ABSTRACT
This article addresses the synthesis of Fe3+doped TiO2nanoparticles with variations of molar concentrations of Fe3+and their adequate use as potential photocatalysts for Photocatalysis applications. Synthesized photocatalysts were characterized thoroughly by different analytical techniques in terms of morphological, chemical, structural, crystalline, optical, electronic structure, surface area etc properties. The occurrence of red shift phenomenon of the energy band gap attributes to the transfer of charges and transition between the d electrons of dopant and conduction band (CB) or valence band (VB) of TiO2. The doping of Fe3+ions generates more trap sites for charge carriers with the surface trap sites. Thorough experimental conclusions revealed that the Fe3+ions necessarily regulate the catalytic property of TiO2nanomaterial. The obtained total degradation efficiency rate of Methylene Blue (MB) was 93.3% in the presence of 0.1 M Fe3+in the host material and for Malachite Green Oxalate the efficiency was 100% in the presence of 0.05 M and 0.1 M Fe3+in the host material. In both the cases the total visible light irradiation time was 90 min. The adsorption properties of the photocatalysts have been also performed in a dark for 90 min in the presence of MB dye. However, till now there are hardly reported photocatalysts which shows complete degradation of these toxic organic dyes by visible light driven photocatalysis. of potential values of valence and conduction band shows the production of active oxidizing species for hydrogen yield and the possible mechanism of the Schottky barrier has been proposed. A schematic diagram of visible light driven Photocatalysis has been pictured showing degradation activity of Fe3+-TiO2catalysts sample.
ABSTRACT
Pure ZnO and a group of Cu2+-ion-doped (4, 6, and 8 wt%) ZnO nanomaterials are synthesized using the co-precipitation technique. X-ray diffraction and Fourier transform infrared spectroscopy confirm both the substitution of Zn2+ ions by Cu2+ ions in the ZnO lattice and formation of the ZnO/CuO composite. The divalent oxidation state of Cu is confirmed using X-ray photoelectron spectroscopy. A suppression in the oxygen vacancy density is observed up to a doping level of 6 wt%, but beyond that it increases. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show a cross-linked nanoflower-like structure. The presence of a separate CuO phase is also confirmed via TEM. Absorption spectroscopy yields a reduction in the bandgap up to 6 wt%, after which it is increased for 8 wt%. An enhanced plasmon band in the spectra reveals the presence of CuO. The photoluminescence is quenched for doping up to 6 wt%, and with further doping the emission is enhanced. These observations are explained by the doping-concentration-dependent Förster resonance energy transfer (FRET) phenomenon between the ZnO (donor) and the CuO (acceptor). For the highest doping concentration, the emission profile shows a sudden enhancement resulting from the simultaneous competition of two FRET mechanisms (the intra-acceptor mechanism and the inter-donor-acceptor mechanism). By contrast, for other doped nanomaterials, the inter-donor-acceptor FRET mechanism with doping-concentration dependence is able to explain the suppression of the emission intensity. All doped nanomaterials show an improved visible-light-driven photocatalytic efficiency compared with pure ZnO for methylene blue, which results from the synergistic effects of a reduction in the concentration of bulk defects, enhanced charge separation, and FRET. The highest photocatalytic performance is demonstrated by the 6 wt% nanomaterial due to its optimum doping concentration. However, beyond this concentration, the formation of excessive CuO on the surface of ZnO increases the concentration of bulk defects, and the simultaneous occurrence of the inter-donor-acceptor FRET and intra-acceptor FRET mechanisms takes place leading to the rapid recombination of electron-hole pairs and reduced photocatalytic activity.
ABSTRACT
The effects of surface modification on the defect state densities, optical properties, and photocatalytic and quantum efficiencies of zinc oxide (ZnO) nanoplates were studied in this work. The aim of this study is to identify the photophysical processes that dictate the quenching of emission from defect states upon surface modification and the role of different defects such as zinc interstitials (Zni) or oxygen vacancies (VO) beside the photophysical processes in determining the photocatalytic efficiency of plate-like ZnO nanostructures. For controlling the intrinsic defect state densities of ZnO nanoplates, which is difficult to achieve, their surface was modified using different polymers such as PMMA and PVA. X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) emission spectroscopy were employed to identify and quantify the defect states. The analysis of relative defect state densities of Zni or VO showed that Zni significantly impacts the photocatalytic activity (PCA) besides VO, but it has a lower influence than VO because of the difference in the accessibility and intrinsic nature of these two defects. Synchronous quenching of emission from different defect states with different formation energies and its correlation with the photocatalytic activity led us to conclude that photophysical processes such as concentration-dependent Förster resonance energy transfer (FRET), charge transfer (CT) and Zni defects play a significant role behind PCA, which has been previously reported to be influenced by VO only. FRET and CT also play a critical role behind emission quenching upon surface modification. Upon the surface modification of nanoplates, a drop in the quantum efficiency from 12.14% to 4.44% was observed with the fine-tuning of emission colour from bluish-white to blue. Besides the defect states, FRET and CT phenomena are dominant in reducing the quantum efficiency of hybrid light-emitting diodes (HyLEDs) and photocatalytic efficiency. Therefore, the work outlines the reason behind the suppression of luminescence and photocatalytic efficiency of ZnO nanoparticles after surface modification and how to optimise them for their applications as an emissive layer in HyLEDs and efficient photocatalysts.
ABSTRACT
This study has been carried out to understand the mechanism of charge carrier dynamics and the existence of exciton-dopant energy transfer within Mn-doped ZnS nanomaterials. Improvement in the energy transfer efficiency and electroluminescence properties of these nanomaterials has been investigated for using them as an emissive layer of LEDs. A chemical co-precipitation method has been used to synthesize ZnS with varying Mn contents to achieve enhanced luminescence properties demonstrating the effect of Mn doping on excitonic luminescence intensity. X-ray powder diffraction analysis reveals the prepared materials to be cubic crystallites with size varying between 2 nm and 4 nm. Agglomerated clusters and a nanogranular morphology have been observed in SEM analysis. The UV-Vis spectra reveal that the band gaps slightly decrease with an increase in the Mn content in ZnS samples. The photoluminescence spectra show that upon Mn incorporation, the intensity of blue emission at 420 nm increases due to the surface states in ZnS; an orange emission at 588 nm is also observed due to a transition within Mn2+. The energy transfer efficiency of 3 to 6% was measured theoretically by using the FRET (Förster resonance energy transfer) model. Mn-doped ZnS shows high photoluminescence quantum yield (QY) in comparison with ZnS, where 0.04 mol% Mn-doped ZnS achieved the highest QY of about 28.94%. The CIE chromaticity coordinates accordingly shift toward white from the blue region upon Mn substitution. A kinetic model has been used to determine the energy transfer efficiency, which affects the luminescence properties of ZnS. The FRET model has suitably unraveled the Förster radius and interatomic distance, which make the energy transfer possible in these materials. We have found 0.04 mol% Mn-doped ZnS with an enhanced energy transfer efficiency. The maximum external quantum efficiency of about 0.643% can be achieved at 2.9 volts of operating voltage for this nanomaterial. These highly luminescent materials possess the characteristics of an emissive layer to be used for light-emitting applications.
ABSTRACT
The role of quantum mechanical phenomena such as polaron-exciton quenching interaction and concentration-dependent FRET in determining the luminescence efficiency of PAni-PMMA polymer blends has been investigated. PAni samples prepared in different environments using different acids and bases show different absorbance and emission profiles indicating a direct relation between generated polarons in PAni by acid-base doping-dedoping and photoluminescence spectra of PAni. The observed low luminescence in PAni has been modeled using exciton quenching by polarons through charge transfer. The investigation also reveals that the effect of exciton quenching by polarons becomes pronounced when the polaron concentration in PAni reaches a density of â¼1017-1018 polarons cm-3. To overcome the low emission efficiency of PAni, poly(methyl methacrylate) (PMMA) has been blended with PAni. The blending of donor PMMA (D) with acceptor PAni (A) gives rise to another quantum phenomenon - donor PMMA concentration dependent FRET between PAni (A) and PMMA (D). It is experimentally observed from the photoluminescence measurements of blends that at high donor PMMA concentration above a critical value in the PAni-PMMA polymer blend the emission profile of blends drops sharply. Donor concentration dependent FRET is a contradictory observation with respect to standard concentration independent FRET theory due to competition between inter-layer donor-acceptor and donor-donor intra-layer energy transfer within the donor layer. At high donor concentration intra-donor interaction gradually overtakes inter-layer donor-acceptor FRET which modifies the lifetime of the donor. The modification decreases the quantum yield of the donor and hence emission efficiency of blends above a critical concentration of PMMA by reducing inter donor-acceptor FRET. Thus, polaron exciton quenching and concentration dependent FRET are two dominant physical phenomena controlling luminescence in PAni-PMMA polymer blends. Therefore, optimization of luminescence of PAni-PMMA should be achieved by tuning the factors like reduction of spectral overlap between polarons and excitons in PAni, the density of PAni, diffusion of excitons in blends, and intra donor FRET within the PMMA layer before consideration of the blend being used as an emissive layer in PLEDs.
