Cerium doped MgFe2O4 nanocomposites: highly sensitive and fast response-recoverable acetone gas sensor.

We report a facile synthesis of Cerium doped MgFe2O4 nanocomposite ferrite and its usability as gas-sensor via simple and robust synthesis approach of glycine-combustion-process. The route utilizes metal nitrates (Ce, Mg, Fe -nitrates) and glycine, in aqueous solution. The involved sol-gel concept was explained on the basis of zwitterion characteristic of glycine. The analysis of the developed ferrite was done in two different ways - i) effect of Ce-doping concentration, and ii) effect of sintering temperature. With the ferrite system MgFe2-xCexO4, the doping concentration of Ce was varied from 0.04 to 0.12 with the step x = 0.04, and sintering was done at two different temperatures i.e. 973K and 1173K. As-produced composite system was examined for their gas response towards reducing gases such as LPG, ethanol, acetone and ammonia. The material displayed excellent gas sensing properties towards acetone for wide operating temperature range of 575-675 K. The XRD analysis revealed nanocrystallinity with crystallite size in the range of 28-34 nm. Microstructural analysis confirmed the porous morphology due to auto-ignition during the combustion reaction. The present investigations confirm the produced MgFe2-xCexO4 is a promising candidate for fabricating high performance acetone sensor.

Optical and photocatalytic properties of single crystalline ZnO at the air-liquid interface by an aminolytic reaction.

Crystalline flowerlike ZnO was synthesized by an aminolytic reaction at the air-liquid interface in an aqueous media at an alkaline pH. A thin visible film was formed at the air-liquid interface by self-assembly of flowerlike ZnO. Diffraction studies show rearrangement of the single crystalline units at the air-liquid interface leading to the formation of nanobelts. These nanobelts overlap systematically to form petals of the flowerlike structure; individual petals get curved with time. Each nanobelt is found to be single crystalline and can be indexed as the hexagonal ZnO phase. The organic product formed in the aminolytic reaction and dissolution-reprecipitation mechanism is the driving force for the formation of flowerlike ZnO at the air-liquid interface. A clear relationship between the surface, photocatalytic, and photoluminescent properties of ZnO is observed. The flowerlike structure exhibits a blue shift (3.56 eV) in the band emission as compared to bulk ZnO (3.37 eV). The photodegradation of methylene blue over the flowerlike ZnO catalyst formed at the air-liquid interface and in the sediments shows enhanced photocatalytic activity. The sub-bands formed due to surface defects facilitate separation of charge carriers increasing their lifetime, leading to enhanced photocatalytic activity of flowerlike ZnO.

Controlled synthesis of ZnO from nanospheres to micro-rods and its gas sensing studies.

1D ZnO rods are synthesized using less explored hydrazine method. Here we find, besides being combustible hydrazine can also be used as a structure-directing agent. The ratio of zinc nitrate (ZN) to hydrazine is found to control the morphology of ZnO. At lower concentration of ZN as compared with hydrazine the morphology of ZnO is found to be spherical. As we increase the hydrazine content the morphology changes from spherical (diameter approximately 100 nm) to the elongated structures including shapes like Y, T as well dumbbell (diameter approximately 40 nm and length approximately 150 nm). Interestingly for more than 50% of hydrazine ZnO micro-rods are formed. Such rods are of diameter approximately 120 nm having length of about 1 microm for ZN to hydrazine ratio of 1:9, isolated as well as bundle of rods are seen in scanning electron microscopy (SEM). The X-ray diffraction (XRD) reveals the phase formation with average particle size of 37 nm as calculated using Scherrer's formula. The high-resolution transmission electron microscopy (HRTEM) is also done to confirm the d-spacing in ZnO. Gas sensing study for these samples shows high efficiency and selectivity towards LPG at all operating temperatures. Photoluminescence (PL) study for these samples is performed at room temperature to find potential application as photoelectric material.

Formation of Cu and Cu2O nanoparticles by variation of the surface ligand: preparation, structure, and insulating-to-metallic transition.

Copper and copper (I) oxide nanoparticles protected by self-assembled monolayers of thiol, carboxyl, and amine functionalities [X(CH(2))(n)-CH(3), where X can be -COOH, -NH(2), or -SH] have been prepared by the controlled reduction of aqueous copper salts using Brust synthesis. The optical absorption spectrum (lambda(max)=289 nm) is found to be invariant with the nature of the capping molecule while the particle shape and distribution are found to depend strongly on it. A comparison of the protection efficiency for different capping agents such as dodecanethiol (DDT), tridecylamine (TDA), and lauric acid (LA) suggests that although zerovalent Cu is initially formed for dodecanethiol, all other cases allow oxidation to Cu(2)O nanoparticles. Despite the variation in particle size and relative stability, nanoparticles have been found to form oxides after a few days, especially for the case of LA and TDA surface capping. For all the samples studied, the size has been found to be 4-8 nm by high-resolution transmission electron microscopy. The protective ability is found to be better for dodecanethiol SAM (similar to the case of Au and Ag nanoparticles), while the order of capping efficiency varies as Cu-DDT>Cu-TDA>Cu-LA. In the present study we also demonstrate a reversible metal-insulator transition (MIT) in capped nanoparticles of Cu using temperature-dependent electrical resistivity measurement. However, the LA-capped sample does not show any such transition, possibly due to the oxide formation.