One-step hydrothermal synthesis of Ag/Cu₂O heterogeneous nanostructures over Cu foil and their SERS applications

Abstract

In this paper, Ag/Cu₂O heterogeneous nanostructured films (HNFs) were prepared by a one-step hydrothermal method. It involved only two materials, AgNO₃ and Cu foil, in the aqueous solution to form Ag/Cu₂O HNF. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were used to characterize the Ag/Cu₂O films. Ag nanoparticles and Cu₂O nanocubes were formed by redox reactions and the Ag nanoparticles deposited on the Cu₂O nanocubes via electrostatic attraction. The obtained Ag/Cu₂O HNF was found to be a good candidate for SERS application.

---

1. Introduction

Owing to its highly enhanced vibrational signals, low detection requirements, and good selectivity for adsorbates, surface-enhanced Raman scattering (SERS) is widely applied in chemical and biomolecular sensing.^1–5 An efficient substrate is of great importance for SERS in practical application. SERS substrates can provide strong enhancement factors, and should be stable, reproducible, inexpensive, and easy to be fabricated and handled. In the past few years, SERS-active substrates have been investigated mainly regarding some noble metals and transitional metals.^6–12 Also, it was reported that various semiconductors can directly generate weak SERS activity, including TiO₂,¹³ Cu₂O,¹⁴ ZnO,¹⁵ ZnS,¹⁶ CdS¹⁷ and CdTe.¹⁸ However, such weak SERS activities hampered their practical applications. An effective strategy to enhance the SERS activity is to combine noble metals with semiconductors.

Recently, semiconductor–noble metal heterostructures have been widely studied because some of their excellent properties are improved, such as surface photocatalytic activities and SERS. Semiconductor–metal heterostructures with noble metals like Ag, Pd and Au deposited onto a semiconductor surface have been synthesized by various methods and their obvious SERS effect were observed. For example, Wang et al.¹⁹ prepared nanometer-sized Ag/TiO₂ particles by irradiation to AgNO₃ solution containing TiO₂. Zhao et al.²⁰ obtained site-specific deposition of Ag nanoparticles on ZnO nanorod arrays via galvanic reduction. To date, most of these studies focus on the SERS activity of ZnO and TiO₂-noble metal heterostructures. It remains a major challenge to develop a facile route for the synthesis of new SERS substrate based on semiconductor–noble metal heterostructures.

In this study, the one-step hydrothermal synthesis of Ag/Cu₂O heterogeneous nanostructures grown on Cu foil was first reported. Cu foil and AgNO₃ were the only reactants involved in the preparation process. To surface enhanced Raman scattering, the as-prepared Ag/Cu₂O HNF is a universal substrate.

2. Experimental section

2.1. Materials

All reagents were analytical grade and used without further purification. Silver nitrate (AgNO₃, 99.8%) was purchased from Beijing Chemical Reagent Company. Copper foil was purchased from Changchun Fine Chemical Company. Deionized water was used in all of the experiments.

2.2. Formation of Ag mirror

The details of the Ag mirror preparation protocol are as follow: 10 mL of silver nitrate aqueous solution (2% in mass) was poured into a 50 mL beaker and dilute ammonia was subsequently added in dropwise under stirring. Silver ammonia solution was formed until the previous precipitation was just completely dissolved. Then 3 drops of 10% glucose solution was added under stirring. A cleaned glass plate with 20 mm × 10 mm × 1 mm was dip into the beaker and the solution containing plate was heated at 70 °C. After a few minutes, the solution turned gray and black. Finally, the silver mirror was washed with distilled water.

2.3. Formation of Ag/Cu₂O films

A typical synthesis of Ag/Cu₂O HNF was performed as follows: A copper foil (10 mm × 10 mm × 0.25 mm) was immersed in concentrated HCl for 15 min and ultrasonically cleaned in ethanol for 5 min; then it was rinsed with deionized water and ethanol for 3 times to remove the surface impurities. Meanwhile, 0.1013 g of AgNO₃ was dissolved in 30 mL of deionized water (0.02 mol L⁻¹) under stirring. The above solution was transferred into a 50 mL capacity Teflon-lined autoclave, and then the previously cleaned copper foil was immersed in the solution. The autoclave was kept in an oven at 120 °C for 12 h. After cooling to room temperature in air, the foil was taken out, washed with deionized water and ethanol for several times, and dried in air.

2.4. Characterization

The X-ray powder diffraction (XRD) pattern was recorded by using a Rigaku D/max 2500V PC diffract meter with Cu Kα radiation operated at 40 kV and 30 mA. X-ray photoelectron spectroscopy (XPS) spectrum were measured on a Leybold (Specs) MAX 200XPS system (Specs GmbH, Berlin, Germany) utilizing monochromatized Mg-Ka source operating at 15 kV and 20 mA. The morphology and structure of the product were investigated by field emission scanning electron microscopy (FESEM, Philips XL30 FEG) equipped with dispersive X-ray Spectroscopy (EDX, XLTACHIS-3000N) and high-resolution transmission electron microscopy (HRTEM, JEOL-2100F) at 200 KV. Raman spectrum was obtained with a Renishaw Raman system model 1000 spectrometer. The 633 nm radiation from a 20 mW air-cooled argon ion laser was used as exciting source. The laser power at the sample position was typically 400 μW with an average spot size of 1 μm in diameter. Data acquisition involved the 30 s accumulations for p-aminothiophenol (PATP) that adsorbs on the surface of Ag/Cu₂O nanoheterostructured film. To record the Raman spectrum of the PATP adsorbed on the Ag/Cu₂O surface, the substrate was immersed in 1 mL of PATP (10⁻³ M) ethanol solution for 20 min and rinsed with an ethanol solution for three times. Raman spectrum of 4-pyridinethiol (4-Mpy) adsorbed on Ag/Cu₂O nanoheterostructured film was obtained by the same experimental procedures.

3. Results and discussion

3.1. SEM analysis

Fig. 1 shows the SEM images of the as-synthesized Ag/Cu₂O HNF. The heterogeneous nanostructures Ag/Cu₂O distributes on all available Cu surfaces (Fig. 1a). Clearer observation (Fig. 1b) presents that the Ag/Cu₂O nanoheterostructures are cubes in shape with size ranging from tens to hundreds of nanometers. It is difficult to detect the presence of Ag from the SEM images; therefore, the as-prepared Ag/Cu₂O HNF is further characterized by EDX analysis (Fig. 2). The EDX spectrum with three main peaks (Ag, Cu and O) is achieved, indicating that the as-prepared film is composed of Ag, Cu and O elements.

Fig. 1 SEM images of Ag/Cu₂O HNF with low (a) and high (b) magnifications.

Fig. 2 EDX spectrum of Ag/Cu₂O HNF.

3.2. XRD pattern

Fig. 3 is a typical XRD pattern of the as-obtained Ag/Cu₂O HNF. The diffraction peaks are labeled with “★” can be readily indexed to face-centered cubic Ag (JCPDS 65-2871, a = 0.4086 nm), while the peak marked with “◆” can be indexed to face-centered cubic Cu₂O (JCPDS 05-0667, a = 0.470 nm). The Cu peaks are from Cu substrate. No characteristic peaks of impurity phases are observed from the patterns, such as silver oxide or copper oxide phases. Besides, no remarkable shifts in diffraction peaks are detected, indicating that the silver atoms are not incorporated into the Cu₂O lattice.²¹

Fig. 3 XRD pattern of Ag/Cu₂O HNF.

3.3. XPS analysis

The surface components and chemical states of Ag/Cu₂O HNF were investigated by XPS analysis and the corresponding results are shown in Fig. 4. All peaks in XPS curve for the composites are ascribed to Ag, Cu, O, and C. The appearance of C peak mainly comes from pump oil due to vacuum treatment before the XPS test. No other peaks can be observed, indicating that the film is composed of Cu, Ag and O. High-resolution spectra of Ag, Cu and O species are shown in Fig. 4b–d, respectively. The two peaks centered at 368.1 and 374.1 eV in Fig. 4b can be attributed to Ag 3d_5/2 and Ag 3d_3/2, respectively. Peak positions of Ag 3d are close to that of pure metallic Ag.²² The XPS spectrum shown in Fig. 4c demonstrates the Cu 2p_3/2 and Cu 2p_1/2 peaks located at 932.2 and 952 eV, respectively, which are attributed to Cu₂O.²³ No CuO signal is detected with Cu 2p_3/2 ≈ 933.6.²⁴ In Fig. 4d, the O 1s profile is asymmetric and can be fitted into two symmetrical peaks of α and β with location at 530.4 and 531.7 eV. The α and β peaks originate from the lattice oxygen of Cu₂O and adsorbed oxygen, respectively.²⁵ The EDX, XRD and XPS results consistently prove the formation of Ag and Cu₂O.

Fig. 4 XPS spectrum of Ag/Cu₂O HNF (a) and the high-resolution spectra of Ag 3d (b), Cu 2p (c) and O1s (d).

3.4. TEM analysis

The morphologies and microstructure of the Ag/Cu₂O heterogeneous nanostructures were further observed by TEM. Fig. 5a exhibits some tiny spherical particles that are supposed to be Ag nanoparticles attaching on the surface of nanocubic Cu₂O, which is further confirmed by the HRTEM image (Fig. 5b–d). The observed interplanar spacing of 0.24 nm (Fig. 5c) and 0.20 nm (Fig. 5d) corresponds to (111) lattice planes of Cu₂O and (200) lattice planes of Ag, respectively.

Fig. 5 TEM images of Ag/Cu₂O heterogeneous nanostructures with low (a) and high (b–d) magnifications.

3.5. Possible mechanism for the formation of Ag/Cu₂O NHF

According to previous reports, we anticipate that the Ag and Cu₂O are formed by the following reactions:²⁶

Cu(s) + 2Ag⁺(aq) → Cu²⁺(aq) + 2Ag(s) (1)

Cu + Cu²⁺(aq) + H₂O → Cu₂O + 2H⁺(aq) (2)

Ag and Cu²⁺ are generated when reaction (1) occurred, and Cu²⁺ contacts with Cu substrate to form Cu₂O on the Cu foil by a redox reaction (2). We consider that Cu₂O may be inclined to absorb OH⁻, which makes them negatively charged. Ag particles tend to absorb Ag⁺, and turn positively charged. Therefore, Cu₂O and Ag are easy to interact with each other owing to the electrostatic attraction. Such adsorption of Ag particles inhibits the continued growth of the cuprous oxide. Thus, Ag/Cu₂O heterogeneous nanostructures are formed.

3.6. UV-vis studies

The UV-vis absorption spectrum of the Ag/Cu₂O HNF was collected and shown in Fig. 6. The Ag/Cu₂O HNF has two broad absorption peaks. The peak at 550 nm can be attributed to Cu₂O, while the peak at 390 nm can be attributed to the plasmon resonance of the Ag nanoparticles.²⁷

Fig. 6 UV-visible diffuse reflectance spectrum of Ag/Cu₂O film.

3.7. SERS studies

The as-synthesized Ag/Cu₂O HNF was used as substrate to examine the SERS effect. PATP was chosen as the probe molecule owing to its well-established vibrational features. Fig. 7 shows the SERS spectra of PATP with various concentrations from 1.0 × 10⁻³ to 1.0 × 10⁻⁷ M. The predominant peaks located at 1595 and 1089 cm⁻¹ are assigned to the a₁ modes of the PATP molecules shifting to 1578 and 1077 cm⁻¹, respectively. The first peak is related to the C=C stretch and the other is the C–S stretch. The frequency shifts are due to the strong interaction between the adsorbate and the substrate. The difference among the substrates also causes little difference in the Raman shift.²⁸ It is worth noting that the peaks at 1440, 1390, 1192 and 1142 cm⁻¹ are strongly enhanced. These are all of b₂ symmetry and the bands show strong resonance enhancements due to charge transfer.²⁹ On the other hand, the SERS intensities of PATP decrease with the decreasing concentration of PATP, but the SERS signals are still observable at concentration of 1.0 × 10⁻⁵ M.

Fig. 7 Surface-enhanced Raman spectra of PATP molecules adsorbed of various concentrations: (a) 1.0 × 10⁻³, (b) 1.0 × 10⁻⁴, (c) 1.0 × 10⁻⁵, (d) 1.0 × 10⁻⁶ and (e) 1.0 × 10⁻⁷ M on Ag/Cu₂O HNF.

Besides PATP, the Ag/Cu₂O HNF has also been proven as an effective SERS substrate for the detection of 4-Mpy. Fig. 8 shows the SERS spectra obtained with various 4-Mpy concentrations from 1.0 × 10⁻³ to 1.0 × 10⁻⁵ M. Generally, the strong peaks located at 1580 and 1609 cm⁻¹ can be attributed to the ring stretch mode of the 4-Mpy molecule with deprotonated and protonated nitrogen, respectively. The peak at 1219 cm⁻¹ is attributed to the CH deformation and NH stretching modes, and the peak at 1095 cm⁻¹ is assigned to the X-sensitive modes. X-sensitive modes are described as modes strongly coupled between substitute and aromatic ring modes. The band at 1060 cm⁻¹ and 1009 cm⁻¹ are assigned to the CH deformation and ring breathing modes, respectively.^30,31 All the above bands are similar to those in the SERS of 4-Mpy adsorbed on Ag mirror (Fig. 9b). However, besides the conventional bands of 4-Mpy adsorbed on Ag nanoparticles, another three distinct bands at 1576, 1035 and 1017 cm⁻¹ are also observed from the difference spectrum between the Raman spectra of 4-Mpy adsorbed on the surface of Ag/Cu₂O HNF and Ag mirror (Fig. 9c). We consider one possible reason is attributed to the SERS of 4-Mpy adsorbed on the bare Cu₂O nanocubes surface. From the TEM image, Ag nanoparticles are formed on the surface of Cu₂O nanocubes, but not cover the whole surface. And the enhancement of the SERS signals of 4-Mpy at 1034 and 1212 cm⁻¹ adsorbed on bare Cu₂O nanocubes should come from the long-range electromagnetic enhancement because Ag nanoparticles formed on the surface of Cu₂O nanocubes would excite localized surface plasmon resonance (LSPR) under the irradiation of suitable laser.³² Another possible reason is that the SERS intensities may be determined by the average molecular orientation on the nanostructured substrate, so the observed difference could also be due to a change in this parameter. Further study will be carried out in the future experiments.

Fig. 8 Raman spectra of 4-Mpy with different concentrations adsorbed on Ag/Cu₂O HNF: (a) [4-Mpy] = 1.0 × 10⁻³ M, (b) [4-Mpy] = 1.0 × 10⁻⁴ M and (c) [4-Mpy] = 1.0 × 10⁻⁵ M.

Fig. 9 Raman spectra of 4-Mpy with [4-Mpy] = 1.0 × 10⁻³ M concentrations adsorbed on (a) Ag/Cu₂O HNF, (b) Ag mirror and (c) The difference spectrum of (a) and (b).

4. Conclusions

In summary, we have described a one-step hydrothermal method to prepare Ag/Cu₂O HNFs. The synthetic method is simple, economical, and pollution-free, so it may be adapted for the preparation of new SERS substrates. The Ag/Cu₂O HNF could be used as an effective SERS substrate for PATP and 4-Mpy detection.

Acknowledgements

This work was supported by the Natural Science Fund of China (no. 21073031, 20573017).

Notes and references

1. C. R. Yonzon, C. L. Haynes, X. Zhang, J. T. Walsh and R. P. Van Duyne, Anal. Chem., 2003, 76, 78–85.
2. M. Mulvihill, A. Tao, K. Benjauthrit, J. Arnold and P. Yang, Angew. Chem., Int. Ed., 2008, 47, 6456–6460.
3. M. Lin, L. He, J. Awika, L. Yang, D. R. Ledoux, H. Li and A. Mustapha, J. Food Sci., 2008, 73, T129–T134.
4. A. Barhoumi, D. Zhang, F. Tam and N. J. Halas, J. Am. Chem. Soc., 2008, 130, 5523–5529.
5. L. Chen, L. Luo, Z. Chen, M. Zhang, J. A. Zapien, C. S. Lee and S. T. Lee, J. Phys. Chem. C, 2009, 114, 93–100.
6. Y. Ma, Q. Ding, L. Yang, L. Zhang and Y. Shen, Appl. Surf. Sci., 2013, 265, 346–351.
7. L. Gao, S. Lv and S. Xing, Synth. Met., 2012, 162, 948–952.
8. W. Song, W. Li, Y. Cheng, H. Jia, G. Zhao, Y. Zhou, B. Yang, W. Xu, W. Tian and B. Zhao, J. Raman Spectrosc., 2006, 37, 755–761.
9. X. Li, Y. Wang, H. Jia, W. Song and B. Zhao, J. Raman Spectrosc., 2005, 36, 635–639.
10. B. Ren, X. Lin, Z. Yang, G. Liu, R. F. Aroca, B. Mao and Z. Tian, J. Am. Chem. Soc., 2003, 125, 9598–9599.
11. Z. Tian, B. Ren and D. Wu, J. Phys. Chem. B, 2002, 106, 9463–9483.
12. Z. Tian, B. Ren, J. Li and Z. Yang, Chem. Commun., 2007, 34, 3514–3534.
13. L. Liao, H. Zhou and X. Xiao, Chem. Phys., 2005, 316, 164–170.
14. C. Qiu, L. Zhang, H. Wang and C. Jiang, J. Phys. Chem. Lett., 2012, 3, 651–657.
15. Y. Wang, W. Ruan, J. Zhang, B. Yang, W. Xu and B. Zhao, J. Raman Spectrosc., 2009, 40, 1072–1077.
16. Y. Wang, Z. Sun, H. Hu, S. Jing, B. Zhao, W. Xu and C. Zhao, J. Raman Spectrosc., 2007, 38, 34–38.
17. Y. Wang, Z. Sun, H. Hu, B. Zhao, W. Xu and J. R. Lombardi, Spectrochim. Acta, Part A, 2007, 66, 1199–1203.
18. Y. Wang, J. Zhang, H. Jia, M. Li, J. Zeng, B. Yang, B. Zhao, W. Xu and J. R. Lombardi, J. Phys. Chem. C, 2008, 112, 996–1000.
19. C. Wang, C. Liu, Y. Liu and Z. Zhang, Appl. Surf. Sci., 1999, 147, 52–57.
20. W. Song, X. Han, L. Chen, Y. Yang, B. Tang, W. Ji, W. Ruan, W. Xu, B. Zhao and Y. Ozaki, J. Raman Spectrosc., 2010, 41, 907–913.
21. F. Sun, X. Qiao, F. Tan, W. Wang and X. Qiu, J. Mater. Sci., 2012, 47, 7262–7268.
22. W. Wu, Y. Wang, L. Shi, W. Pang, Q. Zhu, G. Xu and F. Lu, J. Phys. Chem. B, 2006, 110, 14702–14708.
23. J. J. Teo, Y. Chang and H. Zeng, Langmuir, 2006, 22, 7369–7377.
24. Y. Xiong, Z. Li, R. Zhang, Y. Xie, J. Yang and C. Wu, J. Phys. Chem. B, 2003, 107, 3697–3702.
25. X. Lin, R. Zhou, J. Zhang and S. Fei, Appl. Surf. Sci., 2009, 256, 889–893.
26. L. Qu and L. Dai, J. Phys. Chem. B, 2005, 109, 13985–13990.
27. Z. Wang, S. Zhao, S. Zhu, Y. Sun and M. Fang, CrystEngComm, 2011, 13, 2262–2267.
28. Z. Sun, C. Wang, J. Yang, B. Zhao and J. R. Lombardi, J. Phys. Chem. C, 2008, 112, 6093–6098.
29. L. Yang, W. Ruan, X. Jiang, B. Zhao, W. Xu and J. R. Lombardi, J. Phys. Chem. C, 2008, 113, 117–120.
30. Z. Wang and L. J. Rothberg, J. Phys. Chem. B, 2005, 109, 3387–3391.
31. W. Song, Y. Wang and B. Zhao, J. Phys. Chem. C, 2007, 111, 12786–12791.
32. W. Song, J. Wang, Z. Mao, W. Xu and B. Zhao, Spectrochim. Acta, Part A, 2011, 79, 1247–1250.

---