Kaustava Bhattacharyya

Synergetic effect of MoS2–RGO doping to enhance the photocatalytic performance of ZnO nanoparticles

The sunlight driven photocatalytic activity of semiconductor based nanostructures has attracted widespread attention in recent years for environmental remediation and energy applications. Numerous good semiconductors, including ZnO, have wide bandgaps and are active only under ultraviolet light, which comprises only 5% of sunlight. Several strategies, such as noble metal doping, non-metal doping, etc., have been adapted to make ZnO heterostructures active in the visible light region. One other strategy is to dope ZnO with narrow bandgap semiconductors like MoS2. In addition, co-doping with graphene as a support material can enhance pollutant adsorption and can aid in electron transport, thereby leading to pollutant degradation. In this work, we report our investigations on the synergetic role played by MoS2–RGO doping to enhance the photocatalytic activity of ZnO nanoparticles, and especially to utilize both the UV and visible light regions of the solar spectrum. The ZnO–MoS2–RGO heterostructures, having different levels of doping, were prepared by a facile hydrothermal method and were characterized thoroughly using different spectroscopy and microscopy techniques. The photocatalytic performance was evaluated by studying the degradation of methylene blue, a model dye pollutant, and carbendazim, a colorless hazardous fungicide, under natural sunlight irradiation. The results reveal that doping of ZnO nanoparticles with 1 wt% MoS2–RGO was optimal and possessed the highest photocatalytic activity among all the investigated samples. A possible mechanism is proposed and discussed in detail.

Intense red emitting monoclinic LaPO4:Eu3+ nanoparticles: host–dopant energy transfer dynamics and photoluminescence properties

LaPO4 nanoparticles were synthesized using complex polymerization method and characterized systematically using X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. Varied concentration of europium ion is doped in the LaPO4 lattice and optical properties and Judd–Ofelt analysis were investigated. It is observed that LaPO4 nanoparticles give violet–blue emission when irradiated with UV light. On doping europium ion the band gap of LaPO4 decreases. Based on DFT calculations it is proposed that energy range over which d and f states of Eu are distributed is coincident with the valence band of LaPO4 so causing an efficient energy transfer from LaPO4 to europium ion. The actual site symmetry for europium ion in lanthanum orthophosphate was also evaluated as D2d based on the Stark splitting pattern although it is C1 for La3+ in LaPO4. The critical energy-transfer distance for the Eu3+ ions was evaluated, based on which the quenching mechanism was verified to be an electric multipolar interaction. It is also observed that the red emission intensity of LaPO4:Eu3+ (2.0 mol%) is almost 85% of a commercial red phosphor, which clearly demonstrates that the as-prepared samples are promising red phosphors under near-UV for use in white-light emitting diodes.

N-doped ZnO–MoS2 binary heterojunctions: the dual role of 2D MoS2 in the enhancement of photostability and photocatalytic activity under visible light irradiation for tetracycline degradation

In this work, we report the fabrication of binary semiconductor heterojunctions comprising N-doped ZnO nanorods loaded with two-dimensional MoS2 nanoflowers in varying amounts, using a facile hydrothermal synthesis method. These semiconductor heterojunctions have been demonstrated to be highly efficient photocatalysts with enhanced performance under visible light irradiation for the degradation of a pharmaceutical pollutant, tetracycline. The superior photocatalytic activity of the heterojunctions can be attributed to the synergistic effect of N-doping of ZnO and loading of MoS2 leading to higher absorption of visible light, efficient separation of photogenerated charge carriers and rapid charge transfer to reaction sites, as per the conduction band potentials of both N-doped ZnO and MoS2. In addition, the two-dimensional nanoflower morphology of MoS2 provides more reaction sites for the adsorption of pollutants, due to its large surface area. Furthermore, the transfer of holes from the valence band of N-doped ZnO to the valence band of MoS2 prevents the photocorrosion of N-doped ZnO resulting in enhanced photostability of the catalyst during the reaction.

Efficient Electron Transfer across ZnO-MoS2-RGO Heterojunction for Remarkably Enhanced Sunlight Driven Photocatalytic Hydrogen Evolution

The development of noble metal-free catalysts for hydrogen evolution is required for energy applications. In this regard, ternary heterojunction nanocomposites consisting of ZnO nanoparticles anchored on MoS2 -RGO (RGO=reduced graphene oxide) nanosheets as heterogeneous catalysts show highly efficient photocatalytic H2 evolution. In the photocatalytic process, the catalyst dispersed in an electrolytic solution (S2- and SO32- ions) exhibits an enhanced rate of H2 evolution, and optimization experiments reveal that ZnO with 4.0 wt % of MoS2 -RGO nanosheets gives the highest photocatalytic H2 production of 28.616 mmol h-1  gcat-1 under sunlight irradiation; approximately 56 times higher than that on bare ZnO and several times higher than those of other ternary photocatalysts. The superior catalytic activity can be attributed to the in situ generation of ZnS, which leads to improved interfacial charge transfer to the MoS2 cocatalyst and RGO, which has plenty of active sites available for photocatalytic reactions. Recycling experiments also proved the stability of the optimized photocatalyst. In addition, the ternary nanocomposite displayed multifunctional properties for hydrogen evolution activity under electrocatalytic and photoelectrocatalytic conditions owing to the high electrode-electrolyte contact area. Thus, the present work provides very useful insights for the development of inexpensive, multifunctional catalysts without noble metal loading to achieve a high rate of H2 generation.

Photodegradation of Methanol Under UV―Visible Irradiation by Titania Dispersed on Polyester Cloth

Titania supported on polyester fabric (TiO2–PY) with varying titania loadings (2–7 wt%) were prepared via the dip‐coating method at room temperature using an aqueous slurry of anatase titania. Structural and morphological characterizations by X‐ray diffraction and scanning electron microscopy revealed that the titanium dioxide crystallites deposited on the surface of the polyester fabric were in the micrometer range while their phase remained to be anatase. Photocatalytic activity of TiO2–PY fabric catalysts was evaluated for vapor‐phase oxidation of methanol in air as a test reaction in the presence of UV as well as solar radiation under ambient conditions. These catalysts were found to be quite active in both UV and solar irradiation with activity being higher in the former case. CO2 yield from photo‐oxidation of methanol depended on titania content and also on its dispersion over polyester fabric support.

Gas‐Phase Photooxidation of Alkenes by V‐Doped TiO2‐MCM‐41: Mechanistic Insights of Ethylene Photooxidation and Understanding the Structure–Activity Correlation

Nanoparticles of Ti(0.95)V(0.05)O(2) were found to be impregnated in the hexagonal channels of the MCM-41 host, with a distribution of some particles on the surface, thus leading to an effective variation in the particle size as a function of loading host MCM-41 matrix. These catalysts were subjected to the photocatalytic degradation of alkenes under the ambient conditions in which the photocatalytic activity varied as a function of the loading percentage of Ti(0.95)V(0.05)O(2) in the host MCM-41.This is explained in light of the structure-activity correlation, and the better catalytic activity can be attributed to an electronic interaction between the host and guest molecules, as established from X-ray photoelectron spectroscopy. To understand the mechanistic aspect of the photooxidation of ethylene on the vanadium-doped titania dispersed in the MCM-41 matrix, extensive in situ FTIR experiments were undertaken. The intermediate species produced on bare Ti(0.95)V(0.05)O(2) are different from that produced on the Ti(0.95)V(0.05)O(2)/MCM-41 surface. Moreover, different intermediates were produced during ethylene oxidation under UV and visible irradiation, thus leading to different rates. The ethylene decomposition over bare Ti(0.95)V(0.05)O(2) occurs by means of formation of ethoxy groups, transformed to acetaldehyde or enolates, subsequently to acetates, and then to CO(2) under both UV and visible irradiation. However, in the case of Ti(0.95)V(0.05)O(2)/MCM-41 catalyst with UV irradiation, the adsorbed acetaldehyde thus formed undergoes aldol condensation over the Lewis acid sites to lead to the formation of crotonaldehyde, which is subsequently oxidized to acetate and consequently to CO(2). It was observed that during visible irradiation labile ethyl acetate is produced either by the Tischenko reaction or by the reaction between the labile acetic acid and the unreacted ethoxy groups. The ethyl acetate produces acetic acid monomer, which is oxidized to CO(2). Furthermore, in this work the effects of particle size on the intermediate species were also studied.

Correlation of Mo dopant and photocatalytic properties of Mo incorporated TiO2: an EXAFS and photocatalytic study

The present study addresses the quantitative estimation and understanding of the nature of phases present in Mo-incorporated titania through extensive EXAFS measurements (at Mo K-edge and Ti K-edge) and to decipher their role in photodegradation of Methylene Blue (MB) dye under UV and visible irradiation. EXAFS results revealed the presence of both MoO3 nano heterophase phase and substitutional Mo dopants in the TiO2 lattice, the extent of the latter was reduced with the increasing Mo content of the sample. The presence of MoO3 in the Mo-incorporated titania was also revealed by FT-IR and TEM studies. Photocatalytic studies have shown considerable adsorption of the cationic MB-dye, perhaps due to the electronic interaction between the dye and catalytic surface. Under visible irradiation, the photocatalytic activity followed the trend: Mo-5 > Mo-2 > Mo-1 > Mo-10 ≫ TiO2, while the trend for photodegradation of MB dye under UV irradiation was as follows: Mo-5 > Mo-2 > Mo-1 > TiO2 > Mo-10. These results have been explained in the light of the structural properties of the Mo–TiO2 system obtained from EXAFS measurements. It has been observed that the relative ratio of substitutional Mo-dopant to the MoO3 phase in this tri-phasic photocatalyst plays a crucial role in augmenting its oxidative photocatalytic property.

Utilizing non-stoichiometry in Nd2Zr2O7 pyrochlore: exploring superior ionic conductors

To explore materials with high ionic conductivity without any heterovalent substitution, a non-stoichiometric Nd2−xZr2+xO7+x/2 (−0.2 ≤ x ≤ 0.4) pyrochlore system was synthesized via a combustion process. All the compositions were characterized by X-ray diffraction and revealed perfect pyrochlore-type compounds. Interestingly, Raman spectroscopy exhibited an enhanced oxygen disorder in Nd1.6Zr2.4O7.2 (x = 0.4) relative to the rest of the compositions, which was manifested in a substantially high pre-exponential factor for this composition. Detailed AC impedance analysis revealed that the ionic conductivity for Nd1.6Zr2.4O7.2 was higher by two orders of magnitude despite the similar structures adopted by all the other compositions. X-ray photoelectron spectroscopy provided support for the high ionic conductivity observed. Modulus spectroscopic analysis related the microscopic energy barrier to the macroscopic activation energy. The highlight of the study was the high ionic conductivity (1.78 × 10−2 S cm−1 at 973 K) for Nd1.6Zr2.4O7.2, which is comparable to the highest values reported for a zirconia-based system at this temperature. This was attributed to the optimized hybrid structure consisting of fluorite-type disordered zones interspersed in the ordered pyrochlore structure obtained under suitable annealing conditions. The present study highlights the utilization of non-stoichiometry in the pyrochlore structure to tailor its ionic conductivity. Such high ionic conductivity observed in Nd1.6Zr2.4O7.2 makes it a superior candidate for electrolyte materials as it is endowed with the stability of zirconia-based systems and has ionic conductivity comparable to ceria-based systems.

In situ FT-IR studies on mechanistics of heterogeneous photocatalytic oxidation of ethene over uranyl species anchored on MCM-41

The uranyl species encapsulated within the mesopores of siliceous MCM-41 serves as efficient heterogeneous photo-catalyst for the sunlight-assisted direct oxidation of ethene. The mode of oxidation is through abstraction of H-atom from ethene by the photolytically excited uranyl species and the consequent formation of peroxy species. The in situ IR spectroscopy results indicate that these peroxy species give rise to final products such as carbon dioxide and water on further oxidation via formation of formate-type transient species. Furthermore, the silanol groups of the host matrix help in immobilization of these peroxy species through hydrogen bonding and, at the same time, they participate in the subsequent oxidation reactions also.

Study of the oxidation state and the structural aspects of the V-doped TiO2

A modified sol-gel method for synthesizing vanadium doped titania is being reported. These materials were thoroughly characterized for their oxidation states by electron paramagnetic resonance and x-ray absorption near edge structure and the local environment of the V-atom were investigated by the x-ray absorption fine structure. V-doped titania was found to be more active than nanotitania for photo-oxidation of methane in air under ambient conditions using UV-visible irradiation. The vanadium doping in the crystal lattice of titania leads to a mixture of oxidation states of 4+ and 5+ in the crystal lattice sites of the TiO2, which is crucial for its catalytic activity.