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Electrodeposited Cu2O on the {101} facets of TiO2 nanosheet arrays and their enhanced photoelectrochemical performance
Solar Energy Materials & Solar Cells 165 (2017) 27–35
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Electrodeposited Cu2O on the {101} facets of TiO2 nanosheet arrays and their enhanced photoelectrochemical performance
MARK
Lei Yanga,b,c, Weihua Wangb, Hui Zhangb, Shenhao Wangb, Miao Zhanga, Gang Hea, Jianguo Lvd, ⁎ Kerong Zhua, Zhaoqi Suna, a
School of Physics and Material Science, Anhui University, Hefei 230601, PR China Institute of Applied Physics, AOA, Hefei 230031, PR China c Co-operative Innovation Research Center for Weak Signal-Detecting Materials and Devices Integration, Anhui University, Hefei 230601, PR China d School of Electronic and Information Engineering, Hefei Normal University, Hefei 230601, PR China b
A R T I C L E I N F O
A BS T RAC T
Keywords: Cu2O/{101}TiO2 nanosheet Photoelectrochemical properties Band offset value Anatase nanosheet arrays
A novel Cu2O/{101}TiO2 nanosheet (Cu2O/{101}TNS) array film was prepared by the electrodeposition of Cu2O on the {101} facets of anatase nanosheet arrays by varying electrodeposition potential. The crystal structure, morphology, elemental chemical states, optical properties, photoelectrochemical properties, and stability of Cu2O/{101}TNS array films were investigated in detail. For TNS, due to different band structures and band edge positions between {001} and {101} facets, Cu2O/{101} facets of TiO2 have higher band offset value which supply a larger driving force to increase the transport efficiency of carriers. Besides, owing to the directional flow of photo-generated electrons from {001} to {101} facets, the electrodeposition of Cu2O on the {101} facets of TNS will shorten the route length that the electrons must travel, thus reduce recombination of photo-generated electron-hole pairs. In addition, as the applied negative potential is high enough, a part of Cu+ is reduced to Cu, which is beneficial for the photoexcited electrons transfer from CB of Cu2O to that of TiO2. The enhanced photoelectrochemical properties of Cu2O/{101}TNS array films can be attributed to the cocontributions of different band edge positions between {001} and {101} facets, Cu2O-Cu-TiO2 ternary components and vertically aligned single-crystal TiO2 nanosheet structure.
1. Introduction As one of the most important n-type semiconductors, TiO2 has been widely used in solar cells [1], environmental purification [2], selfcleaning and antifogging coatings [3], and electrochemical energy storage [4,5]. It is well-known that the properties of metal oxide semiconductors are not only related to their crystal phase, specific surface area, defects in the lattice, crystal morphology, crystallinity, but also exposed crystal facets. More specifically, both theoretical and experimental studies have shown that the {001} facets of anatase TiO2 exhibited more active than the thermodynamically more stable {010} and {101} facets [6,7]. However, high reactive facets usually diminished during the crystal growth as a result of the minimization of total surface energy [8]. Since Yang et al. synthesized micron-sized anatase crystals with ~47% exposed {001} facets [9], a series of researches have focused on the TiO2 with exposed {001} facets. In early studies, it was found that TiO2 with higher percentage of {001} facets usually exhibit greater reactivity [10,11], due to abundant unsaturated coordination
⁎
sites. In recent studies, Ohno et al. found that stacked octahedral anatase crystals show the high photocatalytic activity due to efficient separation of oxidation and reduction sites [12]. Tachikawa et al. provided definitive evidence that reduction sites are preferentially located on the {101} facets, and they hypothesized that photogenerated charge carriers make directional flow [13]. Based on the DFT calculation, Yu et al. demonstrated that {001} and {101} facets of anatase exhibit different band structures and band edge positions and proposed a “surface heterojunction” concept [14]. With synchronous illumination X-ray photoelectron spectroscopy, Bi et al. directly observated the photo-generated charge separation on anatase single crystals [15]. On the whole, their focus was optimizing the ratio of different exposed facets. However, the practical application of TiO2 is hindered by its wide band gap and high recombination rate of the photo-generated electronhole pairs [16]. Thus, considerable efforts have been dedicated to fabricate {001} exposed TiO2 composite materials [17]. Wang et al. prepared CdS NPs sensitized TiO2 films with oriented {001} facets, the
Corresponding author. E-mail addresses: [email protected] (L. Yang), [email protected] (Z. Sun).
http://dx.doi.org/10.1016/j.solmat.2017.02.026 Received 3 July 2016; Received in revised form 15 February 2017; Accepted 19 February 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
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obtained TiO2 films show a enhanced photoelectrochemical properties [18]. Liu et al. demonstrated that photocatalytic activity can be enhanced by the deposition of Pt on the {101} facets of anatase TiO2 [19]. Etgar et al. reported that interfacial charge separation at PbS/ {001} facets is five times faster than that on {101} facets of TiO2 [20]. More recently, it is worth noting that Li et al. prepared Cu2O/TiO2 heterojunctions on various exposed facets of Cu2O polyhedral, they have demonstrated that photocatalytic performance of the composites is dependent upon Cu2O/TiO2 energy band alignment, as opposed to surface energy alone [21]. Cu2O (direct band gap ~2.0 eV), as a non-toxic p-type semiconductor is a promising material in solar-energy-conversion applications [22]. Because both the conduction band (CB) and valence bands (VB) of Cu2O are more negative than that of TiO2. Thus, when Cu2O-TiO2 heterojunction is formed, the photoexcited electrons transfer from CB of Cu2O to that of TiO2. As already reported [14], the CB level of {101} facets is lower than that of {001} facets of TiO2, thus, Cu2O/{101} facets have a higher band offset value. The higher band offset value implies the larger driving force for photo-generated electrons transfer from Cu2O to {101} facets of TiO2, thus improves the charge separation and inhibits charge carriers recombination. In addition, unlike TiO2 nanoparticles, well-aligned single-crystalline TiO2 nanosheet (TNS) grown directly on the conductive glass without any barrier layer, hence facilitates electron transfer through the TiO2/conductive substrate interface. In this work, dense regular TNS were prepared as our previous work [23]. The TNS are nearly perpendicular to the FTO substrate and the upper surface of film are {101} facets of anatase single crystal. Then, the Cu2O are electrodeposited on the {101} surface of TNS arrays films. Moreover, as the applied negative potential is high enough, Cu deposition will occur, which facilitates the transportation of electrons. The effect of the electrodeposition potential on their microstructure, morphology and photoelectrochemical performance were discussed and a model of electron transfer among Cu2O/{101}TNS was proposed.
2.2. Characterization X-ray powder diffraction (XRD) patterns of Cu2O/{101}TNS array films were identified by the MAC M18XHF X-ray diffractometer using Cu Kα radiation. The morphology of the Cu2O/{101}TNS array films was examined in a Hitachi S4800 field-emission scanning electron microscope (FESEM) and JEM-2100 high-resolution transition electron microscope (HRTEM). Surface compositions and elemental chemical states were analyzed by a Thermo ESCALAB 250 X-ray photoelectron spectroscopy (XPS) electron microscopy. The absorbance spectra were obtained with an UV-2550, Shimadzu ultraviolet-visible spectrophotometer equipped with a diffuse reflectance integrating sphere. Photoluminescence (PL) spectra were measured using a Hitachi F-4500 fluorescence spectrophotometer at the excitation wavelength of 325 nm. The photoelectrochemical experiments, including electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV) and photocurrent were performed by means of a CHI600D electrochemical workstation (ChenHua Instruments Co. Ltd, China). In a standard three electrode system, the prepared films, Pt foil and Ag/AgCl (saturated with KCl), were used as the working, counter and reference electrodes, respectively. The supporting electrolyte was 0.1 M Na2SO4 aqueous solution. A 150 W xenon lamp (SS150A, ZOLIX, China) with an AM 1.5 G cutoff filter was employed as the light source in the photoelectrochemical measurements. 3. Results and discussion Fig. 1 shows the current density versus deposition time curves for Cu2O electrodeposition onto TNS array films under different potentials from the same growth solution. The shapes of these current density curves indicate two main processes: nucleation and growth of crystals [24]. For Cu2O/{101}TNS-1 and Cu2O/{101}TNS-2, current density increases sharply and reaches to a maximal value (nucleation), then, a continuously decreasing occurs until to a relatively stable value (growth). Due to relatively low of applied negative potential, a quick decrease of current happened as Cu2O growing. As the applied potential becomes more negative (−0.3 V, −0.4 V, −0.5 V), the currents have shown a gradual increase, indicating a progressive nucleation process. With the increase of negative voltage, it is proposed that the larger electrical force will apply on the Cu2+ ions, which are acquired more energy to attach on TiO2 surface, leading to more nucleation of Cu2O [25]. However, current density curve of Cu2O/{101}TNS-6 show a different shape. The decrease of deposition current after the first few second reflects the relatively high resistivity due to the fast electrodeposition of Cu2O. In addition, the maximal values increase from 0.19 to 8.85 mA/cm−2 as deposition potential become more cathodic. The results indicate that the nuclei density becomes higher gradually [26]. To study crystalline nature of the Cu2O/{101}TNS thin films, XRD patterns of the thin films were recorded and shown in Fig. 2. Four distinct diffraction peaks at 2θ=25.3°, 37.7°, 48.1° and 55.0° correspond to the (101), (004), (200) and (211) planes of anatase phase (JCPDS no. 21-1272), respectively [11], some weak peaks located at 26.5°, 38.5°, 51.8° and 65.5° can be assigned to the FTO substrate (JCPDS no. 46-1088). More remarkable, the intensity of (004) diffraction peak is enhanced compared with other types of TiO2, implies the exposure of high energy {001} facets [27]. The diffraction peaks at 2θ=36.4°, 42.4°, 61.5° can be indexed to (111), (200) and (220) peaks of the standard powder diffraction patterns for cuprite Cu2O (JCPDS no. 34-1354) [28]. In addition, the Cu phases are observed when the applied negative potential is increased to −0.4 V. The mechanism for the electrodeposition of Cu2O can be described as follows [24,29].
2. Experimental 2.1. Preparation of Cu2O/{101}TNS films In a typical synthesis of TNS films, 12 mL of hydrochloric acid (36.5–38% by weight) was mixed with 18 mL of deionized water. The mixture was stirred for 5 min before 0.5 mL titanium butoxide (97% Aldrich) was added. After stirring for another 5 min, 0.25 g of (NH4)2TiF6 was added, and the mixture was further stirred for 15 min. Subsequently, the solution was transferred into a 50 mL polytetrafluoroethylene-lined stainless steel autoclave, in which a piece of FTO-coated glass slide placed slantly, with the FTO side facing up. The hydrothermal reaction was carried out at 170 ℃ for 16 h in an electric oven. After the reaction was completed, the sample was then taken out, washed with distilled water and ethanol thoroughly. Before the deposition of Cu2O on TNS films, the samples were annealed at 500 ℃ for 2 h. The procedure of the synthesis of Cu2O/{101}TNS heterogeneous array films is as follows: 4.99 g cupric acetate was dissolved in 500 mL of deionized water, then, 4.1 g sodium acetate was added to this solution under magnetic stirred for 1 h, at 40 ℃. The deposition of Cu2O on TNS array films was carried out by electrodeposition method with a standard three-electrode system in the preceding solution. TNS films, Pt foil and Ag/AgCl (saturated with KCl), were used as the working, counter and reference electrodes, respectively. The electrolyte was kept at a constant temperature of 40 ℃ and the deposition time was 8 min. Six samples are obtained by varying electrodeposition potential i.e., −0.1 V, −0.2 V, −0.3 V, −0.4 V, −0.5 V and −0.6 V and denoted as Cu2O/{101}TNS-1, Cu2O/{101}TNS-2, Cu2O/{101}TNS-3, Cu2O/{101}TNS-4, Cu2O/{101}TNS-5 and Cu2O/{101}TNS-6, respectively. 28
Cu2+ + e− → Cu+
(1)
2Cu+ + H2 O → Cu2 O + 2H+
(2)
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Fig. 1. Current density versus deposition time curves for Cu2O/{101}TNS array films deposited under different potentials.
Fig. 3c and d. When the applied negative potential increases to −0.3 V, Cu2O particles have obvious agglomerated and dendritic branching Cu2O are formed. The inset in Fig. 3d and e clearly show that morphology of Cu2O is altered by the deposition potential. At deposition potential −0.4 and −0.5 V, the dendritic branching structure gradually decreases and stacking particles become prominent. When a more negative deposition potential was applied (−0.6 V), TNS films are covered by dense Cu2O particles completely. As shown, the TNSs are nearly perpendicular to the FTO substrate, and the upper surface of film are {101} facets of anatase single crystal, therefore, the Cu2O particles are deposited on the {101} surface of TNS array films. As shown in Fig. 3i, the EDS result of Cu2O/{101}TNS-2 reveals that the thin film consists O, Ti and Cu, C peak from the adventitious contaminants is also present. TEM and HRTEM images of the obtained Cu2O/{101}TNS samples are shown in Fig. 4. These TiO2 sheets have a truncated bipyramid in shape. The SAED pattern of TNS viewed along the [001] orientation confirmed its single crystal structure. As shown in Fig. 4b, interfacial angle between {001} and {101} facets is 68°, this is in line with the theoretical values of anatase [9]. Furthermore, surface angles of 108°, which is consistent with the theoretical value between the {111} and {111} facets of Cu2O [31]. Insets of Fig. 4b are the HRTEM images of the Cu2O and TNS, two different lattice planes with spacings of 0.245 and 0.352 nm, which correspond to the (101) plane of Cu2O and (101) plane of anatase phase, respectively, confirming the successful deposition of Cu2O on the TNS. These TNS with {001} facets exposed are more active, the difference in surface energy levels between {001} and {101} faces facilitates the separation of photoexcited electron-hole pairs on different facets. In addition, the deposition of Cu2O on the {101} faces of TNS is favorable to the transfer of photoexcited charges between them, Therefore, the synthesized Cu2O/{101}TNS array films are expected to exhibit high photoelectrochemical properties. Information on the surface components and elemental chemical states of the Cu2O/{101}TNS were analyzed by XPS. Fig. 5a shows the survey spectra of Cu2O/{101}TNS-2 and Cu2O/{101}TNS-5. Sharp photoelectron peaks of Cu 2p, Ti 2p and O 1s appear in both cases, the C 1s peak at 284.6 eV is also observed due to the adventitious carbon species [32]. The Cu 2p core level spectra in Fig. 5b show two peaks at 932.4 and 952.2 eV, corresponding to Cu 2p3/2 and Cu 2p1/2 of Cu+ or
Fig. 2. XRD patterns for films deposited at different potentials.
Under low potential, Cu2+ is reduced to Cu+ (Reaction 1). Due to the solubility limitation of Cu+, precipitation of Cu+ to Cu2O happened (Reaction 2). However, as the applied negative potential is high enough, Cu deposition will occur, and the process is as Reaction 3.
Cu+ + e− → Cu
(3)
The above reactions represent a combination of the electrochemical reduction of copper ions and chemical precipitation of cuprous oxide. The crystal structure and shape of Cu2O particle strongly depend on the experimental conditions, particularly the pH value, temperature and applied potential. Fig. 3 depicts the morphology of Cu2O/{101}TNS films deposited at different potentials. From Fig. 3a and b, it can be see that the dense TNS are well-ordered and vertically oriented covering the FTO substrate. The side length and thickness of the TNS are ~2.7 µm and ~380 nm, respectively. On the basis of the Wulff construction [30], the small lateral surfaces are {101} facets and two square surfaces are {001} facets of the anatase single crystal. When the applied potentials are −0.1 V and −0.2 V, octahedral Cu2O particles with side length 0.6– 1.0 µm exist mainly on the {101} facets of the TNS films, as shown in 29
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Fig. 3. SEM images of TNS and Cu2O/{101}TNS array films deposited at different potentials. (a) surface and (b) side views of TNS films (c) Cu2O/{101}TNS-1, (d) Cu2O/{101}TNS-2, (e) Cu2O/{101}TNS-3, (f) Cu2O/{101}TNS-4, (g) Cu2O/{101}TNS-5, (h) Cu2O/{101}TNS-6, and (i) EDS of Cu2O/{101}TNS-2.
Cu [33]. Generally, it is difficult to distinguish Cu+ and Cu due to their binding energies are very close and the distinction are only 0.1–0.2 eV [34]. However, as foregoing XRD patterns, which have revealed the coexistence of Cu2O and Cu for Cu2O/{101}TNS-5, and it will be proved by the following UV–vis absorption spectra. In addition, satellite peak of Cu 2p3/2 centered at ~942 eV cannot be seen, indicating no CuO here [25]. The Ti 2p spectra in Fig. 5c show two peaks at the binding energies of 458.8 and 464.5 eV, respectively, corresponding to Ti 2p3/2 and Ti 2p1/2, which are in line with previous studies [35,36]. O 1s region of the Cu2O/{101}TNS in Fig. 5d can be
fitted into two peaks. The main peak at 530.0 eV is assigned to lattice oxygen in TiO2 and Cu2O [37,38] and the other peak at around 531.5 eV is attributed to surface hydroxides [37]. The surface element percentages of Cu2O/{101}TNS-2 and Cu2O/{101}TNS-5 are listed in Table 1. It was found that the peak intensities of Cu 2p increase while Ti 2p, O 1s decrease with the applied potential is more negative, indicating the increase in Cu2O surface coverage. Fig. 6 shows the UV–vis diffuse reflection absorption spectra of the samples deposited at different potentials. BaSO4 dry-pressed disk was used as a reflectance standard in the experiments. Without the
Fig. 4. (a) TEM image and SAED pattern of the TNS. (b) TEM image and HRTEM image of Cu2O/{101}TNS.
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Fig. 5. (a) Survey, (b) Cu 2p, (c) Ti 2p and (d) O 1s XPS spectra of Cu2O/{101}TNS-2 and Cu2O/{101}TNS-5.
the Cu2O content, an enhanced absorption response from 400 to 600 nm is obtained, meanwhile, when the applied negative potential is increased to −0.2 V, two absorption peaks at around 440 and 520 nm can be observed. The first peak at ~440 nm is related to the Cu2O sensitization, which is similar to the previous report [39]. The second peak at ~520 nm can be assigned to the SPR absorption characteristic of the metallic Cu [40]. The exact position of the absorption peaks in our case have slight deviation compared with that reported in other Cu2O/TiO2 composites due to different particle size and shape [41]. For Cu2O/{101}TNS-2 and Cu2O/{101}TNS-3, although the diffraction peak of Cu are not identified in XRD patterns, SPR of the metallic Cu can be seen in UV–vis absorption spectra, implying its small size and low content of Cu species. Thus, the absorption spectrum can be attributed to synergetic effect of TNS, Cu2O and Cu composite structures. Generally speaking, photoelectrochemical property of the electrode is related to the interfacial charge transfer and separation efficiency between the electrode and electrolyte solution [42]. Fig. 7 shows the EIS plots of the Cu2O/{101}TNS films deposited at different potentials under xenon lamp irradiation. The semicircle radius on the plots of Cu2O/{101}TNS films are smaller than that of TNS film without decorated by Cu2O, which suggests that the heterojunction formed between TNS and Cu2O could facilitate the separation of photogenerated electron-hole pairs and the transport of interfacial electron. In addition, the semicircle radiuses decrease with more negative deposition potential was applied (−0.5 V) and then increase when the potential is −0.6 V. The result indicating that Cu2O/{101}TNS-5 exhibited the best interfacial electron transfer performance and conductivity. LSV plots of the samples were measured, as shown in Fig. 8. It can
Table 1 The surface element percentage of Cu2O/{101}TNS-2 and Cu2O/{101}TNS-5. Surface element
Cu2O/{101}TNS-2 (at%)
Cu2O/{101}TNS-5 (at%)
Ti 2p Cu 2p O 1s F 1s
21.25 10.46 65.58 2.71
14.06 62.30 22.25 1.39
Fig. 6. UV–vis absorption spectra of the samples deposited at different potentials.
deposition of Cu2O, TNS films have an intrinsic absorption edge with the threshold of 388 nm. For bare Cu2O film, although its absorption edge is red-shifted, but absorbance is relatively low. With increasing in 31
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generated electron-hole pairs than bare TNS. The Cu2O/{101}TNS-5 displayed the best photocurrent response, which is 64.7 times higher than that of the TNS electrode at 1.2 V (vs Ag/AgCl). When the applied potential was −0.6 V, the {101} facets of TNS was completely covered by excess amount of Cu2O, which may inhibit the photoexcited electrons flow among various facets, resulting in the reduced photocurrent. Because the CB and VB of Cu2O are more negative than that of TiO2, thus, the photoexcited electrons transfer from the CB of Cu2O to that of TiO2, and the holes from the VB of TiO2 move to the VB of Cu2O. Besides, the CB level of {101} facets is lower than that of {001} facets of TiO2, therefore, the Cu2O/{101} facets of TNS exhibited a larger driving force for photo-generated electrons transfer from Cu2O to {101} facets of TiO2. Furthermore, Cu metal is electron storage center that may facilitate the separation of electrons. Therefore, the enhanced photoelectrochemical properties can be attributed to the synergistic effect of the favorable energy level between Cu2O and {101} facets of TiO2, Cu2O-Cu-TiO2 heterojunction composites and well-ordered TNS arrays structure. Fig. 9a shows the photocurrent response of the TNS and Cu2O/ {101}TNS deposited at different potentials. It can be seen that all of the samples have a rapid photocurrent response via on-off cycles. Compared to amorphous TiO2, an initial current spike is observed, which is related to onset of carriers recombination [44]. Obviously, the photocurrents of TNS, Cu2O/{101}TNS-1 and Cu2O/{101}TNS-2 have a fast response speed, excellent stability and repeatability. Interesting, when the applied negative potential is high than −0.3 V, an increase of the photocurrent is observed. At the fourth transient cycles, the photocurrent increases with the deposition negative potential increasing from 0 to −0.5 V and then declines at the potential of −0.6 V. These results are in accordance with the LSV and EIS plots perfectly. In order to test the stability of Cu2O/{101}TNS electrode, we measured photocurrents of the TNS and Cu2O/{101}TNS-5 for 6000 s under the illumination of a 150 W xenon lamp. As shown in Fig. 9b, the bare TNS film has an apparent initial photocurrent spike, which is due to the separation of photo-generated electron-hole pairs between the photosensitive substance and the semiconductor surface [45]. Subsequently, electrons transport to conductive substrate, and holes are trapped by hole scavengers [46]. After ~10 min illumination, generation and recombination rate of the charge reach to an equilibrium state, and photocurrent remains constant thereafter. However, photocurrent of Cu2O/{101}TNS-5 shows a increase during the first 10 min, followed by a gradual decrease and then a relatively stable photocurrent value is achieved under continuous illumination. For the reason of photocorrosion, photocurrent of Cu2O usually decreases during the photoelectrochemical measurement. However, when Cu2O particles reache a certain amount, it will produce dual effects: on the one hand, closepacked particles hinder the electron transfer from Cu2O to the {101}
Fig. 7. EIS response of the samples deposited at different potentials.
Fig. 8. LSV plots of the samples deposited at different potentials.
be seen that the photocurrent improved with the negative deposition potential increasing from 0 to −0.5 V and then decreased with a more negative deposition potential was applied (−0.6 V). The Cu2O/{101} TNS films deposited at different potential (0, −0.1, −0.2, −0.3, −0.4, −0.5 and −0.6 V) exhibited photocurrent density values of 0.28, 0.35, 0.49, 10.09, 16.81, 18.11 and 17.22 mA/cm−2 at 1.2 V (vs Ag/AgCl), respectively. The bare TNS film exhibited the least photocurrent response due to its wide bandgap, which is limited to absorb only ~5% or less of solar spectrum [43]. Under the same condition, Cu2O/ {101}TNS showed higher photocurrent. This suggests that the Cu2O/ {101}TNS exhibited stronger ability for the separation of photo-
Fig. 9. (a) Transient photocurrent response of the samples deposited at different potentials. (b) Photocurrent-time plot of the TNS and Cu2O/{101}TNS-5 under continuous xenon lamp illumination.
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Fig. 10. (a) XRD patterns, SEM images of the Cu2O/{101}TNS-5 (b) before and (c) after illumination for 6000 s.
pairs. Obviously, the PL intensity of Cu2O/{101}TNS-5 shows significant decrease in compared with pure TNS, which suggests that the deposition of Cu2O results in a effectively inhibit in the radiative recombination process. For Cu2O-TiO2 heterostructure, the appropriate band structure between Cu2O and {101} facets of TiO2 driving the electrons from Cu2O to TiO2, facilitates the charge separation and hinders charge carriers recombination, which is the reason for the decrease of PL spectra as Cu2O was deposited. Based on the above observations and the energy level diagram of the multicomponent systems, the charge transfer process is proposed in Fig. 12. First, compared with previous work that anatase nanosheet films were prepared by doctor-blading method, single-crystal TiO2 nanosheets were grown directly on the FTO substrate without any barrier layer, which can decrease resistance originated from grain boundaries and barrier layer, hence facilitates electron transfer through the TiO2/conductive substrate interface. Second, when p-type Cu2O/n-type TiO2 heterojunction is formed, on account of the charge carrier concentration gradient, the transfer of electrons and holes is in reverse direction, thus create an inner electrostatic field [55]. In addition, the CB and VB of Cu2O are more negative than that of TiO2. Thus, when Cu2O-TiO2 heterojunctions are exposed to light, the photoexcited electrons move from CB of Cu2O to that of TiO2, and holes to move in opposite direction. Therefore, inner electrostatic field combined with energy-level alignment are working on promoting the electrons transfer from the Cu2O to TiO2 and then to FTO, while holes transfer from TiO2 to Cu2O and are collected by the hole scavengers (SO42−) [56]. Furthermore, the {001} Facets of singlecrystalline anatase nanosheets usually exhibit greater chemical activity due to a high density of active surface oxygen atoms and unsaturated coordinated Ti atoms. However, the photoexcited electrons transfer from the {001} to {101} facets due to the CB level of {101} facets is lower than that of {001} facets of anatase. For Cu2O/TiO2 composite, photoelectrochemical performance is dependent upon energy band offset value, as opposed to surface energy alone. Cu2O/{101} facets have higher band offset value which supply a larger driving force to accelerate the photo-generated electron-hole pairs separation among Cu2O and {101} facets of TiO2. Third, as the applied negative potential is high enough, a part of Cu+ is reduced to Cu, which serves as an important role in enhancing the photoelectrochemical performance. As reported, the Cu nanoparticles were beneficial for the photoexcited electrons transfer from CB of Cu2O to that of TiO2 and enhance the conductivity of the electrode [57]. Meanwhile, the SPR effect of Cu particles will enhance optical absorption and excite more electron-hole pairs. In addition, due to the photo-
facets of TiO2 and increase the chance of recombination [47], on the other hand, photocorrosion on the particles would reduce particle size and then enhance photoelectrochemical properties temporarily [48]. This transitory increase in the photocurrent was also observed by Zangari et al. [48]. To verify the photocorrosion of Cu2O/{101}TNS, XRD and SEM were measured for Cu2O/{101}TNS-5 after illumination for 6000 s under the AM 1.5 simulated sunlight source. As shown in Fig. 10a, after 6000 s irradiation, (111), (200) and (220) diffraction peaks of Cu2O significantly decrease while diffraction peaks corresponding to TiO2 have little change. From Fig. 10b and c, we observed the surface of Cu2O becomes coarser compared with that without irradiation, while there is no change on the TNS. This is further confirmed that surface corrosion mainly take place on Cu2O. In addition, the photocurrent achieves a steady value of 46 μA/cm−2 for the Cu2O/{101}TNS-5 electrode, which is 7.1 times higher than that of the TNS electrode, indicating that the photocurrent can be enhanced by the formation of Cu2O-TiO2 heterostructure, which has a higher charge separation and transfer efficiency. PL emission spectra, resulting from the recombination of charge carriers, is a useful technique to investigate migration, transfer and separation of charge carriers [49]. As shown in Fig. 11, the TNS displays three main peaks situated at 396, 470 and 535 nm. The first peak is ascribed to the band gap transition of TiO2, which is in line with the published values [50]. The emission peaks at 470 and 535 nm are related to exciton-caused PL resulting from band edge free excitons and oxygen vacancies of TiO2 [51–54]. Generally, the lower PL intensity suggests the lower recombination of photo-generated electron-hole
Fig. 11. PL spectra of TNS and Cu2O/{101}TNS-5.
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Fig. 12. Proposed schematic diagram of Cu2O/{101}TNS heterojunction and charge transfer process.
generated electrons make a directional flow from {001} to {101} facets, the electrodeposition of Cu2O on the {101} surface of TiO2 will shorten the route length that the electrons must travel, thus reduce recombination of photo-generated electron-hole pairs.
[7]
[8]
4. Conclusions [9]
In this work, we demonstrated the electrodeposition of Cu2O on the {101} surface of single-crystalline anatase nanosheet by varying electrodeposition potential. On account of different band structures and band edge positions between {001} and {101} facets of anatase, Cu2O/{101} facets have higher band offset value which supply a larger driving force to increase the transport efficiency of carriers. In addition, due to the photo-generated electrons make a directional flow from {001} to {101} facets, the electrodeposition of Cu2O on the {101} surface of TiO2 will shorten the route length that the electrons must travel, thus reduce recombination of photo-generated electron-hole pairs. Furthermore, the appearance of Cu is beneficial for the photoexcited electrons transfer from CB of Cu2O to that of TiO2. Thus, an ideal construction of Cu2O/{101}TNS array film could be employed not only in environmental applications, but also in solar energy cells.
[10]
[11]
[12]
[13]
[14]
[15]
[16]
Acknowledgements This work is supported by the National Natural Science Foundation of China (Nos. 51272001, 51472003, 91326101, 51576208 and 51572002), and the National Key Basic Research Program (2013CB632705), National Magnetic Confinement Fusion Science Program of China (Nos. 2013GB113004, 2015GB15007 and 2015GB120006), the Doctor Scientific Research Fund and Co-operative Innovation Research Center for Weak Signal-Detecting Materials and Devices Integration of Anhui University (Y01008411). The authors would like to thank Yonglong Zhuang Chao Cheng, Shenqiang Zhao and Zhongqing Lin of the Experimental Technology Center of Anhui University, for the electron microscope test, XRD test and discussion.
[17] [18]
[19]
[20]
[21]
References [22] [1] C.C. Chung, C.S. Lee, E. Jokar, J.H. Kim, E.W.G. Diau, Well-organized mesoporous TiO2 photoanode by using amphiphilic graft copolymer for efficient perovskite solar cells, J. Phys. Chem. C 120 (2016) 9619. [2] S.A. Ansari, M.M. Khan, M.O. Ansari, M.H. Cho, Silver nanoparticles and defectinduced visible light photocatalytic and photoelectrochemical performance of Ag@ m-TiO2 nanocomposite, Sol. Energy Mater. Sol. Cells 141 (2015) 162–170. [3] B. Su, Y. Tian, L. Jiang, Bioinspired Interfaces with superwettability: from materials to chemistry, J. Am. Chem. Soc. 138 (2016) 1727. [4] N. Umezawa, H.H. Kristoffersen, L.B. Vilhelmsen, B. Hammer, Reduction of CO2 with water on Pt-Loaded rutile TiO2(110) modeled with density functional theory, J. Phys. Chem. C 120 (2016) 9160. [5] J. Zhang, X. Jin, P.I. Morales-Guzman, X. Yu, H. Liu, H. Zhang, L. Razzari, J.P. Claverie, Engineering the absorption and field enhancement properties of AuTiO2 nanohybrids via whispering gallery mode resonances for photocatalytic water splitting, ACS Nano 10 (2016) 4496. [6] F.D. Angelis, G. Vitillaro, L. Kavan, M.K. Nazeeruddin, M. Grätzel, Modeling
[23]
[24]
[25]
[26]
[27]
34
ruthenium-dye-sensitized TiO2 surfaces exposing the (001) or (101) faces: a firstprinciples investigation, J. Phys. Chem. C 116 (2012) 18124. J. Yan, G. Wu, W. Dai, N. Guan, L. Li, Synthetic design of gold nanoparticles on anatase TiO2 {001} for enhanced visible light harvesting, ACS Sustain. Chem. Eng. 2 (2014) 1940. Q. Kuang, X. Wang, Z. Jiang, Z. Xie, L. Zheng, High-energy-surface engineered metal oxide micro- and nanocrystallites and their applications, Acc. Chem. Res. 47 (2014) 308. H.G. Yang, C.H. Sun, S.Z. Qiao, J. Zou, G. Liu, S.C. Smith, H.M. Cheng, G.Q. Lu, Anatase TiO2 single crystals with a large percentage of reactive facets, Nature 453 (2008) 638. Z. Zheng, B. Huang, X. Qin, X. Zhang, Y. Dai, M. Jiang, P. Wang, M.H. Whangbo, Highly efficient photocatalyst: TiO2 microspheres produced from TiO2 nanosheets with a high percentage of reactive {001} facets, Chem. Eur. J. 15 (2009) 12576. H.G. Yang, G. Liu, S.Z. Qiao, C.H. Sun, Y.G. Jin, S.C. Smith, J. Zou, H.M. Cheng, G.Q. Lu, Solvothermal synthesis and photoreactivity of anatase TiO2 nanosheets with dominant {001} facets, J. Am. Chem. Soc. 131 (2009) 4078. N. Murakami, Y. Kurihara, T. Tsubota, T. Ohno, Shape-controlled anatase titanium (IV) oxide particles prepared by hydrothermal treatment of peroxo titanic acid in the presence of polyvinyl alcohol, J. Phys. Chem. C 113 (2009) 3062. T. Takashi, Y. Soichiro, M. Tetsuro, Evidence for crystal-face-dependent TiO2 photocatalysis from single-molecule imaging and kinetic analysis, J. Am. Chem. Soc. 133 (2011) 7197. J. Yu, J. Low, W. Xiao, P. Zhou, M. Jaroniec, Enhanced photocatalytic CO2reduction activity of anatase TiO2 by coexposed {001} and {101} facets, J. Am. Chem. Soc. 136 (2014) 8839. X. Liu, G. Dong, S. Li, G. Lu, Y. Bi, Direct observation of charge separation on anatase TiO2 crystals with selectively etched {001} facets, J. Am. Chem. Soc. 138 (2016) 2917. M.V. Sofianou, M. Tassi, V. Psycharis, N. Boukos, S. Thanos, T. Vaimakis, J. Yu, C. Trapalis, Solvothermal synthesis and photocatalytic performance of Mn4+-doped anatase nanoplates with exposed {0 0 1} facets, Appl. Catal. B: Environ. 162 (2015) 27. M. Dahl, Y. Liu, Y. Yin, Composite titanium dioxide nanomaterials, Chem. Rev. 114 (2014) 9853. X. Wang, G. Liu, L. Wang, J. Pan, G.Q. Lu, H.M. Cheng, TiO2 films with oriented anatase {001} facets and their photoelectrochemical behavior as CdS nanoparticle sensitized photoanodes, J. Mater. Chem. 21 (2011) 869. C. Liu, X. Han, S. Xie, Q. Kuang, X. Wang, M. Jin, Z. Xie, L. Zheng, Enhancing the photocatalytic activity of anatase TiO2 by improving the specific facet-induced spontaneous separation of photogenerated electrons and holes, Chem. Asian J. 8 (2013) 282. E. Ghadiri, B. Liu, J.-E. Moser, M. Grätzel, L. Etgar, Investigation of interfacial charge separation at PbS QDs/(001) TiO2 nanosheets heterojunction solar cell, Part. Part. Syst. Char. 32 (2015) 483. L. Liu, W. Yang, W. Sun, Q. Li, J.K. Shang, Creation of Cu2O@TiO2 composite photocatalysts with p-n heterojunctions formed on exposed Cu2O facets, their energy band alignment study, and their enhanced photocatalytic activity under illumination with visible light, ACS Appl. Mater. Interfaces 7 (2015) 1465. S. Sun, Recent advances in hybrid Cu2O-based heterogeneous nanostructures, Nanoscale 7 (2015) 10850. L. Yang, Q. Zhang, W. Wang, S. Ma, M. Zhang, J. Lv, G. He, Z. Sun, Tuning the photoelectronic and photocatalytic properties of single-crystalline TiO2 nanosheet array films with dominant {001} facets by controlling the hydrochloric acid concentration, J. Mater. Sci. 51 (2016) 950. S. Bijani, R. Schrebler, E. Dalchiele, M. Gabas, L. Martínez, J. Ramos-Barrado, Study of the nucleation and growth mechanisms in the electrodeposition of microand nanostructured Cu2O thin films, J. Phys. Chem. C 115 (2011) 21373. L. Zhao, W. Dong, F. Zheng, L. Fang, M. Shen, Interrupted growth and photoelectrochemistry of Cu2O and Cu particles on TiO2, Electrochim. Acta 80 (2012) 354. M. Zhang, K. Xu, X. Jiang, L. Yang, G. He, X. Song, Z. Sun, J. Lv, Effect of methanol ratio in mixed solvents on optical properties and wettability of ZnO films by cathodic electrodeposition, J. Alloy. Compd. 615 (2014) 327. X. Wu, Z. Chen, G.Q. Lu, L. Wang, Nanosized anatase TiO2 single crystals with tunable exposed (001) facets for enhanced energy conversion efficiency of dye-
Solar Energy Materials & Solar Cells 165 (2017) 27–35
L. Yang et al.
[43] M. Santamaria, G. Conigliaro, F. Di Franco, F. Di Quarto, Photoelectrochemical evidence of Cu2O/TiO2 nanotubes hetero-junctions formation and their physicochemical characterization, Electrochim. Acta 144 (2014) 315. [44] H. Tsuchiya, J.M. Macak, A. Ghicov, A.S. Räder, L. Taveira, P. Schmuki, Characterization of electronic properties of TiO2 nanotube films, Corros. Sci. 49 (2007) 203. [45] Y. Wang, W. Chu, S. Wang, Z. Li, Y. Zeng, S. Yan, Y. Sun, Simple synthesis and photoelectrochemical characterizations of polythiophene/Pd/TiO2 composite microspheres, ACS Appl. Mater. Interfaces 6 (2014) 20197. [46] Y. Xu, M. Zhang, J. Lv, M. Zhang, X. Jiang, X. Song, G. He, Z. Sun, Preparation and characterization of heat-assisted PbS/TiO2 thin films, Appl. Surf. Sci. 317 (2014) 1035. [47] G. Hodes, Comparison of dye- and semiconductor-sensitized porous nanocrystalline liquid junction solar cells, J. Phys. Chem. C 112 (2008) 17778. [48] L.-K. Tsui, G. Zangari, Modification of TiO2 nanotubes by Cu2O for photoelectrochemical, photocatalytic, and photovoltaic devices, Electrochim. Acta 128 (2014) 341. [49] F. Dong, Q. Li, Y. Sun, W.-K. Ho, Noble metal-like behavior of plasmonic Bi particles as a cocatalyst deposited on (BiO)2CO3 microspheres for efficient visible light photocatalysis, ACS Catal. 4 (2014) 4341. [50] J. Long, H. Chang, Q. Gu, J. Xu, L. Fan, S. Wang, Y. Zhou, W. Wei, L. Huang, X. Wang, P. Liu, W. Huang, Gold-plasmon enhanced solar-to-hydrogen conversion on the {001} facets of anatase TiO2 nanosheets, Energ. Environ. Sci. 7 (2014) 973. [51] C. Hsu, Y. Shen, Z. Wei, D. Liu, F. Liu, Anatase TiO2 nanobelts with plasmonic Au decoration exhibiting efficient charge separation and enhanced activity, J. Alloy. Compd. 613 (2014) 117. [52] L. Yang, D. Chu, Y. Chen, W. Wang, Q. Zhang, J. Yang, M. Zhang, Y. Cheng, K. Zhu, J. Lv, G. He, Z. Sun, Photoelectrochemical properties of Ag/TiO2 electrodes constructed using vertically oriented two-dimensional TiO2 nanosheet array films, J. Electrochem. Soc. 163 (2016) H180. [53] Y. Lei, L.D. Zhang, G.W. Meng, G.H. Li, X.Y. Zhang, C.H. Liang, W. Chen, S.X. Wang, Preparation and photoluminescence of highly ordered TiO2 nanowire arrays, Appl. Phys. Lett. 78 (2001) 1125. [54] N. Wu, J. Wang, D.N. Tafen, H. Wang, J.-G. Zheng, J.P. Lewis, X. Liu, S.S. Leonard, A. Manivannan, Shape-enhanced photocatalytic activity of single-crystalline anatase TiO2 (101) Nanobelts, J. Am. Chem. Soc. 132 (2010) 6679. [55] X. Xu, Z. Gao, Z. Cui, Y. Liang, Z. Li, S. Zhu, X. Yang, J. Ma, Synthesis of Cu2O octadecahedron/TiO2 quantum dot heterojunctions with high visible light photocatalytic activity and high stability, ACS Appl. Mater. Interfaces 8 (2016) 91. [56] C. Cao, C. Hu, W. Shen, S. Wang, Y. Tian, X. Wang, Synthesis and characterization of TiO2/CdS core–shell nanorod arrays and their photoelectrochemical property, J. Alloy. Compd. 523 (2012) 139. [57] Q. Yang, M. Long, L. Tan, Y. Zhang, J. Ouyang, P. Liu, A. Tang, Helical TiO2 nanotube arrays modified by Cu–Cu2O with ultrahigh sensitivity for the nonenzymatic electro-oxidation of glucose, ACS Appl. Mater. Interfaces 7 (2015) 12719.
sensitized solar cells, Adv. Funct. Mater. 21 (2011) 4167. [28] W. Wu, K. Feng, B. Shan, N. Zhang, Orientation and grain shape control of Cu2O film and the related properties, Electrochim. Acta 176 (2015) 59. [29] A.S. ELmezayyen, S. Guan, F.M. Reicha, I.M. El-Sherbiny, C. Xu, Effect of conductive substrate (working electrode) on the morphology of electrodeposited Cu2O, J. Phys. D: Appl. Phys. 48 (2015) 175502. [30] M. Lazzeri, A. Vittadini, A. Selloni, Structure and energetics of stoichiometric TiO2 anatase surfaces, Phys. Rev. B 63 (2001) 15. [31] S. Sun, C. Kong, S. Yang, L. Wang, X. Song, B. Ding, Z. Yang, Highly symmetric polyhedral Cu2O crystals with controllable-index planes, CrystEngComm 13 (2011) 2217. [32] L. Yang, H. Jiang, W. Wang, D. Chu, J. Yang, M. Zhang, J. Lv, B. Wang, G. He, Z. Sun, Enhanced photoelectronic properties of single crystal TiO2 nanosheet array films by selective deposition of CdS nanoparticles on their {101} facets, CrystEngComm 18 (2016) 496. [33] L. Li, J. Lei, T. Ji, Facile fabrication of p-n heterojunctions for Cu2O submicroparticles deposited on anatase TiO2 nanobelts, Mater. Res. Bull. 46 (2011) 2084. [34] Z. Ai, L. Zhang, S. Lee, W. Ho, Interfacial hydrothermal synthesis of Cu@Cu2O core−shell microspheres with enhanced visible-light-driven photocatalytic activity, J. Phys. Chem. C 113 (2009) 20896. [35] Y. Zhu, Y. Wang, Z. Chen, L. Qin, L. Yang, L. Zhu, P. Tang, T. Gao, Y. Huang, Z. Sha, G. Tang, Visible light induced photocatalysis on CdS quantum dots decorated TiO2 nanotube arrays, Appl. Catal. A 498 (2015) 159. [36] S. Yousefzadeh, M. Faraji, Y.T. Nien, A.Z. Moshfegh, CdS nanoparticle sensitized titanium dioxide decorated graphene for enhancing visible light induced photoanode, Appl. Surf. Sci. 320 (2014) 772. [37] S. Xu, A.J. Du, J. Liu, J. Ng, D.D. Sun, Highly efficient CuO incorporated TiO2 nanotube photocatalyst for hydrogen production from water, Int. J. Hydrog. Energy 36 (2011) 6560. [38] E. Liu, L. Qi, J. Bian, Y. Chen, X. Hu, J. Fan, H. Liu, C. Zhu, Q. Wang, A facile strategy to fabricate plasmonic Cu modified TiO2 nano-flower films for photocatalytic reduction of CO2 to methanol, Mater. Res. Bull. 68 (2015) 203. [39] S. Zhang, C. Liu, X. Liu, H. Zhang, P. Liu, S. Zhang, F. Peng, H. Zhao, Nanocrystal Cu2O-loaded TiO2 nanotube array films as high-performance visible-light bactericidal photocatalyst, Appl. Microbiol. Biot. 96 (2012) 1201. [40] L. Manca, B.K. Ines, P.N. Anton, J.A. Bostjan, Cu and CuO/titanate nanobelt based network assemblies for enhanced visible light photocatalysis, Langmuir 30 (2014) 4852. [41] J. Xing, Z.P. Chen, F.Y. Xiao, X.Y. Ma, C.Z. Wen, Z. Li, H.G. Yang, Cu-Cu2O-TiO2 nanojunction systems with an unusual electron-hole transportation pathway and enhanced photocatalytic properties, Chem. Asian J. 8 (2013) 1265. [42] X. Cheng, H. Liu, Q. Chen, J. Li, P. Wang, Preparation and characterization of palladium nano-crystallite decorated TiO2 nano-tubes photoelectrode and its enhanced photocatalytic efficiency for degradation of diclofenac, J. Hazard. Mater. 254–255 (2013) 141.
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Electrodeposited Cu2O on the {101} facets of TiO2 nanosheet arrays and their enhanced photoelectrochemical performance
MARK
Lei Yanga,b,c, Weihua Wangb, Hui Zhangb, Shenhao Wangb, Miao Zhanga, Gang Hea, Jianguo Lvd, ⁎ Kerong Zhua, Zhaoqi Suna, a
School of Physics and Material Science, Anhui University, Hefei 230601, PR China Institute of Applied Physics, AOA, Hefei 230031, PR China c Co-operative Innovation Research Center for Weak Signal-Detecting Materials and Devices Integration, Anhui University, Hefei 230601, PR China d School of Electronic and Information Engineering, Hefei Normal University, Hefei 230601, PR China b
A R T I C L E I N F O
A BS T RAC T
Keywords: Cu2O/{101}TiO2 nanosheet Photoelectrochemical properties Band offset value Anatase nanosheet arrays
A novel Cu2O/{101}TiO2 nanosheet (Cu2O/{101}TNS) array film was prepared by the electrodeposition of Cu2O on the {101} facets of anatase nanosheet arrays by varying electrodeposition potential. The crystal structure, morphology, elemental chemical states, optical properties, photoelectrochemical properties, and stability of Cu2O/{101}TNS array films were investigated in detail. For TNS, due to different band structures and band edge positions between {001} and {101} facets, Cu2O/{101} facets of TiO2 have higher band offset value which supply a larger driving force to increase the transport efficiency of carriers. Besides, owing to the directional flow of photo-generated electrons from {001} to {101} facets, the electrodeposition of Cu2O on the {101} facets of TNS will shorten the route length that the electrons must travel, thus reduce recombination of photo-generated electron-hole pairs. In addition, as the applied negative potential is high enough, a part of Cu+ is reduced to Cu, which is beneficial for the photoexcited electrons transfer from CB of Cu2O to that of TiO2. The enhanced photoelectrochemical properties of Cu2O/{101}TNS array films can be attributed to the cocontributions of different band edge positions between {001} and {101} facets, Cu2O-Cu-TiO2 ternary components and vertically aligned single-crystal TiO2 nanosheet structure.
1. Introduction As one of the most important n-type semiconductors, TiO2 has been widely used in solar cells [1], environmental purification [2], selfcleaning and antifogging coatings [3], and electrochemical energy storage [4,5]. It is well-known that the properties of metal oxide semiconductors are not only related to their crystal phase, specific surface area, defects in the lattice, crystal morphology, crystallinity, but also exposed crystal facets. More specifically, both theoretical and experimental studies have shown that the {001} facets of anatase TiO2 exhibited more active than the thermodynamically more stable {010} and {101} facets [6,7]. However, high reactive facets usually diminished during the crystal growth as a result of the minimization of total surface energy [8]. Since Yang et al. synthesized micron-sized anatase crystals with ~47% exposed {001} facets [9], a series of researches have focused on the TiO2 with exposed {001} facets. In early studies, it was found that TiO2 with higher percentage of {001} facets usually exhibit greater reactivity [10,11], due to abundant unsaturated coordination
⁎
sites. In recent studies, Ohno et al. found that stacked octahedral anatase crystals show the high photocatalytic activity due to efficient separation of oxidation and reduction sites [12]. Tachikawa et al. provided definitive evidence that reduction sites are preferentially located on the {101} facets, and they hypothesized that photogenerated charge carriers make directional flow [13]. Based on the DFT calculation, Yu et al. demonstrated that {001} and {101} facets of anatase exhibit different band structures and band edge positions and proposed a “surface heterojunction” concept [14]. With synchronous illumination X-ray photoelectron spectroscopy, Bi et al. directly observated the photo-generated charge separation on anatase single crystals [15]. On the whole, their focus was optimizing the ratio of different exposed facets. However, the practical application of TiO2 is hindered by its wide band gap and high recombination rate of the photo-generated electronhole pairs [16]. Thus, considerable efforts have been dedicated to fabricate {001} exposed TiO2 composite materials [17]. Wang et al. prepared CdS NPs sensitized TiO2 films with oriented {001} facets, the
Corresponding author. E-mail addresses: [email protected] (L. Yang), [email protected] (Z. Sun).
http://dx.doi.org/10.1016/j.solmat.2017.02.026 Received 3 July 2016; Received in revised form 15 February 2017; Accepted 19 February 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
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obtained TiO2 films show a enhanced photoelectrochemical properties [18]. Liu et al. demonstrated that photocatalytic activity can be enhanced by the deposition of Pt on the {101} facets of anatase TiO2 [19]. Etgar et al. reported that interfacial charge separation at PbS/ {001} facets is five times faster than that on {101} facets of TiO2 [20]. More recently, it is worth noting that Li et al. prepared Cu2O/TiO2 heterojunctions on various exposed facets of Cu2O polyhedral, they have demonstrated that photocatalytic performance of the composites is dependent upon Cu2O/TiO2 energy band alignment, as opposed to surface energy alone [21]. Cu2O (direct band gap ~2.0 eV), as a non-toxic p-type semiconductor is a promising material in solar-energy-conversion applications [22]. Because both the conduction band (CB) and valence bands (VB) of Cu2O are more negative than that of TiO2. Thus, when Cu2O-TiO2 heterojunction is formed, the photoexcited electrons transfer from CB of Cu2O to that of TiO2. As already reported [14], the CB level of {101} facets is lower than that of {001} facets of TiO2, thus, Cu2O/{101} facets have a higher band offset value. The higher band offset value implies the larger driving force for photo-generated electrons transfer from Cu2O to {101} facets of TiO2, thus improves the charge separation and inhibits charge carriers recombination. In addition, unlike TiO2 nanoparticles, well-aligned single-crystalline TiO2 nanosheet (TNS) grown directly on the conductive glass without any barrier layer, hence facilitates electron transfer through the TiO2/conductive substrate interface. In this work, dense regular TNS were prepared as our previous work [23]. The TNS are nearly perpendicular to the FTO substrate and the upper surface of film are {101} facets of anatase single crystal. Then, the Cu2O are electrodeposited on the {101} surface of TNS arrays films. Moreover, as the applied negative potential is high enough, Cu deposition will occur, which facilitates the transportation of electrons. The effect of the electrodeposition potential on their microstructure, morphology and photoelectrochemical performance were discussed and a model of electron transfer among Cu2O/{101}TNS was proposed.
2.2. Characterization X-ray powder diffraction (XRD) patterns of Cu2O/{101}TNS array films were identified by the MAC M18XHF X-ray diffractometer using Cu Kα radiation. The morphology of the Cu2O/{101}TNS array films was examined in a Hitachi S4800 field-emission scanning electron microscope (FESEM) and JEM-2100 high-resolution transition electron microscope (HRTEM). Surface compositions and elemental chemical states were analyzed by a Thermo ESCALAB 250 X-ray photoelectron spectroscopy (XPS) electron microscopy. The absorbance spectra were obtained with an UV-2550, Shimadzu ultraviolet-visible spectrophotometer equipped with a diffuse reflectance integrating sphere. Photoluminescence (PL) spectra were measured using a Hitachi F-4500 fluorescence spectrophotometer at the excitation wavelength of 325 nm. The photoelectrochemical experiments, including electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV) and photocurrent were performed by means of a CHI600D electrochemical workstation (ChenHua Instruments Co. Ltd, China). In a standard three electrode system, the prepared films, Pt foil and Ag/AgCl (saturated with KCl), were used as the working, counter and reference electrodes, respectively. The supporting electrolyte was 0.1 M Na2SO4 aqueous solution. A 150 W xenon lamp (SS150A, ZOLIX, China) with an AM 1.5 G cutoff filter was employed as the light source in the photoelectrochemical measurements. 3. Results and discussion Fig. 1 shows the current density versus deposition time curves for Cu2O electrodeposition onto TNS array films under different potentials from the same growth solution. The shapes of these current density curves indicate two main processes: nucleation and growth of crystals [24]. For Cu2O/{101}TNS-1 and Cu2O/{101}TNS-2, current density increases sharply and reaches to a maximal value (nucleation), then, a continuously decreasing occurs until to a relatively stable value (growth). Due to relatively low of applied negative potential, a quick decrease of current happened as Cu2O growing. As the applied potential becomes more negative (−0.3 V, −0.4 V, −0.5 V), the currents have shown a gradual increase, indicating a progressive nucleation process. With the increase of negative voltage, it is proposed that the larger electrical force will apply on the Cu2+ ions, which are acquired more energy to attach on TiO2 surface, leading to more nucleation of Cu2O [25]. However, current density curve of Cu2O/{101}TNS-6 show a different shape. The decrease of deposition current after the first few second reflects the relatively high resistivity due to the fast electrodeposition of Cu2O. In addition, the maximal values increase from 0.19 to 8.85 mA/cm−2 as deposition potential become more cathodic. The results indicate that the nuclei density becomes higher gradually [26]. To study crystalline nature of the Cu2O/{101}TNS thin films, XRD patterns of the thin films were recorded and shown in Fig. 2. Four distinct diffraction peaks at 2θ=25.3°, 37.7°, 48.1° and 55.0° correspond to the (101), (004), (200) and (211) planes of anatase phase (JCPDS no. 21-1272), respectively [11], some weak peaks located at 26.5°, 38.5°, 51.8° and 65.5° can be assigned to the FTO substrate (JCPDS no. 46-1088). More remarkable, the intensity of (004) diffraction peak is enhanced compared with other types of TiO2, implies the exposure of high energy {001} facets [27]. The diffraction peaks at 2θ=36.4°, 42.4°, 61.5° can be indexed to (111), (200) and (220) peaks of the standard powder diffraction patterns for cuprite Cu2O (JCPDS no. 34-1354) [28]. In addition, the Cu phases are observed when the applied negative potential is increased to −0.4 V. The mechanism for the electrodeposition of Cu2O can be described as follows [24,29].
2. Experimental 2.1. Preparation of Cu2O/{101}TNS films In a typical synthesis of TNS films, 12 mL of hydrochloric acid (36.5–38% by weight) was mixed with 18 mL of deionized water. The mixture was stirred for 5 min before 0.5 mL titanium butoxide (97% Aldrich) was added. After stirring for another 5 min, 0.25 g of (NH4)2TiF6 was added, and the mixture was further stirred for 15 min. Subsequently, the solution was transferred into a 50 mL polytetrafluoroethylene-lined stainless steel autoclave, in which a piece of FTO-coated glass slide placed slantly, with the FTO side facing up. The hydrothermal reaction was carried out at 170 ℃ for 16 h in an electric oven. After the reaction was completed, the sample was then taken out, washed with distilled water and ethanol thoroughly. Before the deposition of Cu2O on TNS films, the samples were annealed at 500 ℃ for 2 h. The procedure of the synthesis of Cu2O/{101}TNS heterogeneous array films is as follows: 4.99 g cupric acetate was dissolved in 500 mL of deionized water, then, 4.1 g sodium acetate was added to this solution under magnetic stirred for 1 h, at 40 ℃. The deposition of Cu2O on TNS array films was carried out by electrodeposition method with a standard three-electrode system in the preceding solution. TNS films, Pt foil and Ag/AgCl (saturated with KCl), were used as the working, counter and reference electrodes, respectively. The electrolyte was kept at a constant temperature of 40 ℃ and the deposition time was 8 min. Six samples are obtained by varying electrodeposition potential i.e., −0.1 V, −0.2 V, −0.3 V, −0.4 V, −0.5 V and −0.6 V and denoted as Cu2O/{101}TNS-1, Cu2O/{101}TNS-2, Cu2O/{101}TNS-3, Cu2O/{101}TNS-4, Cu2O/{101}TNS-5 and Cu2O/{101}TNS-6, respectively. 28
Cu2+ + e− → Cu+
(1)
2Cu+ + H2 O → Cu2 O + 2H+
(2)
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Fig. 1. Current density versus deposition time curves for Cu2O/{101}TNS array films deposited under different potentials.
Fig. 3c and d. When the applied negative potential increases to −0.3 V, Cu2O particles have obvious agglomerated and dendritic branching Cu2O are formed. The inset in Fig. 3d and e clearly show that morphology of Cu2O is altered by the deposition potential. At deposition potential −0.4 and −0.5 V, the dendritic branching structure gradually decreases and stacking particles become prominent. When a more negative deposition potential was applied (−0.6 V), TNS films are covered by dense Cu2O particles completely. As shown, the TNSs are nearly perpendicular to the FTO substrate, and the upper surface of film are {101} facets of anatase single crystal, therefore, the Cu2O particles are deposited on the {101} surface of TNS array films. As shown in Fig. 3i, the EDS result of Cu2O/{101}TNS-2 reveals that the thin film consists O, Ti and Cu, C peak from the adventitious contaminants is also present. TEM and HRTEM images of the obtained Cu2O/{101}TNS samples are shown in Fig. 4. These TiO2 sheets have a truncated bipyramid in shape. The SAED pattern of TNS viewed along the [001] orientation confirmed its single crystal structure. As shown in Fig. 4b, interfacial angle between {001} and {101} facets is 68°, this is in line with the theoretical values of anatase [9]. Furthermore, surface angles of 108°, which is consistent with the theoretical value between the {111} and {111} facets of Cu2O [31]. Insets of Fig. 4b are the HRTEM images of the Cu2O and TNS, two different lattice planes with spacings of 0.245 and 0.352 nm, which correspond to the (101) plane of Cu2O and (101) plane of anatase phase, respectively, confirming the successful deposition of Cu2O on the TNS. These TNS with {001} facets exposed are more active, the difference in surface energy levels between {001} and {101} faces facilitates the separation of photoexcited electron-hole pairs on different facets. In addition, the deposition of Cu2O on the {101} faces of TNS is favorable to the transfer of photoexcited charges between them, Therefore, the synthesized Cu2O/{101}TNS array films are expected to exhibit high photoelectrochemical properties. Information on the surface components and elemental chemical states of the Cu2O/{101}TNS were analyzed by XPS. Fig. 5a shows the survey spectra of Cu2O/{101}TNS-2 and Cu2O/{101}TNS-5. Sharp photoelectron peaks of Cu 2p, Ti 2p and O 1s appear in both cases, the C 1s peak at 284.6 eV is also observed due to the adventitious carbon species [32]. The Cu 2p core level spectra in Fig. 5b show two peaks at 932.4 and 952.2 eV, corresponding to Cu 2p3/2 and Cu 2p1/2 of Cu+ or
Fig. 2. XRD patterns for films deposited at different potentials.
Under low potential, Cu2+ is reduced to Cu+ (Reaction 1). Due to the solubility limitation of Cu+, precipitation of Cu+ to Cu2O happened (Reaction 2). However, as the applied negative potential is high enough, Cu deposition will occur, and the process is as Reaction 3.
Cu+ + e− → Cu
(3)
The above reactions represent a combination of the electrochemical reduction of copper ions and chemical precipitation of cuprous oxide. The crystal structure and shape of Cu2O particle strongly depend on the experimental conditions, particularly the pH value, temperature and applied potential. Fig. 3 depicts the morphology of Cu2O/{101}TNS films deposited at different potentials. From Fig. 3a and b, it can be see that the dense TNS are well-ordered and vertically oriented covering the FTO substrate. The side length and thickness of the TNS are ~2.7 µm and ~380 nm, respectively. On the basis of the Wulff construction [30], the small lateral surfaces are {101} facets and two square surfaces are {001} facets of the anatase single crystal. When the applied potentials are −0.1 V and −0.2 V, octahedral Cu2O particles with side length 0.6– 1.0 µm exist mainly on the {101} facets of the TNS films, as shown in 29
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Fig. 3. SEM images of TNS and Cu2O/{101}TNS array films deposited at different potentials. (a) surface and (b) side views of TNS films (c) Cu2O/{101}TNS-1, (d) Cu2O/{101}TNS-2, (e) Cu2O/{101}TNS-3, (f) Cu2O/{101}TNS-4, (g) Cu2O/{101}TNS-5, (h) Cu2O/{101}TNS-6, and (i) EDS of Cu2O/{101}TNS-2.
Cu [33]. Generally, it is difficult to distinguish Cu+ and Cu due to their binding energies are very close and the distinction are only 0.1–0.2 eV [34]. However, as foregoing XRD patterns, which have revealed the coexistence of Cu2O and Cu for Cu2O/{101}TNS-5, and it will be proved by the following UV–vis absorption spectra. In addition, satellite peak of Cu 2p3/2 centered at ~942 eV cannot be seen, indicating no CuO here [25]. The Ti 2p spectra in Fig. 5c show two peaks at the binding energies of 458.8 and 464.5 eV, respectively, corresponding to Ti 2p3/2 and Ti 2p1/2, which are in line with previous studies [35,36]. O 1s region of the Cu2O/{101}TNS in Fig. 5d can be
fitted into two peaks. The main peak at 530.0 eV is assigned to lattice oxygen in TiO2 and Cu2O [37,38] and the other peak at around 531.5 eV is attributed to surface hydroxides [37]. The surface element percentages of Cu2O/{101}TNS-2 and Cu2O/{101}TNS-5 are listed in Table 1. It was found that the peak intensities of Cu 2p increase while Ti 2p, O 1s decrease with the applied potential is more negative, indicating the increase in Cu2O surface coverage. Fig. 6 shows the UV–vis diffuse reflection absorption spectra of the samples deposited at different potentials. BaSO4 dry-pressed disk was used as a reflectance standard in the experiments. Without the
Fig. 4. (a) TEM image and SAED pattern of the TNS. (b) TEM image and HRTEM image of Cu2O/{101}TNS.
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Fig. 5. (a) Survey, (b) Cu 2p, (c) Ti 2p and (d) O 1s XPS spectra of Cu2O/{101}TNS-2 and Cu2O/{101}TNS-5.
the Cu2O content, an enhanced absorption response from 400 to 600 nm is obtained, meanwhile, when the applied negative potential is increased to −0.2 V, two absorption peaks at around 440 and 520 nm can be observed. The first peak at ~440 nm is related to the Cu2O sensitization, which is similar to the previous report [39]. The second peak at ~520 nm can be assigned to the SPR absorption characteristic of the metallic Cu [40]. The exact position of the absorption peaks in our case have slight deviation compared with that reported in other Cu2O/TiO2 composites due to different particle size and shape [41]. For Cu2O/{101}TNS-2 and Cu2O/{101}TNS-3, although the diffraction peak of Cu are not identified in XRD patterns, SPR of the metallic Cu can be seen in UV–vis absorption spectra, implying its small size and low content of Cu species. Thus, the absorption spectrum can be attributed to synergetic effect of TNS, Cu2O and Cu composite structures. Generally speaking, photoelectrochemical property of the electrode is related to the interfacial charge transfer and separation efficiency between the electrode and electrolyte solution [42]. Fig. 7 shows the EIS plots of the Cu2O/{101}TNS films deposited at different potentials under xenon lamp irradiation. The semicircle radius on the plots of Cu2O/{101}TNS films are smaller than that of TNS film without decorated by Cu2O, which suggests that the heterojunction formed between TNS and Cu2O could facilitate the separation of photogenerated electron-hole pairs and the transport of interfacial electron. In addition, the semicircle radiuses decrease with more negative deposition potential was applied (−0.5 V) and then increase when the potential is −0.6 V. The result indicating that Cu2O/{101}TNS-5 exhibited the best interfacial electron transfer performance and conductivity. LSV plots of the samples were measured, as shown in Fig. 8. It can
Table 1 The surface element percentage of Cu2O/{101}TNS-2 and Cu2O/{101}TNS-5. Surface element
Cu2O/{101}TNS-2 (at%)
Cu2O/{101}TNS-5 (at%)
Ti 2p Cu 2p O 1s F 1s
21.25 10.46 65.58 2.71
14.06 62.30 22.25 1.39
Fig. 6. UV–vis absorption spectra of the samples deposited at different potentials.
deposition of Cu2O, TNS films have an intrinsic absorption edge with the threshold of 388 nm. For bare Cu2O film, although its absorption edge is red-shifted, but absorbance is relatively low. With increasing in 31
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generated electron-hole pairs than bare TNS. The Cu2O/{101}TNS-5 displayed the best photocurrent response, which is 64.7 times higher than that of the TNS electrode at 1.2 V (vs Ag/AgCl). When the applied potential was −0.6 V, the {101} facets of TNS was completely covered by excess amount of Cu2O, which may inhibit the photoexcited electrons flow among various facets, resulting in the reduced photocurrent. Because the CB and VB of Cu2O are more negative than that of TiO2, thus, the photoexcited electrons transfer from the CB of Cu2O to that of TiO2, and the holes from the VB of TiO2 move to the VB of Cu2O. Besides, the CB level of {101} facets is lower than that of {001} facets of TiO2, therefore, the Cu2O/{101} facets of TNS exhibited a larger driving force for photo-generated electrons transfer from Cu2O to {101} facets of TiO2. Furthermore, Cu metal is electron storage center that may facilitate the separation of electrons. Therefore, the enhanced photoelectrochemical properties can be attributed to the synergistic effect of the favorable energy level between Cu2O and {101} facets of TiO2, Cu2O-Cu-TiO2 heterojunction composites and well-ordered TNS arrays structure. Fig. 9a shows the photocurrent response of the TNS and Cu2O/ {101}TNS deposited at different potentials. It can be seen that all of the samples have a rapid photocurrent response via on-off cycles. Compared to amorphous TiO2, an initial current spike is observed, which is related to onset of carriers recombination [44]. Obviously, the photocurrents of TNS, Cu2O/{101}TNS-1 and Cu2O/{101}TNS-2 have a fast response speed, excellent stability and repeatability. Interesting, when the applied negative potential is high than −0.3 V, an increase of the photocurrent is observed. At the fourth transient cycles, the photocurrent increases with the deposition negative potential increasing from 0 to −0.5 V and then declines at the potential of −0.6 V. These results are in accordance with the LSV and EIS plots perfectly. In order to test the stability of Cu2O/{101}TNS electrode, we measured photocurrents of the TNS and Cu2O/{101}TNS-5 for 6000 s under the illumination of a 150 W xenon lamp. As shown in Fig. 9b, the bare TNS film has an apparent initial photocurrent spike, which is due to the separation of photo-generated electron-hole pairs between the photosensitive substance and the semiconductor surface [45]. Subsequently, electrons transport to conductive substrate, and holes are trapped by hole scavengers [46]. After ~10 min illumination, generation and recombination rate of the charge reach to an equilibrium state, and photocurrent remains constant thereafter. However, photocurrent of Cu2O/{101}TNS-5 shows a increase during the first 10 min, followed by a gradual decrease and then a relatively stable photocurrent value is achieved under continuous illumination. For the reason of photocorrosion, photocurrent of Cu2O usually decreases during the photoelectrochemical measurement. However, when Cu2O particles reache a certain amount, it will produce dual effects: on the one hand, closepacked particles hinder the electron transfer from Cu2O to the {101}
Fig. 7. EIS response of the samples deposited at different potentials.
Fig. 8. LSV plots of the samples deposited at different potentials.
be seen that the photocurrent improved with the negative deposition potential increasing from 0 to −0.5 V and then decreased with a more negative deposition potential was applied (−0.6 V). The Cu2O/{101} TNS films deposited at different potential (0, −0.1, −0.2, −0.3, −0.4, −0.5 and −0.6 V) exhibited photocurrent density values of 0.28, 0.35, 0.49, 10.09, 16.81, 18.11 and 17.22 mA/cm−2 at 1.2 V (vs Ag/AgCl), respectively. The bare TNS film exhibited the least photocurrent response due to its wide bandgap, which is limited to absorb only ~5% or less of solar spectrum [43]. Under the same condition, Cu2O/ {101}TNS showed higher photocurrent. This suggests that the Cu2O/ {101}TNS exhibited stronger ability for the separation of photo-
Fig. 9. (a) Transient photocurrent response of the samples deposited at different potentials. (b) Photocurrent-time plot of the TNS and Cu2O/{101}TNS-5 under continuous xenon lamp illumination.
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Fig. 10. (a) XRD patterns, SEM images of the Cu2O/{101}TNS-5 (b) before and (c) after illumination for 6000 s.
pairs. Obviously, the PL intensity of Cu2O/{101}TNS-5 shows significant decrease in compared with pure TNS, which suggests that the deposition of Cu2O results in a effectively inhibit in the radiative recombination process. For Cu2O-TiO2 heterostructure, the appropriate band structure between Cu2O and {101} facets of TiO2 driving the electrons from Cu2O to TiO2, facilitates the charge separation and hinders charge carriers recombination, which is the reason for the decrease of PL spectra as Cu2O was deposited. Based on the above observations and the energy level diagram of the multicomponent systems, the charge transfer process is proposed in Fig. 12. First, compared with previous work that anatase nanosheet films were prepared by doctor-blading method, single-crystal TiO2 nanosheets were grown directly on the FTO substrate without any barrier layer, which can decrease resistance originated from grain boundaries and barrier layer, hence facilitates electron transfer through the TiO2/conductive substrate interface. Second, when p-type Cu2O/n-type TiO2 heterojunction is formed, on account of the charge carrier concentration gradient, the transfer of electrons and holes is in reverse direction, thus create an inner electrostatic field [55]. In addition, the CB and VB of Cu2O are more negative than that of TiO2. Thus, when Cu2O-TiO2 heterojunctions are exposed to light, the photoexcited electrons move from CB of Cu2O to that of TiO2, and holes to move in opposite direction. Therefore, inner electrostatic field combined with energy-level alignment are working on promoting the electrons transfer from the Cu2O to TiO2 and then to FTO, while holes transfer from TiO2 to Cu2O and are collected by the hole scavengers (SO42−) [56]. Furthermore, the {001} Facets of singlecrystalline anatase nanosheets usually exhibit greater chemical activity due to a high density of active surface oxygen atoms and unsaturated coordinated Ti atoms. However, the photoexcited electrons transfer from the {001} to {101} facets due to the CB level of {101} facets is lower than that of {001} facets of anatase. For Cu2O/TiO2 composite, photoelectrochemical performance is dependent upon energy band offset value, as opposed to surface energy alone. Cu2O/{101} facets have higher band offset value which supply a larger driving force to accelerate the photo-generated electron-hole pairs separation among Cu2O and {101} facets of TiO2. Third, as the applied negative potential is high enough, a part of Cu+ is reduced to Cu, which serves as an important role in enhancing the photoelectrochemical performance. As reported, the Cu nanoparticles were beneficial for the photoexcited electrons transfer from CB of Cu2O to that of TiO2 and enhance the conductivity of the electrode [57]. Meanwhile, the SPR effect of Cu particles will enhance optical absorption and excite more electron-hole pairs. In addition, due to the photo-
facets of TiO2 and increase the chance of recombination [47], on the other hand, photocorrosion on the particles would reduce particle size and then enhance photoelectrochemical properties temporarily [48]. This transitory increase in the photocurrent was also observed by Zangari et al. [48]. To verify the photocorrosion of Cu2O/{101}TNS, XRD and SEM were measured for Cu2O/{101}TNS-5 after illumination for 6000 s under the AM 1.5 simulated sunlight source. As shown in Fig. 10a, after 6000 s irradiation, (111), (200) and (220) diffraction peaks of Cu2O significantly decrease while diffraction peaks corresponding to TiO2 have little change. From Fig. 10b and c, we observed the surface of Cu2O becomes coarser compared with that without irradiation, while there is no change on the TNS. This is further confirmed that surface corrosion mainly take place on Cu2O. In addition, the photocurrent achieves a steady value of 46 μA/cm−2 for the Cu2O/{101}TNS-5 electrode, which is 7.1 times higher than that of the TNS electrode, indicating that the photocurrent can be enhanced by the formation of Cu2O-TiO2 heterostructure, which has a higher charge separation and transfer efficiency. PL emission spectra, resulting from the recombination of charge carriers, is a useful technique to investigate migration, transfer and separation of charge carriers [49]. As shown in Fig. 11, the TNS displays three main peaks situated at 396, 470 and 535 nm. The first peak is ascribed to the band gap transition of TiO2, which is in line with the published values [50]. The emission peaks at 470 and 535 nm are related to exciton-caused PL resulting from band edge free excitons and oxygen vacancies of TiO2 [51–54]. Generally, the lower PL intensity suggests the lower recombination of photo-generated electron-hole
Fig. 11. PL spectra of TNS and Cu2O/{101}TNS-5.
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Fig. 12. Proposed schematic diagram of Cu2O/{101}TNS heterojunction and charge transfer process.
generated electrons make a directional flow from {001} to {101} facets, the electrodeposition of Cu2O on the {101} surface of TiO2 will shorten the route length that the electrons must travel, thus reduce recombination of photo-generated electron-hole pairs.
[7]
[8]
4. Conclusions [9]
In this work, we demonstrated the electrodeposition of Cu2O on the {101} surface of single-crystalline anatase nanosheet by varying electrodeposition potential. On account of different band structures and band edge positions between {001} and {101} facets of anatase, Cu2O/{101} facets have higher band offset value which supply a larger driving force to increase the transport efficiency of carriers. In addition, due to the photo-generated electrons make a directional flow from {001} to {101} facets, the electrodeposition of Cu2O on the {101} surface of TiO2 will shorten the route length that the electrons must travel, thus reduce recombination of photo-generated electron-hole pairs. Furthermore, the appearance of Cu is beneficial for the photoexcited electrons transfer from CB of Cu2O to that of TiO2. Thus, an ideal construction of Cu2O/{101}TNS array film could be employed not only in environmental applications, but also in solar energy cells.
[10]
[11]
[12]
[13]
[14]
[15]
[16]
Acknowledgements This work is supported by the National Natural Science Foundation of China (Nos. 51272001, 51472003, 91326101, 51576208 and 51572002), and the National Key Basic Research Program (2013CB632705), National Magnetic Confinement Fusion Science Program of China (Nos. 2013GB113004, 2015GB15007 and 2015GB120006), the Doctor Scientific Research Fund and Co-operative Innovation Research Center for Weak Signal-Detecting Materials and Devices Integration of Anhui University (Y01008411). The authors would like to thank Yonglong Zhuang Chao Cheng, Shenqiang Zhao and Zhongqing Lin of the Experimental Technology Center of Anhui University, for the electron microscope test, XRD test and discussion.
[17] [18]
[19]
[20]
[21]
References [22] [1] C.C. Chung, C.S. Lee, E. Jokar, J.H. Kim, E.W.G. Diau, Well-organized mesoporous TiO2 photoanode by using amphiphilic graft copolymer for efficient perovskite solar cells, J. Phys. Chem. C 120 (2016) 9619. [2] S.A. Ansari, M.M. Khan, M.O. Ansari, M.H. Cho, Silver nanoparticles and defectinduced visible light photocatalytic and photoelectrochemical performance of Ag@ m-TiO2 nanocomposite, Sol. Energy Mater. Sol. Cells 141 (2015) 162–170. [3] B. Su, Y. Tian, L. Jiang, Bioinspired Interfaces with superwettability: from materials to chemistry, J. Am. Chem. Soc. 138 (2016) 1727. [4] N. Umezawa, H.H. Kristoffersen, L.B. Vilhelmsen, B. Hammer, Reduction of CO2 with water on Pt-Loaded rutile TiO2(110) modeled with density functional theory, J. Phys. Chem. C 120 (2016) 9160. [5] J. Zhang, X. Jin, P.I. Morales-Guzman, X. Yu, H. Liu, H. Zhang, L. Razzari, J.P. Claverie, Engineering the absorption and field enhancement properties of AuTiO2 nanohybrids via whispering gallery mode resonances for photocatalytic water splitting, ACS Nano 10 (2016) 4496. [6] F.D. Angelis, G. Vitillaro, L. Kavan, M.K. Nazeeruddin, M. Grätzel, Modeling
[23]
[24]
[25]
[26]
[27]
34
ruthenium-dye-sensitized TiO2 surfaces exposing the (001) or (101) faces: a firstprinciples investigation, J. Phys. Chem. C 116 (2012) 18124. J. Yan, G. Wu, W. Dai, N. Guan, L. Li, Synthetic design of gold nanoparticles on anatase TiO2 {001} for enhanced visible light harvesting, ACS Sustain. Chem. Eng. 2 (2014) 1940. Q. Kuang, X. Wang, Z. Jiang, Z. Xie, L. Zheng, High-energy-surface engineered metal oxide micro- and nanocrystallites and their applications, Acc. Chem. Res. 47 (2014) 308. H.G. Yang, C.H. Sun, S.Z. Qiao, J. Zou, G. Liu, S.C. Smith, H.M. Cheng, G.Q. Lu, Anatase TiO2 single crystals with a large percentage of reactive facets, Nature 453 (2008) 638. Z. Zheng, B. Huang, X. Qin, X. Zhang, Y. Dai, M. Jiang, P. Wang, M.H. Whangbo, Highly efficient photocatalyst: TiO2 microspheres produced from TiO2 nanosheets with a high percentage of reactive {001} facets, Chem. Eur. J. 15 (2009) 12576. H.G. Yang, G. Liu, S.Z. Qiao, C.H. Sun, Y.G. Jin, S.C. Smith, J. Zou, H.M. Cheng, G.Q. Lu, Solvothermal synthesis and photoreactivity of anatase TiO2 nanosheets with dominant {001} facets, J. Am. Chem. Soc. 131 (2009) 4078. N. Murakami, Y. Kurihara, T. Tsubota, T. Ohno, Shape-controlled anatase titanium (IV) oxide particles prepared by hydrothermal treatment of peroxo titanic acid in the presence of polyvinyl alcohol, J. Phys. Chem. C 113 (2009) 3062. T. Takashi, Y. Soichiro, M. Tetsuro, Evidence for crystal-face-dependent TiO2 photocatalysis from single-molecule imaging and kinetic analysis, J. Am. Chem. Soc. 133 (2011) 7197. J. Yu, J. Low, W. Xiao, P. Zhou, M. Jaroniec, Enhanced photocatalytic CO2reduction activity of anatase TiO2 by coexposed {001} and {101} facets, J. Am. Chem. Soc. 136 (2014) 8839. X. Liu, G. Dong, S. Li, G. Lu, Y. Bi, Direct observation of charge separation on anatase TiO2 crystals with selectively etched {001} facets, J. Am. Chem. Soc. 138 (2016) 2917. M.V. Sofianou, M. Tassi, V. Psycharis, N. Boukos, S. Thanos, T. Vaimakis, J. Yu, C. Trapalis, Solvothermal synthesis and photocatalytic performance of Mn4+-doped anatase nanoplates with exposed {0 0 1} facets, Appl. Catal. B: Environ. 162 (2015) 27. M. Dahl, Y. Liu, Y. Yin, Composite titanium dioxide nanomaterials, Chem. Rev. 114 (2014) 9853. X. Wang, G. Liu, L. Wang, J. Pan, G.Q. Lu, H.M. Cheng, TiO2 films with oriented anatase {001} facets and their photoelectrochemical behavior as CdS nanoparticle sensitized photoanodes, J. Mater. Chem. 21 (2011) 869. C. Liu, X. Han, S. Xie, Q. Kuang, X. Wang, M. Jin, Z. Xie, L. Zheng, Enhancing the photocatalytic activity of anatase TiO2 by improving the specific facet-induced spontaneous separation of photogenerated electrons and holes, Chem. Asian J. 8 (2013) 282. E. Ghadiri, B. Liu, J.-E. Moser, M. Grätzel, L. Etgar, Investigation of interfacial charge separation at PbS QDs/(001) TiO2 nanosheets heterojunction solar cell, Part. Part. Syst. Char. 32 (2015) 483. L. Liu, W. Yang, W. Sun, Q. Li, J.K. Shang, Creation of Cu2O@TiO2 composite photocatalysts with p-n heterojunctions formed on exposed Cu2O facets, their energy band alignment study, and their enhanced photocatalytic activity under illumination with visible light, ACS Appl. Mater. Interfaces 7 (2015) 1465. S. Sun, Recent advances in hybrid Cu2O-based heterogeneous nanostructures, Nanoscale 7 (2015) 10850. L. Yang, Q. Zhang, W. Wang, S. Ma, M. Zhang, J. Lv, G. He, Z. Sun, Tuning the photoelectronic and photocatalytic properties of single-crystalline TiO2 nanosheet array films with dominant {001} facets by controlling the hydrochloric acid concentration, J. Mater. Sci. 51 (2016) 950. S. Bijani, R. Schrebler, E. Dalchiele, M. Gabas, L. Martínez, J. Ramos-Barrado, Study of the nucleation and growth mechanisms in the electrodeposition of microand nanostructured Cu2O thin films, J. Phys. Chem. C 115 (2011) 21373. L. Zhao, W. Dong, F. Zheng, L. Fang, M. Shen, Interrupted growth and photoelectrochemistry of Cu2O and Cu particles on TiO2, Electrochim. Acta 80 (2012) 354. M. Zhang, K. Xu, X. Jiang, L. Yang, G. He, X. Song, Z. Sun, J. Lv, Effect of methanol ratio in mixed solvents on optical properties and wettability of ZnO films by cathodic electrodeposition, J. Alloy. Compd. 615 (2014) 327. X. Wu, Z. Chen, G.Q. Lu, L. Wang, Nanosized anatase TiO2 single crystals with tunable exposed (001) facets for enhanced energy conversion efficiency of dye-
Solar Energy Materials & Solar Cells 165 (2017) 27–35
L. Yang et al.
[43] M. Santamaria, G. Conigliaro, F. Di Franco, F. Di Quarto, Photoelectrochemical evidence of Cu2O/TiO2 nanotubes hetero-junctions formation and their physicochemical characterization, Electrochim. Acta 144 (2014) 315. [44] H. Tsuchiya, J.M. Macak, A. Ghicov, A.S. Räder, L. Taveira, P. Schmuki, Characterization of electronic properties of TiO2 nanotube films, Corros. Sci. 49 (2007) 203. [45] Y. Wang, W. Chu, S. Wang, Z. Li, Y. Zeng, S. Yan, Y. Sun, Simple synthesis and photoelectrochemical characterizations of polythiophene/Pd/TiO2 composite microspheres, ACS Appl. Mater. Interfaces 6 (2014) 20197. [46] Y. Xu, M. Zhang, J. Lv, M. Zhang, X. Jiang, X. Song, G. He, Z. Sun, Preparation and characterization of heat-assisted PbS/TiO2 thin films, Appl. Surf. Sci. 317 (2014) 1035. [47] G. Hodes, Comparison of dye- and semiconductor-sensitized porous nanocrystalline liquid junction solar cells, J. Phys. Chem. C 112 (2008) 17778. [48] L.-K. Tsui, G. Zangari, Modification of TiO2 nanotubes by Cu2O for photoelectrochemical, photocatalytic, and photovoltaic devices, Electrochim. Acta 128 (2014) 341. [49] F. Dong, Q. Li, Y. Sun, W.-K. Ho, Noble metal-like behavior of plasmonic Bi particles as a cocatalyst deposited on (BiO)2CO3 microspheres for efficient visible light photocatalysis, ACS Catal. 4 (2014) 4341. [50] J. Long, H. Chang, Q. Gu, J. Xu, L. Fan, S. Wang, Y. Zhou, W. Wei, L. Huang, X. Wang, P. Liu, W. Huang, Gold-plasmon enhanced solar-to-hydrogen conversion on the {001} facets of anatase TiO2 nanosheets, Energ. Environ. Sci. 7 (2014) 973. [51] C. Hsu, Y. Shen, Z. Wei, D. Liu, F. Liu, Anatase TiO2 nanobelts with plasmonic Au decoration exhibiting efficient charge separation and enhanced activity, J. Alloy. Compd. 613 (2014) 117. [52] L. Yang, D. Chu, Y. Chen, W. Wang, Q. Zhang, J. Yang, M. Zhang, Y. Cheng, K. Zhu, J. Lv, G. He, Z. Sun, Photoelectrochemical properties of Ag/TiO2 electrodes constructed using vertically oriented two-dimensional TiO2 nanosheet array films, J. Electrochem. Soc. 163 (2016) H180. [53] Y. Lei, L.D. Zhang, G.W. Meng, G.H. Li, X.Y. Zhang, C.H. Liang, W. Chen, S.X. Wang, Preparation and photoluminescence of highly ordered TiO2 nanowire arrays, Appl. Phys. Lett. 78 (2001) 1125. [54] N. Wu, J. Wang, D.N. Tafen, H. Wang, J.-G. Zheng, J.P. Lewis, X. Liu, S.S. Leonard, A. Manivannan, Shape-enhanced photocatalytic activity of single-crystalline anatase TiO2 (101) Nanobelts, J. Am. Chem. Soc. 132 (2010) 6679. [55] X. Xu, Z. Gao, Z. Cui, Y. Liang, Z. Li, S. Zhu, X. Yang, J. Ma, Synthesis of Cu2O octadecahedron/TiO2 quantum dot heterojunctions with high visible light photocatalytic activity and high stability, ACS Appl. Mater. Interfaces 8 (2016) 91. [56] C. Cao, C. Hu, W. Shen, S. Wang, Y. Tian, X. Wang, Synthesis and characterization of TiO2/CdS core–shell nanorod arrays and their photoelectrochemical property, J. Alloy. Compd. 523 (2012) 139. [57] Q. Yang, M. Long, L. Tan, Y. Zhang, J. Ouyang, P. Liu, A. Tang, Helical TiO2 nanotube arrays modified by Cu–Cu2O with ultrahigh sensitivity for the nonenzymatic electro-oxidation of glucose, ACS Appl. Mater. Interfaces 7 (2015) 12719.
sensitized solar cells, Adv. Funct. Mater. 21 (2011) 4167. [28] W. Wu, K. Feng, B. Shan, N. Zhang, Orientation and grain shape control of Cu2O film and the related properties, Electrochim. Acta 176 (2015) 59. [29] A.S. ELmezayyen, S. Guan, F.M. Reicha, I.M. El-Sherbiny, C. Xu, Effect of conductive substrate (working electrode) on the morphology of electrodeposited Cu2O, J. Phys. D: Appl. Phys. 48 (2015) 175502. [30] M. Lazzeri, A. Vittadini, A. Selloni, Structure and energetics of stoichiometric TiO2 anatase surfaces, Phys. Rev. B 63 (2001) 15. [31] S. Sun, C. Kong, S. Yang, L. Wang, X. Song, B. Ding, Z. Yang, Highly symmetric polyhedral Cu2O crystals with controllable-index planes, CrystEngComm 13 (2011) 2217. [32] L. Yang, H. Jiang, W. Wang, D. Chu, J. Yang, M. Zhang, J. Lv, B. Wang, G. He, Z. Sun, Enhanced photoelectronic properties of single crystal TiO2 nanosheet array films by selective deposition of CdS nanoparticles on their {101} facets, CrystEngComm 18 (2016) 496. [33] L. Li, J. Lei, T. Ji, Facile fabrication of p-n heterojunctions for Cu2O submicroparticles deposited on anatase TiO2 nanobelts, Mater. Res. Bull. 46 (2011) 2084. [34] Z. Ai, L. Zhang, S. Lee, W. Ho, Interfacial hydrothermal synthesis of Cu@Cu2O core−shell microspheres with enhanced visible-light-driven photocatalytic activity, J. Phys. Chem. C 113 (2009) 20896. [35] Y. Zhu, Y. Wang, Z. Chen, L. Qin, L. Yang, L. Zhu, P. Tang, T. Gao, Y. Huang, Z. Sha, G. Tang, Visible light induced photocatalysis on CdS quantum dots decorated TiO2 nanotube arrays, Appl. Catal. A 498 (2015) 159. [36] S. Yousefzadeh, M. Faraji, Y.T. Nien, A.Z. Moshfegh, CdS nanoparticle sensitized titanium dioxide decorated graphene for enhancing visible light induced photoanode, Appl. Surf. Sci. 320 (2014) 772. [37] S. Xu, A.J. Du, J. Liu, J. Ng, D.D. Sun, Highly efficient CuO incorporated TiO2 nanotube photocatalyst for hydrogen production from water, Int. J. Hydrog. Energy 36 (2011) 6560. [38] E. Liu, L. Qi, J. Bian, Y. Chen, X. Hu, J. Fan, H. Liu, C. Zhu, Q. Wang, A facile strategy to fabricate plasmonic Cu modified TiO2 nano-flower films for photocatalytic reduction of CO2 to methanol, Mater. Res. Bull. 68 (2015) 203. [39] S. Zhang, C. Liu, X. Liu, H. Zhang, P. Liu, S. Zhang, F. Peng, H. Zhao, Nanocrystal Cu2O-loaded TiO2 nanotube array films as high-performance visible-light bactericidal photocatalyst, Appl. Microbiol. Biot. 96 (2012) 1201. [40] L. Manca, B.K. Ines, P.N. Anton, J.A. Bostjan, Cu and CuO/titanate nanobelt based network assemblies for enhanced visible light photocatalysis, Langmuir 30 (2014) 4852. [41] J. Xing, Z.P. Chen, F.Y. Xiao, X.Y. Ma, C.Z. Wen, Z. Li, H.G. Yang, Cu-Cu2O-TiO2 nanojunction systems with an unusual electron-hole transportation pathway and enhanced photocatalytic properties, Chem. Asian J. 8 (2013) 1265. [42] X. Cheng, H. Liu, Q. Chen, J. Li, P. Wang, Preparation and characterization of palladium nano-crystallite decorated TiO2 nano-tubes photoelectrode and its enhanced photocatalytic efficiency for degradation of diclofenac, J. Hazard. Mater. 254–255 (2013) 141.
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