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n-ZnO microspheres with enhanced photocatalytic H2 production
Chemical Physics Letters 734 (2019) 136748
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Facile synthesis of hollow p-Cu2O/n-ZnO microspheres with enhanced photocatalytic H2 production
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Yong-Hui Zhang, Xiao-Li Cai1, Yu-Liang Li1, Ming-Ming Liu, Cheng-Long Ding, Jun-Li Chen, Shao-Ming Fang College of Materials and Chemical Engineering, Collaborative Innovation Center of Environmental Pollution Control and Ecological Restoration, Zhengzhou University of Light Industry, Zhengzhou 450002, PR China
H I GH L IG H T S
Cu O/ZnO heterojunction with novel hollow microspheres was prepared. • The CZ-20 composite exhibits excellent photocatalytic performance. • The • The photocatalytic H production could be improved due to the p-n heterojunction. 2
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A R T I C LE I N FO
A B S T R A C T
Keywords: Cu2O/ZnO p-n junction Photocatalytic H2 production
The Cu2O/ZnO heterojunction with novel hollow microspheres was prepared via a facile one-step hydrothermal method. The morphology was significantly changed from octahedron to hollow microsphere after adding zinc acetate. The microsphere Cu2O/ZnO materials exhibited superior photocatalytic hydrogen yields (129.6 μmol/ g), almost 10.3 times higher than pure Cu2O. The separation of photogenerated electron-hole pairs were effectively promoted due to the formation of p-n heterojunction between ZnO and Cu2O. In addition, the hollow structure and shell permeability of Cu2O/ZnO microsphere could also improve the utilization of visible light.
1. Introduction With the consumption of fossil fuel and increasing environmental problems, the developing of clean and efficient energy is urgent [1]. As a renewable and carbon-free power source, solar energy could be converted into chemical or electrical energy by photoelectric effect [2–7]. Cu2O with a band gap of 2.17 eV, is a typical p-type semiconductor material with excellent visible light absorption capacity [8–10]. It has been widely used in solar cells, photocatalytic application, and gas sensors due to their unique environmental friendliness and low cost [11–16]. However, the electron-hole pairs will be combined rapidly due to its own thermal vibration in the reaction process, which restrains the photocatalytic efficiency of the materials [17,18]. According to previous investigations, Cu2O could combine with other semiconductor materials (TiO2, CdS, WO3, ZnO), noble metal (Au, Ag, Pt) or carbon materials to improve its photocatalytic performance [19–25]. ZnO, a wide band gap (3.37 eV), is an excellent n-type semiconductor photocatalytic material with the advantages of good
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chemical stability, high reaction activity, low price and low toxicity [26–28]. It has been widely used in photocatalytic and sensing fields [29,30]. Li et al. reported the flower-like ZnO hierarchical structure exhibited an enhanced photocatalytic performance for photodegradation of RhB near the visible light region [31]. Eseoghene et al. synthesized Pd-ZnO-exfoliated graphite photocatalyst for degradation the acid orange 7 dye by a one-pot hydrothermal method [32]. Piña-Pérez et al. prepared a novel ZnS-ZnO composites with the efficient photocatalytic hydrogen via a solvothermal method at low temperatures [33]. However, ZnO can only absorb ultraviolet light, and its photocatalytic efficiency is not high, which is mainly due to its low conductivity band position and wide band gap [34,35]. When Cu2O is combined with n-type semiconductor ZnO, the n-p junction can form a built-in electric field. The photogenerated electrons produced by Cu2O can be efficiently transmitted to the conduction band of ZnO. It is beneficial to the separation of photogenerated electron-hole pairs [36–38]. Furthermore, previous investigates based on Cu2O/ZnO composite material have been also reported. Kung et al. [2] reported Cu2O nanostructure of truncated octahedral exhibits photonic Fano
E-mail addresses: [email protected] (Y.-H. Zhang), [email protected] (J.-L. Chen), [email protected] (S.-M. Fang). These authors contributed equally to this work and should be considered co-second authors.
https://doi.org/10.1016/j.cplett.2019.136748 Received 15 August 2019; Received in revised form 4 September 2019; Accepted 5 September 2019 Available online 06 September 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics Letters 734 (2019) 136748
Y.-H. Zhang, et al.
resonance compared with the other shapes by FDTD simulation. Kramm et al. [39] investigates the band alignments of the Cu2O/ZnO heterointerface. However, few literatures have reported the photocatalytic performance of Cu2O/ZnO composite with hollow structure. Therefore, the exploration of strategies to effectively tune the hollow structure is important and meaningful for photocatalytic material. In this work, Cu2O/ZnO microspheres with hollow structure were synthesized by a facile one-step hydrothermal method. The structure and optical performance of the Cu2O/ZnO microspheres were investigated systematically by XRD, FESEM, TEM, UV-DRS and PL spectra. The morphology of the Cu2O/ZnO heterojunction exhibits hollow microspheres with the size of about 1.03 μm after adding zinc acetate. The highest H2 production for Cu2O/ZnO microspheres (129.6 μmol/g) was 10.3 times higher than that of pure Cu2O (12.5 μmol/g). It could be attributed to synergistic effects between p-n heterojunctions and the unique hollow structures.
Fig. 1. X-Ray diffraction of truncated octahedral Cu2O (red line) and spherical Cu2O/ZnO-0.2 (black line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2. Experiment
(Beijing Perfect Light, Microsolar 300) was used as the light source. The intensity remains between 52 mW/cm2 and 55 mW/cm2.
2.1. Materials All the reagents including Copper acetate hydrate (Cu(Ac)2·H2O), Zinc acetate dihydrate (Zn(Ac)2·2H2O), sodium bis (2-ethylhexyl) sulfosuccinate (NaAOT, 97%, Aldrich) and L-ascorbic acid (C6H8O6, 99.7%, Kermel). The deionized water (18.2 MΩ cm−1) was used throughout the experiments.
3. Results and discussion Fig. 1 illustrates the XRD patterns of the pure truncated octahedral Cu2O and spherical CZ-20 heterojunction. For the pure Cu2O, the major diffraction peaks are indexed at 29.6°, 36.4°, 42.5°, 61.3°, 73.5° and 77.3°, which are associated with the (1 1 0), (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) facets of the cubic Cu2O phase (JCPDS No. 05-0667). For the CZ-20 heterojunction, the weak peaks located at 34.4°, 47.5° and 56.6°, corresponding to (0 0 2), (1 0 2) and (1 1 0) planes of hexagonal wurtzite ZnO (JCPDS No. 36-1451). Besides, the ZnO peaks of the CZ-20 were weak due to the low content [40]. The surface morphologies and structure of the truncated octahedral Cu2O and spherical CZ-20 heterojunction are shown in Fig. 2a–d. The FESEM image of the pure Cu2O shows homogeneous distribution of the truncated octahedral particles with an average edge length of approximately 0.75 μm (Fig. 2a–b). The surface of the pure Cu2O is smooth. After the formation of the Cu2O and ZnO heterojunction, the material exhibits spherical and the average edge length is 1.03 μm. (Fig. 2c–d). After the hydrothermal reaction, it is found that the content of zinc acetate is vital to the morphology of the materials, which is mainly due to the effect of Zn2+. Fig. 3e and g shows the TEM images of the pure Cu2O and Cu2O/ZnO heterojunction, respectively. The HRTEM images (Fig. 2f) of the pure Cu2O exhibited the crystal lattice spacing is 0.21 nm, corresponding to the {2 0 0} plane of Cu2O crystal. In addition, the lattice spacing of CZ-20 is 0.26 nm (Fig. 2h), which was consistent with the {0 0 2} plane of ZnO crystal [41]. The insert in Fig. 3f and h show the selected area electron diffraction (SAED) of the Cu2O and Cu2O/ZnO heterojunction. The as-prepared truncated octahedral Cu2O demonstrated single crystal behavior, while Cu2O/ZnO composites exhibited single crystal diffraction points of ZnO and Cu2O, respectively. Fig. S1 shows the EDS mapping spectra of CZ-20, which confirm the existence of Cu, Zn and O elements. The Zn, Cu, and O elements are uniformly distributed in the whole hollow Cu2O/ZnO microsphere. The growth mechanism of Cu2O/ZnO hollow microspheres is shown in Scheme S1e. In the initial stage of hydrothermal process, the metal salts (Cu(Ac)2 and Zn(Ac)2) slowly dissolve and release the corresponding metal cations (Cu2+ and Zn2+). In the second step, metal cations (Cu2+ and Zn2+) react with OH− to form hydroxide (Cu (OH)42− and Zn(OH)2 under alkaline conditions. Subsequently, [Cu (OH)4]2− and Zn(OH)2 can be translate into Cu2O and ZnO due to the strong reducibility of ascorbic acid. The specific reaction is as follows:
2.2. Preparation of spherical Cu2O/ZnO heterojunction Typically, 0.0205 g Cu(Ac)2·H2O and a certain amount of Zn (Ac)2·2H2O (0.0033 g-15 wt%, 0.0044 g-20 wt% and 0.0066 g-30 wt%) were dispersed and sonicated in 80 mL deionized water to form a homogeneous solution. 0.1335 g AOT was dissolved in 10 mL n-butyl alcohol. Then the mixed aqueous solution was added to AOT solution drop by drop and kept stirring for 1 h. After that, 0.0705 g ascorbic acid was introduced to above solution and kept stirring for 30 min. Then, 2 mL of 1.0 M NaOH was dripped into the solution and stirred for 2 h. Finally, the suspension was transferred to a 100 mL Teflon-lined reactor and reacted at 80 °C for 3 h. The solid products were centrifuged, washed and dried at 60 °C for 6 h. The as-prepared Cu2O/ZnO samples were denoted as CZ-15, CZ-20 and CZ-30, respectively. The synthesis of pure truncated octahedral Cu2O was supplied in the Supporting Information. 2.3. Characterization X-ray diffraction patterns (German Bruker D8) were analyzed the phase structure of the materials. The microstructures of samples were characterized by field emission scanning electron microscopy (FESEM, JSM-7001F), transmission electron microscopy (TEM, JEM-2100) and high-resolution transmission electron microscopy (HRTEM, 200 kV). The X-ray photoelectron spectrometer (XPS) was performed by an ESCALAB-250Xi spectrometer. The optical properties of the samples were analyzed by a UV–vis absorption spectra (UV-U3900, Hitachi) and photoluminescence spectra (PL F-7000, Hitachi). The efficiency of photocatalytic H2 production was measured using Agilent Technologies 7890A GC-MS analyzer. 2.4. Photoelectrochemical measurements All photoelectrochemical tests were carried out in a three-electrode system on a CHI660D electrochemical workstation. Electrochemical impedance spectroscopy (EIS) was performed in a 1:1 mixture ratio of 5 mM of K3[Fe(CN)6]/K4[Fe(CN)6] and 0.1 M of KCl solution at a frequency range from 100 kHz to 0.01 Hz. Photocurrent experiments were tested in 0.1 M Na2SO4 electrolyte solution. A 300 W Xenon lamp
Cu2+ + 4OH− → [Cu(OH)4]2− 2
(1)
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Fig. 2. FESEM of the truncated octahedral Cu2O (a, b), and spherical CZ-20 composites (c, d). TEM images (e, g) and the corresponding SAED patterns (inset, SAED pattern of Cu2O and CZ-20). High-resolution TEM images of the square regions revealing the (2 0 0) and (0 0 2) planes of Cu2O (white) and ZnO (black), respectively.
Zn2+ + 2OH− → Zn(OH)2
uniform octahedral without zinc salt. After adding zinc salt (including zinc acetate, zinc sulfate, zinc nitrate) in Fig. S2, the composites become a uniform spheres. The evolution of the hollow structure is usually related to the Ostwald ripening mechanism, which can lead to the formation of hollow shell by consuming internal material and recrystallizing process [42]. (2) The ratio of Zn(Ac)2 to Cu(Ac)2 is vital to the formation of hollow Cu2O/ZnO microspheres (in Fig. S3 and Fig. S4). With the increase of zinc salt content, the morphology of the composites gradually become hollow spheres. However, when further increase of zinc salt content, the small particles of ZnO could be precipitated and gradually become irregular particles.
(2)
2[Cu(OH)4]2− + 3C6H8O6 (ascorbic acid) → Cu2O + 3C6H6O6 + 7H2O (3) Zn(OH)2 → ZnO + H2O
(4)
Scheme S1a–d shows the formation process of various morphology of Cu2O/ZnO composites. It is worth noting that Zn2+ is vital to the growth process of the Cu2O/ZnO hollow structure: (1) No Cu2O/ZnO hollow microspheres were formed without zinc salt on the experiment. Under the same other reaction conditions, the morphology of Cu2O is
Fig. 3. (a) XPS of survey spectra of spherical Cu2O/ZnO heterojunction, (b) Cu 2p; (c) Zn 2p XPS spectrum of Cu2O/ZnO. 3
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Fig. 4. (a) UV-DRS absorption spectra (the inset is the curves of (αhν)2 vs. hν) and (b) Photoluminescence (PL) emission spectra of the truncated octahedral Cu2O and spherical Cu2O/ZnO heterojunctions.
separation capacity of as-prepared materials. As shown in Fig. 5b, four samples of Cu2O/ZnO were prepared with different loading amount of ZnO (0, 15, 20, and 30 wt%) and performed for their photoelectrochemical test. The prompt increase in the photocurrent response could be ascribed to that the generation of photogenerated electrons induced by illumination, and the electrons directional movement was driven by external voltage. The higher photocurrents were generated for the Cu2O/ZnO samples than that of the pure Cu2O. CZ-20 possesses a maximum current density, which is about 1.5 times compared to pure Cu2O. The CZ-20 catalyst exhibits excellent separation of photogenerated electron-hole pairs, which was also confirmed by the PL results. The photocatalytic performance of pure Cu2O and Cu2O/ZnO heterojunction were investigated by using CH3OH aqueous solution (20 vol %) as holes sacrificial reagent under the visible light irradiation. Fig. 6a shows comparable photocatalytic H2 production of all the materials. As expected, the photocatalytic hydrogen-producing performance of pure Cu2O is poor (only 12.5 μmol/g in 6 h), which is mainly due to the recombination of the electron-hole pairs. With increasing the ZnO (15 wt%, 20 wt% and 30 wt%), the H2 yield of Cu2O/ZnO heterojunction was 44.5 μmol/g, 129.6 μmol/g and 36.9 μmol/g, respectively. It is clearly found that CZ-20 displays the highest photocatalytic activity than those with a low or high content of ZnO. This fact could be explained that excessive ZnO can restrain the energy transformation on the surface of Cu2O. Fig. 6b shows the rates of H2 generation for all photocatalysts. The H2 generation rate of Cu2O is 2.1 μmol·g−1·h−1. For comparison, Cu2O/ZnO heterojunction exhibits an evidently enhanced activity. In particular, the hollow CZ-20 gives the maximum H2 generation rate of 21.6 μmol·g−1·h−1, that is about 10.3 times higher than that of Cu2O. It could be attributed to more active sites of p-n heterojunction in CZ-20 than that of CZ-15, which induces the material have excellent photocatalytic properties. With the increase of ZnO, a larger grain boundary barrier could inhibit the transport of electron. The stability of the CZ-20 photocatalyst was further investigated by the recycling test for three repeated cycles. From Fig. 6c, the H2 generation of the CZ-20 heterojunction only decreased by 7% after three cycles. It indicates that CZ-20 possesses high stability and excellent activity under the visible light irradiation. Fig. 6d shows the XPS spectra for Cu 2p of CZ-20 heterojunction before and after the photocatalytic reaction. It shows that the peaks area of Cu2+ had a slightly increase from 10.07% to 15.84%, illustrating a few of Cu+ may be oxidized during the photocatalytic reaction. VB-XPS spectroscopy of Cu2O and CZ-20 heterojunction are shown in Fig. 7a and b. From the VB-XPS spectra, the VB potentials of pure Cu2O is determined to be 1.18 eV. For CZ-20 heterojunction, the VB potential is 1.39 eV, which is 0.21 eV more positive than the pure Cu2O. Combining the results from UV-DRS and VB-XPS, the approximate band positions of the CZ-20 heterojunction was shown in Fig. 7c. Based on the previously experimental discussions, the photocatalytic mechanism
XPS spectra is executed to investigate the chemical composition and elemental analysis of the CZ-20 heterojunction. Fig. 3a shows the XPS survey spectrum of Cu2O/ZnO composites The peaks centered at 284.6, 569.6 and 932.6 eV are assigned to C 1s, O 1s and Cu 2p, respectively. It indicates the existence of Cu2O and ZnO in the Cu2O/ZnO composite. For Cu 2p XPS spectrum (Fig. 3b), two peaks at 932.2 and 952.1 eV can be assigned to the Cu 2p3/2 and Cu 2p1/2 of Cu+, respectively [43]. The weaker signals at 933.2 and 953.3 eV are attributed to Cu2+, which demonstrate that few Cu2O were oxidized to CuO on the surface of the composite. Fig. 3c shows the Zn 2p spectrum in Cu2O/ZnO heterojunction. The peaks located at 1021.2 and 1044.3 eV corresponded to binding energies of the Zn 2p3/2 and Zn 2p1/2, respectively [44]. Thus, Zn2+ exists mainly chemical states in the Cu2O/ZnO composite. The UV–vis diffuse reflectance spectroscopy (UV-DRS) of the truncated octahedral Cu2O and Cu2O/ZnO heterojunctions are compared in Fig. 4a. The pure truncated octahedral Cu2O exhibits an absorption wavelength at 640 nm (1.94 eV). The optical absorption edge and absorption intensity of Cu2O/ZnO heterojunction is substantially enhanced. It could be attributed to the increase of grain size of Cu2O/ZnO composites and the surface defects on the Cu2O induced by ZnO. The defect energy levels are produced, which can reduce the energy of the electron transition. It demonstrates that the solar energy could be used more effectively by Cu2O/ZnO composites [45]. The band gap estimation of the truncated octahedral Cu2O and CZ-20 heterojunction are calculated by the Kubelka–Munk equation. As shown in the inset of Fig. 4a, the band gap of pure Cu2O is approximately 1.94 eV in accordance with the previous report [46]. With increasing content of ZnO (15 wt% to 30 wt%), the band gap of Cu2O/ZnO heterojunction significantly reduced from 1.93, 1.88 to 1.85 eV. The narrower band gaps are mainly ascribed to the intimate interfacial contact between Cu2O and ZnO, which is essential to gain the visible light and promote the transfer of photoelectrons. PL spectra is widely performed to investigate the separation and transfer capacity of photo-induced charge carriers in the photocatalysts. Fig. 4b shows the PL spectra of pure Cu2O and Cu2O/ZnO heterojunctions under 460 nm (2.70 eV) excitation. It is clear that Cu2O displays a strong fluorescence emission peak centered at ~570 nm (2.17 eV). The PL intensity of CZ-20 significantly quenched after introducing ZnO, indicating the lowest recombination rate of photogenerated electron-hole pairs. To investigate the interfacial electron transfer behavior of the photocatalysts, the EIS of Cu2O and Cu2O/ZnO composites are shown in Fig. 5a. The EIS curves are composed of a semicircle in the high-frequency region and a linear in the low-frequency region [47]. The diameter of Cu2O/ZnO electrode is smaller than the electrode of Cu2O, indicating that the Cu2O/ZnO electrode exhibits a lower electrochemical resistance. The formation of the ZnO/Cu2O p-n junction can lead to the higher electron migration efficiency and the lower resistance of interfacial charge transfer. The transient photocurrent responses were carried out by five cycles irradiation to investigate the charge 4
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Fig. 5. (a) EIS of Cu2O and Cu2O/ZnO in a 1:1 mixture ratio of 5 mM of K3[Fe(CN)6]/K4[Fe(CN)6] and 0.1 M of KCl (the inset is the Nyquist plots of bare Au electrode); (b) photocurrent response versus time of as-prepared Cu2O and Cu2O/ZnO heterojunction in 0.1 M Na2SO4 under xenon lamp irradiation.
4. Conclusion
of the Cu2O/ZnO heterojunction was illustrated in Fig. 7c. Upon the visible light irradiation, the photoinduced electrons from the valence band (VB) of the Cu2O is transferred to conduction band (CB), leaving holes on the valence band. Subsequently, the excited electrons transfer from CB of Cu2O to that of ZnO, and the generate holes in the VB of ZnO transfer to the VB of Cu2O [48,49]. The photoinduced electrons on the CB of ZnO tend to combine with H2O/H+ to generate H2, while the holes on the VB of Cu2O reacts with methanol to produce oxidation production [50,51]. Compared with the pure Cu2O, the superior photocatalytic performance of the Cu2O/ZnO heterostructure can be attributed to the following three aspects: (i) The Cu2O/ZnO microspheres with the hollow structure can provide more active sites for the photocatalytic reaction to promote the effective utilization of visible light. (ii) The introduction of ZnO could play the active role in the process of photocatalytic H2 production. ZnO can serve as electron acceptor to capture and gather electrons from Cu2O, which is beneficial to suppress the recombination of electron-hole pairs. (iii) The synergistic effect between hollow structure and p-n heterojunction can effectively promote the separation and transfer of the photo-generated electrons, and be more beneficial to the progress of photocatalytic reaction.
The spherical Cu2O/ZnO heterostructure was successfully prepared by one-step hydrothermal method. The photocatalytic hydrogen production performance of the pure Cu2O and Cu2O/ZnO heterojunction was tested under visible light. Among all the photocatalysts, CZ-20 exhibited the highest hydrogen production of 129.6 μmol/g which more than 10.3 times than the pure Cu2O (12.5 μmol/g). This is mainly due to the formation of p-n junction between the surface of Cu2O and ZnO, which can effectively promote the separation of photogenerated electron-hole pairs. In addition, the hollow structure of Cu2O/ZnO heterostructure can provide more active sites for the photocatalytic reaction. All of these results have an important effect on the photocatalytic properties of the materials.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Fig. 6. (a) The performance of photocatalytic H2 production by the truncated octahedral Cu2O and Cu2O/ZnO heterojunction under visible light irradiation; (b) Photocatalytic hydrogen production rate of different samples under visible-light irradiation; (c) repeated time courses of photocatalytic H2 evolution on CZ-20 sample; (d) The Cu 2p peaks of the CZ-20 before and after the photocatalytic experiment.
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Chemical Physics Letters 734 (2019) 136748
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Fig. 7. Valence band XPS spectra of (a) Cu2O and (b) CZ-20 heterojunction, (c) schematic diagram of the separation and transfer of photogenerated charges in the Cu2O/ZnO under visible light irradiation.
Acknowledgements
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Facile synthesis of hollow p-Cu2O/n-ZnO microspheres with enhanced photocatalytic H2 production
T
Yong-Hui Zhang, Xiao-Li Cai1, Yu-Liang Li1, Ming-Ming Liu, Cheng-Long Ding, Jun-Li Chen, Shao-Ming Fang College of Materials and Chemical Engineering, Collaborative Innovation Center of Environmental Pollution Control and Ecological Restoration, Zhengzhou University of Light Industry, Zhengzhou 450002, PR China
H I GH L IG H T S
Cu O/ZnO heterojunction with novel hollow microspheres was prepared. • The CZ-20 composite exhibits excellent photocatalytic performance. • The • The photocatalytic H production could be improved due to the p-n heterojunction. 2
2
A R T I C LE I N FO
A B S T R A C T
Keywords: Cu2O/ZnO p-n junction Photocatalytic H2 production
The Cu2O/ZnO heterojunction with novel hollow microspheres was prepared via a facile one-step hydrothermal method. The morphology was significantly changed from octahedron to hollow microsphere after adding zinc acetate. The microsphere Cu2O/ZnO materials exhibited superior photocatalytic hydrogen yields (129.6 μmol/ g), almost 10.3 times higher than pure Cu2O. The separation of photogenerated electron-hole pairs were effectively promoted due to the formation of p-n heterojunction between ZnO and Cu2O. In addition, the hollow structure and shell permeability of Cu2O/ZnO microsphere could also improve the utilization of visible light.
1. Introduction With the consumption of fossil fuel and increasing environmental problems, the developing of clean and efficient energy is urgent [1]. As a renewable and carbon-free power source, solar energy could be converted into chemical or electrical energy by photoelectric effect [2–7]. Cu2O with a band gap of 2.17 eV, is a typical p-type semiconductor material with excellent visible light absorption capacity [8–10]. It has been widely used in solar cells, photocatalytic application, and gas sensors due to their unique environmental friendliness and low cost [11–16]. However, the electron-hole pairs will be combined rapidly due to its own thermal vibration in the reaction process, which restrains the photocatalytic efficiency of the materials [17,18]. According to previous investigations, Cu2O could combine with other semiconductor materials (TiO2, CdS, WO3, ZnO), noble metal (Au, Ag, Pt) or carbon materials to improve its photocatalytic performance [19–25]. ZnO, a wide band gap (3.37 eV), is an excellent n-type semiconductor photocatalytic material with the advantages of good
1
chemical stability, high reaction activity, low price and low toxicity [26–28]. It has been widely used in photocatalytic and sensing fields [29,30]. Li et al. reported the flower-like ZnO hierarchical structure exhibited an enhanced photocatalytic performance for photodegradation of RhB near the visible light region [31]. Eseoghene et al. synthesized Pd-ZnO-exfoliated graphite photocatalyst for degradation the acid orange 7 dye by a one-pot hydrothermal method [32]. Piña-Pérez et al. prepared a novel ZnS-ZnO composites with the efficient photocatalytic hydrogen via a solvothermal method at low temperatures [33]. However, ZnO can only absorb ultraviolet light, and its photocatalytic efficiency is not high, which is mainly due to its low conductivity band position and wide band gap [34,35]. When Cu2O is combined with n-type semiconductor ZnO, the n-p junction can form a built-in electric field. The photogenerated electrons produced by Cu2O can be efficiently transmitted to the conduction band of ZnO. It is beneficial to the separation of photogenerated electron-hole pairs [36–38]. Furthermore, previous investigates based on Cu2O/ZnO composite material have been also reported. Kung et al. [2] reported Cu2O nanostructure of truncated octahedral exhibits photonic Fano
E-mail addresses: [email protected] (Y.-H. Zhang), [email protected] (J.-L. Chen), [email protected] (S.-M. Fang). These authors contributed equally to this work and should be considered co-second authors.
https://doi.org/10.1016/j.cplett.2019.136748 Received 15 August 2019; Received in revised form 4 September 2019; Accepted 5 September 2019 Available online 06 September 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
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resonance compared with the other shapes by FDTD simulation. Kramm et al. [39] investigates the band alignments of the Cu2O/ZnO heterointerface. However, few literatures have reported the photocatalytic performance of Cu2O/ZnO composite with hollow structure. Therefore, the exploration of strategies to effectively tune the hollow structure is important and meaningful for photocatalytic material. In this work, Cu2O/ZnO microspheres with hollow structure were synthesized by a facile one-step hydrothermal method. The structure and optical performance of the Cu2O/ZnO microspheres were investigated systematically by XRD, FESEM, TEM, UV-DRS and PL spectra. The morphology of the Cu2O/ZnO heterojunction exhibits hollow microspheres with the size of about 1.03 μm after adding zinc acetate. The highest H2 production for Cu2O/ZnO microspheres (129.6 μmol/g) was 10.3 times higher than that of pure Cu2O (12.5 μmol/g). It could be attributed to synergistic effects between p-n heterojunctions and the unique hollow structures.
Fig. 1. X-Ray diffraction of truncated octahedral Cu2O (red line) and spherical Cu2O/ZnO-0.2 (black line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2. Experiment
(Beijing Perfect Light, Microsolar 300) was used as the light source. The intensity remains between 52 mW/cm2 and 55 mW/cm2.
2.1. Materials All the reagents including Copper acetate hydrate (Cu(Ac)2·H2O), Zinc acetate dihydrate (Zn(Ac)2·2H2O), sodium bis (2-ethylhexyl) sulfosuccinate (NaAOT, 97%, Aldrich) and L-ascorbic acid (C6H8O6, 99.7%, Kermel). The deionized water (18.2 MΩ cm−1) was used throughout the experiments.
3. Results and discussion Fig. 1 illustrates the XRD patterns of the pure truncated octahedral Cu2O and spherical CZ-20 heterojunction. For the pure Cu2O, the major diffraction peaks are indexed at 29.6°, 36.4°, 42.5°, 61.3°, 73.5° and 77.3°, which are associated with the (1 1 0), (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) facets of the cubic Cu2O phase (JCPDS No. 05-0667). For the CZ-20 heterojunction, the weak peaks located at 34.4°, 47.5° and 56.6°, corresponding to (0 0 2), (1 0 2) and (1 1 0) planes of hexagonal wurtzite ZnO (JCPDS No. 36-1451). Besides, the ZnO peaks of the CZ-20 were weak due to the low content [40]. The surface morphologies and structure of the truncated octahedral Cu2O and spherical CZ-20 heterojunction are shown in Fig. 2a–d. The FESEM image of the pure Cu2O shows homogeneous distribution of the truncated octahedral particles with an average edge length of approximately 0.75 μm (Fig. 2a–b). The surface of the pure Cu2O is smooth. After the formation of the Cu2O and ZnO heterojunction, the material exhibits spherical and the average edge length is 1.03 μm. (Fig. 2c–d). After the hydrothermal reaction, it is found that the content of zinc acetate is vital to the morphology of the materials, which is mainly due to the effect of Zn2+. Fig. 3e and g shows the TEM images of the pure Cu2O and Cu2O/ZnO heterojunction, respectively. The HRTEM images (Fig. 2f) of the pure Cu2O exhibited the crystal lattice spacing is 0.21 nm, corresponding to the {2 0 0} plane of Cu2O crystal. In addition, the lattice spacing of CZ-20 is 0.26 nm (Fig. 2h), which was consistent with the {0 0 2} plane of ZnO crystal [41]. The insert in Fig. 3f and h show the selected area electron diffraction (SAED) of the Cu2O and Cu2O/ZnO heterojunction. The as-prepared truncated octahedral Cu2O demonstrated single crystal behavior, while Cu2O/ZnO composites exhibited single crystal diffraction points of ZnO and Cu2O, respectively. Fig. S1 shows the EDS mapping spectra of CZ-20, which confirm the existence of Cu, Zn and O elements. The Zn, Cu, and O elements are uniformly distributed in the whole hollow Cu2O/ZnO microsphere. The growth mechanism of Cu2O/ZnO hollow microspheres is shown in Scheme S1e. In the initial stage of hydrothermal process, the metal salts (Cu(Ac)2 and Zn(Ac)2) slowly dissolve and release the corresponding metal cations (Cu2+ and Zn2+). In the second step, metal cations (Cu2+ and Zn2+) react with OH− to form hydroxide (Cu (OH)42− and Zn(OH)2 under alkaline conditions. Subsequently, [Cu (OH)4]2− and Zn(OH)2 can be translate into Cu2O and ZnO due to the strong reducibility of ascorbic acid. The specific reaction is as follows:
2.2. Preparation of spherical Cu2O/ZnO heterojunction Typically, 0.0205 g Cu(Ac)2·H2O and a certain amount of Zn (Ac)2·2H2O (0.0033 g-15 wt%, 0.0044 g-20 wt% and 0.0066 g-30 wt%) were dispersed and sonicated in 80 mL deionized water to form a homogeneous solution. 0.1335 g AOT was dissolved in 10 mL n-butyl alcohol. Then the mixed aqueous solution was added to AOT solution drop by drop and kept stirring for 1 h. After that, 0.0705 g ascorbic acid was introduced to above solution and kept stirring for 30 min. Then, 2 mL of 1.0 M NaOH was dripped into the solution and stirred for 2 h. Finally, the suspension was transferred to a 100 mL Teflon-lined reactor and reacted at 80 °C for 3 h. The solid products were centrifuged, washed and dried at 60 °C for 6 h. The as-prepared Cu2O/ZnO samples were denoted as CZ-15, CZ-20 and CZ-30, respectively. The synthesis of pure truncated octahedral Cu2O was supplied in the Supporting Information. 2.3. Characterization X-ray diffraction patterns (German Bruker D8) were analyzed the phase structure of the materials. The microstructures of samples were characterized by field emission scanning electron microscopy (FESEM, JSM-7001F), transmission electron microscopy (TEM, JEM-2100) and high-resolution transmission electron microscopy (HRTEM, 200 kV). The X-ray photoelectron spectrometer (XPS) was performed by an ESCALAB-250Xi spectrometer. The optical properties of the samples were analyzed by a UV–vis absorption spectra (UV-U3900, Hitachi) and photoluminescence spectra (PL F-7000, Hitachi). The efficiency of photocatalytic H2 production was measured using Agilent Technologies 7890A GC-MS analyzer. 2.4. Photoelectrochemical measurements All photoelectrochemical tests were carried out in a three-electrode system on a CHI660D electrochemical workstation. Electrochemical impedance spectroscopy (EIS) was performed in a 1:1 mixture ratio of 5 mM of K3[Fe(CN)6]/K4[Fe(CN)6] and 0.1 M of KCl solution at a frequency range from 100 kHz to 0.01 Hz. Photocurrent experiments were tested in 0.1 M Na2SO4 electrolyte solution. A 300 W Xenon lamp
Cu2+ + 4OH− → [Cu(OH)4]2− 2
(1)
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Fig. 2. FESEM of the truncated octahedral Cu2O (a, b), and spherical CZ-20 composites (c, d). TEM images (e, g) and the corresponding SAED patterns (inset, SAED pattern of Cu2O and CZ-20). High-resolution TEM images of the square regions revealing the (2 0 0) and (0 0 2) planes of Cu2O (white) and ZnO (black), respectively.
Zn2+ + 2OH− → Zn(OH)2
uniform octahedral without zinc salt. After adding zinc salt (including zinc acetate, zinc sulfate, zinc nitrate) in Fig. S2, the composites become a uniform spheres. The evolution of the hollow structure is usually related to the Ostwald ripening mechanism, which can lead to the formation of hollow shell by consuming internal material and recrystallizing process [42]. (2) The ratio of Zn(Ac)2 to Cu(Ac)2 is vital to the formation of hollow Cu2O/ZnO microspheres (in Fig. S3 and Fig. S4). With the increase of zinc salt content, the morphology of the composites gradually become hollow spheres. However, when further increase of zinc salt content, the small particles of ZnO could be precipitated and gradually become irregular particles.
(2)
2[Cu(OH)4]2− + 3C6H8O6 (ascorbic acid) → Cu2O + 3C6H6O6 + 7H2O (3) Zn(OH)2 → ZnO + H2O
(4)
Scheme S1a–d shows the formation process of various morphology of Cu2O/ZnO composites. It is worth noting that Zn2+ is vital to the growth process of the Cu2O/ZnO hollow structure: (1) No Cu2O/ZnO hollow microspheres were formed without zinc salt on the experiment. Under the same other reaction conditions, the morphology of Cu2O is
Fig. 3. (a) XPS of survey spectra of spherical Cu2O/ZnO heterojunction, (b) Cu 2p; (c) Zn 2p XPS spectrum of Cu2O/ZnO. 3
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Fig. 4. (a) UV-DRS absorption spectra (the inset is the curves of (αhν)2 vs. hν) and (b) Photoluminescence (PL) emission spectra of the truncated octahedral Cu2O and spherical Cu2O/ZnO heterojunctions.
separation capacity of as-prepared materials. As shown in Fig. 5b, four samples of Cu2O/ZnO were prepared with different loading amount of ZnO (0, 15, 20, and 30 wt%) and performed for their photoelectrochemical test. The prompt increase in the photocurrent response could be ascribed to that the generation of photogenerated electrons induced by illumination, and the electrons directional movement was driven by external voltage. The higher photocurrents were generated for the Cu2O/ZnO samples than that of the pure Cu2O. CZ-20 possesses a maximum current density, which is about 1.5 times compared to pure Cu2O. The CZ-20 catalyst exhibits excellent separation of photogenerated electron-hole pairs, which was also confirmed by the PL results. The photocatalytic performance of pure Cu2O and Cu2O/ZnO heterojunction were investigated by using CH3OH aqueous solution (20 vol %) as holes sacrificial reagent under the visible light irradiation. Fig. 6a shows comparable photocatalytic H2 production of all the materials. As expected, the photocatalytic hydrogen-producing performance of pure Cu2O is poor (only 12.5 μmol/g in 6 h), which is mainly due to the recombination of the electron-hole pairs. With increasing the ZnO (15 wt%, 20 wt% and 30 wt%), the H2 yield of Cu2O/ZnO heterojunction was 44.5 μmol/g, 129.6 μmol/g and 36.9 μmol/g, respectively. It is clearly found that CZ-20 displays the highest photocatalytic activity than those with a low or high content of ZnO. This fact could be explained that excessive ZnO can restrain the energy transformation on the surface of Cu2O. Fig. 6b shows the rates of H2 generation for all photocatalysts. The H2 generation rate of Cu2O is 2.1 μmol·g−1·h−1. For comparison, Cu2O/ZnO heterojunction exhibits an evidently enhanced activity. In particular, the hollow CZ-20 gives the maximum H2 generation rate of 21.6 μmol·g−1·h−1, that is about 10.3 times higher than that of Cu2O. It could be attributed to more active sites of p-n heterojunction in CZ-20 than that of CZ-15, which induces the material have excellent photocatalytic properties. With the increase of ZnO, a larger grain boundary barrier could inhibit the transport of electron. The stability of the CZ-20 photocatalyst was further investigated by the recycling test for three repeated cycles. From Fig. 6c, the H2 generation of the CZ-20 heterojunction only decreased by 7% after three cycles. It indicates that CZ-20 possesses high stability and excellent activity under the visible light irradiation. Fig. 6d shows the XPS spectra for Cu 2p of CZ-20 heterojunction before and after the photocatalytic reaction. It shows that the peaks area of Cu2+ had a slightly increase from 10.07% to 15.84%, illustrating a few of Cu+ may be oxidized during the photocatalytic reaction. VB-XPS spectroscopy of Cu2O and CZ-20 heterojunction are shown in Fig. 7a and b. From the VB-XPS spectra, the VB potentials of pure Cu2O is determined to be 1.18 eV. For CZ-20 heterojunction, the VB potential is 1.39 eV, which is 0.21 eV more positive than the pure Cu2O. Combining the results from UV-DRS and VB-XPS, the approximate band positions of the CZ-20 heterojunction was shown in Fig. 7c. Based on the previously experimental discussions, the photocatalytic mechanism
XPS spectra is executed to investigate the chemical composition and elemental analysis of the CZ-20 heterojunction. Fig. 3a shows the XPS survey spectrum of Cu2O/ZnO composites The peaks centered at 284.6, 569.6 and 932.6 eV are assigned to C 1s, O 1s and Cu 2p, respectively. It indicates the existence of Cu2O and ZnO in the Cu2O/ZnO composite. For Cu 2p XPS spectrum (Fig. 3b), two peaks at 932.2 and 952.1 eV can be assigned to the Cu 2p3/2 and Cu 2p1/2 of Cu+, respectively [43]. The weaker signals at 933.2 and 953.3 eV are attributed to Cu2+, which demonstrate that few Cu2O were oxidized to CuO on the surface of the composite. Fig. 3c shows the Zn 2p spectrum in Cu2O/ZnO heterojunction. The peaks located at 1021.2 and 1044.3 eV corresponded to binding energies of the Zn 2p3/2 and Zn 2p1/2, respectively [44]. Thus, Zn2+ exists mainly chemical states in the Cu2O/ZnO composite. The UV–vis diffuse reflectance spectroscopy (UV-DRS) of the truncated octahedral Cu2O and Cu2O/ZnO heterojunctions are compared in Fig. 4a. The pure truncated octahedral Cu2O exhibits an absorption wavelength at 640 nm (1.94 eV). The optical absorption edge and absorption intensity of Cu2O/ZnO heterojunction is substantially enhanced. It could be attributed to the increase of grain size of Cu2O/ZnO composites and the surface defects on the Cu2O induced by ZnO. The defect energy levels are produced, which can reduce the energy of the electron transition. It demonstrates that the solar energy could be used more effectively by Cu2O/ZnO composites [45]. The band gap estimation of the truncated octahedral Cu2O and CZ-20 heterojunction are calculated by the Kubelka–Munk equation. As shown in the inset of Fig. 4a, the band gap of pure Cu2O is approximately 1.94 eV in accordance with the previous report [46]. With increasing content of ZnO (15 wt% to 30 wt%), the band gap of Cu2O/ZnO heterojunction significantly reduced from 1.93, 1.88 to 1.85 eV. The narrower band gaps are mainly ascribed to the intimate interfacial contact between Cu2O and ZnO, which is essential to gain the visible light and promote the transfer of photoelectrons. PL spectra is widely performed to investigate the separation and transfer capacity of photo-induced charge carriers in the photocatalysts. Fig. 4b shows the PL spectra of pure Cu2O and Cu2O/ZnO heterojunctions under 460 nm (2.70 eV) excitation. It is clear that Cu2O displays a strong fluorescence emission peak centered at ~570 nm (2.17 eV). The PL intensity of CZ-20 significantly quenched after introducing ZnO, indicating the lowest recombination rate of photogenerated electron-hole pairs. To investigate the interfacial electron transfer behavior of the photocatalysts, the EIS of Cu2O and Cu2O/ZnO composites are shown in Fig. 5a. The EIS curves are composed of a semicircle in the high-frequency region and a linear in the low-frequency region [47]. The diameter of Cu2O/ZnO electrode is smaller than the electrode of Cu2O, indicating that the Cu2O/ZnO electrode exhibits a lower electrochemical resistance. The formation of the ZnO/Cu2O p-n junction can lead to the higher electron migration efficiency and the lower resistance of interfacial charge transfer. The transient photocurrent responses were carried out by five cycles irradiation to investigate the charge 4
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Fig. 5. (a) EIS of Cu2O and Cu2O/ZnO in a 1:1 mixture ratio of 5 mM of K3[Fe(CN)6]/K4[Fe(CN)6] and 0.1 M of KCl (the inset is the Nyquist plots of bare Au electrode); (b) photocurrent response versus time of as-prepared Cu2O and Cu2O/ZnO heterojunction in 0.1 M Na2SO4 under xenon lamp irradiation.
4. Conclusion
of the Cu2O/ZnO heterojunction was illustrated in Fig. 7c. Upon the visible light irradiation, the photoinduced electrons from the valence band (VB) of the Cu2O is transferred to conduction band (CB), leaving holes on the valence band. Subsequently, the excited electrons transfer from CB of Cu2O to that of ZnO, and the generate holes in the VB of ZnO transfer to the VB of Cu2O [48,49]. The photoinduced electrons on the CB of ZnO tend to combine with H2O/H+ to generate H2, while the holes on the VB of Cu2O reacts with methanol to produce oxidation production [50,51]. Compared with the pure Cu2O, the superior photocatalytic performance of the Cu2O/ZnO heterostructure can be attributed to the following three aspects: (i) The Cu2O/ZnO microspheres with the hollow structure can provide more active sites for the photocatalytic reaction to promote the effective utilization of visible light. (ii) The introduction of ZnO could play the active role in the process of photocatalytic H2 production. ZnO can serve as electron acceptor to capture and gather electrons from Cu2O, which is beneficial to suppress the recombination of electron-hole pairs. (iii) The synergistic effect between hollow structure and p-n heterojunction can effectively promote the separation and transfer of the photo-generated electrons, and be more beneficial to the progress of photocatalytic reaction.
The spherical Cu2O/ZnO heterostructure was successfully prepared by one-step hydrothermal method. The photocatalytic hydrogen production performance of the pure Cu2O and Cu2O/ZnO heterojunction was tested under visible light. Among all the photocatalysts, CZ-20 exhibited the highest hydrogen production of 129.6 μmol/g which more than 10.3 times than the pure Cu2O (12.5 μmol/g). This is mainly due to the formation of p-n junction between the surface of Cu2O and ZnO, which can effectively promote the separation of photogenerated electron-hole pairs. In addition, the hollow structure of Cu2O/ZnO heterostructure can provide more active sites for the photocatalytic reaction. All of these results have an important effect on the photocatalytic properties of the materials.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Fig. 6. (a) The performance of photocatalytic H2 production by the truncated octahedral Cu2O and Cu2O/ZnO heterojunction under visible light irradiation; (b) Photocatalytic hydrogen production rate of different samples under visible-light irradiation; (c) repeated time courses of photocatalytic H2 evolution on CZ-20 sample; (d) The Cu 2p peaks of the CZ-20 before and after the photocatalytic experiment.
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Fig. 7. Valence band XPS spectra of (a) Cu2O and (b) CZ-20 heterojunction, (c) schematic diagram of the separation and transfer of photogenerated charges in the Cu2O/ZnO under visible light irradiation.
Acknowledgements
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