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Direct observation of enhanced plasmon-driven catalytic reaction activity of Au nanoparticles supported on reduced graphene oxides by SERS† Xiu Liang,a Tingting You,a Dapeng Liu,a Xiufeng Lang,b Enzhong Tan,c Jihua Shi,a Penggang Yin*a and Lin Guo*a Graphene-based nanocomposites have recently attracted tremendous research interest in the field of catalysis due to their unique optical and electronic properties. However, direct observation of enhanced plasmon-driven catalytic activity of Au nanoparticles (NPs) supported on reduced graphene oxides (Au/rGO) has rarely been reported. Herein, based on the reduction from 4-nitrobenzenethiol (4-NBT) to p,p 0 -dimercaptoazobenzene (DMAB), the catalytic property of Au/rGO nanocomposites was investigated
Received 12th February 2015, Accepted 9th March 2015
and compared with corresponding Au NP samples with similar size distribution. Our results show that
DOI: 10.1039/c5cp00908a
chemical reactions. In addition, systematic comparisons were conducted during power- and time-
Au/rGO nanocomposites could serve as a good catalytic and analytic platform for plasmon-driven dependent surface-enhanced Raman scattering (SERS) experiments, which exhibited a lower power
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threshold and higher catalytic efficiency for Au/rGO as compared to Au NPs toward the reaction.
1. Introduction When new catalysts are introduced, characterized, and developed, investigation of the catalytic activity and reaction kinetics becomes an essential issue. A variety of molecular surface spectroscopic techniques, including ultraviolet-visible,1 fluorescence,2 Raman,3 and infrared spectroscopy,4 have been employed to study reactions occurring at the surface of metal structures. However, limited spatial and time resolution hinders their wide application in the nanoscale. Fortunately, surface-enhanced Raman scattering (SERS), which was discovered in 1974,5 is advantageous because of its high sensitivity and selectivity and has become an attractive and powerful tool for analysis and detection.6–9 Based on the mechanisms of electromagnetic enhancement (near-field effect) and chemical enhancement (long-range effect),10,11 SERS plays a vital role in probing the reaction process and kinetics through real time and in situ fingerprint spectra of reactants or intermediates in a variety
a
Key Laboratory of Bio-inspired Smart Interfacial Science and Technology of the Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing, China. E-mail: [email protected] b Department of Physics, Hebei Normal University of Science and Technology, Qinhuangdao, 066004, China c Department of Mathematics and Physics, Beijing Institute of Technology Petrochemical, Beijing, 102617, P. R. China † Electronic supplementary information (ESI) available: EDX spectrum and timedependent SERS spectrum of Au/rGO, Au NPs and GO. See DOI: 10.1039/ c5cp00908a
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of reactions occurring at metal surfaces or electrochemical interfaces.12,13 Integrating high plasmonic properties and catalytic activity, Au NPs are considered to be a good SERS substrate candidate for monitoring catalytic reactions. However, Au NPs aggregate easily in order to disperse high surface energy. Consequently, various substances, for example, metal oxides,14 polymers,15 and carbon materials,16 are introduced to anchor Au NPs. As recently demonstrated, graphene has been employed as an excellent burgeoning support to stabilize and disperse nanoparticles in virtue of its large surface area, extraordinary electronic transport properties, and strong mechanical strength in catalytic fields. For example, Li et al. reported using hydrothermal conditions to prepare Au/rGO sheets that exhibited excellent catalytic performance towards the reduction of 4-nitrophenol to 4-aminophenol.17 Additionally, Au/rGO SERS studies have also recently attracted much attention, due to the coupled electromagnetic field present on Au/rGO surfaces.18 Plasmon-driven chemical reactions have seen a surge of interest because they create a new path to control and monitor the catalytic reactions on metallic substrates along with the development of SERS19 and TERS (tip-enhanced Raman scattering).20 It has been ascertained by direct experimental and theoretical evidence that the molecule p,p0 -dimercaptoazobenzene (DMAB) was produced from PATP (p-aminothiophenol) by a plasmon-driven surface-catalyzed reaction.21 After scrutinizing the selective oxidation process of PATP to DMAB on Au and Ag nanostructures via SERS, SPR-assisted (surface plasmon resonance) activation of the 3O2 mechanism was introduced to sufficiently explain catalytic reactions.22 More recently,
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the plasmon-driven chemical reaction of 4-NBT into DMAB molecules on Ag and Cu films was further confirmed by SERS.23 Sun and co-workers demonstrated that the dimerization reaction from 4-NBT to DMAB possessed substrate-, wavelength-, and timedependent features.24 Very recently, further study of the kinetics of this plasmon-driven chemical reaction was conducted in situ using spatiotemporal SERS on a single peony-like silver microflower particle.25 Furthermore, multifunctional magnetic composite nanomaterials, such as Fe3O4/C/Au NPs26 and Fe3O4@C@Ag NPs,27 which possess more than two functional properties, have been attracting significant research interest in the fields of SERS plasmon-driven catalytic monitoring or SERS detection. To the best of our knowledge, no study has been reported that used SERS to investigate the plasmon-driven chemical reaction of 4-NBT dimerizing into DMAB on Au/rGO nanocomposite substrates until now. Herein, Au/rGO nanocomposites were synthesized through an in situ chemical reduction method and applied as a SERS substrate for monitoring the plasmon-driven surface-catalyzed reaction from 4-NBT to DMAB. Furthermore, we compared the catalytic activities of Au/rGO nanocomposites with Au NPs of the same size by utilizing power- and time-dependent SERS experiments that determined the enhanced catalytic activity of the Au/rGO sample.
2. Experimental section 2.1
Materials
Graphite powder (300 mesh, 99.9999%) was purchased from Alfa Aesar; hydrogen tetrachloroaurate(III) tetrahydrate (HAuCl44H2O) was purchased from the Kermel Chemical Regent Co. Ltd (Tianjin, China); trisodium citrate was obtained from the Beijing Chemical Regent Company (China); 4-NBT was purchased from J&K Chemical Company (China). All other reagents were of analytical grade and used without further purification. Highly purified water (Millipore Milli-Q System) was used throughout the experiments. 2.2
Synthesis of samples
Synthesis of graphene oxide (GO). GO was produced by chemical exfoliation and oxidization from natural graphite powder using Hummer’s method with some modification.28 In brief, 7.2 mL of concentrated H2SO4 was mixed with 0.9 g of graphite, 1.5 g of K2S2O8, and 1.5 g of P2O5 at 80 1C in a roundbottom flask. After stirring for 4.5 h, the mixture was thoroughly washed with water. Afterwards, the pre-oxidized graphite was maintained at 0 1C in an ice bath while 0.5 g of NaNO3 and 23 mL of concentrated H2SO4 was slowly added, followed by the slow addition of 3 g of KMnO4. Subsequently, the solution was warmed and kept at 35 1C with stirring for 2 h. Then, 86 mL of deionized water and 5 mL of H2O2 (30%) was slowly added, and the color of the solution changed to brilliant yellow with bubbling. Finally, the mixture was thoroughly washed with water and subjected to dialysis, followed by lyophilization. The exfoliation of the as-prepared graphene oxide was accomplished by dispersion in water (0.63 mgmL1) under ultrasonication for 2 h, and the mixture was then centrifuged to produce a homogeneous yellow dispersion.
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Synthesis of Au/rGO. The synthesis of Au/rGO nanocomposites was based on an in situ reduction method of the Au(III) complex by sodium citrate as previously reported with some modification.29 Briefly, 5.95 mL of the as-prepared GO aqueous suspension (0.63 mg mL1) was added to 50 mL of HAuCl4 solution (0.01%) in a round-bottom flask with stirring. The resultant suspension was aged for at least 1 h at room temperature. Then, the solution was heated to 80 1C, and 940 mL of sodium citrate (0.085 M) was added dropwise. After reaction for one hour, the mixed solution was cooled to room temperature. The precipitate was washed with water and centrifuged at 6000 rpm for more than three times to remove any free Au NPs. Finally, the Au/rGO was added to water to form a stable dispersion. Synthesis of Au NPs. For comparison, the citric-capped Au NPs with the same diameter were prepared by the typical method reported by Frens.30 In brief, 50 mL of 0.01% HAuCl44H2O aqueous solution was heated to boiling under vigorous stirring. Subsequently, 1 mL of 1% sodium citrate solution was rapidly added. The solution was refluxed for another 30 min to complete the crystal nucleus formation and growth process of gold before cooling to room temperature. The final concentration of the as-obtained Au was kept under the same conditions as the Au/rGO solution. Preparation of samples for SERS. Au/rGO and Au NPs were functionalized with 4-NBT via self-assembly. 1 mL of a 2 103 M 4-NBT ethanol solution was mixed with 1 mL of Au/rGO or Au colloid dispersion for 30 min. Afterwards, centrifugation and ethanol washing were used to remove the unbound 4-NBT molecules. The products were re-dispersed in the same volume of water to form a stable suspension. Finally, the dip-coating method was used to assemble the 4-NBT-functionalized Au/rGO and Au NPs onto the SiO2/Si substrate for immediate SERS measurements. 2.3
Characterization
TEM images were obtained using a JEM-2100F transmission electron microscope operated at 200 kV. FTIR spectra were recorded on an FTIR spectrometer-733 (iN10MX). The UV-vis spectrophotometer and fittings were provided by Shanghai Lab-Spectrum Instruments (Shanghai, China) Co., Ltd Raman spectra were recorded with a JY HR800 Raman spectrometer (HORIBA Jobin Yvon), equipped with a 50 objective (NA = 0.5) and an Ar–Kr laser with 647 nm wavelength; the laser power values measured in the experiments were obtained from the power meter (mW at the samples with a spot area of approximately 1.4 mm2). The Raman band of a silicon wafer at 520.8 cm1 was used to calibrate the spectrometer.
3. Results and discussion 3.1
Formation and characterization of Au/rGO
FTIR spectra were used to confirm the existence of oxygen functionalities at the GO surface that served as the nucleation sites for the growth of Au NPs in the synthetic procedure of Au/rGO nanocomposites.29 As seen in Fig. 1, the characteristic
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Fig. 1
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FTIR spectra of prepared GO nanosheets.
peaks for the CQO carbonyl stretching vibration appear at 1725 cm1. For the epoxy C–O–C group, the stretching vibration peaks appear at 1227 cm1, and the peaks at 1050 cm1 can be assigned to the C–O stretching vibration. Another obvious peak at 1622 cm1 denotes the main CQC sp2 character of the carbon framework.31,32 Consequently, the FTIR results provide direct evidence of the successful oxidation of graphite powder with abundant oxygen groups on the surface of the GO nanosheets. The TEM image (Fig. 2A) of the GO morphology shows translucent sheets with wrinkles and folds, indicating the thin layered structure of the well exfoliated GO. Fig. 2B shows the TEM images of Au/rGO nanocomposites in which Au NPs with an average diameter of 14.5 nm were loaded on the surface of the GO nanosheets. The corresponding energy-dispersive X-ray (EDX) spectroscopy (Fig. S2, ESI†) of Au/rGO shows the peaks of elemental C, O, and Au, firmly confirming the existence of AuNPs on the surface of the GO nanosheets. For comparison, the sample of Au NPs (d = 14.7 nm) without GO was also characterized by TEM, as shown in Fig. 2C. It is clear that in
both samples, the Au NPs exhibit a narrow particle size distribution. The successful preparation of Au/rGO was further confirmed by UVvis absorption spectroscopy. As shown in Fig. 2D, the absorption spectrum of the GO suspension (blue line) shows a broad peak at 230 nm, which is attributed to the p–p* electron transition of the CQC bands, as well as a shoulder peak at approximately 300 nm that is attributed to an n–p* electron transition of the CQO bands.33 After decoration with Au NPs onto the surface of GO nanosheets, a new obvious peak at 520 nm (black line) appeared that was due to the SPR of Au NPs. Such a change in the absorption spectrum further characterizes the formation of Au/rGO nanocomposites. Furthermore, it was noted that the position of the absorption peak of the prepared bare Au NPs remained the same as that of the Au/ rGO due to the same Au size distribution for both that results in consistency of Au-related SPR properties.34 Raman spectroscopy is a useful, non-destructive tool that can be used to detect significant structural changes of the carbon framework. Fig. 3 shows the Raman spectra of pristine graphite, GO, and Au/rGO nanocomposites. The D peak at 1340 cm1 and the G peak at 1570 cm1 are the most prominent signals observed in graphite samples. GO exhibits two characteristic peaks of D and G bands at 1351 and 1600 cm1, which could be attributed to the symmetry A1g vibration mode and the E2g vibration mode of sp2 carbon atoms, respectively. As for the Au/rGO, the corresponding D and G bands appear at 1340 and 1589 cm1, respectively. The redshift of the G band for Au/rGO nanocomposites from 1600 to 1589 cm1 relative to GO is assigned to the recovery of the hexagonal network of carbon atoms.35 In addition, after the reduction of sodium citrate, Au/rGO shows an increased intensity ratio of D/G (1.42) than that of GO (0.92), indicating a decrease in the size of the in-plane sp2 domains and a relatively ordered crystal structure of Au/rGO. This indicates that GO is relatively reduced in Au/rGO nanocomposites.33 Furthermore, the Raman intensities of Au/rGO were found to be enhanced by five-fold compared to pure GO, which could be attributed to the enhanced local electromagnetic field induced by the anchored Au NPs on the GO surface.36 3.2
Surface plasmon-driven catalytic reaction activity
Sample Au NPs and Au/rGO with the same Au size of approximately 15 nm were systematically investigated by comparing the catalytic activity toward the reduction of 4-NBT to DMAB on both kinds of substrates by SERS. Fig. 4 shows the scheme to
Fig. 2 TEM images of the as-prepared (A) GO nanosheets, (B) Au/rGO nanocomposites, and (C) Au NPs; (D) UV-vis spectra of the abovementioned three samples.
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Fig. 3
Raman spectra of graphite, GO, and Au/rGO nanocomposites.
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monitor the reaction on Au/rGO using the SERS technique. Firstly, 4-NBT molecules were adsorbed onto Au/rGO via a selfassembly method. Then, direct observation of SERS spectra was carried out on the Au/rGO to monitor the reduction process in situ. Fig. 5a is the normal Raman spectrum (NRS) of 4-NBT powders in solid state for comparison, and there exists three main strong Raman peaks at 1099, 1332, and 1574 cm1, characterizing the S–C stretching vibration, the symmetric NO2 stretching vibration, and the C–C stretching vibration of the benzene ring, respectively.37 Fig. 5b and c show the SERS spectra of 4-NBT adsorbed on Au NPs and Au/rGO nanocomposites under 4 mW laser irradiation after 50 s, respectively. Both spectra have identical features that are distinctively different from the NRS of 4-NBT. New peaks at 1141 cm1 (bCH + nCN), 1390 cm1 (nNN + nCC + bCH), and 1441 cm1 (nNN + nCC + bCH) appear, which have been demonstrated to be the characteristic peaks of the produced p,p0 -dimercaptoazobenzene (DMAB).23,24 This result firmly indicates that the photoreaction of 4-NBT to DMAB takes place on the surfaces of Au NPs and Au/rGO nanocomposites while being exposed to 4 mW from the 647 nm laser. However, obvious differences of the Raman spectra between Fig. 5b and c can be also observed. On one hand, the measured relative Raman intensities at 1141, 1390 and 1441 cm1 to 1332 cm1 of 4-NBT adsorbed on the Au/rGO nanocomposites are much higher than those adsorbed on the Au NPs, indicating a larger degree of reaction conversion from 4-NBT to DMAB on Au/rGO substrate as compared to the Au NPs. On the other hand, all peaks of Au/rGO
Fig. 4 Schematic illustration of the plasmon-driven surface-catalyzed reaction of 4-NBT dimerizing to DMAB that is monitored on Au/rGO nanocomposites.
Fig. 5 Normal Raman spectrum of 4-NBT solid (a) and the SERS spectra of 4-NBT adsorbed on Au NPs (b) and Au/rGO nanocomposites (c). The laser power was 4 mW and the acquisition time was 50 s.
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possess significantly higher intensities than those of the pure Au NP sample. These SERS effects are consistent with previously reported results, which originate from chemical effects improved with the addition of GO substrates.38–40 It was reported that the plasmon-driven conversion of 4-NBT into DMAB on Ag or Cu was influenced by the power of an incident laser.23 Fig. 6(a) shows the NR spectra of 4-NBT, and Fig. 6(b–f) shows the SERS spectra of 4-NBT-functionalized Au/ rGO nanocomposites (A) and Au NPs (B) under the laser power of 0.04 (b), 0.4 (c), 1.2 (d), 4 (e), and 8 mW (f), respectively. As for Au/rGO, the spectral pattern at the laser power of 0.04 mW is nearly the same as that in the neat solid state except for a small SERS peak at 1110 cm1 assigned to the vibrational mode of d(C–H) for 4-NBT. When the laser power was increased to 0.4 mW (Fig. 6c in A), three characteristic peaks of DMAB at 1142, 1390, and 1441 cm1 appeared. If the laser power was further increased, the three peaks of DMAB became more obvious, and the peak at 1332 cm1 decreased as 4-NBT molecules were reduced to DMAB. A similar tendency was also observed in the case of pure Au NPs substrates (Fig. 6B). Consequently, a power threshold of the incident laser existed for observation of the phenomenon in our system. The power threshold for triggering the conversion from 4-NBT to DMAB on Au NPs increased to 4 mW (Fig. 6e in B). The bands at 1441 cm1 were produced by new species of DMAB, and the band at 1332 cm1 was mainly produced by the initial reactant 4-NBT.23 Thus, the two bands were chosen to represent the reaction efficiency. As shown in Fig. 7, the 1441 cm1/1332 cm1 intensity ratios as a function of the laser power were plotted as a quantitative measure of the reaction progress from 4-NBT to DMAB. It was found that the relative Raman intensity steadily increased with the laser power on both Au/rGO nanocomposites and Au. Based on well-known SPR-mediated reaction mechanisms, the additional hot electrons created by a higher laser power may cause more 4-NBT molecules to convert into DMAB.24 When the laser power was varied from 0.04 to 8 mW, the intensity ratios ranged from 0.02 to 0.6 for Au/rGO, while values from 0.01 to 0.15 were obtained for pure Au NP samples, indicating a larger reaction conversion from 4-NBT into DMAB on Au/rGO. To further monitor the reaction over time, the reaction kinetics of 4-NBT dimerizing into DMAB was investigated and
Fig. 6 Normal Raman spectrum of 4-NBT solid (a) and the SERS spectra of 4-NBT adsorbed on Au/rGO (A) and Au NPs (B) under the laser power of (b) 0.04, (c) 0.4, (d) 1.2, (e) 4, and (f) 8 mW, respectively. All Raman spectra were collected after a 20 s laser exposure.
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Fig. 7 Obtained DMAB/4-NBT 1441/1332 cm1 intensity ratios as a function of the laser power under 647 nm excitation.
comparatively studied using time-dependent SERS spectra by continuously illuminating the samples with a laser power of 1.2 mW. Fig. S3 (ESI†) shows the characteristic peaks of the changing tendency of DMAB and 4-NBT with time elapsing. The three characteristic peaks of DMAB at 1142, 1390, and 1441 cm1 started to emerge within 10 s, and their intensities gradually increased until achieving the maximum for Au/rGO nanocomposites. The relative Raman intensities of bands at 1142, 1390, and 1441 cm1 to 1332 cm1 for Au/rGO and Au substrates were plotted as a function of laser irradiation time in Fig. 8. As for Au/rGO, the relative Raman intensities of 1142, 1390, and 1441 cm1 to 1332 cm1 increased drastically from 0 to approximately 0.6, 0.55, and 0.7, respectively, within 120 s. In the next 300 s, changes in the relative Raman intensity were less than 10%, which demonstrates that more and more DMAB molecules were produced from 4-NBT in a short time, and then the transformation approached a stable maximum. In contrast, the relative Raman intensities of 1142, 1390, and 1441 cm1 to 1332 cm1 on Au NP samples without rGO substrates required at least 250 s to reach a maximum of 0.15 to 0.18. Therefore, a higher conversion rate and catalytic efficiency of 4-NBT were achieved on Au/rGO substrates under the same conditions. It is worth mentioning that after the maximum conversion, the fingerprint spectra revealed that 4-NBT and DMAB coexisted in this system. The complete conversion from 4-NBT to DMAB can be achieved with the addition of NaBH4 reductant as previously reported.41,42 However, one dispute exists in time-dependent, plasmondriven catalytic reactions detected by SERS, which states that
illumination with high laser power over a long period of time might cause the thermal effect on the focal spot of substrates to accelerate the conversion reaction.43 To avoid this effect, timedependent SERS experiments with Au/rGO and Au NPs catalysts were conducted under the lower laser power of 0.4 mW. As shown in Fig. S4A (ESI†), it took approximately 300 s for the catalytic reaction of 4-NBT to reach a stable conversion rate of 0.35–0.43 on Au/rGO (Fig. S4C, ESI†), while no obvious characteristic peaks of DMAB were observed within 500 s on Au NPs without graphene (Fig. S4B, ESI†). An enhanced catalytic efficiency and conversion rate of 4-NBT to DMAB on Au/rGO as compared to that of bare Au NPs were confirmed as well as the exclusion of the impact of thermal effects in this system. Separate experiments were performed to exclude the influence of GO on the characteristic bands. Fig. S5 (ESI†) depicts the time-dependent SERS spectra of 4-NBT molecules (103 M) adsorbed on blank GO nanosheets under a 4 mW 647 nm laser. The spectra shows only characteristic peaks of GO, and no characteristic band related to DMAB was observed, suggesting that GO itself is unlikely to catalyze the dimerization reaction. It has been previously reported that catalytic activity for carbon-supported metal nanocomposites was greatly enhanced as compared to that of metal NPs. For example, Huang et al. conducted a comparative catalytic activity study of Au and Au–rGO nanocomposites in the reduction reaction of 4-nitroaniline to 1,2-benzanediamine by NaBH4, and it was observed that the Au–rGO nanocomposites exhibited higher catalytic activities than the Au NPs alone.18 They explained that the higher catalytic performance of the Au–GO and Au–rGO should be attributed to the electronic interactions between the metal and graphene. In this study, the aforementioned higher reaction efficiency and faster reaction rate for the Au/rGO as compared to that of the bare Au nanospheres under the same conditions may be explained as follows. It was demonstrated that the surfaceinduced photoreduction reaction from 4-NBT to DMAB involves charge transfer from the substrates to the adsorbed molecules,24 and thus it is very likely that the enhanced electrical conductivity of rGO accelerated the transfer of the electrons from the Au NPs to the molecules, resulting in an enhanced catalytic efficiency and rate. Furthermore, the saturation adsorption of 4-NBT molecules in the preparation of SERS substrates was easily achieved. In our system, rGO, which served as a molecular enricher,44 can adsorb more 4-NBT molecules by p–p stacking interactions to provide more 4-NBT near the Au nanoparticles, resulting in a higher reaction efficiency of Au/rGO due to highly efficient contact between them in the context of the same Au NP density distribution on the two substrates.
4. Conclusion 1
Fig. 8 Relative Raman intensity of bands at 1142, 1390, and 1441 cm to 1332 cm1 under excitation with a laser power of 1.2 mW on Au/rGO and Au NPs.
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In summary, we have successfully synthesized Au/rGO nanocomposites through a simple in situ chemical reduction method, which was confirmed by characterization of TEM, UV-vis, FTIR, and Raman spectra. The prepared Au/rGO nanocomposites can
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be used as SERS substrates to monitor the process of the plasmondriven, surface-catalyzed reaction from 4-NBT to DMAB. Powerdependent SERS spectra for Au/rGO revealed a lower threshold power of 0.4 mW as compared to that of 4 mW on Au NPs under the same conditions. In addition, a larger reaction conversion from 4-NBT to DMAB was obtained on Au/rGO. Excluding the impact of a thermal effect, enhanced reactivity was achieved on the Au/rGO nanocomposites during the time-dependent SERS spectra. We suggest that excellent electrical conductivity as well as molecular adsorption ability of graphene makes the Au/rGO a more efficient catalyst than 15 nm Au NPs in the plasmon-catalysis reduction from 4-NBT to DMAB by SERS. The results of this study can assist with increasing our understanding of the interaction between metal and graphene as well as plasmon-driven chemical reactions.
Acknowledgements The work was supported by the National Natural Science Foundation of China (51272013).
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16 G. Lawson, F. Gonzaga, J. Huang, G. de Silveira, M. A. Brook and A. Adronov, J. Mater. Chem., 2008, 18, 1694. 17 J. Li, C.-y. Liu and Y. Liu, J. Mater. Chem., 2012, 22, 8426. 18 J. Huang, L. Zhang, B. Chen, N. Ji, F. Chen, Y. Zhang and Z. Zhang, Nanoscale, 2010, 2, 2733. 19 K. N. Heck, B. G. Janesko, G. E. Scuseria, N. J. Halas and M. S. Wong, J. Am. Chem. Soc., 2008, 130, 16592. 20 M. Sun, Z. Zhang, H. Zheng and H. Xu, Sci. Rep., 2012, 2, 647. 21 Y. Fang, Y. Li, H. Xu and M. Sun, Langmuir, 2010, 26, 7737. 22 Y. F. Huang, M. Zhang, L. B. Zhao, J. M. Feng, D. Y. Wu, B. Ren and Z. Q. Tian, Angew. Chem., Int. Ed., 2014, 53, 2353. 23 B. Dong, Y. Fang, L. Xia, H. Xu and M. Sun, J. Raman Spectrosc., 2011, 42, 1205. 24 B. Dong, Y. Fang, X. Chen, H. Xu and M. Sun, Langmuir, 2011, 27, 10677. 25 X. Tang, W. Cai, L. Yang and J. Liu, Nanoscale, 2014, 6, 8612. 26 W. Cai, X. Tang, B. Sun and L. Yang, Nanoscale, 2014, 6, 7954. 27 L. Yang, P. Li and J. Liu, RSC Adv., 2014, 4, 49635. 28 W. S. Hummers Jr and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339. 29 G. Goncalves, P. A. Marques, C. M. Granadeiro, H. I. Nogueira, M. Singh and J. Gracio, Chem. Mater., 2009, 21, 4796. 30 G. Frens, Nature, 1973, 241, 20. 31 J. Paredes, S. Villar-Rodil, A. Martinez-Alonso and J. Tascon, Langmuir, 2008, 24, 10560. ´, A. Szeri 32 A. B. Bourlinos, D. Gournis, D. Petridis, T. Szabo ´ka ´ny, Langmuir, 2003, 19, 6050. and I. De 33 Z. Zhang, F. Xu, W. Yang, M. Guo, X. Wang, B. Zhang and J. Tang, Chem. Commun., 2011, 47, 6440. 34 P. N. Njoki, I.-I. S. Lim, D. Mott, H.-Y. Park, B. Khan, S. Mishra, R. Sujakumar, J. Luo and C.-J. Zhong, J. Phys. Chem. C, 2007, 111, 14664. 35 X.-J. Zhang, G.-S. Wang, W.-Q. Cao, Y.-Z. Wei, M.-S. Cao and L. Guo, RSC Adv., 2014, 4, 19594. 36 Y. Qin, J. Li, Y. Kong, X. Li, Y. Tao, S. Li and Y. Wang, Nanoscale, 2014, 6, 1281. 37 Z. Dai, X. Xiao, Y. Zhang, F. Ren, W. Wu, S. Zhang, J. Zhou, F. Mei and C. Jiang, Nanotechnology, 2012, 23, 335701. 38 K. Jasuja and V. Berry, ACS Nano, 2009, 3, 2358. 39 Y. K. Kim, H. K. Na, Y. W. Lee, H. Jang, S. W. Han and D. H. Min, Chem. Commun., 2010, 46, 3185. 40 J. Lee, S. Shim, B. Kim and H. S. Shin, Chemistry, 2011, 17, 2381. 41 X. Ren, E. Tan, X. Lang, T. You, L. Jiang, H. Zhang, P. Yin and L. Guo, Phys. Chem. Chem. Phys., 2013, 15, 14196. ¨cker, J. Am. Chem. Soc., 42 W. Xie, B. Walkenfort and S. Schlu 2012, 135, 1657. 43 P. Tan, Y. Deng, Q. Zhao and W. Cheng, Appl. Phys. Lett., 1999, 74, 1818. 44 W. Xu, J. Xiao, Y. Chen, X. Ling and J. Zhang, Adv. Mater., 2013, 25, 928.
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Direct observation of enhanced plasmon-driven catalytic reaction activity of Au nanoparticles supported on reduced graphene oxides by SERS† Xiu Liang,a Tingting You,a Dapeng Liu,a Xiufeng Lang,b Enzhong Tan,c Jihua Shi,a Penggang Yin*a and Lin Guo*a Graphene-based nanocomposites have recently attracted tremendous research interest in the field of catalysis due to their unique optical and electronic properties. However, direct observation of enhanced plasmon-driven catalytic activity of Au nanoparticles (NPs) supported on reduced graphene oxides (Au/rGO) has rarely been reported. Herein, based on the reduction from 4-nitrobenzenethiol (4-NBT) to p,p 0 -dimercaptoazobenzene (DMAB), the catalytic property of Au/rGO nanocomposites was investigated
Received 12th February 2015, Accepted 9th March 2015
and compared with corresponding Au NP samples with similar size distribution. Our results show that
DOI: 10.1039/c5cp00908a
chemical reactions. In addition, systematic comparisons were conducted during power- and time-
Au/rGO nanocomposites could serve as a good catalytic and analytic platform for plasmon-driven dependent surface-enhanced Raman scattering (SERS) experiments, which exhibited a lower power
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threshold and higher catalytic efficiency for Au/rGO as compared to Au NPs toward the reaction.
1. Introduction When new catalysts are introduced, characterized, and developed, investigation of the catalytic activity and reaction kinetics becomes an essential issue. A variety of molecular surface spectroscopic techniques, including ultraviolet-visible,1 fluorescence,2 Raman,3 and infrared spectroscopy,4 have been employed to study reactions occurring at the surface of metal structures. However, limited spatial and time resolution hinders their wide application in the nanoscale. Fortunately, surface-enhanced Raman scattering (SERS), which was discovered in 1974,5 is advantageous because of its high sensitivity and selectivity and has become an attractive and powerful tool for analysis and detection.6–9 Based on the mechanisms of electromagnetic enhancement (near-field effect) and chemical enhancement (long-range effect),10,11 SERS plays a vital role in probing the reaction process and kinetics through real time and in situ fingerprint spectra of reactants or intermediates in a variety
a
Key Laboratory of Bio-inspired Smart Interfacial Science and Technology of the Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing, China. E-mail: [email protected] b Department of Physics, Hebei Normal University of Science and Technology, Qinhuangdao, 066004, China c Department of Mathematics and Physics, Beijing Institute of Technology Petrochemical, Beijing, 102617, P. R. China † Electronic supplementary information (ESI) available: EDX spectrum and timedependent SERS spectrum of Au/rGO, Au NPs and GO. See DOI: 10.1039/ c5cp00908a
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of reactions occurring at metal surfaces or electrochemical interfaces.12,13 Integrating high plasmonic properties and catalytic activity, Au NPs are considered to be a good SERS substrate candidate for monitoring catalytic reactions. However, Au NPs aggregate easily in order to disperse high surface energy. Consequently, various substances, for example, metal oxides,14 polymers,15 and carbon materials,16 are introduced to anchor Au NPs. As recently demonstrated, graphene has been employed as an excellent burgeoning support to stabilize and disperse nanoparticles in virtue of its large surface area, extraordinary electronic transport properties, and strong mechanical strength in catalytic fields. For example, Li et al. reported using hydrothermal conditions to prepare Au/rGO sheets that exhibited excellent catalytic performance towards the reduction of 4-nitrophenol to 4-aminophenol.17 Additionally, Au/rGO SERS studies have also recently attracted much attention, due to the coupled electromagnetic field present on Au/rGO surfaces.18 Plasmon-driven chemical reactions have seen a surge of interest because they create a new path to control and monitor the catalytic reactions on metallic substrates along with the development of SERS19 and TERS (tip-enhanced Raman scattering).20 It has been ascertained by direct experimental and theoretical evidence that the molecule p,p0 -dimercaptoazobenzene (DMAB) was produced from PATP (p-aminothiophenol) by a plasmon-driven surface-catalyzed reaction.21 After scrutinizing the selective oxidation process of PATP to DMAB on Au and Ag nanostructures via SERS, SPR-assisted (surface plasmon resonance) activation of the 3O2 mechanism was introduced to sufficiently explain catalytic reactions.22 More recently,
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the plasmon-driven chemical reaction of 4-NBT into DMAB molecules on Ag and Cu films was further confirmed by SERS.23 Sun and co-workers demonstrated that the dimerization reaction from 4-NBT to DMAB possessed substrate-, wavelength-, and timedependent features.24 Very recently, further study of the kinetics of this plasmon-driven chemical reaction was conducted in situ using spatiotemporal SERS on a single peony-like silver microflower particle.25 Furthermore, multifunctional magnetic composite nanomaterials, such as Fe3O4/C/Au NPs26 and Fe3O4@C@Ag NPs,27 which possess more than two functional properties, have been attracting significant research interest in the fields of SERS plasmon-driven catalytic monitoring or SERS detection. To the best of our knowledge, no study has been reported that used SERS to investigate the plasmon-driven chemical reaction of 4-NBT dimerizing into DMAB on Au/rGO nanocomposite substrates until now. Herein, Au/rGO nanocomposites were synthesized through an in situ chemical reduction method and applied as a SERS substrate for monitoring the plasmon-driven surface-catalyzed reaction from 4-NBT to DMAB. Furthermore, we compared the catalytic activities of Au/rGO nanocomposites with Au NPs of the same size by utilizing power- and time-dependent SERS experiments that determined the enhanced catalytic activity of the Au/rGO sample.
2. Experimental section 2.1
Materials
Graphite powder (300 mesh, 99.9999%) was purchased from Alfa Aesar; hydrogen tetrachloroaurate(III) tetrahydrate (HAuCl44H2O) was purchased from the Kermel Chemical Regent Co. Ltd (Tianjin, China); trisodium citrate was obtained from the Beijing Chemical Regent Company (China); 4-NBT was purchased from J&K Chemical Company (China). All other reagents were of analytical grade and used without further purification. Highly purified water (Millipore Milli-Q System) was used throughout the experiments. 2.2
Synthesis of samples
Synthesis of graphene oxide (GO). GO was produced by chemical exfoliation and oxidization from natural graphite powder using Hummer’s method with some modification.28 In brief, 7.2 mL of concentrated H2SO4 was mixed with 0.9 g of graphite, 1.5 g of K2S2O8, and 1.5 g of P2O5 at 80 1C in a roundbottom flask. After stirring for 4.5 h, the mixture was thoroughly washed with water. Afterwards, the pre-oxidized graphite was maintained at 0 1C in an ice bath while 0.5 g of NaNO3 and 23 mL of concentrated H2SO4 was slowly added, followed by the slow addition of 3 g of KMnO4. Subsequently, the solution was warmed and kept at 35 1C with stirring for 2 h. Then, 86 mL of deionized water and 5 mL of H2O2 (30%) was slowly added, and the color of the solution changed to brilliant yellow with bubbling. Finally, the mixture was thoroughly washed with water and subjected to dialysis, followed by lyophilization. The exfoliation of the as-prepared graphene oxide was accomplished by dispersion in water (0.63 mgmL1) under ultrasonication for 2 h, and the mixture was then centrifuged to produce a homogeneous yellow dispersion.
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Synthesis of Au/rGO. The synthesis of Au/rGO nanocomposites was based on an in situ reduction method of the Au(III) complex by sodium citrate as previously reported with some modification.29 Briefly, 5.95 mL of the as-prepared GO aqueous suspension (0.63 mg mL1) was added to 50 mL of HAuCl4 solution (0.01%) in a round-bottom flask with stirring. The resultant suspension was aged for at least 1 h at room temperature. Then, the solution was heated to 80 1C, and 940 mL of sodium citrate (0.085 M) was added dropwise. After reaction for one hour, the mixed solution was cooled to room temperature. The precipitate was washed with water and centrifuged at 6000 rpm for more than three times to remove any free Au NPs. Finally, the Au/rGO was added to water to form a stable dispersion. Synthesis of Au NPs. For comparison, the citric-capped Au NPs with the same diameter were prepared by the typical method reported by Frens.30 In brief, 50 mL of 0.01% HAuCl44H2O aqueous solution was heated to boiling under vigorous stirring. Subsequently, 1 mL of 1% sodium citrate solution was rapidly added. The solution was refluxed for another 30 min to complete the crystal nucleus formation and growth process of gold before cooling to room temperature. The final concentration of the as-obtained Au was kept under the same conditions as the Au/rGO solution. Preparation of samples for SERS. Au/rGO and Au NPs were functionalized with 4-NBT via self-assembly. 1 mL of a 2 103 M 4-NBT ethanol solution was mixed with 1 mL of Au/rGO or Au colloid dispersion for 30 min. Afterwards, centrifugation and ethanol washing were used to remove the unbound 4-NBT molecules. The products were re-dispersed in the same volume of water to form a stable suspension. Finally, the dip-coating method was used to assemble the 4-NBT-functionalized Au/rGO and Au NPs onto the SiO2/Si substrate for immediate SERS measurements. 2.3
Characterization
TEM images were obtained using a JEM-2100F transmission electron microscope operated at 200 kV. FTIR spectra were recorded on an FTIR spectrometer-733 (iN10MX). The UV-vis spectrophotometer and fittings were provided by Shanghai Lab-Spectrum Instruments (Shanghai, China) Co., Ltd Raman spectra were recorded with a JY HR800 Raman spectrometer (HORIBA Jobin Yvon), equipped with a 50 objective (NA = 0.5) and an Ar–Kr laser with 647 nm wavelength; the laser power values measured in the experiments were obtained from the power meter (mW at the samples with a spot area of approximately 1.4 mm2). The Raman band of a silicon wafer at 520.8 cm1 was used to calibrate the spectrometer.
3. Results and discussion 3.1
Formation and characterization of Au/rGO
FTIR spectra were used to confirm the existence of oxygen functionalities at the GO surface that served as the nucleation sites for the growth of Au NPs in the synthetic procedure of Au/rGO nanocomposites.29 As seen in Fig. 1, the characteristic
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Fig. 1
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FTIR spectra of prepared GO nanosheets.
peaks for the CQO carbonyl stretching vibration appear at 1725 cm1. For the epoxy C–O–C group, the stretching vibration peaks appear at 1227 cm1, and the peaks at 1050 cm1 can be assigned to the C–O stretching vibration. Another obvious peak at 1622 cm1 denotes the main CQC sp2 character of the carbon framework.31,32 Consequently, the FTIR results provide direct evidence of the successful oxidation of graphite powder with abundant oxygen groups on the surface of the GO nanosheets. The TEM image (Fig. 2A) of the GO morphology shows translucent sheets with wrinkles and folds, indicating the thin layered structure of the well exfoliated GO. Fig. 2B shows the TEM images of Au/rGO nanocomposites in which Au NPs with an average diameter of 14.5 nm were loaded on the surface of the GO nanosheets. The corresponding energy-dispersive X-ray (EDX) spectroscopy (Fig. S2, ESI†) of Au/rGO shows the peaks of elemental C, O, and Au, firmly confirming the existence of AuNPs on the surface of the GO nanosheets. For comparison, the sample of Au NPs (d = 14.7 nm) without GO was also characterized by TEM, as shown in Fig. 2C. It is clear that in
both samples, the Au NPs exhibit a narrow particle size distribution. The successful preparation of Au/rGO was further confirmed by UVvis absorption spectroscopy. As shown in Fig. 2D, the absorption spectrum of the GO suspension (blue line) shows a broad peak at 230 nm, which is attributed to the p–p* electron transition of the CQC bands, as well as a shoulder peak at approximately 300 nm that is attributed to an n–p* electron transition of the CQO bands.33 After decoration with Au NPs onto the surface of GO nanosheets, a new obvious peak at 520 nm (black line) appeared that was due to the SPR of Au NPs. Such a change in the absorption spectrum further characterizes the formation of Au/rGO nanocomposites. Furthermore, it was noted that the position of the absorption peak of the prepared bare Au NPs remained the same as that of the Au/ rGO due to the same Au size distribution for both that results in consistency of Au-related SPR properties.34 Raman spectroscopy is a useful, non-destructive tool that can be used to detect significant structural changes of the carbon framework. Fig. 3 shows the Raman spectra of pristine graphite, GO, and Au/rGO nanocomposites. The D peak at 1340 cm1 and the G peak at 1570 cm1 are the most prominent signals observed in graphite samples. GO exhibits two characteristic peaks of D and G bands at 1351 and 1600 cm1, which could be attributed to the symmetry A1g vibration mode and the E2g vibration mode of sp2 carbon atoms, respectively. As for the Au/rGO, the corresponding D and G bands appear at 1340 and 1589 cm1, respectively. The redshift of the G band for Au/rGO nanocomposites from 1600 to 1589 cm1 relative to GO is assigned to the recovery of the hexagonal network of carbon atoms.35 In addition, after the reduction of sodium citrate, Au/rGO shows an increased intensity ratio of D/G (1.42) than that of GO (0.92), indicating a decrease in the size of the in-plane sp2 domains and a relatively ordered crystal structure of Au/rGO. This indicates that GO is relatively reduced in Au/rGO nanocomposites.33 Furthermore, the Raman intensities of Au/rGO were found to be enhanced by five-fold compared to pure GO, which could be attributed to the enhanced local electromagnetic field induced by the anchored Au NPs on the GO surface.36 3.2
Surface plasmon-driven catalytic reaction activity
Sample Au NPs and Au/rGO with the same Au size of approximately 15 nm were systematically investigated by comparing the catalytic activity toward the reduction of 4-NBT to DMAB on both kinds of substrates by SERS. Fig. 4 shows the scheme to
Fig. 2 TEM images of the as-prepared (A) GO nanosheets, (B) Au/rGO nanocomposites, and (C) Au NPs; (D) UV-vis spectra of the abovementioned three samples.
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Fig. 3
Raman spectra of graphite, GO, and Au/rGO nanocomposites.
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monitor the reaction on Au/rGO using the SERS technique. Firstly, 4-NBT molecules were adsorbed onto Au/rGO via a selfassembly method. Then, direct observation of SERS spectra was carried out on the Au/rGO to monitor the reduction process in situ. Fig. 5a is the normal Raman spectrum (NRS) of 4-NBT powders in solid state for comparison, and there exists three main strong Raman peaks at 1099, 1332, and 1574 cm1, characterizing the S–C stretching vibration, the symmetric NO2 stretching vibration, and the C–C stretching vibration of the benzene ring, respectively.37 Fig. 5b and c show the SERS spectra of 4-NBT adsorbed on Au NPs and Au/rGO nanocomposites under 4 mW laser irradiation after 50 s, respectively. Both spectra have identical features that are distinctively different from the NRS of 4-NBT. New peaks at 1141 cm1 (bCH + nCN), 1390 cm1 (nNN + nCC + bCH), and 1441 cm1 (nNN + nCC + bCH) appear, which have been demonstrated to be the characteristic peaks of the produced p,p0 -dimercaptoazobenzene (DMAB).23,24 This result firmly indicates that the photoreaction of 4-NBT to DMAB takes place on the surfaces of Au NPs and Au/rGO nanocomposites while being exposed to 4 mW from the 647 nm laser. However, obvious differences of the Raman spectra between Fig. 5b and c can be also observed. On one hand, the measured relative Raman intensities at 1141, 1390 and 1441 cm1 to 1332 cm1 of 4-NBT adsorbed on the Au/rGO nanocomposites are much higher than those adsorbed on the Au NPs, indicating a larger degree of reaction conversion from 4-NBT to DMAB on Au/rGO substrate as compared to the Au NPs. On the other hand, all peaks of Au/rGO
Fig. 4 Schematic illustration of the plasmon-driven surface-catalyzed reaction of 4-NBT dimerizing to DMAB that is monitored on Au/rGO nanocomposites.
Fig. 5 Normal Raman spectrum of 4-NBT solid (a) and the SERS spectra of 4-NBT adsorbed on Au NPs (b) and Au/rGO nanocomposites (c). The laser power was 4 mW and the acquisition time was 50 s.
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possess significantly higher intensities than those of the pure Au NP sample. These SERS effects are consistent with previously reported results, which originate from chemical effects improved with the addition of GO substrates.38–40 It was reported that the plasmon-driven conversion of 4-NBT into DMAB on Ag or Cu was influenced by the power of an incident laser.23 Fig. 6(a) shows the NR spectra of 4-NBT, and Fig. 6(b–f) shows the SERS spectra of 4-NBT-functionalized Au/ rGO nanocomposites (A) and Au NPs (B) under the laser power of 0.04 (b), 0.4 (c), 1.2 (d), 4 (e), and 8 mW (f), respectively. As for Au/rGO, the spectral pattern at the laser power of 0.04 mW is nearly the same as that in the neat solid state except for a small SERS peak at 1110 cm1 assigned to the vibrational mode of d(C–H) for 4-NBT. When the laser power was increased to 0.4 mW (Fig. 6c in A), three characteristic peaks of DMAB at 1142, 1390, and 1441 cm1 appeared. If the laser power was further increased, the three peaks of DMAB became more obvious, and the peak at 1332 cm1 decreased as 4-NBT molecules were reduced to DMAB. A similar tendency was also observed in the case of pure Au NPs substrates (Fig. 6B). Consequently, a power threshold of the incident laser existed for observation of the phenomenon in our system. The power threshold for triggering the conversion from 4-NBT to DMAB on Au NPs increased to 4 mW (Fig. 6e in B). The bands at 1441 cm1 were produced by new species of DMAB, and the band at 1332 cm1 was mainly produced by the initial reactant 4-NBT.23 Thus, the two bands were chosen to represent the reaction efficiency. As shown in Fig. 7, the 1441 cm1/1332 cm1 intensity ratios as a function of the laser power were plotted as a quantitative measure of the reaction progress from 4-NBT to DMAB. It was found that the relative Raman intensity steadily increased with the laser power on both Au/rGO nanocomposites and Au. Based on well-known SPR-mediated reaction mechanisms, the additional hot electrons created by a higher laser power may cause more 4-NBT molecules to convert into DMAB.24 When the laser power was varied from 0.04 to 8 mW, the intensity ratios ranged from 0.02 to 0.6 for Au/rGO, while values from 0.01 to 0.15 were obtained for pure Au NP samples, indicating a larger reaction conversion from 4-NBT into DMAB on Au/rGO. To further monitor the reaction over time, the reaction kinetics of 4-NBT dimerizing into DMAB was investigated and
Fig. 6 Normal Raman spectrum of 4-NBT solid (a) and the SERS spectra of 4-NBT adsorbed on Au/rGO (A) and Au NPs (B) under the laser power of (b) 0.04, (c) 0.4, (d) 1.2, (e) 4, and (f) 8 mW, respectively. All Raman spectra were collected after a 20 s laser exposure.
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Fig. 7 Obtained DMAB/4-NBT 1441/1332 cm1 intensity ratios as a function of the laser power under 647 nm excitation.
comparatively studied using time-dependent SERS spectra by continuously illuminating the samples with a laser power of 1.2 mW. Fig. S3 (ESI†) shows the characteristic peaks of the changing tendency of DMAB and 4-NBT with time elapsing. The three characteristic peaks of DMAB at 1142, 1390, and 1441 cm1 started to emerge within 10 s, and their intensities gradually increased until achieving the maximum for Au/rGO nanocomposites. The relative Raman intensities of bands at 1142, 1390, and 1441 cm1 to 1332 cm1 for Au/rGO and Au substrates were plotted as a function of laser irradiation time in Fig. 8. As for Au/rGO, the relative Raman intensities of 1142, 1390, and 1441 cm1 to 1332 cm1 increased drastically from 0 to approximately 0.6, 0.55, and 0.7, respectively, within 120 s. In the next 300 s, changes in the relative Raman intensity were less than 10%, which demonstrates that more and more DMAB molecules were produced from 4-NBT in a short time, and then the transformation approached a stable maximum. In contrast, the relative Raman intensities of 1142, 1390, and 1441 cm1 to 1332 cm1 on Au NP samples without rGO substrates required at least 250 s to reach a maximum of 0.15 to 0.18. Therefore, a higher conversion rate and catalytic efficiency of 4-NBT were achieved on Au/rGO substrates under the same conditions. It is worth mentioning that after the maximum conversion, the fingerprint spectra revealed that 4-NBT and DMAB coexisted in this system. The complete conversion from 4-NBT to DMAB can be achieved with the addition of NaBH4 reductant as previously reported.41,42 However, one dispute exists in time-dependent, plasmondriven catalytic reactions detected by SERS, which states that
illumination with high laser power over a long period of time might cause the thermal effect on the focal spot of substrates to accelerate the conversion reaction.43 To avoid this effect, timedependent SERS experiments with Au/rGO and Au NPs catalysts were conducted under the lower laser power of 0.4 mW. As shown in Fig. S4A (ESI†), it took approximately 300 s for the catalytic reaction of 4-NBT to reach a stable conversion rate of 0.35–0.43 on Au/rGO (Fig. S4C, ESI†), while no obvious characteristic peaks of DMAB were observed within 500 s on Au NPs without graphene (Fig. S4B, ESI†). An enhanced catalytic efficiency and conversion rate of 4-NBT to DMAB on Au/rGO as compared to that of bare Au NPs were confirmed as well as the exclusion of the impact of thermal effects in this system. Separate experiments were performed to exclude the influence of GO on the characteristic bands. Fig. S5 (ESI†) depicts the time-dependent SERS spectra of 4-NBT molecules (103 M) adsorbed on blank GO nanosheets under a 4 mW 647 nm laser. The spectra shows only characteristic peaks of GO, and no characteristic band related to DMAB was observed, suggesting that GO itself is unlikely to catalyze the dimerization reaction. It has been previously reported that catalytic activity for carbon-supported metal nanocomposites was greatly enhanced as compared to that of metal NPs. For example, Huang et al. conducted a comparative catalytic activity study of Au and Au–rGO nanocomposites in the reduction reaction of 4-nitroaniline to 1,2-benzanediamine by NaBH4, and it was observed that the Au–rGO nanocomposites exhibited higher catalytic activities than the Au NPs alone.18 They explained that the higher catalytic performance of the Au–GO and Au–rGO should be attributed to the electronic interactions between the metal and graphene. In this study, the aforementioned higher reaction efficiency and faster reaction rate for the Au/rGO as compared to that of the bare Au nanospheres under the same conditions may be explained as follows. It was demonstrated that the surfaceinduced photoreduction reaction from 4-NBT to DMAB involves charge transfer from the substrates to the adsorbed molecules,24 and thus it is very likely that the enhanced electrical conductivity of rGO accelerated the transfer of the electrons from the Au NPs to the molecules, resulting in an enhanced catalytic efficiency and rate. Furthermore, the saturation adsorption of 4-NBT molecules in the preparation of SERS substrates was easily achieved. In our system, rGO, which served as a molecular enricher,44 can adsorb more 4-NBT molecules by p–p stacking interactions to provide more 4-NBT near the Au nanoparticles, resulting in a higher reaction efficiency of Au/rGO due to highly efficient contact between them in the context of the same Au NP density distribution on the two substrates.
4. Conclusion 1
Fig. 8 Relative Raman intensity of bands at 1142, 1390, and 1441 cm to 1332 cm1 under excitation with a laser power of 1.2 mW on Au/rGO and Au NPs.
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In summary, we have successfully synthesized Au/rGO nanocomposites through a simple in situ chemical reduction method, which was confirmed by characterization of TEM, UV-vis, FTIR, and Raman spectra. The prepared Au/rGO nanocomposites can
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be used as SERS substrates to monitor the process of the plasmondriven, surface-catalyzed reaction from 4-NBT to DMAB. Powerdependent SERS spectra for Au/rGO revealed a lower threshold power of 0.4 mW as compared to that of 4 mW on Au NPs under the same conditions. In addition, a larger reaction conversion from 4-NBT to DMAB was obtained on Au/rGO. Excluding the impact of a thermal effect, enhanced reactivity was achieved on the Au/rGO nanocomposites during the time-dependent SERS spectra. We suggest that excellent electrical conductivity as well as molecular adsorption ability of graphene makes the Au/rGO a more efficient catalyst than 15 nm Au NPs in the plasmon-catalysis reduction from 4-NBT to DMAB by SERS. The results of this study can assist with increasing our understanding of the interaction between metal and graphene as well as plasmon-driven chemical reactions.
Acknowledgements The work was supported by the National Natural Science Foundation of China (51272013).
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