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Abstract
Three kinds of reusable surface enhanced Raman scattering (SERS) substrates based on Ag-TiO2-G, Ag-G-TiO2 and G-Ag-TiO2 hybrid structures were prepared by a combination method of simple sol-gel self-assembly and annealing. The composites were confirmed by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). Raman mapping spectra of the three hybrids were analyzed in order to study the difference of charge transfer between Ag, TiO2 and graphene. In addition, rhodamine 6G (R6G) was used as the probe analyte, and the SERS activity of the three structures decreased orderly in the sequence of Ag-G-TiO2, G-Ag-TiO2, and Ag-TiO2-G, with the enhancement factor of about 106, which was due to the high electromagnetic enhancement of the Ag nanoparticles (AgNPs). The photo-catalytic properties of the three kinds of composites were experimentally studied via Raman mapping measurements through UV irradiation. The photocatalytic ability decreased also orderly in the sequence of Ag-G-TiO2, G-Ag-TiO2, and Ag-TiO2-G, and the reason was discussed in details.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface-enhanced Raman scattering (SERS) has attracted considerable attention since its discovery in 1974, which is a powerful tool for ultrasensitive, single-level, and real-time detection [1–3]. Traditionally used SERS substrates are Au, Ag and Cu in the formation with rough surfaces or nanostructures, in which the surface roughness or hot spots in nanostructures produce an enhancement of the electromagnetic field through a laser excited localized surface plasmon resonance [4–6]. This enhancement is known as electromagnetic enhancement (EM), which is proportional to fourth power of the intensity of the electromagnetic field. Besides EM, the chemical enhancement (CM) also contributes to SERS effect, which comes from charge transfer between analyst molecule and the substrate [6–8].
However, one of the most restrictive limitations is that frequently the SERS substrates made of noble metals are discarded after a single detection [4,7,8], which would result in wasting resources and hindering its commercial application aspects of SERS. Therefore, from the viewpoint of practical application, developing a SERS substrate with high sensitivity, stability, uniformity, reproducibility, recyclability and especially low cost, plays vital significant for the extension of SERS research.
Among semiconductor materials, TiO2 has been widely studied due to its good physical function, chemical stability and high photo-catalytic activity compared to ZnO or other oxide semiconductors, and it has been extensively used as a photo-catalyst on the degradation of organic pollutants [9,10]. TiO2 stands out for its high photo-catalytic activity and remarkably chemical stability [11,12]. Nevertheless, in practical applications, TiO2 still has several drawbacks [13], such as a weak light efficiency and a high-recombination rate of photo-generated electron-hole pairs.
Graphene is a good candidate for scavenging photo-generated electrons mainly according to its two-dimensional planar structure [14–18]. Graphene-TiO2 hybrid materials have been studied in applications of highly sensitive photodetector [19], thin film perovskite solar cells [20], plasmonic photocatalytic reaction [21,22], and photocatalytic properties as reusable Raman measurement substrates [23]. Especially, in our previous work [23], we have discussed the photocatalytic performance of graphene-TiO2 (graphene on the top layer) and TiO2-graphene (TiO2 on the top layer) with different preparation methods. For a purpose of SERS applications using metal nanoparticles/TiO2/graphene (MNPs/TiO2/G), it is natural to raise some questions. Firstly, due to the properties of TiO2, are MNPs/TiO2/G composites similar in improving the photocatalytic performance when we prepare these composites using different assemble methods? Secondly, are these MNPs/TiO2/G composites similar in Raman enhancement properties?
Bearing these two questions mentioned above in mind, a deep study on MNPs/TiO2/G composites has been carried out in this paper. Firstly, taking Ag nanoparticles (AgNPs) as example, we have synthesized three types of Ag/TiO2/G with different graphene positions, using the sol-gel and additional thermal approach to ensure the interfacial contact between different layers. Secondly, their characterization, photocatalytic performance and Raman enhancement properties have been investigated in details. Thirdly, the corresponding theoretical and numerical analysis has been discussed. This work could be more effective understanding on the interfacial function between graphene and AgNPs/TiO2, and on reusable SERS substrates using Ag/TiO2/G composites.
2. Experimental
2.1. Materials
Graphene was grown on a copper foil (Alfa Aesar, 99.8%) in ethylene (C2H4) with a flow rate of 5 sccm as the carbon source, under a temperature of 1000 °C for 10 min, and transferred to a conductive glass substrate using a sacrificial PMMA supported layer [24]. A TiO2 solution was prepared by dispersing 1 mg of commercial TiO2 powder (Nanostructured and Amorphous Materials, Inc.) in 10 mL deionized water [23]. A Ag sol solution was prepared by reducing silver nitrate (AgNO3) via sodium citrate [25,26].
2.2. Sample preparation
Preparation of TiO2/G substrates: The steps of the sample preparation are shown in Fig. 1. Firstly, a TiO2 solution was orderly dealt with an ultrasonic treatment, multiple centrifuged, and filtered to remove large NPs agglomerates. Secondly, a 5 µL drop was dispensed onto a conductive glass or graphene-conductive glass support, then dried to form TiO2 substrate or TiO2-G substrate (graphene on the lower layer). Thirdly, another graphene transferred to the as-prepared TiO2 substrate to form G-TiO2 substrate (graphene on the upper layer).
Preparation of Ag-TiO2-G and Ag-G-TiO2 substrates: The previously prepared TiO2-G, G-TiO2 and TiO2 substrates were immersed in the Ag sol solution for 12 hours respectively, then washed with deionized water three times to remove the non-adsorbed AgNPs on the surface of the substrates, and naturally dried to obtain the Ag-TiO2-G, Ag-G-TiO2 and Ag-TiO2 substrates.
Preparation of G-Ag-TiO2 substrate: First, the graphene was transferred onto the prepared Ag-TiO2 substrate by wet transfer method, the specific transfer steps refer to our previous study [24]. Second, an additional annealing is used to keep a better interfacial contact of graphene and AgNPs, specifically, the prepared sample was put into a vacuum quartz tube and heated to 400 °C for 20 min, H2 (20 sccm) and Ar (40 sccm) were introduced continuous during annealing process with temperature increasing at 10 °C/min, and the quartz tube was cooled down to room temperature, and we obtained the G-Ag-TiO2 substrate.
2.3. Photo-catalytic experiments
The photo-degradation of rhodamine 6G (R6G) was carried out to evaluate the photo-catalytic activity of three photo-catalyst substrates (Ag-G-TiO2, Ag-TiO2-G and G-Ag-TiO2). There were four steps, shown in Fig. 2. Firstly, the photo-catalyst substrates (areas of 10 × 10 mm2) were immersed in a R6G aqueous solution with a concentration of 10−6 mol/L and kept in a black box for 1 h to establish an adsorption-desorption equilibrium. Secondly, they were carried on with Raman mapping test, with a mapping region of 160 × 160 μm2 and a 10 μm scanning interval. Thirdly, they were exposed to an ultraviolet light (UV-light). A 500 W Xenon light source (Solar 500, NbeT) with a KenKo L41 UV filter (λ > 410 nm) was used as an UV-light source and fixed 10 cm away from the reaction system. Fourthly, Raman mapping measurements were again taken at the desired time intervals to achieve Raman intensity of R6G at each instant of photo-catalytic time. The SERS intensity of R6G was used to determine the concentration of residual dye and evaluate the photo-catalytic efficiency of each sample.
Fig. 2 Experimental steps of photo-catalytic performance of R6G solution on three kinds of Ag/TiO2/G composite substrates.
2.4. Instruments and measurements
The morphology of samples was characterized by transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN) and energy dispersive spectrometer (EDS, Oxford Instruments 150). Raman mapping experiments were taken with a laser confocal Raman spectrometer (Horiba JY LabRAM HR Evolution) equipped with a 100 × (0.9 NA) objective (laser focus spot diameter of ~0.72 µm), and an air cooled double-frequency Nd:Yag green laser (λ = 532 nm, 50 mW with a 10% neutral density filter). An integration time of 2 s was used in all Raman measurements to prevent hot damage.
3. Results and discussion
3.1. Characterization of three samples
Figure 3(a1) shows the TEM and high-resolution TEM (HRTEM) image of Ag nanoparticles, from the HRTEM image in the inset of Fig. 3(a1), the diameter of AgNPs is ~30 nm, the measured fringe lattice of AgNPs is ~0.24 nm, which corresponds to the (111) crystal plane. Figure 3(a2) shows the selected area electron diffraction (SAED) image of a single AgNP, which indicates SAED patterns with rings and an inter-planar distance corresponding to the fcc phase of Ag. Figure 3(b1) shows a HRTEM image of TiO2 nanoparticle, from the HRTEM image in the inset of Fig. 3(b1), the measured fringe lattice of TiO2 is 0.352 nm, indexed to (101) crystal plane. The corresponding SAED pattern (in Fig. 3(b2)) is indexed to the (101), (004), (200) planes of TiO2, respectively. Figure 3(c1) shows a Raman spectrum of graphene, the inset image is a picture of graphene on a Cu foil, from which we know the quality and layers of graphene by checking the intensity of D band (~1348 cm−1), the ratio of the intensity of 2D (~2683 cm−1) and G (~1585 cm−1). The calculated ratio of I2D/IG is about 0.73, corresponding to two or three layers [27]. Meanwhile, an absorption of graphene is also given in Fig. 3(c2), and the optical microscope image of graphene on SiO2 substrate is inserted in Fig. 3(c2).
Fig. 3 (a1) TEM image of AgNPs, the inset is an enlarged image; (a2) the corresponding SAED patterns; (b1) TEM image of TiO2 nanoparticles, the inset is an enlarged image; (b2) the corresponding SAED patterns; (c1) the Raman spectrum of graphene, the inset is the picture of graphene sample on a Cu foil; (c2) the absorption of graphene, the inset is the optical microscope image of graphene on the SiO2 substrate.
Further, the presence of three Ag/TiO2/G composites was confirmed by the energy-dispersive X-ray spectroscopy (EDS) as shown in Fig. 4, including the element stratification, general area spectra of elements concentration, and the distribution diagram of elements for Ag, C and Ti, respectively. The EDS results show that the main components of Ag/TiO2/G composites are Ti, C and Ag, where the Ti signal originates mainly from TiO2 nanoparticles, the C signal is derived from graphene, and the Ag signal confirms the existence of Ag nanoparticles. Apart from these elements, O and Si peaks have also emerged. In addition, for different structures, the content of three elements of Ti, C and Ag are different. Specifically, the content of Ti decreases orderly in the sequence of Ag-TiO2-G, Ag-G-TiO2, G-Ag-TiO2, and the corresponding contents are 47.3 wt%, 30.7 wt% and 29.6 wt%, respectively. While the content of C decreases orderly in the sequence of G-Ag-TiO2, Ag-G-TiO2, Ag-TiO2-G, and Ag content decreases by the order of Ag-G-TiO2, G-Ag-TiO2 and Ag-TiO2-G.
Fig. 4 Energy dispersive spectrum analysis results of samples (a) G-Ag-TiO2, (b) Ag-G-TiO2, (c) Ag-TiO2-G, and the distribution diagram of element for Ag, C and Ti is given out accordingly.
3.2. Comparison analysis of the Raman enhancement performance
In order to comparatively study the Raman enhancement performance of the three kinds of Ag/TiO2/G composites, Raman mapping analysis was used to obtain the averaged Raman spectra of each sample. R6G (as probe molecule with a concentration of 10−6 mol/L) Raman mapping was carried out to evaluate the Raman enhancement performance of each substrate.
A. Comparative analysis of Raman spectroscopy of graphene: The averaged Raman spectra of the three composite substrates were shown in Fig. 5(a). The Raman spectra of Ag/TiO2/G composites not only contained a typical Raman peak of TiO2 at 140 cm−1, but also three peaks (D, G and 2D peaks) of graphene. At 140 cm−1, the Raman intensity of Ag-TiO2-G substrate was higher than that of G-Ag-TiO2 and Ag-G-TiO2 substrates, which could be that Ag nanoparticles and graphene deposited or transferred on the top of TiO2 nanostructures reduced the intensity of the excitation light reaching the TiO2 nanoparticles, thereby diminished the Raman scattering intensity of TiO2 [28–30]. In addition, the lowest intensity of D band and largest intensity ratio of 2D to G band indicated the good quality and low doping level of graphene on G-Ag-TiO2 substrate. Meanwhile, the Raman intensity of 2D band of graphene on G-Ag-TiO2 substrate was higher than that on Ag-G-TiO2. There are two reasons: one is that graphene located on the top of the nanostructure allowed the excitation light irradiating onto graphene without any hindrance, the other is the interaction between graphene and Ag nanoparticles.
Fig. 5 The averaged Raman spectra of (a) three different samples at range of 100 to 3000 cm−1, and (b) six different samples at range of 1200 to 2800 cm−1.
For further study Raman spectra of graphene, we also took Raman mapping of all samples in our preparation process, including G-TiO2, TiO2-G, pure graphene, and Ag/TiO2/G composites. The averaged Raman spectra were shown in Fig. 5(b), and the details of the band position and intensity of Raman characteristic peaks were calculated and given in Table 1. We could see that the intensity of G band on three kinds of Ag/TiO2/G composite substrates was enhanced significantly compared with that on G-TiO2, TiO2-G and graphene substrates, and the intensity of 2D band on both Ag-G-TiO2 and Ag-TiO2-G substrates was reduced. In the absence of Ag, the ratio value of I2D to IG (I2D/IG) was 1.53, 1.42 and 0.91 for graphene, G-TiO2 and TiO2-G, respectively. Meanwhile, the value of I2D/IG was 0.64, 0.17 and 0.24 for G-Ag-TiO2, Ag-G-TiO2 and Ag-TiO2-G, respectively. For Ag-G-TiO2 and Ag-TiO2-G substrates, the value of I2D/IG is 0.17 and 0.24, respectively, the reasons could be: (1) the presence of Ag nanoparticles causes the background Raman signal of the substrate itself increase, and the superimposition of the background peak and the G peak causes the G peak intensity to be significantly enhanced; (2) the introduction of Ag nanoparticles causes the electromagnetic enhancement of Ag-G-TiO2 is stronger than that of Ag-TiO2-G structure, which is confirmed in the simulation section. For G-Ag-TiO2 substrate, the value of I2D/IG is 0.64, which is higher than that of Ag-G-TiO2 and Ag-TiO2-G. As a result of the annealing process, the AgNPs and graphene fit more closely, and making the 2D peak intensity of graphene on G-Ag-TiO2 is stronger, in addition, the introduction of Ag and TiO2 nanoparticles that cause both types of doping of graphene. Therefore, the value of I2D/IG of graphene on different substrates is different.
Table 1. Details of the position and the intensity of Raman characteristic peaks for graphene and TiO2 (140 cm−1) on different substrates
B. Comparative analysis of Raman spectroscopy of TiO2: The averaged Raman spectra of these composites at the range of 100-800 cm−1 were shown in Fig. 6. We selected TiO2, TiO2-G and Ag-TiO2 substrates as reference substrates to characterize the effect of Ag nanoparticles and graphene on the Raman spectra of TiO2. We analyzed as follows:
- (1) Compared with pure TiO2 substrate, a band shift of TiO2 Raman peak at 140 cm−1 of other composites samples was observed. It may be due to the introduction of AgNPs and graphene, making AgNPs, graphene and TiO2 nanoparticles interact with each other. Meanwhile, with the change of the substrate structure, the level of Raman peak position is changed [31].
- (2) The intensity at 140 cm−1 gradually decreased by the order of TiO2-G, pure TiO2, Ag/TiO2/G, and Ag-TiO2. The graphene could cause an agglomeration of TiO2 nanoparticles [23], resulting in the enhancement of TiO2 Raman signal on TiO2-G substrate. For the three kinds of Ag/TiO2/G composites, the Raman signal of TiO2 was weakened, the reasons could be: on one hand, the interaction between Ag, TiO2 and graphene increased the TiO2 intensity, and on the other hand, AgNPs and graphene had an absorption effect on the incident light. The above two phenomena coexisted to determine the Raman intensity of TiO2 [32].
C. Comparative analysis of Raman spectrum of R6G: The Raman mapping spectra of R6G molecule on Ag-G-TiO2, G-Ag-TiO2 and Ag-TiO2-G were shown in Figs. 7(a), 7(b) and 7(c), respectively. The averaged spectra were given in Fig. 7(d). The averaged Raman spectra of Ag-TiO2 was also shown in order to be comparative discuss. Raman peaks of R6G molecule were seen clearly at 610, 772, 1182, 1311, 1362, 1510, 1574 and 1650 cm−1, and the detailed assignment of the Raman spectral features has been reported previously [33,34]. In our experiments, the relative intensities decreased orderly in the sequence of Ag-G-TiO2, G-Ag-TiO2, Ag-TiO2 and Ag-TiO2-G, which was highly consistent with the order of Ag content in EDS characterization, indicating that the electromagnetic enhancement of AgNPs directly affected the SERS activities. Specifically, based on a main Raman peak at ~1362 cm−1, the signals for Ag-G-TiO2, G-Ag-TiO2, Ag-TiO2 and Ag-TiO2-G are about 20699, 13213, 10465 and 7132 a.u., respectively, and the other positions and corresponding intensities of the R6G characteristic Raman peaks are given in Table 2.
Fig. 7 Raman mapping of R6G solution with a concentration of 1 × 10−6 mol/L on (a) Ag-G-TiO2, (b) G-Ag-TiO2 and (c) Ag-TiO2-G substrates, (d) the corresponding averaged Raman spectra.
Table 2. Raman characteristic peak intensity and position analysis results for R6G molecules on SERS substrates with different structures
To quantitatively further demonstrated the enhancement effect of our substrates, the representative peaks of R6G are selected to calculated the enhancement factor (EF) values by the formula [35]: EF = (ISERS/IRS)/(CSERS/CRS), where CSERS and ISERS are the concentration and the averaged intensity for the SERS measurement, respectively, CRS and IRS are the concentration and intensity for normal Raman measurement with 0.1 mol/L R6G solution. The calculated EF values of Ag/TiO2/G and Ag-TiO2 substrates are shown in Table 2, the EF values are different for different substrates and even different for different Raman peaks of the same substrate, and the EF values are mainly in the range of 105~106. There are some reasons to discuss the small variation of EF values. One is due to the different size, shape, agglomeration, and distribution of AgNPs in different substrates. Different size AgNPs lead to different absorption spectrum, thus leading to different enhancement effect. The different vibration mode (low-energy of high-energy mode) of R6G also attributes the different Raman intensity [36].
3.3. Comparative analysis of photo-catalytic efficiency
One of the main advantages of Ag/TiO2/G in comparison to other SERS substrates is their potential recyclability. In this case, the photocatalytic activity of TiO2 nanoparticles could be exploited to remove the organic molecules. In order to study the photocatalytic activity, R6G with a concentration of 10−6 mol/L was used as probe molecule. For better comparative analysis, all Raman intensity data are the averaged spectra from Raman mapping.
A. Analysis of photocatalytic experiments: The averaged Raman spectra of R6G molecule at the beginning time and after different irradiation UV light time of 10 min to 60 min at the 10 min interval were given in Figs. 8(a), 8(b) and 8(c) for samples Ag-G-TiO2, G-Ag-TiO2 and Ag-TiO2-G, respectively. Figure 8(d) showed the Raman characteristic peak intensity at 1650 cm−1 for R6G on the three samples. The fitting curves of the normalized Raman intensity at different UV irradiation time were shown in Fig. 8(d), and some specific data were listed in Table 3.
Fig. 8 The averaged Raman spectra of R6G on samples (a) Ag-G-TiO2, (b) G-Ag-TiO2 and (c) Ag-TiO2-G, at the beginning time and after different UV light illumination time; (d) the relationship between the normalized intensity at 1650 cm−1 and the illumination time, the fitting curves were also given out.
Table 3. Raman characteristic peak intensity at 1650 cm−1 on the three samples under different illumination time
The Raman intensity was gradually weakened with an increase of the UV irradiation time. When the UV irradiation time reached 50 min, the Raman characteristic peaks of R6G almost cannot be distinguished on samples Ag-G-TiO2 and G-Ag-TiO2, which meant that these two samples had a good photocatalytic degradation of R6G. While on Ag-TiO2-G substrate, the Raman characteristic peaks of R6G can still be detected. The slope of the fitting curve in Fig. 8(d) represented the photocatalytic rate of the sample, in the first 40 min with UV-irradiation, the photocatalytic ability decreased orderly in the sequence of Ag-G-TiO2, G-Ag-TiO2 and Ag-TiO2-G. In order to quantify the photocatalytic activity of the Ag/TiO2/G composites, the photocatalytic rate is calculated by the formula: -ln(I/I0), where I0 is the initial SERS intensity of a certain Raman peak, and I is the intensity of the corresponding Raman peak after different UV irradiation time, so the calculated photocatalytic rates are about 0.0371, 0.0301, 0.0111 min−1 for Ag-G-TiO2, G-Ag-TiO2 and Ag-TiO2-G at a R6G Raman peak of 1650 cm−1 respectively. The detailed analysis will be discussed in the later section B.
B. Photo-catalytic theory: Taking G-Ag-TiO2 composite structure as an example, we must pay more attention to two points. Firstly, a contact of the AgNPs and TiO2 resulted in the formation of a Schottky barrier [28]. With an irradiation, the surface plasmon resonance (SPR) of AgNPs can excite the electrons to overcome the Schottky barrier, thus to transfer from AgNPs to TiO2 [29,30,32]. These electrons after a series of reactions could bring the reactive oxygen species (ROS), which provided a strong oxidizing agent for the organic R6G degradation [28–30,32]. Secondly, after the contact of graphene and AgNPs, another part of the SPR excited electrons would transfer from AgNPs to graphene, because the work function of graphene (Wgraphene = 4.8 eV) is greater than that of Ag (WAg = 4.2 eV). The electrons can transfer to the electron acceptor (oxygen molecules), thus to form the reactive oxygen species. Note that the energy level structure of three samples are shown in Fig. 9. The detailed process and reaction can be given by the following discuss:
- (2) Electrons transfer: with an irradiation, the generation of surface charge and the transfer of interface electrons are induced by the SPR of AgNPs [37].
- (6) Oxidation products generation: there is a reaction between ROS, holes on the surface of TiO2 and R6G, which results in the effective degradation of R6G [38].
3.4. Simulation
In order to give a theoretical Raman enhancement of our samples, we used finite-difference time-domain (FDTD) method to calculate the electric field intensity. We set the same parameters, including nanoparticle size, gap, and layers of graphene and so on, only changed the positions of Ag, graphene or TiO2, and the simulation models were shown in the Figs. 10(a)-10(c). The simulations were carried out in vacuum (nmedium = 1.0), and the wavelength of incident light was 532 nm and the incident electric field (E0 = 1 V/m) with X-polarization propagated along the -Z direction. The optical properties of TiO2 were taken from anatase TiO2, and the refractive index was set to 2.4335 + 0.0001i, which was corresponding to the excitation wavelength of 532 nm. Meanwhile, the refractive indices of Ag and graphene were 0.13 + 3.19i and 2.63 + 1.28i [39], respectively. The thickness of graphene was set to 1 nm (three layers), and the diameter of Ag and TiO2 nanoparticles were set to 30 nm and 10 nm, respectively, which is basically consistent with our experimental parameters. In addition, the nanogap between two AgNPs was set to 2 nm, while the nanogap between two TiO2 NPs was set to 1 nm. The electric field distributions of Ag, TiO2 and graphene with different combinations were shown in Figs. 10(d)-10(f). We found that the hot spots existing between the adjacent AgNPs, and the local electric fields have reached the maxima of 24.96, 24.63 and 24.57 V/m for Ag-G-TiO2, G-Ag-TiO2 and Ag-TiO2-G, respectively. According to the mechanism of electromagnetic enhancement, the value of the enhancement factor can be estimated by |Eout|4/|E0|4, where E0 is the incident electric field intensity, Eout is the electric field intensity of the position of hot spots caused by the incident light. So the calculated EF value is ~3 × 105, which is consistent to the experimental results. The FDTD calculations clearly show that the SERS enhancement of the substrate is mainly due to the extremely strong electric fields at the inter-AgNP nanogaps.
Fig. 10 Simulation models of (a) Ag-G-TiO2, (b) G-Ag-TiO2 and (c) Ag-TiO2-G composite structures, the corresponding electric field distributions of (d) Ag-G-TiO2, (e) G-Ag-TiO2 and (f) Ag-TiO2-G.
4. Conclusions
In summary, we prepared Ag/TiO2/G composites by a sol-gel self-assembly method, and studied their SERS activities and photocatalytic properties. The substrates showed effective SERS activity, and the EF values were calculated to be about 106. In addition, the photocatalytic ability decreased orderly in the sequence of Ag-G-TiO2, G-Ag-TiO2 and Ag-TiO2-G. The hybrids exhibited good photocatalytic activity and could clean itself via UV photocatalytic degradation. Future work will be focus on the uniformity, stability and long life of these composites.
Funding
National Natural Science Foundation of China (No. 61376121); National High-tech R/D Program (No. 2015AA034801); National Natural Science Foundation of Chongqing (No. CSTC2015JCYJBX 0034).
Acknowledgments
We would like to thank Analysis and Test Center of Chongqing University. We also thank Prof. H. F. Shi and D. P. Wei in Chongqing Green and Intelligent Technology Chinese Academy of Sciences, for graphene sample help, and Mr. X. N. Gong for Raman spectrometer help.
References and links
1. M. Fleischmann, P. J. Hendra, and A. J. McQuillan, “Raman spectra of pyridine adsorbed at a silver electrode,” Chem. Phys. Lett. 26(2), 163–166 (1974). [CrossRef]
2. D. L. Jeanmaire and R. P. Van Duyne, “Surface Raman spectroelectrochemistry part I. heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver electrode,” J. Electroanal. Chem. 84(1), 1–20 (1977). [CrossRef]
3. M. G. Albrecht and J. A. Creighton, “Anomalously intense Raman spectra of pyridine at a silver electrode,” J. Am. Chem. Soc. 99(15), 5215–5217 (1977).
4. J. Kneipp, H. Kneipp, and K. Kneipp, “SERS-a single-molecule and nanoscale tool for bioanalytics,” Chem. Soc. Rev. 37(5), 1052–1060 (2008). [CrossRef] [PubMed]
5. X. X. Han, B. Zhao, and Y. Ozaki, “Surface-enhanced Raman scattering for protein detection,” Anal. Bioanal. Chem. 394(7), 1719–1727 (2009). [CrossRef] [PubMed]
6. M. Fan, G. F. Andrade, and A. G. Brolo, “A review on the fabrication of substrates for surface enhanced Raman spectroscopy and their applications in analytical chemistry,” Anal. Chim. Acta 693(1-2), 7–25 (2011). [CrossRef] [PubMed]
7. S. Shanmukh, L. Jones, J. Driskell, Y. Zhao, R. Dluhy, and R. A. Tripp, “Rapid and sensitive detection of respiratory virus molecular signatures using a silver nanorod array SERS substrate,” Nano Lett. 6(11), 2630–2636 (2006). [CrossRef] [PubMed]
8. A. Otto, “The ‘chemical’ (electronic) contribution to surface-enhanced Raman scattering,” J. Raman Spectrosc. 36(6–7), 497–509 (2005). [CrossRef]
9. Q. W. Huang, S. Q. Tian, D. W. Zeng, X. X. Wang, W. L. Song, Y. Y. Li, W. Xiao, and C. S. Xie, “Enhanced photocatalytic activity of chemically bonded TiO2/graphene composites based on the effective interfacial charge transfer through the C–Ti bond,” ACS Catal. 3(7), 1477–1485 (2013). [CrossRef]
10. D. Naumenko, V. Snitka, B. Snopok, S. Arpiainen, and H. Lipsanen, “Graphene-enhanced Raman imaging of TiO2 nanoparticles,” Nanotechnology 23(46), 465703 (2012). [CrossRef] [PubMed]
11. J. Xu, X. Xiao, F. Ren, W. Wu, Z. Dai, G. Cai, S. Zhang, J. Zhou, F. Mei, and C. Jiang, “Enhanced photocatalysis by coupling of anatase TiO2 film to triangular Ag nanoparticle island,” Nanoscale Res. Lett. 7(1), 239 (2012). [CrossRef] [PubMed]
12. X. Wang, Y. Tang, Z. Chen, and T. T. Lim, “Highly stable heterostructured Ag–AgBr/TiO2 composite: a bifunctional visible-light active photocatalyst for destruction of ibuprofen and bacteria,” J. Mater. Chem. 22(43), 23149–23158 (2012). [CrossRef]
13. M. Pelaez, N. T. Nolan, S. C. Pillai, M. K. Seery, P. Falaras, A. G. Kontos, P. S. M. Dunlop, J. W. J. Hamilton, J. A. Byrne, and K. O. Shea, “A review on the visible light active titanium dioxide photocatalysts for environmental applications,” Appl. Catal. B 125(33), 331–349 (2012). [CrossRef]
14. Y. Xie and Y. Meng, “SERS performance of graphene oxide decorated silver nanoparticle/titania nanotube array,” RSC Advances 4(79), 41734–41743 (2014). [CrossRef]
15. K. C. Hsu and D. H. Chen, “Highly sensitive, uniform, and reusable surface-enhanced Raman scattering substrate with TiO2 interlayer between Ag nanoparticles and reduced graphene oxide,” ACS Appl. Mater. Interfaces 7(49), 27571–27579 (2015). [CrossRef] [PubMed]
16. Q. Liu, Z. F. Liu, X. Y. Zhang, L. Y. Yang, N. Zhang, G. L. Pan, S. G. Yin, Y. S. Chen, and J. Wei, “Polymer photovoltaic cells based on solution-processable graphene and P3HT,” Adv. Funct. Mater. 19(6), 894–904 (2009). [CrossRef]
17. H. Zhang, X. Lv, Y. Li, Y. Wang, and J. Li, “P25-graphene composite as a high performance photocatalyst,” ACS Nano 4(1), 380–386 (2010). [CrossRef] [PubMed]
18. L. Gu, J. Wang, H. Cheng, Y. Zhao, L. Liu, and X. Han, “One-step preparation of graphene-supported anatase TiO2 with exposed {001} facets and mechanism of enhanced photocatalytic properties,” ACS Appl. Mater. Interfaces 5(8), 3085–3093 (2013). [CrossRef] [PubMed]
19. F. X. Liang, D. Y. Zhang, J. Z. Wang, W. Y. Kong, Z. X. Zhang, Y. Wang, and L. B. Luo, “Highly sensitive UVA and violet photodetector based on single-layer graphene-TiO2 heterojunction,” Opt. Express 24(23), 25922–25932 (2016). [CrossRef] [PubMed]
20. J. T. Wang, J. M. Ball, E. M. Barea, A. Abate, J. A. Alexander-Webber, J. Huang, M. Saliba, I. Mora-Sero, J. Bisquert, H. J. Snaith, and R. J. Nicholas, “Low-temperature processed electron collection layers of graphene/TiO2 nanocomposites in thin film perovskite solar cells,” Nano Lett. 14(2), 724–730 (2014). [CrossRef] [PubMed]
21. Y. Wen, H. Ding, and Y. Shan, “Preparation and visible light photocatalytic activity of Ag/TiO2/graphene nanocomposite,” Nanoscale 3(10), 4411–4417 (2011). [CrossRef] [PubMed]
22. H. J. Huang, S. Y. Zhen, P. Y. Li, S. D. Tzeng, and H. P. Chiang, “Confined migration of induced hot electrons in Ag/graphene/TiO2 composite nanorods for plasmonic photocatalytic reaction,” Opt. Express 24(14), 15603–15608 (2016). [CrossRef] [PubMed]
23. X. Wang, J. Zhang, X. Zhang, and Y. Zhu, “Characterization, uniformity and photo-catalytic properties of graphene/TiO2 nanocomposites via Raman mapping,” Opt. Express 25(18), 21496–21508 (2017). [CrossRef] [PubMed]
24. T. C. Gong, Y. Zhu, J. Zhang, W. J. Ren, J. M. Quan, and N. Wang, “Study on surface-enhanced Raman scattering substrates structured with hybrid Ag nanoparticles and few-layer graphene,” Carbon 87, 385–394 (2015). [CrossRef]
25. P. C. Lee and D. Meisel, “Adsorption and surface-enhanced Raman of dyes on silver and gold sols,” J. Phys. Chem. 86(17), 3391–3395 (1982). [CrossRef]
26. X. Zhang, J. Zhang, J. Quan, N. Wang, and Y. Zhu, “Surface-enhanced Raman scattering activities of carbon nanotubes decorated with silver nanoparticles,” Analyst (Lond.) 141(19), 5527–5534 (2016). [CrossRef] [PubMed]
27. A. Reina, S. Thiele, X. T. Jia, S. Bhaviripudi, M. S. Dresselhaus, J. A. Schaefer, and J. Kong, “Growth of large-area single-and bi-layer graphene by controlled carbon precipitation on polycrystalline Ni surfaces,” Nano Res. 2(6), 509–516 (2009). [CrossRef]
28. A. Sclafani and J. M. Herrmann, “Influence of metallic silver and of platinum-silver bimetallic deposits on the photocatalytic activity of titania (anatase and rutile) in organic and aqueous media,” J. Photoch. Photobio. A 113(2), 181–188 (1998). [CrossRef]
29. A. Furube, L. Du, K. Hara, R. Katoh, and M. Tachiya, “Ultrafast plasmon-induced electron transfer from gold nanodots into TiO2 nanoparticles,” J. Am. Chem. Soc. 129(48), 14852–14853 (2007). [CrossRef] [PubMed]
30. Y. Tian and T. Tatsuma, “Mechanisms and applications of plasmon-induced charge separation at TiO2 films loaded with gold nanoparticles,” J. Am. Chem. Soc. 127(20), 7632–7637 (2005). [CrossRef] [PubMed]
31. T. C. Gong, J. Zhang, Y. Zhu, X. Y. Wang, X. L. Zhang, and J. Zhang, “Optical properties and surface-enhanced Raman scattering of hybrid structures with Ag nanoparticles and graphene,” Carbon 102, 245–254 (2016). [CrossRef]
32. K. Awazu, M. Fujimaki, C. Rockstuhl, J. Tominaga, H. Murakami, Y. Ohki, N. Yoshida, and T. Watanabe, “A plasmonic photocatalyst consisting of silver nanoparticles embedded in titanium dioxide,” J. Am. Chem. Soc. 130(5), 1676–1680 (2008). [CrossRef] [PubMed]
33. J. Zhang, T. Fan, X. Zhang, C. Lai, and Y. Zhu, “Three-dimensional multi-walled carbon nanotube arrays coated by gold-sol as a surface-enhanced Raman scattering substrate,” Appl. Opt. 53(6), 1159–1165 (2014). [CrossRef] [PubMed]
34. J. Zhang, X. Zhang, C. Lai, H. Zhou, and Y. Zhu, “Silver-decorated aligned CNT arrays as SERS substrates by high temperature annealing,” Opt. Express 22(18), 21157–21166 (2014). [CrossRef] [PubMed]
35. E. C. Le Ru, E. Blackie, M. Meyer, and P. G. Etchegoin, “Surface enhanced Raman scattering enhancement factors: a comprehensive study,” J. Phys. Chem. C 111(37), 13794–13803 (2007). [CrossRef]
36. J. Zhang, S. M. Chen, T. C. Gong, X. L. Zhang, and Y. Zhu, “Tapered fiber probe modified by Ag nanoparticles for SERS detection,” Plasmonics 11(3), 743–751 (2016). [CrossRef] [PubMed]
37. J. M. Herrmann, H. Tahiri, Y. Ait-Ichou, G. Lassaletta, A. R. González-Elipe, and A. Fernández, “Characterization and photocatalytic activity in aqueous medium of TiO2 and Ag-TiO2 coatings on quartz,” Appl. Catal. B 13(3), 219–228 (1997). [CrossRef]
38. N. Yang, J. Zhai, D. Wang, Y. Chen, and L. Jiang, “Two-dimensional graphene bridges enhanced photoinduced charge transport in dye-sensitized solar cells,” ACS Nano 4(2), 887–894 (2010). [CrossRef] [PubMed]
39. S. Cheon, K. D. Kihm, H. Kim, G. Lim, J. S. Park, and J. S. Lee, “How to reliably determine the complex refractive index (RI) of graphene by using two independent measurement constraints,” Sci. Rep. 4(1), 6364 (2015). [CrossRef] [PubMed]
