Sunkara V. Manorama

SEMICONDUCTING GAS SENSOR FOR CHLORINE BASED ON INVERSE SPINEL NICKEL FERRITE

Abstract Nickel ferrite, a p-type semiconducting oxide with an inverse spinel structure has been used as a gas sensor to selectively detect chlorine in air. This compound was prepared by two different routes namely, the citrate and co-precipitation method and sensor properties of the resulting compounds from both the methods were compared. X-ray diffraction was used to confirm the structure. The gas sensing characteristics were obtained by measuring the sensitivity as a function of various controlling factors like dopant, concentration of the dopant, operating temperature, concentration of the gas and finally the response time. The sensitivity to chlorine has been compared with that of other interfering gases. A probable explanation has been proposed to explain the selective sensitivity to oxidising gases like chlorine.

A Facile and Green Approach for the Controlled Synthesis of Porous SnO2 Nanospheres: Application as an Efficient Photocatalyst and an Excellent Gas Sensing Material

A facile and elegant methodology invoking the principles of Green Chemistry for the synthesis of porous tin dioxide nanospheres has been described. The low-temperature (∼50 °C) synthesis of SnO₂ nanoparticles and their self-assembly into organized, uniform, and monodispersed porous nanospheres with high surface area is facilitated by controlling the concentration of glucose, which acts as a stabilizing as well as structure-directing agent. A systematic control on the stannate to glucose molar concentration ratio determines the exact conditions to obtain monodispersed nanospheres, preferentially over random aggregation. Detailed characterization of the structure, morphology, and chemical composition reveals that the synthesized material, 50 nm SnO₂ porous nanospheres possess BET surface area of about 160 m²/g. Each porous nanosphere consists of a few hundred nanoparticles ∼2-3 nm in diameter with tetragonal cassiterite crystal structure. The SnO₂ nanospheres exhibit elevated photocatalytic activity toward methyl orange with good recyclability. Because of the high activity and stability of this photocatalyst, the material is ideal for applications in environmental remediation. Moreover, SnO₂ nanospheres display excellent gas sensing capabilities toward hydrogen. Surface modification of the nanospheres with Pd transforms this sensing material into a highly sensitive and selective room-temperature hydrogen sensor.

HYDROGEN SULFIDE SENSOR BASED ON TIN OXIDE DEPOSITED BY SPRAY PYROLYSIS AND MICROWAVE PLASMA CHEMICAL VAPOR DEPOSITION

Spray pyrolysis and microwave plasma chemical vapor deposition techniques have been employed successfully for the deposition of CuO impregnated SnO2 films suitable for sensing hydrogen sulfide and methyl mercaptan. The observed change in conductivity of these films upon exposure to H2S gas in air has been explained on the basis of the band theory of solids.

High sensitivity and selectivity of an SnO2 sensor to H2S at around 100 °C

Abstract The effect of preparation conditions on the sensitivity of tin-oxide-based semiconducting gas sensors has been studied by appropriately engineering the base material. The incorporation of CuO as an additive has significantly improved the sensing character of the sensor element. These sensor elements have been made sensitive to low concentrations (about 10 ppm and less) of hydrogen sulphide in air. Further, the operating temperature for maximum sensitivity has been considerably reduced to about 90–100 °C. These elements have been tested for cross sensitivity to other gases and confirmed unambiguously to be specifically sensitive to H 2 S, which is detected with maximum sensitivity. A plausible mechanism is proposed to explain this behaviour.

Further insights in the conductivity behavior of nanocrystalline NiFe2O4

Control on the conductivity behaviors has been achieved in nanoparticles of NiFe2O4, synthesized by the hydrothermal route at a fixed temperature of 225°C by varying the pH of the starting solution. Particles synthesized at pH 7 and 8 behave as n-type semiconductors, while those synthesized at pH 9 and above behave as p-type semiconductors. The observed conductivity behavior has been confirmed by gas sensing and thermo-emf studies, and the mechanism has been established by x-ray photoelectron spectroscopic studies. Complete physicochemical characterizations of their phase and morphology have been carried out by x-ray diffraction and transmission electron microscopy. The thermal and electrical characteristics, elemental composition, and magnetic properties have been evaluated by thermogravimetry and differential thermogravimetry, dc conductivity, ac impedance studies, atomic absorption spectroscopy, and vibrating sample magnetometry. Gas sensing studies reveal that the resistance across the samples synthes...

Viable method for the synthesis of biphasic TiO2 nanocrystals with tunable phase composition and enabled visible-light photocatalytic performance.

Here we demonstrate a facile method to synthesize high-surface-area TiO(2) nanoparticles in aqueous-ethanol system with tunable brookite/rutile and brookite/anatase ratio possessing high surface area that exhibits enhanced photoactivity. Titanium tetrachloride (TiCl(4)) is used as the metal precursor of choice and the tuning of phase compositions are achieved by varying the water:ethanol ratio, used as mixed solvent system. The synthesized samples were characterized in detail using X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), BET nitrogen sorption measurements, and UV-vis diffuse reflectance spectroscopy (UV-DRS). The photocatalytic activity of biphasic TiO(2) nanocrystals was evaluated by following the degradation kinetics of rhodamine-B dye in aqueous solution and under visible light. Mixed-phase TiO(2) nanostructures composing 83% brookite and 17% of rutile exhibited superior photoactivity when compared to Degussa P25 and phase-pure anatase nanocrystals. The exceptional photocatalytic activity of the synthesized nanostructures can be elucidated on the account of their large surface area and biphasic composition. On the basis of the detailed investigation reported herein, we conclude that tuning the ethanol volume in the mixed-solvent reaction system holds the key to tailor and control the final TiO(2) phase obtained.

Tailored conductivity behavior in nanocrystalline nickel ferrite

In this letter, we report an important issue in nanoparticle synthesis by the “bottom up” approach. By controlling the pH of the starting mixture of the salts we have been successful in obtaining the desired conductivity of nanosized nickel ferrite. X-ray diffraction and transmission electron microscopy confirmed the size, structure, and morphology of the nanoferrites. All the materials are typical semiconducting oxides whose conductivity depends on the pH of the starting salt solution. Direct current and alternating current conductivity studies coupled with thermoelectric measurements and the resultant activation energies help us to propose the mechanism of conductivity in these ferrites. X-ray photoelectron spectroscopy studies are indicative of Ni3+ presence in p-type ferrite. The n- and p-type conductivity in these materials is attributed to the hopping due to the presence of Fe3+ and Ni3+ ions, respectively.

SnO2:Bi2O3 based CO sensor: Laser-Raman, temperature programmed desorption and X-ray photoelectron spectroscopic studies

Abstract The sensitivity and selectivity of tin oxide (SnO2) based gas sensors towards carbon monoxide (CO) was improved by doping the base material with Bi2O3 and Sb2O3. Laser-Raman studies of the compound confirm the formation of bismuth stannate (Bi2Sn2O7) above 800°C, which seems to be acting as a molecular sieve allowing only CO gas to react with the sensor surface, thereby imparting selectivity to the sensor. The chemisorption of oxygen on SnO2:Bi2O3 was investigated over a wide range of temperatures from room temperature to 800°C by means of Temperature Programmed Desorption (TPD) and X-ray Photoelectron Spectroscopic (XPS) studies which were carried out to establish the exact chemical species present on the sensor surface before and after the reaction.

LIQUID-PETROLEUM-GAS SENSOR BASED ON A SPINEL SEMICONDUCTOR, ZNGA2O4

Abstract A liquid-petroleum gas sensor has been developed with high selectivity and sensitivity based on an oxygen deficient spinel semiconductor, ZnGa2O4. The operating temperature of the sensor element has been optimised under different operating conditions. Palladium doped zinc gallate sensor, on exposure to LPG at about 320°C showed a change of 4–5 orders in the resistance. The response time also decreases from about 20 min in virgin ZnGa2O4 to less than 1 min in the palladium doped element. The electronic interaction between additive and semiconductor is proposed to account for the sensitisation effects in the sensor element.

SnO2 / Bi2 O 3: A Suitable System for Selective Carbon Monoxide Detection

An SnO 2 based gas sensor composition with Bi 2 O 3 and Sb 2 O 3 is shown to respond selectively to CO gas even in the presence of other gases. X-ray diffraction and laser-Raman studies of the compound confirm the formation of bismuth stannate (Bi 2 Sn 2 O 7 ) above 800°C, which seems to act as a molecular sieve. The operating temperature for maximum sensitivity is around 200°C. Temperature programmed desorption studies were conducted to determine the nature of oxygen species adsorbed on the semiconductor surface responsible for the gas sensing.