Electrodeposition of High Density Silver Nanosheets with Controllable

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Electro-deposition of high density silver nanosheets with controllable morphologies served as effective and reproducible SERS substrates Yiqing Xia, Yunwen Wu, Tao Hang, Jiaming Chang, and Ming Li Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00101 • Publication Date (Web): 22 Mar 2016 Downloaded from http://pubs.acs.org on March 23, 2016

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Electro-deposition of high density silver nanosheets with controllable morphologies served as effective and reproducible SERS substrates Yiqing Xia,† Yunwen Wu,† Tao Hang,* Jiaming Chang, Ming Li

†These authors contributed equally to this work.

State Key Laboratory of Metal Matrix Composites, School of Material Science and Engineering, Shanghai Jiao Tong University, No. 800 Dongchuan Rd., Shanghai 200240, China.

ABSTRACT: Silver nanosheets with a nanogap smaller than 10 nm and high reproducibility were constructed through simple and environmental friendly electro-deposition method on copper plate. The sizes of the nanogaps can be varied from around 7 nm to 150 nm by adjusting the deposition time and current density. The nanosheets with different nanogaps exhibited varied surface-enhanced Raman scattering (SERS) properties due to electromagnetic mechanism (EM). The optimized high density silver nanosheets with a nanogap smaller than 10 nm showed effective SERS ability with an enhanced factor as high as 2.0×105. Furthermore, the formation mechanism of the nanosheets during the electro-deposition process has been investigated by discussing the influence of boric acid and current density. This method has proved to be

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applicable on different metal substrates, which exhibits the potential to be widely used in different fields.

KEYWORDS: silver nanosheets; sub-10 nm nanogap; electro-deposition; surface-enhanced Raman scattering (SERS).

1. INTRODUCTION Silver nanostructures has special physical, optical properties, which lead to the applications in different fields, such as efficient optical catalysis,1, 2 superhydrophobic substrates,3, 4 bonding materials5, 6 and sensors.10, 11 It is widely shared that the silver nanostructures are the most effective surface-enhanced Raman scattering (SERS) substrates since this phenomenon was first reported in 1974.7,8 In recent years, many studies have demonstrated its prospects for non-damaging single-molecule detection applied in various areas, including biology,9-11 chemistry,12,

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agriculture,14,

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food safety16 and so on.17-19 Nanogaps existing in the

nanostructures can form hot spots when its localized surface plasmon resonance (LSPR) is excited by light20, which are considered as the effective enhanced area, due to the electromagnetic enhancement effect.21-26 This is the major contribution to the SERS ability. Many studies have demonstrated that the sub-10 nm nanogaps can generate the most powerful EM field. And the silver nanostructures with the existence of sub-10 nm nanogaps are the most effective SERS substrates.14, 15


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Different silver nanostructures like nanoshell,27 nanorods,28 nanotriangles,29 nanocubes,30 have been successfully prepared in the colloid suspension, which is easy to aggregate that will result in the invalidation of SERS signals. In recent years, researchers have poured attention into directly deposit nanostructured silver films on a platform. This is a more practical substrate for the SERS detection. Silver dendrite,31-33 silver nanoplates,34 and many other silver nanostructures35, 36 have been prepared on different substrates by simple surface replacement reaction. But the silver nanostructures are always growing randomly on the substrate because that the surface replacement reaction is too fast to be controlled. This will lead to large nanogaps and low reproducibility of SERS signals. In order to fabricate substrates with nanostructures in a controlled manner, a combination of lithographic techniques, physical vapor deposition and other techniques has been used. However, in these methods, the process is often time consuming and high cost unique equipment is required. In our previous report, we have successfully deposited quasi- periodical microball-nanosheets silver films as effective SERS substrates by chemical deposition method.37 The existing of sub-10 nm nanogaps has intensively improved the sensitivity of SERS. However, the uniformity of the silver film is still not satisfied due to the uncontrollability of chemical deposition. And the chemical deposition method is too sensitive to the deposition substrate, which hinders the practical application of the product. Compared with the above preparation method, electro-deposition is more controllable, effective and has the potential to be widely used. In 2014, Shikuan Yang et al.34 have reported the successful synthesis of silver nanosheets by electro-deposition method. The deposited 3


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dimensional structured silver nanosheets were demonstrated to have good SERS ability by providing substantial hot spots. However, the nanogap between the reported silver nanosheets was as large as 1 μm and the nanosheets were mostly disordered, which limits the further improvement on their SERS ability. Consequently, in order to provide more effective and reproducible SERS substrate, electro-deposition of uniform silver nanosheets structures with controllable size of nanogap are expected to be developed. In this paper, we have raised a high reproducible electro-deposition method to fabricate silver nanosheets on copper plate with sub-10 nm nanogaps. By adjusting the deposition time and the current density, we can design the nanosheets with controllable morphology. SERS properties of silver nanosheets with different length and density have been measured to investigate the influence of morphologies on SERS. The formation mechanism of the typical silver nanosheets has been discussed by investigating the influence of boric acid and current density. This deposition method has proved to be widely used on different metal substrates, such as silver plates and aluminum surface of can rings. This electro-deposited silver nanosheets can provide effective and reproducible SERS signals，which exhibits the potential to be widely used in different fields.

2. EXPERIMENTAL SECTION 2.1. Materials


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All chemicals including silver nitrate (AgNO3), ammonium citrate, orthoboric acid (H3 BO3), and copper (Cu) plate were obtained from Sinopharm Chemical Reagent Co.,Ltd. and used without further purification.

2.2. Preparation of the Silver Films First the Cu foil was pretreated by degreasing and acid pickling. After being rinsed by deionized water, the copper foil was immersed into the electroplating bath, which composed of AgNO3 (0.02 M), ammonium citrate (0.01 M) and H3BO3 (0.5 M). Electro-deposition was carried out in a double electrode system under a galvanostatic mode. A piece of silver plate was used as cathode with a working area of about 15 cm2. The current density is 0.2 A/dm2 (hereafter defined as ASD).

2.3. Characterization and SERS measurement Scanning electron microscopy images were obtained on a Zeiss ULTRA 55 scanning electron microscope. Transmission electron microscopy (TEM) images were acquired using an FEI Tecnai 20 transmission electron microscope, with 200kV accelerating voltage. X-ray diffraction (XRD) was measured using Rigaku D/MAX-IIIA X-ray polycrystalline diffractometer, recorded from 20° to 100°, with Cu Ka radiation (λ=0.15418 nm).

SERS spectra were collected by LabRAM HR Evolution, (HORIBA Jobin Yvon, France) equipped with a Ar ion laser (λ=532 nm, beam size: ca. 2μm, using a 50×short focal length


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objective). Rhodamine 6G (R6G) dye was used as probe molecules. For Raman experiment, the irradiation power of laser is set as 10%, 5 mW, while the integration time is adjusted to 3s.

3. RESULTS AND DISCUSSION 3.1. Controllable Fabrication Figure 2 presents typical SEM images of the deposited silver on the copper plate at current density of 0.2 A/dm2. When the copper plate was immersed into the plating solution for 5 min, the 300 nm disordered silver nanosheets were deposited on the copper surface. From Figure 2a and b, we can measure the nanogap between the disordered silver nanosheets was as large as 150 nm. After 15 min's deposition, in Figure 2c we can see the nanosheets became denser, in Figure 2d, we can calculate the nanogap between nanosheets was about 37 nm. The density of silver nanosheets became higher with the increase of deposition time. From Figure 2d and e, we can find that at the deposition time of 30 min, the silver nanosheets grew closest with each other, and the nanogap was calculated to be between 6 nm to 8 nm. Based on the electromagnetic enhancement mechanism, this nanostructured silver film is supposed to be the highly sensitive SERS substrate. While keeping the deposition time to 60 min, as shown in Figure 2g and h, the silver sheets grew much longer and thicker, which may be caused due to the nearby silver nanosheets growing into a thicker nanosheet. As a result, the nanogap has increased to 20 nm. The nanosheets with different nanogaps can be easily fabricated by controlling the


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electro-deposition time. We believe these nanosheets will be used in different areas due to their different nanostructures.

3.2. Crystal Analysis In the TEM image (Figure 3a), we can see that at the beginning of deposition process, hundreds of extremely thin silver nanosheets grew vertically against the copper substrate. We can observe the 1/3 {422} planes with a plane spacing of 0.25 nm in the HRTEM image of silver nanosheets in Figure 3b. Such 1/3 {422} forbidden reflections were ascribed to {111} stacking faults parallel to the {111} surface and extending across the nanoplates.38 The surface energies of crystals having these facets decrease in the order: {110} 0.953 eV > {100} 0.653 eV > {111} 0.553 eV.27, 39, 40 To reach the state of thermodynamic stability, the normal reduction of silver ions results in the formation of stable faceted particles exposing the low surface energy {111} facets. It is reasonable we find the 1/3 {422} plane defect in the silver nanosheet's surface. Four distinct peaks are observable at (200), (220), (311) and (004) in the XRD pattern of silver nanosheets (Figure S1, Supporting Information). At 5min' deposition, the dominant peak was located at (111), which has the lowest surface energy. After 30 min' deposition, the dominant peak changed to the high index plane (004), which indicates the growth of the high index crystal plane.

3.3. Growth Mechanism 3.3.1. Influence of Current Density on Silver Nanostructure


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In our dual–electrode system, Ag+ moves to the cathode and the reduction reaction of silver ions happens at the surface of the cathode (Figure 1): Ag+

+

e → Ag

(1)

We have combined both the current density and the capping agent views to investigate the growth mechanism of the high density silver nanosheets. From Figure 4a and b, we can see that without any outer current, the disordered silver nanosheets with large nanogaps formed after 30 min deposition. This structure is deposited only by surface exchange reaction between Ag+ and copper substrate due to the difference of chemical potential. And the replacement reaction will be terminated after the surface copper has been replaced. This disordered structure is almost the same as the electro-deposited silver nanosheets at 5 min. We speculated that in the electro-deposition process, the procedure was induced by the surface replacement reaction, because the surface replacement reaction took place as soon as the copper substrate contacted the plating bath. When the current density increased, the silver nanosheets grew denser and longer (Figure 4c to 3f) because of the reduction reaction of Ag+. When the current density reached 0.2 A/dm2 (ASD), the nanosheets with the highest density were obtained. In Figure 4g and h, the silver nanosheets aggregated due to the current density as high as 0.3 A/dm2 (ASD). This will cause the decrease of the uniformity of the silver nanosheets. If the current density reached 0.5 A/dm2 (ASD), a brunt deposit was obtained due to the excessive current density and no nanosheets existed. 3.3.2. Influence of Boric Acid on Silver Nanostructure


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In Figure 5, we have adjusted the amount of boric acid in the plating bath to investigate the influence of boric acid on silver structures. The controlled deposition time was 30 min. When the plating bath contained no boric acid, the silver nanoparticles appeared instead of nanosheets. When we added 0.25 M in the plating bath, the nanoparticles changed to nanosheets, which were about 500 nm thick. From Figure 5b and c, we can observe the thick nanosheets morphology. When we continued to increase the concentration of boric acid to 0.5 M, the silver nanosheets with the thickness less than 50 nm were fabricated (shown in Figure 5d). When we increased the boric acid to 1 M, the silver nanosheets would be too thin, and nanogap between each nanosheet was too small to be distinguished. This phenomenon is corresponding with the previous research in electrochemical depositing silver nanosheets.35 We believe that the boric acid acted as the capping agent on the silver nanosheets' surface, and guided the silver in growing along the direction of the sheets’ surface, illustrated in Figure 5.

3.4. Structural Controlled SERS Measurements 3.4.1 R6G served as probe molecules To examine and compare the SERS abilities of silver plates with different nanostructures, we used 1 μM Rhodamine 6G (R6G) ethanol solution. The SERS property of the silver nanosheets obtained at different deposition time have been shown in Figure 6. The silver nanosheets at 5 min showed the worst SERS ability, while the nanogap between nanosheets was measured to be 150 nm in Figure 2. At 15 min, the nanogap was 37 nm, and the nanogap narrowed to sub-10 nm


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at 30 min. The SERS spectra intensity corresponds well with the nanogap change. When the deposition time reached 60 min, the silver nanosheets got thicker, and the nanogap slightly increased, causing the decline of the SERS peaks. Raman spectra on silver nanosheets fabricated under different current densities in Figure 7, also been tested under 1 μM R6G, shows the typical peaks of R6G. The 606 and 760 cm−1 peaks are respectively assigned to C–C–C ring in-plane, out-of-plane bending, and C–H in-plane bending vibration. And the seven sharp peaks at 1106, 1162, 1288, 1342, 1494, 1570 and 1645 cm−1 are associated with the symmetric modes of C–C stretching vibrations.41-43 From Figure 7, we can find that with the current density increasing from 0 A/dm2 to 0.2 A/dm2, the intensity of the SERS specrtra has increased by 5 times. This result matches with the SEM image of the different silver nanosheets (Figure 4). When the current density increases, the nanogaps between silver nanosheets become smaller, which leads to the intense surface enhanced electromagnetic field. The ability of SERS can be characterized quantitatively by calculation of the enhancement factor (EF) of silver substrates, according to eqn (2).44 R6G, as the probe molecule, was taken to measure the SERS performance and the intensities of the 609 cm-1, 771 cm-1, 1342 cm-1, 1508cm-1 peaks (the four strongest peaks) at the concentration of 10–6 M. Nbulk is the average number of molecules in the scattering volume for the Raman measurement and NSERS represents the average number of the SERS surface-adsorbed molecules. In addition, we take the intensity of the solid R6G spectra (Figure S3 Supporting Information) and the SERS spectra into account,


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the EF of silver nanosheets with different nanogaps of prominent peaks of R6G are calculated and listed in Table 1. EF = (ISERS/Ibulk)(Nbulk /NSERS)

(2)

As discussed above, we can conclude that the silver nanosheet deposited at a current density of 0.2 A/dm2 (ASD) for 30 min is the typical nanostructure which has the highest SERS ability, with an enhanced factor (EF) as high as 2.0×105. The nanogap is the dominant element that influences the SERS ability. The existence of sub-10 nm nanogap silver nanosheets can provide highly enhanced surface electromagnetic field (Figure S4 Supporting Information). It indicates that the electromagnetic enhancement mechanism is the main mechanism of SERS enhancement.

3.4.2 4-ATP served as probe molecules To compare the SERS efficiency of the silver nanosheets substrate, non-resonance 4-ATP was also served as probe molecules. The spectra in Figure S5 Supporting Information were obtained by dropping 10-4 M 4-ATP on flat silver and the substrates prepared with an area of 1 cm2. It is clearly seen that the silver nanosheets can provide relatively high Raman enhancement. The Raman spectra of 4-ATP weakened with the decreasing concentration, shown in Figure S6 Supporting Information. The main peaks of 4-ATP can still be clearly observed as the 4-ATP concentration was reduced down to 10-8 M, which demonstrates the high sensitivity of silver nanosheets.


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3.5. Reproducibility of the High Enhancement SERS Substrate

Unlike other silver nanostructures prepared by electroplating, the as prepared typical silver nanosheets have the advantage of highly uniformity and can provide stable and reproducible SERS signals. The SERS spectra of R6G from 15 spots, shown in Figure S7 Supporting Information, were randomly selected from one single typical silver nanosheets plate under the same Raman experimental conditions. The obtained standard deviation in 1342 cm-1 peak intensity was smaller than 20%, which reflects the high reproducibility for the spectra of different spots in the substrate. 45 The excellent SERS reproducibility of the typical silver nanosheets demonstrates that the substrate is a desired SERS substrate.

3.6. Universal Application of this Electroplating Method In order to expand the application filed of this electro-deposition method, we have tried to deposit silver nanosheets on different substrates, like silver surface and can rings. After been acid cleaned, the silver plate was immersed into the plating bath. In Figure 9a we can see that the high density silver nanosheets have been deposited with uniform nanogap about 7 nm. The nanosheets are thicker than that deposited on copper plate. From Figure 9 b and c we can see clearly that a uniform layer of silver nanosheets has formed on the surface of the silver plate. We have also tried to deposit silver nanosheets on a can ring. Figure 9e is the photo of the can ring after being easily surface cleaned. After being immersed into the plating bath for 30 min, the uniform silver nanosheets with the nanogap about 10 nm, shown in Figure 9d, have been formed on the can ring.


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The photo of the can ring is showed in Figure 9f. We believe the deposited silver nanosheets on different substrates have the potential to be used in different fields other than SERS, such as the low-temperature bonding.

4. CONCLUSIONS In conclusion, by simple electro-deposition process, the silver nanosheets with high density can be prepared on copper plate. By carefully adjusting the deposition time, we can fabricate the typical silver nanosheets with the smallest nanogap around 7 nm. These sub-10 nm nanogap silver nanosheets can provide highly enhanced surface electromagnetic field with an EF of 2.0×105. The single crystal feature of silver nanosheets has been characterized by TEM and XRD. The formation mechanism of high density silver nanosheets has been investigated to be the high outer electron density and the capping effect of boric acid. The as-prepared typical silver nanosheets can provide not only high intensity SERS signals, but also high reproducible SERS spectra. Based on the SERS ability and the reproducibility of the typical silver nanosheets, we believe this product can be used as applicable SERS substrates. In addition, we have successfully deposited silver nanosheets on silver plates and can rings by this method. This indicates this electro-deposition method has the potential to be widely used in different fields. AUTHOR INFORMATION Corresponding Authors *(Tao Hang) E-mail: [email protected]. Fax: +86-21-3420-2748. Tel: +86-21-3420-2748.


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Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

This work is sponsored by National Natural Science Foundation of China (No. 21303100) and Shanghai Natural Science Foundation (No. 13ZR1420400).

Supporting Information Available

Illustration of SERS calculation, XRD patterns, SEM images and SERS spectra of silver nanosheets fabricated under different conditions. This information is available free of charge via the Internet at http://pubs.acs.org/.

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(30) Sun, Y.; Xia, Y. Shape-Controlled Synthesis of Gold and Silver Nanoparticles. Science 2002, 298, 2176-2179.

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LIST OF FIGURE CAPTIONS Figure 1. Schematics of the high density nanosheets deposition. Figure 2. SEM images of the silver films electro-deposited with different deposition time (a) 5 min, (c) 15 min, (e) 30 min, (g) 60 min at 0.2 A/dm2 . (b) (d) (f) (h) are the magnified SEM images corresponding with (a) (c) (e) and (g) respectively. Figure 3. (a) TEM image and (b) HRTEM image of the silver nanosheets deposited with 0.01M AgNO3 at the current density of 0.2 A/dm2. Figure 4. SEM images of the silver films electro-deposited with different current density of (a) 0 A/dm2, (c) 0.1 A/dm2, (e) 0.2 A/dm2, (g) 0.3 A/dm2 for 30 min. (b) (d) (f) (h) are the magnified SEM images corresponding with (a) (c) (e) (g) respectively. Figure 5. SEM images of different silver films prepared with the concentration of boric acid of (a) 0 M, (b) (c) 0.25 M, (d) 0.5 M, (e) (f) 1 M. (c) and (f) are the small magnification images corresponding with (b) and (e) respectively. Figure 6. SERS spectra of 1 μM R6G on the as-deposited silver nanosheets fabricated with different deposition time. Figure 7. SERS spectra of 1 μM R6G on the as-deposited silver nanosheets fabricated under different current densities. Table 1. Enhanced Factor of silver SERS substrates with different nanogaps at 609cm-1, 771cm-1, 1342cm-1, 1508cm-1 peaks


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Figure 8. (a) SEM image of the silver nanosheets deposited on silver plate. Photos of silver plate before (b) and after (c) plating. (d) SEM image of the silver nanosheets deposited on can ring. Photos of can ring before (e) and after (f) plating.


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Figure 1. Schematics of the high density nanosheets deposition. 164x136mm (149 x 149 DPI)


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Figure 2. SEM images of the silver films electro-deposited with different deposition time (a) 5 min, (c) 15 min, (e) 30 min, (g) 60 min at 0.2 A/dm2. (b) (d) (f) (h) are the magnified SEM images corresponding with (a) (c) (e) and (g) respectively. 370x613mm (96 x 96 DPI)


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Figure 3. (a) TEM image and (b) HRTEM image of the silver nanosheets deposited with 0.01M AgNO3 at the current density of 0.2 A/dm2. 415x208mm (96 x 96 DPI)


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Figure 4. SEM images of the silver films electro-deposited with different current density of (a) 0 A/dm2, (c) 0.1 A/dm2, (e) 0.2 A/dm2, (g) 0.3 A/dm2 for 30 min. (b) (d) (f) (h) are the magnified SEM images corresponding with (a) (c) (e) (g) respectively. 142x236mm (219 x 219 DPI)


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Figure 5. SEM images of different silver films prepared with the concentration of boric acid of (a) 0 M, (b) (c) 0.25 M, (d) 0.5 M, (e) (f) 1 M. (c) and (f) are the small magnification images corresponding with (b) and (e) respectively. 143x79mm (219 x 219 DPI)


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Figure 6. SERS spectra of 1 µM R6G on the as-deposited silver nanosheets fabricated with different deposition time. 287x202mm (150 x 150 DPI)


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Figure 7. SERS spectra of 1 µM R6G on the as-deposited silver nanosheets fabricated under different current densities. 464x328mm (96 x 96 DPI)


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Table 1. Enhanced Factor of silver SERS substrates with different nanogaps at 609cm-1, 771cm-1, 1342cm1, 1508cm-1 peaks 1830x419mm (96 x 96 DPI)


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Figure 8. (a) SEM image of the silver nanosheets deposited on silver plate. Photos of silver plate before (b) and after (c) plating. (d) SEM image of the silver nanosheets deposited on can ring. Photos of can ring before (e) and after (f) plating. 202x198mm (149 x 149 DPI)


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Electrodeposition of High Density Silver Nanosheets with Controllable

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