Sayan Bayan

Narrowing of band gap and effective charge carrier separation in oxygen deficient TiO2 nanotubes with improved visible light photocatalytic activity.

Oxygen vacancies are introduced into hydrothermally processed TiO2 nanotube by vacuum calcination. Formation of oxygen vacancies modifies the local coordination in TiO2 as evident from Raman spectroscopy and secondary ion mass spectrometry (SIMS) results. The surface area is increased from 172.5m(2)/g in pure to 405.1m(2)/g in defective TiO2 nanotube. The mid-band gap electronic states created by oxygen vacancies are mostly responsible for the effective narrowing of band gap. Charge carrier separation is sufficiently prolonged as the charged oxygen defect states inhibit facile carrier recombination. With high surface area, narrowed band gap and separated charge carriers defective TiO2 nanotube is a suitable candidate in the photodegradation of methylene blue (MB) and phenol under visible light illumination. Photosensitized electron transfer from MB to the conduction band of TiO2 and the photodegradation of MB is facilitated in presence of high density of oxygen vacancies. Unlike MB, phenol absorbs in the UV region and does not easily excited under visible light. Phenol shows activity under visible light by forming charge transfer complex with TiO2. Defect trapped carriers become available at the phenol-TiO2 interface and finally interact with phenol molecule and degrade it.

Role of cohesive energy on the interparticle coalescence behavior of dispersed nanoparticles subjected to energetic ion irradiation

The present work reports on the conditions of nanoparticle growth and splitting under energetic ion irradiation. Cohesive energy that determines the thermal stability of a given nanoparticle system was calculated by extending surface area difference (SAD) and liquid drop model (LDM). Based on the size-dependent cohesive energy calculations, the interparticle coalescence mechanism is discussed for a ZnS-based nanoparticle system with special reference to a variety of matrices. The interparticle separation is found to play key role in particle–particle coalescence leading to nanoparticle growth or partial evaporation that results in splitting.

Enhanced vacuum-photoconductivity of chemically synthesized ZnO nanostructures

We report on the enhanced ultraviolet (UV) photoconductivity of zinc oxide (ZnO) nanostructures in vacuum. Nanoparticles and nanorods of ZnO were fabricated using a simple cost-effective solid state grinding method. Morphology of the nanostructures was studied using transmission electron microscopy, while the optical properties were investigated using UV–visible absorption and photoluminescence spectroscopy. The emission spectra of the nanostructures revealed the existence of various native defect states of ZnO and also indicated the presence of surface adsorbed water molecules. In the photoconductivity measurements, although the ZnO nanoparticles exhibited lower photoconductivity in comparison to the nanorods, a similar trend of photoresponse was observed for both the cases. An initial decrease in the photoconductivity followed by a large enhancement was observed in vacuum compared to that in ambient condition. Such unusually increased photoconductivity has been correlated to the desorption of physisorbed water molecules from nanostructure surfaces under vacuum. This desorption is responsible for the rise in dark current and an initial decrease in photoconductivity. Continual UV irradiation in vacuum leads to the desorption of chemisorbed water molecules from the defect sites of the nanostructures, resulting in the occurrence of high photoconductivity.

A comprehensive secondary ion mass spectrometry analysis of ZnO nanowalls: Correlation to photocatalytic responses

We report on the visible light induced photocatalytic responses of zinc oxide (ZnO) nanostructures in the form of nanowires and nanowalls grown on aluminum substrates. Morphological and microstructural characteristics of these nanostructures have been analyzed using scanning electron microscopy (SEM) and high resolution electron microscopy (HRTEM). The presence of surface-adsorbed H+, O2−, and OH− species on ZnO nanostructures has been established through secondary ion mass spectrometry (SIMS). The relative change in substrate coverage under varying reaction time has also been evidenced through SIMS and is in agreement with SEM observation. Compared to nanowires, oxygen adsorption on ZnO surfaces and subsequent oxygen in-diffusion are found to be prominent for the nanowall-like structures and are seen to be highest for nanowalls grown in lower reaction time. In contrast to nanowires, nanowalls are found to exhibit higher photocatalytic activity and this can be attributed to higher adsorption of oxygen. Th...

Modified photoluminescence and photodetection characteristics of chemically grown SnO coated ZnO nanoneedles

The authors report on the synthesis of tin oxide (SnO) coated zinc oxide (ZnO) needlelike nanostructures and their modified light emission and detection features. The formation of SnO phase on ZnO surface has been revealed from energy dispersive x-ray analysis and secondary ion mass spectrometry studies. The luminescence response of the SnO-coated ZnO nanoneedles gets lowered compared to that of bare ZnO and is assigned to the lowering of radiative emission due to the occurrence of charge-carrier separation. Again in the heterostructured system, due to SnO led surface passivation, the band-edge emission becomes prominent and defect-related emission gets lowered. The photoconductivity response is found to be significantly enhanced for the SnO-ZnO heterostructured material formed with lower Sn:Zn molar ratio. The enhancement of photocurrent has been understood in the light of carrier separation and carrier multiplication processes occurring at the SnO-ZnO heterojunctions.

Crystallographic, luminescence and photoconductive characteristics of chemically tailored ZnO nanorods

The optoelectronic properties of zinc oxide (ZnO) nanorods synthesized using two different chemical methods have been explored in the light of microstructural features. The presence/absence of band edge emission in the luminescence spectra of the nanorods is found to be governed by the crystallographic properties. Moreover, we observed a pronounced effect of variation in crystallite size on the UV photoconductivity of the nanorods. Understanding the influence of microstructural aspects on the optical and electronic properties of the nanostructures may help in the fabrication of prototype, miniaturized optoelectronic devices.