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Pulsed laser deposited Ag nanoparticles on nickel hydroxide nanosheet arrays for highly sensitive surface-enhanced Raman scattering spectroscopy
Accepted Manuscript Title: Pulsed laser deposited Ag nanoparticles on nickel hydroxide nanosheet arrays for highly sensitive surface-enhanced Raman scattering spectroscopy Author: Yuting Jing Huanwen Wang Xiao Chen Xuefeng Wang Huige Wei Zhanhu Guo PII: DOI: Reference:
S0169-4332(14)01697-3 http://dx.doi.org/doi:10.1016/j.apsusc.2014.07.169 APSUSC 28416
To appear in:
APSUSC
Received date: Revised date: Accepted date:
21-6-2014 14-7-2014 27-7-2014
Please cite this article as: Y. Jing, H. Wang, X. Chen, X. Wang, H. Wei, Z. Guo, Pulsed laser deposited Ag nanoparticles on nickel hydroxide nanosheet arrays for highly sensitive surface-enhanced Raman scattering spectroscopy, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.07.169 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights >Silver nanoparticles (NPs) were deposited on nickel hydroxide nanosheet (NS) arrays by pulsed laser deposition (PLD) for surface-enhanced Raman scattering
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(SERS) spectroscopy. >the Ag/Ni(OH)2 composite film exhibits very high Raman
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scattering enhancement ability, possessing an enhancement factor as high as 5 106.
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>The enhancement ability of the substrate was strongly dependent on the size and interparticle gap of Ag NPs. >the 3D structure of Ni(OH)2 NS arrays and the charge
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transfer of Ag NPs may be responsible for this high sensitivity Raman phenomenon.
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Pulsed laser deposited Ag nanoparticles on nickel hydroxide nanosheet arrays for highly sensitive surface-enhanced
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Raman scattering spectroscopy
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Yuting Jing, Huanwen Wang, Xiao Chen, Xuefeng Wang*
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Department of Chemistry, Tongji University，Shanghai 200092, China
Huige Wei, Zhanhu Guo*
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Integrated Composites Laboratory (ICL), Dan F. Smith Department of Chemical
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Engineering, Lamar University, Beaumont, Texas 77710, United States
* Email: [email protected];
* Email: [email protected]
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Abstract In the present work, silver nanoparticles (NPs) were deposited on nickel hydroxide
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nanosheet (NS) arrays by pulsed laser deposition (PLD) for surface-enhanced Raman scattering (SERS) spectroscopy. The effective high specific surface area with silver
(FESEM)
and
transmission electron
microscopy
(TEM).
The
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microscopy
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NPs decorated on the NS arrays was revealed by field-emission scanning electron
microstructure and optical property of this three-dimensional (3D) substrate were
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investigated by X-ray diffraction (XRD) and UV-vis spectra, respectively. Using
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rhodamine 6G (R6G) as probe molecules with the concentration down to 10–5 M, the Ag/Ni(OH)2 composite film exhibits very high Raman scattering enhancement ability,
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possessing an enhancement factor as high as 5 106. It has been found that the
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enhancement ability of the substrate was strongly dependent on the size and
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interparticle gap of Ag NPs rather than the testing position on the film surface. In addition, the 3D structure of Ni(OH)2 NS arrays and the charge transfer of Ag NPs may be responsible for this high sensitivity Raman phenomenon. Keywords: Ag nanoparticles; nickel hydroxide nanosheet arrays; pulsed laser
deposition; surface-enhanced Raman scattering
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1. Introduction Surface-enhanced Raman scattering (SERS) has been recognized as one of the
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most useful analytical techniques for identifying molecules due to its large enhancement of normally weak Raman signals [1]. The extremely high sensitivity,
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selectivity and fast response make it possible to characterize molecular species in
environmental analysis [5-7] and optical fibers [8,9].
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many areas, including biological sensors [2,3], electrochemical systems [4],
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During last ten years, the design of SERS substrates that raise super
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electromagnetic enhancement has been a major research interest. Large amounts of SERS studies have been reported on the planar metal nanostructures by controlling
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the shape and size [10,11], chemical nature [12] and the surrounding medium [12-14]
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of nanoparticles (NPs). Efficient SERS substrates such as silver and gold have been
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prepared by many available techniques including electrochemical deposition [15], electroless deposition [16], electron beam lithography [17], nanosphere lithography [18], and so on. An alternative method to fabricate metal nanostructures is pulsed laser deposition (PLD) [19,20], which provides clean metal NPs with controllable dimensions and even distributions but without any impurity [21]. The PLD can produce energetic metal atomic species with energy up to a few keV with initial velocities >106 cm s−1 [22], which undergo a highly directional expansion perpendicularly to a surface on which metal atoms are seeded with tight binding. Moreover, it is possible to change various parameters, such as laser energy, the
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number of laser pulses and deposition time, to control the properties of metal NPs [23-26]. Recently, it has been proved that three dimensional (3D) nanostructures with
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high specific surface area (SSA) can provide giant enhancement of Raman scattering
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[27-29]. Zhang et al. reported an increased Raman intensity of 2-8 times than usual
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2D nanostructures by large-area silver-coated silicon nanowire arrays while extremely hazardous hydrofluoric acid is demanded during the synthesis of silicon nanowire
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arrays [27]. Porous alumina membranes that may be fragile were also utilized for the fabrication of 3D silver nanostructure arrays [28]. Besides, metal NPs deposited by
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conventional techniques are difficult to access the bottom ends of the 3D nanostructures [29].
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Ni(OH)2 nanosheet (NS) arrays are considered to be an especially promising candidate as SERS substrates because of the large specific surface area, easy
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fabrication and modest cost. Herein, Ag NPs/Ni(OH)2 NS arrays were fabricated as a highly efficient SERS substrate by chemical bath deposition process to form Ni(OH)2 NS arrays and pulse laser deposition to obtain Ag NPs on the NS array surface. The morphology, SERS enhancement ability, reproducibility of Raman signal, and optical property of the composite substrate were investigated. Hierarchically porous structure of Ni(OH)2 NS arrays have supplied more hot spots for SERS activity and the charge transfer between Ag NPs and Ni(OH)2 NSs has facilitated an additional enhanced Raman scattering. 2. Experiment
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2.1 Fabrication of Ni(OH)2 NS arrays and planar Ni(OH)2 film Nickel hydroxide thin films were prepared by a chemical bath deposition method and the experimental details are displayed in detail [30,31]. Briefly, the glass slide was
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washed with deionized water and then placed vertically in the fresh solution that was
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mixed with 80 mL of 1 M nickel sulfate, 60 mL of 0.25 M potassium persulfate and
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20 mL of aqueous ammonia. The sample was kept at room temperature for 20 min to deposit the NS film. After rinsing with deionized water, the cleaned Ni(OH)2 NS
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arrays were dried in air.
The planar Ni(OH)2 films were deposited by electrochemical synthesis on a CHI
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660 C electrochemical workstation in 0.08 M Ni(NO3)2·6H2O aqueous solution at a potential of -0.90 V vs. Ag/AgCl (saturated KCl solution) electrode [32,33]. All the
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electrodeposition was done in a three-electrode cell consisting of an ITO glass
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working electrode (4 cm2 in area), a platinum foil counter electrode and an Ag/AgCl
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(saturated KCl solution) reference electrode. 2.2 Deposition of silver NPs
The technique for generating nanoparticles by PLD has been described in detail
previously [19,20]. Before deposition, the chamber was pumped down to a residual pressure of 10–4 Pa. The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate with 8 ns pulse width) was focused on a rotating silver metal target and the generated Ag atoms spread uniformly to the surface of Ni(OH)2 NS arrays. The deposition
process was carried out at room temperature. The distance between the target and the substrate was 3 cm. Similarly, the silver NPs were also deposited on planar Ni(OH)2
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film or glass slide under the same condition. In order to obtain better SERS enhancement, different deposition time periods were applied to control the size of
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silver particles. 2.3 Characterization
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Field emission scanning electron microscopy (FESEM; Philips XSEM30,
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Holland) and transmission electron microscopy (TEM; JEOL, JEM-2010, Japan) were performed to reveal the morphology and microstructure of the samples. The structure
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of deposited nickel hydroxide film was characterized by X-ray diffraction (XRD; Philips PCAPD with Cu Kα radiation). Raman spectra were collected using a 514.5
size of about 1 μm using a 50
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nm laser under ambient conditions. The laser beam was focused to a laser spot with a microscope objective. The incident power was 10.3
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mW and the data acquisition time was 10 s for one accumulation. UV-visible diffuse reflectance spectroscopy (UV-Vis DRS, BWS002) was used to determine the optical
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absorption behaviors of the composite films. 3. Result and Discussion
3.1 Structural characterization of Ag NPs on Ni(OH)2 NS arrays The XRD pattern of Ni(OH)2 NS arrays is presented in Figure 1. All the
diffraction peaks in Figure 1 are perfectly indexed to α-type Ni(OH)2 (JCPDS
22-0444). Besides, the formation of the Ni(OH)2 NS arrays is also accompanied by a minor amount of NiOOH crystalline phase (JCPDS, 06-0044). The inset of planar Ni(OH)2 indicates the formation of α-type Ni(OH)2 (JCPDS, 38-0715), which is closer to amorphous Ni(OH)2 than Ni(OH)2 NS arrays.
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Figure 2a shows the FESEM images of Ni(OH)2 NS arrays. It can be observed that Ni(OH)2 NSs with ultrathin walls are grown uniformly on the glass substrate. These NSs lie perpendicular to the support, and are interconnected with each other,
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forming an ordered nanoarray with a highly porous structure, which provides a large
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specific surface area for the deposition of Ag.
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Meanwhile, Figure 2b clearly shows that silver NPs located on the surface of planar Ni(OH)2 film are coalesced with each other. The FESEM images in Figure 2c
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reveal the morphology and microstructure of Ag coated on Ni(OH)2 NS array, respectively. These particles are homogeneously distributed on both sides of the
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Ni(OH)2 NSs. The Ag-coated Ni(OH)2 NS arrays are further characterized by energy-dispersive X-ray spectrum (EDS) (Figure 2d), which shows strong Ag and Ni
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peaks, corresponding to the silver particles and Ni(OH)2 NSs, respectively. Figure 3(a-d) shows transmission electron microscopy (TEM) images of the Ag
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NPs deposited on Ni(OH)2 NSs with increasing the diameter of NPs from 55 to 212 nm. As the deposition time varied from 10 to 60 min, the density of Ag NPs seems unchanged but interparticle gaps becomes smaller, which can be attributed to the benefit of PLD for growth of Ag NPs. The special structure of Ag NPs is available for the formation of hot spots, which enhances the SERS for the adsorption of probe molecules. 3.2 Optimization of experimental conditions To evaluate the SERS activity, Rhodamine 6G (R6G) was selected as a probe molecule, which is a typically good SERS analyte [34]. Every substrate was immersed
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in a 10-5 M solution of R6G for 30 min and dried in N2 gas. For all samples, SERS signals were obtained under identical conditions and the baselines were corrected by polynomial fitting [35]. It is observed that the peaks corresponding to R6G
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characteristic Raman signals can be clearly observed in Figure 4. The strong peaks
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around 1649, 1508 and 1361cm-1 can be attributed to the aromatic C-C stretching
mode. The peaks at 1574 and 1309 cm-1 are assigned to the N-H in plane bending
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mode and the bands at 1183 and 1127 cm-1 to the C-H bond in plane bending mode.
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Additionally, the peaks at 772 and 611cm-1 correspond to the C-C-C ring in plane vibrating mode and C-H bond out of plane bending mode, respectively [36].
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The deposition time of Ag NPs is a key factor to influence the magnitude of SERS signal enhancement and the sensitivity. It is an important task to optimize the
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experimental conditions for obtaining high sensitive SERS signals. The deposition time of Ag NPs on the NS arrays was varied to provide different SERS enhancement
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(Figure 4). The SERS intensity of the corresponding bands for R6G is enhanced with increasing the silver deposition time and the SERS enhancement reaches maximum at 50 min, demonstrating that much smaller sized gaps are needed to create hot spots among Ag NPs. Nevertheless, with further increasing the deposition time, the intensities of the bands of R6G decrease markedly because of the particle aggregations. The plasmon resonance properties of metal NPs are reported to be influenced by their size and shape [37]. Hence, the size control of Ag NP plays an important role in SERS studies because the enhancement effect declines rapidly as the interparticle gap
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increases [28]. The Ag NPs sizes on the Ni(OH)2 NSs are increased with longer deposition time, which can lead to smaller interparticle gap (Figure 3). The Raman intensity is largely enhanced due to the stronger interaction of the NPs as they get
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closer to each other. However, the Ag NPs start to aggregate at the deposition time of
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60 min (Figure 3d). As a consequence, the optimum condition is found at the
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deposition time of 50 min. 3.3 SERS activity of the silver-coated Ni(OH)2 NS arrays
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To evaluate the SERS performance on different substrates, Raman spectra were measured on three different substrates, i.e., silver-coated Ni(OH)2 NS arrays,
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silver-coated planar Ni(OH)2 film and uncoated Ni(OH)2 NS arrays (Figure 5). Figure 5a shows the strongest SERS enhancement on silver-coated Ni(OH)2 NS arrays when
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using the R6G probe; however, the silver-coated planar Ni(OH)2 film (Figure 5b ) is much less active. Besides, the Ni(OH)2 NS arrays without silver decoration (Figure 5c)
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apparently show no SERS signal even with the R6G concentration as high as 10-4 M,
which can be supposed by the absence of electromagnetic field enhancement induced by silver [38].
It demonstrates that the 3D Ni(OH)2 NS arrays can produce more Ag NPs
decorated on the NSs so that more hot spots are generated around or between the Ag NPs. The special geometric structure of the 3D NS arrays can improve the density of hot spots in a unit area and the local electromagnetic field intensity around each individual hot spot, which is thought to be the primary reason for the enhanced SERS performances.
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Subsequently, the SERS enhancement factor (EF) can be estimated by the formula: (ISERS/INR) (CNR/CSERS) [39,40], where ISERS and INR are the SERS integrated intensities of R6G on optimized silver-coated nickel hydroxide composite film and
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normal Raman (NR) scattering intensity of R6G solution, respectively; CSERS and CNR
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are the concentrations of the R6G molecules in the SERS and NR scattering samples.
Using 10-2 M R6G on the glass slide as a reference (Figure 5d), ISERS (Figure 5a) and
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INR were measured at 1361 cm-1, hence the EF would be at least 5 106.
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Additionally, signal reproducibility is also a vital element to evaluate an effective
SERS substrate in practical applications. The SERS spectra of the R6G molecules
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were collected from 14 randomly selected positions on the optimized silver-coated
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Ni(OH)2 NS arrays under the same experimental conditions as shown in Figure 6.
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Raman spectra with similar intensity clearly demonstrate the good reproducibility on this composite film. The calculated relative standard deviation (RSD) values of major SERS peaks to estimate the reproducibility of SERS signals are below 0.15 (illustrated in Table 1), revealing a good reproducibility on the optimized silver-coated Ni(OH)2 NS arrays.
3.4 Optical properties of the silver coated on the Ni(OH)2 NS arrays The optimized composite substrate exhibits a strong absorption in the UV and
visible region as shown in Figure 7. It is found that the absorption of pure Ag NPs is observed at about 306 nm; however, the absorption shifts to 320 nm for the optimized composite film (Figure 7a). The plasmon absorption of silver is given by 11
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, where
is the effective mass of the free electron of the metal and N is the
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electron density of the metal [41]. In this case, the location of plasmon absorption is connected with the electron density of the metal. Therefore, the red shift of the
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plasmon absorption band from 306 to 320 nm reveals that the electron density of
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silver is decreasing. The reduction of electron density of silver may be caused by the electron transfer from Ag NPs to Ni(OH)2. In fact, the α-type Ni(OH)2 is a
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hydroxyl-deficient compound and consists of a stacking of positively charged
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Ni(OH)2-x layers, which intercalate charge balancing anions in the interlayer region [42]. Consequently, positively charged Ni(OH)2-x layers promote the charge transfer
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between Ag NPs and Ni(OH)2 NSs. As a result of the charge redistribution, the
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positive charges are left on Ag NPs, while Ni(OH)2 NSs are negatively charged, and
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the adjacent surface of Ag NPs and Ni(OH)2 NSs receives the higher charge density as illustrated in Scheme 1. When the substrate is irradiated with a visible light, the local electromagnetic field can be strongly improved at the interface between Ni(OH)2 and
Ag ascribing to the surface plasmon resonance [41,43]. Thus the Raman signal is strongly increased because of the enhanced local electromagnetic field arising from the charge transfer. 4. Conclusions In summary, an efficient and facile strategy has been demonstrated to fabricate Ni(OH)2 NS arrays with silver NPs decoration by taking advantage of PLD method for the creation of an efficient SERS substrate. The SERS enhancement of the 12
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substrates can be controlled by deposition time due to the influence of size and interparticle gap of Ag NPs. The composite substrate using R6G molecule as a model probe has proved to exhibit excellent SERS performance and reproducibility. The 3D
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nanostructures make a contribution to the formation of hot spots for remarkable SERS
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performances. The interaction between Ni(OH)2 and Ag induces the enhancement of
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the local electromagnetic field. Furthermore, this method is by no means limited to the NS arrays; instead it might allow feasible construction of metal NPs deposited
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nanotubes, nanowires, or nanocubes for future SERS studies. Acknowledgements
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This work was supported by the National Natural Science Foundation of China (No.21173158 and No.21373152) and the Ministry of Science and Technology of
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China (No.2012YQ220113-7).
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Figure captions Figure 1 XRD patterns of the Ni(OH)2 NSs. Inset shows the XRD pattern of the planar Ni(OH)2 film.
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Figure 2 FESEM images of (a) Ni(OH)2 NS arrays, (b) silver-coated planar Ni(OH)2
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film, and (c) silver-coated Ni(OH)2 NS arrays; and (d) EDS of the silver-coated
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Ni(OH)2 NS arrays shown in (c).
Figure 3 TEM images of the silver-coated Ni(OH)2 NS arrays at a deposition time of
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(a) 10, (b) 30, (c) 50 and (d) 60 min.
Figure 4 SERS spectra of 10-5 M R6G adsorbed on silver-coated Ni(OH)2 NS arrays
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at a deposition time of (a) 10, (b) 30, (c) 50 and (d) 60 min. The laser wavelength was 514.5 nm, the irradiation power was 10.3 mW, and data acquisition time was 10 s.
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Figure 5 SERS spectra of R6G molecules in different concentrations on the structure of (a) optimized silver-coated Ni(OH)2 NS arrays, (b) silver-coated planar Ni(OH)2
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film, (c) uncoated Ni(OH)2 NS arrays and (d) glass slide with the laser wavelength of
514.5 nm, the irradiation power of 10.3 mW, data acquisition time of 10 s. The deposition time of (a &b) was 50 min and the concentration of R6G molecules was (a &b) 10-5, (c) 10-4 and (d) 10-2 M, respectively. Figure 6 SERS spectra of 10-5 M R6G collected on the randomly selected 14
positions of the optimized silver-coated Ni(OH)2 NS arrays at the deposition time of 50 min. The laser wavelength was 514.5 nm, the irradiation power was 10.3 mW, data acquisition time was 10 s.
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Figure 7 UV-vis spectra of Ag NPs, Ni(OH)2 NS arrays and optimized silver-coated Ni(OH)2 NS arrays at the deposition time of 50 min. Scheme 1 Schematic image of charge transfer between Ni(OH)2 NS and Ag NPs.
Ac ce p
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d
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an
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cr
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Table 1 RSD values for the major R6G characteristic peaks of the SERS spectra.
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40
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30
50
60
70
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20
cr
Intensity (a.u.)
10
20
30
40
50
60
NiOOH
70
80
M
10
an
Ni(OH)2
2(deg)
Ac ce p
te
d
Figure 1.
19
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ip t cr us an M d te Ac ce p Figure 2
20
Page 20 of 27
ip t cr us an M d te Ac ce p Figure 3
21
Page 21 of 27
d
1649
1508
ip t
1361
cr
1574 611 1183
600
800
an
1127
1309
us
772
1000
1200
M
Raman Intensity (a.u.)
5000 counts
1400
1600
c b a
1800
-1
Raman shift (cm )
Ac ce p
te
d
Figure 4
22
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5000 counts 1508
1574
772 -5
10 M
1127
-5
10 M -4
10 M -2 10 M 800
1000
1200
M
600
1309
us
1183
cr
611
an
Raman Intensity (a.u.)
1361
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1649
1400
a b c d
1600
1800
-1
Ac ce p
te
Figure 5
d
Raman shift (cm )
23
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1649
1508 1574
1183
600
800
1000
1309
us
772
cr
611
ip t
1361
1200
1400
an
Raman Intensity (a.u.)
5000 counts
1600
1800
-1
Raman shift (cm )
Ac ce p
te
d
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Figure 6
24
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Ag/Ni(OH)2
cr
Intensity (a.u.)
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Ni(OH)2
200
an
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Ag
300
400
×1.4
500
M
Wavenumber (nm)
Ac ce p
te
d
Figure 7
25
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ip t cr us
Ac ce p
te
d
M
an
Scheme 1
26
Page 26 of 27
1649
1574
1508
1361
1309
1183
772
611
RSD value
0.12
0.11
0.11
0.10
0.10
0.11
0.10
0.10
cr
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Peak position (cm-1)
Ac ce p
te
d
M
an
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Table 1
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Page 27 of 27
S0169-4332(14)01697-3 http://dx.doi.org/doi:10.1016/j.apsusc.2014.07.169 APSUSC 28416
To appear in:
APSUSC
Received date: Revised date: Accepted date:
21-6-2014 14-7-2014 27-7-2014
Please cite this article as: Y. Jing, H. Wang, X. Chen, X. Wang, H. Wei, Z. Guo, Pulsed laser deposited Ag nanoparticles on nickel hydroxide nanosheet arrays for highly sensitive surface-enhanced Raman scattering spectroscopy, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.07.169 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights >Silver nanoparticles (NPs) were deposited on nickel hydroxide nanosheet (NS) arrays by pulsed laser deposition (PLD) for surface-enhanced Raman scattering
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(SERS) spectroscopy. >the Ag/Ni(OH)2 composite film exhibits very high Raman
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scattering enhancement ability, possessing an enhancement factor as high as 5 106.
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>The enhancement ability of the substrate was strongly dependent on the size and interparticle gap of Ag NPs. >the 3D structure of Ni(OH)2 NS arrays and the charge
Ac ce p
te
d
M
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transfer of Ag NPs may be responsible for this high sensitivity Raman phenomenon.
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Pulsed laser deposited Ag nanoparticles on nickel hydroxide nanosheet arrays for highly sensitive surface-enhanced
cr
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Raman scattering spectroscopy
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Yuting Jing, Huanwen Wang, Xiao Chen, Xuefeng Wang*
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Department of Chemistry, Tongji University，Shanghai 200092, China
Huige Wei, Zhanhu Guo*
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Integrated Composites Laboratory (ICL), Dan F. Smith Department of Chemical
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Engineering, Lamar University, Beaumont, Texas 77710, United States
* Email: [email protected];
* Email: [email protected]
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Page 2 of 27
Abstract In the present work, silver nanoparticles (NPs) were deposited on nickel hydroxide
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nanosheet (NS) arrays by pulsed laser deposition (PLD) for surface-enhanced Raman scattering (SERS) spectroscopy. The effective high specific surface area with silver
(FESEM)
and
transmission electron
microscopy
(TEM).
The
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microscopy
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NPs decorated on the NS arrays was revealed by field-emission scanning electron
microstructure and optical property of this three-dimensional (3D) substrate were
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investigated by X-ray diffraction (XRD) and UV-vis spectra, respectively. Using
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rhodamine 6G (R6G) as probe molecules with the concentration down to 10–5 M, the Ag/Ni(OH)2 composite film exhibits very high Raman scattering enhancement ability,
d
possessing an enhancement factor as high as 5 106. It has been found that the
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enhancement ability of the substrate was strongly dependent on the size and
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interparticle gap of Ag NPs rather than the testing position on the film surface. In addition, the 3D structure of Ni(OH)2 NS arrays and the charge transfer of Ag NPs may be responsible for this high sensitivity Raman phenomenon. Keywords: Ag nanoparticles; nickel hydroxide nanosheet arrays; pulsed laser
deposition; surface-enhanced Raman scattering
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1. Introduction Surface-enhanced Raman scattering (SERS) has been recognized as one of the
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most useful analytical techniques for identifying molecules due to its large enhancement of normally weak Raman signals [1]. The extremely high sensitivity,
cr
selectivity and fast response make it possible to characterize molecular species in
environmental analysis [5-7] and optical fibers [8,9].
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many areas, including biological sensors [2,3], electrochemical systems [4],
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During last ten years, the design of SERS substrates that raise super
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electromagnetic enhancement has been a major research interest. Large amounts of SERS studies have been reported on the planar metal nanostructures by controlling
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the shape and size [10,11], chemical nature [12] and the surrounding medium [12-14]
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of nanoparticles (NPs). Efficient SERS substrates such as silver and gold have been
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prepared by many available techniques including electrochemical deposition [15], electroless deposition [16], electron beam lithography [17], nanosphere lithography [18], and so on. An alternative method to fabricate metal nanostructures is pulsed laser deposition (PLD) [19,20], which provides clean metal NPs with controllable dimensions and even distributions but without any impurity [21]. The PLD can produce energetic metal atomic species with energy up to a few keV with initial velocities >106 cm s−1 [22], which undergo a highly directional expansion perpendicularly to a surface on which metal atoms are seeded with tight binding. Moreover, it is possible to change various parameters, such as laser energy, the
4
Page 4 of 27
number of laser pulses and deposition time, to control the properties of metal NPs [23-26]. Recently, it has been proved that three dimensional (3D) nanostructures with
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high specific surface area (SSA) can provide giant enhancement of Raman scattering
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[27-29]. Zhang et al. reported an increased Raman intensity of 2-8 times than usual
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2D nanostructures by large-area silver-coated silicon nanowire arrays while extremely hazardous hydrofluoric acid is demanded during the synthesis of silicon nanowire
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arrays [27]. Porous alumina membranes that may be fragile were also utilized for the fabrication of 3D silver nanostructure arrays [28]. Besides, metal NPs deposited by
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conventional techniques are difficult to access the bottom ends of the 3D nanostructures [29].
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d
Ni(OH)2 nanosheet (NS) arrays are considered to be an especially promising candidate as SERS substrates because of the large specific surface area, easy
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fabrication and modest cost. Herein, Ag NPs/Ni(OH)2 NS arrays were fabricated as a highly efficient SERS substrate by chemical bath deposition process to form Ni(OH)2 NS arrays and pulse laser deposition to obtain Ag NPs on the NS array surface. The morphology, SERS enhancement ability, reproducibility of Raman signal, and optical property of the composite substrate were investigated. Hierarchically porous structure of Ni(OH)2 NS arrays have supplied more hot spots for SERS activity and the charge transfer between Ag NPs and Ni(OH)2 NSs has facilitated an additional enhanced Raman scattering. 2. Experiment
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2.1 Fabrication of Ni(OH)2 NS arrays and planar Ni(OH)2 film Nickel hydroxide thin films were prepared by a chemical bath deposition method and the experimental details are displayed in detail [30,31]. Briefly, the glass slide was
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washed with deionized water and then placed vertically in the fresh solution that was
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mixed with 80 mL of 1 M nickel sulfate, 60 mL of 0.25 M potassium persulfate and
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20 mL of aqueous ammonia. The sample was kept at room temperature for 20 min to deposit the NS film. After rinsing with deionized water, the cleaned Ni(OH)2 NS
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arrays were dried in air.
The planar Ni(OH)2 films were deposited by electrochemical synthesis on a CHI
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660 C electrochemical workstation in 0.08 M Ni(NO3)2·6H2O aqueous solution at a potential of -0.90 V vs. Ag/AgCl (saturated KCl solution) electrode [32,33]. All the
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electrodeposition was done in a three-electrode cell consisting of an ITO glass
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working electrode (4 cm2 in area), a platinum foil counter electrode and an Ag/AgCl
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(saturated KCl solution) reference electrode. 2.2 Deposition of silver NPs
The technique for generating nanoparticles by PLD has been described in detail
previously [19,20]. Before deposition, the chamber was pumped down to a residual pressure of 10–4 Pa. The Nd:YAG laser fundamental (1064 nm, 10 Hz repetition rate with 8 ns pulse width) was focused on a rotating silver metal target and the generated Ag atoms spread uniformly to the surface of Ni(OH)2 NS arrays. The deposition
process was carried out at room temperature. The distance between the target and the substrate was 3 cm. Similarly, the silver NPs were also deposited on planar Ni(OH)2
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Page 6 of 27
film or glass slide under the same condition. In order to obtain better SERS enhancement, different deposition time periods were applied to control the size of
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silver particles. 2.3 Characterization
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Field emission scanning electron microscopy (FESEM; Philips XSEM30,
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Holland) and transmission electron microscopy (TEM; JEOL, JEM-2010, Japan) were performed to reveal the morphology and microstructure of the samples. The structure
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of deposited nickel hydroxide film was characterized by X-ray diffraction (XRD; Philips PCAPD with Cu Kα radiation). Raman spectra were collected using a 514.5
size of about 1 μm using a 50
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nm laser under ambient conditions. The laser beam was focused to a laser spot with a microscope objective. The incident power was 10.3
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mW and the data acquisition time was 10 s for one accumulation. UV-visible diffuse reflectance spectroscopy (UV-Vis DRS, BWS002) was used to determine the optical
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absorption behaviors of the composite films. 3. Result and Discussion
3.1 Structural characterization of Ag NPs on Ni(OH)2 NS arrays The XRD pattern of Ni(OH)2 NS arrays is presented in Figure 1. All the
diffraction peaks in Figure 1 are perfectly indexed to α-type Ni(OH)2 (JCPDS
22-0444). Besides, the formation of the Ni(OH)2 NS arrays is also accompanied by a minor amount of NiOOH crystalline phase (JCPDS, 06-0044). The inset of planar Ni(OH)2 indicates the formation of α-type Ni(OH)2 (JCPDS, 38-0715), which is closer to amorphous Ni(OH)2 than Ni(OH)2 NS arrays.
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Figure 2a shows the FESEM images of Ni(OH)2 NS arrays. It can be observed that Ni(OH)2 NSs with ultrathin walls are grown uniformly on the glass substrate. These NSs lie perpendicular to the support, and are interconnected with each other,
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forming an ordered nanoarray with a highly porous structure, which provides a large
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specific surface area for the deposition of Ag.
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Meanwhile, Figure 2b clearly shows that silver NPs located on the surface of planar Ni(OH)2 film are coalesced with each other. The FESEM images in Figure 2c
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reveal the morphology and microstructure of Ag coated on Ni(OH)2 NS array, respectively. These particles are homogeneously distributed on both sides of the
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Ni(OH)2 NSs. The Ag-coated Ni(OH)2 NS arrays are further characterized by energy-dispersive X-ray spectrum (EDS) (Figure 2d), which shows strong Ag and Ni
te
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peaks, corresponding to the silver particles and Ni(OH)2 NSs, respectively. Figure 3(a-d) shows transmission electron microscopy (TEM) images of the Ag
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NPs deposited on Ni(OH)2 NSs with increasing the diameter of NPs from 55 to 212 nm. As the deposition time varied from 10 to 60 min, the density of Ag NPs seems unchanged but interparticle gaps becomes smaller, which can be attributed to the benefit of PLD for growth of Ag NPs. The special structure of Ag NPs is available for the formation of hot spots, which enhances the SERS for the adsorption of probe molecules. 3.2 Optimization of experimental conditions To evaluate the SERS activity, Rhodamine 6G (R6G) was selected as a probe molecule, which is a typically good SERS analyte [34]. Every substrate was immersed
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in a 10-5 M solution of R6G for 30 min and dried in N2 gas. For all samples, SERS signals were obtained under identical conditions and the baselines were corrected by polynomial fitting [35]. It is observed that the peaks corresponding to R6G
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characteristic Raman signals can be clearly observed in Figure 4. The strong peaks
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around 1649, 1508 and 1361cm-1 can be attributed to the aromatic C-C stretching
mode. The peaks at 1574 and 1309 cm-1 are assigned to the N-H in plane bending
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mode and the bands at 1183 and 1127 cm-1 to the C-H bond in plane bending mode.
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Additionally, the peaks at 772 and 611cm-1 correspond to the C-C-C ring in plane vibrating mode and C-H bond out of plane bending mode, respectively [36].
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The deposition time of Ag NPs is a key factor to influence the magnitude of SERS signal enhancement and the sensitivity. It is an important task to optimize the
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experimental conditions for obtaining high sensitive SERS signals. The deposition time of Ag NPs on the NS arrays was varied to provide different SERS enhancement
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(Figure 4). The SERS intensity of the corresponding bands for R6G is enhanced with increasing the silver deposition time and the SERS enhancement reaches maximum at 50 min, demonstrating that much smaller sized gaps are needed to create hot spots among Ag NPs. Nevertheless, with further increasing the deposition time, the intensities of the bands of R6G decrease markedly because of the particle aggregations. The plasmon resonance properties of metal NPs are reported to be influenced by their size and shape [37]. Hence, the size control of Ag NP plays an important role in SERS studies because the enhancement effect declines rapidly as the interparticle gap
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increases [28]. The Ag NPs sizes on the Ni(OH)2 NSs are increased with longer deposition time, which can lead to smaller interparticle gap (Figure 3). The Raman intensity is largely enhanced due to the stronger interaction of the NPs as they get
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closer to each other. However, the Ag NPs start to aggregate at the deposition time of
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60 min (Figure 3d). As a consequence, the optimum condition is found at the
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deposition time of 50 min. 3.3 SERS activity of the silver-coated Ni(OH)2 NS arrays
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To evaluate the SERS performance on different substrates, Raman spectra were measured on three different substrates, i.e., silver-coated Ni(OH)2 NS arrays,
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silver-coated planar Ni(OH)2 film and uncoated Ni(OH)2 NS arrays (Figure 5). Figure 5a shows the strongest SERS enhancement on silver-coated Ni(OH)2 NS arrays when
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using the R6G probe; however, the silver-coated planar Ni(OH)2 film (Figure 5b ) is much less active. Besides, the Ni(OH)2 NS arrays without silver decoration (Figure 5c)
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apparently show no SERS signal even with the R6G concentration as high as 10-4 M,
which can be supposed by the absence of electromagnetic field enhancement induced by silver [38].
It demonstrates that the 3D Ni(OH)2 NS arrays can produce more Ag NPs
decorated on the NSs so that more hot spots are generated around or between the Ag NPs. The special geometric structure of the 3D NS arrays can improve the density of hot spots in a unit area and the local electromagnetic field intensity around each individual hot spot, which is thought to be the primary reason for the enhanced SERS performances.
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Page 10 of 27
Subsequently, the SERS enhancement factor (EF) can be estimated by the formula: (ISERS/INR) (CNR/CSERS) [39,40], where ISERS and INR are the SERS integrated intensities of R6G on optimized silver-coated nickel hydroxide composite film and
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normal Raman (NR) scattering intensity of R6G solution, respectively; CSERS and CNR
cr
are the concentrations of the R6G molecules in the SERS and NR scattering samples.
Using 10-2 M R6G on the glass slide as a reference (Figure 5d), ISERS (Figure 5a) and
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INR were measured at 1361 cm-1, hence the EF would be at least 5 106.
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Additionally, signal reproducibility is also a vital element to evaluate an effective
SERS substrate in practical applications. The SERS spectra of the R6G molecules
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were collected from 14 randomly selected positions on the optimized silver-coated
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Ni(OH)2 NS arrays under the same experimental conditions as shown in Figure 6.
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Raman spectra with similar intensity clearly demonstrate the good reproducibility on this composite film. The calculated relative standard deviation (RSD) values of major SERS peaks to estimate the reproducibility of SERS signals are below 0.15 (illustrated in Table 1), revealing a good reproducibility on the optimized silver-coated Ni(OH)2 NS arrays.
3.4 Optical properties of the silver coated on the Ni(OH)2 NS arrays The optimized composite substrate exhibits a strong absorption in the UV and
visible region as shown in Figure 7. It is found that the absorption of pure Ag NPs is observed at about 306 nm; however, the absorption shifts to 320 nm for the optimized composite film (Figure 7a). The plasmon absorption of silver is given by 11
Page 11 of 27
, where
is the effective mass of the free electron of the metal and N is the
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electron density of the metal [41]. In this case, the location of plasmon absorption is connected with the electron density of the metal. Therefore, the red shift of the
cr
plasmon absorption band from 306 to 320 nm reveals that the electron density of
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silver is decreasing. The reduction of electron density of silver may be caused by the electron transfer from Ag NPs to Ni(OH)2. In fact, the α-type Ni(OH)2 is a
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hydroxyl-deficient compound and consists of a stacking of positively charged
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Ni(OH)2-x layers, which intercalate charge balancing anions in the interlayer region [42]. Consequently, positively charged Ni(OH)2-x layers promote the charge transfer
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between Ag NPs and Ni(OH)2 NSs. As a result of the charge redistribution, the
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positive charges are left on Ag NPs, while Ni(OH)2 NSs are negatively charged, and
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the adjacent surface of Ag NPs and Ni(OH)2 NSs receives the higher charge density as illustrated in Scheme 1. When the substrate is irradiated with a visible light, the local electromagnetic field can be strongly improved at the interface between Ni(OH)2 and
Ag ascribing to the surface plasmon resonance [41,43]. Thus the Raman signal is strongly increased because of the enhanced local electromagnetic field arising from the charge transfer. 4. Conclusions In summary, an efficient and facile strategy has been demonstrated to fabricate Ni(OH)2 NS arrays with silver NPs decoration by taking advantage of PLD method for the creation of an efficient SERS substrate. The SERS enhancement of the 12
Page 12 of 27
substrates can be controlled by deposition time due to the influence of size and interparticle gap of Ag NPs. The composite substrate using R6G molecule as a model probe has proved to exhibit excellent SERS performance and reproducibility. The 3D
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nanostructures make a contribution to the formation of hot spots for remarkable SERS
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performances. The interaction between Ni(OH)2 and Ag induces the enhancement of
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the local electromagnetic field. Furthermore, this method is by no means limited to the NS arrays; instead it might allow feasible construction of metal NPs deposited
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nanotubes, nanowires, or nanocubes for future SERS studies. Acknowledgements
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This work was supported by the National Natural Science Foundation of China (No.21173158 and No.21373152) and the Ministry of Science and Technology of
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China (No.2012YQ220113-7).
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[31] X.H. Xia, J.P. Tu, X.L. Wang, C.D. Gu, X.B. Zhao, Hierarchically porous NiO film grown by chemical bath deposition via a colloidal crystal template as an electrochemical pseudocapacitor material, J. Mater. Chem. 21 (2011) 671-679. [32] G.R. Fu, Z.A. Hu, L.J. Xie, X.Q. Jin, Y.L. Xie, Y.X. Wang, Z.Y. Zhang, Y.Y. Yang, H.Y. Wu, Electrodeposition of nickel hydroxide films on nickel foil and its electrochemical performances for supercapacitor, Int. J. Electrochem. Sci. 4 (2009) 1052-1062. [33] H.Y. Wu, H.W. Wang, Electrochemical synthesis of nickel oxide nanoparticulate films on nickel foils for high-performance electrode materials of supercapacitors, Int. J. Electrochem. Sci. 7 (2012) 4405-4417. [34] E.C. Le Ru, E. Blackie, M. Meyer, P.G. Etchegoin, Surface enhanced Raman scattering enhancement factors: A comprehensive study, J. Phys. Chem. C 111 (2007) 13794-13803. [35] X.W. Feng, Z.L. Zhu, M.Y. Shen, P.S. Cong, The method of baseline drift correction of Raman spectrum based on polynomial fitting, Comput. Appl. Chem. 26 (2009) 759-762. [36] X.Z. Sun, L.H. Lin, Z.C. Li, Z.J. Zhang, J.Y. Feng, Fabrication of silver-coated silicon nanowire arrays for surface-enhanced Raman scattering by galvanic displacement processes, Appl. Surf. Sci. 256
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(2009) 916-920. [37] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, The optical properties of metal nanoparticles: The influence of size, shape, and dielectric environment, J. Phys. Chem. B 107 (2003) 668-677. [38] M. Moskovits, Surface-enhanced spectroscopy, Rev. Mod. Phys. 57 (1985) 783-826. [39] X. He, C. Yue, Y.S. Zang, J. Yin, S.B. Sun, J. Li, J.Y. Kang, Multi-hot spot configuration on urchin-like Ag nanoparticle/ZnO hollow nanosphere arrays for highly sensitive SERS, J. Mater. Chem.
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SERS performance of silver-nanoparticle-decorated Si/ZnO nanotrees in ordered arrays, ACS Appl.
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[42] P.V. Kamath, G.H.A. Therese, J. Gopalakrishnan, On the existence of hydrotalcite-like phases in the absence of trivalent cations, J. Solid State Chem. 128 (1997) 38-41.
[43] H. Tang, G. Meng, Q. Huang, Z. Zhang, Z. Huang, C. Zhu, Arrays of cone-shaped ZnO nanorods
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decorated with Ag nanoparticles as 3D surface-enhanced Raman scattering substrates for rapid
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te
d
M
detection of trace polychlorinated biphenyls, Adv. Funct. Mater. 22 (2012) 218-224.
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Figure captions Figure 1 XRD patterns of the Ni(OH)2 NSs. Inset shows the XRD pattern of the planar Ni(OH)2 film.
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Figure 2 FESEM images of (a) Ni(OH)2 NS arrays, (b) silver-coated planar Ni(OH)2
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film, and (c) silver-coated Ni(OH)2 NS arrays; and (d) EDS of the silver-coated
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Ni(OH)2 NS arrays shown in (c).
Figure 3 TEM images of the silver-coated Ni(OH)2 NS arrays at a deposition time of
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(a) 10, (b) 30, (c) 50 and (d) 60 min.
Figure 4 SERS spectra of 10-5 M R6G adsorbed on silver-coated Ni(OH)2 NS arrays
M
at a deposition time of (a) 10, (b) 30, (c) 50 and (d) 60 min. The laser wavelength was 514.5 nm, the irradiation power was 10.3 mW, and data acquisition time was 10 s.
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d
Figure 5 SERS spectra of R6G molecules in different concentrations on the structure of (a) optimized silver-coated Ni(OH)2 NS arrays, (b) silver-coated planar Ni(OH)2
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film, (c) uncoated Ni(OH)2 NS arrays and (d) glass slide with the laser wavelength of
514.5 nm, the irradiation power of 10.3 mW, data acquisition time of 10 s. The deposition time of (a &b) was 50 min and the concentration of R6G molecules was (a &b) 10-5, (c) 10-4 and (d) 10-2 M, respectively. Figure 6 SERS spectra of 10-5 M R6G collected on the randomly selected 14
positions of the optimized silver-coated Ni(OH)2 NS arrays at the deposition time of 50 min. The laser wavelength was 514.5 nm, the irradiation power was 10.3 mW, data acquisition time was 10 s.
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Figure 7 UV-vis spectra of Ag NPs, Ni(OH)2 NS arrays and optimized silver-coated Ni(OH)2 NS arrays at the deposition time of 50 min. Scheme 1 Schematic image of charge transfer between Ni(OH)2 NS and Ag NPs.
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d
M
an
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cr
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Table 1 RSD values for the major R6G characteristic peaks of the SERS spectra.
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40
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30
50
60
70
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20
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Intensity (a.u.)
10
20
30
40
50
60
NiOOH
70
80
M
10
an
Ni(OH)2
2(deg)
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te
d
Figure 1.
19
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ip t cr us an M d te Ac ce p Figure 2
20
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ip t cr us an M d te Ac ce p Figure 3
21
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d
1649
1508
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1361
cr
1574 611 1183
600
800
an
1127
1309
us
772
1000
1200
M
Raman Intensity (a.u.)
5000 counts
1400
1600
c b a
1800
-1
Raman shift (cm )
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te
d
Figure 4
22
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5000 counts 1508
1574
772 -5
10 M
1127
-5
10 M -4
10 M -2 10 M 800
1000
1200
M
600
1309
us
1183
cr
611
an
Raman Intensity (a.u.)
1361
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1649
1400
a b c d
1600
1800
-1
Ac ce p
te
Figure 5
d
Raman shift (cm )
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1649
1508 1574
1183
600
800
1000
1309
us
772
cr
611
ip t
1361
1200
1400
an
Raman Intensity (a.u.)
5000 counts
1600
1800
-1
Raman shift (cm )
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d
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Figure 6
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Ag/Ni(OH)2
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Intensity (a.u.)
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Ni(OH)2
200
an
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Ag
300
400
×1.4
500
M
Wavenumber (nm)
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te
d
Figure 7
25
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Ac ce p
te
d
M
an
Scheme 1
26
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1649
1574
1508
1361
1309
1183
772
611
RSD value
0.12
0.11
0.11
0.10
0.10
0.11
0.10
0.10
cr
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Peak position (cm-1)
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te
d
M
an
us
Table 1
27
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