Arun Kumar Sinha

Multifunctional Au-ZnO Plasmonic Nanostructures for Enhanced UV Photodetector and Room Temperature NO Sensing Devices

In this study we report the enhancement of UV photodetection and wavelength tunable light induced NO gas sensing at room temperature using Au-ZnO nanocomposites synthesized by a simple photochemical process. Plasmonic Au-ZnO nanostructures with a size less than the incident wavelength have been found to exhibit a localized surface plasmon resonance (LSPR) that leads to a strong absorption, scattering and local field enhancement. The photoresponse of Au-ZnO nanocomposite can be effectively enhanced by 80 times at 335 nm over control ZnO. We also demonstrated Au-ZnO nanocomposites application to wavelength tunable gas sensor operating at room temperature. The sensing response of Au-ZnO nancomposite is enhanced both in UV and visible region, as compared to control ZnO. The sensitivity is observed to be higher in the visible region due to the LSPR effect of Au NPs. The selectivity is found to be higher for NO gas over CO and some other volatile organic compounds (VOCs), with a minimum detection limit of 0.1 ppb for Au-ZnO sensor at 335 nm.

Fabrication of Large‐Scale Hierarchical ZnO Hollow Spheroids for Hydrophobicity and Photocatalysis

We report here the preparation of a crystalline, pure hexagonal phase of ZnO as hollow 500-800 nm spheroids in the presence of organic bases, such as pyridine, using zinc acetate as the precursor salt. The spheroids exhibit unique 3D hierarchical architectures, like cocoons, and demonstrate improved superhydrophobic (water contact angle, 150 degrees) character due to the inherited air-trapped capillarity within the cocoon structure. The simple synthetic strategy used in this process is modified hydrothermolysis (MHT), which represents a general approach and may contribute to the formation mechanism of the hollow nanostructures with highly improved porosity. Depending on the concentration of the precursor salt, it has been possible to cover glass plates or the inner wall of a reaction vessel with ZnO nanocrystals. A low salt concentration (<0.01 M) allows the easy preparation of a superhydrophobic glass surface, whereas a high salt concentration (>0.01 M) results in the precipitation of cocoons at the bottom of the reaction vessel as a solid mass together with a deposited thin film of ZnO nanocrystals covering the inner wall of the glass vessel. The thickness of the film successively grows through repetitive hydrothermolysis processes for which a low salt concentration (<0.01 M) was employed. Because of the hollow cocoon-like morphology, the surface area of the film is greatly increased, which makes it accessible for functionalization by incoming substrates from both sides (internally and externally) and helps to drive a competent photocatalytic dye degradation pathway. The heterocyclic base pyridine exclusively develops cocoons. Thus, the mechanism of self-aggregation of ZnO nanocrystals under MHT reaction conditions has been studied and the characterization of the compounds has been supported with physical measurements.

A one pot synthesis of Au–ZnO nanocomposites for plasmon-enhanced sunlight driven photocatalytic activity

To enhance the photocatalytic efficiency of ZnO nanomaterials, plasmonic Au nanoparticles (NPs) have deliberately been introduced into ZnO through a facile, inexpensive one pot hydrothermal approach. The enhanced photocatalytic efficiency was ascribed to surface plasmon resonance induced local electric field enhancement of Au. Thus the photoproduced e−–h+ pair in ZnO under sunlight irradiation is reluctant to recombine. The as-synthesized photocatalyst with an excellent sunlight driven photocatalytic activity can effectively decompose various kinds of organic dyes and maintain a high level of photoactivity even after four cycles. The photocatalytic activity of the Au–ZnO nanocomposites was examined by the photodegradation of a series of cationic and anionic dye molecules such as rhodamine B, Congo red, methyl orange, methylene blue, and Rose Bengal. It is interesting to note that with a reasonable increase in the Au concentration the shape of the nanocomposites remains unaltered but the visible light driven photocatalytic activity is enhanced. This observation and the result are promising for plasmonic photocatalysis. The presence of Au nanoparticles makes the Au–ZnO nanocomposite a superior photocatalyst over the ZnO nanomaterial and the nanocomposite presents altogether a different scenario. In a nut-shell the present study reports not only a new insight into the gram level synthesis of a plasmonic photocatalyst from one pot but also its application in waste water treatment through the degradation of toxic dye molecules upon direct sunlight exposure.

Transparent and flexible resistive switching memory devices with a very high ON/OFF ratio using gold nanoparticles embedded in a silk protein matrix

The growing demand for biomaterials for electrical and optical devices is motivated by the need to make building blocks for the next generation of printable bio-electronic devices. In this study, transparent and flexible resistive memory devices with a very high ON/OFF ratio incorporating gold nanoparticles into the Bombyx mori silk protein fibroin biopolymer are demonstrated. The novel electronic memory effect is based on filamentary switching, which leads to the occurrence of bistable states with an ON=OFF ratio larger than six orders of magnitude. The mechanism of this process is attributed to the formation of conductive filaments through silk fibroin and gold nanoparticles in the nanocomposite. The proposed hybrid bio-inorganic devices show promise for use in future flexible and transparent nanoelectronic systems.

Tin oxide with a p–n heterojunction ensures both UV and visible light photocatalytic activity

Tuning of the tin oxide (SnO/SnO2) heterojunction (TOHJ) has been made possible by heating the as-prepared p-type SnO semiconductor in air in a controlled fashion. Thus a better photocatalytic activity for dye degradation under UV, visible as well as solar light irradiation was achieved. Multiple reflection of light and the TOHJ of SnO plates facilitate the photocatalysis reactions.

Selective and Sensitive Recognition of Cu2+ in an Aqueous Medium: A Surface-Enhanced Raman Scattering (SERS)-Based Analysis with a Low-Cost Raman Reporter

In the present study, surface-enhanced Raman spectra of a bifunctional Raman reporter, 2-mercaptobenzimidazole, has been found to be responsive exclusively towards Cu(2+) ions while the reporter remains anchored on the Au nanoparticle surface. Thus a specific Cu(2+)-ion-detection protocol emerges. The simplicity, sensitivity, and reproducibility of the method allow routine and quantitative detection of Cu(2+) ions. An interference study involving a wide number of other metal ions shows the procedure to be uniquely selective and analytically rigorous. A theoretical study was carried out to corroborate the experimental results. Finally, the method is promising for real-time assessment of Cu(2+) ions in aqueous samples and also has the ability to discriminate Cu(I) and Cu(II) ions in solution.

Synthesis of gold nanochains via photoactivation technique and their catalytic applications.

The article reports a simple photoactivation technique for the synthesis of chain like assembly of spherical Au nanocrystals using a nontoxic biochemical, β-cyclodextrin under ~365 nm UV-light irradiation. Under UV irradiation, β-cyclodextrin acts as a reducing as well as capping agent and eventually becomes a stabilizing linker for Au nanoparticles. The UV-visible spectroscopy, transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD), and X-ray photoelectron spectroscopic techniques are employed to systematically characterize the Au nanochains. Additionally, it is shown that the Au nanocrystals act as an effective catalyst for the reduction in nitrobenzene to aniline and methylene blue to leuco methylene blue in presence of suitable reducing agent. The catalytic reduction reactions and kinetic parameters are evaluated from UV-visible spectroscopy.

New hydrothermal process for hierarchical TiO2nanostructures

Hydrothermolysis of inorganic Ti(IV) compounds (peroxo or sulfate) yields hierarchical titania nanocrystals of controlled morphologies on either a glass or resin surface.

Synthesis of Superparamagnetic β-MnO2 Organosol : a Photocatalyst for the Oxidative Phenol Coupling Reaction

Superparamagnetic monodispersed spherical beta-MnO 2 nanoparticles of approximately 10 nm size with a band gap of 2.52 eV have been synthesized in toluene and support the oxidative phenol coupling reaction as a photocatalyst.

Fabrication of a ZnO nanocolumnar thin film on a glass slide and its reversible switching from a superhydrophobic to a superhydrophilic state

A simple, cost effective way to deposit ZnO thin films on glass slides has been observed under modified hydrothermal (MHT) conditions. The nanocolumnar growth of ZnO on glass surfaces takes place as a result of the hydrolysis of zinc sulfate by ethanolamine at 100 °C without employing any templates or surfactants. The evolved superhydrophobic thin film becomes hydrophilic upon UV light exposure and the film reverts back to its original superhydrophobic character upon storage in the dark. The water contact angle switches back and forth from 154° ± 1° to 15°. Characterization of the ZnO thin films and the nanocolumnar growth mechanism have been discussed.