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Ag–Cu2O composite microstructures with tunable Ag contents: synthesis and surface-enhanced Cite this: RSC Adv.,2014,4,17249 (resonance) Raman scattering (SE(R)RS) properties†

Lihua Yang,a Jian Lv,a Yongming Sui,a Wuyou Fu,a Xiaoming Zhou,b Jinwen Ma,a Qian Li,a Meiling Sun,a Yannan Mu,ac Yanli Chen,a Jun Wanga and Haibin Yang*a

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 Received 23rd January 2014 Cu O can be tuned through the variation of the concentration of AgNO , which further greatly aﬀected Accepted 26th March 2014 2 3 the surface-enhanced (resonance) Raman scattering (SE(R)RS) performance. The Ag–Cu2O CMSs DOI: 10.1039/c4ra00675e prepared with 0.4 mM AgNO3 show the optimum SE(R)RS properties, better than pure Ag NPs.

www.rsc.org/advances Furthermore, the enhancement mechanism and uniformity of Ag–Cu2O CMSs are investigated in detail.

1. Introduction as Fe3O4,Ag2O, NiO, MoO3 and a-Fe2O3 have also been investigated.44–48 However, it is found that semiconductors can only

Cu2O is a p-type semiconductor oxide with a band gap of 2.17 directly generate weak SERS activity, much lower than metals eV, which makes it an excellent candidate for catalysis, gas such as Ag and Au. It is widely accepted that composite nano- sensors, chemical templates, CO oxidation, solar driven water structures can incorporate multiple functions into one system splitting, fuel cells and solar cells.1–8 Over the past decades, an for specic applications and can induce fascinating new prop- 49 – impressive eﬀort has been devoted to the synthesis of Cu2O with erties by the heterointerfaces. Therefore, semiconductor designed shapes and desired functions, including polyhedral metal composites are studied for their improved SERS eﬀect. To 9–12 13–16 nanoparticles, nanosheets/rods/wires/tubes, hollows, further improve the SE(R)RS activity on Cu2O semiconductor, for nanocages and nanoframes.17–21 However, just a few studies practical applications, Ag is chosen as the other domain to

Published on 28 March 2014. Downloaded by Jilin University 31/12/2014 02:23:06. – – have reported the use of Cu2O as Raman active substrate and prepare Ag Cu2O composite microstructures (Ag Cu2O CMSs). 22–25 – Cu2O can usually generate weak Raman activity. The Ag Cu2O CMSs may exhibit the following advantages: (i) Ag Surface-enhanced (resonance) Raman scattering (SE(R)RS) is is a more eﬀective plasmonic material which can exhibit excel- a powerful analytical tool for determining chemical information lent Raman enhancement ability; (ii) Cu2O provide advantages of molecules on substrates for surface science, analytical in terms of chemical stability (inhibiting the aggregation of Ag chemistry, and biology.26–30 With SE(R)RS, extremely small nanostructures) and tune the localized surface plasmonic reso- – amounts of substances can be detected; even single molecule nance (LSPR) of metallic nanostructures; (iii) the Ag Cu2O CMSs detection has been reported.31–33 During the past few decades, with heterostructures will show fascinating SE(R)RS properties SE(R)RS active substrate has been developed signicantly. Noble better than both Cu2O and pure Ag nanoparticles (NPs). metals and transition metals, such as Au,34 Ag,35 Cu,36 Pt,37 Pd,38 In this paper, we report a facile in situ method for the 39 40 Co and Ni have been extensively employed owing to their homogeneous growth of Ag NPs on the surfaces of Cu2O trun- large enhancement. Semiconductor materials have been used as cated octahedra by directly adding AgNO3 into Cu2O-containing SE(R)RS active substrate and caused increasing attention due to mother solution. Ag+ is reduced to Ag, and then Ag NPs are in the widespread application in both SE(R)RS spectroscopy and situ deposited onto primary Cu2O nanomaterials (shown in – material elds. Zhao's group got the SERS signal on the surface Scheme 1). The SE(R)RS properties of Ag Cu2O CMSs using 41–43 of TiO2 and ZnO nanoparticles. Other semiconductors such 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 aState Key Laboratory of Superhard Materials, Jilin University, Changchun, 130012, tuned, which further greatly aﬀect the SE(R)RS performance. China. E-mail: [email protected]; Fax: +86 431 85168763; Tel: +86 431 85168763 – When 0.4 mM AgNO3 is used, the prepared Ag Cu2O CMSs bCollege of Physics, Beihua University, Jilin 132013, PR China exhibit good sensitivity SE(R)RS enhancements and excellent cDepartment of Physics and Chemistry, Heihe University, Heihe 164300, PR China † Electronic supplementary information (ESI) available. See DOI: uniform response. Furthermore, the SE(R)RS enhancement 10.1039/c4ra00675e mechanisms is discussed.

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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 trun- Scheme 1 A schematic illustration of the procedure for synthesis of cated octahedra composed of eight {111} planes and six {100} – the Ag Cu2O CMSs. 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 – 2. Experimental 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 2.1. Preparation of Ag–Cu O composite microstructures 2 good agreement with the (111) lattice spacing of Ag NPs, – The preparation of Ag Cu2O CMSs was based on our previous whereas the fringes with value of 0.30 nm is in good agreement  50  report with little modi cation. Brie y, 0.68 g copper sulfate with the (110) lattice spacing of Cu2O. Fig. 1d displays the XRD

was dispersed in 76 ml of deionized water, followed by addi- pattern of as-prepared Ag–Cu2O CMSs, the diﬀraction peaks can

tion of 4 ml of sodium mixture solution (0.74 M sodium citrate be assigned perfectly to cubic phase Cu2O (standard card JCPDS and 1.2 M sodium carbonate mixed solution) slowly. Aer the no. 05-0667; space group: Pn3m, a ¼ 0.4269 nm) and cubic ¼ mixture was stirred for 10 min, 6 g PVP (K-30; Mw 30 000) phase Ag (JCPDS no. 04-0783; space group: Fm3m, a ¼ 0.4086 was added with vigorous stirring. Aer the complete dissolu- nm). The results indicate that Ag NPs have been coated on the

tion of the PVP powder, 4 ml of 1.4 M glucose solution was surface of Cu2O, consistent with the FESEM and HRTEM slowly dropped into them. The solution was kept in a water observations. bath at a temperature of 80 C for 15 min (mother solution). In ff order to coat walls of the Cu2O with di erent density of Ag, ff 3.1. E ect of AgNO3 AgNO3 was directly added into the mother solution with vigorous stirring for 20 min. Immediate color changed from It is found that AgNO3 plays a crucial role in the formation of – – deep red to deep gray, suggesting uniform Ag Cu2O CMSs were the Ag Cu2O CMSs. As shown in Fig. 2a, only Cu2O truncated formed. 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 concen- 2.2. Characterization tration of AgNO3 to 0.4 mM, the density of Ag and the effective

X-ray powder diﬀraction (XRD) analysis was conducted on a coverage area of Ag on Cu2O truncated octahedra are Rigaku D/max-2500 X-ray diﬀractometer with Cu Ka radiation (l enhanced signicantly (Fig. 2c). When the concentration of

Published on 28 March 2014. Downloaded by Jilin University 31/12/2014 02:23:06. ˚ ¼ 1.5418 A). Field-emission scanning electron microscopic AgNO3 is further increased to 0.6 mM, FESEM allowed us to (FESEM) images were performed on a JEOL JEM-6700F micro- identify the existence of Ag NPs aggregation, and the size of Ag scope 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. Aer 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 spec- trometer. 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 mW. Data acquisition involved 30 s accumulations. For comparison purposes, the Ag Fig. 1 FESEM image (a), TEM image (b), HRTEM image (c) and XRD – NPs with the similar size as that coated on Cu2O were prepared pattern (d) of as-prepared Ag Cu2O CMSs by 0.4 mM AgNO3.

17250 | RSC Adv.,2014,4,17249–17254 This journal is © The Royal Society of Chemistry 2014 View Article Online Paper RSC Advances

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.

NPs further increased to 300–500 nm (Fig. 2d). These results Ag NPs due to the favorable interparticle spacing produced by

imply that Ag NPs coverage on the surfaces of Cu2O can be the reaction conditions yield a higher density of SE(R)RS hot conveniently controlled by tuning the concentration of the Ag spots.55,56 Furthermore, the SE(R)RS signals of (curve c) Ag–

precursor. 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 3.2. Growth mechanism CMSs prepared by 0.4 mM AgNO3 exhibit higher SE(R)RS signals even than pure Ag. This enhanced performance may On the basis of our previously development, at the beginning of derive from the synergistic eﬀect between Ag and Cu2O, and the reaction, the truncated octahedral Cu2O precursor particles Published on 28 March 2014. Downloaded by Jilin University 31/12/2014 02:23:06. more Raman hot spots are introduced not only due to the Ag hot were formed.50 Aer AgNO was directly added into the mother 3 spots but also the hot spots formed at the interface between Ag solution, glucose and sodium citrate in the mother solution are NPs and the Cu O.57 capable of reducing Ag+ to Ag.51–53 At an initial stage, large 2 amounts of small Ag nucleate, and then they quickly deposited – onto the Cu2O to form Ag Cu2O CMSs in order to reduce the 3.4. Enhancement mechanisms surface energy in the system. With the increase of AgNO 3 There are two main SE(R)RS enhancement mechanisms: elec- concentrations, the evolution of Ag contents on the Cu O 2 tromagnetic (EM) and chemical. The EM enhancement is due to particle from sparse dispersion to appropriate density and a large increase of the electric eld caused by localized surface favourable interparticle spacing of Ag nanoparticles and nally plasmon resonance (LSPR) induced by the laser light in nano- entire coverage. sized metal clusters on the surface. LSPR makes a major contribution to the electromagnetic eld enhancement and – 3.3. SE(R)RS of Ag Cu2O CMSs and Ag NPs therefore SE(R)RS.58 In order to investigate the LSPR eﬀect in

The SE(R)RS spectra of RB adsorbed on these Ag–Cu2O CMSs Ag–Cu2O CMSs substrate, the Ag–Cu2O CMSs prepared by 0.4

with diﬀerent density of Ag NPs are compared in Fig. 3A. It is mM AgNO3 was recorded by UV-vis spectroscopy, as shown in

found that no SE(R)RS signal of RB is observed for Cu2O trun- Fig. 4. It is noticed that the plasmon peak of Ag–Cu2O CMSs,

cated octahedra (curve a), indicating no SE(R)RS eﬀect is compared to the Cu2O, shows an observed red shi. Shan et al.

observed on Cu2O. But, distinct SE(R)RS signals are observed show that the surface plasmon absorption may change due to from curve b, c and d. All of above bands are associated with the the interaction between Ag and semiconductor ZnO quantum 59  – RB at around 618, 1196, 1280, 1357, 1504, 1526, 1563, 1599, and dots. A er the deposition of Ag NP onto Cu2O, a metal 1 54 – 1649 cm . Obviously, (curve c) the Ag Cu2O CMSs prepared semiconductor heterostructure is formed. As the Fermi energy by 0.4 mM AgNO3 exhibits the highest SE(R)RS performance. level of Ag (4.26 eV) is lower than that of Cu2O (5.1 eV), electrons

This may be due to the fact that the plasmonic interactions of will transfer from Ag to Cu2O, leading to a consequent red shi

This journal is © The Royal Society of Chemistry 2014 RSC Adv.,2014,4,17249–17254 | 17251 View Article Online RSC Advances Paper

5 Fig. 3 (A) Raman spectra of RB (10 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) 5 Raman spectra of RB (10 M) adsorbed on (c) Ag–Cu2O CMSs prepared by 0.4 mM AgNO3 and bare Ag NPs.

The peak at 1649 cm 1 can be observed as low as 1 10 8 M. 4 The EF of Ag–Cu2O CMSs substrate is 7.8 10 , which is

calculated using (ISERS/INR)(CNR/CSERS). The ISERS could be 8 obtained from 10 M RB on the Ag–Cu2O CMSs sample. The 2 INR was obtained from the normal Raman intensity of RB (10

M) solution. CNR and CSERS represent the concentrations of RB solution. These results indicate that the prepared substrate has good sensitivity. 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. Published on 28 March 2014. Downloaded by Jilin University 31/12/2014 02:23:06. – Fig. 4 UV-vis spectra of (a) Cu2O truncated octahedra, (b) Ag–Cu2O These results suggest that Ag Cu2O CMSs substrate CMSs prepared by 0.4 mM AgNO3. prepared by 0.4 mM AgNO3 has a good SE(R)RS enhancements and a uniform response, which are important for practical assays. 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 eld, as validated by the XPS spectra and related density functional theory (DFT) calculations of Li's group.60 Under the larger electromagnetic eld, 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 eld and consequently exhibit a stronger Raman signal.

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 Fig. 5 Raman spectra of RB with diﬀerent concentrations adsorbed on

peaks decreases with the decreasing of the RB concentration. Ag–Cu2O CMSs prepared by 0.4 mM AgNO3.

17252 | RSC Adv.,2014,4,17249–17254 This journal is © The Royal Society of Chemistry 2014 View Article Online Paper RSC Advances

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