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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Synthesis of 0D/3D CuO/ZnO Heterojunction with Enhanced Photocatalytic Activity Linyu Zhu, Hong Li, Zirui Liu, Pengfei Xia, Yahong Xie, and Dehua Xiong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01933 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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The Journal of Physical Chemistry

Synthesis of 0D/3D CuO/ZnO Heterojunction with Enhanced Photocatalytic Activity

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

†

State Key Laboratory of Silicate Materials for Architectures Wuhan University of

Technology,

122

Luoshi

Road,

Wuhan,

430070,

P.

R.

China.

E-mail:

[email protected], [email protected]. ‡

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

Angeles, CA, 90095-1595, USA.

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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: Construction of heterojunction has aroused great interest recently in the photocatalysis field due to the special electronic band structure and unique physicochemical properties. In this work, a novel 0D/3D CuO/ZnO heterojunction was obtained via in-situ deposition of CuO nanoparticles on flower-like ZnO surface using wet chemistry method. After depositing CuO nanoparticles onto the ZnO, the CuO/ZnO heterojunction exhibits enhanced visible-light harvesting and effective separation of the photogenerated electron-hole pairs compared with those in the pure ZnO. The photocatalytic removal efficiency of phenol over CuO/ZnO heterojunction is up to 78% under the irradiation of the light, which is ~2 and ~4 times higher than those of the pristine ZnO and CuO, respectively. This composite also presents good durability and stability for phenol degradation in the photocatalytic reactions. Additionally, in the photodegradation system of CuO/ZnO heterojunction, the superoxide radicals (•O2−) and hydroxyl radicals (•OH) are confirmed as the active species by the trapping experiments. This research provides a promising way to achieve 0D/3D heterojunctions for the application in environmental purification and remedy.

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INTRODUCTION In recent years, various environmental damages, especially for the water pollutants, gradually threaten our humanity healthy as well as hinder economic development. Therefore, exploring efficient strategies to handle these environmental problems is urgent for the sustainable development of our society. Satisfactorily, photocatalytic technology acts as a green and promising method to mineralize organic pollutants into nontoxic inorganic substances, without producing any other pollutants.1,2 Driven by light irradiation, photocatalysts can produce electron-hole pairs and then these charge carriers participate in a series of redox reactions to generate reactive species, which are regarded as the efficient oxidizing agents for pollution removal.3 Since TiO2 was investigated as the photocatalyst for water splitting in 1972, many accessible semiconductor photocatalysts including WO3, V2O5 and ZnO have been well explored for photodegraded organic pollutants.4-6 Among these materials, ZnO, an important n-type semiconductor, has aroused tremendous interests for its noteworthy advantages of nontoxicity, easily controlled morphology and high redox potential.7 Many advances have been achieved in the synthesis of well-developed ZnO photocatalysts with various morphologies and structures.8,9 Nevertheless, ZnO is commonly accepted as a typical semiconductor with wide-bandgap (Eg=3.37 eV), giving rise to its photocatalytic activity in ultraviolet (UV) region.10 That is, the most portion of sunlight cannot be utilized, which severely restricts the improvement of degradation efficiency for ZnO photocatalysts. Besides, the fast recombination of charge carriers is also unfavorable to improve the photocatalytic activity of ZnO.11 3



Hence, for ZnO-based photocatalysts, attempts to expand visible absorption range and to facilitate separation of photogenerated electron-hole pairs would benefit the enhancement of photocatalytic property. Formation of the heterojunctions of ZnO and other narrow band-gap semiconductors is considered as a promising strategy to solve the drawbacks.12-13 Two main advantages of these heterojunctions are as follows: (1) Once the electronic field is built near the interface of the heterojunction, the photogenerated electron-hole pairs would be effectively separated, indicating that more charge carriers can take part in the photocatalytic reactions.14,15 (2) The heterojunctions between ZnO and narrow band-gap semiconductors exhibit photoresponse over the wide range from UV to visible region as well as the enhancement of the light absorption ability. 16 Among all types of heterojunctions, the p-n heterojunction, as one of the most prospective heteroblocks, also attracts great attention for its favorable potential to realize the above two merits.17 Cupric oxide materials is one of the promising candidates for fabricating p-n heterojunctions with ZnO, since it has the narrow band gap, visible light response, nontoxicity and stable properties.18 Several works on preparing CuO/ZnO p-n heterojunction have been reported to investigate the gas-sensor or photocatalytic activity.16,19,20

Nevertheless, the complex synthesis strategies used in

these works would result in uncontrollable micro-structure of CuO/ZnO p-n heterojunction.16,20 Moreover, for the photocatalytic system of CuO/ZnO p-n heterojunction, uncovering photocatalytic mechanism based on the detailed experiments is still lacking and challenging. 4


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In this work, we prepare the 0D/3D CuO/ZnO p-n heterojunction by in-situ deposition of p-type CuO nanoparticles on the surface of flower-like ZnO nanorods. At the interface of CuO/ZnO p-n heterojunction, the internal electric field would be generated due to the difference of Femi levels. As a result, the reverse transfer of electrons and holes across p-n heterojunction interface can be easily achieved, leading to improved dynamics of photogenerated charge carriers. Hence, on the basis of this favorable electronic structure, the CuO/ZnO heterojunction exhibits better performance of photocatalytic phenol degradation than that of pristine components. During the photocatalytic reactions, the active species of the CuO/ZnO heterojunction for dye degradation can be confirmed as •OH and •O2−, and the photocatalytic mechanism has been investigated and expounded based on the experimental results.

EXPERIMENTAL SECTION Synthesis of flower-like ZnO. Flower-like ZnO was obtained by hydrothermal method. Specifically, 1.09 g of znic acetate dihydrate (Zn(C2H3O2)2·2H2O) was dissolved in 30 mL distilled water. To dissolve completely, the solution was stirred rapidly. Next, 2 mL hydrazine hydrate (50 % mass fraction) was added by dropwising into the above mentioned solution of znic acetate dihydrate. The mixture was then sealed into a teflon-lined reaction kettle. The hydrothermal reaction for 24 h at 150 ℃ was done. The resultant white precipitates were fully washed with deionized water and ethanol for several times. Finally, the as-prepared sample was thoroughly dried at 60 ℃ for 12 h in an oven under the atmospheric conditions. 5



Synthesis of CuO/ZnO p-n heterojunction. The CuO/ZnO p-n heterojunction was synthesized by wet chemistry method. 0.26 g of the as-prepared flower-like ZnO and 0.05 g of acetate copper monohydrate (Cu (C2H3O2)2·H2O) were dispersed in 50 mL dimethylformamide (DMF) and continuously stirred for 10 min. Subsequently, the beaker involving the above mentioned mixed solution was put in a water bath kept at 90℃ and continually stirred for 3.5 h. After that, the sample was centrifuged from the above mentioned solution and adequately washed with deionized water and ethanol for several times. Finally, the as-fabricated composite was thoroughly dried at 60 ℃ for 12 h. The pristine CuO was prepared on the basis of the above method without adding flower-like ZnO.

RESULTS AND DISCUSSION The possible formation mechanism can be illustrated in Fig. 1. When the flower-like ZnO was dipped in the copper salt solution, the Cu2+ could absorb the surface of ZnO because of the presence of abundant dangling bonds and surface oxygen with unsaturated coordination. With increasing temperature in DMF solution, these absorbed Cu ions bonded with the dangling bonds or surface oxygen could combine with the oxygen atoms and consequently form CuO crystal nucleuses on the surface of ZnO. Then these nucleuses gradually grew up and finally generated the p-n heterojunction with a close contact.

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Figure 1. Schematic illustration of the in-situ deposition method of CuO/ZnO heterojunction photocatalyst.

The crystalline structures of flower-like ZnO, CuO and CuO/ZnO composite were analyzed by X-ray diffraction (XRD) measurements. As shown in Figure 2, the hydrothermally prepared ZnO is indexed to the hexagonal wurtzite structure (JCPDS card No.36-1451) and the sharp diffraction peaks reveal its good crystallinity.21 In addition, all the diffraction peaks of pure CuO are well assigned to those for the monoclinic phase (JCPDS card No.48-1548).22 The peaks on the XRD pattern of CuO show large full width half maximum (FWHM) and low intensity, most probably due to the small grain size and poor crystallinity of CuO nanoparticles.23 Additionally, it should be noted that a weak diffraction peak of CuO phase, corresponding to (111) plane, can be observed on the XRD pattern of the CuO/ZnO composite as presented in Figure 2 (inset), suggesting that a small number of CuO nanoparticles are loaded on 7



the ZnO by the in-situ deposition method.21 Moreover, the peak intensity of ZnO component on the XRD pattern of the composite becomes lower compared with that of pure ZnO, probably resulting from the coating of CuO nanoparticles on the ZnO sureface.24

Figure 2. X-ray diffraction (XRD) patterns of ZnO, CuO/ZnO heterojunction and CuO, inset shows the expanded view of CuO/ZnO heterojunction from 2θ = 30° to 2θ =50°.

To investigate the morphologies and chemical constitution of the as-prepared samples, the FESEM observations and elemental mapping measurements were performed. Figure 3a shows the flower-like ZnO nanostructures with the micron-scale diameter of 1-3 μm. Each ZnO micro-flower is composed of the secondary nanorods with 100-200 nm in diameter as shown in Figure 3b and their surface is clearly smooth as indicated by the high magnification SEM image in Figure 3c (inset). 8


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Apparently, after the growth of CuO nanoparticles on the pristine ZnO, the color of the sample is changed from uniformly white for the pristine ZnO (inset, Figure 3a) to brown for the CuO/ZnO composite (inset, Figure 3d). Nevertheless, the morphology of the CuO/ZnO composite has negligible changed in comparison with that of the pristine ZnO as shown in Figure 3d, implying that the CuO nanoparticles are loaded on the surface of ZnO. This can be further confirmed by the FESEM images as presented in Figure 3e and 3f. The elemental mappings of CuO/ZnO heterojunction in Fig. 3g-i verify the composition of Zn, O and Cu elements. In addition, the distribution of Cu element is well corresponding to the elemental Zn distribution and also in accordance with elemental O mapping, suggesting the well distribution of CuO on the surface of ZnO in this composite.

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Figure 3. (a) Low-magnification and (b,c) high-magnification SEM images of ZnO, and (d) low-magnification and (e,f) high-magnification SEM images of CuO/ZnO heterojunction. Insets of (a) and (d) are digital photos of ZnO and CuO/ZnO heterojunction, respectively. (g-i) Elemental mapping images of CuO/ZnO heterojunction.

Figure 4 shows the TEM images and energy dispersive spectrum analysis (EDS) of CuO/ZnO composite, as well as the particle size distribution (PSD) of CuO nanoparticles on the surface of the ZnO nanorod. As seen from Figure 4a, CuO 10


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nanoparticles are loaded on the surface of ZnO secondary nanorods. Meanwhile, it should be noted that the composite in Figure 4b shows the obvious lattice fringes with the interplanar spacings of 0.247 and 0.232 nm, corresponding to the (101) of ZnO and (111) of CuO, respectively.25 These results imply a close contact formed between the CuO nanoparticles and flower-like ZnO substrate. In Figure 4c, the particle size distribution of CuO nanoparticles on the surface of the ZnO nanorod shows the various diameters in the range of 20-40 nm. As shown in Figure 4d, a weak peak of Cu element appears in the energy dispersive spectrum (EDS) of CuO/ZnO composite, revealing a small number of CuO nanoparticles loaded on the surface of the ZnO nanorod. This is in good accordance with the analyses of XRD patterns and TEM images.

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Figure 4. (a) Low-magnification and (b) high-magnification transmission electron microscope (TEM) images of CuO/ZnO heterojunction. (c) Particle size distribution (PSD) of CuO nanoparticles on the surface of the ZnO nanorod. (d) Energy dispersive spectrum analysis (EDS) of CuO/ZnO heterojunction using aluminum foil as substrate.

The formation of the CuO/ZnO heterojunction on the surface of the ZnO nanorod could be evidenced by the chemical states of surface elements in the XPS spectra of the as-prepared samples as shown in Figure S1 and Figure 5. In Figure S1, the survey spectra of the pure ZnO and CuO are dominated by the peaks due to Zn and O and those due to Cu and O, respectively. The survey spectrum of the CuO/ZnO composite exhibits the peaks due to the Zn, O and Cu elements. In Figure 5a, the high-resolution XPS spectrum of Zn 2p for the pure ZnO and that for the CuO/ZnO composite are compared. For the pure ZnO, the Zn 2p1/2 and Zn 2p3/2 peaks are observed at 1044.1 and 1021 eV, respectively. Therefore, the bonding energy difference between these two peaks is estimated to be about 23.1 eV, which is in accordance with the previous literature.26 For the CuO/ZnO composite, both the Zn 2p1/2 and Zn 2p3/2 peaks shift to the higher energies by 0.2 eV in comparison with those for the pure ZnO in Figure 5a, revealing that ZnO acts as the electron donor.27 Additionally, Figure 5b shows the high-resolution Cu 2p XPS spectra of the pure CuO and CuO/ZnO composite. For the pure CuO, two peaks at 933.9 and 953.8 eV correspond to the Cu 2p3/2 and Cu 2p1/2, respectively. In addition, there are two satellite peaks centered at about 944.0 and 12


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962.3 eV, demonstrating the bivalence oxidation state of elemental Cu.28 Moreover, for the CuO/ZnO composite, these peaks show slightly negative shift of 0.2 eV, revealing that CuO acts as the electron acceptor.29 In Figure 5c, the O1s XPS spectra of the ZnO, CuO/ZnO composite and CuO are well decomposed into two or three peaks. The peak due to the Zn-O bonding for the pure ZnO and that due to the Cu-O bonding for pure CuO are centered at 529.9 and 530.0 eV, respectively, which are very close to the absorbed oxygen peak located at 531.7 eV.

30

For the CuO/ZnO

composite, the positive shift of 0.2 eV for the Zn-O bonding and the negative shift of 0.3 eV for the Cu-O bonding can be observed. All the above mentioned peak shifts can be reasonably explained by the formation of p-n heterojunction between the CuO and ZnO components.

Figure 5. High-resolution XPS spectra of (a) Zn 2p, (b) Cu 2p and (c) O 1s for ZnO, 13



CuO/ZnO heterojunction and CuO. (d) Mott-Schottky plots of CuO/ZnO heterojunction. To further confirm the formation of p-n heterostructure between the ZnO and CuO nanoparticles, the Mott-Schottky measurement of the CuO/ZnO composite was carried out as shown in Figure 5d. Typically, the n-type or p-type semiconductors can be determined by the slope of their Mott-Schottky plots in the linear portion, where the positive slope indicates the n-type semiconductor and negative one indicates the p-type semiconductor.31 The appearance of both the positive and negative slopes in the linear portion in the Mott-Schottky plots of the CuO/ZnO composite is a typical p-n junction feature.32 This is fully consistent with the XPS and TEM analyses. The pore structure and specific surface areas of the ZnO, CuO/ZnO heterojunction and CuO were measured via Brunauer-Emmett-Teller (BET) method as presented in Figure 6. The parameters obtained from the BET method are also summarized in Table 1. In Figure 6a, the nitrogen adsorption-desorption isotherms of all the as-synthesized samples show the IV type with a H3 hysteresis loop according to BDDT (Brunauer-Deming-Deming-Teller) classification, revealing the presence of the slit-like pore structure formed by the aggregation of plate-like particles in those samples.22,33 In particular, CuO nanoparticles exhibit the largest amount of nitrogen adsorption and micropore volume among all the as-prepared samples, most probably due to the nanostructure of CuO. More importantly, the specific surface areas of the ZnO, CuO/ZnO heterojunction and CuO nanoparticles are 13, 92 and 226 m2/g, respectively. This indicates that the increased specific surface area of ZnO can be 14


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achieved by the modification of CuO nanoparticles.34

Figure 6. (a) N2 adsorption-desorption isotherm and (b) Barrett-Joiner-Halenda (BJH) pore size distribution plots of CuO, CuO/ZnO heterojunction and ZnO. Table 1 Experimentally obtained values of the total pore volume(Vpore), average pore width (Dpore) and specific surface area (SBET). Sample

Vpore(cm3/g)

Dpore(nm)

SBET (m2/g)

ZnO

.0.0

06011

06

CuO/ZnO

.0.6

70.8

29

CuO

.0.2

9022

991

15



The UV-Vis absorption spectra were measured and recorded to investigate the optical absorption processes in the ZnO, CuO/ZnO heterojunction and CuO. As illustrated in Figure 7, the pristine ZnO exhibits an absorption band edge at ~390 nm. The optical gap energy is estimated to be ~3.18 eV by Kubelka-Munk function (Figure 7b).35 Comparatively, CuO nanoparticles have the absorption band edge at ~1020 nm, corresponding to the photon energy of ~1.21 eV (see in Figure 7b). The CuO/ZnO heterojunction greatly enhances visible-light harvesting compared with the pristine ZnO, which is quite beneficial for utilizing the light and producing more photogenerated charge carriers.

Figure 7. (a) UV-Vis absorption spectra of ZnO, CuO/ZnO heterojunction and CuO. (b) Pots of the (αhν)1/2 νs photon energy (hν) for ZnO and CuO.

Photocatalytic property. The photocatalytic properties of the as-prepared samples were evaluated by photocatalytic degradation of phenol under the irradiation of the light. The photodegraded behaviors of blank experiment and as-prepared samples towards phenol solution were collected by the UV-vis absorption spectra as shown in 16


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Figure S2 and Figure 8a-c. For the blank experiments as shown in Figure S2, the intensity of the characteristic absorption peak due to the phenol molecules at 270 nm only changes a little with the extension of irradiation time, suggesting that the photolysis of phenol molecules is very weak. In addition, for all the samples, the concentration changes of the phenol solution with the irradiation time are measured at 270 nm and plotted in Figure 8d. The degradation efficiency for the CuO/ZnO heterojunction is up to the 78% after 180 min irradiation, while the degradation efficiencies for the pure ZnO and CuO samples are 38% and 20%, respectively. Furthermore, their kinetic behaviors for phenol photodegradation are quite complied with the pseudo-first order kinetic model: Ln(C0/C) = Kapt. Where, C0 and C represent the original (t = 0) and residual (at time t) concentration of phenol solution, as shown in Figure 8e.24,36 The slope of the kinetic curve represents the degradation rate constant (Kap). Besides, the larger Kap value means higher degradation efficiency for phenol. Specifically, the Kap value of the CuO/ZnO heterojunction is up to 7.5×10-3 min-1, which is much higher than those of the ZnO (2.8×10-3 min-1) and CuO (6.8×10-4 min-1). This result suggests that the as-prepared CuO/ZnO heterojunction photocatalysts can greatly improve the photodegraded activity of phenol in comparison with the ZnO and CuO under the light irradiation. More importantly, the cycling photodegradation experiments in Figure 8f show that the CuO/ZnO heterojunction still maintains good degradation efficiency for phenol after three cycling experiments. This indicates that ZnO could be durable and stable in the photocatalytic reactions after loading CuO nanoparticles on its surface. 17



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Figure 8. UV-vis absorption spectra of phenol photodegradation for (a) ZnO, (b) CuO and (c) CuO/ZnO heterojunction under solar light irradiation. (d) Photocatalytic phenol degradation activity and (e) fitted curves with pseudo-first order kinetic model of blank experiment, ZnO, CuO and CuO/ZnO heterojunction under the light irradiation.

(f)

Cycling

photodegradation

experiments

heterojunction.

18


for

the

CuO/ZnO

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Photocatalytic mechanism. To investigate the dynamics of the photogenerated charge carriers over as-prepared samples, the time-resolved PL (TRPL) spectra, photocurrent transient responses and electrochemical impedance spectroscopy (EIS) were measured as shown in Figure 9. The decay curves of the TRPL spectra for all the samples are well fitted using the bi-exponential function as presented in Figure 9a.37 The fitted parameters are summarized in Table 2. The lifetimes of 1 and 2 reflect the radiative process caused by combination of charge carriers and nonradiative energy transfer process, respectively.38,39 The average lifetimes for the ZnO, CuO/ZnO heterojunction and CuO are 4.1, 8.0 and 6.8 ns, respectively. Compared with the pristine ZnO and CuO, the CuO/ZnO heterojunction has the longest average lifetime, suggesting that the recombination of the photogenerated electron-hole pairs is effectively inhibited.40,41 This result means that the photoelectrons generated from CuO/ZnO heterojunction have more probability to participate in photocatalytic reactions, which would highly benefit the enhancement of photocatalytic activity.42

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Figure 9. (a) Time-resolved fluorescence spectra (PL), (b) transient photocurrent responses and (c) electrochemical impedance spectroscopy (EIS) for the CuO, CuO/ZnO heterojunction and ZnO.

Table 2 Kinetic parameters of the emission decay of ZnO, CuO/ ZnO heterojunction and CuO. Sample

PL(ns)

1(ns)(Rel.%)

2(ns)(Rel.%)

CuO/ZnO

8.0

3.9(83.1)

15.6(14.9)

CuO

6.8

3.0(87)

12.7(13)

ZnO

4.1

2.3(94.5)

10.8(5.5)

Figure 9b shows the transient photocurrent responses of pure ZnO, CuO/ZnO 20


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heterojunction and CuO, respectively. The transient photocurrent responses value is recorded with several turn on-off cycles. In comparison with the pure ZnO and CuO, the CuO/ZnO heterojunction shows obviously higher photocurrent density, suggesting more effective separation and faster transfer rate of the photogenerated electron-hole pairs.43 Electrochemical impedance spectra (EIS) were measured to evaluate the interfacial charge transfer resistance of the as-synthesized samples as illustrated in Figure 9c. The electrochemical impedance spectroscopy plots can be well fitted according to the analog circuit in the inset of Figure 9c, where the Rsr is a solution resistance, CPE is equivalent to the constant phase element and Rctr represents the interfacial transfer resistance. The Rctr values of the CuO/ZnO heterojunction, pure ZnO and pure CuO can be calculated to be 240 Ω, 680 Ω and 1250 Ω, respectively. The lower Rctr value of the CuO/ZnO heterojunction indicates that the photogenerated charge carriers encounter smaller transfer resistance during the migration process from the bulk to the surface, compared with each components.44 This suggests that construction of p-n heterojunction can efficiently facilitate effective transfer of photogenerated charge carriers via the built-in electric field, which would endow the photogenerated charge carriers with more probability involved in photocatalytic reactions. To explore the reactive species generated in the photocatalytic reactions, the EPR spin-trapping experiments were employed in the photocatalytic system of the ZnO, CuO/ZnO heterojunction and CuO under the irradiation of the light. The generated hydroxyl radicals (•OH) and superoxide radicals (•O2−) can be easily trapped by the 21



5,5-dimethyl-1-pyrroline N-oxide (DMPO) and form the adducts of DMPO-•OH and DMPO-O2•, respectively. As presented in Figure 10a, no EPR signals for the blank experiment can be observed. Once the as-prepared samples are exposed under the light irradiation, the EPR signals of DMPO-•O2− adduct are obviously observed, suggesting that the •O2− are generated during the photocatalytic reactions. In particular, the EPR signals caused by the CuO/ZnO heterojunction are remarkably stronger than those of the pure ZnO and CuO, revealing that much more •O2− are produced under the light irradiation. Meanwhile, the EPR spectra of the DMPO-•OH adduct for all the as-prepared samples are shown in Figure 10b. The negligible EPR signals are detected for the CuO sample, suggesting that the CuO cannot produce the •OH in the photocatalytic reaction. Nevertheless, the CuO/ZnO heterojunction shows the strongest EPR signal intensity among all the as-prepared samples, indicating that a maximum amount of •OH can be achieved in the CuO/ZnO heterojunction system. All the above mentioned results confirm that the CuO/ZnO heterojunction can generate more reactive species than those of the ZnO and CuO under the light irradiation, which is particularly favorable to the pollutant photodegradation. Furthermore, the trapping experiments of reactive species for phenol degradation over the ZnO, CuO/ZnO heterojunction and CuO photocatalysts were also carried out as shown in Figure 10c. Isopropanol (IPA), benzoquinone (BQ) and ammonium oxalate (AO) are employed as the scavengers to the remove •OH, •O2− and photogenerated h+ during the photocatalytic reactions.45 For all the three photocatalysts, the addition of BQ causes an obvious decrease for phenol 22


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photodegradation, while the photodegradation efficiency reduces a little in the presence of AO and IPA. These results reveal that the •O2− are the primary species, whereas the •OH and the photogenerated h+ contribute less to the phenol degradation in these photocatalytic system.

Figure 10. EPR spectra of (a) DMPO−•O2−and (b) DMPO-•OH adducts in the photocatalytic systems of ZnO, CuO and CuO/ZnO heterojunction after 60 s irradiation of the light. (c) Photodegraded efficiencies of phenol over ZnO, CuO/ZnO heterojunction and CuO photocatalysts with the addition of IPA, BQ and AO. (d) Proposed photocatalytic mechanism of CuO/ZnO heterojunction for phenol degradation.

To determine the electronic band structure of the CuO/ZnO bandstructure, the 23



Mott-Schottky measurements were also performed for the pure ZnO and CuO. The Mott-Schottky plots of pure ZnO and CuO are shown in Figure S3. The as-prepared ZnO sample shows a positive slope peculiar to the n-type semiconductor , while the CuO sample shows a negative slope peculiar to the p-type semiconductor.31 Based on the Mott-Schottky plots, the flat-band potential (Vfb) of the as-prepared ZnO and CuO can be evaluated to be -0.62 V and 0.50 V (vs. NHE, pH=7), respectively. For the n-type semiconductor, the Vfb and Fermi level are close to its conduction band (CB) bottom, whereas the Vfb and Fermi level of the p-type semiconductor are near to its valence band (VB) top.32 Considering the band gap (Eg = EVB-ECB) of the as-prepared samples, their CB and VB edges can be calculated and summarized in Table S1. According to the above mentioned results, the thermodynamic processes of producing the reactive species over the as-prepared samples can be reasonably described as follows: For the ZnO component, the photoexcited electrons in the CB can reduce the oxygen molecules into the •O2− because the CB bottom (−0.62 V vs. NHE) of ZnO is more negative than the potential of the O2/•O2−(−0.33V vs. NHE).46 On the other hand, the holes left in the VB of ZnO would react with the H2O molecules to produce the •OH because the VB top (2.57 V vs. NHE) of ZnO is more positive than the potential of the OH−/•OH and H2O/•OH (1.19 and 2.27 V vs. NHE).45 For the CuO component, the photoexcited electrons in the CB can produce the •O2− via reacting with the absorbed O2, since the CB bottom (−0.71 V vs. NHE) of CuO is more negative than the potential of the O2/•O2−. However, the photogenerated holes in the VB of CuO would not oxidize the H2O molecules into the •OH because the potential of the 24


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OH−/•OH or H2O/•OH (1.19 and 2.27 V vs. NHE) is more positive than that of CuO VB top (0.5 V vs. NHE). The above mentioned thermomechanical analyses are in good accordance with the results obtained by the trapping experiments. On the basis of all the above mentioned results and discussions, most probable mechanism for the enhanced photocatalytic activity in the CuO/ZnO heterojunction is proposed in Figure 10d. In-situ deposition method would endow the intimate interface between the ZnO nanorod and the CuO nanoparticle with stronger interfacial interaction. Thus, in the dark, the electrons in the ZnO nanorod would migrate to the CuO nanoparticle because ZnO has higher Fermi level than that of CuO. As a result, a positively charged depletion layer near the ZnO component and a negatively charged accumulation layer near the CuO component are formed. Hence, the built-in electric field is finally generated between the n-type ZnO and p-type CuO. Under the light irradiation, the electrons in ZnO and CuO are excited into their CBs, resulting in the rich-electron regions in their CBs and the rich-hole regions in their VBs. After that, the migration of photoelectrons in the CB of CuO to the CB of ZnO and that of the photogenerated holes in the VB of ZnO to the VB of CuO are driven by the built-in electric field, leading to the spatial separation of electron-hole pairs. Additionally, the CB bottom of CuO is more negative than that of ZnO, which is also beneficial to the injection of the photoelectrons from the CB of CuO into that of ZnO, resulting in the efficient separation of photogenerated charge carriers via the p-n junction. Therefore, more photoelectrons accumulated in the CB of ZnO in the CuO/ZnO heterojunction can be used for producing the •O2− in comparison with the single ZnO and CuO. In 25



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addition, a small number of photogenerated holes remaining in the VB of ZnO component can also generate the •OH. Although the CuO/ZnO heterojunction would be considered as no advantages in producing the •OH based on the thermodynamic analyses, the EPR and trapping experiments demonstrate that the CuO/ZnO photocatalytic system produces the most amount of the •OH among all the as-prepared samples. Hence, these •OH in the photocatalytic system most probably originate from further reduction of partial •O2−, as follows: 47,48 e− +O2→ •O2−

(1)

•O2− +e− +2H+→ H2O2

(2)

H2O2→ 2•OH

(3)

•O2− +H2O → •OH + OH− + O2

(4)

Some of the •O2− are finally reduced into the •OH by combining with the photogenerated electrons. These •O2− and •OH in the CuO/ZnO system actually act as the reactive species to directly degrade the dye molecules under the continuous irradiation of the light. Thus, the CuO/ZnO composite with the p-n heterojunctions could generate more reactive species and enhance the photocatalytic activity.

CONCLUSIONS In summary, a novel CuO/ZnO p-n heterojunction was prepared by in-situ deposition of CuO nanoparticles on the surface of flower-like ZnO. The photocatalytic removal efficiency towards phenol over the CuO/ZnO heterojunction is up to the 78% under 26


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the irradiation of the light, which is ~2 and ~4 times higher than those of the pristine ZnO and CuO, respectively. The enhanced photocatalytic activity can be reasonably attributed to the formation of p-n heterojunction, which exhibits enhanced visible-light harvesting and fast separation of the photogenerated electron-hole pairs compared with the pure ZnO. Moreover, the active species are also evidenced as the •O2− and •OH by the trapping experiments. This work provides an effective strategy to construct 0D/3D heterojunctions for high-efficiency photocatalysts.

ASSOCIATED CONTENT Supporting Information Characterization details, photocatalytic experiments, photoelectrochemical measurements, trapping experiments, EPR measurement, XPS survey spectra, blank experiment for phenol photodegradation, Mott-Schottky plots. The Supporting Information is available free of charge on the ACS Publications website.

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

27



ACKNOWLEDGEMENTS The authors would like to express our sincere thanks for the financial supports by the National Natural Science Foundation of China (51372179, 51772224), Hubei Province Foreign Science and Technology Project (2016AHB027) and Science and Technology Planning Project of Hubei Province (2014BAA136).

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