Room Temperature Ferromagnetic Ni Nanocrystals - ACS Publications

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J. Phys. Chem. C 2009, 113, 6022–6032

Room Temperature Ferromagnetic Ni Nanocrystals: An Efficient Transition Metal Platform for Manifestation of Surface-Enhanced Raman Scattering Sougata Sarkar, Surojit Pande, Subhra Jana, Arun Kumar Sinha, Mukul Pradhan, Mrinmoyee Basu, Sandip Saha, S. M. Yusuf,† and Tarasankar Pal* Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India ReceiVed: December 10, 2008; ReVised Manuscript ReceiVed: February 11, 2009

A simple solid-phase synthetic approach has been deliberately exploited for the synthesis of room temperature ferromagnetic, phase pure, fcc Ni nanocrystals on resin matrix. Self-assembly directed chainlike hierarchical nanostructures on the matrix could be engendered from magnetic dipole-dipole interaction between the nanocrystallites. Then, a practical virtue of the transition metal nanoparticle, Ni, was expressed from the rich and high-quality vibrational information of a chelating ligand, 1,10-phenanthroline (phen), onto the magnetically separated metal particles. Thus, surface-enhanced Raman scattering (SERS) has emerged exclusively from the time-dependent surface complexation of the chemically adhered probe molecule. Finally, kinetic effect has bestowed Ni(II)-phen chelate which later on demonstrates unique SERS activity on fcc Ni nanocrystals. The results provide a benchmark illustration of the value of transition metal for aiding interpretation of the vibrational signature of the adsorbate attainable from SERS studies. 1. Introduction During recent years, the development of nanotechnology enabled us to synthesize functional materials in low dimensions, including metals,1 semiconductors,2 hybrid materials,3 and so forth.4 Scientists have been paying more and more attention to design and fabrication of functional materials by ordered organization of nanostructures across extended dimensions.5 As previously described in multiple publications, the controlled synthesis of magnetic nanoparticles is of high scientific and technological interest.6 This sustained interest was of course motivated by the manifold crucial applications of these systems in the fields of high-density magnetic storage devices, contrast enhancement in magnetic resonance imaging (MRI), biomedicine, and catalysis.7 Moreover, properly conjugated with molecules that have an affinity to tumor cells, magnetic nanoparticles could also be advantageous for drug delivery, therapy, and regenerative medicine.8 Superparamagnetism, a unique and important aspect of magnetism in nanoparticles, is of broad interest for potential applications including ferrofluid technology and magnetocaloric refrigeration and has been studied extensively involving nanoparticles of Fe, Co, and Ni. The use of dipolar nanoparticles (NPs) as building blocks to prepare organized hierarchical materials is an emerging area of great potential in materials chemistry. Ferromagnetic colloids are of immense interest for this application, as the inherent dipole moment of these materials enables one- and two-dimensional assembly into novel mesostructures. One of the basic challenges of studying and utilizing these materials is obtaining appreciable quantities of ferromagnetic NPs possessing uniform size and well-defined magnetic properties. * Corresponding author. E-mail: [email protected]. † Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India.

Undoubtedly, as an important magneto-anisotropic material, nickel has also attracted attention and been the focus of intense research. The intriguing potential applications of magnetic nickel nanostructures have stimulated rapid development of the synthetic techniques. To date, a variety of synthetic protocols encompassing sonochemistry,9 thermal decomposition of organometallic precursor,10 chemical reduction,11 and electrochemical reduction12 have been successfully employed in generating nickel nanocrystallites with different morphological architectures.13 Recently, the fabrication of one-dimensional Ni nanostructures principally engrosses the template-based techniques: electrochemical deposition or MOCVD of metals within the nanopore of template materials like anodic aluminum oxide (AAO) or carbon nanotube.14 Still, little colloidal chemistry approach has been accounted to date on the preparation of Ni 1D nanostructures.15 Polystyrene template has been recently taken into account for the synthesis of bowl-like Co nanocrystals.16 Thus, such a template (polystyrene)-based synthetic protocol may provide a more promising technique for preparing 1D nanostructure than conventional method in terms of cost and potential for large-scale production.17 Following initial observation by Fleischman et al. in 197418 and subsequent identification,19 surface-enhanced Raman scattering (SERS) paved the way for aiding interpretation of adsorbate vibrational spectra. The typical Raman cross section of a molecule is about 106 and 1014 times smaller than the infrared and fluorescence cross sections, respectively.20 For in situ Raman studies of most adsorbates, the intensities of the Raman signals were so low that they were at (or under) the detection limits of the instruments. Therefore, one must use surface-enhanced Raman spectroscopy (SERS) to boost the detection sensitivity enormously. The recent advent of SERS has provided the detailed vibrational information for species present at trace concentrations down to the single molecule level.21 Recent findings have made SERS a versatile technique having a diverse field of applications not only in analytical sciences but also in biomedicine, environmental monitoring,

10.1021/jp8108732 CCC: $40.75  2009 American Chemical Society Published on Web 03/20/2009

Ferromagnetic Ni Nanocrystals artwork conservation, corrosion, lubrication, explosives, nanotechnology, and heterogeneous catalysis.22 Despite all of these advantages, SERS has limitations in analytical figures of merit, such as reproducibility and dynamic range. The general consensus attributes the observed enhancement to contributions from two mechanisms: a long-range electromagnetic (EM) effect and a short-range chemical (CHEM) effect. Over the years, several different techniques have been developed to create or fabricate functional noble/coinage metal SERS substrates. However, little endeavor has been successfully achieved for transition metals which severely limits the breadth of practical applications. This is explainable as, for coinage metals (Cu, Ag, and Au), the local field enhancement is maximized when the laser frequency is in resonance with the collective electron absorption of nanoparticles (surface plasmon resonance) whereas for transition metal surfaces, the coupling between conduction and interband electron transitions decreases the surface plasmon resonance and consequently results in poor SERS intensity. Tian’s group has made various approaches to optimize the detection sensitivity of a confocal Raman microscope and, further, to develop surface roughening procedures for different transition metals, such as Pt, Rh, Pd, Fe, Co, and Ni, through which good-quality surface Raman signals have been obtained.23 As a consequence, it has become feasible to carry out SERS investigations on diverse molecules on many transition metal substrates.24 1,10-Phenanthroline, a planar, nitrogen-containing bidentate aromatic heterocycle, is capable of forming stable and reducible complexes with transition metal ions due to its efficient chelating ability and π-acidity and has been widely employed as a competent SERS analyte. Several research groups have extensively examined the vibrational nature of this probe molecule both experimentally and theoretically but again the success was achieved either in coinage metal colloids/electrodes or in transition metal electrodes.25 Given the key importance of transition metals (VIIIB group elements) in electrochemistry, it is of great interest to develop means by which Raman enhancements can be imparted to such materials. Till date, very few reports have the SERS spectra on Ni nanocrystallites.26 In this present approach, we intend to focus on the Raman activity on straightforwardly synthesized Ni nanocrystallites. So, an effortless chemical reduction route has been accounted to synthesize hierarchical chainlike architectures of Ni nanocrystallites at ambient conditions on polystyrene beads. The particles were isolated readily by stirring with a laboratory magnet. Associated characterizations (PXRD, EDAX, HRTEM, and SAED) have identified it to be the pure fcc Ni without any signature of surface contamination. However, photoelectron spectroscopy suggests surface oxidation in open-air/aerobic condition. The temperature- and field-dependent magnetization (M-T and M-H) studies for the magnetically isolable nanoparticles were performed. The data investigated the confinement of room-temperature ferromagnetism within the sample. Finally, Ni nanoparticles were elegantly introduced to read out the attachment and surface complexation of phenanthroline with the metal nanoparticles (a time-dependent phenomenon) by surface-enhanced Raman spectroscopy. During the course of investigation, we characterize an unusual substrate, Ni(0), and observe a neat SERS spectrum out of 1 × 10-5 M 1,10-phen. On standing, the SERS spectrum of 1,10-phen successively changes its profile and finally becomes normal Raman spectra, NRS, of Ni(II)-1,10-phen even at the 10-5 M concentration. This spectrum matches well with the NRS of the as-prepared

J. Phys. Chem. C, Vol. 113, No. 15, 2009 6023 red-colored Ni(II)-1,10-phen complex. However, under normal condition, Ni(II)-1,10-phen complex at 10-5 M concentration fails to display the signature NRS of 1,10-phen. Thus, SERS of Ni(II)-1,10-phen complex on Ni(0) substrate illustrates a new SERS probe that too on the Ni(0) substrate. 2. Experimental Section 2.1. Materials. All the reagents used were of AR grade and were used as received without further purification. NiCl2 · 3H2O, cation-exchange resin (R-H+) {Seralite-SRC-120 (with ionexchange capacity 4.5 mequiv/g)}, pyridine, and ethylenediamine were purchased from Sisco Research Laboratory, India. Sodium borohydride (NaBH4) was received from Sigma-Aldrich, and an aqueous solution was prepared freshly in ice-cold water. 1,10-Phenanthroline was purchased from Sigma-Aldrich. Acetonitrile (CH3CN) was purchased from E-Merck, India. All glassware were cleaned using aqua regia, subsequently rinsed with a copious amount of double-distilled water, and dried well prior to use. Double-distilled water was used throughout the course of the investigation. 2.2. Synthesis and Isolation of Ni Nanoparticles. 2.2.1. Synthesis oWer Cation-Exchange Resin. In a conical flask, ∼0.5 g of cation-exchange resin beads were taken and soaked overnight in water and finally washed several times with water. Then water-soluble Ni(II) precursor, aqueous NiCl2 solution (25 mL of 0.05 M), was allowed to exchange with the H+ ion of the resin in portions, i.e., in small aliquots (5 mL), for complete immobilization. After each step, the supernatant became colorless, indicating the ready exchange of Ni2+ with H+. A gradual change in color of the resin beads from pale-yellow to Kelly green substantiates the completion of the binding process. The Ni2+-immobilized resin beads were further washed with plenty of water to drain out the liberated HCl and unexchanged NiCl2. Then ice-cold aqueous solution of borohydride was introduced into the wet resin beads in aerobic condition which results in the reduction of Ni2+ to Ni0 and their subsequent deposition onto the polystyrene beads. After completion of the reduction process, the black nickel-coated beads [R(Ni0)]-H+, were washed thoroughly with water and dried under vacuum. 2.2.2. Magnetic-Field-Induced Isolation of Ni Nanoparticles from Resin Matrix. The as-prepared Ni0-coated dried beads were then taken in CH3CN and stirred magnetically for ∼1 h. The key strategy for the protocol described here is the introduction of a PTFE-coated magnetic bar. This leads to the trouble-free and efficient separation of the magnetic nanoparticles from the resin matrix onto the bar, leaving apart the metalfree resin beads. Finally, the product was isolated from the bar mechanically, washed several times with water and ethanol, and dried in vacuum. Ni nanoparticles were obtained in batches so long as the resin beads remained loaded with Ni0. However, the quantity of Ni0 nanoparticles successively decreased in amount. The exposed resin beads were repetitively used for the gram level synthesis of Ni nanoparticles. 2.3. Analytical Measurements. 2.3.1. X-ray Diffraction (XRD) Study. X-ray diffraction (XRD) patterns of the product were recorded on a Philips PW-1710 X-ray diffractrometer (40 kV, 20 mA) using Cu KR radiation (λ ) 1.5418 Å) in the 2θ range of 30°-80° at a scanning rate of 0.5° min-1. The XRD data was analyzed using JCPDS software. 2.3.2. X-ray Photoelectron Spectroscopy (XPS). The chemical state of the element on the surface was analyzed by a VG Scientific ESCALAB MK II spectrometer (UK) equipped with a Mg KR excitation source (1253.6 eV) and a five-channeltron detection system. The samples were prepared by placing one

6024 J. Phys. Chem. C, Vol. 113, No. 15, 2009 drop of the prepared nanoparticle suspension in ethanol onto a clean glass slide and then allowing them to dry in air. 2.3.3. Field Emission Scanning Electron Microscopy (FESEM). The morphology of the samples was analyzed by field emission scanning electron microscopy (FESEM) using a JEOL (JSM-5800) microscope at an accelerating voltage 20 kV. Compositional analysis of the sample was done by an energydispersive X-ray microanalyzer (Oxford ISI 300 EDAX) attached to the FESEM. 2.3.4. Transmission Electron Microscopy (TEM). Transmission electron microscopic (TEM) study was carried out on a Hitachi H-9000 NAR transmission electron microscope, operating at 100 kV. Samples were prepared by sonicating the powders with alcohol and then placing a drop of solution on a carboncoated copper grid followed by solvent evaporation in vacuum. 2.3.5. Magnetization Measurement. Temperature and magnetic field dependent dc magnetization measurements were carried out using a vibrating sample magnetometer (VSM; Oxford Instruments, UK). For the zero-field-cooled (ZFC) magnetization measurements, the sample was first cooled from room temperature down to 5 K in zero field. After applying the magnetic field of 0.05 T at 5 K, the magnetization was measured in the warming cycle with the field on. For the field-cooled (FC) magnetization measurements, the sample was cooled in the same field (0.05 T) down to 5 K and the FC magnetization was measured in the warming cycle under the same field. For magnetization as a function of field, field-dependent magnetization measurement was carried out at 1.5 K, 100 K, and room temperature up to a maximum field of 7 T. 2.4. Normal Raman Scattering (NRS) and SERS Measurement. Raman spectrum of the samples were obtained with a Renishaw Raman microscope, equipped with a He-Ne laser excitation source emitting at a wavelength of 632.8 nm, and a Peltier-cooled (-70 °C) charge-coupled device (CCD) camera. A Leica DMLM microscope was attached and was fitted with three objectives (5×, 20×, 50×). For our experiments, the 20× objective was used. Laser power at the sample was 15 mW and the data acquisition time was 30 s. The holographic grating (1800 grooves/mm) and the slit provided a spectral resolution of 1 cm-1. 2.4.1. Synthesis of Nickel(II)-1,10-Phenanthroline Complex for Normal Raman Scattering. Reaction of NiCl2 · 6H2O (1 mmol) and 1,10-phenanthroline (3 mmol) in a water-methanol (5:5 v/v) medium under continuous stirring and on slow evaporation of the resulting solution in a fused CaCl2 charged desiccator yields red crystals of [Ni(1,10-phenanthroline)3]Cl2. The crystals were collected by filtration, washed with methanol, and dried. Finally, they were taken for NRS analysis. 2.4.2. Preparation of the Samples for Surface-Enhanced Raman Scattering. 0.05 g of the as-prepared nickel nanopowder was placed in a small glass vial into which 2 mL of the stock solution of 1,10-phenanthroline (10-5 M ethanolic solution) was added and this substrate-probe assembly was incubated overnight to ensure binding of phenanthroline on the Ni surface. 15 µL of the above suspension was dropped on a microscope slide and SERS measurements were started as soon as the solvent evaporated. For a controlled time-dependent kinetics of adsorption of phenanthroline on the nanoparticle surface, a new set was prepared each time and SERS spectra were recorded at seven different time domains. 3. Results and Discussion Cation-exchange resin, a cross-linked polystyrene polymer, consists of aryl sulfonate moiety as integral part of the polymer

Sarkar et al. SCHEME 1: Schematic Representation of Synthesis of Ni Nanoparticles Coated Resin Beads and Their Isolation Using a Laboratory Magnet

matrix with a mobile ion (H+) which can be exchanged with metal ions present in the solution to be treated.27 Now the immobilization of Ni2+ is achievable due to its ready exchange with the H+. The sulfonate functionality (-SO3-) of the polymer lattice offers electrostatic attachment and therefore stabilization of the cation and an equivalent amount of H+ is thus released into the solution. Reduction of Ni(II) ions on resin matrix with aqueous borohydride solution leads to fabricate metallic nickel (Ni0)-coated [R(Ni0)]-H+ beads. Finally, stirring these beads in a magnetic environment conduces facile and quantitative retrieval of the magnetic Ni0 particles. The synthetic protocol has been illustrated mechanistically in Scheme 1. Here it is worth mentioning that borohydride is known to reduce some metal ions to metal borides;28 this facile synthetic methodology has alleviated the bewildering complexity of such reactions and in every case it leaves behind metallic nickel as primary product. Here the solid support definitely facilitates the evolution of stable Ni nanoparticles leaving the lacunae of the solution phase reduction strategy even from surfactant solution. The phase structure and purity of the as-synthesized sample were examined by powder X-ray diffraction (XRD) and the pattern is displayed in Figure 1. Three peaks at 2θ ) 44.4°, 51.8°, and 76.4° are assigned to be the diffraction from the {111}, {200}, and {220} facets of face-centered cubic (fcc) Ni nanocrystal (NC) with lattice constant a ) 3.521 Å which is in good agreement with the reported data {JCPDS File No. 040850; a ) 3.524 Å; space group: Fm3m (225)}. No other secondary phases of the Ni NC system29 or absence of other characteristic peaks of impurities (such as peaks from nickel oxide or nickel hydroxide) in the diffraction profile authenticates the exclusive availability of phase-pure cubic Ni NCs under the current synthetic protocol. The Scherrer equation, t ) 0.91λ/ (B cos θ), was employed in getting an idea about the average crystallite size based on the line width of the diffraction peak corresponding to the {111} reflection. In the aforementioned equation t, λ, B, and θ stand for average crystallite size, the wavelength of the X-ray radiation, full width at half-maximum

Ferromagnetic Ni Nanocrystals

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Figure 1. XRD pattern of the synthesized Ni nanocrystallites using Cu KR radiation (λ ) 1.5418 Å).

(fwhm) of the diffraction peak, and the Bragg diffraction angle. Based on this analysis, the average crystallite size was determined to be ∼20 nm, briefly agreeing with the primary particle size (from TEM). To decipher the nature of surface composition of the nickel nanoparticles, XPS was employed as the surface monitoring tool. The survey scan for the nanoparticles (Figure 2a) exhibits main core-level peaks for C 1s, O 1s, and Ni 2p, centered at binding energies (BEs) 285.7, 532.1 and 852.7 eV, respectively. The binding energy (BE) scale was corrected for specimen charging effects by assigning a value of BE ) 285.7 eV to the C 1s peaks from the surface contamination. Figure 2b and 2c, shows the core-level XPS spectra of Ni 2p and O 1s region. The Ni 2p spectrum is shown to split by core-hole spin-orbit interaction into a 2p3/2 and 2p1/2 peak with an energy separation of 17.5 eV. Both peaks have an input of satellite structure at 3-9 eV higher energy30 to the main/principal peaks. The binding energy of 852.7 eV can be assigned to nickel atoms.31 The strong broad satellite bands consistently have definable maxima at 3.9 and 9.4 eV above the principal line. Bands in this energy region have been ascribed to a charge transfer multielectron transition.32 The O 1s core level spectrum is shown to comprise three peaks positioned at binding energies 529.7, 532.1, and 534.2 eV, respectively. The first two stand for the oxygen element in Ni-O33 and adsorbed oxygen, respectively.34 These O 1s energy components corroborate the assumption that parts of the nickel surface were oxidized to nickel oxide maybe because of exposure of the sample in air before characterization and XRD was incompetent for its detection due to its quite low percentage composition. Field emission scanning electron microscopy (FESEM) images of the as-synthesized Ni products are displayed in Figure 3. It is evident from Figure 3a that the Ni product on the polystyrene surface consists of microspheres with an average diameter of 100-200 nm. These spheres are in close proximity with each other (inset of Figure 3a). Close examination can disclose that the building blocks of these superstructures have a chain/fiber-like morphology in micrometer length scale and the fibers are self-assembled by orderly arranged spherical Ni nanoparticles (Figure 3b). Here it is worth noting that the vague boundaries among the particles at the chain surface allude to a growth mechanism of particle attachment. In having an additional query regarding the possible route of formation of such superstructures, hydrazine hydrate was introduced as a reductant in lieu of borohydride. However, there was no perceptible alteration in the particle morphology (Figure 3c); this fact

Figure 2. (a) Survey scan for the Ni nanoparticles exhibiting main core-level peaks for C 1s, O 1s, and Ni 2p; (b) XPS spectrum of Ni 2p core-level; and (c) XPS spectrum of O 1s core-level with a Mg KR excitation source (1253.6 eV).

provides direct evidence of particle self-assembled growth mechanism. A plausible proposition for the self-assembly directed growth process involves the reduction of Ni2+ to tiny Ni0 nuclei and their subsequent attachment by oriented aggregation leaving aside random aggregation. Such self-assemblyassisted 1D growth has been also reported for organic molecules such as micromolecules and surfactants as organic connectors to link inorganic building blocks to afford nanoparticle-based superstructures.35 The gluing of resulted Ni nanospheres to 1D nanochain could be an effect of magnetic dipole-dipole interaction as stated in earlier literature.36 Here it is worth pointing out that the branching within the hierarchical architectures might be an outcome of stronger anisotropic magnetic forces.37 Compositional analysis of the freshly prepared Ni nanoparticles synthesized by borohydride reduction method was achieved with energy-dispersive X-ray (EDX) analysis which left behind the conclusion that the product was essentially pure metallic nickel (Figure 3d). To figure out the interior of the hierarchical chainlike architectures, the product was further examined by transmission

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Figure 3. FESEM images of the Ni nanoproducts (synthesized by borohydride reduction) on the polystyrene matrix in (a) low magnification (inset: medium magnification) and (b) high magnification. Panel (c) is the representative FESEM image after hydrazine reduction. Panel (d) is the EDX spectrum of the synthesized product.

electron microscopy (TEM) after being processed by sonication for ∼1 h. The image (Figure 4a) shows the presence of nearly uniform and monodisperse spherical nanoparticles having size in the range ∼15-20 nm. Additional characterizations were made through high-resolution TEM (HRTEM) and selected area electron diffraction (SAED) analyses. The fringe spacing, in correspondence to the HRTEM image in Figure 4b, is observed to have a value of 0.205 nm, and closes to the interplanar {111} distance of fcc Ni (0.203 nm), further ascertaining the cubic phase structure of the synthesized nanocrystals. The SAED pattern shown in Figure 4c is evocative of the polycrystalline nature of the sample and the concentric rings therein could be indexed as the {111}, {200}, {220}, {311}, {222}, and {420} planes of the fcc Ni NCs corresponding to the plane distances of 0.197, 0.174, 0.126, 0.106, 0.924, and 0.761 nm, respectively. Thus, by regulating the growth environment and growth dynamics, control of the phase structure of Ni NCs can be easily realized. The magnetic properties of nanomaterials have been believed to be highly dependent on the structural and physicochemical properties of the nanoparticulates. Hence the average particle size and its distribution, shape anisotropy, crystal order, surface characteristics, and presence or absence of bonded molecules could have had a significant influence to regulate the properties.38 This paper has elucidated the magnetic properties of the synthesized fcc Ni nanocrystals through the measurement of both temperature- and field-dependent magnetization. Figure 5a displays the temperature dependency of zero-field-cooled (ZFC) and field-cooled (FC) magnetization, applying a magnetic field strength of 500 Oe for the cubic phase Ni NCs. The sample

behaves as expected for an ensemble of magnetic nanoparticles with a volume distribution, f(V).39 Thus, there is a distribution for both energy barriers, U (U ) KV), and relaxation times, τ ) τ0 exp(U/KBT), which gives rise to different magnetization values depending on whether the sample is cooled in the absence or the presence of a magnetic field.40 The ZFC magnetization curves present two relatively sharp maxima; the first one is observed at low temperature (T1) whereas the second one is at higher temperature (T2). The T1 and T2 were estimated to be 12 and 315 K (Figure 5b), respectively. In addition to these two sharper maxima, one broad maximum is also observed centered around 97 K (T3). The observed three maxima are indicative of the presence of particle size distributions with three mean sizes corresponding to these three mean blocking temperatures (TB) that corresponds to the blocking of particles’ magnetic moments with random orientation. The bifurcation between the ZFC and FC magnetization plots, observed below 318 K (the maximum measured temperature), indicates that the cubic phase Ni NCs have ferromagnetic behavior and its Curie temperature (TC) is above 318 K, which is beyond the measuring range of the magnetometer. Analogous result could also account for the measurement carried out by Jeon et al.41 In the FC curves the magnetization continues to increase below the peak temperature T1, without a tendency toward saturation. It is known that the temperature dependence of the FC magnetization becomes saturated below the peak temperature T1 for spin glasses and continues to increase below that temperature for superparamagnets.42 However, the glassy behavior has also been claimed in magnetic nanoparticles, in which the FC susceptibility continues to increase with decreasing temperature.43 The cusp in the ZFC



Figure 5. (a) Magnetization (M) vs temperature (T) curve (field-cooled and zero- field-cooled) at an applied field of 500 Oe; (b) enlarged M vs T plot near room temperature to show that an irreversibility is very much present right up to 318 K, the highest measured temperature; and (c) field-dependent magnetization over (7 T at 1.5, 100, and 300 K.

Figure 4. (a) TEM image, (b) HRTEM image, and (c) SAED pattern of the Ni nanocrystals.

plot indicates a narrow size distribution. Above TB, Ni nanoparticles show superparamagnetic behavior that follows the Curie-Weiss law. Similar behavioral nature has also been noticed in nanocrystalline Fe2O3 particles.44 The bifurcation of magnetization below the blocking temperature (315 K) in the ZFC-FC curve in Figure 5a resulted from the existence of magnetic anisotropy barriers.45 TB is correlated with magnetocrystalline anisotropy constant (K) and volume of nanoparticles (V) in accordance with the equation K ) 25kBTB/V, where kB stands for Boltzmann constant.

The field-dependent magnetization of the sample was measured at 1.5, 100, and 300 K and is displayed in Figure 5c. At 1.5 K, the sample presents a hysteretic behavior. The saturation magnetization (Ms), remanent magnetization (Mr), coercivity (Hc), and squareness (Sr ) Mr/Ms) could be determined to be about 20.6 emu g-1, 6.1 emu g-1, 600 Oe and 0.3 respectively from this plot. The Ms, Mr, and Hc values of the bulk Ni at 300 K were about 55 emu g-1, 2.7 emu g-1, and 100 Oe, respectively.46 The Ms at 300 K was observed to be nearly 8 emu g-1, far from the saturation value for bulk Ni. Such behavior is most likely the outcome of surface effects and has already been observed in the CoFe2O4 system.47 In general, Ms for nanoscale magnetic materials is lower than that for the corresponding bulk material because the spin disorder on the surface, magnetic interaction between the particles, and surface oxidation would significantly reduce the total magnetic moment.48 The much enhanced coercivity in comparison to that of bulk symbolizes the confinement of ferromagnetism within the


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TABLE 1: Observed Raman Bands and Their Symmetry Assignments NRS spectrum of solid 1, 10-phenanthroline

Ni(II)-1, 10-phenanthroline complex

symmetry species

253 408 412 428 470 510 545 553 605 624 711 810 856 884 1038 1096 1140 1187 1203 1219 1250 1273 1297 1348 1403 1422 1453 1505 1564 1588 1601 1619

242 268 302 413 429 484 510 559 734 812 835 872 905 965 1058 1111 1144 1200 1256 1310 1349 1427 1453 1520 1587 1606 1632

B1 A2 A1 A2 B2 A2 B1 A2 A1 A2 B2 A1 A2 A1 B2 A1 A1 B2 A1 B2 A1 B2 A2 A1 A1 B2 B2 A1 A1 B2 B2 A1 A1

nanocrystallites below the blocking temperature. And its weak Hc signature at room temperature signifies that, at this temperature, the nanoparticles show a behavior that is close to superparamagnetism as the blocking temperature could be marginally higher than the room temperature. The reason behind such enhanced ferromagnetism might emerge from the onedimensional architectures of the synthesized material.49 Surface-enhanced Raman scattering (SERS) has revolutionized the thought about the nature and orientation of adsorbed molecular species and the metal-adsorbate interaction mechanism at surfaces and interfaces.50 Long effort has been achieved successfully in employing SERS as a tool for characterization of molecular adsorption on coinage metal surfaces.51 However, an extension of this technique for analyzing molecules on transition metal surfaces was done only few years ago;52 the main points that made this possible were the accumulated knowledge about the effect, the efforts of several groups in achieving good activation procedures for transition metal surfaces, and the development of highly efficient Raman spectrometers. These improvements allowed the study of adsorption processes of chemically significant species on transition metal surfaces, such as Fe, Co, Ni, Pt, Pd, Rh, and others. Here the study comprises the SERS spectra of 1,10phenanthroline on Ni nanoparticles surface and looks into the time-dependent variation of different vibrational modes.

Figure 6. Normal Raman spectra of (a) 1,10-phenanthroline in the solid state and (b) Ni(II)-phenanthroline complex. Excitation wavelength: 632.8 nm. Laser power: 20 mW. Acquisition time: 30s.

Free phenanthroline is a planar aromatic molecule with 22 atoms and belongs to the C2V point group. To identify the nature of time-dependent SERS spectrum, to get a quantitative idea of the vibrational band flipping and to have an analytical understanding regarding the adsorbate-metal surface interaction, normal Raman spectra (NRS) for free molecule and its Ni(II) complex have been diagnosed by sharp and well-resolved vibrational bands and these observations harmonize well with the theoretically computed one based upon the work by different groups.53 Table 1 lists the Raman frequencies of free and Ni(II)bound phenanthroline. As we can see from the table and from Figure 6, NRS of solid phenanthroline significantly differ from that of Ni(II)-phen complex in vibrational figures of merit. Noticeable changes are observed in frequencies. For instance, blue shift occurs for most of the peaks in case of the chelated complex, e.g., 711 to 734 cm-1, 1038 to 1058 cm-1, 1297 to 1310 cm-1, and 1619 to 1632 cm-1, etc. except for the peaks at 1348 and 1453 cm-1. Here it is discernible that the relative intensities of the bands at ∼1310 and ∼1453 cm-1, with respect to the most enhanced peak in the normalized spectrum of the coordination complex, increase considerably, while the most intense band at 1403 cm-1 in the NRS of pure phen loses its intensity significantly. These features collectively suggest the perturbation of the adsorbate to a certain extent during the course of complexation. The isolated magnetic nanoparticles of Ni(0) do not show any Raman scattering profile. On the other hand, (10-1 M) phenanthroline scatter and provide unique Raman spectrum, i.e., NRS. However, Ni surface-adsorbed phen (10-5 M) does not provide any vibrational signature even in ∼5 h time. Again, we failed to observe NRS bands from the phen as well as from as-prepared Ni(II) complex at ∼10-5 M concentration. Upon incubation for days together, the Ni-adsorbed phen (10-5 M) successively show up vibrational bands of the



Figure 7. Time-dependent normalized SERS spectra of 1,10-phenanthroline (1 × 10-5 M) over Ni nanoparticles after (a) 24 h, (b) 48 h, (c) 72 h, (d) 96 h, (e) 120 h, (f) 144 h, and (g) 168 h (or 7 days).

adsorbate. Hence, we see the SERS spectrum on isolated magnetic Ni nanoparticles. Interestingly enough, the SERS spectrum changes its profile and finally represents the NRS of the Ni(II)-phen complex. Hence, surface oxidation of Ni and in situ chelation by phen are proved beyond doubt.

The time-dependent SERS spectra at 10-5 M adsorbate concentration are shown in Figure 7. As can be observed, significant and distinguishable changes in the spectral pattern are yielded with increase in incubation time of probe-nanoparticles assembly. Initial inspection of the spectra in Figure 7a


Sarkar et al. SCHEME 2: Schematic Representation of Manifold Application of Ni Nanoparticles and the Investigated SERS Phenomena

Figure 8. SERS spectrum of 10-5 M Ni(II)-phenanthroline complex on Ni nanoparticles’ surface.

reveals some resemblance in its band position and frequencies with that of pure solid phenanthroline. However, they also differ in view of few other peak positions and intensities. The vibrational bands at 734 and 1058 cm-1 have no counterpart in the NRS spectrum of solid phenanthroline; bands originating at 409, 1308, and 1422 cm-1 leave their very weak signature in the NRS spectrum and the bands at ∼711 cm-1, and ∼1038 cm-1 lose their intensity in the SERS spectra. The most pronounced differences between the SERS and normal Raman spectra of the bulk sample are found in the 1400-1460 cm-1 frequency range. The most intense Raman band in the spectrum of the phenanthroline monohydrate observed at 1403 cm-1 and assigned to the B2 mode which gradually faded away in the SERS spectra. Instead, the 1422 cm-1 band assigned to the vibration of A1 symmetry is considerably enhanced. This band is also rather strong and slightly blue-shifted in the Raman spectrum of the solid Ni(II)-phenanthroline complex. A comparison of the relative intensities of representative bands of phenanthroline in the NRS (Figure 6, a and b) and SERS spectra (Figure 7a-g) can be made to understand the nature of the surface species. The 1300 and 1509 cm-1 bands become substantially more intense in the SERS spectra than the spectrum of the complex. These bands are also rather strong in the normal Raman spectrum of phenanthroline monohydrate (bands at 1295 and 1505 cm-1). On the contrary, some bands, mainly those corresponding to ring stretching modes in the 1300-1600 cm-1 region, exhibit small downshifts. This evidence was previously observed in the case of benzo-[c]cinnoline54 adsorbed on Ag colloid and related to a decreasing of the ring bond strength due to the chemical interaction with the metal surface. Thus, we may infer that the SERS spectra can diagnose the less perturbed surface complex of 1,10-phenanthroline with metal sites at nanoparticles surface than in its Ni (II) complex. The low-frequency window of the Raman spectra prescribes more diagnostic features related to the nanoparticles-analyte interaction. The SERS spectra in that frequency range exhibit a couple of Raman bands that are not observed in the normal Raman spectra of phenanthroline. These are the bands at ∼242, ∼268, and ∼302 cm-1, all visible in the spectrum of Ni(II) complex. Other bands in the low-frequency part of the SERS spectra under discussion, observed at ∼409, ∼428, ∼485, ∼510, and ∼555 cm-1, have their counterparts in the NRS of the free probe molecule. The band at ∼409 cm-1 has been ascribed by Thornton53a as A2 out-of-plane, in-phase mode. The journey through the time-dependent SERS spectra (Figure 7, a-g) identified the peak intensity reversal for a couple of vibrational modes. For example, the band at ∼1426 cm-1 has its very weak complement in the SERS spectrum recorded after 24 h of

incubation (Figure 7a). The peak gets intensified progressively and reaches the highest intensity after 7 days (Figure 7g). On the contrary, the vibrational band at ∼1405 cm-1 which loses its intensity gradually (Figure 7, a to e) and finally gets mislaid (Figure 7, f and g). Analogous remarks may well be made for the bands at ∼708 and ∼734 cm-1 as well as for ∼1036 and ∼1058 cm-1. Alternatively, the vibrational bands in the higher frequency region, at ∼1587, ∼1606, and ∼1625 cm-1, steadily increase in their intensity with the elapse of time (Figure 7a-g). Surprisingly, we observed that the SERS spectrum after 7 days of incubation (Figure 7g) was superimposable on the NRS spectrum of the complex. So, on behalf of the deliberate interaction of bidentate phenanthroline on powdered nanoparticles surface, we may conclude that the observations made so far are apparently due to the change in surface-adsorbed state of phen (SERS effect) to a surface-adsorbed Ni(II)-phen (in situ produced) chelated species. We have also performed SERS measurement with pyridine (molecule which cannot give strong resonance Raman effect) on Ni substrate at concentration 10-5 M (Figure S1, Supporting Information) and the observed enhancement for different vibrational bands harmonized well with the reported data. Furthermore, the laser excitation used to get the SERS spectra (632.8 nm) is way above from the electronic absorption band of [Ni(phen)3]2+ (Figure S2), precluding any resonance Raman enhancement of the bands. Thus, a likely explanation for this behavior is that the efficient Raman scattering occurs from specific surface sites.55 A similar situation has also been observed by Zawada and Bukowska during SERS investigation of phen on the roughened Cu electrode.56a A few other SERS reports for coordination complexes on noble metal substrates are also known.56b,c A recent report by Andrade and Temperini has also highlighted the monitoring of adsorption behavior of phen on Fe, Co, and Ni electrodes with the variation in applied potential by SERS study.56d Finally, SERS investigation with the synthesized Ni(II)-phen complex (10-5 M) on Ni(0) nanoparticles (Figure 8) and its identity with the Figure 7g gives a full proof evidence of in-house generation of a new Raman probe on the nanoparticles surface (Scheme 2). To the best of our knowledge such SERS effect of a metal complex on the same metal nanoparticles substrate is still unknown. Ethylenediamine was used in lieu of phenanthroline, and a similar observation was noticed in the SERS spectral profile (Figure S3) leading to the same episode of surface complexation. The literature was reinvestigated to contemplate the nature of adsorption orientation of the probe molecule25a on the nanoparticles’ surface. Phenanthroline, being a chelating bidentate planar molecule, can assemble itself on the Ni surface either through “edge-on” or through “face-on” pattern. According to Creighton’s surface selection rules,57 for edge-on adsorp-

Ferromagnetic Ni Nanocrystals tion the A2 vibrations, spanning in the xy plane, parallel to the metal surface, should be the least enhanced. For “flat” or faceon adsorption, the B2 vibrations should be the least enhanced. Now, the most intense SERS bands belong to A1 or B2 symmetry species. The SERS spectra involve the B2 symmetry arrested vibrational bands in major. These spectral observations lead to the conclusion that a flat orientation, involving the π electrons of the aromatic rings, can be ruled out. Hence, an edge-on orientation can be proposed. The present investigation closely agrees with the SERS experiments performed in freshly prepared Ag colloid by Moskovits and co-workers58 who proposed a bidentate chelation of 1,10-phenathroline with respect to the metal surface. But the occurrence of bands at ∼268 and ∼428 cm-1 in the SERS spectra are also indicative of a possibility of “tilted” orientation of the molecule. So, in conclusion, whatever may be the orientation, the interfacial attachment of phenanthroline is justified by its chemisorption on Ni surface followed by metal-to-ligand charge transfer (MLCT), i.e., surface complexation based on the chemical interaction of the molecule with an atomic-scale active-site or “adatom”.59 In a true sense, the chemical and electromagnetic mechanisms collectively contribute to SERS enhancement and are not readily separable.60 Any theoretical approach that challenges to separate them will probably fail in one or more limits. Thus, electromagnetic factor can also share in the enhancement observed in our case due to the different environment factors. 4. Conclusions In summary, a straightforward, reliable, and tailored synthetic protocol has been depicted to fabricate Ni nanoshell coated functionalized polystyrene beads. The immobilization of the precursor ions to the resin moiety could be elucidated through exploitation of electrostatic field force. Of special interest is the easy magnetic field induced isolation of the nanoparticles from the resin matrix. Self-assembly directed growth was observed to afford hierarchical chainlike architectures of Ni nanocrystallites on the polystyrene beads. Temperature- and field-dependent magnetization study revealed the retention of room temperature ferromagnetism within the sample. The most intriguing feature of the synthesized nanoparticles was their successful implementation to probe SERS analyte. Thorough analyses of the SERS spectra of phenanthroline on the Ni nanocrystallites suggest (i) metal-analyte interaction is a timedependent phenomenon, i.e., a kinetic effect; (ii) surface adsorption of phen is chased by its coordinative complexation; and (iii) tilted orientation of the probe molecule on the nanoparticles surface. The surface complexation was authenticated by the superimposition of the SERS spectra (after 7 days) with the normal Raman spectrum of Ni(II)-phen complex. Finally, in situ generation of the new Raman probe (Ni(II)-phen complex) was put forward through SERS investigation with the synthesized complex (10-5 M) on Ni(0) nanoparticles. Therefore, in situ specificity of vibrational spectroscopy for monitoring metal-analyte interaction at their interface makes SERS an ideal tool for probing such interfaces. Acknowledgment. The authors are thankful to the CSIR, UGC, NST, DST, New Delhi, and Indian Institute of Technology, Kharagpur for financial assistance. Supporting Information Available: SERS spectrum of pyridine, UV-visible spectrum of [Ni(phen)3]Cl2, and normalized NRS spectra of ethylenediamine (en) and its Ni(II) complex and SERS spectrum of en on Ni nanoparticles in the same

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