Abstract
ZnO nanorods and ZnO/Cu2O nanocomposites were fabricated by a simple hydrothermal method. The morphology, crystalline structure and optical properties of the samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectra, Fourier transform infrared spectra (FTIR) and photoluminescence (PL) spectra. Both samples were applied to degrade methylene blue (MB). Under light irradiation, ZnO nanorods exhibit high photocatalytic activity, and the degradation rate of ZnO/Cu2O nanocomposites is obviously higher than that of ZnO nanorods. The possible photocatalytic mechanism for both samples has been discussed comparatively.
Introduction
Zinc oxide (ZnO) as an important semiconductor photocatalyst has attracted much attention and its photocatalytic mechanism is similar to that of TiO2 (a popular and universally recognized catalyst) [1]. However, ZnO can only be activated by ultraviolet (UV) light because of its large energy band gap of 3.2 eV. It is well known that UV light is limited to a small fraction (about 4 to 5%) of solar spectrum [2]. Therefore, the effective improvement of absorption coefficient in the visible region still remains a challenge in photocatalytic application. Up to now, various methods, such as dye-sensitizing, coupling with narrow band gap semiconductors, doping with metals ions or non-metal ions, have been carried out to extend the optical absorption of ZnO [3]. Cuprous oxide (Cu2O) is a natural p-type direct band gap semiconductor with a band gap of 2.17 eV, which has favorable absorption in the visible range and an appropriate band alignment with ZnO [4, 5]. The ZnO/Cu2O coupled semiconductors can be applied in gas sensors [6], UV photodetectors [7], solar cells [8] and so on.
In recent years, many methods have been developed to synthesize ZnO/Cu2O nanocomposites as photocatalysts. Cao et al. prepared ZnO/Cu2O compound photocatalysts by soak-deoxidize-air oxidation method with different Cu2+ concentrations. ZnO/Cu2O compounds exhibited better photocatalytic activity than ZnO [9]. Komarneni et al. fabricated ZnO/Cu2O nanocomposites by a coprecipitation method. The composite samples demonstrated higher photocatalytic activity than either pure Cu2O or ZnO [10]. Zhang et al. reported similar results in ZnO microrods modified by Cu2O nanocrystals [11].
In this paper, we report on a low-cost and fast fabrication of ZnO nanorods and ZnO/Cu2O nanocomposites. The morphology, microstructure, and optical properties of samples were characterized. The photocatalytic activity of samples was investigated by degradation of methylene blue (MB) under light irradiation. The possible photocatalytic mechanism has been discussed comparatively.
Experimental details
Synthesis of catalyst
ZnO nanorods and ZnO/Cu2O nanocomposites were fabricated by hydrothermal method. All the chemicals used were of Analar grade. Zinc acetate dihydrate (Zn(Ac)2·2H2O) and sodium hydroxide (NaOH) were used as the precursor. First, 2.2 g Zn(Ac)2·2H2O and 2.0 g NaOH were dissolved in 100 mL and 50 mL ethanol, respectively. Subsequently, NaOH solution was dropwise added into Zn(Ac)2·2H2O solution under vigorous stirring at room temperature for 40 min, and then the mixture solution was transferred into Teflon-lined autoclaves. The autoclave was put into a laboratory oven and heated to 150C. The temperature was maintained for 4 h. For ZnO/Cu2O nanocomposites, additional Cu(Ac)2·2H2O was added in the initial Zn(Ac)2·2H2O solution. The atom ratio of Zn:Cu was 1 : 0.03. The precipitates were alternately washed with deionized water and ethanol for five times, and dried in air at 80C for 12 h. The final products were obtained.
Characterization of catalyst
The phase and structural analysis of the samples were carried out by X-ray diffraction (XRD) with Cu Ka radiation and Raman spectra on LabRam microRaman spectrometer with the wavelength of 532 nm laser excitation. The morphology of samples was observed by a field-emission scanning electron microscope (FE-SEM). Fourier transform infrared (FTIR) spectra were characterized in the transmittance mode for the flakes of samples pressed with KBr powder. The optical absorption was obtained by a UV-Vis spectrometer with the wavelength range 300–800 nm. Photoluminescence (PL) spectra were performed on a spectrometer using a 450 W Xe lamp as the excitation source.
Photocatalytic test
The photocatalytic activities of samples were evaluated through measuring the decomposition rate of methylene blue (MB) aqueous solutions (10 mg/L) under light irradiation. The light irradiation was carried out using a 150 W Xe lamp as the light source of simulated sunlight. The experiments were conducted at room temperature. Typically, 0.05 g of powder products was added into 100 mL of MB solution. Before light illumination, the suspensions were magnetically stirred in dark for 30 min to ensure the adsorption/desorption equilibrium. During the course of light irradiation and stirring, 5 mL suspensions aliquot was collected at regular intervals and centrifuged to remove the photocatalyst powders. The optical absorption of the obtained filtrate was measured via a UV–Vis spectrophotometer (Hitachi V4100) and the concentration of MB was determined. The degradation efficiency was calculated using the following equation [3]:
Morphology and microstructure of catalyst
Figure 1(a) and (b) present the SEM images of ZnO nanorods and ZnO/Cu2O nanocomposites. It is evident that the both samples are rod-like crystals with a non-uniform length from 10–100 nm and a diameter about 10 nm. No significant difference was observed between the both samples.
XRD patterns of ZnO nanorods and ZnO/Cu2O nanocomposites are shown in Fig. 2 (a). Figure 2 (b) shows the magnified portions of the XRD patterns. The strong (100), (002) and (101) peaks of ZnO indicate that the major phase of both samples is wurtzite structure ZnO. Compared with ZnO nanorods, ZnO/Cu2O nanocomposites present typical (111), (200) and (220) diffraction peaks of Cu2O. ZnO diffraction peaks have no obviously shift for the nanocomposites, which indicates that ZnO and Cu2O are separated phases and no alloy is formed.
Raman spectra of ZnO nanorods and ZnO/Cu2O nanocomposites are shown in Fig. 3. For ZnO nanorods, the two strong Raman-active peaks located at about 99 and 437 cm–1 are assigned as E2 (low) and E2 (high), which are associated with the vibration of heavy Zn sub-lattice and the motion of oxygen atoms [12]. Two bands at 379 and 329 cm–1 are due to A1(TO) and overtone of the acoustic modes of ZnO, respectively [13]. For the ZnO/Cu2O nanocomposites, an additional peak at 218 cm–1 is attributed to the second-order of Cu2O. The characteristic peak of Cu2O suggests the presence of Cu2O separated phase [14].
Figure 4 shows the FTIR spectra of ZnO nanorods and ZnO/Cu2O nanocomposites. The strong absorption band at <540 cm–1 is attributed to Zn-O stretching vibration mode [15–17]. Another broad band at <3430 cm–1 is ascribed as the bending vibration of surface O-H stretching mode, indicating the presence of OH groups [18]. For as-grown samples, the intensity of the band for ZnO nanorods is much stronger than that of ZnO/Cu2O nanocomposites, suggesting larger amounts of OH groups on the surface of ZnO nanorods. For both samples after photocatalytic degradation, the intensity of the band obviously decreases, revealing that OH groups can be removed in photocatalytic process.
Optical properties of catalyst
Figure 5 presents the UV-Vis absorption spectra of ZnO nanorods and ZnO/Cu2O nanocomposites. ZnO nanorods show no absorption in the visible region. However, there is an obvious peak located at <500 nm and absorption in the visible region for ZnO/Cu2O nanocomposites besides the typical absorption of ZnO located at 370 nm. Similar case has been observed in previous reports [19]. It indicates the presence of separated phases Cu2O. The results are consistent with the XRD data and Raman spectra.
Figure 6 demonstrates the PL spectra of ZnO nanorods and ZnO/Cu2O nanocomposites at room temperature with excitation wavelength of 340 nm. Both samples exhibit a weak UV emission peak located at ∼ 380 nm and a strong broad emission band in the visible range. The UV emission is typical band-edge transition. Comparing with ZnO nanorods, the visible emission band of ZnO/Cu2O blue-shifts from yellow (∼ 565 nm) to green (∼ 530 nm) and the intensity decreases. The yellow emission is generally attributed to OH groups on ZnO surface, which is universal in PL spectra of ZnO prepared from aqueous solution [20, 21]. The change of the visible band indicates the decrease of OH groups in ZnO/Cu2O.
Photodegradability of MB
Figure 7(a) and (b) show the change in optical absorption spectra of MB by ZnO nanorods and ZnO/Cu2O nanocomposites catalyst under simulated sunlight irradiation for different time intervals, respectively. With the increase of irradiation time, the intensity of the band at 664 nm decreases, indicating that MB has been photodegraded by ZnO nanorods and ZnO/Cu2O nanocomposites. Figure 7(c) shows the percentage of MB on irradiation solution of MB in the presence of ZnO nanorods or ZnO/Cu2O nanocomposites under different irradiation time from the equation (1). Moreover, a photolysis experiment (in the absence catalysts) was carried out and the results were shown in the inset of Fig. 7(c). After light illumination for 380 min, only slight degradation of MB occurred in the absence of catalysts. Compared with the ZnO/Cu2O nanocomposites, ZnO nanorods exhibit higher photocatalytic activity in 200 min. However, when irradiation time exceeds 200 min, the degradation rate of the ZnO/Cu2O nanocomposites is obviously higher than ZnO nanorods. After irradiation is conducted for 380 min, the degradation efficiency of MB reaches 82% and 92% for ZnO nanorods and ZnO/Cu2O nanocomposites, respectively.
Mechanism of degradation
The band gap of ZnO is 3.2 eV, corresponding to the UV region radiation and having a flat absorbance spectrum in the visible region. When UV region radiation of light source is supplied to the ZnO nanorods sample, Surface adsorbed oxygen will trap photogenerated electrons on the conduction band (CB). The hydroxide ion derived from water will capture photogenerated holes in its valence band, and then and •OH will be formed [22, 23]. They will react with MB, leading to subsequent degradation. Thus the rate of degradation is correlated with the population of and •OH, which can be produced from the ZnO surface. When visible region light of source is irradiated, the photo-generated electron-hole pairs can be excited in Cu2O and separated in the interface of ZnO/Cu2O nanocomposites. The generated electrons in the CB of ZnO will react with adsorbed oxygen species on surface and produce . Meanwhile, the remained holes in valence band of Cu2O may react with OH– and introduce •OH. The increase of the quantity of and •OH benefits the improvement of photocatalytic activity. The possible mechanism scheme is illustrated in Fig. 8. However, due to the separation phase of Cu2O on the ZnO, ZnO/Cu2O nanocomposites have smaller exposed surface of ZnO, resulting in the decreasing of and •OH. In the initial period, the exposed surface of ZnO is a dominated factor for the degradation. After 200 min, the enhanced photocatalytic activity for ZnO/Cu2O nanocomposites can be ascribed to an effective electron transfer from Cu2O to ZnO under light irradiation.
Conclusions
ZnO/Cu2O nanocomposites and ZnO nanorods have been fabricated using hydrothermal method. The XRD and Raman spectra indicated that the samples are ZnO nanorods and ZnO/Cu2O nanocomposites, respectively. FTIR and PL spectra exhibit OH groups on the surface of samples. Coupled Cu2O can extend the optical absorption of ZnO. Under light irradiation, ZnO nanorods exhibit higher photocatalytic activity in initials period. The degradation rate of the ZnO/Cu2O nanocomposites is obviously higher than ZnO nanorods after photocatalytic conduction for a period time due to the effective electron transfer from Cu2O to ZnO.
Footnotes
Acknowledgments
This work was supported by the Natural Science Foundation of Tianjin (Nos 14JCZDJC31200, 15JCYBJC16700 and 15JCYBJC16800), the Natural Science Foundation of Tianjin University of Technology and Education (No KJY12-08) and International Cooperation Program from Science and Technology of Tianjin (No 14RCGHGX00872).