Open Access
Add to BibSonomy
Abstract
Three types of anatase TiO2, graphene-TiO2, TiO2-graphene composites (G/TiO2) were developed, synthesized via a combination of simple sol-gel self-assembly method and additional thermal annealing process. Their structures and properties are determined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM) and Raman spectrum analysis. In addition, Raman spectra of TiO2 and graphene, the band shift and intensity of G and 2D band were analyzed, in order to verify the mutual coupling between TiO2 and graphene. Combined Raman mapping with AFM analysis, the agglomeration effect of TiO2 nanoparticles was figured out by quantitative analysis. Finally, the photo-catalytic properties of three kinds of composites were experimentally studied via Raman mapping measurements. The results reveal that graphene with high electron mobility, as an acceptor through interfacial interactions, was certificated to enhance the photo-catalytic effect of TiO2.
© 2017 Optical Society of America
1. Introduction
Graphene has attracted an enormous amount of interests since its discovery in 2004 [1], due to its special properties, such as excellent electron mobility [2], two dimensional large surface area of ~2600 m2/g [3], and excellent optical transparency [4]. Graphene plays an important role on the horizon of materials science, condensed matter physics and optics [5]. With regard to the domain of photocatalysis, graphene also promotes great interest to synthesize graphene-semiconductor composites as photo catalysts for target applications [6,7] and surface-enhancement Raman scattering (SERS) along with metal nanoparticles [8,9].
We notice that, on the one hand, the coupling of graphene with semiconductor is expected to accelerate the photo-generated charge separation in a more favorable way. Most of the research works are inclined to highlight that the enhanced photocatalytic activity of graphene-semiconductor composites is due to the additional properties of graphene [10]. On the other hand, we have investigated carbon/metal nanoparticles (C/MNPs) composites for SERS applications, such as carbon nanotubes arrays/Ag nanoparticles [11] (CNTAs/AgNPs), CNTAs/AuNPs [12], CNTs films/AgNPs [13], and graphene/AgNPs [14–16]. In order to find effective SERS substrates with reusable activity, we have studied graphene/ZnO/AgNPs [17]; its reusable activity is determined by the photocatalytic of the composites. In order to find an effective SERS structure with reusable activity, the structure must have high performance of photocatalytic activity. Research has shown that TiO2 stands out for its high photo-catalytic activity and remarkable chemical stability [18, 19]. Meanwhile, graphene-TiO2 hybrid materials have been studied in applications of lithium-ion batteries [20], highly sensitive photodetector [21], thin film perovskite solar cells [22], enhanced photocatalytic H2-production activity [23], and plasmonic photocatalytic reaction [24, 25]. Reduced graphene oxide-TiO2 has also been developed to enhance visible light photocatalysis [26], and photoinactivation of bacteria in solar light irradiation [27]; and graphene oxide-TiO2 was used in photodegradation of methyl orange [28]. However, in practical application, TiO2 still has several drawbacks [29], such as the weak solar efficiency and high-recombination rate of photo-generated electron-hole pairs.
Thus, it is natural to raise some questions as follows. Firstly, due to the properties of TiO2, are TiO2 and graphene/TiO2 (G/TiO2) similar in the photocatalytic performance? Secondly, are G/TiO2 composites similar in improving the photocatalytic performance while we use them to assemble graphene-TiO2 (graphene on the top layer) and TiO2-graphene (TiO2 on the top layer) with different preparation method? Thirdly, are graphene/TiO2/MNPs composites similar in improving the photocatalytic performance while we use them to assemble graphene/TiO2/MNPs with different preparation method?
Bearing the questions mentioned above in mind, a comparison study has been carried out in this work, mainly for the first two questions. By taking TiO2 as example, we have synthesized three types of graphene/TiO2 composites with different graphene position, using the same sol-gel and additional thermal approach to guarantee the interfacial contact between graphene and TiO2. Their structures and properties are determined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM) and Raman spectrum analysis. Furthermore, we investigated the TiO2 agglomerates effects and photocatalytic property via combination of AFM and Raman mapping theoretically and experimentally. Our results demonstrate the presence and position of graphene affect the photocatalytic performance of G/TiO2 composite. It is hoped that our work could be more effective understanding on the photoactivity improvement of G/TiO2 composite and G/TiO2/MNPs studied in the next step.
2. Structures and preparation
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, at 1000 °C for 10 min, and transferred to a conductive glass ((SiO2 with a conductive film of ITO, Guluo glass company, China)) substrate using a sacrificial PMMA support layer [15,16]. The TiO2 solution was prepared by dispersing 1 mg of commercial TiO2 powder (Nanostructured and Amorphous Materials, Inc.) in 10 mL deionized water.
2.2. Sample preparation
The TiO2 solution was dealed with ultrasonic treatment, and then multiple centrifuged and filtered to remove large NPs agglomerates. A 5 µL drop was dispensed onto a conductive glass or graphene-conductive glass support and dried to form TiO2 and TiO2-G substrate. Another graphene was transferred to the as-prepared TiO2 substrate to form G-TiO2 substrate. Ag SERS substrate used in photo-catalytic experiments (in section 2.3) was prepared through vacuum deposition of silver film and the following anneal process: Ag film with thickness of 30 nm was deposited on to SiO2/Si substrate, and then the as-prepared substrate was put in a gas mixture of H2 20 sccm and Ar 40 sccm, annealed at 400 °C for 20 min. The steps of sample preparation are shown in Fig. 1.
2.3. Photo-catalytic experiments
The photo-degradation of Rhodamine 6G (R6G) was carried out to evaluate the photo-catalytic activity of three samples (G-TiO2, TiO2-G and TiO2). Photo-catalytic experiments were done in a homemade reactor, which was surrounded with a cooling system to keep the photo-catalytic reaction system at ambient temperature (in Fig. 2). A 500 W Xenon light source (Solar 500, NbeT) with a KenKo L41 UV filter (λ > 410 nm) was used as an ultraviolet light (UV-light) source and fixed 10 cm away from the reaction system. The photo-catalyst substrates (10 × 10 mm2) were immersed in 10 mL of R6G aqueous solution with a concentration of 10−6 mol/L. As a comparison, a 10 mL R6G aqueous solution without photo-catalyst substrate in it was set as the reference group. The reaction system was firstly kept in the dark for one hour to establish an adsorption-desorption equilibrium, and then exposed to the UV-light. At the desired time intervals, 5 μL aliquots from each sample were taken, then dropped to an as-prepared Ag substrate and dried. The SERS intensity of R6G was used to determine the concentration of residual dye in solution.
2.4. Instruments and measurements
The morphology of samples was characterized by field emission SEM (JEOL, JSL-7800F), TEM (Tecnai G2 F20 S-TWIN), XRD (D8 advance diffractometer, Bruker, Germany), AFM (Dimension Edge), and EDS (Oxford Instruments 150). The Raman spectra were collected using a laser confocal Raman spectrometer (Horiba JY LabRAM HR Evolution) equipped with a 100 × (0.9 NA) objective, resulting in spot diameter of ~0.72 µm at the laser focus, 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 in order to reduce the thermal effect. Rhodamine 6G (Sigma-Aldrich) is used as photo-catalytic de-colorization molecule.
3. Results and discussion
3.1. Characterization of photo-catalyst substrates
SEM, TEM, XRD and EDS: SEM images of our samples were shown in Figs. 3(a), 3(c) and 3(d). Transmission electron microscopy was used to further investigate the prepared crystals and nanostructure of TiO2, shown in insert part of Fig. 3(a), and a typical micrograph of TiO2 nanoparticles is presented. The size of TiO2 nanoparticle ranging from 5 nm to 20 nm, can be observed, and the averaged particle size is ~15 nm. Figure 3(b) shows a HRTEM image of a TiO2 nanoparticle and its corresponding selected area electron diffraction (SAED) pattern (insert). From the distance between the adjacent fringes, the lattice spacing of 0.352 nm can be indexed to (101) plane of TiO2. The concentric circles which can be seen in the inserted SAED pattern are indexed to the (101), (004), (200) planes of TiO2, respectively.
Fig. 3 SEM images of (a) TiO2, with the inset showing the TEM image of TiO2 nanoparticles; (b) HRTEM image of a TiO2 nanoparticles, with the inset showing the corresponding SAED patterns; SEM images of (c) TiO2-G and (d) G-TiO2 substrate (the yellow dotted line represents the border of graphene sheets) after annealing process; (e) XRD spectrum of TiO2; EDS analysis of (f) TiO2-G and (g) G-TiO2, the distribution diagram of element for Ti, O and C is given out accordingly.
For G-TiO2 substrate dealt with annealing process in Fig. 3(d), graphene sheets at the top of the micro-nanostructures were packed densely with TiO2 nanoparticles (the yellow dotted line represented the border of graphene sheets), which displayed a combination of graphene and TiO2 nanoparticles.
XRD spectrum of TiO2 with a high peak at ~25.4° and a small peak at ~27.5° was shown in Fig. 3(e), indicating that the TiO2 substrate prepared in our experiment mostly composed by anatase phase TiO2 and a small part of the ingredients of rutile phase TiO2 [30]. The crystallite sizes (D) is about 14.2 nm, which was estimated from the half band width of the corresponding X-ray spectral peak by the Scherrer formula: D = kλ/(βcosθ), where λ is the X-ray wavelength, β is the half width of the (101) peak, θ is the Bragg diffraction angle, and k is a correction factor, which is taken as 0.89 [31].
Analysis results of EDS are shown in Figs. 3(e) and 3(f), including the element stratification, general area spectra of elements concentration, and the distribution diagram of element for Ti, O and C, respectively.
3.2 Raman mapping analysis
Here, Raman position mapping was used to make further investigation on three samples within 160 × 160 μm2 regions, at a scanning interval of 10 μm.
Analysis on Raman intensity: For TiO2 substrate, the contour plots of 256 spots and averaged Raman spectrum were shown with the Raman shift from 100 to 800 cm−1 in Figs. 4(a1) and 4(a2), respectively. The Raman mapping results of G-TiO2 and TiO2-G substrates are shown in Figs. 4(b1), 4(b2), 4(c1) and 4(c2). A continuous line with high brightness around 140 cm−1 and three lines with relatively lower brightness around 393, 513, and 635 cm−1 are observed. These bright lines with high continuity represent the main Raman vibrations of TiO2. The characteristic peaks around 140, 193, and 635 cm−1 are Eg modes, 393 cm−1 belongs to B1g modes, and 513 cm−1 is A1g mode [32–34].
Fig. 4 The Raman contour plot and Raman mappings of the 256 data sets on (a1) TiO2 and (b1) G-TiO2 and (c1) TiO2-G substrate; the averaged Raman spectra of (a2) TiO2, (b2) G-TiO2 and (c2) TiO2-G substrate obtained from Raman mapping analysis.
Analysis on Raman peak position: For the further study on the interaction between TiO2 and graphene on G/TiO2 composites, the position histograms and the corresponding Gaussian fitting curves of G and 2D band were given in Fig. 5. We analyze as follows:
Fig. 5 The position histograms and the corresponding Gaussian fitting curves of (a) G and (b) 2D band for TiO2, G-TiO2 and TiO2-G substrate.
- (1) In Fig. 5(a), compared with pure graphene substrate, the G peak position of graphene on G/TiO2 composites always produce a small frequency shift (<1 cm−1) which can be ignored, because the Raman shift is less than the accuracy of our instrument measurement (1 cm−1).
- (2) As shown in Fig. 5(b), compared with pure graphene substrate, the position of 2D band of composite is up-shifted from 2687.7 to 2693.2 cm−1 for G-TiO2 sample, but downshift to 2684.2 cm−1 for TiO2-G substrate. In order to analyze the phenomenon of band shift, we should know that: firstly, the position of G (ωG) and 2D (ω2D) peaks is affected by strain (tensile or compressive) and doping (n-type or p-type) effects [15, 35, 36]; secondly, without considering the strain effect, p-type doping of graphene leads to up-shift (phonon stiffening) of the G band and the 2D band; while n-type doping of graphene leads to up-shift of G band and down-shift of 2D band [37, 38]; thirdly, without considering doping effect, compressive strain causes Raman peaks of graphene to shift to higher frequencies, while tensile strain causes them to shift to lower frequencies [37]. In our G/TiO2 system, TiO2 nanoparticles could produce n-type doping in graphene. At the same time, the predominant tensile strain (compressive strain) in graphene [38, 39] could lead to the up-shift (downshift) of 2D band in G-TiO2 (TiO2-G) structure. Above all, there exists electron transfer between graphene and TiO2 to induce the band shift of graphene on both G-TiO2 and TiO2-G. Moreover, the interaction of stress and doping results in the different shift situation on different G/TiO2 structures [15, 40, 41], which can be reflected through the two kinds of shifting for 2D band.
3.3 Uniformity of TiO2 effect via Raman mapping
To further study the uniformity of TiO2 intensity on TiO2-G composites, the results of Raman mapping are shown in Figs. 6(a)-6(c). Figure 6(d) shows the intensity histogram and the corresponding Gaussian fitting curves of TiO2 Raman characteristic peak at 140 cm−1 on the three samples, in which x-axis represents the TiO2 Raman mapping intensity. In order to be clearly comparative analysis, y-axis represents the normalized number of mapping points in the corresponding intensity interval for three samples. We can see that the intensity distribution of TiO2 on TiO2-G substrate is less uniform than that on TiO2 and G-TiO2 substrates; an area enhancement effect can be observed on TiO2-G sample. The fitting intensity is 1025 for TiO2 substrate, 1301 for G-TiO2 substrate and 3381 for TiO2-G substrate. In addition, for the three fitting curves, the FWHM (full width at half maximum) value of TiO2-G substrate is largest, which means that the intensity distribution of TiO2 on TiO2-G substrate is wider than that of TiO2 and G-TiO2 substrate.
Fig. 6 Raman intensity mapping of TiO2 Raman characteristic peak at 140 cm−1 on (a) TiO2, (b) G-TiO2 and (c) TiO2-G samples; (d) the intensity histograms of TiO2 intensity with Gaussian fitting curves for the three samples.
For the above experimental results, our analysis is as follows:
- (1) For TiO2-G substrate, TiO2 nanoparticles were deposited onto graphene sheets; due to its relatively high surface energy [42], anatase TiO2 nanoparticles would incline to agglomerate on the surface of graphene. The variability in the volume of TiO2 agglomerates in close proximity to the graphene surface would influence the Raman signal of TiO2, due to the chemical-enhanced mechanism in GERS [43], which will induce the enhancement of TiO2 Raman signal on TiO2-G substrate. In addition, due to an effect of electron and hole puddles, whereby the Dirac point deviates spatially, leading to the strong local reactivity of single-layer graphene sheets [44], which results in a higher Raman signal of TiO2 (deeply analysis in section 3.4).
- (2) For G-TiO2 substrate, with graphene transferred onto TiO2 nanoparticles and the following annealing process, the redistribution of TiO2 under graphene may reduce the degree of agglomerate, resulting in a lower intensity of TiO2 Raman signal.
3.4 Agglomeration effect of TiO2 nanoparticles via Raman mapping and AFM
AFM image of TiO2-G sample is shown in Fig. 7(a), within the same scanning region with Raman mapping in Fig. 6(c) (160 × 160 μm2). For better understanding and comparative analysis, four obvious areas are marked with #1 to #4. In order to investigate the relationship between the agglomeration of TiO2 and Raman intensity, there are three-step Calculations.
Fig. 7 (a) AFM image of TiO2-G substrate in mapping area, (b) the enlarged views and agglomeration profiles of marks in the AFM image, (c) the volume distribution of TiO2 agglomerates at each mapping point, and (d) the relationship of TiO2 intensity at 140 cm−1 and volume of agglomerates on TiO2-G substrate.
- (1) Gwyddion as an AFM analysis software is employed to obtain the profile f (x, y, z) of TiO2 agglomerates at each scan points. The profiles of TiO2 agglomerates marked in the AFM are also given in Fig. 7(b). The height of agglomerate marked in #1 is ~10 nm which is equal to the diameter of single TiO2 nanoparticle, indicating a single-layer agglomeration of TiO2 nanoparticles.
- (2) Secondly, the volumes () in each position are obtained from the integral calculation of profile functions within areas of 10 × 10 μm2 (for #1 and #2) and 30 × 10 μm2 (for #3 and #4, due to large agglomeration). The volume mapping is plotted accordingly in Fig. 7(c). Calculated by integral at the same area of 10 × 10 μm2, the agglomeration volume of TiO2 marked from #1 to #4 are ~1.0 × 109, ~1.5 × 109, ~3.0 × 109, ~5.0 × 109 nm3 and the agglomerate marked in #4 has the highest agglomeration volume.
- (3) Combined with Raman mapping data in Fig. 6(c), the relationship between TiO2 intensity and the agglomeration volume is plotted in Fig. 7(d). The slope and standard deviation of the linear fitting curve are 1.29 × 10−6 and 4.9. Higher Raman intensity occurred at areas with higher agglomeration level of TiO2 nanoparticles.
3.5 Photo-catalytic efficiency
Evaluation of the photo-catalytic efficiency: The photo-catalytic activity of three samples was studied through R6G degradation, and the photo-catalytic efficiency was evaluated through the remaining R6G Raman intensity of 1364 cm−1.
The analysis is as follows:
- (1) Before the dark treatment, the initial Raman spectra of R6G (without UV irradiation) in beaker a, b and c were drawn with black solid line in Figs. 8(a)-8(c). Three Raman spectra are different although the concentration of R6G in the four beakers is theoretically equal. The non-uniformity of Ag substrate prepared by Ag film annealing process is the major factor, resulting in the differences in R6G Raman spectra. For better comparative analysis, the normalized intensity is calculated for next analysis.
- (2) When we discuss the photocatalytic activity the point 0 at x-axis in Fig. 8(d) is set at the original situation. After dark treatment for 1 hour, the Raman intensity of remaining R6G in three beakers decreased with different degrees, which is caused by different adsorption capacity of photo-catalysts with different structures. From Fig. 8(d), we can see that about 38% of the R6G molecules adsorbed on G-TiO2 and 21% on TiO2-G is far higher than 11% on bare TiO2. It could be due to two factors. (a) The large specific surface of TiO2 plays a minor role in adsorbing R6G, because of its high hydrophilicity, while the hydrophobic interaction between graphene and R6G is responsible for the good absorbability for G/TiO2 composites [45]. (b) The enhanced absorbability of G-TiO2 and TiO2-G did not merely originate from simple physical adsorption but was largely the result of selective adsorption of the aromatic dye on the catalyst. The adsorption was non-covalent and driven by π−π stacking between R6G and the aromatic regions of the graphene.
- (3) The level of the photo-catalytic activity is described by the slope of the function of normalized intensity and time in Fig. 8(d). The photo-catalytic activity of the G/TiO2 composite is higher than that of TiO2 under UV-light illumination. There would be three factors. (a) It is reported that graphene is a competitive acceptor material because of its two-dimensional π-conjugated structure [46]. (b) The two-dimensional planar structure endows it with unexpectedly good conductivity and benefits the transport of charge carriers [47]. (c) The photocatalytic performance of G/TiO2 is affected by the preparation methods. The difference in preparation methods leads to the different structural composition and synergetic interaction between graphene and TiO2, which influences the photocatalytic performance of G/TiO2 composites [10]. Therefore, photo-excited electrons of TiO2 could rapidly transfer from R6G to graphene through interfacial interaction [48]. The recombination of electron−hole (e−−h+) pairs could therefore be greatly suppressed in the composite system, leaving more charge carriers to react with water, dissolved O2, and dye molecules.
Fig. 8 Photo-catalytic activities of (a) TiO2, (b) TiO2-G and (c) G-TiO2 as a function of time for degradation of R6G under UV-light illumination; (d) the relationship between R6G intensity and illumination time of UV-light: the vertical axis is the normalized intensity and the first one hour from −1 h to 0 h is for dark treatment.
Enhancement mechanism of the photo-catalytic properties in G/TiO2 system: The enhancement of photo-activity for G/TiO2 nanocomposites is ascribed to the fact that incorporation of C contents into the matrix of TiO2 will increase the absorptivity and the life span of photoexcited e−−h+ pairs [10]. Under UV-light irradiation, TiO2 particles directly absorb photons, exciting electrons from the valence band (VB) to the conduction band (CB), shown in Fig. 9(a). The generated e−−h+ pairs are unstable and tend to recombine quickly, leading to a serious decrease in the quantum efficiency and impairment of the photo-activity [49]. However, in the presence of graphene, with a π-conjugated structure, these photo-induced electrons can transfer from the CB of TiO2 to the surface of graphene and then react with oxygen to yield reactive oxygen species such as O2•−, HO2•, and •OH [39, 50]. The main oxidative species, holes left in the VB of TiO2, will transfer to the surface and directly oxidize the adsorbed dye molecules, as well as reacting with water to produce •OH radicals, shown in Fig. 9(b).
Fig. 9 (a) Energy level structure of G/TiO2 composites, (b) reaction process, where ROS and M + • represent the reactive oxygen species and oxidation products.
4. Conclusions
In summary, we have prepared three types of G/TiO2 nanocomposites with different position of graphene by a sol-gel process along with additional annealing post-treatment. We have investigated the photocatalytic performance of the synthesized G/TiO2 composites via Raman mapping. The results have demonstrated a higher agglomeration volume of TiO2 nanoparticles results in a higher Raman intensity of TiO2. The additional graphene can induce the increased light adsorption intensity; it can also promote an efficient separation of the photoexcited electron-hole pairs. More efforts will be carried on to study the role and mechanism of graphene on affecting the photocatalytic properties. In addition, by taking the advantages of TiO2, graphene, and noble metal nanoparticles/nanostructures (such as Ag or Au), our work will further on the preparation, characterization, performance of G/TiO2/MNPs composites as SERS structures with reusable properties using photocatalytic activity.
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, Dr. J. M. Quan for help of the SEM, XRD, AFM and Raman measurements.
References and links
1. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, “Electric field effect in atomically thin carbon films,” Science 306(5696), 666–669 (2004). [CrossRef] [PubMed]
2. S. V. Morozov, K. S. Novoselov, M. I. Katsnelson, F. Schedin, D. C. Elias, J. A. Jaszczak, and A. K. Geim, “Giant intrinsic carrier mobilities in graphene and its bilayer,” Phys. Rev. Lett. 100(1), 016602 (2008). [CrossRef] [PubMed]
3. M. D. Stoller, S. Park, Y. Zhu, J. An, and R. S. Ruoff, “Graphene-based ultracapacitors,” Nano Lett. 8(10), 3498–3502 (2008). [CrossRef] [PubMed]
4. X. Li, Y. Zhu, W. Cai, M. Borysiak, B. Han, D. Chen, R. D. Piner, L. Colombo, and R. S. Ruoff, “Transfer of large-area graphene films for high-performance transparent conductive electrodes,” Nano Lett. 9(12), 4359–4363 (2009). [CrossRef] [PubMed]
5. A. K. Geim and K. S. Novoselov, “The rise of graphene,” Nat. Mater. 6(3), 183–191 (2007). [CrossRef] [PubMed]
6. N. Zhang, Y. H. Zhang, X. Y. Pan, M. Q. Yang, and Y. J. Xu, “Constructing ternary CdS-graphene-TiO2 hybrids on the flatland of graphene oxide with enhanced visible-light photoactivity for selective transformation,” J. Phys. Chem. C 116(34), 18023–18031 (2012). [CrossRef]
7. O. Akhavan and E. Ghaderi, “Photocatalytic reduction of graphene oxide nanosheets on TiO2 thin film for photoinactivation of bacteria in solar light irradiation,” J. Phys. Chem. C 113(47), 20214–20220 (2009). [CrossRef]
8. 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 graphene/TiO2 composites based on the effective interfacial charge transfer through the C–Ti Bond,” ACS Catal. 3(7), 1477–1485 (2013). [CrossRef]
9. 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]
10. M. Q. Yang, N. Zhang, and Y. J. Xu, “Synthesis of fullerene-, carbon nanotube-, and graphene-TiO2 nanocomposite photocatalysts for selective oxidation: a comparative study,” ACS Appl. Mater. Interfaces 5(3), 1156–1164 (2013). [CrossRef] [PubMed]
11. J. Zhang, X. L. Zhang, S. M. Chen, T. C. Gong, and Y. Zhu, “Surface-enhanced Raman scattering properties of multi-walled carbon nanotubes arrays-Ag nanoparticles,” Carbon 100, 395–407 (2016). [CrossRef]
12. 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]
13. 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]
14. Z. Jie, W. Xinyu, Z. Pengyue, Q. Jiamin, and Z. Yong, “Surface-enhanced Raman scattering activities of graphene-wrapped Cu particles by chemical vapor deposition assisted with thermal annealing,” Opt. Express 24(21), 24551–24566 (2016). [CrossRef] [PubMed]
15. T. C. Gong, J. Zhang, Y. Zhu, X. Y. Wang, X. 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]
16. T. C. Gong, Y. Zhu, J. Zhang, W. B. Xie, W. J. Ren, and J. M. Quan, “Study on surface-enhanced Raman scattering substrates structured with hybrid Ag nanoparticles and few-layer graphene,” Carbon 87, 385–394 (2015). [CrossRef]
17. J. Zhang, X. Zhang, Y. Ding, and Y. Zhu, “ZnO/graphene/Ag composite as recyclable surface-enhanced Raman scattering substrates,” Appl. Opt. 55(32), 9105–9112 (2016). [CrossRef] [PubMed]
18. 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]
19. 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]
20. B. Qiu, M. Xing, and J. Zhang, “Mesoporous TiO2 nanocrystals grown in situ on graphene aerogels for high photocatalysis and lithium-ion batteries,” J. Am. Chem. Soc. 136(16), 5852–5855 (2014). [CrossRef] [PubMed]
21. 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]
22. 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]
23. Q. Xiang, J. Yu, and M. Jaroniec, “Enhanced photocatalytic H2-production activity of graphene-modified titania Nanosheets,” Nanoscale 3(9), 3670–3678 (2011). [CrossRef] [PubMed]
24. 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]
25. 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]
26. K. H. Leong, L. C. Sim, D. Bahnemann, M. Jang, S. Ibrahim, and P. Saravanan, “Reduced graphene oxide and Ag wrapped TiO2 photocatalyst for enhanced visible light photocatalysis,” APL Mater. 3(10), 104503 (2015). [CrossRef]
27. O. Akhavan and E. Ghaderi, “Photocatalytic Reduction of Graphene Oxide Nanosheets on TiO2 Thin Film for Photoinactivation of Bacteria in Solar Light Irradiation,” J. Phys. Chem. C 113(47), 20214–20220 (2009). [CrossRef]
28. Y. Gao, X. Pu, D. Zhang, G. Ding, X. Shao, and J. Ma, “Combustion synthesis of graphene oxide–TiO2 hybrid materials for photodegradation of methyl orange,” Carbon 50(11), 4093–4101 (2012). [CrossRef]
29. 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]
30. X. Xue, W. Ji, Z. Mao, H. Mao, Y. Wang, X. Wang, W. Ruan, B. Zhao, and J. R. Lombardi, “Raman investigation of nanosized TiO2: effect of crystallite size and quantum confinement,” J. Phys. Chem. C 116(15), 8792–8797 (2012). [CrossRef]
31. V. Swamy, B. C. Muddle, and Q. Dai, “Size-dependent modifications of the Raman spectrum of rutile TiO2,” Appl. Phys. Lett. 89(16), 163118 (2006). [CrossRef]
32. J. Zhang, M. Li, Z. Feng, J. Chen, and C. Li, “UV Raman spectroscopic study on TiO2. I. Phase transformation at the surface and in the bulk,” J. Phys. Chem. B 110(2), 927–935 (2006). [CrossRef] [PubMed]
33. T. Ohsaka, F. Izumi, and Y. Fujiki, “Raman spectrum of anatase, TiO2,” J. Raman Spectrosc. 7(6), 321–324 (1978). [CrossRef]
34. W. Zhang, Y. He, M. Zhang, Z. Yin, and Q. Chen, “Raman scattering study on anatase TiO2 nanocrystals,” J. Phys. D Appl. Phys. 33(8), 912–916 (2000). [CrossRef]
35. Z. Osváth, A. Deák, K. Kertész, G. Molnár, G. Vértesy, D. Zámbó, C. Hwang, and L. P. Biró, “The structure and properties of graphene on gold nanoparticles,” Nanoscale 7(12), 5503–5509 (2015). [CrossRef] [PubMed]
36. T. M. G. Mohiuddin, A. Lombardo, R. R. Nair, A. Bonetti, G. Savini, R. Jalil, N. Bonini, D. M. Basko, C. Galiotis, and N. Marzari, “Uniaxial strain in graphene by Raman spectroscopy: G peak splitting, Gruneisen parameters, and sample orientation,” Phys. Rev. B Condens. Matter 79(20), 196–198 (2008).
37. S. Pisana, M. Lazzeri, C. Casiraghi, K. S. Novoselov, A. K. Geim, A. C. Ferrari, and F. Mauri, “Breakdown of the adiabatic Born-Oppenheimer approximation in graphene,” Nat. Mater. 6(3), 198–201 (2007). [CrossRef] [PubMed]
38. A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S. K. Saha, U. V. Waghmare, K. S. Novoselov, H. R. Krishnamurthy, A. K. Geim, A. C. Ferrari, and A. K. Sood, “Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor,” Nat. Nanotechnol. 3(4), 210–215 (2008). [CrossRef] [PubMed]
39. F. Ding, H. Ji, Y. Chen, A. Herklotz, K. Dörr, Y. Mei, A. Rastelli, and O. G. Schmidt, “Stretchable graphene: a close look at fundamental parameters through biaxial straining,” Nano Lett. 10(9), 3453–3458 (2010). [CrossRef] [PubMed]
40. S. Heeg, R. Fernandez-Garcia, A. Oikonomou, F. Schedin, R. Narula, S. A. Maier, A. Vijayaraghavan, and S. Reich, “Polarized plasmonic enhancement by Au nanostructures probed through Raman scattering of suspended graphene,” Nano Lett. 13(1), 301–308 (2013). [CrossRef] [PubMed]
41. X. Y. Wang, N. Wang, T. C. Gong, Y. Zhu, and J. Zhang, “Preparation of graphene-Ag nanoparticles hybrids and their SERS activities,” Appl. Surf. Sci. 387, 707–719 (2016). [CrossRef]
42. U. Diebold, “The surface science of titanium dioxide,” Surf. Sci. Rep. 48(5–8), 53–229 (2003). [CrossRef]
43. X. Ling and J. Zhang, “First-layer effect in graphene-enhanced Raman scattering,” Small 6(18), 2020–2025 (2010). [CrossRef] [PubMed]
44. R. Sharma, J. H. Baik, C. J. Perera, and M. S. Strano, “Anomalously large reactivity of single graphene layers and edges toward electron transfer chemistries,” Nano Lett. 10(2), 398–405 (2010). [CrossRef] [PubMed]
45. Y. Wang, Y. Tang, Y. Chen, Y. Li, X. Liu, S. Luo, and C. Liu, “Reduced graphene oxide-based photocatalysts containing Ag nanoparticles on a TiO2 nanotube array,” J. Mater. Sci. 48(18), 6203–6211 (2013). [CrossRef]
46. Q. Liu, Z. Liu, X. Zhang, L. Yang, N. Zhang, G. Pan, S. Yin, Y. Chen, and J. Wei, “Polymer photovoltaic cells based on solution-processable graphene and P3HT,” Adv. Funct. Mater. 19(6), 894–904 (2009). [CrossRef]
47. 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]
48. 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]
49. J. Yu, W. Wang, B. Cheng, and B. L. Su, “Enhancement of photocatalytic activity of mesporous TiO2 powders by hydrothermal surface fluorination treatment,” J. Phys. Chem. C 113(16), 6743–6750 (2009). [CrossRef]
50. Q. Xiang, J. Yu, and M. Jaroniec, “Enhanced photocatalytic H2-production activity of graphene-modified titania nanosheets,” Nanoscale 3(9), 3670–3678 (2011). [CrossRef] [PubMed]
