Photocatalytic activity of Cu2O/ZnO nanocomposite for the decomposition of methyl orange under visible light irradiation

Abstract ZnO is modified by Cu2O by the process of precipitation and calcination. X-ray diffraction has shown that Cu2O/ZnO catalysts are made of highly purified cubic Cu2O and hexagonal ZnO. Scanning electron microscopy and transmission electron microscopy have shown that ZnO adhered to the surface of Cu2O. Due to the doping of Cu2O, the absorption range of the Cu2O/ZnO catalyst is shifted from the ultraviolet to the visible region due to diffuse reflection. X-ray photoelectron spectroscopy and photoluminescence spectra have confirmed that there is a substantial interaction between the two phases of the resultant catalyst. The degradation efficiency of Cu2O/ZnO on methyl orange solution is obviously enhanced compared to Cu2O and ZnO. The maximum degradation efficiency is 98%. The degradation efficiency is affected by the pH of the solution and initial concentration. After three rounds of recycling, the degradation rate is almost same. This shows a consistent performance of Cu2O/ZnO. The increase in catalytic ability is related to the lattice interaction caused by the doping of Cu2O.


Introduction
In recent years, the semiconductor-based photocatalytic technique has been widely used [1] as a promising and effective approach to solve global energy and organic pollutant problems. There are many kinds of the usual photocatalysts, including TiO 2 , ZnO, and WO 3 . Most of them can react to ultraviolet (UV) light, although the UV light in (MO). The microstructure and performance of the composite are studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and diffuse reflection. X-ray photoelectron spectroscopy (XPS) is carried out to explore the mechanism behind the process. The results show that there is the p-n heterostructure in hybrid powder and the composite has excellent photocatalytic performance. The mechanism of Cu 2 O/ZnO is in accordance with the mechanism of photoinduced electron transfer.

Preparation
Zn(NO 3 ) 2 ·5H 2 O (5.95 g) and Na 2 CO 3 (14.10 g) were both dissolved in 100 ml deionized water. With magnetic stirring, the Na 2 CO 3 solution was added drop by drop to the solution of 100 ml Zn(NO 3 ) 2 ; a white precipitate is then formed. The mixed solution was washed and filtered with deionized water several times to obtain a white precipitate. The white precipitate was dried in a blast oven at 100°C for 2 h. Then, the dried precipitate was fired at a muffle furnace at 400°C for 2 h; the ZnO nanomaterial was then obtained.
Cu(AC) 2 ·H 2 O was dissolved in 50 ml absolute ethanol to obtain different concentration solutions of Cu(AC) 2 (0, 0.028, 0.056, 0.084, and 0.112 mol/l), and 0.3 g ZnO was dispersed in the above solution under sonication for 30 min. HAC (2 ml) was added drop by drop to the above solution and the above solution was kept in a constant temperature water bath at 60°C with magnetic stirring. Then, 0.17 mol/l glucose solution (50 ml) was added to the above solution at a rate of 6 ml/min, and 1 mol/l NaOH solution (60 ml), deionized water (25 ml), and absolute ethanol (35 ml)

Instrumental analysis
XRD patterns were measured with XRD (D8 Advanced XRD, XD-3, Japan) with CuKα radiation source (λ = 0.15418 nm). XRD patterns were obtained with diffraction angles 2θ ranging from 20°to 80°. The microstructure of the catalyst was observed by SEM (Philips XL-30, Japan). TEM (ASAP2010M, Norcross, GA, USA) images of the samples were obtained on a FEI-Tecnai F20ST electron microscope by fixing the powder of the catalyst to a 3 mm copper grid. UV-visible (UV-vis) spectrum was obtained on a UV-vis Cintra-10e spectrometer (UV-1700, Japan) with a wavelength range of 200-800 nm. The basic element composition and binding state of the catalyst were studied by XPS (ESCALAB 250, Thermo Fisher Scientific, Waltham, MA, USA). The photoluminescence (PL) analysis was performed on a spectrometer (F-4500, Japan), using a Xe lamp (excitation at 365 nm) as the light source.

Photocatalytic studies
The effect of the sample was evaluated by degrading MO [13]. At room temperature, 0.100 g of samples I-IV, pure ZnO, and pure Cu 2 O were added to 100 ml MO solution of different concentration. The pH was controlled by the addition of HCl. The solution was stirred for 30 min without illumination. After sampling, the solution was kept under a 300 W hernia lamp using JZ-420 filter to filter out the light below 420 nm. The MO solution with the catalyst was irradiated and stirred for 4 h; in the process, samples were taken every 30 min. After centrifugation, the supernatant was collected and analyzed by UV spectrophotometry, and the concentration of MO was calculated by 464 nm absorbance. The photodegradation rate of MO (D) was measured using the following formula for calculation: where C 0 and C are the MO concentrations at time 0 and t (irradiation time).   Figure 2. It reveals that the prepared ZnO is hexagonal wurtzite nanoparticle with an average particle size of 60 nm. The prepared Cu 2 O is a homogeneous spherical nanoparticle with an average particle size of about 500 nm. In Figure 2B and C, it can be found that after combining with ZnO the smooth surface of spherical Cu 2 O becomes rough because the Cu 2 O surface is covered by a lot of ZnO nanoparticles. The result of EDX shows that no any other secondary phase exits. Figure 3 shows the mapping of Cu 2 O/ZnO. This illustrates that there are O, Zn, and Cu elements on the surface of Cu 2 O/ZnO. The O element is basically distributed in the whole SEM image and the Zn element is also scattered in the picture. However, in the position where the sphere is more, the Cu element is more concentrated and the O and Zn elements are sparser compared to other locations. Cu exists in the form of Cu 2 O, and ZnO is present on the surface of Cu 2 O.

Results and discussion
The TEM and high-resolution TEM (HR-TEM) images of Cu 2 O/ZnO (sample II) are shown in Figure 4. ZnO nanoparticles adhered to the surface of Cu 2 O with a diameter of about 500 nm, which is consistent with the SEM image. In Figure 4B, the close contact of Cu 2 O and ZnO could be observed clearly, where the stripes interval at 0.151 nm and 0.148 nm can be attributed to the lattice face value of cubic phase Cu 2 O (220) and hexagonal wurtzite (103). Figure 4B obviously shows the existence of an interaction between Cu 2 O/ZnO nanoparticles. The TEM result further confirms the XRD result. Figure 5A-C shows the Cu 2p , Zn 2p , and O 1s XPS spectra of Cu 2 O, ZnO, and Cu 2 O/ZnO (sample II), respectively. In Figure 5A, the peaks of pure Cu 2 O at 932.4 and 952.0 eV are related to the binding energy of Cu 2P3/2 and Cu 2P1/2 , which is consistent with the Cu + species [18,[24][25][26]. Compared to the peaks of pure Cu 2 O, the binding energy of Cu 2 O/ZnO peaks shift at 0.1 eV to lower binding energy. In Figure 5B, the characteristic peaks of pure ZnO at 1022.2 and 1045.4 eV are mainly consisted of Zn 2P3/2 and Zn 2P1/2 in ZnO [27]. In Figure 5, the binding energies of Zn 2P3/2 and Zn 2P1/2 in Cu 2 O/ZnO are obviously higher than that in pure ZnO. Figure 5C shows the O 1s XPS spectra of pure ZnO, Cu 2 O/ZnO, and pure Cu 2 O. The characteristic peak of Cu 2 O/ZnO at 529.6 eV corresponds to the lattice of Cu 2 O and ZnO. The pure ZnO peak at 527.6 eV corresponds to the oxygen in the lattice of ZnO. The peak of pure Cu 2 O at 529.8 eV corresponds to the oxygen in the Cu 2 O lattice. The source of the characteristic peak of pure Cu 2 O at 531.8 eV, the peak of pure ZnO at 529.4 eV, and the peak of Cu 2 O/ZnO at 531.2 eV are more complex and may correspond to oxygen deficiencies and surface-adsorbed oxygen. The binding energy of the oxygen in the Cu 2 O/ZnO lattice is higher and lower than the binding energy of the oxygen in the ZnO and Cu 2 O lattice, which illustrates that the reaction between lattice O and Zn has become weaker and the one between lattice O and Cu has become stronger. This is obvious evidence of lattice interaction.
The UV-vis spectra of Cu 2 O/ZnO prepared with different concentrations of Cu 2 O are shown in Figure 6. UV-vis spectra are studied to compare to pure ZnO and The relationship between degradation efficiency, time, and doping ratio under visible light is shown in Figure 7A. The corresponding kinetic plots are shown in Figure 7B. In Figure 7, all Cu 2 O/ZnO catalysts have better photocatalytic activity than pure ZnO and Cu 2 O in visible light irradiation. ZnO has nearly no degradation effect on  the visible region of λ > 420 nm. This may be due to the adsorption of ZnO on the MO. On the contrary, the number of defective sites is the most important factor leading to the result, such as oxygen vacancy and interstitial zinc atom, and to the acceptor states that arise from zinc vacancies and interstitial oxygen atoms [29]. The degradation rate of Cu 2 O is 30% in 120 min, but it does not have extinct change after 120 min, which may ascribe to the recombination of photoinduced electrons and holes [30].
The best ratio of Cu 2 O to ZnO is 0.78, which agrees with the result of UV-vis. This could be explained by the utilization of photons. The lower ratio did not have enough p-n heterojunctions and the higher ratio has caused the decrease of the interface area between Cu 2 O and photons. To describe quantitatively, we have used the kinetic model fitting the process of MO degradation. Results show that photocatalysis is first-order reaction kinetics: k II > k III > k IV > k I > k Cu2O > k ZnO , k II = 2.75k III . Thus, when the Cu 2 O concentration is 0.056 mol/l and the MO concentration is 50 mg/l, the photocatalytic degradation of the prepared Cu 2 O/ZnO catalyst is the best, and MO has been completely degraded at 180 min. The UV-vis spectrum of the photodegradation efficiency of MO by Cu 2 O/ZnO (sample II) with degradation time is shown in Figure 8. The curve [31] demonstrates that the absorption of MO gradually decreases to 0 as the reaction progresses, which indicates that MO has been completely degraded at 3 h. Figure 9 shows the degradation efficiency of the initial concentrations of different MO with Cu 2 O/ZnO (sample II). The curve reveals that the initial concentration of MO has a great effect on the photodegradation. To describe quantitatively, we have used the kinetic model fitting the process of MO degradation. Results show that k 50 > k 100 > k 200 > k 300 , k 50 = 2.79k 100 .
Thus, as the initial concentration increases from 50 to 300 mg/l, the rate of photodegradation gradually decreases, because the larger MO concentration causes the heavier solution color; thus, light transmittance is affected. Furthermore, MO and its intermediates have occupied the active sites of the Cu 2 O/ZnO catalyst, which decreases the reactivity.
The degradation efficiency of 100 mg/l MO with Cu 2 O/ZnO (sample II) in different pH is shown in Figure 10. The pH of the solution has a great effect on the photodegradation efficiency. To describe quantitatively, we have used the kinetic model fitting the process of MO degradation. Results show that k 3.8 > k 4.8 > k 6.8 > k 5.8 > k 2.8 , k 3.8 = 1.89k 4.8 , With the higher pH of the solution, the degradation rate sharply increases first and then decreases gradually. When the pH of solution is equal to 3.8, the reaction speed is the highest.
Thus, the photocatalytic degradation of pH (=3.8) is the best when the catalyst is sample II, and the MO concentration is 100 mg/l and the largest degradation efficiency is 98%.    It is well known that benzoquinone (BQ) and isopropyl alcohol (IPA) can be used to remove ·O 2− and ·OH, respectively. EDTA is an effective scavenger for photoinduced holes (h + ) [32][33][34].
In Figure 12, EDTA and IPA have a stronger inhibitory effect on the photodegradation of MO with Cu 2 O/ZnO than BQ, which indicates that h + and ·OH are the main active groups. To describe quantitatively, we have used the kinetic model fitting the process of MO degradation. Results show that k normal > k BQ > k EDTA > k IPA , and BQ, EDTA, and IPA decrease normal degradation rates by 39.98%, 66.73%, and 80.36%, respectively.    According to the results of this trapping experiment, the possible process of MO photodegradation with the Cu 2 O/ZnO catalyst can be proposed by the following equations [35]: Theoretical analysis shows that the main reason for the enhanced photocatalytic activity of Cu 2 O/ZnO can be attributed to the absorption band edge of ZnO and Cu 2 O and the heterogeneous structure formed between them. Figure 13 shows the diagram of the absorption band edge of ZnO Figure 13: Diagram of the absorption band edge of ZnO and Cu 2 O and the charge transfer under visible light irradiation. −1.6 and +0.4 eV, respectively, and Cu 2 O is more negative than ZnO (the conduction band and valence band of ZnO are −0.3 and +2.9 eV, respectively), which result in the generation of electrons from the Cu 2 O conduction band transferred to the ZnO conduction band. At the same time, the p-n heterogeneous structure is formed by the contact between the n-type semiconductor ZnO and the p-type semiconductor Cu 2 O, which further improves the transmission of the photogenerated electron-hole pairs by the difference of potential energy.
The PL spectra results of pure ZnO and Cu 2 O/ZnO (sample II) are shown in Figure 14. With the doping of Cu 2 O, the peaks in the range of 350-400 nm shifted to visible range, which demonstrates the decreasing of bandgap. In the range of 400-470 nm, compared to pure ZnO, a weaker peak can be observed in Cu 2 O/ZnO (sample II), which indicates that the charge transfer between ZnO and Cu 2 O would suppress the recombination of electron-hole pairs [27]. The peaks at about 520 nm can be attributed to vacancy and defect of O and the addition of Cu 2 O increased the vacancies and defects. The vacancy and defect also can inhibit the recombination of electron-hole pairs. Thus, the PL spectra are consistent with the previous mechanism.

Conclusions
Cu 2 O/ZnO is prepared by precipitation and calcination. ZnO covers the surface of spherical Cu 2 O, and the structures of Cu 2 O and ZnO are not deformed during the doping process. There is a strong interaction existing between Cu 2 O and ZnO. The visible light photocatalytic ability is obviously enhanced because of the presence of lattice interaction. When the ratio of Cu 2 O to ZnO is 0.78, the degradation of MO is the best. The maximum degradation efficiency is 98%, and Cu 2 O/ZnO has good repeatability. h + and ·OH are the main active groups in the process of degradation. This concludes that the lattice interaction can enhance the photocatalytic ability.