Novel bilayer 2D V2O5 as a potential catalyst for fast photodegradation of organic dyes.

Two-dimensional (2D) materials have recently drawn interest in various applications due to their superior electronic properties, high specific surface area, and surface activity. However, studies on the catalytic properties of the 2D counterpart of V2O5 are scarce. In the present study, the catalytic properties of 2D V2O5 vis-à-vis bulk V2O5 for the degradation of methylene blue dye are discussed for the first time. The 2D V2O5 catalyst was synthesized using a modified chemical exfoliation technique. A massive increase in the electrochemically active surface area of 2D V2O5 by one order of magnitude greater than that of bulk V2O5 was observed in this study. Simultaneously, ~ 7 times increase in the optical absorption coefficient of 2D V2O5 significantly increases the number of photogenerated electrons involved in the catalytic performance. In addition, the surface activity of the 2D V2O5 catalyst is enhanced by generating surface oxygen vacancy defects. In the current study, we have achieved ~ 99% degradation of 16 ppm dye using the 2D V2O5 nanosheet catalysts under UV light exposure with a remarkable degradation rate constant of 2.31 min-1, which is an increase of the order of 102 from previous studies using V2O5 nanostructures and nanocomposites as catalysts. Since the enhanced photocatalytic activity emerged from the surface and optical properties of the catalyst, the current study shows great promise for the future application of 2D V2O5 in photo- and electrocatalysis.

Spectroscopic determination of the role of vanadyl oxygen in room temperature NH3 sensing by V2O5 nanoparticles.

In the present study, a multi-modal approach consisting of in-situ photoluminescence, Raman, and UV-Vis absorption spectroscopic studies is carried out along with chemiresistive sensing to unveil the mechanism of NH3 gas sensing by V2O5 nanoparticles in ambient air. V2O5 nanoparticles with an average size of 49 nm show a superior sensor response of 17 ± 1.5 % towards 1 ppm of NH3 gas with a response and recovery time of 96 s and 45 s, respectively. The photoluminescence and UV-Vis absorption studies in the presence of NH3 reveal electron doping to a new energy level at 1.84 eV, resulting in conduction band filling and increase in the optical band gap. The intensity of the photoluminescence spectrum shows an increase in the presence of NH3 gas as a result of this electron doping. The sensor response from the optical sensing carried out by in-situ photoluminescence study is 43 % for 40 ppm of NH3 gas. The vanadyl oxygen site is the most active in the sensing process, as evident by a selective enhancement in the intensity of V-O (vanadyl) bond vibration. This study gives an experimental evidence for the changes in optical and electronic properties of V2O5 on the adsorption of NH3 gas molecules.

Electronic and Vibrational Decoupling in Chemically Exfoliated Bilayer Thin Two-Dimensional V2O5.

The synthesis of high-quality two-dimensional (2D) transition metal oxides is challenging compared to 2D transition metal dichalcogenides as a result of the exotic surface changes that can appear during formation. Herein, we report the synthesis of bilayer 2D V2O5 nanosheets with a thickness of ∼1 nm using the chemical exfoliation method and a comprehensive study on the vibrational and optical properties of bilayer 2D V2O5. We report, for the first time, a thickness-dependent blue shift of 1.33 eV in the optical bandgap, which signifies the emergence of electronic decoupling in bilayer 2D V2O5. In addition, a thickness-dependent vibrational decoupling of phonon modes observed via Raman spectroscopy fingerprinting was verified by computing the lattice vibrational modes using the density functional perturbation theory. We demonstrate that the manifestation of the electronic and vibrational decoupling can be used as a benchmark to confirm the successful formation of bilayer 2D V2O5 from its bulk counterpart.

Gas sensing materials roadmap.

Gas sensor technology is widely utilized in various areas ranging from home security, environment and air pollution, to industrial production. It also hold great promise in non-invasive exhaled breath detection and an essential device in future internet of things. The past decade has witnessed giant advance in both fundamental research and industrial development of gas sensors, yet current efforts are being explored to achieve better selectivity, higher sensitivity and lower power consumption. The sensing layer in gas sensors have attracted dominant attention in the past research. In addition to the conventional metal oxide semiconductors, emerging nanocomposites and graphene-like two-dimensional materials also have drawn considerable research interest. This inspires us to organize this comprehensive 2020 gas sensing materials roadmap to discuss the current status, state-of-the-art progress, and present and future challenges in various materials that is potentially useful for gas sensors.

Investigation on CH4 sensing characteristics of hierarchical V2O5 nanoflowers operated at relatively low temperature using chemiresistive approach.

Methane (CH4) gas, the second most potent greenhouse gas share a substantial role in contributing to the global warming and it is a necessary pre-requisite to detect the release of CH4 into the environment at its early stage to combat climate change. In that front, this work is focussed to develop an effective CH4 gas sensor using vanadium pentoxide (V2O5) thin films that works at an operating temperature of ∼100 °C. To understand the effect of sputtering power towards the structural characteristics of V2O5 films, X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) analysis were performed from which the orthorhombic polycrystalline structure of V2O5 thin films was confirmed with varied texture co-efficient. Further, the surface elemental studies using X-ray photoelectron spectroscopy (XPS) confirmed the prominence of V+5 oxidation state from the binding energy of V2p3/2 and O1s peak. The effect of sputtering power on the growth of different nanostructures were observed using field-emission scanning electron microscopy (FE-SEM). The critical role of adsorption and desorption kinetics of the deposited nanostructures were explained through first order kinetics based on Elovich model and the phase stability of different nanostructures were evaluated using Raman spectral analysis. This work achieved the sensor response of about ∼8% towards CH4 at an operating temperature of 100 °C towards 50 ppm concentration.

The Role of In-Plane Oxygen Vacancy Defects in SnO2 Nanoparticles for CH4 Sensing.

The utility of different types of surface defects in SnO2 nanoparticles (NPs) for the detection of low concentration (50 ppm) of methane (CH4) at relatively low temperature of 50 °C is established. Chemically synthesized SnO2 quantum dots are annealed in air and Ar environments at 800 °C to make two different sets of SnO2 NPs. Variation in dimension, morphology and optical properties due to the annealing conditions are elaborated using X-ray diffraction, transmission electron microscopy and UV Visible spectroscopy respectively. Electron energy loss spectroscopy provides an insight of defect distribution in NPs. Detailed temperature dependent photoluminescence and the Raman studies allow understanding the interplay of in-plane oxygen and bridging oxygen vacancies in above two samples for low concentration CH4 detection at low temperature. The sensor response was about 1-2% due to low operating temperature. The decisive role of in-plane oxygen vacancy to detect low concentrations of gas and utility of bridging oxygen vacancy for improved response at high temperature are further corroborated from the analysis of sensor response and Arrhenius type plots.

Native defect-assisted enhanced response to CH4 near room temperature by Al0.07Ga0.93N nanowires.

Gas sensors at low operating temperature with high sensitivity require group III nitrides owing to their high chemical and thermal stabilities. For the first time, Al0.07Ga0.93N nanowires (NWs) have been utilized in CH4 sensing, and it has been demonstrated that they exhibit an improved response compared to GaN NWs at the low operating temperature of 50 °C. Al0.07Ga0.93N NWs have been synthesized via the ion beam mixing process using inert gas ion irradiation on the bilayer of Al/GaN NWs. The sensing mechanism is explained with the help of native defects present in the system. The number of shallow acceptors created by Ga vacancies (VGa) is found to be higher in Al0.07Ga0.93N NWs than in as-grown GaN NWs. The role of the O antisite defect (ON) for the formation of shallow VGa is inferred from photoluminescence spectroscopic analysis. These native defects strongly influence the gas sensing behaviour, which results in enhanced and low-temperature CH4 sensing.