Manoj Neergat

Oxygen Reduction Reaction and Peroxide Generation on Shape-Controlled and Polycrystalline Platinum Nanoparticles in Acidic and Alkaline Electrolytes

Shape-controlled Pt nanoparticles (cubic, tetrahedral, and cuboctahedral) are synthesized using stabilizers and capping agents. The nanoparticles are cleaned thoroughly and electrochemically characterized in acidic (0.5 M H2SO4 and 0.1 M HClO4) and alkaline (0.1 M NaOH) electrolytes, and their features are compared to that of polycrystalline Pt. Even with less than 100% shape-selectivity and with the truncation at the edges and corners as shown by the ex-situ TEM analysis, the voltammetric features of the shape-controlled nanoparticles correlate very well with that of the respective single-crystal surfaces, particularly the voltammograms of shape-controlled nanoparticles of relatively larger size. Shape-controlled nanoparticles of smaller size show somewhat higher contributions from the other orientations as well because of the unavoidable contribution from the truncation at the edges and corners. The Cu stripping voltammograms qualitatively correlate with the TEM analysis and the voltammograms. The fractions of low-index crystallographic orientations are estimated through the irreversible adsorption of Ge and Bi. Pt-nanocubes with dominant {100} facets are the most active toward oxygen reduction reaction (ORR) in strongly adsorbing H2SO4 electrolytes, while Pt-tetrahedral with dominant {111} facets is the most active in 0.1 M HClO4 and 0.1 M NaOH electrolytes. The difference in ORR activity is attributed to both the structure-sensitivity of the catalyst and the inhibiting effect of the anions present in the electrolytes. Moreover, the percentage of peroxide generation is 1.5-5% in weakly adsorbing (0.1 M HClO4) electrolytes and 5-12% in strongly adsorbing (0.5 M H2SO4 and 0.1 M NaOH) electrolytes.

Removal of surfactant and capping agent from Pd nanocubes (Pd-NCs) using tert-butylamine: its effect on electrochemical characteristics

Synthesis of shape-controlled nanoparticles of precious metals with defined size is well-established in the literature and the control over shape and size is achieved using surfactants and capping agents. However, a clean surface without impurities is required for realistic applications. In the present investigation, palladium nanocubes are synthesized using poly(vinylpyrrolidone) and potassium bromide. A novel method for cleaning the nanoparticle surface, i.e., treatment with tert-butylamine is reported. For comparison, a part of the untreated sample is subjected to the commonly used method of heat-treatment in an oxygen atmosphere for surface cleaning. The XPS and FTIR spectra of the heat-treated sample show incomplete removal of PVP and complete removal of Br− and the XRD pattern suggests oxide formation on the Pd surface. Treatment with tert-butylamine provides a clean surface free of PVP and Br−. Cleanliness of the surface is further confirmed by the voltammograms and ORR activities in 0.1 M HClO4. We conclude that tert-butylamine can be an effective solvent for the removal of PVP and a reagent for Br− ions because of its ability to form a quaternary ammonium salt.

Stability issues in Pd-based catalysts: the role of surface Pt in improving the stability and oxygen reduction reaction (ORR) activity

Carbon-supported Pd and Pd3Co catalysts have been electrochemically characterized in 0.1 M HClO4 solution and we found that both catalysts were unstable. On repeated potential cycling, the electrochemical surface area of the catalysts decreases and the oxygen reduction reaction (ORR) activity suffers. To stabilize surface Pd atoms of both Pd and Pd3Co catalysts, we deposited Pt using adsorbed hydrogen on the catalytically active Pd sites. The Pt : Pd ratio of Pt-coated Pd and Pt-coated Pd3Co catalysts suggests half-a-monolayer coverage of Pt (two hydrogen atoms required for reducing a Pt(2+) ion). The Pt : Pd ratio of Pt-coated Pd3Co catalyst obtained from the simple geometrical hard sphere model, energy-dispersive X-ray spectroscopy (EDS) line scan and bulk EDS agrees very well with that calculated from the hydrogen desorption (H(des)) charge of Pd3Co. At the same time, the Pt : Pd ratio of Pt-coated Pd calculated from the H(des) charge of Pd catalyst is significantly lower than the ratio obtained from the other methods. Thus, the Pt : Pd ratio of the Pt-coated Pd catalyst estimated from the H(des) region of Pd is an underestimation of the composition. This suggests that Pd forms an electrochemically inactive species from the H(upd) region itself and Co in Pd3Co seems to stabilize Pd against oxidation by delaying the formation of electrochemically inactive species to higher potentials above the H(upd) region. The voltammograms along with the peroxide formation characteristics of the catalysts support the above observations. The deposited Pt on the surface of the Pd and Pd3Co catalysts masks active Pd sites from the electrochemical environment and even partial coverage with Pt improves the stability and ORR activity of the catalysts when compared to that of the respective Pt-free counterparts.

Oxygen Reduction Reaction and Peroxide Generation on Ir, Rh, and their Selenides – A Comparison with Pt and RuSe

Carbon-supported Ir, Rh, IrSe, and RhSe were synthesized and subjected to electrochemical and physical (XRD, TEM) characterizations. ORR activity, peroxide generation, and methanol tolerance on carbon-supported Ir, Rh, IrSe, and RhSe were compared to a similarly prepared Pt and RuSe catalysts at comparable loading. Electrochemical characteristics of selenides studied using cyclic voltammetry and Cu stripping demonstrated complete suppression of the formation of oxygenated species and Hupd on the catalyst surface by Se. In presence of methanol, all selenides exhibited high methanol tolerance, and their oxygen reduction activities were in the order of Ir<Pt<Rh<IrSe<RhSe<RuSe. Potential dependant peroxide generation features on Ir and Rh differed significantly from that on Pt. Peroxide generation on selenides were also different from those of the corresponding bare metal catalysts (without Se). On all catalysts the peroxide generation in the reverse scan was higher than that in the forward scan. VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3604744] All rights reserved.

The effects of process parameters on yield and properties of iron nanoparticles from ferrocene in a low-pressure plasma

The effects of process parameters on iron nanoparticle formation and properties while using ferrocene as a precursor in a low-pressure capacitively coupled plasma are investigated. The L18 array of the Taguchi method, followed by the L4 array, is used with the notional objective of increasing the yield of nanoparticles. A study of the size, shape and composition of the particles (using transmission electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, x-ray diffraction, CHON and inductively coupled plasma-atomic emission spectroscopy analysis) gives an insight into the role played by various process parameters. Pressure is the most critical parameter in increasing nanoparticle yield, whereas hydrogen flow plays a key role in determining the nanoparticle size and composition. Atomic hydrogen helps in removing amorphous carbon and reducing the nanoparticle size. RF power plays an important role in the dissociation of ferrocene thus also affecting the composition. Nanoparticles obtained using optimized conditions are a mixture of Fe3O4 and Fe2O3 with cluster size 25?40?nm in diameter that are further made up of 2?4?nm crystallites. Magnetic property measurements indicate that the nanoparticles are super-paramagnetic in nature.

Chloride (Cl−) ion-mediated shape control of palladium nanoparticles

The shape control of Pd nanoparticles is investigated using chloride (Cl(-)) ions as capping agents in an aqueous medium in the temperature range of 60-100 °C. With weakly adsorbing and strongly etching Cl(-) ions, oxygen plays a crucial role in shape control. The experimental factors considered are the concentration of the capping agents, reaction time and reaction atmosphere. Thus, Pd nanoparticles of various shapes with high selectivity can be synthesized. Moreover, the removal of Cl(-) ions from the nanoparticle surface is easier than that of Br(-) ions (moderately adsorbing and etching) and I(-) ions (strongly adsorbing and weakly etching). The cleaned Cl(-) ion-mediated shape-controlled Pd nanoparticles are electrochemically characterized and the order of the half-wave potential of the oxygen reduction reaction in oxygen-saturated 0.1 M HClO4 solution is of the same order as that observed with single-crystal Pd surfaces.

Reduction of graphene oxide – a comprehensive electrochemical investigation in alkaline and acidic electrolytes

Graphene synthesized by the reduction of graphene oxide (GO) features in a myriad of applications ranging from sensors to batteries and catalysts to dye-sensitized solar cells. The exceptional physical and electrochemical properties of graphene originate from the presence of several residual functional groups and the non-stoichiometry in its structure. But, investigating the evolution of graphene from GO has been a daunting task. In this manuscript, simple electrochemical methods are reported to characterize GO subjected to thermal, electrochemical, and chemical reduction. The electrochemical features of these samples along with their FTIR spectra and XRD patterns help to identify the functional groups and provide compelling evidence for the transformation among them during the reduction of GO. The redox features of the voltammograms suggest the conversion of epoxides to carbonyl, carbonyl to carboxylic acid groups, and their subsequent removal with potential cycling. Thermal treatment of GO in the range of 80–150 °C causes the conversion of some of the epoxides to carbonyls and removal of water content. At the same time, epoxides are more prevalent in chemically reduced GO. The double layer capacitance – one of the figure of merits that distinguishes graphene from other carbon allotropes – gives an indication of the reduced graphene oxide content in the sample. Thus, electrochemical characterization sheds significant light onto the nature of oxygen moieties in non-heat-treated GO (n-HT-GO), thermally reduced GO (t-GO), chemically reduced GO (c-RGO) and electrochemically reduced GO (e-RGO), besides explaining the range of reported electrochemical capacitance.

Synthesis of Iron Oxide Nanoparticles from Iron Acetylacetonate and Cyclopentadienyliron Dicarbonyl Dimer in Low Pressure Plasma — Effect of Plasma Parameters on Morphology and Magnetic Properties

Iron oxide nanoparticles are synthesized using organometallic precursors namely, iron (III) acetylacetonate and cyclopentadienyliron dicarbonyl dimer in a capacitively coupled low pressure plasma system. They are characterized using High Resolution Transmission Electron Microscopy (HRTEM), X-ray Diffraction (XRD), magnetization studies and Raman spectroscopy. The role of hydrogen and RF (Radio Frequency) power on the crystalline and magnetic properties of the nanoparticles is studied. Incorporation of hydrogen to the Plasma-Enhanced Chemical Vapor Deposition (PECVD) chamber during the synthesis facilitates both crystallization of iron oxide nanoparticles and reduction of carbon content in the product. The saturation magnetization of iron oxide nanoparticles synthesized using iron (III) acetylacetonate and cyclopentadienyliron dicarbonyl dimer in presence of hydrogen at 200 W RF power is higher than that synthesized in the absence of hydrogen at 50 W RF power. In case of nanoparticles synthesized using iron (III) acetylacetonate, the saturation magnetization increases from 1.5 emu g-1 to 19 emu g-1, and for the same synthesized from cyclopentadienyliron dicarbonyl dimer it increases from 3.2 emu g-1 to 22.4 emu g-1.