Anita Swami

New approaches to the synthesis of anisotropic, core–shell and hollow metal nanostructures

The synthesis of nanomaterials with control over size, shape and chemical composition continues to be a major challenge in nanoscience. The requirements of nanomaterial synthesis are becoming more sophisticated and, in addition to anisotropic structures, there is much excitement surrounding the development of recipes for the synthesis of core–shell and hollow nanostructures. Much of the motivation for research in this direction stems from the unusual optoelectronic and chemical properties exhibited by such nanostructures. In this article, we review the work from this laboratory on the synthesis of flat gold nanostructures at the air–water interface, either by confining the reductant or the precursor metal ions to the air–water interface. We also describe the synthesis of phase-pure core–shell nanoparticles by immobilizing UV- and pH-dependent reducing agents on the surface of the core nanoparticles as well as the synthesis of organically soluble hollow-shell nanostructures via transmetallation reactions.

Variation in morphology of gold nanoparticles synthesized by the spontaneous reduction of aqueous chloroaurate ions by alkylated tyrosine at a liquid–liquid and air–water interface

We demonstrate the formation of gold nanocrystals of different morphologies using alkylated tyrosine (AT) as a reducing agent at a liquid–liquid and air–water interface. The reduction of aqueous chloroaurate ions occurs in a single step wherein the AT molecule plays the multifunctional role of a phase transfer, reducing and capping agent. Gold nanoparticles formed at the air–water interface are very thin, flat sheet or ribbon-like nanostructures, which are highly oriented in the (111) direction. On the other hand, reduction of aqueous chloroaurate ions at a liquid–liquid interface by AT molecules present in the organic phase yielded nanoparticles having predominantly spherical morphology but with no specific crystallographic orientation. The difference in morphology of the nanoparticles may be due to the different orientational and translational degrees of freedom of the AT molecules and gold ions at these two interfaces. The AT-capped gold nanoparticles were characterized by UV-vis spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and nuclear magnetic resonance spectroscopy ( 1H NMR), while the LB films of flat gold sheets were also studied by X-ray photoemission spectroscopy (XPS).

Langmuir–Blodgett films of laurylamine-modified hydrophobic gold nanoparticles organized at the air–water interface

The organization of hydrophobized colloidal gold nanoparticles at air-water interface and the formation thereafter of lamellar, multilayer films of the gold nanoparticles by the Langmuir-Blodgett technique is described in this paper. The hydrophobization of the colloidal particles was accomplished by the direct chemisorption of laurylamine molecules on aqueous colloidal gold nanoparticles during a phase-transfer process. While monolayers of the laurylamine-capped gold nanoparticles at the air-water interface were not amenable to layer-by-layer transfer onto solid supports, it was observed that addition of the water-insoluble amphiphile octadecanol to the gold nanoparticle solution improved the stability of the monolayer at the interface as well as the multilayer assembly protocol. The organization of the gold nanoparticles at the air-water interface was followed by surface pressure-area isotherm measurements while the formation of multilayer films of the nanoparticles by the Langmuir-Blodgett technique was monitored by quartz crystal microgravimetry, UV-vis spectroscopy, Fourier transform infrared spectroscopy, and transmission electron microscopy.

Flat gold nanostructures by the reduction of chloroaurate ions constrained to a monolayer at the air–water interface

Formation of flat gold nanostructures occurs by the reduction of a Langmuir monolayer of hydrophobized chloroaurate ions using anthranilic acid as a reducing agent present in the subphase. Vigorous shaking of the aqueous chloroauric acid solution with a solution of surfactant, such as octadecylamine (ODA) or benzyldimethylstearylammonium chloride (BDSAC) in chloroform results in rapid transfer of chloroaurate ions (AuCl4−) from the aqueous phase to the chloroform phase. Strong electrostatic interactions between negatively charged chloroaurate ions and cationic head groups of ODA and BDSAC molecules, making the gold ions sufficiently hydrophobic, are believed to be responsible for the transfer of AuCl4− ions from the aqueous to the organic phase. Surface pressure–area (π–A) isotherm measurements reveal that these hydrophobized chloroaurate ions behave as amphiphilic molecules and form stable Langmuir monolayers on the acidic aqueous subphase. Spreading of hydrophobized chloroaurate ions on the surface of aqueous anthranilic acid solution results in the immobilization of AuCl4− ions strictly at the two dimensional surface. Hence, further reduction of these AuCl4− ions by anthranilic acid molecules from the subphase leads to the formation of highly anisotropic, flat gold nanostructures at the air–water interface. The capping of gold nanoparticles formed at the air–water interface by ODA and BDSAC enables their facile transfer as multilayers onto suitable solid substrates by the Langmuir–Blodgett (LB) technique. Multilayer Langmuir–Blodgett films were characterized by UV-vis spectroscopy, transmission electron microscopy (TEM), electron diffraction and X-ray photoelectron spectroscopy.

Enhanced methanol electrooxidation at Pt skin@PdPt nanocrystals

Pt skin growth over PdPt alloy nanocrystals has been described using a simple wet chemical method, where a layer-by-layer epitaxial deposition of Pt on PdPt could be understood by the Stranski–Krastanov mechanism. Initial PdPt alloy nanocrystals grown in a simple wet-chemical method, in the presence of a reducing solvent like N-methyl pyrrolidone (NMP) and a stabilizer like polyvinyl pyrrolidone (PVP), have been used as the substrate for secondary growth of a Pt thin layer. Surface changes have been observed during step-by-step growth of polyhedral Pt skin@PdPt nanocrystals originating from nearly octahedral geometries of PdPt. The methanol electrooxidation activities of two different Pt skin@PdPt nanostructures have been compared with PdPt nanocrystals with similar compositions but without skin structures and commercial RuPt catalysts. A gain factor of 8 towards electrooxidation of methanol in acidic media with activities of 1950 mA mgPt−1 and 3.1 mA cmPt−2 (with lower onset potential compared to the RuPt commercial catalyst), which is believed to be much higher compared to that of previous reports and state-of-art RuPt/C catalysts, indicating better surface properties and core-alloy formation along with improved intraparticle active interfacial sites. Additionally, exciting results of electrooxidation of ethanol and ethylene glycol with 70% and 58% activity retention respectively, after 5000 cycles are also found, demonstrating a facile C–C breaking in such C2 type alcohols.

Carbon Nanotube/Boron Nitride Nanocomposite as a Significant Bifunctional Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions

It is an immense challenge to develop bifunctional electrocatalysts for oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) in low temperature fuel cells and rechargeable metal-air batteries. Herein, a simple and cost-effective approach is developed to prepare novel materials based on carbon nanotubes (CNTs) and a hexagonal boron nitride (h-BN) nanocomposite (CNT/BN) through a one-step hydrothermal method. The structural analysis and morphology study confirms the formation of a homogeneous composite and merging of few exfoliated graphene layers of CNTs on the graphitic planes of h-BN, respectively. Moreover, the electrochemical study implies that CNT/BN nanocomposite shows a significantly higher ORR activity with a single step 4-electron transfer pathway and an improved onset potential of +0.86 V versus RHE and a current density of 5.78 mA cm-2 in alkaline conditions. Interestingly, it exhibits appreciably better catalytic activity towards OER at low overpotential (η=0.38 V) under similar conditions. Moreover, this bifunctional catalyst shows substantially higher stability than a commercial Pt/C catalyst even after 5000 cycles. Additionally, this composite catalyst does not show any methanol oxidation reactions that nullify the issues due to fuel cross-over effects in direct methanol fuel cell applications.

Water-dispersible nanoparticles via interdigitation of sodium dodecylsulphate molecules in octadecylamine-capped gold nanoparticles at a liquid-liquid interface

This paper describes the formation of water-dispersible gold nano-particles capped with a bilayer of sodium dodecylsulphate (SDS) and octadecylamine (ODA) molecules. Vigorous shaking of abiphasic mixture consisting of ODA-capped gold nanoparticles in chloroform and SDS in water results in the rapid phase transfer of ODA-capped gold nanoparticles from the organic to the aqueous phase, the latter acquiring a pink, foam-like appearance in the process. Drying of the coloured aqueous phase results in the formation of a highly stable, reddish powder of gold nanoparticles that may be readily redispersed in water. The water-dispersible gold nanoparticles have been investigated by UV-Vis spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FTIR). These studies indicate the presence of interdigitated bilayers consisting of an ODA primary monolayer directly coordinated to the gold nanoparticle surface and a secondary monolayer of SDS, this secondary monolayer providing sufficient hydrophilicity to facilitate gold nanoparticle transfer into water and rendering them water-dispersible.

Confinement of DNA in Water-in-Oil Microemulsions

The study of systems that allow DNA condensation in confined environments is an important task in producing cell-mimicking microreactors capable of biochemical activities. The water droplets formed in water-in-oil emulsions are potentially good candidates for such microcompartments. The anionic surfactant AOT was used here to stabilize the droplets. We have studied in detail the DNA distribution and the structural modifications of these microemulsion drops by varying the concentration and molecular weight of DNA and using various techniques such as light, X-ray, and neutron scattering, electrical conductivity, and surface tension. DNA induces the formation of large drops into which it is internalized. The size of these drops depends on the amount of DNA dissolved in water as well as on its molecular weight. The local DNA concentration is very high (>100 mg/mL). The large drops coexist with small empty drops (not containing DNA), similar to those found in the DNA-free microemulsion.

Lamellar multilayer hexadecylaniline-modified gold nanoparticle films deposited by the Langmuir-Blodgett technique

Organization of hexadecylaniline (HDA)-modified colloidal gold particles at the air-water interface and the formation thereafter of lamellar, multilayer films of gold nanoparticles by the Langmuir-Blodgett technique is described in this paper. Formation of HDA-capped gold nanoparticles is accomplished by a simple biphasic mixture experiment wherein the molecule hexadecylaniline present in the organic phase leads to electrostatic complexation and reduction of aqueous chloroaurate ions, capping of the gold nanoparticles thus formed and phase transfer of the now hydrophobic particles into the organic phase. Organization of gold nanoparticles at the air-water interface is followed by surface pressure—area isotherm measurements while the formation of multilayer films of the nanoparticles by the Langmuir-Blodgett technique is monitored by quartz crystal microgravimetry, UV-Vis spectroscopy, Fourier transform infrared spectroscopy and transmission electron microscopy.

Unusual enhancement in the electroreduction of oxygen by NiCoPt by surface tunability through potential cycling

Chemically ordered interconnected nanostructures of NiCoPt alloy have been prepared using a simple solvothermal process and studied for oxygen reduction reaction (ORR) kinetics. NiCoPt/C catalyst has demonstrated an interesting trend of enhancement in the ORR activity along with long-term durability. The specific activity of 0.744 mA cm−2 for NCP10/C (NiCoPt/C prepared at reaction time of 10 h) is ∼3.7 times higher than that of Pt/C (0.2 mA cm−2). The durability of the catalyst was evaluated over 30k potential cycles in the lifetime regime. More significantly, a novel trend in the enhancement in the ORR activity during stability cycles has been observed for the first time, where a remarkable enhancement of 82% in the specific activity has been observed after 30k potential cycles. Thus, ∼7-fold higher activity of NCP10/C@30k over initial activity of commercial Pt/C would make a tremendous impact on fuel cell technology. Systematic X-ray diffraction studies were performed to supplement subsequent improvement in the ORR activity during potential cycling, where structural changes due to alloying and de-alloying taking place with formation of tetrahexahedron-like surfaces after 15k cycles. Furthermore, transmission electron microscopy (TEM) analysis after 30k durability cycles reveals better stability of NCP10/C nanostructure signifying the retention of Ni and Co due to the chemically ordered structures of NiCoPt alloy catalyst. The observed enhancement in durability might be due to the ordered arrangement of Pt and Ni/Co within the alloy.