K. Suresh Babu

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.

Enhancing the dual magnetic and optical properties of co-doped cerium oxide nanostructures

In spite of the potential biomedical application, ceria nanoparticles suffer from poor optical and magnetic properties. Here we report the structural, optical and magnetic properties of iron (8 at%) and europium (1 at%) co-doped ceria nanoparticles with respect to annealing temperature. A cubic fluorite structure of ceria was observed on co-doping but an additional α-Fe2O3 phase emerged on annealing at 700 °C and above. An increase in the mean crystallite size from 6.5 to 37 nm and a corresponding reduction in strain was observed with annealing temperature. Surface area and scanning electron microscopic studies indicated a porous structure which became dense upon annealing. Raman spectroscopic studies reveal the presence of oxygen vacancy defects arising from the combination of intrinsic and extrinsic defects due to the presence of Ce3+ and dopant, respectively. X-ray photoelectron spectroscopic results confirm the oxidation of Ce3+ to Ce4+ state while the trivalent state of iron was retained on annealing at higher temperatures. On annealing above 500 °C, reduction in defect concentration improved the emission intensity primarily through magnetic dipole transition. A typical ferromagnetic behaviour was noticed in all the samples and shift a from soft to hard magnetic behaviour upon annealing.

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.

Synergistic effect of gold supported on redox active cerium oxide nanoparticles for the catalytic hydrogenation of 4-nitrophenol

Recently, the conversion of 4-nitrophenol to 4-aminophenol via metal-supported oxide catalysts has attracted significant attention because of its potential applications. The main objective of this study was to demonstrate the crucial role of support materials in heterogeneous catalysts with inherent redox cycling features. The metals supported on these redox active materials can significantly alter the catalytic performance in a synergistic manner. For this, the catalytic hydrogenation of Au–ceria, prepared by altering the annealing–impregnation procedures, was investigated in terms of the size of gold and support material and their redox chemistry. The study clearly shows the variation in the catalytic activity of gold supported on the as-prepared and annealed ceria surface. This variation was due to the fact that the mutual interactions at the interface between absorbed Au and cerium oxide were certainly different in the as-prepared and annealed ceria surfaces with respect to surface chemistry and size of the particles. The redox nature of ceria and partial oxidation of gold on the surface of CeO2 were involved in the catalytic reduction process.

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.

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.

Role of oxygen vacancy tuning in EBPVD deposited LaxCe1−xO2−δ films in high temperature oxidation protection

Cerium oxide based nanostructure coatings are shown to be promising for high temperature oxidation protection of AISI 304 stainless steel due to the presence of oxygen vacancy defects. The present work focuses on the correlation of oxygen vacancies generated by varying lanthanum concentration in ceria to high temperature oxidation protection. LaxCe1−xO2−δ (x = 0, 0.05, 0.1, 0.2 & 0.4) synthesized using a chemical co-precipitation method was sintered at 1473 K for 5 hours and used as a target in electron-beam physical vapour deposition (EBPVD). The thickness of the coatings over the AISI 304 substrate was maintained at 2000 nm and all the samples were isothermally oxidized at 1243 K for 24 hours. The La3+ doping and the presence of oxygen vacancies were confirmed by using X-ray diffraction and Raman spectroscopy, respectively, in the as coated condition. Though the target had larger mean crystallite size (77 to 52 nm) but varied marginally (6.7 to 7.3 nm) in the as coated condition emphasizing the role of physical process during deposition. The rate constant value for the oxidation process was found to be 3–4 orders lower than that of bare AISI 304 indicating the effect of coating against high temperature oxidation. 5% La doped ceria coating provided better oxidation protection than pure ceria while 40% doping resulted in one order lower rate. This can be attributed to the higher oxygen vacancy concentration present in the sample. The presence of La in the coatings also helped in the retention of oxygen vacancy concentration after oxidation. The reported study indicates the importance of oxygen vacancy tuning and need for uniform coatings towards designing coatings for high temperature oxidation protection.

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.

Modulation of biomimetic properties of cerium oxide nanoparticles by hypoxic tumor microenvironments: steering towards tumor specificity

The low oxygen tension characterizing the core glioblastomas (GBMs) and other solid tumors is known to cause a metabolic shift within tumors. This lowers the pH and simultaneously increases the ROS (O2˙−, HO˙, H2O2etc.,) accumulation which eventually facilitates the tumor resistance to chemo/radiotherapy. Herein, we report the biomimetic antioxidant properties of cerium oxide nanoparticles (CNPs) under physiological and tumor microenvironment-like conditions in vitro using a GBM cell line. CNPs were synthesized in a single step combustion method and exhibited selective cytotoxicity under tumor microenvironment-like conditions. Furthermore, the peroxidase-like activity of the CNP nanoparticles was modulated by tumor microenvironment-like conditions (pH 6.5 and 1.5% O2) due to the redox transformation between the Ce3+ ↔ Ce4+ states of the cerium oxide nanoparticles under different conditions. When incubated with CNPs, a remarkable variation in the oxidative stress was observed from the measured MDA levels. Combined results from a peroxidase assay, MDA assay and TBARS assay revealed size and pH dependent activity which could have significant implications towards the tumor specific activity. The results are promising for the use of CNPs for tumor therapy and warrant further studies using both in vivo and in vitro models on their possible therapeutic potential.

Structure dependent luminescence, peroxidase mimetic and hydrogen peroxide sensing of samarium doped cerium phosphate nanorods

Rare earth phosphates have been used extensively in luminescent phosphors, bio-imaging, catalysis, and sensors. However, there is a need to correlate the structural-chemical changes associated with stability and performance. In the present work, hydrothermally synthesized CePO4:Smx (x = 0, 5 and 10 mol%) nanorods were annealed at different temperatures to understand the modulations in structure as well as optical and enzyme mimetic properties. As prepared samarium doped cerium phosphate (SCP) nanorods crystallized in a hydrated hexagonal structure transformed into an anhydrous hexagonal and a monoclinic structure on annealing at 400 °C and 800 °C, respectively. Though temperature did not affect the rod-like morphology of the SCP, the lattice strain changed from compressive to tensile. Monoclinic SCP exhibited excellent emission until 5% Sm3+ doping while the quenching effect dominated at 10% Sm3+. Monoclinic SCP samples demonstrated higher peroxidase-like enzymatic activity in comparison to natural enzyme HRP and hexagonal SCP. A mechanism for the enhanced peroxidase-like activity of the monoclinic structure was proposed based on the fluorescence property of terephthalic acid and the surface peroxo complex using Raman spectroscopy. Fluorimetric detection based on the luminescent quenching effect of the monoclinic SCP nanorods treated with different concentrations of hydrogen peroxide showed a linear response from 0 to150 μM concentration with a detection limit (LOD) of 3.17 μM H2O2. Our results demonstrate the importance of structure for enzyme mimetic activity.