Microstructure, optical properties, and catalytic performance of Cu2O-modified ZnO nanorods prepared by electrodeposition

Cu2O-modified ZnO nanorods are prepared by a two-step electrodeposition method on ITO substrates, and the deposition time of Cu2O is 0, 1, 5, and 10 min, respectively. Cu2O particles are embedded in the interspaces of the ZnO nanorods, and the amounts of the Cu2O particles increase obviously when the deposition time lasts longer. The peaks corresponding to ZnO nanorods and Cu2O particles are detected from scanning electron microscope (SEM) and X-ray diffraction (XRD) results. UV-vis absorption spectra measurements have shown the bandgaps of ZnO nanorods shift from 3.22 to 2.75 eV. The methyl orange (MO) concentration can be reduced to around 15% in 100 min with Cu2O electrodeposition time for 10 min.


Background
Zinc oxide (ZnO), a typical n-type semiconductor with a direct bandgap of 3.37 eV, is an attractive material that could be suitable for a window layer [1][2][3][4]. Lowdimensional nanostructural ZnO shows unique physical and chemical properties [5][6][7][8]. Up to now, great attention has been focused on the investigation of ZnO nanostructures including thin film, nanowires, nanorods, and nanoparticles [9][10][11][12][13]. Due to the high specific surface area and excellent optical and electrical properties, ZnO nanorods have attracted much attention for their applications in solar cells [14][15][16][17][18]. Unfortunately, the absorption of ZnO in the visible-light region is very low due to its wide bandgap [19,20]. To extend the absorption of ZnO into the visible region, narrow bandgap semiconductors, such as CdS, CdSe, and Cu 2 O, have been used to construct heterostructures with 1D ZnO [21][22][23][24]. Cuprous oxide (Cu 2 O), with a direct bandgap of 2.17 eV, is a natural p-type semiconductor owning good mobility and high minority carrier diffusion length [25,26]. Low-cost producibility, abundance, nontoxicity, and high absorption coefficient in the visible light region make Cu 2 O a promising material for photovoltaic application [27,28]. Cu 2 O is considered to be a promising partner with ZnO for p-n heterojunction due to its narrow energy band [29,30].
The Cu 2 O/ZnO heterojunction has always been synthesized by several methods, such as thermal oxidation, sputtering, pulsed laser deposition, chemical vapor deposition, and electrodeposition. To our knowledge, much attention is focused on the Cu 2 O/ZnO heterojunction for solar cells, and there are few reports involving Cu 2 O/ZnO nanorods for photocatalysis studies [31][32][33]. Jeong et al. [34] reported that interface recombination is the dominant carrier transport mechanism, and Cu 2 O/ZnO heterojunction solar cells have high potential as solar cells if the recombination and tunneling at the interface can be suppressed at room temperature. In the current work, we prepared Cu 2 O-modified ZnO nanorods by a two-step electrodeposition method. The amount of Cu 2 O is controlled by the deposition time. The effects of the deposition time on the morphological, microstructural, optical properties, and catalytic performance of the Cu 2 O-modified ZnO nanorods have been investigated in detail.

Preparation of Cu 2 O-modified ZnO nanorods
The Cu 2 O-modified ZnO nanorods were prepared by a two-step electrodeposition method on ITO substrates.
Prior to the deposition, ITO substrates were ultrasonically cleaned in acetone, alcohol, and deionized water, sequentially. Firstly, an aqueous solution composed of 0.01 mol/L zinc nitrate (Zn(NO 3 ) 2 ) was used to prepare ZnO nanorods. The ITO substrates served as the working electrode, platinum worked as the counter electrode, and an Ag/AgCl electrode was the reference electrode. The electrodeposition procedure was conducted in a water bath for 1 h at a temperature of 70°C under the constant potential of −0.7 V vs the reference electrode. Consequently, the electrochemical deposition of Cu 2 O on the ZnO nanorods was performed in an aqueous solution composed of 0.05 mol/L copper acetate (Cu(CH 3 COO) 2 ) and 0.1 mol/L sodium acetate (NaCH 3 COO). The ZnO/ITO films served as the working electrode, and the temperature of water bath was 40°C. The applied potential was controlled at −0.2 V vs the reference electrode and lasted for different times (1, 5, and 10 min) for each Cu 2 Omodified ZnO nanorods. The samples were labeled as Cu 2 O(1)-ZnO, Cu 2 O(5)-ZnO, and Cu 2 O(10)-ZnO, respectively. The pure Cu 2 O film was deposited at the same condition, and the deposition time is 30 min.

Characterization
The phase and crystalline structure of the as-deposited films were examined by X-ray diffraction (XRD; MAC Science, Yokohama, Japan) with an X-ray diffractometer employing Cu-Kα radiation. The surface morphology of the Cu 2 O-modified ZnO nanorod films was observed with a field-emission scanning electron microscope (FESEM; S4800, Hitachi, Ltd., Chiyoda, Tokyo, Japan). A UV-visible (UV-vis) spectrophotometer (UV-2550, Shimadzu, Tokyo, Japan) was used to measure the UV-vis absorption spectra of the as-deposited films. The surface composition was analyzed by X-ray photoelectron spectrometer (XPS; ESCA-LAB 250, Thermo Fisher Scientific, Waltham, MA, USA). The Raman spectra and photoluminescence (PL) spectra were recorded by micro-Raman spectroscope system. The photocatalytic activity of the as-prepared samples was evaluated by the photodegradation of methyl orange (MO) solution under visible light irradiation. The visible light source was obtained using a 420-nm cutoff filter. The samples (15 mm × 10 mm) were immersed in 10 mL 15 ppm MO solutions. The distance between the samples and the light source was fixed at 5 cm. After the given time interval, the UV-vis absorption spectra of MO were recorded by UV-vis spectrophotometer (Shimadzu, UV-2550).

Results and discussion
Surface morphology analysis  and the amounts of the Cu 2 O particles increase obviously when the deposition time increases [35]. It can also be found that as the Cu 2 O deposition time increases, the diameter and length of the nanorods decreased, which can be affected by electrolyte corrosion during the Cu 2 O deposition process.  (Figure 2b) can barely be detected, and this can be ascribed to an insufficient amount. In a word, the peaks of Cu 2 O particles are relatively weaker due to the shorter deposition time compared with ZnO nanorods.      [38]. The energy bandgap (E g ) is measured by linear extrapolation to the hv-axis. The inset of Figure 4 shows (ahv) 2 versus hv for the Cu 2 O films, and the estimated direct bandgaps are listed in Table 1. Due to variation in deposition time, the value of absorption spectra changed. For Cu 2 O-modified ZnO nanorods, when the Cu 2 O deposition time increases from 1 to 10 min, the corresponding bandgaps of Cu 2 O particles are 2.43, 2.38, and 2.30 eV, respectively. In addition, the bandgaps of ZnO nanorods shift from 3.22 to 2.75 eV, which is also consistent with previous SEM and XRD results.

UV-vis absorbance and XPS analysis
XPS measurements were conducted for surface analysis of the Cu 2 O-modified ZnO nanorods with Cu 2 O deposition time for 5 min ( Figure 5). As shown in Figure 5, the peaks of Zn2p and Cu2p are detected from the XPS spectrum. Two peaks of Zn2p located at 1,045.1 and 1,021.9 eV are assigned to Zn2p3/2 and Zn2p1/2, respectively, which can be assigned to Zn 2+ in ZnO nanorods (Figure 5a) [39]. The typical XPS peaks of Cu (2p) at 952.2 and 932.4 eV for the Cu 2 O-modified ZnO nanorods indicate the existence of Cu + during the deposition of Cu 2 O particles ( Figure 5b). Furthermore, the characteristic peaks for Cu 2+ at 953.6 (2p1/2) and 933.7 eV (2p3/2) were not observed [40]. This result confirms that the sample contains Cu + rather than Cu 2+ or Cu.  Raman and PL spectra analysis Figure 6 shows the Raman and PL spectra of Cu 2 Omodified ZnO nanorods with Cu 2 O deposition for 0 and 5 min. The peak at 437 cm −1 is due to the E 2 high vibration mode of ZnO hexagonal crystal structure as shown in Figure 6a [41]. Figure 6a also shows that the stronger Raman peak of 218 cm −1 , which corresponds to 2Γ 12− vibration modes of Cu 2 O. Meanwhile, the relative weaker peaks of 146 and 626 cm −1 can be contributed to infrared vibration mode Γ 15 , which are excited from oxygen vacancy [42]. Figure 6b shows the PL spectra of Cu 2 O-modified ZnO nanorods. There appeared two component peaks of the UV emission in the PL spectra of Cu 2 O-modified ZnO nanorods. The predominant sharp peak appeared at about 380 nm could be assigned to the near band emission of the ZnO nanorods [43]. In addition, a wide emission with a peak at 600 nm was detected and regarded as defectrelated emissions of ZnO nanorods. The existence of Cu 2 O particles has little impact on peak position of ZnO nanorods. The emissions at 380 and 600 nm were diminished when Cu 2 O particles were deposited on the ZnO nanorods, which may originate from the random multiple scattering in such structure.

Photocatalytic degradation of MO
The photocatalytic activities of the as-prepared samples were carried out by the degradation of MO solution under visible light irradiation, and the experimental results are shown in Figure 7. Here, C 0 and C are the absorbance of the characteristic absorption peak (464 nm) of MO solution before and after irradiation. As indicated in Figure 7, the pure ZnO nanorods exhibit a weak ability for the degradation of MO. The poor degradation ability of the pure ZnO nanorods can be ascribed to the fact that the visible light cannot provide energy to excite electrons from the valance band to the conduction band. All the Cu 2 Omodified ZnO nanorods have strong degradation ability of MO than the pure ZnO nanorods [44]. With increasing Cu 2 O electrodeposition time, the degradation abilities of the Cu 2 O-modified ZnO nanorods enhanced. The reason is that Cu 2 O has higher degradation ability than ZnO. Meanwhile, the amount of Cu 2 O particles on the ZnO nanorods increases when increasing the Cu 2 O electrodeposition time. Furthermore, the Cu 2 O-modified ZnO nanorods have a large specific surface area than pure ZnO nanorods. It is worth mentioning that the MO concentration can be reduced to around 15% in 100 min with Cu 2 O electrodeposition time of 10 min. As a result, the photocatalytic activity of the Cu 2 O-modified ZnO nanorods depends on the Cu 2 O electrodeposition time.

Conclusions
In summary, the Cu 2 O-modified ZnO nanorods are prepared by electrodeposition method on ITO substrates. XRD measurement shows the coexistence of Cu 2 O with cubic structure and ZnO with hexagonal structure. SEM images reveal that Cu 2 O particles embed in the interspaces of the ZnO nanorods and the amounts of the