Soubantika Palchoudhury

Water-Soluble Iron Oxide Nanoparticles with High Stability and Selective Surface Functionality

The water dispensability and stability of high quality iron oxide nanoparticles synthesized in organic solvents are major issues for biomedical and biological applications. In this paper, a versatile approach for preparing water-soluble iron oxide nanoparticles with great stability and selective surface functionality (-COOH, -NH(2), or -SH) was demonstrated. The hydrophobic nanoparticles were first synthesized by the thermal decomposition of an iron oleate complex in organic solvent. Subsequently, the hydrophobic coatings of nanoparticles were replaced with poly(acrylic acid) , polyethylenimine, or glutathione, yielding charged nanoparticles in aqueous solution. Two parameters were found to be critical for obtaining highly stable nanoparticle dispersions: the original coating and the surfactant-to-nanoparticle ratio. These charged nanoparticles exhibited different stabilities in biological buffers, which were directly influenced by the surface coatings. This report will provide significant practical value in exploring the biological or biomedical applications of iron oxide nanoparticles.

Synthesis and Growth Mechanism of Iron Oxide Nanowhiskers

Iron oxide nanowhiskers with dimensions of approximately 2 × 20 nm were successfully synthesized by selectively heating an iron oleate complex. Such nanostructures resulted from the difference in the ligand coordination microenvironments of the Fe(III) oleate complex, according to our electronic structure calculations and thermogravimetric analysis. A ligand-directed growth mechanism was subsequently proposed to rationalize the growth process. The formation of the nanowhiskers provides a unique example of shape-controlled nanostructures, offering additional insights into nanoparticle synthesis.

Flexible and High Performance Supercapacitors Based on NiCo2O4for Wide Temperature Range Applications

Binder free nanostructured NiCo2O4 were grown using a facile hydrothermal technique. X-ray diffraction patterns confirmed the phase purity of NiCo2O4. The surface morphology and microstructure of the NiCo2O4 analyzed by scanning electron microscopy (SEM) showed flower-like morphology composed of needle-like structures. The potential application of binder free NiCo2O4 as an electrode for supercapacitor devices was investigated using electrochemical methods. The cyclic voltammograms of NiCo2O4 electrode using alkaline aqueous electrolytes showed the presence of redox peaks suggesting pseudocapacitance behavior. Quasi-solid state supercapacitor device fabricated by sandwiching two NiCo2O4 electrodes and separating them by ion transporting layer. The performance of the device was tested using cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. The device showed excellent flexibility and cyclic stability. The temperature dependent charge storage capacity was measured for their variable temperature applications. Specific capacitance of the device was enhanced by ~150% on raising the temperature from 20 to 60 °C. Hence, the results suggest that NiCo2O4 grown under these conditions could be a suitable material for high performance supercapacitor devices that can be operated at variable temperatures.

Controlled synthesis of iron oxide nanoplates and nanoflowers

An effective approach to synthesizing crystalline iron oxide nanoplates (~3 nm thick) and nanoflowers composed of ~5 nm small grains was reported. The formation of different-shaped nanoparticles in a similar system was achieved by controlling the nucleus concentration and growth rate.

Make conjugation simple: a facile approach to integrated nanostructures.

We report a facile approach to the conjugation of protein-encapsulated gold fluorescent nanoclusters to the iron oxide nanoparticles through catechol reaction. This method eliminates the use of chemical linkers and can be readily extended to the conjugation of biological molecules and other nanomaterials onto nanoparticle surfaces. The key to the success was producing water-soluble iron oxide nanoparticles with active catechol groups. Further, advanced electron microscopy analysis of the integrated gold nanoclusters and iron oxide nanoparticles provided direct evidence of the presence of a single fluorescent nanocluster per protein template. Interestingly, the integrated nanoparticles exhibited enhanced fluorescent emission in biological media. These studies will provide significantly practical value in chemical conjugation, the development of multifunctional nanostructures, and exploration of multifunctional nanoparticles for biological applications.

Layered ternary sulfide CuSbS2 nanoplates for flexible solid-state supercapacitors

Layer-structured materials are advantageous for supercapacitor applications owing to their ability to host a variety of atoms or ions, large ionic conductivity and high surface area. In particular, ternary or higher-order layered materials provide a unique opportunity to develop stable supercapacitor devices with high specific capacitance values by offering additional redox sites combined with the flexibility of tuning the interlayer distance by substitution. CuSbS2 is a ternary layered sulfide material that is composed of sustainable and less-toxic elements. We report the results of a systematic study of CuSbS2 nanoplates of varying thickness (4.3 ± 1.4 to 105 ± 5.5 nm) for use as supercapacitors along with the effect of ionic size of electrolyte ions on the specific capacitance and long-term cycling performance behavior. We have obtained specific capacitance values as high as 120 F g−1 for nanoplates with thickness of 55 ± 6.5 nm using LiOH electrolyte. Electronic structure calculations based on density functional theory predict that with complete surface coverage by electrolyte ions a specific capacitance of over 1160 F g−1 is achievable using CuSbS2, making it a very attractive layer-structured material for supercapacitor applications. Additionally, the calculations indicate that lithium ions can be intercalated between the van der Waals layers without significantly distorting the CuSbS2 structure, thereby further enhancing the specific capacitance by 85 F g−1. Quasi-solid-state flexible supercapacitor devices fabricated using CuSbS2 nanoplates exhibit an aerial capacitance value of 40 mF cm−2 with excellent cyclic stability and no loss of specific capacitance at various bending angles. Moreover, the supercapacitors are operable over a wide temperature range. We have further compared the electrochemical behavior of CuSbS2 with other non-layered phases in the system, namely Cu3SbS3, Cu3SbS4 and Cu12Sb4S13 that clearly highlight the importance of the layered structure for enhancing charge storage.

Synthesis of multiple platinum-attached iron oxide nanoparticles

Multiple, small platinum (Pt, ∼2 nm) attached iron oxide nanoparticles were successfully synthesized using two different approaches. One approach involves growing Pt nanoparticles onto iron oxide seeds in an organic solvent followed by a phase transfer process. We discovered that the surfactant packing density on the seed NPs played a critical role in multiple Pt nucleation. The presence of Pt nanoparticles on the seeds was confirmed using energy dispersive X-ray spectroscopy and high angle annular dark-field imaging. Alternatively, multiple Pt nanoparticles were deposited onto polyacrylic acid coated iron oxide nanoparticles in aqueous solution where the Pt nanoparticles were linked to the polymer coatings. The capability of growing multiple Pt nanoparticles on oxide seeds will be directly beneficial to both catalytic research and nanomedicine.

New insight into high-temperature driven morphology reliant CoMoO4 flexible supercapacitors

A facile hydrothermal method has been successfully developed for the synthesis of cobalt molybdate (CoMoO4). The morphology of the CoMoO4 was tailored by varying the growth conditions, and as a result different morphologies have been achieved such as cauliflower, brick and nano-sphere structures. The proposed potential use of the CoMoO4 as an electrode material for flexible supercapacitor applications was examined using cyclic voltammetry (CV) and galvanostatic charge–discharge measurements. It was observed that the specific capacitance of CoMoO4 depends on its morphology. A specific capacitance of 169 F g−1 in 3 M KOH at a current of 1 mA was observed for the nano-sphered CoMoO4. The effect of the electrolyte (LiOH, NaOH and KOH) on the electrochemical properties of the CoMoO4 was also investigated. The specific capacitance depends on the type of electrolyte and showed the highest value of 259 F g−1 in a 3 M NaOH electrolyte. Furthermore, these electrodes showed excellent cyclic stability. We have fabricated a flexible supercapacitor device by sandwiching two electrodes separated by an ion-transporting layer. The device shows no degradation in its capacitive properties upon bending and shows improved stability with the number of cyclic CV performances. The effect of temperature on the charge storage properties of the device was also investigated for high temperature applications. The specific capacitance of the device significantly increased when the operational temperature of the device was elevated from 10 to 70 °C. Hence, this study provides an ultimate facile method to synthesize morphology controlled cobalt molybdate for applications in the next generation of flexible energy storage devices, which can perform more efficiently at a higher temperature.

Synthesis of iron oxide nanoworms

We report on the synthesis of highly crystalline iron oxide nanoworms via a modified “heat-up” method using iron oleate as the precursor. According to a detailed nanoparticle growth study, we proposed that the nanoworms resulted from the aggregation of spherical iron oxide nanoparticles. The aggregation was induced by the high percentage coverage of a weakly bound ligand (trioctylphosphine oxide) on the iron oxide surfaces. A time dependent study clearly demonstrated the evolution of these nanostructures from spheres to one-dimensional nanoworms. The diameter of the nanoworms was similar to the spherical nanoparticles observed at an early stage of the reaction, and the length of the nanoworms changed from 50–200 nm during the reaction. The spheres and the nanoworms were both maghemite crystal structures, but the magnetic properties changed from superparamagnetic for the spheres to ferromagnetic for the elongated nanoworms. These one dimensional structures will offer additional opportunities for biomedical...

Platinum Attachments on Iron Oxide Nanoparticle Surfaces

Platinum nanoparticles supported on metal oxide surfaces have shown great potential as heterogeneous catalysts to accelerate electrochemical processes, such as the oxygen reduction reaction in fuel cells. Recently, the use of magnetic supports has become a promising research topic for easy separation and recovery of catalysts using magnets, such as Pt nanoparticles supported on iron oxide nanoparticles. The attachment of Pt on iron oxide nanoparticles is limited by the wetting ability of the Pt (metal) on ceramic surfaces. A study of Pt nanoparticle attachment on iron oxide nanoparticle surfaces in an organic solvent is reported, which addresses the factors that promote or inhibit such attachment. It was discovered that the Pt attachment strongly depends on the capping molecules of the iron oxide seeds and the reaction temperature. For example, the attachment of Pt nanoparticles on oleic acid coated iron oxide nanoparticles was very challenging, because of the strong binding between the carboxylic groups ...