ZnO/Ag porous nanosheets used as substrate for surface-enhanced Raman scattering to detect organic pollutant

Abstract

Self-assembled porous ZnO nanosheets were fabricated through an one-step solvent method which is convenient and environmentally friendly, and then silver nanoparticles were deposted on to it to make a type of hybrid material. The ZnO/Ag composite was used as the substrate for surface enhanced Raman scattering (SERS) to measure the Rhodamine 6G (R6G) molecules. Strong SERS signals were obtained when the concentration of the probe molecules was as low as 10⁻¹³ M. Besides, the substrate exhibited light-assisted self-cleaning properties under UV irradiation. Thus, the as-prepared nanomaterials showed potential application for environmentally friendly and economical organic pollutant detection.

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Introduction

As we know, organic pollutants heavily affect the health of human beings, and also damage the growth of plants; therefore, they interfere or destroy the ecological balance seriously.^1–4 Thus, many researchers has developed a variety of methods to detect organic pollutants, like gas chromatography, liquid chromatography and ultraviolet detection.^5–7 As one of the developing detection technologies in current analytical chemistry, surface-enhanced Raman scattering (SERS) has attracted more and more attention.

The surface-enhanced Raman scattering, due to its high sensitivity, rapid response, and spectroscopic precision, has been widely used in various fields. SERS holds promising potential not only for chemical, physical, medical, biomolecular sensing, but also for sensing environmental pollution and application in life science. The Raman cross section is inherently small, comparatively, SERS renders an obviously increase in Raman signals. In order to expand the advantages and application in the detection of organic pollutants, researchers attempted to improve the SERS effects in various nanostructures, and obviously it is necessary to develop a stable SERS substrate to provide stronger signal enhancement.^8–10

For many years, noble metals like Au, Ag, Cu nanostructures were used as traditional SERS substrates, due to the electromagnetic enhancement caused by surface plasmon resonance mechanism of the metal.^11–16 Recently, it was found that many semiconductors, such as TiO₂, Fe₃O₄, ZnO, also possess SERS activity.^17–19 Particularly, ZnO, as an important semiconductor material with high chemical stability, has a wide direct band gap of 3.37 eV and a large exciton binding energy of 60 meV. Thus, ZnO has received great attention, due to its potential application as SERS substrate.^20–23 For example, Terakawa et al. used ZnO nanorods as substrate to detect the Raman signals of Rhodamine 6G (R6G) at 532 nm; the concentration of detection was as low as 1 μM.²⁴

In recent years, a lot of investigations have addressed the huge advantages of using “hybrid materials” as SERS substrates, and composite semiconductors (ZnO, and TiO₂) and noble metals (Au and Ag) as substrates have attracted great attention.^25–38 According to previous reports, noble metals possess SERS activity owing to the electromagnetic enhancement caused by surface plasmon resonance mechanism.^39–41 Whereas, semiconductors have SERS activity because of chemical enhancement, which is caused by charge transfer between the noble metal and the semiconductor.^42,43 Recently, several synthesized ZnO/noble metals nanocomposites as hybrid SERS substrates have also been reported. For example, urchin-like Ag nanoparticle/ZnO hollow nanosphere arrays were fabricated as a SERS substrate, using Rhodamine 6G (R6G) as the probe with the detection limit as low as 10⁻¹⁰ M, and the enhancement was as high as 10⁸.⁴⁴

Herein, we report the preparation of a porous ZnO nanosheets decorated with Ag nanoparticles as a hybrid SERS substrate, using Rhodamine 6G (R6G) as the SERS active molecule pumped at 633 nm. The limit of detection is as low as 10⁻¹³ M. The results demonstrate that our work reveals a simple and convenient method to synthesize a hybrid SERS substrate, and the substrate possesses high sensitivity for potential SERS applications.

Experimental

Chemicals

Zinc acetate dehydrate, sodium hydroxide, silver nitrate, didodecyldimethylammonium bromide (DDAB), ethanol (Beijing Chemical Works), ethylene glycol (Xilong Chemical Co.) were used without any further purification.

Preparation of ZnO nanosheets

The porous ZnO nanosheets were fabricated using a one-step solvent method which is convenient and environmentally friendly.⁴⁵ The synthesized porous nanosheets were self-assembled by the ZnO nanoparticles. In this method, the precursor not only acts as the template generators, but also as the building blocks at the same time. Addition of didodecyldimethylammonium bromide (DDAB) was used to achieve better monodispersity of the nanosheets, but did not destroy the growth process. Then, ZnO/Ag nanocomposites were synthesized by the deposition of Ag onto the ZnO nanosheets.

Preparation of ZnO/Ag nanosheets

ZnO/Ag nanocomposites were synthesized by the growth of Ag on the ZnO nanosheets.⁴⁶ Firstly, ZnO production should be heated in a muffle furnace at 500 °C for 2 h, in order to achieve better crystallinity and stability, and then 30 mg ZnO nanosheets was dissolved in ethylene glycol with sonication. 0.5 mL of AgNO₃ (0.01 g mL⁻¹) aqueous solution was dropped into the ZnO suspension under strong stirring. The dropping rate is about one drop per 30 seconds. The colour of the mixture changed slowly from white to yellow to claybank. After 4 h stirring, the final product was collected, washed with ethanol and deionized water several times, and dried in air at 60 °C for 12 h. Finally, after drying, the production was heated under a nitrogen atmosphere in a quartz tube furnace at 500 °C for 2 h.

Preparation of sample for SERS measurement

For the SERS substrate preparation, ZnO nanosheets were dispersed on a cleaned silicon plate, and the dried silicon was then immersed in R6G ethanol solution for 6 h. The above substrate with sample was rinsed with ethanol several times to make sure no free R6G molecules were left on the surface of the ZnO before Raman measurements.

Materials characterization

The morphologies of the nanosheets were obtained by scanning electron microscope (SEM, Hitachi S4800 cold field-emission) and transmission electron microscope (TEM, JEOL JEM-2100F microscope). The measurements of Raman spectra were performed with a Jobin Yvon Raman spectrometer model HR800. The 514.5 nm line from an Ar–Kr ion laser and 633 nm line from He–Ne laser were used as excitation source. The laser power on the surface of the sample was typically 1.5 mW. The accumulation time was 10 s.

Results and discussion

The morphology of the as-fabricated ZnO nanosheets was examined by Scanning electron microscopy (SEM) images. From Fig. 1, we can clearly see that the ZnO nanosheets are porous parallelograms with sharp edges. The side length of the ZnO sheets is about 200 nm to 600 nm, and the thickness is up to 50 nm. The surface of the nanosheets is rough. This character is important for the following progress. After the nanosheets were immersed into the AgNO₃ solution, ZnO nanosheets were covered with silver ions, and the silver ions were reduced to Ag nanoparticles after the calcination process, thus, the ZnO/Ag substrate was finally obtained.

Fig. 1 Low-magnification (a) and high-magnification (b) SEM images of porous ZnO.

From the TEM images of the porous ZnO nanosheets and the ZnO/Ag nanocomposite, shown in Fig. 2, we can see the surface of the nanosheets has Ag nanoparticles deposited on to it, and we can find that the surface of the ZnO/Ag nanosheets are rougher than that of the pure ZnO nanosheets before deposition. We believe the deposition reaction may have some effect on the morphology of the nanosheets. From the HRTEM image, the interplanar spacing of lattice fringes is 0.283 nm and 0.228 nm, corresponding to the (100) crystal plane of ZnO nanoparticle and (111) crystal plane of Ag nanoparticle. We also found that the distance between the Ag nanoparticles is less than 10 nm (Fig. S2†).

Fig. 2 (a and b) TEM images of porous ZnO nanosheets and ZnO/Ag nanocomposites; (c) HRTEM image of ZnO/Ag nanocomposites.

Fig. 3 shows the EDS spectrum of the ZnO/Ag hybrid nanosheets, in which we can see that Zn, O and Ag are all presented, demonstrating that Ag nanoparticles mixed well with ZnO nanosheets. According to the EDS spectrum and HRTEM image, the good deposition of Ag nanoparticles on the surface of ZnO nanosheets can be confirmed.

Fig. 3 EDS spectra of the obtained ZnO/Ag nanocomposite.

We measured the UV-vis diffuse reflection spectrum of porous ZnO nanosheets and ZnO/Ag nanocomposite, as shown in Fig. 4. The ZnO nanosheets showed an appreciable peak at 365 nm, and the reflectance of the ZnO/Ag nanocomposite has been improved in the visible region. There was an inconspicuous peak from 400 nm to 500 nm, which could be assigned the transverse plasmon mode of Ag nanoparticles. The improvement was not obvious because of the light concentration of the Ag nanoparticles.

Fig. 4 UV-vis diffuse reflection spectrum of porous ZnO nanosheets (black line) and ZnO/Ag nanocomposite (red line).

The porous ZnO nanosheet and ZnO/Ag nanocomposite material was used as substrate to detect SERS activity. R6G probe molecule was used to analyse the difference of the two SERS substrates. As shown in Fig. 5, the ZnO/Ag nanocomposite presented a high enhancement compared with the porous ZnO nanosheets. As a potential substrate, the hybrid composite substrate should reveal high sensitivity and stability. In our SERS experiment, the SERS spectrum of R6G probe molecule absorbed on ZnO/Ag substrate clearly shows characteristic bands at 1 × 10⁻¹³ M concentration as shown in Fig. 6. The band around 611 cm⁻¹ is assigned to out-of-plane deformation vibration of the xanthene ring, and vibrations at 1360 cm⁻¹ and 1648 cm⁻¹ are assigned to in-plane stretch vibrations of the xanthene ring. The result indicates the ZnO/Ag nanocomposite has significant SERS effect in molecule detection.

Fig. 5 The SERS spectra of R6G detected by porous ZnO nanosheets (red) and ZnO/Ag nanocomposite (black).

Fig. 6 SERS spectra of R6G on ZnO/Ag nanocomposite substrate with different various concentrations. The inset picture is the SERS spectra at low concentration (10⁻¹¹ M, 10⁻¹² M, 10⁻¹³ M). Excitation wavelength: 633 nm; power: 1.5 mW; lens: 50× long distance objective; acquisition time: 10 s.

The enhancement mechanism for semiconductor–noble metal substrate has been reported frequently in the literature. There are two main explanations for the mechanism: (i) The Fermi level of noble material is doped into the band gap of the semiconductor, served as doping level, accelerating the charge-transfer process. (ii) The semiconductor acts as a template for noble metal nanoparticles, adjusting and controlling the effect of near-field coupling between nanoparticles, aiming at forming more “hot spots” for high enhancement of SERS scattering. The second mechanism is dominant for the high sensitivity in the SERS experiment, because the low concentration (1 × 10⁻¹³ M) can be detected by Ag nanoparticles, which serves as many “hot spots”.⁴⁷

By comparing the SERS signals for 10⁻⁷ M and 10⁻¹³ M in Fig. 6, a clear difference in peak intensity is observed, which can be explained by the following three reasons. (i) The Raman signal comes from the relaxation process when the molecules come back from the excited state the ground state, which is a nonradiative process by phonons. However, at low concentration of R6G, only several molecules can stimulate the SERS signal, different transition channels result in nonreproducible Raman signal. (ii) SERS enhancement is not only contributed by well-documented electromagnetic “hot spot”, but also influenced by the chemical interactions of ground state and dynamic charge transfer process. (iii) Raman signal enhancement is significantly high when probe molecule absorbed at “hot spot”, nonlinear optics processes will take Raman active molecules to the semi-state, switching Raman signal off and resulting in the blinking or peak splitting phenomenon as shown in Fig. 6 (10⁻⁷ M; 1507 cm⁻¹). In addition, the SERS signal enhanced by 514.5 nm laser wavelength is weaker than 633 nm (Fig. S1†), which can be explained by the surface plasma excitation of Ag nanoparticles under laser irradiation, making electron transfer from Ag to ZnO and forming plenty of electron–hole and electron pairs. These well-separated electron–hole pairs will photochemically decompose the probe molecule absorbed on Ag surface, which in turn reducing Raman signal at 514.5 nm laser excitation.

Except for acting as an effective SERS substrate, ZnO/Ag nanocomposite also exhibited light-assisted self-cleaning properties. The mechanism for self-cleaning activity under UV irradiation can be described as follows: a charge-transfer process from ZnO to Ag excited by UV light would produce highly active oxidative species (˙O₂⁻, ˙OH) on the surface of porous ZnO nanosheets, decomposing the absorbed probe molecules on the surface of ZnO/Ag composite.^48,49 Three different molecules absorbed on ZnO/Ag composite are successfully decomposed one after another by UV irradiation and the SERS effect of the substrate remained effective, which is clearly illustrated in Fig. 7(a). To further investigate the relationship between the self-cleaning property and the substrate SERS activity, Fig. 7(b) shows that the ZnO/Ag substrate could be regenerated without loss of SERS activity by self-cleaning performance under UV irradiation. In situ recycling SERS mapping of ZnO/Ag substrate absorbed by 4-MBA (the peak mapped is 1586 cm⁻¹) and methyl blue is shown in Fig. S2,† which confirms the composite material is a kind of easily recyclable and highly efficient SERS material.

Fig. 7 (a) Recycling SERS experiment of three different molecules absorbed on ZnO/Ag substrate by self-cleaning under UV irradiation. (b) SERS response of methylene blue molecule (peak signal at 1620 cm⁻¹) under repeated UV irradiation. Laser wavelength: 633 nm; power: 1.5 mW; lens: 50× long distance objective; acquisition time: 2 s.

Conclusions

In conclusion, a simple and convenient method was used to fabricate the self-assembled porous ZnO nanosheets, and the ZnO/Ag nanosheets were deposited with silver nanoparticles to make hybrid materials for surface enhanced Raman scattering (SERS) applications. Raman analyses showed that the ZnO/Ag composite nanosheets are highly SERS active substrates to measure Rhodamine 6G (R6G) molecules. The enhancement factor was as high as 10⁸ when the concentration of the probe molecules was as low as 10⁻¹³ M. The SERS effects are not only contributed to electromagnetic “hot spot”, but also influenced by the chemical interactions of ground state and dynamic charge transfer process. Besides, the substrate exhibited light-assisted self-cleaning properties under UV irradiation. The recyclability of the substrate makes SERS detection more environmentally friendly and economical, showing great potential for the detection of organic pollutants and application in life science.

Acknowledgements

The project was supported by the National Natural Science Foundation of China (51272012 & 21471013).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra06735a

‡ These authors contributed equally to this work.

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