Ag–Cu₂O composite microstructures with tunable Ag contents: synthesis and surface-enhanced (resonance) Raman scattering (SE(R)RS) properties

Abstract

Metal–semiconductor composite microstructures have recently been demonstrated to possess potential applications due to their unique structures. Ag–Cu₂O composite microstructures (Ag–Cu₂O CMSs) with tunable silver content have been synthesized with a facile in situ method. Ag contents on the surface of Cu₂O can be tuned through the variation of the concentration of AgNO₃, which further greatly affected the surface-enhanced (resonance) Raman scattering (SE(R)RS) performance. The Ag–Cu₂O CMSs prepared with 0.4 mM AgNO₃ show the optimum SE(R)RS properties, better than pure Ag NPs. Furthermore, the enhancement mechanism and uniformity of Ag–Cu₂O CMSs are investigated in detail.

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1. Introduction

Cu₂O is a p-type semiconductor oxide with a band gap of 2.17 eV, which makes it an excellent candidate for catalysis, gas sensors, chemical templates, CO oxidation, solar driven water splitting, fuel cells and solar cells.^1–8 Over the past decades, an impressive effort has been devoted to the synthesis of Cu₂O with designed shapes and desired functions, including polyhedral nanoparticles,^9–12 nanosheets/rods/wires/tubes,^13–16 hollows, nanocages and nanoframes.^17–21 However, just a few studies have reported the use of Cu₂O as Raman active substrate and Cu₂O can usually generate weak Raman activity.^22–25

Surface-enhanced (resonance) Raman scattering (SE(R)RS) is a powerful analytical tool for determining chemical information of molecules on substrates for surface science, analytical chemistry, and biology.^26–30 With SE(R)RS, extremely small amounts of substances can be detected; even single molecule detection has been reported.^31–33 During the past few decades, SE(R)RS active substrate has been developed significantly. Noble metals and transition metals, such as Au,³⁴ Ag,³⁵ Cu,³⁶ Pt,³⁷ Pd,³⁸ Co³⁹ and Ni⁴⁰ have been extensively employed owing to their large enhancement. Semiconductor materials have been used as SE(R)RS active substrate and caused increasing attention due to the widespread application in both SE(R)RS spectroscopy and material fields. Zhao's group got the SERS signal on the surface of TiO₂ and ZnO nanoparticles.^41–43 Other semiconductors such as Fe₃O₄, Ag₂O, NiO, MoO₃ and a-Fe₂O₃ have also been investigated.^44–48 However, it is found that semiconductors can only directly generate weak SERS activity, much lower than metals such as Ag and Au. It is widely accepted that composite nanostructures can incorporate multiple functions into one system for specific applications and can induce fascinating new properties by the heterointerfaces.⁴⁹ Therefore, semiconductor–metal composites are studied for their improved SERS effect. To further improve the SE(R)RS activity on Cu₂O semiconductor, for practical applications, Ag is chosen as the other domain to prepare Ag–Cu₂O composite microstructures (Ag–Cu₂O CMSs). The Ag–Cu₂O CMSs may exhibit the following advantages: (i) Ag is a more effective plasmonic material which can exhibit excellent Raman enhancement ability; (ii) Cu₂O provide advantages in terms of chemical stability (inhibiting the aggregation of Ag nanostructures) and tune the localized surface plasmonic resonance (LSPR) of metallic nanostructures; (iii) the Ag–Cu₂O CMSs with heterostructures will show fascinating SE(R)RS properties better than both Cu₂O and pure Ag nanoparticles (NPs).

In this paper, we report a facile in situ method for the homogeneous growth of Ag NPs on the surfaces of Cu₂O truncated octahedra by directly adding AgNO₃ into Cu₂O-containing mother solution. Ag⁺ is reduced to Ag, and then Ag NPs are in situ deposited onto primary Cu₂O nanomaterials (shown in Scheme 1). The SE(R)RS properties of Ag–Cu₂O CMSs using Rhodamine B (RB) as probing molecules is investigated. Through the variation of the concentration of AgNO₃, Ag contents on the Cu₂O truncated octahedra can be controllably tuned, which further greatly affect the SE(R)RS performance. When 0.4 mM AgNO₃ is used, the prepared Ag–Cu₂O CMSs exhibit good sensitivity SE(R)RS enhancements and excellent uniform response. Furthermore, the SE(R)RS enhancement mechanisms is discussed.

Scheme 1 A schematic illustration of the procedure for synthesis of the Ag–Cu₂O CMSs.

2. Experimental

2.1. Preparation of Ag–Cu₂O composite microstructures

The preparation of Ag–Cu₂O CMSs was based on our previous report with little modification.⁵⁰ Briefly, 0.68 g copper sulfate was dispersed in 76 ml of deionized water, followed by addition of 4 ml of sodium mixture solution (0.74 M sodium citrate and 1.2 M sodium carbonate mixed solution) slowly. After the mixture was stirred for 10 min, 6 g PVP (K-30; M_w = 30 000) was added with vigorous stirring. After the complete dissolution of the PVP powder, 4 ml of 1.4 M glucose solution was slowly dropped into them. The solution was kept in a water bath at a temperature of 80 °C for 15 min (mother solution). In order to coat walls of the Cu₂O with different density of Ag, AgNO₃ was directly added into the mother solution with vigorous stirring for 20 min. Immediate color changed from deep red to deep gray, suggesting uniform Ag–Cu₂O CMSs were formed.

2.2. Characterization

X-ray powder diffraction (XRD) analysis was conducted on a Rigaku D/max-2500 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). Field-emission scanning electron microscopic (FESEM) images were performed on a JEOL JEM-6700F microscope operating at 5 kV. Transmission electron microscopic (TEM) images and high-resolution transmission electron microscopic (HRTEM) images were obtained on a JEOL JEM-2000EX microscope with accelerating voltage of 200 kV and a JEOL JEM-3010 microscopy operated at 200 kV, respectively. UV-vis absorption spectra were recorded using a spectrophotometer (Shimadzu, 3100 UV-vis-NIR).

2.3. SE(R)RS measurements

For SE(R)RS experiments, Rhodamine B (RB) dye was used as a Raman probe. The Ag–Cu₂O CMSs substrate was incubated in the dark for 10 h in an aqueous solution containing 1 × 10⁻⁵ M RB. After the precipitate was centrifuged and dried, the Raman spectrum of the samples drop-casted onto glass slides was measured with a Renishaw Raman system model 1000 spectrometer. The 514.5 nm radiation from a 20 mW air-cooled argon ion laser was used as the exciting source. The laser power at the samples position was typically 400 μW. Data acquisition involved 30 s accumulations. For comparison purposes, the Ag NPs with the similar size as that coated on Cu₂O were prepared in similar conditions by sodium citrate reducing AgNO₃ aqueous solution (ESI†).

3. Results and discussion

Fig. 1a and b show representative FESEM and TEM images of the product. It is quite intriguing to observe that Cu₂O truncated octahedra composed of eight {111} planes and six {100} planes is formed, and Ag NPs with size of 80–100 nm are deposited on the surface of Cu₂O truncated octahedra. To further study the structures of the samples, a typical HRTEM image of Ag and Cu₂O taken from the interface of Ag–Cu₂O CMSs is shown in Fig. 1c. The fringes with value of 0.24 nm is in good agreement with the (111) lattice spacing of Ag NPs, whereas the fringes with value of 0.30 nm is in good agreement with the (110) lattice spacing of Cu₂O. Fig. 1d displays the XRD pattern of as-prepared Ag–Cu₂O CMSs, the diffraction peaks can be assigned perfectly to cubic phase Cu₂O (standard card JCPDS no. 05-0667; space group: Pn3̄m, a = 0.4269 nm) and cubic phase Ag (JCPDS no. 04-0783; space group: Fm3m, a = 0.4086 nm). The results indicate that Ag NPs have been coated on the surface of Cu₂O, consistent with the FESEM and HRTEM observations.

Fig. 1 FESEM image (a), TEM image (b), HRTEM image (c) and XRD pattern (d) of as-prepared Ag–Cu₂O CMSs by 0.4 mM AgNO₃.

3.1. Effect of AgNO₃

It is found that AgNO₃ plays a crucial role in the formation of the Ag–Cu₂O CMSs. As shown in Fig. 2a, only Cu₂O truncated octahedra is formed when no AgNO₃ is used. When 0.2 mM AgNO₃ is used, less Ag NPs are formed on surfaces of Cu₂O truncated octahedral (Fig. 2b). Upon increasing the concentration of AgNO₃ to 0.4 mM, the density of Ag and the effective coverage area of Ag on Cu₂O truncated octahedra are enhanced significantly (Fig. 2c). When the concentration of AgNO₃ is further increased to 0.6 mM, FESEM allowed us to identify the existence of Ag NPs aggregation, and the size of Ag NPs further increased to 300–500 nm (Fig. 2d). These results imply that Ag NPs coverage on the surfaces of Cu₂O can be conveniently controlled by tuning the concentration of the Ag precursor.

Fig. 2 FESEM images of the Ag–Cu₂O CMSs prepared by (a) 0 mM (b) 0.2 mM, (c) 0.4 mM and (d) 0.6 mM AgNO₃.

3.2. Growth mechanism

On the basis of our previously development, at the beginning of the reaction, the truncated octahedral Cu₂O precursor particles were formed.⁵⁰ After AgNO₃ was directly added into the mother solution, glucose and sodium citrate in the mother solution are capable of reducing Ag⁺ to Ag.^51–53 At an initial stage, large amounts of small Ag nucleate, and then they quickly deposited onto the Cu₂O to form Ag–Cu₂O CMSs in order to reduce the surface energy in the system. With the increase of AgNO₃ concentrations, the evolution of Ag contents on the Cu₂O particle from sparse dispersion to appropriate density and favourable interparticle spacing of Ag nanoparticles and finally entire coverage.

3.3. SE(R)RS of Ag–Cu₂O CMSs and Ag NPs

The SE(R)RS spectra of RB adsorbed on these Ag–Cu₂O CMSs with different density of Ag NPs are compared in Fig. 3A. It is found that no SE(R)RS signal of RB is observed for Cu₂O truncated octahedra (curve a), indicating no SE(R)RS effect is observed on Cu₂O. But, distinct SE(R)RS signals are observed from curve b, c and d. All of above bands are associated with the RB at around 618, 1196, 1280, 1357, 1504, 1526, 1563, 1599, and 1649 cm⁻¹.⁵⁴ Obviously, (curve c) the Ag–Cu₂O CMSs prepared by 0.4 mM AgNO₃ exhibits the highest SE(R)RS performance. This may be due to the fact that the plasmonic interactions of Ag NPs due to the favorable interparticle spacing produced by the reaction conditions yield a higher density of SE(R)RS hot spots.^55,56 Furthermore, the SE(R)RS signals of (curve c) Ag–Cu₂O CMSs prepared by 0.4 mM AgNO₃ are compared with that of bare Ag NPs as show in Fig. 3B. It is striking that the Ag–Cu₂O CMSs prepared by 0.4 mM AgNO₃ exhibit higher SE(R)RS signals even than pure Ag. This enhanced performance may derive from the synergistic effect between Ag and Cu₂O, and more Raman hot spots are introduced not only due to the Ag hot spots but also the hot spots formed at the interface between Ag NPs and the Cu₂O.⁵⁷

Fig. 3 (A) Raman spectra of RB (10⁻⁵ M) adsorbed on Ag–Cu₂O CMSs prepared by (a) 0 mM (b) 0.2 mM (c) 0.4 mM and (d) 0.6 mM AgNO₃; (B) Raman spectra of RB (10⁻⁵ M) adsorbed on (c) Ag–Cu₂O CMSs prepared by 0.4 mM AgNO₃ and bare Ag NPs.

3.4. Enhancement mechanisms

There are two main SE(R)RS enhancement mechanisms: electromagnetic (EM) and chemical. The EM enhancement is due to a large increase of the electric field caused by localized surface plasmon resonance (LSPR) induced by the laser light in nanosized metal clusters on the surface. LSPR makes a major contribution to the electromagnetic field enhancement and therefore SE(R)RS.⁵⁸ In order to investigate the LSPR effect in Ag–Cu₂O CMSs substrate, the Ag–Cu₂O CMSs prepared by 0.4 mM AgNO₃ was recorded by UV-vis spectroscopy, as shown in Fig. 4. It is noticed that the plasmon peak of Ag–Cu₂O CMSs, compared to the Cu₂O, shows an observed red shift. Shan et al. show that the surface plasmon absorption may change due to the interaction between Ag and semiconductor ZnO quantum dots.⁵⁹ After the deposition of Ag NP onto Cu₂O, a metal–semiconductor heterostructure is formed. As the Fermi energy level of Ag (4.26 eV) is lower than that of Cu₂O (5.1 eV), electrons will transfer from Ag to Cu₂O, leading to a consequent red shift of the plasmon absorption. The charge redistribution results in a positively charged Ag NPs and negatively charged Cu₂O (as shown in Scheme 2 ESI†), which induces a larger electromagnetic field, as validated by the XPS spectra and related density functional theory (DFT) calculations of Li's group.⁶⁰ Under the larger electromagnetic field, Ag NPs would excite a more intense LSPR under the irradiation of a suitable laser, and therefore makes a major contribution to the SE(R)RS enhancement. The RB molecules adsorbed on the Ag surface are located in this enhanced electromagnetic field and consequently exhibit a stronger Raman signal.

Fig. 4 UV-vis spectra of (a) Cu₂O truncated octahedra, (b) Ag–Cu₂O CMSs prepared by 0.4 mM AgNO₃.

3.5. Uniformity, sensitivity and enhancement factor

To test the sensitivity of Ag–Cu₂O CMSs substrate, SE(R)RS spectra of RB with different concentrations (10⁻³ to 10⁻⁸ M) adsorbed on Ag–Cu₂O CMSs substrate prepared by 0.4 mM AgNO₃ were obtained. As shown in Fig. 5, the intensity of the peaks decreases with the decreasing of the RB concentration. The peak at 1649 cm⁻¹ can be observed as low as 1 × 10⁻⁸ M. The EF of Ag–Cu₂O CMSs substrate is ∼7.8 × 10⁴, which is calculated using (I_SERS/I_NR)(C_NR/C_SERS). The I_SERS could be obtained from 10⁻⁸ M RB on the Ag–Cu₂O CMSs sample. The I_NR was obtained from the normal Raman intensity of RB (10⁻² M) solution. C_NR and C_SERS represent the concentrations of RB solution. These results indicate that the prepared substrate has good sensitivity.

Fig. 5 Raman spectra of RB with different concentrations adsorbed on Ag–Cu₂O CMSs prepared by 0.4 mM AgNO₃.

The uniform response is another essential parameter for quantitative SE(R)RS substrate. Fig. 6 displays the Raman spectra recorded from seven randomly selected positions on Ag–Cu₂O CMSs substrate prepared by 0.4 mM AgNO₃. The peak at 1649 cm⁻¹ is chosen to calculate the relative standard deviation (RSD) of Raman intensity of spot-to-spot. The RSD is lower than 9.3% from seven points, which reveals that the uniform response of the measurements is excellent. These results suggest that Ag–Cu₂O CMSs substrate prepared by 0.4 mM AgNO₃ has a good SE(R)RS enhancements and a uniform response, which are important for practical assays.

Fig. 6 The uniform response of Ag–Cu₂O CMSs substrate prepared by 0.4 mM AgNO₃, the inset shows the intensity change of peak at 1649 cm⁻¹.

4. Conclusion

In summary, Ag–Cu₂O CMSs have been synthesized with a facile in situ method. The density of Ag NPs coated on Cu₂O can be controlled by tuning the concentration of AgNO₃. The SE(R)RS property of Ag–Cu₂O CMSs was investigated. There was an optimum amount of Ag NPs. When 0.4 mM AgNO₃ was used, the prepared Ag–Cu₂O CMSs showed the highest SE(R)RS properties, even better than pure Ag. Besides the good sensitivity SE(R)RS enhancements, our substrates display good reproducibility suggest that this substrate has a potential application in SE(R)RS detection. It is expected that this work can provide valuable information for design of new composites.

Acknowledgements

This work was financially supported by the Science and Technology Development Program of Jilin Province (20100417), the Postdoctoral Science Foundation of China (no. 801130120411) and the National Natural Science Foundation of China (51272086, 3A512Q191460).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra00675e

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