N. Ravishankar

Rensselaer Polytechnic Institute, Troy, New York, United States

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Publications (120)378.98 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Extensive work from both scientific and engineering fields have been devoted to CO oxidation reaction, in response both pollution control and renewable energy development. This is extensively used reaction that has various industrial applications in automotive catalytic converter and fuel cells. CeO2, SnO2, Fe2O3, TiO2, are commonly utilized for their application in CO oxidation. The metal oxides which are reducible have advantages in releasing the lattice oxygen and promoting the CO oxidation. A key factor is that the material should have high oxygen storage capacity at low temperatures and thus a high activity of CO oxidation. In these ionic-substituted oxides, a weakening of “metal-oxygen” bond results in lattice oxygen release at very low temperatures during reaction and that are better as compared to metal dispersed oxides.1One of the limitations of CO oxidation catalyst is that it shows poor performance in hydrogen rich atmosphere. Many efforts have been focused on preferential CO oxidation catalysts that are inactive towards undesired H2 oxidation.2 Few studies revealed that compared to other oxides, zinc stannate sensors showed high selectivity towards CO in H2 atmosphere at low temperatures.3 Inspired by these studies, we hypothesize that zinc stannate may be an important material for CO oxidation, which has not been reported so far. In this report, we synthesized new and novel oxygen storage Pd-substituted Zn2SnO4 through solution combustion method that exhibits good oxygen storage capacity resulting in a low temperature CO oxidation. Encouragingly, this system showed higher activity towards CO oxidation at low temperatures. Our results indeed indicate that, when Pd is substituted in zinc stannatethe metal-oxygen bond weakens, which benefits the lattice oxygen release.We proposed the mechanism based on reaction rates, both CO and O2 are competitive for the same adsorption sites, according to the data collected in kinetic regime. References 1. Hegde, M.; Madras, G.; Patil, K., Noble metal ionic catalysts. Accounts of chemical research 2009,42(6), 704-712. 2. Pozdnyakova, O.; Teschner, D.; Wootsch, A.; Kröhnert, J.; Steinhauer, B.; Sauer, H.; Toth, L.; Jentoft, F.; Knop-Gericke, A.; Paál, Z., Preferential CO oxidation in hydrogen (PROX) on ceria-supported catalysts, part I: Oxidation state and surface species on Pt/CeO2 under reaction conditions. Journal of Catalysis 2006,237(1), 1-16. 3. Yu, J. H.; Choi, G. M., Selective CO gas detection of Zn2SnO4 gas sensor. Journal of electroceramics 2002,8 (3), 249-255.
    14 AIChE Annual Meeting; 11/2014
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    ABSTRACT: Fuel cells are believed to be promising technology to challenge the increasing energy demand particularly direct alcohol oxidation reaction displays high energy density. This work presents the synthesis of hybrid ZnO/ultrathin Au nanowires1 and their support and geometry-dependent electrocatalytic performances towards alcohol oxidation upon illumination. Geometry control of Au nanostructures is a useful method to modify its electrocatalytic properties– by tuning electrochemical active surface area, exposed facets and aspect ratio.2 Grown ultrathin Au nanowires exhibited outstanding electrochemical activity that is much higher than that exhibited by Au nanoparticles. ZnO support provides an excellent stability for Au nanowires during reaction and also produced reactive charge carriers under excitation, thus assisting in oxidation current generation. Since both ZnO and Au nanowires exhibited ethanol oxidation reaction independently via photoirradiation and applied potential, respectively, the direct coupling of Au nanowires and ZnO, hybrid displayed impressive photoassisted electrocatalytic performance and excellent stability.3The mechanism of the synergetic effect of ZnO supported Au nanowires towards photoelectrocatalytic activity is proposed based on electrochemical measurements. We envision that the as-synthesized semiconductor-ultrathin nanowires hybrids hold great possibility for integration as catalyst in future fuel cell technology. References 1. Halder, A.; Ravishankar, N., Ultrafine Single‐Crystalline Gold Nanowire Arrays by Oriented Attachment. Advanced Materials 2007,19(14), 1854-1858. 2. Porter, N. S.; Wu, H.; Quan, Z.; Fang, J., Shape-control and electrocatalytic activity-enhancement of Pt-based bimetallic nanocrystals. Accounts of chemical research 2013,46(8), 1867-1877. 3. Drew, K.; Girishkumar, G.; Vinodgopal, K.; Kamat, P. V., Boosting fuel cell performance with a semiconductor photocatalyst: TiO2/Pt-Ru hybrid catalyst for methanol oxidation. The Journal of Physical Chemistry B 2005,109 (24), 11851-11857.
    14 AIChE Annual Meeting; 11/2014
  • Subhajit Kundu, A Leelavathi, Giridhar Madras, Narayanan Ravishankar
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    ABSTRACT: Although ultrathin Au nanowires (~ 2 nm diameter) are expected to demonstrate several interesting properties, their extreme fragility has hampered their use in potential applications. One way to improve the stability is to grow them on substrates; however there is no general method to grow these wires over large areas. The existing methods suffer from poor coverage and associated formation of larger nanoparticles on the substrate. Herein, we demonstrate a room temperature method for growth of these nanowires with high coverage over large areas by in-situ functionalization of the substrate. Using control experiments, we demonstrate that an in-situ functionalization of the substrate is the key step in controlling the areal density of the wires on the substrate. We show that this strategy works for a variety of substrates ranging like graphene, borosil glass, kapton and oxide supports. We present initial results on catalysis using the wires grown on alumina and silica beads and also extend the method to lithography-free device fabrication. This method is general and may be extended to grow ultrathin Au nanowires on a variety of substrates for other applications.
    Langmuir : the ACS journal of surfaces and colloids. 10/2014;
  • A Leelavathi, Giridhar Madras, Narayanan Ravishankar
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    ABSTRACT: Oxidation of small organic molecules in a fuel cell is a viable method for energy production. However, the key issue is the development of suitable catalysts that exhibit high efficiencies and remain stable during operation. Here, we demonstrate that amine-modified ZnO nanorods on which ultrathin Au nanowires are grown acts as an excellent catalyst for the oxidation of ethanol. We show that the modification of the ZnO nanorods with oleylamine not only modifies the electronic structure favorably but also serves to anchor the Au nanowires on the nanorods. The adsorption of OH species on the Au nanowires that is essential for ethanol oxidation is facilitated at much lower potentials as compared to bare Au nanowires leading to high activity. While ZnO shows negligible electrocatalytic activity under normal conditions, there is significant enhancement in the activity under light irradiation. We demonstrate a synergistic enhancement in the photoelectrocatalytic activity of the ZnO/Au nanowire hybrid and provide mechanistic explanation for this enhancement based on both electronic as well as geometric effects. The principles developed are applicable for tuning the properties of other metal/semiconductor hybrids with potentially interesting applications beyond the fuel cell application demonstrated here.
    Journal of the American Chemical Society 09/2014; · 10.68 Impact Factor
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    ABSTRACT: Due to the ease of modification of electronic structure upon analyte adsorption, semiconductors have been preferred materials as chemical sensors. At reduced dimension, however, the sensitivity of semiconductor-based sensors deteriorate significantly due to passivation, and often by increased band gap caused by quantum confinement. Using first principles density functional theory combined with Boltzmann transport calculations, we demonstrate semiconductor-like sensitivity towards chemical species in ultrathin gold nanowires (AuNW). The sensing mechanism is governed by the modification of electronic structure of the AuNW as well as scattering of the charge carriers by analyte adsorption. Most importantly, the sensitivity exhibits a linear relationship with the electron affinities of the respective analytes. Based on this relationship, we propose an empirical parameter, which can predict an analyte-specific sensitivity of a AuNW, rendering them as effective sensor for a wide range of chemical analytes.
    The Journal of Physical Chemistry C 07/2014; · 4.84 Impact Factor
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    ABSTRACT: A detailed understanding of structure and stability of nanowires is critical for applications. Atomic resolution imaging of ultrathin single crystalline Au nanowires us-ing aberration-corrected microscopy reveals an intriguing relaxation whereby the atoms in the close-packed atomic planes normal to the growth direction are displaced in the ax-ial direction leading to wrinkling of the (111) atomic plane normal to the wire axis. First principles calculations of the structure of such nanowires confirm this wrinkling phe-nomenon whereby the close-packed planes relax to form saddle-like surfaces. Molecular dynamics studies of wires with varying diameters and different bounding surfaces point to the key role of surface stress on the relaxation process. Using continuum mechanics arguments, we show that the wrinkling arises due to anisotropy in the surface stresses and in the elastic response, along with the divergence of surface-induced bulk stress near the edges of a faceted structure. The observations provide new understanding on the equilibrium structure of nanoscale systems and could have important implications for applications in sensing and actuation.
    Nano letters. 07/2014;
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    ABSTRACT: It is well known that metals with higher electron affinity like Au tend to undergo reduction rather than cation-exchange. It is experimentally shown that under certain conditions cation-exchange is dominant over reduction. Thermodynamic calculation further consolidates the understanding and paves the way for better predictability of cation-exchange/reduction reactions for other systems.
    Small 05/2014; · 7.82 Impact Factor
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    ABSTRACT: Gold–silica hybrids are appealing in different fields of applications like catalysis, sensorics, drug delivery, and biotechnology. In most cases, the morphology and distribution of the heterounits play significant roles in their functional behavior. Methods of synthesizing these hybrids, with variable ordering of the heterounits, are replete; however, a complete characterization in three dimensions could not be achieved yet. A simple route to the synthesis of Au-decorated SiO2 spheres is demonstrated and a study on the 3D ordering of the heterounits by scanning transmission electron microscopy (STEM) tomography is presented—at the final stage, intermediate stages of formation, and after heating the hybrid. The final hybrid evolves from a soft self-assembled structure of Au nanoparticles. The hybrid shows good thermal stability up to 400 °C, beyond which the Au particles start migrating inside the SiO2 matrix. This study provides an insight in the formation mechanism and thermal stability of the structures which are crucial factors for designing and applying such hybrids in fields of catalysis and biotechnology. As the method is general, it can be applied to make similar hybrids based on SiO2 by tuning the reaction chemistry as needed.
    Angewandte Chemie International Edition 03/2014; · 11.34 Impact Factor
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    ABSTRACT: Gold–silica hybrids are appealing in different fields of applications like catalysis, sensorics, drug delivery, and biotechnology. In most cases, the morphology and distribution of the heterounits play significant roles in their functional behavior. Methods of synthesizing these hybrids, with variable ordering of the heterounits, are replete; however, a complete characterization in three dimensions could not be achieved yet. A simple route to the synthesis of Au-decorated SiO2 spheres is demonstrated and a study on the 3D ordering of the heterounits by scanning transmission electron microscopy (STEM) tomography is presented—at the final stage, intermediate stages of formation, and after heating the hybrid. The final hybrid evolves from a soft self-assembled structure of Au nanoparticles. The hybrid shows good thermal stability up to 400 °C, beyond which the Au particles start migrating inside the SiO2 matrix. This study provides an insight in the formation mechanism and thermal stability of the structures which are crucial factors for designing and applying such hybrids in fields of catalysis and biotechnology. As the method is general, it can be applied to make similar hybrids based on SiO2 by tuning the reaction chemistry as needed.
    Angewandte Chemie 03/2014;
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    ABSTRACT: N anoscale gold has been a material of interest for decades owing to its large and controllable variation of functionality depending on its shape, form and size. 1À3 It has been exploited suitably in various fields of application including catalysis, optics, plasmonics and biotech-nology. 4À6 Gold nanowires of molecular scale dimensions are of fundamental as well as technological interest owing to their tunable electrical transport characteristics which can lead to ballistic conduction. The-oretical models predict the onset of quan-tized conductance upon decrease of the wire diameter and a deviation of its struc-ture from bulk, implying a structural depen-dence on the electron transport charac-teristics. 7,8 This quantization effect due to downsizing is a direct consequence of their electronic states which are dependent on their atomic structure. These tunable elec-tronic properties open up possibilities for the exploitation of Au nanostructures as highly active catalysts 9,10 and sensors, where the mechanism is electron sen-sitive 11 and structure dependent. 12 Several protocols are reported for wire synthesis in solution; 13À16 however, exploring/exploiting them remained challenging until the growth of single crystal ultrafine wires (∼2 nm diameter) with [111] growth direc-tion could be achieved in a controlled way on various substrates. 17 This controlled synthesis allowed to probe the electronic properties of such fine wires and it was demonstrated that ultrathin Au nanowires of ∼2 nm diameter exhibit an insulating state, unlike metallic bulk Au, due to a strongly correlated electron transport mechanism. 18 However, a detailed structur-al investigation has not been reported to date. Atomic level defects acting as scatter-ing barriers 19 or any morphological changes induced by interaction with the substrate could largely influence the electron trans-port behavior. 20,21 Besides, surface strain can be induced in the crystal upon down-sizing, influencing its electronic properties and hence its catalytic behavior as demon-strated for twinned Au nanoparticles. 22 It is therefore essential to investigate the stabil-ity and structural characteristics of these * Address correspondence to paromita.kundu@uantwerp.be.
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    ABSTRACT: Theoretical studies exist to compute the atomic arrangement in gold nanowires and the influence on their electronic behavior with decreasing diameter. Experimental studies, for e.g. by transmission electron microscopy, on chemically synthesized ultrafine wires are however lacking owing to the unavailability of suitable protocols for sample preparation and the stability of the wires under electron beam irradiation. In this work, we present an atomic scale structural investigation on quantum single crystalline gold nanowires of 2 nm diameter, chemically prepared on a carbon film grid. Using low dose aberration-corrected high resolution (S)TEM we observe an inhomogeneous strain distribution in the crystal, largely concentrated at the twin boundaries and the surface along with the presence of facets and surface steps leading to a non-circular cross-section of the wires. These structural aspects are critical inputs needed to determine their unique electronic character and their potential as a suitable catalyst material. Furthermore, electron-beam-induced structural changes at the atomic scale having implications on their mechanical behavior and their suitability as interconnects, are discussed.
    ACS Nano 11/2013; · 12.03 Impact Factor
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    ABSTRACT: Ceria, because of its excellent redox behavior and oxygen storage capacity, is used as a catalyst for several technologically important reactions. In the present study, different morphologies of nano-CeO2 (rods, cubes, octahedra) were synthesized using the hydrothermal route. An ultrafast microwave-assisted method was used to efficiently attach Pt particles to the CeO2 polyhedra. These nanohybrids were tested as catalysts for the CO oxidation reaction. The CeO2/Pt catalyst with nanorods as the support was found to be the most active catalyst. XPS and IR spectroscopy measurements were carried out in order to obtain a mechanistic understanding and it was observed that the adsorbed carbonates with lower stability on the reactive planes of nanorods and cubes are the major contributor to this enhanced catalytic activity.
    Dalton Transactions 09/2013; · 3.81 Impact Factor
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    ABSTRACT: We present a non-hydrolytic sol–gel combustion method for synthesizing nanocomposites of PbO quantum dots on anatase TiO2 with a high surface area. XRD, electron microscopy, DRS, cathodoluminescence and BET were employed for structural, microstructural and optical characterization of the composites. The photocatalytic activity of TiO2 and PbO/TiO2 was investigated and compared with Degussa P-25. The results indicate that the photocatalytic activity of quantum dot dispersed TiO2 is higher than that of bare TiO2 and much higher than that of commercial Degussa P-25. The origin of enhanced photoreactivity of the synthesized material can be assigned to a synergetic effect of high surface area, higher number of active sites and an engineered band structure in the heterostructure. The mechanisms for photocatalytic activity are discussed based on production of photogenerated reactive species. The knowledge gained through this report open up ideal synthesis routes for designing advanced functional heterostructures with engineered band structure and has important implications in solar energy based applications.
    RSC Advances 08/2013; 3(43):19671 to 21128. · 3.71 Impact Factor
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  • A Leelavathi, Giridhar Madras, N Ravishankar
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    ABSTRACT: Faceted ZnO nanorods with different aspect ratios were synthesized by a solvothermal method by tuning the reaction time. Increased reaction leads to the formation of high aspect ratio ZnO nanorods largely bound by the prism planes. The high aspect ratio rods showed significantly higher visible light photocatalytic activity when compared to the lower aspect ratio structures. It is proposed that the higher activity is due to better charge separation in the elongated 1D structure. In addition, the fraction of unsaturated Zn(2+) sites is higher on the {101[combining macron]0} facets, leading to better adsorption of oxygen-containing species. These species enhance the production of reactive radicals that are responsible for photodegradation. The photocurrent for these ZnO nanostructures under solar light was measured and a direct correlation between photocurrent and aspect ratio was observed. Since the underlying mechanisms for photodegradation and photocurrent generation are directly related to the efficiency of electron-hole creation and separation, this observation corroborates that the charge separation processes are indeed enhanced in the high aspect ratio structures. The efficiency of photoconduction (electron-hole pair separation) could be further improved by attaching Au nanoparticles on ZnO, which can act as a sink for the electrons. This heterostructure exhibits a high chemisorption of oxygen, which facilitates the production of highly reactive radicals contributing to the high photoreactivity. The suggested mechanisms are applicable to other n-type semiconductor nanostructures with important implications for applications relating to energy and the environment.
    Physical Chemistry Chemical Physics 05/2013; · 4.20 Impact Factor
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    Paromita Kundu, E A Anumol, N Ravishankar
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    ABSTRACT: Capping-free and linker-free nanostructures/hybrids possess superior properties due to the presence of pristine surfaces and interfaces. In this review, various methods for synthesizing pristine nanomaterials are presented along with the general principles involved in their morphology control. In wet chemical synthesis, the interplay between various reaction parameters results in diverse morphology. The fundamental principles behind the evolution of morphology including nanoporous aggregates of metals and other inorganic materials, 2D nanocrystals of metals is elucidated by capping-free methods in aqueous medium. In addition, strategies leading to the attachment of bare noble metal nanoparticles to functional oxide supports/reduced graphene oxide has been demonstrated which can serve as a simple solution for obtaining thermally stable and efficient supported catalysts with free surfaces. Solution based synthesis of linker-free oxide-semiconductor hybrids and capping-free metal nanowires on substrates are also discussed in this context with ZnO/CdS and ultrathin Au nanowires as examples. A simple and rapid microwave-assisted method is highlighted for obtaining such hybrids which can be employed for high-yield production of similar materials.
    Nanoscale 05/2013; · 6.73 Impact Factor
  • E. A. Anumol, C. Nethravathi, N. Ravishankar
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    ABSTRACT: Hollow nanostructures are used for various applications including catalysis, sensing, and drug delivery. Methods based on the Kirkendall effect have been the most successful for obtaining hollow nanostructures of various multicomponent systems. The classical Kirkendall effect relies on the presence of a faster diffusing species in the core; the resultant imbalance in flux results in the formation of hollow structures. Here, an alternate non-Kirkendall mechanism that is operative for the formation of hollow single crystalline particles of intermetallic PtBi is demonstrated. The synthesis method involves sequential reduction of Pt and Bi salts in ethylene glycol under microwave irradiation. Detailed analysis of the reaction at various stages indicates that the formation of the intermetallic PtBi hollow nanoparticles occurs in steps. The mechanistic details are elucidated using control experiments. The use of microwave results in a very rapid synthesis of intermetallics PtBi that exhibits excellent electrocatalytic activity for formic acid oxidation reaction. The method presented can be extended to various multicomponent systems and is independent of the intrinsic diffusivities of the species involved.
    Particle and Particle Systems Characterization 03/2013; 30(7):590. · 0.86 Impact Factor
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    ABSTRACT: Using first principles based density functional calculation we study the mechanical, electronic and transport properties of single crystalline gold nanowires. While nanowires with the diameter less than 2 nm retain hexagonal cross-section, the larger diameter wires show a structural smoothening leading to circular cross-section. These structural changes significantly affect the mechanical properties of the wires, however, strength remains comparable to the bulk. The transport calculations reveal that the conductivity of these wires are in good agreement with experiments. The combination of good mechanical, electronic and transport properties make these wires promising as interconnects for nano devices.
    AIP Advances 03/2013; 3(3). · 1.35 Impact Factor
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    ABSTRACT: The electronic state in ultrathin gold nanowires is tuned by a careful engineering of the device architecture via a chemical methodology. The electrons are localized to an insulating state (showing a variable range hopping transport) by simply bringing them close to the substrate, while the insertion of an interlayer leads to a Tomonaga Luttinger liquid state.
    Advanced Materials 03/2013; · 14.83 Impact Factor
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    ABSTRACT: We present a green method for the synthesis of ZnO-Au hybrids using an ultrafast microwave-based technique. This method provides good control over the nucleation of the metal nanoparticles on the oxide support, which governs the morphology and microstructure of the hybrids. The hybrids exhibit good catalytic activity for CO oxidation compared to similar hybrids reported in the literature. Detailed XPS investigation reveals the presence of Au-Zn and Au-O bonds at the interface. This surface doping leads to the formation of anionic and cationic Au sites that contribute to the enhanced activity. Our method is general and can be applied for designing other supported catalysts with controlled interfaces.
    Dalton Transactions 06/2012; 41(29):8762-6. · 3.81 Impact Factor

Publication Stats

526 Citations
378.98 Total Impact Points

Institutions

  • 2009–2011
    • Rensselaer Polytechnic Institute
      • Department of Materials Science and Engineering
      Troy, New York, United States
    • University of Connecticut
      • Department of Chemical and Biomolecular Engineering
      Storrs, CT, United States
  • 2008
    • Bangalore University
      • Department of Chemistry
      Bengalore, State of Karnataka, India
  • 2006–2008
    • St. Joseph's College of Bangalore
      • Department of Chemistry
      Bengalore, State of Karnataka, India
  • 2004
    • Indian Institute of Science
      • Department of Solid State and Structural Chemistry Unit
      Bengalore, State of Karnataka, India
  • 2001–2004
    • University of Minnesota Twin Cities
      • Department of Chemical Engineering and Materials Science
      Minneapolis, MN, United States
  • 2000–2002
    • University of Minnesota Duluth
      Duluth, Minnesota, United States