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Electrodeposition of Cu2O nanocrystalline on TiO2 nanosheet arrays by chronopotentiometry for improvement of photoelectrochemical properties
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Electrodeposition of Cu2O nanocrystalline on TiO2 nanosheet arrays by chronopotentiometry for improvement of photoelectrochemical properties ⁎
Jinbo Xuea,b,c, , Mingzhe Shaoa,c, Qianqian Shena,b,c, Xuguang Liua,c, Husheng Jiaa,c,
⁎
a
Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of Technology), Ministry of Education, Taiyuan 030024, PR China Research Centre of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, PR China c College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Chronopotentiometry method Cu2O/TiO2 composite Size-dependent effect Photoelectrochemical properties
A nano-sized Cu2O/TiO2 nanosheet arrays (TiO2 NSAs) heterojunction film was fabricating by electrodepositing Cu2O onto TiO2 NSAs using chronopotentiometry. The cyclic alternating current on working electrode is the main contribution to nano-sized Cu2O, which in turn improve the photoelectrochemical properties of the Cu2O/ TiO2 NSAs heterojunction film because nano-sized Cu2O, with better electron transfer ability and smaller change transfer resistance in electrolyte, increases the separation and transportation efficiencies of photo-induced electron-hole pairs. Furthermore, electrolyte pH value and deposition current density have significant influence on Cu2O particles size. The results show that the sample prepared with pH = 8 and current density j = 0.25 mA/ cm2 exhibits best photoelectrochemical properties because of the smaller sized Cu2O and uniform distribution on TiO2 NSAs.
1. Introduction Semiconductor nano-materials have drawn great attention owing to their outstanding physicochemical properties and potential applications including materials production and environmental purification [1]. Among these, TiO2-based nanomaterial has been widely studied and considered as a promising candidate because of its non-toxicity, superhydrophilicity, photo-stability and strong oxidation ability [2]. However, the photo-catalytic performance of TiO2 was restricted by its intrinsic defects. On one hand, bare TiO2 can only be excited by UV light, a small fraction of the solar spectrum. On the other hand, the high recombination of photo-induced electron and hole (e−/h+) pairs lowers the quantum efficiency of TiO2 [3]. Thus, a lot of strategies have been proposed to challenge the defects. Cubic cuprous oxide (Cu2O), as a nonstoichiometric p-type semiconductor with a direct forbidden band gap of 2.17 eV and a high optical absorption coefficient, has found significant applications in lithium battery [4], solar energy conversion [5–7], gas sensing [8,9], and photo-catalysis [10–13]. Since Cu2O powder was discovered by Hara et.al. As a photo-catalyst to effectively split water into H2 and O2 under solar irradiation [14], its photo-catalysis becomes an important hotspot of research. Han and co-workers prepared Cu2O/TiO2 composite films, which exhibited better photo-catalytic efficiencies than TiO2 and Cu2O
did alone under visible light irradiation [15]. Shi's group reported Cu2O and TiO2 nano-rod array composite photo-catalysts for solar water splitting anode [16]. The efficient photo-generated electron-hole transformation and separation in composite sample led to the improvement of photo-catalytic performance. Anderson reported that Cu2O/TiO2 photo-catalyst composites were synthesized by an ethanol reduction method, which exhibited excellent photo-catalytic performance for removal of nitrate and oxalic acid [17]. These previous studies [18–20] mainly focused on the tuning of orientation and architecture of electrodeposited films and the control of crystal size and morphology. However, nano-sized Cu2O has rarely been studied. Generally, the overall photo-catalytic activity of a semiconductor is mainly decided by three factors: (1) The adsorption ability, (2) photo-absorption ability in UV–visual light region, (3) the separation and transporting rate of photo-generated electrons and holes in the catalyst [21]. Nano-sized structures with large surface area have prospective applications in photo-catalysis, because the large surface area can provide more surface sites for adsorption of reactant molecules, which makes the adsorption process more efficient. Furthermore, nanosized Cu2O particles may further improve the photo-degradation ability because electrons and excitons can transport efficiently through the smallest dimension. Electrochemical deposition is one of the most attractive methods for
⁎ Corresponding authors at: Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of Technology), Ministry of Education, Taiyuan 030024, PR China. E-mail address: [email protected] (J. Xue).
https://doi.org/10.1016/j.ceramint.2018.03.078 Received 15 January 2018; Received in revised form 22 February 2018; Accepted 10 March 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xue, J., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.03.078
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Fig. 1. Deposition potential (vs. SCE) versus deposition time curves for nano-sized Cu2O/TiO2 NSAs heterojunction films deposited under different pH values: (a, b). pH = 7, (c, d) pH = 8 and (e, f) pH = 9.
2. Experimental sections
the synthesis of nanostructure materials. It provides advantages such as low synthesis temperature, low manufacturing cost, and high level of purity in the products. Also, electrodeposition allows the stoichiometry and microstructure of nano-materials to be controlled by adjusting the deposition parameters. The aim of the present study is to investigate the controllable synthesis of nano-sized Cu2O through chronopotentiometry. The morphology and structure of nano-sized Cu2O/TiO2 nano-sheet arrays (TiO2 NSAs) heterojunction films were studied in detail. The size-dependent photoelectrochemical properties of Cu2O/TiO2 composite reveal that the smaller sized Cu2O has better electron transfer ability and smaller change transfer resistance in electrolyte, which increases the photoinduced electron-hole pair separation and transportation efficiencies.
2.1. Preparation of TiO2 nano-sheet arrays (TiO2 NSAs) films 30 mL of hydrochloric acid was mixed into 30 mL of deionized water. Under magnetic stirring, 1.5 mL of tetrabutyl titanate was added dropwise. After stirring for 10 min, 0.4 g of CO(NH2)2 (99.0%) was added and stirring was continued for 10 min. Then, 0.4 g of (NH4)2TiF6 was added to the solution and further magnetically stirred for 10 min. Subsequently, the formulated solution was transferred to a 25 mL polytetrafluoroethylene lined stainless steel autoclave, in which a 1 cm × 3 cm of an FTO conductive glass was placed obliquely in the diagonal direction, with the conductive surface facing downward. The hydrothermal synthesis reaction conditions were 180 °C for 18 h in a heating furnace. After completion of the preparation, the sample was
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Fig. 2. X-ray diffraction (XRD) patterns of nano-sized Cu2O/TiO2 NSAs heterojunction films deposited under different pH values: (a) 2θ = 20–80° (b) 2θ = 35–45°.
of Cu2O/TiO2 heterojunction thin films were measured using a PerkinElmer Lambda 950 UV–Vis spectrophotometer with diffuse reflection integrating sphere. Photoluminescence (PL) spectra were detected by employing a Horiba fluoromax-4 spectrofluorometer at the excitation wavelength of 314 nm.
removed from the autoclave, thoroughly rinsed with deionized water and ethanol absolute, and dried at 45 °C for 2 h. Before the electrochemical deposition of Cu2O, the pure TiO2 NSAs films were annealed at 450 °C for 2 h with increasing and decreasing rate of 2 °C/min. 2.2. Electrodeposition of Cu2O nanoparticles on TiO2 NSAs films
2.4. Photoelectrochemical performance test Preparation of deposition solution: 4.49 g of copper sulfate was dissolved in 49.8 mL of deionized water and then 12.28 g of lactic acid was added dropwise to the solution under magnetic stirring and stirred for an additional half hour at room temperature. Subsequently, 8 g of sodium hydroxide was dissolved in 50 mL of deionized water under magnetic stirring. The pH of the copper sulfate and lactic acid solution was adjusted to 7, 8, 9 with 4 M sodium hydroxide solution, and the solution was stabilized by continuous magnetic stirring for 5 h at room temperature. The electrodeposition of Cu2O onto pure TiO2 NSAs was carried out using a standard three-electrode electrochemical deposition method in the prepared deposition solution. Pure TiO2 NSAs, Pt sheet (1 cm × 3 cm) and saturated calomel electrode (SCE) were used as the working electrode, counter electrode and reference electrode, respectively. The electrolyte was always maintained at a temperature of 25 °C and the deposition time was 500 s. In order to investigate the influence of pH on electrodeposition of Cu2O, different samples were achieved by the chronopotentiometry (CP) method with a constant current density of j = 0.25 mA/cm2 in electrolytes of pH= 7, 8, 9, separately. The obtained samples are noted as Cu2O/TiO2-CP-7, Cu2O/TiO2-CP- 8 and Cu2O/TiO2-CP-9. In addition, in order to investigate the influence of current density on electrodeposition of Cu2O, different samples were prepared by the chronopotentiometry (CP) method with current density j = 0.15, 0.20, 0.25, 0.30 and 0.35 mA/cm2 at pH = 8, and these samples are noted as Cu2O/TiO2 − 0.15 to 8, Cu2O/TiO2 − 0.20 to 8, Cu2O/TiO2 − 0.25 to 8, Cu2O/TiO2 − 0.30 to 8 and Cu2O/TiO2 − 0.35 to 8.
In the photoelectrochemical performance test, the photocurrent density versus testing time curve (Amperometric i-t Curve), electrochemical impedance spectra (EIS) and open circuit potential (OCP) versus testing time curve were detected using a CHI660E electrochemical workstation (Chenhua Instruments Co. Ltd, China). The electrochemical test electrolyte is 0.5 M Na2SO4 aqueous solution. The standard three-electrode system, Cu2O/TiO2 NSAs, Pt (1 cm × 3 cm) and SCE electrode were used as the working electrode, counter electrode and reference electrode, respectively. A CHF-XM500 xenon lamp (Beijing Changtuo Technology Co. Ltd, China) with an AM 1.5 G cutoff filter was adopted as the light source in photoelectric detection. 3. Results and discussion Fig. 1 displays the deposition potential (vs SCE) versus deposition time curves for nano-sized Cu2O electrodeposited onto pure TiO2 NSAs heterojunction films in electrolytes of different pH values with an identical deposition current(j = 0.25 mA/cm2). The shapes of these deposition potential (vs SCE) curves comprise two major steps: crystal nucleation and growth [22]. The range of deposition potential is − 0.1 V to − 0.6 V (vs SCE), as Cu2O can only be deposited in a certain potential range at different pH values [23,24]. Fig. 1a, c and e are deposition curves of nano-sized Cu2O/TiO2 NSAs heterojunction thin films by the chronopotentiometry (CP) with an identical current density j = 0.25 mA/cm2 in electrolytes of pH = 7, 8, 9 during 500 s, respectively. Fig. 1b, d and f are the partially enlarged view from 0 to 10 s of Fig. 1a, c and e curves while the shapes of the three curves indicate the time applied on the working electrode and counter electrode is 0.9 s and 0.1 s in one step, respectively. The deposition potential of nano-sized Cu2O/TiO2-CP-7, Cu2O/TiO2-CP-8 and Cu2O/TiO2-CP-9 is − 0.37, − 0.41 and − 0.53 V (vs SCE) in the early stages, respectively. X-ray diffraction patterns of pure TiO2 NSAs and Cu2O/TiO2-CP specimens are shown in Fig. 2. The diffraction angle range of Fig. 2a is from 20° to 80° while Fig. 2b is the partially enlarged view from 35° to 45° that represents the red square fraction. Four weak diffraction peaks at 2θ = 26.5°, 33.8°, 51.5°, 65.5° are consistent with SnO2 of FTO substrate (JCPDS 46-1088). The diffraction peaks at 2θ = 25.3°, 37.8°, 48.1°, 55.1° can be matched with (101), (004), (200) and (211) planes of anatase crystal with a space group of I41/amd (JCPDS 21-1272) [25].
2.3. Characterization The morphology of Cu2O/TiO2 heterojunction thin films was observed on JEOL JSM-6700F field emission scanning electron microscope (FESEM). The energy dispersive X-ray spectrometry (EDS) analysis was performed with an energy dispersive detector of Oxford Corporate equipped on the FESEM. X-ray diffraction (XRD) patterns of Cu2O/TiO2 heterojunction thin films were detected by the Rigaku Ultima IV X-ray diffractometer using Cu Kα as radiation source. The surface elemental compositions and elemental valence of Cu2O/TiO2 heterojunction thin films were analyzed by a KRATOS AMICUS X-ray photoelectron spectroscope (XPS) with Al Kα as the X-ray source. The absorbance spectra 3
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Fig. 3. FESEM images of pure TiO2 NSAs and nano-sized Cu2O/TiO2 NSAs heterojunction films deposited under different pH values: (a) pure TiO2 NSAs, (b) Cu2O/TiO2-CP-7, (c) Cu2O/ TiO2-CP-8, (d) Cu2O/TiO2-CP-9 and (e) the EDS of Cu2O/TiO2-CP-7.
Three characteristic peaks appear in the XRD patterns at 36.4°, 42.4°, 61.5°, which can be identified as (111), (200), (220) crystal facets of Cu2O with a space group of Pn-3m (JCPDS 05-667) [26]. Fig. 2b shows clearly the position and intensity of (111), (200) crystal facets of Cu2O prepared in the electrolytes with pH = 7, 8, 9. The peaks of impurities, as Cu, CuO and Cu(OH)2, are not detected in the patterns. By Compared with the XRD patterns of anatase TiO2 nano-sheets reported by Yang et al. and Sun et al. [25,27], the intensity of (004) diffraction peak is higher, which indicates that the proportion of exposed (001) crystal planes of the specimen is increased. Fig. 3a is the SEM morphology of pure TiO2 nanosheets, which is consistent with standard anatase TiO2 model comprised of eight isosceles trapezoidal (101) facets on sides. Two flat, square surfaces can be attributed to (001) facets [25]. It can be observed that TiO2 nanosheets of proper density are vertically covering the FTO substrate. TiO2
nanosheets have a thickness between 200 and 300 nm and a length between 2.2 and 2.6 µm. When the pH of electrolytes is 7, 8, 9, as shown in Fig. 2b, c and d, the size of Cu2O particles maintains between 90 and 120 nm. And the amount of Cu2O particles on the (101) crystal facets is larger than that on the (001) crystal facets. The size of Cu2O nanoparticles electrodeposited at pH = 8 is much smaller than that at pH = 7 and 9 [25,26]. Fig. 3e, the EDS spectrum of Cu2O/TiO2-CP-7, reveals that the heterojunction films consists of O, Ti and Cu, both Sn peaks from the FTO substrate and C peak from adventive impurities also appear. The relevant information of surface elemental compositions and elemental chemical environment of Cu2O/TiO2 NSAs heterojunction thin films were analyzed by XPS. Fig. 4a displays O 1s spectra. The O 1s peaks at 530.0, 530.2 and 532.0 eV can be assigned to the oxygen species in TiO2, Cu2O and -OH of surface, respectively [25–31]. The Cu 4
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Fig. 4. XPS patterns of O 1s (a), Cu 2p (b) for nano-sized Cu2O/TiO2 NSAs heterojunction films deposited at different pH values: (1) pH = 7, (2) pH = 8 and (3) pH = 9.
gap energy of anatase TiO2 crystal [35,36]. The values of intercepts 2.288, 2.291 and 2.295 eV, correspond to Cu2O/TiO2-CP-7, Cu2O/TiO2CP-8, Cu2O/TiO2-CP-9, respectively. The band gap energy of about 2.29 eV for the Cu2O in Cu2O/TiO2-CP samples is larger than that for the pure Cu2O [37]. The formation of Cu2O and TiO2 heterojunction films may change the band gap energy of Cu2O by changing the band gap structure in it [38]. At the same time, the particle size of Cu2O ranges from 90 to 120 nm, as shown in Fig. 3, this relatively small particle size may also affect its band gap energy [35]. The results of the photocurrent performance measurement of samples are shown in Fig. 6. The photo-generated current of the samples was tested for 300 s, and the time interval between the light-on and the light-off was 20 s in 0.5 M Na2SO4 aqueous solution by employing a xenon lamp with an AM 1.5 G cutoff filter as light source. The photocurrent density is 0.40, 17.87, 27.40 and 4.35 μA/cm2 for pure TiO2 NSAs, Cu2O/TiO2-CP-7, Cu2O/TiO2-CP-8 and Cu2O/TiO2-CP-9,
2p spectra in Fig. 4b show two peaks at 932.7 and 952.5 eV, identical to Cu 2p3/2 and Cu2p1/2 of Cu+ [26,27,32,33]. Fig. 5a shows the UV–vis diffuse reflection absorption spectra of specimens. BaSO4 standard whiteboard was employed as the reflection standard in the experiment. Owing to inherent characteristics of wide band gap anatase TiO2, pure TiO2 NSAs possess an inherent absorption edge while the threshold of optical absorption band is less than 400 nm. Since the deposition of Cu2O on the TiO2 NSAs, the optical absorption edges of Cu2O/TiO2-CP are red-shifted and absorption edge from 400 to 600 nm. Two absorption edge, at about 390 and 550 nm are observed obviously, the first one is employed as the excitation peak of TiO2 and the second one is the absorption peak of Cu2O [27,34,35]. The band gap energy spectra of specimens are exhibited in Fig. 5b. By drawing the tangent in the linear part of these curves, the intercepts of the tangent and the abscissa are the values of the band gap of the specimens. The intercept of 3.17 eV for TiO2 NSAs is consistent with the 3.2 eV band 5
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Fig. 5. UV–vis absorption spectra and Band gap energy spectra of pure TiO2 NSAs and nano-sized Cu2O/TiO2 NSAs heterojunction films deposited under different pH values: (a) pure TiO2 NSAs, (b) Cu2O/TiO2-CP-7, (c) Cu2O/TiO2-CP-8 and (d) Cu2O/TiO2-CP-9.
Fig. 8. The open circuit potential (vs SCE) versus time curves of pure TiO2 NSAs and nano-sized Cu2O/TiO2 NSAs heterojunction films deposited under different pH values in 0.5 M Na2SO4 electrolyte: (a) pure TiO2 NSAs, (b) Cu2O/TiO2-CP-7, (c) Cu2O/TiO2-CP-8 and (d) Cu2O/TiO2-CP-9.
Fig. 6. Photocurrent density versus testing time curve of pure TiO2 NSAs and nano-sized Cu2O/TiO2 NSAs heterojunction films deposited under different pH values: (a) pure TiO2 NSAs, (b) Cu2O/TiO2-CP-7, (c) Cu2O/TiO2-CP-8 and (d) Cu2O/TiO2-CP- 9.
Fig. 7. PL spectra of nano-sized Cu2O/TiO2 NSAs heterojunction films deposited under different pH values: (a) pure TiO2 NSAs, (b) Cu2O/TiO2-CP-7, (c) Cu2O/TiO2-CP-8 and (d) Cu2O/TiO2-CP-9.
Fig. 9. The electrochemical impedance spectroscopy (EIS) of pure TiO2 NSAs and nanosized Cu2O/TiO2 NSAs heterojunction films deposited under different pH values in 0.5 M Na2SO4 electrolyte: (a) pure TiO2 NSAs, (b) Cu2O/TiO2-CP-7, (c) Cu2O/TiO2-CP-8 and (d) Cu2O/TiO2-CP-9.
respectively. The optical absorption spectrum of TiO2 shows that its optical response band is less than 400 nm. The xenon lamp source has a 420 nm filter, so a relatively low photocurrent density is obtained. The photocurrent density of Cu2O/TiO2-CP-8 is 68.5 times higher than that of pure TiO2 NSAs, and also is more steady and higher than the samples obtained at pH = 7, 9 [27,39]. The values of photocurrent density indicate that the photoelectric conversion efficiency of Cu2O/TiO2-CP-8 is
higher than that of the specimens deposited in the electrolytes with pH = 7, 9 while all other conditions are kept the same [23,24]. The different pH values mainly reflect the concentration state of -OH in the electrolytes. Since the amounts of copper sulfate and lactic acid are fixed, the deposition state of Cu2+ gradually stabilizes and then decreases with increasing -OH concentration of and pH from 7 to 9 6
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Scheme 1. Size-dependent charge transfer between Cu2O nanocrystals and TiO2 NSAs.
Fig. 10. FESEM images of nano-sized Cu2O/TiO2 NSAs heterojunction films deposited in electrolyte of pH = 8 under different deposition current: (a) 0.15 mA/cm2, (b) 0.20 mA/cm2, (c) 0.25 mA/cm2, (d) 0.30 mA/cm2 and (e) 0.35 mA/cm2.
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crystals from 4.2 to 2.8 nm, while the VB shifts from − 6.13 to − 6.16 eV [48]. Charge transfer between QDs and TiO2 is driven mainly by the differences in the CB of the QDs and TiO2. Thus, the size of QDs is one of the most critical factors to determine the charge transfer rate. As presented in Scheme 1, the energy difference in the conduction band of Cu2O (CB-Cu2O) and TiO2 (CB-TiO2) is larger when smaller Cu2O nanoparticles are attached to TiO2. Electron injection from the Cu2O to TiO2 is enhanced when smaller-sized Cu2O are used because of the high energy level of CB-Cu2O. Therefore, photo-generated electrons can be transferred more rapidly from smaller Cu2O to TiO2. In addition, the effect of deposition current density on the structure and properties of nano-sized Cu2O/TiO2 NSAs heterojunction films was also investigated. These samples were prepared with different deposition current density in the electrolyte of pH = 8 and the morphology of the SEM are shown in Fig. 10. The increase of current density from 0.15 to 0.25 mA/cm2 increases the deposition amount of Cu2O particles [49,50]. As the current density increases to 0.30 and 0.35 mA/cm2, the size of Cu2O particles on the (001) crystal plane of TiO2 nano-sheets becomes larger [22,28,29]. Fig. 11 show the photocurrent density versus testing time curves of the specimens prepared with different currents density. The photocurrent density of 15.07, 23.93, 27.40, 13.30 and 8.12 μA/cm2 corresponds to the specimens prepared at 0.15, 0.20, 0.25, 0.30 and 0.35 mA/cm2, respectively. The results indicate that the size-dependent effect is obvious in Cu2O/TiO2-CP samples and Cu2O/TiO2-CP-0.25-8 heterojunction has smaller Cu2O size and higher photoelectric conversion efficiency.
Fig. 11. Photocurrent density versus testing time curves of nano-sized Cu2O/TiO2 NSAs heterojunction films deposited in electrolyte of pH = 8 under different deposition current: (a) 0.15 mA/cm2, (b) 0.20 mA/cm2, (c) 0.25 mA/cm2, (d) 0.30 mA/cm2 and (e) 0.35 mA/cm2.
[28,29]. The photoluminescence (PL) spectra of the specimens excited at 314 nm are shown in Fig. 7. The excitation wavelength of 314 nm, corresponding to photon energy of 3.95 eV, is larger than the band gap of anatase TiO2 [40]. The peaks in the PL spectra are due to the recombination of the carriers and the greater peak intensity indicates a higher recombination efficiency of electron-hole pairs [36]. The peak of pure TiO2 NSAs at 389 nm, corresponding to 3.20 eV, is attributable to direct electron-hole pair recombination, which should be equal to or slightly bigger than anatase TiO2 band gap [40,41]. Owing to the electrodeposition of Cu2O nanoparticles on TiO2 NSAs, Cu2O/TiO2 heterojunction is formed and the PL peak intensity at 389 nm is reduced, which indicates that the formation of heterojunction increases the separation efficiency of the electron-hole pairs. The PL spectra of the samples prepared in the electrolytes with pH = 7, 8, 9 show roughly similar line shape and peak position, which indicates that the electronic states of the specimens are similar in band gap [40]. The open circuit potential versus time curve for the specimens was tested for 400 s, and light-on and light-off time interval was 50 s, as shown in Fig. 8. The open circuit potential of 0.00340, − 0.160, − 0.173 and − 0.134 V corresponds to pure TiO2 NSAs, Cu2O/TiO2-CP7, Cu2O/TiO2-CP-8 and Cu2O/TiO2-CP-9, respectively. The optical response band of pure TiO2 NSAs is less than 400 nm, while the xenon lamp source has a 420 nm filter, so low open circuit potential value is obtained. The open circuit potential of Cu2O/TiO2-CP-8 is much more negative than that of the sample prepared at pH = 7, 9, which suggests high separation efficiency of electron-hole pairs and strong photoelectric conversion ability under simulated sunlight conditions. The frequency range of EIS test is 0.01–105 Hz under the irradiation of a xenon lamp. The electrochemical impedance spectroscopy (EIS) spectra of the specimens are shown in Fig. 9. The semicircular diameters of EIS curves of Cu2O/TiO2-CP samples are smaller than that of pure TiO2 NSAs, indicating that the heterojunction formed between Cu2O and TiO2 promotes the separation efficiency of electron-hole pairs and transports electron on the interface [42–45]. With increase of pH value from 7 to 9, the semicircular diameter of EIS curves first decreases and then increases. This is consistent with the change trend of photogenerated current density of the specimens prepared with pH = 7, 8 and 9. The above results show that Cu2O/TiO2-CP-8 has better electron transfer ability and conductivity in electrolyte [43,45]. In particular, the energy level of the conduction band (CB) is influenced by their sizes more than the energy level of the valence band (VB) because the effective mass of an electron is smaller than that of a hole [46–48]. Kamat et. al. recently showed that the CB of CdSe tends to shift from − 3.94 to − 3.65 eV with decreasing size of the CdSe nano-
4. Conclusion Nano-sized Cu2O was electrodeposited onto TiO2 NSAs for fabricating the Cu2O/TiO2 NSAs heterojunction films by using chronopotentiometry method. The cyclic alternating anodic and cathodic current with 0.9 s and 0.1 s, respectively, applied on the TiO2 NSAs working electrode was the mainly reason for preparation of nano-sized Cu2O. The pH of electrolyte and deposition current density have obvious effects on Cu2O particles size, which is the main contribution to increased photoelectrochemical properties of the Cu2O/TiO2 NSAs heterojunction films. The sample prepared with pH = 8 and deposition current density j = 0.25 mA/cm2 exhibits best photoelectrochemical properties. This is because that the smaller sized Cu2O has better electron transfer ability and smaller charge transfer resistance in electrolyte, which increases the separation and transportation efficiencies of photo-induced electron-hole pairs. In all, by incorporating Cu2O nano-crystals onto TiO2 NSAs films, the composite exhibits enlarged optical response region and improved photoelectrochemical properties. We believe this work could provide an important and self-contained methodology for fabricating nano-sized Cu2O photo-catalysts. Acknowledgements This study was funded by Natural Science Foundation of China [51402209], Natural Science Foundation of Shanxi Province [201603D121017, 201601D102020, 2015021075, 201701D221083], Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi [2016124], Program for Science and Technology Development of Shanxi [20140321012-01], Shanxi Provincial Key Innovative Research Team in Science and Technology [201605D131045-10], Zhejiang Provincial Science and Technology Key Innovation Team [2011R50012], and Key Laboratory [2013E10022], and Foundation of Taiyuan University of Technology [2015MS046]. References [1] X. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications, Chem. Rev. 107 (2007) 2891–2959. [2] J. Yu, L. Qi, M. Jaroniec, Hydrogen production by photocatalytic water splitting
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Electrodeposition of Cu2O nanocrystalline on TiO2 nanosheet arrays by chronopotentiometry for improvement of photoelectrochemical properties ⁎
Jinbo Xuea,b,c, , Mingzhe Shaoa,c, Qianqian Shena,b,c, Xuguang Liua,c, Husheng Jiaa,c,
⁎
a
Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of Technology), Ministry of Education, Taiyuan 030024, PR China Research Centre of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, PR China c College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Chronopotentiometry method Cu2O/TiO2 composite Size-dependent effect Photoelectrochemical properties
A nano-sized Cu2O/TiO2 nanosheet arrays (TiO2 NSAs) heterojunction film was fabricating by electrodepositing Cu2O onto TiO2 NSAs using chronopotentiometry. The cyclic alternating current on working electrode is the main contribution to nano-sized Cu2O, which in turn improve the photoelectrochemical properties of the Cu2O/ TiO2 NSAs heterojunction film because nano-sized Cu2O, with better electron transfer ability and smaller change transfer resistance in electrolyte, increases the separation and transportation efficiencies of photo-induced electron-hole pairs. Furthermore, electrolyte pH value and deposition current density have significant influence on Cu2O particles size. The results show that the sample prepared with pH = 8 and current density j = 0.25 mA/ cm2 exhibits best photoelectrochemical properties because of the smaller sized Cu2O and uniform distribution on TiO2 NSAs.
1. Introduction Semiconductor nano-materials have drawn great attention owing to their outstanding physicochemical properties and potential applications including materials production and environmental purification [1]. Among these, TiO2-based nanomaterial has been widely studied and considered as a promising candidate because of its non-toxicity, superhydrophilicity, photo-stability and strong oxidation ability [2]. However, the photo-catalytic performance of TiO2 was restricted by its intrinsic defects. On one hand, bare TiO2 can only be excited by UV light, a small fraction of the solar spectrum. On the other hand, the high recombination of photo-induced electron and hole (e−/h+) pairs lowers the quantum efficiency of TiO2 [3]. Thus, a lot of strategies have been proposed to challenge the defects. Cubic cuprous oxide (Cu2O), as a nonstoichiometric p-type semiconductor with a direct forbidden band gap of 2.17 eV and a high optical absorption coefficient, has found significant applications in lithium battery [4], solar energy conversion [5–7], gas sensing [8,9], and photo-catalysis [10–13]. Since Cu2O powder was discovered by Hara et.al. As a photo-catalyst to effectively split water into H2 and O2 under solar irradiation [14], its photo-catalysis becomes an important hotspot of research. Han and co-workers prepared Cu2O/TiO2 composite films, which exhibited better photo-catalytic efficiencies than TiO2 and Cu2O
did alone under visible light irradiation [15]. Shi's group reported Cu2O and TiO2 nano-rod array composite photo-catalysts for solar water splitting anode [16]. The efficient photo-generated electron-hole transformation and separation in composite sample led to the improvement of photo-catalytic performance. Anderson reported that Cu2O/TiO2 photo-catalyst composites were synthesized by an ethanol reduction method, which exhibited excellent photo-catalytic performance for removal of nitrate and oxalic acid [17]. These previous studies [18–20] mainly focused on the tuning of orientation and architecture of electrodeposited films and the control of crystal size and morphology. However, nano-sized Cu2O has rarely been studied. Generally, the overall photo-catalytic activity of a semiconductor is mainly decided by three factors: (1) The adsorption ability, (2) photo-absorption ability in UV–visual light region, (3) the separation and transporting rate of photo-generated electrons and holes in the catalyst [21]. Nano-sized structures with large surface area have prospective applications in photo-catalysis, because the large surface area can provide more surface sites for adsorption of reactant molecules, which makes the adsorption process more efficient. Furthermore, nanosized Cu2O particles may further improve the photo-degradation ability because electrons and excitons can transport efficiently through the smallest dimension. Electrochemical deposition is one of the most attractive methods for
⁎ Corresponding authors at: Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of Technology), Ministry of Education, Taiyuan 030024, PR China. E-mail address: [email protected] (J. Xue).
https://doi.org/10.1016/j.ceramint.2018.03.078 Received 15 January 2018; Received in revised form 22 February 2018; Accepted 10 March 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xue, J., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.03.078
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Fig. 1. Deposition potential (vs. SCE) versus deposition time curves for nano-sized Cu2O/TiO2 NSAs heterojunction films deposited under different pH values: (a, b). pH = 7, (c, d) pH = 8 and (e, f) pH = 9.
2. Experimental sections
the synthesis of nanostructure materials. It provides advantages such as low synthesis temperature, low manufacturing cost, and high level of purity in the products. Also, electrodeposition allows the stoichiometry and microstructure of nano-materials to be controlled by adjusting the deposition parameters. The aim of the present study is to investigate the controllable synthesis of nano-sized Cu2O through chronopotentiometry. The morphology and structure of nano-sized Cu2O/TiO2 nano-sheet arrays (TiO2 NSAs) heterojunction films were studied in detail. The size-dependent photoelectrochemical properties of Cu2O/TiO2 composite reveal that the smaller sized Cu2O has better electron transfer ability and smaller change transfer resistance in electrolyte, which increases the photoinduced electron-hole pair separation and transportation efficiencies.
2.1. Preparation of TiO2 nano-sheet arrays (TiO2 NSAs) films 30 mL of hydrochloric acid was mixed into 30 mL of deionized water. Under magnetic stirring, 1.5 mL of tetrabutyl titanate was added dropwise. After stirring for 10 min, 0.4 g of CO(NH2)2 (99.0%) was added and stirring was continued for 10 min. Then, 0.4 g of (NH4)2TiF6 was added to the solution and further magnetically stirred for 10 min. Subsequently, the formulated solution was transferred to a 25 mL polytetrafluoroethylene lined stainless steel autoclave, in which a 1 cm × 3 cm of an FTO conductive glass was placed obliquely in the diagonal direction, with the conductive surface facing downward. The hydrothermal synthesis reaction conditions were 180 °C for 18 h in a heating furnace. After completion of the preparation, the sample was
2
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Fig. 2. X-ray diffraction (XRD) patterns of nano-sized Cu2O/TiO2 NSAs heterojunction films deposited under different pH values: (a) 2θ = 20–80° (b) 2θ = 35–45°.
of Cu2O/TiO2 heterojunction thin films were measured using a PerkinElmer Lambda 950 UV–Vis spectrophotometer with diffuse reflection integrating sphere. Photoluminescence (PL) spectra were detected by employing a Horiba fluoromax-4 spectrofluorometer at the excitation wavelength of 314 nm.
removed from the autoclave, thoroughly rinsed with deionized water and ethanol absolute, and dried at 45 °C for 2 h. Before the electrochemical deposition of Cu2O, the pure TiO2 NSAs films were annealed at 450 °C for 2 h with increasing and decreasing rate of 2 °C/min. 2.2. Electrodeposition of Cu2O nanoparticles on TiO2 NSAs films
2.4. Photoelectrochemical performance test Preparation of deposition solution: 4.49 g of copper sulfate was dissolved in 49.8 mL of deionized water and then 12.28 g of lactic acid was added dropwise to the solution under magnetic stirring and stirred for an additional half hour at room temperature. Subsequently, 8 g of sodium hydroxide was dissolved in 50 mL of deionized water under magnetic stirring. The pH of the copper sulfate and lactic acid solution was adjusted to 7, 8, 9 with 4 M sodium hydroxide solution, and the solution was stabilized by continuous magnetic stirring for 5 h at room temperature. The electrodeposition of Cu2O onto pure TiO2 NSAs was carried out using a standard three-electrode electrochemical deposition method in the prepared deposition solution. Pure TiO2 NSAs, Pt sheet (1 cm × 3 cm) and saturated calomel electrode (SCE) were used as the working electrode, counter electrode and reference electrode, respectively. The electrolyte was always maintained at a temperature of 25 °C and the deposition time was 500 s. In order to investigate the influence of pH on electrodeposition of Cu2O, different samples were achieved by the chronopotentiometry (CP) method with a constant current density of j = 0.25 mA/cm2 in electrolytes of pH= 7, 8, 9, separately. The obtained samples are noted as Cu2O/TiO2-CP-7, Cu2O/TiO2-CP- 8 and Cu2O/TiO2-CP-9. In addition, in order to investigate the influence of current density on electrodeposition of Cu2O, different samples were prepared by the chronopotentiometry (CP) method with current density j = 0.15, 0.20, 0.25, 0.30 and 0.35 mA/cm2 at pH = 8, and these samples are noted as Cu2O/TiO2 − 0.15 to 8, Cu2O/TiO2 − 0.20 to 8, Cu2O/TiO2 − 0.25 to 8, Cu2O/TiO2 − 0.30 to 8 and Cu2O/TiO2 − 0.35 to 8.
In the photoelectrochemical performance test, the photocurrent density versus testing time curve (Amperometric i-t Curve), electrochemical impedance spectra (EIS) and open circuit potential (OCP) versus testing time curve were detected using a CHI660E electrochemical workstation (Chenhua Instruments Co. Ltd, China). The electrochemical test electrolyte is 0.5 M Na2SO4 aqueous solution. The standard three-electrode system, Cu2O/TiO2 NSAs, Pt (1 cm × 3 cm) and SCE electrode were used as the working electrode, counter electrode and reference electrode, respectively. A CHF-XM500 xenon lamp (Beijing Changtuo Technology Co. Ltd, China) with an AM 1.5 G cutoff filter was adopted as the light source in photoelectric detection. 3. Results and discussion Fig. 1 displays the deposition potential (vs SCE) versus deposition time curves for nano-sized Cu2O electrodeposited onto pure TiO2 NSAs heterojunction films in electrolytes of different pH values with an identical deposition current(j = 0.25 mA/cm2). The shapes of these deposition potential (vs SCE) curves comprise two major steps: crystal nucleation and growth [22]. The range of deposition potential is − 0.1 V to − 0.6 V (vs SCE), as Cu2O can only be deposited in a certain potential range at different pH values [23,24]. Fig. 1a, c and e are deposition curves of nano-sized Cu2O/TiO2 NSAs heterojunction thin films by the chronopotentiometry (CP) with an identical current density j = 0.25 mA/cm2 in electrolytes of pH = 7, 8, 9 during 500 s, respectively. Fig. 1b, d and f are the partially enlarged view from 0 to 10 s of Fig. 1a, c and e curves while the shapes of the three curves indicate the time applied on the working electrode and counter electrode is 0.9 s and 0.1 s in one step, respectively. The deposition potential of nano-sized Cu2O/TiO2-CP-7, Cu2O/TiO2-CP-8 and Cu2O/TiO2-CP-9 is − 0.37, − 0.41 and − 0.53 V (vs SCE) in the early stages, respectively. X-ray diffraction patterns of pure TiO2 NSAs and Cu2O/TiO2-CP specimens are shown in Fig. 2. The diffraction angle range of Fig. 2a is from 20° to 80° while Fig. 2b is the partially enlarged view from 35° to 45° that represents the red square fraction. Four weak diffraction peaks at 2θ = 26.5°, 33.8°, 51.5°, 65.5° are consistent with SnO2 of FTO substrate (JCPDS 46-1088). The diffraction peaks at 2θ = 25.3°, 37.8°, 48.1°, 55.1° can be matched with (101), (004), (200) and (211) planes of anatase crystal with a space group of I41/amd (JCPDS 21-1272) [25].
2.3. Characterization The morphology of Cu2O/TiO2 heterojunction thin films was observed on JEOL JSM-6700F field emission scanning electron microscope (FESEM). The energy dispersive X-ray spectrometry (EDS) analysis was performed with an energy dispersive detector of Oxford Corporate equipped on the FESEM. X-ray diffraction (XRD) patterns of Cu2O/TiO2 heterojunction thin films were detected by the Rigaku Ultima IV X-ray diffractometer using Cu Kα as radiation source. The surface elemental compositions and elemental valence of Cu2O/TiO2 heterojunction thin films were analyzed by a KRATOS AMICUS X-ray photoelectron spectroscope (XPS) with Al Kα as the X-ray source. The absorbance spectra 3
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Fig. 3. FESEM images of pure TiO2 NSAs and nano-sized Cu2O/TiO2 NSAs heterojunction films deposited under different pH values: (a) pure TiO2 NSAs, (b) Cu2O/TiO2-CP-7, (c) Cu2O/ TiO2-CP-8, (d) Cu2O/TiO2-CP-9 and (e) the EDS of Cu2O/TiO2-CP-7.
Three characteristic peaks appear in the XRD patterns at 36.4°, 42.4°, 61.5°, which can be identified as (111), (200), (220) crystal facets of Cu2O with a space group of Pn-3m (JCPDS 05-667) [26]. Fig. 2b shows clearly the position and intensity of (111), (200) crystal facets of Cu2O prepared in the electrolytes with pH = 7, 8, 9. The peaks of impurities, as Cu, CuO and Cu(OH)2, are not detected in the patterns. By Compared with the XRD patterns of anatase TiO2 nano-sheets reported by Yang et al. and Sun et al. [25,27], the intensity of (004) diffraction peak is higher, which indicates that the proportion of exposed (001) crystal planes of the specimen is increased. Fig. 3a is the SEM morphology of pure TiO2 nanosheets, which is consistent with standard anatase TiO2 model comprised of eight isosceles trapezoidal (101) facets on sides. Two flat, square surfaces can be attributed to (001) facets [25]. It can be observed that TiO2 nanosheets of proper density are vertically covering the FTO substrate. TiO2
nanosheets have a thickness between 200 and 300 nm and a length between 2.2 and 2.6 µm. When the pH of electrolytes is 7, 8, 9, as shown in Fig. 2b, c and d, the size of Cu2O particles maintains between 90 and 120 nm. And the amount of Cu2O particles on the (101) crystal facets is larger than that on the (001) crystal facets. The size of Cu2O nanoparticles electrodeposited at pH = 8 is much smaller than that at pH = 7 and 9 [25,26]. Fig. 3e, the EDS spectrum of Cu2O/TiO2-CP-7, reveals that the heterojunction films consists of O, Ti and Cu, both Sn peaks from the FTO substrate and C peak from adventive impurities also appear. The relevant information of surface elemental compositions and elemental chemical environment of Cu2O/TiO2 NSAs heterojunction thin films were analyzed by XPS. Fig. 4a displays O 1s spectra. The O 1s peaks at 530.0, 530.2 and 532.0 eV can be assigned to the oxygen species in TiO2, Cu2O and -OH of surface, respectively [25–31]. The Cu 4
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Fig. 4. XPS patterns of O 1s (a), Cu 2p (b) for nano-sized Cu2O/TiO2 NSAs heterojunction films deposited at different pH values: (1) pH = 7, (2) pH = 8 and (3) pH = 9.
gap energy of anatase TiO2 crystal [35,36]. The values of intercepts 2.288, 2.291 and 2.295 eV, correspond to Cu2O/TiO2-CP-7, Cu2O/TiO2CP-8, Cu2O/TiO2-CP-9, respectively. The band gap energy of about 2.29 eV for the Cu2O in Cu2O/TiO2-CP samples is larger than that for the pure Cu2O [37]. The formation of Cu2O and TiO2 heterojunction films may change the band gap energy of Cu2O by changing the band gap structure in it [38]. At the same time, the particle size of Cu2O ranges from 90 to 120 nm, as shown in Fig. 3, this relatively small particle size may also affect its band gap energy [35]. The results of the photocurrent performance measurement of samples are shown in Fig. 6. The photo-generated current of the samples was tested for 300 s, and the time interval between the light-on and the light-off was 20 s in 0.5 M Na2SO4 aqueous solution by employing a xenon lamp with an AM 1.5 G cutoff filter as light source. The photocurrent density is 0.40, 17.87, 27.40 and 4.35 μA/cm2 for pure TiO2 NSAs, Cu2O/TiO2-CP-7, Cu2O/TiO2-CP-8 and Cu2O/TiO2-CP-9,
2p spectra in Fig. 4b show two peaks at 932.7 and 952.5 eV, identical to Cu 2p3/2 and Cu2p1/2 of Cu+ [26,27,32,33]. Fig. 5a shows the UV–vis diffuse reflection absorption spectra of specimens. BaSO4 standard whiteboard was employed as the reflection standard in the experiment. Owing to inherent characteristics of wide band gap anatase TiO2, pure TiO2 NSAs possess an inherent absorption edge while the threshold of optical absorption band is less than 400 nm. Since the deposition of Cu2O on the TiO2 NSAs, the optical absorption edges of Cu2O/TiO2-CP are red-shifted and absorption edge from 400 to 600 nm. Two absorption edge, at about 390 and 550 nm are observed obviously, the first one is employed as the excitation peak of TiO2 and the second one is the absorption peak of Cu2O [27,34,35]. The band gap energy spectra of specimens are exhibited in Fig. 5b. By drawing the tangent in the linear part of these curves, the intercepts of the tangent and the abscissa are the values of the band gap of the specimens. The intercept of 3.17 eV for TiO2 NSAs is consistent with the 3.2 eV band 5
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Fig. 5. UV–vis absorption spectra and Band gap energy spectra of pure TiO2 NSAs and nano-sized Cu2O/TiO2 NSAs heterojunction films deposited under different pH values: (a) pure TiO2 NSAs, (b) Cu2O/TiO2-CP-7, (c) Cu2O/TiO2-CP-8 and (d) Cu2O/TiO2-CP-9.
Fig. 8. The open circuit potential (vs SCE) versus time curves of pure TiO2 NSAs and nano-sized Cu2O/TiO2 NSAs heterojunction films deposited under different pH values in 0.5 M Na2SO4 electrolyte: (a) pure TiO2 NSAs, (b) Cu2O/TiO2-CP-7, (c) Cu2O/TiO2-CP-8 and (d) Cu2O/TiO2-CP-9.
Fig. 6. Photocurrent density versus testing time curve of pure TiO2 NSAs and nano-sized Cu2O/TiO2 NSAs heterojunction films deposited under different pH values: (a) pure TiO2 NSAs, (b) Cu2O/TiO2-CP-7, (c) Cu2O/TiO2-CP-8 and (d) Cu2O/TiO2-CP- 9.
Fig. 7. PL spectra of nano-sized Cu2O/TiO2 NSAs heterojunction films deposited under different pH values: (a) pure TiO2 NSAs, (b) Cu2O/TiO2-CP-7, (c) Cu2O/TiO2-CP-8 and (d) Cu2O/TiO2-CP-9.
Fig. 9. The electrochemical impedance spectroscopy (EIS) of pure TiO2 NSAs and nanosized Cu2O/TiO2 NSAs heterojunction films deposited under different pH values in 0.5 M Na2SO4 electrolyte: (a) pure TiO2 NSAs, (b) Cu2O/TiO2-CP-7, (c) Cu2O/TiO2-CP-8 and (d) Cu2O/TiO2-CP-9.
respectively. The optical absorption spectrum of TiO2 shows that its optical response band is less than 400 nm. The xenon lamp source has a 420 nm filter, so a relatively low photocurrent density is obtained. The photocurrent density of Cu2O/TiO2-CP-8 is 68.5 times higher than that of pure TiO2 NSAs, and also is more steady and higher than the samples obtained at pH = 7, 9 [27,39]. The values of photocurrent density indicate that the photoelectric conversion efficiency of Cu2O/TiO2-CP-8 is
higher than that of the specimens deposited in the electrolytes with pH = 7, 9 while all other conditions are kept the same [23,24]. The different pH values mainly reflect the concentration state of -OH in the electrolytes. Since the amounts of copper sulfate and lactic acid are fixed, the deposition state of Cu2+ gradually stabilizes and then decreases with increasing -OH concentration of and pH from 7 to 9 6
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Scheme 1. Size-dependent charge transfer between Cu2O nanocrystals and TiO2 NSAs.
Fig. 10. FESEM images of nano-sized Cu2O/TiO2 NSAs heterojunction films deposited in electrolyte of pH = 8 under different deposition current: (a) 0.15 mA/cm2, (b) 0.20 mA/cm2, (c) 0.25 mA/cm2, (d) 0.30 mA/cm2 and (e) 0.35 mA/cm2.
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crystals from 4.2 to 2.8 nm, while the VB shifts from − 6.13 to − 6.16 eV [48]. Charge transfer between QDs and TiO2 is driven mainly by the differences in the CB of the QDs and TiO2. Thus, the size of QDs is one of the most critical factors to determine the charge transfer rate. As presented in Scheme 1, the energy difference in the conduction band of Cu2O (CB-Cu2O) and TiO2 (CB-TiO2) is larger when smaller Cu2O nanoparticles are attached to TiO2. Electron injection from the Cu2O to TiO2 is enhanced when smaller-sized Cu2O are used because of the high energy level of CB-Cu2O. Therefore, photo-generated electrons can be transferred more rapidly from smaller Cu2O to TiO2. In addition, the effect of deposition current density on the structure and properties of nano-sized Cu2O/TiO2 NSAs heterojunction films was also investigated. These samples were prepared with different deposition current density in the electrolyte of pH = 8 and the morphology of the SEM are shown in Fig. 10. The increase of current density from 0.15 to 0.25 mA/cm2 increases the deposition amount of Cu2O particles [49,50]. As the current density increases to 0.30 and 0.35 mA/cm2, the size of Cu2O particles on the (001) crystal plane of TiO2 nano-sheets becomes larger [22,28,29]. Fig. 11 show the photocurrent density versus testing time curves of the specimens prepared with different currents density. The photocurrent density of 15.07, 23.93, 27.40, 13.30 and 8.12 μA/cm2 corresponds to the specimens prepared at 0.15, 0.20, 0.25, 0.30 and 0.35 mA/cm2, respectively. The results indicate that the size-dependent effect is obvious in Cu2O/TiO2-CP samples and Cu2O/TiO2-CP-0.25-8 heterojunction has smaller Cu2O size and higher photoelectric conversion efficiency.
Fig. 11. Photocurrent density versus testing time curves of nano-sized Cu2O/TiO2 NSAs heterojunction films deposited in electrolyte of pH = 8 under different deposition current: (a) 0.15 mA/cm2, (b) 0.20 mA/cm2, (c) 0.25 mA/cm2, (d) 0.30 mA/cm2 and (e) 0.35 mA/cm2.
[28,29]. The photoluminescence (PL) spectra of the specimens excited at 314 nm are shown in Fig. 7. The excitation wavelength of 314 nm, corresponding to photon energy of 3.95 eV, is larger than the band gap of anatase TiO2 [40]. The peaks in the PL spectra are due to the recombination of the carriers and the greater peak intensity indicates a higher recombination efficiency of electron-hole pairs [36]. The peak of pure TiO2 NSAs at 389 nm, corresponding to 3.20 eV, is attributable to direct electron-hole pair recombination, which should be equal to or slightly bigger than anatase TiO2 band gap [40,41]. Owing to the electrodeposition of Cu2O nanoparticles on TiO2 NSAs, Cu2O/TiO2 heterojunction is formed and the PL peak intensity at 389 nm is reduced, which indicates that the formation of heterojunction increases the separation efficiency of the electron-hole pairs. The PL spectra of the samples prepared in the electrolytes with pH = 7, 8, 9 show roughly similar line shape and peak position, which indicates that the electronic states of the specimens are similar in band gap [40]. The open circuit potential versus time curve for the specimens was tested for 400 s, and light-on and light-off time interval was 50 s, as shown in Fig. 8. The open circuit potential of 0.00340, − 0.160, − 0.173 and − 0.134 V corresponds to pure TiO2 NSAs, Cu2O/TiO2-CP7, Cu2O/TiO2-CP-8 and Cu2O/TiO2-CP-9, respectively. The optical response band of pure TiO2 NSAs is less than 400 nm, while the xenon lamp source has a 420 nm filter, so low open circuit potential value is obtained. The open circuit potential of Cu2O/TiO2-CP-8 is much more negative than that of the sample prepared at pH = 7, 9, which suggests high separation efficiency of electron-hole pairs and strong photoelectric conversion ability under simulated sunlight conditions. The frequency range of EIS test is 0.01–105 Hz under the irradiation of a xenon lamp. The electrochemical impedance spectroscopy (EIS) spectra of the specimens are shown in Fig. 9. The semicircular diameters of EIS curves of Cu2O/TiO2-CP samples are smaller than that of pure TiO2 NSAs, indicating that the heterojunction formed between Cu2O and TiO2 promotes the separation efficiency of electron-hole pairs and transports electron on the interface [42–45]. With increase of pH value from 7 to 9, the semicircular diameter of EIS curves first decreases and then increases. This is consistent with the change trend of photogenerated current density of the specimens prepared with pH = 7, 8 and 9. The above results show that Cu2O/TiO2-CP-8 has better electron transfer ability and conductivity in electrolyte [43,45]. In particular, the energy level of the conduction band (CB) is influenced by their sizes more than the energy level of the valence band (VB) because the effective mass of an electron is smaller than that of a hole [46–48]. Kamat et. al. recently showed that the CB of CdSe tends to shift from − 3.94 to − 3.65 eV with decreasing size of the CdSe nano-
4. Conclusion Nano-sized Cu2O was electrodeposited onto TiO2 NSAs for fabricating the Cu2O/TiO2 NSAs heterojunction films by using chronopotentiometry method. The cyclic alternating anodic and cathodic current with 0.9 s and 0.1 s, respectively, applied on the TiO2 NSAs working electrode was the mainly reason for preparation of nano-sized Cu2O. The pH of electrolyte and deposition current density have obvious effects on Cu2O particles size, which is the main contribution to increased photoelectrochemical properties of the Cu2O/TiO2 NSAs heterojunction films. The sample prepared with pH = 8 and deposition current density j = 0.25 mA/cm2 exhibits best photoelectrochemical properties. This is because that the smaller sized Cu2O has better electron transfer ability and smaller charge transfer resistance in electrolyte, which increases the separation and transportation efficiencies of photo-induced electron-hole pairs. In all, by incorporating Cu2O nano-crystals onto TiO2 NSAs films, the composite exhibits enlarged optical response region and improved photoelectrochemical properties. We believe this work could provide an important and self-contained methodology for fabricating nano-sized Cu2O photo-catalysts. Acknowledgements This study was funded by Natural Science Foundation of China [51402209], Natural Science Foundation of Shanxi Province [201603D121017, 201601D102020, 2015021075, 201701D221083], Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi [2016124], Program for Science and Technology Development of Shanxi [20140321012-01], Shanxi Provincial Key Innovative Research Team in Science and Technology [201605D131045-10], Zhejiang Provincial Science and Technology Key Innovation Team [2011R50012], and Key Laboratory [2013E10022], and Foundation of Taiyuan University of Technology [2015MS046]. References [1] X. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications, Chem. Rev. 107 (2007) 2891–2959. [2] J. Yu, L. Qi, M. Jaroniec, Hydrogen production by photocatalytic water splitting
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