S. Ismat Shah

Size dependence of thermal stability of TiO2 nanoparticles

Anatase TiO2 nanoparticles with average particle size ranging between 12 and 23 nm were synthesized by metalorganic chemical vapor deposition. The structure and particle size were determined by x-ray diffraction and transmission electron microscopy. The specific surface areas were measured by Brunauer-Emmett-Teller and ranged from 65 to 125m2∕g. The size effects on the stability of TiO2 in the air were studied by x-ray diffraction and transmission electron diffraction for isochronally annealed samples in the temperature range of 700–800 °C. Only anatase to rutile phase transformation occurred. With the decrease of initial particle size the onset transition temperature was decreased. An increased lattice compression of anatase with the raising of temperature was observed by the x-ray peak shifts. Larger distortions existed in samples with smaller particle size. The calculated activation energy for phase transformation decreased from 299 to 180 kJ∕mol with the decrease of initial anatase particle size from ...

Band gap tailoring of Nd3+-doped TiO2 nanoparticles

Undoped and Nd3+-doped TiO2 nanoparticles were synthesized by chemical vapor deposition in order to tailor the band gap of TiO2. The doping reduced the band gap. The band gap was measured by ultraviolet-visible light absorption experiments and by near-edge x-ray absorption fine structure. The maximum band gap reduction was 0.55 eV for 1.5 at. % Nd-doped TiO2 nanoparticles. Density functional theory calculations using the generalized gradient approximation with the linearized augmented plane wave method were used to interpret the band gap narrowing. The band gap narrowing was primarily attributed to the substitutional Nd3+ ions which introduced electron states into the band gap of TiO2 to form the new lowest unoccupied molecular orbital.

Metallorganic chemical vapor deposition and characterization of TiO2 nanoparticles

TiO2 nanoparticles were synthesized using the metallorganic chemical vapor deposition process. Particles with and without metal ion dopants were obtained. X-ray photoelectron and energy dispersive X-ray spectroscopic measurements confirmed the stoichiometry of the TiO 2 nanoparticles. X-ray diffraction patterns showed a polycrystalline anatase structure of TiO 2. Transmission electron microscopy revealed that these particles are of nanoscale dimensions. Exact particle size and size distribution analyses were carried out by dynamic light scattering. The average particle size was determined to be 22 nm. The nanosize particles provided large surface area for photocatalysis and a large number of free surface-charge carriers, which are crucial for the enhancement of photocatalytic activity. To improve the photocatalytic activity, metal ions, including transition metal ions (Pd 2+ , Pt 4+ , Fe 3+ ) and lanthanide ion (Nd 3+ ) were added to pure TiO2 nanoparticles. The effects of dopants on photocatalytic kinetics were investigated by the degradation of 2-chlorophenol under an ultraviolet light source. The results showed that the TiO 2 nanoparticles with the metal ion dopants have higher photocatalytic activity than undoped TiO 2. The Nd 3+ ion of these dopant metal ions showed the highest catalytic activity. The difference in the photocatalytic activity with different dopants is related to the different ionic radii of the dopants.

Enhanced solubility Ag-Cu nanoparticles and their thermal transport properties

Ag-Cu alloy nanoparticles were prepared by the inert gas condensation (IGC) process. X-ray diffraction (XRD) patterns show that particles were phase separated as pure Cu and Ag with some Cu incorporated in the Ag matrix. The particle size obtained either from Scherer’s formula or electron microscopy images shows no systematic change of the size of either pure Cu or Ag-Cu particles in the evaporation temperature range between 800 °C and 1400 °C. By using lattice constant values and Vegard’s law, the composition of Cu in Ag particles was calculated to be 6.6 vol pct. Analyses of the alloy nanoparticles suspended in hydrocarbon rotary pump oil were also carried out in order to determine the changes in thermal conductivity and viscosity of nanofluids. Thermal transport measurements have shown that there is a limit to the nanoparticle loading for the enhancement of the thermal conductivity. This maximum value was determined to be 0.006 vol pct Ag-Cu nanoparticles, which led to the enhancement of the thermal conductivity of the pump oil by 33 pct. Beyond this maximum loading, thermal conductivity decreased and reached back to the pure oil thermal conductivity value.

Role of vacancies in transport and magnetic properties of nickel ferrite thin films

Nickel ferrite thin films were synthesized by pulsed laser deposition. It was determined that the monotonic increase in saturation magnetization and the non-monotonic increase in electrical conductivity depend on the oxygen partial pressure during the growth of the thin films. A substantial reduction in magnetization was found which ranged between 0.4% and 40% of the bulk value as the oxygen partial pressure increased from 0.2 × 10−6 Torr to 500 mTorr during the deposition of the films. There was a three orders of magnitude increase in conductivity for the sample prepared under the most oxygen deficient environment (partial pressure of oxygen 0.2 × 10−6 Torr). These variations in saturation magnetization and conductivity are described within the framework of cation/oxygen vacancies in an inverse spinel nickel ferrite structure. The changes in the electronic structure due to the presence of the vacancies were investigated using x-ray photoelectron spectroscopy, which confirmed the formation of lower valent Ni for the samples prepared in an oxygen deficient atmosphere.

Effect of particle size on the magnetic properties of core-shell structured nanoparticles

Effect of particle size on exchange bias observed in Fe∕Fe oxide core/shell structured nanoparticles was investigated. Inert gas condensation was utilized for the synthesis of samples. Two sets of different particle size samples were prepared and the structural and magnetic properties were probed. It was found that the small particles show superparamagnetic behavior and exhibit high exchange bias field, 1574±25Oe at 5K, when field cooled in the presence of 2T magnetic field. Structural analyses of the particles in correlation with the magnetic measurements show that the smaller particle size (6nm Fe core, 1.5nm Fe oxide shell) favors amorphous oxide shell structure, and this in turn causes high magnetocrystalline anisotropy and enhanced exchange bias. Furthermore, we have also observed a vertical shift of the hysteresis loop related to the pinned spins at the ferromagnetic/antiferromagnetic (AFM) interface of the small particles. Decreased core size, high AFM anisotropy, and pinned spins observed from the...

The magnetic behavior of iron oxide passivated iron nanoparticles

Iron oxide passivated iron nanoparticles were synthesized in an inert gas condensation system. The nanoparticles were single domain but not superparamagnetic. Varying amounts of oxide passivation of the nanoparticles were achieved. The oxide-passivated particles exhibit an exchange bias when cooled below a blocking temperature characteristic of the thickness of the oxide layer present. It was found that the exchange bias and blocking temperature both increase with oxide thickness with the blocking temperature in all cases being much lower than the Neel temperature for Fe oxides. We find that the oxide shell-core spin interaction leaves its imprint on the low field dc and ac magnetizations as well. Furthermore, below a characteristic freezing temperature a sharp increase in the field cooled magnetic moment of the samples is evidenced and is suggestive of a spin freezing process at the surface of the particles.

Electrical and optical properties of point defects in ZnO thin films

We show that the deposition of ZnO films under varying oxygen partial pressure and annealing conditions allows for the controllable formation of specific defects. Using x-ray diffraction and photoluminescence, we characterize the defects formed and show that these defects are responsible for changes in film carrier density, carrier type, sheet resistivity and mobility.

Visible light photocatalysis with nitrogen-doped titanium dioxide nanoparticles prepared by plasma assisted chemical vapor deposition

Nitrogen-doped TiO2 nanoparticles were synthesized via plasma assisted metal organic chemical vapor deposition. Nitrogen dopant concentration was varied from 0to1.61at.%. The effect of nitrogen ion doping on visible light photocatalysis has been investigated. Samples were analyzed by various analytical techniques such as x-ray diffraction, transmission electron microscopy, x-ray photoelectron spectroscopy, and near-edge x-ray absorption fine structure. Titanium tetraisopropoxide was used as the titanium precursor, while rf-plasma-decomposed ammonia was used as the source for nitrogen doping. The N-doped TiO2 nanoparticles were deposited on stainless steel mesh under a flow of Ar and O2 gases at 600°C in a tube reactor. The photocatalytic activity of the prepared N-doped TiO2 samples was tested by the degradation of 2-chlorophenol (2-CP) in an aqueous solution using a visible lamp equipped with an UV filter. The efficiency of photocatalytic oxidation of 2-CP was measured using high performance liquid chrom...

Studies of the growth parameters for silver nanoparticle synthesis by inert gas condensation

Silver nanoparticles were synthesized by an inert gas condensation method using flowing helium in the process chamber. Nucleation, growth mechanism, and the kinetics of nanoparticle formation in vapor phase are studied. Effect of process parameters, such as evaporation temperature and inert gas pressure, on the particle crystallinity, morphology, and size distribution are examined. Particles were synthesized at evaporation temperatures of 1123, 1273, and 1423 K and at helium pressures of 0.5, 1, 5, 50, and 100 Torr. Synthesized silver nanoparticles were characterized by x-ray diffraction (XRD) and transmission electron microscopy (TEM). The particle size ranged from 9 to 32 nm, depending on the growth conditions. At lower evaporation temperature and inert gas pressure, smaller particles with spherical shape showing less agglomeration are formed. Based on the experimental results and theoretical model of surface free energy and undercooling as a function of evaporation temperature and inert gas pressure, particle formation is analyzed. A simple operating map for nanoparticle synthesis is presented. The theoretical model is well supported by the experimental data.