Pawan Chetri

Oxygen defects and formation of Ce3+ affecting the photocatalytic performance of CeO2 nanoparticles

Here we report the photocatalytic activity of CeO2 nanoparticles. This is carried out with methyl orange as the reference pollutant. Annealing of ceria under vacuum generates oxygen deficient CeO2 nanoparticles with defects such as oxygen vacancies and formation of Ce3+. This is evident from the characterization results of X-ray diffraction, Raman spectroscopy, N2 adsorption–desorption and X-ray photoelectron spectroscopy. The band gap is red shifted due to the creation of intermediate energy states of Ce3+ and oxygen vacancies in the band gap. The reduced photoluminescence (PL) intensity of defective ceria indicates that the electron–hole separation is substantially enhanced by the surface trap centers. Air annealed ceria not only has relatively low surface area but also has fewer surface defects. Thus, it is expected to display less photocatalytic activity. Vacuum annealed CeO2 indeed displays better photocatalytic activity in the degradation of methyl orange under UV and visible light as compared to the air annealed samples.

Structural and optical properties of Cu doped SnO2 nanoparticles: An experimental and density functional study

The paper investigates, both theoretically and experimentally, the structural and optical changes in SnO2 system brought about by introduction of Cu in a SnO2 system. On the experimental front, a cost effective sol-gel technique is used to prepare hexagonal shaped Cu doped SnO2 nanoparticles. The prepared pristine SnO2 nanoparticle is found to be of random shape by transmission electron microscope (TEM) studies. A structural and morphological study is carried out using X-ray diffraction and TEM techniques. The different phonon interaction in the system is observed by Raman spectroscopy while electron paramagnetic resonance and UV-Visible spectroscopy confirms the presence of Cu in 2+ state. First principle calculations have been performed using “density functional theory”-based MedeA Vienna Ab Initio Simulation package on a SnO2 system where Cu is introduced. The introduction of Cu in the SnO2 system brings distortion which is corroborated by the variation in the corresponding bond lengths. The Density of State calculation of Sn16O32 and CuSn15O32 is also performed. Finally, a correlation is established between the experiment and the theory.

Room temperature ferromagnetism in SnO2 nanoparticles: an experimental and density functional study

Room temperature ferromagnetism is observed in pristine SnO2 nanoparticles prepared via sol–gel method. Microstructural studies with XRD and Raman reveal the coexistence of SnO and SnO2 phases in vacuum annealed SnO2. Electron paramagnetic resonance (EPR) results confirm the presence of paramagnetic trapped hole in lattice oxygen atoms and superoxide radical species in air and vacuum annealed SnO2. It is observed that growth conditions have significant influence on the magnetism of the nanoparticle. Air and vacuum annealed SnO2 display clear ferromagnetic behavior at room temperature. However, the saturation magnetization decreases in vacuum annealed SnO2. We have compared the experimental results with ab initio density functional calculations and observed that the Sn vacancies are the main reason of magnetism in SnO2. The pattern of theoretical formation energy values shows that it is easier to form Sn vacancies in SnO2 under O rich limit (similar to air annealing). This is in conformity with the experimentally determined higher saturation magnetization values in air annealed SnO2. The spin polarization of lattice oxygen atoms by the presence of Sn vacancy gives rise to magnetism.

Annealing temperature and oxygen-vacancy-dependent variation of lattice strain, band gap and luminescence properties of CeO2 nanoparticles

CeO2 nanoparticles are annealed in vacuum at 200°C and in air at 200°C, 600°C and 1000°C, respectively. Vacuum-annealed CeO2 contains high concentration of oxygen vacancies and exhibits very high lattice strain, whereas the corresponding values decrease on air annealing. Oxygen-deficient CeO2 has redshift in band gap with high Urbach energy. The magnitude of this energy decreases as CeO2 is annealed in air at 600°C. At 1000°C, thermal disorder increases the Urbach energy. Photoluminescence property of the samples depends on the presence of radiative and non-radiative oxygen vacancy centres. Vacuum-annealed ceria have large numbers of non-radiative oxygen vacancies that act as emission quencher. CeO2 annealed at 600°C contains requisite amount of oxygen vacancies to show better luminescence property.

Enhanced visible light photocatalytic activity of Gadolinium doped nanocrystalline titania: An experimental and theoretical study.

HYPOTHESIS Undoped TiO2 nanoparticles are considered as a poor photocatalytic candidate in visible light due to the wide band gap. Incorporation of Gd ions is expected to modulate the electronic structure of the material and thereby enhance the photocatalytic properties of the material. EXPERIMENTS Gadolinium doped TiO2 nanoparticles were fabricated via a simple sol-gel method. FINDINGS The surface area of Gd doped TiO2 (225m(2)/g) nanoparticles is much higher than that of undoped TiO2 (95m(2)/g). Doping of Gadolinium enhances the visible light absorption property of TiO2 nanoparticles. Photoluminescence intensity increases at 0.03 and 0.05mol and thereafter reduces at 0.07mol. The photocatalytic efficiency of these nanoparticles is evaluated by observing degradation of phenol in aqueous solution under visible light. The doped nanoparticles are found to exhibit better photocatalytic activity. This enhancement has been attributed to the introduction of the Gd 4f energy levels in the band gap of TiO2. The presence of these states has been further confirmed by theoretical study based on density functional theory (DFT). It is speculated that the 4f states of Gd act as efficient electron trap centers. These 4f states facilitate electron migration to the surface making available free carriers to take part in photocatalysis.

Daylight photocatalytic activity of TiO2/SnO2 core/shell nanostructures: An experimental and density functional study

TiO2/SnO2 core/shell nanostructures is prepared via a simple sol-gel process and compared with bare TiO2 nanoparticles. We carried out XRD, TEM and UV-Visible characterization for evaluating structural and optical properties. A better and promising day light photocatalytic activity is observed for TiO2/SnO2 in comparison to TiO2 in the degradation of methyl orange (MO). We have also done DFT calculation based VASP 5.2 to calculate Density of States of both the system. Finally, a correlation is established between theory and experiment.