover, Figure 10c shows the electrochemical impedance spectroscopy (EIS) with the analog circuit (inset) of ZnO, CeO2 and ZnO/CeO2-3, where Rctr, CPE and


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Rsr correspond to the transfer resistance, the constant phase element and the solution resistance, respectively. After fitting the EIS plots based on the analog circuit, the Rctr values are calculated to be 2.1, 4.9 and 7.0 kΩ for ZnO/CeO2-3, ZnO and CeO2, respectively. The ZnO/CeO2-3 has the lowest Rctr, suggesting that the heterojunction encounters fewer obstacles and greatly facilitates the migration of photogenerated charge carriers.44 The effective separation and transfer of charge carriers in ZnO/CeO2-3

heterojunction

are

quite

favorable

for

the

enhancement

of

photodegradation property. The electron paramagnetic resonance (EPR) spectra were measured to analysis those active species generated in the photodegradation systems of ZnO, CeO2 and ZnO/CeO2-3. Typically, the superoxide radicals (•O2−) and hydroxyl radicals (•OH) produced

in

the

photodegradation

system

can

be

easily

captured

by

5,5-Dimethyl-l-pyrroline N-oxide (DMPO) in aqueous solution and methanol to produce the DMPO-•OH and DMPO-•O2− adducts, respectively.45,46 As shown in Figure 11a, after 60 s irradiation, the EPR signals of DMPO-•O2− adducts can be examined in the presence of ZnO, CeO2 and ZnO/CeO2-3, revealing that the •O2− are produced in the photocatalytic systems of those samples. The weak signals of DMPO-•O2− can be observed in the pure ZnO photocatalytic system, suggesting the ZnO is unfavorable for producing the active species of •O2− radicals. Meanwhile, the CeO2 shows a relatively stronger EPR signal compared with pure ZnO component, revealing that the CeO2 could produce more active species of •O2− radicals than that of ZnO. Importantly, ZnO/CeO2-3 exhibits the strongest signals of DMPO-•O2−



among all as-prepared samples, which convincingly reveals that ZnO/CeO2-3 heterojunction could produce more active species of •O2− radicals than pristine components under identical experimental condition. Additionally, the EPR signal intensity of DMPO-•OH adducts for ZnO/CeO2-3 is much stronger than that of ZnO component, together with the negligible EPR signal of CeO2 as shown in Figure 11b. Those results suggest that pure CeO2 component cannot produce •OH radicals and the ZnO component can produce a small amount of •OH radicals, most probably due to different oxidation ability of the photogenerated holes in their valence bands. In particular, the ZnO/CeO2-3 heterojunction achieves the maximum amount of •OH radicals among all these as-obtained samples. Based on the abovementioned results, constructing the heterojunction between ZnO and CeO2 can obviously promote the production of •O2− and •OH radicals, which are considered as main reactive species to degrade the dye molecules during photodegradation process. To further certify the reactive species produced in the photdegradation reaction, several scavengers were added into the photocatalytic systems of ZnO, CeO2 and ZnO/CeO2-3. Typically, isopropanol (IPA), ammonium oxalate (AO) and benzoquinone (BQ) were employed to remove the photogenerated •OH, h+ and •O2−, respectively.

47,48

In Figure 11c, for ZnO and ZnO/CeO2-3, the addition of IPA and

AO cause slightly decrease of their degradation efficiencies, suggesting that •OH and h+ play crucial roles in the photodegradation system. Their photodegradation efficiencies are drastically reduced with the addition of BQ into those systems, revealing that the •O2− is the predominant oxidative species in photodegradation


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systems of ZnO and ZnO/CeO2-3. For pure CeO2, the degradation efficiency almost remains unchanged when the IPA and AO were dispersed in the solution, suggesting that •OH and h+ contribute less in the photocatalytic system. By contrast, when BQ was dispersed in the reaction system, a sharp decrease of the degradation efficiency is observed, implying that •O2− play a crucial role in the degradation system.

Figure 11. EPR spectra of the photodegradation systems for ZnO, CeO2 and ZnO/CeO2-3: (a) DMPO-•O2− and (b) DMPO-•OH. (c) Photocatalytic efficiencies of RhB for ZnO, CeO2 and ZnO/CeO2-3 with the addition of IPA, BQ, and AO. (d) The MS plots of ZnO and CeO2. The MS plots of pristine ZnO and CeO2 were obtained to investigate the band structures of ZnO/CeO2 composites. As presented in Figure 11d, the n-type



characteristic of both ZnO and CeO2 is verified by their positive slopes of MS curves. From Figure 11d, the Vfb values of CeO2 and ZnO are about -0.87 and -0.61 V (vs. NHE, pH 7), respectively. The Vfb position of n-type semiconductor is known to be approximately equal to their conduction band (CB) bottoms. Hence, the CB bottoms of CeO2 and ZnO are evaluated to be -0.87 and -0.61 eV, respectively. Considering their calculated band-gap energies, the valence band (VB) tops of ZnO and CeO2 are estimated to be 2.52 and 1.99 eV, respectively, based on Eg = EVB - ECB. According to the thermodynamic constraints, both •OH and •O2− radicals can be generated on ZnO for its suitable band edges, whereas for CeO2 only •O2− radicals can be produced.49 The result quite accords with the analysis of the trapping experiments. According to the band structures of ZnO and CeO2, there are two possible migration mechanisms of photoinduced charge carriers on the ZnO/CeO2 heterojunction as shown in Figure 12. If the migratory behaviors of photogenerated charge carriers in the ZnO/CeO2 composites comply with the mechanism of typeⅡ heterojunction, the photoinduced electrons would migrate to the CB of ZnO from the CB of CeO2 (Figure 12a). However, this transfer process of photoelectrons is not easy to achieve because it could cause the strong Coulomb repulsion between the photoelectrons in their CBs.50,51 The photoinduced holes on the VB of ZnO would migrate to the VB of CeO2 according to the typeⅡmechanism. Nevertheless, the photoinduced holes on the VB of CeO2 are incapable of oxidizing OH- to produce •OH radicals, since the VB edge potential of CeO2 (1.99 V) are more negative than the potential of OH-/•OH (2.24 V) as presented in Figure 12a. Actually, it should be


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noted that this typeⅡmechanism could simultaneously weaken the reducibility of photoelectrons and oxidizability of photoinduced holes, because the reduction reactions occur on ZnO with lower reduction potential and the oxidation reactions occur on CeO2 with lower oxidation potential. As a result, the ability to generate the active species in this typeⅡ photocatalytic system can be weakened. However, based on the analysis of EPR trapping experiments, the EPR signal intensity of DMPO-•O2− adducts for ZnO/CeO2-3 are stronger than intensity sum of DMPO-•O2− adducts for pure ZnO and CeO2, revealing that the number of •O2− radicals on the composites are greater than the total number of those on the ZnO and CeO2. Similarly, the number of •OH radicals on the ZnO/CeO2 composites are also larger than the total number of those on the pure components. These results are contradict with the abovementioned analysis of the type Ⅱ mechanism. Contrarily, these EPR experimental results evidence the construction of direct Z-scheme heterojunction in ZnO/CeO2 composites (Figure 12b). The XPS spectra of ZnO/CeO2-3 composites were tested before and after the light illumination to further validate the construction of Z-scheme heterojunction between ZnO and CeO2, as presented in Figure S4. Before the light illumination, the shift of Ce4+/Ce3+ peaks for ZnO/CeO2-3 to higher binding energies are observed compared with that for CeO2 (Figure S4a), while the peaks of Zn 2p1/2 and Zn 2p3/2 for ZnO/CeO2-3 shift to lower binding energies in comparison with those in pristine ZnO (Figure S4b). The results reveal that the photoelectrons of CeO2 transfer to the ZnO because of higher Fermi level of CeO2 in darkness. Interestingly, when the sample is



exposed on the light, the binding energies of Ce4+/Ce3+ peaks in ZnO/CeO2-3 show negative shift and those of Zn 2p1/2 and Zn 2p3/2 peaks display positive shift, compared with those of pure components. It can be inferred that the photoelectrons transfer from ZnO to CeO2 under the excitation. The result manifests that the heterojunction formed between ZnO and CeO2 belongs to the Z-scheme heterojunction. According to the abovementioned discussion and the analysis of trapping experiments, a direct Z-scheme mechanism over the ZnO/CeO2 composites can be proposed in Figure 12b. Upon the light irradiation, both pristine ZnO and CeO2 component in the ZnO/CeO2 composites are excited to generate the charge carriers in their CBs and VBs, respectively. Those pristine components present poor photodegradation property owing to the direct recombination of most photoinduced electron-hole pairs. After the construction of Z-scheme heterojunction, the photoelectrons of ZnO in the CB position could readily migrate to the VB position of CeO2 though the interface electronic field, resulting in effective separation of charge carriers through the heterojunction. Meanwhile, via this Z-scheme migration of photoinduced charge carriers, the photoelectrons and holes are gathered on more negative CB position of CeO2 and more positive VB position of ZnO, respectively. Thus, the ZnO/CeO2 composites simultaneously possess the strong reduction and oxidation ability in comparison with single component. In addition, compared with pristine ZnO and CeO2 photocatalysts, more photogenerated electrons in the CB of CeO2 would reduce oxygen moleculars into •O2− radicals and the photoinduced holes


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in the CB of ZnO would react with the H2O molecules to generate the •OH radicals for their favorable band edges. Those •O2− and •OH radicals would finally serve as reactive species to remove the RhB molecules during the photocatalytic reaction.52 Hence, with the formation of Z-scheme heterojunction, the photodegradation properties of ZnO/CeO2 composites are dramatically enhanced compared with pure ZnO and CeO2 components.

Figure 12. Schematic diagram of photogenerated electron-hole separation process for ZnO/CeO2 composites (a) typeⅡ heterojunction, (b) Z-scheme heterojunction.

CONCLUSIONS In summary, the novel ZnO/CeO2 Z-scheme heterojunction has been synthesized through the wet chemistry method and then heat-treatment process. After the deposition of CeO2 nanoparticles, all the ZnO/CeO2 composites exhibit promoted photodegradation efficiency of RhB compared with pristine ZnO and CeO2. The ZnO/CeO2-3 (mass fraction of CeO2, 30%) shows good durability and the best



photocatalytic RhB decomposition, which is almost 2.5 and 1.7 times than pure ZnO and CeO2, respectively. The improved photodegradation performance can be reasonably ascribed to the construction of direct Z-scheme heterojunction, which promotes the effective separation of photoinduced charge carriers and the enhanced redox capacity. This study offers a novel strategy to promote the photocatalytic property of ZnO via constructing the Z-scheme heterojunction for environmental remediation.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Characterization of photocatalysts, photocatalytic experiment, photoelectrochemical measurements, trapping experiments, EPR measurements, DFT calculations, SEM, EDS and UV-vis absorption spectra of photocatalytic RhB degradation.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail:[email protected]. Notes The authors declare no competing financial interest.


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ACKNOWLEDGEMENTS The authors of this work are deeply grateful for these financial supports by National Natural Science Foundation of China (51372179, 51772224).

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Hierarchical ZnO Decorated with CeO2 Nanoparticles as Direct Z

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