TiO 2 doped with Cu 2+ initiates the formation of brookite phase along with anatase. Doping of Cu 2+ introduces structural defects into TiO 2 . The direct evidence is the low intense and broad diffraction peaks. Raman peaks of doped TiO 2 are also broad and are blueshifted. Pure TiO 2 exhibits an absorption in the UV region, the position of which is shifted towards the visible region on incorporation of Cu into it. The visible absorption peaks arise due to the
Here we report the photocatalytic activity of CeO 2 nanoparticles. This is carried out with methyl orange as the reference pollutant. Annealing of ceria under vacuum generates oxygen deficient CeO 2 nanoparticles with defects such as oxygen vacancies and formation of Ce 3+ . This is evident from the characterization results of X-ray diffraction, Raman spectroscopy, N 2 adsorption-desorption and X-ray photoelectron spectroscopy. The band gap is red shifted due to the creation of intermediate energy states of Ce 3+ 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 CeO 2 indeed displays better photocatalytic activity in the degradation of methyl orange under UV and visible light as compared to the air annealed samples.
Defects play a pivotal role in the device performance of a photocatalytic, light-emitting, or photovoltaic system. Herein, graphitic carbon nitride (g-C 3 N 4 ) nanosheets are prepared at different calcination temperatures, and the evolution of defects in the system is studied by positron annihilation spectroscopy (PAS) and photoluminescence (PL) spectroscopy. Steady-state PL spectra show that free and defect-bound excitonic emission peaked at 2.78, 2.58, and 2.38 eV are dominant with above-band-gap excitation. Timeresolved PL studies reveal a significant enhancement of excitonic lifetime from 17.4 ns for free exciton to 27.4 ns in case of defect-bound exciton. We provide a direct correlation between the defects observed by PAS and those of the excitonic lifetime found from PL studies. Below-band-gap excitation activates defect emission, and it is characterized by a short carrier lifetime (∼0.14 ns). An excitation power-dependent PL study with 405 nm laser shows a progressive red shift and narrowing of the emission line. We have interpreted the different PL features with defect band filling of exciton, interplanar, intraplanar, interchain exciton migration, etc. These results are significant for tuning the optoelectronic properties of g-C 3 N 4 nanosheets and exploiting their applications in various emerging areas.
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