ABSTRACT
We report the structure of defect and the oxygen vacancy-induced optical phonon confinement in phase pure tetragonal rutile crystal structured ultrananocrystalline BixSn1-xO2 (x = 0, 0.03, 0.05, 0.08) with high surface area synthesized by sonochemical method. As the Bi ion incorporates into the SnO2 host lattice, it replaces the Sn ions marked by the lattice expansion, which leads to the formation of oxygen vacancies so as to maintain charge neutrality. The grain size reduces from 6 nm to 3 nm with increase in Bi content from 0% to 8%. The size effect and the increased oxygen vacancy concentration were found to induce phonon confinement within the grain. This has led to interesting changes in the vibrational spectra of the ultrananocrystalline BixSn1-xO2 as the size reduces below 9 nm. Absence of periodicity beyond this critical particle size relaxes the zone-centre optical phonon selection rule, causing the Raman spectrum to have contributions also from phonons away from the Brillouin-zone centre. The structure of defects, such as the in-plane, bridging and sub-bridging oxygen vacancies present, was confirmed using Raman spectroscopic analysis. The reason for enhancement in photoluminescence behaviour with increased Bi content is discussed. The energy band gap was found to be wider (â¼4 eV) compared to the bulk and reveals an increasing trend as a function of Bi%. The increase in band gap with decrease in particle size marks the quantum confinement effect. The variation of band gap upon doping is due to the BM shift effect, which arises as a result of the increase in carrier concentration.
ABSTRACT
High quality nanocrystalline pristine and Cu-doped SnO2 hollow nanofibers were successfully prepared through simple and effective electrospinning technique. Nanofibers calcined at 600 °C for 3 h were characterized with different analytical techniques such as X-ray diffraction (XRD), Transmission electron Microscope (TEM) and Vibrating sample magnetometer (VSM). Observed TEM images and XRD patterns were corroborate to the formation of tetragonal crystalline SnO2 hollow nanofibers with rutile phase. Excellent optical behaviour was observed for Cu-doped SnO2. Highly intense near band edge emission at 3.58 eV for Cu-doped SnO2 evidences the free exciton decay process in the hollow nanofibers. For the first time we have reported here the near band edge PL emission in Cu-doped SnO2 tubular hollow nanostructure. This study substantiates that material potential for UV-lasing application. In addition to the above, magnetic measurement ascribes that Cu-doped SnO2 exhibit the intrinsic room temperature ferromagnetism within the low field strength. The occurrence of ferromagnetism in Cu-doped SnO2 is directly related to the p-d ferromagnetic exchange coupling between the local magnetic moment of Cu2+ and the polarized valence electrons of surrounding oxygen. Over all this study provides the primary information about tunable multifunctionality of SnO2 hollow nanostructures by adding the non-magnetic Cu ions.
ABSTRACT
Electrospinning was employed to synthesize tubular TiO2 nanofibers. The as-spun fibers were subjected to heat treatment at 800 degrees C for 1 h in the air. By controlling the polymer concentration, pores measuring 30-60 nm were formed on the side walls of the tubular nanofibers. During annealing, the average nanofiber diameter shrank from 150 nm to 120 nm. The structural properties were characterized by XRD, Raman and FTIR spectroscopy. Further porous and tubular structures were confirmed by SEM and HRTEM. The specific surface area of porous tubular nanofibers (PTNFs) was measured using the Brunauer-Emmett-Teller (BET) method, which revealed a high surface area of 63 m2 g(-1). Photodegradation of methyl orange demonstrated that the PTNFs have higher photocatalytic activity than nonporous nanofibers. This enhanced photocatalytic activity can be attributed to the high surface area of the porous and tubular structures.
ABSTRACT
The hollow nanofibers of Mn-doped SnO2 were fabricated by electrospinning method. The structural and magnetic properties of the electrospun fibers calcined at 600 degrees C were studied. X-ray diffraction patterns of the nanofibers showed broad diffraction peaks and were indexed to the characteristic diffraction pattern of tetragonal SnO2. The hollow fiber micro-structure of Mn-doped and pure SnO2 were confirmed from the observed HRSEM and TEM analysis. Typical diameter of the hollow nanofibers was found to be around 150 nm. Strong emission peak in the visible region of the PL spectra characteristic of the optical activity of the SnO2 is obtained. Surface composition of the nanofiber and successful incorporation of Mn into SnO2 were confirmed from intense peaks recorded in the XPS spectra. Finally, a reasonable ferromagnetic transition observed at 10 K in the Mn-doped SnO2, substantiates that the presence of undetectable Sn-Mn solid solution or the formation of Mn based oxide secondary phases. It concludes that the induced ferromagnetism is only due to the precipitated impurity phases and does not arise from any intrinsic pure SnO2 or the dopant.
