A single-source precursor route to a Cu/V₂O₅ composites as surface-enhanced Raman scattering substrates and catalysts for cross coupling

Abstract

A Cu/V₂O₅ composite was obtained via a simple and time-saving microwave-assisted reduction method from a single-source precursor, copper vanadate. This composite exhibited excellent SERS sensitivity and reproducibility for both Rhodamine 6G (1.0 × 10⁻⁷ M) and 4-mercaptobenzoic acid (1.0 × 10⁻⁵ M) probe molecules, with the enhancement factor of 1.9 × 10⁶ for Rhodamine 6G. The Lewis acid–base properties of V₂O₅ help improve the chemical enhancement of SERS via Lewis acid–base interaction. Furthermore, the Cu/V₂O₅ composite was also employed to catalyze C–N and C–C bond cross coupling of the Ullmann reaction. The results demonstrated that Cu/V₂O₅ catalysts had a better catalytic ability toward the C–N bond (yield of 92.5%) than the C–C bond (yield of 68.8%). This SERS-active composite was hopeful in monitoring the catalytic reactions with in situ SERS technology.

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1. Introduction

The surface-enhanced Raman scattering (SERS) phenomenon was discovered in the mid-1970s on roughened Ag surfaces.¹ It has attracted increasing attention due to its broad applications in surface science, analytical chemistry, electrochemistry and biomedical sciences, and so forth.^2–5 Its high sensitivity and resolution make it ideal for investigating the interaction between molecules and metal surfaces, such as surface information, molecular orientation, and interface reactions.^6,7

To date, there are two major mechanisms for SERS enhancement: the long-range electromagnetic mechanism (EM) and short-range chemical mechanism (CM).⁸ EM enhancement is due to the large local fields caused by localized surface plasmon resonance (LSPR) of metallic nanostructure. The shape, composition and surrounding environment of metals could affect the LSPR.⁹ Thus, it is of great importance to design proper parameters of materials for strong SERS enhancement. CM is considered to be a resonance Raman-like process caused by the formation of a ‘surface complex’ between adsorbed molecules and substrates. EM is believed to play a dominant role in the enhancement of the Raman signal while CM is generally limited to only the first layer of adsorbed molecules.¹⁰

During the past few decades, SERS active substrates have been developed significantly.^11,12 Noble metals Au and Ag could provide excellent SERS signals due to their LSPR properties.¹³ Meanwhile, copper as a relatively abundant material was a cheaper alternative with less investigation than Ag and Au. Especially, copper attracted considerable attentions for its applications as catalysts in organic synthesis. It was an excellent catalyst for Ullmann reaction, which could catalyze C–X (X = C, N, O, S etc.) bonds' cross coupling.^14,15 As copper played a key role both in adsorption process and catalytic reaction, it was particularly suitable for studying various surface phenomena in these systems by SERS techniques.^16,17

However, copper is prone to oxidation, especially in small size. Various routes have been proposed to solve this problem, such as conventional microemulsion techniques, sonochemical reduction, and chemical reduction.^18–20 Yet, controlled synthesis and stabilization of copper particles was still challenging.

In this work, microwave-assisted reduction method was employed because it was a homogeneous process, which showed several merits compared with conventional one, such as rapid volumetric heating, high reaction rate, short reaction time, and high reaction selectivity.²¹ By this way, the copper vanadate precursor could be employed to obtain copper nanoparticles and vanadium pentoxide (V₂O₅). Especially, previous reports suggested that V₂O₅ could strongly enhance the microwave adsorption capability,²² which could promote the reduction rate of copper. Moreover, V₂O₅ was important to improve surface adsorption due to its redox and Lewis acid–base properties. The Lewis base sites (V=O and V–O–V) and Lewis acid site (V⁵⁺) on the surface of the V₂O₅ (ref. 23 and 24) would probably exhibit synergy with copper to SERS and catalysis.

2. Experimental section

2.1 Synthesis of copper vanadate nanoribbons

All chemicals were analytical grade without further purification. A simple hydrothermal method was used to synthesize copper vanadate nanoribbons. In a typical procedure: 1 mmol CuCl₂·2H₂O and 0.5 mmol NH₄VO₃ were dissolved in 20 mL deionized water, respectively. Then the NH₄VO₃ solution was dropwised into the CuCl₂·2H₂O solution under magnetic stirring. The mixture was transferred into a 50 mL Teflon-lined autoclave, maintained at 180 °C for 10 h, then cooled to room temperature naturally. The products were collected and washed several times with deionized water as well as absolute ethanol, and dried under vacuum at 60 °C for 6 h.

2.2 Synthesis of Cu/V₂O₅ composite

The as-prepared copper vanadate (40 mg) was dispersed into 50 mL ethylene glycol. The suspension was placed in a microwave oven and irradiated for 5 min. The color of the suspension changed from dark yellow to pale red, which demonstrated the formation of copper. When the suspension was cooled to room temperature, it was filtered and washed several times with deionized water and absolute ethanol, respectively. The products were collected after dried under vacuum at 60 °C for 6 h.

2.3 Catalytic process

A mixture of aryl iodide (PhI, 1 mmol), o-phenylenediamine (1.2 mmol), K₂CO₃ (1.5 mmol), benzyltrimethyl-ammonium chloride (BTMA, 0.5 mmol) were dissolved in 10 mL deionized water under magnetic stirring by adding 8 mg Cu/V₂O₅ composite prepared by microwave-assisted reduction, while the K₂CO₃ was used as a base and BTMA as phase transfer catalysts. The mixture was transferred into a 15 mL Teflon-lined autoclave, maintained at 135 °C for 16 h, and then cooled to room temperature. Ethyl acetate (5 mL) was added into the mixture with shaking, and the supernatant was collected after centrifugation. The organic layer was separated after deionized water was added into the supernatant, and the supernatant was extracted twice more with ethyl acetate while each extract was sequentially collected. Finally, anhydrous sodium sulfate (Na₂SO₄) desiccant was added into the extract and stayed for overnight to remove moisture. The yields of final products were measured by gas chromatograph-mass spectrometer (GC-MS).

2.4 SERS measurement

The as-prepared Cu/V₂O₅ composite was dispersed on a silicon wafer after ultrasonic treatment, and served as SERS substrates. Rhodamine 6G (R6G) (1.0 × 10⁻⁷ M) and 1.0 × 10⁻⁵ M 4-mercaptobenzoic acid (MBA) aqueous solution were used as SERS probe molecules, respectively. Then the composite SERS substrate was immersed in R6G and MBA aqueous solution in a quartz cell of 150 μL with a quartz window, respectively. A Labram-HR 800 Raman spectroscopy (J Y, France) equipped with a synapse CCD detector and a confocal Olympus microscope was used to collect Raman spectra. The whole SERS measurement was measured at room temperature. A 633 nm He–Ne laser was employed while the SERS spectra were collected at 50 × objective lens (Olympus) with a numerical aperture of 0.5 and the accumulation time of 1 s, and the spectrograph used 600 g mm⁻¹ gratings.

2.5 Characterization

The as-prepared products were characterized by X-ray diffraction (XRD, a Philips X'pert PRO MPD diffractometer) with Cu Kα radiation (λ = 0.15406 nm) and a scanning rate of 0.05° s⁻¹. The morphology and size of the products were examined by field emission scanning electronic microscopy (FESEM, FEI Co., model Quanta-200). The transmission electron microscopy (TEM), high-resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) images were taken with HRTEM analyzer (FEI Tecnai G2 F20) with an accelerating voltage of 200 kV. The microwave irradiated process was conducted in a Galanz microwave oven with the output power of 800 W (P80D23N1P-G5-WO). In the process of organic reactions, the yields of the products were measured by GC-MS (Trace-ISQ, Thermo Fisher).

3. Results and discussion

3.1 Characterization of the copper vanadate and Cu/V₂O₅ composite

The XRD pattern in Fig. 1a reveals that the as-prepared products with hydrothermal method are matched well with monoclinic copper vanadate phase (JCPDS no. 80-1169) without impurity peaks. The sharp and narrow diffraction peaks reveal their high crystallinity. Fig. 1b presents the XRD pattern of the microwave-irradiated products: the strong peaks at 2θ = 43.39, 50.51 and 74.19° are ascribed to the (111), (200) and (222) crystal planes of the cubic phase copper (JCPDS 89-2838), which confirmed that copper vanadate could be successfully reduced to copper by ethylene glycol via microwave irradiation. The peak at 2θ = 30.99° with the d spacing of 0.2885 nm may be assigned to (400) crystal plane of orthorhombic V₂O₅ phase (JCPDS 89-2483). There is only (400) crystal plane of V₂O₅ could be detected in the XRD pattern, which may due to the preferred growth direction of V₂O₅, and V₂O₅ phase will be further confirmed in the next discussion.

Fig. 1 XRD patterns of (a) as-prepared copper vanadate nanoribbons, (b) and products after microwave reaction.

From the SEM images of copper vanadate (Fig. 2a and b), it can be observed that the samples are composed of uniform nanoribbons with the length of several micrometers and width of ca. 200 nm.

Fig. 2 SEM images of the as-prepared copper vanadate nanoribbons.

Fig. 3a shows the SEM image of Cu/V₂O₅ composite. It can be clearly observed that copper particles with the diameters of about 100 nm were uniformly covered on the surface of V₂O₅ nanoribbons. The width and length of V₂O₅ nanoribbon were estimated from the size distribution histogram in Fig. S3 (ESI†). The width statistics shows that the average width of V₂O₅ nanoribbon is 220 ± 82.1 nm with the standard deviation of 34.7%. Meanwhile, the heterogeneous length of V₂O₅ nanoribbon indicates that the nanoribbon is broken under the microwave irradiation from the length distribution histogram (Fig. S3b†).

Fig. 3 (a) SEM image; (b) TEM image and SAED pattern (inset); (c) statistics of particle size of copper from TEM image based on 108 particles; and (d) HRTEM image of Cu/V₂O₅ composite.

The morphology and structure of the Cu/V₂O₅ composite were further characterized by TEM, HRTEM and SAED patterns. Fig. 3b and d show the TEM and HRTEM images of the composite. It can be seen that the copper particles were obviously dispersed on the surface of V₂O₅ nanoribbon (Fig. 3b), which is consistent with the aforementioned SEM analyses. The average size of copper particle could be estimated from the size distribution histogram (Fig. 3c) based on 108 particles. The calculation basically presents the Gauss normal distribution which indicates that the average size of copper is 94.5 ± 22.8 nm with the standard deviation of 22.8%. This result is consistant with the SEM image.

The HRTEM in Fig. 3d shows the examined region is perfectly free of dislocation and distortion. There are two lattice fringes with the d spacing of 0.5648 and 0.3443 nm, corresponding with the (200) and (101) lattice planes of orthorhombic V₂O₅, respectively. And the intersection angle between (200) and (101) planes is about 71.6°, which is according with the SAED pattern (inserted in Fig. 3b). Associated with SAED pattern, it can be concluded that the V₂O₅ nanoribbon shows a preferred [100] growth direction and has a well-crystallized character. The reduced V₂O₅ nanoribbon not only strongly adsorbed microwave to accelerate the reduction rate of copper vanadates,⁵ but also could be used as framework to support Cu particles and help increase its anti-oxidation under the SERS detection. Fig. 3 confirms that Cu/V₂O₅ composite was obtained after microwave irradiation treatment.

3.2 SERS detection

To demonstrate the SERS efficiency of the Cu/V₂O₅ composite, Raman experiments were conducted employing it as substrate. And 1 × 10⁻⁷ M R6G was used as the probe molecules owing to its well-established vibrational features.²⁹

Fig. 4 shows the SERS spectra of 1 × 10⁻⁷ M R6G adsorbed on the surface of Cu/V₂O₅ substrate in aqueous solution. It presents good spectral resolution and high Raman signals. And all the Raman peaks can be assigned to R6G molecule according to normal signals. The strong peaks at 1511, 1365 and 1651 cm⁻¹ are corresponding to the aromatic C–C stretch modes, and the peaks centered at 1577 and 1182 cm⁻¹ are attributed to the C=O and C–H bonds, respectively, while those at 1128 and 1311 cm⁻¹ are assigned to the C–H in-plane bending and C=C stretching vibration, correspondingly.^25,26

Fig. 4 A typical Raman spectrum of 1 × 10⁻⁷ M R6G aqueous solution on the Cu/V₂O₅ substrate (upper part) and the SERS contour (lower part).

Here, we employed the main peak at 1511 cm⁻¹ to calculate the enhancement factor (EF) through the equation:⁸ , where I_SERS and N_SERS are the peak intensity of Raman and probe molecule number under SERS conditions, respectively. I₀ and N₀ are the peak intensity of the normal Raman and probe molecule number measured with 0.01 M R6G aqueous solution (Fig. S1†). According to equation, the EF was calculated to be 1.9 × 10⁶ (3897 × 10⁻²/205 × 10⁻⁷, the details were shown in the ESI†), which confirms that the Cu/V₂O₅ substrate shows excellent SERS enhancement for adsorbed R6G molecules.

To further evaluate the SERS performance of Cu/V₂O₅ substrate, it was similarly used as substrate to detect 1 × 10⁻⁵ M MBA probe molecules. Fig. 5 shows the SERS spectrum of 1 × 10⁻⁵ M MBA adsorbed on the surface of Cu/V₂O₅ substrate. All Raman peaks in the upper part of Fig. 5 are characteristic SERS signals of MBA molecules:^27,28 the strong vibrational modes at about 1586 and 1071 cm⁻¹ are assigned to ν8a aromatic ring vibration and ν₁₂ aromatic ring vibration, respectively. Yet, the modes at 1391 cm⁻¹ assigned to the ν(COO⁻) symmetric stretching vibration is obviously stronger and broader than previous reports²⁹ which indicates the carboxyl group and the metal surface has a strong interaction. Other weak peaks at about 1142 (ν₁₅) and 1184 cm⁻¹ (ν₉) are attributed to the C–H bending modes. All the modes are consistent with previous reports^29,30 and the normal Raman spectra of MBA powder (Fig. S2†). Another important feature shown in the lower part of Fig. 5 is the reproducibility of the SERS signals. The SERS contour was plotted for the mapping mode in 1 × 10⁻⁵ M MBA aqueous solution from 100 points. It is obvious that most of the plots show distinctive Raman intensity, revealing excellent capability to enhance the Raman signals of MBA molecules. It is known that copper is easy to oxidation. The XRD pattern of Cu/V₂O₅ substrate after SERS detection was collected and presented in Fig. S4 (ESI†) in order to test the stability of copper substrate. The peak at about 2θ = 61.8° is resulted from Si wafer while other peaks can be attributed to copper. There was no obvious peak of copper oxide, which showed that the copper has good stability in the SERS detection.

Fig. 5 A typical Raman spectrum of 1 × 10⁻⁵ M MBA aqueous solution on the Cu/V₂O₅ substrate (upper part) and the SERS contour (lower part).

The SERS enhancement of Cu/V₂O₅ composite may be due to the synergy of two effects: firstly, the electromagnetic fields caused by localized surface plasmon resonance (LSPR) of Cu would result in the enhanced Raman signal; In addition, the chemical enhancement also may be expressed to the Lewis acid–base properties of V₂O₅, which promote the chemical adsorption capacity for probe molecules. It is speculated that Lewis base R6G molecules could be chemisorbed on Lewis acid sites V⁵⁺. On the other hand, Lewis acid MBA molecules could be chemisorbed on the terminal oxygen of Lewis base sites V=O and on the bridging oxygen in V–O–V bonds. When the V₂O₅ combined with Cu, all these chemical adsorption could increase the concentrations of probe molecules on the surface of Cu. Therefore, the SERS enhancement was considered to be the synergetic effect of V₂O₅ and Cu.

3.3 Catalytic performance study

It is known that both the V₂O₅ and Cu have excellent catalytic activity based on previous reports.^31,32 To investigate their activity in organic catalytic reaction, the as-prepared Cu/V₂O₅ composite were firstly employed as catalysts to the Ullmann reaction with C–N and C–C bonds' cross coupling. During the course of optimization of the reaction conditions, the coupling of iodobenzene and o-phenylenediamine was initially studied as a typical reaction. The reaction is conducted in a simple and mild hydrothermal system following the below eqn (i):

After the measurement of GC-MS, the yields of the goal products were calculated and shown in Table 1. All results were obtained without the protection of inert atmospheres or the use of ligands. Entry 1 suggested that iodobenzene could be successfully coupled with o-phenylenediamine in the presence of Cu/V₂O₅ catalysts to produce 2-aminodiphenylamine in a yield of 92.5%.

Table 1 Investigation of the Cu/V₂O₅-catalyzed coupling reaction at 135 °C for 15 h

Entry	Aryl halogen	Substrate	Product	Yield (%)
1				92.5
2				83.6
3				75.4
4				68.8

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Previous reports demonstrated that the electronic properties of the substituent on aryl and the type of aryl halide could play important roles in the reactivity of C–N coupling reactions.³³ Based on these theories, some basic experiments have been further studied in the same condition. It is found that the yield of the products will decrease when the phenylamine (entry 2) replaced the o-phenylenediamine with two strong electron-donating groups (NH₂). Entry 3 showed lower yield when the iodobenzene was replaced by bromobenzene which has a weaker reactivity. The C–C bond couping reaction also conducted in a simple iodobenzene system. The yield of the products was only 68.8% which suggested that Cu/V₂O₅ catalysts have a better catalytic ability to C–N bond than C–C bond. To determine the oxidation state of the copper, the XRD pattern of the products after catalytic reaction was collected and shown in Fig. S5.† Judged from the XRD pattern, it could be seen that cuprous oxide (Cu₂O) was generated by oxidation of copper under the high temperature reaction.

All these preliminary tests showed that the Cu/V₂O₅ composite has an excellent catalytic performance in Ullmann reactions. However, its catalytic property needs to be further studied to expand its application areas in other organic reactions.

4. Conclusions

In summary, Cu/V₂O₅ composite has been successfully synthesized via a simple and time-saving microwave-assisted reduction. The products V₂O₅ not only strongly adsorbed microwave to accelerate the reduction rate, but also could be used as framework to support copper and help increase its anti-oxidation property. In addition, V₂O₅ also exhibited Lewis acid–base properties which was important for increasing surface adsorption and synergetic effect to SERS. This substrate exhibited excellent SERS sensitivity and reproducibility for both MBA and R6G probe molecules. Furthermore, Cu/V₂O₅ composites also could be employed as catalysts for Ullmann reaction for C–N and C–C bonds' cross coupling. The results demonstrated that C–N bond couping reaction was more complete than C–C bond couping.

Acknowledgements

Financial support from by the National Basic Research Program of China (973 Program) (Grant no. 2012CB932903), the National Natural Science Foundation of China (21071106), and the Priority Academic Program Development of Jiangsu Higher Education Institutions are appreciated.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Normal Raman spectrum of 0.01 M R6G methanol solution and MBA powder; Distribution histograms of the width and length of V₂O₅ nanoribbon; XRD pattern of Cu/V₂O₅ composite after SERS detection. See DOI: 10.1039/c3ra46696e

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