P. Arunkumar

Texturing of pure and doped CeO2 thin films by EBPVD through target engineering

In this paper, we report the effect of annealing temperature of target on the texture of thin films coated by electron beam physical vapor deposition method. Nanocrystalline cerium oxide (CeO2) and 20 mol% samarium doped cerium oxide (SDC) powders, compacted into pellets, were used as targets after annealing at 300, 500 and 800 °C. Grain size analysis of the target by X-ray diffraction showed a size range of 12–52 nm and 9–22 nm for CeO2 and SDC, respectively. Texture coefficient calculation from glancing incident X-ray diffraction showed a preferential orientation of (111) in CeO2 films. However SDC films exhibited (200) orientation grown at the expense of (111) which resulted in higher residual strain with annealing temperature. The pole figure analysis elucidated smaller in-plane misorientation in CeO2 than in SDC films. Under similar deposition conditions, difference in textured growth between CeO2 and SDC is primarily induced by vapor pressure modifications associated with the annealing temperature of the target. Raman and X-ray photoelectron spectroscopic studies of the films indicate the presence of higher oxygen vacancy concentration in SDC as well as a decrease in Ce3+ concentration with target annealing temperature.

In situ generated nickel on cerium oxide nanoparticle for efficient catalytic reduction of 4-nitrophenol

Efficient and economic catalysts are required for the large scale degradation of hazardous pollutants. In the present work, two nickel (5 wt%) based compounds, Ni(NO3)2 and NiO, immobilized over a CeO2 surface were tested for the reduction of 4-nitrophenol. Size, structural and surface properties of the catalyst were characterized by XRD, SEM & TEM – EDX, FTIR and Raman spectroscopy. UV-visible spectroscopic results indicated the better catalytic performance of the Ni(NO3)2 support than that of NiO supported CeO2. The reduction rate of 4-nitrophenol in the presence of the Ni(NO3)2 support was found to be 12 times faster than that of NiO supported CeO2. The time-dependent Raman spectroscopic investigation demonstrated that the performance of Ni(NO3)2 supported CeO2 arises from the in situ generation of nickel in the presence of an excess of sodium borohydride in the reduction of 4-nitrophenol. Further, the reversible conversion of nickel to nickel nitrate enabled the recyclability of the Ni(NO3)2 supported CeO2. The formation of nickel was found to be important for the reduction of 4-nitrophenol as NiO supported CeO2 did not form nickel thereby exhibiting poor catalytic activity. Thus, the present work showcases the in situ generation of nickel as a novel strategy for the catalytic reduction of 4-nitrophenol.

Role of iron addition on grain boundary conductivity of pure and samarium doped cerium oxide

The present paper reports the effect of iron doping (0.5, 1.5 mol%) on the densification and electrical properties of cerium oxide (CeO2) and 20 mol% samarium-doped cerium oxide (SDC) electrolytes for intermediate temperature solid oxide fuel cell (ITSOFC) applications. A single-step solution combustion method was used for doping and the resultant powder was compacted into green pellets and subsequently sintered at 1200 °C. X-ray diffraction (XRD) studies indicated the presence of a cubic fluorite CeO2 structure without the formation of a secondary phase and the stoichiometry was confirmed by X-ray fluorescence spectroscopy. In the as-compacted green pellets, the XRD peak position shifted to lower or higher angles depending on the ionic radii of the dopants due to lattice level mixing. Addition of iron resulted in smaller crystallite sizes ( 40 nm) after sintering. Densification was found to be higher (95%) in iron-doped samples than in bare samples (<90%) due to viscous flow sintering. Upon sintering the calculated strain value showed a lower value due to the segregation of iron from the lattice. Raman spectroscopic studies indicate that sintering marginally modifies the oxygen vacancy concentration in the SDC system, and found it to be higher than in CeO2. Addition of iron into the SDC improved the grain boundary conductivity 1.8 fold, but only a minor change was noticed for CeO2. The activation energy for the grain boundary conductivity was found to be lower for 1.5 mol% (1.06 eV) iron-doped SDC than for pure SDC (1.24 eV). Our results indicate that lattice level mixing of iron in SDC improves the density at relatively lower sintering temperatures and scavenges the grain boundary impurities, thereby increasing the grain boundary conductivity.

Effect of fuel ratio on combustion synthesis and properties of magnetic nanostructures

We report a simple one-step solution combustion method for the preparation of ultrafine Ni, NiO and Ni/NiO nanostructures. The fuel-to-oxidizer ratio can be tuned to control the formation of either nanoparticles (Ni or NiO) or nanocomposites (Ni/NiO). The resultant nanostructures are characterized by x-ray diffraction, scanning electron microscopy, Raman spectroscopy and vibrating sample magnetometer. Processing under fuel rich conditions leads to the formation of Ni/NiO nanocomposite while fuel lean environment results in primarily NiO. The processing conditions influence the stoichiometry and the size (6.8 to 39.5 nm) of NiO nanoparticles. The intensity of one- phonon Raman longitudinal optical mode is found to be higher than two-phonon optical mode for non-stoichiometric NiO. The saturation magnetization and coercivity of the as-synthesized powders are influenced by the ratio of Ni to NiO, stoichiometry of NiO and pore size.

Tunable transport property of oxygen ion in metal oxide thin film: Impact of electrolyte orientation on conductivity

Quest for efficient ion conducting electrolyte thin film operating at intermediate temperature (~600 °C) holds promise for the real-world utilization of solid oxide fuel cells. Here, we report the correlation between mixed as well as preferentially oriented samarium doped cerium oxide electrolyte films fabricated by varying the substrate temperatures (100, 300 and 500 °C) over anode/ quartz by electron beam physical vapor deposition. Pole figure analysis of films deposited at 300 °C demonstrated a preferential (111) orientation in out-off plane direction, while a mixed orientation was observed at 100 and 500 °C. As per extended structural zone model, the growth mechanism of film differs with surface mobility of adatom. Preferential orientation resulted in higher ionic conductivity than the films with mixed orientation, demonstrating the role of growth on electrochemical properties. The superior ionic conductivity upon preferential orientation arises from the effective reduction of anisotropic nature and grain boundary density in highly oriented thin films in out-of-plane direction, which facilitates the hopping of oxygen ion at a lower activation energy. This unique feature of growing an oriented electrolyte over the anode material opens a new approach to solving the grain boundary limitation and makes it as a promising solution for efficient power generation.

Thin Film: Deposition, Growth Aspects, and Characterization

Thin film science and technology plays an important role in the development of devices in the future ranging from energy-efficient display devices to energy-harvesting and storage devices such as solar cell, fuel cell, batteries, super capacitor, etc. Thin films have properties that can be different from that of their corresponding bulk structures. A film is considered as thin, as long as its surface properties are different from its bulk behaviour. Thin films have larger surface to volume ratio, hence the surface and near surface characteristics decide the properties of the thin film. As a result thin film properties generally depend on the thickness of the film which extends from few micrometre to nanometre, substrate nature on which the films are grown and deposition methodology/conditions used in the fabrication of thin films. Thin film fabrications are generally carried out by depositing the required material in the atomistic deposition (atom by atom) over the required substrate, which may result in either single crystalline, polycrystalline, or amorphous structure depending on the deposition conditions. Thin film technology has the potential to engineer the various properties such as porosity, surface morphology, surface roughness, and crystallite size. These advantages in thin film assist in the development of new products and minimize the waste as in the conventional manufacturing techniques. This chapter provides an overview of various thin film processing methods, mechanism behind the growth and important tools used for the characterization of thin films.

Strontium mediated modification of structure and ionic conductivity in samarium doped ceria/sodium carbonate nanocomposites as electrolytes for LTSOFC

The structural changes on the addition of strontium in samarium doped ceria/Na2CO3 nanocomposites were investigated with respect to sintering temperature. The nanocomposites prepared by a co-precipitation method in the presence (SrSDS) and absence (SDS) of strontium were sintered at 500, 600 and 700 °C. XRD results indicated an increase in crystallite size and lattice parameter with respect to sintering temperature in the presence of strontium. Raman, SEM and FT-IR studies confirmed the presence of Na2CO3 and CeO2 phases. The observed changes in crystallinity and oxygen vacancy concentrations indicate the beneficial role of strontium upon sintering up to 600 °C. The impedance spectral analysis clearly shows the beneficial effect of adding strontium to the composite. The lowest activation energy (0.61 eV) with the highest conductivity (3.8 × 10−3 S cm−1) for SrSDS sintered at 600 °C arises due to the strong interaction between the Na2CO3 and CeO2 phase. However, sintering the composites at 700 °C indicated a negligible effect of strontium due to the decomposition of Na2CO3, thereby limiting the operational temperature of the nanocomposites for potential fuel cell applications.