Unusual functionalization of reduced graphene oxide for excellent chemical surface-enhanced Raman scattering by coupling with ZnO
Introduction
Graphene is a novel carbon nanomaterial of a two-dimensional conjugated structure, offering unique electronic properties, and has been intensively studied for many applications, such as electronic devices [1], solar cells [2], photocatalysts [3], and sensors [4]. There has been growing interest in graphene-based surface-enhanced Raman scattering (SERS) substrates for bio-chemical sensing due to the high surface area and great electrical conductivity of graphene. For example, Au nanoparticle (NP)–graphene oxide (GO) [5] and Ag–graphene [6] composites were recently synthesized for chemical sensing and cell imaging by taking advantage of the electromagnetic enhancement of plasmonic metal NPs. In addition, an ultraviolet/ozone treatment [7] has been used to oxidize graphene and demonstrated an efficient improvement in SERS compared with pristine graphene, due to charge transfer-induced chemical enhancement. However, these high-performance SERS substrates usually need expensive noble metals to enhance plasmonic resonance or toxic O3 gas to activate graphene. It thus remains a challenge to fabricate low-cost graphene-based SERS substrates with excellent enhancement performance.
In this work, an excellent SERS substrate comprised of annealed low-cost reduced GO (rGO)/ZnO composites is firstly demonstrated. The morphologies, structures, and functional groups of the rGO/ZnO composites after different heat treatments show significant variations, indicative of strong reactions between rGO and ZnO. The high SERS enhancement factor (EF) of about 104, even better than those reported for Ag/GO [8], Au/GO [5], Au/rGO [9], and Au/graphene [10] composites, is mainly attributed to the abundant oxygen-containing groups unusually generated on the surface of rGO by coupling with ZnO NPs at moderate temperature, as seen in the Fourier transform infrared spectroscopy (FTIR) spectra. This work demonstrates the preparation of low-cost SERS substrates with excellent performance by reactions of rGO with ZnO, which have good potential for bio-chemical sensing applications.
Section snippets
Preparation of rGO/ZnO composites
GO flakes were prepared following Hummers method [11]. The GO dispersion was prepared by dispersing 0.5 g GO flakes in 500 ml distilled water using centrifugal separation at a speed of 8000 rpm for 40 min. GO dispersion of 0.5 ml was then dropped onto a ZnO nanorod (NR)/Si substrate, synthesized by a hydrothermal route [12], and dried at 90 °C for 5 min. The dropping/drying processes were repeated for 10 cycles to obtain a pristine GO/ZnO composite. The prepared GO/ZnO composites were then annealed at
Morphologies, microstructures, and crystal structures of the rGO/ZnO composites
Based on the SEM results, the morphologies of the GO/ZnO composites show significant variations after annealing at different temperatures. Fig. 1(a) and (b) show the SEM images of the ZnO NRs and GO/ZnO composites before annealing, respectively. The GO flakes are uniformly attached to the surface of the NRs. After annealing at 150–200 °C for 1 h, the GO flakes were transformed into film-like structures, and small dots of lighter contrast were generated on the films, as shown in Fig. 1(c) and (d).
Conclusion
This work presents a novel and efficient SERS substrate based on low-cost rGO/ZnO composites. The high EF value, about 104 for R6G, is mainly attributed to the unusually abundant oxygen-containing groups generated on rGO by coupling with ZnO at a moderate temperature. The SEM, TEM, and XRD results show numerous ZnO NPs were generated on the surface of rGO, and the d-spacing of GO gradually decreased after the annealing treatments. Unusually, the functional groups of rGO/ZnO increased
Acknowledgement
This work was supported by the National Science Council of Taiwan under the Grant (NSC100-2221-E-390-009-MY3).
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