b. Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, China
Metal oxide semiconductors with micro-nano structure, such as TiO2, ZnO, and Cu2O, Co3O4 particles, can act as common photocatalytic materials to degrade organic pollutants into water, small molecules, and CO2 under the irradiation of ultraviolet or visible light [1-7]. These materials are generally photo-chemically stable and nontoxic. This technology not only can reduce environmental pollutants effectively but also making full use of solar energy, which is in line with the development trend of green chemistry, and has been widely focused on. However, large gaps between the semiconductor metal atoms in the valence band and conduction band limit the absorbed sunlight in the visible light, leading to the low efficiency of solar light and thus reducing the ability to degrade dye pollutants in the past decades [8, 9].
To effectively utilize solar light, noble metal nanoparticles, such as gold and silver, are often decorated on the surface of the semiconductor metal oxide to form a composite material . When the composite material is irradiated by the light, the metal semiconductor's electrons are driven from the valence band to the conduction band. These electrons can promptly leave the semiconductor surface, with the noble metal nanoparticles capturing them and effectively preventing the electrons from falling again to the valence band. Therefore, the photocatalytic degradation properties of this composite material under sunlight are increased significantly [11-13]. Lu and co-workers synthesized a Au nuclear Cu2O composite for photocatalytic degradation of methyl orange under visible light using the UV-Vis spectrum . Lu et al. synthesized Ag/ZnO hollow microspheres and studied its catalytic properties. These results demonstrated that Ag nanoparticles can not only facilitate the light given to the electronic principle hole but also increase the O2 adsorption, produce more active hydroxyl groups, and increase the catalytic activity .
In the last decade, noble metal nanoparticles of gold and silver were commonly used in the surface enhanced Raman spectroscopy (SERS) and their effect on the Raman signal enhancement of the probe molecules was obvious [16, 17]. Liu et al. synthesized a highly controllable morphology of spiny Au particles and Au/Ag bimetallic nanoparticles by reducing a gold solution with sodium citrate and different amounts of silver nitrate . The highly ordered Ag nanowire was prepared by using alumina as a template and the high SERS activity was measured by Sun group . The noble metal nanoparticles, which are modified on the surface of the template, are not easy to agglomerate and the gap between the nanoparticles can be adjusted by the experimental parameters, so the stability and sensitivity of the nanoparticles can be improved . Our group has performed some studies on the assembly of noble metal nanoparticles on the template [21, 22].
In this work, the Cu2O micro-nano materials with polyhedral morphology are designed as the template, which is modified in situ with silver nanoparticles by reducing silver nitrate and the density of silver nanoparticles can be adjusted by adding different amounts of silver nitrate. The obtained Cu2O/Ag composites have the foremost advantage of adsorption performance of a polyhedral Cu2O crystal that has a large specific surface area. Second, the aggregation of silver nanoparticles can be avoided by modifying them on the surface of the template. Furthermore, the density of silver can also be controlled, all of which can allow this composite material to detect high molecular signals as a SERS substrate. Finally, Cu2O and Ag nanoparticles both have catalytic activity in the composite and the absorption efficiency of the visible light is higher because of their synergistic effects. The study provides new, efficient, and catalytic micro/nano materials using sunlight directly; more importantly the photocatalytic degradation process can be monitored in situ by using the SERS technique, which will provide some experimental basis on the research of the photocatalytic mechanism.Ⅱ. EXPERIMENTS A. Reagents and apparatus
All chemicals used were analytical-reagent grade and used as received. Copper sulfate pentahydrate (CuSO4·5H2O), silver nitrate, glucose, ethanol, anhydrous sodium carbonate, sodium citrate (Na3C6H5O7), polyvinylpyrrolidone (PVP), 4-mercaptopyridine (4-Mpy), malachite green (MG), and crystal violet (CV) were purchased from Shanghai Chemical Reagent Ltd. Co. of China. All solutions were prepared using ultrapure water (resistance > 8 MΩ·cm).
Scanning electron microscopy (SEM) images were obtained using a SU8000 field emission scanning electron microscope. X-ray diffraction (XRD) images were obtained using a Bruker D8 Advance X-ray diffractometer. Raman spectra images were generated using a LabRAM HR800 confocal microscope Raman system (Horiba Jobin Yvon). Using a 50 × microscope objective, the laser beam focused on a sample that was approximately 1 μm in size. The laser power was approximately 1 mW and the recording time was 1 s for each spectrum.B. Sample preparation 1. Cu2O particles
For the synthesis of Cu2O particles, 0.68 g CuSO4·5H2O was completely dissolved in 76 mL of deionized water, followed by injecting 4 mL of a sodium mixture solution (0.74 mol/L sodium citrate and 1.2 mol/L sodium carbonate mixed solution) into the glass vial with vigorous stirring. The color of the solution changed from the light blue to dark blue transparent. Finally, after the addition of 3 g of PVP and 4 mL of a 1.4 mol/L glucose solution, the reaction was proceeded for 15 min at 80 ℃ to be completed and cooled down to room temperature under stirring.2. Cu2O/Ag composites
The decorating of Cu2O with Ag nanoparticles was based on the following method. A volume of 21 mL of the above Cu2O reaction liquid, without centrifugation and washing after cooling down to room temperature, was added into a beaker. Different aliquot of a AgNO3 solution (8 mmol/L) (1.0, 1.2, and 1.4 mL) were slowly dripped into the reaction liquid under vigorous magnetic stirring, and the reaction liquid color changed from brown to dark gray. The mixture was stirred for 20 min to complete the reaction. The product was collected by filtration using nanofiltration membrane and washed with distilled water and ethanol for three times. Finally, filtered samples were dried in an oven for 30 min at 50 ℃, the obtained composites were named as Cu2O/Ag (1-3), and dispersed in distilled water in accordance with the proportion of 1:20.C. Samples detection 1. SERS sensitivity detection of composite materials
SERS sensitivity detection of composite materials were recorded for 4-Mpy and MG. First, 20 μL of Cu2O/Ag-2 composites were added into 20 μL of 4-Mpy at five different concentrations until each reached a final concentration of 10-5, 10-6, 10-7, 10-8, 10-9 mol/L. Then, 10 μL of the corresponding suspensions was cast on a silicon slide, after which each sample was collected at a 532 nm excitation. Using the same method, SERS measurements for MG at different concentrations were obtained.2. Visible light photocatalytic degradation properties for in situ SERS analysis
The photocatalytic degradation of the Cu2O/Ag-2 composite for CV was monitored by SERS in situ. A final mixture concentration of 10-5 mol/L CV in distilled water and 2 × 10-5 mol/L in real water sample with Cu2O/Ag-2 composite was obtained by using the same method as above. Then, 20 μL of the corresponding suspensions was cast on a silicon slide, after which, the Raman signal was continuously collected at 532 nm excitation.Ⅲ. Results and discussion
The formation of Cu2O and Cu2O/Ag composites were characterized by SEM. Figure 1(a) exhibits the Cu2O crystal template which indicates they are polyhedron and the size is estimated to be 1 μm. When adding different amounts of AgNO3 solution (1.0, 1.2, and 1.4 mL) into the template, the obtained Cu2O/Ag (1-3) composites are shown in Fig. 1(b-d), in which the size of the Ag nanoparticles is approximately 60 nm. By changing the amount of AgNO3 solution, the distribution density of the Ag nanoparticles modified on the surface of the template can be achieved and the original morphology of the Cu2O crystal template is not destroyed during the reduction. This allows the distribution of Ag on the Cu2O/Ag-2 composite material to be the most uniform as shown in their high-magnification images on the top right corner of Fig. 1(b-d), respectively, so only the Cu2O/Ag-2 composites are used in the following detection without special instructions.
XRD was carried out to determine the phases present in the Cu2O crystal and Cu2O/Ag composite. The corresponding diffraction patterns are displayed in Fig. 2. The formation of a cubic phase Cu2O is confirmed (Fig. 2(a)) by the presence of characteristic peaks at 2θ values of 29.55°, 36.4°, 42.3°, 61.4°, and 73.55° associated with (110), (111), (200), (220), and (311) planes, respectively (JCPDS No.05-0667). After Ag NPs are coated on the Cu2O, the peaks of Cu2O become a little weak and new sharp peaks, attributed to Ag NPs, appear. These new diffraction peaks are located at 2θ=38.0°, 44.3°, 64.4°, 77.4°, which can be indexed to the (111), (200), (220), and (311) planes of the face-centered cubic structure of silver (JCPDS No.04-0783) (Fig. 2(b)) [23, 24]. These results indicate that Ag NPs have been coated on the surface of the Cu2O template. Additionally, there were no other miscellaneous peaks, which showed that the composite was of high purity.
To further confirm the presence of Ag nanoparticles on the surface of the Cu2O template, we analyzed the Cu2O/Ag composite by electron mapping image analysis (Fig. 3). The images were acquired by visualizing the inelastically scattered electrons in the energy loss windows for elemental O, Cu, and Ag. The different color areas shown in Fig. 3(b-d) represent O, Cu, and Ag enriched areas of the sample, respectively, which indicate the presence of Ag in the outer surface of the Cu2O template. The images also show that the Ag is well dispersed on the surface of the Cu2O template.
In this work, our aim is to have optimum photocatalytic materials for the degradation of organic pollutants under visible light. Furthermore, the photocatalytic degradation process can be detected by the SERS technique, so SERS enhanced activity for this type of optimum photocatalytic materials is important. To evaluate the potential application of the as-obtained Cu2O/Ag composite as a SERS substrate, 4-Mpy was chosen as the probe molecule. Figure 4(a) reveals the series of SERS spectra of 4-Mpy of 10-5, 10-6, 10-7, 10-8, 10-9 mol/L with Cu2O/Ag as the substrates. The results clearly show that the determination capability of our SERS probe was below 10-9 mol/L, that is, the Cu2O/Ag exhibits good SERS activity and sensitivity as a SERS substrate. The primary vibrations of 4-Mpy conformed the results in literature. Generally, the strong peaks located at 1587 cm-1 can be attributed to the ring stretch mode of the 4-Mpy molecule. The peak at 1217 cm-1 is attributed to the CH deformation and NH stretching modes, the peak at 1100 cm-1 is assigned to the X-sensitive and 1062 cm-1 is assigned to the CH deformation modes, respectively. All the above bands are similar to those in the SERS of 4-Mpy adsorbed on the Ag mirror. Lastly, 1030 cm-1 is attributed to the SERS of 4-Mpy adsorbed on the bare Cu2O template surface [25, 26]. Therefore, we can assume that chemical enhancement is the second main factor that enhanced the SERS signals, other than electromagnetic enhancement mechanism. In addition to 4-Mpy, the Cu2O/Ag composite has also been proven as an effective SERS substrate for the detection of MG as shown in Fig. 4(b) .
To further evaluate the SERS activity of this composite, a quantization relationship between the relative signal intensity of Raman and the concentration. Figure 5(a) illustrated the corresponding plot of ISERS versus-log[4-Mpy], in which ISERS is the SERS intensity recorded for the band at 1587 cm-1 which is one of the strongest bands in the 4-Mpy spectrum. The results show that the data can be fitted by a linear plot and the limit of detection (R6G) is about 10-9 mol/L, which is considered as the lowest concentration leading the SERS intensity of the marker band at 1587 cm-1. The relative standard deviation (RSD) of major peaks is often used to estimate the reproducibility of SERS signals. Figure 5 (b) and (c) show the RSD of the integrated Raman intensity of 10-6 mol/L 4-Mpy at 1217 and 1587 cm-1 collected on 20 randomly selected places of Cu2O/Ag particles substrate are 0.1466 and 0.1525, respectively, which clearly reveals the high reproducibility of the substrate. 4-Mpy was also chosen as the probe molecule to evaluate the intensity of the as-obtained Cu2O/Ag-1 and Cu2O/Ag-3 composites as a SERS substrate. The results show the limit of detection (LOD) were both only 10-8 mol/L, furthermore the integrated Raman intensity of 10-6 mol/L 4-Mpy at 1217 cm-1 collected on 20 randomly selected places of Cu2O/Ag-1 and particles Cu2O/Ag-3 substrate are 0.3005 and 0.3526, respectively. That indicates SERS activity and reproducibility of Cu2O/Ag-2 are higher than Cu2O/Ag-1 and particles Cu2O/Ag-3 which are quite in accord with the SEM images (SERS spectra here are not given).
We evaluated the effect of the photocatalytic activity of Cu2O/Ag composites by photocatalytic degradation of a CV (10-5 mol/L) aqueous solution under visible-light irradiation . The changes in the concentration of CV were monitored by examining the variations in the SERS absorption. Figure 6(a) shows SERS spectra changes of CV (10-5) mol/L over the Cu2O/Ag composite. With the passage of time, the Raman signal faded quickly, which means the reaction speed of CV degradation on the surface of the composite is rapid, and the concentration was reduced to the SERS detection limit (about < 10-9 mol/L) at approximately 250 s. This indicates that the degradation of CV is nearly completed. The other parallel experiment was carried out under the same conditions but the duration was only 350 s, as shown in Fig. 6(b). The degradation reaction speed of CV is similar to the above experiment and after approximately 300 s, a wide peak in the 1250 cm-1 to 1500 cm-1 appears, which may be resulted from the formation of amorphous carbon after the extended laser irradiation time on the surface of the Cu2O/Ag composite material. The detection results show that the composite material has a high efficiency for the degradation of CV under visible light irradiation in a short amount of time.
To evaluate its practical photocatalytic ability, the Cu2O/Ag composite was applied to detect CV in real water samples by the standard addition method . The water samples, which were collected from Dongpu Reservoir of Hefei without any pretreatment except filtration, were added with CV standard sample and the finial concentrations of CV was 2 × 10-5 mol/L. Despite of the complexity of the real samples, the photocatalytic degradation of 2 × 10-5 mol/L CV was monitored, the results are shown in Fig. 7. It indicates that the Cu2O/Ag composites have a higher photocatalytic ability in the determination of CV in real water samples.
Photo-catalytic degradation of CV in situ and rapid detection with the Cu2O/Ag composites as SERS substrate, using a laser at 532 nm, were accomplished. We speculate there are three main reasons. First, the mechanism was proposed to be the formation of hydroxyl radical species (·OH) as shown in Fig. 8. The electron of the Cu2O on the electronic band is excited into the conduction band under the irradiation of visible light. Then, O2-· are generated because the excited electrons are captured by the absorbed O2, and ·OH are generated by the surface hydroxyl trapping the corresponding holes. On the other hand, the interaction between the above generated O2-· and the absorbed H2O will further generate ·OH. It was known that the final ·OH radicals are able to oxidize pollutants because of their high oxidative capacity (the reduction potential of ·OH is approximately 2.8 V) [30, 31]. Second, the mechanism of the photocatalytic activity in the metal-semiconductor of the Cu2O/Ag composite lies mainly in the transfer of an electron and hole, rapidly and effectively. Ag, with a high value of electron work function, can make the electron transfer process from the conducting band of the Cu2O to Ag more rapidly, meanwhile, effectively preventing the electrons from falling back into the valence band of Cu2O . The charge separation in turn prevents the recombination of electrons and holes and thus, enhances the photocatalytic activity of Cu2O . Third, the charge redistribution results in positively charged Ag and negatively charged Cu2O with the highest charge density region located adjacent to the junction, which may induce a larger electromagnetic field, so that the Cu2O/Ag composite can act as a high sensitivity SERS substrate [24, 34].Ⅳ. CONCLUSION
In summary, we have developed a facile, successive, one-step procedure for producing tunable Ag NPs on Cu2O nanoframes to obtain a Cu2O/Ag composite with highly sensitive SERS signals and an excellent photo-catalytic degradation material for organic pollution. The results show that the composite material has a highly efficient degradation ability for organic pollution, such as CV, under visible light irradiation in a short amount of time, in which the photocatalytic degradation process was monitored in situ by the SERS technique. We believe that such a strategy can not only effectively utilize solar light to degrade organic pollution but also could be extended to other systems, such as the mechanistic study of degrading organic pollution by an in situ SERS technique.Ⅴ. ACKNOWLEDGMENTS
This work was supported by the Key Projects of Natural Science Research of Universities in Anhui Province (No.KJ2015A183, No.KJ2015A201) and Talents Foundation of Hefei University (No.15RC05), Anhui Province Natural Science Foundation (No.1608085MD78), the Key Projects of Anhui Province University Outstanding Youth Talent Support Program (gxyqZD2016274), the National Natural Science Foundation of China (No.21305142, No.51403048).
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b. 中国科学院智能机械研究所, 合肥 230031