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Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode
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Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode Jirong Chen, Ying Jia*, Wenzhong Wang, Junli Fu, Honglong Shi, Yujie Liang College of Science, Minzu University of China, Beijing 100081, China
highlights ZnO/Cu2O nanotube arrays with cubic and spherical Cu2O microcrystals were prepared. The morphology of Cu2O deposited on ZnO can be adjusted by the deposition potential. The morphology and size of Cu2O can influence the PEC performance of ZnO/Cu2O.
article info
abstract
Article history:
ZnO nanotube arrays were synthesized by the electrodeposition method and Cu2O mi-
Received 21 November 2019
crocrystals with two kinds of morphologies were deposited on ZnO nanotube arrays suc-
Received in revised form
cessfully. At the deposition potential of 0.5 or 0.7 V, the cubic or spherical Cu2O
12 January 2020
microcrystals were selectively deposited on ZnO nanotube arrays. By adjusting the depo-
Accepted 17 January 2020
sition time, Cu2O microcrystals with different sizes were obtained. The optical properties
Available online xxx
and photo-electrochemical performance of ZnO/Cu2O were measured. The results showed
Keywords:
and enhanced photocurrent due to the excellent ability of Cu2O microcrystals for har-
ZnO/Cu2O nanotube arrays
vesting visible light, and the effective separation and transfer of photo-generated electrons
that the as-prepared ZnO/Cu2O heterojunction exhibited improved visible light absorption
Photo-electrode
and holes owing to p-n junction between ZnO and Cu2O. The experimental results
Morphology
demonstrate that the photo-electrochemical performance of ZnO/Cu2O heterojunction nanotube arrays can be manipulated by controlling the morphology and the size of Cu2O microcrystals. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the development of the world economy, the energy crisis and environmental pollution problems have become increasingly prominent. Therefore, the application of clean
energy is an irresistible trend. As a mean of solar energy conversion technology, photo-electrochemical (PEC) water splitting has received great attention [1e5]. Among the available photosensitive semiconductor materials, ZnO is one of the most promising photo-electrode materials because of its special features, such as low cost, chemical inertness and high
* Corresponding author. E-mail address: [email protected] (Y. Jia). https://doi.org/10.1016/j.ijhydene.2020.01.114 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114
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electron mobility (~120 cm2V1s1) [6]. However, the wide band gap (~3.37 eV) of ZnO leads to the poor light harvesting capability in the visible light region, which hints its extensive application [7e10]. The construction of heterostructure by coupling with narrow band gap p-type semiconductor is an effective way to extend the application of ZnO, because this kind of heterostructure not only extends the absorption range of ZnO to visible light region, but also reduces the recombination rate of photo-generated carriers, which will be helpful to improve the PEC performance of the photo-electrode [11e16]. Cu2O is a typical p-type semiconductor material with narrow band gap (~2.0 eV), which matches well with the visible spectrum, and its theoretical power conversion efficiency is about 20% [17,18]. As a promising material, Cu2O shows great potential for the p-n junction construction because of its efficient light absorption and photo-induced electron provision [19e25]. Recently, ZnO/Cu2O heterostructure has attracted extensive interest, because ZnO and Cu2O have an appropriate band match. The conduction and valence band edge of Cu2O are both above those of ZnO [8,26]. The suitable band gap structure follows the typical type-II mode, which can effectively suppress the photo-generated carrier recombination and enhance the PEC performance [27e33]. Different ZnO/Cu2O heterostructures have been synthesized successfully [34e36]. However, to the best of our knowledge, the deposition of Cu2O on ZnO nanotube arrays (NTAs) has been rarely reported. ZnO/Cu2O heterojunction structure prepared by depositing Cu2O microcrystals with different morphologies on ZnO NTAs is promising to improve the PEC performance of the photoelectrode. On one hand, the highly ordered ZnO NTAs have high aspect ratio and electron mobility, which is beneficial to the light absorption and the electron transfer. On the other hand, different morphologies of Cu2O can influence the absorption of light and the transfer of photo-generated carriers of the heterogeneous photo-electrode, which has been proved in the Cu2O/TiO2 PEC cell [37]. In this study, we provide a convenient electrochemical deposition method to deposit Cu2O microcrystals with cubic and spherical morphologies on the ZnO NTAs to prepare ZnO/ Cu2O heterojunction photo-electrode. The PEC performance of the heterogeneous photo-electrode is studied, and the ZnO/ Cu2O photo-electrode exhibits excellent PEC performance compared to that of pure ZnO photo-electrode in the visible light region.
Preparation of ZnO NTAs First of all, the Fluorine-doped tin oxide (FTO) conductive glass was ultrasonically cleaned by deionized water, ethanol and acetone for 15 min, respectively. ZnO NTAs were prepared by through the following two processes. Firstly, ZnO nanorod arrays (NRAs) were synthesized on a FTO conductive glass by a three-electrode electrochemical deposition system, in which platinum, Ag/AgCl and FTO glass were used as the counter electrode, the reference electrode and the working electrode, respectively. The electrolyte was the mixed solution of 200 mL polyethylene glycol 400, 50 mL anhydrous ethylenediamine and 150 ml 0.0125 M Zn(NO3)2 aqueous solution. The temperature of the electrolyte was maintained at 70 C. The anode electrode potential was controlled at 1.1 V, and the deposition time was 1.5 h. Secondly, the FTO conductive glass deposited with ZnO NRAs was immersed in KOH solution (0.18 M). The etching time was 1 h, and the solution temperature was fixed at 80 C [38]. Then the sample was taken out and rinsed with deionized water. Finally, the sample was annealed at 400 C for 30 min.
Deposition of Cu2O microcrystals on ZnO NTAs In this step, the electrochemical method was used for the deposition of Cu2O microcrystals on ZnO NTAs. The electrolyte solution was a mixture of 0.05 M Cu(NO3)2 and 0.3 M lactic acid. The pH value of the electrolyte solution was tuned to 12 by using 4 M NaOH. The temperature of the electrolyte solution was fixed at 60 C and the deposition voltage was kept at a stable voltage of 0.5 or 0.7 V to synthesize Cu2O microcrystals with cubic or spherical shape, respectively. Then the sample was rinsed by deionized water and dried at 60 C for half an hour in a vacuum drying oven.
Samples characterizations The scanning electron microscopy (SEM, Hitachi S-4800, Japan) was used to characterize the morphologies of the prepared samples. The crystallinity and phase of the nanostructures were characterized by X-ray diffraction (XRD) on a XD-3 diffractometer (Beijing Pepsee, China) with Cu-Ka radiation (l ¼ 1.5406 A) as the excitation source. X-ray photoelectron spectroscopy (XPS, ThermoFisher, UK) was used to analyze the elemental composition. Lambda 950 UV/VIS/NIR spectrometer (Perkin-elmer, USA) was used to record the UVevis absorption spectra of the samples.
Material and methods
PEC measurements
Zinc nitrate hexahydrate (Zn(NO3)2$6H2O, 99%), polyethylene glycol 400 (H(OCH2CH2)nOH), anhydrous ethylenediamine (H2NCH2CH2NH2, 99.0%), copper nitrate (Cu(NO3)2$3H2O, 99.0%), lactic acid (CH3CH-(OH)COOH, 85.0%), potassium hydroxide (KOH, 85.0%) and sodium hydroxide (NaOH, 96%) were purchased from Beijing Chemical Company. All of the reagents were of analytical grade and used as received without further purification.
Photo-electrochemical measurements of ZnO and ZnO/Cu2O NTAs were carried out by using a three-electrode electrochemical analyzer. Platinum, Ag/AgCl and the FTO conductive glass were used as the counter electrode, reference electrode and working electrode, respectively. NaOH aqueous solution (0.5 M) was used as the electrolyte solution. The light source was 300-W Xenon lamp (Perfect Light, China), and a visible light filter was used to obtain visible light. The power density
Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114
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Fig. 1 e (a) Low- and (b) high-magnification SEM images of ZnO NTAs. (cee) SEM images of ZnO/Cu2O NTAs synthesized at deposition potential of ¡0.5 V under the deposition time of 5, 10 and 15 min. (feh) SEM images of ZnO/Cu2O NTAs synthesized at deposition potential of ¡0.7 V under the deposition time of 3, 5 and 10 min.
Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114
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Fig. 2 e XRD patterns of pure ZnO NTAs and ZnO/Cu2O NTAs loaded with cubic Cu2O microcrystals (a) and spherical Cu2O microcrystals (b).
Fig. 3 e (a) The survey XPS spectrum of ZnO/Cu2O NTAs with spherical Cu2O microcrystals obtained by the deposition for 5 min. (b) The high-resolution XPS spectrum of zinc 2p. (c) The high-resolution XPS spectrum of oxygen 1s. (d) The highresolution XPS spectrum of copper 2p. near the position of the electrode was 100 mW/cm2. The dissolved oxygen in the electrolyte solution was removed by injecting N2 for 20 min. The experiments were carried out in a quartz window by using an electrochemical workstation (CHI 660c, CH Instruments Inc., USA). The linear sweep voltammetry (LSV), I-t curve and electrochemical impedance (EIS) were collected under visible light (l > 420 nm) irradiation.
Results and discussion Morphology and composition Fig. 1 shows the surface morphologies of pure ZnO and ZnO/ Cu2O NTAs. Fig. 1a and b are low-magnification and high-
Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114
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Fig. 4 e The absorption spectra of ZnO/Cu2O NTAs loaded with cubic Cu2O microcrystals (a) and spherical Cu2O microcrystals (b). magnification SEM images of bare ZnO NTAs. The ZnO NTAs exhibit ordered arrangement, and they are nearly perpendicular to the FTO surface. It can be found from Fig. 1b that a single ZnO nanotube displays a hollow six-square structure with an average diameter of 420 nm. The surface of ZnO nanotube is smooth and the wall thickness is about 50 nm. Fig. 1cee shows the surface morphologies of ZnO/Cu2O NTAs synthesized at deposition potential of 0.5 V. It can be found that the Cu2O microcrystals with cubic shape are deposited on ZnO NTAs, and the Cu2O microcrystals exhibit smooth surface and regular cube structure. When the deposition time is chosen as 5 min, the side length of the Cu2O cube is about 150 nm as seen in Fig. 1c. When the deposition time increases to 10 and 15 min, the side length of the Cu2O cubes increases to 300 and 700 nm, respectively, as seen in Fig. 1d and e. It can be seen that the size of the Cu2O microcrystals increases significantly with the extension of deposition time. Fig. 1feh shows the SEM images of ZnO/Cu2O NTAs synthesized at deposition potential of 0.7 V. It can be seen that the Cu2O
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microcrystals with spherical shape are deposited on ZnO NTAs. Fig. 1f is the SEM image of the prepared ZnO/Cu2O NTAs prepared under the deposition time of 3 min. It can be found that the diameter of the spherical Cu2O microcrystal is about 300 nm. With extending deposition time to 5 and 10 min, the diameter of the Cu2O microcrystal increases to 400 and 600 nm, respectively. Additionally, the SEM images of Fig. 1ceh clearly demonstrate that the deposition of Cu2O microcrystals does not damage the structure of the ordered ZnO NTAs. Fig. 2a shows the XRD patterns of pure ZnO NTAs and ZnO/ Cu2O NTAs loaded with cubic Cu2O microcrystals, and Fig. 2b is the XRD patterns of pure ZnO NTAs and ZnO/Cu2O NTAs loaded with spherical Cu2O microcrystals. In the XRD pattern of the pure ZnO NTAs, six main diffraction peaks correspond to the crystal planes of (100), (002), (101), (102), (103) and (112), respectively, which is in accordance with the diffraction peaks of ZnO with hexagonal phase reported in the literature [39]. The intensity of the diffraction peak corresponding to (002) crystal plane is obviously higher than those of other diffraction peaks, indicating that the ZnO NTAs has preferential growth along the c-axis. For the heterogeneous structure of ZnO/Cu2O, three diffraction peaks located at 36.5 , 42.3 and 62.1 correspond to the crystal planes of (111), (200) and (220) of cubic phase Cu2O, respectively, With the extension of deposition time, the intensity of the three diffraction peaks increases. The composition of ZnO/Cu2O NTAs was further confirmed by X-ray photoelectron spectroscopy. Fig. 3aed is the XPS results of the survey spectrum and the high resolution spectra of ZnO/Cu2O NTAs with spherical Cu2O microcrystals obtained by the deposition for 5 min. The measured spectrum in Fig. 3a shows that the peaks indexed to Zn, Cu and O elements are found. The high resolution spectra of ZnO/Cu2O NTAs are shown in Fig. 3bed. As shown in Fig. 3b, the two peaks at the binding energies of 1021.7 and 1044.8 eV correspond to Zn 2p3/ 2 and 2p1/2, respectively. The interval of 23.1 eV of the two peaks indicates the Zn species with þ2 valence state exist in ZnO/Cu2O NTAs. Fig. 3c shows the binding energies of O in ZnO/Cu2O NTAs, which can be fitted to two peaks located at 530.8 and 531.8 eV attributing to O in Cu2O and ZnO. Two peaks at 932.6 and 952.2 eV of Fig. 3d attribute to Cu 2p3/2 and 2p1/2, respectively, indicating the existence of cuprous in ZnO/ Cu2O NTAs [40]. The UVevis absorption spectra of ZnO/Cu2O NTAs are presented in Fig. 4. It can be found that the pure ZnO NTAs have good ultraviolet absorption, and there is hardly any visible light absorption. The absorption edge of pure ZnO NTAs is about at 395 nm, which is in accordance with the intrinsic bandgap absorption of ZnO crystals. By comparison with pure ZnO NTAs, the visible light absorption of ZnO/Cu2O NTAs loaded with both cubic and spherical Cu2O microcrystals is significantly enhanced. It can be found that there is an obvious absorption peak at about 470 nm after deposition of Cu2O on ZnO, and the absorption spectra show a wide absorption range. Fig. 4a is the absorption spectra of ZnO/Cu2O NTAs with cubic Cu2O microcrystals. Upon careful observation, the ZnO/Cu2O NTAs with cubic Cu2O microcrystals, which was obtained by the deposition for 10 min, show better absorbance performance than the samples, in which the Cu2O
Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114
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Fig. 5 e The photocurrent densities of ZnO/Cu2O NTAs loaded with cubic Cu2O microcrystals (a) and spherical Cu2O microcrystals (b) at 0 V versus Ag/AgCl. The LSV curves of ZnO/Cu2O NTAs with cubic Cu2O microcrystals (c) and spherical Cu2O microcrystals (d).
microcrystals are obtained by the deposition for 5 and 15 min, respectively. Fig. 4b is the absorption spectra of ZnO/Cu2O NTAs with spherical Cu2O microcrystals. It can be found that the sample with spherical Cu2O microcrystals obtained by the deposition for 5 min shows better absorbance performance.
PEC performance Fig. 5a and b shows the instantaneous visible light response and photocurrent densities of bare ZnO and ZnO/Cu2O NTAs photo-electrode at 0 V bias voltage versus Ag/AgCl. Fig. 5a is the photocurrent densities of ZnO/Cu2O NTAs with cubic Cu2O microcrystals, and Fig. 5b is the photocurrent densities of ZnO/Cu2O NTAs with spherical Cu2O microcrystals. It can be found that the pure ZnO NTAs photo-electrode almost does not produce photocurrent by visible light illumination at 0 V versus Ag/AgCl, while ZnO/Cu2O NTAs exhibit obvious visible light response. When the ZnO/Cu2O NTAs photo-electrode is under visible light irradiation, the photocurrents are generated rapidly, showing that the deposition of Cu2O microcrystals can significantly improve the PEC performance of ZnO
photo-electrode under visible light irradiation. The results are consistent with the strong visible light absorption of ZnO/ Cu2O NTAs as mentioned above. In Fig. 5a, when the deposition time for the deposition of cubic Cu2O microcrystals is 10 min, the ZnO/Cu2O NTAs achieve higher photocurrent than the samples, in which the Cu2O microcrystals are obtained by the deposition for 5 and 15 min, respectively. The highest photocurrent density of the ZnO/Cu2O NTAs with cubic Cu2O microcrystals is 0.12 mA/cm2. In Fig. 5b, when the spherical Cu2O microcrystals are deposited on ZnO NTAs under deposition time of 5 min, the photocurrent density of ZnO/Cu2O NTAs photo-electrode reaches to higher value. While the deposition time increases to 10 min or decreases to 3 min, the photocurrent density decreases. These results indicate that the amount and size of Cu2O particles obviously affect the photo-electricity performance of ZnO/Cu2O NTAs photoelectrode, which is similar to the results of the previous report [41]. Through comparing Fig. 5a and b, it can be found that the morphologies of Cu2O also affect the photocurrent of ZnO/Cu2O NTAs photo-electrode, which is similar to the results shown in the previous report [37]. Thus, the
Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114
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Fig. 6 e The EIS curves of pure ZnO (a) and ZnO/Cu2O NTAs (b) photo-electrodes under visible light irradiation.
experimental measurements clearly demonstrate that the PEC water splitting performance of the ZnO/Cu2O NTAs photo-electrode can be manipulated by controlling the morphology and size of Cu2O microcrystals. The photocurrent is decreased with the increase of time, which could be due to the slight photo-corrosion of Cu2O. The similar PEC water splitting property was reported in the previous study [42]. The linear sweep voltammograms (LSVs) were used to analyze the relationship between the photocurrent densities of visible light response and bias voltage. Fig. 5c and d shows the LSV curves of ZnO/Cu2O NTAs with cubic and spherical Cu2O microcrystals, respectively. The bias voltage is from 0 to 0.6 V versus Ag/AgCl and the ZnO/Cu2O NTAs photo-electrode is illuminated by visible light. It can be found that the photocurrent densities hardly change when the pure ZnO photoelectrode is under visible light irradiation. The reason is that the wide band-gap ZnO has no response to visible light and it can not be excited by visible light to generate electron-hole pairs. After the Cu2O microcrystals are deposited on ZnO NTAs, the photocurrent densities of the ZnO/Cu2O NTAs
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Fig. 7 e (a) Mott-Schottky plots of ZnO NTAs and ZnO/Cu2O NTAs with cubic Cu2O microcrystals obtained by the deposition for 10 min. (b) Mott-Schottky plots of ZnO NTAs and ZnO/Cu2O NTAs with spherical Cu2O microcrystals obtained by the deposition for 5 min.
photo-electrodes increase with the applied bias voltage. The results show that the deposition of Cu2O microcrystals not only enhances the absorption of visible light but also improves the PEC performance of the ZnO based photo-electrode. The charge separation and transfer process of the obtained photo-electrodes are characterized by electrochemical impedance spectroscopy. Fig. 6 shows the Nyquist plots of pure ZnO and ZnO/Cu2O NTAs photo-electrodes obtained under visible light illumination at open circuit potential (OCP) and the measured range is from 1 to 100,000 Hz. The value of OCP for pure ZnO NTAs is 0.352 V. The ZnO/Cu2O NTAs with cubic Cu2O microcrystals obtained by the deposition for 5, 10 and 15 min are 0.531, 0.542 and 0.533 V, respectively. The ZnO/Cu2O NTAs with spherical Cu2O microcrystals obtained by the deposition for 3, 5 and 10 min are 0.514, 0.522 and 0.517 V, respectively. The high-frequency part of the EIS
Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114
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Fig. 8 e (a) Schematic diagram of band gap structures and the proposed charge separation and transfer in ZnO/Cu2O NTAs under visible light irradiation. (b) Schematic diagram of carrier transmission in an external circuit.
curves has been given as the inset of Fig. 6. The arc diameter of the Nyquist curve reflects the resistance of the charge transfer [43,44]. The smaller diameter indicates smaller charge transfer resistance and higher separation efficiency of photogenerated carriers. As shown in Fig. 6, the diameters of the semi-circle arcs of ZnO/Cu2O NTAs photo-electrodes are obviously smaller than that of ZnO NTAs photo-electrode, which indicates that deposition of Cu2O microcrystals can reduce the resistance of charge transfer. In Fig. 6a, it can be found that the ZnO/Cu2O NTAs photo-electrodes with cubic Cu2O microcrystals obtained by the deposition for 10 min has the smallest diameter of semi-circle arc, so its charge transfer resistance is the smallest. The result is in accordance with its highest photocurrent as seen in Fig. 5a. In Fig. 6b we can see that the ZnO/Cu2O NTAs photo-electrodes with spherical Cu2O obtained by the deposition for 5 min has the smallest diameter of semi-circle arc, so its photocurrent is the highest as seen in Fig. 5b.
Mechanism To further understand the mechanism of the enhanced PEC performance for ZnO/Cu2O NTAs photo-electrodes, Mott Schottky (M S) curves of the obtained photo-electrodes are recorded. The M S plots of pure ZnO and ZnO/Cu2O NTAs photo-electrodes are collected by conducting impedancepotential spectroscopy at an AC frequency of 1000 Hz under visible light irradiation. Fig. 7a shows the M S curves of ZnO NTAs and ZnO/Cu2O NTAs with cubic Cu2O microcrystals obtained by the deposition for 10 min. It shows that the M S curve of ZnO NTAs photo-electrode has a positive slope, which exhibits n-type semiconductor behavior. However, for the ZnO/Cu2O NTAs photo-electrode, an obvious inverted Vshape can be observed, which shows the p-n junction characteristic and indicates that n-type ZnO and p-type Cu2O have successfully constructed p-n junction at their interface. Fig. 7b shows the M S curves of ZnO NTAs and ZnO/Cu2O NTAs with spherical Cu2O microcrystals obtained by the deposition for 5 min. An obvious inverted V-shape can also be observed, which indicates that the p-n junction has been constructed
successfully at the interface between ZnO and Cu2O. The Mott-Schottky equation is as follows: 1 2 kB T ; ¼ Vappl VFB 2 2 C ε0 εqND A q where C is the capacitance of the space charge layer, ε0 and ε are the vacuum permittivity and the dielectric constant of the semiconductor, respectively, q is the elementary charge, ND is the donor density, A is the surface area of the electrode exposed to the electrolyte, Vappl and VFB are the applied potential and the flat-band potential, respectively, kB is the Boltzmann constant and T is Kelvin temperature. It can be obtained from the above formula that the donor density ND is ascribed to the slope of the M S curves. The donor densities of ZnO/Cu2O NTAs with cubic Cu2O obtained by the deposition for 10 min and ZnO/Cu2O NTAs with spherical Cu2O obtained by the deposition for 5 min are 2.30 1020 and 7.96 1019 cm3, respectively, which are both higher than that of pure ZnO (4.79 1019 cm3). The results indicate that the charge separation is enhanced due to the formation of p-n heterojunction between ZnO and Cu2O. Based on the above experimental results, the proposed charge separation and transfer in ZnO/Cu2O NTAs under visible light irradiation is presented in Fig. 8. Fig. 8a is the schematic energy band diagram of ZnO/Cu2O and the possible charge transfer in ZnO/Cu2O NTAs. Fig. 8b is the schematic diagram of carrier transmission in an external circuit. The Fermi energy level of Cu2O is lower than that of ZnO, so after Cu2O microcrystals are deposited on ZnO NTAs, the Fermi level of Cu2O moves up and that of ZnO moves down until the Fermi level between ZnO and Cu2O reaches an equilibrium state. Consequently, an electric field is built in the space charge region between ZnO and Cu2O, and the p-n junction is constructed. When ZnO/Cu2O NTAs photo-electrode is under visible light irradiation, some electrons are excited from the valence band (VB) of Cu2O to its conduction band (CB). With the help of the inner electric field at the interface between ZnO and Cu2O, the electrons will transfer from the CB of Cu2O to the CB of ZnO, then move to Pt electrode through an external circuit and react with the Hþ near the Pt electrode to take part
Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114
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in water splitting reaction. The inner electric field can effectively promote the separation and transfer of photo-generated electrons and holes, resulting in the enhancement of the PEC performance.
Conclusions In summary, we prepared ZnO/Cu2O NTAs with cubic and spherical Cu2O microcrystals. The morphologies of Cu2O microcrystals can be controlled by adjusting deposition potential. The cubic and spherical Cu2O microcrystals are obtained at the deposition potential of 0.5 and 0.7 V, respectively. With the increase of deposition time, the PEC performance of ZnO/Cu2O NTAs changes significantly. By contrast, the ZnO/ Cu2O NTAs with cubic Cu2O microcrystals obtained by the deposition for 10 min have higher PEC performance than the samples in which the Cu2O microcrystals are obtained by the deposition for 5 and 15 min, respectively. While the ZnO/Cu2O NTAs with spherical Cu2O microcrystals grown by the deposition for 5 min have higher PEC performance than the samples in which the Cu2O microcrystals are grown by the deposition for 3 and 10 min, respectively. As a photoelectrode, the PEC performance of heterogeneous ZnO/Cu2O NTAs p-n junction is significantly improved compared to pure ZnO NTAs. Moreover, the PEC water splitting performance of the ZnO/Cu2O NTAs photo-electrode can be manipulated by controlling the morphology and size of Cu2O microcrystals. The findings provide useful guidance for enhancing PEC performance of ZnO-based photo-electrode materials.
Acknowledgements This work was supported by National Natural Science Foundation of China (NSFC, Grants No. 11604394, 11474174, 61575225, 11404414, 51132002 and 51372282), project to develop the scientific capacity of young teachers of Minzu university of China (Grant No. 2019QNPY73) and the Undergraduate Innovative Test Program of China (Grant No. GCCX 2019110004, GCCX 2019110026).
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Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114
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Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode Jirong Chen, Ying Jia*, Wenzhong Wang, Junli Fu, Honglong Shi, Yujie Liang College of Science, Minzu University of China, Beijing 100081, China
highlights ZnO/Cu2O nanotube arrays with cubic and spherical Cu2O microcrystals were prepared. The morphology of Cu2O deposited on ZnO can be adjusted by the deposition potential. The morphology and size of Cu2O can influence the PEC performance of ZnO/Cu2O.
article info
abstract
Article history:
ZnO nanotube arrays were synthesized by the electrodeposition method and Cu2O mi-
Received 21 November 2019
crocrystals with two kinds of morphologies were deposited on ZnO nanotube arrays suc-
Received in revised form
cessfully. At the deposition potential of 0.5 or 0.7 V, the cubic or spherical Cu2O
12 January 2020
microcrystals were selectively deposited on ZnO nanotube arrays. By adjusting the depo-
Accepted 17 January 2020
sition time, Cu2O microcrystals with different sizes were obtained. The optical properties
Available online xxx
and photo-electrochemical performance of ZnO/Cu2O were measured. The results showed
Keywords:
and enhanced photocurrent due to the excellent ability of Cu2O microcrystals for har-
ZnO/Cu2O nanotube arrays
vesting visible light, and the effective separation and transfer of photo-generated electrons
that the as-prepared ZnO/Cu2O heterojunction exhibited improved visible light absorption
Photo-electrode
and holes owing to p-n junction between ZnO and Cu2O. The experimental results
Morphology
demonstrate that the photo-electrochemical performance of ZnO/Cu2O heterojunction nanotube arrays can be manipulated by controlling the morphology and the size of Cu2O microcrystals. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the development of the world economy, the energy crisis and environmental pollution problems have become increasingly prominent. Therefore, the application of clean
energy is an irresistible trend. As a mean of solar energy conversion technology, photo-electrochemical (PEC) water splitting has received great attention [1e5]. Among the available photosensitive semiconductor materials, ZnO is one of the most promising photo-electrode materials because of its special features, such as low cost, chemical inertness and high
* Corresponding author. E-mail address: [email protected] (Y. Jia). https://doi.org/10.1016/j.ijhydene.2020.01.114 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114
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electron mobility (~120 cm2V1s1) [6]. However, the wide band gap (~3.37 eV) of ZnO leads to the poor light harvesting capability in the visible light region, which hints its extensive application [7e10]. The construction of heterostructure by coupling with narrow band gap p-type semiconductor is an effective way to extend the application of ZnO, because this kind of heterostructure not only extends the absorption range of ZnO to visible light region, but also reduces the recombination rate of photo-generated carriers, which will be helpful to improve the PEC performance of the photo-electrode [11e16]. Cu2O is a typical p-type semiconductor material with narrow band gap (~2.0 eV), which matches well with the visible spectrum, and its theoretical power conversion efficiency is about 20% [17,18]. As a promising material, Cu2O shows great potential for the p-n junction construction because of its efficient light absorption and photo-induced electron provision [19e25]. Recently, ZnO/Cu2O heterostructure has attracted extensive interest, because ZnO and Cu2O have an appropriate band match. The conduction and valence band edge of Cu2O are both above those of ZnO [8,26]. The suitable band gap structure follows the typical type-II mode, which can effectively suppress the photo-generated carrier recombination and enhance the PEC performance [27e33]. Different ZnO/Cu2O heterostructures have been synthesized successfully [34e36]. However, to the best of our knowledge, the deposition of Cu2O on ZnO nanotube arrays (NTAs) has been rarely reported. ZnO/Cu2O heterojunction structure prepared by depositing Cu2O microcrystals with different morphologies on ZnO NTAs is promising to improve the PEC performance of the photoelectrode. On one hand, the highly ordered ZnO NTAs have high aspect ratio and electron mobility, which is beneficial to the light absorption and the electron transfer. On the other hand, different morphologies of Cu2O can influence the absorption of light and the transfer of photo-generated carriers of the heterogeneous photo-electrode, which has been proved in the Cu2O/TiO2 PEC cell [37]. In this study, we provide a convenient electrochemical deposition method to deposit Cu2O microcrystals with cubic and spherical morphologies on the ZnO NTAs to prepare ZnO/ Cu2O heterojunction photo-electrode. The PEC performance of the heterogeneous photo-electrode is studied, and the ZnO/ Cu2O photo-electrode exhibits excellent PEC performance compared to that of pure ZnO photo-electrode in the visible light region.
Preparation of ZnO NTAs First of all, the Fluorine-doped tin oxide (FTO) conductive glass was ultrasonically cleaned by deionized water, ethanol and acetone for 15 min, respectively. ZnO NTAs were prepared by through the following two processes. Firstly, ZnO nanorod arrays (NRAs) were synthesized on a FTO conductive glass by a three-electrode electrochemical deposition system, in which platinum, Ag/AgCl and FTO glass were used as the counter electrode, the reference electrode and the working electrode, respectively. The electrolyte was the mixed solution of 200 mL polyethylene glycol 400, 50 mL anhydrous ethylenediamine and 150 ml 0.0125 M Zn(NO3)2 aqueous solution. The temperature of the electrolyte was maintained at 70 C. The anode electrode potential was controlled at 1.1 V, and the deposition time was 1.5 h. Secondly, the FTO conductive glass deposited with ZnO NRAs was immersed in KOH solution (0.18 M). The etching time was 1 h, and the solution temperature was fixed at 80 C [38]. Then the sample was taken out and rinsed with deionized water. Finally, the sample was annealed at 400 C for 30 min.
Deposition of Cu2O microcrystals on ZnO NTAs In this step, the electrochemical method was used for the deposition of Cu2O microcrystals on ZnO NTAs. The electrolyte solution was a mixture of 0.05 M Cu(NO3)2 and 0.3 M lactic acid. The pH value of the electrolyte solution was tuned to 12 by using 4 M NaOH. The temperature of the electrolyte solution was fixed at 60 C and the deposition voltage was kept at a stable voltage of 0.5 or 0.7 V to synthesize Cu2O microcrystals with cubic or spherical shape, respectively. Then the sample was rinsed by deionized water and dried at 60 C for half an hour in a vacuum drying oven.
Samples characterizations The scanning electron microscopy (SEM, Hitachi S-4800, Japan) was used to characterize the morphologies of the prepared samples. The crystallinity and phase of the nanostructures were characterized by X-ray diffraction (XRD) on a XD-3 diffractometer (Beijing Pepsee, China) with Cu-Ka radiation (l ¼ 1.5406 A) as the excitation source. X-ray photoelectron spectroscopy (XPS, ThermoFisher, UK) was used to analyze the elemental composition. Lambda 950 UV/VIS/NIR spectrometer (Perkin-elmer, USA) was used to record the UVevis absorption spectra of the samples.
Material and methods
PEC measurements
Zinc nitrate hexahydrate (Zn(NO3)2$6H2O, 99%), polyethylene glycol 400 (H(OCH2CH2)nOH), anhydrous ethylenediamine (H2NCH2CH2NH2, 99.0%), copper nitrate (Cu(NO3)2$3H2O, 99.0%), lactic acid (CH3CH-(OH)COOH, 85.0%), potassium hydroxide (KOH, 85.0%) and sodium hydroxide (NaOH, 96%) were purchased from Beijing Chemical Company. All of the reagents were of analytical grade and used as received without further purification.
Photo-electrochemical measurements of ZnO and ZnO/Cu2O NTAs were carried out by using a three-electrode electrochemical analyzer. Platinum, Ag/AgCl and the FTO conductive glass were used as the counter electrode, reference electrode and working electrode, respectively. NaOH aqueous solution (0.5 M) was used as the electrolyte solution. The light source was 300-W Xenon lamp (Perfect Light, China), and a visible light filter was used to obtain visible light. The power density
Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114
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Fig. 1 e (a) Low- and (b) high-magnification SEM images of ZnO NTAs. (cee) SEM images of ZnO/Cu2O NTAs synthesized at deposition potential of ¡0.5 V under the deposition time of 5, 10 and 15 min. (feh) SEM images of ZnO/Cu2O NTAs synthesized at deposition potential of ¡0.7 V under the deposition time of 3, 5 and 10 min.
Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114
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Fig. 2 e XRD patterns of pure ZnO NTAs and ZnO/Cu2O NTAs loaded with cubic Cu2O microcrystals (a) and spherical Cu2O microcrystals (b).
Fig. 3 e (a) The survey XPS spectrum of ZnO/Cu2O NTAs with spherical Cu2O microcrystals obtained by the deposition for 5 min. (b) The high-resolution XPS spectrum of zinc 2p. (c) The high-resolution XPS spectrum of oxygen 1s. (d) The highresolution XPS spectrum of copper 2p. near the position of the electrode was 100 mW/cm2. The dissolved oxygen in the electrolyte solution was removed by injecting N2 for 20 min. The experiments were carried out in a quartz window by using an electrochemical workstation (CHI 660c, CH Instruments Inc., USA). The linear sweep voltammetry (LSV), I-t curve and electrochemical impedance (EIS) were collected under visible light (l > 420 nm) irradiation.
Results and discussion Morphology and composition Fig. 1 shows the surface morphologies of pure ZnO and ZnO/ Cu2O NTAs. Fig. 1a and b are low-magnification and high-
Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114
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Fig. 4 e The absorption spectra of ZnO/Cu2O NTAs loaded with cubic Cu2O microcrystals (a) and spherical Cu2O microcrystals (b). magnification SEM images of bare ZnO NTAs. The ZnO NTAs exhibit ordered arrangement, and they are nearly perpendicular to the FTO surface. It can be found from Fig. 1b that a single ZnO nanotube displays a hollow six-square structure with an average diameter of 420 nm. The surface of ZnO nanotube is smooth and the wall thickness is about 50 nm. Fig. 1cee shows the surface morphologies of ZnO/Cu2O NTAs synthesized at deposition potential of 0.5 V. It can be found that the Cu2O microcrystals with cubic shape are deposited on ZnO NTAs, and the Cu2O microcrystals exhibit smooth surface and regular cube structure. When the deposition time is chosen as 5 min, the side length of the Cu2O cube is about 150 nm as seen in Fig. 1c. When the deposition time increases to 10 and 15 min, the side length of the Cu2O cubes increases to 300 and 700 nm, respectively, as seen in Fig. 1d and e. It can be seen that the size of the Cu2O microcrystals increases significantly with the extension of deposition time. Fig. 1feh shows the SEM images of ZnO/Cu2O NTAs synthesized at deposition potential of 0.7 V. It can be seen that the Cu2O
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microcrystals with spherical shape are deposited on ZnO NTAs. Fig. 1f is the SEM image of the prepared ZnO/Cu2O NTAs prepared under the deposition time of 3 min. It can be found that the diameter of the spherical Cu2O microcrystal is about 300 nm. With extending deposition time to 5 and 10 min, the diameter of the Cu2O microcrystal increases to 400 and 600 nm, respectively. Additionally, the SEM images of Fig. 1ceh clearly demonstrate that the deposition of Cu2O microcrystals does not damage the structure of the ordered ZnO NTAs. Fig. 2a shows the XRD patterns of pure ZnO NTAs and ZnO/ Cu2O NTAs loaded with cubic Cu2O microcrystals, and Fig. 2b is the XRD patterns of pure ZnO NTAs and ZnO/Cu2O NTAs loaded with spherical Cu2O microcrystals. In the XRD pattern of the pure ZnO NTAs, six main diffraction peaks correspond to the crystal planes of (100), (002), (101), (102), (103) and (112), respectively, which is in accordance with the diffraction peaks of ZnO with hexagonal phase reported in the literature [39]. The intensity of the diffraction peak corresponding to (002) crystal plane is obviously higher than those of other diffraction peaks, indicating that the ZnO NTAs has preferential growth along the c-axis. For the heterogeneous structure of ZnO/Cu2O, three diffraction peaks located at 36.5 , 42.3 and 62.1 correspond to the crystal planes of (111), (200) and (220) of cubic phase Cu2O, respectively, With the extension of deposition time, the intensity of the three diffraction peaks increases. The composition of ZnO/Cu2O NTAs was further confirmed by X-ray photoelectron spectroscopy. Fig. 3aed is the XPS results of the survey spectrum and the high resolution spectra of ZnO/Cu2O NTAs with spherical Cu2O microcrystals obtained by the deposition for 5 min. The measured spectrum in Fig. 3a shows that the peaks indexed to Zn, Cu and O elements are found. The high resolution spectra of ZnO/Cu2O NTAs are shown in Fig. 3bed. As shown in Fig. 3b, the two peaks at the binding energies of 1021.7 and 1044.8 eV correspond to Zn 2p3/ 2 and 2p1/2, respectively. The interval of 23.1 eV of the two peaks indicates the Zn species with þ2 valence state exist in ZnO/Cu2O NTAs. Fig. 3c shows the binding energies of O in ZnO/Cu2O NTAs, which can be fitted to two peaks located at 530.8 and 531.8 eV attributing to O in Cu2O and ZnO. Two peaks at 932.6 and 952.2 eV of Fig. 3d attribute to Cu 2p3/2 and 2p1/2, respectively, indicating the existence of cuprous in ZnO/ Cu2O NTAs [40]. The UVevis absorption spectra of ZnO/Cu2O NTAs are presented in Fig. 4. It can be found that the pure ZnO NTAs have good ultraviolet absorption, and there is hardly any visible light absorption. The absorption edge of pure ZnO NTAs is about at 395 nm, which is in accordance with the intrinsic bandgap absorption of ZnO crystals. By comparison with pure ZnO NTAs, the visible light absorption of ZnO/Cu2O NTAs loaded with both cubic and spherical Cu2O microcrystals is significantly enhanced. It can be found that there is an obvious absorption peak at about 470 nm after deposition of Cu2O on ZnO, and the absorption spectra show a wide absorption range. Fig. 4a is the absorption spectra of ZnO/Cu2O NTAs with cubic Cu2O microcrystals. Upon careful observation, the ZnO/Cu2O NTAs with cubic Cu2O microcrystals, which was obtained by the deposition for 10 min, show better absorbance performance than the samples, in which the Cu2O
Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114
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Fig. 5 e The photocurrent densities of ZnO/Cu2O NTAs loaded with cubic Cu2O microcrystals (a) and spherical Cu2O microcrystals (b) at 0 V versus Ag/AgCl. The LSV curves of ZnO/Cu2O NTAs with cubic Cu2O microcrystals (c) and spherical Cu2O microcrystals (d).
microcrystals are obtained by the deposition for 5 and 15 min, respectively. Fig. 4b is the absorption spectra of ZnO/Cu2O NTAs with spherical Cu2O microcrystals. It can be found that the sample with spherical Cu2O microcrystals obtained by the deposition for 5 min shows better absorbance performance.
PEC performance Fig. 5a and b shows the instantaneous visible light response and photocurrent densities of bare ZnO and ZnO/Cu2O NTAs photo-electrode at 0 V bias voltage versus Ag/AgCl. Fig. 5a is the photocurrent densities of ZnO/Cu2O NTAs with cubic Cu2O microcrystals, and Fig. 5b is the photocurrent densities of ZnO/Cu2O NTAs with spherical Cu2O microcrystals. It can be found that the pure ZnO NTAs photo-electrode almost does not produce photocurrent by visible light illumination at 0 V versus Ag/AgCl, while ZnO/Cu2O NTAs exhibit obvious visible light response. When the ZnO/Cu2O NTAs photo-electrode is under visible light irradiation, the photocurrents are generated rapidly, showing that the deposition of Cu2O microcrystals can significantly improve the PEC performance of ZnO
photo-electrode under visible light irradiation. The results are consistent with the strong visible light absorption of ZnO/ Cu2O NTAs as mentioned above. In Fig. 5a, when the deposition time for the deposition of cubic Cu2O microcrystals is 10 min, the ZnO/Cu2O NTAs achieve higher photocurrent than the samples, in which the Cu2O microcrystals are obtained by the deposition for 5 and 15 min, respectively. The highest photocurrent density of the ZnO/Cu2O NTAs with cubic Cu2O microcrystals is 0.12 mA/cm2. In Fig. 5b, when the spherical Cu2O microcrystals are deposited on ZnO NTAs under deposition time of 5 min, the photocurrent density of ZnO/Cu2O NTAs photo-electrode reaches to higher value. While the deposition time increases to 10 min or decreases to 3 min, the photocurrent density decreases. These results indicate that the amount and size of Cu2O particles obviously affect the photo-electricity performance of ZnO/Cu2O NTAs photoelectrode, which is similar to the results of the previous report [41]. Through comparing Fig. 5a and b, it can be found that the morphologies of Cu2O also affect the photocurrent of ZnO/Cu2O NTAs photo-electrode, which is similar to the results shown in the previous report [37]. Thus, the
Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114
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Fig. 6 e The EIS curves of pure ZnO (a) and ZnO/Cu2O NTAs (b) photo-electrodes under visible light irradiation.
experimental measurements clearly demonstrate that the PEC water splitting performance of the ZnO/Cu2O NTAs photo-electrode can be manipulated by controlling the morphology and size of Cu2O microcrystals. The photocurrent is decreased with the increase of time, which could be due to the slight photo-corrosion of Cu2O. The similar PEC water splitting property was reported in the previous study [42]. The linear sweep voltammograms (LSVs) were used to analyze the relationship between the photocurrent densities of visible light response and bias voltage. Fig. 5c and d shows the LSV curves of ZnO/Cu2O NTAs with cubic and spherical Cu2O microcrystals, respectively. The bias voltage is from 0 to 0.6 V versus Ag/AgCl and the ZnO/Cu2O NTAs photo-electrode is illuminated by visible light. It can be found that the photocurrent densities hardly change when the pure ZnO photoelectrode is under visible light irradiation. The reason is that the wide band-gap ZnO has no response to visible light and it can not be excited by visible light to generate electron-hole pairs. After the Cu2O microcrystals are deposited on ZnO NTAs, the photocurrent densities of the ZnO/Cu2O NTAs
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Fig. 7 e (a) Mott-Schottky plots of ZnO NTAs and ZnO/Cu2O NTAs with cubic Cu2O microcrystals obtained by the deposition for 10 min. (b) Mott-Schottky plots of ZnO NTAs and ZnO/Cu2O NTAs with spherical Cu2O microcrystals obtained by the deposition for 5 min.
photo-electrodes increase with the applied bias voltage. The results show that the deposition of Cu2O microcrystals not only enhances the absorption of visible light but also improves the PEC performance of the ZnO based photo-electrode. The charge separation and transfer process of the obtained photo-electrodes are characterized by electrochemical impedance spectroscopy. Fig. 6 shows the Nyquist plots of pure ZnO and ZnO/Cu2O NTAs photo-electrodes obtained under visible light illumination at open circuit potential (OCP) and the measured range is from 1 to 100,000 Hz. The value of OCP for pure ZnO NTAs is 0.352 V. The ZnO/Cu2O NTAs with cubic Cu2O microcrystals obtained by the deposition for 5, 10 and 15 min are 0.531, 0.542 and 0.533 V, respectively. The ZnO/Cu2O NTAs with spherical Cu2O microcrystals obtained by the deposition for 3, 5 and 10 min are 0.514, 0.522 and 0.517 V, respectively. The high-frequency part of the EIS
Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114
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Fig. 8 e (a) Schematic diagram of band gap structures and the proposed charge separation and transfer in ZnO/Cu2O NTAs under visible light irradiation. (b) Schematic diagram of carrier transmission in an external circuit.
curves has been given as the inset of Fig. 6. The arc diameter of the Nyquist curve reflects the resistance of the charge transfer [43,44]. The smaller diameter indicates smaller charge transfer resistance and higher separation efficiency of photogenerated carriers. As shown in Fig. 6, the diameters of the semi-circle arcs of ZnO/Cu2O NTAs photo-electrodes are obviously smaller than that of ZnO NTAs photo-electrode, which indicates that deposition of Cu2O microcrystals can reduce the resistance of charge transfer. In Fig. 6a, it can be found that the ZnO/Cu2O NTAs photo-electrodes with cubic Cu2O microcrystals obtained by the deposition for 10 min has the smallest diameter of semi-circle arc, so its charge transfer resistance is the smallest. The result is in accordance with its highest photocurrent as seen in Fig. 5a. In Fig. 6b we can see that the ZnO/Cu2O NTAs photo-electrodes with spherical Cu2O obtained by the deposition for 5 min has the smallest diameter of semi-circle arc, so its photocurrent is the highest as seen in Fig. 5b.
Mechanism To further understand the mechanism of the enhanced PEC performance for ZnO/Cu2O NTAs photo-electrodes, Mott Schottky (M S) curves of the obtained photo-electrodes are recorded. The M S plots of pure ZnO and ZnO/Cu2O NTAs photo-electrodes are collected by conducting impedancepotential spectroscopy at an AC frequency of 1000 Hz under visible light irradiation. Fig. 7a shows the M S curves of ZnO NTAs and ZnO/Cu2O NTAs with cubic Cu2O microcrystals obtained by the deposition for 10 min. It shows that the M S curve of ZnO NTAs photo-electrode has a positive slope, which exhibits n-type semiconductor behavior. However, for the ZnO/Cu2O NTAs photo-electrode, an obvious inverted Vshape can be observed, which shows the p-n junction characteristic and indicates that n-type ZnO and p-type Cu2O have successfully constructed p-n junction at their interface. Fig. 7b shows the M S curves of ZnO NTAs and ZnO/Cu2O NTAs with spherical Cu2O microcrystals obtained by the deposition for 5 min. An obvious inverted V-shape can also be observed, which indicates that the p-n junction has been constructed
successfully at the interface between ZnO and Cu2O. The Mott-Schottky equation is as follows: 1 2 kB T ; ¼ Vappl VFB 2 2 C ε0 εqND A q where C is the capacitance of the space charge layer, ε0 and ε are the vacuum permittivity and the dielectric constant of the semiconductor, respectively, q is the elementary charge, ND is the donor density, A is the surface area of the electrode exposed to the electrolyte, Vappl and VFB are the applied potential and the flat-band potential, respectively, kB is the Boltzmann constant and T is Kelvin temperature. It can be obtained from the above formula that the donor density ND is ascribed to the slope of the M S curves. The donor densities of ZnO/Cu2O NTAs with cubic Cu2O obtained by the deposition for 10 min and ZnO/Cu2O NTAs with spherical Cu2O obtained by the deposition for 5 min are 2.30 1020 and 7.96 1019 cm3, respectively, which are both higher than that of pure ZnO (4.79 1019 cm3). The results indicate that the charge separation is enhanced due to the formation of p-n heterojunction between ZnO and Cu2O. Based on the above experimental results, the proposed charge separation and transfer in ZnO/Cu2O NTAs under visible light irradiation is presented in Fig. 8. Fig. 8a is the schematic energy band diagram of ZnO/Cu2O and the possible charge transfer in ZnO/Cu2O NTAs. Fig. 8b is the schematic diagram of carrier transmission in an external circuit. The Fermi energy level of Cu2O is lower than that of ZnO, so after Cu2O microcrystals are deposited on ZnO NTAs, the Fermi level of Cu2O moves up and that of ZnO moves down until the Fermi level between ZnO and Cu2O reaches an equilibrium state. Consequently, an electric field is built in the space charge region between ZnO and Cu2O, and the p-n junction is constructed. When ZnO/Cu2O NTAs photo-electrode is under visible light irradiation, some electrons are excited from the valence band (VB) of Cu2O to its conduction band (CB). With the help of the inner electric field at the interface between ZnO and Cu2O, the electrons will transfer from the CB of Cu2O to the CB of ZnO, then move to Pt electrode through an external circuit and react with the Hþ near the Pt electrode to take part
Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114
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in water splitting reaction. The inner electric field can effectively promote the separation and transfer of photo-generated electrons and holes, resulting in the enhancement of the PEC performance.
Conclusions In summary, we prepared ZnO/Cu2O NTAs with cubic and spherical Cu2O microcrystals. The morphologies of Cu2O microcrystals can be controlled by adjusting deposition potential. The cubic and spherical Cu2O microcrystals are obtained at the deposition potential of 0.5 and 0.7 V, respectively. With the increase of deposition time, the PEC performance of ZnO/Cu2O NTAs changes significantly. By contrast, the ZnO/ Cu2O NTAs with cubic Cu2O microcrystals obtained by the deposition for 10 min have higher PEC performance than the samples in which the Cu2O microcrystals are obtained by the deposition for 5 and 15 min, respectively. While the ZnO/Cu2O NTAs with spherical Cu2O microcrystals grown by the deposition for 5 min have higher PEC performance than the samples in which the Cu2O microcrystals are grown by the deposition for 3 and 10 min, respectively. As a photoelectrode, the PEC performance of heterogeneous ZnO/Cu2O NTAs p-n junction is significantly improved compared to pure ZnO NTAs. Moreover, the PEC water splitting performance of the ZnO/Cu2O NTAs photo-electrode can be manipulated by controlling the morphology and size of Cu2O microcrystals. The findings provide useful guidance for enhancing PEC performance of ZnO-based photo-electrode materials.
Acknowledgements This work was supported by National Natural Science Foundation of China (NSFC, Grants No. 11604394, 11474174, 61575225, 11404414, 51132002 and 51372282), project to develop the scientific capacity of young teachers of Minzu university of China (Grant No. 2019QNPY73) and the Undergraduate Innovative Test Program of China (Grant No. GCCX 2019110004, GCCX 2019110026).
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Please cite this article as: Chen J et al., Morphology selective electrodeposition of Cu2O microcrystals on ZnO nanotube arrays as efficient visible-light-driven photo-electrode, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.114