No abstract

Root like structured Ni-doped zinc oxide [Zn(1-x)NixO (x = 0.09)] thin films were deposited on a non-conducting glass substrate by indigenously developed spray pyrolysis system at optimized substrate hotness of 573±5 K. Thus obtained Ni-doped ZnO thin films were characterized by UV-visible spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Atomic Force Microscopy (AFM). XRD result revealed that Ni-doped ZnO has a polycrystalline nature with a hexagonal wurtzite structure. For pure ZnO and Ni-doped ZnO thin films, the particle sizes were 60.9 and 53.3 nm while lattice strain values were 1.56×10−3 and 1.14×10−3, respectively. The film surface showed characteristic root-like structure as observed by the SEM. It was observed that the Ni-doped ZnO thin films were grown in high density along with more extent of branching as compared to pure ZnO thin films but retained the root-like morphologies, however, the branches were more-thinner and of shorter lengths. AFM analysis showed that the surface grains of the Ni-doped samples are homogeneous with less RMS roughness values compared with the undoped ZnO samples. The photocatalytic activity of the prepared thin films was evaluated by the degradation of methyl orange (MO) dye under UV light irradiation. Pure ZnO and Ni-doped ZnO thin films took 150 min and 100 min to degrade about 60% MO dye, respectively.

The falling temperature of the photosphere with height is responsible for the effect known as limb darkening. The Sun is not equally bright all over the disc. When we observe the Sun towards the limbs, it appears to get darker. Light from the photosphere travels through an absorptive medium. Therefore, one can see only so far into the photosphere. This is the optical depth (Fig. 1). The line of sight at the center sees deeper into the Sun to an optical depth of about 2/3 into the photosphere. In other words, photons that escape from a smaller radius of the photosphere have originated in a hotter region; therefore, they will have a higher intensity. Photons that originate from a larger radius come from a cooler part of the photosphere; therefore, this results in lower intensity. This effect is known as solar limb darkening. In addition, the solar photosphere displays various phenomena that can easily be observed in images obtained with small telescopes, such as sunspots, faculae, and granulation. Limb darkening results from the fact that we are looking into hot gas when we look at the Sun and, as a consequence of this, the brightness of the Sun decreases as one looks from the center of the disc (where we see deeper inside the Sun) towards the limb (where we will not see as deeply into the solar atmosphere because we are looking at a slant through the photospheric material).