Redox Transmetalation of Prickly Nickel ... - ACS Publications

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Redox Transmetalation of Prickly Nickel Nanowires for Morphology Controlled Hierarchical Synthesis of Nickel/Gold Nanostructures for Enhanced Catalytic Activity and SERS Responsive Functional Material Sougata Sarkar,† Arun Kumar Sinha,† Mukul Pradhan,† Mrinmoyee Basu,† Yuichi Negishi,‡ and Tarasankar Pal*,† † ‡

Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India Tokyo University of Science, Japan

bS Supporting Information ABSTRACT: A nonpolar surfactant assisted mild wet chemistry approach has been presented for controlled fabrication of ferromagnetic ultralong (several micrometers in length) prickly nickel nanowires in gram scale with the assistance of hydrazine hydrate as the reducing agent and nickel chloride as the metal ion precursor. Nanowire structures analogous to the natural plant Euphorbia milii resulted due to the magnetic dipole driven self-assembly, and their alignment was oriented desirably with an external magnetic field. Systematic microscopic characterizations identified the nanowire to be pure fcc-Ni (i.e., face-centered cubic Ni) without any signature of contamination, though X-ray photoelectron spectroscopy (XPS) and magnetization measurements refer to the existence of an ultrathin nickel oxide (NiO) layer over the nanostructures. The as-synthesized nanowires were used as a singlesource precursor for the evolution of nanometric black NiO when calcined. Again, the Ni nanowires act as a sacrificial template that addresses deposition of metallic gold over the nanowire with variable structural hierarchy through their quantitative oxidative dissolution. Then, the composite material serves as a heterogeneous catalyst for reduction of 4-nitrophenol, and a probable reaction mechanism has been suggested. Additionally, the materials were proved to furnish a full-proof enhanced field effect for prolific surface-enhanced Raman scattering (SERS) activity. In a nutshell, the strategy provides a new horizon to design need-based functional material with much practical implication.

’ INTRODUCTION The research area with nanostructured magnetic materials has been enlightened and therefore expectedly received considerable interest due to its multifaceted and promising applications in diverse areas.1 Encouraged by the demanding magnetic transport properties2 with other crucial performances, anisotropic magnetic nanostructures have recently been the subject of sustained interest. Preferentially, the one-dimensional nanoscale building units have been paid more care, engrafted with new physical and chemical insights. Thus, a spectacular variety of different magnetic nanocrystals3 continue to evoke interest with their subsequent implementation as building blocks for the fabrication of materials with complex architectures to meet different needs. Being a prime member of the celebrated magnetic materials, nickel has been extensively investigated in the horizon of catalysis, magnetic storage media, medical diagnosis, etc.4 Therefore, during the last two decades, significant progress has been made toward the artwork of assorted superstructures including r 2010 American Chemical Society

nanorods, nanobelts, nanotubes, hollow microspheres, nanoflowers, oriented nanocolumns, and even more.5 A variety of “bottom up” synthetic protocols in aqueous and nonaqueous media have been believed to be promising options for large-scale fabrications of these hierarchical structures.6 Recently, the presence of an external magnetic field during the nucleation and growth of magnetic nanocrystallites is being paid attention as a real solution to address their ordered-aligned self-assembly.7 An increasingly deserving approach employing galvanic replacement reactions/transmetalation reactions has recently become a key and novel means toward syntheses of a broad spectrum of hollow/porous metal nanoframeworks, metal alloy nanostructures, etc. A group of the materials' research community has exploited the technique both in aqueous and in organic Received: October 6, 2010 Revised: December 13, 2010 Published: December 30, 2010 1659

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The Journal of Physical Chemistry C media to design engineered nanomaterials with intriguing properties.8-11 However, from the published reports, it is understandable that the area has been very little exemplified with onedimensional transition metal nanostructures,11h-j and thus there is still an essential requirement to further exploit the protocol with their one-dimensional hierarchical nanostructures even endangering field effect. The employment of a variety of supported/unsupported transition metal nanoparticles as heterogeneous catalysts to facilitate chemical reactions is now well admitted and also beneficial over the homogeneous congeners which have drawbacks particularly in their separation from the reaction medium or in terms of reusability.12 The study of gold-based heterogeneous catalysis dates back to the low-temperature oxidation of CO by Haruta et al.,13 and since then, considerable efforts have been made to explore its capability in several catalytic aspects.14 Aminoaromatics like 4-aminophenol are significant intermediates for different industrial products,15 and therefore a number of catalytic methods for reduction of nitroaromatics have been used to accomplish this transformation. Nanoparticle-catalyzed reduction of 4-nitrophenol has been diagnosed well by different groups including ours,16 and it has conveyed a new research impetus for heterogeneous catalytic reaction to understand the reaction mechanism. In retrospect, the first ground-breaking landmark in surfaceenhanced Raman scattering (SERS) stemmed from the silver surface,17 and since then, coinage metal substrates, preferentially silver and gold in their nanoregime, have been celebrated as the supreme avenue to probe SERS. Tian's group has made an assortment of significant contributions for granting SERS from the so-called “ill-defined” transition metal substrates.18 Therefore, further SERS study with a transition metal derived nanocomposite would be of potential interest to assimilate rich vibrational information for nearby molecules located on the surface. Herein, we have taken into account a simple approach for fabrication of prickly Ni nanowire that looks like the plant Euphorbia milii in a trouble-free and cost-effective way. The approach was reduction of aqueous Ni(II) ions by hydrazine in a surfactant medium where the inherent magnetic property of the formed tiny nanocrystals leads to their dipolar gluing where an externally applied magnetic field was observed to motivate their self-aggregation. Every related reaction parameter (surfactant concentration and its polarity, hydrazine concentration, metal ion concentration, presence and absence of external magnetic field, counterion effect of precursor metal salt, and heating source) was systematically studied to understand their possible function for the observed hierarchical nanostructures. The synthesized prickly nanowires served as a single source precursor for the formation of robust NiO nanotubes with surface porosity. This phenomenon has been qualitatively explained on the basis of a literature survey. The spines over the surface were observed to serve as active sites for chemical reaction to occur, and this was substantiated by their oxidative etching with CN-, F-, and S2O82- ions. Treatment of the nanowires with these ions resulted in the appearance of densely pitted surface. Benefitted by the typical difference in the reduction potential value of the Ni2þ/Ni and AuCl4-/Au redox pair, the nanowires served as a sacrificial template for galvanic replacement reaction/redox transmetalation with chloroauric acid (HAuCl4). On treatment with HAuCl4, Ni dissolved out gradually, and metallic Au was deposited in turn over the prickly nickel tubular surface. The

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composite materials (Ni@Au) were prepared in succession in a different time domain and also with a different concentration of HAuCl4, and the materials bear a specific surface morphology. A plausible mechanism has been proposed to explain the results. Thus, here we have presented a detailed study of a transmetalation reaction of one-dimensional Ni nanowire with HAuCl4 to anchor nanosized Au(0). Finally, the materials were found to be active catalysts for 4-nitrophenol reduction and pertinent candidates with enhanced field effect suited for SERS studies. Kinetic aspects of the catalytic and SERS activities were then analyzed to figure out the underlying mechanism.

’ EXPERIMENTAL SECTION Materials. Nickel chloride hexahydrate [NiCl2 3 6H2O], Triton X-100 [(C14H22O(C2H4O)n), polyethylene glycol p-(1,1,3, 3-tetramethylbutyl)-phenyl ether; TX-100], hydrazine hydrate (N2H4 3 H2O; 90%), sodium fluoride, potassium cyanide, sodium hydroxide, sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), 4-nitrophenol (4-NP), and 1,10-phenanthroline were purchased from Sisco Research Laboratory, India. Chloroauric acid (HAuCl4 3 3H2O) and sodium borohydride (NaBH4) were purchased from Aldrich. All glassware was 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. Synthesis. All chemicals were of analytical grade and were used without further purification. In a typical and selective synthesis of a 1D Ni nanowire with a sharp slender spiny surface, a precursor aqueous solution of nickel chloride (10 mL; 0.05 M) was poured into an aqueous solution of TX-100 (10 mL; 0.05 M) under continuous stirring conditions, and the resulting green transparent solution was transferred to a 50 mL flask along with the magnetic bar followed by slow heating at ∼80 °C on a water bath. After a while, 50 μL of 1 M NaOH solution was added with a subsequent addition of 1 mL of 99% N2H4 3 H2O. This results in the formation of a blue colored solution which progressively turns gray-black. Finally, a black, floating, and fluffy product resulted inside the solution, wrapping the magnet, leaving aside the clear supernatant solution. The product was collected, washed several times with water to drain out excess surfactant or hydrazine, and then washed with ethanol by magnetic decantation; finally, this was dried in vacuum for further characterizations. The as-synthesized nanowires were incubated separately with an aqueous solution of HAuCl4 of known concentration for different periods of time (10 min, 30 min, 4 h, and 12 h) at room temperature under occasional shaking for fabrication of the Ni@Au transmetalated composite material. Characterization. The crystalline nature of the samples was ascertained through X-ray diffraction (XRD) analyses and 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 data were analyzed using JCPDS software. X-ray photoelectron spectroscopy (XPS) analysis was performed with 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 morphology of the products obtained under different reaction conditions was analyzed by field emission scanning 1660

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The Journal of Physical Chemistry C electron microscopy (FESEM) using a (Supra 40, Carl Zeiss Pvt. Ltd.) microscope at an accelerating voltage of 20 kV. Compositional analysis of the sample was completed with an energydispersive X-ray microanalyzer (OXFORD ISI 300 EDAX) attached to the scanning electron microscope. Transmission electron microscopic (TEM) analyses of the samples were 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 carbon-coated copper grid followed by solvent evaporation in a vacuum. Nitrogen adsorption and desorption measurements for the porous oxide sample were performed at 77 K using a Micromeritics Automated Gas Sorption System utilizing BarrettEmmett-Teller (BET) calculations for surface area and BarretJoyner-Halenda (BJH) calculations for pore size distribution after the samples were degassed in a vacuum overnight. Raman spectra 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. The laser power at the sample was 20 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. Study of Catalytic Activity. The catalytic reduction of 4-NP was carried out in a well-stoppered quartz cuvette in ambient conditions. To a 3 mL aqueous solution of 10-4 M 4-NP, 0.5 mL of 0.1 M aqueous solution of NaBH4 was added. The color of the solution immediately changed to deep yellow from a faint yellow color. At this stage, the catalyst was added, and the reaction was spectrophotometrically monitored in a Spectrascan UV 2600 spectrophotometer (Chemito, India). The study was conducted separately with four catalysts, viz., with prickly Ni nanowire and morphologically different composites of Ni@Au prepared in different time domains. Sample Preparation for Surface-Enhanced Raman Scattering (SERS). Measured amounts (0.03 g) of the as-prepared samples, i.e., nickel nanowires and Ni@Au bimetallic nanocomposites, were placed separately 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 substrate surface. The above materials were then taken on microscope slides, and SERS measurements were done as soon as the solvent evaporated.

’ RESULTS AND DISCUSSION Here, entirely simple, cost-effective, completely aqueous solution based `bottom-up' chemistry has accounted for the anisotropic growth of hierarchical Ni nanostructures. Controllable synthesis of the magnetic nanomaterial was achieved with the assistance of nickel chloride as the metal ion precursor, TX-100 as the stabilizer and also as a soft template, hydrazine hydrate as reducing agent, and a trace amount of NaOH. The key strategy to be addressed here is the deliberate introduction of a polytetrafluoroethylene (PTFE) coated magnetic bar within the reaction mixture to achieve magnetic field assisted dipolar assembly of molecularly interconnected magnetic nanoparticles. The

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Scheme 1. Schematic Presentation of Successive Evolution of Ni Nanowires on a Water Batha

a

Conditions: [Ni2þ]/[N2H4] = 0.026; [TX-100] = 0.025 M; with a magnetic stir bar (inducing magnetic field) in the reaction mixture.

nonionic surfactant presumably endures complexation with Ni2þ ions in solution through its hydrophilic polyethylene oxide (PEO) group and thus offers the slow release of metal ions available for further complexation and successive reduction. Upon addition of N2H4, the clear green solution immediately turns blue owing to the complexation between Ni2þ and hydrazine. A series of hydrazine linked complexes might form in solution with a variable composition like [Ni(N2H4)2Cl2] and [Ni(N2H4)3]Cl2 where hydrazine serves as the bridging bidentate ligand toward the metal center. With progression in reaction time, the blue color gradually changes to gray-black under the reaction conditions suggesting the onset of the evolution of Ni nanocrystallites resulting from the reduction of the coordination complex where, at the solution temperature (∼80 °C), the excess hydrazine serves the purpose of a reducing agent where the reduction potential of the reductant is strongly pH dependent. At room temperature, in acidic pH, the predominant form of hydrazine in solution is the hydrazinium ion (N2H5þ ion), and the standard reduction potential has been recorded as -0.23 V (N2 þ 5Hþ þ 4e- = N2H5þ). However, in alkaline pH, it serves as a more powerful reducing agent where the reduction potential is -1.16 V (N2H4 þ 4OH- = N2 þ 4H2O þ 4e-). However, in those pH environments, reduction potentials for the metal ion are observed to be -0.25 and -0.72 V, respectively, according to the following half-cell equilibrium Ni2þ þ 2e - ¼ Ni

ð - 0:25VÞ

NiðOHÞ2 þ 2e - ¼ Ni þ 2OH -

ð - 0:72VÞ

Therefore, hydrazine can solve the purpose of reduction only in alkaline pH though the pH of the medium should not be very high to cause bulk precipitation of nickel hydroxide which may render the homogeneity of the reduction process, and thus, in turn, the nucleation and growth of the metal nanoparticles may not be processed controllably. Thus, these dual functionalities (coordination and reduction)19 of the amine result in homogeneous nucleation of Ni nanoparticles. The overall reaction could be addressed well as below Ni2þ þ xN2 H4 f ½NiðN2 H4 Þx 2þ ½NiðN2 H4 Þx 2þ þ N2 H4 f Ni0 þ ð4x=3ÞNH3 v þ fðx=3Þ þ 1gN2 v þ 2H2 v and the protocol has been illustrated mechanistically in Scheme 1. Here, it is worth mentioning that in solution borohydride is known to reduce some metal ions but with the subsequent formation of metal borides even from a surfactant medium;20 1661


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Figure 1. XRD pattern of the as-synthesized prickly Ni nanowires.

therefore, this facile synthetic route with hydrazine in lieu of borohydride has alleviated the bewildering complexity of such reactions, and in every case it leaves behind pure metallic nickel as the primary product. Figure 1 illustrates the crystallographic structural characteristics of the as-synthesized samples investigated by XRD analysis. All the reflection peaks could be indexed entirely to the face-centered cubic (fcc) structure {space group: Fm3m (225)} of a Ni nanocrystal having a unit cell dimension: a = 3.521 Å. Three peaks at 2θ = 44.5°, 51.9°, and 76.5° correspond to the diffraction from the {111}, {200}, and {220} facets, and these statistics are in good agreement with the reported data {JCPDS File No. 04-0850; a = 3.524 Å}. Also, the broadening of the peaks in the pattern indicates the nanoscale dimensionality of the crystallites. XPS was employed as the quantitative technique to figure out the chemical nature and surface atomic composition of the prickly nickel nanochains. The survey scan presented in Figure 2a exhibits main core-level peaks for C1s, O1s, and Ni2p centered at binding energies (BEs) of 282.9, 532, and 854.5 eV, respectively. The binding energy (BE) scale was corrected with reference to the spurious C1s peak (282.9 eV) in view of specimen charging effects from the surface contamination. Detailed energy analysis of the Ni2p (Figures 2b) core-level spectrum shows its characteristic spin-orbit splitting of the nickel ion into 2p3/2 and 2p1/2 peaks (at 852.9 and 870.6 eV, respectively) with an energy separation of 17.7 eV.21a Satellite peaks at higher energies (at the 858-862 eV energy region for 2p3/2 and the 873-877 eV energy region for 2p1/2) than these “2p” principal peaks are also noticed, but the smaller intensities and bands in this energy region have been ascribed to a charge transfer multielectron transition.21b These features corroboratively suggest the surface oxidation of the nanowires though their low intensity profile is indicative of a smaller extent of the oxidation. A shoulder peak at ∼849 eV could be assigned to Ni0 in the metallic nanowires. The presence of trace oxygen is obvious from the general scan, and the O1s core level spectrum is observed (Figure 2c) to be comprised of a maxima at 532 eV with an adjoining peak at 531.3 eV which could have been associated with Ni-O of surface NiO.21c However, the ultrathin layer (NiO) does not hamper the stability and also the morphology of the nanowires. Archetypal FESEM images of the synthesized Ni nanowires are presented in Figure 3. The panoramic image in Figure 3a is an

Figure 2. (a) Survey scan for Ni nanowires exhibiting main core-level peaks for C1s, O1s, and Ni2p. (b) XPS spectrum of Ni 2p core-level. (c) XPS spectrum of O 1s core-level with the Mg KR excitation source (1253.6 eV).

overview of the samples with teeming but disorderly assembled wirelike morphology at the surfactant concentration (0.025 M) adequately higher than its critical micelle concentration (CMC; 2.7 10-4 M). This image displays that the high-yield grown architectures are favorably comprised of all but uniform nanowires with a diameter of about ∼1 μm and lengths up to several hundreds of micrometers. The enlarged view in Figure 3b discloses that the evolved network is built-up with plentiful patterned and aligned prickly nanowires with densely packed spiky branches over their surface, and more intriguingly, these spearlike parts grew outwardly from the surface of the nanowires. The conical growth of the branches results in their morphology, constructed with a wide root and sharp tip with a length of ∼100 nm from the root to the tip. A high-magnification SEM micrograph in Figure 3c represents a well-defined and discrete nanowire with distinct spiky texture, and these perpendicularly grown nanodimensional branches are standing along the radial directions from the nanowire backbone. Compositional analysis of the freshly prepared nanowires was achieved with energy-dispersive X-ray (EDX) (Figure 3d) which suggests the product was essentially pure metallic nickel. The tool was also unable to detect the presence of oxygen in the sample. Thus, once more it is practical to believe the presence of oxygen as only a skinny oxide layer over the surface of the samples which is only noticeable by XPS analysis.22 The texture and crystalline orientation of the as-prepared nanostructured sample were further executed with TEM. Figure 4a is the low-magnification TEM image of the products with surfactant concentration of 0.025 M, illustrating the formation of network structure through interconnected nanowires having diameter in the range of 0.3-0.4 μm and length up to several tenths of micrometers. This figure also reveals that the nanowires (Figure 4a) were in association with each other to construct the network. The spiny characteristics of the surface of the synthesized nanowires are also evident from Figure 4b where the sharp contrasts among the dark central axis and the overgrown faded branches substantiate the protruding nature of the 1662



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Figure 3. Panoramic FESEM images of Ni nanowires at (a) lower and (b) medium magnification. (c) An enlarged view of a well-defined and discrete nanowire [conditions: [Ni2þ]/[N2H4] = 0.026; [TX-100] = 0.025 M; with a magnetic stir bar (inducing magnetic field) in the reaction mixture] and (d) EDX spectrum of the prickly sample.

spikes. These well-defined exterior parts have a length of ∼0.1 μm from their basement to the tip. These overall features are in good agreement with our FESEM observation. The TEM observations also acquaint with the evidence of no obvious crack or destruction of the magnetic superstructures. This evidently indicates that the ultrasonication force (applied during TEM sampling as stated in the Experimental Section) was inadequate to overcome the inherent magnetic dipolar force, and therefore, though the nanostructures were built up with an assemblage of particles, we could not excavate the feature from the presented TEM images. Figure 5a,b is the typical higher-magnification TEM images taken on a piece of the extended nanospike rooted from the

surface of the nanowire (Figure 5a). The 2D projection of the canonical-shaped spike gives the impression of a “trigonally grown-up nanopetal”. Close inspection of Figure 5b (the encircled area surrounded by the sky frame in Figure 5a) brings out the lattice resolved fine structures (HRTEM images) of different regional parts on the nanowhisker surrounded by the black frames, and the insets therein explicitly illustrate the nearly parallel atomic planes with a fringe spacing of 0.20 nm. The separation is in excellent agreement with the spacing of {111} crystal planes of fcc nickel. This message also supports the preferential growth of the branches along the {111} direction of the fcc nanocrystals. Appearance of periodic diffraction spots 1663



Figure 4. Bright-field TEM images of Ni nanowires at (a) low and (b) medium magnifications.

in the selected area electron diffraction (SAED) pattern as shown in Figure 5c identified the sample to be polycrystalline in nature, and the concentric rings therein could be indexed as the {111}, {220}, {311}, and {331} crystal planes of the fcc crystal system with the corresponding “d” spacing calculated to be 2.0, 1.26, 1.1, and 0.80 Å, respectively. Of special interest was the appearance of a weakly intense diffraction spot, e.g., at d = 0.52 Å, besides the above which could be addressed as the diffraction from a plane described as 1/2{311}.23 Now, a plausible mechanistic interpretation is essential for the investigated anisotropic growth of the above nanostructures. As supported from XRD and HRTEM analyses, the favored growth orientation of these nanowires could be anticipated as along the {111} planes of the fcc crystal. In the hydrazine-rich environment, poorly crystallized magnetic nanoparticles are first formed owing to the kinetic advantage through the reduction of the blue complex (Figure 6a) (step II of Scheme 1) and result in the formation of spherical Ni particles homogenized (step III of Scheme 1) throughout the solution. These multidomain structures, being magnetic in nature, are inclined to be self-assembled as expected and thus act as the intriguing structure building units (SBUs) for the construction of magnetic superstructures through the inherent dipole-dipole interaction as was observed from Figure 6b. Subsequently, the spontaneous assembly among these nanoparticles results in formation of nanowires, and the mechanism of congregation may find an explanation from several parameters including the velocity of the preformed nucleus, magnetic dipolar interactions, the presence of stabilizer, and also other related kinetic aspects. The shape anisotropy thus causes the {111} axis to be the magnetic easy axis, and as reported in different literature,24 the anisotropic energy is minimum when the magnetization is along this axis direction. Hence, the introduction of the external magnetic field in the present synthetic environment directs the alignment of the easy axis of all the nanocrystallites along the magnetic line of force and inturn puts together the crystallites directionally to reach the conditions of attenuation of both magnetic anisotropic energy and surface energy. However, the attachment of the nanowires within themselves in portion is due to their inherent magnetic dipolar interaction, but the introduced field does not lead to the aggregation of the Ni wires. To have a comprehensive understanding concerning the role of the applied magnetic field on the morphology of the evolved one-dimensional nanostructures, a magnetic stir bar having different field strength was introduced, and a distinctive alteration in product morphology was observed (Figure 7) at the

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standard synthesis conditions. As shown in Figure 7a, in the absence of any external magnetic field, the magnetic nanowires were observed to be oriented randomly while the field effect makes certain the alignment of them. Figure 7b and Figure 7c evidently bring out the difference where we could notice the field strength tempted/motivated deliberate alignment of the wires {(Figure 7b) small magnetic stir bar, .agnetic field strength, 100 G; length, 3.3 cm; and diameter, 1.6 cm; (Figure 7c) large magnetic stir bar, magnetic field strength, 225 G; length, 5 cm; and diameter, 1.5 cm}. Therefore, it was evident that superior field strength could entirely make possible the unidirectional arrangement of the nanowires. Inherent dipolar affinity is thus responsible for the oriented growth of the nanocrystals in the dynamic reaction environment, but their stipulated one-dimensional arrangement is strictly favored by increasing the external field strength. The more compact nature of the wires as observed in Figure 7c over the others might be a result of increased number of nuclei density participating in wire formation with the increase in density of magnetic lines of force. Such controlled patterning of the alignment with and without the presence of an external magnetic field has recently been reported where the authors have publicized the formation of straight nanowires only with applied magnetic field, and only spherical nanoparticles resulted.7 We also noticed that the diameter of the Ni nanowires synthesized under a “small” magnetic stir bar (Figure 7b) is higher than the Ni nanowires synthesized under a “large” magnetic stir bar (Figure 7c). The applied external field directs the alignment of magnetic easy axis of the nuclei as stated earlier. Therefore, in a more enhanced magnetic field, with the increase in density of magnetic lines of force, the kinetics of longitudinal alignment (occurs along the magnetic easy axis) over radial attachment becomes rapid. However, when the field strength is comparatively less, the kinetics becomes slow and hence radial attachment of the nuclei becomes more favorable which results in higher diameter. The mechanistic interpretation has been schematically presented in Scheme 2. Thus, the kinetics of nucleation as well as the effective magnetic field would have a bearing on the observed wire diameters. To excavate the function of TX-100 more precisely beyond its believed responsibility as a stabilizer, we have extended our study with its further lower concentration where we could observe the perceptible change in product morphology. Nanowires with welldefined spiny overgrowth from the surface were obtained with concentration (0.025 M) adequately higher than its critical micelle concentration (CMC), as has been elaborated already in our preceding discussion. The surfactant, when introduced with its concentration of 1.35 10-5 M (5 times higher than CMC), was able to uphold the nanowire formation (Figure S1a, Supporting Information), but the surface of the wires came across with an entirely new look. The overgrown prickly branches skip town with a flowery appearance all the way through plentiful thin, patterned, and radiating nanopetals with an average thickness of about 15-20 nm (Figure S1b, Supporting Information; enlarged view of yellow encircled area of Figure S1a). These petals are interlocked with each other to result in the said morphology having a porous interior. The reaction environment without any trace of TX-100 was still observed to be sympathetic for the one-dimensional growth (Figure S1c, Supporting Information), but as stated above, well-nurtured surface patterning was absent, and an “undergrown-nanospike studded” surface can only be produced in this regard (inset of Figure S1c, Supporting Information). A good understanding of the 1664



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Figure 5. HRTEM image of Ni nanowire: (a) nanospike and (b) lattice resolved HRTEM image of different regional parts (surrounded by the black frames) of the nanospike with a fringe spacing of 0.20 nm. (c) SAED pattern of the nanowire.

Figure 6. FESEM images of (a) the blue Ni-N2H4 complex {inset: digital image of the blue complex (step II of Scheme 1) in solution} and (b) initiation of the magnetic dipole induced self-assembled nanocrystals formation {inset: digital image of the dispersed Ni particles (step III of Scheme 1) in the solution}. Conditions: [Ni2þ]/[N2H4] = 0.026; [TX-100] = 0.025 M and with a magnetic stir bar (inducing magnetic field) in the reaction mixture.

morphologies implied the prerequisite of a critical concentration level of the stabilizer for regulating the surface structural arrangements. It is tentative to assume that there occurs diffusion of tiny nanocrystals within the growth medium toward the surface of the nanowires to assist a preorganized system for growth of the spiny

branches where, most likely, the growth-controlling agent provides kinetic control over the growth of the nanocrystals. Actually, in higher concentration of TX-100, reduction kinetics of metal ions is more confined, and crystal growth is more habitual and therefore results in confined slender spiny spikes. 1665



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Figure 7. FESEM images of Ni nanowires: (a) synthesized in the absence of a magnetic stir bar (no external magnetic field) and synthesized in the presence of a (b) small magnetic stir bar and (c) large magnetic stir bar. Conditions: [Ni2þ]/[N2H4] = 0.026; [TX-100] = 0.025 M. Small magnetic stir bar: magnetic field strength, 100 G; length, 3.3 cm; and diameter, 1.6 cm. Large magnetic stir bar: magnetic field strength, 225 G; length, 5 cm; and diameter, 1.5 cm.

Scheme 2. Schematic Presentation of the Effect of Magnetic Field Strength over Nanowire Diametera

a

Conditions: [Ni2þ]/[N2H4] = 0.026; [TX-100] = 0.025 M. Small magnetic stir bar: magnetic field strength, 100 G; length, 3.3 cm; and diameter, 1.6 cm. Large magnetic stir bar: magnetic field strength, 225 G; length, 5 cm; and diameter, 1.5 cm.

However, in lower concentration, the nuclei are kinetically free to assemble, and so the less-confined leaflike surface results, though in a real state of affairs this might be more complicated. Again, we have introduced charged surfactant separately (CTAB as cationic and SDS as anionic) in place of the neutral one, keeping other reaction parameters unaltered to further ascertain their effect. We observed the formation of micrometer

long nanowires (Figure 8a) with a roughly textured surface (as shown in the inset of Figure 8a) for CTAB, but for SDS, no such nanowires were formed. Instead, only irregular spherical nanoparticles resulted (Figure 8b). Here, it is worth mentioning that on addition of hydrazine within the Ni2þ-SDS mixed solution for reduction an instantaneous bluish-violet gluey precipitation occurs which was observed to be reduced much more slowly than 1666



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Figure 8. FESEM images of Ni nanostructures synthesized in the presence of (a) CTAB and (b) SDS. Conditions: [Ni2þ]/[N2H4] = 0.026; [CTAB] or [SDS] = 0.025 M, and with a magnetic stir bar.

Figure 9. Transformation of prickly Ni nanowires into porous NiO nanotubes by calcination at 800 °C for 3 h. (a) Low and (b) high (tubular nature) magnification FESEM images. (c) Enlarged view of the porous NiO nanotubes indicating a “blister” surface.

what is observed for TX-100 or CTAB. This incident was again verified with other types of anionic surfactants like SDBS (sodium dodecylbenzene sulfonate) where even no reduction occurs. This could be explained on the basis of favorable formation of layered nickel hydroxides in the presence of the amine, intercalated with DS/DBS anions.25 Figure S2 (Supporting Information) represents the XRD patterns in the same window of the one-dimensional nanowires

synthesized in different reaction conditions, from which we also observed that the preferential growth of these entire unidirectional nanowires was along the (111) direction. The as-synthesized nanowires after being calcined at 800 °C for 3 h in a tube furnace under ambient pressure were completely transformed into gray-black NiO. The morphological change of the thermally treated product is clearly visible from the corresponding FESEM images (Figure 9) where we could observe 1667



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Figure 10. FESEM images of Ni@Au nanocomposite materials synthesized from redox transmetalation reaction at different time intervals using aqueous 10-2 M HAuCl4 solution: (a) 10 min, (b) 30 min, (c) 4 h, and (d) 12 h. Inset shows the digital image of the composite material.

large-scale formation of nanotubes (Figure 9a) with an inner diameter of ∼500 nm and outer diameter of ∼1 μm (Figure 9b). In comparison to the nanowires, the tubes were shorter in length which was mostly due to the breakdown of the ultralong materials in portions by the thermal effect. The spiky nature was completely lost from the architectures with a neat appearance of their “blister” surface (Figure 9c). In retrospect, as stated previously, a galvanic replacement reaction has been appreciated as an advanced protocol for needbased fabrication of hollow metal and metal alloy nanostructures with diverse hierarchy. We implemented the strategy for our synthesized prickly Ni nanowires with chloroauric acid (HAuCl4; 10-2 M) where their standard reduction potential values (E0AuCl4-/Au = þ0.99 V vs SHE and E0Ni2þ/Ni = -0.25 V vs SHE) prescribe the redox favorable dissolution of the nanowires with successive formation of Au0 in assent with the following reaction 3Ni0 ðsÞ þ 2AuCl4 - ðaqÞ f 3Ni2þ ðaqÞ þ Au0 ðsÞ þ 8Cl - ðaqÞ We have deliberately studied the above interaction in different time domains, and the results have been presented in Figure 10 (FESEM images). Just 5 min incubation (Figure 10a) resulted in nanowires with a prickly surface together with the presence of

embryonic nanospherical gold (encircled by the greenish dotted line) grown in a different position over the nanowires. We could observe the growth of gold clusters in colonial fashion all through the nanowires' surface when the incubation time was 30 min (Figure 10b). The enlarged area specified that the flowery overgrown patterns stayed well separated from each other. Similarly, an incubation for 4 h proportionately has an effect on making complete coverage of the nanowires (Figure 10c) where the encircled area shows the presence of densely packed spherical superstructures. When the aging time was prolonged to 12 h, surprisingly we observed the high yield and uniform formation of dendritic nanostructures (Figure 10d). Close inspection suggested that these fractal structures were preferably made up with a micrometer long stem with 2-5 μm long highly ordered branches, distributed on both sides of the stem. Also, the structures were predominated with a smooth surface. EDX analysis suggested that the dendritic structures were essentially made of pure Au, whereas both Ni and Au were present for the other derivatives. A plausible mechanistic interpretation was thus necessary to explain the results we obtained. As we have mentioned earlier, the spikes with sharp tips stand with relatively high surface energy, and after addition of HAuCl4, a redox transmetalation reaction 1668



Figure 11. Representative XRD presentation of the Ni@Au nanostructures after stepwise and steady redox transmetalation reaction.

was initiated at these sites. Consequently, pitting over the surface was initialized and resulted in nucleation of small gold nanoclusters still remaining in the embryonic stage (Figure 10a). Such redox-driven corrosion was continued with time and propagated throughout the whole surface of the nanowires. Then, these pinholes function as an anode where metallic Ni was further oxidized with continuous stripping out of electrons from the surface which were transferred to AuCl4- ions following their reduction to Au atoms. A large difference in lattice constant value of metallic Ni and Au retarded their interatomic diffusion, and hence these preformed nuclei grew with time, resulting in bumpy overgrowth in place (Figure 10b). With time, the whole nanowire's surface was covered with such spherical superstructures (Figure 10c). When the intimate contact time was sufficiently extended, an exaggerated increase in atomic density of gold throughout the surface took place which in-turn diffused through the outer surface of the nanowires with the involvement of a number of different mass transfer kinetics as stated earlier, and in due course, morphological reconstruction happened to eliminate high surface energy to make such dendritic structures (Figure 10d).26 An epitaxial relationship between the deposited gold and Ni core might be imperative and could not be avoided in understanding of the complete coverage with a smooth Au shell. Once again, the transmetalation kinetics was verified with XRD analyses of different sets of the products (only Ni nanowire and Ni@Au nanocomposites prepared from 30 min and 12 h transmetalltion reaction (Figure 11)) where we observed the stepwise and steady evolution of gold nanostructures at the expense of the sacrificial Ni template. A further treatment with 10-3 and 10-4 M HAuCl4 was also performed independently, and the product morphologies are given in Figure S10 (Supporting Information). Here, we also observed effective surface etching of Ni nanowires followed by surface overgrowth of Au nanospheres for 10-3 M, but for 10-4 M, the effect was insignificant. The composite materials were also characterized by HRTEM where (Figure 12) for every material we observed the presence of Au (fringe spacing = 0.24 nm) within the Ni (fringe spacing = 0.20 nm) crystallite domain, and this results in a thin shell over the magnetic nanowires. Lattice-resolved HRTEM images of different regional parts also justified the sheathed structure of the

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Figure 12. Elemental mapping of the Ni@Au nanocomposite materials from HRTEM images showing segregated Au distribution on the Ni nanowires after stepwise and steady redox transmetalation reaction with aqueous 10-2 M HAuCl4 solution for 12 h.

materials. A large difference in lattice constant values of Ni and Au (0.352 vs 0.408 nm) suggests embedding but not alloying between the two. The elemental mapping profiles (inset of the figures) also provided exclusive evidence of the presence of both of the metals, their distribution pattern, and the progressive wrapping of Au over Ni nanostructures. From EDAX and XRD characterizations, it seemed to us that the Au encapsulated Ni nanowire was purely gold though the mapping suggested the presence of both even after a long transmetalation. Finally, we explored the possible usefulness of the nanowire and its transmetalated derivatives for two beneficial applications, for instance, in reduction of nitroaromatic and SERS studies. Catalytic reduction of 4-nitrophenol has been diagnosed well by different groups including ours,16 and thus, now this reaction has become a benchmark illustration for understanding the role of a couple of its concerned parameters. It is now well-known that 4-nitrophenol solution exhibits a strong absorption peak at 317 nm which when treated with an aqueous solution of NaBH4 is remarkably red-shifted to 400 nm. This is because of the formation of 4-nitrophenolate ions owing to an increase in solution alkalinity upon the addition of NaBH4. It was found that the reaction was kinetically inert at this stage and could only proceed in the presence of metal nanoparticles where the kinetics could be easily monitored spectrophotometrically associated with its perceptible color change with a concomitant appearance of a new peak at ∼300 nm, addressing the formation of 4-aminophenol. A better understanding often comes from comparison and contrast, and thus our study has quantified the catalytic activity of the monometallic and the composites materials. Initial concentrations of 4-nitrophenol and NaBH4 were kept at 8.6 10-5 M and 1.5 10-2 M throughout the course of all the reactions. The time-dependent reduction kinetics at room temperature (26 °C) has been presented in Figure 13. We observed the completion of the reduction reaction in 35 min to a couple of hours using the Ni@Au nanocomposites. The presence of isobastic points in every case clearly authenticated the sole formation of aminophenol with no other byproduct. Figure S12 (Supporting Information) represents the change in concentration of 4-nitrophenol with time, and in every case it was observed to follow first-order rate law. Here we also observed an 1669



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Figure 13. Spectrophotometric study of the successive reduction of 4-nitrophenol with NaBH4 at room temperature (26 °C) using (a) only Ni nanowire and (b, c, and d) Ni@Au composite materials as catalysts. The composites (here catalysts) obtained from the transmetalation reaction at different time (30 min, 4 h, and 12 h) intervals. Conditions: [4-NP] = 8.6 10-5 M; [NaBH4] = 1.5 10-2 M. Amount of catalyst, 8 mg.

induction period for Ni nanowires as has been highlighted in another report,16a whereas this was not observed for the composite with a Au-encapsulated Ni nanowire. Nonetheless, after the induction period, the kinetics was much faster for the Ni system. Most likely, the Ni surface was renewed and in turn reactivated by borohydride. This surface regeneration certainly needed a definite time (induction period), but such an oxide layer was absent for Au for which the reaction started from the very first, i.e., with the Au encapsulated Ni nanowire. Again, in Raman spectra of 4-NP over the two samples (only the Ni nanowire and the Au encapsulated set), we observed Raman bands (in place little shifted) only for the latter sample. Also, there was a new peak, ∼241 cm-1. This was assigned as the Au-O band (spectra not shown). These features collectively suggested that 4-NP favorably reacts with the gold surface. Therefore, a higher surface coverage by this molecule may trim down the effective coverage by another nucleophile, borohydride. Therefore, successive electron transfer from borohydride ions became slow, and consequently the rate was poor for the Au encapsulated samples. Literally, in the presence of excess BH4-, the reduction is pseudofirst-order wrt 4-nitrophenol which was also justified for our case from a linear correlation between ln(At/A0) with time (t) (Figure S13, Supporting Information), and rate constants were determined to be (3.5 10-2 min-1), (4.1 10-2 min-1), (7.6 10-2 min-1), and (4.2 10-3 min-1), respectively. Catalytic efficiency of the materials in terms of apparent rate constant has been bar-diagrammatically presented in Figure 14.

Here it is worth mentioning that the reaction did not proceed even for an extended exposure with bulk Ni foil suggesting the necessary presence of a nanostructured surface for the catalytic success. A thorough literature survey specifies a set of different reaction mechanisms which could be principally addressed either with the Langmuir-Hinshelwood (L-H) model16a or the Eley-Rideal model.16f The first one considers the transfer of hydride from BH4- to a nanoparticle surface followed by adsorption of nitrophenolate ions. Both these processes of adsorption are reversible, and in r.d.s, reduction of the nitro group is achieved with hydride transfer from the surface to the adsorbed species. Hence, the mechanism depends on the nature of the surface of the adsorbent, i.e., nanoparticles and adsorption constant of both of the species. The second mechanism considers only the adsorption of hydrogen. Again, some other groups have reported that the rate is diffusion controlled (ref 25 of 16a). Thus, whatever may be the accurate pathway, the difference in rate constant (as observed from Figure S13, Supporting Information) clearly indicated the reaction was surface controlled where a continuous adsorption-desorption kinetics of the reactants also affected the rate of our reaction. We observed a progressive increase in reaction rate with successive increase in the amount of gold deposition through a transmetalation (Figure S13a-c, Supporting Information) process and anticipated a much faster kinetics with the Au-encapsulated Ni nanowire Ni@Au. However, remarkably the rate was too slow in the case where Au 1670



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Figure 14. Bar-diagrammatic representation of the efficiency of the catalysts in terms of the experimentally determined rate constant (min-1) of 4-NP reduction with respect to sequential Au loading.

completely encapsulated Ni (Figure S13d, Supporting Information). This clearly pointed out that the presence of a bimetallic interface was more favorable and made the electron relay system facile. A recent report by Haruta et al. (ref 28 of 16a) also recommended an effective decrease in catalytic efficiency of gold when it is in the aggregated stage. This could be a justification for this observation as the surface was decorated with pure gold atoms. We also believed that redox etching of the Ni nanowires by stepwise transmetalation increased the surface roughness and generated more defect sites that accelerated the reaction rate, but complete coverage of Ni by a gold shell could not rejuvenate a catalytic rate because a smooth surface resulted in the materials that slowed down the reaction. Catalysis by metal nanoparticles and their controlled morphology contribute to the observed rate law. We have executed SERS of 10-5 M 1,10-phenanthroline (phen) with the pure Ni nanowire and the composite nanomaterials. The as-synthesized Ni nanowire and the fully Au covered nanocomposite failed to give any SERS signature out of the probe molecule, phen. Similarly, we did not observe any NRS spectrum with only 10-5 M phen solution. SERS could only be observed with the other transmetalated Ni@Au substrates, and the spectra are shown in Figure 15 in one window. Our previous observation22 of SERS for the same probe molecule over spherical Ni nanoparticles indicated slow and progressive surface complexation. Finally, the formation of a Ni-phen trischelate complex resulted in over the surface, and we have reported a perceptible change in spectral profile compared to pure phenanthroline. However, some significant amount of nickel oxide layers for surface complexation with phen is needed, and that is absent from the surface of the asprepared prickly nanowire. So we have no SERS spectrum of phen from the prickly nanowire. Analyzing these observations with other reports on SERS from phen,27 we demonstrated the change in different band positions upon complexation. Here also we could observe an excellent correspondence of the spectra over the nanocomposites with our previously reported spectra over Ni nanoparticles, and hence in this case also, chelation by phen is again proved beyond doubt. For instance, all the vibrational bands at 734, 1054, 1308, 1426, 1454, and ∼1520 cm-1 clearly demonstrate the band positions for the chelate complex. Here we observed the best enhanced spectrum with (Ni@Au_30 min) composite after which the intensity gradually died down. Again, the band at ∼301 cm-1, preferably occurring due to the Ni-phen complex, was evident in the 10 and 30 min composites, whereas its

Figure 15. SERS spectra of 10-5 M 1,10-phenanthroline over different Ni@Au nanocomposites prepared from transmetalation reaction at different time intervals.

signature was absent for the 4 h composite. This information authenticated the favorable surface complexation by phen for the first two. Presumably, an increased oxidative etching of the Ni surface with HAuCl4 opened up a higher possibility of such surface complexation through chelation. This complex, remaining in the electromagnetic field of the deposited Au, gave enhanced SERS spectra. A recent study28 on SERS from phthalic acid has been shown to be effective only in the presence of a nearby gold nanodisk where the Raman active molecule remains completely associated with the Ni nanowire segment. No SERS was attained from the free nanodisk or from only the Ni nanowire. This was reported as the long-range SERS effect from the noble metal part owing to their worthy contribution of defined surface plasmon resonance toward EM part of SERS enhancement. The observation was also supported from theoretical modeling studies. We could also explain our above observation in relation to this report where the presence of the Au regime was necessary to stimulate the enhancement of the chelated species, solely anchored over the nearby Ni part of the nanowires. Hence, SERS intensity increased with a gradual increase in Au domain over the Ni nanowire. However, after a critical limit of surface coverage by Au(0), the SERS intensity died down completely as a result of greater extent of coverage by Au. This was mostly due to the less available Ni sites for complexation. Thus, for the Au encapsulated Ni nanowire nanocomposite with maximum Au covering, a complete sheath structure of Au(0) failed to make any complex with the N-donor ligand, and we failed to get any SERS from the material. Therefore, the above study helps us to realize that a chelating probe could be a better alternative in getting SERS from Ni nanowire; but, of course, the proximal presence of Au(0) was necessary, and the enhancement is definitely Au(0) dose dependent. Complete dissolution of the sacrificial transition metal template was not achieved by any means (as being confirmed from elemental mapping) during the redox transmetalation, and this residual presence may impart a significant contribution to our observed catalytic and SERS performance by the materials.

’ CONCLUSIONS In summary, an aqueous solution-based straightforward protocol has been exploited for syntheses of highly anisotropic ferromagnetic Ni nanostructures. Explicit regulation in magnetic 1671


The Journal of Physical Chemistry C dipolar assembly of the nanocrystals was successfully achieved with the deliberate introduction of a magnetic bar within the reaction medium. The alignment of the nanowires was observed to be field-strength dependent, and of course every concerned reaction parameter truly affected the reaction dynamics. However, in every case, the synthesized Ni nanostructures were observed to be crystallized in the fcc phase. A critical surfactant concentration was observed to be successful to result in the prickly surface of the nanowires which looks like Euphorbia milii. The conical growth of the spines was built-up with a wide root and sharp tip. When the nanowires were calcined, they formed nickel oxide nanotubes with comparatively shorter length, and this was due to the thermal breakdown of the ultralong materials in portions. Mesoporous behavior of the tubular structures has been ascertained by BET gas sorption measurements. We observed that the spikes with their sharp tips served as the primary sites for reaction with suitable etching agents and eventually dissolved out from the surface to form nanowires with a densely pitted surface when we employed CN-, F-, or S2O82- for the purpose. Finally, for these prickly nanowires, when treated with an aqueous solution of 10-2 M HAuCl4 for different time periods, metallic Ni dissolved out with the deposition of metallic gold over the nanowires, and this transmetalation process opens up the possibility of fabrication of functional composite (Ni@Au) materials. Time-dependent surface coverage of the nanowires with a gold shell has been substantiated with XRD patterns, though complete dissolution of the transition metal template was not achieved by any means. The derived composites were found to be potent catalysts for hydrogenation reaction of 4-nitrophenol and also served as active substrates to exhibit fruitful SERS enhancement from nearby molecules. Thus, this simple approach with cost-effective experimental setup provides a way to fabricate hierarchical Ni nanostructures which may readily be scaled-up for making composite nanomaterials for further application in catalysis and spectroscopy.

’ ASSOCIATED CONTENT

bS

Supporting Information. (Figure S1): FESEM images of Ni nanowire with [TX-100] = 1.35 10-5 M concentration and without TX-100. (Figure S2): XRD patterns of the nanowires synthesized in different reaction conditions. (Figure S3): Fieldand temperature-dependent magnetization of the nanowires with explanation. (Figure S4): FESEM images of Ni nanowire surface synthesized from (a) NiSO4 and (b) Ni(OOCCH3)2 with explanation. (Figure S5): (a) TEM image and (b) XRD pattern of the NiO nanotube with explanation. (Figure S6): FESEM images of prickly Ni nanowires after oxidative etching using 0.01 M aqueous (a) NaF, (b) KCN, and (c) K2S2O8 solutions with explanation. (Figure S7): Ni nanoplatelets synthesized with 1.31% of hydrazine with explanation. (Figure S8): Ni nanowires with variable [Ni2þ]/[N2H4] ratio with explanation. (Figure S9): Ni nanostructures under reflux and microwave irradiation with explanation. (Figure S10): FESEM images of Ni@Au nanocomposites synthesized by transmetalation reaction with variable concentrations [(a) 10-3 and (b) 10-4 M] of aqueous HAuCl4. (Figure S11): Elemental mapping of the Ni@Au nanocomposite materials (from HRTEM images) at different time intervals: (a) 10 min, (b) 30 min, and (c) 4 h. (Figure S12): Kinetic study of the reduction of 4-nitrophenol. (Figure S13): Evaluation of rate constants from the

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ln(A/A0) vs time (t) plot. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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’ NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on December 30, 2010. The Supporting Information associated with this manuscript has been modified. The correct version was published on January 7, 2011.

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