V. Biju

DC conductivity of consolidated nanoparticles of NiO

Abstract DC conductivity of consolidated nanoparticles of NiO, having different average particle sizes (2.5nm-17nm) was measured in the temperature range 313 K to 423 K. The conductivity of NiO nanoparticles are found to be enhanced by six to eight orders of magnitude over that of NiO single crystals. This large enhancement in conductivity is attributed to the high density of Ni 2+ vacancy in the nanoparticles. Calculation of activation energy, leads to the conclusion that the most prominent conductivity mechanism over the temperature range of observation is the large polaron conduction associated with holes in the 2p band of O 2- . The decrease in activation energy for the samples in comparison with that of the bulk material is explained on the basis of the enormously defective nature of nanoparticles. In the last part of the discussion, the effect of interfacial region on the electrical conductivity of the samples is analyzed, by taking into account the contributions due to grain boundaries and triple junctions. A semi-quantitative explanation for the observed variation of conductivity with particle size is presented on the basis of the assumption that the role of the triple junctions is to reduce the conductivity. The importance of triple junctions in determining the transport properties of nanoparticles is emphasized.

Effect of finite size on the magnetization behavior of nanostructured nickel oxide

Nanostructured nickel oxide samples having different average particle sizes are synthesized through a wet chemical route. Room temperature magnetic hysteresis of the samples are recorded using a vibrating sample magnetometer. The magnetic properties of the samples are found to be markedly different from those of single crystalline nickel oxide. The sample with an average particle size of 2-3 nm showed superparamagnetism with magnetization curves defined by the Langevin function. Anomalously large uncompensated magnetic moment associated with this sample is attributed to the multisublattice magnetic structure. Interestingly, samples with larger average particle sizes of 13 and 18 nm exhibited superantiferromagnetism with the magnetization curves varying linearly with applied field and susceptibility values larger than that of bulk nickel oxide. The results highlight the importance of surface atoms and surface driven spin rearrangements in determining the magnetic properties of nanostructured nickel oxide.

Fourier transform infrared spectroscopy study of nanostructured nickel oxide

Nanostructured nickel oxide having different average particle sizes ranging from 3 to 16 nm were synthesized and Fourier transform infrared (FTIR) spectra of the samples were recorded in the far infrared (IR) region. The spectra were found to be dominated by surface mode absorptions with no distinct absorption corresponding to the bulk transverse optical mode. IR absorption coefficient, alpha, for the nanostructured NiO samples were calculated as a function of frequency using a macroscopic approach devised by Fuchs. The effects of crystalline geometry, numerical values of optical constants, filling factor and increased damping on the spectral features of the samples were analyzed. Though the simulations approximately reproduced the occurrence of a shoulder in the experimental spectra, the most intense peak in the simulated spectra was found to be about 50 cm(-1) above the corresponding experimentally observed peak. It was shown that the experimentally observed absorption maximum of all the samples were in close agreement with that determined using a microscopic theory based on the rigid ion model. The weak absorption peaks in the frequency region 60-100 cm(-1) appearing in the spectra of all the samples were identified as surface induced transverse acoustical modes, omegaTA, which became IR active due to the breakdown of translational symmetry in the nanocrystallites.

Electronic Structure of Nanostructured Nickel Oxide Using Ni 2p XPS Analysis

We examine the electronic structure of nanostructured nickel oxide with an average particle size of 4–5 nm using Ni 2p X-ray photoelectron spectrum. The most striking features of the spectrum are the Ni 2p main line broadening and an increase in the relative intensity of the ∼ 1.5 eV satellite. We explain the observations as due to an enhancement in the non-local screening process on the basis of Ni7O36 cluster model by taking into account the large surface area to volume ratio and high Ni2+ vacancy concentration in nanostructured nickel oxide. The important contribution of an enhanced Ni 3d–O 2p hybridization and the origin of shake-up peaks above the ∼ 7 eV satellite is also discussed. The study underlines the importance of factors such as the actual local environment of the core hole site, defect density and distribution, possible structural transitions, etc. in determining the Ni 2p core level X-ray photoelectron spectrum of nanostructured nickel oxide and shows that the actual synthesis routes and the thermal history greatly influence the electronic structure of nanostructured nickel oxide.

AC conductivity of nanostructured nickel oxide

AC conductivity of consolidated nanoparticles of NiO, having different average particle sizes (2.5 nm–17 nm) was measured in the temperature range 313 K to 413 K and over the frequency range 50 Hz to 3 MHz. The ac conductivity of the samples was found to be enhanced by six to eight orders of magnitude over that of NiO single crystals reported in the literature. This large enhancement in conductivity is attributed to the high density of Ni2+ vacancies in the nanoparticles. The variation of ac conductivity with frequency of the applied signal and temperature is discussed on the basis of the Correlated Barrier Hopping (CBH) Model. The observed spread in activation energy and slope of the log σac Vs log ω plots is attributed to the distribution of the charge carrier hopping distance and localization energies in the nanoparticle samples. The effect of the interfacial region on the electrical conductivity of the samples is analyzed by taking into account the contributions due to grain boundaries and triple junctions. The observed variation of measured ac conductivity with average particle size is semi-quantitatively explained based on the reasoning that the role of the triple junctions is to reduce the conductivity.

Influence of soil moisture content on surface albedo and soil thermal parameters at a tropical station

Half hourly data of soil moisture content, soil temperature, solar irradiance, and reflectance are measured during April 2010 to March 2011 at a tropical station, viz., Astronomical Observatory, Thiruvananthapuram, Kerala, India (76°59’E longitude and 8°29’N latitude). The monthly, seasonal and seasonal mean diurnal variation of soil moisture content is analyzed in detail and is correlated with the rainfall measured at the same site during the period of study. The large variability in the soil moisture content is attributed to the rainfall during all the seasons and also to the evaporation/movement of water to deeper layers. The relationship of surface albedo on soil moisture content on different time scales are studied and the influence of solar elevation angle and cloud cover are also investigated. Surface albedo is found to fall exponentially with increase in soil moisture content. Soil thermal diffusivity and soil thermal conductivity are also estimated from the subsoil temperature profile. Log normal dependence of thermal diffusivity and power law dependence of thermal conductivity on soil moisture content are confirmed.

Effect Of Finite Size On One‐magnon And Two‐magnon Excitations In Nanocrystalline Nickel Oxide Studied Using Infrared And Raman Spectroscopy

One‐magnon and two‐magnon excitations in superparamagnetic nanocrystalline nickel oxide (NiO) with an average crystallite size of ∼2–3 nm is studied using infrared and raman spectroscopy. Nanocrystalline NiO with an average crystallite size of ∼2–3 nm was synthesized through a two‐step process. An infrared transmission spectrum of the sample was recorded in the range 15–60 cm−1. Over and above the one‐magnon excitation at 34 cm−1 the spectra shows a strong low frequency absorption peak at ∼23 cm−1 which is identified as a surface magnon excitation. Room temperature Raman spectra of the sample shows that the two‐magnon excitation is shifted by about 50 cm−1 towards the lower wavenumber side. The observations are explained in terms of the large surface to volume ratio of the samples together with the varied spin arrangements at the surface.