High-Yield Synthesis of 1D Rh Nanostructures from Surfactant

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High-Yield Synthesis of 1D Rh Nanostructures from Surfactant Mediated Reductive Pathway and their Shape Transformation Mukul Pradhan, Sougata Sarkar, Arun Kumar Sinha, Mrinmoyee Basu, and Tarasankar Pal* Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India ReceiVed: February 22, 2010; ReVised Manuscript ReceiVed: August 8, 2010

Precise control over the size and shape of nanoparticles from solution-phase synthetic approach is currently a major objective in nanoscience. Metal nanorods and nanowires have attracted much attention because of their outstanding catalytic, magnetic, optical, and electrical properties. We have reported here the microwave (MW) assisted gram quantity synthesis of 1D nanostructures composed of Rh(0) and Rh(I). Thus, the rodlike assembly evolves through Rh(I)-Rh(I) interaction as the building blocker for the 1D nanostructure by using cetyltrimethylammonium bromide (CTAB) as a reducing agent as well as soft template. Reduction of Rh(III) to Rh(0)/Rh(I) occurred on the glass surface as a result of decomposition of CTAB upon MW heating without any other reducing agent. Uniform heating and the presence of CTAB, a face selective adsorption additive, helped the formation of 1D Rh nanostructures. Here, CTAB upon decomposition produced ammonia which in turn acted as a reducing agent ((a) Bal, R.; Tada, M.; Iwasawa, Y. Chem. Commun. 2005, 3433. (b) Huang, Y.; Wang, W.; Liang, H.; Xu, H. Cryst. Growth Des. 2009, 9, 858), and the undecomposed CTAB stabilized the nanostructure moiety. Hydrothermal condition produced only spherical Rh(0) nanoparticles, and boiling condition prompted anisotropic growth of the Rh(0)/Rh(I) nanoparticles with ill-defined morphology. The presence of impurity such as NH4Cl or CsCl produced distinct 1D nanorods or nanowires in microwave heating condition. Interestingly, the syntheses of different morphology of Rh nanomaterials have been obtained by keeping the Rh(III) ion precursor to CTAB molar ratio unaltered. It has been observed that the pH has a remarkable influence on the alteration of aspect ratio and sharpening of the edges of Rh nanorods. The evolved nanostructures in different stages were characterized ex situ by different physical methods. How and why thermodynamically rather unstable Rh nanorods and nanowires changed their shapes in a chosen redox environment is reported. Interesting shape transformation has also been shown in a selective redox environment for nanorods and nanowires to octahedral and spherical particles, respectively. Isolated intermediates, identified by FESEM and TEM measurements, supported and guarantee the rod-to-octahedral shape transformation. Cyclic voltammetric measurement shows that as-prepared nanorods can be used as a potential candidate for oxygen evolution. Introduction The synthesis of noble-metal nanoparticles (NPs) has a great importance because of all the possible applications in the emerging field of nanotechnology and for the improvement of catalytic properties.2 Among all the noble-metal NPs, rhodium(0) particles are known to be an efficient catalyst for hydrogenation and C-C coupling reactions.3 Template-assisted nanostructure synthesis is a straightforward route to one dimensionality. In this approach, the template simply serves as a scaffold within (or around) which a different material is generated in situ and shapes a nanostructure with a morphology complementary to that of the template. Shape-control synthesis of a thermodynamically unstable 1D nanocrystal is a real challenge, and more information is needed to ascertain the general principles that determine the structure. Among all the parameters, shape control has been proved to be as effective as size control in fine-tuning the properties and functions of metal nanostructures.4 Despite its fundamental and technological importance, the challenge of synthetically and systematically controlling the shape of metal nanostructures has been met with limited success. This situation did not change until several solution-phase approaches were demonstrated very recently. Gas-phase approaches can only * Corresponding author. E-mail: [email protected].

generate metal NPs with well-defined shapes in low yield.5 Solution-phase methods have the potential to process metals into nanostructures with a range of well-defined morphologies and in bulk quantities. For instance, rod-shaped micellar assembly from cetyltrimethylammonium bromide (CTAB) have been demonstrated as a template to grow nanorods/nanowires of silver (or gold) with controllable diameters and aspect ratios.6 Such one-dimensional (1D) nanostructures and nanocubes could also be synthesized by introducing a small molecule or polymer as capping reagent that selectively increases or decreases the growth rates of different crystallographic planes.7 Onedimensional material can be formed via electrical interaction between molecular building blockers.8 These challenges and opportunity have motivated us to synthesize shape-controlled rhodium NPs and nanostructures. In the consideration of possible future applications, an important concern is to offer the stability to these 1D nanostructures in different conditions. The higher surface energy makes the 1D nanostructure relatively unstable. Its importance comes from another prospective when transformation causes evolution of different shapes. The intrinsic properties of a metal nanostructure can be tailored-made by controlling the size, shape, composition, and crystallinity. Morphology control during NP synthesis is now a hot topic because small morphology

10.1021/jp101585y  2010 American Chemical Society Published on Web 09/13/2010

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changes bring spectacular effects.9 Shape transformation of NPs may occur for different reasons. For example, transformation may be due to thermal and photothermal shape transformation, seed-mediated shape evolution due to over growth, alteration of the gas composition, impurity effect, chemical reaction, and so forth. Xia et al. observed that because of thermal fluctuation silver nanobelt brakes into buckle and finally transform to small segment.10 El-Sayed et al. have demonstrated structural transformation from nanorod to nanodot owing to thermal effect.11 There are several reports for laser-induced shape transformation due to photothermal effect.12 Yang and his group synthesized monodisperse colloidal solutions of silver nanocrystals with regular polyhedral shapes, and they have isolated different intermediates depending on the sequential addition of silver nitrate and polyvinylpyrrolidone (PVP) into the preformed nanocrystals.13 Wang et al. have demonstrated glutathione and cysteine-induced transverse overgrowth on Au NRs.14 LizMarzan et al. demonstrated rod-to-octahedron transition15a and Au nanodecahedra15b synthesis by seed-mediated overgrowth. In the past, transmission electron microscopy (TEM) has shown that metal NPs undergo shape changes as a function of the gas composition.16 Choi et al. observed the effect of additives on the growth and stability of Cu2O surfaces via shape transformation of pregrown crystals.17 Murphy et al. have used unpurifiedgold nanorod solutions for the preparation of dogbone-like structures.18 Xu et al. have observed the effect of chloride ions in the shape transformation from triangular to discal silver nanoplates.19 The shape transformation of colloidal platinum nanocatalyst has been demonstrated by El-Sayed et al.20 Structural transformation in ceria NPs during redox processes was observed by Wang et al.21 Very recently, Zubarev and his co-worker established judicious manipulative redox reaction dependent reversible transformation of Au nanoplatelets into more soluble smaller nanodisks and vice versa by the way of Oswald ripening.22 Kruse et al. explored oxygen-induced reconstruction and surface oxidation of rhodium single crystal by using field ion microscopy (FIM).23 Stierle et al. showed microscopic insight into how and why catalytically active NPs change their shape during oxidation and reduction reactions.24 The group also reported an oxygen-induced shape transformation of rhodium NPs but on magnesium-oxide (001) support that is lifted upon carbon-monoxide exposure. A Wulff analysis of high-resolution in situ X-ray diffraction, combined with TEM, shows that this phenomenon is driven by the formation of a oxygen-rhodium-oxygen surface oxide at the rhodium nanofacets. The present work is one of the very few in the area of shape transformation under precise control of pH. Unlike the seeded growth but in a chosen redox reaction, octahedral particles are obtained exclusively from pentagonal faceted nanorods in neutral condition. However, shape transformation from nanorods and nanowires to octahedral or spherical particles has been examined categorically by deploying a redox reaction. Experimental Section Reagents and Instruments. All the reagents used were of AR grade and were used as received without further purification. RhCl3 · 3H2O and CTAB were received from Sigma-Aldrich, and the aqueous solution was prepared freshly in double-distilled water. All glassware were cleaned by using aqua-regia, subsequently rinsed with a plentiful amount of double-distilled water, and dried well prior to use. Experimental Procedure. Preparation of Stock Solution of Rhodium Chloride. Stock rhodium chloride solution (0.96 × 10-2 M) was prepared by dissolving 1 g rhodium chloride

Pradhan et al. (RhCl3 · 3H2O) in 500 mL of an aqueous solution containing 300 µL of concentrated hydrochloric acid. Synthesis. Pentagonal Faceted Rh Nanorods. To an aliquot of 1.2 mL RhCl3, 10 mL water was added; the solution was heated in a water bath for 2 h and then neutralized with 2 mL ∼10-4 M ammonia solution. During heating, the volume was kept constant by pouring water from time to time. Here, the color of the solution changed from reddish-yellow to yellow. The final volume of the solution was made up to 7 mL, and then 2.5 mL 0.1 M CTAB was added. The mixture was heated for ∼90 s in an ordinary MW oven with 800 W power. Cylindrical Rh Nanorods of Higher Aspect Ratio. To an aliquot of 1.2 mL RhCl3, 5 mL water was added, and the solution was neutralized with 2 mL ∼10-4 M ammonia under boiling condition. After neutralization, the final volume was made up to 7 mL. Then, 2.5 mL 0.1 M CTAB was added. For the preparation of nanorods with higher aspect ratio, an increasing amount (50-100 µL) of concentrated HCl was added. In the case of the preparation of nanorods of 5-10 µm length and nanorods of 15-20 µm length, 50 µL and 100 µL concentrated HCl were introduced. Then, the solution was heated for ∼90 s in a MW oven again with 800 W power. Cylindrical Rh Nanorods and Nanowires as Mixture. To an aliquot of 1.2 mL RhCl3, 6 mL water and 0.1 mL 10-4 M NH4Cl were added, and the solution was boiled on a small bare flame for 5 min. Then, 2.5 mL 0.1 M CTAB was added, and the mixture was heated for ∼90 s in a MW oven again with 800 W power. Rh Nanonetwork Structures. To an aliquot of 1.2 mL RhCl3, 6 mL water was added. To this solution, 2.5 mL 0.1 M CTAB and 150 µL 1.0 M HCl were added. Then, the solution was heated for ∼90 s in a microwave oven with 800 W power. All the synthesized one-dimensional Rh nanostructures were separated from the solution by centrifugation (twice) for 10 min at 10 000 rpm, and redispersion of the precipitate was done in double-distilled water. Preparation of Nanorods, Nanowires, and Nanonetwork Structures for Smart Shape Transformation. The solutions containing nanorods (pentagonal and cylindrical), nanowires, and nanonetwork structures for shape transformation were obtained individually from the above-mentioned methods but with ∼70 s microwave heating (not 90 s heating). Then, the solution was left for seven days, and the aged solution was used for the study of shape transformation on carbon-coated Cu grid dispensing 30 µL solutions all at a time. Electrochemical Studies. Cyclic voltammetric (CV) measurements were carried out by using a conventional system comprising a Glassy carbon working electrode, a Pt wire as a counter, and a quasi reference electrode. CV measurements of all the samples (10-3 M Rh-nanorod, nanonetwork, and spherical NP) were performed after coating the glassy carbon electrode with the samples and taking 1.0 M aq KCl solution as supporting electrolyte. CV was recorded within the potential window from -0.8 to +0.8 V. The scan rate for all the measurements was 50 mV/Sec. All electrochemical measurements were carried out on an Autolab PGSTAT30 (Eco chemie) instrument. Results and Discussion Shape-controlled syntheses are generally achieved with variable amount of surfactant-to-precursor-ion molar ratio.25 The present report of synthesis of different 1D nanostructures of Rh(0)/Rh(I) has been possible under MW heating, by keeping the Rh(III)-to-CTAB molar ratio constant. Thus, the effects of only two variables, impurity effect and pH, were studied

High-Yield Synthesis of 1D Rh Nanostructures judiciously. Under the MW heating condition, Rh(III) ions are reduced by CTAB. Only recently, CTAB-assisted reduction has been reported for the synthesis of metallic NPs.1 CTAB, TEAB, TBAB, and other amines along with Rh(III) moiety under MW heating generated ammonia in the glass reaction vessel. Evolution of ammonia has been confirmed by litmus solution and Nessler’s reagent. As a consequence of ammonia evolution, the solution pH was increased. Higher concentration of CTAB in the reaction vessel increased the ammonia evolution greatly. But a 30-fold excess of CTAB above its critical micellization concentration (CMC) value was necessary to obtain the required morphology of Rh(0)/Rh(I) NPs. Here, CTAB acted as reductant, and the undecomposed CTAB performed the capping action. Simply boiling the stock RhCl3 solution with water and dil HCl produced inert [Rh(H2O)6)]3+ (yellow) and labile [RhCl6]3(rose-pink) complexes, respectively.26 Unique pentagonal faceted Rh(0)/Rh(I) nanorods were synthesized from neutral medium by MW heating exploiting the inert [Rh(H2O)6)]3+complex with impurity. Because of the inertness of the complex, the product yield was low. On the other hand, labile [RhCl6]3- complex readily produced ion associate with CTAB which in turn was reduced in the MW to cylindrical nanostructures. But the reaction mixture under hydrothermal condition, heating on a water bath or heating over a naked flame, could not produce well-defined 1D nanostructures. Sometimes, ill-defined nanostructures (see Supporting Information Figure S1A-B) or spherical (see Supporting Information Figure S1C-F) Rh(0) NPs were produced. It is worth mentioning that the aquo complex or the chloro complex of Rh(III) alone in water solution were not reduced by MW heating. Therefore, ion association, micellar solution, and MW heating cojointly evolved the asprepared NPs of Rh(0)/Rh(I) reproducibly with well-defined morphologies. Interestingly, variable acidic conditions produced 1D nanostructures in high yield. Impurity-driven acidic conditions produced cylindrical nanorods and nanowires. Finally, the shape transformation of a Rh(0) nanostructure has been proved to remain associated with the Rh(I) ions for always as described in the following section. UV-Visible Spectroscopic Study. The aqueous reddish solution of rhodium chloride contains varying proportions of RhCl3(H2O)3, [RhCl2(H2O)4]+, and [RhCl(H2O)5]2+ species. It showed strong absorption peaks at 374 and 470 nm due to ligand-to-metal charge transfer (LMCT, Curve A, Figure 1).27 Upon the addition of CTAB to the Rh(III) solution, both peak intensities for Rh(III) species increased, whereas the peak positions remained almost same (Curve B, Figure 1). But the synthesized 1D nanostructured species as colloidal solution showed two intense peaks at 308 and 234 nm before the removal of remnant (Curve C, Figure 1). Curve D demonstrates two distinct peaks at 312 and 234 nm for the same species after purification. Generally, the color for Rh(0) NPs in aqueous solution is reported to be black or brown.28 The synthesized 1D nanomaterial in solution is greenish (Figure 1). A similar kind of absorption profile for green solution has been observed by Son et al. for Rh(I) 1D organometallic framework. There exist a few reports in the literature for the formation of green solution of Rh(0) NPs.29 Spherical Rh(0) shows a featureless absorption profile. However, Xia et al. have reported a single hump at 250 nm in UV-vis absorption spectrum for the Rh multipod nanocrystals.30 The synthesized green 1D nanostructure remains dispersed in different noncoordinating organic solvent; but the green color disappears in coordinating solvents, and the solution becomes yellow. The absorption profile of the green

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Figure 1. UV-vis absorption spectra indicating Rh NP synthesis from aqueous RhCl3 (1.2 mM) solution. Curve A shows the absorption spectrum of aqueous rhodium chloride containing two bands at 374 and 470 nm due to LMCT. Curve B is the spectrum of the mixed solution of CTAB (25 mM) with Rh(III). Curve C is the spectrum of synthesized 1D Rh nanostructures in aqueous CTAB media before the removal of remnant. Curve D shows the UV-vis absorption spectrum of synthesized 1D nanostructures after purification by centrifugation followed by washing and redispersion in distilled water. Curve E shows the surface plasmon band of spherical Rh NPs obtained after the reaction of Cu wire with Rh 1D nanostructures in solution after 45 days.

colloidal solution decreases (Figure S2 in the Supporting Information) gradually with unaltered peak positions upon dilution with different noncoordinating solvents, and this is probably due to the insolubility of the nanostructure. Otherwise, dilution would cause a blue shift of the peak postions.8 The peak at 234 nm has been observed to be very susceptible (blue shift) to nucleophilic (iodide, thiocynate, borohydride) attack (Figure S3 in the Supporting Information). Even iodine evolution was observed with KI solution. When letting the solution stand for a long time with KI solution, we observed a gradual evolution of a featureless absorption profile (Curve E, Figure 1) for the 1D nanostructure dispersed solution. This relates to the spherical Rh(0) NPs in solution from the 1D nanostructure, and consequently, the double-hump curve disappeared. We incubated metallic Cu wire into the solution containing 1D nanostructures and kept the solution for 45 days. When the solution was left standing for a long time, a featureless characteristic absorption profile (Curve E, Figure 1) for the metallic spherical Rh(0) NPs was observed. Here also, the double-hump curve disappeared. XPS Analysis. To interpret the nature of surface composition of the rhodium NPs, XPS analysis was done as the surface monitoring tool. Figure 2a shows the full-scan XPS spectra of Rh NPs synthesized and in situ stabilized by CTAB. Figure 2b shows the core-level XPS spectra of Rh3d region. The Rh3d spectrum is shown to split by core-hole spin-orbit interaction into a 3d3/2 and a 3d5/2 peaks with an energy separation of 3.5 eV. The binding energies of the major signal at 308.2 eV lies between the value of the Rh(I) and of the reduced Rh(0) species while stabilized in ammonium salt.31 Generally, Rh(0) NPs shows 3d5/2 XPS signal at 307.2 eV.32 One recent report supports that the binding energies of the major signal appears at 308.2 eV for Rh(0) if stabilized in ammonium salt, which lies slightly above the theoretical value.31 In our case, the synthesized 1D nanostructre in ammonium salt shows a binding energy of the major signal (3d5/2) at 308.2 eV. Such values are typical for Rh(I).31,33 Therefore, it is pertinent to conceive that Rh(0)

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Figure 3. XRD spectra of 1D Rh nanostructures after removal of CTAB by heating at 270 °C.

Figure 2. (a) Overview and (b) expanded form of XPS spectra of rhodium signals for 1D rhodium nanostructures generated from RhCl3 in CTAB under MW irradiation. The theoretical value of Rh(0) is (307.2 eV), as marked in the figure.

remains in association with Rh(I), and the authentication comes from other studies, for example. UV-visible, XRD, and fringe spacing. XRD Study. The phase structure and purity of the samples were examined by powder X-ray diffraction (XRD), and the pattern is displayed in Figure 3. The Rh(0)/Rh(I) nanostructures remained highly dispersed in CTAB. We have tried to separate excess CTAB by centrifugation and also by employing different types of binary solvent systems. We are able to remove the adsorbed CTAB from the Rh(0)/Rh(I) surface to a degree by using CHCl3 as extractant. Then, the samples show small humps with reasonable noise. The hump appears at the position of the face-centered cubic (fcc) Rh(0) along with an extra peak at ∼26° (Figure S4 in the Supporting Information). All the samples show similar types of high-angle XRD pattern (Figure S5 in the Supporting Information). Interestingly, there appeared altogether four characteristic peaks below 20° for the synthesized 1D nanomaterials (Figure 4). The peaks authenticate the presence of Rh(I) in the 1D nanostructure and are quite similar to the reported low-angle XRD pattern of the Rh(I) organometallic framework.8 The chain length of the associated amine and the absence of π acidic ligand presumably affect the less intense XRD peaks of the nanometerial. There exist many reports for

Figure 4. XRD spectra of 1D nanostructures after removal of CTAB by washing with chloroform.

fcc lattice of Rh(0) with diffraction peaks all above 40°. Vijayamohanan et al. for the first time reported a diffraction peak at ∼26° for monolayer-protected Rh clusters prepared by using tridecylamine (TDA) as stabilizer and NaBH4 as a reducing agent.34 We have also observed similar diffraction peaks at ∼26° for all the samples. Then, we tried to examine the thermal effect on the CTAB capped 1D nanostructures. We observed two decomposition steps from the thermogravimetric analysis, which revealed the presence of physisorbed and capped CTAB (Figure 5) on the Rh matrix. The XRD pattern of the nanorod after removal of adsorbed CTAB by heat treatment at 270 °C is shown in Figure 3. Three peaks at 2θ ) 41, 48, 70, 84, and 89° are assigned to be the diffraction from the {111}, {200}, {220}, {311}, and {222} facets of fcc Rh nanocrystal, which is in good agreement with the reported data {JCPDS File no. 04-0850; a ) 3.524 Å; space group: Fm3m (225)}. The absence of other characteristic peaks of the impurities (such as peaks from rhodium oxide or rhodium hydroxide) in the diffraction profile authenticates the exclusive availability of phase-pure cubic Rh nanocrystals under the current synthetic protocol. The as-prepared samples with CTAB were then dispensed onto a carbon-coated Cu grid (TEM grid) for further structure analysis. Figure 6b shows the low-resolution TEM images of the nanonowire, and Figure 7 shows the images at

High-Yield Synthesis of 1D Rh Nanostructures

Figure 5. (a) Thermogravimetric (TG) and (b) derivative of the TG curve of Rh-CTAB revealing two inflections from uncapped and capped CTAB molecules.

high-resolution. It can be seen that the nanostructures are well crystallized with clear lattice fringes. The interplanar distance is 0.220 nm for the (111) plane. Therefore, synthesized 1D nanomaterials contain both oxidized species (possibly Rh(I) organic framework) and Rh (0). TEM Analysis of 1D Rh Nanostructures. Additional characterizations of the prepared nanonetwork (Figure 6a), mixture of nanowire and nanorod structures (Figure 6b), were made through TEM and selected area electron diffraction (SAED) analysis. The SAED patterns shown in Figure 6a,b indicating the polycrystalline nature of the samples and the concentric rings could be assigned as the {111}, {200}, {311}, and {222} planes of the fcc Rh nanocrystals. Surprisingly enough, when we obtained the fringe spacing of the nanomaterials, we noticed very tiny (1-2 nm) spherical particles instead of ID nanostructures. There are several reports where the 1D nanostructures have been shown to be composed of spherical NPs.35 This readily vouches that the reported 1D structures were composed of small spheres (Figure 7), which has been observed upon high-energy electron impact during the analysis. Again, the lattice gap matches with the {111} crystal plane of the Rh(0) nanomaterials. Energy Dispersive X-ray Spectroscopy Analysis. Figure 8 represents energy dispersive X-ray spectroscopic (EDS) analysis of the as-synthesized 1D nanostructures. The EDS analysis was typically used for the determination of elements present in the reaction product. The EDS spectrum consisted of different peaks for Rh, Cu, C, Br, and N. The moderately intense peak of Rh was due to the 1D Rh nanostructures, and the Cu peak was observed from the used Cu grid. The C peak also came from the carbon-coated grid (TEM grid). The highly intense peak for Br and less intense peak for N came from the surfactant CTAB used during the synthesis of Rh(0)/ Rh(I) nanostructrures. FESEM and TEM Analysis of Rh Nanooctahedra. The solution bearing the pentagonal faceted nanorods finally transformed into octahedra while as-prepared solution (with CTAB) was dispensed on the copper grids. Further characterizations of the octahedra were made through TEM and SAED (Figure 9a) and FESEM (Figure 9b) analysis. The FESEM analysis was done in a straightforward way from the TEM grids itself. The SAED patterns shown in Figure 9a indicating the polycrystalline nature of the samples and the concentric rings could be assigned as the {111}, {200}, and {311} planes of the fcc Rh nanocrystals.

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16133 Increase of Aspect Ratio and Sharpening of the Edges of Nanorods. Evolution of spherical NPs is generally achieved from a simple solution of a precursor compound by using a fast-reaction protocol. Strong reducing agents make the process of evolution of spherical particles feasible. The presence of suitable capping agent with high diffusibility offers protection to the evolved metal particles at their early stage of nucleation. Thus, inter-particle coalescence does not take place, and a tight size distribution of NPs may be achieved. Otherwise, the seedmediated-growth36 mechanism is evinced, bringing uncontrolled growth which results in polydispersity of the particles in the solution phase. On the other hand, controlled growth of seed particles is altogether a different art of manipulating spheres into rods or facating wires out of metallic NPs. This strategy of course is considered to be important to increase the aspect ratio of the particles. All the above factors are easily verified experimentally from the UV-vis spectra when taking coinage metals into consideration because they have rich plasmon band in the visible region. Again, for gold, it is fascinating because of its noblest nature in the series and interesting optical property which makes it suitable for varied applications. But when it comes to platinum metal NPs, the work become dull, taking electronic spectral profile into consideration while dealing with monotonous variation of the plasmon absorption in the visiblewavelength region. But the applications of platinum metal particles in the nanoregime deserve special attention. Rhodium especially has shown unique catalytic property. Crystal growth in the nanoscale seems to follow a behavior similar to that of the bulk phase37 with a marked dependence on pH. The pH is particularly important when some impurities are present in the growth medium because it influences the growth processes by controlling the formation of either zwitterions (ions having both positive and negative charges) or complex ions. The presence of these various species during the nanocrystals growth modifies the growth of certain crystal faces. Most of the changes are based on the existence of a more or less epitaxial adsorption layer on the crystal. This layer is composed of solvent, impurity, or salts, the precise roles of which are as yet to be understood because of the complexity. The changes in shape are due to the differences between the growth rates of the various crystallographic faces. There are several reports where one metal acts as an impurity in the controlling nanocrystal shape of other metal. Introduction of silver impurity during gold-particle formation resulted in the control of the nanocrystal shape.38 In another case, silver ions were used to control the aspect ratio of the gold nanorods produced by electrochemistry and photochemistry.39 It has been shown that the mean aspect ratio of the nanorods in solution decreases with increasing temperature and that the rods becomes shorter.40 Multifunctional additives are often applied to modify the nanocrystal growth in certain crystal faces. They are capable of forming bonds with cationic species at crystal-liquid interfaces with their oxygen donors. Choi et al. observed the effect of additives on the growth and stability of Cu2O surfaces via shape transformation of pregrown crystals.17 An advantage of these types of additives is their performance at concentrations as low as 10-50 ppm; the exact dosage depends on the process conditions such as the pH, concentrations, and so forth. It has been reported that anisotropic nanocrystal formation is more related to selective adsorption of ions during the crystal growth rather than to the nature of the templates as has been shown by Marie-Paule Pileni.9 When keeping all the above factors in mind, we have presented the effect of CTAB for 1D growth of Rh(0)/Rh(I) nanostructures

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Figure 6. (a) TEM and selected area electron diffraction (SAED) analysis of the prepared nanonetwork structures.(b) TEM images of mixture of nanorod and nanowire in the presence on ammonium-chloride impurity.

Figure 7. (a) TEM image of a single nanowire showing that it is composed of small spherical NPs. (b) Fringe spacing of the spherical NPs.

Figure 8. EDS of the CTAB capped 1D Rh nanostructures.

by taking into account the effect of pH and impurity for the change in aspect ratio and sharpness of the edge of Rh nanorods (Figure 10).

Impurity Effect. A nanonetwork structure was obtained in the absence of impurity just by boiling the stock solution of RhCl3, HCl,and CTAB in the MW oven for 90 s. Then, the



Figure 9. TEM and SAED (a) and FESEM (b) images of Rh nanooctahedra.

Figure 10. FESEM images of (a) Rh nanonetwork structure prepared in the absence of NH4Cl impurity but in acidic condition (1.8 × 10-6 M HCl media), (b) pentagonal faceted nanorod of 1-2 µm length prepared in the presence of in situ generated NH4Cl impurity but from neutral solution, (c) cylindrical nanorod of 5-10 µm length prepared in the presence of in situ generated NH4Cl impurity but from acidic (70 × 10-6 M HCl) condition, (d) cylindrical nanorod of 10-20 µm length prepared in the presence of in situ generated NH4Cl impurity but in higher acidic (140 × 10-6 M HCl) condition.

effect of impurity was confirmed by introducing different salt solutions in the micro molar concentration for the formation of distinct 1D nanostructures. We were able to produce a mixture of nanowires and nanorods (see Supporting Information Figure S6a,b) in the solution phase under impurity-driven condition; but then, the nanonetwork structure (Figure 10a) was not obtained. To substantiate our observation, we have introduced salts, such as NH4Cl, CsCl, NaCl, LiCl, and so forth, individually to the solution of RhCl3, HCl, and CTAB. We found that the evolution of the mixture of nanowires and nanorods was

evident with CsCl and NH4Cl, and there was no distinct structure formation while NaCl and LiCl (see Supporting Information Figure S6c-d) were introduced. It may be mentioned that the effect of an added salt from outside brings two changes: first, it markedly changes the phase behavior of the nanostructures,41 and second, we observe the incorporation of the residue as an impurity in crystal lattice.9 From XRD analysis, we observed that the phase behavior of nanowires obtained after the addition of a salt from outside remained the same. In our case, it was due to the incorporation of the residue (but in micromolar level)

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as an impurity in crystal lattice which was indirectly related with the hydrated radius of metal ions. The cation with a larger size is less hydrated; that is, the hydrated radii of Cs+ is at a minimum in the series and at a maximum for Li+ case. From this result, it may be concluded that cations of smaller hydrated radii catalytically altered the network structures. Reaction Mechanism of Shape Transformation of Nanorod. We have reported for the first time nucleophile-induced reversible dissolution of NPs to obtain small spherical Ag particles.42 There are only a few reports for redox reaction dependent shape transformation. Zubarev and his co-worker showed redox reaction dependent reversible transformation of Au nanoplatelets into more soluble smaller nanodisks or vice versa via Oswald ripening.22 It has been shown that Au(III) can oxidize Au NPs in the presence of CTAB. The reaction occurred preferentially at the surface sites with a higher curvature during the oxidation of large NPs with a marked ellipticity. The redox reaction took place between Au0 and AuCl4- ions on the CTAB micellar surface where the oxidant was confined to the positively charged micelles. Hence, the spatially directed oxidation was attributed to the differences in the flux of micelles to highly curved and flat surfaces of the different gold NPs.22 Yang and his group13 synthesized monodisperse colloidal solutions of silver nanocrystals of regular polyhedral shapes entirely bound by {100} and {111} facets of the fcc crystal lattice. In a typical synthesis, silver nitrate and PVP were dissolved separately and then injected periodically onto the preformed nanocrystal seeds; those were synthesized by using the polyol method. Seedmediated shape transformation due to overgrowth was the planed way in order to know the mechanism of shape transformation through the isolation of the intermediates. Depending on how long these sequential additions were continued, specific polyhedral shapes were obtained in high yield. Initially, small silver particles ( (100) > (111).31,47 In an effort to minimize the total surface energy of the prepared nanorods and nanowires, the faces were slowly etched from the tips, and the resultant Rh ions were reduced (by metallic copper) and deposited on (100) facets to increase the percentage of (111) facets on the surface causing the evolution of larger paprticles. The nanorods and nanowires prepared by us remained stable in surfactant solution for several months. When we dispensed the solution containing rhodium nanorods on the carbon-coated copper grid, an interesting shape transformation was been observed. It is worth mentioning that the nanorods devoid of surfactant solution were stable on the copper grids for months. But the metallic copper of the grids changed the nanorods into Rh(0) particles with octahedral shape in the nanoregime through cubes and several other intermediate shapes while surfactant remained associated along with the

nanorods. It is pertinent to believe that Rh(I) species associate with CTAB was the main reason for this type of shape transformation. We have isolated several intermediates by drop casting with an increasing amount of nanorod solution on the TEM grids. From Figure 13, we presumed that the oxidized rhodium species get adsorbed on (100) facets of the nanocubes and were reduced during the oxidation of metallic Cu present in the grids. Therefore, copper was oxidized by the Rh(I) ions. When we deliberately introduced Cu wire in the Rh nanorod solution, Rh nanorods were transformed into small spherical particles even in the solution phase and were deposited on the Cu wire surfaces. From the above discussion, as well as from our experimental observation, that is, ex situ analysis, we can conclude that the oxide layer, Rh(I), and the surface of redox environment, carbon-coated Cu grid surface, were the main factors for rod-to-octahedral shape transformation.

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Figure 12. Transformation of as-prepared cylindrical nanorods of different sizes (a, b) to spherical particles (c) from acidic solution on a Cu grid.

Figure 13. Mechanistic pathway showing the ex situ transformation of pentagonal faceted nanorod to octahedra.

Transformation of As-Prepared Nanorods and Nanonetwork Structures to Spherical Particles. The shape transformation of Rh 1D nanostructure on Cu grids in the presence of

surfactant is really a redox phenomenon. To have a better demonstration when we dipped Cu wires into the solution containing pentagonal faceted Rh nanorods prepared in neutral



Figure 14. Transformation of as-prepared nanorod to spherical particles with Cu wire.

medium, we have observed that nanorods gradually broke down into small spherical NPs (Figure 14). All this had happened because of the association of Rh(I) ions along with Rh(0) nanostructures. We have recorded FESEM images periodically with increased incubation time. First, the tips of the rods broke down into small spherical particles, and within about 45 days, the nanorods were completely converted into spherical particles. The overall size of the nanocrystal also decreased in this process. The thermodynamically controlled transformation of the rods into spherical particles could be interpreted again in terms of oxidative etching and Ostwald ripening. To minimize the total surface energy, the tips with relatively higher surface energy were preferentially etched through oxidation, and the resultant Rh ions were subsequently reduced. Thus, the percentage of (111) facets on the surface was increased. Another observation was that, when the cylindrical nanorods were prepared in an acidic medium and were drop-casted on carbon-coated Cu grid, nanorods segregated in small spherical particles (Figure 12). A similar observation has been made when we drop-casted a solution containing Rh nanonetwork structures (Figure 15c,d). Formation of CuO Nanoplates and Nanowires on TEM Grid. As mentioned above, the as-prepared pentagonal faceted rhodium nanorod solution on TEM grid oxidized Cu into CuO; as a result, nanoplates (Figure 16c,d) and nanowires (Figure 16e) of CuO were obtained. This happens because of the coexistence of Rh(I) ions with Rh(0) particles and nanostructures. Another interesting observation is that, when we dropcasted Rh nanowires or cylindrical nanorods (instead of pentagonal faceted Rh nanorods) solution onto the carbon-coated Cu grid, also oxidation of Cu was observed. However, in this case, the evolved morphology of the CuO (Figure 16a) becomes ill-defined unlike the previous nanorod case. This may be due to the lower pH condition of the nanowire solution. To the best of our knowledge, this is the first report of this kind of

phenomenon, that is, oxidation of TEM grid during shape transformation of Rh NPs. We have primarily characterized the redox reaction generated CuO nanoplates and nanowires from the EDX spectra (Figure 16f). Application CV with all the three samples (Rh-spherical NP (Figure 17A), nanonetwork (Figure 17B), and nanorod (Figure 17C)) were done after coating the glassy carbon electrode individually with sample solutions. It is customary to centrifuge all the Rh solutions for the removal of surfactant. Electrolysis was done in aqueous solution by taking 1.0 M aq KCl solution as supporting electrolyte. Under these experimental conditions, we observed much higher current generation for Rh nanorods at lower potential compared to the blank glassy carbon electrode. Thus, the result reveals the capability of Rh-nanorods (Figure 17C) for O2 evolution48 from water. But Rh nanonetwork and spherical NPs behaved differently and showed almost the behavior of the glassy carbon electrode. Therefore, the asprepared Rh nanorods can be used as a potential candidate for oxygen evolution. Conclusion A new CTAB-mediated bottom-up process of reduction of Rh(III) ions from boiling aqueous solution has been presented. It has been demonstrated that MW heating, pH, and s impurty well account for the size- and shapeselective synthesis of rhodium NPs. The unique and reasonably stable double -hump curve as the electronic spectral profile for the 1D structure is presented. Shape transformation of rhodium NPs has also been demonstrated ex situ in selectively chosen redox environment in the presence of CTAB and Rh(I) ions. A new insight of stabilization of Rh(I) species in the absence of any π acidic

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Figure 15. FESEM (a) and TEM (b) image of nanonetwork structures after removal of remnant. Transformation of the as-prepared nanonetwork to spherical particles when nanonetwork structures were dispensed on the Cu grid with remnant after 2 h (c) and after 1 day (d).

Figure 16. The sample for FESEM analysis was prepared by placing a drop of the solution containing Rh nanostructure onto a carbon-coated Cu grid (TEM grid) followed by slow evaporation of the solvent at ambient condition. (a) FESEM images of CuO nanowire formed when the solution of cylindrical Rh nanorod was dispensed onto the Cu grid. (b) Digital image of a TEM grid. (c,d) FESEM images of CuO nanoplates and (e) nanowires formed when pentagonal faceted nanorod solution was dispensed onto the Cu grid. (f) EDX spectra of the evolved CuO nanostructures.

ligand is reported. The general conclusion is that copper wire in solution produced spherical particles from 1D nanostructures. However, pentagonal faceted rods from neutral solution and cylindrical rods from acidic solution on carbon-coated Cu grids selectively produced octahedral and spherical Rh(0) particles,

respectively. Thus, Cu surface (coated and uncoated), pH, CTAB, and Rh(I) cojointly influence the shape transformation of rhodium 1D nanostructure. It has been shown that water oxidation has been observed exclusively with the nanorods. Now, it would beis possible to study the shape-selective catalysis


Figure 17. CVs of all the samples [spherical NP (A), nanonetwork (B), and 10-3 M Rh-nanorod (C)] were measured after coating the samples on glassy carbon electrode by taking 1.0 M aq KCl solution as supporting electrolyte. Cyclic voltammograms were recorded within the potential window from -0.8 to +0.8 V. The scan rate for all the measurements was 50 mV/Sec. The inset shows the corresponding magnified plot of spherical NP (A) and nanonetwork (B).

involving Rh NPs because of the availability of plentiful supply of Rh nanorods and nanowires from the above-mentioned simple synthetic procedures. Acknowledgment. DST, New Delhi for financial assistance, and Indian Institute of Technology, Kharagpur for research facilities. Supporting Information Available: Synthetic procedure for 1D microstructures and spherical NPs of Rh(0) and their UV-vis spectroscopic study, HRMS, FESEM, and TEM analysis. XRD spectra of 1D nanostructures before and after heat treatment and that of spherical particles before heat treatment. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Bal, R.; Tada, M.; Iwasawa, Y. Chem. Commun. 2005, 3433– 3435. (b) Huang, Y.; Wang, W.; Liang, H.; Xu, H. Cryst. Growth Des. 2009, 9, 858–862. (2) (a) Andres, R. P.; Datta, S.; Janes, D. B.; Kubiak, C. P.; Reifemberger, R.; Naiwa, H. S. (Ed.). Handbook of Nanostructured Material and Nanotechnology, Vol. 4; Academic Press: New York, 2000. (b) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; Wiley: New York, 1994. (c) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690–1693. (3) (a) Roucoux, A.; Philippot, K. Handbook of Homogenous Hydrogenations; de Vries, J. G., Elsevier, C. J., Eds.; Wiley-VCH: Weinheim, 2006; pp 217-255. (b) Leger, B.; Nowicki, A.; Roucoux, A.; Rolland, J. P. J. Mol. Catal. A: Chem. 2007, 266, 221–225. (4) (a) Shi, A. C.; Masel, R. I. J. Catal. 1989, 120, 421–431. (b) Pellegatta, J. L.; Blandy, C.; Colliere, V.; Choukroun, R.; Chaudret, B.; Cheng, P.; Philippot, K. J. Mol. Catal. A: Chem. 2002, 178, 55–61. (c) Borsla, A.; Wilhelm, A. M.; Delmas, H. Catal. Today 2001, 66, 389–395. (d) Schulz, J.; Roucoux, A.; Patin, H. AdV. Synth. Catal. 2003, 345, 222– 229. (e) Grzelczak, M.; Perez-Juste, J.; Mulvaney, P.; Liz-Marzan, L. M. Chem. Soc. ReV. 2008, 37, 1783–1791. (5) Harris, P. J. F. Nature 1986, 323, 792–794. (6) (a) Jana, N. R.; Gearheart, L.; Murphy, C. J. AdV. Mater. 2001, 13, 1389–1393. (b) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 617–618. (c) Yu, Y. Y.; Chang, S. S.; Lee, C. L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661–6664. (7) (a) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165–168. (b) Sun, Y.; Xia, Y. AdV. Mater. 2002, 14, 833–837. (c) Zhou, Y.; Yu, S. H.; Wang, C. Y.; Li, X. G.; Zhu, Y. R.; Chen, Z. Y. AdV. Mater. 1999, 11, 850–852. (d) Sun, Y.; Xia, Y. Science 2002, 298, 2176–2179.

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JP101585Y

High-Yield Synthesis of 1D Rh Nanostructures from Surfactant

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